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Biomass, Bioengineering of 151

2. New Liquid Fuels from Biomass 3. Land Requirements for Fuel Production
Total consumption of gasoline and diesel fuel in the United However, assuming no improvement in vehicle efficiency
States is about 150 billion gallons per year. Assuming an and that only half of the tonnage of the three major crop
average density of these liquid fuels of six pounds per gal- residues (corn stover, rice straw and sugarcane bagasse) is
lon, the total mass of raw material that would need to be available for conversion to fuel ethanol (because of pos-
processed just to supply the U.S. with liquid fuel is about sible constraints of collection, erosion, alternative uses,
450 million tons per year, assuming complete conversion etc), the approximate worldwide liquid fuel replacement
of the mass of the raw material into fuel. In practice, sig- thatmightberequiredfrombiomass-derivedethanolcould
nificantly less than 100% raw material conversion to fuel be as high as one thousand billion gallons per year. At 100
products will be obtained. gallons per ton, roughly 3.0 billion acres of land world-
In the case of biomass raw materials, however, the sit- wide would need to be devoted to liquid fuel production
uation is even more constrained. If the entire domestic from biomass to produce the total required biomass ton-
corn crop of about 10 billion bushels per year were con- nage of 10 billion tons per year, assuming that the aver-
verted to fuel ethanol, the total gallons of fuel produced age U.S. hay production of 3 tons per acre per year were
would be less than 20% of domestic diesel and gaso- obtained.
line consumption. However, ethanol has an energy con- However, as pointed out earlier, there is great potential
tent about 70% of that of gasoline, a consequence of its to increase crop yields. Several tropical grasses, including
oxygenated character. Thus a gallon of ethanol will pro- sugarcane, have been shown to yield as much as 25–35
vide lower vehicle mileage than a gallon of gasoline, even tons of dry matter per acre per year. Furthermore, most
when burned in high compression engines designed to take land currently used for animal pasture (about 7 billion
advantage of its high octane value. Thus grain-derived acres worldwide) is not managed for intensive biomass
ethanol can never meet more than a small fraction of our production. For example, the United States has about 600
liquid fuel needs, however important corn ethanol may be million acres in permanent pasture. Grazing animals ob-
as a bridge to a biomass fuel industry based on lignocel- tain about 150 million tons of forage annually from this
lulosics. What is the potential of the lignocellulosics for pasture, for an average yield of less than 0.3 tons per acre
liquid fuel production? per year, versus about 3 tons per acre per year from our
The United States produces at least 300 million tons managed hay lands. Without a demand, there is no incen-
per year of lignocellulosic crop and forest residues that tive to increase forage grass production on the available
might be available for conversion to liquid fuels. Current pasture land.
laboratory best yields of ethanol from such residues are Therefore the demand for land and other agricultural re-
about 100 gallons per ton. The total ethanol that might be sources required to support biobased industrial products
produced from these lignocellulosic residues is therefore is probably not a factor for chemicals and materials, but
approximately 30 billion gallons per year, a significant will be an issue for biobased fuels. The demand for land
fraction of our total domestic liquid fuel demand. World- to supply liquid fuels depends on the yields of biomass
wide, if all of the corn stover, rice straw and sugarcane from the land, the yield of fuel from the biomass and the
bagasse now produced were converted to ethanol at these miles traveled per unit of fuel. All three factors are im-
yields, approximately 200 billion gallons of ethanol could portant and must be considered in conjunction. Increasing
be generated annually, versus an approximate worldwide the efficiency (yield) of each step will increase the over-
consumption of petroleum-derived liquid transportation all system efficiency. In particular, biomass conversion to
fuels of 800 billion gallons per year. fuel ethanol must be highly efficient and low cost if the
The requirements for liquid fuels obviously depend product is to compete with petroleum-derived fuels.
strongly on vehicle mileage. For example, if average ve-
hicle fuel efficiency were to increase by a factor of two,
4. Cost of Liquid Fuel Production from Biomass
demand for liquid fuels would be cut in half. Were that to
happen, the fuel ethanol that could be produced just from It is well known that fuel ethanol derived from corn must
the residues of these three major crops might satisfy nearly be subsidized to compete with gasoline. Raw material
half of the worldwide demand for liquid transportation fu- costs (primarily the corn) alone are in the neighborhood
els, without the need to plant additional crops dedicated of $1.00 per gallon of ethanol produced, without any al-
for energy uses. We must not lose sight of the need to lowance for processing costs. Therefore, it seems unlikely
work on the demand side of the fuel equation, as well as that corn ethanol will ever be able to compete economi-
the supply side. cally with petroleum-derived fuels. Nonetheless, a large