Page 161 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

138 Biomass Gasification, Pyrolysis and Torrefaction

a. Solid moves under gravity with an acceptable space velocity U s , that will
provide the required residence time in the drying sections.
b. Upward gas velocity is not too high to blow fine particles away.
1. Data for solid space velocity in a torrefier is not available at the moment but a
value of 0.5 5.0 m/h is used in an updraft cool gasifier (see Section 8.8.1.1).
For a chosen value of space velocity and the designed feed rate of biomass,
W f , one calculates the minimum cross-sectional area, A r , of the reactor as:

W f
A r 5 (4.34)
U s ρ bulk
where ρ bulk is the bulk density of solids. It is to be noted that the calcula-
tion is based on W f , instead of W t to allow easier flow of solids.
2. The superficial velocity of the gas, U g , needs to be chosen to avoid fluidi-
zation or entrainment of fine solids. It is calculated from the volume
flow-rate of gas products W through this section:
0
g
W 0 g
U g 5 (4.35)
ρ A r
g
where ρ g is the density of the gas at average temperature of the section
which, for the preheating section, is (T g3 1 T g0 )/2.
The gas solid relative space velocity is

(4.36)
U rel 5 U g 1 U s
The value of U rel should not exceed the entrainment velocity of fine bio-
mass particles or minimum fluidization velocity of average particles.

Drying and Postdrying Sections A similar method is used for the design of
these two successive sections. The log mean temperature difference is calcu-
lated from the temperatures shown in Figure 4.12.

Torrefier Section The torrefaction zone is designed in the same way but
with an additional stipulation that it must provide the specified solid resi-
dence time for the biomass feed and required yield.
The volume, V tor , of the torrefaction zone for the given residence time of
and production rate, W t , is:

τW t
V tor 5 (4.37)
ρ bulk
The height of the torrefaction zone is therefore calculated from V tor :

V tor
L tor 5 (4.38)
A r