Energy Fuels 2010, 24, 1781–1787 Published on Web 01/07/2010
: DOI:10.1021/ef901238c
Relationships between the Critical Properties of Gases and Their High Pressure Sorption Behavior on Coals Richard Sakurovs,* Stuart Day, and Steve Weir CSIRO Energy Technology, PO Box 330, Newcastle 2300, Australia Received October 28, 2009. Revised Manuscript Received December 9, 2009
Sequestration of carbon dioxide in coal seams is a potential method of reducing atmospheric emissions of carbon dioxide. If carbon dioxide can be sequestered in coal seams and it simultaneously results in enhanced coal bed methane (ECBM) production, some of the sequestration costs can be recovered in the value of the methane produced. This requires knowledge of both the carbon dioxide and methane sorption behavior of coal at high pressures. However, the relationship between methane and carbon dioxide sorption at high gas pressure is not well understood. To elucidate their relationship, we investigated the sorption of carbon dioxide, methane, ethane, nitrogen, argon, krypton, xenon, carbon tetrafluoride, and sulfur hexafluoride by dry coals at 55 °C at pressures up to 20 MPa; all of these gases have critical temperatures below 55 °C. Sorption isotherms for the different gases were very different but all could be fitted by a modified Dubinin-Radushkevich model to within about 1% of their calculated maximum adsorption capacity. We found that the maximum adsorption capacity, determined from the isotherms, of the coals investigated for a supercritical gas increases linearly with the critical temperature of the gas when the maximum adsorption capacity is expressed on a (van der Waals) volume basis, except for gases that could not penetrate the coal as effectively as the other gases: carbon tetrafluoride and sulfur hexafluoride. This behavior is consistent with the idea that, in sorption of supercritical gases, the surface phase is not condensed but acts as a compressed gas. This provides a simple explanation of why the molar maximum sorption capacity at temperatures near ambient decreases in the order carbon dioxide > methane > nitrogen: their critical temperature decreases in the same order. The heats of sorption of different gases on a given coal, calculated from the isotherm, were closely related to their van der Waals attraction constant and were similar to those reported for sorption of these gases onto graphite at low pressure. The calculated heats of sorption for xenon and ethane on coal were higher than that for carbon dioxide on the corresponding coal. For the three bituminous coals examined, carbon tetrafluoride and sulfur hexafluoride did not penetrate the coal as completely as the other gases. About 3-5% (by volume) of each coal was calculated to be inaccessible to these two gases that were accessible by the other gases, which we attribute to their greater molecular diameter. Except for these two gases, the volume of each coal accessible by each gas was found to be similar (to within 1.5% of the coal volume).
saturation and carbon dioxide is adsorbed more strongly than methane, which would increase the ratio especially at very low pressures. However, comparisons of the maximum adsorption capacity (maximum possible sorption at a given temperature) of the two gases have not been so intensively investigated. From basic monolayer models, it would be expected that the maximum adsorption capacities of gases should be roughly the same, when considered on a volume basis, since the surface area and pore volume of the coal should be constant. Simple pore-filling models would also reach a similar conclusion. In fact, the expectation that the pore volume occupied by condensable gases and liquids is constant is the basis of the Gurvitsch rule.5 This simple expectation is complicated by the possibilities that larger molecules may not penetrate some pores that are accessible to smaller molecules;this issue has long been recognized as a potential problem in coals6;and that coals swell to some extent when exposed to gases that are strongly sorbed,7 which could change surface area and
1. Introduction Unmineable coal seams are one option for sequestration of carbon dioxide, because they can store 6-12% by weight of carbon dioxide.1 Often coal seams contain methane. If carbon dioxide can be sequestered in such coal seams and the process simultaneously results in enhanced coal bed methane (ECBM) production, some of the sequestration costs can be recovered in the value of the methane produced.2 It has long been known that coals can adsorb more carbon dioxide than methane,3 although published values for the ratio of molar sorption capacity for coals vary from 2:1 to 10:1.4 This variation is partly because the ratios were measured at pressures below
*To whom correspondence should be addressed. E-mail:Richard.
[email protected]. (1) Day, S.; Duffy, G.; Sakurovs, R.; Weir, S. Int. J. Greenhouse Gas Control 2008, 2 (3), 342–352. (2) White, C. M.; Smith, D. H.; Jones, K. L.; Goodman, A. L.; Jikich, S. A.; LaCount, R. B.; DuBose, S. B.; Ozdemir, E.; Morsi, B. I.; Schroeder, K. T. Energy Fuels 2005, 19 (3), 659–724. (3) Ettinger, I.; Chaplins, A; Lamba, E.; Adamov, V. Fuel 1966, 45 (5), 351–&. (4) Harpalani, S.; Prusty, B. K.; Dutta, P. Energy Fuels 2006, 20 (4), 1591–1599. r 2010 American Chemical Society
(5) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: London, 1982; p 303. (6) Mahajan, O. P. Carbon 1991, 29 (6), 735–742. (7) Day, S.; Fry, R.; Sakurovs, R. Int. J. Coal Geology 2008, 74 (1), 41–52.
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pubs.acs.org/EF
Energy Fuels 2010, 24, 1781–1787
: DOI:10.1021/ef901238c
Sakurovs et al.
Table 1. Analytical Data for the Coals Investigateda coal 1 2 3 4 5 6
moist ad, %
ash db, %
VM db, %
C db, %
H db, %
N db, %
S db, %
Rv,max, %
vitrinite, vol %, mmf
liptinite, vol %, mmf
He density, db, kg m-3
1.1 2.7 1.7 8.9 2.8 4.7
16.9 17.6 11.4 20.3 8.3 18.7
18.0 23.4 29.2 25.9 27.2 25.3
73.9 69.1 61.5 56.9 75.7 67.9
3.78 3.75 4.00 2.73 4.11 3.88
1.34 1.60 1.04 0.82 1.52 1.58
0.25 0.40 0.34 0.19 0.33 0.41
1.40 0.81 0.80 0.62 0.90 0.80
28.1 10.6 64.0 23.9 33.9 20.2
0.0 5.0 1.6 1.6 2.3 4.9
1.47 1.49 1.37 1.64 1.39 1.51
a Notes: Sample 6 is from the same source as sample 2. Helium density reported is that for the coal after exposure to carbon dioxide. Coals 1-3 were used in series 1 and coals 4-6 were used in series 2. ad = air-dried, db = dry basis, Rv,max = mean maximum vitrinite reflectance in oil; mmf = mineral matter-free - normalized so that vitrinite þ liptinite þ inertinite = 100%.
micropore volume and perhaps the internal structure.8 However, the difference in maximum sorption of methane and carbon dioxide is too great to be explained by either effect. If the difference in maximum sorption capacity by methane and carbon dioxide was explicable by the difference in their molecular size, then it would require that most of the pore necks in coal are of a narrow size range that is penetrable by carbon dioxide but not by methane. If the difference was due to coal swelling, then the surface area or internal volume must more than double by swelling. Others have suggested that there could be a specific interaction between carbon dioxide and coal that does not exist between methane and coal.2 Many groups have found that the sorption for gases on materials such as coals and charcoals at a given pressure decreases with increasing temperature,2,9,10 but whether they found this to be also true of maximum adsorption capacity was not clear, nor was this effect quantified. Recently it was found that for nitrogen sorption on charcoal, the maximum sorption capacity, determined from the sorption isotherm, decreased with increasing temperature, in fact the maximum sorption capacity was proportional to inverse temperature over the range 100-300 K.11 This is inconsistent with expectations from monolayer or pore-filling models of sorption. If maximum adsorption capacity for supercritical gases is proportional to inverse temperature, then the maximum adsorption capacity for different supercritical gases at a given temperature would be expected to be proportional to their critical temperature, if they all interact with coal in the same way and pore penetration is not an issue. This would provide a simple explanation of why the maximum adsorption capacity of coals for carbon dioxide is substantially greater than that for methane: the critical temperature of carbon dioxide is almost double that of methane. In this paper we determine the isotherms for a number of gases that have critical temperatures below 55 °C on coals to determine relationships between them.
Table 2. Physical Properties of the Gases Used in the Sorption Experiments gas
MW
Tcrit, Pcrit, vdW density, vdW a, kinetic K MPa kg m-3 Pa m6 mol-2 diameter,21 nm
44.0 304.2 CO2 CH4 16.0 190.5 28.0 126.2 N2 Ar 40.0 150.9 Kr 83.8 209.4 Xe 131.3 289.7 88.0 227.7 CF4 C2H6 30.1 305.4 SF6 146.1 318.6
7.4 4.6 3.4 4.9 5.5 5.8 3.7 4.9 3.8
1028 372 726 1248 2119 2546 1392 463 1658
0.366 0.230 0.137 0.136 0.232 0.419 0.404 0.557 0.654
0.33 0.38 0.36 0.34 0.36 0.40 0.47 0.38 0.55
Sorption measurements were made using a nominal sample mass of 200 g at pressures up to 20 MPa (accurate to 0.01 MPa) and at a temperature of 55 ((0.2) °C. Samples were maintained at each pressure step for sufficient time to allow equilibrium to establish. The actual time required for equilibration depended on the gas being used. For CO2, CH4, C2H6, Ar, Kr, and N2, measurements were continued for at least 4 h after each pressure change. However, CF4 and Xe had relatively slow equilibration times and in some cases required several days to stabilize. The equilibration time for SF6 was of the order of weeks; two weeks were used between measurements and complete equilibration may not have been achieved. Neon was also examined but it was not found to sorb significantly (
