Effects of Water Vapor Addition on NO Formation in Nonpremixed and Premixed Flames of NH ₃ and CH ₄

Ammonia/methane cofiring is a practical route for decarbonizing high-temperature industrial combustion, but NO control remains challenging because NH 3 introduces strong fuel-NO chemistry. H 2 O addition is an established NO-mitigation strategy in hydrocarbon combustion; however, its role in NH 3 -containing flames remains unclear, especially in nonpremixed flames for which spatially resolved data are still scarce. In this work, spatially resolved NO measurements were performed in pure CH 4 and NH 3 –CH 4 nonpremixed flames using a counterflow configuration by a self-developed calibration-free diagnostic based on microprobe sampling and tunable diode laser absorption spectroscopy. Premixed flames using the same counterflow configuration were analyzed numerically for comparison. The effects of H 2 O fraction, NH 3 blending ratio, and equivalence ratio on NO formation were evaluated, and sensitivity and pathway analyses were used to separate the thermal-dilution and chemical effects of H 2 O addition. In pure CH 4 flames, H 2 O addition suppresses NO in both configurations, mainly through thermal dilution, although this suppression weakens under rich premixed conditions. In NH 3 –CH 4 nonpremixed flames, however, H 2 O addition increases NO because H 2 O-induced radical changes offset the temperature reduction. By contrast, in NH 3 –CH 4 premixed flames,H 2 O addition reduces NO, and the suppression becomes stronger at higher equivalence ratio because the thermal-dilution effect remains dominant. These results show that the NO response to H 2 O addition in NH 3 –CH 4 flames is strongly configuration-dependent and that NO-control strategies effective for hydrocarbon flames cannot be transferred directly to ammonia-containing systems.