Hydrodynamic scaling and solids mixing in pressurized bubbling fluidized bed combustors

A quarter-scale cold model of American Electric Power’s 70 MW. Tidd pressurized fluidized bed combustor (PFBC) has been constructed based on a simplified set of scaling parameters. Time-varying pressure drop data from the hot combustor and the cold model were used to compare the hydrodynamics of the two beds. Excellent agreement between the dimensionless probability density functions, the mean solid fraction profiles, and the bed expansions, provide a verification of the scaling parameters for commercial bubbling PFBC. Some controversy has surrounded the importance of matching the solid-to-gas density ratio when scaling bubbling beds. Hydrodynamic scaling comparisons were conducted with all the scaling parameters matched with the exception of the density ratio. The comparisons indicate that to reliably scale the hydrodynamics of bubbling beds it is essential to match the solid-to-gas density ratio. Bubbles provide the motive force for solids mixing in bubbling fluidized beds, prompting an investigation of the bubble characteristics in the cold model of the Tidd PFBC. A unique optical bubble probe design was used to measure bubble rise velocities, mean pierced lengths, and bubble frequency. Gas through-flow and bubble-growth rates appear to be significantly lower in pressurized beds than in atmospheric fluidized beds. A thermal tracer technique has been implemented in the cold model of the Tidd PFBC. The technique involves thermally tagging bed particles, injecting them into the bed, and tracking their motion using an array of thermistors, The thermal tracer data suggest that the tube bank within the bed restricts solids mixing, making adequate mixing in the tubefree zone at the bottom of the bed of paramount importance. Increasing gas superilcial velocity is shown to increase both axial and lateral mixing beneath the tube bank. A mechanistic model of solids mixing in bubbling fluidized beds has been developed. Axial solids mixing is attributed to bubbles transporting solids vertically as they rise to the surface of the bed, while lateral mixing is associated with the lateral motion of bubbles as

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