Tar Production and Destruction - Biomass Gasification, Pyrolysis and Torrefaction - page 192

Cyclone

Gas, tar

Freeboard

Ash

Tar

800 - 900°C

Fluid bed

Biomass

Plenum

Air stream

Temperature

FIGURE 6.6 Bubbling fluidized-bed gasifier. Here tar is not produced at any specific location.
As such it passes through average temperature zone of the fluidized-bed gasifier.

Though the solids are back-mixed, the gases flow upward in plug-flow
mode. This means that further up in the bed neither older char particles nor
fresh pyrolyzing biomass particles come in contact with the oxygen. Any
tar released moves up in the bed and leaves along with the product gas.
For this reason, tar generation in a fluidized-bed gasifier is between the
two extremes represented by updraft and downdraft gasifiers, averaging
about 10 mg/Nm 3 .

Entrained-Flow Gasifier
Tar production is negligible, as whatever is released passes through a very-
high-temperature (

1000 C) combustion zone and is therefore nearly all

.

converted into gases.

6.3.1.5 Design Modifications for Tar Removal
Modification of a reactor design for tar removal involves the following:

Secondary air injection
Separation of the pyrolysis zone from the char gasification zone
Passage of pyrolysis products through the char.

We saw earlier that char is effective in aiding tar decomposition. A
moving-bed two-stage gasifier that uses the first stage for pyrolysis and the
second stage for conversion of tar in a bed of char succeeds in reducing the
tar by 40 times (Bui et al., 1994). Air addition in the second stage increases
the temperature and thereby reduces the tar (Knoef, 2005, p. 170).