Biomass and municipal solid wastes are two of the sustainable energy resources. Countercurrent, substoichiometric conversion of these fuels into energy in packed beds has the benefits of lower tar and dust carryover in flue gases, and therefore gas-treatment procedures can be simplified and the running cost is reduced. In this paper, mathematical simulation is performed on a large-scale plant furnace for both municipal solid wastes and biomass fuels in a range of operating conditions. The temperature profile inside the moving packed bed of the same system was measured by an in situ electronic device for the baseline case to partially validate the simulation results. Mathematical simulation indicates that by using variable grate speed to obtain constant bed height, a typical moving-bed furnace can achieve over 99% of conversion efficiency with only 40-50% of the normal primary air supply, without loss in throughput; the char conversion rate is significantly lower than the devolatilization rate in the bed and the char conversion process occupies over half of the total bed length, whereas fuel devolatilization occupies only one-third or more of the bed length.