Experimental study and equilibrium analysis on thermal reduction of CO2 by CFB gasification

Abstract

CO2 can be involved in the CFB gasification process as one of the gasifying agents and be reduced to CO. In this paper, the feasibility of using CFB gasification process to reduce CO2 was investigated by using a bench-scale CFB gasifier. The effect of gasifying agent parameters including the CO2 substitution for N2, the O2 concentration and O2/C were studied. The result of experiments was compared with the chemical equilibrium calculations. The experimental result shows that the reduction of CO2 can be realized in CFB gasification, and the yield of CO in O2/CO2 atmosphere is higher than that that in air atmosphere. When the O2 concentration is 30%, the CO2 conversion reached the maximum of 0.017 Nm3(CO2)/kg(coal). However, the conversion of CO2 is limited by temperature and char reactivity at higher CO2 concentration. The gradual increase in CO2 yield with increasing O2/C suggests that at approximate temperature, the increase in O2/C enhances the oxidation of C and CO. When O2/C is greater than 0.45, the gasification results reach equilibrium. Under other conditions, the gasification results differ from the chemical equilibrium significantly. This is due to the change in the dominant reaction of the gasification process from the combustion reactions to the Boudouard reaction.