Contribution of Particle Size and Moisture Content to Flowability of Fractionated Ground Loblolly Pine

Abstract

The use of woody biomass requires feedstock materials in particulate form; therefore there is a need for size reduction of the feedstock materials. Ground biomass typically has a non-uniform particle size and as a biological material, biomass will exchange moisture with the environment. Thus, ground biomass feedstock will have typical flow problems associated with bulk materials during storage. The objective of this research is to investigate the contribution of particle size and moisture content to flowability of fractionated ground loblolly pine. In this study, ground loblolly pines were fractionated into six size classes at five moisture content levels. The physical and flow characteristics of the fractionated samples that are related to flowability and design of the storage vessels were quantified. The geometric mean diameters for the fractions were in the range 0.10 - 2.38 mm. Bulk and tap densities of the fractions increased with fraction size but particle density was not affected by fraction size. The densities (bulk, particle and tap) decreased with increase in moisture content. Porosity, Hausner ratio and compressibility increased with increase in moisture content and reduction in fraction size. The flowability of the fractions decreased with decrease in fraction size and increase in moisture content. Flow index values of 4.11, 4.17 and 4.29 were recorded for 1.40 mm fractions at moisture levels of 4.78%, 8.69% and 16.53%, respectively which implies easy flowing. However, a reduction in flowability from easy flowing to cohesive flowing was observed when 1.40 mm fractions were dosed with 0.50 mm fractions at 10:1, 10:2 and 10:3 mass ratio respectively. Cohesive strength and angle of internal friction decreased with increase in fraction size. Moisture content caused an increase in angle of internal friction but had no significant effect on the cohesive strength of the fractions. Particle size and moisture content had a significant effect on the angle of wall friction of the fractions. There was a reduction in wall friction angle with increase in fraction size. Lower wall friction angles were obtained at lower moisture content when stainless and mild steel surfaces were used while Tivar 88 surface had a consistent low angle of wall friction at all moisture levels. The adjusted hopper outlet size varied between 1.20 and 28.56 mm with 1.40mm fraction having the highest minimum hopper outlet size. The wall normal and vertical pressure acting on the cylindrical section of the silo increased from 9.35 to 45.42 kPa and 15.34 to 48.91 kPa, respectively with increase in fraction size and decrease in moisture content. The initial fill and flow induced pressures acting on the hopper section of the silo increased from 15.34 to 48.91 kPa and 24.71 to 78.79 kPa, respectively with increase in fraction size and decrease in moisture content. The heating values (18-19 MJ/kg) and volatile matter (84.91 - 87.48% d.b) were not affected by the fraction size while ash content was found to increase with reduction in the fraction size.