Energy & Fuels 2005, 19, 1759-1760
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Communications Structure Changes in Pittsburgh No. 8 Coal Caused by Sorption of CO2 Gas A. L. Goodman,*,† R. N. Favors,†,‡ M. M. Hill,§ and John W. Larsen# U.S. Department of Energy, National Energy Technology Laboratory, Pittsburgh, Pennsylvania 15236, Chemistry Department, Purdue University, West Lafayette, Indiana 47907, Environmental Science, Indiana University, Bloomington, Indiana 47405, and The Energy Institute, 209 Academic Projects, The Pennsylvania State University, University Park, Pennsylvania 16802 Received February 28, 2005. Revised Manuscript Received May 10, 2005 After Pittsburgh No. 8 coal has been exposed once to CO2 at 55 °C and 0.35 MPa, subsequent CO2 sorption is much faster than the initial uptake and the amount of CO2 sorbed increases. Therefore, the exposure of Pittsburgh No. 8 coal to CO2 under these conditions results in changes in coal physical structure. The sorption of CO2 by Pittsburgh No. 8 coal was studied using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). The initial CO2 uptake is Fickian. For coals that rearrange, the information gained by studying fresh coals may not be useful to CO2 sequestration, because their structure will be different after exposure to CO2. It is necessary to gather data on coals already rearranged, because of CO2 exposure. For CO2 sequestration in coal seams and for enhanced coal bed methane recovery, it is crucial to understand the amount of CO2 that dissolves in coals, the amount of CO2 that is adsorbed on coal surfaces, and the permeability of coal seams to gases. It is also necessary to identify and characterize any changes in coal structure caused by the CO2. At the pressures we are using, it is expected that more CO2 will be dissolved in the coal than will be adsorbed on the coal surface.1 It is well-established that the dissolution of organic fluids in coals plasticizes them and enables the occurrence of physical structure changes.2,3 These structure changes can, in turn, affect both diffusion and solubility in coals. Three pathways for the movement of CO2 through a coal seam exist. There is Darcy flow through cleats and cracks. This is the most rapid pathway. This pathway can be blocked if the CO2 dissolves in coals and swells them, thereby closing the cracks and cleats.4,5 There also exists a relatively rapid Fickian diffusion of CO2 in coals to reach the coal pores.6,7 The third diffusion pathway in coals is Case II diffusion, in * Author to whom correspondence should be addressed. Telephone: 1-412-386-4962. Fax: 1-412-386-5920. E-mail address:
[email protected]. † U.S. Department of Energy, National Energy Technology Laboratory. ‡ Purdue University. § Indiana University. # The Pennsylvania State University. (1) Reucroft, P. J.; Sethuraman, A. R. Energy Fuels 1987, 1, 72-75. (2) Hsieh, S. T.; Duda, J. L. Fuel 1987, 66, 170-178. (3) Larsen, J. W.; Flowers, R. A., II; Hall P. J.; Carlson, G. Energy Fuels 1997, 11, 998-1002. (4) Karacan, C. O. Energy Fuels 2003, 17, 1595-1608. (5) Skawinski, R. Arch. Min. Sci. 1999, 44, 425-434. (6) Larsen, J. W.; Wernett, P. C. Energy Fuels 1988, 2, 719-720. (7) Larsen, J. W.; Hall, P.; Wernett, P. C. Energy Fuels 1995, 9, 324330.
Figure 1. Time dependence of the CO2 concentration in the top 1 µm of Pittsburgh No. 8 coal at 55 °C and 0.35 MPa, obtained by measuring the area of the 2333 cm-1 CO2 IR band with the Spectra Tech ATR cell: (b) first exposure to CO2 and (O) second exposure to CO2 after evacuation for 1 h and nitrogen passivation for 2 h. Lines are the best least-squares fit of a polynomial to the data points.
which the rate-determining step is the motion of the coal macromolecules.8 The concentration of CO2 sorbed was measured using ATR-FTIR. Two different ATR cells were used. The use of a modified high-pressure ATR cell with a ZnSe crystal has been described (Spectra Tech ATR Cell),9 and a diamond ATR cell was also used (Specac ATR Cell). Experimental details are provided in the Supporting Information. It is important that the depth sampled is at least 1 µm (see Supporting Information for the calculation of this depth). This ensures that both the bulk and surface of the sample are interrogated. Concentrations of CO2 in the top 1 µm of the coal were measured using the peak area of the CO2 infrared (IR) band at 2333 cm-1. Diffusion rates were measured using the time dependence of the area of the 2333 cm-1 band. Surface adsorption is effectively instantaneous, in comparison to dissolution. The time dependence of the CO2 concentration in the top 1 µm of Argonne Premium Pittsburgh No. 8 coal at 55 °C and a pressure of 0.35 MPa is shown in Figure 1. These data were obtained using the Spectra Tech ATR cell (see Supporting Information for details). The lower points were obtained using a coal sample that had not (8) Ritger, P. L.; Peppas, N. A. Fuel 1987, 66, 815-826. (9) Goodman, A. L.; Campus, L. M.; Schroeder, K. T. Energy Fuels 2005, 19, 471-476.
10.1021/ef050051n CCC: $30.25 © 2005 American Chemical Society Published on Web 06/01/2005
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Figure 2. Time dependence of the CO2 concentration in the top 1 µm of Pittsburgh No. 8 coal at 55 °C and 0.35 MPa, obtained by measuring the area of the 2333 cm-1 CO2 IR band with the Specac ATR cell: (9) first exposure to CO2 and (0) second exposure to CO2 after 30 min of evacuation, followed by exposure to 8.78 MPa CO2 for 1650 min, followed by evacuation for 30 min. Lines are the best least-squares fit of a polynomial to the data points.
been previously exposed to CO2 gas. The upper set of points was obtained on the second exposure of the coal sample to CO2 under the same conditions. Between the two exposures, the ATR cell was evacuated for 3 h and all of the CO2 was removed from the coal, as evaluated by the disappearance of the 2333 cm-1 IR band. Two major changes are obvious. First, much more CO2 is sorbed on the second exposure. This higher sorption capacity is likely due to the loss of coal moisture between the first and second exposures, as discerned by the decrease of the IR hydroxyl band intensity. It is well accepted that the CO2 adsorption capacity of drier coals is higher than those of as-received coals.10-12 Second, the diffusion of the CO2 into the coal is much faster, instantaneous on the time scale of this experiment. Exposure of this coal to 0.35 MPa of CO2 gas at 55 °C is sufficient to cause structure changes that alter both the rate and amount of CO2 sorbed. A similar experiment was performed using the Specac ATR cell (see Supporting Information for details). The time dependence and spectral data are shown in Figures 2 and 3, respectively. The results are the same. The transport mechanism of CO2 in the Pittsburgh No. 8 coal was better understood by applying the mathematical theory developed by Peppas and Ritger.8 The initial uptake data were plotted as Mt/M∞ vs t1/2 and Mt/M∞ vs t, where Mt and M∞ are the peak areas of the 2333 cm-1 IR band at time t and ∞, respectively. Fickian diffusion is diagnosed by a straight-line plot of Mt/M∞ vs t1/2, whereas Case II diffusion generates a straight-line plot of Mt/M∞ vs t.8 Fickian diffusion was observed in both cases (see Supporting Information for details). The diffusion of CO2 out of swollen Pittsburgh seam coal is Fickian.13 If the coal was rearranging as part of the CO2 diffusion process, the diffusion kinetics would be Case II.8 The fact that they are not demonstrates that the rearrangement occurs after the initial uptake. The rearrangement is not reversed as the CO2 is removed from the coal.
Communications
Figure 3. Time-dependent spectral data of sorbed CO2 on Pittsburgh No. 8 coal after the first exposure of CO2 at 0.35 MPa and 55 °C measured with the Specac ATR cell.
This observed structure change is not specific to CO2. The probable mechanism is plasticization of the coal by dissolved CO2, which enables a rearrangement in the coal’s physical structure. The sorption of toluene at low pressures is known to cause a structure rearrangement that alters subsequent toluene adsorption isotherms.2 The temperature dependence of the heats of wetting of coals in tetralin requires a fast coal structure rearrangement.14 Refluxing Pittsburgh No. 8 coal in chlorobenzene causes large structure changes.3 Solvent swelling causes structure changes in Pittsburgh No. 8 coal.15,16 CO2 is known to dissolve in Pittsburgh No. 8 coal and an elegant X-ray study demonstrates structure changes in the vitrinite as a result of CO2 dissolution.4 Coal structure alterations as a result of plastization by dissolved CO2 are probably not universal; however, data exist to show that they occur with many coals.17 A recent study of CO2 adsorption isotherms in several Argonne Premium coals revealed that the first and second isotherms measured on the same sample are different.18 It was suggested that either extraction or a macromolecular structure change might be responsible for this effect. A change in macromolecular structure caused by dissolved CO2 is the best explanation. CO2 is a good plasticizer for many coals. Exposure to CO2 gas changes subsequent CO2 diffusion rates, CO2 adsorption isotherms, and CO2 solubility. The measurement on a single coal sample of sequential adsorption isotherms, sequential diffusion rates measured, or sequential measurements of CO2 may be affected by changes in the coal structure caused by the initial exposure of the coal to CO2. Data obtained using fresh (unexposed to CO2) coal samples may be irrelevant to CO2 sequestration because exposure to CO2 may induce structure changes and the coal may be different before and after the initial CO2 exposure. Supporting Information Available: Experimental details of ATR-FTIR analyses of Pittsburgh No. 8 coal, using the Spectra Tech ATR cell and the Specac ATR cell, including figures and a list of cited references (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.
EF050051N (10) Clarkson, C. R.; Bustin, R. M. Int. J. Coal Geol. 2000, 42, 241271. (11) Krooss, B. M.; van Bergen, F.; Gensterblum, Y.; Siemons, N.; Pagnier, H. J. M.; David, P. Int. J. Coal Geol. 2002, 51, 69-92. (12) 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. (13) Nandi, S. P.; Walker, P. L., Jr. Fuel 1964, 43, 385-393.
(14) Larsen, J. W.; Kuemmerle, E. W. Fuel 1978, 57, 59. (15) Cody, G. D., Jr.; Larsen, J. W.; Siskin, M. Energy Fuels 1988, 2, 340-344. (16) Yang, X.; Larsen, J. W,; Silbernagel, B. G. Energy Fuels 1993, 7, 439-445. (17) Larsen, J. W. Int. J. Coal Geol. 2004, 57, 63-70. (18) Busch, A.; Gensterblum, Y.; Krooss, B. M. Int. J. Coal Geol. 2003, 55, 205-224.
