Direct Evidence of Carbon Dioxide Sorption on Argonne Premium

Dec 21, 2004 - Migration of metamorphic CO 2 into a coal seam: a natural analog study to assess the long-term fate of CO 2 in Coal Bed Carbon Capture,...
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Direct Evidence of Carbon Dioxide Sorption on Argonne Premium Coals Using Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy A. L. Goodman,*,† L. M. Campus,‡ and K. T. Schroeder† National Energy Technology Laboratory, U.S. Department of Energy, Pittsburgh, Pennsylvania 15236, and Chemical Engineering Department, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 Received May 11, 2004. Revised Manuscript Received October 22, 2004

The direct interaction between CO2 and two Argonne Premium Coals [Pocahontas No. 3 (lowvolatile bituminous) and Beulah-Zap (lignite)] was probed by use of attenuated total reflectanceFourier transform infrared (ATR-FTIR) spectroscopy at 328 K and pressures up to 8.0 MPa. Sorbed CO2 on Argonne Coals was detected at 2335 cm-1 for Beulah-Zap coal and 2332 cm-1 for Pocahontas No. 3 coal. The energy of adsorption (18.8-20.5 kJ/mol), estimated from the Langmuir equation, was consistent with physisorption. The spectral data indicated that only one type of site was available for sorption. No evidence could be found for specific interactions between CO2 and oxygen functional groups in the coals. The CO2-coal sorption isotherm was derived without estimating the CO2 compressibility and adsorbed layer density, both of which are needed in manometric techniques.

Introduction Carbon dioxide (CO2) sequestration in coal seams has recently been proposed as a viable option for the mitigation of greenhouse gas emissions.1-5 An understanding of the mechanism by which CO2 is stored in coals would provide a scientific foundation on which to base predictions of long-term CO2 storage stability. Although one report in the literature has implicated specific interactions between CO2 and oxygen functional groups in coals,6 most of the literature considers gases in coals to be stored as adsorbed species on the coal surface, both external and within the pore structure, and absorbed within the organic matrix of the coals.1 Spectroscopic techniques can provide mechanistic information about the type of interaction(s) responsible for the CO2 sorption phenomenon. In FTIR spectroscopic * To whom correspondence should be addressed: Telephone: 1-412386-4962. Fax: 1-412-386-5920. E-mail: [email protected]. † National Energy Technology Laboratory. ‡ ORISE undergraduate student from the Chemical Engineering Department, Carnegie Mellon University. (1) White, C. W.; Strazisar, B. R.; Granite, E. J.; Hoffman, J. S.; Pennline, H. W. J. Air Waste Manage. Assoc. 2003, 52, 645-715. (2) Reeves, S. R. Geological Sequestration of CO2 in Deep, Unmineable Coalbeds: An Integrated Research and Commerical-Scale Field Demonstration Project. Presented at the Annual Technical Conference and Exhibition, New Orleans, LA, September 30-October 3, 2001; SPE Paper No. 71749. (3) Richardson, R. J.; Hughes, J. D.; Rottenfusser, B. A.; Gentzis, T.; Gunter, W. D.; Bachu, S. Coalbed Methane and CO2 Disposal; The Answer for Alberta?; Geological Society of America, 28th Annual Meeting, Boulder, CO, October 28, 1996; Geological Society of America: Boulder, CO, 1996; Vol. 28, Issue 7, p 41. (4) Gentzis, T. Int. J. Coal Geol. 2000, 43, 287-305. (5) Allis, R.; Chidsey, T.; Gwynn, W.; Morgan, C.; White, S.; Adams, M.; Moore, J. Natural CO2 Reservoirs on the Colorado Plateau and Southern Rocky Mountains: Candidates for CO2 Sequestration; DOE/ NETL-2001/1144 [CD-ROM]; National Energy Technology Laboratory Publications, Pittsburgh, PA, May 14, 2001, pp 1-19. (6) Nishino, J. Fuel 2001, 80, 757-764.

10.1021/ef0498824

studies of CO2 dissolved in polymers, for example, it was found that polymers possessing electron-donating functional groups exhibited specific interactions with CO2.7 A similar understanding of the interactions between CO2 and coals would provide mechanistic information on which to base predictions of sequestration long-term stability. Also of interest is the CO2 coal seam storage capacity, which is typically estimated from adsorption isotherm measurements by volumetric or manometric techniques. Calculation of absolute adsorption requires two major parameters to be estimated. First, to account for the nonideality of CO2 at elevated pressures, the compressibility factor must be calculated from an equation of state (EOS). The Span and Wagner8 EOS is the most recent and most widely accepted but others have been used. Second, and perhaps more importantly, the change associated with the volume occupied by the surface adsorbed CO2 must be estimated from the CO2 adsorbed layer density. A number of different values have been proposed for subcritical and supercritical conditions.9 There appears to be little experimental justification for choosing one value over another beyond better isotherm curve fits. More recently, Ozdemir et al.10,11 have addressed the problem of volumetric changes that may occur during the development of the adsorption isotherm. They considered the additional complication of (7) Kazarian, S. G.; Vincent, M. F.; Bright, F. V.; Liotta, C. L.; Eckert, C. A. J. Am. Chem. Soc. 1996, 118, 1729-1736. (8) Span, R.; Wagner, W. J. Phys. Chem. 1996, 25, 1509-1596. (9) Sudibandriyo, M.; Pan, Z.; Fitzgerald, J. E.; Robinson, R. L.; Gasem, K. A. M. Langmuir 2003, 19, 5323-5331. (10) Ozdemir, E.; Morsi, B. I.; Schroeder, K. Fuel 2004, 83, 10851094. (11) Ozdemir, E.; Morsi, B. I.; Schroeder, K. Langmuir 2003, 19, 9764-9773.

This article not subject to U.S. Copyright. Published 2005 by the American Chemical Society Published on Web 12/21/2004

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Goodman et al. Table 1. Comparison of the Langmuir Constants and Energies of Adsorption Obtained by ATR-FTIR to Those Obtained by the Manometric Technique

technique

moisture content (%)

energy of adsorption, Q (kJ/mol)

Langmuir constant, b (MPa-1)

ATR-FTIR ATR-FTIR manometric

Beulah-Zap Coal 8.0a 19.9 0.2b 20.5 4.0c 20.2

0.711 0.878 0.787

ATR-FTIR ATR-FTIR manometric

Pocahontas No. 3 Coal 0.5a 20.0 0.1b 18.8 0.5c 19.9

0.706 0.471 0.708

a Dried at 295 K in nitrogen, as determined by TGA. b Dried at 353 K in nitrogen, as determined by TGA. c Dried at 353 K under vacuum, as determined by TGA.

Figure 1. Schematic of the modified ATR-FTIR cell assembly.

the swelling behavior of coals and attempted to correct for it by modifying the equation used to obtain the absolute adsorption from the excess adsorption. A few papers have reported the isotherms for CO2 adsorption on coals at pressures above 8 MPa. In these reports, a variety of curve shapes have been reported.12,13 A technique that could directly measure the amount of sorbed CO2 would aid in determining if such differences are real or due to an experimental or calculation artifact. Here, we report the direct observation of the interaction of CO2 with two of the Argonne Premium Coals by use of attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy. Upon exposure of Argonne Coals to CO2, a positive absorption band due to sorbed CO2 appeared in the spectrum. As the coals were exposed to increasing CO2 pressure, the CO2 absorption band grew in intensity. Because this technique allowed for the direct observation of sorbed CO2, both the chemical interaction between CO2 and coals and the isotherm shape were determined. This method of determining the adsorption isotherm shape avoided assumptions about the compressibility and the adsorbed layer density needed in manometric techniques. Experimental Section All experiments were conducted in a modified ATR CIRCLE cell from Spectra Tech (Figure 1). The modified cell assembly consisted of two stainless steel high-pressure cells each containing a ZnSe ATR crystal. One cell contained a ZnSe crystal with no coal present (blank) and the other cell contained a ZnSe crystal coated with the coal (sample). The two cells were connected in tandem via 1/16-in. stainless steel tubing so that each cell experienced the same CO2 pressure. The entire ATR cell assembly was enclosed in a heating jacket. (12) Krooss, B. M.; van Bergen, F.; Gensterblum, Y.; Siemons, N.; Pagnier, H. J. M.; David, P. Int. J. Coal Geol. 2002, 51, 69-92. (13) Hall, F. E.; Zhou, C.; Gasem, K. A. M.; Robinson, R. L., Jr.; Yee, D. Adsorption of Pure Methane, Nitrogen, and Carbon Dioxide and Their Binary Mixtures on Wet Fruitland Coal. Conference Proceeding: Presented at SPE Eastern Regional Conference and Exhibition, Charleston, WV, November 8-10, 1994; SPE 29194, pp 329-344.

High-pressure experiments were performed by connecting the ATR cell assembly to a gas handling system consisting of a syringe pump (ISCO model 260D), a port for gas introduction, and a pressure transducer (OmegaDyne Inc., PX01K1-5KGV). The total volume of the system was estimated to be 1 mL. Carbon dioxide (99.998% purity by supercritical-fluid chromatography, SFC) and ultra-high-purity (UHP) nitrogen were used as supplied from Butler Gas Co. Infrared spectra were recorded with a single-beam FTIR spectrometer (Thermo Electron Nexus 670 FTIR ESP) equipped with a wide-band mercury cadmium telluride (MCT) detector. Unless otherwise noted, 500 scans were collected with an instrument resolution of 2 cm-1 in the spectral range extending from 4000 to 600 cm-1. Spectra were distorted below 650 cm-1 because the ZnSe crystal became opaque. The following two Argonne Premium Coals were investigated: Pocahontas No. 3 (low-volatile bituminous) and BeulahZap (lignite). The Argonne Premium Coals provide the highest quality samples available for basic research. These coals are well-characterized, providing the research community with a large database of physical and chemical properties.14,15 The samples were obtained as powders (-100 mesh). In these experiments, coals were prepared in a nitrogen-filled glovebag to avoid surface oxidation. The coals were immersed in nitrogen-sparged water to form a slurry. A ZnSe ATR crystal was then coated with a thin layer of the slurry (∼10 mg) and allowed to dry at 295 K in the nitrogen-filled glovebag for 20 min. The crystal was loaded into the ATR cell and sealed under nitrogen. The ATR-FTIR spectra of the interaction of CO2 with the coals were measured under one of two conditions: the coals were equilibrated at 328 K and then exposed to CO2 pressure, or the coals were additionally dried at 353 K for 36 h under flowing N2, equilibrated at 328 K, and then exposed to CO2. The moisture contents of the nitrogen-dried coals used in this study are listed in Table 1. All spectral data were collected at 328 K. The ATR cell assembly was mounted on a linear translator inside the FTIR spectrometer. The translator allowed each cell of the assembly, the blank side for CO2 measurements and the sample side for CO2-coal measurements, to be scanned by moving the cell of interest into the IR beam path. In the ATR-FTIR experiments, reference spectra were collected of the blank and sample cells. Then a known pressure of CO2 was applied to both cells. The pressure of the gas reached equilibrium after the first few minutes. No changes in the sorbed CO2 spectrum were observed even when the gas was allowed to equilibrate with the coal for 24 h. This is consistent with CO2 sorption studies on Argonne Coals reported in the literature in that 30 min was sufficient for the sorption of CO2 (14) Vorres, K. S. Users Handbook for the Argonne Premium Coal Sample Program, 1993: accessed December 9, 2003 at URL http://www.anl.gov/ PCS/pcshome.html. (15) Vorres, K. S. Energy Fuels 1990, 4, 420-426.

Carbon Dioxide Sorption on Argonne Premium Coals

Figure 2. (a) ATR-FTIR spectrum of gaseous CO2 (3.5 MPa) interacting with Pocahontas No. 3 coal at 328 K. Both sorbed and gaseous CO2 are present in the spectrum. (b) ATR-FTIR spectrum of gaseous CO2 (3.5 MPa) at 328 K. (c) Difference spectrum (a - b) to remove gaseous CO2 from spectrum a. on powdered coal samples to reach equilibrium.10,16 However, the equilibrium conditions were not monitored for 5 days or more as suggested by Larsen and Hsieh in order to account for diffusion of CO2 into the coal.17-19 After 30 min, two spectra were recorded: one from the cell containing both CO2 and coal (Figure 2a) and one from the cell containing CO2 (Figure 2b). By calculating the difference between the two spectra, an absorption spectrum of CO2 sorbed on the coal was obtained. The spectral subtraction result for CO2 sorption on Pocahontas No. 3 coal at 328 K and 3.5 MPa is illustrated in Figure 2c. CO2 pressures up to 8.0 MPa were investigated by this technique. Although the ATR cell assembly is rated to 10.0 MPa, the CO2 absorptions were too intense at pressures above 8.0 MPa and saturated the wide-band MCT detector.

Results and Discussion The interaction of CO2 with coals was examined by ATR-FTIR spectroscopy, a technique that is well suited to study high-pressure gas systems and coals.7,20-24 Infrared spectra of coals are complex with many overlapping absorption bands.25,26 The ATR-FTIR spectra of Beulah-Zap (lignite) and Pocahontas No. 3 (lowvolatile bituminous) coals were typical of complex, heterogeneous coals containing multiple functionalities (Figure 3). Upon exposure of Beulah-Zap coal (0.2% moisture) to CO2, two major positive absorption bands at 2335 and 654 cm-1 appeared in the difference spectrum (Figure 4a). A shoulder band at 2322 cm-1 also appeared in the spectrum. A similar spectrum was obtained when a Beulah-Zap sample containing more (16) Goodman, A. L.; Busch, A.; Duffy, G. J.; Fitzgerald, J. E.; Gasem, K. A. M.; Gensterblum, Y.; Krooss, B. M.; Levy, J.; Ozdemir, E.; Pan, Z.; Robinson, R. L.; Schroeder, K.; Sudibandriyo, M.; White, C. M. Energy Fuels 2004, 18, 1175-1182. (17) Larsen, J. W.; Hall, P.; Wernett, P. C. Energy Fuels 1995, 9, 324-330. (18) Hsieh, S. T.; Duda, J. L. Fuel 1987, 66, 170-178. (19) Larsen, J. W. Prepr. Symp.sAm. Chem. Soc., Div. Fuel Chem. 2003, 48, 112-113. (20) Kazarian, S. G. Macromol. Symp. 2002, 184, 215-228. (21) Yarwood, J. Anal. Proc. 1993, 30, 13-18. (22) Yokoyama, C.; Kanno, Y.; Takahashi, M.; Ohtake, K.; Takahashi, S. Rev. Sci. Instrum. 1993, 64, 1369-1370. (23) Thomasson, J.; Coin, C.; Kahraman, H.; Fredericks, P. M. Fuel 2000, 79, 685-691. (24) Jacques, F. Physical Adsorption: Experiment, Theory, and Applications; Kluwer Academic: Dordrecht and Boston, 1997. (25) Charcossett, H. Advance Methodologies in Coal Characterization; Elsevier: New York, 1990; pp 399-417. (26) Thomasson, J.; Coin, C.; Kahraman, H.; Fredericks, P. M. Fuel 2000, 79, 685-691.

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Figure 3. ATR-FTIR spectra of Beulah-Zap (lignite) and Pocahontas No. 3 (low-volatile bituminous) coals at 298 K.

Figure 4. Difference spectra of sorbed CO2 at 0.8 MPa and 328 K: (a) Beulah-Zap, 0.2% moisture; (b) Beulah-Zap, 8.0% moisture; (c) Pocahontas No. 3, 0.1% moisture; (d) Pocahontas No. 3, 0.5% moisture.

moisture (8.0% moisture) was exposed to CO2 (Figure 4b). The higher moisture content of the coal did not result in new or shifted absorption bands. Similar spectra were obtained when Pocahontas No. 3 coals of different moisture contents were exposed to CO2 (Figure 4c,d). The absorption band frequencies, centered at 2332 and 656 cm-1, were slightly shifted from those obtained with Beulah-Zap samples. After desorption in flowing nitrogen, spectral absorption bands were not detected, indicating that the interaction between CO2 and the coals was reversible. Gaseous CO2 is a linear molecule with two infraredactive absorption bands at 2349 cm-1 (ν3 antisymmetric stretching mode) and 667 cm-1 (ν2 bending mode) (Figure 2b).27 Upon interaction with a surface, molecules can attach to surfaces in two ways. In physisorption, there is a van der Waals interaction between the adsorbate and the substrate. In chemisorption, adhesion occurs with the formation of a chemical bond, usually covalent. Infrared spectroscopy can be used to distinguish between these mechanisms because only a slight shift in the absorption band frequency accompanies physisorption, whereas chemisorption is accompanied by a much larger shift to lower energy. Physisorbed CO2 on carbon-containing surfaces such as C60, graphite, diamond,28 and carbon29 has been reported to display (27) Freund, H. J.; Roberts, M. W. Surf. Sci. Rep. 1996, 25, 225273. (28) Fastow, M.; Kozirovski, Y.; Folman, M. J. Electron Spectrosc. Relat. Phenom. 1993, 64, 843-848. (29) Mawhinney, D. B.; Rossin, J. A.; Gerhart, K.; Yates, J. T., Jr. Langmuir 1999, 15, 4617-4621.

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absorption bands between 2340 and 2328 cm-1 for the ν3 antisymmetric stretching mode and between 663 and 652 cm-1 for the ν2 bending mode. Chemisorbed CO2 has main absorption bands at lower energy near 1400 cm-1.30 In this study, the observed absorptions bands between 2335 and 2332 cm-1 (ν3 antisymmetric stretching mode) and 654-656 cm-1 (ν2 bending mode) detected upon exposure of the coals to CO2 were consistent with physisorption. The CO2 absorption band frequency and shape can help elucidate the chemical interaction between CO2 and coal. That fact that there was one distinct absorption band suggests that there is one type of sorption site for the CO2. If more than one distinct type of site were available for CO2 to sorb, one would expect several distinct absorption bands corresponding to each type of sorption site.29,31 The presence of inherent moisture does not appear to change the fundamental chemistry of the CO2-coal interaction because intentional removal of essentially all of the inherent moisture does not affect the absorption band position, although it does appear to affect the adsorption capacity (vide infra). In his survey of the interaction of CO2 with several polymers, Kazarian reported that the wavenumber corresponding to the anti-symmetric stretch of sorbed CO2 was between 2338.0 and 2339.5 cm-1 when the polymer contained carbonyl or carboxyl functionality. In the absence of these functionalities, the wavenumber was between 2334.6 and 2335.3 cm-1. In this study, the wavenumber values of 2332 cm-1 for the low-volatile bituminous coal and 2335 cm-1 for the lignite coal tend to indicate that specific interactions between CO2 and oxygen functional groups were not responsible for the sorption phenomenon. The shoulder absorption band at 2322 cm-1 (BeulahZap) and 2320 cm-1 (Pocahontas No. 3) (Figure 4) has been reported for CO2 interacting with polymers,7 carbon nanotubes,32 and solutions.33,34 This absorption band has been tentatively assigned as either a hot band transition or as an absorption band representing a distinct sorption site. We are unable to make a strong argument for the origin of this band but simply note that it has been reported only for CO2 sorbed to carbonaceous materials and not for CO2 sorbed to inorganic materials.31,35,36 This supports the view that CO2 sorption is important primarily for the carbonaceous material in coal, not the mineral matter. The ν2 bending mode of sorbed CO2 has also been used to provide evidence of specific interactions between CO2 and polymer functional groups.7,37 Upon sorption of gaseous CO2, the degeneracy of the ν2 bending mode is lost and the sorbed CO2 absorption splits into two bands because of a change in symmetry. Kazarian et al.7 and (30) Davydov, A. A. Infrared Spectroscopy of Adsorbed Species on the Surface of Transmission Metal Oxides; Wiley and Sons: Chichester, U.K., 1990. (31) Gallei, E.; Stumpf, G. J. Colloid Interface Sci. 1976, 55, 415420. (32) Matranga, C.; Chen, L.; Smith, M.; Bittner, E.; Johnson, J. K.; Bockrath, B. J. Phys. Chem. B 2003, 107, 12930-12941. (33) Kazarian, S. G.; Sakellarios, N.; Gordon, C. M. Chem. Commun. 2002, 1314-1315. (34) Cunliffe-Jones, D. B. Spectrochim. Acta 1968, 25A, 779-791. (35) Ueno, A.; Bennett, C. O. J. Catal. 1978, 54, 31-41. (36) Tan, H. S.; Jones, W. E. Can. J. Spectrosc. 1989, 34, 35-37. (37) Nelson, M. R.; Borkman, R. F. J. Phys. Chem. 1998, 102, 78607863.

Goodman et al.

Figure 5. Difference ATR-FTIR spectra of sorbed CO2 on Pocahontas No. 3 coal (0.5% moisture) as a function of increasing CO2 pressure up to 8.0 MPa at 328 K.

Nelson and Borkman37 showed that the ν2 split was large when CO2 interacted with carbonyl functionalities (∼6 cm-1) found in polymers but that the ν2 split was small when CO2 interacted with aromatic rings (∼1 cm-1). They concluded that CO2 interaction with carbonyl groups was favored over aromatic structures in the polymers they examined. The ν2 CO2 absorption bands we observed were broad (Figure 4). If the absorption band was splitting, the two bands were not clearly separated. According to Nelson, the presence of several surface sites can make the observed splitting of the bending mode of CO2 complicated.37 In our case, the heterogeneous nature of the coals may mask the ν2 splitting. Although there is some ambiguity in the ν2 region for coals, the ν3 and ν2 absorption bands taken together tend to support surface sorption without specific interactions with the oxygen functional groups in the coal. This conclusion is in agreement with Walker et al.,38 who did not find any significant chemical interaction of the CO2 molecule with oxygen functionalities in coal. This is in contrast to the results of Nishino,39 who found a correlation between the carboxyl functionality and CO2 adsorption. The lack of ATR-FTIR evidence for specific interactions may suggest that the observed correlation was due to both surface area and carboxyl functionality increasing with decreasing rank. The interaction of the Argonne Coals with increasing CO2 pressure was also examined to determine the shape of the CO2 sorption isotherms. Pressures up to 8.0 MPa were investigated at 328 K. Absorption spectra of CO2 sorbed on Pocahontas No. 3 coal are shown in Figure 5 as a function of increasing CO2 pressure. As the CO2 pressure increased, the intensity of the absorption band at 2332 cm-1 due to sorbed CO2 increased. The BeulahZap coal produced similar sorbed-CO2 spectra as a function of pressure (spectra not shown). Because ATRFTIR allows for the direct observation of sorbed CO2, the isotherm shape can be derived by plotting the net spectral absorbance of sorbed CO2 versus pressure. The ν3 stretching mode of sorbed CO2 was integrated from 2360 to 2300 cm-1 and the area was plotted as a function of pressure.21 The sorption isotherms generated for Pocahontas No. 3 and Beulah-Zap coals of two (38) Walker, P. L., Jr.; Verma, S. K.; Rivera-Utrilla, J.; Khan, M. R. Fuel 1988, 67, 719-726. (39) Nishino, J. Fuel 2001, 80, 757-764.

Carbon Dioxide Sorption on Argonne Premium Coals

Figure 6. ATR-FTIR sorption isotherms of CO2 on Argonne Premium Coals at 328 K and pressure up to 8.0 MPa: (4) Coals that were nitrogen dried at 295 K (0.5% moisture for Pocahontas No. 3, 8.0% moisture for Beulah-Zap); (9) coals that were dried at 353 K (0.1% moisture for Pocahontas No. 3, 0.2% moisture for Beulah-Zap); (s) sorption isotherm predicted by the Langmuir equation (eq 1).

moisture contents are shown in Figure 6. The plots show the shape of the CO2 isotherm curve as a function of pressure by use of integrated spectral area units. For both Pocahontas No. 3 and Beulah-Zap coals, more CO2 was sorbed on the drier coal, although this did not become obvious for the high-rank coal until pressures over 3.5 MPa were attained. These results are consistent with the reports that moisture plays a role in determining the CO2 adsorption capacity of coals.40-42 If the sorption sites for the coals with different moisture contents are the same, water may compete for adsorption sites, resulting in less CO2 sorption. Typically, volumetric or manometric techniques are used to measure the CO2 sorption isotherm for coals.43 The amount of CO2 sorbed on the coal, also known as the Gibbs9 excess sorption, is calculated from the real gas law. The gas compressibility coefficient is required to determine the mass balance. This parameter describing the nonideality of the gas must be known with the highest degree of precision because the shape of the CO2 sorption isotherm and the calculated amount of excess sorption are affected by the choice of the compressibility factor.16 The most recent and widely accepted method (40) Clarkson, C. R.; Bustin, R. M. Int. J. Coal Geol. 2000, 42, 241271. (41) Ozdemir, E.; Schroeder, K.; Morsi, B. I.; White, C. W. Mechanism of Carbon Dioxide (CO2) Adsorption on Moist Coals. Prepr. Symp.sAm. Chem. Soc., Div. Fuel Chem. 2003, 47 (1). (42) Krooss, B. M.; Gensterblum, Y.; Siemons, N.; van Bergen, F.; Pagnier, H. J. M.; David, P. High-Pressure Methane and Carbon Dioxide Adsorption on Dry and Moisture-Equilibrated Carboniferous Coals. Proceedings: International Coalbed Methane Symposium, University of Alabama, Tuscaloosa, AL, May 14-18, 2001; pp 177191. (43) Mavor, M. J.; Owen, L. B.; Pratt, T. J. Measurement and Evaluation of Coal Sorption Isotherm Data. Presented at the 65th Annual Technical Conference and Exhibition, Society of Petroleum Engineers, New Orleans, LA, September 23-26, 1990; SPE Paper No. 20728, pp 157-70.

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for computing the compressibility coefficients of carbon dioxide is the formulation published by Span and Wagner.8 To obtain the absolute adsorption, the increasingly larger volume occupied by the sorbed CO2 as the pressure increases must be calculated. Typically the adsorbed layer density is assumed to be constant, and different authors use various values.9 In this study, the spectral absorption intensity of the sorbed CO2 is directly proportional to the absolute amount of CO2 sorbed. Therefore, neither a compressibility factor nor an adsorbed layer density is needed to determine the shape of the absolute sorption isotherm. Ideally, it would be preferable to be able to convert the spectral absorption intensity to molar absolute sorption (millimoles per gram (mmol/g)) by applying Beer’s law.44-46 In our study, the experimental apparatus does not provide sufficient control over all of the parameters needed to do this. First, uniform coal coverage on the ZnSe rod is difficult to achieve. While this does not change qualitative information such as the peak position and the shape of the isotherm curve, it does affect the quantitative properties of the absorption peak. Second, the extinction coefficient of the sorbed CO2 and the refractive index of the coal must be estimated. While these approximations have been successfully applied when calculating the amount of CO2 dissolved in polymers by ATR-FTIR,47 the errors are too large to apply Beer’s law to the ATR-FTIR data reported here. Thus, at present sorption isotherm measurements will still have to be carried out with traditional techniques for calibration purposes. However, knowledge of the shape of the absolute sorption isotherm itself provides useful information. Figure 7 compares the CO2-coal sorption isotherms generated by a manometric technique16 and ATR-FTIR spectroscopy. The data were fit to the Langmuir equation (solid lines, Figure 7):

θ)

bP 1 + bP

(1)

where b is the Langmuir constant, P is pressure, and θ is the fractional sorption.48 The unitless parameter θ was used to directly compare the two techniques because the ATR-FTIR data are in units of integrated absorbance, which cannot be converted to millimoles per gram without additional information. The similarity of the curves provides confidence in the values selected for the gas-phase density (Span and Wagner EOS)8 and adsorbed layer density (23.45 mmol/cm3) that were used to calculate the absolute adsorption from the manometric data. The energy of adsorption (Q) was estimated from the Langmuir constant b in eq 1 by use of

b ) b0 exp

Q (RT )

(2)

(44) Urban, M. W. Attenuated Total Reflectance Spectroscopy of Polymers; American Chemical Society: Washington, DC, 1953. (45) Mirabella, F., Jr. Internal Reflection Spectroscopy Theory and Applications; Marcel Dekker: New York, 1993. (46) Harrick, N. J. Internal Reflection Spectroscopy; Harrick Scientific Corporation: New York, 1979. (47) Flichy, N. M. B.; Kazarian, S. G.; Lawrence, C. J.; Briscoe, B. J. J. Phys. Chem. B 2002, 106, 754-759.

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Goodman et al.

by the ATR-FTIR technique compared favorably with the 20.0 kJ/mol average calculated from the data obtained by the manometric technique. These values are consistent with CO2 physisorption. The values compare well with CO2 isosteric heats of adsorption that have been reported to range between 25 and 27 kJ/mol for the Argonne Coals.10,50 Isosteric heats are obtained under conditions approaching infinite dilution and are expected to be higher than the Langmuir values because the most exothermic sites are expected to be occupied first. The fact that the adsorption energies are similar for the two different coal types even though lignite coals (Beulah-Zap) are known to have larger concentrations of organic oxygen than low-volatile bituminous coals (Pocahontas No. 3)51,52 further supports the ATR-FTIR data that CO2 sorption is not related to oxygen functionality. In summary, we find that sorption of CO2 is energetically similar for the two coal types, is due only to London forces and quadrupole interactions, and occurs preferentially on a hydrocarbon site. Conclusion Figure 7. Comparison of ATR-FTIR and manometric sorption isotherms of CO2 on Argonne Premium Coals at 328 K and pressures up to 8.0 MPa. All samples were dried prior to CO2 sorption for 36 h at 353 K. (9) ATR-FTIR data; (O) manometric data; (s) sorption isotherm predicted by the Langmuir equation (eq 1).

where R is the gas constant, T is the temperature, and b0 is a constant defined as

b0 )

Nστ0 (2πMRT)1/2

(3)

where N is Avogadro’s constant, σ is the molecular area49 of CO2 (0.22 nm2), τ0 equals 1 × 10-13 s, and M is the molecular weight of CO2. The energy of adsorption values are listed in Table 1. Nearly identical values were obtained regardless of coal type, moisture content, or technique. The average value of 19.8 kJ/mol obtained (48) Adamson, A. W. Physical Chemistry of Surfaces; Wiley and Sons: New York, 1990; p 662. (49) McClellan, A. L.; Harnsberger, H. F. J. Colloid Interface Sci. 1967, 23, 577-599.

The direct interaction between CO2 and two Argonne Premium Coals was probed by ATR-FTIR spectroscopy at 328 K. The sorbed CO2 absorption bands at 2335 cm-1 (Beulah-Zap) and 2332 cm-1 (Pocahontas No. 3) and an energy of adsorption of ∼20 kJ/mol were consistent with a mechanism involving physical adsorption. The spectral data for CO2 sorbed on the coals indicated that there was one type of sorption site for CO2. No evidence for specific interactions between CO2 and oxygen functionalities was found. The pressure dependence of the IR data was used to calculate the CO2-coal adsorption isotherm while avoiding assumptions about the gas compressibility and the adsorbed layer density needed in manometric techniques. Acknowledgment. We are grateful to Dr. John W. Larsen for his help and advice. EF0498824 (50) Glass, A. S.; Larsen, J. W. Energy Fuels 1994, 8, 284-285. (51) Solum, M. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989, 3, 187-193. (52) Kelemen, S. R.; Kwiatek, P. J. Energy Fuels 1995, 9, 841-848.