Probing the reactivity of ammonia/C1 mixtures using shock tube coupled with laser absorption spectroscopy

Abstract

Ignition delay times (IDT) and speciation profiles (NH₃, NO, and CO) were measured for NH₃/C₁ fuel blends (NH₃/CO, NH₃/CH₄, NH₃/CH₃OH) in a shock tube using laser absorption spectroscopy. Experiments spanned equivalence ratios of 0.5–1.5, 5–20 % C₁ additives, and temperatures of 1477–2236 K at around 2.5 bar. The experimental data were validated against the simulation results from the PTB-NH₃/C₂ 1.1 mechanism, which demonstrated robust performance across all mixtures. Methanol significantly enhances ignition reactivity, resulting in the shortest IDTs among the three C₁ additives. Combining the findings from our prior studies, the IDT reduction order by different hydrocarbons at high temperatures is: C₂H₅OH ≈ C₂H₆ > CH₃OH > CH₄ > CO, indicating that high temperature favors C₂ compounds. While at intermediate temperatures and high pressures, where the functional groups dominate, the reactivity order is: C₂H₅OH > CH₃OH > C₂H₆ > CH₄, as alcohols enhance reactivity stronger than alkanes. Kinetic modeling analysis identified NH₂ as a key intermediate in NH₃ oxidation, following the primary pathway NH₃ → NH₂ → NH → N → NO. For NH₃/CO, CO contributed to secondary branching intermediates like HNCO through reactions like NH₂ + CO ≤> HNCO + H, influencing nitrogen-carbon interactions. In NH₃/CH₄, hydrocarbon oxidation promoted CO and CH₂O formation, with limited CN cross-reactions. NH₃/CH₃OH pathways exhibited unique CH₃O and CH₂OH radical dynamics, facilitating prolonged CO formation and unique broader CO peaks under fuel-rich conditions. While the PTB-NH₃/C₂ 1.1 mechanism captured most trends, discrepancies emerged at lower temperatures and fuel-rich conditions, underscoring the need for further improvement in future. Measuring more intermediate species such as N₂O, NO₂, and CH₂O would also benefit model validation.