Abstract
An internally circulating fluidized bed (ICFB) reactor was used to study annulus bed-to-wall heat transfer. The inner diameter and height of the fluidized bed were 9 cm and 2.5 m, respectively. A concentric draft tube was located at the bottom of the bed. The heat transfer coefficient was measured by a heat transfer probe system similar to that of Basu et al. (1987). The effects of the physical properties of the bed materials, orifice diameter, orifice number at the same total orifice area, distance of the draft tube distributor to the orifice, static bed height, and superficial gas velocity at the annulus inlet on the annulus bed-to-wall heat transfer coefficient were investigated.It was found that at a fixed gas velocity to the annulus, the annulus bed-to-wall heat transfer coefficient increased initially, reached a maximum value, and then decreased with increasing gas velocity in the draft tube. In addition, the heat transfer coefficient increased with decreasing distance of the draft tube distributor to the orifice, and particle diameter. A higher static bed height caused more solid backflowing to the draft tube, which decreased the solids circulation rate and the annulus bed-to-wall heat transfer coefficient. Besides, the annulus bed-to-wall heat transfer coefficient increased with increasing particle thermoconductivity coefficient and orifice number. A larger orifice diameter had a lower resistance for the solids circulation between the draft tube and the annulus, which resulted in a higher annulus bed-to-wall heat transfer coefficient. When the annulus inlet superficial gas velocity was greater than umf, excess gas might form bubbles, which reduced frequency of the solids contacting with the heat transfer surface. As a consequence, the heat transfer coefficient decreased. Furthermore, there was a maximum annulus bed-to-wall heat transfer coefficient at Red=14.75. An empirical correlation of the annulus bed-to-wall heat transfer in terms of Nusselt number was obtained as follows：,