Advancements in jet engine design have led to improvements in efficiency and power output. As a result the operating temperature of the engine has steadily increased over the years. To counter this increased thermal load on the engine, aviation fuel is used as a heat sink, which leads to it becoming thermally stressed. The increase in temperatures in the fuel causes it to undergo autoxidation. In these conditions, insoluble material, such as gums and sediments, are formed in the fuel systems. eventually this build up of insoluble material leads to in-service issues and an increase in operating costs for engine manufacturers and airlines. Not all fuels behave the same in thermally stressed conditions, with the fuels chemical composition affecting this property, referred to as thermal stability. The importance of environmental concerns and security of supply issues have led to numerous alternative fuels to be exploited in recent years. However, the effect that their chemical composition has on their autoxidation behaviour is not well understood, as these novel fuels can have very different chemistry to historical stocks.
These novel fuels are chemically purer than conventionally derived fuels with lower concentrations of polar and sulfur species. As such, it is important to understand the role that an aviation fuels hydrocarbon composition has on its thermal stability, as this will become the dominant factor effecting their thermal stability. The work in this thesis focuses on understanding the mechanisms that govern the radical autoxidation of hydrocarbons and how their chemical and electronic structure affects these processes. Aviation fuels with well defined chemical composition, as well as individual hydrocarbon components, such as n-alkanes, cycloalkanes and aromatics are investigated in this study.
This thesis uses computational and experimental techniques to investigate the role that hydrocarbon composition has on the thermal stability of an aviation fuel. The PetroOxy oxidation stability tester is demonstrated as an effective research tool for investigating the oxidation mechanism, with good repeatability of the data collected, and can study three distinct regimes of hydrocarbon oxidation. The induction period, rapid radical oxidation and slower oxidative coupling regimes are studied for fuels and mono-component hydrocarbons, with the results linked to findings in the literature. From this study, it is clear that aromatic hydrocarbon can act as antioxidants in the fuel by increasing the induction period, but eventually led to bigger issues with deposition in the fuels tested. As such, cycloalkanes offer an alternative, as they can alter the density of the fuel without sacrificing the thermal stability properties.
Density functional theory (DFT) has been used to construct pseudo-detailed mechanisms for the initial oxidation of three distinct classes of hydrocarbons, n-alkanes, cycloalkanes and mono-aromatics. These mechanism are able to demonstrate the fundamental differences between the three classes of hydrocarbons, and how these structural and electronic differences affect the kinetics of their autoxidation. However, this work also highlighted the need to correctly describe these radical reaction mechanism, as single reference method like DFT fail to model the bi-radical nature of many of these reac- tions. This phenomena is investigated further, by applying multi-reference and unrestricted electronic structure methods to the reactions of peroxides and peroxyl radicals. These reactions are shown to be vital to understand autoxidation, and the inclusion of computational calculated rate parameter for these reactions into pseudo-detailed mechanisms, allows them to be able to predict the oxidation rate of an industrial solvent in an isothermal tube reactor.
A key finding of this thesis is the importance of using multi-reference computational chemistry methods to investigate the radical reactions that occur in the autoxidaiton process, and using these calculations to construct a pseudo- detailed kinetic mechanism, in order to correctly model thermal stability and better understand the fundamental chemistry at play. This also offers an interesting challenge, as this work is at the cutting edge of applications for theoretical chemistry, and can provide a link from first principles to real world applications.
