Turning renewable forestry biomass into bioenergy.

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.

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.

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.

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.

Thermal energy includes the heating of buildings, heating of water, and heating that is required for certain manufacturing processes. It also includes the production of heat for drying materials, such as lumber, paper, clay, and other products. The heating is produced by direct combustion within boilers, gasifiers and furnaces. Steam boilers and hot water boiler systems fired by wood chips or bark are often used. Wood pellets can be used in furnaces, heaters, and in the larger boiler systems.

Electricity can be generated through the use of steam turbines powered by steam from boilers using wood chips, bark, or wood pellets as fuel. Cogeneration is achieved by producing electricity in this manner while also using the steam for a thermal energy need. The wood fuel can also be combusted within gasification systems to produce a synthetic gas, similar to natural gas. The synthetic gas is then combusted to produce steam and electricity, or the gas can be used directly in a gas turbine for electricity production.

Ethanol, methanol, butanol, and BTL diesel are all liquid fuels that can be produced from cellulosic biomass. The two general conversion technologies used are enzymatic hydrolysis and thermo chemical. Clean wood chips can be converted to alcohol fuels (ethanol, etc.) using enzymatic hydrolysis. Biomass-to-liquid diesel and alcohol fuels can be produced through thermo chemical conversion of any forest biomass material. Gasification and pyrolosis can be used within the thermo chemical conversion methods.

Although wood pellets are a product, they are actually a fuel that can be used to produce the energy types above. They are most easily produced from sawdust and other mill residues. However, small diameter trees can also be chipped and utilized for wood pellets. Wood pellets are produced using pellet presses that force the sawdust through holes in a die using rollers. The natural resins in the wood provide the “glue’ necessary to hold the pellets together. Wood moisture content must be in the 10-12% range, so drying is often required during the process. Wood pellets can be used in pellet heaters, furnaces, and boilers. The major use of wood pellets is by European-based utilities that co-fire wood pellets with coal to produce electricity. Large quantities of pellets are also used in residential heaters and furnaces in Europe, Canada, and the northern U.S. Georgia has the potential to use pellets within several industries, such as the poultry industry to provide heat to poultry houses. Pellets have the advantages of being uniform for ease of fuel handling (conveying), dense for ease of transport, and low in moisture to produce more energy.