Options for reducing moisture and ash content in forest biomass harvesting systems
Cutshall, Jason Blaine
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Renewable energy sources are increasingly important as part of a 21st century energy solution. Wood energy facilities prefer clean, dry raw material to maximize energy content and minimize ash remaining after combustion. This dissertation assessed properties and production costs of chipping and grinding systems processing whole trees and residues from roundwood and clean chipping operations. Moisture content was reduced from 53% for freshly felled trees to 43% and 39% for trees allowed to dry transpirationally for 4 and 8 weeks, respectively prior to a whole‐tree chipping operation. Drying did not significantly change the ash content (<0.7%) associated with this feedstock. Truck payloads were 16% and 24% lower for material dried 4 and 8 weeks compared to green material, thus increasing hauling costs by $0.80 per ton (field condition) for each subsequent 4‐week drying period. Transpirational drying increased the energy content of delivered chips by approximately 1,200 BTU/lb. While drying stems up to eight weeks increased on‐board costs $0.68 on a field ton basis, it reduced those costs by $7.83 on a bone dry ton basis and by $0.47 on a million BTU basis. Data on production and the properties of chip and wood residue products were used to estimate costs per million BTU for forest biomass harvesting systems including conventional roundwood, clean chipping, whole-tree chipping, and screened and unscreened grinders processing logging and clean chip residue. Delivered costs for these systems were evaluated for a range of values for moisture content, ash content, tract size, tons of biomass removed per acre and at grinding decks, truck payload, haul distance, and diesel fuel price. Delivered cost per mmBTU decreased by over 50% for all systems as moisture content decreased from 55% to 30%. Whole tree chipping provided the lowest cost option ($4.39 per mmBTU) at ash content levels less than 1%, and unscreened grinding of clean chip residue produced the least expensive option at 5% ash ($2.87 per mmBTU). Tract size had minimal effects on any operation until the acreage declined below 40 acres. Clean chipping and roundwood systems were considerably more expensive than whole-tree chipping operations on all tract sizes. Costs declined significantly as truck payload increased and/or haul distance decreased. Optimizing truck payload to attain legal weight limits by lowering tare weight and/or increasing trailer volume capacity is an opportunity for forest biomass harvesting system managers to achieve lower costs especially when dealing with drier material or low‐density screened roundwood grindings. Fuel price increases directly increase cut and haul costs and limit economical haul distances accordingly.