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Biomass gasification in a circulating fluidized bed Li, Xuantian


This work is devoted to the experimental study of biomass gasification in a pilot-scale circulating fluidized bed, and development of an equilibrium model of the process based on Gibbs free-energy minimization. Biomass gasification has considerable potential for reducing greenhouse gas emissions. In the present study, six types of sawdust were gasified in a pilotscale air-blown circulating fluidized bed gasifier to produce low-calorific-value gases. The pilot gasifier employs a riser 6.5 m high and 0.1 m in diameter, a high-temperature cyclone for solids recycle and a ceramic fibre filter unit for gas cleaning. The riser temperature was maintained at 970-1120 K (700-850°C), while the sawdust feed rate varied from 16-45 kg/h, corresponding to a superficial gas velocity of 4-10 m/s. It was found that gas composition and heating value depended heavily on the air or O/C ratio, and to a lesser extent on operating temperature. The higher heating value of the product gas decreased from 5.6 to 2.1 MJ/Nm³ as the stoichiometric air ratio increased from 0.22 to 0.54. The gas heating value was increased by increasing the overall suspension density in the riser. Fly ash re-injection and steam injection led to increases in gas heating value for the same Q/C molar ratio. Tar yield from biomass gasification was found to decrease drastically from 15 to 0.54 g/Nm³ as the average suspension temperature increased from 970 to 1090 K. Elevating the operating temperature provides the simplest solution for tar removal in the absence of any catalyst. Secondary air had only a very limited effect on tar removal with the total air ratio maintained constant. A nickel-based, catalyst proved to be effective in reducing the tar yield and in adjusting the gas composition. The cold gas efficiency decreased with increasing air ratio (or O/C molar ratio), though the carbon conversion increased. The cold gas efficiency provides a better criterion for evaluating the gasification process than the carbon conversion. Experimental data showed that the gasification efficiency can be maximized within an optimum range of air ratio (a = 0.30-0.35, or O/C = 1.5-1.7), while keeping the tar yield acceptably low. A non-stoichiometric equilibrium model based on Gibbs free energy minimization was developed for biomass gasification. Five elements (C, H, O, N and S) and 44 species were considered in the model. Both pure equilibrium and situations where kinetic factors cause a partial approach to equilibrium are considered. The equilibrium model predicts that the product gas composition from gasification of woody biomass (e.g. sawdust) depends primarily on the air ratio. An air ratio of 0.2-0.3 is predicted to be most favourable for producing CO-rich gas, while temperatures of 1200-1400 K and an air ratio of 0.15-0.25 are predicted to be optimum for H₂ production. The predicted cold gas efficiency reached a maximum at an air ratio of about 0.25. The model successfully predicts the onset of carbon formation in a C-H-O-dominated system when the relative abundance of carbon exceeds a certain level. When a system is C-saturated, the gas composition is insensitive to the elemental abundance of carbon in the total feed streams. The equilibrium model successfully predicts the limiting behaviour of the system with changes in different operating parameters and provides an in-depth understanding of the underlying thermodynamic principles governing biomass gasification. The model was modified to take non-equilibrium factors into account. The modified model successfully predicts product gas compositions, heating value, gas yield and cold gas efficiency in good qualitative agreement with the experimental data.

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