Biomass pretreatment for steam gasification toward H2-rich syngas production – An overview

Steam gasification of biomass is considered a promising technology for producing H2-rich syngas that can be utilized for the synthesis of liquid fuels and chemicals. Pretreatments prior to the gasification process are generally carried out to improve the physicochemical properties and structure characteristics of the raw biomass, which further contribute to increasing the bulk and energy density, enhancing the thermal conversion efficiency and improving the products quality. In particular, the tar can be eliminated as well as the product gas upgraded with the objective of mitigating the requirements of tar removal and syngas reforming during biomass gasification. The biomass pretreatment methods include mechanical pretreatment, biological pretreatment, chemical pretreatment, and thermal pretreatment. The mechanical pretreatment such as grinding and pelletization is almost required for the biomass conversion processes, since the raw biomass usually has a poor bulk density, irregular shape and high-water content. The biological process of anerobic digestion can modify the components and structure of biomass for enhancing steam gasification. The chemical pretreatments such as water or acid leaching process can remove the alkali and alkaline earth metallic species in biomass to mitigate the ash-related issues during high temperature gasification. Among these pretreatments, thermal pretreatments such as torrefaction and hydrothermal carbonization have attracted more concern for converting raw biomass into torrefied biomass and hydrochar for improving the efficiency and performance of the process. Co-gasification is normally carried out by feeding the blended feedstock or the co-feeding, so blending biomass with other feedstock can also be considered as an important pretreatment process. This review summarizes the pretreatment processes for enhancing the biomass steam gasification. Finally, several challenges have been proposed in biomass pretreatment to enhance the steam gasification for H2-rich syngas production.