Abstract
In order to clarify oxidation behavior of NO to NO2 immediately after emission into the atmosphere, a chemical reaction model involving oxidation of NOx and methane has been developed. To examine its validity at atmospheric temperature, irradiation experiments have been conducted under various conditions, and calculated NO and NO2 values were compared with experimental values. As for the validity of the model at high temperature, modeling calculation results were compared with flow tube reactor experiments conducted in the literature. As a result, it was shown that the reaction model should adequately describe the oxidation behavior of NO to NO2 atatmospheric temperature and high temperature condition.
In addition, important reactions which may influence oxidation behavior of NO to NO2 at 298K and 873K have been examined by conducting sensitivity calculation. As a result, at 298K under dry condition, eight inorganic reactions including reaction 2NO+ 02→2NO2 were suggested to be important, while at 873K, oxidation to NO occurs not through reaction 2NO+O2→2NO2, but mainly through NO+ HO2→NO2+ OH during the process of the oxidation of CH4, while the reaction path involving the oxidation of C2H6 produced by combination of CH3 is not important under the reaction condition studied. It was also suggested that elementary reactions NO2 + hν→N0+ O (3P), NO2+ 0 (3P)→NO + 02 and NO + OH +M→HNO2 + M are important for NO2 reduction, as well as NO2 + H→NO +OH.
Using the reaction model, reaction calculations were carried out, assuming constant temperature and volume, to calculate the time profiles of NO2 within a short reaction time. As a result, it was suggested that, within the temperature range of 500-700°C, presence of CH4 in exhaust gases causes prompt NO oxidation and that the conversion behavior of NO to NO2 in the vicinity of the emission duct cannot be ignored.
