Evaluation of the Thermoplasticity of Upper Freeport Coal and Its

Energy & Fuels 2000, 14, 929-935

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Evaluation of the Thermoplasticity of Upper Freeport Coal and Its Extracts Using Dynamic Viscoelasticity Koyo Norinaga* and Masashi Iino Institute for Chemical Reaction Science (ICRS), Tohoku University, Katahira, Aoba-ku, Sendai 980-8577, Japan Received March 10, 2000. Revised Manuscript Received May 2, 2000

The viscoelastic properties of Upper Freeport coal and extracts, including carbon disulfide/Nmethyl-2-pyrrolidinone mixed solvent (1:1 by volume) solubles and pyridine solubles, were characterized by controlled strain oscillatory rheometry to evaluate the thermally induced relaxation of coal molecules. The dynamic viscoelastic modulus of the pelletized sample was measured at temperatures where the samples plasticize, while the frequency was increased from 0.05 to 5 Hz, to yield a data set suitable for analysis by time-temperature superposition. Although the applicability of the time-temperature superposition principle to the coal samples may be doubtful, it allows the empirical construction of master curves at reduced temperatures, yielding data for the frequency dependencies of the modulus over a wide range, i.e., 10-3-103 Hz. However, the temperature dependencies of the shift factor are not explainable by means of the assumptions that underlie the William-Landall-Ferry equation. Arrhenius-type plots of the shift factors show that the apparent activation energy (∆Ha) is temperature dependent. The approximate distribution ranges of ∆Ha are from 50 to 230 kJ/mol for pyridine solubles, and 50 to 370 kJ/mol for raw coal. The multiexponential behaviors of the shift factors suggest that the thermorheological characteristics of coal molecules in the plastic phase are not simple, but involve multiple viscoelastic mechanisms with different temperature dependencies.

Introduction The thermoplastic property of bituminous coals has been extensively studied, since this property is considered important for predicting the coking potential. Rheological properties, such as the apparent viscosity of the coal during softening, are the most useful parameters for assessing this property, and can be measured by Gieseler plastometry1-4 and needle penetrometry.5-7 These methods focus mainly on the fluid character of softening coal under relatively high shear rates. Proton nuclear magnetic resonance thermal analysis technique is also used to evaluate the thermoplasticity, on the basis of molecular mobility of coal, and can yield quantitative information about the relative abundance of hydrogen atoms, which exist in significantly different physical and chemical environments in coals.8-10 Sakurovs et al.8 measured the transverse relaxation signals of 33 Australian bituminous coals in situ while * Author to whom all correspondence should be addressed. Fax: +81-22-217-5655. E-mail: [email protected]. (1) van Krevelen, D. W.; Huntjens, F. J.; Dormans, N. M. Fuel 1956, 35, 462. (2) Fitzgerald, D. Fuel 1956, 35, 178. (3) Waters, P. L. Fuel 1962, 41, 3. (4) Brown, H. R.; Waters, P. L. Fuel 1966, 45, 41. (5) Matsuoka, K.; Kumagai, T.; Chiba, T. ISIJ Int. 1996, 36, 40. (6) Matsuoka, K.; Hayashi, J.; Chiba, T. ISIJ Int. 1997, 37, 566. (7) Hayashi, J. i.; Denma, D.; Takahashi, H.; Kumagai, H.; Chiba, T. Fuel 2000, 79, 391. (8) Sakurovs, R.; Lynch, L. J.; Maher, T. P.; Banerjee, N. Energy Fuels 1987, 1, 167. (9) Sakurovs, R.; Lynch, L. J.; Maher, T. P. Fuel Process. Technol. 1994, 37, 255. (10) Sakurovs, R. Fuel 1997, 76, 615.

heating at 4 K/min. The signals were resolved into a slowly relaxing exponential or mobile component and rapidly relaxing Gaussian or rigid components. In addition, they estimated the fraction of mobile hydrogen in the coal specimen as a function of temperature. The fraction of mobile hydrogen of most coals increases with temperature and is maximal at temperatures between 710 and 760 K, at which significant thermal relaxation of coal macromolecules takes place due to softening. However, the maximum fraction of mobile hydrogen is approximately 0.6, at most. This indicates that a considerable amount of hydrogen remains immobile, even during softening. Therefore, it is reasonable to consider that under most conditions softening coals show characteristics of both elastic and viscous behavior, and can be treated as a viscoelastic material. To characterize such materials accurately, both elastic and viscous responses must be measured. Dynamic mechanical analysis is therefore a uniquely powerful method because it measures both properties simultaneously. Studies of viscoelastic properties of coal employing the dynamic mechanical analysis are nothing new and have been reported by several workers.11-17 Nomura et al.16 measured the dynamic viscoelastic modulus of six coals (11) Wert, C.; Weller, M. J. Appl. Phys. 1982, 53, 6505. (12) Wert, C.; Weller, M.; Caulfield, D. J. Appl. Phys. 1984, 56, 2453. (13) Weller, M.; Wert, C. Fuel 1984, 63, 891. (14) Yun, Y.; Suuberg, E. M. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1993, 38 (2), 489. (15) Takanohashi, T.; Yoshida, T.; Iino, M.; Katoh, K. Fuel 1999, 78, 863. (16) Nomura, S.; Kato, K.; Komaki, I.; Fujioka, Y.; Saito, K.; Yamaoka, I. Fuel 1999, 78, 1583.

10.1021/ef000049q CCC: $19.00 © 2000 American Chemical Society Published on Web 06/28/2000

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Norinaga and Iino Table 1. Properties of Samples

sample

yield

C

UF UFMS UFPI UFPS

59 29 21

85.5 85.9 85.8 86.6

a

[wt % dafbcoal] H N 4.7 5.5 5.0 5.4

1.6 1.8 2.1 1.8

By difference. b Dry-ash-free. c Moisture-free.

S

Oa

ash [wt % mfccoal]

0.7 2.9 1.1 1.0

7.5 3.9 6.2 5.2

13.2 0 0 0

d

[g/mol] Mnd Mwe 1086 1306 1069

1658 1897 1523

Mw/Mn[-]

bulk density [g/cm3]

1.53 1.45 1.42

1.21 1.00 1.08 1.00

Number-averaged molecular weight. e Weight-averaged molecular weight.

during heating at 3 K/min from 573 to 773 K. A continuous sinusoidally varying strain with an amplitude of 0.1% and a frequency of 1 Hz was applied to the pelletized coal samples placed between two parallel plates. They found that near the softening temperature, determined by the conventional Gieseler plastometry, the storage and loss moduli start to decrease and the loss tangent starts to increase. The moduli as well as the complex viscosity are at a minimum and the loss tangent is at a maximum at around the temperature at which the coal shows maximum fluidity. These findings indicate that a close relationship exists between the coking properties measured by conventional methods and the dynamic rheological characteristics. Yoshida et al.17 applied this technique more comprehensively to characterize the coking properties of various coals, including slightly coking coals. Although the dynamic mechanical analysis is recognized as an alternative method for characterizing coal thermoplasticity, few systematic studies have examined the frequency dependence of the measured moduli. The dynamic viscoelastic characteristics depend not only on the temperature but also on the frequency. By studying the frequency dependence of the moduli over a wide range of temperatures and employing the time-temperature superposition principle,18 data can be generated for short and long time intervals outside the range of measurement. This also affords a valuable simplification for separating the two principle variables of time (frequency) and temperature on which the viscoelastic properties depend, and for expressing the properties in terms of a single function. This study attempted to follow the viscoelastic responses of a coal and its extracts during melting or softening over a wide range of time scales. Pelletized Upper Freeport coal was subjected to controlled stress oscillatory rheometry. The solvent-extractable portions of Upper Freeport coal were also used to investigate the thermoplasticities of the different fractions. The frequency sweep test was carried out as a function of temperature to obtain a data set suitable for analysis by time-temperature superposition. The mechanism of the thermal relaxation of the molecules in coal during softening is discussed on the basis of the reduced variables that are empirically employed to obtain master curves of the samples. It is clear that the application of the time-temperature superposition principle to the coal is questionable, since a chemical reaction always accompanies coal softening, i.e., the change in its internal structure. The softening coal is a ternary system involving gas, liquid, and solid. The thermorheological properties of the softening coal are therefore

strongly influenced by the composition of the system, which also depends on the operational variables, such as the heating rate. Therefore, the viscoelastic parameters, such as the modulus of rigidity presented here, are not the constants belonging to the samples; they are independent of operational variables, but are determined by the experimental conditions.

(17) Yoshida, T.; Iino, M.; Takanohashi, T.; Katoh, K. Fuel 2000, 79, 399. (18) Ferry, J. D. Viscoelastic Properties of Polymers; Wiley: New York, 1961.

(19) Iino, M.; Takanohashi, T.; Ohsuga, H.; Toda, K. Fuel 1988, 67, 1639. (20) Huang, H.; Wang, K.; Bodily, D. M.; Hucka, V. J. Energy Fuels 1995, 9, 20.

Experimental Section Sample. Argonne Premium Upper Freeport coal was dried under a pressure of less than 1 Pa at 333 K for 48 h, which was long enough for it to attain a constant weight. Solvent extraction of the dried Upper Freeport coal (hereafter referred to as UF) followed the procedure of Iino et al.19 UF was extracted with a 1:1 mixture (by volume) of N-methyl-2pyrrolidinone (NMP) and carbon disulfide (CS2). The mixed solvent extract (UFMS) was extracted with acetone to remove NMP and CS2 that were strongly retained, and the acetoneinsoluble UFMS was further fractionated into pyridine solubles (UFPS) and insolubles (UFPI). These were finely pulverized (particle sizes were finer than 75 µm) in a mortar with a pestle and then pelletized by applying 1-t mechanical stress under evacuated dies. The respective mass and diameter of the cylindrical pellets were 0.4 g and 13.5 mm. The molecular mass distribution of the extracts was measured by laser desorptionionization mass spectrometry (LD/MS) on a spectrometer (Thermoquest Co. Ltd. Vision 2000). The extract sample was dissolved in CS2-NMP without any matrix, since the spectra obtained in the absence of matrix were as intense as those obtained using it. The spectra were observed in a linear mode under the following conditions: acceleration voltage, 20 kV; mass range, 1-6000; pressure, less than 5 × 10-5 Pa. A nitrogen laser operating at 337 nm was used for the laser desorption. The laser was scanned across a sample coated on a target slide, and the spectra of up to 20 laser pulses were summed. For each of the measurements, the laser power was set at a value slightly above the threshold for the appearance of the spectrum assigned to the extract, to avoid fragmentation of the molecules. Preliminary measurements revealed that the molecular masses of the extracts range from 100 to 6000. The relevant properties of the samples are listed in Table 1. The packing fraction of the pellet can be calculated from the true density measured by helium pycnometry20 and the bulk density of the pellet. The packing fraction of a UF pellet is 0.85, which is much larger than that of the closest-packed structure of identical spheres, i.e., 0.74 for hexagonal or cubic closest-packed structures. Thus, the pelletized coal samples behave as a coal block and therefore the viscoelastic property of the pellet does not seem to reflect slippage between individual particles. Apparatus and Procedure. Rheological measurements were performed using a rheometer (Rheometric Scientific Inc., ARES-2KSTD). All the measurements were made with a 25mm parallel plate geometry. The pellet was placed between the plates. After purging the samples with a nitrogen atmo-

Thermoplasticity of Upper Freeport Coal and Its Extracts

Energy & Fuels, Vol. 14, No. 4, 2000 931

sphere for 30 min, the temperature was raised to 573 K at a rate of approximately 10 K/min. During the course of each experiment, the sample was maintained under an inert nitrogen atmosphere, conveniently achieved by running the rheometer air bearing, which exhausts gas at 80 L/min into the sample area, from a supply of liquid nitrogen fitted with a vaporizer system. Measurements were made at temperatures from 573 to 653 K for UFPS, from 573 to 693 K for UFMS, from 573 to 723 K for UFPI, and from 573 to 693 K for UF, in 10-K steps, to yield a data set suitable for analysis by timetemperature superposition. Beyond the peak temperature employed for each measurement, the modulus of the sample was increased due to resolidification. At each temperature, the moduli were recorded as a function of frequency at equilibrated temperatures, in which a shear rate was selected within the linear viscoelastic region, while the frequency increased from 0.05 to 5 Hz in 10 logarithmic steps. The frequency sweep test at each temperature took 3 min. The temperature was stable within 0.2 K over the range used in this study. Strain sweeps were previously performed to ensure that the viscoelastic response was linear, and strain values less than 0.015% were consequently chosen. The normal stress applied to the samples was controlled at 500 g to avoid slippage at the interface between the plates and the pellet, as long as the pellet did not soften and maintained its shape at relatively low temperatures.

Results and Discussion Oscillatory rheometry is a dynamic measurement in which the sample is subjected to a sinusoidally varying strain. If the amplitude of the strain lies within the linear viscoelastic region, where the peak stress varies linearly with the peak strain, the resulting stress response is also a sine wave, and the values of the measured rheological parameters are independent of the strain amplitude. The viscous and elastic components of the shear modulus can be resolved by monitoring the amplitude and phase lag, δ, of the resultant stress response of the material. If the shear modulus is written in the complex form, G*, then

G* ) G′ + iG′′

(1)

tan δ ) G′/G′′

(2)

and

where G′ is the storage (elastic) modulus, the portion of the oscillation energy that is stored elastically, G′′ is the loss (viscous) modulus, representing the energy dissipated by the system, and δ is the phase lag, or angular displacement, between the applied strain and the stress response. For an elastic material (Hookean solid), G′ . G′′, and the stress response is exactly in phase with the applied strain, whereas a Newtonian fluid is characterized by G′ , G′′ and thus a δ value equal to π/2 rad is found. Any material having a δ value between 0 and π/2 is classified as viscoelastic. The results of the frequency sweep experiments for UFPS, UFPI, UFMS, and UF are shown in the form of log-log plots of G′ and G′′ against frequency (ω), in Figures 1-4, respectively. Generally, G′ and G′′ decrease with increasing temperature and decreasing ω. At low temperatures, G′′ passes through a maximum, but G′ tends to become constant. The ω dependence of the moduli becomes more significant with increasing temperature. Figure 5 plots G′ measured at ω ) 1 Hz as a function of

Figure 1. Storage (G′, top) and loss (G′′, bottom) moduli of UFPS as a function of both the temperature and the oscillation frequency (ω).

temperature. G′ begins to decrease at around 600, 650, and 660 K for UFPS, UF, and UFMS, respectively. G′ decreases from 108 to 105 Pa for UFPS and UF, and from 108 to 106 Pa for UFMS, while the temperature dependence of G′ for UFPI is less significant. The data were analyzed using the reduced variable time-temperature superposition procedure of Ferry18 to yield so-called master curves. In this procedure the measured moduli data, G[T,ω], determined at temperature T and frequency ω, are first modified by a small corrective factor to account for changes in temperature and specific gravity, F, to yield data reduced to a single temperature Ts. Then the reduced moduli are plotted logarithmically against ω, and shift factors, aT, are determined graphically from either the loss or storage modulus data. The value of aT represents a factor by which the frequency must be changed in order to make all the data sets continuous at the desired reduced temperature Ts.

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Figure 2. Storage (G′, top) and loss (G ′′, bottom) moduli of UFMS as a function of both the temperature and the oscillation frequency.

Figure 3. Storage (G′, top) and loss (G′′, bottom) moduli of UFPI as a function of both the temperature and the oscillation frequency.

Master curves are then plotted using the following expression:

measurements performed for the samples and plotted in Figure 6, in which the reduced frequency scales extend from 10-3 to 103 Hz. All the samples can be classified as viscoelastic materials at the individual reduced temperatures, since the values of tan δ range from 0.1 to 10 over the whole range of reduced frequencies. A computer determined aT graphically using custom-written software that allows the average separation of adjacent temperature data to be determined. Ferry18 described three criteria for the applicability of the time-temperature superposition principle to the measured viscoelastic data. They are the following: (a) the shapes of adjacent curves should match exactly, (b) the same values of aT must superpose all the viscoelastic functions, and (c) the temperature dependence of aT must have a reasonable form consistent with experience. As seen in Figure 6, G′, G′′, and tan δ are found to superpose well, using aT determined from the storage modulus data. Therefore, the curve for each sample is

G[Ts,ω‚aT] )

G[T,ω]‚Ts‚Fs T‚F

(3)

where Fs is the specific gravity of the sample at Ts. However, the change in F with T is unknown for the coal samples. It seems safe to ignore the corrective factor to account for changes in both temperature and specific gravity, since the increase in T accompanies the expansion of the samples, namely, a decrease in F. Therefore, in practice, we consider only the frequency shift for the application of the time-temperature superposition principle to the coal samples using

G[Ts,ω‚aT] ) G[T,ω]

(4)

Data are reduced to the midrange temperatures of the



Figure 5. Change in storage modulus (G′) of the samples with temperature, measured at ω ) 1 Hz.

Figure 4. Storage (G′, top) and loss (G′′, bottom) moduli of UF as a function of both the temperature and the oscillation frequency.

continuous, and criteria (a) and (b) are satisfied. The temperature dependence of aT was examined to address criterion (c). aT is often described in terms of two empirical parameters, c1 and c2, by means of the Williams-Landel-Ferry (WLF) equation:21

ln aT )

-c1(T - Ts) (c2 + T - Ts)

(5)

Rearrangement of this equation indicates that a plot of {(T - Ts)/ln aT} vs (T - Ts) will yield a straight line with a negative slope. As shown in Figure 7, however, a plot of these two parameters for the present data reveals complex behavior, not explainable by means of the assumptions that underlie the WLF equation. Chemical reactions, such as gas evolution and degrada(21) Williams, M. L.; Landel, R. F.; Ferry, J. D. J. Am. Chem. Soc. 1955, 77, 3701.

tion of the network structure, always accompany the softening of coal, and the structure of each coal- and solvent-derived fraction evolves irreversibly with time at elevated temperatures. Therefore, it is not surprising that the WLF plots are not successful, since this equation is appropriate for treating the viscoelastic data when the internal structure of the system does not change with temperature. Another possible reason for the failure is connected to the upper limit of application of the WLF equation, which is estimated to be 100 K higher than the glass transition temperature of the sample. Mackinnon et al.22-26 reported the existence of second-order phase transitions in coals, which are revealed by differential scanning calorimetry. When dried coals are heated to about 473 K, a first-order transition occurs, which is probably the release of stored paleostress. Subsequent heating shows the existence of second-order phase transitions, which are fully reversible and are characteristic of glass transitions. They occur at around 383 K and the intensity varies with coal rank. Thus, the temperature range at which softening of the samples takes place seems to be higher than the upper limit for the application of the WLF equation. Bueche27 has discussed the temperature dependence of aT over a wide range on the basis of fluctuations in the thermal vibrations of concentric shells surrounding a central molecular segment, with results that are consistent with the WLF form at low temperature and the Arrhenius form at high temperature. In Figure 8, ln aT is plotted against 1000/T using the Arrhenius equation. Although the plots are never approximated by a straight line, the data vary smoothly with temperature, unlike WLF-type plots. The lines in the figure are drawn by assuming that the apparent activation energies depend (22) Mackinnon, A. J.; Hall, P. J. Fuel 1992, 71, 974. (23) Mackinnon, A. J.; Antxustegi, M. M.; Hall, P. J. Fuel 1994, 73, 113. (24) Mackinnon, A. J.; Hall, P. J. Energy Fuels 1995, 9, 25. (25) Mackinnon, A. J.; Hall, P. J.; Snape, C. E.; Burchill, P. Fuel 1995, 74, 136. (26) Mackinnon, A. J.; Hall, P. J. Fuel 1996, 75, 85. (27) Bueche, F. J. Chem. Phys. 1959, 30, 748.

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Figure 7. Plots for the samples based on eq 5 (WLF equation).

Figure 8. Plots for the samples based on theArrhenius equation. The lines represent the best fit to the data using a Gaussian function.

Figure 6. Master curves for the samples. (a) UFPS, (b) UFMS, (c) UFPI, and (d) UF.

on the temperature, using Gaussian functions. The apparent activation energy (∆Ha) can be calculated from

d ln aT

∆Ha ) R

d(1/T)

(6)

where R is the gas constant. Figure 9 shows the change in ∆Ha with temperature for the samples. The curves have maxima at 620, 680, 690, and 660 K for UFPS, UFMS, UFPI, and UF, respectively. For the extracts samples, the temperature at which ∆Ha shows the maximum increases with the molecular mass. The maximum ∆Ha is approximately 220, 330, 240, and 370 kJ/mol for UFPS, UFMS, UFPI, and UF, respectively. The temperature at which ∆Ha of UF is maximal is almost identical with the temperature at which UF starts to soften, as evaluated by conventional Gieseler

plastometry.28 Unlike the extracts samples, UF contains solvent-insoluble materials. These would bind the components with relatively low masses, i.e., the solventextractable portion, and inhibit the molecular motion of these components. This is why UF requires such a large activation energy before it starts to soften. Sanada and Honda29 studied the creep compliances of a pellet made of the vitrain portion of a bituminous coal as a function of both time and temperature and made a creep master curve with the appropriate translations along the time and compliance axes. They also analyzed the temperature dependence of the shift factors along the time axis using the Arrhenius equation, and found that the apparent activation energy was temperature dependent and showed a maximum at around 603 K. Combined with our observations, this suggests that the structural relaxation characteristics (28) Vorres, K. S. User’s Handbook for the Argonne Premium Coal Sample Program; Argonne National Laboratory: Argonne, IL, 1993. (29) Sanada, Y.; Honda, H. Fuel 1963, 42, 479.



fractionation convert multicomponent coal into a somewhat simpler material, thereby narrowing the ∆Ha distributions. Conclusions

Figure 9. Temperature-dependent changes in the apparent activation energies (∆Ha) of the samples.

of coal molecules during heating are not rheologically simple, but involve multiple viscoelastic mechanisms with different temperature dependencies. This result reveals what we already know or suspect about the thermal decomposition of coals, namely, that the multiexponential description of thermal changes in coal will also exhibit time-temperature superposition behavior. The magnitude of the derived shift factors in the present experiment, therefore, reflects the combined effects of a changing structure with time-averaged (3 min) viscoelastic properties of the sample obtained at a given temperature. Such complicated behavior would also be attributed to the structurally complex and chemically heterogeneous nature of coal. The approximate distribution ranges of ∆Ha are from 50 to 230 kJ/mol for UFPS and UFPI, 50 to 330 kJ/mol for UFMS, and 50 to 370 kJ/mol for UF. It appears that the solvent extraction or

Controlled strain oscillatory rheometry was used to characterize the viscoelastic properties of UF and its extracts, UFPS, UFMS, and UFPI. Frequency sweep tests were carried out over temperatures from 573 to 703 K in 10-K steps, to yield a data set suitable for analysis by time-temperature superposition. The horizontal shifts on logarithmic plots empirically achieve a single composite curve from the curves for a given viscoelastic function. However, the temperature dependencies of the shift factor are not explainable by means of the assumptions that underlie the WLF equation. Arrhenius-type plots of the shift factors show that the apparent activation energy is temperature dependent. These atypical observations suggest that the thermorheological characteristics of coal molecules in the plastic phase are not simple, but involve multiple viscoelastic mechanisms with different temperature dependencies. In the range at which coal or coal extracts soften, the method of reduced variables is wholly inapplicable; therefore, a theoretical guide or empirical model is necessary to perform such a manipulation. Acknowledgment. The authors are grateful to Dr. Takahiro Yoshida of Tohoku University for his useful advice on the dynamic viscoelastic measurements and helpful discussion on the manuscript. The authors are also grateful to Dr. Hiroyuki Seki of the Petroleum Energy Center for providing molecular mass distribution data of the extracts. The authors thank the anonymous reviewers for their useful comments. This work was supported in part by a “Research for the Future Project” grant from the Japan Society for the Promotion of Science (JSPS), through the 148 Committee on Coal Utilization Technology. EF000049Q

Evaluation of the Thermoplasticity of Upper Freeport Coal and Its

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