Taking off with sustainable kerosene
One of the big hopes of the air transport industry as it pursues its proclaimed goal of achieving climate neutrality by 2050, is sustainable aviation fuel, SAF for short. The kerosene currently used consists of a mix of certain hydrocarbons derived from crude oil. During combustion in jet turbines, carbon dioxide (CO₂) is released along with the energy. Its concentration in the atmosphere increases, and the climate heats up.
SAF is made up of the same hydrocarbons and could, unlike electric power and hydrogen, immediately replace fossil fuel-based kerosene. “SAF can be directly integrated into the existing airport infrastructure and, with a few adaptations, used in conventional engines,” explains Marco Ranocchiari, a PSI chemist and head of the Energy System Integration (ESI) Platform. ESI is a test facility for environmentally friendly energy sources of the future.
The advantage of SAF for the climate is that the hydrocarbons are not extracted from the earth in the form of crude oil and thus do not place an additional burden on the atmosphere. Instead, biological material from the surface serves as the source – to date, this has mainly been vegetable and animal cooking oils and fats. The carbon they contain comes from the atmosphere, so the CO₂ concentration remains the same. The aircraft’s propulsion is climate-neutral – provided SAF production and transportation are done exclusively with renewable energy instead of fossil fuels. This is not currently the case, which is why SAF cannot yet cut the CO₂ emissions of a flight to zero, though it can at least reduce them by around 80 percent.
However, producing the required quantities of SAF at an affordable cost is a major challenge. Around 600 million litres of SAF were produced worldwide in 2023, a tiny fraction of the approximately 325 billion litres required. The International Air Transport Association (IATA) estimates that 450 billion litres will be needed in 2050 to fully replace fossil kerosene.
PSI researchers, together with a number of different partners, are investigating, developing, and optimising several options for producing kerosene without petroleum. For example, in the three-year SynFuel Initiative, together with the Swiss Federal Laboratories for Materials Science and Technology Empa and with financial support from the ETH Board. Or with the climate start-up Metafuels.
The available biomass is not enough
One option is based on biomass. SAF made from cooking oils and fats is already in use, and to date this is the only certified product. In a process known as hydrolysis, oils are turned into fatty acids. Further processing then transforms these into a product that resembles crude oil. Finally, this is refined with hydrogen to become bio-kerosene. At present, this first-generation SAF can be blended up to 50 percent with conventional kerosene.
But humanity cannot consume enough fried food to generate sufficient amounts of used cooking oil to produce the quantity of SAF that is needed. So researchers are exploring other ways to convert biomass into kerosene. Sawdust and other plant waste from gardening, agriculture, and forestry are suitable, as is sewage sludge. Using various hydrothermal processes – involving heat, pressure, and water – the researchers can convert these materials into the desired hydrocarbons, which are then refined with hydrogen to make kerosene.
Yet even that will not be sufficient, so the researchers are looking at other options to replace fossil kerosene. One variant of SAF could be produced artificially using renewable electricity (power-to-liquid) or directly using the power of the sun (sun-to-liquid) from the simple precursors hydrogen and carbon dioxide. This second generation of SAF is also known as e-kerosene. Considerably greater quantities could be produced this way than from used cooking oils.
Artificial fuels are especially friendly to the environment
One method of producing fuels from hydrocarbon-rich solids and gases has been known for a long time: It is called Fischer-Tropsch synthesis, named after the German chemists Franz Fischer and Hans Tropsch, who filed a patent for the process nearly 100 years ago. In this process, carbon monoxide is hydrogenated with hydrogen at temperatures of 150 to 350 degrees Celsius on the surface of catalysts containing cobalt or iron, which regulate the chemical reactions. The process is well established and is used today on a large industrial scale.
“Kerosene, however, is particularly challenging,” says PSI chemical engineer Jörg Roth, project coordinator for SynFuel. This very energy-rich fuel consists of a special combination of light and heavy hydrocarbons. It’s this combination that gives kerosene the specific viscosity, boiling point, flash point, and other parameters that must be maintained to ensure the necessary level of safety in an aircraft. But the Fischer-Tropsch process produces a wild mixture containing many types of hydrocarbons that are not wanted in kerosene. “In order to obtain high-quality kerosene, the product has to be treated at great expense, with a corresponding loss of efficiency,” Roth acknowledges.
Methanol route is promising
PSI researchers are working on developing alternatives. One possibility is known as methanol synthesis. In this process, methanol is produced from carbon monoxide and hydrogen at high pressure using catalysts made of zinc oxide and copper. This too is a long-established process, but only recently has research revealed how longer-chain hydrocarbons such as ethylene (C₂H₄), propylene (C₃H₆), butylene (C₄H₈), and finally kerosene can be synthesised from methanol (CH₃OH) in a series of additional steps. Reactor type, temperature, pressure, and the relative proportions of hydrogen and carbon oxides are just a few of the parameters that have to be tuned to each other to accomplish this. “But the critical thing is the choice of catalyst,” says ESI head Ranocchiari. A catalyst is a material that promotes certain chemical reactions; some reactions are not possible without the catalyst. In the present instance, the catalyst controls the increasing length of the hydrocarbon chains. The size of the pores on its surface determines which molecular chains are formed. The right catalyst therefore ensures that there are fewer unwanted by-products. Above all, it stops the reactions at a specific point. “Otherwise, the carbon chains would become endless, and we’d just end up with wax,” Jörg Roth explains.
The insights gained on the ESI Platform are also the basis for a promising industrial collaboration that started at the beginning of last year: “We were approached by the Swiss start-up Metafuels,” says Ranocchiari says. The company believes that there is a huge market for SAF, and it already had a business plan and had secured powerful investors in the green technology sector. What it still lacked, however, was proof that SAF could be produced more efficiently than in conventional Fischer-Tropsch plants. Only then would production be economically viable.
Catalyst ensures the right mix
Together, the project partners tested different ideas and eventually found a functional configuration, including an effective catalyst. In the laboratory, the synthesis already works as intended. Now it’s time to scale up the process: the partners are currently assembling the components for a pilot plant, roughly the size of a house, slated to begin operation on the PSI site next year and produce 50 litres of SAF per day. The construction is being supported by the Swiss Federal Office of Energy (SFOE) with nine million Swiss francs. By 2028, Metafuels wants to build its first industrial-scale commercial plant with around a hundred times the capacity. “First of all, we need to use the pilot plant to find out how best to design production on such a large scale,” says Ranocchiari.
In any case, everyone involved hopes their newly developed technology, which they call aerobrew, will spur others to follow their lead and launch similar projects to achieve the ultimate goal of becoming climate neutral: “First and foremost, such transfer projects between research and industry can point the way,” says Thomas J. Schmidt, Head of the PSI Center for Energy and Environmental Sciences.
Correctly designed, SAF can reduce formation of particulates
Those involved believe that the chances of SAF helping achieve climate neutrality are very high. Not only could it reduce CO₂ emissions to a minimum without subjecting the entire aviation industry to a technical overhaul, it could also address the non-CO₂ effects of flying, which are even more relevant for global warming. Synthetic kerosene can be designed in such a way that it reduces cloud formation during operation and thus also curbs global warming.
A new initiative called reFuel.ch has been set up to investigate by 2032 the extent to which this is reasonable and feasible. The project will focus on another important aspect besides the composition of SAF: “The starting materials such as methanol, carbon monoxide, and ethylene are essentially the same as those used in the chemical industry to produce all kinds of plastics as well as fine chemicals for medicines,” says Thomas J. Schmidt. Up to now the basis for this has mainly been natural gas, another fossil resource that is harmful to the climate. This versatility is one of the reasons why the SFOE has granted reFuel.ch 15 million Swiss francs in funding.
One factor remains, however: Even with further gains in efficiency, the production of SAF will require significantly more electricity than the production of conventional kerosene. This is mainly due to the energy-intensive production of hydrogen by electrolysis and energy losses during each production step – electrolysis, CO₂ extraction, and synthesis. As a result, today SAF remains four to seven times more expensive than conventional kerosene. “Ultimately, the investment cost of the new technologies we are currently developing is not the critical factor,” says Jörg Roth. “Rather, it’s the cost of the energy that will be used to operate them.” Calculations show that meeting the aviation industry’s current demand for kerosene synthetically would need at least three times as much solar and wind energy as was produced worldwide in 2021. The demand is huge. Accordingly, the whole idea of SAF as a kerosene substitute becomes all the more worthwhile, the cheaper it is to produce the energy needed.
