Effect of Particle Size Distribution on Loose-filled and Tapped Densities of Selected Biomass after Knife Mill Size Reduction

Abstract

Unique particle size distributions were created with a knife mill using four different classifying screen sizes ranging from 12.7 to 50.8 mm, plus other mill operating conditions. Mathematical descriptors of these distributions were then correlated with loose-filled and tapped densities. A forage size distribution standard (ASABE Standard S424.1) was used to determine the mass fractions on five sizes of sieves and a pan. Weakness of the forage standard was noted for coarse particle distributions that were not normally distributed across sieve sizes. Resulting particle distributions were modeled using both unconstrained and constrained Sigmoid ('S') curves. A wider range of increased sieve sizes for the standard would aide particle distribution modeling. Mass fractions retained on the bottom sieve and pan correlated significantly with biomass densities, likely due to packing of fine particles in void space. Fitting of the cumulative particle size distributions using unconstrained 'S' curves indicated a strong linear correlation between curve fit parameters such as asymptote value and slope factor with densities of biomass. Asymptote values correlated significantly with loose-filled and tapped bulk densities with Pearson correlation coefficients ranging from -0.668 to -0.765 for chopped switchgrass, wheat straw, and corn stover. Slope factors correlated significantly with loose-filled and tapped bulk densities with correlation coefficients ranging from -0.712 to -0.879. Regression models developed using slope factor predicted the loose filled and tapped bulk density with greater accuracy than a regression model using asymptote value. Mean loose-filled bulk densities were 67.5 18.4 kg/m3 for switchgrass, 36.1 8.6 kg/m3 for wheat straw, and 52.1 10.8 kg/m3 for corn stover. Mean tapped bulk densities were 81.8 26.2 kg/m3 for switchgrass, 42.8 11.7 kg/m3 for wheat straw, and 58.9 13.4 kg/m3 for corn stover. On average, tapping increased bulk density by 21.2% for switchgrass, 18.7% for wheat straw, and 13.0% for corn stover. These results can be used to design efficient size reduction, handling, storage, and transportation systems for chopped biomass.