Comminution Energy Consumption of Biomass in Knife Mill and its Particle Size Characterization

Abstract

Current research is driven by the need to reduce the cost of ethanol production from biomass. Preprocessing research is focused on developing processes that would result in reduced bioconversion time, minimize enzymes usage, and/or maximize ethanol yields. Size reduction is an important preprocessing unit operation of biomass, which utilizes major portion of input energy. It changes the particle size, shape, and bulk density, and increases the total surface area of biomass and number of contact points for chemical reaction. Objectives of the present study were to chop switchgrass, wheat straw, and corn stover in knife mill and analyze the particle size distribution. Direct power inputs were determined for different knife mill screen openings from 12.7 to 50.8 mm, rotor speeds between 250 and 500 rpm, and mass feed rates from 1 to 11 kg/min. During the experiment, data were collected for determining effective and total specific energy for chopping. The chopped samples were analyzed for particle size distribution using ASABE sieve analyzer. For knife mill screen size of 25.4 mm a speed of 250 rpm gave the optimum performance. The optimum feed rates at these conditions were 7.6, 5.8, and 4.5 kg/min for switchgrass, wheat straw, and corn stover, respectively. The corresponding total specific energies were 27.3, 37.9, and 31.9 MJ/Mg and effective specific energies were 4.6, 5.4, and 0.9 MJ/Mg for switchgrass, wheat straw, and corn stover, respectively. Mathematical equations adequately fitted the total specific energy consumption and particle size distribution data. knife mill chopping of switchgrass/wheat straw/corn stover resulted in ‘well-graded’ ‘strongly fine-skewed mesokurtic’/’fine-skewed mesokurtic’/’fine-skewed mesokurtic’ particles with reduced size screens (12.7 to 25.4 mm) and ‘well-graded’ ‘fine-skewed mesokurtic’/’strongly fine-skewed mesokurtic’/’fine-skewed mesokurtic’ particles with increased size screen (50.8 mm).