POWER July 2015 - 39

ENVIRONMENTAL ISSUES
Table 1. Drift data. This table shows 7-day drift data for PCME 181WS from an electric
generating unit that uses wet flue gas desulfurization (FGD) (span reference of 10,000). Source:
Altech PCME
Date
12/21/12
12/22/12
12/23/12
12/24/12
12/25/12
12/26/12
12/27/12
Daily span
10017
10117
10116
10085
10094
10105
10091
periodically in the measurement path, providing
a full check of the instrument's capability to
measure scattered light.
The self-checks that the PCME STACK
181 WS employs are useful for complying
with PS-11 calibration drift test requirements.
PS-11 requires that a PM CEMS perform
a daily zero and upscale calibration test
and that as a part of the initial certification
process a 7-day drift test must demonstrate
calibration drift results within 2% of the upscale
reference. Some field data supporting
the PCME STACK 181 WS's ability to meet
this requirement can be found in Table 1.
The STACK 181 WS was evaluated by TUV
in 2011 for suitability as an Automated Measuring
System (AMS) according to EN 14181
(Stationary Source Emissions: Quality Assurance
of Automated Measuring Systems) and
was found to be compliant. Approval to this
standard for a particulate monitoring device
involves lab and field testing of relative accuracy
versus an isokinetic gravimetric Standard
Reference Method (SRM) of two co-located
devices as well as evaluation of measurement
uncertainties associated with linearity of response
to reference materials, ambient temperature,
and line voltage variation.
Lab testing of response to dust was performed
on a wind tunnel apparatus using a
calibrated particulate injection device. The
field test of the co-located instrument responses
versus a gravimetric SRM, in this
case EN 13284-1, was conducted twice at
widely separated intervals in time on the exhaust
stream from an industrial solvents manufacturing
process in Europe with abatement
that included a wet electrostatic precipitator
(ESP) and a wet FGD. Figures 4 and 5 summarize
the field test results.
Response to reference materials was tested
with PCME audit filters (see Figure 6). These
reference materials are inserted in the optical
path of the STACK 181 sensor and work by
creating a controlled amount of light scatter that
corresponds to a known particulate reading from
the sensor. Audit filters can be used as quarterly
audit materials for the Absolute Correlation AuJuly
2015 | POWER
Span drift (% of
reference)
0.17
1.17
1.16
0.85
0.94
1.05
0.91
Daily zero
46
46
45
41
41
43
42
Zero drift (% of
reference)
0.46
0.46
0.45
0.41
0.41
0.43
0.42
dit requirements in Appendix F Procedure 2.
Effects of Wet FGD and Other
Abatement Processes on PM
CEMS
Although wet FGD is deployed primarily, as
the name indicates, to remove gaseous sulfur,
it can have ancillary abatement benefits:
■ Removal of oxidized mercury due its water
solubility.
■ Reduction of particulate emissions.
The latter being germane to the scope of
this article, a brief and general discussion of
FGD particulate abatement mechanisms and
their effects on PM CEMS and PS-11 correlation
testing is in order.
The particulate removal process at work
in most wet FGDs relies on impaction of
the particle in question with a water droplet.
Efficiency of such a process depends on
the particle and droplet diameters and their
relative velocities. Many wet FGDs have water
droplets at least 50 microns in diameter,
though designs with low liquid-to-gas ratios
probably have much smaller droplets.
Although some newer sources incorporate
fabric filters, most U.S. EGUs with wet FGD
incorporate ESPs upstream of the FGD, so
for brevity, only this particulate abatement
technique will be discussed.
The volume weighted particle size distributions
found at ESP outlets have volume
means variously reported as between 2 to 3.1
microns, with a preponderance of submicron
particulate on a number basis. As particles
become smaller than 1 micron, their tendency
to follow flow stream lines prevails over their
momentum, thus they do not impact, and get
absorbed by FGD water droplets efficiently at
the typical relative velocities present. Hence,
the particle size distribution at an FGD outlet
usually skews toward the submicron from a
number standpoint for scrubber designs with
larger water droplets. Volume means have
been difficult to know with confidence at wet
FGD outlets due to limited empirical data.
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160
140
120
100
80
60
40
20
-20
0 20 40 60 80 100 120 140 160
PCME Stack 181WS AMS response (mg/m3)
Note: y=0.9991x+0.0217
R2=0.9989
6. Audit filter inserted into PCME
STACK 181. Courtesy: Altech PCME
Audit unit
Laser
Scattered light
4. Field test 1. Courtesy: Altech PCME
◆ AMS device 1, mg/m3(ntr) ✕ AMS device 2, mg/m3/
(ntr)
Linear (AMS device 2, mg/m3(ntr)
200
150
100
50
-50
50
100
150
200
PCME Stack 181WS AMS response (mg/m3)
Note: y=0.9992x+0.05
R2=0.9996
5. Field test 2. Courtesy: Altech PCME
What does this mean to PM CEMS installed
on wet FGD? Because wet FGD are very effective
at removing large particles, it is often
difficult to create relatively constant particulate
concentrations in excess of or close to the
MATS PM limit, even when inlet opacity is relatively
high, because most of the mass is due to
the larger particles that FGD removes so well.
The 30-day average MATS filterable PM
limit of 0.03 lb/MMBtu equates to a stack
particulate concentration of 26.3 mg/Am3,
assuming typical wet FGD stack conditions,
to wit, a wet basis CO2 concentration of 11%,
a stack temperature of 130F, a stack pressure
of 1 atmosphere, and a fuel constant consistent
with bituminous and certain common
subbituminous coals.
It is not unusual for wet FGD stacks to have
particulate concentrations of 5 mg/Am3 or less
when operating normally provided post ESP
39
SRM response (mg/m3)
SRM response (mg/m3)

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POWER July 2015

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