Circulating fluidised bed hydrodynamics

Analysis is given for flow regimes in the riser of a circulating fluidised bed (CFB) in the range of gas velocities (2-8 m/s) and particulate solids f luxes (15-90 kg/m(2) s) appropriate for a CFB combustor. It is shown that t he simplest version of the core-annulus model gives the correct magnitudes of slip velocity and average solids hold-up. The anomaly in the core-annulu s model, whereby diffusion of particles is towards the region of high solid s concentration, is ascribed to the presence of clusters, diffusion towards the wall occurring in the dilute regions between clusters. The clusters ar e conjectured to be fragments of the relatively dense particle film falling near the wall: this film is shown to have features in common with a film o f ordinary liquid flowing down a vertical surface. Implications for scale-u p are discussed, using published data showing that very large CFBs exhibit core-annulus flow. (C) 2000 Elsevier Science S.A. All rights reserved.