COMPUTATION OF NOX EMISSION OF A METHANE-AIR DIFFUSION FLAME IN A 2-DIMENSIONAL LAMINAR JET WITH DETAILED CHEMISTRY

NOx formation from a methane-air diffusion frame in a two-dimensional jet involving highly preheated air, which has recently become an impor tant topic in industrial furnaces, is investigated numerically using a full chemistry approach including C-2, prompt and thermal mechanisms. Effects of increased air temperature on NOx formation are examined. N umerical results show that both NO formation mechanisms increase drama tically with increasing air temperature. A C-shaped production zone of NO2, corresponding to the fuel-lean and fuel-rich regions of triple f lame, is identified. It is shown that NO formation with high air tempe rature can be suppressed efficiently by decreasing the oxygen concentr ation in the airstream. Production rate analyses of elementary reactio ns are made. Formation paths of NOx at low and high temperatures are o btained and compared. The results show that the NOx formation path dep ends strongly on the air temperature. In addition to the thermal route and the HCN --> NO route, the HCN --> CN and NO --> CN recycling rout es are greatly enhanced at high air temperature. The results show that the prompt mechanism and the thermal mechanism are strongly coupled a t high air temperature. Calculations of prompt NO and thermal NO in a two-dimensional jet and in the counterflow configuration reveal that t he conventional method cannot give a correct prediction of prompt NO a nd thermal NOx particularly at high air temperature. A method using th e concept of fixed nitrogen is presented. Numerical results indicate t hat the formation process of prompt NO and thermal NO can be evaluated properly by the present method.