Effect of H₂O dilution on NO_x emissions from the oxidation of NH₃/H₂ fuel mixture

During NH₃/H₂ combustion, reactant jets entrain the burnt gas, resulting in a combustion zone diluted with H₂O. Therefore, the effect of H₂O dilution on NO_x formation should be carefully examined. In this study, NH₃/H₂ oxidation with N₂ and H₂O dilutions was investigated in a jet-stirred reactor (JSR). A systematic experimental analysis was conducted to investigate the effects of different factors, including temperature (T), equivalence ratio (Φ), and H₂O volume fraction (X_H2O). A numerical simulation was conducted using a modified mechanism to interpret the measured results and examine the kinetic pathways. NO formation is enhanced as T increases but inhibited as Φ increases. The effect of H₂O dilution on NO formation is related to T and Φ. Under fuel-lean conditions, NO generation is hindered by H₂O dilution. NO formation is reduced by 43 % at T = 1375 K and Φ = 0.5. In contrast, under fuel-rich conditions (Φ > 1), H₂O dilution promotes NO formation at high temperatures (e.g., T > 1300 K at Φ = 2.5). The N₂O concentration peaks at approximately 1250 K in cases of N₂ dilution, and 25 % H₂O dilution increases the T of the peak N₂O concentration by approximately 100 K. At 1375 K, the promoting effect of H₂O on N₂O generation is significant at X_H2O = 10 %. This study provides new insights into the effect of H₂O dilution on NO_x formation characteristics during NH₃/H₂ oxidation.