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/Nm 3 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/Nin 3 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. Ill 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 H 2 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 abundan...

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In an investigation of in situ CO2 removal for fluidized-bed combustion processes, seven calcium-based sorbents were tested for simultaneous CO2 and SO2 removal using both an atmospheric thermogravimetric reactor and a pressurized thermogravimetric analyzer. SO2 was found to impede cyclic CO2 capture because of pore blockage by sulfate products, resulting primarily from direct sulfation during the later stage of each cycle. The sorbents showed similar patterns during cocapture. Loss in sorbent reversibility could not be prevented but was improved by higher CO2 partial pressures.

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Simultaneous CO 2 /SO 2 capture characteristics of three limestones were investigated in a pilot scale fluidizedbed reactor. For each of these sorbents, the measured CO 2 capture capacity decreased as the number of cycles increased and as the SO 2 concentration increased. On the other hand, the SO 2 capture increased with the number of cycles and the SO 2 concentration. The total calcium utilization decreased as the number of cycles increased, but the effect of SO 2 concentration on the total calcium utilization depended on the sulfation pattern of limestone. For one limestone (with unreacted-core-type sulfation), the total calcium utilization decreased with increasing SO 2 concentration. However, for the other two limestones (with uniform-type sulfation), the total calcium utilization was almost independent of SO 2 concentration for the range investigated. The results show that SO 2 reduces the CO 2 capture capacity of limestone and indicate that the sulfation patterns affect the CO 2 capture capacity.