After the secondary air is used in circulating fluidized bed, generally the bed is artificially divided into two areas: the dilute phase zone at the upper part and the dense-phase zone at the lower part. 

The secondary air outlet is arranged above the dense-phase area. The functions of dense-phase area are: 

1. Hold some big particle fluidization and complete combustion. 

2. Provide a device for storing heat, in order to enable the cold fuel into the combustion chamber and then stably warm and fire. The existing of dense-phase enables the stability of the bed when the load is increased or decreased.