Bottom Zone Flow Properties of a Square Circulating Fluidized Bed with Air Staging

Abstract

The recycled solids fed to the circulating fluidized bed (CFB) riser on one wall have a limited lateral dispersion rate, which results in an uneven solids distribution in the bottom zone of the bed. Using a reflective optical fiber solids concentration measuring system and visual observation, the flow properties in the bottom zone of a fluidized bed was investigated. The cold model CFB riser has a square cross-section of 0.25m×0.25m and a height of 6.07m with quartz sand as bed material (dp=276μm). At the condition of u 0=4m/s, Gs=21kg/m2s, the back feeding particles can penetrate the gas-solid flow in the riser and get to the opposite wall. It makes a denser region near the front wall of the bed. When air-staging is adopted, solids holdup increases below the secondary air (SA) injection ports with the increase of the rate of the secondary air to the total air (SAR). The solids holdup increase also exists above the secondary air injection ports but the incremental amount decreases quickly along the riser. At SAR=0.3, a denser bottom zone limits the back feeding particles’ penetration and a relatively even solids distribution is formed in the bottom bed. A W-shaped solids concentration profile is formed just above the SA injection level. At SAR=0.5, uneven solids concentration profile exists in the bottom bed and no W-shaped solids concentration profile is formed for the deeper penetration of SA jets.