Numerical investigations of burning and emission characteristics of laminar methane diffusion flames in a coflowing preheated oxidizer are presented. The preheated oxidizer stream contains hot products of combustion, such as carbon dioxide and water vapor, along with oxygen and nitrogen, and is equivalent to that obtained in an exhaust gas recirculation process. Numerical simulations are carried out using a numerical model incorporated with a C2 chemical mechanism having 25 species and 121 reactions and an optically thin radiation submodel. The numerical results are validated against the experimental results for methane-air coflow flames reported in the literature. Parametric studies have been carried out by changing the temperature and composition of the coflowing oxidizer stream. The effects of these parameters on flame height, flame stability, and emission characteristics are analyzed. It is observed that the increase in the temperature of the oxidizer has a stabilizing effect on the flame, even with a lesser concentration of oxygen in the oxidizer. By investigating the stable flame cases in which a maximum temperature of around 2000 K is achieved, it can be concluded that the preheated oxidizer stream (higher temperature and reduced oxygen) helps in significant reduction of emissions such as nitric oxide and carbon monoxide. © 2010 by Begell House, Inc.