Physicochemical and Ignition Properties of Dust from Loblolly Pine Wood

Abstract

Energy derived from biomass has attracted considerable attention because of increasing energy demand, high crude oil prices and environment and climate concerns. Biomass must be harvested and processed before conversion to a desirable form of energy. However, dust particles can be generated during biomass processing with humans working in dusty environments potentially causing health problems such as skin and eye allergies, respiratory issues and lung cancer. Additionally, the risk of fire and explosion also exists with biomass dust due to its combustible nature. Loblolly pine tree in southern forestland of the United States serves as a potential biomass feedstock for the bioenergy industry. Therefore, this research was conducted to study the ignition risk associated with dust generation during the grinding process of loblolly pine chips. The specific objectives were to: 1) determine the effect of moisture content (5%, 15% and 25% on w.b.) and screen size (1.20, 3.18 and 6.35 mm) of a hammer mill on energy required to grind loblolly pine chips and the amount of dust generated during grinding, 2) quantify the physical and chemical properties of ground material and associated dust, and 3) quantify the physicochemical and ignition properties of fractionated pine dust (<90, 90-180, 180-420 µm). Results suggested that the grinding energy, amount of dust generated and the physicochemical properties of ground material and dust were significantly affected by wood chip moisture content and hammer-mill screen size. Grinding energy increased from 39.65 to 360.00 kJ/kg when moisture content and screen size increased from 4.7% and 6.35 mm to 23.6% and 1.20 mm, respectively. The amount of dust generated decreased with increase in moisture content and decrease in screen size. About 21.5 % of the dust was observed in ground material when wood chips at 4.7% moisture content was ground through a 1.20 mm hammer mill screen. Further, the fine fraction (<90 µm) of dust had the higher bulk density (208.33 kg/m3), and ash content (1.70% d.b.) compared to the medium (90-180 µm) and coarse (180-420 µm) fractions. Other physicochemical properties such as particle density, volatile matter and energy content were not significantly different (p>0.05). Fine dust fraction had lower hot surface ignition temperature than the other dust fractions. Temperature of volatilization, temperature of maximum mass loss rate and temperature of oxidation were not significantly affected by particle size. The exothermic reaction started at a significantly (p<0.05) lower temperature for fine dust fractions compared to other fractions. The maximum temperature during exothermic reaction was almost the same for all three fractions. According to the plot of activation energy versus oxidation temperature, three fractions (<90, 90-180, 180-420 µm) were at high to medium risk of ignition. The hot surface temperature and temperature of rapid exothermic reaction indicated that the risk of ignition increased with decrease in particle size.