Analysis of grinding of pelletized wood waste with the Szego Mill

Abstract

Biomass, in particular wood and wood residues, is of increasing importance as an energy source, in particular for direct combustion to produce electricity. Size reduction is a key step in biomass applications, and is generally energy intensive, such as when wood residues are pulverised to sizes below 1000 μm for suspension firing. Wood residues may be pelletized for easier handling and transportation, and pelletized wood waste is frequently used as secondary fuel in coal fired furnaces, thus its grinding characteristics are of interest. For this study, Energex™ pelletized wood waste consisting of coniferous bark and wood was ground to fine sizes with a Szego Mill, a type of ring roller mill, at material rates up to about 500 kg/h on a dry basis in a pilot (22-cm diameter) mill, and 2000 kg/h in a commercial-scale (46-cm diameter) mill. Energy–product particle size relationships were determined, e.g., the specific energy E was of the order of 20 kWh/t oven-dry wood (ODW) for x 80 (80 wt.% passing mesh size) = 1000 μm. The specific energy required for the pelletized wood waste is an order of magnitude higher than for brittle materials such as quartz, coal, limestone and gypsum, but roughly one-quarter of that for newsprint, a fibrous material. Mathematical relationships were derived from first principles for the mill power P and μ r , the coefficient of friction for rolling resistance of the rollers through the particle bed. Experiments show that as a first approximation, the resistance to roller motion, and hence mill power and specific energy, is independent of the type of material that is ground. For dry grinding of pelletized wood waste, moist aspen chips, air-dry sawdust, wheat, gypsum and limestone at 1000 rpm in the pilot-size mill, E = 3.14 × 10 3 (MR) − 0.764 , where MR is the material rate, from which rolling resistance and mill power are roughly proportional to material rate to the exponent one-quarter. Mill power at constant material rate varied roughly with mill speed squared. • Pelletized wood waste was ground to fine sizes in the Szego Mill. • The specific energy E for particulate 80 wt.% < 1000 μm ranged from 8 to 30 kWh/t. • E was ~ 10 times that for the same size products of brittle materials such as coal. • As a first approximation, mill power is independent of the type of material ground. • Bark fractured between cells; wood broke down by cell wall fracture