(23a) Mechanical Properties and Bulk Flow Characterization of Loblolly Pine

The inhomogeneous, elastic and cohesive nature of biomass poses several feeding and handling challenges during granular flow which create issues when transporting material through biorefinery unit operations (hoppers, silos, screw conveyors, etc.). It is therefore essential to study the mechanical and physical properties of these materials and accurately characterize bulk flow to ensure uniform handling and prevent issues of segregation, agglomeration, plugging, ratholing, stagnant material, arching, etc. This work is aimed at understanding and solving the flow challenges that biorefineries are faced with, through extensive experimental investigation of the mechanical properties and flow behavior (resulting from shear-failure of the bulk sample) of industrially relevant mature loblolly pine wood grown on an established plantation. Shear tests are conducted on a rotary shear tester (Schulze-style tester) and a direct axial shear tester (similar to the flow indicizer developed by Johanson) at the different consolidation stresses (approximately 1 – 10 kPa) that the material might experience in an industrial scale hopper and the following mechanical properties are recorded and analyzed: Young’s modulus, Poisson’s ratio, major principal stress, unconfined yield strength, cohesion, flowability, effective angle of friction, internal angle of friction and wall friction angle. Resultant flow performance is also established in an instrumented, adjustable flow hopper with measurements of discharge rate and profilometery, as well as static and dynamic wall stress measurements. This project investigates the linkages between material attributes (particle size and shape, mechanical properties arising largely from the biological cellular structure and biopolymer composition, and moisture content manifesting as enhanced particle cohesion due to capillary forces, etc.), and resulting flow performance. The impact of altering size, shape, and friction (surface roughness) on mechanical properties was studied by using different samples prepared using various comminution and size discrimination techniques. Preliminary results suggest that modest changes in particle size and distribution can, for example, almost change cohesion and bulk unconfined yield strength by a factor of 2. Smaller particles are seen to have higher cohesion and lower flowability. This is hypothesized to be an observational result of the interparticle forces exceeding gravitational forces in finer particles. Further, the measured properties depend on the consolidation forces. These results indicate that material might stagnate and cause mechanical locking (plugging, arching) at certain particle sizes and consolidation stresses, preventing flow. These observations imply careful consideration of operational conditions and material properties on equipment design and operation is essential to avoid material flow issues. This work is an experimental investigation and validation of biomass flow that will be used to verify particle-scale DEM models to extract microscale physics interactions and bulk phenomena needed for FEM model development to accurately represent biorefinery processing of biomass at a pilot and industrial scale. The current state of the project as well as experimental data linking mechanical properties to flow performance and observations will be presented.