A glimpse into the complex Sustainable Aviation Fuel sector

SAF nozzle and plane
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Aviation’s #1 Challenge is reduction of CO emissions

Sustainable Aviation Fuel has great promise

Some of the technical aspects, prospects and players

The global aviation industry is seeking ways to replace jet fuel with greener alternative(s). There are a myriad dimensions to this search. The challenge is further exacerbated by difficulties in economical production, distribution. infrastructure and incredible technological research efforts both in terms of fitting the new energy source to the aircraft and of exploring new “fuels” in all meanings of that word. The risks of each are unknown, potentially massive and may vary over time.

Below is a very informative article by two scholars and the subject is the evaluation of all of the twists and turns the greening of aviation faces. The 23 page exposition has been excerpted. The last section contains a number of links that offer further insights into Sustainable Aviation Fuels.

An excellent research paper providing an expert, objective view of SAF’s potential and practicability. article headliner

[1] [2]


Abstract: As the push for carbon-neutral transport continues, the aviation sector is facing increasing pressure to reduce its carbon footprint. Furthermore, commercial air traffic is expected to resume the continuous growth experienced until the pandemic, highlighting the need for reduced emissions. The use of alternative fuels plays a key role in achieving future emission goals, while also lowering the dependency on fossil fuels. The so-called sustainable aviation fuels (SAF), which encompass bio and synthetic fuels, are currently the most viable option, but hydrogen is also being considered as a long-term solution.

The present paper reviews the production methods, logistical and technological barriers, and potential for future mass implementation of these alternative fuels.

In general, biofuels currently present higher technological readiness levels than other alternatives. Sustainable mass production faces critical feedstock-related challenges that synthetic fuels, together with other solutions, can overcome. All conventional fuel replacements, though with different scopes, will be important in meeting long-term goals. Government support will play an important role in accelerating and facilitating the transition towards sustainable aviation.”

[The scholarly paper is 23 pages long; so, below are excerpts from the article focusing on SAF developments.]

“  Gains in fuel efficiency amount to the most significant improvement in recent decades, with the current generation of aircraft being 20% more efficient than the previous generation, and 85% more than those of the 1960s . However, the rate of improvement is progressively slowing down and future progress is not expected to match the projected increase in air traffic, hence will therefore be insufficient, as this Figure indicates…

future trends SAF

Hydrogen- and battery-powered aircraft, as alternatives, are sustained on eventual breakthroughs that can overcome the current technological barriers…Thus, these technologies are only viable as long-term solutions.

“Drop-in“ alternatives to conventional jet kerosene emerge as the best short- and medium-term choice, with different pathways for production of sustainable aviation fuels already being certified for commercial use up to specific blend levels. Currently, biogenic feedstocks are most used, but sustainability poses as a major obstacle to larger-scale applications, and so power-to-liquids (PtL) options could present themselves as a better solution. However, the latter are currently too expensive in comparison with the former, and other methods, such as using solar energy, are still in the early development phases.

Against this background, this paper seeks to further analyze the current state of alternative aviation fuels, their potential to become both short- and long-term solutions towards carbon-neutral aviation, together with the challenges that oppose their future mass implementation in the industry…

2.1. Certification Process Overview

In order to be “drop-in ready“, SAF must meet the qualities and characteristics of conventional jet fuel… The international standard most used to define the kerosene-based fuel used in commercial aviation is ASTM International’s D1655,Standard Specification for Aviation Turbine Fuels…Table A1 in

Appendix A summarizes ASTM specifications for a selection of critical fuel propertiesASTM SAF


2.2. Biofuels

Biofuels are alternative fuels that can be produced from any renewable carbon-based material, or feedstock. Since plants are the most common sources for biofuel production, associated carbon life cycle emissions can be significantly reduced, as some of the CO2 will be reabsorbed by the next generation of crops. Figure 5shows a comparison of carbon life cycle emissions between fossil-based jet fuel and bio SAF

carbon life cycle



2.1. Renewable Feedstocks

It is important to notice that not all biofuels can be used in the aviation sector. Biodiesel, for example, does not meet the performance requirements to enable its use in aircraft, mainly due to lower energy density and higher freezing point than Jet-A fuel. Some bioalcohols, on the other hand, have shown potential in studies conducted with internal combustion engines—an example is bioethanol However, in order for a biofuel to be classified as an SAF, it needs to meet further emissions and sustainability criteria. That is not the case for biofuels made from first-generation feed-stocks (edible crops), which is why these are not considered viable alternatives for future implementation

[The headings of the article’s SAF descriptions:]





Waste Oil…


2.2.2. Certified Pathways

Presently, ASTM has approved eight technology platforms, or conversion processes, for SAF production, specified in as many annexes to the already mentioned standards, mainly D7566. These are related to different feedstock types, and have specific maximum

blending ratios associated.

[some of the subheadings under Certified Pathways:]

      • Hydroprocessed Esters and Fatty Acids Synthetic Paraffinic Kerosene (HEFA-SPK)
      • Hydroprocessed Fermented Sugars to Synthetic Isoparaffins (HFS-SIP)
      • Alcohol to Jet Synthetic Paraffinic Kerosene (ATJ-SPK)
      • Catalytic-Hydrothermolysis-Synthesized Kerosene (CH-SK, or CHJ)
      • Hydroprocessed Hydrocarbons, Esters, and Fatty Acids Synthetic Paraffinic Kerosene


saf timeline and processe



2.2.3. Other Pathways

In addition to the aforementioned approved pathways, other methods are currently seeking future certification in different phases of the D4054 process. Other candidate pathways are in the pre-qualification stages for ASTM approval.


2.3. Synthetic Fuels

Unlike biofuels, synthetic or e-fuels do not rely on biomass to produce liquid hydrocarbons. This means that sustainability issues adjacent to large-scale production of renewable bio feedstocks, such as land usage disputes or food–feed–fuel ethical and economical problems would be significantly avoided , which in turn implies that, though research is not as advanced as is the case with bio SAFs, e-fuels could potentially become the best choice for short- and medium-term decarbonization efforts…


2.3.1. Power-to-Liquids (PtL)

Production Pathways

There are two established pathways for production of PtL fuels—the Fischer–Tropsch(FT) pathway and the methanol pathway [ They present similarities in that both require a supply of captured CO2 and of H2, with the latter being obtained through electrolysis of water; from then on, though, they differ in how hydrocarbons are synthesized and upgraded into fuel. Figure 8presents a generic scheme of the PtL production chain.

ElectrolysisSAF PTL chart



2.3.2. Sun-to-Liquid Process

Sun-to-liquid processes differ from PtL pathways in that solar energy is directly used to synthesize liquid hydrocarbon fuels… This paper will focus on the thermochemical process to produce fuels from concentrated sunlight that is being developed by the European Union-backed consortium SUN-to-LIQUID.


3.1. Hydrogen

Hydrogen consumption has been progressively increasing in recent decades, but not on a sustainable manner, being mostly produced from gasification of coal or reforming of natural gas, with associated CO2sun to liquid emissions exceeding 800 million tons per year…Hydrogen could be used to power aircraft as combustion fuel or in fuel cells. If used as a liquid fuel replacement, and though the same combustion principles as with current aircraft apply,

there would have to be engine reconfigurations to due to variations in the combustion gases and properties between the kerosene and hydrogen  hydrogen fuel cell generates electricity

via an electrochemical reaction between hydrogen and oxygen…

3.2. Battery Electric

Aircraft propulsion through the use of batteries has the highest potential to reduce not only CO2 emissions, but emissions of other GHGs, such as NOx as well  However, it is also affected by limited range barriers. Current research has been focusing on developing vertical take-off and landing (VTOL) technologies for urban mobility and other small-range applications that are scheduled to enter commercialization in upcoming years. Still, even by 2050, large battery-powered aircraft are not expected to cover distances of more than approximately 500km

SAF Poster



Some Useful Reference Materials


Sustainable Aviation Fuels Summit Remarks

Wednesday, June 1, 2022

Acting FAA Administrator Billy Nolen (April 1, 2022 – present)

Acting Administrator Nolen








Sustainable Aviation Fuel: Review of Technical Pathways Report

DoE SAF report


New Handbook Provides Expert Guidance on Using High-Integrity Sustainable Aviation Fuels (SAF) to Decarbonize AviationEDF logo




An Introduction to Sustainable Aviation Fuels

Part 1 of a series, “Sustainable Aviation Fuels: A Critical Emissions Mitigation Strategy Gaining Momentum”

EESI logo



 Click for full page



[1] J.M.M. Sousa (0000-0002-0310-3115) (orcid.org)

[2] Eduardo CABRERA | University of Lisbon, Lisbon | UL (researchgate.net)



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