Thermal Stability of Jet Fuels: The Kinetics of Forming Deposit Precursors, 03-9020

Principal Investigators
David W. Naegeli
Clifford A. Moses

Inclusive Dates: 04/01/97 - 04/01/99

Background - The thermal stability of fuels is of concern in the maintenance of all types of combustion engines. Thermal stability is measured in terms of the fuel’s tendency to form deposits on heat exchangers and nozzles of jet engines as well as on intake valves, fuel injectors, and combustion chamber surfaces in gasoline and diesel engines. Several fuel additives have been marketed over the years to alleviate fuel-deposit problems in engines. Despite extensive research geared to developing better fuel additives, the mechanism of deposit formation remains an enigma. An elucidation of the problem could provide a more direct and confident approach to additive development and fuel formulation.

It is well known that thermal deposits originate from the autoxidation of fuels. However, beyond that generalization, the mechanism is speculative. Not long ago, it was assumed that fuels would form deposits rapidly if they autoxidized fast. Since then, it has been realized that fuels that autoxidize rapidly have a much lower tendency to form deposits than fuels that show great resistance to autoxidation. Literature surveys show some variation in the rates at which hydrocarbons autoxidize, for example, the rate of hydro peroxide formation in hydrocarbons varies as benzylic > allylic > tertiary > secondary > primary. However, there is no evidence indicating that some hydrocarbons are vastly more resistant to autoxidation than others. For example, the difference in the rates of oxidation of alkyl benzenes and normal paraffins is not that great. Therefore, it has been concluded that naturally occurring oxidation inhibitors are responsible for the exceptionally slow oxidation of some fossil fuels.

Approach - The present investigation shows that two oxidation-inhibiting mechanisms are in fossil fuels. The first is well known, and is caused by naturally occurring antioxidants that forestall the oxidation process until the antioxidant is depleted. The second type of inhibition is caused by antioxidants created by the oxidation process. These antioxidants reduce the rate of oxidation. This second form of inhibition seems to be observed only in fuels that contain aromatics and sulfur. The purpose of this investigation was to understand better the mechanism of formation of the peroxides, gums, and deposits that form when jet fuels are thermally stressed. The goal was to create a clearer understanding of the mechanism, which would then open up new avenues for the development of fuel additives that would inhibit or prevent the formation of deposits. Test fuels consisting of Jet A, dodecane, and a blend of dodecane and 15 percent cumene were doped with sulfur compounds and stressed in an isothermal reactor at 120 to 180°C. Fuel samples were sealed in glass ampoules with an oxygen-filled headspace. Deposit precursor was determined by laser-induced fluorescence, hydroperoxides were measured spectrophotometrically, and oxygen consumption was determined by measuring the pressure change in the ampoule.

Accomplishments - This study shows that specific sulfur compounds, namely thiols and disulfides, play an important role in the formation of jet fuel deposits. The results indicate that thiols and disulfides oxidize faster than other fuel components, including other sulfur compounds. It is proposed that thiols and disulfides oxidize to sulfones, which liberate sulfur dioxide, SO₂, as they decompose. SO₂ reacts with fuel oxidation products to make gums and deposits. SO₂ also oxidizes to SO₃ and combines with water to form sulfuric acid. The acid acts as a catalyst that causes aromatic hydro peroxides to decompose into phenols. The phenols act as antioxidants, which is the reason fossil fuels are so much more resistant to oxidation than pure hydrocarbons.

A possible explanation of the anomalous relationship between deposit formation and the oxidation rate is that phenols act not only as oxidation inhibitors, but they are also involved in the deposit-forming process. Other studies have shown that phenols increase the formation of gum in thermally stressed fuels.

It was found that adding an organic base such as tridodecyl amine prevented the formation of phenols. The base neutralizes the acid produced from sulfur in the fuel, so the aromatic hydro peroxides decompose into oxygen-containing compounds other than phenols. If phenols are essential to the formation of high-temperature deposits, the addition of organic bases may improve the thermal stability of the fuel.

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