Optimisation of the downstream sustainable aviation fuel production process
The Fischer-Tropsch synthesis is a common process used for the production of synthetic fuels. The reaction converts carbon monoxide and hydrogen gases into hydrocarbons via a catalyst.
It is a very well-established process dating back to 1925 and has been adopted by major businesses to produce fuel and other carbon products. However, achieving the desired products from the Fischer-Tropsch reaction requires thorough research and analysis for process optimisation.
Evaluation of syncrude upgrading pathways to produce aviation-grade kerosene
The syncrude produced by the Fisher-Tropsch (FT) synthesis is a mixture consisting mainly of linear alkanes, or straight-chain hydrocarbons, according to the classic Florey-Schulz distribution. The remainder of the mixture is made up of a smaller proportion of alkenes, or hydrocarbons containing carbon-carbon double bonds, and oxygenated compounds. The process for upgrading the syncrude mixture is similar to the traditional refinery process for maximising the yield of kerosene (hydrocarbons containing 9 to 14 carbons, C9-C14) from crude oil but with some important differences due to the fact that syncrude has a significantly different composition to fossil crude despite their similarities.
Avioxx is developing a lab-scale system to validate and extend existing upgrading pathways common to both crude oil and syncrude, specifically focusing on hydrogenation of alkenes, cracking of larger molecules (longer-chain hydrocarbons that exist as waxes), isomerisation to increase branched/cyclic molecules to increase fuel density, and separation by distillation to maximise C9-C14 paraffins. In this way, the trade-off between carbon efficiency and upgrading cost and complexity can be quantified to inform the design of a commercial production plant. Put simply, more carbon from the feedstock can be converted to fuel at the expense of increased upgrading steps.
Strategies for fuel blending and regulatory compliance
In contrast to diesel for ground transportation, stringent demands are required for Jet A1 fuel to ensure flight safety. Aviation fuel containing synthesised hydrocarbons follows the ASTM D7566 standard, set by ASTM International, an organisation that develops international standards for various products, materials, and systems. Pre-screening carried out on kerosene produced in the Avioxx Advanced Fuels Laboratory will provide key fuel properties that are pre-requisites for ASTM D7566 compliance such as density at 15°C, surface tension at 22°C, viscosity at -20°C and -40°C, distillation curve, flash point, net heat of combustion, thermal stability, smoke point, and so on.
As of January 2025, the ASTM has approved 11 pathways for SAF production or processing, listed in the Annexes to ASTM D7566. Annex A1: “Fischer-Tropsch Synthetic Paraffinic Kerosene (FT-SPK)” is the route Avioxx is pursuing to transform household waste into jet fuel. Importantly, no SAF production technology, including the Annex A1 FT route, is approved for commercial use above a 50% blend with fossil-derived jet fuel due to the lack of certain components such as aromatics that are required to be absorbed into elastomer seals to swell them and prevent fuel leaks.
SAFs are approved for usage via a Tiered Test or, for a 10% blend, a less onerous Fast Track programme which is quicker and requires less product volume. There is therefore a compromise between blending levels, cost of approval and design and operation of the upgrading pathway. Avioxx is combining lab results and their team’s process engineering expertise to create a quantitative and predictive model to inform the design of the syncrude upgrading pathway for their commercial SAF plant.
Analysis and testing of FT-derived Jet A1 product samples produced at Avioxx’s Advanced Fuels Lab is supported by industry-leading test facilities and the UK SAF Clearing House. The results will enable the Avioxx engineers to optimise both the Fischer-Tropsch reaction and syncrude upgrading processes as well as to ensure the fuel complies with ASTM D7566 specifications.
To learn more about Avioxx’s novel waste-to-SAF process, please get in touch at info@avioxx.com.