Types of Forest Biomass
Merchantable stem wood
The wood in the stems of sound trees greater than 5" in diameter at breast height (dbh) has been traditionally used for lumber, wood panels, paper, and chemical products. Trees with dbh of 5" - 9" are typically used for paper and chemical products and are lower in value. Thinning of forest stands is a major source of these smaller trees. Trees greater than 9" dbh can be used for lumber and other building products, and are higher in value. Defects in trees sometimes will cause larger diameter trees to be used for lower-value products. These trees are termed "cull trees." Much of Georgia's 7 million acres of planted timberland and 17 million acres of naturally regenerated timberland is managed for these "traditional" products. The stem wood in lower value small diameter trees will be used by some bioenergy facilities for energy production. The higher cost of wood from larger diameter trees will limit their use to higher value products, such as lumber and plywood.
Tops and branches of harvested trees
The normal whole-tree timber harvesting methods used in Georgia result in large volumes of wasted wood and bark in the tops and branches of trees harvested for their stem wood. These tops and branches are often referred to as logging residues. They are left in piles at the log loading decks or they are scattered out across a timber harvest area, at additional cost. Piles are often burned as a disposal means. Although some branches and tops are broken and scattered during the harvesting operation, much of them could be chipped and used for bioenergy operations.
Understory and Cull Trees
Trees less than 5" dbh and other cull trees can be harvested during normal harvesting or thinning operations and used for bioenergy. These smaller trees do not meet the standard for higher-value products and are normally disposed of at an additional expense to the landowner, when preparing a forest site for reforestation. These trees must be harvested during a traditional harvest and not at a separate time, to take advantage of the cost efficiencies. In addition, cost of harvesting volumes of biomass is inversely related to tree size. Therefore, the portion of the understory in very small trees and shrubs cannot be harvested with traditional forestry equipment because of high costs.
Naturally regenerated forest stands of southern pine species are often overly dense, sometimes with thousands of trees growing on each acre. A thinning operation that results in a forest stand of 500 - 700 trees per acre and less understory shrubs will improve residual tree growth and reduce the hazard of damage from wildfire. It is best to perform these thinnings at early ages, usually when the forest stand is less than 10 years old. The small removed trees and underbrush can be used for bioenergy. These thinnings can be done in a variety of ways and with a variety of equipment types. Traditional forestry equipment can be used in conjunction with a chipper, if trees and shrubs to be removed are not too small. The development of various types of biomass baling systems is also occurring to reduce transportation costs and improve efficiencies for smaller understory harvests and forest improvement operations.
Mills within the forest industry produce residues as a by-product of their manufacturing process. These residues include: wood chips, bark, sawdust, shavings, and other coarse wood pieces. Almost 100% of these residues are currently used to produce additional products, including 49% going to energy products.
Urban Wood Waste
There are several sources of waste wood from metropolitan and other developing areas. These sources include construction waste, wood from building demolition, disposal of wooden pallets, and others. These materials are usually low in moisture content and provide an excellent fuel for bioenergy combustion systems. Use of these materials also prevent their disposal in landfills. Care should be taken to ensure that wood treated with chemicals and hazardous materials are not utilized.