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
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.
[email protected]. (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
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Table 1. Description of the Tested Units facility C C O 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.
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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.
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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
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
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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
d
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.
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
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Table 7. Results of Long-Term Sorbent Trap Sampling sorbent traps unit(s) 1/2
3
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
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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
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Table 8. Comparison of Speciating Sorbent Trap and OH Method HgT (μg/dscm)
Hgo /HgT (%)
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
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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. 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.
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