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Microwave-assisted catalytic pyrolysis of biomass for improving bio-oil and biochar properties Mohamed, Badr Ali 2018

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MICROWAVE-ASSISTED CATALYTIC PYROLYSIS OF BIOMASS FOR IMPROVING BIO-OIL AND BIOCHAR PROPERTIES  by  Badr Ali Mohamed  B.Sc., CAIRO UNIVERSITY, CAIRO, EGYPT, 2005 M.Sc., CAIRO UNIVERSITY, CAIRO, EGYPT, 2012  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF  DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (CHEMICAL AND BIOLOGICAL ENGINEERING)  THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)  April 2018  © Badr Ali Mohamed, 2018     ii  Abstract This thesis evaluates K3PO4, clinoptilolite, bentonite and their combinations as potential additives for enhancing microwave absorption, catalyzing pyrolysis of biomass and improving bio-oil and biochar qualities. Catalyst load ratio, pyrolysis temperature, liquid and solid product yields, bio-oil and biochar properties are examined to screen selected catalysts in terms of their effectiveness in increasing microwave absorption and improving bio-oil and biochar qualities. Thermogravimetric analysis (TGA) was also used to study the catalytic behaviour of those catalysts to interpret its performance in microwave-assisted catalytic pyrolysis and to study the catalytic pyrolysis kinetics for each of the three major biomass components, i.e., hemicellulose, cellulose and lignin, using the lumped three parallel reactions model. The performance of the produced biochars is evaluated in terms of their ability to improve soil water holding capacity (WHC), cation exchange capacity (CEC) and fertility of loamy sand soil. The capacity of those biochars in reducing bioavailability, phytotoxicity and uptake of heavy metals by wheat plants and the efficacy of those biochars in increasing soil fertility and plant growth in contaminated soil were also investigated. K3PO4, clinoptilolite and bentonite all showed good catalytic activities in microwave-assisted pyrolysis, resulting in reduced acidity, viscosity and water content of bio-oil product and catalyst loading and combination of different catalysts are controlling parameters on heating rate and product quality. The synergistic effects were observed in the combination of K3PO4 and clinoptilolite or bentonite, resulting in higher-than-expected microwave heating rate, in conjunction with improved bio-oil and biochar quality. Biochar produced from mixing K3PO4 and clinoptilolite or bentonite with biomass showed better performance in reducing toxicity and uptake of heavy metals than biochars produced from single catalyst. Catalytic microwave-assisted     iii  pyrolysis could be one potential approach for tailoring biochar quality to improve soil physiochemical properties. High microwave absorption, high water and nutrient affinity, desirable plant nutrients and high catalytic performance are the four key features of an effective additive for microwave-assisted biomass pyrolysis for making high quality bio-oil and biochars.          iv  Lay Summary This research is focused on improving the quality of the liquid biofuel and biochar produced from thermochemical conversion of biomass using microwave heating and natural additives. The natural additives are mixed with biomass to simultaneously increase microwave absorption to speed up the heating rate so as to improve the quality of the liquid biofuel and biochar, and they will remain in the produced biochar as a nutrients/soil conditioner to increase its performance as a fertilizer or soil remediate. The selected natural additives are found to increase biomass heating rate and improve the liquid biofuel and biochar qualities in microwave pyrolysis. The produced biochars possess high contents of the essential plant nutrients, which are beneficial for improving soil productivity and increasing crops yield, and these biochars could thus be used as a balanced fertilizer for plants and crops.         v  Preface Chapter 1 is a literature review and part of the literature review has been published in: Mohamed, B.A., Kim, C.S., Ellis, N., Bi, X. 2016. “Microwave-Assisted Catalytic Pyrolysis of Switchgrass for Improving Bio-Oil and Biochar Properties.” Bioresource Technology 201: 121–32. Mohamed, Badr A., Naoko Ellis, Chang Soo Kim, and Xiaotao Bi. 2017. “The Role of Tailored Biochar in Increasing Plant Growth, and Reducing Bioavailability, Phytotoxicity, and Uptake of Heavy Metals in Contaminated Soil.” Environmental Pollution 230: 329–38. A version of Chapter 2 has been published. Mohamed, Badr A., Chang Soo Kim, Naoko Ellis, and Xiaotao Bi. 2016. “Microwave-Assisted Catalytic Pyrolysis of Switchgrass for Improving Bio-Oil and Biochar Properties.” Bioresource Technology 201: 121–32. I conducted all the experiment and sample characterization and wrote the manuscript and the supervisors corrected and revised the manuscript.     Chapter 3 presents the results obtained from the experimental work conducted in a TGA. I carried out all the experimental work, data analysis and manuscript preparation. A version of Chapter 3 has been submitted to a peer-reviewed journal. The co-authors provided comments and feedback and edited the manuscript. Chapter 5 is the experimental work on evaluating performance of biochars produced from microwave catalytic pyrolysis for improving soil water holding capacity, cation exchange capacity and fertility of loamy sand soil. I was responsible for all the experimental work, biochar characterizations, data collection and analysis, and paper preparation. A version of Chapter 5 has been published. Mohamed, B.A., Ellis, N., Kim, C.S., Bi, X., Emam, A.E., 2016. “Engineered Biochar from Microwave-Assisted Catalytic Pyrolysis of Switchgrass for Increasing Water-    vi  Holding Capacity and Fertility of Sandy Soil.” Science of The Total Environment 566–567: 387–97.). The co-authors provided comments and feedback and edited the manuscript. Chapter 6 is the result on the performance of produced biochars in terms of their capacity to reduce bioavailability, phytotoxicity and uptake of heavy metals by wheat plants. I was responsible for all the experimental work, biochar characterization, heavy metals and nutrients extraction, plants acid digestion and characterization, data collections and analysis, and paper preparation. A version of Chapter 6 has been published. Mohamed, Badr A., Naoko Ellis, Chang Soo Kim, and Xiaotao Bi. 2017. “The Role of Tailored Biochar in Increasing Plant Growth, and Reducing Bioavailability, Phytotoxicity, and Uptake of Heavy Metals in Contaminated Soil.” Environmental Pollution 230: 329–38. The co-authors provided comments and feedback and edited the manuscript.      vii  Table of Contents  Abstract .......................................................................................................................................... ii  Lay Summary ............................................................................................................................... iv  Preface .............................................................................................................................................v Table of Contents ........................................................................................................................ vii  List of Tables .............................................................................................................................. xiii List of Figures ...............................................................................................................................xv List of Abbreviations ................................................................................................................. xix Glossary ........................................................................................................................................xx  List of Symbols ........................................................................................................................... xxi Acknowledgements .................................................................................................................. xxiv Dedication ................................................................................................................................. xxvi  Chapter 1: Introduction ........................................................................................................... 1 1.1 Background ................................................................................................................. 1 1.2 Pyrolysis of biomass ................................................................................................... 4 1.3 Microwave heating and catalytic pyrolysis................................................................. 5 1.4 Microwave heating mechanism .................................................................................. 7 1.5 Catalytic pyrolysis .................................................................................................... 10 1.6 Kinetics and factors affecting the solid yield of biomass pyrolysis.......................... 12 1.7 Pyrolytic bio-oil properties ....................................................................................... 16 1.8 Biochar properties and applications .......................................................................... 18 1.8.1 Effect of different heating methods on biochar properties ................................... 18     viii  1.8.2 Biochar and heavy metals remediation ................................................................. 21 1.8.3 Biochar and water holding capacity ...................................................................... 23 1.9 Thesis scope and objectives ...................................................................................... 25 1.9.1 The contents of the dissertation ............................................................................ 28 Chapter 2: Microwave-assisted catalytic pyrolysis for improving bio-oil and biochar properties. ................................................................................................................................ 32  2.1 Experimental ............................................................................................................. 32 2.1.1 Samples ................................................................................................................. 32 2.1.2 Preparation of mixture of biomass and catalysts .................................................. 33 2.1.3 Experimental apparatus and procedures of microwave-assisted pyrolysis ........... 34 2.1.4 Analysis of bio-oil and biochar ............................................................................. 35 2.2 Results and discussion .............................................................................................. 36 2.2.1 Effect of different catalysts on microwave heating of biomass ............................ 36 2.2.2 Microwave-assisted pyrolysis products ................................................................ 42 2.2.3 Effect of catalysts on bio-oil properties ................................................................ 48 2.2.3.1 Bio-oil acidity ............................................................................................... 48 2.2.3.2 Bio-oil water content..................................................................................... 48 2.2.3.3 Bio-oil viscosity ............................................................................................ 51 2.2.4 Effects of catalysts on biochar properties ............................................................. 52 2.2.4.1 Biochar elemental composition ..................................................................... 52 2.2.4.2 Biochar specific surface area and porosity ................................................... 53 2.2.4.3 Scanning electron microscopy (SEM) of biochars ....................................... 56 2.3 Summary ................................................................................................................... 58     ix  Chapter 3: Catalyst-biomass interactions and kinetics of catalytic pyrolysis. .................. 60 3.1 Experimental ............................................................................................................. 60 3.1.1 Biomass sample and thermogravimetric analysis ................................................. 60 3.2 Potential interaction mechanisms between biomass and catalyst particles .............. 61 3.2.1 Catalyst particles as a heat transfer medium: ........................................................ 61 3.2.2 Solid-catalyzed biomass decomposition: .............................................................. 62 3.2.3 Catalyzed vapour cracking:................................................................................... 62 3.3 Reaction kinetics ....................................................................................................... 65 3.4 Results and discussion .............................................................................................. 66 3.4.1 Catalytic effect on pyrolysis reaction rate and final solids yield .......................... 68 3.4.2 Effect of catalyst particle size ............................................................................... 70 3.4.3 Detailed thermal catalytic decomposition of biomass pseudo-components ......... 72 3.4.4 Reaction pathways for biomass thermal catalytic pyrolysis using bentonite and clinoptilolite ...................................................................................................................... 84  3.5 Summary ................................................................................................................... 86 Chapter 4: Effects of catalysts mixtures on microwave heating behaviour of biomass catalytic pyrolysis .................................................................................................................... 87  4.1 Experimental ............................................................................................................. 88 4.1.1 Microwave and TGA catalytic pyrolysis .............................................................. 88 4.2 Results and discussion .............................................................................................. 89 4.2.1 Effects of catalysts mixtures on microwave heating behaviour ............................ 89 4.2.2 Effects of catalysts mixtures on coke formation and coke properties................... 94 4.3 Summary ................................................................................................................. 101     x  Chapter 5: The role of tailored biochar in improving soil fertility and water retention.................................................................................................................................................. 102  5.1 Experimental ........................................................................................................... 103 5.1.1 Biochar preparation ............................................................................................. 103 5.1.2 Biochar characterization ..................................................................................... 103 5.1.3 Biochar and soil WHC and CEC ........................................................................ 104 5.1.4 Soil and biochar incubation ................................................................................ 105 5.1.5 Statistical analysis ............................................................................................... 106 5.2 Results and discussion ............................................................................................ 106 5.2.1 Effect of biochar on soil water holding capacity (WHC) ................................... 110 5.2.2 Effect of incubation period on soil water holding capacity (WHC) ................... 111 5.2.3 Effect of biochar on soil cation exchange capacity (CEC) ................................. 114 5.2.4 Effect of incubation period on soil CEC and extractable cations ....................... 115 5.2.5 Biochar and soil water holding capacity ............................................................. 118 5.2.6 Biochar and soil CEC.......................................................................................... 120 5.2.7 Biochar production and integration .................................................................... 122 5.3 Summary ................................................................................................................. 124 Chapter 6: The role of tailored biochar in reducing bioavailability, phytotoxicity and uptake of heavy metals. ........................................................................................................ 125 6.1 Experimental ........................................................................................................... 126 6.1.1 Catalytic microwave pyrolysis and biochars preparation ................................... 126 6.1.2 Biochar, heavy metals and soil incubation ......................................................... 127 6.1.3 Heavy metals and nutrients extraction ................................................................ 128     xi  6.1.4 Germination test, heavy metals, and nutrients uptake ........................................ 129 6.1.5 Statistical Analysis .............................................................................................. 130 6.2 Results and discussions ........................................................................................... 130 6.2.1 Biochar properties ............................................................................................... 130 6.2.2 Effect of engineered biochar on soil CEC and pH .............................................. 130 6.2.3 Effect of engineered biochar on nutrients extractability ..................................... 131 6.2.4 Effect of engineered biochar on heavy metals bioavailability ............................ 134 6.2.5 Effect of engineered biochar on wheat germination and shoot length ................ 136 6.2.6 Effect of engineered biochar on heavy metals and nutrients uptake .................. 139 6.3 Summary ................................................................................................................. 145 Chapter 7: Conclusion .......................................................................................................... 147  7.1 Overall Conclusion ................................................................................................. 147 7.2 Recommendations for Future Research .................................................................. 151 References ...................................................................................................................................154  Appendices ..................................................................................................................................173 Appendix A Biochar pore size distribution ......................................................................... 173 A.1 BJH Adsorption Pore Distribution for the biochar produced from 30wt.% clinoptilolite .................................................................................................................... 173 A.2 BJH Adsorption Pore Distribution for the biochar produced from 10wt.% K3PO4 + 10wt.% clinoptilolite ....................................................................................................... 173 A.3 Isotherm linear plot for 30Clino biochar. ................................................................. 174 A.4 Isotherm linear plot for 20Clino biochar. ................................................................. 174 A.5 Isotherm linear plot for 10KP/10Clino biochar. ...................................................... 175     xii  Appendix B Chemical composition of bio-oil produced from SG and SG mixed with 30wt.% K3PO4 using GC-MS ............................................................................................................ 176 B.1 The lumped product composition of bio-oil (peak area %) produced from SG and SG mixed with 30wt.% K3PO4 (30KP) under microwave-assisted pyrolysis using GC-MS………………………………………………………………………………………176 B.2 Chemical composition of total bio-oil (organic and aqueous phases) produced from SG and SG mixed with 30wt.% K3PO4 (30KP) under microwave-assisted pyrolysis using GC-MS. ........................................................................................................................... 177       xiii  List of Tables  Table 1.1 Advantages and disadvantages of microwave pyrolysis versus conventional pyrolysis.6......................................................................................................................................................... 7  Table 2.1 pH, water content and viscosity of bio-oil produced from different catalysts with different loads. .............................................................................................................................. 50  Table 2.2 Effect of different catalysts on biochar elemental composition, ash content and elemental analysis. ........................................................................................................................ 53  Table 2.3 The average heating rate (25 to 400°C), heating time, BET surface area, average diameter, micropore area and pore volume of different biochars produced from microwave heating and conventional heating. ................................................................................................. 55  Table 3.1 Thermal properties of switchgrass, char, silica sand and different catalysts. ............... 63 Table 3.2 The time required for the core of a switchgrass particle to nearly (99%) reach the temperature of 450°C. ................................................................................................................... 64  Table 3.3 Solid yield at 650°C and mass loss rates at drying (1st peak temperature) and pyrolysis (2nd peak temperature) for different catalysts with different particles size at different heating rates under TGA. ................................................................................................................................... 70  Table 3.4 Peak temperatures, mass loss rates and conversion percentage for biomass pseudo-components between SG, SG mixed with silica sand and SG mixed with bentonite or clinoptilolite using TGA at 100°C/min. ........................................................................................ 73  Table 3.5 Kinetic parameters of different pseudo-components for switchgrass and different catalysts at 100°C/min. ................................................................................................................. 77      xiv  Table 3.6 Fitted two-step reaction kinetic parameters for switchgrass and switchgrass mixed with 30Clino. ......................................................................................................................................... 82  Table 3.7 The lumped products composition of bio-oil (peak area %) produced from SG and SG mixed with 30%wt clinoptilolite under microwave-assisted pyrolysis using GC-MS. ................ 83 Table 4.1 Heating rates for switchgrass mixed with different catalysts or catalyst mixtures at different heating stages under microwave-assisted pyrolysis. ...................................................... 90  Table 4.2 Dielectric properties and microwave heat flow for different materials and spent catalysts. ........................................................................................................................................ 96  Table 4.3 Raman bands, band position and bond type for Raman Spectroscopy. ........................ 98 Table 4.4 Peak areas of the Raman bands corresponding to the coke deposit on different spent catalysts. ...................................................................................................................................... 100  Table 5.1 BET surface area, average diameter, micropore area and pore volume of different biochars produced from microwave heating and conventional heating. ..................................... 109 Table 6.1 Cation exchange capacity (CEC), pH and extractable elements from soil, soil with heavy metals (Soil+HM) and soil with heavy metals mixed with different biochars. ................ 133      xv  List of Figures  Figure 1.1 Staged degasification for value-added chemicals and fuels,20 reproduced with permission from Elsevier. ............................................................................................................... 5  Figure 1.2 Different criteria for selecting and designing biochars,64 reproduced with permission from Springer Nature. ................................................................................................................... 20  Figure 2.1 Heating behaviour of pure switchgrass and switchgrass with 10, 20 and 30 wt.% load of: (a) clinoptilolite; and (b) K3PO4 in microwave reactor under pyrolysis conditions. ............... 37 Figure 2.2 Heating behaviour of pure switchgrass and switchgrass with mixtures of different catalysts. ........................................................................................................................................ 40  Figure 2.3  Heating behaviour of switchgrass mixed with 10 % and 20 wt.% bentonite. ............ 41 Figure 2.4 Heating behaviour of switchgrass with 10 wt.% K3PO4 + 10 wt.% bentonite under microwave heating (MH) and conventional heating (CH). .......................................................... 42 Figure 2.5 Pyrolysis products distribution of microwave-assisted pyrolysis of switchgrass with: (a) 20 wt.% SiC, 10, 20 or 30 wt.% of K3PO4 and clinoptilolite; and (b) 20 wt.% SiC, 10 wt.% K3PO4 + 10 wt.% bentonite, 10 wt.% K3PO4 + 20 wt.% bentonite, 10 wt.% K3PO4 + 10 wt.% clinoptilolite and 20 wt.% of activated carbon (Error bars represent standard deviation of 3 replicates). ..................................................................................................................................... 43  Figure 2.6 SEM micrographs of 10% K3PO4 + 10% bentonite biochar produced under conventional heating and microwave heating. .............................................................................. 57  Figure 2.7 SEM micrographs of different types of biochars produced under microwave heating of switchgrass mixed with different catalysts. .................................................................................. 58     xvi  Figure 3.1 Weight loss and DTG for different catalyst particles of: (a) 30Clino; (b) 30Bento at 100°C/min. .................................................................................................................................... 67  Figure 3.2 The weight loss and DTG curves at slow (10°C/min) and fast (150°C/min) heating rates for SG and SG+30Sand. ....................................................................................................... 68  Figure 3.3 Final solid yield (ash-free basis) and the mass loss rate for different samples at different heating rates. .................................................................................................................. 69  Figure 3.4 Weight loss for SG and SG mixed with silica sand and different catalysts with different particles sizes using TGA at 100°C/min. ....................................................................... 72 Figure 3.5 The deconvoluted curves of the pseudo-components for switchgrass (SG) and SG-30Clino and pseudo-components comparison between SG, SG mixed with silica sand or with different catalysts using TGA at 100°C/min. ................................................................................ 76  Figure 3.6 Predicted and reported gas and oil yield for (a) switchgrass and (b) SG+30Clino as well as the measured DTG curves in this study. ........................................................................... 81 Figure 3.7 Catalytic reaction pathway for biomass mixed with a catalyst with high thermal conductivity. .................................................................................................................................. 86  Figure 4.1 Temperature rise profiles of 30KP compared to different samples under microwave catalytic pyrolysis. ........................................................................................................................ 91  Figure 4.2 Heat flow for SG and SG mixed different catalysts at heating rates of: (a) 25°C/min and (b) 100°C/min. ....................................................................................................................... 93  Figure 4.3 DTG for different spent catalysts produced from microwave catalytic pyrolysis. ...... 95 Figure 4.4 Raman spectroscopy for different spent catalysts produced from microwave catalytic pyrolysis: (a) 10KP/10Bento and 10KP/20Bento; and (b) 30KP-1.5L/min and 30KP-5L/min. .. 97     xvii  Figure 4.5 Deconvoluted peaks of Raman spectra for spent catalysts of 10KP/10Bento and 10KP/20Bento. .............................................................................................................................. 99  Figure 4.6 Deconvoluted peaks of Raman spectra for the spent catalysts of 30KP-1.5L/min and 30KP-5L/min. ............................................................................................................................... 99  Figure 5.1 SEM micrographs of different types of biochars produced under conventional heating and microwave heating. .............................................................................................................. 107  Figure 5.2 Water holding capacity of the loamy sand soil incorporated with biochars at 0, 1 and 2 wt.% before and after the incubation. Error bars are the standard deviation of three replicates. 110 Figure 5.3 Effect of heating method on the ability of the produced biochars through microwave-assisted pyrolysis (MAP) and conventional pyrolysis (CP) on increasing water holding capacity of the loamy sandy soil compared to control. Error bars are the standard deviation of three replicates. .................................................................................................................................... 112  Figure 5.4 Cation exchange capacity of the loamy sand soil incorporated with biochars at 0, 1 and 2 wt.% before and after incubation. Error bars are the standard deviation of three replicates...................................................................................................................................................... 115  Figure 5.5 Effect of 0, 1 and 2 wt.% load of different biochars on the extractable Ca, K, Mg and Na from the loamy sand soil before and after incubation. Error bars are the standard deviation of three replicates. ........................................................................................................................... 117  Figure 6.1 Germinated wheat plants for pure soil, soil spiked with heavy metals (Soil+HM) and heavy metal spiked soil amended with different biochars inside the environmental chamber. .. 129 Figure 6.2 Extracted heavy metals (Pb, Ni, and Co) from soil spiked with heavy metals (S+HM), and heavy metal spiked soil incorporated with different biochars at 1 and 2 wt.% loads. Error bars are the standard deviation of three replicates. ..................................................................... 135      xviii  Figure 6.3 Effect of different types of the engineered biochars at different loads (1 and 2 wt.%) on wheat germination percentage (a) and wheat shoot length (b) in comparison with pure soil and soil spiked with heavy metals (S+HM). Error bars are the standard deviation of three replicates...................................................................................................................................................... 137  Figure 6.4 Elements concentrations in wheat shoots for soil, soil spiked with heavy metals (S+HM) and heavy metal spiked soil incorporated with different biochars at 1 and 2 wt.% loads. Error bars are the standard deviation of three replicates............................................................. 141  Figure 6.5 Heavy metals (Pb, Ni and Co) concentrations in wheat shoots for soil with heavy metals (S+HM) and heavy metal spiked soil incorporated with different biochars at 1 and 2 wt.% loads. Error bars are the standard deviation of three replicates. ................................................. 144        xix  List of Abbreviations Bento Bentonite BET Brunauer-Emmet-Teller BJH Barrett-Joyner-Halenda CEC Cation exchange capacity  CH Conventional heating Clino Clinoptilolite CP Conventional pyrolysis DTA Differential thermal analysis DTG Differential thermogravimetry  GHG Greenhouse gas HM Heavy metals KP K3PO4 MAP Microwave assisted pyrolysis MH Microwave heating  PAH Polycyclic aromatic hydrocarbon PID Proportional–integral–derivative S Soil SEM Scanning electron microscopy SG Switchgrass TGA Thermogravimetric analysis WHC Water holding capacity     xx  Glossary Biochar A charcoal material obtained from biomass pyrolysis.  Bio-oil Bio-oil or pyrolysis oil is the main product from biomass pyrolysis which is a brown colored with smoky odor and contains a complex mixture of hydrocarbons and oxygenates. Catalytic coke A carbon material formed by repolymerization or condensation of hydrocarbons and oxygenates on catalyst surfaces.    Coke A carbon material formed by repolymerization or condensation of hydrocarbons and oxygenates.    Graphitic coke  A carbon material deposited on catalyst surfaces which has more graphitic carbon. Oxygenated coke A carbon material deposited on catalyst surfaces which has high oxygen-containing species.  Pyrolysis Pyrolysis is a thermochemical decomposition of a material under inert conditions at temperature > 350°C to produce bio-oil, biochar and non-condensable gases (i.e., H2, CO, CO2, CH4). Torrefaction Torrefaction is a mild thermochemical decomposition of a material under inert conditions at temperature 200-300°C.      xxi  List of Symbols A The frequency factor  ADTG The total peak area of the DTG curve between 200 and 600°C  Ai The peak area of each pseudo-component between 200 and 600°C Bi Biot number (r h/Kp) Cj Is related to the initial compositions and called pseudo-components of lignin, cellulose, and hemicellulose Cp Specific heat (J/kg K)  E Activation energy (kJ/mole) Ef Electric field (V/m) Fi The fraction of each pseudo-component from the devolatilized vapours occurred between 200 and 600°C h External heat transfer coefficient of particles (W/m2 K) kC Rate constant for primary char  kC2 Rate constant for secondary char kG Rate constant for primary gas kG2 Rate constant for secondary gas kL Rate constant for primary liquid Kp Thermal conductivity (W/m K) Mpeak Mass loss rate at the highest peak (%/min) n Reaction order P Absorbed microwave power per unit volume (W/m3)     xxii  r Particle radius (m) R The universal gas constant (J/mol. K) T0 Initial temperature of particle (°C) Tc Temperature of the core (°C) Tpeak Temperature at the highest peak (°C) Ts Temperature of the surroundings (°C) w∞ Final solid yield (mg) wB Weight for biomass (mg) WBi The weight of the corresponding component in biomass (i.e., cellulose, hemicellulose and lignin) based on the dry basis weight (mg) wC2 Weight for primary char (mg) wC2 Weight for secondary char (mg) WDV(200-600) The total weight of the devolatilized vapours occurred between 200 and 600°C (mg) WDVi The weight of the devolatilized vapours from each pseudo-component (mg) wG Weight for primary gas (mg) wG2 Weight for secondary gas (mg) wL Weight for primary liquid (mg) YG Gas yield (g) YL Liquid yield (g) YV Volatile yield (g)      xxiii  Greek symbols 𝜀ᇱ Dielectric constant  𝜀଴ Permittivity of free space (F/m) 𝜆଴ The microwave wavelength (cm) α Conversion ratio  αd Thermal diffusivity (m2/sec) αr Ratio between the gas and oil (%) β Heating rate (°C/min) ρ Material density (kg/m3) 𝜀″ Dielectric loss 𝜎 Effective conductivity (W/m K) 𝛾௜ The conversion percentage for each pseudo-component (%)      xxiv  Acknowledgements In the name of Allah, the Most Gracious and the Most Merciful,  The most and foremost, All praises be to Allah for the faith, hope and strength that Allah Almighty granted to me, and His blessing in completing the thesis “My Lord, enable me to be grateful for Your favour which You have bestowed upon me, Qur’an, 46.15”.     I would like to express my appreciation to my supervisors, Dr. Xiaotao Bi, Dr. Naoko Ellis and Dr. Chang Soo Kim for their encouragements and guidance on my research work and study. Without their patient supervision, support and enlightening advice I would not have the chance to pursue my study in this interesting research area. I owe special thanks to Dr. Xiaotao Bi for his continuous support and help and also for giving me the chance to conduct this research work and providing me the financial support throughout my PhD program. I would like also to thank my committee members Dr. Jim Lim and Dr. Mark Johnson for their time, guidance and support. I am thankful to my previous supervisors at Cairo University Dr. Ahmed El-raie and Dr. Mohamed Ezz Hasan for their support and advice and I also thank Dr. Gamal Nasr for his great help. During my studies I have received many helps and advice from many people which I cannot thank them all by names. For that reason, I would like to thank all my colleagues and friends at UBC and at my home university “Cairo University” for their help and friendship during my study. I am grateful to the financial supports from Egyptian Ministry of Higher Education, Egypt in the form of a research scholarship, Misr El Kheir, Egypt in the form of supplemental scholarship, Korea Institute of Science and Technology (KIST), South Korea in the form of a KIST-UBC collaborative research program, and Natural Science and Engineering Council (NSERC) of Canada in the form of a Discovery Grant.      xxv  My wholehearted thanks to my family, my wife and my children for their continuous encouragements, support and love.      xxvi  Dedication  This dissertation is dedicated to the memory of my beloved father, Ali Mohamed I miss you every day “To the righteous it will be said, “O reassured soul, Rerun to your Lord, well-pleased and pleasing [to Him], And enter among My [righteous] servants, And enter My Paradise, Qu’ran 89:27-30”       1  Chapter 1: Introduction 1.1 Background The increasing demand for energy derived from fossil fuels with the soaring population increased the negative effects of global warming. Thus, there is an urgent need to find alternative energy sources to produce renewable and low-carbon energy. Various forms of energy can be produced from biomass residues through a number of distinctive processes including, mechanical (physical), thermochemical, and biological processes. Biological processing is highly selective and only small numbers of distinctive products are produced. On the other hand, among all thermochemical conversion processes, pyrolysis is usually non-selective, which can produce multiple products over a very short reaction time, including biochar, bio-oil and non-condensable gases.1 In pyrolysis, a material is decomposed under inert conditions at temperature > 350°C to produce bio-oil, biochar and non-condensable gases (i.e., H2, CO, CO2, CH4). Bio-oil, or pyrolysis oil, is the main product from biomass pyrolysis, which is brown coloured with smoky odor, and contains a complex mixture of hydrocarbons and oxygenates with a low heating value compared to diesel.1,2 Biochar is a charcoal material obtained from biomass pyrolysis. Pyrolysis is considered as one of the feasible routes that can produce bio-oil from wood and agricultural wastes.2 Fast pyrolysis has been explored mainly for the production of bio-oil, with a maximum yield > 70 wt.%. However, the challenge remains on upgrading bio-oil to drop-in liquid biofuels because of its high oxygen content, high acidity and viscosity and low stability. Bio-char, as a main by-product, also needs to be further treated to increase its specific surface area and to remove the unwanted impurities (e.g., heavy metals).3,4 To improve the qualities of bio-oil and biochar, several technologies have been explored including the use of catalyst and microwave heating.5-7 However, due to poor absorption of microwave by dry biomass, microwave absorbers are often added to     2  increase the microwave absorption rate, and thus accelerate the heating rate to reach pyrolysis temperatures at reduced microwave power requirement.5  In addition, many soils are becoming less suitable for cultivation of food crops and bioenergy plants because of water deficiency and low soil fertility.8-10 Increase in drought conditions also poses severe negative impacts on sandy soils through water loss by evaporation, and facilitates the degradation of organic matter. Furthermore, water drains rapidly through sandy soils, leaching soluble nutrients and contaminating groundwater.9-11 To minimize the competition between crop productions for bioenergy and food, bioenergy crops should be cultivated in low productivity areas or marginal lands. Soil limitations (salinization, desertification and water shortage) need to be alleviated to ensure food security and increase the production of biofuel crops. One of the major reasons for improving soil fertility is to address the issue of food security, particularly in South Asia and sub-Saharan Africa, with malnutrition in 22 and 32% of the total population, respectively.12 Increasing the ability of soil to retain water and nutrients is crucial for achieving higher growth of bioenergy raw materials in desert land, arid and semi-arid areas and to provide enough food for the soaring population.8-11 Biochar application to soil may mitigate some of the portended deficiency in water as a consequence of climate change effects, in order to retain the future productivity of bioenergy crops.10,13 Biochars produced from microwave-assisted pyrolysis possess higher surface area than those biochars produced from conventional pyrolysis.14,15 Furthermore, biochars could be produced at lower temperatures with a quality higher than that produced from conventional heating.6 Soil contaminations in many areas around the world pose severe environmental problems (e.g., toxicity, water pollution, food pollution, human and animal health, etc.) and increase the risks to humans and biota.4,16 High concentrations of heavy metal(loid)s (e.g., As Cd, Cu, Pb, Co,     3  Ni and Zn) have been reported in many countries around the world.4,17 To improve soil physiochemical properties of contaminated soils, biochars need to be further treated by activation or blending with other nutrients to increase cation exchange capacity (CEC), pH and surface area.16,17 An engineering approach to remediate heavy metals cost-effectively is to immobilize pollutants and heavy metals while improving plant growth through providing nutrients to plants in situ, and promoting ecological restoration.16 It is known that phosphate as a major nutrient for plants is a good immobilizer for heavy metals and metalloids such as As, Pb, Cd, Co, Ni and Zn.4 When additives are introduced to biomass, either for the purpose of enhanced microwave absorption or catalytic pyrolysis, those additives will mostly remain with the biochar, which may serve as a source of nutrients for plants or as a remediate for soils which are afflicted by heavy metals. Thus, selecting a proper additive in microwave-assisted catalytic pyrolysis is key in improving the quality of bio-oil and biochars for soil amendment and remediation applications.  According to the literature, none of the reported studies have considered selecting a catalyst for biomass pyrolysis which may simultaneously improve: (1) microwave absorption for increasing the heating rate; (2) the quality of bio-oil and biochar; and (3) the benefits of biochar as a nutrients/soil conditioner, fertilizer or soil remediation. This thesis evaluates K3PO4, clinoptilolite, bentonite and their combinations as potential additives for enhancing microwave absorption, catalyzing pyrolysis of biomass and improving bio-oil and biochar qualities. Catalyst load ratio, pyrolysis temperature, liquid and solid product yields, bio-oil and biochar properties are examined to screen selected catalysts in terms of their effectiveness in increasing microwave absorption and improving bio-oil and biochar qualities. Thermogravimetric analysis (TGA) was also used to study the catalytic behaviour of those catalysts to interpret its performance in microwave-assisted catalytic pyrolysis and to study the catalytic pyrolysis kinetics for each of the     4  three major biomass components, i.e., hemicellulose, cellulose and lignin, using the lumped three parallel reactions model. The performance of the produced biochars is evaluated in terms of their ability to improve soil water holding capacity (WHC), cation exchange capacity (CEC) and fertility of loamy sand soil. The capacity of those biochars in reducing bioavailability, phytotoxicity and uptake of heavy metals by wheat plants and the efficacy of those biochars in increasing soil fertility and plant growth in contaminated soil were also investigated. 1.2 Pyrolysis of biomass Pyrolysis is a thermochemical decomposition of material under inert conditions that can produce multiple products over a very short reaction time, including biochar, bio-oil and non-condensable gases.1,18 Lignocellulosic biomass materials are mainly composed of cellulose, hemicellulose and lignin which can be selectively decomposed for different value-added chemicals and products (Fig. 1.1).19,20 Each component has different thermochemical stability in which hemicellulose is well known to be more reactive and less stable which makes it more prone to decompose at much lower temperatures (200-300°C) than cellulose and lignin.19-21 While cellulose is more stable and decomposes within 300 to 400°C. Lignin is the most difficult to decompose, which occurs under a wide range of temperatures of up to 900°C.1,21 In addition, the pyrolysis of lignin is extremely slower than cellulose and hemicellulose because lignin contains more thermally stable bonds between aromatic rings which require higher activation energy to break these bonds.1,21,22 It is known that hemicellulose is the more reactive and less stable component which makes it more prone to condensation and re-polymerization which give the second highest solid yield after lignin (46 wt.% at 900°C) which can contribute more toward final solid yield with up to 20 wt.% obtained at 900°C, while cellulose gives the lowest solid yield less than 3 wt.% at the     5  same temperature.19,21,22 In addition, most of coke is formed from acetic acid, acetaldehyde, acetyl, phenolic compounds and complex aromatics.23,24  Figure 1.1 Staged degasification for value-added chemicals and fuels,20 reproduced with permission from Elsevier.  1.3 Microwave heating and catalytic pyrolysis Microwave-assisted pyrolysis has been explored as an effective tool to improve the quality of bio-oil, biochar and syngas for different biomass materials such as wood wastes,25 wheat straw,5 corn stover,7,26 and sewage sludge.27 Biochar produced from microwave-assisted pyrolysis has been reported to have higher surface area and pore volume than that from conventional heating processes.6,28 Due to poor absorption of microwave by dry biomass, microwave absorbers are often     6  added to increase the microwave absorption rate, and thus accelerate the heating rate to reach pyrolysis temperatures at reduced microwave power requirement.14 Many microwave absorbing materials have been tested for increasing microwave heating and for in-situ upgrading of the organic vapours, including chemical solutions (NH3, H2SO4 and HCl),5 inorganic compounds (MgCl2, Na2HPO4, KAc and Al2O3),26 catalysts (KOH, FeSO4, H3BO3, ZnCl2 and H2SO4),27 and char.28 Thus, selecting a proper additive in microwave-assisted catalytic pyrolysis is key in improving the quality of bio-oil and producing biochars for soil amendment and remediation applications. However, there are many challenges related to microwave catalytic pyrolysis such as selecting the proper additive/catalyst and their mixtures, which is the key for promoting different pyrolysis reactions, increasing the microwave heating rate and scaling up the microwave catalytic pyrolysis reactor. The properly selected catalyst must also be a good microwave absorber in order to avoid the use of additional microwave absorber such as activated carbon. The catalyst with good microwave absorption capacity can be applied to biomass by mechanical mixing or impregnation, with the former preferred.     The scalability of microwave reactors to a continuous-flow reactor is one of major challenges for microwave biomass pyrolysis because of the limited penetration depth for microwave radiation through the sample.30 In addition, there have been no reports on the economic viability of microwave reactors for large-scale applications of biomass pyrolysis for bio-oil and biochar production. The auger reactor using multiple microwave generators distributed along the reactor can be potentially designed with continuous biomass feed.  This will improve the mixing of catalysts with samples, improve microwave energy uniformity and thus improve temperature uniformity throughout the sample.            7  Comparing microwave pyrolysis with conventional pyrolysis, microwave pyrolysis has many advantages and the most important ones are that no size reduction is required, moist biomass can be used without pre-drying and high-quality products can be obtained. These advantages will reduce energy consumption for the process and reduce the operational cost required for size reduction, drying and storage, which are main requirements for conventional pyrolysis.6  Table 1.1 Advantages and disadvantages of microwave pyrolysis versus conventional pyrolysis.6 Microwave pyrolysis Conventional pyrolysis Advantages  Disadvantages Advantages Disadvantages Size reduction of biomass not required, flexibility of feedstocks and products and high-quality products   Difficult for temperature measurement Flexibility of feedstocks and products    Lower quality products (e.g., high content of polycyclic aromatic hydrocarbons “PAH”)  Energy savings by reduced operating temperatures (150-300°C)  Scaling-up more complicated   Well-developed and easy to scale-up Energy consumption due to higher temperature (600°C) Possibility for continuous processing using multiple units   Products reproducibility issues and inhomogeneities   Possibility for continuous processing   1.4 Microwave heating mechanism The ability of a material to absorb microwave radiation is highly determined by its dielectric properties, namely, dielectric constant (𝜀′) and dielectric loss (𝜀″). Dielectric constant depicts the capacity of molecules to be polarized when it is under electric field and measures the efficiency of the material to convert the electromagnetic radiation into heat. Dielectric loss denotes to the     8  amount of input microwave energy which is dissipated through heat.29,30 The loss factor is defined as the ratio of dielectric loss to dielectric constant (tan 𝛿 = 𝜀″ 𝜀′⁄ ). Depending on the dielectric properties, materials can be classified into three types: a) conductors (reflective); b) insulators (transparent); and c) dielectrics (absorptive). A susceptor with a high tan δ might be needed to achieve the rapid heating.30 The absorbed power per unit volume of a material and the heating rate can be determined using the following equations:   𝑃 = 𝜎 |𝐸|ଶ = 2𝜋 𝑓 𝜀଴ 𝜀ᇱᇱ ห𝐸௙หଶ (1.1) ∆𝑇∆𝑡=2𝜋 𝑓 𝜀଴ 𝜀ᇱᇱ ห𝐸௙หଶ𝜌 . 𝐶௣ (1.2) Where, P is the absorbed power per unit volume, 𝜆଴ is the microwave wavelength, 𝜀ᇱ dielectric constant, 𝜀″ dielectric loss, 𝜎 effective conductivity, 𝜀଴ permittivity of vacuum, Ef is the electric field, ∆்∆௧ is the microwave heating rate, ρ is material density and Cp is the specific heat.  A possible mechanism for microwave heating of biomass premixed with a microwave absorber is that: microwave radiation is first absorbed by added microwave absorbing material and then the absorbed energy is transferred to biomass by heat conduction.28 During pyrolysis, the temperature of a good microwave absorber is much higher than the biomass particles, and the biomass particles are heated mainly through conduction. As a result, the real temperature of the microwave absorbing particles is probably higher than the biomass particles and the measured surface temperature. There are two types of charges: free charges and bound charges, while the movements of the free charges result in polarization. A restriction in the rotational motion of charges during the polarization of molecules can lead to a lag between the polarization and the electric field, and this lag is known as the relaxation time. Relaxation time results from the     9  dissipation of energy as heat inside the material.31 The structure of the material affects the relaxation time, and its ability to be heated is mainly related to the ability of the dipoles to be aligned to the electromagnetic field. This ability of orientation defines the dielectric properties of a material.31 The maximum rate of the microwave absorption energy occurs at the frequency of  𝑓 = 1 𝜏ൗ  , where τ is the relaxation time of the molecule.25 Thus, when the dipoles of the material responsible for the polarization are unable to follow the oscillation of the electric field at specific frequencies, the electric field reversal and the reorientation of dipoles become out-of-phase, which in turn gives a noticeable increase in energy dissipation.31 Increasing the temperature of biomass allows molecules of polymers to move readily due to the decrease in biomass viscosity or rigidity,25 thus resulting in increased heating rate and reduced microwave heating time. Another important aspect in microwave heating is the type of pyrolysis reactions. Endothermic chemical reactions are energy consuming, which will result in increasing microwave irradiation time;15,28 while, exothermic reactions such as water-gas shift reaction play an important role in microwave pyrolysis by generating heat, which in turn raises the temperature of the sample rapidly and reduces the microwave energy demand. It has been reported that the high concentration of potassium promotes endothermic Reactions (1.3), (1.4) and (1.5).15,28,32  C(s) + COଶ(g)  ↔ 2CO(g),       ∆Hଶଽ଼ ୏ = 173 kJ molିଵ                                        (1.3)      C(s) + HଶO(g)  ↔ CO(g) + Hଶ(g)   ∆Hଶଽ଼ ୏ = 132 kJ molିଵ (1.4)      CHସ(g) + COଶ(g)  ↔ 2CO(g) +  2Hଶ(g),   ∆Hଶଽ଼ ୏ = 260.5 kJ molିଵ (1.5)      CHସ(g) → C(s) +  2Hଶ(g),   ∆Hଶଽ଼ ୏ = 75.6 kJ molିଵ                                           (1.6)      CO(g) + Hଶ O(g) ↔ COଶ(g) + Hଶ(g)  ∆Hଶଽ଼ ୏ = −41.5 kJ molିଵ            (1.7)      10  1.5 Catalytic pyrolysis Catalytic pyrolysis has also been explored extensively for improving the quality of pyrolysis vapours or, more specifically, the quality of bio-oil and noncondensable gases.33 Many catalysts including natural and synthetic zeolites (clinoptilolite, mordenite, ZSM-5, Fe-H-MORD-20-IE, Fe-H-ZSM-5-IE and H-MORD-20-IE), and natural clays (bentonite, kaoline, cambrian clays, mergel clays, sepiolite and attapulgite) have been investigated.34-37 It was found that the use of catalyst mixed with biomass could improve the quality of bio-oil and noncondensable vapours. Zeolites can catalytically crack pyrolysis vapours to produce aromatic compounds and other hydrocarbon products, which can be further upgraded to produce transport fuels such as diesel and gasoline.1,36 Clinoptilolite exhibits a better catalytic performance and increases bio-oil yield in comparison with other natural and synthetic zeolites due to large pore structures (7.6 Å) which allow many types of large hydrocarbon molecules to be directly cracked to produce light olefins.36 Furthermore, the addition of clinoptilolite resulted in a sharp reduction of highly oxygenated groups (polar groups and asphaltenes) and significant increase in aliphatics.34,36 It was found that bentonite reduced heavy bio-oil compounds and raised bio-oil yield compared with other natural clays and synthetic zeolites;35 while, the stability of bio-oil organic fraction was also improved after accelerated aging tests.37 The improved catalytic performance in the production of upgraded bio-oil, low cost and the abundance of these natural catalysts make them favourable for bio-oil upgrading and value-added chemicals production.36,37 High gas yield is obtained from H-ZSM-5 because it has intermediate-sized pores with intersecting channels with pore sizes of 5.4×5.6 Å and 5.1×5.5 Å.38 It should also be noted that some ZSM-5 catalysts can have larger pores > 10-20 Å. While natural zeolite reduces gas products and increases bio-oil product compared to H-ZSM-5 because it has a clinoptilolite type structure     11  with two main channels of pore sizes of 7.6×3.0 Å and 3.3×4.6 Å.38 It has been reported that the bio-oil yield of cotton-seed cake increased by > 26% compared to the yield of bio-oil produced without clinoptilolite.34 The catalytic role of potassium in biomass pyrolysis has shown that it lowers the temperatures of initial and maximum decompositions, promotes the formation of char, gas, H2O and phenols, and suppresses the formation of levoglucosan, furans, pyrans, acetol, acids and aldehydes,39-42 reducing the liquid yield markedly.40,43 The significant increase in phenolic compounds using potassium catalysts result from the cleavage of different linkages (β-O-4 aryl ether bond, etc.) in addition to side-chain elimination reactions.41,43 Potassium also is known to promote crosslinking reactions that can result in increased solid yield with exothermic behaviour.41,42,44 Phosphorus was also found to promote the formation of char, levoglucosenone and furfural, and also shifted the maximum devolatilization peak to 1ower temperatures. The major difference in the solid yield was found for cellulose and hemicellulose, while small impact was found for lignin.45 It has been found that K3PO4 produced more phenolic compounds with a peak over 60% and promoted the pyrolytic decomposition of lignin, while inhibited the devolatilization of hemicellulose to create organic volatile compounds and produced more solid yield, leading to reduced acids compounds remarkably in bio-oil produced from poplar wood, based on experiments using a Pyroprobe at a very high heating rate > 10,000°C/min.43 It is known that hemicellulose is more reactive and less thermally stable, which makes it more prone to condensation and re-polymerization, while cellulose is more stable and lignin is considered the most difficult component to decompose.19,21,42 Coke deposition is a major concern for catalyst deactivation which reduces catalytic activity and affects product distribution markedly.46,47 Catalytic coke can form from the oxygenated     12  volatile intermediates and the dehydrated species.24 Catalytic coke formation on catalyst surfaces includes surface reaction, precipitation of carbon and diffusion. A hydrocarbon molecule is chemisorbed onto the catalyst surface and then converted to carbon by a surface reaction.48,49 Coke strongly adsorbs on the active sites, and blocks catalyst pores entrance. Coke forms at law temperatures (< 350°C) mainly through vapour condensation and repolymerization.47 The primary oxygenated vapours from hemicellulose such as acetic acid, acetaldehyde and acetyl are main precursors for catalytic coke formation. The catalytic transformation of acetic acid occurs through decarboxylation and dehydration.23,24 It is known that oxygenated coke has very low microwave absorption ability compared to graphitic carbon.30 1.6 Kinetics and factors affecting the solid yield of biomass pyrolysis  Pyrolysis is considered as one of the successful routes that can produce bio-oil from wood and agricultural wastes.2 However, there are many challenges associated with this technology such as the poor quality of the produced bio-oil because of the complexity of the pyrolysis process and the heterogeneous nature of biomass materials.50 Fundamental studies for the kinetics of the pyrolysis reactions will help to better understand the reaction pathways and to help to predict the pyrolysis behaviour of biomass materials at different operating conditions. As a result, more efficient pyrolytic reactors for industrial applications can be designed.2,51,52 Different kinetic models have been used to understand biomass pyrolysis such as single component model and segmented model. The simplest model is the single component one-step model, which cannot describe multi-step reactions as the case with catalytic pyrolysis.2,21       13  The general lumped models for biomass catalytic pyrolysis and secondary vapour cracking pathways are as follow:      Gas Biomass                      Oil                                                                                                  (Model-1) Char A global model of primary biomass decomposition                                     Gas (p)                    Gas (s)   Biomass                      Oil                                                                                                  (Model-2) Char (p)                     Char (s) A global model of primary and secondary biomass decomposition        Gas (p)                           Gas  Biomass                                Oil                                                                                        (Model-3)  Intermediate char                             Char A model of primary and secondary biomass decomposition considering intermediate biochar stage (Model-3) was proposed by.53 It was claimed by the author that this model gave the best fit with experimental results. To apply this two-step global model of primary and secondary biomass decomposition reactions to catalytic pyrolysis of biomass, the yields for gas and/or liquid need to be measured. Coats-Redfern integral method has been used by many researchers to determine the kinetic parameters for catalytic and non-catalytic pyrolysis.54 According to Coats-Redfern method the following equations can be derived for 𝑛 = 1 and 𝑛 ≠ 1:  When 𝑛 ≠ 1: ko ko ko     14  ln ቈ1 − ln(1 − 𝛼௜)ଵି௡೔𝑇ଶ (1 − 𝑛௜)቉ = ln ൤𝐴௜𝑅𝛽𝐸௜൬1 −2𝑅𝑇𝐸௜൰൨ ∙𝐸௜𝑅𝑇 (1.8) When 𝑛 = 1: ln ቈ−ln(1 − 𝛼௜)𝑇ଶ቉ = ln ൤𝐴௜𝑅𝛽𝐸௜൬1 −2𝑅𝑇𝐸௜൰൨ ∙𝐸௜𝑅𝑇 (1.9) Where α is the conversion ratio; A is the frequency factor; β is the heating rate; E is the activation energy; R is the universal gas constant; and n is the reaction order.  The activation energy and pre-exponential can be determined using linear regressions between the left-hand-side of equations (1.7) and (1.8) versus ଵ் , while the expression of 𝑓(𝛼௜) or g(𝛼௜) will depend on the mathematical model being used to determine the best fit with the experimental data according to Table 1.1.54 The pyrolysis process is divided into different stages to attain the well-fitted reaction model with high coefficient of determination (R2) using linear regressions.  Table 1.1. Expressions for pyrolysis reaction mechanisms in solid state reactions.  Reaction model Reaction mechanism  𝒇(𝜶𝒊)  𝐠(𝜶𝒊) Chemical reaction First-order reaction (F1) 𝟏 − 𝜶 −𝐥𝐧(𝟏 − 𝜶) Second-order reaction (F2) (𝟏 − 𝜶)𝟐 (𝟏 − 𝜶)ି𝟏 − 𝟏 Third-order reaction (F3) (𝟏 − 𝜶)𝟑 [(𝟏 − 𝜶)ି𝟏 − 𝟏]/2 Diffusion-controlled reaction  One-dimensional diffusion (1D) 𝟏/𝟐𝜶 𝜶𝟐 Two-dimensional diffusion (2D) [−𝐥𝐧(𝟏 − 𝜶)]ି𝟏 (𝟏 − 𝜶)𝐥𝐧(𝟏 − 𝜶) + 𝜶 Three-dimensional diffusion (3D) 𝟑/𝟐(𝟏 − 𝜶)𝟐/𝟑[𝟏 − (𝟏 − 𝜶)𝟏/𝟑]ି𝟏 [𝟏 − (𝟏 − 𝜶)𝟏/𝟑]𝟐 Four-dimensional diffusion (4D) 𝟑/𝟐[(𝟏 − 𝜶)ି𝟏/𝟑 − 𝟏]ି𝟏 𝟏 − (𝟐𝟑𝜶)−(𝟏 − 𝜶)𝟐/𝟑      15  Biomass pyrolysis is a complex process and it mostly cannot be described by a single step reaction model. In addition, lignocellulosic materials contain at least three main components which are cellulose, hemicellulose and lignin, which decompose differently, as reflected by partially overlapping peaks in the mass loss rate curves, and the deconvoluted peaks can be used to study the kinetics for each pseudo-component.2 The term “pseudo-component” is used to represent a group of compounds of similar molecular structures of different chain lengths which decompose following similar kinetics as identified in the weight loss curve.21 Three-parallel reaction model has been used widely to describe the reaction kinetics of lignocellulosic materials in which it is assumed that the three major pseudo-components (i.e., lignin, cellulose, and hemicellulose) decompose independently.21,55,56 Using biomass three parallel pseudo-components reactions approach will help to understand the catalytic effect on the three pseudo-components so as to identify possible catalytic mechanisms. It is known that heat and mass transfers play important role on determining pyrolysis products distribution and the quality of products by affecting the extent and selectivity of secondary reactions of primary vapours.2,21,57 In general, biomass has low thermal conductivity, limiting heat transfer especially in fixed bed reactors. Heating medium can be used to improve the heat transfer; while, catalyst with a high thermal conductivity can also serve as both a heating medium and a catalyst to promote pyrolysis reactions at lowered temperatures which in turn will improve heat and mass transfer. There are many factors that affect the solid yield of biomass pyrolysis and the most important ones that can affect to a great extent the final solid yield are: heat and mass transfer limitations, particles size, catalyst activity and coke deposition, vapour residence time and thermal properties of biomass and catalysts. When hot vapours migrate from a hot region toward low-temperature regions, re-condensation may occur that also can be triggered with the     16  increased vapour residence time.21 The quick release of volatiles from biomass will reduce the solid yield in the absence of internal heat and mass transfer limitation by reducing the occurrence of condensations and repolymerization reactions due to the shortened residence time of volatiles.1,2 Small biomass particles sizes with small Biot number (< 0.1) are recommended for kinetics study due to less mass and heat transfer limitations.58 Catalyst particles sizes also play an important role on biomass decomposition and finer catalyst particles will have higher contact area with biomass particles. Due to high thermal conductivity and heat capacity of catalyst particles, the presence of catalyst will increase the heat transfer between biomass particles and also improve temperature uniformity.   1.7 Pyrolytic bio-oil properties  Bio-oil or pyrolysis oil is the main product from biomass pyrolysis which is a brown colour with smoky odor and contains a complex mixture of hydrocarbons and oxygenates with a low heating value compared to diesel.1 Pyrolytic bio-oil produced from lignocellulosic materials has higher water, oxygen and ash contents than heavy petroleum oil. The challenge remains on the upgrading of bio-oil to drop-in liquid biofuels because of the high oxygen content, high acidity and viscosity. The acidity of bio-oil is related to the presence of carboxylic compounds such as acetic and formic acids.59 The pH of typical fast pyrolysis liquids is very low, e.g., 2.5 for wood materials (birch, pine and poplar) under conventional pyrolysis.59 Reducing the acidity of bio-oil is considered as a major task because high bio-oil acidity leads to many problems associated with bio-oil transfer in pipes and use in engines, and the strong acidity of bio-oil also reduces the stability of bio-oil and prompts polymerization reactions.37 Acetic acid was found to be the major acid product from deacetylation of hemicellulose.43     17  Another problem with the pyrolytic bio-oil is the high-water content (> 30 wt.%) that in turn will reduce the heating value of the fuel. One possible reason for the high water content in some bio-oil is the low heating rate which prolongs the heating time and likely increases the dehydration reactions.15 High heating rates are expected to result in better bio-oil and biochar qualities.6 In the microwave heating process, water inherent in the feedstock will evaporate quickly, making the possibility for water to be involved in the decomposition reactions to be low. On the other hand, drying and the release of volatiles in conventional pyrolysis occur simultaneously, making water to co-exist with the cracking reactions during conventional pyrolysis.15  High viscosity of bio-oil which ranges from 40 to 100 cP is one of the bio-oil properties that often limits bio-oil being used directly as a fuel, as viscosity affects the atomization of fuel; while, the viscosity for No. 2 diesel fuel ranges from 2 to 2.7 cP.1,30 It has been reported that microwave-assisted pyrolysis produces much more noncondensable gases and less bio-oil than conventional pyrolysis, because the microwave heating promotes secondary cracking reactions during pyrolysis which, at the same time, also increases the amount of lighter compounds in the bio-oil which in turn reduced bio-oil viscosity down to 15 cP.30,60 Bio-oil instability is one of the major problems during bio-oil storage in which viscosity, density and high heating value can be dramatically affected. Bio-oil instability is a result of the presence of highly reactive organic compounds such as aldehydes, organic acids and ketones. However, it has been reported that aldehydes are most unstable compounds which can react with other compounds such as phenolics, alcohols and water. Eventually, bio-oil properties will be changed remarkably, and the quality of bio-oil will be affected markedly as a function of storage time and result in phase separation.1,61 These characteristics make pyrolytic bio-oil difficult to be     18  used particularly as a fuel. For that reason, improving the quality of bio-oil through the use of catalyst or through bio-oil upgrading is required.      1.8 Biochar properties and applications  Biochar, a co-product of thermochemical conversion of biomass into valuable biofuel, can be applied to soil as an amendment to improve the sustainability of biomass and crop production, enrich fertility and quality of agricultural soils.9,10,62 Nevertheless, the information for identifying feedstock and proper pyrolysis conditions to maximize the capability of soils is sparse.62 Thus, more research is needed to develop suitable approaches to improve biochar characteristics with respect to soil applications and increase biofuel crop yield and food production. The use of bioenergy would be an efficient avenue to replace fossil fuels to address related environmental problems.9 However, food crops should not be used as raw materials for bioenergy because of soaring population and lack of food production resources such as farmland and water. Biochar production cost is one of the limiting factors, because producing biochar is an endothermic process which requires a significant amount of energy, particularly through conventional heating.  1.8.1 Effect of different heating methods on biochar properties  Biochar produced from fast pyrolysis (high heating rates) have different physical properties from biochars produced under slow pyrolysis conditions (low heating rates).6 In microwave-assisted pyrolysis, moisture is vaporized from the depth of the particle prior to the organic contents being volatilized. The steam from vaporized water is swiftly released, and not only sweeping volatiles from the pores, but also creates preferential channels in the biochar, which in turn increases the biochar porosity. As a result, the volatiles releasing rate at low temperatures is expected to be higher than conventional heating.14 Moreover, the porosity of biochars produced from microwave-assisted pyrolysis is higher than those produced from conventional pyrolysis, and     19  also the steam gasification reactions.15 Biochars produced under low heating rates are predominated by micropores; while, macropores are largely found under high heating rates due to larger pores created from the collapse of the cell structure.63 In microwave heating, particles are directly heated. As a result, heterogeneous reactions are favoured in microwave heating compared to conventional heating and the higher heating rate might affect pore structure of biochar.28 In conventional pyrolysis, volatiles produced during pyrolysis are released from outer layer first, and the thermal decomposition of volatiles released from the sample is expected to increase as the reaction develops toward the centre of the particle because of the slow heating rate.63 Because of the cracking volatiles, it will deposit inside the pores to block them. However, the micropores of biochar produced from microwave-assisted pyrolysis are clean and have more pores due to the uniform release of the volatile matters across the whole particle.25 At low heating rate during pyrolysis, the inherited porosity of feedstock would allow volatiles to release without morphological changes.63 However, higher heating rates quickly release the volatiles, and modify the pore structure of biochar associated with increased yield of liquid and gas fractions.6,28 Therefore, microwave-assisted pyrolysis can provide a new strategy for creating highly porous biochars, which can be used in sorption applications or as a precursor for producing activated carbon. Biochar can be designed for specific applications based on the selection of feedstock materials and pyrolysis properties as mentioned in Figure 1.2.64              20   Figure 1.2 Different criteria for selecting and designing biochars,64 reproduced with permission from Springer Nature.  Biochars produced from organic feedstocks such as woody biomass, crop residues, manures, etc. have the potential to increase the long-term soil carbon sequestration, promote soil aggregations formation and restore fertility. There is no “one-size fits all”, but biochars can be designed to target certain deficiencies in soil.64 In order to design biochar for carbon sequestration, the produced biochars should have high aromatics content and black carbon with O/C ratios of 0.2 – 0.4.3,64 To obtain these properties, materials should be pyrolyzed at high temperatures (500 – 700°C) which will produce biochar composed of poly-condensed aromatic structures and O/C     21  ratios between 0.2 and 0.4. These properties are important for carbon sequestration because biochar can resist microbial mineralization, which is good for long term carbon storage.3,64,65      1.8.2 Biochar and heavy metals remediation  Significant amounts of heavy metals in contaminated soils are associated with mining and mineral processing.66 Previous studies have also found that heavy metals such as Pb, Ni, and Co can reduce the plant germination and pose significant inhibitory effects on roots, stems and leaves of various plant species.67,68 One inexpensive rehabilitation method is to stabilize mine tailings through vegetation. However, because of the toxicity, low soil fertility, low water holding-capacity and unfavourable soil structure, amendments should also be used to improve soil physiochemical properties.69 An engineering approach to remediate heavy metals cost-effectively is to immobilize pollutants and heavy metals while improving plant growth through providing nutrients to plants in situ, and promoting ecological restoration.16,70,71 Biochar can be applied to soil to serve as an amendment and remediate agent to immobilize and reduce the bioavailability and toxicity of heavy metals in contaminated soils through ion-exchange, metal ion surface complexation, co-precipitation and physical adsorption.4,69,71 Many natural and synthetic amendments can reduce extractability of heavy metals from contaminated soils. However, they cannot improve soil productivity, microbial activity, and plant growth.4,72 It is easier to reduce extractable metals in contaminated soils than to improve plant growth.73 Biochars have been demonstrated to retain both nutrients and organic/inorganic contaminants.71 Biochars having high cation-exchange capacity (CEC) are considered the most suitable for soil remediation in contaminated soils.16 Most biochars used in heavy metal adsorption/removal are produced from either slow pyrolysis with low surface area (5 – 25 m2/g) or fast pyrolysis without any pre-treatment or modification.4,16,71 To improve soil physiochemical     22  properties of contaminated soils, biochars need to be further treated by activation or blending with other nutrients to increase CEC, pH and surface area.16,74 Many low-cost natural adsorbents have been used for removing pollutants such as natural zeolites which can effectively immobilize many heavy metals (e.g., Cd, Pb, Fe, Co, Cr, Ni, Hg and Zn) from contaminated sites.66,72,75,76 It was also found that natural zeolite (clinoptilolite) possesses a great potential for heavy metals immobilization in contaminated soils and improves plant productivity.77,78 The unique structure of clinoptilolite makes it an ideal candidate for sorption and ion-exchange processes, to be used as a carrier for slow-release fertilizers, and as a remediation agent in contaminated soils.77 Clinoptilolite is also the most common natural zeolite used in agriculture due to its high CEC, the relatively high absorption rate of moisture and dehydration capacities.78 Bentonite also plays an important role in detoxifying heavy metals, chlorinated hydrocarbons, and oxyanions from contaminated sites.79,80 It is known that phosphate as a major nutrient for plants is a good immobilizer for heavy metals and metalloids such as As, Pb, Cd, Co, Ni and Zn.4,81,82 Phosphates immobilizes heavy metals and organic contaminants through forming insoluble or sparingly-soluble metal phosphates with enhanced geochemical stability.81,83,84 A laboratory study found that Phosphates considerably reduced the phytotoxicity of Pb2+, Cd and Zn in a highly contaminated soil, and increased plant yields compared to soil without treatment.73 An experimental field study also found that the addition of phosphate to a contaminated soil with Pb under field conditions reduced Pb bioavailability and improved the stability of Pb-compounds.81    Soil pH plays a major role in the bioavailability of heavy metals (Pb, Co and Ni) in the contaminated soils, and the adsorption capacity for some heavy metals can increase up to three times per unit increase in soil pH.16,85 Soil pH is the main factor that controls the potential release     23  of the immobilized heavy metals or metalloids and surface precipitation.4,85,86 The precipitants of metal oxy/hydroxides are formed due to increased hydrolysis of heavy metals after pH is increased.68,87 The mobile fraction of heavy metals in soil is greatly affected by soil pH and generally increases as soil pH decreases.90 Thus, increasing soil pH will reduce heavy metals mobility in contaminated soil, and thus reduce their bioavailability to plants.90,91 Lead immobilization could occur as a result of the formation of very low soluble pyromorphite-like minerals in the presence of phosphate ion, which reduces Pb bioavailability in soils.82,83 It was reported that Pb-phosphates compounds are less soluble than naturally-occurring Pb compounds such as cerussite (PbCO3) by at least 44 times.88 It was evident that pyromorphite can be easily formed during extraction steps when Pb and P react, and the rate of pyromorphite formation increases with increasing phosphate content.89 The rate of pyromorphite formation is affected by the presence of both ions Pb2+ and PO43- in the contaminated soil.82 At the presence of sufficient amounts of phosphate, the solubility of Pb will rapidly decrease and Pb-P precipitation could complete after 60 minutes.83 It was found that the application of K3PO4 to soil reduced Pb extractability to 34.1% compared to 84.9% for untreated soil.73 The sorption of heavy metal ions highly depends on the concentrations of Ca, K, Mg, and Na, which indicates that ion-exchange mechanism is dominant.72  1.8.3 Biochar and water holding capacity  The ability of biochar to improve soil water holding capacity (WHC) are associated with many factors: surface functional groups, total pore volume, porosity structure and specific surface area.9 It has been found that the presence of polar oxygen-containing groups raises hydrophilicity of carbon materials, aiding the formation of hydrogen bonds.92 Another important factor is biochar porosity structure, where the adsorption process occurs mostly in micropores. Macro and meso-    24  pores play important roles in the adsorption process where they act as conduits for adsorbate to reach the micropores.9,92 The specific surface area of biochar is generally higher than sand and comparable to clay. Blending biochar with soil would raise the total soil specific surface area.62 Specific surface area and cation exchange capacity (CEC) are considered indirect measures of the ability of soils to hold water and retain nutrients (e.g., ammonium, nitrate, P, Mg and Ca), which will improve soil fertility, and bind different contaminants.93 Improving N-fertilizer use efficiency would lead to reduced fertilizer application rates, and reduced GHG emissions from the whole process, starting from the production of fertilizer to the application to soil.93 Therefore, there is an increasing need for new methods to create oxygenated biochar possessing high CEC to increase soil CEC. Such biochar could retain soil nutrients, reduce fertilizer leaching, sequester carbon and improve soil hydrological properties.93,94 Most biochars created from virgin biomass without catalysts possess fewer nutrients compared to conventional fertilizers. In addition, most of biochar studies focused on biochars produced from slow conventional heating methods which are not suitable for industrial production.62,95,96 Producing biochar capable of releasing nutrients for plants is thus in great need.3,62 Clinoptilolite is a natural zeolite which can retain water up to 60% of their weight due to the high porosity of its crystalline structure, high CEC, relatively high absorption rate and dehydration capacities. It can also reduce the amount of water required for irrigation, improve water use efficiency in semiarid and arid areas and can serve as a carrier of agricultural pesticides due to their high absorption capacity which in turn will reduce groundwater contamination and increase plant growth by ameliorating the value of fertilizer. Thus, clinoptilolite will diminish the costs of water and fertilizer significantly by holding useful plant nutrients.78,96 Clinoptilolite also possesses a great potential for heavy metal immobilization in contaminated soils.77 Bentonite improves soil     25  fertility, increases soil water and nutrient retention, and improves the agrochemical and physical properties of soil, resulting in significantly increased sorghum grain yield compared to soil without bentonite.97 Furthermore, bentonite increased the surface charge and CEC of different types of soils, increased the availability of nutrients in low fertility soils and improves fertilizer use efficiency.98,99 K3PO4 is used as a soil fertilizer for providing plants with two essential nutrients (potassium and phosphorus). Phosphorus is considered as a major nutrient for plants and also a good immobilizer for heavy metals such as As, Pb, Cd, Co and Ni which are extremely harmful for plants.4 As a consequence, mixing these additives/catalysts (K3PO4, clinoptilolite and bentonite) to biomass would greatly improve the physical and chemical properties of the produced biochars, and increase their sorption affinity for water, nutrients, and heavy metals.  Biochar surface chemistry can contribute to the hydrophobicity of biochar surfaces, which in turn prevents water from getting into pores by creating a negative capillary pressure.100 It is believed that these hydrophobic compounds might be oxidized over the incubation period to form hydrophilic compounds, which might result in a considerable increase in WHC.80,101,102 It was found that the continuing water uptake might decrease the surface hydrophobicity by increasing the moisture content within biochar pores.95 Biochar porosity plays also an important role in water retention, and pyrogenic nanopores which are the voids that compose within the carbon structure due to chemical changes during pyrolysis. These pores contain a majority of biochar pore volume, and the diameter of these pores is generally smaller than 50 nm with the vast majority of these pores being < 2 nm.100  1.9 Thesis scope and objectives   Pyrolytic bio-oil produced from lignocellulosic materials has higher water, oxygen and ash contents than heavy petroleum oil. The challenge remains on the upgrading of bio-oil to drop-in     26  liquid biofuels because of the high oxygen content, high acidity and viscosity.1,15,37,59,61 Bio-char, as a main by-product, also needs to be further treated to increase its specific surface area and cation exchange capacity (CEC) to remove the unwanted impurities (e.g., heavy metals).3,4,16,70 To improve the qualities of bio-oil and biochar, several technologies have been explored including the use of catalyst and the microwave heating.1,5,6 It was found that bentonite reduced heavy bio-oil compounds and raised bio-oil yield compared with other natural clays and synthetic zeolites;35 while, the stability of bio-oil organic fraction was also improved after accelerated aging tests.37 Clinoptilolite exhibits a better catalytic performance and increases bio-oil yield in comparison with other natural and synthetic zeolites.1,34,36 Furthermore, K3PO4 has been identified as a good microwave absorber, while markedly reducing bio-oil acidity.43 To improve soil physiochemical properties of contaminated soils, biochars need to be further treated by activation or blending with other nutrients to increase CEC, pH and surface area.16,74 Potassium phosphate mixed with biomass will remain in the biochar after pyrolysis and provides plants with the two essential macro-nutrients of potassium and phosphorous. It is known that phosphate as a major nutrient for plants is a good immobilizer for heavy metals and metalloids such as As, Pb, Cd, Co, Ni and Zn.4,81,82 Clinoptilolite also possesses a unique structure which makes it an ideal candidate for sorption and ion exchange processes, to be used as a slow releasing carrier of fertilizers, and as a remediation agent in contaminated soils.77,78 Bentonite plays an important role in environmental protection by acting as a catalyst for detoxifying specific contaminants, such as heavy metals, chlorinated hydrocarbons, and oxyanions.80 The fact that most biomass materials are poor in absorbing microwaves requires the addition of microwave absorbers to increase microwave heating rate and efficiency. When additives are introduced to biomass, either for the purpose of enhanced microwave absorption or catalytic pyrolysis, those additives will mostly remain with the     27  biochar, which may serve as a source of nutrients for plants or as a remediate for soils which are afflicted by heavy metals. Thus, selecting a proper additive in microwave-assisted catalytic pyrolysis is key in improving the quality of bio-oil and producing biochars for soil amendment and remediation applications.  According to the literature, none of reported studies has considered selecting a catalyst for biomass pyrolysis which may simultaneously improve: (1) microwave absorption for speeding up the heating rate; (2) the quality of bio-oil and biochar; and (3) the benefits as a nutrients/soil conditioner in the biochar to increase its performance as a fertilizer or soil remediate. It is thus desirable to identify solid additives which can serve as a catalyst to improve the bio-oil quality, a microwave absorber to improve microwave absorption rate, and a nutrient to improve the quality of biochar byproduct. Therefore, based on their potential effectiveness for microwave absorption and catalytic performance according to the literature, three catalysts/additives, namely K3PO4, clinoptilolite and bentonite, have been selected and mixed with switchgrass to improve bio-oil and biochar properties and speed up microwave heating. The main objectives of this study are to improve the qualities of bio-oil and biochar produced from microwave pyrolysis and to increase microwave heating rate through the use of multi-functional catalysts. Attempts are also made to elucidate the interactions between catalysts activities and microwave heating and to decouple the effects of microwave heating rate from the catalytic effects. The specific research tasks to be conducted to achieve those objectives include:    Examine bentonite, clinoptilolite and K3PO4 as potential solid additives for microwave absorption and catalyst for biomass pyrolysis, which have not been tested before in microwave heating.     28   Investigate the potential synergistic effects from different combinations of these selected additives to increase microwave heating and improve bio-oil and biochar properties.  Study the effect of heating rate and additives activities on microwave heating and decouple their interactions through measured dielectric, thermal and physical properties and endo/exothermicity, kinetics and selectivity data from TGA/DTA.   Study the effects of the produced biochars on improving soil fertility, water retention, and heavy metals immobilization.   Examine the desirable additives proportions for producing bio-oil and biochar with high qualities at reduced microwave power requirement.   1.9.1 The contents of the dissertation The dissertation will include seven chapters. The first chapter will be a general introduction about pyrolysis, microwave heating mechanism, biomass catalytic pyrolysis, biochar applications and biomass pyrolysis kinetics. The second to six chapters report the main results of the research work. The seventh chapter is a summary of the research work and recommendations for the future research.  The summaries of the five main chapters are as follow: Chapter two: Microwave-assisted catalytic pyrolysis for improving bio-oil and biochar properties. In this chapter, the focus is on the role of the selected solid catalysts on improving the low microwave absorption of biomass materials and improving the bio-oil and biochar properties. The effect of different catalyst loads on microwave heating rate and bio-oil and biochar qualities are examined. The synergistic effects from different combinations of the selected catalysts on increasing microwave heating and improving bio-oil and biochar qualities are also discussed. The microwave heating behaviour, bio-oil and biochar physical and chemical properties for the samples     29  with catalyst are compared to the inert sample. This chapter has been published in Bioresource Technology (2016) 201: 121–32. Chapter three: Study on catalyst-biomass interactions and kinetics of catalytic pyrolysis. The central inquiry of this chapter is how the catalyst particles interact with biomass particles in terms of increased heating rate of biomass particles via solid-solid contact (in which catalyst acts as a heat transfer medium only) and catalyzed cracking of organic vapours from biomass cracking through different reaction pathways. The thermal decomposition behaviour using TGA for switchgrass and switchgrass mixed with different catalysts are studied and compared to the base line silica sand. The catalytic pyrolysis kinetics for each of the three major biomass components using the lumped three-parallel reaction model are discussed. In addition, the two-stage biomass pyrolysis are studied to confirm the findings obtained from the lumped three-parallel reaction model. This chapter has been submitted to a peer-reviewed journal. Chapter four: Effects of Catalysts Mixtures on Microwave Heating behaviour of Biomass Catalytic Pyrolysis. Results in Chapters 2 and 3 showed that K3PO4 possesses good microwave absorption but inhibited the devolatilization of hemicellulose, leading to significant increase in the catalytic coke yield compared to other samples, which can then affect microwave absorption and biomass decomposition. Bentonite, on the other hand, has a high thermal conductivity, promotes hemicellulose decomposition, yet possesses poor microwave absorption. Thus, it is expected that mixing the two catalysts can potentially increase the microwave absorption rate and reduce formation of oxygenated coke type that can further affect microwave absorption. Since some catalyst can promote microwave absorption while some catalyst shows promising catalytic effect, it is desirable to use mixtures of catalysts to improve high microwave heating rate and catalyze biomass pyrolysis reactions. Thus, this chapter studied the catalytic effects of     30  mixtures of different catalysts on biomass catalytic pyrolysis compared to the single catalyst under microwave heating and TGA. To understand the effect of catalyst and catalyst mixture on coke formation, coke deposited on catalyst particles is sampled and characterized. Chapter five: The role of tailored biochar in improving soil fertility and water retention. The focus of this chapter is on evaluating the performance of the produced biochars in terms of their ability to improve soil water holding capacity (WHC), cation exchange capacity (CEC) and fertility of loamy sand soil, investigating the synergistic effects of the combinations of two different catalysts on biochar properties in contrast with the addition of one catalyst, and examining the effect of the incubation period on WHC and CEC of the incorporated soil with the biochar. The correlation between pH, cation exchange capacity, biochar micropore surface area and water retention are also studied. This chapter has been published in Science of The Total Environment (2016) 566–567: 387–97. Chapter six: The role of tailored biochar in reducing bioavailability, phytotoxicity and uptake of heavy metals. This chapter studies the performance of produced biochars from microwave catalytic pyrolysis by assessing the capacity of these biochars to reduce bioavailability, phytotoxicity and uptake of heavy metals by wheat plants and determining the efficacy of these biochars to increase soil fertility and plant growth in contaminated soil. To test the ability of these biochars on reducing phytotoxicity on plant growth and uptake of Pb, Co, and Ni, a germination test is conducted using wheat seeds, and heavy metals and nutrients concentrations are measured. The correlation between pH, cation exchange capacity, biochar micropore surface area and heavy metals adsorption is examined. The synergistic effects of catalysts combinations on reducing toxicity and uptake of heavy metals are also studied and contrasted to the control and the samples     31  with a single catalyst. This chapter has been published in Environmental Pollution (2017) 230: 329–38.      32  Chapter 2: Microwave-assisted catalytic pyrolysis for improving bio-oil and biochar properties. The objectives of this chapter are to identify these potential solid catalysts (K3PO4, clinoptilolite and bentonite) for microwave absorbing and catalytic microwave pyrolysis of biomass, which have not been tested before in microwave heating, and to investigate the potential synergistic effects from different combinations of these selected catalysts to increase microwave heating, and to improve bio-oil and biochar qualities. Catalyst load ratio, pyrolysis temperature, liquid and solid product yields, bio-oil and biochar properties are examined to screen selected catalysts in terms of their effectiveness in increasing microwave absorption. Another important aspect in elucidating in microwave heating is the type of pyrolysis reactions. Endothermic chemical reactions are energy consuming, which will result in increasing microwave irradiation time;28 103 while, exothermic reactions play an important role in microwave pyrolysis by generating heat, which in turn raises the temperature of the sample rapidly and reduces the microwave energy demand. It is believed that the synergistic effects in microwave heating will be triggered by mixing two or more catalysts which might promote and maintain the occurrence of exothermic reactions such as the water-gas shift reaction.  2.1 Experimental 2.1.1 Samples Switchgrass from Manitoba, Canada, was used in this study for its bioenergy application potential. Switchgrass can be grown in a wide geographic area with a high productivity, high ability to improve land quality and low water and nutrients requirements.104 The United States of America, Department of Energy has identified switchgrass as a model energy crop.104 The ultimate and proximate analyses of switchgrass were performed, and the moisture, ash content, volatile     33  matter, fixed carbon and higher heating value are found to be 5.1, 6.3, 76.9, 11.7 wt.% and 19.6 MJ/kg, respectively. The C, H, N, S and O contents are 47.9 wt.%, 6.2 wt.%, 0.8 wt.%, 0.1 wt.% and 38.7 wt.% (by difference), respectively. Carbon, hydrogen, nitrogen and sulfur contents of switchgrass samples were determined using a Perkin-Elmer 2400Series II CHNS/O analyzer, and oxygen content was determined by difference. Moisture, volatile matter, fixed carbon and ash content were measured according to ASTM D1762 standard using a muffle furnace (Thermo Scientific, Lindberg/BlueM). Higher heating value was measured using an Oxygen Bomb Calorimeter (Parr 6100, USA).    2.1.2 Preparation of mixture of biomass and catalysts To screen different catalysts for microwave-assisted pyrolysis of switchgrass, a total of 20.0 g of pure switchgrass (600-300 micron) or switchgrass mixed with different loads (10, 20 and 30 wt.%) of natural zeolite (clinoptilolite), bentonite or K3PO4 was pyrolyzed in a tubular reactor in each test run with bio-oil and biochar properties determined. Different catalysts with particles size <50 μm were mechanically mixed with switchgrass using a mixer at different fractions in order to investigate the synergistic effect of the mixture in increasing microwave heating rate and maximizing the catalytic effects.  Bentonite (Al2O34SiO2H2O) and K3PO4 (Potassium Phosphate Tribasic, reagent grade, ≥98%) were purchased in powder form from Sigma-Aldrich, Canada Co. Commercial grade clinoptilolite ((K,Ca,Na)2O-Al2O3-10SiO2-6H2O) was purchased from Bear River Zeolite Co, USA. The natural zeolite mineral contains 85% clinoptilolite, and has a high cation exchange capacity (CEC) of 140-165 meq/100 g. Since switchgrass has a poor microwave absorption rate, silicon carbide (SiC), which is a chemically inert microwave absorber, was used as a benchmark     34  to contrast the catalytic performance of other catalysts. All catalysts were used as received without any pretreatment.  2.1.3 Experimental apparatus and procedures of microwave-assisted pyrolysis   The microwave-assisted pyrolysis of switchgrass was carried out using a single mode microwave oven manufactured by EnWave Corporation, Vancouver, Canada. The reactor consists of five major parts: a microwave source, a microwave transmission duct, a quartz tube reactor, a data acquisition system and a microwave leakage detection system. The frequency of the microwave generator is 2.45 GHz and the maximum power output is 1.2 kW. Water cooling system is used to absorb the un-absorbed microwave to avoid reflection. A microwave leakage detection system was used to monitor microwave leakage. A cylindrical quartz tube with an inner diameter of 44 mm and a height of 250 mm was placed vertically inside the microwave transmission duct. Both ends of the quartz tube were screwed tight with O-ring and stainless-steel sealers. Because microwave field could interfere with the measurement of thermocouples made of metals,6 an infrared pyrometer was used to measure the surface temperature of particles inside the reactor. The measured temperature was also used by a proportional–integral–derivative (PID) controller to control the reactor temperature via controlling the microwave power output, and more details about the microwave reactor can be found elsewhere.105 Nitrogen gas from a nitrogen cylinder with 99.99% purity, Praxair Canada Inc., was used to maintain the inert atmosphere in the system and to prevent the infrared pyrometer from fouled by condensed bio-oil. Temperatures of biomass sample and microwave power output were recorded by a data acquisition system. The pyrolysis volatiles were condensed in a three-stage condensation system quenched by cold tap water, with the pyrolytic liquid being collected from bottles connected to the bottom of condensers. Ethanol was used to wash and collect the condensates adhering to the interior wall of the condensers and     35  the quartz tube reactor. A rotary vacuum evaporator was used to evaporate ethanol at 40°C until constant weight was reached, with the final weight being recorded.  Premixed biomass-catalyst sample was loaded into the quartz tube reactor and N2 was used as inert gas at a flow rate of 1.5 L/min for about 30 min to drive the air out to create an anoxic state. 750 W microwave power setting was used to heat up the sample to the set temperature of 400°C, with the averages of microwave heating rates provided in Table 2.3. Once 400°C was reached, the microwave power was decreased to maintain the temperature for ~10 min. At the end of each experiment, the microwave generator was shut down and N2 flow was kept until the biochar (solid residue) cooled down to about 25°C before biochar was removed from the reactor. Each biochar sample was weighed and then put into a sealed bag. For each experimental condition, three test runs were conducted and the average of the three experimental results was reported.  2.1.4 Analysis of bio-oil and biochar Bio-oil dynamic viscosity was measured by a Cambridge VISCOlab3000, Cambridge Viscosity PAC, USA, an instrument complying with ASTM D7483 standard and correlates well with ASTM D445 standard. The instrument measures multiple times of the sample until the standard deviation becomes less than 0.2% before a viscosity value is recorded per ASTM 7483 standard. Measurements were done in triplicate at 40ºC. Before measurements, the instrument was calibrated by a standard solution. Water content was measured by Karl Fischer (KF) titrator Metrohm AG, according to ASTM E203: Standard Test Method for Water Using Volumetric KF Titration. Bio-oil pH was measured by a standard pH meter. The pH meter was calibrated by different standard pH buffer solutions. The measurements were done in triplicate, and average values were reported. Specific surface area was measured by nitrogen adsorption at -77 K using a Micromeritics ASAP 2020 instrument. Samples were degassed first at 120ºC for 7 h before     36  analysis. Specific surface area was determined based on Brunauer-Emmet-Teller (BET) method and Barrett-Joyner-Halenda (BJH) method was used to determine the micropore volume. The scanning electron microscopy (SEM) of biochars was carried out by a Hitachi S3000N VP-SEM with EDX. Biochar ash content was measured according to ASTM D1762 standard using a muffle furnace (Thermo Scientific, Lindberg/BlueM).  Carbon, hydrogen and nitrogen contents of biochar samples were determined using a Perkin-Elmer 2400Series II CHNS/O analyzer operated in the CHN mode, and oxygen content was determined by difference. Elemental compositions of biochar samples were measured at an ISO17025 accredited laboratory (ALS Group, Vancouver, BC, Canada). Briefly, a prepared sample (0.25 g) was digested with HF, HClO4, NH3 and HCl, then the residue was topped up with dilute HCl and the resulting solution was analyzed by ICP-AES. Results were corrected for spectral inter-element interference and measurements were conducted in duplicate and average results were recorded.  2.2 Results and discussion  2.2.1 Effect of different catalysts on microwave heating of biomass Fig. 2.1 shows the heating behaviour of pure switchgrass, compared to switchgrass with 10, 20 or 30 wt.% of clinoptilolite or K3PO4. It can be clearly seen that the temperature of pure switchgrass increased very quickly to about 110°C during the first 3 minutes of heating because of the inherent moisture content in switchgrass (~5 wt.%), which is considered as a good absorber for microwave radiation. However, after the moisture has escaped, the dry switchgrass became very poor in absorbing the microwave radiation.       37   (a)  (b) Figure 2.1 Heating behaviour of pure switchgrass and switchgrass with 10, 20 and 30 wt.% load of: (a) clinoptilolite; and (b) K3PO4 in microwave reactor under pyrolysis conditions.      38  The maximum temperature recorded for pure switchgrass is about 159°C after ~30 min of microwave irradiation, which is well below the desired pyrolysis temperature. On the other hand, the temperature of biomass + 10 wt.% K3PO4 sample increased to 400°C in 15 min under microwave irradiation, and 20 min for biomass + 10 wt.% clinoptilolite. Increasing the percentage of clinoptilolite promoted the heating rate of the sample and reduced the microwave heating time required to reach the target pyrolysis temperature. 400°C was reached after 6.9 min of microwave radiation for the sample with 30 wt.% clinoptilolite. Therefore, 30 wt.% of clinoptilolite is considered as the best compared to other loads of clinoptilolite in terms of microwave absorption. However, the addition of K3PO4 exhibited slightly different trend. As shown in Fig. 2.1b, the sample with the highest K3PO4 load (i.e., 30 wt.%) gave the fastest initial increase in temperature, but took a longer time to reach 400°C, which was confirmed after repeated experiments. It has been reported that the high concentration of potassium promotes endothermic Reactions (2.1), (2.2) and (2.3).28,103 It was also found that K3PO4 promoted decarbonylation reactions in conventional pyrolysis.43 Thus, it is likely that the higher K3PO4 load may have promoted the endothermic reactions in this study, leading to a slower heating rate of the sample. C(s) + COଶ(g)  ↔ 2CO(g),       ∆Hଶଽ଼ ୏ = 173 kJ molିଵ                                                                (2.1) C(s) + HଶO(g)  ↔ CO(g) + Hଶ(g)   ∆Hଶଽ଼ ୏ = 132 kJ molିଵ                                                        (2.2) CHସ(g) + COଶ(g)  ↔ 2CO(g) +  2Hଶ(g),   ∆Hଶଽ଼ ୏ = 260.5 kJ molିଵ                  (2.3) CHସ(g) → C(s) +  2Hଶ(g),   ∆Hଶଽ଼ ୏ = 75.6 kJ molିଵ                                           (2.4)  Furthermore, alkali metals at higher percentage ≥ 20 wt.% are shown to promote coke production,103 which may deactivate the catalyst and reduced its microwave absorption efficiency, which in turn may reduce the conversion rate and produce more solid. Low heating rates and long     39  residence time could further drive the pyrolysis process in the direction of the formation of solid product.6 Therefore, it is expected that the highest biochar yield will be produced form 30 wt.% K3PO4, as will be further discussed in Section 2.2.2. Another reason that the endothermic Reaction (4) of methane dry reforming might be triggered is related to high temperatures of K3PO4 particles compared to biomass particles, and the accumulation of coke on the catalyst surface which further enhances this reaction.28  Fig. 2.2 illustrates the heating behaviour of pure switchgrass, with an addition of different combinations of catalysts, namely K3PO4, bentonite, or clinoptilolite, compared with switchgrass with 20 wt.% activated carbon. The shortest microwave radiation time was recorded for the sample with 10 wt.% K3PO4 + 10 wt.% bentonite, which is the mixture used in this study. Bentonite, a natural aluminum phyllosilicate (mostly montmorillonite), can serve as a heat-carrier, leading to more uniform heating.35 However, note that adding bentonite only to switchgrass at 10 or 20 wt.% loads did not increase the microwave heating rate compared to its combination with K3PO4, and the maximum temperatures recorded well below 200°C (Fig. 2.3). Furthermore, the temperature curves for the sample mixed with 10 wt.% K3PO4 + 10 wt.% bentonite show the highest heating rate compared to the samples mixed with three different ratios of K3PO4 (Fig. 2.1b) or bentonite only (Fig. 2.3). This phenomenon may be related to that a higher temperature is required for bentonite to activate the exothermic pyrolysis reaction (water-gas shift reaction) and increase the dipolar molecules rotation,35 which will increase the dielectric loss of the sample and in turn raise the heating rate. In addition, the average heating rate of the mixture of 10 wt.% K3PO4 + 10 wt.% clinoptilolite is also higher than the samples mixed with clinoptilolite and K3PO4 at different loads from 10 to 30 wt.%. These observed phenomena prove the existence of synergistic effects in microwave heating by mixing two or more catalysts. Thus, the energy efficiency of microwave     40  heating can be improved via selecting proper catalysts, which promote the exothermic reactions. In order to reduce microwave power consumption, the mixture of 10 wt.% K3PO4 + 10 wt.% bentonite is recommended as a good microwave absorber due to its high microwave absorption rate which resulted in reduced microwave irradiation time compared to other combinations of catalysts.   Figure 2.2 Heating behaviour of pure switchgrass and switchgrass with mixtures of different catalysts. A possible mechanism for microwave heating of biomass premixed with a microwave absorber is that: microwave radiation is first absorbed by added microwave absorbing material and then the absorbed energy is transferred to biomass by heat conduction.28 During pyrolysis, the temperature of a good microwave absorber is much higher than the biomass particles, and the biomass particles are heated mainly through conduction. As a result, the real temperature of the microwave absorbing particles is probably higher than the biomass particles and the measured surface temperature.      41   Figure 2.3  Heating behaviour of switchgrass mixed with 10 % and 20 wt.% bentonite.  Fig. 2.4 compares microwave heating and conventional heating using the same mixture of catalysts. A total of 20.0 g switchgrass and 10 wt.% K3PO4 + 10 wt.% bentonite was heated in a fixed bed reactor inside an electrical furnace to compare the heating behaviour with that in a microwave reactor. 750 W of microwave power was used to reach the designated pyrolysis temperature at 400°C; while, the heat capacity of the electrical furnace was 800 W. Based on the 10-fold difference in the time required to heat the sample to 400°C, a significant amount of energy can be saved by reducing the time of microwave irradiation as a result of using a proper microwave absorber.      42   Figure 2.4 Heating behaviour of switchgrass with 10 wt.% K3PO4 + 10 wt.% bentonite under microwave heating (MH) and conventional heating (CH).  2.2.2 Microwave-assisted pyrolysis products  Fig. 2.5 shows the products distribution of microwave-assisted pyrolysis of switchgrass with additives of SiC, activated carbon, K3PO4, or clinoptilolite. Gas yield was taken by difference. Since the microwave-assisted pyrolysis of pure switchgrass did not reach the desirable temperature of 400°C (Fig. 2.1a), SiC, a chemically inert additive, 20 wt.% was mixed with switchgrass to speed up the microwave heating and promote pyrolysis reactions. 20 wt.% SiC load was selected as an average for the other catalysts loads (10, 20 and 30 wt.%). However, depending on the mixed percentage of SiC with switchgrass, the heating rate might change because of changing microwave absorption rate of the sample. This in turn might affect the final pyrolysis products distribution of switchgrass. It is shown that the gas yield produced from the sample mixed with SiC is the highest     43  which proves the hypothesis that self-gasification reaction between carbon and carbon dioxide favours the gas production in microwave-assisted pyrolysis.15,28    (a)  (b) Figure 2.5 Pyrolysis products distribution of microwave-assisted pyrolysis of switchgrass with: (a) 20 wt.% SiC, 10, 20 or 30 wt.% of K3PO4 and clinoptilolite; and (b) 20 wt.% SiC, 10 wt.% K3PO4 + 10 wt.% bentonite, 10 wt.% K3PO4 + 20 wt.% bentonite, 10 wt.% K3PO4 + 10 wt.% clinoptilolite and 20 wt.% of activated carbon (Error bars represent standard deviation of 3 replicates).        44  The catalysts mixed with switchgrass showed catalytic effects on pyrolysis products distribution with remarkable difference between K3PO4 and clinoptilolite at the three different proportions. At 10 wt.% ratio, the bio-oil and biochar yields for the sample mixed with 10 wt.% clinoptilolite are slightly higher than 10 wt.% K3PO4, while the gas yield is moderately higher for 10 wt.% of K3PO4. Because of the lower heating rate and longer heating or reaction time for 10 wt.% clinoptilolite which favours biochar production, more biochar and less amount of gas were resulted. However, at 20 wt.% ratio, K3PO4 showed the highest gas yield among all samples of different proportions of K3PO4. The decrease of bio-oil yield and gas yield is probably due to the high average heating rate of the samples, which might increase the catalytic cracking reactions of more noncondensable gases compared with clinoptilolite sample.32 Another possible reason is that potassium might catalyze the gasification reaction and promote the cracking reaction of high molecular components at high heating rates, creating small molecules of gases and coke and resulting in increased percentage of noncondensable gases (e.g., CO, H2) according to Reactions (2.1) and (2.2).15,28,103 Secondary cracking reactions of volatiles is favoured at higher temperatures (> 800ºC) in conventional pyrolysis, which would shift the equilibrium towards CO production.28,103 However, the secondary cracking of volatiles is considerably different compared with conventional pyrolysis due to the unique nature of microwave heating.103 Due to the lack of local temperature measurement, this cannot be confirmed in our current study. It can still be speculated that increased occurrence of hotspots due to the presence of metals in the catalysts, may result in considerably higher local temperatures than the measured temperature of the bed surface.28,103 The temperature of the hotspots may be high enough to promote heterogeneous reactions between the solid and the gases produced.15 Therefore, it is expected that the heterogeneous reactions between CO2, H2O     45  and carbon particles in Reactions (2.1) and (2.2) could be triggered at low temperatures or even at the early stages of the microwave pyrolysis process.103  As noted earlier, 30 wt.% K3PO4 showed an opposite trend (i.e., reduced heating rate) compared to 10 and 20 wt.% K3PO4, with increased biochar yield and reduced gas yield. It can be postulated that 30 wt.% K3PO4 promotes Reactions (2.3) and (2.4) because of the low average heating rate, which increased the shift of the equilibrium towards carbon and CO2 production. Consequently, produced CO2, which is believed to be responsible for increasing the gas yield because of self-gasification reaction according to Reaction (2.1), might consume CO2 in the dry reforming reaction with methane and resulted in reduced gas yield according to Reaction (2.3).15,28 However, gas composition was not measured in this study to confirm this phenomenon, but the typical CH4 concentration from non-catalytic conventional pyrolysis of switchgrass can be ~39 %.106 Therefore, the highest biochar yield at 30 wt.% K3PO4 is probably due to the low heating rate and the presence of higher percentage of potassium, which is believed to be responsible for increasing the secondary carbon formation according to Reaction (2.1), which is favoured for producing more solid product.103 The second reason for these phenomena is that increasing the ratio of K3PO4 up to 30 wt.% might inhibit the devolatilization reaction for hemicellulose during pyrolysis process which results in high biochar yield and low gas and bio-oil yields compared with the sample mixed with SiC or clinoptilolite.43 Increasing the ratio of K3PO4 from 10 to 30 wt.% reduced the weight loss markedly from 69.2 wt.% to 55.8 wt.%, which results in lowered gas yield at 30 wt.% load. This leads us to propose that the addition of higher load of K3PO4 changed the reactions slightly toward bio-oil and markedly for biochar production, i.e., higher bio-oil yield leading to lower gas yield and vice versa, which greatly depends on the heating rate and the microwave irradiation time.        46  A maximum bio-oil yield was obtained at 36.2 wt.% from the sample mixed with 30 wt.% clinoptilolite compared with other catalysts. The bio-oil yield at 30 wt.% clinoptilolite is almost equal to the highest bio-oil yield for the switchgrass at 36.3 wt.% which was produced at 650ºC after 18 min of residence time under microwave irradiation.107 Clinoptilolite catalyst produced more liquid product than H-ZSM-5, but less gaseous product.38 H-ZSM-5 generates products that contain more gases because it has intermediate-sized pores with intersecting channels with pore sizes of 5.4×5.6 Å and 5.1×5.5 Å. Therefore, more gas yield can be generated as a result of the diffusion process of cracked fragments through the pores of H-ZSM-5 and of further cracking of those fragments in the cavities created at the intersection of two channels, producing more gaseous products.38 While natural zeolite has a clinoptilolite type structure with two main channels of pore sizes of 7.6×3.0 Å and 3.3×4.6 Å, which decreases gas products and increases bio-oil product.38 It has been reported that the bio-oil yield of cotton-seed cake increased by > 26% compared to the yield of bio-oil produced without clinoptilolite.34 It can thus be proposed that the most effective ratio of clinoptilolite is 30 wt.% for switchgrass to achieve a high bio-oil yield.  Fig. 2.5b illustrates the pyrolysis product distribution of microwave-assisted pyrolysis of switchgrass with the addition of SiC, activated carbon and combinations of K3PO4, bentonite or clinoptilolite. A maximum gas yield of 43.8 wt.% was obtained from the sample mixed with 20 wt.% activated carbon, while the bio-oil yields are almost the same for 10 wt.% K3PO4 + 10 wt.% bentonite and 10 wt.% K3PO4 + 10 wt.% clinoptilolite.   It was suggested that activated carbon favoured gas production due to the increased devolatilization rate of the organic materials, and a partial gasification of the biochar also likely contributed to the increased gas yield.15 From our results, increasing the mixed amount of bentonite from 10 to 20 wt.% with 10 wt.% of K3PO4 slightly reduced the gas yield and partially increased     47  the bio-oil yield. Adding 10 or 20 wt.% bentonite, however, increased bio-oil yield and reduced gas yield compared with the sample mixed with only 10 wt.% K3PO4. These phenomena confirm that bentonite has an impact on the occurrence of secondary cracking reactions.35 It should be noted that the average heating rate of the sample mixed with 10 wt.% K3PO4 and 10 wt.% bentonite is considerably higher than the sample mixed with only 10 wt.% K3PO4. This seems to be supported by the reported findings that bentonite could promote exothermic reactions such as water-gas shift reaction due to the presence of Fe and Ca, and inhibit the endothermic self-gasification reaction between the organic carbon of the biochar and CO2.28,35,103 It was found that bentonite significantly increased the bio-oil yield by 48% compared to the non-catalytic process, which is much higher than other tested catalysts.35 Consequently, bentonite clay was proposed to be the most effective catalyst in contrast with synthetic zeolites. Furthermore, its abundance and low price increased its potential to be used as a catalyst.35  Biochar yield for the same feedstock is highly dependent on the production conditions during pyrolysis, namely, temperature, heating rate, heating time and particle size.3 As shown in Fig. 2.5a, biochar yield decreased with increasing clinoptilolite load from 10 to 30 wt.% with a corresponding average heating rate of 20 and 59ºC/min, while keeping the pyrolysis temperature of particles constant. However, K3PO4 showed an opposite trend, with biochar yield increasing remarkably with increasing K3PO4 loading possibly due to the inhibition of devolatilization of hemicellulose.43 If the goal is to produce biochar, 30% K3PO4 might be a good option since it would result in a high yield of biochar rich in K and P, which are considered as the essential-nutrients for plants.       48  2.2.3 Effect of catalysts on bio-oil properties 2.2.3.1 Bio-oil acidity The pH value is considered as one of the most important bio-oil properties, and is used as an indicator of corrosiveness. The high acidity of bio-oil is a major concern particularly if the goal is to use it as a fuel. The acidity of bio-oil is related to the presence of carboxylic compounds such as acetic and formic acids.59 The pH of typical fast pyrolysis liquids is very low, e.g., 2.5 for wood materials (birch, pine and poplar) under conventional pyrolysis.59 As shown in Table 2.1, both clinoptilolite and K3PO4 significantly increased the pH of bio-oil compared with the pH of bio-oil from switchgrass with SiC. In general, increasing the catalyst load increased the pH except for bio-oil produced at 10 wt.% clinoptilolite. It has been revealed that higher percentage of K3PO4 inhibits the devolatilization of hemicellulose to create organic volatile compounds, leading to reduced linear acids in bio-oil when 50 wt.% K3PO4 was added to poplar wood in conventional heating.43 Acetic acid was found to be the major acid product from deacetylation of hemicellulose.43 It should be noted that clinoptilolite has a high content of CaO, which is responsible for reducing the formation of acid compounds as a result of reaction between CaO particles and CO2-like compounds to form calcium salts.108 Reducing the acidity of bio-oil is considered as a major task because high bio-oil acidity leads to many problems associated with bio-oil transfer in pipes and use in engines, and the strong acidity of bio-oil also reduces the stability of bio-oil and prompts polymerization reactions.37 2.2.3.2 Bio-oil water content Table 2.1 shows that increasing the ratio of clinoptilolite from 10 to 30 wt.% reduced the bio-oil water content markedly. This is in line with the water content of bio-oil produced from cotton-seed cake, which decreased by 31.2% as a result of increasing the load of clinoptilolite from     49  1 to 20 wt.% at 400ºC.34 Mixing 10 wt.% K3PO4 with 10 wt.% clinoptilolite dramatically reduced the water content of bio-oil by 39.5% and 25.7%, respectively, compared with the bio-oil produced at 10 wt.% clinoptilolite only and 10 wt.% K3PO4 only, demonstrating a potential synergistic effect of catalyst mixture. Mixing K3PO4 with bentonite showed the lowest water content compared to SiC and other catalysts, but no significant difference was observed after increasing the percentage of bentonite from 10 to 20 wt.% mixed with 10 wt.% K3PO4. These findings confirmed that these catalysts possess good catalytic activities, leading to changes in bio-oil properties. One possible reason for the high water content in some bio-oil is the low heating rate which prolongs the heating time and likely increases the dehydration reactions.103 Conventional pyrolysis process is generally classified into three subclasses: slow, fast and flash pyrolysis in which heating rates are 0.1-1, 10-200 and > 1000 K/s, respectively. Slow pyrolysis produces bio-oil with high water content due to dehydration reactions which are favoured at low temperatures < 325ºC and slow heating rate.103 While, high heating rates result in better bio-oil and biochar qualities.1,6 This seems to be supported by the low water content in some samples obtained at high average heating rates over short heating time. Furthermore, water is considered a strong microwave absorber. In the microwave heating process, water inherent in the feedstock will evaporate quickly, making the possibility of water to be involved in the conversion reactions to be low. On the other hand, drying and the release of volatiles in conventional pyrolysis occur simultaneously, making water to co-exist with the cracking reactions during conventional pyrolysis.103 It is believed that bound water will be slowly released during pyrolysis process at higher temperatures due to the strong chemical bonding, with addition to water produced from dehydration reactions.15 As a result, the exothermic water-gas shift reaction can occur more likely in microwave-assisted pyrolysis at lower temperatures or during the initial stages of the reactions.103 It has been found that bentonite fraction     50  in the sample is an important factor influencing the rate of chemical reactions during pyrolysis.35 It was found that the increase of bentonite content increased the formation of hydrocarbons, carbon dioxide and hydrogen, meanwhile decreased carbon monoxide concentration due to the high concentrations of Ca and Fe in bentonite, which promoted the water-gas shift reaction.35 A hypothesis has thus been proposed that the exothermic reaction of carbon monoxide with water (water-gas shift reaction) would result in the formation of CO2 and H2:15,35 CO(g) + Hଶ O(g) ↔ COଶ(g) + Hଶ(g)   ∆Hଶଽ଼ ୏ = −41.5 kJ molିଵ   (2.5) Table 2.1 pH, water content and viscosity of bio-oil produced from different catalysts with different loads. Bio-oil properties 20 wt.% SiC 10 wt.% Clino 20 wt.% Clino 30 wt.% Clino 10 wt.% K3PO4 20 wt.% K3PO4 30 wt.% K3PO4 10 wt.% K3PO4 +10 wt.% Bento 10 wt.% K3PO4 +10 wt.% Clino 10 wt.% K3PO4 +20 wt.% Bento pH 3.08 3.12 3.65 4.19 4.53 5.30 5.61 4.68 4.46 4.89 SD ±0.061 ±0.045 ±0.065 ±0.075 ±0.087 ±0.090 ±0.102 ±0.080 ±0.083 ±0.085 Water content (wt.%) 22.7 25.1 23.7 21.8 22.6 20.8 21.8 15.1 18.0 14.6 SD ±0.51 ±0.48 ±0.45 ±0.36 ±0.41 ±0.40 ±0.43 ±0.28 ±0.31 ±0.29 Viscosity (40°C) (cP) 14.8 4.63 5.61 6.11 6.16 5.18 4.97 6.82 7.43 8.18 SD ±0.64 ±0.14 ±0.23 ±0.28 ±0.26 ±0.22 ±0.25 ±0.24 ±0.25 ±0.36 Values represent the mean of triplicate samples ± SD (standard deviation).  Based on this hypothesis, it can be interpreted that the low water content of bio-oil and the high heating rate (141ºC/min) with the bentonite catalyst were caused by the exothermic reaction of CO with H2O. Microwave heating rate and heating time are important factors because of their indirect impact on bio-oil quality. The bio-oil produced from 10 wt.% K3PO4 + 10 wt.% bentonite showed a low water content, possibly related to its higher average heating rate and short microwave     51  heating time. Furthermore, reducing heating time through selecting a proper microwave absorber would likely decrease the water content of bio-oil under microwave-assisted pyrolysis by minimizing the dehydration reactions and promoting the consumption of H2O in the exothermic water-gas shift reaction.   2.2.3.3 Bio-oil viscosity Viscosity is one of the bio-oil properties that often limits bio-oil being used directly as a fuel, as viscosity affects the atomization of fuel. It is seen in Table 2.1 that the catalysts showed a remarkable effect in lowering bio-oil viscosity compared with the bio-oil produced from switchgrass with SiC. However, increasing the load of clinoptilolite from 10 to 30 wt.% slightly increased the viscosity because of the reduced bio-oil water content. It has been reported that microwave-assisted pyrolysis produces much more noncondensable gases and less bio-oil than conventional pyrolysis, because the microwave heating promotes secondary cracking reactions during pyrolysis which, at the same time, also increases the amount of lighter compounds in the bio-oil.60 The molecular weight of the produced bio-oil was not measured. However, a linear relationship was found between bio-oil viscosity and its average molecular weight.59 As the addition of catalysts reduced bio-oil viscosity compared to the control sample, the average molecular weight of bio-oil is expected to decrease. This suggests that the addition of catalysts upgrades the bio-oil quality by cracking down the heavy components in bio-oil, lowering the viscosity of bio-oil, which is considered as a favourable attribute for handling and transporting bio-oil, as well as using bio-oil as a fuel.  The yield of bio-oil in microwave catalytic pyrolysis from this study is lower than the bio-oil yield in conventional fast pyrolysis which can give a bio-oil yield up to 75%. However, the quality of the bio-oil from conventional fast pyrolysis is very poor as a result of high water content     52  > 25 wt.%, high acidity with pH less than 3 and high viscosity that ranges from 40 to 100 cP.1 While microwave catalytic pyrolysis remarkably improved the bio-oil quality compared to the bio-oil produced from conventional fast pyrolysis, it is expected that the market value of bio-oil produced from this study is higher than the market value of conventional bio-oil which, in turn, will offset the higher capital cost of microwave catalytic pyrolysis. 2.2.4 Effects of catalysts on biochar properties  2.2.4.1 Biochar elemental composition Catalysts at different loading resulted in biochars of various ash content and elemental compositions. From Table 2.2, it can be seen that ash content significantly increased with increased catalyst loading, and consequently carbon content decreased. Understandably, potassium and phosphorus percentages increased because of the increase in the ratio of K3PO4. The high contents of the essential plant nutrients (K, P, Ca, Mg and S) and micronutrients (Fe, Cu, Mn and Zn) in biochars are favoured for improving soil productivity and increasing crops yield, and these biochars could thus be used as a balanced fertilizer for plants and crops.109  Furthermore, some biochars contain high amounts of phosphorus compounds which are considered to be a good immobilizer for heavy metals by changing and altering the form of metals from soluble into insoluble phosphate compounds.17,110 In addition, bentonite and clinoptilolite remaining in the produced biochars will improve biochar sorption properties for chemical pollutants and heavy metals, increase soil water and nutrient retention and provide plants with important nutrients.77,80,111 Consequently, it may be possible to create a highly nutrient-rich biochar for soil applications by choosing the right catalyst that should be a good microwave absorber to speed up microwave heating within a few minutes and the catalyst would remain in biochar to serve as a slow release fertilizer.      53  Table 2.2 Effect of different catalysts on biochar elemental composition, ash content and elemental analysis. Biochar type 10 wt.% Clinoptilolite 30 wt.% Clinoptilolite  10 wt.% K3PO4 30 wt.% K3PO4 10 wt.% K3PO4 +10 wt.% Bentonite 10 wt.% K3PO4 +10 wt.% Bentonite (CH)a 10 wt.% K3PO4 +10 wt.% Clinoptilolite Elemental composition (wt.%)b      C  57.3 35.8 42.9 25.9 37.3 35.8 34.2 H  0.38 0.10 0.36 1.39 0.10 1.10 0.21 N  1.10 0.82 0.54 0.31 0.41 0.44 0.31 Oc 12.2 14.6 20.4 12.8 14.2 16.6 14.1 Ash (wt.%)b 28.9 48.6 35.8 59.7 47.9 46.1 51.2 Elemental analysis (g/kg)d K2O  65.2 109 150 463 366 - 260 P2O5  2.43 18.3 159 446 385 - 158 MgO  9.51 38.8 5.02 11.5 7.51 - 21.5 CaO  30.4 58.1 13.2 32.6 19.9 - 26.6 SO3  0.37 1.75 1.67 2.27 2.27 - 3.77 Fe2O3 11.7 38.9 0.73 0.43 0.86 - 23.8 MnO  0.12 0.48 0.09 0.09 0.10 - 0.12 ZnO  0.05 0.11 0.04 0.04 0.04 - 0.06 CuO  0.01 0.04 0.04 0.03 0.03 - 0.03 a Conventional heating. b Dry basis c Calculated by difference  d Calculated from elements content based on dry basis.  2.2.4.2 Biochar specific surface area and porosity  Table 2.3 shows that the highest BET surface area is associated with biochar produced by adding 10 wt.% K3PO4 + 10 wt.% bentonite, which also showed the highest heating rate. In addition, there is an increase in BET surface area, micropore area and pore volume with increasing     54  catalyst load except for the biochar produced from 30 wt.% K3PO4, while the average pores diameters remaining the same. Furthermore, by comparing different heating methods with the same mixture of catalysts (10 wt.% K3PO4 + 10 wt.% bentonite), biochar produced from microwave-assisted pyrolysis has a significantly higher BET surface area (i.e., 76.3 m2/g compared to 0.33 m2/g for sample from conventional heating), micropore area and pore volume than those produced from conventional heating. Note that samples would normally contain more additives than biochar after pyrolysis, and that Table 2.3 would represent the average measurements of these samples. From Table 2.3 it is noticed that the specific surface area for clinoptilolite is 24.9 m2/g and the content of catalyst in 30Clino biochar is about 50%. If we consider that the specific surface area of the catalyst remained the same after pyrolysis, the specific surface area for solid biochar without catalyst could be estimated to be about 79 m2/g, which is higher than 46.9 m2/g for the biochar-catalyst mixture in Table 2.3. It is also well-known that the specific surface area for catalysts could be reduced by up to 35% after pyrolysis because of coke deposition,49 which is especially true under slow heating which promotes condensation and repolymerization reactions as shown for the biochar produced from conventional pyrolysis 10 wt.% K3PO4 +10 wt.% Bentonite (CH). Thus, the specific surface area for pure biochar from the current study is expected to be higher than the reported values for biochar-catalyst mixtures in Table 2.3.  Biochar produced from fast pyrolysis (high heating rates) have different physical properties from biochars produced under slow pyrolysis conditions (low heating rates).6 In microwave-assisted pyrolysis, moisture is vaporized from the depth of the particle prior to the organic contents being volatilized. The steam from vaporized water is swiftly released, and not only sweeping volatiles from the pores, but also creates preferential channels in the biochar, which in turn increases the biochar porosity. As a result, the volatiles releasing rate at low temperatures is     55  expected to be higher than conventional heating.14 Moreover, the porosity of biochars produced from microwave-assisted pyrolysis is higher than those produced from conventional pyrolysis, and also the steam gasification reactions.103 Biochars produced under low heating rates are predominated by micropores; while, macropores are largely found under high heating rates due to larger pores created from the melting of the cell structure.63 In microwave heating, particles are directly heated. As a result, heterogeneous reactions are favoured in microwave heating compared to conventional heating and the higher heating rate might affect pores structure of biochar.28  Table 2.3 The average heating rate (25 to 400°C), heating time, BET surface area, average diameter, micropore area and pore volume of different biochars produced from microwave heating and conventional heating. Biochar type Average heating rate (°C/min) Heating time  400°C (min) BET Surface Area (m²/g) Average diameter (nm) Micropore Area  (m²/g) Pore volume cm3/g 10 wt.% Clinoptilolite  20 20.0 33.1 2.15 10.9 0.0091 20 wt.% Clinoptilolite  25 16.1 34.1 2.43 14.7 0.0107 30 wt.% Clinoptilolite  59 6.89 46.9 2.22 26.8 0.0141 10 wt.% K3PO4 26 15.1 33.5 2.15 20.1 0.0107 20 wt.% K3PO4 60 6.66 53.3 2.11 37.9 0.0275 30 wt.% K3PO4 26 15.3 39.2 2.19 24.3 0.0128 10 wt.% K3PO4 +10 wt.% Bentonite (MH) 141 2.83 76.3 2.34 44.6 0.0332 10 wt.% K3PO4 +10 wt.% Bentonite (CH) 14 28.8 0.33 2.10 2.01 0.0068 Pure Clinoptilolite a - - 24.9 - - -  MH; microwave heating, CH; conventional heating. a Surface area for clinoptilolite was provided by the supplier.  Increasing the catalyst load increased the average heating rate, and thus increased BET surface area. This can be noticed by increasing the load of clinoptilolite from 10 to 30 wt.%, which     56  increased the average heating rate considerably, which in turn, resulted in increased BET surface area, micropore area and pore volume of biochar. Further evidence is observed at different percentage of K3PO4, as 20 wt.% K3PO4 showed the highest specific surface area, micropore area and pore volume than 10 and 30 wt.% K3PO4 because of the highest average heating rate of the sample at 20 wt.% K3PO4. More information about the pore size distribution and isotherm plot for some of the produced biochars can be found in Appendix A.  2.2.4.3 Scanning electron microscopy (SEM) of biochars Heating method had a significant effect on the morphology of the biochar structure. Biochars produced from microwave heating had a more porous structure than biochar produced under conventional heating due to the higher heating rate and the unique heating process of microwave heating. In conventional pyrolysis, volatiles produced during pyrolysis are released from outer layer first, and the thermal decomposition of volatiles released from the sample is expected to increase as the reaction develops toward the centre of the particle because of the slow heating rate.63 Because of the cracking volatiles, it will deposit inside the pores and block them. However, the micropores of biochar produced from microwave-assisted pyrolysis are clean and have more pores due to the uniform release of the volatile matters across the whole particle.25 At low heating rate during pyrolysis, the inherited porosity of feedstock would allow volatiles to release without morphological changes.63 However, higher heating rates quickly release the volatiles, and modify the pore structure of biochar associated with increased yield of liquid and gas fractions.6,28 This can be noticed for the biochar produced from 10% K3PO4 + 10% bentonite under microwave pyrolysis which has more clean pores, compared to the biochar produced from conventional pyrolysis using the same catalysts mixtures, as shown in Fig 2.6. Therefore, microwave-assisted pyrolysis can provide a new strategy for creating more porous biochars, which can be used in     57  sorption applications or as a precursor for producing activated carbon (Figs. 2.6 and 2.7). From the SEM images taken at 100-micron scale which is bigger than catalyst particles (< 50-micron), it can be seen that the structure of biochar appears to contain more pores which contribute mostly to the high BET surface area, supporting previous discussion on high biochar specific surface area.      Conventional heating  Microwave heating Figure 2.6 SEM micrographs of 10% K3PO4 + 10% bentonite biochar produced under conventional heating and microwave heating.        58   Figure 2.7 SEM micrographs of different types of biochars produced under microwave heating of switchgrass mixed with different catalysts.   2.3 Summary K3PO4, clinoptilolite and bentonite showed good catalytic activities in microwave-assisted pyrolysis, resulting in reduced yield, acidity, viscosity and water content of bio-oil product. Catalyst loading, and combination of different catalysts are important in controlling average heating rate and product quality. Mixing 10 wt.% K3PO4 with 10 wt.% clinoptilolite considerably reduced the water content of bio-oil by 39.5%; while pH of bio-oil increased by 43% compared to 10 wt.% clinoptilolite only, demonstrating a potential synergistic effect of catalyst mixtures. In 30% K3PO4  10% K3PO4  10%K3PO4 +10%clinoptilolite 30% clinoptilolite  30% clinoptilolite  30% clinoptilolite   20% clinoptilolite 20% clinoptilolite 20% clinoptilolite     59  addition to catalytic effect, those results are partly attributed to the increased heating rate and the internal-heating of biomass particles in the microwave reactor.                    60  Chapter 3: Catalyst-biomass interactions and kinetics of catalytic pyrolysis. Most of the recent research has focused on the kinetics of biomass in the absence of catalysts with limited studies on biomass catalytic pyrolysis. Furthermore, there are no clear studies on the kinetics of the selected catalysts (clinoptilolite and bentonite) and the effect of catalyst particles sizes on conversions as relates to the final solid yield. These catalysts have been previously tested under microwave heating in Chapter 2 and indicated good performance as catalysts to improve the quality of bio-oil and biochar, and as a microwave absorber to increase the microwave heating rate and reduce microwave energy consumption. Because of the difficulties associated with studying the intrinsic kinetics under microwave heating, TGA was selected to study the catalytic behaviour of those catalysts which will help us to interpret microwave catalytic pyrolysis and design efficient pyrolysis microwave reactors. The aims for this chapter are therefore to study: (1) the catalytic behaviour for these selected catalysts in comparison with results using pure switchgrass and switchgrass mixed with silica sand; (2) the effect of heating rate and particles size of the selected catalysts on the solid yield; and (3) the catalytic pyrolysis kinetics for each of the three major biomass components using the lumped three-parallel reaction model. These data can be used for design and optimization of the microwave catalytic pyrolysis reactors. 3.1 Experimental 3.1.1 Biomass sample and thermogravimetric analysis   Switchgrass (SG) was crushed and sieved to small particles (100–150 μm) to keep a low Biot number (< 0.01) in order to minimize the internal heat and mass transfer resistances. Clinoptilolite or bentonite with particle sizes < 50 μm were mixed with SG at 30 wt.%. Switchgrass was mixed with silica sand, as an inert material, which has also a thermal conductivity close to     61  bentonite (Table 3.1) to differentiate between the thermal and thermal catalytic decompositions. More information about switchgrass and catalysts can be found in Chapter 2.  Thermogravimetric analysis (TGA) of the samples was conducted using a SDT Q600 TGA with a nitrogen flow rate of 100 mL/min. The sample was loaded into a crucible made of alumina and the weight was recorded every 0.5 s and the accuracy of the balance is 0.0001 mg. The experiments were performed at heating rates of 10, 25, 50, 100 and 150°C/min and the experiments were repeated two to four times to ensure the reproducibility of the results with a standard deviation less than 5%. It should be noted that TGA heating rate cannot go beyond 150°C/min, while the microwave heating rate under microwave catalytic pyrolysis for some sample can reach more than 400 °C/min at the heating stage (110-260°C), although the average overall heating rate for the samples only reached about 141 °C/min (25-400°C) under the microwave catalytic pyrolysis.      3.2 Potential interaction mechanisms between biomass and catalyst particles The central inquiry of this chapter is to understand how the catalyst particles interact with biomass particles in terms of increased heating rate of biomass particles via solid-solid contacting heat transfer (in which catalyst acts as a heat transfer medium only); and catalyzes cracking of organic vapours from biomass cracking through different reaction pathways. 3.2.1 Catalyst particles as a heat transfer medium:  Due to a high thermal conductivity and heat capacity of catalyst particles, the presence of catalyst will increase the heat transfer between biomass particles and also improve temperature uniformity. It is thus expected that the finer particles will have higher contact surface area between catalyst particles and biomass particles, leading to faster heat up of biomass particles which, in turn, will increase the mass loss rate and reduce the final solid yield under the assumption of no     62  other influencing factors such as porosity, packing density and internal heat and mass transfer limitations of particles.  3.2.2 Solid-catalyzed biomass decomposition:  Biomass decomposition reaction may be catalyzed by direct contact with catalyst particles in addition to convective heat transfer. As a result, biomass particle cracking reaction will be accelerated, leading to a lower final solid yield like in fast pyrolysis, and changes in vapour product composition reflected in the intermediate products from direct cracking of biomass. Because of the relatively large particles with limited direct contact between catalyst and biomass surfaces, such solid catalyzed biomass cracking mechanism will unlikely be dominant in our system.     3.2.3 Catalyzed vapour cracking:  In this mechanism, it is expected that catalyst will expedite the reactions of gaseous intermediates released from biomass decomposition. The intermediates will be cracked in-situ once they are released from biomass cracking, preventing re-condensation or polymerization of those intermediates, equivalent to quick removal of volatiles as cracking products from the catalyst surface. In which case, a lowered solid yield from catalytic pyrolysis and a higher yield of light hydrocarbons are expected, given limited heat or mass transfer resistances, because of the catalytic vapour cracking reactions. Such a mechanism is anticipated to be the most dominated mechanism for catalytic pyrolysis at high temperatures (> 300°C). If the catalytic vapour cracking is dominant, the changing of catalyst particle size will not be expected to have a significant effect on the mass loss rate and/or the final solid yield. In general, the contact heat transfer and vapour cracking mechanisms may dominate but at different stages, in which conductive heat transfer is expected to dominate during the drying and heating (torrefaction) stages at relatively low temperatures (> 280°C) when there is little biomass cracking; while,     63  catalyzed vapour cracking may dominate during the pyrolysis stage (280-600°C) when most volatiles are released from biomass particles. It should be noted that although catalytic vapour cracking may dominate, it does not eliminate the existence of the solid-solid catalytic biomass cracking and vice-versa because some catalytic vapour reactions can also increase the final solid yield (e.g., char formation reactions).  As mentioned earlier, many factors can affect the final solid yield. For that reason, relying only on the change in solid yield with changing the catalyst particles size to determine which mechanism is dominant is not conclusive. Moreover, the mass loss rate should also be examined as an important indicator, as it is more sensitive to the change in heat transfer and the chemical activity of the catalyst particles. Thus, the mechanisms are examined by comparing the weight loss curves for catalyst particles of different sizes mixed with biomass.  Table 3.1 Thermal properties of switchgrass, char, silica sand and different catalysts. Thermal Properties Value References Thermal conductivity of char 0.105 (W/m K) 112 Thermal conductivity of switchgrass 0.096 (W/m K)  Thermal conductivity of clinoptilolite 0.61 (W/m K) 113 Thermal conductivity of bentonite 1.15 (W/m K) 114 Thermal conductivity of fine sand (< 0.362 mm) 1.32 (W/m K) 115 Thermal capacity of bentonite 1150 (J/kg K) 114 Thermal capacity of clinoptilolite 1123 (J/kg K) 113 Thermal capacity of biomass 1500 + T (J/kg K) 116 Thermal capacity of char 420 + 2.09 × 𝑇 + 6.85 × 10ିସ × 𝑇ଶ (J/kg K) 116  To study the effect of catalyst particle size, original catalyst particles (< 50 μm) and fine (< 20 μm) were mixed with the same biomass particles (100-150 μm). The relationship between thermal properties of biomass and catalyst particles, particles size and the time required for the     64  core of biomass particles to reach the temperature of the surroundings can be expressed by Heisler method in the following form:      (3.1)  where: Ts, temperature of the surroundings; Tc, temperature of the core; T0, the initial temperature of particle; Bi, Biot number; αd, thermal diffusivity; r, particle radius and t, is the time required for the core of a particle to reach 99% of the surrounding temperature, t, can be determined from the Heisler charts.  Table 3.2 shows that the thermal properties and size of catalyst particles play important roles in expediting the release of volatiles from the core of biomass particles which may increase the mass loss rate and thus reduce the activation energy compared to biomass without catalyst. Furthermore, quick release of volatiles from biomass will reduce the solid yield in the absence of internal heat and mass transfer limitation by reducing the occurrence of condensations and repolymerization reactions due to the shortened residence time of volatiles.1 Our experimental results showed significant increase in the mass loss rate for the finer catalyst particles compared to the original catalyst (Table 3.3), which reveals the importance of the thermal conductivity of catalysts in increasing biomass devolatilization. Table 3.2 The time required for the core of a switchgrass particle to nearly (99%) reach the temperature of 450°C.  Sample Bi-number (< 0.1, recommended) 𝜶𝒕𝒓𝟐 t (s) SG (150-100 micron) 0.012 0.6563 0.0171 SG (300-150 micron) 0.029 0.9375 0.1469 30Clino+SG-150-100 micron 0.0038 0.5938 0.0032 30Bento+SG-150-100 micron 0.0023 0.5625 0.0019 ೞ்ି ೎்ೞ்ି బ் = 𝑓 ቀ𝐵𝑖,  𝛼ௗ௧௥మቁ     65  3.3 Reaction kinetics  Pyrolysis is a complex process which involves many parallel and series chemical reactions, lumped models can be used to describe the effect of catalytic and non-catalytic reactions. It is assumed that the material will decompose through independent parallel and competitive reactions following nth order reaction and it is assumed that the reaction rate constant follows Arrhenius equation.  𝑘௜ =  𝐴௜  𝑒𝑥𝑝ିா೔ோ்  (3.2) 𝑑𝛼௜𝑑𝑡= 𝑘௜ ∙ 𝑓(𝛼௜) (3.3) 𝑓(𝛼௜) =  (1 − 𝛼௜)௡೔ (3.4) 𝛼௜ =𝑚௜௢ − 𝑚௜௧𝑚௜௢ − 𝑚௜௙ (3.5) 𝑑𝛼௜𝑑𝑡= 𝐴௜ ∙ ൬𝑒𝑥𝑝ିா೔ோ் ൰ ∙ (1 − 𝛼௜)௡೔ (3.6) To make the model independent of the heating rate (β = dT/dt), equation (3.6) is divided by β and then rearranged to the following form:   𝑑𝛼௜𝑑𝑇= ൬𝐴௜𝛽൰ ∙ ൬𝑒𝑥𝑝ିா೔ோ் ൰ ∙ (1 − 𝛼௜)௡೔ (3.7) Lignocellulosic materials contain three main components which are cellulose, hemicellulose and lignin. The average values for the three components reported for switchgrass are 0.321, 0.360 and 0.256 wt.% for cellulose, hemicellulose and lignin, respectively.117,118 Using biomass three parallel pseudo-components reactions approach will help to understand the catalytic effect on the three pseudo-components so as to identify possible catalytic mechanisms.  𝑑𝛼௝𝑑𝑇= ෍ 𝐶௝௡௝ୀଵ൬𝐴௝𝛽൰ ∙ ൬𝑒𝑥𝑝ିாೕோ் ൰ ∙ ൫1 − 𝛼௝൯௡ೕ (3.8)     66  Where Cj is related to the initial compositions and called pseudo-components of lignin, cellulose, and hemicellulose; α the conversion ratio; A the frequency factor; β the heating rate; E the activation energy; R the universal gas constant; and n is the reaction order.  3.4 Results and discussion  Figs. 3.1 and 3.2 show typical TGA mass loss curves for SG, SG+30Sand (i.e., SG with 30 wt.% Sand), SG+30Clino (i.e., SG with 30 wt.% Clinoptilolite), and SG+30Bento (i.e., SG with 30 wt.% Bentonite) samples. The two distinctive peaks in the DTG curve correspond to the drying (lower than 150°C) and pyrolysis (> 300°C) stages. Based on the TGA and DTG curves, the final solids yield (ash-free basis) and mass loss rates at drying stage (at 1st peak temperature) and pyrolysis stage (at 2nd peak temperature) can be determined for each sample. To differentiate between thermal and thermal catalytic decomposition, silica sand was tested as an inert material with a thermal conductivity close to bentonite (Table 3.1). Heating rate was varied from 10 to 150°C/min to investigate the heating rate effect on mass loss since the catalyst particles are expected to increase the heat transfer rate for biomass samples. On the other hand, it will take shorter heating time to reach a given temperature at a faster heating rate in a non-isothermal TGA test, making it difficult to compare the TGA curves obtained at different heating rates.       67   (a)  (b) Figure 3.1 Weight loss and DTG for different catalyst particles of: (a) 30Clino; (b) 30Bento at 100°C/min.  As can be seen in Fig. 3.2, the weight loss at 350°C is 51.5% at a heating rate of 10°C/min and 19.85% at 150°C/min because the heating time required at 10°C/min to reach 350°C was 30.9 min compared to 2.1 min at 150°C/min. Such a difference in heating time correspond to a temperature shift of the weight loss curve by 51°C. When sand is added into the biomass sample, it acts as a heat conducting media to increase the heat transfer rate among biomass particles, equivalent to an increased heating rate. As expected, Figure 3.3 shows an accelerated weight loss and drying of the biomass in the presence of silica sand at the same heating rate, with the peak temperature difference between the two curves for SG and SG+30Sand by up to 40°C at 150°C/min heating rate. This shows that sand improves the heat transfer for switchgrass and accelerated the drying and pyrolysis of biomass samples. The difference in the final solid yield for SG between 10 and 150°C/min was about 14.5%. There is also significant difference in final solid yield between SG and SG+30Sand samples.      68   Figure 3.2 The weight loss and DTG curves at slow (10°C/min) and fast (150°C/min) heating rates for SG and SG+30Sand.  3.4.1 Catalytic effect on pyrolysis reaction rate and final solids yield As shown in Table 3.3, final solids yield only slightly depends on heating rates for all samples tested; while, the mass loss rate at peak temperatures of drying and pyrolysis strongly depends on the heating rate, increasing with increasing the heating rate due to shorter heating time to reach the peak temperature. As shown in Figure 3.3, the final solid yield for SG at 50, 100 and 150°C/min was similar except for the slowest heating rate at 10°C/min which is slightly higher. The final solid yields for SG+30Sand, SG+30Bento and SG+30Clino are comparable for all heating rates. Because of the enhanced heat transfer, the average final solid yield for SG+30Sand is about 12 wt.%, compared to ~17 wt.% for SG. The insignificant effect of heating rates on the final solid yield for silica sand and the other two catalysts is probably due to the high heat transfer rates with improved temperature uniformity for these samples associated with their high thermal conductivity. Compared to the catalyzed system, the solid yields for SG+30Bento and SG+30Clino     69  are lower than SG+30Sand by about 42%. This was expected and likely due to the catalytic effect of the catalysts assisting the in-situ cracking of pyrolysis vapours released from biomass and preventing the vapour condensation and repolymerization to form biochar.    Figure 3.3 Final solid yield (ash-free basis) and the mass loss rate for different samples at different heating rates.   The catalytic effects of vapour cracking and prevention of vapour condensation/repolymerization are also expected to increase the peak pyrolysis mass loss rate, which is supported by the results in Figure 3.3. The pyrolysis peak mass loss rate for the catalysts was markedly higher than SG+30Sand and SG. At low heating rates the difference in mass loss rate was less pronounced. The enhancement of mass loss rate tended to increase with increasing heating rate, with the highest differences at 150°C/min, especially between SG and SG+30Sand. This is because the improvement in heat transfer becomes more important when the heat transfer limitation is more important at high heating rate.     70  Table 3.3 Solid yield at 650°C and mass loss rates at drying (1st peak temperature) and pyrolysis (2nd peak temperature) for different catalysts with different particles size at different heating rates under TGA.  Samples Heating rate °C/min  Solid yield  (wt.%) ash-free basis Mass loss rate (%/min) at drying stage Mass loss rate (%/min) at pyrolysis stage SG 150 17.1 ±0.71 0.81 10.69 100 17.4 ±0.86 0.56 6.24 50 15.8 ±0.67 0.32 3.59 10 20.0 ±0.27 0.10 0.91 SG+30Sand < 50 μm (Original) 150 11.7 ±0.58 1.14 15.01 SG+30Sand < 50 μm (Original) 100 11.5 ±0.49 0.75 9.61 SG+30Sand < 20 μm (Fine) 100 11.4 ±0.52  1.18 10.42 SG+30Sand < 50 μm (Original) 50 12.2 ±59 0.65 5.41 SG+30Sand < 50 μm (Original) 10 12.2 ±0.55 0.14 1.23 SG+30Bento < 50 μm (Original) 150 7.31 ±0.35 2.01 17.9 SG+30Bento < 50 μm (Original) 100 6.81 ±0.24 0.90 11.5 SG+30Bento < 20 μm (Fine) 100 6.64 ±0.34 2.04 11.5 SG+30Bento < 50 μm (Original) 50 6.90 ±0.33 0.81 6.63 SG+30Bento < 50 μm (Original) 10 6.68 ±0.23 0.17 1.42 SG+30Clino < 50 μm (Original)  100 6.78 ±0.14 0.83 11.0 SG+30Clino < 20 μm (Fine)  100 7.18 ±0.24 1.17 11.1 SG+30Clino < 50 μm (Original) 25 7.35 ±0.19 0.25 2.77 Error bars are the standard deviation from 2-4 replicates.  3.4.2 Effect of catalyst particle size The catalyst particle size and the percentage of finer particles (< 20 μm) can significantly affect the thermal conductivity of the packed bed of particles,115 thus influencing the heat transfer between biomass particles in a biomass-catalyst mixture. The fine catalyst particles may also increase the contact surface between biomass particles and biomass particles, increasing the catalytic activity for solid catalyzed biomass pyrolysis reactions. To elucidate those effect, original     71  particles and fine particles of bentonite, clinoptilolite and silica sand were tested and compared in Table 3.3.   Mixing SG with the fine particles (< 20 μm) compared to the original sand particles (< 50 μm) did not show any significant difference in the final solid yield (Table 3.3). The mass loss rate at the pyrolysis stage, i.e., the second peak, only changed marginally. However, the mass loss rate during the drying stage (at the first peak) increased significantly compared to the original particle size for the sand and catalyst particles. The higher drying rate with finer particles can be attributed to the increased heat transfer rate; while, the negligible effect of particle size on biomass pyrolysis rate can be partially explained by the insignificant catalyzed biomass pyrolysis resulting from surface-surface contact between biomass particles and catalyst particles. For catalyzed vapour cracking reactions, the total surface area including both internal and external surface area is not expected to be much different between fine and original porous catalyst particles. The insensitivity of the final solid yield to catalyst and sand particle size also supports the speculation that the catalytic vapour cracking is not changed by slight increase in the external surface area of catalyst particles. The catalyst particles served as a heat transfer medium to promote heat transfer to biomass particles, leading to increased heating and drying at low temperatures. The solid-catalyzed biomass pyrolysis may still exist but is unlikely to be in dominant.    Mass loss curves in Fig. 3.4 indicate that there is no weight loss for pure bentonite and clinoptilolite within the range of the temperatures examined, because the melting point for the catalysts is higher than 1000°C. As discussed above, the mass loss at the drying stage is enhanced by the presence of heat conductive sand and catalyst particles, and the drying rate is highest for finer particles. In the pyrolysis stage at the intermediate temperature, the catalysts contributed little to the biomass decomposition reaction, with similar weight loss curves for all tested biomass     72  samples. At temperatures above 350°C, which falls into the effective temperature window of those catalyst particles, the mass loss curves start to deviate, with higher mass loss rate for bentonite and clinoptilolite particles. Since most vapours are released between 250 and 550°C at heating rate of 100°C/min for all biomass samples, the catalysts mostly affected biomass pyrolysis between these temperatures, with little difference observed beyond 550°C.   Figure 3.4 Weight loss for SG and SG mixed with silica sand and different catalysts with different particles sizes using TGA at 100°C/min.   3.4.3 Detailed thermal catalytic decomposition of biomass pseudo-components  Table 3.4 shows the peak temperature and mass loss rate of samples at 100°C/min. These data were obtained from DTG peak deconvolution of the experimental results with a coefficients of determination (R2) ranged from 0.997 to 0.999 (Fig. 3.5). In general, the mass loss rates of the pseudo-cellulose and pseudo-lignin components in the presence of catalysts are significantly higher than silica sand and the highest values were found for 30Bento, while no significant     73  difference in the mass loss rate was found for pseudo-hemicellulose. The most significant difference in the mass loss rate was found for the pseudo-lignin in the SG+30Bento sample, with an increase by 152 and 86% compared to SG and SG+30Sand, respectively. The peak temperature for the pseudo-cellulose and pseudo-hemicellulose was similar for all samples with no observable shift with the addition of sand or bentonite/Clinoptilolite; while, the peak temperature for the pseudo-lignin was significantly shifted to lowered temperatures (up to 53°C lower) in the presence of catalyst particles compared to silica sand and pure SG.  Table 3.4 Peak temperatures, mass loss rates and conversion percentage for biomass pseudo-components between SG, SG mixed with silica sand and SG mixed with bentonite or clinoptilolite using TGA at 100°C/min.  Pseudo-components  SG SG+30Sand SG+30Clino SG+30Bento Cellulose Peak temperature (°C) 407 405 400 402 Mass loss rate (%/min) 5.11 8.39 9.09 10.15 Conversion percentage (%) 86.6 85.9 93.7 93.6 Hemicellulose + extractives Peak temperature (°C) 365 359 362 361 Mass loss rate (%/min) 4.20 7.10 7.18 7.14 Conversion percentage (%) 82.0 97.2 88.7 86.6 Lignin Peak temperature (°C) 448 461 416 408 Mass loss rate (%/min) 0.71 0.96 1.49 1.79 Conversion percentage (%) 65.5 68.9 93.6 93.5  Table 3.4 also shows the conversion percentage for each biomass pseudo-component (between 200 and 600°C) for SG and SG mixed with different catalysts or sand. The conversion percentage for each pseudo-component was determined according to the following equations: 𝛾௜ =𝑊஽௏௜𝑊஻௜ (3.9)     74  𝑊஽௏௜ = 𝐹௜ ∙ 𝑊஽௏(ଶ଴଴ି଺଴଴) (3.10) 𝑃𝐴௜ =𝐴௜𝐴஽்ீ (3.11) Where: 𝛾௜ the conversion percentage for each pseudo-component, 𝑊஽௏௜ the weight of the devolatilized vapours from each pseudo-component, 𝑊஻௜ the weight of the corresponding component in biomass (i.e., cellulose, hemicellulose and lignin) based on the dry basis weight, 𝑊஽௏(ଶ଴଴ି଺଴଴) the total weight of the devolatilized vapours occurred between 200 and 600°C, 𝐹௜ the fraction of each pseudo-component from the devolatilized vapours occurred between 200 and 600°C, 𝐴௜ the peak area of each pseudo-component between 200 and 600°C, 𝐴஽்ீ the total peak area of the DTG curve between 200 and 600°C. The highest conversion percentage for SG was found for the pseudo-cellulose, with the pseudo-lignin being the lowest because it is the most difficult component to decompose and can contribute more toward final solid yield with up to 46 wt.% of its original weight compared to cellulose and hemicellulose.19 Bentonite and clinoptilolite significantly increased the percentage conversion of all three pseudo-components, while the major difference was found for the pseudo-lignin. Silica sand increased the conversion percentage for the pseudo-hemicellulose + extractives, but no significant difference was found for the pseudo-cellulose and the pseudo-lignin. Bentonite and clinoptilolite increased the percentage conversion of pseudo-lignin by 43 and 36% compared to SG and silica sand, respectively.  Figure 3.5 also compares the mass loss of pseudo-components for different tested samples at 100°C/min. As shown earlier, the mass loss rates for SG+30Sand, SG+30Bento and SG+30Clino samples are considerably higher than pure SG. The major difference was found for the pseudo-lignin in which the addition of catalysts lowered the decomposition temperatures significantly. The addition of silica sand increased slightly the decomposition rate of pseudo-lignin, but did not shift     75  the peak temperature, but bentonite and clinoptilolite lower the peak temperature significantly. It is well known that among the three major pseudo components, lignin is the most difficult to decompose, which occurs under a wide range of temperatures of up to 900°C.2,19 In addition, the pyrolysis of lignin is extremely slower than cellulose and hemicellulose because lignin contains more thermally stable bonds between aromatic rings which require high activation energy to break these bonds.2,119 The results in Figure 3.5 and Table 3.4 suggest that bentonite and clinoptilolite promoted the decomposition of pseudo-lignin because of their higher reactivity with vapours generated from decomposed lignin at temperatures higher than the thermal decomposition of cellulose and hemicellulose.  Hemicellulose, on the other hand, is well known to be more reactive and less stable which makes it more prone to decompose at much lower temperatures than cellulose and lignin.21 As a result, the bentonite and clinoptilolite catalysts have very low reactivity in cracking the organic vapours released from the decomposition of hemicellulose, inserting negligible effect.   It is known that hemicellulose is more reactive and less stable which makes it more prone to condensation and re-polymerization reactions.21 The primary oxygenated vapours from hemicellulose such as acetic acid, acetaldehyde and acetyl are main precursors for catalytic coke.23 The increased conversion of the pseudo-hemicellulose + extractives by silica sand can thus be explained by the fact that the silica sand acts only as an inert heating medium which promoted biomass decomposition at low temperatures but did not possess any catalytic effect on vapour cracking and catalytic coke formation.     76  Switchgrass  SG+30Clino  Figure 3.5 The deconvoluted curves of the pseudo-components for switchgrass (SG) and SG-30Clino and pseudo-components comparison between SG, SG mixed with silica sand or with different catalysts using TGA at 100°C/min.     77  Table 3.5 shows the kinetic parameters for the three pseudo-components at different heating rates for SG and other catalysts. The coefficient of determination (R2) was high for all samples and the ranges were between 0.980 to 0.997 which means that the kinetic models can be used to describe the pyrolysis reactions. The values for n were ranged between 0.89 and 1.53 for all samples. The frequency factor was changed markedly within the different heating rates and among the different catalysts. The activation energy of the pseudo-cellulose for SG at 100°C/min was within the range of the values reported for cellulose at high heating rates (260-285 kJ/mole).2,120 This shows the reliability of the peak deconvolution approach and the pseudo-cellulose may represent the major components of cellulose. All catalysts reduced the activation energy of the pseudo-cellulose, while no significant difference was found for the silica sand compared to SG. The lowest value for the activation energy of the pseudo-cellulose was for 30Bento which reduced the activation energy by 22% compared to SG. Table 3.5 Kinetic parameters of different pseudo-components for switchgrass and different catalysts at 100°C/min.     pseudo-component Heating rate 100°C/min SG SG-30Sand SG-30Clino SG-30Bento Cellulose  E kJ/mole 281 273 230 218 A (1/min) 3.80E+22 7.56E+21 2.26E+18 3.18E+17 n R2 1.53  (0.994) 1.39 (0.995) 1.15  (0.994) 1.09  (0.994) Hemicellulose + extractives  E kJ/mole 120 112 102 100 A (1/min) 5.30E+09 6.45E+09 6.10E+08 4.96E+08 n R2 1.09  (0.992) 1.02 (0.992) 0.89  (0.990) 0.90  (0.991) Lignin  E kJ/mole 57 53 44 41 A (1/min) 2.12E+04 6.80E+03 2.33E+03 1.42E+03 n R2 1.28  (0.997) 1.35 (0.980) 1.25  (0.988) 1.32  (0.981)     78  The activation energy for the pseudo-hemicellulose of SG and was within the reported values for hemicellulose.121 All catalysts and silica sand reduced the activation energy of the pseudo-hemicellulose significantly compared to pure SG (Table 3.5). The activation energy of the pseudo-lignin for SG was also within the reported values for lignin,55,121 with catalyst addition lowering the activation energy of the pseudo-lignin. The activation energy for lignin is found to be very low, while the peak temperature is high compared to cellulose and hemicellulose, because lignin is decomposed over a wide temperature range of 200  to 600°C while cellulose and hemicellulose are decomposed within narrow temperature ranges, as shown in Fig. 3.5, which have much sharper peaks than the decomposition peak for lignin, resulting in increased activation energy.2,21 When a segment from lignin decomposition peak from 400 to 600°C was taken for analysis, the activation energy was found to be 144 kJ/mole.2 These results show that the catalysts affect lignin decomposition to a greater extent through solid-vapour catalytic reactions. A global two-step model of primary and secondary biomass decomposition was then applied to further examine the reaction mechanism:                                       Gas (p)                             Gas (s)   Biomass                       Liquid                                                                                                                                    Char (p)                               Char (s)  It is assumed that the material will decompose through independent, parallel and competitive reactions following a first order reaction and it is further assumed that the reactions follow Arrhenius equation for catalytic and non-catalytic pyrolysis, according to the following equations: 𝑘௜ =  𝐴௜  𝑒𝑥𝑝ିா೔ோ்  (3.12) kL     79  𝐵𝑖𝑜𝑚𝑎𝑠𝑠 ∶  −𝑑𝑤஻(𝑡)𝑑𝑡= (𝑘௅ + 𝑘ீ + 𝑘஼)(𝑤஻(𝑡) − 𝑤ஶ) (3.13) 𝑘ீ : 𝑑𝑤ீ(𝑡)𝑑𝑡= 𝑘ீ(𝑤஻(𝑡) − 𝑤ஶ) (3.14) 𝑘௅ : 𝑑𝑤௅(𝑡)𝑑𝑡= 𝑘௅(𝑤஻(𝑡) − 𝑤ஶ) (3.15) 𝑘஼ : 𝑑𝑤஼(𝑡)𝑑𝑡= 𝑘஼(𝑤஻(𝑡) − 𝑤ஶ) (3.16) 𝑘ீଶ : 𝑑𝑤ீଶ(𝑡)𝑑𝑡= 𝑘ீଶ(𝑤௅(𝑡) − 𝑤ஶ) (3.17) 𝑘஼ଶ : 𝑑𝑤஼ଶ(𝑡)𝑑𝑡= 𝑘஼ଶ(𝑤௅(𝑡) − 𝑤ஶ) (3.18) Where: wB, w∞, wG, wL, wC, wC2 and wG2 are the weight for biomass, final yield, primary gas, primary liquid, primary char, secondary gas and secondary char; kG, kL, kC, kC2 and kG2 are the rate constants for primary gas, primary liquid, primary char, secondary char and secondary gas. Solid weight loss data have been successfully applied to study the primary biomass decomposition reaction and the secondary vapour cracking reaction kinetics due to the complexity of secondary cracking reactions. In addition, the global approach is preferred to obtain the formation rates of the three lumped products (char, gas and liquid) and tar compounds.21,122,112 Secondary reactions of tar compounds are classified as homogeneous and heterogeneous which include multiple reactions such as partial oxidation, cracking, repolymerization and condensation. Thus, large number of chemical reactions are required to describe the detail transformation of intermediates for each compound due to the complexity of liquid product composition. For that reason, the most cited mechanism for secondary tar cracking reactions focuses on two reactions, which are adopted in this study.21,112,123 The solid weight loss data are then used to fit the two-step global kinetics model of primary and secondary biomass decomposition reactions.     80  To fit this two-step global model of primary and secondary biomass decomposition reactions to the solids weight loss curve for catalytic pyrolysis of biomass, the yields for gas and/or liquid need to be measured. However, TGA test only provides the solid yield. To overcome the problem, we used literature data to determine the relationship between the gas and liquid yields, defined by the ratio between the gas and oil (𝛼௥), according to the following equations:122 𝛼௥ =𝑌𝑌௅ (3.19) Where αr is the ratio between the gas yield (YG) and oil yield (YL). It has been reported that αr did not change significantly at different pyrolysis temperatures and a fixed value could be assumed.122,124 αr for switchgrass was also reported to change only slightly over a wide range of temperatures up to 600°C.106 αr is thus assumed to be a fixed value over the temperatures tested in the current study.  αr was determined to be 1.35 (wt./wt.) using our recent data from microwave pyrolysis of switchgrass.125 Fig. 3.6(a) shows that the calculated data for liquid and gas for switchgrass at different temperatures (400, 450, 500 and 600°C) are in line with other reported data using different heating methods for pure switchgrass with a standard deviation less than 6%.107,126,127 This confirms that there is no significant difference in αr over a range of temperatures using different heating methods. αr was determined to be 0.91 (wt./wt.) using the data from microwave catalytic pyrolysis of switchgrass mixed with clinoptilolite.125 This αr value is close to what was obtained from catalytic pyrolysis of cotton-seed cake, which has chemical properties similar to switchgrass in terms of ash content, volatile matter, and carbon content (5.2, 79.3, 52.0 wt.%, respectively, compared to switchgrass at 6.3, 76.9, 47.9 wt.%, respectively),34 with a standard deviation < 6% (see Fig. 3.6b). This confirms that catalyst affected not only final solid yield but also the ratio between gas and liquid.     81   (a)  (b) Figure 3.6 Predicted and reported gas and oil yield for (a) switchgrass and (b) SG+30Clino as well as the measured DTG curves in this study.  Table 3.6 shows the fitted kinetic parameters for switchgrass and switchgrass mixed with 30Clino using the two-step kinetic model. It is seen that the activation energy for the primary and     82  secondary products for SG are in the same order as the reported data in the review.128,129 There are no major differences found between SG and 30Clino for the activation energy of the primary products as the catalyst acting mostly as a heating medium at low temperatures for primary reactions. However, there is a remarkable difference between SG and SG+30Clino in the activation energy of the secondary reactions, which are reduced by up to 50% due to the catalyzed solid-vapour reactions for the heavy volatiles. The activation energy for the secondary gas and secondary char are similar for both SG and SG+30Clino, which also agree with the trends reported data in the review.129 First reaction order did not fit well with the experimental data for the secondary reactions for both SG and SG+30Clino and thus different reaction orders were fitted with the experimental data to obtain high coefficient of determination. Those results confirm the speculation based on one-step 3-components kinetics that catalysts are mostly responsible for promoting the catalytic cracking of vapours released from lignin decomposition at temperatures higher than cellulose and hemicellulose.   Table 3.6 Fitted two-step reaction kinetic parameters for switchgrass and switchgrass mixed with 30Clino.      Kinetic parameters kL kG kC kG2 kC2 Review E kJ/mole 112 88.6 107 108 108 SG E kJ/mole 105 95.2 101 100 100 n R2 1 (0.997) 1 (0.991) 1 (0.995) 2 (0.988) 2 (0.989) 30Clino E kJ/mole 97.3 96.6 96.5 49.5 50.1 n R2 1 (0.994) 1 (0.995) 1 (0.992) 1.5 (0.986) 1.5 (0.990)     83  From our previous study, a maximum bio-oil yield of 36.2 wt.% was obtained from the SG mixed with 30 wt.% clinoptilolite and the bio-oil acidity and viscosity was reduced under microwave catalytic pyrolysis compared to SG.125 The produced bio-oil was further characterized in this study to understand the catalytic mechanism using GC-MS Agilent, with the chemical composition of total bio-oil (organic and aqueous phases) produced from SG and SG mixed with 30% clinoptilolite under microwave-assisted pyrolysis. Clinoptilolite altered the chemical composition of bio-oil as expected from the kinetics in which acids, aldehydes, furans were significantly reduced or completely eliminated; while phenolic and other aromatic compounds remarkably increased compared to SG, as shown in Table 3.7 where the lumped products composition of bio-oil products are given. Alkylated phenols increased by up to 49%, while acids compounds reduced by 27% compared to SG. These results support the findings from the kinetics study that clinoptilolite mainly catalyzed lignin decomposition and tar cracking. Table 3.7 The lumped products composition of bio-oil (peak area %) produced from SG and SG mixed with 30%wt clinoptilolite under microwave-assisted pyrolysis using GC-MS.  Acids Phenolics Furans Ketones  Aldehydes  Anhydrosugars SG 24.6 11.0 4.81 16.5 1.23 0.63 Clinoptilolite 17.9 16.5 3.01 14.9 0 0 Percentage change -27% +49% -38% -10% -100% -100%  Literature data appear to be consistent with our results obtained from the kinetic study of bentonite and clinoptilolite, which showed that they catalyzed the cracking of vapours released from lignin decomposition as reflected by an increase in the conversion rate, shift in the decomposition peak for lignin to lower temperature and the reduced activation energy of the pseudo-lignin.130 reported that clinoptilolite catalyzed hardwood lignin decomposition, which produced a bio-oil rich in phenolic compounds. It has been reported that clinoptilolite reduced     84  guaiacols, eugenols and the heavy pyrolyzates while increased the amounts of alkylphenols, pyrocatechols and BTEX (Benzene, toluene, ethylbenzene and xylenes) resulting from lignin.131 Clinoptilolite also enhanced the dealkylation reaction activity of guaiacols, eugenols and heavy pyrolyzates, leading to increased formation of olefins.131 Clinoptilolite also promoted decarboxylation, decarbonylation and dealkylation reactions, as supported by increased amounts of CO, CO2 and olefins such as ethene and propene produced from lignin.131 The decrease in the acid compounds and the increase in pH for bio-oil produced from clinoptilolite may be due to catalyzed decarboxylation of carboxylic groups which is responsible for the formation acidic compounds.131 It should also be noted that clinoptilolite has a high content of CaO, which is responsible for reducing the formation of acid compounds as a result of reaction between CaO particles and CO2-like compounds to form calcium salts.108  3.4.4 Reaction pathways for biomass thermal catalytic pyrolysis using bentonite and clinoptilolite Based on the systematic experimental design and analyses as presented above, we can speculate how bentonite and clinoptilolite interact with biomass pyrolysis. At temperatures < 150°C, the addition of catalyst particles promotes heat transfer from a heat source to biomass particles, leading to increased drying rate of biomass particles. Further increase in reactor temperature, organic volatiles in biomass start to be released and reactive hemicellulose and cellulose start to decompose, releasing organic vapours. However, because of the low reactor temperature (e.g., < 300°C), the vapour cracking reactions are very slow, and the catalyst is not effective in catalyzing cracking reactions at such temperatures. When temperature is further increased to beyond ~350°C, lignin decomposition starts to become dominant; while, the catalyst also starts to become effective in catalyzing the organic vapour cracking reactions, leading to     85  increased removal of vapours which potentially decreases condensation and repolymerization for coke formation. This biomass-catalyst interaction mechanism is supported by the peak temperature shift of lignin in deconvoluted mass loss curves and the decrease of final solid yield in comparing bentonite/clinoptilolite and inert silica sand. The insensitivity of final solid yield and mass loss rate to the sand and catalyst particle size further suggest that solid-solid directly catalyzed biomass decomposition reactions unlikely play an important role in catalytic biomass pyrolysis, because of low contact areas and low reaction temperatures.  The implication of this identified catalytic pyrolysis mechanism, shown in Fig. 3.7, is that those catalysts will be more effective at high temperatures for the pyrolysis of lignin or biomass with high lignin contents. For pyrolysis reactions taking place at low temperatures (e.g., torrefaction at < 300°C), those catalyst particles will serve only as a heat transfer medium but not as a catalyst. More reactive catalyst is expected to promote pyrolysis reactions to occur at lowered temperatures and will expedite the reaction of vapours intermediates released from biomass decomposition that will diffuse onto the catalyst surface for reactions to take place. The intermediates will be immediately cracked in-situ once they are released from the biomass cracking, preventing re-condensation or polymerization of those intermediates, equivalent to quick removal of cracking products from the catalyst surface. It is expected a lowered solid yield from catalytic pyrolysis if there is no heat or mass transfer limitations and higher yield of light hydrocarbons, because of the vapour cracking reaction on catalyst surface.      86   Figure 3.7 Catalytic reaction pathway for biomass mixed with a catalyst with high thermal conductivity.  3.5 Summary To differentiate between thermal and thermal catalytic decompositions for biomass catalytic pyrolysis using bentonite and clinoptilolite, silica sand was tested as an inert material with a thermal conductivity close to bentonite. Bentonite and clinoptilolite significantly increased the percentage conversion of all three pseudo-components, while the major difference was found for pseudo-lignin. Silica sand significantly increased the conversion for pseudo-hemicellulose only. Addition of catalysts increased the percentage conversion of pseudo-lignin by 43 and 36% compared to SG and sand. The lowest value for the activation energy of pseudo-lignin was found for 30Bento which was reduced by 29 and 25% compared to SG and sand, respectively. Clinoptilolite produced bio-oil rich in phenolic compounds, particularly alkylated phenols which increased by 49%, while acids compounds were reduced by 27%, compared to SG under microwave-assisted pyrolysis. The results obtained from the two-step kinetic model confirmed the speculation based on one-step 3-components kinetics that catalysts are mostly responsible for promoting the catalytic cracking of vapours released from lignin decomposition at temperatures higher than cellulose and hemicellulose.       87  Chapter 4: Effects of catalysts mixtures on microwave heating behaviour of biomass catalytic pyrolysis It was observed in Chapter 2 that microwave heating rates were increased significantly when catalyst mixtures were used. In order to elucidate the effects of catalysts mixtures on increasing the microwave heating rates, the effect of microwave heating rates should be decoupled from the effect of catalyst activity by maintaining at similar heating rates. A TGA will be thus used in this chapter to measure the heat flow at constant heat rates for SG and SG mixed with different catalysts or different catalyst mixtures. Can coke characteristics elucidate the effect of microwave heating rate? It is known that oxygenated coke has very low microwave absorption ability compared to graphitic carbon and graphite.30 Coke can also cause catalyst deactivation which reduces catalyst activity and affects pyrolysis products distribution.46,47 Coke can form from the oxygenated volatile intermediates and the dehydrated species.24 Results in Chapters 2 and 3 showed that K3PO4 possesses good microwave absorption but inhibited the devolatilization of hemicellulose significantly, leading to significant increase in the catalytic coke yield compared to other samples, which can affect microwave absorption and biomass decomposition. Bentonite, on the other hand, has a high thermal conductivity and did not inhibit hemicellulose decomposition, but possesses poor microwave absorption. Thus, it is expected that mixing the two catalysts can potentially increase the microwave absorption rate and reduce formation of oxygenated coke type that can further affect microwave absorption. It is hypothesized that the type of coke deposited on catalyst surfaces can play an important role on affecting the microwave absorption, which determines microwave heating rate of biomass catalytic pyrolysis. To understand the effect of catalyst and catalyst mixture     88  on coke formation, coke deposited on catalyst particles is sampled and characterized. These data can be used to design and optimize the microwave reactors for biomass catalytic pyrolysis.  4.1 Experimental 4.1.1 Microwave and TGA catalytic pyrolysis  A total of 20.0 g of pure switchgrass or switchgrass mixed with different loads (10, 20 and 30 wt.%) of natural zeolite (clinoptilolite), bentonite or K3PO4 (≤50 μm) was pyrolyzed in a tubular reactor. Different catalysts were mixed at different fractions in order to investigate the effect of catalysts mixtures in increasing microwave heating rate and maximizing the catalytic effects. More information about the microwave reactor and experimental setup can be found in Chapter 2. The spent catalysts for the samples contained K3PO4 from microwave pyrolysis were quantified and characterized using temperature programmed oxidation (TPO) and Raman spectroscopy to investigate the different functional groups on the catalyst surfaces and the dielectric properties were measured to study the effect of catalytic coke type deposition on microwave heating and solid yield. In order to measure the heat flow at constant heat rates for SG and SG mixed with different catalysts or different catalysts mixtures, Thermogravimetric analysis of the samples was conducted using SDT Q600 TGA-DTA with a flow rate of nitrogen 100 mL/min. Switchgrass was sieved to small particles (100-150 μm) to obtain a low Biot number (< 0.01) in order to reduce the internal heat and mass transfer issues and obtain more accurate data. K3PO4, clinoptilolite and bentonite with particles size ≤ 50 μm were mixed at 30 wt.% load and combinations (10/10 wt.%) between different catalysts were also studied. The sample was loaded into a crucible made of alumina and the weight was recorded every 0.5 s and the accuracy of the balance is 0.0001 mg and the experiments were performed at 25 and 100°C/min.      89  4.2 Results and discussion   4.2.1 Effects of catalysts mixtures on microwave heating behaviour  The heating behaviour of pure switchgrass and switchgrass with different loads of different catalysts and different catalysts combinations was discussed previously in Chapter 2 (Figs. 2.1 and 2.2). Based on the heating behaviour for different samples under microwave catalytic pyrolysis, the whole process can be divided into three main stages; drying (room temperature to 110°C), heating and torrefaction (110-260°C, depolymerization, and partial devolatilization) and pyrolysis (260-400°C, devolatilization). A possible mechanism for microwave heating of biomass premixed with a microwave absorber is that the microwave radiation is first absorbed by microwave absorbing material and then the absorbed energy is transferred to biomass through heat conduction.28 Under microwave catalytic pyrolysis, catalysts that possess high microwave absorption ability will be heated faster than biomass particles and then will serve as a heating medium and transfer heat to biomass particles via mostly conduction (solid-solid contact heat transfer). Released vapours from biomass particles will then be diffused into catalyst pores where they are catalytically cracked (solid-vapour interactions). Table 4.1 shows the microwave heating rates for switchgrass mixed with different catalysts or catalysts combinations at different stages (torrefaction and pyrolysis) and total heating rates under microwave-assisted catalytic pyrolysis. It can be seen that the heating rate is mostly affected between 260 to 400°C (pyrolysis stage) in whi.ch the heating rate with catalyst mixtures are much higher than what is expected from the simple combination of the two catalysts, confirming the existence of a synergistic effect. In general, the microwave heating rates at the pyrolysis stage (260-400°C) are much lower than at the torrefaction stage (110-260°C) and the lowest was found     90  for 30KP (Fig. 4.1), likely due to the occurrence of endothermic biomass pyrolysis reactions. However, the highest heating rates for pyrolysis stage were found for the catalyst mixtures, with the highest values for 10KP/10Bento. It is noted that adding bentonite only to switchgrass at different loads up to 30 wt.% did not increase the microwave heating rate, with the maximum temperatures recorded well below 200°C. This further confirms the existence of a synergistic effect of catalyst mixtures on increasing microwave heating rates compared to single catalysts.  Table 4.1 Heating rates for switchgrass mixed with different catalysts or catalyst mixtures at different heating stages under microwave-assisted pyrolysis.  Sample Abbreviations  Heating rate, °C/min (110-260°C, torrefaction)  Heating rate, °C/min (260-400°C, pyrolysis) Overall heating rate, °C/min (110-400°C) 10wt.% clinoptilolite 10Clino 145 9 19 20wt.% clinoptilolite  20Clino 169 10 21 30wt.% clinoptilolite  30Clino 391 39 81 10wt.% K3PO4 10KP 173 10 20 20wt.% K3PO4  20KP 380 38 79 30wt.% K3PO4  30KP 219 9 19 10wt.% K3PO4 + 10wt.% bentonite 10KP/10Bento 409 179 271 10wt.% K3PO4 + 20wt.% bentonite  10KP/20Bento 200 35 73 10wt.% K3PO4 + 10wt.% clinoptilolite 10KP/10Clino 230 141 193 20wt.% activated carbon  20AC 265 22 46  Increasing the percentage of clinoptilolite increased the heating rate at both the torrefaction and pyrolysis stage. However, the addition of K3PO4 exhibited different trends. As shown in Table 4.1, heat rate increased when percentage of K3PO4 increased from 10 to 20%, but decreased when the percentage further increased from 20 to 30%. Fig. 4.1 further shows that the sample with 30%     91  K3PO4 load gave the fastest initial temperature increase at temperature < 230°C, but the trends changed noticeably beyond 250°C and it took the longest time for the sample to reach 400°C. It has also been reported that higher percentage of K3PO4 inhibits the devolatilization of hemicellulose to create organic volatile compounds, leading to higher solid yield using Pyroprobe at a very high heating rate > 10,000°C/min.43 Thus, it is believed that 30wt.% of K3PO4 shifted the decomposition of hemicellulose, and catalytic coke started to be formed at lower temperatures.  This will be discussed later in more details in the following section.      Figure 4.1 Temperature rise profiles of 30KP compared to different samples under microwave catalytic pyrolysis.      92  It is noticed in Table 4.1 that 10KP/10Clino showed a low heating rate at the torrefaction stage but the highest heating rate after 10KP/10Bento at the pyrolysis stage compared to other single catalysts with different loads. This indicates that the combined catalysts may have strongly influenced biomass pyrolysis reactions. At the pyrolysis stage, many chemical reactions with different reaction pathways and heat of reactions (endothermic or exothermic) take place simultaneously, leading to the formation of different types of catalytic coke on catalyst surfaces. As catalyst particles absorb microwaves and transfer to biomass particles through different heat transfer modes, the microwave absorption of catalyst particles could be affected by the deposited coke on the catalysts surfaces, which in turn will affect the microwave heating rates of biomass particles.  It was speculated previously in Chapter 2 that the synergistic effects for catalysts combinations in terms of increasing microwave heating rates were due to exothermic reactions triggered by catalysts mixtures. In order to show the effect of catalysts combinations on exothermic reactions, DTA analysis was conducted at different heating rates (25 and 100°C/min) in a TG unit. It was found that the 10KP/10Bento sample was more endothermic compared to 30KP and SG at different heating rates under TGA (Fig. 4.2). In addition, the decomposition stage from 300 to 400°C is almost neutral for SG and other catalysts at different heating rates, but significant fluctuations in the heating rate under microwave heating were observed between 300 and 400°C which may be caused by the change in microwave absorption (Fig. 4.2). These findings are also in agreement with the reported data in which cellulose decomposition was endothermic while decompositions of hemicellulose and lignin were exothermic between 300 and 400°C,19 and the overall heat flow associated decomposition of whole biomass can be neutral at the pyrolysis stage. Potassium is the most influential element on pyrolysis products distribution and is known to     93  promote crosslinking reactions that can result in increased solid yield with exothermic behaviour.41,42   (a)  (b) Figure 4.2 Heat flow for SG and SG mixed different catalysts at heating rates of: (a) 25°C/min and (b) 100°C/min.      94  Our results on heat flow under TGA and the literature data seem to suggest that the exothermic reactions hypothesis proposed in Chapter 2 cannot be the main cause of the synergistic effects on increasing microwave heating rate. Instead, the change of dielectric properties of catalyst particles caused by deposited coke, which determine microwave absorption ability, may be the major cause of the high microwave heating rate at the microwave catalytic pyrolysis stage. 4.2.2 Effects of catalysts mixtures on coke formation and coke properties  Fig. 4.3 shows the DTG curves for the spent catalyst of 30KP using N2 flow at 1.5 and 5 L/min and 10KP/10Bento at 1.5 L/min using temperature programmed oxidation (TPO) procedure under air at a heating rate of 20°C/min. The sample 30KP-1.5L/min showed two peaks at low temperatures (160 and 285°C). The other two samples were removed at temperature < 360°C, called oxygen-containing coke (oxygenated coke), which have a lower combustion temperature, while the catalytic coke removed at temperature > 360°C may correspond to aromatic hydrocarbons which possess graphitic properties as confirmed later based on the determined functional groups of the spent catalysts.47  It was found that the amount of deposited coke on catalyst surfaces for the three samples is similar (about 31 wt.%), which means that the amount of coke deposition is not responsible for the difference in microwave heating rate. Rather, the type of coke deposit with different dielectric properties may be important. DTG curves in Fig. 4.3 for the three samples showed major peaks at low temperatures (160 and 285°C) for sample 30KP-1.5L/min with ~51% of coke being removed at temperature < 360°C compared to about 19% for 30KP-5L/min. The coke removed at temperature < 360°C, called oxygenated coke, has a lower combustion temperature as mentioned before. These findings were also confirmed by the data obtained from Raman Spectroscopy in which disordered carbon/graphitic carbon ratio (D/G ratio) for 30KP-5L/min was 20% lower than     95  the two other samples. That means that more graphitic carbon and less disordered carbon are formed on catalyst surfaces, which will have a high microwave absorption ability as to be confirmed by measured dielectric properties of the spent catalysts.   Figure 4.3 DTG for different spent catalysts produced from microwave catalytic pyrolysis. It is known that oxygenated coke has very low microwave absorption ability compared to graphitic carbon.30 Thus, covering catalyst surfaces and active sites with the oxygenated coke will reduce microwave absorption and reduce heating rate. The spent catalyst for 30KP-1.5L/min having more oxygenated coke and less graphitic coke are expected to have lower dielectric loss than those of 10KP/10Bento and 30KP-5L/min catalysts.  To confirm the effect of coke deposition and the coke type on microwave absorption and microwave heating rate, dielectric properties of the spent catalysts were measured, and the heating rate can be calculated using the following equations:   𝑷 = 𝝈 |𝑬|𝟐 = 𝟐𝝅 𝒇 𝜺𝟎 𝜺ᇱᇱ |𝑬|𝟐 (4.1) ∆𝑻∆𝒕=𝟐𝝅 𝒇 𝜺𝟎 𝜺ᇱᇱ |𝑬|𝟐𝝆 . 𝑪𝒑 (4.2)     96  Where, P is the absorbed power per unit volume, 𝜆଴ is the microwave wavelength, 𝜀ᇱ dielectric constant, 𝜀″ dielectric loss, 𝜎 effective conductivity, 𝜀଴ permittivity of free space, E is the electric field, ∆்∆௧ is the microwave heating rate, ρ is material density and Cp is the specific heat. The ability of a material to absorb microwave radiation is highly determined by dielectric constant (𝜀′) and dielectric loss (𝜀″). Dielectric constant depicts the capacity of molecules to be polarized when it is under electric field and represents the efficiency of the material to convert the electromagnetic radiation into heat. Dielectric loss denotes the amount of input microwave energy which is dissipated through heat.29,30 Table 4.2 Dielectric properties and microwave heat flow for different materials and spent catalysts.   Samples  Dielectric constant 𝜀′ Dielectric loss 𝜀″ Penetration depth DP (cm) Tan α Microwave heat flow kW/m3 switchgrass 1.45 0.10 22.54 0.071 52.7 Biochar + 10KP + 10Bento 27.81 35.12 0.29 1.263 18518.3 Biochar + 30% K3PO4, 5L/min N2 11.37 14.36 0.45 1.262 7570.8 Biochar + 30% K3PO4, 1.5L/min N2 7.01 1.59 3.18 0.227 838.3 K3PO4 3.25 1.48 2.33 0.455 780.3  Microwave heat flow calculated at room temperature using 750 W microwave power output.  The heating rate for 30KP-1.5L/min was 219°C/min, which increased to 327°C/min for 30KP-5L/min and the biochar yield was reduced by 37% using a high flow rate of N2 (5L/min), despite that high flow rate of N2 (5L/min) will remove more heat from the reaction zone. This illustrates the importance of the dielectric properties of coke deposition on catalyst surfaces on determining the microwave heating rate.      97  To further confirm the hypothesized effect of oxygenated coke on microwave heating, the coke deposited on the catalyst surfaces for the spent catalysts was characterized and semi-quantified using Raman Spectroscopy. As seen in Fig. 4.4, there are two major peaks and the first peak represents the disordered carbon “D” and the second peak corresponding to graphitic carbon “G”. From the figure there is a clear difference between the four samples, and to better explain the difference between the four samples the peaks were deconvoluted into different bands using the data provided in Table 4.3.49  (a)  (b) Figure 4.4 Raman spectroscopy for different spent catalysts produced from microwave catalytic pyrolysis: (a) 10KP/10Bento and 10KP/20Bento; and (b) 30KP-1.5L/min and 30KP-5L/min.     98  Table 4.3 Raman bands, band position and bond type for Raman Spectroscopy.   band name band position (cm−1) description bond type GL 1700 carbonyl group C=O sp2 G 1590 graphite E2g2, aromatic ring quadrant breathing, alkene C=C sp2 GR 1540 aromatics with 3−5 rings, amorphous carbon structures sp2 VL 1465 methylene or methyl, semicircle breathing of aromatic rings, amorphous carbon structures sp2, sp3 D 1340 D band on highly disordered carbonaceous materials, C−C between aromatic rings and aromatics with no less than six rings sp2 SL 1230 aryl−alkyl ether, para-aromatics sp2, sp3 S 1185 Caromatic−Calkyl, aromatic (aliphatic) ethers, C−C on hydroaromatic rings, hexagonal diamond carbon sp3, C−H on b aromatic rings sp2, sp3 SR 1060 C−H on aromatic rings, benzene (ortho-disubstituted) ring sp2 R 960−800 C−C on alkanes and cyclic alkanes, C−H on aromatic rings sp2, sp3   From the deconvoluted peaks the ratio between the peak areas for D band and G band (D/G) was determined and shown in Table 4.4. A significant decrease in D/G ratio by 20% was found after N2 flow rate was increased from 1.5 to 5 L/min for 30KP, which means less disordered carbon or more graphitic carbon was deposited onto the catalyst surface. This agrees with the trends in their dielectric properties which was considerably higher for 30KP-5L/min than for 30KP-1.5L/min. Also, 10KP/10Bento has lower D/G ratio than 10KP/20Bento by ~18% which also agrees with the higher heating rate for 10KP/10Bento (179°C/min) than 10KP/20Bento (35°C/min) at the pyrolysis stage. In addition, there is a 38% increase in VL band which represents the amorphous carbon, and 33% increase in SL band which refers to aryl−alkyl ether for 10KP/20Bento compared to 10KP/10Bento. These findings may explain the significant decrease in the heating rate at the pyrolysis stage (260-400°C) under microwave heating due to the different types of coke deposited on the catalysts surface. These findings clearly show that catalyst mixtures     99  increased the microwave heating rate by producing less oxygenated coke and more graphitic coke, which changed the microwave absorption rate compared to the case with a single catalyst.  Figure 4.5 Deconvoluted peaks of Raman spectra for spent catalysts of 10KP/10Bento and 10KP/20Bento.    Figure 4.6 Deconvoluted peaks of Raman spectra for the spent catalysts of 30KP-1.5L/min and 30KP-5L/min.   Table 4.4 also shows a great decrease on different bands such as GL band, which represents the carbonyl group, GR band, which represents the amorphous carbon, and SL band, which refers to aryl−alkyl ether, by 89, 50 and 35%, respectively, and the R band, which represents C−H on aromatic rings, was eliminated after N2 flow was increased from 1.5 to 5 L/min. It should also be     100  noted that biochar 30KP-1.5L/min showed the highest H/C ratio compared to other samples which may due to the low aromaticity of the biochar (Table 2.2).64 Those compounds mostly contain the C=O group. It was reported that hemicellulose contains much more C=O contained organic compounds than cellulose and lignin, which are released at temperature 200-400°C,19 and may correspond to the two peaks of the DTG at low temperatures (160 and 285°C) for the spent catalyst of 30KP-1.5L/min (Fig. 4.3). Those observations are also supported by the properties of bio-oil produced from 30KP-1.5L/min under microwave heating, which has a higher pH (by 82%) and lower acid compounds content (by 76%) than bio-oil produced from pure SG, resulting from condensation and repolymerization of those oxygenated compounds (see Appendix B). Table 4.4 Peak areas of the Raman bands corresponding to the coke deposit on different spent catalysts.    In conclusion, coke type plays an important role on determining microwave heating rate and products distribution under microwave catalytic pyrolysis, and these different coke types may be formed as a result of different catalysis mechanisms. This implies that different catalyst mixtures may catalyze different reactions which will lead to the formation of less oxygenated coke or more graphitic coke. Deposit on catalyst surfaces, they will affect microwave heating rate. It should be noted that small amount of deposited oxygenated coke on catalyst surfaces can result in a significant decrease in microwave heating because of their poor dielectric properties. This explains Spent catalyst sample G D D/G SL VL GR GL 10KP-10Bento 2042 3292 1.61 1164 1664 1316 462 10KP-20Bento 2862 5358 1.87 1543 2294 1622 452 30KP-1.5L/min 1752 2453 1.40 921 544 435 214 30KP-5L/min 1133 1268 1.12 607 518 220 24     101  the data in Table 4.1, where changes in the oxygenated and amorphous carbon affected the microwave heating rate at the pyrolysis stage (260-400°C) for catalyst mixtures and single catalyst. 4.3 Summary This chapter studied the catalytic effects of mixtures of different catalysts on biomass catalytic pyrolysis compared to the single catalyst under microwave heating and TGA. Since some catalyst can promote microwave absorption while some catalyst shows promising catalytic effect, it is desirable to use mixtures of catalysts to improve high microwave heating rate and catalyze biomass pyrolysis reactions. The highest microwave heating rates for torrefaction (110-260°C) and pyrolysis stages (260-400°C) were found for 10KP/10Bento. It was also found that the catalysts mixtures affected the type of coke deposited on catalyst surfaces which play an important role on determining the microwave heating rate under microwave catalytic pyrolysis by affecting the dielectric properties of the sample and then microwave absorption. Coke deposit on catalyst surfaces has a greater effect on determining the microwave heating rate and the heating behaviour of the sample than the pyrolysis reactions type (endothermic or exothermic). Oxygenated coke on catalyst surfaces reduces the microwave heating rate, while graphitic coke deposit increases the microwave heating rate considerably. Finally, catalyst combinations could improve the catalytic performance and reduce the catalyst loads which in turn will reduce the catalytic pyrolysis process cost compared to single catalyst, particularly under microwave catalytic pyrolysis.      102  Chapter 5: The role of tailored biochar in improving soil fertility and water retention.  Increasing the ability of soil to retain water and nutrients is a crucial need for achieving higher growth of bioenergy raw materials in desert land, arid and semi-arid areas and to provide enough food for the soaring population.8-11,132 Biochar application to soil may mitigate some of the portended deficiency in water as a consequence of climate change effects in order to retain the future productivity of bioenergy crops.10,132 Biochar, a co-product of thermochemical conversion of biomass into valuable biofuel, can be applied to soil as an amendment to improve the sustainability of biomass and crop production, enrich fertility and quality of agricultural soils.9,10,62,133 Nevertheless, the information on distinguishing feedstocks and proper pyrolysis conditions to maximize the capability of soils to retain water and nutrients, and increase plant productivity is sparse.62 Thus, more research is needed to develop suitable approaches to improve biochar characteristics with respect to soil applications and increase biofuel crops yield and food production.  The overall goal of this research is to use the multi-functional catalysts for biomass pyrolysis: (1) as a good microwave absorber for speeding up the heating rate; (2) as a catalyst to improve the quality of bio-oil and biochar; and (3) as a nutrients/soil conditioner imbedded in biochar to increase its performance as a fertilizer or soil remediate. This chapter is focused on evaluating the performance of the produced biochars in terms of their ability to improve soil water holding capacity, cation exchange capacity and fertility of loamy sand soil, investigating the synergistic effects of the combinations of two different catalysts on biochar properties in contrast with the addition of one catalyst, and examining the effect of the incubation period on WHC and CEC of the incorporated soil with the biochar.       103  5.1 Experimental 5.1.1 Biochar preparation  Biochars used in this study were produced from microwave-assisted catalytic pyrolysis of switchgrass at 400°C in a 750 W 2.45 GHz single mode microwave reactor and further details on the microwave reactor reported in Chapter 2. K3PO4 and clinoptilolite were mixed with switchgrass at different loads (10, 30 wt.% of each K3PO4, clinoptilolite and combinations of 10 wt.% K3PO4 with 10 wt.% clinoptilolite or 10 wt.% bentonite) to expedite microwave heating, promote pyrolysis reactions and improve the quality of the produced biochars (10KP, 30KP, 10Clino, 30Clino, 10KP/10Bento, 10KP/10Clino, 300-30KP). Adding bentonite at 10 and 30 wt.% to switchgrass did not increase the temperature significantly compared to pure switchgrass where no biochar was produced (Fig 2.3). Another biochar (300-30KP) sample was produced at 300°C to study the effect of temperature on biochar properties, particularly the sorption affinity. This biochar was produced under the same heating power of 750 W using the same reactor, once the desired temperature of 300°C was reached, the microwave power was decreased to maintain the temperature for ~10 min before power was turned off. Biochar sample from conventional pyrolysis was produced using an electric furnace, in which 20 g of switchgrass were mixed with 10 wt.% K3PO4 + 10 wt.% bentonite (CP-10KP/10Bento) and then pyrolyzed at 400°C using an 800 W electrical furnace in a fixed-bed reactor under a flow of heated N2 at ~200°C. More information about biochar production process can be found in Chapter 2. Biochar from conventional pyrolysis was then compared with biochars from microwave heating.  5.1.2 Biochar characterization  Biochar surface area was measured by a Micromeritics ASAP 2020 using nitrogen adsorption at -77 K. Brunauer-Emmet-Teller (BET) method was applied to determine surface area.     104  Because N2 molecules cannot diffuse effectively to micropores < 1.7 nm while CO2 can diffuse effectively and rapidly for narrow micropore (< 0.4 nm),92 CO2 adsorption isotherm was performed at 273 K using Quantachrome Autosorb at low relative pressure to determine micropore surface area, micropore volume and average pore diameter within a pore diameter range of 0.31 to 1.93 nm. Dubinin-Radusckevich (DR) equation was applied to determine micropore surface area and micropore volume, which is more suitable than BET equation and other calculation methods at CO2 adsorption pressure range of P/Po <10-4 to 10-2.63,92 Samples were degassed first at 120ºC for 7 h before BET measurement and for 24 h before CO2 adsorption isotherm measurement. The scanning electron microscopy (SEM) of biochars was carried out by Hitachi S3000N VP-SEM with EDX.  Biochar ash content was measured according to ASTM D1762 standard. Carbon, hydrogen and nitrogen contents of biochar samples were determined using a Perkin-Elmer 2400Series II CHNS/O analyzer, and oxygen content was determined by difference. Elemental compositions of biochar samples were measured at an ISO17025 accredited laboratory (ALS Group, Vancouver, BC, Canada) using acid digestion method, analyzed by ICP-AES. Measurements were conducted in duplicate and average results were recorded. As shown in Table 2.2 (Chapter 2), biochars possess high ash, high potassium and phosphorus contents, and other plant nutrients due to the use of catalysts which resulted in increased nutrients contents in the produced biochar. 5.1.3 Biochar and soil WHC and CEC Loamy sand soil (76.8% sand, 18.4% silt and 4.8% clay) was collected from the UBC farm located at the University of British Columbia, Vancouver, BC, Canada. Samples were collected from the top soil at 150 mm depth, then air dried and sieved to ≤2 mm through a stainless-steel mesh and stored in sealed containers.      105  For measuring water holding capacity of the samples, different types of the prepared biochars (biochars particles size ≤ 500 micron) were mixed well at 1 or 2 wt.% ratios with the air-dried soil. A control sample of soil with no biochar load was conducted to compare with the other samples incorporated with different types of biochars. It has been reported that the use of conventional pressure plate methods for observing soil moisture properties in mixed soil with biochar is not appropriate because the higher possibility of blocked ceramic plates micropores. Instead, gravimetric soil-moisture content can be used to avoid the issues of other methods.10,62 Therefore, soil water holding capacity was measured by gravimetric method. All samples were packed into PVC columns, which were then tamped down to achieve the same bulk density as the loamy sand soil at 1570 kg/m3. The bottoms of the PVC columns were covered by a precision filter mesh with 5 microns openings to prevent the loss of soil or biochar. All columns were saturated with deionized water and the top was covered by a plastic wrap to prevent the loss of water by evaporation. Then, the saturated columns were drained for 48 h at room temperature.10,133 For consistency, all columns were drained for the same time period and all experiments were conducted in triplicate, with the average determined. A sample was taken after 48 h and then dried at 105°C until constant weight. WHC was determined by the difference between the mass of the oven-dried and the wet sample. The remaining soil sample was air dried and stored for CEC calculation. Effective CEC was measured as the summation of exchangeable cations Ca, K, Mg and Na. Exchangeable cations (Ca, K, Mg and Na) were extracted and measured using ICP-AES.8,134 Extraction was performed in triplicate, with the average determined and reported. 5.1.4 Soil and biochar incubation  Biochar surface chemistry can contribute to the hydrophobicity of biochar surfaces, which in turn prevents water from getting into pores by creating a negative capillary pressure.100 It is     106  believed that these hydrophobic compounds might be oxidized over the incubation period to form hydrophilic compounds, which might result in a considerable increase in WHC.80,101,102 Thus, after WHC determination, all columns were incubated for 6 weeks at room temperature and the moisture content of each column was maintained during incubation at the soil field capacity of the loamy sand soil gravimetrically. At the end of the incubation period all columns were saturated again with deionized water following the same procedure mentioned above to measure soil WHC after the incubation period. Subsamples were taken from each column, and then air dried and stored for measuring effective CEC using the ammonium acetate method as stated above. Cations extraction and WHC measurements after incubation were performed in triplicate. 5.1.5 Statistical analysis Three-way analysis of variance ANOVA and Tukey’s honest significant difference (HSD) were performed to compare the effect of catalysts loads, biochar application ratios and the effect of incubation period on WHC and CEC with a 95% confidence level, i.e., P < 0.05 level of significance. Pearson correlations were performed to check the relationships between WHC with CEC, BET surface area, micropore surface area and micropore volume at different biochars additions at P < 0.05 level. A correlation was also performed between average microwave heating rate and biochar surface area. All statistical tests were performed using IBM SPSS Statistics Version 22.0.0.   5.2 Results and discussion  As shown in Figure 5.1, heating method and catalyst type had significant effects on the morphology of the biochar structure. Biochars produced from microwave heating had a more porous structure than biochar produced under conventional heating due to the higher heating rate and the unique heating process of microwave heating.      107    Conventional heating 10% K3PO4 + 10% bentonite   Microwave heating 10% K3PO4 + 10% bentonite   Microwave heating, 30% clinoptilolite   Microwave heating, 30% K3PO4 Figure 5.1 SEM micrographs of different types of biochars produced under conventional heating and microwave heating.      108  Table 5.1 shows that the highest BET surface area is attained for 10KP/10Bento biochar sample. In addition, there is a significant increase in the surface area, micropore surface area and micropore pore volume by increasing catalyst load, which also resulted in an increased heating rate (Table 4.1); while, the average pore diameter slightly changed. However, increasing K3PO4 catalyst load from 10 to 30 wt.% showed decreased micropore area and micropore volume, which may relate to that high percentages of K3PO4 promoted coke formation at high temperatures (> 300°C) which might block or reduce the width of some micropores, resulting in reduced micropore volume and area (Figure 5.1). It was found that alkali metals at ≥20 wt.% are shown to promote coke production.103 Coke is mostly deposited inside the internal pores and/or the catalyst surface. The formation of char and coke will lead to reduced catalytic conversion efficiency and deactivate the catalyst irreversibly.24 This can be noticed by looking at the temperature curve, where temperature increased rapidly to 300°C within ~2.5 mins and then took > 12 mins to reach 400°C. More explanation on those phenomena can be found in Chapter 4. By comparing different heating methods with the same mixture of catalysts, biochar produced from microwave heating has a significantly higher surface area, micropore area and micropore volume than biochar produced from conventional heating method. Microwave heating increased the BET surface area and micropore surface area of biochar from 0.33 m2/g to 76.29 m2/g, and 26.41 to 401.7 m2/g, respectively, compared to conventional heating. A remarkable difference was found between BET surface area and micropore surface area of the biochars due to their microporosity structure with an average pore diameter <1.8 nm (Table 5.1). A strong positive correlation was found between the average heating rate (calculated for the first 3.5 mins of heating phase) with BET surface area and micropore surface area with Pearson correlation coefficients of 0.909 (P = 0.002) and 0.927     109  (P = 0.01), respectively. This shows that the microwave heating rate plays an important role in forming biochar structure. Table 5.1 BET surface area, average diameter, micropore area and pore volume of different biochars produced from microwave heating and conventional heating. CP: Conventional pyrolysis.  In microwave heating, thermochemical reactions may take place at lower temperatures because of the low temperature gradients across the particles, compared to convective heating where heat needs to be transferred from surface to the core via conduction. This difference in heating has been shown to contribute to producing biochar with a high specific surface area and pore volume compared to that from conventional heating.25 In addition, switchgrass with small particles (600-300 micron) with low Biot number (0.10) were used in this study in order to minimize the internal heat and mass transfer resistances which in turn will reduce the formation of catalytic coke that block the pores of biochars. Consequently, it may result in higher devolatilization rate and removes tar from the pores of switchgrass to create more porous structure. Microwave heating thus presents a great potential for making more porous biochar. Biochar type BET surface Area (N2)  (m²/g) Micropore surface Area (CO2)  (m²/g) Average diameter (CO2)  (nm) Micropore Volume (CO2)  (cm³/g) 10Clino  33.1 132.6 1.58 0.0462 30Clino  46.9 371.4 1.78 0.1295 10KP 33.5 337.5 1.71 0.1177 30KP 39.2 329.7 1.58 0.1150 300-30KP 37.5 372.6 1.62 0.1299 10KP/10Clino 56.8 404.9 1.70 0.1412 10KP/10Bento  76.3 401.7 1.65 0.1401 CP-10KP/10Bento   0.33 26.41 1.50 0.0092     110  5.2.1 Effect of biochar on soil water holding capacity (WHC) Fig. 5.2. Shows the WHC before and after incubation period for the samples of soil mixed with 0, 1 and 2 wt.% load of different types of biochars which were produced using different catalysts at different loads. The WHC of soil without biochar was 14.1% (wt./wt.). The highest WHC before incubation at 2 wt.% biochar load was achieved with 30KP biochar, which considerably increased WHC of the soil by 60.6%. The lowest WHC at 2 wt.% biochar load was observed with 10Clino biochar, which moderately increased (P < 0.001) the WHC of the soil by 30.1% compared to the control soil sample. On the other hand, there was no significant difference (P > 0.05) between WHC of the samples mixed with 30Clino or 10Clino biochars.    Figure 5.2 Water holding capacity of the loamy sand soil incorporated with biochars at 0, 1 and 2 wt.% before and after the incubation. Error bars are the standard deviation of three replicates.      111  At 1 wt.% biochar load the highest WHC was obtained with 10KP/10Bento biochar, while was observed with 10Clino biochar, which slightly increased the soil WHC by 22.9% compared to the control sample. There was also a significant (P < 0.05) increase in soil WHC after clinoptilolite load increased from 10 to 30 wt.% that was more pronounced at low biochar application rates (i.e., 1 wt.%) compared to 2 wt.% biochar load, which may relate to the hydrophobicity effect of some biochars. The proposed reason for decreased WHC with increasing biochar load for some soil samples with added biochars is likely associated with the hydrophobic nature of these biochars, causing water repellency and decrease in soil WHC. This phenomenon could be noticed when biochar load was increased from 1 to 2 wt.%, such as 10KP biochar which showed a significant (P < 0.05) decline in WHC after biochar load was increased from 1 to 2 wt.%.   5.2.2 Effect of incubation period on soil water holding capacity (WHC)  It is seen from Fig. 5.2 that incubation period resulted in different trends of WHC for each biochar incorporated with the soil at 1 and 2 wt.% loads, while there was no significant (Tukey HSD, P = 1) change observed in WHC for the control sample after incubation period. The highest increase in WHC after incubation was observed for 300-30KP biochar produced at 300°C, compared to the same biochar produced at 400°C. However, there was no significant (P = 1) difference between the two biochar conditioned soil samples before incubation. The increases in soil WHC after incubation for samples with different biochar contents varied widely between 12 and 60% compared to the case before incubation; while, the variation was higher for all samples with 2 wt.% biochar load than 1 wt.% load. These phenomena may be related to the effects of hydrophobic compounds, which are responsible for water repellency and thus decreased WHC. Biochar surface chemistry can contribute to the hydrophobicity of biochar surfaces, which in turn prevents water from getting into pores by creating a negative capillary pressure.100 It is believed     112  that these hydrophobic compounds may be oxidized over the incubation period to form hydrophilic compounds, resulting in a considerable increase in WHC.101,102 Fig. 5.3 shows the effect of biochar addition to WHC by comparing microwave and conventional heated biochars against the loamy sand soil as the base line. It shows the highest WHC was observed for both 1 and 2 wt.% biochar loads for biochars produced by microwave heating. At 1 wt.% load, 10KP/10Bento biochar produced by microwave heating increased the WHC of soil by 18.5% compared to the CP-10KP/10Bento biochar produced from conventional heating. It should be noted that the biochar produced from microwave heating possessed higher micropore surface area and micropore volume than the one produced from conventional heating (Table 5.1). It can be thus concluded that WHC of loamy sand soil can be improved through engineered biochars using a proper catalyst which could enhance biochar properties during the pyrolysis process.   Figure 5.3 Effect of heating method on the ability of the produced biochars through microwave-assisted pyrolysis (MAP) and conventional pyrolysis (CP) on increasing water holding capacity of the loamy sandy soil compared to control. Error bars are the standard deviation of three replicates.     113  The ability of these biochars to hold more water is probably related to direct and indirect mechanisms. Direct effects may be related to the enhanced physical adsorption/absorption of water molecules onto/into biochar; and chemical adsorption in which water molecules form hydrogen bonds with the oxygenated-compounds (e.g., carboxyl, hydroxyl) in biochar surfaces, between hydrogen atoms from water molecules with oxygen lone pairs in the catalyst, and/or with anions from the catalysts remained in the biochars. A covalent bond can be formed between cations (e.g., K+, Ca2+) from the catalysts and with the water oxygen lone pairs.135 Indirectly biochar can ameliorate soil structure due to the binding ability of the catalysts used in biochar production, which can act as binding agents. Thus, they can collect and adhere many soil particles with each other and filling the large spaces between those newly formed large aggregates. This would reduce soil bulk density and water permeability, and increase soil porosity, which allow more water to be held either physically or chemically. It is well known that sandy soils have larger pores and lower surface area than clayey soils, allowing water to move rapidly through the gravity force. Soil macropores (> 80 µm) can participate in decreasing water holding capacity by increasing the rapid flow of water by gravity through soil pores, and an obvious leaching events can be noticed after heavy rainfall, while, micropores (< 30 µm) can hold water in place.93 Reducing the large pore spaces and voids between soil particles and increase soil surface area through adding biochar will in turn reduce water loss through gravitational force, thereby increasing soil ability to retain more water molecules which might have soluble nutrients. These phenomena will likely indirectly reduce groundwater contamination by reducing leaching of chemical fertilizers especially in sandy soils. Therefore, biochar porosity plays an important role in water retention, and pyrogenic nanopores which are the voids that compose within the carbon structure due to chemical changes during pyrolysis. These pores contain a majority of biochar pore     114  volume, and the diameter of these pores is generally smaller than 50 nm with the vast majority of these pores being < 2 nm.100 Comparing the physical properties of biochars, particularly the BET surface area, the highest BET surface area was 76.3 m2/g for 10KP/10Bento biochar, while that for 30KP biochar was 39.2 m2/g. However, the highest WHC at 2 wt.% load was observed for 30KP biochar. As a consequence, no correlation (Pearson coefficient = 0.214, P = 0.463) was found between biochar BET surface area and WHC, which agrees also with Zhang and You (2013).9 This suggests that the BET surface area of biochar may not give a good indication for water retention capacity and there might be other influencing parameters such as the bulk behaviour and the chemical composition of biochar. Strong positive correlations were found between WHC with micropore surface area and micropore volume with Pearson correlation coefficients of 0.812 (P < 0.0001) and 0.814 (P < 0.0001), respectively. These findings show that micropore surface area and micropore volume of biochar have a more significant impact on biochar water retention ability than BET surface area, and high heating rate will likely increase the pyrogenic nanopores in biochar as confirmed by the strong correlation between the average heating rate and biochar microporosity. 5.2.3 Effect of biochar on soil cation exchange capacity (CEC) Fig. 4.4 shows soil CEC before and after incubation period for soil samples mixed with 0, 1 and 2 wt.% of different types of the engineered biochars. The CEC of the loamy sand soil without biochar addition is 6.69 cmol/kg, which is very low compared to soils with high clay and/or high organic matter contents. However, addition of biochars to the loamy sand soil resulted in significant increases in soil CEC. As can be seen in Fig. 5.4, there is a wide variation of CEC values between different biochar types. Generally, biochars produced from K3PO4 mixed with switchgrass showed higher CEC values than those biochars produced from clinoptilolite mixed     115  with switchgrass. Comparing different temperatures, 300-30KP biochar produced at 300°C showed the highest CEC values compared with the control and other biochar added samples. However, there was no significant difference (P > 0.05) in CEC values before incubation, compared to 30KP biochar produced at 400°C. Comparing two different biochar loads, there were significant differences in CEC values between 1 and 2 wt.% biochar loads for all samples mixed with biochars, with 2 wt.% biochar load showing higher CEC values.  Figure 5.4 Cation exchange capacity of the loamy sand soil incorporated with biochars at 0, 1 and 2 wt.% before and after incubation. Error bars are the standard deviation of three replicates.  5.2.4 Effect of incubation period on soil CEC and extractable cations It is seen from Fig. 5.4 that the incubation period had significant (P < 0.05) impacts in soil CEC for some biochar added samples, particularly for samples 300-30KP at 1 and 2 wt.% biochar loads, 10KP/10Bento, 10KP/10Clino, 30Clino, and 30KP at 1 wt.% biochar loads. However, the increase in CEC values was more pronounced for the samples containing 1 and 2 wt.% of 300-30KP biochar, which resulted in > 30% increase in soil CEC compared with the same samples     116  before incubation. The control sample and other samples treated with different biochars did not show any significant (P > 0.05) differences in CEC value. These phenomena might result from the short incubation period compared to other studies with long incubation periods. It is thus expected that after a long period of incubation the CEC values for samples of other biochar contents could increase significantly.95   After incubation, the highest CEC values were 16.96 and 22.04 cmol/kg for 1 and 2 wt.% of 300-30KP biochar, respectively, which increased soil CEC by 146.0% and 219.5%, respectively, compared to soil without biochar. The CEC of soil would increase after incubating with biochar as a result of oxidized biochar surfaces leading to the formation of carboxylate and different ionizable functionalities.101,102 The addition of biochars produced from clinoptilolite (i.e., 10Clino and 30Clino) as a catalyst slightly increased soil CEC compared to other biochars produced from K3PO4 or the mixture of K3PO4 with clinoptilolite or bentonite. The lowest CEC value was observed for 1 wt.% 10Clino biochar which only increased soil CEC by 16.63% compared to the control sample. Adding 10 wt.% K3PO4 and 10 wt.% clinoptilolite as catalysts to switchgrass considerably (P < 0.0001) increased CEC compared with all biochars produced from 10 or 30 wt.% clinoptilolite, while the difference was slightly higher than that biochar produced from 10 wt.% K3PO4. These phenomena revealed the synergistic effects of combing different catalysts on increasing biochar sorption capacity, and that adding suitable catalysts to biomass will considerably improve soil CEC. The wide variations in CEC values at different biochar contents prove that these catalysts have a great impact on improving biochar physiochemical properties.       117      Figure 5.5 Effect of 0, 1 and 2 wt.% load of different biochars on the extractable Ca, K, Mg and Na from the loamy sand soil before and after incubation. Error bars are the standard deviation of three replicates.  The effect of incubation time on extractable Ca, K, Mg and Na before and after incubation period is shown in Fig. 5.5, where the extractable amounts for these cations widely varied between different types of biochars. The highest extractable amounts were obtained for Ca and K, while Mg and Na remained low and did not affect soil CEC significantly (P > 0.05) as much as K and Ca. Potassium was the highest extractable cation from the samples with 2 wt.% load for biochars produced from 30 wt.% K3PO4 at different temperatures, which resulted in an extremely significant     118  increase in CEC of the sandy soil compared to the control soil. This is because of high K concentration in those biochars produced from K3PO4 as a catalyst. Ca was the highest for all other particularly for 2 wt.% load of biochar produced from 30 wt.% clinoptilolite as a result of high CaO concentration in clinoptilolite.   The extractable cations showed different trends after incubation. All samples showed a significant (P < 0.05) increase in Na extractable amount after incubation, although this increase did not affect soil CEC significantly (P > 0.05) after incubation. Meanwhile, other cations did not increase significantly (P > 0.05) after incubation for most samples with different biochars except for 300-30KP biochar samples at 1 and 2 wt.% loads. While potassium decreased significantly for some samples and that resulted in a significant increase in extracted Ca for the same samples, which may be related to either binding K onto the negatively charged sites on the surfaces of these biochars and releasing more Ca, or the reciprocal relationship between Ca and K.   5.2.5 Biochar and soil water holding capacity The results showed that the addition of the catalysts to biomass increased biomass microwave heating rate, increased biochar surface area and pore volume, enriched biochar with important plant nutrients, improved the sorption affinity of the produced biochars and resulted in a considerable increase in WHC and CEC of the loamy sandy soil after the addition of the engineered biochars. It can be noticed from the results that catalysts play a major role in increasing biochar sorption affinity to water. This phenomenon is more pronounced after catalysts load was increased from 10 to 30 wt.% after incubation. The fact that clinoptilolite can retain water into its pores more than half of its weight would likely have a great impact on increasing water retention capacity of the produced biochars from mixing clinoptilolite with biomass.111  This is more pronounced after     119  clinoptilolite load was increased from 10 to 30 wt.%. Adding 10 wt.% clinoptilolite with 10 wt.% K3PO4 to switchgrass increased the retention ability of the produced biochar, which is significantly higher than 10KP, 10Clino and 30Clino biochars, and comparable to 30KP biochar. This indicates the synergistic effects for the combination of two catalysts compared to the biochars produced from single catalyst, which may due to increased biochar microporosity as a result of enhanced microwave heating rate (Table 4.1 and Fig. 5.1). These findings agree well with Polat et al. (2004),111 who reported that clinoptilolite behaved like a permanent water reservoir, providing a prolonged moisture periods through the drought periods. As a consequence, biochar produced from mixing clinoptilolite with biomass will reduce the irrigation water requirements particularly in arid and semi-arid climates. It was found that bentonite clay improves soil fertility, increases soil water and nutrient retention, and improves the agrochemical and physical properties of soil.97 K3PO4 also increased the sorption affinity of the loamy sandy soil due to its chemical structure, which may contribute to the formation of different chemical bonds (i.e., hydrogen and covalent bonds) with water molecules. When biochar is water saturated, hydrogen bonding between hydrogen atoms on biochar surface and the oxygen of water forms.135 However, water-biochar interactions can happen between the orbitals, which have the unpaired electrons, and the electron-deficient orbitals of the hydrogen atoms of the water.135 It was observed that WHC for 300-30KP biochar was lower than 30KP biochar (produced at 400°C) before incubation and then was the highest after incubation period. The likely reason is that biochars produced at low temperatures have less affinity towards water because the existence of aliphatic functional groups, which trigger the hydrophobicity of biochar.136 However, many researchers confirmed that hydrophobicity would be reduced after a long incubation period due to biochar surface oxidation. It was found that the continuing water uptake might decrease the surface     120  hydrophobicity by increasing the moisture content within biochar pores.95 However, because of the difficulty of separating biochar particles from soil samples after incubation, the surface oxidation of biochar surface was not confirmed analytically in this study. Another mechanism was proposed in the literature that the oxidation of biochar during incubation period could reduce biochar hydrophobicity by increasing the formation of oxygen-containing functional groups such as carboxylic which in turn make biochar more hydrophilic.95,100 This means that hydrophobicity of biochars might not be a big issue after long term of biochar application. Consequently, if the aim is to enhance soil quality, the produced biochars at low pyrolysis temperature and from certain biomass materials might be a good candidate for improving soil retention capacity.  A strong correlation was found between biochar hydrophobicity and the presence of alkyl functional groups. However, when biochar interacts with water the alkyl groups may rearrange, leading to reduced overall particle hydrophobicity.96 Hydrophobicity is considerably dominant in external pores and residual macropores. When biochars are submerged in water the pyrogenic nanopores are likely filled by water vapour.100 The percentage of pyrogenic nanopores is very small compared to total porosity. Nevertheless, the predominance of the critical biochar surface area for contaminants and nutrient sorption is provided by pyrogenic nanopores.100,137 This supports the strong correlation found in this study between biochar microporosity and increased soil WHC.  5.2.6 Biochar and soil CEC  The high CEC in biochar reflects the magnitude of the biochar in cation exchanging ability. It is believed that biochar which possesses a high CEC will have a high nutrient retention capability, reducing fertilizer run-off.138 A significant amount of hydrophilic oxygen-containing groups such as carboxylic and phenolic compounds, which is reflected by the higher cation     121  exchange capability, can be found in biochars with high CEC. It is hypothesized that the oxidation of biochar will lead to increased CEC, which is responsible for improved soil fertility in a soil amended with biochar. The increase in CEC of the soil mixed with biochar after a few months of incubation even with no microbial activity reveals that the application of biochar as a soil conditioner is important for enhancing the fertility of highly weathered soils.101 Therefore, cation-exchanging ability of biochar is considered as an important attribute.  It was found that the addition of clinoptilolite to soil in arid and semi-arid zones resulted in increased soil CEC by 37.6% compared with untreated soil, and increased water use efficiency by 40.4% in drought conditions.139 These findings are in line with the biochar produced from clinoptilolite in this study, which proves the ability of clinoptilolite in improving biochar sorption affinity. These improvements are considered important benefits for applying this engineered biochar to soil particularly when water resources become limited. Moreover, more food and energy crops can be cultivated in arid and semi-arid regions with less water resources.139  Clinoptilolite addition to soil resulted in an considerable increase in the yield of bean by up to 57.3% at lower irrigation levels compared to the control soil.139 Similar positive effects in plant yields were reported for different crops such as eggplant, carrots, cowpea and potatoes. The positive impacts of clinoptilolite on physiochemical properties, which increase the nutrient use efficiency by raising the uptake and availability of macro and micronutrients, have resulted in increased yield for these crops.111,139 Bentonite increased the surface charge and CEC, and improved fertilizer use efficiency considerably of different types of soils. This would reduce the environmental detrimental effects of fertilizer application to soils markedly.99 These improvements were reflected on the engineered biochar after 10 wt.% bentonite and 10 wt.% K3PO4 were added, which resulted in increased biochar sorption affinity to water.       122  Biochar surface oxidation will lead to increased CEC with higher charge density.102 The prevalence of basic reactions over acid reactions could result in increased water retention to biochar surface in which water protons can be exchanged with Na+, K+ and Ca2+.140 These phenomena support the strong correlation (R2 = 0.82) between CEC and WHC found in this study, which suggests that higher CEC would lead to increased soil water retention capacity.  5.2.7 Biochar production and integration Biochar production cost should be low and reasonable for farmers, and the benefits of biochar application should offset the total cost of biochar application to soils or the cost of using additional irrigation water.3 Every soil has certain constraints, which need to be enhanced by adding amendments. It is known that the WHC would change significantly from one soil to another, as a result of changing soil texture, particularly the proportions of sand and clay in a soil. The impact of biochar on increasing WHC of soil will be more pronounced in sandy soils compared to other soils. Generally, sandy soils are more hydrophobic compared with clayey soils due to the higher content of alkyl carbon in their organic matter which is believed to be responsible for low water retention in sandy soils.141 Based on soil properties, biochar can be tailored to its task. For instance, sandy soils have very low water and nutrient retention capacity. Consequently, biochar with high CEC and high sorption affinity could be added to these soils to improve their ability to hold water and nutrients, particularly in arid and semi-arid zones. By doing so, groundwater will be preserved from being contaminated by leached chemical fertilizers. Moreover, water irrigation requirements will be significantly reduced due to improved soil water retention ability.  Another criterion, which needs to be taken into account, is carbon sequestration efficiency, with the maximum biochar yield in terms of mass and carbon content achieved at a low pyrolysis temperature of 300°C. However, a trade-off was observed between carbon sequestration efficiency     123  (low pyrolysis temperature) and the optimum hydrological characteristics (450-550°C).96 This trade-off may relate to biochar hydrophobicity, which may be persist in the soil-biochar mixture.96 When the longer biochar application in soil is considered, biochar hydrophobicity will likely decrease, while its hydrological properties may improve. Thus, there may be no trade-offs between carbon sequestration and the optimum hydrological properties. Producing a superior biochar which possesses several functions (e.g., increase water and nutrient retention, immobilize heavy metals and other contaminants, source for plant nutrients, increased soil pH for acidic soil, low production cost, etc.) is crucial to overcome the high cost associated with the application of biochar to soil in a large scale. Most of biochar studies were conducted using biochars produced from slow pyrolysis by conventional heating, which is not favourable for industrial biochar production because of many constraints such as large equipment needed, low productivity, and high production cost.3 Biochar production cost can be reduced through utilizing proper catalysts in which these catalysts would serve for many functions: such as increasing microwave heating rate to reduce energy consumption, producing a high grade bio-oil that can be used as a fuel, and remaining in the produced biochar to serve as a source of nutrients.125 Meanwhile, biochar sorption affinity can be increased by increasing its surface area and CEC. Catalytic microwave pyrolysis could produce biochars at low temperatures capable of improving soil hydrological properties and more efficient for carbon sequestration. This can be noticed for 300-30KP biochars, which was produced at 300°C with low microwave power consumption and showed the highest WHC, exchangeable cations and CEC compared to biochars produced at 400°C with K3PO4 or with other catalysts. While comparing biochars (10KP/10Bento and CP-10KP/10Bento) produced from microwave pyrolysis and conventional pyrolysis using the same catalyst proportions, microwave heating can be utilized to produce biochar with high sorption     124  affinity and rich in nutrients through selecting proper catalysts that can increase microwave heating and reduce energy consumption. This integrated system would likely reduce the total production cost of biochar.  Biochars produced from this study contain significant amounts of potassium, phosphorus and other essential plant nutrients that make them a potential amendment and remediate agent for soil. However, it should be noted that this study was performed in a lab scale. To confirm these findings at large scale, those biochars need to be tested in a field scale with cultivated plants for a long period of time, in which they would be more exposed to biotic and abiotic effects. The interaction between biochar and soil might also have an impact on biochar performance.  5.3 Summary Catalytic microwave-assisted pyrolysis could be one potential approach for tailoring biochar quality to improve soil physiochemical properties. Favourable microwave absorption, high water and nutrient affinity and desirable plant nutrients are the key features of an effective catalyst for microwave pyrolysis for making high quality biochars. The experimental results from this study showed that biochar produced from microwave pyrolysis was more effective than biochar produced from conventional pyrolysis in increasing soil water holding capacity, while high quality biochar for soil applications can be produced at a low temperature (i.e., 300ºC) from catalytic microwave pyrolysis. Biochars produced from mixing switchgrass with two or more catalysts showed superior performance in increasing soil WHC and CEC, and improving soil fertility markedly compared with the biochar produced from one catalyst only. This is probably due to increased biochar microporosity resulting from increased microwave heating rate and improved biochar chemical properties.       125  Chapter 6: The role of tailored biochar in reducing bioavailability, phytotoxicity and uptake of heavy metals.  Soil contaminations by organic and/or inorganic contaminants in many areas around the world pose severe environmental problems (e.g., toxicity, water pollution, food pollution, human and animal health, etc.) and increase the risks to humans and biota.16,17 An engineering approach to remediate heavy metals cost-effectively is to immobilize pollutants and heavy metals while improving plant growth through providing nutrients to plants in situ, and promoting ecological restoration.16,70,71 Many natural and synthetic amendments can reduce extractability of heavy metals from contaminated soils. However, they cannot improve soil productivity, microbial activity, and plant growth.17,72 It is easier to reduce extractable metals in contaminated soils than to improve plant growth.73 Biochar can be applied to soil to serve as an amendment and remediate agent to immobilize and reduce the bioavailability and toxicity of heavy metals in contaminated soils through ion-exchange, metal ion surface complexation, co-precipitation and physical adsorption.17,69,71 Most biochars used in heavy metal adsorption/removal are produced from either slow pyrolysis with low surface area (5 - 25 m2/g) or fast pyrolysis without any pretreatment or modification.16,71 To improve soil physiochemical properties of contaminated soils, biochars need to be further treated by activation or blending with other nutrients to increase cation exchange capacity (CEC), pH and surface area.16,74 The fact that most biomass materials are poor in absorbing microwaves requires the addition of microwave absorbers to increase microwave heating rate and efficiency.125 It is thus desirable to identify solid catalysts which can serve as a catalyst to improve the bio-oil quality, a microwave absorber to improve microwave absorption rate, and a nutrient to improve the quality of biochar byproduct. Those catalysts are pre-mixed with biomass samples and remain within biochar to serve     126  as a source of slow-release nutrients for plants and act as a remediate agent. From our previous work, it is noted that microwave catalytic pyrolysis could produce biochars with higher quality (e.g., high surface area, rich in nutrients and high sorption affinity) at reduced microwave heating time. The overall goal of this study is to use the multi-functional catalysts for biomass pyrolysis: (1) as a good microwave absorber for accelerating the microwave heating rate; (2) as a catalyst to improve the quality of bio-oil and biochar; and (3) as a nutrients/soil conditioner embedded in biochar to increase its performance as a fertilizer and soil remediate. This chapter reports the performance of produced biochars by assessing the capacity of these biochars to reduce bioavailability, phytotoxicity and uptake of heavy metals by wheat plants and determining the efficacy of these biochars to increase soil fertility and plant growth in contaminated soil. 6.1 Experimental 6.1.1 Catalytic microwave pyrolysis and biochars preparation  Biochars used in this study were produced from microwave-assisted pyrolysis of switchgrass at 400°C and 750 W in a 2.45 GHz single mode microwave oven. Different catalysts with different loads were used to accelerate heating and promote the pyrolysis reactions. K3PO4 and clinoptilolite ((K,Ca,Na)2O-Al2O3-10SiO2-6H2O) were mixed with switchgrass at different percentages (10 or 30 wt.%) to promote pyrolysis reactions and improve the quality of the produced biochars (10KP, 30KP, 10Clino, 30Clino). Combinations of 10 wt.% K3PO4 and 10 wt.% clinoptilolite (10KP/10Clino) and 10 wt.% K3PO4 and 10 wt.% bentonite (10KP/10Bento) were performed to explore the synergistic catalytic effects between the two catalysts on improving biochar performance in reducing toxicity and uptake of heavy metals compared to biochars produced from     127  single catalyst. More details about the microwave reactor and biochar production can be found in Chapter 2. 6.1.2 Biochar, heavy metals and soil incubation  For measuring plant nutrient and heavy metal retention, loamy sandy soil (76.8% sand, 18.4% silt and 4.8% clay) was collected from the UBC farm located at the University of British Columbia, Vancouver, BC, Canada. Samples were collected from the topsoil at 0-150 mm depth, air dried and sieved to ≤ 2 mm through a stainless-steel mesh and stored in sealed containers. Heavy metals in powder form were purchased from Alfa Aesar; Pb(II), Ni(II), and Co(II) in oxide form (PbO, NiO and CoO) which is one of the forms in which these heavy metals are present in contaminated soils.83 Heavy metals were directly added to the air-dried soil samples and mixed thoroughly at concentrations of 1000, 500 and 300 mg/kg, respectively, and blank soil samples without heavy metals (pure soil) were tested for comparison. Soils spiked with heavy metals were then mixed thoroughly with biochars at 1 or 2% (wt./wt.) and soil samples spiked with heavy metals without biochar (Soil+HM) were also tested for comparison. The concentrations of heavy metals for the doped soil samples belong to soil type C as regulated by the Ministry of Environment: British Columbia Standards for Managing Contamination at the Pacific Place Site (1990). This type of soil is restricted to agricultural and all other usages until proper amendments are applied to the contaminated sites in order to reduce the high level of heavy metals toxicity. Thus, testing these engineered biochars using this soil type may reduce the level of toxicity and increase soil fertility, so that this soil type can be used for cultivating energy crops.   All samples were packed into PVC columns, and then tamped to achieve the same bulk density as the loamy sand soil at 1570 kg/m3. The bottom of the PVC columns was covered by a precision mesh of 5-micron openings to prevent the loss of soil or biochar. All experiments were     128  conducted in triplicate. The moisture content of each sample was kept at field condition during the incubation by gravimetric method.62 All columns were incubated for 6 weeks at room temperature. After the incubation, a subsample was taken from the top of each column for characterization: cation exchange capacity (CEC) was measured using ammonium acetate method;142 and soil pH was measured using 1:1 (wt./wt.) soil/deionized water according to ASTM D4972-2013 standard. The pH meter was calibrated regularly by standard pH buffer solutions; while the measurements were done in triplicate with the average results reported.   6.1.3 Heavy metals and nutrients extraction For heavy metals and nutrients extraction, a multi-element extraction method, Mehlich-3, was used to identify the bioavailability and extractability of heavy metals, and to assess the availability of other plant nutrients. Mehlich-3 extraction solution is composed of 0.2 M CH3COOH + 0.25 M NH4NO3 + 0.15 M NH4F + 0.01 M HNO3 + 0.001 M EDTA. Mehlich-3 method has been commonly used in the evaluation of the mobility and bioavailability of heavy metals in contaminated soils and in the determination of available plant nutrients. Mehlich-3 also shows a good correlation between the extracted amounts from the soil with nutrients uptake and the crop response.143,144 A good agreement was also found between Mehlich-3 and the standard USEPA Method 3051A (United States Environmental Protection Agency).144 A subsample was taken from each column after incubation, then the subsamples were extracted using 1:10 (w/v) Mehlich-3 in a 50-mL centrifuge tube, shaken for 5 min, then filtered and analyzed by ICP-OES.144,145 All glassware and tubes were washed by ultra-pure water followed by Mehlich-3 extractant several times, and extractions were done in triplicate and average results were recorded.      129  6.1.4 Germination test, heavy metals, and nutrients uptake To test the ability of these biochars to reduce phytotoxicity on plant growth and uptake of Pb, Co, and Ni, a germination test was conducted using wheat seeds. First, wheat seeds with a uniform shape were chosen, and for each sample, 30 seeds were sown in a Petri dish filled with 50 g soil sample collected after incubation. Then, all Petri dishes were put randomly and kept inside an environmental chamber controlled at a constant temperature of 25°C in dark conditions (Fig. 6.1). After five days, germinated seeds were counted with 2 mm long emergence of radicle being counted as a germinated seed,67 and the length of the wheat shoot for each seedling was measured. To measure nutrients and heavy metals concentration in wheat shoots, plants were carefully washed with ultra-pure water for two times. Then all samples were oven dried at 60°C for 72 hr, with the dried weight of each sample being recorded. The dried plant samples were ground and digested using NH3/H2O2 in a hot block digestion system, with the element concentrations being determined by ICP-OES.146,147  Figure 6.1 Germinated wheat plants for pure soil, soil spiked with heavy metals (Soil+HM) and heavy metal spiked soil amended with different biochars inside the environmental chamber. Soil + HM Soil Soil + HM + Biochar      130  6.1.5 Statistical Analysis  Two-way analysis of variance ANOVA and Tukey’s honest significant difference test (HSD) were performed to compare the effect of different catalysts loads (10 or 30 wt.%) and biochar application loads (1 or 2 wt.%) on heavy metals bioavailability and uptake at a 95% confidence level, i.e., P < 0.05 level of significance. Pearson correlation (r) was performed to check the relationships between extracted heavy metals and soil pH, CEC and micropore area. Another Pearson correlation analysis was performed between heavy metals uptake and heavy metals extraction to identify the correlation between heavy metals bioavailability and uptake. All statistical tests were performed using IBM SPSS Statistics Version 22.0.0.   6.2 Results and discussions 6.2.1 Biochar properties  As mentioned earlier in Chapter 2 that the produced biochars from catalytic pyrolysis contain high concentrations of potassium, phosphorus, and other important plant nutrients. A summary of biochar properties used in this chapter is given in Table 2.2. 6.2.2 Effect of engineered biochar on soil CEC and pH  Table 6.1 shows CEC, pH and extractable elements from the pure soil (Soil), soil spiked with heavy metals (Soil+HM) and incorporated with different biochars (Soil+HM+Biochar) at 1 and 2 wt.% loads. The pH of soil only was 5.73 which is moderately acidic, with no significant difference in pH for Soil+HM compared to the pure soil (P > 0.05). However, the highest pH of 8.06 was found at 2 wt.% biochar load for the sample treated with 30KP biochar. On the other hand, the lowest pH at 2 wt.% biochar load was found for 10Clino biochar and the lowest pH for 10Clino biochar at 1 wt.% biochar load among all biochar samples.      131  Increasing catalyst load from 10 to 30 wt.% resulted in a significant increase in soil pH (P < 0.05) for most biochar samples except 10Clino biochar. This shows that catalyst type and load have a great effect on soil pH. Soil pH plays a major role in the bioavailability of heavy metals (Pb, Co and Ni) in the contaminated soils, and the adsorption capacity for some heavy metals can increase up to three times per unit increase in soil pH.16,85 Furthermore, soil pH is the main factor that controls the potential release of the immobilized heavy metals or metalloids and surface precipitation.17,85 The precipitants of metal oxy/hydroxides are formed due to increased hydrolysis of heavy metals after pH is increased.68,87 Thus, increasing soil pH especially for acidic soil will reduce heavy metals bioavailability, which is discussed later.   Table 6.1 also shows that the addition of the engineered biochar considerably increased soil CEC compared to the soil without biochar. The CEC for Soil+HM was slightly higher than the pure soil but not significant (P > 0.05) based on Tukey test. All biochar applications increased soil CEC markedly compared to pure soil and Soil+HM, and the highest CEC was recorded for 10KP/10Clino biochar at 2 wt.% load, which increased CEC by 226% and 142% compared to pure soil and Soil+HM, respectively. The increased soil CEC after biochar addition will result in increased soil sorption affinity due to the increased soil negative charges, which will raise soil adsorption affinity for heavy metals.16,69,85  6.2.3 Effect of engineered biochar on nutrients extractability As shown in Table 6.1, biochars addition resulted in increased extractable nutrient contents compared to the pure soil and soil+HM. Generally, biochars produced from 30 wt.% K3PO4 showed the highest extracted K and P amounts compared to all the other samples including control samples, which increased the extractable amounts of K and P by 20.9-fold and 3.5-fold, respectively, at 2 wt.% biochar load; while, 10Clino biochar showed the lowest extracted amounts     132  and negative trends compared to the pure soil. Spiking soil with heavy metals significantly affected the extractability of other soil nutrients, and some elements showed increased extractability while others showed the opposite trend. The major changes were for K, Ca and P of which the extractability greatly changed after heavy metals were spiked to the soil. This may result from ion-exchange and co-precipitation mechanisms (discussed later in Section 6.2.6). There is also a significant reduction in the extractable P for Soil+HM and samples treated with 10Clino biochar by 95.7 and ~75 %, respectively, compared to the pure soil, which may due to co-precipitation of P with Pb, Co, and Ni to form insoluble compounds. Lead immobilization could occur as a result of the formation of very low soluble pyromorphite-like minerals in the presence of phosphate ion, which reduces Pb bioavailability in soils.82 83 It was reported that Pb-phosphates compounds are less soluble than naturally-occurring Pb compounds such as cerussite (PbCO3) by at least 44 times.88 It is plausible that pyromorphite be easily formed during extraction steps when Pb2+ and PO43- react; while, the rate of pyromorphite formation increases with increasing phosphate ion.89 The rate of pyromorphite formation is affected by the presence of both ions Pb2+ and PO43- in the contaminated soil.82 At the presence of sufficient amounts of phosphorus, the solubility of Pb will rapidly decrease and Pb-P precipitation could complete after 60 minutes.83 However, because of the concentrations of Pb (1000 mg/kg) in the current study being much lower than 10,000 mg/kg which is the threshold identifiable by XRD,88 XRD analysis did not detect these phases in the current study. The highest extractable amounts of Ca and S were observed for 10KP/10Clino biochar at 2 wt.% load, while the highest extractable amounts of Fe and Mn were given by 30KP biochar at 2 wt.% load. In general, biochars produced with two catalysts revealed adequate extractability of multiple plant nutrients, which probably resulted in better performance than other biochars produced from single catalysts.     133  Table 6.1 Cation exchange capacity (CEC), pH and extractable elements from soil, soil with heavy metals (Soil+HM) and soil with heavy metals mixed with different biochars.  Treatment CEC cmol/kg pH K mg/kg P mg/kg Ca mg/kg Mg mg/kg S  mg/kg Fe mg/kg Mn mg/kg Soil 8.1 ±0.62a* 5.73 ±0.14a 132  ±10.7a 95.5  ±4.06a 1493 ±179a 40.2 ±3.68abc 42.7 ±4.83bcd 189 ±4.01abc 18.7 ±1.98a Soil+HM 10.9  ±0.74a 5.79 ±0.15a 88.1  ±5.90a 48.8  ±3.02b 2162 ±212ab 43.6 ±2.55a 36.0 ±2.91abc 151 ±7.33a 25.9 ±1.11bc 300-30KP-1% 22.6 ±1.2ef 7.58 ±0.15 2003 ±74.4c 279 ±1.32de 2866 ±145bc 40.7 ±1.47abc 42.3 ±1.25bcd 182 ±4.24abc 26.8 ±1.03bcd 300-30KP-2% 25.6 ±1.54f 8.10 ±0.09 3645 ±273f 559 ±15.6 2580 ±158bc 49.4 ±1.73bcd 49.0 ±3.24d 195 ±2.39bcd 28.8 ±1.08cde 30KP-1% 21.0  ±1.14cde  7.52 ±0.16f 1798 ±40.5cd 335 ±1.85e 2952 ±176c 49.5 ±0.86bcd 39.3 ±2.61abc 234 ±4.33de 30.6 ±0.73def 30KP-2% 23.8 ±1.59ef 8.06 ±0.18g 2897 ±410e 425 ±39.4f 2748 ±332bc 59.2 ±7.53de  35.8 ±2.97abc 260 ±29.6e 36.5 ±3.17f 10KP-1% 20.2  ±0.94cd 7.41 ±0.12f 1529 ±82.6bc 255 ±3.48d 2893 ±282bc 57.7 ±3.10de 33.4 ±1.88a 211 ±23.4cd 29.5 ±2.53cde 10KP-2% 21.4  ±1.21de  7.63 ±0.08f 2046 ±114c 329 ±14.1e 3000 ±215c 62.1 ±2.38e 44.2 ±4.80cd 205 ±11.1cd 31.7 ±3.11def 10Clino-1% 16.6  ±0.86b 6.45 ±0.14b 118 ±7.03a 52.3 ±2.01ab 2985 ±245c 42.1 ±1.60ab 38.3 ±3.20bcd 165 ±6.35ab 32.5 ±2.17ef 10Clino-2% 16.8  ±0.71b 6.82 ±0.14bc 135  ±6.07a 54.6 ±4.26ab 3298 ±212c 42.8 ±0.06ab 43.5 ±2.61bcd 174 ±12.8abc 34.9 ±0.73ef 30Clino-1% 18.0  ±1.00bc 6.94 ±0.14cd 162  ±11.5a 56.1 ±2.62ab 3273 ±363c 46.7 ±3.47abc 33.1 ±1.43a 165 ±6.35ab 35.0 ±0.39ef 30Clino-2% 18.7  ±1.10bcd 7.12 ±0.11cde 187  ±12.3a 56.0 ±6.04ab 3393 ±301c 52.4 ±1.79cde 44.4 ±2.39cd 196 ±6.77bcd 36.1 ±1.74f 10KP/10Clino-1% 21.4 ±0.90de 7.23 ±0.14def 1175 ±94.5b 154 ±4.89c 3331 ±290c 51.3 ±1.11bcd 34.7 ±1.98bc 160 ±6.14ab 23.1 ±0.23ab 10KP/10Clino-2% 26.4  ±1.22f 7.42 ±0.16ef 1987 ±250c 239 ±26.5d 3511 ±323c 59.1 ±5.95de 49.7 ±3.70d 180 ±11.3abc 26.8 ±2.19bcd 10KP/10Bento-1% 20.0 ±1.08de 7.29 ±0.08def 864 ±31.1d 175 ±2.04 3154c ±330 54.0 ±0.76de 46.4 ±2.23d 191 ±7.03bcd 33.2 ±2.07ef 10KP/10Bento-2% 23.3 ±1.08ef 7.49 ±0.16ef 1780 ±122cd 310 ±15.9e 3280c ±314 70.5 ±2.63f 48.3 ±4.10d 205 ±6.94cd 34.5 ±1.09ef *Values represent the mean of triplicate samples ± SD (standard deviation) and the values followed by the same letter are not significantly different (P = 0.05) using Tukey HSD test.     134  The increased extractable amounts of Ca and Mg for Soil+HM compared to pure soil suggests that ion-exchange is the dominant mechanism for reducing heavy metals mobility in the contaminated soil without biochar addition. This phenomenon was also found between Mg2+ with Pb or Cd ions.148 It was found that the combinations of Ca and P resulted in a significantly reduced Pb extractability compared to the case with Ca or P alone.81 This may be due to Pb precipitation with P and ion-exchange with Ca ion. It was also found that ion-exchange took place significantly between heavy metals and Ca and Mg in soil amended with clinoptilolite,149 and between heavy metals with K+, Ca2+, Na+ and Mg2+ triggered at high pH levels.150  The significant reduction in extractable Fe from the Soil+HM sample compared to pure soil suggests that Fe may have an impact on reducing heavy metals bioavailability through precipitation. It has been reported that the drop in Fe extraction was caused by the precipitation of amorphous Fe(OH)3, which emphasizes the occurrence of surface complexation with iron hydroxides.66  6.2.4 Effect of engineered biochar on heavy metals bioavailability Figure 6.2 shows the amounts of extractable heavy metals from Soil+HM mixed with different types biochars. In general, biochar additions to the contaminated soil resulted in a remarkable decrease in heavy metals extraction, which in turn reduced the heavy metals bioavailability in the soil. The extractable percentages for Pb, Ni, and Co with respect to the total concentrations (1000, 500 and 300 mg/kg, respectively) vary widely among different biochars applications and among the three heavy metals. The highest extractable percentage was from Soil+HM, ~63% for Pb and Co and as low as 5% for Ni. This shows the high extractability of Pb and Co and low extractability of Ni in the soil. The low extractability of Ni agrees with previous studies, which reported a range of extractable percentage of 1.5–6.2%.143,145     135   Figure 6.2 Extracted heavy metals (Pb, Ni, and Co) from soil spiked with heavy metals (S+HM), and heavy metal spiked soil incorporated with different biochars at 1 and 2 wt.% loads. Error bars are the standard deviation of three replicates.  As seen in Fig. 6.2, increasing biochar load from 1 to 2 wt.% resulted in a remarkable decrease in the extractable Pb for some samples, while others such as 30KP and 10Clino biochars did not show any significant decrease (P > 0.05) in Pb extractability. These trends were also shown by Co and the differences of extractable Co between 1 and 2 wt.% for 30KP, 10Clino and 30Clino biochars were also not significant. It was found that the application of K3PO4 to soil reduced Pb     136  extractability to 34.1% compared to 84.9% for untreated soil.73 However, biochars produced from K3PO4 were more efficient than K3PO4 in reducing Pb extractability.  Biochar micropores may have a great impact on heavy metals adsorption due to their greater affinity to retain heavy metals and thus prevent heavy metals from leaching. The effect of micropores may be more pronounced after a long period of exposure when heavy metals diffuse into micropores and undergo chemisorption inside the pores forming oxygenated functional groups. As a result, strong negative correlations were observed between micropore area and the extracted heavy metals. The correlation coefficient ranged from -0.63 to -0.88 (P < 0.002) at 2 wt.% biochar load, and -0.68 to -0.73 (P < 0.001) at 1 wt.% biochar load. 6.2.5 Effect of engineered biochar on wheat germination and shoot length   Figure 6.3a shows that the addition of the engineered biochars to the contaminated soil considerably increased wheat germination percentage compared to Soil+HM. The soil spiked with heavy metals (Soil+HM) showed the lowest germination percentage compared to the pure soil and Soil+HM mixed with different biochars. The lowest germination percentage for contaminated soil without biochar might result from the toxic effects of these heavy metals, which inhibited the germination of wheat seeds. It is also seen that the sample with 10Clino biochar showed the lowest germination percentage among all biochars applied; while, 10KP/10Clino and 10KP/10Bento biochars showed the highest germination percentage at 1 and 2 wt.% biochar loads. 10Clino biochar at 1 wt.% biochar load did not improve germination percentage compared to Soil+HM, which may be due to the increased phytotoxicity of heavy metals as confirmed by the highest extractability and uptake of heavy metals in Sections 6.2.4 and 6.2.6. This indicates that catalysts in biochar have a significant (P < 0.05) impact on wheat germination and on reducing heavy metals phytotoxicity in contaminated soil.      137   (a)  (b) Figure 6.3 Effect of different types of the engineered biochars at different loads (1 and 2 wt.%) on wheat germination percentage (a) and wheat shoot length (b) in comparison with pure soil and soil spiked with heavy metals (S+HM). Error bars are the standard deviation of three replicates.     138  Another observation is that the germination percentage for 30KP at 2 wt.% biochar load had an opposite trend compared with 1 wt.% load, which may be caused by the excessive potassium concentration and the elevated soil pH, as shown in Table 6.1. High K concentrations could also affect the uptake of other plant nutrients, which will be discussed in Section 6.2.6. It was also found that high pH (> 8) greatly reduced wheat germination and crop yield, which agrees with the results from current work.151 Wheat shoot length widely varied between different biochar types at different loads. Heavy metals reduced the shoot length of wheat markedly compared to the pure soil (Fig. 6.3b). The largest shoot length at 2 wt.% biochar load was obtained from 10KP/10Clino and 10KP/10Bento biochars, while the smallest shoot length was given by 10Clino biochar at 1 wt.% biochar load. Using two catalysts showed a great improvement on wheat shoot length compared to the samples treated with biochars produced from using single catalyst. This may be due to the synergies between the two catalysts on reducing heavy metals toxicity and providing the plant with sufficient nutrients because excessive K could lead to the adverse effect on shoot length. Spiking soil with heavy metals also greatly reduced the shoot length by 70.9% compared to the pure soil, confirming that heavy metals can greatly affect plant growth and reduce crop yield. The main symptom of Pb toxicity in plants is a fast inhibition in root growth as a result of reduced cell division,152 which was observed by the extreme decrease in wheat germination and shoot length compared to the pure soil because of the high mobility of Pb as confirmed by the highest extracted Pb (Fig. 6.2). The same effect may also be reflected in the uptake of Pb by wheat plants as in Section 6.2.6. Cobalt and nickel have also great toxic effects on plant growth, in which they can inhibit the growth of roots, leaves and stems of plants and reduce germination percentage markedly, thus reducing the crop yield.67,68 However, it is very difficult to judge which heavy     139  metal has the greatest toxicity because many parameters can interfere and reduce the plant growth in the current study. 6.2.6 Effect of engineered biochar on heavy metals and nutrients uptake   Figure 6.4 shows plant nutrients uptake by wheat plants for pure soil, Soil+HM, and Soil+HM incorporated with different types of biochars at 1 and 2 wt.% loads. It is clear that spiking soil with heavy metals affected the uptake of other elements compared to the pure soil, as reflected by a significant decrease in the uptake of some macro- and micro-nutrients. The samples treated with biochars produced with K3PO4 showed the highest concentrations of K and P compared to all other samples due to their high extractable amounts of K and P (Table 6.1). However, high K and P concentrations adversely affected the uptake of other metals especially for Ca, resulting from the reciprocal effect between K and Ca.  Phosphorus content in wheat shoots for Soil+HM was also significantly lower than the pure soil, and this was consistent with the data on extractable P from soil, as a result of the formation of phosphate compounds with the heavy metals. Fe uptake for Soil+HM without biochar was seen to be higher than pure soil, which was opposite to the extractable Fe data (Table 6.1). It was found that the roots of some plants can secrete chelating amino-acids called phytosiderophores such as deoxymugineic acid from wheat, which can dissolute and transport Fe to plant roots.68   The key factor affecting the adsorption and desorption capacity of soils on heavy metals is the pH. Increasing soil pH will increase the negative surface charge, which in turn will increase soil and biochar surface affinity for cations.74,87 The sorption of heavy metal ions highly depends on the concentrations of Ca, K, Mg, and Na, where the ion-exchange mechanism is dominant.72 The mobile fraction of heavy metals in soil is greatly affected by soil pH and increases as soil pH decreases.90 Increasing soil pH will reduce heavy metals mobility in contaminated soil, and thus     140  reduce their bioavailability to plants.90,91 Such a mechanism is supported by the strong negative correlations between soil pH and heavy metals uptake with r ranging from -0.87 to -0.90 and P < 0.0001 in this study. The sorption capacity of 10Clino biochar was considerably improved after 10 wt.% K3PO4 was added to clinoptilolite (10KP/10Clino biochar). This is because adding K3PO4 increased K and P contents in the produced biochar, in addition to increasing biochar micropore area because of high microwave heating rate, while increased the soil CEC and pH considerably (Tables 6.1, 5.1 and 4.1). As a result, adsorption of heavy metals was improved either through the physical adsorption, forming stable phosphate compounds and through ion-exchange between K and heavy metals. Strong negative correlations were found between CEC and extracted amounts of Pb, Ni, and Co, with r= -0.81 to -0.92, and with heavy metals uptake, with r= -0.85 to -0.86 at P values < 0.0001.  Potassium uptake by wheat also showed good negative correlations with the total uptake amounts of Pb, Ni, and Co (r= -0.76 to -0.83). Those strong correlations confirm the importance of CEC in reducing bioavailability and bioaccessibility of heavy metals through ion-exchang in a contaminated soil. Similarly, it was found that ions retention capacity of clinoptilolite increased sharply after increasing alkali metal cations through chemical modifications,78 which improved its sorption affinity due to a remarkable increase in CEC of clinoptilolite.76 Phosphorus plays a major role in improving plant growth and crop yield, as confirmed by the samples with 10KP/10Clino biochar which showed a higher shoot length than the samples with 10Clino or 30Clino biochars (Fig. 6.3b). These findings agree with Brown et al., 2005,73 who found that the addition of phosphates compounds to soil resulted in decreased heavy metal concentration in plant tissue and increased plant growth.      141     Figure 6.4 Elements concentrations in wheat shoots for soil, soil spiked with heavy metals (S+HM) and heavy metal spiked soil incorporated with different biochars at 1 and 2 wt.% loads. Error bars are the standard deviation of three replicates.      142  Strong positive linear correlations were found between the extracted amounts of heavy metals and the plant uptake of Pb, Ni, and Co with r = 0.89, 0.90 and 0.85, respectively, at P values < 0.0001. This suggests that Mehlich-3 extraction method may provide a good indication of the amounts of heavy metals that will be up-taken by plants. However, this suggestion should not be generalized for all soil types with different conditions and may only apply for the similar experimental conditions (i.e., sandy soil spiked with heavy metals).  It is known that K+ uptake is necessary for cell elongation when K accumulates in plant cells.152 Potassium is the main solute in the cell vacuoles which causes cell extension and also keeps the pH in the cytoplasm stable.152 More than 1000 mg/kg of extractable K is considered excessive and thus may become the dominant soluble cation in soil and may affect the uptake of other plant nutrients.153 This means a suitable but not excessive amount of K+ available in the soil will increase shoot length. A moderate positive correlation was observed between K uptake and shoot length with r = 0.68 and P < 0.001. This may be caused by excessive amounts of extractable K in some samples which created an adverse effect on the uptake of other plant nutrients such as Ca, resulting in a significant decrease in Ca concentrations in wheat shoots compared to pure soil and Soil+HM. The moderate correlation between K uptake and shoot length may also be contributed from other factors such as soil pH and uptake competition between elements and heavy metals. Figure 6.5 shows the uptake of heavy metals by wheat plants for pure soil, Soil+HM, and Soil+HM incorporated with different types of biochars at 1 and 2 wt.% loads. It is clear that biochars affected the bioavailability of heavy metals compared to the pure soil, as reflected by a significant decrease in the uptake of heavy metals. 10Clino biochar showed the highest heavy metals uptake compared to other biochars, while the uptake amounts of heavy metals for 10Clino     143  biochar was significantly lower than the soil without biochar. The lowest uptake amounts of heavy metals were found for the biochars produced from catalysts mixtures (10KP/10Clino and 10KP/10Bento biochars) indicating the synergistic effects of catalysts combinations on reducing the bioavailability of heavy metals and improving the plant productivity of the contaminated soil with multiple heavy metals.  The accumulation of Pb may also affect the uptake of K and Ca because of the interrelations between Pb2+ and the concentrations of K+ and Ca2+.152 A positive correlation between Pb2+ and Ca2+ was observed in which Pb2+ increased the uptake of Ca2+ while reduced the uptake of K+ significantly.152 In this study, a weak negative correlation (r = -0.365 and P = 0.014) was observed between K and Ca uptakes when all samples were compared. This is probably because the biochar produced from clinoptilolite has a higher concentration of CaO than those produced from K3PO4 (Table 5.1), which resulted in increased Ca extractability and uptake (Table 6.1 and Fig. 6.4). When samples treated with biochar produced from K3PO4 are considered only, a moderate negative correlation was found between K uptake and Ca uptake with r = -0.66 and P = 0.003. This may prove the reciprocal relationship between Ca and K, as potassium has lower ionization energy than Ca and can be easily ionized to K+ which is the suitable form for plant uptake. This may reveal that high K concentration in soil may reduce Ca uptake by plant because K is more mobile and easier to be ionized than Ca. The reduction in Co bioavailability and uptake may result from ion-exchange with other soil metals, precipitation and the formation of insoluble or sparingly soluble complexes, as well as through physical adsorption due to the microporous structure of biochars. The adsorption of Co2+ on a number of metal oxide, hydroxide and (alumino) silicate surfaces has been studied and confirmed extensively using XAFS.154 The accumulation of Co was found to be in hydrous oxides     144  form with Fe and Mn in soils, and the capacity of the soil for Co sorption is highly correlated with Co content and soil surface area and slightly with Mn.85 Soil pH has a great influence on Co phytotoxicity and plant uptake, with high pH leading to a remarkable reduction in Co extractability and uptake as confirmed in this study by a strong negative correlation with soil pH.     Figure 6.5 Heavy metals (Pb, Ni and Co) concentrations in wheat shoots for soil with heavy metals (S+HM) and heavy metal spiked soil incorporated with different biochars at 1 and 2 wt.% loads. Error bars are the standard deviation of three replicates.  Some plants may stabilize contaminants through precipitating and accumulating toxic elements in their roots or through adsorption on their root surfaces. In another possible mechanism,     145  changing soil chemical properties around the root zone, such as pH and redox potential, will alter the chemical form of the toxic elements and likely reduce heavy metals uptake.70 This implies the importance of selecting the suitable plant for contaminated soil. In combination with the use of a proper remediate agent, soil restoration can be achieved with improved plant productivity.16,71 Engineered biochars from microwave catalytic pyrolysis can be used to reclaim and remediate contaminated sites by reducing heavy metals phytotoxicity and improving soil fertility and plant growth with reduced groundwater contamination. It is thus suggested that energy crops (e.g., switchgrass, corn, canola, etc.) could be cultivated in contaminated sites after applying efficient amendment and remediate agent to produce biomass, which can be pyrolyzed to produce bio-oil, syngas, and biochar through catalytic microwave pyrolysis. The produced biochars can be reapplied to the contaminated sites to further reduce heavy metals bioavailability and phytotoxicity, improve soil productivity and plant growth, and preserve groundwater from contamination. Such an integrated system will provide a sustainable approach to producing biofuel, reclaim contaminated soil and ensure food security. However, heavy metals bioaccumulation in in the energy crops and heavy metals concentration in the produced bio-oil should be monitored.  6.3 Summary Biochars produced from catalytic-microwave pyrolysis considerably increased CEC, pH and nutrients content of the loamy sandy soil. Improved soil physicochemical properties and reduced bioavailability and phytotoxicity of Pb, Co, and Ni. Biochar addition also resulted in reduced heavy metals uptake by the wheat plants and increased wheat germination percentage and shoot length. Strong correlations were observed between heavy metals bioavailability and uptake with soil pH, CEC and biochar micropore area. Engineered biochars not only reduced heavy metals     146  bioavailability and uptake but also improved soil fertility and increased plant growth. Adding two catalysts to biomass showed synergistic effects in reducing microwave heating requirement and improving biochar performance in reducing toxicity and uptake of heavy metals compared to biochars produced from single catalyst. However, the results from this study may not necessarily be extrapolated to all soil types and conditions.            147  Chapter 7: Conclusion 7.1 Overall Conclusion  The switchgrass sample mixed with 10wt.% K3PO4 + 10wt.% bentonite was heated from room temperature to 400ºC over 2.8 min, compared to 28.8 min in conventional electrical heating, and the produced biochar has a BET surface area of 76.3 m2/g, compared to 0.33 m2/g from conventional electrical heating. The addition of catalysts not only increased biomass heating rate but also improved bio-oil and biochar quality. As a result, biochar production cost is expected to be reduced through the selection of proper additives/catalysts in which those additives or catalysts would serve for multiple functions such as increasing microwave heating rate to reduce energy consumption, producing a high-grade bio-oil that can be used easily upgraded to drop-in biofuels, and remaining in the biochar product to serve as a source of nutrients  Synergistic effects were observed in the presence of a mixture of K3PO4 and clinoptilolite or bentonite, resulting in a higher-than-expected microwave absorption rate, and improved bio-oil and biochar quality, in comparison with single catalyst.  K3PO4, clinoptilolite and bentonite showed good catalytic activities in microwave-assisted pyrolysis, resulting in reduced yield, acidity, viscosity and water content of bio-oil product, and catalyst loading, and combination of different catalysts are controlling parameters on heating rate and product quality.  Clinoptilolite reduced bio-oil acidity and viscosity, and increased bio-oil yield with a maximum bio-oil yield obtained at 36.2 wt.% from the biomass sample mixed with 30 wt.% clinoptilolite among all tested catalysts; while K3PO4 reduced bio-oil acidity markedly but increased the solid yield significantly.     148   It is possible to create a highly nutrient-rich biochar for soil applications by choosing the right catalyst that should be a good microwave absorber to speed up microwave heating and would remain in biochar to serve as a slow-release fertilizer.  The final solid yields for 30Sand, 30Bento and 30Clino are comparable at all heating rates in a thermogravimetric analyzer (TGA). Because of the enhanced heat transfer rate, the average final solid yield for 30Sand is about 12 wt.%, lower than ~17 wt.% for SG. The solid yields for 30Bento and 30Clino are ~42% lower than 30Sand, which is expected due to the catalytic effect of the catalysts assisting the in-situ cracking of pyrolysis vapours released from biomass and preventing the vapour condensation and repolymerization to form coke. The results emphases the importance of using inert sand as a heating medium in the tests to contrast the catalytic effect of bentonite and clinoptilolite, because one cannot observe the catalytic effect from comparing the TGA curves of switchgrass and switchgrass + catalyst samples at a given heating rate, because of the different heat transfer characteristics of the samples.   Using three-parallel reaction mechanism, it is shown that sand increased the conversion of pseudo-hemicellulose only. Bentonite and clinoptilolite significantly increased the conversion of all three biomass pseudo-components, and the major difference was on the pseudo-lignin with 43 and 36% increase in the conversion compared to SG and sand, respectively. Clinoptilolite produced bio-oil rich in phenolic compounds, with alkylated phenols increased by 49%, but acids compounds were reduced by 27%, compared to SG under microwave-assisted pyrolysis. This is likely due to the catalytic effect of the catalysts on the in-situ cracking of pyrolysis vapours released from lignin decomposition at temperatures > 350ºC, preventing the vapour condensation and repolymerization to form     149  coke, which is confirmed by the measured two-stage biomass pyrolysis and vapour-cracking kinetics.  The lowest activation energy of pseudo-lignin was found for 30Bento which was 29 and 25% lower than SG and 30Sand, respectively. The results obtained from the two-step kinetic model confirmed the speculation based on one-step 3-components kinetics that catalysts are mostly responsible for promoting the catalytic cracking of vapours released from lignin decomposition at temperatures higher than cellulose and hemicellulose decomposition temperatures.    The highest microwave heating rates for torrefaction (110-260°C) and pyrolysis stages (260-400°C) were found for 10KP/10Bento. It was also found that the catalyst mixtures affected the type of coke deposited onto catalyst surfaces which play an important role on determining the microwave heating rate under microwave catalytic pyrolysis by affecting the dielectric properties of the sample and then microwave absorption. Coke deposit on catalyst surfaces has a greater effect on determining the microwave heating rate and the heating behaviour of the sample than the pyrolysis reactions type (endothermic or exothermic). Oxygenated coke on catalyst surfaces has a low dielectric constant and thus a microwave heating rate, while graphitic coke deposit increases the microwave heating rate considerably.   Catalytic microwave-assisted pyrolysis could be a potential approach for tailoring biochar quality to improve soil physiochemical properties. High microwave absorption, high water and nutrient affinity and desirable plant nutrients are the key features of an effective catalyst for microwave pyrolysis for making high quality biochars.     150   Biochars produced from mixing switchgrass with two or more catalysts showed superior performance in increasing soil water holding capacity (WHC), cation exchange capacity (CEC) and improving soil fertility compared to the biochar produced from one catalyst only. This is probably due to increased biochar microporosity resulting from increased microwave heating rate and improved biochar chemical properties.   Biochar produced from microwave pyrolysis was more effective than biochar produced from conventional pyrolysis in increasing soil water holding capacity, while high quality biochar for soil applications can be produced at a low temperature (i.e., 300ºC) from catalytic microwave pyrolysis.  Engineered biochars produced from microwave catalytic pyrolysis not only reduced heavy metals bioavailability and uptake but also improved soil fertility and increased plant growth. Adding two catalysts to biomass showed synergistic effects in reducing microwave heating requirement and improving biochar performance in reducing toxicity and uptake of heavy metals compared to biochars produced from single catalyst. Strong negative correlations were also observed between biochar micropore surface area, CEC and the extracted amounts of heavy metals.  Using two catalysts showed a great improvement on wheat shoot length and germination percentage compared to the samples treated with biochars produced from using single catalyst. Engineered biochars from microwave catalytic pyrolysis can be used to reclaim and remediate contaminated sites by reducing heavy metals phytotoxicity and improving soil fertility and plant growth with reduced groundwater contamination.  To obtain synergistic effects under microwave catalytic pyrolysis using catalysts mixtures that will result in a higher-than-expected microwave absorption rate, and producing high     151  quality bio-oil and biochars, at least each catalyst from the catalysts mixtures should possess two or more from the following criteria: high microwave absorption, high water and heavy metals affinity, desirable plant nutrients and high catalytic performance to producing a high grade bio-oil that can be used as a fuel, these are the four key features of an effective catalyst for microwave-assisted biomass pyrolysis for making high quality bio-oil and biochars and increasing the microwave absorption rate.  Finally, catalyst combinations could improve the catalytic performance and reduce the catalyst loads which in turn will reduce the catalytic pyrolysis process cost compared to single catalyst, particularly under microwave catalytic pyrolysis. 7.2 Recommendations for Future Research  The biochars produced from mixing K3PO4 and clinoptilolite or bentonite with biomass showed synergistic effects in reducing microwave heating requirement and improving biochar performance in reducing toxicity and uptake of heavy metals compared to biochars produced from single catalyst. Future research is required to understand the synergistic mechanism taking place in the microwave pyrolysis reactor and the immobilization mechanism of these biochars for the heavy metals which in turn will further help to design biochars with optimum performance for remediation and amendment of contaminated soil.     Biochars produced from this study contain significant amounts of potassium, phosphorus and other essential plant nutrients that make them a potential amendment and remediate agent for soil. However, it should be noted that the test on heavy metal immobilization using the engineered biochars was performed in a lab scale. To confirm these findings at large scale, those biochars need to be tested in a field scale with cultivated plants for a long     152  period of time, in which they would be more exposed to biotic and abiotic effects. The interaction between biochar and soil might also impact on biochar performance.   The findings from this study confirmed the synergistic effects of a mixture of K3PO4 and clinoptilolite or bentonite, resulting in a higher-than-expected microwave absorption rate in comparison with single catalyst. However, the study was conducted under lab scale using a fixed bed microwave reactor and thus the catalysts need to be further tested under microwave reactor in a larger scale with different operating conditions in order to confirm the effects of the catalysts mixtures on increasing microwave heating rate.  The effect of different operating conditions on microwave heating rate, bio-oil and biochar properties under microwave catalytic pyrolysis need to be further investigated such as, using different operating temperatures, microwave power and different flow rate of the purging gas. It is expected that high flow rate of the purging gas “but not too high” will reduce the oxygenated coke deposition on catalyst surfaces and that could increase the microwave heating rate which in turn could improve the quality of the produced bio-oil and biochar.  Combinations of K3PO4 and clinoptilolite or bentonite can be mixed at lower loads < 20wt.% to investigate the synergistic effects of catalysts mixtures on microwave heating, bio-oil and biochar properties using lower loads and this will reduce the cost of using high catalysts loads. In addition, more low cost and natural additives with high microwave absorption ability can be tested to investigate their catalytic effect on microwave heating and the quality of bio-oil and biochar. As each catalyst of K3PO4 and clinoptilolite or bentonite has different microwave absorption ability, different catalytic behaviours and different sorption capacity, thus it is expected mixing the three catalysts at lower loads will     153  result in synergistic effects that could result in further increase in the microwave heating rate and improved quality of the produced bio-oil and biochar.     The cost of the selected catalysts is relatively inexpensive, which is about a few hundred dollars per ton compared to synthetic zeolites and other commercial catalysts. However, the cost can be further reduced by recycling the spent catalysts as the spent catalysts from the catalysts mixtures showed high microwave absorption ability compared to the fresh catalysts mixtures because of the deposited graphitic coke type. It is expected that the spent catalysts produced from catalysts mixtures will have higher microwave absorption ability. Thus, the spent catalysts from the catalysts mixtures can be recycled multiple times and that will reduce the overall cost of producing bio-oil and biochar, and the cost of the catalysts by maximizing the benefits from using the catalysts multiple times compared to single use of the catalysts. However, the catalytic activity and reactivity, and microwave absorption ability of the catalysts should be monitored.  As nitrogen is the one of the most important nutrients for plants (i.e., N P K) and thus feedstock that rich in nitrogen such as chicken, cattle manures or other manures from poultry can be further investigated and that will produce biochars rich in most of the macro and micro nutrients which are required for plants. However, care should be taken as nitrogen will be evaporated at lower temperatures during pyrolysis as nitrogen oxides.      154  References (1)  Bridgwater, A. V. Review of Fast Pyrolysis of Biomass and Product Upgrading. Biomass and Bioenergy 2012, 38, 68–94. 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Rev. 2001, 43 (11), 963–1073.      173  Appendices Appendix A   Biochar pore size distribution  A.1 BJH Adsorption Pore Distribution for the biochar produced from 30wt.% clinoptilolite  Pore Diameter Range (nm) Average Diameter (nm) Incremental Pore Volume (cm³/g) Cumulative Pore Volume (cm³/g) Incremental Pore Area (m²/g) Cumulative Pore Area (m²/g) 2.7 -2.4 2.5 0.002138 0.002138 3.365 3.365 2.4 – 2.1 2.2 0.002365 0.004502 4.242 7.607 2.1 -1.9  2.0 0.002255 0.006757 4.577 12.184  A.2 BJH Adsorption Pore Distribution for the biochar produced from 10wt.% K3PO4 + 10wt.% clinoptilolite  Pore Diameter Range (nm) Average Diameter (nm) Incremental Pore Volume (cm³/g) Cumulative Pore Volume (cm³/g) Incremental Pore Area (m²/g) Cumulative Pore Area (m²/g) 2.7 - 2.5 2.6 0.000774 0.000774 1.200 1.200 2.5 - 2.2 2.3 0.001010 0.001783 1.736 2.936 2.2 - 2.0 2.1 0.001234 0.003017 2.365 5.301 2.0 - 1.7 1.8 0.001803 0.004821 3.908 9.209       174  A.3 Isotherm linear plot for 30Clino biochar.  A.4 Isotherm linear plot for 20Clino biochar.      175  A.5 Isotherm linear plot for 10KP/10Clino biochar.               176  Appendix B  Chemical composition of bio-oil produced from SG and SG mixed with 30wt.% K3PO4 using GC-MS  Table B.1 shows the lumped products composition of bio-oil produced from SG and SG mixed with 30wt.% K3PO4 (30KP) under microwave-assisted pyrolysis. Acetic acid was found to be the major acid product from deacetylation of hemicellulose.43 The primary oxygenated compounds derived from hemicellulose such as acetic acid, acetaldehyde and acetyl are main precursors for catalytic coke formation through decarboxylation and dehydration.23 There is also 3 folds increase in the phenolic compounds compared to SG. These findings reveal that potassium has a great inhibitory effect for the primary oxygenated compounds originated from hemicellulose, which was also confirmed by.43         B.1 The lumped product composition of bio-oil (peak area %) produced from SG and SG mixed with 30wt.% K3PO4 (30KP) under microwave-assisted pyrolysis using GC-MS.  Acids Phenolics Furans Ketones  Aldehydes  Anhydrosugars SG 24.6 11.0 4.81 16.5 1.23 0.63 30KP 5.95 45.4 1.15 7.12 0 0 Percentage change -76% +312% -76% -57% -100% -100%  Table B2 shows the chemical composition of total bio-oil (organic and aqueous phases) produced from SG and SG mixed with 30wt.% K3PO4 under microwave-assisted pyrolysis. It is seen that K3PO4 significantly altered the chemical composition of bio-oil and, as expected, acids, ketones, aldehydes, furans contents were markedly reduced or completely eliminated, while phenolics and other aromatics compounds dramatically increased, compared to SG. In addition, some hemicellulose derivatives such as 2-Furaldehyde, aldehydes and anhydrosugars were completely eliminated.      177  B.2 Chemical composition of total bio-oil (organic and aqueous phases) produced from SG and SG mixed with 30wt.% K3PO4 (30KP) under microwave-assisted pyrolysis using GC-MS. Switchgrass Switchgrass + 30wt.% K3PO4 (30KP)  Retention time Area percentage Compound name Retention time Area percentage Compound name  7.3999 5.9391 2-Propanone 7.4061 0.497 2-Propanone  8.0414 0.8682 2-Cyclopenten-1-one 8.1223 0.6436 2-Cyclopenten-1-one 8.1099 0.8813 Cyclopenten-3-one 8.2157 0.4996 1-Hydroxy-2-butanone 8.1473 1.2209 1-Hydroxy-2-butanone 8.2406 0.3925 Methyl-2-cyclopentenone 8.2033 1.7803 1 - hydroxy - 2 - butanone 8.8571 4.873 Acetic acid    9.9407 1.0809 Propanoic acid 8.8136 21.9542 Acetic acid 10.1151 1.1322 3-Methyl-2-cyclopentenone 9.2059 3.0913 Furfural 10.3455 0.7821 2-Cyclopenten-1-one, 2,3-dimethyl- 9.3367 0.5907 Propanal 11.0056 0.9357 1,2-Ethanediol 9.9221 2.3104 Propanoic acid 11.5287 0.6435 Furfuryl alcohol 10.6196 0.5158 5-Methyl-2-furfural 11.6284 0.5075 2(3H)-Furanone, dihydro- 10.9933 0.2783 1,2-Ethanediol 13.3472 1.3969 Azulene 11.1676 0.3448 Butanoic acid 13.6648 1.1085 3,5-Dimethyl cyclopentenolone 11.5164 0.6781 Furfuryl alcohol 14.2876 0.9398 2-Cyclopenten-1-one, 2-hydroxy-3-methyl- 11.616 0.522 2(3H)-Furanone, dihydro- 14.8667 3.3454 Phenol, 2-methoxy- 14.2752 1.1449 2-Cyclopenten-1-one, 2-hydroxy-3-methyl- 15.1407 0.5274 Naphthalene, 2-methyl- 14.8606 1.5445 Phenol, 2-methoxy- 15.6078 0.489 2,6-Dimethylphenol 16.486 0.5791 Creosol 15.7635 0.4139 Methylnaphthalene 17.1399 0.447 2-Cresol 16.4983 1.2266 p-Cresol 17.2333 1.6525 Phenol 17.1398 2.6468 o-Cresol     178  Switchgrass Switchgrass + 30wt.% K3PO4 (30KP)  Retention time Area percentage Compound name Retention time Area percentage Compound name  17.7689 0.3999 Phenol, 4-ethyl-2-methoxy- 17.2394 6.9458 Phenol 18.5847 0.4987 p-Cresol 17.7812 1.5856 Benzeneethanol, 2-methoxy-  18.7279 0.4137 m-Cresol 18.3106 0.5151 Phenol, 2-ethyl-  20.2474 0.7164 4-Ethylphenol 18.4725 0.9008 2,4-Dimethylphenol  20.7394 1.2891 p-Vinylguaiacol 18.5285 1.139 2,4-Dimethylphenol  21.9102 0.9598 Syringol 18.5846 2.6686 Benzenemethanol  24.1272 1.3908 4-vinylphenol 18.734 3.0444 p-Hydroxytoluene  24.3639 0.6271 1,4:3,6-Dianhydro-.alpha.-d-glucopyranose 19.7616 0.4538 Phenol, 2,4-dimethyl-  26.0142 0.6353 2-Furaldehyde 20.0232 0.6332 Phenol, 4-(1-methylethyl)-  27.297 1.1169 Vanillin 20.2536 4.973 4-Ethylphenol     20.3906 0.7065 1-Hydroxy-3-ethylbenzene     20.7518 1.2885 p-Vinylguaiacol     21.1192 0.8928 Phenol, 2-ethyl-     21.2313 0.8672 Biphenylene     21.7669 0.5506 Phenol, 3-ethyl-5-methyl-     21.9163 2.1449 Phenol, 2,6-dimethoxy-     23.411 0.5763 4 - methyl - syringol     23.4608 0.7476 Phenol, 2-methoxy-4-(1-propenyl)-, (E)-     23.7597 0.4675 9H-Fluorene     24.1334 3.2627 4-vinylphenol     24.3949 0.9871 Benzene, 1,2,5-trimethoxy-3-methyl-     

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