Power Requirement and Energy Consumption of Small Bale Recompression

Abstract

Highlights Compression pressure has a nonlinear relationship with bale density. Increasing compression speed increased the energy, power, and specific energy required to recompress bales. The effect of speed on compression pressure and density was dependent on type of crop. Abstract. Biomass densification can reduce space needs during storage and transportation. It is important to quantify the amount of energy needed to densify biomass. Small bales (0.36 × 0.46 × 0.86 m) of herbaceous biomass crops, corn stover, indiangrass, switchgrass, and wheat straw, were recompressed in a compression chamber to determine their compression behavior in baled form. This chamber had the same cross-sectional area as a small baler. On average, the volume of all bales that were recompressed was reduced by 67.4%. Mean bale densities (d.b.) for corn stover, indiangrass, switchgrass, and wheat straw were increased from 94 to 264 kg m-3, 117 to 352 kg m-3, 139 to 384 kg m-3, and 98 to 277 kg m-3, respectively. The baled crops follow a nonlinear density vs. pressure relationship that can be accurately predicted using an exponential equation. When increasing loading or compression speed, the energy, power, and specific energy required to recompress the baled crops also increased. However, since each crop behaved differently during recompression, the effect of speed on compression pressure and density was dependent on the type of crop. At 60% reduction in bale volume and at the lowest compression speed (2.54 mm s-1), switchgrass required 15.6%, 61.7%, and 55.4% more energy than indiangrass, corn stover, and wheat straw, respectively. There were no significant differences in power required to compress different crops at the slowest speed (a = 0.05), but wheat straw required a lower specific energy of 0.725 kJ kg-1(d.b.) compared to between 0.734 and 0.814 kJ kg-1(d.b.) for the other crops. At the fastest compression speed (106.7 mm s-1), switchgrass required 32.4%, 59.4%, and 73.0% more energy, 12.9%, 20.5%, and 27.0% more power, and 15.7%, 5.4%, and 20.7% more specific energy than indiangrass, corn stover, and wheat straw, respectively. The purpose of this study was to determine the power requirement and energy consumption required to densify small bales of biomass crops using recompression to high enough densities to allow tractor trailers to be fully loaded to legal weight capacities within legal dimensions for highway transportation. Results from this study will be helpful when calculating energy balances related to biomass bale recompression for use as a feedstock in biofuel production and also for structural design and component selection for bale recompression machinery. Keywords: Bale, Biomass logistics, Bulk densification, Recompression.