Pore Accessibility by Methane and Carbon Dioxide in Coal as

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Pore Accessibility by Methane and Carbon Dioxide in Coal as Determined by Neutron Scattering Lilin He,*,† Yuri B. Melnichenko,† Maria Mastalerz,‡ Richard Sakurovs,§ Andrzej P. Radlinski,‡,∥ and Tomas Blach∥ †

Neutron Scattering Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6393, United States Indiana Geological Survey, Indiana University, Bloomington, Indiana 47405-2208, United States § Commonwealth Scientific and Industrial Research Organisation (CSIRO) Energy Technology, 11 Julius Avenue, North Ryde 2113, Sydney, New South Wales, Australia ∥ Queensland Micro and Nanotechnology Centre, Griffith University, Nathan 4111, Brisbane, Queensland, Australia ‡

ABSTRACT: Contrast-matching ultrasmall-angle neutron scattering (USANS) and small-angle neutron scattering (SANS) techniques were used for the first time to determine both the total pore volume and the fraction of the pore volume that is inaccessible to deuterated methane, CD4, in four bituminous coals in the range of pore sizes between ∼10 Å and ∼5 μm. Two samples originated from the Illinois Basin in the U.S.A., and the other two samples were commercial Australian bituminous coals from the Bowen Basin. The total and inaccessible porosity were determined in each coal using both Porod invariant and the polydisperse spherical particle (PDSP) model analysis of the scattering data acquired from coals both in vacuum and at the pressure of CD4, at which the scattering length density of the pore-saturating fluid is equal to that of the solid coal matrix (zero average contrast pressure). The total porosity of the coals studied ranged from 7 to 13%, and the volume of pores inaccessible to CD4 varied from ∼13 to ∼36% of the total pore volume. The volume fraction of inaccessible pores shows no correlation with the maceral composition; however, it increases with a decreasing total pore volume. In situ measurements of the structure of one coal saturated with CO2 and CD4 were conducted as a function of the pressure in the range of 1−400 bar. The neutron scattering intensity from small pores with radii less than 35 Å in this coal increased sharply immediately after the fluid injection for both gases, which demonstrates strong condensation and densification of the invading subcritical CO2 and supercritical methane in small pores.

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INTRODUCTION The increasing concentration of carbon dioxide (CO2) in the atmosphere linked to the use of fossil fuels is of global concern. One approach to reduce the increase of the atmospheric CO2 level is to capture and store the generated CO2 in deep, porous, geologic formations, including unminable coal seams.1−3 Because of its high internal surface area, coal can store an order of magnitude more CO2 than the same volume of a conventional aqueous reservoir. Another advantage of injecting CO2 into unminable coal seams is that methane gas adsorbed in the coal matrix may be recovered and used to offset the cost of geological storage.2−4 Both methane and CO2 are supercritical at temperatures and pressures of potential storage sites for carbon dioxide. The effectiveness of storage strongly relies on the penetrability of gas into the pores of the coal and the behavior of fluids confined in the pores, both of which are dependent to a large extent upon the reservoir temperature and pressure.5−7 It is well-known that the phase behavior of confined fluids may differ dramatically from that of the same fluids in the bulk, owing to the molecule−surface interactions.8,9 In addition, the density, compressibility, and flow efficiency of confined fluids is known to be dependent upon the pore size, shape, and tortuosity, as well as the chemical composition of the solid matrix.10 Traditionally, the sorption capacity and microstructure of porous media, including man-made materials, coals, sandstones, © 2011 American Chemical Society

and shales, have been determined using techniques such as adsorption measurements and mercury porosimetry.11,12 Mercury porosimetry is highly invasive and only provides information about “open” porosity, whereas gravimetric and volumetric sorption methods only give information on the excess fluid adsorption integrated over the total sample volume of pores accessible to the fluid.13 During the last 2 decades, the small-angle neutron scattering (SANS) technique has been developed and refined for structural characterization of various natural and engineered porous materials.14−22 Because of the high penetration power and relatively short wavelength of neutrons, SANS and ultrasmall-angle neutron scattering (USANS) techniques are ideally suited for characterization of the phase behavior of fluids in fine pores under pressure. Moreover, SANS and USANS measurements record the scattering from all pores, including pores that are inaccessible to fluids and, therefore, immeasurable by other techniques. It has been recently demonstrated that SANS/USANS can be used to quantitatively assess both accessible and inaccessible porosity in porous media.23 We define inaccessible (or closed) porosity as a fraction of the total pore volume in coal that is inaccessible to the fluids at a certain pressure and temperature Received: November 1, 2011 Revised: December 15, 2011 Published: December 18, 2011 1975

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the particle size of 1−0.5 mm and loaded into thin-wall aluminum holders with the internal thickness of 1 mm. Use of particulates is advantageous because SANS is measured from a representative sample of coal, in which macerals are positioned in all possible orientations. The 1−0.5 mm particles are large enough to avoid scattering from interparticle voids, which might contribute to the scattering from coal pores and bias data analysis. In addition, use of 1−0.5 mm particulates reduces the effective thickness of coal samples, which helps to minimize the contribution of multiple scattering. Putting SANS and USANS data on an absolute scale requires normalizing the data to the effective rather than nominal (1 mm) sample thickness. The effective thickness of the equivalent solid coal platelets was calculated using both helium density and packing density of coal particulates (the latter was determined by weighing coal samples of a known volume).28 Effective thickness was found to be between ∼0.3 and 0.4 mm for all studied samples. Prior to measurements, the samples were dried for a minimum of 8 h under vacuum at 60 °C to remove moisture and immediately mounted inside a SANS/USANS pressure cell. The SANS and USANS scattering profiles of coals were collected in situ in the pressure range from 0 (vacuum) to about 400 bar at room temperature (23 °C). The pressure was increased stepwise using a newly constructed pressure intensifier. Each measurement began 10 min after the pressure was changed to allow for equilibrium saturation of the pores with the injected fluid. The equilibration time was determined in separate kinetics experiments, in which the intensity of scattering was repeatedly measured as a function of time for 16 h after changing the pressure. No changes in scattering patterns were revealed 10 min at any pressure step after the fluid injection. Neutron Scattering Measurements. SANS experiments were conducted using the General-Purpose SANS instrument at Oak Ridge National Laboratory (ORNL). The neutron wavelength λ was set to 4.75 Å with a wavelength spread, Δλ/λ, of 0.13. Scattered neutrons were detected using a 1 × 1 m2 helium-filled two-dimensional (2D) position-sensitive detector with 192 × 192 pixels.29 Three sample− detector distances (18, 12, and 0.3 m) were chosen to cover an overall range of scattering vectors (Q) of 0.0016 < Q < 0.7 Å−1, where Q = (4π/λ)sin(θ/2) and θ is the scattering angle. The data acquisition time from empty and fluid-saturated coals varied from approximately 10 min to 1 h at each detector position to ensure sufficient data statistics. The raw 2D data were corrected for the detector pixel efficiency and dark current, which represents the ambient radiation background and electronic noise, and azimuthally averaged to produce a one-dimensional (1D) profile I(Q). Data were placed on an absolute scale in units of cm−1 through the use of precalibrated secondary standards.30 The reduced 1D profiles from the three detector distances were merged using software developed by the National Institute of Standards and Technology (NIST) implemented in Igor Pro 6.1 (WaveMetrics, Lake Oswego, OR).31 USANS measurements were carried out at NIST, using the BT-5 perfect crystal SANS instrument (λ = 2.4 Å).32 The slit-smeared USANS data were converted into the pinhole geometry. Application of the GP-SANS and BT-5 instruments in a tandem allowed for a broad range of pore sizes, from approximately 4 to 40 000 Å (0.0004−4 μm), to be probed by neutrons. The elemental and maceral composition of the studied coals is presented in Table 1. The SLD of each coal was calculated on the basis

on the time scale of the experiment (tens of hours in this work). Information on the inaccessible porosity is obtained using the technique of contrast matching. In this case, the system consists of two components: the solid coal matrix and the pore-invading fluid. The scattering intensity from such a two-component system is proportional to the square of the difference in the scattering length densities (SLDs) of the two phases.21 If SLDs of the two components are equal [the zero average contrast (ZAC)-matching condition], the scattering of all pores accessible to the contrast-matching fluid is annulled, leaving scattering only from inaccessible pores. Thus, contrastmatching SANS (CM-SANS)24,25 and contrast-matching USANS (CM-USANS) techniques provide a unique method to quantify both the accessible and inaccessible pores in a range of linear scales from ∼10 to ∼105 Å. Coal is a chemically and structurally heterogeneous material, and the degree of accessibility of coal pores to the invading fluids has been under debate for some time. Bond26 proposed a structural framework that is based on the assumption that all pores in coal belong to the interconnected pore network. This model was later challenged by Larsen et al.,8,9 suggesting that most of the coal pores are not connected to the external surface and, thus, are inaccessible to the invading fluids. The latter observation was supported by Hall et al.,25 who measured CMSANS on samples of the Pittsburgh No. 8 coal and found that the scattering profiles were characteristic of a low-porosity solid, which did not have a fully connected micropore structure. However, the contrast-matching experiment was performed using solvents, such as deuterated benzene, that, because of their molecular size, may not have fully penetrated fine pores that are accessible by smaller molecules, such as CD4 or CO2. Recently, Melnichenko et al.23 used CM-SANS/CM-USANS to investigate inaccessible porosity as a function of the pore size in a suite of coal samples. They found that the volume of pores inaccessible to these fluids may vary between ∼90% in the microporous region to ∼30% in the mesoporous region and the variation was distinctive for each of the examined coals. In situ small-angle scattering studies of CO2-saturated coals also helped to characterize aspects of the fluid adsorption process, including determination of the density of the adsorbed fluid and the kinetics of filling pores of different sizes.16,27 In this paper, we use CM-SANS and CM-USANS to determine the volume fraction of inaccessible pores in four different bituminous coal samples and relate this inaccessibility to chemical, maceral, and physical properties of the coal matrix.

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MATERIALS AND METHODS

Samples. Four coal samples were investigated in this study: two from the Illinois Basin, U.S.A., and two from the Bowen Basin, Australia. Coals had been received dry and stored in a refrigerator at 4 °C. The coal samples were prepared in the form of particulates with

Table 1. Elemental Composition, Ash Yield, Maceral Composition, and Vitrinite Reflectance (R0) of the Coals Studied maceral composition (vol %) coal

H (%, daf)

C (%, daf)

N (%, daf)

S (%, daf)

O (%, daf) (by difference)

ash (%, dry)

R0 (%)

VIa

INa

LIa

1 2 3 4

5.5 5.8 3.9 5.7

82.0 79.4 80.7 84.1

1.8 1.6 1.0 2.3

1.6 5.6 0.2 0.6

8.6 7.6 14.2 7.3

7.4 8.5 20.3 5.6

0.62 0.53 0.62 0.95

85.0 91.3 23.9 82.6

10.1 3.9 74.5 13.3

4.9 4.8 1.6 4.1

a

VI, vitrinite; IN, inertinite; LI, liptinite. 1976

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of their chemical composition and matrix (helium) density d21

ρn =

NAd M

major effects: the incoherent background scattering from hydrogen atoms present in the coal matrix and a possible contribution from the scattering on physical and chemical inhomogeneities on a sub-nanometer scale.35 Deviation of scattering from the power law in the low-Q limit may reflect a change in the fractal nature of the coal that leads to a smaller number of pores than predicted from extrapolation from the results for the smaller pore sizes but could also be due to effects of multiple scattering.36 Figure 2 shows scattering patterns from all four samples obtained after subtracting the background. The curves for coals 1, 2, and 4 overlap in the low-Q domain, thus indicating

∑ pj ( ∑ sibi)j j

i

(1)

where NA is Avogadro’s constant, si is the proportion of the number of nucleus i in compound j, pj is the proportion of the molecular number of the compound j in the mixture, and bi is the coherent scattering amplitude for nucleus i. Calculated SLD values are 2.2 × 1010 cm−2 (coals 1 and 2), 3.44 × 1010 cm−2 (coal 3), and 2.33 × 1010 cm−2 (coal 4). ZAC pressure (PZAC) for CO2 and CD4 is defined as a pressure at which neutron contrast kn2 between the SLDs of the solid matrix of coal (ρ*s) and a fluid (ρ*f) is equal to zero: kn2 = (ρ*s − ρ*f)2 = 0. The SLDs of CO2 and CD4 may be calculated using the following equations:16,25 ρ*CO2 = [2.49(ρCO2)] × 1010 cm−2, and ρ*CD4 = [10(ρCD4)] × 1010 cm−2. Pressures at which the fluid density of CO2 and CD4 corresponds to the ZAC condition were calculated using equations of state with REFPROP software obtained from NIST (http:// www.nist.gov/data/nist23.htm). Densities of CD4 were calculated from densities of normal methane by multiplying them by a factor of 1.25 (the ratio of atomic weights of the deuterated and protonated methane).

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RESULTS AND DISCUSSION Total Inaccessible Porosity. Radially symmetric 2D SANS patterns were obtained for all four samples. Each 2D SANS pattern was azimuthally averaged to provide 1D data for further analysis. The neutron cross-section I(Q) in units of cm−1 obtained from the four coals is shown in Figure 1. Experimental

Figure 2. Scattering profiles of the four coal samples after subtraction of the incoherent scattering background.

structural similarity on this scale. Coal 3 exhibits higher scattering intensity than other coals in the whole Q range. The (negative) slope, s, determined in the Q range of 10−4 < Q < 10−2 Å−1 is −3.7 ± 0.1 (coal 1), −4.1 ± 0.1 (coal 2), −2.9 ± 0.1 (coal 3), and −3.4 ± 0.1 (coal 4), indicating surface fractal geometry of the coal matrix−pore interface over this Q range. The surface fractal dimension of the interface is determined by the relation D = s + 6.37 Schmidt38,39 demonstrated that scattering from a system of uncorrelated three-dimensional (3D) objects with a power law size distribution is mathematically equivalent to scattering from surface fractals. In the present work, the pore size distribution (PSD, also known as the pore number density) was calculated using the polydisperse spherical particle (PDSP) model that may be used to fit scattering from an arbitrary PSD to measured data, including scattering by surface fractals. Our data were fitted to the PDSP model using PRINSAS software.40 Fits obtained for all samples are shown in Figure 3 as solid lines. The PSDs for all samples are very broad (see Figure 4), covering the pore size from about 3 nm to about 5 μm. There is no evidence of maxima in the PSD for any of the studied coals. The combination of SANS and USANS techniques provides access to micro-, meso-, and macropores in one experiment. The total porosity of the samples was determined via the Porod invariant (Z).

Figure 1. Combined SANS and USANS profiles for four coals measured in vacuum.

data span the Q range from 5 × 10−5 to 0.4 Å−1, which corresponds to a wide range of pore sizes (R ≈ 2.5/Q) between Rmax ∼ 5 μm and Rmin ∼ 10 Å.33 Scattering intensity varies by about 10 orders of magnitude within this Q range. For coals 3 and 4, a nearly linear dependence of the scattering intensity, I, versus Q value is observed on the log−log scale over 3 orders of magnitude in Q, indicating a power law scattering, which is expected for a fractal system.34 Coals 1 and 2 show deviation from a power-law behavior for Q > 10−2 Å−1, which may be related to a certain correlation in the spatial position of pores with the correlation length around 125 Å and/or presence of additional scattering objects with size of about 125 Å. The deviation from the power law scattering in the large-Q limit (Q > 0.2 Å−1) for all samples may be attributed to two

Z= 1977

∫0

∞

Q 2I(Q )dQ

(2)

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Figure 3. Fits of the PDSP model to scattering data for four coals.

Z = 2π 2(ρ*s − ρ*f )2 φs(1 − φs) =

∫0

∞

Q 2I(Q )dQ

(3)

where ϕs is the volume fraction of the solid material. Because I(Q) and the SLDs of the solid matrix of coal (ρ*s) and fluid (ρ*f) are known, the volume fraction of solid material can be calculated. Previously, SANS was used to quantify the pore volume in coal that is inaccessible to contrast-matching liquids, such as isotopic mixtures of light and heavy water, as well as normal and deuterated toluene.24,25 In this work, the scattering data were acquired from coal samples saturated with CD4 and CO2 at pressures corresponding to the ZAC condition. In the twophase approximation, the neutron contrast between the solid matrix of coal and all pores accessible to a fluid is zero and any residual scattering is attributed to pores that are not accessible to the contrast-matching fluid. In our previous paper,23 we demonstrated that the ZAC condition for different coals (i.e., minimum in the variation of scattering as a function of the fluid pressure) indeed occurs at the scattering length density of fluid corresponding to precalculated SLD of the coal matrix

Figure 4. PSD determined using the PDSP model for four coals.

In the limit of two-phase approximation, the following relationship holds: 1978

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Figure 5. Scattering patterns acquired in vacuum and at the contrast-matching point with CD4 fluid for four coals.

estimated using chemical composition of coal and the equation of state of the invading fluid. Scattering profiles acquired at vacuum and the ZAC condition for the four coals are plotted in Figure 5. As expected, saturation of the accessible pores by CD4 fluid results in the substantial decrease of the scattering intensity for all four coals. The SANS data measured under vacuum and at ZAC pressure were used to evaluate the total porosity (in the range of pore sizes between ∼10 Å and ∼5 μm) as well as the pore volume inaccessible to CD4 using two different approaches: (1) Porod invariant analysis and (2) the PRINSAS implementation of the PDSP model. The results obtained by these two methods are shown in Tables 2 and 3. The porosity results obtained using both methods agree reasonably well. Coal 3 exhibits the highest total porosity (13%) and the lowest fraction of inaccessible pores, whereas coal 4 is the least porous sample (6.7%) with the highest proportion of inaccessible porosity (34.7% of total porosity). To the best of our knowledge, this is the first experimental evaluation of the pore volume inaccessible to methane in coal. The fact that 12−36% of all pores may not be accessible to injected methane suggests that, if methane were present in

Table 2. Total Porosity Obtained from the Porod Invariant coal

total porosity under vacuum (%)

fraction of total porosity inaccessible to CD4 (%)

1 2 3 4

7.8 10.4 13.0 6.7

26.3 17.2 12.5 36.4

Table 3. Total Porosity Obtained from the PDSP Model coal

total porosity under vacuum (%)

fraction of total porosity inaccessible to CD4 (%)

1 2 3 4

8.7 10.7 12.0 6.7

24.7 20.4 14.0 34.7

closed pores originally, it would be difficult to release it from these pores. Studies of the gas release from coals often find that “residual gas” (gas not desorbed before the sample is pulverized) can constitute a considerable fraction of the total gas present.41 1979

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This residual gas requires extensive crushing of the coal to release. Our results suggest that crushing helps to release the gas trapped in the initially closed (i.e., inaccessible) pores. Mercury Porosimetry. Mercury porosimetry measurements were performed on coals 3 and 4. Porosity (differences in volume between mercury penetration at 10 μm pore size and helium penetration) was calculated as 12.9% for coal 3 and 5.9% for coal 4, which are comparable to the values calculated for these coals using the Porod invariant and the PDSP model in Tables 2 and 3. Many researchers have argued that helium does not reach all of the pores in coal.8,42,43 Their arguments were based on the fact that more CO2 (and other fluids, such as water and methanol) penetrates coals than could be accounted by pore volume calculated by helium density. However, because SANS detects all pores in coals and the pore volume measured by SANS is the same as that determined from helium density, we conclude that helium must be able to penetrate all of the pores, at least in the coals studied in the current work. Moreover, because helium appears to penetrate all of the pores and methane does not and other evidence suggests that the ability of carbon dioxide to penetrate coal is similar to that of methane,23,44 CO2 cannot penetrate coal micropores as well as helium. Thus, the observation that the volume of CO2 adsorbed is greater than the available pore volume is not due to CO2 penetrating the coals better than helium. One of the possible explanations of this effect might be superdensification of CO2 in coal pores.16 Surface Area. For porous systems with the PSD covering multiple length scales, the internal specific surface area (SSA) depends upon the size of the measuring probe. In this work, the SSA of the studied samples for a probe size r was calculated from the PSD by summing the surface areas of all pores of radius larger than r and dividing by the sample volume. Figure 6

Table 4. SSA Obtained Using Various Methods coal

accessible SSA using the PDSP modela (m2/g)

accessible SSA measured by N2 adsorptionb (m2/g)

1 2 3 4

6.4 24.6 36.4 2.0

10.6 23.0

a

Using extrapolation to a probe size of 4 Å (as illustrated in Figure 6). Adsorption method only measures SSA accessible to N2 (molecular dynamic radius of 4 Å). b

the largest and coal 4 has the lowest SSA of the studied samples. The SSA of coal 2 is ∼24 m2/g, which is close to the BET value from nitrogen adsorption (23.0 m2/g) with a probe size of 4 Å.16 At the same time, the SSA of coal 1 from neutron scattering analysis and BET are substantially different (∼6.4 and 10.6 m2/g, respectively). Given the extrapolation-related uncertainties for SANS data, the agreement between the two methods is satisfactory. Fluid Condensation in Nanopores. Coal 2 was selected for more detailed examination in the nanopore region. Figure 7

Figure 6. Total SSA versus probe size obtained from fits to the PDSP model for four coals.

shows the calculated SSA versus the pore size and the extrapolation to a molecular probe size of r = 4 Å from fits to the PDSP model.45 Because the extrapolation procedure is only based on a few experimental points for probe sizes smaller than about 100 Å, the uncertainty is significant, generally of the order of ±50% of the quoted values. The values of SSA obtained using the PDSP model, as well as Brunauer−Emmett−Teller (BET) data, for coals 1 and 2 are shown in Table 4 for comparison. Coal 3 has

Figure 7. SANS patterns for coal 2 measured in vacuum and after injection of (a) CO2 and (b) CD4 fluid at various pressures. The insets show scattering curves with the incoherent background subtracted.

shows the SANS data in the high-Q region at various pressures of CO2 and CD4 at the temperature of 23 °C. We note that, at this temperature, CD4 is a supercritical fluid, whose density varies continuously with pressure, whereas subcritical bulk CO2 1980

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gas condenses into a liquid at the pressure of about 59 bar. The variation of I(Q) as a function of the CO2 pressure is different in the high- and low-Q regions. The decreasing contrast between the coal matrix and saturating fluids, imposed by increasing fluid pressure, results in a significant decrease of scattering intensity in the low-Q region, Q < 0.07 Å−1 (R > 2.5/0.07 ≈ 36 Å), as expected from the two-phase model.16 The decrease in scattering intensity from larger pores is stepwise for CO2 and continuous for CD4, which correlates with the corresponding density variation of bulk fluids. The observed increase of scattering intensity in the high-Q region, Q > 0.07 Å−1, for both fluids is caused by condensation and strong densification of the fluids in pores smaller than ∼36 Å. Strong densification of both sub- and supercritical fluids in small pores has been observed before for several systems: hydrogen in activated carbon,46 CO2 in silica aerogels,47 and coal,26 and methane in carbon aerogel.48 Quantitative calculation of the density of fluids adsorbed in micropores would only be possible if the scattering contribution of inaccessible pores was taken into account. However, such information cannot be extracted from the available data. After initial condensation, a further increase in pressure from 71 to 142 bar forces liquid CO2 to progressively penetrate the pores ranging in size from 13 to 80 Å, as indicated by the minor change of scattering intensity in the Q region from 0.03 to 0.2 Å−1. The scattering profiles acquired at the pressure of 142 and 382 bar are practically identical, which is attributed to the low compressibility of liquid CO2. The scattering pattern obtained in vacuum after the exposure to the high pressure of CO2 overlaps well with the scattering data acquired before the exposure, except for some variation in the highQ region. This indicates that the coal structure is not significantly affected by the high pressure and that the adsorption−desorption process is reversible. This work shows that SANS can be used to directly determine the maximum pore size for which condensation can occur. This provides exact limits to methods using sorption to determine PSDs (for instance, through the density functional theory) and can thus be used for explaining some of the discrepancies seen when different gases are used to determine surface areas of materials. Relationships between the inaccessible porosity and vitrinite reflectance (R0, as a rank indicator), as well as the maceral composition, are shown in Figures 8 and 9, respectively. There is a tendency for both an increase of inaccessible porosity (Figure 8)

Figure 9. Volume of pores inaccessible to CD4 fluid (percentage of the total pore volume) as a function of the vitrinite/inertinite ratio in studied coals.

and a decrease of total porosity (Tables 2 and 3) with an increasing vitrinite reflectance, suggesting a trend toward tightening in coals with an increasing rank. Whereas a decrease of total porosity with rank is an expected phenomenon for bituminous coals,4,49 this is the first time when the evolution of inaccessible porosity with rank has been documented as well. We acknowledge, however, that more coals of various rank originating from different regions need to be studied to further test this relationship and to determine to what extent it could be influenced by the maceral composition or mineral matter content. This study does not show any regular trend for inaccessible porosity with maceral composition (Figure 9). It has been demonstrated that accessible micro- and mesoporosity of coal to a large extent depend upon the maceral composition,50−52 and we expect that a similar dependence may extend to inaccessible (closed) porosity as well. Both maceral composition and coal rank are key parameters that influence gas adsorption capacity.53,54 Determination of the interplay between these parameters and inaccessible porosity would help to better understand not only adsorption capacities but also gas transport in coal. Figures 10 and 11 indicate a clear trend for

Figure 10. Volume of pores inaccessible to CD4 fluid (percentage of the total pore volume) as a function of the total porosity.

inaccessible porosity to decrease with an increased total porosity and the accessible SSA. It is interesting to note that

Figure 8. Volume of pores inaccessible to CD4 fluid (percentage of the total pore volume) as a function of the vitrinite reflectance Ro. 1981

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were supported in part by the National Science Foundation under agreement DMR-0454672.

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Figure 11. Volume of pores inaccessible to CD4 fluid (percentage of the total pore volume) as a function of the accessible SSA.

the observed trend is less pronounced on an absolute scale; without normalization to the total porosity, the fraction of inaccessible pores varies between 1.6% (coal 3) and 2.4% (coal 4).55 Evidently, more coals need to be studied to verify the trend. However, if such a relationship is confirmed, the total porosity and SSA could be used as a proxy for quantifying accessibility of pores to various gases and liquids.

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CONCLUSION In this work, we applied CM-SANS/CM-USANS to characterize the structure of four different samples of coal and to determine their inaccessible porosity. Our experiments have demonstrated for the first time that a considerable fraction of the porosity of some coals is inaccessible to CD4 and the fraction of closed porosity varies significantly for coals from different depositional environments and diagenetic histories. The volume of inaccessible pores shows no evident correlation with rank or maceral composition (see Figures 8 and 9). At the same time, the proportion of inaccessible porosity decreases as the total porosity and SSA increase (Figures 10 and 11). Our data indicate that most of the pores in low-porosity coals are inaccessible to fluids but even highly porous coals have a significant proportion of closed porosity. CM-SANS/CM-USANS studies of the inaccessible porosity in a number of various bituminous coals are underway, and the results of the ongoing work should provide more information on the possible correlation between the closed porosity and the chemical as well as structural parameters of coal.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

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ACKNOWLEDGMENTS Research at Oak Ridge National Laboratory (ORNL)’s High Flux Isotope Reactor was sponsored by the Laboratory Directed Research and Development Program and the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. This research was supported in part by an appointment to the ORNL Postdoctoral Research Associates Program, administered jointly by the ORNL and the Oak Ridge Institute for Science and Education. The elements of this work using the BT-5 instrument at the National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR) 1982

dx.doi.org/10.1021/ef201704t | Energy Fuels 2012, 26, 1975−1983

Energy & Fuels

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dx.doi.org/10.1021/ef201704t | Energy Fuels 2012, 26, 1975−1983