NO_<i>x</i> Formation/Destruction during Moderate or Intense Low-Oxygen Dilution Combustion of NH₃/CH₄/H₂ Tertiary Fuel Blends with a Reduced Mechanism

Ammonia is a promising carbon-free fuel for moderate or intense low-oxygen dilution (MILD) combustion, offering potential as an alternative to fossil fuels. Recent research has focused on ammonia as a replacement fuel, yet the optimum parameters for NH3/CH4/H2 blends during MILD combustion, particularly for minimizing NOx emissions, remain unclear. In this study, a well-stirred reactor model is used to identify optimal operating conditions for various NH3/CH4/H2 tertiary blends by fixing one component at 30% (H2, NH3, or CH4) and adjusting the other two fuels across equivalence ratios of 0.8 to 1.2. The Okafor mechanism is reduced, optimized, and validated for the simulations. Results indicate that fuel blends with 30% H2, decreasing CH4, and increasing NH3 achieve NOx and CO emissions around 60 ppm and 20 ppm, respectively, at the equivalence ratio of 0.8. In the case of these fuel blends, NH3 decomposition exceeds 90%, contrasting with lower decomposition rates in blends with 30% NH3 or CH4. Under stoichiometric and fuel-rich conditions, NH3 oxidation is limited (20%–50%) while CO emissions increase (up to 1200 ppm). Optimal parameters are also simulated with eight other mechanisms from recent literature, showing approximately similar NOx emissions between 1300 K and 1375 K across all mechanisms. Kinetic analysis revealed that NOx formation is dominated by the HNO and NNH pathways when NH3 exceeds 40% in the fuel blend, with key reactions identified as NH3 + OH → NH2 + H2O, HNO + O → NO + OH, NH2 + O → HNO + H, and NH2 + NO → N2 + H2O. The findings support the reduced mechanism as a reliable and efficient tool for NH3-based fuel modeling in MILD combustion applications. In the case of tertiary fuel blends, low NOx emissions are achieved under fuel-lean conditions as compared to binary fuel blends where the equivalence ratio is to be maintained above 1.0 to ensure low NOx emissions.