A study on mechanisms of nitrous oxide formation in post-combustion flue gases

N2O formation in low-concentration NO and SO2 mixtures was examined and the promotional effects of O2 and water vapor on the formation were studied. At 25°C N2O formed in dry gas mixtures with NO and SO2 concentrations as low as 10 ppm. With the addition of water vapor N2O formation rate increased significantly. The effect of water vapor was more pronounced when O2 was also present. The results suggest that SO2 participates in the gas phase reaction that forms N2O, and forms H2SO4 which enhances N2O formation rate. The temperature dependence of N2O formation appears to be weak and at −25°C N2O still forms in appreciable quantities, suggesting that N2O could form in severe winter conditions in power plant plumes. Such formation can be attributed to gas phase and surface reactions but not liquid phase reactions. A study of the effects of solid materials on the rate of surface reaction indicated that carbon steel enhanced N2O formation whereas lime particles inhibited the formation. Possible mechanisms are discussed along with the implication of the results in predicting N2O formation levels in post-combustion flue gases from large combustion units.