Radial gas mixing in the upper dilute core of a circulating fluidized bed

Abstract

Based on the assumption of a core-annulus structure, radial gas mixing characteristics were experimentally studied in the core of the upper dilute zone of a pilot scale circulating fluidized bed (CFB), 0.4 m in diameter and 9 m in height. C0 2 as a tracer was injected continuously into the center of the bed by a point source. Radial tracer gas concentration profiles were measured in several planes downstream of the injection point. An analytical solution derived by Klinkenberg et al. ( Ind. Eng. Chem., 6 (1953) 1202) [1] for the description of gas mixing in turbulent single phase flow was successfully applied to determine the Peclet number, Pe r,c, for radial gas mixing in the core zone. Pe r,c was found to be independent of the superficial gas velocity u, which is in agreement with the gas mixing behaviour in turbulent single phase flow. A mean value of Pe r,c=465, which is also in agreement with other authors' measurements in single phase flow, was calculated from measurements at different solids rates G s, up to 70 kg m −2 s −1. The fact that Pe r,c was found to be independent of G s is attributed to the low mean solids concentration in the core zone and to the size of the particles, which is too small to cause a significant influence on the turbulent intensity of the gas phase.