Fluidized-bed combustion is a very popular technology used to convert solid biomass into thermal energy, which has many advantages over other conversion technologies. However, these advantages strongly depend on the freeboard combustion. This study focuses on the effect of turbulence of the freeboard via developing a comprehensive mathematical model including four-way coupling, biomass thermal decomposition, and turbulent combustion by means of an Eulerian-Lagrangian framework. The validated model predicts with significant accuracy for all gas species when turbulence modeling is improved. The optimum amount of the secondary air ratio and the optimum excess air ratio were determined, which give a higher freeboard temperature while maintaining minimum emission.