Modelling of the flow structure in circulating fluidized beds

Abstract

Radial solids flux profiles, at different axial positions, gas velocities, particle sizes and solids circulating fluxes, in a cold circulating fluidized bed were measured with two solids (sand and coal), type B of the Geldart classification. These profiles showed a core-annulus flow structure with upward solids flux in the core in a dilute suspension and downward solids flux in the annulus in a denser suspension. The upward and downward solids fluxes decreased with the height in the dilute region. This demonstrated there was a net solid transfer from the core to the annulus. A mathematical model for prediction the solids flow structure in the circulating fluidized bed was developed. The proposed model is based on a core-annulus flow structure, with solids rising in the core in a dilute suspension and solids descending in the annulus in a denser suspension with a constant solid velocity and voidage changing with the height. The model allows us to predict the external solid circulation flux in a riser as a function of the operating conditions. Moreover, it gives predictions on the internal flow structure at different riser heights. To solve the model it was necessary to know the solids flux rising from the dense region and the constant for the solid dispersion from the core to the annulus. Two equations are proposed to determine them as a function of operating conditions. The developed model together with the equations proposed in this work gave a good fit of experimental results.