Combustion characteristics of preheated and diluted methane-air mixtures

Preheating air to high temperatures and dilution of fuel-air mixtures with combustion gases are thought to support high performance combustion, or, in other words, large fuel fluxes and, simultaneously, low emission levels of NO. These combustion characteristics are known to result from complicated interactions between temperature and composition effects, but the mechanisms leading to these characteristics have not yet been elucidated. In this work, combustion of preheated mixtures of methane+air+combustion gases was simulated using a one-dimensional model. In order to analyze separately the temperature and the composition effects, four different values of adiabatic flame temperature T_a were chosen a priori; then, the preheating temperature was determined as for satisfying T_a for fixed amounts of combustion gases added to the fuel-air mixture. The above cited combustion characterisics could be basically reproduced with the premixed flame model used in this study. Whatever added to the fuel-air mixture, preheating to about 1800 K was seen to be necessary for allowing the mixture to ignite spontaneously and burn efficiently with large burning velocities; at such high preheating temperature combustion, the burning velocity, and then the fuel flux were found to be very large and the NO concentration much lower than that obtained when methane burns with room temperature air.