Biomass potential
In his Aug. 1 letter, Ted Lofstrom correctly concludes that the entire US corn and soybean crops, if converted to ethanol and biodiesel, would offset only a very small fraction of US petroleum consumption.
Environmentalists are now arguing that new enzyme fermentation processes that convert cellulose into fuel alcohol will dramatically increase the supply of biofuels. In order to understand why biomass can never provide anything more than a very small fraction of the US energy supply, let's look at the production of wood on commercial forest lands.
Forests cover about 731 million acres of the US.1 However, commercial timberland, defined as forest land which produces or is capable of producing at least 20 cu ft/acre (ft3/ac) per year and has not been withdrawn for other purposes (e.g., parks, recreation areas, urban development, etc.), comprises about 483 million acres, or about 20% of the land area of the US.1-3 The net annual growth of growing stock on this commercial timberland averages about 47 ft3/ac.4 Note that this growth rate applies to growing stock on land that was logged at some time in the past, not to mature stands of old-growth timber, which exhibit very little net growth.
In order to avoid questions about the efficiencies of alcohol conversion processes, energy inputs for enzyme production, processing, distillation, product separation, waste disposal, etc., let's look at a very straightforward process, namely the use of wood to fuel electrical power generating plants. Modern coal-fired electrical generating plants have an overall conversion efficiency of about 40%. A plant using wood would have a somewhat lower efficiency, but let's assume the conversion efficiency is 40%. Since the growing stock on commercial timberland consists of both softwoods and hardwoods, let's assume the wood has an average density of about 40 lb/ft3. The heat of combustion of all wood is about 6,500 btu/lb. Last year the total electrical energy generated in the US was 3,953 billion kw-hr.5 (This total includes both the electric power and the commercial and industrial sector generation.) Now let's pose the following question. Suppose that 20% of this electrical energy were produced using wood-fired generating plants (and several states have passed legislation that will supposedly require 20% of electrical energy to be produced from "renewable" energy sources). If the net annual growth of 47 ft3/ac is to be used each year to fuel the generating plants, how much land must be set aside to produce the necessary amount of wood each year? The answer is 552 million acres, which exceeds the acreage of commercial timberland by almost 70 million acres.
Let's rephrase the question. Suppose the entire 483 million acres of commercial timberland were used for fuel-wood production. What fraction of US electrical energy generation would it provide? Using the same assumption, we find the answer is 17.5%.
The notion that biofuels are going to end US dependence on foreign energy supplies is an illusion.
References
1."Land use of the US by section and type of land, 1987," Table 36, An Analysis of the Timber Situation in the US: 1989-2040, Tech. Rept. RM-199, Rocky Mountain Forest and Range Experiment Station, US Forest Service, Fort Collins, Colo.
2. Domestic timber resources, An Analysis of the Timber Situation in the US: 1952-2030, Forest Service Rept. No. 23, Forest Service, US Dept. of Agriculture, Washington, DC.
3."Land Utilization, United States, 1940-1997," Table 9-13, Agricultural Statistics 2003, National Agricultural Statistics Service, US Dept of Agriculture, Washington, DC.
4. Ibid., p. 54.
5."Electricity Net Generation: Total All Sectors," Table 7.2a, Monthly Energy Review, June 2005, DOE/EIA-0035(2005/06), Energy Information Administration, June 27, 2005.
Donald F. Anthrop
Professor
San Jose State University
