Detailed gasification process of woody biomass-derived char with H2O and CO2 gasifying agents

Abstract

Most gasification processes including updraft/downdraft gasifiers and entrained flow gasifiers involve partial combustion, wherein the combustion heat (approximately 800–1100 °C) is used. In the reduction zone of gasifiers, biomass pyrolysis and biochar gasification proceed in the presence of the H2O vapor and heat, and CO2 in the combustion area. (Thurner et al., 1981; Di-Blasi et al., 2001; Matsumoto et al., 2009; Guo et al., 2014; Ram et al., 2019.) Because the pyrolysis rate is much higher than the gasification rate of biochar, the overall conversion rate is controlled by the gasification rate of char. That is, the overall gasification rate is limited by the slow gasification rate of char. (Zhang et al., 2008; Okumura et al., 2009; Seo et al., 2010.) Many studies have been performed on the gasification of biochar. The gasification rate of biochar varies greatly with the experimental device and the type of biomass, even when the same gasification temperature is used. In addition, pore development in the biochar during gasification with H2O and CO2 agents, especially the relation between gasification reaction rate and interface area, has been rarely studied. The present study compared in detail the gasification rate of biochar in H2O gasifying agent with that in CO2 gasifying agent at 1073–1273 K using the same experimental apparatus and the same char samples, under the same pyrolysis conditions. The results showed that the gasification rate constant of the biochar was approximately 3–10 times higher under H2O compared to the CO2 gasifying agent. The specific surface area of woody biochar during H2O gasification was compared with that observed during CO2 gasification. When the overall gasification rates were almost same, the increase in the specific surface area of the pores under the H2O atmosphere was similar to that processed under the CO2 atmosphere i.e., no significant differences in pore development or physical shape were observed between the CO2 and H2O gasifying agents. * Faculty of Engineering and Design, Kagawa University Hayashi-cho, Takamatsu,761-0396, Japan E-mail: okumura.yukihiko@kagawa-u.ac.jp