SOLIDS CIRCULATION IN THE JETTING REGION OF A FLUIDIZED BED.

摘要

The purpose of this study was to analyze particle circulation, and reactions in the jet region of a fluidized bed. The two fundamental questions addressed here related to the measurement of particle velocities in the grid region of a fluidized bed, and to the relevance of the jet zone to the operation of a fluidized bed reactor.; An optical fiber probe anemometer, capable of measuring the velocities of solids moving in two-phase flows, was constructed. Three optical fiber probes were designed: one suitable for use in measuring the local average velocities of solid particles moving in flat beds; another, to measure instantaneous velocity; and a third, having high directional sensitivity.; Particle velocities were measured at corresponding points with a jet in a two-dimensional bed, using the optical fiber probe, and also using a Laser Doppler Velocimeter (LDV). The probe measurements compare well with the LDV results. Particle velocities were also measured using the probe, in the dense phase region of the bed, where measurements were not possible using the LDV. The resulting flow maps clearly indicate solids circulation patterns around jets, and identify dead zones formed on the distributor plate. The measurements also show that the assumption of two-dimensional behavior in flat beds is valid, except in the immediate neighborhood of grid orifices, since no significant velocity differences were observed across the thickness of the bed.; A mathematical model for solids motion in the inlet jet was developed from first principles, based on the particle momentum balance and continuity equations. The simulation results compare well with the experimentally measured particle velocities, when gas percolation from the spout to the annulus is taken into account.; A gas jet entering a fluidized bed coal combustor, was modeled. Typical conditions for a fluidized bed combustor were used, and the model includes the reactions between carbon and carbon dioxide and oxygen, and the homogeneous oxidation of carbon monoxide. The results indicate that considerable reaction occurs in the gas jets entering a combustor, and that they should be considered in any careful analysis of its operation.