There was a lot of excitement about the potential boost to farm income when the US introduced a national initiative to exponentially increase the production of “sustainable aviation fuel”, aka SAF. That excitement has turned to frustration, though, with farmers largely left on their own to navigate the crop possibilities. Researchers at the University of Illinois have now done some of the leg work for producers looking for the right fit for their fields. They also hope the results will help guide policymakers.
The U.S. currently consumes 23 billion gallons of jet fuel per year, and aviation fuel accounts for roughly 13% of domestic transportation carbon dioxide emissions, the researchers report in their analysis in the journal Environmental Science and Technology. The so-called “Sustainable Aviation Fuel Grand Challenge,” a joint effort by the U.S. Department of Energy (DOE), the U.S. Department of Transportation (DOT), the U.S. Department of Agriculture (USDA), and other federal government agencies, is aimed at expanding production of SAF to 3 billion gallons by 2030 and 35 billion gallons by 2050.
The mix of bioenergy crop feedstocks that will be produced to meet this challenge, their relative costs and carbon intensities will depend on how the goals of the policy are specified, said Madhu Khanna, a professor of agricultural and consumer economics at the University of Illinois Urbana-Champaign and the director of the U. of I. Institute for Sustainability, Energy and Environment. Khanna led the study with Xinxin Fan, a postdoctoral researcher at iSEE.
“It’s a huge task to weigh all the factors that make a particular biofuels feedstock economically or environmentally viable,” notes Khanna. “You have to consider all other potential uses for the land used to grow the crop, the costs of establishing a new crop, and numerous other factors like weather, soil carbon and the productivity of a given crop in a particular location.” Fan notes that the costs of converting different feedstocks into biofuels and the greenhouse gas emissions associated with growing and transporting them to a refinery also need to be consider.
Khanna’s team analyzed the financial and environmental costs and benefits of four biofuels crops used to produce sustainable aviation fuels in the U.S. They found that each feedstock — corn stover, energy sorghum, miscanthus or switchgrass — performed best in a specific region of the rainfed United States. The goal was to identify feedstocks with the lowest “break-even price” for a grower switching from another viable crop, the lowest carbon intensity and cost of carbon abatement, and the highest biomass produced per unit of land.
The researchers divided the rainfed zones of their study area — encompassing most of the continental U.S. from the Dakotas, Nebraska, Kansas, Oklahoma and Texas eastward — into 1.5-mile-square plots. They focused on four zones: the Great Plains, Midwest, Northeast and Southeast. They then determined the break-even costs for a grower switching from the next most viable crop to a biofuels crop. The scientists also modeled the different growing conditions, carbon emissions, and the costs and benefits across the life cycle of each feedstock.
Miscanthus and switchgrass were notable for their low carbon intensities, particularly in the Midwest and South, making them favorable for regions prioritizing environmental benefits. Conversely, corn stover showed the lowest cost of production but higher carbon abatement costs, suggesting its suitability where production volume is prioritized.
Different policies would favor some feedstocks over others, Khanna said. Corn stover would win out if policymakers prioritized the volume of production over the total reduction in greenhouse gas emissions. However, use of this feedstock would reduce soil carbon stores, making it more carbon-intensive than the other energy crops. Miscanthus and switchgrass increase soil carbon and would do much more to lower greenhouse gas emissions than corn stover. However, these feedstocks are more expensive to produce, requiring an incentive like a carbon tax credit to make them economically viable.
Ultimately, the researchers conclude, “either carbon prices would need to rise or the cost of producing sustainable aviation fuels will need to fall to make SAFs an economically attractive alternative to jet fuel.” The full study is HERE.
