Hydrous pyrolysis reactions of sulphur in three Australian brown coals

PII:

Org. Geochem. Vol. 29, No. 5±7, pp. 1469±1485, 1998 # 1998 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain S0146-6380(98)00137-5 0146-6380/98 $ - see front matter

Hydrous pyrolysis reactions of sulphur in three Australian brown coals SONG ZHIGUANG, BARRY D. BATTS* and JOHN W. SMITH School of Chemistry, Macquarie University, Sydney, NSW 2109, Australia AbstractÐHydrous pyrolysis of three Australian brown coals in the presence of cadmium acetate (CdOAc) indicates that as much as 77% of the original organic sulphur may be generated as H2S. In the absence of CdOAc much of the generated H2S was re-adsorbed back into the insoluble coal residues. GC±MS analyses of the solvent extracts of the coals and of the solvent extracts of the hydrous pyrolysates at 110±3308C both in the presence and absence of cadmium acetate reveal a little parent thiophene and benzothiophene totally dominated by a broad range of methyl derivatives. Yields of these sulphur compounds tend to be greater when cadmium acetate is absent. Benzothiophenes are favoured over thiophenes at the higher hydrolysis temperatures. Analyses of the solvent extracted residues by FP±GC±MS and of mixtures of low sulphur coal and elemental sulphur, produced a similar range of parent thiophene and benzothiophene and their methylated derivatives. The ubiquity of these sulphur compounds suggests these to be the thermodynamically favoured products of sulphur/organic matter interaction. The only other class of sulphur compounds seen, the longer chain n-alkyl thiophenes, occurred in the soluble 2008C hydrous pyrolysis products obtained in the absence of cadmium acetate. Only these appear to re¯ect structures in the original brown coals. The implication of this study is that the earliest formed organic sulphur compounds in immature sediment will, with increasing temperature, continue to decompose and release H2S and elemental sulphur. The released sulphur species will, in turn, undergo reaction with the organic matter to form more stable sulphur compounds. Sulphur incorporation is likely to be a continuing process throughout diagenesis and catagenesis. # 1998 Published by Elsevier Science Ltd. All rights reserved Key wordsÐhydrous pyrolysis, brown coals, organic sulphur, hydrogen sulphide, secondary reaction, sulphur compounds, ¯ash pyrolysis

INTRODUCTION

The occurrence and mode of incorporation of organic sulphur compounds in coals continue to command extensive literature coverage. By way of contrast, this study considers the reactivity of these various sulphur forms and in particular the changes these sulphur species undergo both during laboratory simulated and natural coal maturation processes. With these objectives in mind three Australian brown coals were investigated by hydrous pyrolysis (HyPy) under a wide range of temperatures and conditions. Products were analysed and identi®ed by gas chromatography±mass spectrometry (GC±MS) and ¯ash pyrolysis GC±MS (FP±GC±MS) analysis. Hydrous pyrolysis was chosen as the main degradative approach as this technique brings about the cleavage of the weaker hetero-atom bonds by hydrolysis/hydrogenation without a€ecting C±C linkages (Brooks and Smith, 1969; Lewan, 1985). Particular attention was paid to the nature of any secondary reactions between the newly released hydrogen sulphide, the residual coal and new compounds generated during the HyPy process. *To whom correspondence should be addressed. Tel.: +61-2-9850-8275; Fax: +61-2-9850-8313.

Previous studies have focused on sulphur addition to the coaly organic matter at the early stages of diagenesis (Nissenbaum and Kaplan, 1972; Casagrande and Ng, 1979; Casagrande et al., 1979; Francois, 1987; Philp et al., 1992) and it is widely accepted that the major pathway for the formation of organic sulphur compounds at that stage is via reaction of the reduced sulphur species, H2S, sulphur and/or polysulphide reactions with organic material (Orr, 1978; Vairavamurthy and Mopper, 1987). However, a detailed knowledge of the actual mechanism(s) for incorporation of sulphur into the coal, the chemical changes undergone during maturation and the formation of sulphur-rich coals and kerogen are lacking. Earlier studies of the thermal behaviour of coals show that the hydrogen sulphide or sulphur released during high temperature pyrolysis interacts with the residual coal (Attar, 1978; Calkins, 1987; Ibarra et al., 1987). Furthermore some inorganic species, such as dolomite and limestone, will also capture hydrogen sulphide freed when pyrolysing or partially gasifying coal (Liliedahl et al., 1992; Fenouil et al., 1994; Zevenhoven et al., 1996). An inevitable question is, do reactions of this type occur during low temperature hydrous pyrolysis and in natural geological systems? With this concern in mind, hydrous pyrolysis experiments, both

1469

1470

Z. Song et al.

in the presence and absence of cadmium acetate (CdOAc), were carried out at 110, 200 and 3308C, respectively. The aim of using CdOAc was to capture the hydrogen sulphide as it was formed and to allow the thermally evolved H2S to be quanti®ed. In the absence of CdOAc the chemical behaviour of the hydrogen sulphide released from the coal during HyPy is demonstrated. EXPERIMENTAL

Three Australian brown coals investigated in this study were from Victoria (Loy Yang and Anglesea) and South Australia (Lochiel). The chemical analysis data of these brown coals are quoted on an airdried-basis and presented in Table 1. As a ®rst step, all three coals were Soxhlet extracted for 3 days with a methyl alcohol (MeOH)/dichloromethane (DCM) mixture (1:9) and the recovered solvent extracts concentrated, weighed and retained. The insoluble residues were washed with HCl to remove traces of sulphate, either originally present or resulting from oxidation of other sulphur species, and dried. In essence, as shown in Table 1, the bulk of the sulphur in the coals is in the form of organic sulphur. The procedure used for hydrous pyrolysis was to transfer 0.5 g of the extracted coal together with 0.5 ml of water in a Pyrex tube of 2±3 ml capacity. Calculations show that water in the liquid phase was present in the reaction tube at all temperatures used in these experiments. Cadmium acetate (CdOAc), in slight excess of the amount needed to convert all the sulphur in the coal to cadmium sulphide, was added, if required, at this stage. After sealing the reactor tube with a small ¯ame, the tube was placed in a steel bomb of 10 ml volume with 2 ml of water to equalise pressure and the bomb was sealed. The bomb was held at the desired reaction temperature for 66 h, after which it was cooled to room temperature and opened. The glass reactor was recovered and cooled in liquid nitrogen. Whilst frozen, it was broken under DCM. Finally, the total reaction products were washed

with methanol and DCM into a Soxhlet extractor where they were extracted with MeOH:DCM (1:9) for 16 h. Copper turnings were added to remove any elemental sulphur generated during hydrous pyrolysis. At this stage the soluble extract was evaporated to a small volume and retained for further analysis, as was a weighed portion of the insoluble residue. The remainder of the residue was reacted under water at 1008C with excess hydrochloric acid in a stream of nitrogen to form hydrogen sulphide. The gas stream was ®rst washed in a water trap to remove hydrochloric acid vapour and then passed into a 10% silver nitrate solution, where the hydrogen sulphide was precipitated as the silver salt. This was recovered by ®ltration, washed with ammonia and water, dried and weighed. To check whether any additional elemental sulphur had been formed, or released during acidi®cation, 0.5 g of iron powder was added to the reactants in the ¯ask and any additional liberated hydrogen sulphide was collected and weighed, as described previously. Additions of aqueous barium chloride to the ®ltered, acidi®ed solution from the reaction vessel allowed the presence or absence of sulphate to be con®rmed. After hydrous pyrolysis, the sulphur content of the residual coal was determined by combustion of a portion of the residue with Eschka mixture and recovered as barium sulphate. The organic solvent extracts of both the original coal and of the products of hydrous pyrolysis were separated by column chromatography on silica gel/alumina. The paranic, aromatic and polar fractions were displaced by sequential elution using pentane, pentane:DCM (1:4) and DCM:MeOH (1:1) respectively, concentrated and retained. In this study the bulk of the organic sulphur compounds (OSCs) were detected in the aromatic fraction. Only traces of sulphur species were found in the aliphatic fraction. Products were ®nally identi®ed by GC±MS using a Fisons GC8000 Gas Chromatograph linked to a Fisons MD800 Mass Spectrometer with a Masslab data acquisition system. The mass spectrometer operating parameters were: an ionisation

Table 1. Chemical compositions of brown coals samples Coal samples

Air-dried basis Moisture Ash Carbon (uncorr. for CO2) Hydrogen Nitrogen Oxygen (di€erence) Total sufur Pyritic sulphur Sulphate sulphur Organic sulphur (Sorg) (di€erence) Sorg content of extracted and HCl treated coals (%)

Loy Yang

Anglesea

Lochiel

9.0 1.0 62.3 4.3 0.5 22.7 0.2 <0.01

11.1 2.6 59.1 4.0 0.6 19.7 2.85 0.03 0.34 2.58 2.58

9.5 14.1 51.8 3.8 0.6 17.9 2.95 0.11 0.46 2.38 2.74

0.2 0.20

Except for the data in the bottom row, all others are original CSIRO data.

Hydrous pyrolysis reactions of sulfur

energy of 70 eV, a source temperature of 2008C, emission current 100 mA, a scan rate of 35±600 Da in 2 s with an interface temperature of 2508C. The gas chromatograph was operated in a splitless mode using helium gas as carrier, a (J&W) DB1 column (50 m  0.33 m) and an injector temperature of 2508C. The oven was held at 408C for an initial 4 min and then heated at 48C per minute to 3008C and held at that temperature for 30 min. Hydrous pyrolysis experiments were carried out using either one of two heating regimes. In one, a set temperature was employed and a fresh coal sample was used at each HyPy experiment. The reaction products were isolated and analysed as above. In the other, a sequential or incremental process was used, that is the fresh coal sample was only used in the initial temperature of hydrous pyrolysis at 2008C. After removal of H2S and soluble products, the insoluble pyrolysate (solid residue) was placed back into the bomb and the experiment repeated at 230, 270 and 3308C. The initial extracted coals and insoluble residues remaining from HyPy were also characterised by FP±GC±MS using an SGE Pyrojector coupled to a splitless injector. The assembly was ®tted to the GC8000 Gas Chromatograph interfaced with the MD800 mass spectrometer and the same mass spectrometer conditions as described above were used. Injection was via a SGE solids injector at a temperature of 5508C.

1471

RESULTS AND DISCUSSION

H2S release/entrapment The sulphur content of the insoluble pyrolysates from the HyPy at 110, 200 and 3308C of coals, both in the presence and absence of cadmium acetate (CdOAc), are shown in Fig. 1. The much higher sulphur content in the absence of CdOAc is clearly evident at the higher temperatures. Changes in the amounts of hydrogen sulphide generated from the extracted coals and of sulphur remaining in the residual coals after HyPy in the presence of CdOAc, at temperatures from 200 to 3308C, are presented in Fig. 2. The ®gure shows that H2S and the sulphur species in the coal residue account for up to 95% of the total sulphur in the original coal. The soluble hydrous pyrolysates together with other unidenti®ed losses (such as SO2, COS, sulphur and sulphate) account for 1±14%. These di€erences in the sulphur content of the coal residues, with and without CdOAc, may be readily explained. The H2S or sulphur produced by HyPy of the organic sulphur containing compounds, if not trapped by CdOAc, reacts with the residual coal. The extent of this secondary reaction is also shown in Fig. 1. In the absence of CdOAc, the sulphur content of the residual coals is signi®cantly greater at 200 and 3308C than that in the residual coals from the HyPy in the presence of CdOAc for all three coals. In other words, after

Fig. 1. Comparison of organic sulphur contents in coal residues of HyPy both in the presence and absence of CdOAc at 110, 200 and 3308C.

1472

Z. Song et al.

Fig. 2. Sulphur balance of three brown coals after HyPy in the presence of CdOAc at temperature ranges of 200±3308C.

HyPy at 3308C without CdOAc, approximately 70% of the sulphur in the Lochiel and Anglesea coal residues results from this secondary reaction. Neither the nature of this secondary reaction for in-

corporation of sulphur, nor the nature of the reaction product is known but, on subjecting the residues from the 3308C pyrolysis with CdOAc to further HyPy at the same temperature, only 3±5%

Hydrous pyrolysis reactions of sulfur

1473

Fig. 3. Mass chromatograms of m/z 97 + 98 + 111 + 112 + 125 + 126 + 139 + 140 + 154 of thiophenes found in aromatics of pyrolysates of sequential HyPy of three coals in the presence of CdOAc at the temperature range of 200±3308C (see Table 2 for numbering system, NP = naphthalene).

of the sulphur retained in the coal was released as hydrogen sulphide. This demonstrates the thermodynamic stability of the secondary products formed under the conditions employed. In seam coal, H2S

is a very rare component of seam gas and elemental sulphur only occurs in traces (Smith and Philips, 1990). Therefore it appears that the survival of the sulphur in coal may re¯ect, at least in part, this re-

Table 2. Alkylthiophenes and alkylbenzothiophenes identi®ed in the solvent extracts of coals and their hydrous pyrolysis products No.

Alkylthiophenes

No.

Alkylbenzothiophenes

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

thiophene 2-methylthiophene 3-methylthiophene 2-ethylthiophene 2,5-dimethylthiophene 3,4-dimethylthiophene 2,3-dimethylthiophene 2,4-dimethylthiophene trimethylthiophene trimethylthiophene trimethylthiophene trimethylthiophene trimethylthiophene tetramethylthiophene tetramethylthiophene tetramethylthiophene pentylmethylthiophene pentylmethylthiophene

1 2 3 4 5 6 7 8 9 10 11 12 13 14±23 24±27

Benzothiophene 3-methyl benzothiophene 4-methylbenzothiophene 2-methylbenzothiophene 6-methylbenzothiophene 2,5-dimethylbenzothiophene 7-ethyl-benzothiophene dimethylbenzothiophene dimethylbenzothiophene dimethylbenzothiophene dimethylbenzothiophene dimethylbenzothiophene dimethylbenzothiophene tri-methylbenzothiophene tetra-methylbenzothiophene

1474

Z. Song et al.

incorporation reaction throughout diagenesis and possibly also catagenesis. The percentages of hydrogen sulphide released by hydrous pyrolysis of the coals at 110, 150 and 1758C in the presence of CdOAc are 1.2, 6.3 and 12.0% for Anglesea and 2.0, 6.6 and 17.0% for Lochiel, respectively. These data indicate that the two higher sulphur coals release traces of sulphur as H2S at temperatures as low as 1108C, with signi®cant amounts liberated at 1508C. From Fig. 1 it

can be seen that the sulphur content of Lochiel coal residues from hydrous pyrolysis at 1108C, both in the presence and absence of CdOAc is similar. Identi®cation and distribution of OSCs in the soluble extracts and pyrolysates In the solvent extracts of initial coals. The only sulphur compounds detected in the solvent extracts of the original coals by GC±MS analysis were traces of C1±C2 methylated thiophenes.

Fig. 4(a,b).

Hydrous pyrolysis reactions of sulfur

1475

Fig. 4. Benzothiophenes identi®ed in aromatics of soluble pyrolysates of sequential HyPy of (a) Lochiel coal, (b) Anglesea coal and (c) Loy Yang coal in the presence of CdOAc at temperature range of 200± 3308C (see Table 2 for numbering system, # = 3,4-dihydro-2,2-dimethyl-2H-1-benzopyran, * = examethylbenzene).

In the soluble pyrolysates. Analysis of soluble pyrolysates from separate HyPy of Lochiel at 110, 150 and 1758C in the presence of CdOAc indicate that no organic sulphur compounds are formed in the pyrolysates of 1108C. Only two thiophenic sulphur compounds were identi®ed in the pyrolysate of 1508C while methyl-benzothiophenes initially occurred in the pyrolysate of 1758C. Dibenzothiophene was identi®ed only in the pyrolysates of the two higher sulphur coals at 3308C in the absence of CdOAc. Figure 3 and Table 2 are devoted to the distribution and identi®cation of alkyl thiophenes in the soluble pyrolysates of sequential HyPy of three coals from 200 to 3308C in the presence of CdOAc. Thiophene is absent, and generally C1-thiophenes are present in trace quantities. The di-, tri- and, on occasion, tetramethyl-thiophenes dominate the chromatograms and reach their maximum at 2008C for Lochiel and Loy Yang coals and 2308C for Anglesea coal. However these methyl-thiophenes are not uniformly distributed throughout the incremental fractions. The soluble pyrolysates from Loy Yang at 230 and 2708C, from Anglesea at 200 and 2708C and from Lochiel at 2708C, are devoid of thiophenes and short chain thiophenes. Figure 4 and Table 2 similarly display the distributions of benzothiophene and their methylated derivatives in these soluble incremental pyrolysates. The Lochiel and Anglesea data sets (Fig. 4a and b respectively)

show little variation with pyrolysis temperature in the distribution of parent and C1±C3 benzothiophenes. However, a relative increase in the abundance of these C3-benzothiophenes and the appearance of trace C4-benzothiophenes in the 2708C fraction are evident. The Loy Yang data (Fig. 4c) are generally less well-de®ned, but continues to support the same distinctive distribution feature at 2708C. In the soluble pyrolysates formed in the presence and absence of CdOAc. The total sulphur compounds identi®ed in the soluble pyrolysates from two sets of hydrous pyrolysis experiments, both with and without CdOAc, are comparably displayed in Figs 5±8 and listed in Tables 2 and 3. These show that the occurrences of methylated thiophenes and benzothiophenes in all pyrolysates of these two sets of experiments are generally similar, except for the longer chain (C4±C24) alkylthiophenes (Table 4) and organic polysulphides and elemental sulphur (Fig. 7) in the soluble pyrolysates at 2008C. The ratios of total thiophenes to naphthalene (NP) at 3308C and benzothiophenes to indene at 2008C and benzothiophene to C4-benzene (tB) at 3308C are much lower in the soluble pyrolysates formed from HyPy in the presence of CdOAc than those formed in the absence of CdOAc (Fig. 5b, Fig. 6a and b). In the absence of CdOAc in the hydrous pyrolysis experiments, the longer-chain monoalkylthiophenes and di- and trimethyl and longer chain

1476

Z. Song et al.

Fig. 5. Thiophenes identi®ed in the soluble hydrous pyrolysates of Lochiel and Anglesea coals both in the presence and absence of CdOAc at (a) 200 and (b) 3308C (see Table 2 for numbering system, dmS3 indicates dimethyltrisulphide, dms4 dimethyltetrasulphide, NP naphthalene and total T/NP the ratios of thiophenes to naphthalene).

Hydrous pyrolysis reactions of sulfur

1477

Fig. 6. Mass chromatograms of m/z 134 + 147 + 148 + 161 + 162 of benzothiophenes identi®ed in the aromatics of Anglesea and Lochiel soluble pyrolysates from two sets of HyPy in the presence and absence of CdOAc at (a) 200 and (b) 3308C (see Table 2 for numbering system, tB = tetramethyl-benzene and total-BTs/tB is the ratio of total benzothiophenes to tB).

alkylthiophenes were detected in the soluble pyrolysates at 2008C (Table 4), as was dibenzothiophene in the 3308C soluble pyrolysates. The resultant

organic polysulphides, for example dimethyl-disulphide, trisulphide and tetrasulphide, diethyltrisulphide and propyldisulphide [(CnH2n + 1)2Sx,

1478

Z. Song et al.

Fig. 7. Mass chromatograms of m/z 64 + 65 + 79 + 94 + 108 + 150 + 158 + 182 + 256 showing organic polysulphides and elemental sulphur in the soluble pyrolysates from HyPy at 2008C of Anglesea and Lochiel coals and of a mixture of Loy Yang coal and sulphur in the absence of CdOAc.

1 R n R 3, 2 Rx R 4] and elemental sulphur formed in the soluble pyrolysates at 2008C, are depicted in Fig. 7. Various linear alkylthiophenes have been found in immature sediments and crude oils and it

has been suggested that they were formed by the addition of sulphur to unsaturated functional groups (Sinninghe Damste and de Leeuw, 1989 and references therein). Because the longer chain

Fig. 8. Thiophenes identi®ed from extracted coals by FP±GC±MS (see Table 2 for numbering system).

Hydrous pyrolysis reactions of sulfur

1479

Table 3. OSCs identi®ed in the soluble hydrous pyrolysates of Anglesea and Lochiel coals In the absence of CdOAc 2008C Thiophene (T) C1-Ts C2-Ts C3-Ts C4-Ts C5-Ts Longer chain thiophenes Polysulphides

Elemental sulphur (S8) Benzothiophene (BT) C1-BTs C2-BTs C3-BTs C4-BTs Dibenzothiophene (DBT) Thiophene (T) C1-Ts C2-Ts C3-Ts C4-Ts C5-Ts Longer chain thiophenes Polysulphides

Elemental sulphur (S8) Benzothiophene (BT) C1-BTs C2-BTs C3-BTs C4-BTs Dibenzothiophene (DBT)

In the presence of CdOAc

3308C

2008C

3308C

trace trace

C1-Ts C2-Ts C3-Ts

Anglesea coal C1-Ts C2-Ts C3-Ts C4-Ts trace C4-C24 alkylthiophenes dimethyldisulphide dimethyltrisulphide propyldisulphide dimethyltetrasulphide diethyltrisulphide S8 BT C1-BTs C2-BTs trace

C1-Ts C2-Ts C3-Ts trace

S8 BT C1-BTs C2-BTs C3-BTs C4-BTs DBT

BT C1-BTs C2-BTs trace

BT C1-BTs C2-BTs C3-BTs C4-BTs

C1-Ts C2-Ts C3-Ts C4-Ts trace

trace C2-Ts C3-Ts

BT C1-BTs C2-BTs trace

BT C1-BTs C2-BTs C3-BTs trace

Lochiel coal C1-Ts C2-Ts C3-Ts C4-Ts C4-C21 alkylthiophenes dimethyldisulphide dimethyltrisulphide propyldisulphide dimethyl-tetrasulphide diethyltrisulphide S8 BT C1-BTs C2-BTs trace

C1-Ts C2-Ts C3-Ts trace

S8 BT C1-BTs C2-BTs C3-BTs C4-BTs DBT

alkylthiophenes in this study were found only in the pyrolysate of HyPy at lower temperatures (2008C), it is possible that unsaturated alkenes, acids, esters or ketones of various chain lengths are likely precursors for these sulphur compounds. Sulphur compounds in the insoluble products by FP± GC±MS In the solvent extracted coals. FP±GC±MS analysis of the solvent-extracted coals reveal that thiophene and C1±C3 thiophenes are present in the Anglesea and Lochiel coals, but in trace quantities only in the Loy Yang coal. In contrast, a little benzothiophene and C1-benzothiophenes are found in the two higher sulphur coals and hardly any in the Loy Yang coal. Only alkyl-thiophenes detected in the solvent-extracted coals are presented in Fig. 8 where the C1±C3 thiophenes were dominant sulphur compounds in all three coals. It should be noted that thiophene is found only in the initial extracted coals by FP±GC±MS. The distribution range of methylated thiophenes (C1±C3) in the extracted coals is wider than that of initial solvent extracts of

these coals (C1±C2) and narrower than that of soluble hydrous pyrolysates (C1±C5 in maximum). In the insoluble hydrous pyrolysates. In the Lochiel and Anglesea coals, methylated thiophenes were found to be signi®cant in the insoluble pyrolysates of 2008C, while benzothiophenes were readily observed in the insoluble pyrolysates from the 200 and 3308C experiments (Figs 9 and 10). The presence or absence of CdOAc appears not to a€ect the chromatograms produced by ¯ash pyrolysis of the insoluble pyrolysates of each coal, but there was a relatively higher concentration of these sulphur compounds in the insoluble hydrous pyrolysates when CdOAc was absent (Figs 9 and 10). C1±C3 methylthiophenes dominate the aliphatic sulphur compounds in the coal residues produced at 2008C. The benzothiophenes are dominated by the parent and C1-benzothiophenes. In the 3308C insoluble pyrolysates, benzothiophene and C1±C2 benzothiophenes, (although in relatively lower concentrations) are major sulphur compounds, while thiophenes are virtually absent, or present in trace quantities only. In Table 5 the sulphur compounds identi®ed in the

1480

Z. Song et al. Table 4. Alkylthiophenes identi®ed in the soluble pyrolysates from HyPy at 2008C in the absence of CdOAc of Lochiel, Anglesea and of a mixture of Loy Yang coal and elemental sulphur Sulphur compounds identi®ed C1-Ts(2 peaks) C2-Ts (4 peaks) C3-Ts (3 peaks) C4-Ts (3 peaks) C5-T C6-T C7-T Methyl-C7-T C8-T Methyl-C8-T C9-T Methyl-C9-T C10-T Methyl-C10-T C11-T Methyl-C11-T C12-T Methyl-C12-T C13-T Methyl-C13-T Isomer(a)-C13-T C14-T Methyl-C14-T Unknown-isomer(c) Isomer(b)-C13-T Isomer(d)-C13-T C15-T Methyl-C15-T C16-T Methyl-C16-T C17-T Methyl-C17-T C18-T Methyl-C18-T C19-T Methyl-C19-T C20-T Methyl-C20-T C21-T Methyl-C21-T C22-T Methyl-C22-T C23-T Methyl-C23-T C24-T Methyl-C24-T

Retention time (min) 14.9±15.1 19.2±20.4 23.9±24.9 27.7±28.3 32.6 37.0 40.0 40.4 43.6 43.9 46.7 47.3 50.0 50.6 52.8 53.2 51.8 56.2 58.4 58.9 59.3 60.7 61.1 61.5 62.1 62.8 63.6 64.0 66.0 66.3 68.0 68.4 70.3 70.7 72.2 72.6 74.3 74.7 76.3 76.7 78.4 78.8 80.1 80.5 82.7 83.0

Loy Yang coal + sulphur (%)* trace 15±25 10±25 15 16 10 15 10 20 49 21 42 31 17 38 17 35 20 40 130 20 39 83 90 98 19 33 20 47 24 36 35 72 55 84 46 99 67 69 trace 98 trace 45 trace 82

Anglesea brown coal (%)* 37±60 50±63 20±25 25±30 23 21

Lochiel brown coal (%)* 6±10 20±29 35±50 20±30 20 18

25 trace 36 21 30

23 15 32 28 31

trace

trace

36 trace 30 trace trace 51 trace trace 60 41 39 trace 20

45 trace 37 trace trace 56 trace trace 50 60 61 trace 35

trace trace

trace trace

trace trace 34 trace 53 trace trace trace 50

50 trace trace trace 40 trace trace trace 42

% = relative peak height in mass chromatograms, T = thiophene, Ts = thiophenes, and trace = alkylthiophene peak overlayed by co-eluted peaks.

insoluble pyrolysates in two parallel sets of HyPy experiments, one in the presence of and one in the absence of CdOAc, are compared. Hydrous pyrolysis and ¯ash pyrolysis of Loy Yang coal in the presence of sulphur Laboratory studies of sulphur (elemental sulphur and H2S) addition to organic matter at temperatures less than 508C has demonstrated that sulphur/ sulphide incorporation is the major pathway for the formation of organic sulphur compounds in recent sediments (Vairavamurthy and Mopper, 1987; Philp et al., 1992). The role these incorporation mechanism(s) play in the formation of organic sulphur rich coal and kerogen is not clear. It seems that accumulation of organic sulphur in the coals is more or less the result of a direct reaction (Casagrande and Ng, 1979; Casagrande et al., 1979; Idiz et al., 1990).

For a better understanding of organic sulphur transformation under the conditions of HyPy, the low-S Loy Yang coal was hydrous pyrolysed with added elemental sulphur. The results were presented in Table 5. The data indicate that, under hydrous pyrolysis in the absence of CdOAc, reaction with 2.5% and 3.3% of elemental sulphur resulted in the organically bound sulphur content of the Loy Yang coal (0.2%) being increased to 1.8% after HyPy at 2008C and 2.8% at 3308C. Further analyses (Table 4 and Fig. 7) reveal that the sulphur compounds found in the pyrolysates of Loy Yang at 200 and 3308C in the presence of sulphur are similar to those present in the soluble pyrolysates of Lochiel and Anglesea coals at 200 and 3008C in the absence of CdOAc. The polysulphides and long-chain alkylthiophenes were not detected in these coals when they were hydrous pyrolysed at

Hydrous pyrolysis reactions of sulfur

1481

Table 5. OSCs identi®ed in the insoluble hydrous pyrolysates of Anglesea and Lochiel coals by FP±GC±MS In the absence of CdOAc Original coal Thiophene (T) C1-Ts C2-Ts C3-Ts C4-Ts C5-Ts Benzothiophene (BT) C1-BTs C2-BTs C3-BTs

T C1-Ts C2-Ts C3-Ts C4-Ts

Thiophene (T) C1-Ts C2-Ts C3-Ts C4-Ts C5-Ts Benzothiophene (BT) C1-BTs C2-BTs C3-BTs C4-BTs

T C1-Ts C2-Ts C3-Ts C4-Ts

trace

trace

2008C

In the presence of CdOAc

3308C

2008C

3308C

trace trace trace

C1-Ts C2-Ts C3-Ts C4-Ts trace BT C1-BTs C2-BTs

trace trace trace

Anglesea C1-Ts C2-Ts C3-Ts C4-Ts C5-Ts BT C1-BTs C2-BTs C3-BTs Lochiel coal C1-Ts C2-Ts C3-Ts C4-Ts C5-Ts BT C1-BTs C2-BTs C3-BTs

110 and 3308C. In contrast, both families of compounds were found when Loy Yang coal was hydrous pyrolysed with elemental sulphur at 2008C. The commonality of this range of active sulphur species compounds suggests a similarity in mechanisms for the formation of these sulphur compounds. The chromatograms obtained by ¯ash pyrolysis of the original coal (Fig. 8) are dominated by the same thiophenes as are those of the insoluble residue for 2008C pyrolysis (Fig. 9). Accordingly, the products of ¯ash pyrolysis of coal do not re¯ect necessarily the speciation of sulphur in the coal

BT C1-BTs C2-BTs C3-BTs trace

C1-Ts C2-Ts C3-Ts C4-Ts trace BT C1-BTs C2-BTs

BT C1-BTs C2-BTs C3-BTs trace

molecule. This was shown when the Loy Yang insoluble residue from 200 and 3308C hydrous pyrolysis was ¯ash pyrolysed at 5508C with 10% elemental sulphur. From Fig. 11 it can be seen that the insoluble pyrolysate of Loy Yang coal at 2008C does not generate any thiophenes. After the addition of elemental sulphur, a similar range and types of simple thiophenes and longer chain alkylthiophenes was generated from both insoluble hydrous pyrolysates of 200 and 3308C, but clearly in a lower abundance in the latter. Benzothiophene and its methylated derivatives were also produced but in

Fig. 9. Thiophenes identi®ed by FP±GC±MS in the insoluble pyrolysates from HyPy at 2008C of Lochiel and Anglesea coal both in the presence and absence of CdOAc (see Table 2 for numbering system).

1482

Z. Song et al.

Fig. 10. Benzothiophenes identi®ed by FP±GC±MS in the insoluble hydrous pyrolysates of Lochiel and Anglesea both in the presence and absence of CdOAc at 200 and 3308C (see Table 2 for numbering system, tB = tetramethylbenzene).

much lower abundance. According to the mass spectra, an appreciable quantity of elemental sulphur has been converted to H2S, CS2 and SO2 during ¯ash pyrolysis. Therefore, these sulphur compounds were clearly formed by reaction between at least some of these sulphur species

(including elemental sulphur) and organic molecules under the ¯ash pyrolysis conditions. Figure 11 also clearly shows that the relative abundance of alkene to alkane (Fig. 11, peaks a and b for C15) is much higher in the chromatogram of Loy Yang coal residue alone than that of coal residues with added el-

Table 6. Hydrous pyrolysis of the mixtures of Loy Yang coal (low sulphur) and elemental sulphur in the absence of CdOAc at 2008C and 3308C HyPy temperature (8C) 2008C 3308C

Weight of coal used

Amount of sulphur in original coal

Weight of elemental sulphur added

Amount of sulphur in coal residue after HyPy

Sulphate after HyPy

2.0 g 1.5 g

4 mg/0.2% 3 mg/0.2%

100 mg 100 mg

35.5 mg/1.8% 42 mg/2.8%

trace trace

Fig. 11. Flash pyrolysis±GC±MS of insoluble pyrolysates from HyPy at 2008C of Loy Yang coal residue and of the mixtures of insoluble pyrolysates from HyPy of Loy Yang coal at 200 and 3308C and 10% of elemental sulphur revealed that thiophene (1), methylated thiophenes (peaks 2±6) and longer chain monoalkythiophenes(marked as ± inside the ®gure) were present in the latter ones (± indicates C3±C12 longer chain monoalkylthiophenes, C15 stands for both of C15 alkene (a) and alkane (b)).

Hydrous pyrolysis reactions of sulfur 1483

1484

Z. Song et al.

emental sulphur, thereby indicating that some alkenes may react with the sulphur species to generate longer chain alkylthiophenes. Clearly then, the sulphur compounds identi®ed in both soluble and insoluble pyrolysates need not be directly related to the original sulphur speciation in the original coal. Origin of sulphur compounds in the soluble and insoluble pyrolysates As noted above, methylated thiophenes and methylated benzothiophenes are the two major groups of organic sulphur compounds identi®ed in the coal extracts and the soluble and insoluble hydrous pyrolysates. Their distribution patterns in the coal extracts and pyrolysates are strikingly similar regardless of the initial sulphur content (Figs 3±6 and 8±10) of the coals. C1±C2 thiophenes usually dominate the thiophenic fraction and C1-benzothiophenes dominate the benzothiophenic fraction. A similar overall pattern has been reported previously for the ¯ash pyrolysis products of bitumen and immature oil fractions (Sinninghe Damste et al., 1989; Eglinton et al., 1990; Bakel et al., 1990). It seems that the sulphur compounds identi®ed in the soluble hydrous pyrolysates and by ¯ash pyrolysis of coal and kerogen from various sources are universally similar (Sinninghe Damste et al., 1990). They suggested that this implies a ubiquitous sulphur incorporation process in the early stage of formation, regardless of the type of environment. They further suggested that these sulphur compounds were generated from related thiophenic and benzothiophenic moieties present in the organic macromolecules. However, this study suggests that in nature there are geologically common sulphur moieties, the occurrence of which is largely controlled by thermal stability and sediment maturity, as indicated by the formation of a common range of structures by secondary reactions. The ease with which the ubiquitous sulphur compounds are generated by reaction with sulphur is illustrated by the relatively higher concentration of simple thiophenes and benzothiophenes in the pyrolysate fractions when CdOAc was absent and thus sulphur species (mainly H2S) were in greater concentration. Similarly, dibenzothiophene was only formed in the soluble hydrous pyrolysates of the Lochiel and Anglesea coals at 3308C in the absence of CdOAc and only in Loy Yang 3308C pyrolysate when sulphur was added to the hydrous pyrolysis reactants. Because Loy Yang coal itself generated only few sulphur compounds at low levels, the sulphur compounds identi®ed from the pyrolysis of Loy Yang with sulphur at 200 and 3308C were surely the products of reaction between the added sulphur and the organic matter. The similarity of the organic sulphur compounds produced in this way to those naturally occurring in the two high

sulphur coals possibly indicates some similarity in the pathway of formation. It has been suggested that the structure of the sulphur containing organic compounds will undergo a series of reactions according to the sequence: sulphide 4 thiophene 4 benzothiophene 4 dibenzothiophene (Schmid et al., 1987; Sinninghe Damste and de Leeuw, 1989). In contrast, evidence from this study suggests that thiophenes, benzothiophenes and dibenzothiophene can be formed also by direct reaction between H2S and organic substrates. In this way thiophenes and benzothiophenes can be formed at temperatures below 2008C and dibenzothiophene at 3308C. Therefore, no simple genetic linkages between these main types of sulphur species are demanded. CONCLUSIONS

In the hydrous pyrolysis of brown coals at 3308C as much as 77% of the organic sulphur in two initial higher-S coals is thermally unstable but about 50% in the low-S Loy Yang coal is unstable. This unstable sulphur can be liberated and trapped as H2S when the CdOAc was added to the reactants, while in the absence of CdOAc this gas is re-incorporated to a large extent into the residual coal to re-inhabit as organic sulphur through secondary reaction. Under the hydrous pyrolysis conditions, the hydrogen sulphide and sulphur generated species in the coal residue are two major forms of organic sulphur compounds involved in the organic sulphur transformations. The hydrogen sulphide reincorporation (or secondary reaction) seems to be the dominant form of organic sulphur transformation during maturation processes. On the other hand, the re-incorporation of sulphur at temperatures from 110 to 3308C implies that the sulphur incorporation into organic matter is likely to be a lasting process throughout diagenesis and possibly also catagenesis. GC±MS analysis of the solvent extracts of three brown coals and of the solvent extracts of the residues from hydrous pyrolysis at 110±3308C, both in the presence and absence of CdOAc, revealed no thiophene and only benzothiophene, whereas a wide range of methyl derivatives was observed. Yields of thiophenes and benzothiophenes are reduced in the hydrous pyrolysates when CdOAc was present. An analysis of the corresponding solvent extracted residues by FP±GC±MS and of mixtures of sulphur-depleted coals with added elemental sulphur produced a similar range of thiophene and benzothiophene and their methylated derivatives. The ubiquity of these sulphur compounds suggests they are the thermodynamically favoured products of sulphur/organic matter interactions. Benzothiophenes are favoured over thiophenes at the higher hydrous pyrolysis temperatures. The

Hydrous pyrolysis reactions of sulfur

other class of sulphur compounds seen, the longchain n-alkyl thiophenes, occurred in the soluble products of hydrous pyrolysis at 2008C in the absence of CdOAc. Only these compounds appear to re¯ect organic sulphur changes in the original brown coals, while a number of organic polysulphides may play a important role in the secondary reaction of sulphur re-incorporation. The above results imply that the earliest formed organic sulphur compounds in immature sediment will, with increasing temperature, continue to be decomposed and release H2S and elemental sulphur. The released sulphur species will, in turn, undergo reaction with the organic matter to form more stable sulphur compounds. AcknowledgementsÐThe authors would like to thank the reviewers S. Killops and L. Stalker for their helpful comments and suggestions. The assistance of Dr Daniel Jardine in the mass spectrometric analyses is also appreciated. REFERENCES

Attar, A. (1978) Chemistry, thermodynamics and kinetics of reactions of sulfur in coal-gas reactions: a review. Fuel 57, 201±212. Bakel, A. J., Philp, R. P. and Galvez-Sinibaldi, A. (1990) Characterization of organosulfur compounds in Oklahoma coals by pyrolysis±gas chromatography. In Geochemistry of Sulfur in Fossil Fuels, eds L.W. Orr and C. M. White. ACS Symposium Series 429, Washington, DC, pp. 326±344. Brooks, J. D. and Smith, J. W. (1969) The diagenesis of plant lipids during the formation of coal, petroleum and gas. II. Coali®cation and the formation of oil and gas in the Gippsland Basin. Geochima et Cosmochimica Acta 33, 1183±1194. Calkins, W. H. (1987) Investigation of organic sulfur-containing structures in coal by ¯ash pyrolysis experiments. Energy and Fuels 1, 59±64. Casagrande, D. J. and Ng, L. (1979) Incorporation of elemental sulfur in the coal as organic sulfur. Nature 282, 598±599. Casagrande, D. J., Idowu, G., Friedman, A., Rickert, P., Siefert, K. and Schlenz, D. (1979) H2S incorporation in coal precursors: origins of organic sulfur in coal. Nature 282, 599±600. Eglinton, T. I., Sinninghe Damste, J. S., Kohnen, M. E. L., de Leeuw, J. W., Larter, S. R. and Patience, R. L. (1990) Analysis of maturity-related changes in the organic sulfur composition of kerogen by ¯ash pyrolysis± gas chromatography. In Geochemistry of Sulfur in Fossil Fuels, eds L. W. Orr and C. M. White. ACS Symposium Series 429, Washington, DC, pp. 529±567. Fenouil, L. A., Towler, G. P. and Lynn, S. (1994) Removal of H2S from coal gas using limestone: kinetics considerations. Industrial and Engineering Chemistry Research 33, 266±272. Francois, R. (1987) A study of sulfur enrichment in the humic fraction of marine sediments during early diagenesis. Geochimica et Cosmochimica Acta 51, 17±27.

1485

Ibarra, J. V., Mirand, J. L. and Perez, A. J. (1987) Product of distribution and sulfur in the low temperature pyrolysis of a Spanish subbituminous coal. Fuel Processing Technology 15, 31±34. Idiz, E. F., Tannenbaum, E. and Kaplan, I. R. (1990) Pyrolysis of high sulfur monterey kerogens: stable isotopes of sulfur, carbon and hydrogen. In Geochemistry of Sulfur in Fossil Fuels, eds L. W. Orr and C. M. White. ACS Symposium Series 429, Washington, DC, pp. 565±591. Lewan, M. D. (1985) Evalution of petroleum generation by hydrous pyrolysis experimentation. Philosophical Transaction of The Royal Society of London A 315, 123± 134. Liliedahl, T., SjoÈstroÈm, K. and Wiktorsson, L. P. (1992) A thermodynamic study of dolomite as hydrogen sulphide adsorbent when pyrolysing or partially gasifying coal. Fuel 71, 797±803. Nissenbaum, A. and Kaplan, I. R. (1972) Chemical and isotopic evidence for the in situ origin of marine humic substances. Limnology Oceanography 17, 570±582. Orr, W. L. (1978) Sulfur in heavy oils, oil sands and oil shales. In Oil Sand and Oil Shale Chemistry, eds O. P. Strausz and E. M. Lown. Verlag Chemie, New York, pp. 223±243. Philp, R. P., Suzuki, N. and Galvez-Sinibaldi, A. (1992) Early-stage incorporation of sulfur into protokerogens and possible kerogen precursors. In Organic Matter: Productivity, Accumulation, and Preservation in Recent and Ancient Sediments, eds J. K. Whelan and J. W. Farrington. Columbia University Press, New York, pp. 264±282. Schmid, J. C., Connan, J. and Albrecht, P. (1987) Occurrence and geochemical signi®cance of long-chain dialkthiacyclopentanes. Nature 329, 54±56. Sinninghe Damste, J. S. and de Leeuw, J. W. (1989) Analysis, structure and geochemical signi®cance of organically-bound sulfur in the geosphere: state of the art and future research. In Advances in Organic Geochemistry 1989, eds B. Durand and F. BeÂhar, Organic Geochemistry 16, 1077±1101. Sinninghe Damste, J. S., Eglinton, T. I., de Leeuw, J. W. and Schenck, P. A. (1989) Organic sulfur in macromolecular sedimentary organic matter: I. Structure and origin of sulfur-containing moieties in kerogen, asphaltenes and coal as revealed by ¯ash pyrolysis. Geochimica et Cosmochimica Acta 53, 873±889. Sinninghe Damste, J. S., Eglinton, T. I., Rijpstra, W. I. C. and de Leeuw, J. W. (1990) Characterization of organically bound sulfur in high-molecular-weight, sedimentary organic matter using ¯ash pyrolysis and Raney Ni desulfurization. In Geochemistry of Sulfur in Fossil Fuels, eds L. W. Orr and C. M. White. ACS Symposium Series 429, Washington, DC, pp. 486±528. Smith, J. W and Philips, R.(1990) Isotopic study of coalassociated hydrogen sul®de. In Geochemistry of Sulfur in Fossil Fuels, eds L. W. Orr and C. M. White. ACS Symposium Series 429, Washington, DC, pp. 568±574. Vairavamurthy, A. and Mopper, K. (1987) Geochemical formation of organosulphur compounds (thiols) by addition of H2S to sedimentary organic matter. Nature 329, 623±625. Zevenhoven, C. A. P., Yrjas, P. K. and Hupa, M. M. (1996) Hydrogen sul®de capture by limestone and dolomite at elevated pressure. 2. Sorbent particle conversion modeling. Industrial and Engineering Chemistry Research 35, 943±949.