Virtually no human activity is as difficult to decarbonise as flying. Relatively light, energy-dense aviation fuel — or Jet A-1, as the most common type is known — packs vast amounts of energy into a tiny amount of space and weight. Finding an equally efficient alternative is a daunting prospect.
“Jet A-1 verges on being perfect in terms of weight for power,” explains Andrew Charlton of aviation consultancy Aviation Advocacy.
Right now, the leading alternatives are “sustainable aviation fuels”. These are produced from non-fossil sources — including used cooking oil, animal fats, or even carbon from the air — and can be used as a direct replacement for Jet A-1. But there are many questions about how quickly the production of such fuels can be scaled up, their cost, and the effect of their production on other activities, such as agriculture.
How does it work?
The main “feedstock” for SAF, at present, is used cooking oil. The oil is filtered and “hydrogenated” — a process in which the oxygen is replaced with hydrogen, to turn it into a hydrocarbon. The resulting mixture is distilled.
Producers are also working on ways to turn material from municipal waste into SAF. There is considerable excitement, also, about the potential for a technology called “power-to-liquids” or “e-fuels”, which uses electrolysis to turn carbon pulled from the air into a carbon-based fuel.
Lauren Riley, chief sustainability officer for the US’s United Airlines, describes that technology as particularly exciting.
“You avoid that whole concern about constraints on feedstocks,” Riley says, referring to concerns about the availability of other material to turn into SAF. “Power-to-liquid, literally, is pulling carbon out of the atmosphere and you do that using sustainable power.”
What are the pros and cons?
SAFs should have many of the advantages of Jet A-1 and other conventional aviation fuels, while generating far lower carbon emissions. Although burning the fuel produces carbon dioxide in the same way as with conventional aviation fuels, far less is added to the atmosphere.
Burning fossil fuels emits carbon that has been buried underground into the atmosphere, whereas SAFs mostly release carbon that is already part of the earth’s carbon cycle — already stored in plants that capture carbon from the atmosphere as they grow.
However, there are concerns about access to the feedstocks for SAFs. Supplies of used cooking oil are constrained by the amount used in restaurants and other places where the oil can be collected. There are still greater concerns about the effect on agriculture if large amounts of land are turned over to growing plants for SAF feedstocks.
“The huge risk is: are you using feedstock that could otherwise be used for food?” asks Charlton.
The production of power-to-liquids fuels, meanwhile, could require vast amounts of renewable electricity that could otherwise be used as a clean energy source elsewhere.
Will it save the planet?
SAFs do not entirely eliminate carbon emissions. To be used in many aircraft, they still have to be mixed with some proportion of conventional jet fuel. There are also carbon emissions related to their production and transport. The carbon dioxide produced by their burning will still contribute to global warming until it is reabsorbed by growing plants or other processes that absorb carbon.
Advocates for the technology predict that it will reduce carbon emissions by up to about 80 per cent, but critics suggest that figure may be over-optimistic. And that is on the assumption that large quantities of SAF actually come into use.
Then there are the costs to passengers. Germany’s Lufthansa Group — owner of airlines including Austrian Airlines and Swiss, as well as German flag carrier Lufthansa — announced plans last month for ticket surcharges of up to €72, after new European rules came in mandating that carriers use some SAFs to power their aircraft. However, the mandated proportion is at first only 2 per cent. United Airlines currently uses SAF for only 0.1 per cent of its fuel.
Has it arrived yet?
Some SAFs are already in commercial production. But Riley at United Airlines points out that the total worldwide production of the fuel last year was only 150mn gallons. United, alone, used 4.2bn gallons of conventional aviation fuel in the same year.
Power-to-liquids fuels, meanwhile, are being produced in only tiny amounts, on an experimental basis.
And sustainable fuels remain expensive. Although a litre of SAF typically contains the same energy as a litre of conventional fuel, SAFs generally costs at least twice as much as the same amount of conventional hydrocarbon fuel — and often far more.
There is some optimism that SAF costs may fall as higher production levels bring economies of scale. But there is still likely to be a large price difference for the foreseeable future. United Airlines’ Riley said last month that the biggest risk for the transition to SAFs was the timeline that the industry had to meet.
Who are the winners and losers?
The losers are likely to be passengers — who will face paying far more for tickets as use of SAFs spread — and airlines, whose operations may have to shrink if prices go up.
The beneficiaries are likely to be companies producing the new types of fuel.
Who is investing in it?
Traditional producers of hydrocarbon fuels are mostly not, with some exceptions, involving themselves in SAF production. “The big oil companies have effectively walked off the field,” Charlton says.
There are instead a series of small, specialist start-up producers, many of them publicly listed. They include Finland’s Neste and Gevo, a producer based in Colorado.
There is also the possibility that some airport owners, such as the UK’s Heathrow, will invest in production near their own sites. Some airlines have also suggested they could invest in SAF production, too.