Kinetic coupling effects on the extinction limits of diffusion flames of hydrocarbons blended with ammonia

Chemical kinetic coupling effects between NH₃ and liquid hydrocarbon fuels on extinction limits of diffusion flames are experimentally and numerically investigated. Three n-alkanes (n-heptane, n-decane, and n-dodecane), isooctane as a representative fully branched isoalkane, and toluene as a representative mono-aromatic are each tested, along with their blends with NH₃. The measured extinction strain rates are analyzed, employing transport-weighted enthalpy and radical index to demonstrate relative changes of chemical kinetic potentials for each fuel evaluated. The results show a significantly lower chemical kinetic potential for NH₃, compared to hydrocarbon fuels. Comparison of extinction limits as a function of transport-weighted enthalpy multiplied by radical index for fuel/NH₃ mixtures shows potential promotive effects for n-alkanes and no significant coupling for isooctane and toluene. Planar laser-induced fluorescence is applied to quantify OH concentrations for n-heptane, isooctane, and their mixtures with NH₃, and data are modeled to test the fidelity of chemical kinetic model predictions. It is found that including interaction reactions of n-alkyl and isoalkyl fragments with NH₂, and the reactions involving methylamine and cyanide are critical to predicting OH production rates, as well as extinction limits for hydrocarbon/NH₃ blends. Promotive effects of NH₃ blending on n-alkanes diffusion flame extinction limits are primarily from higher flame temperatures due to the reduced fraction of CO₂ found in the flame products. In the case of isooctane blended with NH₃, the formation of two main isoalkyl fragments, CH₃ and C₃H₆, are found to interact with NH₂, resulting in the suppression of the reactive radical pool population and kinetic inhibition. A significantly weaker reactive radical pool in toluene oxidation leads to more significant inhibitive kinetic coupling from NH₃-related reactions.