Soot Formation Benefits of Sustainable Aviation Fuels Characterized with a Yield- Based Approach to Sooting Tendency

Sustainable aviation fuels (SAF) can potentially reduce emissions of carbonaceous soot particles from air travel. Soot is the second largest source of global warming, and it contribute to ambient PM2.5 that causes millions of deaths worldwide each year. SAF are likely to produce less soot since they usually consist of alkanes and cycloalkanes, with only low levels of aromatics. The sooting propensities of jet fuels are characterized with the ASTM D1322 smoke point (SP) test; indeed, the ASTM 7566 jet fuel specifications require that SP exceed 25 mm. The SP test works well for repetitive testing of conventional jet fuels, but it suffers from significant drawbacks that affect the development of new fuels: it requires large sample volumes (10 mL), it has a limited dynamic range, and the results cannot be simulated from first principles.

This work proposes a method to derive SP from a novel yield-based soot propensity approach that overcomes the disadvantages of the smoke point methodology. In the yield-based approach, the maximum soot concentration is measured in a methane/air coflow nonpremixed flame after a small concentration -- typically 1000 ppm -- of the test fuel is added to the flame’s fuel. The results are then linearly rescaled into a Yield Sooting Index (YSI) that is analogous to an octane or cetane rating. This method requires less than 100 microliters of sample due to the small concentration added to the test flame. The dopant concentration can be varied to accommodate high or low sooting test fuels. The results can be readily simulated since there is just one well-defined flame, and the low dopant concentration allows the computations to be greatly simplified through perturbation methods. Simulations are critical for understanding the kinetic pathways that cause emissions reductions, and to validate the benefits for real engines without extensive testing.

Despite their differences, YSI and SP characterize the same fuel property and they can be directly related. In particular, we have found good correlation between YSI/MW and 1/SP for a set of about 100 pure hydrocarbons for which we have measured YSI and SP data is available in the literature. This correlation allows the soot reduction benefits of SAF to be quantified without the challenges of measuring smoke point.