Creating High-Density Large Square Bales by Recompression

Abstract

Abstract. Creating high-density biomass bales would reduce the number of bales handled, stored, and transported to biorefineries, thereby reducing costs. Recompression of large square bales is one approach to increase bale density; however, recompression research on large bales is limited. Recompression of common biomass crops in 80 cm × 90 cm nominal cross-section bales was used to quantify pressure-density and stress relaxation relationships as well as restraining forces required to maintain compressed densities. Linear, power, and exponential models were fit to the pressure-density data, with the linear model providing the best representation of the recompression process. When bales were recompressed soon after bale formation, density increased by an average of 134%, and the target dry-basis density of 205 kg m-3 was exceeded at the end of recompression with all crops. While still compressed, bales were wrapped with four steel cables configured with load cells to measure the restraining force after release of pressure followed by bale re-expansion. Total restraining forces ranged from 18 to 40 kN. Bale density decreased by an average of 17% due to re-expansion after pressure was released. Strain relief induced by allowing the bale to re-expand by approximately 10% before placing the cables around the bales reduced the required restraining force by an average of 35%. Although the forces applied were great (>560 kN), the dry-basis specific energy requirements were comparatively low (0.18 to 0.29 kWh Mg-1) because recompression took place over a relatively long duration of approximately 20 to 25 s. Recompressing large square bales is a low-energy method to achieve bale densities that should ensure weight-limited biomass transport. Keywords: Bales, Biomass, Compression, Energy.