Energy & Fuels 1992,6,411-413
411
Comparison of Pyrolytic and X-ray Spectroscopic Methods for Determining Organic Sulfur Species in Coal W. H. Calkins,* R. J. Torres-Ordonez, and Bongjin Jung Center for Catalytic Science and Technology, University of Delaware, Newark, Delaware 19716
M.L.Gorbaty,* G. N. George, and S. R. Kelemen Exxon Research and Engineering Company, Annandale, New Jersey 08801 Received February 24, 1992. Revised Manuscript Received April 2, 1992
Three methods for speciating and quantifying the forms of organically bound sulfur in coal, one based on chemical reactivity, and the other two based on direct measurement by X-ray absorption spectruscopies, are described and compared. One is based on the isothermal flash pyrolysis of coals as a function of temperature. The other two are based on spectra taken by X-ray absorption near-edge structure (XANES) spectroscopy and X-ray photoelectron spectroscopy (XPS). Data on aliphatic sulfur content of Rasa coal determined by the pyrolysis method are reported. With minor modification in the pyrolysis method, these three methods, used to characterize organically bound sulfur for the same suite of coal samples in terms of sulfidic and aromatic or thiophenic forms, give similar values. The pyrolysis method utilizes a specialized coal feeder, temperature-controlled fluidized sand bed, and a suitable analyzer such as a gas chromatograph with a sulfur-sensitive photodetector for H2S, CS2,and COS. The XANES method requires the availability of a synchrotron light source, and the XPS method requires the appropriate spectrometer. All three methods show that sulfur-containing low-rank coals tend to be high in sulfidic sulfur, whereas higher rank coals tend to contain mostly thiophenic sulfur forms.
Introduction A major problem in the utilization of coal, which is our largest fossil energy resource, is the preaence of sulfur. The efficient and complete removal of this sulfur from the coal prior to combustion or processing into liquid or gaseous fuels is an important objective. Sulfurin coal exists as both inorganic and organic forms. Although the inorganic forms are predominantly iron pyrite, other sulfides and sulfates may be present. While physical and chemical methods are available for removal of much of the inorganic sulfur species, the organic sulfur has proved to be more difficult to remove. Much research and development effort has been devoted to this objective, but as yet, no entirely satisfactory or economical solution has been found. A serious limitation to devising a suitable organic sulfur elimination process is the lack of knowledge of the forms of the organic sulfur that exist in coal. ASTM tests'J provide methods for the determination of both the total sulfur contant of coals and the inorganic sulfur forms present. The total sulfur in organic structures is usually determined by difference, although electron microprobe techniques can be used to determine it diUntil recently, there has been only limited information in the literature on the chemical structures in the organic sulfur components of coal. Attar4 has developed an interesting approach to determining the organic chemical structures of sulfur in coals by temperature-programmed catalytic reduction to HzS. While discrete peaks that (1) Total Sulfur in the Analyeia of Coal and Coke", ASTM D3177-75, American Society for Testing and Materials, Philadelphia, PA, 1975. (2) 'Forme of Sulfur in Coal", ASTM D2492-68; American Society for Testing and Materials, Philadelphia, PA, 1968. (3) Hsieh, K. C.; Wert, C. A. Fuel 1985,64, 255-262. (4) Attar, A. Analytical Methods for Coal and Coal Products; Academic: New York, 1979; Vol. 111, Chapter 56.
presumably come from different sulfur entities are obtained at various temperatures, recovery of the sulfur has not been quantitative, and interpretation of the results is difficult. Likewise, LaCount5 has developed a similar approach based on the temperatureprogrammed oxidation to SOz. SO2 peaks are obtained a t specific temperatures that have been associated with certain sulfur groups and iron pyrite. In 19856 a method for speciating and approximately quantifying the organically bound sulfur based on the temperature at which volatile sulfur products (such as H a , COS, and CS2)are emitted from coal was described. Since these products can also evolve from inorganic sulfur compounds, correction had to be made for the pyrite content of the coal. The pyrolysis was carried out with very rapid heat-up (ca. lo4 OC/s) in a fluidized sand bed, and at very short contact times (ca. 0.5 8) to minimize secondary reactions from occurring either in the coal or in the coal volatiles. This work provided a clear indication of the presence of labile sulfur structures in coal and, by comparison with pyrolysis of model compounds under identical conditions, suggested that they were aliphatic sulfur structures. That these aliphatic sulfur structures are thioether groups and not mercaptans is suggested by the work of Ignasiaklg who made a study of Rasa coal. Unpublished work by the authors on other coals tend to support this conclusion. It is interesting to note that Rasa coal has been investigated by White et al.,18 who showed it to be a rather unusual coal and likely to be atypical. In that work they characterizedthe volatile pyrolysis products (5) La Count, R. B.; Gapen, D. K.; King, W. P.; Dell, D. A.; Simpson, F. W.; Helms, C. A. New Approaches in Coal Chemistry; ACS Symposium Series 169; American Chemical Society: Washington, DC, 1981; pp 415-426. (6) Calkins, W. H. Prepr. Pap.-Am. Chem. Soc., Diu.Fuel Chem. 1985, 30(4), 45C-465.
0887-0624/92/2506-0411$03.00/00 1992 American Chemical Society
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412 Energy &Fuels, Vol. 6, No. 4, 1992
Table I. Percent of Organic Sulfur as Aliphatic Sulfur by Pyrolysis and X-ray Methods % aliphatic sulfur country coal Meauinenza Bueiah-Zap Wyodak-Anderson Charming Creek Rasa
of origin
% carbon (maf basis)
% total S
68.6 72.9 75.0 78.7 80.2
12.6 0.88 0.63 5.85 11.8
% organic S by pyrolysis ~~
SDain USA USA
NZ Yugoslavia
from Rasa coal by mass spectrometry to be entirely thiophenic in nature. However, with the pyrolysis mass spectrometer method they used, it is unlikely that any aliphatic sulfur products would have survived the pyrolysis and reached the mass spectrometer. Pyrolysis of coals of a broad range of ranks indicated that the fraction of the total sulfur which was labile is high in low-rank coals and decreases rapidly as rank increases. Coalification apparently removes aliphatic sulfur in coal and/or converta it to the more thermally stable aromatic or heterocyclic sulfur structures (e.g., thiophene^).'^.'^ Subsequently, X-ray methods for determining the organically bound sulfur species in coal using X-ray absorption near-edge spectroscopy (XANES) and X-ray photoelectron spectroscopy (XPS) have been developed.'+'6 With careful analysis and interpretation, these methods allowed the speciation and approximate quantification of the organic sulfur forms in coal. In that work, the trend of increasing aromatic sulfur forms and decreasing aliphatic sulfur with increasing rank was also observed. Thus three different methods are now available to the coal science community for determining the forms of organic sulfur in coals. Two of the methods involve surface and bulk X-ray spectroscopies, XPS and XANES, respectively, and measure sulfur species directly, and one (pyrolysis) relies on selective reactivity. Accurate results from XANES assume no or minimal spectral attenuations due to reabsorption effects, while those from XPS assume that the distributions of sulfur forms on the surface are representative of those in the bulk of the sample. Accuracies of both X-ray techniques also are highly dependent on the choice of model systems spectra used in the curve reconstructions. Accuracy of results from the selective reactivity method depends on no or minimal mass transport limitations or secondary reactions of the pyrolysis products and on the assumption that the sulfur forms in coals behave during the pyrolysis like the model systems. It, therefore, seemed appropriate to compare the results of these methods on the same coal samples in order to determine how well they agree. Coals were selected on the basis of low pyrite content, to avoid the substantial correction involved in both methods. Coals of both low and (7)Calkins, W. H., Energy Fuels 1987,1, 59. (8)Torres-Ordonez, R. J.; Calkins, W. H.; Klein, M. T. ACS Symp. Ser. 1990,No.429,Chapter 17. (9)Kelemen, S . R.;Gorbaty, M. L.; George, G. N.; Kwiatek, P. J.; Sansone, M. Fuel 1991,70(3), 396. (10)George, G. N.;Gorbaty, M. L. J.Am. Chem. SOC.1989,I l l , 3182. (11)Gorbaty, M. L.; George, G. N.; Kelemen, S. R. Fuel 1990,69,945. (12)George, G. N.;Gorbaty, M. L.; Kelemen, S. R.; Sansone, M. Energy Fuels 1991,5,93. (13)Kelemen, S.R.;George, G. N.; Gorbaty, M. L. Fuel 1990,69,939. (14)Huffman, G. P.; Huggins, F. E.; Mitra, S.; Shah, N.; Pugmire, R. J.; Davis, B.; Lytle, F. W.; and Greegor,R. B. Energy Fuels 1989,3,200. (15)Huffman, G. P.; Huggins, F. E.; Francis, H. E.; Mitra, S.; Shah, N. In Processing and Utilization of High-Sulfur Coals III; Markuszewski, R., Wheelock, T. D., Eds.; Elsevier: Amsterdam, 1990; pp 21-32. (16)Huffman, G. P.; Mitra, S.; Huggins, F. E.; Shah, N.; Vaidya, S.; Lu, F. Energy Fuels 1991,5 , 574.
by X-ray XANES XPS
mol % H2S by TPDg
~
11.8 0.70 0.47 5.76 11.4
67 39 36 26 47
48 37 33 28 30
66 45 37 38 26
75 29 40
high organic sulfur content were also selected to determine the effect of sulfur level. Experimental Section Pyrolysis Method. The pyrolysis experimentswere conducted at 625-930 "C with 105-149-1m fractions in the continuous flow pyrolyzer (described in detail in ref 7). The coals used were ground in a SPEX mill or mortar and pestle in a nitrogen atmosphere, sieved, and then vacuum-dried a t 104-107 O C for approximately 20 h. The coal particles were entrained into a nitrogen stream in the coal feeder and carried over into the fluidized sand bed at various flash pyrolysis temperatures. The coal feeder was suspended from a Mettler balance and the change in i h weight with time was monitored by a Linseis recorder. Coal was fed at uniform rates of 0.02-0.10 g/min, and approximately 1-2 g total was fed. Upon entering the pyrolyzer, the coal was rapidly heated to the bed temperature at a rate of approximately lo4 OC/s and residence time (assuming full utilization of the expanded fluidized sand bed) was about 0.5 s. The gases were collected and analyzed in a Perkin-Elmer Sigma 3B gas chromatograph, equipped with a flame photometric detector for sulfur gases. They were assumed to be derived from labile aliphatic sulfidic structures in coal, based on model compound studies. The tars were collected in Soxhlet thimbles, recovered by Soxhlet extraction with methylene chloride, and subsequently analyzed in an HP5880A gas chromatograph/mass spectrometer. Sulfur recovery varied from 85-92%. Data for flash pyrolysis of Rasa coal and phenyl disulfide were obtained by the methods previously described for all other coals and model X-ray Methods. The XANES spectra were recorded a t the National Synchrotron Light Source at Brookhaven National Laboratory on line X-1OC. The detailed procedures for recording and interpreting XANES spectra used in this work were described previous1y.lwl2 The spectra were interpreted using a method involving curve reconstructionof their third derivatives to provide approximate quantification of aliphatic and aromatic as well as thiophenic sulfur species. (The X-ray methods classify sulfur bonded to two sp2carbons as aromatic, which include aromatic sulfides as well as thiophenic sulfur structures.) XPS spectra were taken on a Vacuum Generator (VG) ESCA Lab System using M g K radiation and the experimental procedures have been described in detail.I3 To avoid ambient temperature oxidation, samples were prepared in a nitrogen glovebag or drybox, and kept under nitrogen, helium (XANES),or ultrahigh vacuum (XPS). In all casea, the coal samples were 4 0 0 mesh as received and were dusted on to sulfur-free Mylar tape mounted on an aluminum holder for XANES analysis and for XPS were mounted on to the sample block using double-sided tape. The accuracy of both X-ray methods is estimated to be f10 mol %.
Results and Discussion Table I shows the comparative resulta for aliphatic sulfur content on five low-pyrite coals determined by the pyrolysis and X-ray methods showing total sulfur and organic sulfur contents and the maf carbon contents as a measure of rank. The table includes previously weported aliphatic sulfur values for h a coal determined by the flash pyrolysis method. Except for the Mequinenza sample, the XANES and XPS results agree within the experimental error. The source of the discrepancy in the X-ray data seta for the Mequinenza sample is not known at this time and is a subject of future studies; however, it is believed that
Organic Sulfur Species in Coal
Energy & Fuels, Vol. 6,No. 4, 1992 413
Aliphatic
A l U d C
Sulfides, Mercaptans,
Sulfides,Mercaptans
Disulfides
f
.-
V
'"I-j."
nyl msdfi& ~,
mIho iPhcr
20
-0Phm
&hl!uuMhiophW
0
600
700
800
900
1000
Temperature O C
Figure 1. Pyrolysis of model sulfur compounds (% conversion vs temperature).
reabsorption effects in the XANES spectrum are partly responsible. It is apparent that, except for the Rasa sample, the pyrolysis values fairly closely track those of the X-ray measurements as far as the aliphatic sulfur contents are concerned. It is also of interest to note the close agreement in the levels of hydrogen sulfide evolved by the rapid pyrolysis method and that evolved below 750 "C obtained by a temperature-programmed decomposition (TPD) method, which involved a much slower rate of heating? Finally, the previously observed trend of decreasing aliphatic sulfur content with increase in coal rank appears to be borne out in the low-pyrite coals. The notable difference in the data was for the Rasa coal, with the pyrolysis method showing a significantly higher amount of aliphatic sulfur than either of the X-ray methods. One hypothesis is that there are forms of organically bound sulfur which would be classified as aromatic by XANES and XPS and which are labile enough to produce Hfi and CS2under conditions of the pyrolysis experiment. Such behavior was not found in any of the aromatic model compounds studied previously.e8 However, in a separata study of the mild oxidation of Rasa coal, the presence of disulfides was invoked to account for the large quantities of sulfonic acid seen in the oxidation product." Thus it appeared possible that, if Rasa coal contained aromatic disulfides, they would be detected as aromatic sulfur by XANES and XPS, but might be reactive enough to evolve a significant amount of H2S and CS2 at a lower temperature than other aromatic sulfur compounds and thus be interpreted as aliphatic sulfur. To test this, the behavior of phenyl disulfide in the pyrolysis method was determined. The results are shown (17) Kelemen, S. R.; Gorbaty,.M. L.; George, G. N.; Kwiatek, P. J. R e p r . Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1991,36(3), 1213; Fuel, in press. (18) White, C. M.;Douglae, L. J.; Anderson, R. R.; Schmidt, C. E.; Gray, R. J ACS Symp. Ser. 1990, No. 429,261-286. (19) Ignaeiak, B. S.; Fryer, J. F.; Jadernik, F. Fuel 1978, 578-584.
in Figure 1, in which sulfur as H2S and CS2 evolution is plotted along with data previously determined for other aromatic and aliphatic sulfides. It is clear that phenyl disulfide behaves more like other aromatic sulfides than aliphatic sulfides and thiophenes, but the line for phenyl disulfide is shifted enough to the left of the figure that there is some overlap with other aliphatic sulfides. It is interesting too that apparently only one sulfur atom is eliminated per aromatic disulfide molecule. Not knowing the precise disulfide content of the Rasa coal, it is not possible to accurately estimate the aliphatic sulfide content of that coal, based on the pyrolysis experiments. Since the disulfide content appears to be high it would be reasonable to expect the aliphatic sulfide content to be in the range of 30-40% or less, in closer agreement with the X-ray data. Since high disulfide content appears to be unusual in coal, this should not usually present a problem. However, based on the model compound data, it appears appropriate to change the cutoff point in the pyrolysis of the aliphatic structures to 700-750 OC from our customary 750-800 "C, which should eliminate most of the interference from aromatic disulfides. Conclusions Results from three methods for speciating and quantifying organically bound sulfur forms in low-pyrite-containing coals have been compared. Agreement was found to be good within the experimental accuracy of the methods, and results for the pyrolysis method should be taken as an upper limit of the amount of aliphatic sulfur forms present. To obviate the problem of interference by aromatic disulfides, a slightly different cutoff point in the pyrolysis curve appears adequate to minimize interference by these sulfur compound types which apparently do not commonly occur in significant amount and still account for the majority of the aliphatic sulfur compounds present. Data from all three methods confirm the trend of increasing aromatic sulfur content with increasing coal rank.
Acknowledgment. The pyrolysis work was sponsored in part by Amoco Oil Co. and in part by the State of Delaware as authorized by the State Budget Act of Fiscal Year 1988. The X-ray absorption spectra were recorded at the National Synchrotron Light Source (Brookhaven National Laboratory) which is funded by the Division of Material Sciences, US. Department of Energy, under contract DEAC02-76CH-00016. We also acknowledge the help of Professor Harold H. Schobert of the Pennsylvania State University for providing a sample of the Spanish Lignite, Drs. Richard Markwzewski and Glenn A. Norton of Iowa State University for providing the New Zealand Coal, and Dr. Curt White of the Pittsburgh Energy Technology Center for the sample of Rasa Coal. Registry No. S, 7704-34-9.
