| Oregon's Biomass Energy Resources |
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| Wood |
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There are three primary sources of woody biomass in Oregon. Wood products residue is the wood waste generated at Oregon sawmills and wood products mills. Forest biomass is residual biomass material generated from logging or thinning activities on forests in Oregon. Urban wood waste includes discarded wood and yard debris. In addition, hybrid poplar plantations in Oregon represent a small woody biomass resource for energy production.
The Department of Energy estimates that these sources generate approximately 12.7 million bone dry tons of woody biomass on an annual basis, but not all of the woody biomass resource is available for energy uses due to the cost of collecting and transporting the materials and other factors. The available woody biomass resource may be as much as 9.8 million bone dry tons.
Much of the available resource is currently used. About 67 percent of the available resource is used for purposes other than energy production. The primary use is in the pulp and paper industry. About 26 percent of the available resource is already used for energy production. This amounts to about 2.5 million bone dry tons annually. The energy value of this resource amounts to about 43 trillion Btu.
About 7 percent of the available resourse is not being used for either energy production or other purposes. This resource amounts to about 0.7 million bone dry tons of woody biomass on an annual basis. This resource is potentially available for energy production and has a gross energy value of approximately 12 trillion Btu.
Wood Products Residue
Wood products residue is a byproduct of timber milling and wood products manufacturing. Sawmills convert harvested timber into wood products through debarking, chipping, sawing, peeling, planing, shaving, trimming and sanding. Sawmills process logs into lumber, veneer and plywood. Other wood products mills and factories process wood into manufactured goods, such as furniture, cabinets, containers and pallets. Residue from the wood products industry includes trim, planer shavings, sawdust and bark.
Lower-quality mill waste is put through a grinding machine (called a "hog") and is used for fuel. "Hogged fuel" is a convenient fuel supply for boilers and electric power generation at mills that produce the material. The moisture content of hogged fuel is about 50 percent. Its energy content is 4,500 Btu per pound. It has a bulk density of 16 to 22 pounds per cubic foot.
Modern and efficient manufacturing processes at Oregon mills have resulted in less wood waste. Some of the waste is of high quality and is suitable for use in the pulp and paper industry. New composite wood products made from residual materials have further reduced the amount of wood products residue available for energy production.
Forest Biomass
Tree tops, limbs and cull material left over from logging activity represents a large potential resource for biomass energy. Forest biomass has a bulk density of 18 to 22 pounds per cubic foot and a typical moisture content of 50 percent. It has an energy value of 4,500 Btu per pound. Timber harvest variability and uncertain market conditions make long-term supplies of forest biomass from logging operations unpredictable.
Nevertheless, the potential energy resource is large. The Department of Energy estimates that 3.3 million bone dry tons of forest biomass residue was generated from timber harvest activity in 2004. An estimated 0.63 million bone dry tons of forest biomass was economically available to be used for energy production. The available forest biomass resource had an energy value of 10.8 trillion Btu.
Leaving some dead wood in the forest is good for forest ecosystems. Standing snags and dead wood on the forest floor provide habitat for wildlife. Woody debris on the ground deters erosion and, by its decomposition, helps maintain soil fertility and tilth. Although dead trees and woody debris play an important role in forest ecosystems, excessive accumulation of forest biomass becomes a threat to the health of live trees by making the forest susceptible to disease, insect infestations and high-intensity forest fires.
Reduced timber harvest activity and suppression of forest fires have caused an unnatural surplus of dead wood in many Oregon forests. Selective thinning in these areas could remove the excess biomass that poses a risk to sustainable forests.
According to Western Forest Health and Biomass Energy Potential, a study prepared for the Oregon Department of Energy, the cost to the public for fighting forest fires averages about $216 per acre. However, the cost of a thinning operation ranges from $50 to $150 per acre. The cost of thinning varies and depends largely on the location and topography of the site and the type of equipment used.
Urban Wood Waste
A significant amount of wood is discarded from individual households, commercial businesses, and construction and demolition sites. Urban wood waste includes lumber, pallets, crates, discarded wood furniture and other wood products. Yard debris contains additional wood waste in the form of tree and brush prunings, limbs, trunks and stumps.
Manufacturers of composite wood products can use clean, high quality urban wood waste in products such as flakeboard, oriented strandboard, particleboard and fiberboard. Pulp mills can convert clean wood waste into paper products. Urban and industrial wood waste that is not suitable for higher-value products has value as hogged fuel or compost.
The Oregon Department of Energy estimates that Oregonians discarded 0.56 million bone dry tons of urban wood waste in 2004. Approximately 0.32 million bone dry tons of wood was recovered from the waste stream. Much of the wood recovered from the waste stream was used for energy production. About 0.19 million bone dry tons of recovered urban wood waste was used for energy. This resource had a gross energy value of about 3 trillion Btu.
By increasing the rate of recovery of urban wood waste, an additional 14,000 bone dry tons of urban wood waste could be kept out of landfills. This amount of available wood waste has a gross energy value of about 0.24 trillion Btu.
Hybrid Poplar Plantations
There are more than 34,000 acres of hybrid poplar (cottonwood) trees growing on plantations in Oregon. The trees were originally planted to be a source of wood fiber for the pulp and paper industry, but some of the plantations will be harvested for lumber.
Although commonly called "hybrid poplar," the plantation trees grown in Oregon are a cross between black cottonwood and Eastern cottonwood. The hybrid is a fast-growing tree that thrives in the Pacific Northwest. Properly managed hybrid cottonwood plantations are ready for harvest six to eight years after cuttings are planted. At harvest, 70 to 80 percent of the biomass in each tree is suitable for pulp.
The U.S. Department of Energy anticipates that dedicated feedstock supply systems, including cultivation of short-rotation woody crops such as hybrid poplar, will be a major fuel source for the biomass power industry in the future. Harvest of these trees for pulp chips produces a biomass residue of bark, leaves and stumps that are usable for fuel. Residue yield varies depending on the hybrid variety, management practices and the age of the stand at the time of harvest as well as weather and other environmental factors. Residue yield ranges from 7 to 15 bone dry tons of fuel per acre. Thus, the gross energy value of the residue per 1,000 acres of harvested hybrid poplar ranges from 0.12 to 0.26 trillion Btu.
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| Pulping Liquor |
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The pulping process produces a waste stream of spent pulping liquor. Pulp mills burn the pulping liquor to recover and recycle the chemicals used in the pulping process. These chemical recovery boilers supply pulp mills with process steam. Two Oregon pulp mills use the boilers to cogenerate steam and electricity. The energy content of the pulping liquor consumed in Oregon in 2004 was approximately 35 trillion Btu.
The entire pulping liquor resource is already being used for energy in the form of steam and electricity. However, when older boilers reach the end of their operating life, there will be opportunities to install new, more efficient technology. There is a potential to add up to 57 average megawatts of generating capacity to those mills that do not currently produce electricity.
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| Municipal Solid Waste |
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Approximately 70 percent of the waste disposed of in landfills is biomass material, including food waste, waste paper, cardboard and wood waste. Municipal solid waste (MSW) has a moisture content of 30 to 40 percent and an energy content of about 4,500 Btu per pound. Its bulk density is 12 to 20 pounds per cubic foot.
Oregonians recycle or recover 37 percent of the MSW generated in the state, but every day, about 8,100 tons of MSW is dumped in landfills. In 1983, the Oregon Legislature established priorities for solid waste management, establishing a legislative policy that the use of solid waste for energy production should take precedence over landfill disposal.
In 2004, Oregonians disposed of an estimated 3.0 million tons of MSW in landfills. Of that, an estimated 70 percent could have value as an energy source. Excluding the Department of Energy´s estimate of recoverable urban wood waste, the potential energy value of the MSW discarded in 2004 was approximately 18 trillion Btu. This amount of biomass could be used to generate 121 average megawatts of electricity.
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| Biogas |
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Wastewater Treatment
Anaerobic digesters reduce the organic content of wastewater and decrease the amount of sludge disposal required at wastewater treatment facilities. The biogas generated in the process is often used as boiler fuel to supply heat for the digesters and for other treatment facility uses. However, nine wastewater treatment facilities in the state also use the gas to produce electricity.
The Department of Energy estimates that, overall, as much as 36 percent of the biogas produced at Oregon´s wastewater treatment facilities is unused. This surplus biogas is a potential energy source. In 2004, the unused gas had an energy value of approximately 0.3 trillion Btu. This amount of biogas could be converted to about 2 average megawatts of electricity.
Organic Waste Digesters
Manure from livestock on Oregon farms is a resource for the production of biogas through anaerobic digestion technology. Other organic wastes, such as agricultural and food-processing wastes, also could be used as digester feedstock. In local areas where there are many dairy farms, development of a centrally-located digester may be feasible.
The minimum size of dairy herd required to make a digester an economical investment for an individual farm depends on the local climate, the amount of manure collected, the type of technology used and the value of revenues from co-products and offsets. In general, large dairy operations (500 or more cows) may find it economically feasible to install a plug-flow digester.
In 2003, there were 111 dairies in the state that were licensed for herds of 500 or more cows. Based on the cumulative number of cows on-site at these dairies, the Department of Energy estimates that approximately 3,400 million cubic feet of biogas is potentially available annually through anaerobic digestion technology. This amount of biogas would have an energy value of about 1.7 trillion Btu, which could produce up to 13 average megawatts of electricity.
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| Landfill Gas |
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Anaerobic digestion of organic materials in landfills produces landfill gas. The rate of landfill gas generation varies depending on moisture content, temperature, the quantity of organic matter in the waste and the depth and density of the landfill. In general, production of landfill gas begins six months to two years after the waste is deposited. Production increases until the landfill closure and then declines rapidly, although landfill gas production can continue for 10 years.
The US Environmental Protection Agency´s Landfill Methane Outreach Program has identified five landfills in Oregon as "candidate" landfills for production of electricity from landfill gas. The EPA selected these candidate landfills based on national data sources rather than on-site evaluation. More detailed assessment would be needed to determine the economic feasibility of developing a power generating facility at any of the state´s landfills.
Based on EPA estimates of landfill gas available at candidate landfills, about 4,600 million cubic feet of landfill gas is potentially available on an annual basis. The energy value of this quantity of landfill gas is about 2.3 trillion Btu. The available landfill gas could produce up to 22 average megawatts of electricity.
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| Agricultural Residue |
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The harvest of field crops and grass seed generates a residue of straw, stalks and stubble. The average moisture content of agricultural residues is 15 percent and the average heat content is 7,500 Btu per pound. Bulk density of these wastes is low, ranging from 10 to 16 pounds per cubic foot. Due to the annual cycles of crop growth and harvest, the supply of agricultural residue is not constant throughout the year. After a few months of storage, agricultural residue will begin to decompose and lose energy content.
The primary consideration in agriculture is maintaining the productivity of the soil where crops are grown. After harvest, crop residue has a vital role to play in controlling erosion from wind and water and in deterring runoff. A 30-percent covering of crop residue can reduce soil erosion from water and wind by 50 to 75 percent. Soil type and variations in slope length and steepness affect the amount of residue needed for control of water erosion. Crop residue reincorporated into the soil helps maintain soil carbon and nutrients and improves soil tilth and porosity. For these reasons, the amount of agricultural residue available as a biomass energy resource is limited to the residue that is not needed to maintain soil productivity.
The amount of biomass economically available from agricultural residues cannot be estimated with much precision. There are many varieties of agricultural products grown in Oregon and annual variations in yield. Weather, soil type, fertility, topography and cultivation practices affect the quantity of agricultural residues generated and available. To approximate the magnitude of the resource, the Oregon Department of Energy estimated the available residue from winter wheat and grass seed production.
In 2003, approximately 1.5 million dry tons of agricultural residue was available from farming activities in Oregon. The energy content of this resource was about 27 trillion Btu. This amount of agricultural residue could be converted into approximately 213 average megawatts of electricity.
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| Summary of Biomass Energy Resources |
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Resources
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Quantity Available (2004)
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Energy Value (TBtu)
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Wood
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0.7 million bdt
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12
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96
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Pulping Liquor
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2.0 million bdt
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25
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57
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MSW
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1.3 million bdt
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18
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121
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Wastewater Treatment
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460 mcf
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0.3
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2
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Organic Waste Digesters
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3,400 mcf
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1.7
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13
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Landfill Gas
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4,600 mcf
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2.3
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22
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Agricultural Residue
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1.5 million bdt
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27
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213
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Total
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86.3
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524
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