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Energy & Fuels 2005, 19, 2500-2508
Speciation of Chromium in Feed Coals and Ash Byproducts from Canadian Power Plants burning Subbituminous and Bituminous Coals Fariborz Goodarzi*,† and Frank E. Huggins‡ Geological Survey of Canada-Calgary Division, 3303 33rd Street N.W., Calgary, Alberta, Canada T2L 2A7, and Department of Chemical and Materials Engineering, University of Kentucky, 533 South Limestone Street, Lexington, Kentucky 40506-0043 Received July 19, 2005. Revised Manuscript Received September 8, 2005
The chromium species in the feed coals and ash byproducts from seven Canadian coal-fired power plants that were burning local subbituminous or bituminous coals with sulfur contents in the range of 0.30-3.5 wt % have been examined using Cr X-ray absorption near-edge spectroscopy (XANES). Chromium in the Canadian feed coals is always found as Cr3+ but generally has a dual occurrence, as Cr3+ is distributed to varying degrees between the clay mineral illite (Cr3+/ illite) and a poorly crystallized chromium oxyhydroxide (CrOOH) phase associated with the organic fraction. In two subbituminous feed coals from Alberta, chromium is present largely as Cr3+/ illite, whereas in two other such coals, it is present predominantly as CrOOH. Chromium in a low-sulfur (0.50 wt %) bituminous feed coal from Alberta is found mostly as Cr3+/illite, whereas for feed coals from Nova Scotia with high sulfur contents (2.60-3.56 wt %), chromium is distributed between both Cr3+/illite and CrOOH. Very little chromium was found in the limestone used in a fluidized-bed combustor. The chromium species in most bottom ash samples from all seven combustion units is predominantly, if not entirely (>95%), Cr3+ associated with aluminosilicate phases. Chromium speciation for subbituminous electrostatic precipitator (ESP) fly ash is mostly Cr3+ (>95%), but in some cases, it is slightly less (>80%) and varies by sampling location at the plant. Chromium in fly ash from the combustion of bituminous feed coals is predominantly (>95%) Cr3+. A unique species of chromium found in one feed coal and an unrelated fly ash is metallic chromium (Cr0), similar to that in stainless steel. The occurrence of this form of chromium in these materials indicates contamination from machinery, such as the coal milling machine or possibly wearing down of stainless steel parts by the coal or ash. The observation of this unexpected contamination demonstrates the power and usefulness of X-ray absorption finestructure (XAFS) spectroscopy for speciation determination.
Three stable forms of chromium, Cr0, Cr3+, and Cr6+, are found in nature.1 The Cr3+ oxidation state is the most widespread form of chromium in nature. Chromium is perceived as one of the most critical elements to monitor in pulverized coal combustion, because hexavalent chromium (Cr6+) compounds, such as chromates (CrO42-), are toxic and can be present in feed coal.2 Chromium can cause lung cancer, when it enters the lungs in the form of chromates in dusts or other inhaled fine particulate matter.3 In larger quantities, chromates are also known to cause ulceration of the skin.1,4 Biologically, chromium is important in both its trivalent and hexavalent forms. In its trivalent (Cr3+) form,
chromium is an essential element to mammals.5 Chromium presents an interesting dilemma because Cr3+ is essential for carbohydrate metabolism, and chromium deficiency can cause diabetes, whereas Cr6+ is carcinogenic,6,7 as previously mentioned. There is no known microbial activity associated with chromium.7 Chromium does not bioaccumulate in the food chain. However, a high concentration of Cr in soil can result in soil infertility.1 Generally, the adverse effects of Cr6+ are short-lived, as it rapidly changes to a Cr3+ form.1 The chemistry and geochemistry of chromium are dictated in large part by the high energies and oxidation potentials necessary to convert it from one chemical state to another.8 As a result, Cr exists geochemically either as Cr3+ in oxide or silicate minerals in igneous
* To whom correspondence should be addressed. E-mail: fgoodarz@ nrcan.gc.ca. † Geological Survey of Canada-Calgary Division. ‡ University of Kentucky. (1) Harte, J.; Holden, C.; Schneider, R.; Shirley, C. Toxics A to Z (A Guide to Everyday Pollution Hazards); University of California Press: Berkeley, CA, 1991; pp 276-278. (2) Lamarre, L. EPRI J. 1995, 20, 6-15. (3) Piperno, E. In Trace Elements in Fuel; Babu, S. P., Ed.; Advances in Chemistry Series No. 141; American Chemical Society: Washington, D.C., 1975; pp 192-209.
(4) Tietz, N. W. Clinical Guide to Laboratory Tests, 2nd ed.; Saunders: Philadelphia, 1990. (5) Beliles, R. P. In ToxicologysThe Basic Science of Poisons; Casarett, L. J., Doull, J., Eds.; Macmillan: New York, 1975; pp 585597. (6) Anderson, R. A. Sci. Total Environ. 1981, 17, 13-29. (7) Vela, N. P.; Olson, L. K.; Caruso, J. A. Anal. Chem. 1993, 65, 585-597. (8) Mahan, B. H. University Chemistry; Addison-Wesley: Palo Alto, CA, 1965.
Introduction
10.1021/ef050221w CCC: $30.25 © 2005 American Chemical Society Published on Web 10/12/2005
Speciation of Cr in Feed Coals and Ash Byproducts
or metamorphic rocks or as chromate minerals in highly oxidized sediments. With the exception of a single Russian locality, near Lake Baikal,9 there are no known terrestrial occurrences of Cr in sulfide minerals; the most common chromium sulfide mineral, daubreelite (Fe2+Cr2S4), is found only in iron meteorites. In 1993, Huggins et al.10 stated that the oxidation state of chromium does not appear to change between the feed coal and the power plant ashes. Any Cr6+ in the United States anthracite and bituminous coals and related ashes that they examined at that time was below the limit of detection, estimated to be less than 5% of the total chromium.10 However, in more recent studies, minor to significant (up to 40%) fractions of chromium have been observed as Cr6+ in ash and in particulate matter derived from the combustion of coals from the United States, principally those from the western United States.11-13 A study by Electric Power Research Institute (EPRI)14 indicates that the carcinogenic risk due to emission of elements, including Cr, from 594 power-plants burning fossil fuels is estimated to be less than 0.1 cancer occurrence per year for the entire population of the United States.2 Chromium in Coal. The amount of chromium in world coal is in the range of 0.5-60 mg/kg.15 Coals with higher Cr contents are uncommon.14 In general, high contents of Cr (>100 mg/kg) in coal are due to the inclusion of chromium-bearing minerals such as chromite (FeCr2O4).16-18 One of the earliest reported high Cr contents was found in Japanese coal (80 mg/kg) by Mingaye in 1907.19 Some Canadian subbituminous coals have chromium contents between 910 and 2000 mg/kg in a seam with greater than 45 wt % ash due to the presence of chromite-magnetite.20 These extreme cases do not occur frequently, and in most cases, seams with high Cr contents either are not mined (partings/high ash coal) or are washed and cleaned. For example, the feed coal produced from a mine with a high chromium content in one seam could be washed to a chromium content of 11.7 mg/kg.21 Mode of Occurrence of Chromium in Coal. Until recently, the mode of occurrence of chromium in coal (9) Reznickij, L. Z.; Sklarov, E. V.; Ustchapovskaya, Z. F. Zapiski Vses. Mineralog. Obshch. 1985, 114, 622-627 (in Russian); as referenced in Am. Mineral. 1987, 72, 223. (10) Huggins, F. E.; Shah, N.; Zhao, J.; Lu, F.; Huffman, G. P. Energy Fuels 1993, 7, 482-489. (11) Huggins, F. E.; Najih, M.; Huffman, G. P. Fuel 1999, 78, 233242. (12) Huggins, F. E.; Shah, N.; Huffman, G. P.; Kolker, A.; Crowley, S.; Palmer, C. A.; Finkelman, R. B. Fuel Process. Technol. 2000, 6, 79-92. (13) Shoji, T.; Huggins, F. E.; Huffman, G. P.; Linak, W. P.; Miller, C. A. Energy Fuels 2002, 16, 325-329. (14) Electric Utility Trace Substance Synthesis Report; Report EPRI TR-104614-VI; Electric Power Research Institute (EPRI): Palo Alto, CA, 1994. (15) Swaine, D. J. Trace Elements in Coal; Butterworths: London, 1990. (16) Ruppert, L. F.; Finkelman, R. B.; Boti, E.; Milosavljevic, M.; Kaluderovic, M.; Kolinovic, R. Geol. Soc. Am. Abstr. 1991, 23 (5), A144. (17) Ruppert, L. F.; Finkelman, R. B.; Boti, E.; Milosavljevic, M.; Tewalt, S.; Simon, N.; Dulong, F. Int. J. Coal Geol. 1996, 29, 235258. (18) Brownfield, M. E.; Affolter, R. H.; Stricker, G. D.; Hildebrand, R. T. Int. J. Coal Geol. 1995, 27, 153-169. (19) Mingaye, J. C. H. Rec. Geol. Surv. NSW 1907, 8, 251-257. (20) Pollock, S. M.; Goodarzi, F.; Riedeger, C. L. Int. J. Coal Geol. 2000, 43, 259-286. (21) Gentzis, T.; Goodarzi, F. Energy Sources 1997, 19, 493-505.
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was not very clear.22 However, more recent studies indicate that chromium has principally a dual mode of occurrence, as it can be found associated with either minerals or organic matter in many coals. Chromium in minerals in coal can be associated with both clay minerals and organic matter.23 It can be associated with the clay mineral illite in low-pyrite coal and with mixed clays, pyrite, and hematite in high-pyrite coal.24,25 In addition, Cr has been reported to be present in coal as chromite and crocoite (PbCrO4), generally in coals of higher Cr contents and/or unusual geological settings.16-18,20,26 Chromium X-ray absorption near-edge spectroscopy (XANES) spectroscopy of coal indicates that Cr can be associated with clay minerals such as illite and chlorite, with oxides (e.g. chromite), and also with organic matter in float fractions.12 For the last occurrence, X-ray absorption fine-structure (XAFS) studies suggest that Cr is present as a poorly crystalline CrOOH phase based on systematics in Cr XANES spectra.10,12 The current study was carried out to determine the speciation of chromium in various Canadian subbituminous and bituminous feed coals and their bottom and fly ashes from coal-fired power plants operating in Alberta and Nova Scotia. Analytical Section Feed coal, bottom ash, electrostatic precipitator (ESP) ash, and stack ash were sampled, following the recommendations of EPRI14 for a period of 3 days (one sample per day). Initially, individual samples of feed coal were examined using Cr XANES spectroscopy. The result of this procedure showed no real distinction in the speciation of chromium between the three samples. Subsequently, all three daily samples of feed coal from the individual stations were combined and then analyzed. The concentrations of chromium in the feed coals and power plant ashes were determined using neutron activation analysis (NAA) and inductively coupled plasma atomic emission spectrometry (ICP-AES). These methods are recommended by EPRI14 for the study of chromium emissions from coal-fired power plants. Sulfur and ash contents of feed coals were determined using ASTM methods.27 Relative enrichment index (RE) values of the chromium were determined according to Meij.28 XAFS Spectroscopic Speciation. The XAFS spectra of chromium were obtained from the coal and ash samples, with minimal modification of their as-received state, by suspending them in the monochromatic X-ray beam in ultrathin (6-µm) polypropylene bags. The XAFS spectra were collected in fluorescence geometry, using a 13-element germanium detector either at beamline 4-3 at Stanford Synchrotron Radiation (22) Finkelman, R. B. In Environmental Aspects of Trace Elements in Coal; Swaine, D. J., Goodarzi, F., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1995; pp 24-50. (23) Finkelman, R. B. Open File Report No. OFR-81-99; U.S. Geological Survey, U.S. Government Printing Office: Washington, D.C., 1981. (24) Davidson, R. M. Modes of Occurrence of Trace Elements in Coal; IEA Report CCC/36; IEA Clean Coal Centre: London, 2000. (25) Galbreath, K. C.; DeWall, R. A.; Zygarlicke, C. J. IEA Coal Research Collaborative Program on Modes of Occurrence of Trace Elements in CoalsResults from Energy and Environment Research Center. Final Report; Energy and Environmental Research Center, University of North Dakota: Grand Forks, ND, Nov 1999; p 21. (26) Ward, C. R. Int. J. Coal Geol. 2002, 50, 135-168. (27) Annual Book of ASTM Standards, Forms of Sulfur in Coal; D2492-77; ASTM: Philadelphia, PA, 1978; Part 26, p 332. (28) Meij, R. In Environmental Aspects of Trace Elements in Coal; Swaine, D. J., Goodarzi, F., Eds. Kluwer Academic Publishers: Dordrecht, The Netherlands, 1995; pp 111-127.
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Goodarzi and Huggins the determination of forms of occurrence information because, first, the Cr contents of coal and ash are relatively low, giving rise to noisy EXAFS spectra, and, second, the EXAFS spectral region for Cr also contains significant L absorption edges due to omnipresent Ce and Nd at 6164 and 6208 eV, respectively. Such edges occurring just 175 eV above the Cr K-edge severely limit the extent of the Cr EXAFS region and basically render it useless for analysis. Analysis of the Cr XANES spectra of coal and ash generally proceeds by comparison of the spectra with those for standard compounds. Figure 1a shows the Cr XANES spectra of metallic chromium, chromium sulfide, and three ionic Cr3+ compounds. The three ionic Cr3+ compounds, although they clearly differ from each other, have a number of features in common, including a very small and sharp preedge peak at ∼2 eV, a two-component major absorption peak with maxima at ∼18 and 20 eV, and an obvious minimum in the absorption between ∼45 and 50 eV. Such features are also observed for many other ionic Cr3+ compounds. In contrast, metallic chromium and Cr2S3 have more prominent and complex structure at the edge between 0 and 10 eV and smaller but more complex fluctuations in absorption in the 20-120 eV region. In addition, it is easy to discriminate between the Cr3+ and Cr6+ oxidation states. As shown in Figure 1b, Cr6+ compounds have an intense preedge peak at ∼4 eV, a maximum in absorption between about 40 and 50 eV, and a minimum between ∼70 and 80 eV, in contrast to the general features listed above for ionic Cr3+ compounds. Mineralogy. Semiquantitative and qualitative mineralogy was performed on low-temperature ash (LTA, 90 °C) for coal and for bottom and fly ashes using scanning electron microscopy/energy-dispersive X-ray analysis (SEM/EDX) on carboncoated samples. Quantitative crystalline mineral phase identification was provided by X-ray diffraction (XRD).29-31
Results and Discussion
Figure 1. (a) Chromium XANES spectra of various standard materials. Note how the overall spectral shapes for the metallic and sulfide chromium compounds differ from those of compounds in which chromium is octahedrally coordinated by oxygen. (b) Chromium XANES spectra of various standard materials (two Cr3+ oxide minerals and two Cr6+ oxide compounds). Note the prominent preedge peak at about 4 eV that is highly characteristic of hexavalent chromium. Laboratory (SSRL) at Stanford University (Stanford, CA) or at beamline X-19A at National Synchrotron Light Sources (NSLS) at Brookhaven National Laboratory (Upton, NY). Each Cr XAFS spectrum consists of typically ∼400-600 points collected at energies between ∼100 eV below the Cr K absorption edges (at 5989 eV) to ∼500 eV above the edge. A thin metal foil of stainless steel was used as the primary standard for chromium; generally, separate calibration spectra were run periodically during the data collection. The spectra were collected and stored on a Pentium PC computer at NSLS or on a MicroVAX computer at SSRL. The spectra were then transferred electronically to a MicroVAX computer at the University of Kentucky for analysis. The raw XAFS data were first calibrated with respect to the primary standard and then normalized to the edge step, corrected for background slope above and below the edge, and finally divided into separate XANES and extended X-ray absorption fine-structure (EXAFS) regions. For Cr, the XANES region was the principal region of interest that was used for
The majority of coal-fired power plants in Alberta use locally mined subbituminous coal, but one plant included in this study did fire bituminous coal.32 In Nova Scotia, all coal-fired power plants burn bituminous coal32 (see Table 1). Subbituminous Feed Coals. The averages of the three daily samples taken from each station are shown in Table 1. The spectra of subbituminous feed coals 1-4 (see Figure 2a) indicate that the chromium oxidation state is entirely trivalent (Cr3+), as there is no significant enhancement of the Cr6+ preedge peak at ∼4 eV. Inspection of the four spectra indicates close similarities between the spectra for 1 and 3 and between those for 2 and 4. Comparison of the Cr XANES data and the corresponding first-derivative spectra (see Figure 2b) with standard spectra indicates that the chromium in coals 1 and 3 is present mainly as Cr3+/illite; conversely, the Cr3+/illite features (Figure 2d) are relatively weak in the spectra of 2 and 4, and it is likely that a second form of Cr is dominant in these latter two samples. The spectra for these latter coals appear more like those noted for certain bituminous coals (see Figure 2c) and organic-rich fractions separated from bituminous coals in float/sink tests.10,12 Such spectra are thought to derive largely from a poorly crystallized chromium oxyhydroxide (CrOOH) mineral10,12 trapped within organic mac(29) Chung, F. H. J. Appl. Crystallogr. 1974, 7, 519-525. (30) Chung, F. H. J. Appl. Crystallogr. 1974, 7, 526-531. (31) Chung, F. H. J. Appl. Crystallogr. 1975, 8, 17-19. (32) Goodarzi, F. Fuel 2002, 81, 1199-1213. (33) Huggins, F. E.; Huffman, G. P. Int. J. Coal Geol. 2004, 58, 193224.
Speciation of Cr in Feed Coals and Ash Byproducts
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Figure 2. (a) Comparison of Cr XANES spectra of subbituminous milled coals 1-4. (b) XANES and first-derivative XANES spectra for chromium in feed coal 3. (c) Comparison of XANES and first-derivative XANES spectra for chromium in the milled coal no. 1 and in Kentucky No. 9 coal. (d) XANES and first-derivative XANES spectra for chromium in illite showing the characteristic triple-peak grouping between 10 and 20 eV in the first-derivative spectrum. Table 1. Total Chromium (mg/kg), Sulfur (%), Iron (mg/kg), and Ash Contents of Feed Coals subbituminous
a
feed coal
1
2
ash content (%) chromium (mg/kg) sulfur (%) iron (mg/kg) pyritic sulfur nickel (mg/kg)
22.20 11.70 0.24 6736 0.04 6.30
17.80 6.00 0.22 4016 0.03 4.30
Cr3+ Cr6+ Cr0
>90 95 0 95 >90 >95
