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Consideration on Coal Plasticity by Referring to the Distribution and Structural Properties of Skeletal and Volatile Fractions from Two Different Coals in Both Open and Closed Heat-Treating Systems Koh Kidena, Masataka Hiro, 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 October 25, 2004
Coal plasticity was investigated using two coals, a strongly coking coal (Goonyella; GNY) and a slightly coking coal (Witbank; WIT) by the heat treatment followed by solvent extraction and the analyses of the resulting fractions. The two different methods of heat treatment were employed: one was heat treatment under nitrogen flow and the other one was heat treatment in a batch container. The heat-treatment followed by pyridine/CS2 extraction gave volatile, solventsoluble fraction and solvent-insoluble fraction, the last one corresponding to the skeletal component during development of plasticity while the volatile + soluble fraction is believed to act as a lubricant against the skeletal component. The elemental analyses, the measurements of FT-IR and solid-state 13C NMR spectra, the measurement of swelling index, and the estimation of the amount of transferable hydrogen were conducted for the samples obtained to elucidate how coal plasticity appears and develops. In the open and closed systems, WIT tended to be pyrolyzed at the early stage of plasticity; however, GNY was pyrolyzed little in the open system and a little in the closed system, respectively. The amount of transferable hydrogen and swelling index of solvent-insoluble fractions (PI) strongly depended on the heat-treatment temperature in the open and closed heat-treating systems, respectively. In the closed system, WIT showed the relatively higher swelling index of PI along with fairly small amount of transferable hydrogen while GNY coal showed lower swelling index and could keep a lot of transferable hydrogen and low-molecularweight component, which significantly released at the latter stage of plasticity.
Introduction Plastic property of coal is so complex that it has not been completely understood yet. Since it cannot be described by only physical structural changes, it should be explained in view of chemical reactions such as bond cleavage, hydrogen transfer, and cross-linking reactions. Two principal mechanisms of coal plasticity have been proposed so far:1-8 metaplast theory and γ-compound (the inherent solvent-soluble fraction in coal) theory. The authors have been investigating coal plasticity consecutively from a chemical point of view.9-11 In a previous study,9,10 the authors explained plastic phenomena in terms of the amount of transferable hydrogen * Corresponding author tel: +81-6-6879-7361; fax: +81-6-68797362; e-mail:
[email protected]. (1) Elliot, M. A. Chemistry of Coal Utilization, 2nd Suppl. Vol.; Wiley-Interscience: New York, 1981; p 348. (2) Fitzgerald, D. Trans. Faraday Soc. 1956, 362. (3) Chermin, H. A. G.; van Krevelen, D. W. Fuel 1957, 36, 85. (4) van Krevelen, D. W. Coal; Elsevier: Amsterdam, 1993. (5) Ouchi, K. Fuel 1961, 40, 485. (6) Ouchi, K.; Itoh, H.; Itoh, S.; Makabe, M. Fuel 1989, 68, 735. (7) Fortin, F.; Rouzaud, J. N. Fuel 1993, 72, 245. (8) Fortin, F.; Rouzaud, J. N. Fuel 1994, 73, 795. (9) Nomura, M.; Kidena, K.; Hiro, M.; Murata, S. Energy Fuels 2000, 14, 904. (10) Kidena, K.; Katsuyama, M.; Murata, S.; Nomura, M.; Chikada, T. Energy Fuels 2002, 16, 1231. (11) Katsuyama, M.; Kidena, K.; Yokoyama, T.; Matsumoto, K.; Murata, S.; Nomura, M.; Chikada, T. Fuel (in press).
that is applicable for decomposition reaction occurring between softening temperature and maximum fluidity temperature. On the basis of these researches, the appearance of coal plasticity can be considered as follows: the solvent-soluble fraction extracted from heat-treated coal acts as material-like lubricant against the large-molecular-weight fraction (skeletal component). In other words, the skeletal component can show plasticity with the assistance of low-molecular-weight component under thermal conditions. This concept is based on metaplast theory; however, the theory does not refer to a chemical view. In the present study, the authors discussed the chemistry of plasticity by analyzing metaplast and solid materials. The fractionation of heat-treated coal into a soluble fraction and a residue was performed by Takanohashi et al.12 They showed a very good correlation between the amount of soluble fraction and coal fluidity and argued that the lighter fraction could solublize the heavier fraction, in turn. Recently, the other experiments concerning heat treatment of coking coal were conducted by Maroto-Valer et al.13,14 using NMR spectroscopy. High-temperature insitu 1H NMR and solid-state 13C NMR of partially (12) Takanohashi, T.; Yoshida, T.; Iino, M.; Katoh, K.; Fukada, K. Energy Fuels 1998, 12, 913. (13) Maroto-Valer, M. M.; Andre´sen, J. M.; Snape, C. E. Energy Fuels 1997, 11, 236.
10.1021/ef0499022 CCC: $30.25 © 2005 American Chemical Society Published on Web 12/15/2004
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Table 1. Properties of the Sample Coals (wt %, daf) Goonyella (GNY) Witbank (WIT) a
(wt %, db)
C
H
N
S
Oa
H/C
ash
87.3 82.5
5.3 5.0
1.9 2.0
0.7 0.7
4.8 9.8
0.73 0.72
8.8 8.1
By difference. Table 2. Gieseler Properties of the Sample Coals Gieseler temp (°C)a
GNY WIT
ST
MFT
RT
Gieseler max fluidity/log(ddpm)
397 412
456 432
498 446
2.99 0.60
a ST, softening temperature; MFT, maximum fluidity temperature; RT, resolidification temperature, which are determined by Gieseler plastometer.
carbonized coal samples were measured independently. The former was very effective to observe the ratio of rigid components to fluid components, this measurement being known as PMRTA (proton magnetic resonance thermal analysis),15,16 while the latter one focused on the analysis of structural features of the semicokes. The present paper aims at the detailed analyses of skeletal and low-molecular-weight components derived from slightly coking coal and strongly coking coal. The skeletal and low-molecular-weight components were obtained by the heat treatment followed by the solvent extraction of respective heat-treated two coals. In a theory concerning coal plasticity, chemical structural information and chemical reactions of both skeletal and low-molecular-weight components are very important since plastic phenomena essentially reflect chemical transformation of coal organic materials. Experimental Section Coal Samples. Coal samples used in this work are a strongly coking coal, Australian Goonyella (GNY) coal, and a slightly coking coal, South African Witbank (WIT) coal. GNY coal has much higher Gieseler maximum fluidity than WIT coal. Carbon content and Gieseler properties of these coals are summarized in Tables 1 and 2, respectively. These coals had been characterized well in the previous paper.9 All samples for the experiments were ground under 100 mesh and dried under reduced pressure at 60 °C for 6 h or more. Heat Treatment of Coal. Two different methods of the heat treatment (HT) were applied: an open system under nitrogen flow in a quartz tube and a closed system under nitrogen atmosphere in a batch container. HT in the open system was performed with a tubular electric furnace. Three grams of coal was placed at the center of the electric furnace. After the inside of quartz tube was purged with nitrogen flow, the coal sample was heated to the determined temperature at 3 K/min of heating rate. The HT was conducted up to Gieseler plastometer related temperature such as softening temperature (ST), maximum fluidity temperature (MFT), and resolidification temperature (RT). These temperatures are shown in Table 2. When the sample reached the determined temperature, the quartz tube was removed from the electric furnace immediately, and then it was quenched under a continuous flow of nitrogen to obtain semicoke. (14) Maroto-Valer, M. M.; Atkinson, C. J.; Willmers, R. R.; Snape, C. E. Energy Fuels 1998, 12, 833. (15) Lynch, L. J.; Webster, D. S.; Sakurovs, R.; Barton, W.. A.; Maher, T. P. Fuel, 1988, 67, 579. (16) Lynch, L. J.; Sakurovs, ; Webster, D. S.; Redlich, P. J. Fuel 1988, 67, 1036.
HT in the closed system was performed with an autoclave. Five grams of coal sample was placed into a hastelloy autoclave container, and then the inside was pressurized to 1.0 MPa of nitrogen. This container was put into an electric furnace and heated in a similar way to the procedure mentioned above. After HT, the semicoke sample was recovered from the container, which had been quenched in cold water. Solvent Extraction. Coal samples and semicoke samples obtained by the HT were extracted with pyridine/CS2 mixed solvent (1:1 by volume) under ultrasonic irradiation at room temperature for 30 min. Insoluble portion was separated from soluble portion by a centrifugation and a decantation. The procedure for the extraction was repeated more than three times until the solution phase decolorized. A soluble fraction was obtained as a solid material after solvent removal. However, contaminated amount of pyridine was remained in both insoluble and soluble fractions. To remove the trace amount of pyridine, both fractions were treated with a mixture of methanol and water (4:1 by volume) under ultrasonic irradiation for 30 min. The solids obtained were dried under reduced pressure and submitted to the following experiments and analyses. In this paper, the insoluble material was denoted as PI, and the soluble material was called as PS. Estimation of the Amount of Transferable Hydrogen. A mixture of coal sample or PI fraction and anthracene (1:1, wt/wt, total 200 mg) was heated to 420 °C for 5 min in a sealed glass tube. The resulting mixture was analyzed by GC to determine the yields of hydrogenated anthracenes. The amount of transferable hydrogen in the coal or PI fraction was estimated from the yields of hydrogenated anthracene according to the equation cited elsewhere.17 Solvent Swelling. A volumetric swelling index of the samples was measured according to the conventional method. After a 250 mg of the sample was packed into a glass tube, it was centrifuged at 4000 rpm for 10 min, and then the initial height of the sample (h1) was evaluated. Excess amount of pyridine was added into the glass tube, and it was kept at 40 °C for 24 h. After the time passed, the tube was removed from the water bath, and the sample was centrifuged again. The resulting height of the sample (h2) was determined, and the volumetric swelling index (Qv) was calculated by the equation of Qv ) h2/h1. Measurement of 13C NMR Spectra. All 13C NMR spectra of the solid samples were recorded on a Chemagnetics CMX300 spectrometer. The sample was placed into a rotor with 5 mm diameter. The sample mass was 80-120 mg. A 10.5 kHz of magic angle spinning was achieved using compressed air. The pulse width and the delay time for the next pulse were 1.5 µs (45° pulse) and 100 s, respectively. The FIDs were accumulated 3000 times or more; the resulting data being treated with GRAMS/32 (Galactic Industries Corp.) software in order to determine a distribution of various types of carbon in the samples. Measurement of FT-IR Spectra. Spectra were recorded on a Shimadzu FTIR-8100M spectrometer with DRS-8000 using diffuse reflectance method. The sample was mixed with dried KBr in the ratio of 1:8 by weight; the mixture being submitted to the measurement.
Results and Discussion HT in Open System Followed by Pyridine/CS2 Extraction. To separate skeletal component and lowmolecular-weight component except gas and tar, the resulting semicoke was submitted to the extraction by pyridine/CS2. In the present paper, PI is supposed to be skeletal component and PS is low-molecular-weight component as discussed in the latter section. Figure 1 (17) Kidena, K.; Murata, S.; Nomura, M. Energy Fuels 1996, 10, 672.
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Figure 1. Yields of low-molecular-weight components (volatile + pyridine/CS2-soluble: PS) and skeletal components (pyridine/CS2-insoluble: PI) heat-treated coal samples under nitrogen flow (open system). Table 3. Elemental Analyses (wt %, daf) of Pyridine/ CS2-Insoluble Fraction of Virgin or Heat-Treated Coals in the Open System GNY-virgin PI GNY-ST PI GNY-MFT PI GNY-RT PI WIT-virgin PI WIT-ST PI WIT-MFT PI WIT-RT PI a
C
H
N
O + Sa
H/C
85.3 85.3 86.3 87.8 81.0 82.9 82.2 86.3
4.9 4.5 4.3 4.0 5.0 4.4 4.2 3.7
1.8 2.4 2.1 2.1 2.2 2.3 2.4 2.4
8.0 7.8 7.3 6.1 11.8 10.4 11.2 7.6
0.69 0.63 0.60 0.55 0.74 0.64 0.61 0.51
Figure 2. FT-IR spectra of the fractions obtained from heattreatment in an open system and solvent extraction. The values next to the sample ID are the ratios of aliphatic C-H to aromatic C-H.
By difference.
shows the distribution of volatile (gas and tar), PS and PI of semicokes obtained by heating two coals up to ST, MFT, and RT along with the extraction results of virgin coals. PS of GNY-virgin coal is almost retained in the HT at ST while volatile materials (gas and tar) in WIT coal was lost before softening. This finding with WIT is indicating that decomposition reaction took place significantly up to ST. With GNY coal, such the decomposition reactions occur at the temperature range between ST and MFT based on the finding that volatile materials produced mostly from ST to MFT. Analyses of PI Fraction Obtained from HT in Open System. Table 3 shows the elemental analyses of PI fractions obtained from virgin coals and from heated coals up to ST, MFT, and RT, respectively. As described above, PI is considered to be skeletal component. The results clearly indicated that H/C ratio decreases greatly from WIT-virgin PI to WIT-ST PI and from WIT-MFT PI to WIT-RT PI. FT-IR spectra of these PI fractions were shown in Figure 2. The value cited at each spectrum is the height ratio of aliphatic C-H (2956 cm-1) to aromatic C-H (3040 cm-1). In the case of WIT, the ratio decreased steeply from WIT-virgin PI (3.9) to WIT-ST PI (1.8), this being in good agreement with the fact that relatively great amount of volatile materials released before softening (Figure 1) because volatile material contains alkane and/or alkene in greater amount than the residue. This estimation is true in considering the change observed from GNY-virgin PI to GNY-ST PI: the ratio of aliphatic C-H to aromatic C-H varied from 2.3 to 1.8 even with a small amount of tar fraction. It is interesting to note that this ratio in WIT-virgin PI (3.9) is very close to those in GNY-ST PS (3.8) and WIT-ST PS (3.6). Solid-state 13C NMR spectra of PI fractions were measured as shown in Figure 3. Deconvolution of these
Figure 3. Solid-state 13C NMR spectra of insoluble fraction (PI) of coals or semicokes.
spectra was conducted to give the distribution of eight different types of carbon. The distribution and fa values were shown in Table 4. The error of these values could be estimated as 5% of each value so that the detailed structural changes are difficult to be mentioned; however, the tendency for the structural changes would be addressed. A great increase of fa was observed for WIT coal from WIT (0.79) to WIT-ST (0.90) while for GNY coal, a relatively great increase was observed from GNY-ST (0.87) to GNY-MFT (0.93). The corresponding significant decrease of the methylene group was confirmed: from 13.0 to 4.2 with WIT to WIT-ST and from 7.1 to 2.3 with GNY-ST to GNY-MFT. As for fa value, it is interesting to note that GNY-MFT and GNY-RT show the same value as WIT-MFT and WIT-RT, respectively. By the close look at Table 4 the authors suppose that dealkylation in WIT coal occurs with both homolytic bond cleavage reaction giving benzylic radicals and alkyl radicals, and dealkylation reaction of alkoxy group also occurs because decrease of methyl group is not so large as compared with the great decrease of methylene, and OCHn decreases to some extents. So the cleavage reaction that gives radical species requires transferable hydrogen. It is also suggested that a great decrease of methylene group from GNY-ST to GNY-MFT along with small change of the amount of methyl group during this heating is indicative of main reaction being dehydro-
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Energy & Fuels, Vol. 19, No. 1, 2005 227 Table 4. Distribution of Carbon Functional Groups by Solid-State
13C
NMR
distribution of carbon functionalities (mol %) PI samples
CdO
COOH COOR
Ar-O
Ar-C
bridgehead + Ar-H
-OCHn-
-CH2-
-CH3
faa
GNY-virgin GNY-ST GNY-MFT GNY-RT WIT-virgin WIT-ST WIT-MFT WIT-RT
∼0 ∼0 ∼0 ∼0 ∼0 ∼0 ∼0 ∼0
∼1
