Energy & Fuels 1995,9, 324-330
324
Pore Structure of the Argonne Premium Coals John W. Larsen," Peter Hall, and Patrick C. Wernett Department of Chemistry, Lehigh University, 6 E. Packer Avenue, Bethlehem, Pennsylvania 18015, and Exxon Research and Engineering Co., Rt. 22E, Clinton Township, Annandale, New Jersey 08801 Received August 23, 1994@
Data are presented that are inconsistent with the accepted model of coal pores existing as an interconnected network of bottlenecked pores. It is proposed that pores in coals are isolated and can be reached only by diffusion through the solid. The fractal dimensionality of the pores in five Argonne Premium Coals were measured by small-angle X-ray scattering and vary between 2.35 and 2.87. The adsorption of the following gases has been measured on all of the Argonne coals: N2, CO2, ethane, cyclopropane, cyclobutane, cyclopentane, and cyclohexane. Except for Zap lignite, there is a very steep dependence of BET surface area on adsorbate size which is rationalized as being due to the size dependence of the adsorbate diffusion rates through coals.
Introduction In this paper we will endeavor to show that coals do not contain networks of interconnected pores and propose that coal pores are isolated from one another and can be reached only by diffusion through solid coal. Papers on the subject of coal surfaces generally begin with statements of the importance of surface characterization to coal chemistry. We will honor this tradition briefly. The surface is where a coal initially encounters the rest of the world. Under many circumstances, that contact is limited to the surface and is totally controlled by the area, geometry, and chemical nature of the surface. Most coal cleaning technology falls into this category. An accurate description of coal surfaces is crucial to the development of this technology. Our concern now is only with the surface area and geometry and not its chemical identity or characteristics. A more complex situation exists when a reagent contacts the surface only to pass through it and enter the coal. Temporal considerations often dominate. Figure 1, taken from a paper by Hsiu and Duda, illustrates the situation and the prob1em.l A fluid (gas in this case) first rapidly adsorbs on the coal surface. If soluble in the coal, it immediately begins to diffuse into the coal usually swelling it. Depending on the diffusion rate, it may or may not be possible to differentiate between the rapid adsorption and the early absorption as the fluid begins t o diffuse into the coal. This distinction becomes crucial in the gas adsorption measurements to be described. No matter how fast o r slow, diffusion begins at the surface and an accurate surface area is necessary t o a fundamental understanding of the surface and its properties. It is difficult t o imagine any quantitative study of coal reactivity in which the area and characteristics of the surface will not be important parameters. Coals are porous. They have generally been thought to be high surface area materials having a network of Abstract published in Aduance ACS Abstracts, February 15, 1995. ( 1 )Hsieh, S. T.; Duda, J. L. Fuel 1987, 66, 170-177. @
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m( s"2 ) Figure 1. Rate of toluene uptake by a pyridine extracted bituminous coal (71.0% C, daf). (Reprintedwith permission from ref 1. Copyright 1987 Butterworth-Heinemann.)
slitlike pores interconnected by narrow capillary constrictions.2-6 This model was originally proposed by Bond and has come t o be d ~ m i n a n t . ~ A good example of its use is the usual explanation for the great difference in coal surface areas measured with N2 at 77 K and CO2 measured at 195 K to 298 K.8 Nz is smaller than COz, yet COa surface areas are often 1-2 orders of magnitude greater. The larger molecule gives a much greater surface area in apparent contradiction to the prediction of a rigid interconnected network. The measurements were made by static adsorption techniques and the application of the BET or Dubinin data treatments. This problem with the (2) Gregg, S. J.; Pope, M. I. Fuel 1959,38, 501-505. (3) Ganguli, N. C.; Mukherjee, P. N.; Lahiri, A.Fuel 1961,40, 525-
526. (4)Sharkey, A. G., Jr.; McCartney, J. T. In Chemistry of Coal Utilization, Second Supplementary Volume; Elliot, M.A,, Ed.; John Wiley and Sons: New York, 1981. (5) Mahajan, 0. P.; Walker, P. L., Jr. InAnalytical Methods for Coal and Coal Products; Karr, C., Jr., Ed.; Academic Press: New York, 1979; VOl. 1. (6) Marsh, H. Carbon 1987, 25, 49-58. (7) Bond, R. L. Nature 1956,178,104-105. ( 8 ) Gan, H.; Nandi, S. P.; Walker, P. L. J r . Fuel 1972,51, 272-277.
0 1995 American Chemical Society
Pore Structure of the Argonne Premium Coals
model was rationalized away using the idea of activated diffusion. At 77 K, N2 does not have enough thermal energy to diffuse through the narrow constrictions in the pore network. It thus remains outside and reports the area of the external accessible surface. CO2 is capable of forcing the narrow passages because of its greater energy (higher temperature) and thus reports the area of the entire pore network. A variety of gasses have been used to determine the surface areas of coals by adsorption. They include methane, argon, xenon, hydrogen, and the mentioned nitrogen and carbon dioxide.*-ll Surprisingly, no extended series of gases systematically varying in size had been used to determine coal surface areas. This paper corrects that deficiency. We desired to know the surface roughness, in addition to the surface areas. Small-angle X-ray scattering (SAXS)was used to determine the fractal dimensionality of the pore surfaces. The fractal dimension of a surface must lie between 2 and 3 and is a measure (among other things) of the roughness of a surface. A perfectly flat surface has a dimensionality of 2. As the roughness of the surface increases, so does the dimensionality which approaches the limiting value of 3 at which point the surface fills a three-dimensional volume. Both SAXS and neutron scattering have been used to investigate the pore structure of a few Bale was the first t o use scattering to measure the fractal dimensionality of a coal.16
Experimental Section All coals were Argonne Premium Samples received in sealed 5 g ampules and all manipulations were carried out in a NZ atmosphere to prevent air oxidation. The BET Procedure. The gasses used and their sources were cyclohexane (Aldrich gold label, 99.9% minimum purity); cyclopentane (TCI American, 99% minimum purity); cyclobutane (Columbia Organic, 99.9% minimum purity); cyclopropane (Matheson Gas Co., 99.9% minimum purity); carbon dioxide (Blue Valley Co., 99.998%);nitrogen (Blue Valley Co., 99.999%); ethane (Matheson Gas Co., 99.99%);helium (Blue Valley Co., 99.995%). A Micromeritics Digisorb 2500 adsorption apparatus was used for all coals except Illinois No. 6. In an Nzfilled glovebag, aliquots of the liquids cyclohexane, cyclopentane, and cyclobutane were placed in stainless steel bulbs prerinsed with five 20 mL aliquots of the liquid. These were attached to the apparatus and subjected to five freeze-pumpthaw cycles before use. All other gasses were used without further purification. Coals were degassed overnight at room Torr. Standard instrutemperature under a vacuum of ment procedures were used. An equilibration time of 24 h was used for the first data point and 6 h was allowed between all other points for all gasses except Nz. NZ surface areas were determined using the Digisorb automated five-point Nz BET program. Illinois No. 6 surface areas were determined on an all-glass vacuum apparatus equipped with a diffusion pump and a (9)Kini, K.A.Fuel 1964,43, 173-180. (10)Yang, R. T.;Saunders, J. T. Fuel 1985,64, 616-620. (11)Van der Sommen, J.; Zwietering, P.; Eillebrecht, B. J. M.; Van Krevelen, D. W. Fuel 1955,34, 444-448. (12)Spitzer, Z.;Ulicky, L. Fuel 1977,55, 212-224. (13)Setek, M.;Snook, I. K.; Wagenfeld, H. K. In The Chemistry of Low Rank Coals; Schobert, H. H., Ed.; American Chemical Society: Washington, DC, 1984. (14)Gethner, J. S.J. Appl. Phys. 1986,59, 1068-1085. (15)Winans, R. A.;Thiyagarajan, P. Energy Fuels 1988,2 , 356358. (16)Bale, H.D.;Schmidt, P. W. Phys. Reu. Lett. 1984,53, 596599.
Energy & Fuels, Vol. 9, No. 2, 1995 326
I
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Figure 2. X-ray scattering intensity as a function of the X-ray scattering vector for Beulah Zap lignite.
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Figure 3. X-ray scattering intensity as a function of the X-ray scattering vector for Wyodak-Anderson subbituminous coal.
Barocel pressure sensor t o measure gas uptake. Coal samples were degassed overnight at Torr. Equilibration for 6 h was allowed between each point. This apparatus was used for Illinois No. 6 coal and all gasses except Nz for which the Digisorb was used. SAXS. Coals were selected from the Argonne Premium Coal Sample program and were used untreated in the scattering experiments. The SAXS was performed at the National Synchrotron Light Source at the Brookhaven National Laboratory. X-ray beam collination was by Bonse-Hart apparatus. The sample consisted of 2 mm coal powder held between two sheets of Kapton film. Multiple scattering becomes a problem for sample sizes greater than 2 mm. Blank experiments showed that scattering from the Kapton was at most 3 orders of magnitude less than scattering from the coal. The scattering curves are therefore uncorrected for background scattering. X-ray scattering experiments were measured from scattering wave vectors between and 0.15 A-l with measurements being made at intervals of A-l.
Results Small-angle X-ray scattering (SAXS) curves for five coals are shown in Figures 2-6. A power law dependence between the scattering intensity ( I ( q ) )and the scattering wave vector h was found where
I ( q ) DC h-'6-D' 4n h =; 1sin(fY2)
II is the X-ray wavelength and 8 is the scattering angle. Our results are similar to Bale's.16 It has been dem-
Larsen et al.
326 Energy & Fuels, Vol. 9, No. 2, 1995
lE6
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Table 1. Cross-SectionalAreas of the Adsorbate Gases adsorbate gas mol wt (g/mol) density (g/cm3) nb (A2) S l o p e = -3 13
A
0 = 2.87
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0.795 0.760 0.725 0.630 0.448 0.662 0.813
34.3 31.2 27.8 25.2 25.3 25.1 16.2
Liquid densities of the adsorbate gases were obtained from ref 23. The cross-sectional areas of the adsorbate gases were where M is the determined using u = (4)(0.866)(M/4J2AD)2'3, molecular weight, D is the density, and A is Avagadro's number.
. 8
'
84.2 70.1 56.1 42.1 30.1 44.0 28.0
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Figure 4. X-ray scattering intensity as a function of t h e X-ray scattering vector for Illinois No. 6 coal. "'
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.
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-
cyclohexane cyclopentane cyclobutane cyclopropane ethane carbon dioxide nitrogen
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1.3 1.4 1.5 Log Adsorbate Cross-SectionalArea (h)
1.6
Figure 7. Fractal plot of t h e adsorption of gasses on Pocahontas No. 3 coal.
i-11
Figure 5. X-ray scattering intensity a s a function of t h e X-ray scattering vector for Pittsburgh No. 8 coal. iEe
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Figure 6. X-ray scattering intensity a s a function of t h e X-ray scattering vector for Upper Freeport coal.
onstrated that the dimensionality measured by SAXS is equal to the fractal dimensionality." The linear plots demonstrate that the pores in these coals have fractal surfaces over the h range. The dimensionality of those surfaces is equal to 6 the slope. Slopes and dimensionalities are recorded on the plots. The scattering angles are such that the scale length range covered is 30-1000 A. For all of these coals, the pore surfaces over this range are fractal and the dimensionalities range between 2.35 and 2.87. An alternative experimental method for determining the dimensionality of a fractal surface consists of monitoring the number of like molecules adsorbed on a surface as a function of the size of the adsorbate
+
(17) Hall, P. J. Chem. Phys. Lett. 1986, 124, 467-469.
molecules. Small molecules will follow the contours of a rough surface more closely than will larger molecules. The differences between the surface areas measured by small and large molecules increase with the roughness (fractal) dimensionality) of the surface. A plot of the number of molecules required for monolayer surface coverage (n,obtainable from the BET equationj vs the cross-sectionalarea (a)of the adsorbate will have a slope of -012, where D is the fractal dimensionality of the surface. This technique has been applied to many surfaces.1s-22 It requires knowledge of the cross-sectional areas of the adsorbate molecules. These were determined using the method of Emmett and Brunauer from the densities of the liquid adsorbate^.^^^^^ The data are contained in Table 1. The use of the BET equation and its assumptions are well u n d e r ~ t o o d . ~ ~ Figures 7-12 contain log-log plots of the moles of adsorbate in the surface monolayer vs the crosssectional area of the adsorbate for six bituminous coals. As determined by SAXS, the surfaces are fractals, so the slopes of the plots should lie between -1 and -1.5. The slopes vary between -5.5 and -11.6 (see Table 2). The process responsible for these results clearly cannot (18) Farin, D.; Peleg, S.; Yavin, D.; Avnir, D. Langmuir 1985, I , 399-407. (19) Farin, D.; Volpert, A,; Avnir, D. J . A m . Chem. Soc. 1985, 107, 3368-3370. (20) Pfeifer, P.; Avnir, D. J . Chem. Phys. 1983, 79, 3558-3565. (21) Avnir, D.; Farin, D.; Pfeifer, P. Nature (London) 1984,308,261263. (22)Ross, S. B.; Smith, D. M. Langmuir 1988, 4 , 977-982. (23)Emmett, P. H.; Brunauer, S. J.Am. Chem. SOC. 1937,59,15531564. (24) Gallant, R. W. Physical Properties of Hydrocarbons; Gulf Publishing Co.: Houston, TX,1968. (25) Lowell, S. Introduction to Powder Surface Area; John Wiley and Sons: New York, 1979.
Pore Structure of the Argonne Premium Coals
.
8
Energy & Fuels, Vol. 9,No. 2, 1995 327
l
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be simple surface adsorption. The highly polar lignite shows different behavior: all of the hydrocarbons give essentially the same surface area. The subbituminous coal is intermediate between the lignite and the bituminous coals. Plots for these two materials are shown in Figures 13 and 14. The surface areas for the coals measured with all of the gases are collected in Table 3. Discussion The data cannot be explained using a network of interconnected bottle-necked pores in a rigid framework. We believe it can best be rationalized using a model in which pores in coals are closed, isolated bubbles in a solid which are reachable only by diffusion through the
1.3
1.4
1.5
1.6
Log Adsorbate Cross-SectionalArea (h)
Figure 12. Fractal plot of the adsorption of gases on Illinois No. 6 coal. Table 2. Comparison of the Fractal Dimensionalitiesof the Argonne Coals Determined by SAXS and Gas Adsorption of Cyclic Hydrocarbon Gases (Cyclopropane-Cyclohexane) D %C slope correln dmmf SAXS ads ads coeffb coal Pocahontas No. 3 91.8 12 -5.8 -0.998 Upper Freeport 11 88.1 2.58 -5.5 -0.995 Lewiston-Stockton 15 85.5 -7.4 -0.942 85.0 -0.987 Pittsburgh No. 8 2.69 16 -7.9 14 -0.990 Blind Canyon 81.3 -6.7 Illinois No. 6 -11.6 80.7 2.87 23 -0.999 Wyodak-Anderson 76.0 2.56 11 -5.3 -0.867 Beulah Zap 74.1 2.35 a Linear correlation coefficient of the fractal analysis based on the cyclic hydrocarbon gas adsorption.
solid. This model results from the following four conclusions: (1) The dependence of measured surface area on the structure of the adsorbate is inconsistent with adsorption in a bottlenecked pore network. (2) The surface areas measured by gas adsorption are not controlled by surface adsorption. (3) The dependence of surface area on adsorbate size can be explained by diffusion of the adsorbates into the bituminous coals. (4) The constant surface area measured for Zap lignite is due to the insolubility (no diffusion) of the hydrocarbons used in the lignite. Ethane and C02 have the same overall cylindrical shape. Ethane has a diameter about 16% bigger than C02. The mean molecular diameters of ethane and CO2 are respectively 4.42 and 3.95 A.26 If adsorption in a
Larsen et al.
328 Energy & Fuels, Vol. 9, No. 2, 1995
fn
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Figure 13. Fractal plot of the adsorption of gasses on Wyodak-Anderson coal. I
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Figure 14. Fractal plot of the adsorption of gasses on BeulahZap lignite.
pore network is occurring, these two gases should report similar surface areas. They do not. For the bituminous coals, the CO2 surface areas are between 2 and 5 times larger than ethane surface areas. For the lignite, the difference is a factor 25. It is not reasonable that these pore systems contain bottlenecks so precisely sized that a 16%difference in diameter should lead to surface area differences greater than a factor of 2. Comparing surface areas measured with the other gases confirms that the interconnected pore network model is not satisfactory. The cross-sectional areas of ethane and cyclopropane are respectively 25.3 and 25.2 A2, essentially identical. Their shapes are different. One is planar and the other cylindrical. They should have different abilities t o pass through a system of slitlike pores and should thus report different surface areas. If the pores are slitlike as proposed by Bond, cyclopropane should penetrate more easily. For Pittsburgh No. 8 and Illinois No. 6 coals, ethane and cyclopropane give the same surface areas. For the other bituminous coals, the surface areas measured with ethane are larger than those measured with cyclopropane contradicting slitlike pores. C 0 2 has the same cross-sectional area as cyclopropane and reports very much larger surface areas. Cyclobutane should have about the same ability to diffuse through slitlike pores as cyclopropane but reports much lower surface areas. It is not possible to rationalize these data using a network of slitlike, bottlenecked pores. These surface areas measured by gas adsorption cannot (except for the lignite) be due to adsorption on surfaces. The SAXS data demonstrate that the coal
surfaces are fractals. The slopes of the lines (-012) in Figures 7-14 must therefore be between -1 and -1.5 if adsorption on fractal surfaces is solely occurring. The slopes for the bituminous coals are between -5.5 and -11.6. Simple adsorption on fractal surfaces cannot be responsible for these data. What is responsible for these data is diffusion of the gasses into the coals. Adsorption on the surface is immediately and so closely followed by diffusion into the bituminous coals (see Figure 1)that the two processes cannot be separated. The diffusion is so slow that the individual BET gas adsorption points are stable and seem to have reached equilibrium. But they have not. This was demonstrated by following the uptake of ethane by Illinois No. 6 coal for 5.5 days. The pressure is given in Table 4 and is seen to drop continuously over the entire time. It is now easy to rationalize the gas adsorption on the bituminous coals. It is controlled by the rates of diffusion of the various gases into the coals. It is wellknown that COZis soluble in and swells ~ o a l s It. ~ ~ ~ ~ ~ diffuses rapidly through the coals reaching all of the pores and reports an approximately correct surface area based on the agreement between coal surface areas measured by COz adsorption and by SAXS or SANS.12J4 The rapid diffusion must be due to a specific interaction with the coals. We do not know the nature of this interaction, but a thermodynamic demonstration of its existence exists.zQThe hydrocarbons used are very poor swelling solvents for bituminous coal and interact very ~ e a k l y . ~ OTheir - ~ ~ diffusion through the coals will be slow. The extraordinary steep dependence of apparent surface area on adsorbate size is due to the dependence of diffusion rate on molecular size. The slopes of the log-log plots if surface area vs adsorbate cross-sectional area in Figures 7-12 range between -5.5 and -11.6. The measured surface area of these coals varies with the adsorbate cross-sectional area raised to between the 5th and 11th power. There are few things in nature which show such an enormous size dependence. Diffusion through glassy polymers shows size dependences that are this l a ~ - g e . *A~n, ~ ~ example of this large size dependence is shown in Figure 15. We have already documented that these are not equilibrium measurements. The apparent surface areas are functions of the diffusion rates of the hydrocarbons into the coals and show appropriate size sensitivities for diffusion into glassy polymers. Coals are glasses at the temperatures at which the surface area measurements were made.34b35 If the hydrocarbons are insoluble in a coal, they should report surface areas that are nearly the same (26) Berens, A. R.; Hopfenberg, H. B. J . Membr. Sci. 1982,10,283303. (27) Reucroft, P. J.; Patel, H. Fuel 1986,65, 816-820. (28) Walker, P. L., Jr.; Verma, S. K.; Rivera-Utrilla, J.; Khan, M. R. Fuel 1988,67,719-726. (29) Glass, A. S.; Larsen, J . W. Energy Fuels 1994,8, 629-636. (30) Lucht, L. M.; Peppas, N. A. Erdoel Kohle, Petrochem. 1987,40, 483-485. (31) Larsen, J. W.; Green, T. K.; Kovac, J. J . Org. Chem. 1985,50, 4729-4735. (32) Glass, A. S.; Larsen, J. W. Energy Fuels 1993,7 , 994-1000. (33) Holley, R. H.; Hopfenberg, H. B.; Stannet, V. Polym. Eng. Sci. 1970,10,376-382. (34) Brenner, D. Fuel 1985,64,167-173. (35) Lucht, L. M.; Larsen, J. M.; Peppas, N. A.Energy Fuels 1987, 1, 56-58.
Pore Structure of the Argonne Premium Coals gas measurment temp "(C) coal (% C, dmmf) Pocahontas (91.8) Upper Freeport (88.1) Stockton (85.5) Pittsburgh No. 8 (85.0) Blind Canyon (81.3) Illinois No. 6 (80.7) Wyodak (76.0) Beulah Zap (74.1) a
Energy & Fuels, Vol. 9, No. 2, 1995 329
Table 3. Surface Areas (ma/g)Measured by Gas Adsorptiona COZ ethane cyclopropane cyclobutane cyclopentane -196 -78 -78 -78 -10 0
Nz
10 6 4 9 2 29 4 24
202 166 175 177 239 132 330 274
69 72 34 37 122
38 106 11
26 18 20 31 28 34 14
23 13 27 18 27 14 26
5
8
cyclohexane 0
15
10 8
9 13
5
13
5
14
8 2 9 7
5 7 5
BET equation, f 3 m2/g.
Table 4. Ethane Uptake as a function of time on 0.1190 g of Argonne Illinois No. 6 Coal at 177 K ethane gas ethane gas time (h) press. (Torr) time (h) Dress. (Torr) 0 0.02 0.08 0.17 0.33 0.75 1.0 1.5 2.0
99.429 40.090 36.748 35.502 34.162 32.098 30.923 30.068 29.360
3.0 4.5 6.3 8.0 27 54 108 131
-4
-
-6 -
\
-5
28.431 27.650 25.320 25.268 24.083 23.262 23.143 23.098
-7-
-a
-
\
-9-
for low dimensionality surfaces because no diffusion into the coal can occur. This occurs with Zap lignite (D= 2.35). The hydrocarbons do not dissolve in this polar high oxygen lignite. Carbon dioxide does dissolve and diffuse and so reports a high surface area. The hydrocarbons are limited to the surface and report a surface area of 8 f 3 m2/g. The expected fractal dependence of surface area on molecular size is smaller than our experimental error (f3m2/g). These data confirm the uselessness of the interconnected pore model. There cannot be a pore network for CO2 and no network for the hydrocarbons. Wyodak subbituminous coal shows behavior intermediate between the lignite and the bituminous coals and is clearly transitional. Coals are porous materials. The pores are not open t o the surface but can only be reached by diffusion through the solid coal. Diffusion rates are steep functions of molecular size. The existence of specific interactions with coals can also result in rapid diffusion rates. Helium is small enough to diffuse rapidly into coals and we have no reason to doubt the many published helium densities or the pore volumes derived from them. Likewise, C02 surface areas should be in error only by the small amount of C02 that is dissolved in the coal. But the CO2 surface area is a special case. Coals present only very small external surfaces to molecules which do not dissolve in them. They are not high surface area materials, although they are porous. Overview. The behavior of coals contacted with fluids covers a continuum established by the type and magnitude of the interactions between the fluid and the coal. At one end of the continuum are the fluids that strongly interact with a coal disrupting most of the noncovalent interactions in a coal. Pyridine is a good example. These solvents swell coals, often by more than 200%. They diffuse rapidly through the bulk coal. Diffusion in such systems has been well studied, especially by Peppas and his group. The diffusion kinetics are normally Case I1 showing that relaxation of the coal structure is occurring during d i f f ~ s i o n .This ~ ~ behavior is typical of polymeric systems. Diffusion through a
0
0
-16 .2
1
I
1
.3
.4
.5
h .6
d.nm
Figure 15. Dependence of diffusion rates through glassy polystyrene on the size of the penetrating gas. (Reprinted with permission from ref 26. Copyright 1982 Elsevier.)
pore network is not necessary to explain this behavior and, t o the best of our knowledge, has never been suggested. This diffusion and swelling alters the coal.37 In the expanded coal, reduced interchain interactions and increased free volume enable the coals to rearrange to a more stable conformation which is not that of the asmined coal. Diffusion through the coal alters it, while diffusion through an interconnected pore network does not. This provides one means of distinguishing between the two. Further along the continuum come more weakly interacting fluids, a good example being toluene. A very complete and prescient study of its interactions with coals was carried out by Duda and Hsieh.l Toluene dissolves in coals and is a weakly swelling solvent.31 (36)Barr-Howell,B. D.;Peppas, N. A,; Winslow, D. N. Chem. Eng. Commun. 1986,43,301-315. (37)Larsen, J. W.; Mohammadi, M. Energy Fuels 1990, 4 , 107110. Hall, P. J.; Larsen, J. W. Energy Fuels 1993,7,42-46. Nishioka, M.; Larsen, J. W. Energy Fuels 1990, 4 , 100-106.
330 Energy & Fuels, Vol. 9, No. 2, 1995
Duda and Hsieh demonstrated that toluene diffusion into a bituminous coal is accompanied by a rearrangement of the coal. Its properties are different before and after contact with toluene vapor. The toluene is diffusing through the bulk coal and coal macromolecular segments are moving during the course of this diffusion. Toluene diffuses much more slowly than does pyridine, but the general behavior is the same. Diffusion through an interconnected pore network is not necessary to explain these data nor, to our knowledge, has it been suggested. The end of the continuum is established by fluids that are insoluble in a coal. This will be established by the properties of both materials, the coal and the fluid. Our data for hydrocarbons on Zap lignite is an example. There is no penetration of the coal because the hydrocarbons are insoluble and there is no pore network. The insoluble hydrocarbons all report the same low surface area because they cannot diffuse into a material in which they are not soluble. COn is soluble in Zap lignite and so reports a large surface area. There is one circumstance in which an insoluble molecule can diffuse rapidly into coals. The molecule must be very small. Helium is an excellent example. We believe that He densities of coals are correct. Helium is so small that its diffusion rates through the glassy solid coal (cf. Figure 15) are rapid enough to allow penetration during the time required for the density experiment. Unlike Walker and Mahajan, we find no contradiction between the experimental results with He and the closed pore model presented here.38 Indeed, the results are nicely consistent with it. It does raise the interesting case of the origin of helium’s rapid diffusion. Does it move rapidly because coal molecular segments must move only very slightly to allow its passage or is the micropore network continuous on the small size scale of the He atom? We prefer the former because of its consistency with the model presented here but know of no data permitting an unequivocal choice between the two models at the small size dimensions of He. For larger dimensions, experiments are consistent with closed pores and inconsistent with the existence of a network. This extreme case of fluid insolubility has been adopted by Walker and Mahajan and used to explain (38)Walker, P. L., Jr.; Mahajan, 0. P. Energy Fuels 1993,7, 559560.
Larsen et al. the “adsorption” behavior of permanent gasses.38 The coal is claimed t o be a rigid framework holding a set of interconnected pores. The dependence of surface area on molecular size is ascribed to discrimination between molecules based on the sizes of the openings in the pore network. This model cannot explain our data for Zap lignite and is thereby demonstrated not to be universal. It is most difficult to explain the remainder of our hydrocarbon adsorption data using a pore network model and the simpler single picture of diffusion through a bulk solid is preferred. There is no compelling reason to use an extremum of a continum as the only description of adsorption behavior. The crucial case is CO2. It has been known since the 1930s that C 0 2 swells coals and must therefore interact significantly with them.26,27Specific interaction has been d e m ~ n s t r a t e d .On ~ ~ the continuum, COz is less interacting than toluene. But it is an interacting, swelling fluid and a small molecule. This combination of size and interactions allows it to diffuse rapidly through bulk coal and adsorb on pore surfaces. We have no argument with surface areas measured with COz and agree with Walker and Mahajan that these surface areas are irrelevant to much of coal chemistry. Tetralin will experience a much smaller surface than that reported by CO2 because its initial adsorption will be on an external surface much smaller than the surface measured by C02. Because coal surfaces are fractal, each adsorbate will report a different surface area. Because most molecules of interest are soluble in coals, their accessibility will be controlled by their diffusion rates through the bulk and not by diffusion through a pore system. There exist data that are inconsistent with the existence of an interconnected pore network in coals. Those data can be explained by the existence of a continum of behaviors dependent on the magnitude of ‘‘adsorbate” interactions with the coal and adsorbate molecular size. Coals behave as organic macromolecular systems and not as rigid inorganic rocks.
Acknowledgment. We thank the US.Department of Energy for financial support of this work. Enlightening discussions with Dr. Om Mahajan were helpful and enjoyable. We are grateful to Dr. Ken Liang (Exxon) for his advice concerning the SAXS measurements. EF940165R
