Radial and axial voidage distributions in circulating fluidized bed with ring-type internals

Non-uniform radial and axial flow structures are characteristic of circulating fluidized beds (CFB). As a result, gas solids contact is still relatively poor in the riser although much better than in the bubbling bed. In this study, the influence of ring-type internals on the reduction of nonuniformity in a riser of 7.6 cm in diameter and 3 m in height was investigated. Three ring internals with open areas of 70%, 90% and 95% were installed in five axial positions in the riser. Measurement results for solids hold-up distribution of sand particles obtained using a fibre optic probe indicate that the internals can reduce radial nonuniformity and evenly redistribute solids in the radial direction. Under high superficial gas velocities and low solids circulation rates, reversed radial solids distribution profiles with more dilute flow in the wall region than in the core region were also observed. A detailed study on the radial solids distributions around the ring internals shows the formation of a denser region above the 70% opening ring as compared to above the 90% and 95% opening internals and a more dilute region below all three rings. Results of the axial voidage distribution with the presence of internals show the formation of a zigzag type axial profile instead of the regular S-shape profile.