Numerical study of the combined effect of radiation reabsorption and pressure on NOx formation in Ch4/Air/CO2 counterflow diffusion flames

In this study, the combined effects of radiation reabsorption and pressure on flame structure and NO formation in CH4/Air/CO2 counterflow diffusion flames were numerically investigated by using detailed chemistry ( GRI 2.11 mechanism) and transport properties. A statistical narrow-band model ( SNB) was adopted to evaluate the spectral gas radiative properties and to consider the reabsorption of radiative species. The radiative transfer equation was solved by means of the discrete ordinate method. It is found that the radiation reabsorption effect on NO emission index ( EINO) increases with increasing pressure. Contrary variations of EINO with pressure were indicated by the calculation results with and without consideration of radiation reabsorption, respectively. It is indicated that the non-reabsorption model such as the optically thin model is unable to correctly predict the NO formation in flames at high pressures and with the addition of CO2 since the radiation heat loss was overestimated and the spectral properties of radiative gases were not taken into consideration. For high-pressure flames and flames including high CO2 concentration ( for example, the flames with exhaust gas recirculation), it is suggested that the radiation reabsorption should be considered in predicting NO formation.