Kinetic modeling study of autoignition, laminar burning velocity, and emission characteristics of n-heptane/ammonia mixtures

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Introduction The mixed combustion of ammonia (NH 3 ) and high activity fuel in internal combustion engines is an effective way to achieve global decarbonization. However, there remains a lack of detailed high-precision numerical models and basic theoretical research on this topic. Objective This study aims to explore the autoignition characteristics, premixed laminar flame characteristics, and NO X generation characteristics of n-heptane/ammonia blended fuel under different initial conditions (equivalence ratio, pressure, temperature) and energy substitution ratio of added ammonia (ESR). Methods Using n-heptane as a representative fuel, this study coupled the combustion mechanism of n-heptane/ammonia blended fuel and comprehensively analyzed the combustion characteristics of the blended fuel from the perspective of chemical reaction kinetics. Results As the ESR increases, the ignition delay time (IDT) increases. As the temperature increases, the IDT of the blended fuel is shortened. There is a negative temperature coefficient (NTC) region during the ignition delay. The energy ratio of ammonia weakens the ability of CO to combine with OH, and more CO tends to react with HNO to form CO 2 . The increase in ESR leads to a decrease in the concentration of H, O, OH, and HO 2 free radicals, which suppresses the laminar burning velocity (LBV) of the blended fuel. As the energy ratio of ammonia increases, the sensitivity of ammonia chemistry to LBV increases. The NO X content of blended fuel is mainly affected by ammonia. When the ESR is greater than 20%, increasing the ESR can reduce the generation of NO X in laminar flames of blended fuel. Conclusion This study investigated in detail the combustion reaction process of n-heptane/ammonia mixed fuel over a wider range of temperature, pressure, equivalence ratio, and ESR. The findings provide critical insights for optimizing the blended combustion of NH 3 and hydrocarbon fuels.

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