6. Biomass: Wood

B. Wood Resources

Timberland Harvests

Before its colonization, the land now occupied by the United States included more than 1 billion acres of forests. By 1907, U.S. forests had been reduced to 759 million acres. In the 1920s and 1930s, forested land area began to stabilize, and it has remained relatively constant since then, totaling 737 million acres in 1992. In the case of fuelwood, no statistical linkage can be made between available resources (supply) and actual consumption. Very little information is available on fuelwood supply. Nevertheless, this chapter provides a qualitative profile and rough measure of fuelwood resources, with the caveat that the resulting data are only approximate. Consumption data (see Chapter 3) are more reliable, and some survey-supported numbers are available.

Analysis readily reveals the underlying difficulty of quantifying fuelwood supplies. The forest products industry is made up of thousands of suppliers and thousands of consumers■sometimes even within a given State. While some operators are large, many are very small, scattered, and autonomous. Most of the statistical resource data used here are either provided by or derived from forest resource assessment data published by the U.S. Forest Service. The most recent assessment by the Forest Service was for statistical year 1991.(3)

U.S. forest acreage currently represents 7 percent of the world's forests.(4) U.S. timberland contains the equivalent of about 858 billion cubic feet of roundwood,(5) which is timber stock that typically consists of 92 percent live, sound trees (called ■growing stock■) and 8 percent rotten, cull, or "salvable" dead trees (Table 19). While a percentage of rotten, cull, or salvable dead timber may be suitable for lumber or veneer logs (which are used mainly in plywood manufacturing), most is used for pulp, fuel, and products that require only low-quality wood.

According to the last comprehensive Forest Service estimate, for statistical year 1991, roundwood harvested from U.S. timberland totaled 17.9 billion cubic feet. This set of Forest Service statistics specifically reports only fuelwood of roundwood class that was commercially harvested for use by primary wood-using mills(6) or for fuel. About 18 percent of this quantity "about 62 million tons or 3.2 billion cubic feet" was used as fuelwood (Table 20), with an equivalent energy value of 871 trillion Btu (Table 21). Almost three times more nongrowing roundwood stock than growing roundwood stock was used for fuel, because, under prevailing market conditions, higher quality logs are used for value-added products. Some 28 percent of the roundwood harvest, or about 5 billion cubic feet, was used as pulpwood. Pulpwood is an important resource category of industrial wood energy, because about 30 percent of the volume of pulpwood consumed in the kraft pulp and paper manufacturing process is recoverable for energy in the form of black liquor. A rough approximation of the energy recovered from roundwood pulpwood in 1991 is about 370 trillion Btu. Roundwood is only one source of pulpwood and this estimate does not include energy recovered from both forest and nonforest residues that are used for pulpwood.

Other important wood resource categories reported by the Forest Service are the salvable wood from (1) living and dead stock cut or knocked down and left at the harvest site, (2) cull trees, (3) growing stock tops consisting of wood less than 4 inches in diameter, (4) growing stock trees less than 5 inches in diameter, and (5) growing stock removed from forestland by cultural operations or timberland clearing.(7) The Forest Service reports these categories as logging residues and other removals. Table 22 indicates that total wood supply available from these sources in 1991 was about 5 billion cubic feet. While there are few hard statistics that shed any light on the disposition of these resources, anecdotal evidence indicates that most of them are used for industrial boiler fuel, pulpwood, residential fuelwood, compost, mulch, and animal bedding. If wood chips used for pulpwood and the resulting black liquor byproduct are also considered, an energy value of at least 600 trillion Btu is represented by logging residues and other removals. Stumpage prices vary regionally and may fluctuate by as much as 40 percent over the short term, even though demand remains flat over the long term.(8) Fuelwood normally has a lower value than saw timber, veneer wood, or pulpwood, and its price varies according to available wood supply and demand for such higher valued commodities.

Nonforest Residues

Private Clearing and Silviculture

Wood is salvaged for fuel by private owners from woodlots, farm fence rows, cropland clearing, orchards, and various operations of private urban silviculture. Comprehensive information on the disposition of wood from these sources is not available, while anecdotal evidence indicates that this wood represents an important component of residential fuelwood supply and is also commonly chipped for boiler fuel or pulpwood.(9) A Forest Service study of residential fuelwood use in Michigan revealed that rural woodlands in heavily forested areas supplied more than 94 percent of the residential firewood for those areas. In addition, 84 percent of the firewood supply of a more heavily populated region of the State was cut locally within that region. While general conclusions cannot be drawn from evidence of this type, it does appear that cutting from private land and various types of urban clearing are probably more important sources of residential firewood than cutting by commercial harvesters on forestland. The Michigan study indicated that production of firewood by households was 20 times greater than production by commercial harvesters.(10)

Urban Tree and Landscape Residues

Urban tree and landscape residues consist of tree limbs, tops, brush, leaves, stumps, and grass clippings. They are generated by commercial tree care firms, municipal tree trimming operations, electric utility power line maintenance departments, municipal park and recreation departments, orchards, and landscapers. These residues, unrecovered, make up about 18 percent of the municipal solid waste stream and represent a serious disposal problem for landfills.(11) Alternative uses include processing for mulch, compost, wood products such as animal litter and bedding, and fuel. It has been estimated that about 200 million cubic yards a year are produced by all the sources generating this category. Firewood and boiler fuel (as products) and wood burned for energy by the producer of the residue are recovered from this resource. Total recovered energy from urban tree and landscape residues amounts to about 45 trillion Btu, or the equivalent of about 7 million barrels of oil per year.(12) Additionally, recovery of these residues for fuel and other products avoids the economic and environmental costs to society of landfilling.

Waste Wood

Waste from Primary and Secondary Wood Mills

Primary mills include sawmills, veneer mills, and pulp mills. Secondary mills include manufacturers of dimension lumber, trusses and building components, flooring, windows and doors, cabinets, pallets, poles and fencing, barrels, boats, highway transport trailers, manufactured homes, musical instruments, etc. Waste products include chips, slabs, edges, sawdust, and planer shavings. Because many of the mill waste residues are clean■in contrast with such wastes as construction and demolition debris or treated lumber■they are often a source of the wood chips used by pulp and paper mills.(13)

Mill waste residues are used primarily as fuel, with the next most important application being use as pulp and fiber for making paper products. The paper production process yields a byproduct known as "black liquor," which is also a source of energy. Pulp and paper mills use large quantities of wood bark, edgings, and residues from their own log-stripping operations for fuel, and also buy them from facilities such as sawmills. Total annual estimated energy production from primary mill wood residues used for fuel is 744 trillion Btu (Table 23).

A comprehensive survey by the Tennessee Valley Authority (TVA) of mill residues in the Tennessee Valley region provides a good profile of residue uses (Table 24). Sawmills are the largest generator of mill wood waste, and finding uses for excess sawdust is a problem for sawmills. Some entrepreneurs have taken advantage of this resource as a raw material in the manufacture of densified wood fuel products, such as briquettes and pellets, which are used in both residential stoves and industrial boilers.

Construction and Demolition Debris

Construction and demolition debris, which makes up 10 to 15 percent of the municipal solid waste stream, includes wood, ferrous and nonferrous metals, corrugated cardboard, plastics such as wire and cable sheathing, brick, rock, and concrete. Combustible construction and demolition debris includes materials such as dimension lumber, plywood, appliance packing cartons, cardboard, wire and cable sheathing, old railroad ties, and demolished wooden bridges. Wood typically makes up about 40 percent of total construction and demolition wastes, and its uses include serving as boiler fuel and providing raw material for wood pellets.(14) Of the 31 million tons of construction and demolition debris generated each year, 8 million tons is wood, representing an energy potential of about 150 trillion Btu. The amount of wood contained in construction and demolition debris is expected to reach 9.5 million tons by the year 2000.(15) However, environmental regulations restrict the recovery of fuelwood from such debris to processing sites that can separate clean wood from treated wood.

Wood from Pallets and Containers

Pallets represent a large percentage of the wood used in shipping. Point sources for pallet wastes are harbor and port authorities, redistribution centers, furniture movers, common carriers, computer manufacturers, major department and retail stores, and warehouses. Because of their construction, pallets and containers recovered for fuel are most likely to be hammermilled to remove metal fasteners and then chipped for boiler fuel. (The same applies to wood from construction and demolition debris.)

In 1990, according to the Forest Service, one-third to one-half of all hardwood lumber consumption was by the pallet industry. The National Wooden Pallet and Container Association (NWPCA) estimates that 540.7 million wooden pallets were manufactured in 1991, 565.6 million in 1992, and 599.0 million in 1993.(16) About 60 percent of the pallets made in the United States in 1990 were of heavy-duty construction and were intended to be used for as long as possible; about 40 percent were of lighter construction and were intended to be "one-way."(17) More recently, according to the NWPCA, environmental considerations are leading to an ongoing decrease in the production of one-way or single-use pallets, and the reuse of heavy-duty pallets and the establishment of user pools are growing. The NWPCA believes that one-way pallets probably will disappear by the year 2000.

A recent survey of the pallet manufacturing industry by the Virginia Polytechnic Institute indicated that 44 percent of the respondents conducted recycling operations, and that heavy-duty or ■multi-use■ pallets constituted 90 percent of their activity in 1992.(18) The survey data indicated that more than 3 million wooden pallets, equivalent to 48.5 million board feet of lumber, were ground or chipped by pallet manufacturers in 1992 for use as fuel.

Wood Pellets

(19) See the next edition for updated information.

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