Are Green Jet Fuels Finally Ready for Takeoff?

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.”