Necessary parameters for specifying the hydrodynamics of circulating fluidized bed risers - a review and reiteration

Gas velocity U-g and solids circulation rate G(s) are still being commonly treated as the only parameters determining the hydrodynamics of circulating fluidized bed (CFB) risers with valve-controlled solids circulation. Through reviewing research literature about the influence of solids holding height in the downcomer, the downcomer bed height, and that of solids flow control valve, the present article confirms that different axial voidage profiles might prevail in a riser even under specified U-g and G(s). New experiments were conducted to further clarify when and why U-g and G(s) alone are insufficient to define the gas-solid flows in such CFB risers. Three different flow patterns, i.e. dilute suspension, axially nonuniform flow and fully dense flow, and their respective parametric dependencies of hydrodynamics are clarified. As a consequence, the article reiterates that the downcomer bed height or solids inventory, and the operational status of the solids flow control valve, including its opening and aeration, have to be specified in addition to U-g and G(s) when analyzing the gas-solid flows in CFB risers. This results in the general steps necessary to compute the axial voidage profiles of such risers. (C) 2003 Elsevier B.V. All rights reserved.