Horizontal gas and solids mixing processes may play a dominant role in circulating 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 fluidized bed with a rectangular cross section. Since the combustion process is rather complex, the underlying gas-solid flow structure was modeled in a simplified semi-empirical way to keep the computational time within reasonable 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. Since no momentum balances are made up in the model, additional information is needed from measurements of the up and down flowing local solid mass fluxes. In the bottom zone a bubbling fluidized bed is assumed. As an application the combustion chamber of the 12 MWth,, CFB boiler of the Chalmers University of Technology is modeled. The results demonstrate the st:strong influence 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 future modeling of gaseous emissions.