Various approaches look to reduce the costs of sustainable jet fuel while maintaining performance.
By Dr. Raj Shah, Beau Eng, Mrinaleni Das and Dr. Vikram Mittal
Sustainable aviation fuels (SAFs), biofuels exhibiting properties similar to conventional jet fuel, are growing rapidly in popularity as countries across the globe seek emission-free sustainable energy sources. Over the past three years, SAFs have experienced significant innovations and advancements, prompted by international efforts to reduce greenhouse gas emissions. In this article, we will explore the notable developments in SAFs, remarking on their importance and positive implications for the further development of sustainable aviation fuel as a whole.
The primary fault of SAFs and the main reason they haven’t been widely implemented is their prohibitively high cost. Researchers sought to remedy this by expanding upon the number of chemical production pathways for SAFs, as the implementation of more approved methods of creating SAFs would allow more production companies to produce them. Ideally, the greater variety in SAF production would lead to increased production and greater stock, which would drive down demand and lower the price of SAF to reasonably compete with traditional jet fuels.
From that metric, SAF development started off on a very strong foot in 2020, as the American Society for Testing and Materials (ASTM) approved two technical certifications, effectively increasing the number of approved technical pathways to seven. While this may seem like a fantastic start, not all pathways have found use in generating SAFs for civil use. This may be due to changing priorities, a mismatch of materials required for that pathway or some other issue that renders those paths unsuitable to produce “economically viable” SAFs.
Still, this does not render the discovery of two new pathways useless. On the contrary, both pathways present opportunities to branch from their processes to arrive at entirely new pathways and even improvements to the existing pathways. If these pathways are experimented with further, it could be possible to salvage them for alternative use, or perhaps these pathways could be replicated, using biofuel or other techniques that may yet render them viable for commercial production.
Alongside this discovery, the Sathyabama Institute of Science and Technology in India released a study regarding two SAFs, reporting that they could be “the potential replacement” for Jet A-1, a kerosene-type fuel and one of the most commonly used fuels for commercial aviation. The two alternative fuels are blends of Jet-A fuel, one being a blend of 70% Jet-A, 20% canola oil and 10% ethanol, while the other is an almost identical blend with a 1:1 substitution of canola oil with pyrolysis oil. In the experiment, standard Jet-A fuel was tested in a gas engine turbine at various loads for performance, combustion and emission. The two blends were then tested separately under the same conditions, and the results were compared.
The study found that the SAF mixtures exhibited greater static thrust than the standard Jet-A fuel due to the addition of ethanol, with the pyrolysis oil blend exhibiting higher static thrust than the canola. Moreover, the pyrolysis blend registered the lowest fuel consumption at all speeds, while the canola blend showed no significant difference. Finally, the study found that the average emission of NOx, CO and CO2 was reduced significantly compared to the Jet-A blend, with the canola oil blend experiencing a greater reduction than the pyrolysis blend. From this experiment, the researchers concluded that pyrolysis oil could work well for higher-performance engines, while the canola oil blend works well to mitigate carbon emissions. With further experimentation on canola oil and pyrolysis oil, future SAF pathways could find greater success in mitigating the emission of greenhouse gases while also outpacing the fuel efficiency of traditional jet fuels.
In keeping with the idea of sustainable pathways, a study called “Toward Net-Zero Sustainable Aviation Fuel with Wet Waste-Derived Volatile Fatty Acids” was conducted, testing a potential SAF pathway that creatively utilized wet waste as a feedstock source. Wet waste is a slurry of food waste, manure, sludge and various waste fats, oils and greases. Due to its high moisture content and relative complexity, it is often relegated to simple methane production through anaerobic digestion.
If used as is, the high moisture would not allow for conventional thermochemical approaches for biofuels like pyrolysis or gasification. However, if the anaerobic digestion of wet waste were to be paused before methanogenesis occurs, volatile fatty acids (VFAs) are produced. From there, VFAs can be reworked into SAFs through carbon coupling and deoxygenation to create long-chain hydrocarbons suitable for jet fuel. This experiment then puts this theory into practice, producing n-paraffin and testing it using the ASTM’s Fast Track qualifications for a SAF. This testing would allow the potential SAF blend to bypass years of testing and potentially see production within one to two years.
The study reports that the paraffin SAF blend met the specifications for this qualification, proving that the method works and is feasible. Furthermore, the study enhanced the mixture by blending it further with isoparaffin VFA-SAF. This new blend increased the upper blend limit to 70 vol%, meaning that 70% of the blend can consist of the VFA-SAF mixture, with the remainder being standard jet fuel. On top of the significantly reduced soot production compared to standard fossil fuels, this new pathway could prove to be very significant. Wet waste is very prominent in many landfills and is currently not very useful as a resource. If this pathway is approved, wet waste could very well be a new source of SAFs. Using wet waste as a resource could lead to a drastic increase in the supply of SAFs, reducing the high price of SAFs while simultaneously reducing the amount of clutter and pollution in landfills.
Thus, the development of SAFs has come very far in the past three years, and it shows no signs of stopping anytime soon. As of 2022, five new SAF pathways are working towards qualification according to the International Civil Aviation Organization, and the number of proposed pathways is only increasing. New laboratories have opened up for the express purpose of SAF testing, including one in Washington funded by Washington State University that promises to digitalize and revolutionize SAF research, development and transport.
In terms of recent corporate advancements, as of 2022, 38 of the world’s top airlines have committed to net-zero emissions and almost 30 of those airlines have set a target for SAF adoption, according to BloombergNEF. Most promising of all is Virgin Atlantic’s first-ever net-zero transatlantic flight, which will be fueled by a 100% SAF blend composed of waste oils and fats and combined with biochar credits, which is a material that traps and stores carbon from the atmosphere. The flight is scheduled for 2023, and if all goes well, this could be yet another landmark in SAF history.
While SAFs remain more expensive than standard fuel, the gap is gradually narrowing, and with the market for SAFs growing steadily, we will see the price of SAF fall below jet fuel eventually. At such a crucial juncture, SAF engineers will continue to push ever forward, towards a cheaper fuel source, and towards a cleaner future.
Dr. Raj Shah is a director at Koehler Instrument Company. He is an elected Fellow by his peers at IChemE, CMI, STLE, AIC, NLGI, INSTMC, Institute of Physics, The Energy Institute and The Royal Society of Chemistry. An ASTM Eagle award recipient, Dr. Shah recently coedited the bestseller, “Fuels and Lubricants handbook.”
Dr. Vikram Mittal is an assistant professor at the United States Military Academy in the Department of Systems Engineering. Mittal’s current research interests include system design, model-based systems engineering and modern engine technologies.
Beau Eng and Mrinaleni Das are part of a thriving internship program at Koehler Instrument Company in Holtsville, New York, and are students of chemical engineering at Stony Brook University, Long island, where Dr. Shah and Dr. Mittal are on the external advisory board of directors.
