Numerical study on the effects of oxygen enrichment on methane/air flames

• Effects of oxygen on flame thermal properties and NO production were studied.
• A bit of oxygen increased NO formation, but more oxygen decreased NO emission.
• Oxygen shows it significant effects in the pathways of CH3 oxidation.

In the study, we numerically simulated premixed laminar methane/air flames enriched by oxygen at flame equivalence ratios of 0.7, 1.0 and 1.2, and over a wide range of enrichment ratios from 0 to 0.79. The effects of oxygen enrichment on flame characteristics were studied through comparing global thermal properties, NO production and flame kinetics. With the addition of oxygen, the flame temperature gradient, inner layer temperature, and average heat release rate are consistently increased, and the flame thickness decreases. According to the predictions of NO emission, a small amount of oxygen addition increased NO production, whereas a large quantity of oxygen decreased NO emission. Moreover, we explored the kinetic effect of oxygen enrichment on flame reaction routes via flame kinetic analyses. The effect of oxygen enrichment was inconspicuous in the pyrolysis routes from CH4 to CH3, in which the related reactions were promoted coequally. Significant differences were found in the pathways of CH3 oxidation, in which the reactions related to the consumption of O, OH, and H had been promoted significantly. Based on the analyses of the variations of reaction progress rate, we found the primary affecting routes of oxygen on methane/air flame. R38 and R84 played central roles in methane oxidation and they were promoted by oxygen addition and consequently contributed to the acceleration of the global reaction of enriched flames.