By Bjorn Fehrm.

April 25, 2025, ©. Leeham News: We do a Corner series about the state of developments to improve the emission situation for Air Transport. We try to understand why development has been slow.

Over the last Corners, we examined the non-CO2 effects of Air Transport that contribute to global warming. Contrails had the largest effects, even larger than the CO2 emissions from Air Transport.

To complete the review of our progress with lower emission actions for Air Transport, we will examine the present situation with SAF, Sustainable Aviation Fuel.

Figure 1 The main production pathways to SAF. Source: Clean Skies for Tomorrow report. Click to enlarge.

 Jet Fuel

I did a comprehensive run-down of Jet Fuel and SAF a few years ago; you can find it here. Here, we summarize the key points and describe where we are with SAF and SAF blends in our Jet-A1 aviation fuel.

Jet Fuel, or more precisely Jet A1, is a mixture of over 2,000 different hydrocarbons. It’s classified as a Kerosene-type fuel. It’s produced in a cracking process from crude oil, which was, in turn, created from dead zooplankton and algae sediments that changed into oil under heat and pressure over millions of years.

The plankton and algae absorbed CO2 from the atmosphere as they grew. When we burn refined crude oil, we release the fuel’s carbon atoms as CO2. The time separation of absorption and release of millions of years creates our CO2 problem.

Sustainable Aviation Fuel, SAF

Sustainable Aviation Fuel, SAF, is a hydrocarbon fuel in which CO2 is absorbed from the atmosphere near the time of its CO2 release when burning the fuel.

There are a wide variety of processes where capturing CO2 into a hydrocarbon mix can create a SAF, Figure 1. The HEFA process is dominant today, where different biomasses are processed into SAF.

Other possible process chains (called Pathways in the industry) involve fermenting alcohols from agricultural or other waste materials and then converting the alcohol into SAF. There is also a waste pathway where waste gasification and Fischer-Tropsch processing form an SAF. Both pathways are of little importance today, but have long-term potential.

The final SAF pathway is called E-fuels, which involves producing hydrogen from water by electrolysis. Carbon atoms are then added to the hydrogen in a Fischer-Tropsch process, where the carbon comes from CO2/CO air capture. The result is an SAF hydrocarbon produced from water and energy (thus the name E-fuel).

SAF volumes and costs, today and tomorrow

The HEFA biofuel process is the lower-cost alternative, resulting in biofuels that cost four to five times the current price of Jet A1. The technology with no restrictions on feedstock, the E-fuels are highly dependent on the cost of the used energy. Until new energy sources such as Solar Power in desert areas are established, the energy costs for E-fuels will make these more costly than biofuels.

The SAF problem areas

SAF has two main problems: ramping up production and aircraft compatibility. Of these, aircraft compatibility is the easiest one.

Classical rubber seals used in aircraft fuel systems rely on aromatic compounds in the fuel to keep the seals supple. Pure biofuel or E-fuels lack such compounds. Therefore, SAFs lacking such compounds must be mixed with Jet fuel to keep the seals from shrinking and generating leaks.

This is the background to the need for blends for aircraft that don’t use modern seal materials for their fuel systems. Right now, most aircraft and engines are approved for up to a 50% SAF blend.

The bigger problem is the ramp-up of production. During 2024, we produced and consumed around 300 million tonnes of Jet Fuel. Of these, 1 million tonnes were SAF, or 0.3%. For 2025, the production of SAF is planned to increase to 1.5 million tonnes, or 0.5%.

We are a long way from producing enough SAF to satisfy a 2030 target of a 10% blend of SAF in Jet Fuel. For that, we need 30 tonnes of SAF (if the consumption stays the same), which is a growth rate of 30 times in five years, or 6 tonnes per year, or 600% in the first year. We have around 50% as the projected SAF production is 1.5 million tonnes in 2025.