Simulation of 2-D Temperature Distributions in Fluidized Bed Reactors for Highly Exothermic Reactions

Abstract

Fluidized bed reactors are generally known for their temperature uniformity, but with highly exothermic reactions, significant temperature gradients may occur. The generation of temperature distributions inside the fluidized bed is the result of a competition between two mechanisms: local feeding of reactants creates heat of reaction locally, and this heat must be dispersed inside the bed by solids mixing. The present work investigates these mechanisms for the case of coal combustion in pressurized bubbling fluidized beds. With a simple one-dimensional carbon mass balance, the governing parameters for the generation of temperature profiles are deduced. For the quantitative calculation of temperature, carbon and oxygen distributions, a two-dimensional model was formulated. In pressurized fluidized beds, the fuel is often fed as a coal-water slurry, so the drying of the fuel also has been considered in this model. The model has been validated with measurements made in the pressurized fluidized bed combustor (PFBC) pilot-scale test unit of Deutsche Babcock. The results are in good agreement with measurements for full-load as well as part-load conditions. Feeding the coal as a slurry is shown to be a means to flatten the temperature distribution because the slurry agglomerates are dispersed in the fluidized bed while drying. Further simulation results point at and quantify critical parameters that have to be considered in the scaleup of pressurized fluidized bed combustors.