The most widespread and practical process for capture solar radiation as organic fuels is the growth of virgin biomass. Extremely large quantities of carbon are fixed each year in the form of terrestrial and aquatic biomass.
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 global energy consumption of these 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 re-growth.
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. natural gas demand 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 commercial 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%. Relatively large areas are required, but not so much as to make the use of land or freshwater biomass for energy applications impractical. 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.
This approach to the preliminary assessment of the potential of virgin biomass energy presumes that suitable conversion processes are available for conversion of biomass to SNG. 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 CO2 and the lithospheric carbonates. Electrical power and thermal energy can be supplied by nonfossil-powered nuclear reactors, and by means of hydroelectric and wind energy systems, ocean thermal gradients, wave action, and solar-actuated devices. Hydrogen production can also be manufactured from biomass and by direct action of solar energy benefit 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. 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.
