Lateral non-uniformities of solids and gas concentrations in fluidized bed reactors

Abstract

It is the interrelation between mixing of solid particles and gas—solid reactions in fluidized beds which is of particular significance for the design and operation of fluidized bed reactors for non-catalytic gas—solid reactions. A model for the calculation of the distribution of a solid reactant over the cross-sectional area of the fluidized bed under conditions of gas—solid reactions is developed. The general tendencies are illustrated with the examples of coal gasification and coal combustion, respectively. Detailed calculations have been carried out for the combustion of coal in an atmospheric fluidized bed combustor. As reactions, the combustion of char and volatiles and the release and capture of sulfur dioxide are taken into account. The steady state size distribution of the coal particles in the combustor and an effective reaction rate constant based on this size distribution are evaluated. The influence of the mechanisms of attrition and elutriation as well as that of the Sherwood number on this rate constant are shown. The performance of the model is demonstrated by an application of the 20-MW pilot plant of Tennessee Valley Authority to the geometry and the operating conditions. On this plant detailed measurements of oxygen, carbon dioxide and sulfur dioxide profiles above the bed surface have been carried out. A good agreement between these measurements and the predictions of the model is obtained.