Characteristics of NOx emission with flue gas dilution in air and fuel sides

Flue gas recirculation (FGR) is a method widely adopted to control NO_x in combustion system. The recirculated flue gas decreases flame temperature and reaction rate, resulting in the decrease in thermal NO production. Recently, it has been demonstrated that the recirculated flue gas in fuel stream, that is, the fuel induced recirculation (FIR), could enhance a much improved reduction in NO_x per unit mass of recirculated gas, as compared to the conventional FGR in air. In the present study, the effect of FGR/FIR methods on NO_x reduction in turbulent swirl flames by using N₂ and CO₂ as diluent gases to simulate flue gases. Results show that CO₂ dilution is more effective in NO reduction because of large temperature drop due to the larger specific heat of CO₂ compared to N₂ and FIR is more effective to reduce NO emission than FGR when the same recirculation ratio of dilution gas is used.