High-Pressure Adsorption of Methane and Carbon Dioxide on Coal

Energy & Fuels 2006, 20, 2599-2607

2599

High-Pressure Adsorption of Methane and Carbon Dioxide on Coal Jun-Seok Bae and Suresh K. Bhatia* DiVision of Chemical Engineering, The UniVersity of Queensland, Brisbane, QLD 4072, Australia ReceiVed July 13, 2006. ReVised Manuscript ReceiVed August 17, 2006

Adsorption isotherms of methane and carbon dioxide on two kinds of Australian coals have been measured at three temperatures up to pressures of 20 MPa. The adsorption behavior is described by three isotherm equations: extended three-parameter, Langmuir, and Toth. Among these, the Toth equation is found to be the most suitable, yielding the most realistic values of pore volume of the coals and the adsorbed phase density. Also, the surface area of coals obtained from CO2 adsorption at 273 K is found to be the meaningful parameter which captures the CO2 adsorption capacity. A maximum in the excess amount adsorbed of each gas appears at a lower pressure with a decrease in temperature. For carbon dioxide, after the appearance of the maximum, an inflection point in the excess amount adsorbed is observed close to the critical density at each temperature, indicating that the decrease in the gas-phase density change with pressure influences the behavior of the excess amount adsorbed. In the context of CO2 sequestration, it is found that CO2 injection pressures of lower than 10 MPa may be desirable for the CH4 recovery process and CO2-holding capacity.

1. Introduction Recognizing the fact that a major contributor to climate change is the emission of greenhouse gases such as carbon dioxide (CO2) and methane (CH4) through human activities, particularly the combustion of fossil fuels, a great emphasis has been placed on the questions of how to reduce this emission and of how to capture and store the discharged gases effectively, economically, and safely. As Australia is one of the top five countries in coal production,1 its contribution to CH4 emissions is expected to be substantial. Coal mining, for example, is responsible for up to a third of the total emission in the coal industry,2 which is an unfortunate side effect from operating mines as well as from abandoned sites. Another concern, that of carbon dioxide release, stems from the combustion of coal, oil, and natural gas. Along with industrial efforts to deduce the CO2 emissions, it is necessary to capture CO2 from the emission sources and reuse or store it in a safe place for a long time. In terms of CO2 storage, geological sequestration is considered to have the greatest potential for the safe and effective storage of CO2,3 among which CO2 sequestration in coal seams is a promising method because CO2 can adsorb on coals more strongly than CH4, thereby replacing adsorbed CH4. It is also therefore more likely to remain stored within the seam (if there is no disturbance), while enhancing CH4 recovery, which offsets the costs of capture, transportation, and storage of CO2.4,5 There * Author to whom correspondence should be addressed. Phone: +61 7 3365 4263. Fax: +61 7 3365 4199. E-mail: [email protected]. (1) Murray, D. K. In Coalbed Methane and Coal Geology; Gayer, R. A., Harris, I., Eds.; Geological Society: London, 1996; Vol. 1, p 344. (2) Fong, C. Ph. D. dissertation, The University of Queensland, Brisbane, Australia, 1999. (3) Thomas, D. C.; Benson, S. M. Carbon Dioxide Capture for Storage in Deep Geologic Formations - Results from the CO2 Capture Project; Elsevier: Amsterdam, 2005. (4) Mastalerz, M.; Gluskoter, H.; Rupp, J. Int. J. Coal Geol. 2004, 60, 43-55. (5) 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.

are, however, some concerns in relation to CO2 sequestration in coal seams, including long-term effects of the injected CO2 on coal geochemistry and effects of the presence of water. The candidate sites should be carefully chosen by considering coal seam characteristics such as geometry, structure, permeability, and optimum depth range.6 The complexity of coal structure due to the presence of a mixture of heterogeneous organic and inorganic matters (depending on coalification processes) has made fundamental sorption studies on coal less interesting than those on other porous media such as activated carbon and zeolite. To describe the high-pressure adsorption of supercritical fluids mostly on activated carbons, a number of empirical and theoretical approaches have been reported in the literature, including Langmuir-type isotherms,7 Dubinin-Radushkevitch (DR) or Dubinin-Astakhov (DA) forms,8 Ono-Kondo equations,9 grand canonical Monte Carlo simulations,10 and density functional theory (DFT).11 For coals, the description of CH4 and CO2 isotherms up to pressures of 20 MPa has not been reported. In this paper, we investigate three isotherm equations such as the extended three-parameter equation, the Langmuir equation, and the Toth equation to describe the absolute adsorptions and, consequently, the excess adsorptions. A real issue related to the characterization of coals is that, at low temperatures (77 or 87 K), N2 (or Ar) cannot penetrate small micropores, while CO2 at 273 K does not fill the large pores (but fills most ultramicropores). For the former case, N2 (or Ar) at the low cryogenic temperature, its adsorption in very small pores is severely diffusionally limited (i.e., activated diffusion) within a reasonable equilibrium time scale,12 giving (6) Gale, J.; Freund, P. EnViron. Geosci. 2001, 8, 210-217. (7) Malbrunot, P.; Vidal, D.; Vermesse, J.; Chahine, R.; Bose, T. K. Langmuir 1992, 8, 577-580. (8) Aoshima, M.; Fukasawa, K.; Kaneko, K. J. Colloid Interface Sci. 2000, 222, 179-183. (9) Benard, P.; Chahine, R. Langmuir 1997, 13, 808-813. (10) Cao, D.; Wang, W.; Duan, X. J. Colloid and Interface Sci. 2002, 254, 1-7. (11) Nguyen, T. X.; Bhatia, S. K.; Nicholson, D. Langmuir 2005, 21, 3187-3197.

10.1021/ef060318y CCC: $33.50 © 2006 American Chemical Society Published on Web 09/20/2006

2600 Energy & Fuels, Vol. 20, No. 6, 2006

Bae and Bhatia

Table 1. Physical Properties of the Selected Coal Samples14 analysis

coal A

Proximate Analysis (wt %, adb) ash content 9.5 moisture content 2.0 volatile content 25.0 fixed carbon 63.5 carbon hydrogen nitrogen oxygen

Ultimate Analysis (wt %, adb) 86.5 5.00 2.00 5.95

coal B 14.5 2.3 26.3 56.9 84.43 4.98 1.71 8.40

rise to unrealistically low surface areas (in the range of 1-9 m2/g). Therefore, a good system is needed to probe the coal structure. For sequestration, the pore volume is most important, which has been approximately calculated from density measurements with mercury and helium.13 In this paper, the pore volume is obtained from CH4 and CO2 adsorption at high pressures (up to 20 MPa) with the Toth equation. Here, we provide a unique set of experimental data of CH4 and CO2 on coals at three different temperatures and up to pressures of 20 MPa. For consistent results, a reliable technique for degassing prior to adsorption is also demonstrated in the article, as a result of which free moisture is desorbed and its effect on adsorption avoided. Further, the experimental isotherms are interpreted, on the basis of fits of three different models, among which the Toth isotherm is found to yield the most physically reasonable parameters. The results suggest that the high-pressure adsorption of CO2 at room temperature can be a useful method for characterization. In addition, the results provide insight on the high-pressure adsorption of CO2 and CH4, useful for the sequestration of CO2 while displacing CH4 from coal seams. 2. Experimental Section Two kinds of Bowen Basin coals in Australia were collected for this adsorption study of methane and carbon dioxide. The coal samples were ground to a size between 180 and 212 µm prior to adsorption experiments. The physical properties14 of the selected coals are listed in Table 1. The gases used here were ultrahigh-purity grade, supplied by BOC gas Australia. Adsorption experiments were carried out using a gravimetric sorption system (Rubotherm Pra¨zisionsmesstechnik GmbH, Bochum, Germany) which is designed to be operated up to 40 MPa and 523 K. The system consists of three parts: a gas dosing system, magnetic suspension balance, and adsorption chamber. The inlet gas lines are connected to a gas reservoir to dose high pressure to the adsorption chamber by using an air-driven gas booster (AG-152 Haskel, U. S. A.) for methane and a high-pressure liquid pump (Gilson) for carbon dioxide. The magnetic suspension balance with an absolute accuracy of 0.01 mg is separated from the adsorption chamber. Additionally, equilibrium measurements of argon at 87 K and carbon dioxide at 273 K were carried out using a Micromeritics ASAP 2010 volumetric adsorption analyzer, with autoanalysis using DFT. 2.1. Sample Preparation. Prior to adsorption measurements, the adsorbent should be outgassed properly to eliminate any (12) Marsh, H. Fuel 1965, 44, 253-268. (13) Walker, P. L., Jr.; Verma, S. K.; Rivera-Utrilla, J.; Davis, A. Fuel 1988, 67, 1615-1623. (14) Coxhead, B. A. Queensland Coals: Physical and Chemical Properties, Colliery and Company Information, 12th ed.; Queensland Department of Mines and Energy: Brisbane, Australia, 1999.

complication caused by physically adsorbed gases. Further, for adsorption studies, coal samples should be outgassed in such a way that their physical and chemical structures remain unchanged. As the degree of outgassing affects the adsorption capacity and the surface area of coals, especially for low-rank coals whose structure may experience a microscopic shrinkage due to moisture loss,15,16 ambiguity in outgassing conditions gives rise to difficulties in comparing the adsorption data of a coal with those of another kind. In the literature, the effect of outgassing methods on the structure of various coals has been studied with NMR measurements,15 suggesting that thermal outgassing at 378 K, which is used in this study, does not alter the physical structure of sub-/bituminous coals. This was confirmed here for methane adsorption, as discussed in a later section. It is, in general, considered that the moisture content of coals is removed at 378 K, where physically adsorbed moisture (most of the inherent moisture that interferes with gas adsorption) is driven off. Outgassing conditions for coal characterization in most literature15-23 fall into the temperature range of 353-378 K under a vacuum (10-4 Pa) for adsorption measurements, which were applied in this work. 2.2. Measurement of Excess Amount Adsorbed. There are three points between an electromagnet and a permanent magnet: the zero point (M0), measuring point 1 (M1), and measuring point 2 (M2). Zero points (M0) are taken within a relatively short interval to correct the measuring points even with small changes. Here, M1 includes the weights of the permanent magnet, metal suspension, sample basket, and sample, and M2 includes the weight of sinker additional to M1. Because there are, however, no direct methods to measure both the absolute amount adsorbed and the adsorbed phase density (or the adsorbed phase volume), the only measurable quantity is the excess amount adsorbed, nexc, at a given bulk density (Fb) and temperature (T):

nexc(Fb,T) ) Mad - FbVad ) M1(Fb,T) - (Mm + Ms) + Fb(Vm + Vs) ) M1(Fb,T) - M1(0,∞) + Fb(Vm + Vs)

(1)

where M1(0,∞) is the apparent weight measured at a high temperature under a vacuum. Mm, Ms, and Mad are the masses of all metal parts, the sample, and the adsorbed molecules, respectively. Vm, Vs, and Vad are the corresponding volumes. It is noted that the adsorbent volume (Vs) may change during the course of adsorption, especially for coal samples. Thus, it is necessary to measure Vs to correctly calculate the excess amount adsorbed. Knowing the measured bulk density (Fb) and the difference in weight of the metal parts due to the buoyancy effect, the sample volume (Vs) can be obtained. Provided that the sample volume does not change in the course of adsorption (15) Miknis, F. P.; Netzel, D. A.; Turner, T. F.; Wallace, J. C.; Butcher, C. H. Energy Fuels 1996, 10, 631-640. (16) Ozdemir, E.; Morsi, B. I.; Schroeder, K. Fuel 2004, 83, 10851094. (17) Ruppel, T. C.; Grein, C. T.; Bienstock, D. Fuel 1972, 51, 297303. (18) Joubert, J. I.; Grein, C. T.; Bienstock, D. Fuel 1973, 52, 181185. (19) Reucroft, P. J.; Patel, K. B. Fuel 1983, 62, 279-284. (20) Roberts, D. L. Int. J. Coal Geol. 1991, 17, 297-311. (21) Vance, W. E.; Chen, X. D.; Scott, S. C. Combust. Flame 1996, 106, 261-270. (22) Busch, A.; Gensterblum, Y.; Krooss, B. M.; Littke, R. Int. J. Coal Geol. 2004, 60, 151-168. (23) Fitzgerald, J. E.; Pan, Z.; Sudibandriyo, M.; Robinson, R. L., Jr.; Gasem, K. A. M.; Reeves, S. Fuel 2005, 84, 2351-2363.

High-Pressure Adsorption

Energy & Fuels, Vol. 20, No. 6, 2006 2601

Figure 1. Excess amount adsorbed of CH4 on sample A degassed at two different temperatures (357 and 378 K): “Duplicate” indicates subsequent adsorption measurements with the same sample.

and that the adsorbed amount of helium on coals is negligible, we can estimate Vs from measurements with helium.

Vs(T) )

M1(0,∞) - M1(FHe b ,T) FHe b

- Vm(T)

Figure 2. Argon adsorption isotherms on coals A and B, at 87 and 313 K. Table 2. Apparent and Helium Densities and Pore Volumes of Coals A and Ba analysis

(2)

However, because it has been known that at high bulk densities helium adsorption is no longer negligible, one should take into account the adsorbed amount of helium. In this study, however, the sample volume was measured at 105 °C with helium less than 2 MPa, where the error in the sample volume measurement due to helium adsorption is small.24 3. Results and Discussion 3.1. Sample Degassing. Degassing conditions should be reasonable and consistent, minimizing the alteration of coal pore structure. In the present work, degassing temperatures of 353 and 378 K are chosen, as they are frequently used in the literature, as mentioned in section 2.1. Two similar samples were degassed at 353 and 378 K under a vacuum (10-4 Pa) with their weight being measured. For the medium volatile bituminous coal used here, about 24 h under a vacuum at 378 K is found to be sufficient for degassing, with degassing at 353 K taking somewhat longer. Figure 1 demonstrates that no structural changes occur as a result of degassing under these conditions. Coal A, degassed at 353 K for 2 days, exhibits an almost identical excess amount adsorbed as that degassed at 378 K for 1 day. In addition, when the coal A sample previously degassed at 378 K was subsequently again degassed at this temperature, the readsorption isotherm matched the previous result after the first degassing. The same was observed for coal B. This is clear evidence that the coal pore structure of both coals does not undergo any permanent change as a result of methane adsorption. 3.2. Coal Characteristics. Coal can be physically divided into organic (macerals) and inorganic fractions whose compositions and ratio indicate the coal type, whereas the degree of diagenesis or coalification determines the coal rank, which has been used mainly for commercial purposes.25 To relate the coal (24) Sircar, S. In Fundamentals of Adsorption 7; Kaneko, K., Kanoh, H., Hanzawa, Y., Eds.; IK International, Ltd.: Nagasaki, Japan, 2002; pp 656-663. (25) Thomas, L. Coal Geology; Wiley: Chichester, West Sussex, England, 2002.

(g/cm3)

ash density apparent density (g/cm3 dry) apparent density (g/cm3 daf) helium density (g/cm3 dry) helium density (g/cm3 daf) a

coal A

coal B

3.724 1.263 1.181 1.416 1.330

2.992 1.363 1.248 1.452 1.336

Note: daf ) dry, ash-free basis.

properties to the adsorption behavior of methane and carbon dioxide, here, we consider only the density and the surface area of coals. It is also noted that the moisture content of coal and its effect on the adsorption behavior of CH4 and CO2 are not considered, following our degassing procedure discussed earlier. 3.2.1. Coal Density. The inorganic fraction of both coals studied consists of mostly SiO2 and Al2O3 with similar content for each coal (i.e., coal A: 24.31 and 56.18 and coal B: 26.00 and 65.64 wt % of ash, respectively). Additionally, coal A has 14.3 wt % of Fe2O3 in ash while coal B has 3.2 wt % ash. As coal A contains more iron, its ash density is greater than that of coal B (i.e., 3.7244 and 2.9912 g/cm3, respectively). As the mineral content is variable, depending on the coalification processes and the environment, being regarded as contaminants of coal gasification, the carbon content (based on dry, mineralmatter-free or dry, ash-free) has been related to the corresponding density. The measured densities of coals A and B containing ash constituents can be corrected to the densities based on dry, ash-free material26 and are listed in Table 2. It is, however, noted that all adsorption data in later sections are presented on the basis of dry coal. 3.2.2. Surface Area Determination. To determine the surface areas of coals studied, argon adsorption measurements were initially conducted at 87 and 313 K, the isotherms for which are depicted in Figure 2. Coal A shows lower adsorption at 87 K than coal B. Analysis of the 87 K isotherms yielded Brunauer-Emmett-Teller (BET) areas of 2.95 and 4.74 m2/g for coals A and B, respectively. The corresponding BoppJancso-Heinzinger pore volumes were found to be 9.67 × 10-3 and 17.56 × 10-3 cm3/g, respectively. Most interestingly, however, as seen in Figure 2, the order of the isotherms is (26) Singh, K. P.; Kakati, M. C. Coal Preparation (Philadelphia, PA, U.S.) 1998, 19, 1-8.


Bae and Bhatia

Figure 3. Pore size distributions obtained by DFT analysis (assuming slit pores) of CO2 adsorption at 273 K and argon adsorption at 87 K (inset) for coals A and B.

Figure 4. CO2 density in the gas phase with respect to pressure: the solid lines are calculated by the Span and Wagner equation of state.30

Table 3. Surface Areas Obtained from Different Methods

for CH4 and CO2, respectively. All EOS constants were taken from published values.29,30 Excellent correspondence between the calculated densities and experimental data are found, as shown in Figure 4 for CO2. As the critical density of CO2 is 0.4676 g/cm3, a clear inflection point appears at the corresponding density for each temperature. After the inflection point, one expects the value of dFb/dP to decrease. Consequently, the change of the excess amount adsorbed with respect to pressure (i.e., dnexc/dP) will be reduced according to eq 4, suggesting that an inflection point may appear in the value of the excess amount adsorbed, provided that the density of adsorbed phase does not change with pressure as much as that of the gas phase near the inflection point. In this regard, more will be discussed in the section on CO2 adsorption. 3.4. Description of CH4 and CO2 Adsorption on Coals. All experimental methods for isotherm measurements give the excess amount adsorbed, which has to be related with its absolute amount adsorbed to evaluate thermodynamic functions such as the isosteric heat of adsorption31 and to compare with theoretical approaches such as grand canonical Monte Carlo and DFT.32 In this paper, because of the complexity of coal structure due to the presence of a mixture of heterogeneous organic and inorganic matters, we consider three semiempirical equations to describe the adsorption behavior of methane and carbon dioxide on coals. The absolute amount adsorbed refers to the actual number of molecules present in pores. In an adsorption volume V where the density outside this volume is equal to the bulk density, the absolute amount adsorbed is determined by integration of the adsorption density distribution Fad(r) with respect to the adsorption volume as follows:

method

coal A

coal B

BET (m2/g) Dubinin-Radushkevich (m2/g) Dubinin-Astakhov (m2/g) DFT (m2/g)

99.223 133.028 233.663 126.293

86.139 116.908 193.239 114.368

reversed at 313 K, with coal A adsorbing a larger amount. Such results suggest strong effects of pore mouth constrictions and blockage at the lower temperature, caused by polyaromatic hydrocarbon units in the coal structure.27 Indeed, such results are consistent with the observation of Maggs,28 who suggested that enormous activation energy is required for nitrogen or argon to penetrate into very small pores at very low temperatures (7787 K). Another possible reason could be shrinkage of the mouths of pores to less than 10 Å at 87 K, leading to a diffusional problem. After the argon adsorption, CO2 adsorption at 273 K was carried out for both coals, as it has been recommended for measurements of coal surface areas. The results are listed in Table 3. The surface areas from CO2 adsorption at 273 K are much higher than those from argon at 87 K, but they depend on analysis methods. Which value better represents the surface area of coals will be clarified later with CH4 and CO2 adsorption at higher temperatures. Although CO2 at 273 K cannot fill large micropores (>10 Å) at pressures less than 1 atm, it would appear that the surface areas of both coals studied are predominantly from pores smaller than 10 Å. The pore size distributions (PSDs) for both coals were obtained from DFT and are shown in Figure 3. The micropore size distributions of both coals are found to be similar. As seen in the inset, argon molecules at 87 K do not have access to the pores smaller than about 12 Å in width within a practical equilibrium time. Although the PSD obtained from argon adsorption at 87 K shows how the large micropore and mesopore sizes are distributed, it is not very meaningful. 3.3. Bulk Density Measurements. As the bulk density values of CH4 and CO2 are required to estimate the excess amount adsorbed from gravimetric experiments, they were measured experimentally and compared with those calculated from the Bender equation of state (EOS)29 and the Span/Wagner EOS30 (27) Radovic, L. R.; Menon, V. C.; Leon y Leon, C. A.; Kyotani, T.; Danner, R. P.; Anderson, S.; Hatcher, P. G. Adsorption 1997, 3, 221-232. (28) Maggs, F. A. P. Nature (London) 1952, 169, 793-794. (29) Puziy, A. M.; Herbst, A.; Poddubnaya, O. I.; Germanus, J.; Harting, P. Langmuir 2003, 19, 314-320.

nabs )

∫V

ad

Fad(r) dV

(3)

where r is a distance away from the pore surface within the adsorption volume and Vad is the adsorbed phase volume. For determination of the absolute amount adsorbed, it is common to use approximate values of the adsorbed phase density or its volume, because they cannot be directly measured. The adsorbed (30) Span, R.; Wagner, W. J. Phys. Chem. Ref. Data 1996, 25, 15091596. (31) Ustinov, E. A.; Do, D. D.; Herbst, A.; Staudt, R.; Harting, P. J. Colloid Interface Sci. 2002, 250, 49-62. (32) Murata, K.; El-Merraoui, M.; Kaneko, K. J. Chem. Phys. 2001, 114, 4196-4205.



phase volume Vad is often replaced with the volume of small pores where the adsorption density distribution at any position is greater than the bulk density due to the overlapping of the potential energies exerted by opposite pore walls in a confined geometry. The excess amount adsorbed refers to the difference between the absolute amount adsorbed and the amount of molecules at the bulk density displaced by the volume of the adsorbed phase:33

nexc ) nabs - FbVad ) (Fjad - Fb)Vad

(4)

where Fjad is the average adsorbed phase density. For the description of high-pressure adsorption, because the chemical potentials of equilibrium phases are proportional to the logarithm of fugacity, the absolute amount adsorbed for type I isotherms may be considered as a function of gas-phase fugacity.34 Here, we consider three different semiempirical isotherm equations to describe the absolute adsorption of CH4 and CO2 on coals, treating the adsorbed phase volume as a constant for a given system. The first is an extended threeparameter equation:35

nexc ) nmax

f - FbVad f + k exp[(λP)/(RT)]

(5)

where nmax is the maximum adsorbed amount, f is the fugacity of the gas phase, k is the Henry’s law constant, λ is the molar volume of the adsorbed phase, and Vad is the adsorbed phase volume. The second is the Langmuir equation, which is suggested by the fact that the fluid-solid interaction is greater than the fluid-fluid interaction under supercritical conditions. For this model, the excess amount adsorbed is given by

nexc ) nmax

bf - FbVad 1 + bf

(6)

where b is the adsorption affinity. Finally, the Toth equation with gas-phase fugacity is also considered. In a study considering three different isotherm equations (Langmuir, LangmuirFreundlich, and Toth isotherms), the Toth equation has been found to best describe methane adsorption on activated carbon with a high-surface area, because the adsorbed density obtained from the Langmuir equation significantly exceeded the liquid density of methane while the Langmuir-Freundlich does not yield a Henry constant at low pressures.36 The Toth equation gives the excess amount adsorbed as

nexc ) nmax

bf - FbVad [1 + (bf)t]1/t

(7)

For coals studied in this paper, these three equations (eqs 5-7) were fitted to the excess amount adsorbed obtained from experiments and parameter values determined by minimizing the residuals: (33) Sircar, S. Ind. Eng. Chem. Res. 1999, 38, 3670-3682. (34) Myers, A. L. Adsorption Science and Technology. Proceedings of the Pacific Basin Conference, 3rd, Kyongju, Republic of Korea, May 2529, 2003; pp 44-50. (35) Herbst, A.; Harting, P. Adsorption 2002, 8, 111-123. (36) Do, D. D.; Do, H. D. Carbon 2003, 41, 1777-1791.

m

residual )

predicted observed 2 (nexc,j - nexc,j ) ∑ j)1

with j ) 1, 2, ..., m (8)

where m is the total number of data points at a given temperature. Once the adsorbed phase volume is obtained by fitting the experimental data with the above equations, the adsorbed phase density can be determined from eq 4. It is noted that all adsorption data are presented on a dry coal basis. 3.4.1. Methane Adsorption. Methane adsorption isotherms on both dried coals were measured at 313, 323, and 333 K, up to pressures of 20 MPa. Figure 5 compares the experimental data with the fitted results using eqs 5-7. Also plotted are results of the corresponding absolute adsorbed amounts versus the fugacity. The extended three-parameter equation (eq 5) is relatively insensitive to the molar volume (λ) of the adsorbed phase as the exponential term in eq 5 is always less than 1.5 within the given conditions of this work. Thus, to avoid any unphysical value of λ, we fixed the adsorbed phase volume as 34 cm3/mol (the liquid density of methane at its boiling point) during the parameter optimization process. The fitted adsorbed phase volumes Vad for coal A at 313 K are found to be 0.0233, 0.0302, and 0.0559 cm3/g, and those for coal B are 0.0221, 0.0245, and 0.0333 cm3/g from eqs 5-7, respectively. Fits of the three isotherm models shows that the Toth equation is the most successful over the temperature range covered here. The optimum parameters for the three isotherm models are listed in Table 4. It can be seen that the adsorbed phase volume decreases with an increase in temperature. Considering the adsorbed phase density, it is expected to be less than the liquid-phase density because of incomplete molecular packing in pores at supercritical conditions used in this study. The adsorbed phase density can be obtained from the relationship between the absolute amount adsorbed and the adsorbed phase volume (see eq 4). As seen in Figure 5, the absolute amount adsorbed from the Toth equation is higher than that from the other expressions. The adsorbed phase density of methane at 313 K for coal A, determined from the results of eqs 5-7, is plotted against fugacity in Figure 6. As seen in this figure, only the Toth equation exhibits an adsorbed phase density below the liquid methane density. Because the Toth equation describes well the excess amount adsorbed of methane on both coals and yields reasonable values of the adsorbed phase density, it was considered the most suitable among the three isotherm equations tested. The adsorption isotherms of methane on both coals at three temperatures are plotted in Figure 7 according to the Toth equation, eq 8. For both coals at 313 K, the excess amount adsorbed is found to decrease beyond a fugacity of around 12 MPa. The pressure at which the maximum appears is known to vary with the adsorbate as well as the adsorbent. For methane on Norit R1 activated carbon, for example, the maximum appears at a fugacity of around 8.7 MPa at 313 K, whereas the maximum for nitrogen appears at around 15 MPa.29 At higher temperatures, a maximum in the excess amount adsorbed generally appears at higher pressures, which has been observed for other porous media such as activated carbon.7,29,31 Because there is a limit to the number of molecules that can be packed into the solid pores at high pressures, the adsorbed phase density (Fad) initially increases with the bulk pressure and then levels off at a certain maximum density (Fad*), whereas the bulk density (Fb) increases monotonically with an increase in the bulk pressure above the critical temperature of the gas.33 Consequently, above the critical temperature of a gas at high pressures, the excess amount


Bae and Bhatia

Figure 5. Fits of methane adsorption data on coal A at 313 K with three equilibrium isotherm models, following eqs 5-7.

Figure 7. Excess and absolute adsorption isotherms of methane on coals A and B at three temperatures (313, 323, and 333 K): the dashed lines represent the excess amount adsorbed, and the solid ones represent the absolute amount adsorbed, obtained from the Toth equation (eq 7).

Figure 6. Variation of adsorbed phase density of methane with fugacity at 313 K, based on the three equilibrium isotherm equations (eqs 5-7).

adsorbed exhibits a maximum at a threshold fugacity (f*) at which the increased rates of both densities (Fad and Fb) are equal with respect to the pressure (i.e., dFad/df ) dFb/df).9,31 Above the threshold fugacity, dFad/df will be less than dFb/df and gradually decrease with an increase in the bulk-phase fugacity. Kross et al. have recently reported CH4 isotherms on dried coals at three different temperatures (313, 333, and 353 K) up to pressures of 20 MPa.37 Although the coal samples used by them have slightly different physical and chemical properties from those used here, a common trend can be drawn in that the excess amount adsorbed decreases more slowly than that for activated carbon, after the appearance of its maximum (for example, at 313 K), indicating that the coals studied have wide PSDs. Figure 7 indicates a decreasing temperature dependence on a further increase in fugacity, after the appearance of the maximum, which has been observed for activated carbon up to a certain threshold fugacity (f*) at which the excess amount adsorbed has no temperature dependence.29,38 3.4.2. Carbon Dioxide Adsorption. Given the surface areas of coals A and B in Table 3, one would expect that coal A has a higher CO2 adsorption capacity than coal B. Figure 8 depicts the excess and absolute adsorbed amounts of CO2 on both coals (37) Krooss, B. M.; van Bergen, F.; Gensterblum, Y.; Siemons, N.; Pagnier, H. J. M.; David, P. Int. J. Coal Geol. 2002, 51, 69-92. (38) Herbst, A.; Harting, P. Adsorption 2002, 8, 111-123.

at four temperatures. As in the case of methane, the Toth equation describes the CO2 isotherms better than the others and, therefore, is used in Figure 8. At the critical density of CO2 (0.4676 g/cm3), the corresponding fugacities of CO2 in the gas phase at 313, 323, 333, and 343 K are 5.631, 6.544, 7.535, and 8.601 MPa, respectively (for the corresponding pressures, see Figure 4). It is interesting to see that the inflection points of the excess amount adsorbed appear close to the critical gas fugacity at each temperature. Thus, below the critical fugacity, the adsorbed phase density increases more rapidly with an increase in the fugacity than the bulk density. However, with an increase in the fugacity beyond the critical value, the value of |dFb/df| is greater than that of |dFad/df|, giving rise to the decrease in the excess amount adsorbed. Because the adsorption affinity of CO2 toward coals is higher than that of CH4, small pores are expected to be filled with CO2 molecules faster than with CH4 molecules. At 313 K, for example, the adsorbed phase volumes (Vad) of coals A and B are found to be 0.0826 and 0.0688 cm3/g, respectively. The optimum parameters of the Toth isotherm for CO2 adsorption on both coals are listed in Table 5. The parameter t, which is usually less than unity, is related with the system heterogeneity. The value of t for coal A is similar to that for coal B, suggesting that both coals have a similar degree of heterogeneity. As can be seen in Tables 4 and 5, CO2 adsorption is more heterogeneous than CH4 adsorption, caused by the fact that the solid-fluid interaction potential of CO2 is more attractive than that of CH4. Moreover, the adsorbed phase volume of CO2 is found to be consistently greater than that of CH4 (for example, 0.0826 versus 0.0559 cm3/g, respectively, for coal A at 313 K), suggesting



Table 4. Optimum Fitting Parameters of Three Isotherm Models for Methane Adsorption on Coals A and B at Three Temperatures (313, 323, and 333 K) coal A equilibrium isotherms extended three parameters Langmuir Toth

coal B

parameters

313 K

323 K

333 K

313 K

323 K

333 K

nmax (mmol/g)

1.4116

1.2380

1.0967

1.0993

0.9944

0.9004

k (MPa) Vad (cm3/g) nmax (mmol/g) b (MPa1-) Vad (cm3/g) nmax (mmol/g) b (MPa1-) t (-) Vad (cm3/g)

2.2407 0.0233 1.4006 0.4648 0.0302 4.0154 0.7363 0.3533 0.0559

2.2549 0.0132 1.2900 0.4030 0.0228 2.2570 0.5428 0.5014 0.0388

2.2560 0.0055 1.1453 0.3996 0.0143 1.6560 0.4755 0.6089 0.0264

2.2519 0.0221 1.0813 0.4432 0.0245 1.9688 1.0351 0.4201 0.0333

2.2485 0.0170 1.0200 0.4047 0.0223 1.9438 0.7318 0.4362 0.0343

2.2505 0.0130 0.9218 0.4072 0.0167 1.1413 0.6000 0.6450 0.0195

Table 5. Optimum Fitting Parameters of the Toth Isotherm for CO2 Adsorption on Coals A and B at Four Temperatures (313, 323, 333, and 343 K) coal A

coal B

parameters

313 K

323 K

333 K

343 K

313 K

323 K

333 K

343 K

nmax (mmol/g) b (MPa-1) t (-) Vad (cm3/g)

9.1013 5.6833 0.2297 0.0826

7.1684 2.9586 0.2639 0.0858

6.0014 1.6675 0.2999 0.0690

5.5756 1.1281 0.3178 0.0650

5.8110 1.7030 0.3176 0.0688

5.5554 1.7004 0.3090 0.0510

5.2510 1.7001 0.3028 0.0530

4.6128 1.6999 0.3044 0.0467

that there are pores inaccessible to methane molecules at the given temperatures. From CO2 adsorption at 273 K based on the DA equation, the micropore volumes for coals A and B, obtained, are found to be 0.0744 and 0.0600 cm3/g, respectively, which are slightly smaller than the adsorbed phase volumes. This could be due to the fact that CO2 adsorption at 273 K only covers pores less than 9 Å, as seen in Figure 3. These data at high pressures up to 20 MPa allow a more accurate determination of pore volumes and therefore lead to improved characterization. In addition, high-resolution transmission electron microscopy with image analysis can be used to improve the structural analysis of narrow micropores.39 The surface area obtained from CO2 adsorption at 273 K, discussed in section 3.2.2, can be now validated. For example, considering an average value of pore size as 6 Å for coals A and B, from Figure 3, and the adsorbed phase volumes as 0.0826 and 0.0688 cm3/g, respectively, the surface areas for coals A and B can be approximately obtained as 275 and 229 m2/g, respectively, which are close to the values (234 and 193 m2/g, respectively) based on the DA equation in Table 3. Knowing the respective pore volumes as 0.0559 and 0.0333 cm3/g obtained from CH4 adsorption at 313 K, we estimate the corresponding surface areas as 186 and 111 m2/g, which are somewhat smaller than the above values based on CO2 adsorption. Therefore, it can be seen that the surface area obtained from CO2 adsorption at 273 K is a meaningful parameter to characterize porous media having small micropores.40 It is well-known that the adsorbed amount of CO2 is always higher than that of CH4 on coals and that the adsorbed CO2/ CH4 ratio (Ω) decreases with an increase in pressure (or fugacity).4 The value of Ω on high volatile bituminous coals (such as coals A and B used in this study) is generally assumed to be approximately 2, decreasing with increasing coal rank.4 Figure 9 compares the absolute and excess adsorbed amounts of CO2 and CH4 on both coals at 313 K. The ratio Ω decreases with pressure but exhibits three distinct regions. In region I, the average values of Ω are 2.32 and 2.53 for coals A and B, respec(39) Lozano-Castello, D.; Cazorla-Amoros, D.; Linares-Solano, A.; Oshida, K.; Miyazaki, T.; Kim, Y. J.; Hayashi, T.; Endo, M. J. Phys. Chem. B 2005, 109, 15032-15036. (40) Cazorla-Amoros, D.; Alcaniz-Monge, J.; de la Casa-Lillo, M. A.; Linares-Solano, A. Langmuir 1998, 14, 4589-4596.

tively, which are higher than 2. This is due to the higher Henry’s law constant for CO2, resulting from the higher adsorption affinity. At around 0.5 MPa, Ω starts to decrease sharply and then decreases slowly up to the pressure where the maximum in CO2 amount adsorbed appears, which is in region II. Beyond this, Ω starts to decrease sharply and follows the pattern of region II as the maximum in CH4 amount adsorbed is approached. It is interesting to see that the ratio of excess amounts adsorbed is less than 1 at pressures above 10 MPa, suggesting that during CO2 sequestration in coal seams CO2 pressures lower than 10 MPa may provide highest efficiencies. This is consistent with the recent simulation study of Kurniawan et al.41 suggesting an optimum pressure of about 9-10 MPa. To confirm this observation, it is necessary to have binary adsorption data of CH4 and CO2, which will be published in a subsequent paper. 3.5. Solid Volume Change. Using dual-energy X-ray computed tomography, Karacan42 has observed that the swelling and CO2 adsorption in coals are heterogeneous processes, depending on the maceral composition of the coal material. In the course of CO2 adsorption, an expansion-contraction behavior was observed in the macromolecular structure of mainly vitrites (which contain not less than 95% of vitrinite and not more than 5% of liptinite or inertinite), indicating that coals with higher vitrinite contents would yield a higher degree of swelling and possibly giving a reason for experimental observations43 that a lower carbon content correlates with a higher degree of swelling. According to a report from the Queensland Department of Mines and Energy,14 an inverse relationship between vitrinite and carbon contents is found among thermal coals in the Queensland region of Australia. Here, it is of interest to examine changes in coal volumes in the course of CO2 adsorption and the effect on the amount adsorbed. Because the sample volume increase leads to a decrease in the apparent weight (M1) (i.e., balance reading) of gravimetric adsorption measurements, it may be expected that the amount adsorbed will be greater than that shown in Figure 8 (which did not take into account the volume change). In the absence of exact knowledge on the volume changes of coals A (41) Kurniawan, Y.; Bhatia, S. K.; Rudolph, V. AIChE J. 2006, 52, 957967. (42) Karacan, C. O. Energy Fuels 2003, 17, 1595-1608. (43) Reucroft, P. J.; Patel, H. Fuel 1986, 65, 816-820.


Bae and Bhatia

Figure 10. Estimation of coal volume change in the course of CO2 adsorption at 313 K: filled circles represent experimental data at 298 K taken from Walker et al.13

Figure 8. Excess and absolute adsorption isotherms of carbon dioxide on coals A and B at four temperatures (313, 323, 333, and 343 K): the dashed lines represent the excess amount adsorbed, and the solid ones represent the absolute amount adsorbed, obtained from the Toth equation (eq 7).

Figure 11. Predicted effect of swelling on the excess amount adsorbed of CO2.

Figure 9. Ratio of the adsorbed amount of CO2 and CH4 on coals A and B at 313 K, based on fits of the Toth equation.

and B due to CO2 adsorption, data on a similar coal from the literature were used to estimate the effect on the amount adsorbed. Walker et al.13 used a microdilatometer to measure the volume expansion of various coals, one of which has similar properties to those of coal A (carbon, 85.71; oxygen, 6.84; nitrogen, 1.81; and volatile matter, 38.46 wt %, dmmf). The volume change (∆Vs, %) with respect to pressure at 298 K is shown in Figure 10 and is extended up to 20 MPa. It is noted that the volume change at 298 K in the literature is directly used to estimate the volume change of coal A at 313 K. Such

extrapolation is subject to uncertainty, in particular, whether the coal volume will increase further with a further increase in pressure beyond 5 MPa. Nevertheless, on the basis of the results from Walker et al.13 and the extrapolation in Figure 10, we estimate the effect of CO2-induced swelling on the excess amount adsorbed of CO2 on coal A at 313 K as shown in Figure 11. Here, we assumed that the fractional swelling of the solid skeleton mirrored the measured fractional swelling of the particle itself, as it was the latter that was measured by Walker et al. It can bee seen that the effect is negligible at low pressures (i.e., 10 MPa) may decrease the efficiency of CH4


recovery and CO2 storage. Thus, it is recommended to keep the CO2 injection pressure within region II in Figure 9. The effect of coal volume change on the excess amount adsorbed during CO2 adsorption is estimated from the literature without direct measurements. The effect is found to be substantial after the appearance of a maximum in the excess amount adsorbed, whereas it is insignificant within the pressure region of interest (region II of Figure 9). To confirm this behavior, binary adsorption data of CH4 and CO2 will be reported in a subsequent paper. EF060318Y

High-Pressure Adsorption of Methane and Carbon Dioxide on Coal

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