Energy & Fuels 1992,6, 97-103
97
Mass Spectrometric and Chemometric Studies of Thermoplastic Properties of Coals. 1. Chemometry of Conventional, Solvent Swelling, and Extraction Data of Coals Anna Marzec,* Sylwia Czajkowska, and Jan Moszydski Institute of Coal Chemistry, Polish Academy of Sciences, 1 Maja 62, 44-100 Gliwice, Poland
Hans-Rolf Schulten Department of Trace Analysis, Fachhochschule Fresenius, Dambachtal20, 0-6200 Wiesbaden, FRG Received April 22, 1991. Revised Manuscript Received October 18, 1991
Twenty-seven coals from Carboniferous seams in Poland were studied with the aim to find links between thermoplastic properties and chemical characteristics of the coals. Three sets of data were obtained for all the coals: (1)thermoplastic properties measured using the Gieseler plastometer: (2) yields of pyridine extractables and swelling measurements for pyridine residues; (3) ultimate, proximate, and petrographic analyses. The three data sets were evaluated using chemometric techniques with the purpose of looking for significant correlations between all the data. Temperature of softening is a linear regression of pyridine extractables and hydrogen content in coals as well as of swelling data. Temperatures of maximum fluidity and resolidification are correlated with each other and with oxygen, exinite, and moisture contents of the coals as well as with the swelling data. It has been concluded that temperature of softening is a colligative property and indicates a phase transition resulting in an increase of thermal induced mobility of coal material; the energy demand of the transition is dependent on contents of bulk components of coal system that were specified in this study. Temperatures of maximum fluidity and resolidification appear to have the same chemical background; i.e., the temperatures depend on the content of the same structural units or components. However, the means of chemical characterization of coal material used in this study were not capable of identifying them. Volatile matter and petrographic composition showed rather limited value as predictive means for some (TF(”) and T R ) and no predictive value for the other thermoplastic properties.
Introduction Thermoplastic behavior of coals and coal-derived materials such as pitch is important from the point of view of their utilization in coke-making, carbonization, and briquetting, and in modern gasification and combustion processes as well as in production of carbon fibers. Research on coal thermoplasticity has been aimed at (a) finding means of prediction of thermoplastic behavior of coals in their processing and (b) understanding basic chemical and physical phenomena that result in coal thermoplastic properties. As was recently pointed out,l most companies have been still using deceptive relationships based on coal rank and maceral composition for preliminary evaluation of coal thermoplastic properties. An example of an unsuccessful attempt to predict coking properties of pitch materials on the basis of their various analytical data was published recentlyS2 It seems that progress in this field could be made if better understanding of basic phenomena of thermoplasticity is achieved. The existing theories of coal thermoplasticity have been recently reviewed by Khan and J e n k i n ~such , ~ as homogeneous melting, partial melting, thermobitumen theory, (1) Menendez, R.;Alvarez, R. Prepr. Pap.-Am. Chem. SOC., Diu. Fuel Chem. (Storch Award Symposium) 1989,34(1),263-270. (2) Abel, 0.; Bosse, R.; Bratek, K. Kopsch, H. Lorson, H.; Oelert, H. H. Erdoel Kohle 1988,41(5), 211-221. (3) Khan, M. R.;Jenkins, G. Influence of Weathering and Oxidation on the Thermoplastic Properties of Coal. In Chemistry of Coal Weathering; Nelson, Ch. R.; Ed.; Elsevier: New York, 1989; Chapter 5; pp 107-132.
and the theory that may be called “inner liquefaction theory”. The first three theories were derived from experimental observations and data were obtained with the use of analytical techniques that cannot be considered at present time to be up-to-date techniques. Moreover, all the theories referred to thermoplasticity in rather general terms, and neglected separate explanations for individual thermoplastic properties such as temperature of softening, fluidity, temperatures of maximum fluidity and of resolidification, swelling, and dilatation. This Introduction section has been limited to papers published in 1980-90 that provide experimental data for the separate terms of thermoplasticity. The inner liquefaction theory4+ is also included since some implications concerning the separate Gieseler plastic properties of coals can be drawn from it. Marsh and Neavel; Spiro,S and NeaveP expressed the view that “smaller molecules” arising from the coal and formed pyrolytically serve as a necessary solvating vehicle (plasticizer) and as hydrogen donors that stabilize radicals and prevent their polymerization. Thus, the amount of “smaller molecules” and their hydrogen donor ability have been viewed as decisive factors that determine coal thermoplasticity. The experimental results of Grint’ and Ouchis question whether hydrogen donor (4) Marsh, H.; Neavel, R. Fuel 1980, 59,511-513. (5) Spiro, C. L. Fuel 1981, 60, 1121-1128. (6) Neavel, R.C. Coal Plasticity Mechanism;Inferences from Liquefaction Studies. In Coal Science; Gorbaty, M. L., Larsen, J. W., Wender, I., Eds; Academic Press: New York, 1982; Vol. 1, pp 1-19.
0887-0624/92 /2506-0097$03.00/ 0 0 1992 American Chemical Society
98 Energy & Fuels, Vol. 6,No. 1, 1992
ability of the molecules plays any role in coal thermoplasticity. Softening (Ts).Temperature of softening measured with the Gieseler plastometer for various coals is observed in the 330-400 "C range. It has been assumed4 that softening commences at a temperature at which van der Waals forces and hydrogen bonding become insufficient to hold some of the molecular units together. Lynch et al.9 have found that heating rate has little if any effect on molecular mobility in the temperature region where initial softening occurs. Thus, their datag imply that initial softening is a consequence of physical changes in coal material. Research on oxidized and weathered coals has shown that oxidation results in an increase of softening temperature and may also be responsible for the entire loss of plastic proper tie^.^ Wachowska et a1.l0J1 stated that cleavage of ether linkages in the coals led to a lowering of softening temperature. As has been recently pointed out by Khan and J e n k i n ~ ,introduction ~ of relatively few cross-links,one every 40-50 main chain atoms, reduces the mobility of various polymers to the extent that a previously thermoplastic polymer can no longer go through a plastic stage. All these data3J0J1indicate that cross-linking of coal material, inherent or induced by oxidation, influences temperature of coal softening. Saki et al.12 measured Gieseler plasticity of three coals and their extraction residues obtained by stepwise extraction with increasing yields of extractables. They found that the softening temperature of the residues increases with increase in the amount of extracted material. Other characteristics of coal material were found by Lloyd et al.13 to have an influence on softening temperatures of 40 hvb coals; they were oxygen and sulfur content as well as yields of T H F and DMF extractables. Hence, the concurrent data12J3show that amount of extractables also have an impact on temperature of coal softening. Fluidity and Maximum Fluidity (F(max)). The maximum fluidity of coals, as measured by Gieseler plastometer, falls in a very wide range from several units (ddpm) to several thousand units; the temperature range at which the maximum fluidity is observed, is from -380 to 460 "C. Fluidity has been viewed4+as a result of melting of low molecular components and of chemical reactions that involve thermal cracking to form volatile products and a reactive residue containing nascent free radicals which are subsequently stabilized by hydrogen atom transfer. The progressive consumption of transferable hydrogen leads to decay of fluidity. This theory can hardly explain why low-rank coals are not fluid at any temperature range, although they readily undergo thermal decomposition and form volatile products and contain more aliphatic hydrogen compared with high-rank coals; they also contain some amount of low molecular components. Moreover, if the hypothesis is true one should expect a relationship between maximum fluidity of coals and their aliphatic (or total) hydrogen content. (7) Grint, A.; Mechani, Sh.; Trewhela, M.; Crook, M. J. Fuels 1985,64, 1335-1342. (8) Ouchi, K.; Itoh, S.; Makabe, M.; Itoh, H. Fuel 1989, 68, 735-740. (9) Lynch, L. J.; Webster, D. S.; Sakurova, R.; Maher, T. P. Fuel 1988, 67, 579-583. (10) Wachowska, H. M.; Nandi, B. N.; Montgomery, D. S. Fuel 1974, 53, 212-219. (11) Wachowska, H. M.; Pawlak, W. Fuel 1977, 56, 422-426. (12) Saki, H.; Kumagai, J.; Matsuda, M.; Ito, 0.;Iino, M. Fuel 1989, 68, 978-982. (13) Lloyd, W. G.; Reasoner, J. W.; Hower, J.; Yates, L. P. Fuel 1990, 69, 1257-1270.
Marzec et al. Senftle et al.14 determined maximum fluidity and aliphatic hydrogen content for a set of 21 coals and 21 vitrinites isolated from them. It was concluded3J4that their results showed distinct relationship between maxF and aliphatic hydrogen. The conclusion is questionable; the results rather show that there are two different populations of coals and vitrinites and in none of them the relationship is evident. David6 investigated fluidity of several coals; they observed that the primary influence of oxidation is to decrease the maximum fluidity. On the other hand, Weber et a1.,16 in studies of blends of a coal and various coalderived materials, found that maximum fluidity increases with the increase of hydroxyl group content. In the work already described12it was shown that maximum fluidity was influenced by the amount of extractables; the higher amount of extractable material was withdrawn the lower was maxF of the extraction residues. Solomon1' has shown that fluidity at any temperature can be predicted on the basis of kinetic rates of bond breaking and temperature induced cross-linking. The bond breaking rate is calculated from tar evolution data and the cross-linking rate from methane evolution; both were measured with the use of TG/FT lR instrument. Another set of chemical characteristics of coals was found to form a linear regression of maximum fluidity; they are Sorg, yields of DMF and THF extractables, and yields of some pyrolysis product.13 Temperature of maximum fluidity (TF(-)) indicates the temperature at which fluidity begins to decrease. From the theory of thermoplasticit94 already described in the preceding paragraph, it may be concluded that TF(") should be related to aliphatic hydrogen content in coals. According to the theory, the hydrogen stabilizes radicals and prevents their polymerization; the reaction that leads to progressive decay of fluidity and, finally, to resolidification of coal. Thus, the higher the hydrogen content in a coal the longer should be the time needed for its consumption by radicals. This would be manifested by the increase of TF(max) since standard Gieseler measurements are carried out at uniformly increasing temperature. Our literature survey for experimental data referring to hydrogen content and TF(") of coals found no such data. Lloyd et al.13 showed that TF(*=) is a linear regression of Sorg, vitrinite reflectance and yields of DMF and pyridine extractables of coal. Saki et a1.12 showed that TF(") of extraction residues increased with the increase in the amount of extractables removed. Temperature of Resolidification (TR!. TR determined with the Gieseler plastometer for various coals occurs in the 420-500 "C range. Davis et al.15 found that oxidation of coals reduced their resolidification temperatures. Lloyd13 stated that TR of coals was a linear regression of So,, vitrinite reflectance, yield of THF extractables, a n 8 yield of some pyrolysis products. A decrease of TR was also observed12 when coal extraction residues that contained decreasing amount of extractable material were measured. Iyama et a1.18 showed that there was a relation between TRand the ratio of H donor and H acceptor capabilities of coals. These capabilities were (14) Senftle, J. T.; Kuehn, D.; Davis, A.; Painter, P. C. Fuel 1984,63, 245-250. (15) Davis, J. D.; Reynolds, D. A.; Brewer, R. E.; Naugle, B. W.; Wolfson, D. E. U.S.Bureau Mines Tech. Pap. 1947, 702, 18. (16)Weber, J. W.; Schneider, M.; Darif, M.; Yax, E.; Bertau, R. Fuel Process. Technol. 1990, 24, 27-33. (17) Solomon, P. R.; Best, P. E.; Yu, Z. Z.; Deshpande, G.V. Prepr. Pap.-Am. Chem. SOC.,Diu.Fuel Chem. 1989,34(3), 895-906. (18) Iyama, S.; Yokono, T.; Sanada, Y. Carbon 1986,24(4), 423-428.
Energy & Fuels, Vol. 6, No. 1, 1992 99
Thermoplastic Properties of Coals Table I. Properties of the Coals
coal no. 100 lo1 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126
F-1 F-2 F-3 F-4 F-5 F-6
coal mine" Rvdultowv RidultoG Rydultowy Jankowice Knurow Knurow Nowy Wirek Nowa Ruda Gliwice Gliwice XXX-Iecia XXX-Iecia XXX-Iecia 1-go Maja 1-go Maja Debiensko Debiensko Szczyglowice Szczyglowice Manifest Lipcowy Halemba Gliwice Gliwice Moszczenica Moszczenica Wawel Pstrowski range for 27 coals Henri Robert Permac Merlebach Norwich Sommerset County Pinanacle range for Fl-F6 coals
Ts 367 370 355 373 372 353 370 350 345 350 352 366 374 367 380 360 350 370 358 377 373 400 382 364 357 377 365 345-400 n.d n.d n.d n.d n.d n.d
F(max) 15 13 10 6 6 20 18 5400 35000 39300 200 4000 1950 1580 1600 460 27 15 6 760 8 70 550 75 15 9 12 6-39300 218 4138 90 28 195 112 28-4138
-
C 85.0 84.4 87.3 85.9 85.3 85.7 85.7 86.4 82.8 88.4 88.3 86.7 90.6 91.9 89.4 82.0 81.5 84.5 85.7 89.5 84.4 91.4 88.7 82.5 85.1 82.9 82.9
H 5.1 4.9 5.1 5.0 4.9 5.1 4.9 5.3 5.3 5.2 5.2 5.1 4.6 4.8 4.7 5.0 5.0 4.7 5.1 5.0 4.6 4.3 4.5 4.9 5.0 5.1 4.9
393-458 417-490
82-92
4.3-5.3 0.5-3.8 0.8-9.5
ma cerals,d % dmmf daf V E 37.4 3.0 76.2 8.4 39.8 2.7 76.6 9.2 41.2 4.2 81.3 4.6 42.3 5.5 87.5 5.5 74.1 33.9 1.6 4.5 35.6 1.7 81.6 8.6 63.3 33.7 1.6 10.8 79.5 36.2 1.5 3.3 40.6 2.0 94.8 2.8 35.5 1.7 66.9 8.8 32.7 1.4 69.3 5.4 30.4 0.8 82.1 5.7 28.8 2.9 75.2 2.1 24.2 1.0 69.6 2.6 76.5 23.8 0.9 0.7 87.2 40.0 2.6 3.0 73.5 41.9 2.0 5.3 39.1 2.0 69.9 4.2 33.4 2.0 62.5 7.4 27.4 1.0 69.6 3.5 39.8 2.1 67.1 7.8 18.7 0.6 75.6 0.0 22.6 0.8 70.9 1.5 26.1 1.9 81.6 3.3 70.5 35.5 2.3 9.0 38.9 2.8 84.4 4.3 36.9 2.5 78.6 4.9 19-42 0.6-5.5 63-95 0-11
463 451 438 476 471
n.d n.d n.d n.d n.d
89.4 87.0 84.1 90.3 89.4
5.0 5.1 5.3 4.6 4.5
1.0 1.1 1.0 1.1 1.2
4.6 6.8 9.6 4.0 4.9
27.0 30.5 37.5 19.4 20.0
0.7 0.7 1.2 0.7 0.6
88.2 77.6 72.0 77.6 80.0
480 438-480
n.d
91.0 84-91
4.4 4.4-5.3
1.0 1-1.2
3.6 3.6-9.6
17.5 18-37
0.7 0.6-1.2
77.8 74-89
thermoDlastic
elemental!
TR
TFc-) 420 407 393 409 424 422 427 428 427 434 427 430 445 448 455 430 413 433 418 452 409 456 458 416 405 413 407
433 430 417 428 451 440 451 464 466 471 458 465 478 487 490 460 437 454 448 479 430 488 490 445 430 443 434
VM,
5% daf
S 1.6 0.9 1.3 0.8 1.6 1.3 1.3 3.8 1.7 1.1 0.7 1.1 0.9 0.5 0.8 1.9 2.0 1.5 1.8 0.6 1.3 1.7 3.3 2.1 1.4 1.2 1.1
0 6.9 8.2 4.6 6.6 6.9 6.9 6.6 2.8 8.4 3.8 3.6 5.2 1.9 0.8 3.2 9.1 9.5 7.5 5.3 2.7 7.7 0.8 1.7 8.5 6.9 9.2 9.2
M,
m %
~~
0.6 3.6 11.4 0.0 0.2
0.0 0-12
swel, vol%
~ y - ~ x , % daf
95 95 90 60 55 100 70 100 105 90 100 95 60 30 25 100 85 85 90 40 100 10 10 50 100 90 70 10-105
24.1 18.5 21.6 18.1 19.1 19.3 21.0 24.7 31.4 25.8 28.0 31.7 22.9 17.9 11.9 27.2 27.0 16.0 18.3 17.8 17.4 3.4 7.4 18.8 19.8 17.8 18.8 3-32
110 100 80 40 50
19.1 20.6 18.7 1.9 2.5
35 35-110
1.1 1-21
'If coal samples are from the same mine, they are from different seams. *Nitrogen content is from 1 to 2.2% daf. cAsh content is from 5 to 20 wt % dry basis. dInertinite content (% dmmf): 100 - (V + E); inertinite components (% dmmf): semifusinite, 1-16; fusinite, 0.4-9; inertodetrinite, 0.2-7; micrinite, 0.2-4; macrinite, 0 . 2 . Vitrinite components (% dmmf): collinite, 62-92; tellinite, 0-9. Exinite components, (% dmmf): sporinite, 0.7-77; cutinite, 0-1; resinite, 0-2.
determined in reactions of coals with anthracene and dihydroanthracene, respectively. Studies of thermoplastic properties of 24 coals carried out at elevated pressures (nitrogen, up to 3 MPa) indicated that resolidification temperatures for all coals increased with the increase in pressure.lg This has been attributed to increased residence time of volatiles that play a role of "plasticizers" in the coal melt? Alternatively, it might be attributed to a delay of diffusion-controlled reactions of volatiles with nonvolatile melt, due to lower diffusion rates of volatiles at high pressure. The objective of the study presented in part 1 is the systematic examination of Gieseler thermoplastic properties of a large set of coals and identification of the relationships between these properties and conventional coal characteristics as well as nonconventional data referring to the content of relatively low molecular components and cross-linking of macromolecular part of coal material.
Experimental Section Coals. Twenty-seven carboniferous coals from Poland were studied that represent a wide variety of thermoplastic properties. Freshly mined coal samples were ground in a stepwise manner until the whole sample passed an appropriate sieve for analytical procedure. The samples were kept in a glass sealed ampules until used for analyses. Results of proximate, ultimate, petrographic analyses of the coals are shown in Table I. In the final step of this study six (19) Khan, M. R.; Jenkins, R. G. Fuel 1986,65, 725-731.
thermoplastic coals from outside Poland (shown at the end of Table I) were also included with the aim of checking whether correlations found for the coals derived from Upper Silesia region in Poland can be attributed to other coals. Coal Thermoplastic Properties. The properties were determined using a constant-torque Gieseler plastometer and the Polish standard method PN-62/04536. The instrument provided temperatwe and fluidity measurements at 60-5 intervals, a uniform heating rate of 3 "C/60 s was applied. The fluidity readings are in angle units. According to this standard method the softening temperature Ts is defined as the temperature at which the dial pointer moves from zero to 1angle unit position. Tsis calculated as an arithmetic average of at least two measurements that may be different by not more than 10 "C. The temperature of maximumfluidity TRm, is the temperature at which the dial movement reaches the maximum rate. The resolidification temperature TR is a temperature at which the dial movement stops. The two temperatures, Le., TF(-)and TR,are reported in the same way: they are arithmetic averages of at least two measurements that may be different by not more than 3 "C. The maximum fluidity F(max) is the maximum rate of dial movement expressed in angle units per minute and is calculated as an arithmetic average of at least two measurements that may be different not more than 20% of the lower value. Thus, according to this standard method two measurements of the thermoplastic properties of a coal sample are sufficient if the results meet the respective requirements. However, in this study the measurements were carried out in quadruplicates. The separate averages for the first and second pair of measurements were calculated according to the standard method. When the two averages agreed within 10 "C for Ts,within 3 "C for TF(") as well as for TR and within 20% for maxF, the arithmetic averages were calculated from the two respective averages and reported
Marzec et al.
100 Energy & Fuels, Vol. 6, No. 1, 1992
Table 11. Correlation Coefficients" between the Gieseler Thermoelastic Proeerties of the 27 Coals temperature of max Softening fluidity. max softening -~temp ( Ts) maximum fluidity (log F(max)) temp of max fluidity; TF(-) temp of resolidificn (TR)
0.72
0.97
0.88 0.72 plastic range (TR-Ts) Coefficients I[0.4] are not shown in the table.
1 0.82
in Table I. If the criteria were not met, the results were discarded and a second set of quadruplicates run. In general, there was no need to run the second set of quadruplicates except for a few coals showing maximum fluidity above 3000 angle units/min. All the thermoplastic measurements for the 27 coals were carried out by the same operator with the same apparatus. There are differences between the Polish standard applied and the ASTM D 2639-89. One of them refers to fluidity units. The fluidity unit of one dial division used in the ASTM method corresponds to 3.6 angle units applied in the Polish standard. Moreover, the definitions of softening temperatures are different (Ts according the ASTM standard in question is the temperature at which the dial movement reaches a rate of 1 dial division per minute). Thus, Ts values determined by the ASTM standard would be higher compared with the Ts values reported in Table I. Extraction of the Coals Using Pyridine. The extraction was carried out in an extrador unit (1043 Model, Tecator, Sweden) in boiling pyridne (Py);the samples were placed in magnetically sealed thimblea that prevented any removal of dparticles during extraction. Each coal sample was subjected to extraction as long as was needed to obtain colorless filtrate, which was from 70 to 100 h. Every 8 h, the solution of extracted material was removed and fresh solvent used. The solutions were freed from Py in a rotary evaporator under reduced pressure, 2 Torr, at 50 OC, until constant weight was attained. The extracted coal samples were placed in a Sohxlet and again extracted with water/HCl mixture, 100 mL/mL of 1N HC1 ratio, and finally, with water until neutral filtrate was attained. The washed residues were dried in a vacuum drier, at 1Torr and 50 "C, until a constant weight was stated. These Py residues were then subjected to swelling measurements. Material balances of extraction (extract plus Py residue) were in the range from 99 to 105 wt % of the coal samples. The yields of extracts were calculated as wt % daf of a coal. Differences of duplicate runs were in the range from 0.3 to 3% daf. When the difference was over 1% daf, a third extraction run was carried out. The arithmetic averages of duplicates or triplicates are shown in Table I. Swelling Measurements. The Py-residue samples were placed in cylindrical graduated glass tubes, up to -20 mm height. The tubes were centrifuged and then the initial height of the sample layer (h,) was measured. Pyridine was added up to 100 mm height. The tubes were sealed with glass stoppers, shaken
vigorously, and placed in a thermostat at 20 "C. After 48 h, the tubes were centrifuged and the height of swollen sample layer (h,)was measured. The swelling was calculated as 100[hl- h O ] / b ; the calculated averages from duplicate runs are shown in Table I. Chemometric Evaluation of Thermoplastic, Conventional, Swelling, and Extraction Data. The chemometric calculations, that is correlation as well as linear regression analyses, were carried out using SPSS/PC software package. Over 100 correlation coefficients were calculated between every possible pair of the coal properties. Eventual nonsystematic errors of determination of some coal properties might lead to lower values of correlation coefficients compared with those that could be obtained if the experimental determinations would have been correct. In such case, a significant correlation might have been evaluated as insignificant. With the aim to avoid such erroneous evaluation as far as possible, correlation coefficients below the level of significance ( r < [0.5]) are not shown or discussed.
Results and Discussion All the characteristics of the twenty-seven coals taken into consideration in this study are presented in Table I. Swelling data included in the table provide a relative measure of cross-linking density of macromolecular part of coal material. On the basis of Flory-Rehner equationz0 and the assumption that interaction parameters between pyridine and coal macromolecular system are not significantly different for the studied coals, one can say the higher the swelling of a coal the lower is its cross-linking density. The yield of material extracted with the use of pyridine from a coal (Table I) is an approximate measure of the content of molecular components in coal material in contrast to the extraction residues which represent macromolecular and cross-linked material. Correlation coefficients between all the thermoplastic properties of the twenty-seven coals are shown in Table 11. The softening temperature, Ts, is very weakly correlated with maximum fluidity (r = 0.49) and with none of the other properties which occur in the higher temperature range, from 390 to 490 "C.Thus, the nature of softening must be far away from the other thermoplastic phenomena. In contrast to Ts,extremely high correlation is observed between temperature of maximum fluidity, TFc,-,, and temperature of resolidification, TR(r = 0.97). The correlations found between the thermoplastic properties and the conventional data i.e., elemental composition of the coals, their volatile matter (VM) and moisture (M) content, are shown in Table 111. Moreover, the table shows correlations for exinite content; other petrographic components, vitrinite, and inertinite as well as their constituents (shown a t the end of Table I) were not significantly correlated and, therefore, they are not included in Table 111. The table also shows correlation coefficients for swelling data and Py extract yields. Some of the analytical data used in this study are correlated to each other, as can be seen in Table IV; this may pose a difficulty in the search for causative links between chemical and thermoplastic properties of the coals. Some of these intercorrelations are trivial; an example is the high
Table 111. Correlation Coefficients between Thermoplastic Properties and Analytical Data for the 27 Coals elemental composn" VM C H 0 moisture exiniteb swelling Py extract temp of softening (Ts) (-0.57)d -0.82 -0.74 -0.79 temp of max fluidity (TF(ma.u)) (-0.78) (0.68) -0.71 -0.68 -0.58 -0.68 C temp of resolidificn (TR) (-0.77) (0.67) -0.72 -0.70 -0.64 -0.63 c plastic range (TR-TS) (0.52) -0.57 -0.64 No correlations were found for N and S contents. No correlation were found for the other macerals. The coals were divided into two populations when the thermoplastic properties were plotted versus pyridine-extract yields (see Figure as an example). dFor values in parentheses see the text. (I
Thermoplastic Properties of Coals
Energy & Fuels, Vol. 6, No.1, 1992 101
Table IV. Correlation Coefficients between Analytical Data for the 27 Coals' C
H 0 VM moisture exinite swelling Py extract a
C 1
H
0
VM
moisture
exinite
swelling
Py extract
1 -0.98 -0.58 -0.47 -0.52
0.52
1 0.56
0.51 0.67 0.77
1 0.68 0.52 0.81 0.55
1 1
1 0.74
1
Coefficients r 5 [0.4] are not shown. Table V. ExDerimental and Calculated Softening Temwratures of the Coals softening temp, OC softening temp, oc exptl Ts calcd Ts, Ts - TG coal no. exptl T s cdcd Tb 114 367 358 9 380 380 4 115 370 366 360 358 116 360 -5 355 350 360 117 367 6 373 370 373 369 3 118 372 358 362 -7 360 119 353 377 369 3 367 120 370 373 373 -3 353 121 350 400 395 -4 349 122 382 345 388 -5 355 364 123 350 369 -1 124 353 357 352 362 12" 354 377 125 366 362 1 373 126 374 365 368 -7 374 367 ~
coal no. 100 101 102 103 104 105 106 107 108 109 110 111 112 113
~~
TE- Ta 0 2 -1W -3 -4 8 0 5 -6 -5 -5 15O -3
High mineral matter content in these coals (ash from 13 to 18 w t % dry basis) may lead to erroneous Tsdetermination.
correlation (Table IV)between carbon and oxygen content (r = -0.98) which is a consequence of the fact that they are major elements of coal material constituting 90-95 wt % daf of coals. Thus, the correlations of thermoplastic properties with carbon content shown in Table I11 (in parentheses) are of no causative meaning compared with the higher coefficients for oxygen content. The data of Table IV indicate that all the chemical properties are correlated with volatile matter content VM and imply that all of them have an impact on thisstandard property. Thus, VM is a summary symptom of various chemical characteristics of coal structure such as elemental composition, exinite content, pyridine extractables content, and cross-linking density of coal material. Therefore, its correlation coefficients with thermoplastic properties shown in Table 111(in parentheses) cannot be useful in the search for causative links between a given chemical property and thermoplasticity of coals. Temperature of Softening. As the data of Table I11 show, the temperature of softening is significantly correlated with content of hydrogen in coals (r = 4.82), Pyextract yields (r = 4-79),and swelling (r = -0.74). Hence, the higher the content of hydrogen and Py-extract yields and the swelling the lower is the temperature of softening. Linear regression analysis produced an equation
Ts = 495.19966 - 0.5599Py-E - 0.08603Sw 22.624088
RZ = 0.837 (1)
Softening temperatures calculated with the use of this equation for each of the twenty seven coals are shown in Table V. The differences between the experimental and calculated values are in the range from -7 to +8 OC (except for three coals). Thus, strong links are evident between softening and the three chemical characteristics. The present results are consistent with the results of other (20) Green, T.;Kovac, J.; Brenner, D.; Larsen, J. W. In Coal Structure; Meyers, R. A., Ed.; Academic Press: New York, 1982; pp 199-282.
works (see Introduction) showing that the content of ext r a c t a b l e ~ ' ~and J ~ cross-linking of coal materia13J0J1influence softening. However, our results indicate that if one aims to get a quantitative relationship (eq l),one has to include the third parameter which is hydrogen content. It is concluded that the initial softening of coal indicated by the temperature of softening is a phase transition that results in an increase of thermal-induced mobility of the coal system. The energy demand of the transition is proportional to the softening temperature. The softening is a colligative property; i.e., it depends only on the amount and not the nature of system components. The present results indicate that it depends on (a) content of molecular part (Py-extract yield) since the energy demand for ita thermal induced mobility is lower compared with the demand of macromolecular part; (b) the density of crosslinking of macromolecular part (which is inversely proportional to ita swelling) for the reason that the less cross-linked the macromolecular system is, the lower its energy demand for thermal mobility; (c) content of aliphatic and alicyclic structural fragments in coal material (which is directly related to coal hydrogen content) for the reason that energy demand of these structures is lower compared with aromatic structural units. Maximum Fluidity, F(max), is correlated with the temperatures of maximum fluidity (r = 0.59) and of resolidification (r = 0.72); thus, the higher the maximum fluidity the higher these temperatures are (Table 11). Strong correlation was found between F(max) and the plastic range ( r = 0.88;Table 11)which indicates that the development of high fluidity can occur if a coal is characterized by a wide plastic range. No correlations were found between maximum fluidity, as well as ita logarithmic values, and the coal characteristics used in this study. For this reason, F(max) is not present in Table III. Thus,the correlation analysis carried out for the large set of coals does not show links that were found or could be inferred from other studies, between maximum fluidity and amount of e~tractables;~f'~~J~J~ sulfur ~ontent,'~
Marzec et al.
102 Energy & Fuels, Vol. 6, No. 1, 1992
51
I
5
40
45
$0
25
Py- Extract; % daf
30
*
Figure 1. Maximum fluidities versus pyridine-extractyields of the 21 coals. Lines show a relationhsip between these two characteristicswithin each of the two populations of the coals.
hydrogen content,14and oxygen ~ 0 n t e n t . l ~ When logarithmic values of maximum fluidities of all the coals were plotted versus the yields of Py extracts, the coals were divided into two populations (Figure 1); within each of the populations, a relationship between F(max) and Py-extract yields can be seen. However, Figure 1 clearly demonstrates that the Py extract yields are of minor importance compared with another, yet unknown property of the coals. Due to this property, the coals with the same Py-extract yields show maximum fluidities differentiated by a factor of 100 (see for example coals 113,119, and coals 101, 103, 118, 125). This result as well as the lack of correlation of F(max) with the other coal characteristics imply that F(max) is dependent on some fine properties of chemical structure of coals that are not manifested by the analytical methods used in this study. Temperatures of Maximum Fluidity (TF(")) and Resolidification ( TR).The two temperatures are correlated with the same chemical properties (Table 111)and values of their correlation coefficient are close to each other. Thus, TF(mar) and TR are correlated with oxygen content (r = -0.71 and -0.72); moisture (r = -0.68 and -0.70); exinite (r = -0.58 and -0.64);and swelling (r = -0.68 and -0.63). The correlations found for the temperatures and oxygen content in coals are consistent with results3J5 showing that oxidation decreases the temperatures. Thus, TF(") and TR are dependent on oxygen, exinite, and moisture content in coals. The higher their contents in coal material, the lower temperatures of maximum fluidity and resolidification; in contrast to this, the high content of cross-links in coal material results in high values of TF(mcu)and TR. These correlations do not explain, however, whether all oxygen compounds, all exinite components, and all cross-links, or perhaps some of them, have an impact on the temperatures. The role of moisture is also obscure. No correlations were found between TF(") (as well as TR)and Py-extract yields for the set of twenty-seven coals. When the temperatures in question had been separately plotted versus Py-extract yields, the coals were divided into two populations. These plots are not shown here, since the two populations can be seen from the plot already shown in Figure 1for maximum fluidity. This result indicates, as in the case of maximum fluidity, that the two temperatures are influenced by some, yet unknown, fine properties of chemical structure of coal material. Summing up, the correlations shown in Table 111 between the two temperatures in question and the coal characteristics, as well as strong correlation between the
temperatures themselves (Table 11), indicate that temperatures of maximum fluidity and resolidification have the same background. They depend on the same chemical structures of coal material which are yet unknown. On the other hand, it can be reasonably assumed that the temperatures are symptomatic for condensation and/or polymerization reactions; the first temperature is related to commencing and the second one to ending of these reactions. Thus, the structures in question must be involved in these reactions. Volatile Matter Content and Petrographic Composition of the Coals as Predictive Means of the Thermoplastic Properties. Our results showed that softening temperature of the coals is not correlated with any of the petrographic components. The temperature is correlated with VM content however, the correlation coefficient is rather low (r = -0.57 in Table 111). Maximum fluidity as well as the plastic range of the coals is not correlated with VM content or with any of the petrographic components. Thus, softening temperatures, maximum fluidity, and the plastic range of the studied coals cannot be predicted on the basis of their volatile matter content and petrographic composition. In contrast, the temperatures of maximum fluidity and resolidification are correlated with VM and exinite contents in the coals (Table 111). The linear regression analysis produced the following equations: TF(max)
- 492.517 - 1.4963 - 1.732 VM
(R2= 0.657)
TF(") values were calculated using the equation and compared with the experimental values. The differences (experimental - calculated) were in the range from -26 to +16 "C. TR = 534.363 - 2.4483 - 1.992 VM (R2= 0.675) The differences (experimental - calculated) were in the -29 to +28 "C range. The results indicate that VM and petrographic composition cannot serve as predictive means for the softening temperature, maximum fluidity, and the plastic range, and show rather limited prediction capability of the temperatures of maximum fluidity and of resolidification. Correlation Analysis for Enlarged Set of Coals. In the find step, correlation analysis was carried out for the set of 33 coals that consisted of 27 Polish coals and 6 thermoplastic coals from other countries. Characteristics of the foreign coals are shown (see coals from F-1 to F-6 in Table I). Comparison of the correlation coefficients obtained for the set of 27 (Polish) and of 33 coals (foreign coals included) showed that the coefficients are essentially the same. This indicates that correlations found for carboniferous coals derived from one coal region may be also valid for coals from other regions. Conclusions
Chemometric studies of thermoplastic coals that were characterized in terms of molecular and macromolecular systems and with use of proximate, ultimate, and petrographic data, revealed the following: 1. Quantitative links between softening temperature and some of the bulk characteristics of the coals; these links form a reasonable basis for understanding the nature of initial softening which has been viewed as a phase transition. Energy demand of the transition, indicated by the temperature of softening, depends on the content of molecular (Py extractable) part; cross-linking density of the macromolecular part; and the content of aliphatic and alicyclic structures.
Energy & Fuels 1992, 6, 103-108 2. Significant correlations between temperatures of maximum fluidity and resolidification as well as between the temperatures and some of the coal characteristics. It has been concluded that the temperatures have the same chemical background; however, the applied analytical means were not capable of identifying it. Another, more specific approach for chemical characterization of coal material on a molecular basis should be used for identification of structural units or compounds that have an impact on these thermoplastic temperatures and on fluidity. 3. Quantitative links between some of the thermoplastic properties and volatile matter content and petrographic composition of the coals. These two conventional param-
103
eters showed rather limited value as predictive means of temperatures of maximum fluidity and of resolidification and no value in prediction of the other Gieseler thermoplastic properties.
Acknowledgment. We gratefully acknowledge financial support of the Committee of Scientific Research in Poland and of the Deutsche Forschungsgemeinschaft, Bonn-Bad Godesberg/Project Schu 416/ 15-1, Germany. A.M. expresses appreciation to Emil Yax, Centre de Pyrolyse de Marienau, France, for providing the F1-F6 coal samples. Registry No. Py, 110-86-1.
Mass Spectrometric and Chemometric Studies of Thermoplastic Properties of Coals. 2. Field Ionization Mass Spectrometry of Coals Hans-Rolf Schulten* Department of Trace Analysis, Fachhochschule Fresenius, Dambachtal20, D - 6200 Wiesbaden, FRG
Anna Marzec and Sylwia Czajkowska Institute of Coal Chemistry, Polish Academy of Sciences, 1 Maja 62, 44-100 Gliwice, Poland Received April 22, 1991. Revised Manuscript Received October 18, 1991
The aim of the study was to find chemical structures of coal material that determine its thermoplasticity. Twenty-seven coals (carboniferous, Poland) showing a wide range of thermoplastic properties were analyzed. Three sets of data were obtained for all the coals pyrolysis-field ionization mass spectra of the coals; (2) thermoplastic properties measured using the Gieseler plastometer; and (3) amounts of hydrogen consumed by the coal samples in reaction with tetralin as H donor. Chemometric techniques were applied to evaluation of quantitative links between the three sets of data. The results show that two groups of coal constituents, arising from a coal or formed pyrolytically, influence thermoplasticity in opposite ways. Group A consists of a large variety of unsubstituted aromatic hydrocarbons as well as sparsely substituted multiring aromatics and hydroxy compounds; the higher their content the lower are thermoplastic properties of a coal. It is concluded that components of this group undergo condensation or polymerization reactions that bring about lowering of fluidity and temperatures of maximum fluidity and resolidification. Group B components include alkylated aromatic hydrocarbons; the higher their content the higher are the thermoplastic properties. It has been assumed that the species are inert in these reactions, dilute the active structures, and impose a delay of condensation and polymerization in this way. Links are shown between reactivity of the structures in thermally induced condensation or polymerization and in reaction of hydrogen transfer from the H donor. In conclusion of the results presented in parts 1and 2 as well as of works referring to mechanism of thermal decomposition of coal, a general view of coal thermoplasticity is presented.
Introduction The work presented in part 2 aims to identify components of coal material that have an impact on thermoplastic properties of coals. As a means of identification, pyrolysis (PY) field ionization (FI) mass spectrometry (MS) has been applied. FI MS is a soft ionization mode which in combination with temperature-programmed and time-resolved pyrolysis carried out in situ in the ion source of the mass spectrometer produces almost exclusively molecular ions of thermal degradation products in mass 0887-0624/92/2506-OlO3$03.00/0
range up to m / z 10oO. Recently an improved experimental setup for PY-FI MS has been described and its application to studying various biomaterials has been shown.' The specific role of PY-FI MS of coals lies in that the technique provides information about individual components of volatiles generated by heating coal. These volatiles play an important role in coal reactivity in all kinds of coal (1) Schulten, H.-R.; Simmleit, N.; Mueller, R. Anal. Chem. 1987,59, 2903-2907.
0 1992 American Chemical Society
