Hydrodynamic characterization of a tapered gas–solid bed without a gas distributor

Abstract

The tapered fluidized bed (TFB) without a gas distributor has long been used as an atmospheric coal gasifier by taking advantage of its simplicity in configuration and maintenance and also its ease in discharging bottom ash. This study investigated the hydrodynamic characteristics of such a bed fluidizing quartz sand of 156μm in mean diameter to understand if it enables the fully uniform fluidization of its particle bed. The changes in pressure drop and voidage in the bed were measured under varied conditions. Three flow regimes were identified by gradually increasing the superficial gas velocity through the bed. The gas–solid flow in the bed can be divided vertically into three gross sections of gas convergence, gas diffusion and particle elutriation, and laterally into two zones of annulus and core. Vertically along the bed the shape of the radial voidage profile changed gradually from an “M” shape to a “parabola” by increasing the superficial gas velocity from 0.17m/s to 0.35m/s. The axial voidage profile at the bed center showed the “S” shape to distinguish the above-mentioned three vertical sections. These indicated the existence of non-uniformity of particle fluidization in the bed, and changing the arrangement of gas jets into the bed did not fully resolve the problem. The article found also that the major hydrodynamic parameters of the tested TFB using only one layer of gas jet to supply the fluidizing gas could be predicted by modifying their empirical correlations known for the TFB with a gas distributor.