This effort describes laboratory evaluations of six alternative (nonpetroleum) jet fuel candidates derived from coal, natural gas, camelina, and animal fat. Three of the fuels were produced via Fischer-Tropsch (FT) synthesis, while the other three were produced via extensive hydroprocessing. The thermal stability, elastomer swell capability, and combustion emissions of the alternative jet fuels were assessed. In addition, detailed chemical analysis was performed to provide insight into their performance and to infer potential behavior of these fuels if implemented. The fuels were supplied by Sasol, Shell, Rentech, UOP, and Syntroleum Corporation. Chemical analyses show that the alternative fuels were comprised of mostly paraffinic compounds at varying relative concentrations, contained negligible heteroatom species, and were mostly aromatic-free. The six paraffinic fuels demonstrated superior thermal oxidative stability compared to JP-8, and therefore, have increased resistance to carbon formation when heated and can be exposed to higher temperatures when used to cool aircraft systems. Material compatibility tests show that the alternative fuels possess significant seal swelling capability in conditioned nitrile O-rings; however, elastomer swelling was significantly lower than for JP-8, which may likely result in fuel leaks in aircraft systems. Engine tests with the alternative fuels demonstrated no anomalies in engine operation, production of significantly lower nonvolatile particulate matter (soot), and moderately lower unburned hydrocarbons and carbon monoxide emissions compared to baseline JP-8 fuel. Also, no penalty (i.e., increase) in fuel flow requirement for equal engine power output was observed. In general, this study demonstrates that paraffinic fuels derived from different feedstocks and produced via FT synthesis or hydroprocessing can provide fuels with very similar properties to conventional fuels consisting of excellent physical, chemical, and combustion characteristics for use in turbine engines. These types of fuels may be considered as viable drop-in replacement jet fuels if deficiencies such as seal swell, lubricity, and low density can be properly addressed.

The use of synthetic fuels produced via the Fischer−Tropsch (FT) process is being considered for U.S. military applications as an approach to provide secure and stable fuel resources. Many potential operational benefits have been observed while employing this type of paraffinic fuel for aviation applications. However, operational limitations with respect to seal-swell and lubricity may need to be improved for ultimate implementation. Studies were performed to investigate the feasibility of adding aromatic solvents as an option to achieve “fit-for-purpose” (FFP) requirements. Aromatics are known to improve seal-swell characteristics, but also increase engine soot emissions. Three aromatic solvents, which encompass the molecular weight range typically found in military jet fuel (JP-8), were added both individually and as a blend to an FT fuel at varying concentrations. The seal-swell capability of the solvents was inferred using nitrile, fluorosilicone, and fluorocarbon O-rings while the solvent effect on combustion emissions was evaluated using a T63 engine. The studies showed that particulate matter (PM) emissions increased with increasing aromatic molecular weight and concentration, which was attributed to an increase in soot precursors. The seal-swell of nitrile rubber was most strongly affected by the addition of aromatics in the form of alkyl-naphthalenes as compared to alkyl-benzenes. This result was attributed to the decreasing molar volume and increasing polarity and hydrogen-bonding potential for the larger aromatics, primarily naphthalene and corresponding derivatives. Results showed that the desired swell characteristics and relatively low PM emissions can be achieved with the solvents evaluated in this study. The overall relation between PM emissions production and volume swell showed that all solvents considered had comparable dependence, which suggests that it may be possible to predict these interdependent variables. Overall, this study provided an initial assessment and basis for subsequent evaluations of potential feedstocks for blending in FT fuels to achieve FFP requirements. Future studies will evaluate additional elastomer materials and individual aromatic species while characterizing the emissions production on varying combustion platforms.

To examine the contributions of different types of aromatics, the volume swell of nitrile rubber O-rings was determined in aromatic-free synthetic JP-5 fuel and in synthetic JP-5 fuel blended with selected aromatics. Additionally, partition coefficients between the O-ring and fuel phases were measured for the fuel and aromatic species. Volume swell was measured using an in situ optical dilatometry technique that provided temporal data, while partition coefficients were measured using direct thermal desorption GC-MS analysis of swollen O-ring samples. For the hydrocarbons studied, the data indicate a correlation between partition coefficient and volume swell. The propensity to swell nitrile rubber was found to increase with the polarity and hydrogenbonding character of the aromatics, suggesting that swelling of nitrile rubber requires disrupting the attractive forces between cyano groups on adjacent polymer chains and replacing them with cyano group-aromatic interactions. Volume swell was also found to decrease with increasing molecular weight.

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