One of the most promising techniques of sulfur dioxide removal from flue gas is sorption using a fluidized bed of limestone. Two commonly considered schemes are the passing of combustion gases through the fluidized bed of limestone or actually burning fuel, e.g., coal, within such a bed (Zielke et al., 1970). Although the latter alternative seems to be more attractive and has been under intensive investigation (Horio and Wen, 1976;Zheng et at., 1982) the former scheme is also desirable since it can be designed to operate in conjunction with existing combustors. Recently, Wormser Engineering, Inc. (Kaplan, 1982) has developed a two-stage fluidized-bed coal combustor with the bottom stage being the coal combustor and the upper stage the limestone desulfurization reactor. The design appears to provide easier temperature control and better desulfurization efficiency compared to a single-stage, fluidized-bed coal-limestone combustor.The objective of this study was to model and simulate a fluidized-bed limestone desulfurization reactor by coupling a grain model for reaction kinetics with a fluidized-bed model based on the two-phase theory. The stack gas from a combustor containing sulfur dioxide was used as the fluidizing gas. The effects of the gas and solid flow rates, bed height, limestone properties, and operating temperature on the desulfurization efficiency were investigated. MechanismThe mechanism of the desulfurization reaction involves two consecutive steps, calcination of calcium carbonate followed by reaction of calcium oxide with sulfur dioxide, i.e., and It has been observed that reaction I is usually fast a t high temperature; however, reaction I1 decreases its reaction rate rapidly as sulfation proceeds. Mathematical FormulationThe simulation model developed in this study included a grain model and a fluidized-bed model. The grain model is useful for analysis of a reaction involving a gas reactant and porous particles. It generates the conversion data with respect to reaction time within a particle. The grain model used is essentially the one proposed by Hartman and Coughlin (1976) and is not described here. Fluidized-bed modelThe fluidized-bed model proposed is based on the two-phase theory and is designed for large particle systems (d, > 1 mm). The model has the following assumptions:1. The bed is composed of two phases, the bubble and the emulsion phases.2. The volumetric flux through the bubble phase is constant throughout the bed, and the rest of the gas flows through the emulsion phase.3. The gaseous reactant disappears according to a first-order reaction Based on these assumptions, steady state material balances on the gaseous reactant A in the bubble phase and emulsion phase have the following forms:

Dynamic characteristics of solids mixing between coal and limestone were studied. Experiments were carried out in a 0.203 m diameter cold fluidized bed with coal and limestone of different sizes as the fluidized particles. Experimental parameters examined included operation time, air Row rate, bed height, initial bed setup, relative particle size and relative amount of the two particles. The coal concentration profile was analyzed through bed sectioning, and a mixing index was used to describe the profile. The observed results were presented graphically and in correlation forms. The steady state results were also compared with those available in the literature.