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Energy & Fuels 2001, 15, 170-175
Dynamic Viscoelastic Measurement of Coal Extracts and Residues Takahiro Yoshida and Toshimasa Takanohashi* National Institute for Resources and Environment, Tsukuba, 305-8569, Japan
Masashi Iino Institute for Chemical Reaction Science, Tohoku University, Sendai, 980-8577, Japan
Kenji Katoh Steel Research Laboratories, Nippon Steel Corporation, Futtsu, 293-8511, Japan Received July 10, 2000. Revised Manuscript Received October 24, 2000
Goonyella coking coal was extracted with tetrahydrofuran (THF), chloroform, N-methyl-2pyrrolidinone (NMP), pyridine, and a 1:1 (v/v) carbon disulfide/N-methyl-2-pyrrolidinone (CS2/ NMP) mixed solvent. Temperature-variable dynamic viscoelastic measurement was carried out for the extracts and residues obtained. The thermoplasticity in residues obtained by chloroform or NMP extractions significantly decreased compared to that of the raw coal despite the fact that the extraction yields, 2.7 and 5.4 wt % (daf), respectively, were low. A further decrease in thermoplasticity was observed for residues obtained in high yield with pyridine (20.6 wt %) and CS2/NMP (42.8 wt %). On the other hand, the thermoplastic state was observed in the range 260-500 °C for the pyridine soluble (Sox-PS) fraction obtained from pyridine Soxhlet extraction, while the thermoplasticity for the pyridine insoluble fraction (PIMS) obtained from pyridine fractionation of the CS2/NMP extract fraction was significantly smaller and similar in value to that for residues left after chloroform or NMP extractions, despite the fact that the PIMS originally was a component of a solvent-soluble fraction. The role of solvent-soluble components on the thermoplasticity of coals is discussed based on these results and results of an earlier study.
Introduction Novel, efficient processes are needed to convert into coke low-quality coals that show little thermoplasticity during heating. These processes must be based on an understanding of the mechanism of coal thermoplasticity. It is well-known that coal thermoplasticity is affected by the presence of low-molecular-weight components that are soluble in chloroform or pyridine. Several theories have been proposed to explain the role of these components. One, the γ component theory,1-3 holds that chloroform solubles in the original coals (the γ component) disperse heavier components. While, the metaplast theory4,5 holds that low-molecular-weight components (metaplasts) generated by cleavage of covalent bonds during pyrolysis disperse coal fragments. The hydrogen-donating model6,7 holds that low-molecular* To whom correspondence should be addressed. (1) Shimmura, T. Fuel 1933, 12, 204. (2) Brown, H. R.; Waters, P. L. Fuel 1966, 45, 17. (3) Ouchi, K.; Tanimoto, K.; Makabe, M.; Itoh, H. Fuel 1983, 62, 1227. (4) Van Krevelen, D. W.; van Heerden, C.; Huntjens, F. J. Fuel 1951, 30, 253. (5) Chermin, H. A. G.; van Krevelen, D. W. Fuel 1957, 36, 85. (6) Neavel, R. C. Coal Science; Gorbaty, M. L., et al., Ed.; Academic Press: New York, 1982; p 1. (7) Yokono, T. Report of the Section of Coking Properties of Coals; The Iron and Steel Institute of Japan: Tokyo, 1985; p 83.
weight compounds such as the γ and metaplast components donate hydrogens to coal fragments generated during pyrolysis. Addition of hydrogen-donating compounds to coals was reported to increase their fluidities.8,9 Sakurovs and Lynch reported10 that coal fluidity is enhanced by aromatic-rich containing additives and proposed a model in which the additives solvate coal fragments. Seki et al. also reported11 that the addition of petroleum pitch to the extraction residue or original coal increased its thermoplasticity. Iino et al. found12,13 that a carbon disulfide/N-methyl2-pyrrolidinone (CS2/NMP) mixed solvent gives high extraction yields (40-80%) at room temperature for bituminous coals, which indicates that a significant amount of solvent-soluble constituents exist in the coals. Seki et al. reported11 that coal residues obtained from the CS2/NMP extraction showed almost no thermoplas(8) Clemens, H.; Matheson, T. W. Fuel 1995, 74, 57. (9) Maroto-Valer, M. M.; Andresen, J. M.; Snape, C. E. Fuel 1998, 77, 921. (10) Sakurovs, R.; Lynch, L. J. Fuel 1993, 72, 743. (11) Seki, H.; Kumagai, J.; Matsuda, M.; Ito, O.; Iino, M. Fuel 1989, 68, 987. (12) Iino, M.; Takanohashi, T.; Ohsuga, H.; Toda, K. Fuel 1988, 67, 1639. (13) Iino, M.; Takanohashi, T.; Obara, S.; Tsueta, H.; Sanokawa Y. Fuel 1989, 68, 1588.
10.1021/ef000150v CCC: $20.00 © 2001 American Chemical Society Published on Web 12/15/2000
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Energy & Fuels, Vol. 15, No. 1, 2001 171
Table 1. Ultimate and Proximate Analyses and Giesler Fluidity for Goonyella Coal ulitmate anlaysis, wt% (daf) coal
C
Goonyella
88.1
a
H
N
S
4.6
1.5
0.3
b
proximate analysis, wt% (db)
Oa
VM
ash
FC
5.5
23.4
9.8
66.8
c
Gieseler plastometry STb
(°C)
407
MFTc (°C)
RTd (°C)
MFe (log ddpm)
456
498
3.0
d
e
By difference. Softening temperature. Maximum fluidity temperature. Resolidification temperature. Maximum fluidity (Logarithm of the dial divisions per minute). Table 2. Extraction Yield and Various Analyses for the Residues ultimate analysis (wt%, daf)
c
extraction solvent
extraction yield (wt %, daf)
C
H
N
O + Sa
ash (wt %, db)
Aar/Aalb
raw coal THF chloroform N-methyl-2-pyrrolidinone (NMP) pyridine CS2/NMP mixed solvent
1.3c 2.7d 5.4c 20.6d 42.8c
88.1 85.9 86.3 85.2 85.4 84.3
4.6 4.9 5.0 5.0 4.9 4.6
1.5 2.1 2.1 2.2 2.1 2.1
5.8 7.6 6.6 7.6 7.6 9.0
9.8 8.7 9.0 9.2 10.0 14.2
0.35 0.42 0.44 0.48 0.70 0.52
a By difference. b The ratio of intensity of aromatic C-H stretching (3030 cm-1) to that of aliphatic C-H stretching (2920 cm-1). Extraction at room temperature. d Sohxlet extraction.
ticity. Takanohashi et al. found14 that the CS2/NMP extraction yields of heat-treated coals increased greatly at the initial softening stage and then decreased toward the resolidification stage. Furthermore, CS2/NMP extracts obtained from coking coals contained a considerable amount of heavy solvent-soluble componentss pyridine insoluble components in the CS2/NMP mixed solvent extractssthat showed a linear relationship with Gieseler fluidity.15 In contrast, the heavy components were present in low concentrations in low-quality, lowcoking coals.14,15 The mechanism by which heavy components affect coal thermoplasticity and the relation between the heavy and low-molecular-weight components are still unclear. Gieseler plastometry has been widely used to estimate coal thermoplasticity, which is related to the ability of a stirrer immersed in a coal to rotate under a constant torque at a heating rate of 3 °C/min in the range 300500 °C. Although this method is convenient, it is not sufficiently sensitive to use for non- or low-coking coals. In addition, this method cannot give information about viscoelastic properties at the thermoplastic state. A proton magnetic resonance thermal analysis (PMRTA) method16 can be used to measure mobilities of aliphatic and aromatic hydrogens in coals at the thermoplastic state. Although this method is very sensitive to changes of chemical structure, it cannot evaluate physical properties such as the viscoelasticity of coal. Recently, Takanohashi et al. measured17 the dynamic viscoelasticity of various coals using a rheometer. This method can be used to estimate the thermoplasticity of various coals, including low-coking coals, allows both viscous and elastic properties to be evaluated, and also gives information about the dilatation of coal. The maximum thermoplasticity temperatures estimated by this method were found to correlate well with maximum reflectance and carbon % of the coals.18 In the present study, the temperature-variable dynamic viscoelastic technique was used to investigate the role of solvent-soluble components on the thermoplasticity of extracts and residues obtained from Australian (14) Takanohashi, T.; Yoshida, T.; Iino, M.; Katoh, K.; Fukada, K. Energy Fuels 1998, 12, 913. (15) Yoshida, T.; Iino, M.; Takanohashi, T.; Katoh, K. International Journal of The Society of Materials Engineering for Resources 1999, 7, 301.
Goonyella coking coal by extraction with various solvents. Experimental Section Coal Sample. Australian Goonyella coking coal was ground to 4.5), which may be related to the presence of low-molecular-weight components. On the other hand, PIMS fraction had a very low fluidity, which is almost same as residues extracted with pyridine or the mixed solvent, despite the fact that the PIMS is part of the CS2/NMP extract (MS). Dynamic Viscoelastic Behaviors for Residues. Figure 2 show the dynamic viscoelastic behaviors for the raw coal (a) and the residues (b-f), and results are summarized in Table 4. Results for raw coal (Figure 2a) have been described elsewhere.17,18 Dynamic viscoelastic behaviors could be measured for all residues, unlike the case of Gieseler plastometry. In the case of THF residue (Figure 2b), compared to raw coal, broad peaks of G′ and G′′ decreased in the range 450-480 °C, which correspond to the thermoplastic stage. For the THF residue maximum tan δ, which shows the degree of thermoplasticity of the sample,17,18 was lower than that of raw coal. As shown in Figure 2c,d, and Table 4, results for chloroform and NMP residues were similar. The peaks for G′, G′′ and tan δ observed over the range of the thermoplastic state were greatly decreased compared to raw coal (Figure 2a) and the THF residue (Figure
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Energy & Fuels, Vol. 15, No. 1, 2001 173
Table 3. Giesler Plastometry for Raw Coal, its Residues, and Extracts Gieseler plastometry sample
extraction yield (wt%, daf)
Goonyella raw coal residues extracted with THF chloroform NMP pyridine CS2/NMP mixed solvent extracts MSe PSf PIMSg
STa (°C)
MFTb
RTc (°C)
(°C)
MFd (log (ddpm))
407
456
498
3.0
1.3 2.7 5.4 20.6 42.8
420 451 446
465 n.d.h n.d.
498 475 458
1.8 0.0 (1 ddpm) 0.3 (2 ddpm)
42.8 30.4 12.4
4.5 0.0 (1 ddpm)
a Softening temperature. b Maximum fluidity temperature. c Resolidification temperature. d Maximum fluidity. e CS /NMP mixed solvent 2 soluble fraction. f Pyridine soluble fraction. g Pyridine insoluble and CS2/NMP mixed solvent soluble fraction. h n.d. ) not detectable.
Figure 2. Dynamic visoelastic behaviors for (a) Goonyella raw coal, (b) its residues extracted with THF, (c) chloroform, (d) NMP, (e) pyridine, and (f) CS2/NMP mixed solvent. Table 4. Parameters of Thermoplasticity Estimated by Dynamic Visoelastic Measurement residue tan δ max temperature (°C) (max fluidity) crossover temperaturec of G′ and G′′ (°C) max value of tan δ a
raw coal
THF
438 436, 454 1.51
450 444, 453 1.39
chloroform 458 0.63
NMP 461 0.62
extract pyridine 461 0.25
CS2/NMPa
Sox-PSb
475 0.18
PIMS 466
278, 470 .2
0.55
CS2/NMP mixed solvent. Extract obtained by Sohxlet extraction. Temperature of G′ ) G′′ b
2b). The extraction yields for both residues were still low, 2.7 and 5.4 wt % (daf) for chloroform and NMP, respectively. Thus, thermoplasticity was decreased greatly by the removal of small amounts of extract, independent of the solvent used. Figure 2e,f show the profiles for pyridine and CS2/ NMP residues, respectively. Peaks for G′, G′′ and tan δ are broadened compared to the other residues. The maximum tan δ values were observed for both residues at around 460 °C, and the values, 0.25 and 0.18, respectively, were much smaller than that of raw coal. The decrease is reasonable because more solvent-soluble components have been extracted: extraction yields are 20.6% (daf) for pyridine and 42.8% (daf) for the CS2/ NMP mixed solvent.
c
The values of maximum tan δ and its temperature are plotted against the extraction yield in Figure 3a,b, respectively. As the extraction yield increases, maximum tan δ decreases (Figure 3 a) and the temperature increases (Figure 3b), suggesting that the thermoplasticity of the residues is reduced as more components are extracted from raw coal. The large changes in the small extraction yields show that removal of a small amount of solvent-soluble components has a significant effect on thermoplasticity. The general increase in the temperature at maximum tan δ with increasing extraction yield can also be the result of the removal of lighter components that soften at lower temperature, although the value with pyridine deviates from the curve.
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Figure 3. Value of maximum (a) tan δ and (b) its temperature against the extraction yield.
Figure 4. Dynamic visoelastic behaviors for extracts (a) SoxPS and (b) PIMS.
Dynamic Viscoelastic Behaviors for Extracts. The dynamic viscoelastic behavior for the pyridine soluble (Sox-PS) fraction, which was obtained from pyridine Soxhlet extraction, is shown in Figure 4 a. The large peaks for G′, G′′ and tan δ were observed in the range 260-500 °C, which shows that the Sox-PS fraction becomes thermoplastic over a wide temperature range, presumably because the Sox-PS fraction includes lots of lighter components. The measurements for the chloroform-soluble fraction, and PS fraction obtained from pyridine fractionation of the CS2/NMP extract fraction could not be performed because the extract was too viscous to form a pellet. Results for the PIMS, which is heavier than PS and Sox-PS, are shown in Figure 4b. The profile was rather different from that of Sox-PS (Figure 4a): peaks for G′ and G′′ at thermoplastic state were small, although the PIMS fraction is part of extract obtained from CS2/NMP extraction. The viscoelastic behavior was similar to residues extracted with chloroform and NMP (Figure 2c,d). The maximum value of tanδ, 0.55, was similar to those for the chloroform and NMP residues. We have
Yoshida et al.
reported14,15 that coking coals contained a considerable amount of PIMS, which correlated well with the maximum fluidity determined by Gieseler plastometry. Thus, although its own thermoplasticity is quite low, PIMS seems to play an important role in the development of thermoplasticity. The role of heavy components such as PIMS can be different from that of low-molecular-weight components as reported so far.1-9 Role of Solvent-Soluble Components. In an earlier study we found19 that the PIMS fraction obtained from the CS2/NMP extract of several coking coals was only partly soluble in the same mixed solvent: that is, fractionation of the extract with pyridine resulted in the insolubilization of a heavy component, PIMS. Addition of the lighter components such as PS, or electron acceptors (tetracyanoethylene (TCNE)) or donors (pphenylenediamine) greatly increased the solubility of the PIMS fraction in the mixed solvent.19-21 Thus, the solubility of PIMS is greatly enhanced by the presence of lighter components. While the PS fraction was almost completely soluble in pyridine, the THF insoluble (TIPS) fraction obtained from fractionation of the PS with THF was only partly soluble in pyridine.22 Thus, removal of the lighter components by fractionation led to a decrease in the solubility of the remaining heavier components.22 The lighter components may serve as a part of the “solvent” for the dissolution of the heavier components. Similarly, coal thermoplasticity may be related in part to the “solvating” action of lighter components. Saito et al. estimated23 the quantity of “mobile components” in Goonyella coal during heating using an in situ NMR imaging technique. They have found that, as the temperature was increased in the range 300-425 °C, the mobile components gradually increased around the initial mobile components present at 25 °C. This result suggests that the initial lighter components corresponding to the extracts obtained from extractions with THF, chloroform, and NMP (1-5 wt % extraction yields) gradually make heavier components in proximity mobilesthe lighter components disperse and gradually dissolve the surrounding heavier component. Consequently, PIMS may become mobile because of interaction with lighter components during heating. Furthermore, the CS2/NMP mixed solvent insoluble fraction may become mobile because of the presence of lighter components, especially PIMS, since their chemical structures are relatively similar13,22 and so PIMS has a stronger affinity for the CS2/NMP mixed solvent insoluble fraction. In this concept low-molecular-weight components such as the γ-component have an important role, and the effects of heavy components such as PIMS on coal thermoplasticity can be explained. Conclusion Goonyella coking coal was extracted with various solvents, and temperature-variable dynamic viscoelastic (19) Sanokawa, Y.; Takanohashi, T.; Iino, M. Fuel 1990, 69, 1577. (20) Ishizuka, T.; Takanohashi, T.; Ito, O.; Iino, M. Fuel 1993, 72, 579. (21) Liu, H.; Ishizuka, T.; Takanohashi, T.; Iino, M. Energy Fuels 1993, 7, 1108. (22) Takanohashi, T.; Fengjuan, X.; Saito, I.; Sanokawa, Y.; Iino, M. Fuel 2000, 79, 955. (23) Saito, K.; Komaki, I.; Hasegawa, K.; Tsuno, H. Fuel 2000, 79, 405.
Coal Extracts and Residues
measurement was carried out for the extracts and residues obtained. The thermoplasticity in residues obtained by chloroform or NMP extractions significantly decreased compared to that of the raw coal despite the fact that the extraction yields, 2.7 and 5.4 wt % (daf), respectively, were low. Thermoplasticity further decreased for residues with higher yields obtained by extraction with pyridine (20.6 wt %) and CS2/NMP (42.8 wt %). The thermoplastic state was observed in wide temperature range (260-500 °C) for the pyridinesoluble (Sox-PS) fraction obtained from Soxhlet extraction with pyridine, while thermoplasticity for the heavier pyridine-insoluble (PIMS) fraction obtained from pyridine fractionation of the CS2/NMP extract fraction was small and similar to that for residues
Energy & Fuels, Vol. 15, No. 1, 2001 175
obtained from chloroform or NMP, even though the PIMS is part of solvent-soluble fraction. The roles of both lighter and heavier components can be explained by the continuous “solvating” action of lighter components in the dissolution and/or dispersion of heavier components. Acknowledgment. The authors greatly appreciate the Iron and Steel Institute of Japan for financial support. We are grateful to Dr. T. Kajiwara of Kyushu Institute of Technology and Mr. F. Munekane of Mitsubishi Chemical Co., Ltd. for discussions on the dynamic viscoelastic measurement. EF000150V
