Trace element migration and structure evolution are related to biomass gasification performance, which varies to a certain extent during torrefaction. This substantial summary of the integrated process based on biomass classification has practical significance for the consumption and commercialization of a bio-refinery. Herein, biomass is divided into two categories and an understanding of the relevant variations is presented during the integrated two processes according to the classification. The effects of torrefaction on the organic (hemicellulose, cellulose, and lignin) and inorganic components (trace elements), and the physicochemical structural evolution (morphology, pore structure, functional groups) of biomass are reviewed. The differences (syngas generation and tar formation) between raw and torrefied biomass gasification are compared. Meanwhile, the effects of torrefaction on gasification performance and the possible gasification mechanisms are reviewed regarding pore structure, carbon structure, surface area, and alkali and alkaline earth metals (AAEMs). The gasification of the torrefied biomass materials on a pilot-scale is outlined. Finally, future directions and technological challenges associated with the integrated technologies are proposed. More scaling-up experiments should be conducted to investigate the mass and heat transfer in reactors during amplification and to improve the reactor's adaptability to biomass species in the contexts of economy and society.
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