Physical characterization of wet and dry wheat straw and switchgrass - Bulk and specific density

Bulk density of biomass is a major factor in determining the cost and logistics requirements of handling and moving biomass from farm to biorefinery. Bulk density is a strong function of size and shape and individual particle density. In this research we study experimentally the effect of particle length, moisture content, and particle density on bulk density of wheat straw and switchgrass. Wheat straw and switchgrass stems were cut to exact nominal lengths of 6, 12, 25, and 50 mm. The moisture contents of biomass samples were adjusted upward from an original 8% to 20, 40, and 60%. Three particle densities were measured assuming two structural geometries for the stems; i.e a hollow cylinder and a solid cylinder. The particle densities were measured using a gas pycnometer at a gas pressure of 40 kPa. The bulk density of both loose-fill and packed-fill were examined. The bulk density of wet straw and switchgrass increased with moisture content from 24.16 to 111.13 kg/m³ for straw and from 49.44 to 266.52 kg/m³ for switchgrass. The corresponding tapped bulk density was 33.75 to 130.43 kg/m³ for straw and 67.68 - 323.10 kg/m³ for switchgrass. The increase in bulk density due to tapping the container was from 10% for short 6 mm particles to more than 50% for long 50 mm particles. It is concluded that the following mixture equation can be used for estimating bulk density of the same size material from particle moisture content = 1/ρ_b = 1-M_w/ax^-b + M_w/ρ_w where ρ_b is the wet bulk density of biomass at a moisture content of M_w, (decimal fraction wet basis), x is particle length, a and b are biomass species constants, and ρ_w is the density of water (roughly 1000 kgm^-3).