FIGURE 4.6 Published estimates of range of impacts on net greenhouse gas (GHG) emissions (A) and oil inputs (B) for grain-based ethanol. For additional discussion and information, see http:www.sciencemag.org/cgi/content/full/sci;312/5781/1748b. SOURCE: Farrell et al. (2006). Reprinted with permission of the American Association for the Advancement of Science.

FIGURE 4.7 Primary energy inputs and net greenhouse gas (GHG) emissions for gasoline and ethanol (primary energy inputs are in megajoules per megajoule of product energy; GHG emissions are in kilograms of CO2 equivalent per megajoule of fuel). SOURCE: Farrell et al. (2006). Reprinted with permission of the American Association for the Advancement of Science.

TABLE 4.4 CO2 Emissions from Today’s Conventional Light-duty Gasoline and Diesel Engines in a Typical Family Sedan and from Fuels from Less Conventional Sources

Vehicle Technology or Fuel Source

Well-to-Wheels CO2 Emissions (g CO2/km)

Conventional gasoline 2005

165

Conventional diesel 2005

143

Ethanol from corn

131

Ethanol from cellulose

15

SOURCE: Heywood (2007).

Production Potential of Biofuels

Production of ethanol from grain is fully commercial. Figure 4.8 shows the corn-ethanol production capacity growth from 1990 to 2007. U.S. production capacity grew from 4.3 billion gallons annually at year-end 2004, to 5.9 billion gallons annually by year-end 2006, and about 7 billion gallons annually by year-end 2007. Considering only current plant construction under way, ethanol capacity will be at least 8 billion gallons per year by year-end 2008 and could be as much as 10 billion gallons per year if all proposed projects are completed. This is a doubling of capacity in 4 years and



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