ELECTROSTATIC PRECIPITATION 291
Electrostatic PrecipitationExisting electrostatic precipitator performance may be
improved by the use of wide plate spacing (replacing
weighted wire discharge electrodes with rigid type), by
using intermittent energization (i.e., blocking selected half-
cycles of power to the transformer–rectifier sets powering
the ESP) and by applying flue gas conditioning.^7 The impor-
tance of modeling ESP performance via 3 different types of
computer models enables utilities to optimize their upgrade
taking into account hot-to-cold side conversion, fuel switch-
ing and sizing for bidding and licensing purposes.^8
The effects of sulfur and sodium on fly ash resistivity
and performance have been discussed in the literature. 9,10
Daub^11 discusses the effects of computer controlled
energization of the tranformer–rectifier sets of a precipitator.
Typical results in various European situations are presented
in Table 1. Greico and Wedig^12 discuss the performance and
economics of ESP’s for removal of heavy metals in coal, oil,
and orimulsion fired units.
Good design provides for a mass flow 12% above that
anticipated, and for a 5% variation in flow distribution
between the precipitator boxes.
For particulate control application at Ravenswood Unit 3,
the reasons for choosing to upgrade the hotside precipitator
included.1) Only minimal modification work is required for
upgrading the existing precipitator.
2) Electrostatic precipitators will meet performance
criteria on either coal or oil firing.3) The low pressure drop across the system elimi-
nate the need for additional booster fans or ID fan
modifications.
4) Electrostatic precipitators are a comparatively low
maintenance system.
5) Electrostatic precipitators performance is not
adversely affected by a rapidly changing gas flow
or boiler load.
6) Performance of the precipitator is not affected by
changing ash characteristics or coal sulfur content
since ash resistivity is low due to high gas tem-
peratures (Figure 12b, 13).
7) Power requirements are comparable to a bag-house
filter.
8) Ash removal is more reliable due to the higher
temperatures.
9) Air heater performance and maintenance is
improve because of the cleaner flue gas.Fabric Filters: Alternative to Proposed PrecipitatorFabric filters (baghouses) can be installed at Ravenswood
Unit 3 if the existing precipitators are removed. They offer
no advantage in collection efficiency and opacity over the
proposed precipitation upgrading (Figures 14 and 15), but
will increase the reconversion costs by about $90 million
more than the proposed precipitator upgrading.
Fabric filters, as applied in the utility industry, operate by
drawing dust-laden flue gas through a porous fabric bag woven
of multifilament glass yarn. During operation, a fly-ash cake is
formed over the cloth pores (with the glass filaments forming aFLY ASH PARTICLE SIZE IN MICRONS
m
ACTUAL DIAM.0.10 0.2 0.3 0.4 0.5 1.0 2.0 3.0 4.0 5.0 10 20 30 40 50 100COAL CHARACTERISTICS
EASTERN BITUMINUS
SULFUR NOMINALLY 1.0% RANGE 0.6–1.0%
ASH RANGE 10.0–15.0%MIN PRECIPITATOR DESIGN CRITERIA
SCA (A/V)
AREA SQ FT
VOLUME CFM
W (MIGRA VEL) cm/sec.
GAS TEMP °F90.095.096.097.098.099.099.599.699.799.899.999.9599.9699.9799.98PRECIPITATORCOLLECTION EFFICIENCY PERCENTEFFICIENCY=^1–eWA VA V3048X...COLLECTING AREAGAS VOLUMESCAPrecipitator Design Chart
DESIGN CRITERIA FOR RAVENSWOOD 30
EXPECTED PARTICLESIZE DISTRIBUTION PREDICTED PRECIP.COLLECTION EFFIC.
m %%%
MICRON BY MASS PREDICTED WEIGHTED0 – 2
2 – 5
5 – 10
1 – 3
3 – 5
5 – 10
10.08 x
x x x x x x= = = = = = =
.62
2.10
11.20
10.00
20.00
56.0099.60
99.20
97.50
99.65
99.77
99.61
99.63.079
.615
2.048
11.161
9.977
19.962
56.906
AVERAGE PRECIP EFFICIENCY = 99.75%BLR. 30 329
1,318,000
4,007,000
9.4
600COLLECTION EFFICIENCY -- PARTICLE SIZEDESIGN EFFICIENCYμμFIGURE 13 Precipitator collection efficiency as a function of particle size.C005_005_r03.indd 291C005_005_r03.indd 291 11/18/2005 10:22:04 AM11/18/2005 10:22:04 AM