Influence of the staged-air declination angle on flow-field deflection in a down-fired pulverized-coal 300 MWe utility boiler with direct-flow split burners

Cold airflow experiments were conducted within a small-scale furnace of a down-fired pulverized-coal 300MWe utility boiler. With focus on the large combustion difference between the zones near the front and rear walls in down-fired pulverized-coal boilers, we investigate the aerodynamic field at different staged-air declination angles of 0°, 15°, 30°, 45°, and 55°. For declination angles of 0°, 15°, and 30°, a deflected flow field appeared in the lower furnace, with downward airflow velocities near the rear wall decaying more rapidly than velocities near the front wall. In addition to the downward airflow reach into the lower furnace, the turbulence intensity and longitudinal-velocity components at certain cross sections were lower near the rear wall than near the front wall. Through an increase of the declination angle from 0° to 30°, the flow-field deflection diminished, which was accompanied by a slower decay in the downward airflow near the rear wall and an increase in the reach (as measured by the dimensionless depth) of the downward airflow near the rear wall as well as longitudinal-velocity components within the associated cross section. Those near the front wall changed only slightly. For larger angles of 45° and 55°, the deflected flow field disappeared. Turbulence intensities in the staged-air zones near the front and rear walls increased steadily as the declination angle increased from 0° to 55°. The optimal setting for staged air would necessitate a declination angle of 45°.