Wishful Thinking in the Stratosphere

Jet Fuel from Corn Sounds Sustainable, but Isn’t Much Cleaner

By Matthew L. Wald

Technologies intended to save the climate can be categorized three ways: solutions that will probably work, with a little more research & development; solutions that probably can’t work; and solutions that probably work but may not help solve the problem even if they do.

Welcome to Sustainable Aviation Fuel, known as SAF, which is, alas, in the third category. The Biden administration recently tightened the rules on what corn- and soybean-based fuel would be eligible, a recognition that current production methods don’t have much of a carbon advantage. 

There are good reasons for the airlines to want to very publicly embrace SAF and argue that they are reducing aviation’s contribution to climate change. Chief among them: a lot of air travel is discretionary, and taking a flight is high on the list of personal choices that the enviro-conscious like to feel guilty about. The airlines are responding with an initiative to shift from petroleum to ethanol-based fuel. But this will only squander scarce resources on a tactic that has low value for climate, but will improve the industry’s image. 

The airlines are the poster-villain of hard-to-abate industries. They have an installed base of very expensive engines that require fuel that meets narrow specifications; jet fuel must reliably turn to a vapor, no matter the temperature or air pressure, and it cannot freeze or gel at stratospheric altitudes. And the fuel must weigh very little per unit of energy because much of the work an airplane does is to carry the fuel that it will need to burn enroute. (A fully loaded, fully fueled Boeing 727 is about 22 percent fuel, by weight, when it takes off.) So, while cars can get away with substituting heavy batteries for a gasoline tank filled with a light liquid, long-haul aircraft can’t.

And the problem isn’t just Hollywood stars jetting to Sun Valley or Patagonia. The grapes from Chile that you buy in February at a Chicago supermarket, the snail-mail letter delivered to Seattle from Nashville, the fat FedEx envelope you send to your tax accountant before April 15, all have a carbon footprint tied to aviation. 

The most effective tactic for shrinking that footprint to date has been switching to better jet engines and improvements to air frames, like the up-pointed fins on the ends of the wings. The industry says that it has improved fuel efficiency by about 2 percent a year in recent years. Air traffic control improvements have helped, too. The prime reason for these changes is not to save the climate, but to save the airlines money. In fact, they do both. 

But that isn’t cutting emissions. As the price of flying has fallen and global living standards have increased, so has the number of planes in the sky. Efficiency improvement on that order could be overtaken by more air traffic. And as other sectors decarbonize, aviation’s emissions will be a larger slice of a shrinking pie. And demand for air travel is growing, up 9.1 percent in June 2024, compared with June last year. 

So the airlines are touting SAF. 

Earlier this month, the British Advertising Standards Authority banned an ad run by Virgin Atlantic because it implied that the carrier’s use of SAF eliminated carbon emissions, and this was misleading. (Virgin Atlantic’s fuel included used cooking oil and other bio products.) 

And so far, sustainable aviation fuel (SAF) accounts for less than 0.1 percent of fuel burned by U.S. airlines, according to the Government Accountability Office. For years, there have been ambitious expectations about larger volumes. And there is recent growth; JetBlue just signed a contract for a million gallons over the next year at Kennedy International Airport. United Airlines has lined up 2.9 billion gallons, with contracts extending out as long as 20 years, according to BloombergNEF.

United insists that its sustainable aviation fuel has a smaller carbon footprint. Its website suggests that the biomass is waste material and that creation of alcohol is done with renewable energy, which would be very helpful in lowering the fuel’s footprint. Unfortunately, it’s not the usual way of making alcohol from biomass. 

Some of it may be window dressing. The airlines do not want you to feel bad about flying. They want to be seen as doing their part for sustainability. The airlines want a marketing tool and they’ve been  promoting alternative fuels as a sustainability solution for years. 

Their target has changed because the initial efforts were in the second category (solutions that can’t work.) They were focused on non-food crops like Jatropha, but the bloom seems to have faded. Cellulosic ethanol (ethanol from the non-food parts of crops) was all the rage for a while. What if you could make ethanol from corn cobs and stalks, as well as from kernels? Or from wheat straw or lots of other agricultural byproducts. But for years now, nobody can seem to get the recipe right, despite lots of research and generous incentives. 

For ethanol, we’re back to corn kernels. On average, American farmers plant 90 million acres of corn every year, and 45 percent of that goes into ethanol, according to the Department of Agriculture. That land could have been used to plant crops for human consumption, or to feed animals raised for protein. 

Domestic jet fuel consumption is in the range of 1.5 million barrels a day. SAF could be burned in a 50/50 mix with conventional jet fuel, but even that would take a lot of corn. 

A variety of major media have picked up on SAF from ethanol but they don’t answer the key question: would it reduce CO₂ emissions? The New York Times did a front page story with some percentage estimates but only one hard number in it: a $100 million venture capital fund started by United Airlines and others. (The article concentrated on another problem, whether the water needed to grow the corn can be sustainably supplied.)

The farm lobby used to make the argument that corn ethanol would reduce the carbon footprint of cars, but the studies never confirmed much of a carbon impact either way. A key difficulty is accounting for land use changes. The farmers then fell back to the argument that it made the United States energy independent, which doesn’t make as much sense now that we’re at record levels of oil production. 

Why would a strategy that doesn’t reduce the carbon footprint of cars be more helpful in reducing the carbon footprint of jets? (We have to continue using it in cars now because the refineries need it as an octane enhancer. It has an octane of about 109.) 

A white paper issued by Consumer Reports found that the carbon intensity of ethanol can range from - 60 percent to 150 percent of gasoline or diesel. Ethanol makers at one point were using coal as the heat source in ethanol plants, for example. A gallon of ethanol has an energy value of around 80,000 BTU, but making it requires 24,000 BTU of process heat, which almost always comes from fossil fuel.  

Given the very wide range of estimates for the carbon footprint of ethanol, we can acknowledge that we don’t know the actual carbon benefit, if any.  

Last December the Treasury Department ruled that the GREET model (“Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation,” widely used in “well-to-wheels” and similar analyses) wasn’t appropriate for calculating greenhouse gas intensities for the Sustainable Aviation Fuel tax credit. It has too many unexplored assumptions that make a big difference. In April, it released new guidance, under which corn and soybean growers can get credit for no-till farming, using cover crops, and using fertilizer more efficiently. But so-called “climate smart” agricultural practices also have their own problems. Ethanol that is calculated to have a 50 percent benefit can get a tax break of $1.25 a gallon, which is probably a little less than half of the price premium for SAF over kerosine fuel.

And the airlines are highly sensitive to fuel prices; it is their largest expense. At the moment, jet fuel sells for about $6.50 a gallon, and sustainable aviation fuel, about $9.50. At those prices, a voluntary switch is unlikely. It isn’t clear when that alternative will be remotely competitive with conventional jet fuel. 

It will require new fueling systems at airports, among other changes, and we could go to a great deal of effort to take jet fuel out of the system and replace it with something that is almost as bad, or maybe even worse. A 2022 study published by the National Academy of Sciences found that counting land-use changes, the carbon footprint of a gallon of ethanol was 24 percent larger than for gasoline, which implies that it’s not saving carbon when used as jet fuel, either. Other studies have been more positive to ethanol, but there is no consensus that justifies embracing ethanol as a carbon solution. 

There are other possibilities; various industries (including ethanol refineries) produce carbon dioxide, which could be chemically linked to hydrogen that was made by splitting water molecules. Marrying the carbon and the hydrogen would create new hydrocarbons. The fuel then becomes a beneficial re-use of the captured carbon. It makes sense only if the hydrogen has no carbon footprint, and nuclear reactors are good candidates for doing that work. That’s because the clean way to split water molecules is with electricity, but the amount of electricity needed is reduced by about one third if the water is heated first, and the reactors can produce both heat and electricity. (Electricity from wind and sun is also low carbon, but those technologies don’t make heat.) 

But the next step—turning carbon and hydrogen into long-chain hydrocarbons, which are liquid at room temperature and can meet the other specifications for jet fuel—isn’t a commercially viable technology yet. 

And commercializing artificial hydrocarbons might not be sensible. In a perfect world, it might turn out that the price of ethanol, and the ancillary environmental damage incurred by making it, don’t make that pathway a good one for carbon abatement. The same resources put into other carbon abatement technologies, like small modular reactors that would squeeze coal and gas out of the electricity system, or displace gas in heavy industry, might very well abate more carbon dioxide.  

But dollars-per-ton-abated aren’t what’s driving the airline fuel effort. It’s the desire to look responsible. This might be a climate tactic, but it certainly isn't a strategy.