Mixing and reaction in the circulating fluidized bed - A three-dimensionalcombustor model

Horizontal gas and solids mixing processes may play a dominant role in circ ulating fluidized bed (CFB) reactors with a small height-to-diameter ratio, which is typical for CFB combustors. A semi-empirical approach was chosen to describe the three-dimensional combustion of coal in a circulating fluid ized bed with a rectangular cross section. Since the combustion process is rather complex, the underlying gas-solid flow structure was modeled in a si mplified semi-empirical way to keep the computational time within reasonabl e limits. In the upper part of the CFB the flow domain is divided into two phases, a dilute up-flowing suspension and a dense downward flowing cluster phase. For both phases the mass balances for gas and solids are solved. Si nce no momentum balances are made up in the model, additional information i s needed from measurements of the up and down flowing local solid mass flux es. In the bottom zone a bubbling fluidized bed is assumed. As an applicati on the combustion chamber of the 12 MWth,, CFB boiler of the Chalmers Unive rsity of Technology is modeled. The results demonstrate the st:strong influ ence of the coal feed and the solids return on the spatial distributions of volatiles, char, oxygen and carbon monoxide. Depending on the contents of volatiles and on the reactivity of the char the distributions of reactants and reaction products are far from being uniform over the cross section of the combustion chamber. The model is intended to serve as a basis for futur e modeling of gaseous emissions. (C) 1999 Elsevier Science Ltd. All rights reserved.