A numerical code to calculate the burn-out time of a single biomass particle in a fluidized bed reactor was developed. Combustion modeling was performed by numerical calculation of energy and mass balances on the particle. Kinetic data for pyrolysis and char combustion stages, as determined by thermogravimetric analysis, were used. The results showed that the time needed for the completion of each partial combustion stage is less for biomass than for brown coal. The same remarks hold for all biomass species studied, whereas the predictions of the simulation model were in agreement with the results of the efforts of other researchers. The effect of uncertain parameters on model predictions was investigated, and it was proved that the particle diameter, moisture content, oxygen concentration, and especially the gas temperature have the greater influence on the calculated results. The combined use of the experimental data and the theoretical calculations provide a complete view of biomass/waste combustion behavior, which is absolutely necessary for the design and operation of large-scale units.