A kinetic study on the blending behavior of sustainable and conventional aviation fuels: Soot formation processes

Together with electrification, the introduction of short-term carbon cycle alternatives to fossil fuels represents one of the most promising strategies towards a significative reduction in GHG emissions. For sectors such as air transport this option is particularly appealing, as aviation strongly depends on the availability of high energy-density carriers. These needs justify the current interest in sustainable aviation fuels (SAF). In particular, paraffinic kerosenes from Fischer-Tropsch (FT) processes, hydro-processed esters and fatty acids (HEFA) and alcohol-to-jet (ATJ) are within the SAF that are attracting major interests. Their introduction as drop-in fuels, pure or in blends with conventional jet-fuels from fossils, may also have a positive impact on non-CO2 emissions such as soot, whose formation is known to strongly correlate with fuel composition. This study addresses a set of three standard (Jet-A, JP5 and JP8) and three sustainable jet fuels (HEFA-Camelina, FT-SPK, ATJ) unraveling the mechanisms behind their soot emission tendency and that of their blends. Suitable surrogate fuels are formulated from a palette of 11 components using an in-house code targeting physical and chemical properties of the real fuels. The sooting behavior of selected jet-fuels and blends (HEFA/Jet-A, FT-SPK/Jet-A, ATJ/Jet-A) is then calculated and compared with soot volume fraction data in laminar counterflow diffusion flames. A targeted kinetic analysis highlights kinetic pathways governing soot inception in standard jet-fuels in comparison to their sustainable alternatives supporting explanation of the observed trends. Morphological aspects such as the average particle diameters and particles size distribution from HEFA, Jet-A and HEFA/Jet-A flames are also discussed, identifying characteristic behaviors informing effective blending strategies and motivating targeted experimental studies.