Effect of Declination Angle of Vent Air on Flow Characteristics of a Scale Model of a Down-Fired Utility Boiler with Swirl Burners

Abstract

W-shaped furnaces equipped with swirl burners suffered from high carbon content in fly ash and high NOx emission. Using an IFA 300 constant-temperature anemometer system, cold air experiments on a scale model of a 300-MW of electrical output down-fired pulverized-coal utility boiler with swirl burners were performed to investigate the effect of the declination angle of vent air on flow characteristics of the furnace. When the angle of vent air was 28°, 35°, and 40°, the flow field in the furnace has good symmetry, which was beneficial to the stable combustion of the flame. As the angle of vent air increased, the mixing of downward airflow and vent air was delayed. At a vent-air angle of 45°, the flow field in the furnace was deflected. The deflected flow field would result in a flame short circuit, the local high temperature, an increase in the carbon content in fly ash, and a reduction of boiler efficiency. The angle of vent air had little effect on the mixing of downward airflow and staged air. Downward airflow began to turn upward in the region of the staged air nozzle and mixed weakly with the staged air. As the declination angle of vent air increased, the mixing of arch downward airflow and staged air was postponed, which extended the residence time of the pulverized coal under strong reducing atmosphere and was beneficial to reducing the generation of fuel NOx. Meanwhile, the dimensionless penetrating depth of downward airflow increased, which helped extend the residence time of the pulverized coal in the furnace, reduced the carbon content in fly ash, and increased the boiler efficiency. Considering the symmetry of the flow field and the dimensionless penetrating depth, the best angle for the vent air is 40°.