Promoting Effect of Syngas Addition on Ammonia Consumption and NOx Emission at High Pressure

Understanding the microspecies behavior of ammonia–syngas mixtures is crucial for cutting NOx emissions in engine applications of ammonia. Experiments were carried out at 5.0 MPa, with the mixture volume fraction varying from 0–50%. Concentrations of reactants, key intermediate species, and products were measured during the oxidation of ammonia–syngas mixtures. A detailed reaction mechanism was fully validated against the measured data. Experimental data indicated that the onset temperature of NH3, H2, and CO decreased with more syngas addition as well as the consumption temperature window, indicating an apparent improvement in the overall reactivity. An N-shape trend can be observed in NO concentration profiles under higher syngas blending ratios and fuel-lean conditions. The model in this paper can accurately predict the measured concentration data. Kinetic analysis showed that ammonia oxidation is first sensitive to NH2-involved reactions and the increase in radicals due to the blending of syngas enhances the sensitivity of these reactions. Then, ammonia oxidation is more sensitive to reaction NH2 + H2 = NH3 + H and reactions related to H2O2 at a higher syngas blending ratio. Third, reaction H + O2(+M) = HO2(+M) exhibits a strong promoting effect on NH3 consumption at 10% syngas addition but instead shows an inhibiting effect at 50% syngas addition. As to NO formation, the chemical kinetic effect of syngas addition is more dominant than the dilution effect. Meanwhile, the reduction of NO is not significant. It eventually results in a significant rise of NO concentration. As for N2O, the dilution effect and chemical kinetic effect of syngas are comparable, resulting in a slight increase in the peak concentration of N2O with more syngas blending.