Viewpoint: Bio-Based Aromatics Point The Way To Burning 100% SAF

The need for aromatics in aviation fuel currently limits the percentage of sustainable aviation fuel (SAF) that can be added. However, a new process that produces bio-based aromatics has made it possible to fuel aircraft with greater volumes of SAF, including 100% SAF, which has been tested by United Airlines, Emirates Airline, Gulfstream, Bell Textron and now Virgin Atlantic, with the first 100% SAF, trans-Atlantic crossing by a commercial airline. But how was this made possible, and what does it mean for the industry?

While new fuels and technologies, such as hydrogen, offer potential pathways to net zero, SAF offers the most practical solution to decarbonizing global aviation because it can be a “drop-in” solution. Modification of global aircraft fleets, airports, and fueling infrastructure, a great deal of which is underground, would be a huge undertaking in terms of cost, time, logistics, and disruption to air travel. Jet fuel has precise specifications for quality and composition, and, like shipping, air transportation is reliant on this global network being available. After all, there is no point in having a sustainably fueled aircraft that can fly from London to New York if it cannot be refueled at its destination.

SAF, on its own, will not pave the way to decarbonization. The term “SAF” has become an umbrella for varied SAF blends with traditional jet fuels. Ultimately, if the fuel blend reduces emissions by a certain percentage, it can be referred to as SAF. However, this is muddying the waters, as the result is only a small reduction in emissions and far from what is needed if the aviation industry is going to meet its 2050 emission targets.

Up until now, only a maximum of 50% SAF could be blended with traditional jet fuel. This is because the various forms of SAF do not contain aromatics, which are present in traditional jet fuels. Paraffinic hydrocarbons, which make up most of the fuel, are straight chained molecules; a visual analogy would be to think of them shaped like breadsticks. Aromatics, on the other hand, are ring-shaped: imagine them like bagels. While these are a relatively small percentage of the fuel, they are an important part of the fuel “breadbasket” as they have lower freeze points, lower viscosity, and interact with polymer seals in a jet’s fueling system, making them swell and helping prevent fuel leakage and the associated fire risk this presents.

To meet the global Jet A fuel specification, synthetic fuels must contain between 8-25% aromatics. While some believe new specifications should be established for 100% Hydroprocessed Esters and Fatty Acids (HEFA), or Fischer-Tropsch (FT) and Alcohol-to-Jet (A2J) products, using these fuels without aromatics presents a variety of challenges, and would almost certainly require at least recalibration of instrumentation to account for the different density of the fuel.

Bio-Based Aromatic Blend Component
A new process, originally invented by Virent and further developed in collaboration with Johnson Matthey, has enabled the creation of a bio-based aromatic blend component: BioForm Synthesized Aromatic Kerosene (SAK). This breakthrough means that achieving the percentage of aromatics within jet fuel (as defined by ASTM international standards), is no longer reliant on the inclusion of conventional jet fuel. This clears a pathway to using increasing percentages of SAF within current fleets and fueling infrastructure and has enabled the 100% SAF trans-Atlantic flight by Virgin Atlantic. No modification is required to engines and the fuel performs as well as traditional jet fuel.

The BioForming process converts sugars to aromatics (S2A). While the initial Virgin Atlantic flight used BioForm SAK produced from dextrose from corn, the synthetic aromatics can be created using a wide range of plant sugars, including beet sugar, sugar cane, and corn starch, as well as cellulosic sugars produced from bagasse, corn stover, grasses, sorghum, and wood, and other soluble carbohydrates. This means it is not reliant on a limited source of feedstocks and the process can be adapted to the different resources available in different regions of the world. The result is a drop-in bio-based aromatic component that can be blended with various paraffinic blendstocks produced using the different process techniques defined in current or future international standards, such as HEFA or FT and A2J products.

SAK supplies the necessary aromatics for jet fuel and can be blended with a range of other SAF, accelerating the shift from conventional fuel to sustainable alternatives compatible with existing aviation technology. The various demonstration flights using BioForm SAK have also shown that SAK can be a “normalizer” to ensure different SAF blendstocks can be brought within specification and meet aviation performance criteria.

What does all this mean in terms of emissions? Ultimately, burning SAF to power aircraft still emits carbon dioxide into the atmosphere. After all, it is still a hydrocarbon, and the basic rules of chemistry still apply. Indeed, SAF offers such a great solution because it acts in the same way as traditional jet fuel.

However, SAF can be made from renewable biomass (agriculture sources), waste resources, or from captured carbon dioxide and green hydrogen. Whichever feedstock is used, it is either removing or stopping amounts of CO2 being released into the atmosphere. i.e., the crops that provide the feedstocks absorb CO2 from the atmosphere as they grow, waste that would have gone to landfill and released CO2 is better used, or CO2 is directly removed from the atmosphere. In other words, lifecycle greenhouse gas (GHG) emissions are significantly lower than traditional jet fuel. By adding bio-based aromatics into the SAF blend, up to 100% SAF can be used to power airplanes.

As well as enabling the use of greater volumes of SAF, BioForm SAK offers additional environmental benefits compared with traditional jet fuel. This is because the aromatics found in conventional jet fuel contain large molecules with multiple rings fused together, which are significant contributors to the fine particulate matter released when the fuel is burned. The feedstock and processes used to produce BioForm SAK produces smaller, single-ringed molecules, which provide a cleaner-burning alternative that can reduce particulate emissions by up to 80%.

Increasing Percentages Of SAF
SAF is the strongest contender to reduce lifecycle aviation GHG emissions as it can be used as a drop-in solution within current fleets and fueling infrastructure. To date, its use has been mainly constrained by the need to blend SAF with traditional jet fuel to meet stringent jet fuel standards and achieve the required percentage of aromatics in the fuel.

BioForm SAK removes the need to incorporate traditional jet fuel in the fuel mix and provides an overall cleaner-burning alternative. It opens the pathway to using increasing percentages of SAF to significantly reduce the impact of air travel on climate change. Indeed, depending on the feedstock and processes used to produce SAF, BioForm SAK can achieve net zero emissions today.

Of course, the production volumes of SAF need to increase dramatically if we are to move to 100% SAF-powered flights, but the Virgin Atlantic 100% SAF trans-Atlantic crossing has shown the immediate power and potential of SAF.

Iain Gilmore is senior manager of Catalyst Technologies at Johnson Matthey, a developer of sustainable technologies. David Kettner is president and general counsel of Virent, a wholly owned subsidiary of Marathon Petroleum Corp.