Radar-Based Measurements of the Solids Flow in a Circulating Fluidized Bed - Back-Mixing and Clustering

This work provides an experimental investigation of the solids back-mixing in the freeboard of circulating fluidized beds. The use of radar-based measurements (a novel technique for fluidized beds) provides detailed data on the velocity distribution of the solids phase, which allows quantification of the two main solids back-mixing mechanisms: clustering mainly in the core region of the riser and downward flow of solids at the riser walls forming wall layers. The experiments are carried out in a circulating fluidized bed riser with a height of 3.1 m and 0.45 m² in cross section operated with Geldart B solids at room temperature and involve both radar diagnostics (for the solids velocity distributions) and pressure sampling (for the vertical profile of solids concentration). With this data, mass balance closure of the solids flow can be applied from which both the mass transfer coefficient for the core-wall layer exchange and the volumetric rate of cluster formation/destruction can be derived. Having this, the effect of flow parameters (gas superficial velocity and solids concentration) on the transfer of solids from the core to the walls and the clustering rate are investigated and discussed.

Keywords: Circulating fluidized bed; Radar technique; solids mixing; Discrete phase flow; Cluster phase flow