Effect of Flue Gas Recirculation (FGR) on NOx Emission in a Pilot-Scale Vortexing Fluidized-Bed Combustor

The combustion experiments were carried out in a 0.75 m inner diameter and 4.6 m high pilot-scale vortexing fluidized-bed combustor (VFBC). The secondary air was injected tangentially at the bottom of the freeboard, 2.05 m above the distributor plate. Coal of particle sizes from 1.4 to 8 mm was used as the fuel. Silica sand was used as the bed material. The effects of operating conditions, such as the bed temperature, excess oxygen ratio, secondary air preheating, and stoichiometric oxygen in the combustion chamber, on the temperature distribution within the VFBC, CO emission, and NOx emission were investigated. The experimental results show that NOx emission can be effectively decreased using flue gas recirculation (FGR) in the VFBC. The lower NOx emission can be attributed to the following reasons: first, FGR sends NOx back to the reactor and increases its chance of further reduction, and second, FGR supplies more CO2 to help suppress NOx formation or promote NOx reduction through its conversion to CO from reacting with char.