EPR Studies on Petrographic Constituents of Bituminous Coals, Chars

Energy & Fuels 1997, 11, 951-964

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EPR Studies on Petrographic Constituents of Bituminous Coals, Chars of Brown Coals Group Components, and Humic Acids 600 °C Char upon Oxygen and Solvent Action Franciszek Czechowski* Institute of Chemistry and Technology of Petroleum and Coal, Technical University of Wrocław, 7/9 Gdan˜ ska St., 50-344 Wrocław, Poland

Adam Jezierski Faculty of Chemistry, Wrocław University, 14 F. Joliot Curie St., 50-383 Wrocław, Poland Received November 19, 1996. Revised Manuscript Received June 30, 1997X

Free radicals were characterized in petrographic constituents of bituminous coals and in chars from brown coal components by EPR. The change in free radical concentration on oxidation of humic acid 600 °C char from humodetritous brown coal with gaseous 10% oxygen at 330 °C was evaluated. Spin concentration in the petrographic constituents from a bituminous coal increases in the order liptinite < vitrinite , inertinite. Increasing maturity of each petrographic constituent correlates with an increase in spin concentration and decrease in g-value. Heat treatment of brown coal group components (bitumens, cellulose, lignin, humic acids, and residual coal) leads to the formation of free radicals up to a temperature of about 550 °C and to their sharp disappearance at higher temperatures. Strongly marked maxima in the spin concentration of the chars are observed at atomic H/C ratios in the range 0.40-0.42 and O/C ratios in the range 0.08-0.10. Increasing heat treatment temperature causes a steady decrease in g-value from about 2.0040 to 2.0025. Treatment of the 600 and 700 °C chars with organic solvents resulted in an increase in their spin concentration. This effect was particularly strong in the case of treatment with tetralin and naphthalene. Burning off humic acid 600 °C char with 10% oxygen at 330 °C leads to a decrease in spin concentration in oxidized products. Structural units associated with elevated char free radicals density were more prone to oxidation and presumably are peripheral polyaromatic skeletons of lower structural ordering.

Introduction Electron paramagnetic resonance is a well-established technique for the study of free radical systems in carbonaceous materials. It is applied to characterize the organic stable free radical signal (EPR line around g-value of 2.00). Detailed examinations of coals and their macerals revealed the presence of different groups of paramagnetic centers associated with various chemical environments, which are characterized by distinct g-values and line widths.1-13 Delocalized unpaired Abstract published in Advance ACS Abstracts, August 1, 1997. (1) Petrakis, L.; Grand, D. W. Free Radicals in Coals and Synthetic Fuels; Coal Science and Technology 5; Elsevier: Amsterdam, 1983, and references therein. (2) Retkofsky, H. L.; Stark, J. M.; Friedel, R. A. Anal. Chem. 1968, 40, 1699-1704. (3) Schlick, S.; Narayana, M.; Kevan, L. Fuel 1983, 62, 1250-1254. (4) Dack, S. W.; Hobday, M. D.; Smith, T. D.; Pilbrow, J. R. Fuel 1984, 63, 39-42. (5) Doetschman, D. C.; Mustafi, D. Fuel 1986, 65, 684-693. (6) Ito, O.; Seki, H.; Iino, M. Bull. Chem. Soc. Jpn. 1987, 60, 29672975. (7) Wieˆckowski, A. B. Exp. Tech. Phys. 1988, 36, 299-303. (8) Jeunet, A.; Nickel, B.; Rassat, A. Fuel 1989, 68, 883-889. (9) Nickel-Pepin-Donat, B.; Jeunet, A.; Charcosset, A.; Jamond, M. Fuel 1990, 69, 856-890. (10) Brezgunov, A. Yu.; Dubinskii, A. A.; Poluektov, O. G.; Vorob’eva, G. A.; Lebedev, Ya. S. J. Chem. Soc., Faraday Trans. 1990, 86, 31853189. (11) Pilawa, B.; Trzebicka, B.; Wie¸ ckowski, A. B. Fuel 1991, 70, 1109-1110. X

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π-electrons of polyaromatic structures with strong exchange interactions are characterized by narrow components of the resonance absorption spectra, while the broad lines are due to paramagnetic centers of aliphatic structures or are composed of smaller-condensed aromatic structures.3-9,11-13 Retkofsky at al.2 noted a composite structure of the EPR lines for a wide variety of American coals and concluded that in some cases the sharp component is due to fusain. Changes in the g-values, line widths, and intensity of the EPR signals depend on coal’s chemical constitution and rank.1,2,8,14-20 With increasing coalification, (12) Pilawa, B.; Trzebicka, B.; Wie¸ ckowski, A. B.; Hana, B.; Komorek, J.; Pusz, S. Erdo¨ l Kohle Erdgas Petrochem. 1991, 44, 421425. (13) Pilawa, B.; Trzebicka, B.; Wieˆckowski, A. B. Mol. Phys. Rep. 1994, 5, 245-247. (14) Retcofsky, H. L. In Coal Science; Gorbaty, M. L., Larsen J. W., Wender, I., Eds; Academic Press: New York, 1982; Vol. 1, pp 51-82. (15) Kwan, C. I.; Yen, T. F. Anal. Chem. 1979, 51, 1225-1229. (16) Silbernagel, B. G.; Gebhard, L. A.; Dyrkacz, G. R. Fuel 1985, 65, 558-562. (17) Bakr, M. Y.; Akiyama, M.; Sanada, Y. Org. Geochem. 1990, 15, 595-599. (18) Requejo, A. G.; Gray, N. R.; Freud, H. Energy Fuels 1992, 6, 203-214. (19) Retcofsky, H. L.; Hough, M. R.; Maquire, M. M.; Clarkson, R. B. Coal Structure; Gorbaty, M. L., Ouchi, K., Eds.; American Chemical Society Advances in Chemistry Series; American Chemical Society: Washington, DC, 1981; Vol. 192, pp 37-58. (20) Petrakis, L.; Grandy, D. W. Anal. Chem. 1978, 50, 303-308.

© 1997 American Chemical Society

952 Energy & Fuels, Vol. 11, No. 5, 1997

coals show a steady increase in overall spin concentration.1,19,20 This follows the generally accepted view on the coal metamorphism, where condensation and size of aromatic lamellas increase with the maturation. Particularly intensive development of aromatic units in the coals occurs over the carbon content range 82-86%, and at higher coalification their condensation takes place.21 Schmidt and Van Krevelen22 concluded that coalification is a slow thermal process occurring at low temperature and under high pressure. EPR parameters were found to be sensitive to the chemical environment of the unpaired electron and therefore may differ considerably for coals of similar rank depending on their petrographic constitution. This is because the g-value, line width, and line shape of a given coal are composite properties of the radical types present in the vitrinite, liptinite, and fusinite. In general, the chemical composition of vitrinite relates to the humic-originated material, which is the most abundant in heteroatoms. Liptinite is mainly composed of waxy, aliphatic structures, and fusinite is composed of aromatic skeletons.23 Spin concentrations differ among the maceral groups in the order liptinite < vitrinite , inertinite.21,24 In the more recent work fusains have been found to have approximately 5 times greater spin concentration compared to vitrains of the same coal.2 We present data concerning the characteristics of free radicals in petrographic constituents separated from bituminous coals of widely differing rank. The lignites are characterized by a significant concentration of heteroatoms (mainly oxygenated functionalities) in the structure and exhibit much higher g-values compared to the bituminous coals owing to participation of these groups in spin-orbit coupling.1,25 The nature of low-maturity lignites is highly heterogeneous, since they consist of complex mixtures of biologically synthesized biomacromolecules of different plant tissues that have undergone limited diagenetic alterations. The following major polymeric constituents (group components) contribute to the formation of lignite structures: bitumens, cellulose, lignin, and humic acids. They can be selectively separated by chemical means. Therefore, it was intriguing to establish whether or not they exhibit noticeable differences in free radical characteristics. Much attention in the literature has been devoted to investigations of free radical formation and recombination upon heat treatment of coals, petrographic constituents, and biomaterials.1,2,8,14,26-32 It is well recog(21) Van Krevelen, D. W. Coal; Elsevier: Amsterdam, New York, 1993. (22) Schmidt, J.; Van Krevelen, D. W. Fuel 1959, 38, 355-368. (23) Winans, R. E.; Crelling, J. C. Chemistry and Characterization of Coal Macerals; Winans, R. E., Crelling, J. C., Eds.; Amercian Chemical Society: Washington, DC, 1984; pp 1-20. (24) Kro¨ger, C. Brennst. Chem. 1958, 39, 62-67. (25) Yen, T. F.; Sprang, S. R. Prepr.sAm. Chem. Soc., Pet. Chem. 1970, 15 (3), A65-A76. (26) Austen, D. G. E.; Ingram, D. J. E.; Given, P. H.; Binder, C. R.; Hill, L. W. Adv. Chem. Ser. 1966, 55, 344-362. (27) Lewis, I. C.; Singer, L. S. In Chemistry and Physics of Carbon; Walker, P. L., Eds.; Marcel Dekker: New York, 1981; Vol. 17, pp 1-88. (28) Obara, T.; Yokono, T.; Sanada, Y. Liquid Fuels Technol. 1983, 1, 59-65. (29) Mrozowski, S. Carbon 1988, 26, 521-529. (30) Mrozowski, S. Carbon 1988, 26, 531-541. (31) Seehra, M. S.; Ghosh, B.; Zondlo, J. W.; Mintz, E. A. Fuel Process. Technol. 1988, 18, 279-286. (32) Pilawa, B.; Wiłckowski, A. B.; Lewandowski, L. Fuel 1995, 74, 1654-1657.

Czechowski and Jezierski

nized that all carbonaceous substances at some stage of heat treatment go through a phase involving formation and subsequent disappearance of stable radicals due to thermal cracking and condensation reactions occurring in these materials while they are heated. The most intensive development of new radicals occurs at temperatures from 400 to 600 °C, whereas at higher temperatures, a sharp decrease in their concentrations is observed. Heating rate and residence time at the final temperature have an influence on the free radical concentration.1,30 The aim of this work was to detect the above trends, which are common to all carbonaceous materials in the process of carbonization, for lignite group components regardless of their chemical preparation. There have also been many attempts to establish links between EPR parameters and coal oxidation at low temperature.4,33-41 Three major steps of this process are recognized: (1) up to 70 °C, formation of coaloxygen complexes involving reactions of oxygen and free radical sites in the coal structure; (2) the decomposition of these complexes between 70 and 150 °C; (3) the exothermic formation of new oxygen-coal complexes above 150 °C. The objective of the present work was also to explore the effects of humic acids char (HTT 600 °C) burn off with 10% oxygen at a temperature close to ignition (330 °C) and treatment of the oxidation products with benzene on their EPR characteristics. Materials and Methods Separation of Petrographic Constituents. Petrographic concentrates were separated from the three Upper Silesian bituminous coals derived from Janina, Halemba and 1 Maja collieries. Macerals separation has been achieved from preliminarily hand-picked lithotypes: durain, vitrain, and fusain. They were further powdered under nitrogen in a planetary ball mill to grain diameters below 100 µm. The maceral concentrates were obtained by a further enrichment of the lithotypes using multiple separation in heavy liquids.42,43 Petrographic analysis was performed with the universal Zetopan microscope (Reichard, Austria) equipped with a Rathenow (Germany) object stage counter. Observations were made on polished cross sections in a white (halogen) reflected light. The maceral reflectances were determined for the parent coals. Volatile matter and carbon contents of the parent coals, as well as petrographic analyses (1000 point counts), are presented in Table 2. Vitrinite reflectance from 0.51 to 1.08% covers the catagenetic range of coalification. Reflectances of liptinites are about half the value of the corresponding vitrinites, while fusinites reflectances are twice as high as for the corresponding vitrinites. Preparation of Brown Coal Group Components. Group components were prepared from the two basic lithotypes of low-maturity brown coals, that is, xylitic (XC) and humodetritous (HC) Tertiary brown coals from Polish deposits: Turow Open Cut and Konin Open Cut, respectively. XC consisted mainly of textinite components of the humotelinite and hu(33) Seki, H.; Ito, O.; Iino, M. Fuel 1990, 69, 317-321. (34) Kudynska, J.; Bruckmaster, H. A.; Duczmal, J.; Bachelor, F. W.; Majumdar, A. Fuel 1992, 71, 1127-1135. (35) Buckmaster, H. A.; Kudynska, J. Fuel 1992, 71, 1137-1140. (36) Kudynska, J.; Buckmaster, H. A. Fuel 1992, 71, 1141-1145. (37) Buckmaster, H. A.; Kudynska, J. Fuel 1992, 71, 1147-1151. (38) Kudynska, J.; Buckmaster, H. A. Fuel 1993, 72, 1733-1738. (39) Kudynska, J.; Buckmaster, H. A. Fuel 1994, 73, 526-530. (40) Carr, R. M.; Kumagai, H.; Peake, B. M.; Robinson, B. H.; Clemens, A. H.; Matheson, T. W. Fuel 1995, 74, 389-394. (41) Kudynska, J.; Buckmaster, H. A. Fuel 1996, 75, 872-878. (42) Jasienko, S.; Kidawa, H. Chem. Stosow. 1985, 29, 315-328. (43) Kidawa, H.; Jasienko, S. Koks Smola Gaz 1989, 34, 259-267.

Petrographic Constituents modetrinite subgroups, accounting for 57% and 29%, respectively. Resinite (14%) occurred in the form of isolated grains or duct fillers in textinite. HC was composed of attrinite (40%) and densinite (29%). Within the gelified mass, other macerals are present in smaller amounts. The humotelinite subgroup (11%) was represented by texto-ulminite and ulminite, the liptinite subgroup by resinite (6%), and the inertinite subgroup (9%) by fusinite, macrinite, and sclerotinite. Levigelinite and pirogelinite constituted only 5% of the humocollinite subgroup. Group components were prepared from these brown coals in the following way. Bitumens were removed from vacuumdried raw coals by exhaustive benzene extraction in a Soxhlet apparatus and evaporation of the solvent on a rotary evaporator. Benzene-pre-extracted coals were subjected to exhaustive alkaline extraction with 0.1 N NaOH at 80 °C (five times). The resulting alkaline solutions were combined and neutralized with dilute HCl. The humic acids precipitates obtained were collected by filtration and washed on a filter funnel with distilled water until the Cl- ion could no longer be detected. The insoluble portion of HC, remaining after bitumen and humic acids extraction, contained mainly ligneous plant constituents. It was not further separated and, after neutralization and Cl- washing, it constituted the residual coal. The remaining organic material of XC consisted of preserved ligneous and unaltered cellulose remains. This material was divided into two parts. One was used to separate the cellulose, which was achieved by selective ligneous material solubilization via nitration of its polyaromatic units with a mixture of 20% HNO3 and 4% H2SO4 in ethyl alcohol. The remaining cellulose suspension was washed with ethyl alcohol. The second part was subjected to hydrolysis with 72% H2SO4, which led to the removal of cellulose. The lignin obtained was washed with distilled water, neutralized with dilute NaOH, and further washed until SO42- could no longer be detected. Yields and proximate and elemental analyses of the prepared group components are presented in Table 1 (respective data for HTT 100 °C). Carbonization Experiments. Preparation of the chars was carried out in a vertical quartz tube (60 cm long, 4 cm diameter) reactor. A 1 g sample was placed in a stainless steel basket (3 cm diameter) within the uniform heating zone inside the reactor tube. Charring was performed under pure argon flowing at 0.1 mol cm-2 h-1. The samples were heated at 10 °C min-1 rate to a temperature between 100 and 800 °C at intervals of 100° and with 30 min hold time at each final temperature. To avoid cumulative effects, fresh samples were used for each carbonization experiment. Change in the char mass upon heat treatment was controlled by an automatic mass-compensating system.44 Yields, proximate and elemental analyses of the chars obtained, are presented in Table 1. Oxidation of Humodetritous Brown Coal Humic Acids 600 °C Char. Gasification with 10% (v/v) oxygen in argon was carried out on 600 °C humodetritous brown coal humic acids char to different burn off values at 330 °C. For these experiments, the same reactor was used as for the carbonization process. Gasification used 1 g of raw humic acids. The sample was initially charred at 600 °C under argon flow, and after the reactor was cooled to 330 °C, it was further oxidized with a mixture of oxygen and argon flowing through the reactor tube at rates of 0.01 and 0.09 mol cm-2 h-1, respectively. The change of the char mass upon oxidation was controlled by an automatic mass-compensating system.44 EPR Measurements. Quantitative EPR studies were carried out on powdered samples of coals. The EPR spectra were recorded at 25 °C using X-band microwave frequencies: SE-Radiopan and ESP 300E-Bruker. The g-factor and the line width of the signals for the coals and their fractions as well as for the chars were determined. Nuclear magnetometer, (44) Tomko´w, K. Prace Naukowe Instytutu Chemii i Technologii Nafty i We¸ gla Politechniki Wrocławskiej Nr 25/2, Wrocl-aw; 1975; pp 183-192.

Energy & Fuels, Vol. 11, No. 5, 1997 953 Table 1. Yield, Proximate and Elemental Analyses, of Chars from Xylitic and Humodetritous Brown Coals and Their Group Components Brown coal and its group components Xylitic brown coal

bitumens (1.8%)

cellulose (31.2%)

lignin (71.4%)

humic acids (7.9%)

Humodetritous brown coal

bitumens (9.6%)

humic acids (72.8%)

residual coal (16.2%)

char HTT yield ash Ad carbon hydrogen [°C] [%] [%] Cdaf [%] Hdaf [%] 100 200 300 400 500 600 700 800 100 200 300 400 500 100 200 300 400 500 600 700 800 100 200 300 400 500 600 700 800 100 200 300 400 500 600 700 800 100 200 300 400 500 600 700 800 100 300 400 500 100 200 300 400 500 600 700 800 100 200 300 400 500 600 700 800

100 99.2 86.8 58.2 45.3 43.3 41.5 40.2 100 86.1 45.8 11.3 8.9 100 97.0 51.2 29.9 26.7 25.7 24.4 23.7 100 98.9 95.2 71.7 58.6 56.3 52.2 51.2 100 95.3 92.0 74.7 58.6 55.5 52.8 51.6 100 96.9 89.4 70.5 54.2 51.4 47.7 46.5 100 79.0 8.1 5.4 100 95.2 84.5 73.5 61.3 58.8 56.4 54.8 100 97.9 84.9 70.7 60.5 58.2 56.2 55.2

1.2 1.5 2.1 2.9 3.7 3.8 3.9 4.0 0.01 0.01 0.05 0.09 0.10 0.8 1.1 2.6 3.8 4.7 4.8 4.9 5.0 2.5 2.8 3.0 3.6 4.5 4.7 5.0 5.4 2.5 2.6 3.0 3.4 4.1 4.3 4.5 4.6 8.4 8.8 9.5 12.4 13.6 14.4 15.4 16.5 0.01 0.01 0.13 0.19 0.7 0.8 0.9 1.1 1.2 1.4 1.6 1.7 9.3 9.5 10.4 12.0 13.2 14.8 15.9 16.9

60.6 61.2 66.2 77.0 89.5 91.5 93.4 94.8 82.3 80.3 84.7 87.1 89.2 44.4 45.0 65.3 76.1 85.9 88.0 91.7 92.4 65.6 66.9 69.4 77.3 88.4 90.4 92.0 93.7 66.3 67.1 69.0 77.4 86.3 90.1 92.6 93.8 63.2 63.7 67.7 73.6 81.3 84.2 86.7 88.6 79.6 80.3 84.3 94.0 63.3 65.8 67.8 78.9 87.4 89.8 92.2 93.4 66.2 66.5 71.0 77.9 85.9 87.6 88.9 98.1

6.0 5.8 5.1 4.5 3.2 2.3 1.7 1.3 9.8 9.9 9.9 6.5 3.5 6.4 6.1 4.7 4.0 3.0 2.8 2.1 1.5 5.3 5.1 4.9 4.0 3.0 2.6 1.4 1.1 5.4 5.2 4.7 4.2 3.2 2.6 1.7 1.5 5.3 5.1 5.0 4.3 3.1 2.6 2.3 1.5 11.1 11.8 9.8 5.8 4.9 4.6 4.0 3.7 3.1 2.5 2.1 1.2 5.1 4.9 4.3 4.0 2.9 2.5 1.7 1.2

frequency counter, and the EPR standards were used to calculate g-factors and magnetic field calibration. Quantitative techniques were used (20.0 mg samples, quartz tubes of constant volume, constant microwave power, constant modulation amplitude, and frequency, etc.). The concentrations of free radicals in the samples were calculated using standard integration of the derivative signal and by comparing the area of the free radical EPR signal with the area determined with the free radical standards (DPPH, TEMPO, ultramarine).45 Although resonance lines have a complex character (several



Table 2. Technical, Microscopic, and EPR Spectroscopic Characterization of Investigated Bitumnous Coals and Their Petrographic Constituents microscopic analysis

sample Janina coal Halemba coal 1 Maja coal Janina liptain Halemba liptain 1 Maja liptain Janina vitrain Halemba vitrain 1 Maja vitrain Janina fusain Halemba fusain 1 Maja fusain

volatile matter Vdaf %

carbon content Cdaf %

40.6 35.7 27.9

77.0 83.9 87.8

EPR data

reflectance %

Liptinite content % (v/v)

Vitrinite content % (v/v)

Inertinite content % (v/v)

spin concn (×10-18)

g ( 0.0001

∆H (G)

0.22 0.35 0.62 0.51 0.95 1.08 1.13 2.54 2.00

14.7 6.9 3.2 88.2 85.0 23.4 0.3 0.9 0.4 0.7 0.0 0.0

61.1 65.4 80.1 8.6 9.9 73.6 99.3 97.5 98.7 0.6 1.0 7.2

23.2 26.7 16.3 3.2 5.1 3.0 0.4 1.6 0.9 98.7 98.6 92.0

1.76 5.67 6.00 0.60 2.67 3.99 0.68 3.42 6.00 9.69 11.19 11.29

2.0029 2.0027 2.0027 2.0028 2.0028 2.0027 2.0033 2.0029 2.0028 2.0028 2.0027 2.0026

4.4 4.7 4.9 4.8 4.9 5.1 6.0 6.1 6.4 2.8 3.3 3.0

Gaussian and Lorentzian components), for the comparative evaluations total spin concentration was calculated using the above procedure. Values of the g-parameter were determined on the basis of magnetic field intensity and a microwave frequency. Additionally, they were controlled using Li atoms in LiF as a g-parameter standard.

Results and Discussion Maturation Trends of EPR Signals in Petrographic Constituents of Bituminous Coals. The spin concentration, g-value, and line width of the bituminous coals and their petrographic constituents are reported in Table 2. As shown on a diagram (Figure 1A), the spin concentrations of the petrographic constituents of a given bituminous coal increase in the order liptain < vitrain < raw coal < fusain. Intermediate values of spin concentrations in raw coals, compared to the values in their petrographic constituents, result from the cumulative effect of the respective constituents in a given coal. Higher concentrations of carbon radicals are observed in more mature samples. Correlation of radical spin densities with the level of maturation, expressed as a reflectance, is illustrated in Figure 1B. Within the maturity range investigated, there is a continuous increase in the spin density with vitrain and liptain maturation. In fusain maturation, despite the biggest reflectance range (from 1.13 to 2.54%), the increase in the spin density is less pronounced (only 20%). Above 2% fusain reflectance, the spin concentration reaches a nearly constant value of around 11.2 × 1018 spins/g. For clarity, it should be noted that the reflectance of fusain from the less mature Halemba bituminous coal is higher (2.54%) than from the more mature 1 Maja bituminous coal (2.00%). This fact can be rationalized by a different early depositional thermal history of the above lithotype in both coals. These data support the view that upon coalification, the major structural changes, reflected by generation of new carbon radicals, are taking place in the less coalified lithotypes i.e., liptinite and vitrinite. Liptinite is characterized by significant aliphatic character and the presence of isolated condensed aromatic structures. In vitrinite, the condensed aromatic structures are separated by a great deal of heteroatom linkages between the aromatic sheets of low ordering degree. Fusinites (45) Janczak, J.; Kubiak, R.; Jezierski, A. Inorg. Chem. 1995, 34, 3505-3509.

Figure 1. Correlation of spin concentration with parent coals and petrographic constituents maturity (A) and with petrographic constituents reflectance (B).

are dominated by condensed polyaromatic structures. Upon coalification, the loss of chain hydrocarbons and heteroatoms in coal lithotypes causes increases in ordering of the polyaromatic units and in their sizes. This process is especially intensive in liplinite and vitrinite, and hence, the sharp increase in carbon radicals density is detected.

Petrographic Constituents

Energy & Fuels, Vol. 11, No. 5, 1997 955

Figure 2. Correlation of g-value with petrographic constituents reflectance (A) and spin concentration (B).

Figure 3. Correlation of ∆H value with petrographic constituents reflectance (A) and spin concentration (B).

The g-value is correlated with the maturity level of the petrographic constituents. Dependence of this parameter on either the reflectance (Figure 2A) or spin concentration (Figure 2B) of the petrographic constituents clearly shows a decreasing shift of the g-value toward the free electron value. The most pronounced decrease of the g-value with increasing maturation is observed for vitrain. It is rationalized by the higher content of heteroatoms in this petrographic constituent, which falls drastically with its maturation. The loss of the heteroatom linkages between aromatic units upon coalification causes a gradual increase in the ordering of the polyaromatic skeleton and hence a shift of the g-value toward the free electron value. For liptain and fusain, the g-values also decrease with increasing maturation. The observed reversal in the g-value for fusain at 2.0% reflectance probably results from the enhanced reflectance of fusain in the medium-maturity Halemba coal compared to the more mature 1 Maja coal. This trend reflects both the decrease in the H/C ratio and an increase in the core size of polynuclear aromatic moieties with increasing maturation. Changes of the g-value for vitrain and fusain follow the same general trend, while a different pathway is observed in the case

of liptain. This observation is explained by the different chemical nature of the respective coal macerals and not by their postdepositional thermal history, which is the same for all constituents during the formation of a given coal. The correlations found between the g-values and the maturation level agree with earlier studies on macerals from bituminous coal and kerogens.16,18 Petrographic constituents exhibit distinct values of line widths as illustrated in Figure 3. Values of this parameter for the petrographic constituents of each coal decrease in the order vitrain > liptain > raw coal > fusain (Table 2). This is because line widths are sensitive to the radicals’ chemical environment. For vitrains, the line width is about 6 G, for liptains about 5 G, and for fusains 3 G. The magnetization recovery rate depends on both the molecular properties of the polyaromatic units and the intermolecular interactions between them, that is, on local structural ordering. The observed line width monotonically broadens with increasing maturation. This effect is due to the gradual decrease of heteroatom content as well to a decrease of the H/C ratio, which causes the T1e value to increase because the exchange interaction becomes more significant within more ordered aromatic moieties, as expected



Figure 4. Van Krevelen diagram of chars from the xylitic (A) and humodetritous (B) brown coals and their group components.

for the more mature samples. The measured maturation trend in the line width for petrographic constituents of bituminous coals is opposite to the trend observed for kerogen.17 The most plausible explanation for the lack of consistency is the difference in summary effect of molecular structure and intermolecular interactions in these materials. Namely, with maturation there is a concomitant decrease of heteroatom content as well as an increase in the average aromatic core size accompanied by a higher degree of association of aromatic moieties. An exchange-narrowing mechanism has been invoked to account for the decrease in EPR line width with increasing kerogen maturation.17 However, an increase in the association of the polyaromatic core would disable electron delocalization, which leads to the increased electron-electron interaction reflected in line broadening. This effect probably prevails in coaly materials. Influence of Brown Coals Group Components Heat Treatment Temperature (HTT) on Chars EPR Signals. Thermal maturation pathways of brown coal lithotypes and their group components are illustrated on Van Krevelen diagram in Figure 4 (Table 1). It shows that with an increasing HTT up to 400 °C, samples preferentially lose their oxygen functional groups (mainly in the form of water, carbon dioxide, and carbon monoxide). Then at higher temperatures, their hydrogen functional groups are lost in the form of liquid and gaseous hydrocarbons and finally (above 600 °C) in the form of gaseous hydrogen. Irrespective of the chemical composition of the starting material, atomic H/C and atomic O/C ratios fall within the same correlation line, with only a little scatter of points. The novel aspect of the EPR studies on these chars is an attempt to integrate and interrelate all the parameters on a suite of chemically different materials isolated from the same sample of raw brown coal. It makes it possible to assess the susceptibility to radical formation in different chemical environments. The EPR signals of these chars, determined over the HTT range from 100 to 800 °C in the presence of oxygen, consist of an absorption line of varying intensity centered at a position typical of organic radicals. We observe a strong increase of the signal up to 550-600 °C and above 700 °C its sharp decrease. It is connected with the Curie law for the carbon radicals in HTT’s up to 700 °C. As

has been demonstrated earlier for similar materials, with an increasing contribution of conduction electrons, a deviation from the Curie law appears at HTT’s above 750 °C.46,47 The EPR data for chars are presented in Table 3. Dependencies of the spin density upon the carbonization process are illustrated in Figure 5. The coals and their group components with increasing HTT exhibit formation and subsequent decay of stable radicals with the most intensive transformations occurring between 500 and 600 °C. Lower thermal stability is observed only for bitumens isolated from xylitic brown coal, where the respective maximum is shifted to around 400 °C (triangles in Figure 5A). Only small changes in the concentrations of paramagnetic centers are observed up to 300 °C. The formation of carbon radicals begins around 400 °C with a sharp increase in total concentration of paramagnetic centers after heating above this temperature. The maximum values of radical densities are reached around 550 °C. They exceed 2 × 1019 spins/g for all of the investigated materials except for bitumens from xylitic brown coal. Above 550 °C a very steep decrease of radical concentration occurs. A possible explanation of radical decay at increased HTT’s is the formation of more favorable positions in which the contact between two radicals may occur. In this macromolecular condensation process larger polyaromatic units are gradually formed, exhibiting an enhanced electric conductivity that makes the EPR measurements difficult. All materials, irrespective of their chemical composition, show surprising similarities in the spin concentration changes upon HTT. The spin density maximum occurs in the temperature range typical for coals or their components.29,31 Evidently, the radical concentrations in chars from humic acids of both brown coals above the HTT range are enhanced relative to the values for chars of raw coals and other their group components. The starting materials differ significantly in chemical composition. Therefore, it was intriguing to correlate changes in spin concentration with the atomic H/C and O/C ratios of the chars (Figures 6 and 7, respectively). Generally, with decreasing atomic H/C and O/C ratios, (46) Mrozowski, S.; Gutsze, A. Carbon 1977, 15, 335-342. (47) Mrozowski, S. Carbon 1981, 19, 365-373.



Figure 5. Variation of the free radical concentration with HTT of the xylitic (A) and humodetritous (B) brown coals and their group components. Table 3. EPR Data on Group Components Chars from Xylitic and Humic Brown Coal Lithotypes HTT, °C raw sample

100

200

300

400

500

Spin Concentration per Gram (×10-18) 0.07 2.20 2.50 15.0 0.07 0.19 8.66 6.32 0.26 0.21 2.16 14.4 0.02 0.10 1.65 16.4 0.06 0.19 3.81 22.1 0.50 0.93 2.02 8.83 0.02 21.1 0.48 1.08 2.73 13.8 0.16 0.49 5.14 22.8

xylitic brown coal bitumens cellulose lignin humic acids humodetritous brown coal bitumens residual coal humic acids

0.05 0.03 0.01 0.02 0.04 0.52 0.01 0.64 0.15

xylitic brown coal bitumens cellulose lignin humic acids humodetritous brown coal: bitumens residual coal humic acids

7.1 6.5 3.2 5.0 4.5 5.8 5.6 5.1 4.3

7.2 7.8 3.2 5.1 4.8 5.8

xylitic brown coal bitumens cellulose lignin humic acids humodetritous brown coal bitumens residual coal humic acids

2.0038 2.0045 2.0033 2.0034 2.0036 2.0036 2.0034 2.0031 2.0036

2.0034 2.0045 2.0029 2.0034 2.0036 2.0034

4.8 4.3

2.0031 2.0036

∆H Value [G] 6.5 5.4 7.1 6.7 4.8 4.9 5.5 5.3 5.2 6.1 5.9 5.8 5.7 5.7 5.7 5.2 6.3 g-Value, (0.0001 2.0033 2.0031 2.0035 2.0029 2.0030 2.0030 2.0034 2.0031 2.0034 2.0032 2.0034 2.0034 2.0040 2.0031 2.0031 2.0034 2.0032

the chars’ spin concentrations steadily increase to maximum values. All maxima occur at 0.40-0.42 atomic H/C ratio and 0.08-0.10 atomic O/C ratio except for the chars from bitumens of humodetritous brown coal. This starting material mainly has aliphatic character, and therefore, their atomic O/C ratios at maximum spin concentration are very low. There is an abrupt fall in spin concentration below 0.40-0.42 atomic H/C ratio and below 0.08-0.10 atomic O/C ratio. These values can be interpreted as critical, below which intensive molecular association proceeds upon further HTT increase.

600

700

13.7

1.10

13.3 16.3 15.0 10.2

2.40 0.31 0.48 2.70

10.5 17.2

0.55 0.01

5.0 6.2 4.5 5.0 5.7 6.5 5.4 5.1 5.6

5.4

6.0

5.0 5.4 5.6 5.3

4.6 6.9 8.2 4.8

4.5 5.8

7.5 3.7

2.0029

2.0027

2.0027

2.0030 2.0029 2.0028 2.0029 2.0029 2.0028 2.0029

2.0029 2.0027 2.0028 2.0028

2.0028 2.0027 2.0027 2.0028

2.0027 2.0027

2.0027

800

0.04 0.01 0.02 0.01

8.1 5.3 7.7 6.1

2.0027

The dependencies of the g-value on HTT are presented in Figure 8. In raw coals and their group components, the g-values of free radicals are rather high (about 2.0035). It is typical for the organic materials containing oxygen functional groups. Additionally, the line shape of the EPR signals lends further evidence for the presence of such functionalities in the samples. The highest g-values, however, were found in bitumens of xylitic coal (Figure 8A), which indicates delocalization of the unpaired electrons on heteroatoms. In contrast, the cellulose from this coal, despite high oxygen content, exhibits the lowest g-value; probably the hydroxyl



Figure 6. Correlation between the free radical concentration and atomic H/C ratio of the chars from xylitic (A) and humodetritous (B) brown coals and their group components.

Figure 7. Correlation between the free radical concentration and atomic O/C ratio of the chars from xylitic (A) and humodetritous (B) brown coals and their group components.

groups present in cellulose are not associated with the free radicals. Generally, an increase of HTT up to 700 °C results in a gradual g-value decrease to a level slightly above the free electron value (around 2.0026). Similar trends were reported for other artificially matured coal samples.1 This trend is explained by the thermal destruction of the radicals associated with the oxygen functionalities originally present in raw materials due to reactions taking place below 400 °C. At higher temperatures, formation of radicals in the increasing condensed polyaromatic hydrocarbons is responsible for the observed trend. For clarity it should be noted that the individual materials behave somewhat differently. This is clearly seen in the plots of g-values vs atomic H/C and O/C ratios (Figures 9 and 10, respectively). The g-values in bitumen chars up to HTT 300 °C from both coals, from cellulose, and from residual coal fall outside the typical zone. The first show

elevated g-values, while the latter two have g-values lower than those of the other samples following the general trend. Much less consistency is found in the line width variation with HTT (Figure 11). The line width scatters within a wide zone, around 6 ( 2 G for xylitic brown coal and its group components, and is a little narrower, around 5 ( 1.5 G for humodetritous brown coal and its group components. Wider dispersion of these values is observed at both ends of HTTs, below 300 °C and exceeding 600 °C. The dipole interactions of unpaired electrons and the unresolved hyperfine structure of the interactions of unpaired electrons with neighboring protons are the reasons for the larger line widths of these signals. However, the differences in the trends of line width dependencies on HTT for individual materials, particularly at carbonization temperatures up to 400 °C for xylitic coal and its bitumens, show a



Figure 8. Correlation between the g-value and HTT of the chars from xylitic (A) and humodetritous (B) brown coals and their group components.

Figure 9. Correlation between the g-value and atomic H/C ratio of the chars from xylitic (A) and humodetritous (B) brown coals and their group components.

lowering trend compared to the other group components of this coal. This is probably caused partly by a g inhomogenity of the materials resulting from differences in their chemical composition and is partly affected by the chemical preparation of group components (Figure 11A). Interactions of Chars from Brown Coals and Their Group Components with Organic Solvents. Our approach was to assess the effect of chars penetration by different solvents under atmospheric pressure, as reflected in the change of spin concentration. A 2-fold (mass) excess of solvents was added to char and boiled for 15 min. Addition of benzene to char samples obtained at HTTs below 500 °C had no effect on the stable radicals density. However, in the chars from higher HTTs, benzene caused an increase in the concentration of paramagnetic centers. Therefore, the studies using other solvents were performed on the

chars of 600 and 700 °C HTTs. Results of the solvent effect on the stable radical density in these chars are listed in Table 4. The data indicate that more pronounced enhancement of stable radical density is achieved in 700 °C chars by the flatter and bigger molecules. The experimental order of free radical density increase in relation to its original level is as follows: cyclohexane < n-hexane < benzene < toluene< dimethyl-substituted benzenes < tetralin < naphthalene. This phenomenon is due to the presence of a large proportion of free radicals in the chars of higher HTTs, which are not completely recombined yet.29,30 These radicals exist mainly between the adjacent aromatic sheets of lower ordering degree. The introduction of diamagnetic molecules such as benzene or others between these aromatic sheets causes separation of the radicals. The number of these radicals is particularly high in the chars of HTTs, just exceeding



Figure 10. Correlation between the g-value and atomic O/C ratio of the chars from xylitic (A) and humodetritous (B) brown coals and their group components.

Figure 11. Correlation between line width and temperature of thermal treatment of the xylitic (A) and humodetritous (B) brown coals and their group components.

the maximum radical density. The observed boundary effect is higher for the chars of HTT 700 °C compared to the chars of HTT 600 °C (Table 4). Justification of the observed enhancing effect is probably the spatial separation of the adjacent aromatic sheets in char, which is higher when the “intercalating” molecule is flatter and bigger in size. Accessibility of the benzene molecules to the pore structure of the investigated materials is the highest at HTT 600 °C;48 their desorption isotherms are characteristic of partial chemisorption of benzene molecules in chars structure. At higher HTTs the pore structure accessible to benzene molecules is gradually closing. However, further increase in its accessibility to smaller carbon dioxide molecules up to

800 °C HTT is explained as an increase in the molecular sieve effect. Considering the pure sorption of molecules in pore structure only, the molecular sieve effect would be even higher for larger molecules such as tetralin. The observed EPR data indicate, however, that despite the pore structure closing and an increase of the molecular sieve effect, to some extent the polyaromatic structure of chars can be “intercalated” by the bigger organic molecules. The presence of “intercalated” molecules is detected by the increased concentrations of free radicals in 700 °C chars (Table 4). The g-values of the chars after treatment with solvents are practically the same, while the observed line widths show little scatter with no significant change.

(48) Czechowski, F.; Jankowska, A.; Marsh, H.; Siemieniewska, T.; Tomkow, K. In Characterization of porous solids; Gregg, S. J., Sing, K. S. W., Stoeckli, H. F., Eds.; Society of Chemical Industry: London, 1979; pp 67-78.

Influence of Burn Off of 600 °C Char from Humodetritous Brown Coal Humic Acids with Oxygen on EPR Signals. Oxygen burn off effects on



Table 4. Effect of Solvent Addition on Spin Concentration in Selected Chars from Xylitic and Humic Brown Coal Lithotypes and Their Group Components before solvent addition

raw sample xylitic brown coal cellulose lignin humic acids

humodetritous brown coal residual coal humic acids

HTT °C

solvent added

spin concentration per gram (×10-18)

600 700 600 700 600 700 500 500 500 500 500 600 600 700 700 700 700 700 700 600 700 600 700 600 700

tetralin tetralin tetralin tetralin tetralin tetralin benzene toluene pyridine p-xylene o-xylene benzene tetraline cyclohexane n-hexane benzene toluene tetralin naphthalene tetralin tetralin tetralin tetralin tetralin tetralin

13.7 1.10 13.3 2.40 16.3 0.31 22.1 22.1 22.1 22.1 22.1 15.0 15.0 0.48 0.48 0.48 0.48 0.48 0.48 10.2 2.70 10.5 0.55 17.2 0.01

Figure 12. FT-IR spectra of 600 °C HTT humic acids from humodetritous brown coal oxidized with 10% of oxygen at temperature of 330 °C to different burn off’s.

the char on EPR signals were studied on the 600 °C HTT char from humic acids of humodetritous brown coal. Reaction of oxygen at a temperature of 330 °C resulted in the loss of char mass as well as formation of surface oxygenated groups in solid char, as evidenced by FT-IR spectra (Figure 12). Mainly carbonyl and ether groups are formed as revealed by increases in the absorbance intensity at 1720-1740 and 1270 cm-1 proportional to the burn off value. To assess the influence of oxygenated groups in the char oxidation products on the EPR signals, the samples were reheated to their final charring temperature in an inert atmosphere. The reheating caused thermal decomposition

after solvent addition

∆H (G)


∆H (G)

multiplicity of spin concentration increase after solvent addition

5.4 6.0 5.0 4.6 5.4 6.9 5.7 5.7 5.7 5.7 5.7 5.6 5.6 8.2 8.2 8.2 8.2 8.2 8.2 5.3 4.8 4.5 7.5 5.8 3.7

13.7 22.0 22.6 13.2 84.8 6.80 22.1 22.1 24.3 26.5 28.7 16.5 18.0 0.72 1.10 1.35 1.86 5.03 5.29 7.40 7.20 16.9 0.65 24.1 0.09

5.4 5.6 5.0 4.8 5.5 7.3 5.7 5.5 5.6 5.6 5.3 5.5 5.5 8.2 8.2 8.2 8.1 8.4 8.4 5.2 4.8 4.5 7.5 5.7 3.5

1.0 20.0 1.7 5.5 5.2 22 1.0 1.0 1.1 1.2 1.3 1.1 1.2 1.5 2.3 2.8 3.9 10.5 11.0 1.4 1.5 1.6 1.2 1.4 8.5

of the formed oxygenated groups and additional mass loss (see Table 5). The EPR data obtained for the products are presented in Table 5. Spin concentrations in 600 °C char continuously decrease with an increasing oxygen burn off as illustrated in Figure 13A (solid line). We suppose that one explanation of the observed decrease due to the oxidation probably results from the preferential reaction of oxygen with char centers bearing unpaired electrons and consequently their partial removal in a form of volatile oxidation products. The oxidized products (AO2) were subjected to the secondary heat treatment at 600 °C under argon flow (AO2 + HTT). This resulted in further spin density decreases (Figure 13B; compare results for oxidation products of 2.0 and 7.2% burn off with their respective products after reheat). This change is interpreted as being due to the presence of free radicals in the structures of formed oxides, which readily decompose upon HTT producing CO and CO2. It has also been found that introduction of benzene to the samples, either AO2 or their reheat AO2 + HTT products, highly enhances the number of free radicals (Table 5 and Figure 13). The amount of benzene introduced into the structure of analyzed samples is rather small. The weight percentage of benzene introduced under vacuum, which is not removable from the materials upon secondary vacuum evacuation at room temperature (benzene molecules trapped in the char structure), was determined to be below 0.1%. An overall spin density change of the samples containing benzene (Figure 13B) before and after thermal removal of the oxides proceeds in the same direction as observed in the respective oxygen burn off samples and their further HTT products, but at different levels. The benzene-enhancing effect is burn off dependent. It exhibits its maximum value (a 2-fold enhancement) for 7% burn off (Figure 14A). This enhancing effect is much higher if the benzene is introduced under vacuum



Figure 13. Spin concentration in 600 °C char from humic acids of humodetritous brown coal, dependence on burn off of oxidation products and method of their treatment with benzene (A), and reheat effect of oxidation products at 600 °C under argon atmosphere (B). Table 5. EPR Data on Chars from Turo´ w Humic Acids (600 °C) Activated with 10% of Oxygen in Argon at 330 °C to Different Burn Off’s and on Activation Products Reheated at 600 °C EPR data

sample weight loss, % activation products activation products saturated with C6H6 at atmospheric pressure activation products saturated with C6H6 at atmospheric pressure and conditioned at room temperature activation products saturated with C6H6 in vacuum activation products saturated with C6H6 in vacuum and conditioned at room temperature activation products saturated with C6H6 in vacuum and evacuated at room temperature in vacuum


∆H (G)


Burn Off Products with 10% of Oxygen in Argon 2.0 7.2 12.70 (g ) 2.0029) 4.5 9.09 (g ) 2.0030) 13.27 4.3 15.34

4.1 3.9

3.90 (g ) 2.0030) 5.05

3.5 3.8

12.90

4.2

11.69

3.5

5.12

3.9

22.62

3.6

20.49

3.5

7.36

3.6

20.82

3.5

17.91

3.1

7.52

3.9

21.16

3.8

15.10

3.4

6.95

3.6


∆H (G)

∆H (G)

22.4

Burn Off Products with 10% of Oxygen in Argon and Further Heated in Argon to HTT of 600 °C weight loss, % 12.2 19.8 HTT activation products 11.63 (g ) 2.0027) 6.7 8.08 (g ) 2.0028) 5.4 HTT activation products saturated 15.36 4.1 13.88 3.7 with C6H6 at atmospheric pressure HTT activation products saturated 11.80 3.1 9.84 3.8 with C6H6 at atmospheric pressure and conditioned at room temperature HTT activation products saturated 21.81 4.0 19.36 3.9 with C6H6 in vacuum HTT activation products saturated 20.38 3.8 15.74 4.1 with C6H6 in vacuum and conditioned at room temperature HTT activation products saturated 13.75 4.6 12.20 4.7 with C6H6 in vacuum and evacuated at room temperature in vacuum

than under atmospheric pressure (compare solid and dashed lines in Figure 14A). Such dependence is

associated with the pore structure accessibility to benzene molecules and the extent of char “intercalation”



Figure 14. Spin density enhancement caused by benzene in oxidation products with 10% oxygen at 330 °C of 600 °C char from humic acids of humodetritous brown coal (A) and in their reheat products at 600 °C under argon atmosphere (B).

Figure 15. EPR line width in oxidation products with 10% oxygen at 330 °C of 600 °C char from humic acids of humodetritous brown coal (A) and in their reheat products at 600 °C under argon atmosphere (B).

under defined conditions. We found that this effect is higher if benzene is introduced under vacuum. The absolute values of the benzene-enhancing effect on spin concentration are almost the same for oxidized chars and for the respective samples in which surface oxides were thermally removed (the case particularly clearly illustrated for the product of 7.2% burn off and its further HTT counterpart; Figure 14B). Surprisingly, as the density of oxides increases, which parallels higher burn off, the EPR line narrows (Figure 15A). This is attributed to the formation of oxides

within the char structure of the lower ordering degree, i.e., around condensed polyaromatic units, which causes their progressive isolation with burn off. It is reflected by the narrowing of the signals. Thermal removal of oxides lends further evidence for such a conclusion. Namely, the products obtained upon reheating (Figure 15B) have lines about 2.5 times broader than their oxidized counterparts. After the decomposition of oxides, the polyaromatic units are exposed to contact with molecular oxygen, which is known to broaden the EPR spectra of free radicals.19,22


The novel aspect of the data is the evaluation that under experimental conditions (10% O2, 330 °C) the more prone to oxidation are char structural units associated with an elevated density of free radicals, presumably peripheral polyaromatic skeletons of lower degree of structural ordering. Therefore, oxidation causes a continuous decrease of the total concentration of free radicals and the prevention of the formation of carbon radicals of more delocalized nature. The presented data indicate that the EPR technique can be successfully used for assessment of the char oxidation. Conclusions EPR quantitative methods were used to assess the total concentration of free radicals in coals and chars. Although a free radical signal in coal has a complex nature, the measured average g-parameter and spin concentration depend on the structural changes occurring upon natural coalification and carbonization processes. The steady decrease of the g-parameter proceeds in parallel to the loss of oxygenated functionalities and to an increase of aromaticity. It is evidenced by the linear correlation of atomic H/C as well as atomic O/C with the g-parameter in the chars of brown coals group components. With the maturation of bituminous coals and their petrographic constituents, the above parameter decreases. The change of spin concentration during the coalification of bituminous coals depends also on the nature


of the petrographic constituents. Namely, liptain, vitrain, and fusain having reflectances within the range from 0.22% to 2.54% exhibit increasing spin concentrations with their maturation on different pathways. Furthermore, the EPR line width is also characteristic of the individual petrographic constituents. The total spin concentration (free radical concentration) can be treated as a measure of structural changes in the chars of brown coal group components induced by heat treatment, solvent sorption, and formation of surface oxides. Maxima of spin concentration in the chars are found at 550 °C HTT. They correspond to atomic H/C and O/C ratios of 0.42 and 0.09, respectively. Addition of solvent molecules into the char pore structure or its oxidation products results in the pronounced spin concentration increase. The oxidation process of 600 °C humic acids char from humodetritous brown coal with gaseous oxygen at 330 °C causes gradual elimination of free radicals. Acknowledgment. The authors express gratitude to Dr. Halina Kidawa (Technical University of Wrocław) for providing us with the samples of the petrographic constituents of bituminous coals and their elemental and microscopic analyses. EF960209R

EPR Studies on Petrographic Constituents of Bituminous Coals, Chars

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