Radar-based measurement of solids back-mixing in the freeboard of a circulating fluidized bed

Abstract

This work investigates solids back-mixing in the freeboard of a circulating fluidized bed based on THz-radar measurements of the concentrations and velocity distributions of the solid particles along the height of the riser. These data allow height-resolved closure of the solids mass balance and, thereby, quantification and further insight of the two main mechanisms for the back-mixing of the solids entrained from the bottom region of the fluidized bed: (i) solids disengagement and backmixing within the core region of the riser cross-section,; and (ii) solids lateral transfer of from the core region to the wall layers. The experiments were carried out in a circulating fluidized bed riser (3.1 m in height and 0.45 m2 in cross-section), which was operated with Geldart B solids fluidized with air at room temperature and for different gas velocities. The experimentally-derived data are expressed in terms of the disengagement rate and a lateral core-to-wall layer mass transfer coefficient. From the results, it is estimated that the presence of disengagement-based solids back-mixing is significant all along the 3-m riser. The disengagement rate shows a non-linear dependency on the solids concentration, with the lateral solids transfer to the walls (which follows a linear dependency on the solids concentration) eventually becoming the dominant form of back-mixing at upper heights.