Effect of ammonia fiber expansion (AFEX™) pretreatment on compression behavior of corn stover, prairie cord grass and switchgrass

Understanding the fundamental mechanisms involved in densification of bulky lignocellulosic feedstocks is imperative. This study was carried out to understand the effect of ammonia fiber expansion (AFEX™) pretreatment on the compression behavior of corn stover, prairie cord grass, and switchgrass. Samples were ground using three different hammer mill screen sizes (2, 4, and 8mm) and were subjected to AFEX™ pretreatment. Untreated and AFEX™ pretreated samples were moisture adjusted to four levels (8, 12, 16, and 20% wb) and were compressed using a single pelleting unit. Physical properties comprising bulk density, particle density of the samples and unit density of pellets were determined for each combination. AFEX™ pretreated corn stover with moisture content of 20% at screen size of 2mm produced pellets with 21% higher unit density compared to untreated corn stover pellets. AFEX™ pretreated prairie cord grass and switchgrass with 20% moisture content at a screen size of 2mm produced pellets having 25% and 21% higher unit density. The decrease in hammer mill screen size and the increase in moisture content and applied pressure increased the pellet unit density. Data obtained from the compression experiments were fitted with different compaction models viz. Jones, Walker, and Kawakita and Ludde. The Kawakita and Ludde model exhibited high degree of accuracy (R2 – 0.99 and 1.00) in all the samples. The constant value ‘1/b’ in Kawakita and Ludde model represents the yield strength of the compacts, and the lower 1/b values were obtained for AFEX™ pretreated samples compared to untreated samples. This implies the impact of pretreatment, which in turn made the biomass easier to compress. Lower values of yield strength were obtained at high moisture content (16–20% wb) signifying that AFEX™ pretreated biomass at high moisture content leads to onset of deformation at relatively low pressure to produce highly compacted pellets.