Lower NO emission conditions of NH3–H2 mixtures under the oxygen-enriched premixed combustion mode

Compared with pure NH₃ fuel, the lower NO emission conditions of NH₃–H₂ mixtures under the oxygen-enriched combustion mode have been identified quantitatively. Furthermore, the effects of the H₂ addition on the NO productions of premixed oxygen-enriched NH₃–H₂ flames are studied numerically, while the major reactions responsible for the variation of total NO are identified. Results show that the H₂ addition is eligible to reduce the NO emissions of the oxygen-enriched NH₃ combustion if the laminar burning velocity is fixed to a larger value (>23 cm/s). The quantitative equations are obtained to determine the optimum oxygen-enriched conditions of the NH₃–H₂ mixture, aiming to achieve lower NO emission and similar or improved combustion stability compared to pure NH₃ fuel by restricting the minimum and maximum O₂ percentages for the NH₃–H₂ mixtures. Based on the rate of production (ROP) analysis, for the oxygen-enriched NH₃–H₂ combustion, R144 (HNO + H<=>H₂+NO) is consistently the most significant NO production reaction, while R85 (NH + NO<=>N₂O + H) and R91 (N + NO<=>O + N₂) play significant roles in the NO consumption. When the laminar burning velocity (S_L) is kept to a lower value, the increased total NO of the oxygen-enriched NH₃ flames with the H₂ addition is ascribed to more efficient suppressions on the NO consumption reactions of R85, R91 and R77 (NH₂+NO<=>NNH + OH). In contrast, thanks to the extra impact of the higher O₂ percentage on the H/O/OH radical pool at larger S_L, the NO production reactions begin to suffer stronger suppressions of the H₂ addition, resulting in the reduced total NO of NH₃–H₂ mixtures. Furthermore, R144 is identified as the key reaction influencing the variation trend of total NO at both lower and higher S_L values.