Sustainable aviation fuel (SAF) is renewable or waste-derived fuel that reduces CO2 and non-CO2 emissions. SAFs can be made from used oils, dedicated crops, or by capturing carbon dioxide from the atmosphere and mixing it with green hydrogen. SAFs are being produced as drop-in fuels so they can be used with existing engines and within the existing fuel supply chain, without modification. Companies started developing SAFs over a decade ago, but SAF supply is limited and will remain so until production is up to scale. It will take years, if not decades, before SAF supply is fully scaled to fuel the aviation industry.
Video: https://youtu.be/sdYe9R95noM?si=CsH7MgocYJ153EGu
SAFs are like conventional jet fuel with a few exceptions. SAFs have a lower aromatic content and sulfur content. When conventional jet fuel combusts during a flight, it’s the aromatic content, specifically naphthalene, and sulfur content, that leads to the emission of small carbon soot particles. The soot particles cause ice particle formation, and that causes warming contrails. With SAFs, there are less soot particles, and therefore less warming contrails.
In a 2021 study published in the journal Nature, researchers from NASA and German Aerospace Center (DLR) concluded that airplanes flown using SAF blends at cruising altitudes result in a 50% to 70% reduction in soot and ice number concentrations. This could translate to 50 to 70% less warming contrails. As the above video shows, NASA, and DLR are partnering up again, this time with Boeing, to run more flights to test what happens when flights are operated on 100% SAFs, rather than just a blend of SAFs. It will be exciting to see the potential climate impact if we fly using only SAFs. We will continue to follow their study to see the results.
However, with current production levels, the industry is still a long way from operating all flights using only SAFs. Currently, airlines are blending the limited available supply of SAFs with conventional fuel to slowly work SAFs into their operations. So, if there’s a limited supply, how much SAF should be blended or used on a given flight? Should airlines use lower amounts of SAFs to spread them out across all flights or a higher concentration used in fewer targeted flights? In a 2022 study published in Environmental Science & Technology, researchers answered these questions. They analyzed data for flights in the North Atlantic. They concluded we can have the biggest impact on climate if we use available SAFs on flights at night, during the winter when warming contrails are most likely to form. They propose using higher blends of SAFs on the small number of flights most likely to cause the worst warming contrails. This targeted use strategy maximizes the overall climate benefits of SAFs. If airlines implement targeted SAF use and flight diversion strategies they can avoid contrail formation to have an immediate impact on the climate.