Flue gas recirculation for NOx control includes gas recirculation into the furnace or into the burner. In this technology 20-30% of the flue gas (at 350-400°C) is re-circulated and mixed with the combustion air. The resulting dilution in the flame decreases the temperature and availability of oxygen therefore reducing thermal NOx formation. When flue gas recirculation into the burner is used in low NOx burners, the flue gas is usually re-circulated subject to the operational constraints of flame stability and impingement, as well as boiler vibration. Flue gas recirculation in combination with other primary measures for NOx control is installed at 49 pulverised coal-fired units on a total capacity of >15 GWe.
Retrofitting an existing coal-fired unit with flue gas recirculation involves installation of a system to extract the flue gas from the boiler unit, additional ductwork, fan and a fly ash collecting device. The fly ash control device is necessary in order to clean the flue gas of particulate matter prior to recirculation. Heat distribution in the furnace may be affected due to the increase in throughput. Excessive flue gas recirculation can also result in flame instability problems and increased steam temperatures.
Flue gas recirculation alone in coal-fired boilers achieves a low NOx reduction efficiency (<20%). This is because the ratio of thermal-NOx to total NOx emissions is relatively low in coal-fired plants. The technique is being used on coal-fired units in combination with other primary measures for NOx control.
Click on relevant text for details of other primary measures for NOx control: