Corn ethanol has been the standard for biofuels in the United States for nearly 20 years since the Renewable Fuel Standard (RFS) was introduced in 2005.
Carbon pipelines have been positioned as the next step the ethanol industry needs to sequester carbon, receive tax credits, and be poised for growth, including in new areas such as sustainable aviation fuel. In our series on carbon pipelines, we have covered how companies are working to establish pipelines across the Midwest. We also looked at how farmers can navigate eminent domain use when pipeline construction comes closer to reality.
For this story, we set out to directly compare the life cycle of greenhouse gas (GHG) emissions — from well to wheels — of ethanol and gasoline, and how carbon pipelines factor into the future of ethanol as a viable, low-emission fuel.
This turned out to be more difficult than expected. Few recent independent studies have been conducted removed from the goal of adopting ethanol as a clean energy source in the U.S.
There are so many different approaches to measure well-to-wheel emissions, it’s almost impossible to find an agreed-upon carbon intensity (CI) score among various studies and stakeholders. The U.S. government uses the Department of Energy’s Greenhouse Gases, Regulated Emissions, and Energy Use in Technologies (GREET) model.
Mark Heckman is vice president of sales and marketing at Edeniq, Inc., a California-based biotechnology company, and formerly ethanol services director at EcoEngineers, an Iowa-based engineering consultant. “In the carbon world, people talk about system boundaries — the handoff of where emission measurement starts and when it stops,” Heckman said. “The GREET model is a quick tool that pulls all these emission factors into it, and uses the science that’s coming out of the Department of Energy and Argonne National Laboratory, to state acceptable practices or levels to use, if you take farm practices as a whole.”
Emissions From “Well to Wheels”
Congress created the RFS clean fuel policy in 2005 and expanded it with the Energy Independence and Security Act of 2007, with the goal of reducing GHG emissions and U.S. reliance on imported oil. President Biden’s Inflation Reduction Act (IRA) of 2022 made reducing CI via carbon capture and sequestration (CCS), either on-site or by pipeline transport, vital to access tax credits for ethanol production.
Typically, carbon dioxide emissions are standardized to Carbon Intensity (CI) scores. CI is measured in grams of carbon dioxide equivalent per megajoule of energy.
To comply with the RFS’s required GHG emission reductions, renewable fuels derived from corn must have a CI at least 20% below the gasoline baseline of 93.1 CI the RFS established in 2005. The RFS’s 2010 regulatory impact analysis projected ethanol fuel’s life cycle carbon emissions would have a 73.2 CI by 2022, a 21.4% reduction from gasoline, in line with the reduction target. The baseline CI estimate for ethanol’s life cycle emissions is 53.6, which aligns with the Renewable Fuels Association (RFA) assessment, a 2024 study by the nonprofit think tank EFI Foundation, and a 2019 study by the USDA.
The RFA breaks down the life cycle emissions for gasoline and ethanol from “well to wheels” as follows:
Gasoline: The oil extraction, pre-processing, and transportation to the refinery generates 11 CI. Refining generates 14 CI. Distribution is another 0.5 CI. Combustion in end-use generates 73 CI. This results in a total 98.5 CI.
Ethanol: Corn production generates 22 CI. Transporting the corn generates 1.5 CI. Production at the ethanol refinery generates 28 CI. Distribution of ethanol generates 1.5 CI. Combustion in end-use generates 0.3 CI. This results in a total 53.3 CI.
The EFI Foundation reported ethanol facilities’ CI scores ranging from 48–68 CI. Ethanol fermentation emissions CCS could decrease this by 30.5 CI.
The study estimates ethanol emissions’ CCS cost to be $37 per ton of carbon dioxide (CO2) — lower than the $85 per ton the 45Q tax credit offers.
Heckman added that he believes pipelines are imperative for ethanol to eventually bring the carbon cycle to net-zero.
“If you’re burning ethanol, you’re taking carbon that’s been trapped someplace and putting it into the atmosphere,” Heckman said. ”Somehow, some way, we’ve gotta get it back into the ground.”
Tax Credit Refresher
The 45Q tax credit was expanded under President Biden’s Inflation Reduction Act (IRA) in 2022. This credit incentivizes carbon capture and sequestration (CCS), with the government providing $85 per metric ton (17 cents per gallon of ethanol) for carbon dioxide (CO2) permanently sequestered underground, and $60 per metric ton (12 cents per gallon of ethanol) for CO2 used for enhanced oil recovery or other industrial uses, according to documentation from the Iowa Corn Grower’s Association.
The 45Z tax credit specifically incentivizes fuel producers to lower their CI scores to 50 or lower. For on-road vehicles, 45Z offers 2 cents per gallon up to $1 per gallon for every CI point reduction below 50 CI. For sustainable aviation fuel, the 45Z offers 3.5 cents per gallon, up to $1.75 per gallon for every CI point reduction below 50 CI.
Research Studies
A USDA study published in 2019 found greenhouse gas emissions from corn ethanol were 39%–43% lower than gasoline, about a 56.6 CI score compared with the gasoline baseline. The score could be reduced as much as 43% when refined at natural-gas-powered refineries. A major contributor to the reduction was an updated assessment of land use change (LUC).
While previous estimates projected farmers bringing more land into production as a result of increased corn prices, the USDA study found only a modest increase. The study takes into account additional refinery improvements, as well as farm practices such as reduced tillage and cover crops. The study also projects possible scenarios showing ethanol life cycle emissions becoming 47%–70% lower than gasoline.
US Department of Energy
Conversely, ethanol production spurred by the RFS likely had at least 24% higher GHG emissions than standard gasoline, according to a National Academy of Sciences (NAS) study published in 2022. The study solely looked at the effects of LUC, and the causal change that would have on emissions projected by the RFS’s 2010 regulatory impact analysis.
This research found the RFS increased corn prices by 30%, and the prices of other crops by 20%. This expanded U.S. corn production by 6.9 million acres, and total cropland by 5.1 million acres. Both were higher than the USDA’s 2019 research of 3.65 million acres and 1.44 million acres, respectively. These changes also led to 3%–8% increased annual nationwide fertilizer use and increased water quality degradants 3%–5%.
With their analysis, the score for ethanol increased to 115.7 CI, 24% higher than the baseline 93.1 CI score for gasoline established in 2005 — failing to meet the RFS’s proposed 20% reduction. Even accounting for a 30.5 CI reduction from CCS, this would only lower ethanol to 85.2 CI, about an 8.5% reduction from gasoline.
“Domestic LUC emissions spurred by the RFS undermine the GHG benefits of using ethanol as transportation fuel,” the NAS study states.
“In our case, we looked at the RFS policy associated with ethanol production and said, ‘What impact did that policy in particular have on crop prices?’” said Tyler Lark, a co-author of the article and a scientist at the University of Wisconsin-Madison’s Center for Sustainability and the Global Environment. “Then, we looked at the influence crop prices had on LUC and resulting emissions, as well as the carbon intensity in the fuel.”
Lark said this research centered on the impacts from the growth of land use in ethanol production. This means maintaining the current production level won’t induce additional land use nor further alter the product’s CI. The U.S. has increased ethanol production capacity from 13.6 billion gallons per year in 2011 to 18 billion gallons per year as of January 2024, according to the U.S. Energy Information Administration.
U.S. renewable fuel associations largely discredited the National Academy of Science study. The RFA cited a 2021 study by the Argonne National Laboratory, showing a reduction in ethanol emissions from 58 CI in 2005 to 45 CI in 2019 — a 44% reduction from baseline gasoline emissions. The RFA also sourced farmland data from the USDA showing the amount of land planted for corn in 2021 was essentially the same as in 2007, before the RFS revision was adopted.
The RFA’s largest criticism fell on the NAS study’s methodology, saying its use of “error-prone” USDA Cropland Data Layer satellite imagery provided unreliable data. The study claims farmers converted native grasslands with high carbon storage into corn production because of RFS, but the imagery cannot distinguish land covered in grass from pastureland, hay grass, Conservation Reserve Program land, or native grassland.