Effects of H2O on NH3 combustion in air: A reactive molecular dynamics study

Ammonia (NH₃) emerges as a new clean energy alternative to fossil fuels, with incomplete combustion resulting in the formation of nitrogen oxides (NOx). The present study investigates the potential of water (H₂O) to mitigate NOx emissions during the NH₃ combustion. Reactive force field molecular dynamics simulations are employed to systematically analyze the impact of H₂O on NH₃ combustion in air, and the results are compared with the oxidation of NH₃ under dry air conditions. The research systematically analyzes the effects of H₂O blending ratios on NH₃ consumption rates, intermediate species, and NOx formation, with a particular focus on the influence of H₂O on the NOx formation pathways. Key findings reveal that the addition of H₂O significantly enhances the oxidation rate of NH₃ in the temperature range of 2400–2800 K, while the promotional effect can be neglected at high temperatures. Optimal H₂O/NH₃ blending ratios (0.5–0.75) effectively promote the generation of OH radicals, thereby accelerating the oxidation of NH₃. Notably, the introduction of H₂O initiates new nitrogen transformation pathways, modifying the NOx formation mechanism and influencing the NOx emissions. Detailed analysis indicates that the optimal H₂O/NH₃ mixing ratios suppress the NOx emissions by inhibiting the HNO-to-NO conversion. This study provides theoretical support and scientific insights into the regulation of NOx emissions through H₂O modulation in NH₃ combustion processes.