A model to predict the carbon combustion efficiencies in circulating fluidized beds (CFB) previously developed was validated with experimental data obtained in the combustion of different coals in a CFB pilot plant with a 20 cm internal diameter and 6.5 m height. The model gave a good fitting without using any adjustable parameters. A simulation, from the point of view of carbon combustion efficiencies, to analyze the behavior of circulating fluidized bed combustors depending on different design and operating variables was made. The variables most affecting the carbon combustion efficiencies were the pressure drop or the solid inventory in the combustor, the coal reactivity, and the particle size distribution through the coal particles of small size fed in the combustor. Likewise, a good design of the cyclone is necessary to obtain high carbon combustion efficiencies.