Energy & Fuels 1991,5,574-581
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Quantitative Analysis of All Major Forms of Sulfur in Coal by X-ray Absorption Fine Structure Spectroscopy G. P. Huffman,* S. Mitra, F. E. Huggins, N. Shah, S. Vaidya, and Fulong Lu Consortium for Fossil Fuel Liquefaction Science, 233, Mining and Mineral Resources Bldg, University of Kentucky, Lexington, Kentucky 40506-0107 Received December 21, 1990. Revised Manuscript Received April 3, 1991
A method has been developed for quantitative analysis of all major sulfur forms in coal, both organic and inorganic, from sulfur K-edge X-ray absorption fine structure (XAFS) spectroscopy. The method is based on least-squares analysis of the X-ray absorption near-edge structure, or XANES, into a series of peaks that represent 1s np photoelectron transitions. Because the major sulfur forms occurring in coal (pyrite, organic sulfide, thiophene, sulfoxide, sulfone, and sulfate) have characteristic s p transition energies, the relative peak area contributed to the XANES by each sulfur form can be determined. These peak areas are converted to weight percentages of sulfur by using calibration constants derived from XANES data from standard compound mixtures. This method has been applied to determine the sulfur forms in the Argonne Premium Coal Sample Bank (APCSB) coals, a number of additional whole coals, and a suite of maceral separates from coals of various ranks. Several interesting trends are observed (1)the organic sulfide content is generally observed to increase with decreasing rank, although thiophenic sulfur is normally the dominant organic component, regardless of rank; (2) exinite macerals consistently exhibit a higher organic sulfide percentage than vitrinite or inertinite; (3) sulfur-oxygen bonded phases increase with decreasing rank for the maceral separates; and (4) good agreement is observed between pyritic sulfur content determined from XANES analysis and that measured by Mossbauer spectroscopy.
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I. Introduction Removing sulfur from coal, thereby reducing environmental pollution through the release of sulfur compounds, has long been a topic of great importance. Though inorganic sulfur forms (principally pyrite) have been removed successfully by physical cleaning methods, an effective and economical procedure to remove organic sulfur from coal remains to be found. The removal of organic sulfur by both chemicalIg and biologicalh7reactions is an active area of current research. Such removal procedures require a knowledge of the structural forms of organic sulfur in coal. In a recent review article,* Stock has emphasized the importance of quantifying the measurement of the functional forms of organic sulfur. While the amounts of pyritic sulfur and inorganic sulfates have been quantitatively measured by both chemical methodss and Mossbauer spectroscopy1&l2for some time, little progress has been ~
(1) Lee, S.; Kesavan, S.K.; Lee, B. G.; Ghosh, A. "Proceedings: 13th Annual EPRI Conference on Fuel Science and Conversion", Electric Power Research Institute Report, GS-6219, 1989, 1-1 to 1-21. (2) Buchanan, D. H.; Coombs, K. J.; Chaven, C. C.; Kruse, C. W.; Hacley, K. C. In Processing and Utilization of High Sulfur Coals III; Markuszewski, R., Wheelock, T. D., Eds.; Coal Science and Technology 16; Elsevier: Amsterdam, 1990; pp 79-87. (3) Chatterjee, K.; Wolny, R.; Stock, L. M. Energy Fuels 1990, 4, 402-406. (4) Dugan, P. R. Biotechnol. Bioeng. Symp. 1986, 16, 185-203. (5) Khalid, A. M.; Bhattacharyya, D.; Hsieh, M.; Kermode, R. I.; Aleem, M. I. H. In Processing and Utilization of High Sulfur Coals III; Markuszewski, R., Wheelock, T. D., Eds.; Coal Science and Technology 16; Elsevier: Amsterdam, 1990; pp 469-480. (6) Cohen, M. S.; Bowers, W. C.; Arnson, H.; Gray, E. T. Appl. Enuiron. Microbiol. 1987, 53, 2840-2842. (7) Isbister, J. D.; Kobylinski, E. A. In Processing and Utilization of High Sulfur Coals; Attia, Y. A., Ed.; Elsevier: Amsterdam, 1985; pp 627-641. (8)Stock, L. M.; Wolny, R. Energy Fuels 1989, 3, 651-660. (9) Annual Book of ASTM Standards, Part 26, Gaseous Fuels; Coal and Coke; Atmos. Anal. 1976, 319-323.
made on measurement of the functional forms of organic sulfur until recently. As noted by Stock: X-ray absorption fine structure (XAFS) spectroscopy is perhaps the most promising direct, nondestructive method for this purpose. Following early XAFS studies of sulfur in coal by Hussain et al.I3 and Spiro et al.,14 several groups have developed methods for quantitative analysis of the X-ray absorption near-edge structure, or XANES, spectra of sulfur in both oil and coal. Gorbaty and c o - ~ o r k e r s ~ ~ ~ ~ have developed a differential treatment of the XANES and have shown that the amplitude of certain features in the third derivative are proportional to the relative amounts of sulfidic and thiophenic sulfur. Our group has adopted a least-squares analysis approach,*= fitting the XANES (IO) Huffman, G. P.; Huggins, F. E. Fuel 1978,57,592. (11) Huggins, F. E.; Huffman, G. P. Analytical Methods for Coal and Coal Products III; Karr, Jr., Clarence, Ed.; 1979; pp 372-421. (12) Huffman, G. P.; Lin, M. C.; Huggins, F. E.; Dunmyre, G. R.; Pienocco. A. J. Fuel 1985.64.849-55. "(13) Hussain, Z.; Umbach,'E.; Shirley, D. A.; Stohr, J.; Feldhaus, J. Nucl. Instrum. Methods 1982,195, 115. (14) Spiro, C. E.; Wong, J.; Lytle, F.; Greegor, R. B.; Maylotte, D.; Lampson, S. Science 1984, 226, 48. (15) Gorbaty, M. L.; George, G. N.; Kelemen, S. R. Fuel 1990, 69, 939-944. (16) Gorbaty, M. L.; George, G. N.; Kelemen, S. R. Fuel 1990, 69, 945-949. (17) Gorbaty, M. L.; George, G. N.; Kelemen, S. R. Fuel 1990, 69. 1065-1067. (18)George, G. N.; Gorbaty, M. L. J . Am. Chem. SOC. 1989,111,3182. (19) George, G. N.; Gorbatv, M. L.: Kelemen, S. R.: Sansone, M. Energy Fuels 1%1,5,93-97. (20) Huffman, G. P.; Huggins, F. E.; Mitra, S.; Shah, N.; Pugmire, R. J.; Davis, B.; Lytle, F. W.; Greegor, R. B. Energy Fuels 1989,3,200-205. (21) Huffman, G. P.; Huggins, F. E.; Mitra, S.; Shah, N. Proceedings of the 1989Internotional Conference on Coal Science; Int. Eng. Agency, 1989; pp 47-50. (22) Huffman, G. P.; Huggins, F. E.; Francis, H. E.; Mitra, S.; Shah, N. In Processing and Utilization of High Sulfur Coals II4 Markuezeweki, R., Wheelock, T. D., Eds.; Elsevier: Amsterdam, 1990, pp 21-32. ~
0887-0624/91/2505-0574$02.50/00 1991 American Chemical Society
Energy & Fuels, Vol. 5, No.4, 1991 575
Analysis of Sulfur in Coal a
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Table I. White Line (1s np) Energies (eV, i0.2 eV) of Various Sulfur Standard Compounds formal oxidation standard state E, eV -1 -0.5 pyrite (FeSd elemental sulfur 0 0.0
Dibenzothiophene
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Figure 1. XANES of several sulfur standards illustrating the effect of sulfur valence.
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directly with a series of s p photoelectron transition peaks and resonance scattering peaks, and arctangent step functions representing the transition of the photoelectron to the continuum. Very recently, Waldo et ales have developed a mathematical method of calculating the X-ray absorption coefficient from experimental spectra that removes self-absorptioneffects from the XANES, which they have applied to quantification of the sulfur in crude oil.% In the current paper, we present new calibration data obtained from standard compound mixtures. Calibration constants derived from these data allow us to convert s p peak area percentages obtained by least-squares analysis of the XANES to weight percentages. The method is then applied to determine quantitative forms of sulfur percentages for the Argonne Premium Coal Sample Bank (APCSB) coals as well as a number of additional whole coals and maceral separates. As discussed below, the least-squares analysis method has the advantage that it is capable of determining all of the major functional forms of sulfur.
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11. Experimental Procedure Most of the XAFS experimentswere conducted at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL) using the bending magnet beamline X-19A. Some experiments were also conducted at the Stanford Synchrotron Radiation Laboratory (SSRL) using wiggler beamlines VII-3 and IV-1. At the NSLS, electron energies were 2.5 GeV and beam currents usually 90-200 mA, while at SSRL, the electron energy was 3.0 GeV and beam currents were typically 30-90 mA. Si(ll1) dou(23) Mitra, S.; Huggins, F. E.; Shah, N.; Huffman, G. P. Prepr. Pap.-Am. Chem. SOC..Diu. Fuel Chem. 1990,325(2), 364-369. (24) Huggins, F. E.; Huffman, G.P.; Shah,N.; Mitra, S.;Ganguly, B.; Shah,A.; Taghiei, M. M.; Vaidya, S . R o c . First Int. Symp. Biol. Process. Coal; EPRI GS-6970; 1990, 1-43 to 1-65. (25) Waldo, G. S.; Penner-Hahn, J. E. Normalization and Self-Absorption Correction in XANES Spectra. Manuscript in preparation. (26) Waldo, G. S.; Carbon, R. M. K.; Moldowan, J. M.; Petere, K. E.; Penner-Hahn, J. E. Geochim. Cosmochim. Acta, in press.
dodecyl SO,, Na salt inorganic SO,
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ble-crystal monochromators were used to vary the incident X-ray energy. To minimize harmonica, 80% detuning was used. Scans were obtained for incident energies varying from 100 eV below to 300 to 600 eV above the sulfur K-edge energy (2472 eV). Beamline X-19 A, which has the X-ray pathway entirely maintained in storage ring vacuum all the way to the beryllium window in the experimental hutch in order to minimize absorption, is particularly suited for such soft X-ray experiments. Very thin (2-6 rm) Mylar and polypropylene windows were used on ionization counters and sample chambem, also to "heabsorption. X-ray fluorescent experiments were performed using a SternHeald type detector as described elsewhere.n Samples of standard compounds were prepared by grinding them to very fine size and mixing them thoroughly with boric acid (HB08) in a mechanical "wig-1-bug" and then cold pressing the resulting mixtures into 3-5 mm thick pellets. In the case of the Argonne coals used in this study, the samples were transferred as quickly as possible from gas-tight, nitrogen-fded ViaLs obtained from the Argonne Premium Coal Sample Bank (APCSB) into thin Mylar sample bags that are positioned in a helium-fied chamber for the XAFS experiments. The total time of exposure in air of these samples was leas than 5 min. Oxidation during this transfer process is believed to be negligible. Other coal and coal macer& samples were prepared by pressing the coal powder into thin (3-5 mm thick) boric acid pelleb or, in some cases, into pure coal pellets. The maceral separates used in this study were prepared by density gradient centrifugation by research groups at the University of Utah,28Southern Illinois and the (27) Lytle,F. W.; Greegor, R. B.; Sandetrom, D. R.; Marquies, E. C.; Wong, J.; Spiro, C. L.; Huffman, G.P.; Huggins, F. E. Nucl. Inetrum. Methods 1988,226, 542.
576 Energy & Fuels, Vol. 5, No. 4, 1991
Huffman et al.
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Figure 2. Typical least-squaresfits of the XANES of an Amtralian brown coal (top) and the APCSB Beulah-Zap coal (bot-
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18
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Figure 3. Leaet-squareafits of the XANES of two APCSB c& Illinois No. 6 (top) and Upper Freeport (bottom).
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University of Kentucky Center for Applied Energy Research Center.81s32 111. Least-Squares Analysis Procedures
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As discussed elsewhere,20and illustrated by Figure 1,the energy of the s p transition or "white line" of sulfur increases with the formal oxidation number of the sulfur ion. This occurs because of a loss of screening of the inner-shell electrons from the nuclear charge, which produces a larger decrease in the energy of the s level than the p level. I t is this change of s p transition energy with valence that makes quantitative analyais of the sulfur XANES of fossil fuels possible. In Table I, we have summarized the s p transition energies of a large number of sulfur compounds, taking the s p transition energy of elemental sulfur as 0.0 eV and measuring all other energies relative to it. [This is slightly different than the convention adopted in our previous papers," where the zero of energy was taken as the peak of the first derivative of the elemental sulfur spectrum. Zero energy is now taken to be the first peak of the XANES, or fmt zero of the first derivative of the XANES, of elemental sulfur. This method of energy calibration has been found to be more re-
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(28) Karas, J.; puemire, R. J.; Woolfenden, W. R; Grant, D. M.; Blair, S. Int. J. Coal Geol. 1986,6, 315. (29) Palmer, S. R.; Hippo, E. J.; Kruge, M.A,; Crelling, J. C. In Geochemistry of Sulfur in Fossil Fueb; Om,W. L., White, C. M., Eds.;ACS Symp. Seriw No. 429; American Chemical Society: Washington, DC, 1990; pp 296-315. (30) Hi po, E.; Palmer, S. R; Crelling, J.; Kruge, M.; White, C. h o c . First Int. &mp. Biol. Process. CoaI; EPRI GS-6970; 1990,1-15 to 1-31. (31) Keogh, R.;.Poe, S.; Chawla, B.;Davis, B. In Coal Science and Technology; Moulqin, J. A., Nater, K. A., Chemin, H. A. G., Eds.; Eleevier: Amsterdam, 1987; Vol. 11, p 289. (32) "Cooperativeh e a r c h in Coal Liquefaction Infratachnologyand Generic Technology Development." Final Report, DOE Contract No. DEFC22-86PC90017; Consortium for Fossil Fuel Liquefaction Science, Lexington, KY, 1988.
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Figure 4. Least-squaresfits of the vitrinite maceral separates of a hvAb coal, (PSOC 733, PA, Appale, top) and of a S m C cod (PSOC 1110, UT So.-W, bottom).
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producible; it causes a small shift of -1.4 eV in the s p energies relative to the previous method.] The accuracy is f0.2 eV. Our method of fitting the XANES uses an extensively modified version of a program called EDGFIT, provided to us originally by R. B. Greegor of the Boeing Co. As illustrated by Figures 2-4, it fits the XANES to a series of
Analysis of Sulfur in Coal
Energy & Fuels, Vol. 5, No. 4, 1991 577
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peaks representing s p transitions and scattering resonances, and to several arctangent step functions representing the transition of ejected photoelectrons to the continuum. While such least-squares fits are straightforward, it is necessary to introduce a number of conditions and constraints in order to obtain meaningful fits. First, although each sulfur phase should, in principle, have its own white line and arctangent step function, we have found that the best results are obtained by using a two-step function approximation, one step representing the transition to the continuum for unoxidized sulfur forms, the second representing the same feature for oxidized sulfur forms, if significant. Additionally, the program converges more rapidly and gives more reasonable results if the width of the arctangent step functions is held constant at 1.1 eV. Both the height and position of the step functions are allowed to vary freely. Second, the peaks are fitted by a function that is 50% Lorentzian and 50% Gaussian. Pure Lorentzian, pure Gaussian, and various percentages of each have been tried, and the 50-50 combination has been found to give the best results. Constraints on the widths of the peaks are dependent on their intensity and resolution. If a peak is reasonably well resolved, its width is allowed to vary; if not, its width is held constant and/or constrained to be equal to the width of a more intense peak. In general, the best results for XANES deconvolution have been obtained by holding the widths of the pyrite, sulfide, and thiophene equal to one another (1.3-1.5 eV), and the widths of the sulfone and sulfate peaks equal to each other (2.5-3.0 eV). All peak heights and positions are allowed to vary freely. Some typical XANES fits obtained by using these conditions are shown in Figures 2-4. In the case of the Australian brown coal, Mossbauer spectroscopy shows that neither pyrite nor iron sulfate is present. Similarly, the maceral separates are also expected to contain little or no pyrite or inorganic sulfates. The XANES peaks for those samples are therefore due primarily to organic sulfur forms. The Illinois No. 6 and Upper Freeport coals from the APCSB also contain a peak derived from pyrite, while the Beulah-Zap XANES exhibits a sulfate peak that is believed to arise from gypsum. Referring to Table I, the primary s p transition peak energies for the principal sulfur forms observed are as follows: pyrite -0.5 eV; organic sulfide 0.7 eV; thiophenic sulfur 1.3-1.8 eV; sulfoxide 3.4 eV; sulfone 7.5 eV; and sulfate 9.9-10.1 eV. As we have noted previ~usly,~"~ there is a problem in distinguishing the sulfone and sulfate s p peaks from resonant scattering peaks in the coal XANES, and in distinguishing the sulfoxide s p peak from the sharp peak at approximately the same energy in the XANES of thiophenic sulfur. This may be seen from the spectra of the two APCSB coals in Figure 3. These samples were run in a helium atmosphere immediately after removal from a nitrogen atmosphere in sealed ampules, and independent XPS measurements showed no indication of any oxidized sulfur. Therefore, the peaks at approximately 3.5,7.5, and 10.1 eV are indicative of the peak sizes that must be exceeded before sulfoxide, sulfone, and sulfate can be identified. This is quantified below. In order to convert peak area percentages to atomic percentages, the relative transition probabilities of different sulfur forms must be considered. As noted elsewhere,%the s p transition probability should be roughly proportional to the number of 3p vacancies and should increase with increasing valence. To quantify this, the XANES of a number of standard compound mixtures were obtained. Since thiophenic sulfur was dominant in most
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50 % mixtures with DBT
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Figure 5. XANES spectra of 50-50 mixtures of DBT and several other sulfur standard compounds (top), and least-squares fit of the 50-50 mixture of FeSOl and DBT (bottom).
of the coals examined, dibenzothiophene (DBT) was chosen as the base compound, and mixtures were prepared containing 25, 50, and 75% of the total sulfur in one of several compounds and the balance in DBT. The mixtures were diluted to 5% total sulfur in boric acid, carefully homogenized and ground to fine particle size in a wig-1-bug, and pressed into pellets for fluorescent XAFS measurements. It is well-known that self-absorption effects can distort fluorescent XANES spectra when either concentration levels or particle sizes are too 1arge.251B*99Specifically, the s p transition peaks are decreased in intensity and broadened by such self-absorption effects. For the calibration samples, our aim was to minimize these effects to the extent possible, while generating calibration data that are realistic for typical coal samples and maintaining a good signal to noise ratio for the XANES spectra. At a total sulfur level of 5%, the concentration of any single sulfur species in the calibration samples varied from 1.25 to 3.75%,which would seem to be a reasonable calibration range for the sulfur functionalities in coal. Regarding particle size, the wig-1-bug grinding procedure produced samples which were 100%
