Over past years, a number of test flights have been carried out using 100% sustainable aviation fuel, therefore unblended with conventional jet fuel. Boeing and other aircraft and engine manufacturers have committed to delivering commercial airplanes ready to fly on 100% SAF by 2030. In December, United Airlines flew the world’s first passenger flight with 100% SAF fuelling one engine. While it may seem that flying on unblended SAF may soon be the norm, the situation is significantly more complex. ASTM standards do not allow SAF to be used neat today and is mostly limited to a 50% blend, with a maximum of 10% in some cases. SAF production technologies produce fuels that are chemically distinct, even though the universal term SAF implies that they are all the same. Susan van Dyk unpacks ASTM standards, the meaning of 100% SAF and ‘drop-in’ fuel, and the future prospects for commercial flights operating on unblended SAF.
Jet fuel must comply with strict standards to ensure safe operability under the conditions encountered during a flight. ASTM D1655 defines the minimum property requirements for Jet A and Jet A-1. With the advent of synthetic fuels, a dedicated standard, ASTM D7566, was developed to specify requirements and definitions for manufacturing and blending synthetic jet fuel blending components with conventional jet fuel. Not all synthetic fuels are sustainable and classified as SAF, for example coal-based fuels, as the standard neither determines nor requires the synthetic components to be procured sustainably. Seven types of synthetic jet fuel blending components – based on different production process technologies – are currently approved under ASTM D7566, which sets out the specifications and blending limits under separate Annexes. At present, a blending limit of 50% maximum is applicable for most types of synthetic jet fuel.
Questioned about the 50% blend limit under ASTM, Steve Csonka, Executive Director of the Commercial Aviation Alternative Fuel Initiative (CAAFI), whose members have played an important role in the standard setting process, explained the sector is still in its infancy. The current ASTM specification (D7566) for synthetic jet fuel is based on the initial understanding in 2009. At the time, a conservative approach was taken, and in many cases the 50% blend limit reflected industry caution to ensure aircraft and engine manufacturers (OEMs) had confidence in introducing synthetic fuels into use, recalled Csonka. The alcohol-to-jet technology, for example, had an initial blend limit of 30%, which was then increased to 50% after more testing.
The industry continues to progress in developing the ASTM standard based on new knowledge, he added. However, a decision was made early on that a drop-in approach was needed where no new infrastructure would be required since, he pointed out, the industry does not want segregated fuel systems, so higher blend levels will be pursued while remaining with the standard for a drop-in fuel.
The term ‘drop-in’ is used to denote fuels that can serve as a full substitute for the fossil-equivalent fuel and must be fully formulated (paraffins and aromatics) and fit-for-purpose. While currently specified unblended SAF in D7566 is generally perceived as a drop-in fuel, this is not correct for all types, as it is the blended fuel (SAF plus conventional jet fuel) that is a drop-in fuel for all types. Consequently, as a final step in producing SAF, D7566 requires blending of SAF blend components with conventional jet fuel and retesting to meet jet fuel property requirements.
The term SAF creates the impression that all SAFs are equal, but this is not the case as different types of technologies produce compositional variations across SAF blending components. Even using the same technology, different producers will not produce identical products, just as different refineries, using different crude oil, today produce jet fuel with varying compositions. Current ASTM-approved SAF types are either compositional subsets of jet fuel or fully formulated jet fuel compositions. The degree of compositional similarity determines their drop-in potential and therefore their maximum allowed blending percentage. SAF technologies such as Catalytic Hydrothermolysis Jet (CHJ) have a fully formulated jet fuel composition and therefore potentially could produce a drop-in SAF unblended, and it is expected that the blend level will eventually be increased to allow 100% of this type to be used unblended, said Csonka. Similarly, the SAF type FT-SPK/A (Fischer-Tropsch synthetic paraffinic kerosene and aromatics) is a fully formulated jet fuel obtained by blending synthetic aromatics with synthetic paraffins from the Fischer-Tropsch process.
Aromatics essential in jet fuel
Aromatics in jet fuel are needed for maintaining seal compatibility. They are reactive molecules that infiltrate the seal elastomeric material to promote seal swelling, and a lack of aromatics results in seal shrinkage which can lead to fuel leakage and seal failures with certain types of seals that exist in older legacy engine products, airframes, facilities and fuelling trucks. Fuel-seal compatibility is a key consideration for SAF certification and blending limits. The ASTM D1655 specification allows for an aromatic content of <25%, and fuel surveys indicate perhaps an average level of aromatics in petro-jet fuel worldwide is around 17%. Thus, mixing a 0% aromatic blending component, for example SPK, with a 17% aromatic petro-jet fuel at 50/50 ratio will result in a blended SAF having at least an 8% aromatics level. Such a level is viewed as being sufficient to maintain requisite fuel characteristics.
Several test flights on 100% neat SAF have used unblended HEFA-SPK, showing that it is possible to operate aircraft without any technical concerns. However, the engines and aircraft used for these flights have either been modified to include seals that do not require aromatics to prevent leakage, or their current designs and configurations have recently eliminated such materials. Should unblended HEFA-SPK be used in an incompatible jet engine and aircraft, seal leakage could occur, which may compromise the safe operation of the aircraft.
According to Csonka, there are currently more than 25,000 ‘legacy’ aircraft in use over the world, and widespread use of unblended paraffinic SAF at every airport will require modification of most of these aircraft and engines, as well as the infrastructure, so making it a daunting task. Aircraft are in use for decades and the natural replacement of aircraft is a very slow process. These legacy aircraft will need jet fuel certified to current standards, meaning that it must contain some level of aromatics, even if reduced, or other fuel molecules that can replicate the function of aromatics.
In order to achieve the climate benefits of zero or low aromatics, said Csonka, researchers are exploring the possibility of using alternative compounds such as cycloparaffins to serve the same purpose as aromatics.
If SAF must have aromatics, why are OEMs carrying out test flights on 100% SAF without aromatics?One of the main reasons for test flights using 100% HEFA was to test the impact of 100% SAF on non-CO2 climate effects through persistent contrail formation. A NASA/DLR study, flying on 100% SAF and monitoring emissions through a chaser aircraft, demonstrated that using 100% HEFA-SPK, non-CO2 climate effects from aviation could be reduced as the absence of aromatics reduces particulate formation and persistent contrails that are tied to particulates.
As Csonka explained, doing test flights with 100% SPK has allowed researchers to measure the full impact of zero aromatics. They determined aromatic compounds are the biggest culprits in particulate formation and persistent contrails. In the process, airframers have demonstrated that much of their current production equipment can use 100% HEFA-SPK, although some additional work is still needed to verify complete compatibility.
Boeing has made a commitment to ensure compatibility of its current and future airplanes with 100% SPK (or 100% SAF without aromatics) by 2030. Speaking at the IEA Bioenergy end-of-triennium conference in November 2021, Sean Newsum, Director of Environmental Strategy at Boeing, explained the objective is to ensure airplane compatibility issues do not limit long-term growth of SAF supply.
Although the legacy fleet will not be able to use 100% SPK for decades to come, test flights with 100% HEFA-SPK are being carried out by OEMs to remove the barriers to higher blends and enable deeper carbon reductions, said Csonka.
ASTM 100% SAF certification
Will it be possible to certify 100% SAF under ASTM?Theoretically, this is possible, said Csonka, but it will depend on the type of SAF and the functional characteristics of the unblended SAF, which will be measured through extensive testing and evaluations according to ASTM procedures. Two SAF production process technologies, CHJ and FT-SPK/A, can likely already produce a drop-in equivalent SAF unblended and it is expected that the blend level would be increased to allow 100% of this type to be used unblended, he reported.
A drop-in 100% SAF could potentially also be obtained using a mixture of different types of SAF types, said Csonka. It means that a SAF technology producing only paraffinic kerosene will have to be blended with aromatic or certain cycloparaffinic compounds. The blend could still be 100% renewable if the aromatic compounds are derived from biobased feedstocks. The ASTM standard does not currently allow the blending of two or more approved types of SAF blending components (although distinct SAF blends themselves could be blended as they are reidentified as Jet A/A-1) for commercial flights, but studies have been carried out to show that multi-blends can be used without any concerns, for example, the United Airlines flight in December 2021 used a blend of HEFA-SPK and synthetic aromatics produced by the company Virent’s technology. A research and demonstration project was carried out under the German Mobility and Fuel Strategy and investigated the use of a multi-blend of SAF (HEFA-SPK and ATJ-SPK, alcohol-to-jet synthetic paraffinic kerosene) at Leipzig/Halle Airport and demonstrated that this could be feasible.
According to Csonka, a multi-blend mix could potentially be approved under ASTM within the next one-and-a-half to two years.
Why can’t two types of jet fuel be supplied at airports, one with aromatics and one without aromatics?While ASTM issues specifications for jet fuel, transport and quality assurance is carried out under the Joint Inspection Group (JIG) and Airlines for America (A4A) Standards which permits only one type of fuel (certified Jet A/A-1) at an airport, explained Csonka. Where SAF is used, blending must take place outside the airport and ASTM compliance demonstrated before the blended fuel may enter the airport to become part of the common fuel hydrant system. To supply two types of fuel at an airport will therefore require extensive changes to the entire fuel supply chain infrastructure and logistics. The industry does not want segregated fuel systems, insisted Csonka, as there is a potential for mistakes in refuelling where two different types of fuel are available.
Using 100% SAF will be essential in the future if the sector is to meet its long-term climate goals, he said. Aircraft manufacturers are anticipating this need and developing aircraft that can use 100% SAF, and demonstration flights have tried to show that this can be done without any technical concerns.
“Drop-in SAF is something our industry can adopt now to begin making inroads on our commitment to be net-zero emissions by 2050,” said Gaël Méheust, CEO of CFM International, whose LEAP-1B engines powered the United Airlines test flight.
Gurhan Andac, GE Aviation’s engineering leader for aviation fuels and additives, chairs a task force under ASTM to modify the relevant standards to allow certification of 100% drop-in SAF, for example, produced by the CHJ process. The possible adoption of non-drop-in SAF will be considered later, he said at a CAAFI webinar in October 2021.
While 100% SAF is not an immediate need, it is time to start the process to get ready from a technological, operational and standardisation perspective, said Andac.
Photo: In December, United Airlines flew the world’s first passenger flight with 100% SAF fuelling one engine
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