An experimental and modeling study of the auto-ignition of NH₃/syngas mixtures in a shock tube

Ammonia (NH₃) holds promise as an ideal zero-carbon fuel for modern energy systems. Co-combustion NH₃ with syngas can enhance the reactivity and improve the combustion efficiency of NH₃. In the current study, the auto-ignition and speciation experiments for NH₃/syngas mixtures were conducted at pressures of 0.86 and 4.0 atm, within a temperature range of 1234–1620 K, and across three equivalence ratios of 0.5, 1.0, and 2.0, with syngas content of 20 %, 30 % and 50 %, respectively. To the best of our knowledge, the experimental study for NH₃/syngas mixtures using shock tubes and laser absorption spectroscopy is the first in the literature. The experimental results show a similar reactivity under fuel-lean and stoichiometric conditions, which is slightly higher than the reactivity observed under fuel-rich conditions. The reactivity of the mixture is enhanced as the syngas content and pressure increase. The sensitivity analyses were conducted based on various kinetic models, and three key elementary reactions were identified that predominantly influence the oxidation of NH₃/syngas mixtures under experimental conditions: R1 (N₂H₂+M<=>NNH+H+M), R2 (NH₃+OH<=>NH₂+H₂O) and R3 (N₂H₂+H<=>NNH+H₂). The rate constants of these three crucial reactions were updated based on literature data, and an updated detailed kinetic model (NH₃-syngas model) was proposed based on our previous work (NH₃-C₂H₄ model). Simulated results by the NH₃-syngas model agree well with experimental data of the ignition delay times and time-histories of ammonia across different equivalence ratios, various syngas contents and pressures, as well as a series of literature data. Rate of production and sensitivity analyses were employed to elucidate the reaction pathways and crucial elementary reactions of NH₃/syngas mixtures. This investigation may contribute to optimizing kinetic models between ammonia and higher carbon number fuels in the future.