Sustainable fuels: SAFs as a lever for clean flight

AEROREPORT: Mr. Donus, sustainable aviation fuels, or SAFs for short, are a key issue in aviation. What are their characteristics?

Fabian Donus: SAFs can already be used in today’s air traffic, as they meet the same standards. But they have a much smaller impact on the climate than kerosene from fossil sources. The logic behind SAFs is that the CO₂ emitted in flight is recycled in the production of the fuel, thus creating a CO₂ cycle that is as closed as possible. To this end, the CO₂ must first be extracted from the atmosphere in the best possible way. Another positive effect is that the combustion of SAFs produces fewer soot particles. These are partly responsible for the formation of contrails, which in turn contribute to global warming alongside CO₂ and nitrogen oxides. Today’s SAF production processes are based either on biomass or on renewable energy and CO₂. However, the quantity of SAF actually used is currently still far too low.

AEROREPORT: How much SAF does aviation need?

Donus: If we want to achieve climate-neutral aviation by 2050, then the widespread introduction of SAF in the existing fleet is a fundamental prerequisite. Aircraft entering service today and in the years ahead will continue to use kerosene-powered gas turbines. The only way to significantly reduce the climate impact of these aircraft is with SAFs. Current forecasts assume a demand of around 600 million metric tons per year. However, only around 0.1 percent of the industry’s global fuel requirements can currently be met by sustainable fuels. That’s why the European Union (EU) adopted the ReFuelEU aviation initiative, which sets a minimum proportion of SAFs for all flights from EU airports. ReFuelEU aviation obligates fuel suppliers to ensure that the share of SAFs reaches 2 percent by 2025, 6 percent by 2030, and 70 percent by 2050. For synthetic fuels, a quota of 1.2 percent will apply starting in 2030; this will increase to 35 percent by 2050. Other countries are also making efforts to increase the proportion of SAFs. In the U.S., the main focus is on providing incentives through the Inflation Reduction Act, whereby fuel suppliers receive tax breaks if they sell SAF.

The aim of all these measures is to create a binding framework to stimulate demand. Another reason why SAFs aren’t yet a true alternative on the market is that they’re significantly more expensive than fossil kerosene.

AEROREPORT: How can these quotas be achieved?

Donus: The industry needs to attain the required production volumes very quickly. Synthetic fuels, known as synfuels, are a particularly interesting prospect, but they aren’t yet available on the market. Although the manufacturing processes for them have been developed and approved, there are very few demonstration plants worldwide. An analysis by the Potsdam Institute for Climate Impact Research suggests that of the 60 or so new synfuel projects planned by 2035, only around 1 percent have secured the investment commitments they need to proceed. Even a single project on an industrial scale requires an investment of several billion euros. No producer is going to build such a plant without being sure that there will be demand for the fuel in the long term. And that, of course, puts fulfillment of the quota at risk.

AEROREPORT: What SAF manufacturing processes have already been approved?

Donus: SAFs are currently manufactured mainly from biogenic residues. There are various certified manufacturing methods based on different raw materials and processes. One well-known process, which is employed to produce almost the total volume of today’s SAFs, is to hydrogenate vegetable or animal fats and oils into kerosene, also known as hydroprocessed esters and fatty acids (HEFA). Other approved methods, which in turn process other biomasses, can be used to maximize the amount of SAFs. Always on the condition of not competing with food production, of course. But abiding by that condition makes it impossible to establish a sustainable supply of enough SAFs to power the global aircraft fleet.

AEROREPORT: Power-to-liquid (PtL) is considered a very promising process. Why is that?

Donus: In the PtL process, hydrogen is produced using electricity from renewable sources such as wind, water, or solar power. It is then synthesized with CO₂ to form hydrocarbons, which are processed into liquid fuel. To make the cycle carbon-neutral, this CO₂ must first be extracted from the atmosphere. Given these requirements, it makes most sense to produce large quantities of synthetic fuels in regions with plenty of wind and sun. According to an estimate by the Bauhaus Luftfahrt think tank, it would take less than 1 percent of the area covered by the world’s deserts to meet the aviation industry’s global demand for green energy for the production of synthetic fuels. In the short term, offering PtL processes economically on an industrial scale calls for large, subsidized demonstration plants. These would allow experience to be gathered on the optimal arrangement of the individual process steps. A subsequent market ramp-up would require sufficient green electricity and CO₂ to be available, preferably from sustainable sources.

AEROREPORT: Aircraft are already allowed to use SAFs, but not fuel that is 100 percent SAF. Why is that?

Donus: We have to differentiate here between drop-in and non-drop-in fuels. Drop-in fuels can already be used today with no technical modifications to engines, aircraft, or airport infrastructure. They are mixed with at least 50 percent conventional kerosene, as this contains substances that are still required for safe operation. These include aromatics, which are organic compounds that improve the lubricity, density, and material compatibility of aviation fuel. They are needed to ensure that particular seals used in engines swell correctly. Most SAFs are free of aromatics, which means 100 percent pure SAF is currently regarded as non-drop-in.

AEROREPORT: What are the disadvantages of aromatics?

Donus: Aromatics lead to increased formation of soot particles in the course of the combustion process in the engine. These, in turn, serve as condensation nuclei for water vapor, which then freezes into ice crystals. Under certain conditions, these crystals can form long-lasting contrails, the significant climate impact of which is well known.

AEROREPORT: Initial studies show that SAFs greatly reduce the formation of contrails.

Donus: And that can also reduce the impact of what are known as non-CO₂ effects. This is due to the lower aromatics content of SAFs compared to fossil kerosene, which I’ve already mentioned. DLR flight tests have shown that a reduction in particle numbers also reduces the number of ice crystals. This means that increasing the proportion of SAF in fuel has great potential to reduce its climate impact. As predicted by all manufacturers, the next generation of engines will no longer require aromatics.

AEROREPORT: But even if future engines manage without aromatics, there will still be older aircraft in use that rely on fuel with aromatics.

Donus: That’s right. After all, aircraft have a service life of 25 years or more, so an aircraft that goes into operation today will still be flying in 2050 and beyond. As a result, we’ll see a mix of older and new aircraft in the skies in the coming decades. And that’s why I believe the first 100 percent SAF fuel won’t be free of aromatics.

AEROREPORT: What additional technologies could help reduce climate impact?

Donus: To make the production of synthetic fuels sustainable, the CO₂ required must be extracted from the atmosphere. This is where carbon capture technology comes into play, the purpose of which is to remove CO₂ from the atmosphere. In the case of SAFs, this CO₂ is then recycled in fuel production. As the absolute amount of CO₂ in the atmosphere is already too high, thoughts are now generally also turning to its long-term storage in reservoirs.

AEROREPORT: But won’t this act as a brake on the development of technological innovations in aviation?

Donus: Absolutely not. We need to keep working to reduce the energy consumption and climate impact of aircraft engines. Especially in times when the amount of green energy available is limited, it’s important to use as little of it as possible. On top of that, energy consumption will always have a significant influence on price. That means we’re dependent on technological innovations in aviation if we want to make flight climate-neutral in the future. Nevertheless, I believe that carbon capture technology is an important building block for achieving the ambitious climate targets across all sectors.