A model was previously developed to predict the combustor performance of an internally circulating fluidized bed reactor (ICFB). The model was validated with experimental data obtained during the thermal treatment of different industrial wastes in a 20 kW ICFB pilot unit. The model gave good agreement with experiments without the use of adjustable parameters. Simulation is made to analyze the behavior of reactor depending on different design and operating variables as well as waste properties. The variables most affecting the ICFB performance are solids circulation flux, gas-solid feedrates, riser diameter, working bed temperatures and waste properties. The ratio, G(s)A(r)/W-feed, is a critical parameter for design and operation. A small annular bed volume is most appropriate for rapid bed heatup and to rapidly reach steady-state operation, which is important for a unit that needs frequent shut-downs and start-ups or load changes. Practically, very high aeration has an adverse effect on the formation of thermal NOx as a consequence of enriched oxygen concentration. Hence, it is necessary to find optimum operating air flow rates and aeration rates accounting for organic destruction, gaseous emissions and bed temperatures.