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, paper and pulp mills, and veneer mills.
The USDA Forest Service, Forest Inventory and Analysis Program (FIA) tracks the wood harvested from U.S. forests, and collects mill data (http://www.fia.fs.fed.us) through surveys and questionnaires sent to processors and harvesters. 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 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. Only one study estimates secondary mill residue quantities (12.5 million dry tons generated, but only 1.2 million dry tons available due to contamination, inaccessibility, etc.). In addition to wood residues generated in mill operations, pulp and paper mills also generate black liquor (about 52 million dry tons) as a by-product of the kraft pulping process. Black liquor is currently used by paper and pulp mills to produce heat, steam, and electricity and is the single largest source of bioenergy produced today.