Nuclear Magnetic Resonance and Ruthenium Ion Catalyzed

Heat treatment of coal was performed at 500, 600, and 700 °C to obtain .... on a Chemagnetics CMX-300 spectrometer with a 13C frequency of 75.55 MHz...
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Energy & Fuels 2004, 18, 1709-1715

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Nuclear Magnetic Resonance and Ruthenium Ion Catalyzed Oxidation Reaction Analysis for Further Development of Aromatic Ring Size through the Heat Treatment of Coking Coals at >500 °C Koh Kidena,* Koji Matsumoto, Satoru Murata, and Masakatsu Nomura Department of Applied Chemistry, Faculty of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan Received April 20, 2004. Revised Manuscript Received August 5, 2004

The development of aromatic cluster size in heat-treated coals by heating above 500 °C is discussed. Heat treatment of coal was performed at 500, 600, and 700 °C to obtain semicoke samples. The temperature range was above the resolidification temperature of the coal samples. For comparison, a strongly coking coal, Australian Goonyella coal, and a slightly coking coal, Chinese Enshu coal, were used as the sample coals. Analysis of the virgin coals and the semicoke samples with solid-state 13C NMR indicated that the strongly coking coal tended to develop the aromatic ring size to a greater degree than the slightly coking coal as the heat-treatment temperature increased. To examine this tendency, a ruthenium ion catalyzed oxidation reaction was applied to the semicoke samples to obtain information concerning the distribution of the aromatic ring size. The products from this reaction also implied that the semicoke samples obtained from the strongly coking coal had an aromatic ring of larger size on average. A schematic representation of the behavior of molecules in the two coal samples during heating was established.

Introduction In Japan, coal consumption for metallurgical coke production in the steel industry is still predominant although, recently, several new coal-fired power plants entered into operation with consumption of increasing amounts of coal. High-quality coking coal is indispensable for the production of coke with excellent properties such as high mechanical strength and adequate porosity. However, the strongly coking coal is becoming higher priced than others, and should be less minable. Japan must import coking coals from various countries; therefore, blended coals are usually submitted to the coke-making process. The strategy for coal blending is based on many empirical viewpoints, while scientific findings are believed to develop new criteria for obtaining high-quality coke from not only coking coals but also coking coals coupled with noncoking coals. Scientific clarification of plastic properties has been performed so far.1 Since thermoplastic properties of coal involve various chemical reactions of its organic portion, coal scientists focus on the explanation of coal plasticity in various ways.2-18 The importance of mobile * Author to whom correspondence should be addressed. Phone: +816-6879-7361. Fax: +81-6-6879-7362. E-mail: [email protected]. (1) Elliot, M. A. Chemistry of Coal Utilization, Second Supplementary Volume; Wiley-Interscience: New York, 1981; Chapter 6. (2) Fitzgerald, D. Trans. Faraday Soc. 1956, 362. (3) Solomon, P. R.; Best, P. E.; Yu, Z. Z.; Charpenay, S. Energy Fuels 1992, 6, 143. (4) van Krevelen, D. W. Coal; Elsevier: Amsterdam, 1993. (5) Ouchi, K.; Itoh, H.; Itoh, S.; Makabe, M. Fuel 1989, 68, 735. (6) Neavel, R. C. In Coal Science I; Gorbaty, M. L., Larsen, J. W., Wender, I., Eds.; Academic Press: London, 1982; p 1.

hydrogen and effect of oxidation on plasticity are one of the topics. An explanation with the knowledge of the chemical structure of coal was also focused on. Thus, the thermoplastic stage of coal has been widely investigated; however, studies concerning semicokes are limited. On the basis of the microscopic observations of semicoke, Fortin’s illustrative model of the coking process clearly showed molecular interaction.11 Chemical structural analysis of semicokes was conducted by several researchers.19-21 They utilized NMR spectroscopy to obtain structural information on semicoke (7) Oxley, G. R.; Pitt, G. J. Fuel 1958, 37, 19. (8) Yokono, T.; Obara, T.; Iyama, S.; Yamada, J.; Sanada, Y. Nenryo Kyokaishi 1984, 63, 239. (9) Clemens, A. H.; Matheson, T. W. Fuel 1992, 71, 193. Clemens, A. H.; Matheson, T. W.; Rogers, D. E. Fuel 1991, 70, 215. Clemens, A. H.; Matheson, T. W.; Lynch, L. J.; Sakurovs, R. Fuel 1989, 68, 1162. (10) Spiro, C. L. Fuel 1981, 61, 1121. Spiro, C. L.; Kosky, P. G. Fuel 1982, 61, 1080. (11) Fortin, F.; Rouzaud, J. N. Fuel 1993, 72, 245; 1994, 73, 795. (12) Solomon, P. R.; Best, P. E.; Yu, Z. Z.; Charpenay, S. Energy Fuels 1992, 6, 143. (13) Marzec, A.; Czajkowska, S.; Moszynski, J.; Schlten, H.-R. Energy Fuels 1992, 6, 97. Schulten, H.-R.; Marzec, A.; Czajkowska, S. Energy Fuels 1992, 6, 103. (14) Nakamura, K.; Takanohashi, T.; Iino, M.; Kumagai, H.; Sato, M.; Yokoyama, S.; Sanada, Y. Energy Fuels 1995, 9, 1003. (15) Kidena, K.; Murata, S.; Nomura, M. Energy Fuels 1996, 10, 672. (16) Nomura, M.; Kidena, K.; Hiro, M.; Murata, S. Energy Fuels 2000, 14, 904. (17) Kidena, K.; Katsuyama, M.; Murata, S.; Nomura, M. Energy Fuels 2002, 16, 1231. (18) Kidena, K.; Murata, S.; Nomura, M. Energy Fuels 1998, 12, 782. (19) Freitas, J. C. C.; Bonagamba, T. J.; Emmerich, F. G. Energy Fuels 1999, 13, 53. (20) Maroto-Valer, M. M.; Atkinson, C. J.; Willmers, R. R.; Snape, C. E. Energy Fuels 1998, 12, 833.

10.1021/ef049901+ CCC: $27.50 © 2004 American Chemical Society Published on Web 10/23/2004

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Table 1. Properties of the Sample Coals

a

coal

[C] (wt %, daf)

Gieseler maximum fluidity (ddpm)

ST

Gieseler Temperaturesa (°C) MFT

RT

GNY ENS

85.7 79.8

912 21

406 396

457 431

496 459

ST ) softening temperature, MFT ) maximum fluidity temperature, and RT ) resolidification temperature.

samples. Although many other methods such as X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, and FT-IR were applied to the analysis of semicoke,22-28 the information concerning the structural changes after resolidification was not enough. In this study, we used solid-state NMR to analyze semicoke samples prepared at different temperatures in a chemical view. Subsidiary information about the chemical structure and oxidative degradation (ruthenium ion catalyzed oxidation reaction) was obtained to analyze the products. Both experiments led to a conclusion about chemical structural changes from coal to semicoke during heat treatment above the resolidification temperature. Then, a comparison of the coking behavior of two kinds of coal samples, a strongly and a slightly coking coal, was also done. Experimental Section Coal Samples. Two kinds of coal samples were used in this study: Australian Goonyella coal (abbreviated as GNY in the following text) and Chinese Enshu coal (ENS). The carbon contents and the plastic properties of these coals are summarized in Table 1. GNY coal is representative of a strongly coking coal that is very effective in producing high-quality coke. On the other hand, ENS coal is a slightly coking coal, which is required in the coke-making industry to be utilized in larger quantities. Preparation of Semicoke. Coal samples (4 g) were placed in a quartz tube with an inner diameter of 8 mm and a length of 200 mm. The quartz tube was packed into a stainless steel container consisting of Swagelok parts under a nitrogen atmosphere. The container was heated to the determined temperature (500, 600, and 700 °C) at a heating rate of 3 °C/ min. When the temperature reached the determined value, the stainless steel container was quenched in an ice bath to stop the reactions during heating. A solid residue was recovered as semicoke, and a tarry fraction was obtained by washing the inside of the reactor with acetone. Volatile material was not collected in this study. The semicoke samples were submitted to NMR measurements and ruthenium ion catalyzed oxidation reaction, respectively, as shown below. Solid-State 13C NMR Measurements and Spectral Analyses. The solid-state 13C NMR spectra were recorded on a Chemagnetics CMX-300 spectrometer with a 13C frequency of 75.55 MHz. For the quantitative measurement, the singlepulse excitation/magic angle spinning (SPE/MAS) method was employed with 83 kHz of proton decoupling. The sample was (21) Hu, J. Z.; Solum, M. S.; Taylor, C. M. V.; Pugmire, R. J.; Grant, D. M. Energy Fuels 2001, 15, 14. (22) Lu, L.; Sahajwalla, V.; Kong, C.; Mclean, A. ISIJ Int. 2002, 42, 816. (23) Yoshizawa, N.; Maruyama, K.; Yamada, Y.; Ishikawa, E.; Kobayashi, M.; Toda, Y.; Shiraishi, M. Fuel 2002, 81, 1717. (24) Davis, K. A.; Hurt, R. H.; Yang, N. Y. C.; Headley, T. J. Combust. Flame 1995, 100, 31. (25) Johnson, C. A.; Patric, J. W.; Thomas, K. M. Fuel 1986, 65, 1284. Green, P. D.; Johnson, C. A.; Thomas, K. M. Fuel 1983, 62, 1013. (26) Jones, J. M.; Pourkashanian, M.; Rena, C. D.; Williams, A. Fuel 1999, 78, 1737. (27) Villegas, J. P.; Valle, C. J. D.; Calahorro, C. V. Serrano, V. G. Carbon 1998, 36, 1251. (28) Marzec, A. Carbon 2000, 38, 1863.

put into a rotor with a 5 mm diameter, and its mass was 80120 mg. MAS was performed with a speed of 10.5 kHz. The pulse width was 1.5 µs (45° pulse), and 3000 scans of the free induction decays were accumulated for each measurement. The pulse delay time for each accumulation was determined by the spin-lattice relaxation time of carbon nuclei (T1C), which was briefly measured with the conventional Torchia method.29 Each data point is not shown here, but the pulse delay was determined as 5T1C. Manipulation of the spectra was conducted using a Sun workstation with Spinsight (version 3.5.2.3) and GRAMS/32 (Galactic Industries Corp.) software. Deconvolution of the spectra was performed with the latter software with a Gaussian curve, using the following assignments and parameters: carbonyl carbon (CdO, CHO; peak center, >200; full width at half-maximum, 12-15; the values are given in parts per million referred to TMS), carboxyl carbon (COOH, COOR; 187, 178; 12-15), oxygen-bonded aromatic carbon (Ar-O; 167, 153; 15-16), carbon-bonded aromatic carbon (Ar-C; 140; 16-17), bridgehead aromatic carbon and protonated aromatic carbon (bridgehead, Ar-H; 126, 113, 100; 17-18), oxygen-bonded aliphatic carbon (aliphatic C-O; 93, 70, 56; 16-18), methylene and methyne carbon (CH2; 40, 31; 16-17, 11-13), and methyl carbon (CH3; 20, 13; 10-12). Spinning sidebands (SSB) appeared on each spectrum, these being taken into account in calculating the amount of aromatic carbon. A series of measurements with the dipolar dephasing (DD) method were also performed.30-32 In this case, the pulse width was 1.5 µs, 400 scans of FID were accumulated, and 190n (n ) 1-10) µs of dephasing time was applied. Ruthenium Ion Catalyzed Oxidation (RICO) Reaction. This reaction has been applied to various kinds of coals.33-36 The basic procedure is similar to that in previous papers, but the strategy for recovering the products was a little different. Carbon dioxide was captured by an adsorbent during the reaction, and three fractions were recovered from the reaction mixture: ether-soluble, water-soluble, and water-insoluble fractions. The water-soluble and water-insoluble fractions were submitted to solid-state 13C NMR measurements.

Results and Discussion Elemental Composition of Semicokes. Semicoke samples were obtained by heat treatment of two kinds of coal at three different temperatures. The elemental composition of the virgin coals and semicoke samples (29) Torchia, D. A. J. Magn. Reson. 1978, 30, 613. (30) Opella, S. J.; Frey, M. H. J. Am. Chem. Soc. 1979, 101, 5854. (31) Murphy, P. D.; Gerstein, B. C.; Weinberg, V. L.; Yen, T. F. Anal. Chem. 1982, 54, 522. Murphy, P. D.; Cassady, T. J.; Gerstein, B. C. Fuel 1982, 61, 1233. (32) Love, G. D.; Law, R. V.; Snape, C. E. Energy Fuels 1993, 7, 639. (33) Stock, L. M.; Tse, K.-T. Fuel 1983, 62, 974. Stock, L. M.; Wang, S. H. Fuel 1985, 64, 1713; 1986, 65, 1552; 1987, 66, 921; Energy Fuels 1989, 3, 533. Muntean, J. V.; Stock, L. M. Energy Fuels 1991, 5, 767; 1993, 7, 704. (34) Mallya, N.; Zingaro, R. A. Fuel 1984, 63, 423. Ilsley, W. H.; Zingaro, R. A.; Zoeller, J. H., Jr. Fuel 1986, 65, 1216. (35) Standen, G.; Boucher, R. J.; Eglinton, G.; Hansen, G.; Eglinton, T. I.; Larter, S. R. Fuel 1992, 71, 31. (36) Murata, S.; Uesaka, K.; Inoue, H.; Nomura, M. Energy Fuels 1994, 8, 1379. Artok, L.; Murata, S.; Nomura, M.; Satoh, T. Energy Fuels 1998, 12, 391. Murata, S.; Tani, Y.; Hiro, M.; Kidena, K.; Artok, L.; Nomura, M.; Miyake, M. Fuel 2001, 80, 2099.

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Table 2. Elemental Analyses of the Coals and Semicoke Samples Elemental Analyses (wt %, daf) sample

C

H

N

S

O(diff)

H/C

O/C

GNY GNY-500 GNY-600 GNY-700 ENS ENS-500 ENS-600 ENS-700

85.7 87.7 91.0 92.3 79.8 82.9 89.2 91.4

5.2 4.4 3.2 2.7 5.3 4.7 3.8 2.8

1.9 2.0 2.2 2.0 1.6 1.7 1.7 1.8

0.5 0.5 0.4 0.7 0.3 0.6 0.5 0.5

6.7 5.4 3.2 1.9 13.0 10.1 4.8 3.5

0.73 0.60 0.42 0.35 0.75 0.68 0.51 0.36

0.06 0.05 0.03 0.02 0.12 0.09 0.04 0.03

Table 3. Yields of Semicoke, Tar, and Volatile Materials Yield (wt %) coal-HTTa

semicoke

tar

volatile materials

GNY-500 GNY-600 GNY-700 ENS-500 ENS-600 ENS-700

92.9 89.6 84.8 84.4 78.4 71.9

2.4 2.0 3.8 5.6 8.3 8.3

4.7 8.4 11.4 10.0 13.3 20.8

a

HTT ) heat-treatment temperature.

and the yield of semicoke, tar, and volatile material are shown in Tables 2 and 3, respectively. For both coals, the carbon contents increased with increasing heattreatment temperature, while the hydrogen and oxygen contents decreased. A comparison of semicokes from two coals indicated that ENS-derived semicoke apparently retained a greater amount of oxygen than GNY-derived semicoke at the same heat-treatment temperature. The virgin ENS coal is richer in oxygen functionalities than GNY coal, some of these oxygen functionalities reacting more in the case of ENS than GNY. Hydroxyl and carboxyl groups are candidates for the oxygen functionalities reacted up to 700 °C. However, at 700 °C, less reactive oxygen functionalities such as in furan-type structures might be retained more in ENS-derived semicoke. As a whole, differences in elemental composition were small but are meaningful for discussion. The yields of semicoke are utilized in the following section to calculate the change of the amount of carbon after heat treatment. NMR Analysis of Semicokes. Solid-state 13C NMR measurements gave invaluable information concerning the chemical structure. In the present paper, the semicoke samples obtained by closed-system heat treatment were submitted to solid-state NMR analyses to evaluate chemical structural changes during heat treatment. SPE/MAS measurements can give the quantitative carbon distribution of the samples; however, such an analysis cannot provide clear information about the fractions of bridgehead and tertiary (protonated) aromatic carbons. These types of carbons appeared at 100, 113, and 126 ppm (the position of the center of the peaks); therefore, it could be assumed that the sum of the peak area at these positions showed the sum of the fractions of the bridgehead and the tertiary aromatic carbons. To discriminate these types of carbons, the authors measured a series of NMR spectra with the DD method. According to the DD method,30-32 tertiary aromatic carbon and quaternary aromatic carbon can be separated by the difference in dephasing behavior originating from neighboring hydrogen atoms. Although NMR measurements with conventional SPE/MAS and DD methods were successful for the

Figure 1. Solid-state 13C NMR spectra of the virgin coals and semicoke samples. “/” indicates spinning sideband.

samples obtained by heat treatment at 700 °C, it became difficult for the samples obtained at 750 °C or higher temperature. The difficulties of the measurements probably come from the fact that the semicoke samples obtained by high-temperature treatment came to have electron conductivity. In the experiments, electrical tuning for minimizing the reflected pulse and highspeed spinning of the sample had some trouble. Dilution with KBr by mixing was effective for measuring spectra of the semicoke samples obtained at 750 °C. However, the signal-to-noise ratio was not enough; therefore, the obtained spectra were not appropriate for getting structural information. Under these circumstances, this study dealt with the semicoke samples obtained by heat treatment up to 700 °C. Solid-state 13C NMR spectra of the virgin coals and the semicokes are shown in Figure 1. The fact that a significant decrease of the aliphatic peak intensity was observed for the semicokes obtained from both coals confirms that aliphatic portions of the coal molecule decompose under heating. That is, at 700 °C, most of the volatile material had been split off, so the remaining semicoke samples were rich in aromatic carbon. To discuss detailed structural changes from the virgin coal to the semicoke, the carbon distribution of each sample was calculated as shown in Figure 2. At this time, the yield of the semicoke and carbon content was taken into account so that the net changes of the amount of each carbon could be estimated. Thus, the basis of all values in Figure 2 was the total amount of carbon in the virgin coal. A decrease of a certain type of carbon originated from not only structural changes during heating but also the volatilization of the tar fraction or gases. Therefore, at the initial stage of heat treatment, up to 500 °C, the volatile fractions with high aliphaticity escaped. An initial increase of Ar-H in the case of GNY coal indicated that a significant amount of transferable hydrogen was lost from the naphthenic portions to give a new aromatic structure since GNY has a relatively large amount of transferable hydrogen which acted as a radical stabilizer in the chemical reactions that occurred during the plastic range. At 600 °C, significant structural changes were observed: bridgehead aromatic carbon increased apparently. Since the volatile materials escaped from coal, any type of carbon must decrease if there is no structural change. Even at 700 °C, several Ar-O carbons remained while oxygen-bonded aliphatic carbons (OCHn) almost disappeared. As was speculated in the previous section, this implies that semicoke has a furan-type structure with Ar-O carbon.

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Figure 2. Change of the amount of each type of carbon during heat treatment of GNY and ENS coals. The total amount of carbon in the virgin coal is equivalent to 100% for each coal.

Figure 3. Average number of aromatic carbons in a cluster for the virgin coals and the semicokes.

On the basis of the data in Figure 2, the average size of the aromatic ring could be estimated according to the method presented by Pugmire et al.37 The parameter χb, the ratio of bridgehead aromatic carbon to total aromatic carbon, is known to correlate with the number of carbons in an aromatic cluster; for example, χb ) 0.2 for naphthalene (2 bridgehead aromatic carbons and 10 aromatic carbons), and χb ) 0.375 () 6/16) for pyrene. Figure 3 shows the average number of carbons in the aromatic clusters for the virgin coals and the semicoke samples. At 500 °C, the number of aromatic carbons in an average cluster in the semicoke obtained from ENS coal was almost the same as that from GNY coal although GNY virgin coal has merely a larger aromatic ring compared with ENS virgin coal. In the case of ENS coal, a lot of tar and volatile materials were split off; therefore, the apparent size of the aromatic ring for ENS-500 increased. Nevertheless, the following heat treatment brought about significant differences: semicokes obtained by heat treatment at 600 °C had a larger aromatic ring than those obtained at 500 °C, and there were clear differences between GNY-derived semicoke (37) Solum, M. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989, 3, 187.

and ENS-derived semicoke especially when the heat treatment temperature was 700 °C. For GNY-700 and ENS-700, the average number of aromatic carbons in a cluster was 38.5 and 32.6, respectively. These values correspond to the polycondensed aromatic hydrocarbon with 12-13 and 9-10 rings, respectively. Thus, at higher heat-treatment temperature, the discrepancy between the strongly and the slightly coking coals became clear, the aromatic ring in the strongly coking coal tending to develop more than that in the slightly coking coal. Similar analysis of the semicoke samples obtained by heat treatment at a temperature higher than 700 °C seems to be interesting; however, NMR measurements were not successful for those samples, unfortunately. As supporting information, XRD of the semicokes (heat-treatment temperature up to 1000 °C) was measured, the results being shown in Figure 4. XRD measurements were performed with a MacScience M18XHF-SRA diffractometer with Cu KR X-ray radiation. A conventional Scherrer equation was applied for evaluating the crystallite parameter: peak treatments (broadening and deconvolution) were conducted manually, and the peak width and position of the (002) band were taken into account for evaluating Lc (average height of a stack of layers consisting of aromatic rings) and d (layer spacing), respectively. Both parameters, Lc and d, supported that GNY-derived semicokes had a highly stacked structure of aromatic rings. Large aromatic rings make highly stacked structures; therefore, GNY-derived semicokes seemed to have a larger aromatic ring than ENS-derived semicokes. The parameter La, layer size, is well-known to achieve direct comparison of the aromatic ring size with NMR data; however, the La values seem to have an inherent error since the peak height was too small and the sharpness was too poor for the calculation of La. RICO Products Obtained from Semicokes. NMR measurements performed in this study gave us information on the average structural parameter of the

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Figure 5. Solid-state 13C NMR spectra of the water-insoluble fraction of the RICO product of semicoke samples. “/” indicates spinning side band. Table 4. Parameter Concerning the Ring Size of the Water-Insoluble Fraction of the RICO Product of Semicoke Samples

Figure 4. XRD-derived parameters of the virgin coal and semicokes: (a) Lc ) average size of the stack of layers; (b) d ) layer spacing.

semicoke samples, namely, the size of the average aromatic cluster in the semicoke, while there is no information about the aromatic size distribution in these samples. Therefore, as supplementary knowledge, we tried to apply ruthenium ion catalyzed oxidation reaction to the semicoke samples. This reaction is very effective to analyze aliphatic side chains or bridges because sp2 carbons are selectively attacked by ruthenium catalyst to give carbon dioxides and carboxylic acids. For example, 1 mol of toluene gives 5 mol of carbon dioxide and 1 mol of acetic acid, whose methyl carbon is derived from the methyl group in the toluene molecule. On the other hand, a highly condensed polyaromatic hydrocarbon decomposes partly to give an aromatic polycarboxylic acid. This behavior can be explained as follows: carboxyl groups attached to an aryl group deactivate the aromatic molecule toward oxidation reaction, aromatic carboxylic acids with many carboxyl groups thus surviving. A polyaromatic hydrocarbon gives aromatic polycarboxylic acids as watersoluble and water-insoluble fractions. The water-soluble fraction contains carboxylic acids with lower molecular weight, which were derived from a smaller polyaromatic hydrocarbon in the semicoke samples. On the other hand, the water-insoluble portion corresponds to the oxidation product with higher molecular weight. In the present study, both water-soluble and -insoluble fractions were analyzed by solid-state 13C NMR spectroscopy to obtain their structural information. NMR spectra of the water-insoluble fraction obtained as the RICO product of the semicoke samples are shown in Figure 5. Judging from the peak intensity at around

sample

χb

average number of carbons in a cluster

GNY-600 GNY-700

0.47 0.63

23 44

ENS-600 ENS-700

0.42 0.61

21 42

200 ppm, the water-insoluble fraction had a very small amount of carboxyl groups. This fraction might be derived from the extremely large aromatic cluster portion of the semicoke. The parameter χb and the average number of aromatic carbons in a cluster (Table 4) indicate that this fraction had very large aromatic rings compared with the semicoke samples themselves. Additionally, GNY-derived samples had a larger aromatic ring than ENS-derived samples. Therefore, it could be concluded that GNY-derived semicoke had a more developed aromatic ring system than ENS also for the heavier portion of the semicoke. Next, we analyzed the water-soluble fraction of the oxidation product from the semicoke samples. Figure 6 shows their NMR spectra. Three distinct peaks are observed. These peaks can be assigned as Ar-C + Ar-H (110-140 ppm) and COOH bonded to aromatic carbon (160-170 ppm). The peak intensity for COOH bonded to aliphatic carbon (180-190 ppm) is not significant. The carbon distribution, which was calculated by each peak area, gave us structural images of the watersoluble fraction. Here, we focused on the ratio of COOH(-Ar) to Ar-C + Ar-H since this ratio can be an index of the aromatic ring size. On the assumption that similar numbers of carboxylic groups are on the peripheral position of the aromatic cluster in water-soluble molecules of the RICO products, a small ratio of COOH(-Ar) to Ar-C + Ar-H means that the core part of the water-soluble fraction consisting of an aromatic ring system is large. For example, benzenehexacarboxylic acid has six Ar-C and six COOH(-Ar) groups so that the ratio becomes 1.0, and biphenyldecacarboxylic acid gives a ratio of 0.83 () 10/12). The ratios are shown in Table 5, the data indicating that the water-soluble fraction of the RICO product from GNY coal had a larger aromatic ring system than that from ENS coal.

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Figure 6. Solid-state

13C

Kidena et al.

NMR spectra of the water-soluble fraction of the RICO product of semicoke samples.

Table 5. Ratio of COOH(-Ar) to Ar-C + Ar-H in the Water-Soluble Fraction of the RICO Product of Semicoke Samples sample

COOH(-Ar)/(Ar-C + Ar-H)

GNY-600 GNY-700

1.10 0.91

ENS-600 ENS-700

1.16 1.01

As mentioned above, not only the average size of the aromatic ring of the whole semicoke but also the larger and smaller portions of the semicoke samples showed that GNY-derived semicoke tended to have a larger aromatic ring than ENS-derived semicoke. Structural Changes after Resolidification. On the basis of the information concerning the semicoke structure as mentioned above, the structural changes after the resolidification step of heat treatment of the two coals are discussed. It is difficult to describe the

detailed chemical structure of coal and its changes. Therefore, structural images were established for helping one to understand the difference in structural changes between two coal samples, GNY and ENS coals. Figure 7 represents the scheme of molecular images of the virgin coals and the semicoke samples. According to the NMR data shown in Figure 2, GNY virgin coal has slightly larger aromatic rings but a smaller amount of aliphatic side chains and bridges than ENS virgin coal, these being represented on the left side of Figure 7. In this figure, the size of the circle indicates the size of the aromatic cluster. A step of the heat treatment up to 500 °C corresponds to the plastic range of each coal. This step was discussed in previous papers in detail.15,16 GNY coal has higher plasticity than ENS coal, these properties making the molecular arrangements different: GNY-derived semicoke showed well-arranged aromatic clusters because of the high fluidity, while the

Figure 7. Schematic representation of molecular images after resolidification of (a) Goonyella and (b) Enshu coals.

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aromatic clusters in ENS-derived semicoke did not align well. Large-sized molecules were released as tar and volatile materials in the following heat treatment. Then, at a temperature of 700 °C, condensation reaction between aromatic nuclei accompanying the dehydrogenation of aromatic hydrogen begins to occur. Such a reaction strongly helps further development of the aromatic cluster size. However, since the condensation reaction is a solid-phase reaction, the arrangement of the aromatic clusters becomes important. In this sense, the GNY-derived semicoke sample tended to have a larger aromatic cluster than the ENS-derived semicoke. GNY coal is well-known to produce metallurgical coke with very high strength. One factor for making such coke can be the development behavior of large aromatic clusters as mentioned in the present paper.

treatment of the two kinds of coal up to 500, 600, and 700 °C was conducted. The resulting semicoke samples were analyzed by solid-state NMR and submitted to ruthenium ion catalyzed oxidation reaction. The results from both experiments indicated that GNY-derived semicoke samples had larger aromatic rings than ENS-derived semicoke samples. Therefore, one of the factors to produce good-quality coke is to form a wellrearranged structure just after resolidification. By considering the analytical results, we compared the structure of semicoke derived from a strongly coking coal with that from a slightly coking coal, this viewpoint lacking in previous studies. To help one to understand the difference in the coking process for the two coals, integrated schematic images for structural changes after resolidification were proposed.

Summary

Acknowledgment. This research was supported by a Grant-in-Aid provided by the Iron and Steel Institute of Japan.

To discuss the difference in thermal behavior, especially after resolidification, of a strongly coking coal, GNY coal, and a slightly coking coal, ENS coal, heat

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