Forest Residue
Ratings Summary Table:
What is Forest Residue?
Forest harvest and management generates large amounts of low value wood material, including sawdust, chips, bark, branches, and other forms of woody material that is "left over" after the forest management activity or harvest have occurred. While a portion of that material is useful within the forest ecosystem, much of the material can be utilized for energy or other purposes. Many farms include woodlots or other forested areas that have potential as be a source of marketable woody material.
How is it produced?
Most farm woodlots consist of a mix of species that naturally regenerate from the existing trees. Some woodlots are carefully managed, but most would benefit from selective harvests to remove undesirable species and encourage growth of trees that are economically and ecologically beneficial. Some farms also utilize treelines between fields as a windbreak, and these treelines also tend to vary widely in level of management and productivity. Some farms may produce fast growing woody crops as a "short rotation woody crop (SRWC)" - see the SRWC page of this site for more information.
Cordwood is a common biomass fuel that can be produced and used on the farm (Photo D Ciolkosz, Penn State).
How is it harvested?
Woody material from forest operations can be produced in several ways, at several points in the supply chain. In the woodlot, cordwood can be cut for splitting and drying, or whole tree chips can be created - usually using a "flail debarker" to remove most of the bark prior to chipping. Bark-rich "flail debris" are also created as part of this process a mulch. "Hog fuel" is another woody product, created by pulverizing the wood into more of a shredded form, rather than cutting it into chips. Often the term "dirty chip" is used to refer to wood chips that include bark, whereas a "clean chip" is one without bark (soil and other debris from a forest harvest often stick to the bark). At the sawmill, bark is separated from the logs, and either "slabwood" or mill chips are created when the curved portion of the log is trimmed away prior to sawing. Sawdust is created when the logs are cut into boards, and if the facility produces any finished wood, then sander dust or wood flour (which is much finer than sawdust), is also created.
How is it Used?
Energy uses for woody material include cordwood for wood stoves, and wood chips for combustion. Cordwood is typically used at the residential scale, whereas chips are the more common fuel type for commercial-scale or utility-scale combustion systems. Cordwood is usually supplied cut to size, split, and either "seasoned" (air dried to ~20% moisture content) or kiln dried to 10% moisture or less. Chips are usually sold as "green chips" (recently harvested, with moisture in the 45-50% range) - dried chips are not as common.
Logs, chips or sawdust can also be supplied to a pellet mill, where they are ground and then extruded into wood pellet fuel. Wood for energy is expected to meet a performance specification for its end use. While there are many potential ways to measure wood properties, the most important performance characteristics for energy uses are moisture content, energy content, size, bulk density and ash content. Cordwood is typically sold by the "cord" - a unit of measure equivalent to 128 cubic feet (about 11.5 cubic meters), whereas chips are sold by the ton.
Liquid biofuel production from wood is technically possible, but a commercial market has yet to emerge.
Non-energy uses for woody material are extremely varied. Shavings and sawdust may be reground into wood flours, or the wood flour may be recovered as sized “dust” materials that have been screened and separated. Wood flour has major industrial markets in industrial fillers, binders and extenders in industrial products like epoxy resins, fertilizers, adhesives, absorbent materials, felt roofing, inert explosive components, ceramics, floor tiles, cleaning products, wood fillers, caulks and putties, soil extenders and a vast array of plastics. Some wood flours like mesquite may be used in edible flavorings for human or pet consumption.
Shavings and sawdust can be marketed for use in molded or laminated composite wood products (e.g., toilet seats, countertops) in automotive materials and in oil and water isolation and solidification products for the environmental control industry. Other uses include fillers, bulk shavings, sawdust, hog fuel (dried bark shavings), meat-smoking chips, barbeque cooking fuels and composite fireplace logs. Landscaping applications include playground “footing,” equestrian arena and other “wood edge footing” (safety margin and walkway material) and some exhibit and tradeshow applications. A few manufacturers are using post-consumer plastic waste mixed with a sawdust extender to make high-value extruded composite decking lumber and similar products for the home improvement market.
Currently, a primary use of baled dry shavings is for equine and livestock bedding or small pet bedding applications. This usually takes the form of aromatic material from eastern red cedar, soft ponderosa pine, chlorophyll pine and aspen. In some cases, these wood products are mixed with “non-wood” agricultural residues like corncobs. These products may be prepared for specific pets such as dogs, cats, gerbils and hamsters or for other small animals. Related products may include litter box liners, all natural wipes, and odor and stain eliminators.
One hybrid "energy plus material" use for woody material is biochar. The biochar manufacturing process generates a significant amount of heat as the wood is transformed into biochar, and the use of that heat for energy is an important part of a biochar production facility's overall efficiency. The biochar can be used for a wide variety of industrial and commercial products, as well as being used as a soil amendment on degraded and low quality land.
Market opportunities
At this time and in the near future, wood wastes are and probably will be the most commonly used biomass fuel for heat and/or power.
When compared to grasses and crop residues, woody biomass is usually available at lower cost, has higher density, is more easily transported and handled, and produces less ash. The most economic sources of wood fuels are usually wood residues from manufacturers (mill residues), discarded wood products or woody yard trimmings diverted from landfills, and non-hazardous wood debris from construction and demolition activities. A significant environmental benefit of using these materials for generating electricity is that their energy value is utilized while landfill disposal is avoided. As long as clean-burning combustion technologies are employed, carbon emissions to the atmosphere can be minimized.
Market opportunities for woody materials usually depend on the availability of local buyers of this material. Cutting and splitting cordwood for direct sale to local customers is one common way that farmers can utilize their woodlots to generate biomass for sale. However, the generally small size of most farms' woodlots makes it difficult for farmers to generate wood chips for the larger commercial, industrial and utility scale markets. Instead, farmers are more likely to find opportunities by contacting local forestry companies, who are often in a position to serve as a "broker" of sorts. They will provide a "stumpage" payment to the farmer for any wood removed from their site and re-sell the material to the end user.
When compared to traditional energy sources, some of the attractive features of woody biomass for energy are that its cost tends to be lower than that of many fossil fuels, and its price tends to be stable from year to year (as opposed to the wide fluctuations that are common in the fossil fuel market). The "local" and "renewable" attributes of wood fuel are often attractive to end users, and the carbon benefits of wood fuel are giving it more value in the eyes of many. On the negative side, cordwood and chips require significant handling and storage space, and in some communities that are not acquainted with sustainable forest management practices, there may be a perception that cutting a tree is morally "bad".
The market for non-lumber woody materials tends to vary widely from year to year, as the lumber sector swings from boom to bust to boom again, creating a glut or shortage of woody residues. Thus, farmers can often benefit from monitoring the market and only harvesting their woodlots in years when the market is strong.
Large, commercial scale woodchip boilers can be used to heat greenhouses, businesses, and schools (Photo: D Ciolkosz, Penn State)
Links
Wood Biomass for Energy, Techline, Forest Product Laboratory, 2004.
Forest Biomass & Wood Wastes, Energy Efficiency & Renewable Energy, U.S. Department of Energy (DOE).
U.S. Billion-Ton Update. Biomass Supply for a Bioenergy & Bioproducts Industry, Energy Efficiency & Renewable Energy, U.S. DOE.
Wood Energy Markets - 2010-2011, UNECE/FAO Forest Products Annual Market Review.
PA Fuels for Schools and Communities – Resources for woody biomass energy production and utilization
Wisconsin State Wood Energy Team – Resources for woody biomass energy production and utilization
Minnesota State Wood Energy Team – Resources for woody biomass energy production and utilization
Michigan Wood Energy – Resources for woody biomass energy production and utilization
Vermont Wood Energy – Resources for woody biomass energy production and utilization
Colorado Wood Energy Program – Resources for woody biomass energy production and utilization
Alliance for Green Heat – Resource for wood heat: cordwood and pellets, with a focus on high efficiency, low emissions technologies.
Topics:
> Corn Stover as a biomass product
> Short Rotation Woody Crops
Bio-Based Energy
