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Structural Ordering of Pennsylvania Anthracites on Heat Treatment to 2000-2900 °C Joseph V. Atria,† Frank Rusinko, Jr., and Harold H. Schobert* The Energy Institute, The Pennsylvania State University, University Park, Pennsylvania 16802 Received December 18, 2001
Three Pennsylvania anthracites from the Penn State Coal Sample Bank and Data Base were heat-treated to temperatures to 2900 °C. The products were characterized by X-ray diffraction. These anthracites clearly differ in the extent to which they transform to a graphitic structure by heat treatment at ambient pressure. Graphitizability, based on approach of interlayer spacing to the ideal value for graphite and development of crystallite height, is in the order DECS 21 > PSOC 1461 > PSOC 1468. By 2700 °C the interlayer spacing is largely established, since improvements by increasing graphitization temperature to 2900 °C are small. The crystallite stacking continues to develop; significant changes are achieved at 2900 vs 2700 °C. The structural changes can be related to composition via two factors. In heat treatment to 2000 °C, the structural ordering may be impeded by a “locking” of aromatic sheets in place by cross-linkers, such as oxygen atoms. At the higher temperatures, i.e., 2700 and 2900 °C, the ease of rearrangement of aromatic sheets, which is related to their size, is the dominant issue. The relative sizes of aromatic sheets can be approximated from the net hydrogen content of the anthracites. DECS-21, which has both the highest oxygen content and the highest net hydrogen value, shows the least order after heat treatment to 2000 °C, but the best structural development after reaction at 2700 or 2900 °C.
Introduction Currently, the carbon and graphite industry uses petroleum coke as the main filler constituent in the manufacture of synthetic graphite. One of the major applications is the manufacture of electrodes for electric steel-making furnaces. Anecdotal reports from the carbon and graphite industry provide concern for the slowly declining quality of petroleum coke, particularly with respect to nickel, vanadium, and sulfur contents. Concern is also raised about the large, and growing, dependence of the United States on imported petroleum and petroleum products. Anthracites are high-carbon coals, typically greater than 90% carbon, and, by definition, greater than 92% fixed carbon on a dry, mineralmatter-free basis,1 with virtually all of the carbon present in aromatic structures.2 Further, anthracites are of substantially lower cost than premium petroleum cokes, and the possible use of lower cost materials has sparked occasional industrial interest in this application of anthracite. Therefore, we were interested in examining the possibility of using anthracites as a replacement for petroleum coke in production of synthetic graphite materials. The production of these materials involves a number of processing operations.3 A first step in assessing the suitability of anthracites in this application
is assessing the graphitizability of anthracites when reacted alone. Doing so would avoid complications from other effects, such as, for example, the graphitization of a pitch binder that would also occur in the production of a molded graphite artifact. There have been various accounts of the graphitizing behavior of anthracites in the literature. Franklin established the classification of carbons into graphitizable and nongraphitizable materials.4 She showed the relationship of layer diameter to the number of layers in a parallel group for graphitizable and nongraphitizable carbons. At temperatures to 2290 °C, anthracites followed the layer diameter vs parallel layers behavior of nongraphitizable carbons. Other work showed that anthracites are nongraphitizable at temperatures to 2000 °C.5 Although Franklin’s pioneering study classified anthracites as being nongraphitizable, she also found that above 2500 °C they became even more highly graphitized than did carbons that were originally classified as graphitizing.4 Evans et al. observed graphite formation in Welsh anthracites at 1200-1370 °C, promoted by catalysis of mineral particles.6 Russian anthracites from the Donbas were reported to show an increase in structural order to 2200 °C, but then a disordering of the structure in the region 2200-2600 °C.7,8
* Corresponding author. † Present address: Vesuvius Crucible Company, Pittsburgh, PA. (1) Berkowitz, N. An Introduction to Coal Technology; Academic Press: San Diego, 1994; Chapter 3. (2) Gerstein, B. C.; Murphy, P. D.; Ryan, L. N. In Coal Structure; Meyers, R. A., Ed.; Academic Press: New York, 1982; Chapter 4.
(3) Redmouunt, M. B.; Heintz, E. A. In Introduction to Carbon Technologies; Marsh, H., Heintz, E. A., Rodriguez-Reinoso, F., Eds.; University of Alicante: Alicante, 1997; Chapter 11. (4) Franklin, R. E. Proc. R. Soc. A 1951, 209, 196. (5) Hirsch, P. B. Proc. R. Soc. A 1954, 226, 143. (6) Evans, E.; Jenkins, J.; Thomas, J. Carbon 1972, 10, 637.
10.1021/ef010295h CCC: $22.00 © 2002 American Chemical Society Published on Web 10/03/2002
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The effects of microtexture and pore shape in the graphitization of anthracites have been investigated by Oberlin and her group,9,10 Evans et al.,6 and Rouzaud.11 Flattened pores, and the flattening of pores upon heat treatment, may distinguish the graphitizing behavior of anthracites from that of the so-called hard carbons that are nongraphitizable.5,9 Various workers have suggested that areas around flattened pores undergo relatively sudden phase changes into graphitic lamellae,9,12 and that the flattened pores may be necessary for anthracites to be graphitizable.11 Graphitization of Korean anthracites indicated that a lack of flat pores may inhibit formation of graphitic lamellae.10 Several groups have investigated the role of shear strain on the graphitization of carbons.13-17 Much of this work has focused on understanding graphitization in natural processes and has, therefore, involved pressures much higher than those used in laboratory or industrial graphitization, e.g., 1 GPa. The present study is the initial effort in a long-range program that has the dual goals of elucidating some of the reaction chemistry of anthracites and of evaluating the potential of using anthracites as feedstocks for production of high-value carbon artifacts. Three Pennsylvania anthracites were heat-treated to temperatures up to 2900 °C and the products were characterized by X-ray diffraction. Results were related to factors of elemental composition of the anthracites. Experimental Section Sample Selection. Three Pennsylvania anthracites, along with data on proximate and ultimate analyses, were obtained from the Penn State Coal Sample Bank and Data Base. Properties of these anthracites are summarized in Table 1. Density and Pore Characterization. The characterization tests described here were conducted on 60-100 mesh (150-250 µm) samples that were dried at 40 °C in a vacuum oven for 24 h. Helium pycnometry was done using a Quantichrome Multipycnometer MVP-1. Measurements were performed at room temperature on ≈1.5 g of sample. The so-called true densities were calculated by standard methods.18 Mercury porosimetry was performed using a Quantichrome Autoscan instrument, also at room temperature. Samples of ≈0.5 g were used. The maximum mercury intrusion pressure was 60 000 psi. Particle densities were calculated from data at 60 psi using standard methods.18 Both nitrogen and carbon dioxide apparent surface areas were determined using a Quantachrome Autosorb-1 instrument. For nitrogen adsorption, measurements were obtained at -196 °C and apparent surface areas were calculated using the multi-point BET method.19 For
Atria et al. Table 1. Properties of the Anthracites Used in This Studya anthracite ash volatile matter fixed carbon
DECS 21
PSOC 1461
PSOC 1468
Proximate, dry basis 11.16 24.18 4.50 4.01 84.21 71.70
6.83 3.66 89/75
Ultimate, daf basis 90.33 93.42 4.02 1.91 0.80 1.15 0.57 0.98 4.32 1.17 0.533 0.246 0.030 0.003 1.615 1.794 1.530 1.549 5.3 13.6 4.13 2.56 460.4 319.7 3.7 4.2
95.36 1.38 0.83 0.52 1.88 0.174 0.012 1.653 1.631 1.3 0.92 392.8 1.0
carbon hydrogen nitrogen sulfur, total oxygen (diff.) atomic H/C, dmmf atomic O/C, dmmf He density, g/cm3 Hg density, g/cm3 open porosity, vol % N2 surface, m2/g CO2 surface, m2/g N2 total pore volume, cm3/g × 10-3 Hg total pore volume, cm3/g × 10-3 mesopore volume, cm3/g × 10-3 macropore volume, cm3/g × 10-3
40.1
48.2
28.9
39.2
48.0
27.7
0.9
0.2
1.2
a The anthracites are from the Lykens Valley (DECS 21), Mammoth (PSOC 1461), and Buck Mountain (PSOC 1468) seams in Pennsylvania.
carbon dioxide, measurements were done at 0 °C and calculations, by the Dubinin-Radushkevich method.19 Graphitization. Reactions at 2000 °C were performed in our laboratory using a graphite tube furnace. Samples were placed in graphite crucibles. A continuous flow of argon was maintained through the furnace throughout the reaction. Samples were held at 2000 °C for 5 h. Reactions at 2700 and 2900 °C were conducted at facilities of The Carbide/Graphite Group, St. Marys, PA, using an induction furnace. Samples were again contained in graphite crucibles. Reactions involved a 1 h hold at 700 °C and a 5 h hold at the peak graphitization temperature. X-ray Diffraction. Graphitized samples were prepared for measurement of interlayer spacing and crystalline height by mixing 40 mg of 200 mesh sample with 10 mg of silicon (as an internal standard). Measurements were performed from 22° to 58° 2θ at 0.5°/min with 0.02 chopper increment. Interlayer spacings were obtained from the (004) reflection using the Bragg equation.20 The crystalline height was calculated from the Scherrer equation from the full width at half-maximum of the (002) peak, corrected for instrumental peak broadening.21
Results and Discussion (7) Gilyazo, V. Khim. Tverd. Top. 1979, 13, 46. (8) Gilyazo, V.; Yurkovskii, I.; Konstantinova, D.; Rogaita, M. Khim. Tverd. Top. 1981, 15, 84. (9) Oberlin, A.; Terriere, G. Carbon 1975, 13, 367. (10) Deurbergue, A.; Oberlin, A.; Oh, J.; Rouzaud, J. Int. J. Coal Geol. 1987, 8, 375. (11) Rouzaud, J.; Duber, S.; Beny, C.; Dumas, D. Extended Abstr. Biennial Carbon Conf., 21st. 1993, 316. (12) Rouzaud, J.; Oberlin, A.; Trichet, J. Adv. Org. Geochem. 1979, 505. (13) Mastalerz, M.; Wilks, K.; Bustin, R.; Ross, J. Org. Geochem. 1993, 20, 315. (14) Wilks, K.; Mastalerz, K.; Bustin, R.; Ross, J. Int. J. Coal Geol. 1993, 22, 247. (15) Inagaki, M. Fuel 1979, 58, 741. (16) Wilks, M.; Mastalerz, K.; Bustin, R.; Ross, J. Int. J. Coal Geol. 1993, 24, 347. (17) Ross, J.; Bustin, R.; Rouzaud, J. Org. Geochem. 1991, 17, 585. (18) Gan, H.; Nandi, S.; Walker, P. L., Jr. Fuel 1972, 51, 272.
Characteristics of Anthracites. Table 1 provides a summary of the proximate and ultimate analyses from the Penn State Coal Sample Bank and Data Base, as well as results of our measurements of surface area and density properties. Although the anthracite rank range is relatively narrow, these three samples do provide a span of elemental compositions, apparent surface areas, and porosities. (19) Gregg, S.; Sing, K. Adsorption, Surface Area, and Porosity; Academic Press: New York, 1982. (20) Hower, J.; Levine, J.; Skehan, J.; Daniel, E.; Lewis, S.; Davis, A.; Gray, R.; Altaner, S. Org. Geochem. 1993, 20, 619. (21) Cullity, B. D. Elements of X-ray Diffraction; Addison-Wesley: Reading, 1978.
Structural Ordering of Pennsylvania Anthracites
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Table 2. Principal Characteristics of the Reaction Products of the Three Anthracites at the Three Reaction Temperatures
DECS 21 2000 °C 2700 °C 2900 °C PSOC 1461 2000 °C 2700 °C 2900 °C PSOC 1468 2000 °C 2700 °C 2900 °C
interlayer spacing, d002, Å
stacking height, Lc, Å
random dis-orientation, p
degree of graphitization, g
3.413 3.359 3.359
250 400 560
0.686 0.058 0.058
0.314 0.942 0.942
3.401 3.361 3.359
200 330 490
0.547 0.081 0.058
0.453 0.919 0.942
3.408 3.364 3.362
220 260 330
0.638 0.116 0.093
0.372 0.884 0.907
(net H/C)at 0.435
0.187
0.118
Effect of Graphitization Temperature. Table 2 summarizes the results on interlayer spacing, d002, and the crystalline height, Lc. In addtion, we have calculated the degree of graphitization, g, from the equation of Seehra and Pavlovic, who have defined g from the expression
g ) (3.440 - d002)/(3.440 - 3.354) where 3.354 Å is the interlayer spacing in graphite and 3.440, the value for a carbon with no graphitic order.22 The effect of increasing temperature is, in all three cases, to reduce the interlayer spacing, to increase the crystalline height, and to increase the degree of graphitization. The ideal interlayer spacing for graphite, 3.354 Å, is approached but not attained. (However, natural graphites occurring in some igneous rocks display interlayer spacings of 3.36 Å.23) In terms of the values of the three parameters d002, Lc, and g, we rank the graphitizability of these three anthracites in order DECS 21 > PSOC 1461 > PSOC 1468. Our results obtained for graphitization at 2000 °C are comparable to those reported recently by Tyumentsev and his colleagues for two anthracites.24 After reaction at this temperature, a “low-metamorphized anthracite” showed d002 of 3.408 Å and Lc of 210 Å; and a “highmetamorphized anthracite,” d002 of 3.429 Å and Lc of 69 Å. (Unfortunately the provenance and compositions of these anthracites were not reported.) Also, our results are reasonably similar to those for an otherwise unidentified Pennsylvania anthracite (95.5% C, 1.87% H) graphitized at 2800 °C, for which 3.38 Å d002 and 228 Å Lc were reported.25 The data consistently indicate that the largest changes in structure occur between 2000 and 2700 °C. For example, values of g calculated for the three anthracites at the three graphitization temperatures (Table 2) show a substantial increase in the 2000-2700 °C range, with a smaller increase from 2700 to 2900 °C. We did not perform graphitizations at intermediate temperatures in this range. However, these results suggest that there may be a point below 2700 °C at which a significant structural change occurs in anthracites, and that graphitization at, say, 2400 or 2500 °C would be worth pursuing. (22) Seehra, M. S.; Pavlovic, A. S. Carbon 1993, 31, 557. (23) Kwiecinska, B. Prace Mineralogiczne 1980, 67, 87. (24) Tyumentsev, V. A.; Belenkov, E. A.; Shveikin, G. P.; Podkopaev, S. A. Carbon 1998, 36, 845. (25) Bustin, R. M.; Rouzaud, J.-N.; Ross, J. V. Carbon 1995, 33, 679.
Compositional Factors Affecting Graphitization. Since this study examined only three anthracites, detailed statistical analysis of the data is not appropriate. However, the relative ranking of graphitizability, DECS 21 > PSOC 1461 > PSOC 1468, as established from X-ray data is similar to the relative trends in hydrogen content (or atomic H/C ratio). Hydrogen content relates to graphitizability of these anthracites in at least two ways. First, in anthracites, a high hydrogen content may be indicative of aliphatic groups or amorphous carbon at the peripheries of large polycyclic aromatic structures. This concept dates at least from the pioneering X-ray work of Hirsch.5 Thus, a high hydrogen content may reflect a structure that is more disordered to begin with. Among our anthracites, the one of highest hydrogen content, DECS 21, even after reaction at 2000 °C is still the least ordered in terms of interlayer spacing (3.413 Å) and degree of graphitization (0.314). The probability of random disorientation between any two neighboring layers, p, is given by
d002 ) 3.354 + 0.086p based on Ruland’s modification26 of Franklin’s original work.27 (A useful and more recent discussion of applications of this equation is also given by Babu and Seehra.28) In essence, p provides an alternative way of considering the ordering or disordering, and is a useful concept for focusing on the latter. Normally p and g sum to 1. After reaction at 2000 °C, DECS 21 also has the highest value of p, 0.69. A study of the carbonization and graphitization of mesophase powders has shown that localized molecular orientation is dependent on the extent of cross-linking.29 This behavior could be related to a factor, (F)at, which is the atomic ratio of the cross-linkers (i.e., heteroatoms) to hydrogen. The dominant heteroatom in these anthracites is oxygen. To simplify, we take (F)at to be equivalent to the O/H atomic ratio. After reaction at 2000 °C, the anthracite having the lowest O/H atomic ratios PSOC 1461, with a ratio of 0.038sdisplays the highest degree of graphitization, 0.453. It appears that up to at least 2000 °C “locking” of aromatic units in place by cross-linkers is an important factor in affecting the (26) Ruland, W. In Chemistry and Physics of Carbon, Vol. 4; Walker, P. L., Jr., Ed.; Dekker: New York, 1968; 1. (27) Franklin, R. E. Acta Cryst. 1951, 4, 253. (28) Babu, V. S.; Seehra, M. S. Carbon 1996, 34, 1259. (29) Horr, N.-E.; Bourgerette, C.; Oberlin, A. Carbon 1994, 32, 1035.
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ability of the anthracites to rearrange into a more ordered structure. Despite having the most disordered material after reaction at 2000 °C, DECS 21 nevertheless becomes the best graphitized anthracite at higher temperatures, with lowest d002, and p, and highest values of Lc, and g, As graphitization proceeds, heteroatoms are removed as their respective hydrogen compounds, i.e., H2O, NH3, and H2S. Consequently, some of the hydrogen originally in the anthracite is depleted by this process.30 The hydrogen remaining from these reactions is thought to be located on aromatic molecules that can form a solvolytic, or, more likely, suspensive, medium that helps facilitate rearrangement of the large aromatic structural units.31 One approach to quantifying the residual hydrogen is the concept of “net hydrogen” originally introduced by Donath.32 Net hydrogen is the amount of hydrogen expressed per hundred grams of carbon after stoichiometric correction for the hydrogen that would be removed with the heteroatoms. Our previous work has shown the utility of the net hydrogen concept in explaining various phenomena related to direct coal liquefaction.33-35 Among these anthracites, DECS 21 has significantly the highest net hydrogen, 3.64 g/100 g C, while the values for PSOC 1461 and 1468 are 1.56 and 0.98, respectively. Thus there is a definite relationship, with the anthracite having the highest net hydrogen being the most graphitizable, and the lowest net hydrogen being the least graphitizable. An alternative, but related, view of the dependence on net hydrogen considers the thermally induced structural rearrangements that occur around 2500 °C. Aromatic layers can exist in vertical stacking but with disordered structure, the so-called turbostratic structure.4,36,37 Turbostratic carbons have an interlayer spacing of 3.44 Å. After reaction at 2000 °C, these anthracites still have significant structural disorder, as, for example, indicated by the relatively low values of g, and the d002 spacingss3.40-3.41 Åsare not much smaller than the value for a completely turbostratic carbon. At higher temperatures, around 2500 °C, the aromatic layers can, in principle, rotate to form the graphitic structure.38 However, the larger the aromatic layer, the more energy is needed to get it to rotate. Conceivably, extremely large aromatic layers may not have enough energy to rotate at any reasonable graphitization temperature (i.e., up to 3000 °C). Therefore, the size of the aromatic layers can also affect graphitizability. A similar concept has been suggested by Gilyazo, and coworkers, who suggested that side chains or other groups holding aromatic sheets in place are destroyed in the 2200-2600 °C range, with a reorientation of the freed aromatic units occurring around 2600 °C.7,8 (30) Oberlin, A.; Bonnamy, S.; Bourrat, X.; Menthioux, M.; Rouzaud, J. N. In Petroleum-Derived Carbons; Bacha, J. D., Newman, J. W., White, J. L., Eds.; American Chemical Society: Washington, DC, 1986; Chapter 3. (31) Chermin, H. A. G.; van Krevelen, D. W. Fuel 1957, 36, 85. (32) Donath, E. E. In Chemistry of Coal Utilization, Supplementary Volume; Lowry, H. H., Ed.; Wiley: New York, 1963; Chapter 22. (33) Burgess, C. E.; Schobert, H. H. Energy Fuels 1996, 10, 718. (34) Burgess, C. E.; Huang, L.; Martin, S. C.; Tomic, J.; Schobert H. H. Proc. 9th Int. Conf. Coal Sci. 1997, 3, 1373. (35) Burgess, C. E.; Schobert, H. H. Energy Fuels 1998, 12, 1212. (36) Houska, C. R.; Warren, B. E. J. Appl. Phys. 1954, 25, 503. (37) Oberlin, A. In Chemistry and Physics of Carbon, Vol. 22; Thrower, P., Ed.; Dekker: New York, 1989; p 134. (38) Oberlin, A. Carbon 1984, 22, 521.
Atria et al.
In the absence of an external hydrogen source, as was the case in our reactions, the removal of the crosslinkers as H2O (along with NH3 and H2S) depletes the available hydrogen, as discussed above. The size of the aromatic sheets remaining can then be estimated from the atomic ratio of net hydrogen to carbon, (net H/C)at. For DECS 21, this ratio is 0.435. If the aromatic sheets were comprised of fully condensed rings, ovalene, C32H14, (H/C ) 0.438) would be reasonably comparable. (We are not implying that all of the aromatic sheets in this anthracite are ovalene-like, but only use this as an illustrative example.) To reach the much smaller values of (net H/C)at of the other two anthracites would require considerably larger aromatic units. As two examples, dodecabenzocoronene, C54H18 has H/C of 0.333, and the hypothetical C78H22, 0.282. For the anthracites and reaction conditions in this study, two compositional factors appear to be important. Up to 2000 °C, the development of an ordered structure is impeded by cross-linking in which oxygen (as aromatic ether linkages) has an important role. At higher temperatures, i.e., 2700-2900 °C, structural development is easier when the aromatic units are smaller, requiring less energy for their rotation, or providing an suspensive medium that facilitates rearrangement, or both. Removal of the heteroatomic cross-linkers, along with some hydrogen, “unlocks” the aromatic sheets and removes a major barrier to their realignment. The size of the aromatic sheets can be estimated from the net hydrogen, or the ratio (net H/C)at. Therefore, the graphitizability relates to the heteroatom content, especially oxygen, and to the net hydrogen. In this study, DECS 21 happened to have both the highest oxygen content and the highest net hydrogen. After reaction at 2000 °C, DECS 21 has the least ordered structure, as indicated by d002, p, and g, consistent with the idea that the aromatic sheets have, at least in part, been “locked” in place by the oxygen atoms. After reaction at 2700 or 2900 °C, DECS 21 provides the most ordered structure, as indicated by these same parameters as well as Lc. This is consistent with its having the highest net hydrogen, implying smaller and more mobile aromatic sheets. Conclusions Graphitizability, based on approach of interlayer spacing to the ideal value for graphite and development of crystallite height, is in the order DECS 21 > PSOC 1461 > PSOC 1468. These anthracites clearly differ in their ability to graphitize by heat treatment at ambient pressure. By 2700 °C the interlayer spacing is largely established (degree of graphitization 0.88-0.94), since improvements by increasing graphitization temperature to 2900 °C are small at best. However, the crystallite stacking continues to develop and significant changes are achieved at 2900 vs 2700 °C. The significant change in structure in the temperature interval 2000-2700 °C suggests that there may be a “graphitization jump” at some temperature
