Low NOx emissions area major benefit of FBC boilers.When burning fuel in FBC boilers, approximately
up to 70% of the combustion air flow enters through the FLUIDISING AIR NOZZLES as primary air. The sub stoichiometric amount of air suppresses volatile nitrogen oxidation to NOx by creating a fuel-rich zone in the fuel de-volatilization region ( with temperature 600 to 850 Deg C).
The secondary air is added further above the lower reducing zone. Since the fuel nitrogen is already transformed into molecular nitrogen, formation of NOx above this zone is controlled. The relatively low combustion temperature 750 to 900 °C) also play a important role to reduce NOx formation.
NOx emissions in FBC boilers are influenced by various factors including nitrogen and volatile matter in the fuel, furnace temperature, excess air, bed stoichiometry and limestone feed rate.
Additional NOx reduction other than 50 % happen in FBC furnace can be achieved by injecting ammonia (NH3) either in Free Board before super heater / after super heater. The factors influencing additional NOx reduction are NH3/NOx molar ratio, initial NOx Concentration, furnace temperature, degree of NH3 mixing and gas residence time.