IDENTIFICATION OF MERCURY SPECIES IN UNBURNED CARBON FROM PULVERIZED COAL COMBUSTION. AWMA Paper 99-72

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1 SR-977 IDENTIFICATION OF MERCURY SPECIES IN UNBURNED CARBON FROM PULVERIZED COAL COMBUSTION AWMA Paper Frank E. Huggins, Nora Yap and Gerald P. Huffman University of Kentucky Lexington, KY Constance L. Senior 20 New England Business Center Physical Sciences, Inc. Andover, MA Presented to the Air and Waste Management Association 1999 Annual Meeting and Exhibition Copyright 1999

2 Identification of Mercury Species in Unburned Carbon from Pulverized Coal Combustion Frank E. Huggins, Nora Yap and Gerald P. Huffman University of Kentucky, Lexington, KY Constance L. Senior Physical Sciences, Inc., Andover, MA ABSTRACT Coal-fired power plants are a significant source of atmospheric mercury. In coal combustion flue gases, mercury can be adsorbed on fly ash, reducing the emission of mercury to the environment since fly ash is efficiently removed from the flue gas. The mechanism for adsorption on fly ash is not known. New analytical methods are being developed to identify the mercury species adsorbed on fly ash as well as on sorbent materials. In this paper, we present some results of X-Ray Absorption Fine Structure Spectroscopy (XAFS) measurements of unburned coal (char) exposed to mercury compounds in a simulated flue gas. When the gaseous mercury species was HgCl 2, the adsorbed form on char appeared to be a mercury-chlorine compound. When elemental mercury was the gaseous form, the form of mercury adsorbed on the char showed a dependence on the char composition. Preliminary analysis of the data indicates that chars exposed to elemental mercury may contain a mixture of elemental mercury and mercury chloride, depending on the amount of chlorine in the coal. INTRODUCTION A recent report by the U.S. Environmental Protection Agency (EPA) on emission of hazardous air pollutants by electric utilities predicted that emissions of air toxics from coal-fired utilities would increase by 10 to 30% by the year Mercury from coal-fired utilities was identified as the hazardous air pollutant of greatest potential public health concern. Anthropogenic emissions of mercury account for 10 to 30% of the world-wide emissions of mercury. 2 EPA has estimated that during the period annual emissions of mercury from human activities in the United States were 159 tons. 1 Approximately 87% of these emissions were from combustion sources. Coal-fired utilities in the U.S. were estimated to emit 51 tons of mercury per year into the air during this period. Based on these data, it is possible that mercury emissions from coal-fired power plants will be regulated under the Clean Air Act Amendments of Should regulations be imposed on mercury emissions from coal-fired power plants, a sound understanding of the fundamental principles controlling the formation and partitioning of mercury during coal combustion will be needed to aid in developing methods for controlling mercury emissions. A fraction of the mercury in flue gases is converted to adsorbed species on particulate matter. Mercury measurements made at power plants have shown that as much as 20% of the total 1

3 mercury entering the plant can be removed in the ESP ash. 3,4 The mechanism for mercury capture on particulate matter in coal combustion flue gas seems to be chemical or physical adsorption on the fly ash and depends on the oxidation state of the mercury and the composition of the fly ash. Gas-phase oxidized mercury is readily captured by activated carbon, while elemental mercury has a much lower affinity for carbon. The surface of the carbon is crucial to mercury sorption; adding sulfur or iodine can dramatically increase the capacity of activated carbon for elemental mercury. 5-8 Residual carbon from coal combustion is not the same as activated carbon. The pore structure, surface properties, and inorganic content may be strikingly different. There is evidence for the adsorption of mercury on coal fly ash even in the absence of significant unburned carbon, 9,10 although the specific species which are adsorbed is not known. In a previous study, 11 chars generated from three coals were used as sorbent material for both elemental mercury and mercuric chloride in a laboratory fixed bed reactor. The temperatures of the source as well as that of the char sorbent were carefully controlled in the range of 343 to 433 K ( o C). These three chars showed consistently higher mercury capture in the case of mercury chloride (over elemental mercury) as the mercury source. Elemental mercury appeared to react chemically with sulfur in the char, particularly with organic sulfur at lower gas-phase mercury concentrations. For mercury chloride, a physical adsorption process seemed to be indicated based on the correlation with char surface area. The organic sulfur content of the char appeared to be the better predictor of the affinity of the char for elemental mercury, while the char surface area appeared to be the better indicator of affinity of the char for mercuric chloride. The chars that were exposed to mercury were analyzed using X-Ray Absorption Fine Structure (XAFS) spectroscopy. XAFS spectroscopy is basically a measurement of the variation (or fine structure) of the X-ray absorption coefficient with energy associated with one of the characteristic absorption edges of the absorbing element. Analysis of XAFS spectra provides information on the bonding of the element in question and can be used to distinguish different compounds from one another. In this work, XAFS is used to determine the speciation of mercury adsorbed on chars and various sorbent materials. An understanding of the speciation of mercury on solids will help clarify the mechanisms for mercury adsorption. EXPERIMENTAL METHODS Preparation of Chars Three coals were chosen for this study, two bituminous coals from the Pittsburgh and Illinois No.6 seams and a sub-bituminous coal from the Powder River Basin, Wyodak seam. The ash compositions of the bituminous coals are fairly typical of American bituminous coals, containing primarily silica, alumina, and iron oxide. The ash from the Wyodak sub-bituminous contains significantly less iron and more calcium than the bituminous coals. Results of the analysis of the forms of sulfur in the coals are presented in Table 1. The data indicate that the Pittsburgh and Illinois No. 6 coals contain significant amounts of both pyritic and organic sulfur. The Wyodak coal contains primarily organic sulfur. Chars were made by reacting the coals in the PSI Entrained Flow Reactor (EFR) as described in Reference 11. The ash content of the chars and surface area of the chars are summarized in Table 2. The forms of sulfur in the chars were determined using XAFS spectroscopy. Sulfur 2

4 XANES spectra were analyzed according to least-squares fitting procedure developed for sulfur in coals and chars. 12 Estimates of the percentage of sulfur in different forms in the chars are given in Table 3. The bituminous chars have a mixture of organic sulfur, elemental sulfur, and pyrrhotite (Fe 1-x S). The latter two forms of sulfur are the result of decomposition of pyrite during devoloatilization. The Wyodak char has primarily organic sulfur with some sulfate and elemental sulfur. Sorption experiments were performed using a small fixed bed reactor. 11 Mercury or mercury chloride sources were placed in an oven. A small fixed bed of char was placed in another oven. The fixed bed was heated to either 343 K or 443 K. The temperature of the mercury source was controlled to deliver mercury concentrations in the gas phase from 0.4 ppmw to 60 ppmw. All the experiments were done under oxidizing conditions where the gas composition was 15% CO 2, 3% O 2, 2% H 2 O and balance N 2. This gas composition contains the major species in coal combustion flue gas, but none of the minor species such as SO 2, NO, NO 2, HCl or Cl 2. Recent work has demonstrated that these species affect the interaction of gaseous mercury compounds and activated carbon. 13 Therefore, the results of this study may not be directly applicable to the behavior of unburned coal in power plants. Future experiments will employ more realistic flue gas compositions. Chars were exposed to the simulated flue gas for times ranging from 5 to 25 hours. At the end of the experiment, the concentration of mercury in the char was determined by cold vapor atomic absorption. The data were used to determine the sorption ability and capacity of sorbent. Char samples were analyzed by XAFS for forms of mercury. XAFS Spectroscopy Mercury XAFS spectroscopy was conducted principally at beam-line IV-3 at the Stanford Synchrotron Radiation Laboratory (SSRL), Palo Alto, CA. Virtually identical experimental practice was carried out at both synchrotrons. The mercury L III edge at 12,284 ev was used for the absorption experiments and all measurements on char samples were carried out in fluorescence geometry. The mercury contents of the chars varied from as much as 8.5 wt% to as little as 8 parts per million (ppm) and a Lytle-type detector 13 was used for the more concentrated samples, whereas a 13-element germanium detector 14 was used to record the spectra from less concentrated samples. A 6µ gallium filter and Soller slits 15 were used to enhance the signal/noise ratio with both types of detector. For the most dilute samples (< 500 ppm Hg), multiple scanning was also used to improve the signal/noise ratio. Additionally, in the course of this investigation, the spectra of a number of mercury compounds were measured. These spectra were measured in absorption geometry using pressed pellets of the compound in a plastic medium or thin smear mounts on tape. A droplet of elemental mercury pressed and sealed between two thin plastic sheets served as the calibration standard. The single inflection point in the XAFS spectrum of elemental mercury was taken to be the zero-point of energy for calibrating the XANES spectra shown in this investigation. Where possible, this spectrum was obtained simultaneously in an absorption experiment after the main experiment, so that each XAFS spectrum would be individually calibrated. Analysis of the calibrated spectral data consisted of dividing the XAFS spectrum into separate 3

5 X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) regions. Analysis of the XANES region consisted of smoothing and differentiating the spectrum to obtain (Figure 1) the d 2 Abs/(dE) 2 spectrum. The XANES spectrum pictured at the top of Figure 1 exhibits two inflection points. The location of these inflection points appeared to vary systematcially with the character of the mercury bond in the sample. In order to determine more precisely the spacing or inflection point difference (IPD), the second derivative of the spectrum was calculated as shown in the bottom section of Figure 1. The IPD is the spacing between the inflection points as indicated by the arrows in the figure. The isolated EXAFS region was analyzed in the standard manner 16,17 by fitting a five-region cubic spline to the data to obtain the chi spectrum and then converting from real space to reciprocal (k) space. A Fourier transform was then applied to the k 3 -weighted chi vs. k spectrum to obtain the radial structure function (RSF). The RSF is a one-dimensional representation of the local structure around the absorbing mercury atom. RESULTS AND DISCUSSION XAFS data for various mercury compounds are summarized in Table 4. Included in this table are values for IPD from each XANES spectrum and the position of the major peak in the corresponding RSF. As can be seen from this table the mercury compounds with the smallest and most ionic anions (O 2-, -O - ) have the largest values of IPD, whereas those with the largest anions (I - ) have the smallest values of IPD. Hence, it appears that the IPD value correlates with differences in the Hg-X bond ionicity and/or length. The value of IPD appears not to depend on the oxidation state of the mercury because both Hg(I) and Hg(II) sulfates and chlorides have effectively the same IPD value. It is well known that features in the XANES region can reflect bond lengths and other structural details, and, as can be readily seen in Table 4, there appears to be an inverse correlation between the separation of the mercury L III edge inflection points and the RSF peak position, as might be expected based on previous investigations of systematics in XANES spectra. 18,19 Similar trends can be seen in other compilations of mercury compound data. 20 Values for the IPD determined from the XANES spectra of various chars after exposure to mercury, as Hg 0 or as HgCl 2, in simulated flue gases (SFG) are displayed in Table 5. When the IPD values for the chars are compared with those for standards (Figure 2), we find that most of the char samples have values comparable to chlorides and sulfides. Experiments carried out with mercuric chloride (HgCl 2 ) in the vapor phase give rise to higher IPD values than those carried out with Hg 0 vapor. In chars exposed to elemental mercury (Hg 0 ), there is clearly a trend towards lower values for the experiments carried out with the Wyodak char than with the Illinois No.6 and Pittsburgh chars. Such differences may reflect the different chlorine contents of the coals. The Wyodak coal is known to have a much lower content of chlorine than the other two coals. The data listed in Table 5 and shown in Fig. 2 make a further interesting point in that they appear not to show a bimodal distribution around the values for HgS and HgCl 2. The XANES spectra for Hg in the chars do not resemble crystalline mercuric chloride or mercuric sulfide, even though the IPD values for the chars are very similar to that for Hg chloride or Hg sulfide. 4

6 Such trends reflect either a mixture of distinct mercury species or possibly a single amorphous mercury species of variable and mixed composition. We have explored one possible reason for this variation in terms of a mixture of chemisorption, involving formation of a HgCl 2 or HgS species on the char surface, and physisorption of elemental mercury. This was done by simulation of a suite of Hg XANES spectra from endmember standard spectra for metallic (liquid) mercury and either mercuric chloride or mercuric sulfide. Examples of such simulated spectra are shown in Figure 3 for Hg-HgCl 2. Also shown are the derivative spectra for the same suite of XANES spectra. It can be seen that the peakheight ratio of the derivative spectra changes systematically with the amount of Hg in the mixture. Hence, the combination of the IPD parameter and the derivative peak-height ratio provides a two-dimensional plot (Figure 4) that appears to have the potential to discriminate between chemisorption and physisorption. Also shown in Figure 4 are the Hg-XANES data obtained for the chars and it can be seen that these data are parallel but displaced relative to the IPD/Peak-height Ratio trend for Hg-HgCl 2. Hence, a possible explanation for the Hg XANES data trend shown by the char samples is that it is due to the occurrence of chemisorption of Hg as HgCl 2 and physisorption of metallic Hg as competing processes. Further, the relative fraction of the chemisorption process is likely to reflect the Cl/Hg ratio in the system, as we speculated previously. Further analysis of these trends will be undertaken and new spectral data will be obtained for the standards as well as for new samples to be prepared. CONCLUSIONS The results of this investigation have shown that the difference in energy between the two principal inflection points (IPD) on the mercury L III absorption edge measured in XAFS spectra is a sensitive indicator of the structure and/or chemistry of the mercury adsorbed onto unburned carbon and sorbent materials. Unburned coal (char) samples were exposed to mercury compounds in a simulated flue gas. When the chars were exposed to gaseous HgCl 2, the adsorbed species on char appeared to be a mercury-chlorine compound. When elemental mercury was the gaseous form, the adsorbed mercury species on the char showed a dependence on the char composition. Preliminary analysis of the data indicate that chars exposed to elemental mercury may contain a mixture of elemental mercury and mercury chloride, depending on the amount of chlorine in the coal. This investigation implies rather conclusively that there are likely to be a number of competing mechanisms for low-temperature mercury sorption by a given material in a real flue gas. Such mechanisms will depend on the surface chemistry, the species in the flue gas, etc. Hence, any mechanistic interpretation of mercury sorption by carbon-based materials in terms of a specific individual mercury form is likely to be insufficient. Rather, a more general approach to mercury sorption must be developed. ACKNOWLEDGEMENTS Financial support was received from the U.S. Department of Energy through Contract No. DE- AC22-95PC95101 and from the Electric Power Research Institute (EPRI). Char samples were prepared by Mr. Joseph Morency of Physical Sciences, Inc. Mercury exposure experiments were carried out by Dr. Baochun Wu, Prof. Thomas Peterson, and Prof. Farhang Shadman at the 5

7 University of Arizona. We also acknowledge the U.S. Department of Energy for its support of the synchrotron facilities at SSRL, where the XAFS measurements were made. REFERENCES 1. Keating, M.H.; et al. Mercury Study Report to Congress, Volume I: Executive Summary, EPA-452/R , December Stein, E.D.; Cohen, Y.; Winer, A.M. Environmental Distribution and Transformation of Mercury Compounds, Critical Rev. Environ. Sci. Technol., 1996, 26, DeVito, M.S.; Rosenhoover, W.A. Flue Gas Mercury and Speciation Studies at Coal-Fired Utilities Equipped with Wet Scrubbers, presented at 15 th International Pittsburgh Coal Conference, Pittsburgh, PA, September 15-17, Fahlke, J; Bursik, A. Impact of the State-of-the-Art Flue Gas Cleaning on Mercury Species Emissions from Coal-Fired Steam Generators, Water, Air, Soil Poll. 1995, 80, Dunham, G.E.; Miller, S.J. Evaluation of Activated Carbon for Control of Mercury from Coal-Fired Boilers presented at the First Joint Power and Fuel Systems Contractors Conference, Pittsburgh, PA, July 9-11, Krishnan, S.V.; Gullett, B.K.; Jozewicz, W. Sorption of Elemental Mercury by Activated Carbon Env.Sci.Tech. 1994, 28, Vidic, R.D.; McLaughlin, J.B.; Uptake of Elemental Mercury Vapors by Activated Carbons J.Air Waste Manage.Assoc. 1996, 46, Otani, Y.; Emi, H.; Kanoaka, I.; Nishiro, H. Removal of Mercury Vapor from Air with Sulfur-Impregnated Adsorbents, Env.Sci.Tech. 1988, 22, Carey, T.R.; Hargrove, O.W.; Brown, T.D.; Rhudy, R.G. Enhanced Control of Mercury in Wet FGD Systems Paper 96-P64B.02 presented at Air & Waste Management Association s 89 th Annual Meeting, Nashville, TN, June, Broderick, T.; Haythornthwaite, S.; Bell, W.; Selegue, T.; Perry, M. Determination of Dry Carbon-Based Sorbent Injection for Mercury Control in Utility ESP and Baghouses Paper 98-WP79A.09, presented at the Air & Waste Management Association 91 st Annual Meeting, San Diego, CA, June 14-18, Senior, C.L.; Morency, J.R.; Huffman, G.P.; Huggins, F.E.; Shah, N.; Peterson, T.; Shadman, F.; Wu, B. Interactions Between Vapor-Phase Mercury and Coal Fly Ash Under Simulated Utility Power Plant Flue Gas Conditions, Paper 98-RA79B.04, AWMA 91 st Annual Meeting, San Diego, CA, June,

8 12. Huffman, G.P.; Mitra, S.; Huggins, F.E.; Shah, N. Energy & Fuels, 1991, 5, p Hsi, Miller Lytle, F.W.; Greegor, R.B.; Sandstrom, D.R.; Marques, E.C.; Wong, J.; Spiro, C.; Huffman, G.P.; Huggins, F.E. Nucl. Instrum. Methods 226 (1984) Cramer, S.P.; Tench, O.; Yocum, N.; George, G.N. Nucl. Instrum. Methods A266 (1988) Stern, E.A.; Heald, S.M. Rev. Sci. Instrum. 50 (1979) Eisenberger, P.; Kincaid, B.M. Science 200 (1978) Lee, P.A.; Citrin, P.H.; Eisenberger, P.A.; Kincaid, B.M. Rev. Mod. Phys. 53 (1981) Bianconi, A.; Fritsch, E.; Calas, G.; Petiau, J. Phys. Rev. B 32 (1985) Lytle, F.W.; Greegor, R.B.; Panson, A. Phys. Rev. B 37 (1988) Åkesson, R.; Persson, I.; Sandström, M.; Wahlgren, U. Inorg. Chem. 33 (1994)

9 Table 1. Forms of Sulfur in Coals Sulfur Pittsburgh Illinois No. 6 Wyodak (wt%, dry basis) Forms of sulfur: Wt% in Coal Percent of Total Sulfur Wt% in Coal Percent of Total Sulfur Wt% in Coal Percent of Total Sulfur Sulfate 0.01 < Pyritic Organic

10 Table 2. Properties of Chars for Mercury Sorption Experiments Pittsburgh Illinois 6 Wyodak Carbon content, wt% LOI 69% 39% 83% BET surface area, m 2 /g Total sulfur, wt% 2.07% 4.09% 0.96% 9

11 Table 3. Forms of Sulfur in Chars by XAFS Analysis Pittsburgh Illinois 6 Wyodak Sulfate 5% 2% 20% Pyrrhotite 20% 35% Elemental sulfur 25% 35% 17% Organic sulfide 50% 29% 63% 10

12 Table 4. Hg XAFS systematics for mercury compounds Mercury Compound IPD, ev RSF, Å HgO (yellow) 13.3 HgO (red) Hg acetate Hg 2 SO HgSO Hg 2 Cl HgCl HgS (metacinnabar) HgS (cinnabar) HgCH 3 I Hg diphenyl 6.8 HgI KHgI Hg (liquid)

13 Table 5. Mercury XANES systematics for char samples Sample Identity Parent Coal Description Hg Vapor HCP-5 Pitt HgCl HCY-5 Wyo HgCl CIL-5 Ill 6 HgCl HIL-25 Ill 6 Hg 8.4 HP-25 Pitt Hg 8.1 HIL-25 Ill 6 Hg 8.0 HY-25 Wyo Hg 7.9 HY-25 Wyo Hg 7.6 P8-10 Pitt Hg 8.0 HIL-25 Ill 6 Hg 8.0 HLP-25 Pitt Hg 8.0 HHIL-25 Ill 6 Hg 7.9 HHP-25 Pitt Hg 7.8 HHY-25 Wyo Hg 7.7 IPD EV HLY-25 Wyo Hg

14 1.0 Normalized Absorption dabs/de x d 2 Abs/(dE) 2 x Energy (ev) Figure 1. Determination of inflection point difference (IPD) from Hg XANES spectra of a carbon-based sorbent. Arrows indicate measurement positions. 13

15 10 HgSO4 9 IPD Value 8 HgCl2 HgS 7 6 Hg Gas Phase Compound HgCl2 Figure 2. IPD values for bituminous (s) and PRB (x) chars exposed to Hg or HgCl 2 in simulated flue gas. Range of IPD values for mercury standards are indicated on right-hand side. 14

16 % HgCl 2 90:10 100% HgCl :20 70:30 60:40 50:50 40:60 30:70 20:80 10:90 100% Hg(0) 90:10 80:20 70:30 60:40 50:50 40:60 30:70 20: :90 100% Hg(0) Energy (ev) Energy (ev) Figure 3. Simulated XANES spectra and derivative spectra for mercuric chloride (HgCl 2 ) and liquid mercury (Hg 0 ).`` 15

17 HgS HgS/Hg(0) HgCl2/Hg(0) Chars HgCl IPD (ev) Figure 4. Plot of the Inflection Point Difference (IPD) against the peak-height ratio determined from the derivative spectra shown in Figure 3. Solid lines represent data trends from simulated mixtures of Hg 0 and either HgS or HgCl 2. Individual data points are experimental points determined for the char samples. 16