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Thermochemical and geochemical characteristics of sulphur coals
Fuel Processing Technology 77 – 78 (2002) 45 – 52 www.elsevier.com/locate/fuproc
Thermochemical and geochemical characteristics of sulphur coals Achim Bechtel a,*, Ludmila Butuzova b, Oksana Turchanina c, Reinhard Gratzer a b
a Institut fu¨r Geowissenschaften, Montanuniversita¨t Leoben, Peter-Tunner-Str. 5, Leoben A-8700, Austria L.M. Litvinenko Institute of Physical Organic and Coal Chemistry, National Academy of Sciences of the Ukraine, 70 R. Luxemburg str., Donetsk 83114, Ukraine c Donetsk State Technical University, 48 Artema str., Donetsk 83000, Ukraine
Received 31 January 2002; received in revised form 27 March 2002; accepted 29 March 2002
Abstract Gas chromatography – mass spectrometry (GC-MS) method was applied for investigation of the extracts obtained from three pairs of Donets bituminous coals (76 – 79% of Cdaf) of similar rank but differing in sulphur content. The elemental characteristic of the coals and hydrocarbon composition of their extracts reflect the differences in the environments of sulphur coals formation and differences in their structure. The thermal and natural coalification pathways of low- and high-sulphur coals formed under low-reduced and reduced conditions during early diagenesis were determined. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Sulphur coals; Structure; Biomarkers; Coalification; Carbonification
1. Introduction A high sulphur content in coals is an effect of the postdepositional history of coal bed formation and one of the most important criterion of its use as a fuel [1]. It is known that coal properties are determined by genetic type, petrographic composition and rank. Sulphur coals are common in Europe [2], Africa and America. In the Donets basin (Ukraine), coal deposits are multifacial, which were formed under marine transgressions and regressions resulting in variation of marine and terrestrial inputs. Coal seams that have *
Corresponding author. Fax: +43-3842-402640. E-mail address: [email protected] (A. Bechtel).
0378-3820/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 3 8 2 0 ( 0 2 ) 0 0 0 5 5 - 3
46
A. Bechtel et al. / Fuel Processing Technology 77 – 78 (2002) 45–52
been influenced by marine transgressions dominate in the basin (72.7% of total coal deposit) [3]. They are overlain by marine strata and are characterised, especially in the upper parts of the seams, by a particularly high content of fine pyrite (commonly in the form of fromboids) and organic sulphur. Formation of these coals during early diagenesis (peat-formation period) proceeded under more reductive conditions (flooding of peat by seawater) with high bacterial degradation of plant remnants and bacterial reduction of seawater sulphates. The coals formed under reductive conditions (RCs) are typically enriched in sulphur and hydrogen contents. The early diagenesis of coals formed under less reductive conditions (LRCs) took place in a freshwater environment, nourishing peatland, which burial was accomplished by fluvial sediments. A high content of sulphur in coals is causing a serious environmental and technological problem during their utilisation. The high proportion of coal in coal seams containing RCs is susceptible to self-ignition [4], while coals in seams containing LRCs are characterised by a elevated susceptibility to sudden ejection of coal and gas [5]. The aim of the paper is to establish the differences in the geochemical characteristic of the RCs and LRCs and their behaviour in the processes of carbonisation and coalification.
2. Experimental 2.1. Samples Three pairs of Donets bituminous coals of similar rank (Rm = 0.49– 0.71%), differing genetic types, and tendency to self-ignition were investigated (see Table 1 in the further text). They derive from Pennsylvanian (Moscovian stage) and have uniform petrographic composition: 80– 87% vitrinite, 5 –8% liptinite, 5 –12% inertinite. The samples of reduced and low reduced coals were collected from coal seams within lateral distances between stratigraphic columns smaller than 100 m. Limestone layers at the top of the coal seams and finely crystalline pyrite presence were reliable signs of a reduced type of Donets coals. Microlithotypes with a fine pyrite (carbopyrite) content were used as indicators of RCs and LRCs, respectively [6].
Table 1 Proximate and ultimate analyses of parent coals No. Coal, seam
Type Rm W a Ad V daf C daf H daf N daf Std Ssd Spd Sodaf (%) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%)
1 2 3 1V 2V 3V
LRC LRCa LRC RC RCa RC
Cheluskintsev, l4 Trudovskaya, l4 Kurahovskaya, l4 Ukraine, k8 Trudovskaya, k8 Kurahovskaya, l2 a
0.71 0.55 0.66 0.57 0.49 0.52
Susceptible to self-ignition.
0.8 1.0 9.4 1.5 0.9 5.5
2.4 1.6 5.3 9.9 4.6 8.6
35.6 37.3 37.2 41.8 46.2 43.0
79.3 78.4 79.3 77.9 76.1 76.1
4.94 4.95 5.07 5.30 5.43 5.22
2.32 1.90 – 2.10 1.97 –
2.17 1.05 1.04 2.87 5.85 5.60
0.04 0.04 0.12 0.11 0.05 0.02
0.11 0.17 0.07 0.80 0.71 2.44
2.07 0.85 0.90 2.18 5.34 3.44
A. Bechtel et al. / Fuel Processing Technology 77 – 78 (2002) 45–52
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2.2. Thermal analyses The thermal behaviour of coals was studied by differential thermal analyses and classical Fisher method. Differential thermal analysis of the samples was carried out in a Paulik-Paulik-Erdei Q-1500D thermobalance in a closed platinum crucible at a heating rate of 10 jC min 1. 2.3. Chemical analyses The proximate and ultimate analyses of the samples, including the total (St), organic (So), pyritic (Sp) and sulphate (Ss) sulphur were determined by standard procedures. The total organic carbon content was measured on a Leco carbon analyser on the samples pretreated with concentrated hydrochloric acid and calculated on a dry and ash-free basis (Cdaf, wt.% of the sample; Table 1). 2.4. Extraction The pulverised samples (about 7 g) were extracted using dichloromethane in a Dionex ASE 200 accelerated solvent extractor at 75 jC and 50 bar. The solvent was evaporated in a Zymark TurboVap 500 closed cell concentrator. Then the asphaltenes were precipitated from n-hexane-dichloromethane solution (80:1 v/v) and separated by centrifugation. 2.5. Liquid chromatography The medium-pressure liquid chromatography (Ko¨hnen – Willsch) was used for the separation of the n-hexane-soluble fraction of coal organic matter into saturated, aromatic hydrocarbons and polar heterocompounds. 2.6. Gas chromatography – mass spectrometry Saturated and aromatic hydrocarbon fractions were analysed on a gas chromatograph equipped with a 25-m DB-1 fused silica capillary column (diameter 0.25 mm) and coupled to a Finnigan MAT GCQ ion trap mass spectrometer. The oven temperature was programmed from 70 to 300 jC at a rate of 4 jC min 1 followed by an isothermal period of 15 min.
Table 2 Elemental composition of the solid products during thermal analyses of the coals T (jC)
20 520 650 850
Sample 1
Sample 2
Sample 1V
Sample 2V
C daf (wt.%)
H/C (atm)
C daf (wt.%)
H/C (atm)
C daf (wt.%)
H/C (atm)
C daf (wt.%)
H/C (atm)
79.3 87.8 89.5 92.5
0.75 0.49 0.39 0.19
78.4 88.8 89.9 93.5
0.76 0.43 0.4 0.18
77.9 80.8 82.5 90.3
0.82 0.76 0.5 0.18
76.1 81.2 84.9 92
0.86 0.76 0.38 0.14
48
A. Bechtel et al. / Fuel Processing Technology 77 – 78 (2002) 45–52
Fig. 1. Variations in the H/C atomic ratio versus carbon content for RCs (samples 1V, 2V; dotted lines) and LRCs (samples 1, 2; solid lines) during carbonisation (a, b) and coalification (c). Symbols in (a, b) refer to pyrolysis temperatures indicated below (b).
A. Bechtel et al. / Fuel Processing Technology 77 – 78 (2002) 45–52
49
Helium was the carrier gas. The samples ionisation in mass spectrometer was achieved by the electron impact (70 eV) and a scan range of 50– 650 daltons (0.7 s total scan time). Data were processed with a Finnigan data system. Identification of the individual compounds was accomplished by their retention times in the total ion current chromatogram and by comparison of the mass spectra with those of published data. Relative percentages and absolute concentrations of the compounds in the saturated and aromatic fractions were calculated using peak areas from the gas chromatograms in relation to that of internal standards. The concentrations were normalised to the Cdaf content in the sample.
3. Results and discussion The ultimate and proximate analysis of the coals investigated is given in Table 1. The reduced coals (RCs), compared with respective low reduced coals (LRCs) of corresponding coalification, have lower value of mean vitrinite reflectance (Rm), higher organic, pyritic and total sulphur contents as well as higher H/C ratio and yield of volatile matter. Organic sulphur is the main form of sulphur in coals under study. The content of lithotypes containing finely dispersed pyrite is high in the RCs (50 –63 vol.%), while in the LRCs it is much smaller (0– 6 vol.%). The pathways in atomic H/C ratio versus carbon content of the coals (natural metamorphism with depth) and carbonisates (obtained by heat alteration – carbonisation; Table 2) are presented in Fig. 1. The carbon content Cdaf in coals and carbonisates vary within range of 76– 93%. With an increase of the heating temperature (Table 2) Table 3 Concentrations (Ag/g Cdaf) and their ratios of specific compounds in the aliphatic and aromatic fractions Compounds
n-Alkanes CPIa Prb/n-C17 Phc/n-C18 Pr/Ph Regular steranes Diasteranes 4a-Methylsteranes Hopanes Steranes/Hopanes Diterpenoids (saturated + aromatic) Naphthalene + alkylated naphthalenes Phenanthrene + methylphenanthrenes Alkylated biphenyls Dibenzofuran Dibenzothiophene a b c
CPI = Carbon Preference Index. Pr = Pristane. Ph = Phytane.
Parent coal 1
2
1V
2V
60.06 1.43 5.58 1.10 5.64 4.93 1.36 2.90 37.80 0.24 12.75 116.13 40.12 44.61 4.71 6.77
27.93 1.42 6.82 1.41 5.45 3.49 0.75 1.26 24.52 0.22 22.00 91.94 44.36 47.88 5.03 9.09
36.54 1.22 6.40 1.71 4.24 15.30 5.73 0.64 34.22 0.63 31.81 170.58 100.08 86.98 16.03 17.91
73.36 1.13 2.34 0.93 2.99 35.08 13.35 7.34 97.16 0.57 37.38 116.85 61.02 116.88 14.74 20.34
50
A. Bechtel et al. / Fuel Processing Technology 77 – 78 (2002) 45–52
Fig. 2. Geochemical correlations for the studied coals: (a) yield of extractable organic matter versus atomic Hdaf/ Cdaf ratio, (b) atomic Sodaf/Cdaf ratio versus hopanes/organic carbon contents and (c) atomic Sodaf/Cdaf ratio versus steranes/hopanes contents. So = organic sulphur.
A. Bechtel et al. / Fuel Processing Technology 77 – 78 (2002) 45–52
51
and progression of coalification, the discussed dependencies proceed on different pathways for the RCs and LRCs. The more pronounced decrease of hydrogen content is manifested by quicker fall in value of atomic H/C ratio during carbonisation and metamorphism of the RCs in comparison with corresponding data of the LRCs (compare dotted and solid lines in Fig. 1, traces a, b and c, respectively). The relative course of the corresponding curves is similar for all LRC and RC pairs presented. It can be suggested that during natural or thermal maturation loss of hydrogen bearing compounds is easier from RCs enriched in organic sulphur. It should be noted that caking properties of the coals are different. Carbonisates from the LRCs are powders while these from the RCs are quality cokes. The gas chromatography –mass spectrometry (GC-MS) analysis of the aliphatic and aromatic fractions revealed a marked quantitative difference in compounds extracted from the LRC and RC coals (Table 3). Particularly pronounced differences are in content of the sulphur-bearing compounds. The RCs have indeed considerably higher absolute concentrations of dibenzothiophene (17.9 –20.3 Ag/g Cdaf) as compared with the LRCs (6.8 –9.1 Ag/g Cdaf; Table 3). Also the overall content of aromatic hydrocarbons is nearly twofold higher in the RCs, which is in accord with their good caking properties, even for lower rank coals. Lower values of pristane/phytane ratio for respective RCs counterparts can be explained by their formation under more reductive conditions during early diagenesis. The following correlations were determined: (i) between the yield of extractable (soluble) organic matter (SOM) and atomic H/C ratio (Fig. 2, trace a), (ii) atomic Sodaf/Cdaf ratio versus content of hopanes (Fig. 2, trace b), and (iii) atomic Sodaf/Cdaf ratio (dry and ashfree basis) versus steranes/hopanes ratio (Fig. 2, trace c). The predominant aliphatic biomarkers are bacterial source hopanes (Table 2). The elevated microbial activity was associated with sulphate reduction and hence leading to the positive correlation between atomic Sodaf/Cdaf ratio and hopanes concentration. The higher concentration of steranes relative to hopanes is a characteristic feature of marine influence. Therefore, the found higher steranes/hopanes ratio for the RCs indicates the increased abundance of marine photosynthetic organisms relative to aerobic bacteria and are interpreted to reflect enhanced inflow of seawater [7]. The differential thermal analysis allowed to recognise the susceptibility to self-ignition of the coals studied. It can be shown that for the coals 1, 2, 1V, 2V values of the highest rate of gaseous matter evolution (temperature range, 390– 420 jC) are 9, 12, 17, 24 mg/(min g), accordingly, i.e. increase in the series: 1 (LRC) < 2 (LRC) < 1* (RC) < 2* (RC). Carbonisation of RCs revealed 1.8– 2.0-fold higher values of the highest rate of gaseous matter evolution compared to the values for LRC counterparts. Additionally, these rates are always higher for the coals susceptible to self-ignition. The above-presented data parallel the content of methyl-biphenyls and dibenzothiophenes in the coals.
4. Conclusions The thermal and metamorphic maturation pathways for the LRCs and RCs are established. The organic geochemical parameters allowed to differentiate the early diagenetic environments of the LRCs and RCs formation. Revealed differences in the
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A. Bechtel et al. / Fuel Processing Technology 77 – 78 (2002) 45–52
coals elemental constitution and chemical composition of their extracts are proposed to be responsible for the higher reactivity of the RCs. The maximum rate of the gaseous matter evolution during coal carbonisation can be used for assessment the nature of environment of coal formation and coal susceptibility to self-ignition.
Acknowledgements This article benefited from the critical remarks of two anonymous reviewers. Financial support of the Austrian Science Foundation (FWF project no. P14245-CHE) is gratefully acknowledged.
References [1] P.L. Padgett, S.M. Rimmer, J.C. Ferm, J.C. Hower, C.F. Eble, M. Mastalerz, Sulfur variability and petrology of the Lower Block Coal Member (Pennsylvanian) in Southwest Indiana, Int. J. Coal Geol. 39 (1999) 97 – 120. [2] D.A. Spears, J.H. Rippon, P.F. Cavender, Geological controls on the sulphur distribution in British Carboniferous coals: a review and reappraisal, Int. J. Coal Geol. 40 (1999) 59 – 81. [3] I.I. Ammosov, Geology of coal and oil deposits in the USSR, Gos. NTI literatury po geologii i okhrane nedr, Moscow, 1963, 1220 pp. [4] G.P. Matsenko, V.I. Saranchuk, Donbass coal inclination to self-ignition depending on genesis, Jour. Khimiya tv’ordogo topliva 5 (1980) 23 – 28. [5] A.P. Bakaldina, Geology and reconnaissance, Izv. vyschikh uchebnykh zavedenij 6 (1969) 72 – 77. [6] G.P. Matsenko, Finely dispersed pyrite concretions as a petrographic parameter of types by reductivity of Donets coals, Jour. Khimiya tv’ordogo topliva 1 (1983) 13 – 19. [7] A. Bechtel, W. Pu¨ttmann, Palaeoceanography of the early Zechstein Sea during Kupferschiefer deposition in the Lower Rhine Basin (Germany): A reappraisal from stable isotope and organic geochemical investigations, Palaeogeogr., Palaeoclimatol., Palaeoecol. 136 (1997) 331 – 358.
Thermochemical and geochemical characteristics of sulphur coals Achim Bechtel a,*, Ludmila Butuzova b, Oksana Turchanina c, Reinhard Gratzer a b
a Institut fu¨r Geowissenschaften, Montanuniversita¨t Leoben, Peter-Tunner-Str. 5, Leoben A-8700, Austria L.M. Litvinenko Institute of Physical Organic and Coal Chemistry, National Academy of Sciences of the Ukraine, 70 R. Luxemburg str., Donetsk 83114, Ukraine c Donetsk State Technical University, 48 Artema str., Donetsk 83000, Ukraine
Received 31 January 2002; received in revised form 27 March 2002; accepted 29 March 2002
Abstract Gas chromatography – mass spectrometry (GC-MS) method was applied for investigation of the extracts obtained from three pairs of Donets bituminous coals (76 – 79% of Cdaf) of similar rank but differing in sulphur content. The elemental characteristic of the coals and hydrocarbon composition of their extracts reflect the differences in the environments of sulphur coals formation and differences in their structure. The thermal and natural coalification pathways of low- and high-sulphur coals formed under low-reduced and reduced conditions during early diagenesis were determined. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Sulphur coals; Structure; Biomarkers; Coalification; Carbonification
1. Introduction A high sulphur content in coals is an effect of the postdepositional history of coal bed formation and one of the most important criterion of its use as a fuel [1]. It is known that coal properties are determined by genetic type, petrographic composition and rank. Sulphur coals are common in Europe [2], Africa and America. In the Donets basin (Ukraine), coal deposits are multifacial, which were formed under marine transgressions and regressions resulting in variation of marine and terrestrial inputs. Coal seams that have *
Corresponding author. Fax: +43-3842-402640. E-mail address: [email protected] (A. Bechtel).
0378-3820/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 3 8 2 0 ( 0 2 ) 0 0 0 5 5 - 3
46
A. Bechtel et al. / Fuel Processing Technology 77 – 78 (2002) 45–52
been influenced by marine transgressions dominate in the basin (72.7% of total coal deposit) [3]. They are overlain by marine strata and are characterised, especially in the upper parts of the seams, by a particularly high content of fine pyrite (commonly in the form of fromboids) and organic sulphur. Formation of these coals during early diagenesis (peat-formation period) proceeded under more reductive conditions (flooding of peat by seawater) with high bacterial degradation of plant remnants and bacterial reduction of seawater sulphates. The coals formed under reductive conditions (RCs) are typically enriched in sulphur and hydrogen contents. The early diagenesis of coals formed under less reductive conditions (LRCs) took place in a freshwater environment, nourishing peatland, which burial was accomplished by fluvial sediments. A high content of sulphur in coals is causing a serious environmental and technological problem during their utilisation. The high proportion of coal in coal seams containing RCs is susceptible to self-ignition [4], while coals in seams containing LRCs are characterised by a elevated susceptibility to sudden ejection of coal and gas [5]. The aim of the paper is to establish the differences in the geochemical characteristic of the RCs and LRCs and their behaviour in the processes of carbonisation and coalification.
2. Experimental 2.1. Samples Three pairs of Donets bituminous coals of similar rank (Rm = 0.49– 0.71%), differing genetic types, and tendency to self-ignition were investigated (see Table 1 in the further text). They derive from Pennsylvanian (Moscovian stage) and have uniform petrographic composition: 80– 87% vitrinite, 5 –8% liptinite, 5 –12% inertinite. The samples of reduced and low reduced coals were collected from coal seams within lateral distances between stratigraphic columns smaller than 100 m. Limestone layers at the top of the coal seams and finely crystalline pyrite presence were reliable signs of a reduced type of Donets coals. Microlithotypes with a fine pyrite (carbopyrite) content were used as indicators of RCs and LRCs, respectively [6].
Table 1 Proximate and ultimate analyses of parent coals No. Coal, seam
Type Rm W a Ad V daf C daf H daf N daf Std Ssd Spd Sodaf (%) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (wt.%)
1 2 3 1V 2V 3V
LRC LRCa LRC RC RCa RC
Cheluskintsev, l4 Trudovskaya, l4 Kurahovskaya, l4 Ukraine, k8 Trudovskaya, k8 Kurahovskaya, l2 a
0.71 0.55 0.66 0.57 0.49 0.52
Susceptible to self-ignition.
0.8 1.0 9.4 1.5 0.9 5.5
2.4 1.6 5.3 9.9 4.6 8.6
35.6 37.3 37.2 41.8 46.2 43.0
79.3 78.4 79.3 77.9 76.1 76.1
4.94 4.95 5.07 5.30 5.43 5.22
2.32 1.90 – 2.10 1.97 –
2.17 1.05 1.04 2.87 5.85 5.60
0.04 0.04 0.12 0.11 0.05 0.02
0.11 0.17 0.07 0.80 0.71 2.44
2.07 0.85 0.90 2.18 5.34 3.44
A. Bechtel et al. / Fuel Processing Technology 77 – 78 (2002) 45–52
47
2.2. Thermal analyses The thermal behaviour of coals was studied by differential thermal analyses and classical Fisher method. Differential thermal analysis of the samples was carried out in a Paulik-Paulik-Erdei Q-1500D thermobalance in a closed platinum crucible at a heating rate of 10 jC min 1. 2.3. Chemical analyses The proximate and ultimate analyses of the samples, including the total (St), organic (So), pyritic (Sp) and sulphate (Ss) sulphur were determined by standard procedures. The total organic carbon content was measured on a Leco carbon analyser on the samples pretreated with concentrated hydrochloric acid and calculated on a dry and ash-free basis (Cdaf, wt.% of the sample; Table 1). 2.4. Extraction The pulverised samples (about 7 g) were extracted using dichloromethane in a Dionex ASE 200 accelerated solvent extractor at 75 jC and 50 bar. The solvent was evaporated in a Zymark TurboVap 500 closed cell concentrator. Then the asphaltenes were precipitated from n-hexane-dichloromethane solution (80:1 v/v) and separated by centrifugation. 2.5. Liquid chromatography The medium-pressure liquid chromatography (Ko¨hnen – Willsch) was used for the separation of the n-hexane-soluble fraction of coal organic matter into saturated, aromatic hydrocarbons and polar heterocompounds. 2.6. Gas chromatography – mass spectrometry Saturated and aromatic hydrocarbon fractions were analysed on a gas chromatograph equipped with a 25-m DB-1 fused silica capillary column (diameter 0.25 mm) and coupled to a Finnigan MAT GCQ ion trap mass spectrometer. The oven temperature was programmed from 70 to 300 jC at a rate of 4 jC min 1 followed by an isothermal period of 15 min.
Table 2 Elemental composition of the solid products during thermal analyses of the coals T (jC)
20 520 650 850
Sample 1
Sample 2
Sample 1V
Sample 2V
C daf (wt.%)
H/C (atm)
C daf (wt.%)
H/C (atm)
C daf (wt.%)
H/C (atm)
C daf (wt.%)
H/C (atm)
79.3 87.8 89.5 92.5
0.75 0.49 0.39 0.19
78.4 88.8 89.9 93.5
0.76 0.43 0.4 0.18
77.9 80.8 82.5 90.3
0.82 0.76 0.5 0.18
76.1 81.2 84.9 92
0.86 0.76 0.38 0.14
48
A. Bechtel et al. / Fuel Processing Technology 77 – 78 (2002) 45–52
Fig. 1. Variations in the H/C atomic ratio versus carbon content for RCs (samples 1V, 2V; dotted lines) and LRCs (samples 1, 2; solid lines) during carbonisation (a, b) and coalification (c). Symbols in (a, b) refer to pyrolysis temperatures indicated below (b).
A. Bechtel et al. / Fuel Processing Technology 77 – 78 (2002) 45–52
49
Helium was the carrier gas. The samples ionisation in mass spectrometer was achieved by the electron impact (70 eV) and a scan range of 50– 650 daltons (0.7 s total scan time). Data were processed with a Finnigan data system. Identification of the individual compounds was accomplished by their retention times in the total ion current chromatogram and by comparison of the mass spectra with those of published data. Relative percentages and absolute concentrations of the compounds in the saturated and aromatic fractions were calculated using peak areas from the gas chromatograms in relation to that of internal standards. The concentrations were normalised to the Cdaf content in the sample.
3. Results and discussion The ultimate and proximate analysis of the coals investigated is given in Table 1. The reduced coals (RCs), compared with respective low reduced coals (LRCs) of corresponding coalification, have lower value of mean vitrinite reflectance (Rm), higher organic, pyritic and total sulphur contents as well as higher H/C ratio and yield of volatile matter. Organic sulphur is the main form of sulphur in coals under study. The content of lithotypes containing finely dispersed pyrite is high in the RCs (50 –63 vol.%), while in the LRCs it is much smaller (0– 6 vol.%). The pathways in atomic H/C ratio versus carbon content of the coals (natural metamorphism with depth) and carbonisates (obtained by heat alteration – carbonisation; Table 2) are presented in Fig. 1. The carbon content Cdaf in coals and carbonisates vary within range of 76– 93%. With an increase of the heating temperature (Table 2) Table 3 Concentrations (Ag/g Cdaf) and their ratios of specific compounds in the aliphatic and aromatic fractions Compounds
n-Alkanes CPIa Prb/n-C17 Phc/n-C18 Pr/Ph Regular steranes Diasteranes 4a-Methylsteranes Hopanes Steranes/Hopanes Diterpenoids (saturated + aromatic) Naphthalene + alkylated naphthalenes Phenanthrene + methylphenanthrenes Alkylated biphenyls Dibenzofuran Dibenzothiophene a b c
CPI = Carbon Preference Index. Pr = Pristane. Ph = Phytane.
Parent coal 1
2
1V
2V
60.06 1.43 5.58 1.10 5.64 4.93 1.36 2.90 37.80 0.24 12.75 116.13 40.12 44.61 4.71 6.77
27.93 1.42 6.82 1.41 5.45 3.49 0.75 1.26 24.52 0.22 22.00 91.94 44.36 47.88 5.03 9.09
36.54 1.22 6.40 1.71 4.24 15.30 5.73 0.64 34.22 0.63 31.81 170.58 100.08 86.98 16.03 17.91
73.36 1.13 2.34 0.93 2.99 35.08 13.35 7.34 97.16 0.57 37.38 116.85 61.02 116.88 14.74 20.34
50
A. Bechtel et al. / Fuel Processing Technology 77 – 78 (2002) 45–52
Fig. 2. Geochemical correlations for the studied coals: (a) yield of extractable organic matter versus atomic Hdaf/ Cdaf ratio, (b) atomic Sodaf/Cdaf ratio versus hopanes/organic carbon contents and (c) atomic Sodaf/Cdaf ratio versus steranes/hopanes contents. So = organic sulphur.
A. Bechtel et al. / Fuel Processing Technology 77 – 78 (2002) 45–52
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and progression of coalification, the discussed dependencies proceed on different pathways for the RCs and LRCs. The more pronounced decrease of hydrogen content is manifested by quicker fall in value of atomic H/C ratio during carbonisation and metamorphism of the RCs in comparison with corresponding data of the LRCs (compare dotted and solid lines in Fig. 1, traces a, b and c, respectively). The relative course of the corresponding curves is similar for all LRC and RC pairs presented. It can be suggested that during natural or thermal maturation loss of hydrogen bearing compounds is easier from RCs enriched in organic sulphur. It should be noted that caking properties of the coals are different. Carbonisates from the LRCs are powders while these from the RCs are quality cokes. The gas chromatography –mass spectrometry (GC-MS) analysis of the aliphatic and aromatic fractions revealed a marked quantitative difference in compounds extracted from the LRC and RC coals (Table 3). Particularly pronounced differences are in content of the sulphur-bearing compounds. The RCs have indeed considerably higher absolute concentrations of dibenzothiophene (17.9 –20.3 Ag/g Cdaf) as compared with the LRCs (6.8 –9.1 Ag/g Cdaf; Table 3). Also the overall content of aromatic hydrocarbons is nearly twofold higher in the RCs, which is in accord with their good caking properties, even for lower rank coals. Lower values of pristane/phytane ratio for respective RCs counterparts can be explained by their formation under more reductive conditions during early diagenesis. The following correlations were determined: (i) between the yield of extractable (soluble) organic matter (SOM) and atomic H/C ratio (Fig. 2, trace a), (ii) atomic Sodaf/Cdaf ratio versus content of hopanes (Fig. 2, trace b), and (iii) atomic Sodaf/Cdaf ratio (dry and ashfree basis) versus steranes/hopanes ratio (Fig. 2, trace c). The predominant aliphatic biomarkers are bacterial source hopanes (Table 2). The elevated microbial activity was associated with sulphate reduction and hence leading to the positive correlation between atomic Sodaf/Cdaf ratio and hopanes concentration. The higher concentration of steranes relative to hopanes is a characteristic feature of marine influence. Therefore, the found higher steranes/hopanes ratio for the RCs indicates the increased abundance of marine photosynthetic organisms relative to aerobic bacteria and are interpreted to reflect enhanced inflow of seawater [7]. The differential thermal analysis allowed to recognise the susceptibility to self-ignition of the coals studied. It can be shown that for the coals 1, 2, 1V, 2V values of the highest rate of gaseous matter evolution (temperature range, 390– 420 jC) are 9, 12, 17, 24 mg/(min g), accordingly, i.e. increase in the series: 1 (LRC) < 2 (LRC) < 1* (RC) < 2* (RC). Carbonisation of RCs revealed 1.8– 2.0-fold higher values of the highest rate of gaseous matter evolution compared to the values for LRC counterparts. Additionally, these rates are always higher for the coals susceptible to self-ignition. The above-presented data parallel the content of methyl-biphenyls and dibenzothiophenes in the coals.
4. Conclusions The thermal and metamorphic maturation pathways for the LRCs and RCs are established. The organic geochemical parameters allowed to differentiate the early diagenetic environments of the LRCs and RCs formation. Revealed differences in the
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coals elemental constitution and chemical composition of their extracts are proposed to be responsible for the higher reactivity of the RCs. The maximum rate of the gaseous matter evolution during coal carbonisation can be used for assessment the nature of environment of coal formation and coal susceptibility to self-ignition.
Acknowledgements This article benefited from the critical remarks of two anonymous reviewers. Financial support of the Austrian Science Foundation (FWF project no. P14245-CHE) is gratefully acknowledged.
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