Virtually no human activity is harder to decarbonize than flying. Relatively light, energy-dense aviation fuel (the most common type is known as Jet A-1) packs a huge amount of energy into a tiny amount of space and weight. Finding a comparably efficient replacement is a daunting prospect.
“The A-1 jet is almost perfect in terms of power to weight,” explains Andrew Charlton of aviation consultancy Aviation Advocacy.
Currently, the main alternative fuel is “sustainable aviation fuel”. These are produced from non-fossil resources, including used cooking oil, animal fats, and even atmospheric carbon, and can be used as a direct replacement for A-1 jetliners. However, there are many questions about how quickly production of such fuels can be scaled up, the cost, and the impact on other activities such as agriculture.
How does it work?
Currently, SAF’s main “feedstock” is used cooking oil. The oil is filtered and then “hydrogenated,” a process that replaces oxygen with hydrogen to turn it into hydrocarbons. The resulting mixture is then distilled.
Producers are also working on ways to produce SAF from municipal waste, and there is great promise in a technology called “Power-to-Liquids,” or “e-fuels,” which uses electrolysis to turn carbon extracted from the air into carbon-based fuels.
Lauren Riley, chief sustainability officer at United Airlines in the US, said the technology was particularly interesting.
“It avoids all of the concerns about feedstock constraints,” Riley says, referring to concerns about the availability of other materials that can be converted into SAF. “Power-to-liquids literally takes carbon out of the air, and it does it using sustainable electricity.”
What are the pros and cons?
SAF has many of the benefits of Jet A-1 and other conventional aviation fuels, but with much lower carbon emissions. When burned, the fuel produces the same amount of carbon dioxide as conventional aviation fuels, but much less of it is released into the atmosphere.
Burning fossil fuels releases carbon buried in the ground into the atmosphere, but SAF primarily releases carbon that’s already stored in plants, which capture carbon from the atmosphere as they grow and are therefore part of the Earth’s carbon cycle.
However, there are concerns about access to feedstock for SAF. The supply of used cooking oil is limited by usage in restaurants and other places where the oil can be collected. There are even greater concerns about the impact on agriculture if large amounts of land were to be diverted to grow plants for SAF feedstock.
“The big risk is whether you’re using ingredients that could actually be used in food,” Charlton asks.
Meanwhile, producing liquid fuels could require large amounts of renewable electricity that could be used as a clean energy source elsewhere.
Will it save the planet?
SAF does not completely eliminate carbon emissions; it must be blended with traditional jet fuel for use in many aircraft, and its production and transportation also have carbon emissions. The carbon dioxide produced by combustion continues to contribute to global warming until it is reabsorbed by plant growth or other carbon-sucking processes.
Proponents of the technology estimate it could reduce carbon dioxide emissions by up to about 80 percent, but critics say that figure may be too optimistic — and it assumes that large amounts of SAF will actually be used.
Then there are the passengers: Germany’s Lufthansa Group, which owns Austrian Airlines, Swissair and Germany’s flag carrier Lufthansa, last month announced plans to impose a ticket surcharge of up to 72 euros in response to new European rules that will require airlines to use SAF in their planes’ fuel, but the requirement will initially be just 2%; United Airlines currently uses SAF in just 0.1% of its fuel.
Has it arrived yet?
Some SAF is already being commercially produced, but United’s Riley points out that global production of the fuel last year was just 150 million gallons. United alone used 4.2 billion gallons of conventional aviation fuel that year.
Meanwhile, Power-to-Liquids fuel is being produced in very small quantities on an experimental basis.
And sustainable fuels remain expensive: Although one liter of sustainable fuel typically contains the same energy as one liter of conventional fuel, sustainable fuels typically cost at least twice as much, and sometimes more, than the same amount of conventional hydrocarbon fuel.
There is optimism that rising production levels could bring down the cost of SAF as economies of scale become possible, but large price differences are likely to remain for the time being.United Airlines’ Riley said last month that the biggest risk to the transition to SAF is the deadline the industry must meet.
Who are the winners and losers?
The losers are likely to be passengers who will pay more for tickets as SAF use becomes more widespread, and airlines who will be forced to scale back operations if prices rise.
The beneficiaries are likely to be companies that produce new types of fuel.
Who is investing?
Traditional hydrocarbon fuel producers, with a few exceptions, have had little involvement in SAF production. “Big oil has essentially exited the sector,” Charlton says.
In their place are a number of smaller, more specialized emerging manufacturers, many of which are publicly traded, including Finland’s Neste and Colorado-based manufacturer Gebo.
Some airport owners, such as Heathrow Airport in the UK, may also invest in production near their sites. Some airlines have also indicated they may invest in SAF production.
