High pressure and temperature premixed laminar NH3/CH4 oxygenated flames: from experiments to kinetic models

Achieving carbon neutrality will require the integration of renewable, carbon-free fuels, with ammonia (NH₃) emerging as a promising hydrogen carrier. However, its low laminar flame speed (S L 0 < 10 cm/s at 298 K, 1 bar) limits stable combustion, motivating the use of methane (CH 4 ) blending and oxygen enrichment to enhance reactivity. In this work, laminar flame speeds of NH 3 /CH 4 /O 2 /N 2 mixtures were measured under isochoric conditions in a constant-volume spherical chamber over pressures of 2–5 bar, temperatures of 365–470 K, and oxygen mole fractions of 21–40%. A detailed kinetic mechanism incorporating 112 species and 1058 reactions was developed and evaluated against the measurements. Flame speed increased strongly with oxygen enrichment, with 40% O 2 resulting in flame speeds five times the values measured at 21% O 2 . Kinetic analyses showed that oxygen enrichment accelerates H-abstraction in both NH 3 and CH 4 oxidation, strengthens N-C coupled pathways, and promotes conversion of intermediates toward N 2 , CO 2 , and H 2 O, whereas increasing pressure redistributes the reaction fluxes and reduces the relative importance of the most reactive radical-mediated channels. A thermal/chemical contribution analysis further showed that CH 4 promotes NH 3 flame propagation through both effects, with the relative chemical contribution decreasing during isochoric evolution, primarily because of pressure rise. The pressure exponent β captures this competition, shifting toward zero with oxygen enrichment but remaining strongly dependent on equivalence ratio for NH 3 -rich flames. These results provide new insight into the coupled effects of oxygen enrichment, pressure, and blend composition on NH 3 /CH 4 flame propagation under isochoric conditions, and supply useful validation targets for kinetic model development in high-pressure, oxygen-enriched combustion systems. Novelty and significance statement Available laminar flame-speed data for NH 3 /CH 4 mixtures remain largely confined to near-ambient, air-based conditions, leaving the combined effects of oxygen enrichment, elevated pressure, elevated temperature, and isochoric flame evolution insufficiently documented. The present work addresses this gap by providing laminar flame-speed measurements for NH 3 /CH 4 /O 2 /N 2 mixtures at pressures up to about 5 bar, temperatures up to about 470 K, and oxygen mole fractions from 21 to 40%, obtained in a spherical constant-volume chamber with full optical access. A detailed NH 3 /CH 4 kinetic mechanism comprising 112 species and 1058 reactions is further developed and assessed against the measurements. The significance of the work lies not only in the new dataset, but also in the accompanying mechanistic interpretation. The results clarify how oxygen enrichment, pressure, and blend composition jointly govern flame propagation, quantify the thermal and chemical contributions of CH 4 addition, and show that oxygen enrichment progressively reduces pressure sensitivity while NH 3 -rich flames remain more sensitive under lean conditions. These findings provide useful validation targets for detailed kinetic mechanisms and support the development of ammonia-based combustors under practically relevant elevated-pressure conditions.