How do we clean up air travel? Fuel from fast-food grease is just the start

With demo flight, United Airlines helps chart a path for using 100% sustainable aviation fuels.

A December 2021 United flight from Chicago to Washington, D.C. carrying more than 100 people was the first passenger flight to use only "sustainable aviation fuels" in one of its two engines. (United Airlines)
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When United Airlines flew a plane using sustainable aviation fuels” earlier this month, the company took a small but meaningful step toward cleaning up the aviation sector. With more than 100 people on board, the Boeing jet flew from Chicago to Washington, D.C. burning only alternative fuels in one of its two engines — a first for any aircraft full of passengers.

The December 1 flight revealed two things about the global effort to curb the carbon emissions that result from hauling people and cargo in the sky. First, the tiny amounts of sustainable aviation fuels” (SAFs) that airlines use today are made from surprisingly mundane materials: used cooking oils and discarded animal fats. One major fuel supplier, named Neste, gathers its waste ingredients from some 45,000 fast-food kitchens across North America.

Second, much more innovation is needed both to boost supplies of SAFs and to ensure more of the fuels can be used in existing jet engines. Old burger grease and other alternatives don’t meet the criteria for jet fuels on their own, so fuel companies are required to blend them with petroleum-based kerosene before they can supply commercial airlines.

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United avoided this conundrum on the Dec. 1 flight by combining greasy, fatty fuels with synthetic compounds made from corn sugar. The compounds performed the role of petroleum in the fuel mix, allowing United to claim 100 percent SAF” for the 500 gallons used in that particular engine. The second engine, meanwhile, burned only fossil fuels.

The milestone comes as airlines, manufacturers and policymakers in the United States and European Union are pushing to drastically boost adoption of sustainable aviation fuels for commercial flights starting this decade. 

Commercial air travel accounts for roughly 3 percent of U.S. greenhouse gas emissions and about 2.5 percent of global emissions every year. As the industry bounces back from Covid-related disruptions, aviation emissions are on track to triple by 2050 if nothing changes, according to an analysis by the International Council on Clean Transportation.

Experts say batteries are unlikely to be able to power anything but small planes on shorter routes. Other zero-emissions solutions — such as liquid hydrogen made using renewable electricity — are still many years from takeoff at commercial scale. (Relatedly, this week United acquired a stake in ZeroAvia, a startup developing hydrogen-electric engines for regional aircraft.) 

Although SAFs aren’t entirely emissions-free, fuels made from waste materials have the potential to start curbing carbon dioxide pollution using the aircraft and fuel infrastructure we have now and will continue to use in the coming decades.

Eventually, We want to just use [SAF] 100 percent so that we can actually take a significant step toward decarbonizing our operations,” Lauren Riley, United’s managing director of global environmental affairs, told Canary Media. 

The seven shades of SAF

Today, sustainable aviation fuels represent less than 1 percent of total jet fuel demand, with about 26 million gallons produced worldwide every year. Both the Biden administration and European officials aim to increase production to several billions of gallons a year by 2030.

Seven types of SAF are now approved for drop-in” use by ASTM International, which sets technical standards for the aviation industry. These include fuels made from landfill garbage, corn stalks, sugar beets and algae, though the most common fuel is hydroprocessed esters and fatty acids (HEFA) — made from pork fat, beef tallow and the yellow grease collected from fast-food kitchens. 

But used onion-ring oil and other SAFs are missing a key element: aromatics. 

These compounds help lubricate jet engines and reduce fuel leakage around the seals, and today they’re primarily made from hydrocarbons. To ensure fuels are compatible with engines and infrastructure worldwide, industry standard-setters limit the amount of alternatives that can be blended with conventional jet fuel. For instance, producers can’t use more than 50 percent HEFA in a given fuel mix. 

The recent United flight was part of a larger initiative to develop new aromatic compounds and also demonstrate that engines can handle higher shares of SAF. Virent, a subsidiary of Marathon Petroleum, provided synthesized aromatic kerosene” made from corn dextrose and other plant-based sugars at its facility in Madison, Wisconsin. World Energy produced the remaining sustainable HEFA fuels, while Boeing and CFM International supplied the jet and engines, respectively.

An aircraft fueler in Chicago supplies the sustainable aviation fuels for United's December 1 flight. (United Airlines)

In October, with federal approval, United conducted a passenger-free test flight using a mix of 80 percent HEFA and 20 percent Virent’s sugary aromatics. According to Riley, the test pilots said the plane operated as expected. You would not know the difference between SAF and conventional jet fuel,” she said, though the pilots did notice the smell of french fries in the galley because of the used cooking oil. 

The December demonstration flight, which used the same proportion of fuels, was the next step in proving the new fuel’s compatibility with jet engines. Riley, who was a passenger on the second flight, said it felt like a wedding party — and she didn’t catch a whiff of fries.

Next-generation engines, hold the soot

Aromatics are a mixed blessing for the climate, however. While they enable jets to burn sustainable fuels, the compounds are also a source of soot emissions and contribute to the condensation trails (“contrails”) from aircraft exhaust — which researchers say are making the planet warmer.

Aromatics are the chemicals we need to get to 100 percent SAF…but they’re also very harmful to the environment,” said Pratik Chandhoke, the technical services manager for renewable aviation at Neste, the world’s largest producer of sustainable fuels from waste and residues.

So engine manufacturers are working to develop new models that bypass the need for aromatics altogether, he said. 

Headquartered in Finland, Neste is part of a research initiative in Europe to test the use of only HEFA fuels in newly designed Rolls-Royce engines on an Airbus passenger jet. A series of test flights this spring over the Mediterranean Sea showed no operational issues or engineering obstacles to using the greasy waste oil in the next-generation engines, according to companies and researchers participating in the Emission and Climate Impact of Alternative Fuels study.

As this technology gets adopted and a lot of the [commercial jet] fleet starts changing over to new aircraft, I think we’ll see a lot more 100 percent SAF demand then,” Chandhoke said from his office in Houston. 

Beyond the fast-food fry vat

Both the United flight and European engine tests are largely working to solve a future problem. Even if jets were technically capable of running on 100 percent alternative fuels today, there’s not enough supply of SAFs right now to meet demand. 

It’s irrelevant today because we’re nowhere near that limit. But it could be important in 2050,” said Dan Rutherford, director of the aviation and maritime programs at the International Council on Clean Transportation. (Rutherford recently joined Shayle Kann on Canary Media’s Catalyst podcast.)

As the fuel industry works to boost production of SAFs, much of the supply will continue to come from HEFA, which is considered a mature technology and is also commonly used for renewable diesel in trucks. In 2020, Neste acquired Illinois-based Mahoney Environmental to support its expansion into North America. Mahoney collects and recycles used cooking oil from fast-food spots like Burger King, Wendy’s and Buffalo Wild Wings. After being processed into fuel in Houston and transported to California, the HEFA is delivered to a hub at San Francisco International Airport. 

Yet the world discards only so much french fry oil and beef fat. If all the HEFA available today were used solely for the aviation sector, it would still only cover about 2 percent of the annual jet fuel supply, Rutherford said. Beyond that point, producers will need to look for new materials. A concern that Rutherford and other experts share is that those feedstocks could include crops like corn or soy, which might result in displacing food supplies or driving deforestation and land-use change.

For its part, United is also investing in fuels made from other types of waste, such as leftover corn stalks or fallen branches and leaves. In 2015, the airline bought a $30 million stake in Fulcrum BioEnergy, which recently finished building a commercial-scale plant in Nevada that will turn household landfill garbage into synthetic fuels. Neste is likewise looking to develop fuels made from municipal solid waste, forest residues and, potentially, hydrogen. 

Given that a typical jet can operate for 25 years, planes and engines built today will likely face increasingly stringent emissions standards over their lifetimes. In many ways, the push to use 100 percent sustainable aviation fuels is an attempt to future-proof” commercial jets, especially as airlines pledge to achieve net-zero emissions by 2050, Rutherford said.

If you’re a manufacturer,” he added, you kind of have to have an argument for why this will never be a stranded asset.”

Maria Gallucci is a clean energy reporter at Canary Media, where she covers hard-to-decarbonize sectors and efforts to make the energy transition more affordable and equitable.