Tar Production and Destruction - Biomass Gasification, Pyrolysis and Torrefaction - page 195

FIGURE 6.7 Mechanism of separation of dust in a barrier filter.

Besides their high-pressure drop, barrier filters also suffer from the prob-
lem that if a filter is broken or cracked, dust and tar-laden gas preferentially
flow through that passage, adversely affecting downstream equipment. The
condensation of tar on the filter elements can block the filter, and this is a
major concern. Ceramic filters can be designed to operate in temperatures as
high as 800 900 C.
Fabric filters are made of woven fabric as opposed to porous materials as
in candle filters. Unlike candle filters, they can operate only in lower tem-
peratures (

350 C). Here, the filter cake is removed by either back-flushing
as with a candle filter or shaking. Condensation of tar on the fabric is a prob-
lem here if the gas is cooled excessively.
One could use a fabric filter with a precoat, which is removed along with
the dust cake formed on the filter. Such precoat can effectively remove unde-
sired substances from the product gas.

,

Wet Electrostatic Precipitators
Wet ESPs are used in some gasification plants. The gas is passed through a
strong electric field with electrodes. High voltage charges the solid and liq-
uid particles. As the flue gas passes through a chamber containing anode
plates or rods with a potential of 30 75 kV, the particles in the flue gas pick

up the charge and are collected downstream by positively charged cathode
collector plates. Grounded plates or walls also attract the charged particles
and are often used for design simplicity. Although collection efficiency does
not decrease as particles build up on the plates, periodic mechanical wrap-
ping is required to clean the plates to prevent the impediment of the gas flow
or the short-circuiting of the electrodes through the built-up ash.
The collected solid particles are cleaned by mechanical means, but a liq-
uid like tar needs cleaning by a thin film of water. Wet ESPs have very high
(

90%) collection efficiency over the entire range of particle size down to
about 0.5

.

m, and they have very low pressure drop (few inches water
gauge). Sparking due to high voltage is a concern with an ESP, especially

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