Insights into the NO removal mechanism by hydrazine
Menglei Zheng, Xiaoyuan Zhang
Abstract
Hydrazine (N<sub>2</sub>H<sub>4</sub>) can be a better reductant than ammonia (NH<sub>3</sub>) for NO removal during the selective non-catalytic reduction (SNCR) process due to its wider temperature window. To better understand the DeNO<sub>x</sub> chemistry by N<sub>2</sub>H<sub>4</sub>, this study conducted a kinetic modeling study for N<sub>2</sub>H<sub>4</sub>/NO/O<sub>2</sub> based on recent NH<sub>3</sub> 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), N<sub>2</sub>H<sub>4</sub> is more favorable to produce NH<sub>2</sub> via N1 pathways, which mainly reacts with NO to proceed DeNO<sub>x</sub> pathways. In Stage II (848−1,048 K), the reaction sequence N<sub>2</sub>H<sub>4</sub> → N<sub>2</sub>H<sub>3</sub> → H<sub>2</sub>NN → NO<sub>2</sub> → NO prevails, decreasing NO<sub>x</sub> removal ratios. In Stage III (1,048−1,248 K), the branching fraction of H<sub>2</sub>NN = N<sub>2</sub>H<sub>2</sub> increases, resulting in less NO<sub>2</sub> and more N<sub>2</sub>H<sub>2</sub> being produced. The NO removal efficiency is further increased. When the temperature exceeds 1,248 K, i.e. Stage IV, NH<sub>2</sub> 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.