An Experimental Investigation of the Conversion of NO to NO2 at High Pressure

Unexpectedly high concentrations of NO2 have been noted in stack emissions from industrial combustors. NO2 formation has been reported to occur through the so called “HO2 mechanism” in which NO combines with HO2 to produce NO2 and OH. In this study, the formation of NO2 was investigated at super-atmospheric pressures through experiments and computer modeling. Computer modeling utilized the CHEMKIN chemical kinetics program and a subset of a previously published C-H-O-N system mechanism. Experimental work was conducted using a high pressure flow reactor designed and built in the course of the study. The effects of pressure, temperature, and the presence of a NO2 promoting hydrocarbon, methane, were investigated. It was discovered that as pressure increased from 1 atm. to 8.5 atm., the rate and amount of NO converted to NO2 also increased. The results also show a temperature “window” between approximately 800 K and 1000 K in which NO to NO2 conversion readily occurred. The presence of methane was seen to enhance NO conversion to NO2, and a ratio of [CH4]/[NO] was found to be a useful parameter in predicting NO2 formation. Significant NO conversion to NO2 was noted for [CH4]/[NO]>1 at the hydrocarbon injection point. Experimental results validated those trends obtained from modeling with a modified C-H-O-N mechanism.