Colloquia

Summary

This research is performed to find out the effects of flue gas recirculation (FGR) on the emission of NOx in a domestic condensing boiler. With increasingly stringent environmental regulations, it is valuable to investigate techniques for reduction of NOx emissions. Therefore, the physics and chemistry that play a role in combustion are investigated first, followed by modeling of different NO formation mechanisms and effects of FGR. In order to measure the effects of FGR a boiler suitable for FGR measurements is made. The parametric research is performed at constant fan speed and by varying the air factor and the FGR ratio. Different cases are investigated. These include adding wet flue gas to the methane/air mixture, adding dry flue gas and replacing air with wet flue gas. Also, measurements over the modulation range for hydrogen/air are performed. The investigations show a decrease in NOx with increasing air factor and FGR ratio. However, for most cases, an increase in CO emissions is observed. The NOx emissions that are performed over the modulation range of the boiler are calculated according to the EN15502 norm. Furthermore, 1D flame simulations using Cantera software are executed. The NOx values are in good agreement with the performed measurements and provide therefore valuable insights into NO formation. Adding wet flue gas to a fuel/air mixture mainly affects the formation of thermal NOx for both: methane and hydrogen combustion. This is due to lowering the flame temperature as well as shifting the chemical kinetics because of water vapor in the flue gas.