Torrefaction integrated with steam gasification of agricultural biomass wastes for enhancing tar reduction and hydrogen-rich syngas production

Steam gasification is considered as a promising technology for conversion of various biomass wastes to valuable hydrogen (H₂)-rich gas products that can be applied for the sustainable production of green hydrogen and methanol. However, some inevitable problems such as high tar content and low cold gas efficiency greatly hinder its broad application. Torrefaction has been widely employed for upgrading low-rank biomass sources that favors the follow-up gasification process, resulting in low tar yield and high syngas yield. Torrefied biomass usually shows higher energy density, improved grindability characteristics, and lower O/C and H/C ratios. This research work studies the effect of torrefaction on steam gasification of corncob (CC) and rice husk (RH). The mechanisms of biomass torrefaction integrated with steam gasification are also given. Biomass torrefied at a relatively high temperature (280 °C) is more efficient to extract the oxygenated volatiles, reducing the generation of tar and particulate matters during the gasification process. The increase of torrefaction temperature resulted in an increase of H₂ yield and a decrease of CO yield, corresponding to an increase of H₂/CO ratio. Particularly, the H₂ yield in the CC-derived syngas increased from 6.38 mmol/g (raw) to 12.01 mmol/g (280 °C), and the H₂ yield in the RH-derived syngas increased from 4.33 mmol/g (raw) to 12.97 mmol/g (280 °C). Steam gasification of RH torrefied at 280 °C achieved a maximum H₂/CO ratio of 2.84. After torrefaction of CC and BB at 280 °C, the tar yield of steam gasification was below 1% [gasification temperature: 800 °C, mass ratio of steam to biomass (S/B): 1]. In general, the torrefaction pretreatment of biomass at relatively high temperatures (i.e., 280 °C) favors the steam gasification process under an appropriate S/B (i.e., 1) in terms of improving the syngas quality and reducing the tar production.