Tar Production and Destruction - Biomass Gasification, Pyrolysis and Torrefaction - page 193

A large commercial unit (70 MW fuel power) uses this concept, where
biomass dries and pyrolyzes in a horizontal moving bed, heated by waste
heat from a diesel engine. The tar concentration of the product gas is about
50 g/Nm 3 . This gas passes through the neck of a vertical chamber, where
injection of preheated gas raises the temperature above 1100 C, reducing the
tar amounts to 0.5 g/Nm 3 . It then passes through a fixed bed of char or
carbon being gasified. Tar in the gas leaving the gasifier is very low
( 0.025 g/Nm 3 ). It is further cleaned to 0.005 g/Nm 3

in a bag filter (Knoef,

,

2005, p. 159).
Another design involves twin fluidized beds. Biomass fed into the first
bed is pyrolyzed. The pyrolysis product can be burnt to provide heat. The
char then travels to a parallel fast fluidized-bed combustor that burns part of
it. A commercial unit (8 MW fuel power) operates on this principle, where
gas leaving the gasifier contains 1.5 4.5 g/Nm 3 tar. A fabric filter that sepa-

rates dust and some tar reduces its concentration to 0.75 g/Nm 3 , which is
finally reduced to 0.010 0.04 g/Nm 3 in a scrubber.

6.3.2 Postgasification—Secondary Reduction of Tar
As indicated earlier, the level of cleaning needed for the product gas depends
greatly on its end use. For example, combustion in an engine or a gas turbine
needs a substantially cleaner product gas than that required by a boiler. Most
commercial plants use particulate filters or scrubbers to attain the required
level of cleanliness. A substantial amount of tar can be removed from the
gas in a postgasification cleanup section. It can be either catalytically con-
verted into useful gases like hydrogen or simply captured and scrubbed
away. The two basic postgasification methods are physical removal and
cracking (catalytic or thermal).

6.3.2.1 Physical Tar Removal
Physical cleaning is similar to the removal of dust particles from a gas. It
requires the tar to be condensed before separation. Tar removal by this
means could typically vary between 20% and 97% (Han and Kim, 2008).
Table 6.6 shows some typical values of extent of tar removal by different
methods.
The energy content of the tar is often lost in this process such that it
remains as mist or drops on suspended particles in gas. Physical tar removal
can be accomplished by cyclones, barrier filters, wet electrostatic precipita-
tors (ESP), wet scrubbers, or alkali salts. The choice depends on the
following:

Inlet concentration of particulate and tar
Inlet particle size distribution
Particulate tolerance of the downstream application of the gas.