Bjorn’s Corner: Sustainable Air Transport. Part 53. Sustainable Aviation Fuel

By Bjorn Fehrm.

January 13, 2023, ©. Leeham News: Last week, we could see in an example how effective Sustainable Aviation Fuel, SAF, blended into our regular Jet fuel, would be in reducing CO2 emissions until 2050.

It’s the only technology that can substantially influence our emissions over the next 30 years, as alternate technologies like hydrogen-fueled airliners need to ramp to thousands of aircraft before it affects emissions.

To understand SAF and how it can be produced and blended into Jet fuel, we first need to understand what Jet fuel is.


Figure 1. An Iberia A340 filled up with Jet-A1. Source: Wikipedia.

Jet Fuel

A gas turbine can run on a wide variety of fuels. The initial fuel for the German Jumo jet engines used in WW2 was diesel or synthetic fuel. Since then, about 20 different specifications have defined the characteristics of the jet fuel used in civil air transport and military aircraft.

The fuel used in civil air transport today is Jet-A1;

  • Wikipedia: Jet fuel is a mixture of over 2,000 different hydrocarbons. Because the exact composition of jet fuel varies widely based on the petroleum source, it is impossible to define jet fuel as a ratio of specific hydrocarbons. Jet fuel is therefore defined as a performance specification rather than a chemical compound.

The performance specification for Jet-A1 used in air transport is ASTM D1655 (Jet A-1). There are also specifications for civil airliner Jet fuel variants with a higher freezing point than -47°C (Jet-A at -40°C) and an extra low freezing point (Russian TS-1 with -50°C freeze point).

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

The feedstock of crude oil absorbed CO2 from the atmosphere when it grew. We release these carbon atoms as CO2 when we burn refined crude oil. This time separation of absorption and release creates our CO2 problem.

Sustainable Aviation Fuel

Sustainable Aviation Fuel or SAF is a hydrocarbon mix where the carbon absorption from the atmosphere is near in time with the release of the atoms as CO2 when burned. SAF is a renewable fuel, meaning its cycle is the absorption of CO2, then release, and it’s a repeatable process.

There are a wide variety of processes where capturing CO2 into a hydrocarbon mix can create a SAF. In general, we can define two big groups;

  • Biofuels, where the feedstock of the process is a biologically grown material such as oil plants, algae, different biomasses, or fermented alcohols from agricultural or other waste materials. The list of possible feedstocks and their processing into hydrocarbons is constantly growing.
  • E-fuels are synthesized from hydrogen; then carbon atoms are added in a CO2/CO air capture process to generate hydrocarbons.

The different biofuel processes are the lower cost alternative, resulting in biofuels that cost three to four times the current price of Jet Fuel. E-fuels are highly dependent on the cost of the used energy and will, with classical energy sources, cost more than biofuels.

The SAF problem areas

SAF has two main problems, the ramp of production and its aircraft compatibility. Of these, aircraft compatibility is the easier one.

Classical rubber seals used in aircraft fuel systems rely on aromatic compounds in the fuel to keep the seals supple. Pure biofuels lack such compounds, and unless these are added or biofuel is mixed with Jet fuel, the seals can shrink and start to leak. It’s the background to the need for blends for aircraft that don’t use modern seal materials.

Blends up to 50% are OK with these older seal materials and have been certified by ASTM as safe for use in airliners.

The SAF production ramp problem has made a global 50% blend unachievable for decades, so the problem we have is not the blend tolerance of our airliners; it’s the world production of SAF. More on this in next week’s Corner.

14 Comments on “Bjorn’s Corner: Sustainable Air Transport. Part 53. Sustainable Aviation Fuel

  1. “The different biofuel processes are the lower cost alternative, resulting in biofuels that cost three to four times the current price of Jet Fuel”

    Can the process ever been streamlined enough to get this price substantially down? There’s no such thing as “cheap electricity” when you take into account the costs of building new sustainable generating sources (such as nuclear, tidal, wind, solar), buffering/storage to cope with fluctuating output, and upgrading distribution networks.

    Will there be enough appetite for air travel if tickets double (or more) in price as a result of higher fuel costs?

    • Fuel is less than 20% of the ticket price, so given more fuel-efficient aircraft it should be Ok even with biofuel. The E-fuels need to be produced where energy is in abundance and can’t be routed to consumer areas. We’ll come to that.

  2. The bioethanol market is pretty big and mature. The bioethanol market is expected to grow to $52.51 billion in 2026 at a CAGR of 9.7% mainly for gasoline cars (E10 to E85 fuel), as cars goes electric it can be more economical to convert ethanol to SAF and diesel fuel. “Today, nearly every gallon of gasoline in the U.S. contains a minimum of 10% ethanol derived from corn”.

      • There are many routes. LanzaTech use catalyst and microreactor developed by PNNL to convert ethanol to n-butene. n-butene can be pretty much converted to anything via polymerization over catalysts in a long-established process. Creating n-butene of used to be a two step process.
        I think they are also using some interesting processes of taking CO2 from a blast furnace and hydrogen and fermenting to produce the ethanol.
        -In WW2 the German production preceded along the route of reacting syngas over a chromium catalyst. This produced 22% butanol and 78% methanol. The methanol was simply recirculated as only butanol was wanted. This was dehydrated to n-butalyene and rearranged to n-butene which was polymerised to create iso-octane. It couldn’t have been more than 30% efficient. In theory they could have fermented the butanol using a different strain of bacteria to those producing ethanol.

    • Still waiting for someone to explain to me why are they adding corn to gasoline when it does not reduce emissions. Per some reports cause premature engine wear.

      Easy to see how and elixir of Kerosene and bio or synthetic fuel might cause some rubber wear on some important gaskets that were not designed to interact with such new fuel.

      • The maize derived ethanol is there for a number of reasons. It was supposed to replace the oxygenate MTBE with something much more benign in case of leakage and to reduce emissions. It is meant to help provide subsidies to the American farmer to keep him on the land. I don’t regard this as cynical politics to appease the farm lobby. This is a matter of food security. A 20% over production is always a good buffer in case of crop failure. In Australia the source of ethanol is wheat. It provides a second market for the farmer in particular for substandard crops. I would think this ethanol is best used in SAF rather than in gasoline since gasoline that is clean is easily produced by the industry. The judgement of whether these bioethanols are worth it in energy terms is not clear so it is quite marginal but seems slightly positive. There are expectations of significant improvements through reverse membrane concentration of the alcohol. The production of bioethanol in the US is now so high that if it were converted to jet fuel instead of added to gasoline the USA would already have all of its jet fuel.

  3. I suspect that Jet fuel far exceeds the freeze point, its a minimum spec , Jet fuel was long used in Alaska in the winter time and temps often exceed -70 F (-56C) .

    I never saw diesel freeze either but the wax would come out and plug the filter.

  4. The concern we have with biofuel, (if it is “manufactured” in significant volume), is the competition with food crops (unless the available agricultural surfaces are increased through additional deforestation).
    But increased deforestation (either for growing biofuel or food crops) means less capacity for capture of CO2 without mentioning impact on biodiversity.
    We do not like that at all (I have loved flying in my small morane-saulnier), but the problem of fuel for planes (compliance with climate protection requirements) seems to be without possible solution for overseas travels (where electrical high speed trains cannot be installed). Unfortunateley, long distance electrical driven planes is presently science fiction (for the least very problematic) for the reasons well analysed in some Leeham articles. For “sunday’s flight lover”, short distance “electrical” flying (and “touch and go training”) remains possible, depending the way the electrical power is produced.

    • -I fully agree with your concerns. The word “Sustainability” in the SAF acronym “Sustainable Aviation Fuel” is there to emphasize that rules and standards are in place to prevent the issues you speak of becoming a problem. Some of us still have bad memories of the effect EU subsidies for biodiesel had in promoting rainforest clearing in Malaysia and habitat destruction for the orangutan. Palm kernel produces nearly 0.6kg of oil per square meter per year.
      -The main way is to obviously use waste oil, mainly from the food industry I imagine.
      -Other ways are to use oil seed crops using species that can grow on marginal land, with limited water and no need for fertilizer. These plants produce good fuel oil but do not produce edible oils due to toxins or bad tastes.
      -Another is to ferment agricultural waste, sewage, domestic waste to alcohols and biogas and convert these to SAF.
      -I don’t know who actually controls the SAF standards. Is it IATA?

      • @William You are right, some “sustainable” subsitute to kerosene may be produced, but probably not with the proper order of magnitude.
        I don’t know the accurate figures, but for commercial aviation, we are presently burning, worldwide, probably several hundred thousand of tons ( 500,000,000 tons ? more ?). We can suspect the required surface to grow the “fuelplants” is not marginal !
        ( Btw, spent cooking oil is already burn in diesel cars, but even if you are fond of french fries, the available volume is quite limited)
        – process do exist to manufacture fuel, but they are energy intensive and with poor efficiency. Some need imput of CO2 that it would be nice to extract from ambiant air, but there also, it is very energy intensive. Use of CO2 directly out of “big producers” ( cement ..) may be part of the solution but I think these industries are (at least in western countries) more and more trapping “their” CO2 for long terme disposal through underground storage.
        – Producing and reburning hydrogen (H2) is also of poor energy efficiency but without CO2 concern but probably it remains the only viable solution to sustainably power our commercial planes. But with very large limitations in autonomy.
        May be in next decades we will have to fly from Paris to New York with “again” stopovers in Iceland and Newfoundland for “re-hydrogenate” the tanks ? (smiling)

        • -I don’t see biologically obtained SAF as a full solution but certainly it is a substantial partial solution if the material is genuine waste material.
          -Cryogenic hydrogen production including electrolysis and liquefaction is potentially 80% efficient but I think we would be starting at closer to 70%. There are substantial transport costs.
          -Production of Electrofuel SAF is potentially 65% efficient using certain high temperature processes such as thermochemical water splitting and integrating waste heat to regenerate CO2 absorbers. I think we would be looking at closer to 55%.
          -So about 40% more energy for electro SAF versus LH. Maybe only 20% if concentrated sources of CO2 or very advanced direct air capture is used. The margin will be less when transport is considered.
          -France famously has 70% of its energy from nuclear. If France were to double its nuclear electricity generation to 140% it would have sufficient to cover all of its electrical and road transport emissions. Since these are both 30% each in a non nuclear country we’ve covered 60% of emissions. There should be enough to cover cement calcination, iron or smelting, aluminum production, ammonia production so we’ve likely covered 70% of emissions.
          -Since aviation emissions are only 2.4% of the remainder its almost not worth bothering.

    • Same issues as SAF not to mention while viable ground transport wise the negative factor with putting into an aircraft makes some kind of SAF the right choice.

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