Sustainable aviation fuels (SAFs) are a critical part of aviation’s decarbonisation puzzle, especially for existing aircraft and for longer flights. But measuring their emissions equivalents and sustainability is remarkably complicated, depending on much more than just the feedstocks used and how the SAF itself is produced. To start diving into the details, we spoke with Dr Helge Sachs, senior vice president at Sasol ecoFT, the division of energy and chemicals company Sasol that focusses on producing SAF via the Fischer-Tropsch (FT) process.
“Measuring sustainability is surprisingly complex,” Sachs tells us. “The primary disadvantage of fossil kerosene here is the CO2 emissions that contribute to climate change. To replace it with more sustainable alternatives we have to ensure that the new fuel not only reduces net CO2 emissions, but also that the feedstocks used in its production don’t harm the environment in other ways.”
This would include, among other issues, ensuring that plant-based feedstock is not grown in ways that have negative impacts on the environment, and that plants — or arable land — that might otherwise be used for human consumption are not diverted to SAF feedstocks.
“We clearly position against the use of food like grain or corn as feedstock for the SAF production, as it is using food and increasing the land consumption,” Sachs says. “The HEFA [hydro-processed esters and fatty acids] route is a feasible option in our view as long as virgin vegetable oils are avoided to prevent food turned into fuels. However, long supply chains and sources of the oil must be examined critically. A lot of feedstock has a long journey from southeast Asia to consumers in Europe. For local collection from restaurants, sustainability standards have to be considered in order to avoid generating issues at the start.”
As part of its research and production, Sasol ecoFT is also working with aircraft manufacturer Deutsche Aircraft on green hydrogen-based PtL [power-to-liquid] SAF for the D328eco programme. The programme, based on a Dornier 328 turboprop, is aiming to produce an environmentally conscious 40-seater aircraft.
“One example for sustainable biomass feedstock is forest residues like wood pellets. These are then converted via gasification and Sasol’s advanced FT technology into SAF. The PtL-SAF pathway is especially attractive,” Sachs says, “because the production process is based on renewable electricity instead of burning fossil fuel. Other pathways only achieve GHG emissions reductions in varying degrees. For example, a HEFA-based SAF achieves a GHG reduction of up to 84 percent. Also, in comparison to fossil kerosene the supply chain for PtL-SAF from well to wing will be much shorter and yield an additional reduction of the GHG footprint.”
But the way in which fuel is burned in jet engines — as well as the altitude and area of the atmosphere in aircraft fly — also has an impact.
“Another key aspect for the reduced impact of our PtL-SAF is the reduced climate impact of aircraft flying with SAF,” Sachs notes. “Fossil kerosene contains a significant percentage of aromatic hydrocarbons. These don’t burn completely and form soot particles, which are released into the atmosphere. The soot works as a seed for the formation of contrails, which reflect heat back to Earth. Aircraft flying with SAF and therefore reduced aromatic content will produce less contrails and subsequently contribute less to global warming.”
This kind of non-direct emission warming effect also includes the water vapor from contrails, which will depend on the amount of water both in the atmosphere and in the SAF itself, as well as other factors like temperature, time of day, existing high cloud cover, and insolation.
Overall, there are no easy answers, and the complexities remain — at least for the time being. Almost all SAF will inherently be more sustainable than using fossil kerosene, but aviation needs to accelerate its research into the more sustainable pathways and processes to ensure the maximum decarbonisation benefits of these new fuels.
Author: John Walton
Published 06 June 2023
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