WHEN UNITED AIRLINES test pilot Ryan Smith took off from Houston earlier this month for a 90-minute flight over the Gulf of Mexico, he wasn’t carrying any passengers, but he did have a special fuel powering the Boeing 737. One engine was burning standard petroleum-based aviation fuel from a Texas refinery, while the other was running on gas produced entirely from leftover cooking oil and grease from a factory in Los Angeles.

Each engine burned about 600 gallons during the flight, according to United, and created about the same carbon emissions (12,660 pounds). But because the sustainable fuel is made from plant-based sources instead of petroleum, and because plants consume carbon dioxide during photosynthesis, it has a carbon footprint that’s about 70 percent smaller.

“What we were trying to do is demonstrate that the aircraft can operate in the same capacity with sustainable fuel as with blended fuel,” says Lauren Riley, United’s managing director for global environmental affairs and sustainability. “It did. This is a true step in the path of decarbonization.”

Imagine sustainable aviation fuel, or SAF, as part of a big plant-fuel-engine carbon recycling loop, rather than a one-way ticket that sends carbon from a subterranean oil patch directly to the atmosphere. In fact, federal government and industry estimates hold that using SAF can reduce lifetime carbon emissions from 50 to 80 percent depending on the feedstock and type of energy used during manufacturing. The Houston test flight was the first time a commercial aircraft ran at least one engine on 100 percent SAF, which is currently limited to a 50/50 blend on passenger flights.

SAF is produced by refining various plant or animal feedstocks, waste oils from cooking, or solid waste. The Environmental Protection Agency has certified seven kinds so far, although others are in the works. SAF is a drop-in fuel, meaning it can be used without making modifications to existing jet engines. The refining process uses heat and chemical catalysts to turn these feedstocks into fuel, and many companies are using renewable solar or wind energy as their power sources to keep the process low-carbon. In order to reach their carbon reduction goals, SAF manufacturers have to keep track of the energy used in each step of the process, and they even hire auditors to certify their carbon footprint.

The Biden administration is encouraging airlines to use more of this fuel, but there’s just not enough to go around. Only two plants in the United States make SAF: the World Energy waste oil plant in Paramount, California, and the Gevo facility in Silsbee, Texas, which takes an alcohol-based compound made from corn called isobutanol and distills it into aviation fuel.

Because it’s so scarce, SAF costs two to four times as much as regular aviation fuel. Of the 4 billion gallons of jet fuel United buys each year, only about 1 million are SAF. “We’ve got some work to do,” says Riley. “But there’s not enough [SAF] to go around.”

Flying is dirty, at least in terms of global warming. A round-trip flight from New York to LA generates up to 2.4 metric tons of carbon dioxide, depending on the airline. That’s about the same emissions as you’d get from driving a passenger car 6,000 miles or burning 2,653 pounds of coal, according to EPA calculations. Longer flights are even worse. Jetting from Denver to Paris generates 4.9 metric tons of carbon dioxide, while Miami to Shanghai spews out nearly 10 tons of CO₂, according to the Berlin-based Atmosfair carbon flight calculator.

Carbon emissions from jet travel make up only 2.5 percent of global greenhouse gases, but experts worry that as the demand for air travel grows in developing parts of the world, so will that figure. Jet engines burning conventional aviation fuel also produce sulfur and nitrogen pollutants, water vapor, and contrails. These compounds amplify the overall contribution of jet travel to climate warming to an estimated 7 to 8 percent of global emissions, according to a study by US and European researchers published in January 2021 in the journal Atmospheric Environment.

Last month, the Biden administration announced it would include a $4.3 billion research program as well as tax credits for SAF manufacturers in the proposed $3.5 trillion reconciliation bill that is currently being debated in Congress. The White House also said three federal agencies would work with a consortium of US airlines to boost the use of sustainable fuels to 3 billion gallons by 2030.

Right now, the government limits airlines to a 50 percent blend of SAF and conventional fuel, something that United officials are hoping to change following the results of the test flight. “We are showcasing that the aircraft can handle sustainable aviation fuel and the performance is technically the same,” Riley says. “This blending limit is perhaps not needed. We would like to see 100 percent of our airplanes fly on 100 percent SAF.”

Patrick Gruber, the CEO of Gevo, says that after a decade of manufacturing SAF, the technology is ready for a big scale-up that might happen if the Biden tax credits are passed and airlines continue their demand for his green fuel. “The technology is well developed. The products are proven and certified to work,” Gruber says. “Right now, it’s about deploying capital, and making sure everybody can make money.”

Gevo announced plans for a new $800 million facility in Lake Preston, South Dakota, that by the time it opens in 2024 will be able to make 45 million gallons of SAF a year, along with 350 million pounds of animal feed. To keep its carbon footprint low, the plant will run on renewable energy from a nearby wind farm, Gruber says.

Instead of using plants or waste oils, some entrepreneurs want to go even further by manufacturing SAF out of thin air. Well, actually by a neat chemistry trick that combines a carbon molecule pulled from CO₂ in the atmosphere with a hydrogen molecule split from water. The result is a sustainable hydrocarbon fuel, according to Nicolas Flanders, cofounder of Twelve, a startup spun out of a Stanford University chemical engineering lab.

Twelve’s core technology is a suitcase-sized electrochemical reactor that uses a proprietary catalyst to transform CO₂ into fuel using water and electricity. Their idea is to line up stacks of these reactors next to an airborne stream of carbon dioxide being emitted from cement, steel, or other industrial facilities, then turn it into hydrocarbon fuel. “You just combine as many of those modules together as you need to get to whatever jet fuel production capacity is required for a particular site,” Flanders says.

Flanders says this method of producing fuel could also work by pulling CO₂ right out the atmosphere, something called direct air capture, or DAC. A study by UC San Diego researchers published earlier this year postulated that DAC might put a dent in greenhouse gases if there’s a global effort to build tens of thousands of these carbon-sucking machines by the end of the century. But they also warned that even with a massive investment in DAC, this would still only remove a fraction of the carbon dioxide needed to meet the Paris Climate Agreement’s goal of halting global warming at no more than 1.5 degrees Celsius over preindustrial levels.

This method might be a good option a decade from now, says Steve Csonka, executive director of the Commercial Aviation Alternative Fuels Initiative, a public-private partnership between the federal government and the aviation industry. But companies like Twelve aren’t far enough along in the commercialization curve to make the quantities needed by the airline industry. Direct air capture for fuel “is not ready for prime time yet,” says Csonka. “They will be at some point in the future. And if and when we do get those perfected, that basically obviates the need for terrestrial-based feedstocks.”

Until then, airlines will continue to look for new sources of SAF as new manufacturing plants come online. Later this year, United expects to fly the first passenger flight using 100 percent SAF from Chicago to Washington, D.C. Purchasing these new green fuels is more expensive for the airline than using conventional fossil fuels, Riley notes. “But it’s important to send a demand signal to make this transition to lower-carbon flying,” she says.