The Cost of Abating Greenhouse Gas Emissions with Sustainable Aviation Fuel from Cellulosic Feedstocks
Madhu Khanna, C-FARE Board Member and Professor at University of Illinois, Urbana-Champaign
The aviation sector in the US accounted for about 13% of domestic transportation carbon dioxide emissions in 2022. There is growing interest in producing sustainable aviation fuels (SAFs) to meet the growing demand for aviation fuel while reducing greenhouse gas (GHG) emissions. The production of SAF has grown from 5 million gallons in 2021 to about 16 million gallons in 2022; this is largely based on production that uses waste oils, greases and vegetable oils as feedstocks. The SAF Grand Challenge aims to expand domestic consumption to 3 billion gallons in 2030 and 35 billion gallons in 2050, while achieving at least a 50% reduction in lifecycle GHG emissions.
A promising category of feedstocks for scaling up SAF production is cellulosic feedstocks, such as crop residues, annual crops like energy sorghum, and perennial crops such as miscanthus and switchgrass that can be grown without irrigation. These feedstocks are appealing because they can be grown productively on low quality land and are thus less likely to compete with food crops. They are also higher yielding compared to using food crops for biofuels. There is considerable heterogeneity in the yields and life-cycle carbon intensities of these feedstocks across locations due to differences in suitability of local growing conditions.
In a recent paper, Fan et al. (2024)* show that the preferred feedstock for each location will differ depending on the policy objective, whether this is lowering the breakeven price of SAF, lowering its carbon intensity (or cost of carbon abatement with biomass) or achieving the highest biomass production per unit land. Thus, different types of policies, such as volumetric targets, tax credits and low carbon fuel standards, will incentivize different feedstocks and production locations. A simple SAF mandate is likely to incentivize corn stover-based SAF because it has the lowest breakeven price per gallon, even though it has a relatively high GHG intensity. A ‘low carbon’ fuel standard, on the other hand, is likely to incentivize feedstocks with the lowest cost of GHG abatement. And a SAF tax credit will incentivize low GHG-intensity feedstocks, provided that the gap between the resulting cost and the market price of SAF is lower than the amount of the tax credit.
The breakeven price of SAF from cellulosic feedstocks varies with the choice of feedstock and production location. But it is estimated as currently being at least three to four times higher than the corresponding cost of petroleum aviation fuel. Thus, the cost of GHG abatement with cellulosic feedstocks is currently very high, ranging from $181 Mg-1 CO2e to more than $444 Mg-1 CO2e. The lowest costs are achieved with miscanthus in the Midwest, switchgrass in the south, and energy sorghum in a small part of the Great Plains. However, with further research and development, more ‘learning by doing,’ and the scaling up of production, the cost of producing SAF with cellulosic feedstocks can be expected to fall.
*Fan, Xinxin, Madhu Khanna, Yuanyao Lee, Jeffrey Kent, Rui Shi, Jeremy S. Guest and DoKyoung Lee, 2024, “Spatially Varying Costs of GHG Abatement with Alternative Cellulosic Feedstocks for Sustainable Aviation Fuels” Environmental Science and Technology, 2024, 58, 26, 11352–11362; https://pubs.acs.org/doi/abs/10.1021/acs.est.4c01949
