Hammer milling switchgrass from weathered bales

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Considering that grasses have been targeted as raw material for bioenergy and will have to be handled in mass flow manner to be efficient, there is need to understand how grasses can be ground into easily flowing particles. For a biorefinery, the biomass will inevitably have to be stored, more than likely in an environment exposed to the elements. Consequently, there is a need to understand how the weathered material will interact with processing equipment like a hammermill. Grinding tests with switchgrass stored for a year in the open (weathered) and undercover (unweathered) were undertaken. The switchgrass was ground in an 18.64 kW hammermill with screen openings 9.52, 31.75, and ∞ mm (no screen). Results from this preliminary evaluation show that both weathered switchgrass (WSG) and unweathered switchgrass (USG) had grinding rates similarly increasing as the screen openings size increased with the unweathered material having a slightly higher throughput. When analyzed as a volume distribution of ground particles in machine vision methodology, switchgrass condition did not affect the particle size distribution parameters of the ground material. However, the geometric mean length for WSG increased from 6 mm to 37 mm with increasing screen size for WSG, whereas the geometric mean length for USG increased but only from 6 mm to 31 mm. While future studies are needed, the results however serve as a reference for preprocess grinding of biomass, either fresh or weathered (common in large-scale storage), for bioenergy/industrial use in obtaining the desired particle size at a given production rate.