Gas-particle two-phase turbulent flows are numerically studied in a 50 MWe circulating fluidized bed combustor. The dense flow of particles is modeled by the frictional stress models adopted from solid mechanics theory. and the dilute flow in the so-called rapid granular flow regime is modeled from the kinetic theory of granulat flow. At low concentrations the viscosity due to the effect of the presence of particles is modeled by means of a semiempirical viscosity for fluidized catalytic cracking particles The distribution of the velocity and concentration of par tidies in a circulating fluidized bed combustor is predicted Simulations indicate that the dense regime with a high concentration of particles is in the bottom part and the dilute regime with a low concentration in the upper part of the furnace The effect of secondary air on the flow behavior is analyzed, and the penetration length of the secondary air jet is computed in a 50 MWe circulating fluidized bed combustor.