Flow behaviors in a circulating fluidized bed with various bubble cap distributors

Solids volume concentration was determined in an 8.5-m-high circulating fluidized bed (CFB) riser with two types of bubble cap distributors by using capacitance probes. In the bottom region of the CFB, the lateral solids volume concentration in the center region is low with slight change, whereas lateral solids volume concentration increases significantly toward the wall with the highest solids concentration at wall. Furthermore, lateral solids volume concentration at high-pressure drop of a bubble cap is lower than that at low-pressure drop of a bubble cap. Horizontal solids volume concentration in the center region remains flat and rises toward the wall. However, horizontal solids volume concentration at entrance region ranges from 0.43 to 0.5 at low superficial gas velocity. The pressure drop of a distributor has little influence on lateral profiles of solids volume concentrations in CFB upper dilute region. It has been found that the pressure drop of the distributor has little influence on the axial apparent solids concentration in upper dilute region. The bubble cap-type design with a duct where gas is flowing downward to the gas outlet is the recommended structure to avoid solids backflow.