4831r 2009 American Chemical Society pubs.acs.org/EF

Energy Fuels 2009, 23, 4831–4839 : DOI:10.1021/ef900294sPublished on Web 08/19/2009

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, 2413Nashville 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 tomeasure total vapor phase mercury (Hg), was carried out at two coal-fired power plants. The objective ofthe study was to evaluate the effects (if any) on data quality when the following aspects of the sorbent trapmethod are varied: (a) sorbent trap configuration; (b) sampling time; and (c) analytical technique. Also, theperformance of a speciating sorbent trap (i.e., a trap capable of separating elementalHg fromoxidizedHg),developed by the Western Kentucky University’s Institute for Combustion Science and EnvironmentalTechnology (ICSET), was evaluated by direct comparison against the Ontario Hydro (OH) referencemethod. Flue gas samples were taken using both “regular” and modified sorbent trap measurementsystems. The regular sorbent trap systems used a dual-trap, in-stack sampling technique. The modifiedsystems 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 runconcurrently 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 thesorbent trap sampling runs. After sampling, the conventional (nonspeciating) sorbent traps were analyzedfor Hg, using either a direct combustion method or a wet-chemistry analytical method (i.e., microwave-assisted digestion coupled with cold vapor atomic absorption spectroscopy). The speciating traps wereanalyzed only by the direct combustion method. All of the sorbent trap data collected in the study wereevaluated 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 boththe short-term and long-term testing. For the short-term tests, the in-stack sampling results comparedmore favorably to the OH method than did the out-of-stack results. The relative deviation between thepaired 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) betweenthe direct combustion and wet-chemistry analytical methods used in the study; the results from the directcombustion method were consistently higher than the wet-chemistry results. The evaluation of thespeciating mercury sorbent trap demonstrated that the trap is capable of providing reasonably accuratetotal mercury concentrations and speciation data that are somewhat comparable to data obtainedwith theOHmethod. Although the results of the study were informative and promising, further evaluation of boththe out-of-stack sampling methods and the speciating sorbent trap is recommended.

I. Introduction

Coal combustion processes may result in the emission ofhazardous air pollutants (HAP), which include mercurycompounds. Currently, the largest source of mercury pollu-tion in America is from coal-burning power plants. Approxi-mately 50 tons of mercury are emitted annually by the utilityindustry as a result of coal use.1 Mercury emissions frompower plants can pollute rivers and lakes, contaminating the

fish living in these waters, and has also been known to causeneurological and developmental damage in humans.2,3 OnMay 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 innationwide Hg mass emissions from coal-fired electricitygeneration units (EGUs) by 2018. CAMR would have re-quired affected sources to continuously monitor and report

† 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. USEnvironmental Protection Agency, Technical Report, EPA-452/R-96-001b,Office of Air Quality Planning and Standards: Washington, DC, 1996.

(2) DOE/EIA U.S. Coal Reserves: 1997 Update. US Department ofEnergy, Energy InformationAdministration, Office of Coal, Nuclear, ElectricandAlternate Fuels,Office of IntegratedAnalysis and ForecastingDOE/EIA-0529 (97): Washington, DC, 1999.

(3) National Research Council Toxicological Effects of Methylmer-cury.Committee on the Toxicological Effects ofMethylmercury Board onEnvironmental Studies and Toxicology, Commission on Life Sciences;National Academy Press: Washington, DC, 2000.

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their cumulative annual Hg mass emissions. However,CAMRwas challenged on legal grounds, and the U.S. Courtof Appeals for the District of Columbia vacated the rule in2008. EPA is expected to propose a more restrictive Hgcontrol regulation (specifically, a maximum achievable con-trol technology, or “MACT” standard) for coal-fired EGUsin the near future.

Despite the status of the Federal mercury rule, approxi-mately 20 states (e.g., Illinois, Pennsylvania, Connecticut,Maine, Massachusetts, and others) have adopted regulationsto limit Hg emissions from power plants. To ensure that theHg emission reduction goals can be met, many of these statesrequire continuous emission monitoring (CEM) systems orsorbent-trap systems to be installed and operated by affectedelectric utility units.

Currently, sorbent-trap sampling is one of the few suitablemethods for continuously monitoring total vapor phase Hgemissions. It is a viable alternative to a mercury continuousemission monitoring system (Hg CEMS), particularly whenthe Hg concentration in the flue gas is very low. InMay 2005,EPA first published a continuous sorbent trap samplingmethod in support of the CAMR rule. This method, whichwas found in Appendix K of 40 CFR Part 75, was latervacatedby theDCCourt ofAppeals. InSeptember 2007,EPApublished Reference Method 30B, a stack test method thatuses sorbent traps tomeasure total vapor phaseHg emissions.Method 30B is similar in principle to vacated Appendix K,and the basic sampling equipment is the same, but Method30B hasmuchmore rigorous quality assurance procedures. Ina previous study4 that evaluated the Appendix K method-ology, a number of issues arose concerning some of thesampling and analytical procedures and the sample collectiontime. In reviewing the results of that study, ICSET concludedthat further investigation and refinement of the sorbent trapsampling method is needed.

In view of this, ICSET, in collaboration with the IllinoisCleanCoal Institute (ICCI), initiated a full-scale investigationof the sorbent trap monitoring method, using in-stack andout-of-stack sampling techniques and two different analyticalmethods (i.e., direct combustion and microwave-assisteddigestion coupled with cold vapor atomic absorption spectro-scopy). A speciating sorbent trap was also tested, to assess itsability to provide credible total Hg concentrations and spe-ciated Hg emissions data., with a view toward using it as apossible alternative to the cumbersome Ontario Hydro (OH)method. The following sections describe the experimentalprocedures that were used in the investigation and presentthe results of the study.

II. Experimental Procedures

A. Testing Sites and Sampling Setup. 1. Conventional(Nonspeciating) Sorbent Trap Sampling. In this part of thestudy, two coal-fired sources (i.e., units 1/2 and unit 3 at

Facility C) were tested. A description of the tested units ispresented in Table 1. The flue gases generated from units 1and 2 at Facility C are fed into a common duct, then passthrough a flue gas desulfurization (FGD) system, and arefinally emitted through a common stack. However, due to anoutage, unit 1 was not in service during the testing period;therefore, only emissions from unit 2 were sampled. Sche-matic diagrams of the sampling sites at each of the two testedstacks are shown in Figure 1. At each stack, two sorbent trapsampling systems were set up. Sampling probes in which twosorbent traps were installed were used for in-stack measure-ments. Out-of-stack sampling was also employed, using aninertial probe (at the common stack serving units 1 and 2)and a cyclone probe (at unit 3). All sampling systems wereoperated in accordance with EPA Method 30B. In theout-of-stack sampling runs, particulate matter was first

Table 1. Description of the Tested Units

facility boiler(s) boiler type capacity (MWe) fuel type emission controls

C units 1 and 2 cyclone 180 (combined) bituminous coal SCRa þ ESPb þ FGDc

C unit 3 wall-fired 205 bituminous coal SCR þ ESP þ FGDO units 4 and 5 unit 4 cyclone unit 5 T-fired 425 (combined) bituminous coal SCR þ ESP þ FGD

a Selective catalytic reduction. bElectrostatic precipitator. cFlue gas desulfurization.

Figure 1. Sampling Configurations for the Tested Units.

(4) Pan, W.-P.; Cheng, C.-M; Cao, Y. Long-Term Evaluation ofMercury 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 deliveredthrough a heated transportation line to two sorbent trapslocated outside of the stack. The purpose of using the out-of-stack approach was to minimize the deposition of particu-lates at the tip of the sorbent traps, which can cause opera-tional difficulties, especially during long-term sampling. In aprevious study carried out at the common stack serving units1 and 2,4 trap fouling was observed when in-stack sorbenttraps were placed in service for long-term sampling. Due tothe fouling, a high vacuum was built up inside the measure-ment system, which led to an unexpected termination of thesampling. The potential advantage of out-of-stack samplingis that it can be applied at locations where there is a highparticulate loading (e.g., at an ESP inlet).

In addition to the in-stack and out-of-stack sorbent trapsystems, mercury continuous emission monitoring systems(CEMS) and Ontario Hydro (OH) method sampling trainswere also set up at each of the tested stacks to providereference values.

2. Speciating Sorbent Trap Sampling. The possibility ofusing speciating sorbent traps as an alternative to the OHmethod was studied at units 4/5 of Facility O. A descriptionof units 4/5 is presented in Table 1, and a schematic diagramof the sampling locations is shown in Figure 1. The evalua-tion of the speciating traps was carried out at the inlet andoutlet of the FGD system. Out-of-stack sampling (with aninertial probe) was used at the FGD inlet, and in-stackmeasurement was used at the outlet. The OH method wasrun concurrently at both locations to provide referencemercury concentrations and speciation information. Theduration of each sorbent trap sampling run was more than1 hour. After sampling was completed, the Hg samples werebrought back to ICSET’s analytical laboratory for recoveryand analysis.

B. Sorbent Trap Sampling and Analysis. All sorbent trapsampling systems used in this study were provided by ApexInstruments (Raleigh, NC). These systems continuouslyextract a known volume of dry flue gas from the stack at aconstant flow rate of 0.2-0.6 L/min and capture vapor phaseHg in the gas sample with a pair of sorbent traps. Thenonspeciating sorbent traps used in the study consisted oftwo separate sections filled with activated carbon. The firstsection was designed to capture the vapor phase Hg in theflue gas. The second section was used for QA/QC purposes.The speciating sorbent traps developed by ICSET containedthree sections. The first and second sections captured oxi-dized mercury and elemental mercury, respectively. Thethird section was used for QA/QC purposes.

After sampling, each section of the nonspeciating trapswas analyzed for Hg, using either a direct combustiontechnique or a wet-chemistry analytical method (i.e., micro-wave-assisted digestion coupled with cold-vapor atomicabsorption spectroscopy). The speciating traps were ana-lyzed only by the direct combustion method. The OhioLumex Mercury Analyzer (Ohio Lumex Co., Twinsburg,OH) that was used for the direct-combustion decomposedthe sorbent material at a temperature between 600 and800 �C. The Hg concentration was then determined usingZeeman atomic absorption spectroscopy to measure themercury vapor released during the decomposition. For thewet-chemistry method, the sorbent material recovered fromthe two sections of each trap was digested separately. Thedigestion was carried out as follows. Six milliliters of con-centrated nitric acid, 1.5 mL of concentrated hydrofluoric

acid, and 1.5 mL of 30% (v/v) hydrogen peroxide weretransferred to a digestion vessel and mixed with the sorbentmaterial that was recovered from the sorbent trap. The vesselwas then heated in a microwave digestion system (Ethos EZ,Milestone Inc., Shelton, CT), which was programmed toincrease the solution temperature to 225 �C in 30 min andmaintain that temperature for another 30 min. After diges-tion, the liquid sample was recovered from the digestionvessel. The vessel was then rinsed with 5% nitric acid, andthe rinse was added to the digested sample. Demineralizedwater was added to achieve a final solution volume of 50mL.The concentration of mercury in the solution was analyzedusing cold vapor atomic absorption spectroscopy (CVAAS,Leeman Lab Hydra, Teledyne Leeman Laboratories,Hudson, NH).

C. Ontario Hydro Method Sampling and Analysis. Aspreviously mentioned, during the short-term sampling runs,the mercury concentration in the stack gas was concurrentlymeasured by the OH method, to provide reference values.OH measurement systems provided by Apex Instruments(Raleigh, NC) were used for the sampling. Each systemincluded a probe with glass linear, a heated filter box, a setof glass impingers, an umbilical cord, and a metering con-sole. The equipment setup for the OH method has beendescribed elsewhere.5 Flue gas samples were extracted iso-kinetically from the source and passed through a series ofimpingers in an ice bath. Particle-bound mercury was col-lected in the front half of the sampling train on a quartz fiberfilter. Oxidizedmercury was collected in a series of impingerscontaining a chilled aqueous potassium chloride solution.Elemental mercury was collected in subsequent impingers(one impinger containing a chilled aqueous acidic solution ofhydrogen peroxide, and three impingers containing chilledaqueous acidic solutions of potassium permanganate). Aftersampling, all solutions were recovered and digested using anautomatedmercury preparation system (LeemanLabHydraPrep, Teledyne Leeman Laboratories, NH). A 4 mL aliquotof each Hg absorbing solution (i.e., KCl, H2O2/HNO3,and KMnO4/H2SO4) was recovered from the impingersand transferred to a 15 mL digestion cup. Then, 0.2 mL ofconcentrated H2SO4, 0.1 mL of concentrated HNO3, 1.2 mLof 5% KMnO4, and 0.32 mL of 5% K2S2O8 were addedautomatically to each cup by means of a dispenser. The cupswere heated in a water bath at a constant temperature of95 �C for two hours. After cooling, 1.333 mL of 12%:12%NaCl/hydroxylamine sulfatewasadded toprepare the solutionfor mercury analysis by CVAAS (Leeman Lab Hydra, Tele-dyne Leeman Laboratories, NH). During analysis, 5%HNO3

was employed as the rinse solution and a 10%SnCl2/10%HClsolution was utilized as the reducing agent. The peristalticpump was controlled at 5 mL/min, while the carrier gas wasultrahigh-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 aThermo 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 spectro-scopy 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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between the Tekran and Thermo analyzers is the applicationof gold traps. Gold traps are used by the analyzer in theTekran 3300 system to selectively capture the elementalmercury in the sample gas prior to the detector. In theThermo system, flue gas is continually delivered into thedetector without passing through gold traps, and therefore,generates continuous readings of total vapor phase Hg.

III. Quality Assurance

The following procedures, which are described in sections8.2.2.1, 8.2.3.1.1, and 8.2.6 of EPA Method 30B, were per-formed to quality-ensure the data obtained with the sorbenttrap sampling systems.

A. Method Detection Limit Determination. The methoddetection limits (MDL) of the direct combustion and wet-chemical analytical methods were determined by using themethods to analyze a National Institute of Standards andTechnology (NIST) traceable mercury standard with a massor concentration level five times higher than the instrumentnoise. TheMDL is defined as the minimum concentration ofa substance that can be measured and reported, with 99%confidence that the concentration is greater than zero. Sevenanalyses of the Hg standard were performed with eachmethod. The MDL was determined by multiplying thestandard deviation of the measurements by a t-test value.The results are presented in Table 2. TheMDL values for thedirect combustion andwet-chemistrymethods were found tobe 3.5 and 0.9 ng, respectively. Although the MDL value ofthe wet-chemistry method is lower than the direct combus-tion method, it is not as sensitive due to dilution occurredduring sample preparation.

B. Analytical Bias Test.An analytical bias test was carriedout in the lab to demonstrate the ability of the two analyticalprocedures to recover and to accurately quantify mercuryfrom the sorbentmaterial. The test was performed by spikingthe 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 forthe spiking. The results are presented in Table 3, and indicateexcellent spike recoveries, ranging from 94.2 to 99.2%.

C. Field Recovery Test. A field recovery test, using threesets of paired sorbent traps, was conducted to verify theperformance of the in-stack sampling and the direct combus-tion mercury analysis procedures adopted in this study. Oneof the traps in each pair was spiked with a known level ofmercury (i.e., 240 ng). Then, flue gas was sampled with eachpair of traps and the Hg was recovered from the traps andanalyzed. For each sample run, the spike recovery wascalculated by comparing the analytical results of the spiked

and unspiked traps. The difference between the Hg massrecovered from the spiked and unspiked traps was assumedto be the mass of the spiked Hg. The results of the three fieldrecovery test runs are shown in Table 4. Satisfactory spikerecoveries were obtained, ranging from 99.4 to 105%, indi-cating that: (1) the sampling and analytical procedures usedin this study effectively recovered the captured mercury; (2)there were no adverse effects from the flue gasmatrix; and (3)there was no contamination during the sampling, transpor-tation, and analytical processes.

IV. Discussion of Results

A. Overall Monitoring Results. In September and October2008, a total of 13 and 9 short-term (1.5 h) “in-stack”sampling runs were conducted at units 1/2 and 3, respec-tively, using nonspeciating sorbent traps. During the short-term sampling, concurrent OH method measurements weremade to provide reference values. The results of the short-term sorbent trap and OH method measurements are sum-marized in Table 5, along with the Hg concentrationsmeasured by the CEMS during the sampling runs.

The relative accuracies of the sorbent trap systems and theCEMS are also shown in Table 5. Relative accuracy (RA)was calculated according to section 7.3 of Part 75, AppendixA. The sorbent-trap systems and the CEMS provided satis-factory RA results when compared against the performancespecifications that were developed for the CAMR regulation(i.e.,e 20%RAor an absolutemean differencee1.0 μg/m3).In cases where the RA exceeded 20%, the alternative speci-fication for low-emitting sources, that is, absolute meandifference e1.0 μg/m3, was met. No statistically significantbias was observed for either the regular sorbent trap systemsor the CEMS. Due to operational difficulties, valid datafrom the CEMS installed on the units 1/2 stack were notrecorded for four of the sampling runs. However, nine validCEMS runs, which is the number of runs traditionally usedfor RA calculations, were still obtained.

Short-term sampling was also performed using two out-of-stack sorbent trap methods (i.e., one using an inertialprobe, and the other, a cyclone probe). The results of the out-of-stack test runs are summarized in Table 6, along with thecorresponding OH values. The results obtained with the twoout-of-stack sampling methods exceeded 20% RA, but theyare satisfactory when compared against the alternative RAspecification. Both out-of-stack methods had higher percent

Table 2. Method Detection Limits (MDL) of the Direct Combustion

and Wet Chemistry Methods

direct combustion cold vapor atomic adsorption

run No. standard, ng standard, ng/mL ng

1 18.0 0.028 1.42 19.0 0.031 1.553 21.0 0.023 1.154 19.0 0.019 0.955 19.0 0.018 0.96 18.0 0.015 0.757 20.4 0.022 1.1standard deviation, S 1.12 0.0056 0.28MDLa(ng) = 3.143*S 3.5 0.9

aMethod detection limit.

Table 3. Analytical Bias Tests Results

concentrationlevel spiked amount (ng)

measured(ng) % recoveryaverage

high 1000 908 90.8 94.2995 99.5924 92.4

5000 4430 88.6 99.25070 101.45380 107.6

8000 7720 96.5 96.57670 95.97760 97.0

low 100 91.8 91.8 94.998.0 98.095.0 95.0

250 238.0 95.2 99.2246.0 98.4260.0 104.0

500 486.0 97.2 98.1510.0 102.0475.0 95.0

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RA values than the in-stack sampling method. This waslikely due, at least in part, to the small body of valid samples

obtained with the out-of-stack methods. Only six and threevalid sampling runs, respectively, were obtained with the

Table 5. Relative Accuracy Results for Sorbent Traps and CEMS

sorbent traps cems

OHM data sorbent trap data db relative accuracy results CEMS data db relative accuracy results

run No. date run Time μg/dscma μg/dscma |d|c RAd μg/dscma |d|c RAd

Units 1/21 9/12/08 11:00-12:30 2.00 1.55 0.45 0.05 12.48 1.12 0.88 0.23 35.582 9/12/08 14:15-15:45 2.49 2.84 -0.35 3.39 -0.903 9/13/08 09:17-10:47 2.10 2.01 0.10 na na4 9/13/08 11:37-13:07 2.55 1.82 0.73 na na5 9/13/08 14:46-16:16 1.85 1.90 -0.05 2.39 -0.546 9/13/08 16:58-18:28 1.94 1.83 0.12 1.96 -0.027 9/30/08 10:32-12:02 2.33 2.53 -0.20 1.40 0.938 9/30/08 16:16-17:46 2.36 2.62 -0.26 1.29 1.079 10/1/08 09:22-10:52 1.35 1.68 -0.33 1.05 0.3010 10/1/08 12:46-14:16 1.56 1.88 -0.32 1.37 0.1911 10/1/08 14:45-16:15 1.50 1.54 -0.04 1.32 0.1812 10/2/08 10:00-11:30 1.48 1.79 -0.31 na na13 10/2/08 11:50-13:20 1.35 1.49 -0.14 na na

Unit 31 9/12/08 11:00-12:30 2.44 2.76 -0.32 0.09 13.80 2.92 -0.48 0.10 14.632 9/12/08 14:15-15:45 3.37 2.81 0.56 2.99 0.383 9/13/08 09:17-10:47 2.72 2.43 0.30 2.50 0.224 9/13/08 11:37-13:07 2.64 2.75 -0.10 2.67 -0.035 9/13/08 14:46-16:16 2.39 2.96 -0.56 2.64 -0.256 9/13/08 16:58-18:28 2.45 2.53 -0.08 2.69 -0.247 9/30/08 10:32-12:02 2.89 3.04 -0.15 2.18 0.718 9/30/08 16:16-17:46 2.63 3.13 -0.50 2.26 0.37

aMicrograms per dry standard cubic meter in 20 �C and 3%O2.bDifference between results from the method and OHM. cAbsolute mean difference

between results from the method and OHM. dRelative 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

OHM sorbent trap data, Avg. db RA results

run No. date run time μg/dscma μg/dscma μg/dscma |d|c RAd

Inertial Probe at Units 1/21 9/25/08 12:30-14:00 2.36 2.41 -0.05 0.08 33.872 9/25/08 15:40-17:10 2.12 2.80 -0.683 9/30/08 10:32-12:02 2.33 1.35 0.984 9/30/08 16:16-17:46 2.36 2.23 0.135 10/1/08 12:46-14:16 1.56 1.80 -0.246 10/1/08 14:45-16:15 1.50 2.13 -0.63

Cyclone Probe at Unit 31 9/25/08 10:00-11:30 2.89 3.92 -1.03 0.47 71.302 9/25/08 12:50-14:20 2.63 3.14 -0.513 9/25/08 14:50-16:20 2.50 2.37 0.13

aMicrograms per dry standard cubicmeter in 20 �C and 3%O2.bDifference between results from the sorbent trap andOHmethods. cAbsolutemean

difference between results from the sorbent trap and OH methods. dRelative accuracy was calculated according to section 7.3 of 40 CFR Part 75,Appendix A.

Table 4. Field Recovery Test Results from Regular Sorbent Trap Sampling Method

trap ID Hg spiked (μg) section No. Hg (μg) spiked Hg recovered (μg) % recovery

run 1 trap A: 004804 0.2400 S1 0.4290 0.2360 99.4S2 0.0009

trap B: 004931 S1 0.1930S2 0.0003

run 2 trap A: 004832 0.2400 S1 0.4310 0.2450 103.0S2 0.0007

trap B: 004947 S1 0.1860S2 0.0003

run 3 trap A: 004807 0.2400 S1 0.4440 0.2490 105.0S2 0.0008

trap B: 004806 S1 0.1950S2 0.0003

aMicrograms per dry standard cubic meter in 20 �C and 3% O2.

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inertial and cyclone methods. Several sampling runs per-formed on 9/12/08 and 9/13/08 were contaminated by theconnection assembly between the sampling probes and sor-bent traps, and had to be discarded

Long-term (18 h or longer) sorbent trap sampling wasconducted at both the units 1/2 stack and unit 3, to evaluatethe effects of sample duration on the test results. In-stack andout-of-stack sampling was performed at both test locations.However, the in-stack and out-of-stack methods were runconcurrently 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 thelong-term tests were compared against concurrent datarecorded by the CEMS. The results of these comparisonsare summarized in Table 7. No OH method measurementswere made during the long-term testing. Table 7 shows thatthe results from the out-of-stack samplingmethods agreed towithin (0.5 μg/m3 of both the CEMS data and the con-current in-stack results. Unlike the previous study carriedout at the units 1/2 stack,4 no trap fouling was observed fromthe in-stack sampling during this study, even for the longer(4-7 days) sampling durations. The trap fouling thatoccurred during the previous study was likely due to thedeposition of fine droplets of FGD slurry at the tip of thesorbent trap inlet. The absence of trap fouling during thisstudy may have been the result of lower stack gas flow rates.Due to the outage of unit 1, the stack gas flow rate was about13 million standard cubic feet per hour (scfh) duringthe testing period, which is about one-half the flow rateobserved in the previous tests. The lower flue gas flow rates inthis study may have reduced the slurry droplet carry-overphenomenon.

B. Comparison of Sorbent Trap Analytical Methods.

Twenty two (22) sorbent traps were analyzed using directcombustion, and another 22 traps were digested and ana-lyzed for mercury using CVAAS. The relative differencebetween the results from each sorbent trap analysis and itscorresponding OH reference value was calculated using thefollowing equation:

Relative difference ¼ ðCHg,OH -CHg,TrapÞCHg,OH

� 100%

whereCHg,OH is theHg concentration obtained from the OHmeasurement and CHg,Trap is the Hg concentration obtainedfrom the sorbent trap.

The “box and whisker” plots shown in Figure 2 graphi-cally illustrate several important statistical features of thedata set. Each box encloses the interquartile range with thelower edge at the first quartile and the upper edge at the thirdquartile. The horizontal line drawn through the box repre-sents the median value at the second quartile. The whisker,which is a vertical line extending from each end of the box,

represents the value within the 1.5 interquartile range fromeither the first or the third quartile. The points outside of thetwo whiskers are outliers.

On average, the results from the direct combustion wereabout 8.44% higher than the OH method, while the averagewet chemistry results were in near-perfect agreementwith theOH method. In view of this, one might have concluded thatthe direct combustion analytical method has an inherenthigh bias compared to the wet chemistry method. However,when the data were examined more closely, a wide scatter ofvariations was found. It is therefore possible that the differ-ence observed between the two analytical methods was dueto random sampling or analytical errors. To test this hypoth-esis, a one-way analysis of variance (ANOVA) method wasperformed to determinewhether the difference between thesetwo data sets is statistically significant. The calculatedp value was 0.06, which is less than the selected standard of0.1. Therefore, the difference between the two data sets issignificant. With the standard of 0.1, based on PowerAnalysis, the power of the experiment is about 73% with asample size of 22. The “power” is the ability to reject the nullhypothesis when it is not true. Therefore, the sample size is inan acceptable range.

C. Evaluation of Relative Deviation and Breakthrough.Therelative deviation (RD) is a measure of the agreementbetween the analytical results from a pair of sorbent traps.The following equation was used to calculate the RD valuesin this study:

RD ¼ jCa -CbjCaþCb

� 100%

Table 7. Results of Long-Term Sorbent Trap Sampling

sorbent traps

unit(s) sampling dates and times in-stack (μg/dscma) out-of-stack (μg/dscm) CEMS

1/2 9/14/08 (08:37) - 9/21/08 (07:43) 1.95 NAχ 1.289/21/08 (08:18) - 9/24/08 (16:20) 1.43 NA 0.889/14/08 (08:50) - 9/24/08 (16:10) NAb 1.75 1.309/26/08 (15:46) - 9/30/08 (08:23) NAb 0.29 0.41

3 9/14/08 (15:00) - 9/16/08 (08:45) 0.67 NA 0.639/16/08 (09:30) - 9/17/08 (10:43) 1.26 1.28 1.009/17/08 (11:00) - 9/18/08 (08:15) NAb 1.09 1.149/20/08 (15:40) - 9/25/08 (08:00) 1.22 1.19 1.13

aMicrograms per dry standard cubic meter in 20 �C and 3% O2.bNot participated in the testing.

Figure 2. Relative difference between OH method measurementsand the results obtained from the digestion and direct combustionsorbent trap analytical methods.

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where Ca and Cb are the Hg concentrations measured withsorbent traps “a” and “b,” respectively. Ca and Cb werecalculated using the following equation:

Ca ¼ ðm1þm2ÞVt

where m1 and m2 are the mass of Hg measured on sorbenttrap sections 1 and 2, respectively; andVt is the total volumeof dry gas measured during the sampling period.

According to EPA Method 30B, for Hg concentrationsgreater than 1 μg/dscm, the RD value of each sorbent trapsampling run must be less than 10% to validate the run.

Figure 3 illustrates the relative deviation results for thesampling methods used in this study. For the in-stackmeasurements, the agreement between the total mercuryreadings from the paired traps was excellent for bothshort-term and long-term sampling. All of the RD valueswere within 10%, with the exception of one measurement.The relative deviation results were not as good for the out-of-stack sampling methods, particularly for the inertial probemethod, in the short term sampling. However, theRDvaluesfor both out-of-stackmethods were significantly lower in thelong term sampling, suggesting that the out-of-stack sam-pling setup may have created unevenly distributed flowwithin the flue gas stream during the early stages of thesampling, causing higher relative deviation between the twotraps during the short test runs. The variation apparentlybecame insignificant as sampling progressed, and the RD forthe long-term out-of-stack sampling met the 10% criterion.

Another sampling QA/QC parameter, that is, break-through, was also evaluated. The breakthrough of each trapafter sampling was evaluated by the following equation:

B ¼ m2

m1� 100%

where m1 and m2 are the mass of Hg measured in sections 1and 2 of the sorbent trap, respectively. The results of thebreakthroughmeasurements for all sorbent traps analyzed inthe study are presented in Figure 4.

It was found that for short-term in-stack sampling, 6 (outof 50) traps had more than 10% breakthrough. The break-through was relatively low for both out-of-stack methodsduring short-term sampling. Only one out-of-stack sorbenttrap (out of 18) exceeded 10% breakthrough when theinertial probe was used. None of the 6 cyclone method trapsshowed more than 10% breakthrough. However, as shownin Figure 4, significant breakthrough was observed for thetraps used in the long-term cyclone sampling. It was likelycaused 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 capturedmercury to migrate through the sorbent trap column fromsection 1 to section 2. Only minimal breakthrough wasobserved for the other two sampling approaches duringlong-term sampling. It is therefore believed that precisetemperature control of sorbent trap sampling systems isimportant to ensure that good data are obtained, particu-larly for long sample runs.

D. Evaluation of Speciating Sorbent Traps. The speciatingsorbent trap was evaluated in January and February 2009.The results of total mercury concentration measurements(i.e., HgT) and mercury speciation data from the evaluationare summarized in Table 8. The out-of-stack inertial sam-pling approach was used at the FGD inlet location. Theconcentrations and speciation of mercury at the FGD inletand at the outlet stack were also measured using theOH method to provide reference values. The OH methodmeasurements were carried out concurrently with selectedsorbent trap runs.

For all test runs shown in Table 8, excellent agreement wasobtained between the total Hg concentrations measured bythe A and B sorbent traps. For two other runs carried out on2/4/09 at the FGD inlet and a sampling run conducted on2/9/09 at the outlet stack (data not shown), the relative

Figure 3. Relative deviation of paired sorbent trap measurementfrom the in-stack and out-of-stack sampling methods, during:(a) short-term sampling and (b) long-term sampling.

Figure 4. Sorbent trap breakthrough during short-term and long-term sampling periods when using regular in-stackmethod and out-of-stack inertial, and cyclone methods.

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deviations of paired speciation traps were higher than 10%.Also included in Table 8 are the ratios of elemental mercuryto total mercury, that is, Hg0/HgT, which ranged from 6.8 to27.7% at the FGD inlet and from 46.3 to 90.1% at the stack.The Hg0/HgT ratios observed from speciating sorbent trapmeasurements at both FGD inlet and stack are comparableto the speciation data obtained from the OH method mea-surements. A greater variation in the Hg0/HgT ratio wasobserved at the stack, possibly due to nonhomogeneousdistribution of mercury species in the flue gas or nonopti-mized sampling conditions. Although the measuredmercuryspeciation varied among the test runs, the results from thesame 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 thesampling systems.

Results from both sorbent trap and OH measurementsshow that the stack mercury emission varied significantlyduring the testing period. The concentration levels of totalmercury were actually as high or higher at the outlet stack asthe concentrations observed at the FGD inlet, for several ofthe sampling runs on 2/9/09 and 2/10/09. Similar variation inthe outlet stack mercury concentration was also seen in aprevious study carried out at the same stack, in which themercury concentration was continuously monitored using aThermo Mercury Freedom CEM system. The apparent lowmercury removal by the FGD system was most likely due tothe re-emission of elemental mercury, which may have beencaused 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 hascurrently being carried out by ICSET at the same stack totry to correlate changes in boiler and FGD operating condi-tions with the re-emission phenomenon.

From the summarized test results in Table 8, it is clear thatthe mercury speciating trap evaluated in this study is able toprovide useful information on the change of mercury speciesacross a wet-scrubber. In addition, the paired traps from thesame sampling run at a given sampling location showedsatisfactory relative deviations for both mercury speciationand total mercury concentration, with the exception of testruns carried out at the early stages of the study, when thesampling system operational parameters had not yet beenoptimized. Using the all of the data shown in the Table,excluding the 11th runs at the FGD inlet, the relativeaccuracy (RA) of the speciating sorbent traps was calculatedto be 14.8% on a total Hg basis, indicating the speciatingsorbent trap can provide satisfactory results for total mer-cury measurement.

V. Conclusions

The conventional in-stack sorbent trap sampling methodand the twomodified out-of-stack samplingmethods tested inthis study provided satisfactory measurements of total vaporphase mercury, when compared against the OH referencemethod. The relative deviation of the paired traps was foundto be higher for the short-term, out-of-stack sampling than forthe short-term, in-stack sampling. The long-term results fromthe out-of-stack sampling methods agreed with both in-stackmeasurements and data recorded by Hg CEMS. The use of

Table 8. Comparison of Speciating Sorbent Trap and OH Method

HgT (μg/dscm) Hgo /HgT (%)

Speciating sorbent trap Speciating sorbent trap

date run time trap A trap B avg. RDa OH trap A trap B OH RDTrap-OHb

Units 4/5 FGD Inlet at Plant O01/22/09 10:00-11:30 18.5 18.0 18.2 1.2% 13.2 26.6 29.1 16.0 6.9802/03/09 09:30-11:35 17.2 18.2 17.7 3.1% 16.6 7.5 8.7 15.7 4.4402/04/09 10:25-11:58 18.7 16.8 17.7 5.4% 15.9 8.0 9.8 14.7 3.4502/04/09 12:14-13:36 17.5 15.6 16.6 5.7% 16.0 6.8 9.3 14.5 3.9702/05/09 13:50-15:20 18.6 17.0 17.8 4.5% NAc 11 12.1 NA NA02/06/09 10:30-11:40 17.8 18.9 18.4 3.1% NA 15.8 12.1 NA NA02/06/09 11:45-12:45 17.8 17.8 17.8 0.0% NA 13.1 16.1 NA NA02/06/09 12:50-13:50 19.3 19.9 19.6 1.6% NA 14.8 18.8 NA NA02/09/09 10:51-11:56 24.1 23.2 23.7 2.0% NA 16.1 20.0 NA NA02/09/09 13:10-14:15 17.7 18.2 18.0 1.5% 24.7 26.6 27.7 26.7 0.5902/09/09 16:08-17:08 16.2 16.8 NA 1.7% NA 15 15.4 NA NA

Units 4/5 Outlet Stack at Plant O Stack

01/20/09 11:30-13:00 14.2 14.0 14.1 0.8% 14.4 65.3 70.0 62.1 3.9601/22/09 10:00-11:30 17.4 17.5 17.5 0.3% 15.7 66.6 76.3 55.8 10.2501/22/09 11:55-13:30 14.6 14.7 14.6 0.2% 18.2 69.5 65.6 44.8 13.3001/23/09 09:25-11:30 9.6 9.6 9.6 0.3% 9.5 74.1 73.3 50.0 13.6801/23/09 11:25-13:00 10.0 10.0 10.0 0.4% 11.8 64.5 62.7 44.9 10.8002/04/09 11:45-13:00 9.6 8.7 9.1 5.4% NA 64.9 90.1 NA NA02/04/09 13:30-14:40 2.1 2.3 2.2 4.5% NA 52.3 62.9 NA NA02/04/09 15:35-16:35 7.8 7.6 7.7 1.6% NA 51.3 73.2 NA NA02/05/09 15:30-16:30 4.4 4.5 4.4 1.0% NA 46.3 65.4 NA NA02/06/09 12:20-13:50 7.0 7.1 7.1 0.6% NA 68.1 78.2 NA NA02/09/09 13:25-14:55 19.1 19.8 19.5 1.8% NA 84.3 86.3 NA NA02/09/09 15:11-16:41 14.7 14.9 14.8 0.9% NA 71.0 78.2 NA NA02/10/09 11:25-12:30 20.6 18.3 19.4 5.7% 17.4 81.4 82.5 53.7 14.3702/10/09 12:54-13:54 19.0 17.8 18.4 3.4% NA 79.8 77.2 NA NA02/10/09 14:02-15:07 19.0 17.9 18.5 3.0% NA 81.5 81.7 NA NA02/10/09 15:18-16:23 19.3 19.6 19.4 0.6% NA 85.9 81.3 NA NA

aRD: relative deviation of paired sorbent traps results in HgT measurement. bRDTrap-OHM: relative difference of sorbent trap and OHM results inHg0/HgT ratio. cNot available: OH method measurement was not carried out.

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out-of-stack methods appears to be feasible for long-termsampling, and is potentially useful at sampling locations wheretrap fouling is a concern.However, further investigationof out-of-stack sorbent trap sampling is needed to optimize theoperational parameters and conditions for this approach.

For the in-stack measurements, the duration of sampling(ranging from1 to 7 days) did not have anobservable effect ondata quality. No detectable breakthrough was observed forthe longest (7 days) sampling run, and the relative deviation ofthe paired traps was less than 10%. Therefore, it is feasible toperform in-stack sorbent trap sampling for up to a week at atime. This finding reinforces the technicalmerit of the vacatedcontinuous in-stack sorbent trapmonitoringmethod that wasdeveloped for theCAMRrule, that is, the formerAppendixKof 40 CFR Part 75.

Results of the direct combustion and wet chemistry ana-lytical methods used in this study were compared using theone-way analysis of variance (ANOVA) method. The calcu-lated p value was less than 0.1, indicating that there is astatistically significant difference between the results provide

by the two analyticalmethods.Results from thewet chemistrymethod agreed well with the OH reference method, but thedirect combustion results were about 8% higher than the OHmethod measurements.

The mercury speciating trap developed by the ICSETgenerally provided accurate total mercury concentration dataand was able to detect changes in Hg species across a wetscrubber. Therefore, the speciating trap shows promise as apotential alternative to the OH reference method. However,further study of the trap’s performance under precisely con-trolled sampling conditions is needed to establish its equi-valency to the OH method.

Acknowledgment. This paper was prepared by ICSET withsupport, in part, by grants made possible by the Illinois Depart-ment of Commerce and Economic Opportunity through theOffice of Coal Development and the Illinois CleanCoal Institute.The authors thank Mr. Robert Vollaro of US EPA for hisvaluable and stimulating comments during preparation of themanuscript.

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