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The importance of size reduction to the commercial deployment of biomass utilization technologies cannot be overemphasized. Size reduction is necessary to create good flowing, easily digestible lignocellulosic biomass. However, inefficiencies and inconsistencies have not been well addressed for the process of size reduction. This has motivated quite a few research studies, some of which have reported the influence of process variables and pretreatment on the size reduction performance as well as the development of novel size reduction equipment and empirical mathematical models. In this Perspective, we present a systematic and critical evaluation of the state of lignocellulosic biomass size reduction research to provide insights for engineers and scientists for future development and investigation. We first briefly discuss the structural biology and failure mode of lignocellulosic biomass. Then, we present a comprehensive, data-driven picture of the interactions among lignocellulosic biomass attributes, size reduction process variables, pretreatment processes, and size reduction process performance (as defined by product characteristics and size reduction energy consumption). The sizing equipment tool wear issues are also summarized. Finally, we highlight some existing gaps and future research opportunities for lignocellulosic biomass size reduction to inform the development of newer technologies that could effectively produce high-value lignocellulosic biomass particles.

Feedstock variability is a significant barrier to the scale-up and commercialization of lignocellulosic biofuel technologies. Variability in feedstock characteristics and behavior creates numerous challenges to the biorefining industry by affecting continuous operation and biofuels yields. Currently, feedstock variability is understood and explained largely on the basis of chemical composition. Physical and mechanical properties and behavior of lignocellulosic feedstock in various unit operations, studied through advanced analytical methods, can further explain variability. Such studies will enable us in developing processes and designing equipment to improve operation and conversion performance. In this perspective, we review several advanced analytical methods that measure density, moisture content, thermal properties, flowability, grindability, rheology properties, and micromorphological characteristics. We also discuss the correlations and interactions among these properties that reflect the complexity of lignocellulosic biomass as a feedstock and the associated quality metrics and logistics of supplying consistent quality feedstock to a biorefinery. We also examine methods that have not traditionally been used to characterize lignocellulosic feedstocks but have the potential to bridge the gap in our explanation of feedstock variability.

Between the molecular and reactor scales, which are familiar to the chemical engineering community, lies an intermediate regime, here termed the “mesoscale,” where transport phenomena and reaction kinetics compete on similar time scales. Bioenergy and catalytic processes offer particularly important examples of mesoscale phenomena owing to their multiphase nature and the complex, highly variable porosity characteristic of biomass and many structured catalysts. In this review, we overview applications and methods central to mesoscale modeling as they apply to reaction engineering of biomass conversion and catalytic processing. A brief historical perspective is offered to put recent advances in context. Applications of mesoscale modeling are described, and several specific examples from biomass pyrolysis and catalytic upgrading of bioderived intermediates are highlighted. Methods including reduced order modeling, finite element and finite volume approaches, geometry construction and import, and visualization of simulation results are described; in each category, recent advances, current limitations, and areas for future development are presented. Owing to improved access to high-performance computational resources, advances in algorithm development, and sustained interest in reaction engineering to sustainably meet societal needs, we conclude that a significant upsurge in mesoscale modeling capabilities is on the horizon that will accelerate design, deployment, and optimization of new bioenergy and catalytic technologies.

Specific grinding energy and particle size distribution of grinds are usually used to assess the efficiency of grinding equipment and material grindability. The objective of this study was to quantify the effects of moisture content and storage time on the specific grinding energy of loblolly pine woodchips and the physical properties of the resulting grinds. Samples were ground in a hammer mill fitted with 3.18 and 6.35 mm screen sizes. The specific grinding energy, moisture loss during grinding, particle size distribution, and bulk density of the grinds were measured. Moisture loss during grinding increased with increase in moisture content of woodchips and decreased with increase in hammer mill screen size. Bulk density of grinds reduced from 273.64 to 106.03 kg/m 3 and from 251.14 to 131.40 kg/m 3 (for 3.18 mm and 6.35 mm hammer mill screen size, respectively), when the moisture content of woodchips was increased from 13.6 to 100.0 % (dry basis). Results also showed that storage time did not significantly affect (p \ 0.05) the specific grinding energy. However, specific grinding energy was significantly affected (p \ 0.05) by the moisture content of woodchips and hammer mill screen size. The specific grinding energy of woodchips ground through 3.18 mm hammer mill screen size increased with increase in moisture content of woodchips. However when loblolly pine woodchips were ground through 6.35 mm hammer mill screen size, the specific grinding energy initially increased as moisture of woodchips increased from 13.6 to 42.9 % (dry basis), then decreased with further increase in moisture content of woodchips from 42.9 to 100.0 % (dry basis).