The range of fluidizing velocity, Ug, in a bubbling bed depends on the mean particle size of the bed materials. The choice is made in the same way as for a fluidized-bed combustor. The range should be within the minimum fluidization and terminal velocities of the mean bed particles. The particle size may be within group B or group D of Geladart’s powder classification (see Basu, 2006, Appendix I). The typical fluidization velocity for silica sand of about 1 mm mean diameter may, for example, vary between 1.0 and 2.0 m/s.

If the gasifier reactor is a circulating fluidized-bed type, the fluidization velocity in its riser (Figure 6.12) must be within the limits of fast fluidization, which favors groups A or group B particles. Typical fluidization velocity for particle size in the range 150 to 350 microns is 3.5 to 5.0 m/s in a CFB. This type of bed has another important operating condition to be satisfied for opera­tion in the CFB regime. Solids, captured in the gas-solid separator at the gasifier exit, must be recycled back to the gasifier at a rate sufficiently high to create a “fast-fluidized” bed condition in the riser. Additional details about this are available in Basu (2006) or Kunii and Levenspiel (1991).