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

Highlights: • A novel high-efficiency low-NO_x technology is applied to a down-fired boiler with swirl burners. • Gas/particle flow characteristics at different vent-air inclination angles are investigated. • With increasing angle, recirculation zone and downward depth of gas/particle flows increase. • Primary air is more effectively ejected by under-arch air with increasing inclination angle. • The appropriate inclination angle of vent air for the improved boiler varies from 40° to 50°. -- Abstract: The eccentric-swirl-secondary-air combustion technology has been confirmed to comprehensively solve high NO_x emission and poor coal burnout for down-fired boilers with swirl burners. The influence of vent-air inclination angle (β_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 β_v (i.e., 10°, 20°, 28°, 40° and 50°) are performed by using a particle dynamic analyser in a 1:10-scale model of the full-scale improved boiler. With increasing β_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 β_v of 40° and 50° are still as high as 2–4 m/s, however, the vertical velocities of gas/particle flows basically decay to negative values for smaller β_v of 10° and 20°. At furnace section Y/Y₀ from 0.220 to 0.369, the vertical fluctuation velocities of gas/particle flows near water-cooled wall for β_v of 40° and 50° are significantly higher than those with β_v of 10°, 20° and 28°, and the particle volume flux at the same horizontal position X/X₀ constantly increases with increasing β_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 β_v. The appropriate inclination angle of vent air for the improved boiler varies from 40° to 50°.