Multiscale Shear Properties and Flow Performance of Milled Woody Biomass

Abstract

One dominant challenge facing the development of biorefineries is achieving consistent system throughput with highly variant biomass feedstock quality and handling performance. Current handling unit operations are adapted from other sectors (primarily agriculture), where some simplifying assumptions about granular mechanics and flow performance do not translate well to a highly compressible and anisotropic material with nonlinear time- and stress-dependent properties. This work explores the shear and frictional properties of loblolly pine at multiple experimental test apparatus and particle scales to elucidate a property window that defines the shear behavior over a range of material attributes (particle size, size distribution, moisture content, etc.). In general, it was observed that the bulk internal friction and apparent cohesion depend strongly on both the stress state of the sample in granular shear testers and the overall particle size and distribution span. For equipment designed to characterize the quasi-static shear stress failure of bulk materials ranging from 50 to 1,000 ml in test volume, similar test results were observed for finely milled particles (50% passing size of 1.4 mm) with a narrow size distribution (span between 10 and 90% passing size of 0.9 mm), while stress chaining and over-torque issues persisted for the bench-scale test apparatus for larger particle sizes or widely dispersed sample sizes. Measurement of the anisotropic particle–particle friction ranged from coefficients of approximately 0.20 to 0.45 and resulted in significantly higher and more variable friction measurements for larger particle sizes and in perpendicular alignment orientations. To supplement these laboratory-scale properties, this work explores the flow of loblolly pine and Douglas fir through a pilot-scale wedge-shaped hopper and a screw feeder. For the gravity-driven hopper flow, the critical arching distance and mass discharge rate ranged from approximately 10 to 30 mm and 2 to 16 tons/hour, respectively, for both materials, where the arching distance depends strongly on the overall particle size and depends less on the hopper inclination angle. Comparatively, the auger feeder was found to be much more impacted by the size of the particles, where smaller particles had a more consistent and stable flow while consuming less power.

Effects of the Moisture Content on the Flow Behavior of Milled Woody Biomass

“…Biomass to biofuel has become one of the most promising energy resources to replace fossil aviation fuels. − However, the economic competitiveness of the biofuel industry still suffers from the unstable flow and jamming of granular biomass materials in various feedstock handling equipment like hoppers and feeders. − Solving these issues centers on the fundamental understanding and accurate prediction of milled biomass’s mechanical and rheological properties. Recent multi-scale experimental and numerical investigations promoted the knowledge of the flow behavior of dry or room-moisture woody biomass particles. − However, biorefineries often have to process wet materials with water from the original plants or due to outdoor transport and storage. Wet biomass materials usually have worse flowability than dry particles − due to particle swelling, softening, and capillarity.…”

“…Current studies on the influence of the moisture content on biomass flowability include feeding and handling testing via hopper, ,,,, silo, ,, and screw feeder. , Laboratory characterizations of the moisture content influence on material compressibility, ,,− angle of repose (AoR), ,,,, shear responses, ,,,,− ,− and wall friction, ,,,− were also conducted. However, the link between the moisture-induced particle-scale and meso-scale property changes with macro-scale flow behavior has not been established for woody biomass.…”

“…Current studies on the influence of the moisture content on biomass flowability include feeding and handling testing via hopper, 11,12,15,18,19 silo, 16,17,20 and screw feeder. 21,22 Laboratory characterizations of the moisture content influence on material compressibility, 12,15,23−28 angle of repose (AoR), 19,21,26,28,29 shear responses, 11,12,15,19,25−28,30−32 and wall friction, 23,24,28,31−33 were also conducted.…”

Wedge-Shaped Hopper Design for Milled Woody Biomass Flow

“…Material handling issues such as particle segregation, surging flow, and jamming in hoppers, feeders, or conveyors often result in severe economical disadvantages and safety concerns. − Addressing these processing failures demands an in-depth understanding of milled biomass material flowability, efficient flow prediction tools, and renovations of equipment design . Efforts have been made to probe the complex flow behavior of various biomass materials ,− and to explore their flow and jamming physics in handling equipment. ,− However, equipment design and operation taking woody biomass flow behaviors (e.g., flow pattern, flow rate) into consideration are still needed. For example, hoppers constitute the most widely used material handling equipment and their design still relies on the early work of Jenike and colleagues in the 1960s. ,, Use of hoppers with an outdated design in biorefineries often results in arching and jamming, , indicating the need for a modified design guide that incorporates state-of-the-art knowledge on biomass flow behavior.…”