Detailed gas/particle flow characteristics of an improved down-fired boiler with respect to a critical factor affecting coal burnout: Vent-air inclination angle

The eccentric-swirl-secondary-air combustion technology has been confirmed to comprehensively solve high NOx emission and poor coal burnout for down-fired boilers with swirl burners. The influence of vent-air inclination angle (beta(v)), a critical factor that affects coal burnout, on gas/particle flow characteristics is investigated to further decrease unburned combustible in fly ash. Gas/particle two-phase flow experiments under different beta(v) (i.e., 10 degrees, 20 degrees, 28 degrees, 40 degrees and 50 degrees) are performed by using a particle dynamic analyser in a 1:10-scale model of the full-scale improved boiler. With increasing beta(v), the under-arch recirculation zone and the recirculation velocities of gas/particle flows continuously increase. In the staged air region, the vertical velocities of gas/particle flows near water-cooled wall for beta(v) of 40 degrees and 50 degrees are still as high as 2-4 m/s, however, the vertical velocities of gas/particle flows basically decay to negative values for smaller beta(v) of 10 degrees and 20 degrees. At furnace section Y/Y-0 from 0.220 to 0.369, the vertical fluctuation velocities of gas/particle flows near water-cooled wall for beta(v) of 40 degrees and 50 degrees are significantly higher than those with beta(v) of 10 degrees, 20 degrees and 28 degrees, and the particle volume flux at the same horizontal position X/X-0 constantly increases with increasing beta(v). Furthermore, the downward ejection ability of vent air to gas/particle flows from burners continuously improves, and the downward depth of gas/particle flows and the space utilization ratio of lower furnace both increase with increasing beta(v). The appropriate inclination angle of vent air for the improved boiler varies from 40 degrees to 50 degrees. (C) 2019 Elsevier Ltd. All rights reserved.