42 



year in the Woods Hole Oceanogrpahic Institution's publication Oceanus. The fol- 

 lowing remarks have been taken from that manuscript. 



Of the various existing and proposed methods of utilizing solar energy, the pro- 

 duction of fuels from new, photosynthetically-produced organic matter — "biomass" 

 in the current terminology — is, at once, one of the simplest and most complicated. 

 The technology itself is simple. Dried plant biomass may be burned directly. That, 

 indeed, is man's original source of energy, and firewood is still a familiar fuel in 

 much of the United States — one that appears destined to revive in its importance 

 and significance. 



But plant biomass must be relatively dry for direct combustion. Unlike seasoned 

 wood, most freshly harvested plant material contains 85-95 percent water which 

 cannot be easily or economically removed and which may cost more in energy for its 

 removal than the energy content of the final product. 



An alternative method for obtaining power, or at least fuel, from wet plant 

 biomass is that of anaerobic digestion. Sugar plants (cane, beets, sorghum) and 

 grains, particularly corn in the United States, have a distinct advantage in this 

 respect because much of their biomass is directly fermentable to liquid fuel, ethanol. 

 Some U.S sugar crops are already contributing to the production of "gasohol". 

 However, careful analyses of the existing technology for producing corn and con- 

 verting it to ethanol indicates a negative energy output-input balance, so the pros- 

 pects of fuel production from this source would appear to be limited to the use of 

 sugar crops from the limited area in the U.S. where they may be grown. 



Virtually all wet plant biomass also readily undergoes a more complete anaerobic 

 decomposition or fermentation, with the ultimate production of gas that is a mix- 

 ture of carbon dioxide and methane. The gases produced from such anaerobic 

 digestion have heating values of 500-800 Btu per standard cubic foot and can be 

 readily upgraded to pipeline-quality gas by established processes. 



The difficulty with all of these approaches lies in the fact that vast quantities of 

 biomass are needed to make a significant contribution to the U.S. energy budget. 

 The energy content of most organic matter, including seasoned firewood, is in the 

 range of 20-30 million Btu per dry metric ton. The best yelds from short rotation 

 silviculture are roughly 10 dry metric tons per acre per year. To provide the energy 

 equivalent in firewood of a single 1,000 megawatt fossil-fuel or nuclear power plant 

 would thus require a managed energy farm of the order of 100,000 acres, over 

 300,000 acres, (ca. 500 square miles) if that energy were in the form of electricity 

 generated from direct combustion of the wood. 



With respect to the anaerobic digestion of wet plant biomass, only about half of 

 most organic matter is capable of being fermented to the above-mentioned low grade 

 (50-60 percent methane) gas mixture. Agricultural crops, grass lands, and other 

 forms of terresterial vegetation in continental U.S. are, on the average, less produc- 

 tive than the forest trees cited above. The mean annual yield of corn, the most 

 productive temperate U.S. crop, is no more than 5 dry tons/acre including residues 

 (about 45 percent of the total plant biomass). About one billion acres in this country 

 are presently used for the production of 1.2 billion tons of grains and grasses — an 

 overall average of just about one ton per acre per year. The energy potential of 

 these relatively low yields, converted to methane by the rather inefficient process of 

 anaerobic digestion, means that some 200 million acres of cropland would be needed 

 to produce just one quad (10'^ Btu) of energy, about 1 percent of the projected U.S. 

 energy budget for the year 2000 — that being the gross output uncorrected for the 

 energy input for growing, harvesting, tranporting, processing, and fermenting the 

 biomass and for upgrading, transmitting and/or storing the product gas. Such an 

 area is roughly 20 percent of the total now in use in the United States for agricul- 

 ture and grazing and twice the area that has been designated as unused, available 

 cropland. 



Not only do the above areal requirements appear to be unreasonably high for 

 either economic or energy-based cost effectiveness, but the more important consider- 

 ation is that good agricultural land available in the United States — that capable of 

 producing even the modest agricultural yields discussed above — is already almost 

 fully in use for the production of food and fiber crops that, for the most part, are 

 worth 10 to 100 times the value of the corresponding biomass for fuel. Even at a 

 deregulated price of $5.00 per thousand cubic feet, the amount of methane that 

 could be produced by anaerobic digestion from one ton of a typical agricultural crop 

 would be worth no more than about $25.00, roughly one cent per pound. 



In short, with the exceptions of wood and certain agricultural wastes, that may be 

 burned directly, and a few special crops, surpluses of which may be converted 

 directly to liquid fuel, the conventional agricultural crops, grasses, and other forms 

 of terrestrial vegetation do not appear to hold much promise as a major source of 

 U.S. energy. 



