SIMULATION OF 2-D TEMPERATURE DISTRIBUTIONS IN FLUIDIZED-BED REACTORSFOR HIGHLY EXOTHERMIC REACTIONS

Fluidized bed reactors are generally known for their temperature unifo rmity, but with highly exothermic reactions, significant temperature g radients may occur. The generation of temperature distributions inside the fluidized bed is the result of a competition between two mechanis ms: local feeding of reactants creates heat of reaction locally, and t his heat must be dispersed inside the bed by solids mixing. The presen t work investigates these mechanisms for the case of coal combustion i n pressurized bubbling fluidized beds. With a simple one-dimensional c arbon mass balance, the governing parameters for the generation of tem perature profiles are deduced. For the quantitative calculation of tem perature, 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 measuremen ts 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 distributio n because the slurry agglomerates are dispersed in the fluidized bed w hile drying. Further simulation results point at and quantify critical parameters that have to be considered in the scaleup of pressurized f luidized bed combustors.