It is important to examine the potential amounts of energy and biofuels that might be produced from biomass carbon resources and to compare these amounts with fossil fuel demands. This would make it possible to estimate the percentage of energy demand that might be satisfied by particular bio­mass types.

A. Virgin Biomass

Consider first the incident solar radiation, or insolation, that strikes the earth’s surface. At an average daily insolation worldwide of about 220 W/m2 (1676 Btu/ ft2), the annual insolation on about 0.01% of the earth’s surface is approximately equal to all the primary energy consumed by humans each year. For the United States alone, the insolation on about 0.1 to 0.2% of its total surface is equivalent to its total annual energy consumption.

The most widespread and practical process for capture of this energy as organic fuels is the growth of virgin biomass. As already discussed, extremely large quantities of carbon are fixed each year in the form of terrestrial and aquatic biomass. Using the figures in Table 2.2, the energy content of standing biomass carbon; that is, the renewable, above-ground biomass reservoir that in theory could be harvested and used as an energy resource, is about 100 times the world’s annual energy consumption. At a nominal biomass heating value of 18.6 GJ/dry t (16 X 106 Btu/dry ton) and assuming that the world’s total annual coal, oil, and natural gas consumption is about 315 EJ (1993), the solar energy trapped in 16.9 Gt of dry biomass, or about 7.6 Gt of biomass carbon, would be equivalent to the world’s consumption of these fossil fuels. Since it is estimated that about 77 Gt of carbon, or 171 Gt of dry virgin biomass equivalent, most of which is wild and not controlled by humans, is fixed on the earth each year, it is certainly in order to consider biomass as a raw material for direct use as fuel or for conversion to large supplies of substitute fossil fuels. Under controlled conditions, dedicated biomass species might be grown specifically as energy crops or for multiple uses including energy. Relatively rapid replacement of the biomass utilized can take place through regrowth.

A more realistic assessment of biomass as an energy resource can be made by calculating the average surface areas needed to produce sufficient biomass at different annual yields to meet certain percentages of fuel demand for a particular country, and then to compare these areas with those that might be made available. Such an assessment for the United States could, for example, address the potential of biomass for conversion to SNG as shown in Table 2.5. For this analysis, the annual U. S. demand for natural gas is projected to reach 26.5 EJ (25.1 quad) by 2010 at an annual growth rate in consumption of 1.2% (U. S. Dept, of Energy, 1994). It is assumed that biomass, whether it be trees, plants, grasses, algae, or water plants, has a heating value of 18.6 GJ/dry t, is grown under controlled conditions in “methane plantations” at yields of 20 and 50 dry t/ha-year, and is converted in integrated biomass planting, harvest­ing, and conversion systems to SNG at an overall thermal efficiency of 50%. These conditions of biomass production and conversion either are within

the range of present technology and agricultural practice, or are believed to be attainable in the near future. The average total plantation areas were then calculated to meet 1.42, 10, 50, and 100% of the projected U. S. demand in 2010 for natural gas. A percentage of 1.42 is equal to a daily pro­duction of 26.9 X 106 m3 at normal conditions (1 X 109 SCF) of dry SNG. The range of areas required at the low yield level is between 2,023,000 and 142,500,000 ha, or 0.2 and 14.9% of the 50-state U. S. area. At the high yield level, the areas are between 809,000 and 57,000,000 ha, or 0.08 and 6.0% of the 50-state U. S. area. To put this analysis in the proper perspective, the results are shown in graphical form in Fig. 2.2 at the two yield levels together with the percentage area of the United States needed at any selected gas demand supplied by SNG from biomass. Relatively large areas are required, but not so much as to make the use of land or freshwater biomass for energy applications impractical. When compared with the area distribution pattern of the United States (Table 2.6) (USDA Forest Service, 1989), it is seen that selected areas or combinations of areas might be utilized for biomass energy. Areas that are not used for productive purposes might be suitable, or possibly biomass for both energy and foodstuffs or energy and forest products applications can be grown simultaneously or sequentially in ways that would benefit both. Also, relatively small portions of the bordering oceans might supply the needed biomass growth areas, in which case, marine plants would be grown and har­vested.

AREA REQUIRED (million ha) 50-STATE AREA (%)

“Adapted from USDA Forest Service (1989). The data for forest, rangeland, and other land are for 1982. The data for inland water are for 1990. The data for other water are for 1970. Forest areas are at least 10% stocked by trees of any size, or formerly having such tree cover and not currently developed for nonforest use. Transition land is forest land that carries grasses or forage plants used for grazing as the predominant vegetation. Climax vegetation on rangelands is predominantly grasses, grass-like plants, forbs, and shrubs suitable for grazing and browsing. Other land areas include crop and pasture land and farmsteads, strip mines, permanent ice and snow, and land that does not fit any other land cover.

This approach to the preliminary assessment of the potential of biomass energy presumes that suitable conversion processes are available for conversion of biomass to SNG. Other processes could be used to manufacture other synfuels such as synthesis gas, alcohols, esters, and hydrocarbons. The direct route, alluded to in Fig. 2.1 as natural production of hydrocarbons, can possibly bypass the harvesting-conversion routes. As already mentioned, some biomass species produce hydrocarbons as metabolic products. Natural rubber, glycer­ides, and terpenes from selected biomass species, for example, as well as other reduced compounds could be extracted and refined to yield conventional or substitute fossil fuels.

A second source of renewable carbon is the deposits and reservoirs of essentially non-energy carbon forms—ambient C02 and the lithospheric car­bonates. The availability of such raw materials cannot be questioned, although low-cost separation and energy-efficient recovery of very small concentrations of C02 from the atmosphere present technological challenges. Another basic problem resides in the fact that all of the energy must be supplied by a sec­ond raw material, such as hydrogen. Hydrogen would have to be made avail­able in large quantities from a nonfossil source or the purpose of the synfuel system to produce renewable fuels would be defeated. Conceptually, there is no difficulty in developing such hydrogen sources. Hydrogen can be produced by water electrolysis and thermochemical and photolytic splitting of water. Electrical power and thermal energy can be supplied by nonfossil-powered nu­clear reactors, and by means of hydroelectric and wind systems, ocean thermal gradients, wave action, and solar-actuated devices. Hydrogen can also be man­ufactured from biomass and by direct action of solar energy on certain cat­alytic surfaces.

As already pointed out, about 16.9 Gt of dry biomass, or about 7.7 Gt of biomass carbon, would have approximately the same energy content as the total global consumption of coal, oil, and natural gas (in 1993). This amount of carbon corresponds to less than 1.0% of the total standing biomass carbon of the earth. Under present conditions of controlled and natural production of fixed carbon supplies, the utilization of some of this carbon for energy applications seems to be a logical end use of a renewable raw material. Forest biomass is especially interesting for these applications because of its abundance. The expansion of controlled production of virgin biomass in dedicated energy crop systems should also be considered because this would result in new additions to natural biomass carbon supplies. For example, the biomass carbon supplies in marine ecosystems might conceivably be increased under controlled conditions over the current low levels by means of marine biomass energy plantations in areas of the ocean that are dedicated to this objective. Unused croplands and federal lands might also be used for the production of herbaceous or woody biomass energy crops.