Measurement of overall solids mass flux in a gas–solid Circulating Fluidized Bed

It is important to know the overall solids flux and solids Cycle Time Distribution (CTD) to properly characterize Circulating Fluidized Bed (CFB) systems. Although conceptually simple, estimating the solids mass flux in these closed loop systems, either via experiments or by using a model, is a non-trivial problem. In the present work, a non-invasive in situ technique is applied, which is sensitive, and can be used on a large-scale circulating flow systems. The technique is based on estimating the solids velocity and volume fraction distribution in a section (downcomer) of the CFB loop. Solids velocity is obtained by tracking a single radioactive particle as tracer, using a two detector setup, and the volume fraction distribution is obtained by γ-ray densitometry measurements. In the downcomer section of a 2-in.-diameter column, the solids flow was found to be close to a moving packed bed condition, with minimal random fluctuations in the solids volume fraction. Hence, only a negligible error is incurred by using the cross-sectionally averaged solids velocity and holdup values for the estimation of the solids mass flow. A calibration curve was established for the overall solids mass flux as a function of superficial gas velocity, and the mass flux values compared with those obtained from a timing and weighing method were within 4%.