External flue gas recirculation: flue gas from an outlet of the boiler is re-added to the furnace through an external pipe.According to the study, external flue gas recirculation can reduce NOx production by 70%.
Internal flue gas recirculation: the return of flue gas to the combustion zone is mainly through the gas dynamics of the burner.The internal flue gas recirculation is mainly through the entrainment of the high-speed jet flame or the swirl burner to make the airflow rotate and achieve the circulation effect.The hot gas from the central recirculating area will return to the burner throat, which ensures ignition of the cold unburned gas while reducing NOx production by lowering the flame temperature and oxygen partial pressure.The ratio of external circulation also has a great influence on the control effect of Nox. With the increase of the ratio of external circulation, the reduction of Nox is more obvious, but the power consumption of the circulating fan will also increase.
——Author：ebico