Frictional contacts between individual woody biomass particles under wet and dry conditions

Abstract

Friction between wood particles can be a critical property when designing processing equipment for creating cellulosic biofuels and similar natural products. Also, as wood processing is likely to occur year-round, understanding the friction between wet as well as dry wood particles can help design and model equipment, such as screw feeders. Here, a tribo-rheometry method was developed using a commercial rheometer that measured the torque required to rotate two contacting millimeter-scale wood chip particles. First, a range of normal forces were used to find the limits of the experimental method. Torque values were measured for seven pairs of wood chip particles at 1 N normal force under both dry and wet conditions, at both ambient conditions and in a humidity-controlled chamber. The torque required to rotate the wet particles was consistently higher than that of the dry particles, although differences were not quite statistically significant when measurements were performed at ambient conditions. Using humidity control resulted in torque values for wet particles of 1900 μNm and 900 μNm for dry particles. Finally, the friction coefficient was calculated as 0.23 for wet particles and 0.11 for dry particles when humidity was controlled. These frictions coefficients agreed with values for wood reported in the literature. Overall, controlling humidity is strongly recommended to quantify the range of friction for particles that uptake water, including granular systems from biomass to food to powders. Friction higher for wet vs dry wood particles at constant humidity from bar graph on left. On right parallel plate rheomter shown with two wood particles between the plates and axial force pushing plates/particles together and shear force rotating bottom plate. • Tribo-rheometric method developed for controlled moisture friction experiments. • Individual particle friction measured for millimeter-scale wood particles. • Friction coefficients higher for wet versus dry wood particles.