Energy & Fuels 2008, 22, 433–439
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Effect of the Reaction Temperature and Fuel Treatment on the Deposit Formation of Jet Fuels Ömer Gül,*,† Leslie R. Rudnick,‡ and Harold H. Schobert† The PennsylVania State UniVersity, The Energy Institute, UniVersity Park, PennsylVania 16802, and Ultrachem, Inc., 900 Centrepoint BouleVard, New Castle, Delaware 19720 ReceiVed July 16, 2007. ReVised Manuscript ReceiVed September 20, 2007
In the present paper, the thermal stability behavior and amount of carbon deposit from thermally stressed jet fuel candidates were examined as functions of fuel treatment and reaction temperatures. The jet fuel candidates were refined chemical oil, a byproduct of coke manufacture saturated by high-pressure hydrogenation (HPSat-RCO); light cycle oil, a petroleum product saturated by high-pressure hydrogenation (HP-Sat-LCO); 1:1 blends of these two materials; two (1:1 vol) RCO/LCO blends saturated by low-pressure hydrogenation; and JP-8. They were tested in a flow reactor on an Inconel 718 surface to assess their thermal stability. All fuels showed marked improvements in stability when the fuels were alumina-treated. Alumina treatment decreased the amount of carbon deposit even if carbon deposition was already very low for the untreated fuel. Chemical compositions of jet fuel samples and proton distributions were determined using gas chromatography–mass spectrometry (GC/MS) and nuclear magnetic resonance (NMR). These results showed that alumina treatment did not change chemical composition and proton distribution significantly. Nitrogen-containing compounds were totally removed by alumina treatment, whereas sulfur- and oxygen-containing compounds were only partially removed. Alumina treatment essentially removed peroxides. When peroxides, nitrogen-bearing compounds, and some sulfur-containing compounds were removed from the fuels, the carbon deposition was retarded.
1. Introduction In general, the thermal stability of fuels is of concern for the maintenance of combustion engines. Thermal stability is measured in terms of the tendency of fuel to form deposits on fuel lines, valves, injectors, and combustion-chamber surfaces in engines. Fuel degradation can cause engine failure and engine malfunction and affects engine performance. Solid deposits from fuel degradation can attach to the surface of the flow lines or plug filters and create problems with the fuel system operations. Future high-speed aircraft will be challenged to meet their onboard cooling requirements, because of higher heating loads. Given weight limitations that preclude adding separate cooling systems, the fuel will be required to serve as both the propellant and coolant and, thus, the fuel will need to be thermally stable. We use the term thermal stability to refer to resistance to decomposition at elevated (>400 °C) temperatures to form deleterious solid deposits. Poor thermal or oxidative stability results in the formation of higher molecular-weight compounds with limited fuel solubility. Instability of a fuel also involves the formation of peroxides and hydroperoxides, soluble gums, and most critically, insoluble material that may either coat surfaces or form particulates. The development of insolubles depends upon both trace and bulk fuel properties. Trace quantities of peroxides and/or hydroperoxides are capable of initiating deleterious reactions, which can result in high-molecular deposits. Reactive hydrocarbon components of the jet fuel can also contribute to * To whom correspondence should be addressed. E-mail: omergul@ psu.edu and/or
[email protected]. † The Energy Institute. ‡ Ultrachem, Inc.
undesired deposit formation. The composition of the fuel, which determines its solvency, will determine the solubility of these deleterious components.1 Carbonaceous deposit formation depends upon a combination of different conditions, such as temperature, pressure, the reactivity of starting fuel, and the nature of the substrate surface. Temperature is one of the most important parameters that affect the rate and reaction mechanisms of fuel degradation. Three regimes for deposit formation have been defined2,3 and are widely accepted. Hazlett et al.2,3 worked on the stressing of n-dodecane using a flow apparatus, and they attributed fuel degradation and solid formation to different reactions in three temperature regimes: autoxidation at
