Bjorn’s Corner: Sustainable Air Transport. Part 54. Sustainable Aviation Fuel Production

By Bjorn Fehrm

January 20, 2023, ©. Leeham News: Having introduced Sustainable Aviation Fuel, SAF, as essential for reducing Green House Gases (GHG) until 2050, we now look at how SAF is produced, what pathways from feedstock to SAF are the main ones, and what are the short and long-term potentials of the different pathways.

I was helped to grasp the area by Neste’s EVP of Renewable Aviation fuel, Thorsten Lange, who, after we met when he presented at the Airbus Summit in November (Neste is a leading SAF producer), took the time to explain the subject to me last week. He also recommended a report we use to understand SAF.

Figure 1. The mebers of the Clean Skies for Tomorrow forum. Source: The World Economic Forum.

Sustainable Aviation Fuel

The report is by the Clean Skies for Tomorrow forum organized by the World Economic Forum. Clean Skies for Tomorrow now includes 80 organizations (Figure 1) working towards the goal of net zero emissions from Air Transport by 2050.

The forum produced a report: Sustainable Aviation Fuels as a Pathway to Net-Zero Aviation (highly recommended reading for the interested). It’s useful as it gives a baseline in the tricky field of SAF. There are many ways to produce the hydrocarbons that make a SAF, and not all are important ones.

Who decides what is an acceptable SAF and not?

The body that certifies aviation fuels and, therefore, SAF and its acceptable blend ratios is The American Society of Testing and Materials (ASTM). All aircraft and engine OEMs rely on ASTM to say what goes in the fuel tanks and what does not.

ASTM has approved seven alternative jet fuels for blending with conventional fossil jet fuel up to a specific blend ratio in their standard D7655. The resulting blends meet the Jet A/A-1 specifications D1655 and can thus safely be used in commercial aviation without further adjustments.

The presently accepted SAFs and their importance

Below we list the key SAF variants accepted by ASTM and their importance. Figures 2 and 3 are from the Clean Skies for Tomorrow report.

HEFA: Hydroprocessed Esters and Fatty Acids, also labeled HEFA-SPK. It’s the classical oils and fats route to SAF, Figure 2. It’s over 95% of today’s SAF and will stay above 80% by 2030, according to experts. The feedstock is larger than one thinks. Rigorous care is taken by the SAF industry to not compete with feedstock for food production. There are many ways to avoid that; read the report for details.

Figure 2. The main pathways to SAF. Source: Clean Skies for Tomorrow report. Click to enlarge.

To explain the line ” % LCA GHG reduction vs. fossil jet “: It looks at the complete pathway and measures all energy use and their GHG emissions, including what the fuel contributes, and compares it to the emissions from Jet A1.

The Alcohol-to-jet pathway converts biomass to ethanol, isobutanol, or methanol and then into SAF. The ethanol route was added to the ASTM-approved fuels in 2018; the others are not certified. Look at Figure 3 to understand the importance of Alcohol-to-jet and other SAF variants in the near term.

Figure 3. The production volumes of the different SAF types to 2025. Source: Clean Skies for Tomorrow report.

The SAF report was issued in November 2020. We have the result for 2022 today. The SAF production volume was at the dark blue area level, about 0.2m tonnes, compared with a Jet-A1 consumption of 300m tonnes.

Gasification+Fisher Tropsch is a biomass but, more importantly, a waste pathway. It has little importance until 2030 but has high long-term potential. The feedstock potential is vast as society produces abundant waste, but the waste collection process must be established that results in pyrolysis that produces syngas and then SAF.

Power to Liquid or e-fuel is a long-term source, but we must have cheap energy that can’t easily be routed to consumers to motivate this pathway. Direct capture of Sun energy and production of SAF in reactors is one long-term possibility that doesn’t compete with other energy uses; there are others. They have in common that until 2030 and possibly until 2050, this is a parenthesis because of the energy costs of grid energy and the time it takes to build alternatives dedicated to Syngas production (from which you can produce hydrogen or SAF).

The grounding of the SAF discussion

The work by the Clean Skies for Tomorrow forum and this SAF report is invaluable as it sets the scene for SAF. The importance of HEFA is evident, and the minor roles of other pathways (at least near term) are important to understand.

An example of the value of the forum and the report: The role of algae as feedstock is classified by the report (and thus the experts) as unclear, and the approved ASTM blend ratio is only 10% at the report’s writing. As a consequence, it’s not part of the main pathways to SAF. Good to know, as it always surfaces as an important pathway in discussions.

Next Corner, we dig deeper around SAF feedstocks, SAF production fractions from feedstock, and the cost of SAF.

25 Comments on “Bjorn’s Corner: Sustainable Air Transport. Part 54. Sustainable Aviation Fuel Production

  1. Tangentially related to the present story:
    “Scientists calculate how much CO2 is being removed from the atmosphere for the first time”

    “About two billion tonnes of carbon dioxide are being removed from the atmosphere every year, scientists have calculated for the first time.

    “But nearly all of it is down to forests, despite growing investments in new technologies.

    “The University of Oxford led report is the first to assess how much CO2 removal the world is already achieving – and how much more is needed.

    “Writing in CarbonBrief today, the authors describe the 2bn GtCO2 estimate as “small” relative to current CO2 emissions of 36.6 GtCO2 per year from fossil fuels and cement. But add the figure is “perhaps larger than many might expect.”

    “The report finds that roughly 1,300 times more carbon dioxide removal from new technologies – and twice as much from trees and soils – are needed by 2050 to limit global heating to well below 2C, as set out in the Paris Agreement.”

  2. Fischer and Tropsch developed this process in the 1920ties.
    ( based on research beginning much earlier ( 1900+ ) )
    Germany has sufficient coal but lacks oil resources.
    Major use case was coal to fuel. Peak during WWII.
    Later South Afrika’s Sasoil worked large Fischer Tropsch plants for synthetic fuel ( embargo on oil but they again had coal available : sanctions are so productive 🙂

    “fresh” Biomass as input is rather new.

    • South African coal to fuel was also driven by huge increase in oil prices from the 70s and to save the foreign currency spend, which it continues to do so.
      Embargos only leak like a sieve and the UN only had a ‘voluntary’ ( general Assembly) oil embargo and a mandatory ( security Council) arms embargo. Capital flight of western money caused bigger problems for the apartheid government.
      Other synthetic fuel plants from that era are in Malaysia , New Zealand. ( both from natural gas)
      Theres a new plants under going commissioning in Qatar ( gas ) and Turkey (coal)

      • Coal to Oil technology has been competitive with mineral oil for quite a time, maybe 30-40 years. The break even point was about $50/barrel in the 2000’s. (It’s probably closer to $90 now). The issue has been that anyone investing a few billion dollars would be faced with the oil industry lowering its costs to drive the coal to oil industry to ruin as Saudi Producers easily can. It would require Government protection against oil price drops. Many of the plants under construction try to use propane and butane as a source for syn gas since these are in excess. In some cases conversion on the oil rig where it would be otherwise flared off. Natural gas is rather valuable now.

  3. Back in the late 1990ties I remember research reports
    that used CO2 ladden diesel exhaust (plus sunlight) to grow algae with nutritional value. ) Nothing seen on that topic in later years.

    • Every plant in the world requires CO2 to grow. The more there is, the more growth you get. This is why greenhouses significantly boost CO2 in the air; the results are dramatic.

      Atmospheric air contains approximately the following proportions of gases:

      Nitrogen (N2): 78%
      Oxygen (O2): 21%
      Argon (Ar): 0.93 %
      Carbon dioxide (CO2) : 0.04% (400 ppm)

      For human beings, there are a lot of oxygen molecules in the air to breath. Plants are limited to a very small amount of carbon dioxide. If you are sharing a room with your plant, you can think of it this way: one fifth (20%)of the volume of the room is usable for you to breathe (oxygen) when only 0.04% (400 ppm) is carbon dioxide available for the plant.

      lower levels of carbon dioxide than ambient can decrease plant growth 30-40% (at 150 ppm)

      Conversely, with a CO2 level about 500 ppm plant growth increased by 15-25%.

      Between 340 ppm – 700 ppm, CO2 can increase growth by 30-40%.

      • If you look at my comment above, you’ll see that plants are only removing a tiny fraction of the CO2 that humanity is putting into the atmosphere: it’s 2 billion tons versus 36.6 billion tons per year.

        Nice for the plants that they’re getting a little more lush (if they’re not burning), but not enough to prevent a runaway greenhouse effect.

        • plants are not the only carbon sink.

          Actually the oceans are the major absorber.

          ( Easy: look at where and how carbon is stored in fossil environments. Over time a lot of existing Carbon has been sequestered underground )

          cement, concrete: what seems to be overlooked is that over time the released CO2 from manufacture is reabsorbed. ( That (helped by that idiot Steve Bannon) hampered the “Biosphere 2” project.
          O2 levels dropped from 20% to 14% by way of CO2 being absorbed in the unsealed concrete surfaces.

        • This kind of comment makes it crystal clear that nobody has a clear idea how the earth actually works. Any quick internet search suggests that 25% of CO2 produced is absorbed by plants, and another 25% by ocean plants. That is 50% – a very far cry from “2 billion tons versus 36.6 billion tons.”

          If the error bars are THIS huge, then there is no clarity on the actual data. Your 1/18th versus the commonly-understood 1/2?

          Garbage in, garbage out.

          • Garbage in, garbage out.

            The numbers you compare are different domains.

            Before industrialization we had a slightly negative carbon balance.

          • “Any quick internet search suggests that 25% of CO2 produced is absorbed by plants, and another 25% by ocean plants.”

            Go ahead and give us some authorative links…

  4. As bad as the models are (remember: “All models are wrong. Some models are Useful”), we have actual DATA on what has happened so far.

    “a quarter to half of Earth’s vegetated lands has shown significant greening over the last 35 years largely due to rising levels of atmospheric carbon dioxide. The findings are based on computer models and data collected by NASA and NOAA satellites. The greening represents an increase in leaves on plants and trees equivalent in area to two times the continental United States.”[]

    There are hundreds of studies, papers, and real world experiences that prove how beneficial increased CO2 is to many kinds of vegetation. Not models. Not projections. Actual results. CO2 is plant food, and thee is far more life on earth because of CO2 emissions than there would be without it.

  5. Sustainable fuel is a laudable goal.
    How about Sustainable management of the commercial aviation industry in the US. What happens when a company quits investing in its product line and instead buy more of its own stock? Dated product line, shrinking market share, reduced footprint and employment…..
    Is this business model “sustainable”? No more so than a farmer eating his own seed corn.
    No need to name names here, but thinking of a company whose name rhymes with “Going “.

  6. To put the discussion around the argument of Bryce in perspective: the total carbon uptake (and emissions) from the plant system is much higher as 2 Gt/yr (in CO2), but something like 120 Gt/yr (as carbon) uptake from photosynthesis and a similar number respiration from plants and microbes living on decayed vegetation/soil carbon. See:

    The 2 Gt/yr in the Oxford report covers carbon story by active human management: ‘CDR involves capturing CO2 from the atmosphere and storing it durably on land, in the ocean, in geological formations or in products. Examples include reforestation, biochar, Bioenergy with Carbon Capture and Storage (BECCS) and Direct Air Carbon Capture and Storage (DACCS)’. That is a fraction of what currently is used in photosynthesis (and also a fraction of fossil fuel emissions). The issue is that though the flows going in and out of the vegetation/soil system are large, they are in equilibrium – as it should be, since otherwise no stable temperature on Earth (massive carbon sink -> snowball Earth; massive carbon emission -> hothouse Earth). By adding carbon from fossil fuels (long cycle), even in relatively small numbers, we distort this dynamic equilibrium leading to the current CO2 concentrations in air that are close to 450 ppm from a much lower pre-industrial level.

    • Quoting from the Oxford report that I posted above (emphasis added):

      “About two billion tonnes of carbon dioxide are being removed from the atmosphere every year, scientists have calculated for the first time.
      “**But nearly all of it is down to forests…**”

      One needs to distinguish between “absorption” and “removal”.
      – Plants absorb CO2 while they’re photosynthesizing during the day — but they emit CO2 while they’re respiring during the night. The (very small) difference between these two represents *net absorption*…but how much of that is subsequently “stored”?
      – When herbaceous (i.e. non-woody) plants die off each autumn, their decomposition returns most of their net-absorbed carbon to the atmosphere (only a tiny quantity ends up in soil). Same applies to herbaceous parts of woody plants, e.g. leaves and fruits.
      – When animals eat plant material, the net-absorbed carbon in that plant material gets returned to the atmosphere via digestion (only a tiny quantity ends up in soil). For the record: animals eat *a lot* of plant material each day.

      The Oxford report looks at the net, long-term removal of CO2 — predominantly by plants. That amount is only a fraction of what plants hold in short-term storage (and return to the atmosphere typically on a timescale of less than a year).

      So, all that CO2 is nice for the plants — but it’s far more than they can absorb **and store in wood** on the timescale on which it’s emitted.

  7. The US Nuclear Regulatory Commission (NRC) has certified a design for an advanced small modular reactor (SMR) of NuScale.
    It’s a 50MW rating but will soon be 77MW.
    The reactor, steam generator are all inside one convection circulation housing as in a nuclear submarine. No circulation pumps are required. The Unit can be factory assembled and transported by road and rail thus avoiding costs on site.

    • Isn’t that 50MW, the electrical output , which could suggest the thermal output of the reactor is around 200MW or so.
      Nuscale plants would come in 4 , 6 or 12 module sets

      They seem to be quite tall ‘chimneys’ with the nuclear core at the bottom of a 76 ft by 15 ft unit which includes steam generator inside containment shell.

      It will be interesting when they build an actual pilot unit

Leave a Reply

Your email address will not be published. Required fields are marked *