Airlines, oil and gas companies and some political leaders are placing big bets on sustainable aviation fuel as a pathway to cut greenhouse gas emissions and meet climate targets, but it remains unclear what mix of crops will prove to be the most effective feedstocks.
“Sustainable aviation fuel is an agricultural issue,” said Darrin Qualman, director of climate crisis policy and action at Canada’s National Farmers Union. “It’s going to affect everything.”
SAF demand could be one of the top drivers of how agriculture is shaped in the coming century, and will affect every farm in every country in the world, Qualman said at a recent SAF conference in Houston.
In the near term, the focus will be on feedstocks from corn, soybeans and canola. From there, feedstocks could expand to residues, straw, corn stover and purpose-grown energy crops.
Corn-based ethanol has emerged as a leading option for jet fuel. In January, LanzaJet opened the world's first ethanol alcohol-to-jet fuel facility with support from the federal government. The plant is expected to produce 9 million gallons of SAF and 1 million gallons of renewable diesel in its first year and contribute millions to a local economy, according to the Department of Energy.
The DOE’s 2023 Billion-Ton Report (BT23), released earlier this year, assessed the availability of several biomass resources. It found 1.1-1.5 billion tons of annual biomass production potential in a mature market. It also found that corn-to-ethanol production is already responsible for about 150 million tons of biomass per year.
There’s a lot of ethanol available which makes it an “easy” feedstock, and corn production is becoming more efficient through using multiple parts of corn, said Sean Arians, vice president of sustainable production and value chain engagement at the National Corn Growers Association.
The ethanol industry views SAF has a critical new market with gasoline consumption expected to continue shrinking as cars become more efficient and purchases of electric vehicles increase.
The DOE report identified a number of potential feedstocks and there is research to explore new biomass beyond corn and soy.
Dedicated energy feedstocks like switchgrass that can be grown quickly and with low nutrient inputs are an emerging feedstock opportunity, said Adam Guss, genetic and metabolic engineer at Oak Ridge National Laboratory.
These crops can also be altered to be more environmentally friendly, by increasing the carbon going back into the soil and reducing the level of nitrogen, phosphorus and water required.
Adam Guss
Guss said research centers are looking at other high-yield options like eucalyptus and pine as feedstocks. Crops that grow faster can accumulate more biomass more quickly, which is critical for producing the feedstocks necessary for the SAF transition.
On top of being able to produce sufficient biomass, researchers are trying to understand the genetics of some of these crops to better “engineer them in the future” to make them cheap and the conversion process efficient, Guss said.
“There’s just a lot of opportunity across the U.S. to grow a variety of feedstocks not limited just to the corn belt where so much of agriculture is today,” said Guss.
There are some options like winter camelina that tout water quality, biodiversity and carbon sequestration. It’s also an option for farmers working to adapt to climate change and may benefit from a crop they can plant in the fall and harvest in June.
One of the main challenges of renewable energy crops is the understanding of the agronomics of particularly new crops like switchgrass and miscanthus, said Damon Hartley, group and technical lead of operations research and analysis at Idaho National Laboratory.
Getting both the markets and processing settled while also conducting more demonstration plots and trials will be important to propelling these crops, said Ariel Kagan, director of climate and working lands at Minnesota Farmers Union.
Keeping aviation fuel sustainable
While SAF offers new markets for farmers, Qualman believes the potential impact on land use needs to be considered. He said if by 2050 all SAF were produced from crops, this would require over 400 million tons of SAF, which equates to 1.7 billion tons of crops and 1.4 billion acres of farmland.
As SAF production ramps up, it’s unlikely to rely solely on crop feedstocks. Qualman suggests that even a significant portion of crops devoted to SAF could take a significant amount of land.
In addition to putting pressure on cropland, he said transitioning to SAF could increase agricultural greenhouse gas emissions from increased nitrogen fertilizer use. The demands could also limit the ability of soil to sequester carbon.
He added that the agricultural sector is not prepared to meet the demands of both SAF and bioenergy with carbon capture and storage, or BECCS. This also draws from the same feedstocks, residues and energy crops.
Darrin Qualman“We’re already taking too much, and SAF is a plan to take much much more,” Qualman said.
Others at the conference echoed concerns about canceling out the environmental potential of SAF by potentially upping agricultural greenhouse gas emissions.
“We’re in a global context of increasing population, a wealthier population, higher demand for food and expanding agricultural land globally,” said Audrey Denvir, U.S. lands data and analysis associate at World Resources Institute. “So if the point of scaling and creating sustainable aviation fuel is to decarbonize this sector, corn-based SAF, or any crop-based SAF, really will not achieve that.”
Ethanol creates byproducts with climate benefits, like feed for livestock. However, this does not always offset the carbon cost of producing ethanol from corn and the possible displayed food production, Denvir said.
She said that if land or corn that could go to the food market is diverted to fuel, that causes indirect land use change as that food must be produced elsewhere. This is happening less in the United States but is a concern in other countries.
Denvir said that even if the core of the corn plant is going to fuel and the proteins are separated out for livestock feed, it could still displace sugars and starches that could go to human foods. These environmental concerns are not necessarily limited to corn and ethanol, she said.
She did point to promising areas involving agriculture in SAF, like the use of corn stover. Restricting biofuel production to waste biomass, exploring synthetic e-fuels and supporting hydrogen-powered flights are some alternatives, according to a WRI post Denvir co-authored.
Kagan pointed to the number of products that come from corn biorefining plants, and the economic opportunities it can provide for farmers and rural communities.
Hartley pushed back on assertions that growing crops for fuel production displaces land for farming intended for food. He said energy crops can be grown on land that is less productive and in lieu of crops that require a lot of fertilizer.
“We’re not proposing taking the majority of your prime cropland and turning it into fuel,” Hartley said. “It’s on the fringes. It’s the areas that likely won’t be the highest production food and that case would have the minimum impact on the food supply chain at least on a national level.”
Denvir said the conversation about energy crops is more complex. She said that these present an “interesting opportunity” but it requires a clear definition of marginal or unproductive lands.
Arians pointed to life cycle assessment models to show that improvements can be made to reduce carbon impact in agriculture. The equipment and practices farmers are now using show that things are moving in the right direction, he said.
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