Effect of pellet die diameter on density and durability of pellets made from high moisture woody and herbaceous biomass

Densified products produced from pellet mill are commercially used as a commodity type product for energy applications that are transported nationally and internationally. The quality of the pelletized biomass produced depends on the process variables such as die diameter, length to diameter (L/D) ratio, die speed, preheating, and steam conditioning; and feedstock variables such as feedstock type, moisture content, and particle size and shape. In the present study, pelleting tests were conducted with both woody (i.e., lodgepole pine and pinyon-juniper) and herbaceous (i.e., corn stover, wheat straw, and energy sorghum) biomass. A high level of feedstock moisture content of 33% (w.b.) was selected, while the die speed and preheating temperature process variables were kept at 60 Hz (380 rpm) and 70 °C. Results indicated that during the pelleting and cooling process, an approximate 10–13% (w.b.) moisture loss in both the woody and herbaceous biomass was observed. The high moisture pellets produced were further dried in a laboratory oven at 70 °C for three hours to reduce the moisture content of the pellets to <10% (w.b). The dried pellets were then evaluated further for other quality attributes including unit, bulk, and tapped density; and durability. The pellets that resulted in the highest unit, bulk, and tapped densities following this process were the herbaceous biomass corn stover (e.g., >1133, >580, >620 kg/m3) and the woody biomass lodgepole pine (e.g., >1037, >568, >641 kg/m3), respectively. In the case of durability for the 8 mm diameter pellets, wheat straw and corn stover recorded a maximum of about 96%, respectively, while the lodgepole pine and pinion juniper recorded a maximum of >96%, respectively.