Effects of riser height and total solids inventory on the gas–solids in an ultra-tall CFB riser

Abstract

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 s 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.