Circulating fluidized beds (CFB) operate with a heterogeneous structure consisting of a solid-rich dense phase and a gas-rich dilute phase, the variational criterion and the critical condition of which are indispensable for CFB quantification. With such a strategy, a comprehensive model is developed on the basis of the energy-minimization multi-scale (EMMS) approach. The model is capable of predicting the saturation carrying capacity and the steady states at both the top and the bottom of an S-shaped axial profile. Comparison between the model prediction and available experimental data shows reasonable agreement. In addition, the overall strategy adopted in the EMMS approach is also discussed with updated concepts. It is concluded that the variational criterion (or something equivalent) in analyzing heterogeneous two-phase systems is indispensable unless the observation scale can be sufficiently small.