Evaluation of Dry Steam Preconditioning on Switchgrass Pellet Quality Metrics

Abstract

Abstract. The utilization of dry steam as a low energy preprocessing step in the pelletization of switchgrass was evaluated in this work. Ground switchgrass was moisture-conditioned at 16% and 18% wet basis (w.b.) and then subjected to 100°C and 120°C steam treatment cycles in an autoclave before being formed into pellets. Similarly, pellets were formed from switchgrass moisture-conditioned at 16% and 18% without steam treatment (control). Determined metrics included particle density, particle size distribution, pellet density, diametral and axial strengths, and durability using both tumbling box and friabilator. Pellet density values were similar to particle density for all treatments suggesting pelleting produced highly compressed biomass with little voids remaining. Neither particle density nor pellet density significantly changed (p>0.05) due to steam conditioning. Although not significant (p>0.05), an increase in pellet diametral strength was observed for the 120°C dry steam conditioning compared to the non-steam conditioned and 100°C dry steam conditioning pellets. Axial strength of single pellets for control and dry steam conditioned materials were not significantly different (p>0.05). Pellet durability analysis resulted in an average friability of 93.8% for control and all treatments. The more aggressive box durability test indicates that dry steam conditioned pellets had a 7% to 10% lower durability compared to pellets formed using unconditioned switchgrass. Overall, the dry steam conditioning did not result in improved pellet quality for the tested moisture contents compared to the control. Future work is required to test the dry steam conditioning at other moisture and/or particle size levels. Keywords: Biomass, Densification, Durability, Pelletization, Pellet density, Pellet strength, Preprocessing.