Coefficient of Dynamic Wall Friction for Hardwood Fuel Pellets

The design of storage and handling systems for bulk materials requires an understanding of their frictional properties. While these properties are well-studied for many agricultural commodities, they need to be better defined for pelletized biomass, which present unique challenges due to their particle sizes and nontypical aspect ratios. This study evaluated the coefficient of dynamic wall friction for hardwood fuel pellets against two common handling surfaces (ultra-high molecular weight polyethylene (UHMW-PE) and galvanized steel). Effects of confining pressure and shear displacement were investigated using a direct shear wall friction tester, which allowed for up to 20 cm of displacement. Three confining pressures (5 kPa, 10 kPa, and 15 kPa), and two loading methods were evaluated. Fuel pellets against UHMW were most impacted by displacement, with friction decreasing up to 9.5% over the length of the test. Additionally, sequentially increasing the confining pressure over the length of a single test resulted in shear stress differences of up to 14.2%, compared to independent tests. The lowest confining pressure was most affected by these trends and showed higher friction values, relative to the higher confining stresses. Linear functions for stress-dependent friction were estimated, and the overall wall friction angle across methods was 13.2° for UHMW and 11° for steel. Overall, this study explored several factors that influence wall friction test methods, and the results can help improve the design of pellet storage and handling systems.