CO2–steam mixture for direct and indirect gasification of rice straw in a downdraft gasifier: Laboratory-scale experiments and performance prediction

This paper explored the feasibility of CO2 as alternative gasifying agent of steam to obtain higher thermal efficiency in biomass gasification. The effect of the gasifying agent on gas evolution and thermal efficiency was examined by using a lab scale downdraft gasifier. The results were also compared with the output of pyrolysis with N2. The reaction atmosphere in indirect and direct gasification was simulated by carrying out the experiments without and with O2 presence. The reaction conditions were varied in the temperature range from 750 °C to 950 °C by changing the CO2 molar fraction in gasifying agent to the ratio of 0, 30 and 60 vol.% in balance with steam. 40 vol.% of N2 fraction was kept during O2 free experiments, while 31.7 vol.% of N2 and 8.3 vol.% O2 were kept during the experiments with the presence of O2. At all examined condition, gasification yielded more combustible gas than pyrolysis. Furthermore, high CO2 fraction in gasifying agent generally resulted in low H2 yield and high CO yield. These substitutions mostly lowered the energy yield of the producer gas, but in the other hand also reduced the preheating energy of gasifying agent. Thus, optimizations of these changes were investigated for gaining highest thermal efficiency of the gasification process. Highest thermal efficiency of the process without O2 was 52% under N2 (40 vol.%)–CO2 (60 vol.%) atmosphere at the temperature of 850 °C. For the process with O2, where the part of gasifying agent preheating energy supplied by the biomass partial combustion, the highest thermal efficiency was 60% under the CO2 (60 vol.%)–O2 (8.3 vol.%)–N2 (31.7 vol.%) atmosphere at the temperature of 950 °C. CO2 utilization as a gasifying agent potentially increases the thermal efficiency of biomass conversion process compared with pyrolysis or conventional indirect and direct gasification, i.e. pure-steam gasification and O2–steam gasification.