Bulk density and compaction behavior of knife mill chopped switchgrass, wheat straw, and corn stover

Bulk density of comminuted biomass significantly increased by vibration during handling and transportation, and by normal pressure during storage. Compaction characteristics affecting the bulk density of switchgrass, wheat straw, and corn stover chopped in a knife mill at different operating conditions and using four different classifying screens were studied. 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. Percentage changes in compressibility due to variation in particle size obtained from a knife mill ranged from 64.3 to 173.6 for chopped switchgrass, 22.2–51.5 for chopped wheat straw and 42.1–117.7 for chopped corn stover within the tested consolidation pressure range of 5–120 kPa. Pressure and volume relationship of chopped biomass during compression with application of normal pressure can be characterized by the Walker model and Kawakita and Ludde model. Parameter of Walker model was correlated to the compressibility with Pearson correlation coefficient greater than 0.9. Relationship between volume reduction in chopped biomass with respect to number of tappings studied using Sone’s model indicated that infinite compressibility was highest for chopped switchgrass followed by chopped wheat straw and corn stover. Degree of difficulty in packing measured using the parameters of Sone’s model indicated that the chopped wheat straw particles compacted very rapidly by tapping compared to chopped switchgrass and corn stover. These results are very useful for solving obstacles in handling bulk biomass supply logistics issues for a biorefinery.