Investigations into the Formation of Carbonaceous Deposits via Autoxidation of Jet Fuels

Jet fuel is used as a coolant for lubricants and oils, and receives heat from the burner feed arm before combustion. The thermal stress received by the fuel, in combination with dissolved oxygen, results in reactions yielding carbonaceous deposits on fuel-wetted surfaces. These deposits can cause efficiency and safety issues as they grow on critical components. The initial stage of the deposition process involves a well understood autoxidation mechanism, leading to a series of well characterized oxidized molecules. However, it is less clear what chemical mechanisms lead to the agglomeration of these oxidized molecules producing deposits. The aim of this thesis is to gain a more fundamental understanding of the agglomeration and deposition process. Density functional theory (DFT) and experimental techniques including gas/liquid chromatography mass spectroscopy (GC/LC-MS) were used to investigate these processes. Additionally, kinetic analysis using pseudo-detailed mechanisms were performed throughout to explore competing pathways. The Soluble Macromolecular Oxidatively Reactive Species (SMORS) mechanism has previously been proposed as a universal mechanism for deposition, but has received little mechanistic scrutiny. Our DFT calculations performed here showed that the originally proposed SMORS mechanism involving electrophilic aromatic substitution (EAS) was kinetically and thermodynamically prohibited. Instead, it was found a homolytic aromatic substitution (HAS) mechanism could allow C-C bonds to form to lead to agglomerated species. Additionally, the SMORS mechanism neglects to elucidate the role of indigenous fuel sulfur compounds, which have a major influence on deposit formation. Using surrogate fuels and deposit characterization with GC/LC-MS, it was shown that the addition of sulfur containing compounds to 5- membered fuel nitrogen heterocycles led to oligomers containing nitrogen-nitrogen compounds. This implied sulfur compounds were able to catalyze the coupling. Indigenous fuel sulfurs were also was shown to play a crucial role in the early stage of deposition. DFT calculations showed that compared to other fuel oxygenated compounds, sulfur acids formed from autoxidation of indigenous sulfurs have the highest adsorption energy on stainless steel surface oxides. Exploring the heterogeneity of stainless steel surface oxides, sulfur acids had a higher adsorption energy on Cr2O3 and carboxylic acids had a higher adsorption energy on Fe2O3. Other oxygenated and indigenous fuel components were found to physisorb at lower adsorbtion energies. Finally, using the mechanistic findings from this thesis, 6 pseudo-detailed kinetic mechanisms were built using DFT calculate barriers, each representing a fuel containing a common antioxidant/heteroatom. The calculated mass of deposit dimer from our mechanisms correlated well with the measured mass of deposit formed from 6 equivalent surrogate fuels. The results here show how DFT can be used to build deposition mechanisms from first principles. The chemical mechanistic findings throughout this whole thesis will help guide future researchers to build new predictive models for deposition from first-principles.