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Modeling of Solids and Gas Mixing Effects in Large-Scale CFB Combustors

[+] Author Affiliations
Karsten Luecke, Ernst-Ulrich Hartge, Joachim Werther

Technical University Hamburg-Harburg, Hamburg, Germany

Paper No. FBC2003-028, pp. 245-256; 12 pages
doi:10.1115/FBC2003-028
From:
  • 17th International Conference on Fluidized Bed Combustion
  • 17th International Conference on Fluidized Bed Combustion
  • Jacksonville, Florida, USA, May 18–21, 2003
  • Conference Sponsors: Advanced Energy Systems
  • ISBN: 0-7918-3680-0 | eISBN: 0-7918-3675-4
  • Copyright © 2003 by ASME

abstract

In a CFB combustor the reacting solids are locally fed into the combustion chamber. These reactants have to be dispersed across the reactor’s cross-sectional area. Since the rate of mixing is limited this leads to a mal-distribution of the reactants and to locally varying reaction conditions. In order to describe the influence of mixing a three-dimensional model of the combustion chamber is suggested here. The model is divided into three sub-topics. First, the flow structure in terms of local gas and solids velocities and solids volume concentrations is described. Second, mixing of the solids and the gas phase has to be quantified by defining dispersion coefficients, and finally the combustion process itself, i.e. the reaction kinetics, has to be modeled. Employing the information of the three sub-models mass balances for the reactants at each finite control volume inside the CFB combustion chamber can be formulated. The model was validated against data from measurements in the large-scale combustor of Chalmers University of Technology in Göteborg/Sweden. Concentration gradients concerning the char phase are only moderate. However, the spatial distribution of the oxygen shows strong non-uniformities, especially under conditions of staged combustion. In further predictive calculations, the influence of the fuel supply arrangement on the emissions of industrial sized CFB boilers was studied. Furthermore, the influence of the fuel composition on the feeding technique has been examined. High volatile fuels tend to form plumes of unburned hydrocarbons near the fuel feed point, and might therefore need more feed points per square meter cross-section area. Since the average gas residence time in the primary cyclone of a CFB plant is about 30–40% of the total gas residence time, a considerable burn-off of not completely oxidized gas species may occur here. An effectively used cyclone may remedy to a certain extent the negative impacts of incomplete mixing in the combustion chamber.

Copyright © 2003 by ASME

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