Transpirational Drying Effects on Energy and Ash Content from Whole-Tree Southern Pine Plantation Chipping Operations
Wood energy facilities prefer clean, dry raw material to maximize energy content and minimize the ash remaining after combustion. We allowed felled loblolly pine trees to transpirationally dry in-field for 4 and 8 weeks during late summer in central Georgia. We then compared costs of whole-tree chipping and transportation, as well as the energy, ash, bark, and nutrient contents of the chips produced. Moisture content was reduced from 53% for freshly felled trees to 43% and 39% for trees allowed to dry for 4 and 8 weeks, respectively. Drying did not change the ash content (<0.7%). Truck payloads were 16% and 24% lower for material dried 4 and 8 weeks compared with green material, thus increasing hauling costs by $0.80 per ton (field condition) for each subsequent 4-week drying period. Drying increased the energy content of delivered chips by approximately 1,200 BTU/lb. Whereas drying stems increased on-board and delivered costs on a field ton basis, it reduced those costs by 20% when measured on a bone dry ton or million BTU basis. Transpirational drying for 4 weeks or more appears to be a promising method for increasing the energy content without increasing the ash content of woody feedstocks.
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