Experimental study of gas–solid flow over a novel heating surface in a CFB furnace

Abstract

As the capacity of a circulating fluidized bed (CFB) boiler is increased, the surface area of the wing walls must be increased to maintain the heat balance in the furnace. The minimum spacing requirement between adjacent wing walls limits their total area. A novel suspended heat transfer surface is proposed to address this problem. The suspended surface is designed with a “U” shape and is referred to as U-wall. U-walls are suspended from the top of the furnace and may provide more than 25% greater heat transfer surface area. In this study, the flow of solids over U-walls in a CFB was measured using a fiber optical probe. The effects of the superficial gas velocity, the bed material inventory, and the width of the U-walls on the solid volume fraction and particle velocity were investigated. The solid volume fraction had a U-shaped profile inside the U-wall except in the upper section, where the profile had an approximate straight-line shape. Their values increased with the superficial gas velocity and the bed material inventory. The solid vertical velocity had an inverted-U-shaped profile inside the U-wall. The solids were found to flow downward at the U-wall surface in the upper section, where the velocity increased with the superficial gas velocity and the solid suspension density. The flow of solids at the U-wall surface in the middle and lower sections changed from downward to upward as the superficial gas velocity increased. In a CFB furnace, there is an optimal width for U-walls that provides the best heat absorption characteristics; larger or smaller widths are both undesirable.