Forest Harvest Residue Moisture Management in the Pacific Northwest, USA Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/jq085n325

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  • Moisture content management is a key requirement to improve forest harvest residue economics for bioenergy production. This dissertation aims to contribute towards better management through these three general objectives (1) Determine average moisture content of fresh forest harvest residues and its changes over the different seasons of the year, focusing on the three main commercial forest species growing in the Pacific Northwest, (2) Determine in-forest stored drying rates of this material for two harvest systems and the same species specified in objective (1) and (3) Determine the cost effectiveness of in-forest drying for two harvest systems and the advantages of drier material when its energy content is considered at a cogeneration facility. A repeated measures experimental design was conducted to determine average branch moisture content in live trees during each season of the year in four different locations in Oregon. At the same time, an innovative sampling protocol was employed to determine moisture content for in-forest stored of piled and scattered harvest residues for one year in four different Oregon sites. These data were used to calibrate finite element analysis (FEA) models to predict residue drying rates based on weather information such as temperature, relative humidity, precipitation and wind velocity. Finally, one of the FEA models was used to determine drying rates on real Douglas-fir units harvested with different harvest systems (a case study). These harvest units were employed to set a mixed integer linear program to optimally deliver harvest residues to a hypothetical cogeneration plant over 24 periods (months) and determine processing and transport costs. Major findings indicate that from all sites, the highest moisture recorded was 50% (wet basis) in ponderosa pine during the winter; the lowest was 43% in the summer for both the same ponderosa pine and Willamette Valley Douglas-fir. When compared by season, ponderosa pine had significantly higher moisture content in the winter than in other seasons (1.6 to 9.8% higher). Summer moisture content was also significantly lower than fall moisture content for ponderosa pine (5.4 to 2.5% lower). Willamette Valley Douglas-fir had significantly lower moisture content during summer than during other seasons (0.8 to 3.9% lower). FEA models were successfully developed to determine drying rates for four different climate regions in Oregon. These models were compared with data obtained in the field and statistical tests show model agreement with correlations between 0.56 and 0.92 (Kendall’s tau) on all sites. The harvest residue generated from the case study was sufficient to optimally deliver the necessary volume to supply 63% of a hypothetical 6 MW-hr cogeneration plant. Approximately 98% of the harvest residue generated with cable logging system was delivered to the plant compared with only 56% of the residue generated with a ground-based system. By considering the energy content of drier residues, the amount of ODMT needed to supply the plant can be reduced by 13.3% without affecting the energy output over a 6-period planning horizon. A lower ODMT demand and shifting to drier material results in 16.5% lower cost, which represents a more accurate estimate of the production cost. We conclude that forest harvest residues that are mainly composed of branches should not have moisture content levels greater than 50%. Seasonality should not affect the average moisture content of this material unless it is composed of ponderosa pine. After harvesting, piling residues in a berm (windrow) shape will promote drying in the summer and re-wetting in the winter. It is best to reduce pile size to facilitate drying in summer, and increase pile size if material will be left in the field over the winter. Drying times can be reduced up to 1/3 if the material is cut and left to dry during the dry, warm summer months versus starting in the winter. I this case study, residues coming from cable harvest units present a cost advantage compared to ground-based harvest units. Collection cost from the drier ground-based units was too large to offset the higher moisture content of piled residue in the cable harvest units. Recognizing the energy value of drier material has potential to improve the supply and cost estimates of the utilization of forest harvest residues for power generation.
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