Clearing the Air with SAF

Understanding Sustainable Aviation Fuel

Jan 09, 2025

The aviation industry has always prized efficiency. Whether that’s more efficient aircraft design, flight operations, or engine function, the progress of flight has relied on efficiency to drive innovations and improvements. As the sector embraces the goal of achieving net-zero carbon emissions by 2050, the next, and crucial, step for efficiency involves fuel. As such, SAF has become an important piece of the sustainability puzzle, with many airlines seeing it as the key for unlocking cleaner flight.

While SAF will be integral in the reduction of carbon emissions, it will also have a key role in addressing aviation’s non-CO2 emissions. With lower aromatic content, SAF is being researched as a potential tool for contrail mitigation, as fewer emitted particles become fewer nuclei around which the ice may condensate. Not all SAF is created equal, however, as different methods of production can have different outcomes and impacts. We’ll get into all of that, but first, let’s discuss what SAF is and how it is produced.

What is SAF?

SAF, which stands for Sustainable Aviation Fuel or Synthetic Aviation Fuel, is a renewable fuel alternative to the fossil-based jet fuels currently powering aviation. It can be produced by two different methods: (1) using biomass feedstocks to create biofuels or (2) combining CO2 and clean hydrogen to create synthetic fuels or e-SAF.

Sustainable Aviation Fuel Image | ResourceWise

A 2023 report by the Air Transport Action Group (ATAG) highlights that the chemical and physical properties of SAF are practically identical to conventional Jet A1. Meaning SAF can be blended seamlessly into the existing fuel supply and certified to the same standard as traditional jet fuel. As such, SAF is the perfect drop-in fuel to streamline the transition away from aromatically-heavy fossil fuels and towards the emissions-reducing, cleaner burning biofuels and e-fuels. Experts estimate these sustainably produced fuels could reduce CO2 emissions by as much as 80% across their lifecycle.

What are Drop-in Fuels? Drop-in fuels are alternatives to conventional fuels which can be safely infused into the existing distribution infrastructure without the need to alter the fuel systems on the aircraft or storage facilities at the airport.

Despite the robust findings of the beneficial impact SAF can have on aviation emissions, the SAF market is still in its infancy. Currently, cost analyses show waste-based SAF is 2x the price of conventional jet fuel while synthetic fuels can cost 6-10x more. This cost disparity is a result of limited supply due to production capacity, making it difficult for airlines to jump onboard the SAF train. However, there is a willingness to invest in SAF that will grow in the years to come – as more investors are attracted to the SAF market and regulations are crafted to facilitate adoption, production will scale to meet the growing demand.

How is SAF Produced?

As mentioned, SAF is developed from various sustainable feedstocks which are converted to aviation fuel through several conversion processes. At the time of writing, ICAO has a list of 11 approved processes and 11 more under review by ASTM International, an organization which sets standards worldwide to ensure safe and reliable products and services. The conversion processes, also known as production pathways, are vital to increasing the supply of SAF while endeavoring to retain the ‘sustainable’ quality that makes it such a tantalizing option for decarbonizing aviation.

SAF Feedstocks | Environmental Energy and Study Institute

A report by Twelve highlights four of the most prominent pathways currently being used which will be crucial for the expansion of the SAF market:

HYDROPROCESSED ESTERS AND FATTY ACIDS (HEFA) – A process which utilizes feedstocks such as waste and residue from vegetable oil, used cooking oils, and animal fats as well as dedicated crops like jatropha and camelina, hydrocarbon fuel components are produced by removing oxygen from the feedstock with hydrogen (hydrodeoxygenation). HEFA is arguably the most advanced technology for commercial SAF production, but is heavily limited by the availability of feedstocks and industrial competition.
ALCOHOLS TO JET (AtJ) – This method takes agricultural and forest residues (lignocellulosic feedstocks) along with crops such as corn, sugarcane, and wheat to produce ethanol, which is then refined into jet fuel. AtJ fuels are just beginning to enter the market, but production is expected to grow quickly. Though the feedstocks are considered more sustainable, their availability presents a challenge to meeting the needs of the entire aviation sector (not unlike HEFA).
BIOMASS GASIFICATION + FISCHER TROPSCH (GAS + FT) – By using a Fischer-Tropsch reactor, synthetic gas (syngas) is converted into liquid fuel which can be upgraded to make SAF. The syngas is produced by the burning of municipal solid waste and other residues in the gasification process. The biofuels derived from this method are considered to have one of the lowest lifecycle emissions, but are not yet available commercially. They too are limited by the supply of feedstocks.
POWER TO LIQUID (PtL) – Unlike the other pathways, this process does not rely solely on feedstocks, producing syngas with hydrogen, CO2, and electricity (preferably renewable). After electrolysis, the syngas is converted through Fischer-Tropsch, methanol synthesis, or gas fermentation to a liquid product. PtL technologies are currently the most expensive pathway, but promise to be the best method for achieving a zero-emission fuel, as Direct Air Capture (DAC) could provide the CO2 necessary to feed the process.

SAF and Non-CO2

The relationship between SAF and non-CO2 is in its nascency, but is evolving as more studies and trials demonstrate the challenges and benefits of using SAF to curb the impact of non-CO2 emissions. Traditional jet fuel is rife with gases, particles, and aromatics which play a role in diminishing air quality, forming persistent contrails, and harming the environment. On the other hand, cleaner burning SAF promises to lessen and even eliminate those emissions as the technology matures and understanding improves. Research is ongoing to provide a broader perspective, testing the efficacy of SAF to reduce non-CO2’s climate impact.

In 2021, Airbus – working with Rolls-Royce, the German Aerospace Center (DLR), and Neste – took part in the ECLIF3 campaign, studying flights using 100% SAF to see how it may impact contrail creation. The results show the use of SAF can significantly reduce the warming effects of contrail formation and lower soot and other particulate emissions. Similarly, Virgin Atlantic’s historic Flight100 tested the use of 100% SAF in a transatlantic flight, which proved successful and provided important insights into the emissions-reducing properties of SAF. These trials not only displayed the advantages of SAF use but some of the potential drawbacks and limitations that may be encountered along the supply chain.

Although it will take years for SAF production to meet the needs of a growing aviation industry set on sustainability, the potential benefits (and limitations) are readily apparent. To be considered truly ‘sustainable’, great thought must be given as to which feedstocks and pathways to embrace for the long-haul, which to utilize in the interim, and which to abandon altogether. While not the only tool in aviation’s decarbonization toolbelt, Sustainable Aviation Fuel will indeed have a critical function in cleaning up both the CO2 and non-CO2 emissions of aircraft.