Radical overshoots and dissociation in methane/air flames

A discussion is given of the fact that observed radical overshoots in methane combustion imply the existence of dissociation reactions. The fraction of methane molecules which oxidize via a path including a dissociation has been found to approach 0.8 in very rich methane/air flames by observing that radical concentrations tend toward zero as the flames are made richer and approach a limiting equivalence ratio, φl. This finding provides an interpretation of the observation that methane is converted entirely to CO and CO2 at equivalence ratios below φl, whereas above φl unreacted methane persists into the burned gases.