The role of hydrocarbon composition on the thermal stability of aviation fuels

Advances in jet engine efficiencies and environmental concerns have lead to increased demands being placed on aviation fuel. This has driven interest in the use of alternative fuels, such as those derived from synthetic processes, in the aviation sector. This development however requires a greater understanding of the dependence of a fuels chemical composition on its thermal stability. Previous work on the oxidation mechanisms of hydrocarbons has been restricted to treating them as a general class of species. With the increased control of fuel composition that alternative fuels can achieve, it is necessary to research the role that chemical composition has on the oxidation process, allowing for the development of fuels with increased thermal stability, for use in more efficient engines. This work investigates the oxidation mechanisms for three classes of hydrocarbon commonly found in fuel with quantum chemistry techniques, using methods established for use in mechanistic organic chemistry studies. A straight chain alkane, dodecane, a cyclic alkane, decalin, and an aromatic hydrocarbon, toluene, were used to model the three classes of hydrocarbons. Our calculations indicate that aromatic and aliphatic hydrocarbons oxidise through different routes. Aromatic hydrocarbons act as antioxidants in the fuel, donating hydrogen to other hydrocarbons, thus stabilising radicals formed during the oxidation process. However this increases their susceptibility to oxidation, and as a consequence they undergo aromatic substitution reactions to form deposits, while aliphatic alkanes oxidise through different mechanisms. The theoretical modelling work has been supported by experiments carried out on small-scale thermal stability test devices.