Energy & Fuels 1994,8, 360-368
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Mass Spectrometric and Chemometric Studies of Thermoplastic Properties of Coals. 3. Optical Anisotropy and Isotropy of Carbonized Coals Anna Marzec* and Sylwia Czajkowska Institute of Coal Chemistry, Polish Academy of Sciences, Sowinskiego 5, 44-100 Gliwice, Poland
Hans-Rolf Schulten Department of Trace Analysis, Fachhochschule Fresenius, Dambachtal20, W-6200 Wiesbaden, FRG Received February 17,1993. Revised Manuscript Received November 5, 199P
The aim of the study was to find coal pyrolysates that contribute to the formation of the optically anisotropic phase during carbonization and those which form isotropic char. Twenty-seven coals (the same as already studied in parts 1and 2) were analyzed. Two sets of data were obtained for each coal: (i) pyrolysis-field ionization mass spectra of vacuum pyrolysates formed on heating coals to 600 "C at 1W Pa and (ii) the content of optically anisotropic phase in a coal sample carbonized to 600 "Cunder atmospheric pressure. Correlation analysis was applied to evaluate the quantitative links between the two seta of data. Additionally, the previously obtained set of Gieseler thermoplastic properties of the coals were included in this evaluation. The analysis resulted in the selection of two groups (An and Is) of vacuum pyrolysates. Group An consists of aromatic and partially hydrogenated aromatic hydrocarbons; the higher their content in coals the higher is the content of anisotropic phase on carbonization. Group Is consists of oxygen and nitrogen compoundsas well as some aromatic hydrocarbons. The higher ita content, the higher is the content of isotropic char in the carbonized coals. Significant correlations also exist between anisotropy content and the Gieseler temperatures of maximum fluidity and resolidification. It has been concluded that group An pyrolysates are inert in intermolecular condensation and are fusible and capable of forming liquid droplets in a wide temperature range in contrast to the group Is pyrolysates which undergo polymerization and condensation at temperatures corresponding to the temperatures of their generation. The direct link between capabilities of coals to develop thermoplasticity and to form anisotropic phase lies in the fact that the two phenomena are generated by the same species.
Domains of carbonaceous materials showing optical anisotropy contain systems that consist of parallelly oriented planar mo1ecules.l Formation of an anisotropic phase (Le., mesophase in coal-derived materials) takes place in the 350-550 "C temperature range.a The initial formation of anisotropic spherule8 is observed from 350 to 450 "C,depending on coal rank.4 An increase of temperature promotes the developmentof anisotropy, but in general, no further development occurs above 525 "Cm4 The formation of mesophase is strongly related to thermoplastic properties. Coals which on heating cannot soften, melt, turn into semisolid or fluid, and finally resolidify,are not capable of forming an anisotropic phase.' Moreover, the thermoplasticity temperature range of coals is from about 340to 500 "C,coinciding with the range that is characteristic for formation of anisotropy.
The two characteristicphenomena of thermoplastic coals (i.e., thermoplasticity and anisotropy formation) fall into the temperature range of Yactiventhermal decomposition that occurs in all coals.s The active decomposition starts at about 350 "Cfor low-rank coals and from about 400 "C for higher rank coals. It significantly decreases at around 550 "C.5-8 At favorable conditions (vacuum, fine coal particles, and low amount of an analyzed sample), the decomposition is manifested by massive weight loss which is due to the generation and discharge of a complex condensable liquid, gaseous products such as C& hydrocarbons, Hz,COz, and CO.&V Yields of carbonaceous residues are definitely much higher compared with the yields of gaseous and liquid products; this indicates that aromatic structures which dominate in coal are the main precursors of the residues. It was recently pointed out that beside aromatic hydrocarbons, polyhydroxybenzenes also contribute to formation
Abstract published in Aduance ACS Abstracts, January 1, 1994. (1) Marnh, H.; Menendez, R. Mechanism of Formation of Isotropic and Anisotropic Carbons. In Introduction to CarbonScience; Marsh, H., Ed.;Butterworths: London, 1989; Chapter 2, pp 38-73. (2) Crelling, J. C. The Nature of Coal Material. In Introduction t o Carbon Science; Marsh, H., Ed.;Butterworthe: London, 1989; Chapter 8, pp 269-284. (3) Honda, H. Carbon 1988,26, 139-1155. (4) Patrick, J. W.; Walker, A. Fuel 1991, 70, 465-470.
(6) Berkowitz, N. The Chemhtry of Coal;Elsevier: Amsterdam, 1985; Chapter 7, pp 213-267. (6) Serio, M. A.; Hamblen, D. G.; Markham, J. R.; Solomon, P. R. Energy Fuels 1987,1,138-153. (7) Stock, L. M. Acc. Chem. Rea. 1989,22, 427-433. (8) Solomon, P. R.; Hamblen, D. G. Pyrolysis. In Chemistry of Coal Conversion; Schlosberg, R. H., Ed.;Plenum: New York, 1986; pp 121248. (9) Karcz, A.; Porada, S. Fuel Process. Technol. 1990, 26, 1-13.
Introduction
0887-0624/94/2508-0360$04.50/0
0 1994 American Chemical Society
Thermoplastic Properties of Coals
of the residues.1° Studies of model compounds showed that aromatic hydrocarbons on heating in the 350-550 "C range undergo a complex set of reactions including formation of thermal degradation products as well as products of inter- and intramolecular condensation. An example of this complex behavior may be found in thermolysis experiments of perylenell and thermal reactivity studies of model dihydroxy derivatives of benzene and naphthalene.12J3 The characteristic reaction of unsubstituted benzenoid hydrocarbons is intermolecular ~ondensation~~J6 that produces oligoaryls (mainly biaryls) and hydrogen. Due to hydrogen generation, partially hydrogenated hydrocarbons are also formed.16 The most thermally stable arene, benzene, condenses at temperatures above 550 OC. However, condensation of polycyclic aromatics can occur below this temperature.15 Methylated aromatics such as dimethylnaphthalenes react in essentially the same way as unsubstituted hydrocarbons.16 Results of studies of model hydrocarbons imply that numerous aromatics present or formed in coals on heating in the temperature range in question undergo intermolecular condensation to oligoaryls. However, the degree of their conversion may be strongly differentiated. This conclusion can be derived from studies of carbonization rates of individual kata- as The well as peri-condensed aromatic hydrocarbon~.~~Js hydrocarbons were isothermally heated at 430 "C for 4 h in sealed glass tubes. Out of 10 hydrocarbons studied, the highest reactivity was found for tetracene and the lowest (by a factor of 10) for triphenylene. The reactivities of the set of hydrocarbons studied appeared to be functions of minimum Dewar localization energies as well as of first ionization potential.'* Other studies1@ showed that pyrene (this hydrocarbon was not included in ref 18) does not undergo condensationreaction around 500 "C. It scarcely leads to a carbonaceous residue in contrast to katacondensed aromatics. However, these studies showed that partial hydrogenationof pyrene makes the intermolecular condensationpossible.1@$20 The example of hydrogenated pyrenes may be not an exceptional case and we should assume that hydrogen formed as a result of condensation of some aromatics in coal material can hydrogenate other aromatics and convert them to partially hydrogenated species that are capable of intermolecular condensation. Further condensation of oligoaryls can proceed via intramolecular dehydrocyclization.ls~l*~zl The crucial point is that intramolecular condensation is much more specific with respect to the structure of a reactant than the intermolecular reaction. Thus, 1,l'-binaphthyl undergoes intramolecular dehydrocyclization due to its favorable topology while 2,2'-binaphthyl does not. The dehydrocyclization is strongly accelerated by hydrogen donors such (10) Stein, S. P r e p . Pap-Am. Chem. Soc., Diu. Fuel Chem. 1992, 37(1\. - .-,,393-394. - - - -(11) Zander, M.; Haaee, J.; Dreskamp, H. Erdoel Kohle, Erdgas Petrochem. 1982, 35,65-69. (12) McMillen, D. F.; Chang, S. J.; Nigenda, S..;Malhotra, R. P r e p . Pap-Am. Chem. Soc., Diu. Fuel Chem. 1985,4,414-426. (13) Stein, S. Ace. Chem. Res. 1991,24,250-260. (14) Zander, M. Top. Curr. Chem. 1990,153,102-122. (15)Poutama, M. L. Energy Fuels 1990,4, 113-130. (16) Lewis, I. C. FueZ 1987,66, 1527-1531. (17) Zander, M. Fuel 1986, 65, 1019-1020. (18) Zander, M. Fuel 1987, 66, 1459-1466. (19) Mochida, I.; Matauoka, H.; Fuiitau, H.; Korai, Y.;Takeshita, K. Carbon 1981,19, 213-216. (20) Mochida, I.; Tamaru,K.; Korai, Y.; Fuiitau, H.; Takeshita, H. Carbon 1982,20, 231-236. (21) Lewis, I. C. Carbon 1980,18,1901-196. ~
Energy & Fuels, Vol. 8, No. 2, 1994 361
as 9,lO-dihydroanthracene or fluorene.22 It was shown that the arylating species in the dehydrocyclization is a stabilized r-cyclohexadienyl radical formed by hydrogen addition to an aromatic ring.22 One should expect that the presence of hydrogen as well as hydroaromatics in carbonized coals enhances not only inter- but also intramolecular condensation. It seems that intramolecular dehydrocyclization plays asignificant role in formation of mesophase. As Zander18P pointed out, molecules showing oligoarylic structures such as 9,9'-bianthracenyl hamper mesophase formation due to their nonplanarity. Planar aromatics had to be formed by intramolecular dehydrocyclization before orientation could occur. The same conclusion may be derived from studies on catalytic dehydrocyclization of model hydroc a r b o n ~ .Thus, ~ ~ for example, 1,l'-binaphthyl produced a pitch that contained 100%of mesophase in contrast to 2,2'-binaphthyl which produced a pitch with 0% mesophase because its intramolecular dehydrocyclization is not possible. Catalytic carbonization of naphthalene and CHa-naphthalenes also resulted in mesophase pitches.% It is noteworthy that the components of the mesophase contained methyl substituents as well as hydroaromatic rings.2s This result indicated that neither methyl groups nor hydroaromatic rings disturbed parallel orientation of molecules. From studies on thermal behavior of model compounds in the 350-550 "C temperature range that is characteristic for formation of anisotropic phase in coals, it may be concluded that (a) numerous aromatic and partially hydrogenated species of coal material can undergo intermolecular condensation, but their conversion in this reaction may be strongly differentiated when compared at the same temperature; (b) further condensation may proceed by intramolecular dehydrocyclization; in a few cases this dehydrocyclization is hampered due to the specific topology of molecules. However, any direct extrapolation from thermal behavior of individual model compounds to coal cannot be made. The eventual complications that prevent such extrapolation are the limited number of model compounds studied compared with the number of coal structural features, interactions among individual structural features simultaneously present in coal, and effecta of restricted molecular mobility in semisolid decomposing coal. In the present study numerous coals were investigated with the aim of (a) finding those structural componenta that contribute to formation of optically anisotropic phase as well as those which generate isotropic char and (b) seeking an answer to the question: why are thermoplasticity and the ability to generate anisotropic phase inseparable phenomena? Correlation analysis has been applied to two sets of data: (1)pyrolysis-field ionization mass spectra of the coals which show the vacuum pyrolysates formed on heating coals to 600 "C and (2) the content of optically anisotropic phase in samples of the coals that were carbonized in the same temperature range. The studies are based on the following concept. Pyrolysis-FI MS provides information about individual ~~~
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(22) Senthilnathan, V. P.;Stein, S. J. Org. Chem. 1988,63,3000-3021. (23) Boenigk, W.; Haenel, M. W.; Zander, M. Fuel 1990, 69, 12261232. (24) Lewis,I.C.Proc. TwentiethBiennial CarbonConf.,SantaBarbom 1991,156-157. (25) ,Korai,Y.;N~wa,M.;Mochida,I.;Sakai,Y.;F~iyama,S.A.oc. Twentreth Bzennral Carbon Conf., Santa Barbara 1991, 168-159.
362 Energy & Fuels, Vol. 8, No. 2,1994
Marzec et al.
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Figure 1. (a) Averaged FI mass spectrum of four coals (no. 114,119,121,and 122)that showed the highest capabilities of generating anisotropicphase during carbonization (63-73%);(b) averaged FI mass spectrum of four coals (no. 100,102,103,and 107)that showed low abilities of forming anisotropic phase during carbonization (3-10% ).
products of thermal decomposition (vacuum pyrolysates) that are generated by heating coal in a certain temperature range and are made volatile due to high vacuum in the mass spectrometer. The same thermal decomposition products are formed when coal is subjected to the same temperature range during carbonization (or other processes). However, when no vacuum is applied, the majority of thermal decomposition products instead of being volatilized are trapped in grains of processed coal, diffuse through the coal material, and may react with it. Therefore, the "vacuum pyrolysates" may play an important role in formation of anisotropic or isotropic phases during carbonization. If the pyrolysates detected by FI MS do contribute in some way to formation of the phases, there has to be a relationship between intensities of their mass signals and a content of anisotropic (or isotropic) phase in carbonized coals. The applied methodology is analogous to the approach already used for identification of coal components that are reactive in formation of tar,26 in reaction of hydrogen transferred from H 620nors,27~~~ and in the developmentof Gieseler thermoplastic properties.28929 The same set of 27 coals were the subject of the previous28?29 parts (1and 2) of this paper and of the present studies.
Experimental Section Coals. Twenty-sevencoals were studied. The characteristics of the coals such as ultimate, proximate, and maceral analyses and Gieseler thermoplasticproperties were already reported (see ~~~
(26) Schulten, H.-R.; Marzec, A.; Dyla, P.; Simmleit, N.; Mueller, R. Energy Fuels 1989,3,481-487. (27) Marzec, A.; Czajkowska, S.; Simmleit, N.; Schulten, H.-R. Fuel Process. Technol. 1990, 26, 53-66. (28) Schulten, H.-R.; Marzec, A.; Czajkowska, S. Energy Fuels 1992, 6,103-108. (29) Marzec, A.; Czajkowska,S.;Moszynski,J.; Schulten,H.-R. Energy Fuels 1992,6,97-103.
Table 1 in ref 29). Reactivities of the coals with tetralin have also been published.% Pyrolysis-field ionizationmass spectrometryof the coals has already been described in detail.% Briefly, for each of the coals the FI MS spectrum was obtained which shows molecular ions in the n l z 90-600range of the pyrolysates formed when the coal was heated directly in the mass spectrometer in the 50-600 "C temperature range. The spectrumrepresentsall the volatilized compounds that were detected by succesive mass scans taken at each 10 "C increase of temperature. For sets of selected coals, averaged FI mass spectra were prepared by summing their individual spectra. The averaged spectraare presented in Figure 1 which illustrates the composition of volatiles of coals showing the highest (part a of Figure 1)and the lowest capabilities (part b of Figure 1) of forming an anisotropic phase during carbonization (for the carbonization procedure see the next section). Next, the two averaged spectra were normalized to the same level of total ion intensity and the spectra were subtracted. The difference spectrum is presented in Figure 2. Identification of the pyrolysates was carried out for four coals with the use of high-resolution (HR), high-precision mass measurements,and GC/MS techniques. The four coals (no.100, 111, 122, and 123) represent a wide range of thermoplastic properties (TRfrom 433 to 490 "C; anisotropy from 4 to 73%) and carbon content from 82.5 to 89 %C. A summary of the results is presented in Table 3. Carbonization of the Coals. The coal samples were carbonized in the Gray-King apparatus using the procedure of Polish standard PN-84/G-04519 (which is equivalent to British IS0 5021982) except that no anthracite sample was added to any coal sample. Briefly, an 8-g coal sample was placed in a quartz tube, the tube was introduced into an electric furnace preheated to 300 "C, and then the temperature was raised to 600 "C at 5 "C/ min and kept constant at 600 "C for 15 min. Uniform heating of coal samples was controlled by two thermocouples. For each coal, three parallel carbonizationswere performed. After cooling, the samples of carbonized coal were subjected to microscopic determination of the content of the optically anisotropic phase. MicroscopicDetermination of Anisotropic and Isotropic Phases. The samples of carbonizedcoals were mixed with liquid
Energy & Fuels, Vol. 8, No. 2, 1994 363
Thermoplastic Properties of Coakr
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Table 1. Content of Optically Anisotropic Phase (An) and Ratio of Selected Pyrolysates (sum An:sum Is) in Coal Samdes Heated UP to 600 OC s u m An/ sum Ant coal no.' % Anb sum Isc coal no.' % Anb s u m ISC 0.7 107 10 0.9 101 3 1.0 0.7 116 10 103 3 1.3 0.7 110 12 125 3 1.0 0.9 108 14 4 100 1.1 109 16 1.2 4 104 32 1.1 1.2 123 4 118 1.6 0.8 111 44 4 126 56 2.5 0.8 113 102 5 1.8 1.1 112 61 106 5 63 2.4 0.9 114 5 124 66 2.2 1.1 119 105 6 70 3.2 0.8 121 120 6 73 2.1 I 1.0 122 116 0.7-3.2 0.9 range 3-73 117 I 0 Correspondstocoal numbers in refs 28 and 29;various properties of the coals are presented in Tables 1-111 of ref 28 and in Tables I-IV of ref 29. b 100 - %An = % content of isotropic phase. For explanation see Results and Discussion section.
epoxy resin, cured, and polished. Three preparations were obtained for each of the carbonized coals. The determinations of anisotropic and isotropic phase contents were carried out using a metallographic microscope (OlympusPMG-3) under polarized light at 500X magnification. A 500-point count was made of microscopically identifiable components which were categorized as anisotropic or isotropic component. The arithmetic average was calculatedwhen the differencebetween minimal and maximal value of three measurements of anisotropy content was less than 6%. When the difference was higher, another set of three measurements was carried out for newly prepared microscopic specimens. The results are shown in Table 1. Correlation Analysis of Mass Spectrometry and Anisotropy Data. The evaluation of the data was carried out using a modified ARTHUR statistical software package, and correlation analysis was made in the same way previously used for chemometric handling of mass spectrometric data versus thermoplastic properties as well as hydrogen transfer data.% Summing up, the
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Py-FIMS spectra of the coals were quantified by mlz values of mass signals and by the signal intensities. Correlation analysis was carried out for pairs of two variables (y and x i ) . One of them (i.e., y values) was always the same: this was a set of 27 anisotropy contents in the 27 carbonized coals. The other member of a pair waa a set of 27 intensities of a given mlz mass signal. Thus, the number of x sets equals the number of even-masssignalsdetected. Correlation coefficients (r) were calculated for every possible pair of y and x i . The coefficient r is by definition a quantitative evaluation of the relationship between two variables and is independent of the scales used for measuring variables. The value of r is bounded between -1 and +l. When two variables are independent, r = 0. Maximal absolute [r] = 1is found when the values of one variable is perfectly linearly related with the other. Positive values of r < 1 indicate that there is an increasing trend between the two variables. The negative r values show that there is a decreasing trend. An arbitrary threshold value of coefficient r between significant and weak correlations is [rl = 0.56. Results of the correlation analyses are presented in Table 2. With the aim of clearly showing links among anisotropy, thermoplasticity and H-transfer reactivity of the coals, Table 2 also includes the correlation coefficients that already were reported.%
Results and Discussion Averaged and Difference Mass Spectra of Selected Coals. A set of four coals (no. 114, 119,121, and 122 in Table 1)which generated the highest content of anisotropic phase (63-73 % ) during carbonization was selected. Their averaged FI mass spectrum is presented in part a of Figure 1. T h e other four coals (no. 100, 102, 103, and 107 in Table 1) were selected from those coals that produced a low content of anisotropic phase (3-10 % and have carbon contents as similar as possible (85-87 % C ) t o t h e first set of the coals (89-92 % C). The averaged FI mass spectrum is presented in part b of Figure 1. Figure 2 shows a difference FI spectrum obtained by subtracting the two averaged spectra. T h e difference spectrum (Figure 2)
Marzec et al.
364 Energy & Fuels, Vol. 8, No. 2, 1994 Table 2. FI Mass Signals Correlated with Content of Optically Anisotropic Phase (An)in Carbonized Coals, Gieseler Temperature of Resolidification ( fi),and of the Coals Hydrogen Transfer (HT,) correln coeff for correln coeff for m/z %An T R ~ H n b mlz %An TRO Hnb Group An: Pyrolysates Contributing to Formation of Anisotropic Phase 166 180 192 194 206 208 216 218 230 242 244 256 258 280 284
0.63 0.89 0.86 0.97 0.90 0.73 0.73 0.79 0.91 0.60 0.90 0.72 0.80 0.85 0.56
0.70 0.87 0.85 0.85 0.90 0.77 0.68 0.74 0.90 0.64 0.90 0.76 0.85 0.85 0.71
-0.70 -0.80 -0.78 -0.83 -0.91 -0.71 -0.72 -0.86 -0.90 -0.69 -0.90 -0.75 -0.74
294 298 304 306 308 318 320 330 332 334 344 346 358 368 382
0.88 0.57 0.66 0.76 0.74 0.89 0.76 0.74 0.85 0.68 0.91 0.80 0.77 0.60 0.66
0.91 0.71 0.66 0.79 0.84 0.85 0.93 0.70 0.87 0.79 0.79 0.85 0.68
-0.86 -0.72 -0.57 -0.75 -0.87 -0.81 -0.88 -0.57 -0.83 -0.87 -0.83 -0.87 -0.72
Group Is: Pyrolysates Contributing to Formation of Isotropic Char 158 160 162 172 174 176 184 186 188 190 198 200 212 214 222 226 236 238 240 246 250 252
-0.69 -0.61 -0.74 -0.75 -0.68 -0.74 -0.57 -0.67 -0.68 -0.60 -0.64 -0.74 -0.82 -0.76 -0.66 -0.80 -0.65 -0.83 -0.80 -0.58 -0.69 -0.83
-0.82 -0.56 -0.75 -0.83 -0.63 -0.71 -0.64 -0.70 -0.59 -0.63 -0.67 -0.85 -0.83 -0.91 -0.60 -0.88 -0.94 -0.81 -0.84
0.85 0.59 0.81 0.85 0.69 0.76 0.79 0.53 0.70 0.91 0.80 0.93 0.82 0.91 0.82 0.81
254 260 264 288 300 302 314 324 328 336 338 342 350 352 364 366 374 378 388 390 392
-0.64 -0.63 -0.79 -0.79 -0.83 -0.84 -0.91 -0.68 -0.80 -0.60 -0.89 -0.68 -0.66 -0.88 -0.87 -0.73 -0.82 -0.80 -0.77 -0.76 -0.71
-0.75 -0.64 -0.90 -0.70 -0.65 -0.85 -0.86
0.77 0.57 0.85 0.69 0.62 0.87 0.86
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-0.77 -0.77 -0.72 -0.92 -0.88 -0.83 -0.76 -0.90
0.81 0.83 0.71 0.90 0.85 0.88 0.67 0.88 0.74 0.87 0.84
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a Thermoplastic properties of the coals were already shown;29 results of correlation analysis between FI MS data and thermoplastic properties were presented in Table I1 of ref 28. Determination of Hn values for the coals as well as results of correlation analysis between FI MS data and H n values were reported.%
clearly shows that the composition of vacuum-volatilized material is different for coals that are capable of generating high content of anisotropic phase during carbonization compared with the material of the other coals which produce a negligible amount of anisotropy. FI Mass Signals Correlated with Content of Anisotropic Phase. As Table 2 shows, there are 73 FI mass signals in the 160-400 m/z range that are significantly correlated ( [ r l > 0.56) with the content of optically anisotropic phase. The FI mass signals shown in group An of Table 2 are positively correlated with anisotropic phase content. This means the higher the intensity of a signal in the FI mass spectrum of a coal the higher is the content of anisotropic phase in the carbonized coal. These data indicate that pyrolysates of group An contribute to formation of anisotropic phase during carbonization. All the mass signals of group An are shown in the difference spectrum as characteristic signals for coals capable of generating high amount of anisotropic phase (upper part of Figure 2).
8o
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In contrast to group An, group Is of Table 2 includes mass signals that are correlated negatively with respect to anisotropic phase content. Since anisotropy plus isotropy phases make loo%, the negative sign indicates that the higher the intensity of a signal of group Is in a coal, the higher is the content of isotropic phase in the carbonized coal. Thus, pyrolysates of group Is contribute to formation of isotropic char. All the mass signals of group Is are present in the difference spectrum on the side of coals showing very low capability of generation of anisotropy phase (see bottom part of Figure 2). Concluding the correlation analysis, it is worth pointing out that positive and negative correlation coefficienta signs definitely discriminate vacuum pyrolysates which contribute to formation of anisotropic phase (+r) and the others that contribute to formation of isotropic char (-r). For each coal sample, the intensities of the positively correlated mass signals were summed (sum An), as well as the intensities of the negatively correlated signals (sum Is). The ratios sum An/sum Is for all the coals are given in Table 1. A plot of anisotropic phase content versus the ratio for all the studied coals is presented in Figure 3. The figure indicates that there is negligible possibility of formation of anisotropic phase for coals having the ratio
