Assessing the sustainability implications of alternative aviation fuels
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Hungry Hungry Airlines
Growing up in the late 1970s, my brothers and I played a popular if ridiculous game called “Hungry Hungry Hippos”. In it, players would compete to snatch as many marbles as possible using little plastic hippos with retractable heads. There’s zero strategy involved—just a mad, loud feeding frenzy lasting a minute at most, followed by repeated games and eventual exhaustion.
My brothers and I graduated from Hungry Hungry Hippos long ago, but a similar, real-life, high-stakes game may now be unfolding. Here, the marbles are sustainable aviation fuels (SAFs)—alternatives to fossil jet fuel that are produced from biological or renewable energy feedstocks. In the Hungry Hungry Airlines version, US carriers compete to snag as much corn and soybeans as possible to secure supplies of SAFs. And in the end, people, not airplanes, may end up tightening their belts.
Policy—some of which seems at odds from its original, laudable intent—is creating this new game. In November 2021, the US government committed to cutting emissions from US carriers to net-zero by 2050 and joined a coalition of countries pushing for a greenhouse gas (GHG) target for international aviation consistent with a 1.5º C future. And in September, the US announced the “Sustainable Aviation Fuel Grand Challenge”, a whole-of-government initiative to spur production of 3 billion gallons of SAF by 2030.
While SAFs will likely play a key role in the decarbonization of aviation, the care needed to promote scalable SAFs with low life-cycle emissions is missing from the administration’s approach. The Grand Challenge’s production target for 2030 is startlingly high—more than 100 times the global production of SAFs in 2020 – suggesting an emphasis of quantity over quality. Crop-based biofuels that rely on off-the-shelf conversion technologies are likely to be deployed to meet this aggressive target. But producing transport fuels from corn, soy, and palm is problematic for several reasons.
Substantial emissions. Crop-based biofuels generate both direct and indirect emissions that undermine their life-cycle performance. Direct emissions come from energy used to grow, harvest, and process crops into fuel. Indirect emissions result when forests are felled to open new land to replace food crops diverted to fuel production. The UN’s International Civil Aviation Organization (ICAO) estimates that SAF produced from US soy reduces GHG emissions by an average of only 27% over its full life cycle compared to fossil jet fuel. Corn alcohol-to-jet SAF fares even worse: The isobutanol-to-jet pathway yields a modest 12% reduction in emissions, while the ethanol-to-jet pathway results in a 2% increase in emissions.
Crucially, these emissions outcomes fall well short of the 50% minimum reduction in life-cycle GHG emissions required by the Grand Challenge. Given the poor emissions performance of biofuels for aircraft, biofuel lobbyists are turning to sleight-of-hand, appealing for changes to carbon accounting rules to permit the use of poorly performing crops. They also seek to sideline US EPA in favor of agencies friendlier to biofuels like USDA and FAA. Changing the rules and players may be good for agricultural interests, but they do little to stabilize the climate.
A big bite out of food stocks. In 2020, the US already devoted about a third of its corn and soybean harvests to production of gasoline and diesel for road transport. Biofuels for aviation would divert even more food into fuel production. Consider soybeans. Assuming 27 kg of soy per bushel, and 16% of fuel yield per kg of soy, production of 3 billion gallons of SAF using soybeans would require about 2 billion bushels of soybeans. That’s about half (49%) of 2020 US soy production, the equivalent of 90% of US soy exports for farm year 2020. Yet it would reduce GHGs from US flights by only 3%, well short of the SAF Grand Challenge goal of a 20% reduction. Devoting all of US 2020 soy production to jet fuel would replace about one-quarter of 2030 jet fuel and reduce emissions by just 6%.
What about corn? Assuming 25 kg of corn per bushel and a fuel yield of 20% (on a mass/mass basis), producing 3 billion gallons of SAF would require 9% of 2020 US corn production, and a bit less than half (45%) of US exports. But the emissions picture is actually worse than for soy. Via an isobutanol-to-jet pathway, 3 billion gallons of SAF would reduce GHG emissions from US flights by only 1.4%; for corn ethanol, emissions would be largely flat (+0.2%). It’s also costly: at an estimated production cost of more than $6 US per gallon of fuel, corn alcohol-to-jet fuels are an expensive way to achieve only minimal reductions in GHG emissions.
Been there, done that. The story of large crop diversions for small environmental benefit is a familiar one. Congress’s General Accounting Office concluded in 2019 that the US Renewable Fuel Standard (RFS), the main federal policy promoting biofuels for road transport, had provided little to no GHG benefit, failed to promote the advanced biofuels needed to drive deeper decarbonization, and increased food prices, to boot. And newer research has concluded that the RFS led to a net increase in GHG emissions compared to petroleum fuels due to accelerated land conversion in the US.
Soaring food prices. If food price inflation made the RFS poor policy in the relatively price-stable 2005–2020 period, imagine how irrational a similar Grand Challenge policy is today, as food prices gallop upward. Supply constraints drove food prices up by 8% from February 2021 to January 2022 (due to COVID-driven supply chain disruptions), and are expected to push prices up another 3 to 6.5% this year (due in part to the Ukraine war). Add to the equation a new source of crop demand—jet fuel production—and upward pressure on prices will likely worsen.
These challenges suggest that the Biden administration should refocus the Grand Challenge on the highest quality fuels that provide large GHG benefits, rather than on food crops. High-quality fuels include the aviation equivalent of the advanced biofuels that the RFS failed to generate. Because advanced biofuels will require additional time and effort to mature, a more modest goal of 1.5 billion gallons of SAF using the best feedstocks – those that provide an 80% reduction in lifecycle emissions—would actually reduce emissions more than a higher volume met with crop-based fuels.
The figure below compares the projected performance of SAFs made from food crops (a 50/50 mix of soy and corn butanol, red dotted line) with that of advanced biofuels produced from cellulosic wastes like agricultural and forestry residues (blue). As shown, a lower volumetric target that promotes higher quality feedstocks would actually cut emissions more in 2030 than a crop-heavy ambitious target. Such a policy could also help avoid a feeding frenzy for low-quality feedstocks that eat into limited global reserves, exacerbating food cost shocks for vulnerable consumers today.
Figure. CO2 emissions from US civil aviation by SAF type, 2010 to 2030
Right now, governments worldwide are struggling to contain inflation and to ensure sufficient food supplies for their citizens. This includes moves to restrict the export of vegetable oils that can also be used to generate transportation fuels. It would be sadly ironic if the US did the same—in order to prioritize feeding its planes over people abroad. That’s a game we should refuse to play.
