Document ID: EPA-HQ-OAR-2017-0655-0092
Agency: epa
Document Type: Supporting & Related Material
Title: 
Posted Date: 2018-08-02T04:00Z

MEMORANDUM
SUBJECT:	Summary for the Final Rule of Key Assumptions for EPA's Analysis of the Lifecycle Greenhouse Gas Emissions Associated with Biofuels Produced from Distillers Sorghum Oil
DATE: 		July 20, 2018
TO:		Air Docket EPA-HQ-OAR-2017-0655

Lifecycle Greenhouse Gas Calculations
This memo accompanies a spreadsheet in the docket that includes our calculations of the lifecycle greenhouse gas (GHG) emissions associated with biofuels produced from distillers sorghum oil (DSO). The spreadsheet includes the following tabs:
 Oil Extraction Rate: Calculates pounds of DSO extracted per pound of dried distillers grains and solubles (DDGS)
 Corn Substitution: Calculates pounds of corn substituted for each pound of DSO extracted
 Corn upstream GHGs: Calculates the upstream GHG emissions factor per pound of corn substituted (explained in detail below)
 Livestock Sector: Calculates the livestock sector GHGs based on the tabs listed above
 Oil extraction GHGs: Calculates the GHGs from thermal and electrical energy used to extract DSO from DDGS via centrifuge
 Distillers Sorghum Oil Transport: Calculates average GHG emissions associated with transporting DSO from an ethanol plant to a biofuel production facility
 Biodiesel Production: Calculates GHGs associated with biodiesel and heating oil production
 Hydrotreating: Calculates GHGs associated with renewable diesel, jet fuel, naphtha and LPG production via a hydrotreating process
 LCA Results: Calculates the lifecycle GHG emissions as a sum of the emissions calculated on prior tabs
The key assumptions in this spreadsheet are discussed in section III.B of the preamble to the final rule.
Calculation of upstream emissions factor for corn used as livestock feed
As discussed in the preamble, based on comments on the proposed rule EPA undertook additional analysis of the greenhouse gas (GHG) emissions associated with replacing distillers sorghum oil in livestock feed with corn. The `Oil Extraction Rate' and `Corn Substitution' tabs in the spreadsheet `Distiller Sorghum LCA' in the docket, show the analysis conducted to determine how many pounds of corn substitute for each pound of distillers sorghum oil used for biofuels instead of animal feed. To determine the GHG emissions associated with this substitution, we needed a GHG emissions factor per pound of corn used as animal feed. The calculation of this emissions factor relied on EPA's previous corn starch ethanol lifecycle analysis for the RFS program, as explained below.
EPA evaluated the lifecycle GHG emissions associated with corn starch ethanol for the March 2010 RFS2 rule (75 FR 14670). The results from that analysis were presented in terms of kilograms carbon dioxide-equivalent emissions (kgCO2e) per million British thermal units (mmBtu), lower heating value (LHV) of undenatured ethanol (see Table 1). We used the following steps to convert those results to kgCO2e per pound of corn.
Table 1: Corn Starch Ethanol Lifecycle GHG Emissions Results from the March 2010 RFS2 Final Rule
Lifecycle Stage
GHG emissions (kgCO2e/mmBtu)
U.S. Crop Production
8.3 
U.S. Rice CH4
(0.2)
U.S. Livestock
(3.7)
U.S. Land Use Change
(4.0)
Non-U.S. Crop Production
6.6 
Non-U.S. Rice CH4
2.1 
Non-U.S. Livestock
3.5 
Non-U.S. Land Use Change (Mean)
31.8 
Feedstock Transport
2.4 
Feedstock Processing
-   
Fuel Production
27.9 
Co-Product Transport
0.6 
Fuel Distribution
1.2 
Fuel Use
0.9 
Total (Mean)
77.2
Upstream (Mean)
46.6 
Fuel Production
27.9 
Downstream
2.8 
                                       
Since we were only interested in the GHG emissions associated with corn production and delivery, we only took the "upstream" emissions results from Table 1 (i.e., 46.6 kgCO2e/mmBtu).
Converting the upstream result from Table 1 to GHG per pound of corn required the following steps:
 Convert the upstream emissions from kgCO2e per mmBtu of ethanol to kgCO2e per pound of corn based on the corn ethanol yields assumed for the March 2010 RFS2 lifecycle analysis.
 Account for the corn displaced by distillers grains and solubles (DGS).
The corn starch ethanol lifecycle analysis for the March 2010 RFS2 rule assumed corn starch ethanol yield of 2.71 gallons per bushel, which converts to 0.0037 mmBtu of ethanol per pound of corn. Based on a simple conversion, we obtained the result: 0.17 kgCO2e per pound of corn. However, this underestimates the GHG emissions per pound of corn used as livestock feed because corn used for ethanol results in ethanol and DGS co-product that in turn displaces the amount of corn needed for animal feed.
The analysis for the March 2010 RFS2 rule assumed DGS yield of 0.30 pounds per pound of corn used as ethanol feedstock (on dry basis). The models also included DDGS displacement rates for corn. The March 2010 RFS2 analysis assumed that all of this DGS was fed to livestock, and displaced corn at different rates for different livestock.
Two agricultural models were used for the March 2010 RFS2 analysis: FASOM and FAPRI (see the March 2010 RFS2 preamble and RIA for more information). The FAPRI results show that approximately 75% of DGS was fed to cows. The FASOM results included the total amount of DGS used for feed, but not the breakdown by animal type. An Iowa State University study reported that in the U.S. 86% of DGS was consumed by cows in 2016/17. Weighting DGS consumption more heavily towards cows results in a greater DGS corn displacement estimate, and a higher GHG per pound of corn emissions factor. For this distillers sorghum oil analysis, we have used the Iowa State data as a conservative estimate. Based on this weighting, each pound of DGS used displaced 0.36 pounds of corn (see Figure 1).
Figure 1: Corn Use and Displacement in the March 2010 RFS2 Corn Starch Ethanol Lifecycle Analysis
                                       
Thus, the intermediate result of 0.17 kgCO2e per lb of corn (see above), became 0.17 kgCO2e per 0.64 lbs of corn, which converts to 0.266 kgCO2e per lb of corn.
This corn emissions factor (0.266 kgCO2e/lb) was used in our distillers sorghum oil lifecycle analysis to calculate the GHG emissions associated with each pound of corn needed to substitute for distillers sorghum oil used for biofuel instead of livestock feed.
The calculations explained above in this section are included in the lifecycle analysis spreadsheet described in the first section of this memorandum.