Design and experimental study of the centrally-located secondary air of a circulating fluidized bed

Described were two types of secondary air structure, i.e. horizontally arranged rod type and vertically arranged pillar type, both of which are located in the center of a furnace. On a cold-state circulating fluidized bed (CFB), the influence of the arrangement pattern, height and secondary air flow rate of the centered secondary air on the particle concentration distribution along the height direction inside the bed was studied and compared with that of the conventional wall-surface secondary air at the same height. It has been found that after the division of air into stages, the particle concentration below the staged secondary air increases with an increase of the secondary air flow rate. An increase in the secondary air height will be accompanied by the dense-phase zone at the bottom extending upwards. Among the secondary air arrangement patterns, the horizontal rod type secondary air arrangement pattern can make the concentration at the bottom attain a maximal value. A trajectory tracking of the jet flow shows that when the secondary air speed is high, the centrally located secondary air can be diffused to the wall-surface zones, but in the central area there is a lack of secondary air. This is unfavorable for the oxygen replenishment to the central area. The horizontal rod type centered secondary air jet flow, however, will swerve and deflect under the action of a lateral momentum when it enters into the bed and can form a relatively large diffusion area. The authors have also mentioned the problems meriting attention and requiring solutions when the centered secondary air is further applied in practical CFB boilers.