Document ID: EPA-HQ-OAR-2017-0655-0004
Agency: epa
Document Type: Supporting & Related Material
Title: 
Posted Date: 2017-12-27T05:00Z

November 21, 2017
MEMORANDUM
TO: 		Air Docket EPA-HQ-OAR-2017-0655
SUBJECT:	Summary of Key Assumptions for EPA's Analysis of the Lifecycle Greenhouse Gas Emissions Associated with Biofuels Produced from Distillers Sorghum Oil

Fuel Use
Following the methodology developed for the March 2010 RFS rule, after notice, public comment and peer review, the carbon in the finished fuel derived from renewable biomass is treated as biologically derived carbon originating from the atmosphere. The uptake of this carbon from the atmosphere by the renewable biomass and the CO2 emissions from combusting it cancel each other out.  Instead of presenting both the carbon uptake and tailpipe CO2 emissions, we leave both out of the results.  Note that our analysis also accounts for all significant indirect emissions, such as from land use changes, meaning we do not simply assume that biofuels are "carbon neutral."
Biodiesel
As for the March 2010 RFS rule, this analysis treats all of the biodiesel fuel carbon as biologically derived carbon originating from the atmosphere. This would be the case where the methanol used in the trans-esterification process was produced from biomass.  If the methanol instead is produced from natural gas, the net carbon dioxide emissions from the combustion of fossil derived methanol in biodiesel would result in a small change to the lifecycle GHG emissions for biodiesel.  Based on our modeling of the trans-esterification process in the March 2010 RFS final rule we estimate that the lifecycle GHG emissions from combustion of biodiesel made with fossil methanol rather than methanol derived from biomass would include approximately 4 kg of additional carbon dioxide-equivalent (CO2e) emissions per mmBtu of biodiesel.  If we included the additional GHG emissions associated with the use of fossil methanol in biodiesel production in our lifecycle GHG assessment of distillers sorghum oil biodiesel it would not affect our determination that such fuel meets the 50% lifecycle GHG reduction threshold.
Renewable Diesel
Our analysis uses the emissions factors for non-CO2 GHGs for baseline diesel fuel.  Available data suggests the non-CO2 emissions factors for renewable diesel are small and not significantly different.  For example, GREET-2016 uses the same methane and nitrous oxide emissions factors for conventional and renewable diesel used in a heavy or medium duty trucks.
Jet Fuel
Our analysis uses the emissions factors for non-CO2 GHGs for baseline diesel fuel.  This is a conservative assumption (i.e., results in higher emissions) because available information suggests that renewable jet fuel use in aircraft results in less methane and nitrous oxide emissions, per mmBtu of fuel used, than the emissions factor for baseline diesel fuel.  For example, GREET-2016, reports non-CO2 GHG emissions of 0.07 kgCO2e/mmBtu for hydroprocessed renewable jet fuel used in a single aisle passenger aircraft, compared to 0.7 kgCO2e/mmBtu for the diesel baseline.  
The emissions factors used in our analysis apply the same global warming potentials (GWPs) as used in the March 2010 RFS rule for ground-level emissions.  There is some uncertainty about the most appropriate GWPs for emissions released by aircraft flying at cruising altitudes.  However, available information indicates that emissions for renewable jet fuels, such as hydrotreated jet fuel produced from vegetable oils, and petroleum-based jet fuels are very similar. Thus, regardless of the GWPs applied, renewable jet fuel is unlikely to result in tailpipe emissions with significantly greater climate impacts relative to conventional jet fuels.