Effect of the separated overfire air location on the combustion optimization and NOx reduction of a 600 MWe FW down-fired utility boiler with a novel combustion system

•A novel combustion system is applied to reduce NOx emissions of a down-fired boiler.•Influence of SOFA locations is evaluated.•The upper-furnace SOFA is advisable with considering burnout rate and NOx emissions.•NOx reduction of 50% is obtained without producing negative effects.

A novel combustion system has been applied to a 600 MWe down-fired boiler to reduce NOx emissions without producing an obvious increase in the carbon content of fly ash. The system mainly includes moving fuel-lean nozzles from the arches to the front/rear walls, and re-arranging the staged air, as well as introducing separated-over-fire air (SOFA). This paper evaluates the effects of the SOFA locations (on the arches, on the throat, and on the upper furnace) on the combustion and NOx emissions characteristics using simulations. The numerical results are in good agreement with the measured results. Compared to the original combustion system, significant NOx reduction (approximately 50%) is found for all three SOFA location settings. Taking economic efficiency and NOx emissions into account, the SOFA on the upper furnace is adopted in the actual modification, and no negative effects are observed.

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