Chemical effect of H2 on NH3 combustion in an O2 environment via molecular dynamics simulations

Due to the imperative to reduce carbon emissions, NH3-H2 mixed combustion has garnered significant attention. However, the knowledge of the reaction kinetics of NH3-H2-O2 mixtures at the molecular level remains limited. In this paper, the reactive force-field (ReaxFF) method was employed to study the microscale reaction kinetics of the NH3-H2-O2 mixture with varying hydrogen ratios (HR). The results of the microcanonical ensemble simulations indicated that the addition of H2 significantly increased the combustion performance of NH3. However, H2 did not change the two primary reaction paths of NH3 oxidation (NH3 → N2H2 → N2 and NH3 → NO → N2). The addition H2 not only led to a shift in the initiation reactions from NH3-O2 to H2-O2 reactions, but also notably enhanced the concentration of HO2 and OH radicals (e.g., O2 + H2 → HO2 + H), which accelerated the extraction of H from NxHy and subsequently facilitated the immediate formation of N2. Furthermore, the ignition delay decreased from 120 ps to 40 ps, and the activation energies decreased from 295.46 kJ/mol to 162.12 kJ/mol as HR increased from 0 to 0.3 under the equivalence ratio of 0.4. These key findings can guide the development of clean NH₃ combustion technology.