Lagrangian simulations are performed to investigate the process of fuel mixing in fluidized-bed energy converters. The computations are carried out for a narrow (0.4 m) bed and a wide (1.2 m) bed. Movement of a limited number of large and light particles in a bulk of heavy and small particles is studied using a multigrid technique that our group proposed earlier [M. Farzaneh et al. Chem. Eng. Sci. 2011, 66, 5628-5637]. Preferential positions and the dispersion coefficient of the fuel particles are obtained under different operating conditions. In addition, detailed information on the motion of the fuel particles in the form of upward and downward velocity is obtained. Furthermore, in an attempt to investigate the effect of the inlet boundary conditions on the process of fuel mixing, two boundary conditions are employed: a uniform velocity profile at the air distributor and a nonuniform velocity profile obtained by including the air supply system in the computational domain. It is observed that the numerical simulations that include the air supply system in the computational domain improve the prediction of the hydrodynamic behavior of the bed. However, regarding the averaged movement pattern of the fuel particles, the effect of the boundary condition employed is not significant in the 0.4-m bed. As for the wide (1.2 m) bed, the simulation results differ substantially from the experiments when the uniform velocity profile is employed as the inlet boundary condition. Including the plenum in the simulations considerably improves the results; however, they are still not in perfect agreement with the experiments.