Tar Production and Destruction - Biomass Gasification, Pyrolysis and Torrefaction - page 196

when it is used to clean highly combustible syngas. Thus, the savings from
lower fan power due to low pressure drop is offset by a higher safety cost.
Additionally, the capital cost for ESP is 3

4 times higher than that for a wet

scrubber.

Wet Scrubbers
Here, water or an appropriate scrubbing liquid is sprayed on the gas. Solid
particles and tar droplets collide with the drops, forming larger droplets
because of coalescence. Such larger droplets are easily separated from the
gas by a demister like cyclone. The gas needs to be cooled until it is below
100 C before cleaning. The tar-laden scrubbing liquid may be fed back into
the gasifier or its combustion section. Alternatively, stripping the tar away
may regenerate the scrubbing liquid.
Some commercial methods, such as the OLGA and TARWTC technolo-
gies, use proprietary oil as the scrubbing liquid. The tar-laden liquid is then
reinjected into the gasifier for further conversion (Knoef, 2005, p. 196).
Figure 6.8 shows a schematic of the OLGA process.
Wet scrubbers have a high ( 90%) collection efficiency, but the effi-

.

ciency drops sharply below 1 μ m-sized particles. They consume much fan

power owing to the large ( 50 in water gauge) pressure drop across the

scrubber. While their operating cost is high, their capital cost is much less
than that for ESPs.
A system with a tar removal scrubber produces cleaned gas with a lower
outlet temperature and higher energy content, but it contains tars that are

B

Tar-free gas

To engine

To gasifier
Tar loaded air
and scrubbing liquid

Tar laden gas

Scrubbing
liqu id makeup

Tar and scrubbing
liquid bleed

Stripper air

Collector Absorber Stripper

FIGURE 6.8 Schematic of the OLGA process. Source: Redrawn from Han and Kim (2008).