Regulation of Fuels and Fuel Additives: Renewable Fuel Standard ...

efficiency and to reduce air emissions. This technology, in the face of increasing natural
gas prices, may make coal a more attractive energy source for new ethanol plants.
GREET assumes that 20 percent of plants will be powered by coal. However, our
review of plants under construction and those planned for the near future indicates that
coal will only be used for approximately 10% of the plants. This is the value we assumed
in GREET for our analysis. However, as new plants are constructed to meet the demands
of the RFS, this percentage is expected to go up. Future work in preparation for the final
rule will evaluate the potential trends for combined heat and power and coal as process
fuel.
c. Ethanol Production Yield
It is generally assumed that 1 bushel of corn yields 2.7 gallons of ethanol.
However, the development of new enzymes continues to increase the potential ethanol
yield. We used a value of 2.71 gal/bu in our analysis. This value represents pure ethanol
production (i.e. no denaturant). This value is consistent with the cost modeling of corn
ethanol discussed in Section VII.
2. Controversy Concerning The Ethanol Energy Balance
Although we have made use of lifecycle impact estimates from ANL's GREET
model, there are a variety of lifecycle impact analyses from other researchers that provide
alternative and sometimes significantly different estimates. The lifecycle energy balance
for corn-ethanol, in particular, has been the subject of numerous and sometimes
contentious debates.
Several metrics are commonly used to describe the energy efficiency of
renewable fuels. We have chosen to use displacement indexes for this proposal because
they provide the least ambiguous and most relevant mechanism for estimating the
impacts of renewable fuels on GHGs and petroleum consumption. However, other
metrics, such as the net energy balance and energy efficiency, have more commonly been
used in the past. The use of these metrics has served to complicate the issue since they
do not involve a direct comparison to the gasoline that the ethanol is replacing.
Among researchers who have studied the lifecycle energy balance of cornethanol,
the primary differences of opinion appear to center on fossil energy associated
with fertilizers, the energy required to convert corn into ethanol, and the value of coproducts.
As a result of these differences, the net energy balance has been estimated to
be somewhere between -34 and + 31 thousand Btu/gal, and the energy efficiency has
been estimated to be somewhere between 0.6 and 1.4 89 . A concern arises in cases where
89 A net energy balance of zero, or an energy efficiency of 1.0, would indicate that the full lifecycle fossil
fuels used in the production and transportation of ethanol are exactly equal to the energy in the ethanol
itself.
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