The solid flow structure in a circulating fluidized bed riser/downer of 0.42-m diameter

Local transient solid fraction and particle velocity signals were measured in a 0.418-m I.D. and 18-m high large-scale circulating fluidized bed (CFB) by a dual-optical fiber density probe and a Laser Doppler Velocimeter (LDV) system. The probability density distributions (PDD) of transient solid fraction and particle velocity show bimodal distributions in the riser, while in the downer, the PDD curves only exhibit single peak distribution. The results indicate that two-phase solid flow structure exists in the riser: i.e. a dense cluster phase and a dispersed particle phase. Although there are solid aggregations in the downer, the solid concentration of which is much lower than that in the riser, and they could not form a dense cluster phase with solid fraction 1−εmf as those in the riser. From the analysis and comparison of transient solid fraction signal and the PDD of solid fraction and particle velocity in the riser and downer, it is clear that significant difference exist in the solid flow structure and the particle cluster mechanism between the gravity-assisted gas–solid flow in the downer and the against-gravity flow in the riser.