Effect of stem crushing on the uniaxial bulk compression behaviour of switchgrass and miscanthus

The bulk density with current baling technology is too low to utilise the full transportation capacity. For square bales, bulk density is limited by the pressure generated by the baler, density settings, and the ability of baling twine to withstand the expansion forces shortly after ejection from the bale chamber. This study explores crushing the stem on the pressure required to compact the material and the rebounding forces generated after compression. Uniaxial compression tests were conducted on bulk samples of switchgrass at three moisture levels (nominally 10%, 20%, and 45%) and miscanthus at two moisture levels (nominally 10% and 20%) to evaluate crushing. The pressure required to compress the material to 256 dry kg m−3 and the materials' stress relaxation behaviour were used as evaluation metrics. Crushing the material resulted in a significant decrease in the compression required for both crops. The peak stress was reduced from 577 to 441 kPa (a decrease of 23.6%) and from 581 to 365 kPa (a decrease of 37.2%) for switchgrass and miscanthus, respectively. Moisture level was also significant for both crops at the 10% and 20% moisture level with the low moisture material requiring higher pressures to reach the target densities. The final pressure after 5 min of relaxation was significantly lower for crushed material by 37.6% for miscanthus and 24.2% for switchgrass. However, this result from the reduction in peak stress at the end of the compression cycle, and relaxation behaviour was more affected by moisture level than by processing treatment.