Flame extinction in interacting CO-air and syngas-air premixed flames

Chemical interaction on flame extinction in downstream interaction of syngas-air and CO-air premixed flames was investigated. Flame stability diagrams mapping the extinction limits for various fuel concentrations as functions of the strain rate were developed for the downstream interactions between syngas-air and CO-air premixed flames. Of particular concern was the impact of the chemical interaction on the flame extinction characteristics. The results showed that the lean extinction boundary was precipitously slanted for extremely small amount of syngas, finally causing the extinction boundary to open up in further increase of strain rate. There existed a critical syngas concentration below which the flame could not be sustained and above which the extinction boundary was double-valued. On the upper branch, hydrogen was produced via the reaction steps CO + H + M -> HCO + M and HCO + H -> H-2 + CO near the highly rich CO-air premixed flame to participate in CO-oxidation. For highly stretched interacting flames, only a flammable island existed. The pure syngas-air diffusion flame was extinguished at a critical strain rate. Further increase of strain rate forced the flammable island to be shrunk and finally to be a point. For highly stretched interacting flames, a minimum CO concentration existed below which the flame could not be survived. In such flames, the diffusion flame can act as a parasite to the lean CO-air premixed flame through sharing hydrogen penetrated from the rich syngas-air premixed flame. Such chemical interactions were examined in detail to describe the flame extinction characteristics.