A pressure balance model for a circulating fluidized bed (CFB) unit that incorporates a downer has been proposed. A comparison of the operation stability between a CFB downer and a CFB riser showed that, although only one critical operating gas velocity exists in the CFB riser for a given riser solids flux, there can be many critical gas velocities for the operation of a CFB downer. Therefore, it is possible to achieve high solids concentration in a CFB downer if appropriate operating conditions are used. Parameters that affect the solids concentration in the CFB downer have been studied using the model and have been validated with experimental data obtained from a special CFB unit that incorporates a downer. Measures to increase the solids concentration in the downer have been proposed.