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A computational fluid dynamics model of a 660 MW down-fired boiler with multiple layers of secondary air has been developed. The plausibility of CFD model has been determined by performing a grid independence test and demonstrated by comparing the temperatures of the furnaces. in simulation and experiment. The validated CFD model was used to investigate the effect of different downdip angles of SA(F) on the coal combustion performance and NOx emission characteristics at full load. The result shows that as the downdip angle of SA(F) rises from0° to 60°, then traveling distance of primary air/fuel in the lower furnace is significantly increased. The peak temperature occurs in the burnout zone, violating the initial design concept. When the downdip angle of SA(F) is set below45°, most of the air/fuel cannot cross the secondary air which is burned by the mixing intensity of the upward flowing pulverized coal and secondary air. In contrast, for SA(F) downdip angle of greater than 30°, primary air/fuel can penetrate the secondary air to reach the hopper, where combustion is controlled by the mixing intensity of the pulverized coal and secondary air, forming recirculation zones at the hopper. Considering the carbon content of fly ash and NOx emission at the furnace outlet, the downdip angle of SA(F) is preferable to 15°.
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