The near wall dense ring in a large-scale down-flow circulating fluidized bed

Abstract

Radial profiles of local solids fraction at different axial positions were obtained through a dual-optical density probe in a 0.418 m ID and 18 m high large-scale circulating fluidized bed (CFB) downer under different operating conditions. Radial distribution of local solids fraction in large-scale downer is uniform in the central region but a relatively high concentration dense ring exists near the wall. The near wall dense ring is a typical characteristic in the downer and exists under all the examined operating conditions. Although the peak value and its radial position of the near wall dense ring will slightly change with operating conditions, the overall shape of radial solids fraction profiles remains unchanged. The absolute distance from the radial position of the peak value of solids fraction to the wall is relatively constant in different scale downers, and the proportion of the area of core region where solids distributed uniformly to the whole cross-section increases with the scale-up of downer. Experiments were carried out in a specially designed wall structure, and results show that the existence of the near wall dense ring is related with the wall of downer.