Energy & Fuels 2006, 20, 2537-2543
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Argonne Coal Structure Rearrangement Caused by Sorption of CO2 A. L. Goodman* U.S. Department of Energy, National Energy Technology Laboratory, Pittsburgh, PennsylVania 15236
R. N. Favors Chemistry Department, Purdue UniVersity, West Lafayette, Indiana 47907
John W. Larsen The Energy Institute, 209 Academic Projects, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802 ReceiVed April 28, 2006. ReVised Manuscript ReceiVed August 24, 2006
The exposure of powdered unconfined coals to CO2 results in changes in the coals’ physical structures. The presence of water changes the behavior of the coals on exposure to CO2. The sorption of CO2 on seven Argonne premium coals was measured by using attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy as a function of time at constant CO2 pressure (∼0.62 MPa) and temperature (55 °C). The depth sampled is at least 1 µm, and this ensures that both the bulk and surface of the sample were interrogated. Concentrations of CO2 in the top 1-7 µm of the coal were measured by using the peak area of the CO2 ATR-FTIR band near 2333 cm-1. Diffusion rates were measured by using the time dependence of the area of the 2333 cm-1 band. Surface adsorption is effectively instantaneous. The coals were either extensiVely or briefly dried. The coals were exposed to CO2, evacuated, and then exposed to CO2 a second time. For the extensively dried coals, removal of the CO2 under vacuum was much faster than CO2 sorption, indicating a coal structure change caused by CO2 sorption. The diffusion rate of the CO2 into the coal was much faster for the second exposure, confirming that the coal underwent a physical structure rearrangement. Structure rearrangement was observed for all seven extensively dried coals and for the Pittsburgh No. 8 briefly dried coal. The presence of residual moisture in the briefly dried coal samples appeared to inhibit or block CO2 uptake as equilibrium was reached within minutes. The second exposure of the briefly dried coals universally resulted in greater CO2 uptake that was again instantaneous on our experimental time scale.
Introduction In many circumstances, it is acceptable to think of a coal as a brittle combustible rock. However, there are situations where this view will lead to error. One of those situations occurs whenever a coal comes into contact with a fluid that is soluble in the coal. Then, the coal is best regarded as a glassy, organic, polymeric solid, requiring the application of polymer chemistry concepts. Because CO2 is soluble in coals and capable of swelling coals, treating the coal as a more-or-less rigid solid is inappropriate and will often lead to errors.1-5 We will begin by sketching the effects of soluble fluids on coals and then proceed to the specific case of CO2. Figure 1 shows what happens when a coal is exposed to the vapors of a compound that is soluble in the coal. It behaves as a glassy strained polymer.6 First, there is very fast adsorption of the compound onto the coal surface. The adsorbed molecules * To whom correspondence should be addressed. Tel.: 1-412-386-4962. Fax: 1-412-386-5920. E-mail address:
[email protected]. (1) Sethuraman, A. R.; Reucroft, P. J. Prepr. Pap., Am. Chem. Soc., DiV. Fuel Chem. 1987, 32, 259-264. (2) Reucroft, P. J.; Sethuraman, A. R. Energy Fuels 1987, 1, 72-75. (3) Reucroft, P. J.; Patel, H. Fuel 1986, 65, 816-820. (4) Reucroft, P. J.; Patel, K. B. Fuel 1983, 62, 279-284. (5) Walker, P. L., Jr.; Verma, S. K.; Rivera-Utrilla, J.; Khan, M. R. Fuel 1988, 67, 719-726. (6) Hsieh, S. T.; Duda, J. L. Fuel 1987, 66, 170-178.
Figure 1. Rate of toluene uptake by a pyridine extracted bituminous coal (71.0%, daf; from ref 6).
immediately begin to diffuse into the coal making it difficult to distinguish between pure adsorption and a mixture of adsorption and absorption.7 At significantly longer times, the coal begins to undergo a rearrangement of its physical structure. Because this rearrangement involves large-scale motion of the coal macromolecules, it is usually a slow process. For materials (7) Glass, A. S.; Larsen, J. W. Energy Fuels 1994, 8, 284-285.
10.1021/ef060188t CCC: $33.50 © 2006 American Chemical Society Published on Web 10/05/2006
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that are not soluble in a coal, adsorption can be treated using the techniques well-developed for a material of fixed surface area.8 We will now address diffusion through coals and coal structure rearrangements. The mathematics of the two diffusion pathways through coals have been adequately developed.9,10 We differentiate coal permeability that involves Darcy flow through cleats and cracks and diffusion through solid coal that involves the pore system and molecules dissolved in the solid. For diffusion through solid coal, there are two diffusion processes: Fickian and case II. In Fickian diffusion, the rate-determining step is movement of the diffusing molecule. There is a broad concentration gradient between “pure” coal and coal containing the equilibrium amount of the dissolved molecule. The mass uptake of the diffusing molecule increases as t1/2 (t is time) if the solid is a slab. Case II diffusion is quite different. Now, the rate-determining step is movement of the coal macromolecules that must be displaced to make room for the diffusing molecule. The concentration gradient is a sharp boundary. On one side of the boundary is pure coal, and on the other side is essentially fully swollen coal. The mechanical strains caused by this sharp boundary are large and capable of fragmenting or tearing coal particles. The increase in mass of a polymer slab is directly proportional to time for case II diffusion. There is also the inevitable mixture of the two diffusion mechanisms called anomalous diffusion. All three diffusion mechanisms, Fickian, case II, and anomalous diffusion have been observed as organic molecules diffuse into coals, swelling them.11-13 Coals are glassy solids, and the temperature at which they become rubbery (Tg) lies above their decomposition temperature.15-17 When swollen by a dissolved molecule, Tg decreases, coal properties change, and the coal may undergo a structural rearrangement.6,14 When greatly swollen, rubbery coals have been observed and the rubbery state has been partially characterized.18,19 Molecules dissolved in coals are plasticizers. By creating additional free volume in the solid, they enable molecular motion and a rearrangement of the coal physical structure, even when present at low concentrations.6 The rate at which the rearrangement occurs depends on the concentration of the dissolved molecule, the temperature, and the identity of the coal.10,11,20,21 Large structure changes are possible.11 This rearrangement remains largely uncharacterized. Carbon dioxide dissolves in coals and swells them.1-5,22 This complicates measurements of CO2 “adsorption” on coals at any (8) Lowell, S. Introduction of Powder Surface Area; Wiley-Interscience: New York, 1979. (9) Ritger, P. L.; Peppas, N. A. Fuel 1987, 66, 1379-1388. (10) Brenner, D.; Hagan, P. S. Prepr. Pap., Am. Chem. Soc., DiV. Fuel Chem. 1985, 30 (1), 71-82. (11) Otake, Y.; Suuberg, E. M. Fuel 1989, 68, 1609-1612. (12) Olivares, J. M.; Peppas, N. A. Chem. Eng. Commun. 1995, 132, 91-106 and references therin. (13) Green, T.; Kovac, J.; Brenner, D.; Larsen, J. W. In Coal Structure; Meyers, R. A., Ed.; Academic Press: New York, 1995; pp 199-282. (14) Cody, G. D. J.; Larsen, J. W.; Siskin, M. Energy Fuels 1988, 2, 340-344. (15) Painter, P. C.; Graf, J.; Coleman, M. M. Energy Fuels 1990, 4, 379-384. (16) Painter, P. C.; Park, Y.; Sobkowiak, M.; Coleman, M. M. Energy Fuels 1990, 4, 384-393. (17) Painter, P. C.; Graf, J.; Coleman, M. M. Energy Fuels 1990, 4, 393-397. (18) Brenner, D. Nature 1984, 306 (5945), 772-773. (19) Yang, X.; Larsen, J. W.; Silbernagel, B. G. Energy Fuels 1993, 7, 439-445. (20) Barr-Howell, B. D.; Howell, J. M.; Peppas, N. A. Thermochim. Acta 1987, 116, 153-159. (21) Suuberg, E. M.; Otake, Y.; Langner, M. J.; Leung, K. T.; Milosavljevic, I. Energy Fuels 1994, 8, 1247-1262. (22) Karacan, C. O. Energy Fuels 2003, 17, 1595-1608.
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pressure. Carbon dioxide has long and extensively been used to measure coal surface areas.23 At the low pressures required by the BET technique,8 the solubility of CO2 in coals is so low that the errors introduced by its solubility are probably acceptably low. But, the solubility of CO2 provides a diffusion pathway not available to molecules insoluble in coals, for example, ethane. The differences between the surface areas measured using CO2 and ethane, molecules of closely similar size and shape, are large and are the basis for the claim that coal pores do not form an extended interconnected system.24 This view has been criticized25,26 and supported,27,28 but the critics have never proposed a satisfactory explanation for the large differences in surface area measured by CO2 and ethane under identical conditions. A recent paper includes an estimate of the degree of pore interconnectivity.28 At the pressures used in sequestration, CO2 dissolves in coals and swells them.22 Confined coals have been studied by using X-ray tomography.22 Different macerals absorb different amounts of CO2 and some swell, compressing the others in this confined system. Structure rearrangements of the confined coals have also been observed. The reasons for the greater coal solubility of CO2 than ethane are not clear, unless it is due to the CO2 quadrupole moment. The proton affinity of CO2 is close to that of methane, and it is not protonated in superacid.29,30 It is not a basic molecule. Its interaction with amphoteric molecules such as water is limited to nucleophilic attack on the electron deficient CO2 carbon as in the formation of carbonic acid in water.31 This interaction is easily detected by changes in the CO2 IR carbonyl band.32 There is no evidence for this interaction in coals.33 By elimination, CO2 does not participate in acid-base interactions with coals. The enthalpy of its interaction with Illinois No. 6 coal surfaces is similar to ethane’s, a nonpolar molecule of similar size and shape, but where ethane interacts solely by London dispersion interactions, the CO2 interaction is composed of a dispersion component and another component.34 The only possibility for that other component seems to be the CO2 quadrupole moment. We are not entirely satisfied by this and believe that work to establish the origin of the interactions between CO2 and coals is desirable. The fact that CO2 swells coals complicates volumetric and gravimetric measurements of CO2 sorption.35 The coal volume change affects the sample weight because the size change results in an unknown buoyancy change that is significant. The change in solid sample volume has a deleterious effect on volumetric sorption measurements by altering the free volume in the measurement apparatus. For this reason, we use infrared spectroscopy to measure CO2 uptake by coals.33 This measure of CO2 uptake is not subject to errors caused by coal swelling. For reliable CO2 sequestration in coals, the transport (including diffusion) of CO2 in coals, the solubility of CO2 in coals, (23) Larsen, J. W. Intl. J. Coal Geol. 2004, 57, 63-74. (24) Larsen, J. W.; Wernett, P. C. Prepr. Pap., Am. Chem. Soc., DiV. Fuel Chem. 1992, 37 (2), 849-855. (25) Walker, P. L. J.; Mahajan, O. P. Energy Fuels 1993, 7, 559-560. (26) Mahajan, O. P. Carbon 1991, 29, 735-742. (27) Hall, P. J.; Brown, S. D.; Calo, J. M. Fuel 2000, 79, 1327-1332. (28) Kelemen, S. R.; Kwiatek, L. M.; Siskin, A. Energy Fuels 2006, 20, 205-213. (29) Fehsenfeld, F. C.; Lindinger, W.; Schift, H. I.; Hemsworth, R. S.; Bohme, D. K. J. Chem. Phys. 1976, 64, 4887-4891. (30) Olah, G. A.; Shen, J. J. Am. Chem. Soc. 1973, 95, 3582-3584. (31) Falk, M.; Miller, A. G. Vib. Spectr. 1992, 4, 105-108. (32) Maiella, P. G.; Schoppelrei, J. W.; Brill, T. B. Appl. Spectrosc. 1999, 53, 351-355. (33) Goodman, A. L.; Campus, L. M.; Schroeder, K. T. Energy Fuels 2005, 19, 471-476. (34) Glass, A. S.; Larsen, J. W. Energy Fuels 1994, 8, 629-636.
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Energy & Fuels, Vol. 20, No. 6, 2006 2539
and the effect of dissolved CO2 on coal structure must all be at least characterized and preferably understood. Temporal, thermal, and pressure effects must all be characterized. Because the structure changes maybe very slow, the task may be difficult. In this work, we report on the kinetics of CO2 uptake by several coals and report that the initial exposure to CO2 results in alteration of the coal structure and that this alteration depends on the presence of water. Experimental Section The following seven Argonne premium coals were investigated: Upper Freeport (medium volatile bituminous), Pittsburgh No. 8 (high volatile bituminous), Lewiston-Stockton (high volatile bituminous), Blind Canyon (high volatile bituminous), Illinois No. 6 (high volatile bituminous), Wyodak (sub-bituminous), and Beulah Zap (lignite). The Argonne premium coals are the highest quality samples available.36,37 These coals are well characterized, providing the research community with a large database of physical and chemical properties. The samples were obtained as powders (-100 mesh). Experiments were conducted using a modified ATR (attenuated total reflectance) CIRCLE cell from Spectra Tech as described previously.33 All ATR-FTIR data were collected with a single beam Fourier transform infrared (FTIR) spectrometer (Thermo Electron Nexus 670 FTIR ESP) equipped with a wide-band mercury cadmium telluride (MCT) detector. Unless otherwise noted, 250 scans were collected with an instrument resolution of 4 cm-1 over the spectral range extending from 4000 to 600 cm-1. The Spectra Tech cell uses a ZnSe crystal, which allows 11 reflections. The penetration depth38 (dp) of the infrared light into the coal sample is estimated by eq 1 where λ1 is the dp )
λ1 2π(sin θ - n212)1/2 2
(1)
wavelength of the incident beam, which ranges between 2335 and 2332 cm-1, divided by the refractive index of the ATR-FTIR crystal (2.4), θ is the incident angle of the infrared beam (48°), and n21 is the refractive index of the coal divided by the refractive index of ATR-FTIR crystal (2.4). The refractive index of the coal depends on the carbon content of the coal. On the basis of data for coal slabs, the refractive index measured with infrared light ranges from 1.6 for coal containing 78.4% carbon to 1.9 for coal containing 90.6% carbon.39,40 The maf carbon content of the Argonne premium coals used in this study ranges from 72.9 to 85.5%.32 Figure 2 shows how the penetration depth changes as the refractive index of the coal changes. Lower rank coals have a low refractive index and a small penetration depth, whereas higher rank coals have a high refractive index and a large penetration depth. On the basis of Figure 2, the minimum penetration depth of the infrared beam into the coal particles is expected to be 1 µm. This ensures that both the bulk and surface of the sample are interrogated. To obtain a distortion free spectrum, it is necessary to for the angle of incidence (θ ) 48°) to be greater than the critical angle (θc). The critical angle is defined by eq 238 θc ) arcsin(n2/n1)
(2)
where n2 is the refractive index of the coal and n1 is the refractive (35) Romanov, V. N.; Goodman, A. L.; Larsen, J. W. Energy Fuels 2006, 20, 215-216. (36) Vorres, K. S. Energy Fuels 1990, 4, 420-426. (37) Vorres, K. S. Users Handbook For The Argonne Premium Coal Sample Program, ANL/PCSP-93/1; U.S. Dept of Energy: 1993. (38) Harrick, N. J. Internal Reflection Spectroscopy; Harrick Scientific Corporation: New York, 1979. (39) Foster, P. J.; Howarth, C. R. Carbon 1968, 6, 719-729. (40) Van Krevelen, D. W. Coal; Elsevier Scientific Publishing Company: The Netherlands, 1981.
Figure 2. Estimated penetration depth (dp) of the infrared light into the coal sample versus coal refractive index (n2) calculated according to eq 1 where n1 ) 2.4, θ ) 48°, and n2 ranges from 1.55 to 1.8.
index of the ATR crystal. Thus, for coals with refractive indices greater than 1.8, the incident angle (θ ) 48°) will be less than the critical angle (θ ) 49°) and the spectra will no longer follow ATR theory. The refractive indices of the Argonne coals are not known using infrared light. To check whether the critical angle conditions were met for these ATR experiments, coal spectra were measured with a Ge ATR (n1 ) 1.9, n2 ) 4.0, θc ) 28°) crystal where the critical angle would never be exceeded and a ZnSe (n1 ) 1.9, n2 ) 2.4, θc ) 52°) crystal were the critical angle is likely to be exceeded. Comparison of the ZnSe and Ge spectra showed the expected peak shifts to lower wavenumbers for the ZnSe spectra versus the Ge spectra.38 Using the ZnSe crystals is justified for the coals selected for this study. However, for future CO2-coal infrared measurements, it is recommended that an ATR crystal with a refractive index greater than 2.4 be chosen for spectral measurements in order to avoid any problems with meeting the critical angle condition. In these experiments, the coals were prepared in a nitrogen-filled glovebag to avoid surface oxidation. The coals were immersed in nitrogen-sparged water to form slurries and mixed for 2 h. The ATR crystal was then coated with a thin layer of the slurry (∼10 mg). The ATR crystal was removed from the glovebag and inserted into the high-pressure stainless steel cell which was sealed by tightening bolts against two Teflon O-rings. The ATR cell was transferred to the FTIR instrument. During the transfer, the coal samples were exposed to air for less than 30 min. Then, the coals were either extensiVely dried or briefly dried. The extensively dried samples were dried under vacuum at room temperature. The dryness of the coals was evaluated by collecting infrared spectra as a function of time to monitor the hydroxyl stretching region between 3800 and 3000 cm-1. When the hydroxyl region showed no changes at room temperature, the samples were dried further at 80 °C under vacuum until the hydroxyl region baseline between 3800 and 3000 cm-1 remained constant. The full drying procedure lasted between 1 and 3 days. Lower rank coals took longer to dry than the higher rank coals. The briefly dried samples were evacuated (1 × 10-2 torr) for 30 min and then stored under nitrogen. After the coal samples were either extensively or briefly dried, the coals were thermally equilibrated at 55 °C in the ATR cell and then exposed to CO2 pressure ranging between 0.21 and 0.62 MPa (30 and 90 psig). The CO2 sorption was monitored with ATR-FTIR by following the growth of the ν3 stretching band of sorbed CO2 (2335-2332 cm-1) versus time at 55 °C. For the first CO2 exposure, the CO2-coal interaction was monitored until equilibrium was reached. In some cases, this took up to 4 days. The sample was then evacuated (1 × 10-2 torr) to remove the sorbed CO2. All of the CO2 was removed from the coal as evaluated by the disappearance of the ν3 stretching band of sorbed CO2 near 2333 cm-1. Complete CO2 removal took between 4 min and 4 h as judged by the disappearance of the CO2 IR band. The sample was then exposed
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Figure 3. Time-dependent spectral data of sorbed CO2 at 2334 cm-1 on extensively dried Wyodak-Anderson coal after the first exposure of CO2 at 0.68 MPa (84.6 psig) and 55 °C.
Figure 4. Evacuation of CO2 from extensively dried WyodakAnderson coal after the first exposure of CO2. Spectra were recorded at 0 and 1 min, 1.1, 2.1, 3.4, and 4.5 h.
to the same pressure of CO2 a second time, and the sorbed CO2 (2335-2332 cm-1 ) was monitored until equilibrium was reached. The CO2 was again removed under vacuum until the sorbed CO2 peak was no longer detected. Between the first and second CO2 exposure, the ATR cell was removed from the FTIR instrument and the bolts were retightened. This was done in order to verify that the Teflon O-rings were sealed. Teflon does not contract well when pressure is cycled between atmosphere and vacuum, so retightening was necessary. During this procedure, it is possible that coal could be lost off of the ATR crystal due to any jarring during the retightening procedure.
Table 1. Approximate Time Required to Remove Half of the Sorbed CO2 from Extensively Dried Coals
Results The interaction of CO2 with the extensively dried Argonne coals was monitored as a function of time by using ATR-FTIR spectroscopy. First, the extensively dried Argonne coal samples were exposed to a known pressure of CO2 (0.62 MPa [90 psig]) at 55°C and then sealed off from the CO2 source. Upon exposure of the coals to CO2, a positive absorption bands between 2335 and 2332 cm-1 appeared in the spectrum. This absorption band has been observed before and is indicative of sorbed CO2 (ν3 antisymmetric stretching mode).33 No evidence for formation of carbonates or any other product of reactions between CO2 and these coals was visible in the IR spectra. Spectra were then recorded until equilibrium was reached, until the ν3 absorption band no longer changed with time. For some Argonne coals, equilibrium was not reached until 4 days had passed. Figure 3 shows representative time-dependent CO2 spectra for WyodakAnderson coal. The amount of CO2 sorbed in the coal continued to increase until the forth day when equilibrium was reached. Next, the CO2 was removed under vacuum. The disappearance of the sorbed CO2 band between 2335 and 2332 cm-1 was monitored as a function of time. But for most coals, the majority of CO2 was lost within the few minutes when the first ATRFTIR spectrum was collected. Thus, only approximate kinetics could be obtained. Figure 4 shows typical ATR spectra of Wyodak-Anderson coal after removal of CO2. The absence of the peak at 2334 cm-1 shows that all of the CO2 has been removed. The approximate times required for removal of half of the CO2 from the coals are given in Table 1. Then, the sample was re-exposed to the same CO2 pressure. The amount of sorbed CO2 was monitored until equilibrium was reached as discussed before. In this set of experiments, the amount of CO2 sorbed reached its maximum value within minutessnot days. The diffusion rates for CO2 in the extensively dried coals were investigated by recording the concentration of the sorbed
coal
t1/2 (min)
Upper Freeport Pittsburgh No. 8 Lewiston Stockton Blind Canyon Illinois No. 6 Wyodak Beulah Zap
1.5
