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.

High-performance liquid chromatography (HPLC) based techniques are used to investigate the role of polar species in deposit formation during jet fuel autoxidation and to explore the relative contributions of the various species classes which compose the polar fraction. More specifically, HPLC with UV-vis absorption detection was employed to quantify the polar species in jet fuel as a class, and a technique which combines solid-phase extraction (SPE) with HPLC and gas chromatography with mass spectrometric detection (GC-MS) was used to identify the species classes which compose the polar fraction in typical jet fuels. The analytical results were combined with surface deposit data obtained in a quartz crystal microbalance (QCM) system for a series of twenty jet fuels. The results indicate a relationship between the total amount of polar species measured and the amount of surface deposits produced. Results also suggest that phenols, various other oxygenated polar species, indoles, and carbazoles have a significant positive correlation with jet fuel surface deposit formation, while pyridines, anilines, and quinolines do not demonstrate a strong correlation with the tendency of a fuel to form surface deposits.

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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.