Moisture and Storage Time Effects on Grinding Characteristics of Loblolly Pine Woodchips

Abstract

In the processing of biomass to bio-products, grinding is unavoidable. Grinding is used to produce particulate materials and enhance handling, storage, and conversion processes. Performance of grinding operation is usually evaluated based on specific grinding energy requirements and physical properties of resulting grinds. The objective of this work was to quantify the effects of moisture and storage time on the specific grinding energy requirement and some physical properties of grinds. Loblolly pine woodchips were ground using a hammer mill with two screen sizes (3.175 mm and 6.350 mm). Moisture content of woodchips was adjusted to 12%, 20%, 30%, 40%, and 50% for storage times of 0 month, 2 month, and 4 month. To further investigate the effect of moisture on loblolly pine wood, the specific toughness and bending stress of loblolly wood was measured using three point bending test. Also, the bulk density, aspect ratio, flow index, particle size distribution, specific grinding energy requirements, and minimum drying energy requirements were measured and analyzed for different drying-grinding sequences. Results indicated that specific grinding energy ranged from 54.341 kJ/kg d.b. to 818.818 kJ/kg d.b.. Storage time did not have significant effect on grinding difficulty. However, for both screen sizes, grinding difficulty increased with increase in moisture content to a threshold of 30%. Further increase in moisture content resulted in decrease in grinding difficulty for screen size 6.350 mm. Although this was not obvious for screen size 3.175 mm, a reduction in slope was observed above the threshold moisture content. Bond model produced the best fit (r2 = 0.984) when the three grinding equations were fitted. Geometric mean diameter of grinds initially increased with increase in moisture content of woodchips from 12% to 20%, then reduced with further increase to 50%. It was also significantly affected (p < 0.05) by storage time and hammer mill screen size. The mean oven-dried bulk density decreased with increase in moisture content of feedstock and with increase in screen size. However, an interaction was observed between the moisture content of feedstock and hammer mill screen size. The three point bending test showed that jaggedness in the force-displacement curve disappeared with increase in moisture content. The toughness of loblolly pine wood increased with increasing moisture as well as with increasing tree radius and with decreasing tree height. Although, bending stress followed the same trend with tree height and radius, it reduced as moisture content increased. For the drying-grinding study, the highest oven-dried bulk density of grinds was 267.083 kg/m3 and lowest was 97.947 kg/m3. Generally, high aspect ratio, with mean values ranging from 4.30 to 6.36, was recorded. The specific grinding energy requirement varied from 211.639 kJ/kg d.b. to 818.162 kJ/kg d.b. With the consideration of moisture loss during grinding, there was a trade-off between specific grinding energy requirement and minimum drying energy requirement. Two-phase grinding had lower cumulative energy consumption compared to one-phase grinding.