Reducing the carbon footprint and climate impact of contrails
From today's perspective, synthetic liquid fuels represent the most realistically viable alternative to the kerosene conventionally used for energy-intensive long-haul flights. For medium-haul routes, sustainable aviation fuels (SAFs) constitute technology that can be made available relatively quickly. By comparison, alternative propulsion systems based on fuel cells and hydrogen will only become ready for use in the 2030s, and only for regional flights at first. It is also worth bearing in mind that over 75 percent of CO2 emissions from aviation occur on routes over 1,500 km, making this an area of urgent concern for climate protection.
Hydrogen and carbon dioxide for e-fuels
Sustainable fuels are obtained from renewable sources that do not rely on crude oil. They have a far smaller carbon footprint than fossil kerosene. Fuels based on plant material or waste are already being manufactured today and 'e-fuels' will also enter the frame in the near future, synthesised using renewable forms of energy and sustainably derived green hydrogen.
Electricity-based synthetic fuels, also known as power-to-liquid (PtL) fuels, are chemically produced from hydrogen and carbon dioxide, with the hydrogen obtained with electricity in water electrolysis. The carbon dioxide comes from the air or other sources, including biomass, waste incineration units or cement works. The resulting hydrocarbons are then processed into gasoline or kerosene. These fuels can be produced in a climate-neutral process using electricity obtained from renewable forms of energy.
Today, sustainable fuels are already being used on a one-off basis as 'drop-in fuels' or mixed in small quantities with standard kerosene. Admixtures of up to 50 percent with conventional kerosene are permitted and researchers are currently testing the use of pure SAFs. Existing aircraft can use such fuels without any issues; only minor technical modifications are required.
Fewer ice crystals in contrails
What all of these sustainable fuels have in common is that they can be produced without cyclic hydrocarbons, which are described as 'aromatic'. Fewer aromatic hydrocarbons in the fuel means less soot in emissions. As soot particles act as condensation nuclei for ice crystals in contrails, less soot in aircraft exhaust gases means fewer ice crystals in contrails, thus reducing their climate impact. Contrails and the resulting contrail cirrus only stay in the sky for a few hours. If they have a lower number of ice crystals, their warming effect is lowered immediately. The use of sustainable fuels on flight routes that see frequent contrail formation is particularly promising as a means of quick and targeted climate protection.
Availability is key
If SAFs are to be made available as widely and as quickly as possible as a climate-friendly technology for aviation, they need to be produced sustainably in large quantities without delay. This is where current research, technical developments and initial demonstration systems come into play. The further development of sustainable fuels with detailed characterisation of their emissions and atmospheric effects – from basic testing through to test flights – are all planned as part of the research. As a result, the use of SAFs is increasingly the subject of attention.
The outlook for hydrogen combustion
Direct combustion of sustainably produced hydrogen in gas turbines offers good prospects for the long term. However, aircraft will need to be reconfigured for this purpose, as they require a tank for liquid hydrogen in the fuselage. Using hydrogen completely eliminates CO2 emissions and largely avoids by-products like sulphur oxides (SOx) and nitrogen oxides (NOx). Water vapour, another by-product of kerosene combustion, is, however, the main emission from hydrogen combustion. The effect of this on the climate, including in terms of the for