Spatial Variation of Organic Sulfur in Coal - ACS Publications

Chapter 19

Spatial Variation of Organic Sulfur in Coal 1

Elizabeth Ge and Charles Wert

Downloaded by UNIV LAVAL on February 25, 2013 | http://pubs.acs.org Publication Date: June 29, 1990 | doi: 10.1021/bk-1990-0429.ch019

Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801

Transmission electron microscopy has been used to determine the concentration of organic sulfur in coal. Because the electron beam can be focused to a fine spot on the coal specimen, the variation of organic sulfur in the macerals can be measured over distances as short as 1μmor less. Thus, spatial variation can be determined within a particular maceral and across maceral boundaries in thinned coal foils. The excellent spatial resolution has also permitted us to measure the organic sulfur concentration in close proximity to sulfides; we find that the organic sulfur concentration is constant over the maceral to within 1μmof the pyrite. Techniques of transmission electron microscopy have proved valuable in many areas of solid state science. Use of electron diffraction permits identification of crystal types, determination of unit cell sizes and characterization of crystal defects in the phases. Measurement of Energy Dispersive X-ray (EDS) line intensity allows calculation of the elemental composition of the phases. It is difficult to overestimate the value of such applications to metallic alloys, ceramic materials and electron-device alloys (1-4). Applications to coal and other fuels are far fewer, but the studies also show promise, both in characterization of mineral phases and in determination of organic constituents (5-9). This paper reports measurements on a particular feature of coal, the spatial variation of the organic sulfur concentration. Method The method depends on measurement of the X-ray radiation from atoms when a solid is irradiated with electrons. X-rays of particular energy are emitted-the energies depend on transitions between specific atomic states. The most prominent X-ray line for sulfur has energy of about 2300 eV, corresponding to a wavelength of about 5.4 Angstroms. That radiation may be detected and the intensity determined by crystal detectors, as in the electron microprobe, wavelength dispersive spectroscopy, or by detectors which a^scriminate in energy, energy dispersive microscopy, used in scanning electron microscopy or 1

Current address: Shenyang College of Architectural Engineering, Shenyang, China 0097-6156/90/0429-0316$06.00/0 © 1990 American Chemical Society

In Geochemistry of Sulfur in Fossil Fuels; Orr, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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in transmission electron microscopy. Fortunately, no other elements in coal provide interferences with the S-line, so corrections from that source are unnecessary. The apparatus used in electron microscopy is shown in Figure 1. The beam of electrons is focused to a fine spot on a thinned coal foil or on particles of powdered coal supported on a thin carbon film. The X-ray radiationfromthe sulfur (and other impurities) is detected and counted by a detector and associated multi-channel analyzer. The background radiationfromthe coal matrix is also counted for a predetermined period of time. The area irradiated by the electron beam is typically 0.1 um . Since very little dispersion of the electrons occurs in the foil (or powder particle) the volumefromwhich the X-rays come is typically less than 0.1 cubic um. Accurate calculation of the organic sulfur concentration depends on accurate determination of this volume. Such determination is difficult for solids, in general, but for matrices of light elements, such as coal, a special technique eases the problem. The mass of material present in the volume is proportional to the Bremsstrahlung radiation-the background counts far away in energyfromany characteristic line of elements present in the specimen. Thus the volume can be determined from the X-ray measurement. The accuracy of this determination is within a few %, depending on how reliably the organic composition of the coal is known. A sketch of a typical spectrum is shown in Figure 2. Such spectra have long been known in X-ray measurements, but their simple use for quantitative calculation of elements present in metallic alloys and ceramic systems has been difficult-interpretation of the Bremsstrahlung radiation almost demands prior knowledge of the alloy composition, the very feature sought. This difficulty is greatly reduced for specimens of light elements such as coal. A simple expression can be written for the concentration of an impurity element, such as S, in a matrix of light elements (in this case, mainly carbon, oxygen and hydrogen).


2

c(S) = ACs/Cb

(1)

In this equation, c(S) is the concentration of the sulfur, Cs is the area under the X-ray line for S and Cb the Bremsstrahlung counts over a specific energy range, see the dark areas in Figure 2. For C we use the area at halfmaximum and for Cb the area under the curve from 10 to 18 keV. The parameter A is a constant which takes into account the geometry of the microscope, the counting efficiency of the detectors and certain characteristics of the matrix. It can be evaluated using known sulfur compounds-we typically use pyrite, a thiophene, a sulfone or films of pure S. For our system, a value of A of 1.5 gives the organic sulfur concentration in wt% for a bituminous coal. The value of A changes only by a few % for lignite or for anthracite (10). Thus the technique permits reliable determination of organic sulfur concentration for any coal. The basis for the simple expression of Equation 1 is found in many articles on quantitative analytical-electron-microscopy (11). The original application to light matrices, though, was made by Hall for biologically important materials (12,13). Understanding of details of calculation of parameters making up the constant A can be obtainedfromhis work. s

Applications Three applications of this technique are described in the following sections: (1) measurement of variation of c(S) across macérais, (2) determination of the


318

GEOCHEMISTRY OF SULFUR IN FOSSIL FUELS

Multi- Channel Analyzer Detector


Incident Electrons! Bremsstrahlung X-rays 1

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Figure 1. Sketch of apparatus.


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Spatial Variation ofOrganic Sulfur in Coal

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average value of c(S) for whole coal and (3) variation of c(S) in the vicinity of pyrite. Spatial Variation of Organic Sulfur. The excellent spatial resolution of focused electron beams offers the possibility of examining variation of organic sulfur within macérais. The electron microprobe and scanning electron microscope allow resolution of a few microns (14-16). The transmission electron microscope allows even better resolution (less than 1 um) because the thin foils and powders produce less electron scattering. We have used this capability to measure the distribution of S in a number of coals. The first such study was reported by Hsieh and Wert (10). Measurements on a thinned vitrinite maceral of an Illinois #6 coal at a spacing of about 0.5 μτη showed a variation of about ±5% of the average value of about 2.6 wt%. Other (unreported) measurements using the TEM technique gave about the same results for this coal. Later Tseng et al. measured a traverse across a sporinite maceral embedded in a large vitrinite maceral (17). We have made another such measurement; it is shown in Figure 3. Some 68 measurements were made along a traverse of about 50 mm. In this maceral the average in the vitrinite is about 3.5% with local variation about ±5%. The average in the sporinite maceral is about 5%, again with a variation of about ±5%. Importantly, the jump at the maceral boundary is sharp, a micron or two. Solid solutions in metallic alloys are normally compositionally very uniform; random variations of 5% would be unusual. Also, a two phase layer normally is found between two solid solutions. The sulfur distribution in coal seems not to behave this way. Apparently, the distribution pattern established at some early stage of coal formation is frozen-in and the organic sulfur is bound so tightly to its hydrocarbon sites that it cannot diffuse until the temperature of the coal is raised to 400°C or above (18). The Average Organic Sulfur Concentration. We have used TEM measurements to determine the average organic sulfur concentration in whole coals. Such determination requires that a series of measurements be made over all the maceral types. Use of foils, as described in the previous section, is difficult and expensive, since many foils would be required to give assurance that all maceral types were sampled. Consequently, we have found it more convenient to use finely ground powders. Fine coal powder is further ground in a mortar and pestle (with a little alcohol) to produce micron size powders. A drop of the alcohol is transferred to a carbon film supported on a carbon mesh so that a thin sprinkling of ultra-fine powder is deposited grain by grain. We have found that the average of 50 individual measurements gives a good value for the organic sulfur concentration. A typical measurement is shown in Figure 4 for Illinois #6 coal. The range of individual measurements is from 0.5 to 4.5 wt% S with an average around 2.5 wt%. This average is close to the value of 2.4 wt% determined by the ASTM standard technique. Wert and his colleagues have reported several such comparisons in previous publications (10.17.20). We have just made 21 new TEM measurements and have collected datafromthe literature for additional SEM-type measurements (either Sem or Electron Microprobe) (21.22). For all of these coals, ASTM values were also available (19). The entire group is plotted in Figure 5. Remarkably good agreement exists for these coals between the electron-optical technique and the ASTM standard method. (The full set of data exists in a Lotus 123 file in format wk.l. The interested reader may obtain


320


5

wt %

4L


org Sporinite

Vitrinite

Vitrinite

2L 35 Measurement

-iy Number

Figure 3. Variation of Sorg across maceral types.

3

Figure 4. Distribution of organic sulfur within volumes of about 1 μπι as a function of sulfur content. Illinois #6.



19. GE & WERT


321

Figure 5. Comparison of measured values of Sorg between the ASTM and electron-optical techniques. Not all 56 points can be differentiated because some are coincident this file by sending a diskette to CAW. It identifies the coal, the investigator, the two values and the reference for the original data. A printout of part of the file is listed in Appendix I.) Organic Sulfur in the Vicinity of â Pyrite Particle. Distribution of organic sulfur is not uniform through a coal, even within a particular maceral. But the variations about the average value in a particular maceral are small (fractionally) and seemingly random. But the question may be asked as to the "constancy" of the organic sulfur content in the immediate vicinity of a pyrite particle. Yurovskii claimed (in 1960) that "pyrite concretions in coals are surrounded by coal layers richer in organic sulfur which decreases as the distance from the center of the pyrite concretion increases" (23). He found the enrichment to be enormous, up to 25 times the average value. His measurements were made on "bulk" coal fragments extracted from the vicinity of pyrite particles. Electron-optical methods, though, can make such measurements in situ and have better spatial resolution than his bulk methods allowed. We have made such measurements using transmission electron microscopy on two coals-an Illinois #5 and an Illinois #2. The transmission electron micrograph showing pyrite framboids in an otherwise "clean" maceral of a specimen of Illinois #5 coal is shown in Figure 6: The organic sulfur was measured along 3 directions, as shown. The measurements are shown in Figure 7. (Note that the plots are offset along the y-axis for clarity of presentation). No increase in organic sulfur occurs to within 1 μπι of the framboids. Then an increase occurs in the signal for sulfur—but the X-ray line for Fe also increases. This may indicate that part of the S-signal has its origin in pyritic-sulfur.



322


Figure 6. View of maceral containing pyrite showing traces along which Sorg was measured. Illinois #5.

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Statistical Counting Error 0

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Figure 7. Measured values along the three directions shown in Figure 6. The measurements are offset in the y-coordinate for ease of interpretation.


19. GE & WERT


1.5 wt%

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Figure 8. Measured values along two directions in a specimen of Illinois #2. The measurements are offset in the y-coordinate for ease of interpretation.

A similar measurement for a specimen of Illinois #2 shows the same result, Figure 8. No measurable increase in organic sulfur occurs to within 1 um of the pyrite. One must conclude, therefore, that there is no gradient in organic S concentration in the vicinity of the pyrite at distances greater than Ιμπι. Consequently, the pyrite and adjacent maceral are in a two-phase state with no intervening phase. Since the activity of S in pyrite has a specific value (at a given temperature) independent of the coal, and since the organic sulfur concentration varies among the coals, the sulfur in the maceral and the sulfur in pyrite must not be in thermodynamic equilibrium. Summary Transmission electron microscopy has been used to measure the spatial variation of organic sulfur within and between macérais. The spatial resolution of the measurements is superior to any other reported technique.



subB BLIND-C hvAb-5 WYODAK-A PSOC-1020 Fu X i n hvAb-2 CRCII1PRC BEULAH PSOC-1015 POCAHONTAS PSOC-1014 TANG SHAN hvAb-3 SRC-I STOCKTON SRC-I-RES hvAb-4 U-FREEPORT PSOC-1012 hvAb-1 CRCII5PRC KCER-8055 KCER-8055-V PSOC-1013 PITTS-8 SUbC Ρ aANTHRACITE

COAL

RAYMOND GE RAYMOND GE Clark Ge RAYMOND Ge GE Clark GE Clark Ge RAYMOND Clark GE Ge RAYMOND GE Clark RAYMOND Ge CLARK CLARK Clark GE RAYMOND Hsieh

PERSON

0 .22 0 .37 0.39 0 .46 0 .48 0 .49 0 .52 0 .52 0 .53 0 .55 0 .57 0 .58 0 .61 0 .61 0 .62 0 .64 0 .65 0 .66 0 .70 0 .77 0 .78 0 .79 0 .81 0 .81 0 .86 0 .96 0 .96 0 .99

%S 0 .16 0 .37 0 .35 0 .47 0 .09 0 .49 0 .57 0 .49 0.70 0 .54 0.50 0.42 0 .59 0 .63 0 .65 0.65 0.65 0.63 0.74 0.94 0 .64 0 .71 0.55 0 .59 0.84 0 .89 1 .01 1 . 10

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21 NEW 21 NEW 22 NEW 21 NEW NEW 22 NEW 22 NEW 21 22 NEW NEW 21 NEW 22 21 NEW 22 22 22 NEW 21 jj jj 1j 11 11 11 j1 11 11 11 11 11 ι 1

11 J1 11 J j 1I J1 11 11 11 11 1I 11 11 1j 11

REFERENCE | 1 SHAN X i - 1 SHAN Xi-2 CRCCI07PRC KCER-9127 KCER-9127-V PSOC-1018 KCER-9419 KCER-9419-V PSOC-1019 RM-181 Rm-184 RM-181-A RM-184-A LECO KCER-9121-V KCER-9121 LECA-A KCER-9418-V KCER-9418 Ill#5 KCER-9335 KCER-9335-V Rm-183 ILL#6 RM-183-A PSOC664 RM-182-A RM-182

COAL GE GE Ge CLARK CLARK Clark CLARK CLARK Clark Clark Clark Ge Ge Clark CLARK CLARK Ge CLARK CLARK Ge CLARK CLARK Clark HSIEH Ge Hsieh Ge Clark

PERSON 1 .09 1 .16 1 . 19 1 .35 1 .35 1 .50 1 .52 1 .52 1 .58 1 .59 1 .61 1 .62 1 .72 1 .75 1 .76 1 .76 1 .89 1 .90 1 .90 2 .02 2 .21 2 .21 2 .52 2 .54 2 .78 4 .50 4 .70 4 .78

%S 1 .16 1 .58 1 . 19 1 .35 1 .35 1 .59 1 .72 1 .72 1 .44 1 .81 1 .58 1 .48 1 .58 1 .86 1 .93 1 .93 1 .86 1 .91 1 .91 1 .97 2 .05 2 .05 2 .48 2 .38 2 .48 4 .20 4 .80 4 .80

%S NEW NEW NEW 22 22 22 22 22 22 22 22 NEW NEW 22 22 22 NEW 22 22 NEW 22 22 22 10 NEW 10 NEW 22

REFERENCE

APPENDIX I. Comparison of E l e c t r o n O p t i c a l and ASTM Values of Organic S u l f u r f o r 56 Coals TEM ASTM TEM ! ASTM [


19. GE & WERT


Acknowledgments We acknowledge support of the Ivan Racheff Memorial Fund of the University of Illinois. Literature Cited 1. 2. 3.


4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.

Dollar, M.; Bernstein, I.M.; Daeubler, M.; Thompson, A.W. Met. Trans. 1989, 20A, 447-451. Baker, I.; Huang, B.; Schulson, E.M. Acta Met. 1988,36,493-9. Bruemmer, S.M.; Fluhr, C.B.; Beggs, D.V.; Wert, C.A.; Fraser, H.L. Met. Trans. 1980, 11Α, 693-9. Viswanadham, R.K.; Wert, C.A. J. Less Common Metals 1976, 48, 135-150. Wert, C.A.; Hsieh, K.C. Scanning Electron Microscopy 1983, III. 1123-1136. Harris, L.A.; Yust, C. S. Fuel 1976, 55, 233-6. Hsieh, K.C.; Wert, C.A. Mat. Sci. and Eng. 1981, 50, 117-125. Allen, R.M. ; VanderSande, J. Fuel 1984, 63, 24-29. Lauf, R.J. Amer. Cer. Soc. Bull. 1982, 61, 487-90. Hsieh, K.C.; Wert, C.A. Fuel 1985, 64, 255-262. Goldstein, J.I. In Introduction to Analytical Microscopy; Hren, J.J.; Goldstein, J.I.; Joy, D.C., Eds.; Plenum Press: NY, 1979; pp 83-120. Hall, T.A.; Anderson, H.C.; Appleton, T. J. of Microscopy 1973, 99, part 2 , 177-182. Hall, T.A. J. of Microscopy 1979, 117, 146-163. Raymond, R. Jr. Amer. Chem. Soc. Symp. Series 1982. 205. pp 191203. Solomon, P.R.; Manzoine, A.V. Fuel 1977, 56, 393-6. Greer, R.T. In Coal Desulfurization; Wheelock, Thomas, D., ed.; Amer. Chem. Soc. Symp. Series, No. 64; American Chemical Society: Washington, DC 1977; pp 3-15. Tseng, B.H.; Buckentin, M.; Hsieh, K.C.; Wert, C.A.; Dyrkacz, G.R Fuel 1986, 65, 385-9. Tseng, B.H.; Ge, Y-P; Hsieh, K.C.; Wert. C.A. In Processing and Utilization of High Sulfur Coals II, Chugh, Y.P.; Caudle, R.D., Eds.; Elsevier, Amsterdam, NY, 1987, pp 33-40. Standard Test Method for Forms of Sulfur in Coal, Annual Book of ASTM Standards, ASTM D2442, 1983, 347. Wert, C.A.; Hsieh, K.C.; Buckentin, M.; Tseng, B.H. Scanning Electron Microscopy 1988, 88, , 83-96. Raymond, R.; Hagan, R.C. SEMII1982, 619-627. Clark, C.P.; Freeman, G.B.; Hower, J.C. SEM II 1984, 537-547 Yurovskii, A.Z. Sulfur in Coals, First published in USSR, 1960. Translated by the Indian National Scientific Documentation Center, New Delhi. Published in 1974 by the U.S. Bureau of Mines and the NSF. NTIS Document TT-70-57216, 116-120.

R E C E I V E D March 14,

1990


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Spatial Variation of Organic Sulfur in Coal - ACS Publications

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