AN AMERICAN CHEMICAL SOCIETY JOURNAL VOLUME 2, NUMBER 6
NOVEMBERIDECEMBER 1988
0 Copyright 1988 by the American Chemical Society
Articles Evaluation of the Gas-Phase Thermal Decomposition Behavior of Future Jet Fuels Philip H. Taylor* and Wayne A. Rubey University of Dayton Research Institute, Environmental Sciences Group, 300 College Park, Dayton, Ohio 45469-0001 Received December 17, 1987. Revised Manuscript Received March 16, 1988 Thermal stability is of particular concern for future hypersonicjet fuels due to prolonged exposure, the need for heat dissipation, and the possible formation of undesirable deposita at elevated temperatures. The gas-phase thermal decomposition of four future jet fuel candidates (methylcyclohexane, toluene, trans-decalin, and naphthalene) has been measured under precisely controlled conditions of reaction atmosphere, exposure temperature, and residence time. Differences in fuel thermal decomposition were analyzed with respect to molecular structure and thermochemical bond energy analysis. The thermal decomposition data for these compounds have also been used to evaluate their potential heat-sink capacities. In a separate but related study, the gas-phase thermal degradation behavior of trans-decalin was examined for a broad range of reaction conditions. The results indicate that removal of oxygen from the fuel-handling system and a reduction in high-temperature exposure time will greatly enhance the fuel’s gas-phase stability. The formation of degradation products was found to be most dependent on the composition of the flowing gaseous atmosphere. Under pyrolytic reaction conditions, the nature of trans-decalin reaction products was consistent with an initial degradation pathway involving C-H bond homolysis followed by H atom abstraction reactions.
Introduction High-performance aircraft of the future will require fuels and fuel-handling systems that can withstand significantly higher temperatures than those currently enc0untered.l Previous experimental studies of reaction systems have shown that thermal reactions of hydrocarbons have limited heat sinks; however, select cycloparaffinic hydrocarbons react catalytically by endothermic reaction and furnish much greater cooling capability.2 Consequently, the approach of using a cycloparaffin fuel as a heat-dissipating mechanism for hypersonic aircraft is receiving increased attention by aircraft designers. To ensure continued availability,feasibility studies have been conducted for producing fuels from precursors such as coal, oil shale, and tar sands, which are more commensurate with projected world-wide energy reserves. The heavier refinery products of these alternative energy *Towhom correspondence should be addressed.
sources are hydrotreated to cycloparaffins and saturated naphthenic fuels. The molecular structure of these classes of compounds is such that selective catalytic dehydrogenation processes to aromatic products produce heat absorption capabilities on the order of 1.10 k ~ a l / g . ~An important fundamental property of these endothermic fuels is their overall heat-sink potential, i.e., the sum of the contributions of the physical and chemical enthalpy. Measurement of the thermal decomposition of these cycloparaffin fuels and their aromatic dehydrogenation (1) Henderson, H. T.; Nixon, A. C. ‘Studies on the Measurement of Thermal Stability of Advanced Aircraft Fuels”. Presented at the 171st Meeting of the American Chemical Society, New York, 1976; Preprints-American Chemical Society, Division of Petroleum Chemistry; American Chemical Society, Division of Petroleum Chemistry: Washington, DC, 1976; Vol. 14, No. 3, pp B163-Bl61. (2) Faith, L. E.; Ackerman, G. €3.; Henderson, H. T.; Ritchie, A. W.; Ryland, L. B. “Hydrocarbon Fuels for Advanced Systems”; Technical Report AFAPL-TR-70-71,Part 111; Air Force Aero Propulsion Laboratory, July 1972. (3) Lander, H.; Nixon, A. C. J . Aircr. 1971, 8, 200-207.
0887-0624/88/2502~O723$01.50/00 1988 American Chemical Society
Taylor and Rubey
724 Energy & Fuels, Vol. 2, No. 6, 1988
products allows computation of this property. The formation of insoluble carbonaceous deposits can be especially deleterious to high-speed aircraft employing hydrocarbon fuels. Flow-reactor studies have shown that air-saturated jet fuels are prone to deposit formation at temperatures up to 250 "C at reduced pressures.G Similar studies on deoxygenated fuels at temperatures up to 649 "C and pressures up to 69 atm demonstrated that, with most fuels, removal of molecular oxygen markedly reduced the rate of d e m i t formation? It was noted, however, that the poorest quality fuel did not exhibit lower deposit formation rates with deoxygenation? Subsequently, the presence of certain sulfur-containingcompounds was found to be highly deleterious to the stability of deoxygenated fuels at elevated temperatures.'~~ The mechanism of deposit formation in air-saturated fuels has been discussed by a number of workers.B*+12 The consensus is that deposits at low temperatures (
