Comminution Properties of Biomass in Hammer Mill and its Particle Size Characterization

Abstract

Biomass particle size impacts handling, storage, conversion, and dust control systems. Size reduction mechanical energy was directly measured for switchgrass (Panicum virgatum L.), wheat straw (Triticum aestivum L.), and corn stover (Zea mays L.) in an instrumented hammer mill. Direct power inputs were determined for different operating speeds from 2000 to 3600 rpm for 3.175 mm integral classifying screen and mass input rate of 2.5 kg/min with 90o and 30o edges on hammers. Overall accuracy of power measurement was calculated to be ±0.003 kW. Particle sizes were examined for hammer mill operating factors using ISO sieve sizes from 4.75 to 0.02 mm in conjunction with Ro-tap sieve analyzer. A total specific energy (kWh/Mg) was defined as size reduction energy expended for a particular mill design. Effective specific energy was defined as the energy that can be assumed to reach the biomass. The difference is parasitic or idle energy. Total specific energy for switchgrass, wheat straw, and corn stover grinding increased with hammer mill speed for both 90o and 30o hammers. Effective specific energy decreased marginally for switchgrass and considerably for wheat straw and it increased for corn stover with 90o hammers. However, effective specific energy increased with speed to certain extent and then decreased for 30o hammers. Rosin-Rammler equation fitted the switchgrass, wheat straw, and corn stover grind size distribution data with R2 > 0.995. Mass relative span was greater than 1, which indicated a wide distribution of particle sizes. Uniformity coefficient was less than 4.0 for wheat straw, which indicated uniform mix of particles, and it was about 4.0 for switchgrass and corn stover, which indicated a large assortment of particles and also represented a well graded particle size distribution. Geometric mean diameter had strong correlations with Rosin-Rammler size parameter, median diameter, and effective size. Distribution related parameters, namely, mass relative span, Rosin-Rammler distribution parameter, inclusive graphic skewness, graphic kurtosis, uniformity index, uniformity coefficient, coefficient of gradation and distribution geometric standard deviation had strong correlation among themselves and a weak correlation with mill speed. Results of this extensive analysis of specific energy and particle sizes can be applied to selection of hammer mill operating parameters to produce a particular size of switchgrass, wheat straw, and corn stover grind.