4. Heat transfer in a circulating fluidized bed

Circulating fluidized-bed heat transfer takes place either at the riser's vertical walls, via tubed surfaces (membrane wall), or via tubular or platular exchangers arranged vertically at the top of the riser, or finally in an external bed, arranged on the solid's return circuit (external leg). These external beds are dense, bubbling beds.

Here, we will specify the transfer coefficients available for the vertical wall (membrane wall) and the tubular exchanger immersed in the riser core.

The associated transfer coefficients are essentially dependent on the local volume concentration of divided solids at the exchange surface.

As a first approximation, the bulk density of the gas-solid suspension is assumed to decrease between the dilute zone at the top of the bed and the dense zone at the bottom. The density ρ...