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

Susan van Dyk

GreenAir Correspondent

www.svdconsulting.ca

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The recent virtual symposium hosted in the US by CAAFI, the Commercial Aviation Alternative Fuels Initiative, illustrated the increasing momentum in sustainable aviation fuel (SAF) commercial development, with companies at different stages of development outlining ambitious plans for multiple facilities over the coming years. Many facilities are already under construction or nearing completion, and the increased availability of SAF is expected to become a reality over the next few years. Significantly, many producers are pursuing net zero SAF from initial construction by integrating technologies such as renewable energy, green hydrogen and carbon capture and storage into their fuel production strategy, reports Susan van Dyk. By incorporating net zero targets as part of the initial engineering design of a facility, any SAF pathway can potentially be net zero if the right policies are in place to incentivise the greatest emission reductions.

Gevo’s first fully net zero project is planned for its Lake Preston, South Dakota, facility and across its entire feedstock supply chain, CEO Pat Gruber told the symposium. Onsite electricity used in the facility will be derived from biogas, offsite electricity from wind turbines and renewable hydrogen. As Gevo uses corn for the production of isobutanol, the feedstock supply chain is addressed explicitly through better agricultural practices and land management, improved tillage practices and sustainable fertiliser. Gevo works closely with farmers and plans to reward them according to the sustainability of their corn, according to Gruber. Adds the company: “Gevo designs our entire business with carbon value in mind from the beginning, and carbon value has an impact on everything we do. By focusing on carbon value, Gevo is set up to maximise the value of renewable energy sources. When we aim towards that goal, everything we do in developing our plans, building our facilities, working with airlines, fuel companies, farmers and other partners, becomes focused on sustainability.”

Other companies pursuing a holistic approach to net zero SAF are Velocys, Aemetis, Red Rock Biofuels, and SkyNRG Americas. The Velocys Bayou Fuels project, according to Jeff McDaniel, VP New Projects, can achieve a negative -144 gCO2/MJ carbon intensity based on the production of the facility’s electricity with solar power and further use of carbon capture and sequestration for emissions from the facility, to achieve a significant reduction in carbon intensity.

Eric McAfee, CEO of Aemetis, told the CAAFI symposium the company’s strategy for net-zero includes renewable natural gas, cellulosic hydrogen, and carbon capture and storage. The proposed facility of SkyNRG Americas aims for maximum emission reductions by producing hydrogen from electrolysis, stated CEO John Plaza, while Red Rock Biofuels is implementing engineering changes to its Lakeview facility to lower the carbon intensity of the fuels. According to CEO Terry Kulesa, the Lakeview facility will use solar power for electricity and undertake carbon capture of any emissions.

The ability of SAF to reduce emissions, reflected in the carbon intensity (CI) of the specific fuel, is a central characteristic of its environmental benefits and sustainability, and based on a life cycle assessment across the entire supply chain of the fuel production process. Under CORSIA, eligible fuels are given default CI values, termed life cycle emission factors (LSf), based on the type of technology and feedstock used, although CORSIA provides a methodology to determine the unique LSf of a SAF pathway. The default LSf under CORSIA for isobutanol-to-jet based on a corn feedstock (similar to the Gevo pathway), is 77 gCO2eq/MJ compared to conventional jet fuel of 89 gCO2eq/MJ. However, the actual calculated value for Gevo fuel is -5 gCO2/MJ, according to Gruber.  

If any type of SAF can deliver net zero, all technologies can potentially be used to meet net zero targets for the aviation sector to 2050. Proponents of synthetic e-fuels, such as Andrew Murphy from Transport and Environment, argue that power-to-liquid (PtL) fuels should be the main SAF technology in the long term as it is the only pathway that can achieve net zero. In contrast, Andreea Moyes, Global Aviation Sustainability Director at BP in a presentation at the CAAFI Symposium, argues “multiple SAF pathways are required, and all should be allowed to compete on their own merits within societal preference.” According to Moyes, the GHG profile of SAF, rather than the volume, should be the focus.

An important driver for aggressive targeting of maximum emissions reductions is placing a value on carbon. This type of policy is already in action in California’s Low Carbon Fuel Standard and has created a strong incentive for low carbon intensity fuels. The proposed Sustainable Skies Act, recently introduced in the US House of Representatives by Congressman Brad Schneider, creates exactly this type of policy in the aviation sector (see article). The Act proposes a blenders tax credit of $1.50 per gallon of SAF that provides a 50% reduction in emissions. SAF that provides greater emission reductions can earn an additional credit of $0.01 per gallon for each percentage the fuel reduces emissions over 50% up to a maximum of $2 per gallon for a 100% reduction, in order to incentivise greater reductions in emissions.

In contrast, the European ReFuelEU Aviation policy proposes a volumetric blending mandate which is not directly linked to emission reductions (see article). Although this will create a strong demand signal, it seems unlikely to drive aggressive carbon reductions of SAFs. Bryan Stonehouse, General Manager Aviation Sustainability & Risk at Shell Aviation, speaking at the recent IATA SAF Symposium, explained the role of policy for driving investment in SAF and contrasted the blenders tax credit with the ReFuelEU mandate. According to Stonehouse, the mandate is important for creating a demand but does not give the whole picture. The blenders tax credit will be the carrot for the industry, he said. According to Stonehouse, SAF is incredibly expensive and needs affordability support and argued the SAF industry needs aspects of both the ReFuelEU and the US blenders tax credit for development and scale-up.

While carbon intensity is only one component of sustainability, achieving net zero carbon intensities of SAF pathways should be a critical focus in the sector. Several US companies are engineering new facilities with a goal of achieving net-zero carbon intensity of fuels, and this seems to be driven by policies in the US that incentivise greater reductions. Designing and engineering a facility from the get-go to produce net zero fuels makes sense, and the right policies are crucial at this critical stage of investment and scale-up. While multiple policies are needed, rewarding carbon intensity reductions should play a central role in policymaking.

Photo courtesy of Alaska Airlines and Gevo

Susan van Dyk

GreenAir Correspondent

www.svdconsulting.ca

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