The Search for a Somewhat Sustainable Aviation Fuel

There’s no quick fix for decarbonizing medium- and long-distance flights. Batteries are typically too heavy, and hydrogen fuel takes up too much space to offer a practical solution, leaving sustainable aviation fuels made from plants and other biomass, recycled carbon, or captured carbon as the primary options. Traditionally, this fuel is much more expensive — and the feedstocks for it much more scarce — than conventional petroleum-based jet fuel. But companies are now racing to overcome these barriers, as recent months have seen backers throw hundreds of millions behind a series of emergent, but promising solutions.

Today, most SAF is made of feedstocks such as used cooking oil and animal fats, from companies such as Neste and Montana Renewables. But this supply is limited by, well, the amount of cooking oil or fats restaurants and food processing facilities generate, and is thus projected to meet only about 10% of total SAF demand by 2050, according to a 2022 report by the Mission Possible Partnership. Beyond that, companies would have to start growing new crops just to make into fuel.

That creates an opportunity for developers of second-generation SAF technologies, which involve making jet fuel out of captured carbon or alternate biomass sources, such as forest waste. These methods are not yet mature enough to make a significant dent in 2030 targets, such as the EU's mandate to use 6% SAF and the U.S. government’s goal of producing 3 billion gallons of SAF per year domestically. But this tech will need to be a big part of the equation in order to meet the aviation sector’s overall goal of net zero emissions by 2050, as well as the EU’s sustainable fuels mandate, which increases to 20% by 2035 and 70% by 2050 for all flights originating in the bloc.

“That’s going to be a massive jump because currently, SAF uptake is about 0.2% of fuel,” Nicole Cerulli, a research associate for transportation and logistics at the market research firm Cleantech Group, told me. The head of the airline industry’s trade association, Willie Walsh, said in December at a media day event, "We’re not making as much progress as we’d hoped for, and we’re certainly not making as much progress as we need.” While global SAF production doubled to 1 million metric tons in 2024, that fell far below the trade group’s projection of 1.5 million metric tons, made at the end of 2023.

Producing SAF requires making hydrocarbons that mirror those used in traditional jet fuel. We know how to do that, but the processes required — electrolysis, gasification, and the series of chemical reactions known as Fischer-Tropsch synthesis — are energy intensive. So finding a way to power all of this sustainably while simultaneously scaling to meet demand is a challenging and expensive task.

Aamir Shams, a senior associate at the energy think tank RMI whose work focuses on driving demand for SAF, told me that while sustainable fuel is undeniably more expensive than traditional fuel, airlines and corporations have so far been willing to pay the premium. “We feel that the lag is happening because we just don’t have the fuel today,” Shams said. “Whatever fuel shows up, it just flies off the shelves.”

Twelve, a Washington-based SAF producer, thinks its e-fuels can help make a dent. The company is looking to produce jet fuel initially by recycling the CO2 emitted from the ethanol, pulp, and paper industries. In September, the company raised $645 million to complete the buildout of its inaugural SAF facility in Washington state, support the development of future plants, and pursue further R&D. The funding includes $400 million in project equity from the impact fund TPG Rise Climate, $200 million in Series C financing led by TPG, Capricorn Investment Group, and Pulse Fund, and $45 million in loans. The company has also previously partnered with the Air Force to explore producing fuel on demand in hard to reach areas.

Nicholas Flanders, Twelve’s CEO, told me that the company is starting with ethanol, pulp, and paper because the CO2 emissions from these facilities are relatively concentrated and thus cheaper to capture. And unlike, say, coal power plants, these industries aren’t going anywhere fast, making them a steady source of carbon. To turn the captured CO2 into sustainable fuel, the company needs just one more input — water. Renewable-powered electrolyzers then break apart the CO2 and H2O into their constituent parts, and the resulting carbon monoxide and hydrogen are combined to create a syngas. That then gets put through a chemical reaction known as “Fischer-Tropsch synthesis,” where the syngas reacts with catalysts to form hydrocarbons, which are then processed into sustainable jet fuel and ultimately blended with conventional fuel.

Twelve says its proprietary CO2 electrolyzer can break apart CO2 at much lower temperatures than would typically be required for this molecule, which simplifies the whole process, making it easier to ramp the electrolyzers up and down to match the output of intermittent renewables. (How does it do this? The company didn’t respond when I asked.) Twelve’s first plant, which sources carbon from a nearby ethanol facility, is set to come online next year, producing 50,000 gallons of SAF annually once it’s fully scaled, with electrolyzers that will run on hydropower.

While Europe may have stricter, actually enforceable SAF requirements than the U.S., Flanders told me there’s a lot of promise in domestic production. “I think the U.S. has an exciting combination of relatively low-cost green electricity, lots of biogenic CO2 sources, a lot of demand for the product we’re making, and then the inflation Reduction Act and state level incentives can further enhance the economics.” Currently, the IRA provides SAF producers with a baseline $1.25 tax credit per gallon produced, which gradually increases the greener the fuel gets. Of course, whether or not the next Congress will rescind this is anybody’s guess.

Down the line, incentives and mandates will end up mattering a whole lot. Making SAF simply costs a whole lot more than producing jet fuel the standard way, by refining crude oil. But in the meantime, Twelve is setting up cost-sharing partnerships between airlines that want to reduce their direct emissions (scope 1) and large corporations that want to reduce their indirect emissions (scope 3), which include employee business travel.

For example, Twelve has offtake agreements with Seattle-based Alaska Airlines and Microsoft for the fuel produced at its initial Washington plant. Microsoft, which aims to reduce emissions from its employees’ flights, will essentially cover the cost premium associated with Twelve’s more expensive SAF fuel, making it cost-effective for Alaska to use in its fleet. Twelve has a similar agreement with Boston Consulting Group and an unnamed airline

Eventually, Flanders told me, the company expects to source carbon via direct air capture, but doing so today would be prohibitively expensive. “If there were a customer who wanted to pay the additional amount to use DAC today, we'd be very happy to do that,” Flanders said. “But our perspective is it will maybe be another decade before that cost starts to converge.”

No sustainable fuel is even close to cost parity yet — Cerulli told me that it generally comes with a “roughly 250% to over 800%” cost premium over conventional jet fuel. So while voluntary uptake by companies such as Microsoft and BCG are helping drive the emergent market today, that won’t be near enough to decarbonize the industry. “At the simplest level, the cost of not using SAF has to be higher than using it,” Cerulli told me.

Pathway Energy thinks that by incorporating carbon sequestration into its process, it can help the world get there. The sustainable fuels company, which emerged from stealth just last month, is pursuing what CEO Steve Roberts told me is “probably the most cost-efficient long-term pathway from a decarbonization perspective.” The company is building a $2 billion SAF plant in Port Arthur, Texas designed to produce about 30 million gallons of jet fuel annually — enough to power about 5,000 carbon-neutral 10-hour flights — while also permanently sequestering more than 1.9 million tons of CO2.

Pathway, a subsidiary of the investment and advisory firm Nexus Holdings, has partnered with the UK-based renewable energy company Drax, which will supply the company with 1 million metric tons of wood pellets, to be turned into fuel using a series of well-established technologies. The first step is to gasify the biomass by heating the pellets to high temperatures in the absence of oxygen to produce a syngas. Then, just as Twelve does, it puts the syngas through the Fischer-Tropsch process to form the hydrocarbons that become SAF.

The competitive advantage here is capturing the emissions from the fuel production process itself and storing them permanently underground. Since Pathway is burying CO2 that’s already been captured by the trees from which the wood pellets come, that would make Pathway’s SAF carbon-negative, in theory, while the best Twelve and similar companies can hope for is carbon neutrality, assuming all of their captured carbon is used to produce fuel.

The choice of Drax as a feedstock partner is not without controversy, however, as the BBC revealed that the company sources much of its wood from rare old-growth forests. Though this is technically legal, it’s also ecologically disruptive. Roberts told me Drax’s sourcing methodologies have been verified by third parties, and Pathway isn’t concerned. “I don't think any of that controversy has yielded any actually significant changes to their sourcing program at all, because we believe that they're compliant,” Roberts told me. “We are 100% certain that they’re meeting all the standards and expectations.”

Pathway has big growth plans, which depend on the legitimacy of its sustainability cred. Beyond the Port Arthur facility, which Roberts told me will begin production by the end of 2029 or early 2030, the company has a pipeline of additional facilities along the Gulf Coast in the works. It also has global ambitions. “When you have a fuel that is this negative, it really opens up a global market, because you can transport fuel out of Texas, whether that be into the EU, Africa, Asia, wherever it may be,” Roberts said, explaining that even substantial transportation-related emissions would be offset by the carbon-negativity of the fuel.

But alternative feedstocks such as forestry biomass are finite resources, too. That’s why many experts think that within the SAF sector, e-fuels such as Twelve’s that could one day source carbon via direct air capture and then electrolyze it have the greatest potential for growth. “It’s extremely dependent on getting sustainable CO2 and cheap electricity prices so that you can make cheap green hydrogen,” Shams told me. “But theoretically, it is unlimited in terms of what your total cap on production would be.”

In the meantime, airlines are focused on making their planes and engines more aerodynamic and efficient so that they don’t consume as much fuel in the first place. They’re also exploring other technical pathways to decarbonization — because after all, SAF will only be a portion of the solution, as many short and medium-length flights could likely be powered by batteries or hydrogen fuel. RMI forecasts that by 2050, 45% of global emissions reduction in the aviation sector will come from improvements in fuel efficiency, 37% will be due to SAF deployment, 7% will come from hydrogen, and 3.5% will come from electrification.

If you did the mental math, you’ll notice these numbers add up to 92.5% — not 100%. “What we have done is, let's look at what we are actually doing today and for the past three, four, five years, and let's see if we get to net zero or not. And the answer is, no. We don't get to net zero by 2050,” Shams told me. And while getting to 92.5% is nothing to scoff at, that means that the aviation sector would still be emitting about 700 million metric tons of CO2 equivalent by that time.

So what’s to be done? “The financing sector needs to step up its game and take a little bit more of a risk than they are used to,” Shams told me, noting that one of RMI’s partners, the Mission Possible Partnership, estimates that getting the aviation sector to net zero will require an investment of around $170 billion per year, a total of about $4.5 trillion by 2050. These numbers take a variety of factors into account beyond strictly SAF production, such as airport infrastructure for new fuels, building out direct air capture plants, etc.

But any way you cut it, it’s a boatload of money that certainly puts Pathway’s $2 billion SAF facility and Twelve’s $645 million funding round in perspective. And it’s far from certain that we can get there. “Increasingly, that goal of the 2050 net-zero target looks really difficult to achieve,” Shams put it simply. “Commitments are always going up, but more can be done.”