Thermal conversion of biomass is often carried out in packed-bed furnaces. Optimization of thermal efficiency and furnace emissions is an important goal, which requires accurate understanding of all physical and chemical effects in the reactor. A combined transient single particle and fuel-bed model is presented. The fuel-bed model is discretized, and a representative particle is chosen and discretized in a radial direction at each grid point. Mass, momentum and energy balances are solved for the entire system. Drying is modeled using an equilibrium approach, and primary pyrolysis is described by independent parallel-reactions. Secondary tar cracking, homogeneous gas reactions, and heterogeneous char reactions are modeled using kinetic data from literature. Simulations validated for single particles agree well with experimental studies. Simulation results for the combustion of a biomass bed are Presented for one set of furnace conditions.