SAF

In 2021, aviation accounted for over 2% of global energy-related CO2 emissions, having grown faster in recent decades than road, rail, or shipping [IEA, 2022].

  

Sustainable aviation fuels (SAF) are drop-in aviation fuels made from renewable or waste resources such as biomass, solid, liquid, and gaseous wastes (MSW, CO, CO2 waste streams), and atmospheric CO2. They can reduce life cycle CO2 emissions by up to 80% compared to conventional fuel [IPB, 2021].

   

Sustainability criteria & pathways

 

SAF have to meet stringent sustainability standards with respect to land, water, and energy use, avoiding direct & indirect land use change impacts.  Also, not displace or compete with food crops.  There are many different pathways to produce SAF, transforming a wide range of biomass & waste feedstocks into jet fuel.  At the current stage, different tech platforms are certified to produce SAF for use in commercial aviation, such as FT, HEFA, and AtJ [SK, 2022].

  

Fischer-Tropsch (FT)

 

The FT process takes any carbon containing material and breaks it into individual building blocks in a gas form (syngas), and then combines these building blocks into SAF.  Two different FT processes have been certified by ASTM (American Society for Testing and Materials): straight paraffinic jet fuel (SPK) and synthesized aromatic kerosene (SAK).  Maximum blend ratio for both is 50% (ASTM for SPK: D7566-Annex 1; for SAK: D7566-Annex 4).

  

Hydrotreated esters & fatty acids (HEFA) - by far the most used

  

HEFA refines vegetable oils, waste oils, or fats into SAF thru hydrogenation.  In the first step of the process, the oxygen is removed by hydrodeoxygenation.  Next, the straight paraffinic molecules are cracked & isomerized to jet fuel chain length.  Maximum blend ratio is 50% (D7566-Annex 2).

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Alcohol to Jet (AtJ)

 

AtJ converts alcohols into SAF by removing the oxygen and linking the molecules together to get the desired carbon chain length, a process historically utilized by the petroleum refining & petrochemical industry.  Two feedstocks approved for use in the AtJ technology: ethanol & iso-butanol.  The maximum blend ratio is 50% (ASTM spec: D7566-4-Annex 5).

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Key takeaways

  

Chemical & physical characteristics of SAF are almost identical to those of conventional jet fuel and they can be safely mixed with the latter to varying degrees, use the same supply infrastructure, and do not require the adaptation of aircraft or engines [ISCC, 2023].

  

The hydrocarbon fuel is the only option for aviation for now, and HEFA-based biofuels are the only product that is commercially available today and powered over 95% of all SAF flights [SK, 2022].

  

Figure 1 shows the SPK (Synthetic Paraffinic Kerosene) types.