Reactive molecular dynamics simulation and chemical kinetic modeling of ammonia/methane co-combustion

Under the vision of "low carbonization", ammonia (NH3), as a carbon-free hydrogen-rich fuel, blended with reactive additives, is an effective means to achieve low carbon emissions and reduce the consumption of traditional fossil fuels. In this work, the combustion behaviors of NH3 and CH4, the distribution of main products and the formation and reduction characteristics of NO at different temperatures, CH4 mixing ratios, and O2 equivalence ratios are explored by means of reactive molecular dynamics simulation to reveal the microscopic mechanism of NH3/CH4 co-combustion. The calculated results show that CH4 takes the lead in producing H radicals which react with O2 to generate OH radicals to promote the oxidation of NH3, and it is found that the cocombustion process of NH3/CH4 in the radical chain transfer stage is similar to the respective pyrolysis and oxidation reactions of CH4 and NH3. Accordingly, an optimized kinetic mechanism of NH3/CH4 co-combustion is constructed based on the mechanisms proposed by Glarborg et al. and Okafor et al. by incorporating the NH3 pyrolysis sub-mechanisms related to NH2, HNO, and N2H2 and the skeletal mechanism of CH4 oxidation. The optimized mechanisms of NH3/CH4 co-combustion are found to improve the prediction of the ignition delay time (IDT) and laminar burning velocity (LBV) of NH3/CH4 mixture under wide ranges of conditions compared to the original mechanisms of Glarborg et al. and Okafor et al. To decrease the computational cost of three-dimensional combustion numerical simulations, the skeletal mechanism models are established by simplification of the optimized mechanisms of NH3/CH4 co-combustion using direct relation graph method with error propagation and sensitivity analysis, which can provide nearly the same estimates of IDTs and LBVs as the optimized mechanisms mentioned above.