GAS-SOLIDS MIXING AND FLOW STRUCTURE MODELING OF THE UPPER DILUTE ZONE OF A CIRCULATING FLUIDIZED-BED

A steady-state model of the flow structure and mixing processes in the upper dilute zone of a circulating fluidized bed (CFB), where variati ons in flow parameters with height are neglected, is presented. The mo del is based on a two-phase structure, consisting of dense particle cl usters or strands and a lean gas-particle suspension as observed in se veral investigations. It accounts for vertical flow of gas and particl es in both phases, as well as for gas and particle dispersion in the l ean phase and interphase mass transfer. With the aid of fibre optical measurements of local solids volume concentration, a set of equations is developed that allows calculation of gas and particle velocities an d local volume fractions of the phases for given operating conditions of the CFB. Dispersion and interphase transfer coefficients are determ ined by fitting measured and calculated temperature profiles that were generated by local electrical heating. The results indicate that part icle dispersion is significantly influenced by particle-particle colli sions and that an intensively coupled heat and mass transfer between t he dense clusters and the lean phases exists. The complete model, with all parameter values identified, is applied to two experiments in whi ch heat transport in the upper dilute zone of the bed is investigated under electrical and radiant heating, respectively.