The U.S. forestry industry, in the process of harvesting and converting wood into consumer products, generates a number of residue and waste materials that could be used for bioenergy and bioproducts. Forest industry resources include those resulting from operations within forest and timberland areas, as well as materials generated in the conversion of wood into intermediate and final products such a lumber, paper, and furniture (i.e., mill residues). Primary mills are those that convert roundwood products (i.e., tree trunks and logs) into other wood products and include sawmills that produce lumber, pulp mills, veneer mills, etc. In the process of converting trees into wood products, waste residues are generated consisting of bark, fine wood residues, and coarse wood residues.
The USDA Forest Service Forest Inventory and Analysis Program (FIA) is charged with tracking and reporting on the fate of wood that is harvested from U.S. forests. The FIA collects mill data through surveys and questionnaires sent to processors and harvesters. Data on mill residues are contained in the Timber Product Output (TPO) database and are available on-line at http://www.fia.fs.fed.us. Primary mill residues are classified as bark, coarse wood residues suitable for chipping (i.e., slabs, edgings, veneer cores, etc.), and fine wood residues (i.e., planar shavings, sawdust) not suitable for chipping due to their small particle size and the large proportion of fibers that are cut or broken. End use categories include fuel, fiber uses, and other uses (e.g., mulch and bedding). Bark is primarily used as a hog fuel and increasingly for mulch. Fine wood residues are used mostly to produce particleboard and for bedding. Coarse residues are used to produce a variety of products including pulp for paper and cardboard, and engineered wood products such as fiberboard, oriented strandboard, medium density fiberboard, and other fiber uses. An estimated 86.7 million dry tons of primary mill residues were produced in RPA survey year 2007, but because the residues are relatively clean and concentrated at the site of production, all but 1.3 million dry tons were used to produce energy or other products (table 1). The geographic distribution of residues generated in 2007 is shown in figure 1.
The FIA data form the basis of the estimated available forest resource quantities found or used in a number of studies including Perlack, 2005; Southern States Energy Board, 2006; Western Governors’ Association, 2006; Encyclopedia of Southern Bioenergy Resources, 2006; and Walsh, 2006. Independent assessments of mill residue quantities are generally for small geographic areas and are based on surveys of local producers (e.g., Buehlmann, 2001). Walsh (2000) estimated that 41.5 million dry tons of primary mill residues could potentially be available at a delivered price of $30/dry ton. Given the extensive use of primary mill residues, most studies assume that the only quantities available for bioenergy and bioproducts are those not already used.
Walsh (2007) assumes that residues currently used to produce products are still available for bioenergy uses if the price offered is sufficiently high (table 2). Current quantities are based on the TPO database supplemented with data from state reports to delineate residue sources (i.e., hardwood and softwood) and to further refine their uses. Future year quantities are based on projected timber harvests contained in the USDA Forest Service Resource Policy Act (RPA) assessment (Haynes, 2003; Haynes, 2007). The minimum prices that must be paid for the mill residue to attract it away from its existing use is approximated as the value of the wood in existing uses, additional processing costs, and disposal costs of materials of insufficient quality to be used. The analysis does not account for structural rigidities, and the analysis is static and does not account for price adjustments that will occur as residues shift to alternative uses (e.g., mill residues serve as a significant source of wood material for pulp mills and particleboard plants, and these facilities have historically been aggressive in bidding up the price for wood residues when competition exists). Prices substantially higher than break-even prices may be needed to provide sufficient incentive to fully attract the resource away from its current use.
