Carbon dioxide removal in indirect gasification - SGC

SGC Rapport 2013:277
Figure 9. The first classes of tars and their origin [25].
Those organic species that are derived from cellulose, hemi-cellulose and lignin,
are sorted in the first class, while phenols and olefins are classified as secondary
products. The alkylated tertiary products are methylated aromatic species, while
the condensed tertiary products are aromatic compounds such as benzene, naphthalene,
acenaphthylene, anthracene and pyrene.
These tar classes are formed at different temperatures, and rearrangements
from one tar class to another will happen as temperature increases. The primary
tars are present at 500ºC to 800ºC, the secondary tars between 500ºC and
1,000ºC, the alkylated tertiary products are present between 650ºC and 1,000ºC
while the condensable exist above 750ºC. The formation of tar as well as the transition
between the various classes of tar is a function of residence time and temperature.
Removing the tar from the producer gas can be done using any of a number of
available unit operations, but the most important thing is that the gasifier is operated
at optimal conditions to minimize the initial tar formation. In fluid bed and entrained
flow gasification a first step can be catalytic tar cracking after particle removal.
In fluid bed gasification a catalyst, active for tar cracking, can be added to
the fluidizing bed to further remove any tar formed already in the bed. In this kind
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