Fundamental aspects of coal combustion in the two types of atmospheric fluidized combustors, i.e. those based on bubbling and on circulating technologies, have been studied in order to determine those variables which play a dominant role in the reactor performance. The analysis is based on two relatively simple models, which take into account carbon surface reaction, intraparticle and external diffusion as well as phenomena of comminution of coal and its char. Performances of the two types of fluidized-bed combustors are discussed with reference to coal combustion only, without taking into consideration the emission issues. Sensitivities of main output variables to changes in fuel properties, design and operating variables are evaluated.