Chapter 28
Isotopic Study of Coal-Associated Hydrogen Sulfide 1
J. W. Smith and R. Phillips
2
1
Division of Exploration Geoscience, Commonwealth Scientific and Industrial Research Organisation, P.O. Box 136, North Ryde, N.S.W. Australia 2113 Capricorn Coal Management Pty., Ltd., Private Bag, Middlemount, Queensland, Australia 4745
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2
As much as 0.1% by weight of an Australian bituminous coal occurs as hydrogen sulfide. The gas is tightly held within the coal structure and is only detectable by odour on breakage of the coal. 3 4 S / 32S ratios 1) discount sulfate reduction as the mechanism for sulfide production and 2) suggest the gas is generated at an early stage of diagenesis by the cracking of thermally unstable S-S and C-S linkages in organic compounds. Occurrences of hydrogen sulfide in coal seam gases are rare. Within Australia, only hydrogen sulfide at the Collinsville Colliery in the Bowen Basin has previously been investigated isotopically (1). The widespread invasion of that mine by carbon dioxide, derived externally to the coal, precluded definition of precise hydrogen sulfide sources. The opportunity to gain a better understanding of the factors leading to hydrogen sulfide generation within coal seams was provided by its release in mining operations at the Southern Colliery, Qld. This mine, unlike many in the Bowen Basin, is unaffected by externally generated carbon dioxide and thus transport into the mine workings of hydrogen sulfide in association with that gas could be disregarded. At the Southern Colliery, located in the Bowen Basin, Central Queensland, Figure 1, coal is won from the Permian, German Creek Measures at a depth of some 200 to 350 feet. The immediate roof of the seam is comprised of highly quartzose sandstone, strongly cemented by secondary silica overgrowths and most probably of marine origin. The coal is of bituminous rank with a vitrinite reflectance of 1.4%. The quantity of * insignificant in comparison with those encountered in sour natural gas wells, but, as the gas is released in confined workings, it poses a real discomfort and health hazard in mining operations. Interesting facets of this gas occurrence are that 1 ) it is limited to particular sections of the workings, and 2) in hydrogen sulfide-rich zones, this gas is scarcely detectable by odour until breakage of the coal. s
0097-6156/90/0429-0568$06.00/0 © 1990 American Chemical Society
In Geochemistry of Sulfur in Fossil Fuels; Orr, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
Isotopic Study of Coal-Associated Hydrogen Sulfide
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28. SMITH & PHILLIPS
In Geochemistry of Sulfur in Fossil Fuels; Orr, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
569
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570
GEOCHEMISTRY OF SULFUR IN FOSSIL FUELS
In efforts to locate and identify the source of this gas, and more importantly, to be able to predict when and where it might be encountered, geological and scientific staff at the mine have examined coal from the two zones both chemically and petrographically. Specific factors which might be related to the occurrence of the gas have not been identified. Certainly the total sulfur content of the coal is variable to 3.0%, due to the random occurrence of massive pyrite, but, in general, total sulfur contents are less than 1.1%, with organic sulfur contents in the range of 0.4% to 0.8%. In addition, geological factors, including faults, intrusions etc., within the mine, cannot be related to the presence of H^S. Measurements of the hydrogen sulfide content of the coal were made by either tumbling and crushing lump coal in a closed drum containing steel cubes and drawing the gas through a Draeger tube, or by crushing the coal directly in silver nitrate solution. This latter method showed the coal to contain as much as 700 litres of hydrogen sulfide/1000 kg, that is, approximately 0.1% of the coal by weight may be present as hydrogen sulfide. In an attempt to gain some understanding of the source of this gas, the sulfur isotopic composition of the ^ other forms of sulfur (including acidvolatile sulfide) in the coal were measured in samples from the FL^S - rich and H2S -free zones. m
Experimental The H2S was sampled and collected as silver sulfide (Ag2S) by drawing gas released in the gas desorption tests through silver nitrate solution. Sulfate and pyrite sulfur were determined by the sequential treatment of 4-5 g of coal (-72 B.S. Sieve) with boiling 10% hydrochloric acid and boiling 10% nitric acid, each for 30 minutes, respectively. Sulfate in solution after each acid treatment was recovered as barium sulfate (BaSC^). The organic sulfur in the residual acid-treated coal was determined as BaSC>4 after ignition of the coal with Eschka mixture. In detennining acid-volatile sulfide as Ag2S, 100 g of coal were reacted with boiling 10% hydrochloric acid for 2 hours and die gaseous products carried through a silver nitrate solution in a stream of nitrogen. Ag2S was mixed with cuprous oxide and converted to SO2 by heating at 900 C (2). BaSC>4 was directly converted to SO2 by thermal decomposition in quartz at 1600 C (3). Product SO2 was freed from water and COo before isotopic measurement on a Micromass 602D mass spectrometer. ^S/^S ratios are reported to a precision of ±0.2%o relative to troilite from the Canyon Diablo meteorite using the normal 5 ^ S % o notation. o
Results and Discussion The concentration and isotopic composition of sulfur in all measured forms are given in Table I. Previous studies (4-6) have illustrated the difficulties in relating the isotopic composition of the organic and pyritic forms of sulfur in coals. This is particularly so where large additions of secondary pyrite further distort the frail relationship, if
In Geochemistry of Sulfur in Fossil Fuels; Orr, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
28.
Isotopic Study ofCoal-Associated Hydrogen Sulfide
SMITH & PHILLIPS
Table I. Concentrations and Isotopic Compositions of the Forms of Sulfur in Coal
%
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Mine Zone
Organic
Pyritic
Sulfate
S
S
S
%
34
Ô S
3 4
ο // o o
%
S
0/ /oo
Acid Volatile S ppm 8 S
34
34
5 S
HS 2
% o
1 2 3
0.62 0.61 0.56
+14.2 +14.0 + 7.8
2.89 0.21 0.34
+13.7 +0.4 +9.7
0.10 0.03 0.05
+2.1 -6.8 +1.5
2 n.d. 11 +10.4 +9.3 n.d. n.d +7.9
H S- 4 Free 5
0.44 0.46
+6.0 +6.7
0.08 1.42
+11.7
