Measurement of solids circulation rate in a high-temperature dual fluidized bed pilot plant

Abstract

A number of fluidized bed reactor processes operating at high temperature require that solid particles be circulated back and forth between two reactor vessels. Since the circulation rate strongly affects mass and energy balances, and therefore greatly influences hydrodynamics and performance of the system, a reliable technique for its accurate measurement would be helpful in monitoring and modeling the process. However, there are no reported techniques suitable for measuring solid circulation rates at elevated temperatures typical of gasification systems.A novel thermal-tracing technique was developed for measuring the solids circulation rate between two reactors. Particles at room temperatures (cold particles) are injected into a downward-moving packed bed of solids at elevated temperature (hot particles), creating reduced-temperature zones inside the moving bed. The transit time of the cold-particle-clusters between pairs of thermocouples is determined by cross correlation allowing the flux to be estimated. The technique was shown to provide sensitive and reproducible data for a cold model unit with injection of dry ice. The technique was then applied to determine the solids circulation rate between the bubbling bed gasifier and the riser combustor of a pilot scale dual fluidized bed gasification system. A number of conditions are imposed on the data to eliminate unsatisfactory data at high temperatures. Data which satisfy the discrimination criteria are shown to lead to measured solids circulation fluxes up to 133kg/m2-s at temperatures up to 856°C in the gasifier test section. The technique provides high-temperature solids circulation rate information beyond the capability of other techniques.