Energy & Fuels 1996,9, 25-32
25
Observation of First- and Second-Order Transitions during the Heating of Argonne Premium Coals Alexander J. Mackinnon and Peter J. Hall* Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow, GI IXL, United Kingdom Received May 3, 1994
.
Revised Manuscript Received September 20, 1994@
Differential scanning calorimetry (DSC) has been used to investigate the thermal transitions occurring for a series of Argonne Premium Coals. Two sets of experiments were conducted using different heating profiles to further examine a second-ordertransition which had previously been observed for Illinois No. 6 coal. Thermogravimetric analysis (TGA) was also conducted on the samples to support the DSC data. The DSC results show that on initial heating a first-order transition is observed which is a function of the cooling cycle and that subsequent scanning shows a clear and reproducible second-order process with the characteristics of a glass transition process. TGA shows that no mass loss is associated with either of these transitions.
Introduction Coal is generally accepted to have a macromolecular network and numerous studies have supported this.l The structure consists of aromatic and hydroaromatic clusters with cross-linking due to covalent bonds, hydrogen bonds, and macromolecular entanglements. A network structure would therefore explain some coal properties including low solubility in solvent^,^^^ swelling when contacted with certain solvent^,^^^ viscoelasticity,6v7 and the observation of a glass transition process.8-10 As a consequence, common polymer characterization techniques such as solvent swelling,11-16 NMR,17 differential scanning calorimetry (DSC),16p18s19 dielectric spectroscopy,20and dynamic mechanical thermal analysis (DMTAF have all been applied to investigate the nature of these networks. Of particular interest in this work is the glass transition process (Tg). The Tg is a second-order process and represents the a Abstract
published in Advance ACS Abstracts, November 1,1994. (1)Van Krevelen, D. W. Coal; Elsevier: Amsterdam, 1993 and
references contained therein. (2)Boas-Traube, S. G.; Dryden, I. G. C. Fuel 1960,29,260. (3)Larsen, J. W.;Choudhury, P. J. Org. Chem. 1979,44, 415. (4)Sanada, Y.; Honda, H. Fuel 1966,45,295. (5) Kirov, N.Y.; Oshea, J. M.; Sereant, G. D. Fuel 1968.47.415. (6)MacCrae, J. C.; Mitchell, A. R. Fuel 1967,36,1553. (7)Morgans, W.T. A.; Terry, N. B. Fuel 1968,37,201. ( 8 )Lucht, L. M.; Larson, J. M.; Peppas, N. A. Energy Fuels 1987,1,
56. (9)Sakurovs, R.;Lynch, L. J.;Maher, T. P.; Banejee, R. N. Energy Fuels 1987,1, 56. (10)Yurum, Y . ;Karabakan, A. K.; Altuntas, N. Energy Fuels 1991, 5.701. (11)Larsen, J. W.; Green, T. K.; Kovac, J. J . Org. Chem. 1985,50, -
?
4729.
(12)Lucht, L. M.; Peppas, N. A. Fuel 1987,66,815. (13)Lucht, L. M.; Peppas, N. A. J.Appl. Polym. Sci. 1987,33,2777.
(14)Aida, T.;Squires, T. G. Prepr. Pap-Am. Chem. SOC.,Diu. Fuel Chem. 1986,30,95. (15)Suuberg, E. M.; Otake, Y.; Deevi, S. C. P r e p . Pap-Am. Chem. SOC.,Diu. Fuel Chem. 1991,36(1),258. (16)Yun, Y.; Suuberg, E. M. Fuel 1993,72,1245. (17)Sakurovs, R.:Lvnch, -. . L. J.: Maher, T. P.: Baneriee, R. N. Energy Fuels 1987,1, 167. (18)Lucht, L. M.; Larson, J. M.; Peppas, N. A. Energy Fuels 1987, 1, 56. (19)Hall, P. J.;Larsen, J. W. Energy Fuels 1991,5, 228. (20)Mackinnon, A. J.;Hayward, D.; Hall, P. J.;Pethrick, R. A. Fuel
1994,73,731. (21)Howell, J. M.; Peppas, N. A. Fuel 1987,66, 810.
transition from the glassy to the rubbery state. The existence of rubbery states in coals is important in preliquefaction solvent treatments because diffusion of solvents through rubbers is generally several orders of magnitude greater than in the corresponding glass. In the glassy state the macromolecular mobility is impeded and short segmental /3-processes are observed, whereas, in the rubbery state, a significant increase in macromolecular motion is observed. This latter process is classified as the a-transition. The Tg is accompanied by a characteristic change in the physical properties of the material, such as the sudden decrease of the shear modulus, the sudden increase of the specific heat (C,), and a sudden increase in the expansion coefficient. In addition the mechanical and dielectric losses pass through a maximum. This process for coals has been well established in the region 250-360 "C. Lucht et a1.8 determined values from DSC ranging from 307 to 359 "C, depending on the coal structures studied while Yurum et al.1° observed a distinct process a t 342 "C for a bituminous coal. Yun and Suuberg16 examined four Argonne Premium Coals using DSC and noted distinct processes in the prepyrolysis region. Pittsburgh No. 8 displayed an irreversible second-order transition at 250-300 "C whereas Upper Freeport and Pocahontas No. 3 exhibited first-order endothermic processes a t approximately 350 and 430 "C, respectively. A sharp increase in solvent swellability accompanied these transitions. Recent work has indicated that first- and second-order processes exist at significantly lower temperatures for coal. Hall and M a c k i n n ~ n have ~ ~ s ~ob~ served a first-order process on an initial scan for Illinois No. 6 coal which is attributable t o an enthalpy relaxation and has no mass loss associated with it. A secondorder process is observed on second and subsequent scans at -110 "C and this is reversible and reproducible for multiple samples of coal. The second-order process bears the characteristics of a glass transition process and comparison to DSC studies on polystyrene and (22)Hall, P. J.;Mackinnon, A. J. Fuel 1992,71,974. (23)Mackinnon, A.J.;Antxustegi, M. M.; Hall, P. J. Fuel 1994,73, 113.
0887-0624/95/2509-0025$09.00/0 0 1995 American Chemical Society
26 Energy & Fuels, Vol. 9, No. 1, 1995 Table 1. Properties of the Argonne Premium Coals coal water (as received) % % C (ma0 % 0 (ma0 20.3 Beulah-Zap lignite 32.2 72.9 28.1 18.0 Wyodak Anderson 75.1 13.5 Illinois No. 6 8.0 77.7 11.6 4.6 Blind Canyon 80.7 1.7 83.2 8.8 Pittsburgh No. 8 2.4 9.8 Lewis Stockton 82.6 1.1 7.5 Upper Freeport 85.5 0.7 91.1 2.5 Pocahontas No. 3
polypyrrole supports this.23 Dielectric spectroscopy has been used intensively in investigations of the structure of polymers. From low-frequency dielectric measurements on polymeric systems, the presence of an a-transition manifests itself as an increment in real permittivity data and as a peak in imaginary permittivity data. Low-frequency dielectric measurements have been applied to the entire range of Argonne Premium Coals20,24 and these measurements suggest the existence of an a-process in the temperature range 30-130 "C for Upper Freeport and Pocahontas No. 3. The remaining coals may exhibit an a-process but this could be easily masked by competing processes such as interfacial polarization and direct current conduction effects which are a consequence of the heterogeneous nature of coal. Real conductivity measurements from the same work demonstrate a distinct increase in the temperature region 80-130 "C, similar to that found in semiconducting polymers. This increase is associated with a glass transition process. The work of Miura et al.25926has demonstrated for a series of coals that a distinct reversible second-order process was observed in the region of 100-150 "C. They suggested that this transition was associated with the breakage of hydrogen bonds within coal, and as the event was reversible, they concluded that the bonds reformed on cooling. This work correlates with our previous observations and the breakage and reformation of hydrogen bonds appears to be a suitable explanation for the process we have observed. Further evidence of a low-temperature process has been illustrated by oxidation studies on Beulah Zap lignite2' which demonstrate that the second-order process appears to enhance the oxidation process by increasing the accessibility of the coal structure a t this critical point. To date, the nature of these transitions or the structural origin of these transitions has not been investigated. The objective of this present work is to examine in detail the processes occurring and to suggest an explanation in terms of the macromolecular structure of the coal. We have thus extended our previous work on Illinois No. 6 to the entire suite of Argonne Premium Coals using DSC as the principal technique.
Experimental Section Coal samples were selected from the APC sample bank28 and were used as received (Table 1). Calorimetry measurements were conducted on a Mettler DSC 30 system which was (24) Mackinnon,A.J.;Hayward, D.; Hall, P. J.; Pethrick, R. A.Fuel
1077. (25)Miura, K.;Mae, K.; Sakurada, K.; Hashimoto, K. Energy Fuels 1992, 6,16. (26)Miura, K.Personal communication, 1994. (27)Hall, P. J.;Mackinnon, A. J.; Mondragon, F. Energy Fuels 1994,
1993, 72,
8,1002.
(28) Vorres, K.S.Energy Fuels 1990,4, 420.
Mackinnon and Hall used in conjunction with Mettler software TA72PS.2 for data acquisition and processing. The temperature and intensity of the second-order transitions were derived using routines within the Mettler software. The sensor on the instrument consisted of a 5-fold Au-Ni thermopile mounted on a glass disc. The use of this type of sensor was believed to improve the recognition of low-intensity transitions. Temperature calibration was by the melting points of indium, lead, and zinc standards supplied by Mettler and temperatures are accurate to f0.5 "C. Enthalpy calibration was by integration of the melting endotherm of an indium standard supplied by Mettler. It was estimated that enthalpies were accurate to f0.05 J/g. Standard aluminum crucibles with cold welded lids were used with two pin holes pierced through the lids to allow evaporation of water. The sample size was approximately 10 mg. A nitrogen carrier was used at a flow rate of 50 mL min-l. Cooling of the furnace between consecutive runs was carried out using liquid nitrogen from a reservoir directly beneath the furnace. Steady cooling rates could be achieved using this arrangement. Two different series of scans were conducted on each of the APC samples, both a t heating rates of 10 "C min-l and are described as follows: 1. A sample of each fresh, untreated coal was heated from 30 to 400 "C at 10 "C min-' three times in succession. The sample was cooled at 10 "C min-l between successive heating scans. The purpose of this sequence was to examine the coals up to the temperature just before where major pyrolysis occurs. 2. A sample of each fresh, untreated coal was heated to 110 "C at 10 "C min-' and held for 30 min, cooled a t a nominal rate of 200 "C min-' to 30 "C, heated to 250 "C at 10 "C min-l, held for 10 min, and then cooled to 30 "C at 200 "C min-I. This series of scans is referred to as the pretreatment stage. The sample was then heated from 30 t o 250 "C at 10 "C min-l three times in succession with a cooling rate of 10 "C min-' between heating scans. This sequence was designed to extend our previous work on Illinois No. 6 and to investigate the first and second order transitions of samples which had been thoroughly dried. In addition, samples of Illinois No. 6 and Upper Freeport were predried by weighing a n aliquot of coal into a DSC pan and drying under vacuum a t 40 "C for 24 h. This in situ vacuum drylng was designed to remove water without inducing structural rearrangements. The samples were cycled from 30 to 250 "C a t 10 "C min-l with intermediate cooling a t 10 "C min-'. In all series of experiments the sample always remained in the DSC chamber under nitrogen atmosphere and in repeated heatingkooling sequences the same sample was used to minimize the effects of heterogeneity. A blank run with a n empty pan in each of the sample and reference positions gave a straight baseline. Tar deposits within the DSC chamber were removed between every run by heating to 600 "C in flowing air and holding for 5 min. In addition, thermogravimetric measurements were conducted on a Mettler thermogravimetric balance (Model TG50) with data acquisition and processing controlled using Mettler TA72PS.2 software. Approximately 20-30 mg of sample was weighed into alumina crucibles and scanned a t 10 "C min-I from either 30 to 400 "C or 30 to 250 "C twice in succession. A nitrogen carrier gas flowing a t 20 m u m i n was used.
Results and Discussion TGA and DSC studies are considered here in association with each other. The TGA scans were obtained by heating from 30 to 400 "C and are shown in Figure la-h and the DSC scans were obtained either by heating an untreated sample three times in succession from 30 to 400 "C as shown in Figure 2a-h or by heating a pretreated sample (pretreatment details included in the Experimental Section) from 30 to 250 "C three times in
Heating of Argonne Premium Coals
Energy &Fuels, Vol. 9, No. 1, 1995 27
Weight (mg)
Derivative Weight (mg/'C)
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Figure 1. TGA of Argonne Premium Coals heated from 30 to 400 "C at 10 "C min-l. (a)Beulah Zap lignite; (b)Wyodak Anderson; (c) Illinois No. 6;(d) Blind Canyon; (e) Pittsburgh No. 8; (0Lewis Stockton;(g) Upper Freeport; (h) Pocahontas.
succession as shown in Figure 3a-h. All sets of measurements were conducted at a heating rate of 10 "C min-l with intermediate DSC cooling scans at 10 "C min-l. The heat flows for the DSC traces in Figures 2 and 3 are reported on a basis of the dry mass of coal. The first scan in each trace of Figure 2 represents a situation where mass is being continually lost, however on the second and third scans the no further mass losses are encountered. All scans in Figure 3 represent constant mass scans. TGA on each sample showed that
signifkant no further weight loss was encountered aRer a single scan to 400 "C. Considering initially the scans from 30 to 400 "C in Figure 2, the DSC traces reveal up to five processes in total occurring for the series of coals: (a) low-temperature (-60- 180 "C) evaporation of water on first scan only; (b) low-temperature (150180 "C) endothermic activity on first scan only for certain coals which is associated with an enthalpy relaxation; (c) low-temperature (-110 "C) second-order process on second and third scans only; (d) high-
Mackinnon and Hall
28 Energy &Fuels, Vol. 9, No. 1, 1995 Heat Flow (W/g)
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temperature (-360 " C ) exothermic peak; (e) hightemperature ('300 " C )exothermic activity associated with the loss of bound water and the onset of mild pyrolysis. In general, for all coals the first scan is significantly different from the subsequent two scans, primarily due to the evaporation of water in the region 60-180 "C. The magnitude of the endotherm reflects the amount of water present in the as-received samples. The majority of the water, held as weakly bound water, is
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removed from 30 to 130 "C. In the intermediate range from 130 to 280 "C, the more strongly held water is removed, and this accounts for a small proportion of the total water removed. There may also be some decomposition products released such as CO2, although no gas analyses were performed. Above 280 "C,loss of tightly bound water and the removal of low molecular weight organic species occurs and this is reflected in both the TGA and DSC traces. In certain coals (Illinois No. 6, Pittsburgh No.8, and Lewiston-Stockton) it is possible
Energy &Fuels, Vol. 9, No. 1, 1995 29
Heating of Argonne Premium Coals Heat Flow W/g)
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Figure 3. DSC of Argonne Premium Coals heated from 30 to 250 "C at 10 "C min-l: (-) first scan; (- -1 second scan; (- - -) third scan. (a) Beulah Zap Lignite; (b) Wyodak Anderson; (c) Illinois No. 6 (d) Blind Canyon; (e) Pittsburgh No. 8; (f) Lewis Stockton; (g)Upper Freeport, (h) Pocahontas. within the coal. This process disappears on second and to discern on the DSC traces a small but distinct third scans which confirms that it is an irreversible endotherm from 150 to 180 "C adjacent to the upper process. TGA is not able to resolve any individual zones shoulder of the water endotherm of the first scan. This of weight loss associated with this endotherm. In the peak appears to be consistent with the endothermic case of Wyodak Anderson and Beulah-Zap lignite, which activity displayed by Illinois No. 6 in our previous work and is associated with an enthalpy relaxation process. have large water contents, any such relaxation process occurring could be totally masked by the large endotAs the samples have not been treated before DSC, this herm associated with the loss of water. Further conpeak arises directly from structural rearrangements
30 Energy & Fuels, Vol. 9, No. 1, 1995
Mackinnon and Hall
Heat llow (W/g) -0.24
fL
-0.26
material will have been removed after this treatment. Two successive TGA runs t o 250 "C indeed confirm indeed that all volatile material has been removed on pretreatment and so it can be assumed that the residual endotherm is the result of structural changes. Additional scans similar to those involving the pretreatment stage but with a cooling rate of 10 "C min-l immediately after the pretreatment produce a secondorder transition rather than a first-order transition when cooled at 200 "C min-l. It can therefore be assumed that this endothermic feature is a function of the cooling rate of the preceding scan and that slow cooling (-10 "C min-l) induces a second-order transition whereas fast cooling (-200 "C min-'1 induces a firstorder transition. This would also explain why a secondorder feature is observed on the second scan on the series of scans from 30 to 400 "C which have intermediate cooling rates of 10 "C min-l. This first-order feature closely resembles an enthalpy relaxation which is a common feature when a polymer is heated through its glass t r a n ~ i t i o n .The ~ ~ relaxation depends critically on the thermal history of the polymer and on the cooling rate at which the material has previously undergone when it is cooled from a temperature above its glass transition. Slow cooling generally leads to a relatively large relaxation in the form of an endothermic peak and fast cooling will diminish the peak and normally produce a glass transition process. It is possible therefore that the relaxation in this work is a function of the cooling cycle of the pretreatment stage. However, the behavior of the coals is anomalous when compared t o polymers in that when relatively high cooling rates of 200 "C min-' are encountered, relatively large peaks are produced whereas when the coals are subjected to more moderate cooling rates of 10 "C min-l, a distinct secondorder process is observed, at a temperature below that of the initial relaxation process. In addition, the cooling rate does not influence the position of the subsequent enthalpy and glass transition processes in polymers, only the size of the peak. Mackinnon and Hal131 have investigated this phenomenon, to be reported in a future publication, and noted that these transitions in coals are sensitive t o the thermal history and to the rate at which they are cooled during pretreatments. The prominent transition in both sets of DSC scans is a second-order process at -110 "C which is observable for all samples on the second and third scans and its characteristics are shown in Tables 2 and 3. The intensity of the transition, defined as the change in the heat capacity, is calculated directly using the Mettler Graphware TA72PS.2 software. This transition is reversible and reproducible for multiple samples of coal and is able to withstand temperatures up to at least 400 "C. There are two possible interpretations for the second-ordertransition observed in both series of scans. One is the conventional explanation of a glass transition process whereby the free volume in the coal exceeds a certain fraction of the total volume and macromolecular freedom of motion increases. An alternative explanation has been provided by Suuberg32 whereby the hydrogen bonds could dissociate thermally at the onset
8
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Figure 4. DSC of vacuum-dried coals heated from 30 t o 250 "C a t 10 "C min-l: (a) Illinois No. 6; (b) Upper Freeport.
firmation of the existence of the enthalpy relaxation process is illustrated in Figure 4,a and b, for DSC scans on samples of Illinois No. 6 and Upper Freeport coals which had been vacuum dried in situ at 40 "C. On an initial scan up t o 250 "C, the first-order process is extremely distinct and free from the masking effect of the loss of water. Illinois No. 6 shows an endotherm associated with the loss of water, although it is greatly reduced from that of the untreated sample. It is possible that the sample has not been exhaustively dried during vacuum drying and the sample may also have regained water during transfer from the vacuum oven to the DSC chamber. As these samples had not been significantly heat treated before DSC scanning, the relaxation is considered to be a remnant of the coalification process. Subsequent cooling at 10 "C min-' to 30 "C and rescanning up to 250 "C shows a distinct second-order process for both coals. Recent studies on an untreated and a vacuum-dried British coal (Point of Ayr coal)29have shown similar characteristics. For this particular coal, the first-order process is extremely distinct, even in the undried sample. The series of runs for the sample which had been pretreated to 250 "C are illustrated in Figure 3. Cooling after the pretreatment stage is effected at a nominal rate of 200 "C min-l. All of the coals with the exception of Pittsburgh No. 8 exhibit a first-order endothermic event. Blind Canyon and Pocahontas do not exhibit such a distinct first-order process and the process appears to coincide with a second-order transition. The samples have already been exposed to a temperature of 250 "C before the DSC scans proper and it is reasonable to assume that the majority of volatile (29) Mackinnon, A. J.; Hall, P. J.;Snape, C. E.; Burchill, P. Fuel, in press.
(30) Cowie, J. M. G. Polymers: Chemistry and Physics of Modern Materials, 2nd ed.; Blackie Academic and Professional: London, 1991; p 213. (31)Mackinnon, A. J.; Hall, P. J. Submitted to Fuel. (32) Suuberg, E. M. Personal communication, 1992.
Energy &Fuels, Vol.9,No. 1, 1995 31
Heating of Argonne Premium Coals
Table 2. Characteristics of the Second-OrderProcess from DSC Scanned from 30 to 400 "Ca second scan coal Beulah-Zap lignite Wyodak Anderson Illinois No. 6 Blind Canyon Pittsburgh No. 8 Lewis Stockton Upper Freeport Pocahontas No. 3 a
Ttr
("0
121.9 116.9 103.3 99.0 104.9 102.0 100.2 103.3
ATt, PC) 18.3 17.0 12.6 17.9 11.3 14.1 14.1 7.9
third scan Ttr ("0 117.7 113.7 106.0 98.5 102.6 102.7 100.9 99.6
ACp (J/g/K) 0.20 0.17 0.11 0.12 0.08 0.12 0.06 0.05
AC, (JWK) 0.14 0.13 0.11 0.13 0.10 0.12 0.08 0.09
ATt, ("C) 17.1 14.7 14.3 18.8 13.0 16.0 12.7 14.4
Tt, = temperature of the second-order process.
Table 3. Characteristics of the Second-OrderProcesses from DSC Scanned from 30 to 250 "C coal Ttr ("C) ATt, ("0 ACp (J/g/K) Beulah-Zap lignite Wyodak Anderson Illinois No.6 Blind Canyon Pittsburgh No.8 Lewis Stockton Upper Freeport Pocahontas No.3
114.3 111.0 125.0 110.0 105.4 116.0 108.1 111.5
13.1 13.1 9.4 10.9 10.0 9.7 10.4 6.3
0.74 0.36 0.09 0.22 0.12 0.20 0.22 0.06
of Tg and macromolecular degree of cross-linking decreases. Hydrogen bonds could re-form on cooling. This would indeed manifest itself as a second-order process when observed by DSC but in the strictest possible definition this is not a glass transition process. There are two pieces of work which bear a resemblance to our observations. Firstly, Miura et al.25p26have demonstrated using DSC and FTIR that hydrogen bonds are disrupted between 150 and 200 "C during heating of coal, giving rise to a reversible second-order process. The reversible nature of the process suggests that the hydrogen bonds re-form on cooling. In their work they conclude that low-temperature thermal pretreatment of coal increases the conversion and tar yield for several low-rank coals during pyrolysis due to the suppression of water-forming cross-linking reactions. Secondly, Isaacs et a1.33 have studied the range of APC samples and observed a residual endotherm between 25 and 130 "C which was still evident even on successive scans. They noted that this transition had no weight loss associated with it and concluded that it was due t o lowtemperature coal reaction and/or structural rearrangement processes associated with dispersion forces and/ or hydrogen bonding. In addition, low-frequency dielectric and conductivity m e a s u r e m e n t ~ ~on ~ l the ~~ entire range of APC samples have also demonstrated that a significant change is occurring over the range 30180 "C. In particular, the real conductivity shows a distinct break at a temperature corresponding to the glass transition temperature as measured by DSC. This is a common feature in polymers.34 Oxidation studies on Beulah Zap lignite2' also support the idea of a significant change occurring. The oxidation process of the lignite was significantly enhanced at temperatures above those of the second-order process with a distinct break at a temperature coincident with the second-order process. In general, the intensity of the transition, defined as the change in heat capacity, remains unaltered after (33)Isaacs, L. L.; Abhari, R.; Ledesma, R.; Tsafantakis, E. Energy Fuels 1992,6, 242. (34)Sasabe, H.;Sawamura, K.; Saito, S. Polym. J. 1971,2, 518.
Intensity of Trsnsilion (J/g/K)
0.8
o.6 0.4
,
I
I
I
0
0
0
o
I
1
5
I
1
0
0
I 0 '
,
10 %
15
20
25
0 (MAF)
Figure 6. Rank dependence of intensity of second-order transition.
exposure to 400 "C, although the intensities for Wyodak Anderson and Beulah-Zap lignite are considerably lower than those of the corresponding samples which have been exposed to 250 "C (cf. Tables 2 and 3). The intensity of the transition remains unaltered with repeated scanning from 30 to 250 "C and appears to be rank dependent to some extent. The two coals of lowest rank, Beulah-Zap Lignite and Wyodak Anderson, show the largest intensities whereas those coals of highest rank show much weaker transitions. Figure 5 shows the intensity of the glass transition as a function of the oxygen content of the coal. Although the trend is not completely uniform, there is a marked propensity for the intensity to increase with increasing oxygen content. The most reasonable interpretation of this is that the intensities are determined by the density of cross-links within the network. There was no correlation of the temperature of the glass transitions with rank and we speculate that the hydrogen bond strength determines this feature. It has been suggested that low-rank coals have relatively large proportions of hydrogen bonds compared to lower rank ~ o a l s and ~ ~ indeed s ~ ~ Larsen3' suggests that the cross-link density decreases with the rank of the coal, up to about 86% C. If we expect the glass transition temperature and intensity of the transition t o depend on the breakage of hydrogen bonds, then low-rank coals would be expected to exhibit a larger intensity than high-rank coals. Each coal exhibits an exothermic peak at -360 "C which is reproducible, even on successive scanning up to 400 "C. This type of peak would appear to be associated with pyrolysis. The TGA traces show no distinctive weight loss associated with this process at (35)Larsen, J. W.;Green, T. K.; Kovac, J. J. Org. Chem. 1985,50, 4729. (36)Suuberg, E.M.;Lee, L.; Larsen, J. W. Fuel 1985, 64, 1668. (37)Larsen, J. W.Fuel Process. Technol. 1988,20, 13.
Mackinnon and Hall
32 Energy & Fuels, Vol. 9, No. 1, 1995
this temperature, although at this point the coals are undergoing relatively rapid mass loss and small processes could become masked. This transition is also apparent for Pittsburgh No. 8 in the work of Yun and Suuberg.lG Upper Freeport is the one exception to this behavior and it displays an initial endotherm at 350 "C with an enthalpy of 5.3 J g-l on the first scan which is consistent with the observations of Yun and Suuberg and then is superseded by exotherms during the second and third scans. The endothermic behavior is associated with an enthalpy relaxation process caused by the relief of stress initially induced in the coalification process and is a phenomenon generally observed for amorphous polymers.
Conclusions DSC has demonstrated the existence of a first-order and second-order processes for the entire suite of Argonne Premium Coals upon heating. The first-order process appears t o be an enthalpy relaxation which is influenced by the cooling rate immediately after the
pretreatment process. Relatively high cooling rates (-200 "C min-l) induce a first process whereas a low cooling rate (-10 "C min-l) induces a second-order process. The second-order process is fully reversible and reproducible for multiple scans on all coals and bears the characteristics of a glass transition process. The most reasonable explanation for the origin of the secondorder transitions, based on the available evidence, is a process involving the breakage and re-forming of macromolecular hydrogen bonds. A good correlation between coal oxygen content and glass transition intensity supports this hypothesis. Consequently, low-rank coals exhibit a relatively high intensity whereas the highrank coals show relatively weak transitions.
Acknowledgment. This work was funded by SERC grant number GRIH18821. The authors are grateful to the referees for helpful suggestions in improving this paper. EF940068Y
