A fully predictive mathematical model has been developed to describe the behavior of liquid fluidized beds in which the biological solubilization of coal particles is occurring. This model is based on the particle mass-transport mechanisms of dispersion and convection, and accounts for the changes in the size of the particles as they are solubilized. Two different cases are compared : one in which the bed is replenished with large coal particles and one in which small particles are fed to the bed. The simulation results indicate that replenishment with small coal particles enhances the overall solubilization rate without significantly increasing the mass elutriated.