Experimental and kinetic study of ammonia oxidation and NOx emissions at elevated pressures

To investigate the effects of pressure and equivalence ratio on ammonia oxidation and the emission of NOx, experiments were conducted in a flow reactor at atmospheric pressure and 5.0 MPa between 600 and 1250 K, with equivalence ratios ranging from 0.125 to 1.0. In addition, a kinetic model applicable to ammonia oxidation under high-pressure conditions was developed and validated against the performance of other recently published kinetic models with reference to several parameters associated with the ammonia oxidation and the emission of NOx under high-pressure conditions. The developed model was also validated against previously published experimental data collected under high-pressure conditions, and it can accurately capture the data obtained. The study found that the increase in pressure and oxygen content significantly enhanced the reactivity of NH3 and consequently resulted in increased NO and N2O concentrations at low temperatures. Based on a kinetic analysis using the developed model, the HO2 content was identified as the key factor influencing the difference in reactivity between atmospheric and high pressure. Sensitivity analysis revealed that H + O2 (+M) = HO2 (+M) inhibits NH3 consumption at atmospheric pressures but has a promoting effect at high pressures, because at high pressures the reaction generates a significant amount of HO2 radicals at low temperatures, increasing the reaction activity and reducing the onset temperature of ammonia oxidation. The H2NO system becomes even more critical under higher pressure, as more NH2 and O2 are converted to H2NO and NO. H2NO, NO, and N2O act as chain carriers in the reaction sequence, converting NH2 and O2 to OH and NO and accelerating the process of ammonia oxidation. At the same time, higher pressure enhances the NOx formation pathway due to the large amount of HO2 generated. However, as temperatures increase, O and OH radicals increase and DeNOx pathways dominate, resulting in lower NO and N2O contents. In the context of the equivalence ratios, the increase in the O2 content enhances the generation of HO2, leading to a greater tendency for NH2 to be converted in the NH2 + HO2 = HNO + H2O and NH2 + HO2 = H2NO + OH reaction pathways to HNO and H2NO, respectively. The HNO and H2NO produced are subsequently converted to NOx.