Page 219 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

196 Biomass Gasification, Pyrolysis and Torrefaction

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 1μm-sized particles. They consume much fan
power owing to the large (B50 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

Tar-free gas
To engine
To gasifier
Tar loaded air
and scrubbing liquid

Tar laden gas

Scrubbing
liquid 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).