Page 217 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

194 Biomass Gasification, Pyrolysis and Torrefaction

TABLE 6.6 Range of Tar Removal by Different Physical Separation
Methods
Physical Methods Tar Removal (%) References
Sand bed filter 50 97 Hasler (1999)
Venturi scrubber 50 90 Han and Kim (2008)
Rotational particle separator 30 70 Han and Kim (2008)
Wash tower 10 25 Han and Kim (2008)
Wet ESP 50 70 Paasen (2004)
Fabric filter 0 50 Han and Kim (2008)
Catalytic tar cracker .95 Hasler (1999)

The size distribution of the inlet particulates is difficult to measure, espe-
cially for finer particulates, but its measurement is important in choosing the
right collection devices. For example, submicron (,1 μm) particulates need a
wet ESPs, but this device is significantly more expensive than others. A fab-
ric filter may work for fines, but it may fail if there is any chance of
condensation.

Cyclones
Cyclones are not very effective for tar removal because of the tar’s stickiness
and because cyclones cannot remove small (,1 μm) tar droplets (Knoef,
2005, p. 196). It is however effective in removing particulates from the prod-
uct gas.

Barrier Filters
Barrier filters present a physical barrier in the path of tar and particulates
while allowing the clean gas to pass through. One of their special features is
that they allow coating of their surface with appropriate catalytic agents to
facilitate tar cracking. These filters are of two types: candle and fabric.
Candle filters are porous, ceramic, or metallic. The porosity of the mate-
rial is chosen such that the finest particles do not pass through. Particles fail-
ing to pass through the filter barrier deposit on the wall (Figure 6.7), forming
a porous layer of solids called a “filter cake.” Gas passes through the porous
layer as well as through the filter. One major problem with the filter cake is
that as it grows in thickness, the pressure drop across the filter increases.
Thus, provision is made for its occasional removal. A popular means of
removal is pressure pulse in opposite directions.