Page 220 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 6 Tar Production and Destruction 197

more difficult to remove. The main challenge of tar removal is the formation
of “tar balls,” which are long-chained hydrocarbons that have a tendency to
agglomerate and stick together, fouling equipment in the initial stages of tar
condensing, and collecting.
The tar-laden stripper gas, if fed into the gasifier, lowers its dew point
well below that of water. This allows condensation of the tar, while flue gas
containing tar vapor can be recycled back to the combustion section of the
gasifier for combustion.

Alkali Remover
Compared to fossil fuels, biomass is rich in alkali salts that typically vapor-

ize at high gasifier temperatures but condense downstream below 600 C.
Because condensation of alkali salts causes serious corrosion problems,
efforts are made to strip the gas of alkali. If the gas can be cooled to below

600 C, the alkali will condense onto fine solid particles (,5 μm) that can be
captured in a cyclone, ESPs, or filters. Some applications do not permit cool-
ing of the gas. In such cases, the hot gas may be passed through a bed of
active bauxite maintained at 650 725 C.

Disposal of Collected Tar
Tar removal processes produce liquid wastes with higher concentration of
organic compound, which increase the complexity of water treatment.
Wastewater contaminants include dissolved organics, inorganic acids, NH 3 ,
and metals. Collected tars are classified as hazardous waste, especially if
they are formed at high temperatures (Stevens, 2001). Several technologies
are available for treatment of these contaminants before their final disposal.
Some examples include extraction with organic solvent, distillation, adsorp-
tion on activated carbon, wet oxidation, oxidation with hydrogen peroxide
(H 2 O 2 ), oxidation with ozone (O 3 ), incineration, and biological treatment.

6.3.2.2 Cracking
Postgasification cracking could break large molecules of tar into smaller
molecules of permanent gases such as H 2 or CO. The energy content of the
tar is thus mostly recovered through the smaller molecules formed. Unlike in
physical cleaning, the tar need not be condensed for cracking. This process

involves heating the tar to a high temperature (B1200 C) or exposing it to

catalysts at lower temperatures (B800 C). There are two major types of
cracking: thermal and catalytic.
1. Thermal cracking without a catalyst is possible at a high temperature
(B1200 C). The temperature requirement depends on the constituents of

the tar. For example, oxygenated tars may crack at around 900 C
(Stevens, 2001). Oxygen or air may be added to allow partial combustion