CO2 radiation heat loss effects on NOx emissions and combustion instabilities in lean premixed flames

Abstract

The effects of CO2 dilution on NOx emissions and combustion instability are studied in an effort to use biogas as a fuel in dual lean premixed gas turbines. More specifically, the influence of the radiation heat transfer characteristics of CO2, which is a major component of biogas, on the combustion characteristics of a dual lean premixed flame of biogas is investigated. OH* chemiluminescence images are used to observe the flame structure for various CO2 dilution rates. The results show little difference in the flame structure for CO2 dilution rates of up to 40%, while higher dilution rates lower the flame intensity. Also, dual lean premixed flames show very different flame structures and temperatures depending on the pilot fuel mass fraction. The results show a decrease in the flame temperature when the dilution rate is increased, resulting in a reduction in the thermal NOx emissions. The present paper also shows that the radiation heat release, which is due to the high heat release rate of CO2, promotes a further drop in the flame temperature, in addition to the thermal effects of CO2. These findings are numerically modeled, empirically verified, and added to the NOx prediction model. CO2 dilution also changes the combustion oscillation frequency. To estimate this, the flame temperature is calculated as a function of the CO2 dilution rate and the pilot fuel mass ratio. The accuracy of the flame temperature calculation can be improved with a more accurate estimation of the radiation heat loss. In calculating the radiation heat loss for an unstable flame, considering the heat release fluctuation (i.e. the temperature fluctuation) improves accuracy, whereas taking the average temperature results in an underestimation. With an improved estimation of the temperature, the combustion oscillation frequency is more accurately predicted.