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 (logging residues, other removals, fuelwood, and fuel treatment wood), 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).
Logging residues are the portions of harvested trees that are not utilized and that are left in the woods after operations. The USDA Forest Service, Forest Inventory and Analysis Program estimated that 4.51 billion ft3 (~ 63 million dry tons) of logging residues were generated in 2007 (~ 44% from softwood tree species; ~ 8% from national forests and other public lands).
Other removals are the unutilized wood volume from cut or otherwise killed growing stock, from cultural operations such as pre-commercial thinning, or from timberland diversion to other uses such as to crop land, roads, urban development, parks, etc. About 1.66 billion ft3 (~ 24.7 million dry tons) of other removals were generated in 2007 (~ 26% from softwood tree species; ~ 9% from National Forests and other public lands).
Fuelwood is harvested wood used for residential heating and industrial heat and electricity generation. An estimated 35.4 million dry tons are used for these purposes.
Mechanical thinning of overcrowded forests to remove diseased and dead materials (fuel treatment wood) and reduce the risk of forest fires could potentially supply several million dry tons of wood material. Between 100 and 200 million acres of forest lands are estimated to be at high risk for catastrophic wildfires due to overcrowded conditions. Removal costs can be high however.
Potential changes in the management of pine plantations in response to changes in the existing structure of paper and pulp production in the U.S. is also suggested as a potential source of forest wood material.
Mill residues include primary and secondary mill wastes. Primary mills convert roundwood products (e.g., tree trunks and logs) into other wood products and include sawmills that produce lumber, paper and pulp mills, and veneer mills. Secondary mills utilize the products from primary mills to produce other products (e.g., convert board lumber into furniture, cabinets, etc.). In the process of converting trees into wood products, waste residues are generated consisting of bark, fine wood residues, and coarse wood residues. Of the estimated 86.7 million dry tons of primary mill residues produced in 2007, most (~ 98%) are currently used to produce bioenergy, fiber, or other products. An estimated 12.5 million dry tons of secondary mill wood residues are generated, but only 1.2 million dry tons could be available for bioenergy after correcting for contamination and ability to recover the material. 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.
Few studies attempt to estimate supply curves (i.e., quantities available as a function of price) for forest and mill residues either for current or future time frames. Estimated quantities (combined logging residues, other removals, primary mill residues) in 2010, at breakeven-prices of $50/dry ton and $100/dry ton are 49.2 and 60.0 million dry tons respectively, and are 53.2 and 64.7 dry tons respectively in 2025. The distribution of forest industry residues (combined logging residues, other removals, and primary mill residues) for 2007 is shown in figure 1.