Energy & Fuels 2007, 21, 145-156
145
Study of Mercury Oxidation by a Selective Catalytic Reduction Catalyst 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 oxidation of elemental mercury in selective catalytic reduction (SCR) followed by the capture of the oxidized mercury in the wet scrubber. To better understand Hg oxidation chemistry within a SCR, the Institute for Combustion Science and Environmental Technology at Western Kentucky University set up a pilot-scale SCR slipstream facility at a selected utility boiler burning bituminous coal. The greatest benefit of this scaled-down SCR slipstream test is the ability to investigate the effects of Hg oxidation in a SCR using actual flue gas with fly ash included and to isolate and control specific flue-gas compositions with spike gas additions. The average sulfur, 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, are approximately 1.65, 14.6, and 2.6% on average, respectively. The maximum concentrations of spike gases were 500, 25, 2000, 50, and 15 ppm for HCl, Cl2, SO2, SO3, and HBr, respectively. Semicontinuous mercury emission monitors were used to monitor the variation of mercury speciation at the inlet and outlet of the SCR slipstream reactor, and the American Society for Testing and Materials certified Ontario hydro method was used for data comparison and validation. This paper is the first in a series of two in which the validation of the SCR slipstream test and Hg speciation variation in runs with or without SCR catalysts inside the SCR slipstream reactor under special gas additions (HCl, Cl2, SO2, and SO3) are presented. Effects of HBr additions on mercury speciation within the SCR will be presented in the second part of the series. Tests indicate that the use of a catalyst in a SCR slipstream reactor can achieve greater than 90% NO reduction efficiency with a NH3/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 had no apparent adsorption effect. SCR catalyst 2 seemed more sensitive to the operational temperature. The spike gas tests indicated that HCl can promote Hg0 oxidation but not Cl2. The effect of Cl2 on mercury oxidation may be inhibited by higher concentrations of SO2, NO, or H2O in real flue-gas atmospheres within the typical SCR temperature range (300-350 °C). SO2 seemed to inhibit mercury oxidation; however, SO3 may have some effect on the promotion of mercury oxidation in runs with or without SCR catalysts.
1. Introduction Mercury (Hg) compounds released from human activities are one of the most toxic pollutants to human health and the ecosystem.1-2 Hg emissions from coal-fired power plants contribute about 30% to the anthropogenic sources of mercury. † Neither Western Kentucky University, the Electric Power Research Institute, nor any person acting on behalf of either (A) makes any warrant of representation, express or implied, with respect to the accuracy, completeness, or usefulness of the information contained in this paper or that the use of any information, apparatus, method, or process disclosed in this paper may not infringe privately owned rights or (B) assumes any liabilities with respect to the use of or for damages resulting from the use of any information apparatus, method, or process disclosed in this paper. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily state 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).
Coal contains naturally occurring mercury that varies with both the coal rank and its origin. The United States Environmental Protection Agency (U.S. EPA) announced the Clean Air Mercury Rule (CAMR)3on Hg emission control from coal-fired power generation on March 15, 2005, which requires the reduction of Hg emissions from coal-fired utility boilers of nearly 70% from 1999 levels by 2018. This will affect both economic and environmental aspects of America, as well as around the world. The U.S. EPA also announced the Clean Air (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 Study Report to Congress, EPA-452/R-97-003-010; Office of Air Quality Planning and Standard and Office of Research Development, U.S. Environmental 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 December, 2005.
10.1021/ef0602426 CCC: $37.00 © 2007 American Chemical Society Published on Web 11/17/2006
146 Energy & Fuels, Vol. 21, No. 1, 2007
Interstate Rule (CAIR) that will place caps on the emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx) from coal-fired power plants.4 CAIR calls for intensive investigation of mercury emission control by the combined utilization of flue-gas desulfurization (FGD) and selective catalytic reduction (SCR), which were originally equipped for control of SO2 and NOx, respectively. Hg compounds from coal combustion sources mainly consist of particle-bound mercury [Hg(P)], gaseous elemental mercury (Hg0), and gaseous oxidized mercury (Hg2+). Most of the Hg in coal evaporates during the combustion process as Hg0 in the coal-fired utility boiler. In the downstream cooling within the utility boiler, the oxidation of Hg0 to Hg2+, which is mainly a result of HgCl2, is thermodynamically favored. Because of an ash affinity and water solubility, HgCl2 is much easier to be controlled than Hg0 with conventional air pollution control devices (APCDs), such as electrostatic precipitators (ESPs) and fabric filters (FFs) for particle emission control and FGD wet scrubbers for SOx emission control.1,5-6SCR, the most preferable and cost-effective technology for nitrogen oxides (NOx) emission control, is shown to have an enhanced impact on catalytic oxidation of Hg0 by chlorine species in flue gas.7-10 Thus, the combination of SCR together with ESP or FF and FGD in a coal-fired power plant may logically be the most economic means for simultaneous control of SOx, NOx, and mercury emissions. Data on Hg transformation and capture are available in the U.S. EPA’s Information Collection Request (ICR) Hg emission database and other sources regarding the potential effects of SCR.6,11 Tests in several coal-fired utility boilers, with a wet scrubber and a SCR included, showed a significantly higher Hg capture than those boilers with a wet scrubber but without a 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 no discernible difference in Hg capture with or without the use of a SCR. Tests also indicated that a coal-fired utility boiler, (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 Emission from a 100-MW Utility Boiler with Cold-Side Electrostatic Precipitators and Low-NOx Burners. Energy Fuels 2005, 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-Fired Electric 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 Catalytic Reduction Mercury Field Sampling Project, EPA-600/R-04-147; Office of Research 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 Selective Catalytic Reduction Catalyst on Mercury Speciation in Illinois and Powder River 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 Equipped with Various Catalyst Formulations. In Proceedings of the 8th Electric Utilities 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 an Entrained-Flow Reactor under Simulated PRB Conditions. In Proceedings of the V Air Quality Conference, Washington, D.C., September 18-21, 2005. (11) Chu, P.; Laudal, D; Brickett, L; Lee, C. W. Power Plant Evaluation of the Effect of SCR Technology on Mercury. Presented at the Department of Energy-Electric Power Research InstitutesU.S. Environmental Pretection AgencysAir and Waste Management Association Combined Power Pant Air Pollutant Control Symposium; The Mega Symposium, Washington, D.C., May 19-22, 2003; paper number 106.
Cao et al.
equipped with a SCR and a cold-side ESP (CS-ESP), showed increased Hg capture when bituminous coals were burned but not when a powder river basin (PRB) coal with a lower chlorine content was burned. The use of NH3 to assist in NOx reduction in the SCR seems to have no effect on Hg speciation variation. Lab- and pilot-scale SCR tests with simulated flue-gas conditions can isolate factors affecting Hg oxidation.12-14 However, they cannot duplicate all of the conditions present in the flue gas from the full-scale utility boilers, such as adsorption of Hg on the SCR catalyst and the relation of the HCl concentration and Hg oxidation rates in the flue gas. Therefore, efforts have been made to set up a pilot-scale slipstream SCR facility at a working utility boiler, which can simulate real flue-gas conditions as well as flexibly control test conditions.15-17 It is found that coal rank, chlorine content, temperature, and space velocity were major factors affecting Hg oxidation. Several questions have arisen regarding the use of SCR to enhance mercury oxidation. First, what chlorine species in the flue gas can participate in Hg0 oxidation on a SCR catalyst (such as HCl or Cl2)? Second, what is the contribution of the Deacon reaction within a typical SCR catalyst temperature range? Third, what species in the flue gas can enhance or inhibit mercury oxidation within typical SCR conditions (flue-gas chemistry such as 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 SCR catalyst? This paper is the first in a series, which attempts to answer several questions mentioned above regarding the catalytic nature of SCR catalysts on mercury oxidation. Tests were conducted to disseminate the complicated factors in a SCR slipstream reactor with a real flue-gas atmosphere and individual spiking gas additions. The second in this series will present the effects of the HBr addition on Hg speciation and adsorption. 2. Experimental Section 2.1. Site Description and Configuration. The SCR slipstream reactor was installed at a selected coal-fired power station parallel to the economizer with an inlet flue-gas temperature of about 300350 °C to ensure “real world” flue gas was introduced into the SCR slipstream reactor. The greatest benefit of the smaller pilot(12) Lee, C. W.; Srivastava, R. K.; Ghorishi, S. B.; Hastings, T. W.; Stevens, F. M. Study of Speciation of Mercury under Sumulated SCR NOx Emission Control Conditions. Presented at the Department of EnergyElectric Power Plant Research InstitutesU.S. Environmental Protection AgencysAir and Waste Management Association Combined Power Plant Air Pollution Control Symposium; The Mega Symposium, Washington, D.C., May 19-22, 2003. (13) Hocquel, M. The Behaviour and Fate of Mercury in Coal-Fired Power 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 of NOx Control Processes on Mercury Speciation in Flue Gas. Presented at the Air Quality III Meeting, Washington, D.C., September 10-13, 2002. (15) Laudal, D. L.; Pavish, J. H.; Chu, P. Pilot-Scale Evaluation of the Impact of Selective Catalyst Reduction for NOx on Mercury Speciation. Presented at the Air and Waste Management Association, 94th Annual Conference, 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 across an SCR Unit. Presented at the Department of Energy-Electric Power Plant Research InstitutesU.S. Environmental Protection AgencysAir and Waste Management Association Combined Power Plant Air Pollution Control Symposium; The Mega Symposium, Washington, D.C., May 19-22, 2003. (17) Spitznogle, G.; Senior, C. Strategies for Maximizing Mercury Oxidation across SCR Catalysts in Coal-Fired Power Plants. Presented at the Department of Energy-Electric Power Plant Research InstitutesU.S. Environmental Protection AgencysAir and Waste Management Association Combined Power Plant Air Pollution Control Symposium; The Mega Symposium, Washington, D.C., September 18-21, 2005.
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Energy & Fuels, Vol. 21, No. 1, 2007 147
Figure 1. Schematic of the SCR slipstream reactor.
scale SCR slipstream tests is that it provided the ability to control variables and isolate specific factors under actual flue-gas conditions. The typical operating parameters for the selected boiler are as follows: load capacity, 200 MWe; boiler type, B&W, front wall fired with three rows of burners, with a total of nine burners; particulate control type, CS-ESP; SO2 control, none; NOx control type, low-NOx burners; and coal, bituminous coal with medium sulfur and high chlorine contents. 2.2. SCR Slipstream Reactor System. A pilot-scale slipstream SCR reactor has been designed to simulate the “full-scale” applications of a SCR system, as shown in Figure 1. The site setup picture is shown in Figure 2. The SCR reactor was designed and manufactured in a concentric configuration with an inside pass for SCR catalyst loading, where the main stream of flue gas passes through, and an outside pass, where the bypassed flue gas passes through. The flue gas, which is extracted from the well-insulated intake pipe before the SCR slipstream reactor, is split into two streams, whose ratio is controlled by manual flashboard valves to adjust the slot area of the outside flue-gas pass. The bypassed flue gas functions as a “strengthened” heat insulation because of its
higher temperature, which minimizes the heat transfer rate by decreasing the temperature difference between the introduced main stream of flue gas and the bypassed flue-gas stream. Thus, this slipstream reactor was well-insulated, so that the temperature drop across the SCR slipstream reactor was below 20 °C. The area of the inside pass was 0.152 × 0.152 m, and the outside pass was a 0.01 m slot around the inside square. The total height of the reactor was 6.6 m. The pilot-scale SCR had a two-layer catalyst to simulate the variation of the residence time for gas-solid contact. Each catalyst chamber was 1 m in height. The specific locations of the sampling ports were in relation to the locations of the catalysts. There were three sampling ports located at the inlet, middle, and outlet of each SCR catalyst bed. The “inlet” refers to the location before the first catalyst layer; the “middle” refers to the location between the first and second catalyst layers; and the “outlet” is at the outlet of the second catalyst layer. The Hg samples were taken at the inlet and outlet of the SCR slipstream reactor using semicontinuous mercury emission monitors (SCEMs) and all three locations using the Ontario hydro method (OHM) to gain a better understanding of Hg conversion mechanisms.
148 Energy & Fuels, Vol. 21, No. 1, 2007
Cao et al.
Figure 2. Actual setup on site of the SCR slipstream reactor system.
To prevent the fly ash from depositing on the SCR catalysts, an ash blower using compressed air was designed and installed. The ash-blower control allowed each catalyst layer to have the ash purged with high-velocity compressed air independently. Along with the ash blower, their ports have also been adapted to allow for pressure differential monitoring using a manometer. The overall ash-blowing cycle time was determined by the length of time it took for the pressure differential to reach the upper limit. Generally, the first catalyst layer had a blow cycle of 5 s, blowing at 30 min intervals, while the second catalyst layer had a blow cycle of 8 s, blowing at 30 min intervals. With the aid of cross-catalyst differential pressure monitoring, the ash buildup was monitored, and when the predetermined upper pressure level was reached, the ash-blowing sequence was activated to blow the ash, thereby bringing 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 in the SCR slipstream reactor. The first static mixer was located one duct diameter below the spiking gas injection ports to ensure homogeneous distribution of spiking gas before reaching the first catalyst layer. The second and third static mixers were installed at the bottom of each catalyst layer to ensure homogeneous concentrations of Hg and other gases after the flue gas exited each catalyst layer. The precise control of spiking gas addition was achieved through the construction of a multiport mass-flow controller that had the capability of being set to inject a predetermined amount of gas from one to four attached cylinders including SO2, HCl, Cl2, and NH3. SO3 or HBr addition solution injection equipment with a predetermined concentration of H2SO4 or HBr solutions, respectively. The solutions vaporized to generate SO3 or HBr spiking gases inside the SCR reactor with the desired spiking concentration. All injection ports for spiking gases were set up below the first Hg sampling port, which left the “inlet” sampling port unaffected. The injection of NH3 was separated from other spiking gas lines to ensure operational safety. Considering the actual injection ratio of NH3 in a commercial SCR facility, the ratio of NH3 injection was set at NH3/NO ) 1-1.1. Because of the low-NOx burner used, the NOx level was about 250 ppm during tests. To simulate the flue gas of various types of coal ranks, the maximum addition rates of HCl, 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 mercury speciation variation, as indicated in Table 1. On the basis of the chlorine (Cl) and sulfur (S) contents, mass balance, and spiking gas concentrations, the total flue-gas compositions of SO2, SO3, HCl, and Cl2 were calculated and are listed in Table 1. All gas concentrations were corrected to 3% O2 and a dry basis. SO3 and Cl2 concentrations in the raw flue gas were calculated on the basis of 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) SCR catalysts were provided by two vendors. Catalyst 1 had an 8.4 mm pitch, and the square cross-section had an array of 18 × 18 channels. Catalyst 2 had an approximately 7.5 mm pitch, and the square crosssection had an array of 20 × 20 channels. Each catalyst section was 1 m in length; therefore, the total length of the catalyst chamber was 2 m. The SCR catalysts were designed to be operated at a space velocity of 1800 h-1, which is the actual space velocity used on full-scale coal-fired SCR reactors.18 2.4. Coal and Ash Analysis. Bituminous coal was burned during SCR slipstream tests. The coal sample was collected at the coal transport line after the coal pulverizer. The ash sample was collected from the front-row hopper because it captured the majority of fly ash in the flue gas. The key proximate and elemental analysis of the coal samples is shown in Table 1. Hg in all solid samples was analyzed using a LECO AMA-254 [American Society for Testing and Materials (ASTM) method D 6722]. The variation of Hg, Cl, and S contents during the tests were 0.10-0.15 and 354-1186 ppm and 1.30-2.17%, respectively, with averages of 0.11 and 854 ppm and 1.3% during runs with the SCR catalyst, 1.4% and 946 and 0.13 ppm during runs with SCR catalyst 1, and 1.9% and 549 and 0.14 ppm during runs with SCR catalyst 2. Analytical data for fly ash taken from the ESP hopper are shown in Table 2. Loss on ignition (LOI) and Cl and S contents in the ash samples were determined using ASTM methods D 5142, D 5373, and D 5016, respectively. It was found that the LOI content of the fly ash was lower at about 2.6%, which indicated a good combustion performance during testing, even for the boiler equipped with the lowNOx 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 Power Plants Using SCR and SNCR Control Technologies. EM February 22, 2003.
Hg Oxidation by a SCR Catalyst
Energy & Fuels, Vol. 21, No. 1, 2007 149 Table 1. SCR Slipstream Test Conditions and Coal Properties
catalyst information
additives
catalyst type
space velocity (h-1)
catalyst temperature (°C)
HCl addition
none
1837
317
Cl2 addition
none
1850
SO2 addition
none
HCl addition
total flue-gas concentration dry, 3% O2
coal analyses Cl (ppm) dry
Hg (ppm) dry
S (%) dry
NO (ppm)
HCl (ppm)
0
1270
0.1
1.56
290
101 301 501 601
302
0
647
0.12
1.3
280
1890
304
0
647
0.12
1.3
280
1
1935
293
0
1186
0.13
1.39
231
HCl addition
1
1870
313
1.02
1186
0.13
1.39
231
Cl2 addition
1
1885
303
0
963
0.13
1.34
274
SO2 addition
1
1875
302
0
685
0.12
1.48
293
SO3 addition
1
1870
313
0
710
0.15
1.42
168
NH3/NOx ratio
HCl addition
2
1920
293
1.03
571
0.12
1.89
282
HCl addition
2
1880
305
0
842
0.15
1.76
234
HCl addition
2
1865
310
1.07
842
0.15
1.76
243
SO2 addition
Cl2 addition
2
2
1865
1935
310
293
0
0
396
571
0.15
0.12
2.17
1.89
242
2
1850
302
1.02
354
0.16
1.95
215
SO3 addition
2
1883
304
0
423
0.14
1.89
232
SO3 addition
2
1875
310
1.01
396
0.15
2.17
258
SO2 (ppm)
SO3 (ppm)
3 8 13 28 1037 2037 3037 95 295 595 95 595 4 9 14 24 1181 1681 2181 3181 6 56 46 146 246 446 546 67 167 267 467 500 67 167 267 467 567 1731 2481 3231 3731
246
SO2 addition
captured by fly ash in the duct. Over 95% of the sulfur and chlorine in the coal remained in the gas phase. The major and minor element data from the X-ray fluorescence analysis (ASTM method D 4326) of ashes prepared from the coal samples during the test period are also shown in Table 2. A total of 13 oxides were selected for determination, including CaO, Fe2O3, TiO2, and MnO, which were reported to possibly affect Hg transformation. The higher Fe2O3 content at 14.6% and a lower CaO content at about 1.65% in the
Cl2 (ppm)
2 7 12 22 27 1556 2056 2306 3056 3556 8 28 43 58 9 29 59
ash of the tested coal could be attributed to the active function of the fly ash on Hg speciation. 2.5. Instrumentation. The variation of the Hg concentration was monitored continuously by SCEMs at two locations (inlet and outlet) and by OHM at three locations in the SCR slipstream reactor. OHM measurements (ASTM method 6784-02) for each of the sampling locations were applied to confirm the SCEM sampling results at a period of validation of SCR slipstream tests. Only one
150 Energy & Fuels, Vol. 21, No. 1, 2007
Cao et al. Table 2. Analysis Data on Ash Properties
sample name tests without SCR catalysts
tests with SCR catalysts
ash
ash
sulfur (%)
chlorine (ppm)
mercury (ppm)
LOI (%)
0.11
100
0.10
2.57
Na2O (%)
MgO (%)
Al2O3 (%)
SiO2 (%)
CaO (%)
K2O (%)
SO3 (%)
P2O5 (%)
BaO (%)
SrO (%)
Fe2O3 (%)
MnO (%)
TiO2 (%)
1.705
2.348
1.935
0.583
0.152
0.130
17.508
0.023
1.140
0.004
0.895
18.142
38.272
sulfur (%)
chlorine (ppm)
mercury (ppm)
LOI (%)
0.13 Na2O (%)
177 MgO (%)
0.12 Al2O3 (%)
2.73 SiO2 (%)
CaO (%)
K2O (%)
SO3 (%)
P2O5 (%)
BaO (%)
SrO (%)
Fe2O3 (%)
MnO (%)
TiO2 (%)
13.811
32.609
1.608
1.902
1.350
0.574
0.120
0.088
11.688
0.030
0.962
0.041
0.807
modification was made to the SCEMs, by which the higher temperature inertial probe (300-350 °C) was applied to minimize the gas-phase sampling bias. A detailed description of the two Hg test methods and QA/QC procedures can be found in the references.5,19
3. Results and Discussion 3.1. Validation of the SCR Slipstream Test. Before any investigation tests on the variation of Hg speciation in the SCR slipstream reactor, several issues had to be addressed, including the reduction performance of NOx addition, effects of the reactor material on mercury speciation, and the effect of the SCR catalyst on mercury adsorption. 3.1.1. NOx Reduction by NH3 Addition in the SCR Slipstream Reactor. The reduction performance of the SCR slipstream reactor was evaluated by monitoring the NO concentration at the inlet and outlet locations of the SCR slipstream reactor. Because of the low-NOx burner installed in the test unit, NO concentrations at the inlet were found to be about 280300 ppm during the SCR slipstream validation tests. The NO concentration at the outlet location was almost the same as that at the inlet location with the same O2 concentration. After the NH3 addition started with a molar ratio of NO/NH3 at about 1, the NO concentration decreased gradually and finally went down below 20 ppm, as shown in Figure 3A for catalyst 1 and Figure 3B for catalyst 2. Both catalysts in the SCR slipstream reactor worked properly as expected with above 95% NO reduction. 3.1.2. Effect of the Empty Bed of the Slipstream Reactor Material on the Oxidation of Hg0. With the exception of Teflon and glass, all other materials may affect mercury speciation. Thus, the construction material of the SCR slipstream reactor, which was stainless-steel, could have affected the mercury speciation under typical SCR temperatures. Hence, the reactor blank tests by OHM were made at three locations: inlet, middle, and outlet. Results are presented in Figure 4, where the y axis represents the variation of mercury oxidation by an incremental percentage relative to the inlet value based on the equation shown in eq 1.
reactor materials on mercury speciation at this high temperature. The small increase in mercury oxidation that occurred in the SCR slipstream reactor could possibly be attributed to oxidation effects of in-flight fly ash within the typical SCR temperature range, which is thermodynamically favored. 3.1.3. Effect of the SCR Catalyst on the Total Vapor-Phase of Mercury [Hg(VT)]. To evaluate the possible adsorption of mercury on the SCR catalyst, the total vapor-phase mercury concentration was monitored by the SCEM system and OHM tests at the inlet and outlet of the SCR slipstream reactor with SCR catalysts 1 and 2 test runs. Although the total vapor-phase of mercury varied with coal properties and the boiler load, a good agreement between the experimental results of the total Hg(VT) by SCEM and OHM and the predicted Hg(VT) in the flue gas, which was calculated on the basis of the conversion of the total Hg in coal into the flue gas, was reached. Test results
additional oxidation of Hg0 ) 100{1 - [Hg0/Hg(VT)]mid or out/[Hg0/Hg(VT)in]} (1) Blank tests indicated that the percentage of variation of the mercury oxidation was within 5% as the flue gas crossed the empty SCR slipstream reactor between sampling ports. There seemed to be no evidence to show significant effects of SCR (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 in a 100-MW Coal-Fired Boiler with Low-NOx Burners. Energy Fuels 2005, 19, 800-806.
Figure 3. (A) NO reduction performance of the SCR slipstream reactor for catalyst 1. (B) NO reduction performance of the SCR slipstream reactor for catalyst 2.
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Energy & Fuels, Vol. 21, No. 1, 2007 151
Figure 4. Variation of the mercury speciation across the empty bed of the SCR slipstream reactor (runs 1, 2, and 3).
are shown in parts A and B of Figure 5 for SCR catalysts 1 and 2, respectively. There is no evidence to relate mercury adsorption to the SCR catalysts, at least for catalysts tested in this study at a temperature around 300 °C. However, Lee et al. have examined data from a small-scale SCR reactor under simulated flue-gas conditions and have found that there may possibly be some adsorption of Hg0 on the catalyst for certain measurements.12 We noted that the work of Lee et al. is done under conditions to simulate the flue gas of PRB coal, which is lower in chlorine content. Actually, a reasonable assumption may be that a dynamic situation could be established on the SCR catalyst surface. First, Hg0 is attracted and trapped because of active sites on the SCR catalyst surface, then Hg0 reacts with adsorbed chlorine species or other oxidizing species to form Hg2+, and finally, Hg2+ will be liberated from the surface of the SCR catalyst because there has been no report of mercury adsorption on any material under such a high temperature. Another reason for no Hg adsorption on the SCR catalyst may be associated with the deposition of ash, which covers some of the Hg adsorption sites. The different findings on the adsorption of elemental mercury on the surface of catalysts between different sources may be due to the individual experimental conditions (catalyst, flue-gas compositions, and fly-ash deposit) or measurements. 3.2. Effects of Spiking Gases on Hg Oxidation with or without the SCR Catalyst. Hg oxidation in the SCR may occur because 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 a solid surface. In the SCR slipstream reactor during runs without the SCR catalyst, the possible oxidation mechanism was homogeneous oxidation and also heterogeneous oxidation by the interaction with “in-flight” fly ash, which was different from solid-gas contact mode in the fixed bed when fly ash deposited on the sampling probe. After runs with the SCR catalyst, the additional Hg0 oxidation compared to the runs without the SCR catalyst should be solely a result of Hg0 oxidation from the catalytic effect of the SCR catalyst. Gas compositions in the flue gas, such as HCl, Cl2, SO2, SO3, and NH3, may be the
oxidizing or reducing agents to impact Hg0 oxidation chemistry in the Hg transformation process. The SCR catalyst may promote the oxidation of the Hg0 in the flue gas with the participation of the active agents mentioned above. The addition of the individual spiking gases into the SCR slipstream reactor during runs with or without the SCR catalyst could possibly provide the information on the reaction mechanism of the heterogeneous catalytic oxidation of Hg0. For tests of spiking gas additions during both runs with and without SCR catalysts, results are presented as the variation of Hg0 oxidation across the SCR reactor versus the total concentrations of individual flue-gas species, as shown in Figures 6-9. Considering the constant Hg(VT) during runs with SCR catalysts, the variation of Hg0 oxidation across the SCR reactor is represented by eq 2,
percent Hg0 oxidation ) 100[Hg0in - Hg0out]/[Hg0in] (2) which is the incremental percentage variation between the Hg0 concentration at the SCR reactor inlet (Hg0in) and the Hg0 concentration at the SCR reactor outlet (Hg0out). 3.2.1. Effects of HCl and Cl2 on Mercury Oxidation during Runs with or without SCR Catalysts. HCl and Cl2 are the two most important species with regard to mercury oxidation because of the fact that the main oxidized mercury species in coal-fired flue gas is Hg(Cl)2. The effects of the spike gases HCl and Cl2 on Hg0 oxidation during runs with or without SCR catalysts are shown in Figures 6 and 7. With the SCR catalyst, HCl addition could further promote the oxidation of Hg0 even with the coal chlorine content at 1270 ppm in the present study. During tests without the SCR catalyst in the reactor, the percentage of Hg0 oxidation increased by 3, 6.5, 19.4, and 27.9%, with increasing HCl addition concentrations of 100, 200, 400, and 500 ppm (total chlorine concentration of approximately 200, 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 same conditions during runs without the SCR catalyst, which indicates that SCR catalyst 1 is an active catalyst for Hg0 oxidation.
152 Energy & Fuels, Vol. 21, No. 1, 2007
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Figure 5. (A) Investigation of mercury adsorption on SCR catalyst 1 by SCEM and OHM. (B) Investigation of mercury adsorption on SCR catalyst 2 by SCEM and OHM.
Uncertainty existed on how to evaluate the effects of temperature and NH3 addition on Hg0 oxidation because of the simultaneous variation 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 just 5-10% at different HCl addition levels and there was no apparent improvement upon mercury oxidation with HCl additions. During runs with SCR catalyst 2, the percentage of Hg0 oxidation was shown to increase greatly by gradually increasing HCl addition by 0, 200, 400, and 500 ppm (total HCl concentrations of 100, 300, 500, and 600 ppm in the flue gas), as shown in Figure 6. However, the curves become flat with an increasing of HCl addition concentrations. When HCl addition was 400 ppm, the additional oxidation of Hg0 was approximately 25% relative to those runs without the SCR catalyst. Thus, SCR catalyst 2 was also shown to have a catalytic effect on Hg0 oxidation. From Figure 6, the addition of NH3 did not have any 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 was made between two cases with a temperature difference of 20 °C at a similar NH3 addition ratio (NH3/NO ∼ 1). The higher curve in Figure 6 (NH3/NO ) 1.03) corresponds to a temperature of 293 °C, while the lower curve in Figure 6 (NH3/NO ) 1.07) corresponds to a temperature of 310 °C. The shapes of these two curves are similar, but the magnitude of the mercury oxidation was considerably different between the two curves. The difference between these two curves could be associated with the temperature differences. Models of mercury oxidation across SCR catalysts and other sets of slipstream data have shown higher Hg oxidation at lower temperatures.20 The present study confirms that NOx reduction by NH3 and Hg0 oxidation by chlorine species simultaneously occur on the surface of SCR catalyst 1; however, they are competitively (20) Senior, C. L. Oxidation of Mercury across Selective Catalyst Reduction Catalysts in Coal-Fired Power Plant. J. Air Waste Manage. Assoc. 2005, 56, 23-31.
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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.
adsorbed on the active sites on the surface of the SCR catalyst, while NOx reduction by NH3 is apparently predominant. Both factors of temperature and NH3 addition impact the Hg0 oxidation by controlling process kinetics. Observed curves of the Hg0 oxidation process become flat for both SCR catalysts in this study. This may indicate the impact of the NOx reduction reaction by NH3 on Hg oxidation by their competitive nature (HCl and NH3) on active sites of surfaces of SCR catalysts, which result in the independence of HCl addition concentrations on Hg0 oxidation when HCl continuously increases to certain levels. Observed Hg0 oxidation varied greatly between SCR catalysts 1 and 2. Differences in catalyst pitch and formulation might have been responsible for the differences in performance between these two SCR catalysts. Mercury oxidation by HCl across SCR catalyst 2 appeared to be affected by both temperature and HCl concentrations, while SCR catalyst 1 seemed
virtually unaffected by the variation of both temperature and HCl concentrations. Recently, models for mercury oxidation across SCR catalysts have been developed to include the effects of mass transfer and surface chemistry kinetics simultaneously.10-22 The catalyst formula was the key parameter that impacted surface chemistry kinetics across the temperatures used, and the catalyst geometry was the key parameter that impacted mass transfer. Thus, the design of a certain SCR catalyst directly impacts the performance of the SCR catalyst on Hg0 oxidation through relative influences of the mass transfer rate and chemical (21) Niksa, S.; Fujiwara, N. A Predictive Mechanism for Mercury Oxidation on Selective Catalytic Reduction Catalysts under Coal-Derived Flue Gas. J. Air Waste Manage. Assoc. 2005, 55, 1866-1875. (22) Edwards, J. R.; Srivastava, R. K.; Kilgroe, J. D. A Study of GasPhase Mercury Speciation Using Detailed Chemical Kinetics. J. Air Waste Manage. Assoc. 2001, 5, 69-87.
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Figure 8. Variation of the mercury oxidation with SO2 addition for two catalysts.
reaction kinetics. SCR catalyst 1 could possibly be improved by enhancing its mass transfer through geometry design, and SCR catalyst 2 could be improved by modification of its formulation of active sites. The variation of the percentage of oxidation of Hg0 by the addition of Cl2 into the SCR slipstream reactor was much different when compared to that by the addition of HCl, as shown in Figure 7. The addition of Cl2, with a maximum addition concentration of 25 ppm, results in little variation of the percentage of oxidation of Hg0 in this study, which was within (5% under testing conditions during runs with or without the SCR catalyst and thus cannot be regarded to be significant. In comparison with HCl, which is the main chlorine species in the flue gas, Cl2 seemed to have little impact on mercury speciation under current experimental conditions. The difference in performance between the two SCR catalysts in terms of the percentage of oxidation of Hg0 under the same Cl2 addition concentration should be attributed to the difference in catalyst pitch or formulation, as mentioned previously. Previous work on mercury oxidation has suggested that chlorine compounds are intensively involved in mercury oxidation within the typical SCR temperature range. This study clearly demonstrated that HCl and not Cl2 is the chlorine compound that affects mercury oxidation. 3.2.2. Effects of SO2 and SO3 on Mercury Oxidation during Runs with or without SCR Catalysts. The effects of SO2 addition on the variation of Hg0 oxidation is shown in Figure 8. Although the results show a little scatter, it seemed that the percentage of oxidation of Hg0 followed a decreasing trend as the SO2 addition concentration increased. The incremental percentage of oxidation of Hg0 under the condition of 2000 ppm SO2 addition relative to the zero addition levels in the SCR slipstream reactor was found to be approximately 25 and 5% lower for SCR catalysts 1 and 2, respectively. However, Figure 9 indicates that the percentage of oxidation of Hg0 followed an increasing trend when SO3 addition increased to its maximum addition concentration of 50 ppm. The larger extent of the incremental percentage of oxidation of Hg0 by SO3 addition during runs with the SCR catalyst, compared to those 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 addition during runs without the SCR catalyst and with SCR catalyst 2. This may be attributed to the measurement errors or the variation of the boiler performance occurring when tests were conducted. The incremental percentage of oxidation of Hg0 with the addition of 50 ppm SO3 in the SCR slipstream reactor for both SCR catalysts was found to be approximately 20% higher relative to the zero addition levels. Just as with the previous findings, a higher percentage of oxidation of Hg0 for SCR catalyst 1 compared with that of SCR catalyst 2 under the same SO2 or SO3 addition concentration should be attributed to the difference in catalyst pitch and formula. 3.2.3. Mechanisms Discussion. This study clearly demonstrated that it was HCl and not Cl2 that promoted Hg0 oxidation within a typical SCR temperature range in the flue-gas atmospheres. SO3 also had some positive impact on Hg0 oxidation. These oxidation mechanisms involving SCR catalysts can be promoted to a large extent through HCl and to a lesser extent through SO3. SO2 may inhibit Hg0 oxidation, even reducing Hg2+ to Hg0. However, combining the results of investigations on Hg0 oxidation mechanisms in the present study and previous studies5,6,22 leads to several questions. First, the Deacon reaction, which is shown in eq 3,22
2HCl + 1/2O2 ) Cl2 + H2O
(3)
will be favored below 600 °C in the reverse direction to generate HCl by dissipating Cl2, as shown in eq 4.
Cl2 + H2O ) 2HCl + 1/2O2
(4)
If Cl2 has no effect on Hg0 oxidation, as indicated in this study, at least the increasing of the HCl concentration by Cl2 addition should show some impact on Hg0 oxidation. Second, previous studies attributed the active chlorine compound for Hg0 oxidation to Cl2 because of the possible “Cl” pool maintained by Cl2 through eq 5,
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Figure 9. Variation of the mercury oxidation with SO3 addition for two catalysts.
HgCl + Cl2 ) HgCl2 + Cl
(5)
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 into the SCR reactor should result in increasing the SO3 concentration in the flue gas, which should further enhance Hg0 oxidation by producing SO3 as indicated in this study. However, this reasonable assumption is not in agreement with evidence that the possible Hg0 reduction occurred after the addition of the higher concentration of SO2 in the present study. The prevailing mechanisms5,6,22 for Hg0 oxidation validate the important effects or contributions of chlorine compounds (HCl, Cl2, and Cl) and possible interferences of sulfur chemistry in the flue gas. It should be pointed out that the reaction illustrated by eq 6
Hg + Cl ) HgCl
(6)
is the initial step of the Hg0 oxidation process because of its fast rate in reaction kinetics, if Cl2 is available in the flue gas. The slower reactions, represented by eqs 5, 7,
Hg + Cl2 ) HgCl2
(7)
2Hg + 4HCl + O2 ) 2HgCl2 + 2H2O
(8)
and 8,
dominate the overall Hg oxidation kinetics mainly through Cl2. Cl2 provides the important Hg0 oxidation species of Cl and maintains the pool of this active agent. Furthermore, Cl2 is a much stronger oxidizing agent over HCl because of the difference of their valence electron configurations, which will result in the difference of affinity for Hg0. However, several investigators indicated possible depletion of Cl2 within the temperature range of 200-700 °C by flue-gas compositions such as SO2, NO, and H2O.6,23-25 The reaction routines for depletion (23) Agarwal, H.; Stenger, H. G.; Wu, S.; Fan, Z. Effects of H2O, SO2, and NO on Homogeneous Hg Oxidation by Cl2. Energy Fuels 2006, 20, 1068-1075.
of Cl2 in the coal-fired flue gas are shown in eqs 4, 9,
SO2 + Cl2 ) SO2Cl2
(9)
2NO + Cl2 ) 2NOCl
(10)
and eq 10.
Thus, the reaction routine in eq 8, which is presented as the global reaction, and possible reaction steps in eqs 11
Hg + HCl ) HCl + H
(11)
HgCl + HCl ) HgCl2 + H
(12)
and 12,
may dominate Hg0 oxidation through HCl within the temperature range of the typical SCR operation range (300-350 °C), as confirmed 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 SCR catalyst, SO3 was generally produced because of the active site of V2O5 available for conversion of SO2 to SO3, through eq 13.
SO2 + 1/2O2 ) SO3 (with the SCR catalyst)
(13)
Thus, the inhibition by SO2 and promotion by SO3 on Hg0 oxidation may be balanced somehow when SO2 is added into the SCR reactor. In summary, this study provides some clues of different reaction routes for Hg0 oxidation in the real coalfired flue-gas atmosphere within the typical SCR temperature range. For the present study, the reactions represented in eqs (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 Mercury in Flue Gas. Energy Fuels 2006, 20, 1941-1945.
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8-10 are believed to be responsible for Hg0 oxidation within the typical SCR temperature range in real coal-fired flue gas. 4. Conclusion A SCR slipstream reactor was set up to simulate the scaleddown “real world” circumstance of SCR catalysts for investigation of Hg oxidation at a selected coal-fired utility boiler burning a bituminous coal. This system has observed above 90% of NO reduction performance with selected SCR catalysts and NH3 addition. A modified high-temperature inertial probe provided the measurement accuracy for monitoring mercury speciation by the SCEM. Both SCR catalysts used in this study showed catalytic effects on Hg0 oxidation. The different performance of SCR catalysts on Hg0 oxidation may be attributed to their different characterization of chemical formulation and manufacture geometry, which are related to the mass transfer rate and chemical reaction kinetics. SCR catalyst 2 seemed sensitive to operational temperature with regard to Hg0 oxidation, but SCR catalyst 1 did not. Hg0 oxidation by SCR catalyst 1 may be improved by enhancing its mass transfer through geometry
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design, and SCR catalyst 2 may be improved by modification of its formulation of active sites. Tests by additions of gaseous acidic spike gases indicated that it is HCl and not Cl2 that is the major source of chlorine that dominates the Hg0 oxidation process within the typical SCR temperature 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 the present study with real flue-gas atmospheres. This study also demonstrated that SO2 has an inhibiting effect on Hg0 oxidation; however, SO3 can promote mercury oxidation. The developed mechanisms for Hg0 oxidation within the typical SCR temperature range are in agreement with pre-existing Hg0 oxidation mechanisms. Acknowledgment. This paper was prepared by the Western Kentucky University Research Group with support, in part, by grants made possible by the Electric Power Research Institute (EPRI project number EP-P13792/C6821). EF0602426
