Page 200 - Wastewater Solids Incineration Systems

Emission Control and Monitoring 167

the bags from collapsing. The dirty exhaust flows through the fabric bags from out-
side to inside. The cleaned gas flows out of the top of each bag. Particulate matter is
collected on the outside of the fabric. Periodically the bags are cleaned by either
reversing the airflow, mechanically shaking the bags, or sending a pulse of com-
pressed air down the bags which dislodges the particulate matter from the surface.
Particulate matter falls into large hoppers at the bottom of the unit where it is col-
lected for removal.
As shown in Figure 7.14, the fabric filter has three dust tight sections: a plenum at
the top, a collector housing in the center, and a hopper at the bottom. The dust-laden
air enters the bottom of the unit and is distributed across the chamber by either a dif-
fuser plate or inclined baffles. The dirty gas then flows through the bags and exits
through the top of each bag into the outlet plenum. A tubesheet separates the collector
housing from the outlet plenum. The bags are secured to the tubesheet with clamps.

2.8.1 Performance
On industrial, power utility, and refuse incineration facilities, the use of fabric filters
has allowed particulate removal of as great as 99.9%. These high removal efficiencies
are accomplished not only by filtering of particulate by the fabric, but also by buildup
of particulate on the fabric. As particulate matter builds up on the fabric, the built-up
layer acts as a further filtering medium to remove even smaller size particles.
2.8.2 Advantages and Disadvantages
The advantages of fabric filters are as follows:

• Highest collection efficiencies of all particulate control devices available.
• Significantly less electrical power use than low and medium pressure drop
Venturi scrubbers; slightly greater power use than dry ESPs.
• Fly ash is collected dry, requiring less handling than wet ash.

In the past, use of a fabric filters was very limited because of high water-vapor
content in the incinerator flue gas and the overriding concern of moisture in the gas
condensing out and causing muddy bags. The same phenomenon can occur with
VOCs in the exhaust gas; thus incinerator exhaust must be well burnt out and essen-
tially free of VOCs. Despite these concerns, fabric filters have been used on fluid bed
incinerators in England and Europe and recently at the Metro WWTP in St. Paul,
Minnesota. To use a fabric filter, the flue gas must be cooled to less than 260°C
(500°F), which typically requires a waste heat boiler.