Comparison and explanation of predictive capability of pellet quality metrics based on fundamental mechanical properties of ground willow and switchgrass

Willow is a light-weight and uniform-grained hardwood, which is a promising bioenergy crop. In this study, physical and mechanical properties of ground willow and quality of willow pellets were determined and compared with already published properties of more commonly used bioenergy crop, i.e., switchgrass. Rational explanations were provided for why certain mechanical properties were correlated well with pellet qualities. To that end, willow was ground with two different screen sizes (3.175 mm and 6.350 mm) and conditioned at two moisture contents (17.5% and 20.0% w.b.). The physical and mechanical properties of ground willow and pellet qualities were determined. Ground willow and switchgrass exhibited similar trends of mechanical properties, however, the magnitudes were different. For ground willow, compression index correlated well with strength of willow pellets, however, bulk modulus and spring-back index showed high correlations in case of switchgrass. This could be an attribute of different physical properties, i.e., particles size distribution, bulk density, and particle density, and chemical compositions. Higher levels of lignin present in willow than switchgrass might produce significant effect of resistance to compressibility. Ground willow underwent plastic deformation, which resulted in a high correlation to compression index. Ground switchgrass was dominated by elastic responses thus highly correlated to bulk modulus and spring-back index.