Sustainable Aviation Fuel: Part 2
SAF Production and Composition
How can you fly across the Atlantic using vegetable oil? How do you go from solid biomass like wood chips to liquid fuel you can burn in a jet? How would you turn electricity and air into synthetic kerosene? And most importantly, how much would it cost to produce billions of gallons per year?
In my last post, I introduced sustainable aviation fuel (SAF) and how CO2 and water emitted from combustion can be recaptured via biomass processing or using electricity (Power-to-Liquid). In the next couple of posts, we will dive deeper into what it takes to get to a 100% SAF future by reviewing what jet fuel is made of, some of the approaches to producing SAF, and the economics and other challenges to scale.
Jet fuel is a regulated composition
First, it’s important to get an understanding of the target goal since different types of fuel (e.g. gasoline, jet fuel, diesel) have different compositions of hydrocarbons tailored toward certain physical and combustion properties. Crude oil from the ground contains hydrocarbon chains of various shapes and sizes that can be separated by boiling in a distillation column and collecting fractions at different temperatures in the column. Lighter, or more volatile, fractions tend to comprise hydrocarbon molecules with shorter chains. The fraction called kerosene consists mainly of hydrocarbons with ~8–16 carbons and is the main component of jet fuel. It is intermediate between naphtha (~4–12 carbons), which is primarily used in gasoline, and diesel (~12–20 carbons).
Hydrocarbons can also be classified by molecule shape, which has an impact on a fuel’s properties. Jet fuel contains a mixture of paraffins (straight-chained hydrocarbons) and isoparaffins (branched hydrocarbons), as well as aromatics (e.g. benzene, toluene, xylene) and some naphthenes (a.k.a. cycloparaffins). These compounds are all found in crude oil, but fuel composition can also be modified through operations at an oil refinery. Hydrocracking enables the breakdown of heavier paraffins using a small amount of hydrogen, and an isomerization unit can convert paraffins into branched isoparaffins. Aromatics can also be synthesized through catalytic reforming of hydrocarbons in the naphtha range.
ASTM International is the entity that sets standards and performance criteria for aviation fuel composition. One specification, for example, is that the melting/freezing point of jet fuel must be no higher than -40°C. Iso-paraffins in the blend help to meet this spec, as they have much lower freezing points than paraffins of the same weight. An aromatics content between 8–25% is also necessary. Because of the different solvent properties of aromatics versus paraffins, fuel compositions outside this range might cause shrinking or swelling of O-rings fitted in the fuel lines, resulting in fuel leakage, which would obviously be a big problem. Making a fully drop-in SAF is therefore more complicated than producing one type of hydrocarbon.
Many routes to synthetic paraffinic kerosene (SPK)
So what does it look like to make aviation-compliant fuel using biomass or electricity? There are many companies in the space and a variety of approaches, and recent articles review the landscape in detail. But briefly, most approaches can be grouped into one of 3 categories:
- HEFA (Hydroprocessed esters and fatty acids): Conversion of vegetable or animal fats/oils, such as used cooking oil.
- AtJ (Alcohol-to-jet): Conversion of alcohols, such as ethanol or methanol. The alcohols may be from the fermentation of corn or other sugars, but could also be synthesized from CO2 and electricity using new electrolyzer technologies.
- FT (Fischer-Tropsch process): Conversion of syngas, a mixture containing both hydrogen and carbon monoxide in a ~2:1 ratio. The syngas could be produced from the gasification of biomass, or using green hydrogen and CO2.
Each of these approaches uses special catalysts and reaction conditions to effect the respective transformation, and the output of each is primarily paraffins. Refinery operations like hydrocracking, isomerization, and distillation can be used to formulate a product in the kerosene (~8–16 carbon chain) range, but still with little or no aromatics. The resulting “synthetic paraffinic kerosene,” or SPK, is approved for up to 50% blending with fossil jet fuel. Standard fossil jet fuel is ~17% aromatics, so even a 50% blend of SPK retains the minimum 8% needed.
Notably, the low aromatic content of SPK actually makes it cleaner burning than its fossil counterpart in ways that impact both air quality and climate change beyond CO2 emissions. Aromatics don’t always fully combust and may leave behind smog particulates. These can nucleate aviation contrails that function as a kind of thermal insulation in their own right. By some estimates, these aviation contrails may contribute to atmospheric warming to the same extent as the CO2 emissions themselves. So more substitution of fossil jet fuel with SPK brings immediate climate and environmental benefits.
Towards fully drop-in SAF
In the long term, achieving 100% SAF would require new methods of producing or upgrading SAF to contain aromatics, or else upgrading all fuel line O-rings to a new material so that aromatics are not required, a tall order. One company, Virent, already has a process to synthesize aromatics from plant-based sugars, approved as synthetic aromatic kerosene (SAK). Virent supplied SAK for Virgin Atlantic’s recent 100% SAF-based flight from London to New York, conferring 12% aromatics alongside the 88% HEFA supplied by AirBP. The groundbreaking flight offered a demonstration of the aspirational future of air travel.
In Part 3, we’ll explore the costs of producing SAF and how (or whether) SAF could achieve cost-parity with fossil jet fuel.
Prime Movers Lab invests in breakthrough scientific startups founded by Prime Movers, the inventors who transform billions of lives. We invest in seed-stage companies reinventing energy, transportation, infrastructure, manufacturing, human augmentation, and agriculture.
