Design of Biomass Gasifiers - Biomass Gasification, Pyrolysis and Torrefaction - page 294

evolving. This section presents a tentative method for determining size based
on available information.

8.8.2.1 Cross-Sectional Area
The inside cross-sectional area of the fluidized-bed gasifier, A b , is found by
dividing the volumetric flow-rate of the product gas flow, V g , by the chosen
superficial gas or fluidization velocity through it, U g , at the operating tem-
perature and pressure.

V g
U g

A b 5 (8.29)

The volume of gas V g at the operating temperature and pressure is esti-
mated from the mass of air (or other medium), M fa , required for gasification.
This gas flow-rate should also be appropriate for fluidization. Thus, V g is
necessarily the gas passing through the grate and the bed.
In some designs, part of the gasifying medium is injected above the dis-
tributor grid. In that case, V g is only the amount that passes through the grid.
We can use the mass of gasification medium, M fa , required for gasification
for the computation of V g :

M fa
ρ g

V g 5 (8.30)

where ρ g is the density of the medium at the gasifier's operating temperature

and pressure.
Equation (8.29) requires choosing an appropriate value for the superficial
gas (fluidizing) velocity, U g , through the gasification zone. This is critical as
it must be within acceptable limits for the selected particle size to ensure sat-
isfactory fluidization and to avoid excessive entrainment.

8.8.2.2 Fluidization Velocity
The range of fluidizing velocity, U g , 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 fluidi-
zation 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
( 1 mm mean diameter) may, for example, vary between 1.0 and 2.0 m/s.

If the gasifier reactor is a CFB type, the fluidization velocity in its riser
( Figure 8.12 ) 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

B

5.0 m/s in a CFB. This type of bed has
another important operating condition to be satisfied for operation in the

350 μ m is 3.5