Progress in torrefaction pretreatment for biomass gasification

Torrefaction is an effective method to improve the quality of biomass feedstocks for a subsequent thermal conversion process. Torrefaction has been widely developed for biomass upgrading in terms of increasing bulk and energy densities, reducing water and oxygen contents, and improving hydrophobic properties, which are favorable for transportation and storage. Moreover, the torrefied biomass can be easily ground to even particle sizes, which favor the downstream thermochemical conversion process such as gasification. This paper reports on the progress in the dry and wet torrefaction of biomass sources. Key findings and challenges of torrefied biomass gasification are highlighted as well. In general, the properties of biomass feedstocks can be improved by the torrefaction process. Torrefaction for biomass gasification can improve the quality of syngas. In particular, gasification of torrefied biomass involves a merit of volatile matter (tar precursor) removal prior to the gasification process. The properties of torrefied biomass are significantly affected by the torrefaction conditions mainly including atmosphere, temperature, and pressure. The presence of an appropriate amount of oxygen gas leads to a superior performance for biomass torrefaction. The oxygen-containing gas and the waste heat from industrial flue gas can be utilized to reduce the operating cost, thus making the biomass torrefaction pretreatment more commercially feasible.