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