The influence of burner-port geometry in the mechanisms of hydrocarbon oxidation and NOx formation from a 50 kW industrial-type methane-fired burner was investigated experimentally. Imaging and temographic reconstruction techniques were used to assess the effects of port geometry upon flame visible length and C-2 chemiluminescence distribution in the recirculation zone, C-2 emission of methane flames depicts that low fuel jet velocities allow very rich conditions at recirculation zone and lead methane oxidation through O-2-scarcity mechanism. Higher velocities imply that methane oxidises via a path including dissociation into free radicals. In-furnace measurements were performed from a refractory-lined vertical furnace. NOx concentration results revealed that NO formation is closely connected with the dissociation process, suggesting that prompt-NOx mechanism is more important than hitherto supposed.