Insights into the NO removal mechanism by hydrazine

Hydrazine (N₂H₄) can be a better reductant than ammonia (NH₃) for NO removal during the selective non-catalytic reduction (SNCR) process due to its wider temperature window. To better understand the DeNO_x chemistry by N₂H₄, this study conducted a kinetic modeling study for N₂H₄/NO/O₂ based on recent NH₃ oxidation models. Compared with previous kinetic models, the present model is more accurate in predicting a four-stage NO removal phenomenon over 673−1,523 K. In Stage I (673−848 K), N₂H₄ is more favorable to produce NH₂ via N1 pathways, which mainly reacts with NO to proceed DeNO_x pathways. In Stage II (848−1,048 K), the reaction sequence N₂H₄ → N₂H₃ → H₂NN → NO₂ → NO prevails, decreasing NO_x removal ratios. In Stage III (1,048−1,248 K), the branching fraction of H₂NN = N₂H₂ increases, resulting in less NO₂ and more N₂H₂ being produced. The NO removal efficiency is further increased. When the temperature exceeds 1,248 K, i.e. Stage IV, NH₂ is favorable to produce NH via H-abstraction reactions, which can be subsequently oxidized to produce NO. Additionally, the present model is also validated against experimental data at various oxygen contents, suggesting that the NO removal efficiency is less affected by the oxygen concentration.