A team at the University of Pennsylvania is proposing the use of a liquid-organic hydrogen carrier (LOHC)—specifically, 1,2,3,4-tetrahydroquinoline (THQ)—for use as an endothermic fuel for thermal protection of hypersonic aircraft engines. A paper on their work is published in the journal Fuel.
The conversion and endothermic heats of reaction were measured over a Pd-Al2O3 catalyst at 60 bar and temperatures between 500 and 600 °C. Heats of reaction as large as 950 kJ kg−1 were obtained, with conversions greater than 80% at 600 °C. The catalyst was stable over a period of at least several hours and no coking was observed.
For comparison, the reaction of methylcyclohexane formed tars that quickly blocked the reactor, even at 500 °C on this catalyst. These results suggest that LOHC could serve as alternative fuels for endothermic cooling.—Cao et al.
The notion of using endothermic fuels as a thermal management technique for hypersonic aircraft reaches back to the 1960s. The basic notion is to transfer heat to a cold fluid fuel as a heat sink; as the temperature of the fuel increases, it reaches the point at which endothermic reactions occur and the compound converts to simpler products that can be used as fuel for propulsion.
However, despite the interest in the possibility, coke formation and it accumulation in the fuel lines are two of the critical constraints for regenerative cooling using hydrocarbon fuels, according to a critical review by Dinda et al. earlier this year. The new work by Cao et al. suggests an alternative approach to avoid the known problem set.
Tianyu Cao, WooJae Lee, Renjing Huang, Raymond J. Gorte, John M. Vohs (2022) “Liquid-Organic hydrogen carriers as endothermic fuels,” Fuel, Volume 313, 123063 doi: 10.1016/j.fuel.2021.123063
Dinda, S., Vuchuru, K., Konda, S., & Uttaravalli, A. N. (2021). “Heat Management in Supersonic/Hypersonic Vehicles Using Endothermic Fuel: Perspective and Challenges.” ACS omega, 6 (40), 26741–26755 doi: 10.1021/acsomega.1c04218