Measurement and comparison of mechanical properties of ground corn stover, switchgrass, and willow and their pellet qualities

Corn stover, switchgrass, and willow are promising bioenergy crops, which are found in abundance in the Northeast region of the U.S. In this study, mechanical properties of ground corn stover, switchgrass, and willow are measured and compared in addition to the quality of pellets formed. Size reduction was performed with two different screen sizes (3.175 mm and 6.35 mm) and conditioned to obtain biomass at two different moisture contents (17.5% and 20% on wet basis). Physical properties included bulk density, particle density, and particle size distribution. Ground switchgrass has the smallest D₅₀, whereas, willow has the highest D₅₀, when size reduced with the same screen size. Hydrostatic triaxial compression (HTC) tests were performed using the Cubical Triaxial Tester (CTT) to determine the various mechanical properties. Bulk modulus for the three materials at four conditions at specific unloading pressures (20, 45, 70, and 95 kPa) was determined. Durability, pellet density, axial compressive strength, and diametral tensile strength were the indicators of pellet quality. Willow, size-reduced with 3.175 mm screen size at 17.5% w.b., has the highest bulk modulus among different conditions of all three biomass. The diametral tensile strength is 4-5.8 MPa for corn stover, 0.9-3.8 MPa for switchgrass and 0.8-5.7 MPa for willow pellets, respectively. Axial compressive strength is 8-11.5 MPa for corn stover, 0.8-5.7 MPa for switchgrass and 1.5-8.5 MPa for willow pellets, respectively. Corn stover pellets formed with 3.175 mm screen size at 20% w.b. has the highest diametral tensile and axial compressive strengths among different conditions for all three biomass. Pellet durability for all three materials is more than 80%.