In-wood grinding and screening of forest residues for biomass feedstock applications

Logging residues present a substantial near term opportunity as a bioenergy feedstock, but contaminants that can reduce their value can be introduced during collection. We studied the use of a trommel screen to reduce ash levels in ground forest harvest residues at time of production. Eight treatments of initial harvest type, grinder size, residue age, and screen usage were applied to southern pine plantation residues in the coastal plain of South Carolina, USA. Using the screen, the average ash levels of screened roundwood and clean chipped residue was reduced from 4.0% to 1.4% and from 11.9% to 6%, respectively. Average energy density increased 2–5% by screening. Without screening the feedstock, large grinder utilization with roundwood residues was 58% while the addition of a trommel screen reduced utilization to 47%. Screened roundwood residues were consistently more costly to produce than unscreened roundwood or screened clean chipped residue with either grinder size under a number of economic and operational scenarios on either a load weight or energy content basis. The screened clean chip systems and the unscreened roundwood material provided the most competitive residue on a cost per unit of energy basis.