Coal Lithotypes before, during, and after Exposure to CO2: Insights

Apr 30, 2012 - Migration of metamorphic CO 2 into a coal seam: a natural analog study to assess the long-term fate of CO 2 in Coal Bed Carbon Capture,...
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Coal Lithotypes before, during, and after Exposure to CO2: Insights from Direct Fourier Transform Infrared Investigation Maria Mastalerz,*,† Angela Goodman,‡ and Danielle Chirdon§ †

Indiana Geological Survey, Indiana University, 611 North Walnut Grove Avenue, Bloomington, Indiana 47405-2208, United States National Energy Technology Laboratory, United States Department of Energy, P.O. Box 10940, Pittsburgh, Pennsylvania 15236, United States § Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States ‡

ABSTRACT: Vitrain, clarain, and fusain lithotypes of Pennsylvanian age high volatile bituminous coal from the Springfield Coal Member of the Petersburg Formation and the Lower Block Coal Member of the Brazil Formation from Indiana were examined using standard and in situ attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) prior, during, and after exposure to CO2 to investigate any potential physical or chemical alterations of the coal lithotypes. These lithotypes were distinct petrographically (vitrinite content ranged from 94.0 to 11.6 vol %), as well as with regard to surface area, microporosity, mesoporosity, and Langmuir parameters (volumes and pressures). Specifically, BET surface area for the Springfield Coal ranged from 6.0 m2/g in fusain to 10.0 m2/g in vitrain and for the Lower Block Coal ranged from 15.9 m2/g in fusain to 115.4 m2/g in vitrain. For the Lower Block Coal, Langmuir volumes (on an as-received basis) were 51 cm3/g in vitrain, 37 cm3/g in clarain, and 34 cm3/g in fusain; for the Springfield Coal, 42 cm3/g in vitrain, 42 cm3/g in clarain, and 24 cm3/g in fusain. During experiments performed at 17 °C and CO2 pressure up to 4.1 MPa (600 psig), the only observed changes in the infrared spectra were due to sorption of CO2 reflected by bands at 2333 cm−1 (with a shoulder at 2320 cm−1) and at 657 cm−1. These absorption bands increased in intensity as CO2 pressure increased, but they disappeared after desorption of CO2 in flowing nitrogen, suggesting physical sorption of a reversible nature. Absorption bands characteristic of the lithotypes did not change during or after CO2 exposure. Comparing the CO2 sorption capacity among various lithotypes suggests that vitrains adsorb the most CO2 and fusains the least. Also varying amounts of time were needed to fill the available pore space; more time was needed to saturate vitrains with CO2 than the other lithotypes.

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INTRODUCTION Carbon dioxide (CO2) sorption into coal has been widely studied for many years, providing information about rates and volumes of adsorbed CO2.1−5 Over the past decade, this subject has received special attention because of the possibility of storing CO2 in unminable coal seams in order to reduce greenhouse emissions into the atmosphere. For CO 2 sequestration to be successful, there needs to be knowledge about the sorption mechanisms as well as an understanding of potential changes in the structure of the coal in response to the injection of CO2 at specific temperature and pressure conditions. Understanding of CO2 interactions with coal is important not only to predict long-term effects of CO2 storage, but it should also help to properly model injection potential changes over time. However, these interactions between CO2 and coal are complex and depend on many factors including temperature and pressure conditions, coal rank, and pore size distribution. Therefore, in spite of extensive research on the subject, numerous questions remain unanswered. It has not been resolved if CO2 sorption is exclusively a physical phenomenon (adsorption) or if there are also some chemical reactions of CO2 with functional groups in coal. Some studies suggested that there might be some chemical reactions of organic matter with CO2 (absorption) taking place at high pressure,6 including mobilization of polycyclic aromatic hydrocarbons.7 In contrast, other studies suggested that there were © 2012 American Chemical Society

no chemical interactions between the injected CO2 and coal functional groups, supporting the concept of exclusively physical adsorption of CO2.8,9 Another question relates to the influence of coal composition, particularly specific lithotype composition on CO2 injection potential and storage at reservoir conditions. Despite much discussion in the literature on the influence of maceral composition on the CO2 adsorption in coal,10 the significance of lithotype composition has not been adequately researched.11 Modeling CO2 injection into coal beds currently relies on experience from conventional reservoirs, not taking into account the lithological heterogeneity of coal. Yet, coal is a heterogeneous material and its megascopic differences are expressed as different lithotypes, with vitrain (bright), durain (dull), and fusain (fibrous) being the end members with regard to the content of vitrinite and inertinite maceral groups. Lithotypes have different characteristics with regard to porosity, cleating, hardness, and grindabilityproperties that are expected to influence CO2 injection potential. For example, for high volatile bituminous coals from Indiana,11 it has been demonstrated that there are significant differences in micropore and mesopore characteristics among vitrain, clarain, and fusain lithotypes, with fusains having the smallest micropore and Received: March 2, 2012 Revised: April 24, 2012 Published: April 30, 2012 3586

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mesopore volumes and vitrains usually having the highest. Moreover, these lithotypes also have different CO2 adsorption capacities; for a given coal, vitrain usually has the largest adsorption capacities and fusain the smallest. How significant are these differences for CO2 sequestration? Are they of a magnitude that could cause changes in CO2 injection potential or selectively change the physical or chemical properties of the coal matrix? If so, perhaps lithotype composition should be incorporated into CO2-injection models for coals, as suggested in previous studies.12 This study attempts to address some of these questions by investigating selected lithotypes of high volatile bituminous coal before, during, and after CO2 exposure using FTIR techniques. The specific objectives are to further understand: (1) the physical versus chemical nature of CO2 sorption; (2) the selectivity of the lithotypes to CO2 adsorption; and (3) the effects of CO2 exposure on the measurable physical and chemical properties of the lithotype matrix.

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Table 2. Mesopore (Nitrogen Adsorption) and Micropore (CO2 Adsorption) Characteristics of the Lithotype Samples Studieda BET SA [m2/g]

Springfield coal vitrain 0.56 clarain fusain Lower Block coal vitrain A 0.57 clarain fusain a

inertinite [vol %]

MMa [vol %]

89.2 83.8 12.0

6.0 1.2 0.0

3.4 9.8 85.0

1.4 5.2 3.0

94.0 72.2 11.6

2.0 8.4 13.2

4.0 18.2 72.4

0.0 1.2 2.8

0.013 0.015 0.012

125.0 114.2 72.8

0.059 0.055 0.032

0.091 0.052 0.036

144.8 121.6 98.2

0.069 0.058 0.044

Table 3. Langmuir Volume and Pressure of the Lithotype Samples Studied Langmuir volume [scf/t, as received] Springfield coal vitrain 1352 clarain 1330 fusain 765 Lower Block coal vitrain 1631 clarain 1196 fusain 1086

Table 1. Petrographic Composition (vol %) and Vitrinite Reflectance (Ro random) of the Lithotype Samples Used in This Study liptinite [vol %]

DA micropore volume [cm3/g]

a SA, surface area; BET, Brunauer−Emmett−Teller; BJH, Barrett− Joyner−Halenda; DR, Dubinin−Radushkevich; DA, Dubinin−Astakhov.

EXPERIMENTAL SECTION

vitrinite [vol %]

DR micropore surface area [m2/g]

Springfield coal vitrain 10.0 clarain 11.0 fusain 6.0 Lower Block coal vitrain 115.4 clarain 43.1 fusain 15.9

Sample Material. Hand-picked vitrain, clarain, and fusain lithotypes from the Springfield Coal Member (SPR) and the Lower Block Coal Member (LB) (Pennsylvanian age) were used in this study. For each coal, these lithotypes were collected from one location (coal mine) to avoid any geographic or rank influence on coal characteristics. Durain was not present in these locations and therefore is not included in this study. Fresh coal blocks were collected in coal mines and subsequently lithotypes were carefully isolated from those blocks in the laboratory.

Ro [%]

BJH mesopore volume [cm3/g]

Langmuir volume [cm3/g, as received]

Langmuir pressure [psi]

Langmuir pressure [MPa]

42 42 24

540 520 340

3.72 3.59 2.34

51 37 34

502 436 398

3.46 3.00 2.74

Infrared spectra were collected at room temperature. The bare diamond ATR crystal was used as a reference. For each sample, 500 scans were collected at a resolution of 2.0 cm−1 over the spectral range of 4000−600 cm−1. In-situ ATR-FTIR Infrared Spectroscopy. In-situ ATR-FTIR spectroscopy experiments were carried out using two attenuated total reflectance (ATR) cells from Spectra Tech in an assembly that has been described previously.8,13 Briefly, two high-pressure (up to 136 atm) stainless steel cells were connected in tandem so that each cell experienced the same atmosphere. A cylindrical zinc selenide (ZnSe) ATR crystal was placed in each cell, one bare and one coated with a coal lithotype. To prepare the sample, a coal lithotype/water slurry was painted onto the ATR crystal, sealed, and stored under an ultra high purity (UHP) nitrogen flow for 24 h. Desorption of the water was verified by the disappearance of the respective IR adsorption bands near 3600 and 1632 cm−1.14 To perform gas adsorption experiments, the assembly was connected to a gas handling system composed of a syringe pump (ISCO model 260D), a gas introduction port, and a pressure transducer (OmegaDyne Inc. PX01K1−5KGV). Carbon dioxide (CO2) and UHP nitrogen (N2) were used as supplied from Butler Gas. All ATR-FTIR data were collected using a single beam FTIR spectrometer (Thermo Electron Nexus 4700 FT-IR ESP) equipped with a liquid nitrogen cooled wide-band MCT (mercury−cadmium− telluride) detector. Unless otherwise noted, 500 scans were collected at a resolution of 2.0 cm−1 over the spectral range extending from 4000 to 600 cm−1 with at least 24 h of equilibration time between dosing of the reactor and collection of data. Each sample was thermally equilibrated at 17 °C (corresponding to the average reservoir temperature of Indiana coals as indicated by available temperature logs) by recirculating a 50/50 mixture of ethylene glycol and water through a temperature jacket surrounding the high-pressure cell. Coal lithotypes were exposed to a known pressure of CO2 and subsequently sealed from the gas source. Two

Mineral matter.

For both coals, significant differences exist between these three lithotypes in terms of maceral composition (Table 1), surface areas, mesopore and micropore volumes (Table 2), and Langmuir parameters (Table 3). BET surface areas and mesopore volumes were determined by nitrogen adsorption and micropore volumes were determined by carbon dioxide adsorption.11 Both coals represent the same coal rank (high volatile C bituminous with Ro 0.56% for the Springfield and 0.57% for the Lower Block, Table 1), but the lithotypes of the Lower Block Coal have a much higher surface area, micropore and mesopore volume, and CO2 adsorption capacity than those of the Springfield Coal. The only exception is clarain, which has higher Langmuir volume in the Springfield Coal than in the Lower Block Coal (Table 3). Standard Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) Spectroscopy. Standard FTIR spectroscopy experiments were carried out using a standard cell from SensIR Technologies (071-1503). In these experiments, coal lithotypes were pressed directly against a diamond total reflectance ATR crystal. 3587

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situ ATR-FTIR apparatus cannot be firmly pressed against the ATR crystal as is done with the standard ATR-FTIR cell, which, in some cases, may cause spectral distortions. In situ ATR-FTIR spectra were collected of the vitrain sample without CO2 present (Figure 2A, spectrum ii). In this case, the standard and in situ ATR-FTIR spectra are identical except in the region between 1600 and 1500 cm−1, where spectral distortions may be occurring.15 Weak absorptions detected at 1201 and 1146 cm−1 are due to the Teflon o-rings used to make the highpressure seal between the ATR crystal and stainless steel chamber. The interaction of CO2 with the vitrain sample was carefully monitored as a function of increasing pressure by using ATRFTIR spectroscopy (Figure 2A). The sample was exposed to a known pressure of CO2 and then sealed off from the CO2 source. Spectra were then recorded until equilibrium was reached, namely, until CO2 and coal-lithotype absorption bands no longer changed with time. Equilibrium was reached within hours, but adsorption was monitored for a minimum of 24 h at each pressure step. This procedure was repeated by increasing CO2 pressure in the pressure steps near 0.34, 0.69, 1.38, 2.76, and 4.14 MPa (50, 100, 200, 400, and 600 psig). Infrared measurements were collected at 17 °C. An infrared spectrum of the LB vitrain in the presence of CO2 (3.9 MPa) is shown in Figure 2A (spectrum iii). Comparison of the spectrum without CO2 (spectrum ii) to that with CO2 at 3.9 MPa (spectrum iii) demonstrates that all absorption bands between 1800 and 700 cm−1 remained unchanged upon exposure to CO2. After desorption of CO2 in flowing nitrogen, all absorption bands between 1800 and 700 cm−1 still remained unchanged (Figure 2A, spectrum iv), thus indicating that the interaction between CO2 and this vitrain did not induce chemical alterations of the coal that could be detected by FTIR. Similar infrared data were observed for the five remaining lithotype samples; they show that there were no changes in chemical functional groups of the lithotypes before, during, or after CO2 exposure (see Figure 2B−F). These observations are in agreement with previous studies of CO2 injection into a similar rank coal at high pressure using neutron scattering techniques that also suggested no change in coal structure.16 However, a recent study of coal lithotypes before and after (but not during) high-pressure CO2 injection using advanced solidstate 13C nuclear magnetic resonance spectroscopy (NMR) documents changes in the abundance of aliphatic functional groups, aromaticity, and the size of aromatic rings in some postCO2 injection lithotypes.17 It is possible that some subtle molecular changes occur in the coal, but they either do not affect, or are too weak to change, the overall vibrational signature during FTIR analysis; yet, they could be detected by NMR. The sorption of CO2 on the coal lithotypes in situ was also investigated as a function of increasing CO2 pressure. Again, for illustrative purposes, the vitrain lithotype from Lower Block will be discussed in detail. As the vitrain sample was exposed to CO2 as a function of pressure at 17 °C, absorption bands at 2333 cm−1 having a shoulder at 2320 and 657 cm−1 appeared in the spectra (Figure 3). These absorption bands increased in intensity as CO2 pressure was raised from 0.5 to 3.9 MPa. The adsorption bands at 2332, 2320, and 657 cm−1 were observed previously and are indicative of physically adsorbed CO2.18−23 Gaseous CO2 is a linear molecule having two infrared-active absorption bands at 2349 cm−1 (ν3 antisymmetric stretching mode) and 667 cm−1 (ν2 bending mode).24 When CO2

spectra were recorded for each data point; one from the cell containing the coal-lithotype-coated ZnSe crystal and one from the cell with the bare crystal, both under the same atmosphere. Spectra of coal lithotypes and adsorbed gases were determined by taking the difference between the sample cell and the blank cell, thus eliminating any contribution from the gas phase.1 The absorption of sorbed CO2 was monitored by following the growth of the ν3 antisymmetric stretching mode and ν2 bending mode near 2333 and 657 cm,−1 respectively. The functional groups of the coal lithotypes were also monitored between 4000 and 600 cm−1 as a function of increasing CO2 pressure.

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RESULTS AND DISCUSSION Standard ATR-FTIR Spectroscopy. Standard ATR-FTIR spectra of fusain, clarain, and vitrain lithotypes from the Springfield (SPR) and Lower Block (LB) prior to CO2 exposure are shown in Figure 1. Vitrain and clarain spectra

Figure 1. Graph of standard ATR-FTIR spectra of fusain, clarain, and vitrain lithotypes from the Springfield Coal Member (SPR) and the Lower Block Coal Member (LB).

are very similar and, as expected for coal of this rank, are characterized by a distinct aliphatic stretching region (2800− 3000 cm−1), prominent aromatic carbon with a peak at ∼1595 cm−1, bands in the aliphatic bending region (∼1350−1450 cm−1), and aromatic out-of-plane bands in the 700−900 cm−1 region. Fusain, expectedly, differs from vitrain by less prominent aliphatic bands in the aliphatic stretching region and more distinct aromatic bands in the aromatic out-of-plane region (700−900 cm−1). Prominent bands in the region of 1000−1200 cm−1 likely result from the contribution of mineral matter and clay minerals, in particular. In-situ ATR-FTIR Spectroscopy. To understand any chemical or physical alterations in the lithotypes owing to the exposure to CO2 at high pressure, coal lithotypes were examined before, during, and after CO2 exposure using in situ ATR-FTIR spectroscopy because this technique can detect both physically adsorbed CO2 as well as that absorbed into coal functional groups. For illustrative purposes, the vitrain lithotype from Lower Block will be discussed in detail. The in situ ATRFTIR spectra were compared to the standard ATR-FTIR (Figure 2A, spectrum i) to determine if the two techniques result in comparable spectral signatures. The difference between these two techniques is that the samples in the in 3588

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Figure 2. FTIR spectra of coal lithotypes. (A) Standard and in situ ATR-FTIR spectra of vitrain lithotype from Lower Block (LB): (i) Standard ATR-FTIR spectrum with no CO2. (ii) In situ ATR-FTIR before exposure to CO2. (iii) In situ ATR-FTIR with 3.9 MPa CO2 at the end of ∼72 h. (iv) In situ ATR-FTIR after removal of CO2. Graphs B−F show the same type of spectra as in graph A but for SPR vitrain (B), LB clarain (C), SPR clarain (D), LB fusain (E), and SPR fusain (F).

corresponds to the ν3 antisymmetric CO2 stretching mode and the absorption band 657 cm−1 corresponds to the ν2 CO2 bending mode. The shoulder at 2320 cm−1 was also observed

physically interacts with a solid surface, the resulting absorption band frequencies shift slightly from the gas phase values.25 In this study, the observed absorptions band at 2333 cm−1 3589

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Figure 3. Graph of in situ ATR-FTIR spectra of sorbed CO2 on vitrain lithotype from Lower Block (LB) as a function of increasing CO2 pressure (0.5, 0.8, 1.5, 2.8, and 3.9 MPa). The left side shows the ν3 antisymmetric stretching mode of sorbed CO2 between 2348 and 2280 cm−1, whereas the right sides shows the ν2 bending mode of sorbed CO2 between 670 and 640 cm−1.

previously and was assigned to a combination band.26−32 After desorption of CO2 in flowing nitrogen, the physically sorbed CO2 disappeared from the vitrain spectrum, indicating that the interaction between CO2 and this vitrain did not induce chemical alterations of the coal that could be detected by FTIR. Similar infrared data were observed for the five remaining lithotype samples; absorption bands for sorbed CO2 were detected at 2333 cm−1 (with a shoulder at 2320 cm−1) and 657 cm−1, and these bands disappeared after CO2 desorption in flowing nitrogen. To investigate the CO2 sorption on lithotypes further, FTIRderived adsorption isotherms for the six lithotype samples were constructed using infrared data. Isotherms can be derived by plotting the integrated areas of the peak at 2333 or 657 cm−1 of the sorbed CO2 versus CO2 pressure. Typically, the shape of infrared isotherms can be directly compared to traditional volumetric isotherms; however, quantification of infrared isotherms in terms of the amount of CO2 sorbed per gram of coal is difficult. Overall trends in data for comparing various samples can be generated by normalizing CO2 sorption bands to standard coal absorption bands, but this type of normalizing should not be used to quantify the amount of sorption. In this study, gas adsorption isotherms were generated by monitoring the relative changes in the integrated area of the sorbed CO2 at 2333 cm−1 (ν3 antisymmetric mode) normalized to the area of the aromatic out-of-plane bands in the 955−876 cm−1 region (Figure 4A). These bands were chosen to be normalized to because they consistently did not change upon CO2 exposure for all six lithotypes. The normalized FTIR isotherm data show that LB vitrain and SPR vitrain sorb the most CO2, followed by the two clarain and fusain lithotype samples. The smaller absorbance of fusain and clarain than that in vitrain, in turn, testifies for their smaller adsorption capacity, as expected from their surface areas, micropore volumes, and Langmuir parameters (Tables 2 and 3). Interestingly, the relationships among adsorption capacities of lithotypes as reflected by FTIR isotherms are different than those suggested based on standard

Figure 4. (A) In situ ATR-FTIR isotherms of the lithotypes as a function of increasing CO2 pressure at 17 °C generated from the integrated area in terms of arbitrary units (a.u.) of the ν3 antisymmetric stretching mode of sorbed CO2 between 2348 and 2280 cm−1 normalized to that of characteristic sorbent bands between 955 and 876 cm−1. (B) Standard high pressure isotherms of the lithotypes studied in terms of cm3/g. (C) In situ ATR-FTIR integrated area in terms of arbitrary units (a.u.) of the ν3 antisymmetric stretching mode of sorbed CO2 between 2348 and 2280 cm−1 normalized to that of characteristic sorbent bands between 955 and 876 cm−1 as a function of time after dosing with CO2 pressure near 0.5 MPa.

high-pressure adsorption isotherms (Figure 4B). For example at 3 MPa, based on the FTIR-derived isotherms, LB vitrain adsorbs about five times more CO2 than clarain and ten times more than LB fusain (Figure 4A). In the standard high-pressure isotherm generated on the coal crushed to 60 mesh, at a comparable pressure of 3 MPa, LB vitrain adsorbs approximately 20 and 30% more CO2 than clarain and fusain, respectively. In the Springfield Coal, SPR vitrain adsorbs ∼40% more CO2 than SPR fusain (Figure 4B). This comparison confirms that, while FTIR-derived isotherms show similar trends to those of the standard high-pressure isotherms, they cannot be directly compared with regard to ratios between the sorption capacities of individual lithotypes. 3590

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ACKNOWLEDGMENTS We thank John A. Rupp, Indiana Geological Survey, Indiana University, for useful suggestions and discussion. Comments of two anonymous reviewers greatly improved the paper.

Equilibration of the six lithotypes with CO2 was further investigated by monitoring the relative changes of sorbed CO2 over time, beginning from the time (0) when the lithotypes were first exposed to CO2. Specifically, relative changes in the integrated area of the sorbed CO 2 at 2333 cm −1 (ν 3 antisymmetric mode) normalized to that of coal lithotypes bands (955−876 cm−1) were monitored as a function of time at the initial CO2 exposure near 0.5 MPa (Figure 4C). A significant increase in the amount of sorbed CO2 for the LB and SPR vitrains up to 25 h of data collection was observed, whereas there was no increase in sorbed CO2 for fusain and clarain up to 50 and 95 h, respectively. This lack of increase in absorbance for fusain and clarain within the data collection period at a constant pressure may indicate that CO2 accesses all the open porosity almost instantaneously in fusain and clarain, possibly because of better connection between the pores compared to relatively tight vitrinite. Petrographically, these fusains and clarains have high inertinite contents and larger pore sizes compared to vitrain,11 which may contribute to better connectivity and gas transmissibility between the pores.

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REFERENCES

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CONCLUSIONS This study reports on the examination of coal lithotypes (vitrain, clarain, and fusain) from two high volatile bituminous Pennsylvanian age coals (the Springfield Coal Member and the Lower Block Coal Member) from Indiana before, during, and after CO2 injection at high-pressure conditions using in situ FTIR technique. The temperature at which the experiments were performed was 17 °C, and the pressure steps used were 0.34, 0.69, 1.38, 2.76, and 4.14 MPa . The main conclusions of this study are (1) FTIR spectra before, during, and after CO2 adsorption remain identical except CO2 bands characterized by the peaks at 2333 and 657 cm−1 that appeared in response to CO2 injection and increased with increasing pressure. Disappearance of these bands after the desorption of CO2 suggests the physical and reversible nature of CO2 adsorption. (2) Identical functional group distributions in the coal lithotypes before the experiment and after CO 2 desorption suggest that no alteration of the chemical structure of the coal took place that could be detected by FTIR technique. It is possible, however, that very small molecular changes do occur in the coal which might be detected by other techniques. (3) Adsorption isotherms derived from FTIR indicate significantly larger CO2 adsorption by vitrains compared to that of clarains and fusains. Differences in CO2 adsorption over time at 0.5 MPa pressure, in addition to lower adsorption in clarain and fusain compared to vitrain, suggest also faster access to the available pores in clarains and fusains because of the presence of larger pores in inertinite macerals that facilitate pore accessibility by CO2.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 3591

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