Pore Accessibility of Methane and Carbon Dioxide ... - ACS Publications

Two Australian coals were heat-treated, and the accessibility of the pore space to CH4 and CO2 was investigated. Samples heat-treated at 573 and 673 K...
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Energy & Fuels 2009, 23, 3319–3327

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Pore Accessibility of Methane and Carbon Dioxide in Coals Jun-Seok Bae, Suresh K. Bhatia,* Victor Rudolph, and Paul Massarotto DiVision of Chemical Engineering, The UniVersity of Queensland, QLD 4072, Australia ReceiVed February 2, 2009. ReVised Manuscript ReceiVed April 10, 2009

Two Australian coals were heat-treated, and the accessibility of the pore space to CH4 and CO2 was investigated. Samples heat-treated at 573 and 673 K exhibit larger adsorption/desorption hysteresis and smaller surface areas (measured by CO2 adsorption at 273 K) than untreated samples. For samples heat-treated at 773 K, however, the surface area increased by 50% and the hysteresis was lower, compared to untreated samples. These results demonstrate that volatile hydrocarbons at pore mouths are the cause of energy barriers that prevent adsorbing molecules from passing through. A conceptual model is proposed to illustrate changes in activation energy at constricted pore mouths. Also, the results suggest that both adsorption and desorption isotherms should be measured to determine kinetically inaccessible pore spaces in order to correctly estimate CH4 recovery and CO2 storage capacity. The results have importance to the problem of estimating CH4 recovery and CO2 storage capacity for CO2 geosequestration as part of a CO2-enhanced coal bed methane recovery operation.

1. Introduction Carbon dioxide injection into coal seams as part of enhanced coal bed methane (CO2-ECBM) recovery has been proposed as a technically and economically viable method for CO2 geosequestration and greenhouse gas (GHG) mitigation.1 Coals can generally hold two or more molecules of CO2 for every CH4 released at typical field pressures of 5-10 MPa required for CO2-ECBM. An underlying requirement for the estimation of CH4 recovery and CO2 storage capacity is an understanding of their sorption behavior on coals. This must be measured experimentally due to the complex and heterogeneous physical and chemical structures of coals. Very many sorption isotherms of CH4 and CO2 on a wide variety of coals have been reported in the literature, but most of these were conducted below their critical pressures. Recently, at high pressures, important phenomena of a maximum in the excess amount adsorbed2,3 and unclosed hysteresis before the maximum appearance have been reported.4 However, no clear explanation of the phenomena, especially for the latter, has been provided. This paper endeavors to explain why the phenomena take place by heat-treating coal samples and measuring highpressure sorption isotherms on these samples. Furthermore, we have observed that after the appearance of the maximum in the excess amount adsorbed, the hysteresis for CO2 disappears as the gas density approaches its critical density. The high-pressure sorption behavior of CH4 and CO2 on coals cannot be predicted from conventional methods such as N2 adsorption at 77 K (or Ar at 87 K) due to diffusional restrictions * Corresponding author. E-mail: [email protected]; phone: +61 7 3365 4263. (1) Katyal, S.; Valix, M.; Thambimuthu, K. Energy Sources, Part A 2007, 29, 193–205. (2) Ottiger, S.; Pini, R.; Storti, G.; Mazzotti, M.; Bencini, R.; Quattrocchi, F.; Sardu, G.; Deriu, G. EnViron. Prog. 2006, 25, 355–364. (3) Sakurovs, R.; Day, S.; Weir, S.; Duffy, G. Energy Fuels 2007, 21, 992–997. (4) Busch, A.; Gensterblum, Y.; Krooss, B. M. Int. J. Coal Geol. 2003, 55, 205–224.

at cryogenic temperature. In our previous study,5 we reported higher argon adsorption on coals at 313 K than at 87 K, which leads us to suggest that pore entrances or connections are constricted or blocked at cryogenic temperature, and that these blocked pores were increasingly accessible to adsorbing molecules at higher temperatures. Indeed, it bas been recommended to additionally use CO2 isotherms at 273 K to characterize narrow microporosity.6 For the prediction of high-pressure CH4 and CO2 adsorption on coals, CO2-based pore size distribution (PSD) is reported to be the most robust to predict the adsorption of the other gases most accurately.7 However, recently Rios et. al8 have reported that the adsorption of CO2 can be kinetically restricted even at 273 K on porous media having very narrow constrictions for short equilibrium times (i.e., 30 s). As coal is known to have ultra-micropores whose mouths can act as diffusional constrictions, a sufficient equilibrium time should be allowed to get correct equilibrium isotherms of CO2 even at 273 K. We have observed that at very low pressures (i.e., less than relative pressure of about 10-4) the equilibrium time at each pressure was at times more than 1 h. As we allowed sufficient time to reach equilibrium at each pressure point, the above concern on the kinetic restrictions of CO2 molecules at 273 K on coal was avoided. When using the CO2-based PSD, however, one needs to be careful as the presence of polar sites on coals may considerably affect the obtained PSD, especially at low temperatures.9 The microporous networks within coals, in which most of the sorption occurs, can be considered to be imperfectly connected, limiting the accessibility of adsorbing molecules to (5) Bae, J.-S.; Bhatia, S. K. Energy Fuels 2006, 20, 2599–2607. (6) Garrido, J.; Linares-Solano, A.; Martin-Martinez, J. M.; MolinaSabio, M.; Rodriguez-Reinoso, F.; Torregrosa, R. Langmuir 1987, 3, 76– 81. (7) Sweatman, M. B.; Quirke, N. J. Phys. Chem. B 2001, 105, 1403– 1411. (8) Rios, R. V. R. A.; Silvestre-Albero, J.; Seplveda-Escribano, A.; Molina-Sabio, M.; Rodrguez-Reinoso, F. J. Phys. Chem. C 2007, 111, 3803– 3805. (9) Samios, S.; Stubos, A. K.; Papadopoulos, G. K.; Kanellopoulos, N. K.; Rigas, F. J. Colloid Interface Sci. 2000, 224, 272–290.

10.1021/ef900084b CCC: $40.75  2009 American Chemical Society Published on Web 04/24/2009

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some pore spaces that are otherwise large enough to accommodate them.10 For these spaces to become accessible the energy barrier at the entrances must be overcome. That is, the adsorption behavior displays an “activated” character. Only molecules with sufficient energy can enter these pores during adsorption (or leave then during desorption)sa condition that cannot be realized within practical experimental time scales at the cryogenic conditions used in conventional N2 and Ar isotherm determinations. From an experimental perspective, even normal room temperatures may be insufficient to provide molecules with sufficient activity to overcome the potential barriers imposed by the constrictions in the pore networks, and high pressures may be necessary as well, in order to achieve equilibrium in a practical time (i.e., a few hours for each pressure point). Importantly, at practical temperatures and pressures for CO2 and CH4 sorption on coals, the energy of fluid molecules is insufficient for many of the activation barriers, so that some pore space is not available except over quite long equilibration times. We demonstrate this here by showing that the difference between the excess absorption and desorption amounts for CH4 and CO2 is almost constant below the excess sorption maximum. Some pores are not accessible for adsorption under these conditions, but if filled, are subsequently prevented from desorbing by the high energy barriers at the constrictions. It is consequently necessary to measure both adsorption as well as desorption isotherms to reveal the kinetically inaccessible pore spaces, in order to estimate correctly the CH4 recovery and CO2 storage capacity. We hypothesize that the physical constrictions and activation energy barrier at pore mouths or necks arises from volatile components, since it is known that the hysteresis becomes larger with decrease in the coal rank,11 and the rank has a correlation with the volatile matter content of coals. The volatiles usually consist of polyaromatic hydrocarbons. We explore this through the use of coals heat-treated under argon at 573, 673, and 773 K. Microscopic analysis with X-ray diffraction (XRD) and highresolution transmission electron microscopy (HRTEM) was used to reveal any structural alteration of the heat-treated coals. We show here our new observation that the open-hysteresis of CO2 disappears when the gas phase density approaches the critical density. Also, a conceptual model at pore mouths is proposed to explain how hydrocarbons inside coals contribute to the physical constrictions at pore mouths and how their contribution is lessened without significant structural alteration. 2. Experimental Section Two Australian coal samples, whose petrographic, proximate, and ultimate analyses are listed in Table 1, were used to study the open hysteresis phenomena. The carbon contents for these coals, designated as A and B, are 86.5 and 84.4% (air-dry basis), respectively. Coal A has larger vitrinite but lower inertinite content than coal B, suggesting that the former may have more adsorption capacity and smaller density. 2.1. Sample Preparation. The as-received coal samples (named as AO and BO) were ground down to 180-212 µm in size and were heat-treated at 573, 673, and 773 K in a tube furnace for 5 h each under nitrogen flow. The samples were brought to heat treatment temperature at a heating rate of 30 K/min. The resulting samples were named as A573, A673 and A773 for coal A, and B573, B673 and B773 for coal B, respectively. A SETARAM (SETSYS 16/18) thermogravimetric analyzer (TGA) was used to obtain the weight change of samples during heat treatment. (10) Nguyen, T. X.; Bhatia, S. K. J. Phys. Chem. C 2007, 111, 2212– 2222. (11) Ozdemir, E.; Morsi, B. I.; Schroeder, K. Langmuir 2003, 19, 9764– 9773.

Bae et al. Table 1. Physical Properties of Coals A and B component

coal A

coal B

Petrographic Analysis (vol %, mmf) vitrinite 54.7 liptinite 1.3 inertinite 44 mean maximum reflectance 0.85

16.3 4.9 78.9 0.90

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

14.5 2.3 26.3 56.9

carbon hydrogen nitrogen oxygen

Ultimate Analysis (wt %, adb) 86.50 5.00 2.00 5.95

84.43 4.98 1.71 8.40

2.2. Helium Density. The helium densities of the coal samples were measured using a high-pressure gravimetric adsorption analyzer, as well as a Micromeritics AccuPyc 1340 helium pycnometer. The results from both techniques were found to match each other within 1-2%, and we report here only the values from the high-pressure gravimetric measurements. Provided that that the adsorbed amount of helium on coals is negligible, we can estimate the sample volume from the weight change under helium environment, following

M(0, ∞) - M(FHe, T) ) Vtotal(T)FHe(T)

(1)

where M(0, ∞) and M(FHe, T) are the apparent weights measured at a high temperature under a vacuum and at a given helium bulk density, FHe, and temperature, T, respectively. Vtotal is the total volume, which includes the sample solid phase volume as well as the known sample basket volume. At several different helium pressures, the plot of bulk density against the left-hand side of eq 1 gives a slope that is the total volume, Vtotal. Since it is known that helium adsorption at high pressures is significant, the sample volume was measured at 378 K with helium pressure of less than 2 MPa, under which conditions the error caused by helium adsorption is considered to be small.12 2.3. X-ray Diffraction and High Resolution Transmission Electron Microscopy. Two of the most common nondestructive microscopic methods to characterize porous materials are XRD and HRTEM, and they are used in this study to observe any structural changes during heat treatment. A powder defractometer (Bruker D8 Advance XRD), fitted with a Cu-KR radiation source (40 kV, 30 mA, KR1 ) 1.54060 Å) and a graphite monochromator, was used over the 2θ range of 10-110° with a scanning speed of 1.2 °/min. The XRD patterns for the structural information were analyzed according to the conventional Scherrer equation. For HRTEM observation, the coal samples were crushed under ethanol in a mortar and pestle and deposited onto holey carbon film. The HRTEM images were obtained using a cryoelectron microscope (TECHNAI F30 FEGTEM) with 300 kV, which was equipped with a computerized imaging system (Gatan Image Filter). 2.4. Low- and High-pressure Adsorption Measurements. For low-pressure adsorption measurement, argon adsorption at 87 K and CO2 adsorption at 273 K were carried out using a Micromeritics ASAP 2010 volumetric adsorption analyzer to get the pore size distribution (PSD), as well as pore volume and surface area of each coal sample with the built-in density functional theory (DFT) software. A degassing condition of 378 K under a vacuum (10-4 Pa) for 24 h was applied in this study as it has been proven previously that the physical structure of sub/bituminous coals is (12) Sircar, S. Fundamentals of Adsorption 7; Kaneko, K., Kanoh, H., Hanzawa, Y., Eds.; IK International, Ltd.: Nagasaki, Japan, 2002; pp 656663.

Pore Accessibility of CH4 and CO2 in Coals

Figure 1. Adsorption/desorption isotherms of (a) CH4 and (b) CO2 on coal AO at 313 K.

not altered at this condition.5,13 High-pressure adsorption isotherms of argon, CH4, and CO2 on each coal sample were obtained using a gravimetric sorption system (Rubotherm Pra¨zisionsmesstechnik GmbH, Bochum, Germany). Further details of the apparatus are available elsewhere.5

3. Results and Discussion 3.1. Open Hysteresis in CH4 and CO2 Sorption at 313 K. Open hysteresis has been reported for CH4 and CO2 sorption isotherms on coals, most noticibly for low rank coals.4,11 This is known to be related to ultra micropores.14,15 Figure 1 shows the adsorption/desorption isotherms of CH4 and CO2 on coal AO at 313 K, illustrating this hysteresis. Consider first the CH4 isotherm shown in Figure 1a. The lower curve, using the closed symbols, shows adsorption of methane on a coal sample that has been comprehensively degassed (at 378 K under vacuum for 24 h). This curve is reproducible over many cycles. The upper curve (open symbols) shows the desorption isotherm. If the coal is not degassed following desorption, then the isotherm reproducibly follows the upper curve for subsequent adsorption and desorption cycles. Clearly, there is some part of the adsorbed methane that is retained on the coal and is not released during desorption. Figure 1b provides the isotherm on coal AO using CO2 as the sorbent fluid. At lower pressure (to the left of the maximum) the open hysteresis behavior is observed, whereas at high pressures (to the right of the maximum) the adsorption, (13) Miknis, F. P.; Netzel, D. A.; Turner, T. F.; Wallace, J. C.; Butcher, C. H. Energy Fuels 1996, 10, 631–640. (14) Conforti, R. M.; Barbari, T. A. Marcomolecules 1993, 26, 5209– 5212. (15) Ozdemir, E.; Morsi, B. I.; Schroeder, K. Fuel 2004, 83, 1085– 1094.

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desorption, and readsorption isotherms are equivalent. The actual transition, where the hysteresis disappears, occurs at slightly higher pressure than the adsorption excess maximum, at the critical density, which for CO2 at 313 K corresponds to 8.93 MPa. Above the critical density, adsorption, desorption, and readsorption isotherms collapsed into one curve, indicating that there is no significant hindrance for CO2 molecules to enter or leave the pore space. At 313 K the adsorptive pressure corresponding to the CH4 critical density is 22.82 MPa, which is beyond the pressure range considered here, so the corresponding behavior is not as clearly observed in Figure 1a. According to our interpretation, the open hysteresis is related to the pore space (V*) that is accessible only when the adsorbate potential is high enough to overcome the activation barriers, that is, at high pressures (P > P*). To estimate the volume of this initially inaccessible pore space, we consider the difference in the excess amount adsorbed during the adsorption and desorption of CH4 and CO2 (denoted as ∆Nexc). It may be noted that this difference is almost constant. Consequently, we consider that ∆Nexc is directly associated with the pore space (V*) that is inaccessible to CH4 or CO2 molecules below the pressure, P*. From Figure 1, the values of ∆Nexc are about 0.08 and 0.12 mmol/g for CH4 and CO2, respectively, for coal AO. Taking the adsorption capacities of single-center CH4 and CO2 with a slit width of 0.55 nm at 313 K as 0.440 and 1.230 g/cm3, respectively,16 the pore spaces (V*) for CH4 and CO2 can be estimated as 2.92 × 10-3 and 4.30 × 10-3 cm3/g, respectively, which is likely to be an underestimation because the maximum adsorbed phase densities were not reached at the pressure range studied here. The presence of the pore space (V*) may lead to underestimation of the CH4 content of a coal seam based on a canister desorption test, as well as of the CO2 storage capacity. Since it is linear in shape and has a smaller kinetic diameter, one would expect that CO2 can enter more restricted pore spaces than CH4.17 Complete desorption from the pore space (V*) takes place when the sample is heated at 378 K under vacuum for 1 day. Vacuum provides sufficient driving force for desorption of CH4 and CO2 from the pore space (V*), closing the “open hysteresis” loop. The existence of the pore space (V*) suggests that there are physical constrictions that hinder adsorptive molecules from entering the space (V*) at relatively low pressures (P < P*). The accessibility of pore spaces is known to become limited if the pore spaces are connected with narrow necks or have small pore mouths,10 a likely arrangement for coals due to their complex pore structure. Since the pore spaces between the constrictions are large enough to provide relatively little resistance to the movement of adsorptive molecules, the physical constrictions take place at connection points such as pore mouths or necks18 having high activation energy barriers. Figure 2 illustrates the pressure-dependent accessible pore space in coals whose structure includes physically constricted pore spaces in macropores, mesopores, and micropores.17 At pressures above P*, all the pore space becomes accessible to adsorptive molecules because there is sufficient chemical potential difference to provide the driving force for molecules to overcome the activation barrier and enter the pore space.10 This interpretation raises two questions: what are the physical constrictions at pore mouths that bar easy access to V*; and how can these barriers be reduced? On the basis of the (16) Bhatia, S. K.; Tran, K.; Nguyen, T. X.; Nicholson, D. Langmuir 2004, 20, 9612–9620. (17) Cui, X.; Bustin, R. M.; Dipple, G. Fuel 2004, 83, 293–303. (18) Koresh, J. E.; Kim, T. H.; Koros, W. J. J. Chem. Soc., Faraday Trans. 1 1989, 85, 1537–1544.

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Bae et al. Table 2. Weight Loss and Helium Density of Coals A and B by Heat Treatment helium heat helium weight density treatment weight density 3 temperature (K) coal A loss (%) (g/cm ) coal B loss (%) (g/cm3) 573 673 773

Figure 2. Illustration of accessible pore space only at P > P*. The coal pore structure was adapted from Cui, et.al.17

AO A573 A673 A773

3.54 11.46 20.08

1.36 1.38 1.39 1.49

BO B573 B673 B773

4.11 13.74 21.95

temperature range of 673-873 K, extensive evolution of hydrocarbons and hydrogen (which are relatively bulky molecules) takes place, causing an increase in helium density. This increase in helium density is accompanied by changes in helium accessibility of the pore space, and in the orientation of graphitelike layers and the distance between the layers.19 It may be expected that heat treatment increases pore accessibility to some degree, as a result of loss of hydrocarbons, which may otherwise block pore mouths. Although the weight loss for coal B due to heat treatment is generally higher than that for coal A as listed in Table 2, coal B lost relatively more volatile matter at 673 K while coal A lost more at 773 K. This is consistent with the work of Sakurovs et al.,20 who have reported that liptinite, which may consist of long-chain aliphatic materials, has considerably more mobility at lower temperatures than vitrinite. Since coal B has more liptinite and less vitrinite content than coal A, one would expect that more volatile components may evolve from coal B than coal A at 673 K, as confirmed in Figure 3. The helium densities of heat-treated coals obtained by using a gravimetric sorption analyzer are also listed in Table 2. The increase in helium density with heat treatment implies that the volatile matter (which evolved at temperatures up to 773 K) has smaller densities than that of the residual carbon framework and mineral matter, and that their evolution has increased helium accessibility to pore spaces. As will be discussed later, the former contributes more to the slight increase in helium density with heat treatment at temperatures up to 673 K, whereas the latter is more important above 673 K due to increased accessibility of the pore space. 3.3. Structural Alteration with Heat Treatment. 3.3.1. XRD Analysis. Coal has a graphite-like structure (called crystalline carbon) as well as some amorphous carbon, with the former structure being associated with peaks in the XRD pattern.21 Figure 4 depicts XRD profiles of coals AO, A573, A673, and A773. Here we have focused on the (002) peak as the other peaks were found to be insensitive to the heat treatment. The (002) peak is associated with the layer spacing of aromatic rings.

Figure 3. Normalized weight change during heat treatment for 1 h at 373 and 473 K, and for 5 h at 573, 673, and 773 K.

observation that the hysteresis becomes larger with decrease in the coal rank,11 and therefore the volatile matter content of coals, we hypothesize that it may be these volatile constituents that physically hinder adsorbed gas movement. To test this we remove some volatile matter by heat-treating the coals at 573, 673, and 773 K under a flow of nitrogen. Importantly, the heat treatment should remove volatiles but also not result in significant changes in the pore structure of coals. 3.2. Helium Density Change with Heat Treatment. Figure 3 depicts the normalized weight loss of coal samples due to heat treatment at temperatures of 573, 673, and 773 K for 5 h, respectively. The initial sample weight was taken as the one at 373 K. It is known that dehydration mainly occurs below 623 K, together with a small evolution of other gases.19 In the

1.41 1.42 1.48 1.55

Figure 4. XRD profiles of coals AO, A573, A673 and A773.

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Table 3. Parallelism Indicators and Structural Parameters Based on the Scherrer Equations coal sample

R (-)

Lc (nm)

La (nm)

AO A573 A673 A773 BO B573 B673 B773

2.69 2.64 2.63 2.68 2.89 2.89 2.78 2.79

1.17 1.12 1.32 1.42 0.94 1.04 1.07 1.31

1.20 1.27 1.25 1.61 1.43 1.33 1.55 2.07

This peak is highlighted in the inset of Figure 4. The asymmetric feature of the (002) peak, with a wider left-hand side (LHS), indicates the existence of the γ band that is connected to the packing distance of saturated structures such as aliphatic side chains on the edges of crystallites.21 The inset of Figure 4 shows that with an increase in heat treatment temperature, the LHS of the (002) peak moves slightly toward the (002) peak, implying that aliphatic side chains were removed, to some degree, due to heat treatment and that the distance between carbon atoms and their nearest neighbors are reduced. The ratio of the peak height over the background height at the position of the (002) peak is a parallelism indicator (R), which represents the probability of parallel-stacked graphene layers occurring in the crystalline structure.22 Parallel-stacked graphene layers are responsible for the existence of the (002) peak. The conventional Scherrer equations were applied here to study the changes of the crystallite height (Lc) and the crystallite width (La) with heat treatment temperature. Changes in the parallelism indicators and the structural parameters with heat treatment are listed in Table 3. The parallelism indicators are found to be virtually unchanged with heat treatment at temperatures up to 773 K, indicating that the crystalline structures of heat-treated coals were not significantly altered by heat treatment at temperatures up to 773 K. The crystallite width (La) of A773 increased relatively more than that of the other samples, suggesting that crumpled pore spaces are, to some degree, stretched out. The presence of high microporosity with poorly crystalline porous carbons can affect XRD patterns and the Scherrer equations strictly only apply to highly graphitzed carbon, so the interpretations about pore structure are only qualitative. 3.3.2. HRTEM Analysis. HRTEM provides direct images of local microstructure of heat-treated coals. Figure 5 shows that the microstructure of the coals studied was virtually unaltered by heat treatment at temperatures up to 773 K, confirming the XRD observations. Since the coal structure was not noticeably altered, the significant increase in helium density for coals A773 and B773 (as shown in Table 2) is most likely associated with release of volatile matters from the pore space and with removal of aliphatic side chains at pore mouths, increasing the pore accessibility. Recently, Sharma and co-workers23-25 have used a quantitative analysis of HRTEM images with a filtration technique and image analysis to obtain structural information of heat-treated coals. However, as can be seen in Figure 5, it is difficult to determine the number of stacking layers of coals heat-treated at up to 773 K, since for these coals the graphene layers are small, twisted, and lack coherent orientation. Considering the results of Sharma and co-workers,23-25 the ap(19) Toda, Y. Fuel 1973, 52, 99–104. (20) Sakurovs, R.; Lynch, L. J.; Maher, T. P.; Banerjee, R. N. Energy Fuels 1987, 1, 167–172. (21) Lu, L.; Kong, C.; Sahajwalla, V.; Harris, D. Fuel 2002, 81, 1215– 1225. (22) Tran, K. N.; Berkovich, A. J.; Tomsett, A.; Bhatia, S. K. Energy Fuels 2008, 22, 1902–1910.

Figure 5. HRTEM images of coals studied.

propriateness of the image analysis can be increased for heattreated coals at temperatures greater than 1073 K. 3.3.3. Low-pressure Adsorption. Low-pressure adsorption isotherms of argon at 87 K and CO2 at 273 K were obtained to investigate the effect of heat treatment on the pore size distribution, surface area, and pore volume of coals studied. The DFT surface areas obtained from argon adsorption at 87 K are found to be 1.81, 0.96, 0.85, and 0.44 m2/g for coals AO, A573, A673, and A773, respectively, which decrease with an increase in heat treatment temperature, indicating that heat treatment at temperatures up to 773K did not enhance the pore accessibility at cryogenic temperature. In our previous study,5 we have observed that the adsorbed amount of argon on coals is much higher at 313 K than at 87 K, suggesting that the lower pore (23) Sharma, A.; Kadooka, H.; Kyotani, T.; Tomita, A. Energy Fuels 2002, 16, 54–61. (24) Sharma, A.; Kyotani, T.; Tomita, A. Fuel 1999, 78, 1203–1212. (25) Sharma, A.; Kyotani, T.; Tomita, A. Fuel 2001, 80, 1467–1473.

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Table 4. Surface Areas and Pore Volumes of Coals Studied with CO2 Adsorption at 273 K sample

BET surface area (m2/g)

DFT surface area (m2/g)

DFT pore volume (cm3/g)

AO A573 A673 A773 BO B573 B673 B773

104 103 97 155 86 83 91 148

126 125 119 209 114 101 114 202

0.024 0.022 0.021 0.044 0.022 0.018 0.021 0.043

accessibility at cryogenic temperatures is possibly due to pore mouth constrictions and blockage.26 Thus, some pore spaces are inaccessible to argon molecules at 87 K but become accessible at higher temperatures, where the argon molecules have sufficient kinetic energy to overcome the energy barrier at pore mouths.10 Although argon adsorption at 87 K allows us to determine pores including mesopores and large micropores (>1 nm) that are not affected by pore mouth constrictions, the conventional methods such as nitrogen adsorption at 77 K or argon at 87 K are no longer valid for obtaining the surface area and pore size distribution of coals because of diffusional limitation at cryogenic temperatures and the complex network of pore structure in coals. Unlike the poor accessibility of argon molecules at 87 K to the pores smaller than 1.2 nm, CO2 molecules at 273 K can fill pores up to 9 Å in width at 1 bar,5 and has been therefore recommended for measurements of coal surface areas, despite the fact that CO2 adsorption at even 273 K may be kinetically restricted in narrow pores.8 The pore size distribution of any porous medium is an effective representation and is specific to the adsorptive because of the different accessibilities of adsorbing molecules. The surface areas and pore volumes of coals studied were obtained from CO2 adsorption at 273 K and are listed in Table 4. The DFT surface area of coal A decreased with heat treatment temperature up to 673 K, but then increased by about 65% at 773 K. Similarly, coal B exhibits a minimum DFT surface area below 673 K, but then about a 77% increase at 773 K. The same trend is found for the pore volume. An explanation for this behavior along with a proposed conceptual model is provided later. Notably, the DFT pore volume obtained from CO2 adsorption at 273 K is less than the total pore volume of the coals, which are 0.08 and 0.06 cm3/g for coals AO and BO, respectively, based on the helium and mercury densities.5 The increase in pore volumes of coals A and B by about 86 and 93%, respectively, as a result of heat treatment at 773 K, implies that volatile evolution provided additional accessible micropore spaces to the CO2. The pore size distributions shown in Figure 6 indicates a large number of pores in the size range of 0.4-0.7 nm became accessible after heat treatment at 773 K. Taken in conjunction with the increase in helium density, this result suggests that the increased pore spaces are made inaccessible to CO2 molecules at 273 K due to the presence of hydrocarbons at the pore entrances. 3.4. High-pressure Sorption on Heat-treated Coals. 3.4.1. Argon Sorption at 313 K. High-pressure argon sorption isotherms on heat-treated coals at 313 K are depicted in Figure 7, showing that hysteresis is present at 87 K, whereas it disappears at 313 K. Some energy barriers connecting pores prevent access by argon molecules at 87 K but are overcome at 313 K. This supports the concept of kinetically closed pores (26) 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.

Figure 6. Micropore size distribution obtained from CO2 adsorption at 273 K, using DFT.

Figure 7. Argon sorption isotherms on coals AO, A573, A673, and A773 at 313 K and on coal AO at 87 K (open symbols: adsorption; filled symbols: desorption).

(KCP) proposed recently,10 in which pore space that is closed to adsorbing molecules at a certain temperature becomes accessible at higher temperatures, where the adsorbing molecules have sufficient kinetic energy to overcome the energy barrier at pore mouths. It may be noted that the argon gas density at 313 K within the pressure range studied here is well below its critical density (0.538 g/cm3). Thus, similar to the case of high-pressure CO2 sorption on coal, which was discussed in Section 3.1, it is likely that hysteresis appears in argon sorption at 313 K if the pressure range is well above

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Figure 8. Adsorption/desorption isotherms of CH4 on coals AO, A673, and A773 at 313 K.

the pressure at which a maximum in the excess amount adsorbed appears. 3.4.2. Methane Sorption. The CH4 adsorption capacity of coal A, as shown in Figure 8, decreased with heat treatment at temperatures up to 667 K but increased at 773 K, which is consistent with the Ar adsorption isotherms at 313 K in Figure 7. The hysteresis in CH4 sorption isotherms on heat-treated coals is reduced with more aggressive heat treatment temperature, supporting the notion that physical constrictions at pores are reduced by hydrocarbon evolution. The maximum in the excess amount of methane adsorbed appears clearly only for coal A773 in the pressure range investigated. This behavior is comparable with Norit R1 activated carbon, which exhibits the maximum at a pressure of around 10 MPa at 313 K.27 In general, the maximum occurs at a lower pressure for a lower temperature,5 regardless of physical and chemical properties of the porous medium. This is partly because the gas phase density increases more quickly with pressure at lower temperatures. Another cause may arise from the way extra pore space becomes available as the kinetic barriers at the pore entrances are overcome. The appearance of the maximum is indicated but not clearly achieved for coals AO and A673 within the pressure range shown in Figure 8, supporting the hypothesis that some pores remain empty, reducing the overall adsorbed phase density with respect to pressure and leading to the appearance of the maximum at higher pressures than that for A773. This result suggests that the pressure at which the maximum appears may be a barometer for the degree of accessibility of the pore space at a given temperature. 3.4.3. CO2 Sorption. In terms of general trends, the hysteresis in CO2 sorption isotherms on heat-treated coals behaves similarly to CH4. A clear open hysteresis is evident in CO2 sorption isotherms on coals with heat treatment at temperatures up to 673 K, whereas the hysteresis almost disappears for coals heat-treated at 773 K (i.e., A773 and B773), as shown in Figure 9. The adsorbed amount of CO2 on coals A573 and A673 is less than that for coals AO during adsorption, but the hysteresis for P < 8 MPa increased with heat treatment at temperatures up to 673 K. This demonstrates that some CO2 molecules may be trapped in restricted pore spaces during desorption. The maximum in the excess amount of adsorbed fluid occurs when the increase in rate of the adsorbed phase density with (27) Puziy, A. M.; Herbst, A.; Poddubnaya, O. I.; Germanus, J.; Harting, P. Langmuir 2003, 19, 314–320.

Figure 9. Adsorption/desorption isotherms of CO2 on coals A (a) and B (b) at 313 K.

respect to pressure becomes equal to that of the bulk phase density. For example, the maxima in CO2 adsorption on coal A were found to appear at pressures ranging from 6 to 8 MPa. Coal A773 exhibits the maximum at around 6 MPa, indicating a higher degree of pore accessibility compared to the other samples (coals AO, A573, and A673). After the maxima appearance, all coal samples display inflection points at about 8.93 MPa at 313 K, which is shown clearly in Figure 9. This is consistent with our previous observations.5 The bulk density of CO2 at 8.93 MPa and 313 K is equal to its critical density (0.4676 g/cm3). Four important observations arise from the high pressure region of Figure 9, which is shown in greater detail in Figure 10. First, for P < P* the pore space that is inaccessible to CO2 (V*) is almost constant for all heat-treated coals. The difference (∆Nexc) between CO2 adsorption and desorption isotherms on heat-treated coals can be converted to the inaccessible pore space (V*). By way of example, we take the adsorption capacity (1.230 g/cm3) of single-center CO2 with a slit width of 0.55 nm at 313 K.16 The results are listed in Table 5. The real values of V* will probably be higher than those estimated in Table 5 because the adsorbed phase density does not reach its maximum value within the pressure range studied. On this basis, the inaccessible pore space V* decreased by about 96% for both coals A and B by heat treatment at 773 K, that is, almost all pores are made accessible to CO2 molecules. Second, at pressures beyond 8.93 MPa, Figure 10 shows that the CO2 excess amount adsorbed on moderately heattreated coals (i.e., except for coals A773 and B773) decreases more slowly than that for coals AO and BO. Since the gas phase density with respect to pressure are the same for all

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Figure 11. A conceptual model of potential energy changes at pore mouths due to heat treatment.

Finally, the CO2 adsorption curves for coals A773 and B773 are steeper than those for coals AO and BO at the critical gas pressure (8.93 MPa), for coal B, that is,

| dFdP |

a B773 P>P*

Figure 10. Adsorption/desorption isotherms of CO2 on (a) coal A and (b) coal B at 313 K, for pressures greater than 5 MPa. Table 5. Estimation of Inaccessible Pore Space to CO2 Only at P < P* and 313 K for Coals Studied sample

AO A573 A673 A773 BO B573 B673 B773

∆Nexc (mmol/g) × 10 1.20 1.84 V* (cm3/g) × 103 4.29 6.58

1.87 6.69

0.04 0.80 1.80 0.14 2.86 6.44

1.70 6.08

0.03 0.11

coal samples at a given pressure and temperature, the difference must result from the adsorbed phase density change. That is,

| dFdP |

a AO P>P*


P*

(2)

where Fa is the adsorbed phase density, and P is the pressure. This indicates that coal A673, for example, has pore space that is less accessible to CO2 molecules than coal AO at relatively low pressures (P < P*). At high loading (P > P*) there is sufficient driving force for CO2 molecules to enter the restricted pore space for the heat-treated coals, causing the RHS of eq 2 to be higher than the LHS, and consequently giving rise to a slow decrease in the excess amount adsorbed. Third, unlike coals AO and BO, hysteresis was found for coals A573, A673, B573, and B673 at high pressures (P > P*) as seen in Figure 10, confirming that adsorbed CO2 molecules can not easily adsorb/desorb in/from the pore space of these samples. Coals A773 and B773, however, do not exhibit hysteresis at high pressures (P > P*), and show much less hysteresis at low pressures (P < P*) than coals AO and BO, implying that restrictions at pore mouths are significantly reduced. Since no noticeable changes were found in the pore structure of all heat-treated coals, this supports the idea that the hysteresis is associated with pore accessibility of adsorbing molecules at restricted pore mouths.


P*


P*

(3)

This is consistent with our previous observation that the slope of excess amount adsorbed against pressure becomes steeper when the pore volume increases,5 causing the magnitude of dFa/ dp for coal B773 to be less than that for coal BO. The pore volumes obtained from CO2 adsorption at 273 K are 0.043, 0.022, and 0.021 cm3/g for coals B773, BO, and B673, respectively, as listed in Table 4. 3.5. Conceptual Description of Constrictions at Pore Mouths. In Section 3.1, two questions about the nature of the physical constrictions were raised. Since the physical constrictions are significantly reduced by heat treatment at 773 K, and there was no noticeable alteration in the coal structure by heat treatment, we conclude that hydrocarbons play an important role in the degree of pore accessibility. This raises the question as to why the pore accessibility decreased by heat treatment at temperatures up to 673 K. Here we speculate that this is a result of changes to the activation energy at pore connections, which change due to hydrocarbon evolution by heat treatment. Figure 11 illustrates a conceptual model of the opposing interactions that arise from the alteration. Considering, for example, a CO2 molecule moving from space A to space B through a constricted pore mouth of about the size of a CO2 molecule, heat treatment up to 673 K serves to decrease the potential energy of pore space A due to removal of hydrocarbons from space A, so that a CO2 molecule moving from A to B requires a higher activation energy. At the neck itself, removal of some hydrocarbons would remove physical barriers that the CO2 molecule needs to overcome to move through the neck, that is, decrease of the activation energy of the barrier. On the other hand, if hydrocarbons are released from pore space B as a result of heat treatment and seek to escape through C, then the trend is for the physical barrier at the neck to be increased. The final outcome regarding whether transport through the pore is facilitated or retarded depends on the balance between these conflicting tendencies. Up to 673 K (on these coals) heat treatment increases the activation barrier,

Pore Accessibility of CH4 and CO2 in Coals

whereas at high temperatures, illustrated here by 773 K, the barrier is reduced. In addition to the balance discussed above, heat treatment at 773 K increases the miroporosity (as shown in Figure 6), and it may also enlarge the pore necks through stretching, for which Section 3.3 provides some XRD evidence. This is also consistent with argon adsorption on a carbon molecular sieve that was heat-treated at temperatures lower than 973 K by Verma and Walker,28 who suggested that heat treatment enlarged some pore entrances and possibly created some additional pore volume. 4. Conclusions Conventional measurements of sorption isotherms for coal using N2 at 77 K or Ar at 87 K provide incorrect and misleading results regarding the pore capacity because the measurements do not achieve even close to equilibrium conditions within a tractable experimental time. A large amount of pore space is not accessible at these conditions because the sorbing molecules do not have sufficient energy to enter pores that have significant activation energy barriers. Even if the sorption isotherms are measured at higher temperatures, for example at 313 K, there is a substantial volume of inaccessible pores. The inaccessible (28) Verma, S. K.; Walker, P. L., Jr. Carbon 1990, 28, 175–184.

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pore volume may be approximately estimated from the difference between the open hysteresis legs of the adsorption and desorption isotherms. On this basis it is necessary to measure adsorption as well as desorption isotherms for CH4 and CO2 on coals to determine kinetically inaccessible pore spaces in order to correctly estimate CH4 recovery and CO2 storage capacity. At high pressures, beyond the critical gas pressure that corresponds to the critical density, the pores are essentially all accessible, indicating that adsorbing molecules can enter and leave the restricted pore spaces at a given temperature within practical time scales. Results from heat-treated coal samples provide evidence that volatile hydrocarbons at pore mouths are the likely cause of the barriers that prevent adsorbing molecules from entering some pore spaces unless they have sufficient energy to overcome the activation energy. A conceptual model is proposed in this study to illustrate what is happening at consecutive pore spaces with constricted pore mouths. Acknowledgment. The authors gratefully acknowledge Mr. Kien Tran for TGA measurements and Dr. John Barry for HRTEM measurements. The work is supported by grants from the Australian Research Council. EF900084B