As modern hypersonic aircraft designs evolve, increasing heat sink capabilities are required. One potential approach is the use of endothermic fuels, which supplement fuel heat sink via sensible heating with endothermic chemical reactions. Endothermic reactions can proceed by thermal or catalytic decomposition of the fuel. Regardless of the specific methodology, thermal decomposition (pyrolysis) will occur at the expected reaction conditions. Improved understanding of the effect of chemical composition on supercritical pyrolytic reactivity and deposition propensity is needed to provide a basis of understanding for advanced fuel and system development. In this effort, a small-scale flow reactor system was developed to study the reactivity and deposition propensity of various hydrocarbon fuels and solvents of differing chemical composition. Experimental and computational studies were performed to investigate the relationship between fuel chemical composition and reactivity under supercritical conditions.
Nine fuels and solvents of varying chemical composition were studied to evaluate pyrolytic reactivity and decomposition pathways. Chemical classes studied included normal paraffins, iso paraffins, and cycloparaffins in various combinations. Experimental studies were performed at reaction temperatures of 500-650°C and inlet flow rates of 0.50-3.00 mL/min, with a nominal system pressure of 500 psig. Corresponding equivalent residence times at these reaction conditions ranged from 0.2-11.4 sec. A fuel conversion metric was defined to allow comparison of the reactivity of multi-component fuels at equivalent inlet reaction conditions. Normal and iso-paraffinic solvents, as well as a blend of the two, had similar overall reactivities at equivalent inlet reactor conditions. Normal paraffins formed primarily lower-molecular weight normal paraffins and a-olefins, while iso-paraffins formed primarily lower-molecular weight iso-paraffins and branched olefins. It was observed that normal paraffins decomposed at a higher rate than iso paraffins in a blend of the two classes, indicating that iso-paraffins accelerate the relative decomposition rate of the linear compounds. Fuels and solvents with cycloparaffins had lower extents of conversion at equivalent reaction conditions. Aromatic species were formed at higher yields from fuels and solvents with higher cycloparaffin concentrations. It was observed that multi-component solvents decomposed under similar reaction pathways to single-component compounds; likewise, fully formulated fuels were found to decompose under similar pathways to multi component solvents of similar chemical compositions.
The supercritical pyrolysis of the fuels and solvents was modeled as a first-order irreversible reaction to assist with reactivity comparisons among fuels and allow subsequent predictions. Supercritical pyrolysis was found to deviate from first-order reaction behavior at high conversion levels (> 30%), which limited the use of this simple kinetic analysis. The normal paraffinic solvent (Norpar-13) had estimated kinetic parameters of 60.7 kcal/mol activation energy and 1015.1 s-1 pre-exponential factor, while a specification JP-7 fuel sample was found to have kinetic parameters of 56.2 kcal/mol and 1013.5 s-1. Valid kinetic analysis is dependent upon accurate modeling of physical properties for estimation of residence times.
A preliminary investigation of the deposition propensity of seven fuels and solvents was also performed in this effort. Significant differences in deposition rates were observed between fuels and solvents with similar chemical compositions, indicating that deposition propensity may not be solely determined by bulk chemical composition. Further investigation is needed to better understand the mechanisms of deposition formation in supercritical pyrolysis.