Open Access
Covelli, Curtis R
Graduate Program:
Agricultural and Biological Engineering
Master of Engineering
Document Type:
Master Thesis
Date of Defense:
July 25, 2016
Committee Members:
  • Virendra Puri, Thesis Advisor
  • Switchgrass
  • Steam
  • Pellets
  • biomass
  • preprocessing
  • pretreatment
This research investigated the impact of thermal preconditioning with dry steam on the quality of pellets manufactured from ground switchgrass. The intended impact of preconditioning was to utilize the softening of naturally present binders, such as lignin, to improve pelletized biomass quality. An autoclave was used in dry steam preconditioning of ground switchgrass and to investigate the effect of elevated time-temperature treatment on switchgrass pellet quality metrics. Switchgrass was moisture conditioned at 17.5% and 20.0% wet basis (w.b) and preconditioned in the autoclave with dry steam at temperatures of 100°C and 120°C for five minutes. The properties of ground material and the pellets (both control and dry steam preconditioned) were characterized. Determined ground material properties included particle density, bulk density, and particle size distribution. Determined pellet quality metrics included diametral strength, axial strength, friability, durability, and pellet density. Particle density was observed to significantly decrease (p<0.05) when conditioned by the dry steam. This might have been caused by effects by moisture inducing the migration of natural binders to the outside of the ground particles and eventual evaporation. An increase in pellet diametral strength was observed for the 120oC dry steam preconditioning (p>0.05). Axial strength values of single pellets for control and dry steam preconditioned ground switchgrass were not significantly different (p>0.05). Pellet durability analysis in bulk pellets resulted in an average friability around 94% for control and all treatments. The more aggressive box durability tests indicated that dry steam conditioned pellets had a 7-10% lower durability around 78%. Pellet quality metrics of pellet density, strength, friability, and durability were found to be minimally impacted from dry steam preconditioning.