Page 294 - Materials Chemistry, Second Edition

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Incineration of MSW 265
Particles attracted to collector electrode
Corona (electrical field)
Gas flow with particulates + + + + + + Clean gas

Uncharged
particles + + + + + +

Discharge electorde Grounded electorde
negative polarity (positive polarity)
(high tension supply
from rectifier)

FIGURE 9.10 Electrodes within the ESP. (From Williams, 1998. Reproduced with kind permission of John
Wiley & Sons, Inc.)

● Equipment is needed for collection of the captured materials
● Extensive electrical equipment is needed

9.5.2 FABRIC FILTERS (BAGHOUSES)
The baghouse is one of the oldest, simplest, and most efficient methods for removing solid partic-
ulate contaminants from gas streams, using filtration through fabric media. The baghouse is con-
structed as a simple series of permeable bags that capture particulate matter but allow the passage
of gases (Figure 9.11). The filter fabric is composed of heat-resistant materials, ranging from cot-
ton to nylon to glass fibers (Table 9.5). The choice of the fabric is a function of the operating tem-
perature range, chemical composition of the flue gas, moisture, and the physical and chemical
properties of the particles being collected.
The fabric filter bags are usually tubular or flat. The structure in which the bags hang is referred
to as a baghouse, and the number of bags in a baghouse may vary from less than ten to several thou-
sand. The baghouse system can be operated continuously, with airflow to some bags turned off for
cleaning and maintenance. In bottom-feed units, flue gases are introduced through the baghouse
hopper at the base and then to the interior of the bag. In top-feed units, dust-laden gas enters the top
of the filters.
The baghouse filter fabric is typically woven with relatively large spaces, about 50 µm across.
However, these filters are capable of capturing particulates measuring < 1 µm; obviously, more than
simple sieving is taking place. Capture of particulates apparently occurs as a result of electrostatic
attraction as well as entrapment within the fabric weaving. With woven fabrics composing the bag,
a dust cake eventually forms, which, in turn, acts as an effective sieving mechanism. When felted
fabrics are used, this dust cake is minimal or nonexistent and the primary filtering mechanisms are
a combination of inertial forces and impingement (Vesilind et al., 2002).
As particles are collected, the pressure decreases across the fabric filtering media; therefore, the
filter must be cleaned at predetermined intervals. Dust is removed from the fabric by gravity or
mechanical means. When large number of bags are involved, the baghouse is compartmentalized so
that one compartment may be cleaned while others are still in service.