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. 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 addition to wood residues generated in mill operations, pulp and paper mills also generate black liquor as a by-product of the kraft pulping process.
Most of the black liquor produced in pulp and paper mills is used by the paper mill to produce the heat, steam, and electricity used to run the mill. Excess electricity is sold in the electricity markets. The use of black liquor for the production of energy constitutes the largest component of existing bioenergy production (DOE, 2006).
The Forest and Rangeland Renewable Resources Planning Act (RPA) of 1974 directs the Forest Service to conduct a study every 10 years that projects the future cost and availability of timber products, evaluates emerging issues, and examines impacts of policy options. According to this analysis, demand for wood at U.S. paper and pulp mills is projected to decline in the near term (2010) and then increase by 25 percent by 2050 relative to 1996 levels (table 1) (Haynes, 2003). Black liquor yields are equal to 40-50% of the wood used in bleached pulping processes and 50-65% of the wood used in unbleached processes. The average moisture content of black liquor is 25-35% water (General Bioenergy Inc., 2005).
Perlack (2005) estimates that current black liquor generation in the paper and pulp industry is 52.1 million dry tons and that black liquor quantities will increase by about 30 percent by mid-century.