Energy & Fuels 1989,3, 551-556 anisotropic distribution of branch points in a random coil network. This preserves a structure that is nearly isotropic at the molecular level but has highly anisotropic bulk properties. The origin of optical anisotropy may be assigned to a very slight preferred orientation of PNA's in coal. It can be said, however, that this is a trivially small deviation from randomness of polynuclear aromatic molecules in the macromolecular framework.
Summary Polynuclear aromatic and other organic functionalities are almost randomly oriented in Illinois No. 6 and Pittsburgh No. 8 coals. The low-volatile coal, PSOC 875, and to even a lesser degree Pittsburgh No. 8 coal, exhibit a very weak polarizer-position-dependent effect that is observed as a general increase of absorption across the mid-infrared region when the beam is polarized parallel to the bedding plane. A very minor amount of linear dichroism due to preferred orientation of molecular transition moments may be present
55 1
with PSOC 875 coal. The most obvious example is the aromatic C-H out-of-plane wagging vibration at 815 cm-'. Anthracite and to a very slight degree PSOC 875 and Pittsburgh No. 8 coals exhibit a polarizer-dependenteffect, observed across the mid-infrared region, in which there is greater absorption when the polarizer is oriented parallel to the bedding plane. In the case of anthracite, the dichroic ratio is approximately 2, whereas in the case of PSOC 875 and Pittsburgh No. 8 coals, the ratio is slightly greater than 1. This general dichroism presumably results from greater thermal conductivity parallel to the bedding plane and may reflect a slight preferred orientation of pseudographitic crystallites. In the case of anthracite, the average angle these crystallites make with the bedding plane is approximately 35".
Acknowledgment. We are indebted to Dr. G. D. Cody, Sr., for very helpful discussions. G.D.C. thanks Al Ruppert for very helpful discussions and Bill Anderson of Lehigh University for help with the photoacoustic FTIR instrument.
Correlation of Optical Birefringence with Coal Rank. Structural Implications George D. Cody, Jr.,**tJohn W. Larsen, Michael Siskin, and G. D. Cody, Sr. Corporate Research Laboratory, Exxon Research and Engineering Company, Annandale, New Jersey 08801 Received November 30, 1988. Revised Manuscript Received May 22, 1989
Previous investigations of the anisotropic physical properties of coals using solvent-swelling techniques indicated a greater population of cross-links parallel to the bedding plane in medium-rank coals than perpendicular to it. The presence of optical anisotropy in medium- and high-rank coals had previously suggested that a degree of preferred orientation of organic molecules exists in the macromolecular framework. In this report, the degree of preferred orientation is estimated via a complementary theoretical treatment using hypothetical aromatic compounds. It is concluded that a trivially small degree of preferred alignment of aromatic molecules is necessary to yield the birefringence observed in medium-rank coals. For coals with a % C (daf) of 87% or greater, an additional contribution to optical anisotropy exists. Increases in the absorption index and index of refraction above 87% C are shown to be consistent with the presence of anisotropic pseudographitic regions. It is therefore concluded that the anisotropic physical properties of coals up to -87% C (daf) are consistent with a random coil network structure with an anisotropic distribution of branch points. Above 87 % carbon (daf), nucleation of pseudographitic regions occurs in an essentially randomly oriented molecular framework, yielding a material with optical properties intermediate between those of the random coil network of medium-rank coals and those of the pseudographitic structure of anthracite and metaanthracite. This study and others suggest that the observed optical anisotropy in coal is consistent with the observed absence of infrared dichroism associated with organic functionalities in coals as well as the strongly anisotropic solvent swelling of coals. Further characterization of the random coil network structure is planned.
Introduction codsinsolubility makes this geopolymeric substance very difficult to characterize. The average molecular formula of and the distribution of functionalities can be derived through the use of elemental analysis and derivatization chemistry's2 and FTIR, NMR,. and other 'Department of Geology, The Pennsylvania state University, 218 Deike Building, University Park, PA 16802.
spectroscopies. A second set of less specific probes exist, e.g. density determination and reflectivity measurements. Used in conjunction with the information obtained from the first set of probes, the second set yields information about the average number of rings in PNA's in coal, (1) Liotta, R.Fuel 1979,58, 724-728. (2) Souten, E. G.; Siskin, M.; Rose, K. D.; Aczel, T.; Colgrone, S. G.; Pabst, R. E., Jr. In Proceedings of 4th Australian Workshop on Oil Shale; ANSTO: Lucaa Heights, NSW, 1987; pp 94-100.
0887-0624/89/2503-0551$01.50/00 1989 American Chemical Society
Cody et al.
552 Energy & Fuels, Vol. 3, No. 5, 1989 thereby providing insight into structural development of coals on the molecular level. None of these very powerful tools, however, gives much information about the macromolecular framework of coal. Since critical issues such as mass transport, coking, and depolymerization during liquefaction rely heavily on the way coal molecules are put together, structural information on the macromolecular framework is crucial. Probes that yield information about the macromolecular structure of coal rely, in general, on bulk structural properties of coal that reflect the structure of coal on the submicroscopiclevel. Solvent swelling and extractability, for example, provide information about the macromolecular structure in averaged terms. Careful use of these probes in conjunction with information obtained via the molecular-scale probes can set up constraints that ultimately can be used to predict the most reasonable structure for coal. In a recent paper, it was established that coals swelled anisotropically in solvents such as pyridine, tetrahydrofuran, and chlorobenzene; with a greater degree of swelling in the direction perpendicular to the bedding plane of This swelling anisotropy increased as a function of rank for the coals studied, suggesting that the anisotropy reflects differential changes in the network structure of coal during coalification. For the coals used in this study, a positive correlation between solvent-swelling anisotropy and optical anisotropy was observed. Optical anisotropy in coals and its increase with coal rank have been recognized for decade^.^ The common interpretation is that the appearance of optical anisotropy in coals reflects enhanced ordering of planar aromatic molecules with a increasing degree of preferred orientation parallel to the bedding plane. This would appear to be an obvious conclusion, in light of the unidirectional pressure of overburden. Recently, Brenner showed that the optical anisotropy of bituminous coals can be removed via pyridine swelling, suggesting that the optical anisotropy of coals is stress-related.6 It has also been noted that after removal of the swelling solvent, the dried coals exhibited dimensional changes; they had increased in the perpendicular direction and shrunk in the parallel direction. The degree of dimensional change correlated positively with both the swelling and optical anisotropies of the coals studied. All of these observations could be rationalized by a structure in which the differential swelling, optical anisotropy, and resultant dimensional changes were due to a preferred organic structural orientation in coal and a loss of that stress-induced orientation during swelling, respectively. Several problems exist with this hypothesis. First, the lower rank optically isotropic coals exhibit swelling anisotropy, and second, even after the optical anisotropy of the higher rank coals was removed via pyridine immersion, the higher rank coals still exhibited strong swelling anisotropy. Thus, these data suggest no direct connection between the bulk mechanical (swelling) anisotropy and the optical anisotropy. A study that would directly probe the average degree of preferred orientation in coals was undertaken by using polarized photoacoustic FTIR! This work indicated that the degree of preferred orientation of molecules in the macromolecular network was negligible through the (3) Cody, G. D.; Larsen, J. W.; Siskin, M. Energy Fuels 1988, 2, 340-342. (4) van Krevelen, D. W. Coal; Elsevier, Amsterdam, 1960. ( 5 ) Brenner, D. Fuel 1983, 62, 1347. (6) Cody, G. D.; Larsen, J. W.; Siskin, M. Energy Fuels, preceding paper in this issue.
Table I. Characteristics of Coal Samples anal., wt % sample C H N S 0 ash % C ( d a f ) Wandoan 67.0 4.7 0.8 0.5 21.6 5.4 70.8 Cerrejon 81.4 5.4 1.4 0.6 10.6 0.6 81.9 68.4 5.2 1.3 3.1 10.8 11.2 77.0 Illinois No. 6 PSOC 712 77.0 5.2 1.4 1.7 12.6 -2.1 79.0 5.4 83.8 Pittsburgh No. 8 79.3 5.3 1.4 2.8 2.0 PSOC 341 75.3 4.5 14.5 88.1
high-rank bituminous coals up to anthracites. This is not to say that there were no anisotropic features in the highest rank coals, but rather that vibrations associated with dipoles of organic nature were randomly oriented within the sensitivity of this technique. This observation appears to contradict the observations and interpretations of the presence and progressive increase of optical anisotropy during c~alification.~ This paper sets out to resolve this apparent contradiction by using the degree of optical anisotropy to model the amount of preferred orientation in coals. Critical to this paper is the work done by van Krevelen and S~huyer'-~ in modeling the molecular dements of coal using optical data.
Experimental Section The following coals were used in this study: Wandoan subbituminous A or B, Cerrejon high-volatile C, Illinois No. 6 high-volatile C, PSOC 772 high-volatile B, Pittsburgh No. 8 high-volatile A, and PSOC 341 medium-volatile bituminous rank. Analyses of these coals are reported in Table I. These were stored under dry nitrogen and were not dried before use. Higher rank coals, low-volatile bituminous and up, absorb all visible light a t the thickness of these thin-section samples (3-15 wm), and are not included in this part of the study. Birefringence data on the higher rank coals were obtained from previous s t ~ d i e s . ~ Uncontaminated thin-section samples were prepared by using a soluble adhesive to hold the sample to a petrographic slide during the grinding process, following the procedure of Brenner: to obtain thin section samples of the order 3-15 wm. Whereas an exact measurement of birefringence, the numerical difference between the high and low indexes of refraction, can be made on relatively inert, optically anisotropic substances such as quartz, coals swell in many of the immersion oils used to obtain these indices. A rather simple method exista, however, to estimate the birefringence of coal without the use of oil immersion analysis. This method uses the polarized-light microscope and correlates resultant interference colors with sample thickness to yield birefringence. Interference colors result when optically anisotropic substances are properly oriented between two polarizers, oriented orthogonal to each other. An optically anisotropic substance is defined as having two or three indices of refraction, optically uniaxial or biaxial, respectively. When plane-polarized light passes through an optically anisotropic material, the electric vector will split into two polarized orthogonal vectors that vibrate through the substance at different reduced speeds; the degree of retardation of vibration speed is inversely proportional to the corresponding index of refraction. When the vectors leave the sample, they are out of phase; thus, the resultant transmitted light is now eliptically polarized and as such can pass through a second polarizer oriented 90° to the first. The degree of eliptical polarization is observed as interference colors under crossed polarizers. The interference colors are related to sample thickness and birefringence (the difference in the maximum and minimum indices of refraction). Although one cannot measure the exact birefringence of coals using oil immersion analpis, if the sample thickness is known and a specific interference color is observed, this information can be plotted on (7) Schuyer, J.; Bloom, L.; van Krevelen, D. W. Trans. Faraday SOC. 1953, 49,
1391.
( 8 ) Schuyer, J.; van Krevelen, D. W. Fuel 1954, 33, 176. (9) Brenner, D. Coal Macerals; Winans, R. E., Crelling, J. C., Eds.; ACS Symposium Series 252; American Chemical Society: Washington, DC, 1984, pp 47-64.
Correlation of Optical Birefringence
Energy & Fuels, Vol. 3, No. 5, 1989 553
Table 11. Comparison of Estimated Birefringence Values for Medium-Rank Coals SmDh rank % C (daf) birefringence" Wandoan Sub-b 70.8 0.006 Cerrejon Hvb-c 81.9 0.009 Illinois No. 6 PSOC 772
Hvb-c
Hvb-b Hvb-a
77.0 79.0 83.8 88.1
0.031 0.020
Pittsburgh No. 8 0.042 PSOC 341 Mvb 0.029 "The error for these values is estimated to be a maximum of f0.005. See Experimental Section for details. a Michel-L6vy birefringence chart, and a numerical estimate of birefringence can be obtained. Sample thickness was measured by using a microscope with a high numerical aperture objective, 400X magnification, and a calibrated fine focus adjustment knob. The distance from the top to the bottom of the sample was measured and calibrated with stainless-steelshims of known thickness. Several readings were obtained and averaged to yield sample thickness. Identification of interference colors in coal is not a simple or exact task. The natural color of coal in transmitted light is an orange-yellow due to absorption and/or scattering. The combination of this absorption color with the interference color due to the relative retardation difference between fast and slow vectors produces a color that is little different from the coal's natural absorption color. It is, therefore, difficult to estimate the birefringence of coal. The use of a half-wave plate was effective in obtaining quantitative estimates of birefringence for the coals in this study. At a specific sample thickness, with the sample's major and minor indices of refraction oriented 4 5 O to the polarizers and the major index oriented parallel with the major axis (slow direction) of the half-waveplate, the resultant interference color is dark olive green. This color results from the superposition of coal's natural absorption with the blue interference color due to the sum of the retardations of the oriented coal and the half-wave plate. Fortunately, the coals studied are not pleochroic (their absorption of light does not depend upon their orientation with respect to the polarizer light), so that the effects of absorption and polarization can be resolved. Subtractionof 530 nm from the fmt-order blue color band yields the retardation due to the coal and the true interference color of coal at that thickness. Error in these measurements is attributed to two sources. The first is the broadness of the first-orderblue polarization color band (i50 pm),which causes a maximum of h0.003 error in the birefringence values. The second error is in the thickness determination, which is i 3 pm, yielding an error interval of *0.002 for the birefringence values.
Results Birefringence. The estimated birefringence results are presented in Table 11. In general, the birefringence of these coals increases with rank outside expected error. In addition to the birefringence estimates for vitrinite, it was also possible to qualitatively rank birefringence of optically anisotropic exinitic macerals. In every case, it was observed that exinites are slightly less birefringent than the vitrinite, i.e., at sample thicknesses greater than that necessary to yield a first-order blue color band for vitrinite, a first-order blue color band was observed for the optically anisotropic exinites. As will be discussed later, the cause of this observation may be more than simply a difference in composition, e.g. aromaticity. Discussion Vitrinite. Estimates of birefringence may give an indication of the degree of preferred molecular orientation in these coals. The optical properties of coal, e.g. reflectivity, can be predicted from the molar refractivity of molecular constituents within coal! One critical problem, however, is that the exact structure of coal is poorly un-
derstood. Nevertheless, several researchers have proposed models of average coal mole~ulesl"-~~ from which a net molar refractivity can be calculated. This value can then be used to calculate the reflectivity of a model material composed of such mole~ules.'~ Although the preferred orientation of any polarizable group in coal can yield birefringence, it is generally accepted that the preferred orientation of rigid, planar, PNA's are the best candidates for the origin of birefringence. This conclusion is based on the structural information provided in a variety of model coal molecule^.^^'^ Common to all of these models are substituted PNA's with hydroaromatic rings cross-linked to other PNA's by short methylene, ether, and sulfide linkages. In order to calculate the average degree of preferred orientation of PNA's in coal, the following assumptions are made, both of which are supported by a recent IR dichroism study of coals? The aliphatic groups in coal are randomly (isotropically) distributed, such that they contribute to the average index of refraction, but not to the birefringence. This is probably a safe assumption for vitrinite where the fraction of aliphatic carbon is predominantly hydroaromatic and short-chain a l i p h a t i ~ s .For ~ exinites, however, this assumption may not hold, as will be discussed later.3 The contribution to birefringence by heteroatoms, and oxygen functionalities in particular, is considered to be negligible. This assumption is reasonable because the 0, N, and S contents are considerably lower than the aromatic carbon content in the higher rank optically anisotropic coals.4 From these assumptions, the average degree of preferred orientation of PNA's in these coals can be calculated. To accomplish this, a theoretical treatment with hypothetical aromatic compounds was constructed. Following the procedure of Schuyer and van Krevelen: we will approximate the molar refractivity of a single aromatic molecule by an oblate elipsoid of revolution. The axis of rotation (a, the minor axis) is considered constant for all aromatic molecules (a = 1.6 A), and the major axis ( b ) is derived from the aromatic surface area ( S ) of the molecules by using eq 1 of Schuyer ahd van Krevelen? where A = the number of aromatic carbons and B = the number of aromatic hydrogens.
S = 2.197A
+ 1.055B
(1)
From refraction equivalent values for the individual contributions of aromatic carbon, hydrogen, and the presence of double bonds,4115 the molar refractivity (R,) of the molecule in question can be calculated. This is the average of the refractive components parallel and perpendicular to the bedding plane of the aromatic molecule, R, and R,, respectively, where
R , = (2R, + R c ) / 3
(2)
If R,/Rc is set to be proportional to b l a , the respective indices of refraction can be calculated by using the Lorem-Lorentz equation (eq 3), for a hypothetical compound
+
R , = M ( n 2- l ) / D ( n 2 2)
(3)
composed entirely of aromatics, all planar to each other. (10)Given, P.H. Fuel 1960, 39,147. (11)Wiser, W. H. Am. Chem. SOC.Symp. Ser. 1978, No. 71, 29. (12)Solomon, P.In New Approaches in- Coal Chemistry; Blaustein, B. D., Bockrath, B. C., Friedman, S., Eds.; ACS Symposium Series 169; American Chemical Society: Washington, DC, 1981. (13) Shinn, J. H. Fuel 1984,63, 1187. (14) Davis, A. DOE Symp. Ser. 1978, 46. (15)Hartshorn, N.H.; Stuart, A. Crystals and the Polarizing Microscope; 3rd ed.; Elwood Arnold Publishers, Ltd., London, 1960, p 557.
554 Energy & Fuels, Vol. 3, No. 5, 1989
Cody et al.
Table 111. Calculated Birefringence ( B )of a Hypothetical Compound Composed Entirely of Planar Aromatic Molecules molecule B molecule B
benzene naphthalene phenanthrene
anthracene
0.450 0.723 0.844
perylene
.0.934 0.971
1 " " I ' " ' I ' ~ ' ' I " ' '
0.140
0.040 0.020
-
40.0
41.0
42.0 CY
43.0
44.0
45.0
in Degrees
Figure 1. Calculated birefringence vs degree of preferred orientation (a)for hypothetical aromatic compounds.
It should be noted that R J R , is exactly proportional to the component polarizabilities of the ellipsoid. The ratio of component polarizabilities is equal to the ratio of the ellipsoid axes multiplied by a proportionality factor, k, where k is calculated to be 1.07 for benzene and 1.13 for perylene.' In eq 3, M is the molecular weight of the aromatic molecule and D is the density of the hypothetical compound. For a density of 1.35 g/cm3, chosen as representative of a medium-rank coal, the dependence of the calculated birefringence as a function of ring size for planar configurations is shown in Table 111. The range of birefringence of the coals used in this study is 0.006-0.042. If data from van Krevelen' are included, the range extends up to 0.108. These values are considerably less than those derived for the hypothetical compounds (see Table 111). These calculated values were obtained for a fixed density; if the model density is increased, the birefringence increases. Since the coals and hypothetical compounds exhibit uniaxial negative anisotropy, a simple optical diagram (an indi~atrix)'~ can be constructed. The relative orientation of the aromatic molecules in the hypothetical compound can now be adjusted to match the indicatrix obtained for coal, by using eq 4, where n, and n, are derived from eq nL2(cos2a)/n,2 + n',2(sin2 a)/n: = 1 (4) 3 and a is defined as the average angle the planes of the aromatic molecules make to the optic plane (beddingplane, for coal). The values in Table I11 are for the case in which a =0 ' ; therefore, B = O.OO0 when a = 4 5 O . Figure 1 shows the dependence of birefringence on a,for hypothetical compounds composed entirely of the molecules listed in Table 111. As would be expected the larger the ring system the less the preferred orientation necessary to obtain a given birefringence value. As coalification proceeds, the aromaticity increases.' The birefringence also increases with rank.' If one were to envision the "coalification path" through Figure 1, it would originate at a = 4 5 O and then follow a curve that is convex upward to the birefringence axis. Increases in the degree of preferred orientation inferred from enhanced optical anisotropy may not be real but may be due to increases in the ring size of aromatic molecules. Alterna-
tively, if the ring size remained nearly constant, but the volume fraction of aromatic molecules increased raising the density, the birefringence will also rise. It is conceivable, although unlikely, that increases in optical anisotropy are caused entirely by condensation of the macromolecular network, without any enhancement of preferred orientation. Note that, even in high-rank coals, the degree of orientation is not more than 3O. There is very little net orientation of the aromatic rings in coals with respect to the bedding plane. Exinites. The preceding assumptions and calculations are fairly simple for vitrinites, but exinites provide special difficulties when this protocol is applied. Some of the birefringence of exinitic macerals may be due to the preferred orientation of paraffiiic materials. Evidence for this is obtained by observing the comparative behavior of exinites and vitrinites swollen in pyridine at room temperature. In previous work, it was demonstrated that swelling of coal thin-section samples in pyridine removed the optical anisotropy of ~ i t r i n i t e s . ~ This loss of optical anisotropy results from stress-relaxation-driven conformational changes in the macromolecular n e t ~ o r k Exinites, .~~~ on the other hand, maintained optical anisotropy, even in the pyridine-swollen state. This suggests that birefringence in exinites is not stress-induced and/or there exist secondary interactions in exinites that cannot be broken by pyridine. A reasonable candidate for such interactions is van der Waals forces between paraffinic chains. In addition, X-ray diffraction data suggest some ordering in exinites not observed in vitrinites, specifically the y-band, which is caused by paraffinic ordering.16 It is therefore very difficult to estimate a degree of preferred orientation in exinites, because there may be contributions from both aromatic and paraffinic groups to the birefringence. The following observations can be made. If the birefringence of exinites is only due to a weak alignment of aromatics, the degree of preferred orientation might be comparable to the surrounding vitrinite, even though the birefringence is less, because the average ring size of and volume fraction PNA's in exinites is less than vitrinite.' The curves in Figure 1, therefore, clearly show that ring condensation alone can significantly raise birefringence without changing the degree of preferred orientation. If birefringence in exinites is due entirely to paraffinic material, the degree of preferred orientation may be much greater than the surrounding vitrinite. This conclusion is based on the fact that the molar refractivity of paraffinic materials is inherently less than aromatic molecules due to the lack of conjugation.'J5 This additional complexity precludes an attempt to make a numerical estimate of the degree of preferred orientation in exinitic macerals. Qualitatively, however, from either the paraffinic or aromatic argument, it is conceivable that exinites have a greater degree of preferred orientation than surrounding vitrinite, even though the birefringence of exinities is lower. Effect of "Graphitization"on the Optics of HighRank Coals. In addition to the problem with exinites, there also exist problems extending these calculations to the limits of coalification. Van Krevelen showed that up to 87% C daf, the value of the molar increment per gram of aromatic carbon increases: consistent with increasing aromatic surface area. Above 87% C daf, however, the (16)Ergun, S.;Wender, I. In Chemistry of Coal Utilization, Supplementary Volume, Lowry, H. H., Ed.; 1963,p 42. (17) Ebert, L.; Scanlon, J. In Polynuclear Aromatic Compounds; Advances in Chemistry 217, American Chemical Society Washington, DC, 1987.
Correlation of Optical Birefringence molar increment per gram of aromatic carbon decreased. This is inconsistent with what would be predicted from the ellipsoidal approximation for aromatic molecules. Assuming charge transfer between graphitic crystallites in coal, van Krevelen and Schuyer were able to modify their equations to account for the presence of highly condensed regions in higher rank coals. In order to understand the origin of birefringence in the highest rank coals, the effect of graphitic regions on the optics of coal must first be established. In a study of infrared dichroism in coal: it was noted that thermal conductivity, hence electrical conductivity, increased between 85 and 92% carbon, and these data are also consistent with the presence of highly condensed regions in the highest rank coals. Dichroism due to the preferred orientation of smaller PNA systems, however, was not observed for any of the coals, hvC bituminous up to anthracite. The thermal conductivity, on the other hand, was anisotropic, being greater parallel to the bedding plane, consistent with a preferred orientation of graphitic crystallites in the highest rank coals. To call these regions graphitic, however, may not be accurate. There is no unequivocal evidence that the structure of these condensed regions is true graphite.16 Within these regions, carbon is strongly bonded to three neighboring coplanar carbons resulting in relatively large molecular sheets. These sheets are stacked in a lamellar orientation and held by van der Waals interactions. Slight distortions or vacancies on these sheets may exist, adding a degree of disorder to the crystallites. Hence, we chose to label these regions pseudographitic. The presence of pseudographitic regions in high-rank coals is suggested in reflectivity studies of vitrinites in coal in which an obvious increase in the slope dn/dC(daf) occurs around 87% C daf. Also, van Krevelen and Schuyer observed that the dielectric constant of coals, measured at microwave frequencies, increased dramatically for coals with carbon contents greater than 87%.4 Clearly, the presence of pseudographitic regions will greatly affect the optical properties of the highest rank coals. Coals become pseudographitic during prograde metamorphism, although the exact nature of the transformation is not well understood. Evidence, through X-ray investigation,18 suggests that the transformation to graphite occurs via nucleation of small graphitic regions (clusters) rather than a continuous transformation uniformly imposed across the entire macromolecular framework. Accepting this interpretation, one can use the increase in reflectivity4Jg to approximate the volume percent of pseudographitic regions for coals between 87 and 96% C daf. To this end, the effective medium theory (eq 5) may
be used, in which the dielectric constant of an imhomogeneous medium imbedded with metallic and insulating regions varies as a function of the volume fraction of the metallic regions.20 Here, t is the average dielectric constant, t, is the dielectric constant of the medium, tpg is the dielectric constant of the metallic region, ti is the dielectric constant of the insulator, Vpg is the volume fraction of the metallic region, and Vi is the volume fraction of the insulator. (18)Hirsh, P.B.h o c . R . SOC.London 1954, A226, 143. (19)McCartney, J. T.; Teichmuller, M. Fuel 1972, 51, 64. (20)Aspnes, D.E. Thin Solid Films 1982,89, 249.
Energy & Fuels, Vol. 3, No. 5, 1989 555 ~ " " " " ' ~ " " " " ' l ' " " ' l ~ ' ~ ~ l l ~
1.000
0.500
after van Krevelen (3) h = 546 nm
A
b
1
AAA
1
A A Ab
AAAA A
0.000
-
, 8 + , ,
55
I I I t I t I I
65
# A , L , I , , , , , , , , , I ,1 75 85 I
1 1 1 1 , , 1 1 1 , ,
95
Figure 2. Complex part of the dielectric constant for coals up to anthracite.
For t = E, eq 5 rearranges to eq 6,which describes the situation where the dielectric constant of the material varies exclusively as a function of the volume fractions of the metallic and insulating regions. Vpg(tpg- 4 V,(Ei - t)
+ 24
(epg
+-=
(ti
+ 24
0
High-rank coal is neither a true insulator or a true metal. Its semiconductive properties4 may, however, be a product of insulative and at least semimetallic regions on the submicroscopic level. Equation 6 can be used to test if the optical properties of the higher rank coals are caused by the presence of pseudographitic regions that nucleated during prograde metamorphism. To calculate the volume fraction of pseudographitic regions, the dielectric constants required for equation 6 are needed. If it is accepted that below 87% C (daf) no pseudographitic regions exist, the dielectric constant for vitrinite at 87% C (daf) can be used as $. This justification follows from van Krevelen's observation that the dielectric constant of coal is at a minimum at -87% C (daf).4 For lower carbon content coals, the high dielectric values are due to the presence of oxygen. For higher rank coals, the increase in E is due to the presence of graphitic crystallites.4 Above 87% C only small quantities of oxygen exist in the insulating matrix, and the principal changes in the matrix are increases in the aromatic ring number. In principle, slight losses in the insulator dielectric constant due to loss of oxygen should be offset by the slight increases in the dielectric constant with increasing ring size, and the dielectric constant of the insulator should change very little. For the dielectric constant of the pseudographitic regions tpg, data obtained by ErgunZ1for the average index of refraction (n) and the average absorption index ( k ) of graphite in the visible region, 546 nm, will be used. The indices of refraction and absorption indices of coals from 58% to 96% C (daf) have been calculated by van Krevelen et al.4 from reflectivity measurements using the Fresnel relationship. For materials that have an absorption index greater than zero in the spectral region of interest, the dielectric constant is complex and t
= tl
+ it2
where el = n2 - k2 and t2 = 2nk. Plotting t2 vs % C (daf), we obtain Figure 2,which clearly shows an abrupt change in slope in the region 87%-92% C (daf). In contrast, Figure 3 shows a plot of el that exhibits a progressive increase from 70% C up to 96% C. An interesting feature (21)Ergun, S. Chem. Phys. Carbon 1968,3.
Cody et al.
556 Energy & Fuels, Vol. 3, No. 5, 1989 after van Krevelen (3)
A A
A A
3.300 b
A A
A
2.800 55
60
65
70
75
85
80
95
90
100
Figure 3. Real part of the dielectric constant for coals up to anthracite. 100 90
80
-8
&?
40
30 20 10
0
85
'
~
A
'
"
90
~
'
'
'
95
'
'
'
'
100
'
% C (daf)
Figure 4. Calculated volume percent of pseudographitic regions in coal as a function of % C (daf).
in Figure 3 is the change in curvature at 94% C, which will be addressed later. The strong inflection observed in Figure 2 compels us to use the imaginary part of the dielectric constant for our calculations. The complex part of the dielectric constant has been previously used for similar calculations in studies of the optics of Ag-Si02 and Au-Si02 granular films.22 Figure 4 shows the calculated volume fraction of pseudographitic regions vs % C (daf). This simple model shows that the sharp increase in the reflectivity of vitrinite above 87% C (daf) is consistent with the presence of pseudographitic regions in coal. In eq 6, we used the mean of the optical properties of coal and graphite and included no provision for anisotropy within the metallic or insulating regions. It should be possible to extend it to anisotropic shapes and dielectric constants. The presence of pronounced birefringence4indicates that the pseudographitic regions exhibit some degree of preferred orientation, also indicated in studies of coal utilizing small-angle X-ray scatteringI8 and polarized photoacoustic FTIR.6 These other studies indicated that the degree of preferred ori(22) Abeles, B. Appl. Solid State Sci. 1976, 6, 94.
entation was quite small for coals up to anthracites. Since little is known about the structure of the pseudographitic regions, the degree of preferred orientation of these regions cannot yet be estimated on the basis of the optical data alone. What is clear, however, is that the birefringence observed in the higher rank coals is due to the slight preferred orientation of the pseudographitic regions and not to increases in preferred orientation of the insulating organic matrix. One final note about Figure 3. Significantly more microporosity exists in the highest rank coals, presumably caused by poor packing of the pseudographitic regions.23 This pore size is considerably less than the wavelength of light and, therefore, may also affect the bulk optical properties of the highest rank coals. The presence of pores will not affect the absorption index (k),hence their effect may be best observed in the real part of the dielectric constant (Figure 3). In principle, it is possible to calculate the volume change of micropores by using the effective medium theory. One must first establish, however, whether the pores are "air-filled" or "oil-filled" during measurement of reflectivity in air and oil, respectively. Rather than speculate on this point any further, we simply note that a dramatic increase in micropores may effect the bulk optical properties of the highest rank coals, e.g. the inflection in Figure 3 at approximately 94% C (daf).
Conclusions Implications for the Structural Development of Medium- to High-Rank Coals. On the basis of the previously stated points, the structural development of coals can be generalized with regard to the degree of preferred molecular orientation from the first appearance of optical anisotropy at about 70% C (daf) up to anthracite. As coalification proceeds through the high-rank subbituminous coals up to near 87% C (daf), a very slight degree of preferred orientation of aromatic molecules results from compaction-induced stress. Although the resultant birefringence is pronounced, the degree of preferred orientation within the macromolecular framework is trivially small and it can for most purposes be treated as isotropic or completely random. A structure most compatible with the above points for coals below approximately 87% C (daf) is a random coil network with an anisotropic distribution of branch points. At the point that pseudographitic regions form, nucleation occurs in an essentially randomly oriented matrix. Although the transformation of coal to graphite is still not understood on the molecular level, it appears that the randomly oriented structure of coals below 87% C daf leads to the random nucleation of graphitic regions in the higher rank coals. As shown by others, a relatively strong degree of preferred orientation of graphitic regions does occur in metaanthracitesUand is interpreted to result from directed shear stress. (23) Walker, P.L.,Jr. Carbon 1986,24, 379. (24) McCartney, J. T.;O'Donnel, H. J., Ergun, S. Nature 1966,205, 962.
