Biomass fuels: Woody fuels

Bioenergy Processing Articles >> Biomass

1.1 Woody Fuels
Wood wastes of all types make excellent biomass fuels and can be used in a wide variety of biomass technologies. Combustion of woody fuels to generate steam or electricity is a proven technology and is the most common biomass-to-energy process.

Different types of woody fuels can typically be mixed together as a common fuel, although differing moisture content and chemical makeup can affect the overall conversion rate or efficiency of a biomass project. There are at least six subgroups of woody fuels. The differentiators between these subgroups mainly have to do with availability and cost. - Forestry residues—in-forest woody debris and slash from logging and forest management activities.

- Mill residues—byproducts such as sawdust, hog fuel, and wood chips from lumber mills, plywood manufacturing, and other wood processing facilities.
- Agricultural residues—byproducts of agricultural activities including crop wastes, vineyard and orchard prunings or turnings, and rejected agricultural products.
- Urban wood and yard wastes—residential organics collected by municipal programs or recycling centers and construction wood wastes.
- Dedicated biomass crops—trees, corn, oilseed rape, and other crops grown as dedicated feedstocks for a biomass project.
- Chemical recovery fuels (black liquor)—woody residues recovered out of the chemicals used to separate fiber for the pulp and paper industry.

1.1.1 Forestry Residues
Forestry residues have been the focus of many recent biomass studies and feasibility assessments due to increasing forest management and wildfire prevention activities under the National Fire Plan. The USDA Forest Service and the Bureau of Land Management have been tasked with reducing the hazardous fuel loading within the forests and the urban-wildland interface.

Forestry residues are typically disposed of by on-site (in-forest) stacking and burning. This results in substantial air emissions that affect not only the forest lands and nearby populations, but the overall regional air quality as well. Open burning can also cause water quality and erosion concerns.

The Forest Service and other public and private land management entities would like to have viable alternatives for disposing of their forestry residues in a more environmentally benign manner. An ideal situation, from the perspective of forest managers, would be the creation of a market for the forestry residues. The market they envision would generate revenues for the forest managers, which in turn would allow much needed expansion of the forest management programs.

In general terms, the quantity of forestry residues available throughout the Pacific Northwest is more than sufficient to support a biomass project. However, there are also significant barriers to using forestry residues as a biomass fuel. The barriers are due to variations in the supply cycle and to the cost of collecting and transporting the forestry residues.

Variations in forestry residue supplies are driven by the seasonal nature of forest activities, the trends in the commercial logging industry, and changing funding levels for forest management programs such as the National Fire Plan. The logging industry in the Pacific Northwest has experienced a decreasing trend for several decades, and although the timber harvest volume has stabilized in the past few years, there are no indications that it will ever recover to pre-1980 levels. The current focus on forest management initiatives aimed at reducing hazardous fuel loadings has recently increased the availability of residues, but this activity is subject to federal funding fluctuations and shifting political attitudes toward forest management. A major study for the U.S. Department of Energy (DOE) was undertaken in 2001 to quantify logging residue and agricultural residue supply curves for the Pacific Northwest.

A residue supply curve is a supply and demand curve for a biomass fuel. The study systematically mapped forest lands on a county by county basis, and then calculated incremental collection and transportation costs to get forestry residues from those areas to centralized biomass processing facility locations. This study has been an important benchmark in the industry for quantifying forestry residue availability and cost.

Even though Yakima County has heavily forested areas along the western part of the County, this comprehensive study shows the potential forestry residue volumes within Yakima County to be significantly lower than other locations throughout the Pacific Northwest. The study focused its analysis on the 29 counties with the highest timber harvest levels, and Yakima County did not meet the study’s minimum threshold for indepth analysis of forestry residue availability. This does not necessarily mean that insufficient forestry residues within Yakima County preclude a biomass project fueled by forestry residues, but it does indicate that Yakima County is not an optimum location for such a facility from a region-wide perspective.

The costs of recovering the forestry residues are very high. The DOE study calculated that the cost of forestry residue recovery (skidding, yarding, loading, and chipping) starts at $30 per bone-dry ton (BDT) and can increase to almost three times that much. Transportation costs further add to the total cost of the fuel, making the regional average cost of forestry residue fuels on the order of $60 to $80 per BDT or more for an area the size of Yakima County’s heavily forested areas. A recent study of woody fuels availability in the Prineville, Oregon area determined the cost of forestry residues would be $30 to $44 per BDT based on a relatively small 50-mile maximum radius from the proposed biomass project. A study underway for the Lake Tahoe Basin on the California-Nevada border has a preliminary calculation of $56 to $66 per BDT for a similar 50-mile maximum distance.

These costs for forestry residue fuels are prohibitive to a biomass-to-energy facility. The Tacoma Steam Plant lost money and ceased operation even though its wood waste primary fuel cost an average of less than $5 per BDT. The SEDI (Sustainable Energy Development, Inc.) Bioenergy Refinery project under development in La Grande, Oregon anticipates that it will be able to pay about $20 per BDT of delivered forestry residues, but it is also anticipating substantial subsidies for its wood-ethanol product, and a traditional biomass-to-electricity project would probably not be able to pay similar prices for feedstock. More information on these two projects is provided in Section 4 of this report.

1.1.2 Mill Residues
Mill residues are a much more economically attractive fuel than forestry residues, since the in-forest collection and chipping are already included as part of the commercial mill operations. Biomass facilities collocated with and integral to the mill operation have the advantage of eliminating transportation altogether and thus truly achieve a no-cost fuel.

Mill residues have long been used to generate steam and electricity in the Pacific Northwest. In Washington State alone, there are approximately 38 facilities that combust about 3 million BDT of mill residues per year to generate steam and electricity. All but two of these mill-residue-fired biomass projects are owned and operated by the mills or wood products companies that supply their fuel. The in-plant facilities primarily generate steam for lumber drying and processing. Any electricity produced is used to offset plant use, although a few facilities do sell excess electrical power to the local utility.

Yakima County has one such facility: the Boise Cascade Plywood Plant in Yakima. This biomass facility burns approximately 135,000 BDT of mill residues per year in a spreader stoker boiler to generate steam for the plant’s plywood kilns and dryers. It is unusual for a mill to offer its residues to an external biomass project, unless the mill has excess residues compared with its own energy consumption or unless it cannot process the residues themselves due to local environmental restrictions. One example of a mill residue biomass-to-energy facility not owned by a mill is Avista Utility’s Kettle Falls Station in northeastern Washington. The facility is strategically located within an average distance of 46 miles from 15 different mills, and purchases approximately 350,000 BDT per year of residues to generate 46 MW of electrical power. The facility was conceived in the late 1970s when mills were facing stricter pollution regulations that required them to replace their wigwam burners. Rather than invest in new equipment, the mills were willing to enter into long-term contracts with the private electric utility to supply a biomass facility with mill residues. The facility continues to operate successfully, due in large part to its unique location in one of the most heavily forested areas in the Pacific Northwest.

Because Yakima County does not have a significant mill or wood products industry other than the Boise Cascade plywood facility, there are not sufficient mill residues within a practical distance to support a new biomass facility in Yakima County.

1.1.3 Agricultural Residues
Agricultural residues can provide a substantial amount of biomass fuel. Similar to the way mill residues provide a significant portion of the overall biomass consumption in the Pacific Northwest, agricultural residues from sugar cane harvesting and processing provide a significant portion of the total biomass consumption in other parts of the world.

One significant issue with agricultural residues is the seasonal variation of the supply. Large residue volumes follow harvests, but residues throughout the rest of the year are minimal. Biomass facilities that depend significantly on agricultural residues must either be able to adjust output to follow the seasonal variation, or have the capacity to stockpile a significant amount of fuel.

The 2001 DOE study mentioned previously identified 34 counties in the region with significant agricultural industries. Yakima County ranked 31st in this list, with an estimated agricultural residue availability of 62,300 tons per year. For comparison, the average across the 34 counties was 236,500 tons per year, with Whitman County ranking number 1 on the list at 1,173,850 tons per year. The study also identified 13 optimum biomass conversion sites based on the overall distribution of agricultural residue biomass throughout the region. With the exception of The Dalles, Oregon, all of the optimum sites are east of Yakima County. The optimum sites closest to Yakima County were Moses Lake and Richland.

The DOE study primarily focused on in-field residues remaining after harvests of wheat and barley. The study calculated an average minimum cost of about $40 per dry ton to collect and transport the wastes instead of burning or tilling. However, in Yakima County, turnings and prunings of orchards and vineyards make up a substantial portion of the agricultural residue. Unlike wheat and barley, these orchard and vineyard residues must be removed from the field, and the cost of collection and transportation of the residues is already part of the orchard and vineyard operation costs. It is possible that a biomass project in Yakima County could charge a tipping fee for these types of agricultural residues.

Nevertheless, the statistics from the DOE study indicate that Yakima County does not produce enough agricultural residues on its own to support a biomass project fueled entirely on agricultural residues, nor is it located centrally enough to draw residues from a wider geographical area. Agricultural residues could be a supplemental feedstock for a Yakima County biomass project, but not primary fuel.

1.1.4 Urban Wood and Yard Wastes
Urban wood and yard wastes are similar in nature to agricultural residues in many regards. A biomass facility will rarely need to purchase urban wood and yard wastes, and most likely can charge a tipping fee to accept the fuel. Yakima County is already Section 1 10 R. W. Beck Biomass Report.doc 11/6/03 collecting “clean green” wastes at its landfills at a tipping fee of $11.60 per ton, and reselling it or using it for landfill daily cover. This waste could be diverted to a biomass project, and although the volume currently accepted at the landfills would not be enough on its own to fuel a biomass project, it could be an important supplemental fuel and could provide more value to the County through a biomass project than it currently does as daily landfill cover.

1.1.5 Dedicated Biomass Crops
Dedicated biomass crops are grown specifically to fuel a biomass project. The most prevalent example of dedicated biomass crops are corn varieties grown for ethanol production. Fast-growing poplar trees have also been farm-raised for a biomass fuel, but this has not proven to be economically sustainable. Another dedicated crop example is soybean oils used in the production of biodiesel. Because these crops are created intentionally (and thus are not a waste product from another industry) this report does not evaluate the feasibility of using dedicated biomass crops as a fuel source.

1.1.6 Chemical Recovery Fuels
Chemical recovery fuels are responsible for over 60 percent of the total biomass energy consumption of the United States, and therefore must be mentioned in any analysis of biomass. However, the chemical recovery facilities are owned by pulp and paper facilities and are an integral part of the facility operation. Therefore, although this is an important fuel within the overall biomass industry, it does not have application for Yakima County.

by Yakima County Public Works, Solid Waste Division
From 'Review of Biomass Fuels and Technologies', 2003

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