Numerical simulations of flow, combustion characteristics, and NOx emission for down-fired boiler with different arch-supplied over-fire air ratios

This paper presents research on a 660 MWe Foster Wheeler (FW) type down-fired boiler. The arch-supplied over-fire air (OFA) plan and high efficiency measures are proposed for the boiler. The influence of different OFA ratios on flow field, temperature distribution, and O2 and NOx concentration distributions inside the furnace are investigated by numerical simulation using Fluent 6.3.26 software. Also, the average temperature, the carbon content in the fly ash, and the O2 and NOx concentrations at the exit of the furnace are calculated. Three high-temperature zones are identified in the furnace: at the front and rear wall under the arch, and at the bottom of the upper furnace. As the OFA ratio increases, the gas temperature, the carbon content in the fly ash, and the O2 concentration increase, but the NOx concentration decreases significantly at the exit of the furnace. In particular, when the OFA ratio increases from 20% to 25%, the amplitude of the decrease of the NOx emissions is smaller, but the combustible content in the fly ash increases more obviously. Considering the NOx emissions and the combustible content in the fly ash, an OFA ratio of 20% is advisable for the arch-supplied OFA program.