Development and validation of predictive relationships between mechanical properties of ground willow and pellet quality

Willow is a light-weight and uniform-grained hardwood, which is common to the Northeast U.S. Unlike other hardwood such as oak, beech, and cherry, willow is a short rotation crop. It has the ability to resprout after multiple cuts, and has tolerance of dense planting. Therefore, willow is gaining popularity as a biomass. In this study, willow was ground and pelletized. Physical and mechanical properties of ground willow and quality of willow pellets were determined. 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). Determined physical properties of ground willow include bulk density, particle density, and particle size distribution. D₅₀ of willow ground with 6.35 mm screen size was higher by 33.7% than D₅₀ of willow ground with 3.175 mm screen. To determine mechanical properties of ground willow, hydrostatic triaxial compression (HTC) tests were performed using the Cubical Triaxial Tester (CTT). Bulk modulus, compression index, and spring-back index of ground willow for four different conditions at four unloading pressures were determined. Durability, pellet density, axial compressive strength, and diametral tensile strength were the indicators of pellet quality. Pellets formed with the larger screen size, 6.350 mm, at 17.5% (w.b.) moisture content has highest axial and diametral tensile strengths among all four conditions. Compression index was found most suitable for predicting diametral tensile strength and axial compressive strength for willow pellets.