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Kinetics of the Boudouard reaction for low-rank Western-Canadian coals de Carvalho, Roberto José 1986

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KINETICS OF THE BOUDOUARD REACTION FOR LOW-RANK WESTERN-CANADIAN COALS By ROBERTO JOSE DE|CARVALHO Met. Eng., P o n t i f i c i a Universidade Ca t o l i c a do Rio de Janeiro, B r a s l l , 1975 M.Sc. ( M e t a l l u r g i c a l Engineering), P o n t i f i c i a Universidade Catolica do Rio de Janeiro, B r a s i l , 1978 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n THE FACULTY OF GRADUATE STUDIES Department of M e t a l l u r g i c a l Engineering We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1986 ® Roberto Jose de Carvalho, 1986 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f the requirements f o r an advanced degree a t the U n i v e r s i t y o f B r i t i s h Columbia, I agree t h a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and study. I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the head o f my department or by h i s o r her r e p r e s e n t a t i v e s . I t i s understood t h a t copying or p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be allowed without my w r i t t e n p e r m i s s i o n . Department of METALL Uft ftlCftU Mfr The U n i v e r s i t y o f B r i t i s h Columbia 2075 Wesbrook P l a c e Vancouver, Canada V6T 1W5 th D a t e ZJuME 27 , 1986 ABSTRACT The kinetics of gasification of two Alberta sub-bituminous coal chars with CO2 have been investigated in the temperature range of 800-950°C. The reactor utilized in the experimental work was a laboratory-size batch fluidized bed. The overall gasification kinetics were followed by measurements of gas composition and flow rates. Chars in the particle size -841 + 420 ym were gasified with gas mixtures involving CO, C 0 2 and He. Initially i t was determined that beds containing 20.0 g of char (L/D - 0.25), and a total inlet flow rate of 10 1/min were able to provide an adequate mixing of the reactants and near isothermal conditions in most of the experiments. Moreover these operating conditions allowed the measurement of reaction rates with minimum influence of CO2 starvation, elutriation of char particles and fluidization hydrodynamics. Chars prepared with longer soak time and lower heating rate were less reactive due to their lower surface area and more closed pore structure. For the conditions studied, Highvale chars were more reactive than similarly prepared Smoky Tower chars primarily due to differences in surface area. Increasing PCO2 *-n t n e * n l e t Sas caused a non-linear increase in the reaction rate, and CO strongly retarded the reaction, mainly at low temperatures, suggesting that Langmuir-Hinshelwood kinetics are followed. The reaction also was heavily affected by temperature, especially with increasing concentration of CO due to the - i i i -poisoning effect of this gas. The external appearance of char particles and the changes in pore structure and ash characteristics with the extent of reaction were examined by SEM. The ini t i a l rate of reaction follows the LH equation. The apparent activation energies for the rate constants kj and k 2 of this equation are respectively 176 and 286 kJ/mole. However the LH equation was not the most suitable equation to represent the kinetics of the reaction under the conditions investigated. Therefore a power-law rate equation that accounted for the variation in surface area of the chars was employed. The orders of reaction obtained for Highvale and Smoky Tower chars are 0.4 and 0.7 respectively. The initial apparent activation energies for Highvale chars are 143,210 and 255 kJ/mole for C0/C02 ratios of 0, 0.25 and 0.50 respectively. For Smoky Tower chars the ini t i a l apparent activation energy is 202 kJ/mole. The equations of Bhatia and Perlmutter and Dutta et al. were used to correlate the reactivity of both chars with the extent of reaction. The equation of Bhatia and Perlmutter was able to represent the data better at any temperature for Smoky Tower chars and up to 900°C for Highvale chars. For Highvale chars, the equation of Dutta et al. fitted the data better at 950°C. These results and the values of apparent activation energy obtained suggest that, for the conditions investigated, the gasification reaction was mainly chemically controlled with pore diffusion effects increasing for Highvale chars at 950°C. In addition, the increase in apparent activation energy when the carbon conversion and the C0/C02 ratio increased, supports the contention that the reaction was under chemical control. - Iv -T A B L E OF CONTENTS Page ABSTRACT 11 LIST OF TABLES v i i LIST OF FIGURES ix LIST OF SYMBOLS xvi ACKNOWLEDGEMENTS xxii 1 INTRODUCTION 1 2 BACKGROUND 4 2.1 Fundamentals of the Boudouard Reaction 4 2.2 Chemical Factors Affecting the Reaction 9 2.3 Physical Factors Affecting the Reaction 12 2.3.1 Pore Structure of Coals and Chars 13 2.3.2 Measurement of Surface Area 16 2.4 Effects of Coal Type and Charring Conditions 18 2.4.1 Effect of Coal Type 18 2.4.2 Effect of Charring Conditions 20 3 LITERATURE REVIEW 27 3.1 Catalytic Effect of Inorganic Impurities 27 3.2 Changes in Pore Structure and their Effect on Reaction Rates 34 3.3 Rate Equations for the Boudouard Reaction 39 3.4 Mathematical Models 51 3.5 Experimental Measurement of Char Reactivity 63 - v -Page 4 OBJECTIVES AND SCOPE OF THE PRESENT WORK 67 5 EXPERIMENTAL 70 5*1 Introduction , 70 5.2 Materials Used and their Preparation 71 5.3 Coal Charring Apparatus 73 5.4 Gasification Apparatus 77 5.5 Experimental Procedures 84 5.5.1 Coal Charring 8 4 5.5.2 Gasification Experiments 86 6 RESULTS OF CHARRING AND ASSESSMENT OF GASIFICATION EXPERIMENTS 93 6.1 Charring Experiments 93 6.1.1 Temperature Measurements 94 6.1.2 Effect of Charring Conditions 95 6.1.3 Discussion 99 6.2 Assessment of Gasification Experiments 102 6.2.1 Determination of the Minimum Fluidization Velocity 102 6.2.2 Gas Analysis and Flow Rate Measurements 107 6.2.3 Determination of the Fractional Conversion of Carbon 114 6.2.4 Determination of Reaction Rate 119 6.2.5 Mass Balances 121 6.2.6 Bed Temperature 125 6.2.7 Surface Area Measurements 125 7 RESULTS OF GASIFICATION EXPERIMENTS 130 7.1 Gasification with C02 and C02-He Mixtures 130 7.1.1 Effect of Charring Conditions 131 7.1.2 Effect of Bed Depth 131 7.1.3 Effect of Total Inlet Flow Rate 136 7.1.4 Effect of Inert Gas Concentration 136 7.1.5 Effect of Partial Pressure of C02 136 7.1.6 Effect of Temperature 144 7.1.7 Preliminary Discussion 144 - vi -Page 7.2 Gasification with C02-CO-He Mixtures 152 7.2.1 Effect of Gas Composition 152 7.2.2 Retarding Effect of CO 159 7.2.3 Effect of Char Type 164 7.2.4 Effect of Temperature 164 7.2.5 Reproducibility Tests 164 7.2.6 Preliminary Discussion 168 7.3 Scanning Electron Microscope Observations 173 8 OVERALL DISCUSSION OF RESULTS 185 8.1 Proposed Rate Equation and Determination of Kinetics Parameters 185 8.2 Relationship Between Changes in Surface Area and Gasification Kinetics 216 8.3 Langmuir-Hinshelwood Kinetics 229 8.4 Discussion 245 9 SUMMARY AND CONCLUSIONS 253 LIST OF REFERENCES 260 APPENDIX A Calibration Curves for Inlet Flowmeters 273 APPENDIX B Summary of Coal Charring Experiments 279 APPENDIX C Values of Minimum Fluidization Velocity at 21°C 280 APPENDIX D Derivation of the Equations Utilized to Calculate the Fractional Conversion of Carbon 282 APPENDIX E Summary of Gasification Experiments 286 APPENDIX F Listing of Program to Process Gasification Experiments Data and Sample Output........ 289 - v i i -LIST OF TABLES Page Chapter 5 I Proximate and ultimate analyses of Highvale and Smoky Tower coals 72 II Ash composition of Highvale and Smoky Tower coals 74 III Fisher coal analyzer operating conditions 75 IV Composition of certified-grade gas standards 88 V Gas chromatograph operating conditions 89 Chapter 6 VI Chars used in the gasification experiments 98 VII Proximate and ultimate analyses of -841+420 urn Highvale and Smoky Tower chars used in the gasification experiments 100 VIII Minimum fluidization velocities at 21°C 105 IX Ratios between the superficial gas velocity for Qx = 10 1/min and the minimum fluidization velocities 106 X Composition of gas mixtures Gas 1 and Gas 2... 112 XI Composition of gas mixtures used in the gasification experiments (C0/C02 = 0.25 and 0.50) 116 XII Estimated and measured final masses of carbon 124 XIII Specific surface area of chars as a function of carbon conversion for different charring and gasification conditions 129 - v i i i -Page Chapter 8 XIV Orders of reaction for Highvale chars 193-195 XV Orders of reaction for Smoky Tower chars 196 XVI Test values for correlation coefficients 198 XVII Pseudo rate constant for Highvale chars with C0/C02 = 0 and p ^ = 0.50 atm 200 XVIII Average pseudo rate constant for Highvale chars with C0/C02 = 0.25 201 XIX Average pseudo rate constant for Highvale chars with C0/C02 - 0.50 202 XX Average pseudo rate constant for Smoky Tower chars 203 XXI Apparent activation energies for Highvale and Smoky Tower chars 212 XXII Structural parameters for Highvale chars 220 XXIII Structural parameters for Smoky Tower chars 221 XXIV Langmuir-Hinshelwood rate constants for Highvale chars.... 243 - ix -L I S T OF F I G U R E S Page Chapter 5 5.1 Schematic view of coal charring set-up 76 5.2 Schematic view of the experimental apparatus 79 Chapter 6 6.1 Variation of temperature at three locations in the coal bed during charring run CI9 96 6.2 Pressure drop across the bed as a function of superficial gas velocity or gas flow rate showing the fluidization characteristics of Highvale char; conditions as shown 104 6.3 Calibration curves for composition of CO-C02 mixtures in gas chromatograph 109 6.4 Calibration curves for composition of the gas standards presented in Table TV in gas chromatograph 110 6.5 Calibration curves for flow rate of the gas mixtures presented in Table X in flowmeter Gilmont #3 (21°C and 1 atm) 113 6.6 Calibration curves for flow rate of He-C0-C02 mixtures in flowmeter Gilmont #4 (21°C and 1 atm); composition of the mixtures as shown 115 6.7 Exit gas composition (% C02 and % CO) as a function of time for three gasification experiments; conditions as shown 118 6.8 Calculated and measured exit gas flow rate as a function of time for three gasification experiments 122 6.9 Temperature of the char bed as a function of time for six gasification experiments 126 6.10 Specific surface area of char samples as a function of percent carbon conversion; conditions as shown (H = Highvale char; ST = Smoky Tower char; Temp = gasification temperature) 128 - x -Page Chapter 7 7.1 Plot of fractional conversion of carbon versus time showing the effect of charring conditions (soak time) for Highvale char; conditions as shown 132 7.2 Plot of fractional conversion of carbon versus time showing the effect of bed depth at different temperature (Highvale char); conditions as shown 133 7.3 Plot of fractional conversion of carbon versus time showing the effect of bed depth (Highvale char); conditions as shown. 134 7.4 Plot of fractional conversion of carbon versus time showing the effect of bed depth (Highvale char); conditions as shown 135 7.5 Plot of fractional conversion of carbon versus time showing the effect of total inlet flow rate (Highvale char); conditions as shown 137 7.6 Plot of fractional conversion of carbon versus time showing the effect of total inlet flow rate (Highvale char); conditions as shown 138 7.7 Plot of fractional conversion of carbon versus time showing the effects of total inlet flow rate and partial pressure of CO2 (Highvale char); conditions as shown 139 7.8 Plot of fractional conversion of carbon versus time showing the effect of inert gas concentration (Highvale char); conditions as shown 140 7.9 Plot of fractional conversion of carbon versus time showing the effect of partial pressure of C02 (Highvale char); conditions as shown 141 7.10 Plot of fractional conversion of carbon versus time showing the effect of partial pressure of C02 (Highvale char); conditions as shown 142 7.11 Plot of fractional conversion of carbon versus time showing the effect of partial pressure of C02 (Highvale char); conditions as shown 143 7.12 Plot of fractional conversion of carbon versus time showing the effect of temperature for CO/C02 = 0 (Highvale char); conditions as shown 145 - x i -Page 7.13 P a r t i a l pressure of C0 2 as a functon of time for three bed depths (Highvale char); conditions as shown 150 7.14 Plot of f r a c t i o n a l conversion of carbon versus time showing the ef f e c t of gas composition for CO/C02 = 0.25 (Highvale char); conditions as shown 153 7.15 Plot of f r a c t i o n a l conversion of carbon versus time showing the e f f e c t of gas composition for C0/C0 2 = 0.50 (Highvale char); conditions as shown 154 7.16 Plot of f r a c t i o n a l conversion of carbon versus time showing the ef f e c t of gas composition (Smoky Tower char); conditions as shown 155 7.17 Plot of f r a c t i o n a l conversion of carbon versus time showing the effect of p a r t i a l pressure of C0 2 for a constant p a r t i a l pressure of CO (Highvale char); conditions as shown 156 7.18 Plot of f r a c t i o n a l conversion of carbon versus time showing the e f f e c t of p a r t i a l pressure of C0 2 for a constant p a r t i a l pressure of CO (Highvale char); conditions as shown 157 7.19 Plot of f r a c t i o n a l conversion of carbon versus time showing the effect of p a r t i a l pressure of CO for a constant p a r t i a l pressure of C0 2 (Highvale char); conditions as shown 158 7.20 Plot of f r a c t i o n a l conversion of carbon versus time showing the ef f e c t of C0/C0 2 r a t i o (Highvale char); conditions as shown 160 7.21 Plot of f r a c t i o n a l conversion of carbon versus time showing the ef f e c t of C0/C0 2 r a t i o (Highvale char); conditions as shown 161 7.22 Plot of f r a c t i o n a l conversion of carbon versus time showing the e f f e c t of C0/C0 2 r a t i o (Highvale char); conditions as shown 162 7.23 Plot of f r a c t i o n a l conversion of carbon versus time showing the e f f e c t of C0/C0 2 r a t i o (Highvale char); conditions as shown 163 7.24 Plot of f r a c t i o n a l conversion of carbon versus time showing the e f f e c t of char type at d i f f e r e n t temperatures; conditions as shown 165 - x i i -Page 7.25 Plot of fractional conversion of carbon versus time showing the effect of char type at different gas compositions; conditions as shown (H = Highvale char; ST = Smoky Tower char) 166 7.26 Plot of fractional conversion of carbon versus time showing the effect of temperature for CO/CO2 = 0.50 (Highvale char); conditions as shown 167 7.27 Plot of fractional conversion of carbon versus time showing the reproducibility of the experimental results; conditions as shown 169 7.28.a Microstructure of unreacted Highvale char (1000X) 174 7.28.b Microstructure of unreacted Highvale char (4000X) 174 7.28.C Pore structure and ash inclusions in Highvale char 35% converted at 850°C (1000X) 175 7.28.d External appearance of Highvale char particles 57% reacted at 850°C (20X) 175 7.28.e External appearance of unreacted Highvale char particles (100X) 176 7.28.f Slit-like crack and ash inclusions in Highvale char 57% gasified at 850°C (1000X) 176 7.28.g Pore structure and ash inclusions in Highvale char 57% reacted at 850°C (2000X) 177 7.28.h Pore structure and ash inclusions in Highvale char 72% converted at 900°C (2000X) 177 7.28.i Segregation of ash particles in Highvale char 84% reacted at 900°C (1000X) 178 7.28.J Ash agglomerate in Highvale char 84% gasified at 900°C (2000X) 178 7.28.k Microstructure of unreacted Smoky Tower char (1000X) 179 7.28.1 Microstructure of unreacted Smoky Tower char (2000X) 179 7.28.m Slit-like pores in Smoky Tower char 41% converted at 850°C (800X) 180 - x i i i -Page 7.28.n Porosity in Smoky Tower char 41% gasified at 850°C (2000X) 180 7.28.o Porosity in Smoky Tower char 41% reacted at 850°C (4000X) 181 7.28.p External appearance of Smoky Tower char particles 49% gasified at 875°C (10X) 181 7.28.q Porosity in Smoky Tower char 65% converted at 875°C (1000X) 182 7.28.r Porosity in Smoky Tower char 81% reacted at 950°C (2000X) 182 Chapter 8 8.1 Arrhenius plot for the average pseudo rate constant, k'; conditions as shown 204 8.2 Arrhenius plot for the average pseudo rate constant, k'; conditions as shown 205 8.3 Arrhenius plot for the average pseudo rate constant, k'; conditions as shown 206 8.4 Arrhenius plot for the average pseudo rate constant, k'; conditions as shown 207 8.5 Arrhenius plot for the average pseudo rate constant, k'; conditions as shown 208 8.6 Arrhenius plot for the average pseudo rate constant, k'; conditions as shown 209 8.7 Arrhenius plot for the average pseudo rate constant, k'; conditions as shown 210 8.8 Arrhenius plot for the average pseudo rate constant, k'; conditions as shown 211 8.9 Plot of apparent activation energy as a function of percent carbon conversion and CO/CO2 ratio for Highvale c 2 char using the equation E = E + c,f 213 o 1 - xiv -Page 8.10 Plot of apparent activation energy as a function of percent carbon conversion^or Smoky Tower char using the equation E = E + c,f 2 214 o 1 8.11 Initial apparent activation energy as a function of C0/C02 ratio for Highvale char 215 8.12 Plot of An(a-l) versus vfcnf-f for Highvale char 218 8.13 Plot of £n(a-l) versus v£nf-f for Smoky Tower char 219 8.14 Plot of [ST/S°T(l-f)]2 versus - fcn(l-f) for Highvale and Smoky Tower chars 225 8.15 Plot of average pseudo rate constant, k', versus percent carbon conversion using the equations of Bhatia and Perlmutter and Dutta et al.; conditions as shown 226 8.16 Plot of average pseudo rate constant, k', versus percent carbon conversion using the equations of Bhatia and Perlmutter and Dutta et al.; conditions as shown 227 8.17 Plot of average pseudo rate constant, k', versus percent carbon conversion using the equations of Bhatia and Perlmutter and Dutta et al.; conditions as shown 228 8.18 Plot of reaction rate versus carbon conversion using the equation of Bhatia and Perlmutter; conditions as shown. 231 8.19 Plot of reaction rate versus carbon conversion using the equation of Bhatia and Perlmutter; conditions as shown 232 8.20 Plot of reaction rate versus carbon conversion using the equation of Bhatia and Perlmutter; conditions as shown 233 8.21 Plot of reaction rate versus carbon conversion using the equation of Bhatia and Perlmutter; conditions as shown 234 - X V -Page 8.22 Plot of reaction rate versus carbon conversion using the equation of Bhatia and Perlmutter; conditions as shown 235 8.23 Plot of reaction rate versus carbon conversion using the equation of Bhatia and Perlmutter; conditions as shown 236 8.24 Plot of reaction rate versus carbon conversion using the equation of Bhatia and Perlmutter; conditions as shown 237 8.25 Plot of reaction rate versus carbon conversion using the equation of Bhatia and Perlmutter; conditions as shown 238 8.26 Plot of reaction rate versus carbon conversion using the equation of Dutta et al.; conditions as shown 239 8.27 Plot of reaction rate versus carbon conversion using the equation of Dutta et al.; conditions as shown 240 8.28 Plot of inverse initial reaction rate versus inverse inlet partial pressure of CO2; conditions as shown 241 8.29 Arrhenius plots for the rate constants and k 2 of the Langmuir-Hinshelwood rate equation 244 Appendix A A.l Calibration curve for He flow rate in flowmeter Gilmont #3 (21°C and 1 atm) 274 A.2 Calibration curve for He flow rate in flowmeter Gilmont #2 (21°C and 1 atm) 275 A.3 Calibration curve for C02 flow rate in flowmeter Gilmont #3 (21 °C and 1 atm) 276 A.4 Calibration curve for CO flow rate in flowmeter Gilmont #2 (21°C and 1 atm) 277 A.5 Calibration curve for Ar flow rate in flowmeter Gilmont #3 (21°C and 1 atm) 278 - x v i -LIST OF SYMBOLS a S t r u c t u r a l parameter to represent the changes i n surface area of the char p a r t i c l e s , Equations [3.20] and [8.3] A Parameter i n Equation [3.23] and [8.16] Ar Archimedes number, Equation [I] (Appendix C) b Constant i n Equation [7.1], [g/mole] c 1 Constant i n Equation [8.11 ] , [kJ/mole] c 2 Constant In Equation [8.11] C_. Concentration of C0 ? i n Equations [3.18] and [3.20] b Cp„ Concentration of C0 2 i n the bulk of gas phase, Equation 2 [7.1], [mole/m 3] C Concentration of C0 2 at the e x t e r n a l surface of char 2 p a r t i c l e s , Equation [7.1], [mole/m 3] [C^] Concentration of free s i t e s on the carbon surface [C(0)] Concentration of s i t e s on the carbon surface c o n t a i n i n g an adsorbed oxygen atom C F i n a l carbon content of the char, Table X I I C Mass balance on carbon, Table XII m.b. co r r C o r r e l a t i o n c o e f f i c i e n t d Average diameter of the char p a r t i c l e s , Equation [ I ] 5 3 (Appendix C) , [cm] D Diameter of the char bed E^ ( i = l , 3 ) A c t i v a t i o n energies f o r rate constants E| ( i = l , 3 ) A c t i v a t i o n energies f o r rate constants j E A c t i v a t i o n energy i n Equation [3.19] - x v i i -A c t i v a t i o n energy for rate constant k^ A c t i v a t i o n energy for rate constant k' c Apparent a c t i v a t i o n energy for pseudo rate constant k', [kJ/mole] I n i t i a l apparent a c t i v a t i o n energy for pseudo rate constant k' or apparent a c t i v a t i o n energy for rate constant k, Equation [8.11], [kJ/raole] Parameter in Equation [3.22] F r a c t i o n a l conversion of carbon Acceleration due to g r a v i t y , Equation [I] (Appendix C), [cm/s ] R e a c t i v i t y factor i n Equation [3.19], [cm /g*s] Rate constants for the elementary steps comprising the Boudouard reaction mechanism Frequency factors for rate constants Rate constants for LH equation Frequency factors for rate constants k^ Rate constant, Equation [3.10] Rate constant, Equations [3.13] and [8.1] Frequency factor for rate constant k c Rate constant, Equation [3.16] Frequency factor for rate constant k^_ Volumetric rate constant, Equation [3.18] Rate constant, Equations [3.20] and [8.6] - xviii -k Q Frequency factor for rate constant k k D Rate constant, Equation [8.5] k' Pseudo rate constant, Equations [3.2] and [8.9], [ s - 1 atm_n] k' Frequency factor for pseudo rate constant k', ° [ s - r atm-n] k' Average pseudo rate constant, Equation [8.14], [ s - 1 atm_n] k" Pseudo rate constant, Equation [8.23], [s - 1] k Mass transfer coefficient in Equation [7.1], [m/s] K Equilibrium constant for the oxygen exchange reaction, Equation [3.10] L Height of the char bed m Instantaneous mass of carbon in the char (final mass of carbon in the char in Table XII), [g] mQ Initial mass of carbon in the char, [g] m Mass of the char bed at any conversion (final mass of char C in Table XII), [g] ra°. Initial mass of the char bed, [g] char & M Parameter in Equation [3.24] M Amount of carbon per unit volume of the charge, Equation C [3.19], [g/cm3] n Order of reaction, Equations [3.2], [3.20] and [8.6] n(f) Parameter in Equation [3.22] p Partial pressure of CO p Partial pressure of COo CO 2 p Partial pressure of He - xix -p Equilibrium p a r t i a l pressure of C0 2 for the Boudouard 2 reaction, Equation [3.19] Q c Q Inlet flow rate of CO, [£/min] Q C Q Inlet flow rate of C0 2, [£/min] Q Inlet flow rate of He, [Jc/min] He Q T Total i n l e t flow rate, [Jl/min] Total exit flow rate, [Z/min] Q Flow rate, [£/min] Q n Flow rate of c a l i b r a t i o n gas, Equation [6.8], [A/min] C 3.X r Instantaneous rate of reaction (per mass of carbon remaining), [ s - 1 ] r I n i t i a l instantaneous rate of r e a c t i o n , [ s - 1 ] o r Rate of reaction per unit volume of the charge, Equation [3.19], [mole/cm • s] R Gas constant Re Reynolds number for minimum f l u i d i z a t i o n conditions, Equation [II] (Appendix C) S S p e c i f i c surface area of the chars i n Tables XXII and XXIII, [m2/g] S Surface area per unit weight of the chars at any conversion, Equation [8.16], [m /g] S° I n i t i a l surface area per unit weight of the chars, W Equation [8.16], [m /g] S^ Surface area per unit volume o ^ t h ^ chars at any conversion, Equation [8.18], [m /m ] S° I n i t i a l surface area per unit volume of the chars, v Equation [8.18], [m /m ] - X X -Total surface area of the chars at any conversion, Equation [8.21], [m ] Initial total surface area of the chars, Equation [8.21], [m2] Specific external area of char particles in Equation [7.1], [m2/g] Reaction time Temperature Charring temperature in Equation [2.2], [K] Superficial gas velocity, [cm/s] Minimum fluidization velocity, [cm/s] Volume of the char bed at any conversion, Equation [8.19], [m3] Initial volume of the char bed, Equation [8.20], [m ] Fraction of CO2 converted to CO (Appendix D) Mole (or volumetric) fraction of CO in the exit gas Mole (or volumetric) fraction of C0 2 in the exit gas Parameter in Equations [3.21] and [8.16] Pore diameter in Section 2.3.1 Percentual error between the measured and estimated final mass of carbon in the char, Table XII Effectiveness factor, Equations [3.20] and [3.22] Gas viscosity in Equation [I] (Appendix C), [g/cm • s] Parameter in Equations [3.21] and [8.16] - xxi -Density of exit gas, Equation [6.8], [g/cm ] Density of calibration gas, Equation [6.8], [g/cm ] True density of the chars in Equation [I] (Appendix C), [g/cm3] Gas density in Equation [I] (Appendix C), [g/cm ] Total number of reaction sites on the carbon surface Generic function of p and p„-. in Equation [8.3] Structural parameter in Equation [8.18] - x x i i -ACKNOWLEDGEMENTS I wish to express my most sincere gratitude to my supervisor Professor J.K. Brimacombe for his guidance and support throughout the course of t h i s work. I extend my appreciation to Professor A.P. Watkinson f or his assistance and f r u i t f u l discussions and to Professor R.G. Butters f o r h i s assistance i n designing and bu i l d i n g the experimental apparatus. To Mr. Pat Wenman, I am deeply indebted for a l l his i n t e r e s t , help and friendship throughout the duration of th i s research. Professor Fernando Rizzo deserves my gratitude f o r encouraging me to undertake this task. The f i n a n c i a l support of CNPq and PUC/RJ, B r a s i l , and NSERC, Canada, at d i f f e r e n t stages of this work, i s g r a t e f u l l y acknowledged. I am also thankful to my fellow graduate students for t h e i r cooperation and discussions. F i n a l l y to my wife Isabel and my sons Luis Felipe and Eduardo f o r t h e i r help, patience and support, I g r a t e f u l l y dedicate t h i s t h e s i s . - 1 -CHAPTER 1 INTRODUCTION The gasification of coal chars with (X>2 by the Boudouard reaction is of utmost importance in determining the overall performance of a variety of metallurgical processes such as the direct reduction of iron ores in a rotary kiln (SL/RN), zinc fuming and lead converting (QSL). Recently there has been an Increasing interest in the use of domestic low-rank coals as fuel and reductants in these processes due to a more dependable supply, as well as economical and technological advantages. In contrast to the well known shortage of the highly priced metallurgical coking coals, significant low-rank coal resources are available in many countries where they are used mostly as thermal coals for power generation. In Western Canada, there are large amounts of low rank coals, with sub-bituminous coals from Alberta representing about 70% of the total Canadian coal reserves.* The majority of the proven and recoverable reserves of these sub-bituminous coals can be extracted i by surface mining at relatively low cost.* Moreover highly reactive lignites and sub-bituminous coals have been successfully used as j reductant materials in the SL/kN process and in zinc fuming. In the SL/RN process higher throughputs and lower kiln temperature, with a corresponding decrease in heat requirements, have resulted. It becomes evident from the literature that the reactivity of a char depends essentially on the coal type and on the heat treatment used to prepare the char. Generally speaking, the lower the coal rank, the - 2 -higher the reactivity. However, different reactivities have been observed for chars derived from coals of the same rank, and prepared under identical conditions, due to differences in the internal pore structure and ash properties of the chars. Furthermore, l i t t l e information is available on the reactivity of chars originating from different types of Western-Canadian coals. Most of the research on the kinetics of the char-C02 reaction has been carried out with foreign coals, differing from Western-Canadian coals in physical and chemical properties as well as in geological origin. Therefore a specific kinetics behaviour should be expected for low-rank Canadian coals. This work was, therefore, aimed at a fundamental study of the gasification kinetics of two different Western-Canadian coals, of potential metallurgical applications, with CO2 in a laboratory-size fluidized bed reactor. The work also complements other kinetics studies involving low-rank Western-Canadian coals conducted in this department, 2 namely: the reduction kinetics of titaniferous ores, the kinetics of 3 the zinc-slag-fuming process and the kinetics of direct reduction of unagglomerated iron ore with coal char.1^ The thesis is organized in the following way. The theoretical background of the Boudouard reaction is presented in Chapter 2. Chapter 3 reviews the most relevant aspects of the extensive literature on the subject. This is followed by the objectives and scope of the present work, given in Chapter 4. Chapter 5 presents a detailed account of the experimental techniques employed and, in Chapter 6, the results of the charring and the assessment of the gasification experiments are considered. The results of the gasification experiments, together with - 3 -preliminary discussions, are presented in Chapter 7 and the overall discussion of the results, with the proposed kinetics equation, are presented in Chapter 8. Finally, the summary and conclusions of the work are given in Chapter 9. - 4 -CHAPTER 2 BACKGROUND 2 . 1 Fundamentals of the Boudouard Reaction The kinetics of the Boudouard reaction have been extensively investigated in the last decades. Different carbonaceous materials have been studied such as graphite and other forms of "pure" carbons, activated charcoals, cokes and coal chars. Different experimental systems and conditions also have been employed with thermogravimetric and gas analysis methods predominating. There are extensive literature reviews on the subject that survey most of the fundamental work done. 5" 1 5 Early work has emphasized the determination of the reaction mechanism mostly for "pure" carbons and limited temperature ranges. Carbon reactivities were usually determined by weight loss measurements in thermogravimetric devices. More recently, there has been an increasing interest in a better understanding of the reaction for chars produced from low-rank coals and attempts to correlate the reactivity of chars with their intrinsic properties have been made. Accordingly it has been found that pure forms of carbon are much less reactive than chars and moreover that reactivity increases when the rank of the parent coal decreases. The strong dependence of the reactivity of chars on their physical and chemical properties precludes the utilization of results obtained for "pure" carbons and cokes. It is hardly surprising - 5 -that the kinetics behaviour of "pure" carbons is not similar to that of coal chars owing to differences between the well-ordered carbon structure and the interconnected multipore structure of char. Moreover the char pore structure depends on the nature of the parent coal and on the treatment performed to produce the char. Catalytic action of substances present in the ash of chars also plays an important role. This is further affected by the considerable influence of reactor-related phenomena on the reactivity as will be discussed later. A summary of the basic characteristics of the Boudouard reaction is given below. The rate of the C-C02 reaction is reported to be relatively slow, approximately 10~5 times the rate of the C-02 reaction I ^  at 800°C and 1 atm. The reaction also is strongly endothermic (AH° = 172.4 kj/mole). Generally speaking the gasification of carbon occurs in several physical and chemical steps involving, both external and intraparticle diffusion of the reactant and product gas, and chemical reaction with adsorption and desorption from active sites on the carbon surface. Q Walker et al. have postulated the existence of different temperature regimes that determine which reaction step is rate controlling. At low temperatures, the rate depends on the rate of chemical reaction. At intermediate temperatures the overall rate is limited by the rate of diffusion through the porous carbon particles. Finally at high temperatures the rate of mass transfer of C02 to the external surface of the particles governs the reaction. In the first two regimes, different carbons may present different reactivities while - 6 -i n the t h i r d regime the rate i s independent of the carbon r e a c t i v i t y . T r a n s i t i o n regions exist between the temperature regimes where combined e f f e c t s are important. The temperature at which the influence of external mass transfer becomes dominant depends on the process v a r i a b l e s . For instance, i t decreases with a more rea c t i v e char i n comparison with a less reactive carbon and increases when p a r t i c l e s i z e decreases. In addition, an increase i n the gas flow rate through the p a r t i c l e s causes an increase i n t h i s temperature. There i s general agreement**»^ > t h a t the experimental data f o r the reaction follows, i n the absence of external mass transf e r c o n t r o l and at conditions not close to equilibrium, an equation based on the Langmuir-Hinshelwood k i n e t i c s , as follows. k i p c o 2 r = 1 + k 2 p C ( ) + k 3 p C 0 2 [ 2 ' 1 ] where k^ = k^expC-E^/RT), i = 1,3, are generally functions of the elementary steps comprising the reaction mechanism and depend upon the carbon used and temperature. PCO£ a n c * PCO a r e t n e ambient p a r t i a l pressures of CO2 and CO r e s p e c t i v e l y . Equation [2.1] can be s i m p l i f i e d under some conditions of temperature and p a r t i a l pressure of C 0 £ . It has been 8 10 19 observed » > that under conditions of low temperature and/or high p a r t i a l pressure of CO2, a zero-order reaction with respect to PCO2 occurs. At low temperatures and low pco2 o r n*§h temperature and low PCOo» t n e equation reduces to a f i r s t - o r d e r expression. When a l l - 7 -terms in the denominator are significant, the Langmuir-Hinshelwood equation indicates that the measured order with respect to C02 varies from zero to one with percent conversion. This variation in the reaction order is a function of temperature, pressure, gas composition, type of carbon, sample dimensions and type of experimental set-up. Equation [2.1] correctly takes into account the well known poisoning effect of CO. 1 9 - 2 5 This inhibiting effect is not directly 15 related to thermodynamic reversibility and Increases at lower temperatures and for purer carbons. Several mechanisms have been proposed to explain the CO poisoning effect. What seems to occur is the reaction of CO with adsorbed oxygen atoms on active sites on the carbon surface which regenerates C02 as will be seen. Different mechanisms for the Boudouard reaction leading to Equation 17 22 3 0 [2.1] also have been proposed. » ~ In this respect, the paper by Rao and Jalan, 1 6 in addition to most of the fundamental literature already mentioned, gives a comprehensive discussion on the most relevant mechanisms presented. It is now generally accepted that the basic mechanism of the reaction is the formation of a surface carbon-oxygen complex by reaction of C02 and the carbon surface together with liberation of CO. The complex further decomposes with formation of additional CO as follows: (i) Reversible exchange of oxygen between C02 in the gas phase and the carbon surface: - 8 -C f + c o 2 ( g ) = C(0) + CO(g) ( i i ) I r r e v e r s i b l e desorption of CO into the gas phase af t e r reaction between adsorbed oxygen and the carbon surface. C(0) CO(g) + C f where Cf and C(0) are respectively free and occupied s i t e s on the carbon surface. The f i r s t step i s considered to occur very r a p i d l y being v i r t u a l l y at equilibrium while the second step i s regarded as rate c o n t r o l l i n g . D i f f i c u l t i e s i n extrapolating the rate constants of the Langmuir-Hinshelwood equation to d i f f e r e n t carbons and temperatures as well as i t s inconvenient form for design purposes, have led some in v e s t i g a t o r s to consider special cases for which the rate can be expressed i n s i m p l i f i e d form. Some of the expressions that have been used w i l l be reviewed l a t e r . The temperature dependence of the reaction rate also i s not completely established. A large range of values for the a c t i v a t i o n 31 energy has been reported for the reaction i n v o l v i n g chars. Most of the data obtained refers to i n i t i a l rates of reaction. The large d i f f e r e n c e s In r e s u l t s are mainly due to the influence of the various physical and chemical variables on the reaction and to the d i v e r s i t y of experimental conditions that have been used. The r e a c t i v i t y of any char i s determined by i t s i n t r i n s i c properties, i . e . , the chemical structure, - 9 -the ash c h a r a c t e r i s t i c s and the pore structure. These properties control the phenomena that a f f e c t the rate of the heterogeneous char-gas rea c t i o n s , i . e . , the nature and a c c e s s i b i l i t y of surface for reaction in a char p a r t i c l e expressed by the reaction k i n e t i c s at the char surface and the d i f f u s i o n of reactant and product gas through the pores of the p a r t i c l e . Thus the r e a c t i v i t y i s ult i m a t e l y given by the type of the parent coal and by the char preparation procedure since both w i l l be responsible for the physical and chemical properties of the char. The problem i s further complicated by the continuous change in the char pore structure and ash behaviour during reaction. In a d d i t i o n , the r e a c t i v i t y also can depend on a v a r i e t y of experimental f a c t o r s . Some of these factors are the transport of reactants and products within the i n t e r s t i c e s between p a r t i c l e s in a char bed (which depend on the aerodynamics of the gas flow in the bed), the physical dimensions of the bed, the size d i s t r i b u t i o n and packing density of the p a r t i c l e s , the degree of mixing between the reactant gas and the p a r t i c l e s , temperature d i s t r i b u t i o n and the rate of heat transfer for the strongly endothermic Boudouard reactio n . These reactor parameters may be i n t e r r e l a t e d and us u a l l y present a very complex system. 2.2 Chemical Factors Affecting the Reaction From the previous section, the r e a c t i v i t y of a char i s seen to be a function of the inorganic constituents present i n i t s ash, of i t s pore structure and of the experimental conditions. The chemical structure of the char also plays an important role and w i l l be b r i e f l y reviewed i n t h i s section. - 10 -Coal consists of a mixture of organic substances containing carbon, hydrogen and oxygen i n chemical combination plus smaller amounts of nitrogen and sulphur. In addition i t has varying amounts of moisture and minerals associated with i t s organic matrix. In s t r u c t u r a l terms coals are heterogeneous substances comprised of a s e r i e s of aromatic and hydroaromatic b u i l d i n g blocks or c l u s t e r s i n more or less random o r i e n t a t i o n containing a v a r i a b l e amount of loose a l i p h a t i c cross l i n k s between adjacent c l u s t e r s and functional groups at t h e i r periphery. S t r u c t u r a l transformations take place when the material i s heated during charring. A l i p h a t i c , hydroaromatic and h e t e r o c y c l i c bonds are more susceptible to rupture. Thus the f u n c t i o n a l groups are removed as v o l a t i l e matter, some cross l i n k s are broken and the hydroaromatic c l u s t e r s give r i s e to a d d i t i o n a l aromatic c l u s t e r s and further to small c r y s t a l l i t e s . The rupture of the cross l i n k s aids i n a l i g n i n g the b u i l d i n g blocks. The o v e r a l l chemical or i n t r i n s i c r e a c t i v i t y of a char p a r t i c l e depends on the nature and concentration of "active s i t e s or centres" on the carbon surface. Therefore the concept and influence of a c t i v e s i t e s w i l l be discussed i n d e t a i l . It has been observed that the reaction between a carbon and a gas occurs p r e f e r e n t i a l l y at c e r t a i n carbon f r e e - s i t e s d i s t r i b u t e d i n v a r i a b l e concentrations on the carbon surface. These s i t e s are provided by surface i r r e g u l a r i t i e s and r e s u l t i n valence forces that cause transfer of electrons and, therefore, chemisorption of reactant gas constituents to form e i t h e r adsorbed molecules or surface complexes. This adsorption may be followed by - 11 -migration of intermediate compounds and finally by product desorption. The concept of active sites originated from studies performed on graphite and other "pure" carbons. These studies have shown that essentially all surface reactions with non-atomic species occur with the carbon atoms situated at the edge rather than at basal s i t e s . ^ Additional reaction occurs at edge and screw dislocations and point 32 defects such as vacancies. The active sites are considered to constitute unpaired electrons at the edges or defects of the carbon 11 3 2 lattice or geometric/charge imbalances at defects. The ratio between the active sites and the total number of carbon sites in the structure increases when the crystallite or building block size decreases. However it is recognized that edges and dislocations 3 2 constitute the preferred location of inorganic impurities. Accordingly during heat treatment, impurities tend to diffuse and concentrate at crystallite edges.11 Therefore the distinction between edge and impurity effects is not clear. For chars the existence of active sites is attributed to the concentration of carbon edges and defects, inorganic impurities and oxygen and hydrogen functional groups. As mentioned earlier, chars are characterized by highly carbon-rich aromatic structures. In such structures, edge carbon atoms are much more reactive than basal carbon atoms due to the availability of unsaturated chemical bonds and the 3 3 higher frequency of inorganic impurities at crystallite edges. Therefore reactivity depends on the degree of surface heterogeneity which is related to the size and orientation of the crystallographic - 12 -p l a n e s , to the number of c r y s t a l l a t t i c e d i s c o n t i n u i t i e s and to i m p u r i t y atoms i n the l a t t i c e . Impurity atoms, besides i n c r e a s i n g c a t a l y t i c a c t i v i t y , enhance r e a c t i v i t y by c r e a t i n g f u r t h e r d i s l o c a t i o n s . Oxygen and hydrogen are u s u a l l y present i n non-aromatic s i t e s . Chemisorption on these s i t e s i s favored i n r e l a t i o n to aromatic s i t e s . Therefore c o r r e l a t i o n s have been found between char r e a c t i v i t y and oxygen c o n t e n t 3 4 - 3 6 or hydrogen c o n t e n t . 3 7 Oxygen s i t e s , e s p e c i a l l y i n the h i g h l y r e a c t i v e carbonyl and h e t e r o c y c l i c s t r u c t u r e s , seem to i n f l u e n c e 3 8 r e a c t i v i t y v i a e l e c t r o n exchange. Hydrogen s i t e s are supposed to 38 i n c r e a s e r e a c t i v i t y by p r e f e r e n t i a l o x i d a t i o n , w i t h subsequent 3 7 pro d u c t i o n of carbon s i t e s of high a c t i v i t y . In a d d i t i o n n i t r o g e n and sulphur i n h e t e r o c y c l i c s i t e s were observed to enhance r e a c t i v i t y . 1 1 The mechanism f o r the Boudouard r e a c t i o n presented before can now be f u l l y a p preciated. The two r e a c t i o n s i n v o l v e d take place on a c t i v e s i t e s on the s o l i d s u r f a ce. The carbon atoms, c o n s t i t u t i n g the a c t i v e s i t e s , must have free valence e l e c t r o n s to form chemical bonds w i t h the oxygen. These bonds must be stronger than the C-C bonds i n the l a t t i c e i n order that carbon atoms can be desorbed as CO molecules. Only a c e r t a i n f r a c t i o n of the carbon surface i s occupied by these f r e e s i t e s . Therefore d i f f e r e n c e s i n i n t r i n s i c r e a c t i v i t i e s of various carbons can be due to d i f f e r e n c e s i n the c o n c e n t r a t i o n and nature of a c t i v e s i t e s i n these carbons. 2.3 Physical Factors Affecting the Reaction I t has been g e n e r a l l y observed during char g a s i f i c a t i o n that the chemical r e a c t i o n takes place throughout the i n t e r n a l surface area of - 13 -the char p a r t i c l e s . Experiments performed w i t h CO2 and g r a p h i t e have shown that the r e a c t i o n r a t e i s p r o p o r t i o n a l to the a c t i v e i n t e r n a l 3 9 su r f a c e area of the graphite specimens. The a c c e s s i b i l i t y of the i n t e r n a l surface depends on the c o n d i t i o n s under which the r e a c t i o n takes p l a c e . The o v e r a l l chemical r e a c t i v i t y of a char p a r t i c l e depends on the type and concentration of a c t i v e s i t e s as w e l l as on the pore s t r u c t u r e of the p a r t i c l e . The i n t e r n a l pore s t r u c t u r e has a considerable i n f l u e n c e on r e a c t i v i t y during g a s i f i c a t i o n s i n c e i t determines l o c a l concentrations of C0 2 and CO and, t h e r e f o r e , the extent of d i f f u s i o n of these gases to and from a c t i v e s i t e s i n the p a r t i c l e . 2.3.1 Pore Structure of Coals and Chars A f t e r c a r b o n i z a t i o n , the c o a l chars have a pore s t r u c t u r e r e l a t e d to that of the parent c o a l , c o n t a i n i n g a poly-modal pore s i z e d i s t r i b u t i o n , w i t h pores c l a s s i f i e d i n t o three broad r a n g e s : 1 1 ( i ) micropores, 6 < 20 A ( i i ) mesopores, 20 < 6 < 500 A ( i i i ) macropores, 6 > 500 A This c l a s s i f i c a t i o n suggests c y l i n d r i c a l pores. However SEM a n a l y s i s of pore s t r u c t u r e of chars i n d i c a t e c y l i n d r i c a l and c o n i c a l pores as w e l l as f l a t c a v i t i e s . In a d d i t i o n there are small i n t e r s t i c e s connecting these pores. Therefore the concept of pore diameter i s an i d e a l i z a t i o n . Further experimental i n v e s t i g a t i o n s of the hi 4 2 i n t e r n a l pore s t r u c t u r e by various authors » have i n d i c a t e d that char p a r t i c l e s , e s p e c i a l l y from low-rank c o a l s , have a c e l l u l a r pore s t r u c t u r e w i t h many interconnected v e s i c l e s which are open to the - 14 -exterior. These large pores have diameters of the order of several microns. Moreover different coal char samples have pore structures varying greatly from each other. Therefore different gasification behaviour, such as different reaction orders with respect to CO2, should *+ 3 be expected for these chars. Gray and Misra have, accordingly, shown that differences in the initial reactivities for chars obtained from different Australian coals, charred under the same conditions, appear to be primarily a function of the porosity which determines the rate of access of CO2 to the internal structure. The chars used in their study had high carbon contents (94-95%) with only minor differences in the 14 3 chemical composition of the ash. Therefore catalytic effects were ruled out. The trends in the ini t i a l reaction rate were found to be consistent with the trend in the cumulative pore volume over the size range 0.01 to 10 um. A large fraction of the char surface area lies within the micro/ 37 38 mesopore range. > The contribution of macro and large mesopores is practically negligible. However macropores and mesopores account for 20 to 70% of the total porosity. » Pore surface area is only roughly tied to pore size distribution. It should be noted that a large pore volume does not correspond necessarily to a large surface area. Furthermore high char surface areas do not mean necessarily high i n i t i a l reactivities, since most of the char surface area is not accessible to the reactant gas at the beginning of the reaction. Resistance to diffusion of CO2 into the char particles can have a significant effect in retarding the rate of gasification unless the kinetics are slow - 15 -enough to allow sufficient time for diffusion in the micropores. In this regard the amount of macro and meso ("feeder") porosity in the char , _ .„ 4t_ 33 3k 36 i+3 45-51 „, „ has a major effect on its reactivity. > » > » These pores act as channels to supply reactant gas to reaction sites, most of which are located Inside the micropores. These feeder pores also determine the rate of access of CO2 to the active sites. Pores of mean diameter larger than the mean free path for gas diffusion at a given temperature act as a bulk medium for the diffusion. Chars with high concentrations of these pores are highly reactive since there Is an optimum utilization of all the active sites, i.e., a l l active sites are exposed to a reactant gas concentration close to that in the main gas stream outside the particles. Thus the chemical reaction rate is proportional to the surface area exposed to the reactant gas (active or effective surface area) and not, in general, to the total surface area. From the above discussion i t is clear that besides total pore volume and surface area, pore size distribution is a very important parameter in processes involving diffusion of gas and availability of internal surface area. Attempts to correlate the reactivity to internal surface areas are usually complicated by phenomena such as the non-accessibility of large blind pores, due to smaller feeder pores. 1 1 The pore structure of coals is quantitatively characterized by determining the pore volume, surface area and pore size distribution. Extensive studies have been done on this subject and may be found i c o p C O c c elsewhere. » > - Due to the intrinsic complexity of coals and chars, the characterization of their pore structure poses some - 16 -problems. Thus a brief discussion on surface area measurements in these materials is presented below. 2.3.2 Measurement of Surface Area Surface areas are usually determined by adsorption of gases at low or ambient temperatures. For coals and chars, the most commonly adopted procedures are N2 adsorption at 77K and C02 adsorption at 298K. In the first case the B.E.T. method is employed for calculating the surface area, whereas in the second the Dubinin-Polanyi equation is employed. Frequently, It has been argued that C02 is better suited as an adsorbate for surface area determination than N2. Experimental evidence seems to show that N2 is not appropriate for measuring surface areas of highly microporous materials.^ Also the B.E.T. equation is considered 57 inaccurate in such cases. However these arguments are less important when the problem is differently considered. What should be questioned is the precise meaning of absolute values of surface areas for coals and chars. In this context, surface areas should be viewed more as a simple comparative structural parameter that could be related to pore characteristics and reactivities of chars, than an absolute quantity. At the same time the conceptual differences between the values of surface area measured by N2 and C02 should be fully understood. For coals and chars i t is generally accepted that the surface area accessible to N2 at 77K corresponds to the area contained in macro, meso 52 55 and large micropores. » In this case low apparent surface areas are usually obtained for raw coals and unreacted chars. This is due to - 17 -the severe restrictions imposed on the penetration of N2 into micropores of such materials caused by slow, activated diffusion at 77K. 58 Correspondingly, Walker et al. have shown that this occurs for pores smaller that 5A. Partially reacted chars, on the other hand, tend to exhibit unreasonably high apparent surface areas. This is caused by reversible capillary condensation of N2, due to the high pressure penetration of the gas in the micropores that have been enlarged and opened up by reaction. Conversely surface areas accessible to C02 at 298K are considered to represent total surface areas of coals and chars 59 including the area contained in the micropores. However there are indications that even C02 may not give the "true" surface area in certain cases since C02 monolayers can migrate through capillaries due 59 to polar interactions with the coal surface. For coals and unreacted chars, C02 areas are generally larger than those measured by N2 adsorption. The higher temperatures used promote activated diffusion into the micropore structure, and capillary condensation is inhibited by the lower relative adsorption pressures employed. The apparent surface area of unreacted chars is usually lower than the actual values for partially reacted chars. This difference is, in general, greater for N2 than for C02 surface areas. This suggests that even with C02, penetration into the micropore structure is somewhat hindered before the pore structure is enlarged and opened up by gasification. For N2 at low levels of conversion, penetration into the micropores is severely inhibited leading to low surface areas. Apparently high values are obtained at higher conversions due to the capillary condensation mentioned earlier. - 18 -To summarize, the usefulness of CO2 and N2 surface area measurements are mainly to obtain an estimate of the relative accessibility of reactant gas to the internal sites of the char particles. 2.4 Effects of Coal Type and Charring Conditions 2.4.1 Effect of Coal Type It has been observed frequently that the reactivity of chars prepared under similar conditions is primarily determined by the rank of . 33 34 36 45-48 60-63 r, , . , f the parent coal. » > > » Chars derived from low-rank coals exhibit, in general, considerably higher gasification rates and lower apparent activation energies. Moreover the reactivities of these chars usually show a pronounced increase with the extent of rection, which is not the case of high-rank coal chars. The lower the rank of the parent coal, the greater the spread in the experimentally 3 3 4 5 determined values of reactivity, • due mostly to differences in ash properties of these chars. Furthermore coals of the same rank may have very different values of reactivity under the same experimental conditions. Before analyzing these findings it is necessary to understand the concept of coal rank. Coal is a sedimentary organic rock. 1 1 Rank is a practical parameter related to the extent of coalification of the original organic matter, i.e., the degree to which the original material approaches the structure of pure carbon (graphite). In this sense, rank is an oversimplification of the highly heterogeneous nature of the coal - 19 -substance. However s ince rank i s a measure of the metamorphism to the graphi te s t ruc tu re for d i f f e r e n t coa l t y p e s , with inc reas ing rank there i s a l o s s of a l i p h a t i c and hydroaromatic groups. Therefore high rank coa ls are assoc ia ted with a more or iented lamel la r s t ruc ture which i s l e s s porous. Th is i s confirmed by X-ray s c a t t e r i n g work performed by H i r s c h . 1 1 Coals with l ess than 85% carbon have an "open" s t ructure being h i g h l y porous and e x h i b i t i n g molecular s ieve c h a r a c t e r i s t i c s . Therefore the nature of the parent coal and i t s rank are very important parameters to assess the r e a c t i v i t y of c h a r s . According to Walker and h is co-workers, ® J 1 * 6 » 5 ^ » ^ 1 » ^ > ^ high carbon r e a c t i v i t i e s can be re la ted to the prime fac tors promoting high g a s i f i c a t i o n r a t e s : ( i ) high concentra t ion of ac t ive s i t e s in the char ; ( i i ) good a c c e s s i b i l i t y of reactant gas to these s i t e s ; and ( i i i ) s u b s t a n t i a l c a t a l y s i s of the r e a c t i o n by inorganic i m p u r i t i e s . It has been s h o w n 6 1 » 6 5 that these fac to rs are enhanced in low-rank coals as d iscussed b r i e f l y below. Low-rank coals have i n t r i n s i c chemical d i f f e r e n c e s capable of f a v o r i n g high r e a c t i v i t i e s , such as the higher concentrat ions of more r e a c t i v e atoms l i k e 0, S and H . 6 5 They a lso have l a rger surface a r e a s , 5 3 higher pore volume, higher poros i ty and, as shown by Gan et a l . , a l a r g e r percentage of macropores in the t o t a l poros i ty than high-rank c o a l s . Thus the i n t e r n a l pore s t ruc ture of low-rank coa ls i s very prone to provide high reac t ion r a t e s . Indeed, as seen in the previous s e c t i o n , a l a r g e r concentrat ion of feeder pores markedly a f f e c t s the r e a c t i v i t y , s ince the re tard ing e f f e c t caused by the res is tance to C 0 2 - 20 -diffusion inside the char particles is minimized. Experimental results show that structural parameters such as specific surface area and open porosity vary with coal rank in the same way as reactivity. Finally, low-rank coals usually contain a higher concentration and degree of dispersion of specific inorganic impurities that act as catalysts for the gasification reactions thus improving their activity in the chars as will be seen. 2.4.2 Effect of Charring Conditions The charring treatment plays a very important role in determining the reactivity of the char. Different treatments give rise to chars with different physical and chemical properties. The relationship between the pore structure of the char and that of the parent coal is determined by the charring conditions. Chars with different internal pore structures, concentration of active sites and distribution and degree of contact between the ash and their reactive portion may arise due to variation in the carbonization process. 1 1 > 1 5> 2 1> 3 1> 3 3» 3 4> 4 6» 50-52,63,65-70 these properties are very important in determining the reactivity of the char. Therefore any comparative measurement of reactivity must be made under similar and reproducible conditions. Thus the effects of different preparation methods deserve to be further investigated. The devolatilization of coal occurs mainly in the 300/400 - 900°C temperature range,11 according to the following process. - 21 -Coal 0. char(C-rich solid)+tar(vapor/liquid)+CH4+H2+CO, etc...+H20 (H-rich volatile matter) The char formed is a solid residue containing mostly carbon and ash together with small quantities of H, 0, N and S. It usually constitutes 30 to 70% in weight of the original coal, the exact amount, chemical composition and pore structure depending on the variables that affect the charring treatment, discussed below. When coal is heated in an inert atmosphere, the following physical and chemical transformations occur. At about 100°C, the material begins to lose absorbed moisture while at approximately 300 to 400°C i t starts to decompose into a carbon-rich solid residue and a hydrogen-rich volatile matter. The temperature at which this decomposition commences is a function of the coal type. Prior to and/or during volatiles release, the particles may change in size. Three major changes take place during charring. i) Oxygen and hydrogen atoms are lost. Above 700°C all chars have similar C-H-0 content.11 Most of the oxygen is released at lower temperature while release of hydrogen only occurs significantly above 700°C. i i ) Conversion of mineral matter to metal oxides (ash) by thermal decomposition. i i i ) Thermal annealing at temperatures between 700 and 1100°C in which microporosity and carbon edges are destroyed by cluster - 22 -re-organization and the char structure becomes more graphitic. Structural defects also tend to be eliminated; however impurities promote permanent dislocations even at high 8 32 temperatures. » When low-rank coals are carbonized in an inert atmosphere, the resulting chars retain the poly-modal pore size distribution characteristic of the parent coal. 7 0 During carbonization of these coals, two competitive processes occur in the solids according to the material presented earlier. At low temperatures, liberation of volatile matter results in the creation of additional porosity with a corresponding increase in reactivity. When the temperature is increased, re-organization of the internal microstructure of the char takes place accompanied, usually, by shrinkage of the particles. This leads to decreases in both porosity and reactivity. The extent of these processes depends again on the properties of the original coal and on the heat treatment conditions. Knowledge of the rank of the parent coal and nature of its pore structure permits a prediction of the pore structure that will result in the char for a given carbonization procedure. This may be used in the assessment of the coal for a given application. However, whether the final char has a more or less open pore structure than the parent coal depends upon a balance between the above processes. Franklin 1 1 suggests the existence of graphitizing and non-graphitizing coals. Anthracite and bituminous coals, corresponding to Hirsch's "ordered" structure are of the first type while lignites and sub-bituminous coals, Hirsch's "open" structure coals, are of the second - 23 -type. For the l a t t e r , heating induces c r o s s - l i n k a g e and thus m i c r o p o r o s i t y p e r s i s t s to at l e a s t 2000°C. In a d d i t i o n , carbon edges are more prominent i n chars from these c o a l s . High rank c o a l s , on the other hand, g r a p h i t i z e above 1500°C. The most important v a r i a b l e s i n the c h a r r i n g treatment are the temperature, the time at temperature (soak time) and the heating r a t e . G e n e r a l l y the r e a c t i v i t i e s of chars o r i g i n a t i n g from the same c o a l 37 i n c r e a s e w i t h lower c h a r r i n g temperatures, » higher heating C C T l c c 71 r a t e s > and lower soak times. » Temperature during c h a r r i n g l a r g e l y determines the extent of the changes i n the i n t e r n a l pore s t r u c t u r e of the char. The pore s t r u c t u r e undergoes l i t t l e change u n t i l 300-400°C, when d e v o l a t i l i z a t i o n begins. 3 8 A f t e r that the p o r o s i t y and average macropore s i z e i n c r e a s e and the average micro/mesopore s i z e decreases due to v o l a t i l e r e p o l y m e r i z a t i o n , e s p e c i a l l y f o r caking c o a l s . Thus a dramatic change i n pore s i z e d i s t r i b u t i o n i s not a n t i c i p a t e d and molecular sieve e f f e c t s p e r s i s t . A v a i l a b l e experimental evidence supports t h i s behaviour at temperatures lower than 1300°C and at moderate heating r a t e s , i . e . , t y p i c a l c a r b o n i z a t i o n c o n d i t i o n s . 1 1 When higher heating r a t e s are a p p l i e d , v o l a t i l e matter r e l e a s e i s gre a t e r and more r a p i d , and r e p o l y m e r i z a t i o n i s l e s s favored. Therefore higher p o r o s i t y develops w i t h l a r g e r c o n t r i b u t i o n s of micro and mesopores. 1 1 On the other hand pycnometric 66 67 s t u d i e s performed on non-caking coals > have shown that the volume of micro plus mesopores i s not very s e n s i t i v e to c h a r r i n g temperature, - 24 -i.e., development and blockage of these pores offset each other adding to maintain the molecular sieve characteristics. Moreover several investigators 1 1 have found that the internal surface area (CO2 at 298K) increases until the temperature reaches about 600 and 700°C. The increase In internal surface area indicates micro/mesopore development. Besides low temperatures, the internal surface area increases more for high heating rates and non caking coals. 1 1 Chars prepared at 600 to 700°C have the highest internal surface area and, therefore, the higher 71 overall reactivity. Further increasing temperature causes a decrease in the concentration of feeder pores, a loss of accessible active sites and a drop in internal surface area due to graphitization and plugging of micropores. The result is a progressive decrease in reactivity with increasing temperatures. Lower heating rates decrease reactivity due to favorable thermal 71 annealing conditions, increase in tar deposition and decrease in porosity caused by slow devolatilization. 1 1 The effect of soak time also may be explained by changes in the internal pore structure of the char particles during prolonged heat treatment. Charring for longer times at 700 to 1300°C causes thermal annealing of the pores with the corresponding loss of raicroporosity and carbon edges via cluster re-organization. Structural defects also are lost. Therefore longer charring times can reduce the intrinsic reactivity of the char due to the destruction of active sites. The influence of the parent coal decreases for chars treated at 37 45 extreme conditions of temperature, soak time and heating rate. » - 25 -Accordingly Blake et al. have found that the reactivity of these chars is approximately constant due to a more homogeneous carbon structure. They proposed a two-site theory to explain reactivity losses during charring or gasification. According to this theory, two types of active sites are to be considered: one highly active, consisting mostly of oxygen and mineral matter/ash and the other less active, containing carbon edges. The first site type loses reactivity rapidly while the second type loses it slowly. Intrinsic (chemical) reactivity after extended heat treatment probably results from mineral matter/ash-induced dislocations. 11 72 It has been found by different investigators « that reactivity correlates better with the charring temperature than with coal type. The following correlation between these two parameters has been proposed:11 r a exp(-J-) , Tc > 700°C, [2.2] Where r is the overall char reactivity (g/s.g) and Tc is the charring temperature (K). To summarize, differences in reactivity have been observed for chars originating from the same parent coal but treated under different conditions of temperature, heating rates and soak time. However it should be realized that these differences may be due, in some cases, to additional volatile matter combustion for chars that were not totally o - 26 -d e v o l a t i l i z e d . F i n a l l y , char r ing a lso i s a f fec ted by the gaseous atmosphere i n contact with the coa l p a r t i c l e s , the mass and heat t r a n s f e r c h a r a c t e r i s t i c s of the r e a c t o r , the coal type (pore s t r u c t u r e , type and d i s t r i b u t i o n of mineral matter and maceral composi t ion) , the t o t a l pressure and the coal p a r t i c l e s i z e . A c c o r d i n g l y , i t has been reported that char r e a c t i v i t y i s increased when carbon iza t ion occurs i n 47 the presence of reactant gases and that a i r ox ida t ion of coals has a strong e f f e c t on the development of the pore s t ruc tu re of chars during 7 3 heat treatment and, hence, on t h e i r r e a c t i v i t i e s . CHAPTER 3 LITERATURE REVIEW 3.1 Catalytic Effect of Inorganic Impurities During the l a s t decade, i n t e r e s t i n c a t a l y s i s of coal g a s i f i c a t i o n has been renewed. To improve e x i s t i n g processes and to develop more e f f i c i e n t and economical technology, the use of added c a t a l y s t s i n these processes has been widely studied. Addition of c a t a l y s t s increases char g a s i f i c a t i o n r e a c t i v i t i e s , thus allowing operation at lower temperatures and also i n h i b i t s the swelling and agglomeration of caking c o a l s , 7 4 which has a deleterious e f f e c t i n i n d u s t r i a l f l u i d i z e d bed g a s i f i e r s . Perusal of the l i t e r a t u r e on c a t a l y t i c g a s i f i c a t i o n of coals and chars in d i c a t e s that a vast amount of work has been done on t h i s subject, mainly i n recent years. Comprehensive reviews are a v a i l a b l e 6 4 * 7 5 - 7 8 and a number of investigations of m e t a l l u r g i c a l i n t e r e s t , within the extensive research published, can be found elsewhere. From a m e t a l l u r g i c a l standpoint the use of highly reactive coals i n c e r t a i n processes allows t h e i r operation at lower temperatures, with the advantages of minimum f u e l consumption and maximum p r o d u c t i v i t y . However these coals may not be a v a i l a b l e l o c a l l y or obtainable at reasonable costs. Therefore the use of additives to enhance the r e a c t i v i t y of coals may be very i n t e r e s t i n g , being u l t i m a t e l y d i c t a t e d by a balance between the advantages mentioned above and factors l i k e a d d i t i o n a l costs, side e f f e c t s , necessity of catalyst recovery and - 28 -environmental e f f e c t s . Moreover, as pointed out r e c e n t l y by Alam and 79 DebRoy, i n t e r e s t i n t h i s subject a l s o may a r i s e due to the commonly observed decrease i n the minimum temperature and i n c r e a s e i n the r a t e of the s o - c a l l e d " s o l u t i o n l o s s " (Boudouard) r e a c t i o n i n b l a s t furnaces. This i s caused by the presence of a l k a l i s i n these u n i t s which s t r o n g l y c a t a l y z e the C-C0 2 r e a c t i o n . However, as the study of the r e a c t i v i t y of catalyst-impregnated chars i s beyond the scope of t h i s work and a l s o f o r the sake of b r e v i t y , t h i s subject w i l l not be pursued f u r t h e r . In the f o l l o w i n g d i s c u s s i o n only the c a t a l y t i c e f f e c t of m i n erals n a t u r a l l y present i n the char ash w i l l be considered. I t has been reported by O l O O O f I C C O f O O T many authors » » > > > » > that c e r t a i n i n o r g a n i c c o n s t i t u e n t s present i n the ash and t r a c e elements of a char exert s u b s t a n t i a l c a t a -l y t i c e f f e c t on the char-C0 2 r e a c t i o n . The c a t a l y t i c e f f e c t i n f l u e n c e s only the i n t r i n s i c r e a c t i v i t y of the char; t h e r e f o r e i t increases at r e l a t i v e l y low temperatures corresponding to chemical r e a c t i o n or chemical reaction-pore d i f f u s i o n c o n t r o l . Several mechanisms f o r the c a t a l y s i s have been p r o p o s e d . 1 1 » 3 6 » 6 4 » 7 7 • 7 9 - 8 1 » 8 3 » 8 6 > 8 7 The c a t a l y t i c a c t i v i t y of an i n o r g a n i c i m p u r i t y depends on s e v e r a l v a r i a b l e s . The most important are chemical form, amount, d i s p e r s i o n , degree of contact and p a r t i c l e s i z e of the a c t i v e ash components i n the char m a t r i x . Ash composition i s c l e a r l y a very r e l e v a n t f a c t o r . Most metals i n the reduced and/or o x i d i z e d s t a t e as w e l l as t h e i r s a l t s c a t a l y s e the r e a c t i o n . A l k a l i , a l k a l i n e e a r t h and t r a n s i t i o n metals are p a r t i c u l a r l y a c t i v e . C o r r e l a t i o n s between r e a c t i v i t y of chars to C0 2 and CaO content i n t h e i r ash have been found by d i f f e r e n t - 29 -authors. 3 3 » 3 6 »'*5 > 6 2 » 8 7 In a l l cases the r e a c t i v i t y increased with an increase i n the calcium content of the char. Nevertheless no such c o r r e l a t i o n for any single ash constituent n a t u r a l l y present was 88 61 obtained for brown coal chars and for cokes. The qua n t i t a t i v e d e s c r i p t i o n of the c a t a l y t i c action of a s p e c i f i c ash component i s furthe r complicated by the presence of other i m p u r i t i e s . I n t e r a c t i v e e f f e c t s may occur i n th i s case. In addition no general c o r r e l a t i o n was 6 3 found between t o t a l ash content and char r e a c t i v i t y . The chemical, form of the ca t a l y s t plays a very important r o l e . This i s indicated by the high a c t i v i t y of m e t a l l i c i r o n and s i g n i f i c a n t l y lower a c t i v i t y of i t s oxides. The a c t i v i t y of i r o n seems to decrease with the extent of reaction since i n v a r i a b l y the metal becomes oxidized. For the same reason the e f f e c t of i r o n i s enhanced when the CO/CO2 r a t i o increases. Another important factor i s the dispersion or area of contact of the c a t a l y s t i n the char matrix. When the ca t a l y s t i s well d i s t r i b u t e d i n the char i t s a c t i v i t y i s enhanced. The opposite e f f e c t occurs when the c a t a l y s t i s segregated as large agglomerates. In t h i s regard, calcium i s considered the most important i n - s i t u c a t a l y s t f o r the 0 q g a s i f i c a t i o n of U.S. coal chars by C02« Accordingly one of the reasons f o r the high r e a c t i v i t y found i n chars o r i g i n a t i n g from U.S. l i g n i t e s has been a t t r i b u t e d to the high concentration and disp e r s i o n of calcium i n the char surface, associated with carboxyl groups i n these c o a l s . ' » These high concentrations of calcium are due to natu r a l ion exchange between the metal and s i g n i f i c a n t amounts of - 30 -hydrogen in the carboxyl groups. During charring the carboxyl groups decompose liberating CO2 and leaving highly dispersed calcium throughout 90 91 the char particles. Indeed Walker and his co-workers » regard lignite-char gasification as a catalyzed gas-solid reaction with the catalyst dispersion being the relevant reactivity parameter. In 8 9 contrast bituminous coal has a much lower calcium content, and the metal Is present as coarser and more segregated calcite particles. 5 1 CaO derived from the decomposition of calcite during charring is usually 8 9 poorly dispersed in the char. This contributes to the lower reactivity of bituminous coals. In the general case, most of the inorganic impurities are present as discrete particles with a rather low 92 dispersion in the carbon. This picture is not expected to change significantly during gasification. On the other hand, minerals associated with functional groups at the edges of the carbon matrix are highly dispersed. However their distribution tends to decrease markedly 9 3 with the extent of carbon consumption due to sintering via crystallite growth, causing a drop in the catalytic activity. The agglomeration of mineral inclusions also is favored at the usually high temperatures of the gasification processes. Thus gasification may induce considerable agglomeration of the ash particles present in the internal char surface with their corresponding deactivation, mainly for chars derived from low-rank coals. De-activation also may occur by chemical changes in the catalyst (e.g., Fe -*• Fex0y as mentioned earlier). The continued catalytic activity of minerals also depends on their capacity to remain in close contact with the receding carbon matrix. It - 31 -has been argued that once C O 2 a c t i v a t i o n s t a r t s i n the d i r e c t proximity of the ash p a r t i c l e , g a s i f i c a t i o n may proceed r a p i d l y there forming 4 9 broad pores and improving the access to fresh reactant s i t e s . Studies on carbon g a s i f i c a t i o n using s i n g l e p a r t i c l e s and thermogravimetric apparatus have generally shown that the ash formed during the reaction tends to remain as a product layer i n contact with the surface of the p a r t i c l e . However i t has been reported that p a r t i a l l y reacted sub-bituminuous coal char p a r t i c l e s when observed by SEM did not present 3 1 an ash-rich layer on the surface. The absence of t h i s layer suggests that the ash components were not e f f e c t i v e l y c a t a l y z i n g the r e a c t i o n . This seems to support the view that the r e a c t i o n can take place throughout the char p a r t i c l e s i n micropores much smaller than the inorganic i n c l u s i o n s . Furthermore, i f the reaction i s c a r r i e d out i n f l u i d i z e d bed pr rotary k i l n reactors, the ash layer may flake off from the p a r t i c l e s due to the s o l i d s movement and a t t r i t i o n between the p a r t i c l e s , e s p e c i a l l y at high conversions where the r e l a t i v e ash content increases. In t h i s case, a large proportion of the ash w i l l e x i s t only i n the i n t e r s t i c e s between the char p a r t i c l e s without proper contact with them which decreases the c a t a l y t i c e f f i c i e n c y . Despite the work done, the changes i n the concentration, d i s p e r s i o n and degree of contact of a c t i v e impurities during reaction and t h e i r e f f e c t s on the r e a c t i v i t y of the char needs to be further i n v e s t i g a t e d . The p a r t i c l e s i z e of the inorganic substance also must be considered. The s p e c i f i c a c t i v i t y of the c a t a l y s t usually increases with a decrease i n i t s p a r t i c l e s i z e . From the preceding material, the - 32 -particle size and distribution of the Inclusions have an important effect in promoting reactivity. Accordingly, a greater number of small particles is more effective than a few larger particles. Therefore even though most of the inorganic impurities present in chars are contained in the ash, the eventual catalytic effects of trace elements (present as 11 3 3 organo-metallics) cannot be overlooked. » The activity of trace elements is further enhanced by their high degree of dispersion in the char as compared to the ash. The removal of mineral-matter and ash by acid treatment of coal and 15 33 45 94 95 chars has been undertaken. » > > » In general, similar reactivities have been determined for chars prepared from acid-treated coals varying widely in rank. Therefore the influence of ash components is further indicated. The reactivity of chars obtained from acid-treated low-rank coals decreased compared to that of the chars obtained from the corresponding raw coals. For high-rank coals the opposite occurred, mineral-matter removal enhanced the reactivity of these coals. These results show the important roles and opposing effects of catalysis and Internal mass transport resistance in affecting the reactivity of coal chars. For low-rank coal chars, pore diffusion resistance tends to be minimum since these chars usually contain considerable feeder porosity. Introduction of additional porosity by mineral-matter removal has a small effect in decreasing the internal diffusion resistance as compared to the resultant decrease in the catalytic activity. High-rank coals, on the other hand, have l i t t l e feeder porosity. Therefore removal of mineral-matter results in a large - 33 -increase in porosity and, hence, in a substantial decrease in mass transfer retardation of the reaction offsetting completely the loss of catalytic activity. From the above i t can be inferred that acid-treatment besides removing inorganic matter usually alters the micropore structure and surface area of the material. Attention should be directed to the fact that surface area changes are markedly dependent on whether the coal is acid-washed before carbonization or the char, 9 5 prepared from a raw coal, is further washed. In the f i r s t case, mineral matter is removed from the coals before the structural modification introduced by the heat treatment. It was concluded that although acid-treatment modifies coal or char reactivity, interpretation of the results is not straightforward due to the simultaneous changes in the physical structure. It i s not yet f u l l y established whether ash components lower the activation energy of the reaction or merely increase the number of active sites on the carbon surface. This distinction appears to be important since reactivity depends exponentially on the activation energy, but only linearly on the number of active sites. Moreover diffusional resistance also Is manifested by apparently lowering the activation energy. However the effect of catalysis on activation energy may be greater than that of diffusional resistance. Very l i t t l e attention has been paid to ash behaviour at high temperatures. It is recognized that softening and melting of the ash, besides the various operational problems of accretion formation in the reactor wall and agglomerations in the bed, also can influence char - 34 -reactivities. Indeed char particles exposed to temperatures higher than the ash melting point were found to have lower reactivity, smaller surface areas and less porosity than particles exposed to lower temperatures.96 It has been proposed that at sufficiently high temperatures molten ash components could flow in the microstructure of the char shielding the surface active sites as well as plugging the accessible pores as reaction proceeds, and leading to a decrease in reactivity. 1 1 » 6 3 However more work is needed to confirm the validity of such arguments. Finally, for gasification studies, the inorganic species present in the char ash should be characterized. Ash in coal is usually obtained as the solid residue left after complete burning of the organic matter 9 7 of the coal at certain standard conditions. The ash content and 9 7 composition are generally determined by ASTM standard techniques. A detailed microscopic examination of the amount and distribution of the specific catalyst occurrences in chars would be important for a further 21 3 6 87 understanding of their role on the Boudouard reaction. » > 3.2 Changes In Pore Structure and their Effect on Reaction Rates Frequently i t has been found that ash behaviour is, together with changes in the internal pore structure of chars with conversion, the most important factors to determine their reactivities. The relationship between reactivity and the changes in the physical structure of the chars with the extent of reaction will be discussed in some detail in this section. - 35 -Several investigators » > » > have observed that the reactivity of a char, usually rises to a maximum during the gasification reaction and then falls when complete conversion of the solid is approached. During gasification, changes in the pore structure and in the pore size distribution of the char with conversion lead to changes in reactivity, since there is a variation in the effective area 99 available for reaction. Early work has shown that the internal surface area of graphite increases significantly during reaction with 3 9 CC>2. Walker et al. investigated the possible relationship between reactivity and changes in surface area during reaction of graphite with CO-2* It was shown that the reaction develops new surfaces by creating pores due to preferential removal of carbon and by opening up existing pores not previously available to C 0 2 . With an increase in conversion, coalescence of! the pores and thermal annealing causes a decrease in surface area. Therefore the rate of reaction increases up to a point when the rate of formation of surface area is paralleled by the rate of its destruction, decreasing thereafter. Petersen et a l . 1 0 0 observed that for the graphite-CC>2 reaction the rates of reaction were not simple functions of the total available surface area as might be expected for the chemically controlled reaction. For chars, as discussed before, most of the surface area lies within micropores and is not in i t i a l l y accessible for reaction. However the accessible surface area and pore volume rapidly Increases as gasification proceeds because previously closed micro and mesopores are opened up and new pore interconnections are developed. 5 , 3 1 , 3 l t , k 8 , 6 1 , 6 3 , 6 8 ' 7 0 ' 1 0 1 This increase in surface area - 36 -is more pronounced for chars derived from low-rank coals and causes an increase in reactivity. However at a certain conversion level, depending on the pore structure of the individual char, the total number of open pores begins to decrease due to the collapse of the solid walls between adjacent pores. Therefore the specific surface area and reactivity reach maximum values and then drop at higher conversions. The internal surface area increases with pore growth and decreases with 1 0 2 pore combination. According to other investigators, however, the rapid decline in reactivity at high conversions cannot be explained exclusively by pore coalescence, other morphological factors also having an effect. In this regard, reduction both in the particle size and amount of carbon of the char as well as additional factors such as elutriation of solids from the reactor can have an influence. Indeed, the dimensions of non-caking char particles were observed to remain practically constant up to conversions close to 80%.34 After that the particles disintegrate into smaller fractions. In fluidized bed reactors, attrition between the particles can contribute to a more premature degradation and elutriation of the solids. Various investigators have determined the variation in the specific surface area with conversion for different carbonaceous s o l i d s , 1 8 > 3 1 > 3 4 > 1 0 3 - 1 0 7 confirming the behaviour discussed above. SEM examinations of partially reacted cokes 1 8» 1 0 7 and chars 3 4 have shown that the microstructure becomes more open with increasing carbon consumption. Despite the fact that changes in reaction rates with conversion can be qualitatively expressed in terms of changes in available surface - 37 -area, no conclusive quantitative c o r r e l a t i o n has been determined between t o t a l s p e c i f i c surface area and r e a c t i v i t y . 1 5 ' 3 1 * 3 3 ' 3 4 ' 8 8 ' 1 0 4 ' 1 0 5 It i s generally accepted that the rate dependence of the s o l i d phase i s , excluding c a t a l y t i c e f f e c t s , due to the number of active s i t e s a v a i l a b l e P f o r r e a c t i o n . The number of a v a i l a b l e active s i t e s i s not generally r e l a t e d to the t o t a l surface area (TSA) but i s proportional to a f r a c t i o n of this area, the active or e f f e c t i v e surface area (ASA). In t h i s regard i t can be reasoned that slow reactions allow d i f f u s i o n of C 0 2 into the micropores where most of the surface area and active s i t e s are found, while f a s t e r reactions w i l l take place i n the most accessible larger pores. Only i n the f i r s t case can TSA be related to ASA. Furthermore i n many instances TSA changes with conversion while ASA 108 remains p r a c t i c a l l y constant. However the determination of an " e f f e c t i v e " surface area that could be related to char r e a c t i v i t y should be pursued f u r t h e r . It seems that the wide spread i n reported experimental values of r e a c t i v i t y per unit of TSA (up to four orders of magnitude) of d i f f e r e n t carbons obtained under apparently i d e n t i c a l conditions may be accounted f o r by the d i f f e r e n t amounts of ASA i n 91 carbons having the same TSA. 3 k Dutta et a l . have investigated the s t r u c t u r a l changes that take place i n the Boudouard reaction for d i f f e r e n t chars. They observed that the changes i n the available surface area with conversion were independent of temperature i n the range studied except for a highly caking c o a l . The same behaviour was observed by d i f f e r e n t authors and i n d i c a t e s that g a s i f i c a t i o n i s occurring i n the regime of chemical - 38 -control. For the caking coal, the increase in the available surface area was apparently less at higher temperatures at a certain conversion, in agreement with previous work on the activation of anthracite with C 0 2 « 1 1 0 Dutta et a l . also found that the reaction rate of the chars with CO2 has l i t t l e relation with TSA measured by adsorption of N 2 at 77K. Only the fraction of TSA contained in pores above 30A in diameter would be available for reaction. This fraction represents only a minimum portion of the total surface area in the chars as pointed out by 31 Taylor and Bowman. ' These authors argued that CO2 surface areas may correlate better with reactivity. However chars derived from low-rank coals may present higher reactivities than can be attributed solely to variations in surface area. 1 5 Correspondingly the reactivity of a certain lignite char was found to be 40 times larger than that of a 7 8 bituminous coal char under the same conditions ( C 0 2 at 800°C). The CO2 surface area of the lignite char was only 2.5 times larger than the corresponding surface area of the bituminous coal char. This difference in reactivity was too high to be explained on the grounds of surface area effects alone. The larger concentration of feeder pores in the ligni t e char plus enhanced catalytic effects in this char probably had more influence. Changes in the pore structure with reaction also may occur simultaneously with changes in the catalytic activity of ash minerals. These effects are dependent on the nature of the char and are usually coupled so that i t is sometimes d i f f i c u l t to discriminate between them. The variation of the reaction rate with conversion depends on the - 39 -interaction between specific surface area effects and catalytic activity of ash components. Accordingly the reactivity of highly reactive lignite chars, containing substantial amount of potential catalysts, may decrease steadily with an inflection point at intermediate 34 92 conversions. » This behaviour may be explained in terms of a shift 3 8 in the controlling mechanism of gasification. During the i n i t i a l periods of reaction the ash particles are highly dispersed in these chars so that the catalytic effects can overshadow completely the competitive structural influence. As the char is further gasified, the ash particles can sinter together decreasing the active catalytic area, accumulate and plug off some pores or/and undergo a chemical deactivation. Therefore pore growth and coalescence becomes the dominant mechanism. To conclude this section, i t appears that the ini t i a l char pore structure has a predominant effect on the early stages of the gasification reaction. Later, gasification depends more critically on the variations in the pore structure that take place subsequently. 3.3 Rate Equations for the Boudouard Reaction Due to the complexity of the Boudouard reaction caused by the physical and chemical changes occurring in the char particles as they react, many rate equations have been developed to correlate the kinetic data. Some of these equations will be reviewed in this section. The rate of reaction or "reactivity" of the char is defined in terms of the instantaneous mass of carbon in the char as: - 40 -1 dm 1 d f f i \ r = ~ m ' dt " ' d F <8 / s-«> where f = 1 m o As previously mentioned the Boudouard reaction follows the Langmuir-Hinshelwood (LH) equation, i n the absence of external mass transport l i m i t a t i o n s and at conditions removed from equilibrium. k i p c o 2 T = 1 + V e t ) + k 3 P C 0 2 [ 3 ' 1 ] where Y - k° exp(-E /RT), 1=1,3 Taking i n t o account the mechanism discussed i n Section 2.1: J l J2 1. C f + co 2(g) = C(0) + C0(g) J3 2. C(0) + C0(g) + C f then k x = J i EC, k 2 = j 2 / J 3 and k 3 = j 1 / j 3 where ZC = [C(0)] + [Cf] i s the t o t a l number of reaction s i t e s on the carbon surface. The LH equation s a t i s f i e s most of the experimental data on the C-CO2 r e a c t i o n . However, since i t i s consistent with d i f f e r e n t reaction mechanisms, the i n t e r p r e t a t i o n of the rate constants (kj) and a c t i v a t i o n energies (E^) depends on the mechanism or s p e c i a l case - 41 -18 19 81 assumed. It has been determined > > that when the temperature increases, k^  increases while k2 and k3 decrease. In addition to temperature, the k^  are strongly dependent on the particular carbon 18 under study. Therefore they should be determined for each type of carbon. The rate constants are usually obtained by the re-arrangement of Equation [3.1] in order to provide linear relationships when either Pen o r PC02 o r PC()/PC02 a r e held constant or CO is not admitted in the inlet gas. 1 7» 1 8« 8 1» 1 0 8 The kA are then obtained from the slopes and intercepts of these relationships. Power-law rate equations can be derived from the proposed mechanism 10 3 for the Boudouard reaction as shown recently by Fauteux and Chornet. The general form of these equations, frequently used by many investigators is: r - k ' p ^ [3.2] where k'= k Q exp (-E/RT) and E = E Q + C x f C 2 , c x and c 2 being constant, account for the variations in the apparent activation energy with conversion. Equation [3.2] can be regarded as a special case of the LH equation. Generally speaking, the apparent reaction order varies between 0 and 1. It has been established that, In the absence of retarding effects by CO, the reaction approaches first order at pressures below and up to 1 atm and zeroth order at pressures above 15 3k atm. If the reaction occurs at a shrinking surface, some authors take n=2/3. - 42 -When pen in the reacting gas can be neglected Equation [3.1] is reduced to: r • • \"C°J 13.3, 1 + k 3 p C 0 2 In addition when p is very low, 1 + k 3 p - 1, and a first C0£ CO2 order relation is obtained, as mentioned. r - kx p c ( ) 2 [3.4] Conversely for high values of p C Q, k 2P C 0 » 1 + k 3 p C Q and, k, P C 0 ? r = k 2 - p — Z [ 3 ' 5 ]  1 FC0 A similar equation to Equation [3.5] also can be obtained by 103 considering the mechanism of the Boudouard reaction: Step 1 is assumed to be very fast, being virtually at equilibrium, therefore: h t C f ] P C0 2 ~~ j2 t C ( 0 ) ] P C0 [ 3 ' 6 ] and [C(0)] = | l [Cf] ^ 2 2 . [3.7] J2 f P c o Step 2, on the other hand, is assumed to be rate controlling, hence: rate = j 3 [C(0)] [3.8] - 43 -By s u b s t i t u t i n g Equation [3.7] into Equation [3.8]: rate J2 [ C J [3.9] f J P, CO This fact can be j u s t i f i e d since the quasi-equilibrium assumed by Equation [3.6] i s favored by higher p a r t i a l pressures of CO since, as 27 shown by Mentser and Ergun, CO can e f f e c t i v e l y retard the reaction by changing the equilibrium r a t i o of occupied to free s i t e s (step 1). The temperature dependence of Equation [3.5] deserves comment. High values of apparent a c t i v a t i o n energy may be obtained when t h i s equation i s used to represent the data, as pointed out by Austin and W a l k e r . 1 1 1 These authors found an apparent a c t i v a t i o n energy of 110.7 kcal/mole for electrode carbon. This finding i s understood when i t i s r e a l i z e d that the "rate constant" for Equation [3.5] i s a c t u a l l y the r a t i o between the rate constants k^ and k 2. Since kj increases and k 2 decreases with increases i n temperature, the r a t i o between them i n c r e a s -es more pronouncedly than kj by i t s e l f . The apparent a c t i v a t i o n energy w i l l be the r a t i o of a c t i v a t i o n energies for k^ to k 2 and can be roughly 1=1,3, the apparent a c t i v a t i o n energy i s EJ - E^ + E^. Laurendeau assumed EJ - E^ - 20 kcal/mol and suggested E^ = 60 kcal/mol from previous data for a v a r i e t y of carbonaceous s o l i d s . Adopting these values the apparent a c t i v a t i o n energy i s 80 kcal/mol. estimated as follows . If k x / k 2 = J X J 3 ZC/J 2 and j ± = j° exp(-E'/RT), - AA -25 Ergun assumed for the mechanism of the C-CO2 reaction that step 1 attains equilibrium, step 2 is rate controlling, the total number of active sites (EC) is constant, chemical reaction controls and the system is isothermal. He then obtained the following equation: r- [ 3 . 10 ] p c o P C 0 2 + K where k* is a general rate constant and K is the equilibrium constant for the oxygen exchange reaction (step 1) . k* depends on the tempera-ture and on the surface area available when the rate is measured. K is a function of temperature and char type. Equation [3.10] predicts the observed fractional order of the reaction and has two limiting cases. When Pco2 ^ P C o / K t n e reaction is zeroth order and when pco2 ^ P C o / K t n e reaction is first order. It is interesting to note that in the last case the equation obtained, r = k*K p__, /p„_ also c o 2 CO is similar to Equations [3.5] and [3.9] and again is favored by higher PC0/PC0 2 ratios as suggested by Pco /PC0 2 > K« Equation [3.10] also can be derived from the LH equation if k 2 > 1 or k2pQQ + ^3PC0 2 ^ !• Therefore i t is favored at higher pressures. In this case, k* = ^1/^3 and K = k 3/k 2 = J i / j 2 , the equilibrium constant for step 1. Equation [3.10] can be re-written as: k* r [3.11] - 45 -or = -K + k*K (-) r [3.12] Equation [3.12] indicates a linear relationship between and —. r C02 Therefore k* and K can be estimated from the corresponding slope and an isothermal and fully back-mixed fluidized bed and recently by 113 Freund working with a TGA apparatus. It should be observed that in the first case high pressure was used, 10 atm, while in the second case a very low pressure was employed, 0.1 atm. Furthermore Freund argues that Equation [3.1] reduces to Equation [3.11] at low temperatures and low pressures, in disagreement with Johnson.15 The latter author points out that this simplification in the LH equation is justified at sufficiently high total pressure. In thermogravimetric studies, usually for purer carbons and mild reaction conditions, the production of CO can be neglected. If the partial pressure of C02 is kept practically constant and equal to 1 atm, the absolute rate of reaction is assumed to be directly proportional to the mass of carbon remaining in the bed, as follows. intercept. This procedure was used by Katta and Keairns 112 working with dm [3.13] dt " c where the rate constant k c is proportional to the pore surface area per unit mass of the char. Also k - 46 -also involves the assumptions of chemically controlled reaction, isothermal conditions and the pore structure remaining unchanged during the reaction. Re-arranging Equation [3.13] it can be seen that, under these conditions, the rate constant is equal to the specific rate of reaction as shown below. m - - - d T = r = k c t 3 ' 1 4 ^  d t c Integration of Equation [3.13] gives for f = 1 - — : o - in (1-f) = k t [3.15] c If pcOo * s different from 1 atm but s t i l l can be considered constant = k ' P n n "> [ 3 . 1 6 ] dt c rC0 2 where k' = k exp(- E'/RT). c c c Integrating, - in (1-f) = k^ p t [3.17] or - £n (1-f) = k C t [3.18] v C . U 2 where k = RT k' is the volumetric rate constant, v c Equations [3.15] and [3.18] give a linear relationship between -£n(l-f) and the reaction time, t, and are widely used to correlate kinetic data. From these equations i t follows that k = k /C . M v c C02 - 47 -Unfortunately, however, these equations are only v a l i d under conditions i n which PCO2 * s a l m o s t constant and pco can be neglected. Furthermore they were derived based on the assumption that the pore structure does not change during the reactio n . This assumption i s c l e a r l y very u n r e a l i s t i c as discussed i n the previous section. 114 The empirical equation proposed by von Bogdandy and Engell also 2 has been used. This equation can be written as follows: B r v = M cH c exp (-E^/RT) (^0? ^ C O ^ [ 3 # i g ] where r v i s the rate of reaction per unit volume of the charge (moles/cm charge • s ) , MQ i s the amount of carbon per unit volume of the charge (g/cm charge), H c i s the r e a c t i v i t y factor for the carbon (cm /g.s) and p ^ Q i s the equilibrium p a r t i a l pressure of CO2 for the Boudouard reacti o n . This equation accounts for the fact that the rate i s zero when thermodynamic equilibrium i s approached, i . e . , r v = 0 g when p ^ 0 = PQQ^ ' However i t also does not consider changes i n the pore structure of the carbons since HQ i s assumed constant. Semi-empirical equations were proposed to take into consideration v a r i a t i o n s i n the pore structure of the chars with the extent of re a c t i o n . Two of these equations, r e l a t i n g changes i n surface area with conversion by using empirical c o r r e l a t i o n s containing some adjustable parameters, w i l l be discussed below. 34 Dutta et a l . have proposed a rate equation that allows for the change in the ava i l a b l e pore surface area per unit weight of the char - 48 -with conversions lower than 90%. An empirical parameter that represents these changes in surface area of the particles was introduced as well as an effectiveness factor to account for pore diffusion resistance, as follows: r = n.k.a. c" [3.20] L>U2 The terms of this equation will be discussed briefly below. The effectiveness factor n is defined as the ratio between the measured reaction rate and the intrinsic reaction rate (the rate that would be obtained i f the entire pore surface were exposed to the same reactant concentration as the bulk gas phase). Clearly for chemically controlled reactions n=l. For the cases in which n*l the authors derived an expression for its determination. The rate constant k follows the Arrhenius equation, i.e., k=kQ exp(-E0/RT). The structural parameter "a" was defined as the ratio between the available pore surface area per unit weight of the char at any conversion and the ini t i a l l y available pore surface area per unit weight of the char. Generally "a" is dependent on conversion and temperature. Dutta et a l . did not take into consideration temperature effects and concluded that they were relevant only for caking coals. For the other chars "a" was independent of temperature in the range studied (840-1100°C). Changes in "a" with the fractional conversion of carbon were fitted to the following empirical equation, for conversions less or equal to 90%. - 49 -a - 1 ± 100 f exp (-Bf) [3.21] where f < 0.9 and 0 < v < 1. v and 3 are physical parameters characteristic of a given coal or char, v indicates the conversion at which the relative available surface area reaches the maximum or minimum value. Equation [3.21] indicates that "a" may increase, decrease, or may show a maximum or minimum with conversion according to the sign (+ or -) used. The order of reaction, n, was considered one up to atmospheric pressure. 21 Agarwal and Sears have proposed the following rate equation, exclusive of external mass transfer, to predict the rate of reaction as a function of the fractional conversion and C0-C02 partial pressures r = n • n(f) • F(p c o, f).( ^2 ) [3.22] 2 PC0 d FC0 2 where n, k^ (1=1,3), p^ 0 and p^ n are as before. In this equation n(f) is a conversion factor given by the ratio between the reaction rate at any conversion and the initial reaction rate. Obviously n(f) depends on surface area variations during the reaction. Moreover n(f) is related to conversion for the pure C02 reaction. Therefore since C0-C02 mixtures were used, an additional term, F(pQ0»f)» was included in the rate equation and will be - 50 -discussed below. F(pco,f) is a deviation factor which accounts for changes in n(f) when gases containing CO and CO2 are reacted as compared to the reaction with pure CO2• Agarwal and Sears measured F(pQ0,f) for lignite, sub-bituminous and bituminous coal chars at different temperatures and C0/C02 varying from 50:50 to 99:1. They found F(p c o,f) to be approximately 1 for al l conditions, meaning that pore development is independent of the presence of CO in the gas. To represent n(f), Agarwal and Sears initially used Equation [3.21] given by Dutta et al. However this equation did not f i t their data properly. Therefore the authors proposed additional equations. First they slightly modified Equation [3.21] introducing a constant A. v8 n(f) = 1 ± Af exp(-3f) [3.23] However Equation [3.23] provided only a reasonable f i t to the data and was considered cumbersome since it contains three adjustable parameters. Thus, the authors developed the following single parameter correlation. n(f) = 1 - M to (1-f) [3.24] This correlation was able to represent the data very well and was used in the work. The parameter M was assumed to be indicative of surface area changes during the reaction. For lignite and sub-bituminous coals M was practically independent of temperature in the range 850 to 1100°C, while there was a variation for bituminous coal - 51 -chars at high temperatures. Equation [3.24] was e m p i r i c a l l y derived but i t s form can be seen as a l i n e a r i z a t i o n of the surface area development 1 1 5 model of Bhatia and Perlmutter. This model w i l l be discussed i n the next section. However the equation Is not able to predict the observed decrease i n surface area and r e a c t i v i t y a f t e r a c e r t a i n conversion i s reached, since n(f) increases monotonically with f. Other semi-empirical and t o t a l l y empirical " f i t t i n g " equations have been used to i n t e r p r e t the k i n e t i c data. It i s f e l t , however, that a d i s c u s s i o n of these equations i s unnecessary. Furthermore more refined methods of a n a l y s i s , derived from mathematical models capable of giving a more complete and accurate picture of the g a s i f i c a t i o n process, also have been developed and w i l l be discussed in the next s e c t i o n . 3.4 Mathematical Models Mathematical models for the g a s i f i c a t i o n of "carbon" p a r t i c l e s with CO2 have been developed. Early models f a i l e d to take into consideration the experimentally observed v a r i a t i o n i n the pore structure of the p a r t i c l e s with time. However, as discussed before, o v e r a l l char g a s i f i c a t i o n rates are strongly dependent on the i n i t i a l l y a v a i l a b l e pore structure and on the changes in the available pore structure as r e a c t i o n progresses. Knowledge of the pore structure and i t s transport properties i s e s s e n t i a l to predict g a s i f i c a t i o n rates. Since s t r u c t u r a l changes vary widely with d i f f e r e n t coal chars, the use of empirical k i n e t i c s determined for a few chars under li m i t e d experimental conditions, i s of l i t t l e use for p r a c t i c a l a p p l i c a t i o n s . Therefore - 52 -several models for the structural changes in char gasification have been developed, especially in the last few years. Relevant contributions to the understanding of the kinetics of these reactions have resulted from these models although most of them have been successfully applied only to a small set of available experimental data. Variations in the rate of reaction or in the fractional conversion of carbon with time have been invariably predicted in the chemically controlled regime. Correspondingly, in many studies sufficiently low temperatures have been used to ensure that the reaction was either under chemical control or subject to mild diffusional effects. It also has been shown that the presence of catalytically active substances in the char may affect the gasification process significantly. Thus models specifically aimed to represent catalytic gasification reactions will be reviewed. A complete assessment of the models will not be attempted, however general descriptions of the models emphasizing their main features will be presented. Petersen 1 1 6 has proposed a method of analyzing the complete gasification of a porous solid. The reaction was assumed first order with respect to the reactant gas concentration. Pore diffusion effects also were considered. The rate of change of the average radius of a single cylindrical pore, initially of uniform radius, and of a porous pellet, initially containing a network of uniform pores with random intersections, was evaluated. It was assumed that the pores do not coalesce when they grow. The evolution of internal surface area with reaction was determined. For this purpose two parameters were - 53 -introduced to express the surface area as a function of pore radius, and a relationship between porosity and surface area was derived. Mathematical solutions for the pellet case were used to interpret the experimental results on the gasification of graphite rods with C02. However the model is hardly applicable to practical systems due to the highly complex and dynamic pore structure of coal chars. Austin and Walker 1 1 1 have studied the C-CO2 reaction for conditions where the retarding effect of CO is very pronounced. In this case the build up of small concentrations of CO within porous graphite can induce a significant non-uniformity of gasification. The combined differential equation of internal mass transport and chemical reaction was numerically integrated. The LH equation was applied and the corresponding rate constants were taken from previous data. The results indicate that the reaction may proceed in a strongly non-uniform fashion even when the C02 concentration inside the solid is practically constant. This non-uniformity is probably caused by the retarding effect of CO on the reaction and occurs for gasification rates one hundred times lower than the usual Thiele criteria would predict. 117 Yoshida and Kunii have presented a method of analysis for the reaction between a porous solid and a gas, assuming a significant diffusional resistance. The kinetics equation used is first order with respect to the reactant gas concentration. Changes in the pore structure of the solid with reaction were considered. The model was applied in the gasification of graphite spheres with C02. The results obtained represent the experimental data for the early stages of reaction. - 54 -Tien and Turkdogan have published a mathematical analysis to investigate the internal reaction of graphite or coke controlled simultaneously by counter diffusion of CO2 and CO in the pores and chemical reaction on the pore walls. Due to the strong inhibiting effect of CO, incomplete pore diffusion considerably reduces the depth of internal reaction. Effects of temperature, total pressure and particle size on the rate of reaction of graphite and coke in CO2-CO mixtures, predicted by the model, are in reasonable agreement with experimental data. For pore diffusion control, the rate per particle is proportional to the external geometrical surface area, is independent of total pressure and has an apparent activation energy equal to half of that for chemical reaction control. A very important conclusion derived from the model is that with increasing CO concentration, pore diffusion effects occur at higher temperatures and with larger particles. An increase in total pressure has opposing effects. However the model was developed only for the reaction mechanism of Turkdogan and Vinters. In addition, the model is valid only for the i n i t i a l stages of reaction. Rao and Jalan 1 5 obtained the intrinsic LH rate constants from the corresponding experimentally determined apparent values for disc-like pellets of carbon black. To accomplish this they corrected the apparent rate constants for pore diffusion effects using a generalized method for evaluating the effectiveness factor for porous catalysts following LH 119 120 rate expressions, developed by Roberts and Satterfield. » 121 122 Hashimoto and Silveston » have developed a model describing the evolution of specific surface area, volume, porosity and mean pore - 55 -radius of a porous s o l i d with the extent of r e a c t i o n . Due to the complexity of the pore structure, the model uses the general approach of 123 Hulburt and Katz i n formulating a population balance to represent s t r u c t u r a l changes i n the k i n e t i c a l l y c ontrolled g a s i f i c a t i o n process. The population balance accounts for pore size d i s t r i b u t i o n as well as pore growth, i n h i b i t i o n of new pores and coalescence or collapse of adjacent pore walls into each other as reaction proceeds. The authors extend the model to d i f f u s i o n control l n a subsequent paper. However, the model contains up to ten adjustable parameters and the r e s u l t s are very s e n s i t i v e to t h e i r values. Moreover the model seems to be app l i c a b l e only to a l i m i t e d range of carbons. The model s u c c e s s f u l l y predicts the maximum i n s p e c i f i c surface area i n the k i n e t i c a l l y c o n t r o l l e d g a s i f i c a t i o n of d e v o l a t i l i z e d anthracite by C 0 2 » 1 1 0 These chars have a pore structure considerably d i f f e r e n t from that of low-rank coal chars. 124 Wen and Wu have formulated a one-parameter volume reaction model to describe the reaction of a gas and a consumable s o l i d i n a porous p a r t i c l e . The authors used a modified Thiele modulus to take into account the e f f e c t of s o l i d reactant depletion during r e a c t i o n . The model also considers pore d i f f u s i o n e f f e c t s . Nevertheless i t has the drawback of considering the pore structure of the p a r t i c l e s to remain e s s e n t i a l l y unchanged. The model was s a t i s f a c t o r i l y applied to experimental data on the C-C02 reaction. The k i n e t i c reaction rate was found to be f i r s t order with respect to both C and CO2 concentrations. 125 Stephen and McEnaney were concerned with the poisoning of the Boudouard reaction by accumulation of CO i n the pores of the s o l i d , as - 56 -were Austin and Walker and Tien and Turkdogan. They presented a mathematical model for the retarding effect of CO accumulation in bulk samples of graphite during C-CO2 reaction. LH kinetics and a simplified form of this equation were considered as well as slab and spherical geometries. The results were compared with those from previous models. 126—129 Simons and co-workers have proposed a random pore model related to the model of Hashimoto and Silveston but more refined. The model is based on a statistically derived pore size distribution function that describes the combination and branching of pores during gasification. Char pore structures were considered to be like an ordinary river or tree system with small pores feeding into increasingly larger ones until eventually a l l pores lead into main trunks. However, the model uses several empirical correlations to associate the relevant physical properties of chars such as density of pore intersections, length of pore segments, etc . . . to the fundamental pore statistics instead of doing so by first principles. 13 0 Srinivas and Amundson have developed a model for the gasification of a single char particle in an environment of H2O, H 2 , CO2, CO and CH^ , typical of coal gasification reactors. The model consists of the mass and energy conservation equations and the Stefan-Maxwell relations. Convective terms in these relations have negligible influence and may be ignored in reactor modelling. It also includes the effects of pore diffusion and surface area changes In the particles. These effects are found to be relevant and, thus, cannot be neglected mainly with larger particle sizes and higher temperatures. To - 57 -account for surface area changes, the c o r r e l a t i o n for r e l a t i v e a v a i l a b l e 34 surface area suggested by Dutta et a l . was u t i l i z e d as described i n the previous s e c t i o n . Comparison with d i f f e r e n t models and e x i s t i n g empirical equations, show that the model can be used when d i f f u s i o n a l e f f e c t s are minimal, otherwise s i g n i f i c a n t deviations occur. Due to the large discrepancies i n the data the rate constants were treated l i k e parameters. The ultimate objective was to develop a coupled transport and reaction model to determine the e f f e c t of the various process v a r i a b l e s on the g a s i f i c a t i o n process. 13 1 Gavalas has proposed a random c a p i l l a r y model that predicts the frequency of pore i n t e r s e c t i o n s , length of pore segments and development of pore volume and surface area. These properties were derived from a s i n g l e p r o b a b i l i t y - d e n s i t y function defined by pore size measurements. This function characterizes the pore structure of the s o l i d s . The model i n t e r p r e t s k i n e t i c data and s t r u c t u r a l changes occurring i n char g a s i f i c a t i o n . Model predictions agree with some experimental data f o r the chemically controlled C-CO2 re a c t i o n . » Deviations were considered probably due to eit h e r d i f f u s i o n a l l i m i t a t i o n s or c a t a l y t i c e f f e c t s . Despite being intended and tested for chemically c o n t r o l l e d r e a c t i o n s , the model can be applied for reactions involving d i f f u s i o n a l l i m i t a t i o n s . In th i s case numerical solutions would be required for the r e s u l t i n g equations. The c r i t i c a l assumption i s the conversion-independent i n t r i n s i c r e a c t i v i t y of the pore surface. This assumption should be further evaluated experimentally since d i f f u s i o n a l and c a t a l y t i c e f f e c t s may i n t e r f e r e with i t . It was recognized that the - 58 -pore structure of chars may deviate from that proposed because of n o n - c y l i n d r i c a l pores and possible space r e g u l a r i t i e s . Nevertheless the model seems to give an adequate representation of n o n - c a t a l y t i c char g a s i f i c a t i o n . 1 3 2 Bhatia and P e r l m u t t e r 1 1 5 have developed a random pore model which considers an a r b i t r a r y pore size d i s t r i b u t i o n in the reacting s o l i d . The model involves three main parameters: porosity, surface area and pore length, that can be obtained from the i n i t i a l pore structure of the p a r t i c l e s . The rate of reaction i s assumed to be proportional to the i n t e r n a l surface area developed. The model can represent the behaviour of a system that shows a maximum i n the reaction rate as well as one that shows monotonically decreasing reaction rates with the extent of conversion. It also i d e n t i f i e s an optimum pore structure f o r e i t h e r of such systems and r e f l e c t s the marked influence of the i n i t i a l pore structure on the g a s i f i c a t i o n rate. The model was shown to include several former models as s p e c i a l cases. Accordingly a r e l a t i o n s h i p was derived between a pore structure parameter defined by the model and the 13 3 e f f e c t i v e grain shape factor of the grain models. Moreover when the variance of the pore s i z e d i s t r i b u t i o n i s a c t u a l l y zero, the r e s u l t s approach those predicted by the Petersen m o d e l 1 1 6 over a large range of conversion. The model was i n i t i a l l y tested with s a t i s f a c t o r y r e s u l t s , on the char g a s i f i c a t i o n data of Hashimoto et a l . 1 3 4 A f t e r t h i s i t became one of the most popular and successful models for char g a s i f i c a t i o n reactions. It has been reported to give a s a t i s f a c t o r y 132 behaviour for char p a r t i c l e s undergoing non-catalytic reactions. - 59 -Furthermore i t has been widely used with good r e s u l t s i n diverse 135 s i t u a t i o n s as indicated by the works of Debelak et a l . on the 13 6 char-CC>2 r e a c t i o n assuming k i n e t i c c o n t r o l , Tone et a l . on the potassium catalyzed steam g a s i f i c a t i o n of coal char using H2O-H2-CO mixtures under pressure and Su and Perlmutter on the c h a r - 0 2 13 8 r e a c t i o n . In addition, when coupled with d i f f u s i o n a l l i m i t a t i o n s , the model has been successfully applied i n reactions of porous lime with 139 SO2 and CO2. The only instance that the model can be inadequate i n representing experimental data i s when c a t a l y t i c e f f e c t s give r i s e to rate curves with an i n f l e c t i o n point in the region of chemically c o n t r o l l e d rates. Aderibigbe and S z e k e l y 1 8 > 1 4 0 have developed a mathematical model to represent the reaction of slab-shaped p a r t i c l e s with CO-CO2 gas mixtures when external mass transfer i s not rate c o n t r o l l i n g . The authors reviewed the pore d i f f u s i o n and simultaneous pore d i f f u s i o n and chemical reaction phenomena. The model i s thought to be a comprehensive q u a n t i t a t i v e d e s c r i p t i o n of the g a s i f i c a t i o n process, from the i n i t i a l stages to the complete conversion of the porous carbon, because i n i t s development allowance was made for r e a l i s t i c LH k i n e t i c s with evaluated i n t r i n s i c rate constants, pore d i f f u s i o n , bulk flow due to d i f f u s i o n and s t r u c t u r a l changes i n the coke. The governing equations were solved numerically, u t i l i z i n g the concept of the effectiveness f a c t o r . K i n e t i c data was complemented by the c h a r a c t e r i z a t i o n of the pore structure of commercial metallurgical coke samples at d i f f e r e n t conversion l e v e l s . Model predictions were s u c c e s s f u l l y compared with - 60 -the experimental measurements when the changes in the structural parameters with conversion were accounted for. The ultimate objective of the model was to acquire a quantitative understanding of the rate at which metallurgical cokes react with CO-CO2 mixtures. A procedure similar to that of Aderibigbe and Szekely was later utilized by Alam and 10 8 DebRoy to estimate the intrinsic LH rate constants from the corresponding experimentally determined apparent values, for disc-like pellets of metallurgical cokes. 141 Fieldes and Hansen have proposed a very simple model that considered char particles to consist of cylindrical pores. It was assumed that during the reaction the pores kept their diameter constant but increased in length causing an increase in surface area. It was further assumed that the destruction of surface area at higher conversions, due to the merging of adjacent pores was proportional to the mass of carbon remaining in the bed. An equation representing the effects of several process variables was obtained. Coupled with kinetics information for iron sand reduction this equation might be used in generalized models for rotary kilns. 142 Zygourakis et al. have formulated a probabilistic model for the evolution of the internal surface area and pore volume during the reaction of a single char particle. The model was based on the general population-balance approach proposed by Hulburt and Katz. The pore structure of the char was characterized by two populations of pores; large spherical vesicles interconnected through a few large neck pores, representing the macropores and narrow cylindrical micropores. This - 6 1 -conceptualization i s suggested by experimental evidence on the char pore structure. Model predictions were i n good agreement with experimental data obtained i n the chemically c o n t r o l l e d regime by Dutta 3 4 et a l . Minor discrepancies were p a r t i a l l y a t t r i b u t e d to the c a t a l y t i c e f f e c t of inorganic impurities. 143 Hamilton et a l . have found that models which have been successful i n representing the char pore structure for u n c a t a l y t i c reactions were found inadequate for c a t a l y t i c g a s i f i c a t i o n at low conversions since they do not simulate the complex catalyst-char i n t e r a c t i o n s . Therefore these authors derived a model that combines the e f f e c t s of three contributing factors to the v a r i a t i o n i n rate during c a t a l y t i c r e a c t i o n : change i n char surface area, c a t a l y s t loss and change i n the catalyst/carbon r a t i o . The model gives a s a t i s f a c t o r y representation of t h e i r own experimental r e s u l t s . 1 l i L Reyes and Jensen have developed a mathematical model for c a t a l y t i c char g a s i f i c a t i o n i n which n a t u r a l l y occurring inorganic impurities catalyzed the reactions. The model was based on a p r o b a b i l i s t i c d e s c r i p t i o n of the evolution of the c a t a l y t i c a l l y a c t i v e surface area during the g a s i f i c a t i o n of a sin g l e char p a r t i c l e . The population-balance approach of Hulburt and Katz was adopted i n the same 14 2 manner as by Zygourakis et a l . Loss i n c a t a l y t i c a c t i v i t y was at t r i b u t e d to agglomeration of mineral p a r t i c l e s during g a s i f i c a t i o n . Chemical deactivation of the c a t a l y s t s also was considered. However due to the lack of information on the s p e c i f i c chemical mechanisms of d e a c t i v a t i o n , t h i s was done by using an empirical r e l a t i o n s i m i l a r to - 62 -those used to model the deactivation of heterogeneous c a t a l y s t s . An a n a l y t i c a l equation f o r the drop i n the active surface area with the extent of reaction was derived. Coupling t h i s expression with the model by Zygourakis et a l . , a good representation of published experimental data was obtained. The model also indicates that even at low concentrations of c a t a l y t i c components both the development of the i n t e r n a l pore structure and c a t a l y t i c surface area are important to predict the i n d i v i d u a l contributions of c a t a l y t i c and u n c a t a l y t i c r e a c t i o n thus providing a more fundamental understanding of the o v e r a l l process. The model was further s u c c e s s f u l l y applied to the g a s i f i c a t i o n of char p a r t i c l e s impregnated with c a t a l y t i c a l l y active s a l t s . 7k Su and Perlmutter have developed a cat a l y s t deposition model f o r the chemically controlled k i n e t i c s of c a t a l y t i c g a s i f i c a t i o n of coal chars. O v e r a l l r e a c t i o n k i n e t i c s was modelled as a combination of contributions from non-catalytic reaction on the i n t e r n a l pore surfaces and c a t a l y t i c reaction on p a r t i c l e e x t e r i o r s . The model confirms an expectation that the enhanced r e a c t i v i t y would be more s i g n i f i c a n t at low l e v e l s of conversion. It also predicts that the o v e r a l l rate decreases to that of uncatalyzed reaction at higher conversions. Furthermore the model assesses the i n d i v i d u a l contributions of c a t a l y t i c and n o n - c a t a l y t i c steps to the o v e r a l l reaction rate. The model predictions are i n agreement with the experimental r e s u l t s . F i n a l l y i t can be concluded that recent s t r u c t u r a l models give reasonable predictions f o r the changes i n the i n t e r n a l pore structure of coal chars during g a s i f i c a t i o n , e s p e c i a l l y i n the chemically controlled - 63 -regime. The d e v i a t i o n s between model p r e d i c t i o n s and some ex p e r i m e n t a l l y observed behaviour, may be explained both i n terms of c e r t a i n r e s t r i c t i v e assumptions considered as w e l l as c a t a l y t i c e f f e c t s of i n o r g a n i c c o n s t i t u e n t s present or added to the char. Consequently, and due to the i n c r e a s i n g i n t e r e s t i n the c a t a l y t i c g a s i f i c a t i o n of c o a l chars, s p e c i f i c models have been developed to represent these processes. 3.5 Experimental Measurement of Char Reactivity The r e a c t i v i t y of chars has been measured under a broad range of experimental c o n d i t i o n s . However i n the bulk of the work that has been r e p o r t e d , the methods most commonly used can be d i v i d e d i n t o two b a s i c c a t e g o r i e s : ( i ) D i r e c t weighing (thermogravimetric) methods. These methods have been p r i m a r i l y used f o r d e t a i l e d o v e r a l l k i n e t i c s s t u d i e s . They operate by continuous monitoring of sample weight and are more f l e x i b l e f o r using extremely low pressures, d i f f e r e n t sample weights, p a r t i c l e s i z e s , temperatures and gas flow r a t e s . Changes i n r e a c t i v i t y may be followed as r e a c t i o n proceeds. In a d d i t i o n , s p e c i a l techniques can be 13 3 used under mixed c o n t r o l c o n d i t i o n s . ( i i ) Gas a n a l y s i s methods. These methods are very s u i t a b l e f o r continuous comparative t e s t s . The extent of char conversion as a f u n c t i o n of time may be obtained from the measured flow ra t e of the e x i t gas and i t s chemical composition, u s u a l l y determined by gas chromatography or i n f r a r e d a bsorption spectroscopy. There are divergent opinions regarding which method i s most ap p r o p r i a t e f o r measuring char r e a c t i v i t i e s . Some i n v e s t i g a t o r s p r e f e r - 64 -thermogravimetric methods because they can both d i r e c t l y and accurately record small weight changes with time and because measurements are not subject to errors i n gas a n a l y s i s . The main drawback of d i r e c t weighing methods i s that the small amounts of p a r t i c u l a t e samples employed are hardly representative of the whole coal or char under study. This fact should be taken into account i n the analysis of the r e s u l t s obtained. The p h y s i c a l c h a r a c t e r i z a t i o n of these samples also i s c l e a r l y d i f f i c u l t . The small amounts used are i n e v i t a b l e i n order to minimize the influence of undesirable physical e f f e c t s on the rate of re a c t i o n . In t h i s regard, very small i n i t i a l weights of char are used providing a very t h i n bed i n order to minimize bulk d i f f u s i o n e f f e c t s and also to ensure that the bed temperature does not f a l l s i g n i f i c a n t l y during the rea c t i o n . When geometrically shaped s o l i d s l i k e spheres, c y l i n d e r s , slabs and discs are used, the physical e f f e c t s are even harder to be eliminated. The success i n avoiding extraneous influences, when d i r e c t weighing methods are used, has not yet been f u l l y e s tablished. In the case of gas analysis methods, f l u i d i z e d bed reactors have several advantages when compared to fi x e d bed r e a c t o r s . 1 4 5 In fi x e d beds, a s i g n i f i c a n t temperature gradient i s l i k e l y to develop i n the bed due to the endothermic nature of the Boudouard r e a c t i o n . A corresponding CO2 concentration gradient also may be established. Consequently the reaction rates, and then the r e a c t i v i t y of the char bed, may be measured under d i f f e r e n t environments of temperature and reactant gas concentration, unless the height of the bed i s d r a s t i c a l l y reduced. This l a s t procedure may cause problems associated with the - 65 -small amount of sample, already mentioned. Clearly the good gas mixing and the almost isothermal nature of fluidized bed reactors, coupled with their easy and accurate temperature control, make them suitable reactors for measuring char reactivities. Fixed bed reactors, however, allow the utilization of a wider range of particle size. Fixed/fluidized bed reactors can operate in the differential or In the integral mode.11 In the first case, the reaction rate should be constant at a l l points within the reactor. Since the rate depends on the reactant gas concentration, this only occurs for small conversions or for very shallow small reactors unless the reaction is sufficiently slow or zero order. Continuous sampling allows direct determination of elementary kinetics mechanisms, however, the small concentrations of CO produced magnifies the experimental errors. In the second case, the variation of reaction rate within the reactor is so substantial that i t should be accounted for in the analysis. Such large variations in rate may be expected to take place when the composition of reactant gas changes significantly along the reactor. The problems associated with small concentrations of CO are avoided but a plug flow or perfect mixing reactor approximation is required. 1 4 6 A reactor that combines the characteristics of thermogravimetric and gas analysis devices also has been used. The so-called "Packed Bed 147 Balance Reactor continuously measures the weight of a packed-bed flow reactor, including the carbon sample. The product gas is further analyzed by gas chromatography. Therefore reaction rates are calculated from gas analysis, gas flow through the reactor and from the weight decrease of the system. - 66 -F i n a l l y , to evaluate pore d i f f u s i o n e f f e c t s i n char g a s i f i c a t i o n reactions, d i f f e r e n t p a r t i c l e sizes are u s u a l l y , 15 21 33 36 45 63 87 used. » » > » > • i t i s generally accepted that i f by reducing the size of the char p a r t i c l e s , an increase in r e a c t i o n rate i s obtained, d i f f u s i o n a l e f f e c t s are present i n the system since the chemically controlled rate of reaction should be independent of p a r t i c l e s i z e . The use of smaller p a r t i c l e s contributes to minimize d i f f u s i o n a l resistances. Experimental r e s u l t s also show that p a r t i c l e e f f e c t s are more pronounced for high-rank coals than for low-rank c o a l s , since the former probably o f f e r more resistance to i n t e r n a l d i f f u s i o n of 4 5 reactant gas due to a lower porosity. However i t i s important to note that the differences between chemical and pore d i f f u s i o n control are usually not great and may be masked by the uncertainty in the experimental r e s u l t s unless very d i s t i n c t sizes are employed. In a d d i t i o n , smaller p a r t i c l e s usually have higher ash content and higher s p e c i f i c surface area and both factors may contribute to increase the r e a c t i o n rate. For highly porous char p a r t i c l e s at the temperature and pressure ranges most commonly used, i t i s probable that d i f f u s i o n a l e f f e c t s have been present i n the majority of the studies , 16 18 140 reported. • » - 67 -CHAPTER 4 OBJECTIVES AND SCOPE OF THE PRESENT WORK From the foregoing chapters it can be concluded that a significant advance has been made, in the last few years, on the understanding of the gasification of coal chars with C0 2« Nevertheless, there remains a paucity of available information on the reactivities of different low-rank coal chars and a lack of precise knowledge on the influence of the various physical and chemical phenomena affecting the reactivities. In addition, extensive reserves of low-rank coals are available in Western Canada for which the kinetics behaviour have been hardly investigated. This work aims, therefore, to provide the information necessary on the gasification of Western-Canadian coals in order to utilize the results obtained in mathematical models for metallurgical processes using these coals. To accomplish this, the kinetics of gasification of two sub-bituminous coals from Alberta, with CO2, were determined by contacting preheated C0 2 and coal char in a laboratory-size fluidized bed reactor and measuring the flow rate and composition of the gas exiting the reactor. The coals utilized In the kinetics study are suitable for metallurgical utilization. Regarding the fluidized bed reactor, depending on the ratio between the depth and the diameter of the bed, as well as on the superficial gas velocity used, good mixing conditions can be achieved, thus allowing an intimate contact between the solids and the gaseous reactant. Moreover this reactor also is very - 68 -appropriate for the highly endothermic Boudouard reaction, due to the easy and accurate temperature control coupled with high rates of heat transfer. Fixed bed systems, on the other hand, usually present concentration and temperature gradients within the solids bed and, thus, non-uniformity in the gasification conditions. Thermogravimetric devices, in turn, require very small amounts of particulate solids, hardly representative of the highly heterogeneous coals. In view of these considerations, the objectives of this work may be summarized as follows: (i) To study the kinetics of the gasification of representative samples of char originated from two sub-bituminous coals with CO2, under well mixed and isothermal conditions, to minimize the effects of mass and heat transfer, ( i i ) To evaluate the effects of several variables on the gasification kinetics, namely: coal type, charring conditions, bed depth, total inlet flow rate, inert gas concentration, temperature and gas composition, in particular the effects of different CO to CO2 ratios in the gas phase. ( i i i ) To determine how char reactivity varies with the extent of reaction by correlating the changes in reactivity with the changes in surface area occurring in the char during gasification. (iv) To obtain an appropriate overall kinetic equation for the gasification process. - 69 -A char r ing treatment f o r the c o a l s , p r i o r to the g a s i f i c a t i o n exper iments, was deemed necessary s ince the presence of v o l a t i l e s would compl icate considerably the a n a l y s i s of the Boudouard k i n e t i c s . In a d d i t i o n , It was necessary to perform severa l a u x i l i a r y experiments to determine the minimum f l u i d i z a t i o n v e l o c i t y for the char bed and to evaluate the accuracy of gas ana lys is and flow rate measurements. Since mass and heat t rans fe r e f f e c t s were at l eas t minimized in the major i ty of the experiments, the r e s u l t s obtained for the g a s i f i c a t i o n k i n e t i c s are e s s e n t i a l l y dependent on the proper t ies of the chars used. However some t y p i c a l c h a r a c t e r i s t i c s of the f l u i d i z e d bed reactor may have in f luenced the r e s u l t s as d iscussed l a t e r . To reduce the number of experiments, the k i n e t i c s of the g a s i f i c a t i o n reac t ion was f u l l y studied only for one c o a l . The coa l chosen had been reported to be more r e a c t i v e , and has lower sulphur and ash contents be ing , t h e r e f o r e , more s u i t a b l e fo r m e t a l l u r g i c a l a p p l i c a t i o n s . A few comparative experiments were performed with a second c o a l . - 70 -CHAPTER 5 EXPERIMENTAL 5.1 I n t r o d u c t i o n This chapter presents a d e t a i l e d account of the experimental techniques employed i n this study. It includes the c h a r a c t e r i s t i c s of the two coals investigated and t h e i r preparation, a d e s c r i p t i o n of the apparatus used i n the coal charring and i n the g a s i f i c a t i o n experiments and the procedures adopted to perform these experiments. The coals studied were two sub-bituminous coals selected from three reference coals provided by the coal sample bank of the Alberta Research Council. Sub-bituminous coals from Alberta are r e a d i l y a v a i l a b l e and have vast reserves; moreover t h e i r r e l a t i v e l y high r e a c t i v i t i e s make them very s u i t a b l e for several m e t a l l u r g i c a l processes as shown i n the l i t e r a t u r e . 6 0 The reference coals represent the f u l l range of rank, geographical l o c a t i o n and geological o r i g i n present i n the Alberta P l a i n s coal region. They also represent the e n t i r e spectrum of c o a l i f i c a t i o n (sub-bituminous A, B and C) of Alberta low-rank co a l s . A laboratory-size f l u i d i z e d bed reactor was designed to meet the basic requirements of the experimental work. The apparatus was able to char the coals and measure the r e a c t i v i t i e s of the chars under c o n t r o l l e d and reproducible conditions. In this regard the reactor was able i n a l l the cases, except for the most reactive char at the highest temperature studied, of providing a near isothermal environment for the - 71 -reaction, by allowing an accurate temperature control and efficient rates of heat transfer. 5.2 Materials Used and their Preparation The coals used in the experimental work were selected from the reference coals according to the total ash content based on the composition of these coals given by Parkash and du Plessis 1 since, of the properties affecting reactivity, ash content was the most readily available. The coals with lower ash content; Highvale (wabamum), sub-bituminous B (10.7% ash, dry) and higher ash content; Smoky Tower, sub-bituminous A (20.0% ash, dry) were studied. The proximate and ultimate analyses of these coals, provided by the Alberta Research Council, are presented in Table I. One drum (200 1) of each coal, in the as-received condition, was dried overnight in a warmed floor room. After drying, the coals were crushed in a large hammer mill and sampled by coning and quartering. The accepted material was then screened in a Gilson pilot plant facility Into the size fractions -1190 + 595 um (Highvale coal) and -1190 + 420um (Smoky Tower coal). The oversizes after re-crushing in a smaller hammer mill, were again screened into the same size fractions. This was repeated until sufficient material, in the above size ranges, was obtained to perform the large number of experiments planned. Representative samples of the screened coals were obtained with two Jones r i f f l e sample splitters and sent for chemical analysis. The proximate and ultimate analyses of these samples, performed by Stelco T A B L E I P R O X I M A T E AND U L T I M A T E A N A L Y S E S OF H I G H V A L E AND SMOKY TOWER C O A L S Highvale coal Smoky Tower coal -1190 + 595 vim -1190 + 420 pm Stelco batch 1 batch 2 U.B.C. A.R.C. Stelco A.R.C. (%) (X) (%) (%) (%) (%) Proximate Analysis H2O (as received) 15.27 13.14 14.7 19.6 3.30 16.0 Ash 10.37 10.36 9.4 8.6 13.57 16.8 V o l a t i l e s 29.44 30.80 29.0 28.4 39.33 29.2 Fixed C 44.92 45.70 46.9 43.4 43.80 38.0 Ultimate Analysis (d.b.) Carbon 64.94 65.30 67.0 63.05 60.06 Hydrogen 3.81 3.95 3.9 4.20 4.1 Nitrogen 0.85 0.85 1.0 1.52 1.4 Sulphur (0.18) - 0.2 0.28 0.4 Ash 12.25 11.93 10.7 14.03 20.0 Oxygen (by difference) (17.97) - 17.2 16.92 13.5 - 73 -Inc., are presented In Table I, while the ash composition analysis, also performed by Stelco Inc., is presented in Table II. Table I also shows the proximate analysis of samples of Highvale coal performed in a Fisher Coal Analyzer at U.B.C.. The conditions adopted for this analysis are presented in Table III. The sieved coals were stored in tightly sealed plastic bags. The gases used in the coal devolatilization and gasification runs-argon, helium, carbon dioxide and carbon monoxide-were supplied in commercially available cylinders of gases of chemical purity. These gases were used without any further purification. 5.3 Coal Charring Apparatus To obtain an homogeneous char under accurately controlled conditions, batches of coal were initially devolatilized in the fluidized bed reactor. The set-up used in the gasification experiments was slightly modified in order to f i t this purpose. However this method of charring, despite its many advantages, had the limitation of small throughputs. Thus to obtain the amounts of char required for the gasification experiments, a large number of devolatilization runs would be necessary. This would have the disadvantage of increasing considerably the amount of work and time necessary to complete the project. Therefore charring In the fluidized bed was discontinued and the coals were further devolatilized in a top-fired gas furnace using a set-up similar to the one utilized previously ln this department.4 A schematic view of this system is presented in Figure 5.1. - 74 -TABLE I I ASH COMPOSITION OF HIGHVALE AND SMOKY TOWER COALS Compound Highvale Coal (%) Smoky Tower coal (%) SI02 49.14 58.61 A1203 27.81 19.36 Fe 20 3 3.83 4.18 Ti0 2 0.65 n.d. P2O5 0.10 1.97 CaO 13.96 12.21 MgO 0.97 1.59 K20 0.45 0.68 Na20 3.24 0.24 - 75 -T A B L E I I I F I S H E R COAL ANALYZER OPERATING CONDITIONS V o l a t i l e Ash Analysis Analysis I n i t i a l heating rate (°C/min) 10 35 T r a n s i t i o n temperature (°C) 600 600 F i n a l heating rate (°C/min) 35 35 F i n a l temperature (°C) 900 850 Time at f i n a l temperature (min) 7 200 - 76 -Top view T/c 's r L "I f_ - R _i 11 Gas exit Burner Inconel cover purge Bone ash floor Argon Side view Figure 5.1 Schematic view of coal charring set-up - 77 -The system co n s i s t e d of a s t a i n l e s s - s t e e l t r a y w i t h a c a p a c i t y of 1 kg of c o a l . The t r a y was placed i n s i d e the t o p - f i r e d gas furnace, on the bone ash f l o o r , between two s t a i n l e s s s t e e l tubes that c a r r i e d the i n e r t gas. Three inconel-sheathed chromel-alumel type 'K' thermocouples, C, L and R i n Figure 5.1, were placed i n the c o a l bed. Thermocouple C was located i n the centre of the transverse s e c t i o n along the l o n g i t u d i n a l mid s e c t i o n of the bed. Thermocouple L was placed at the l e f t of the transverse s e c t i o n between the bottom and the mid s e c t i o n of the bed. Thermocouple R was p o s i t i o n e d at the r i g h t of the transverse s e c t i o n c l o s e r to the bottom of the bed. The t i p of each thermocouple reached a d i f f e r e n t l o n g i t u d i n a l p o s i t i o n "in the bed. F i g u r e 5.1 shows the d i s p o s i t i o n of the thermocouples. Thus i t was p o s s i b l e to o b t a i n a rough estimate of the temperature gradients w i t h i n the c o a l bed during the c h a r r i n g runs. The c o a l - c o n t a i n i n g t r a y , f i t t e d w i t h the thermocouples, and the i n e r t gas tubes was covered w i t h a l a r g e r , heavy i n c o n e l t r a y . The i n c o n e l t r a y was placed upside down against the bone-ash f l o o r and acted as a l i d to maintain an i n e r t atmosphere around the c o a l bed. V o l a t i l e products escaped from the system through a small opening l e f t i n the bone ash at the bottom of the cover t r a y , as shown i n Figure 5.1, and were combusted by the furnace flame. 5.4 G a s i f i c a t i o n Apparatus The f l u i d i z e d bed r e a c t o r u t i l i z e d i n the experimental work c o n s i s t s e s s e n t i a l l y of a 316 s t a i n l e s s s t e e l pipe about 90 cm long w i t h - 78 -a nominal internal diameter of 5.1 cm. An overall schematic view of the system is presented in Figure 5.2. The main characteristics of the reactor and its auxiliary equipment are as follows: (i) A sintered nickel disc was welded inside the pipe, at about 66 cm above the gas inlet, with, the purpose of supporting the bed of solids and providing a uniform gas distribution. The disc was made by sintering a relatively coarse nickel powder under a hydrogen atmosphere at 1100°C, using a set-up similar to the one utilized for charring the coals, (i i ) The length of the pipe below the distributor was used as a preheater for the incoming gas. This space contained a stainless steel cartridge filled with ceramic balls to mix the gas and increase the heating efficiency. The preheater also was fitted with an inconel-sheathed chromel-alumel type 'K' thermocouple placed inside a stainless steel tube. This tube passed through the ceramic balls and could be evacuated by a water aspirator to provide suction of gas to the thermocouple tip, situated just below the distributor plate. In this way the temperature of the gas entering the solids bed could be monitored, ( i i i ) The fluidized bed reactor was heated externally by an electric furnace. The furnace was made using a translucent fused quartz tube 67.3 cm in length and 7.7 cm in external diameter. It was divided into two heating zones, one corresponding to the preheater section of the reactor, about Temperature controller Chart recorder Reactor - Preheater / mixer - Suction gas t/c Flowmeter Exhaust Thermometer Figure 5.2 Schematic view of the experimental apparatus - 80 -50.6 cm long, and the other to the reaction section, about 2A.1 cm long. Chromel A wires #19 and #17, used as heating elements, were wound around the upper (reaction) and lower (preheater) sections of the quartz tube respectively. Both windings operated from 208V via variable transformers (VARIACs). Each winding delivered a maximum of 1.5 kW. The furnace was designed for a maximum nominal temperature of 1000°C. The upper zone was controlled by a Honeywell temperature controller connected to a 13 Q resistor to minimize temperature fluctuations. The quartz tube, wound with the heating elements, was covered with an alumina cement and insulated with fiberfrax blanket enclosed by a 650°C steam pipe insulation and finally by an aluminum jacket, (iv) Two inconel sheathed chromel-alumel type 'K' thermocouples were inserted into the char bed via the top li d of the reactor. One of these thermocouples was connected to the temperature controller and was used to control the temperature of the bed. The other thermocouple was connected to a Honeywell chart recorder and measured continuously the temperature of the bed. These thermocouples could move vertically in order to be positioned near the centre of the solids bed for the different bed depths studied. In addition, the gas thermocouple mentioned above also was connected to the chart recorder. The cold junction for the thermocouples was a thermos flask filled with crushed ice. - 81 -(v) An inclined 0.79 cm bore u-tube filled with a low specific gravity (0.826) mineral o i l was used to measure the pressure drop across the bed. For this purpose two pressure taps consisting of 0.48 cm stainless steel tubes were placed in the reactor through the top li d , one just above the distributor disc and the other above the bed. These pressure taps were installed in the lid used for the charring runs performed in the fluidized bed reactor and also in the lid to be utilized in the gasification experiments. The objective was to determine the fluidization conditions of the solids beds. After the fluidization conditions were established, the pressure taps were removed and replaced by pipe plugs. The pressure measuring system worked very well in a l l cases, and indicated clearly the onset of fluidization as well as the oscillatory behaviour of the pressure drop, when the bed was fully fluidized. The onset of fluidization was indicated when the pressure drop across the bed ceased to increase appreciably with further increases in the gas flow rate, (vi) Gilmont rotameter flowmeters, series 3 with glass ball and series 2 with stainless steel ball, were utilized to measure the flow rates of the inlet gases. Fine needle valves were used to control these flow rates. Each flowmeter was previously calibrated for the corresponding gas, utilizing a liquid sealed gas meter for the high flows and the soap bubble method for the low flows. The calibration curves are - 82 -presented in Appendix A. The temperature of the gases was measured close to the inlet of the flowmeters with a thermometer. Pressure gauges, capable of measuring low pressures, were placed downstream of the flowmeters. The gases were thoroughly mixed in a cylindrical vessel filled with cotton balls. This vessel contained a septum at its exit end used to sample the inlet gas and check its composition. The gases were further mixed by the bed of ceramic balls in the preheater, before entering the reaction zone. (vii) In the gasification experiments the gas exited the reactor through a 1.27 cm mild steel pipe and was transported to the cooler through 0.95 cm copper tubing. To avoid reversal of the Boudouard reaction, the gas had to be quenched as fast as possible and therefore the steel pipe and the copper tubing were kept short. The cooler used was a water-cooled copper condenser which consisted of a 2.54 cm copper pipe 40.6 cm long enclosing a 0.95 cm copper tube through which the gas passed. Cold water was circulated in the annular space between the external and internal tubes. After quenching, 3 the gas passed through a 250 cm vacuum flask fitted with a cloth filter to trap entrained solid particles, before entering Gilmont ball flowmeters series 3 and 4 (both using glass balls). These flowmeters also were previously calibrated with gas compositions close to those resulting - 83 -from the gasification reaction, utilizing procedures identical to those used for the inlet gas rotameters. The gas flow was diverted, according to its magnitude, by changing the tygon tubing, coming from the solids trap, into the appropriate flowmeter with the help of quick plastic connectors installed at the ends of each flowmeter. A thermometer situated close to the inlet of the flowmeters measured the temperature of the gas. Gas samples were taken 3 periodically during a run with 60 cm syringes through a latex section of the gas line. The gas sampling point was located after the exit flowmeters as shown in Figure 5.2. The retention time from the exit of the reactor up to the sampling point was only a few seconds since the flow rates used were at least 10000 cm /min; the solids trap used had a 3 volume of 250 cm and the gas line was relatively short being comprised mostly of 0.95 cm tube. After sampling the gas was exhausted into a fume hood, (viii) Gaskets made from fiberfrax sheet, placed between the flange welded at the top of the reactor column and the li d , provided a very efficient seal for the system when the reactor and the lid were bolted together. The fiberfrax gasket had to be replaced after every experiment due to its degradation caused by the combined effects of compression and temperature. The apparatus had to be properly sealed since the reaction kinetics were to be determined by measuring the composition - 84 -and flow rates of the exit gas. Obviously gas leaks would lead to errors in the determination of reaction kinetics. 5.5 Experimental Procedures 5.5.1 Coal Charring As mentioned before, some coal batches were charred in the fluidized bed reactor while some others were charred in the gas-fired furnace. In the fluidized bed reactor, charges of Highvale coal weighing 120,150,200 or 300 g in the size fractions -1190 + 595 um or -841 + 595 ym were loaded in the reactor. The apparatus was closed and completely flushed with helium before the power was turned on. Fluidization of the coal bed, checked by the pressure drop across the bed, was achieved by both the flow of helium and the flow of volatiles generated in the bed. The amounts of volatiles were initially very small increasing progressively with heating, therefore the helium flow had to be adjusted periodically in order to avoid an excessive total flow which would elutriate substantial amounts of coal. Since the gases exiting the reactor were combusted the amount of volatiles released could be qualitatively assessed by observing the intensity of the flame produced. The volatile release was maximum at intermediate temperatures, around 500-600°C, and was very small when the temperature of the runs, 950°C, was reached. The average heating rates for these runs varied from 7.1 to 9.4°C/min. When the bed reached 950°C, the flow rate of helium was switched to 10 1/min and held at this level during the entire soak periods of one or two hours. When the soak period was - 85 -completed, the power was turned off, the helium flow rate was decreased and the char was allowed to cool overnight. When the char reached room temperature, i t was removed from the reactor by suction into a vacuum flask and weighed. The average weight loss was 43.3%. In the gas-fired furnace one kilogram of Highvale coal in the size fraction -1190 + 595 um, or one kilogram of Smoky Tower coal in the size fraction -1190 + 420 um was put into the stainless steel tray which was then placed in the set-up described. The system was thoroughly flushed with argon at a flow rate of 1.5 1/min before the furnace was l i t . This flow rate was kept throughout the run. Average heating rates varying from 5.2 to 6.2°C/min were obtained by controlling the valves for gas and air entering the furnace. The devolatilization temperature was reached when thermocouple C shown in Figure 5.1, positioned at the centre of the coal bed, indicated 950°C. The coal was kept at this temperature during ten hours, after which the furnace was turned off and the char was left to cool overnight, under the same flow rate of argon. When the char reached room temperature, it was removed from the tray and weighed. For the Highvale coal the average weight loss, among the runs, was 45.5% while for the Smoky Tower coal the weight loss was 43.4%. The chars obtained in different runs were thoroughly mixed and sieved to -841 + 420 ym in order that enough material to perform several gasification experiments could be obtained. Care was taken not to mix chars produced in the fluidized bed reactor with chars produced in the gas-fired furnace since their characteristics would be compared. The bulk densities of the -841 + 420 ym chars were measured, in the - 86 -random loose pack condition, with a graduated cylinder and averaged; the values are 735 and 712 kg/ra for Highvale and Smoky Tower chars respectively. Samples of the chars were taken for chemical analysis, surface area measurements and to be examined under the scanning electron microscope. The remaining batches were put in tightly closed containers. 5.5.2 Gasification Experiments The main activity of the experimental work was the gasification experiments. The procedure used to perform these experiments, described below, is the optimum achieved after several early trials. In the gasification experiments, helium was initially allowed to flow into the apparatus to check for leaks. This was done by comparing the flow rate of helium entering the reactor, 10 1/min, with the flow rate of helium exiting the reactor, using two identical flowmeters (Gilmont series 3). At the same time, air was flushed from the system. The reactor was then loaded with previously weighed -841 + 420 ym char particles through a vertical loading port attached to the top l i d , with the help of a funnel. The loading port was tightly closed and the power for the electric furnace was turned on. The VARIACs that controlled the power to the two zones were set to pre-deterrained values in order that the char bed could be slowly heated. The temperature controller was set to a temperature above that of the experiments to compensate for the i n i t i a l drop caused by the endothermic Boudouard reaction. The amount of superheat depended on the conditions of the run, increasing with more severe gasification conditions. During this period air and standard - 87 -gas samples were analyzed in the gas chromatograph, a Perkin-Elmer, Sigma 38 dual column gas chromatograph calibrated before each run. The composition of the gas standards used are shown in Table IV and the operating conditions of the chromatograph are presented in Table V. A peak area integrator and a chart recorder were connected to the chromatograph. The columns used were a 0.3 x 244 cm or a 0.3 x 366 cm column filled with Porapak 'Q' and a 0.3 x 300 cm column filled with a molecular sieve, MS 5A. The first column was used for carbon monoxide and carbon dioxide analysis while the second was used to check the carbon monoxide analysis and also to correct the analysis of both gases for air, which sometimes leaked into the syringes containing the gas samples taken during the run. When air leaked into the samples, the amount was variable but, in almost all cases, smaller than two percent. Generally five samples and always at least three samples of each gas standard and air were run into the chromatograph for calibration checks. The respective area counts for each component were averaged. The relative error between the counts was, in most of the cases, smaller than one percent. Before the reaction was started samples of the exit gas, with only helium flowing in the system, were analyzed. Very small area counts were invariably obtained for these samples as compared to the counts obtained for the gas analyzed after the reaction started; they were less than one percent in most of the cases and therefore were neglected. When the temperature of the bed was approaching that of the experiments, temperature checks were made by comparing the control and - 88 -TABLE IV COMPOSITION OF CERTIFIED—GRADE GAS STANDARDS Standard (%> Gas 1 2 3 4 CO 2 15.6 20.10 49.9 80.0 CO - 74.79 50.1 20.0 Ar - 1.07 - -H2 - 4.04 - -0 2 10.5 - - -N2 73.9 - - -- 89 -TABLE V GAS CHROMATOGRAPH OPERATING CONDITIONS Carrier gas Helium Oven temperature 80°C Injector and detector temperatures 105°C Pressure of injection 30 p.s.i, - 90 -measurement thermocouples against each other. The cold junction f l a s k was f i l l e d with crushed ice and the system was again checked for leaks i n the same way as described before, but with a flow rate of helium equal to the t o t a l flow rate to be used i n the runs. Next the power was increased to compensate for the c h i l l i n g of the bed with the s t a r t of the r e a c t i o n . The power input to the bed and to the preheater was adjusted throughout the run, i n order to a t t a i n isothermal conditions for the reaction i n the char bed. The bed temperature was i n v a r i a b l y unstable during" the i n i t i a l stages of the reaction, tending to s t a b i l i z e at the nominal value ±5°C with the progress of g a s i f i c a t i o n . The behaviour of the bed temperature during the experiments w i l l be discussed further i n the next chapter. When the temperature set i n the c o n t r o l l e r was reached, carbon dioxide or carbon dioxide and carbon monoxide were introduced to the bed. The flow rates of the i n l e t gases were adjusted to provide a pre-determined composition. The t o t a l flow rate was equal to the previous flow rate of helium, and was capable of f l u i d i z i n g the bed. The re a c t i o n then was s t a r t e d . The measurements were i n i t i a t e d one or two minutes afterward. The flow rate of the exit gas was measured i n one of the two exit flowmeters depending on the flow r a t e . This was immediately followed by the withdrawal of a gas sample. These measurements were made p e r i o d i c a l l y throughout the run, being more frequent i n the f i r s t minutes of the reaction and spaced at larger i n t e r v a l s up to a maximum of t h i r t y minutes, a f t e r that. At the higher temperature the number of i n i t i a l measurements increased. The samples - 9 1 -of the exit gas were analyzed in the chromatograph before thirty minutes had elapsed from the time they were taken. The period for safe storage 14 8 of gas samples is considered to be one hour. The measurements continued until the counts obtained for carbon monoxide in the chromatograph started to drop considerably indicating a significant decrease in the reaction rate. For the runs performed at low temperatures the decrease in rate was usually very slow. For these runs the measurements were stopped after two hours of reaction in most of the cases. To finish the run the flows of carbon dioxide and carbon monoxide were stopped and the flow rate of helium was increased in order to maintain the same total flow rate. The power was turned off and the system was once more checked for leaks. When the checks for leaks were made with the reactor cold, some minor leaks were occasionally observed and promptly sealed. At high temperatures leaks were not detected confirming the good sealing achieved for the system. The flow rate of helium was decreased to a low value and the reactor was left to cool overnight. When the reactor reached room temperature, the top lid was removed and the solids remaining in the bed were collected by suction into a vacuum flask, then immediately placed in tightly closed containers. For the runs where a substantial amount of solids had been elutriated from the reactor, the solids trap was disassembled and the fines, deposited there were collected. In most of the experiments the amount of fines recovered from the trap was very small representing a few percent in weight of the i n i t i a l char. In these runs the gas line after the reactor was tapped and flushed with - 92 -helium to recover additional amounts of fines. The amount of fines found in the line was always negligible. Very fine char dust that escaped from the trap was sometimes deposited in the exit flowmeters, making it necessary to clean them periodically. The solids remaining after the experiments were weighed for the calculation of mass balances. For some runs, samples of the i n i t i a l char and of the solid product were analyzed for carbon content in order to verify the mass balances. Samples of the products, for selected runs, were stored in sealed vials to be examined under the scanning electron microscope. Additional samples of partially reacted chars at different carbon conversions were sent for chemical analysis and surface area measurements. CHAPTER 6 RESULTS OF CHARRING AND ASSESSMENT OF GASIFICATION EXPERIMENTS Initially the results of the charring runs, performed according to the procedures described in Section 5.5.1 are presented. This is followed by a general assessment of the gasification experiments. In this section, results of subsidiary experiments to determine the fluidization conditions of the char bed and to evaluate the accuracy of gas analysis and flow rate measurements are presented fi r s t . Next the procedures used to determine the fractional conversion of carbon, reaction rate and mass balances are discussed. The change of the temperature of the char bed during the gasification experiments is discussed. Finally the results of surface area measurements, performed on unreacted and partially reacted Highvale and Smoky Tower chars at different carbon conversions, are shown. These results will be analyzed, in conjunction with the kinetics data, to explain the observed variations in char reactivity with the extent of reaction. 6.1 Charring Experiments Since the objective of this work was to study the kinetics of the Boudouard reaction, devolatilization of the coals was a necessary step before the gasification experiments. If raw coals were used in these experiments, the release of volatile constituents, including moisture, would give rise to additional reactions which would complicate - 94 -substantially the assessment of the Boudouard kinetics. Moreover, low melting point compounds (tars) would deposit downstream of the reactor as the exiting gas cooled and could interfere with the gas flow. Therefore charring runs were performed with the purpose of eliminating most of the volatile matter and moisture content of the raw coals. The values of coal particle size, heating rate, temperature and soak time employed in these runs are tabulated in Appendix B. The amount of volatiles released was determined by both weight loss and chemical analysis. The weight loss of the solids was measured after each run; the values obtained are listed in Appendix B, and reveal the consistency of the charring runs. The chemical analyses, proximate and ultimate, provided an accurate measure of the volatile matter, and individual constituents remaining in the char. These analyses were performed by Stelco Inc. on representative samples of chars from individual runs as well as on samples of thoroughly mixed chars from several runs, as indicated in Section 6.1.2. 6.1.1 Temperature Measurements In the fluidized bed reactor, the temperature of the coal bed was uniform with a variation of ±5°C, during the soak period of charring, in most cases. In addition, with the good solids mixing in a fluidized bed, a homogeneous char could be expected from these runs. In the gas-fired furnace, temperature gradients were expected to develop within the coal bed, due to the characteristics of the system utilized. Therefore some inhomogeneity could result in the chars - 95 -prepared in this apparatus. The temperature gradients were evaluated by measuring the temperature at three different locations in the bed. The thermocouple locations and the variation in temperatures with time at these positions are shown in Figure 6.1 for Run C19. The heating rate varied from 4.8°C/min at point R (bottom RHS) in Figure 6.1, up to 6.4°C/rain at the centre of the bed, point C. This variation should not result in a major change in volatiles release and in the structure of the char. The maximum difference in temperature between these points was approximately 50°C, during most of the soak period. This gradient is not regarded significant at 950°C and for a 10-hour treatment. Similar behaviour was observed in the subsequent charring runs performed in the gas-fired furnace. 6.1.2 Effect of Charring Conditions The most important differences in the two schemes utilized to devolatilize the coals, apart from gas-solid contacting and temperature uniformity, were the soak time and heating rate. In the fluidized bed reactor, heating rates of 7.1 to 9.4°C/min and soak times of 1 and 2 hours were utilized while in the gas-fired furnace heating rates of 5.2 to 6.2°C/min were adopted. The 10-hour treatment was based on the results of a previous study using Alberta sub-bituminous coal and a set-up similar to the one employed in this work. The effect of soak time is probably more relevant than the effect of heating rate since the former was varied more widely; therefore i t will be discussed in more detail. - 96 -1 1 1 1 1 1 1 )l I 1 I I I i 1 1 0 200 400 600 800 Charring time (min) Figure 6.1 Va r i a t i o n of temperature at three locations i n the coal bed during charring run C19 - 97 -The influence of soak time is assessed, in this section, by comparing the weight losses and the amounts of volatiles, particularly hydrogen, remaining in the chars originating from runs with different soak times. The effects of heating rate and soak time on the reactivity of the char will be evaluated in the next chapter. The chars used in the gasification experiments are presented in Table VI. These chars originated either from mixtures of the products of several charring runs (char #1 to char #5) or from the products of single runs (char #6 and char #7). Table VI also shows the runs that provided material to the char mixtures, the corresponding soak times and the average of the weight losses obtained for each individual run comprising a mixture. It can be observed that the weight loss increased slightly for the longer soak time runs. The amount of volatiles plus moisture present in the original coals, based on the analysis performed by Stelco Inc. is given from Table I in the previous chapter. For Highvale coal the average of this value, for the two batches analyzed,is 44.3% while for Smoky Tower coal i t is 42.6%. Therefore i t can be concluded that even for the 1 hour treatment (average weight loss 43.2%) the Highvale coal was almost completely devolatilized, in agreement with the contention that the major fraction of the volatiles is released during the heating-up period, as was found previously. For the longer soak time, a slight oxidation of the char at the exposed surface of the bed, due to traces of oxygen present in the argon flush gas, might have occurred and contributed to the excess in weight loss. In this case the oxidation was favored by the very small amount of volatiles present - 98 -T A B L E V I CHARS USED I N THE G A S I F I C A T I O N EXPERIMENTS Char Charring runs Average wt. Soak time loss (%) (h) 1 (H) CI to C4 43.2 1 2 (H) C5 to C7 43.4 2 3 (H) C8 to CI 6 43.4 2 4 (H) CI 7 and CI 8 43.3 2 5 (H) CI 9 and C20 45.6 10 6 (H) C21 45.3 10 7 (ST) C22 43.4 10 H = Highvale char ST = Smoky Tower char - 99 -during most of the soaking period, that otherwise would react with the oxygen. The proximate and ultimate analysis of the chars are given in Table VII. It can be seen that for the extended soak time, the amounts of volatiles remaining in the char do not differ significantly from the amounts corresponding to shorter times (1 and 2 hours). Similar trends are observed for the individual amounts of hydrogen, water and nitrogen. Attention should be directed particularly to the low levels of hydrogen already obtained after a 1- or 2- hour soaking period, 0.28% on the average. This amount is only slightly reduced to 0.25% (Highvale chars) when the soak time increases to 10 hours. 6.1.3 Discussion The charring experiments will be discussed in terms of the heating rates, soak times and temperature employed. The chars prepared in the gas-fired furnace were submitted to lower heating rates than the chars prepared in the fluidized bed reactor as shown in Appendix B. As discussed in Chapter 2, lower heating rates tend to decrease the reactivity of the char due to favorable thermal annealing conditions, increase in tar deposition and decrease in porosity caused by slow volatiles release. However the effect of heating rate is not likely to be critical in the range applied; 5.2 to 9.4°C/min. In the fluidized bed reactor, the solids remained at the charring temperature for a period 5 to 10 times shorter than in the gas-fired furnace. Increasing the soak time from 1 to 10 hours had only a minor TABLE VII PROXIMATE AND ULTIMATE ANALYSES OF -841 + 420 urn HIGHVALE AND SMOKY TOWER CHARS USED IN THE GASIFICATION EXPERIMENTS Char 1 2 3 4 5 5 5 6 6 7 7 Proximate Analysis % % % % % % % % % % % H20 (as received) 0.28 0.36 0.41 0.25 0.15 0.14 0.16 0.63 0.57 0.61 0.55 Ash 20.38 19.74 16.60 18.38 18.41 18.61 18.11 16.79 16.51 31.51 31.13 Volatiles 2.02 2.05 2.03 2.00 1.41 1.33 1.38 3.18 3.18 2.88 3.08 Fixed C 77.32 77.85 80.96 79.37 80.03 79.92 80.35 79.40 79.74 65.00 65.24 Ultimate Analysis (d.b.) Carbon 77.70 80.54 81.36 79.98 80.29 80.55 80. 76 80.40 81.10 65.81 65.84 Hydrogen 0.25 0.28 0.26 0.31 0.16 0.23 0. 17 0.35 0.34 0.34 0.34 Nitrogen 0.32 0.32 0.32 0.32 0.18 0.18 0. 17 0.39 0.42 0.74 0.71 Sulphur 0.17 0.19 - - - - - 0.03 0.04 0.11 0.14 Ash 20.44 19.81 16.67 18.43 18.44 18.64 18. 14 16.90 16.60 31.70 31.30 Oxygen (by difference) 1.12 — — — — - - 1.93 1.50 1.30 1.67 - 101 -influence on both the weight loss of the original coal and the composition of the final char. This result can be explained by the fact that the major fraction of the volatiles are released during the heating up period, the process being almost complete when the charring temperature is reached. Only residual amounts of volatiles remain in the char at the end of the heating stage and further release tapers off asymptotically with time. Nevertheless different soak times can influence the reactivity of the char by changing its pore structure. Charring for longer periods, in the temperature range 700 to 1300°C, causes thermal annealing of the pores with the corresponding loss of microporosity and carbon edges via cluster reorganization and disappearance of structural defects, as shown in Chapter 2. Therefore prolonged heat treatments can reduce the intrinsic reactivity of the char due to loss of active sites. As mentioned before, the fluidized bed reactor was able to char the coals under near isothermal conditions. In the gas-fired furnace, on the other hand, temperature gradients of about 50°C were developed in the coal bed during the soaking period. The temperature of the charring treatments, 950°C, was equal to the maximum temperature of the gasification experiments. However it was lower than the temperatures reached during the superheat (slightly greater than 50°C) of the char bed that occurred in some runs, as shown in Section 6.2.6. It is considered that these temperature differences should not affect the pore structure and reactivity of the char to a major extent. The effect of devolatilization temperature on char properties, discussed in Chapter 2, - 102 -can be summarized as follows. Chars prepared up to a temperature of 600 135 to 700°C or even 1000°C have the highest pore surface area; hence highest reactivity. When the temperature is further increased, there is a decrease in the concentration of feeder pores, a loss of active sites and a reduction in pore surface area due to graphitization and plugging of micropores. This causes a corresponding decrease in the reactivity of the char. Therefore to obtain maximum reactivity, the coals should have been charred at temperatures around 700°C. However the major fraction of hydrogen is only released above this temperature. The presence of appreciable amounts of hydrogen in the char would increase the extent of additional reactions, apart from the Boudouard, complicating unduly the kinetics of the gasification process. Gasification experiments were conducted to assess the effect of different charring treatments, especially soak time, and will be discussed in the next chapter. 6.2 Assessment of Gasification Experiments The results of the gasification experiments, presented in the next chapter, were obtained based on the considerations discussed in this section. 6.2.1 Determination of the Minimum Fluidizatlon Velocity The minimum fluidizatlon velocity was determined experimentally for -841 + 420 um Highvale char at 800 and 900°C using helium as the fluidizing gas. The mass of char employed in these experiments was - 103 -100.0 g. The results are shown in Figure 6.2. In addition the minimum fluidizatlon velocity was calculated using the equations proposed by 14 9 Grace for the conditions given in Appendix C. The values of true densities for the parent coals, measured recently by Parkash 1 5 0 using helium pycnometry, were utilized for the true densities of the respective chars. The particle sizes correspond to the maximum and average size for the fraction used in the gasification experiments. The values obtained for the minimum fluidization velocity, converted to 21°C, also are given in Appendix C, together with the equations used in their calculation. In order to compare the measured with the calculated values, Table VIII presents the calculated minimum fluidization velocities, at 21°C, for 841 and 631 ym particles of Highvale char using helium at 800, 850, 900 and 950°C. It can be observed from Table VIII and Figure 6.2 that, eventhough the minimum fluidization velocities were calculated utilizing the true density of Highvale coal instead of the true density of the char, a good agreement exists between the measured and predicted values at 800 and 900°C. Moreover the values tabulated in Appendix C indicate that only a minor difference exists between the minimum fluidization velocities calculated for the three gases and for the two chars at each temperature and particle size. Therefore the measured values for helium and Highvale char are considered representative of the whole range of fluidization conditions for the gasification experiments. The ratio between the actual superficial gas velocity, calculated for a flow rate of 10 1/min at 21°C, and the minimum fluidization velocity given in Table VIII is presented in Table IX, for Gas flow rate(l/mln) 2 l ° C f l a t m 0 6.1 IOJ0 12.2 183 24.4 0 5 10 15 2 0 Superficial gas velocityXIO (m/s) 2I°C , latm Figure 6.2 Pressure drop across the bed as a function of superficial gas velocity or gas flow rate showing the fluidization characteristics of Highvale char; conditions as shown - 105 -TABLE VII I MINIMUM FLUIDIZATION VELOCITIES AT 21°C char fluidizing gas Highvale Helium "mf (cm/s) T(°C) Size (ym) 800 850 900 950 841 4.48 4.10 3.85 3.62 631 2.53 2.31 2.17 2.04 - 106 -TABLE IX RATIOS BETWEEN THE SUPERFICIAL GAS VELOCITY FOR Qx = 10 A/mln AND THE MINIMUM FLUIDIZATION VELOCITIES f o r Q T = 10 £/min (21°C), u Q = 8.22 cm/s u o / u m f T(°C) 841 um 631 um 800 1.83 3.25 950 2.27 4.03 - 107 -the two extreme temperatures 800 and 950°C. This flow rate was the minimum used in the majority of the gasification experiments. The flow rate and the corresponding superficial gas velocity are shown in Figure 6.2, indicating complete fluidization of the bed. Finally it should be recalled that during gasification, the gas flow rate increased due to the generation of two volumes of CO for each volume of C02 consumed. Furthermore the particles decreased in size due to the gasification reaction and, to a minor extent, attrition. Both factors contributed to fluidization during the experiments. In addition higher flow rates increased the elutriation of the particles after a certain conversion, and decrease in size, was reached. Even for a low gas flow of 10 1/min elutriation of smaller particles occurred, after some time of reaction, in several experiments. 6.2.2 Gas Analysis and Flow Rate Measurements The instantaneous composition of the gas exiting the reactor was employed directly as a measure of the gasification kinetics. The flow rate of the exit gas also was measured to check the values expected from a mass balance considering the changes in the inlet flow rate due to the reaction. Thus the results of the gasification experiments were strongly dependent on the accuracy of gas analysis and flow rate measurements. Regarding the gas chromatographic analysis, it has been 148 determined that the area counts for a component in a gas mixture is directly proportional to the concentration of the component, except for very low (less than 100 ppms) and very high concentrations. Thus only one calibration standard would be necessary to determine the composition - 108 -of the gas mixture. It is recognized, however, that deviations from linearity may occur, especially when the component tends to produce tailing peaks with subsequent reduction in measurable area. Therefore preliminary experiments were performed to evaluate the accuracy of the gas analysis. The only gaseous species analyzed, apart from nitrogen and oxygen from air, were carbon monoxide and carbon dioxide. Helium was always obtained by difference. Due to the small amounts of hydrogen in the char, i t was considered that the only product of the gasification reaction is carbon monoxide. Trace amounts of oxygen and moisture in the incoming gas also were neglected. Therefore no attempt was made to analyze hydrogen and water vapor. Initially a check was made to determine if the gas concentration was directly proportional to the area counts of the peaks. For this purpose mixtures of CO and C 0 2 encompassing the entire range of compositions in the gasification experiments were prepared carefully with highly accurate glass syringes and analyzed in the chromatograph under the operating conditions specified in Table V in the previous chapter. As shown in Figure 6 .3 , the concentrations of the two gases are linearly related to the respective area counts. Next the gas standards presented in Table IV were analyzed several times and plots of percent gas versus area counts, like the one shown in Figure 6 .4 , were obtained. Excellent linear behaviour was always obtained in these plots. Having established the proportionality between gas concentration and area counts, the composition of samples of the gas exiting the reactor, in the gasification experiments, was obtained using only one gas standard. The standard was analyzed before each experiment. - 109 -O 2 4 6 8 10 12 Area counts X I0" 4 Figure 6.3 C a l i b r a t i o n curves for composition of C0-C0 2 mixtures i n gas chromatograph 1 — I — I — f Figure 6 . 4 Calibration curves for composition of the gas standards presented in Table IV in gas " chromatograph - I l l -With respect to flow rate measurements, the inlet flowmeters were carefully calibrated with the pure gases. The calibration curves are shown in Appendix A. The exit flowmeters were initially calibrated with the gas mixtures presented in Table X. The compositions of Gas 1 and Gas 2 are the averages of the compositions used in the experiments with C0-C0 2-He mixtures, for CO/CO2 ratios of 0.25 and 0.5 respectively. In these tests each gas mixture was allowed to flow in the set-up utilized for the gasification experiments, with the reactor empty and at room temperature. The inlet flowmeters were adjusted to provide the nominal compositions given in Table X. The gas was sampled at the same point as during the gasification runs and analyzed in the chromatograph. Readings in the exit flowmeters also were taken. In the absence of leaks, the exit flow rate is equal to the total inlet flow rate. The inlet flowmeters were re-adjusted after each reading and samplings, in order to increase the total flow rate keeping the composition constant. Flow rates from 10 to 20 1/min using the two exit flowmeters were checked. The analyzed concentration of the gases also is presented in Table X, each value being the average of fourteen samples. The calibrations of the Gilmont #3 flowmeter for Gases 1 and 2, at 21°C and 1 atm, are shown in Figure 6.5. It should be noted that these tests besides providing calibration for the exit flowmeters, also checked gas compositions given by the inlet flowmeters as is the case in the gasification experiments. The accuracy of the inlet flowmeters is demonstrated by the results shown in Table X. The exit flowmeters were - 112 -TABLE X COMPOSITION OF GAS MIXTURES GAS 1 AND GAS 2 Gas 1 Gas 2 Nominal Actual Nominal Actual (%) (%) (%) (%) CO 10 9.69 ± 0.14 20 19.65 ± 0.69 CO 2 40 40.08 ± 0.52 40 39.73 ± 0.37 He 50 50.23 ± 0.58 40 40.62 ± 0.59 - 113 -Figure 6.5 Calibration curves for flow rate of the gas mixtures presented in Table X in flowmeter Gilmont #3 (21°C and 1 atm) - 114 -further calibrated with the inlet gas used in each gasification experiment. Depending on the gas density and thus on its composition, one of the two flowmeters was more suitable to measure the flow rate. The calibration curves for the Gilmont #4 flowmeter, for two gas mixtures, are shown in Figure 6.6 together with the composition of the mixtures. Finally the eight different C0-C02~He mixtures used in the gasification experiments were passed through the empty reactor, assembled as for the gasification experiments, at 900°C. The gas mixtures were sampled at the usual location and analyzed in the chromatograph. These tests were performed as a final check for gas analysis, inlet flowmeters calibrations and to investigate the possibility of decomposition of carbon monoxide as it passed through the heated reactor and cooling system. The results are reported in Table XI and show a good agreement between the nominal and measured composition which was the average of five samples. Therefore the accuracy of gas analysis was once more ascertained and the possibility of carbon monoxide decomposition was ruled out. 6.2.3 Determination of the Fractional Conversion of Carbon The results of the gasification experiments are presented in the next chapter as standard plots of fractional conversion of carbon, f, versus time. The fractional conversion of carbon was calculated for each experiment by the amount of C02 consumed or by half the amount of CO produced. The value that provided the best agreement with the amount - 115 -12 8 I 22 CO C 0 2 Me % % % 0 12 48 40 • 25 50 25 1 25 30 Scale reading 34 Figure 6.6 Calibration curves for flow rate of He-C0-C02 mixtures in flowmeter Gilmont #4 (21°C and 1 atm); composition of the mixtures as shown - 116 -TABLE XI COMPOSITION OF GAS MIXTURES USED IN THE GASIFICATION EXPERIMENTS (CO/C02 = 0.25 and 0.50) Gas Nominal (%) Actual (%) CO 4 4.13 CO 2 16 15.91 He 80 79.96 CO 8 8.19 C02 32 31.85 He 60 59.96 CO 12 11.66 C02 48 47.73 He 40 40.60 CO 16 14.96 CO 2 64 64.29 He 20 20.75 CO 10 9.82 CO 2 20 19.77 He 70 70.42 CO 15 14.27 CO 2 30 29.81 He 55 55.93 CO 25 24.75 CO 2 50 51.03 He 25 24.21 CO 30 29.46 C02 60 60.28 He 10 10.26 - 117 -of carbon reacted, determined by the masses of the in i t i a l char and final product and by chemical analysis of the i n i t i a l char, was used for the experiment. From a total of 75 experiments, the kinetics were determined by CO in 45 and by C02 in 30 experiments. The equations used to determine the fractional conversion of carbon are derived in Appendix D and given below. 1) by half the amount of CO produced. / t ^T^CO-QCO ] d t ° 120.5 (2 - X*L) f [6.1] m o i i ) by the amount of C02 consumed, e _ QC0 ~ QT XC0„ II [ — r - ] d t 120.5 (1 + X* ) f - 2 [6.2] o The inlet f l ow rates of CO and C02 and the total inlet flow rate, QC0» QC02 a n <* QT respectively, were obtained from the inlet flowmeters. The volumetric or mole fractions of CO and C02 in the exit e e gas, X and X , were obtained from the results of the gas analysis. CO L<U2 The composition of the gas samples was assumed (excluding air that occasionally leaked into the syringes) to be equal to the composition of the gas leaving the reaction zone. For the apparatus used, this assumption is quite acceptable as shown in the previous chapter, Section 5.4. The gas compositions were corrected when the samples contained air and are shown in Figure 6.7 as a function of Figure 6.7 Exit gas composition (% C0 2 and % CO) as a function of time for three gasification experiments; conditions as shown - 119 -reaction time, for three typical runs. The in i t i a l mass of carbon in the char, m0, is determined by the mass of the char and its fraction of fixed carbon, given by the proximate analysis. To obtain the fractional conversion of carbon as a function of time, the above integrals were numerically evaluated by the U.B.C. Computing Centre routine QINT4P,151 which was incorporated into the computer program written to correlate the experimental data, outlined in Section 6.2.5. 6.2 . 4 Determination of the Reaction Rate The reaction rate was defined as follows: 1 dm 1 df x r „, m - d F = r T * d F f 6 ' 3 J Where m is the mass of carbon remaining in the reactor at any time. Thus r is the specific rate of reaction based on the instantaneous mass of carbon. The reaction rate was calculated for each experiment by the variation in the amount of gas that provided the best agreement with the amount of carbon reacted, estimated from solids weighing and chemical analysis. The following equations were used. (i) by CO produced Q X 6 - Q r - C 0 C 0 [6.4] 120.5 m .(1-f) • (2 - X ® ) o CO - 120 -( i i ) by CO2 consumed r Q C 0 ? " Q T *»>> [6.5] 120.5 u '(1-f) • (1 + x l , ) O CO 2 The exit gas flow rates were determined by the changes in the inlet flow rate due to the reaction according to the following equations. o 2Q - Q Q e _ _ _ ] _ C 0 [ 6 > 6 ]  2 " XC0 or 1 + x c o 2 The actual exit gas flow rate, measured with the exit flowmeters calibrated by the methods described in the previous section, also was obtained. The flow rates were corrected for the difference in density between the exit gas and the gas used in the calibration, by the equation. °- = Q Cal ( ^ ) 1 / 2 [ 6 - 8 ] where Q c a i and P c a l are the flow rate and density of the calibration gas respectively. Q and p are the corresponding values for the exit gas. - 121 -These flow rates were not directly used In the calculations because, apart from being an Indirect measurement, they result from instantaneous readings in rotameters which exhibited some oscillations. These oscillations, and therefore the errors In the readings, increased with the mass of char fed to the bed. For 20 g of char the flowmeters were very stable. However the measured flow rates were compared with the flow rates calculated by equations [6.6] and [6.7], in most of the experiments. The objective was to check the values obtained for fractional conversion of carbon and reaction rate. A good agreement was found between the calculated and measured flow rates, as shown in Figure 6.8 for three typical runs. 6.2.5 Mass Balances Mass balances were performed on the gases and on carbon from the char as follows. For the gases, the total fractional conversion was calculated by both CO2 consumed and half CO formed and the values were compared. For carbon, the mass initially present in the bed was compared with the final mass remaining in the char plus the mass reacted. The mass of carbon initially present was determined as indicated in Section 6.2.3. The mass of carbon remaining in the final product was evaluated by the difference between the total mass of char recovered and the mass of the ash. The mass of ash at the end of the experiment was considered to be equal to the mass of ash originally present, determined by the in i t i a l mass of char and the fraction of ash given by the proximate analysis. Therefore the ash was regarded inert - 122 -16 14 8 QT(i/min) run o calc G34 • meas o calc G23 • meas calc G26 meas 1 1 1 20 40 60 80 100 Time (min) 120 Figure 6.8 Calculated and measured exit gas flow rate as a function of time for three gasification experiments - 123 -and the loss of very fine material that occurred in the experiments was neglected. The final mass of carbon was checked, in the Initial experiments, by analyzing the solid products in the Fisher Coal Analyzer for ash content. The conditions used in these analyses are given in Table III. The mass of carbon was the product of the mass of char collected and the carbon content given by the analysis. The final mass of carbon, both estimated and measured, and the relative difference between them are given in Table XII. The good agreement found allowed the use of estimated values in the carbon mass balances. The mass of carbon reacted was evaluated by either the amount of C02 consumed or half the amount of CO formed. The mass balances were calculated using average operating conditions and therefore did not consider the variations which actually occurred during the experiments. Minor deviations in gas analysis, flow rates and in the final mass of char could have a substantial influence on the balances due to the much smaller amounts of solids relative to the reactant gas in the majority of the runs. The mass balances closed within 6% in a l l cases, for both gases and carbon, and are presented for a l l gasification experiments performed in Appendix E. A computer program was written to calculate the composition and flow rate of the exit gas, thermodynamics and kinetics parameters and mass balances on gases and carbon, from the experimental data obtained. The program listing and a sample output are presented in Appendix F. TABLE XII ESTIMATED AND MEASURED FINAL MASSES OF CARBON Run char (g) m( estimated) (g) C u m. D. <%) C (%) m(measured) (g) e (%) m. D. (%) Gl 117.6 84.96 -0.1 75.8 89.14 4.7 -3.5 G2 22.8 14.64 -2.5 67.5 15.39 4.9 -4.9 G3 87.4 54.76 0.8 66.1 57.77 5.2 -1.7 G4 14.8 6.64 1.1 48.6 7.19 7.6 -0.7 G5 41.2 24.88 -0.9 64.1 26.41 5.8 -3.4 G6 33.7 25.82 -1.7 79.2 26.69 3.3 -4.5 G7 31.5 23.62 -3.4 77.7 24.48 3.5 -6.2 G8 30.0 22.12 -3.4 75.0 22.50 1.7 -4.6 G9 23.1 15.22 5.6 70.5 16.29 6.6 2.2 G10 22.5 14.62 -1.1 67.9 15.28 4.3 -3.3 G i l 22.0 14.12 -3.8 65.3 14.37 1.7 -4.6 - 125 -6.2.6 Bed Temperature Due to the h i g h l y endothermic nature of the Boudouard r e a c t i o n , the temperature of the bed was the most d i f f i c u l t v a r i a b l e to be c o n t r o l l e d . In a d d i t i o n the r e a c t o r , a s t a i n l e s s s t e e l pipe w i t h a nominal diameter of 5.1 cm, and the amounts of char used, mostly 20 g and maximum 160 g, provided a system of small thermal mass, prone to temperature i n s t a b i l i t i e s . When the experimental c o n d i t i o n s favored moderate ra t e s of r e a c t i o n , an isothermal behaviour was a t t a i n e d during the runs, except f o r i n i t i a l i n s t a b i l i t i e s and temperature o s c i l l a t i o n s due to the c h a r a c t e r i s t i c s of the temperature c o n t r o l l e r used. These o s c i l l a t i o n s , however, were minimized to l e s s than ±5°C In the m a j o r i t y of the experiments, by a d j u s t i n g the VARIACs manually. When the conditions favored high r e a c t i o n r a t e s , p a r t i c u l a r l y at 950°C f o r Highvale chars, s u b s t a n t i a l drops occurred i n the bed temperature, immediately a f t e r the i n t r o d u c t i o n of CO2, notwithstanding the preheating of the bed to the maximum c a p a c i t y of the furnace u s i n g a l l the power a v a i l a b l e . Subsequently an o s c i l l a t o r y behaviour w i t h decreasing amplitude was observed i n some runs u n t i l the temperature s t a b i l i z e d at the nominal v a l u e . Temperature versus time p l o t s f o r s i x t y p i c a l runs are shown i n Figure 6.9. 6.2.7 Surface Area Measurements S p e c i f i c surface areas of r e p r e s e n t a t i v e samples of unreacted and p a r t i a l l y reacted Highvale and Smoky Tower chars, at d i f f e r e n t carbon conversions, were measured by n i t r o g e n adsorption at 77K using the - 126 -1000 975 950 ^ 9251 3 2 900\ o> o. E t> 875 850 825 800 J L 1 20 40 60 80 Time (min) 1 1 100 120 Figure 6.9 Temperature of the char bed as a function of time for s i x g a s i f i c a t i o n experiments - 127 -B.E.T. technique at U.B.C. and Micromeritics Instrument Co.. The variation in specific surface areas of the chars with the fractional conversion of carbon is shown in Figure 6.10. The values of specific surface area for the samples and the respective carbon conversions, together with the conditions under which the corresponding charring and gasification experiments took place, are given in Table XIII. - 128 -Figure 6.10 S p e c i f i c surface area of char samples as a function of percent carbon conversion; conditions as shown (H = Highvale char; ST = Smoky Tower char; Temp = g a s i f i c a t i o n temperature) TABLE XIII SPECIFIC SURFACE AREA OF CHARS AS A FUNCTION OF CARBON CONVERSION FOR DIFFERENT CHARRING AND GASIFICATION CONDITIONS Char Charring Gasification Gas Specific Type soak time temperature composition f surface area (h) r c ) (%) (%) (m2/g) - 0 87.1 C0:0, C02:10, He:90 18.88 192.2 Highvale 2 800 CO:0, C02:30, He:70 32.42 239.7 C0:0, C02:100, He:0 58.49 230.5 - 0 61.3 CO:10, C02:20, He:70 47.85 157.6 Highvale 10 875 C0:16, C02:64, He: 20 68.00 182.9 C0:0 , CO2:50, He: 50 98.00 129.0 - 0 61.3 21.4 128.2 Highvale 10 900 C0:10, CO2:40, He: 50 37.9 159.4 68.0 187.7 80.2 164.4 — 0 20.2 C0:10, C02:40, He: 50 34.5 80.0 Smoky Tower 10 900 45.5 80.6 75.3 70.2 - 130 -CHAPTER 7 RESULTS OF GASIFICATION EXPERIMENTS The results of the gasification experiments are presented In this chapter. Section 7.1 gives findings from the tests with CO2 or C02-He mixtures and Section 7.2 presents measurements from the runs with C0-C02-He mixtures. Preliminary discussions of the results obtained are found at the end of each section, emphasizing only the observed trends and the most important aspects of the Boudouard reaction. No attempt is made to correlate the results in a quantitative basis. This will be done in Chapter 8. In addition the results of SEM observations of unreacted and partially reacted samples of chars originating from the two coals studied are presented and analyzed in Section 7.3. 7.1 Gasification with CO? and CO? - He Mixtures Gasification experiments were performed with C0 2 and C02-He mixtures, using -841 + 420 um Highvale chars, to assess the effects of charring conditions (soak time), bed depth, total inlet flow rate, inert gas concentration, partial pressure of C0 2 and temperature on the rate of the Boudouard reaction at 800, 850, 875, 900 and 950°C. The main objective of these tests was to determine operating values of bed depth (ini t i a l mass of char) and total inlet flow rate able to provide kinetic results with minimum influence of phenomena such as C0 2 starvation, temperature non-uniformity, hydrodynamics of fluidization - 131 -and elutriation of fines. The results obtained are presented as plots of the fractional conversion of carbon versus time. The experimental conditions are given in these plots and summarized in Appendix E. 7.1.1 Effect of Charring Conditions The effect of charring soak time on the fraction of carbon gasified is shown in Figure 7.1. The gasification experiments at 800°C were performed with material charred for one and two hours with heating rates of 8.1 and 7.6°C/min respectively while the experiments at 900°C were conducted with chars originating from two and ten hours treatments and submitted to heating rates of 9.1 and 5.6°C/mln respectively. It can be seen that for both gasification temperatures, the chars prepared with longer soak time and lower heating rate are less reactive. However at 900°C, there is only a slight reduction in the reactivity of the char prepared with a long soak time. 7.1.2 Effect of Bed Depth Experiments with different bed depths were performed with pure CO2 and mixtures of 20 and 50% of C02 in He at 800, 850, and 900°C. The effect of bed depth on the gasification rate is illustrated in Figures 7.2 to 7.4, for the conditions given in each graph. It can be observed that, for a l l the cases, deeper beds yield lower fractional conversion and reaction rates. From the value measured for bulk density of -841 + 420 pm Highvale chars, presented in Chapter 5, 80 g of solids provided a bed with a static height-to-diameter ratio approximately equal to one. - 132 -Figure 7.1 Plot of fractional conversion of carbon versus time showing the effect of charring conditions (soak time) for Highvale char; conditions as shown - 133 -1.01 0.8 c o V) k_ I 0.6 o c o o D i i i I 1 i i—I—i—r 80 120 Time (min) Q T« 101/min p c 0ja2atm -T (°C) • 40 800 • 1 60 O 40 — • 60 850 A 160 160 200 Figure 7.2 Plot of fractional conversion of carbon versus time showing the effect of bed depth at different temperatures (Highvale char); conditions as shown - 134 -Figure 7.3 Plot of fractional conversion of carbon versus time showing the effect of bed depth (Highvale char); conditions as shown - 135 -O 20 40 60 80 T i m e ( m i n ) Figure 7.4 Plot of fractional conversion of carbon versus time showing the effect of bed depth (Highvale char); conditions as shown - 136 -7.1.3 Effect of Total Inlet Flow Rate The effect of total Inlet flow rate, at 900°C, is shown in Figures 7.5 and 7.6 for the conditions given. There is a substantial increase in the reaction rate when the total flow rate is doubled while maintaining 10% C02 In the gas. On the other hand, for 50% C02 in the gas, there is a much smaller increase in the in i t i a l rate, when the total flow rate increases from 10 to 15 1/min. Above about 60% conversion the opposite effect is observed, i.e., the rate decreases for the larger flow rate. Figure 7.7 shows the relationship between total inlet flow rate and partial pressure of C02. Decreasing PCO2> causes a drop in the rate even when the total inlet flow rate is increased. 7.1.4 Effect of Inert Gas Concentration To determine the influence of the inert gas on the reaction rate, helium was introduced at two flow rates, one being twice as larger as the other, while the flow rate of C02 and the amount of char remained constant. Figure 7.8 shows the result obtained. It is seen that, up to about 35% conversion, the rate increases only slightly when the helium flow rate was raised from 5 to 10 1/min. For higher conversions, however, the rate is higher for the lower flow rate of helium. 7.1.5 Effect of Partial Pressure of CO? The effect of the partial pressure of C02 on fraction of char converted is presented in Figures 7.9 and 7.10, at 800 and 900°C respectively, for a total flow rate of 10 1/min and in Figure 7.11 for - 137 -Figure 7.5 Plot of fractional conversion of carbon versus time showing the effect of total inlet flow rate (Highvale char); conditions as shown - 138 -Figure 7.6 Plot of fractional conversion of carbon versus time showing the effect of total inlet flow rate (Highvale char); conditions as shown - 139 -Figure 7.7 Plot of fractional conversion of carbon versus time showing the effects of total inlet flow rate and partial pressure of C02 (Highvale char); conditions as shown - 140 -Figure 7.8 Plot of fractional conversion of carbon versus time showing the effect of inert gas concentration (Highvale char); conditions as shown - 141 -Figure 7.9 Plot of fractional conversion of carbon versus time showing the effect of partial pressure of C02 (Highvale char); conditions as shown - 142 -Figure 7.10 Plot of fractional conversion of carbon versus time showing the effect of partial pressure of C02 (Highvale char); conditions as shown - 143 -Figure 7.11 Plot of f r a c t i o n a l conversion of carbon versus time showing the e f f e c t of p a r t i a l pressure of C02 (Highvale char); conditions as shown - 144 -900°C and 15 1/min. As expected the reaction rate Increases with increasing PCO2 * n t n e inlet gas. However, the Increase In the rate with PCO2 * s n o t linear (first order); for higher values of PCO2 the increase in rate is lower, suggesting that CO is exerting a retarding effect and consequently that the reaction follows Langmuir-Hinshelwood kinetics. 7.1.6 Effect of Temperature The results presented in Figures 7.1, 7.2, 7.9, 7.10 and 7.12 show that the rate is strongly affected by the temperature at which the reaction takes place. Of the variables studied, temperature appears to influence the rate most significantly. 7.1.7 Preliminary Discussion The reactivity of the char decreases with an increase in the charring time in agreement with previous findings reported in the literature. According to Figure 7.1, at 800°C, the in i t i a l reactivity of the char prepared for one hour is approximately 4.84 x 10 - 5 s - 1 while that of the char prepared over two hours is about 3.83 x 10 - 5 s - 1. At 900°C, the in i t i a l reactivity of the material submitted to a soak time 4 1 of two hours is approximately 3.24 x 10~ s~ while that of the char u 1 treated for ten hours is 2.72 x 10 s~ . The specific surface areas of samples of chars prepared with soak times of two and ten hours were measured and the values obtained were 87.1 and 61.3 m /g respectively. In addition, as demonstrated in Figure 6.10, the surface areas of both chars increase about three times with conversion. However the maximum - 145 -Figure 7.12 Plot of fractional conversion of carbon versus time showing the effect of temperature for C0/C02 = 0 (Highvale char); conditions as shown - 146 -value is reached at lower conversion for the char soaked for two hours, Indicating that this char has a more open pore structure. As discussed before, this behaviour results from changes in the pore structure of the char particles during heat treatment. Prolonged charring in the temperature range of 700 to 1100°C causes thermal annealing during which microporosity, carbon edges and structural defects are lost and ash particles can sinter and segregate into large inclusions, decreasing their degree of dispersion in the char. Therefore longer charring treatments tend to reduce the intrinsic reactivity of the char due to the decrease in surface area, concentration of active sites and catalytic activity of mineral constituents. For the experiments in which gas mixtures containing CO was fed, all the chars were prepared with the ten-hour treatment. The effects of bed depth, total inlet flow rate, gas composition and temperature will be analyzed next. The concept of space velocity, i.e., the ratio between the total flow rate and the bed volume has been widely used to assess the influence of external mass transport on the reaction rate. If the reaction rate increases with increasing gas velocity at constant temperature, mass transfer to the char particles is limiting the rate provided that the changes in reactant gas concentration through the reactor are minimal, i.e., the system operates under differential conditions. If the exit reactant concentration differs substantially from the inlet, i.e. for integral reactors, increasing the gas velocity can be inconclusive since i t may increase the intrinsic rate of reaction solely by increasing the average concentration of reactant gas, without any influence of mass transfer. - 147 -It has been generally concluded that if the rate does not change at different gas velocities but at identical space velocities, i.e. at different bed depths, external mass transfer does not limit the rate. Q However, as pointed out by Ergun and Mentser , space velocity cannot be used as an independent variable in kinetics analysis since an increase in this parameter, due to an increase in total flow rate, can be accomplished by increasing the flow rate of the inert gas or the flow rate of the reactant gas, maintaining the in i t i a l mass of char and the flow rate of the other gas constant in each case. In the first case the rate is not affected while ln the second there is a strong effect. In addition, the partial pressure of the reactant gas also cannot be treated as an independent variable, since this partial pressure can be varied, with different results on the reaction rate, by changing the flow rate of the reactant gas, by changing the flow rate of the inert gas or by introducing a third gas that affects the reaction (as CO), holding the other variables constant in each case. Therefore the effects of bed depth, total inlet flow rate, gas composition and temperature are interrelated and will be discussed together. The main factors affecting the rate, influenced by the above variables, are starvation of CO2, elutriation of smaller particles from the reactor, hydrodynamics of the fluidization process and the CO poisoning effect. The reaction rate per mass of carbon increases with a decrease in bed depth, as shown in Figures 7.2, 7.3 and 7.4. Certainly one of the major factors responsible for this behaviour is C02 starvation, i.e., the high consumption of C02 within the bed which decreases the driving force for the reaction thus lowering the reaction rate. The starvation - 148 -of CO2 is proportional to the ratio between the rate of CO2 consumption and the rate of C0 2 fed and increases with deeper beds, high temperatures, low CO2 contents in the inlet gas and low total inlet flow rates. An increase in bed depth causes an increase in the absolute rate of carbon consumption (-dm/dt) since this rate is dependent on the amounts of carbon and surface area available for reaction. This explains the effect of bed depth at each temperature. Since the amount of carbon consumed is equal to the amount of C0 2 consumed, deeper beds favor CO2 starvation. Similarly starvation effects are expected to be more pronounced at elevated temperatures due to the higher rates of reaction. Coupling temperature and bed depth effects, at higher temperatures and deeper beds the reactant gas is strongly consumed in its passage through the solids bed. Thus most of the gasification occurs near the bottom of the bed. Obviously the thickness of the reaction zone increases at lower temperatures. Low contents of C0 2 favor starvation but, on the other hand, decrease the reaction rate as shown in Figures 7.9 to 7.11. The higher rate of carbon consumption in deeper beds causes a higher rate of heat consumption and, therefore, temperature gradients and quenching of the bed may develop which would decrease the reaction rate. In addition to starvation of CO2 and temperature non-uniformity, the effect of bed depth also is related to the hydrodynamic characteristics of the fluidized bed reactor. In a batch fluidized bed operating with a fixed amount of solids, attrition and the gasification reaction will gradually reduce the particle size thus increasing the fraction of gas flowing upward in the form of bubbles. Therefore larger - 149 -bubbles and even slugging in deeper beds also may contribute to the drop in carbon conversion due to the poor gas-solid contacting, caused by the larger by-pass of gas through the solids. The results presented in Figures 7.5 to 7.8, showing the effect of total inlet flow rate and gas composition, also can be explained in terms of CO2 starvation. However, elutriation of solids from the bed similarly plays an important role in these results. The steeper drop in rate after a certain conversion is reached, observed in Figures 7.6 and 7.8, is probably linked to the more severe carry over of finer particles when the higher flow rate was used. Elutriation of fines generated during the experiments was observed in most of the runs, starting at about 50% conversion. Another factor that should be taken into account when interpreting the results of the gasification experiments is the CO/CO2 ratio in the gas phase. When the experimental conditions favor higher consumptions of CO2, PCO2 drops considerably after a few seconds of reaction and the amount of CO formed is substantial. Under these conditions CO will begin to exert a poisoning effect, reducing the reaction rate mainly at lower temperatures where CO inhibition is more pronounced. Figure 7.13 shows the variation in PCO2 w * t n t n e reaction time at 900°C, for the three bed depths corresponding to Figure 7.3. The tendency for C0 2 starvation with an increase in bed depth is clearly indicated in Figure 7.13. As experiments at 950°C were planned, i t was decided to perform a l l the subsequent runs, involving C0-C02-He mixtures for Highvale and Smoky Tower chars, with a bed containing 20.0 g of char (L/D = 0.25) to - 150 -E o CM O O o. C M O U 0) w c / > (A 0) O O. * 0.1 0) IB E < 0.2 1 T=900°C Q T=l0t/min p c o =0.5 atm p H e =0.5" 10 20 30 40 Time (min) 50 60 Figure 7.13 Partial pressure of C02 as a functon of time for three bed depths (Highvale char); conditions as shown - 151 -minimize the effects of CO2 starvation and fluidization hydrodynamics on the rate of reaction. For shallower beds, the area/volume ratio becomes larger. Shallow beds also contribute to ease of temperature control since the absolute rate of carbon consumption and, therefore, heat consumption decreases, better temperature and reactant gas concentration uniformity, i.e., smaller longitudinal gradients and higher degree of mixing between char particles and CO2 (better gas-solid contacting). Difficulties In measuring the rates of reaction at lower temperatures when CO is admitted to the inlet gas due to the smaller absolute rates of reaction (and the smaller driving forces PC0out*"PC0in o r PC02in~ PC02Out) ^ s t n e main disadvantage of shallow beds. Due to the particle elutriation, i t was decided to utilize a gas flow rate of 10 1/min in all subsequent runs. As shown, in Chapter 6, this flow rate provided a superficial velocity at least approximately two times larger than the minimum velocity required to fluidize the char bed. The relatively low values of the ratio between the superficial velocity used and the minimum fluidization velocities required together with the low bed-height-to-diameter ratio adopted would lead to bubbling fluidization as opposed to slugging. Therefore, experimental conditions that made possible to study the gasification kinetics under a well mixed (solids and gas) and Isothermal environment were established, minimizing external mass and heat transfer effects. In addition, the influence of C0 2 starvation and elutriation of fines on the kinetic results also was minimized. - 152 -7.2 Gasification with COy-CO-He Mixtures Experiments were conducted with CO2-CO-He mixtures using -841 + 420 ym Highvale and Smoky Tower chars at 850, 875, 900 and 950°C with the aim of investigating the gasification of the chars in the presence of varying amounts of carbon monoxide, as is the case in industrial metallurgical operations. For the experiments with Highvale chars, two CO/CO2 ratios in the inlet gas - 0.25 and 0.50 - were studied; for each ratio, four different gas compositions were adopted. For the experiments with Smoky Tower chars, only the first CO/CO2 ratio (0.25) and three different compositions were tested. In every experiment, the in i t i a l mass of char was 20.0 g and the total inlet flow rate was 10 1/min. The experimental conditions used are summarized in Appendix E together with plots showing the results obtained. The effects of gas composition, char type and temperature are assessed. Reproducibility tests for three experiments also are presented. 7.2.1 Effect of Gas Composition The effect of gas composition is illustrated in Figures 7.14 to 7.19. Figure 7.14 and 7.15 show the effect of the C02-C0-He mixtures on the fractional conversion of Highvale chars for C0/C0 2 ratios equal to 0.25 and 0.50 respectively, at the intermediate temperature of 900°C. It is seen that the rate increases with increasing C0 2 content of the inlet gas and decreases when the C0/C0 2 ratio increases. This behaviour indicates that CO inhibits the gasification reaction. Similar results were found for the other temperatures. Figure 7.16 shows the conversion - 153 -Figure 7.14 Plot of fractional conversion of carbon versus time showing the effect of gas composition for CO/C02 = 0.25 (Highvale char); conditions as shown - 154 -Figure 7.15 Plot of fractional conversion of carbon versus time showing the effect of gas composition for CO/C02 = 0.50 (Highvale char); conditions as shown - 155 -IJO c o ' 5 5 w-o> > c o o o c o o o 0.8 0.6 mchor * 20 A 0 T « I O I / m i n T - 9 0 0 ° C Pco(otm) Pco(atm) A 0.08 032 — • 0.1 2 0.48 O 0.1 6 0.64 Time (min) Figure 7.16 Plot of fractional conversion of carbon versus time showing the effect of gas composition (Smoky Tower char); conditions as shown - 156 -950WC Pco (atm) Pco? (atm) • O.IO O20 o 0.12 0.4 8 • OJO 0.20 • 0.12 0.48 • 0.10 0.20 o 0.1 2 0.4 8 • 0.10 0.20 o 0.12 0.4 8 c h a r = 2 0 a QT c lOf/min 60 80 Time (min) 120 Figure 7.17 Plot of fractional conversion of carbon versus time showing the effect of partial pressure of C02 for a constant partial pressure of CO (Highvale char); conditions as shown - 157 -Time (min) Figure 7.18 Plot of fractional conversion of carbon versus time showing the effect of partial pressure of C02 for a constant partial pressure of CO (Highvale char); conditions as shown - 158 -Figure 7.19 Plot of fractional conversion of carbon versus time showing the effect of partial pressure of CO for a constant partial pressure of C02 (Highvale char); conditions as shown - 159 -of Smoky Tower chars at 900°C, which reveals the expected increase in rate with increasing P C O 2 i n t n e £ a s mixture. Figure 7.17 and 7.18 show the effect of increasing P c o 2 while maintaining p^ o relatively constant. In Figure 7.17 the fractional conversion in plotted against the reaction time for the four temperatures investigated, showing the effect of increasing pco 2 i n t n e inlet gas from 0.20 to 0.48 atm with PCo equal to 0.10 and 0.12 atm respectively. Figure 7.18 shows a similar plot for p c o 2 increasing from 0.30 to 0.64 atm with pen equal to 0.15 and 0.16 atm respectively. It can be seen that the reaction rate increases with pco 2 i n every case. Figure 7.19 reveals the opposite effect for the four temperatures studied. In these experiments, pco increases from 0.08 to 0.15 atm while Pco 2 is maintained relatively constant; 0.32 and 0.30 atm respectively. The poisoning effect of CO is clearly observed in these results since the rate drops when PQQ increases. It is also seen that the poisoning effect seems to be more pronounced at lower temperatures. 7.2.2 Retarding Effect of CO Figures 7.20 to 7.23 provide further evidence that CO strongly retards the gasification reaction mainly at lower temperatures. These results correspond to the experiments run with three C0/C02 ratios; 0, 0.25 and 0.50, and with PC02 having approximately identical values; 0.48 and 0.50 atm, for the four temperatures. As shown in the figures, increasing the C0/C02 ratio leads to a drop in the rate. Moreover the difference among the fractional conversion curves becomes smaller as the temperature is increased. - 160 -Figure 7.20 Plot of fractional conversion of carbon versus time showing the effect of CO/CO2 ratio (Highvale char); conditions as shown - 161 -Figure 7.21 Plot of fractional conversion of carbon versus time showing the effect of CO/CO2 ratio (Highvale char); conditions as shown - 162 -T i m e (m in ) Figure 7.22 Plot of fractional conversion of carbon versus time showing the effect of CO/CO2 ratio (Highvale char); conditions as shown - 163 -0 20 40 60 T i m e ( m i n ) Figure 7.23 Plot of fractional conversion of carbon versus time showing the effect of C0/C02 ratio (Highvale char); conditions as shown - 164 -7.2.3 Effect of Char Type Figure 7.24 shows the kinetics behaviour of the chars originating from the two coals studied for the intermediate composition; 12% CO, 48% CO2 and 40% He, at the four temperatures. The chars were prepared by similar treatments: gas-fired, ten hours soak time. It is seen that the Smoky Tower chars are always less reactive than the Highvale chars. Similar behaviour was found for the other two compositions tested at a l l temperatures, as shown in Figure 7.25 at the intermediate temperature of 900°C. 7.2.4 Effect of Temperature The critical effect of temperature is again demonstrated in Figures 7.17 to 7.24 as well as in Figure 7.26. This last Figure shows the influence of temperature for experiments carried out with pco 2 e c l u a l to 0.50 atm and a C0/C0 2 ratio of 0.50; i t should be compared with Figure 7.12 which shows the effect of temperature for experiments with the same Pco 2 D U t with a CO/C0 2 ratio of 0. Due to the poisoning effect of CO, the dependence of the reaction rate on temperature increases with the C0/C0 2 ratio. 7.2.5 Reproducibility Tests Experiments to test the reproducibility of the results were performed at 875 and 900°C for different conditions of char type, i n i t i a l mass of char and gas composition as shown by runs G12-G13, G36-G51 and G46-G52 in Appendix E. The results are shown in Figure - 165 -O 40 80 120 Time (min) Figure 7.24 Plot of fractional conversion of carbon versus time showing the effect of char type at different temperatures; conditions as shown - 166 -Figure 7.25 Plot of fractional conversion of carbon versus time showing the effect of char type at different gas compositions; conditions as shown (H = Highvale char; ST = Smoky Tower char) - 167 -0 20 40 60 80 100 120 T i m e (m in ) Figure 7.26 Plot of fractional conversion of carbon versus time showing the effect of temperature for C0/C02 = 0.50 (Highvale char); conditions as shown - 168 -7.27. It is seen that the agreement between the repeated experiments is, on the average, within 5%. 7.2.6 Preliminary Discussion The experimental results obtained show that the Boudouard reaction, for the conditions studied, is strongly retarded by CO especially at low temperature in agreement with the literature. Thus as the concentration of CO in the gas phase increases, the dependence of the reaction rate on temperature is enhanced. The reactivity of chars prepared from Smoky Tower coal was found to be lower than that of chars originating from Highvale coal. This result also was obtained by Parkash and du Plessis 1 working with chars of these coals. Since both coals are sub-bituminous, this result indicates that coal rank Is an insufficient parameter to assess the reactivity of the char. Therefore the difference in reactivity for the two chars studied must be related to other properties of the materials. The two most important relevant properties are the specific surface area and the ash characteristics. Regarding the specific surface area, the unreacted Highvale char has an area of 61.3 m /g while for Smoky Tower 2 char the value is about three fold lower, 20.2 m /g. Moreover the surface area of Highvale chars increases during gasification to reach a maximum value more than two times larger than the maximum surface area for Smoky Tower chars, as indicated in Figure 6.10. Therefore, the higher values of surface area for Highvale chars can account, at least partially, for the higher reactivity. It should be emphasized that these surface areas correspond to samples taken from material similarly - 169 -Figure 7.27 Plot of fractional conversion of carbon versus time showing the reproducibility of the experimental results; conditions as shown - 170 -charred (gas-fired furnace, 10 hours); thus they can be properly compared. With respect to the ash characteristics, the two coals have a very similar total ash content as shown in Table I (values analyzed at Stelco Inc.). Nevertheless the coal with lower ash content is more reactive. Therefore the total ash content is not responsible for the difference in 6 3 reactivity, in agreement with previous results. If the ash composition of the two coals is compared, Table II shows that Highvale chars have a slightly higher CaO content, a lower K20 content and a Na20 content 13.5 times larger than the Smoky Tower chars. If the concentrations of the two alkali oxides are added, the value for the Highvale chars is 3.69% while the Smoky Tower chars have an alkali content of 0.92% in the ash. Therefore the higher concentration of potential catalysts in the Highvale chars ash is another factor that could explain the higher reactivity, even recognizing that K20 has been 8 6 generally reported to be a stronger catalyst for the Boudouard reaction than Na20. Besides total content of the ash and the chemical form and concentration of a particular ash constituent, the dispersion, degree of contact and particle size of ash in the carbon matrix also play an Important role in determining its catalytic efficiency. These last parameters are clearly more difficult to characterize. Moreover, as discussed in Chapter 3, for the reaction taking place in fluidized bed reactors there is a tendency for the ash layer to flake off from the particles due to solids movement and attrition between the particles, - 171-especially at high conversions corresponding to relatively high ash contents. This would considerably reduce the degree of contact between the ash particles and the carbon matrix, decreasing their catalytic activity. SEM observations of unreacted and partially reacted samples of both chars at different fractional conversions will be presented in Section 7.3, in an attempt to investigate these factors. In the results presented in this chapter the plots of fractional conversion versus time are observed to be sigmoidal. Thus the reaction rate initially increases with the gasification time to reach a maximum and then drop with further reaction. Similar behaviour was found for the variation in the surface area of the char particles with the fractional conversion and, hence, reaction time as shown in Figure 6.10. Therefore there is a definite connection between the reaction rate and the changes in surface area of the particles during gasification as will be discussed in greater detail in the next chapter. However, as observed in some of the curves of fractional conversion versus time, the very sharp drop in reaction rate at high conversions cannot possibly be explained only by decreases in surface area. Instead the elutriation of solids from the reactor probably plays the dominant role. Finally a qualitative discussion on possible rate controlling step (s) is presented. A more complete analysis will be made in the next chapter. In the absence of external mass transfer, the rate of reaction depends on the intrinsic reactivity of the char, on the amount of carbon available for reaction and on the amount and accessibility of surface - 172 -area of the particles at any instant. If, on the other hand, the rate were limited by external mass transfer of reactant gas, it would be totally independent of the intrinsic reactivity of the char and related only to hydrodynamics parameters and reactant gas concentration. In this case, the equation governing the gasification kinetics would have the form: r - kg * ( CC0 2 " CcV • Sext ' b where r is the instantaneous rate of reaction, kg is the mass transfer b s coefficient, C^Q^ and C ^ 0 ^ are respectively the concentration of C02 in the bulk of the gas phase and at the external surface of the char particles, S e x t is the specific external area of the particles and b is a constant. Thus similar reaction rates would be obtained for equal amounts and particle sizes of Highvale and Smoky Tower chars (assuming the same external area for the particles), and the same gas flow rate, gas composition and temperature. Equation [7.1] also would predict an unchanging rate of reaction since the mass transfer coefficient for the char particles in the fluidized bed reactor is relatively constant. Furthermore the poisoning effect of C O would not be observed since i t is a chemical and not a physical effect. In addition, the temperature dependence of the reaction rate would be very small, typical of transport dominated process. However the experimental results show that there is a substantial difference in the reactivity of the chars, the reaction rate exhibits a maximum value, C O has a strong retarding effect - 173 -mainly at low temperatures and the rate is heavily dependent on temperature. Therefore these results seem to indicate that, for the shallow beds used, the rate is very l i t t l e influenced by external mass transfer, and is probably controlled by chemical reaction or by a combined effect of chemical reaction and pore diffusion. 7.3 Scanning Electron Microscope Observations A SEM study was conducted on Highvale and Smoky Tower char samples, both unreacted and partially gasified under different conditions of gas composition and temperature, at various stages of conversion. The pore structure of the samples was observed at several magnifications from 10X up to 4000X with the objective of examining the external appearance of the char particles, the changes in pore structure with the progress of reaction and ash characteristics (particle size, dispersion and degree of contact). Typical SEM photographs are shown in Figures 7.28.a to 7.28.r and are discussed below. Figures 7.28.a and b and Figures 7.28.k and 1 present the microstructure of unreacted Highvale and Smoky Tower chars respectively. A highly heterogeneous and porous structure, representative of a wide pore size distribution with pores ranging from large cracks or slits to very small cylindrical cavities is observed. The distribution of ash inclusions also is seen. Figure 7.28.C shows the spongy and porous structure as well as ash inclusions in a 35% converted Highvale char. The external appearance of Highvale char Figure 7.28.b Microstructure of unreacted Highvale char (4000X) Figure 7.28.d External appearance of Highvale char particles 57% reacted at 850°C (20X) Figure 7.28.e External appearance of unreacted Highvale char particles (100X) Figure 7.28.f Slit-like crack and ash inclusions in Highvale char 57% gasified at 850°C (1000X) - 177 -Figure 7.28.h Pore structure and ash inclusions in Highvale char 72% converted at 900°C (2000X) - 178 -Figure 7.28.J Ash agglomerate in Highvale char 84% gasified at 900°C (2000X) - 179 -Figure 7.28.n Porosity in Smoky Tower char 41% gasified at 850°C (2000X) - 181 -Figure 7.28.0 Porosity in Smoky Tower char 41% reacted at 850°C (4000X) Figure 7.28.p External appearance of Smoky Tower char particles 49% gasified at 875°C (10X) Figure 7.28.r Porosity in Smoky Tower char 81% reacted at 950°C (2000X) - 183 -particles 57% reacted Is presented in Figure 7.28.d. By comparing this figure with Figure 7.28.e it can be seen that the shape of the particles do not show any evidence of becoming more spherical with conversion. Figure 7.28.f shows clearly a slit-like crack or macropore, typical of 152 sub-bituminous coal chars, plus ash inclusions. The highly heterogeneous, porous and spongy structure of partially reacted Highvale chars as well as the dispersion of ash inclusions are once again clear in Figures 7.28.g to 7.28.J. In particular, Figures 7.28.1 and j illustrate clearly the segregation of ash particles in agglomerates at high conversions, thus possibly reducing their catalytic efficiency. A tendency for an opening in the pore structure with conversion also is observed. Figures 7.28.m, n and o show examples of a highly porous structure with slit-like pores as well as smaller cylindrical cavities in Smoky Tower char. Figure 7.28.p indicates that the char particles can disintegrate into smaller fractions even at 49% conversion favoring their elutriation from the reactor. Finally Figures 7.28.q and r show the highly open and spongy pore structure of Smoky Tower char at higher conversions. By comparing the pore structures of the unreacted chars with those of the partially gasified solids, it seems that as the reaction proceeds the pores grow in size and larger cavities are formed due to the collapse of the walls between adjacent pores. Therefore there is a tendency for an opening in the pore structure caused by the gasification process, i.e., there seems to be an increase in porosity with the progress of the reaction. The major contribution to the total porosity - 184 -can be attributed to the expansion of macropores as shown in the SEM photographs. In addition it seems that no ash layer is present around the char particles suggesting that the reaction possibly occurs throughout the particles in micropores much smaller than the larger 3 1 mineral inclusions. These inclusions, therefore, are not effectively catalyzing the reaction. However the catalytic effect of very fine mineral particles located inside the micropores cannot be disregarded. - 185 -CHAPTER 8 OVERALL DISCUSSION OF RESULTS An overall discussion of the gasification kinetics of the sub-bituminous coal chars in the fluidized bed reactor is provided in this chapter. This is accomplished by applying a proposed kinetics relationship to the reaction rates and gas compositions at different temperatures as well as correlating the changes in the surface area of the char particles with the fractional conversion of carbon. Apparent activation energies for the chars of the two coals studied are determined and discussed as well as the rate controlling steps. Langmuir-Hinshelwood kinetics are shown to represent the Boudouard reaction for the conditions investigated. 8.1 Proposed Rate Equation and Determination of Kinetics Parameters The complexity of the Boudouard reaction under the range of temperature and gas composition studied does not allow a simple equation to properly describe the gasification kinetics. The following phenomena occur during the gasification of the char particles in the fluidized bed reactor which affect the rate of reaction. [1] The surface area of the particles changes continuously with the carbon conversion, leading to variations in the effective area available for reaction, as shown in Figure 6.10. [2] The partial pressures of CO and C02 change continuously with the extent of reaction, as indicated in Figures 6.7 and 7.13. - 186 -[3] CO exerts a strong retarding effect on the reaction, especially at low temperatures, as illustrated in Figures 7.14/7.15, 7.19 to 7.23, and 7.12/7.26. [4] The catalytic activity of inorganic constituents present in the ash is likely to change with the extent of reaction, as seen in Figures 7.28.g to 7.28.J. [5] The char particles are subjected to morphological changes, i.e., they decrease in size, disintegrate, as shown in Figure 7.28.p, and are eventually elutriated from the reactor after a certain conversion of carbon is reached. [6] The amount of carbon in the char decreases continuously. In addition, experimental inaccuracies at low temperature (850°C) when CO is admitted in the inlet gas, and at high temperature (950°C), have an influence on the reaction rate measured. These difficulties preclude the use of simple kinetics relationships such as the rate equation frequently employed, based on the assumption that the rate is directly proportional to the mass of carbon present in the bed at any instant, as follows. dm [8.1] dt c Re-arranging this equation to the form, 1 dm , r = • - j — = k m dt c [8.2] i t is readily seen that the reaction rate would be constant throughout - 187 -the g a s i f i c a t i o n process, which i s not indicated by the plots of f r a c t i o n a l conversion of carbon versus time presented i n the previous chapter. Therefore i n order to describe the complex k i n e t i c s problem under study, a rate equation i s proposed i n i t s general form as follows. r " 3 0 ( p c o ' W l 8 ' 3 ] where r i s the instantaneous rate of reaction defined as i n Section 6.2.4, a i s a parameter defined as the r a t i o between the surface area per unit weight or volume of the char at any conversion and the i n i t i a l surface area per unit weight or volume of the char, and 0(pco» PC0 2) i s a generic function of pco a n d PC0 2 expressing the gas composition dependence of the reaction rate. Thus Equation [8.3] takes into account the changes i n surface area occurring during r e a c t i o n . In addition 0(PCO> PC0 2) must have a r e l a t i v e l y simple mathematical expression which allows i t s immediate u t i l i z a t i o n and p r a c t i c a l conclusions to be drawn while s t i l l i n c l u d i n g the CO retarding e f f e c t . D i f f e r e n t r e l a t i o n s h i p s f or 0(pco> PC0 2) a r e discussed below. The experimental r e s u l t s presented i n Chapter 7 i n d i c a t e that an increase i n pco2 i° t n e i n l e t gas causes a non-linear increase i n the rate of reaction, as shown i n Figures 7.9 to 7.11, that the presence of CO i n the gas retards the rate, as seen i n Figures 7.14/7.15, 7.19 to 7.23 and 7.12/7.26; the higher p c o , the lower the rate, for the same values of the other v a r i a b l e s , as indicated i n Figures 7.19 to 7.23, and that the poisoning e f f e c t of CO i s more pronounced the lower i s the - 188 -temperature, as i l l u s t r a t e d i n Figures 7.19 to 7.23 and 7.12/7.26. These findings indicate that the o v e r a l l rate of reaction possibly follows the Langmuir-Hinshelwood (LH) equation. Thus, k l P C 0 , 0(pco. »co2> • i • t 2 P C 0 • £ 3P C 0 2 [ 8- 4 1 However the LH equation with i t s three rate constants would add complexity to the general rate equation, Equation [8.3], since t h i s equation already contains the parameter a; and the r e l a t i o n s h i p between the rate of reaction and surface area i s not, i n p r i n c i p l e , straightforward. Therefore a simpler r e l a t i o n s h i p w i l l be considered eventhough, s t r i c t l y speaking, the LH equation should be the proper form of t h i s function. The a p p l i c a b i l i t y of the LH equation w i l l be assessed i n the f i n a l part of this analysis and the implications of using a s i m p l i f i e d r e l a t i o n s h i p w i l l be discussed. The f i r s t equation considered to replace the LH equation, which s t i l l includes the i n h i b i t i n g e f f e c t of CO was: 0<PCO' PCO^ = k B - p — 2 [ 8 - 5 ]  z CO However t h i s equation, which stems from the LH equation based on the r e s t r i c t i o n k2PCO»l + k3PC02» has been r e p o r t e d 1 6 * 2 9 to be i n v a l i d f ° r PC02^PC0 r a t i o s greater than 2.0 for the reaction of graphite with CO-CO2 gas mixtures at 1000°C. Obviously t h i s equation i s more l i k e l y to be s a t i s f i e d when VQQ i s s u f f i c i e n t l y high. Also since both - 189 -k 2 and k3 decrease with an increase in temperature, but k 2 generally decreases more pronouncedly than k3, Equation [8.4] is better applied at low temperatures. In addition as kg = kj/k 2 and k^  increases with an increase in temperature, k B will increase with temperature more than k± itself and provide an unusually high activation energy for the Boudouard reaction, i.e., the apparent activation energy would be given by the sum of the activation energy due to kj and that due to k2. Therefore, owing to these restrictions, the use of Equation [8.5] was abandoned. A more common form of £>(pco»PC02)» used by several investigators 1 1 > 1 I +» 1 5 >31* » 1 0 3 is the so-called "power-law" equation: 0 ( pco' pco 2 ) = k pco 2 [ 8' 6 ] where n is the apparent order of reaction. This equation is generally regarded as a special case of the LH 153 equation and, as suggested by Levenspiel, simple nth order rate equations should be used instead of more complex expressions derived from theoretical mechanisms to correlate data for reactions in heterogeneous catalysis. Therefore, since these equations also have been frequently used for char gasification reactions, Equation [8.6] was adopted in this work. Thus, Equation [8.3] can be written as: [8.7] - 190 -the rate constant k is given by the Arrhenius equation: k = k exp (-E /RT) [8.8 o o where E D is the apparent activation energy for the Boudouard reaction. The parameter a is generally dependent on the fractional conversion of carbon and on the temperature, and will be correlated by available expressions in Section 8.2. However, for the present, a pseudo rate constant k', will be defined as. k' = ka [8.9] Thus k' depends on the temperature and on the surface area available for reaction and, hence, on the carbon conversion, k' also is given by the Arrhenius equation. k' = k' exp (-E/RT) [8.10] o However, in this case, the apparent activation energy depends on the fractional conversion of carbon. According to Laurendeau,11 the following equation accounts for the variations in the apparent activation energy E with the conversion. c2 E = E + c. f [8.11] o 1 where cj and c 2 are constants. - 191 -The apparent order of r e a c t i o n , n, v a r i e s between 0 and 1. I t has been e s t a b l i s h e d that i n the absence of r e t a r d i n g e f f e c t s of CO, i . e . , when pco i s very small and can be neglected, the r e a c t i o n approaches f i r s t order at pressures below and up to 1 atm and zero order at 34 pressures g r e a t e r than 15 atm. However when the poisoning e f f e c t of CO i s s u b s t a n t i a l , as i n the present study, a f r a c t i o n a l order of r e a c t i o n can be expected, i . e . , n w i l l be between 0 and 1. Thus the value of n r e f l e c t s the i n h i b i t i n g e f f e c t of CO. In terms of the LH equation, the f i r s t - o r d e r r a t e equation r=kjpco 2 * s obtained when 1 > k2Pco+k3Pco2 t n a t * s when both p^o and PC02 and/or k 2 and k 3 tend to be s m a l l . When none of the terms i n the denominator of Equation [8.4] can be neglected, the equation y i e l d s a f r a c t i o n a l value f o r the measured order w i t h respect to PC02* From the preceding d i s c u s s i o n , the proposed r a t e equation to c o r r e l a t e the experimental data i s f i n a l l y w r i t t e n as. r = k ' p ^ [8.12] The apparent order of r e a c t i o n i s , then, determined by the r e l a t i o n : Sinr = £nk' + n£np_ [8.13] C0 2 where k' i s constant f o r each f i x e d value of temperature and f r a c t i o n a l c onversion. Thus p l o t s of &nr vs &npco 2 were obtained from s e v e r a l experiments, w i t h chars of both coals s t u d i e d , at each temperature and - 192 -values of conversion starting at 10% and increasing by increments of 10% until the maximum conversion in each set of experiments was reached; n was determined from the slopes of the plots obtained by linear regression of the data. Regarding the degree of mixing for the solids and gas, both phases are assumed to be fully back-mixed in the reactor. Therefore, PCO2 * n t n e bed is equal to the measured PCO2 * n t n e exit gas since, as discussed in Chapter 5, the gas retention time from the exit of the reactor up to the sampling point was only a few seconds. The values obtained for n are given in Tables XIV and XV. For Highvale chars, the values of reaction rate and PCO2 corresponding to each inlet C0/C0 2 ratio were plotted separately since at higher CO/CO2 ratios, the rate for similar values of PCO2 * s l° w er due to CO inhibition as shown in Figures 7.19 to 7.23. Thus the lnr versus l n p c o 2 plots for Highvale chars were comprised of three to six points, depending on the experimental conditions, while for Smoky Tower chars these plots contained only three points, due to the smaller number of experiments performed. In general, data corresponding to low and high conversions, at each temperature, tend to be more inaccurate due to experimental instabilities at the initial stages of reaction and morphological changes and elutriation of the particles In the latter stages. At each fractional conversion, data at 850°C are less precise due to the retarding effect of CO especially for the less reactive Smoky Tower chars. It is also recognized that, according to LH kinetics, the order of reaction may vary with CO/C02 in the inlet gas, temperature and T A B L E X I V ORDERS OF R E A C T I O N F O R H I G H V A L E C H A R S jf points corr c o r r80% n CO/CO 2 T(°C) f(%) 6 0.995 0.608 0.49 0 800 10 6 0.995 0.608 0.52 0 800 20 4 0.956 0.800 0.61 0 800 30 3 0.954 0.951 0.40 0 800 40 5 0.941 0.687 0.37 0 900 10 5 0.945 0.687 0.35 0 900 20 5 0.937 0.687 0.32 0 900 30 5 0.967 0.687 0.36 0 900 40 4 0.964 0.800 0.27 0 900 50 4 0.973 0.800 0.35 0 900 60 4 0.968 0.800 0.45 0 900 70 4 0.983 0.800 0.61 0 900 80 4 0.940 0.800 0.35 0.25 850 10 4 0.996 0.800 0.35 0.25 850 20 4 0.952 0.800 0.44 0.25 850 30 4 0.859 0.800 0.33 0.25 850 40 3 0.860 0.951 0.31 0.25 850 50 4 0.995 0.800 0.42 0.25 875 10 4 0.999 0.800 0.41 0.25 875 20 4 0.998 0.800 0.38 0.25 875 30 4 0.998 0.800 0.33 0.25 875 40 4 0.979 0.800 0.29 0.25 875 50 3 0.693 0.951 0.32 0.25 875 60 TABLE XIV - ORDERS OF REACTION FOR HIGHVALE CHARS Cont . )f points corr C O r r80% n C0/C02 T(°C) f(%) 4 0.973 0.800 0.44 0.25 900 10 4 0.997 0.800 0.48 0.25 900 20 4 0.989 0.800 0.51 0.25 900 30 4 0.979 0.800 0.48 0.25 900 40 4 0.981 0.800 0.42 0.25 900 50 4 0.999 0.800 0.43 0.25 900 60 4 0.973 0.800 0.46 0.25 900 70 3 0.987 0.951 0.56 0.25 950 10 4 0.988 0.800 0.57 0.25 950 20 4 0.985 0.800 0.58 0.25 950 30 4 0.982 0.800 0.61 0.25 950 40 4 0.972 0.800 0.60 0.25 950 50 4 0.964 0.800 0.62 0.25 950 60 4 0.949 0.800 0.66 0.25 950 70 4 0.828 0.800 0.71 0.25 950 80 4 0.938 0.800 0.33 0.50 850 10 4 0.770 0.800 0.19 0.50 850 20 4 0.626 0.800 0.24 0.50 850 30 3 - 0.632 0.951 -0.09 0.50 850 40 4 0.917 0.800 0.28 0.50 875 10 4 0.958 0.800 0.32 0.50 875 20 4 0.981 0.800 0.28 0.50 875 30 4 0.955 0.800 0.24 0.50 875 40 4 0.930 0.800 0.33 0.50 900 10 4 0.968 0.800 0.37 0.50 900 20 TABLE XTV - ORDERS OF REACTION FOR HIGHVALE CHARS Cont. No. of points corr C O r r80% n C0/C02 T(°C) f(%) 4 0.987 0.800 0.33 0.50 900 30 4 0.9.58 0.800 0.27 0.50 900 40 4 0.960 0.800 0.21 0.50 900 50 4 0.627 0.800 0.07 0.50 900 60 3 - 0.972 0.951 - 0.12 0.50 900 70 4 0.995 0.800 0.59 0.50 950 10 4 0.989 0.800 0.57 0.50 950 20 4 0.972 0.800 0.56 0.50 950 30 4 0.971 0.800 0.55 0.50 950 40 4 0.968 0.800 0.56 0.50 950 50 4 0.962 0.800 0.59 0.50 950 60 4 0.897 0.800 0.55 0.50 950 70 4 0.705 0.800 0.36 0.50 950 80 For corr 2 c° r r80% : n = 0.44 ± 28% - 196 -T A B L E XV ORDERS OF REACTION FOR SMOKY TOWER CHARS c o / c o 2 No. of points c o r r80% 0.25 = 3 0.951 corr n T(°C) f(%) 0.909 0.21 850 10 -0.962 -0.17 850 20 0.334 0.11 850 30 1.000 0.67 875 10 0.997 0.79 875 20 0.975 0.86 875 30 0.961 1.00 875 40 0.966 0.50 900 10 0.969 0.56 900 20 0.965 0.64 900 30 0.917 0.64 900 40 0.992 0.90 950 10 1.000 0.82 950 20 0.996 0.76 950 30 0.984 0.66 950 40 0.942 0.47 950 50 0.928 0.16 950 60 0.833 0.13 950 70 For corr >^  corrso%: n = 0.74 ± 19% - 197 -fractional conversion of carbon. For instance, n is expected to increase at lower CO/CO2 ratios and higher temperatures since, in both cases, the retarding effect of CO decreases. However, in order to simplify the analysis, fixed values of n were adopted for the different chars. Moreover, as shown in Tables XIV and XV, there is not a conclusive indication that n follows a definite pattern with respect to variations in the inlet CO/CO2 ratio, temperature and conversion, due to the scatter of few data points, the restriction of temperature plus fractional conversion fixed in each plot and the phenomena associated with the gasification process described at the beginning of this section. Instead a large and random variation in n seems to be the overall tendency. Furthermore, negative values obtained are clearly meaningless. Since the orders of reaction for the two chars studied were obtained by averaging the values of n in Tables XIV and XV, a criterion to accept certain values of n and refuse others had to be adopted. Accordingly the values of n included in the averages were the values obtained from regression lines with correlation coefficients with minimum significance or certainty of 80%, i.e., correlation coefficients greater or equal to a test correlation coefficient corresponding to 80% of significance. Table XVI presents the test values for correlation coefficients as a function of the levels of significance and number of points (degrees of freedom). The values of the correlation coefficients for 80% certainty relative to the number of points used to determine n also are included in Tables XIV and XV. The average values of n are TABLE XVI TEST VALUES FOR CORRELATION COEFFICIENTS c o r r t e s t " • level of significance of corr (%) No. of points df 50 70 80 90 95 99 99.9 3 1 0.707 0.891 0.951 0.988 0.997 1.000 1.000 4 2 0.502 0.701 0.800 0.900 0.950 0.990 0.999 5 3 0.402 0.585 0.687 0.805 0.878 0.959 0.991 6 4 0.347 0.511 0.608 0.729 0.811 0.917 0.974 corr test - ( t 2 >"2 t + df df = degrees of freedom = No. of points -• 2 t = values from Student' s table at each df and degree of certainty (level of significance) Source: Reference 156. - 199 -0.44 ± 28% for Highvale chars and 0.74±19% for Smoky Tower chars. Therefore the values of 0.4 and 0.7 were adopted as the orders of reaction for Highvale and Smoky Tower chars respectively. Tables XIV and XV show that the orders of reaction were averaged over a broad range of inlet CO/CO2 ratios, temperature and fractional conversion, and therefore are representative of the kinetics of the two chars. In addition, considering the restrictions discussed above, the ranges of variation in n are acceptable. For each char and inlet CO/C02 ratio, an average pseudo rate constant, k', was calculated at several conversion levels at each temperature as follows. where I is the number of experiments at each fractional conversion and temperature fixed and kj^  is the pseudo rate constant for each experiment at that particular conversion and temperature given by, where n = 0.4 for Highvale chars and n = 0.7 for Smoky Tower chars. The values obtained for k' are listed in Tables XVII to XX. The apparent activation energies for the Boudouard reaction were — hi obtained by constructing Arrhenius plots of £nk' versus 10 /T for each k' = [8.14] [8.15] - 200 -TABLE XVII PSEUDO RATE CONSTANT FOR HIGHVALE CHARS WITH C0/C02 = 0 AND pco, = 0.50 atm k' [s- 1 atm ~°>h] f 850°C 875°C 900°C 950°C 0.05 2.451 X i o - 4 4.484 X i o - 4 0.10 2.937 X i o - 1 * 5.192 X i o ~ 4 5.518 X 10" .1+ 0.20 3.607 X IO"4 6.073 X i o - 4 6.867 X i o -.4 1.439 X 10" • 3 0.30 3.940 X i o - 4 6.769 X i o - 4 8.299 X 10-.4 1.645 X 10" 3 0.40 4.343 X IO"4 7.400 X IO"4 9.463 X 10" .4 1.875 X 10" .3 0.50 4.891 X i o - 4 8.171 X i o - 4 9.805 X i o -.4 2.153 X 10" 3 TABLE XVIII AVERAGE PSEUDO RATE CONSTANT FOR HIGHVALE CHARS WITH CO/CO o = 0.25 k ' l s " 1 atm - 0 - 4 ] f 850°C 875°C 900°C 950°C 0.05 1.365 X i o - 4 + 8% 2.261 X i o - 4 + 6% 3.609 X I O - 4 + 8% -0.10 1.737 X i o - 4 + 7% 2.669 X i o - 4 + 2% 4.823 X i o - 4 + 6% 1.086 X i o - 3 + 12% 0.20 1.997 X IO"4 + 3% 3.166 X i o - 4 + 1% 6.037 X i o - 4 + 5% 1.335 X i o - 3 + 11% 0.30 1.899 X IO"4 + 8% 3.352 X i o - 4 + 2% 6.548 X lO" 4 + 7% 1.514 X 10" 3 + 12% 0.40 1.774 X i o - 4 + 11% 3.367 X 10-" + 4% 7.009 X i o - 4 + 7% 1.675 X IO - 3 + 13% 0.50 1.398 X i o - 4 + 6% 3.174 X i o - 4 + 7% 7.113 X i o - 4 + 5% 1.864 X 10" 3 + 13% 0.60 - 2.554 X i o - 4 + 9% 6.654 X i o - 4 + 2% 2.059 X i o - 3 + 15% 0.70 - - 4.748 X i o - 4 + 7% 2.155 X IO - 3 + 18% TABLE XIX AVERAGE PSEUDO RATE CONSTANT FOR HIGHVALE CHARS WITH CO/COo = 0.50 k ' f s " 1 atm -°> h] f 850°C 875°C 900°C 950°C 0.05 9.752 X i o - 5 + 11% 1.835 X i o - 4 + 5% 3.390 X i o - 4 + 10% -0.10 1.201 X i o - 4 + 6% 2.138 X i o - 4 + 8% 4.058 X i o - 4 + 7% 1.036 X IO - 3 + 9% 0.20 1.327 X i o - 4 + 11% 2.450 X i o - 4 + 5% 5.018 X i o - 4 + 5% 1.167 X i o - 3 + 9% 0.30 1.159 X IO"4 + 14% 2.487 X i o - 4 + 6% 5.427 X i o - 4 + 4% 1.303 X IO - 3 + 10% 0.40 8.828 X IO"5 + 15% 2.245 X i o - 4 + 8% 5.515 X IO"4 + 7% 1.461 X IO - 3 + 10% 0.50 - 1.598 X i o - 4 + 10% 5.112 X IO"4 + 9% 1.602 X i o - 3 + 10% 0.60 - - 4.300 X IO"4 + 15% 1.647 X IO - 3 + 11% 0.70 - - - 1.548 X IO"3 + 14% T A B L E XX AVERAGE PSEUDO RATE CONSTANT FOR SMOKY TOWER CHARS k ' t s - 1 atm - 0- 7] f 850°C 875°C 900°C 950°C 0.05 1.338 X i o - 4 + 6% 2.260 X i o - 4 + 3% 3.494 X i o - 4 + 10% -0.10 1.571 X i o - 4 + 16% 2.482 X i o - 4 + 1% 3.810 X i o - 4 + 7% 9.125 X i o - 4 + 7% 0.20 1.815 X i o - 4 + 28% 2.765 X i o - 4 + 3% 4.179 X i o - 4 + 6% 1.010 X i o - 3 + 4% 0.30 1.391 X i o - 4 + 19% 2.707 X i o - 4 + 8% 4.177 X i o - 4 + 5% 1.085 X i o - 3 + 3% 0.40 7.702 X IO - 5 + 29% 2.376 X i o - 4 + 13% 3.744 X IO"4 + 8% 1.116 X IO - 3 + 4% 0.50 - - - 1.061 X IO - 3 + 9% - 204 -Temperature (°C) 950 900 875 850 -6.0 —7.0 5 c -8.0 — 9.0 8.0 1 — r T Highvale char C0/C02«0.25 8.4 I04/T (K_l) 83 Figure 8.1 Arrhenius plot for the average pseudo rate constant, k'; conditions as shown - 205 -Figure 8.2 Arrhenius plot for the average pseudo rate constant, k'; conditions as shown - 206 -Figure 8.3 Arrhenius plot for the average pseudo rate constant, k'; conditions as shown - 207 -Temperature (•C) Figure 8.4 Arrhenius plot for the average pseudo rate constant, k'; conditions as shown - 208 -Temperoture CO 950 900 875 850 I I 1 I L 8.0 8.4 8.8 IO4/ T ( K~') Figure 8.5 Arrhenius plot for the average pseudo rate constant, k1; conditions as shown - 209 -Temperature (°C) 950 900 875 850 I 0 4 / T ( K H ) Figure 8.6 Arrhenius plot for the average pseudo rate constant, k'; conditions as shown - 210 -950 1 Temperature (*C) 900 875 850 r I04/T(K"') Figure 8.7 Arrhenius plot for the average pseudo rate constant, k'; conditions as shown - 211 -Temperature (°C) 950 900 875 850 8.0 8.4 8.8 I04/T(K"') Figure 8.8 Arrhenius plot for the average pseudo rate constant, k'; conditions as shown - 212 -TABLE XXI APPARENT ACTIVATION ENERGIES FOR HIGHVALE AND SMOKY TOWER CHARS E(kJ/mole) Highvale char Smoky Tower char C 0 / C 0 2 = o f p C0 2 = 0.50 atm C0 /C0 2 = 0.25 C 0 / C 0 2 = 0.50 C0 /C0 2 = 0.25 0.05 - 213 273 210 0.10 - 213 247 201 0.20 151 221 250 196 0.30 157 239 276 231 0.40 162 258 319 292 0.50 164 295 348 -0.60 - 318 - -E (kJ/mole) o 143 210 255 202 (kJ/mole) 44 296 654 5483 C 2 1.00 1.92 2.75 4.48 - 213 -0 20 40 60 % Conversion Figure 8.9 Plot of apparent activation energy as a function of percent carbon conversion and C0/C0 2 ratio for Highvale char using the equation E = E Q + Cif°2 - 214 -Figure 8.10 Plot of apparent activation energy as a function of percent carbon conversion for Smoky Tower char using the equation E = E Q + c i f C 2 - 215 -Figure 8.11 Initial apparent activation energy as a function of CO/CO ratio for Highvale char - 216 -char, at each inlet CO/CO2 ratio and different fractional conversions. These plots are presented in Figures 8.1 to 8.8 for the various conditions given. The values of the apparent activation energy, E, as a function of the carbon conversion are given in Table XXI for all the conditions investigated. These activation energies were fitted by Equation [8.11] in order to determine the apparent activation energies E Q, corresponding to zero conversion, defined as the apparent activation energies for the chars under study. This equation provided a good f i t for the data as seen in Figures 8.9 and 8.10. The values of E 0, ci and c 2 obtained are given in Table XXI. As indicated in this table, the apparent activation energies for the Boudouard reaction with Highvale coal chars are about 143,210 and 255 kj/mole for inlet C0/C0 2 ratios equal to 0, 0.25 and 0.50 respectively, while for Smoky Tower char the apparent activation energy is about 202 kj/mole for a C0/C0 2 ratio equal to 0.25. Those values are well within the range reported in previous works. The dependence of the apparent activation energy on the CO/C02 ratio in the inlet gas, illustrated in Figure 8.11, is thought to be due to the nature of the CO poisoning effect, reflected in the kinetics law employed in place of the LH equation, as will be discussed in Section 8.4. 8.2 Relationship Between Changes in Surface Area and Gasification  Kinetics Several equations have been proposed to relate the changes in the surface area of char particles undergoing gasification with the extent of reaction. Two of these equations were employed to correlate the - 217 -pseudo rate constant k' with the fractional conversion of carbon via the parameter a, according to Equation [8.9]. 2 1 As discussed in Chapter 3, Agarwal and Sears have modified the 3 4 empirical equation proposed by Dutta et al. to relate the changes in the available surface area per unit weight of the char particles with conversions up to 90%. For the case where a maximum in surface area is observed, the following equation was suggested. S vg a = = 1 + Af exp(-Bf) [8.16] s w where the parameters A, v and 3 are defined as in Section 3.3 and S^, and S° are respectively the surface area per unit weight of the chars at any conversion and the initial surface area per unit weight of the chars. Re-arranging Equation [8.16] the following relationship is obtained. £n(a-l) = 3(v Jtnf-f) + £nA [8.17] thus i f the value of v, the fractional conversion corresponding to a maximum in a and, hence, in S„, is obtained from Figure 6.10 for Highvale and Smoky Tower chars (soak time 10 hours, gasification temperature 900°C), the linear plots of £n(a-l) versus (v£nf-f) allow the determination of the parameters 3 and A from the slopes and intercepts respectively. These plots are presented In Figures 8.12 for Highvale chars and 8.13 for Smoky Tower chars. Tables XXII and XXIII present all the structural parameters for Highvale and Smoky Tower chars - 218 -Figure 8.12 Plot of fcn(a-l) versus v£rif-f for Highvale char - 219 -- 0 . 9 0 -0 .85 - 0 . 8 0 - 0 . 7 5 V Inf - f Figure 8.13 Plot of £n(a-l) versus v£nf-f for Smoky Tower char TABLE XXII STRUCTURAL PARAMETERS FOR HIGHVALE CHARS Charring time 10 hours Gasification temperature 900°C f S(m2/g) v Jlnf-f a Jln(a-l) -£n(l-f ) ST(m2) [ST/S°(l-f)]2 0 61.3 - 1 - 0 2452 1 0.214 128.2 -1.2470 2.0914 0.0875 0.2408 4198.55 4.746 0.379 159.4 -1.0290 2.6003 0.4702 0.4764 4364.37 8.215 0.680 187.7 -0.9384 3.0620 0.7237 1.1394 3282.87 17.505 0.802 164.4 -0.9498 2.6819 0.5199 1.6195 2209.54 20.712 Dutta et al. v - 0.67 corr = 0.9931 A = 13.14 & = 2.005 Bhatia and Perlmutter corr = 0.9997 ip = 14.4 = 14 TABLE XXIII STRUCTURAL PARAMETERS FOR SMOKY TOWER CHARS Charring time 10 hours G a s i f i c a t i o n temperature 900°C S(m2/g) v i n f - f a An(a-l) - A n ( l - f ) S T(m 2) [S T/S°(1-f)] 2 0 20.2 - 1 - 0 404 1 0.3452 80.0 -0.7972 3.9604 1.0853 0.4234 1135.20 18.415 0.4545 80.6 -0.7896 3.9901 1.0953 0.6061 1051.02 22.744 0.7525 70.2 -0.8734 3.4752 0.9063 1.3963 640.93 41.087 Dutta et a l . v = 0.425 corr = 0.9994 A = 18.35 B = 2.293 Bhatia and Perlmutter corr = 0.9941 • = 36.8 * 37 - 222 -respectively. For Highvale chars, the value corresponding to 80% was neglected since, according to Dutta et a l . , 3 4 a drastic change in surface area takes place when complete conversion is approached. These authors have observed in a TGA study that the dimensions of non-caking coal particles remain practically unchanged up to a conversion of about 80%. As the reaction proceeds further the particles disintegrate into smaller fractions. However in fluidized bed reactors, due to solids attrition, the disintegration of the particles possibly occurs at conversions lower than 80% as indicated in Figure 7.28.p. Equation [8.17] provided a good linear f i t if the point corresponding to 80% conversion is neglected. The second equation used is based on the random pore model of Bhatia and Perlmutter for chemically controlled reactions,11"' discussed in Section 3.4 and expressed as. S a = -1 = (1-f) / l - i|»£n(l-f) [8.18] S° v where i> is a structural parameter given by the i n i t i a l physical properties of the chars, being constant for a given char, and S^  and S° are the surface area per unit volume of the chars at any conversion and the i n i t i a l surface area per unit volume of the chars respectively. Equation [8.18] was modified based on the fact that surface areas per unit volume can be expressed in terms of surface areas per unit weight as: - 223 -S - - JEga i • S [8.19] v V w o S° = . S ° [8.20] V „o w where m , , V and m°, , V° are r e s p e c t i v e l y the mass and volume of the char char char bed at any conversion and at zero conversion. Neglecting the changes i n volume of the char bed, which i s reasonable for moderate conversions: S m , • S Sm „o , v char w T V = V and — = = — b m , • b b_ v char w T o where S T and S T are re s p e c t i v e l y the t o t a l surface area of the chars at any conversion and the i n i t i a l t o t a l surface area of the chars. Equation [8.18] i s , then, written as. or = (1-f) A - i|>£n(l-f) [8.21] s [ ] 2 = 1 - + 4n(l-f) [8.22] s ° ( i - f ) - 224 -Therefore a linear relationship between [ ] and -£n(l-f) is s T ( i - f ) obtained as shown in Figure 8.14 for both Highvale and Smoky Tower chars. The values of the structural parameter, are determined from the slopes of these plots and are given in Tables XXII and XXIII. The point corresponding to 80% conversion, for Highvale chars, and the point corresponding to 75% conversion for Smoky Tower chars were neglected. For these relatively high conversions, the assumption of constant bed 13 7 volume is not likely to be valid. Moreover, Su and Perlmutter have used only data up to 60% conversion when applying the Bhatia and Perlmutter model for the kinetics of char oxidation. The relationships between k' and f were then obtained by fitting the Equation [8.9] with the two correlations adopted for a, using the structural parameters determined as indicated above, for Highvale chars with C0-C02 ratios of 0.25 and 0.50 and for Smoky Tower chars. The results are presented in Figures 8.15 to 8.17. It is seen that, for Highvale chars, the equation proposed by Bhatia and Perlmutter fits the data better at 850, 875 and 900°C than the equation proposed by Dutta et al. while for 950°C, the opposite is true. For Smoky Tower chars, the equation of Bhatia and Perlmutter always fits the values of k' better. This behaviour will be discussed in Section 8.4. For the cases where each equation provides the best results, a quite satisfactory f i t is obtained. - 225 -Figure 8.14 Plot of [S T /S°T(1-f)] 2 versus - £n(l-f) for Highvale and Smoky Tower chars - 226 -Figure 8.15 Plot of average pseudo rate constant, k', versus percent carbon conversion using the equations of Bhatia and Perlmutter and Dutta et al.; conditions as shown - 227 -2.0 1.5 io O o I E 1.0 0.5 0 ~ i — i — i — i — r — — Bhotla B Perlmutter Dutta et al \ Highvale char CO/C0 2 B0.50 T (°C) O 850 875 a 900 o 950 4 0 % Conversion 80 Figure 8.16 Plot of average pseudo rate constant, k', versus percent carbon conversion using the equations of Bhatia and Perlmutter and Dutta et al.; conditions as shown - 228 -Smoky Tower chor 0 20 40 60 % Conversion Figure 8.17 Plot of average pseudo rate constant, k', versus percent carbon conversion using the equations of Bhatia and Perlmutter and Dutta et al.; conditions as shown - 229 -8.3 Langmuir-Hinshelwood K i n e t i c s In order to investigate if the LH equation is actually followed by the experimental data as a l l the evidence seems to indicate, a different approach was followed in dealing with Equation [8.3], The function 0^PCO»PCO2^ w a s ma&e equal to a second pseudo rate constant, k", as follows. 0 ( P CO' p c o 2 ) - k " [ 8 ' 2 3 ] Thus the reaction rate is expressed as, r = k"a [8.24] The pseudo rate constant k" depends on temperature, gas composition and carbon conversion. However, in order to simplify the calculations and, keeping in mind that the objective here is to demonstrate the applicability of the LH equation, k" initially was assumed to be independent of the conversion but s t i l l a function of temperature and inlet gas composition. When the fractional conversion is zero the parameter a is, by definition, equal to one and the in i t i a l rate of reaction, r Q , is equal to k". r = k" [8.25] o Equation [8.24] was fitted for the case of Highvale chars with inlet CO/CO2 ratios equal to 0.25 and 0.50. Based on the results obtained in the previous section, the parameter a was given by the Bhatia and Perlmutter equation at 850, 875 and 900°C and by the Dutta et al. - 230 -equation at 950°C. The curves obtained are given in Figures 8.18 to 8.27 for the different conditions of temperature and inlet gas composition indicated. It is seen in these figures that the fits obtained are satisfactory in most of the cases. As mentioned earlier the i n i t i a l rates of reaction, r D , are equal to k", I.e., the fitting parameter in Equation [8.24]. The validity of the LH equation for initial rates of reaction then was verified by combining Equations [8.25], [8.23] and [8.4] and rearranging the resulting equation as follows: ro K l PC0 2 1 P C 0 2 1 Thus linear relationships should be obtained between r - 1 and p"1 for o C 0 2 each PCo/PC0 2 ratio used at different temperatures. These lines should be parallel at each temperature, and have different intercepts. Moreover the lines corresponding to lower PCo/PC02 ratios should have smaller intercepts since the rates are higher. Figure 8.28 shows the plots of the inverse of the initial reaction rates versus the inverse of the inlet partial pressures of C0 2 for the two CO/C02 ratios and the four temperatures investigated. The results illustrate that indeed the initial rates of reaction follow the LH equation. Figure 8.28 also shows clearly the inhibiting effect of CO by the difference between the lines corresponding to p /p = 0.25 and 0.50. It is CO C0 2 seen that this effect is very strong at 850°C but decreases with increasing temperature. - 231 -Figure 8.18 Plot of reaction rate versus percent carbon conversion using the equation of Bhatia and Perlmutter; conditions as shown - 232 -Figure 8.19 Plot of reaction rate versus percent carbon conversion using the equation of Bhatia and Perlmutter; conditions as shown - 233 -Figure 8.20 Plot of reaction rate versus percent carbon conversion using the equation of Bhatia and Perlmutter; conditions as shown - 234 -i — i 1 r % Conversion Figure 8.21 Plot of reaction rate versus percent carbon conversion using the equation of Bhatia and Perlmutter; conditions as shown - 235 -Figure 8.22 Plot of reaction rate versus percent carbon conversion using the equation of Bhatia and Perlmutter; conditions as shown - 236 -Figure 8.23 Plot of reaction rate versus percent carbon conversion using the equation of Bhatia and Perlmutter; conditions as shown - 237 -Figure 8.24 Plot of reaction rate versus percent carbon conversion using the equation of Bhatia and Perlmutter; conditions as shown - 238 -Figure 8.25 Plot of reaction rate versus percent carbon conversion using the equation of Bhatia and Perlmutter; conditions as shown - 239 -20 15 o X o> o c o o o o > or Highvale char T* 9 50°C PCO PC02 (atm) (atm) o 0.04 0.16 • 0.0 8 0.32 — A 0.1 2 0.48 o 0.1 6 0.64 6 -T o o -1 0 0 20 40 % Conversion 60 80 Figure 8.26 Plot of reaction rate versus percent carbon conversion using the equation of Dutta et al.; conditions as shown - 240 -20 o X * c o u o or I 5 10 Highvale char T « 9 5 0 ° C Pco Pco 2 (atm) (atm) o 0.10 0.20 O 0.15 0.30 0.25 0.50 O 0.30 0.6 0 J L 20 40 % Conversion 60 80 Figure 8.27 Plot of reaction rate versus percent carbon conversion using the equation of Dutta et al.; conditions as shown - 241 -Figure 8.28 Plot of inverse i n i t i a l reaction rate versus inverse inlet partial pressure of C02; conditions as shown - 242 -An attempt was made to estimate the rate constants , k 2 and k3 as well as to determine their apparent activation energies. The kj were found, at each temperature, from the slopes and intercepts of the straight lines in Figure 8.28, by applying Equation [8.26]. Table XXIV shows the values obtained. It is seen that k^  and k 2 follow the expected temperature dependence, i.e., kj increases and k 2 decreases with temperature. The substantial drop in k 2 with increasing temperature should be particularly observed since this rate constant multiplies PQQ in the denominator of the LH equation and, therefore, gives an indication of the retarding effect of CO. On the other hand, k 3 varies randomly with temperature probably due to errors In the i n i t i a l rates. Therefore the apparent activation energies were determined only for kj and k 2 from the Arrhenius plots presented in Figure 8.29. The values obtained were 176 and 286 kj/mole respectively. The apparent activation energy for k 1 was determined utilizing only values of ln kj corresponding to 850, 875 and 900°C since, as observed in Figure 8.29, there is a pronounced deflection in the Arrhenius plot above 900°C, with a corresponding lowering of the apparent activation energy. This may be caused either by significant pore diffusion effects at these temperatures or, again, by errors in the values of the initial reaction rates. The first alternative would provide an explanation for the fact that the equation of Bhatia and Perlmutter does not f i t the data at 950°C, as will be discussed in the next section. The Arrhenius plot for k2 does not exhibit this behaviour. Finally k" was calculated at different conversions for Highvale - 243 -TABLE XXIV LANGMDTR-HINSHELWOOD RATE CONSTANTS FOR HIGHVALE CHARS T(°C) kL (s- 1 atm"1) k 2 (atm-1) k 3 (atm-1) 850 6.632 x IO"4 19.981 1.158 875 1.091 x IO"3 13.459 2.644 900 1.477 x IO"3 7.031 1.388 950 1.783 x IO"3 1.719 1.943 Figure 8.29 Arrhenius plots for the rate constants and k 2 of the Langmuir-Hinshelwood rate equation - 245 -chars with CO/CO2 equal to 0.25 and 0.50 by computing the ratio between the reaction rate and the value of a at each conversion, obtained as in the previous case, according to Equation [8.24]. The rate constants , k 2, and k 3 then were calculated by combining Equations [8.23] and [8.4] and fitting the resulting equation using the UBC Computing Centre 154 routine NL2S0L for non-linear functions. The partial pressures of CO and C02 used were the values measured at each conversion where k" was calculated. However this procedure provided totally random values of ki , k2 and k3 , i.e., without following any meaningful pattern with respect to temperature and conversion. 8.4 Discussion The apparent activation energy for the gasification reaction increases with carbon conversion as shown in Figures 8.9 and 8.10. This 92 behaviour also was observed by Tomita et al. for the reaction of chars with hydrogen under pressure and has been attributed^ 2* 1 4 1 to a reduction in diffusional control, when the reaction is limited by combined effects of pore diffusion and chemical reaction, and/or a decrease in the catalytic activity of ash constituents with increasing conversion. In the first case, with the progress of gasification and the opening of the structure, there is a decrease in the diffusional resistance for the reactant gas with a corresponding decrease in the pore diffusion influence and an increasing importance of chemical reaction on the overall control mechanism. The higher activation energy of the chemical reaction step is, therefore, responsible for the increase in the apparent activation energy with conversion. In the - 246 -second case, the catalytic activity of the ash constituents decreases for the various reasons discussed in Chapter 7 so that the apparent activation energy progressively increases. In addition, the increase in apparent activation energy with conversion also may have been caused by the retarding effect of carbon monoxide as will be discussed. The ini t i a l values of apparent activation energy presented in Table XXI, indicate that for both chars the rate is likely to be mainly chemically controlled. This finding confirms that the effect of external mass transfer on the overall reaction rate for the experiments analyzed (20.0 g of char, 10 1/min) was minimal, as discussed in the previous chapter. The variation of apparent activation energy with inlet CO/CO2 ratio for Highvale chars, presented in Figure 8.11, suggests that when the concentration of CO in the gas phase increases, chemical reaction is the rate limiting step. This dependence of the apparent activation energy on the CO/CO2 ratio is caused by the nature of the CO poisoning effect, i.e., the retardation of the reaction due to this effect decreases with an increase in temperature. Figure 8.9 shows the predominance of the chemical over the pore diffusion step for high CO/CO2 ratios at any fraction of carbon converted. Similarly Tien and 118 Turkdogan have concluded that with increasing CO concentration, incomplete internal burning (pore diffusion effects) occurs at higher temperatures and with larger particles. In addition, CO may inhibit the 21 catalytic activity of ash constituents. Figure 8.9 shows clearly that the increase in apparent activation energy with the fraction of carbon - 247 -converted is less pronounced when the inlet CO/C02 ratio is 0, which reveals that CO is effective in inhibiting the reaction and/or decreasing pore diffusion and/or catalytic effects and, thus, in raising the apparent activation energies under the conditions investigated in this study. The results presented in Figures 8.15 to 8.27 indicate that the maximum in the reaction rate versus conversion curve is shifted to higher conversions when the temperature increases. The same behaviour was observed by Mehrotra and Brimacombe155, in research conducted in this department, and may be explained as follows. The rate of the char-C02 reaction up to a certain conversion is mainly affected by the changes in available surface area and by the poisoning effect of CO. At low temperatures the increase in rate due to the development of surface area is hindered by the strong retarding effect of CO, as shown in Figures 8.15 to 8.25. In the early stages of reaction, the rate increases with conversion due to the development of surface area; however, after a relatively low conversion is reached the CO action offsets this effect causing a drop in the rate. This fact may explain the premature decline in rate observed at low temperatures that would be hardly justified by catalytic, morphological or elutriation effects. With increasing temperature, the intrinsic reactivity of the chars is higher and the inhibiting effect of CO becomes less important, thus the surface formation is enhanced which causes the shift in the maximum rate to higher values of conversion as shown in Figures 8.15 to 8.27. These phenomena contribute to the increase of apparent activation energy with 21 carbon conversion. On the other hand it was found in a TGA study that - 248 -pore development i s independent of the presence of CO in the gas phase for l i g n i t i c , sub-bituminous and bituminous coals. However, the high degree of mixing and improved gas-solid contacting in the f l u i d i z e d bed reactor may enhance the poisoning e f f e c t of CO, which takes place by a chemical mechanism. The rate of reaction i s not, i n general, proportional to the apparent t o t a l surface area as given by N 2 adsorption at 77 K, but only to a f r a c t i o n of t h i s area c a l l e d the active surface area. This fact s u b s t a n t i a l l y complicates any attempt to c o r r e l a t e v a r i a t i o n s i n the r e a c t i o n rate with changes in surface area occurring during the g a s i f i c a t i o n reaction. Accordingly no conclusive quantitative c o r r e l a t i o n has been obtained for t h i s purpose. The v a r i a t i o n of the r e a c t i o n rate with the extent of reaction i s often explained in terms of surface area changes, the rate increasing due to pore opening and growth and then decreasing due to pore coalescence. However t h i s explanation o v e r s i m p l i f i e s the g a s i f i c a t i o n process. Furthermore the sharp decline i n rate observed at high conversions cannot be explained s o l e l y by surface area changes. Thus the observed change i n reaction rate with the f r a c t i o n of carbon g a s i f i e d r e s u l t s from the combined e f f e c t s of the phenomena described at the beginning of t h i s chapter and discussed i n Chapter 7. From t h i s perspective, the l i m i t a t i o n s of the equations employed to r e l a t e the v a r i a t i o n s in k' and reaction rate with the changes i n surface area during g a s i f i c a t i o n can be f u l l y appreciated. Both equations represent the case where the reaction i s chemically - 249 -co n t r o l l e d ; therefore, they do not allow for the influence of temperature and gas composition on surface area development. Dutta et 34 a l . have determined experimentally that these e f f e c t s were relevant only for caking coals due to the i r swelling and agglomeration. It should be pointed out that these authors have worked under conditions where PQQ was n e g l i g i b l e . Thus i f the reaction rate i s assumed to be proportional to the surface area, i t would change with conversion presenting a maximum at the same l e v e l i r r e s p e c t i v e of temperature. In t h i s case, the a c t i v a t i o n energy obtained would be independent of the carbon conversion. Therefore the equations proposed would f i t the data only f o r chemically controlled reactions, i . e . , at low temperatures. Considerable deviations would occur when d i f f u s i o n a l and c a t a l y t i c e f f e c t s are s i g n i f i c a n t . In addition the poisoning e f f e c t of CO should be included i n these r e s t r i c t i o n s . It can be observed i n Figure 6.10 that the maximum i n surface area development for Smoky Tower chars p a r t i a l l y reacted i n CO-C02 atmospheres occurs around 43% conversion which i s approximately the l e v e l where k' reaches the maximum at 950°C as shown i n Figure 8.17. This may suggest that the g a s i f i c a t i o n rate of th i s char i s governed by surface area changes and the poisoning e f f e c t of CO i s p r a c t i c a l l y n e g l i g i b l e at t h i s temperature. In addition the equation of Bhatia and Perlmutter f i t s the experimental data reasonably well at any temperature, as seen i n Figure 8.17, which indicates that the g a s i f i c a t i o n of Smoky Tower char was subjected mainly to chemical c o n t r o l , as concluded e a r l i e r . - 250 -For Highvale chars (soak time 10 hours) p a r t i a l l y g a s i f i e d i n C0-C0 2 atmospheres, Figure 6.10 shows that the maximum i n surface area takes place at about 68% conversion. In t h i s case, however, scatte r i n the rate data at these conversion l e v e l s and above, at 950°C, observed i n Figures 8.26 and 8.27, makes i t more d i f f i c u l t to e s t a b l i s h where the rate i s a maximum. Nevertheless i t can be seen that the maximum occurs at conversions roughly equal to t h i s value (68%), or s l i g h t l y above i t . This i s better i l l u s t r a t e d i n Figures 8.15 and 8.16 since the average pseudo rate constant k' i s expected to be more accurate than the i n d i v i d u a l rate data. However the equation of Bhatia and Perlmutter provides a poor f i t f o r the data at t h i s temperature which may i n d i c a t e stronger pore d i f f u s i o n e f f e c t s . Pore d i f f u s i o n e f f e c t s at 950°C f or these chars would be j u s t i f i e d by both t h e i r higher r e a c t i v i t y , as compared to Smoky Tower chars, and by the small CO i n h i b i t i o n at t h i s temperature. These e f f e c t s also were suggested when the LH equation was used as shown i n Figure 8.29, although i n t h i s case, the parameters obtained may involve a sub s t a n t i a l degree of uncertainty. On the other hand, the corresponding rate data was s a t i s f a c t o r i l y f i t t e d with the equation of Dutta et a l . also assuming chemical c o n t r o l . In addition when pore d i f f u s i o n retardation a c t u a l l y occurs, the increase i n rate (and surface area) i s apparently less at a c e r t a i n conversion which, ignoring CO i n h i b i t i o n at 950°C, would tend to s h i f t the maximum i n reaction rate to lower conversions. Therefore i t i s not completely c l e a r i f the g a s i f i c a t i o n of Highvale chars at 950°C, i s mainly c o n t r o l l e d by chemical reaction or by pore d i f f u s i o n . At 850, 875 and - 251 -900°C both the good f i t obtained with the equation of Bhatia and Perlmutter and the LH equation suggest that the rate is mostly chemically controlled. Finally, as in many of the studies performed on the kinetics of the Boudouard reaction, the interpretation of the rate constants and activation energies obtained depends on the special case assumed for the LH equation, i.e., on the particular rate equation selected to represent the kinetics. The poisoning effect of CO increasing with a decrease in temperature may give rise to values of apparent activation energy higher than expected if the rate constant given by the equation adopted is not completely devoid of any CO influence. In this regard the only way to obtain unbiased values of apparent activation energies and rate constants is to either use LH kinetics or to work under conditions where PCO would be negligible; these would be suitable for weight loss but not for gas analysis methods of measuring the reaction rate. Nevertheless, in the case of the present work, the use of the LH equation would make the analysis of the experimental data a substantially more complicated task as shown in Section 8.3. In addition the use of this equation, even for ini t i a l rates of reaction and inlet gas composition, can provide results with a great deal of uncertainty although i t is acknowledged that the rates used were estimated by a procedure involving several restrictions. On the other hand, the use of the empirical rate equation r = kap" , with n assumed constant for each coal and determined from averages of certain kinetic situations that provided more reliable results, introduced other limitations. This approach made it possible to avoid - 252 -the complex i t ies that otherwise would a r i s e with the use of the LH equa t ion , but i t led to rate constants and apparent a c t i v a t i o n energies in f luenced by carbon monoxide. This i s shown by the dependency of k' on the CO-C02 r a t i o and by the d i f f e r e n t values of i n i t i a l apparent a c t i v a t i o n energy obtained for s i m i l a r l y prepared Highvale chars g a s i f i e d with d i f f e r e n t C0/C02 r a t i o s . However i t i s considered t h a t , for the c h a r a c t e r i s t i c s of the system invest iga ted in t h i s work, the power-law rate equation i s the best a l t e r n a t i v e to represent the k i n e t i c s . - 253 -CHAPTER 9 SUMMARY AND CONCLUSIONS The k i n e t i c s of g a s i f i c a t i o n of two A l b e r t a sub-bituminous c o a l chars w i t h CO2 have been i n v e s t i g a t e d i n the temperature range of 800°-950°C. The reac t o r u t i l i z e d i n the experimental work was a l a b o r a t o r y - s i z e batch f l u i d i z e d bed. The o v e r a l l g a s i f i c a t i o n k i n e t i c s were followed by measurements of gas composition and flow r a t e s . Chars i n the p a r t i c l e s i z e -841 + 420 ym were g a s i f i e d w i t h d i f f e r e n t mixtures i n v o l v i n g CO, CO2 and He. The mo t i v a t i o n f o r t h i s study was the l a c k of k i n e t i c data f o r chars o r i g i n a t i n g from d i f f e r e n t types of Western-Canadian c o a l s , s u i t a b l e f o r the design and o p t i m i z a t i o n of va r i o u s i n d u s t r i a l m e t a l l u r g i c a l processes such as slag r e d u c t i o n . P r e l i m i n a r y g a s i f i c a t i o n experiments were performed aiming to determine s u i t a b l e values of i n i t i a l mass of char (bed depth) and t o t a l i n l e t flow r a t e , as w e l l as to i n v e s t i g a t e the e f f e c t of c h a r r i n g c o n d i t i o n s on the k i n e t i c s of the g a s i f i c a t i o n r e a c t i o n . These experiments y i e l d e d the f o l l o w i n g r e s u l t s : ( i ) Beds c o n t a i n i n g 20.0 g of char and a t o t a l i n l e t flow r a t e of 10 1/min were able to provide an adequate degree of mixing of the re a c t a n t s and near isothermal c o n d i t i o n s i n most of the experiments, thus minimizing the e f f e c t s of mass and heat t r a n s f e r . Moreover, these o p e r a t i n g c o n d i t i o n s allowed the measurement of r e a c t i o n r a t e s with minimum i n f l u e n c e of v a r i a b l e s such as C0 2 s t a r v a t i o n , e l u t r i a t i o n of char p a r t i c l e s and hydrodynamics of f l u i d i z a t i o n . These beds had a - 254 -static height-to-diameter ratio of about 0.25 and this flow rate was at least approximately two times the minimum fluidization flow rate required. Difficulties in measuring the rates of reaction at lower temperatures when CO is admitted to the inlet gas was the main disadvantage of the shallow beds employed. (i i ) The chars prepared with longer soak time and lower heating rate are less reactive. At 800°C, the ini t i a l reactivity of the chars prepared for one hour is about 1.3 times that of the chars prepared over two hours. At 900°C, chars prepared for two hours have initial reactivities approximately 1.2 times higher than chars prepared over ten hours. Similarly the specific surface area of samples of char prepared with soak times of two and ten hours are 87.1 and 61.3 m /g respectively. The surface areas of both chars increased about three times with conversion. However the maximum was reached at lower conversion for the char soaked for two hours, suggesting that this char has a more open pore structure. These differences in surface area are the main cause for the differences in reactivity of the chars. Having determined suitable values for i n i t i a l mass of char and total inlet flow rate as well as the effects of charring conditions, gasification experiments were carried out to determine the influence of coal type, gas composition and temperature on the kinetics of the Boudouard reaction. In addition, a SEM study was conducted on unreacted and partially gasified Highvale and Smoky Tower chars under different experimental conditions, with the objective of examining the external appearance of the char particles and the changes in pore structure and ash characteristics with the extent of reaction. - 255 -The following results were obtained. (i) The reactivity of Highvale chars was found to be higher than the reactivity of similarly prepared Smoky Tower chars. The values of in i t i a l specific surface area, 61.3 m Ig for Highvale and 20.2 m /g for Smoky Tower chars, as well as the fact that during gasification Highvale chars reached a maximum surface area about 2.3 times larger than the corresponding value for Smoky Tower chars, is considered to be the main cause for the higher reactivity. In addition, despite having a lower total ash content, the Highvale chars have a total alkali oxides (Na20 + K20) content in the ash equal to 3.69% against 0.92% for Smoky Tower chars. These compounds have been found to be effective catalysts for the Boudouard reaction. However, the K20 content of Highvale chars is lower than in the case of Smoky Tower chars and this oxide has been reported to be a stronger catalyst for the reaction. (i i ) The experimental results show that, for the conditions studied, an increase in Pco 2 i° t n e inlet gas causes a non-linear increase in the rate of reaction (for higher values of Pco 2 t n e increase in rate is lower), the reaction is strongly retarded by CO (the higher PCQ» the lower the rate, for the same values of other variables), and the poisoning effect of CO increases the lower is the temperature. These results suggest that the overall rate of reaction follows Langmuir-Hinshelwood kinetics. ( i i i ) The rate of reaction was determined to be strongly affected by the temperature at which the reaction takes place. Due to the poisoning effect of CO, the dependence of the reaction rate on temperature increases with increasing concentration of this gas. - 256 -(iv) SEM observations indicated that the char particles present a highly heterogeneous and porous structure representative of a wide pore size distribution, with pores ranging from large cracks or slits to very small cylindrical cavities, as well as highly dispersed ash inclusions. As the reaction proceeds the pores grow in size and large cavities are formed due to the collapse of the walls between adjacent pores, thus, increasing the porosity, i.e., opening the structure. The major contribution to the total porosity can be attributed to the expansion of macropores. Segregation of ash inclusions in agglomerates at high conversions also was observed, decreasing the degree of contact between the inclusions and the char matrix. Moreover no ash layer seems to be present around the char particles, suggesting that the ash flaked off from the particles. Therefore the catalytic activity of the ash constituents is likely to decrease with the progress of the reaction. Morphological changes, i.e., the decrease in size and disintegration of the particles with the advance of gasification also was in evidence. Finally, a rate equation was proposed to represent the kinetics of the reaction; the apparent activation energies for different experimental conditions were determined and the rate controlling steps comprising, the overall gasification process were established. In addition, the variation of the reactivity of the chars with the extent of reaction was determined by correlating the reaction rates with changes in surface area occurring in the char particles during gasification. The following conclusions can be drawn: (i) The ini t i a l rates of reaction follow the Langmuir-Hinshelwood equation. The apparent activation energies for the rate constants kj - 257 -and k 2 of this equation are respectively 176 and 286 kj/mole. However the use of the LH equation would complicate the analysis of the experimental data unduly. Moreover the use of this equation, even for in i t i a l rates of reaction and inlet gas compositions, involved several restrictions and provided results with a great deal of uncertainty. Therefore a simpler relationship was applied. (ii) The kinetics of the Boudouard reaction were characterized by a power-law rate equation involving a pseudo rate constant that took into account the variation in the specific surface area of the char particles during reaction. Regarding the degree of mixing of gas and solids, both phases were assumed completely mixed. The orders of reaction obtained for Highvale and Smoky Tower chars are 0.4 and 0.7 respectively. The proposed equation was able to describe the rate of reaction adequately, but i t gave rise to rate constants and apparent activation energies that were dependent on the fractional conversion of carbon and on the CO/CO2 ratio. However it is considered that, for the system studied, i t is the most suitable equation to represent the kinetics. ( i i i ) For the shallow beds and total inlet flow rate employed, the influence of external mass transfer was found to be minimal. Thus the gasification reaction was controlled by chemical reaction and pore diffusion within the ranges of the variables studied. (iv) The ini t i a l apparent activation energies for Highvale chars are 143, 210 and 255 kj/mole corresponding to C0/C02 ratios of 0, 0.25 and 0.50 respectively, while for Smoky Tower char the initial apparent activation energy is 202 kj/mole. These values are in good agreement - 258 -with published data, and indicate that the rate is mostly chemically controlled for both chars. (v) The increase in apparent activation energy with increasing CO/CO2 ratio, at any stage of reaction, supports the contention that chemical reaction is the rate controlling step. (vi) The increase in apparent activation energy with the fractional conversion of carbon is caused by the combined effects of increasing influence of the chemical reaction step on the overall controlling mechanism and decreasing catalytic activity of ash constituents with increasing conversion, and the CO poisoning effect. CO favors chemical control, may inhibit catalysis and causes the variation of the reaction rate with conversion to be dependent on temperature. In the absence of CO effects, if the rate were chemically controlled, it would change with conversion presenting a maximum at the same level irrespective of temperature. In this case the activation energy would be independent of conversion. (vii) The less pronounced increase in apparent activation energy with the fractional conversion of carbon when the inlet CO/C02 ratio is 0 reveals that CO is effective in inhibiting the reaction and/or in decreasing pore diffusion and/or catalytic effects and, hence, in raising the apparent activation energy under the conditions investigated. (viii) The reaction rate initially increases with the fraction of carbon gasified to reach a maximum and then drops with further conversion. Similar behaviour was found for the variation in the - 259 -specific surface area of the char particles with conversion, indicating that these phenomena are related. (ix) The maximum in the reaction rate versus conversion curve is shifted to higher conversions when the temperature Increases. This behaviour is considered to be due to the decrease in the poisoning effect of CO and the increase in the intrinsic reactivity of the char at higher temperatures. Both factors contribute to a higher development in surface area and reaction rate at these temperatures. (x) The kinetics data for both chars were fitted by the equations of Bhatia and Perlmutter and Dutta et al.. These equations correlate the changes in specific surface area with carbon conversion for the case of chemically controlled reactions. For Smoky Tower chars, the equation of Bhatia and Perlmutter fitted the experimental data reasonably well at any temperature, and always better than the equation of Dutta et al.. The satisfactory f i t further indicates that the gasification of these chars was mainly chemically controlled. For Highvale chars, the equation of Bhatia and Perlmutter fitted the data better than the equation of Dutta et al. at 850, 875 and 900°C, while at 950°C the opposite was true. Stronger pore diffusion effects may have occurred for these chars at 950°C which would be justified by their higher reactivity as compared to Smoky Tower chars and by the small CO inhibition at this temperature. At lower temperatures, the good f i t obtained with the equation of Bhatia and Perlmutter suggests that the gasification of Highvale chars was mostly chemically controlled. - 260 -LIST OF REFERENCES 1. Parkash, S., and du Plessis, M.P., "Reactivity of Char from Sub-Bituminous Coals," Proc. of the 32nd Canadian Chemical Engineering Conference, Vancouver, Canada, pp. 452-457, October (1982). 2. Sucre-Garcia, G.A., Kinetics of Reduction of Titaniferous Ores  with Lignite Coal, M.Sc. Thesis, The University of British Columbia, Vancouver, Canada (1979). 3. Richards, G.G., Kinetics of the Zinc Slag Fuming Process, Ph.D. Thesis, The University of British Columbia, Vancouver, Canada (1982). 4. 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Perry, R.H., and Chilton, C.H., eds., Chemical Engineers'  Handbook, Fifth Edition, McGraw H i l l , New York. - 273 -APPENDIX A CALIBRATION CURVES FOR INLET FLOWMETERS Scale reading Figure A.l C a l i b r a t i o n curve for He flow rate i n flowmeter Gilmont #3 (21°C and 1 atm) 1 I r I 1 1 1 r — r Figure A.2 C a l i b r a t i o n curve for He flow rate i n flowmeter Gilmont #2 (21°C and 1 atm) Scale reading Figure A.3 Calibration curve for C02 flow rate in flowmeter Gilmont #3 (21°C and 1 atm) i i 1 1 1 1 1 1 r Figure A.4 Calibration curve for CO flow rate in flowmeter Gilmont #2 (21°C and 1 atm) i 1 1 1 1 1 1 1 r Figure A.5 Calibration curve for Ar flow rate in flowmeter Gilmont #3 (21°C and 1 atm) A P P E N D I X B SUMMARY OF COAL C H A R R I N G E X P E R I M E N T S Run Coal Coal Coal particle Apparatus Gas and Temp Soak Average Weight type mass size (um) flow rate (°c) time heating rt. loss (g) U/min) (h) (°C/min) (%) CI H 200 -841 + 595 FB He, 10 950 + 4 1 7.9 42.9 C2 H 200 -841 + 595 FB He, 10 950 + 4 1 8.2 43.1 C3 H 300 -841 + 595 FB He, 10 950 + 6 1 8.1 43.5 C4 H 300 -841 + 595 FB He, 10 950 + 4 1 8.3 43.3 C5 H 300 -1190 + 595 FB He, 10 950 + 4 2 7.1 43.4 C6 H 200 -1190 + 595 FB He, 10 950 + 4 2 7.8 43.3 C7 H 200 -1190 + 595 FB He, 10 950 + 4 2 7.8 43.4 C8 H 120 -1190 + 595 FB He, 10 950 + 4 2 8.3 43.4 C9 H 120 -1190 + 595 FB He, 10 950 + 5 2 9.1 43.4 CIO H 120 -1190 + 595 FB He, 10 950 + 5 2 - 43.5 Cll H 120 -1190 + 595 FB He, 10 950 + 5 2 9.3 43.4 C12 H 120 -1190 + 595 FB He, 10 950 + 5 2 9.4 43.4 C13 H 120 -1190 + 595 FB He, 10 950 + 5 2 9.4 43.3 C14 H 120 -1190 + 595 FB He, 10 950 + 5 2 - 43.1 CI 5 H 120 -1190 + 595 FB He, 10 950 ± 5 2 9.3 43.4 C16 H 120 -1190 + 595 FB He, 10 950 + 5 2 9.2 43.6 C17 H 150 -1190 + 595 FB He, 10 950 + 5 2 - 43.4 C18 H 150 -1190 + 595 FB He, 10 950 + 5 2 9.0 43.1 C19 H 1000 -1190 + 595 GF Ar, 1.5 950 + 50 10 5.6 45.4 C20 H 1000 -1190 + 595 GF Ar, 1.5 950 + 50 10 5.5 45.7 C21 H 1000 -1190 + 595 GF Ar, 1.5 950 + 50 10 5.2 45.3 C22 ST 1000 -1190 + 420 GF Ar, 1.5 950 + 50 10 6.2 43.4 H = Highvale coal ST = Smoky Tower coal FB = Fluidized bed reactor GF = Gas fired furnace - 280 -APPENDIX C VALUES OF MINIMUM FLUIDIZATION VELOCITY AT 21°C The following equations were used to ca l c u l a t e the minimum f l u i d i z a t i o n v e l o c i t i e s . 1 4 9 pe * ( p c " p a ) * g ' d p A r = _S £ _S [I] Re , = (27.2 2 + 0.0408 A r ) 1 / 2 - 27.2 [II] mi Re • p , = m f 6 mf — d • p P g [III] The values of true density of the parent coals were u t i l i z e d f o r the true d e n s i t i e s of the respective chars: P c = 1.449 g/cm3 for Highvale c o a l / c h a r 1 5 0 P c = 1.426 g/cm3 for Smoky Tower c o a l / c h a r 1 5 0 The values of v i s c o s i t y f o r the gases, at the given temperatures, were obtained from reference 157. The values of minimum f l u i d i z a t i o n v e l o c i t y are given i n cm/s, at 21°C. Gas T ( ° C ) ' He C02 CC ) 800 Size ^\(ym) Char 841 631 Size ^ v ^ C u m ) Char 841 631 Size ym) Char 841 631 Highvale 4.48 2.53 Highvale 4.46 2.55 Highvale 4.71 2.68 Smoky Tower 4.41 2.49 Smoky Tower 4.40 2.51 Smoky Tower 4.64 2.64 850 Size Char 841 631 Size Char 841 631 size ym) Char 841 631 Highvale 4.10 2.31 Highvale 4.10 2.34 Highvale 4.41 2.51 Smoky Tower 4.04 2.28 Smoky Tower 4.03 2.30 Smoky Tower 4.34 2.47 900 Size Char 841 631 Size Char ^^s. 841 631 Size ^•vfym) Char 841 631 Highvale 3.85 2.17 Highvale 3.85 2.19 Highvale 4.04 2.30 Smoky Tower 3.79 2.13 Smoky Tower 3.79 2.16 Smoky Tower 3.98 2.26 950 Size Char 841 631 Size ^ V ^ m ) Char 841 631 Size Char 841 631 Highvale 3.62 2.04 Highvale 3.55 2.02 Highvale 3.80 2.16 Smoky Tower 3.56 2.01 Smoky Tower 3.50 1.99 Smoky Tower 3.74 2.12 - 282 -APPENDIX D DERIVATION OF THE EQUATIONS UTILIZED TO CALCULATE THE FRACTIONAL CONVERSION OF CARBON The fractional conversion of carbon was calculated by half the amount of CO formed as follows. The main chemical reaction taking place i s : C(char) + CO^g) = 2C0(g) From the stoichiometry of the reaction: volume of CO formed in At minutes =2. volume of C02 converted in At minutes = 2QCQ2 x At . . . ( £ ) where x is the fraction of C02 converted to CO. Then, ^co + 2 Q c o 9 x x co QC0 + QC0 2 + %e + QC0 2 X S i n c e QT " QC0 + QC0 2 + QHe QC0 + 2 ( V x x e 2 CO QT + QC()2 x and, x = QT XC0 " QC0 ( 2 " xco> %o2 - 283 -e QT x m ~ ^ r o The rate of CO formation is 2Q_n x = 2 ( — - — — — ) ... U/min) 1 1 x c o e 2 T^ XC0 *^C0 ( ) . . . (mole/min) at 21°C and 1 atm. 24.1 v e 1 ~ XC0 In addition the rate of carbon consumption is equal to half the rate < CO formation, hence: 1 T^ CO C^O rate of carbon consumption = —- ( ) . . . (mole/min) 24.1 _ e 1 x c o e 1 ,QTXCO ~ QC0, . . N ( "—) • • • (g/s) 120.5 v _ ve 1 x c o Thus the mass of carbon consumed after t seconds is: 120.5 (2 - X^) The fractional conversion of carbon is equal to the ratio between the mass of carbon consumed and the ini t i a l mass of carbon, hence: jt [ QT XC0-QC0 j d t ° 120.5 (2 - X^Q) ^ m - 284 -In terms of the amount of CO2 consumed the fraction of carbon converted was calculated as follows: C0„ Q c o 2 " ^ c o 2 x Q T + Q c o 2 x and, Q C 0 2 QT X C 0 2  Q c o 2 ( 1 + The rate of CO2 consumption is Q x = CO 2 Q C 0 2 QT X C 0 2 1 + X CO, . . . (£/min) . QC0, QT XC0„ ( : ~) 24.1 . . (mole/min) at 21°C and 1 atm. 1 + X CO, The rate of carbon consumption is equal to the rate of CO2 consumption, hence: j Q C 0 2 QT X C 0 2 rate of carbon consumption = —.—- ( ) . . . (mole/min) 24.1 e C0„ 1 120.5 JC0 2 QT X C 0 2 ) . . (g/s) 1 + X CO, - 285 -The mass of carbon consumed after t seconds is: . QC0„ QT XC0_ J t ( i J _ } d t . . . ( g ) 120.5 (1 + X ^ ) Finally the fractional conversion of carbon is given by: e ^ QC0„ " QT XC0„ l«  [ — 1 7— ] d t 120.5 (1 + X ) f = £ m o APPENDIX E SUMMARY OF GASIFICATION EXPERIMENTS o „ Mass balance Mass balance Run Char char T QT C^O on gases on C origin (g) (°C) (£/min) (Jt/min) ( Jt/min) (%) (%) Gl 1 160.0 800 10 2 - 1.0 -0.1 G2 1 40.0 800 10 2 - 4.7 -2.5 G3 1 160.0 850 10 2 - 5.2 0.8 G4 1 40.0 850 10 2 - 5.6 1.1 G5 1 80.0 850 10 2 - 5.9 -0.9 G6 2 40.0 800 10 1 - 4.4 -1.7 G7 2 40.0 800 10 2 - 4.0 -3.4 G8 2 40.0 800 10 3 - 2.6 -3.4 G9 2 40.0 800 10 5 - 5.5 5.6 G10 2 40.0 800 10 7 - 5.2 -1.1 Gil 2 40.0 800 10 10 - 2.0 -3.8 G12 3 40.0 900 10 1 - 1.1 -0.6 G13 3 40.0 900 10 1 - 3.0 -0.2 G14 3 40.0 900 10 3 - 0.4 5.7 G15 3 40.0 900 10 5 - 1.2 3.3 G16 3 40.0 900 10 7 - 0.2 1.6 G17 3 40.0 900 10 10 - 0.8 1.0 G18 3 80.0 900 10 10 - 0.1 3.1 G19 3 120.0 900 10 10 - 0.6 2.7 G20 4 40.0 900 20 2 - 4.9 -1.7 G21 5 20.0 900 10 5 - 4.1 0.9 G22 5 40.0 900 10 5 - 2.2 0.7 G23 5 60.0 900 10 5 - 1.4 0.8 G24 5 20.0 900 15 7.5 - 5.0 4.6 G25 5 20.0 900 15 5 - 5.3 3.3 0 QT QC0 2 QC0 Mass balance Mass balance Run Char char T on gases on C origin (g) (°C) (JL/min) (Jt/min) U/inin) (%) (%> G26 5 20.0 900 15 2.5 - 4.7 2.2 G27 5 20.0 850 10 1.6 0.4 5.6 2.9 G28 5 20.0 850 10 3.2 0.8 5.8 2.4 G29 5 20.0 850 10 4.8 1.2 3.1 2.4 G30 5 20.0 850 10 6.4 1.6 5.7 5.3 G31 5 20.0 850 10 2.0 1.0 3.4 4.3 G32 5 20.0 850 10 3.0 1.5 4.5 3.0 G33 5 20.0 850 10 5.0 2.5 2.1 3.6 G34 5 20.0 850 10 6.0 3.0 2.0 2.4 G35 5 20.0 875 10 1.6 0.4 3.4 2.6 G36 5 20.0 875 10 3.2 0.8 2.9 2.3 G37 5 20.0 875 10 4.8 1.2 3.2 5.1 G38 5 20.0 875 10 6.4 1.6 4.5 4.4 G39 5 20.0 875 10 2.0 1.0 3.8 3.4 G40 5 20.0 875 10 3.0 1.5 4.5 3.8 G41 5 20.0 875 10 5.0 2.5 4.6 3.5 G42 5 20.0 875 10 6.0 3.0 3.6 4.2 G43 5 20.0 900 10 1.6 0.4 3.0 3.0 G44 5 20.0 900 10 3.2 0.8 0.04 1.2 G45 5 20.0 900 10 4.8 1.2 4.2 4.9 G46 5 20.0 900 10 6.4 1.6 5.3 1.3 G47 5 20.0 900 10 2.0 1 .0 3.2 2.6 G48 5 20.0 900 10 3.0 1.5 2.9 4.9 G49 5 20.0 900 10 5.0 2.5 3.1 4.8 G50 5 20.0 900 10 6.0 3.0 1.3 4.6 Run Char origin o char (g) T (°C) QT ( fc/min) QC0 2 (fc/min) QC0 U/min) Mass balance on gases (%) Mass balance on C (%) G51 5 20.0 875 10 3.2 0.8 3.7 4.7 G52 5 20.0 900 10 6.4 1.6 2.7 5.0 G53 6 20.0 950 10 1.6 0.4 4.2 -0.3 G54 6 20.0 950 10 3.2 0.8 2.8 -1.2 G55 6 20.0 950 10 4.8 1.2 2.1 2.7 G56 6 20.0 950 10 6.4 1.6 3.2 1.4 G57 6 20.0 950 10 2.0 1.0 5.8 -1.0 G58 6 20.0 950 10 3.0 1.5 4.7 3.0 G59 6 20.0 950 10 5.0 2.5 3.7 4.0 G60 6 20.0 950 10 6.0 3.0 5.7 4.6 G61 6 20.0 850 10 5.0 - 5.1 0.5 G62 6 20.0 875 10 5.0 - 5.0 3.2 G63 6 20.0 950 10 5.0 - 4.3 -2.4 G64 7 20.0 850 10 3.2 0.8 2.1 4.6 G65 7 20.0 850 10 4.8 1.2 4.1 1.6 G66 7 20.0 850 10 6.4 1.6 0.5 3.5 G67 7 20.0 875 10 3.2 0.8 4.9 1.7 G68 7 20.0 875 10 4.8 1.2 5.7 3.1 G69 7 20.0 875 10 6.4 1.6 4.2 3.8 G70 7 20.0 900 10 3.2 0.8 5.7 2.5 G71 7 20.0 900 10 4.8 1.2 4.5 2.2 G72 7 20.0 900 10 6.4 1.6 4.3 4.2 G73 7 20.0 950 10 3.2 0.8 3.9 2.7 G74 7 20.0 950 10 4.8 1.2 2.5 4.0 G75 7 20.0 950 10 6.4 1.6 4.6 4.5 - 289 -APPENDIX F LISTING OF PROGRAM TO PROCESS GASIFICATION EXPERIMENTS DATA AND SAMPLE OUTPUT - 290 -L i s t i n g o f KINBR a t 1 1 : 2 2 : 3 5 on APR 2 5 . 1986 f o r CC1d=BETO 1 C EXPERIMENTS TO DETERMINE THE K INETICS OF THE BOUDOUARD 2 C REACTION FOR LOW-RANK WESTERN CANADIAN COALS. 3 C 4 C 5 C THIS PROGRAM CALCULATES THE COMPOSITION AND FLOW RATE OF THE EXIT 6 C GAS. THERMODYNAMICS AND KINETICS PARAMETERS (FRACTIONAL CONVERSION 7 C OF CARBON IN THE CHAR AND INSTANTANEOUS RATE OF REACTION) AND MASS 8 C BALANCES FROM THE EXPERIMENTAL DATA ON THE BOUDOUARD REACTION FOR 9 C CHARS OF SUB-BITUMINOUS WESTERN CANADIAN COALS. 10 C 11 C NOMENCLATURE AND UNITS. 12 C 13 C AIRSCT=STANDARD COUNTS OF A IR . 14 C AIRCT=COUNTS OF AIR IN THE EXIT GAS SAMPLES. 15 C ASH=FRACTION OF ASH IN THE CHAR. 16 C CFIX=FRACTION OF FIXED CARBON IN THE CHAR. 17 C COSCT = STANDARD COUNTS OF CARBON MONOXIDE. 18 C COCTA=APPARENT COUNTS OF CO IN THE EXIT GAS SAMPLES. 19 C C02SCT=STANDARD COUNTS OF CARBON DIOXIDE. 20 C C02CT=C0UNTS OF C02 IN THE EXIT GAS SAMPLES. 21 C C0C02E=CARB0N MONOXIDE TO CARBON DIOXIDE RATIO IN THE EXIT GAS. 22 C C0C02K=EQUILIBRIUM CO TO C02 RATIO AT THE CONDITIONS OF THE RUN. 23 C CMB=MASS BALANCE ON CARBON PERCENTUAL ERROR. 24 C EQK=EQUILIBRIUM CONSTANT FOR THE BOUDOUARD REACTION, [ a t m ] . 25 C F=FRACTIONAL CONVERSION OF CARBON IN THE CHAR. 26 C FTCO=TOTAL FRACTIONAL CONVERSION OF C CALCULATED BY HALF CO FORMED. 27 C FTC02=T0TAL FRACTIONAL CONVERSION OF C CALCULATED BY C02 CONSUMED. 28 C FTOT=TOTAL FRACTIONAL CONVERSION OF CARBON IN THE CHAR. 29 C GASMB=MASS BALANCE ON GASES (CO AND C02) PERCENTUAL ERROR. 30 C ITEMP=TEMPERATURE OF THE RUN. [ K ] . 31 C L=RUN NUMBER. 32 C N=NUMBER OF TIMES THAT MEASUREMENTS WERE MADE DURING THE RUN. 33 C 0C0=STANDARD (21C AND 1atm) INLET FLOWRATE OF CO. [ 1 / m i n ] . 34 C 0C02=STANDARD INLET FLOWRATE OF C 0 2 , [ 1 / m i n ] . 35 C QTI 'STANDARD TOTAL INLET FLOWRATE. [ 1 / m i n ] . 36 C QTE=STANDARD TOTAL EXIT FLOWRATE, [ 1 / m i n ] . 37 C RATE = INSTANTANEOUS RATE OF REACTION, [ g / g . s ] . 38 C TIME=TIME OF REACTION, [ m i n ] . 39 C WCHARO=MASS OF CHAR FED, [ g ] . 40 C WCHARF =MASS OF CHAR AFTER THE RUN, [ g ] . 41 C WO=MASS OF CARBON FED. [ g ] . 42 C WF=MASS OF CARBON AFTER THE RUN, [ g ] . 43 C WRTD=MASS OF CARBON REACTED, [ g ] . 44 C XCOST=STANDARD MOLE FRACTION OF CARBON MONOXIDE. 45 C XC02ST=STANDARD MOLE FRACTION OF CARBON DIOXIDE. 46 C XAIR=MOLE FRACTION OF AIR IN THE EXIT GAS SAMPLES. 47 C XCOIG=MOLE FRACTION OF CO IN THE INLET GAS. 48 C XCO=MOLE FRACTION OF CO IN THE EXIT GAS SAMPLES. 49 C XCOE=MOLE FRACTION OF CO IN THE EXIT GAS SAMPLES (WITHOUT A I R ) . 50 C XC02IG=M0LE FRACTION OF C02 IN THE INLET GAS. 51 C XC02=M0LE FRACTION OF C02 IN THE EXIT GAS SAMPLES. 52 C XC02E=M0LE FRACTION OF C02 IN THE EXIT GAS SAMPLES (WITHOUT A I R ) . 53 C XHEIG=M0LE FRACTION OF HE IN THE INLET GAS. 54 C XHE=MOLE FRACTION OF HE IN THE EXIT GAS SAMPLES. 55 C XHEE =MOLE FRACTION OF HE IN THE EXIT GAS SAMPLES (WITHOUT A I R ) . 56 C XC0EO=EOUILIBRIUM MOLE FRACTION OF CO AT THE CONDITIONS OF THE RUN. 57 C XC02E0=E0UILIBRIUM MOLE FRACTION OF C02 AT THE CONDITIONS OF THE RUN. 58 C - 291 -L i s t i n g o f KINBR a t 1 1 : 2 2 : 1 5 on APR 2 5 . 1986 f o r CC1d=BET0 59 C 60 C 61 C DECLARATION OF THE V A R I A B L E S . 62 C 63 IMPLICIT R E A L * 4 ( A - H . O - Z ) 64 INTEGER C O S C T , C O C T A , C 0 2 S C T , C 0 2 C T , A I R S C T , A I R C T , R C 65 DIMENSION X H E E ( 5 0 ) . X C O E ( 5 0 ) . X C 0 2 E ( 5 0 ) . C O C T A ( 5 0 ) . C 0 2 C T ( 5 0 ) 66 DIMENSION A I R C T ( 5 0 ) , X C 0 A P ( 5 0 ) . X H E ( 5 0 ) , X C O ( 5 0 ) , X C 0 2 ( 5 0 ) 67 DIMENSION Q T E ( 5 0 ) , W C O N ( 5 0 ) . F ( 5 0 ) , R A T E ( 5 0 ) . T I M E ( 5 0 ) , X A I R ( 5 0 ) 68 DIMENSION X C 0 2 E 0 ( 5 0 ) , X C O E O ( 5 0 ) , C 0 C 0 2 E ( 5 0 ) , C 0 C 0 2 K ( 5 0 ) 69 DIMENSION F 1 ( 5 0 ) , F 2 ( 5 0 ) , 0 1 ( 5 0 ) , 0 2 ( 5 0 ) . W 1 ( 5 0 ) , W 2 ( 5 0 ) 70 C 71 C READING AND WRITING OF DATA. 72 C 73 R E A D ( 5 , 9 2 ) N , L . Q T I . Q C O , Q C 0 2 74 R E A D ( 5 . 9 3 ) EQK. ITEMP 75 R E A D ( 5 , 9 4 ) WCHARO,CFIX,ASH 76 R E A D ( 5 , 9 5 ) XCOST.COSCT,XC02ST .C02SCT , AI RSCT 77 R E A D ( 5 , 9 6 ) WCHARF 78 92 F 0 R M A T ( 2 I 3 , 3 F 6 . 2 ) 79 93 F O R M A T ( F 8 . 4 , I 5 ) 80 94 FORMAT(F6. 1 . 2 F 7 . 4 ) 81 95 F 0 R M A T ( F 7 . 4 . I 7 . F 9 . 4 . 2 I 7 ) 82 96 FORMAT(F7 .2 ) 83 DO 1985 1=1,N 84 R E A D ( 5 , 9 7 ) T I M E ( I ) . C O C T A ( I ) , C 0 2 C T ( I ) . A I R C T ( I ) 85 97 F 0 R M A T ( F 6 . 1 , 1 7 , 2 1 9 ) 86 1985 CONTINUE 87 W R I T E ( 6 , 8 8 ) 88 WRITE(6 ,107 ) L 89 W R I T E ( 6 , 1 0 9 ) N ,OT I ,QCO,QC02 90 WRITE(6,-1 16) EQK. ITEMP 91 WRITE(6 .118 ) WCHARO,CFIX,ASH 92 W R I T E ( 6 , 1 3 0 ) X C O S T , C O S C T , X C 0 2 S T , C 0 2 S C T , A I R S C T 93 W R I T E ( 6 , 3 0 7 ) WCHARF 94 88 F O R M A T ( / / 3 6 X , ' D A T A ' / / ) 95 107 F O R M A T ( / / 2 5 X , ' E X P E R I M E N T A L DATA FOR RUN G ' . I 2 / / ) 96 109 F O R M A T ( 6 X , ' N = ' , I 3 / 6 X . ' Q T I = ' , F 5 . 1 , ' 1 / m 1 n ' / 6 X , ' Q C O = ' , F 4 . 1 . ' 1 / m i n ' / 97 1 6 X . ' Q C 0 2 = ' , F 5 . 1 . ' 1 / m i n ' / ) 98 116 FORMAT(6X, 'EQK= ' , F 8 . 4 , ' a t m ' / 6 X , 'I TEMP=' , 1 5 , ' K ' / ) 99 118 F 0 R M A T ( 6 X , ' W C H A R O = ' , F 6 . 1 , ' g ' / 6 X , ' C F I X = ' . F 7 . 3 / 6 X , ' A S H = ' , F 7 . 3 / ) 100 130 F O R M A T ( 6 X . ' X C O S T = ' , F 7 . 3 / 6 X , ' C O S C T = ' , I 6 / 6 X . ' X C 0 2 S T = ' . F 7 . 3 / 6 X , ' C 0 2 S C 101 1 T = ' . I 6 / 6 X , ' A I R S C T = ' , 1 6 / ) 102 307 F0RMAT(6X, 'WCHARF = ' , F 6 . 2 , ' g ' ) 103 W R I T E ( 6 , 5 5 ) 104 55 F 0 R M A T ( / 5 X , ' T I M E ( m i n ) ' , 5 X , ' COCTA' . 5 X , ' C 0 2 C T ' , 5 X , ' A I R C T ' / ) 105 DO 77 I = 1 , N 106 W R I T E ( 6 , 7 8 ) T I M E ( I ) , C O C T A ( I ) , C 0 2 C T ( I ) , A I R C T ( I ) 107 78 FORMAT(F12. 1 ,I 13 .2111 ) 108 77 CONTINUE 109 C 110 C CALCULATION OF THE EXIT GAS COMPOSTION AND FLOWRATES. 111 C 112 XCOE(1)=OCO/OTI 113 XC02E(1)=QC02/QTI 114 X H E E ( 1 ) = 1 , 0 - X C O E ( 1 ) - X C 0 2 E ( 1 ) 115 XCOIG = XC0E( 1 ) 116 XC02IG = XC02E( 1 ) - 292 -L i s t i n g o f KINBR a t 1 1 : 2 2 : 3 5 o n APR 2 5 , 1986 f o r CC i d = BETO 1 17 XHEIG=XHEE(1) 118 DO 110 1=2,N 1 19 X A I R ( I ) = ( A I R C T ( I ) * 1 . 0 ) / A I R S C T 120 X C O A P ( I ) = ( C O C T A ( I ) * X C O S T ) / C O S C T 121 XCO(I )=XCOAP(I ) - X A I R ( I ) 122 X C 0 2 ( I ) = ( C 0 2 C T ( I ) * X C 0 2 5 T ) / C 0 2 S C T 123 X H E ( I ) = 1 . 0 - X A I R ( I ) - X C O ( I ) - X C 0 2 ( I ) 124 X C O E ( I ) = X C 0 ( I ) / ( 1 . 0 - X A I R ( I ) ) 125 X C 0 2 E ( I ) = X C 0 2 ( I ) / ( 1 . 0 - X A I R ( I ) ) 126 XHE E ( I ) = X H E ( I ) / ( 1 .0-XA I R ( I )) 127 C 0 C 0 2 E ( I ) = X C O E ( I ) / X C 0 2 E ( I ) 128 1 10 CONTINUE 129 QTE(1 )=OTI 130 DO 13 1=2.N 131 Q 1 ( I ) = ( 2 . O * Q T I - Q C 0 ) / ( 2 . O - X C 0 E ( I ) ) 132 0 2 ( I ) = ( 0 C 0 2 + 0 T I ) / ( 1 . 0 + X C 0 2 E ( I ) ) 133 13 CONTINUE 134 C 135 C CALCULATION OF EQUILIBRIUM PARAMETERS. 136 DO 35 1=2.N 137 XC02EQ(I ) = (2 . 0 0 * ( 1 . O O - X H E E ( I ) ) + E Q K - S O R T ( 4 . 0 0 0 * ( 1 . O O - X H E E ( I ) )*EQK+E 138 1 Q K * * 2 ) ) / 2 . 0 0 139 XC0EQ(I )=SQRT(EQK*XC02EQ( I ) ) 140 C 0 C 0 2 K ( I ) = X C 0 E Q ( I ) / X C 0 2 E Q ( I ) 141 35 CONTINUE 142 C 143 C CALCULATION OF THE K INETIC PARAMETERS AND MASS BALANCES. 144 C 145 C NUMERICAL INTEGRATION PERFORMED BY UBC COMPUTING CENTRE ROUTINE QINT4P 146 C 147 WO=WCHARO*CFIX 148 DO 50 1=1,N 149 W1(I ) = ( 1 2 . 0 / 2 4 . 1 ) * ( ( Q T I * X C O E ( I ) - Q C O ) / ( 2 . 0 - X C O E ( I ) ) ) 150 W2(I ) = ( 1 2 . 0 / 2 4 . 1 ) * ( ( Q C 0 2 - Q T I * X C 0 2 E ( I ) ) / ( 1 . 0 + X C 0 2 E ( I ) ) ) 151 F 1 ( I ) = Q I N T 4 P ( T I M E , W 1 , I , 1 , I ) / W 0 152 F 2 ( I ) = Q I N T 4 P ( T I M E , W 2 , I , 1 , I ) / W 0 153 50 CONTINUE 154 C 155 C MASS BALANCE ON GASES. 156 C 157 C TOTAL CONVERSION OF CARBON: BY C02 CONSUMED=BY HALF CO FORMED. 158 C 159 I F ( F 1 ( N ) . L T . F 2 ( N ) ) GO TO 85 160 G A S M B = 1 0 0 . 0 * ( F 1 ( N ) - F 2 ( N ) ) / F 1 ( N ) 161 GO TO 86 162 85 G A S M B = 1 0 0 . 0 * ( F 2 ( N ) - F 1 ( N ) ) / F 2 ( N ) 163 C 164 C MASS BALANCE ON CARBON. 165 C 166 C CARBON: WO(SOLIDS)=WF(SOL IDS)+WRTD(GAS) 167 C (ASH IS ASSUMED INERT: WASHO=WASHF=WASH) 168 C 169 86 WASH=WCHARO*ASH 170 WF=WCHARF-WASH 171 WRTD1=W0*F1(N) 172 WRTD2=WO«F2(N) 173 CMB1=1OO.0*(W0-(WF+WRTD1))/W0 174 CMB2-1OO.O*(W0-(WF+WRTD2))/W0 - 293 -L i s t i n g o f KINBR a t 1 1 : 2 2 : 1 5 on APR 2 5 , 1986 f o r CC1d=BET0 175 I F ( A B S ( C M B 1 ) . G T . A B S ( C M B 2 ) ) GO TO 300 176 DO 450 1=2,N 177 0 T E ( I ) = 0 1 ( I ) 178 450 CONTINUE 179 DO 500 1=1,N 180 WCON(I)=W1(I ) 181 F ( I ) = F 1 ( I ) 182 5O0 CONTINUE 183 FTOT=F1(N) 184 WRTD=WRTD1 185 CMB=CMB1 186 RC=1 187 GO TO 600 188 300 DO 460 1=2,N 189 OTE( I ) =02( I ) 190 460 CONTINUE 191 00 550 I=1,N 192 WCON(I)=W2(I) 193 F ( I ) = F 2 ( I ) 194 550 CONTINUE 195 FT0T=F2(N) 196 WRTD=WRTD2 197 CMB=CMB2 198 RC = 2 199 C 20O C CALCULATION OF THE INSTANTANEOUS RATE OF REACTION. 201 C ( PER MASS OF CARBON REMAINING IN THE B E D - ) 202 C 203 600 DO B 1=1,N 204 RATE( I )=WCON( I ) / (W0*60 .0 * (1 . 0 - F ( I ) ) ) 205 8 CONTINUE 206 C 207 C WRITING OF THE RESULTS. 208 C 209 W R I T E ( 6 . 2 0 0 ) L 210 200 F 0 R M A T ( / / 3 0 X , ' R E S U L T S OF RUN G ' , I 2 / / ) 21 1 W R I T E ( 6 . 2 2 2 2 ) X C O I G , X C 0 2 I G , X H E I G 212 2222 FORMAT(6X, 'XCOIG= ' . F 5 . 2 / 6 X , ' X C 0 2 I G = ' . F 5 . 2 / 6 X , 'XHEIG= ' .F5 .2////7) 213 W R I T E ( 6 . 2 0 1 ) 214 201 F O R M A T ( 2 X , ' T I M E ( m l n ) ' , 3 X , ' X A I R ' , 5 X , ' X H E ' , 4 X , ' X H E E ' , 5 X , ' X C O ' . 4 X , 'XC 215 1 0 E ' , 4 X , ' X C 0 2 ' , 3 X . ' X C 0 2 E ' , 4 X , ' Q T E ( 1 / m l n ) ' / ) 216 DO 203 I=2,N 217 W R I T E ( 6 . 2 0 2 ) T I M E ( I ) . X A I R ( I ) , X H E ( I ) . X H E E ( I ) , X C O ( I ) , X C O E ( I ) , 218 1 X C 0 2 ( I ) , X C 0 2 E ( I ) , Q T E ( I ) 219 202 F O R M A T ( F 9 . 1 , F 9 . 4 , 6 F 8 . 4 , F 1 2 . 2 ) 220 203 CONTINUE 221 W R I T E ( 6 , 2 1 2 ) 222 212 F O R M A T ( / 2 X . ' T I M E ( m l n ) ' . 5 X , ' X C O E Q ' , 5 X , ' X C 0 2 E Q ' , 5 X , ' C 0 C 0 2 E ' , 5 X , 'COCO 223 1 2 K ' , 8 X , ' F ' , 6 X , ' R A T E ( g / g . s ) ' / ) 224 DO 214 1=2,N 225 W R I T E ( 6 , 2 1 3 ) T I M E ( I ) . X C O E Q ( I ) , X C 0 2 E Q ( I ) , C 0 C 0 2 E ( I ) , C 0 C 0 2 K ( I ) , 226 1 F ( I ) , R A T E ( I ) 227 2 13 F O R M A T ( F 9 . 1 , F 1 2 . 4 . F 1 1 . 4 , 2 F 1 1 . 3 . F 1 2 . 4 , E 1 5 . 3 ) 228 2 14 CONTINUE 229 WRITE(6 ,2 18) FTOT.WO.WF,WRTD,GASMB.CMB 230 2 18 F 0 R M A T ( / 2 4 X , 'FTOT= ' . F 7 . 4 / 2 4 X , 'W0=' , F 6 . 2 . ' g ' / 2 4 X , ' W F = ' , F 6 . 2 . ' g ' / 2 231 14X, ' W R T D = ' , F 6 . 2 , ' g ' / / 2 4 X , 'GASMB=' , F 6 . 1, ' % ' / 2 4 X , 'CMB= ' , F6 . 1 . ' %' ) 232 W R I T E ( 6 , 7 0 0 ) RC - 294 -L i s t i n g o f KINBR a t 1 1 : 2 2 : 3 5 on APR 2 5 . 1986 f o r CC1d=BET0 233 700 FORMAT( /24X . 'RC= ' , 1 2 ) 234 W R I T E ( 6 , 6 ) F 1 ( N ) . F 2 ( N ) 235 6 F 0 R M A T ( / 2 4 X . 'FTCO * ' .F7 . 4 / 2 4 X , ' F T C 0 2 * ' , F 7 . A ) 236 STOP 237 END - 295 -DATA EXPERIMENTAL DATA FOR RUN G3G N= 15 QTI<= 1 0 . 0 1 /min 0C0= 0 . 8 l /m1n 0C02= 3 . 2 1 /min EQK= 2 6 . 3 0 2 7 atm ITEMP= 1148 K WCHARO= 2 0 . 0 g CFIX= 0 .801 ASH= 0 . 1 8 4 XCOST= 0 .501 COSCT= 47715 XC02ST= 0 . 4 9 9 C02SCT= 54234 AIRSCT= 87490 WCHARF = 9 . 2 5 g T IME(m ln ) COCTA C02CT AIRCT O .0 0 0 0 1 .0 10876 32072 0 3 . 0 1 1789 31370 0 5. O 12291 30353 0 7 .0 13046 30429 599 10 O 12477 30886 0 15. .0 12614 30699 0 20 .0 12928 30522 0 25 , o 12516 30541 0 30 . .0 12567 30700 0 4 0 . o 12335 30794 0 5 0 . .0 1 1778 31221 0 60 .0 1 1261 31809 0 75 .0 10467 32351 0 9 0 . o 9735 32702 0 RESULTS OF RUN G36 XCOIG= 0 . 0 8 XC02IG= 0 . 3 2 XHEIG= 0 . 6 0 - 2 9 6 -T I M E ( m l n ) XAIR XHE XHEE XCO XCOE XC02 XC02E QTE( l / m m ) 1 . 0 0 . 0 0 . 5907 0 .5907 0 . 1142 0 . 1142 0 . 2951 0 . 2951 10. 18 3 . 0 0 .0 0 . 5876 0 . 5 8 7 6 0 . 1238 0 . 1238 0 . 2 8 8 6 0 2886 10 .23 5 . 0 0 . .0 0 . 5917 0 . 5 9 1 7 0 .1291 0 . 1291 0 . 2 7 9 3 0 . . 2793 10. 26 7 . 0 0 . ,0068 0 . 5830 0 .5871 0 . . 1301 0 . 1310 0 . 2 8 0 0 0 . .2819 10 .27 10. .0 0 , ,0 0 . 5848 0 . 5 8 4 8 0 , .1310 0 . 1310 0 . 2 8 4 2 0 . . 2842 10. 27 15 . .0 0 , .0 0 . 5851 0 .5851 0. . 1324 0 . 1324 0 . 2 8 2 5 0 . .2825 10. 28 2 0 . .0 0 , ,0 0 . 5834 0 . 5 8 3 4 0 . 1357 0 . 1357 0 . 2 8 0 8 0 . .2808 1 0 . 3 0 25 . .0 0 , .0 0 . 5876 0 . 5 8 7 6 0 . 1314 0 . 1314 0 . 2 8 1 0 0 . .2810 10 .28 3 0 . .0 0 . .0 0 . ,5856 0 . 5 8 5 6 0 . 1320 0 . 1320 0 . 2 8 2 5 0 . . 2825 10 .28 4 0 . .0 0 . .0 0 . ,5872 0 . 5 8 7 2 0 . 1295 0 . 1295 0 . 2 8 3 3 0 . .2833 10 .26 5 0 . .0 0 .0 0 . 5891 0 .5891 0 . 1237 0 . 1237 0 . 2 8 7 3 0. .2873 10. 23 6 0 . .0 0 .0 0 . .5891 0 .5891 0 . 1 182 0 . 1 182 0 . 2 9 2 7 0 .2927 10. 20 7 5 , .0 0 .0 0 . .5924 0 . 5 9 2 4 0 . 1099 0 . 1099 0 . 2 9 7 7 0 .2977 10. 16 9 0 . .0 0 .0 0 . . 5969 0 . 5 9 6 9 0 . 1022 0 . 1022 0 . 3 0 0 9 0 .3009 10. 12 TIME(m1n) XCOEO XC02E0 C0C02E C0C02K F RATE(g/g.s) 1 . .0 0 . ,4032 0 .0062 0 . 3 8 7 65 .240 0 . 0 0 . 9 3 9 E - 0 4 3 . .0 0 . ,4061 0 .0063 0 . 429 64 . 762 0 . .0 0 . 1 2 1 E - 0 3 5 .0 o. ,4022 0 .0061 0 . 4 6 2 65 .401 0 , .0317 0 . 1 4 0 E - 0 3 7 .0 0 . .4066 0 .0063 0 . 4 6 5 64 .683 0 . .0497 0 . 1 4 9 E - 0 3 10 .o 0 , ,4089 0 .0064 0 .461 64 . 333 0 , .0752 0 . 1 5 3 E - 0 3 15 . 0 0 . .4086 0 .0063 0 . 4 6 9 64 .371 0 . . 1 176 0 . 1 6 5 E - 0 3 20 o 0 , .4101 0 .0064 0 . 4 8 3 64 . 141 0 . 1630 0 . 1 8 5 E - 0 3 25 . 0 0 . .4061 0 .0063 0 . 4 6 8 64 .762 0 .2088 0 . 1 8 0 E - 0 3 30 . 0 0 . .4081 0 .0063 0 . 4 6 7 64 .449 0 .2503 0 . 1 9 2 E - 0 3 40 . 0 0 .4064 0 .0063 0 . 4 5 7 64 . 722 0 , . 3376 0 . 2 0 7 E - 0 3 50 . 0 0 .4047 0 .0062 0 .431 64 .999 0 .4135 0 . 2 0 6 E - 0 3 60 . 0 o .4047 0 .0062 0 . 4 0 4 64 .999 0 .4807 0 . 2 0 3 E - 0 3 75 . 0 o 4014 0 .0061 0 . 369 65 . 524 o .5655 0 . 1 8 9 E - 0 3 90 . 0 o .3972 0 .O060 0 . 3 4 0 66 .228 0 . 6292 0 . 1 6 4 E - 0 3 FT0T= 0 . 6 2 9 2 W0= 16.02 g WF= 5 . 5 7 g WRTD= 10 .08 g GASMB= 2 . 9 % CMB= 2 . 3 % RC= 1 FTCO = 0 . 6 2 9 2 FTC02= 0 . 6 1 1 0 

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