TECHNICAL RELEASE 12-R-17 Biomass: processing May 2012 www.forestresources.org/members/serpub/12-R-17.html
Wood energy facilities tend to prefer clean, dry raw material to maximize energy content and to minimize the ash remaining after combustion. Most traditional forest industries (paper and building products) purchase their raw material today on a green-ton basis. Freshly felled trees have a moisture content of approximately 50% (wet basis), but allowing harvested material to dry for a few weeks before delimbing and processing (transpirational drying) can reduce moisture content significantly, thus increasing the energy content of the wood. However, under green-ton payment systems, drier material results in lower truck payloads, higher per-ton trucking costs, and reduced revenue for the harvesting contractor. The basis of payment—green ton, dry ton, or energy content (million BTU)—has a dramatic impact on whether drying material in the field is worthwhile.
We assessed if increased drying times were correlated with reduced moisture content and increased ash content and how transpirational drying impacted harvesting cost and production.
METHODS: The study was conducted from August through October 2010 on a 113-acre, 14-year-old loblolly pine stand in Jones County, Georgia that was scheduled for a first commercial thinning using a row plus selection method. The harvesting system consisted of a Tigercat 718 rubber-tired fellerbuncher, a Caterpillar 525 grapple skidder, a Tigercat 230B knuckleboom loader, and a 600-horsepower Morbark 40/36 drum-style chipper. Trees from rows (or corridors) on two 37-acre blocks were felled and bunched in the harvested row and allowed to dry transpirationally for four and eight weeks, respectively, prior to chipping. After drying, trees were skidded to the landing by the grapple skidder and chipped during a commercial first thinning of the entire stand. The feller-buncher thinned a third block (approximately 37 acres) of green material concurrently with the chipping operation to provide a comparison of freshly felled trees to the transpirationally dried material.
We used work sampling and time study methods to record time and production data on the harvesting system. During the loading of each truck, we used a 6-inch-diameter PVC pipe with an elbow to collect samples from the chip stream. Samples were collected several times during the loading of a van and mixed to obtain a composite sample. We obtained moisture content from each sample and then submitted subsamples from them for lab analysis to determine energy and ash content. Since this study focused on the impacts of drying, we use the term “field ton” to refer to a ton of wood in its field condition with the observed moisture content for that treatment (green vs. dried for 4 or 8 weeks). A bone dry ton (BDT) is a ton of wood with zero moisture content. We also evaluated costs on an energy content (mmBTU = million BTU) basis, which correlates directly with the BDT basis. We modified a version of the Auburn Harvesting Analyzer to determine delivered costs for each of the three treatments.
FINDINGS: Moisture content of the chips varied significantly based on the length of time that transpirational drying took place. Each four-week drying period significantly decreased the moisture content (p < 0.05), with the largest reduction, from 53% to 43%, occurring during the first four weeks. During the second four-week period, the moisture content fell another 4% to 39%.
Chipper productivity measured in field-tons per productive machine hour (PMH) was significantly lower with the driest (8-week) stems than with either 4-week or green trees (p < 0.05). However, if productivity is considered on a BDT/ PMH basis, then the productivity handling of green trees is significantly lower (34 BDT/PMH) than that of either of the dried treatments (38 each). This result should be viewed with some caution, because a different loader operator fed the chipper for two of the five days during the green treatment chipping period. The replacement loader operator averaged 5 dry tons/PMH lower production than the normal operator (p < 0.05).
BTU per field-pound was significantly lower for chips from stems chipped green (freshly chipped) than from stems that dried for 4 or 8 weeks before chipping (Fig. 1). However, once water was removed there were no significant differences between either energy content per oven-dried pound. Observed ash content was less than 1% for all treatments.
The cost per loaded field ton for harvesting and chipping material dried for 4 and 8 weeks was $18.77 and $18.98, respectively, and $18.30 for green material. Average truck payloads (for a 42-foot trailer) were 26.5 field tons for green material, 22.3 for 4-week material, and 20.2 for 8-week material. Transport costs per field-ton-mile were thus highest for 8-week-old material at $0.17, followed by 4-week-old and green material at $0.15 and $0.13, respectively. Comparing these on a 50-mile haul, cost estimates per field ton for trucking 8-week material were highest at $8.48, followed by $7.68 for 4-week material, and $6.46 for green material.
Conversely, the cost per loaded BDT for chips from stems dried for 4 weeks was 15% less ($32.93) than for chips from green material ($38.94). Chips from stems dried for 8 weeks were 20% less expensive ($31.11) on a BDT basis. Truck payloads did not differ between the three treatments on a BDT basis; thus, transport costs per BDT were the same, roughly $0.28 per BDT-mile or around $14.00 per BDT for a 50-mile haul distance.
The cost per loaded mmBTU was also highest for green material at $2.39, followed by $1.99 for four week material, and $1.92 for eight week material. Truck payloads per mmBTU did not differ between treatments, with each treatment averaging roughly 200 mmBTU per load. Hauling costs were $0.86 per mmBTU in each treatment. Delivered costs on a field-ton basis for eight-week-old material were 10.8% higher ($27.48) than for green material ($24.80), with $2.00 of the cost difference resulting from hauling cost differences (Fig. 2). On a dry ton basis, however, there were no transport cost differences between treatments, and green material delivered cost was 17.4% higher ($52.76) than material dried 8 weeks ($45.06).
The delivered cost per mmBTU was also highest for green material at $3.24, followed by $2.78 for four- and eight-week material. Hourly production efficiency driven by moisture content contributed less to cost differences than truck payload differences on a field-ton basis. Switching to higher volume capacity truck and trailer combinations to maximize legal truck payloads offers an opportunity to offset much of the cost disparity derived from a field ton comparison, while the cost advantages of drier material would increase based on a BDT or mmBTU comparison.
CONCLUSIONS: Using transpirational drying in woody biomass harvesting operations may improve product quality by increasing energy value without increasing ash content. Trees that have been dried in the field to reduce moisture content cost more to harvest and transport than fresh material when based on a wet or field condition ton basis, but will cost less when based on energy content. Harvesting firms that deploy this strategy in a wood supply system that utilizes an energy content payment system, as opposed to a wet weight-based payment system, could take advantage of potential economic advantages of transpirationally drying stems in the field before processing and transportation.
Tony Macleod FRA Counsel