Sustainable Aviation Fuels: Fossil Fuel Use
We continue our discussion to do with Sustainable Aviation Fuels (SAFs). Previous posts in this series are,
In the post Sustainable Aviation Fuel (SAF) Blends we noted that SAFs are currently a blend of biofuels and conventional Jet A. The biofuel component is generally considerably less than 50% of the total. Therefore, to claim that SAFs, at least as they are now being used, are ‘sustainable’ is somewhat disingenuous.
But the use of fossil fuels with respect to Sustainable Aviation Fuels goes beyond blending. The technologies used to manufacture the SAFs generally require large amounts of fossil fuels in their manufacture. For example, one of these technologies is the hydroprocessing of biomass. This process converts biomass feedstocks, such as vegetable oils, animal fats, and waste oils, into liquid hydrocarbons that can be blended with traditional jet fuel.
Hydroprocessing typically involves hydrogenation and deoxygenation steps to remove oxygen and improve the fuel properties.
Hydrogenation is a chemical reaction that involves the addition of hydrogen to unsaturated hydrocarbon molecules to saturate double or triple bonds. In the context of SAF production, hydrogenation can be used to convert unsaturated fatty acids present in vegetable oils or waste fats into saturated hydrocarbons, which improves the stability and oxidative resistance of the resulting aviation fuel.
I emphasized the phrase ‘addition of hydrogen’ in the above statement. Although very small amounts of ‘green’ hydrogen are made from electricity, the bulk of our current production comes the conversion of natural gas. This conversion process results in emissions of large quantities of CO2 (carbon dioxide).
The deoxygenation step is needed to remove oxygenated molecules that can make the SAF unstable. This process also generally requires the use of hydrogen. New ways of producing ‘green’ hydrogen at scale may be developed in the next few years, but that seems to be unlikely. Time is pressing.
The point of the above discussion is clear. If the airline industry, or any other industry for that matter, decides to decarbonize then the emissions that are created in the manufacture of new fuels must be included in the overall Net Zero evaluation. Based on that premise, the civil aviation has a long way to go before it can claim that its fuels are truly sustainable.
