Study of Mercury Oxidation by a Selective Catalytic ... of Mercury Oxidation by a Selective Catalytic Reduction Catalyst in a Pilot-Scale Slipstream Reactor at a Utility Boiler Burning

Study of Mercury Oxidation by a Selective Catalytic ReductionCatalyst in a Pilot-Scale Slipstream Reactor at a Utility Boiler

Burning Bituminous Coal†

Yan Cao,‡ Bobby Chen,‡ Jiang Wu,‡ Hong Cui,‡ John Smith,‡ Chi-Kuan Chen,§Paul Chu,| and Wei-Ping Pan*,‡

Institute for Combustion Science and EnVironmental Technology (ICSET), Western Kentucky UniVersity(WKU), Bowling Green, Kentucky 42101, Mingchi UniVersity of Technology, Taipei, Taiwan, and

Electric Power Research Institute (EPRI), Palo Alto, California 94304

ReceiVed May 30, 2006. ReVised Manuscript ReceiVed October 2, 2006

One of the cost-effective mercury control technologies in coal-fired power plants is the enhanced oxidationof elemental mercury in selective catalytic reduction (SCR) followed by the capture of the oxidized mercuryin the wet scrubber. To better understand Hg oxidation chemistry within a SCR, the Institute for CombustionScience and Environmental Technology at Western Kentucky University set up a pilot-scale SCR slipstreamfacility at a selected utility boiler burning bituminous coal. The greatest benefit of this scaled-down SCRslipstream test is the ability to investigate the effects of Hg oxidation in a SCR using actual flue gas with flyash included and to isolate and control specific flue-gas compositions with spike gas additions. The averagesulfur, chlorine, and mercury contents in the burned coal were 1.67% and 731 and 0.13 ppm, respectively.CaO and Fe2O3 and loss on ignition of the fly ash, which are reported to possibly affect Hg speciation, areapproximately 1.65, 14.6, and 2.6% on average, respectively. The maximum concentrations of spike gaseswere 500, 25, 2000, 50, and 15 ppm for HCl, Cl2, SO2, SO3, and HBr, respectively. Semicontinuous mercuryemission monitors were used to monitor the variation of mercury speciation at the inlet and outlet of the SCRslipstream reactor, and the American Society for Testing and Materials certified Ontario hydro method wasused for data comparison and validation. This paper is the first in a series of two in which the validation ofthe SCR slipstream test and Hg speciation variation in runs with or without SCR catalysts inside the SCRslipstream reactor under special gas additions (HCl, Cl2, SO2, and SO3) are presented. Effects of HBr additionson mercury speciation within the SCR will be presented in the second part of the series. Tests indicate that theuse of a catalyst in a SCR slipstream reactor can achieve greater than 90% NO reduction efficiency with aNH3/NO ratio of about 1. There is no evidence to show that the reactor material affects mercury speciation.Both SCR catalysts used in this study exhibited a catalytic effect on the elemental mercury oxidation but hadno apparent adsorption effect. SCR catalyst 2 seemed more sensitive to the operational temperature. The spikegas tests indicated that HCl can promote Hg0 oxidation but not Cl2. The effect of Cl2 on mercury oxidationmay be inhibited by higher concentrations of SO2, NO, or H2O in real flue-gas atmospheres within the typicalSCR temperature range (300-350 °C). SO2 seemed to inhibit mercury oxidation; however, SO3 may havesome effect on the promotion of mercury oxidation in runs with or without SCR catalysts.

1. Introduction

Mercury (Hg) compounds released from human activities areone of the most toxic pollutants to human health and theecosystem.1-2 Hg emissions from coal-fired power plantscontribute about 30% to the anthropogenic sources of mercury.

Coal contains naturally occurring mercury that varies with boththe coal rank and its origin. The United States EnvironmentalProtection Agency (U.S. EPA) announced the Clean AirMercury Rule (CAMR)3on Hg emission control from coal-firedpower generation on March 15, 2005, which requires thereduction of Hg emissions from coal-fired utility boilers ofnearly 70% from 1999 levels by 2018. This will affect botheconomic and environmental aspects of America, as well asaround the world. The U.S. EPA also announced the Clean Air

† Neither Western Kentucky University, the Electric Power ResearchInstitute, nor any person acting on behalf of either (A) makes any warrantof representation, express or implied, with respect to the accuracy,completeness, or usefulness of the information contained in this paper orthat the use of any information, apparatus, method, or process disclosed inthis paper may not infringe privately owned rights or (B) assumes anyliabilities with respect to the use of or for damages resulting from the useof any information apparatus, method, or process disclosed in this paper.Reference herein to any specific commercial product, process, or serviceby trade name, trademark, manufacturer, or otherwise does not necessarilystate or reflect the endorsement of the Electric Power Research Institute.

* To whom correspondence should be addressed. E-mail:[email protected].

‡ Western Kentucky University (WKU).§ Mingchi University of Technology.| Electric Power Research Institute (EPRI).

(1) Keating, M. H.; Mahaffey, K. R.; Schoeny, R.; Rice, G. E.; Bullock,O. R.; Ambrose, R. B., Jr.; Swartout, J.; Nichols, J. W. Mercury StudyReport to Congress, EPA-452/R-97-003-010; Office of Air QualityPlanning and Standard and Office of Research Development, U.S. Envi-ronmental Protection Agency; Research Triangle Park, NC, 1997; Vol. 1-8.

(2) Brown, T. D.; Smith, D. N.; Hargis, R. A., Jr.; O’Dowd, W. J.Mercury Measurement and Its Control: What We Know, Have Learned,and Need To Further Investigate.J. Air Waste Manage. Assoc.1999, 49,628-640.

(3) U.S. Environmental Protection Agency. Clean Air Mercury Rules,March, 2005; http://www.epa.gov/mercuryrule/index.htm. Available De-cember, 2005.

145Energy & Fuels2007,21, 145-156

10.1021/ef0602426 CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 11/17/2006

Interstate Rule (CAIR) that will place caps on the emissions ofsulfur dioxide (SO2) and nitrogen oxides (NOx) from coal-firedpower plants.4 CAIR calls for intensive investigation of mercuryemission control by the combined utilization of flue-gasdesulfurization (FGD) and selective catalytic reduction (SCR),which were originally equipped for control of SO2 and NOx,respectively.

Hg compounds from coal combustion sources mainly consistof particle-bound mercury [Hg(P)], gaseous elemental mercury(Hg0), and gaseous oxidized mercury (Hg2+). Most of the Hgin coal evaporates during the combustion process as Hg0 in thecoal-fired utility boiler. In the downstream cooling within theutility boiler, the oxidation of Hg0 to Hg2+, which is mainly aresult of HgCl2, is thermodynamically favored. Because of anash affinity and water solubility, HgCl2 is much easier to becontrolled than Hg0 with conventional air pollution controldevices (APCDs), such as electrostatic precipitators (ESPs) andfabric filters (FFs) for particle emission control and FGD wetscrubbers for SOx emission control.1,5-6SCR, the most preferableand cost-effective technology for nitrogen oxides (NOx) emissioncontrol, is shown to have an enhanced impact on catalyticoxidation of Hg0 by chlorine species in flue gas.7-10 Thus, thecombination of SCR together with ESP or FF and FGD in acoal-fired power plant may logically be the most economicmeans for simultaneous control of SOx, NOx, and mercuryemissions.

Data on Hg transformation and capture are available in theU.S. EPA’s Information Collection Request (ICR) Hg emissiondatabase and other sources regarding the potential effects ofSCR.6,11 Tests in several coal-fired utility boilers, with a wetscrubber and a SCR included, showed a significantly higherHg capture than those boilers with a wet scrubber but withouta SCR (about 88% with SCR versus about 45% without SCR).However, a comparison of tests in pulverized-coal boiler units,using a spray dryer absorber (SDA) with a FF, showed nodiscernible difference in Hg capture with or without the use ofa SCR. Tests also indicated that a coal-fired utility boiler,

equipped with a SCR and a cold-side ESP (CS-ESP), showedincreased Hg capture when bituminous coals were burned butnot when a powder river basin (PRB) coal with a lower chlorinecontent was burned. The use of NH3 to assist in NOx reductionin the SCR seems to have no effect on Hg speciation variation.Lab- and pilot-scale SCR tests with simulated flue-gas condi-tions can isolate factors affecting Hg oxidation.12-14 However,they cannot duplicate all of the conditions present in the fluegas from the full-scale utility boilers, such as adsorption of Hgon the SCR catalyst and the relation of the HCl concentrationand Hg oxidation rates in the flue gas. Therefore, efforts havebeen made to set up a pilot-scale slipstream SCR facility at aworking utility boiler, which can simulate real flue-gas condi-tions as well as flexibly control test conditions.15-17 It is foundthat coal rank, chlorine content, temperature, and space velocitywere major factors affecting Hg oxidation.

Several questions have arisen regarding the use of SCR toenhance mercury oxidation. First, what chlorine species in theflue gas can participate in Hg0 oxidation on a SCR catalyst (suchas HCl or Cl2)? Second, what is the contribution of the Deaconreaction within a typical SCR catalyst temperature range? Third,what species in the flue gas can enhance or inhibit mercuryoxidation within typical SCR conditions (flue-gas chemistry suchas SO2, SO3, NOx, NH3, and other active species additions)?Fourth, what is the effect of catalyst geometry and formulation,which may affect mass transfer and reaction kinetics of the SCRcatalyst? This paper is the first in a series, which attempts toanswer several questions mentioned above regarding the cata-lytic nature of SCR catalysts on mercury oxidation. Tests wereconducted to disseminate the complicated factors in a SCRslipstream reactor with a real flue-gas atmosphere and individualspiking gas additions. The second in this series will presentthe effects of the HBr addition on Hg speciation and adsorp-tion.

2. Experimental Section

2.1. Site Description and Configuration.The SCR slipstreamreactor was installed at a selected coal-fired power station parallelto the economizer with an inlet flue-gas temperature of about 300-350 °C to ensure “real world” flue gas was introduced into theSCR slipstream reactor. The greatest benefit of the smaller pilot-

(4) http://www.epa.gov/mercuryrule/basic.htm. Available in December,2005.

(5) Cao, Y.; Duan, Y. F.; Kellie, K; Li, L. C.; Xu, W. B.; Riley, J. T.;Pan, W. P. Impact of Coal Chlorine on Mercury Speciation and Emissionfrom a 100-MW Utility Boiler with Cold-Side Electrostatic Precipitatorsand Low-NOx Burners.Energy Fuels2005, 19, 842-854.

(6) Kilgroe, J. D.; Sedman, C. B.; Srivastava, R. K.; Ryan, J. V.; Lee,C. W.; Thorneloe, S. A. Control of Mercury Emission from Coal-FiredElectric Utility Boilers. Interim Report, number EPA-600/R-01-109; U.S.Environmental Protection Agency, Washington, D.C., December, 2001.

(7) Laudal, D. L.; Thompson, J. S.; Wocken, C. A. Selective CatalyticReduction Mercury Field Sampling Project, EPA-600/R-04-147; Office ofResearch and Development, U.S. Environmental Protection Agency,Washington, D.C., 2004.

(8) Lee, C. W.; Srivastava, R. K.; Ghorshi, S. B.; Karwowski, J.;Hastings, T. W.; Hirschi, J. Pilot-Scale Study of the Effects of SelectiveCatalytic Reduction Catalyst on Mercury Speciation in Illinois and PowderRiver Basin Coal Combustion Flue Gas.J. Air Waste Manage. Assoc.2004,54, 1560-1566.

(9) Spitznogle, G.; McDonald, K.; Lin, C.; Vesanen, A.; Toole, A.;Duellman, D. Oxidation of Mercury across a Slipstream Reactor Equippedwith Various Catalyst Formulations. In Proceedings of the 8th ElectricUtilities Environmental Conference, Tucson, AZ, 2005; paper number A96.

(10) Lee, S. J.; Lee, C. W.; Serre, S. D.; Zhao, Y.; Karwowski, J.;Hastings, T. W. Study of Mercury Oxidation by SCR Catalyst in anEntrained-Flow Reactor under Simulated PRB Conditions. In Proceedingsof the V Air Quality Conference, Washington, D.C., September 18-21,2005.

(11) Chu, P.; Laudal, D; Brickett, L; Lee, C. W. Power Plant Evaluationof the Effect of SCR Technology on Mercury. Presented at the Departmentof Energy-Electric Power Research InstitutesU.S. Environmental PretectionAgencysAir and Waste Management Association Combined Power PantAir Pollutant Control Symposium; The Mega Symposium, Washington,D.C., May 19-22, 2003; paper number 106.

(12) Lee, C. W.; Srivastava, R. K.; Ghorishi, S. B.; Hastings, T. W.;Stevens, F. M. Study of Speciation of Mercury under Sumulated SCR NOxEmission Control Conditions. Presented at the Department of Energy-Electric Power Plant Research InstitutesU.S. Environmental ProtectionAgencysAir and Waste Management Association Combined Power PlantAir Pollution Control Symposium; The Mega Symposium, Washington,D.C., May 19-22, 2003.

(13) Hocquel, M. The Behaviour and Fate of Mercury in Coal-FiredPower Plants with Downstream Air Pollution Control Devises. Forschr.-Ber. VDI Verlag: Dusseldorf, Germany, 2004.

(14) Richard, C; Machalek, T; Miller, S.; Dene, C.; Chang, R. Effect ofNOx Control Processes on Mercury Speciation in Flue Gas. Presented atthe Air Quality III Meeting, Washington, D.C., September 10-13, 2002.

(15) Laudal, D. L.; Pavish, J. H.; Chu, P. Pilot-Scale Evaluation of theImpact of Selective Catalyst Reduction for NOx on Mercury Speciation.Presented at the Air and Waste Management Association, 94th AnnualConference, Orlando, FL, June 24-28, 2001.

(16) Macharlek, T.; Ramavajjala, M.; Richardson, D. C.; Goeckner, B.;Anderson, H.; Morris, E. Pilot Evaluation Flue Gas Mercury Reaction acrossan SCR Unit. Presented at the Department of Energy-Electric Power PlantResearch InstitutesU.S. Environmental Protection AgencysAir and WasteManagement Association Combined Power Plant Air Pollution ControlSymposium; The Mega Symposium, Washington, D.C., May 19-22, 2003.

(17) Spitznogle, G.; Senior, C. Strategies for Maximizing MercuryOxidation across SCR Catalysts in Coal-Fired Power Plants. Presented atthe Department of Energy-Electric Power Plant Research InstitutesU.S.Environmental Protection AgencysAir and Waste Management AssociationCombined Power Plant Air Pollution Control Symposium; The MegaSymposium, Washington, D.C., September 18-21, 2005.

146 Energy & Fuels, Vol. 21, No. 1, 2007 Cao et al.

scale SCR slipstream tests is that it provided the ability to controlvariables and isolate specific factors under actual flue-gas condi-tions. The typical operating parameters for the selected boiler areas follows: load capacity, 200 MWe; boiler type, B&W, front wallfired with three rows of burners, with a total of nine burners;particulate control type, CS-ESP; SO2 control, none; NOx controltype, low-NOx burners; and coal, bituminous coal with mediumsulfur and high chlorine contents.

2.2. SCR Slipstream Reactor System.A pilot-scale slipstreamSCR reactor has been designed to simulate the “full-scale”applications of a SCR system, as shown in Figure 1. The site setuppicture is shown in Figure 2. The SCR reactor was designed andmanufactured in a concentric configuration with an inside pass forSCR catalyst loading, where the main stream of flue gas passesthrough, and an outside pass, where the bypassed flue gas passesthrough. The flue gas, which is extracted from the well-insulatedintake pipe before the SCR slipstream reactor, is split into twostreams, whose ratio is controlled by manual flashboard valves toadjust the slot area of the outside flue-gas pass. The bypassed fluegas functions as a “strengthened” heat insulation because of its

higher temperature, which minimizes the heat transfer rate bydecreasing the temperature difference between the introduced mainstream of flue gas and the bypassed flue-gas stream. Thus, thisslipstream reactor was well-insulated, so that the temperature dropacross the SCR slipstream reactor was below 20°C. The area ofthe inside pass was 0.152× 0.152 m, and the outside pass was a0.01 m slot around the inside square. The total height of the reactorwas 6.6 m. The pilot-scale SCR had a two-layer catalyst to simulatethe variation of the residence time for gas-solid contact. Eachcatalyst chamber was 1 m inheight. The specific locations of thesampling ports were in relation to the locations of the catalysts.There were three sampling ports located at the inlet, middle, andoutlet of each SCR catalyst bed. The “inlet” refers to the locationbefore the first catalyst layer; the “middle” refers to the locationbetween the first and second catalyst layers; and the “outlet” is atthe outlet of the second catalyst layer. The Hg samples were takenat the inlet and outlet of the SCR slipstream reactor usingsemicontinuous mercury emission monitors (SCEMs) and all threelocations using the Ontario hydro method (OHM) to gain a betterunderstanding of Hg conversion mechanisms.

Figure 1. Schematic of the SCR slipstream reactor.

Hg Oxidation by a SCR Catalyst Energy & Fuels, Vol. 21, No. 1, 2007147

To prevent the fly ash from depositing on the SCR catalysts, anash blower using compressed air was designed and installed. Theash-blower control allowed each catalyst layer to have the ashpurged with high-velocity compressed air independently. Along withthe ash blower, their ports have also been adapted to allow forpressure differential monitoring using a manometer. The overallash-blowing cycle time was determined by the length of time ittook for the pressure differential to reach the upper limit. Generally,the first catalyst layer had a blow cycle of 5 s, blowing at 30 minintervals, while the second catalyst layer had a blow cycle of 8 s,blowing at 30 min intervals. With the aid of cross-catalystdifferential pressure monitoring, the ash buildup was monitored,and when the predetermined upper pressure level was reached, theash-blowing sequence was activated to blow the ash, therebybringing the pressure differential back to normal levels.

To ensure the control and even distribution of spike-gas injection,three static mixers were built and installed at different locations inthe SCR slipstream reactor. The first static mixer was located oneduct diameter below the spiking gas injection ports to ensurehomogeneous distribution of spiking gas before reaching the firstcatalyst layer. The second and third static mixers were installed atthe bottom of each catalyst layer to ensure homogeneous concentra-tions of Hg and other gases after the flue gas exited each catalystlayer.

The precise control of spiking gas addition was achieved throughthe construction of a multiport mass-flow controller that had thecapability of being set to inject a predetermined amount of gas fromone to four attached cylinders including SO2, HCl, Cl2, and NH3.SO3 or HBr addition solution injection equipment with a pre-determined concentration of H2SO4 or HBr solutions, respectively.The solutions vaporized to generate SO3 or HBr spiking gases in-side the SCR reactor with the desired spiking concentration. Allinjection ports for spiking gases were set up below the first Hgsampling port, which left the “inlet” sampling port unaffected. Theinjection of NH3 was separated from other spiking gas lines toensure operational safety. Considering the actual injection ratio ofNH3 in a commercial SCR facility, the ratio of NH3 injection wasset at NH3/NO ) 1-1.1. Because of the low-NOx burner used, theNOx level was about 250 ppm during tests. To simulate the fluegas of various types of coal ranks, the maximum addition rates ofHCl, Cl2, SO2, SO3, and HBr spiking gases were 500, 25, 2000,50, and 15 ppm, respectively. The incremental steps for spiking

gas addition were dependent upon the actual response of mercuryspeciation variation, as indicated in Table 1. On the basis of thechlorine (Cl) and sulfur (S) contents, mass balance, and spikinggas concentrations, the total flue-gas compositions of SO2, SO3,HCl, and Cl2 were calculated and are listed in Table 1. All gasconcentrations were corrected to 3% O2 and a dry basis. SO3 andCl2 concentrations in the raw flue gas were calculated on the basisof an assumption of 1% coal sulfur content and 5% coal Cl content.The catalyst average temperature was 305°C.

2.3. SCR Catalysts.Commercial monolith (Honeycomb) SCRcatalysts were provided by two vendors. Catalyst 1 had an 8.4 mmpitch, and the square cross-section had an array of 18× 18 channels.Catalyst 2 had an approximately 7.5 mm pitch, and the square cross-section had an array of 20× 20 channels. Each catalyst sectionwas 1 m inlength; therefore, the total length of the catalyst chamberwas 2 m. The SCR catalysts were designed to be operated at aspace velocity of 1800 h-1, which is the actual space velocity usedon full-scale coal-fired SCR reactors.18

2.4. Coal and Ash Analysis.Bituminous coal was burned duringSCR slipstream tests. The coal sample was collected at the coaltransport line after the coal pulverizer. The ash sample was collectedfrom the front-row hopper because it captured the majority of flyash in the flue gas. The key proximate and elemental analysis ofthe coal samples is shown in Table 1. Hg in all solid samples wasanalyzed using a LECO AMA-254 [American Society for Testingand Materials (ASTM) method D 6722]. The variation of Hg, Cl,and S contents during the tests were 0.10-0.15 and 354-1186 ppmand 1.30-2.17%, respectively, with averages of 0.11 and 854 ppmand 1.3% during runs with the SCR catalyst, 1.4% and 946 and0.13 ppm during runs with SCR catalyst 1, and 1.9% and 549 and0.14 ppm during runs with SCR catalyst 2. Analytical data for flyash taken from the ESP hopper are shown in Table 2. Loss onignition (LOI) and Cl and S contents in the ash samples weredetermined using ASTM methods D 5142, D 5373, and D 5016,respectively. It was found that the LOI content of the fly ash waslower at about 2.6%, which indicated a good combustion perfor-mance during testing, even for the boiler equipped with the low-NOx burner. Only a small portion of sulfur and chlorine was

(18) Laudal, D. L.; Thompson, J. S.; Pavlish, J. H.; Brickett, L.; Chu,P.; Srivastava, R. K.; Lee, C. W.; Kilgore, J. Mercury Speciation at PowerPlants Using SCR and SNCR Control Technologies.EM February 22, 2003.

Figure 2. Actual setup on site of the SCR slipstream reactor system.


captured by fly ash in the duct. Over 95% of the sulfur and chlorinein the coal remained in the gas phase. The major and minor elementdata from the X-ray fluorescence analysis (ASTM method D 4326)of ashes prepared from the coal samples during the test period arealso shown in Table 2. A total of 13 oxides were selected fordetermination, including CaO, Fe2O3, TiO2, and MnO, which werereported to possibly affect Hg transformation. The higher Fe2O3

content at 14.6% and a lower CaO content at about 1.65% in the

ash of the tested coal could be attributed to the active function ofthe fly ash on Hg speciation.

2.5. Instrumentation. The variation of the Hg concentration wasmonitored continuously by SCEMs at two locations (inlet andoutlet) and by OHM at three locations in the SCR slipstream reactor.OHM measurements (ASTM method 6784-02) for each of thesampling locations were applied to confirm the SCEM samplingresults at a period of validation of SCR slipstream tests. Only one

Table 1. SCR Slipstream Test Conditions and Coal Properties

catalyst information coal analysestotal flue-gas concentration

dry, 3% O2

additivescatalyst

type

spacevelocity

(h-1)

catalysttemperature

(°C)NH3/NOx

ratio

Cl(ppm)dry

Hg(ppm)dry

S(%)dry

NO(ppm)

HCl(ppm)

Cl2(ppm)

SO2

(ppm)SO3

(ppm)

HCl addition none 1837 317 0 1270 0.1 1.56 290

101301501601

Cl2 addition none 1850 302 0 647 0.12 1.3 280

38

1328

SO2 addition none 1890 304 0 647 0.12 1.3 280103720373037

HCl addition 1 1935 293 0 1186 0.13 1.39 23195

295595

HCl addition 1 1870 313 1.02 1186 0.13 1.39 23195

595

Cl2 addition 1 1885 303 0 963 0.13 1.34 274

49

1424

SO2 addition 1 1875 302 0 685 0.12 1.48 293

1181168121813181

SO3 addition 1 1870 313 0 710 0.15 1.42 1686

56

HCl addition 2 1920 293 1.03 571 0.12 1.89 282

46146246446546

HCl addition 2 1880 305 0 842 0.15 1.76 234

67167267467500

HCl addition 2 1865 310 1.07 842 0.15 1.76 243

67167267467567

SO2 addition 2 1865 310 0 396 0.15 2.17 246

1731248132313731

Cl2 addition 2 1935 293 0 571 0.12 1.89 242

27

122227

SO2 addition 2 1850 302 1.02 354 0.16 1.95 215

15562056230630563556

SO3 addition 2 1883 304 0 423 0.14 1.89 232

8284358

SO3 addition 2 1875 310 1.01 396 0.15 2.17 2589

2959


modification was made to the SCEMs, by which the highertemperature inertial probe (300-350°C) was applied to minimizethe gas-phase sampling bias. A detailed description of the two Hgtest methods and QA/QC procedures can be found in the refer-ences.5,19

3. Results and Discussion

3.1. Validation of the SCR Slipstream Test.Before anyinvestigation tests on the variation of Hg speciation in the SCRslipstream reactor, several issues had to be addressed, includingthe reduction performance of NOx addition, effects of the reactormaterial on mercury speciation, and the effect of the SCRcatalyst on mercury adsorption.

3.1.1. NOx Reduction by NH3 Addition in the SCRSlipstream Reactor.The reduction performance of the SCRslipstream reactor was evaluated by monitoring the NO con-centration at the inlet and outlet locations of the SCR slipstreamreactor. Because of the low-NOx burner installed in the test unit,NO concentrations at the inlet were found to be about 280-300 ppm during the SCR slipstream validation tests. The NOconcentration at the outlet location was almost the same as thatat the inlet location with the same O2 concentration. After theNH3 addition started with a molar ratio of NO/NH3 at about 1,the NO concentration decreased gradually and finally went downbelow 20 ppm, as shown in Figure 3A for catalyst 1 and Figure3B for catalyst 2. Both catalysts in the SCR slipstream reactorworked properly as expected with above 95% NO reduction.

3.1.2. Effect of the Empty Bed of the Slipstream ReactorMaterial on the Oxidation of Hg0. With the exception ofTeflon and glass, all other materials may affect mercuryspeciation. Thus, the construction material of the SCR slipstreamreactor, which was stainless-steel, could have affected themercury speciation under typical SCR temperatures. Hence, thereactor blank tests by OHM were made at three locations: inlet,middle, and outlet. Results are presented in Figure 4, wherethe y axis represents the variation of mercury oxidation by anincremental percentage relative to the inlet value based on theequation shown in eq 1.

Blank tests indicated that the percentage of variation of themercury oxidation was within 5% as the flue gas crossed theempty SCR slipstream reactor between sampling ports. Thereseemed to be no evidence to show significant effects of SCR

reactor materials on mercury speciation at this high temperature.The small increase in mercury oxidation that occurred in theSCR slipstream reactor could possibly be attributed to oxidationeffects of in-flight fly ash within the typical SCR temperaturerange, which is thermodynamically favored.

3.1.3. Effect of the SCR Catalyst on the Total Vapor-Phaseof Mercury [Hg(VT)]. To evaluate the possible adsorption ofmercury on the SCR catalyst, the total vapor-phase mercuryconcentration was monitored by the SCEM system and OHMtests at the inlet and outlet of the SCR slipstream reactor withSCR catalysts 1 and 2 test runs. Although the total vapor-phaseof mercury varied with coal properties and the boiler load, agood agreement between the experimental results of the totalHg(VT) by SCEM and OHM and the predicted Hg(VT) in theflue gas, which was calculated on the basis of the conversionof the total Hg in coal into the flue gas, was reached. Test results

(19) Kellie, S.; Cao, Y.; Duan, Y. D.; Li, L. C.; Chu, P.; Mehta, A.;Carty, R.; Riley, J. T.; Pan, W. P. Factors Affecting Mercury Speciation ina 100-MW Coal-Fired Boiler with Low-NOx Burners.Energy Fuels2005,19, 800-806.

Table 2. Analysis Data on Ash Properties

sample namesulfur(%)

chlorine(ppm)

mercury(ppm)

LOI(%)

tests withoutSCR catalysts

ash

0.11 100 0.10 2.57

Na2O(%)

MgO(%)

Al2O3

(%)SiO2

(%)CaO(%)

K2O(%)

SO3

(%)P2O5

(%)BaO(%)

SrO(%)

Fe2O3

(%)MnO(%)

TiO2

(%)

0.004 0.895 18.142 38.272 1.705 2.348 1.935 0.583 0.152 0.130 17.508 0.023 1.140

sulfur(%)

chlorine(ppm)

mercury(ppm)

LOI(%)

tests withSCR catalysts

ash

0.13 177 0.12 2.73Na2O(%)

MgO(%)

Al2O3

(%)SiO2

(%)CaO(%)

K2O(%)

SO3

(%)P2O5

(%)BaO(%)

SrO(%)

Fe2O3

(%)MnO(%)

TiO2

(%)

0.041 0.807 13.811 32.609 1.608 1.902 1.350 0.574 0.120 0.088 11.688 0.030 0.962

Figure 3. (A) NO reduction performance of the SCR slipstream reactorfor catalyst 1. (B) NO reduction performance of the SCR slipstreamreactor for catalyst 2.

additional oxidation of Hg0 )100{1 - [Hg0/Hg(VT)]mid or out/[Hg0/Hg(VT)in]} (1)


are shown in parts A and B of Figure 5 for SCR catalysts 1 and2, respectively. There is no evidence to relate mercury adsorptionto the SCR catalysts, at least for catalysts tested in this study ata temperature around 300°C. However, Lee et al. haveexamined data from a small-scale SCR reactor under simulatedflue-gas conditions and have found that there may possibly besome adsorption of Hg0 on the catalyst for certain measure-ments.12 We noted that the work of Lee et al. is done underconditions to simulate the flue gas of PRB coal, which is lowerin chlorine content. Actually, a reasonable assumption may bethat a dynamic situation could be established on the SCR catalystsurface. First, Hg0 is attracted and trapped because of activesites on the SCR catalyst surface, then Hg0 reacts with adsorbedchlorine species or other oxidizing species to form Hg2+, andfinally, Hg2+ will be liberated from the surface of the SCRcatalyst because there has been no report of mercury adsorptionon any material under such a high temperature. Another reasonfor no Hg adsorption on the SCR catalyst may be associatedwith the deposition of ash, which covers some of the Hgadsorption sites. The different findings on the adsorption ofelemental mercury on the surface of catalysts between differentsources may be due to the individual experimental conditions(catalyst, flue-gas compositions, and fly-ash deposit) or mea-surements.

3.2. Effects of Spiking Gases on Hg Oxidation with orwithout the SCR Catalyst.Hg oxidation in the SCR may occurbecause of two processes, including homogeneous oxidation,which occurs in the gas phase, and heterogeneous oxidation,which occurs at the interface between the solid and gas on asolid surface. In the SCR slipstream reactor during runs withoutthe SCR catalyst, the possible oxidation mechanism washomogeneous oxidation and also heterogeneous oxidation bythe interaction with “in-flight” fly ash, which was different fromsolid-gas contact mode in the fixed bed when fly ash depositedon the sampling probe. After runs with the SCR catalyst, theadditional Hg0 oxidation compared to the runs without the SCRcatalyst should be solely a result of Hg0 oxidation from thecatalytic effect of the SCR catalyst. Gas compositions in theflue gas, such as HCl, Cl2, SO2, SO3, and NH3, may be the

oxidizing or reducing agents to impact Hg0 oxidation chemistryin the Hg transformation process. The SCR catalyst maypromote the oxidation of the Hg0 in the flue gas with theparticipation of the active agents mentioned above. The additionof the individual spiking gases into the SCR slipstream reactorduring runs with or without the SCR catalyst could possiblyprovide the information on the reaction mechanism of theheterogeneous catalytic oxidation of Hg0.

For tests of spiking gas additions during both runs with andwithout SCR catalysts, results are presented as the variation ofHg0 oxidation across the SCR reactor versus the total concentra-tions of individual flue-gas species, as shown in Figures 6-9.Considering the constant Hg(VT) during runs with SCRcatalysts, the variation of Hg0 oxidation across the SCR reactoris represented by eq 2,

which is the incremental percentage variation between the Hg0

concentration at the SCR reactor inlet (Hgin0 ) and the Hg0

concentration at the SCR reactor outlet (Hgout0 ).

3.2.1. Effects of HCl and Cl2 on Mercury Oxidation duringRuns with or without SCR Catalysts. HCl and Cl2 are thetwo most important species with regard to mercury oxidationbecause of the fact that the main oxidized mercury species incoal-fired flue gas is Hg(Cl)2. The effects of the spike gasesHCl and Cl2 on Hg0 oxidation during runs with or without SCRcatalysts are shown in Figures 6 and 7. With the SCR catalyst,HCl addition could further promote the oxidation of Hg0 evenwith the coal chlorine content at 1270 ppm in the present study.During tests without the SCR catalyst in the reactor, thepercentage of Hg0 oxidation increased by 3, 6.5, 19.4, and27.9%, with increasing HCl addition concentrations of 100, 200,400, and 500 ppm (total chlorine concentration of approximately200, 400, 500, and 600 ppm in the flue gas), respectively.

During runs with SCR catalyst 1, the percentage of Hg0

oxidation increased greatly to above 60% compared to the sameconditions during runs without the SCR catalyst, which indicatesthat SCR catalyst 1 is an active catalyst for Hg0 oxidation.

Figure 4. Variation of the mercury speciation across the empty bed of the SCR slipstream reactor (runs 1, 2, and 3).

percent Hg0 oxidation) 100[Hgin0 - Hgout

0 ]/[Hgin0 ] (2)


Uncertainty existed on how to evaluate the effects of temperatureand NH3 addition on Hg0 oxidation because of the simultaneousvariation of these two parameters during tests in this study.During two tests with SCR catalyst 1, the percentage of Hg0

oxidation remained almost constant with a variation of just5-10% at different HCl addition levels and there was noapparent improvement upon mercury oxidation with HCladditions.

During runs with SCR catalyst 2, the percentage of Hg0

oxidation was shown to increase greatly by gradually increasingHCl addition by 0, 200, 400, and 500 ppm (total HClconcentrations of 100, 300, 500, and 600 ppm in the flue gas),as shown in Figure 6. However, the curves become flat with anincreasing of HCl addition concentrations. When HCl additionwas 400 ppm, the additional oxidation of Hg0 was approximately25% relative to those runs without the SCR catalyst. Thus, SCRcatalyst 2 was also shown to have a catalytic effect on Hg0

oxidation. From Figure 6, the addition of NH3 did not haveany impact on the Hg0 oxidation process for SCR catalyst 2.

However, results indicated that temperature impacted Hg0

oxidation greatly for SCR catalyst 2 when a comparison wasmade between two cases with a temperature difference of 20°C at a similar NH3 addition ratio (NH3/NO ∼ 1). The highercurve in Figure 6 (NH3/NO ) 1.03) corresponds to a temper-ature of 293°C, while the lower curve in Figure 6 (NH3/NO )1.07) corresponds to a temperature of 310°C. The shapes ofthese two curves are similar, but the magnitude of the mercuryoxidation was considerably different between the two curves.The difference between these two curves could be associatedwith the temperature differences. Models of mercury oxidationacross SCR catalysts and other sets of slipstream data haveshown higher Hg oxidation at lower temperatures.20

The present study confirms that NOx reduction by NH3 andHg0 oxidation by chlorine species simultaneously occur on thesurface of SCR catalyst 1; however, they are competitively

(20) Senior, C. L. Oxidation of Mercury across Selective CatalystReduction Catalysts in Coal-Fired Power Plant.J. Air Waste Manage. Assoc.2005, 56, 23-31.

Figure 5. (A) Investigation of mercury adsorption on SCR catalyst 1 by SCEM and OHM. (B) Investigation of mercury adsorption on SCRcatalyst 2 by SCEM and OHM.


adsorbed on the active sites on the surface of the SCR catalyst,while NOx reduction by NH3 is apparently predominant. Bothfactors of temperature and NH3 addition impact the Hg0 oxida-tion by controlling process kinetics. Observed curves of the Hg0

oxidation process become flat for both SCR catalysts in thisstudy. This may indicate the impact of the NOx reduction reac-tion by NH3 on Hg oxidation by their competitive nature (HCland NH3) on active sites of surfaces of SCR catalysts, whichresult in the independence of HCl addition concentrations onHg0 oxidation when HCl continuously increases to certain levels.Observed Hg0 oxidation varied greatly between SCR catalysts1 and 2. Differences in catalyst pitch and formulation mighthave been responsible for the differences in performancebetween these two SCR catalysts. Mercury oxidation by HClacross SCR catalyst 2 appeared to be affected by both temper-ature and HCl concentrations, while SCR catalyst 1 seemed

virtually unaffected by the variation of both temperature andHCl concentrations. Recently, models for mercury oxidationacross SCR catalysts have been developed to include the effectsof mass transfer and surface chemistry kinetics simultane-ously.10-22 The catalyst formula was the key parameter that im-pacted surface chemistry kinetics across the temperatures used,and the catalyst geometry was the key parameter that impactedmass transfer. Thus, the design of a certain SCR catalyst directlyimpacts the performance of the SCR catalyst on Hg0 oxidationthrough relative influences of the mass transfer rate and chemical

(21) Niksa, S.; Fujiwara, N. A Predictive Mechanism for MercuryOxidation on Selective Catalytic Reduction Catalysts under Coal-DerivedFlue Gas.J. Air Waste Manage. Assoc.2005, 55, 1866-1875.

(22) Edwards, J. R.; Srivastava, R. K.; Kilgroe, J. D. A Study of Gas-Phase Mercury Speciation Using Detailed Chemical Kinetics.J. Air WasteManage. Assoc.2001, 5, 69-87.

Figure 6. Variation of the mercury oxidation with HCl addition by SCEM for two catalysts.

Figure 7. Variation of the mercury oxidation with Cl2 addition by SCEM for two catalysts.


reaction kinetics. SCR catalyst 1 could possibly be improvedby enhancing its mass transfer through geometry design, andSCR catalyst 2 could be improved by modification of itsformulation of active sites.

The variation of the percentage of oxidation of Hg0 by theaddition of Cl2 into the SCR slipstream reactor was muchdifferent when compared to that by the addition of HCl, asshown in Figure 7. The addition of Cl2, with a maximumaddition concentration of 25 ppm, results in little variation ofthe percentage of oxidation of Hg0 in this study, which waswithin (5% under testing conditions during runs with or withoutthe SCR catalyst and thus cannot be regarded to be significant.In comparison with HCl, which is the main chlorine species inthe flue gas, Cl2 seemed to have little impact on mercuryspeciation under current experimental conditions. The differencein performance between the two SCR catalysts in terms of thepercentage of oxidation of Hg0 under the same Cl2 additionconcentration should be attributed to the difference in catalystpitch or formulation, as mentioned previously. Previous workon mercury oxidation has suggested that chlorine compoundsare intensively involved in mercury oxidation within the typicalSCR temperature range. This study clearly demonstrated thatHCl and not Cl2 is the chlorine compound that affects mercuryoxidation.

3.2.2. Effects of SO2 and SO3 on Mercury Oxidationduring Runs with or without SCR Catalysts. The effects ofSO2 addition on the variation of Hg0 oxidation is shown inFigure 8. Although the results show a little scatter, it seemedthat the percentage of oxidation of Hg0 followed a decreasingtrend as the SO2 addition concentration increased. The incre-mental percentage of oxidation of Hg0 under the condition of2000 ppm SO2 addition relative to the zero addition levels inthe SCR slipstream reactor was found to be approximately 25and 5% lower for SCR catalysts 1 and 2, respectively.

However, Figure 9 indicates that the percentage of oxidationof Hg0 followed an increasing trend when SO3 addition increasedto its maximum addition concentration of 50 ppm. The largerextent of the incremental percentage of oxidation of Hg0 bySO3 addition during runs with the SCR catalyst, compared tothose runs without the SCR catalyst, may indicate the possible

promoting effect of the SCR catalyst on Hg0 oxidation by SO3

addition. Two exceptions occurred at 20 ppm SO3 additionduring runs without the SCR catalyst and with SCR catalyst 2.This may be attributed to the measurement errors or the variationof the boiler performance occurring when tests were conducted.The incremental percentage of oxidation of Hg0 with the additionof 50 ppm SO3 in the SCR slipstream reactor for both SCRcatalysts was found to be approximately 20% higher relative tothe zero addition levels. Just as with the previous findings, ahigher percentage of oxidation of Hg0 for SCR catalyst 1compared with that of SCR catalyst 2 under the same SO2 orSO3 addition concentration should be attributed to the differencein catalyst pitch and formula.

3.2.3. Mechanisms Discussion.This study clearly demon-strated that it was HCl and not Cl2 that promoted Hg0 oxidationwithin a typical SCR temperature range in the flue-gasatmospheres. SO3 also had some positive impact on Hg0 oxida-tion. These oxidation mechanisms involving SCR catalystscan be promoted to a large extent through HCl and to a lesserextent through SO3. SO2 may inhibit Hg0 oxidation, evenreducing Hg2+ to Hg0.

However, combining the results of investigations on Hg0

oxidation mechanisms in the present study and previousstudies5,6,22leads to several questions. First, the Deacon reaction,which is shown in eq 3,22

will be favored below 600°C in the reverse direction to generateHCl by dissipating Cl2, as shown in eq 4.

If Cl2 has no effect on Hg0 oxidation, as indicated in this study,at least the increasing of the HCl concentration by Cl2 additionshould show some impact on Hg0 oxidation. Second, previousstudies attributed the active chlorine compound for Hg0 oxida-tion to Cl2 because of the possible “Cl” pool maintained by Cl2

through eq 5,

Figure 8. Variation of the mercury oxidation with SO2 addition for two catalysts.

2HCl + 1/2O2 ) Cl2 + H2O (3)

Cl2 + H2O ) 2HCl + 1/2O2 (4)


which is in conflict with the conclusion of this study. Third,the active species in the SCR catalyst generally contains V2O5,which can convert SO2 to SO3. Thus, the addition of SO2 intothe SCR reactor should result in increasing the SO3 concentra-tion in the flue gas, which should further enhance Hg0 oxidationby producing SO3 as indicated in this study. However, thisreasonable assumption is not in agreement with evidence thatthe possible Hg0 reduction occurred after the addition of thehigher concentration of SO2 in the present study.

The prevailing mechanisms5,6,22 for Hg0 oxidation validatethe important effects or contributions of chlorine compounds(HCl, Cl2, and Cl) and possible interferences of sulfur chemistryin the flue gas. It should be pointed out that the reactionillustrated by eq 6

is the initial step of the Hg0 oxidation process because of itsfast rate in reaction kinetics, if Cl2 is available in the flue gas.The slower reactions, represented by eqs 5, 7,

and 8,

dominate the overall Hg oxidation kinetics mainly through Cl2.Cl2 provides the important Hg0 oxidation species of Cl andmaintains the pool of this active agent. Furthermore, Cl2 is amuch stronger oxidizing agent over HCl because of thedifference of their valence electron configurations, which willresult in the difference of affinity for Hg0. However, severalinvestigators indicated possible depletion of Cl2 within thetemperature range of 200-700°C by flue-gas compositions suchas SO2, NO, and H2O.6,23-25 The reaction routines for depletion

of Cl2 in the coal-fired flue gas are shown in eqs 4, 9,

and eq 10.

Thus, the reaction routine in eq 8, which is presented as theglobal reaction, and possible reaction steps in eqs 11

and 12,

may dominate Hg0 oxidation through HCl within the temperaturerange of the typical SCR operation range (300-350 °C), asconfirmed by the present study. SCR also may catalyze Hg0

mainly by this reaction through HCl.SO2 was regarded as an inhibitor for Hg0 oxidation, as

indicated in the previous studies and this study. With the SCRcatalyst, SO3 was generally produced because of the active siteof V2O5 available for conversion of SO2 to SO3, through eq13.

Thus, the inhibition by SO2 and promotion by SO3 on Hg0

oxidation may be balanced somehow when SO2 is added intothe SCR reactor. In summary, this study provides some cluesof different reaction routes for Hg0 oxidation in the real coal-fired flue-gas atmosphere within the typical SCR temperaturerange. For the present study, the reactions represented in eqs

(23) Agarwal, H.; Stenger, H. G.; Wu, S.; Fan, Z. Effects of H2O, SO2,and NO on Homogeneous Hg Oxidation by Cl2. Energy Fuels2006, 20,1068-1075.

(24) Laudal, D. L.; Brown, T. D.; Nott, B. R.Fuel Process. Technol.2000, 65-66, 157-165.

(25) Albert, A. P.; Evan, J. G.; Andrew, K.; Richard, A. H.; William, J.O.; Henry, W. P. A Kinetic Approach to the Catalytic Oxidation of Mercuryin Flue Gas.Energy Fuels2006, 20, 1941-1945.

Figure 9. Variation of the mercury oxidation with SO3 addition for two catalysts.

SO2 + Cl2 ) SO2Cl2 (9)

2NO + Cl2 ) 2NOCl (10)

Hg + HCl ) HCl + H (11)

HgCl + HCl ) HgCl2 + H (12)

SO2 + 1/2O2 ) SO3 (with the SCR catalyst) (13)

HgCl + Cl2 ) HgCl2 + Cl (5)

Hg + Cl ) HgCl (6)

Hg + Cl2 ) HgCl2 (7)

2Hg + 4HCl + O2 ) 2HgCl2 + 2H2O (8)


8-10 are believed to be responsible for Hg0 oxidation withinthe typical SCR temperature range in real coal-fired flue gas.

4. Conclusion

A SCR slipstream reactor was set up to simulate the scaled-down “real world” circumstance of SCR catalysts for investiga-tion of Hg oxidation at a selected coal-fired utility boiler burninga bituminous coal. This system has observed above 90% of NOreduction performance with selected SCR catalysts and NH3

addition. A modified high-temperature inertial probe providedthe measurement accuracy for monitoring mercury speciationby the SCEM. Both SCR catalysts used in this study showedcatalytic effects on Hg0 oxidation. The different performanceof SCR catalysts on Hg0 oxidation may be attributed to theirdifferent characterization of chemical formulation and manu-facture geometry, which are related to the mass transfer rateand chemical reaction kinetics. SCR catalyst 2 seemed sensitiveto operational temperature with regard to Hg0 oxidation, butSCR catalyst 1 did not. Hg0 oxidation by SCR catalyst 1 maybe improved by enhancing its mass transfer through geometry

design, and SCR catalyst 2 may be improved by modificationof its formulation of active sites.

Tests by additions of gaseous acidic spike gases indicatedthat it is HCl and not Cl2 that is the major source of chlorinethat dominates the Hg0 oxidation process within the typical SCRtemperature range (300-350°C) in a real flue-gas atmosphere.Cl2 may be depleted by flue-gas compositions, such as SO2,NO, and H2O, and thus cannot impact Hg oxidation in thepresent study with real flue-gas atmospheres. This study alsodemonstrated that SO2 has an inhibiting effect on Hg0 oxidation;however, SO3 can promote mercury oxidation. The developedmechanisms for Hg0 oxidation within the typical SCR temper-ature range are in agreement with pre-existing Hg0 oxidationmechanisms.

Acknowledgment. This paper was prepared by the WesternKentucky University Research Group with support, in part, bygrants made possible by the Electric Power Research Institute (EPRIproject number EP-P13792/C6821).

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Study of Mercury Oxidation by a Selective Catalytic ... of Mercury Oxidation by a Selective Catalytic Reduction Catalyst in a Pilot-Scale Slipstream Reactor at a Utility Boiler Burning