Effect of grinder configuration on forest biomass bulk density, particle size distribution and fuel consumption.

The effect of the grinder configuration, bit type and screen size, on bulk density and fuel consumption when processing forest harvest residues for energy purposes is analyzed. Residues were divided in three size classes based on the piece diameter and length and were processed in a six treatment structured randomized test using a horizontal grinder. For each treatment the basic density, moisture content, bulk density, particle size distribution, fuel consumption and bark and other non-wood substances content was estimated. No effect of bit type or screen size on bulk density was found when processing branches-and-tops size class residue. For the pulpwood and butt-log-chunks size classes, the knife-edge bits tend to produce a denser material explained in part by their cutting capabilities across the grain compared to the normal hammering process using carbide hammer bits. Fuel consumption was only affected by screen size when processing the branches-and-tops size class. For pulpwood and butt-log-chunks size classes, the use of carbide hammer bits for processing increased fuel consumption between 42 and 48% compared to knife-edge bits. Bark and other non-wood substances content accounted for 11% of the total grinding mixture in the branches-and-tops size class compared to 2.5% in grindings from pulpwood and butt-log-chunks size classes. The branches-and-tops size class residue produced denser bulk material compared to the other classes and consumed less fuel due in part to the higher basic density and increasing amount of fine particles compared to the other analyzed size classes.