Regime-map analysis of NO formation pathways and transitions in hydrogen–air combustion

Understanding the dominant pathways of NO formation in hydrogen combustion is essential for developing low-emission hydrogen-based energy systems. However, transitions between different NO formation routes under varying thermochemical conditions remain insufficiently understood, particularly in industrial applications. In this study, NO formation pathways in hydrogen–air combustion are systematically investigated using a combined approach based on detailed chemical kinetics, perfectly stirred reactor (PSR) analysis, and large-eddy simulation (LES). Two representative LES configurations under different industrial conditions are first compared, revealing distinct dominant mechanisms: the thermal (Zeldovich) pathway and the NNH-mediated route. To generalize these observations, a series of zero-dimensional PSR simulations is conducted to construct a regime map of NO formation pathways in temperature–equivalence ratio space. This map provides a first-order mechanistic interpretation of the competition and transition between thermal and NNH-related mechanisms. The regime map is further used to interpret LES results from a high-temperature industrial furnace, showing that most NO-producing regions fall within the thermal regime. For engineering applications, a reduced Reynolds-averaged Navier–Stokes (RANS) model coupled with the Eddy Dissipation Concept (EDC) combustion model is employed to evaluate the effects of different fuel injection strategies. The results indicate that NO formation is primarily governed by the thermal mechanism and is highly sensitive to flame temperature. The proposed framework links kinetic analysis with multidimensional simulations and provides a first-order engineering assessment for NO emission mitigation in the investigated hydrogen furnace configuration.