Characterization of wheat straw particle size distributions as affected by knife mill operating factors

Knowing the effect of mill operating factors on biomass size reduction would be useful for predicting or adjusting particle size distributions that affect supply chain efficiency and biomass-to-fuel conversion processes. Wheat straw (Triticum aestivum L.) particle size distributions generated by a knife mill were determined for integral classifying screen sizes from 12.7 to 50.8 mm, operating frequencies from 4.17 to 8.33 Hz, and mass input rates from 2 to 9 kg min⁻¹. Particle distributions were obtained with standardized sieves for forage analysis that included horizontal sieving motion with machined-aluminum sieves of thickness proportional to sieve opening dimensions to reduce inadvertent particle spearing through sieve openings. Several analytical descriptors were examined to mathematically represent the range of particle sizes in the distributions. Screen size and feed rate directly influenced particle size, whereas operating frequency had a weak indirect relation with particle size. The Rosin-Rammler equation adequately fit the knife-milled wheat straw particle size distribution data (R² > 0.982). Mass relative span was >1, which indicated a wide distribution of particle sizes. Uniformity coefficient was <4.0 for 12.7 and 19.0 mm screens, which indicated particles of relatively uniform size. Knife mill chopping of wheat straw produced fine-skewed mesokurtic particles with 12.7-25.4 mm screens and strongly fine-skewed mesokurtic particles with 50.8 mm screen. Other size related parameters, such as, geometric mean length, Rosin-Rammler size parameter, median length, effective length, and size guide number, were well predicted as a function of knife mill screen size, feed rate, and mill frequency. Results showed the relative effects of knife mill operating factors to produce particular particle sizes of wheat straw.