Effects of aromatic type and concentration in Fischer-Tropsch fuel on emissions production and material compatibility

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.