AN AMERICAN CHEMICAL SOCIETY JOURNAL VOLUME 3, NUMBER 6
NOVEMBER/DECEMBER 1989
C Copyright 1989 by the American Chemical Societj
Reviews Sulfur Distribution in American Bituminous Coals Leon M. Stock,* Ryszard Wolny, and Balkrishna Bal Department of Chemistry, The University of Chicago, Chicago, Illinois 60637 Received April 12, 1989. Revised Manuscript Received August 7, 1989
Pristine bituminous coals from the United States principally contain pyrite and organosulfur compounds. Coals that have been exposed to the atmosphere contain these substances as well as elemental sulfur and inorganic sulfates. Although there are only minor uncertainties about the quantities of pyrite and sulfate, the situation for elemental sulfur remains modestly controversial. Further, chemists have not reached agreement upon the most suitable method for the determination of the organic sulfur content. Advanced physiochemical methods and the conventional ASTM procedures provide essentially equivalent results. Nevertheless, research workers continue to voice reservations about the ASTM methods. It is not clear that any major discrepancies remain when the presence of elemental sulfur and the idiosyncratic characteristics of certain samples are taken into consideration. The quantities of sulfur in demineralized macerals can differ. Generally, the macerals that have been separated from low sulfur coals contain comparable amounts of organic sulfur compounds. However, the sulfur contents of the macerals from high sulfur coals can vary appreciably. Sporinite, for example, may contain 50% more organic sulfur than the associated vitrinite from the same coal. The inertinite macerals contain less organic sulfur than the other macerals. These results have not been convincingly explained, and present fascinating geochemical issues. Important qualitative progress has been made in the identification of the organic sulfur compounds in coal and its reaction products. Literally hundreds of different substances including thiols, sulfides, and heterocyclic compounds have been observed. Heterocyclic derivatives are particularly prevalent. Unfortunately, the quantitative aspects of the matter are not secure. Major doubts remain about the distribution of sulfur among the plausible set of functional groups in coal and about the abundances of different kinds of organic sulfur compounds.
Introduction The sulfur in coal presents an enormous impediment to the utilization of this important natural resource, and its efficient removal constitutes a most serious technological challenge for scientists and engineers. Broadly speaking, American bituminous coals contain elemental sulfur, iron pyrite, inorganic sulfate, and organic sulfur compounds. The three inorganic substances are a major annoyance, but these materials can usually be removed by physical methods. Unfortunately, the organic sulfur compounds remain, and their removal is often essential for the preparation of an environmentally acceptable coal. It is apparent that the removal of the organically bonded sulfur from coal constitutes a most pressing problem, and that 0887-0624/89/2503-0651$01.50/0
the discovery of new chemical strategies for coal desulfurization depends upon a knowledge of the organic sulfur compounds in coals. In this review, we have critically examined the literature regarding the forms of sulfur in American bituminous coals. Attention has been focused on the work that has been presented during this decade on the total organic sulfur content, the sulfur content of macerals, the occurrence of elemental sulfur, and the structures of the organic sulfur compounds. Organic Sulfur Content The organic sulfur content of coal is usually determined by the difference between the total sulfur content and the sum of the sulfur in the pyritic and sulfatic forms.' The 0 1989 American Chemical Society
652 Energy & Fuels, Vol. 3, No. 6, 1989
difficulties and limitations in the estimation of the organic sulfur content via ASTM methods and other procedures have recently been discussed by Markuszewski.2 Errors arise because the ASTM method for the total sulfur content (D-3177) as well as the oxidative ASTM methods for pyritic and sulfatic sulfur (D-2492) have inherent limitations that may accumulate to affect the final value for organic sulfur. The reproducibility in a sulfur analysis by the ASTM methods for a coal containing 2% of pyrite are f0.4% for pyrite, f0.04% for sulfate, and f0.2% for total sulfur.' Clearly, the total error may be quite significant. Kuhn and his associates have described an alternate, reductive approach for the determination of pyrite that involves the reaction with lithium aluminum h ~ d r i d e . ~ Careful analyses of their data imply that the reductive method generally gives results that are slightly lower than the ASTM method. Westgate and Anderson have also used this method successfully for several bituminous coals! Work on a pristine Illinois No. 6 coal suggests that prolonged treatment with lithium aluminum hydride can give high results for pyrite.5 Not surprisingly, many research workers have sought new reliable methods for sulfur analysis via advanced physical methods. Among the most commonly used techniques are X-ray photoelectron spectroscopy (XPS) also known as electron spectroscopy for chemical analysis (ESCA), scanning electron microscopy (SEM), energydispersive X-ray microanalysis (XRMA), and transmission electron microscopy (TEM), as well as X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies. X-ray photoelectron spectroscopy has been widely used for the examination of coal and related product^.^-^ However, this method can only distinguish between the di-, tetra-, and hexavalent forms of sulfur. Furthermore, the bulk concentration of sulfur determined by the ASTM method often differs significantly from that established by XPS. Greater success has been realized in the differentiation of inorganic and organic sulfur compounds in coal via other physical methods. Hurley and White attempted to establish the organic and inorganic sulfur content through XRMA.l0 The shift in KP emission for the different forms of sulfur, 1-2 eV, enables this analysis. The results that were obtained by this method for pyritic, organic, and sulfatic sulfur are in fairly good agreement with the results that were realized with the ASTM procedures.1° Markuszewski and his associates have described a method based upon SEM/XRMA that simultaneously monitors the signals of sulfur, iron, calcium, aluminum, and silicon." This method can successfully avoid particles of pyrite, quartz, and calcium sulfate on the coal surface by a simple count rejection procedure. They investigated raw (1)Gaseous Fuels, Coal and Coke. In Annual Book of ASTM Standards, 1986; ASTM Philadelphia, PA, 1986;Vol05.05. (2)Markuszewski, R. J. Coal Qual. 1988,7 , 1. (3)Kuhn, J. K.; Kohlenberger, L. B.; Shimp, N. F. Enuiron. Geol. Notes (Ill. State Geol. Suru.) 1973,66. (4)Westgate, L. M.; Anderson, T. F. Anal. Chem. 1982,54, 2136. (5)Stock, L. M.;Wolny, R.; Balkrishna, B. Proceedings: Thirteenth Annual EPRI Conference on Fuel Science and Conoersion; Electric Power Research Institute: Palo Alto, CA, 1989;p 4-1. (6)Frost, D. C.; Leeder, W. R.; Tapping, R. L. Fuel 1974,53, 206. (7)Frost, D.C.; Leeder, W. R.; Tapping, R. L.; Walbank, B. Fuel 1977,
- - . .. 56 -, 977
(8)Perry, D. L.; Grint, A. Fuel 1983,62,1024. (9)Pillai, K. C.; Young, V.Y.; Bockris, J. M. J.Colloid Interface Sci. 1985,103,145. (10) Hurley, R. G.; White, E. W. Anal. Chem. 1974,46,2234. (11)Straszheim, W. E.; Greer, R. T.; Markuszewski, R. Fuel 1983,62, 1070.
Reviews
and chemically treated coals and, in many cases, obtained good agreement with the ASTM method. A similar technique was used by Timmer and van der Burgh.12 They obtained very reliable data for the organic sulfur content in coal. For example, their analysis of the organic sulfur content of low pyrite coals revealed only a 3-6% relative difference between the ASTM and SEMI XRMA methods. On the other hand, the organic sulfur contents of coals of high pyrite concentrations were significantly lower than the values determined by the ASTM method. The authors' explanation that pyrite does not dissolve completely in nitric acid is very reasonable. Moreover, as discussed later, the transformation of pyrite to elemental sulfur could contribute to the discrepancy. Wert and his research associates developed a method to determine organic sulfur concentrations using TEM.'*15 They also reported that there was a very good linear correlation between the organic sulfur content of coal determined by the ASTM and TEM method~.'~The difference did not exceed 9%. The TEM method has also been successfully applied for the determination of the analysis of organic sulfur content of macerals and for the study of their thermal decomp~sition.'~ In summary, although the ASTM methods require assumptions and need to be carried out by skillful analytical personnel, their relatively low cost makes them attractive. Generally, the results that have been obtained by the ASTM methods and the advanced physical methods are in good agreement. Discrepancies occur when the coals contain large quantities of pyrite or have been weathered. The situation arises because there are certain inherent limitations in the ASTM methods resulting, on the one hand, from the incomplete dissolution of small, occluded pyrite particles and, on the other hand, from the removal of partially oxidized pyrite in form of ferric ion during the prescribed treatment with dilute hydrochloric acid. These errors lead to underestimates in the pyrite content and leave behind elemental sulfur, which is measured as organic sulfur. For coal samples which have been weathered or contain large concentrations of pyrite, methods that are based on sophisticated instrumentation may be required to determine the sulfur distribution quantitatively. The SEM and TEM offer particular advantages. However, the very small volumes of sample that are analyzed (50-100 pm3 for SEM and 1 pm3 for TEM) present some special problems that can only be resolved by appropriate statistical analyses. We conclude that the methods that are now available can be used to obtain accurate information regarding the sulfur forms in coal. When the discrepancies that arise from idiosyncratic samples, for example, weathered coals, are taken into consideration, few uncertainties remain.
Organic Sulfur Content of Macerals Although sulfur analyses have been carried out successfully upon a broad collection of macerals and maceral concentrates over the past 35 years,152owe have elected to focus attention on the results that have been obtained from coals collected in the United States during the past decade. This approach was selected because highly pu(12)Timmer, J. M.; van der Burgh, N. Fuel 1984,63,1645. (13)Wert, C. A.; Hsieh, K. C.; Fraser, H. L. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1986,31(1),122. (14)Tseng, B. H.; Buckentin, M.; Hsieh, K. C.; Wert, C. A.; Dyrkacz, G. R. Fuel 1986,65,385. (15)Wert, C. A,; Ge, Y.; Tseng, B. H.; Hsieh, K. C. J. Coal Qual. 1988, 7,118. (16)Dormans, H.N.M.; Huntjens, F. J.; van Krevelen, D. W. Fuel 1957,36,321. (17)Ghosh, T. K.Fuel 1971,50, 218.
Energy & Fuels, Vol. 3, No. 6, 1989 653
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Table I. Organic Sulfur Content Determined by Electron Probe Microscopy" sulfur, wt % Lower Kittanning lvb sporinite resinite vitrinite pseudovitrinite micrinite macrinite semifusinite fusinite organic sulfur c
Upper Sunnyside hvAb
Blind Canyon Ab
0.56 0.53 0.75 0.53 0.69 0.43 0.47 0.39 0.71
0.70 0.53 0.44 0.40 0.48 0.30 0.30 0.26 0.43
0.48 0.57 0.39 0.36 0.30 0.46
n,
='"I
0
I
40 Measurement Number
20
60
Figure 1. Spatial distribution of organic sulfur in the vitrinite and sporinite macerals of a section of an Illinois Basin
rified maceral fractions have become available through the powerful density gradient centrifuge method that was introduced by Dyrkacz and Horwitz2I and because these materials have been studied by advanced physical methods. In a contribution that presaged these advances, Retcofsky and van der Hart found that there were only small differences in the organic sulfur contents of mineral-free liptinite and vitrinite maceral concentrates of a low, 0.6%, sulfur, high-volatile-A bituminous coal from the Hernshaw bed in West Virginia.22 These macerals, however, contained about twice as much sulfur as the fusinite concentrate from the same coal. Raymond carried out a thorough study of a collection of macerals from eight American coals that contained between 0.5 and 4.0% He used electron probe microscopy to define the organic sulfur content of a broad array of macerals from these coals. Representative findings are displayed in Table I to illustrate the distribution of sulfur in these materials. Several research groups have investigated samples that have been separated by density gradient centrifugation. Dyrkacz and Wert and their associates observed that the sporinites from several bituminous coals contained significantly more sulfur than the vitrinite or inertinites from the same c0a1s.l~ The results for an Illinois No. 5 coal shown in Figure 1provide a rather dramatic demonstration of the discontinuity in the spatial distribution of organic sulfur. Wilson and coworkers determined the sulfur con(18) Given, P. H.; Spackman, W.; Davis, A.; Zoeller, J.; Jenkins, R. G.; Kahn, R. Fuel 1984, 63, 1656. (19) Kroger, C.; Pohl, A.; Kuthe, F.; Hovestadt, H.; Berger, H. Brennst.-Chem. 1957, 38, 33. (20) Rigby, D.; Batts, B. D.; Smith, J. W. Org. Geochem. 1981,3(1-2),
Ohio American hvAb
Tebo hvBb
1.23
4.07 4.99 3.55 2.81 3.87
1.52 1.30 1.07 0.83 0.71 0.57 1.32
1.03 0.75 3.13
subC
Upper Elkhorn hvAB
0.87 1.16 0.35 1.28 0.82 0.32 0.20 1.04
0.94 1.03 0.73 0.45 0.60 0.64 0.38 0.30 0.69
No. 5
Ohio
No. 4 hvBb 3.89 2.93 2.56 2.90 0.92 1.51 0.73 2.80
tent of Elkhorn, PSOC-2, and Dakota, PSOC-858, hvAb coals.24 The liptinite concentrate of PSOC-2 had 0.9% sulfur, the three vitrinite fractions averaged 0.7 % sulfur, and the two inertinite fractions had about 0.5% sulfur. Similar trends were observed for liptinite-free PSOC-858. Hippo and his co-workers used the density gradient method to separate several Illinois Basin coals into their components.25 They found that the sulfur content of the pyrite-free macerals varied significantly. The sporinite from a Brazil Block coal and a Herrin Illinois No. 6 coal contained significantly more organic sulfur compounds than the other liptinite maceral concentrates or the vitrinite derived from the same coal. Fusinite, semifusinite, and the other inertinites contained lesser quantities of sulfur. There are certain clear trends in the available data. Generally, the sulfur content of the macerals separated from low-sulfur coals contained comparable amounts of organic sulfur compounds. However, the sulfur contents of the macerals in high-sulfur coals can vary appreciably. The fact that sporinites tend to exhibit higher sulfur contents than the other macerals has been frequently highlighted. The abrupt discontinuity that can exist between abutting macerals is portrayed in Figure l. Sporinites generally contain more organic sulfur than the associated vitrinites, and the vitrinites usually contain more of this material than the inertinites. Raymond has pointed out that knowledge of the sulfur content of the vitrinite usually is adequate to define the organic sulfur content of the entire coal.23 This situation obtains because the vitrinites are more abundant than the other macerals and because the liptinites tend to be slightly richer in sulfur and the inertinites slightly lower in sulfur. The observations for the American bituminous coals are quite consistent. This consistency implies that the research in this area can now shift from the determination of the organic sulfur content of the macerals toward assessments of the structural differences in the substances in the macerals so that the fascinating geochemical issues can be resolved. Elemental Sulfur The presence of elemental sulfur in coal is a well-established fact. Most coals that have been exposed to a humid, oxygen-containing atmosphere contain elemental sulfur. However, a major controversy has recently developed concerning the m o u n t s of this material that are present. The results that have been obtained in several different laboratories are examined critically in this section. Analysis. The measurement of the quantity of elemental sulfur in coals presents some special problems
29. (21) Dyrkacz, G. R.; Horwitz, P. E. Fuel 1982, 61, 3. (22) Retcofsky, H. L.; van der Hart, D. L. Fuel 1978,57, 421. (23) Raymond, R., Jr. In Coal and Coal Products; Fuller, E. L., Ed.; ACS Symposium Series 205; American Chemical Society: Washington, DC, 1982; p 191.
(24) Wilson, M. A.; Pugmire, R. J.; Karas, J.; Alemany, L. B.; Woolfenden, W. R.; Grant, D. M.; Given, P. H. Anal. Chem. 1984, 56, 933. (25) Hippo, E. J.; Crelling, J. C.; Sarvela, D. P.; Mukeree, J. Process. U t d . High Sulfur Coals, Proc. Int. Conf., 2nd. 1987, 13.
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Table 11. Elemental S u l f u r f o r T w o Illinois Basin Coal Samples elemental sulfur, wt % pristine exposed, anal. method ASCSP-3 IBCSP-1 extraction, GC-Hall detector 0.00 0.07 extraction, GC-MS 0.00 0.10 extraction, GC-MS 0.00 0.23 extraction, spectroscopic 0.00 0.13 bacteriological 0.02
ref 29 31 30 27 28
because sulfur is a reactive oxidizing agent. It is wellknown that the ring-opening reactions of Sa occur rapidly at temperatures greater than 155 Thus, the determination of sulfur by conventional gas chromatography with inlet systems and columns at high temperature can give erratic results. Nevertheless, suitable chromatographic conditions for the analysis of elemental sulfur have been established by several groups. Buchanan, Chaven, and their associates have developed another extractive method of analysis for elemental sulfur that does not require heating of the sample?' and Kelly and his research group have worked out a clever bacteriological assay that can be used for whole coal samples.2a Two coal samples, one a pristine sample of Illinois No. 6 coal that was collected under the auspices of the Premium Coal Sample Program at Argonne National Laboratory, APCSP-3, and the other a sample that was collected by the Illinois Basin Coal Sample Program, IBCSP-1, have been independently analyzed by using five different methods over the span of 4 y e a r ~ . ~ ~The - ~ lresults (Table 11) illustrate the accuracy that was realized in separate laboratories using current methods. These studies also indicate that the authentic coal samples provided by the Illinois Basin Coal Sample Program and the Argonne Premium Coal Sample Program enable research teams to verify the reliability and accuracy of their experimental procedures. Observations of Elemental Sulfur. Chatterjee and Yurovskii presented some of the first information concerning the presence of this substance in c0als,3~,~~ and an array of information has been presented in the past decade. Richard et al. extracted weathered Iowa No. 2 coal with cyclohexane and found 0.2 w t 90 elemental ~ u l f u r .White ~ and Lee confirmed the presence of this material in several American bituminous coals.35 Several more recent investigations of run-of-the-mine samples of Illinois Basin coals have revealed that ordinary samples of such coals can contain 0.1-0.2 wt ?& elemental s ~ l f u r . ~ ~ - ~ ' More exciting observations have been realized with a (26)Schmidt, M.; Siebert, W. Trotman-Dickenson, A. F., Eds. Comprehensiue Inorganic Chemistry; Pergamon Press: Elmsford, NY,1973; Vol. 2,Chapter 25. (27) Buchanan, D. H.; Coombs, K.; Hackley, K. C.; Chaven, C.; Kruse, C. W. Proceedings: Fourteenth Annual EPRI Conference on Fuel Science and Conuersion; Electric Power Research Institute: Palo Alto,CA, 1989. (28)Schicho, R. N.; Brown, S. H.; Kelly, R. M.; Olson, G. J. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1988,33(4), 554. (29)Duran, J. E.; Mahasay, S. R.; Stock, L. M. Fuel 1986,65,1167. (30)Narayan, R.;Kullerud, G.; Wood, K. V. Prepr. Pap.-Am. Chem. SOC.,Diu Fuel Chem. 1988,33(1),193, and private communication. (31)Chamberlin, S.; Wolny, R.; Stock, L. M. 1989,unpublished results (32)Chatterjee, N. N. 8. J . Geol., Min., Metall. SOC.India 1942,14, 1; Chem. Abstr. 1947,41,1410g. (33)Yurovskii, A. Z.In Sulfur in Coals; English translation available from the US Department of Commerce, National Technical Information Service: Springfield, VA, 1960; Ref. No. TT70-57216. (34)Richard, J. J.; Vick, R. D.; Junk, G. A. Enuiron. Sci. Technol. 1977,11, 1085. (35)White, C. M.; Lee, M. L. Geochim. Cosmochim. Acta 1980,44, 1825.
refuse coal that is sometimes referred to as Indiana bog coal. Several analyses of this material establish that it contains about 11% sulfur and about 1% elemental sulfur.3o In addition, Lee and Leehe and their associates have reported that elemental sulfur can be extracted, sometimes in considerable quantity, from bituminous ~ o a l s . ~ ~ ~ ~ ~ In sharp contrast, very careful studies of pristine samples (Le. coal samples that have not been exposed to the atmosphere) from the Upper Freeport (APCSP-l), the Wyodak (APCSP-2), and the Illinois No. 6 seams (APCSP-3) indicate that elemental sulfur is ab~ent.~'-~O These results are in accord with the expectations expressed by Chatterjee.32 It is notable that the pristine sample of Illinois No. 6 coal from the Argonne National Laboratory has been investigated in four different laboratories using a number of different analytical methods. No elemental sulfur has been detected. The sulfur content of this pristine sample is conservatively estimated to be below 0.001 wt %. Research by Buchanan and his co-workers and by our group has demonstrated that elemental sulfur forms when the pristine Illinois No. 6 coal is exposed to the atmosphere (Table III).27331Further, the rate of formation of sulfur is accelerated, in a humid atmosphere. It has also been established that the 32S/34S ratios of the elemental sulfur and pyrite in another of the Illinois Basin Coal Sample Program coals are closely related, thereby indicating that pyrite is the precursor of elemental Although space limitations prevent a thorough discussion of the fascinating geochemical considerations, it is pertinent to mention that elemental sulfur can be formed after the exposure of the coal samples to the atmosphere by chemical and biological pathways. Elemental sulfur may be generated by the chemical oxidation of the pyrite in coal. Bergholm investigated the oxidation of the pyrite in air.39 He reported that this oxidation reaction yields sulfur of different oxidation states including elemental sulfur. The oxidation occurs under ambient conditions, and is appreciably accelerated at elevated temperatures.33 Although most discussions have focused on pyrite, other sulfur minerals such as marcasite also apparently undergo conversion to sulfur under the same experimental condit i o n ~ The . ~ ~results of Bergholm can be related directly to the transformation of pyrite in coal. The experimental results are compatible with the viewpoint that acid-catalyzed, air oxidation of pyrite produces iron sulfates and sulfur in unequal amounts in a rather complex reaction. The true complexity of the oxidation reaction is not well portrayed in an elementary equation such as eq 1. This FeS2 + O2 + H+ Fe2(SO4I3+ S + H20 (1) feature is well illustrated in the XPS spectra of partially oxidized pyrites that were obtained by Pillai and cow o r k e r ~ .They ~ demonstrated that a variety of partially converted substances including hydrated iron oxides and adsorbed sulfur di- and trioxides appeared on the surface of the pyrite particles during oxidation. The microbiological oxidation of pyrite affords a second pathway for the formation of sulfur from pyrite.28~33s40s41
-
(36)Lee, S.; Kesavan, S. K.; Lee, B. G.; Ghosh, A,; Kulik, C. J. Fuel Sci. Technol. Znt. 1989,7 , 443. (37)Leehe, H. Presented at the EPRI Coal Structure Workshop; Monterey, CA, Nov 4, 1987. (38)Hackley, K. C.; Buchanan, D. H.; Coombs, K.; Chaven, C.; Kruse, C. W. Fuel Process. Technol., in press. (39)Bergholm, A. Jernkontorets Ann. 1955,139, 531;Chem. Abstr. 1955,49,15408e. (40) Groudev, S. N.; Genchev, F. N.; Groudeva, V. J. In Proceedings, Traditional Scientific Conference on Mineral Processing; Edited by Institute of Mineral Processing and Technology: Cracow, Poland, 1981.
Energy & Fuels, Vol. 3, No. 6, 1989 655
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Table 111. Elemental Sulfur Content of Pristine Coals after Intentional ExDosure to the AtmosDheresl coal sample Upper Freeport mv, bituminous, APCSP-1 Wyodak subbituminous, APCSP-2 Illinois No. 6 hv, bituminous, APCSP-3 Illinois No. 6 hv, bituminous, APCSP-3, humid atmosphere a Below
total sulfur, wt %
pyritic sulfur, wt %
pristine
2.4 0.7 4.8 4.8
1.8 0.2 2.5 2.5
BDL' BDL BDL BDL
5.1 X lo" 1.4x 10-3 1.7 x 104 2.0 x 10-3
4.1 X 10"
LO x 10-3 4.7 x 10-3 2.8 x 10-1
detection limits.
Most of the bacteria that have been investigated for the desulfurization of coal convert pyrite to ferric sulfate and sulfur. Indeed X-ray diffraction techniques show that significant amounts of elemental sulfur and jarosite, a complex iron sulfate mineral, appear in coals that have been treated with pyrite-oxidizing bacteria.4l Again, it is difficult to portray the reactions in simple chemical equations, but Beyer and his co-workers have pointed out that the elemental sulfur may arise from the oxidation of pyrite by iron(II1) (eq 2).41
+
2Fe3+ FeSz
-
+
3Fe2+ 2s
(2) A third possibility for the formation of sulfur has been advanced by Narayan and his c o - w ~ r k e r s .They ~ ~ suggested that naturally occurring organic polysulfides may decompose to form elemental sulfur via extrusion reactions. I t is difficult to evaluate this suggestion because so little is known about the nature of the organic sulfur compounds in coal. However, coals are formed under reducing conditions, conditions that would not generally be regarded as conducive for the formation and preservation of the higher oxidation state polysulfides. Although experiments specifically designed to test this idea have not yet been reported, the results of experimental measurements of the pyritic sulfur content using lithium aluminum hydride provide cogent information. These observations are pertinent because lithium aluminum hydride also reduces elemental sulfur and polysulfides to thiolates and sulfide If polysulfides were present in coal in measurable amounts, there should be a discrepancy between the pyrite content assessed by the reductive method and the oxidative ASTM procedure. In point of fact, Kuhn and his associates observed that the pyrite content that was determined by the reductive method was consistently lower than the content measured in the usual way? Their results imply that neither sulfur nor polysulfides were present in the suite of coals that they studied. In more recent work, it was shown that prolonged reactions of pristine Illinois No. 6 coal with this hydride removed no more than 0.2% more sulfur than expected on the basis of the ASTM m e t h ~ d This . ~ small difference is consistent with several other interpretations. For example, as Timmer and van der Burgh have pointed out,12the oxidative method yields low values for the pyrite content of high-sulfur coals. On balance, the available evidence favors the view that the polysulfide content of unweathered bituminous coals is quite small.
Organic Sulfur Compounds A broad variety of techniques have been used to investigate the structures of the organic sulfur compounds (41) Beyer, M.; Ebner, H. G.; Assenmacher, H.; Frigge, J. Fuel 1987, 66, 551. (42) Arnold, R. C.; Lien, A. P.; Alm, R. M. J. Am. Chem. SOC.1950, "0
elemental sulfur, wt % 49 days 9 months
"0,
Ik, I J l .
(43) Shah, A. R.; Badma, D. K.; Vasudeva-Murthy, A. R. Indian J. Chem. 1971, 9,885. (44) Gladysz, J. A.; Wong, V. K.; Jick, B. S. J.Am. Chem. SOC.1978, 100, 838.
in coals and in their conversion products. Broadly speaking, hundreds of different sulfur-containing compounds have been detected. Nevertheless, progress in this area has been frustrated by the lack of suitable experimental procedures for the analysis of the components of solid coal. Many convenient experimental techniques such as the pyrolysis-mass spectroscopic methods usually provide only qualitative information. Other procedures that are more amenable for quantitative work, such as gas chromatography, provide the desired information, but the methods are often limited to relatively low molecular weight compounds. Further, the quantities of the individual compounds that have been measured in such experiments are often in the range of micrograms per gram of coal. Although considerable progress has been made in the past decade, it must be acknowledged that we are far from the desired understanding of the structures of the organic sulfur compounds. Functional Group Distributions. Several research groups have sought methods for the quantitative determination of the distribution of sulfur among the plausible functional groups. Direct methods such as 33SNMR spectroscopy and XPS have inherent limitations that have prevented their use for this purpose. To illustrate, sulfur NMR spectroscopy depends upon the detection of 33S nuclei, which are present in low abundance and for which the widths of the signals in divalent molecules such as sulfides and heterocycles are exceedingly As a consequence no progress in functional group analysis has been made by using this technique. XPS does not provide adequate resolution to distinguish between different divalent organic sulfur compound^.^^ Hence, this method is also unsuitable. Hussain and Spiro and their co-workers have reported that XANES and EXAFS can provide useful information about the organic sulfur compounds in ~oa1.~'1**Their reports have not been amplified by further publications, but research teams led by Gorbaty and Huffman have been examining the capabilities of the approach and have obtained encouraging results. Huffman and his co-workers have examined the use of XANES and EXAFS for the examination of the structures of the organic sulfur compounds in coal macerals that had been freed of inorganic constituent^.^^ The XANES transitions that were observed for the K-shell electrons of sulfur in these samples coincide with the transitions for the sulfur atoms in heterocyclic molecules such as dibenzothiophene and benzothiophene. Furthermore, the molecular spacings implied by the EXAFS portion of the spectrum, for example the 1.75-A carbon-sulfur bond distance, are more compatible (45) Harris, R. K. In NMR and the Periodic Table; Harris, R. K., Mann, B. E., Eds.; Academic Press: New York, 1978. (46) Annunziata, R.; Barbarella, G . Org. Magn. Reson. 1984,22, 250. (47) Hussain, Z.; Umbach, E.; Shirley, D. A.; Stohr, J.; Feldhaus, J. Nucl. Instrum. Methods Phys. Res. 1982, 195, 115. (48) Spiro, C. L.; Wong, J.; Lytle, F. W.; Greegor, R. B.; Maylotte, D. H.; Lamson, S. H. Science 1984,226,48. (49) 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.
656 Energy & Fuels, Vol. 3, No. 6, 1989
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c
0
50
100
150
200
250
300
350
Temperature,'C
Figure 2. Reductive kinetogram for Illinois No. 6 coal.52
with the results expected for heterocyclic compounds than for aliphatic substances. These findings prompted the conclusion that the organic sulfur compounds in several bituminous coals were predominantly thiophenic in nat ~ r e . ~ ~ George and Gorbaty have investigated the XANES spectra of many pure sulfur compounds and have suggested that these spectra can provide fingerprints for several kinds of sulfur compounds including the sulfidic extended the work and thiophenic f ~ r m s . ~ They ~ * ~ have l in this area by showing that the third derivative of the XANES spectrum can be used to deconvolute the absorptions of sulfidic and thiophenic compounds in known mixtures. Quantitative analyses of the distribution of the sulfur compounds in petroleum asphaltenes, Rasa lignite, and a bituminous coal from the Illinois No. 6 seam have been r e p ~ r t e d . ~The ~ ~lignite ' contains 30 f 10% sulfidic and 70 f 10% thiophenic sulfur,5owhereas the Illinois No. 6 coals contains 60 f 10% sulfidic and 40 f 10% thiophenic sulfur.51 The results for the American bituminous coals that have been obtained by Gorbaty and Huffman differ very significantly. Whether these differences are real or the consequence of the manner in which the data are being treated is a matter of major concern, because, as already mentioned, so few techniques are really suitable for solid-state studies of sulfur distributions. One feature deserves special mention. The signals for the fossil materials appear to have larger line widths than the signals of the pure compounds. Additional work that is already under way on mixtures of pure compounds and discreetly doped coal macerals should determine whether the spectral resolution is adequate for organic sulfur functional group speciation. Chemical methods have also been examined. Attar and his co-workers tested an indirect, catalytic hydrogenation method that involves the temperature-programmed reduction of the sulfur compounds to hydrogen sulfide to measure the abundances of different kinds of sulfur containing structures such as polysulfides, aliphatic thiols, aromatic thiols, aliphatic sulfides, aromatic sulfides, and so forth (eq 3).52 The method is based upon the idea that
programmed to 450
OC
different rates and produce hydrogen sulfide at distinctly different temperatures. Plots of the hydrogen sulfide yield as a function of temperature can provide well-resolved bands. The results for an Illinois No. 6 coal are shown in Figure 2. Unfortunately, many coals do not provide such well-resolved observation^.^^ Attar and Dupuis discussed results for five coals in 1981.54 They noted that the results depended upon the particle size and stressed the fact that the yields of hydrogen sulfide obtained from bituminous coals and other coals of higher rank may not be quantitative. They found, for example, that Illinois No. 6 coal provided less than 60% of the expected amount of hydrogen sulfide. The kinetogram for this material suggests that the 60% of the organic sulfur that was converted to hydrogen sulfide was distributed as follows: aliphatic thiols, 9%; aromatic thiols, 4 % ; aliphatic sulfides, 1 2 % ; and aromatic sulfides, 35%This analysis suggests that 21% of the sulfur is present in aliphatic structures and that 13% of the sulfur atoms appears in thiols. Calkins and his associates has also discussed the temperature dependent evolution of hydrogen sulfide from coal (eq 4).55956 The examination of an array of pure organic N2
coal
no catalyst
' H2S
(4)
programmed to 1000 O C
sulfur compounds established that rapid pyrolysis did not completely decompose thiophenic or aromatic compounds below 900 "C but that aliphatic sulfides and thiols were readily decomposed below this temperature. A study of eight coals ranging from lignite to anthracite revealed that hydrogen sulfide emission was essentially complete below 850 OC, and Calkins inferred from this observation that aliphatic thiols and sulfides were responsible for its evolution. The results for these coals imply that the aliphatic sulfur atom content ranges from 0% in high-rank anthracite to 30-50% of the total organic sulfur in bituminous coals to 6040% of the total organic sulfur in low-rank material^.^^,^^
Thermal desulfurization studies imply that aliphatic sulfur compounds may be quite abundant. Consequently, Attar and Calkins and their collaborators have attempted to probe this feature of the results. Calkins used methylation to measure the thiol content and found that 22-33% of the organic sulfur in bituminous coal was present as thioethers and that 2-5% was present as thiols.55 Attar and Dupuis also determined the thiol content by this method and observed that about 4 % of the sulfur was present in thiol groups.54 We investigated the methylation products of several Illinois No. 6 coals in a search for thiol groups. The abundances of such materials were at threshold detection limits, about 1%of the organic sulfur content. Hence, it must be noted that no one has yet been able to substantiate the high thiol content, 13%, that is implied for Illinois No. 6 coal by the hydrogenation method of Attar. This is an important point that deserves thorough evaluation. The magnitude of the discrepancy is quite disturbing and suggests that the kinetograms need to be interpreted with special caution. A temperature-programmed oxidation method for the evaluation of the sulfur functional groups in coal has been
different sulfur functional groups undergo reduction at (50) George, G. N.; Gorbaty, M. L. J.Am. Chem. SOC. 1989,111,3182. (51) George, G. N.; Gorbaty, M. L., Kelemen, S. R. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1989, 34(3), 738. (52) Attar, A. In Analytical Methods for Coal and Coal Products; Karr, C., Ed.; Academic Press: New York, 1979; Vol. 1.
(53) Nichols, D. Private communication. (54) Attar, A.; Dupuis, F. In Coal Structure; M. C. Gorbaty, Ouchi, K., Eds.; Advances in Chemistry 192; American Chemical Society: Washington, DC, 1981; Chapter 16. (55) Calkins, W. H.; Energy Fuels 1987, 1, 59. (56) Calkins, W. H. Prepr. Pap-Am. Chem. Soc., Diu. Fuel Chem. 1985, 30(4), 450.
Energy & Fuels, Vol. 3, No. 6, 1989 657
Reviews
120
1000
loo 80
2.
Elemental sulfur
eag
%
FPD
0
0
1
2
3
4
Time, hour
Figure 3. Oxidative kinetogram for Pittsburgh Seam (Ohio No. 8) coaL5' The solid line indicates the SO2 evolution, and the dashed line indicates the temperature.
investigated by LaCount and co-worker~.~'This method (eq 5) is based on the concept that the oxidation of the 10% O2 in Ar woBcatalyst
SO2
programmed to 1000 "C
different functional groups of sulfur may occur preferentially at different temperatures. The kinetogram for a Pittsburgh seam coal is shown in Figure 3. In general, raw coals produce up to four distinctive peaks in the ranges 170-180,295-325,430-450,and 440-550 "C. The study of pure sulfur compounds enabled the assignment of these four peaks: the first to elemental sulfur and volatile sulfides, the second to nonaromatic sulfides, the third to pyrite, and the fourth to thiophenic structures, aryl sulfides and aryl sulfones. Although some information about the organic sulfur in the coal has been obtained by this method, no definitive structural assignments have been developed from the very broad kinetograms. Investigations of Extracts and Soluble Reaction Products. Many of the difficulties that are associated with the investigations of solid coals are not encountered in the investigation of extracts and reaction products. Tetrahydrofuran and pyridine are quite useful for extraction of coals. Buchanan has recently studied the extraction of Illinois Basin coals.58 He established that the sequential extraction of pristine Illinois No. 6 coal, APCSP-3, with toluene, tetrahydrofuran, dimethylformamide, and pyridine provided an extract that contained 28 wt% of the coal and 29% of the organic sulfur. Calkins recently explored the use of other solvents for selective sulfur compound extraction. He found that tetrahydrofuran was superior to acetonitrile and ethylenediamine and, surprisingly, to pyridine for the extraction of organic sulfur compounds from a Pittsburgh No. 8 bituminous ~ o a l . The ~ ~fact * ~that no more than about 30% of sulfur in organic compounds can be extracted limits the information that can be obtained by the study of extracts. This disadvantage is compounded by the fact that only a portion of the extract can be analyzed by current methods such as GC-MS procedures. Thus, although the results obtained for extracts provide important clues about the structures of the sulfur compounds, the results can not be readily extrapolated to infer the constitution of the entire family of sulfur compounds in whole coal. White and Lee were among the first persons to investigate extracts to learn about the organic sulfur com(57) LaCount, R. B.; Anderson, R. R.; Friedman, S.; Blaustein, B. D. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1986,31(1), 70. (58) Buchanan, D. H.; Osborne, K. R.; Warfel, L. C.; Wanping, M.; Lucas, D. Energy Fuels 1988, 2, 163.
I n 75
100
I
I
I
125
150
175
L 200
225
250
Temperature, "C
Figure 4. Dual chromatographicdetector response traces for FID and FPD systems for the sulfur compounds in an extract of a Homestead, KY, coal.35 The sulfur heterocycles occur at A-D.
pounds.35 They studied the aromatic compounds in an extract of a Homestead, KY, coal using gas chromatography with a sulfur-sensitive detection system and mass spectroscopy. The benzothiophene (l),dibenzothiophene (2), and phenanthro[4,5-bcd]thiophene(3) ring systems were shown to be three most common ones in the extract. 0
\
@Jo;S;o@ 1
2
3
The relationship between the signal strengths for these compounds and elemental sulfur is shown in Figure 4. Even though elemental sulfur constitutes only a small fraction of the sulfur in this coal, it provides the predominant signal; thus, the problems faced by the coal chemist who seeks to define the structures of the nonabundant organic sulfur compounds become readily apparent. As a consequence, several different methods for the concentration of the organic sulfur compounds have been developed. These procedures have been focused on the aromatic heterocyclic compounds and have been tested principally by studies of coal reaction products. These contributions are surveyed in the next few paragraphs. Lee and Castle and their collaborators developed a method based on oxidation and reduction to isolate the sulfur-containing heterocycles from polycyclic aromatic hydrocarbons (eq 6).59 This approach was used to study
04-\'o the heterocyclic compounds in solvent-refined coal fract i o n ~ The . ~ ~compounds that were found in the medium distillate ranged from methylated benzothiophenes to methylated benzothiophenes. Dibenzothiophene was the single most abundant heterocycle at 127 ppm by weight in a sample that contained 6000 ppm of polycyclic sulfur compounds. Other compounds with hydroaromatic structures such as the alkylated dihydrobenzothiophenes were also relatively abundant. Naphtho[2,1-b] [ llbenzothiophene (4) was present in the vacuum bottom fraction that contained other higher molecular weight materials. Many of the sulfur heterocyclic compounds in the sol(59) Kong, R. C.; Lee, M. L.; Iwao, M.; Tominaga, Y.; Pratap, R.; Thompson, R. D.; Castle, R. N. Fuel 1984,63, 702.
658 Energy & Fuels, Vol. 3, No. 6, 1989
Reviews Table IV. Quantities of Aromatic Sulfur Compounds in a Subbituminous Coal and an SRC-1Process Solvent" approx concn,b p g of S/g of coal
4
5
vent-refined coal product contain three to five rings about a central thiophene structure. High-boiling fractions contain even larger molecules such as naphtho[2,3-b]phenanthro[ 1,Zdlthiophene (5).59 The principal features of this study were confirmed in a more recent investigation that used palladium(I1) ligand exchange chromatography rather than chemical oxidation and reduction to separate the desired sulfur heterocycles from the more abundant polycyclic aromatic hydrocarbons in a representative coal tar and in a solvent-refined coal liquid vacuum residue.60 The dibenzothiophene structural element was abundant in the coal tar, whereas other larger ring systems were apparent in the vacuum residue. Further work with ligand-exchange chromatography enabled the identification of additional organic sulfur compounds in a distillate, bp 260-450 "C, of a solvent-refinedfraction of a West Virginia coal from the Pittsburgh SeamaGIThis strategy enabled the detection of many additional compounds, They found three isomeric methyldibenzothiophenes, five isomeric dimethyldibenzothiophenes, two isomeric methylphenanthro[4,5-bcd]thiophenes,and naphtho[2,3-b] [1]benzothiophene (6). Certain of these substances were not observed in the first study because they coemerged with hydrocarbons.
6
In a recent report, Nishioka used palladium(I1) ligandexchange chromatography to concentrate the heterocyclic sulfur compounds and gas chromatography with several different detector systems to define the constituents of a benzene extract of a bituminous coal from Rock Springs No. 7, Sweetwater, WY, PSOC-521, and an SRC 1process solvent.62 His work has provided the first quantitative information about the sulfur compounds in an extract of a bituminous coal, and the data are presented in Table IV for convenient inspection. The results for the coal extract of PSOC-521 and the process solvent differ in several ways. The quantitative observations differ enormously. Only about 0.002 pg of sulfur/g of coal have been identified in this careful In contrast, almost 1000000-foldmore sulfur in the process solvent has been speciated. There are also significant differences in the nature of the sulfur compounds. As illustrated in Table IV, the process solvent contains three-ring heterocycles and dihydrodibenzothiophenesthat are absent from the extract. But it should be mentioned that these multiring compounds apparently have been found in another coal extract by Hippo and his co-workers, who recently investigated Illinois No. 6 coal.63 The methods that have been developed to concentrate and analyze the sulfur heterocyclic compounds may dis-
compda dihydrobenzothiophene methylbenzothiophene
PSOC-521 0.02
methyldihydrobenzothiophene 0.04
C2-benzothiophene
C2-dihydrobenzothiophene C2-dihydrobenzothiophene
SRC-1 20 20
D D D 70
D
C%-benzothiophene
C3-dihydrobenzothiophene
D D
D
C,-benzothiophene C3-phenylthiophene C,-phenylthiophene propyl naphthyl sulfide
0.03 0.01 0.02
tetrahydrodibenzothiophene dibenzothiophene 1-methyldibenzothiophene 2- and 3-methyldibenzothiophene 4-methyldibenzothiophene C2-dibenzothiophene C2-dibenzothiophene phenanthro[4,5-bcd]thiophene methylphenanthro[4,5-bcd]thiophene methylphenanthro[4,5- bcd]thiophene
0.05
D D
10 11000 30 3080
0.08
2220 D
0.02
50 80 40 30
" A variety of methylated and ethylated derivatives were detected, but the structures were not determined. In these cases, the ring system is identified and the number of pendant carbon atoms is shown. b A D in this column indicates that the compound was detected but that no quantitative measurements were made.
criminate against aliphatic sulfur compounds and other sulfur-containingmolecules. White and his associates have examined this point by the investigation of the substances in an untreated extract of Rasa This relatively mature coal, which is often referred to as Rasa lignite, contains almost 12% sulfur and virtually no pyrite. Approximately 25% of the coal can be extracted into pyridinetoluene. Since this coal contained about 10-fold more sulfur than an American bituminous coal, White and his co-workers were able to examine the material without concentration by direct vaporization into a high-resolution mass spectrometer. More than 500 different sulfur-containing molecules were detected in the mass range from 100 to 400 Da. Most of the molecules could be grouped into 17 families on the basis of their empirical formulas. These families are shown in Table V. Benzothiophenes, dibenzothiophenes, naphthobenzothiophenes, and phenanthrothiophenes and their alkyl, hydroxy, amino, and sulfur-containing derivatives account for about 50% of the molecules that have been observed. The most numerws alkyl derivatives contained three, four, or five carbon atoms. The other families of molecules that were detected include a broad array of sulfidic derivatives of benzene, naphthalene, and phenanthrene. There are numerous molecules with more than one heteroatom. In point of fact, 7 gives the base peak in the spectrum. The
7 ~~
~
(60)Nishioka. M.: Lee. M. L.: Castle. R. N. Fuel 1986. 65. 390. (61) Nishioka; M.; C&pbell,'R. M.; Lee, M. L.; Castie, R. N. Fuel 1986. ~. 65. 270. (62) NGhioka, M. Energy Fuels 1988,2, 214.
(63) Palmer, S. R.; Kruge, M. A.; Hippo, E. J,; Crelling, J. C. Proceedings Fourteenth Annual EPRI Conference; Annual EPRI Conference on Fuel Science and Conuersion; Electric Power Research Institute: Palo Alto, CA, 1989.
results that have been obtained for this very sulfur-rich (64)White, C. M.; Douglas, L. J.; Anderson, R. R.; Schmidt, C. E.; Gray, R. J. In Ceochemcstry of Sulfur in FOSSL~ Fuels; Orr,W. L., White, C. M., Eds.; ACS Symposium Series; American Chemical Society: Washington, DC, in press.
Energy & Fuels, Vol. 3, No. 6,1989 659
Reviews Table
V. S u l f u r Compounds in a n E x t r a c t of Rasa CoalM
structure' heterocyclic derivatives benzothiophenes dibenzothiophenes
none none
NHZ NHZ,OH naphthobenzothiophenes
none
phenanthrothiophenes
none
OH
OH SR sulfudic derivatives phenyl naphthyl phenanthryl
none none none
SR
6 16 4
90 500
I
55 425 40 200
10 8
I I
none none
pyrite
35
1
2.0
105
11
415
9 9 11 8
60 200 240 115
9 4
220 15
disulfides
CeH,SSCeH, C6H4SZCBH4
X
homologous seriesb functional re1 substituent no. of abungroup compounds dance'
'Most structural assignments are based on mass data and are tentative. b T h e number of compounds in the homologous series are noted; i.e., benzothiophenes with from one to six carbon atoms in alkyl groups and aminodibenzothiophenes with from one to four carbon atoms in alkyl groups were detected. 'The relative abundances are estimated on the basis of the sum of the ion currents for all the compounds in the homologous series.
coal may not be representative of the sulfur distribution in American bituminous coals, but the information certainly provides valuable clues concerning the kinds of organic sulfur compounds that can appear in natural organic matrices. In this connection, it is also very pertinent to note that DeLeeuw and Albrecht and their co-workers have been very vigorously investigating the compositions of the sulfur-containing substances in kerogens.They have detected and proved the structures of a very broad array of sulfur-containingsubstances including sulfidic and thiophenic derivatives of branched and unbranched alkanes, terpenoids, and hopanes. Again, while it must be acknowledged that much of this information does not relate directly to the organic sulfur compounds that now are present in bituminous coals, the information that is being provided by these research teams provides further perspective on the ways in which sulfur can be incorporated into organic matter and on the possible reaction pathways for the chemical evolution of first-formed compounds into more stable sulfur heterocycles.M In summary, the research on the extracts and soluble reaction products of bituminous coals has been carried out at a very high level of analytical sophistication, but there are two major difficulties. First, the sulfur compounds that have been quantitatively identified appear at the ppm level, and there is no secure basis by which this information can be extrapolated to infer the structures of the other organic sulfur compounds in coal. Second, although 25-35% of the sulfur atoms in the processed coal liquids can be speciated, it is very well established that processing alters the structures of the coal molecules by producing new organic sulfur compounds from inorganic sulfur constituents and by altering the structures of the other organic (65) Sinninghe Damsti, J. S.; Eglinton, T. I.; DeLeeuw, J. W.; Schenck, P. A. Geochim. Cosmochim. Acta 1989,53,873. (66) Sinninghe DamstC, J. S. Organically-Bound Sulphur in the Geosphere: A Molecular Approach, Offsetdrukkerij Kanters B.V.: Alblasserdan, The Netherlands, 1988. (67) Valisolalao, J.; Perakis, N.; Chappe, B.; Albrecht, P. Tetrahedron Lett. 1984,25, 1183-1186. (68) Ourisson, G.; Albrecht, P.; Rohmer, M. Sci. Am. 1984,251(2), 44.
300
500 Temperature, "C
400
600
F i g u r e 5. Sulfur content in t h e organic matter t h a t surrounds a pyrite particle in a sample t h a t has been heated. T h e organic sulfur content at t h e remote location, 25 km away, decreases systematically, but at the near position and at the position I pm away, it decreases modestly and then increases as pyritic sulfur enters the organic matrix.16
sulfur compounds. To illustrate, Wert and his associates demonstrated that the sulfur from pyrite enters the organic matrix in thermolytic reactions (Figure 5),15 and many investigators have commented upon the reactions of sulfur with organic molecules and upon the transformations of aliphatic sulfur compounds into aromatic sulfur comp o u n d ~ . ~ " ~Again, it must be stressed the information that has been derived so far by the study of thermally transformed coals cannot be extrapolated to generate a trustworthy picture of coal's sulfur compounds. Coupled Analytical Strategies. A third strategy for the analysis of the structures of the organic sulfur compounds involves the coupling of a relatively selective chemical conversion reaction with an appropriate analytical method. These methods can provide considerable insight about the structural elements in a coal. The dichromate oxidation of coal by the Argonne National Laboratory group provides an excellent early example of the approach. The oxidation reaction provides an array of aliphatic and aromatic carboxylic acids that were converted to their methyl esters for convenient study. Hayatsu and his associates found 32 different aromatic ring structures via mass spectro~copy.'~They identified 20 heterocyclic derivatives, among which there were four heterocyclic sulfur ring systems, 8-11. Their work also
8
9
10
11
revealed that these sulfur heterocyclic structures were absent from lignite, that the compounds with two and three rings, 8 and 9, were present in the bituminous coals, and that the four- and five-ring compounds, 10 and 11, (69) Weitkamp, A. W. J . Am. Chem. SOC.1969,81, 3430. (70) White, C. M.; Douglas, L. J.; Schmidt, C. E.; Hackett, M. Energy Fuels 1988, 2, 220. (71) Philip, R. P.; Bakel, A. Energy Fuels 1988, 2, 59. (72) Horton, A. W. J. Org. Chem. 1949, 14, 761. (73) DeRoo, J.; Hodgson, G. W. Chem. Geol. 1978, 22, 71. (74) Hayatau, R.; Winans, R. E.; Scott, R. G.; Jadernik, P. Fuel 1978, 57, 541.
660 Energy & Fuels, Vol. 3, No. 6,1989
were only present in the anthracite.I4 The ion currents that were reported by Hayatsu and his associates suggest that these heterocyclic ring structures may be quite abundant, but unfortunately, no accurate quantitative data have been obtained. The results of very recent work in which Illinois Basin coals were oxidized with hydrogen peroxide in acetic acid have been reported in preliminary form.63 The esters of benzothiophene- and dibenzothiophenecarboxylic acid, 8 and 9, were observed in relatively high abundance as expected, but these workers also detected an aryl alkyl sulfone fragment of unknown structure and several aliphatic sulfonic acids. These results are very intriguing because they provide additional support for the view that aliphatic carbon-sulfur linkages exist in some bituminous coals. Pyrolysis-mass spectroscopy has been broadly applied for the investigation of coals. Conventional pyrolysis vaporizes approximately 60% of the organic material in most bituminous coals. These volatile products have a broad molecular weight distribution that ranges from less than 100 to more than 2000 Da. Current chemical methods of mass analysis only enable the identification of the sulfur compounds with molecular weights below 400 Da. Clearly, only a small fraction of the material is being examined in experiments of this kind. Further, the fact that the atomic weights of oxygen and sulfur are simple multiples complicates the interpretation of low-resolution mass spectrometric data. Flash-pyrolysis experiments have been carried out elegantly for coals of various ranks by Calkins and his assoc i a t e ~ Pyroprobe . ~ ~ ~ ~ analysis of a high-sulfur Pittsburgh No. 8 coal with about 1.5% organic sulfur and a sample of the same coal from which 90% of the pyrite had been removed gave comparable yields of many products. The results are shown in Table VI because they effectively illustrate the diversity of the products that are formed in pyrolytic experiments of this kind. The information in the table implies that as much hydrogen sulfide was formed from the essentially pyrite-free coal as from the raw coal. The evolution of this material, as already mentioned, has been interpreted by Calkins as an indication of high aliphatic sulfur content. The study also reveals the broad array of heterocyclic compounds that are volatilized in thermal processes. Calkins observed thiophene, benzothiophene, and dibenzothiophene and virtually every mono- and dimethylated derivative. The utility of pyrolysis-mass spectroscopy for the study of the organic sulfur compounds in coal has recently been discussed out by Boudou and co-w0rkers.7~Although their investigations focused upon a Provence coal, their contributions deserve mention here because the technique is being applied for the coals under examination in this review. Their results parallel the results obtained by Calkins in the sense that broad families of thiophenes, benzothiophenes, and dibenzothiophenes with one, two, or three pendant alkyl groups were detected in the pyrolysis products. These workers noted that these substances might not be native in the coal but rather might be the vestige of precursor alkyl sulfides or alkylaryl sulfides that were transformed via cyclization and ar0matizati0n.l~This reservation is compatible with results for the pyrolysis of an Illinois No. 6 coal that had been modified by the incorporation of a thiophen-2-ylbutyl fragment (eq 7).16 (75) Boudou, J. P.; Boulegue, J.; Malechaux, L.; Nip, M.; deleeuw, J. W.; Boon, J. J. Fuel 1987, 66, 1558. (76) Bal, B. S.; Bal, R. B.; Stock, L. M.; Zabransky, R. F. Fossil Energy, DOE, Final Report, Oct 1, 1985-Mar 31, 1988.
Reviews Table VI. Products of the Flash Pyrolysis of a Pittsburgh No. 8 Seam Coal"*"
compd"
raw 3.03 0.48 0.32 2.13 0.27 0.25 0.34 0.23 0.49 0.47 0.38 0.07 0.19 0.22
H2S
cos
CHBSH
so2 CS2 thiophene methylthiophene-1 methylthiophene-2 dimethylthiophene-1 dimethylthiophene-2 dimethylthiophene-3 dimethylthiophene-4 trimethylthiophene-1 trimethylthiophene-2 tetramethylthiophene benzothiophene methylbenzothiophene-1 methylbenzothiophene-2 methylbenzothiophene-3 methylbenzothiophene-4 methylbenzothiophene-5 methylbenzothiophene-6
0.29 0.06 0.13 0.20 0.15 0.17
dimethylbenzothiophene-1 dimethylbenzothiophene-2 dimethylbenzothiophene-3 dimethylbenzothiophene-4 dimethylbenzothiophene-5 dimethylbenzothiophene-6 dimethylbenzothiophene-7 dimethylbenzothiophene-8 dimethylbenzothiophene-9 dibenzothiophene
0.11 0.06 0.12 0.07 0.05 0.07 0.05 0.05 0.03 0.08
yield, % pyrite (90%) removed 4.83 0.31 0.34 0.02 0.06 0.22 0.23 0.10 0.46 0.38 0.24 0.05 0.10 0.16 0.04 0.24 0.04 0.15 0.19 0.14 0.12 0.02 0.11 0.06 0.14 0.10 0.04 0.08 0.04 0.03 0.02
...
"The suffixes 1,2,3, indicate that more than one isomer with the structure was detected, but the structures of these substances have not been determined.
Benzothiophene and the alkylated thiophenes were all present in readily detectable amounts in the pyrolysis products.
R = CH3, CzH5, CH=
CHz, CHzCHzCH=
CHZ
High-resolution mass spectroscopy affords special advantages for the investigation of the sulfur compound distribution. Winans and his co-workers have implemented this method and have discovered a broad array of compounds with multiple heteroatoms in vacuum pyrolysis tars of Illinois No. 6 The elemental compositions that were confidently determined by low-voltage highresolution methods for this coal are very similar to the compositions of the substances that were detected in Rasa coal by White and his group.@ The results for the pyrolysis products of Illinois No. 6 coal are displayed in Table VI1 to illustrate the structural similarity. In summary, the coupling of reaction chemistry and modern instrumental methods of analysis can provide new insights concerning the organic sulfur compounds in coal. A t the present, the chemical strategies lag behind the analytical capabilities. While the results for oxidative degradation are encouraging, only a few methods have been explored and no satisfactory quantitative information has been provided as yet. Analytical pyrolysis is readily ac(77) Winans, R. E.; Neill, P. H. In Geochemistry of Sulfur in Fossil Fuels; Orr, W. L., White, C. M., Eds.; ACS Symposium Series: American Chemical Society: Washington, DC, in press.
Energy & Fuels, Vol. 3, No. 6, 1989 661
Reviews Table VII. Tentative Structural Assignmentsa for Compounds with Several Heteroatoms Observed in Vacuum Pyrolysis Tars of Illinois No. 6 Coal77
O -H
I
S
son S
s
o
0 H
H
The structural assignments are based exclusively upon highresolution mass spectroscopy.
complished, but the results need to be interpreted with special caution because many sulfur-containingcompounds are unstable under the reaction conditions used for pyrolysis. There are major opportunities for the development of more selective methods that are targeted for sulfur compound speciation.
Conclusion Great progress has been made in the definition of the structures of the sulfur compounds in coal in the past decade. Pristine bituminous coals contain two principal sulfur-containing constituents, pyrite and organic sulfur compounds. Samples of these coals that have been exposed to the atmosphere usually contain these substances and the products of oxidation, inorganic sulfate and elemental sulfur. The traditional ASTM methods for the analysis of the sulfur compounds in coal have been critically evaluated during the past 10 years. These methods are neither highly accurate nor highly precise, but the chemical community is accustomed to their limitations. They remain in wide use because the alternatives, which require special chemical techniques or elaborate equipment, provide essentially the same results. One important matter deserves special mention. Specifically, the ASTM approach often overestimates the organic sulfur content of high-sulfur coals. This problem can cause confusion in the assessment of information for heteroatom removal during coal conversion reactions and in coal desulfurization. Generally, the macerals from low-sulfur bituminous coals contain comparable amounts of organic sulfur compounds. In contrast, the macerals of high-sulfur coals contain significantly different amounts of these substances, and liptinites are somewhat richer and inertinites somewhat
poorer in organic sulfur than the associated vitrinites. These findings have not been satisfactorily explained. Powerful analytical methods are currently being developed for the identification of the organic sulfur compounds in coal, but only XANES and EXAFS spectroscopy among all the modern physical methods appear to hold promise for the evaluation of functional group distribution. It must be noted that these methods may prove to have the same limitation, lack of resolution, that restricts the use of other physical methods. In any event, the latest results do not appear to be in close agreement. One group concludes that bituminous coals are predominantly thiophenic, whereas the other believes that the XANES results are compatible with the occurrence of an important quantity of sulfidic sulfur in Illinois No. 6 coal. The latter results appear to be more compatible with the observations that have been obtained by temperatureprogrammed pyrolysis. The data from two different laboratories suggest that bituminous coals contain abundant amounts of aliphatic sulfur compounds as well as sulfur heterocyclic compounds. This major difference deserves full investigation inasmuch as strategies for desulfurization depend on a thorough appreciation of the sulfur species that are present. The study of coal products and coal extracts has advanced considerably. Excellent chromatographic and spectroscopic tools are now available for the determination of the low molecular weight sulfur compounds in these substances. Recent results reveal that as much as 50% of the organic sulfur in coal liquids can be speciated with high confidence. The results for coal extracts are not as encouraging; only part of the organic sulfur can be identified by the same techniques that were successfully used to study the coal liquids. While it is clear that a broad array of heterocyclic compounds are present in extracts of American coals and that some sulfidic materials may also be present, it is also clear that most of the organic sulfur compounds in the extracts are not being detected. Several factors probably contribute. The unobserved sulfur atoms may be present in aliphatic molecules or in polar molecules with more than one heteroatom which are contained in chromatographic fractions that have not yet been studied. The matter is further complicated by the fact that the apparently abundant higher molecular weight sulfur-containing molecules are not readily analyzed by current methods. New work and, more importantly, new strategies are clearly needed to define the structural characteristics of the very large number of organic sulfur compounds that remain unidentified.
Acknowledgment. It is a pleasure to acknowledge the support of the Center for Research on Sulfur in Coal. Registry No. S, 7704-34-9.
