You Can’t Fly with Lead Wings, or Corn Ethanol’s SAF CI Problem

January 16, 2024

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by Steven Slome, Jia Ming Ong and Matthew Morton (NexantECA/Biofueld Digest) ... Accordingly, the main tool for achieving the goal of aviation decarbonization is currently liquid Sustainable Aviation Fuel (SAF). SAF production is poised to grow significantly, supported by governmental measures such as consumption mandates (in the EU) and generous tax credits (in the United States).

At present, SAF production is dominated by lipid feedstock-using Hydrotreated Esters and Fatty Acids (HEFA, also known as HVO) processes, which are technically proven and increasingly commercialized. However, growth of this technology to the scale required to decarbonize global aviation is blocked by a range of barriers, not least by likely constraints on the availability of sufficient low Carbon Intensity biomass feedstocks.[1] Based on current projections of feedstock availability and rate of technological progress, global SAF production by 2050 will fall well short of targets. By 2050, NexantECA sees SAF contributing between 15 and 20 percent of total jet fuel demand[2], significantly short of targets set by governments and industry bodies, many of whom are targeting at least 60-80 percent of SAF use to support their respective net zero goals. If this proportion is to increase significantly, emerging routes to SAF will need to scale up considerably and rapidly.

Among the most promising emerging routes is Alcohol-to-Jet (ATJ) production, which uses bioethanol – already widely in use as a biomass-derived gasoline blendstock – as its primary feedstock. ATJ production entails the dehydration of ethanol to ethylene, followed by oligomerization and subsequent hydrotreating to produce SAF or other drop-in fuel products, ....

ATJ SAF is being viewed as a likely long term SAF option most particularly in the United States, the world’s major producer of ethanol, largely from corn feedstock[4].

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Two key barriers stand in the way of greater corn ethanol use in SAF production (assuming continuing technological progress on production scale-up), both of which are linked to the regulatory drivers of biofuels consumption. Firstly, as a food/animal feed crop, corn – and most other feedstocks for large scale ethanol production such as sugarcane and wheat – are effectively barred from contributing to SAF consumption targets in the European Union, which is likely to be one of the world’s major SAF markets.

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Unless advanced (cellulosic) ethanol production can scale up very significantly, there is therefore little scope for ATJ SAF use to take off in Europe.

No such regulatory barriers on food competition exist for SAF in the United States or in many other markets. However, regulatory requirements are also linked to the second main barrier to increased corn ethanol use in SAF, which is the fact that it has a relatively lower ability to reduce aviation emissions – and therefore to qualify for government support – than some other SAF feedstocks.

As noted, U.S. support mechanisms for SAF and other biofuels are increasingly based on incentivizing fuels with the lowest Carbon Intensity possible.

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Furthermore, for the resultant SAF to qualify for the US$ 1.25 per gallon Blender’s Tax Credit (BTC) offered under the Inflation Reduction Act, the SAF product must be certified to achieve a minimum lifecycle greenhouse gas emissions reduction percentage of at least 50 percent, something of a challenge if corn ethanol based SAF is produced based on current processes without substantial improvements.

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The standard model (the Greenhouse Gases, Regulated Emissions and Energy Use in Technologies, or GREET model used by the Department of Energy) tends to enable ethanol SAF to meet required CI levels, while environmental groups have argued for more stringent modelling and the prioritization of waste-based feedstocks. In late December 2023, the US government reportedly signaled a compromise whereby the GREET model would remain in use, but may be modified in the near future.

Better SAF than Sorry: Advantages of Corn Ethanol as a SAF feedstock

With such a high CI from the start, one might rightly wonder why corn ethanol is worth the trouble as a feedstock. There are several key advantages that ethanol has over other SAF feedstocks that cannot be ignored.

Feedstock Supply

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Corn supply is roughly 4.5 times total oil supply—and while corn also has many existing end-use markets (including food and animal feed), the supply excess of corn is so great that unlike natural oils it actually is larger than global jet fuel consumption currently. Corn ethanol as a feedstock also opens the door to cellulosic ethanol. While cellulosic ethanol has had notable difficulties in commercialization, the allure of lower CI ethanol and great increases in available biofeedstock supply have continued the interest.

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Getting Past the Blendwall

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Ethanol is among the two approved routes currently for mixtures containing aromatics (Annex A4 and Annex A8), as well as having potential integration opportunities with aromatics production (e.g., Ethanol to Aromatics and/or the Vertimass CADO technology).

Integration Potential

Corn ethanol offers potential integration opportunities not necessarily available to other SAF feedstocks and processes:

Carbon Capture Ready: Fermentation to ethanol produces a concentrated stream of CO₂ that can be captured. Many other routes (e.g, HVO) do not have a readily available substantial stream of biogenic CO₂that can be captured easily. A roughly equal amount of CO₂ and ethanol is produced.
Potential for Integration with Aromatics: As stated previously, one approach to getting past the SAF blendwall is to use aromatics. Development of the ethanol to aromatics (and ethanol to gasoline) technology allows for potential integration opportunities and synergies in production of a finished SAF that doesn’t require blending with a fossil fuel. Few other routes still under development are being pursued with this type of integration.

Getting the Lead Out: Shrinking the Carbon Footprint of SAF

SAF production from ethanol can be carbon intensive—though most of the emissions are related to the corn and ethanol production itself (almost two-thirds). Several approaches are being undertaken to reduce the CI of SAF production.

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Approaches aimed at emissions across the value chain include:

Fertilizer decarbonization
Regenerative agriculture / alternative agricultural practices
Electrification with renewable power
Advanced ethanol / alternative feedstocks
BECCS
Biogas integration
Catalyst advances
Blue/green hydrogen

Announced Plans: Low CI Alcohol-Based SAF on the Radar

A few developers have currently planned low CI ethanol-based (and other alcohol-based) SAF. Listed subsequently are the current developers making notable efforts with planned commercial production of SAF and associated products in the next few years:

Gevo: Net-Zero 1 Project for 65 million gallons per year of products
LanzaJet: LanzaJet Freedom Pines Fuels for 10 million gallons per year of products
Vertimass: Vertimass / Ekobenz sustainable fuel plant for eight million gallons per year of product
Swedish Biofuels: Swedish Biofuels for approximately 90 thousand tons per year

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[1] Source: NexantECA’s Market Scenario Planning: Renewable Feedstock Availability to 2050 (2023)

[2] Source: NexantECA’s Market Insight: Sustainable Aviation Fuel (SAF) (2023)

[3] Source: NexantECA’s Biorenewable Insights: Ethanol to Jet (2024)

[4] While minor byproducts from corn processing (e.g. technical corn oil (TCO) or distiller’s corn oil (DCO) can also be hydroprocessed to derive low CI SAF, this discussion specifically refers to the use of corn grain, converted to ethanol via fermentation and subsequently SAF via the ATJ pathway.

[5] Source: CAARB, https://ww2.arb.ca.gov/sites/default/files/2020-09/basics-notes.pdf

[6] Source: ICAO document – CORSIA Default Life Cycle Emissions Values For CORSIA Eligible Fuels https://www.icao.int/environmental-protection/CORSIA/Documents/CORSIA_Eligible_Fuels/ICAO%20document%2006%20-%20Default%20Life%20Cycle%20Emissions%20-%20June%202022.pdf

[7] Source: Argonne GREET, NexantECA Analysis

[8] Source: NexantECA’s Market Scenario Planning: Renewable Feedstock Availability to 2050 (2023)

[9] Source: Argonne GREET, NexantECA Analysis

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used cooking oil (UCO) Transportation Fuels Policy tax policy tallow Sustainability soybeans Regulations-Federal palm oil LCFS (Low Carbon Fuel Standard) Jetfuel (Sustainable Aviation Fuel (SAF)) Investing HEFA (Hydro-processed esters and fatty acids) GREET Greenhouse Gases Regulated Emissions and Energy Use in Transportation Model food vs fuel food and fuel European Union (EU) ethylene ethanol/bioethanol ESG (Environmental Social Governance) environmental policy Department of Energy (DOE) corn/maize corn ethanol Cellulosic ethanol carbon/CO2 sequestration Carbon Intensity (CI)/Carbon Footprint Carbon Dioxide (CO2) carbon capture and storage (CCS) California Blenders Credit Blend wall BioRefineries/Renewable Fuel Production BioChemicals/Renewable Chemicals Aviation Fuel/Sustainable Aviation Fuel (SAF) aviation fuel (SAF) tax credit ASTM aromatics animal fat alcohol-to-jet (ATJ)/ethanol-to-jet (ETJ) ASTM D7566 (Standard Specification for Aviation Turbine Fuel Containing Synthesized Hydrocarbons)

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