Both the German government and the European Union have a clear goal: aviation in Germany and Europe is to become climate-neutral by 2045 at the latest. The European legislative package entitled "Fit for 55" describes how this goal is to be achieved. Sustainable aviation fuels (SAF) play an important role in this. From 2025, a blending quota of two per cent SAF is prescribed for flights departing from European airports. By 2030, the blending quota will already be five per cent. The proportion of synthetic fuels is also specified - it will initially be 1.2 per cent. But the variety of these successors to conventional kerosene is confusing. We explain the current state of research - and the changes these new fuels will bring.
Sustainable Aviation Fuels (SAF) - this term stands for a variety of sustainable aviation fuels made from very different raw materials. They all have two things in common: they are chemically almost identical to kerosene, but are not based on fossil raw materials, and they have a significantly lower carbon footprint than kerosene over their entire life cycle (from source to combustion). SAFs are therefore intended to replace fossil fuels in all those areas of transport that are difficult to electrify - for example in medium and long-haul aviation, heavy goods transport and shipping. The first semi-synthetic SAF from the South African company Sasol was certified in 1998. To date, seven production routes have been approved. The most advanced and widely available are HEFA fuels.
Conventional aviation fuel is a mixture obtained from crude oil by distillation and refining. In addition to kerosene, diesel, petrol and naphtha, a basic material for the petrochemical industry, are also obtained in the same refining process. Kerosene contains various hydrocarbons such as paraffins, cycloparaffins and aromatics. The aromatics group is important for keeping seals elastic, for example. However, they also produce soot particles during the combustion process, which later lead to the formation of unwanted contrails. Other compounds that are important for the safe operation of an aircraft are often added to the fuel - these include substances such as antistatic agents, icing inhibitors, corrosion inhibitors and substances that improve thermal stability. The specifications for fuel do not stipulate how the fuel must be composed. Instead, they define the physical properties that the fuel must have.
There are currently seven approved manufacturing processes: Alcohol-to-Jet, Catalytic Hydrothermolysis Jet (CHJ), Fischer-Tropsch (F-T), Fischer-Tropsch Synthetic Kerosene with Aromatics (F-T SKA), Hydroprocessed Esters and Fatty Acids (HEFA) and Hydrocarbon HEFA (HC-HEFA). The fuels based on the HEFA process currently have the largest market share. Vegetable and animal (waste) oils and fats, for example used fat from deep fryers, are used as the starting material. The fats and oils are first hydrogenated. The resulting oil is refined into kerosene in a similar way to crude oil. However, it will not be possible to sustainably scale up this production process indefinitely.
HEFA fuels will continue to exist in the future, but most of the demand is likely to be met by synthetic fuels, which can also be produced sustainably in very large quantities - something that is not possible with bio-based fuels. Here, water is first separated into hydrogen and oxygen with the help of electrical energy and then processed into crude oil with CO2 in a second synthesis step. Either Fischer-Tropsch synthesis or methanol synthesis can be used for this second step. Modern processes could make it possible to utilise methanol in such a way that an almost closed carbon cycle is created. We are involved in several projects that are developing a modern methanol-to-jet route.
The British start-up Firefly is probably working with the most unusual raw material: it wants to use human waste - in other words, faeces or sewage sludge. According to the calculations of Firefly founder James Hygate, each person produces enough waste per year to be able to produce four to five litres of SAF. If the UK were to utilise these quantities completely, Hygate calculates that this could cover five percent of the UK's fuel requirements.
The term Sustainable Aviation Fuels (SAF) was coined when the search for alternatives to kerosene started. Today, research has progressed further and is more orientated towards the requirements of the future. SAFs are currently categorised into two main groups - fuels of biological or non-biological origin. Fuels of non-biological origin include synthetic fuels. DLR is concentrating on researching, testing and optimising these SAFs: In Jülich, research is being conducted on sun-to-liquid processes in the solar thermal test power plant. In addition, a semi-industrial plant for PtL production (PtL: Power to Liquid) is to be built at the Leuna Chemical Park, the TPP technology platform in Leuna. This plant is intended to accelerate the market ramp-up of power-to-liquid fuels in Germany and Europe.
At the moment, SAFs may only be used in commercial aviation when mixed with kerosene. A maximum blend of 50 per cent is currently permitted. As only small quantities are currently available on the market, the actual blending rate is less than one per cent - across all flights. From 2025, an initial blending of two per cent SAF will be required by law for all flights departing within Europe. This proportion will then rise to five per cent in 2030, and in order to be able to fly largely climate-neutral by 2050, as intended, the proportion would have to rise to 63 per cent by then.
Drop-in-capable fuels are fuels that can be used immediately in the entire fleet without any modifications to the aircraft being necessary. These fuels are therefore compatible with all engines commonly used today, including the engines of older aircraft. Non-drop-in fuels are fuels that require certain adaptations, for example to seals. They have not yet been approved.
During the combustion process, fossil fuels release the carbon that they have bound over a long period of time during their creation. In this way, they continue to accumulate CO2 in the atmosphere and thus contribute to global warming. Sustainable aviation fuels, on the other hand, not only release less CO2 during combustion, but can also be produced in a much more climate-friendly way. This is shown by analysing their life cycle. In addition, many of these new types of fuel can be designed in such a way that they produce practically no soot and therefore cause fewer contrails. The production of synthetic fuels uses less water and land than the production of biogenic fuels. If they are produced using renewable energy and CO2 from the atmosphere, they can be almost completely carbon neutral. However, there is still a long way to go. The big hurdle is that the energy consumption in production is enormous. The necessary capacities still need to be created.
They can be specifically designed in the laboratory for low-emission combustion so that little to no soot particles are produced during combustion in the aircraft turbines. DLR studies show that when SAF is used, up to 80 per cent less soot is released into the atmosphere compared to kerosene. This results in fewer contrails, the formation of which increases the warming of the atmosphere. This means that these fuels can reduce the impact on the climate far beyond CO2 emissions - this also applies to the so-called non-CO2-effects of aviation.
In fact, it would not be necessary to wait for widespread availability in order to benefit from this effect now. Researchers at the DLR have shown that the targeted use of SAF in regions with a high level of contrail formation has a significantly greater effect than adding it to all flights. The reason for this is that contrails do not form to the same extent everywhere; rather, there are certain hot spots. These include some of the busiest regions over Europe. This is why DLR researchers are working on concepts for the "smart use" of SAF. However, this requires a different infrastructure on the ground.
Local air quality would also improve at airports if kerosene were gradually replaced by SAF. This is indicated by initial measurement results from an EU research project at Copenhagen Airport. The emissions of an aircraft fuelled with a mixture containing 35 percent SAF were measured there over several weeks. The data shows a reduction in particles in the order of 30 per cent.
According to the researchers at the institute, this has already begun, triggered by two important political decisions: In 2021, the USA launched the "Grand SAF Challenge", a funding programme for the domestic production of biogenic SAF. The aim is to produce several billion litres per year by 2030. The EU has chosen a different path and set blending quotas in spring 2023. Both measures have created planning security for producers - it is clear that both start-ups and large corporations are now beginning to invest in the development of a new industry.
A world in which climate-friendly flying is a reality also goes hand in hand with a different structure on the ground. There are several reasons for this: Because there are so many different ways to produce non-fossil fuels, there will be different regional solutions. Today, the oil and gas industry is concentrated in 22 countries, which supply 90 per cent of all fossil feedstock. In fact, half of the amount produced comes from just five countries. According to estimates from the DEPA2050 study, 5,000 to 7,000 refineries will be needed worldwide to produce the necessary quantities of SAF. In all likelihood, we will therefore see a rather regionally diverse production. This could create 14 million jobs around production - not only in Europe or other industrialised countries, but also in the countries of the global South.
