Biomass\nTorrefaction for Renewable Energy: From Physicochemical,\nBulk Properties, and Flowability to Future Perspectives and Applications

Biomass and biomass wastes can be a source of renewable\nenergy\nand fuels through valorization in thermochemical processes. Torrefaction\nis a thermal pretreatment often employed for upgrading raw biomass.\nIn addition to providing the status of current techniques used to\ncharacterize raw and torrefied biomasses (in terms of their flowability\nand physicochemical, thermal, and bulk properties), we discuss current\napplications with these bulk solids. The limitations of current characterization\nmethods are also discussed with a view to future scopes with advanced\ntechniques, particularly related to physicochemical properties. This\nReview underscores a lack of systematic studies focused on the importance\nof comprehensive knowledge of raw and torrefied biomass properties\nto achieve better flowability, thereby contributing to more efficient\nand cost-effective industrial processes. Indeed, among the 1320 literature\npapers evaluated in this review, 647 characterized the chemical/thermal\nproperties of raw and torrefied biomasses, while 254 considered physical/bulk\nproperties, and only 11 assessed bulk solids’ flowability.\nThe scarcity of studies on flowability suggests that this parameter\nhas not been considered important by most researchers for the demonstration\nof process feasibility. However, characterizing bulk solids flow behavior\nis critical for the proper design of handling equipment and ensuring\nsmooth plant operation, thus minimizing risks associated with unforeseen\nexpenses and prolonged time for process troubleshooting and equipment\nretrofit. Moreover, even when the flowability was measured in the\n11 papers, it was difficult to compare results between studies as\nmeasurement techniques were not the same, nor were the biomass type\nor torrefaction conditions. This highlights the need for future research\non the flowability of raw and torrefied biomass, intending to obtain\nmore sound and broad conclusions for the flow behavior of such heterogeneous\nmaterials, culminating in the development of standardized protocols\nto improve biomass handling and processing at an industrial scale.\nBesides, most of the studies available in the literature were based\non small-bench torrefaction units, producing only a few grams of torrefied\nbiomass. This is generally a limiting amount of material for complete\nassessment of flowability at different bulk conditions as well as\nto evaluate other important handling aspects at industrial scale,\nsuch as bulk solid segregation, quality of fluidization, and so on.\nIn future works, we also suggest evaluating other heterogeneous feedstocks,\nsuch as municipal wastes or refuse-derived fuels, and performing a\ncomplete characterization for the bulk solids to facilitate technical\ndecision-making in bioenergy and biofuels processes.