two, the increasing volume could be accommodated incrementally in the existing infrastructure, so it is not explicitly discussed here.

In considering ethanol as a transportation fuel, one needs to be aware that ethanol is not a one-to-one replacement for its petroleum-based counterparts. Ethanol contains only two-thirds of the energy of the same volume of gasoline. The corn grain used and the cellulosic biomass to be used to produce ethanol also vary in density and other physical characteristics that affect their costs of transportation from field to conversion plants, as discussed in Chapter 2. Most of the biomass will be produced in the interior of the United States (or from wood in the Northwest). The economics of transporting biomass feedstock versus finished transportation fuel favor biorefineries in the Midwest, but 80 percent of the U.S. population—representing the largest current and future transportation-fuel markets—lives along the coasts. Figure 5.1 maps existing biorefineries and those

FIGURE 5.1 U.S. ethanol and biodiesel plant locations compared with state population density as of July 1, 2007.

Source: Adapted from NBB (2007), U.S. Census Bureau (2007), and RFA (2008).