CO2-O2 gasification of a bituminous coal in circulating fluidized bed

•The gasification experiments were conducted in a bench-scale self-heated circulating fluidized bed.•The effect of the replacement of N2 by CO2 on gasification performance was investigated with O2 volume percentage of 21%.•The Boudouard reaction and gasification performance are enhanced when N2 is replaced by CO2.•The Boudouard reaction is further strengthened by the increase in O2/C molar ratio.•Thermodynamic equilibrium calculation is coherent with the experimental results.

CO2 as a gasifying agent can fully or partly replace N2 for coal gasification. In this study, the effect of the replacement of N2 by CO2 and the effect of O2/C molar ratio on gasification performance were investigated via experiments as well as thermodynamic equilibrium calculations. Experiments were performed in a bench-scale self-heated circulating fluidized bed (CFB) gasifier with O2 volume percentage of 21%. The results showed the Boudouard reaction was strongly enhanced when N2 in air is replaced by CO2. At the O2/C molar ratio of 0.39, the replacement of N2 by CO2 induced an increase in concentration of CO + H2, yield of CO + H2, calorific value of product gas, carbon conversion and cold gas efficiency, which is mainly due to the enhancement of Boudouard reaction. Though the elevated O2/C molar ratio from 0.39 to 0.64 accelerated the combustion proportion, the yield of H2 + CO was improved in CO2-O2 gasification. Meanwhile the cold gas efficiency increased from 43.8% to 58.6% in CO2-O2 gasification, and showed greater improvement than that in air gasification. The enhancement of Boudouard reaction with the replacement of N2 by CO2 was further strengthened by the increase in O2/C molar ratio. The variation of gas composition in CO2-O2 gasification experiments was predicted by thermodynamic equilibrium calculations, and the increase in the yield of CO + H2 was verified when N2 in air was replaced by CO2 in a gasifying agent.