Circulating Fluidized Beds

Abstract

Introduction As discussed in Chapter 9, dense-phase fluidization other than particulate fluidization is characterized by the presence of an emulsion phase and a discrete gas bubble/void phase. At relatively low gas velocities in dense-phase fluidization, the upper surface of the bed is distinguishable. As the gas velocity increases, the bubble/void phase gradually becomes indistinguishable from the emulsion phase. The bubble/void phase eventually disappears and the gas evolves into the continuous phase with further increasing gas velocities. In a dense-phase fluidized bed, the particle entrainment rate is low and increases with increasing gas velocity. As the gas flow rate increases beyond the point corresponding to the disappearance of the bubble/void phase, a drastic increase in the entrainment rate of the particles occurs such that a continuous feeding of particles into the fluidized bed is required to maintain a steady solids flow. Fluidization at this state, in contrast to dense-phase fluidization, is denoted lean-phase fluidization. Lean-phase fluidization encompasses two flow regimes, i.e. , the fast fluidization regime and the dilute transport regime. The fast fluidization regime is characterized by a heterogeneous flow structure, whereas the dilute transport regime is characterized by a homogeneous flow structure. The circulating fluidized bed (CFB) is a fluidized bed system in which solid particles circulate between the riser and the downcomer. The fast fluidization regime is the principal regime under which the circulating fluidized bed is operated. The operating variables for a circulating fluidized bed system include both the gas flow rate and the solids circulation rate, in contrast to the gas flow rate only in a dense–phase fluidized bed system. The solids circulation is established by a high velocity of gas flow. Note that the dilute transport regime is discussed in Chapter 11, in the context of pneumatic conveying.