Investigation of sulfur forms extracted from coal by perchloroethylene

Fuel Processing Technology, 35 (1993) 233-257 Elsevier Science Publishers B.V., Amsterdam

233

Investigation of sulfur forms extracted from coal by perchloroethylene F r a n k E. Huggins*, S h r e e n i w a s V. Vaidya, N a r e s h S h a h a n d G e r a l d P. H u f f m a n Institute for Mining and Minerals Research, 233 Mining and Mineral Resources Building, University of Kentucky, Lexington, K Y 40506-0107 (USA) (Received November 25, 1992; accepted in revised form March 2, 1993)

Abstract

The forms of sulfur present in a number of coals before and after extraction with perchloroethylene (PCE) at its boiling point (121° C) for up to an hour have been examined by a combination of sulfur XAFS and MSssbauer spectroscopies and standard chemical methods. A hybrid formsof-sulfur analysis was devised that included determinations of pyritic sulfur, elemental sulfur, organic sulfide, thiophene, oxidized organic sulfur forms, and sulfate. Of these forms of sulfur, only elemental sulfur was shown to be significantly extracted by PCE in the coals examined. Except for complications involving pyrite segregation during the extraction process in certain experiments, changes in all other sulfur forms with respect to the extraction were less than experimental uncertainties in the hybrid forms-of-sulfur analyses. MSssbauer and CCSEM data shewed that pyrite and all other minerals in the examined coals are not significantly affected by the PCE extraction.

I. INTRODUCTION T h e specific g r a v i t y of t h e c h l o r i n a t e d h y d r o c a r b o n , p e r c h l o r o e t h y l e n e (PCE, C2C14), is 1.62, w h i c h is close to a n o p t i m u m v a l u e for c o n v e n t i o n a l f l o a t / s i n k c l e a n i n g of m o s t coals. C o n s e q u e n t l y , P C E h a s b e e n p r o p o s e d as a h e a v y liquid m e d i u m for s e p a r a t i n g coal m a c e r a l a n d m i n e r a l c o m p o n e n t s by f l o t a t i o n i n t o a clean, a s h - p o o r f r a c t i o n a n d a n ash-rich w a s t e fraction. In addition, P C E a n d o t h e r c h l o r i n a t e d h y d r o c a r b o n s o l v e n t s will r e a d i l y dissolve m a n y simple o r g a n i c s u l f u r c o m p o u n d s , i n c l u d i n g sulfides a n d t h i o p h e n e s , w h i c h a r e b e l i e v e d to be t h e m a j o r o r g a n i c s u l f u r f u n c t i o n a l f o r m s p r e s e n t in c o a l s of b i t u m i n o u s r a n k . H e n c e , s u c h s o l v e n t s a p p e a r to h a v e t h e p o t e n t i a l for r e m o v i n g b o t h i n o r g a n i c s u l f u r a n d o r g a n i c sulfur f o r m s f r o m coal by f l o a t / s i n k a n d c h e m i c a l e x t r a c t i o n processes, r e s p e c t i v e l y . B o t h of t h e s e p r o c e s s e s a r e i n c l u d e d in a P C E - b a s e d pilot-scale coal c l e a n i n g facility b u i l t by M i d w e s t O r e P r o c e s s i n g C o m p a n y , Inc., a n d a r e p r e s e n t in a d e s i g n for a P C E - b a s e d d e m o n s t r a t i o n - s c a l e c o a l - c l e a n i n g plant, p r o p o s e d by t h e s a m e

* To whom correspondence to be addressed. 0378-3820/93/$06.00

© 1993 Elsevier Science Publishers B.V.

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F.E. Huggins et al./ Fuel Processing Technol. 35 (1993) 233-257

company [1-3]. In these PCE-based cleaning operations, the chemical extraction process is operated at close to the boiling point of PCE, 121 ° C. Despite the ease with which PCE dissolves simple organic sulfur compounds, however, the important question of whether the organic sulfur forms that actually exist in coal are significantly extracted by boiling PCE has not been resolved. Advocates of the PCE process claim that up to 70% of the organic sulfur in a coal can be extracted by PCE [2, 4, 5] on the basis of inference from conventional (ASTM) forms-of-sulfur and total sulfur analyses. The exact mechanism of organic sulfur removal has not been clarified, but the observation has been noted [4-7] that the sulfur removed from coal is recovered from PCE in the form of elemental sulfur. However, this focus on elemental sulfur has lead others [8-10] to criticize this method by claiming that PCE is only removing the elemental sulfur already present in the coal. Also, it has been noted that many of the coals which do show a significant reduction in total sulfur as a result of the PCE extraction are oxidized. Since elemental sulfur is known to be an oxidation product of pyrite [11] a n d is known to be extractable with boiling PCE, it was proposed that elemental sulfur was the only sulfur form in coal significantly amenable to extraction in boiling PCE [8, 10]. However, sulfur removal experiments on fresh, unoxidized coals, as well as aged coals have been carried out [4-7]. Clearly, to resolve this question of which sulfur forms are extractable by PCE, a technique is required that determines sulfur forms in coal directly. In recent years, sulfur K-edge X-ray absorption fine structure (XAFS) spectroscopy has been developed for the direct and non-destructive determination of sulfur forms in coal. Although different approaches have been taken by different research groups, a portion of the sulfur K-edge XAFS spectrum, the X-ray absorption near-edge structure (XANES) region [12] or its third derivative [13], can be resolved into contributions from several sulfur forms. Moreover, by means of calibration curves, it is possible to estimate the fraction of total sulfur in each form reasonably quantitatively. In this investigation, we determine the forms of sulfur in various coals before and after extraction in boiling PCE using sulfur K-edge XAFS spectroscopy to ascertain how much of the different sulfur forms are removed during the extraction process. These determinations are complemented and augmented by conventional elemental and forms-of-sulfur analyses and mineralogical analyses using MSssbauer spectroscopy and computer-controlled scanning electron microscopy to examine the inorganic sulfur forms. From these investigations, it appears that the only form of sulfur in the examined coals that undergoes significant extraction by PCE is elemental sulfur.

2. EXPERIMENTAL 2.1. P C E extraction

The PCE extraction was done in at least three different ways by different groups of investigators. Basically, the extraction process involves heating

F.E. Huggins et al./ Fuel Processing Technol. 35 (1993) 233-257

235

a sample of coal in an excess of PCE to the boiling point of PCE and holding it for periods of up to an hour. These treatments are as follows: 1. The static treatment performed at the University of Kentucky was simply to use a water-cooled glass refluxing apparatus that could comfortably hold 20 g of coal in 100 ml of PCE. An electrical heating mantle supplied heat to the contents of the flask and the temperature was monitored using a mercury thermometer. 2. The in situ method was done at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory and the intention was to measure the sulfur XAFS spectrum as quickly as possible on the PCE-treated sample without exposing it to air. A reactor was built that was compatible with the requirements of both XAFS spectroscopy and the PCE extraction process. The reactor (Fig. 1) consists of an upper and lower chamber. The lower chamber, constructed of stainless steel, acts as the reactor vessel and is about 1000 ml in volume. It is heated from underneath by conduction by means of a hot plate.

Te

Ter Prc

Fig. 1. Schematic diagram of in situ apparatus for conducting PCE extraction at the synchrotron and then performing sulfur K-edge XAFS spectroscopy without exposing sample to air.

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A water-cooled copper condenser is attached to one side of the lower chamber. The upper chamber is fabricated from aluminum and closely simulates one of the sample boxes used in conventional fluorescence XAFS spectroscopy. The fluorescence detector is connected on one side and the upper chamber is filled with helium for optimum t r ans m i t t ance of the low-energy sulfur X-ray fluorescence. An aluminum rod of diameter 2.54 cm is used to hold the sample in the PCE bath and to raise it from the lower chamber to the upper chamber for the XAFS measurement after the extraction. F u r t h e r details of this apparatus are described elsewhere [14, 15]. 3. A number of samples were provided to us by Prof. S. Lee, of the University of Akron, and Prof. D.H. Buchanan, of Eastern Illinois University. Samples from the University of Akron were subjected to PCE extraction either in the minipilot plant [4, 5] at the University of Akron or at the facility operated by Midwest Ore Processing Company. F u r t h e r details of these experiments are given in the literature [4-9]. 2.2. Coal samples

Various coal samples were investigated in the course of this study. Most of them were bituminous-rank coals from either the Illinois basin or the eastern Ohio/western Pennsylvania region of the Appalachian basin. Both oxidized and unoxidized coals were investigated. The coals are listed in Table 1, along with an indication of where the PCE extraction was done. More detailed analyses of these samples are presented in subsequent tables. The Pittsburgh samples were also subjected to oxidation prior to t r e a t m e n t with PCE, to promote the formation of possible oxidation products. This oxidation was carried out in a laboratory oven at 105 °C, in 15% relative humidity moist air, for two hours; such conditions are similar to t hat used by Midwest Ore TABLE

1

List of coals investigated, their sulfur contents, and P C E extractions performed on them

Coal sample

S~ (wt%)

Freeport-1 Freeport-2 Glencoe Illinois @6, APCS Illinois @6, [EIU] Illinois @6, III Indiana @5-1 Indiana @5-2 Ohio @5/@6 Pittsburgh-1 Pittsburgh-2 Pittsburgh-3

3.76 3.00 5.70 4.83 4.41 4.46 2.53 7.75 4.58 1.68 2.28 3.52

Static

In situ

Other J U. Akron J U. Akron

J J J E.I.U J U. Akron J

,/

J J d d

J

J U. Akron

a Values determined at University of Kentucky.

F.E. Huggins et al./Fuel Processing Technol. 35 (1993) 233-257

237

Processing Company in their pilot-scale process [3] to increase the release of sulfur from the coal. However, as will be seen below, this oxidation treatment was ineffective as it did not materially alter the forms of sulfur in the Pittsburgh coals, and, although results are shown for measurements made on both oxidized and unoxidized Pittsburgh seam coals, this oxidation treatment will not be discussed further.

2.3. Spectroscopic measurements 2.3.1. Sulfur K-edge X A F S spectroscopy

The sulfur K-edge XAFS spectra for this investigation were obtained at beam-line X-19A at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory, NY. Typically, the synchrotron is operated at 2.53 GeV and a current of 90-200 mA in order to supply an X-ray beam that is six orders of magnitude more intense than a conventional laboratory X-ray tube. Beam-line X-19A is an unfocussed beam-line dedicated to XAFS spectroscopy that is well suited to the soft nature of the sulfur X-rays because the beam is in an ultrahigh vacuum right up to the experimental table. The monochromator in the beam-line, also in vacuum, consists of two parallel silicon (111) crystals. For sulfur, a fluorescent detector is used to record the K-edge absorption spectrum, and the ratio of the sulfur fluorescent X-ray intensity relative to the incident X-ray intensity measured as a function of the X-ray energy constitutes the XAFS spectrum. For sulfur K-edge XAFS spectroscopy, a spectrum is recorded from about 70 eV below the edge, which occurs at 2472 eV in elemental sulfur, to as much as 300-400 eV above the edge. In the XANES region ( ~ 2460 - 2490 eV), the monochromator is stepped very finely over the edge at typically 0.1 eV/step or less. Elemental sulfur (flowers of sulfur) is used as the prime calibration standard, against which the energy scale of all other sulfur K-edge XAFS spectra are adjusted. All spectra displayed in this paper are shown relative to a zero point defined as the position of the maximum of the major absorption feature in the sulfur K-edge XAFS spectrum of elemental yellow sulfur. Sulfur K-edge XAFS spectra of coals were obtained either from samples pressed onto 2.5 cm diameter boric acid pellets or from samples in thin ( ~ 6 ~m) polypropylene bags suspended in the X-ray beam. The spectra are recorded on a MicroVAX computer at NSLS and transferred to a similar computer at the University of Kentucky for analysis. The XANES region is analyzed by means of a least-squares program that fits the spectrum to an arctangent function that represents the absorption associated with the electronic transition from the sulfur ls levels to the continuum states in the material and a number of mixed Lorentzian-Gaussian shaped peaks that represent absorption associated with the l s ~ 3 p electronic transition within the sulfur atom. As discussed in detail elsewhere [12, 14, 16], both the positions of the peaks and edge steps are determined by the oxidation state (chemical form) of the sulfur atoms in the material. By means of the calibration procedure described elsewhere [12, 14],

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the areas under the peaks can be related to the weight percentage of the sulfur present in different sulfur forms. 2.3.2. MSssbauer spectroscopy Iron-57 MSssbauer spectroscopy was performed on most samples as an alternative method of determining the pyritic sulfur content of the coals. MSssbauer spectra of the coals were recorded in transmission geometry in a constant acceleration M6ssbauer spectrometer equipped with a STCo source. The spectra were accumulated for typically 24 hours in 512 channels of a Tracor Northern 4500 MultiChannel Analyzer (MCA). A calibration spectrum from a thin metallic iron foil was acquired simultaneously at the other end of the MSssbauer drive and recorded in a second group of 512 channels in the MCA. The spectral data were transferred to the MicroVAX II computer for analysis. The analysis was achieved by means of a least-squares fitting program that fits the MSssbauer spectrum to a number of Lorentzian-shaped peaks. Following the procedure described elsewhere [17, 18], the area under peaks attributed to pyrite can be used as a measure of the pyritic sulfur in the coal. MSssbauer data were also used to supplement computer-controlled scanning electron microscopy (CCSEM) results in deriving the mineralogy of two coals before and after the PCE extraction process. The CCSEM technique, which is described in detail elsewhere [19], is basically a statistical method that combines chemical and size information from up to 1200 mineral particles in a polished section of pulverized coal mounted in epoxy to obtain a quantitative mineralogical description of the coal. 2.4. Analysis of sulfur forms in coals Three different methods were employed to determine various forms of sulfur in the coal. Chemical analytical methods, following close to ASTM practice, were used to determine the total sulfur and the traditional forms-of-sulfur for the coal samples. MSssbauer spectroscopy was used to determine the pyritic sulfur in the coals, according to the quantitative method described previously [17, 18], and also to test qualitatively for iron sulfate and oxyhydroxide minerals as an indication of coal oxidation [20]. Least-squares analyses of the sulfur K-edge XANES spectra, according to the method described previously [12, 14, 16], were used to determine the other sulfur forms that might be present in the coals. Such forms include organic sulfur functional forms such as organic sulfide, thiophenic sulfur, oxidized organic sulfur forms such as sulfoxide and sulfone and inorganic forms such as elemental sulfur and sulfate sulfur. A complete analysis of the forms of sulfur in all samples before and after PCE extraction was done by combining the total sulfur determined chemically according to ASTM procedures, with pyritic sulfur values determined either by wet chemical methods or by MSssbauer spectroscopy, to determine a nonpyritic sulfur value: Non-pyritic sulfur = Total sulfur - Pyritic sulfur

(1)

F.E. Huggins et al./ Fuel Processing Technol. 35 (1993) 233-257

239

The non-pyritic sulfur includes not only organic sulfur forms, but also the oxidized inorganic sulfur forms, sulfate and elemental sulfur, if present. The non-pyritic sulfur in the coal was then subdivided according to the leastsquares XANES method described originally [12, 14, 16]. Newly revised calibration values [21] were used for converting peak areas to wt% sulfur in the different forms. As discussed elsewhere [22, 23], this combination of techniques gives the most precise determinations of the different sulfur forms in a coal and avoids the large u n c e r t a i n t y and systematic errors that can arise if pyritic sulfur is determined by the XANES method. In some of the coals, it was found that elemental sulfur was a significant contribution to the total sulfur and it was therefore necessary to construct calibration data for this particular sulfur form for use with the sulfur K-edge XANES method. The calibration method is essentially the same as that described previously [12, 14, 16]. However, to avoid the problems of self-absorption of the sulfur-rich phase, the elemental sulfur/dibenzothiophene (DBT) calibration samples were prepared in solution to maximize the dispersion of both phases and a total sulfur content of 0.5 wt% sulfur was employed for the mixtures. This procedure resulted in much narrower lines and better spectral resolution for both elemental sulfur and DBT and a calibration coefficient of 0.9 was computed for elemental sulfur relative to DBT. It should be noted that the data on sulfur forms presented in this paper differ from those for the same spectra presented in previously published preliminary accounts of our examination of the PCE extraction process [14, 16]. Such differences arise largely because of revised calibration coefficients, not only as developed in connection with this paper for elemental sulfur, but also for all other sulfur forms due to a modification of the peak-shape equation in the least-squares fitting procedure [21]. In addition, better spectral fitting models, improved principally by inclusion of peaks for all possible sulfur forms, have also resulted in minor differences in the peak-area determinations from the sulfur XANES spectra. Finally, some previously presented analytical values for total sulfur in PCE-treated coals that were noted [16] to be in obvious conflict with spectroscopic estimations have now been redetermined in duplicate by independent laboratories. The revised values for these coals reported here agree much better with the spectroscopic estimations and indicate that the total sulfur removed by PCE extraction is much less than the original analysis indicated. As a result, changes in sulfur forms that might have been inferred from these earlier data are no longer apparent. Hence, the data presented here update, correct, and supersede information we have previously published elsewhere [14, 16] on the PCE extraction process.

3. RESULTS AND DISCUSSION 3.1. Effect of P C E extraction on inorganic sulfur and minerals in coal

MSssbauer data for a number of coals before and after PCE treatment are summarized in Tables 2 and 3. By and large, the differences between the

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F.E. Huggins et al./ Fuel Processing Technol. 35 (1993) 233-257

TABLE 2 M S s s b a u e r d a t a for P C E - t r e a t e d coals Sample

IS

QS

Fe

P h a s e I.D.

(minis) (minis) (%)

Spyr

(wt%)

Freeport-l, untreated

0.31 1.16

0.62 2.65

83 17

Pyrite Clay

1.95

Freeport-l, treated with PCE

0.32 1.15

0.63 2.66

84 16

Pyrite Clay

1.85

Freeport-2, u n t r e a t e d

0.32 0.39 1.15 --

0.61 1.06 2.81 --

45 38 12 5

Pyrite Jarosite Szomolnokite Clay/siderite

1.1

Freeport-2, t r e a t e d w i t h PCE

0.33 0.39 1.17 --

0.58 1.05 2.83 --

40 42 14 4

Pyrite Jarosite Szomolnokite Clay/siderite

1.1

Illinois 4#6 (III), u n t r e a t e d

0.31 ---

0.59 ---

97 2 1

Pyrite Clay/sulfate? Siderite?

2.2

Illinois 4#6 (III), P C E t r e a t e d

0.31 1.21 --

0.59 2.75 --

97 2 1

Pyrite Fe 2 ÷ sulfate Siderite?

1.9

Illinois 4#6 [EIU], u n t r e a t e d

0.31 0.38 1.25

0.61 1.15 2.71

81 10 9

Pyrite Jarosite Szomolnokite

1.7

Illinois 4~6 [EIU], PCE t r e a t e d

0.31 0.37 1.26

0.60 1.15 2.71

83 9 8

Pyrite Jarosite Szomolnokite

1.8

I n d i a n a 4#5-1, u n t r e a t e d

0.31 0.38

78 19 3

Pyrite Jarosite Clay/sulfate?

0.9

--

0.59 1.10 --

I n d i a n a 4#5-1 P C E - 4 0 min.

0.32 0.39 --

0.60 1.09 --

71 25 4

Pyrite Jarosite Clay/sulfate?

0.7

I n d i a n a 4#5-2, u n t r e a t e d

0.31 1.25

0.61 2.72

73 27

Pyrite Szomolnokite

3.6

I n d i a n a 4#5-2, t r e a t e d w i t h P C E

0.32 1.25

0.61 2.72

73 27

Pyrite Szomolnokite

3.7

Ohio 4#5/4# 6, u n t r e a t e d

0.32 0.37

0.61 1.10

73 23

Pyrite Jarosite

1.8

--

--

0.31 0.38

0.60 1.13

--

--

Ohio 4#5/~6 P C E - 4 0 min.

4

75 21 4

?

Pyrite Jarosite ?

1.6

241

F.E. Huggins et al./ Fuel Processing Technol. 35 (1993) 233-257 TABLE 3 MSssbauer data for Pittsburgh seam samples Sample

IS QS Fe (ram/s) (re_m/s) (%)

Phase I.D.

Spy~ (wt%)

P-l-l, untreated

0.30 1.14 1.24 0.31 1.14 1.26 0.30 1.16 1.27 0.31 1.15 1.24 0.30 1.14 0.30 1.15 0.30 1.15 0.29 1.13 0.30 1.14 0.30 1.14 0.31 1.14 0.29 1.13

Pyrite Clay Siderite Pyrite Clay Siderite Pyrite Clay Siderite Pyrite Clay Siderite Pyrite Clay Pyrite Clay Pyrite Clay Pyrite Clay Pyrite Clay Pyrite Clay Pyrite Clay Pyrite Clay

1.18

P-l-l, untreated PCE-treated P-l-l, oxidized

P-l-l, oxidized PCE treated P-2-1, untreated P-2-1, untreated PCE-treated P-2-1, oxidized P-2-1, oxidized PCE treated P-3-1, untreated P-3-1, untreated PCE-treated P-3-1, oxidized P-3-1, oxidized PCE treated

0.64 2.66 1.71 0.62 2.68 1.79 0.61 2.65 1.73 0.62 2.63 1.78 0.60 2.61 0.61 2.58 0.60 2.65 0.61 2.61 0.62 2.72 0.62 2.71 0.62 2.73 0.62 2.74

72 18 10 71 21 8 72 18 10 69 18 13 90 10 90 10 88 12 89 11 87 13 85 15 85 15 83 17

0.92

1.03

0.94

1.35 1.04 1.17 1.10 1.88a 1.32 1.89a 1.27

a Values determined about 12 months after crushing; spectrum at that time also exhibited appreciable amounts of szomolnokite, in addition to clay, indicating appreciable oxidation of pyrite to FeSO4-H20 during storage. samples before a n d after P C E t r e a t m e n t are w i t h i n e x p e r i m e n t a l u n c e r t a i n t i e s in b o t h the p e r c e n t a g e of i r o n in different phases (+__2%) a n d the w e i g h t p e r c e n t a g e of p y r i t i c sulfur ( _ 0 . 1 0 wt%), with the possible exception of the e x p e r i m e n t s c o n d u c t e d at the U n i v e r s i t y of K e n t u c k y , in w h i c h the PCEt r e a t e d samples a p p e a r to be s o m e w h a t lower in the a m o u n t of pyritic sulfur, b u t w i t h little or no c h a n g e in the a m o u n t s of the i r o n - b e a r i n g phases relative to e a c h other. This effect is most o b v i o u s for the P i t t s b u r g h P-3-1 sample. The p r e s e n c e or a b s e n c e of significant a m o u n t of i r o n sulfates is a r e a s o n a b l e d i a g n o s t i c test for coal o x i d a t i o n [20]. On this basis, all of the coals listed in Tables 2 and 3 w o u l d be classified as significantly oxidized except for the

242

F.E. Huggins et al./ Fuel Processing Technol. 35 (1993) 233-257

Freeport-I, the Illinois ~ 6 (III), and the t h r e e P i t t s b u r g h coals. The Illinois @6 (APCS) coal sample from the A r g o n n e P r e m i u m Coal Sample p r o g r a m as assumed to be unoxidized based on the well d o c u m e n t e d care t a k e n with these samples [24] and o u r own previous e x p e r i e n c e with samples from this p r o g r a m [25]. The o x i d a t i o n t r e a t m e n t on the t h r e e P i t t s b u r g h coals did not seem to a l t e r the iron-bearing minerals appreciably; no sulfates were n o t e d in the MSssbauer spectra of a n y of the P i t t s b u r g h coal samples before or immediately after the o x i d a t i o n t r e a t m e n t . Additional i n v e s t i g a t i o n of the effect of PCE e x t r a c t i o n process on coal minerals was u n d e r t a k e n by c o m p u t e r - c o n t r o l l e d s c a n n i n g e l e c t r o n microscopy (CCSEM). Two coals were e x a m i n e d by this t e c h n i q u e before and after PCE e x t r a c t i o n for 40 min: I n d i a n a @5-1 and Ohio @5/@ 6 coals. D a t a for b o t h coals are s h o w n in Table 4. Again, the differences before and after PCE e x t r a c t i o n are r e l a t i v e l y m i n o r and are w i t h i n e x p e r i m e n t a l e r r o r s for most categories. A l t h o u g h both coals exhibit a small r e d u c t i o n in p y r i t e in b o t h MSssbauer and C C S E M analyses, the difference is barely significant in e i t h e r analysis. The size distribution of all 1200 m i n e r a l particles d e t e r m i n e d in the C C S E M analysis for b o t h coals before and after PCE t r e a t m e n t is s h o w n in Fig. 2. The two d a t a sets show r e l a t i v e l y m i n o r differences in m i n e r a l size d i s t r i b u t i o n with respect to the PCE t r e a t m e n t , however, t h e r e is a t e n d e n c y for the m i n e r a l particles in the P C E - t r e a t e d samples to be s o m e w h a t less coarse. This t r e n d is more a p p a r e n t for the Ohio @5/@ 6 samples t h a n for the I n d i a n a @5 samples. B o t h the MSssbauer and C C S E M d a t a a p p e a r to show t h a t to first order, the PCE e x t r a c t i o n process has little effect on any aspect of the m i n e r a l matter, including the i n o r g a n i c sulfur forms. A possible e x p l a n a t i o n of the s e c o n d a r y differences n o t e d for p y r i t e in the experiments c o n d u c t e d at the U n i v e r s i t y of K e n t u c k y is s e g r e g a t i o n of l a r g e r and d e n s e r particles to the base l a y e r on the TABLE 4 Mineralogical analyses of two coals before and after PCE extraction. Wt% in ash Mineral

Quartz Kaolinite Illite Misc. silicates Pyrite Fe (II) sulfate Misc. sulf. Fe-rich Ca-rich Ti-rich Others, mixed

Indiana @5-1

Ohio @5/@ 6

As Rec'd

After PCE

As Rec'd

After PCE

15 6 22 28 18 1 1 2 1 1 5

18 9 21 26 15 1 2 1 1 1 5

23 1 11 17 23 2 9 -3 -11

27 2 11 27 19 1 6 -1 -7

F.E. Huggins et al./ Fuel Processing Technol. 35 (1993) 233-257

243

Indiana #5

Ohio #5/#6

Size DIstribution All Minerals

Size Distribution All Minerals Wt %

Wt %

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<2.5

(microns)

Before PCE After P C E Tr e _ ~ e r t t T r e . a ~ a n t

25-50 s.o-lo. 10-20 20-40 40-so Size Distribution (microns)

0 >80

Before PCE After PCE T re~ent

T rea~ent

Fig. 2. Size distribution profiles obtained from coal minerals analysis on CCSEM for Indiana #F5-1 and Ohio ~ 5] ~#6 coals.

filter paper during filtration of the PCE-treated coals. Unfortunately, the filter papers were discarded before this hypothesis could be tested by experiment.

3.2. Sulfur K-edge XANES spectroscopy All the samples listed in Table 1 were subjected to sulfur K-edge XAFS spectroscopy at NSLS. Two distinct methods of analysis were devised [23] for determining the effect of PCE on the forms of sulfur: sulfur XANES difference spectra and least-squares fitting of the sulfur XANES spectra.

3.2.1. Sulfur XANES difference spectra Since the same coal was examine({ before and after PCE treatment, it is a simple task to subtract the sulfur K-edge XANES spectrum for the PCEtreated sample from that for the original after normalization of the spectra to the edge-step. Such a normalization procedure is routinely carried out in the analysis of XAFS spectra. Difference spectra for coals before and after PCE treatment are shown in Figures 3-5 for three different coals. Positive peaks in the difference spectra, as shown in Fig. 3 for the Freeport coal, indicate forms of sulfur present in the original coal that are removed by PCE. All samples analyzed in this fashion generally gave one of two results: (i) there was no significant difference between the sulfur XANES spectra before and after the PCE treatment, as indicated in Fig. 4 for the Australian brown coal, Glencoe; or (ii) there was a positive peak at about 0.0 eV, as indicated for the Freeport coal in Fig. 3. There were some "inflection" peaks generated in

F.E. Huggins et al./Fuel Processing Technol. 35 (1993) 233-257

244

3.5

A

Indiana#5 Coal

/ ~

3 .Co 2.5 o

.a

2 1.5

z 0.5

~

Difference(A-S)

-0.5

Energy,eV Fig. 3. Sulfur XANES spectra and difference spectrum for Indiana ~5-1 coal before and after 40 minute PCE extraction in the laboratory.

4.5

GlencoeCoal A 3.5

2.5

~

1.5

Z

1

0.5

~

,~._ ~

_--~

~f r~nce ~ C )

-0.5

Energy,eV

Fig. 4. Sulfur XANES spectra and difference spectrum for Glencoe (Australian brown coal) before and after PCE extraction at the synchrotron.

this type of analysis, as indicated at about 10 eV in Fig. 3; these inflection peaks are attributed to a slight displacement (0.1-0.2 eV) in the energy scale between the two spectra. Such displacements are most likely due to a small difference in temperature of the monochromator crystals between the two spectra that alter the lattice constant of the silicon used for the monochromator crystals in the XAFS experiments. Unfortunately, for the soft X-rays at the sulfur K-edge, it is not yet possible to calibrate each spectrum individually to compensate for this effect.

245

F.E. Huggins et al./Fuel Processing Technol. 35 (1993) 233-257 3.5 Pittsburgh Coal 3.0 2.5 2.0

A: As received

1.5 1.0 :~

B:

PCE treated

0.5 Difference

-

0

4

8

1

16

20

Energy, eV

Fig. 5. Sulfur XANES spectra and difference spectrum for Pittsburgh 3-1 coal before and after 30 minute PCE extraction in the laboratory. Occasionally, other effects were noted in the difference spectra. If the sulfate peak was large, there was sometimes a significant negative peak noted in the difference spectrum at the sulfate peak position, indicating that the PCEtreated sample contained relatively higher amounts of sulfate sulfur than the original sample. Such differences were seen in coals that had undergone a significant time period between the PCE extraction and the XAFS measurement, indicating, perhaps, that pyrite in the treated coal may be somewhat more readily oxidized than that in the original coal. Finally, the difference spectra of the highest sulfur Pittsburgh coal indicated a procedural complication in the PCE extraction done in our laboratory. The difference spectrum for the original coal and the PCE treated coal is shown in Fig. 5. There is a positive difference in this spectrum, but the peak occurs at - 0 . 5 eV, rather than at 0.0 eV. This difference is therefore attributed to pyrite and not elemental sulfur and it appears that segregation has occurred with respect to pyrite during the PCE extraction. This result is consistent with the minor differences noted in pyritic sulfur in the MSssbauer results (Tables 2 and 3). With respect to the PCE extraction process itself, the twelve coals examined fall into one of three groups, depending on whether the elemental sulfur peak at 0.0 eV in the difference XANES spectrum is significant, marginal, or negligible: Significant Indiana ~5-1 Indiana ~5-2 U. Freeport-1 U. Freeport-2

(2.53% (7.75% (3.76% (3.00%

S) S) S) S)

Marginal

Negligible

Ohio # 5 / # 6 Illinois ~ 6 (III)

Glencoe Illinois @6 (APCS) Illinois @6 [EIU] Pittsburgh-l(1.68% S) Pittsburgh-2 (2.28% S) Pittsburgh-3 (3.52% S)

F.E. Huggins et al./ Fuel Processing Technol. 35 (1993) 233-257

246

The main conclusion from the difference XANES spectra is that some coals contain appreciable elemental sulfur that can be readily removed by extraction with PCE. Except for this particular form of sulfur, however, other spectral differences are relatively minor with respect to the extraction process or have their origin elsewhere.

3.2.2. Least-squares fitting of sulfur XANES spectra Least-squares fitting of the sulfur XANES spectra was carried out to quantify the forms of sulfur in the coal samples. This method of analysis has been well described in the literature [12, 16], and the most recently revised set of calibration coefficients [21], supplemented by a newly-derived coefficient for elemental sulfur, was used for converting areas under the peaks to percentages of sulfur in the different forms. Examples of the least-squares fitted sulfur XANES spectra of various samples are shown in Figs. 6 and 7.

& 2

m =m=== FDi?ta Z ~ ..... Edge j i \ . ~ ....... Peaks

Indiana #5 (2.53 wt% S)

~

1.6

j ~ l ~ T h i ° p hene÷ • :: " • lsu~oxido H " ~ lk

1.2

• org. s u l ~ I

0.8

Z

Eiem"S "

0.4

o

Pyrite ¢

~

2'

~"- : ' -

:

#

~ S u ~ n e

:Sulfate

:.~ ~

~ . . . ~ r ~ ' : . . C

-8

3.2 2.8

o

~

"

"-.......

m m n ~ P " ~ ~ •

c

[t ~

,

-4

•

,

0

,

,

,

"..............

,

,

4 8 Energy, eV

Indiana #5 after 40 min PCE (in situ)

2.4

°

12

,

•

16

A I I| i~ Data ~ m o ~ Fit | B ~, ....... ..... Edge I

2

~

1.6

1.2 ~ Z

0.8

0.4 0

I -8

-4

0

4 8 Energy, eV

12

16

Fig. 6. Least-squares fitted sulfur K-edge XANES spectra of Indiana ~ 5-1 coal before and after PCE extraction at the synchrotron.

F.E. Huggins et al./ Fuel Processing Technol. 35 (1993) 233-257

Freeport Coal (3.00 wt% S) before PCE treatment

A r;~

247

=====D~ta :-.::'~ Edges

3 o

2

E Z

•

-8

,

-4

,

,

0

,

,

,

•

4 8 Energy, eV

,

,

12

,

,

16

A = ~ Data Freeport Coal (2.85 wt°/o S) ~.~.] ~ F~ge after PCE extraction | . ~ ....... Peaks

3

o

"

2

E

1

o Z

.............

-8

-4

O

._.-:.......... -..:..

4 8 Energy, eV

12

16

Fig. 7. Least-squares fitted sulfur K-edge XANES spectra of Freeport-2 coal before and after PCE extraction at the University of Akron. With increasing energy, the peaks correspond to the labelled peaks in Fig. 6.

To assess the least-squares sulfur XANES method for forms-of-sulfur analysis, one set of four samples from the Pittsburgh seam (P-l-l) was analyzed in triplicate. The results of the least-squares fitting on these twelve spectra are summarized in Table 5. As can be seen from this table, the reproducibility of the analyses is on the order of _ 0.1 wt% sulfur, which compares favorably with more conventional forms-of-sulfur analyses. However, the pyritic-sulfur values derived from the least-squares fitting are significantly lower t h a n values obtained from wet chemistry or from MSssbauer spectroscopy (Table 6). As discussed elsewhere [22], the determination of pyritic sulfur by the leastsquares method is affected by particle-size considerations and large systematic errors can be introduced as a result of this factor. As a result, the most precise determinations of sulfur forms using the least-squares XANES method are based on independent determinations of pyritic sulfur obtained by alternative methods (ASTM wet chemical methods or MSssbauer spectroscopy). Such hybrid analyses [22, 23], consisting of the total sulfur determined by ASTM

248

F.E. Huggins et al./Fuel Processing Technol. 35 (1993) 233-257

TABLE 5 Triplicate results of least-squares fitting of sulfur K-edge XANES spectra of Pittsburgh coal samples subjected to PCE extraction. W t % sulfur forms Coal

Pyrite

Elem. S

Sulfide

Thiophene

Sulfoxide

Sulfone

Sulfate

As received

0.71 0.76 0.69 0.72

0.00 0.00 0.00 0.00

0.14 0.14 0.13 0.14

0.80 0.74 0.79 0.78

0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00

0.03 0.04 0.07 0.05

0.71 0.68 0.67 0.69

0.00 0.00 0.03 0.01

0.11 0.13 0.11 0.12

0.74 0.77 0.74 0.75

0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00

0.07 0.04 0.07 0.06

0.75 0.73 0.69 0.72

0.00 0.00 0.00 0.00

0.18 0.16 0.12 0.15

0.59 0.62 0.69 0.62

0.00 0.00 0.02 0.01

0.00 0.00 0.00 0.00

0.04 0.04 0.04 0.04

0.70 0.72 0.65 0.69

0.00 0.00 0.01 0.00

0.20 0.10 0.11 0.14

0.59 0.68 0.70 0.66

0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00

0.05 0.04 0.08 0.06

Average Oxidized at 105° C, 2 h Average PCE treated

Average Oxidized and PCE treated Average

TABLE 6 Comparison of forms-of-sulfur determinations by wet chemical methods and MSssbauer spectroscopy. W t % sulfur forms Coal

Pittsburgh Pol-1 As received Oxidized PCE treated Oxid., PCE Pittsburgh P-2-1 As received Oxidized PCE treated Oxid., PCE Pittsburgh P-3-1 As received Oxidized PCE treated Oxid., PCE

Total sulfur

Wet Chem.

MSssbauer

Lab 1

Lab 2

Average

Sulfate

Pyrite

Pyrite

1.73 1.71 1.59 1.63

1.62 1.55 1.51 1.49

1.68 1.63 1.55 1.56

0.03 0.03 0.03 0.02

0.97 0.93 0.88 0.82

1.18 1.03 0.92 0.94

2.39 2.40 2.04 1.85

2.17 2.46 1.96 1.77

2.28 2.43 2.00 1.81

0.01 0.01 0.02 0.02

1.28 1.24 1.05 0.76

1.35 1.17 1.04 1.10

3.39 3.81 2.54 2.40

3.65 3.51 2.21 2.30

3.52 3.66 2.38 2.35

0.05 0.05 0.09 0.10

2.47 2.42 1.23 1.42

1.88~ 1.89" 1.32 1.27

See footnote to Table 3.

F.E. Huggins et al.]Fuel Processing Technol. 35 (1993) 233-257

249

methods, the pyritic sulfur determined by wet chemistry or M5ssbauer spectroscopy, and the remaining non-pyritic sulfur subdivided according to the leastsquares XANES analysis, are summarized in Tables 7 and 8 for the samples investigated in this study. The weight percentages of the different forms of sulfur summarized in these tables are estimated to have uncertainties of about ___0.2 wt% sulfur, largely as a result of the uncertainties in the determination of total and pyritic sulfur. For the coals investigated in the laboratory, these hybrid forms-of-sulfur analyses were obtained on samples before and after the PCE extraction to determine specific forms of sulfur removed by PCE. As shown in Table 1, ten different laboratory extractions were done: five at the University of Kentucky, four at the University of Akron, and the other at Eastern Illinois University. The information on forms of sulfur for these experiments are summarized in Table 7. These results show that the forms of sulfur in some coals are not significantly altered by the PCE extractions, while in other coals, notably the Freeport (Fig. 7) and Indiana @5 coals, significant elemental sulfur is extracted. In some experiments, notably those at the University of Kentucky, there is apparent variation in both pyritic and total sulfur. However, the variation in total non-pyritic sulfur ( = total sulfur - pyritic sulfur) in these samples is much less and, hence, the variation in total sulfur and pyritic sulfur can be attributed to a sampling bias with respect to pyrite in the preparation of small sample aliquots. These forms-of-sulfur analyses in certain instances employ a consistency check on the analyses because the sulfate sulfur was determined independently both by the hybrid forms-of-sulfur analyses and the conventional chemical forms-of-sulfur analyses. A comparison of the values of the sulfate determined by both methods shows reasonable agreement (Table 9) for all samples and indicates that the hybrid forms-of-sulfur analyses are consistent with conventional methods. Four coals were investigated using the in situ apparatus directly at the synchrotron source. As summarized in Table 8, two of the coals, Glencoe and the APCS Illinois @6 coal, showed no significant difference in sulfur forms between the spectrum before and after PCE treatment (Fig. 4), whereas the other two coals, Indiana @5-1 and Ohio @5] @6, exhibited major and minor changes in sulfur forms, respectively, as a result of the PCE extraction. Figure 6 shows the least-squares fitted spectra for the Indiana @5-1 samples before and after 40 min. extraction in boiling PCE using the in situ apparatus at the synchrotron. As was found with the difference spectra, the major difference between these fitted spectra before and after PCE extraction is the decrease in intensity of the peak closest to 0.0 eV. For the Ohio @5] @6 coal, a much smaller difference was discerned for the same peak. It should be noted for the Indiana @5-1 coal that the peak at 0.0 eV was not reduced to zero in the 40 minute in situ extraction, in contrast to the static test (Table 7). Although this experiment performed well and showed qualitatively similar results to the static laboratory experiments, there was a major drawback with this technique that limited its usefulness. This was the requirement that the coal had to be made into a coherent pellet that preserved the sample geometry

Total S

3.76 3.56 3.56

3.00 2.85

4.41 4.39

4.46 4.46 4.06

2.53 2.55 2.55 1.94

7.75 7.61

Coal

Freeport-1 untr. PCE-tr. a PCE-tr. a

Freeport-2 untr. PCE-tr.

III. @6 (EIU) untr. PCE tr.

III. @6 (III) untr. a untr., w/o S a PCE tr.

I n d i a n a ~ 5-1 untr. PCE 20 rain2 PCE 20 rain2 PCE 40 min.

I n d i a n a @5-2 untr. PCE tr.

3.58 3.70

0.88 1.13 1.13 0.84

2.20 2.20 1.90

1.70 1.80

0.96 0.94

1.95 1.85 1.85

Pyrite b

0.51 0.00

0.33 0.10 0.00 0.00

0.11 0.00 0.00

0.00 0.00

0.28 0.00

0.63 0.17 0.00

Elem. S

0.65 0.77

0.27 0.26 0.31 0.25

0.67 0.80 0.62

0.93 0.86

0.08 0.10

0.29 0.32 0.31

Sulfide

Combined h y b r i d forms-of-sulfur a n a l y s e s of coal samples subjected to PCE e x t r a c t i o n . B a t c h r u n s c o n d u c t e d in laboratory. W t % Sulfur forms

TABLE 7

1.11 1.20

0.37 0.45 0.49 0.39

1.40 1.36 1.48

1.16 1.11

0.44 0.44

0.66 0.73 0.82

Thiophene

0.00 0.00

0.01 0.00 0.00 0.00

0.04 0.05 0.01

0.06 0.07

0.00 0.00

0.00 0.04 0.07

Sulfoxide

0.20 0.24

0.05 0.03 0.03 0.02

0.05 0.04 0.05

0.00 0.00

0.02 0.05

0.04 0.06 0.07

Sulfone

1.72 1.75

0.63 0.59 0.60 0.43

0.00 0.00 0.00

0.56 0.55

1.24 1.32

0.20 0.39 0.44

Sulfate

~

"

2. 0~

1.68 1.63 1.56 1.55

2.28 2.43 1.81 2.00

3.52 3.66 2.35 2.38

Pitt-l-l, untr. oxid. PCE ox., PCE

Pitt-2-1, u n t r . oxid. PCE ox., PCE

Pitt-3-1, u n t r . oxid. PCE ox., PCE

2.47 2.42 1.42 1.23

1.28 1.24 0.76 1.05

0.97 0.93 0.82 0.88

2.03 2.03 1.60 1.61

0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00

0.00 0.02 0.00 0.00

0.09 0.00 0.00 0.00

a A l t e r n a t i v e X A N E S fits w i t h a n d w i t h o u t e l e m e n t a l sulfur. b V a l u e s in italics o b t a i n e d from M S s s b a u e r s p e c t r o s c o p y [17, 18].

4.58 4.58 3.96 3.75

Ohio 4~5/@ 6 untr." u n t r . w/o S ~ PCE 20 min. PCE 40 rain.

0.15 0.14 0.08 0.11

0.14 0.16 0.11 0.15

0.10 0.09 0.14 0.11

0.53 0.55 0.59 0.50

0.78 0.95 0.60 0.74

0.78 0.97 0.91 0.72

0.57 0.55 0.56 0.51

1.22 1.29 1.13 1.12

0.01 0.02 0.00 0.02

0.01 0.02 0.00 0.03

0.00 0.00 0.01 0.00

0.00 0.00 0.00 0.00

0.00 0.01 0.04 0.05

0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00

0.10 0.10 0.21 0.23

0.07 0.05 0.04 0.05

0.04 0.04 0.03 0.05

0.70 0.71 0.64 0.52

b~

t~

~.

.~1 .~

5.70 (5.70)

4.83 (4.83)

2.53 (2.39)

4.58 4.58 (4.50)

Glencoe u n t r . ¢ PCE 55 rain.

Ill. @6 (APCS) untr. PCE tr.

I n d i a n a ~5-1 u n t r . P C E 40 vain.

Ohio ~ 5 / ~ 6 u n t r . a u n t r . w/o S a PCE 30 rain.

0.00 0.00

1.70 (1.70) 0.88 (0.88) 2.03 2.03 (2.03) 0.00 0.00

0.09

0.35 0.21

0.00 0.00

Elem. S

0.00 0.00

Pyrite b

0.53 0.55 0.53

0.34 0.27

0.94 0.88

1.83 1.90

Sulfide

1.22 1.29 1.30

0.39 0.38

2.11 2.16

3.09 3.16

Thiophene

0.00 0.00 0.00

0.02 0.03

0.00 0.00

0.23 0.12

Sulfoxide

0.00 0.00 0.00

0.04 0.05

0.00 0.00

0.13 0.13

Sulfone

0.70 0.71 0.64

0.50 0.56

0.08 0.09

0.42 0.38

Sulfate

V a l u e s i n p a r e n t h e s e s are a s s u m e d (see text)• a A l t e r n a t i v e X A N E S fits w i t h a n d w i t h o u t e l e m e n t a l sulfur. b V a l u e s in italics o b t a i n e d from M S s s b a u e r s p e c t r o s c o p y [17, 18]. Based on its lack of r e s p o n s e to t h e PCE e x t r a c t i o n , t h e p e a k in t h e s u l f u r X A N E S s p e c t r u m a t ~0.1 eV of t h i s A u s t r a l i a n b r o w n coal was i n t e r p r e t e d as due to a di- or polysulfide c o m p o n e n t r a t h e r t h a n e l e m e n t a l s u l f u r [21, 22]. T h e c o n t r i b u t i o n of t h i s p e a k is i n c l u d e d in t h e o r g a n i c sulfide form of sulfur.

Total S

Coal

In situ e x p e r i m e n t s at t h e s y n c h r o t r o n . W t % S u l f u r forms

Combined h y b r i d forms-of-sulfur a n a l y s e s of coal samples s u b j e c t e d to P C E e x t r a c t i o n .

TABLE 8

b~ CJ! b~

253

F.E. Huggins et al./ Fuel Processing Technol. 35 (1993) 233-257

TABLE 9 Comparison of determinations of wt% S as sulfate by conventional wet chemical and the hybrid forms-of-sulfur methods Coal sample

Freeport-2, untr. Freeport-2, PCE-tr. III. @6 (EIU), untr. Indiana @5-1 untr. Indiana @5-2 untr. Indiana @5-2 PCE-tr. Ohio @5/@6 untr. Pitt-l-l, untr. Pitt-l-l, ox., PCE Pitt-2-1, untr. Pitt-2-1, ox., PCE Pitt-3-1, untr. Pitt-3-1, ox., PCE

S (wt% as sulfate) Wet chemical

Hybrid

1.21 1.15 0.37 0.52 1.52 1.75 0.78 0.03 0.03 0.01 0.02 0.05 0.10

1.24 1.32 0.56 0.50, 0.63 1.72 1.75 0.70, 0.71 0.04 0.05 0.07 0.05 0.10 0.23

for the XAFS experiment after the PCE extraction. To make such pellets, a sulfur-free epoxy was used to bind the coal into pellet form as the pressed coal by itself would crumble in the boiling PCE. As a result, the PCE cont act with the coal was significantly reduced and the extraction of the sulfur did not appear to be as rapid as in the static l a bo rat ory tests due to either mantling of u n r eacted coal by a thin surface layer of epoxy or t hi cker and less penetrable surface layers of coal. Furthermore, the geometric necessity of preserving a co h er en t pellet, as well as the presence of epoxy, made it impossible to measure sulfur forms after the PCE extraction by wet chemistry or MSssbauer spectroscopy and relate them meaningfully to the extraction, which took place only at the pellet's surface. Despite this complication, the results obtained with the in situ method are consistent with the results of the static l aborat ory extractions. Spectral analysis of samples from both methods indicate t h a t only the peak nearest to 0.0 eV is significantly affected by the PCE extraction. Also, these results show t h a t possible concerns regarding alteration and oxidation of sulfur forms during the interval between the PCE ext r a c t i on in the l aborat ory and the XAFS measurements at the s y n c h r o t r o n are groundless. Although it was impossible to analyze conventional sulfur forms in the in situ pellets after PCE extraction, it is possible to estimate the forms-of-sulphur assuming (i) t h a t the only sulfur form removed is elemental sulfur; (ii) the sum of the weight percentages of sulfur in o t her forms is unaltered; and (iii) the pyritic sulfur c ont e nt remains the same. With these assumptions, it is possible to calculate the average loss of elemental sulfur from the surface of the

F.E. Huggins et al./Fuel Processing Technol. 35 (1993) 233-257

254

extracted pellets. The results of such calculations are shown in Table 8 for the four samples subjected to the in situ extraction procedure. Only the Indiana #e5-1 coal of the four coals so examined exhibited any significant change in sulfur forms.

3.3. Other observations 3.3.1. Sulfur X A F S of PCE extracts

For the Indiana @?5-1 coal, the sulfur XAFS spectrum of the PCE extract itself was measured. Furthermore, the sulfur XAFS spectra of dilute solutions of dibenzothiophene, dibenzylsulfide, and elemental sulfur dissolved in PCE were also recorded. These four spectra are compared in Fig. 8. In the vicinity of the white lines, the individual sulfur XAFS spectra of the solutions of the three compounds dissolved in PCE are very similar to the spectra of the corresponding crystalline solid materials, shown elsewhere [12, 26, 27]. This similarity indicates that the individual molecular structures of dibenzothiophene, dibenzylsulfide, and elemental sulfur persist in the PCE solution, and furthermore,

10 9

DBT/PCE 8 7 t-

.o Q,.

6

DBS/PCE

o .,Q "0

5

E

4

0 Z

Sulfur/PCE

3 2

Coal/PCE 1 0 -8

-4

0

4

8

12

16

20

Energy, eV

Fig. 8. Sulfur K-edge X A N E S spectra of solutions of three standard sulfur forms ( D B T - dibenzothiophene; D B S - dibenzylsulfide; elemental sulfur) and of the extract from the Indiana #?5-1 coal.

F.E. Huggins et al./ Fuel Processing Technol. 35 (1993) 233-257

255

that the sulfur present in the dissolved organic sulfur compounds remains in the organic structure and is not transformed into elemental sulfur during the extraction. The peak position and shape of the white line peak in the spectrum of the sulfur extracted from the Indiana 4~5-1 coal is virtually identical to that of elemental sulfur dissolved in PCE and differs significantly from those of the other two model compounds. These observations would appear to be convincing evidence that the sulfur extracted from Indiana ~ 5-1 coal is elemental sulfur and not an organic sulfur form. Independent GC/MS chromatographic determinations on PCE extracts from other coals have also shown that the major sulfur phase present in the extracts is elemental sulfur [4, 5, 7]. A similar XAFS experiment attempted on the PCE extract from the Pittsburgh P-3-1 coal showed no appreciable sulfur K-edge absorption. As this particular coal contains no elemental sulfur, this observation implies that all other sulfur forms, both inorganic and organic, resist the PCE extraction process. 3.3.2. Observations on the nature of elemental sulfur in coal

Most of the coals that contain significant elemental sulfur appear to be oxidized as they contain appreciable sulfate sulfur as well. Also, absorption peaks due to both ferrous sulfate (szomolnokite) and ferric sulfate (jarosite), which are common oxidation products of pyrite [20], are present in the MSssbauer spectra of such coals (Table 2). However, the MSssbauer spectra do not show evidence for iron oxyhydroxides in these samples. The lack of oxyhydroxide and the presence of ferrous sulfates in the coal suggest that the pyrite oxidation occurs under relatively constant conditions of humidity and temperature [20]. It is probable that such pyrite oxidation to sulfates also gives rise to the formation of elemental sulfur in the coals [11]. In this study, the sulfur forms in the Indiana # 5 coal have been examined by all available techniques: MSssbauer and sulfur XANES spectroscopies, CCSEM analysis, and conventional total sulfur and forms-of-sulfur analyses, and a major apparent inconsistency exists among the data from the different techniques. Although the sulfur XANES data show the presence of significant elemental sulfur in this coal, there was no evidence for elemental sulfur in the CCSEM analysis (Table 2). To reconcile these results, we deduce that the elemental sulfur must be of extremely small particle size (<<1/~m) and well dispersed throughout the coal matrix so that it is not detected in the CCSEM technique, which is designed only to detect discrete (> 1/~m) mineral particles [19]. This deduction is also consistent with the use of solutions of sulfur/DBT mixtures for maximum dispersion of the sulfur standards for the calibration procedure.

4. CONCLUSIONS Despite the fact that PCE will dissolve significant amounts of simple organic sulfur compounds, analysis of sulfur K-edge XAFS spectra indicates that the

256

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organic sulfur forms present in the coals examined in this investigation are not appreciably affected by extraction with boiling PCE. The results of this study show that, within the accuracy of the measurement, elemental sulfur is the only sulfur form found in these coals t hat is significantly extracted by PCE. However, the mechanism of formation of elemental sulfur in these coals has not been investigated and the possibility t hat the elemental sulfur in these coals arises from organic sulfur forms due to an oxidation process prior to analysis is not eliminated. Other researchers [8-10] have discussed whether PCE extraction could be the basis for a specific analytical procedure for elemental sulfur in coal. The results presented here seem to confirm its usefulness for such an analytical procedure because all other major sulfur forms in coals examined in this study do not appear to be extracted to any appreciable extent by PCE.

ACKNOWLEDGEMENTS We are grateful to Prof. S. Lee, University of Akron, and Prof. D.H. Buchanan, Eastern Illinois University, for donation of samples for this investigation, to Dr. B. Ganguly for assistance with the MSssbauer measurements, and to Drs. F. Lu, J. Zhao, and M.M. Taghiei for assistance with the XAFS measurements at NSLS. Conventional total sulfur and forms-of-sulfur analyses were performed at the K e n t u c k y Geological Survey and University of Kentu ck y Center for Applied Energy Research, under the supervision of H enry Francis and Robert Keogh, respectively. This study was supported by the Office of Exploratory Research of the Electric Power Research Institute (Palo Alto, CA) under EPRI Contract No. RP-8003-20. The XAFS spectra were obtained at the National S y n c h r o t r o n Light Source at Brookhaven National Laboratory, which is supported by the U.S. Department of Energy.

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