More and more CFB boilers with large capacity and ultra-tall furnaces are used for power generation. Understanding the fluid dynamics in the ultra-tall furnace is important. However, existing studies on fluid dynamics in the CFB furnace are limited to the risers with rather short height. An experimental study was conducted with a cold CFB test rig of 240 mm in I.D. and 38 m and 54 m in height respectively. The influences of total solid inventory I-v and fluidizing gas velocity U-g on the axial voidage profile along the riser and solid circulation rate G(s) were investigated. Experimental results showed that when U-g exceeded the transport velocity, an S-shaped voidage profile characterized by fast fluidization was established in the riser. In such circumstance, the voidage at top dilute section kept constant and G, reached saturation carrying capacity (G(s) = G(s)*) and inappreciably change with riser height and I-v. Moreover, G(s)* increased from 40 kg to 50 kg when the riser height increased from 38 m to 54 m. The results indicated that even for the 600 MWe supercritical CFB boiler with a 54 m tall furnace, only a modest increase of I-v and power of forced draft fans is needed to obtain high enough G(s) to meet the requirements of heating surfaces arrangement in furnace and the circulation loop. The necessary conditions to form the S-shaped profile of voidage in the riser were also discussed. (C) 2009 Elsevier B.V. All rights reserved.