Physicochemical properties of torrefied biomass at different atmosphere: A review

Biomass torrefaction, a mild thermochemical pretreatment, is crucial for upgrading raw biomass into a more energy-dense, hydrophobic, and grindable solid fuel (biochar). However, the process conditions, particularly the reaction atmosphere, critically govern the structural transformations and ultimate properties of the torrefied products, directly impacting their suitability for downstream energy conversion processes. This review synthesizes the research advancements on the physicochemical structural evolution of biomass during torrefaction, with a primary focus on the decisive influence of the reaction atmosphere on product performance and structure. It examines how diverse torrefaction atmospheres drive alterations in physical structure, including particle morphology, pore development, density, hydrophobicity, and grindability. Concurrently, it explores atmosphere-specific modifications to the chemical structure, encompassing the degradation pathways of major components (cellulose, hemicellulose, lignin), transformations in functional groups, and shifts in elemental composition (C, H, O). A critical comparative analysis evaluates the distinct impacts of oxidative atmospheres (CO₂, O₂, air, flue gas, steam) versus nonoxidative atmospheres (NH₃, N₂, vacuum) on the yield distribution of solid, liquid, and gaseous products. By assessing existing technologies, this review elucidates the potential and challenges of torrefaction, providing insights to enhance biomass energy utilization efficiency and foster the innovation and industrialization of atmosphere-controlled biomass upgrading technologies. • Torrefaction at different atmosphere was reviewed. • N₂/CO₂/air/NH₃/flue gas enhances biomass carbon, hydrophobicity, energy density. • Optimized temp/time improve fuel grindability, calorific value. • Chemical structure changes were summarized based on three components. • Oxygen-containing atmospheres are more suitable for low-temperature torrefaction.