Development of size reduction equations for calculating power input for grinding pine wood chips using hammer mill

Abstract

Size reduction is an unavoidable operation for preparing biomass for biofuels and bioproduct conversion. Yet, there is considerable uncertainty in power input requirement and the uniformity of ground biomass. Considerable gains are possible if the required power input for a size reduction ratio is estimated accurately. In this research, three well-known mechanistic equations attributed to Rittinger, Kick, and Bond available for predicting energy input for grinding pine wood chips by hammer mill were tested against experimental grinding data. Prior to testing, samples of pine wood chips were conditioned to 11.7 % wb, moisture content. The wood chips were successively ground in a hammer mill using screen sizes of 25.4, 10, 6.4, and 3.2 mm. The input power and the flow of material into the hammer mill were recorded continuously. The recorded power input vs. mean particle size showed that the Rittinger equation had the best fit to the experimental data. The ground particle sizes were four to seven times smaller than the size of the installed screen. Geometric mean size of particles were calculated using two methods: (1) Tyler sieves and using particle size analysis and (2) Sauter mean diameter calculated from the ratio of volume to surface that were estimated from measured length and width. The two mean diameters agreed well, pointing to the fact that either mechanical sieving or particle imaging can be used to characterize particle size. Specific energy input to the hammer mill increased from 1.4 kWh t⁻¹ (5.2 J g⁻¹) for large 25.1-mm screen to 25 kWh t⁻¹ (90.4 J g⁻¹) for small 3.2-mm screen.