Tar Production and Destruction - Biomass Gasification, Pyrolysis and Torrefaction - page 190

not easily available in a gasifier's downstream. Design modification is
needed to incorporate char as a catalyst.

6.3.1.4 Gasifier Design
The design of the gasifier can be a major influence on the amount of tar in
the product gas. For example, an entrained flow gasifier can reduce the tar
content to less than 0.1 g/Nm 3 , while in an updraft gasifier, the tar can well
exceed 100 g/Nm 3 . To understand how gasifier design might influence tar
production, we will examine the tar production process.
As we see in Figure 6.2 , primary tar is produced at fairly low tempera-
tures (200

500 C). It is a mixture of condensable hydrocarbons that under-
goes molecular

rearrangement (reforming) at higher temperatures

900 C), producing some noncondensable gases and secondary tar. Tar
is produced at an early stage when biomass (or another fuel) undergoes
pyrolysis following drying. Char is produced further downstream in the pro-
cess and is often the final solid residue left over from gasification. The gas-
ifier design determines where pyrolysis takes place, how the tar reacts with
oxidants, and the temperature of the reactions. This in turn determines the
net tar production in the gasifier.
Updraft, downdraft, fluidized bed, and entrained bed are the four major
types of gasifier with their distinct mode of tar formation. Table 6.2 com-
pares their tar production, and a brief discussion of formation of tar in these
reactors follows here.

(700

Updraft Gasifier
Biomass is fed from the top and a gasifying medium (air) is fed from the
bottom. In this countercurrent reactor, the product gas leaves from the top
while solids leave from the bottom. Figure 6.5 illustrates the motion of bio-
mass, gas, and tar. The temperature is highest close to the grate, where oxy-
gen meets with char and burns the char. The hot gas travels up, providing
heat to the endothermic gasification reactions and meets pyrolyzing biomass
at a low temperature (200

500 C). Primary tar is produced in this tempera-
ture range ( Figure 6.5 ). This tar travels upward through cooler regions and
therefore has no opportunity for conversion into gases and secondary tar. For
this reason, updraft gasifiers generate the highest amount of tar—typically
10

20% by weight of the feed.

Downdraft Gasifier
Figure 6.4 shows the tar production in a downdraft gasifier. It is a cocurrent
reactor where both gas and feed travel downward. The temperature is highest
in the downstream combustion zone. The tar is produced just after drying in
a zone close to the feed point where the temperature is relatively low
(200 500 C). The oxygen in the air, along with the tar, travels downward to