Energy Fuels 2009, 23, 4845–4847 Published on Web 07/14/2009
: DOI:10.1021/ef900274q
Molecular Exchange of CH4 and CO2 in Coal: Enhanced Coalbed Methane on a Nanoscale† Tim J. Tambach,*,‡ Jonathan P. Mathews,§ and Frank van Bergen‡ Business Unit Geo-energy and Geo-information, TNO Geological Survey of The Netherlands, Post Office Box 80015, 3508 TA Utrecht, The Netherlands, and Department of Energy and Mineral Engineering, and the EMS Energy Institute, The Pennsylvania State University, 126 Hosler Building, University Park, Pennsylvania 16802. ‡ TNO Geological Survey of The Netherlands. § The Pennsylvania State University. Received March 31, 2009. Revised Manuscript Received June 26, 2009 Many molecular coal model structures exist,15-17 yet studies on the molecular sorption process in coal are limited.15,18,19 A general bituminous coal model20 was used to create a 3D atomic model of 1910 atoms, representing the matrix of the twocomponent coal structure.21 Molecular dynamics (integration of the equations of motion)22 was performed using Hyperchem to simulate coal interactions with CO2 and/or CH4 within a periodic box. An interface to Hyperchem was used to create random translations of adsorbate molecules using a Monte Carlo Metropolis algorithm,22 to overcome kinetically closed pores.23 The simulations were carried out at 45 °C, representing field conditions of approximately 1 km depth. Further details are in the Supporting Information. As edge effects play a dominant role in the small-scale coal model, it is not straightforward to define adsorbed and free molecules in comparison to crystalline inorganic sorbents, such as zeolites24 or clays.25 The focus is therefore on the potential energy (Upot,hg) between the host (coal) and guest molecule (CO2 or CH4). The guest-guest, coal-coal, and intramolecular interactions were excluded. Coal was modeled as a rigid entity, because the (induced) coal relaxation and the complex interaction with small molecules is computationally expensive and poorly understood. Swelling was not investigated. The results show that Upot,hg becomes less negative with an increasing loading for both CO2 and CH4. Especially at low loadings, Upot,hg is systematically more negative for CO2 compared to CH4 (Figure 1). This indicates that CO2 has a greater affinity, for this coal, than CH4. This is in agreement with theoretical work23,26 and experiments showing CO2 adsorption up to 3.16 times2 higher compared to CH4. The most favorable adsorption regions on the coal were determined using simulations with a single guest molecule interrogating 11 200 grid cells within the periodic simulation cell. The average Upot,hg for each grid cell was sampled by
Coalbed methane (CH4) is an important energy source. With increasing climate change concerns, coalbeds are also considered potential sinks for underground carbon dioxide (CO2) storage.1,2 Roughly twice the amount of CO2 can be adsorbed than CH4 in most bituminous coals.1 Another advantage of CO2 injection into coalbeds is the additional production of CH4. This process is known as CO2-enhanced coalbed methane (ECBM) and is very attractive considering the increasing energy demand. Several ECBM pilot programs have been implemented in several parts of the world;3,4 however, its full potential has yet to be exploited.5 ECBM field operations can be better managed by an improved understanding of laboratory measurements and coalbed reservoir simulations.6 Both CH4 and CO2 are predominantly stored in the micropores of the coal matrix,7 which appear as “isolated” locations.8 No consensus exists on measuring the total surface area,9 the heat of adsorption,1 or the theoretical description of adsorption,10 partially because coal is difficult to characterize as a result of its heterogeneity11 and complex response to CO2 adsorption.12 In this study, molecular simulations are used to identify energetically favorable adsorption regions, thereby testing the molecular exchange of CH4 by CO2.13,14 Our goal is to discuss the adsorption distribution at a molecular scale. † Progress in Coal-Based Energy and Fuel Production. *To whom correspondence should be addressed. E-mail: tim.
[email protected]. (1) 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, 659–724. (2) Busch, A.; Gensterblum, Y.; Krooss, B. M. Int. J. Coal Geol. 2003, 55, 205–224. (3) van Bergen, F.; Krzystolik, P.; van Wageningen, N.; Pagnier, H.; Jura, B.; Skiba, J.; Winthaegen, P.; Kobiela, Z. Int. J. Coal Geol. 2009, 77, 175–187. (4) van Bergen, F.; Pagnier, H.; Krzystolik, P. Environ. Geosci. 2006, 13, 201–224. (5) Gale, J. Energy 2004, 29, 1329–1338. (6) Romanov, V. Energy Fuels 2007, 21, 1646–1654. (7) Gray, I. SPE Reservoir Eval. Eng. 1987, 28–34, paper 12514. (8) Larsen, J. W.; Hall, P.; Wernett, P. C. Energy Fuels 1995, 9, 324– 330. (9) Mahajan, O. P. Carbon 1991, 29, 735–742. (10) Dutta, P.; Harpalani, S.; Prusty, B. Fuel 2008, 87, 2023–2036. (11) Stach, E. Stach’s Textbook of Coal Petrology; Gebrueder Borntraeger: Berlin, Germany, 1982. (12) Larsen, J. W. Int. J. Coal Geol. 2004, 57, 63–70. (13) Hall, F. E.; Zhou, C.; Gasem, K. A. M.; Robinson, R. L., Jr. SPE Tech. Pap. 29194, 1994. (14) Busch, A.; Krooss, B. M.; Gensterblum, Y.; van Bergen, F.; Pagnier, H. J. M. J. Geochem. Explor. 2003, 78-9, 671–674. (15) Domazetis, G.; James, B. D. Org. Geochem. 2006, 37, 244–259. (16) Carlson, G. A. Energy Fuels 1992, 6, 771–778. (17) Mathews, J. P.; Hatcher, P. G.; Scaroni, A. W. Energy Fuels 2001, 15, 863–873.
r 2009 American Chemical Society
(18) Narkiewicz, M. R.; Mathews, J. P. Visualizing CO2 Sequestration in Coal, International Conference on Coal Science and Technology (ICCS&T), Okinawa, Japan, 2005. (19) Takanohashi, T.; Iino, M.; Nakamura, K. Energy Fuels 1998, 12, 1168–1173. (20) Spiro, C. L.; Kosky, P. G. Fuel 1982, 61, 1080–1087. (21) Given, P. H.; Marzec, A.; Barton, W. A.; Lynch, L. J.; Gerstein, B. C. Fuel 1986, 65, 155–163. (22) Frenkel, D.; Smit, B. Understanding Molecular Simulation: From Algorithms to Applications, 2nd ed.; Academic Press: San Diego, CA, 2002; p 638. (23) Nguyen, T. X.; Bhatia, S. K. J. Phys. Chem. C 2007, 111, 2212– 2222. (24) Smit, B. Chem. Rev. 2008, 108, 4125–4184. (25) Tambach, T. J.; Hensen, E. J. M.; Smit, B. J. Phys. Chem. B 2004, 108, 7586–7596. (26) Cui, X. J.; Bustin, R. M.; Dipple, G. Fuel 2004, 83, 293– 303.
4845
pubs.acs.org/EF
Energy Fuels 2009, 23, 4845–4847
: DOI:10.1021/ef900274q
Figure 1. van der Waals and electrostratic (i.e., nonbonded) potential energy (Upot,hg) in kJ/mol, between each guest molecule (CH4 or CO2) and the host (coal). The left panel shows Upot,hg as a function of the pure CH4 or CO2 loading in the simulation box, and the right panel the most favorable adsorption regions (50 of 11 200 grid cells), computed with a single CH4 or CO2 molecule. The trend of lower energies for CO2 is in agreement with that for the heats of condensation of CH4 and CO2 of 8.8 and 16.7 kJ/mol, respectively.27 The energies from -17 to -21 kJ/mol of the five most favorable sites for CO2 are in comparable range with experimentally measured heats of adsorption.1,28,29
Figure 3. Snapshot of a simulation for a mixture of 25 CO2 and 25 CH4 molecules (left). The atoms are colored as follows: C, cyan; O, red; N, blue; S, yellow; and H, gray. The green box represents the periodic boundaries. The pixels in the right picture represent a sampling of the carbon atoms of CO2 (red) and CH4 (cyan) throughout the simulation (the coal structure is left out for clarity). The bright spots in the right panel possibly indicate persorption sites.
surrounded by the host) should be taken into account. Because of dominant edge effects in our relatively small coal model, this is more complicated to define and, thus, only a qualitative comparison is possible. Figure 2 shows that CO2 and CH4 demonstrate both a preference for different adsorption regions and a considerable overlap of adsorption regions. Overall, the associated energies for CO2 are approximately 5 kJ/mol lower (Figure 1), and this trend is also observed for experimentally measured heats of adsorption.1 The computed energies (8-20 kJ/mol) are in the range of physical adsorption of 8-42 kJ/mol.31 The lowest energy regions for both CH4 and CO2 are possibly related to “persorption”, i.e., a special case of adsorption in pores with sizes approaching the atomic scale,32,33 which corresponds to pore sizes in the work presented here. A solid solution of guest molecules has been put forward by others as imbibition,34 while atomic scale pores have also been identified as isolated pores8 or ultramicropores.35,36 Probably, gas molecules in these types of pores are associated with residual gas that remains in coal cores, even after desorption tests of several months at atmospheric pressure. In bituminous coals, the residual gas content can be substantial.4 To further investigate persorption and molecular-scale porosity, the coal model was exposed to a mixture of 25 CH4 and 25 CO2 molecules. Figure 3 shows a snapshot of the system as well as the probability distribution of the molecules location. Adsorption of CO2 and CH4 inside the coal and on the surface is indicated by the bright red and blue spots, respectively. It is visually confirmed that surface adsorption is favored for CO2 over CH4 in this small model. The Upot,hg is -2.88 kJ/mol, which is closer to simulations with 50 molecules of pure CO2 (-3.65 kJ/mol) than to pure CH4 (-1.60 kJ/mol), as given in Figure 1. This further supports favorable CO2 adsorption over CH4.
Figure 2. Projection of the 50 most energetically favorable adsorption regions, computed for a single CH4 (yellow) or CO2 (red) molecule. Shared regions are indicated by violet spheres. Each sphere represents a sampled grid cell. The rigid coal structure is given by the tubes (C, cyan; O, red; N, blue; S, yellow; and H, gray). The green box represents the periodic boundaries of the simulation cell.
computing the center of mass of the molecule at a time t and assigning Upot,hg to the corresponding grid cell. For both CO2 and CH4, 50 adsorption regions (or grid cells) with the lowest Upot,hg are represented by spheres in Figure 2 and the energies are plotted in Figure 1. The greater difference in Upot,hg for CO2 and CH4 at low loading (left panel of Figure 1) is explained by the energetically more favorable adsorption regions (right panel of Figure 1). The adsorption regions are lower in energy for CO2, which results in stronger CO2-coal interactions and a higher probability of being adsorbed. In theory, the heat of adsorption can be computed from simulations with one molecule and the host.30 As shown for zeolites,30 only the molecules inside the host (i.e., completely
(31) Choi, J. G.; Do, D. D.; Do, H. D. Ind. Eng. Chem. Res. 2001, 40, 4005–4031. (32) Alexeev, A. D.; Ulyanova, E. V.; Starikov, G. P.; Kovriga, N. N. Fuel 2004, 83, 1407–1411. (33) Alexeev, A. D.; Vasylenko, T. A.; Ul’yanova, E. V. Solid State Commun. 2004, 130, 669–673. (34) Walker, P. L.; Verma, S. K.; Riverautrilla, J.; Khan, M. R. Fuel 1988, 67, 719–726. (35) Radovic, L. R.; Menon, V. C.; Leon, C.; Kyotani, T.; Danner, R. P.; Anderson, S.; Hatcher, P. G. Adsorption 1997, 3, 221–232. (36) Prinz, D.; Littke, R. Fuel 2005, 84, 1645–1652.
(27) Nodzenski, A. Fuel 1998, 77, 1243–1246. (28) Goodman, A. L.; Campus, L. A.; Schroeder, K. T. Energy Fuels 2005, 19, 471–476. (29) Ozdemir, E. Chemistry of the Adsorption of Carbon Dioxide Premium Coal and a Model To Simulate CO2 Sequestration in Coal Seams; University of Pittsburgh: Pittsburgh, PA, 2004. (30) Vlugt, T. J. H.; Garcia-Perez, E.; Dubbeldam, D.; Ban, S.; Calero, S. J. Chem. Theory Comput. 2008, 4, 1107–1118.
4846
Energy Fuels 2009, 23, 4845–4847
: DOI:10.1021/ef900274q
From our simulations, we conclude that preferential CO2 adsorption over CH4 in certain coals can be explained by adsorption in pores on atomic scales. Such pores have been found experimentally.8,35,36 The results in this work are encouraging for long-term stability of CO2 storage and enhanced CH4 production. A more detailed analysis of the adsorption regions is underway, whereas the influence of moisture and coal-swelling effects,5,11,12 as well as simulations in the grand-canonical (μVT) ensemble, will be included in future work.
Acknowledgment. This research is part of the CATO program. CATO is the Dutch national research program on CO2 capture and storage. CATO is financially supported by the Dutch Ministry of Economic Affairs under the BSIK programme. More information can be found at www.co2-cato.nl. We thank the five anonymous reviewers for helpful suggestions. Supporting Information Available: Details of the software, simulations, and the molecular models of coal, CH4, and CO2. This material is available free of charge via the Internet at http:// pubs.acs.org.
4847
