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Structural characterisation of Middle Jurassic, high-volatile bituminous Walloon Subgroup coals and correlation with the coal seam gas content
Fuel 89 (2010) 3241–3249
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Structural characterisation of Middle Jurassic, high-volatile bituminous Walloon Subgroup coals and correlation with the coal seam gas content Alan L. Chaffee a,b,*, Galinda Lay a,b, Marc Marshall a,b, W. Roy Jackson a,b, Yi Fei a,b, T. Vincent Verheyen c, Peter J. Cassidy d, Steven G. Scott d a
School of Chemistry, Monash University, Clayton, Victoria 3800, Australia Centre for Green Chemistry, Monash University, Victoria 3800, Australia School of Applied Science and Engineering, Churchill Campus, Churchill, Victoria 3842, Australia d QGC Ltd., Level 30, 275 George Street, Brisbane Queensland 4000, Australia b c
a r t i c l e
i n f o
Article history: Received 28 January 2010 Received in revised form 8 May 2010 Accepted 17 June 2010 Available online 30 June 2010 Keywords: Coal seam gas Chromatography Liquefaction Solvent separation
a b s t r a c t The structure of a suite of high-volatile, bituminous Surat Basin, Queensland coals has been investigated by a combination of analytical probes. These included elemental analyses, pyrolysis-gas chromatography-mass spectroscopy and Fourier transform infrared spectroscopy, together with a study of their liquefaction products in both tetralin and solvent free-tin catalysed hydrogenations. Samples were gathered across a 300 m depth interval intersected by the sampling well. Most techniques revealed clear but subtle differences in structure as a function of depth. The oils produced by solvent free-tin catalysed hydrogenation, however, showed very distinct dependence with depth and the waxy content, as indicated by 1HNMR, could be correlated with the coal seam gas content. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction A number of exploration and core drilling programs has been carried out since 2000 by Queensland Gas Company Limited (QGC) to investigate the coal seam gas potential of the Juandah (upper) and Taroom (lower) Coal Measures of the Middle Jurassic Walloon Subgroup in the Surat Basin, Queensland, Australia (Fig. 1). The gas content varies between the upper and lower coal measures in a manner that cannot be explained readily by petrological differences [1]. In this paper the chemical structural properties of these coals have been measured directly by ultimate analysis, pyrolysis-gas chromatography-mass spectroscopy (pygc-ms), and Fourier transform infrared spectroscopy (FTIR). In addition indirect structural probes comprising evaluation of the products obtained by hydrogenation of these coals under two complementary sets of conditions have been considered. The coals were hydrogenated, both without added catalyst in tetralin and also without solvent in the presence of a tin catalyst. The hydrogenations in the donor solvent tetralin allowed a direct comparison with a data set comprising a very wide range of Australian coals to ensure they contained no abnormal features [2]. The hydrogenations of the tin-treated coals [3] in the absence of solvent allowed
* Corresponding author at: School of Chemistry, P.O. Box 23, Monash University, Victoria 3800, Australia. Tel.: +61 3 9905 4626; fax: +61 3 9905 4597. E-mail address: [email protected] (A.L. Chaffee). 0016-2361/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2010.06.027
for a detailed chromatographic examination of the oil product from the coals without solvent interferences. Attempts have been made to seek comparisons between the coal seam gas content and the chemical properties of the coals. Earlier studies on the correlation of chemical, as distinct from petrographic, properties of coals are limited. A study of Upper Silesian coals showed an inverse correlation between methane content and the atomic H/C ratio of the coals [4], whereas an Indian study reported an increase in coal seam gas with the hydrogen content of the coal [5]. In addition, the variation of chemical properties with coal depth has been compared with those found for other studies involving samples from long sections. These include studies of Victorian brown coals where an average 1% increase in carbon content of the coals was found over 120 m for the Morwell Seam [6]. The structural composition of minor compounds present in various solvent extracts was also shown to vary with depth [7]. The aromatic structure of some Silesian coals over a depth range of 1550 m [8] and of some Lorraine coals over a depth range of 4700 m has been reported [9]. FTIR and ESR spectroscopic analysis have been carried out over an interval of 1160 m for a Zonguldak bituminous coal [10]. 13C and IR spectroscopy together with a study of the humic acid content has been reported for a very long section of Mahakam, Kalimantan, Indonesian coals ranging from plant/peat material to bituminous [11]. The coals in this study were recovered from a depth range of over 300 m, intermediate between the 100 m range of the Victo-
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Elemental analysis of acid-washed coals for C, H, N, S and Cl was carried out by HRL Technology Ltd, C, H and N were determined using a LECO CHN 1000 analyser by HRL Method 1.4 and S and Cl by AS1083.6.3.2 and AS1083.8.2. Acid-washed coals were analysed to avoid interference from carbonates. Ash yields were determined essentially following AS2434.8. 2.2. Coal liquefaction
Fig. 1. Map showing the location of the bore from which the coal samples were taken [20].
rian brown coals and the 1000 m or more for the others. The gas yields were from samples taken from the Bellevue #1/1R well, which is located in close proximity to the Bellevue #4 well from which the coal samples were collected.
2. Experimental 2.1. Coal preparation and elemental analysis The coals from Bellevue #4 (samples QGC 1401–1411) were received ground to 10 mm, double-bagged and in a moist state. Fig. 2 shows the geological section intersected by the well. A representative sample from each of the intersected seams was obtained by coning and quartering and dried in a flow of nitrogen at room temperature, then sieved and ground by mortar and pestle to 250 lm, and stored under nitrogen until used. Following the procedure of Redlich et al [2], representative samples of five coals were acid-washed by stirring overnight a mixture of 25:1, 0.1 M H2SO4:coal, for one hour under vacuum to ensure good wetting of the coal, then under nitrogen. The coal was then filtered and washed overnight with stirring in 25:1 distilled water:coal. Filtration and overnight washing with distilled water were repeated as many times as was necessary for the pH of the filtrate to become constant. This was found to ensure that the sulfate ion had been washed out. Tin impregnation was carried out by the same procedure, with SnCl2 solution substituted for H2SO4; the SnCl2 concentration was such as to give ca 1 mole tin per kg db of the original coal. The low pH obtained ensures that the acid-soluble material in the original coal is removed during the tin impregnation.
The coal liquefaction procedure was similar to the tetralin extraction procedure of Redlich et al [2,3] except that the autoclave was held stationary and heated in a sandbath, whereas Redlich et al [2,3] used a rocking autoclave with a heating jacket. For the runs in tetralin, 3 g of raw 250 lm coal dried at 105 °C in a flow of nitrogen for three hours and 9 g of tetralin were loaded into a 70 mL autoclave, which was then sealed and charged with 6 MPa of hydrogen. The autoclave was lowered into the sandbath, heated to 405 °C in 10–15 min and held at temperature for one hour. For most of the runs, after one hour at temperature, the autoclave was taken out of the sandbath, cooled in a blast of air and weighed. The gas was vented into an evacuated, weighed 300 mL autoclave, which was weighed again and vented through a gas meter. In this way the average molecular weight of the product gas and its composition, on the assumption that it consisted only of CO2 and H2, could be determined. The autoclave was vented completely and weighed again. For the runs with tin-treated coals (samples QGC 1408 and QGC 1411), a more complete gas analysis was carried out. After the initial cooling and weighing of the autoclave, the gas was vented through an Agilent 3000 Micro Gas Chromatograph, equipped with four columns: MS 5A PLOT, 10 m 0.32 mm (110 °C column temperature) for N2 and CH4, PLOT U, 8 m 0.32 mm (100 °C column temperature) for CO, CO2, C2 hydrocarbons, Alumina PLOT, 10 m 0.32 mm (140 °C column temperature) for C3–C5 hydrocarbons and OV-1, 10 m 0.15 mm 2.0 lm (90 °C column temperature) for isobutane and n-hexane. The inlet and injector temperatures were 100 °C. After the analysis the autoclave was vented and the total weight of the gas in the autoclave was calculated. On the assumption that the gases detected (and hydrogen by difference) were the only gases present, the yields of all the gases detected and the hydrogen consumption could be calculated. The solid and liquid products were washed and scraped out of the autoclave with dichloromethane into a flask, and subjected to Lundin distillation to remove water, though in fact no water was detected in any run, probably because it evaporated during work-up. The remaining solid and liquid product was fractionated using solvent solubility criteria, as described by Redlich et al [2,3] into dichloromethane insolubles, asphaltene (dichloromethane soluble, hexane insoluble) and oil (hexane soluble), with the amount of oil being determined by difference. The procedure for runs with tin-impregnated coal, carried out to obtain oil samples free of hydrogenation solvent, was similar to that for tetralin runs, except that 9 g (tin-free basis) of coal, no tetralin and 10 MPa of hydrogen were charged into the autoclave before the run. 2.3. Analysis of coal liquefaction product oil Four oils from runs with tin-impregnated coals were analysed by gas-chromatography-mass spectrometry (GC-MS) on a HP6890 instrument in splitless auto-mode. For GC, a HP 19091S433 capillary column (HP-5MS 5% phenylmethyl siloxane), 30 m long, 0.25 mm diameter, 0.25 lm nominal film thickness, was used. The inlet temperature was 230 °C. The oven temperature was initially held at 50 °C for 2 min then raised to 200 °C at a rate of 4 °C/min, held at 200 °C for 2 min, then raised to 300 °C at 8 °C/
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Fig. 2. Stratigraphic correlation Bellevue #1/1R and Bellevue #4 (not to scale).
min and held at 300 °C for 3 min. For MS, the ionizing potential was 70 eV, the accelerating voltage 1.9 kV, the mass range scanned 45–600 m/z and the ion source temperature 200–250 °C. For 1H-NMR, the oil was dissolved in CDCl3 and spectra were obtained using a 300 MHz instrument, with a 90° pulse flip angle (9.5 ls). 2.4. Analysis of coals FTIR spectra were collected in absorbance mode using a Varian Scimitar 1000 FTIR spectrophotometer. The dried (105 °C under N2), finely ground coals were prepared as KBr discs (1.0 mg coal and 200 mg KBr). The py-gc-ms experiments were performed on a Chemical Data Systems 1000 coil pyroprobe unit interfaced to a Varian 3800 GC Saturn 2000 ion trap system. A small amount of the coal sample (3 mg) was loaded into a quartz tube containing quartz wool which was then inserted into the platinum coil of the probe. The samples
were flash pyrolysed at 700 °C for 40 seconds. Separation was effected using a Varian Factor 4 capillary column (VF-5MS 30 M.) 0.25 mm ID DF = 0.25 with helium as the carrier gas at1 ml/min constant flow. The column was held at 0 °C for 3 min then ramped to 80 °C at 6 °C/min; to 200 °C at 8 °C/min and finally to 325 °C at 10 °C/min with a 15 minutes hold time. The ion trap conditions were 70 eV electron impact, transfer line temperature 170 °C, trap temperature 150 °C, scan range 29 to 200 m/z between 1 and 10 minutes; then 29 m/z to 380 m/z to 25 minutes and finally 50– 450 m/z for the rest of the acquisition. 3. Results and discussion 3.1. The coals The ash yields for the raw coals (Table 1) were all high, (19–65 wt% db). Acid washing reduced the ash yield by 0–4 wt% db for all five coals subjected to this treatment (Table 1), indicating the pres-
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Table 1 (a) Ash yields of QGC coal samples (Bellevue #4), (b) Ash yield for Taroom coal. Sample number
(a) QGC1401 QGC1402 QGC1403 QGC1404 QGC1405 QGC1406 QGC1407 QGC1408 QGC1409 QGC1410 QGC1411 (b) ABL14 a
Formation
Seam
Depth interval (m)
Ash yield (wt% db)a Raw coal
Acid-washed coal
Macalister upper Macalister lower Nangram (A) Nangram (B) Wambo (A) Wambo (B) Argyle (A) Argyle (B)
184.14 189.09 200.16 201.25 209.61 210.71 215.94 221.78 225.72 23 l.42 242.32 247.2 305.09 307.14 330.72 331.93
45.1 ± 3.0 44.3 ± 0.9 62.7 ± 2.9 64.7 ± 2.9 54.2 ± 3.7 37.8 ± 1.6 48.5 ± 2.4 45.7 ± 3.1
41.5 ± 3.2 – – – – 33.8 ± 0.3 – 45.1 ± 1.2
Taroom CM
Auburn Condamine (A) Condamine (B)
413.74 414.74 469.83 474.88 477.64 500.22
60.6 ± 2.3 25.6 ± 1.4 18.8 ± 3.6
– 21.5 ± 2.0 15.9 ± 1.7
Taroom CM
Taroom
–
Juandah CM
7.4 ± 0.1
–
The errors are 90% confidence limits based on multiple determinations.
Table 2 Elemental analysis of selected acid-washed coalsa. Sample number
C
H
N
S
O
H/C
5.8 ± 0.5 6.3 ± 0.2 5.6 ± 0.3 6.2 ± 0.2 5.9
1.1 ± 0.2 1.1 ± 0.1 1.3 ± 0.1 1.2 ± 0.1 1.17
1.4 ± 0.1 0.6 ± 0.1 1.5 ± 0.1 0.5 ± 0.1 0.43
19.7 ± 5.3 20.2 ± 1.0 17.8 ± 2.7 16.1 ± 2.3 16.3
0.96 ± 0.04 1.05 ± 0.03 0.91 ± 0.04 0.97 ± 0.02 0.94
wt% daf QGC1401-AW QGC1406-AW QGC1408-AW QGC1411-AW Taroom [11] a
72.0 ± 4.5 71.8 ± 0.7 73.7 ± 2.2 76.0 ± 1.9 76.2
The errors are the sum of the errors in analysis and the 90% confidence limits on the ash yields.
ence of small but still significant quantities of acid-soluble inorganics or minerals such as calcium carbonate. The ash produced in all cases was very light yellow in colour, implying little iron was present. Four coals, roughly equally spaced along the section, were acidwashed and their elemental analysis determined (Table 2). The results suggested a small increase in carbon content and a small decrease in oxygen content, and hence a greater degree of coalification, with increasing depth, as expected. The sulfur content varied but the atomic H/C ratio and nitrogen content were almost constant down the whole intersection. The deepest coal (QGC 1411) is the one closest in elemental analysis to the Taroom coal (ABL14) used in a prior study by Redlich et al [2]. This is as expected because the Taroom coal investigated by Redlich et al came from the Taroom (lower) Coal Measures of the Walloon Subgroup [1]. The coal FTIR spectra were dominated by mineral matter absorbance bands in accord with their high ash yields (Table 1). These absorptions were assigned predominantly to kaolinite [12] a higher surface activity clay mineral. There was also possibly some illite present, more in the shallower sample (QGC 1401) than in the deepest one (QGC 1411). Signals due to C-H stretching were observed in both samples in agreement with the long chain hydrocarbons found in the py-gc-ms data and in the coal liquefaction products. The various aromatic C–H bending modes in the coals’ FTIR spectra are sensitive to aromatic condensation. The degree of coalification can generally be assessed by changes in aromatic ring substitution by carbon or oxygen [13]. An increase in the 880 and 818 cm 1 and reduction in 750 cm 1 (all aromatic C–H bend mode) band intensities is associated with increased aromatic ring condensation as indicated by a loss of adjacent protons on individual rings. However Fig. 3 reveals the intensities of these marker bands is not changing in any regular way with depth, indicating that there is no clearly discernible aromatic ring condensation observable by this method.
3.2. Product yields from raw and tin-treated coal liquefaction Table 3 shows the product yields of nine of the raw coals from Bellevue #4 under the standard autoclave reaction conditions of
Fig. 3. FTIR absorbance spectra ArC-H bend region.
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Table 3 Liquefaction of coals (a) 3:1 tetralin:coal without catalyst and 6 MPa (cold) H2 at 405 °C for 1 h and (b) tin-treated coal (1 mol Sn/kg dry acid-washed coal) without solvent and 10 MPa (cold) H2 at 405 °C for 1ha. Sample number
CH2Cl2 solubles (inc. gas)
Asphaltene
Oil and H2O
CO2 production
H2 consumption
(wt% daf) (a) QGC1401 QGC1403 QGC1404 QGC1405 QGC1406 QGC1408 QGC1409 QGC1410 QGC1411 ABL14
63.7 ± 9.6 67.3 ± 0.6 65.7 ± 12.0 69.0 ± 10.0 72.9 ± 5.0 73.5 ± 5.8 67.3 ± 9.0 71.2 ± 4.0 64.3 ± 4.1 72.0
29.4 ± 3.4 28.7 ± 1.8 30.2 ± 6.0 27.5 ± 3.6 31.1 ± 2.3 39.4 ± 4.6 23.7 ± 3.9 33.6 ± 2.3 27.8 ± 1.7 33.0
26.0 ± 23.0 25.9 ± 18.0 41.5 ± 28.0 45.0 ± 20.0 43.3 ± 12.0 26.1 ± 16.6 47.2 ± 17.0 41.2 ± 10.3 29.9 ± 10.7 36.0
13.0 ± 5.7 16.4 ± 7.9 4.2 ± 1.8 2.6 ± 0.9 5.4 ± 2.0 12.5 ± 3.1 4.4 ± 1.5 4.4 ± 1.5 5.0 ± 2.0 2.6
4.8 ± 4.7 3.7 ± 7.7 10.3 ± 8.0 6.4 ± 5.0 6.9 ± 2.4 4.5 ± 3.1 8.0 ± 2.5 8.0 ± 2.5 1.6 ± 2.9 nd
(b) QGC1401-Sn QGC1406-Sn QGC1408-Sn QGC1411-Sn ABL14-Sn
78.8 ± 5.1 82.0 ± 5.8 87.7 ± 0.3 64.3 ± 0.8 48.2
27.7 ± 1.4 32.6 ± 1.5 26.9 ± 0.5 27.2 ± 1.0 9.0
47.9 ± 8.2 46.2 ± 8.3 59.4 ± 6.7 31.6 ± 4.7 39.4
5.9 ± 0.7 6.6 ± 0.6 2.4 ± 0.2 3.5 ± 0.3 nd
2.8 ± 1.0 3.9 ± 0.4 6.1 ± 0.7 1.4 ± 0.4 nd
a The uncertainties given are those due to uncertainties in ash yield and weighing. There is an additional uncertainty in the calibration of the reference gas mixture which applies to the CO2 production and H2 consumption data for QGC1408-Sn and QGC1411-Sn.
100
Redlich coals coals in the current study
90 80 Conv. (wt% dmif)
Redlich et al [2]. The close similarity of the conversion (CH2Cl2 solubles and gas), asphaltene and oil + water yield of the Bellevue coals and of the Taroom coal, from the same coal measures, are clear. For QGC 1401 and QGC 1403 the CO2 production, even allowing for the large experimental error, appears to be larger than for the other coals. At least for QGC 1401, this could be in part explained by decomposition of MgCO3 or similar easily reacted carbonate during the liquefaction; the difference in ash yield between raw and acid-washed coals was ca 4 wt% daf (cf. Table 1), which would give over 3 wt% of CO2 if the difference in ash yield were due to MgCO3. The overall similarity between the different coals in oil + water and dichloromethane-solubles (including gas), which is total conversion (Table 3) can be understood in terms of the well-known correlations between oil yield and total conversion vs. atomic H/ C ratio for acid-washed coals [2,3]. The coals analysed are of similar atomic H/C ratio (see Table 2) and contain little active catalyst (e.g. Fe; see above), so that they would, on the basis of this correlation, be expected to have a similar oil yield and total conversion, despite any differences in chemical composition due to the range of burial depths or the high (but probably inert) ash yields. Indeed, the total conversion for the four coals in the suite of known atomic H/C ratio fell only just below or coincided with the correlation established by Redlich et al [3] between total conversion and atomic H/C ratio for Australian coals of a wide range of atomic H/C ratio and rank (Fig. 4). The fact that the total conversions sometimes were slightly lower than those found by Redlich et al [3] for coals of the same atomic H/C ratio can be explained by the lack of agitation of the autoclave in the current experiments compared to the rocking motion in the experiments of Redlich et al [3]. The amount of tin added to the tin-treated coals (Table 4) can be estimated from the difference in ash yield between the tin-treated coals (Table 4) and acid-washed coals (Table 1), because all acidsoluble inorganics are removed during the tin treatment (see above). Any differences in removal of inorganics due to differences in the nature of the acid medium will be negligible compared to the uncertainties in ash yields. The results of hydrogenation of tin-treated coals in the absence of solvent are given in Table 3 together with that previously reported for a hydrogenation of tintreated ABL 14 [14]. (Tin-treated ABL 14 gave a lower conversion than QGC 1411 probably because the method of tin addition to ABL 14 was different and would have resulted in poorer dispersion
70 60 50 40 30 20 10 0 0
0.2
0.4
0.6 0.8 Atomic H/C ratio
1
1.2
1.4
Fig. 4. The correlation between total conversion (CH2Cl2 solubles, including gas) and atomic H/C ratio for a suite of Australian coals [3] with four coals from the current study added.
Table 4 Ash yields of QGC coals treated with SnCl2.
a
Sample number
Ash yielda (wt% db)
Sn contenta (mol/kg dry aw coal)
QGC1401-Sn QGC1406-Sn QGC1408-Sn QGC1411-Sn
49.2 ± 1.6 44.0 ± 2.4 52.6 ± 1.2 28.4 ± 1.3
1.10 ± 0.90 1.21 ± 0.06 1.16 ± 0.20 1.05 ± 0.23
The errors are 90% confidence limits based on multiple determinations.
and large particle size of SnO2, the final product of treatment with SnCl2 solution.) The important feature of these results is that the total conversion, oil yield and hydrogen consumption from reaction of QGC 1411, which, like ABL 14, belongs to the Taroom Coal Measures, were much lower than the corresponding yields for the three coals (QGC 1401, 1406 and 1408) belonging to the Juandah Coal Measures. This might be taken to suggest that the more significant structural changes over the whole length of the section occur between the Juandah and Taroom Coal Measures.
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Table 5 Composition of gas from hydroliquefaction of tin-treated coalsa. Sample number
CO
CH4
C2
C3
C4
C5
C6
0.87 ± 0.08 0.90 ± 0.08
0.90 ± 0.08 0.62 ± 0.06
0.25 ± 0.02 0.19 ± 0.02
0.20 ± 0.02 0.10 ± 0.01
– 0.08 ± 0.01
0.07 ± 0.01 0.04 ± 0.00
wt% daf QGC1408-Sn QGC1411-Sn a
2.84 ± 0.26 1.45 ± 0.13
Uncertainties are those due to uncertainties in ash yield, weighing and composition of reference gas mixture.
The composition of the gas from the Sn-treated QGC 1408 and QGC 1411 reactions are given in Table 5. The hydrocarbon gas yields from reactions of the Taroom Coal Measures coal (QGC 1411) were in most cases lower than for the Juandah Coal Measures coal (QGC 1408). This corresponds to the lower total oil yield obtained from the Taroom Coal Measures coal, but is in contrast to the total gas content of the raw coal (see Table 6). The total gas in several closely related coals was almost entirely made up of hydrocarbons (typically methane 94%, ethane 5%) with CO2 levels of about 0.1%. Thus there was no correlation between the hydrocarbon gas yields of the tin-treated coal and the hydrocarbon gas content of the raw coals. 3.3. Analysis of oils The oils derived from hydrogenations in tetralin contained large contributions of tetralin and its dehydrogenated naphthalene analogue together with traces of hexane. The oil from QGC 1410 gave a 1 H-NMR spectrum with a tetralin/naphthalene ratio of 5.9:1 in good agreement with that obtained from GC. The 1H-NMR data for the oils from Sn-treated QGC 1401, 1406, 1408 and 1411 together with the oil from Sn-treated ABL 14 are given in Table 7. All of the spectra indicate a relatively small Har content and a large contribution from methylene groups, Hb, in keeping with the waxy nature of the oil. The value of Hb increased with increasing depth, paralleling the increase in hydrocarbon gas content of the raw coal (compare Table 6 and Table 7). It might be expected that oil derived from liptinite- rich coals, with their higher wax contents, would display higher Hb values. However, this increase in Hb did not correlate with the liptinite content of these seams in a neighbouring well (Table 8). In fact, the maceral distribution in the corresponding seams in the neighbouring well (Table 8) did not vary in any systematic manner with depth and did not change significantly at the boundary between the Juandah and Taroom Coal Measures. The simultaneous fall in terminal methyl groups, Hc that occurs with increasing depth may suggest that these methyl groups, which obviously constitute part of the coal organic matrix at shallow depths, are somehow associated with the formation of seamgas (methane) at greater depths.
Table 7 1 H-NMR for oil (hexane-sol) from hydroliquefaction of tin-treated coal. Sample number
Seam (S)/Coal Measures (CM)
Har
Ha
Hb
Hc
QGC1401-Sn QGC1406-Sn QGC1408-Sn QGC1411-Sn ABL14[11]
Macalister upper (S) Wambo (B) (S) Argyle (B) (S) Condamine (B) (S) Taroom (CM)
0.07 0.15 0.14 0.13 0.19
0.24 0.25 0.23 0.20 0.21
0.37 0.34 0.47 0.52 0.48
0.31 0.26 0.16 0.15 0.12
The oils were further investigated by GC-MS and the total ion chromatograms are shown in Fig. 5. It can be seen that the proportion of low boiling material tended to increase as the depth of the coal increased. Examination of the mass spectra revealed that alkylated aromatic compounds, including phenols, were more abundant in the oils from these deeper coals. It must be recognized that higher molecular weight compounds (>C33) are not eluted from the GC column under the conditions used here, so that the GC-MS data cannot be taken to be representative of the whole of the oil fraction. Even for hydrogenated coal oil, it is known that over 20% of the hexane solubles do not distil up to 450 °C [15], so that much a larger proportion of the oil produced here would not be expected to elute from the GC. This is contrast to the 1HNMR data, which can be considered to be representative of the whole of the hexane soluble material. Nevertheless, by GC-MS a significant change in the profile of the n-alkane distribution with depth was observed. The maximum became much less pronounced with increasing depth and concentrations of the lower boiling n-alkanes increased. Single ion plots showed that there was a slight difference in the C number of the most intense alkane peak. The oil from QGC 1401 showed a maximum at C26 whereas that from QGC 1411 was at C23. In addition the oil from ABL 14 showed an ill-defined maximum from C23–C25. Thus, the oils from Sn-treatment exhibit distinct compositional differences across the depth interval. These differences must, in some way, arise as a result of structural difference between the precursor coals.
Table 6 Gas yields by seam (Bellevue #1/1R) [19,20]. Seam
Total gas yield (m3/tonne daf)
Juandah coal measures Macalister upper Macalister lower Nangram (A) Nangram (B) Wambo (A) Wambo (B) Argyle (A) Argyle (B)
3.30 nd 4.27 4.42 5.56 5.53 7.26 5.53
Taroom coal measures Auburn Condamine (A) Condamine (B)
6.78 8.13 7.76
Table 8 The maceral group content of seams in Bellevue #1/1R [19,20]. Seam
Macalister upper Macalister lower Nangram (A)+(B) Wambo (A) Wambo (B) Argyle (A) Argyle (B) Auburn Condamine (A) Condamine (B)
Maceral group content (mmf, vol%) Vitrinite
Liptinite
Inertinite
70.7 nd 80.6 68.9 78.7 81.8 77.2 71.8 77.9 82.7
20.6 nd 18.2 30.9 21.2 18.1 22.8 28.2 21.5 16.4
8.6 nd 1.2 0.3 0.0 0.0 0.1 0.0 0.6 0.9
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Fig. 5. The total ion GC-MS plots for the oil from four tin-treated QGC coals compared to that from ABL14 [21], (a) QGC1401, (b) QGC1406, (c) QGC1408, (d) QGC1411, (e) ABL14. The GC conditions for (b) were slightly different from those for (a), (c) and (d).The numbers are carbon numbers of n-alkanes. The asterisks mark phthalate contaminant.
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Fig. 6. Total ion py-gc-ms plots for five raw QGC coals. The numbers are the carbon numbers of n-alkanes.
3.4. Pyrolysis GC-MS of the raw coals Fig. 6 shows the total ion plots from the py-gc-ms of five of the raw coals, QGC 1401, 1406, 1408, 1409 and 1411. The first important point to make is that the plots for all five seams were very similar across the whole molecular weight range. The plots are dominated by the alkane/alkene doublets which are resolved at lower molecular weight, but become merged into single peaks at higher molecular weight (greater than about C20). It can be seen that the alkane/alkene peaks with chain lengths between about C19–C29 dominate the plots, again demonstrating the waxy nature of the coals. When one examines the plots in more detail, however, it can be noticed that there are some subtle differences. Therefore, more extensive comparisons of the pyrolysis product distributions were carried out by preparing single or multiple ion plots that are characteristic of particular chemical features. For example, by comparing plots of m/z 57 (or m/z 71), characteristic of n-alkanes it could be seen (Table 9) that n-C27 is the n-alkane of maximum intensity in the shallowest coal sample (QGC1401) whereas this has shifted downwards slightly, to n-C25 for the deepest sample (QGC1411). It is well know that as coal matures, in response to increasing time, temperature and depth in a sedimentary environment, the distribution of hydrocarbon products tends to lower molecular weight ([16]). In the case of the sequence of coal examined here, this effect is only very slight.
The isoprenoid alkene, prist-1-ene, is an easily distinguished peak, even in the total ion plots (Fig. 6). It can be seen that the relative abundance of this peak, by comparison with the n-C17 also decreases with increasing depth (Table 9). The change in the relative abundance of this peak, which is regarded to be a scission product resulting originally from the side chain of chlorophyll ([17]), is a signal that, despite the prevailing similarities between all the samples, subtle geochemical processes are at work. The total ion plots also reveal many peaks at lower molecular weight that could be attributed to isomeric alkylaromatic compounds and, at higher molecular weight, a small number of peaks that could be assigned as homohopanes. Since the relative abundance of isomers within these molecular classes are well known to serve as geochemical maturity indicators [18], it was considered useful to examine the isomeric distribution within some selected classes more closely. By examining plots of characteristic ions, the distributions of several classes of compounds could be readily compared across the sample suite. For alkylbenzenes (78 + 92 + 106 + 120 + 134), alkylnaphthalenes (128 + 142 + 156 + 170 + 184), alkylphenanthrenes/alkylanthracenes (178 + 192 + 206 + 220 + 234) and alkylphenols (94 + 108 + 122 + 136), these summation plots again exhibited very strong similarity. Nevertheless, upon consideration of some specific isomers that are known to be sensitive as geochemical maturity indicators, some subtle differences could be discerned. Table 9 shows how the following ratios:
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A.L. Chaffee et al. / Fuel 89 (2010) 3241–3249 Table 9 Geochemical Maturity Indicator Ratios that can be derived from the Pyrolysis-GC-MS results.
Maximum carbon number n-Alkanes n-Alkenes
QGC1401
QGC1406
QGC1408
QGC1409
QGC1411
27 27
27 27
27 28
26 25
24 25
Alkylbenzenes n-Propylbenzene/(1,2,4- and 1,2,3-trimethylbenzene) 1,2,4-trimethylbenzene/1,2,3-trimethylbenzene
0.31 0.87
0.30 0.89
0.31 0.93
0.34 0.99
0.40 1.10
Alkylnaphthalenes (1,4,6- and1,3,5-trimethylnaphthalene)/2,3,6-trimethylnaphthalene
3.15
2.84
2.46
2.19
2.34
Pristene Pristene/nC17ene
0.73
0.41
0.50
0.38
0.24
Homohopanes S-ab-homohopane/(S-ab- and R-ab-homohopane)
0.25
0.31
0.36
0.36
0.42
(n-propylbenzene)/(1,2,4-trimethylbenzene + 1,2,3trimethylbenzene) (1,2,4-trimethylbenzene)/(1,2,3-trimethylbenzene) (1,4,6-trimethylnaphthalene + 1,2,5-trimethylnaphthalene)/ (2,3,6-trimethylnaphthalene) all exhibit significant and regular increases with depth across the sample suite. Another ratio that has regularly been correlated with maturity of coal or other sedimentary sequences is the ratio of diastereomers 22S/22S + 22R for 17a,21b-homohopane [7,18]. The change in this ratio is indicative of subtle organic geochemical processes that are slowly but surely differentiating the nature of the organic matter across the depth interval. So, there are clear, but only subtle differences between the pyrograms for the five coals. This contrasts to the significant differences shown in the hydrocarbon distribution of the oils obtained from hydroliquefaction of tin-treated coals (See section 3.3). The pyrolysis conditions are more severe (higher temperature), but less extensive (seconds) and are likely to result in fewer secondary reactions. It appears that the hydrogenation of the Sn-treated coals leads to more extensive degradation of the alkyl chains and the concomitant generation of alkylaromatics. This seems to be more facile for the deeper samples, suggesting that there is some underlying impact of subtle changes in the coal structure which are hinted at, but not fully revealed by, the py-gc-ms results. 4. Conclusion Over a 300 m depth interval the organic content of the coals intersected by Bellevue #4 is remarkably constant in composition and is similar to that of a Taroom coal from the same Walloon Subgroup. Regular differences in C and O content and in a variety of geochemical maturity parameters calculated from py-gc-ms data are observable as a function of increasing depth However the atomic H/C ratio was found to be the best predictor of conversion of these coals, as previously demonstrated by Redlich et al [2] for a broad suite of Australian coals (Fig. 4). This emphasizes that for coal hydroliquefaction such parameters as conversion and oil yield depend almost entirely on the atomic H/C ratio and are nearly independent of detailed coal chemistry. Nevertheless, there are clear differences with increasing depth of the coal seam in the nature of the product oil produced by hydroliquefaction in the presence of Sn. GC-MS analyses indicated an increase in the relative
abundance of low molecular weight alkylaromatic compounds and a decrease in the average carbon chain length of (observable) n-alkane products with increasing depth of the coal seam. 1HNMR indicated increased importance of waxy components (methylene groups) in the oils from the deeper coals, and this increased importance of waxy components appeared to correlate with the increased amount of total gas in the raw coals from lower depths, as given in Table 8. Acknowledgments We thank the Queensland Gas Company (now QGC Ltd) for coal samples and financial support. We also thank Alicia Cruickshank for technical support. References [1] Scott S, Anderson B, Crosdale P, Dingwall J, Leblang G. Int J Coal Geol 2007;70:209. [2] Redlich P, Jackson WR, Larkins FP. Fuel 1985;64:1383. [3] Redlich PJ, Jackson WR, Larkins FP. Fuel 1989;68:231. [4] Kedzior S. Zeszyty Naukowe Politechniki Slaskiej Gornictwo 2003; 257: 85. [5] Bhattacharya SN, Bagchi SJ. Mines, metals, fuels. 1964;12:299. [6] Perry GJ, Allardice DJ, Kiss LT. The chemical characteristics of Victorian brown coal. In: Schobert HH, editor. The chemistry of low-rank coals. Washington, DC: American Chemical Society; 1984. p. 3. [7] Johns RB, Chaffee AL, Verheyen TV. Chemical variation as a function of lithotype and depth in Victorian brown coal. In: Schobert HH, editor. The chemistry of low-rank coals. Washington, DC: American Chemical Society; 1984. p. 106. [8] Jasienko S, Matuszewska A, John A. Fuel Process Technol 1995;41:221. [9] Rouzaud JN, Guechchati N, Kister J, Conard J. Bull Soc Geol Fr 1991;162:201. [10] Yurum Y, Bozkurt D, Yalcin MN. Energy Sources 2001;23:511. [11] Boudou JP, Durand B, Oudin JL. Geochim Cosmochim Acta 1984;48:2005. [12] van der Marel HW, Beutelspacher H. Atlas of infrared spectroscopy of clay minerals and their admixtures. Amsterdam: Elsevier Scientific Publishing Co; 1976. [13] Verheyen TV, Johns RB, Esdaile RJ. Geochim Cosmochim Acta 1983;47:1579. [14] Redlich PJ. The chemical and structural characteristics of coals and their relationship to liquefaction, PhD. Clayton: Monash University; 1987. [15] Murase K, Jackson WR, Larkins FP, Marshall M, Watkins ID. Fuel 1984;63:1694. [16] Chaffee AL, Perry GJ, Johns RB. Fuel 1983;62:311. [17] Korkmaz S, Guelbay RK. Int J Coal Geol 2007;70:292. [18] Lewis CA. The kinetics of biomarker reactions. In: Engel MH, Macko SA, editors. Organic geochemistry – principles and applications. New York: Plenum Press; 1983. p. 491. [19] ‘‘QGC Bellevue 1 well completion report ATP610P”. Brisbane: Queensland Gas Co. Ltd; 2002. [20] ‘‘QGC Bellevue #1R well completion report ATP610P”. Brisbane: Queensland Gas Co. Ltd; 2006. [21] Redlich PJ, Jackson WR, Larkins FP, Chaffee AL, Liepa I. Fuel 1989;68:1549.
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Structural characterisation of Middle Jurassic, high-volatile bituminous Walloon Subgroup coals and correlation with the coal seam gas content Alan L. Chaffee a,b,*, Galinda Lay a,b, Marc Marshall a,b, W. Roy Jackson a,b, Yi Fei a,b, T. Vincent Verheyen c, Peter J. Cassidy d, Steven G. Scott d a
School of Chemistry, Monash University, Clayton, Victoria 3800, Australia Centre for Green Chemistry, Monash University, Victoria 3800, Australia School of Applied Science and Engineering, Churchill Campus, Churchill, Victoria 3842, Australia d QGC Ltd., Level 30, 275 George Street, Brisbane Queensland 4000, Australia b c
a r t i c l e
i n f o
Article history: Received 28 January 2010 Received in revised form 8 May 2010 Accepted 17 June 2010 Available online 30 June 2010 Keywords: Coal seam gas Chromatography Liquefaction Solvent separation
a b s t r a c t The structure of a suite of high-volatile, bituminous Surat Basin, Queensland coals has been investigated by a combination of analytical probes. These included elemental analyses, pyrolysis-gas chromatography-mass spectroscopy and Fourier transform infrared spectroscopy, together with a study of their liquefaction products in both tetralin and solvent free-tin catalysed hydrogenations. Samples were gathered across a 300 m depth interval intersected by the sampling well. Most techniques revealed clear but subtle differences in structure as a function of depth. The oils produced by solvent free-tin catalysed hydrogenation, however, showed very distinct dependence with depth and the waxy content, as indicated by 1HNMR, could be correlated with the coal seam gas content. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction A number of exploration and core drilling programs has been carried out since 2000 by Queensland Gas Company Limited (QGC) to investigate the coal seam gas potential of the Juandah (upper) and Taroom (lower) Coal Measures of the Middle Jurassic Walloon Subgroup in the Surat Basin, Queensland, Australia (Fig. 1). The gas content varies between the upper and lower coal measures in a manner that cannot be explained readily by petrological differences [1]. In this paper the chemical structural properties of these coals have been measured directly by ultimate analysis, pyrolysis-gas chromatography-mass spectroscopy (pygc-ms), and Fourier transform infrared spectroscopy (FTIR). In addition indirect structural probes comprising evaluation of the products obtained by hydrogenation of these coals under two complementary sets of conditions have been considered. The coals were hydrogenated, both without added catalyst in tetralin and also without solvent in the presence of a tin catalyst. The hydrogenations in the donor solvent tetralin allowed a direct comparison with a data set comprising a very wide range of Australian coals to ensure they contained no abnormal features [2]. The hydrogenations of the tin-treated coals [3] in the absence of solvent allowed
* Corresponding author at: School of Chemistry, P.O. Box 23, Monash University, Victoria 3800, Australia. Tel.: +61 3 9905 4626; fax: +61 3 9905 4597. E-mail address: [email protected] (A.L. Chaffee). 0016-2361/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2010.06.027
for a detailed chromatographic examination of the oil product from the coals without solvent interferences. Attempts have been made to seek comparisons between the coal seam gas content and the chemical properties of the coals. Earlier studies on the correlation of chemical, as distinct from petrographic, properties of coals are limited. A study of Upper Silesian coals showed an inverse correlation between methane content and the atomic H/C ratio of the coals [4], whereas an Indian study reported an increase in coal seam gas with the hydrogen content of the coal [5]. In addition, the variation of chemical properties with coal depth has been compared with those found for other studies involving samples from long sections. These include studies of Victorian brown coals where an average 1% increase in carbon content of the coals was found over 120 m for the Morwell Seam [6]. The structural composition of minor compounds present in various solvent extracts was also shown to vary with depth [7]. The aromatic structure of some Silesian coals over a depth range of 1550 m [8] and of some Lorraine coals over a depth range of 4700 m has been reported [9]. FTIR and ESR spectroscopic analysis have been carried out over an interval of 1160 m for a Zonguldak bituminous coal [10]. 13C and IR spectroscopy together with a study of the humic acid content has been reported for a very long section of Mahakam, Kalimantan, Indonesian coals ranging from plant/peat material to bituminous [11]. The coals in this study were recovered from a depth range of over 300 m, intermediate between the 100 m range of the Victo-
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Elemental analysis of acid-washed coals for C, H, N, S and Cl was carried out by HRL Technology Ltd, C, H and N were determined using a LECO CHN 1000 analyser by HRL Method 1.4 and S and Cl by AS1083.6.3.2 and AS1083.8.2. Acid-washed coals were analysed to avoid interference from carbonates. Ash yields were determined essentially following AS2434.8. 2.2. Coal liquefaction
Fig. 1. Map showing the location of the bore from which the coal samples were taken [20].
rian brown coals and the 1000 m or more for the others. The gas yields were from samples taken from the Bellevue #1/1R well, which is located in close proximity to the Bellevue #4 well from which the coal samples were collected.
2. Experimental 2.1. Coal preparation and elemental analysis The coals from Bellevue #4 (samples QGC 1401–1411) were received ground to 10 mm, double-bagged and in a moist state. Fig. 2 shows the geological section intersected by the well. A representative sample from each of the intersected seams was obtained by coning and quartering and dried in a flow of nitrogen at room temperature, then sieved and ground by mortar and pestle to 250 lm, and stored under nitrogen until used. Following the procedure of Redlich et al [2], representative samples of five coals were acid-washed by stirring overnight a mixture of 25:1, 0.1 M H2SO4:coal, for one hour under vacuum to ensure good wetting of the coal, then under nitrogen. The coal was then filtered and washed overnight with stirring in 25:1 distilled water:coal. Filtration and overnight washing with distilled water were repeated as many times as was necessary for the pH of the filtrate to become constant. This was found to ensure that the sulfate ion had been washed out. Tin impregnation was carried out by the same procedure, with SnCl2 solution substituted for H2SO4; the SnCl2 concentration was such as to give ca 1 mole tin per kg db of the original coal. The low pH obtained ensures that the acid-soluble material in the original coal is removed during the tin impregnation.
The coal liquefaction procedure was similar to the tetralin extraction procedure of Redlich et al [2,3] except that the autoclave was held stationary and heated in a sandbath, whereas Redlich et al [2,3] used a rocking autoclave with a heating jacket. For the runs in tetralin, 3 g of raw 250 lm coal dried at 105 °C in a flow of nitrogen for three hours and 9 g of tetralin were loaded into a 70 mL autoclave, which was then sealed and charged with 6 MPa of hydrogen. The autoclave was lowered into the sandbath, heated to 405 °C in 10–15 min and held at temperature for one hour. For most of the runs, after one hour at temperature, the autoclave was taken out of the sandbath, cooled in a blast of air and weighed. The gas was vented into an evacuated, weighed 300 mL autoclave, which was weighed again and vented through a gas meter. In this way the average molecular weight of the product gas and its composition, on the assumption that it consisted only of CO2 and H2, could be determined. The autoclave was vented completely and weighed again. For the runs with tin-treated coals (samples QGC 1408 and QGC 1411), a more complete gas analysis was carried out. After the initial cooling and weighing of the autoclave, the gas was vented through an Agilent 3000 Micro Gas Chromatograph, equipped with four columns: MS 5A PLOT, 10 m 0.32 mm (110 °C column temperature) for N2 and CH4, PLOT U, 8 m 0.32 mm (100 °C column temperature) for CO, CO2, C2 hydrocarbons, Alumina PLOT, 10 m 0.32 mm (140 °C column temperature) for C3–C5 hydrocarbons and OV-1, 10 m 0.15 mm 2.0 lm (90 °C column temperature) for isobutane and n-hexane. The inlet and injector temperatures were 100 °C. After the analysis the autoclave was vented and the total weight of the gas in the autoclave was calculated. On the assumption that the gases detected (and hydrogen by difference) were the only gases present, the yields of all the gases detected and the hydrogen consumption could be calculated. The solid and liquid products were washed and scraped out of the autoclave with dichloromethane into a flask, and subjected to Lundin distillation to remove water, though in fact no water was detected in any run, probably because it evaporated during work-up. The remaining solid and liquid product was fractionated using solvent solubility criteria, as described by Redlich et al [2,3] into dichloromethane insolubles, asphaltene (dichloromethane soluble, hexane insoluble) and oil (hexane soluble), with the amount of oil being determined by difference. The procedure for runs with tin-impregnated coal, carried out to obtain oil samples free of hydrogenation solvent, was similar to that for tetralin runs, except that 9 g (tin-free basis) of coal, no tetralin and 10 MPa of hydrogen were charged into the autoclave before the run. 2.3. Analysis of coal liquefaction product oil Four oils from runs with tin-impregnated coals were analysed by gas-chromatography-mass spectrometry (GC-MS) on a HP6890 instrument in splitless auto-mode. For GC, a HP 19091S433 capillary column (HP-5MS 5% phenylmethyl siloxane), 30 m long, 0.25 mm diameter, 0.25 lm nominal film thickness, was used. The inlet temperature was 230 °C. The oven temperature was initially held at 50 °C for 2 min then raised to 200 °C at a rate of 4 °C/min, held at 200 °C for 2 min, then raised to 300 °C at 8 °C/
A.L. Chaffee et al. / Fuel 89 (2010) 3241–3249
3243
Fig. 2. Stratigraphic correlation Bellevue #1/1R and Bellevue #4 (not to scale).
min and held at 300 °C for 3 min. For MS, the ionizing potential was 70 eV, the accelerating voltage 1.9 kV, the mass range scanned 45–600 m/z and the ion source temperature 200–250 °C. For 1H-NMR, the oil was dissolved in CDCl3 and spectra were obtained using a 300 MHz instrument, with a 90° pulse flip angle (9.5 ls). 2.4. Analysis of coals FTIR spectra were collected in absorbance mode using a Varian Scimitar 1000 FTIR spectrophotometer. The dried (105 °C under N2), finely ground coals were prepared as KBr discs (1.0 mg coal and 200 mg KBr). The py-gc-ms experiments were performed on a Chemical Data Systems 1000 coil pyroprobe unit interfaced to a Varian 3800 GC Saturn 2000 ion trap system. A small amount of the coal sample (3 mg) was loaded into a quartz tube containing quartz wool which was then inserted into the platinum coil of the probe. The samples
were flash pyrolysed at 700 °C for 40 seconds. Separation was effected using a Varian Factor 4 capillary column (VF-5MS 30 M.) 0.25 mm ID DF = 0.25 with helium as the carrier gas at1 ml/min constant flow. The column was held at 0 °C for 3 min then ramped to 80 °C at 6 °C/min; to 200 °C at 8 °C/min and finally to 325 °C at 10 °C/min with a 15 minutes hold time. The ion trap conditions were 70 eV electron impact, transfer line temperature 170 °C, trap temperature 150 °C, scan range 29 to 200 m/z between 1 and 10 minutes; then 29 m/z to 380 m/z to 25 minutes and finally 50– 450 m/z for the rest of the acquisition. 3. Results and discussion 3.1. The coals The ash yields for the raw coals (Table 1) were all high, (19–65 wt% db). Acid washing reduced the ash yield by 0–4 wt% db for all five coals subjected to this treatment (Table 1), indicating the pres-
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Table 1 (a) Ash yields of QGC coal samples (Bellevue #4), (b) Ash yield for Taroom coal. Sample number
(a) QGC1401 QGC1402 QGC1403 QGC1404 QGC1405 QGC1406 QGC1407 QGC1408 QGC1409 QGC1410 QGC1411 (b) ABL14 a
Formation
Seam
Depth interval (m)
Ash yield (wt% db)a Raw coal
Acid-washed coal
Macalister upper Macalister lower Nangram (A) Nangram (B) Wambo (A) Wambo (B) Argyle (A) Argyle (B)
184.14 189.09 200.16 201.25 209.61 210.71 215.94 221.78 225.72 23 l.42 242.32 247.2 305.09 307.14 330.72 331.93
45.1 ± 3.0 44.3 ± 0.9 62.7 ± 2.9 64.7 ± 2.9 54.2 ± 3.7 37.8 ± 1.6 48.5 ± 2.4 45.7 ± 3.1
41.5 ± 3.2 – – – – 33.8 ± 0.3 – 45.1 ± 1.2
Taroom CM
Auburn Condamine (A) Condamine (B)
413.74 414.74 469.83 474.88 477.64 500.22
60.6 ± 2.3 25.6 ± 1.4 18.8 ± 3.6
– 21.5 ± 2.0 15.9 ± 1.7
Taroom CM
Taroom
–
Juandah CM
7.4 ± 0.1
–
The errors are 90% confidence limits based on multiple determinations.
Table 2 Elemental analysis of selected acid-washed coalsa. Sample number
C
H
N
S
O
H/C
5.8 ± 0.5 6.3 ± 0.2 5.6 ± 0.3 6.2 ± 0.2 5.9
1.1 ± 0.2 1.1 ± 0.1 1.3 ± 0.1 1.2 ± 0.1 1.17
1.4 ± 0.1 0.6 ± 0.1 1.5 ± 0.1 0.5 ± 0.1 0.43
19.7 ± 5.3 20.2 ± 1.0 17.8 ± 2.7 16.1 ± 2.3 16.3
0.96 ± 0.04 1.05 ± 0.03 0.91 ± 0.04 0.97 ± 0.02 0.94
wt% daf QGC1401-AW QGC1406-AW QGC1408-AW QGC1411-AW Taroom [11] a
72.0 ± 4.5 71.8 ± 0.7 73.7 ± 2.2 76.0 ± 1.9 76.2
The errors are the sum of the errors in analysis and the 90% confidence limits on the ash yields.
ence of small but still significant quantities of acid-soluble inorganics or minerals such as calcium carbonate. The ash produced in all cases was very light yellow in colour, implying little iron was present. Four coals, roughly equally spaced along the section, were acidwashed and their elemental analysis determined (Table 2). The results suggested a small increase in carbon content and a small decrease in oxygen content, and hence a greater degree of coalification, with increasing depth, as expected. The sulfur content varied but the atomic H/C ratio and nitrogen content were almost constant down the whole intersection. The deepest coal (QGC 1411) is the one closest in elemental analysis to the Taroom coal (ABL14) used in a prior study by Redlich et al [2]. This is as expected because the Taroom coal investigated by Redlich et al came from the Taroom (lower) Coal Measures of the Walloon Subgroup [1]. The coal FTIR spectra were dominated by mineral matter absorbance bands in accord with their high ash yields (Table 1). These absorptions were assigned predominantly to kaolinite [12] a higher surface activity clay mineral. There was also possibly some illite present, more in the shallower sample (QGC 1401) than in the deepest one (QGC 1411). Signals due to C-H stretching were observed in both samples in agreement with the long chain hydrocarbons found in the py-gc-ms data and in the coal liquefaction products. The various aromatic C–H bending modes in the coals’ FTIR spectra are sensitive to aromatic condensation. The degree of coalification can generally be assessed by changes in aromatic ring substitution by carbon or oxygen [13]. An increase in the 880 and 818 cm 1 and reduction in 750 cm 1 (all aromatic C–H bend mode) band intensities is associated with increased aromatic ring condensation as indicated by a loss of adjacent protons on individual rings. However Fig. 3 reveals the intensities of these marker bands is not changing in any regular way with depth, indicating that there is no clearly discernible aromatic ring condensation observable by this method.
3.2. Product yields from raw and tin-treated coal liquefaction Table 3 shows the product yields of nine of the raw coals from Bellevue #4 under the standard autoclave reaction conditions of
Fig. 3. FTIR absorbance spectra ArC-H bend region.
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Table 3 Liquefaction of coals (a) 3:1 tetralin:coal without catalyst and 6 MPa (cold) H2 at 405 °C for 1 h and (b) tin-treated coal (1 mol Sn/kg dry acid-washed coal) without solvent and 10 MPa (cold) H2 at 405 °C for 1ha. Sample number
CH2Cl2 solubles (inc. gas)
Asphaltene
Oil and H2O
CO2 production
H2 consumption
(wt% daf) (a) QGC1401 QGC1403 QGC1404 QGC1405 QGC1406 QGC1408 QGC1409 QGC1410 QGC1411 ABL14
63.7 ± 9.6 67.3 ± 0.6 65.7 ± 12.0 69.0 ± 10.0 72.9 ± 5.0 73.5 ± 5.8 67.3 ± 9.0 71.2 ± 4.0 64.3 ± 4.1 72.0
29.4 ± 3.4 28.7 ± 1.8 30.2 ± 6.0 27.5 ± 3.6 31.1 ± 2.3 39.4 ± 4.6 23.7 ± 3.9 33.6 ± 2.3 27.8 ± 1.7 33.0
26.0 ± 23.0 25.9 ± 18.0 41.5 ± 28.0 45.0 ± 20.0 43.3 ± 12.0 26.1 ± 16.6 47.2 ± 17.0 41.2 ± 10.3 29.9 ± 10.7 36.0
13.0 ± 5.7 16.4 ± 7.9 4.2 ± 1.8 2.6 ± 0.9 5.4 ± 2.0 12.5 ± 3.1 4.4 ± 1.5 4.4 ± 1.5 5.0 ± 2.0 2.6
4.8 ± 4.7 3.7 ± 7.7 10.3 ± 8.0 6.4 ± 5.0 6.9 ± 2.4 4.5 ± 3.1 8.0 ± 2.5 8.0 ± 2.5 1.6 ± 2.9 nd
(b) QGC1401-Sn QGC1406-Sn QGC1408-Sn QGC1411-Sn ABL14-Sn
78.8 ± 5.1 82.0 ± 5.8 87.7 ± 0.3 64.3 ± 0.8 48.2
27.7 ± 1.4 32.6 ± 1.5 26.9 ± 0.5 27.2 ± 1.0 9.0
47.9 ± 8.2 46.2 ± 8.3 59.4 ± 6.7 31.6 ± 4.7 39.4
5.9 ± 0.7 6.6 ± 0.6 2.4 ± 0.2 3.5 ± 0.3 nd
2.8 ± 1.0 3.9 ± 0.4 6.1 ± 0.7 1.4 ± 0.4 nd
a The uncertainties given are those due to uncertainties in ash yield and weighing. There is an additional uncertainty in the calibration of the reference gas mixture which applies to the CO2 production and H2 consumption data for QGC1408-Sn and QGC1411-Sn.
100
Redlich coals coals in the current study
90 80 Conv. (wt% dmif)
Redlich et al [2]. The close similarity of the conversion (CH2Cl2 solubles and gas), asphaltene and oil + water yield of the Bellevue coals and of the Taroom coal, from the same coal measures, are clear. For QGC 1401 and QGC 1403 the CO2 production, even allowing for the large experimental error, appears to be larger than for the other coals. At least for QGC 1401, this could be in part explained by decomposition of MgCO3 or similar easily reacted carbonate during the liquefaction; the difference in ash yield between raw and acid-washed coals was ca 4 wt% daf (cf. Table 1), which would give over 3 wt% of CO2 if the difference in ash yield were due to MgCO3. The overall similarity between the different coals in oil + water and dichloromethane-solubles (including gas), which is total conversion (Table 3) can be understood in terms of the well-known correlations between oil yield and total conversion vs. atomic H/ C ratio for acid-washed coals [2,3]. The coals analysed are of similar atomic H/C ratio (see Table 2) and contain little active catalyst (e.g. Fe; see above), so that they would, on the basis of this correlation, be expected to have a similar oil yield and total conversion, despite any differences in chemical composition due to the range of burial depths or the high (but probably inert) ash yields. Indeed, the total conversion for the four coals in the suite of known atomic H/C ratio fell only just below or coincided with the correlation established by Redlich et al [3] between total conversion and atomic H/C ratio for Australian coals of a wide range of atomic H/C ratio and rank (Fig. 4). The fact that the total conversions sometimes were slightly lower than those found by Redlich et al [3] for coals of the same atomic H/C ratio can be explained by the lack of agitation of the autoclave in the current experiments compared to the rocking motion in the experiments of Redlich et al [3]. The amount of tin added to the tin-treated coals (Table 4) can be estimated from the difference in ash yield between the tin-treated coals (Table 4) and acid-washed coals (Table 1), because all acidsoluble inorganics are removed during the tin treatment (see above). Any differences in removal of inorganics due to differences in the nature of the acid medium will be negligible compared to the uncertainties in ash yields. The results of hydrogenation of tin-treated coals in the absence of solvent are given in Table 3 together with that previously reported for a hydrogenation of tintreated ABL 14 [14]. (Tin-treated ABL 14 gave a lower conversion than QGC 1411 probably because the method of tin addition to ABL 14 was different and would have resulted in poorer dispersion
70 60 50 40 30 20 10 0 0
0.2
0.4
0.6 0.8 Atomic H/C ratio
1
1.2
1.4
Fig. 4. The correlation between total conversion (CH2Cl2 solubles, including gas) and atomic H/C ratio for a suite of Australian coals [3] with four coals from the current study added.
Table 4 Ash yields of QGC coals treated with SnCl2.
a
Sample number
Ash yielda (wt% db)
Sn contenta (mol/kg dry aw coal)
QGC1401-Sn QGC1406-Sn QGC1408-Sn QGC1411-Sn
49.2 ± 1.6 44.0 ± 2.4 52.6 ± 1.2 28.4 ± 1.3
1.10 ± 0.90 1.21 ± 0.06 1.16 ± 0.20 1.05 ± 0.23
The errors are 90% confidence limits based on multiple determinations.
and large particle size of SnO2, the final product of treatment with SnCl2 solution.) The important feature of these results is that the total conversion, oil yield and hydrogen consumption from reaction of QGC 1411, which, like ABL 14, belongs to the Taroom Coal Measures, were much lower than the corresponding yields for the three coals (QGC 1401, 1406 and 1408) belonging to the Juandah Coal Measures. This might be taken to suggest that the more significant structural changes over the whole length of the section occur between the Juandah and Taroom Coal Measures.
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Table 5 Composition of gas from hydroliquefaction of tin-treated coalsa. Sample number
CO
CH4
C2
C3
C4
C5
C6
0.87 ± 0.08 0.90 ± 0.08
0.90 ± 0.08 0.62 ± 0.06
0.25 ± 0.02 0.19 ± 0.02
0.20 ± 0.02 0.10 ± 0.01
– 0.08 ± 0.01
0.07 ± 0.01 0.04 ± 0.00
wt% daf QGC1408-Sn QGC1411-Sn a
2.84 ± 0.26 1.45 ± 0.13
Uncertainties are those due to uncertainties in ash yield, weighing and composition of reference gas mixture.
The composition of the gas from the Sn-treated QGC 1408 and QGC 1411 reactions are given in Table 5. The hydrocarbon gas yields from reactions of the Taroom Coal Measures coal (QGC 1411) were in most cases lower than for the Juandah Coal Measures coal (QGC 1408). This corresponds to the lower total oil yield obtained from the Taroom Coal Measures coal, but is in contrast to the total gas content of the raw coal (see Table 6). The total gas in several closely related coals was almost entirely made up of hydrocarbons (typically methane 94%, ethane 5%) with CO2 levels of about 0.1%. Thus there was no correlation between the hydrocarbon gas yields of the tin-treated coal and the hydrocarbon gas content of the raw coals. 3.3. Analysis of oils The oils derived from hydrogenations in tetralin contained large contributions of tetralin and its dehydrogenated naphthalene analogue together with traces of hexane. The oil from QGC 1410 gave a 1 H-NMR spectrum with a tetralin/naphthalene ratio of 5.9:1 in good agreement with that obtained from GC. The 1H-NMR data for the oils from Sn-treated QGC 1401, 1406, 1408 and 1411 together with the oil from Sn-treated ABL 14 are given in Table 7. All of the spectra indicate a relatively small Har content and a large contribution from methylene groups, Hb, in keeping with the waxy nature of the oil. The value of Hb increased with increasing depth, paralleling the increase in hydrocarbon gas content of the raw coal (compare Table 6 and Table 7). It might be expected that oil derived from liptinite- rich coals, with their higher wax contents, would display higher Hb values. However, this increase in Hb did not correlate with the liptinite content of these seams in a neighbouring well (Table 8). In fact, the maceral distribution in the corresponding seams in the neighbouring well (Table 8) did not vary in any systematic manner with depth and did not change significantly at the boundary between the Juandah and Taroom Coal Measures. The simultaneous fall in terminal methyl groups, Hc that occurs with increasing depth may suggest that these methyl groups, which obviously constitute part of the coal organic matrix at shallow depths, are somehow associated with the formation of seamgas (methane) at greater depths.
Table 7 1 H-NMR for oil (hexane-sol) from hydroliquefaction of tin-treated coal. Sample number
Seam (S)/Coal Measures (CM)
Har
Ha
Hb
Hc
QGC1401-Sn QGC1406-Sn QGC1408-Sn QGC1411-Sn ABL14[11]
Macalister upper (S) Wambo (B) (S) Argyle (B) (S) Condamine (B) (S) Taroom (CM)
0.07 0.15 0.14 0.13 0.19
0.24 0.25 0.23 0.20 0.21
0.37 0.34 0.47 0.52 0.48
0.31 0.26 0.16 0.15 0.12
The oils were further investigated by GC-MS and the total ion chromatograms are shown in Fig. 5. It can be seen that the proportion of low boiling material tended to increase as the depth of the coal increased. Examination of the mass spectra revealed that alkylated aromatic compounds, including phenols, were more abundant in the oils from these deeper coals. It must be recognized that higher molecular weight compounds (>C33) are not eluted from the GC column under the conditions used here, so that the GC-MS data cannot be taken to be representative of the whole of the oil fraction. Even for hydrogenated coal oil, it is known that over 20% of the hexane solubles do not distil up to 450 °C [15], so that much a larger proportion of the oil produced here would not be expected to elute from the GC. This is contrast to the 1HNMR data, which can be considered to be representative of the whole of the hexane soluble material. Nevertheless, by GC-MS a significant change in the profile of the n-alkane distribution with depth was observed. The maximum became much less pronounced with increasing depth and concentrations of the lower boiling n-alkanes increased. Single ion plots showed that there was a slight difference in the C number of the most intense alkane peak. The oil from QGC 1401 showed a maximum at C26 whereas that from QGC 1411 was at C23. In addition the oil from ABL 14 showed an ill-defined maximum from C23–C25. Thus, the oils from Sn-treatment exhibit distinct compositional differences across the depth interval. These differences must, in some way, arise as a result of structural difference between the precursor coals.
Table 6 Gas yields by seam (Bellevue #1/1R) [19,20]. Seam
Total gas yield (m3/tonne daf)
Juandah coal measures Macalister upper Macalister lower Nangram (A) Nangram (B) Wambo (A) Wambo (B) Argyle (A) Argyle (B)
3.30 nd 4.27 4.42 5.56 5.53 7.26 5.53
Taroom coal measures Auburn Condamine (A) Condamine (B)
6.78 8.13 7.76
Table 8 The maceral group content of seams in Bellevue #1/1R [19,20]. Seam
Macalister upper Macalister lower Nangram (A)+(B) Wambo (A) Wambo (B) Argyle (A) Argyle (B) Auburn Condamine (A) Condamine (B)
Maceral group content (mmf, vol%) Vitrinite
Liptinite
Inertinite
70.7 nd 80.6 68.9 78.7 81.8 77.2 71.8 77.9 82.7
20.6 nd 18.2 30.9 21.2 18.1 22.8 28.2 21.5 16.4
8.6 nd 1.2 0.3 0.0 0.0 0.1 0.0 0.6 0.9
A.L. Chaffee et al. / Fuel 89 (2010) 3241–3249
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Fig. 5. The total ion GC-MS plots for the oil from four tin-treated QGC coals compared to that from ABL14 [21], (a) QGC1401, (b) QGC1406, (c) QGC1408, (d) QGC1411, (e) ABL14. The GC conditions for (b) were slightly different from those for (a), (c) and (d).The numbers are carbon numbers of n-alkanes. The asterisks mark phthalate contaminant.
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Fig. 6. Total ion py-gc-ms plots for five raw QGC coals. The numbers are the carbon numbers of n-alkanes.
3.4. Pyrolysis GC-MS of the raw coals Fig. 6 shows the total ion plots from the py-gc-ms of five of the raw coals, QGC 1401, 1406, 1408, 1409 and 1411. The first important point to make is that the plots for all five seams were very similar across the whole molecular weight range. The plots are dominated by the alkane/alkene doublets which are resolved at lower molecular weight, but become merged into single peaks at higher molecular weight (greater than about C20). It can be seen that the alkane/alkene peaks with chain lengths between about C19–C29 dominate the plots, again demonstrating the waxy nature of the coals. When one examines the plots in more detail, however, it can be noticed that there are some subtle differences. Therefore, more extensive comparisons of the pyrolysis product distributions were carried out by preparing single or multiple ion plots that are characteristic of particular chemical features. For example, by comparing plots of m/z 57 (or m/z 71), characteristic of n-alkanes it could be seen (Table 9) that n-C27 is the n-alkane of maximum intensity in the shallowest coal sample (QGC1401) whereas this has shifted downwards slightly, to n-C25 for the deepest sample (QGC1411). It is well know that as coal matures, in response to increasing time, temperature and depth in a sedimentary environment, the distribution of hydrocarbon products tends to lower molecular weight ([16]). In the case of the sequence of coal examined here, this effect is only very slight.
The isoprenoid alkene, prist-1-ene, is an easily distinguished peak, even in the total ion plots (Fig. 6). It can be seen that the relative abundance of this peak, by comparison with the n-C17 also decreases with increasing depth (Table 9). The change in the relative abundance of this peak, which is regarded to be a scission product resulting originally from the side chain of chlorophyll ([17]), is a signal that, despite the prevailing similarities between all the samples, subtle geochemical processes are at work. The total ion plots also reveal many peaks at lower molecular weight that could be attributed to isomeric alkylaromatic compounds and, at higher molecular weight, a small number of peaks that could be assigned as homohopanes. Since the relative abundance of isomers within these molecular classes are well known to serve as geochemical maturity indicators [18], it was considered useful to examine the isomeric distribution within some selected classes more closely. By examining plots of characteristic ions, the distributions of several classes of compounds could be readily compared across the sample suite. For alkylbenzenes (78 + 92 + 106 + 120 + 134), alkylnaphthalenes (128 + 142 + 156 + 170 + 184), alkylphenanthrenes/alkylanthracenes (178 + 192 + 206 + 220 + 234) and alkylphenols (94 + 108 + 122 + 136), these summation plots again exhibited very strong similarity. Nevertheless, upon consideration of some specific isomers that are known to be sensitive as geochemical maturity indicators, some subtle differences could be discerned. Table 9 shows how the following ratios:
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A.L. Chaffee et al. / Fuel 89 (2010) 3241–3249 Table 9 Geochemical Maturity Indicator Ratios that can be derived from the Pyrolysis-GC-MS results.
Maximum carbon number n-Alkanes n-Alkenes
QGC1401
QGC1406
QGC1408
QGC1409
QGC1411
27 27
27 27
27 28
26 25
24 25
Alkylbenzenes n-Propylbenzene/(1,2,4- and 1,2,3-trimethylbenzene) 1,2,4-trimethylbenzene/1,2,3-trimethylbenzene
0.31 0.87
0.30 0.89
0.31 0.93
0.34 0.99
0.40 1.10
Alkylnaphthalenes (1,4,6- and1,3,5-trimethylnaphthalene)/2,3,6-trimethylnaphthalene
3.15
2.84
2.46
2.19
2.34
Pristene Pristene/nC17ene
0.73
0.41
0.50
0.38
0.24
Homohopanes S-ab-homohopane/(S-ab- and R-ab-homohopane)
0.25
0.31
0.36
0.36
0.42
(n-propylbenzene)/(1,2,4-trimethylbenzene + 1,2,3trimethylbenzene) (1,2,4-trimethylbenzene)/(1,2,3-trimethylbenzene) (1,4,6-trimethylnaphthalene + 1,2,5-trimethylnaphthalene)/ (2,3,6-trimethylnaphthalene) all exhibit significant and regular increases with depth across the sample suite. Another ratio that has regularly been correlated with maturity of coal or other sedimentary sequences is the ratio of diastereomers 22S/22S + 22R for 17a,21b-homohopane [7,18]. The change in this ratio is indicative of subtle organic geochemical processes that are slowly but surely differentiating the nature of the organic matter across the depth interval. So, there are clear, but only subtle differences between the pyrograms for the five coals. This contrasts to the significant differences shown in the hydrocarbon distribution of the oils obtained from hydroliquefaction of tin-treated coals (See section 3.3). The pyrolysis conditions are more severe (higher temperature), but less extensive (seconds) and are likely to result in fewer secondary reactions. It appears that the hydrogenation of the Sn-treated coals leads to more extensive degradation of the alkyl chains and the concomitant generation of alkylaromatics. This seems to be more facile for the deeper samples, suggesting that there is some underlying impact of subtle changes in the coal structure which are hinted at, but not fully revealed by, the py-gc-ms results. 4. Conclusion Over a 300 m depth interval the organic content of the coals intersected by Bellevue #4 is remarkably constant in composition and is similar to that of a Taroom coal from the same Walloon Subgroup. Regular differences in C and O content and in a variety of geochemical maturity parameters calculated from py-gc-ms data are observable as a function of increasing depth However the atomic H/C ratio was found to be the best predictor of conversion of these coals, as previously demonstrated by Redlich et al [2] for a broad suite of Australian coals (Fig. 4). This emphasizes that for coal hydroliquefaction such parameters as conversion and oil yield depend almost entirely on the atomic H/C ratio and are nearly independent of detailed coal chemistry. Nevertheless, there are clear differences with increasing depth of the coal seam in the nature of the product oil produced by hydroliquefaction in the presence of Sn. GC-MS analyses indicated an increase in the relative
abundance of low molecular weight alkylaromatic compounds and a decrease in the average carbon chain length of (observable) n-alkane products with increasing depth of the coal seam. 1HNMR indicated increased importance of waxy components (methylene groups) in the oils from the deeper coals, and this increased importance of waxy components appeared to correlate with the increased amount of total gas in the raw coals from lower depths, as given in Table 8. Acknowledgments We thank the Queensland Gas Company (now QGC Ltd) for coal samples and financial support. We also thank Alicia Cruickshank for technical support. References [1] Scott S, Anderson B, Crosdale P, Dingwall J, Leblang G. Int J Coal Geol 2007;70:209. [2] Redlich P, Jackson WR, Larkins FP. Fuel 1985;64:1383. [3] Redlich PJ, Jackson WR, Larkins FP. Fuel 1989;68:231. [4] Kedzior S. Zeszyty Naukowe Politechniki Slaskiej Gornictwo 2003; 257: 85. [5] Bhattacharya SN, Bagchi SJ. Mines, metals, fuels. 1964;12:299. [6] Perry GJ, Allardice DJ, Kiss LT. The chemical characteristics of Victorian brown coal. In: Schobert HH, editor. The chemistry of low-rank coals. Washington, DC: American Chemical Society; 1984. p. 3. [7] Johns RB, Chaffee AL, Verheyen TV. Chemical variation as a function of lithotype and depth in Victorian brown coal. In: Schobert HH, editor. The chemistry of low-rank coals. Washington, DC: American Chemical Society; 1984. p. 106. [8] Jasienko S, Matuszewska A, John A. Fuel Process Technol 1995;41:221. [9] Rouzaud JN, Guechchati N, Kister J, Conard J. Bull Soc Geol Fr 1991;162:201. [10] Yurum Y, Bozkurt D, Yalcin MN. Energy Sources 2001;23:511. [11] Boudou JP, Durand B, Oudin JL. Geochim Cosmochim Acta 1984;48:2005. [12] van der Marel HW, Beutelspacher H. Atlas of infrared spectroscopy of clay minerals and their admixtures. Amsterdam: Elsevier Scientific Publishing Co; 1976. [13] Verheyen TV, Johns RB, Esdaile RJ. Geochim Cosmochim Acta 1983;47:1579. [14] Redlich PJ. The chemical and structural characteristics of coals and their relationship to liquefaction, PhD. Clayton: Monash University; 1987. [15] Murase K, Jackson WR, Larkins FP, Marshall M, Watkins ID. Fuel 1984;63:1694. [16] Chaffee AL, Perry GJ, Johns RB. Fuel 1983;62:311. [17] Korkmaz S, Guelbay RK. Int J Coal Geol 2007;70:292. [18] Lewis CA. The kinetics of biomarker reactions. In: Engel MH, Macko SA, editors. Organic geochemistry – principles and applications. New York: Plenum Press; 1983. p. 491. [19] ‘‘QGC Bellevue 1 well completion report ATP610P”. Brisbane: Queensland Gas Co. Ltd; 2002. [20] ‘‘QGC Bellevue #1R well completion report ATP610P”. Brisbane: Queensland Gas Co. Ltd; 2006. [21] Redlich PJ, Jackson WR, Larkins FP, Chaffee AL, Liepa I. Fuel 1989;68:1549.