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

3. Displacement Indexes (DI)
The displacement index (DI) represents the percent reduction in GHG emissions
or fossil fuel energy brought about by the use of a renewable fuel in comparison to the
conventional gasoline or diesel that the renewable fuel replaces. The formula for
calculating the displacement index depends on which fuel is being displaced (i.e. gasoline
or diesel), and which endpoint is of interest (e.g. petroleum energy, GHG). For instance,
when investigating the CO2 impacts of ethanol used in gasoline, the displacement index is
calculated as follows:
DI CO2 = 1 - lifecycle CO2 emitted for ethanol in g/Btu
lifecycle CO2 emitted for gasoline in g/Btu
The units of g/Btu ensure that the comparison between the renewable fuel and the
conventional fuel is made on a common basis, and that differences in the volumetric
energy content of the fuels is taken into account. The denominator includes the CO2
emitted through combustion of the gasoline itself in addition to all the CO2 emitted
during its manufacturer and distribution. The numerator, in contrast, includes only the
CO2 emitted during the manufacturer and distribution of ethanol, not the CO2 emitted
during combustion of the ethanol.
The combustion of biomass-based fuels, such as ethanol from corn and woody
crops, generates CO2. However, in the long run the CO2 emitted from biomass-based
fuels combustion does not increase atmospheric CO2 concentrations, assuming the
biogenic carbon emitted is offset by the uptake of CO2 resulting from the growth of new
biomass. As a result, CO2 emissions from biomass-based fuels combustion are not
included in their lifecycle emissions results and are not used in the CO2 displacement
index calculations shown above.
Using GREET, we calculated the lifecycle values for energy consumed and GHGs
produced for corn-ethanol, cellulosic ethanol, and soybean-based biodiesel. These values
were in turn used to calculate the displacement indexes. The results are shown in Table
IX.B.3-1. Details of these calculations can be found in Chapter 6 of the RIA. As noted
previously, different models can result in different estimates. For example, whereas
GREET estimates a net GHG reduction of about 26% for corn ethanol compared to
gasoline, the previously cited works by Farrell et al. estimates around a 13% reduction.
- 181 -