An innovative combustion technology for a down-fired boiler with swirl burners: Gas/solid flow characteristics with various burner injection angles

Abstract

Abstract: The eccentric-swirl-secondary-air combustion technology (ESSACT) is an innovative technology to comprehensively solve high NOx emission and unburned carbon content in fly ash of the down-fired boiler with swirl burners. In this study, a breakthrough idea (i.e. decreasing burner injection angle (θb)) is proposed to further greatly improve coal burnout of the improved boiler. Using a laser particle dynamic analyzer, the distributions of average and fluctuation velocities of gas/solid (GS) flows, distribution and decay of particle volume flux and jet trajectory of burner primary air in a 1:10 scale model of full-scale furnace at different θb of 0°, 8°, 15° and 25° are acquired. The effects of θb on GS flow characteristics (i.e. downward depth of GS flows, recirculation zones below arches and air flow fullness in the lower furnace) in the furnace are intensively studied. As the θb decreases, the maximum vertical velocity of GS flows increases in the whole lower furnace, and the vertical fluctuation velocity of GS flows and the turbulent intensity and diffusion also increase. With decreasing θb, maximum recirculation velocity and dimension of recirculation zone below arches both increase continuously, which is conducive to improving ignition and burnout of pulverized coal. Especially, the maximum recirculation velocity with θb decreasing from 25° to 15° and the dimension of recirculation zone below arches with θb decreasing from 15° to 8° increase more obviously. With decreasing θb, the ejection effects of vent air and staged air on GS flows from arches are more effective. In the region below staged air, the maximum particle volume flux for the θb of 0° and 8° is more than twice that for the θb of 15° and 25°. As a result, the downward depth of GS flows and air flow fullness in the lower furnace both increase with decreasing θb. To improve coal burnout of the down-fired boiler applying ESSACT, the θb is recommended to be set below 8°.