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