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Flying the low-carbon skies on sustainable aviation fuel

Airlines are eager to switch to renewable, low-carbon fuel—as soon as there’s enough to go around

by Craig Bettenhausen
June 12, 2022 | A version of this story appeared in Volume 100, Issue 21


A tanker truck from Neste marked as containing sustainable aviation fuel fills a commercial airplane's tank.
Credit: Malaysia Airlines
On June 6, Malaysia Airlines flew its first passenger flight using jet fuel made in part by Neste from used cooking oil. It was an hour-long trip from Kuala Lumpur to Singapore. The firms say the fuel will be available at Kuala Lumpur going forward.

The aviation industry is determined to shed its carbon dioxide emissions by ditching fossil fuels. Depending on how you count carbon, airlines are responsible for 2.5–3.5% of global greenhouse gas emissions, according to the nonprofit Our World in Data. Though companies can cut emissions with better routing and aircraft design, there’s no avoiding the need to run planes on some kind of sustainable aviation fuel (SAF).

“We, our customers, this entire industry are absolutely committed to decarbonizing,” said Dale Smith, who leads Boeing’s environmental strategy for commercial flight in North America. He was speaking at the 2022 Advanced Bioeconomy Leadership Conference in March. “We’ve been working on this for a long time. We feel very confident that we can get there and that SAF is the lion’s share of the solution.”

Still, switching to SAF is a tall order that isn’t even beginning to be filled. Global demand for jet fuel is roughly 100 billion gal (379 billion L) per year, according to Jimmy Samartzis, CEO of the SAF technology firm LanzaJet. As of 2021, the biofuel industry was making only about 33 million gal of SAF per year. US president Joe Biden has set a goal for the US to produce 3 billion gal by 2030 and 35 billion gal by 2050. Other countries and regions are setting comparable targets.

To live up to the green ambitions of airlines and governments, SAF production technology needs to mature and scale up, and fast. “We can’t wait the decades that wind and solar took. We have to do this in a 10-year time frame,” Jill Blickstein, an American Airlines managing director for sustainability, said at a recent conference held by the nonprofit Commercial Aviation Alternative Fuels Initiative.

Corporate and academic researchers are developing several chemical routes to SAF. If they can scale up and reduce costs to a reasonable level, airline customers will be lining up. The SAF industry also needs to expand its supply of renewable feedstocks and determine how to compete for production capacity with ground transportation fuels like renewable diesel. SAF won’t succeed unless all that happens.

Chemical conversion
The international standards organization ASTM has approved these four chemical routes to sustainable aviation fuel for use as up to 50% of the fuel volume in commercial airplanes.
A graphic shows chemical routes to biofuels.
Credit: Yang H. Ku/C&EN
Sources: Gaulthier Blangez/Time to Fly; Advanced Biofuels USA; US Federal Aviation Administration; US Department of Energy; Front. Energy Res. 2020, DOI: 10.3389/fenrg.2020.00110; ChemSusChem 2018, DOI: 10.1002/cssc.201801690. a ASTM has not yet approved syngas derived from captured carbon dioxide as a feedstock in the Fischer-Tropsch route.

As is the case for fossil fuels, making biofuels is a matter of getting the right mix of hydrocarbons. A simplified way to think about it is that diesel fuel is mostly straight-chain alkanes with 8–24 carbons. Jet fuel has a similar number of carbons, 8–16. The main difference is that jet fuel contains a much higher proportion of branched and cyclic alkanes, which keep it from getting too thick or freezing at low temperatures high in the sky.

Like all fossil fuels, jet fuel also contains small aromatic molecules. Aromatics cause swelling in rubber and plastic gaskets, O-rings, and hoses in airplanes, so those parts are sized and engineered accordingly. Take the aromatics away, and the parts shrink and start leaking. Because most routes to SAF don’t make aromatics, for now, airplanes are allowed to fly on no more than 50% SAF.

The routes to SAF approved by ASTM, an international standards organization, use one of four main chemical transformations. The most commercially advanced is the hydrotreated esters and fatty acids (HEFA) process, which treats fats, oils, and greases with hydrogen to saturate double bonds and remove oxygen atoms. It then cracks and isomerizes the resulting hydrocarbons to get the right amount of branching for aviation fuel. Most airlines are only accepting HEFA fuel made from used oils to avoid competing with food supplies.

We can’t wait the decades that wind and solar took. We have to do this in a 10-year time frame.
Jill Blickstein, managing director and head of environmental, social, and governance, American Airlines

A second route using fats and oils, called catalytic hydrothermolysis (CH) or hydrothermal liquefaction, hits the lipids with hot, high-pressure water and catalysts to break them down and remove oxygen. The result is a biobased crude oil. Hydrogenation and isomerization refine the biocrude into SAF in a process similar to that used on fossil crude.

Fischer-Tropsch (FT) conversion is a catalytic process that can make SAF from syngas, a mixture of carbon monoxide and hydrogen. Companies such as Sasol have been making fuels and chemicals from coal-derived syngas for decades. If the syngas comes instead from gasifying biomass, garbage, or a papermaking waste called black liquor, it becomes part of a third ASTM-approved route.

FT generally yields straight-chain hydrocarbons, so operators looking to make SAF add an isomerization step to get a healthy amount of branched alkanes.

A fourth route to SAF is called alcohol to jet (ATJ). First, microbes ferment sugars, biogas, or other carbon stocks into alcohols. Then the ATJ process dehydrates that isobutyl alcohol into isobutene or ethanol into ethylene. Those alkenes are oligomerized, or dimerized and then oligomerized, to make SAF.

ASTM has technically approved seven routes to SAF. Beyond the main four, two are versions of HEFA and FT. Another route uses engineered yeast and hydrogenation to make farnesane, a 15-carbon molecule that has proved more valuable as a specialty chemical than as SAF.

Petroleum refiners can also mix up to 5% pyrolysis oil, biocrude, or FT liquids in with their fossil crude when making conventional jet fuel and then sell 5% of their product as biobased. Some start-ups are looking at that approach as a way to scale and survive between their first production of such intermediates and the start of their own internal SAF refining technology.

Almost all the SAF being flown today is made from fats and oils via the HEFA process. The oil company Neste and the biofuel maker World Energy are the main producers, and both firms are expanding significantly. Neste is spending $1.5 billion to expand its Singapore plant from 34 million gal per year to 495 million gal by 2023. World Energy is working with Air Products on a $2 billion project at its site near Los Angeles to grow from 49 million gal to 340 million gal by 2025.

ATJ and FT are neck and neck as the next routes to scale up. Gevo is building and LanzaJet is planning the first commercial-scale ATJ plants; both are targeting 1 billion gal of annual production by 2030. Fulcrum BioEnergy is in the early stages of commissioning a plant in Nevada to make SAF from trash via the FT route.

Further down the road, captured CO2 could be the carbon source for an exciting but unproven method called power to liquids (PTL) or e-fuels. Some combination of hydrogen, heat, or renewable electricity—and lots of it—provide the power to chemically reduce CO2 into more synthetically useful intermediates, usually syngas or alcohols.

Synhelion, a spin-off of the Swiss Federal Institute of Technology (ETH), Zurich, is commercializing a PTL method that extracts CO2 and water from air and passes them over a cerium oxide catalyst, which is heated to 1,500 °C by concentrated solar radiation, to form syngas for use in FT conversion. Synhelion has buy-in from Swiss International Air Lines and Lufthansa, which formed a collaboration with the start-up in March.

Neste and World Energy are also working on PTL. Pratik Chandhoke, an executive at Neste, told the crowd at the Commercial Aviation Alternative Fuels Initiative event that the firm will expand into PTL by 2030. World Energy CEO Gene Gebolys said during the same event that the company has just established a PTL business unit. “Long term, everything points to power-to-liquid technologies,” he said.

PTL, like most routes to SAF, requires hydrogen as an input. Companies would like to get hydrogen from water using carbon-free electricity, but almost all hydrogen today is made from fossil fuels. That conundrum is one reason the SAF now on the market is described as having a lower carbon footprint than petroleum jet fuel, rather than as carbon zero or negative.

Long term, everything points to power-to-liquid technologies.
Gene Gebolys, CEO, World Energy

Even as companies pursue multiple routes to SAF, they are eyeing potential fuel markets other than the aviation industry.

For example, Gevo’s flagship ATJ process uses isobutyl alcohol made by engineered yeast that consumes corn sugar or, eventually, sugar made from nonfood cellulose. Tim Cesarek, the firm’s chief commercial officer, says the technology can swing between making SAF and a renewable gasoline through adjustments to the catalysts used in the reactors. Gevo also has ethanol-based ATJ technology that offers the option to make renewable diesel.

But Gevo is going big on SAF. Of the 1 billion gal per year it hopes to make by 2030, the biggest chunk will come from a deal with the agriculture giant ADM to convert a number of ADM’s fuel ethanol plants to SAF and fit them with carbon capture equipment. Two purpose-built biofuel plants in the US Midwest are to follow.

Cesarek says demand for SAF is strong and growing. “Not only do we have customers that are asking for product, they want a lot of it, and they want it fast,” he said in a presentation at the Advanced Bioeconomy Leadership Conference. “And they want to make certain that it ultimately has a low carbon score, which means hard work to count carbon across the value chain.”

Gevo has already presold much of its SAF. In March, Delta Air Lines agreed to purchase 75 million gal annually for 7 years starting in 2022 or 2023. An airline consortium called Oneworld Alliance—which includes Alaska Airlines, American Airlines, British Airways, Finnair, Japan Airlines, and Qatar Airways—stated its intention to buy 200 million gal annually for 5 years starting in 2027.

In contrast to Cesarek, Paul Schubert, CEO of the start-up Strategic Biofuels, favors renewable diesel as his firm’s flagship product.

Its first plant, to be located in Louisiana, will gasify forestry waste to make syngas and then use FT chemistry to get to renewable diesel. The equipment, licensed from Johnson Matthey and BP, is expected to produce about 32 million gal per year starting around 2025. An undisclosed West Coast fuel distributor has signed a 20-year agreement for the plant’s output.

Schubert says the same reactors could make SAF instead with a simple switch of catalyst along with some changes to tank, pipe, and hose sizes. But for now at least, he says, the revenues from renewable diesel are better, for two main reasons.

The first is technical. Schubert says FT equipment tuned for diesel production yields 87% diesel and 13% naphtha, a lower-value blend of light hydrocarbons. The SAF version of FT puts out only 75% jet fuel and 25% naphtha.

A person fills a car's fuel tank from a dispenser marked as containing renewable diesel.
Credit: Phillips 66
Renewable diesel is available in much larger volumes than sustainable aviation fuel. The two biofuels compete for plant capacity, feedstock, and government incentives.

Secondly, the carbon credits, many of which are based on energy density, are better for renewable diesel. Thanks to government programs and customers’ emission reduction targets, credits for carbon removal are as important as the fuel itself to a producer’s bottom line.

Strategic Biofuels plans to capture substantially all the CO2 created by its plant and inject it into a saline aquifer the firm owns and operates. Overall, the project will remove 42 kg of CO2 from the atmosphere for every gallon of diesel made, according to an analysis by the sustainability consulting firm Life Cycle Associates. This removal includes the CO2 released when the fuel is eventually burned.

Strategic Biofuels is still waiting on approvals for its full-scale CO2 injection well; it expects to have the entire system up and running by the second half of 2025. Gevo, LanzaJet, and others are also working on carbon capture and sequestration at their facilities to drive their biofuels’ carbon footprints below zero.

Renewable gasoline and diesel are useful and profitable today, but SAF has a robust long-term advantage, according to Mike McCurdy, a managing director at ICF, an energy and sustainability consulting firm. Though batteries and hydrogen will take big bites out of the demand for ground transportation fuels, neither energy source works well for long-haul commercial airplanes, he says.


But biofuel consultant Will Thurmond, CEO of Emerging Markets Online, says the superior economics of renewable diesel explain why, despite the strong pull from airlines, biofuel producers made almost 40 times as much diesel as jet fuel in 2021.

Thurmond recently published a report titled Renewable Diesel & SAF 2030 that digs into the two markets. He found that in California, state and federal credits helped make SAF worth about $1 per gallon more than renewable diesel. But SAF’s higher production costs made its profit margin thinner.

As SAF technologies mature and scale up, costs will decrease, Thurmond says. The plant in California that World Energy is building is a big step in that direction, he says. The new feedstocks such as sugar, waste biomass, garbage, and even captured carbon that FT and ATJ will use are also crucial, as the supply of used fats and oils can only support so much scale beyond the current plants and plans.

Andrea Bozzano, a senior director of energy R&D at Honeywell UOP, brought up the feedstock constraints on HEFA at a recent company event. “Used cooking oil isn’t going to get us there; there just isn’t enough,” he said. Honeywell is working with the start-up Alder Fuels to convert forestry and agricultural waste into SAF by way of pyrolysis and petrochemical-style hydrotreatment.

In September, Honeywell and United Airlines made a joint multimillion-dollar investment in Alder that includes an agreement for United to buy 1.5 billion gal of SAF from Alder over 20 years. United also has purchase options on 900 million gal of SAF from Fulcrum.

Thurmond expects the pace of SAF dealmaking and construction to continue as the airline industry races toward aggressive 2030 and 2050 carbon reduction targets. “Over the next year, Neste is going to add 461 million gal, split between their Singapore plant and one of their plants in Europe,” he says. Along with the 33 million gal already being made and other smaller projects, that’s more than 500 million gal by the end of this year or early next year.

“And then from there, we expect to see another 100 to 200 million gal a year until the year 2026, which will give us up to a billion gallons,” Thurmond says. “And that’s a conservative forecast.”


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