Feeding properties and behavior of hammer- and knife-milled pine

Abstract

This work reports the rheological characterization of hammer- and knife-milled lodgepole pine materials as a function of grind size and moisture content (MC). Characterizations included particle size and shape distributions, bulk density, compressibility at 10kPa uniaxial pressure, elastic recovery from 10kPa uniaxial pressure, shear strength, effective angle of internal friction, arching in a custom hopper, and feeding performance in an auger feeder operating separately in volumetric and gravimetric feeding modes. Compressibility and elastic recovery increased as much as 15% and 4%, respectively with increasing MC. Adding 3kPa of pressure increased the minimum hopper outlet width needed to ensure consistent flow (i.e. the critical arching width) for all hopper arching tests, except for the pellets. Interestingly, for materials with smaller particle sizes, the volumetric feed rate tends to decrease as MC increases even though the density of the material increases with MC. Time variability of volumetric feed rate increased dramatically with increasing particle size but the impact of MC was not as clear. For nearly all tests, gravimetric auger feeding resulted in better feeding control and substantially decreased time variability in the material feed rate. Shear tests indicate that material strength generally decreased with increasing particle size; while the impact of MC was not as obvious. Agreement between experiments and predictions of the critical arching width were best for 1.6mm and 6mm grinds and worst for the 13mm grinds. This work also shows that the correlation between experiments and predictions of the critical arching width can be improved in terms of the coefficient of determination (R2) from 0.56 to 0.84 by correlating the respective upper limits of the 95% confidence intervals rather than the mean values.