A study of nitric oxide formation in fuel-rich hydrocarbon flames: Role of cyanide species, H, OH and O

Abstract

The object of this paper is to clarify the mechanism of prompt NO formation in the premixed methane-oxygen-nitrogen flame. To achieve this object, an accurate comparative study of theoretical and experimental results was carried out. In the experiment, a premixed steady one-dimensional laminar flame at about 0.1 atm pressure was chosen. The concentrations of O and H were measured by ESR together with NO, O 2 , HCN, etc. for several stoichiometric ratios between 0.88 and 1.42. The measured concentrations of H, O, cyanide species and stable species were compared with the results obtained by numerical calculation of species conservation and reaction equations for 20 components. It was found experimentally that the O atom concentration determined by ESR is not high enough to explain the mechanism of prompt NO formation by the Zeldovich mechanism and super-equilibrium concentration of O atoms. HCN was found to form prior to the formation of prompt NO, and the measured concentration of prompt NO was about ten times the maximum HCN concentration. Comparing the measured and calculated concentrations of the components, the rate-controlling reaction of methane oxidation was found to be CH 2 O+ M =CO+H 2 + M , and the rate constant of this reaction is k f .11 =2.1×10 15 exp (−17620/T) mol −1 cm 3 s −1 . The 40 elementary reactions and their reaction rates for 20 components were selected to predict the formation of prompt NO and behavior of the intermediate HCN. The concentration distributions obtained from the experiment and the numerical calculation based on the measured temperature distribution were compared and found to be in good agreement for NO and HCN for =1.26. For =0.88, the measured concentration of prompt NO and maximum concentration of HCN were 19 ppm and 1.1 ppm, respectively, while the calculated values were 26 ppm and 3.1 ppm, respectively. It was concluded that the NO formation mechanism in the wide range of stoichiometric ratio from fuel-lean to fuel-rich can be predicted by the 40 elementary reactions proposed in this paper.