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Identification of some sulphur species in a high organic sulphur coal
Identification of some sulphur species in a high organic sulphur coal J. P. Boudou, J. BoulQgue, and J. J. Boon**
L. Makchaux”,
M. Nipt, J. W. de Leeuwt
CNRS-UA 196, Laboratoire de Geochimie et Metallogenie, 4 Place de Jussieu, 75252 Paris, Cedex 05, France * Cerchar, BP No. 2, 60550 Verneuil-En-Halatte, France t Delft University of Technology, Department of Chemistry and Chemical Engineering, Organic Geochemistry Unit, De Vries van Heystplantsoen 2, 2628 RZ Delft, The Netherlands ** FOM Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands (Received 30 October 1986; revised 9 March 1987)
In this paper, a preliminary study is described concerning the characterization of sulphur forms in a subbituminous coal rich with organic sulphur deposited in a lacustrine carbonate environment (Upper Cretaceous, Provence, France). Two American high organic sulphur coals were studied for comparison with the Provence coal. Optical microscopy, scanning electron microscopy and electron microprobe approaches offered a global view of the relations between sulphur, metals and coal petrography. Sulphur occurs in all the macerals and most minerals. Vitrinite contains the major part of organic sulphur and metals. This common occurrence of organic sulphur and metals in vitrinite suggests the presence of organosulphur-metallic species. X-Ray photoelectron spectrometry showed that mineral and organic sulphur is very much reduced (divalent). It would mainly include pyrite, sulphides and thiophenes. Programmed temperature oxidation and programmed temperature reduction (Attar’s test) revealed fragile sulphur compounds such as aliphatic thiols and sulphides and thermal stable sulphur compounds such as aromatic and high molecular weight compounds. Curie-point pyrolysis in combination with mass spectrometry, gas chromatography, and gas chromatography-mass spectrometry indicated the absence of free organic sulphur compounds and of elemental sulphur. Pyrolysis yielded large amounts of low molecular weight products (H,S, COS, etc.) and smaller amounts of thiopenes, benzothiophenes, dibenzothiophenes and their alkylated homologues. These sulphur compounds could result from the thermal degradation of organic sulphur moieties of the coal as well as from secondary reactions. (Keywords: coal; sulphur; instrumental methods)
The behaviour of organic sulphur compounds associated with the world-wide utilization of fossil fuels is of great importance for the quality of the environment’. Studies dealing with the organic sulphur chemistry of fossil fuels have increased over the past decades and new analytical techniques have been applied over the past few years. These techniques, particularly l.c./s.f.c./g.c.-m.s., have brought a fairly good understanding of chemistry of the lighter sulphur species in rock extracts and oilszm6. However, these identifiable extractable sulphur molecules only represent a very minor part of the total sulphur. So we cannot consider that oil sulphur compounds are totally known. Kerogen and coal sulphur have been investigated by several new techniques: solid spectroscopy’, oxidative and reductive slow pyrolysiss9, thermoreduction in the presence of solvent”, flash pyrolysis-(g.c.km.s.’ l, etc. Nevertheless, there appears to be no really satisfactory means of determining forms of organic sulphur in a coal so that little is known about the distribution of sulphur functional groups in kerogen and coal. In this paper we report preliminary results dealing with the identification of sulphur compounds in a 001~2361/87/111558-12$3.00 0 1987 Butterworth & Co.
1558
(Publishers) Ltd.
FUEL, 1987, Vol 66, November
representative complementary
high organic sulphur analytical methods.
coal
using
MATERIAL The French Provence coal sample (analysis in Table 1) studied here was obtained from the coal minibank of Cerchar. This coal was deposited in a fluviolacustrine environment in a humid climate during the Upper Cretaceous. This humid climate of deposition was followed by dry weather conditions as shown by a sapropelic and pyritic limestone aquifer which overlays the coal Provence seam12. A sample was collected in a fresh cutting of the Quartier de l’Etoile, taille 9 of the Grande mine. The coal sample was kept under water until grinding. Grinding was done under argon down to size (r3 mm. In the laboratory, the sample was stored under argon at -25°C. The main chemical characteristics of the sample are displayed in Table 1 and the distribution of vitrinite reflectance in oil is shown in Figure 1. The petrographic analysis showed quite narrow layers (25-100pm) of
Identification Table 1
Volatile
Analysis
matter
of coals studied
(wt %, dry)
T,,, (“Qb Organic C (wt i”,, dry)b H Index (wt % org. C)” 0 Index (wt % ore. 0 ’ Ash (wt i dry)” CaO (wt % dry) Calcite (wt “/, dry) Total S (wt %. dry) Pyrite S (wt %. dry) Sulphate S (wt Y;, dry) Organic S (wt S,. dry)d a b ’ d
of some sulphur species in a high orgainc sulphur coal: J. P. Boudou
Provence
Muskingum
Meigs
45 415 53.1 26.3 0.9 20 8.9 6.0
41 420 65.2 28.5 0.3 15
42 418 53.2 29.5 0.3 35
4.51 1.00 0.06 3.57
5.30 2.85 0.05 2.40
4.80 2.95 0.15 1.70
Measured by ASTM methods Measured by Espitalie’s methods* Measured by Bernard calcimetry Organic S = Total S - Mineral S
vitrinite (60.1 vol. ‘%) mainly impregnated with resinite. Sporinite and cutinite (6.7 vol. %) are concentrated in layers (5-100 pm). The inertinite (14.9 vol. %) is either finely mixed with the liptinite in the vitrinite (inertodetrinite), or in the form of layers of semi-fusinite and fusinite. There is some sclerotinite too. The mineral matter (18.3 vol. %) includes clays, carbonates and sulphides. The clays are very finely dispersed in coal, mainly in the liptinite. The carbonates till holes, cracks and little faults appear in the form of a big heap in the vitrinite or as thin layers in the liptinite. The macroscopic sulphides (pyrite) are rare (1 vol. %). Two American coals (analysis in Table 1) were taken in order to better assess PTR and Curie point pyrolysis methods. Their programmed temperature reduction kinetograms are very different from each other, and they are currently used by Dr A. Attar as coal standards for PTR analysis. These coals from the Appalachian coal basin were deposited in marine deltaic environments during the Carboniferous. They reveal characteristics of marine coals, and allow comparison of the fluviolacustrine Provence coal with organic sulphur rich fluviomarine coals.
of organic sulphur and metal. The accelerating voltage was kept at 20 keV. As our material has a poor thermal conductivity, heat generation under electron beam induced thermal degradation of the sample. This problem was reduced: (a) by defocusing the electron beam to produce a larger spot diameter (2-5 pm); (b) by decreasing the beam current intensity to 1&40nA; and (c) by lowering the counting time to 5-10 s. Pyrite was used as a standard for sulphur and iron. Matrix effects are very important, ZAF corrections give a remained semilow precision, so measurements quantitative and comparative. X-Ray photoelectron spectroscopy
Coal samples were examined in a Surface Science Laboratories (USA) spectrometer using an Al-Ku radiation source with a base pressure of 5 x 10d9 Torr. A monochromator was used to give a narrower X-ray line by taking only a portion of the K band. The spectrometer was run at a pass energy of 25 eV. The specimens were examined as powders deposited in a thin layer in a metallic boat and introduced into the spectrometer. The diameter of the spot analysed was 600 pm. An electron gun was used to compensate the charging of the sample due to photoionization. Szp binding energy was referenced to the C rs peak at 285 eV. Carbon was recorded in 11 min (interval of binding energy 15 eV recorded on 128 channels during 10 cycles) and sulphur was recorded in 90 min (interval of binding energy 15 eV recorded on 128 channels during 80 cycles). Iron (2~) could not be detected because of a lack of sensitivity. Programmed temperature oxidation (PTO)
PTO was performed according to the method of Chantretr3. This method is based on the concept that the differences in reactivity of sulphur, carbon and mineral species can be made by following the evolution of SO,, CO, and the differential thermal analysis curve during combustion. A flow of air (100 ml min-‘) was passed
Average : 0.44 Standard devratian
METHODS Scanning electron microscopy-energy analysis (SEM-EDAX)
et al.
: 0.05
dispersive X-ray
Coal samples were mounted on metallic discs and coated with carbon for SEM examination with a Jeol Model JSM 2 equipped with an EFOS, EG & G Ortec energy dispersive X-ray analyser (EDAX). The instrument had a point to point resolution of 0.1 pm. The resolution of the EDAX, however, is of the order of l20 pm, owing to electron scattering in the region analysed. Optical microscopy-electron
probe microanalyser (EPM)
The sample was prepared as a block (% 2.5 cm on edge), or was mounted on a glass slide in thin section form. The surface was polished and subsequently coated with carbon for examination. The sample could be viewed by means of a light microscope incorporated within the electron microprobe analyser. A Camebax electron microscope was used to make quantitative measurements
R, 1%)
Figure 1 coal
Statistical
distribution
of vitrinite
FUEL, 1987,
reflectance
in the Provence
Vol 66, November
1559
Identification
of some sulphur species in a high orgainc
through the coal sample finely ground and mixed with kaolin fired to 1200°C. The sample was heated from ambient to 1050°C in a stream of air at a rate of Pyrite (2%) was added to the sample to 10°C mini. determine the combustion temperature of pyrite. The reproducibility of the method is good enough to allow for quantification. This method has been used successfully to identify sulphur species in soils and sediments3’q3’. Progrummed
temperature
reduction
(PTR)
Attar” and Attar rt a1.14 have developed a test to characterize the distribution of several organic sulphur groups in coals by a programmed temperature reduction to H,S. Coal is heated in a hydrogen solvent mixture in the presence of catalysers. At various temperatures, discrete H,S peaks are detected which presumably originate from different organic entities in the coal. H,S peak area would be proportional to the sulphur amounts in coal. Pyritic sulphur can be reduced ~ particularly sulphur in line pyrite grains. Hence removal of pyrite before thermoreduction improves the resolution”. In this work we used almost the same experimental conditions as the ones described by Attar’” (sample weight, 1(r30 mg; heating rate, 5°C min - ’ ; gas flow rate, 60ml min-‘. , solvent mixture, HCL washed 120 mesh dry coal, 5G-100 mg of resorcinol, 200 mg of phenanthrene, 50mg of 9,1Odihydroxynaphthalene, 5OOmg of pyrogallol impregnated into the coal from methanol solution; catalyst, sulphided CO-MO Horshaw 402 catalyst). Sulphur-containing polymers can be used to show the kinetogram location where each supposed sulphur entity is reduced. They are also used to measure, by calibration of the PTR curves, the abundance of the organic sulphur forms. The sequence of appearance of functionalities and the temperature ranges within which they appear are: disulphides (not distinguishable from adsorbed H,S and elementary sulphur), < 160°C; aliphatic thiols, 16& 210°C; aryl thiols, 20&24O”C; aliphatic sulphides, 27& 330°C; alicyclic sulphides, 32&36o”C; aryl sulphides, 350-400°C; simple thiophenes, 40&5OOC; complex thiophenes, >45o”C (can be partially determined only when operating under pressure; otherwise they are calculated by difference). Curie point pyrolysis methods Three Curie point pyrolysis methods were applied. Curie point pyrolysis mass spectrometry using the FOMautopyms was used for fingerprint analysis of the coal pyrolysate. Several Curie point temperatures were tried: 358°C for evaporation; 510, 770 and 980°C for pyrolysis. A recent description of the method for analysis of coal is given by Tromp16. The mass range of the mass spectrometer used in this study was 2&220. The pyrolysis chamber and the expansion chamber were heated to 160 and 2OO”C, Pyrolysis gas chromatography was respectively. performed in a reactor described by Van der Meent “. The pyrolysis unit was mounted in the injection block of a Varian 3700 gas chromatograph system, equipped with an FID and FPD detector. The pyrolysates were separated on a fused silica capillary column coated with CP-SIL 5, 50 x 0.32mm i.d., programmed from 0°C (held for an initial 5 min period) to 300°C at 3°C min -I. The final temperature was held for 20 min. Helium was used as a carrier gas. The Curie temperatures of the wires
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FUEL, 1987, Vol 66, November
sulphur
coal: J. P. Boudou
et al.
used were 358, 510, 610 and 770°C. Curie-point pyrolysis-gas chromatography-mass spectrometry was performed on a Varian 3700 gas chromatograph combined with a MAT 44 quadrupole mass spectrometer. Mass spectrometric analyses were performed in the electron ionization mode at 80 eV electron energy, ion source temperature 250°C cycle time 1 s, mass range 255 400 daltons. Since the EI detection is not as specific for sulphur compounds as the negative chemical ionization mode with ammonia l8 , both the TIC and the Curie-point pyrolysis-g.c.-FID traces were used to identify the g.c.FPD peaks.
RESULTS Microscopic
AND DISCUSSION upprouch
Scanning electron microscopy enabled identification of macerals (as shown in Figure 2). It revealed some minerals -such as metallic sulphides ~ mainly in the form of very small grains (0.5 pm) finely scattered in the organic matrix. In the Provence coal, pyrite is so sparsely dispersed in very small submicrometre-size particles that only 27% of the pyrite could be removed by column flotation, which generally efficiently removes >50”/, of pyrite from most bituminous coal. Scanning electron microscopy also showed some rare crystals of anhydrite - or gypsum - in the vicinity of framboidal pyritic concretions (average diameter 1(r 15 pm). These CaSO, crystals may result from oxidation of pyrite in close contact with calcite before or during storage’ 9. Electron microprobe measurements (Table 3) showed that metal concentrations are locally very high in comparison with ASTM measurements (Table 2). The metal distribution determined by petrographic and EPM measurements is shown in Table 3. Some metals such as iron, lead and molybdenum are particularly abundant in pyrite. Other metals such as germanium, with low abundance in pyrite, are particularly abundant in vitrinite. This heterogeneity can be explained by the presence of fine minerals and organometallic complexes which seem to be absent in the EPM spot (2 pm diameter) under the optical microscope. Optically unobservable minerals and macerals may also be probed below the surface of the polished section: with an accelerating voltage of 20 keV, the depth of electron penetration is z 1 pm in the pyrite and 4pm in the organic matter. As seen in Table 3, sulphur levels showed small intramaceral differences. Gradation in sulphur surrounding pyrite particles, which may result from the pyrite formation2’, was not observed. The average concentrations of sulphur in inertinite tends to be about half that in vitrinite, while in liptinite it approaches more closely the level observed in vitrinite. Sulphur, unlike metals, appeared to be homogeneously distributed throughout the macerals, implying an association on a molecular base. Owing to the absence of elemental sulphur present (as shown below in the section Curiepoint pyrolysis) and the inability to locate crystals and amorphous deposits of this phase, no distinction has been made in the reported analyses between elemental sulphur and organically bound sulphur. If we assume hypothetically that all the metals are present in the form of mineral sulphides finely scattered in the organic matrix, these metals could only account for a very small fraction
Identification
of some sulphur
species
in a high
orgainc
sulphur
coal: J. P. Boudou
et al.
building blocks ~ connected by alkyl and heteroalkyl alkyl substituents and functional bridges - bearing groups - mainly those containing oxygen - as described for type III kerogen”. Spectroscopic
Figure 2 SEM micrographs of the Provence fusinite particles; (b) region with vitrinite microcrystals embedded in vitrinite
Table 2 Metals in Provence coal (measured expressed in /Ig/g of the coal as received) ~. -~~~ Fe 8125 Ni MO 36.0 Se CU 5.0 Zn Hg 0.1 co As 2.5 Cd
coal: (a) region with particles; (c) pyrite
by ASTM
methods,
7.0 1.0 10.0 3.0 0.6
of the total sulphur. Therefore we can think that, for the greatest part, sulphur is associated primarily with the organic matrix in the coal macerals. Most of the sulphur and trace metals are associated with vitrinite (Table 4), i.e., with macromolecules including mainly small (poly)aromatic and heterocyclic
approach
(XI’S)
The XPS Sap spectrum of the Provence coal only shows a wide and slightly asymmetrical peak that includes the s s 283,2 peaks (in the ratio l/2). This peak spreads fr%?i62 to 166 eV (Figure 3), a region which includes the pyritic sulphur (162-163 eV), the reduced organic sulphur species between (163-165 eV) and the sulphoxides (165166 eV). The maximum of S,, occurred at a binding energy of 164 eV. A small flat peak is visible centred around 170 eV indicating the presence of mineral sulphates - as shown by elemental and microscopic oxidized organic sulphur forms analyses ~ and/or (sulphones, sulphites, sulphates) in trace amounts. FT-i.r. spectroscopy of the Provence coal did not show any sulphoxides. All attempts to resolve the XPS S,, spectrum peak, maximizing at 164 eV by means of either mathematical deconvolution or increase of the instrumenlal resolution using a monochromator and a multidetector, failed. These data only showed that the Provence coal does not contain, or if so only in small amounts, any oxidized sulphur groups such as RSO,R, ROSOzR or ROSO,OR, which occur above 166 eV. Apart from peat and its humic compounds, where the most of the sulphur would be present in sulphate esters22, coal XPS S,, spectra present only one peak when the coal has not been oxidized 23-2 5. According to Jones23, the peak of the vitrinite (from high-volatile XPS s,, bituminous coal) would mainly arise from sulphides and thiophenes. Programmed
temperature
oxidation
Differential thermal analysis and CO, evolution curves are presented in Figure 4. DTA and CO2 maxima occur in three intervals: the first two exothermic ones correspond
FUEL, 1987, Vol 66, November
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Identification
of some sulphur species in a high orgainc sulphur coal: J. P. Boudou
et al.
Table 3 Electron microprobe traverses showing profiles of sulphur and metal concentrations in the Provence coal (V: vitrinite, E: exinite, P: pyrite. Elements are quoted by decreasing order of average concentration in pyrite in units of rig/g)) Traverse No. 1 Distance brn) from the left-hand spot 0 v g (wt %) Fe Pb MO Bi CU Hg As Ni Se Ge co Zn Cd
7.5 v 5.4
450 0 0 0 90 0 140 420 0 0 0 170 0
16 v 5.3
160 1200 0 0 0 0 0 0 140 0 210 0 0
23 v
5.2 550 0 0 110 180 170 0 0 0 0 210 0 0
30 V 5.1
400 0 0 420 40 1040 0 0 0 0 0 0 0
35 E 5.0
420 0 0 0 0 520 0 50 0 0 260 390 0
38 E 5.2
170 1200 0 1160 0 0 0 0 0 0 40 0 600
Traverse No. 2 Distance (pm) from the left-hand spot 42 V
3.8 0 0 0 320 0 0 50 0 0 150 0 0 660
50 v 5.0
0 2640 0 0 0 870 0 0 80 1370 340 0 0
0 V 5.4
0 1200 350 420 490 0 0 370 10 0 170 170 0
5 v 4.9
50 970 0 0 1010 0 0 310 0 150 130 590 0
11 v 5.0
0 970 0 0 460 0 240 0 60 0 90 110 0
5.4 330 0 0 540 0 0 0 680 0 0 170 0 180
18 v
22 P 5.0
340 1460 0 540 0 180 80 0 70 150 90 60 610
27 V 53.8
36 V 5.5
44.1” 300 3070 3150 0 0 2040 0 1660 0 1920 0 160 0 1330 30 0 120 1320 0 0 380 0 110 0 60
5.5 990 0 0 210 0 530 0 0 0 0 0 260 0
a Content expressed in wt %
Table 4 Petrographic distribution of sulphur and metals in Provence coal. Total content is expressed in wt % of the total coal mass. Element content in the total coal is calculated by multiplying the element content in the pyrite or maceral by the wt % of pyrite or maceral group in the total coal
(I 5 measurements)
Pyrite (wt %)
Vitrinite (wt %) (35 measurements)
Exinite (wt %) (15 measurements)
S
20.8
62.0
10.2
7.0
Fe Pb MO Bi cu Hg As Ni Se Ge Zn co Cd
97.8 13.5 32.7 12.9 9.4 3.5 21.8 6.1 6.2 2.3 3.5 2.8 0.9
1.8 70.8 34.5 63.9 64.1 63.0 37.5 60.2 47.5 74.6 69.0 65.4 67.5
0.1 12.2 16.9 13.4 17.9 21.7 18.7 9.2 17.5 13.8 6.1 7.4 19.8
0.2 3.4 15.7 9.6 8.5 11.6 21.8 24.4 28.7 9.2 21.2 24.2 11.7
to the decomposition of the organic carbon and the last small exothermic one to the decomposition of calcite. SO, maxima occur in three temperature intervals centred near 375°C (coinciding with the first CO, shoulder), 460°C (at an intermediate position between the first CO2 shoulder and the main peak and 500°C (coinciding with the main CO, peak). Another small peak occurs at 580°C. The sharp peak at 460°C increased with addition of pure pyrite. Lacount et u/.~, using slightly different experimental conditions (heating rate 3°C min-’ in a stream of argon containing 10% oxygen) found in addition three main peaks at 320°C 430°C and 480°C from various coals. These authors structurally assigned the SOz peaks by means of model compounds: the first representing the cyclic sulphides (tetrahydrothiophene), the second the pyrite, and the third the aryl sulphides and thiophenic sulphur. Programmed temperature reduction
The sulphur distribution in the Provence coal resulting from this analysis is given in Figure 5 together with the sulphur distribution of the two US coals taken as reference. Sulphur distribution in the Provence coal is
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FUEL, 1987, Vol 66, November
Inertinite (wt %) (15 measurements)
markedly different. Thiophenic compounds and high molecular weight sulphur compounds are less abundant in the Provence coal than in the reference coals. The PTR test showed that the higher the organic sulphur content, the easier it is to convert to H,S. Curie-point pyrolysis
Py-m.s. analysis of the Provence coal was performed at different Curie temperatures (358, 510, 610, 770 and 980°C). Evaporation of components adsorbed on or trapped in the coal matrix takes place at the lowest pyrolysis temperature (358°C) while at higher temperature Py-m.s. of the coals shows peaks characteristic of pyrolytic fragments: phenols, aromatics and aliphatic hydrocarbons give large mass spectral peaks at m/z 34,48, 60, 64 and 76, while heavier sulphur fragments shown lower abundance (Figure 6). The light sulphur fragment peaks are thought to reflect, respectively, H,S, CH,SH, COS, SO, or S,, and CS, in the pyrolysatez6. H,S, which is the most abundant sulphur compound in the pyrolysate, can result from secondary reactions of sulphur species in the gas phase during pyrolysis. These secondary reactions produce
Identification
I
I
297
293
of some sulphur
species
in a high
sulphur
coal: J. P. Boudou
I
1
I
I
orgainc
289
et al
I
285
car. BE(eV)
S2P
174 Figure 3
X-Ray
172 photoelectron
170 spectra
of the Provence
cor!TE
(eV)
coal obtained
CS,, so in most coals the m/z 64 and 76 peaks are intense at pyrolysis temperatures of 770 and 980°C owing to gasphase reactions after pyrolysis. This is not the case with the Provence coal, for which the m/z 34 peak is predominant; H,S would then result from the degradation of thermolabile organic sulphur compounds such as thiols and organic sulphides. Py-g.c. with simultaneous FID/FPD detection yielded a rather complex pyrolysate (Figure 7, FPD traces shown). The virtual absence of pyrolysis products at 358°C indicates that the organic sulphur compounds present in the other Py-g.c. traces (at 510,610and 770°C) are true pyrolysis products generated from higher molecular weight substances. The Py-g.c. traces indicate that the composition of the pyrolysate does not change much using different Curie temperatures. The yield of pyrolysate seems to increase with higher Curie temperatures, probably owing to a higher input of energy. The heaviest sulphur compounds are released at the highest pyrolysis temperatures. Py-g.c.-m.s. analysis was used to identify the pyrolysis products. It showed that no elemental sulphur was present in the coal. This proves that our samples are not oxidized - Stock2’ having recently shown that elemental sulphur is an oxidation artifact in coal. Among the
166
164
162
160
using an X-ray monochromator
compounds released at 770°C we identified numerous sulphur compounds by means of mass spectrometry and g.c. retention time (Figure 8). These compounds are CH,SH, thiophene, benzothiophene, dibenzothiophene and the C,, C, and C, alkyl-, alkylbenzo- and alkyldibenzothiophenes. These thiophenic compounds have been frequently identified in solvent extracts of other coals. Despite the complexity of the sulphur lingerprint, there is almost no qualitative difference ~ at a similar degree of maturation - between the pyrolysates of the Provence coal and several other coals of variable origin, mineral and sulphur composition (Meigs and Muskingum in this paper). However, we noticed that the heaviest volatile sulphur compounds, i.e., alkyldibenzothiophenes, give different g.c.-FPD traces for the Provence coal and the two US reference coals (Figure 9). ‘Normal’ coal generates volatile matter by cleavage of oxygen functions (evolving CO, and H,O) and by carboncarbon cleavages. During pyrolysis, sulphur-rich coals may also break at weak sulphur linkages and produce larger fragments leading to different tar and hydrocarbon composition, as observed for type II-S kerogens 28*29. Also it seems there are some structural differences between type II-S kerogen and coal: Curie point pyrolysis of the Provence coal, as well as of the US
FUEL, 1987,
Vol 66, November
1563
Identification
of some sulphur species in a high
orgainc
sulphur
coal: J. P. Boudou
et al
during slow and flash coal pyrolysis would require a study of a large number of both high and low organic sulphur coals. This point will be examined in a forthcoming paper.
reference coal, did not release any thiophanes, thianes or terpenoid sulphides which occur in high sulphur oils2,4,6 or sediments (Organic Geochemistry Unit, Delft University) as a result of their high degree of reduction. An accurate examination of hydrocarbon generation
GENERAL
DISCUSSION
For (I better assessment of the value and the specificity the methods used to analyse coal sulphur
Orgc
of
Most previous work on coal sulphur has applied one particular technique for different coal samples. In this preliminary study various kinds of techniques have been applied to a single coal sample. The methods used can be classified by increasing order of specificity and by decreasing order of quantification: ASTM elemental analysis, XPS, microscopy, PTO, PTR and Curie-point Each particular method complements pyrolysis. information previously found by other techniques. Methods such as elemental analysis, XPS and microscopy
8
7+8 I 420
210 Temper&we 6iO
240
PC
)
Figure 5 Programmed temperature reduction of the Provence coal and of two reference coals. The diameter of the circle is proportional to the organic sulphur content in the coal. Hypothetical structural assignments: 1, adsorbed H,S and polysulphides; 2, aliphatic thiols; 3, arylthiols; 4, aliphatic sulphides; 5, alicyclic sulphides; 6, aryl sulphides; 7, simple thiophenes; 8, aryl sulphides and thiophenic compounds in condensed high molecular weight structural moieties
Temperature PC)
Figure 4 Programmed temperature oxidation kinetograms of the Provence coal. Hypothetical structural assignments: A, thermolabile organic sulphur structures; B, pyrite; C and D, arylthiophenic structures and aromatic sulphides
34 (H$S) 108
(Ph) 94
(C,) 56 42
I 70
I
40
60
82
80
I
Mass
Figure 6
1564
Curie-point
pyrolysis
FUEL, 1987,
(770”Ctm.s.
of the Provence
Vol 66, November
coal obtained
122
I, 120
136
140
number using FOMautopyms
160
180
of some sulphur species in a high orgainc sulphur
Identification
coal: J. P. Boudou et al. Py (358°Ckg.c.-fPD
* ._.--dl-L
L_--_&&U Py(358’Cbg.c-FID
JL_L_-_ Ph
I
Py(510°C)-qt.-FID
m
Py(61OW-g.c.-
FID
Py(770°C)-g.c.-FID
Py(610°C)-g.c.-
FPD
Py(7700CLg.c.-FPD
Time -
Time -
Figure 7 Curie-pointpyrolysis(258,510,610,770”Ctg.c. traces of the Provence coal. For assigned structures see ~(gurc 8
I I c-2 s
4
2
It a7
S
5
I
Cl &
Musklgum
S 6
Figure 9 Partial Curie-point pyrolysis (770”Ctg.c.-.FPD traces of the alkyldibenzothiophenes of the Provence coal and of two reference coals. For assigned structures see Figure 8
II
12
Figure 8 Structures of sulphur compounds in the pyrolysates: 1, thiophene; 2, methylthiophene; 3, C2 alkylthiophene; 4, C, alkylthiophene; 5, benzothiophene; 6, methylbenzothiophene; 7, C, alkylbenzothiophene; 8, C, alkylbenzothiophene; 9, dibenzothiophene; 10, methyldibenzothiophene; 11, C2 alkyldibenzothiophene; 12, C3 alkyldibenzothiophene
give the distribution of the total sulphur, but only a very poor chemical characterization. Curie-point pyrolysis gives an accurate picture of some organic sulphur species present in the coal, while PTR and PTO offer both some degree of resolution and quantification leading to information about the structure and reactivity of the total coal organic sulphur.
FUEL, 1987, Vol66,
November
1565
Identification
of some sulphur species in a high
orgainc
Relation between structure and thermal stability of organic sulphur compounds in coal are difficult to establish Owing to thecomplexity of coal structure and thermal reactions -combustion, thermoreduction, pyrolysis - the interpretation of thermal technique data is fairly difficult and requires much investigation of the mechanisms of thermal degradation. During coal combustion, SO, peaks may arise from the oxidation of several sulphur species. These sulphur species may not be oxidized independently of the network to which they are bound. The oxidative peak temperature of pyrite is lowered with decrease of particle size30. The exothermic profile of pyrite may be multi-peak and be strongly modified by the presence of iron and sulphates or by the pyrite level in the coal sample”. The SO, profile may be perturbed by strong exo- or endothermic reactions. The SO,, formed by oxidation of sulphur, may be trapped in the solid residue as sulphates, as seen in the Provence coal by X-ray diffraction spectrometry of the char. An example of chemical reactions which may contribute to the sulphur fixation in the coal (as CaS04) can be represented by the following reactions:
sulphur
coal: J. P. Boudou
et al.
coal break down at rates similar to those of the model compounds. Aliphatic and benzylic sulphides, mercaptans and disulphides would extrude H,S and CS,. The thiophenic compounds (thiophene, thianaphtene and dibenzothiophene) show little conversion, even at 950°C. In the second pathway highly reactive hydrocarbon radicals are formed which can react with sulphur to produce volatile secondary sulphur compounds”,3g~40~42. Calkins” has found that during flash pyrolysis pyrite can react with the coal matrix above 775°C. Johnson4’ has shown that benzenethiol is remarkably resistant to thermal reactions in the absence of oxygen. Conversion into products only become significant above 650°C when pyrolysis of benzenethiol yields benzene, diphenyl sulphide and diphenyl disulphide as the major products. Dibenzothiophene, biphenyl, thianthrene and diphenyl trisulphide are minor products. Braekman-Danheux42 has found that the desulphurization yield of sulphur aromatic model compounds such as thiophene, methylthiophene or benzothiophene is very low and the main products obtained are higher molecular weight sulphur compounds.
S(coa1) + 0, + SO,
No reported
CaCO,--+CaO+CO,
Electron microprobe analysis, which can be used to examine unground coal samples, has been proved to be unsuitable to quantify sulphur content in the finer grains of pyrite scattered in the coal matrix. XPS spectroscopy has proved itself to be unable to distinguish pyritic sulphur from organic sulphur. Other heavy spectrometric methods are Miissbauer spectrometry, high-resolution Xray diffraction spectroscopy and synchrotron X-ray spectroscopy’, for example. Much of the pyrite is finely dispersed in the coal and is coated with organic materials that may hinder physical contact with reagents or produce interference during thermal degradation. Pyrolysis cannot selectively degrade pyrite because of interferences between pyrite organic Programmed and matter. temperature because pyrolysis 43 has been shown to be unsatisfactory of an eventual overlapping between the organic sulphur peak and the pyritic sulphur peak. Mild oxidation might help to find a solution to quantify pyritic sulphur in a satisfactory way. Low-temperature ashing (LTA), followed by ash and evolved gas analysis has been used to analyse coal mineral and organic matter, but this technique is very time and sample consuming. However, it is believed, on the basis of our own results and others8,44,45, that slow oxidation may offer a simple solution for rapid pyrite analysis in coal microsamples for monitoring purposes.
SO2 + CaO + l/2 O2 -+ CaSO, These reactions of SO, fixation by minerals may occur above 500”C32. The possible reactions of adsorptiondesorption of the SO, with the char have been shown to occur below 400”C33. Chantret considers that SO, could be trapped at low temperature by ion exchange with the clay in the sample. Thermoreduction of sulphur model compounds has shown that pure organic sulphur species may give several peaks which can overlap in the PTR curve. Thiols would be reduced at low-temperature peaks. Even when thiol groups are removed by oxidation with hydrogen peroxide or nitric acid solutions, the coal kinetogram still shows a which presumably comes from the ‘thiol’ peak3’, reduction of aliphatic or aromatic sulphides, which may give both low- and high-temperature peaks: for instance, and poly(phenylene sulphide)35 give a thianthrene” thermokinetogram indicating one -SH, one sulphide and one thiophene or condensed thiophene. Thiophenes give the highest temperature peaks or no peak at all. Condensed thiophenes and aromatic sulphides do not give any peak at all; their amount is calculated by difference, with the total organic sulphur itself calculated by difference between the total sulphur and the pyritic sulphur. The result of the analysis does not differentiate between the reduced and the oxidized form of a sulphur group. Therefore structural assignment of the H,S peaks in the PTR kinetogram remains speculative. The thermoreduction method can only provide us with a fingerprint of coal sulphur functionalities and reactivities. We think there are two -at least -formation pathways for organic sulphur compounds during pyrolysis: the heteroatomic structures are present as such in the kerogen and are released during pyrolysis or they result from secondary reactions. The first pathway is supported by theories on the origin of oil sulphur compounds2g*36-38. Calkins’ work’ ’ supports the view that the labile sulphur compounds in
1566
FUEL, 1987,
Vol 66, November
technique to quantify pyrite is satisfactory
How abundant are thiol groups in coal? Ignasiak46 showed that the high organic sulphur Rasa coal contains one-third of its sulphur in the form of sulphides, but no thiol at all. According to Jones23, the vitrinite of high-volatile bituminous coal mainly includes sulphides and thiophenes rather than disulphides and thiols. Calkins” suggests that the aliphatic sulphur compounds in coal are sulphides. However thiol sulphur forms have been frequently noted though poorly quantified. According to Attar14, ‘18-30x of the organic sulphur in high-sulphur coals is sulphidic, l&40% thiolic’. Postovski and Harlampovich4’ have measured thiol and thioether contents in coal by reaction with
Identification
of some
sulphur
species in a high orgainc sulphur coal: J. P. Boudou
methyl iodide, but various reports in the literature cast doubts on the utility of this approach, since anhydrous conditions are required for the method to be valid and such conditions are difficult to achieve with coals. Bodzek and Marzec4* have identified indane thiol and naphthalene thiol in solvent extracts of a Carboniferous coal from Poland, but the authors have not quantified their results. Analysis of sulphur species in the Provence coal by the method of Purnell and Doolan49 indicated a larger quantity of pyrite than ASTM method did. Instead, this ‘excess’ sulphur is probably thiolic organic sulphur, which decomposed under the same general conditions as pyrite in the test. Non-thiophenic functions might partly account for the high micro-metal contents of the Provence coal: the complexing role of the thiol groups is well known in biological systems50m52and is thought to be prominent in metal speciation in recent sediments53. Is coal organic sulphur thiophenic? No method gives a quantification of the total thiophenic sulphur content in coal, Its indirectly determined value can be obtained by programmed temperature reduction, using either synthetic thiophenic compounds as models for standardization or calculation of the difference between the supposed non-thiophenic sulphur and the total organic sulphur. The total organic sulphur is calculated as the difference between the ASTM total sulphur and the ASTM pyritic sulphur. Hence these quantifications are fraught with uncertainties. Curie-point pyrolysis emphasizes the thiophenic sulphur, but thiophenic compounds only represent a very small part of the total sulphur compounds released (mainly H,S, which is presumably released by thermal degradation of the thermally unstable non-thiophenic coal components). Other indirect evidence of the predominance of thiophenic compounds in coal has been reported elsewhere. Preoxidation of coal followed by XPS spectroscopy23 of FTi .r .54 has shown that sulphides and thiophenes are the major sulphur species in vitrinite. Comparison of Xray absorption spectroscopy fingerprints of crude coal with XAS fingerprints of pure compounds taken as models show that coal sulphur is mainly pyritic and thiophenic?. It has recently been shown that coal thiophenic compounds (sulphur present in thiophenic rings may be regarded as replacing two aromatic carbon atoms) are so abundant in coal that they could introduce noise into aromaticity measurements by CP-MAS 13C n.m.r.55. Organosulphur-metallic sulphur coals
species may exist in high organic
Our observations that the highest concentrations of sulphur and trace metals occur most frequently in vitrinite and liptinite are in agreement with the conclusions of other authors56,57. Bouska5(j states that most trace elements in coal are bound to the humic substances (possible vitrinite precursors), which have a high sorption capacity, particularly at the early stages of coalification. Humic substances and not living plants are responsible for entrapment and enrichment in peat bog environments5*. On the basis of the study of trace elements in Upper Cretaceous coals of Transbaikalia, Admakin59 has found that the concentrations of some metals in collinite exceed 3-10 times their amounts in
et al.
tellinite, in which the Ge content, for example, only approaches the Clarke value. This implies that the elements tend to concentrate in coal of collinitic types, like the Provence coal, where the decomposition of plant material is well advanced. Therefore part of the metals associated with organic matter may be bound with it 57.60-64 Understanding the mechanisms of sulphur incorporation into coal organic matter is still speculative
The actual chemical mechanism for sulphur enrichment of organic matter during the early stages of coalification is still unknown. Sulphur compounds present in the water column and the pore water would react with plant and peat organic matter bearing reactive groups 22*65m70. These mechanisms seem to be understood in near-shore marine sediments71. In these sediments, plant proteins and lipids, as well as bacterially derived organic sulphur or ester-sulphates, would be unlikely to produce an increase of sulphur during the early diagenesis. Considering the reactivity of reduced inorganic sulphur species (H,S or its oxidation products) and their abundance in sediments, chemical addition of sulphur to the humic matrix during humilication would be the more likely mechanism. Since pyrite and organic sulphur are finely disseminated in the matrix of the Provence coal, we can speculate that sulphur incorporation has occurred during the sediment deposition and the first stages of coalification (syngenetic sulphur). Sulphur incorporation into the coal organic matter of the Provence coal would have been more important in vitrinite and exinite than in inertinite because the plant organic matter precursors of these almost structureless macerals have probably undergone more intense microbial alterations than inertinite. Microbial alterations would have introduced functional groups into the precursors of vitrinite and exinite and increased their reactivity to sulphur and metals. Thus the common occurrence of organic sulphur and metals in collinite of the Provence coal leads us to think that organic sulphur could have played the role of Understanding sulphur selective metal trapping. incorporation into coal organic matter may lead to explanations for metal enrichment. Alkyl substituents of the thiophenic compounds in the coal Curie-point pyrolysates of the Provence coal might be the vestige of former alkyl sulphides or alkylaryl sulphides at the origin of the thiophenic structure by cyclization and aromatization72. Alkylarylthiophene moieties could be linked with the kerogen by alkyl bridges or hydroaromatic units, as seen in coal structures73-75. Sulphur remaining bound in aliphatic sulphide groups is expected to be thermally labile and would mainly form hydrogen sulphide upon thermal decomposition during the bituminous stage (catagenesis)76. Sedimentologically the Provence coal is highly comparable to the Italian Sulcis or the Spanish Berga coals which were deposited in fresh-water, calcium-rich environments with a low rate of deposition. These coals show similar properties as marine-influenced coals. Owing to a high pH, due to calcium, bacterial activity is accelerated, resulting in increased degradation of plant remains, and there is a depletion of soluble iron. Furthermore, slowly accumulating coal layers have had longer time for bacterial generation of sulphides77. H,S
FUEL, 1987, Vol 66, November
1567
Identification
of some sulphur species in a high orgainc sulphur
and/or polysulphides produced in the anoxia have reacted with organic substances. Hence, most calciumrich coals are remarkably high in organic sulphur and syngenetic pyrite78*79. An extreme example of a calciumrich coal is the Rasa coal of Istria (Yugoslavia)*‘, which carries 11 ‘A organic sulphur.
5 6 7 8 9
CONCLUSIONS
10
In this paper we have reported some preliminary data on the analyses of a number of organic sulphur species in Provence coal using complementary methods of investigation - microscopic, spectroscopic and thermal methods. Two US high organic sulphur coals were studied for comparison with the Provence coal. Each particular method has contributed further to an overall picture of the sulphur distribution and functionalities in the Provence coal. This work has evidenced the microheterogeneous character of sulphur distribution in Provence coal where macerals and minerals are very small. The major proportion of organic sulphur and trace metals are associated with vitrinite. They could have been incorporated during deposition, at the peat stage, in humic-like compounds which could have been the possible precursors of vitrinite in the resulting coal. The Provence coal contains thermally labile organic sulphur compounds, such as thiols and aliphatic sulphides, and thermally stable organic sulphur compounds, such as aromatic and high molecular weight compounds. From the results of this preliminary study, we can visualize a model of the structural basic units of the coal network and of the molecular phase” as benzenic, sulphur benzenic and thiophenic building blocks connected (or not) by alkyl or heteroalkyl bridges and substituted (or not) by sulphur side chains, thiol groups and may be oxidized sulphur groups in very low abundance. A small amount of organic sulphur would be bound with trace metals.
ACKNOWLEDGEMENTS
11 12 13 14
15
16 17 18 19 20 21 22
23 24
25 26 27 28 29 30 31
The authors would like to thank Cerchar (Verneuil-enHalatte, France) for supplying the Provence coal sample and for having invited Dr A. Attar (Coal Gas Inc., Raleigh, USA) for a visit to France in 1986. Dr A. Attar and Dr F. Chantret (Cogema, France) are thanked for their contribution to this work. We also thank Science et Surface (‘La Combe de Charbonnitres’, 78 route de Paris, 69260 Charbonnitres, France) for ESCA measurements. This study was partly supported by ATP charbon (CRNS/ PIRSEM, France). This support is gratefully acknowledged. The FOMautopyms work was supported by FOM (Foundation of Fundamental Research) in the Netherlands.
32
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41 42 43 44
1 2 3 4
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Hgfele, Riemer, H., Ausubel, J. H. and Greiss, H. Org. Geochem. 1986, 10, 1 Rail, H. T., Thompson, C. J., Coleman, H. J. and Hopkins, R. L. US Bureau of Mines Bull. 1972,659, 187 Akmadieva, R. G., Yusupova, N. A. and Numanov, U. D&l. Akad. Nauk Tadzh. SSR 1985, 28, 519 Galpern, G. D. In ‘Thiophene and its derivatives’ (Ed. S. Gronowitz), John Wiley & Sons, 1985
FUEL, 1987,
Vol 66, November
33 34 35 36
37 38
39 40
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coal: J. P. Boudou
et al.
Nishioka, M., Whiting, D. G., Campbell, R. M. and Lee, M. L. Anal. Chem. 1986, 58, 2251 Payzant, J. D., Montgomery, D. S. and Strausz, 0. P. Org. Geochem. 1986,6, 357 Spiro, C. L., Wong, J., Lytle, F. W., Greegor, R. B., Maylotte, D. H. and Lamson, S. H. Science 1984, 226, 48 Lacount, R. B., Anderson, R. R., Friedman, S. and Blaustein, B. D. Am. Chem. Sot. Die. Fuel Chem., Prepr. 1986, 31, 70 Carangelo, R. M. and Solomon, P. R. Am. Chem. Sot. Div. Fuel Chem., Prepr. 1986, 31(l), 152 Attar. A., in ‘Analytical methods for coal and coal products’ (Ed. C. Karr), Academic Press, New York, 1979 Calkins, W. H. Energy & Fuels 1987, 1, 59 Durand, J. P., in ‘Les Gisements de charbon du Bassin de I’Arc’. Mdm. BRGM 1984, 122, 13-25 Chantret, F. C. R. Acad. Sci., Paris 1967, 264, 1825 Attar, A. ‘An analytical method for the evaluation of sulfur functionalities in American coals’, DOE report DOE/PC/30145Tl (DE 84007770), North Carolina State University, 1983 Majchrowicz, B.. Yperman. J.. Reggers, G., Franqois, J. P., Celan, J., Martens, H. J., Mullens, J. and Van Poucke, L. C., in ‘Coal Characterisation for Conversion Processes’. First Int. Rodulc Symp. on Coal Science, The Netherlands, 1986 Tromp, P. J. J.. Moulijn, J. A. and Boon, J. J. Fuel 1986,65,960 van de Meent, D., Brown, S. C.. Philp, R. P. and Simoneit, B. R. T. Geochim. Cosmochim. Arra 1980,8X. 999 Guieze, J.. Devant, G. and Loyaux, D. Int. J. Mass Spectrom. Ion PhJ.7. 1983, 46, 313 Huggins, F. E.. Huffman, G. P. and Lin. M. C. Int. J. Co~tl Geol. 1983,3, 157 Straszheim, W. E. and Greer, R. T. Stunning Elecrron Microsc. 1982, 3, 1013 Bthar, F. and Vandenbroucke, M. Reu. Insritut Franc.ais du Pitrole, 1986, 41, 173 Casagrande, D. J., Gronli, K. and Patel. V., in ‘Environmental Biogeochemistry and Geomicrobiology, Vol. 2: The Terrestrial Environment’, (Ed. W. E. Krumbrein), Ann Arbor, Boston, 1978 Jones, R. B.. McCourt, C. B. and Swift. P., Int. Conf. Coal Sci., Pittsburg. 198 1 Frost, D. C.. Wallbank, B. and Leeder. W. R., in ‘Analytical Methods for Coal and Coal Products’ (Ed. C. Karr). Academic Press, New York, 1978 Dutta, S. N.. Dowerah, D. and Frost, D. C. Fuel 1983,62, 840 KGrnig, S., Boon, J. J., Nip, M. and Kistemaker, P. G. Adc. Muss Spectrom. 1985, Part B, 1435 Stock, L. M.. Duran, J. E. and Mahasay, S. R. Fuel 1986,65, 1150 Tannebaum. E. and Aizenshtat, Z. Org. Geochem. 1985, 8, 181 Orr, W. L. Org. Geochem. 1986, 10, 499 Kopp, 0. C. and Kerr, P. F. Am. Miner. 1958, 43, 1079 Earnest, C. M. Am. Chem. Sot. Div. Fuel Chem., Prepr. 1984, 29(l), 135 Jiintgen, H. and van Heek, K. H. ‘Relationsablatiufe unter nichtisothermen Bedingungen’, Springer Verlag, 1970 Dratwa, H. and Jiintgen, H. Staub 1967, 27, 301 Chantret, F. Bull. Sot. Fr. MindraI. Crisiallogr. 1969, 92, 462 Attar, A. and Dupuis, F.. in ‘Coal Structure’ (Eds. M. L. Gorbaty and K. Ouchi), Am. Chem. Sot. Ado. Chem. Ser., 1981 Gransch, J. A. and Posthuma, J., in ‘Advances in Organic Geochemistry’ (Eds. B. Tissot and F. Bienner), Editions Technip, Paris, 1974 Orr, W. L., in ‘Oil Sand and Oil Shale Chemistry’ (Eds. 0. P. Strausz and M. E. Lown), Verlag-Chemie, 1978 Hughes, W. B.. in ‘Petroleum Geochemistry and Source Rock Pot&tial of Carbonate Rocks’ (Ed. J. G. Palacas). AAPG Studies in Geology No. 18, Am. Assoc. Pet. Geol., Tulsa, 1984, p. 181 Attar, A. Fuel 1978, 57, 201 Solli, H., van de Grass, G., Leplat, P. and Krane. J. Org. Geochem. 1984.6, 351 Johnson, D. E. Fuel 1987,66, 255 Braekman-Danheux, C. J. Anal. Appl. Pyrol. 1985, 7, 316 Madec, M. and Espitalie, J. J. Anal. Appl. Pyrol. 1985, 8, 201 Hyman, H. and Rowe, M. W. in ‘New Approaches in Coal Chemistry’ (Eds. B. D. Blaustein, B. C. Bockrath and S. Friedman) Am. Chem. Sot. Symp. Ser. 1981, 169, 389 Thorpe, A. N., Senftle, F. E., Alexander, C. C., Dulong, F. T., LaCount, R. B. and Friedman, S. Fur/ 1987,66, 147 Ignasiak, B. S., Fryer, J. F. and Jadernik, P. Fuel 1978,57, 578 Postovski, J. J. and Harlampovich, A. B. Fuel 1936, 15, 229
Identification 48 49 50 51 52
53 54 55 56 51 58 59 60 61 62 63
64 65
of some sulphur species in a high orgainc sulphur
Bodzek, D. and Marzec, A. Fuel 1981,60, 47 Purnell, A. K. and Doolan, K. J. Fuel 1983,62, 1107 Hall, D. O., Rao, K. K. and Cammack, R. Sci. Prog., Oxford 1975,62, 285 Ibers, J. A. and Holm, R. H. Science 1980, 209, 223 Newton, W. E. in ‘Sulphur, Its Significance for Chemistry, for the Geo-, Bio-, and Cosmosphere and Technology’ (Eds. A. Miiller and B. Krebs), Elsevier, Amsterdam, 1984 Boulegue, J., Lord, J. III and Church, T. M. Geochim. Cosmochim. Acta 1982,46, 453 Venier, C. G., Int. Conf. Coal Sci., Pittsburg, 1981 Neill, P. H., Maciel, G., Given, P. H. and Weldon, D. Fuel 1987, 66,96 Bouska, V. ‘Geochemistry of Coal’, Elsevier, New York, 1981 Minkin, J. A., Chao, E. C. T., Thompson, C. L., Nobiling, R. and Blank, H. Scanning Ekctron Microsc. 1982, 1, 175 Idiz, E. F., Carlisle, D. and Kaplan, I. R. Appl. Geochem. 1986,1, 573 Admakin, L. A. Dokl. Akad. Nauk 1975, 224, 171 Wong, J., Maylotte, D. H., Peters, R. L. S., Lytle, F. and Wilson, T. F. fnt. J. Coal Geol. 1984, 4, 1 Finkelman, R. B. PhD Thesis, University of Maryland, 1980 Hausler, D. W., Hellgeth, J. W., Taylor, L. T., Borst, J. and Cooley, W. B. Fuel 1981, 60, 40 Mraw, S. C., De Neuville, J. P., Freund, H., Baset, Z., Gorbaty, M. L. and Wright, F. J. in ‘Coal Science’ (Eds. M. L. Gorbaty et al.), Academic Press, New York, 1983 Miller, R.N. and Given, P. H. Geochim. Cosmochim. Acta 1986, 50, 2033 Boulegue, J. and Michard, G. C. R. Acad. Sci., Paris 1974,279,
66 67
68 69 70 71 12 73
74 15 76
77 78 79 80 81 82
coal: J. P. Boudou
et al.
13 Hackley, K. C. and Anderson, T. C. Grochim. Cosmochim. Acta 1977,50, 1703 Bestougeff, M. and Combaz, A. in ‘Advances in Organic Geochemistry’ (Eds. B. Tissot and B. Bienner), Editions Technip, Paris, 1973 de Roo, J. and Hodgson, G. W. Chem. Geof. 1978, 22, 71 Prezewocki, K., Malinski, E. and Szafranek, J. Chem. Geol. 1984185, 47, 347 Mango, F. D. Geochim. Cosmochim. Acta 1983,47, 1433 Francois, R. Geochim. Cosmochim. Acta 1987, 51, 17 De Rycke, G. PhD Thesis, Univ. Paris VI, 1983 Deno, N. C., Curry, K. W., Greigger, B. A., Jones, A. D., Ratitsky, W. G., Smith, K. A. S., Wagner, K. and Minard, R. D. Fuel 1980,59, 694 Benjamin, B. M., Douglas, E. C., Hershberger, P. M. and Gohdes, J. W. Fuel 1985.64, 1340 Farcasiu, M. Fuel Process. Techno!. 1986, 14, 161 Le Tran, K., Connan, J. and Van der Weide, B. in ‘Advances in Organic Geochemistry (Eds. B. Tissot and F. Bienner), Editions Technip, Paris, 1973 Davis, A. ‘Sulphur in coal’, Earth and Minerals Sciences, The Penn State University, 1981, p. 51 Petrascheck, W. 2. Dt. Geol. Ges. 1952, 104, 1 Renton, J. J. in ‘Coal Structure’ (Ed. R. A. Meyers), Academic Press, New York, 1982 Kreulen, D. J. W. Fuel 1952, 31, 462 Marzec, A. Fuel Process. Technol. 1986, 14, 39 Espitalie, J., Laporte, J. L., Madec, M., Marquis, F., Leplat, P., Paulet, J. and Boutefeu, A. Reo. Inst. Fr. Pbtr. 1977, 32, 23
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1569
L. Makchaux”,
M. Nipt, J. W. de Leeuwt
CNRS-UA 196, Laboratoire de Geochimie et Metallogenie, 4 Place de Jussieu, 75252 Paris, Cedex 05, France * Cerchar, BP No. 2, 60550 Verneuil-En-Halatte, France t Delft University of Technology, Department of Chemistry and Chemical Engineering, Organic Geochemistry Unit, De Vries van Heystplantsoen 2, 2628 RZ Delft, The Netherlands ** FOM Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands (Received 30 October 1986; revised 9 March 1987)
In this paper, a preliminary study is described concerning the characterization of sulphur forms in a subbituminous coal rich with organic sulphur deposited in a lacustrine carbonate environment (Upper Cretaceous, Provence, France). Two American high organic sulphur coals were studied for comparison with the Provence coal. Optical microscopy, scanning electron microscopy and electron microprobe approaches offered a global view of the relations between sulphur, metals and coal petrography. Sulphur occurs in all the macerals and most minerals. Vitrinite contains the major part of organic sulphur and metals. This common occurrence of organic sulphur and metals in vitrinite suggests the presence of organosulphur-metallic species. X-Ray photoelectron spectrometry showed that mineral and organic sulphur is very much reduced (divalent). It would mainly include pyrite, sulphides and thiophenes. Programmed temperature oxidation and programmed temperature reduction (Attar’s test) revealed fragile sulphur compounds such as aliphatic thiols and sulphides and thermal stable sulphur compounds such as aromatic and high molecular weight compounds. Curie-point pyrolysis in combination with mass spectrometry, gas chromatography, and gas chromatography-mass spectrometry indicated the absence of free organic sulphur compounds and of elemental sulphur. Pyrolysis yielded large amounts of low molecular weight products (H,S, COS, etc.) and smaller amounts of thiopenes, benzothiophenes, dibenzothiophenes and their alkylated homologues. These sulphur compounds could result from the thermal degradation of organic sulphur moieties of the coal as well as from secondary reactions. (Keywords: coal; sulphur; instrumental methods)
The behaviour of organic sulphur compounds associated with the world-wide utilization of fossil fuels is of great importance for the quality of the environment’. Studies dealing with the organic sulphur chemistry of fossil fuels have increased over the past decades and new analytical techniques have been applied over the past few years. These techniques, particularly l.c./s.f.c./g.c.-m.s., have brought a fairly good understanding of chemistry of the lighter sulphur species in rock extracts and oilszm6. However, these identifiable extractable sulphur molecules only represent a very minor part of the total sulphur. So we cannot consider that oil sulphur compounds are totally known. Kerogen and coal sulphur have been investigated by several new techniques: solid spectroscopy’, oxidative and reductive slow pyrolysiss9, thermoreduction in the presence of solvent”, flash pyrolysis-(g.c.km.s.’ l, etc. Nevertheless, there appears to be no really satisfactory means of determining forms of organic sulphur in a coal so that little is known about the distribution of sulphur functional groups in kerogen and coal. In this paper we report preliminary results dealing with the identification of sulphur compounds in a 001~2361/87/111558-12$3.00 0 1987 Butterworth & Co.
1558
(Publishers) Ltd.
FUEL, 1987, Vol 66, November
representative complementary
high organic sulphur analytical methods.
coal
using
MATERIAL The French Provence coal sample (analysis in Table 1) studied here was obtained from the coal minibank of Cerchar. This coal was deposited in a fluviolacustrine environment in a humid climate during the Upper Cretaceous. This humid climate of deposition was followed by dry weather conditions as shown by a sapropelic and pyritic limestone aquifer which overlays the coal Provence seam12. A sample was collected in a fresh cutting of the Quartier de l’Etoile, taille 9 of the Grande mine. The coal sample was kept under water until grinding. Grinding was done under argon down to size (r3 mm. In the laboratory, the sample was stored under argon at -25°C. The main chemical characteristics of the sample are displayed in Table 1 and the distribution of vitrinite reflectance in oil is shown in Figure 1. The petrographic analysis showed quite narrow layers (25-100pm) of
Identification Table 1
Volatile
Analysis
matter
of coals studied
(wt %, dry)
T,,, (“Qb Organic C (wt i”,, dry)b H Index (wt % org. C)” 0 Index (wt % ore. 0 ’ Ash (wt i dry)” CaO (wt % dry) Calcite (wt “/, dry) Total S (wt %. dry) Pyrite S (wt %. dry) Sulphate S (wt Y;, dry) Organic S (wt S,. dry)d a b ’ d
of some sulphur species in a high orgainc sulphur coal: J. P. Boudou
Provence
Muskingum
Meigs
45 415 53.1 26.3 0.9 20 8.9 6.0
41 420 65.2 28.5 0.3 15
42 418 53.2 29.5 0.3 35
4.51 1.00 0.06 3.57
5.30 2.85 0.05 2.40
4.80 2.95 0.15 1.70
Measured by ASTM methods Measured by Espitalie’s methods* Measured by Bernard calcimetry Organic S = Total S - Mineral S
vitrinite (60.1 vol. ‘%) mainly impregnated with resinite. Sporinite and cutinite (6.7 vol. %) are concentrated in layers (5-100 pm). The inertinite (14.9 vol. %) is either finely mixed with the liptinite in the vitrinite (inertodetrinite), or in the form of layers of semi-fusinite and fusinite. There is some sclerotinite too. The mineral matter (18.3 vol. %) includes clays, carbonates and sulphides. The clays are very finely dispersed in coal, mainly in the liptinite. The carbonates till holes, cracks and little faults appear in the form of a big heap in the vitrinite or as thin layers in the liptinite. The macroscopic sulphides (pyrite) are rare (1 vol. %). Two American coals (analysis in Table 1) were taken in order to better assess PTR and Curie point pyrolysis methods. Their programmed temperature reduction kinetograms are very different from each other, and they are currently used by Dr A. Attar as coal standards for PTR analysis. These coals from the Appalachian coal basin were deposited in marine deltaic environments during the Carboniferous. They reveal characteristics of marine coals, and allow comparison of the fluviolacustrine Provence coal with organic sulphur rich fluviomarine coals.
of organic sulphur and metal. The accelerating voltage was kept at 20 keV. As our material has a poor thermal conductivity, heat generation under electron beam induced thermal degradation of the sample. This problem was reduced: (a) by defocusing the electron beam to produce a larger spot diameter (2-5 pm); (b) by decreasing the beam current intensity to 1&40nA; and (c) by lowering the counting time to 5-10 s. Pyrite was used as a standard for sulphur and iron. Matrix effects are very important, ZAF corrections give a remained semilow precision, so measurements quantitative and comparative. X-Ray photoelectron spectroscopy
Coal samples were examined in a Surface Science Laboratories (USA) spectrometer using an Al-Ku radiation source with a base pressure of 5 x 10d9 Torr. A monochromator was used to give a narrower X-ray line by taking only a portion of the K band. The spectrometer was run at a pass energy of 25 eV. The specimens were examined as powders deposited in a thin layer in a metallic boat and introduced into the spectrometer. The diameter of the spot analysed was 600 pm. An electron gun was used to compensate the charging of the sample due to photoionization. Szp binding energy was referenced to the C rs peak at 285 eV. Carbon was recorded in 11 min (interval of binding energy 15 eV recorded on 128 channels during 10 cycles) and sulphur was recorded in 90 min (interval of binding energy 15 eV recorded on 128 channels during 80 cycles). Iron (2~) could not be detected because of a lack of sensitivity. Programmed temperature oxidation (PTO)
PTO was performed according to the method of Chantretr3. This method is based on the concept that the differences in reactivity of sulphur, carbon and mineral species can be made by following the evolution of SO,, CO, and the differential thermal analysis curve during combustion. A flow of air (100 ml min-‘) was passed
Average : 0.44 Standard devratian
METHODS Scanning electron microscopy-energy analysis (SEM-EDAX)
et al.
: 0.05
dispersive X-ray
Coal samples were mounted on metallic discs and coated with carbon for SEM examination with a Jeol Model JSM 2 equipped with an EFOS, EG & G Ortec energy dispersive X-ray analyser (EDAX). The instrument had a point to point resolution of 0.1 pm. The resolution of the EDAX, however, is of the order of l20 pm, owing to electron scattering in the region analysed. Optical microscopy-electron
probe microanalyser (EPM)
The sample was prepared as a block (% 2.5 cm on edge), or was mounted on a glass slide in thin section form. The surface was polished and subsequently coated with carbon for examination. The sample could be viewed by means of a light microscope incorporated within the electron microprobe analyser. A Camebax electron microscope was used to make quantitative measurements
R, 1%)
Figure 1 coal
Statistical
distribution
of vitrinite
FUEL, 1987,
reflectance
in the Provence
Vol 66, November
1559
Identification
of some sulphur species in a high orgainc
through the coal sample finely ground and mixed with kaolin fired to 1200°C. The sample was heated from ambient to 1050°C in a stream of air at a rate of Pyrite (2%) was added to the sample to 10°C mini. determine the combustion temperature of pyrite. The reproducibility of the method is good enough to allow for quantification. This method has been used successfully to identify sulphur species in soils and sediments3’q3’. Progrummed
temperature
reduction
(PTR)
Attar” and Attar rt a1.14 have developed a test to characterize the distribution of several organic sulphur groups in coals by a programmed temperature reduction to H,S. Coal is heated in a hydrogen solvent mixture in the presence of catalysers. At various temperatures, discrete H,S peaks are detected which presumably originate from different organic entities in the coal. H,S peak area would be proportional to the sulphur amounts in coal. Pyritic sulphur can be reduced ~ particularly sulphur in line pyrite grains. Hence removal of pyrite before thermoreduction improves the resolution”. In this work we used almost the same experimental conditions as the ones described by Attar’” (sample weight, 1(r30 mg; heating rate, 5°C min - ’ ; gas flow rate, 60ml min-‘. , solvent mixture, HCL washed 120 mesh dry coal, 5G-100 mg of resorcinol, 200 mg of phenanthrene, 50mg of 9,1Odihydroxynaphthalene, 5OOmg of pyrogallol impregnated into the coal from methanol solution; catalyst, sulphided CO-MO Horshaw 402 catalyst). Sulphur-containing polymers can be used to show the kinetogram location where each supposed sulphur entity is reduced. They are also used to measure, by calibration of the PTR curves, the abundance of the organic sulphur forms. The sequence of appearance of functionalities and the temperature ranges within which they appear are: disulphides (not distinguishable from adsorbed H,S and elementary sulphur), < 160°C; aliphatic thiols, 16& 210°C; aryl thiols, 20&24O”C; aliphatic sulphides, 27& 330°C; alicyclic sulphides, 32&36o”C; aryl sulphides, 350-400°C; simple thiophenes, 40&5OOC; complex thiophenes, >45o”C (can be partially determined only when operating under pressure; otherwise they are calculated by difference). Curie point pyrolysis methods Three Curie point pyrolysis methods were applied. Curie point pyrolysis mass spectrometry using the FOMautopyms was used for fingerprint analysis of the coal pyrolysate. Several Curie point temperatures were tried: 358°C for evaporation; 510, 770 and 980°C for pyrolysis. A recent description of the method for analysis of coal is given by Tromp16. The mass range of the mass spectrometer used in this study was 2&220. The pyrolysis chamber and the expansion chamber were heated to 160 and 2OO”C, Pyrolysis gas chromatography was respectively. performed in a reactor described by Van der Meent “. The pyrolysis unit was mounted in the injection block of a Varian 3700 gas chromatograph system, equipped with an FID and FPD detector. The pyrolysates were separated on a fused silica capillary column coated with CP-SIL 5, 50 x 0.32mm i.d., programmed from 0°C (held for an initial 5 min period) to 300°C at 3°C min -I. The final temperature was held for 20 min. Helium was used as a carrier gas. The Curie temperatures of the wires
1560
FUEL, 1987, Vol 66, November
sulphur
coal: J. P. Boudou
et al.
used were 358, 510, 610 and 770°C. Curie-point pyrolysis-gas chromatography-mass spectrometry was performed on a Varian 3700 gas chromatograph combined with a MAT 44 quadrupole mass spectrometer. Mass spectrometric analyses were performed in the electron ionization mode at 80 eV electron energy, ion source temperature 250°C cycle time 1 s, mass range 255 400 daltons. Since the EI detection is not as specific for sulphur compounds as the negative chemical ionization mode with ammonia l8 , both the TIC and the Curie-point pyrolysis-g.c.-FID traces were used to identify the g.c.FPD peaks.
RESULTS Microscopic
AND DISCUSSION upprouch
Scanning electron microscopy enabled identification of macerals (as shown in Figure 2). It revealed some minerals -such as metallic sulphides ~ mainly in the form of very small grains (0.5 pm) finely scattered in the organic matrix. In the Provence coal, pyrite is so sparsely dispersed in very small submicrometre-size particles that only 27% of the pyrite could be removed by column flotation, which generally efficiently removes >50”/, of pyrite from most bituminous coal. Scanning electron microscopy also showed some rare crystals of anhydrite - or gypsum - in the vicinity of framboidal pyritic concretions (average diameter 1(r 15 pm). These CaSO, crystals may result from oxidation of pyrite in close contact with calcite before or during storage’ 9. Electron microprobe measurements (Table 3) showed that metal concentrations are locally very high in comparison with ASTM measurements (Table 2). The metal distribution determined by petrographic and EPM measurements is shown in Table 3. Some metals such as iron, lead and molybdenum are particularly abundant in pyrite. Other metals such as germanium, with low abundance in pyrite, are particularly abundant in vitrinite. This heterogeneity can be explained by the presence of fine minerals and organometallic complexes which seem to be absent in the EPM spot (2 pm diameter) under the optical microscope. Optically unobservable minerals and macerals may also be probed below the surface of the polished section: with an accelerating voltage of 20 keV, the depth of electron penetration is z 1 pm in the pyrite and 4pm in the organic matter. As seen in Table 3, sulphur levels showed small intramaceral differences. Gradation in sulphur surrounding pyrite particles, which may result from the pyrite formation2’, was not observed. The average concentrations of sulphur in inertinite tends to be about half that in vitrinite, while in liptinite it approaches more closely the level observed in vitrinite. Sulphur, unlike metals, appeared to be homogeneously distributed throughout the macerals, implying an association on a molecular base. Owing to the absence of elemental sulphur present (as shown below in the section Curiepoint pyrolysis) and the inability to locate crystals and amorphous deposits of this phase, no distinction has been made in the reported analyses between elemental sulphur and organically bound sulphur. If we assume hypothetically that all the metals are present in the form of mineral sulphides finely scattered in the organic matrix, these metals could only account for a very small fraction
Identification
of some sulphur
species
in a high
orgainc
sulphur
coal: J. P. Boudou
et al.
building blocks ~ connected by alkyl and heteroalkyl alkyl substituents and functional bridges - bearing groups - mainly those containing oxygen - as described for type III kerogen”. Spectroscopic
Figure 2 SEM micrographs of the Provence fusinite particles; (b) region with vitrinite microcrystals embedded in vitrinite
Table 2 Metals in Provence coal (measured expressed in /Ig/g of the coal as received) ~. -~~~ Fe 8125 Ni MO 36.0 Se CU 5.0 Zn Hg 0.1 co As 2.5 Cd
coal: (a) region with particles; (c) pyrite
by ASTM
methods,
7.0 1.0 10.0 3.0 0.6
of the total sulphur. Therefore we can think that, for the greatest part, sulphur is associated primarily with the organic matrix in the coal macerals. Most of the sulphur and trace metals are associated with vitrinite (Table 4), i.e., with macromolecules including mainly small (poly)aromatic and heterocyclic
approach
(XI’S)
The XPS Sap spectrum of the Provence coal only shows a wide and slightly asymmetrical peak that includes the s s 283,2 peaks (in the ratio l/2). This peak spreads fr%?i62 to 166 eV (Figure 3), a region which includes the pyritic sulphur (162-163 eV), the reduced organic sulphur species between (163-165 eV) and the sulphoxides (165166 eV). The maximum of S,, occurred at a binding energy of 164 eV. A small flat peak is visible centred around 170 eV indicating the presence of mineral sulphates - as shown by elemental and microscopic oxidized organic sulphur forms analyses ~ and/or (sulphones, sulphites, sulphates) in trace amounts. FT-i.r. spectroscopy of the Provence coal did not show any sulphoxides. All attempts to resolve the XPS S,, spectrum peak, maximizing at 164 eV by means of either mathematical deconvolution or increase of the instrumenlal resolution using a monochromator and a multidetector, failed. These data only showed that the Provence coal does not contain, or if so only in small amounts, any oxidized sulphur groups such as RSO,R, ROSOzR or ROSO,OR, which occur above 166 eV. Apart from peat and its humic compounds, where the most of the sulphur would be present in sulphate esters22, coal XPS S,, spectra present only one peak when the coal has not been oxidized 23-2 5. According to Jones23, the peak of the vitrinite (from high-volatile XPS s,, bituminous coal) would mainly arise from sulphides and thiophenes. Programmed
temperature
oxidation
Differential thermal analysis and CO, evolution curves are presented in Figure 4. DTA and CO2 maxima occur in three intervals: the first two exothermic ones correspond
FUEL, 1987, Vol 66, November
1561
Identification
of some sulphur species in a high orgainc sulphur coal: J. P. Boudou
et al.
Table 3 Electron microprobe traverses showing profiles of sulphur and metal concentrations in the Provence coal (V: vitrinite, E: exinite, P: pyrite. Elements are quoted by decreasing order of average concentration in pyrite in units of rig/g)) Traverse No. 1 Distance brn) from the left-hand spot 0 v g (wt %) Fe Pb MO Bi CU Hg As Ni Se Ge co Zn Cd
7.5 v 5.4
450 0 0 0 90 0 140 420 0 0 0 170 0
16 v 5.3
160 1200 0 0 0 0 0 0 140 0 210 0 0
23 v
5.2 550 0 0 110 180 170 0 0 0 0 210 0 0
30 V 5.1
400 0 0 420 40 1040 0 0 0 0 0 0 0
35 E 5.0
420 0 0 0 0 520 0 50 0 0 260 390 0
38 E 5.2
170 1200 0 1160 0 0 0 0 0 0 40 0 600
Traverse No. 2 Distance (pm) from the left-hand spot 42 V
3.8 0 0 0 320 0 0 50 0 0 150 0 0 660
50 v 5.0
0 2640 0 0 0 870 0 0 80 1370 340 0 0
0 V 5.4
0 1200 350 420 490 0 0 370 10 0 170 170 0
5 v 4.9
50 970 0 0 1010 0 0 310 0 150 130 590 0
11 v 5.0
0 970 0 0 460 0 240 0 60 0 90 110 0
5.4 330 0 0 540 0 0 0 680 0 0 170 0 180
18 v
22 P 5.0
340 1460 0 540 0 180 80 0 70 150 90 60 610
27 V 53.8
36 V 5.5
44.1” 300 3070 3150 0 0 2040 0 1660 0 1920 0 160 0 1330 30 0 120 1320 0 0 380 0 110 0 60
5.5 990 0 0 210 0 530 0 0 0 0 0 260 0
a Content expressed in wt %
Table 4 Petrographic distribution of sulphur and metals in Provence coal. Total content is expressed in wt % of the total coal mass. Element content in the total coal is calculated by multiplying the element content in the pyrite or maceral by the wt % of pyrite or maceral group in the total coal
(I 5 measurements)
Pyrite (wt %)
Vitrinite (wt %) (35 measurements)
Exinite (wt %) (15 measurements)
S
20.8
62.0
10.2
7.0
Fe Pb MO Bi cu Hg As Ni Se Ge Zn co Cd
97.8 13.5 32.7 12.9 9.4 3.5 21.8 6.1 6.2 2.3 3.5 2.8 0.9
1.8 70.8 34.5 63.9 64.1 63.0 37.5 60.2 47.5 74.6 69.0 65.4 67.5
0.1 12.2 16.9 13.4 17.9 21.7 18.7 9.2 17.5 13.8 6.1 7.4 19.8
0.2 3.4 15.7 9.6 8.5 11.6 21.8 24.4 28.7 9.2 21.2 24.2 11.7
to the decomposition of the organic carbon and the last small exothermic one to the decomposition of calcite. SO, maxima occur in three temperature intervals centred near 375°C (coinciding with the first CO, shoulder), 460°C (at an intermediate position between the first CO2 shoulder and the main peak and 500°C (coinciding with the main CO, peak). Another small peak occurs at 580°C. The sharp peak at 460°C increased with addition of pure pyrite. Lacount et u/.~, using slightly different experimental conditions (heating rate 3°C min-’ in a stream of argon containing 10% oxygen) found in addition three main peaks at 320°C 430°C and 480°C from various coals. These authors structurally assigned the SOz peaks by means of model compounds: the first representing the cyclic sulphides (tetrahydrothiophene), the second the pyrite, and the third the aryl sulphides and thiophenic sulphur. Programmed temperature reduction
The sulphur distribution in the Provence coal resulting from this analysis is given in Figure 5 together with the sulphur distribution of the two US coals taken as reference. Sulphur distribution in the Provence coal is
1562
FUEL, 1987, Vol 66, November
Inertinite (wt %) (15 measurements)
markedly different. Thiophenic compounds and high molecular weight sulphur compounds are less abundant in the Provence coal than in the reference coals. The PTR test showed that the higher the organic sulphur content, the easier it is to convert to H,S. Curie-point pyrolysis
Py-m.s. analysis of the Provence coal was performed at different Curie temperatures (358, 510, 610, 770 and 980°C). Evaporation of components adsorbed on or trapped in the coal matrix takes place at the lowest pyrolysis temperature (358°C) while at higher temperature Py-m.s. of the coals shows peaks characteristic of pyrolytic fragments: phenols, aromatics and aliphatic hydrocarbons give large mass spectral peaks at m/z 34,48, 60, 64 and 76, while heavier sulphur fragments shown lower abundance (Figure 6). The light sulphur fragment peaks are thought to reflect, respectively, H,S, CH,SH, COS, SO, or S,, and CS, in the pyrolysatez6. H,S, which is the most abundant sulphur compound in the pyrolysate, can result from secondary reactions of sulphur species in the gas phase during pyrolysis. These secondary reactions produce
Identification
I
I
297
293
of some sulphur
species
in a high
sulphur
coal: J. P. Boudou
I
1
I
I
orgainc
289
et al
I
285
car. BE(eV)
S2P
174 Figure 3
X-Ray
172 photoelectron
170 spectra
of the Provence
cor!TE
(eV)
coal obtained
CS,, so in most coals the m/z 64 and 76 peaks are intense at pyrolysis temperatures of 770 and 980°C owing to gasphase reactions after pyrolysis. This is not the case with the Provence coal, for which the m/z 34 peak is predominant; H,S would then result from the degradation of thermolabile organic sulphur compounds such as thiols and organic sulphides. Py-g.c. with simultaneous FID/FPD detection yielded a rather complex pyrolysate (Figure 7, FPD traces shown). The virtual absence of pyrolysis products at 358°C indicates that the organic sulphur compounds present in the other Py-g.c. traces (at 510,610and 770°C) are true pyrolysis products generated from higher molecular weight substances. The Py-g.c. traces indicate that the composition of the pyrolysate does not change much using different Curie temperatures. The yield of pyrolysate seems to increase with higher Curie temperatures, probably owing to a higher input of energy. The heaviest sulphur compounds are released at the highest pyrolysis temperatures. Py-g.c.-m.s. analysis was used to identify the pyrolysis products. It showed that no elemental sulphur was present in the coal. This proves that our samples are not oxidized - Stock2’ having recently shown that elemental sulphur is an oxidation artifact in coal. Among the
166
164
162
160
using an X-ray monochromator
compounds released at 770°C we identified numerous sulphur compounds by means of mass spectrometry and g.c. retention time (Figure 8). These compounds are CH,SH, thiophene, benzothiophene, dibenzothiophene and the C,, C, and C, alkyl-, alkylbenzo- and alkyldibenzothiophenes. These thiophenic compounds have been frequently identified in solvent extracts of other coals. Despite the complexity of the sulphur lingerprint, there is almost no qualitative difference ~ at a similar degree of maturation - between the pyrolysates of the Provence coal and several other coals of variable origin, mineral and sulphur composition (Meigs and Muskingum in this paper). However, we noticed that the heaviest volatile sulphur compounds, i.e., alkyldibenzothiophenes, give different g.c.-FPD traces for the Provence coal and the two US reference coals (Figure 9). ‘Normal’ coal generates volatile matter by cleavage of oxygen functions (evolving CO, and H,O) and by carboncarbon cleavages. During pyrolysis, sulphur-rich coals may also break at weak sulphur linkages and produce larger fragments leading to different tar and hydrocarbon composition, as observed for type II-S kerogens 28*29. Also it seems there are some structural differences between type II-S kerogen and coal: Curie point pyrolysis of the Provence coal, as well as of the US
FUEL, 1987,
Vol 66, November
1563
Identification
of some sulphur species in a high
orgainc
sulphur
coal: J. P. Boudou
et al
during slow and flash coal pyrolysis would require a study of a large number of both high and low organic sulphur coals. This point will be examined in a forthcoming paper.
reference coal, did not release any thiophanes, thianes or terpenoid sulphides which occur in high sulphur oils2,4,6 or sediments (Organic Geochemistry Unit, Delft University) as a result of their high degree of reduction. An accurate examination of hydrocarbon generation
GENERAL
DISCUSSION
For (I better assessment of the value and the specificity the methods used to analyse coal sulphur
Orgc
of
Most previous work on coal sulphur has applied one particular technique for different coal samples. In this preliminary study various kinds of techniques have been applied to a single coal sample. The methods used can be classified by increasing order of specificity and by decreasing order of quantification: ASTM elemental analysis, XPS, microscopy, PTO, PTR and Curie-point Each particular method complements pyrolysis. information previously found by other techniques. Methods such as elemental analysis, XPS and microscopy
8
7+8 I 420
210 Temper&we 6iO
240
PC
)
Figure 5 Programmed temperature reduction of the Provence coal and of two reference coals. The diameter of the circle is proportional to the organic sulphur content in the coal. Hypothetical structural assignments: 1, adsorbed H,S and polysulphides; 2, aliphatic thiols; 3, arylthiols; 4, aliphatic sulphides; 5, alicyclic sulphides; 6, aryl sulphides; 7, simple thiophenes; 8, aryl sulphides and thiophenic compounds in condensed high molecular weight structural moieties
Temperature PC)
Figure 4 Programmed temperature oxidation kinetograms of the Provence coal. Hypothetical structural assignments: A, thermolabile organic sulphur structures; B, pyrite; C and D, arylthiophenic structures and aromatic sulphides
34 (H$S) 108
(Ph) 94
(C,) 56 42
I 70
I
40
60
82
80
I
Mass
Figure 6
1564
Curie-point
pyrolysis
FUEL, 1987,
(770”Ctm.s.
of the Provence
Vol 66, November
coal obtained
122
I, 120
136
140
number using FOMautopyms
160
180
of some sulphur species in a high orgainc sulphur
Identification
coal: J. P. Boudou et al. Py (358°Ckg.c.-fPD
* ._.--dl-L
L_--_&&U Py(358’Cbg.c-FID
JL_L_-_ Ph
I
Py(510°C)-qt.-FID
m
Py(61OW-g.c.-
FID
Py(770°C)-g.c.-FID
Py(610°C)-g.c.-
FPD
Py(7700CLg.c.-FPD
Time -
Time -
Figure 7 Curie-pointpyrolysis(258,510,610,770”Ctg.c. traces of the Provence coal. For assigned structures see ~(gurc 8
I I c-2 s
4
2
It a7
S
5
I
Cl &
Musklgum
S 6
Figure 9 Partial Curie-point pyrolysis (770”Ctg.c.-.FPD traces of the alkyldibenzothiophenes of the Provence coal and of two reference coals. For assigned structures see Figure 8
II
12
Figure 8 Structures of sulphur compounds in the pyrolysates: 1, thiophene; 2, methylthiophene; 3, C2 alkylthiophene; 4, C, alkylthiophene; 5, benzothiophene; 6, methylbenzothiophene; 7, C, alkylbenzothiophene; 8, C, alkylbenzothiophene; 9, dibenzothiophene; 10, methyldibenzothiophene; 11, C2 alkyldibenzothiophene; 12, C3 alkyldibenzothiophene
give the distribution of the total sulphur, but only a very poor chemical characterization. Curie-point pyrolysis gives an accurate picture of some organic sulphur species present in the coal, while PTR and PTO offer both some degree of resolution and quantification leading to information about the structure and reactivity of the total coal organic sulphur.
FUEL, 1987, Vol66,
November
1565
Identification
of some sulphur species in a high
orgainc
Relation between structure and thermal stability of organic sulphur compounds in coal are difficult to establish Owing to thecomplexity of coal structure and thermal reactions -combustion, thermoreduction, pyrolysis - the interpretation of thermal technique data is fairly difficult and requires much investigation of the mechanisms of thermal degradation. During coal combustion, SO, peaks may arise from the oxidation of several sulphur species. These sulphur species may not be oxidized independently of the network to which they are bound. The oxidative peak temperature of pyrite is lowered with decrease of particle size30. The exothermic profile of pyrite may be multi-peak and be strongly modified by the presence of iron and sulphates or by the pyrite level in the coal sample”. The SO, profile may be perturbed by strong exo- or endothermic reactions. The SO,, formed by oxidation of sulphur, may be trapped in the solid residue as sulphates, as seen in the Provence coal by X-ray diffraction spectrometry of the char. An example of chemical reactions which may contribute to the sulphur fixation in the coal (as CaS04) can be represented by the following reactions:
sulphur
coal: J. P. Boudou
et al.
coal break down at rates similar to those of the model compounds. Aliphatic and benzylic sulphides, mercaptans and disulphides would extrude H,S and CS,. The thiophenic compounds (thiophene, thianaphtene and dibenzothiophene) show little conversion, even at 950°C. In the second pathway highly reactive hydrocarbon radicals are formed which can react with sulphur to produce volatile secondary sulphur compounds”,3g~40~42. Calkins” has found that during flash pyrolysis pyrite can react with the coal matrix above 775°C. Johnson4’ has shown that benzenethiol is remarkably resistant to thermal reactions in the absence of oxygen. Conversion into products only become significant above 650°C when pyrolysis of benzenethiol yields benzene, diphenyl sulphide and diphenyl disulphide as the major products. Dibenzothiophene, biphenyl, thianthrene and diphenyl trisulphide are minor products. Braekman-Danheux42 has found that the desulphurization yield of sulphur aromatic model compounds such as thiophene, methylthiophene or benzothiophene is very low and the main products obtained are higher molecular weight sulphur compounds.
S(coa1) + 0, + SO,
No reported
CaCO,--+CaO+CO,
Electron microprobe analysis, which can be used to examine unground coal samples, has been proved to be unsuitable to quantify sulphur content in the finer grains of pyrite scattered in the coal matrix. XPS spectroscopy has proved itself to be unable to distinguish pyritic sulphur from organic sulphur. Other heavy spectrometric methods are Miissbauer spectrometry, high-resolution Xray diffraction spectroscopy and synchrotron X-ray spectroscopy’, for example. Much of the pyrite is finely dispersed in the coal and is coated with organic materials that may hinder physical contact with reagents or produce interference during thermal degradation. Pyrolysis cannot selectively degrade pyrite because of interferences between pyrite organic Programmed and matter. temperature because pyrolysis 43 has been shown to be unsatisfactory of an eventual overlapping between the organic sulphur peak and the pyritic sulphur peak. Mild oxidation might help to find a solution to quantify pyritic sulphur in a satisfactory way. Low-temperature ashing (LTA), followed by ash and evolved gas analysis has been used to analyse coal mineral and organic matter, but this technique is very time and sample consuming. However, it is believed, on the basis of our own results and others8,44,45, that slow oxidation may offer a simple solution for rapid pyrite analysis in coal microsamples for monitoring purposes.
SO2 + CaO + l/2 O2 -+ CaSO, These reactions of SO, fixation by minerals may occur above 500”C32. The possible reactions of adsorptiondesorption of the SO, with the char have been shown to occur below 400”C33. Chantret considers that SO, could be trapped at low temperature by ion exchange with the clay in the sample. Thermoreduction of sulphur model compounds has shown that pure organic sulphur species may give several peaks which can overlap in the PTR curve. Thiols would be reduced at low-temperature peaks. Even when thiol groups are removed by oxidation with hydrogen peroxide or nitric acid solutions, the coal kinetogram still shows a which presumably comes from the ‘thiol’ peak3’, reduction of aliphatic or aromatic sulphides, which may give both low- and high-temperature peaks: for instance, and poly(phenylene sulphide)35 give a thianthrene” thermokinetogram indicating one -SH, one sulphide and one thiophene or condensed thiophene. Thiophenes give the highest temperature peaks or no peak at all. Condensed thiophenes and aromatic sulphides do not give any peak at all; their amount is calculated by difference, with the total organic sulphur itself calculated by difference between the total sulphur and the pyritic sulphur. The result of the analysis does not differentiate between the reduced and the oxidized form of a sulphur group. Therefore structural assignment of the H,S peaks in the PTR kinetogram remains speculative. The thermoreduction method can only provide us with a fingerprint of coal sulphur functionalities and reactivities. We think there are two -at least -formation pathways for organic sulphur compounds during pyrolysis: the heteroatomic structures are present as such in the kerogen and are released during pyrolysis or they result from secondary reactions. The first pathway is supported by theories on the origin of oil sulphur compounds2g*36-38. Calkins’ work’ ’ supports the view that the labile sulphur compounds in
1566
FUEL, 1987,
Vol 66, November
technique to quantify pyrite is satisfactory
How abundant are thiol groups in coal? Ignasiak46 showed that the high organic sulphur Rasa coal contains one-third of its sulphur in the form of sulphides, but no thiol at all. According to Jones23, the vitrinite of high-volatile bituminous coal mainly includes sulphides and thiophenes rather than disulphides and thiols. Calkins” suggests that the aliphatic sulphur compounds in coal are sulphides. However thiol sulphur forms have been frequently noted though poorly quantified. According to Attar14, ‘18-30x of the organic sulphur in high-sulphur coals is sulphidic, l&40% thiolic’. Postovski and Harlampovich4’ have measured thiol and thioether contents in coal by reaction with
Identification
of some
sulphur
species in a high orgainc sulphur coal: J. P. Boudou
methyl iodide, but various reports in the literature cast doubts on the utility of this approach, since anhydrous conditions are required for the method to be valid and such conditions are difficult to achieve with coals. Bodzek and Marzec4* have identified indane thiol and naphthalene thiol in solvent extracts of a Carboniferous coal from Poland, but the authors have not quantified their results. Analysis of sulphur species in the Provence coal by the method of Purnell and Doolan49 indicated a larger quantity of pyrite than ASTM method did. Instead, this ‘excess’ sulphur is probably thiolic organic sulphur, which decomposed under the same general conditions as pyrite in the test. Non-thiophenic functions might partly account for the high micro-metal contents of the Provence coal: the complexing role of the thiol groups is well known in biological systems50m52and is thought to be prominent in metal speciation in recent sediments53. Is coal organic sulphur thiophenic? No method gives a quantification of the total thiophenic sulphur content in coal, Its indirectly determined value can be obtained by programmed temperature reduction, using either synthetic thiophenic compounds as models for standardization or calculation of the difference between the supposed non-thiophenic sulphur and the total organic sulphur. The total organic sulphur is calculated as the difference between the ASTM total sulphur and the ASTM pyritic sulphur. Hence these quantifications are fraught with uncertainties. Curie-point pyrolysis emphasizes the thiophenic sulphur, but thiophenic compounds only represent a very small part of the total sulphur compounds released (mainly H,S, which is presumably released by thermal degradation of the thermally unstable non-thiophenic coal components). Other indirect evidence of the predominance of thiophenic compounds in coal has been reported elsewhere. Preoxidation of coal followed by XPS spectroscopy23 of FTi .r .54 has shown that sulphides and thiophenes are the major sulphur species in vitrinite. Comparison of Xray absorption spectroscopy fingerprints of crude coal with XAS fingerprints of pure compounds taken as models show that coal sulphur is mainly pyritic and thiophenic?. It has recently been shown that coal thiophenic compounds (sulphur present in thiophenic rings may be regarded as replacing two aromatic carbon atoms) are so abundant in coal that they could introduce noise into aromaticity measurements by CP-MAS 13C n.m.r.55. Organosulphur-metallic sulphur coals
species may exist in high organic
Our observations that the highest concentrations of sulphur and trace metals occur most frequently in vitrinite and liptinite are in agreement with the conclusions of other authors56,57. Bouska5(j states that most trace elements in coal are bound to the humic substances (possible vitrinite precursors), which have a high sorption capacity, particularly at the early stages of coalification. Humic substances and not living plants are responsible for entrapment and enrichment in peat bog environments5*. On the basis of the study of trace elements in Upper Cretaceous coals of Transbaikalia, Admakin59 has found that the concentrations of some metals in collinite exceed 3-10 times their amounts in
et al.
tellinite, in which the Ge content, for example, only approaches the Clarke value. This implies that the elements tend to concentrate in coal of collinitic types, like the Provence coal, where the decomposition of plant material is well advanced. Therefore part of the metals associated with organic matter may be bound with it 57.60-64 Understanding the mechanisms of sulphur incorporation into coal organic matter is still speculative
The actual chemical mechanism for sulphur enrichment of organic matter during the early stages of coalification is still unknown. Sulphur compounds present in the water column and the pore water would react with plant and peat organic matter bearing reactive groups 22*65m70. These mechanisms seem to be understood in near-shore marine sediments71. In these sediments, plant proteins and lipids, as well as bacterially derived organic sulphur or ester-sulphates, would be unlikely to produce an increase of sulphur during the early diagenesis. Considering the reactivity of reduced inorganic sulphur species (H,S or its oxidation products) and their abundance in sediments, chemical addition of sulphur to the humic matrix during humilication would be the more likely mechanism. Since pyrite and organic sulphur are finely disseminated in the matrix of the Provence coal, we can speculate that sulphur incorporation has occurred during the sediment deposition and the first stages of coalification (syngenetic sulphur). Sulphur incorporation into the coal organic matter of the Provence coal would have been more important in vitrinite and exinite than in inertinite because the plant organic matter precursors of these almost structureless macerals have probably undergone more intense microbial alterations than inertinite. Microbial alterations would have introduced functional groups into the precursors of vitrinite and exinite and increased their reactivity to sulphur and metals. Thus the common occurrence of organic sulphur and metals in collinite of the Provence coal leads us to think that organic sulphur could have played the role of Understanding sulphur selective metal trapping. incorporation into coal organic matter may lead to explanations for metal enrichment. Alkyl substituents of the thiophenic compounds in the coal Curie-point pyrolysates of the Provence coal might be the vestige of former alkyl sulphides or alkylaryl sulphides at the origin of the thiophenic structure by cyclization and aromatization72. Alkylarylthiophene moieties could be linked with the kerogen by alkyl bridges or hydroaromatic units, as seen in coal structures73-75. Sulphur remaining bound in aliphatic sulphide groups is expected to be thermally labile and would mainly form hydrogen sulphide upon thermal decomposition during the bituminous stage (catagenesis)76. Sedimentologically the Provence coal is highly comparable to the Italian Sulcis or the Spanish Berga coals which were deposited in fresh-water, calcium-rich environments with a low rate of deposition. These coals show similar properties as marine-influenced coals. Owing to a high pH, due to calcium, bacterial activity is accelerated, resulting in increased degradation of plant remains, and there is a depletion of soluble iron. Furthermore, slowly accumulating coal layers have had longer time for bacterial generation of sulphides77. H,S
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Identification
of some sulphur species in a high orgainc sulphur
and/or polysulphides produced in the anoxia have reacted with organic substances. Hence, most calciumrich coals are remarkably high in organic sulphur and syngenetic pyrite78*79. An extreme example of a calciumrich coal is the Rasa coal of Istria (Yugoslavia)*‘, which carries 11 ‘A organic sulphur.
5 6 7 8 9
CONCLUSIONS
10
In this paper we have reported some preliminary data on the analyses of a number of organic sulphur species in Provence coal using complementary methods of investigation - microscopic, spectroscopic and thermal methods. Two US high organic sulphur coals were studied for comparison with the Provence coal. Each particular method has contributed further to an overall picture of the sulphur distribution and functionalities in the Provence coal. This work has evidenced the microheterogeneous character of sulphur distribution in Provence coal where macerals and minerals are very small. The major proportion of organic sulphur and trace metals are associated with vitrinite. They could have been incorporated during deposition, at the peat stage, in humic-like compounds which could have been the possible precursors of vitrinite in the resulting coal. The Provence coal contains thermally labile organic sulphur compounds, such as thiols and aliphatic sulphides, and thermally stable organic sulphur compounds, such as aromatic and high molecular weight compounds. From the results of this preliminary study, we can visualize a model of the structural basic units of the coal network and of the molecular phase” as benzenic, sulphur benzenic and thiophenic building blocks connected (or not) by alkyl or heteroalkyl bridges and substituted (or not) by sulphur side chains, thiol groups and may be oxidized sulphur groups in very low abundance. A small amount of organic sulphur would be bound with trace metals.
ACKNOWLEDGEMENTS
11 12 13 14
15
16 17 18 19 20 21 22
23 24
25 26 27 28 29 30 31
The authors would like to thank Cerchar (Verneuil-enHalatte, France) for supplying the Provence coal sample and for having invited Dr A. Attar (Coal Gas Inc., Raleigh, USA) for a visit to France in 1986. Dr A. Attar and Dr F. Chantret (Cogema, France) are thanked for their contribution to this work. We also thank Science et Surface (‘La Combe de Charbonnitres’, 78 route de Paris, 69260 Charbonnitres, France) for ESCA measurements. This study was partly supported by ATP charbon (CRNS/ PIRSEM, France). This support is gratefully acknowledged. The FOMautopyms work was supported by FOM (Foundation of Fundamental Research) in the Netherlands.
32
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