POWER April 2013 - 41

EMISSIONS CONTROL
In this example, the ESP is operating well
above the acid dew point. If stack testing is
to be conducted at 250F on an unscrubbed
unit, assuming that 10 ppm SO3
is present
at the ESP outlet, Figure 3 indicates that
about 8.5 ppm SO3
will depend
would have condensed
as sulfuric acid aerosol. The extent of positive
bias from condensed SO3
on how much of this aerosol gets retained
on the front half of the EPA Method 5 sampling
train.
Research has shown that the extent of
positive bias from SO3
can vary, depending
on the ash loading and alkalinity of
particulate being caught in the probe.
Testing that was done to measure the accuracy
of SO3
measurements revealed
that as much as 40% of the sulfuric acid
can be captured on the filter probe. This
amount of positive bias can have considerable
impact on the reported test results.
A 40% capture rate across the filter probe
would amount to 4 ppm SO3
. If 4 ppm SO3
is captured on the probe, positive bias on
filterable particulate as a result of condensation
of SO3
on the filter catch can be as
high as 0.015 lb/MMBtu. For ashes that
are more alkaline, such as those derived
from Powder River Basin (PRB) coals or
produced as a result of alkaline sorbents
added for SO3
mitigation on eastern coal
applications, the capture efficiency of the
condensed acid aerosol on filter probes
could be higher than 40%.
It is not uncommon for ESPs operating
conditioning to experience the
with SO3
effects of over-conditioning from time to
time. Process control of SO3
injection can
be difficult if sulfur and alkaline levels in
the coal change considerably. Therefore, it
is possible for flue gas at the ESP outlet to
be saddled with a substantial level of free
SO3
in the vapor phase. EPA-certified PM
continuous emission monitors (CEMs) operate
at a filter probe temperature specified
by Method 5B (320F, plus/minus 25F). At
this temperature, for the stated example,
condensed sulfuric acid aerosol is unlikely
to be present at the filter probe temperature.
Operation of the filter probe at this
temperature has the most probability of
avoiding SO3
-related positive bias on the
filterable test results.
ESP Performance on Various Coals
Estimated ESP performance curves showing
SCA (normalized at 9 in. plate-toplate
spacing) versus collection efficiency
are illustrated in Figure 4. The figure
demonstrates the expected performance
of ESPs when faced with a variety of different
fuels. The prediction curves closely
correspond with performance estimates
April 2013 | POWER
Notes: LOI = loss on ignition, OFA = overfire air.
reported in the past by EPRI. This early
work projected that, for comparable filterable
outlet emissions, ESPs treating flue
gas from the combustion of low-sodium,
low-sulfur western PRB fuels would need
to be more than twice as large as ESPs on
plants firing higher-sulfur coals. For reasons
discussed earlier, the higher resistivity
of ash resulting from the firing of PRB
coals limits the corona current density in
each electrical section to a much lower
level. This would result in lower specific
corona power for an ESP of similar
size designed for eastern coals. The ESP
size, therefore, would need to be larger
to achieve the necessary particulate colwww.powermag.com
lection,
as dictated by the specific corona
power considerations.
ESP Performance and Testing Data
The remainder of this article reports on the
stack filterable emissions from six coal-fired
power plants, as well as the associated ESP
configuration and operating data. The discussion
above will be helpful when interpreting
the descriptions and test data from each
plant. Table 2 shows the configuration of
power plant boilers referenced in the stack
emissions review. Table 3 lists the properties
of coals used at these plants.
Plant and Fuel Descriptions. Plant 1
is an unscrubbed unit that fires a blend
41
Coal type
Coal data, as
rec'd, avg.
values (Btu/lb)
Ash (%)
Moisture (%)
Sulfur (%)
Ash, LOI (%)
NOx control LNB, OFA,
SNCR
SOx control
HCl control
Mercury
control
Boiler type
SO3
conditioning
Ammonia
conditioning/
other SO3
mitigation
No
Yes,
for SO3
mitigation
Yes, for SO3
mitigation
following
SCR
addition
Notes: LNB = low NOx burners, OFA = overfire air, SNCR = selective noncatalytic reduction, FGD = flue gas desulfurization,
SBC = sodium bicarbonate.
Table 3. Coal-fired power plant operational information of tested units.
Source: Babcock & Wilcox Power Generation Group Inc.
Plant 1 Plant 2 Plant 3
Eastern
PRB/
eastern
coal blend
10,225
6.6
20
0.9
2
bituminous
12,070
10.7
8
3.3
NA
Eastern
bituminous
Similar to
Plant 2
-
-
-
NA
8,800
5.4
27
0.3
9 to 10
moderate sodium in
ash (1.3% to 1.7% as
Na2O)
8,800
5.3
27
0.3
5.3 lower
sodium in ash
(1.30% as Na2O)
than Plant 4
Plant 4
PRB
Plant 5
PRB
Plant 6
Eastern
bituminous
Similar to
Plant 2
-
-
-
9 to 10
after
installation
of OFA
Plant 1 Plant 2 Plant 3
LNB, OFA, SCR
None Wet FGD Wet FGD
None Wet FGD Wet FGD
Opposed
wall-fired
No
Opposed
wall-fired
No
No
Plant 4
OFA, SNCR
None
None Wet FGD Wet FGD Dry sorbent injection of
SBC has been tested
Activated carbon
injection
Cyclone-fired
Yes, > 20 ppm
injection rate for ash
conditioning. Non-SO3
conditioning also tested.
No
No
Yes, for SO3
mitigation
following SCR
addition
Plant 5
None
No
No
Tangentialfired
No
Plant
6
LNB, SOFA OFA, SCR
None
None
None
None
None
Table 2. Coal-fired power plant configurations of tested units. Source: Babcock
& Wilcox Power Generation Group Inc.

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POWER April 2013

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