Flowability parameters for chopped switchgrass, wheat straw and corn stover

A direct shear cell to measure the shear strength and flow properties of chopped switchgrass, wheat straw, and corn stover was designed, fabricated, and tested. Yield loci (r2 = 0.99) determined at preconsolidation pressures of 3.80 kPa and 5.02 kPa indicated that chopped biomass followed Mohr–Coulomb failure. Normal stress significantly affected the displacement required for shear failure, as well as the friction coefficient values for all three chopped biomass types. Displacement at shear failure ranged from 30 to 80 mm, and depended on preconsolidation pressure, normal stress, and particle size. Friction coefficient was inversely related to normal stress, and was highest for chopped corn stover. Also, chopped corn stover exhibited the highest angle of internal friction, unconfined yield strength, major consolidation strength, and cohesive strength, all of which indicated increased challenges in handling chopped corn stover. The measured angle of internal friction and cohesive strength indicated that chopped biomass cannot be handled by gravity alone. The measured angle of internal friction and cohesive strength were 43° and 0.75 kPa for chopped switchgrass; 44° and 0.49 kPa for chopped wheat straw; and 48° and 0.82 kPa for chopped corn stover. Unconfined yield strength and major consolidation strength used for characterization of bulk flow materials and design of hopper dimensions were 3.4 and 10.4 kPa for chopped switchgrass; 2.3 and 9.6 kPa for chopped wheat straw and 4.2 and 11.8 kPa for chopped corn stover. These results are useful for the development of efficient handling, storage, and transportation systems for biomass in biorefineries.