Supercritical Pyrolysis of n-Decane

Fuels used in future generations of high-speed jet aircraft will be required to take on increasing heat loads in their role as the primary coolant for the absorption of waste heat from the engine. As a consequence, the hydrocarbon fuels will be exposed to elevated temperatures and pressures prior to their use as fuels, conditions which are beyond the critical point of the fuel and which lead to the formation of carbonaceous solid deposits in the pre-combustion environment. Deposition of solids causes reduced engine performance and eventual failure, so understanding the mechanisms by which thermally stressed hydrocarbons become solids is extremely important to the continued development of high-performance aircraft. Of particular importance—and the focus of this work—are those reactions leading to the formation of polycyclic aromatic hydrocarbons (PAH), which are known to be the precursors to solid deposits. To this end, experiments were performed in which the reactant n-decane, chosen as a representative of the aliphatic components of real-world jet fuels, was pyrolyzed in a flow reactor under supercritical conditions, with temperatures ranging from 530 to 570 °C and pressures ranging from 40 to 100 atm. Products of the pyrolysis experiments were analyzed by high-pressure liquid chromatography (HPLC) with diode-array ultraviolet-visible detection (UV) in series with mass spectrometry (MS). A two-dimensional HPLC technique was developed specifically for this work which allowed the identification of 281 PAH products, 254 of which had never before been reported as products of n-decane pyrolysis. Furthermore, 77 aromatic products consisting of one to nine rings have been quantified, and yields with respect to temperature and pressure are presented. In the supercritical n-decane pyrolysis environment, single-ring aromatic products are formed first by cyclization and then dehydrogenation of the alkane reactant. Addition of alkyl and alkenyl radicals to these one-ring aromatic compounds produces alkylbenzenes, the substituent groups of which subsequently undergo cyclization and dehydrogenation to produce the two-ring PAH naphthalene. Combinations of aromatic molecules along with further additions of alkyl and alkenyl radicals lead to PAH of increasingly high molecular weight; PAH of sufficiently high molecular weight form a distinct phase, carbonaceous solid deposits