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

N 2O formation in low-concentration NO and SO 2 mixtures was examined and the promotional effects of O 2 and water vapor on the formation were studied. At 25°C N 2O formed in dry gas mixtures with NO and SO 2 concentrations as low as 10 ppm. With the addition of water vapor N 2O formation rate increased significantly. The effect of water vapor was more pronounced when O 2 was also present. The results suggest that SO 2 participates in the gas phase reaction that forms N 2O, and forms H 2SO 4 which enhances N 2O formation rate. The temperature dependence of N 2O formation appears to be weak and at −25°C N 2O still forms in appreciable quantities, suggesting that N 2O 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 N 2O formation whereas lime particles inhibited the formation. Possible mechanisms are discussed along with the implication of the results in predicting N 2O formation levels in post-combustion flue gases from large combustion units.