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

the refinery modeling discussed in Section VII will provide some additional information
on the process energy requirements associated with the production of gasoline and diesel
under a renewable fuels mandate. We will use information from this refinery modeling
for the final rule to determine if any GREET input values should be changed.
A summary of the GREET corn ethanol input values we investigated and
modified for this proposal is given below. We also examined several other GREET input
values, but determined that the default GREET values should not be changed for a variety
of reasons. These included ethanol plant process efficiency, corn and ethanol transport
distances and modes, corn farming inputs, CO2 emissions from corn farming land use
change, and byproduct allocation methods. Our investigation of these other GREET
input values are discussed more fully in Chapter 6 of the RIA. The current GREET
default factors for these other inputs were included in the analysis for this proposal.
a. Wet-Mill Versus Dry Mill Ethanol Plants
The two basic methods for producing ethanol from corn are wet milling and dry
milling. In the wet milling process, the corn is soaked to separate the starch, used to
make ethanol, from the other components of the corn kernel. In the dry milling process,
the entire corn kernel is ground and fermented to produce ethanol. The remaining
components of the corn are then dried for animal feed (dried distillers grains with
solubles, or DDGS). Wet milling is more complicated and expensive than dry milling,
but it produces more valuable products (ethanol plus corn syrup, corn oil, and corn gluten
meal and feeds). The majority of ethanol plants in the United States are dry mill plants,
which produce ethanol more simply and efficiently. The GREET default is 70 percent
dry mill, 30 percent wet mill.
For this analysis, we expect most new ethanol plants will be dry mill operations.
That has been the trend in the last few years as the demand for ethanol has grown, and
our analysis of ethanol plants under construction and planned for the near future has
verified this. Therefore, it was assumed that essentially all new ethanol facilities would
be dry mill plants.
b. Coal Versus Natural Gas in Ethanol Plants
The type of fuel used within the ethanol plant for process energy, to power the
various components that are used in ethanol production (dryers, grinders, heating, etc.)
can vary among ethanol plants. The type of fuel used has an impact on the energy usage,
efficiency, and emissions of the plant, and is primarily determined by economics. Most
new plants built in the last few years have used natural gas. Based on specific situations
and economics, some new plants are using coal. In addition, EPA is promoting the use of
combined heat and power, or cogeneration, in ethanol plants to improve plant energy-
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