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Role of coal type and rank on methane sorption characteristics of Bowen Basin, Australia coals
International Journal of Coal Geology 40 Ž1999. 309–325
Role of coal type and rank on methane sorption characteristics of Bowen Basin, Australia coals Chikatamarla Laxminarayana ) , Peter J. Crosdale Coalseam Gas Research Institute, School of Earth Sciences, James Cook UniÕersity, TownsÕille, Queensland 4811, Australia Received 31 March 1998; accepted 20 January 1999
Abstract The effect of coal composition, particularly the organic fraction, upon gas sorption has been investigated for Bowen Basin and Sydney Basin, Australia coals. Maceral composition influences on gas retention and release were investigated using isorank pairs of hand-picked bright and dull coal in the rank range of high volatile bituminous Ž0.78% R o max . to anthracite Ž3.01% R o max .. Adsorption isotherm results of dry coals indicated that Langmuir volume Ž V L . for bright and dull coal types followed discrete, second-order polynomial trends with increasing rank. Bright coals had a minimum V L at 1.72% R o max and dull coals had a minimum V L at 1.17% R o max . At low rank, V L was greater in bright coal by about 10 cm3rg, but as rank increased, the bright and dull trends converged and crossed at 1.65% R o max . At ranks higher than 1.65% R o max , both bright and dull coals followed similar trends. These competing trends mean that the importance of maceral composition on V L varies according to rank. In high volatile bituminous coals, increases in vitrinite content are associated with increases in adsorption capacity. At ranks higher than medium to low volatile bituminous, changes in maceral composition may exert relatively little influence on adsorption capacity. The Langmuir pressure Ž P L . showed a strong relationship of decreasing PL with increasing rank, which was not related to coal type. It is suggested that the observed trend is related to a decrease in the heterogeneity of the pore surfaces, and subsequent increased coverage by the adsorbate, as coal rank increases. Desorption rate studies on crushed samples show that dull coals desorb more rapidly than bright coals and that desorption rate is also a function of rank. Coals of lower rank have higher effective diffusivities. Mineral matter was found to have no influence on desorption rate of these finely crushed samples. The evolution of the coal pore structure with changing rank is implicated in diffusion rate differences. q 1999 Elsevier Science B.V. All rights reserved. Keywords: coalbed methane; methane adsorption; desorption rate; coal rank; coal type
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Corresponding author. E-mail: [email protected]
0166-5162r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 6 - 5 1 6 2 Ž 9 9 . 0 0 0 0 5 - 1
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C. Laxminarayana, P.J. Crosdaler International Journal of Coal Geology 40 (1999) 309–325
1. Introduction Recognition of ‘coalbed methane’ ŽCBM. as an economic energy resource has led to commercial evaluation in many countries. Australia has a significant CBM potential but commercialisation relies on development of appropriate techniques for Australian conditions ŽWyld, 1997.. The Australian CBM industry still lacks an adequate understanding of the reservoir and other technical parameters determining methane producibility ŽBeavers, 1997.. Understanding sorption of gas by coal is extremely important in CBM resource estimation and determining producibility. Methane in coal occurs mostly as adsorbed gas on pore surfaces, but is also present in small quantities as free gas in the fracture network ŽLevine, 1992.. Adsorption and desorption isotherms can be used to evaluate potential storage capacity, methane recovery and factors governing these parameters. Various studies have identified the most important factors influencing methane storage and recovery and include coal type, rank, moisture content, mineral matter content, temperature, depth, natural fracturing and stress ŽKim, 1977; Ayers and Kelso, 1989; Levine, 1992; Crosdale and Beamish, 1993, 1995; Lamberson and Bustin, 1993; Yee et al., 1993; Levy et al., 1997.. Coal type refers to those characteristics which are ‘‘initially determined by the nature of the ingredient matter, the conditions of deposition, and extent of operation of the first or biochemical process of coal making’’ ŽWhite, 1909.. We restrict the use of coal type to refer to maceral composition. Coal rank describes the degree of carbonification of the coal ŽICCP, 1963.. Coal rank may be determined by a variety of physical and chemical properties, some of which are influenced by coal type. To minimise the effects of coal type, we have chosen to use the mean maximum reflectance of collotelinite as the rank parameter. The relationship of coal type and rank to methane sorption is not fully understood. Coal rank is often considered to be the main parameter affecting the methane adsorption capacity and a relationship of increasing capacity to increasing rank has been established ŽKim, 1977; Yee et al., 1993.. However, other studies suggest that the relationship with rank is a second-order polynomial trend, with a minimum occurring at high volatile A bituminous rank ŽLevine, 1993; Yee et al., 1993; Levy et al., 1997.. Coal type effects on methane sorption are also poorly understood. Inertinite-rich coals at low and medium rank have been found to have higher adsorption capacity than vitrinite-rich coals, whereas at higher ranks, both coal types adsorb similar amounts ŽEttinger et al., 1966.. Other studies, in contrast, have found vitrinite to have a greater adsorption capacity than inertinite over a wide range of ranks ŽBeamish and O’Donnell, 1992; Crosdale and Beamish, 1993, 1995; Lamberson and Bustin, 1993; Levine et al., 1993.. Maceral composition has also been postulated to have little influence on methane sorption properties ŽFaiz et al., 1992.. Recent CBM exploration in the northern Bowen Basin ŽFig. 1. by Mitsubishi Gas Chemicals ŽMGC. has provided a unique opportunity to investigate parameters influencing gas adsorption of a wide range of high coal ranks in this basin. This area is considered to have significant CBM potential ŽJohnson and White, 1988.. A total of 13 coals varying from medium volatile bituminous to anthracite were selected from deep boreholes in the Bowen Basin to investigate the effects of rank, type and mineral matter
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Fig. 1. Location of Dartbrook Colliery in the Sydney Basin Ža. and drill holes in the northern Bowen Basin Žb.. Locations: SK1—South Kemis Creek; NE1—North Ellens Field; NM1—North Moranbah; SM1—Smoky Creek; RS1—Ripstone; PH1—Philips Creek.
on methane adsorption and desorption. In order to include lower rank coals, two high volatile bituminous samples of similar coals from Dartbrook Colliery, Sydney Basin ŽFig. 1. were included in the sample suite.
2. Methods 2.1. Coal samples and preparation Coal samples were obtained from MGC CBM exploration drill holes in the Bowen Basin and from Dartbrook Colliery in the Sydney Basin ŽFig. 1a, b; Table 1.. Bowen Basin samples are from the late Permian Moranbah Coal Measures while the Sydney Basin samples are from the late Permian Whittingham Coal Measures. Mitsubishi Gas Chemicals drilling was completed in 1994 and samples were acquired from bulk coal Žy10 mm. in cold storage. Dartbrook samples were hand-picked from 5 cm behind a coal face which had been standing for 1 month. From each location, approximately 10 g of bright and dull coal lithotypes were hand-picked for analysis; i.e., isorank pairs of bright and dull coal were obtained. Samples were crushed to y1 mm and a representative split was taken for petrography. The remaining sample was further crushed to y0.212 mm for other analyses including CH 4 adsorption isotherm, proxi-
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Table 1 Coal samples locations Sample number
Drill holerColliery
Depth Žm.
Lithotype
G102 Ž15CT. B G102 Ž15CT. D G102 Ž14CT. B G102 Ž14CT. D NM1-04 ŽP6. B NM1-04 ŽP6. D NM1-12B NM1-12D PH1-01 B PH1-01 D NM1-21 ŽP10. B NM1-21 ŽP10. D PH1-03 B PH1-03 D RS1-02 Ž249. B RS1-02 Ž249. D PH1-08 B PH1-08 D SM1-05 B SM1-05 D PH1-12 B PH1-12 D SM1-06 ŽB36. B SM1-06 ŽB36. D NE1-09 B NE1-09 D SK1-05 ŽB53. B SK1-05 ŽB53. D SK1-07 B SK1-07 D
Dartbrook Colliery Dartbrook Colliery Dartbrook Colliery Dartbrook Colliery North Moranbah a1 North Moranbah a1 North Moranbah a1 North Moranbah a1 Philips Creek a1S Philips Creek a1S North Moranbah a1 North Moranbah a1 Philips Creek a1S Philips Creek a1S Ripstone a1S Ripstone a1S Philips Creek a1S Philips Creek a1S Smoky Creek a1S Smoky Creek a1S Philips Creek a1S Philips Creek a1S Smoky Creek a1S Smoky Creek a1S North Ellens Field a1S North Ellens Field a1S South Kemmis Creek a1S South Kemmis Creek a1S South Kemmis Creek a1S South Kemmis Creek a1S
350.0 350.0 350.0 350.0 257.5 257.5 300.0 300.0 216.5 216.5 352.5 352.5 244.9 244.9 479.0 479.0 409.1 409.1 850.0 850.0 486.9 486.9 1020.0 1020.0 673.1 673.1 620.3 620.3 674.0 674.0
Bright Dull Bright Dull Bright Dull Bright Dull Bright Dull Bright Dull Bright Dull Bright Dull Bright Dull Bright Dull Bright Dull Bright Dull Bright Dull Bright Dull Bright Dull
Dartbrook Colliery samples are from the Sydney Basin while all other samples are from diamond drill holes Žcored holes. in the Bowen Basin.
mate, CO 2 surface area and mercury porosimetry. All samples were stored at y108C following preparation, to minimise oxidation. Proximate analysis results showed some dull coals to have a very high ash content Žexceeding 40%.. These coals were re-sampled to obtain low-ash, dull coals for evaluation of type influences. However, results from the high ash samples were considered in the evaluation of ash content effects on gas adsorption. 2.2. Methane sorption testing Methane sorption testing was performed using a gravimetric technique with a Sartorius M25 D-P high pressure microbalance ŽBeamish and O’Donnell, 1992; Levine et al., 1993.. Approximately 1 g of y0.212 mm coal was dried under nitrogen at 1108C prior to analysis. Samples were evacuated to less than y100 kPa for 1 h prior to
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313
determination of helium density in the microbalance. Subsequent to density determination, samples were again evacuated to less than y100 kPa for 1 h for methane sorption studies. Methane was introduced into the microbalance at pressure steps of approximately 0.5, 1, 2, 3, 5, 7 and 9 MPa to produce an adsorption isotherm at 23.58C. The weight of gas adsorbed at each pressure step was converted to an equivalent gas volume at 218C and 1 atm. Desorption isotherms were also generated using the same pressure steps. Pressure and weight data were acquired during adsorption and desorption at 1 min intervals by a data logger. Desorption rate data from the desorption step, 0.5 MPa to atmosphere, were modelled to generate an effective diffusivity coefficient Ž De .. 2.3. Modelling methane sorption data Isotherms were modelled using the Langmuir equation, V s V L PrŽ P L q P . and Langmuir coefficients VL and PL were calculated ŽLangmuir, 1918.. The Langmuir volume Ž VL . physically represents the monolayer adsorption capacity of the coal and is an important characteristic of the samples. The Langmuir pressure Ž P L . is mathematically the pressure at half the Langmuir volume and may give some information about the heterogeneity of the pores. An effective diffusivity coefficient was calculated for methane desorption from 0.5 MPa to atmosphere using a unipore spherical model ŽSmith and Williams, 1984.: V V`
s1y
`
6
p
2
Ý ns1
1 n2
ey Ž D ep
2 2
n t.
.
2.4. Petrography Petrographic samples of y1 mm coal were mounted in polyester resin, polished and desiccated for at least 12 h. Reflectance analysis was performed before maceral analysis. A Leitz MPV1 system was used to determine maximum reflectance Ž R o max . of 50 collotelinite grains per bright coal sample for rank evaluation. The mean maximum reflectance of the bright coal was also used for the rank of its dull coal pair. Following reflectance measurements, maceral analysis was undertaken by point counting a minimum of 500 points per sample at a stage interval of 1 mm. 2.5. Proximate analysis Proximate analysis was performed at the University of Auckland using a thermogravimetric method ŽBeamish, 1994. on 15 mg sub-samples. Carbonate CO 2 can be evaluated using this method and corrections were applied to the volatile matter where necessary ŽBeamish, 1997, personal communication..
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3. Results and discussion 3.1. Petrography and geochemistry Maceral analysis of bright and dull coals ŽTable 2. shows the expected results of bright coals being dominated by monomaceral assemblages of vitrinite Žcollotelinite., while dull coals have mixed maceral assemblages dominated by the inertinite group. Dull coals also contain significant amounts of mineral matter and variable amounts of vitrinite. Liptinite is no longer microscopically recognisable in coals of above hvAb rank. Proximate analysis ŽTable 3. and mean maximum reflectance of vitrinite ŽTable 2. indicate the coals’ range in rank from high volatile bituminous A through to anthracite. Table 2 Petrographic analysis results Sample
G1-02 Ž15CT. B G1-02 Ž15CT. D G1-02 Ž14CT. B G1-02 Ž14CT. D NM1-04 ŽP6. B NM1-04 ŽP6. D-1 NM1-12 B NM1-12 D PH1-01 B PH1-01 D NM1-21 ŽP10. B NM1-21 ŽP10. D-1 PH1-03 B PH1-03 D-1 RS1-02 Ž249. B RS1-02 Ž249. D-1 PH1-08 B PH1-08 D SM1-05 B SM1-05 D-1 PH1-12 B PH1-12 D SM1-06 ŽB36. B SM1-06 D-1 NE1-09 B NE1-09 D-1 SK1-05 ŽB53. B SK1-05 ŽB53. D-1 SK1-07 B SK1-07 D-1
Maceral group composition Vitrinite Liptinite Inertinite Ž%. Ž%. Ž%.
Mineral matter Ž%.
Vitrinite Ž%, mmf.
95.9 34.3 96.9 15.4 88.2 21.0 90.6 28.7 85.5 NA 88.5 15.4 92.1 22.6 92.9 25.2 86.0 20.6 86.0 32.0 90.1 20.0 90.6 11.3 89.8 27.3 88.8 6.8 91.4 34.8
2.1 10.6 2.5 7.0 5.8 8.5 2.7 3.1 10.3 NA 3.6 12.6 5.1 8.4 4.1 13.9 4.2 12.8 4.2 5.2 4.9 21.2 4.4 9.4 5.5 6.0 5.0 7.2 5.1 12.0
98.0 38.4 99.4 16.6 93.6 23.0 93.1 29.6 95.3 NA 91.8 17.6 97.0 24.7 96.9 29.3 89.8 23.6 89.8 33.8 94.7 25.4 94.8 12.5 95.0 29.0 93.5 7.3 96.3 39.5
0.0 1.3 0.0 1.0 0.0 1.6 0.0 0.2 0.0 NA 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.4 0.0 0.0 0.0 0.4 0.0 0.4 0.0 0.8 0.0 0.2 0.0 0.0
2.0 53.8 0.6 76.6 6.0 68.9 6.7 68.0 4.2 NA 7.9 72.0 2.8 69.0 3.0 60.9 9.8 66.2 9.8 62.8 5.0 58.4 5.0 78.9 4.7 65.9 6.2 85.8 3.5 53.2
Collotelinite reflectance Ž% R o max . 0.78 0.79 1.14 1.18 1.37 1.47 1.48 1.52 1.72 1.75 1.99 2.08 2.19 2.79 3.01
Mean maximum reflectance was determined only on bright coal samples and has been used as a rank indicator for the dull coal sample from the same seam in the same locality.
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Table 3 Proximate analysis results Sample
Inherent moisture Ž%, ar.
Ash Ž%, ar.
Volatile matter Ž%, ar.
Fixed carbon Ž%, ar.
Volatile matter Ž%, daf.
G1-02 Ž15CT. B G1-02 Ž15CT. D G1-02 Ž14CT. B G1-02 Ž14CT. D NM1-04 ŽP6. B NM1-04 ŽP6. D NM1-04 ŽP6. D-1 NM1-12 B NM1-12 D PH1-01 B PH1-01 D NM1-21 ŽP10. B NM1-21 ŽP10. D NM1-21 ŽP10. D-1 PH1-03 B PH1-03 D PH1-03 D-1 RS1-02 Ž249. B RS1-02 Ž249. D RS1-02 Ž249. D-1 PH1-08 B PH1-08 D SM1-05 B SM1-05 D SM1-05 D-1 PH1-12 B PH1-12 D SM1-06 ŽB36. B SM1-06 ŽB36. D SM1-06 ŽB36. D-1 NE1-09 B NE1-09 D NE1-09 D-1 SK1-05 ŽB53. B SK1-05 ŽB53. D SK1-05 ŽB53. D-1 SK1-07 B SK1-07 D SK1-07 D-1
5.4 3.5 5.4 3.3 1.6 2.3 1.5 1.6 1.3 1.7 1.5 1.5 1.7 1.3 1.5 2.1 1.3 1.2 1.2 1.2 1.3 1.3 1.4 2.4 1.6 1.2 1.2 1.6 2.2 1.5 1.7 1.9 1.5 2.8 2.4 2.3 2.2 1.4 2.1
1.2 11.6 0.5 8.5 9.3 45.7 23.7 3.6 5.9 11.7 33.4 5.3 39.2 20.2 16.0 53.1 29.0 3.8 30.1 31.1 4.0 13.7 6.5 30.7 11.0 4.9 21.2 5.8 37.2 12.8 7.5 39.6 15.3 5.3 40.3 14.4 3.7 48.0 19.3
32.4 28.9 31.1 26.6 24.5 19.9 19.4 22.9 21.4 19.0 13.9 20.5 13.3 16.2 15.7 11.4 14.3 17.2 16.7 12.9 14.0 15.2 13.8 13.6 13.1 13.1 11.8 11.5 10.0 11.3 8.9 6.6 8.7 6.7 6.8 7.1 6.2 15.6 6.6
61.0 56.0 63.0 61.6 64.6 32.1 55.4 71.9 71.4 67.6 51.3 72.7 45.8 62.3 66.8 33.4 55.4 77.8 52.0 54.8 80.7 69.8 78.3 53.3 74.3 80.8 65.8 81.1 50.6 74.4 81.9 51.9 74.5 85.2 50.5 76.2 87.9 35.0 72.0
34.7 34.1 33.0 30.2 27.5 38.3 26.0 24.2 23.1 21.9 21.3 22.0 22.5 20.6 19.0 25.4 20.5 18.1 24.3 19.1 14.8 17.0 14.5 20.3 15.0 14.0 15.2 12.4 16.5 13.2 9.8 11.3 10.5 7.3 11.9 8.5 6.6 30.9 8.4
Carbon dioxide corrections have been applied in calculation of volatile matter Ždaf. where necessary.
Volatile matter Ždaf. is strongly correlated to mean maximum reflectance of vitrinite ŽFig. 2. and shows that the sample suite represents a progressive rank variation. Ash results from proximate analysis ŽTable 3. indicate generally low ash contents for the bright coals, less than 17% Žaverage 5.9%., and variable ash contents for the dull coals, up to 54% Žaverage 26.5%..
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Fig. 2. Relation between mean maximum reflectance of vitrinite and volatile matter content of bright coals. Trendline equation: V M s 5.44 R 2o y32.9R o q56.4; r 2 s 0.98.
3.2. Moisture effects on adsorption capacity The effects of moisture on gas adsorption by coal are well-documented ŽJoubert et al., 1974; Yalc¸in and Durucan, 1991; Levine et al., 1993.. As little as 1% moisture may reduce the adsorption capacity by 25%, and 5% moisture results in a loss of adsorption capacity of 65% ŽLama and Bodziony, 1996.. For Bowen Basin coals, a decrease in methane adsorption capacity Žat 5 MPa pressure. has been reported at a rate of 4.2 cm3rg for each 1% increase in moisture content ŽLevy et al., 1997.. Both water and methane are sorbates and compete with each other for some sorption sites in the coal structure. In dry coals, adsorption sites previously occupied by moisture may become available for methane. This may be particularly the case for vitrinites, which have a relatively more open structure with hydrophilic groups accommodating large quantities of water ŽLevine, 1993.. It should be noted that the very high Langmuir volumes determined in this study are directly related to analysis of the coals in a dry state. 3.3. Ash content (mineral matter) effects on adsorption capacity Ash content correlates strongly to methane adsorption capacity ŽFig. 3., with increasing ash Žmineral matter. content related to a reduction in the Langmuir volume of the raw coal. Linear decrease in adsorption capacity with increasing ash content indicates that the ash Žmineral matter. acts as a simple diluent, thereby reducing the storage capacity. Over the ash range studied Ž1.5% to 54%., the average decrease in methane adsorption capacity was 0.38 cm3rg for each 1% rise in ash content ŽFig. 3., with the adsorption capacity becoming approximately zero at an ash content of 100%.
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Fig. 3. Mineral matter in the coal, as expressed by its ash content, acted as a diluent to decrease the amount of adsorbable gas in the raw coal. The linear relationship, approximately with a zero intercept at 100% ash, indicates a simple two-component mixing of the non-adsorbent mineral matter and the coal organic fraction. Trendline equation: V L sy0.38 ashq39.6; r 2 s 0.79.
3.4. Rank effects on adsorption capacity Adsorption isotherm results ŽTable 4; Fig. 4. indicate that Langmuir volumes Ž VL . for bright and dull coal types follow discrete, second-order polynomial trends with increasing rank. Bright coals have a minimum VL at low volatile bituminous rank Ž1.72% R o max with a corresponding VL of 38.8 cm3rg. and dull coals have a minimum VL of 38.2 cm3rg at medium volatile bituminous rank Ž1.17% R o max .. At lower ranks Žhigh volatile bituminous., good separation occurs between bright and dull isorank pairs. However, at higher ranks, trend line analysis suggests that maceral composition exerts little influence on V L . Previous investigations up to low volatile bituminous rank ŽBeamish and Gamson, 1993; Crosdale and Beamish, 1993. concluded that vitrinite-rich Žbright. coals had the greater adsorption capacity due to their more microporous nature. These new results indicate that a convergence in adsorption capacity occurs at medium to low volatile bituminous ranks. The concurrence in adsorption capacity of dull and bright coals at higher ranks suggests that their microporosities are similar. This finding is consistent with the desorption rate data Ždiscussed below. in which the desorption rate for dull coals slows as rank increases and becomes similar to that of the bright coal. The observed decrease in methane adsorption capacity from high volatile bituminous A to medium volatile bituminous rank may be attributed to ‘plugging’ of the micropore system by low boiling point hydrocarbons ŽLevine, 1993.. As coalification continues, cracking of the occluded oils during debituminisation re-opens the micropore system and increases the availability of adsorption sites. Surface area of coal also varies with rank in a fashion similar to the pore size distribution ŽGan et al., 1972.. Surface areas are high in
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Table 4 Parameters obtained during methane sorption isotherm analysis Sample
Helium density Žgrcm3 .
Langmuir volume Žcm3 rg.
Langmuir pressure ŽMPa.
Langmuir volume Ždmmf.
Effective diffusivity Žsy1 .
G1-02 Ž15CT. B G1-02 Ž15CT. D G1-02 Ž14CT. B G1-02 Ž14CT. D NM1-04 ŽP6. B NM1-04 ŽP6. D NM1-04 ŽP6. D-1 NM1-12 B NM1-12 D PH1-01 B PH1-01 D NM1-21 ŽP10. B NM1-21 ŽP10. D NM1-21 ŽP10. D-1 PH1-03 B PH1-03 D PH1-03 D-1 RS1-02 Ž249. B RS1-02 Ž249. D RS1-02 Ž249. D-1 PH1-08 B PH1-08 D SM1-05 B SM1-05 D SM1-05 D-1 PH1-12 B PH1-12 D SM1-06 ŽB36. B SM1-06 ŽB36. D SM1-06 ŽB36. D-1 NE1-09 B NE1-09 D NE1-09 D-1 SK1-05 ŽB53. B SK1-05 ŽB53. D SK1-05 ŽB53. D-1 SK1-07 B SK1-07 D SK1-07 D-1
1.275 1.394 1.271 1.384 1.328 1.779 1.498 1.295 1.333 1.370 1.580 1.300 1.642 1.470 1.384 1.817 1.510 1.314 1.566 1.560 1.333 1.430 1.350 1.645 1.407 1.348 1.506 1.349 1.650 1.443 1.385 1.687 1.487 1.421 1.770 1.520 1.417 1.998 1.554
45.4 32.9 46.0 34.8 34.9 21.6 27.6 36.1 34.4 33.0 26.6 36.0 23.2 28.6 31.8 19.9 27.8 36.6 26.0 26.2 36.1 31.6 36.8 24.8 34.5 33.4 27.8 41.1 27.6 34.7 41.1 26.7 35.0 45.3 27.8 39.0 42.9 19.0 37.6
2.1 2.0 2.1 2.0 1.9 2.4 2.0 1.7 1.8 1.73 1.84 1.8 1.9 1.7 1.73 2.1 1.84 1.7 1.7 1.7 1.63 1.63 1.63 1.63 1.53 1.73 1.73 1.54 1.54 1.54 1.34 1.54 1.34 1.24 1.35 1.24 1.04 1.14 1.24
46.1 37.9 46.3 38.5 39.0 43.5 37.5 37.6 36.8 8.0 2.4 38.3 41.2 36.9 8.7 47.9 1.1 38.2 39.1 40.1 7.8 7.3 9.7 8.0 9.3 5.4 6.3 3.9 7.4 0.4 4.9 8.1 2.2 8.2 0.9 6.5 4.7 0.9 8.0
4.8Ey04 9.3Ey04 3.4Ey04 1.0Ey03 4.1Ey04 7.9Ey04 9.7Ey04 2.8Ey04 6.6Ey04 3.3Ey04 6.8Ey04 2.0Ey04 4.8Ey04 6.4Ey04 3.5Ey04 5.0Ey04 4.8Ey04 5.8Ey04 1.7Ey04 6.5Ey04 2.7Ey04 3.8Ey04 2.9Ey04 4.0Ey04 6.4Ey04 3.8Ey04 1.1Ey04 3.0Ey04 3.7Ey04 5.2Ey04 3.0Ey04 8.9Ey04 5.1Ey04 2.3Ey04 9.8Ey04 1.1Ey03 7.6Ey05 1.7Ey04 5.4Ey04
Density of the sample in helium is determined using the microbalance prior to methane adsorption and desorption analyses.
low rank coals Ž- 75% C., lower in the 75–85% C range Žhigh to medium volatile bituminous. and increase again through the medium volatile to anthracite. Surface area has also been related to maceral content and it has been shown that vitrinitic coals have
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Fig. 4. Trends in Langmuir volume in relation to coal rank were different for bright and dull coals. Bright coal trendline equation: VL s 5.90 R o2 y20.3 R o q56.2; r 2 s 0.54. Dull coal trendline equation: VL s 2.90 R o2 y 6.77R o q42.1; r 2 s 0.70.
a greater internal surface area than inertinite-rich coals ŽThomas and Damberger, 1976.. For Bowen Basin coals, it has been found that vitrinite-rich coals have higher internal surface area than inertinite-rich coals in the rank range of high volatile bituminous to medium volatile bituminous rank ŽBeamish and Crosdale, 1995.. Our adsorption isotherm results in the low rank range support this view. 3.5. Coal type effects on adsorption capacity Maceral analysis ŽTable 2. indicates that the coals are dominated by vitrinite and inertinite and can, therefore, be considered a two-component system. Simple comparison of Langmuir volume to vitrinite content ŽFig. 5. shows no relationships. However, evaluation of coal type alone is complicated by rank influences. When individual ranks are considered ŽFig. 5., it is found that in high volatile bituminous coals, increasing Langmuir volume is related to increasing vitrinite content; at medium and low volatile bituminous ranks, maceral composition may exert relatively little influence on gas storage capacity; at semianthracite and anthracite ranks, decreasing Langmuir volume is associated with higher vitrinite content. However, more data points are required to confirm the trend in semianthracite and anthracite ranks. Vitrinite-rich coals have been found to have greater adsorption capacity than inertinite-rich coals for isorank samples ŽCrosdale and Beamish, 1993; Lamberson and Bustin, 1993; Faiz, 1995.. However, poor or no correlation was also observed between adsorption capacity and maceral composition ŽFaiz et al., 1992. and in some cases,
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Fig. 5. Trends in Langmuir volume in relation to coal type also show rank influences. Coal type has greatest influence on methane adsorption capacity in high volatile bituminous coal, semianthracite and anthracite.
inertinite-rich coals have been found to have greater methane adsorption capacity ŽEttinger et al., 1966.. Our results for dry coals show that maceral composition is an important control on gas adsorption but the degree of influence that maceral composition exerts is rank-dependent. This is consistent with varying conclusions reached by other workers. The strong influence of maceral composition reported for some Cretaceous Canadian coals ŽLamberson and Bustin, 1993. probably reflects the coal rank Žhigh volatile A to medium volatile.. In contrast, the lack of maceral control on gas content observed for some medium to low volatile Permian coals of Australia Že.g., Faiz et al., 1992. is also consistent with our results. 3.6. Rank and type effects on Langmuir pressure The Langmuir pressure Ž PL . is, in theory, related to the isosteric heat of adsorption but, in practice, PL is an empirical constant which is difficult to reconcile with theory ŽGregg and Sing, 1982.. Mathematically, PL is also the pressure at half the Langmuir volume. The coals studied show a strong relationship of decreasing PL with increasing rank ŽTable 4; Fig. 6., which does not appear to be related to coal type. Due to the uncertain nature of the relationship of PL to the isosteric heat of adsorption, it is not possible to draw firm conclusions as to the physical significance of this trend. The isosteric heat of adsorption should decrease as surfaces become less heterogeneous and the surface coverage increases ŽGregg and Sing, 1982.. Our results may, therefore, suggest that as
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Fig. 6. Langmuir pressure Ž PL . decreased with increasing rank and was not related to type. Trendline equation: PL sy0.42 R o q2.36; r 2 s 0.91.
coal rank increases, the pore surfaces become less heterogenous and the coverage of these surfaces is more complete. We suggest that the decrease in heterogeneity of the pore surfaces may be linked to the increased aromaticity of the coal structure by polymerizationrcondensation and the loss of non-aromatic carbons ŽLevine, 1993.. In contrast, the opposite relationship of increasing PL with increasing rank has also been reported for dry coals over the volatile matter range of 5% to 45% ŽBoxho et al., 1980 cited by Lama and Bodziony, 1996.. Their range of values was approximately 0.25 to 1.4, which is significantly smaller than those reported here ŽTable 4.. 3.7. Desorption rate Desorption rate investigations were undertaken by measuring gas release rates to atmosphere from an initial pressure of 0.5 MPa Žthe final step of the desorption isotherms.. A unipore spherical model was used to calculate an effective diffusivity parameter Ž De .. Mineral infillings Žpredominantly carbonates and clays. have been found to block the coal pore system, including cleats and fractures, and significantly reduce gas flow rates ŽGamson et al., 1993.. No relationship was found between De and ash content ŽFig. 7., which relates to the finely crushed nature of the samples. Crushing would destroy the larger transport pore networks, which control gas movement rates by Darcy flow, and which are most susceptible to blockage by secondary mineralisation. Additionally, much of the mineral matter is primarily detrital which would not cause blockages of transport pores. Both rank and coal type were found to influence effective diffusivity ŽFig. 8.. Dull coals have faster desorption rates Žtwo to three times. than their bright equivalents in
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Fig. 7. Desorption rate of finely crushed coals was not related to the ash content of coal.
most cases. Dull coal desorption is fastest for high volatile coals, gradually reduces as rank increases and becomes stable from low volatile bituminous coals onwards ŽFig. 8.. Bright coal desorption rates also decrease with increasing rank, but the trend is less marked than that of dull coals.
Fig. 8. Desorption rate of finely crushed coals was both rank- and type-dependent. Dull coals generally desorbed more rapidly than their rank equivalent. Bright coal and dull coal desorption rates generally decreased with increasing rank.
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Rank and type effects on De are likely to be related to pore size distribution. Total coal porosity follows a second-order polynomial trend with a minimum at 89% C Žvan Krevelen, 1993.. The initial decrease in porosity is related to a decrease in macro- and mesoporosity, while the subsequent increase is related to an increase in microporosity. Decreasing larger porosity with increasing rank is translated into lower effective diffusivities with rank increase. Microporosity is a limiting factor for diffusion rate, so a relative increase in the amount of microporosity is not translated into progressively slower diffusivity rates. Inertinite-rich dull coals are often more macroporous than vitrinite-rich bright coals ŽCrosdale and Beamish, 1995.. As a result, the desorption in bright coals is slower due to restrictions imposed by micropores. In case of dull coals, the presence of more macroand mesopores at lower ranks allows a faster desorption rate, which slows as these larger pores become less common at higher ranks.
4. Conclusions Methane adsorption isotherms of dry coals of varying ranks from high volatile bituminous to anthracite, from the Bowen and Sydney Basins, Australia, were investigated. The study revealed that the coal type, rank and mineral matter strongly influence the methane sorption capacity of the coals. Mineral matter in coal, as expressed by ash content, acts as a diluent to significantly reduce the gas adsorption capacity. For Bowen Basin coals, the average decrease in methane adsorption capacity was 0.38 cm3rg for each 1% rise in ash content. Langmuir volumes Ž V L . for bright and dull coal types follow discrete, second-order polynomial trends with increasing rank. Bright coals follow a trend with minimum VL at low volatile bituminous rank Ž1.72% R o max . and dull coals exhibit a minimum V L at medium volatile bituminous rank Ž1.17% R o max .. At low rank, VL was greater in bright coal by about 10cm3rg, but as rank increased, the bright and dull trends converged and crossed at 1.65% R o max . At ranks higher than 1.65% R o max , both bright and dull coals followed similar trends. Maceral composition of coals had a variable effect on gas adsorption. In high volatile bituminous coals, increases in vitrinite content were associated with increases in adsorption capacity. In medium to low volatile bituminous coals, changes in maceral composition may exert relatively little influence. The Langmuir pressure Ž PL . showed a strong relationship of decreasing PL with increasing rank, which was not related to coal type. It is suggested that the observed trend is related to a decrease in the heterogeneity of the pore surfaces, and subsequent increased coverage by the adsorbate, as coal rank increases. Desorption rate studies on crushed coals indicate that coal type and coal rank have important influences, although there is a general trend of decreasing desorption rate with increasing rank. Coals of lower rank have higher effective diffusivities and mineral matter was found to have no influence on desorption rate of the finely crushed coals. Variations in pore size distribution with coal type and with rank are implicated in controlling these trends.
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5. Nomenclature Symbols De P PL t V VL V`
the effective diffusivity Žcm2rs. pressure ŽMPa. Langmuir pressure ŽMPa. time Žs. volume of the adsorbedrdesorbed gas Žcm3 . Langmuir volume Žcm3 . total volume of desorbed gas Žcm3 .
Acknowledgements Proximate analyses on the coal samples were conducted at University of Auckland, New Zealand. We thank Basil Beamish for his help in analysing these coals. Reviewers Jeff Levine and Jay Close are thanked for their critical evaluation of the manuscript.
References Ayers, W.B., Kelso, B.S., 1989. Coalbed methane: 2. Knowledge of methane potential for coalbed resources grows, but needs more study. Oil Gas J. 64–67. Beamish, B.B., 1994. Proximate analysis of New Zealand and Australian coals by thermogravimetry. New Zealand J. Geol. Geophys. 37, 387–392. Beamish, B.B., Crosdale, P.J., 1995. The influence of maceral content on the sorption of gases by coal and the association with outbursting. International Symposium Cum Workshop Management and Control of High Gas Emission and Outbursts. Wollongong, 20–24 March 1995, pp. 353–361. Beamish, B.B., Gamson, P.D., 1993. Laboratory studies: sorption behaviour and microstructure of Bowen Basin coals. In: Oldroyd, G.C. ŽEd.., Final Report ERDC Project 1464—Prediction of Natural Gas Production from Coal Seams, Vol. 2. Energy Research Development, Canberra, 128 pp. Beamish, B.B., O’Donnell, G., 1992. Microbalance applications to sorption testing of coal. Proc. Symp. Coalbed Methane Res. Dev. Australia, Vol. 4. Townsville, pp. 31–41. Beavers, W.M., 1997. Critical issues for successful CSM development in Queensland: case study of CONOCO Australia’s Dawson Valley CSM program. Coalseam Methane Commercialising the Clean Fuel for the 21st Century, Brisbane, March, pp. 12–13. Boxho, J., Stassen, P., Mucke, G., Klaus, N., Jeger, C., Lescher, L., Browning, J.E., Dunmore, R., Morris, ¨ I.H., 1980. Firedamp Drainage: Handbook for the Coal Mining Industry. Verlag. Gluckauf, Essen, 415 pp. ¨ Crosdale, P.J., Beamish, B.B., 1993. Maceral effects on methane sorption by coal. Proc. New Dev. Geol. Symp., Brisbane, pp. 95–98. Crosdale, P.J., Beamish, B.B., 1995. Methane diffusivity at South Bulli ŽNSW. and Central ŽQLD. collieries in relation to coal maceral composition. International Symposium-Cum-Workshop on Management and Control of High Gas Emissions and Outbursts in Underground Coal Mines. National Organising Committee of the Symposium, Wollongong, pp. 363–367. Ettinger, I., Eremin, B., Zimakov, Yavovskaya, M., 1966. Natural factors influencing coal sorption properties: I. Petrography and the sorption properties of coals. Fuel 45, 267–275. Faiz, M.M., 1995. Gaseous hydrocarbons in the southern Sydney Basin. 13th Newcastle Symp. Adv. Study Sydney Basin, pp. 175–182. Faiz, M.M., Aziz, N.I., Hutton, A.C., Jones, B.G., 1992. Porosity and gas sorption capacity of some eastern
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Australian coals in relation to coal rank and composition. Proc. Symp. Coalbed Methane Res. Dev. Australia, Vol. 4. Townsville, pp. 9–20. Gamson, P.D., Beamish, B.B., Johnson, D.P., 1993. Coal microstructure and micropermeability and their effects on natural gas recovery. Fuel 72, 87–99. Gan, H., Nandi, S.P., Walker, P.L., 1972. Nature of the porosity in American coals. Fuel 51, 272–277. Gregg, S.J., Sing, K.S.W., 1982. Adsorption, Surface Area and Porosity. Academic Press, London, 303 pp. ICCP, 1963. International Handbook of Coal Petrography, 2nd edn. Centre National de la Recherche Scientifique, Paris. Johnson, D.P., White, M.G.J., 1988. Geological Assessment of Natural Gas from Coal Seams in the Northern Bowen Basin, Australia. Department of Geology, James Cook University, Townsville, 80 pp. Joubert, J.I., Grein, C.T., Bienstock, D., 1974. Effect of moisture on the methane capacity of American coals. Fuel 53, 186–191. Kim, A.G., 1977. Estimating the methane content of bituminous coalbeds from adsorption data. USBM Reports of Investigation RI 8245, 22 pp. Lama, R.D., Bodziony, J., 1996. Outburst of Gas, Coal and Rock in Underground Coal mines. R.D. Lama and Associates, Wollongong, 499 pp. Lamberson, M.N., Bustin, R.M., 1993. Coalbed methane characteristics of gates formation coals, northern British Columbia: effect of maceral composition. AAPG Bull. 77, 2062–2076. Langmuir, I., 1918. The adsorption of gases on plane surfaces of glass, mica and platinum. The Journal of American Chemical Society 40, 1361–1403. Levine, J.R., 1992. Influences of coal composition on coal seam reservoir quality: a review. Symp. Coalbed Methane Res. Dev., Vol. 1, Townsville, 19–21 November 1992, i–xxviii. Levine, J.R., 1993. Coalification: the evolution of coal as source rock and reservoir. In: Law, B.E., Rice, D.D. ŽEds.., Hydrocarbons from Coal. American Association of Petroleum Geologists, AAPG Studies in Geology, 38, pp. 39–77. Levine, J.R., Johnson, P.W., Beamish, B.B., 1993. High pressure microbalance sorption studies. Proc. 1993 Int. Coalbed Methane Symp. The University of Alabama, Birmingham, May 17–21, pp. 187–196. Levy, J.H., Day, S.J., Killingley, J.S., 1997. Methane capacities of Bowen Basin coals related to coal properties. Fuel 76, 813–819. Smith, D.M., Williams, F.L., 1984. Diffusion models for gas production from coals—determination of diffusion parameters. Fuel 63, 256–261. Thomas, J. Jr., Damberger, H.H., 1976. Internal surface area, moisture content and porosity of Illinois coals: variations with coal rank. Illinois State Geological Survey Circular 493, 38 pp. van Krevelen, D.W., 1993. Coal, 3rd. edn. Elsevier, Amsterdam, 979 pp. White, D., 1909. The effect of oxygen in coal. US Geol. Surv. Bull. 382, 71. Wyld, I., 1997. The Commercial Significance to Australia of Gas in Coal. Coal Seam Methane Commercialising the Clean Fuel for the 21st Century, Brisbane, March 12–13. Yalc¸in, E., Durucan, S., 1991. Methane capacities of Zonguldak coals and the factors affecting methane adsorption. Mining Science and Technology 13, 215–222. Yee, D., Seidle, J.P., Hanson, W.B., 1993. Gas sorption on coal and measurement of gas content in hydrocarbons from coal. AAPG Studies in Geology, Vol. 38, pp. 203–218.
Role of coal type and rank on methane sorption characteristics of Bowen Basin, Australia coals Chikatamarla Laxminarayana ) , Peter J. Crosdale Coalseam Gas Research Institute, School of Earth Sciences, James Cook UniÕersity, TownsÕille, Queensland 4811, Australia Received 31 March 1998; accepted 20 January 1999
Abstract The effect of coal composition, particularly the organic fraction, upon gas sorption has been investigated for Bowen Basin and Sydney Basin, Australia coals. Maceral composition influences on gas retention and release were investigated using isorank pairs of hand-picked bright and dull coal in the rank range of high volatile bituminous Ž0.78% R o max . to anthracite Ž3.01% R o max .. Adsorption isotherm results of dry coals indicated that Langmuir volume Ž V L . for bright and dull coal types followed discrete, second-order polynomial trends with increasing rank. Bright coals had a minimum V L at 1.72% R o max and dull coals had a minimum V L at 1.17% R o max . At low rank, V L was greater in bright coal by about 10 cm3rg, but as rank increased, the bright and dull trends converged and crossed at 1.65% R o max . At ranks higher than 1.65% R o max , both bright and dull coals followed similar trends. These competing trends mean that the importance of maceral composition on V L varies according to rank. In high volatile bituminous coals, increases in vitrinite content are associated with increases in adsorption capacity. At ranks higher than medium to low volatile bituminous, changes in maceral composition may exert relatively little influence on adsorption capacity. The Langmuir pressure Ž P L . showed a strong relationship of decreasing PL with increasing rank, which was not related to coal type. It is suggested that the observed trend is related to a decrease in the heterogeneity of the pore surfaces, and subsequent increased coverage by the adsorbate, as coal rank increases. Desorption rate studies on crushed samples show that dull coals desorb more rapidly than bright coals and that desorption rate is also a function of rank. Coals of lower rank have higher effective diffusivities. Mineral matter was found to have no influence on desorption rate of these finely crushed samples. The evolution of the coal pore structure with changing rank is implicated in diffusion rate differences. q 1999 Elsevier Science B.V. All rights reserved. Keywords: coalbed methane; methane adsorption; desorption rate; coal rank; coal type
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Corresponding author. E-mail: [email protected]
0166-5162r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 6 - 5 1 6 2 Ž 9 9 . 0 0 0 0 5 - 1
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1. Introduction Recognition of ‘coalbed methane’ ŽCBM. as an economic energy resource has led to commercial evaluation in many countries. Australia has a significant CBM potential but commercialisation relies on development of appropriate techniques for Australian conditions ŽWyld, 1997.. The Australian CBM industry still lacks an adequate understanding of the reservoir and other technical parameters determining methane producibility ŽBeavers, 1997.. Understanding sorption of gas by coal is extremely important in CBM resource estimation and determining producibility. Methane in coal occurs mostly as adsorbed gas on pore surfaces, but is also present in small quantities as free gas in the fracture network ŽLevine, 1992.. Adsorption and desorption isotherms can be used to evaluate potential storage capacity, methane recovery and factors governing these parameters. Various studies have identified the most important factors influencing methane storage and recovery and include coal type, rank, moisture content, mineral matter content, temperature, depth, natural fracturing and stress ŽKim, 1977; Ayers and Kelso, 1989; Levine, 1992; Crosdale and Beamish, 1993, 1995; Lamberson and Bustin, 1993; Yee et al., 1993; Levy et al., 1997.. Coal type refers to those characteristics which are ‘‘initially determined by the nature of the ingredient matter, the conditions of deposition, and extent of operation of the first or biochemical process of coal making’’ ŽWhite, 1909.. We restrict the use of coal type to refer to maceral composition. Coal rank describes the degree of carbonification of the coal ŽICCP, 1963.. Coal rank may be determined by a variety of physical and chemical properties, some of which are influenced by coal type. To minimise the effects of coal type, we have chosen to use the mean maximum reflectance of collotelinite as the rank parameter. The relationship of coal type and rank to methane sorption is not fully understood. Coal rank is often considered to be the main parameter affecting the methane adsorption capacity and a relationship of increasing capacity to increasing rank has been established ŽKim, 1977; Yee et al., 1993.. However, other studies suggest that the relationship with rank is a second-order polynomial trend, with a minimum occurring at high volatile A bituminous rank ŽLevine, 1993; Yee et al., 1993; Levy et al., 1997.. Coal type effects on methane sorption are also poorly understood. Inertinite-rich coals at low and medium rank have been found to have higher adsorption capacity than vitrinite-rich coals, whereas at higher ranks, both coal types adsorb similar amounts ŽEttinger et al., 1966.. Other studies, in contrast, have found vitrinite to have a greater adsorption capacity than inertinite over a wide range of ranks ŽBeamish and O’Donnell, 1992; Crosdale and Beamish, 1993, 1995; Lamberson and Bustin, 1993; Levine et al., 1993.. Maceral composition has also been postulated to have little influence on methane sorption properties ŽFaiz et al., 1992.. Recent CBM exploration in the northern Bowen Basin ŽFig. 1. by Mitsubishi Gas Chemicals ŽMGC. has provided a unique opportunity to investigate parameters influencing gas adsorption of a wide range of high coal ranks in this basin. This area is considered to have significant CBM potential ŽJohnson and White, 1988.. A total of 13 coals varying from medium volatile bituminous to anthracite were selected from deep boreholes in the Bowen Basin to investigate the effects of rank, type and mineral matter
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Fig. 1. Location of Dartbrook Colliery in the Sydney Basin Ža. and drill holes in the northern Bowen Basin Žb.. Locations: SK1—South Kemis Creek; NE1—North Ellens Field; NM1—North Moranbah; SM1—Smoky Creek; RS1—Ripstone; PH1—Philips Creek.
on methane adsorption and desorption. In order to include lower rank coals, two high volatile bituminous samples of similar coals from Dartbrook Colliery, Sydney Basin ŽFig. 1. were included in the sample suite.
2. Methods 2.1. Coal samples and preparation Coal samples were obtained from MGC CBM exploration drill holes in the Bowen Basin and from Dartbrook Colliery in the Sydney Basin ŽFig. 1a, b; Table 1.. Bowen Basin samples are from the late Permian Moranbah Coal Measures while the Sydney Basin samples are from the late Permian Whittingham Coal Measures. Mitsubishi Gas Chemicals drilling was completed in 1994 and samples were acquired from bulk coal Žy10 mm. in cold storage. Dartbrook samples were hand-picked from 5 cm behind a coal face which had been standing for 1 month. From each location, approximately 10 g of bright and dull coal lithotypes were hand-picked for analysis; i.e., isorank pairs of bright and dull coal were obtained. Samples were crushed to y1 mm and a representative split was taken for petrography. The remaining sample was further crushed to y0.212 mm for other analyses including CH 4 adsorption isotherm, proxi-
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Table 1 Coal samples locations Sample number
Drill holerColliery
Depth Žm.
Lithotype
G102 Ž15CT. B G102 Ž15CT. D G102 Ž14CT. B G102 Ž14CT. D NM1-04 ŽP6. B NM1-04 ŽP6. D NM1-12B NM1-12D PH1-01 B PH1-01 D NM1-21 ŽP10. B NM1-21 ŽP10. D PH1-03 B PH1-03 D RS1-02 Ž249. B RS1-02 Ž249. D PH1-08 B PH1-08 D SM1-05 B SM1-05 D PH1-12 B PH1-12 D SM1-06 ŽB36. B SM1-06 ŽB36. D NE1-09 B NE1-09 D SK1-05 ŽB53. B SK1-05 ŽB53. D SK1-07 B SK1-07 D
Dartbrook Colliery Dartbrook Colliery Dartbrook Colliery Dartbrook Colliery North Moranbah a1 North Moranbah a1 North Moranbah a1 North Moranbah a1 Philips Creek a1S Philips Creek a1S North Moranbah a1 North Moranbah a1 Philips Creek a1S Philips Creek a1S Ripstone a1S Ripstone a1S Philips Creek a1S Philips Creek a1S Smoky Creek a1S Smoky Creek a1S Philips Creek a1S Philips Creek a1S Smoky Creek a1S Smoky Creek a1S North Ellens Field a1S North Ellens Field a1S South Kemmis Creek a1S South Kemmis Creek a1S South Kemmis Creek a1S South Kemmis Creek a1S
350.0 350.0 350.0 350.0 257.5 257.5 300.0 300.0 216.5 216.5 352.5 352.5 244.9 244.9 479.0 479.0 409.1 409.1 850.0 850.0 486.9 486.9 1020.0 1020.0 673.1 673.1 620.3 620.3 674.0 674.0
Bright Dull Bright Dull Bright Dull Bright Dull Bright Dull Bright Dull Bright Dull Bright Dull Bright Dull Bright Dull Bright Dull Bright Dull Bright Dull Bright Dull Bright Dull
Dartbrook Colliery samples are from the Sydney Basin while all other samples are from diamond drill holes Žcored holes. in the Bowen Basin.
mate, CO 2 surface area and mercury porosimetry. All samples were stored at y108C following preparation, to minimise oxidation. Proximate analysis results showed some dull coals to have a very high ash content Žexceeding 40%.. These coals were re-sampled to obtain low-ash, dull coals for evaluation of type influences. However, results from the high ash samples were considered in the evaluation of ash content effects on gas adsorption. 2.2. Methane sorption testing Methane sorption testing was performed using a gravimetric technique with a Sartorius M25 D-P high pressure microbalance ŽBeamish and O’Donnell, 1992; Levine et al., 1993.. Approximately 1 g of y0.212 mm coal was dried under nitrogen at 1108C prior to analysis. Samples were evacuated to less than y100 kPa for 1 h prior to
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determination of helium density in the microbalance. Subsequent to density determination, samples were again evacuated to less than y100 kPa for 1 h for methane sorption studies. Methane was introduced into the microbalance at pressure steps of approximately 0.5, 1, 2, 3, 5, 7 and 9 MPa to produce an adsorption isotherm at 23.58C. The weight of gas adsorbed at each pressure step was converted to an equivalent gas volume at 218C and 1 atm. Desorption isotherms were also generated using the same pressure steps. Pressure and weight data were acquired during adsorption and desorption at 1 min intervals by a data logger. Desorption rate data from the desorption step, 0.5 MPa to atmosphere, were modelled to generate an effective diffusivity coefficient Ž De .. 2.3. Modelling methane sorption data Isotherms were modelled using the Langmuir equation, V s V L PrŽ P L q P . and Langmuir coefficients VL and PL were calculated ŽLangmuir, 1918.. The Langmuir volume Ž VL . physically represents the monolayer adsorption capacity of the coal and is an important characteristic of the samples. The Langmuir pressure Ž P L . is mathematically the pressure at half the Langmuir volume and may give some information about the heterogeneity of the pores. An effective diffusivity coefficient was calculated for methane desorption from 0.5 MPa to atmosphere using a unipore spherical model ŽSmith and Williams, 1984.: V V`
s1y
`
6
p
2
Ý ns1
1 n2
ey Ž D ep
2 2
n t.
.
2.4. Petrography Petrographic samples of y1 mm coal were mounted in polyester resin, polished and desiccated for at least 12 h. Reflectance analysis was performed before maceral analysis. A Leitz MPV1 system was used to determine maximum reflectance Ž R o max . of 50 collotelinite grains per bright coal sample for rank evaluation. The mean maximum reflectance of the bright coal was also used for the rank of its dull coal pair. Following reflectance measurements, maceral analysis was undertaken by point counting a minimum of 500 points per sample at a stage interval of 1 mm. 2.5. Proximate analysis Proximate analysis was performed at the University of Auckland using a thermogravimetric method ŽBeamish, 1994. on 15 mg sub-samples. Carbonate CO 2 can be evaluated using this method and corrections were applied to the volatile matter where necessary ŽBeamish, 1997, personal communication..
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3. Results and discussion 3.1. Petrography and geochemistry Maceral analysis of bright and dull coals ŽTable 2. shows the expected results of bright coals being dominated by monomaceral assemblages of vitrinite Žcollotelinite., while dull coals have mixed maceral assemblages dominated by the inertinite group. Dull coals also contain significant amounts of mineral matter and variable amounts of vitrinite. Liptinite is no longer microscopically recognisable in coals of above hvAb rank. Proximate analysis ŽTable 3. and mean maximum reflectance of vitrinite ŽTable 2. indicate the coals’ range in rank from high volatile bituminous A through to anthracite. Table 2 Petrographic analysis results Sample
G1-02 Ž15CT. B G1-02 Ž15CT. D G1-02 Ž14CT. B G1-02 Ž14CT. D NM1-04 ŽP6. B NM1-04 ŽP6. D-1 NM1-12 B NM1-12 D PH1-01 B PH1-01 D NM1-21 ŽP10. B NM1-21 ŽP10. D-1 PH1-03 B PH1-03 D-1 RS1-02 Ž249. B RS1-02 Ž249. D-1 PH1-08 B PH1-08 D SM1-05 B SM1-05 D-1 PH1-12 B PH1-12 D SM1-06 ŽB36. B SM1-06 D-1 NE1-09 B NE1-09 D-1 SK1-05 ŽB53. B SK1-05 ŽB53. D-1 SK1-07 B SK1-07 D-1
Maceral group composition Vitrinite Liptinite Inertinite Ž%. Ž%. Ž%.
Mineral matter Ž%.
Vitrinite Ž%, mmf.
95.9 34.3 96.9 15.4 88.2 21.0 90.6 28.7 85.5 NA 88.5 15.4 92.1 22.6 92.9 25.2 86.0 20.6 86.0 32.0 90.1 20.0 90.6 11.3 89.8 27.3 88.8 6.8 91.4 34.8
2.1 10.6 2.5 7.0 5.8 8.5 2.7 3.1 10.3 NA 3.6 12.6 5.1 8.4 4.1 13.9 4.2 12.8 4.2 5.2 4.9 21.2 4.4 9.4 5.5 6.0 5.0 7.2 5.1 12.0
98.0 38.4 99.4 16.6 93.6 23.0 93.1 29.6 95.3 NA 91.8 17.6 97.0 24.7 96.9 29.3 89.8 23.6 89.8 33.8 94.7 25.4 94.8 12.5 95.0 29.0 93.5 7.3 96.3 39.5
0.0 1.3 0.0 1.0 0.0 1.6 0.0 0.2 0.0 NA 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.4 0.0 0.0 0.0 0.4 0.0 0.4 0.0 0.8 0.0 0.2 0.0 0.0
2.0 53.8 0.6 76.6 6.0 68.9 6.7 68.0 4.2 NA 7.9 72.0 2.8 69.0 3.0 60.9 9.8 66.2 9.8 62.8 5.0 58.4 5.0 78.9 4.7 65.9 6.2 85.8 3.5 53.2
Collotelinite reflectance Ž% R o max . 0.78 0.79 1.14 1.18 1.37 1.47 1.48 1.52 1.72 1.75 1.99 2.08 2.19 2.79 3.01
Mean maximum reflectance was determined only on bright coal samples and has been used as a rank indicator for the dull coal sample from the same seam in the same locality.
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Table 3 Proximate analysis results Sample
Inherent moisture Ž%, ar.
Ash Ž%, ar.
Volatile matter Ž%, ar.
Fixed carbon Ž%, ar.
Volatile matter Ž%, daf.
G1-02 Ž15CT. B G1-02 Ž15CT. D G1-02 Ž14CT. B G1-02 Ž14CT. D NM1-04 ŽP6. B NM1-04 ŽP6. D NM1-04 ŽP6. D-1 NM1-12 B NM1-12 D PH1-01 B PH1-01 D NM1-21 ŽP10. B NM1-21 ŽP10. D NM1-21 ŽP10. D-1 PH1-03 B PH1-03 D PH1-03 D-1 RS1-02 Ž249. B RS1-02 Ž249. D RS1-02 Ž249. D-1 PH1-08 B PH1-08 D SM1-05 B SM1-05 D SM1-05 D-1 PH1-12 B PH1-12 D SM1-06 ŽB36. B SM1-06 ŽB36. D SM1-06 ŽB36. D-1 NE1-09 B NE1-09 D NE1-09 D-1 SK1-05 ŽB53. B SK1-05 ŽB53. D SK1-05 ŽB53. D-1 SK1-07 B SK1-07 D SK1-07 D-1
5.4 3.5 5.4 3.3 1.6 2.3 1.5 1.6 1.3 1.7 1.5 1.5 1.7 1.3 1.5 2.1 1.3 1.2 1.2 1.2 1.3 1.3 1.4 2.4 1.6 1.2 1.2 1.6 2.2 1.5 1.7 1.9 1.5 2.8 2.4 2.3 2.2 1.4 2.1
1.2 11.6 0.5 8.5 9.3 45.7 23.7 3.6 5.9 11.7 33.4 5.3 39.2 20.2 16.0 53.1 29.0 3.8 30.1 31.1 4.0 13.7 6.5 30.7 11.0 4.9 21.2 5.8 37.2 12.8 7.5 39.6 15.3 5.3 40.3 14.4 3.7 48.0 19.3
32.4 28.9 31.1 26.6 24.5 19.9 19.4 22.9 21.4 19.0 13.9 20.5 13.3 16.2 15.7 11.4 14.3 17.2 16.7 12.9 14.0 15.2 13.8 13.6 13.1 13.1 11.8 11.5 10.0 11.3 8.9 6.6 8.7 6.7 6.8 7.1 6.2 15.6 6.6
61.0 56.0 63.0 61.6 64.6 32.1 55.4 71.9 71.4 67.6 51.3 72.7 45.8 62.3 66.8 33.4 55.4 77.8 52.0 54.8 80.7 69.8 78.3 53.3 74.3 80.8 65.8 81.1 50.6 74.4 81.9 51.9 74.5 85.2 50.5 76.2 87.9 35.0 72.0
34.7 34.1 33.0 30.2 27.5 38.3 26.0 24.2 23.1 21.9 21.3 22.0 22.5 20.6 19.0 25.4 20.5 18.1 24.3 19.1 14.8 17.0 14.5 20.3 15.0 14.0 15.2 12.4 16.5 13.2 9.8 11.3 10.5 7.3 11.9 8.5 6.6 30.9 8.4
Carbon dioxide corrections have been applied in calculation of volatile matter Ždaf. where necessary.
Volatile matter Ždaf. is strongly correlated to mean maximum reflectance of vitrinite ŽFig. 2. and shows that the sample suite represents a progressive rank variation. Ash results from proximate analysis ŽTable 3. indicate generally low ash contents for the bright coals, less than 17% Žaverage 5.9%., and variable ash contents for the dull coals, up to 54% Žaverage 26.5%..
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Fig. 2. Relation between mean maximum reflectance of vitrinite and volatile matter content of bright coals. Trendline equation: V M s 5.44 R 2o y32.9R o q56.4; r 2 s 0.98.
3.2. Moisture effects on adsorption capacity The effects of moisture on gas adsorption by coal are well-documented ŽJoubert et al., 1974; Yalc¸in and Durucan, 1991; Levine et al., 1993.. As little as 1% moisture may reduce the adsorption capacity by 25%, and 5% moisture results in a loss of adsorption capacity of 65% ŽLama and Bodziony, 1996.. For Bowen Basin coals, a decrease in methane adsorption capacity Žat 5 MPa pressure. has been reported at a rate of 4.2 cm3rg for each 1% increase in moisture content ŽLevy et al., 1997.. Both water and methane are sorbates and compete with each other for some sorption sites in the coal structure. In dry coals, adsorption sites previously occupied by moisture may become available for methane. This may be particularly the case for vitrinites, which have a relatively more open structure with hydrophilic groups accommodating large quantities of water ŽLevine, 1993.. It should be noted that the very high Langmuir volumes determined in this study are directly related to analysis of the coals in a dry state. 3.3. Ash content (mineral matter) effects on adsorption capacity Ash content correlates strongly to methane adsorption capacity ŽFig. 3., with increasing ash Žmineral matter. content related to a reduction in the Langmuir volume of the raw coal. Linear decrease in adsorption capacity with increasing ash content indicates that the ash Žmineral matter. acts as a simple diluent, thereby reducing the storage capacity. Over the ash range studied Ž1.5% to 54%., the average decrease in methane adsorption capacity was 0.38 cm3rg for each 1% rise in ash content ŽFig. 3., with the adsorption capacity becoming approximately zero at an ash content of 100%.
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Fig. 3. Mineral matter in the coal, as expressed by its ash content, acted as a diluent to decrease the amount of adsorbable gas in the raw coal. The linear relationship, approximately with a zero intercept at 100% ash, indicates a simple two-component mixing of the non-adsorbent mineral matter and the coal organic fraction. Trendline equation: V L sy0.38 ashq39.6; r 2 s 0.79.
3.4. Rank effects on adsorption capacity Adsorption isotherm results ŽTable 4; Fig. 4. indicate that Langmuir volumes Ž VL . for bright and dull coal types follow discrete, second-order polynomial trends with increasing rank. Bright coals have a minimum VL at low volatile bituminous rank Ž1.72% R o max with a corresponding VL of 38.8 cm3rg. and dull coals have a minimum VL of 38.2 cm3rg at medium volatile bituminous rank Ž1.17% R o max .. At lower ranks Žhigh volatile bituminous., good separation occurs between bright and dull isorank pairs. However, at higher ranks, trend line analysis suggests that maceral composition exerts little influence on V L . Previous investigations up to low volatile bituminous rank ŽBeamish and Gamson, 1993; Crosdale and Beamish, 1993. concluded that vitrinite-rich Žbright. coals had the greater adsorption capacity due to their more microporous nature. These new results indicate that a convergence in adsorption capacity occurs at medium to low volatile bituminous ranks. The concurrence in adsorption capacity of dull and bright coals at higher ranks suggests that their microporosities are similar. This finding is consistent with the desorption rate data Ždiscussed below. in which the desorption rate for dull coals slows as rank increases and becomes similar to that of the bright coal. The observed decrease in methane adsorption capacity from high volatile bituminous A to medium volatile bituminous rank may be attributed to ‘plugging’ of the micropore system by low boiling point hydrocarbons ŽLevine, 1993.. As coalification continues, cracking of the occluded oils during debituminisation re-opens the micropore system and increases the availability of adsorption sites. Surface area of coal also varies with rank in a fashion similar to the pore size distribution ŽGan et al., 1972.. Surface areas are high in
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Table 4 Parameters obtained during methane sorption isotherm analysis Sample
Helium density Žgrcm3 .
Langmuir volume Žcm3 rg.
Langmuir pressure ŽMPa.
Langmuir volume Ždmmf.
Effective diffusivity Žsy1 .
G1-02 Ž15CT. B G1-02 Ž15CT. D G1-02 Ž14CT. B G1-02 Ž14CT. D NM1-04 ŽP6. B NM1-04 ŽP6. D NM1-04 ŽP6. D-1 NM1-12 B NM1-12 D PH1-01 B PH1-01 D NM1-21 ŽP10. B NM1-21 ŽP10. D NM1-21 ŽP10. D-1 PH1-03 B PH1-03 D PH1-03 D-1 RS1-02 Ž249. B RS1-02 Ž249. D RS1-02 Ž249. D-1 PH1-08 B PH1-08 D SM1-05 B SM1-05 D SM1-05 D-1 PH1-12 B PH1-12 D SM1-06 ŽB36. B SM1-06 ŽB36. D SM1-06 ŽB36. D-1 NE1-09 B NE1-09 D NE1-09 D-1 SK1-05 ŽB53. B SK1-05 ŽB53. D SK1-05 ŽB53. D-1 SK1-07 B SK1-07 D SK1-07 D-1
1.275 1.394 1.271 1.384 1.328 1.779 1.498 1.295 1.333 1.370 1.580 1.300 1.642 1.470 1.384 1.817 1.510 1.314 1.566 1.560 1.333 1.430 1.350 1.645 1.407 1.348 1.506 1.349 1.650 1.443 1.385 1.687 1.487 1.421 1.770 1.520 1.417 1.998 1.554
45.4 32.9 46.0 34.8 34.9 21.6 27.6 36.1 34.4 33.0 26.6 36.0 23.2 28.6 31.8 19.9 27.8 36.6 26.0 26.2 36.1 31.6 36.8 24.8 34.5 33.4 27.8 41.1 27.6 34.7 41.1 26.7 35.0 45.3 27.8 39.0 42.9 19.0 37.6
2.1 2.0 2.1 2.0 1.9 2.4 2.0 1.7 1.8 1.73 1.84 1.8 1.9 1.7 1.73 2.1 1.84 1.7 1.7 1.7 1.63 1.63 1.63 1.63 1.53 1.73 1.73 1.54 1.54 1.54 1.34 1.54 1.34 1.24 1.35 1.24 1.04 1.14 1.24
46.1 37.9 46.3 38.5 39.0 43.5 37.5 37.6 36.8 8.0 2.4 38.3 41.2 36.9 8.7 47.9 1.1 38.2 39.1 40.1 7.8 7.3 9.7 8.0 9.3 5.4 6.3 3.9 7.4 0.4 4.9 8.1 2.2 8.2 0.9 6.5 4.7 0.9 8.0
4.8Ey04 9.3Ey04 3.4Ey04 1.0Ey03 4.1Ey04 7.9Ey04 9.7Ey04 2.8Ey04 6.6Ey04 3.3Ey04 6.8Ey04 2.0Ey04 4.8Ey04 6.4Ey04 3.5Ey04 5.0Ey04 4.8Ey04 5.8Ey04 1.7Ey04 6.5Ey04 2.7Ey04 3.8Ey04 2.9Ey04 4.0Ey04 6.4Ey04 3.8Ey04 1.1Ey04 3.0Ey04 3.7Ey04 5.2Ey04 3.0Ey04 8.9Ey04 5.1Ey04 2.3Ey04 9.8Ey04 1.1Ey03 7.6Ey05 1.7Ey04 5.4Ey04
Density of the sample in helium is determined using the microbalance prior to methane adsorption and desorption analyses.
low rank coals Ž- 75% C., lower in the 75–85% C range Žhigh to medium volatile bituminous. and increase again through the medium volatile to anthracite. Surface area has also been related to maceral content and it has been shown that vitrinitic coals have
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319
Fig. 4. Trends in Langmuir volume in relation to coal rank were different for bright and dull coals. Bright coal trendline equation: VL s 5.90 R o2 y20.3 R o q56.2; r 2 s 0.54. Dull coal trendline equation: VL s 2.90 R o2 y 6.77R o q42.1; r 2 s 0.70.
a greater internal surface area than inertinite-rich coals ŽThomas and Damberger, 1976.. For Bowen Basin coals, it has been found that vitrinite-rich coals have higher internal surface area than inertinite-rich coals in the rank range of high volatile bituminous to medium volatile bituminous rank ŽBeamish and Crosdale, 1995.. Our adsorption isotherm results in the low rank range support this view. 3.5. Coal type effects on adsorption capacity Maceral analysis ŽTable 2. indicates that the coals are dominated by vitrinite and inertinite and can, therefore, be considered a two-component system. Simple comparison of Langmuir volume to vitrinite content ŽFig. 5. shows no relationships. However, evaluation of coal type alone is complicated by rank influences. When individual ranks are considered ŽFig. 5., it is found that in high volatile bituminous coals, increasing Langmuir volume is related to increasing vitrinite content; at medium and low volatile bituminous ranks, maceral composition may exert relatively little influence on gas storage capacity; at semianthracite and anthracite ranks, decreasing Langmuir volume is associated with higher vitrinite content. However, more data points are required to confirm the trend in semianthracite and anthracite ranks. Vitrinite-rich coals have been found to have greater adsorption capacity than inertinite-rich coals for isorank samples ŽCrosdale and Beamish, 1993; Lamberson and Bustin, 1993; Faiz, 1995.. However, poor or no correlation was also observed between adsorption capacity and maceral composition ŽFaiz et al., 1992. and in some cases,
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Fig. 5. Trends in Langmuir volume in relation to coal type also show rank influences. Coal type has greatest influence on methane adsorption capacity in high volatile bituminous coal, semianthracite and anthracite.
inertinite-rich coals have been found to have greater methane adsorption capacity ŽEttinger et al., 1966.. Our results for dry coals show that maceral composition is an important control on gas adsorption but the degree of influence that maceral composition exerts is rank-dependent. This is consistent with varying conclusions reached by other workers. The strong influence of maceral composition reported for some Cretaceous Canadian coals ŽLamberson and Bustin, 1993. probably reflects the coal rank Žhigh volatile A to medium volatile.. In contrast, the lack of maceral control on gas content observed for some medium to low volatile Permian coals of Australia Že.g., Faiz et al., 1992. is also consistent with our results. 3.6. Rank and type effects on Langmuir pressure The Langmuir pressure Ž PL . is, in theory, related to the isosteric heat of adsorption but, in practice, PL is an empirical constant which is difficult to reconcile with theory ŽGregg and Sing, 1982.. Mathematically, PL is also the pressure at half the Langmuir volume. The coals studied show a strong relationship of decreasing PL with increasing rank ŽTable 4; Fig. 6., which does not appear to be related to coal type. Due to the uncertain nature of the relationship of PL to the isosteric heat of adsorption, it is not possible to draw firm conclusions as to the physical significance of this trend. The isosteric heat of adsorption should decrease as surfaces become less heterogeneous and the surface coverage increases ŽGregg and Sing, 1982.. Our results may, therefore, suggest that as
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Fig. 6. Langmuir pressure Ž PL . decreased with increasing rank and was not related to type. Trendline equation: PL sy0.42 R o q2.36; r 2 s 0.91.
coal rank increases, the pore surfaces become less heterogenous and the coverage of these surfaces is more complete. We suggest that the decrease in heterogeneity of the pore surfaces may be linked to the increased aromaticity of the coal structure by polymerizationrcondensation and the loss of non-aromatic carbons ŽLevine, 1993.. In contrast, the opposite relationship of increasing PL with increasing rank has also been reported for dry coals over the volatile matter range of 5% to 45% ŽBoxho et al., 1980 cited by Lama and Bodziony, 1996.. Their range of values was approximately 0.25 to 1.4, which is significantly smaller than those reported here ŽTable 4.. 3.7. Desorption rate Desorption rate investigations were undertaken by measuring gas release rates to atmosphere from an initial pressure of 0.5 MPa Žthe final step of the desorption isotherms.. A unipore spherical model was used to calculate an effective diffusivity parameter Ž De .. Mineral infillings Žpredominantly carbonates and clays. have been found to block the coal pore system, including cleats and fractures, and significantly reduce gas flow rates ŽGamson et al., 1993.. No relationship was found between De and ash content ŽFig. 7., which relates to the finely crushed nature of the samples. Crushing would destroy the larger transport pore networks, which control gas movement rates by Darcy flow, and which are most susceptible to blockage by secondary mineralisation. Additionally, much of the mineral matter is primarily detrital which would not cause blockages of transport pores. Both rank and coal type were found to influence effective diffusivity ŽFig. 8.. Dull coals have faster desorption rates Žtwo to three times. than their bright equivalents in
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Fig. 7. Desorption rate of finely crushed coals was not related to the ash content of coal.
most cases. Dull coal desorption is fastest for high volatile coals, gradually reduces as rank increases and becomes stable from low volatile bituminous coals onwards ŽFig. 8.. Bright coal desorption rates also decrease with increasing rank, but the trend is less marked than that of dull coals.
Fig. 8. Desorption rate of finely crushed coals was both rank- and type-dependent. Dull coals generally desorbed more rapidly than their rank equivalent. Bright coal and dull coal desorption rates generally decreased with increasing rank.
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Rank and type effects on De are likely to be related to pore size distribution. Total coal porosity follows a second-order polynomial trend with a minimum at 89% C Žvan Krevelen, 1993.. The initial decrease in porosity is related to a decrease in macro- and mesoporosity, while the subsequent increase is related to an increase in microporosity. Decreasing larger porosity with increasing rank is translated into lower effective diffusivities with rank increase. Microporosity is a limiting factor for diffusion rate, so a relative increase in the amount of microporosity is not translated into progressively slower diffusivity rates. Inertinite-rich dull coals are often more macroporous than vitrinite-rich bright coals ŽCrosdale and Beamish, 1995.. As a result, the desorption in bright coals is slower due to restrictions imposed by micropores. In case of dull coals, the presence of more macroand mesopores at lower ranks allows a faster desorption rate, which slows as these larger pores become less common at higher ranks.
4. Conclusions Methane adsorption isotherms of dry coals of varying ranks from high volatile bituminous to anthracite, from the Bowen and Sydney Basins, Australia, were investigated. The study revealed that the coal type, rank and mineral matter strongly influence the methane sorption capacity of the coals. Mineral matter in coal, as expressed by ash content, acts as a diluent to significantly reduce the gas adsorption capacity. For Bowen Basin coals, the average decrease in methane adsorption capacity was 0.38 cm3rg for each 1% rise in ash content. Langmuir volumes Ž V L . for bright and dull coal types follow discrete, second-order polynomial trends with increasing rank. Bright coals follow a trend with minimum VL at low volatile bituminous rank Ž1.72% R o max . and dull coals exhibit a minimum V L at medium volatile bituminous rank Ž1.17% R o max .. At low rank, VL was greater in bright coal by about 10cm3rg, but as rank increased, the bright and dull trends converged and crossed at 1.65% R o max . At ranks higher than 1.65% R o max , both bright and dull coals followed similar trends. Maceral composition of coals had a variable effect on gas adsorption. In high volatile bituminous coals, increases in vitrinite content were associated with increases in adsorption capacity. In medium to low volatile bituminous coals, changes in maceral composition may exert relatively little influence. The Langmuir pressure Ž PL . showed a strong relationship of decreasing PL with increasing rank, which was not related to coal type. It is suggested that the observed trend is related to a decrease in the heterogeneity of the pore surfaces, and subsequent increased coverage by the adsorbate, as coal rank increases. Desorption rate studies on crushed coals indicate that coal type and coal rank have important influences, although there is a general trend of decreasing desorption rate with increasing rank. Coals of lower rank have higher effective diffusivities and mineral matter was found to have no influence on desorption rate of the finely crushed coals. Variations in pore size distribution with coal type and with rank are implicated in controlling these trends.
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5. Nomenclature Symbols De P PL t V VL V`
the effective diffusivity Žcm2rs. pressure ŽMPa. Langmuir pressure ŽMPa. time Žs. volume of the adsorbedrdesorbed gas Žcm3 . Langmuir volume Žcm3 . total volume of desorbed gas Žcm3 .
Acknowledgements Proximate analyses on the coal samples were conducted at University of Auckland, New Zealand. We thank Basil Beamish for his help in analysing these coals. Reviewers Jeff Levine and Jay Close are thanked for their critical evaluation of the manuscript.
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