CO2 gasification of woody biomass: Experimental study from a lab-scale reactor to a small-scale autothermal gasifier

Rapid depletion of fossil fuel and CO2 mitigation are two major challenges in the modern society. CO2 gasification of carbon-neutral biomass could be an attractive technology to cope up these two emerging problems. Replacing conventional gasifying agent air with CO2 could reduce the fraction of undesired N2 in the gas products because CO2 behaves as inert gasifying agent under low temperature and could be converted to CO at high temperature range. However, most researches on CO2 gasification are limited at the lab-scale level due to the endothermic feature of Boudouard reaction. In this work, a feasibility study of small-scale autothermal gasification using 15% CO2 and 85% air was conducted and compared with traditional air gasification. Prior to that, a lab-scale study of gasification behaviors, under N2, air and CO2 agents, were performed. It was found that using CO2 as gasification oxidant could produce comparable energy (6.67 kJ/g feedstock) to air gasification (7.45 kJ/g feedstock) at equivalent condition (800 °C, 40 min). The pH of biochar obtained under CO2 condition at 800 °C was measured to be 10.63 while the pH of biochar derived by air gasification at the same temperature could reach 12.32. In addition, a small-scale gasification experiment with 15% CO2 addition was successfully conducted in a downdraft autothermal reactor. The results showed CO production was greatly enhanced while CH4 generation was suppressed owing to Boudouard reaction and CO2 dry reforming. Cold gas efficiency and carbon conversion efficiency were both enhanced by 5.8% and 6%, respectively, with CO2 addition. Meanwhile, particulate matters (PM) emitted from both air gasification and 15% CO2 gasification experiments were measured. It was found that under the same equivalence ratio (ER), 75.4% less particle number concentration was emitted during 15% CO2 gasification compared to air gasification.