Identification of Polycyclic Aromatic Hydrocarbons in the Supercritical Pyrolysis Products of Synthetic Jet Fuel S-8 and Methylcyclohexane

Abstract

Inside the pre-combustor fuel lines of future high-speed aircraft, at supercritical conditions, hydrocarbon fuels react to form polycyclic aromatic hydrocarbons (PAH) and eventually solid deposits. These deposits can block fuel lines and lead to undesirable effects for the aircraft operation. To elucidate the pathways that lead to the formation of PAH (and ultimately, solids formation), the identification of the PAH products generated by the reactions is critical. In this context, two fuels have been analyzed: synthetic jet fuel S-8 and methylcyclohexane. Synthetic jet fuel S-8 is important due to its non-petroleum origin. Methylcyclohexane is important due to its endothermic behavior. Therefore, the goal of this study is the identification of the PAH products from the supercritical pyrolysis of synthetic jet fuel S-8 and from the pyrolysis of methylcyclohexane. With the purpose of identifying the unknown PAH, each pyrolysis product mixture has been analyzed by gas chromatography with mass spectrometric detection, and by high-pressure liquid chromatography with ultraviolet-visible diode-array and mass spectrometric detection (HPLC/UV/MS). The HPLC/UV information has allowed the identification of sixty-one and thirty-nine different products from the supercritical pyrolysis of synthetic jet fuel S-8 and from the supercritical pyrolysis of methylcyclohexane, respectively, at the most drastic conditions. Further interpretation of the UV spectra by means of the alkyl substitution effect, the UV solvent based adjustment, and the Annellation Theory, and confirmation of product identities using the MS spectra have provided irrefutable clues for the identification of twenty-four PAH from the synthetic jet fuel pyrolysis and sixteen PAH from the methylcyclohexane pyrolysis, respectively. Out of the eighty-five total PAH products reported from the supercritical pyrolysis of synthetic jet fuel S-8, twenty-nine unsubstituted PAH with six or more rings have never before been identified in the pyrolysis of a long-chain alkane fuel. In addition, out of the fifty-five total PAH products reported from the supercritical pyrolysis of methylcyclohexane, fifteen unsubstituted PAH with six or more rings have never before been identified as products of methylcyclohexane. Finally, it has been observed that the increases of temperature, pressure, and residence time, favor the formation of heavier PAH„Ÿthe precursors to the solid deposits.