Tar Production and Destruction - Biomass Gasification, Pyrolysis and Torrefaction - page 183

6.2.3.3 Tertiary Tar
The alkyl tertiary product includes methyl derivatives of aromatics, such as
methyl acenaphthylene, methylnaphthalene, toluene, and indene (Evans and
Milne, 1997). These are formed at higher temperature.
Condensed tertiary aromatics make up a polynuclear aromatic hydrocar-
bon (PAH) series without substituents (atoms or a group of atoms substituted
for hydrogen in the parent chain of hydrocarbon). This series contains ben-
zene, naphthalene, acenaphthylene, anthracene/phenanthrene, and pyrene.
The secondary and tertiary tar products come from the primary tar. The pri-
mary products are destroyed before the tertiary products appear (Milne et al.,
1998).
Figure 6.2 shows that above 500 C with increasing temperature, the sec-
ondary tar increases at the expense of the primary tar. Once the primary tar
is nearly destroyed, tertiary tar starts appearing with increasing temperature.
At this stage, the secondary tar begins to decrease. Thus, high temperatures
destroy the primary tar but not the tertiary tar products.
6.3 TAR REDUCTION
The tar in coal gasification comprises benzene, toluene, xylene, and coal tar,
all of which have good commercial value and can be put to good use. Tar
from biomass, on the other hand, is mostly oxygenated and has little com-
mercial use. Thus, it is a major headache in gasifiers, and a major roadblock
in the commercialization of biomass gasification. Research over the years
has improved the situation greatly, but the problem has not completely disap-
peared. Tar removal remains an important part of the development and
design of biomass gasifiers.
Several options are available for tar reduction. These may be divided into
two broad groups ( Figure 6.3 ):
1. Postgasification (or secondary) reduction, which strips the product gas of
the tar already produced.
2. In situ (or primary) tar reduction, which avoids tar formation.
In situ reduction is carried out by various means so that the generation of
tar inside the gasifier is less, thereby eliminating the need for any removal to
occur downstream. As this process is carried out in the gasifier, it influences
the product gas quality. Postgasification reduction, on the other hand, does
not interfere with the process in the reactor, and therefore the quality of the
product gas is unaffected ( Figure 6.3A ).
At times, it may not be possible to remove the tar to the desired degree
while retaining the quality of the product gas. In such cases, a combination
of in situ and postgasification reduction can prove very effective. The tar