Fundamental mechanical properties of ground switchgrass for quality assessment of pellets

Pelletization is one of the promising technologies for biomass densification. Many studies have been conducted to determine the optimum conditions for pelletizing. However, no relationship between the mechanical properties of the ground biomass and pellet quality has been published. The goal of this research was to develop and validate a predictive relationship between mechanical properties of the ground biomass in early stages of densification (<100kPa) and the quality metrics of pellets. Switchgrass was ground with two different screen sizes (3.175mm and 6.35mm) and conditioned at two different levels of moisture content (17.5%, and 20%, w.b.). Hydrostatic triaxial compression (HTC) tests were performed using the Cubical Triaxial Tester (CTT) to determine bulk modulus, compression index, and spring back index below 100kPa. Pellets were formed with ground biomass using a farm-scale pellet mill and diametral tensile strength, axial compressive strength, pellet density, and durability were determined. Regression equations between pellet quality and mechanical property having an R² value higher than 0.80 were selected for prediction. The validation against quality of pellets, which were formed at an alternate condition, confirms that the strength quality metrics of switchgrass pellets can be predicted using size reduced materials' bulk modulus and spring-back index.