CFD simulation of the effects of a suspension section on the hydrodynamics of a circulating fluidized bed

Abstract

Gas–solid two-phase flow in a circulating fluidized bed (CFB) is affected by operating conditions (e.g., superficial gas velocity, solids inventory), material properties and geometric factors, such as the entry and exit configuration. In particular, the suspension section, which is located between the riser bottom and the solids recycle inlet, affects the hydrodynamics in the riser significantly. However, the suspension section has received less attention compared with other geometric factors. Most computational fluid dynamics (CFD) simulations, especially two-dimensional simulations do not take this factor into account. We performed three-dimensional, full-loop CFD simulations with a drag coefficient that was determined by the energy-minimization multi-scale model, and investigated the flow behavior of two CFBs with different suspension-section lengths. The simulation results revealed that the axial profiles of voidage in the riser with a longer suspension section are more likely S-shaped, whereas those with shorter suspension sections decay exponentially. The dependences of solids flux on solids inventory differ in the two CFBs. A shorter suspension section may result in a smooth transition from dilute to dense transport without intermediate accumulative choking, whereas a longer suspension section may lead to a choking transition. These simulation results are qualitatively consistent with the flow behaviors described in literature.