Energy & Fuels 2006, 20, 415-416
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Communications Errors in CO2 Adsorption Measurements Caused by Coal Swelling Vyacheslav N. Romanov,† Angela L. Goodman,† and John W. Larsen*,‡ U.S. Department of Energy, National Energy Technology Laboratory, P.O. Box 10940, Pittsburgh, PennsylVania 16802, and The Energy Institute, 209 Academic Projects Building, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802 ReceiVed September 27, 2005. ReVised Manuscript ReceiVed December 3, 2005 CO2 dissolves in coals and swells them. At moderate to high pressures, this may result in significant errors in measurements of CO2 adsorption on coals using gas adsorption techniques that work well with rigid solids. With a volumetric apparatus, the coal swelling will change the ratio of container volume to sample volume, thus introducing error. In a gravimetric apparatus, coal swelling will alter the buoyancy of the sample, thus introducing error. In both techniques, the coal swelling may continuously change the surface area as the CO2 pressure changes. In all apparatus, the amount of CO2 dissolved in the coal may exceed the amount adsorbed on the coal.1 Assuming during data analysis that all of the sorbed CO2 is on the coal surface will introduce error. Since coal swelling is dependent on the CO2 pressure, both the coal surface area and the error due to coal swelling will change with CO2 pressure. Coals are organic macromolecular systems well-known to absorb organic liquids and CO2 and to swell.1-8 The swelling mechanism is complex and not well-characterized. Permeability studies revealed that cracks and cleats responsible for high gas permeability are closed by CO2-induced coal swelling.9 Even when confined, vitrinite swells in CO2 and compresses other macerals.10 The effect of coal swelling on coal surface area and pore volume is not known. It is probably not uniform. Nuclear magnetic resonance studies revealed that appreciable regions of the coals studied are not swollen by strongly interacting liquids such as pyridine that swell coal by more than 100%.2 The molar volume of the CO2 dissolved in the coal is not known. At each equilibrium point, there is a solid, larger than the original coal sample, of unknown pore volume, surface area, and CO2 content. The change in accessible pore volume and * Corresponding author. Phone: 215-257-8617. E-mail:
[email protected]. † National Energy Technology Laboratory. ‡ The Pennsylvania State University. (1) Reucroft, P. J.; Sethuraman, A. R. Energy Fuels 1987, 1, 72-75. (2) Norinaga, K.; Iino, M.; Cody, G. D.; Thiyagarajan, P. Energy Fuels 2000, 14, 1245-1251. (3) Larsen, J. W.; Green, T. K.; Kovac, J. J. Org. Chem. 1985, 50, 47294735. (4) Suuberg, E. M.; Otake, Y.; Langner, M. J.; Leung, K. T.; Milosavljevic, I. Energy Fuels 1994, 8, 1247-1262. (5) Reucroft, P. J.; Patel, K. B. Fuel 1983, 62, 279-284. (6) Reucroft, P. J.; Patel, H. Fuel 1986, 65, 816-820. (7) Walker, P. L., Jr.; Verma, S. K.; Rivera-Utrilla, J.; Khan, M. R. Fuel 1988, 67, 719-726. (8) Larsen, J. W. Int. J. Coal Geol. 2004, 57, 63-70. (9) Ceglarska-Stefanska, G.; Czaplinski, A. Arch. Min. Sci. 1991, 16, 369-378. (10) Karacan, C. O. Energy Fuels 2003, 17, 1595-1608.
the increased size of the solid impact both volumetric and gravimetric measurements. In either a volumetric11,12 or gravimetric13 experiment, a sample of solid coal is brought into contact with CO2 gas at known temperature and pressure. If a nonswelling material is studied, or if the CO2 fugacity is low enough to cause little or no swelling, the amounts of CO2 in the gas phase or adsorbed on the surface can readily be determined by using standard volumetric or gravimetric techniques. With coals at equilibrium with CO2 pressures above a few atmospheres, CO2 is present in three different states: the free fluid phase, adsorbed on the coal surface, and dissolved in the coal, the solid coal having expanded and changed its pore structure due to mixing with the acidic fluid CO2. In the volumetric technique, the free (accessible) volume in the container and the pressure are measured. Usually, the accessible volume is identified with the initial He volume.14 Coal swelling changes the free volume by swollen coal expanding into it. In the gravimetric technique, the necessary correction for sample buoyancy cannot be made because the inaccessible volume of the coal sample has increased by an unknown amount. The amount of swelling will increase with CO2 pressure, and therefore the errors introduced by ignoring swelling will usually increase with increasing CO2 pressure.1 Without knowing the swelling amount, accurate measurements of CO2 sorption by coals are not possible. The magnitude of the effect of sample swelling can be seen in Figure 1. The excess adsorption of CO2 on dried Argonne Pocahontas No. 3 coal measured gravimetrically at 328 K without correction for coal swelling is shown as a function of CO2 pressure (Figure 1, initial volume). Pocahontas No. 3 coal is a thoroughly studied high-rank hydrophobic coal exhibiting little or no effects of moisture.15 For each point, readings were taken after the 100 mesh samples had been stable for an hour. The gravimetric measurements were done using a magnetic suspension balance of Rubotherm GmbH at the Leipzig University, Germany. At the end of the experiment, the volume (henceforth, we mean the volume inaccessible to He) of the (11) Lu, X.-C.; Li, F.-C.; Watson, A. T. Fuel 1995, 74, 599-603. (12) Ruppel, T. C.; Grein, C. T.; Beinstock, D. Fuel 1972, 51, 297303. (13) Humayun, R.; Tomasko, D. L. AIChE J. 2000, 46, 2065-2075. (14) Romanov, V. N.; Soong, Y.; Schroeder, K. Chem. Eng. Technol. 2006, 29, in press. (15) Vorres, K. S. Users Handbook for the Argonne Premium Coal Sample Program NTIS ANL/PCSP-93/1; Argonne National Lab: Argonne, IL, 1993.
10.1021/ef050318i CCC: $33.50 © 2006 American Chemical Society Published on Web 12/27/2005
416 Energy & Fuels, Vol. 20, No. 1, 2006
Figure 1. Gravimetric experimental data reduced to the excess (Gibbs) sorption using the sample volume values measured before the exposure to CO2 (initial volume) and after the CO2 was removed (final volume). Pocahontas No. 3 sample was tested at 55 °C.
Communications
volume, which is at least 10% more than that observed at the end of the gravimetric experiment (Figure 1, final volume). The degree of swelling may depend on the time allowed to reach equilibrium because CO2 uptake by coal can be a slow process. Alternatively, shrinkage of the coal matrix during the volumetric experiment would have a similar effect. The excess sorption plots for these two experiments have inflection points at around 8 and 12 MPa. The observed pattern is similar to some published data sets17-19 and different from several others.20-27 The reason for the discrepancy is not known. Coal swelling causes other problems. At pressures above 1-1.5 MPa, there may be more CO2 dissolved (mixed) in the coal than adsorbed on the surface, and thus the necessary corrections may be similar in magnitude to the actual adsorption.1 The surface area of the coal also changes in an unknown way with CO2 pressure. A method for correcting adsorption isotherms for coal swelling has been published,28 but that work did not address the measurement difficulties caused by coal swelling. Coal swelling of 2-3% has been observed at CO2 pressures of 1 MPa.1 These expansions are large enough to have an impact on isotherms. The size of the measurement irreproducibility increases with increases in fluid density.29 Reasons for this are not known, but CO2 swelling and rates of swelling are one possible contributing factor. Data on coal swelling as a function of CO2 pressure are needed. Once obtained, they can be used to correct the published CO2 sorption data.17-29 If the molar volume of the CO2 dissolved in coals can also be measured, a complete picture of CO2 on and in coals can be assembled. EF050318I
Figure 2. Comparison of the manometric/volumetric and the gravimetric experimental data reduced to the excess (Gibbs) sorption. Pocahontas No. 3 samples were tested at 55 °C.
coal sample was remeasured, and the new coal volume was used with the same data to calculate the second curve, now partially corrected for buoyancy changes caused by coal swelling (Figure 1, final volume). This large (20%) correction is underestimated at the higher pressures because the coal swelling will have decreased as the CO2 pressure dropped. Diffusion of CO2 from coals is relatively rapid.16 The measured coal volume after all the CO2 pressure has been released will be less than its volume at high CO2 pressure. This is a single-point correction, not one based on the changes in coal swelling with changes in CO2 pressure. Figure 2 shows a comparison of the corrected gravimetric excess adsorption with the manometric/volumetric excess adsorption measured on the same coal under the same conditions. The excess adsorption measured by the volumetric technique is larger than the corrected gravimetric values but is similarly underestimated. The apparatus and techniques used have been described.17 The difference between the two sets of experimental data can be reconciled by assuming that during the gravimetric experiment the coal swelling was larger than that in the volumetric experiment by ∼30% of the initial sample (16) Nandi, S. P.; Walker, P. L., Jr. Fuel 1964, 43, 385-393. (17) Ozdemir, E. Chemistry of the adsorption of carbon dioxide by Argonne Premium Coals and a model to simulate CO2 sequestration in coal seams. Ph.D. Dissertation, University of Pittsburgh, Pittsburgh, PA, 2004.
(18) Zhou, C. Modeling and prediction of pure and multicomponent gas adsorption. Ph.D. Dissertation, Oklahoma State University, Stillwater, OK, 1994. (19) 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. Presented at the Eastern Regional Conference & Exhibition, Charleston, WV, November 8-10, 1994; SPE paper 29194, pp 329-344. (20) Busch, A.; Gensterblum, Y.; Krooss, B. M.; Littke, R. Int. J. Coal Geol. 2004, 60, 151-168. (21) Busch, A.; Gensterblum, Y.; Krooss, B. M. Int. J. Coal Geol. 2003, 55, 205-224. (22) Krooss, B. M.; van Bergen, F.; Gensterblum, Y.; Siemons, N.; Pagnier, H. J. M.; David, P. Int. J. Coal Geol. 2002, 51, 69-92. (23) Fitzgerald, J. E.; Pan, Z.; Sudibandriyo, M.; Robinson, R. L.; Gasem, K. A. M. Fuel 2005, 84, 2351-2363. (24) Fitzgerald, J. E.; Sudibandriyo, M.; Pan, Z.; Robinson, R. L.; Gasem, K. A. M. Carbon 2003, 41, 2203-2216. (25) Hartman, R. C.; Pratt, T. J. A. Preliminary Study of the Effect of Moisture on the Carbon Dioxide Storage Capacity in Coal. Proceedings of the 2005 International Coalbed Methane Symposium, Tuscalossa, AL, May 16-20, 2005; Paper 0533. (26) Gasem, K. A. M.; Fitzgerald, J. E.; Pan, Z.; Sudibandriyo, M.; Robinson, R. L., Jr. Modeling of Gas Adsorption on Coals. Proceedings of the Eighteenth Annual International Pittsburgh Coal Conference, Newcastle, NSW, Australia, December 3-7, 2001. International Pittsburgh Coal Conference: Pittsburgh, PA, 2001. (27) Mavor, M. J.; Hartman, R. C.; Pratt, T. J. Uncertainty in Sorption Isotherm Measurements. Proceedings of the 2004 International Coalbed Methane Symposium, Tuscaloosa, AL, May, 2004; Paper 411. (28) Ozdemir, E.; Morsi, B. I.; Schroeder, K. Langmuir 2003, 19, 97649773. (29) 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., Jr.; Schroeder, K.; Sudibandrio, M.; White, C. M. Energy Fuels 2004, 18, 1175-1182.
