ELECTROSTATIC PRECIPITATION 293
substrate), with subsequent ash collection achieved by impac-
tion of suspended particles on previously collected particles,
as the gas flows through the cake. The collected dust cake is
periodically dislodged into a bottom hopper by back flushing
the fabric with clean flue gas or by shaking. This type of opera-
tion necessitates a multi-compartment approach with a typical
installation consisting of 10 to 30 separately isolated compart-
ments, each containing about 500 1 ft × 30 ft cylindrical bags.
The filtering medium (cloth) selected for use in a fabric
filter must be compatible with the temperature and pH of
the effluent. For coal-fired power plants, the present-day fil-
tering medium for a fabric filter is glass fiber bags treated
with either teflon or silicon-graphite for lubrication to avoid
abrasion between the fabric fibers. The coated fiber fabric
is resistant to the chemical attack of flue gas constituents
and is capable of withstanding operating temperatures up to
approximately 500°F. However, glass bags require precau-
tions in handling and cleaning, due to their potential for self-
abrasion. If a spray dryer is used for SO 2 removal, acrylic
fiber bags are a less expensive alternative and are less vul-
nerable to self-abrasion. Their operating temperature limit is
280°F, which while above the typical operating temperature
for gas leaving a spray dryer, is below the temperature of
unscrubbed flue gas. Advances in filter media technology
are continuing, with emphasis on increasing the thermal and
chemical resistance of the bags and extending bag life.
As ash accumulates on the filtering media, the pres-
sure loss across the fabric filter increases. In general bags
are cleaned automatically upon reaching a specific pres-
sure drop across the fabric filter or by a timing cycle. The
compartmentalization of fabric filters on coal-fired boilers
allows for one compartment to be isolated for cleaning whilethe remaining compartments treat the flue gas. To date, the
cleaning of fabric filters on coal-fired boilers has generally
been accomplished by reversing gas flow against the bag
surface, by shaking the bags, or by a combination of the two
approaches. The fly ash is then released from the bags and
falls into a hopper for removal.
Prior to mid-1978, eight coal fired electric utility stations
were equipped with fabric filters. The best documented of
this group include Sunbury Units 1 & 2 for Pennsylvania
Power and Light and Nucla Units 1, 2 and 3 for Colorado
UTE. For the most part, these units were small (less than 50
MW) stoker-fired units with high air heater exit tempera-
tures, (350°F), and fired with low sulfur coal.
EPA assessed the data from these units in mid-1978
and concluded that this technology was capable of reducing
outlet emissions to less than 0.03 lb per million Btu (99.8%
efficiency). This analysis formed the basis in a large part for
the new NSPS for electric utilities of June, 1979.
An additional 1000–1500 MW of fabric filter capacity
came on line during the 1978–1979 period and approximately
4000 MW of capacity has been installed in the 1980–1983
period, including numerous large units in the 350–750 MW
size class. The fabric filter commitments are generally
for use on western coals that have proven very difficult to
control by standard electrostatic precipitator technology.
This is because filter performance is not sensitive to fly ash
composition. Another factor, which contributes to the rapid
growth of fabric filter applications in the West, is that filters
tend to achieve a lower stack opacity than comparable ESPs
operating on western coal. Plume opacity and atmospheric
visibility have historically been of much greater concern in
the Western states.FIGURE 15 Fly ash particle size in microns, actual diameter.0.10 0.2 0.3 0.5 1.0 2.0 3.0 5.0 10 20 30 50 10090959697989999.599.699.799.899.999.9599.9699.9799.98COLLECTION EFFICIENCY PERCENTFABRIC FILTERFABRIC FILTER
COLLECTION EFFICIENCY AS A FUNCTION OF PARTICLE SIZE (54)C005_005_r03.indd 293C005_005_r03.indd 293 11/18/2005 10:22:05 AM11/18/2005 10:22:05 AM