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

Highlights: • Autothermal gasification with 15% CO₂ addition was conducted in an 8-kW reactor. • Biochar obtained under CO₂ condition had a moderate pH. • CO₂ as gasification oxidant could produce comparable energy to air gasification. • Carbon conversion and cold gas efficiency were enhanced with CO₂ addition. • Under the same equivalence ratio, less PM was emitted during 15% CO₂ gasification. -- Abstract: 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.