Residues from forest products manufacturing such as sawdust, black liquor, and bark are commonly used to create on-site energy in the form of heat and power. Char, pellets, particleboard, nonstructural panels, and animal bedding are also derived from wood processing residues. These residues come from primary and secondary wood processing mills and pulp and paper mills. This type of biomass feedstock is highly desirable because it tends to be clean, concentrated, uniform, and low in moisture, and requires little or no transportation. Currently, about 97 percent of this resource is utilized (USDOE and USDA 2005).
Wood residue created when a log is cut to make lumber. It is fairly uniform in size and shape and is commonly referred to as wood flour, which indicates the particles can pass through a 20-gauge mesh screen. Sawdust with high moisture content has relatively limited uses. It can be used for residential heating in special sawdust furnaces as well as for smoking meats. When dried, sawdust is typically either densified into pellets or directly gasified, combusted, or pyrolized (made into an oil) to generate electricity, heat, and oil.
Black liquor is a recycled byproduct formed during the pulping of wood in the paper-making process. More specifically, it is the substance that remains after cellulose fibers have been broken down and removed from the original chemical slurry (a thick mixture of solids and liquid) to form paper. It consists of lignin, water, and other chemicals used in the extraction process. It is an important liquid fuel in the pulp and paper industry, typically recovered and recycled either through combustion or gasification in on-site boilers or gasifiers. The results of these processes are heat energy, carbon dioxide, and recoverable chemicals. The steam that is generated during the black liquor recovery process contributes significantly to the energy needs of pulp and paper mills. Recovered chemicals can be recycled into white liquor, which is the original slurry of chemicals used in the pulping process, reducing the pulping process’s chemical needs by almost 90 percent.
Bark is the outermost part of woody stems and branches and makes up about 9 to 15 percent of a log’s volume. Bark is used to produce tannins, dyes, resins, flavorings, and medicinal products, and other chemical extracts. Bark is also used as mulch, soil amendment, a fuel source. Approximately 10 tons of bone-dry bark is the equivalent of 7 tons of coal when used for energy. In addition, bark is used in building materials such as fiber and particleboard as well as insulation board because it conducts heat less readily than wood.