The national inventory of forestry residues generated in the United States in the mid-1970s is one of the first comprehensive assessments to be done on a county-by-county basis for a large country (Stanford Research Institute, 1976). Data were compiled on residues from logging operations and on wood and bark residues produced at primary wood processing milk. The regional offices of the U. S. Forest Service supplied most of the data on the residues generated by the mills. Most of the data on logging residues were obtained by applying residue factors to industrial roundwood production figures for each country. The factors used for different regions of the country were obtained from published reports of actual logging residue studies conducted in the field. For cases in which specific county data were not provided, it was necessary to apportion multicounty regional data or total state data among the counties having primary wood processing mills. Detailed data on bark residues were sometimes limited. The results of this survey indicated that a total of 105 million dry t/year of forestry residues are generated in the contiguous United States. The percentage distribution of forestry residues by region was 33.2% in the Pacific area, 6.4% in the Mountain area, 13.0% in the West South Central area, 14.2% in the East South Central area, 19.8% in the South Atlantic area, 2.2% in the West North Central area, 5.2% in the East North Central area, and 6.1% in the New England and Mid-Atlantic area. The Southeast and the Northwest are the areas that produce most of the U. S. timber and hence most of the forestry residues. Of the total forestry residues, 35.4 million dry t/year or 33.7% consisted of logging slash, and 69.6 million dry t/year or 66.3% consisted of bark and wood mill residues.

In another inventory of various sources of wood wastes in the United States (McKeever, 1995), it was found that for 1991, 26.0 million dry tonnes of bark and 74.5 million dry tonnes of wood residues were generated at primary lumber processing mills. Only 5% of the bark and 6% of the wood residue were wasted and not used. The projection for 1993 based on these findings was that 5.7 million dry tonnes of bark and wood residues were available for recov­ery and use as an energy resource. The total of 100.5 million dry t/year gener­ated at the mills is about 44% higher than the mill residues found in the Stanford assessment.

Energy Potential

Assuming the results of the Stanford assessment of logging slash residues in the continental United States would provide approximately the same results today, the energy potential of 35.4 million dry t/year of slash is about 0.66 EJ/year unadjusted for availability. Similarly, 100.5 million dry t/year of mill residue is about 1.86 EJ/year.

Availability

Of the forestry residues collected during normal operations in the United States in the mid-1970s according to the Stanford assessment, most of it was wood and bark residues that accumulated at primary lumber mills. It was reported that of the total forestry residues generated, about 33% was sold, 16% was used as fuel, and 51% was wasted. Most of the wasted residue was generated by logging operations. The fuel usage was represented to be an energy contribu­tion of 0.32 to 0.42 EJ/year to primary energy demand. The lumber and wood products industry has consistently used about 0.35 to 0.50 EJ/year of forestry wastes as fuel for the past few decades (Klass, 1990) and is expected to continue to use them at about the same rate up to at least 2040 (Skog, 1993). If adjustments are made to the total energy potential of U. S. forest residues for the amounts not used but available as a waste biomass resource, most of the logging slash is available (0.66 EJ/year) and 5.7 million dry t/year of available, unused mill residues correspond to about 0.11 EJ/year, or a total from available forestry residues of 0.77 EJ/year. Since the mass of logging residue varies from about 25 to 45% of the wood cut (Howlett and Gamache, 1977), it is highly probable that an energy availability estimate of only 0.66 EJ/year from logging residues is too low. A detailed assessment of the recoverable energy potential of forestry residues in only one state, Georgia, is estimated to be 0.20 EJ/year for logging slash and 0.08 EJ/year for wood manufacturing residues, or a total of 0.28 EJ/year (Riall and Bouffier, 1990). These data suggest that a more realistic estimate of the available energy value from logging slash should be at least 1.0 EJ/year.