Introduction to Alcohol-to-Jet (AtJ) SAF
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As the aviation industry advances towards sustainable operations, various pathways have emerged for producing sustainable aviation fuel (SAF). Among these, the Alcohol-to-Jet (AtJ) process represents one of the most promising approaches for converting renewable sources into jet fuel that can be used in today’s aircraft.
Understanding Alcohol-to-Jet fuel
The AtJ process transforms alcohols, primarily ethanol or isobutanol, into sustainable jet fuel through a series of chemical transformations. It is one of seven currently approved “drop-in” SAF production pathways certified by ASTM International, the organisation responsible for certifying aviation fuels.
The result is a synthetic fuel called synthetic paraffinic kerosene (SPK). Because SPKs lack certain components found in conventional jet fuel, such as aromatics, they must be blended before use. AtJ-SPK made from ethanol can be blended at ratios of up to 50% — one of the hig
Starting materials
While specific processes may vary by producer, the AtJ pathway typically involves these steps:
- Dehydration: The alcohol is heated in the presence of a catalyst to remove water molecules, converting it to an alkene (ethylene from ethanol or isobutylene from isobutanol).
- Oligomerization: These alkenes are then combined to form longer hydrocarbon chains suitable for jet fuel.
- Hydrogenation: Hydrogen is added to saturate the hydrocarbons, creating a stable mixture of paraffins.
- Fractionation and separation: The final mixture is separated to isolate the components meeting jet fuel specifications.
The result is a synthetic kerosene that meets all the stringent requirements for use in commercial aircraft.
End product characteristics
The AtJ-SPK product is nearly identical to conventional jet fuel in its performance characteristics. It meets all the stringent specifications for commercial aviation fuel, including energy density, freezing point, flash point, and combustion properties.
The resulting fuel is a “drop-in” replacement, meaning it can be used without modification to aircraft or fuelling infrastructure. However, current certification requires it to be blended with conventional jet fuel, with maximum blend ratios of 50% for ethanol-derived SAF.
Importantly, AtJ fuel contains no aromatic compounds, which can result in cleaner combustion with reduced particulate emissions compared to conventional jet fuel. This characteristic offers environmental benefits beyond just carbon reduction.
Commercial examples
Several companies have pioneered AtJ technology for aviation applications:
- Gevo has been a leader in developing isobutanol-based AtJ fuels, partnering with various airlines for test flights and commercial agreements. Their process utilises agricultural feedstocks and has demonstrated significant carbon reductions compared to fossil fuels.
- LanzaJet has focused on ethanol-to-jet technology, working to scale up production with backing from major aviation companies. Their “Freedom Pines” facility aims to produce significant quantities of AtJ fuel.
Advantages of the AtJ pathway
The AtJ pathway offers several distinct advantages:
- Feedstock flexibility: Can utilise agricultural residues, sugar crops, starch crops, and lignocellulosic biomass, providing numerous options for sourcing raw materials.
- Integration with existing industries: Leverages established ethanol production infrastructure, particularly in regions like the United States, Brazil, and Europe.
- Scalability: With high blend limits of up to 50% and established production techniques, AtJ represents a pathway that could be scaled relatively quickly.
- Carbon reduction: Offers lifecycle emissions reductions of approximately 70-80% compared to fossil jet fuel, particularly when using waste-derived alcohols.
- Rural development: Creates economic opportunities in agricultural regions where feedstock can be grown or collected.
As the aviation industry continues its journey toward net-zero carbon emissions, the AtJ pathway represents a technically viable and increasingly economical route to producing the sustainable fuel needed to power the next generation of air travel.