Advancing biomass pyrolysis by torrefaction pretreatment: Linking the productions of bio-oil and oxygenated chemicals to torrefaction severity

Torrefaction is an efficient pretreatment method to improve the quality of biomass feedstock and resulting bio-oil. Herein, the relationships of torrefaction severity of sweet sorghum bagasse (SSB) with its physicochemical properties and resulting pyrolytic product distributions were systemically investigated. The SSB was first pretreated by torrefaction at different temperatures (230, 260, and 290 °C) with a constant residence time of 30 min, and the resulting torrefied SSB was subsequently pyrolyzed at 500 °C for the production of bio-oil. The carbon enrichment index (CEI) is used as an indicator to quantitatively reflect the torrefaction severity of SSB. The results show that the extents of decarbonization, dehydrogenation, and deoxygenation reactions of SSB increase linearly with the increase of CEI, and the extent of deoxygenation reaction is the most severe among them. Torrefaction can effectively regulate the pyrolytic production distributions of SSB, and severe torrefaction favors the formation of biochar. The productions of bio-oil and major oxygenated chemicals (e.g., ketones, carboxylic acids, and phenols) in bio-oil as the functions of CEI could be fitted well by quadratic polynomial functions. The findings can guide the rational design and optimization of the torrefaction process for improved biomass pyrolysis.