Energy Fuels 2009, 23, 4831–4839
Published on Web 08/19/2009
: DOI:10.1021/ef900294s
Measurement of Vapor Phase Mercury Emissions at Coal-Fired Power Plants Using
Regular and Speciating Sorbent Traps with In-Stack and Out-of-Stack Sampling
Methods†
Chin-Min Cheng,† Chien-Wei Chen,† Jiashun Zhu,† Chin-Wei Chen,‡ Yao-Wen Kuo,‡ Tung-Han Lin,‡
Shu-Hsien Wen,‡ Yong-Siang Zeng,‡ Juei-Chun Liu,‡ and Wei-Ping Pan*,†
†
Institute for Combustion Science and Environmental Technology, Department of Chemistry, Western Kentucky University, 2413
Nashville Road, Bowling Green, Kentucky 42101, and ‡Department of Chemical Engineering, Ming-Chi University of Technology,
84 Gungjuan RD., Taishan, Taipei, Taiwan 243 R.O.C
Received April 5, 2009. Revised Manuscript Received July 30, 2009
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Published on August 19, 2009 on http://pubs.acs.org | doi: 10.1021/ef900294s
A systematic investigation of sorbent-trap sampling, which is a method that uses paired sorbent traps to
measure total vapor phase mercury (Hg), was carried out at two coal-fired power plants. The objective of
the study was to evaluate the effects (if any) on data quality when the following aspects of the sorbent trap
method are varied: (a) sorbent trap configuration; (b) sampling time; and (c) analytical technique. Also, the
performance of a speciating sorbent trap (i.e., a trap capable of separating elemental Hg from oxidized Hg),
developed by the Western Kentucky University’s Institute for Combustion Science and Environmental
Technology (ICSET), was evaluated by direct comparison against the Ontario Hydro (OH) reference
method. Flue gas samples were taken using both “regular” and modified sorbent trap measurement
systems. The regular sorbent trap systems used a dual-trap, in-stack sampling technique. The modified
systems were equipped with either inertial or cyclone probes and used paired, out-of-stack sorbent traps.
Both short-term (1.5 h) and long-term (18 h to 10 days) samples were collected. The OH method was run
concurrently during the short-term test runs, to provide reference Hg concentrations. At one facility,
mercury concentration data from continuous emission monitoring systems were also recorded during the
sorbent trap sampling runs. After sampling, the conventional (nonspeciating) sorbent traps were analyzed
for Hg, using either a direct combustion method or a wet-chemistry analytical method (i.e., microwaveassisted digestion coupled with cold vapor atomic absorption spectroscopy). The speciating traps were
analyzed only by the direct combustion method. All of the sorbent trap data collected in the study were
evaluated with respect to relative accuracy, relative deviation of paired traps, and mercury breakthrough.
The in-stack and out-of-stack sampling methods produced satisfactory relative accuracy results for both
the short-term and long-term testing. For the short-term tests, the in-stack sampling results compared
more favorably to the OH method than did the out-of-stack results. The relative deviation between the
paired traps was considerably higher for the short-term out-of-stack tests than for the long-term tests.
A one-way analysis of variance (ANOVA), showed a statistically significant difference (p < 0.1) between
the direct combustion and wet-chemistry analytical methods used in the study; the results from the direct
combustion method were consistently higher than the wet-chemistry results. The evaluation of the
speciating mercury sorbent trap demonstrated that the trap is capable of providing reasonably accurate
total mercury concentrations and speciation data that are somewhat comparable to data obtained with the
OH method. Although the results of the study were informative and promising, further evaluation of both
the out-of-stack sampling methods and the speciating sorbent trap is recommended.
fish living in these waters, and has also been known to cause
neurological and developmental damage in humans.2,3 On
May 18, 2005, the US Environmental Protection Agency
(EPA) published the Clean Air Mercury Rule (CAMR).
The purpose of CAMR was to achieve a 70% reduction in
nationwide Hg mass emissions from coal-fired electricity
generation units (EGUs) by 2018. CAMR would have required affected sources to continuously monitor and report
I. Introduction
Coal combustion processes may result in the emission of
hazardous air pollutants (HAP), which include mercury
compounds. Currently, the largest source of mercury pollution in America is from coal-burning power plants. Approximately 50 tons of mercury are emitted annually by the utility
industry as a result of coal use.1 Mercury emissions from
power plants can pollute rivers and lakes, contaminating the
(2) DOE/EIA U.S. Coal Reserves: 1997 Update. US Department of
Energy, Energy Information Administration, Office of Coal, Nuclear, Electric
and Alternate Fuels, Office of Integrated Analysis and Forecasting DOE/EIA0529 (97): Washington, DC, 1999.
(3) National Research Council Toxicological Effects of Methylmercury. Committee on the Toxicological Effects of Methylmercury Board on
Environmental Studies and Toxicology, Commission on Life Sciences;
National Academy Press: Washington, DC, 2000.
†
Progress in Coal-Based Energy and Fuel Production.
*To whom correspondence should be addressed. E-mail: wei-ping.
pan@wku.edu.
(1) US EPA Mercury Study Report to Congress. Volume II: An
Inventory of Anthropogenic Mercury Emissions in the United States. US
Environmental Protection Agency, Technical Report, EPA-452/R-96-001b,
Office of Air Quality Planning and Standards: Washington, DC, 1996.
r 2009 American Chemical Society
4831
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Energy Fuels 2009, 23, 4831–4839
: DOI:10.1021/ef900294s
Cheng et al.
Table 1. Description of the Tested Units
facility
C
C
O
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a
boiler(s)
boiler type
capacity (MWe)
fuel type
emission controls
units 1 and 2
unit 3
units 4 and 5
cyclone
wall-fired
unit 4 cyclone unit 5 T-fired
180 (combined)
205
425 (combined)
bituminous coal
bituminous coal
bituminous coal
SCRa þ ESPb þ FGDc
SCR þ ESP þ FGD
SCR þ ESP þ FGD
Selective catalytic reduction. b Electrostatic precipitator. c Flue gas desulfurization.
their cumulative annual Hg mass emissions. However,
CAMR was challenged on legal grounds, and the U.S. Court
of Appeals for the District of Columbia vacated the rule in
2008. EPA is expected to propose a more restrictive Hg
control regulation (specifically, a maximum achievable control technology, or “MACT” standard) for coal-fired EGUs
in the near future.
Despite the status of the Federal mercury rule, approximately 20 states (e.g., Illinois, Pennsylvania, Connecticut,
Maine, Massachusetts, and others) have adopted regulations
to limit Hg emissions from power plants. To ensure that the
Hg emission reduction goals can be met, many of these states
require continuous emission monitoring (CEM) systems or
sorbent-trap systems to be installed and operated by affected
electric utility units.
Currently, sorbent-trap sampling is one of the few suitable
methods for continuously monitoring total vapor phase Hg
emissions. It is a viable alternative to a mercury continuous
emission monitoring system (Hg CEMS), particularly when
the Hg concentration in the flue gas is very low. In May 2005,
EPA first published a continuous sorbent trap sampling
method in support of the CAMR rule. This method, which
was found in Appendix K of 40 CFR Part 75, was later
vacated by the DC Court of Appeals. In September 2007, EPA
published Reference Method 30B, a stack test method that
uses sorbent traps to measure total vapor phase Hg emissions.
Method 30B is similar in principle to vacated Appendix K,
and the basic sampling equipment is the same, but Method
30B has much more rigorous quality assurance procedures. In
a previous study4 that evaluated the Appendix K methodology, a number of issues arose concerning some of the
sampling and analytical procedures and the sample collection
time. In reviewing the results of that study, ICSET concluded
that further investigation and refinement of the sorbent trap
sampling method is needed.
In view of this, ICSET, in collaboration with the Illinois
Clean Coal Institute (ICCI), initiated a full-scale investigation
of the sorbent trap monitoring method, using in-stack and
out-of-stack sampling techniques and two different analytical
methods (i.e., direct combustion and microwave-assisted
digestion coupled with cold vapor atomic absorption spectroscopy). A speciating sorbent trap was also tested, to assess its
ability to provide credible total Hg concentrations and speciated Hg emissions data., with a view toward using it as a
possible alternative to the cumbersome Ontario Hydro (OH)
method. The following sections describe the experimental
procedures that were used in the investigation and present
the results of the study.
Figure 1. Sampling Configurations for the Tested Units.
Facility C) were tested. A description of the tested units is
presented in Table 1. The flue gases generated from units 1
and 2 at Facility C are fed into a common duct, then pass
through a flue gas desulfurization (FGD) system, and are
finally emitted through a common stack. However, due to an
outage, unit 1 was not in service during the testing period;
therefore, only emissions from unit 2 were sampled. Schematic diagrams of the sampling sites at each of the two tested
stacks are shown in Figure 1. At each stack, two sorbent trap
sampling systems were set up. Sampling probes in which two
sorbent traps were installed were used for in-stack measurements. Out-of-stack sampling was also employed, using an
inertial probe (at the common stack serving units 1 and 2)
and a cyclone probe (at unit 3). All sampling systems were
operated in accordance with EPA Method 30B. In the
out-of-stack sampling runs, particulate matter was first
II. Experimental Procedures
A. Testing Sites and Sampling Setup. 1. Conventional
(Nonspeciating) Sorbent Trap Sampling. In this part of the
study, two coal-fired sources (i.e., units 1/2 and unit 3 at
(4) Pan, W.-P.; Cheng, C.-M; Cao, Y. Long-Term Evaluation of
Mercury Monitoring Systems at Illinois Coal Fired Boilers, ICCI 06-1/
4.1C-1 Final Report: Carbondale, IL, 2007.
4832
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Energy Fuels 2009, 23, 4831–4839
: DOI:10.1021/ef900294s
Cheng et al.
separated from the flue gas. The flue gas was then delivered
through a heated transportation line to two sorbent traps
located outside of the stack. The purpose of using the out-ofstack approach was to minimize the deposition of particulates at the tip of the sorbent traps, which can cause operational difficulties, especially during long-term sampling. In a
previous study carried out at the common stack serving units
1 and 2,4 trap fouling was observed when in-stack sorbent
traps were placed in service for long-term sampling. Due to
the fouling, a high vacuum was built up inside the measurement system, which led to an unexpected termination of the
sampling. The potential advantage of out-of-stack sampling
is that it can be applied at locations where there is a high
particulate loading (e.g., at an ESP inlet).
In addition to the in-stack and out-of-stack sorbent trap
systems, mercury continuous emission monitoring systems
(CEMS) and Ontario Hydro (OH) method sampling trains
were also set up at each of the tested stacks to provide
reference values.
2. Speciating Sorbent Trap Sampling. The possibility of
using speciating sorbent traps as an alternative to the OH
method was studied at units 4/5 of Facility O. A description
of units 4/5 is presented in Table 1, and a schematic diagram
of the sampling locations is shown in Figure 1. The evaluation of the speciating traps was carried out at the inlet and
outlet of the FGD system. Out-of-stack sampling (with an
inertial probe) was used at the FGD inlet, and in-stack
measurement was used at the outlet. The OH method was
run concurrently at both locations to provide reference
mercury concentrations and speciation information. The
duration of each sorbent trap sampling run was more than
1 hour. After sampling was completed, the Hg samples were
brought back to ICSET’s analytical laboratory for recovery
and analysis.
B. Sorbent Trap Sampling and Analysis. All sorbent trap
sampling systems used in this study were provided by Apex
Instruments (Raleigh, NC). These systems continuously
extract a known volume of dry flue gas from the stack at a
constant flow rate of 0.2-0.6 L/min and capture vapor phase
Hg in the gas sample with a pair of sorbent traps. The
nonspeciating sorbent traps used in the study consisted of
two separate sections filled with activated carbon. The first
section was designed to capture the vapor phase Hg in the
flue gas. The second section was used for QA/QC purposes.
The speciating sorbent traps developed by ICSET contained
three sections. The first and second sections captured oxidized mercury and elemental mercury, respectively. The
third section was used for QA/QC purposes.
After sampling, each section of the nonspeciating traps
was analyzed for Hg, using either a direct combustion
technique or a wet-chemistry analytical method (i.e., microwave-assisted digestion coupled with cold-vapor atomic
absorption spectroscopy). The speciating traps were analyzed only by the direct combustion method. The Ohio
Lumex Mercury Analyzer (Ohio Lumex Co., Twinsburg,
OH) that was used for the direct-combustion decomposed
the sorbent material at a temperature between 600 and
800 °C. The Hg concentration was then determined using
Zeeman atomic absorption spectroscopy to measure the
mercury vapor released during the decomposition. For the
wet-chemistry method, the sorbent material recovered from
the two sections of each trap was digested separately. The
digestion was carried out as follows. Six milliliters of concentrated nitric acid, 1.5 mL of concentrated hydrofluoric
acid, and 1.5 mL of 30% (v/v) hydrogen peroxide were
transferred to a digestion vessel and mixed with the sorbent
material that was recovered from the sorbent trap. The vessel
was then heated in a microwave digestion system (Ethos EZ,
Milestone Inc., Shelton, CT), which was programmed to
increase the solution temperature to 225 °C in 30 min and
maintain that temperature for another 30 min. After digestion, the liquid sample was recovered from the digestion
vessel. The vessel was then rinsed with 5% nitric acid, and
the rinse was added to the digested sample. Demineralized
water was added to achieve a final solution volume of 50 mL.
The concentration of mercury in the solution was analyzed
using cold vapor atomic absorption spectroscopy (CVAAS,
Leeman Lab Hydra, Teledyne Leeman Laboratories,
Hudson, NH).
C. Ontario Hydro Method Sampling and Analysis. As
previously mentioned, during the short-term sampling runs,
the mercury concentration in the stack gas was concurrently
measured by the OH method, to provide reference values.
OH measurement systems provided by Apex Instruments
(Raleigh, NC) were used for the sampling. Each system
included a probe with glass linear, a heated filter box, a set
of glass impingers, an umbilical cord, and a metering console. The equipment setup for the OH method has been
described elsewhere.5 Flue gas samples were extracted isokinetically from the source and passed through a series of
impingers in an ice bath. Particle-bound mercury was collected in the front half of the sampling train on a quartz fiber
filter. Oxidized mercury was collected in a series of impingers
containing a chilled aqueous potassium chloride solution.
Elemental mercury was collected in subsequent impingers
(one impinger containing a chilled aqueous acidic solution of
hydrogen peroxide, and three impingers containing chilled
aqueous acidic solutions of potassium permanganate). After
sampling, all solutions were recovered and digested using an
automated mercury preparation system (Leeman Lab Hydra
Prep, Teledyne Leeman Laboratories, NH). A 4 mL aliquot
of each Hg absorbing solution (i.e., KCl, H2O2/HNO3,
and KMnO4/H2SO4) was recovered from the impingers
and transferred to a 15 mL digestion cup. Then, 0.2 mL of
concentrated H2SO4, 0.1 mL of concentrated HNO3, 1.2 mL
of 5% KMnO4, and 0.32 mL of 5% K2S2O8 were added
automatically to each cup by means of a dispenser. The cups
were heated in a water bath at a constant temperature of
95 °C for two hours. After cooling, 1.333 mL of 12%:12%
NaCl/hydroxylamine sulfate was added to prepare the solution
for mercury analysis by CVAAS (Leeman Lab Hydra, Teledyne Leeman Laboratories, NH). During analysis, 5% HNO3
was employed as the rinse solution and a 10% SnCl2/10% HCl
solution was utilized as the reducing agent. The peristaltic
pump was controlled at 5 mL/min, while the carrier gas was
ultrahigh-purity nitrogen flowing at a rate of 0.6 L/min.
D. Continuous Mercury Emission Monitoring Systems.
Two mercury CEMS (i.e., a Tekran 3300 system and a
Thermo Mercury Freedom system) were used in this study.
The Thermo instrument was installed on the units 1/2 stack,
and the Tekran monitor was mounted on the unit 3 stack.
Both of the CEMS used inertial-type sampling probes,
allowing for ash-free flue gas to be extracted from the stack.
Both systems also employed atomic fluorescence spectroscopy as the mercury detection method. The difference
(5) Cheng, C.-M.; Lin, H.-T.; Wang, Q.; Chen, C.-W.; Wang, C.-W.;
Liu, M.-C.; Chen, C.-K.; Pan, W.-P. Energy Fuels 2008, 22, 3040.
4833
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Cheng et al.
Table 2. Method Detection Limits (MDL) of the Direct Combustion
and Wet Chemistry Methods
Table 3. Analytical Bias Tests Results
concentration
level
direct combustion cold vapor atomic adsorption
run No.
standard, ng
1
2
3
4
5
6
7
standard deviation, S
MDLa(ng) = 3.143*S
18.0
19.0
21.0
19.0
19.0
18.0
20.4
1.12
3.5
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a
standard, ng/mL
0.028
0.031
0.023
0.019
0.018
0.015
0.022
0.0056
0.9
high
ng
1.4
1.55
1.15
0.95
0.9
0.75
1.1
0.28
spiked amount (ng)
1000
5000
8000
low
Method detection limit.
100
250
between the Tekran and Thermo analyzers is the application
of gold traps. Gold traps are used by the analyzer in the
Tekran 3300 system to selectively capture the elemental
mercury in the sample gas prior to the detector. In the
Thermo system, flue gas is continually delivered into the
detector without passing through gold traps, and therefore,
generates continuous readings of total vapor phase Hg.
500
measured
(ng)
% recoveryaverage
908
995
924
4430
5070
5380
7720
7670
7760
91.8
98.0
95.0
238.0
246.0
260.0
486.0
510.0
475.0
90.8
99.5
92.4
88.6
101.4
107.6
96.5
95.9
97.0
91.8
98.0
95.0
95.2
98.4
104.0
97.2
102.0
95.0
94.2
99.2
96.5
94.9
99.2
98.1
and unspiked traps. The difference between the Hg mass
recovered from the spiked and unspiked traps was assumed
to be the mass of the spiked Hg. The results of the three field
recovery test runs are shown in Table 4. Satisfactory spike
recoveries were obtained, ranging from 99.4 to 105%, indicating that: (1) the sampling and analytical procedures used
in this study effectively recovered the captured mercury; (2)
there were no adverse effects from the flue gas matrix; and (3)
there was no contamination during the sampling, transportation, and analytical processes.
III. Quality Assurance
The following procedures, which are described in sections
8.2.2.1, 8.2.3.1.1, and 8.2.6 of EPA Method 30B, were performed to quality-ensure the data obtained with the sorbent
trap sampling systems.
A. Method Detection Limit Determination. The method
detection limits (MDL) of the direct combustion and wetchemical analytical methods were determined by using the
methods to analyze a National Institute of Standards and
Technology (NIST) traceable mercury standard with a mass
or concentration level five times higher than the instrument
noise. The MDL is defined as the minimum concentration of
a substance that can be measured and reported, with 99%
confidence that the concentration is greater than zero. Seven
analyses of the Hg standard were performed with each
method. The MDL was determined by multiplying the
standard deviation of the measurements by a t-test value.
The results are presented in Table 2. The MDL values for the
direct combustion and wet-chemistry methods were found to
be 3.5 and 0.9 ng, respectively. Although the MDL value of
the wet-chemistry method is lower than the direct combustion method, it is not as sensitive due to dilution occurred
during sample preparation.
B. Analytical Bias Test. An analytical bias test was carried
out in the lab to demonstrate the ability of the two analytical
procedures to recover and to accurately quantify mercury
from the sorbent material. The test was performed by spiking
the sorbent at the lower (100-500 ng) and higher ends
(1000-8000 ng) of expected mercury concentration levels.
A NIST-traceable mercuric chloride standard was used for
the spiking. The results are presented in Table 3, and indicate
excellent spike recoveries, ranging from 94.2 to 99.2%.
C. Field Recovery Test. A field recovery test, using three
sets of paired sorbent traps, was conducted to verify the
performance of the in-stack sampling and the direct combustion mercury analysis procedures adopted in this study. One
of the traps in each pair was spiked with a known level of
mercury (i.e., 240 ng). Then, flue gas was sampled with each
pair of traps and the Hg was recovered from the traps and
analyzed. For each sample run, the spike recovery was
calculated by comparing the analytical results of the spiked
IV. Discussion of Results
A. Overall Monitoring Results. In September and October
2008, a total of 13 and 9 short-term (1.5 h) “in-stack”
sampling runs were conducted at units 1/2 and 3, respectively, using nonspeciating sorbent traps. During the shortterm sampling, concurrent OH method measurements were
made to provide reference values. The results of the shortterm sorbent trap and OH method measurements are summarized in Table 5, along with the Hg concentrations
measured by the CEMS during the sampling runs.
The relative accuracies of the sorbent trap systems and the
CEMS are also shown in Table 5. Relative accuracy (RA)
was calculated according to section 7.3 of Part 75, Appendix
A. The sorbent-trap systems and the CEMS provided satisfactory RA results when compared against the performance
specifications that were developed for the CAMR regulation
(i.e., e 20% RA or an absolute mean difference e1.0 μg/m3).
In cases where the RA exceeded 20%, the alternative specification for low-emitting sources, that is, absolute mean
difference e1.0 μg/m3, was met. No statistically significant
bias was observed for either the regular sorbent trap systems
or the CEMS. Due to operational difficulties, valid data
from the CEMS installed on the units 1/2 stack were not
recorded for four of the sampling runs. However, nine valid
CEMS runs, which is the number of runs traditionally used
for RA calculations, were still obtained.
Short-term sampling was also performed using two outof-stack sorbent trap methods (i.e., one using an inertial
probe, and the other, a cyclone probe). The results of the outof-stack test runs are summarized in Table 6, along with the
corresponding OH values. The results obtained with the two
out-of-stack sampling methods exceeded 20% RA, but they
are satisfactory when compared against the alternative RA
specification. Both out-of-stack methods had higher percent
4834
Energy Fuels 2009, 23, 4831–4839
: DOI:10.1021/ef900294s
Cheng et al.
Table 4. Field Recovery Test Results from Regular Sorbent Trap Sampling Method
run 1
trap ID
Hg spiked (μg)
trap A: 004804
0.2400
trap B: 004931
run 2
trap A: 004832
0.2400
trap B: 004947
run 3
trap A: 004807
0.2400
trap B: 004806
a
section No.
Hg (μg)
spiked Hg recovered (μg)
% recovery
S1
S2
S1
S2
S1
S2
S1
S2
S1
S2
S1
S2
0.4290
0.0009
0.1930
0.0003
0.4310
0.0007
0.1860
0.0003
0.4440
0.0008
0.1950
0.0003
0.2360
99.4
0.2450
103.0
0.2490
105.0
Micrograms per dry standard cubic meter in 20 °C and 3% O2.
Table 5. Relative Accuracy Results for Sorbent Traps and CEMS
sorbent traps
b
OHM data sorbent trap data
relative accuracy results
date
run Time
μg/dscma
1
2
3
4
5
6
7
8
9
10
11
12
13
9/12/08
9/12/08
9/13/08
9/13/08
9/13/08
9/13/08
9/30/08
9/30/08
10/1/08
10/1/08
10/1/08
10/2/08
10/2/08
11:00-12:30
14:15-15:45
09:17-10:47
11:37-13:07
14:46-16:16
16:58-18:28
10:32-12:02
16:16-17:46
09:22-10:52
12:46-14:16
14:45-16:15
10:00-11:30
11:50-13:20
2.00
2.49
2.10
2.55
1.85
1.94
2.33
2.36
1.35
1.56
1.50
1.48
1.35
1.55
2.84
2.01
1.82
1.90
1.83
2.53
2.62
1.68
1.88
1.54
1.79
1.49
Units 1/2
0.45
0.05
-0.35
0.10
0.73
-0.05
0.12
-0.20
-0.26
-0.33
-0.32
-0.04
-0.31
-0.14
1
2
3
4
5
6
7
8
9/12/08
9/12/08
9/13/08
9/13/08
9/13/08
9/13/08
9/30/08
9/30/08
11:00-12:30
14:15-15:45
09:17-10:47
11:37-13:07
14:46-16:16
16:58-18:28
10:32-12:02
16:16-17:46
2.44
3.37
2.72
2.64
2.39
2.45
2.89
2.63
2.76
2.81
2.43
2.75
2.96
2.53
3.04
3.13
Unit 3
-0.32
0.09
0.56
0.30
-0.10
-0.56
-0.08
-0.15
-0.50
run No.
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d
cems
μg/dscma
|d|c
b
CEMS data
d
μg/dscma
RAd
relative accuracy results
|d|c
RAd
12.48
1.12
3.39
na
na
2.39
1.96
1.40
1.29
1.05
1.37
1.32
na
na
0.88
-0.90
na
na
-0.54
-0.02
0.93
1.07
0.30
0.19
0.18
na
na
0.23
35.58
13.80
2.92
2.99
2.50
2.67
2.64
2.69
2.18
2.26
-0.48
0.38
0.22
-0.03
-0.25
-0.24
0.71
0.37
0.10
14.63
a
Micrograms per dry standard cubic meter in 20 °C and 3% O2. b Difference between results from the method and OHM. c Absolute mean difference
between results from the method and OHM. d Relative accuracy was calculated according to section 7.3 of 40 CFR Part 75, Appendix A.
Table 6. Relative Accuracy Results from Out-of-Stack Sorbent Trap Sampling
run No.
OHM
sorbent trap data, Avg.
db
μg/dscma
μg/dscma
μg/dscma
|d|c
RAd
RA results
date
run time
1
2
3
4
5
6
9/25/08
9/25/08
9/30/08
9/30/08
10/1/08
10/1/08
12:30-14:00
15:40-17:10
10:32-12:02
16:16-17:46
12:46-14:16
14:45-16:15
Inertial Probe at Units 1/2
2.36
2.41
2.12
2.80
2.33
1.35
2.36
2.23
1.56
1.80
1.50
2.13
-0.05
-0.68
0.98
0.13
-0.24
-0.63
0.08
33.87
1
2
3
9/25/08
9/25/08
9/25/08
10:00-11:30
12:50-14:20
14:50-16:20
Cyclone Probe at Unit 3
2.89
3.92
2.63
3.14
2.50
2.37
-1.03
-0.51
0.13
0.47
71.30
a
Micrograms per dry standard cubic meter in 20 °C and 3% O2. b Difference between results from the sorbent trap and OH methods. c Absolute mean
difference between results from the sorbent trap and OH methods. d Relative accuracy was calculated according to section 7.3 of 40 CFR Part 75,
Appendix A.
RA values than the in-stack sampling method. This was
likely due, at least in part, to the small body of valid samples
obtained with the out-of-stack methods. Only six and three
valid sampling runs, respectively, were obtained with the
4835
Energy Fuels 2009, 23, 4831–4839
: DOI:10.1021/ef900294s
Cheng et al.
Table 7. Results of Long-Term Sorbent Trap Sampling
sorbent traps
unit(s)
1/2
3
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Published on August 19, 2009 on http://pubs.acs.org | doi: 10.1021/ef900294s
a
sampling dates and times
9/14/08 (08:37) - 9/21/08 (07:43)
9/21/08 (08:18) - 9/24/08 (16:20)
9/14/08 (08:50) - 9/24/08 (16:10)
9/26/08 (15:46) - 9/30/08 (08:23)
9/14/08 (15:00) - 9/16/08 (08:45)
9/16/08 (09:30) - 9/17/08 (10:43)
9/17/08 (11:00) - 9/18/08 (08:15)
9/20/08 (15:40) - 9/25/08 (08:00)
in-stack (μg/dscma)
out-of-stack (μg/dscm)
χ
1.95
1.43
NAb
NAb
0.67
1.26
NAb
1.22
NA
NA
1.75
0.29
NA
1.28
1.09
1.19
CEMS
1.28
0.88
1.30
0.41
0.63
1.00
1.14
1.13
Micrograms per dry standard cubic meter in 20 °C and 3% O2. b Not participated in the testing.
inertial and cyclone methods. Several sampling runs performed on 9/12/08 and 9/13/08 were contaminated by the
connection assembly between the sampling probes and sorbent traps, and had to be discarded
Long-term (18 h or longer) sorbent trap sampling was
conducted at both the units 1/2 stack and unit 3, to evaluate
the effects of sample duration on the test results. In-stack and
out-of-stack sampling was performed at both test locations.
However, the in-stack and out-of-stack methods were run
concurrently for only two of the tests, that is, the 9/16/08-9/
17/08 and 9/20/08-9/25/08 tests at unit 3. The results of the
long-term tests were compared against concurrent data
recorded by the CEMS. The results of these comparisons
are summarized in Table 7. No OH method measurements
were made during the long-term testing. Table 7 shows that
the results from the out-of-stack sampling methods agreed to
within (0.5 μg/m3 of both the CEMS data and the concurrent in-stack results. Unlike the previous study carried
out at the units 1/2 stack,4 no trap fouling was observed from
the in-stack sampling during this study, even for the longer
(4-7 days) sampling durations. The trap fouling that
occurred during the previous study was likely due to the
deposition of fine droplets of FGD slurry at the tip of the
sorbent trap inlet. The absence of trap fouling during this
study may have been the result of lower stack gas flow rates.
Due to the outage of unit 1, the stack gas flow rate was about
13 million standard cubic feet per hour (scfh) during
the testing period, which is about one-half the flow rate
observed in the previous tests. The lower flue gas flow rates in
this study may have reduced the slurry droplet carry-over
phenomenon.
B. Comparison of Sorbent Trap Analytical Methods.
Twenty two (22) sorbent traps were analyzed using direct
combustion, and another 22 traps were digested and analyzed for mercury using CVAAS. The relative difference
between the results from each sorbent trap analysis and its
corresponding OH reference value was calculated using the
following equation:
ðCHg, OH -CHg, Trap Þ
100%
Relative difference ¼
CHg, OH
Figure 2. Relative difference between OH method measurements
and the results obtained from the digestion and direct combustion
sorbent trap analytical methods.
represents the value within the 1.5 interquartile range from
either the first or the third quartile. The points outside of the
two whiskers are outliers.
On average, the results from the direct combustion were
about 8.44% higher than the OH method, while the average
wet chemistry results were in near-perfect agreement with the
OH method. In view of this, one might have concluded that
the direct combustion analytical method has an inherent
high bias compared to the wet chemistry method. However,
when the data were examined more closely, a wide scatter of
variations was found. It is therefore possible that the difference observed between the two analytical methods was due
to random sampling or analytical errors. To test this hypothesis, a one-way analysis of variance (ANOVA) method was
performed to determine whether the difference between these
two data sets is statistically significant. The calculated
p value was 0.06, which is less than the selected standard of
0.1. Therefore, the difference between the two data sets is
significant. With the standard of 0.1, based on Power
Analysis, the power of the experiment is about 73% with a
sample size of 22. The “power” is the ability to reject the null
hypothesis when it is not true. Therefore, the sample size is in
an acceptable range.
C. Evaluation of Relative Deviation and Breakthrough. The
relative deviation (RD) is a measure of the agreement
between the analytical results from a pair of sorbent traps.
The following equation was used to calculate the RD values
in this study:
jCa -Cb j
100%
RD ¼
Ca þCb
where CHg,OH is the Hg concentration obtained from the OH
measurement and CHg,Trap is the Hg concentration obtained
from the sorbent trap.
The “box and whisker” plots shown in Figure 2 graphically illustrate several important statistical features of the
data set. Each box encloses the interquartile range with the
lower edge at the first quartile and the upper edge at the third
quartile. The horizontal line drawn through the box represents the median value at the second quartile. The whisker,
which is a vertical line extending from each end of the box,
4836
Energy Fuels 2009, 23, 4831–4839
: DOI:10.1021/ef900294s
Cheng et al.
Downloaded by WESTERN KENTUCKY UNIV on October 21, 2009
Published on August 19, 2009 on http://pubs.acs.org | doi: 10.1021/ef900294s
Figure 4. Sorbent trap breakthrough during short-term and longterm sampling periods when using regular in-stack method and outof-stack inertial, and cyclone methods.
Another sampling QA/QC parameter, that is, breakthrough, was also evaluated. The breakthrough of each trap
after sampling was evaluated by the following equation:
m2
B ¼
100%
m1
where m1 and m2 are the mass of Hg measured in sections 1
and 2 of the sorbent trap, respectively. The results of the
breakthrough measurements for all sorbent traps analyzed in
the study are presented in Figure 4.
It was found that for short-term in-stack sampling, 6 (out
of 50) traps had more than 10% breakthrough. The breakthrough was relatively low for both out-of-stack methods
during short-term sampling. Only one out-of-stack sorbent
trap (out of 18) exceeded 10% breakthrough when the
inertial probe was used. None of the 6 cyclone method traps
showed more than 10% breakthrough. However, as shown
in Figure 4, significant breakthrough was observed for the
traps used in the long-term cyclone sampling. It was likely
caused by the high operational temperature of the cyclone,
which was maintained at 200 °C during the sampling period.
Temperature effects may have caused some of the captured
mercury to migrate through the sorbent trap column from
section 1 to section 2. Only minimal breakthrough was
observed for the other two sampling approaches during
long-term sampling. It is therefore believed that precise
temperature control of sorbent trap sampling systems is
important to ensure that good data are obtained, particularly for long sample runs.
D. Evaluation of Speciating Sorbent Traps. The speciating
sorbent trap was evaluated in January and February 2009.
The results of total mercury concentration measurements
(i.e., HgT) and mercury speciation data from the evaluation
are summarized in Table 8. The out-of-stack inertial sampling approach was used at the FGD inlet location. The
concentrations and speciation of mercury at the FGD inlet
and at the outlet stack were also measured using the
OH method to provide reference values. The OH method
measurements were carried out concurrently with selected
sorbent trap runs.
For all test runs shown in Table 8, excellent agreement was
obtained between the total Hg concentrations measured by
the A and B sorbent traps. For two other runs carried out on
2/4/09 at the FGD inlet and a sampling run conducted on
2/9/09 at the outlet stack (data not shown), the relative
Figure 3. Relative deviation of paired sorbent trap measurement
from the in-stack and out-of-stack sampling methods, during:
(a) short-term sampling and (b) long-term sampling.
where Ca and Cb are the Hg concentrations measured with
sorbent traps “a” and “b,” respectively. Ca and Cb were
calculated using the following equation:
ðm1 þm2 Þ
Ca ¼
Vt
where m1 and m2 are the mass of Hg measured on sorbent
trap sections 1 and 2, respectively; and Vt is the total volume
of dry gas measured during the sampling period.
According to EPA Method 30B, for Hg concentrations
greater than 1 μg/dscm, the RD value of each sorbent trap
sampling run must be less than 10% to validate the run.
Figure 3 illustrates the relative deviation results for the
sampling methods used in this study. For the in-stack
measurements, the agreement between the total mercury
readings from the paired traps was excellent for both
short-term and long-term sampling. All of the RD values
were within 10%, with the exception of one measurement.
The relative deviation results were not as good for the out-ofstack sampling methods, particularly for the inertial probe
method, in the short term sampling. However, the RD values
for both out-of-stack methods were significantly lower in the
long term sampling, suggesting that the out-of-stack sampling setup may have created unevenly distributed flow
within the flue gas stream during the early stages of the
sampling, causing higher relative deviation between the two
traps during the short test runs. The variation apparently
became insignificant as sampling progressed, and the RD for
the long-term out-of-stack sampling met the 10% criterion.
4837
Energy Fuels 2009, 23, 4831–4839
: DOI:10.1021/ef900294s
Cheng et al.
Table 8. Comparison of Speciating Sorbent Trap and OH Method
HgT (μg/dscm)
Hgo /HgT (%)
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Published on August 19, 2009 on http://pubs.acs.org | doi: 10.1021/ef900294s
Speciating sorbent trap
date
run time
trap A
trap B
01/22/09
02/03/09
02/04/09
02/04/09
02/05/09
02/06/09
02/06/09
02/06/09
02/09/09
02/09/09
02/09/09
10:00-11:30
09:30-11:35
10:25-11:58
12:14-13:36
13:50-15:20
10:30-11:40
11:45-12:45
12:50-13:50
10:51-11:56
13:10-14:15
16:08-17:08
18.5
17.2
18.7
17.5
18.6
17.8
17.8
19.3
24.1
17.7
16.2
18.0
18.2
16.8
15.6
17.0
18.9
17.8
19.9
23.2
18.2
16.8
01/20/09
01/22/09
01/22/09
01/23/09
01/23/09
02/04/09
02/04/09
02/04/09
02/05/09
02/06/09
02/09/09
02/09/09
02/10/09
02/10/09
02/10/09
02/10/09
11:30-13:00
10:00-11:30
11:55-13:30
09:25-11:30
11:25-13:00
11:45-13:00
13:30-14:40
15:35-16:35
15:30-16:30
12:20-13:50
13:25-14:55
15:11-16:41
11:25-12:30
12:54-13:54
14:02-15:07
15:18-16:23
14.2
17.4
14.6
9.6
10.0
9.6
2.1
7.8
4.4
7.0
19.1
14.7
20.6
19.0
19.0
19.3
14.0
17.5
14.7
9.6
10.0
8.7
2.3
7.6
4.5
7.1
19.8
14.9
18.3
17.8
17.9
19.6
Speciating sorbent trap
a
trap A
trap B
OH
RDTrap-OHb
26.6
7.5
8.0
6.8
11
15.8
13.1
14.8
16.1
26.6
15
29.1
8.7
9.8
9.3
12.1
12.1
16.1
18.8
20.0
27.7
15.4
16.0
15.7
14.7
14.5
NA
NA
NA
NA
NA
26.7
NA
6.98
4.44
3.45
3.97
NA
NA
NA
NA
NA
0.59
NA
Units 4/5 Outlet Stack at Plant O Stack
14.1
0.8%
14.4
65.3
17.5
0.3%
15.7
66.6
14.6
0.2%
18.2
69.5
9.6
0.3%
9.5
74.1
10.0
0.4%
11.8
64.5
9.1
5.4%
NA
64.9
2.2
4.5%
NA
52.3
7.7
1.6%
NA
51.3
4.4
1.0%
NA
46.3
7.1
0.6%
NA
68.1
19.5
1.8%
NA
84.3
14.8
0.9%
NA
71.0
19.4
5.7%
17.4
81.4
18.4
3.4%
NA
79.8
18.5
3.0%
NA
81.5
19.4
0.6%
NA
85.9
70.0
76.3
65.6
73.3
62.7
90.1
62.9
73.2
65.4
78.2
86.3
78.2
82.5
77.2
81.7
81.3
62.1
55.8
44.8
50.0
44.9
NA
NA
NA
NA
NA
NA
NA
53.7
NA
NA
NA
3.96
10.25
13.30
13.68
10.80
NA
NA
NA
NA
NA
NA
NA
14.37
NA
NA
NA
avg.
RD
OH
Units 4/5 FGD Inlet at Plant O
18.2
1.2%
13.2
17.7
3.1%
16.6
17.7
5.4%
15.9
16.6
5.7%
16.0
17.8
4.5%
NAc
18.4
3.1%
NA
17.8
0.0%
NA
19.6
1.6%
NA
23.7
2.0%
NA
18.0
1.5%
24.7
NA
1.7%
NA
a
RD: relative deviation of paired sorbent traps results in HgT measurement. b RDTrap-OHM: relative difference of sorbent trap and OHM results in
Hg0/HgT ratio. c Not available: OH method measurement was not carried out.
deviations of paired speciation traps were higher than 10%.
Also included in Table 8 are the ratios of elemental mercury
to total mercury, that is, Hg0/HgT, which ranged from 6.8 to
27.7% at the FGD inlet and from 46.3 to 90.1% at the stack.
The Hg0/HgT ratios observed from speciating sorbent trap
measurements at both FGD inlet and stack are comparable
to the speciation data obtained from the OH method measurements. A greater variation in the Hg0/HgT ratio was
observed at the stack, possibly due to nonhomogeneous
distribution of mercury species in the flue gas or nonoptimized sampling conditions. Although the measured mercury
speciation varied among the test runs, the results from the
same day are similar, suggesting the variation might be due,
at least in part, to the operating conditions of the boiler and/
or the wet scrubbers, rather than the instability of the
sampling systems.
Results from both sorbent trap and OH measurements
show that the stack mercury emission varied significantly
during the testing period. The concentration levels of total
mercury were actually as high or higher at the outlet stack as
the concentrations observed at the FGD inlet, for several of
the sampling runs on 2/9/09 and 2/10/09. Similar variation in
the outlet stack mercury concentration was also seen in a
previous study carried out at the same stack, in which the
mercury concentration was continuously monitored using a
Thermo Mercury Freedom CEM system. The apparent low
mercury removal by the FGD system was most likely due to
the re-emission of elemental mercury, which may have been
caused by changes in scrubber or boiler operation. However,
it is not clear why the re-emission phenomenon was more
noticeable at some periods than at others. A study has
currently being carried out by ICSET at the same stack to
try to correlate changes in boiler and FGD operating conditions with the re-emission phenomenon.
From the summarized test results in Table 8, it is clear that
the mercury speciating trap evaluated in this study is able to
provide useful information on the change of mercury species
across a wet-scrubber. In addition, the paired traps from the
same sampling run at a given sampling location showed
satisfactory relative deviations for both mercury speciation
and total mercury concentration, with the exception of test
runs carried out at the early stages of the study, when the
sampling system operational parameters had not yet been
optimized. Using the all of the data shown in the Table,
excluding the 11th runs at the FGD inlet, the relative
accuracy (RA) of the speciating sorbent traps was calculated
to be 14.8% on a total Hg basis, indicating the speciating
sorbent trap can provide satisfactory results for total mercury measurement.
V. Conclusions
The conventional in-stack sorbent trap sampling method
and the two modified out-of-stack sampling methods tested in
this study provided satisfactory measurements of total vapor
phase mercury, when compared against the OH reference
method. The relative deviation of the paired traps was found
to be higher for the short-term, out-of-stack sampling than for
the short-term, in-stack sampling. The long-term results from
the out-of-stack sampling methods agreed with both in-stack
measurements and data recorded by Hg CEMS. The use of
4838
Energy Fuels 2009, 23, 4831–4839
: DOI:10.1021/ef900294s
Cheng et al.
out-of-stack methods appears to be feasible for long-term
sampling, and is potentially useful at sampling locations where
trap fouling is a concern. However, further investigation of outof-stack sorbent trap sampling is needed to optimize the
operational parameters and conditions for this approach.
For the in-stack measurements, the duration of sampling
(ranging from 1 to 7 days) did not have an observable effect on
data quality. No detectable breakthrough was observed for
the longest (7 days) sampling run, and the relative deviation of
the paired traps was less than 10%. Therefore, it is feasible to
perform in-stack sorbent trap sampling for up to a week at a
time. This finding reinforces the technical merit of the vacated
continuous in-stack sorbent trap monitoring method that was
developed for the CAMR rule, that is, the former Appendix K
of 40 CFR Part 75.
Results of the direct combustion and wet chemistry analytical methods used in this study were compared using the
one-way analysis of variance (ANOVA) method. The calculated p value was less than 0.1, indicating that there is a
statistically significant difference between the results provide
by the two analytical methods. Results from the wet chemistry
method agreed well with the OH reference method, but the
direct combustion results were about 8% higher than the OH
method measurements.
The mercury speciating trap developed by the ICSET
generally provided accurate total mercury concentration data
and was able to detect changes in Hg species across a wet
scrubber. Therefore, the speciating trap shows promise as a
potential alternative to the OH reference method. However,
further study of the trap’s performance under precisely controlled sampling conditions is needed to establish its equivalency to the OH method.
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Published on August 19, 2009 on http://pubs.acs.org | doi: 10.1021/ef900294s
Acknowledgment. This paper was prepared by ICSET with
support, in part, by grants made possible by the Illinois Department of Commerce and Economic Opportunity through the
Office of Coal Development and the Illinois Clean Coal Institute.
The authors thank Mr. Robert Vollaro of US EPA for his
valuable and stimulating comments during preparation of the
manuscript.
4839