The greenhouse gas benefits of corn ethanol – assessing recent evidence

Figures & data

Table 1. Ethanol production market breakdown and animal feed displacement by ethanol plant type.

Ethanol plant type	Ethanol market share (%)	Total displaced animal feed (g/L of ethanol)				Co-product credit
		Corn	Soybean meal	Urea	Soy oil	g CO2e/L ethanol	g CO2e/MMBtu
Dry mill w/o corn oil extraction	17.7	527	207	15.3	–	−262	−12,981
Dry mill w/corn oil extraction	70.9	504	198	14.6	–	−250	−12,417
Wet mill	11.4	857	–	13.1	117	−291	−14,449
Weighted average		–	–	–	–	−257	−12,749

Table 2. Domestic land-use change emissions for GTAP 2013 simulation using emission factors from Century/COLE, Woods Hole, and Winrock International.

	Total direct emissions (Mg CO2e)	Annualized emissions (Mg CO2e/year)	Direct emissions (g CO2e/L)	Direct emissions (g CO2e/MMBtu)
Century/COLE – 30 cm – Reduced till	−52,191,279	−1,739,709	−39.65	−1965
Century/COLE – 100 cm – Reduced till	−62,656,429	−2,088,548	−47.60	−2359
Century/COLE – 30 cm – Conventional till	−45,625,214	−1,520,840.5	−34.66	−1718
Century/COLE – 100 cm – Conventional till	−54,120,694	−1,804,023.1	−41.13	−2038
Woods Hole	48,163,909	1,605,464	36.59	1813
Winrock International	280,879,558	9,362,652	213.4	10,577

Table 3. Methane emission factors from irrigated rice by region (in kg CO₂e/ha).

Study	Corn belt	Pacific Southwest	South central	Southeast	Southwest	United States
EPA RIA^a	4512	4406	5557	N/A	10,811	N/A
Present^b	5928	5310	7500	9222	11,421	7324

Sources: ^a [1, Table 2.4-9]; ^b [21]; N/A = not applicable; kg CO₂e/ha = kilograms of carbon dioxide equivalent per hectare.

Table 4. Changes in population, emission factors, and total emissions by livestock type.

Livestock type	Change in population (in 1000 head)	Enteric emissions		Manure management emissions		Total g CO2e/MMBtu
		per head (g CO2e/head)	Total emissions (g CO2e/MMBtu)	per head (g CO2e/head)	Total emissions (g CO2e/MMBtu)
Dairy	−20^a	3625	−351	2065	−200	−551
Beef	90	1850	807	143	62	869
Poultry	−12,564.6^b	–	NA	3.21	−195	−195
Swine	−220	37.5	−40	378	−403	−443
Total	NA	NA	416	NA	−736	−320

^a Mature cows only.

^b Population changes the same as in [1] except for poultry, which has been reduced to reflect annual average population changes rather than changes in total head slaughtered.

g CO₂e/head = grams of carbon dioxide equivalent per head; g CO₂e/MMBtu = grams of carbon dioxide equivalent per million British thermal units.

Table 5. Reduced Methane Emissions from Distillers Grains as Animal Feed by Ethanol Plant Type.

Ethanol plant type	Ethanol market share (%)	Dried DGS yield (g/L)	Wet DGS yield (g/L)	Emissions reduced (g CO2e/L)	Emissions reduced (g CO2e/MMBtu)
Dry mill with out corn oil extraction	17.7	504.1	661.7	−50.46	−2506
Dry mill with corn oil extraction	70.9	482.2	632.9	−48.34	−2397
Wet mill	11.4	–	–	–	–
Per average liter	–	431.1	565.9	−43.21	−2143

g/L: grams per liter; g CO₂e/L: grams carbon dioxide equivalent per liter; g CO₂e/MMBtu: grams carbon dioxide equivalent per million British thermal units.

Table 6. iLUC emissions by scenario, emissions factor set, and annualized emissions value.^a

Scenario	Emission factor (EF) data set	Emissions (g CO2e/MMBtu)^c
	RIA [1] analysis
FAPRI-CARD	Winrock	31,790
	Scenarios making up composite iLUC emissions value used in this analysis
GTAP 2013	CARB AEZ Model	17,802
GTAP 2013^b	Winrock	5913
GTAP 2013 adjusted with data in [26]	CARB AEZ Model	8464
GTAP 2013 adjusted with data in [26]	Winrock	1326
CARB 2015 [27]	CARB AEZ Model	20,890
Dunn et al. 2015 [11]	Winrock	5286
[11]	Woods Hole	3893

^a Scenarios in [11] and the four scenarios we construct use land conversion results published by [26]. [27] modified some important factors and values within the GTAP-Bio model to produce their own unique land conversion results.

^b Emissions vary in these studies because within each region, [11] used an average of the individual country EFs, while we weight countries’ EFs by their share of regional arable land.

^c All studies assume emissions from land conversions occur over 30 years. This column shows annualized values. The RIA and GTAP-Bio 2013 consider different volume increases in corn ethanol production. Describing emissions in g CO₂e/MMBtu puts all emissions in a comparable metric.

Abbreviations: iLUC = international Land Use Change; g CO₂e/MMBtu = grams carbon dioxide eqivalent per million British thermal units; RIA = Regulatory Impact Analysis; FAPRI- CARD = Food and Agricultue Policy Research Institute-Center for Agricultural and Rural Research model; GTAP = Global Trade Analysis model; CARB = California Air Resources Board; AEZ = Agricultural-Ecological Zone.

Table 7. Mode and distance assumptions.

Mode	Farm to stacks		Stacks to plant		Plant to blending terminal		Terminal to retail station		DGS^a		Corn oil
	% of total shipped	Distance (km)	% of total shipped	Distance (km)	% of total shipped	Distance (km)	% of total shipped	Distance (km)	% of total shipped	Distance (km)	% of total shipped	Distance (km)
Barge	0	0.0	0	0.0	13	837	0	0.0	2	837	0	0
Rail	0	0.0	0	0.0	79	1287	0	0.0	12	1287	20	644
Truck	100	16.1	100	64.4	8	129	100	48.3	86	80	80	161

^a The values shown in these columns reflect a weighted average dry and wet distiller grains and solubles (DGS) co-product.

Table 8. Assumptions and inputs for fuel production modeling in GREET 2015.

Input category	Dry milling plant without corn oil extraction^a	Dry milling plant with corn oil extraction^a	Wet milling plant^b
Total energy use for ethanol production (MJ/L)	28.33	27.88	50.02
Process fuel energy – natural gas, coal, and biomass (MJ/L)	25.66	25.18	50.02
Electricity use (kWh/L)	0.195	0.198	0.00
Co-product yield – dry DGS to animal feed (g/L ethanol)	504	482	0.00
Co-product yield – wet DGS to animal feed (g/L ethanol)	661	633	0.00
Co-product yield – CGM to animal feed (g/L ethanol)	0.00	0.00	162
Co-product yield – CGF to animal feed (g/L ethanolc	0.00	0.00	702
Co-product yield: Corn oil (actual g/L ethanol)	0.00	22.77	117
Ethanol yield (L/bushel)	10.6	10.7	9.88
GHG emissions (g CO2e/MMBtu)	32,114	31,590	53,055

^a These are composite refineries reflecting the 2014 mix of natural gas, coal, and biomass as a process fuel at the respective dry mill refineries.

^b Electricity consumption is included with process fuel energy.

Abbreviations: GREET = The Greenhouse Gas, Regulated Emissions, and Energy Use in Transportation Model; MJ/L =Mega Joules per liter;

kWh/L = kilowatt hours per liter; g/L = grams per liter; L/bushel = liters per bushel; g CO₂e/MMBtu = grams CO2 equivalent per million Britsh thermal unit; RIA = Regulatory Impact Analysis; BAU = Business as Usual Scenario; HEHC = High Efficiency - High Conservation Scenario

Figure 1. Life-cycle GHG emissions for gasoline and corn ethanol by scenario and source category. RIA: Regulatory Impact Analysis; BAU: Business as Usual Scenario; HEHC: High Efficiency - High Conservation Scenario; N₂O: Nitrous Oxide.

Appendix Table A1. Summary of Key Data Sources and Models Used in the Regulatory Impact Analysis (RIA) Life Cycle Analysis and the Current Greenhouse Gas (GHG) Profile.

Source category	RIA [1]	Current GHG profile
General – Global warming potentials (GWPs)	Second Assessment Report (1996) • Methane: 21 • Nitrous oxide: 310	Fourth Assessment Report (2007) • Methane: 25 • Nitrous oxide: 298
Domestic farm inputs and fertilizer Nitrous Oxide (N2O)	• Fertilizer application rates and fuel Consumption: Forestry and Agricultural Sectoral Optimization Model (FASOM version 2010)  • Fertilizer and fuel production Emission factors: The Greenhouse Gas, Regulated Emissions, and Energy Use in Transportation Model (GREET) version 2009  • Fertilizer use emission factors: Daily Century (DAYCENT) Model (version 2010)	• Fertilizer application rates: [6]  • Fuel consumption: [7]  • Fertilizer and fuel production emission factors: GREET (version) 2015  • Fertilizer use emission factors:[10]  • Animal feed co-product credit: GREET (version 2015)
Domestic land use	• Acres: FASOM (version 2010)  • Emission factors: DAYCENT Model (version 2010)	• Acres: [12]  • Emission rates: Century and Cole Models (version 2015)
Domestic rice methane	• Acres: FASOM (version 2010)  • Emission factors: [19]	• Acres: same as [1]  • Emission factors: [21]
Domestic livestock	• Change in livestock populations: FASOM (version 2010)  • Emission factors: [19]	• Change in livestock populations: same as [1] except poultry  • Livestock conventional feed emission Factors: [21] • Emission reductions from Distillers Grains with Solubles as feed: GREET (version 2015)
International livestock	• Populations: Food and Agriculture Policy Research Institute-Center for Agricultural and Rural Development (FAPRI-CARD) (version 2010)  • Emission Factors: [10]	• Populations: same as [1]  • Emission factors: [10] except for Canada (2016) with a country-specific update
International land-use change	• Acres: FAPRI-CARD (version 2010)  • Emission factors: Winrock International (2009)	• Acres: [12]; in two scenarios, acres adjusted with data in [17]  • Emission factors: Winrock International (2009), Woods Hole (2010) and California Air Resources Board (CARB) Agricultural - Ecological Zones (AEZ) (2015)
International farm inputs and fertilizer N2O	• Acres and production: FAPRI-CARD (version 2010)  • Fertilizer application rates: [30] and [31, version 2009]  • Fuel use: [34]  • Fertilizer and fuel production emission factors: GREET (version 2009) • Fertilizer use emission factors: Methodology similar to domestic	• Acres and production: [12]  • Fertilizer application rates: [31, version 2015]  • Fuel use: same as [1]  • Fertilizer and fuel production emission Factors: GREET (version 2015) • Fertilizer use emission factors: [10]
International rice methane	• Acres: FAPRI-CARD (2010)  • Emission factors: [10]	• Acres: same as [1]  • Emission factors: same as [1]
Fuel and feedstock transport	• Transport distances: GREET (version 2009) • Emission factors: GREET (version 2009)	• Transport distances: GREET (version 2015) • Emission factors: GREET (version 2015)
Fuel production	• Process energy: GREET (version 2009) • Emission factors: GREET (version 2009)	• Process energy: GREET (version 2015) • Emission factors: GREET (version 2015)
Tailpipe	• Emissions: [39]	• Emissions: GREET (version 2015)

Appendix Table A2. Emissions by scenario and category.

	Estimated Greenhouse Gas (GHG) emissions (in grams carbon dioxide equivalent per million British thermal units)
Emissions category	RIA (2010)	Current	2022 BAU	2022 HEHC
Domestic farm inputs	10,313	9065	8190	4490
Domestic Land Use Change	−4000	−2038	−2038	−2359
Domestic rice methane	−209	−1013	−1013	−1013
Domestic livestock	−3746	−2463	−2463	−2463
International Land Use Change	31,790	9094	9094	9094
International farm inputs	6601	2217	2217	2217
International rice methane	2089	2482	2482	2482
International livestock	3458	3894	3894	3894
Fuel and feedstock transport	4265	3432	2641	1237
Fuel production	28,000	34,518	31,006	9695
Tailpipe	880	578	578	578
Total	79,441	59,766	54,588	27,852
	Estimated GHG emissions (in grams carbon dioxide equivalent per Mega Joule)
Domestic farm inputs	9.77	8.59	7.76	4.26
Domestic Land Use Change	−3.79	−1.93	−1.93	−2.24
Domestic rice methane	−0.20	−0.96	−0.96	−0.96
Domestic livestock	−3.55	−2.33	−2.33	−2.33
International Land Use Change	30.13	8.62	8.62	8.62
International farm inputs	6.26	2.10	2.10	2.10
International rice methane	1.98	2.35	2.35	2.35
International livestock	3.28	3.69	3.69	3.69
Fuel and feedstock transport	4.04	3.25	2.50	1.17
Fuel production	26.54	32.72	29.39	9.19
Tailpipe	0.83	0.55	0.55	0.55
Total	75.30	56.65	51.74	26.40

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