Experimental and modeling study of the oxidation of NH3/C2H4 mixtures in a shock tube

Abstract

Ammonia is a promising zero-carbon fuel, offering new possibilities for sustainable energy system development. In this study, ignition delay times (IDTs) of NH₃/C₂H₄ mixtures with C₂H₄ contents of 0 %, 5 %, 10 %, and 25 % were measured using a shock tube at temperatures ranging from 1176 to 1904 K, pressures of 1.0–8.5 atm, and equivalence ratios of 0.5, 1.0 and 2.0. A laser absorption diagnostic system was developed to track the temporal evolution of NH₃ concentration during the oxidation process behind the reflected shock waves. The experimental results indicate that the IDTs of the mixtures exhibit non-linear decrease with the addition of ethylene. Specifically, compared to pure ammonia, the addition of 5 %, 10 % and 25 % ethylene significantly increases the reactivity of the mixture, leading to a 36.7 %, 75.9 % and 90.2 % reduction in IDT at a temperature of 1563 K and a pressure of 1.0 atm, respectively. Moreover, the mixture exhibits similar reactivity under fuel-lean and stoichiometric conditions, which remains higher than the reactivity observed under fuel-rich conditions. Overall, the IDTs and the time required for complete consumption of the mixture decreases as temperature, pressure, and ethylene blending ratio increase. In order to simulate and analyze the reaction process of NH₃/C₂H₄ mixtures, a detailed kinetic model was constructed based on previous studies by updating the interaction reaction between C₂H₄ and NH₂ radical and validated against the current experimental results. Rate of production (ROP) and sensitivity analysis were performed to identify the primary consumption pathways of NH₃/C₂H₄ and the significant impact of C₂H₄ on the reactivity. Additionally, due to the addition of C₂H₄, a substantial amount of NH₂ radical participates in the H-abstraction reaction (C₂H₄ + NH₂<=>C₂H₃ + NH₃). This results in a reduced involvement of NH₂ in the DeNO_x process and, consequently, the NH₃/C₂H₄ mixture exhibits a higher tendency to produce NO_x compared to pure ammonia.

Novelty and significance statement

Ammonia offers new possibilities for sustainable energy systems but faces challenges like low combustion rate and mixing with reactive fuels can effectively enhance the ignition characteristics of NH₃. The ignition delay times and speciation NH₃/C₂H₄ mixtures are systemically measured by using shock tube and laser absorption spectroscopy. A newly detailed kinetic NH₃-C₂H₄ model is also developed based on previous studies by updating the interaction reaction between C₂H₄ and NH₂ radical and validated against the current experimental results. The rate of production and sensitivity analysis reveal that the interaction reaction (C₂H₄ + NH₂<=>C₂H₃ + NH₃) have a significant impact on the ignition performance of the binary mixtures. Additionally, the DeNO_x process of binary mixtures is suppressed due to the addition of C₂H₄, resulting a higher tendency to produce NO_x. To our best knowledge, this is the first experimental study to systematically measure the ignition delay times and speciation data of NH₃/C₂H₄ mixtures.