Authored By: C. Mayfield, J. Gan
The primary production areas and manufacturing facilities of the forest sector provide many different sources of forest biomass that can be utilized for bioenergy and bio-based products. These sources include logging residues, mill residues, thinnings (traditional forest management and fuel treatments), stands damaged by natural disturbances (fire, windstorm, pest outbreaks, etc.), energy plantations, stands degraded by poor logging practices, and urban wood wastes. The quantity of forest biomass is usually quantified in two ways, the amount of available biomass and the amount of recoverable biomass. For the purpose of this discussion, biomass supply means the quantity of recoverable biomass unless otherwise specified.
Logging residues constitute one of the largest sources of forest biomass that have not been utilized in the United States. Logging residues - the unused portions of trees cut or killed by logging and left in the woods (Helms 1998) - consist of tops, branches, and downed dead and cull trees. Logging residues amount to 19% (in volume) of total annual removals from the US forest inventory. Based on the 2002 Forest Inventory Analysis (FIA) data (Smith and others 2004), there are approximately 60 million dry tons of logging residues annually available at harvest sites in the U. S. Taking into account the recovery limitations imposed by accessibility and loss during procurement, several studies (Walsh and others 2000; Perlack and others 2005; Gan and Smith 2006) consistently reveal that some 40 million dry tons of logging residues can be recovered annually in the U. S., over 90% of which are from privately owned lands.
Another source of forest biomass is fuel treatment thinnings. Increasing threats of forest fires in recent years have brought attention to the growing problem of hazardous fuel buildup in the forests. The National Fire Plan of 2000 and the Healthy Forest Restoration Act of 2003 were designed to address hazardous fuels reduction and increased utilization of the material for beneficial purposes. Approximately 8.4 billion dry tons of treatable biomass have been identified nationwide, while only a fraction of this biomass is considered available. Considering accessibility, economic feasibility, and recoverability greatly reduces its supply to about 60 million dry tons per year. This amount would come from both public and private lands with some 60% from private lands (Perlack and others 2005). In addition to fuel treatments, thinnings for the purpose of timber production generate biomass as well. Yet, there is no reliable estimate of this potential biomass supply.
Biomass from the trees damaged by natural disasters/disturbances can also be used to produce bioenergy and bio-based products. The natural disturbances that occur most often in the southern U.S. include hurricanes, tornados, wildfires, and pest and disease outbreaks. In 1989, Hurricane Hugo damaged 2.4 billion dry tons of timber (Haymond and others 1996). In 2005, Hurricanes Katrina and Rita damaged over 2.5 billion dry tons of timber along the Gulf Coast (USDA Forest Service 2005; Texas Forest Service 2005). The amount of biomass recoverable from these hurricane-damaged forests, though not accurately estimated, should be quite significant because restoring the damaged forests tends to generate more residues than regular timber harvesting. In the South, the southern pine beetle (Dendroctonus frontalis Zimmermann) also causes significant damage to pine forests. Coulson and others (2005) estimated that on average 1.36 million tons of biomass were killed by the pest annually in eleven southern states. About half of this amount was not salvaged and could be used for bioenergy. Whereas the quantity of biomass generated by natural disturbances is quite large, it varies tremendously over time and space. Such variations may hinder its utilization.
A significant supply of woody biomass can also come from energy plantations--short rotation woody crops (SRWC)--such as willows and poplars. The estimates of biomass supply from energy plantations vary tremendously with different estimation methods and assumptions on land availability, demand for grains and fiber, and biomass yield, among other things. According to Perlack and others (2005), about 5 million dry tons of biomass from SRWC plantations could be available for bioenergy production annually in the U.S. A study by Alig and others (2000) predicted that SRWC plantations would not become significant until 2040 in the South. The availability of smaller trees for pulp fiber and projected large investments in southern softwood production reduced the attractiveness of SRWC in the region. However, Walsh and others (2000) estimated that 188 million dry tons of biomass could be supplied from energy plantations at a delivered price of $50 per dry ton. Obviously, further studies are needed in this area to provide a consensus estimate. The amount shown in the figure entitled "Forest Biomass Resources in the United States" is based on the more conservative estimate by Perlack and others (2005).
Mill residues are one of the most readily available sources of forest biomass. Mill residues come from three primary sources including primary wood processing mills, secondary wood processing mills, and pulp and paper mills. This biomass source is extremely desirable for use in bioenergy and bio-based products applications. It is clean, uniform, concentrated, low in moisture, and close to utilization facilities, thus requiring minimum further processing efforts and representing a cost-effective source of forest biomass for energy. Because of these desirable qualities, most (about 97%) of the primary wood processing mill residues are currently being utilized (Perlack and others 2005). Thus, we do not anticipate that there would be additional, significant amounts of mill residues available for new bioenergy production.
Finally, urban residues include wood and yard waste and construction and demolition debris. Construction scraps are usually very high quality biomass whereas demolition materials are typically contaminated unless highly processed. It is generally perceived that there is about 20% wood in the urban waste stream. There are very limited data on the availability of urban residues. According to Perlack and others (2005), a total of 28 million dry tons could possibly be collected from urban sources. Yet, urban residues may face competing uses such as mulch, and the quality of demolition debris may hinder their utilization.