Exploring NH3 combustion in environments with CO2 and H2O via reactive molecular dynamics

Abstract

In existing power units such as internal combustion engines and gas turbines, the combustion of ammonia mixed with hydrocarbon fuels can effectively enhance the combustion intensity of ammonia. In the duel-fuel combustion scenario, the effect mechanism of the main components of combustion exhaust gas i.e., CO2 and H2O on ammonia combustion is still unclear and needs to be studied. Therefore, this paper uses reactive molecular dynamics simulations to study the combustion and emission processes of ammonia in environments with CO2 and H2O. The effects of the diverse gas environments (N2/O2, O2/CO2, and O2/CO2/H2O environments), high CO2 levels, varying H2O proportions, and O2 concentrations on ammonia combustion and emission were systematically explored. It was found that ammonia consumption was accelerated in the O2/CO2/H2O environments compared to N2/O2 environments, resulting in more NH2, H2, and H2O production. The effect of high CO2 concentration on ammonia oxidation was mainly related to temperature. The simulations found that the high CO2 concentration inhibited ammonia consumption at T < 2800K while enhancing it at T ⩾ 2800K. In addition, the high CO2 concentration could inhibit NO production under stoichiometric conditions but promote the conversion of nitrogen-containing intermediates to NO with the oxidizing capacity of CO2 enhanced significantly under extremely high temperature conditions. It was found that the increasing content of H2O molecules in the environment promoted NO production and inhibited CO production under stoichiometric conditions. In addition, the reaction paths for the conversion of HNO and HNO2 molecules into NO molecules were significantly enhanced with increasing H2O concentration. The increase in oxygen concentration can promote OH radical production and NO formation at high temperatures.