POWER July 2015 - 40

ENVIRONMENTAL ISSUES
7. PCME 181WS installation on
a wet FGD stack at a generating
unit. Courtesy: Altech PCME
8. Optimal grading. The STACK 181WS
enclosure is centered over the sampling port,
and the sample line has a considerable downward
grade (10 degrees or more is ideal).
Courtesy: Altech PCME
STACK 181 WS enclosure is centered over
the sampling port and the sample line has a
considerable downward grade.
In September and October of 2012 a PS-11
correlation test was conducted on the STACK
181 WS on this source. The results met the
PS-11 correlation coefficient, confidence and
tolerance interval criteria with linear, polynomial,
and power models when evaluated with
MATS PM limits (0.03 lb/MMBtu or 18 mg/
Am3 given the 181 WS measurement conditions).
See Tables 2 and 3 for results.
As per PS-11 the correlation must be established
using the pressure and temperature
measurement conditions of the PM CEMS,
and the moisture basis must be consistent
between it and the reference method (wet or
dry). Thus it is important to understand and
allow for the measurement conditions of the
PM CEMS in question in order to meet the
requirements of PS-11.
If the PM CEMS is an in situ analyzer, then
sorbent injection is not in use. So to reach
MATS limit values can require manipulating
abatement processes so as to create particulate
emissions many times those of nominal operation.
This has sometimes been attempted by
changing the number of FGD spray pumps,
FGD absorber modules, or ESP banks in operation
with varying degrees of success. Thus
generating the widely separated particulate
concentrations that satisfy PS-11 correlation
test requirements and approach the emission
limit can be problematic in wet FGD stacks.
Sorbent Injection for SO3 Control
Sorbent injection has been used in wet FGD
plants to reduce SO3 concentration. A significant
level of SO3 in the stack gas (10 or more
ppm) is undesirable, as it can lead to visible,
blue emissions that become evident as the
vapor plume dissipates. It also can interfere
with mercury removal processes by competing
for activated carbon activation sites.
Two popular sorbents for SO3 abatement are
calcium hydroxide (often called hydrated lime),
which is usually injected at the FGD inlet, and
trona, more typically injected prior to the ESP.
The type of sorbent injection may affect
the flue gas chemistry and particle characteristics,
and these need to be considered properly
in relation to specifying a suitable PM
CEMS or PM CPMS.
Extractive Optical Wet PM CEMS
Use with Wet FGD
The PCME STACK 181 WS has been installed
on U.S. coal-fired electric utility wet
FGD stacks, some of which incorporate other
abatement devices such as SCRs for NOx control
or dry sorbent injection for SO3 removal,
40
and some of which do not. Some typical data
and experiences for these sources follow.
On one such stack that did not employ an
SCR or sorbent injection, sampling velocity
for the 181 WS was controlled by a 4-20 mA
signal from the ultrasonic stack flow meter.
As can be seen in the screen shot at the top
of this article, data collected by the 181 WS
control unit during December and January
shows a decrease in the sampling velocity below
that of the stack velocity command. This
was caused by condensation in the sample
line during cold winter temperatures.
Aggravating the sample line condensation
problem was the grading of the mounting
port, sample line, and probe (Figure 7). This
was partially corrected by a new probe and
sample line that compensated for the mounting
port being installed with an upward instead
of downward orientation. Also, sample
line grading was improved somewhat by increasing
the elevation of the 181 WS main
enclosure. Finally, condensate accumulation
was eliminated by removing a bulkhead that
interfaced the internal and external sample
lines and replacing it with a line that ran continuously
from the sample probe to the vaporizer
inlet.
When working with a dual-phase effluent
stream containing water droplets, as is found
in wet FGD stacks, it is very important that
the sample line descend straight to the port
with no bends and that it be installed with
proper grading (at least a 10 degree angle and
no flat spots) so that condensate does not accumulate.
Figure 8 shows an optimal sample
line grading and orientation, one where the
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the pressure and temperature conditions are the
stack temperature and pressure and the moisture
basis is usually wet. However, if the PM
CEMS uses an extractive sampling system and
is monitoring a wet FGD source, which was the
case with the aforementioned 181 WS correlation
test, then the pressure and temperature may
be different than the stack conditions.
In PM CEMS like the 181 WS that extract
a sample using an eductor and heat the sample
to evaporate moisture droplets, the pressure
at the measurement location is typically very
close to stack pressure and the measurement
is wet basis. Note that in this case the absolute
stack pressure was somewhat lower than typiTable
2. Wet FGD stack PS-11 correlation
test data, Sept. to Oct.
2012. Source: Altech PCME
Run
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
PM CEMS
response
0.2
0.2
0.2
0.2
44.6
44.5
40.9
26.7
27.1
26.2
4.7
5.1
5.9
5.8
4.8
Reference method
PM concentration,
mg/m3
0.376365
0.337165
0.130600
0.673510
45.353000
40.872000
38.923000
27.726000
25.077500
20.849800
9.800000
9.326690
7.886000
7.978500
6.654600
POWER | July 2015
http://www.powermag.com
POWER July 2015

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