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
104 Energy & Fuels, Vol. 6, No. 1, 1992
processing that apply heating, for the reason that the species, trapped in grains or lumps of processed coal, are able to move through and penetrate the material and react with it, if not quickly removed from fine particles by high vacuum as in the case of Py-FI MS analysis. Py-FI MS in combination with chemometric techniques was already applied to identification of coal components that are reactive in liquefaction,2 hydrogen t r a n ~ f e rand , ~ low-temperature pyrol~sis.~ In the present study, FI mass signals of coal material pyrolyzed in the mass spectrometer have been related to thermoplastic properties of coals by application of pattern recognition techniques and correlation analysis to three sets of data: (1) PY-FI mass spectra of 27 coals; (2) thermoplastic properties of the coals; and (3) amounts of hydrogen consumed by the coals in reaction with tetralin as a hydrogen donor. The latter set has been included for the reason that links between hydrogen transfer and pyrolysis3 as well as between hydrogen transfer and resolidification temperature5v6were already stated. Thus, the inclusion can perhaps assist in understanding of chemistry of coal thermoplasticity.
Experimental Section
Schulten et al. Table I. Amount of Hydrogen Transferred ( . H-..dfrom Tetralin to thsCoal Samples" Hr,,g of H/100 E of daf coal coal no. run 1 run 2 av 100 0.78 0.77 0.78 101 0.96 0.94 0.95 102 1.05 1.07 1.06 103 1.24 1.15 1.20 104 0.71 0.71 0.71 105 0.70 0.67 0.68 106 0.75 0.78 0.76 107 0.73 0.75 0.74 108 1.10 1.10 1.10 109 0.64 0.63 0.63 110 0.56 0.55 0.55 111 0.43 0.41 0.42 112 0.51 0.48 0.49 113 0.33 0.31 0.32 114 0.12 0.12 0.12 115 0.70 0.65 0.67 116 0.62 0.63 0.62 117 0.77 0.74 0.76 118 0.64 0.59 0.61 119 0.25 0.29 0.27 120 0.82 0.85 0.83 121 0.41 0.42 0.41 122 0.33 0.38 0.36 123 0.41 0.46 0.43 124 0.75 0.69 0.72 125 0.53 0.52 0.52 126 0.57 0.59 0.58
Coals. Twenty-seven carboniferous coals from Poland were studied. Their individual characteristics such as proximate, ultimate, and petrographic composition, pyridine-extract yields, and swelling data, were already presented (Table I in part 1). See the text for the reaction conditions and other details. Field Ionization Mass Spectrometry of the Coals. The analysis of coals by PY-FI MS has been described p r e v i ~ u s l y . ~ ~ ~ ~ ~ test for thermal decomposition of tetralin a t 400 "C. Tetralin Using the direct introduction system of a double-focusing mass was heated in the autoclave at 400 "C/60 min and GC analyzed. spectrometer (Finnigan MAT 731, Bremen, Germany) about 100 The results of analysis, 97.8% tetralin; 1.4% naphthalene; and pg of the ground coal samples were heated in the high vacuum compounds of lower and higher retention times 0.5% and 0.3%, of a combined EI/FI/FD ion source. The samples were heated respectively, were used for calculation of corrected amount of a t a rate of 1 "C/s until the volatilization of the coal sample was hydrogen transferred to the coal samples, Le., HTr values. These complete, which in general occurred a t a temperature in the H T values ~ determined in duplicate for each of the 27 coals are 500-550 "C range. Yields of volatilization of the coals were in shown in Table I. the 13-37% daf range. During heating of a coal sample, 40 mass The range of Hn values is from 0.1 to 1.2 g of hydrogen per scans were recorded electrically. The FI mass signals of each ecan 100 g of coal dry and ash-free material (dafj. Differences between were integrated, processed, and plotted using Spectro-System SS duplicate determinations are in the range of 0.02-0.06 g of hy200 to give a summed spectrum. Thus, the summed spectrum drogen/100 g daf of coal. represents all volatile5 that were successively emerging from the Chemometric Evaluation of Mass Spectrometric, Thercoal sample on its heating u p to 500-550 "C. Four separate runs moplastic, and Hydrogen Transfer Data. The evaluation was for each specimen were recorded and their averaged summed carried out using a modified ARTHURstatistical software package. spectra used for further studies. The chemometric handling of the data has been described preThermoplastic Properties of the Coals. The Gieseler v i o ~ s l y . ~Summing -~ up, the PY-FI MS spectra of all the coals thermoplastic properties of each of the studied coals are shown were quantified by m/z values and intensities of approximately in Table I of part 1. The coals represent a wide range of ther600 mass signals detected in the 100-700 m/z range. These MS moplastic properties: temperature of softening is from 345 to 400 data as well as thermoplastic properties and hydrogen-transfer "C; maximum fluidity is from 6 to 39000 angle unita; temperature values measured for all the coals were subjected to correlation of maximum fluidity, from 393 to 458 "C; and temperature of analysis. The results of correlation analysis are presented in Table resolidification, from 417 to 490 "C. Results of correlation analysis I1 (see Resulb section). between the thermoplastic properties are presented in Table I1 of part 1,and that between the thermoplastic properties and the Results other coal characteristics, in Table I11 of part 1. Reaction of Coals w i t h Tetralin. The reaction was carried FI Difference Mass Spectrum of Selected Coals. out for coal-tetralin mixtures 1:2 w/w, a t 400 "C for 60 min. Four coals were selected (coals no. 100,102,103, and 120 Duplicate testa were run for each coal. The amount of hydrogen in Table I of part 1 ) that show low resolidification temtransferred from tetralin to a coal sample was calculated on the peratures, in the 417-430 OC range; their FI mass spectra basis of GC determination of unreacted tetralin and naphthalene were normalized and summed. Next, the spectra of four ratio. Other details were previously reported3 except for blank coals (no. 114, 119, 121, and 122) showing the highest (2) Schulten, H.-R.; Simmleit, N.; Marzec, A. Fuel 1988,67,619-625. (3) Marzec, A.; Czajkowska, S.; Simmleit, N.; Schulten, H.-R. Fuel Process. Technol. 1990,26, 53-65. (4) Schulten, H.-R.; Marzec, A.; Simmleit, N.; Mueller, R. Energy Fuels 1989, 3, 481-487. (5) Iyama, S.; Yokono, T.; Sanada, Y. Carbon 1986, 24(4), 423-428. (6) Yokono, T.; Uno,T.; Obara, T.; Sanada, Y. Trans. Iron Steel Inst. Jpn. 1986,26, 512-518. (7) Schulten, H.-R.; Marzec, A. Fuel 1986, 65, 855-860. (8) Marzec, A.; Schulten, H.-R. Prepr. Pap.-Am. Chem. Soc., Diu. Diu.Fuel Chem. 1989, 34(3), 668-675.
resolidification temperatures, in the 480-490 "C range, were also normalized and summed. Figure 1 shows the difference spectrum of these two groups of coals. The spectrum indicates that the composition of the volatilized material in the mass spectrometer is different for the two groups of coals. Hence, the PY-FI MS technique can supply information about components that are affecting thermoplastic properties of coals. FI Mass Signals Correlated with Thermoplastic Properties and Hydrogen Transfer. Correlation coef-
Thermoplastic Properties of Coals
Energy & Fuels, Vol. 6, No. 1, 1992 105
= I
2
-50-
156
a,
.->
c,
u
-100-
344
192 206
a, LT
I 332
318
230
-1501
244 280 \ 2 9 4 I
I
I
I
I
,
,
ficients between intensities of FI mass signals and the thermoplastic properties for the set of 27 coals are shown in Table 11,except for softening temperature which is very poorly correlated. The table also shows coefficients between FI mass signals and hydrogen transfer. A correlation coefficient r between a property A and a property B shows that the higher the values of a property A, the higher are the values of a property B (if r is positive) or the lower are the values of the property B (if r is negative). It is noteworthy to point out that the correlation coefficients indicate that there is some monotonic trend between the A and B pair, which may be or may be not a linear trend. FI Mass Signals Correlated with Resolidification Temperature ( TR) and Temperature of Maximum The same 81 mass signals are correFluidity ( TF(mar)). lated with the two temperatures; not one mass signal can be found that would be correlated with one of the temperatures and not with the other. Moreover, the values of correlation coefficients are essentially the same. For this reason, Table I1 shows only the coefficients for one of the temperatures (resolidification). These results indicate that the two temperatures are effected by exactly the same components of coal material. Out of all the correlated signals, 51 mass signals (upper part of Table 11)are negatively correlated with TR and Tp("). This means the higher the intensities are of these mass signals in FI mass spectrum of a coal, the lower are TR and TF(-) of the coal. Contrary to that, 30 signals (lower part of Table 11) are positively correlated with TR and TF("). Hence, the higher the intensities are of the signals the higher are TR and TF(") of a coal. FI Mass Signals Correlated with Maximum Fluidity. All 35 signals that were correlated with logarithm of maximum fluidity are shown in Table 11. It happened that the signals were also correlated with T R and TFI-). Hence, some structures that are affecting the two temperatures have also an influence on maximum fluidity. All other FI mass signals are not related to maximum fluidity since the
,
[
,
,
,
I
,
,
,
,
,
,
respective correlation coefficients are below [0.5]. Structures for all the correlated mass signals are shown in Table III. The structures represent only one of possible isomers and other isomers can be found elsewhere.9 It is worth pointing out that isomers have the same number of aromatic rings. For example, pyrene (m/z 202) is displayed in the table, although there are three other hydroc81;bons, i.e., aceanthrylene, acephenanthrylene, and fluoranthene, all with the same accurate mass and with four aromatic rings as for pyrene. Structures that refer to FI mass signals negatively correlated with the thermoplastic properties (and positively correlated with Hn)are displayed in the upper part of Table 111. The structures are hydrocarbons and hydroxy compounds. The hydrocarbons represent a variety of aromatic structures, except for species that contain one and two aromatic rings. The majority of these hydrocarbons contain no alkyl substituent, or methyl group only. The hydrosy compounds contain from one to four aromatic rings. At least some of them have OH group directly attached to an aromatic ring. Structures of FI mass signals positively correlated with the thermoplastic properties (and negatively with HTr)are shown in the lower part of Table 111. They are entirely different species from those in the upper part of the table. There are hydrocarbons with two aromatic rings. Others contain three, four, and six aromatic rings. However, these higher aromatics show more alkyl carbon atoms compared with the respective aromatics in the upper part of the table.
Discussion It has been assumed that all structures (upper part of Table 111) that are negatively correlated with resolidification temperature represent species that undergo polymerization or condensation reactions; therefore, the higher (9) Lee, M.
L.;Novotny, M. V.; Bartle, K. D.Analytical Chemistry
of Polycyclic Aromatic Compounds, Academic Press: New
pp 363-386.
York, 1981;
Schulten et al.
106 Energy & Fuels, Vol. 6, No. 1, 1992 Table 11. FI Mass Signals Correlated with Temperatures of Resolidification ( TR)and of Maximum Fluidities" ( TF(mu)), Maximum Fluidities (F(max)), and with Hydrogen Transfer (ET,.) of the Coals correln coeff for correln coeff for
TTnnnr Part vyp" - 1
158 160 162 164 172 174 176 188 190 200 202 212 214 224 226 238 240 250 252 254 260 262 264 274 276 278
0.85 0.59 0.81 0.82 0.85 0.69 0.76 0.79 0.53 0.70 0.54 0.91 0.80 0.50 0.93 0.82 0.91 0.82 0.81 0.77 0.57 0.72 0.85 0.73 0.76 0.88
166 168 180 182 192 194 196 206 208 218 220 230 244 256 258
-0.70 -0.58 -0.80 -0.71 -0.78 -0.83 -0.62 -0.91 -0.71 -0.72 -0.80 -0.86 -0.90 -0.69 -0.90
-0.51
-0.82 -0.56 -0.75 -0.74 -0.83 -0.63 -0.71 -0.70 -0.59 -0.67 -0.60 -0.85 -0.83 -0.60 -0.91 -0.88 -0.94 -0.81 -0.84 -0.75 -0.64 -0.75 -0.90 -0.73 -0.76 -0.79
288 290 300 302 314 328 338 340 342 350 352 364 366 374 376 378 380 386 388 390 392 400 404 418 442
0.69 0.68 0.62 0.87 0.86 0.79 0.81 0.64 0.83 0.71 0.90 0.85 0.88 0.67 0.80 0.88 0.59 0.51 0.74 0.87 0.84 0.58 0.75 0.76 0.81
-0.52 -0.53 -0.60 -0.57 -0.57 -0.70 -0.53 -0.60 -0.72 -0.64 -0.55 -0.52 -0.67 -0.75 -0.64 -0.64 -0.64
-0.70 -0.55 -0.65 -0.85 -0.86 -0.83 -0.77 -0.48 -0.77 -0.72 -0.92 -0.88 -0.83 -0.76 -0.86 -0.90 -0.55 -0.66 -0.84 -0.89 -0.88 -0.70 -0.71 -0.57 -0.64
Lower Part 0.52 0.69 0.54 0.64 0.61 0.60 0.55 0.60 0.70
0.70 0.72 0.87 0.80 0.85 0.85 0.55 0.90 0.77 0.74 0.59 0.90 0.90 0.74 0.85
280 284 294 298 306 308 318 320 322 330 332 334 344 346 358
-0.75 -0.74 -0.86 -0.72 -0.75 -0.87 -0.81 -0.88 -0.59 -0.57 -0.83 -0.87 -0.83 -0.87 -0.72
0.75 0.59 0.71 0.62 0.69 0.66 0.68 0.51
0.85 0.71 0.91 0.71 0.79 0.84 0.85 0.93 0.62 0.70 0.87 0.79 0.79 0.85 0.68
Correlation coefficients between FI mass signals and temperatures of maximum fluidity are not shown since they are essentially the same as for temperatures of resolidification.
their content in a coal the lower is its resolidification temperature. The assumption is in accord with studies of carbonization of model aromatic hydrocarbonslOJ1showing for example that perylene and benzopyrene readily undergo condensation reactions at 430-450 "C. Hydroxy compounds, especially phenolic substances, are able to polymerize; an ionic mechanism was suggested for their po1ymerization.l2 As the correlation analysis indicates, all these aromatic and oxygen species (shown in the upper part of Table 111) are also active in reaction with tetralin as H donor. In contrast to that, it has been concluded from correlation analysis that all structures (shown in the lower part of Table 111) that are positively correlated with TR and negatively with HTrare inert in polymerization and con(10)Zander, M. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1989, 34(4), 1218-1222; and references therein. (11) Zander, M. Fuel 1986, 65, 1019-1020. (12) Wang, F. M.; Senthilnathan,V. P.; Stein, S.E. h o c . I989 Coal Sci. Con/.,Tokyo, 1989, 165-168.
densation reactions, as they are in H-transfer reactions. A part of the inactive structures such as Cl-C2 naphthalenes, C1-Cs fluorenes, C1-C2 acenaphthenes represent two-ring aromatics. Our experiments (unpublished data) carried out for model compounds referring to this group were in accord with the conclusion that they are inert. Naphthalene, methylnaphthalenes, fluorene, and acenaphthene did not undergo condensation when they were heated at 440 OC for 60 min nor did they react with tetralin when heated at the H-donor presence. Thus, the two-ring aromatic hydrocarbons are too stable to react. A comparison of the remaining inactive hydrocarbons with the active ones indicates either higher number or longer alkyl substituents in the inactive structures; for example, C2-C,-pyrenes ( m / z 230, 244, and 258 in the lower part of Table I1 and III) are inactive while unsubstituted pyrene ( m / z 202 in the upper part of Tables I1 and 111) is active. Clearly, steric hindrance exerted by alkyl substituents on aromatic rings is important. Whatever the reason why the species are inactive, their presence imposes a diluting effect on the concentrations of the reactive species. This brings about a delay of condensation and polymerization. The delay is manifested by higher temperatures of maximum fluidity and resolidification in standard measurements carried out at a constant heating rate with the use of Gieseler plastometer. Thus, the ratio of all the active and inactive species in a coal seems to determine its TF(-) and TU. Exactly the same results of correlation analysis obtained for temperature of maximum fluidity and of resolidification (see also Table 111in part 1)show strong link between the two temperatures which lies in that one of them (TF(")) indicates an initial step and the other one (TR),the final step of polymerization and condensation in a coal system. The results of correlation analysis in Table I1 show that there is a common background for chemistry of thermoplasticity and of hydrogen transfer from H donor to coal material. The same structures are active in condensation and polymerization and in hydrogen transfer when coal is exposed to an H donor. Another group of structures is inactive in both types of reactions. These findings bring about some implications. Thus, a measurement of HTr values for coals can be useful in comparing coals with respect of their capability to undergo condensation and polymerization during carbonization in a temperature range of their thermoplasticity. Another implication is that liquefaction of a coal should he carried out at a temperature below TF(max) of the coal, if one aims to avoid retrogressive reactions resulting from condensation and polymerization. Contrary to TF(") and TR, maximum fluidity is correlated with much lower number of FI mass signals (only 35 signals in Table II). The negatively correlated mass signals with maximum fluidity (the signals starting from m/z 278 in upper part of Table 11) correspond to hydrocarbons (shown in Table 111) that contain at least five aromatic rings. Hence, the results indicate that such hydrocarbons as picene, dibenzopyrene, etc., tend to undergo condensation before TF(,-) is reached and that they suppress the development of a high fluidity in this way. Since all components of coal material have to be either reactive or unreactive in the reactions in question and "tertium non datur", one should expect that each mass signal should be (either negatively or positively) correlated with TR(-) and TR;the same conclusion refers to reaction of hydrogen transfer and correlation of the signals with HTr values. However, this is not the observed case. Only 81 signals are correlated out of several hundred signals that
Thermoplastic Properties of Coals
Energy & Fuels, Vol. 6, No. 1, 1992 107
Table 111. Structures in the Correlated FI Mass Signals mlz
108 110 146 134 158 198 224 190 202 250 226 252 260 264 276 302' 300 340 350 3526 374b 386' 4046 142 166' 192 218 168 230' 256 280 306 304 330
122 124 160 148 172 212 238
136 138 174 162
and 240
262 254
274 278' 290
288
314
328'
364b 366' 38Bb 400 418'
378' 380' and
156 180' 206' 182' 244' 294' 3206 318 344
194 2206 196' 25S6 2846 30Sb 334' 332 358
proposed structures Upper Part: Structures Active in Condensation, Polymerization, and H Transfer 150 164 C,-C,- hvdroxvbenzenes" C&;-dihydroxybenzenesn 188 C1-C4-hydroxyindenes" 176 Co-C4-dihydroxyindenes" 190 214 200 Cl-Cz and C4-C5-hydroxynaphthalenes" C2-C3-hydroxyacenaphthenesa C3-C4-hydroxyfl~orenes~ and Cl-C2-octahydropyrenesu 4H-cyc1openta[deflphenanthrenea pyrenen C3-hexahydropyrene"and C4-tetrahydropyrene" Co-C2-benzo[ghi]fluoranthenes;"Co-Cl-cyclopenta[deflchrysenes;n Co-cyclohexa[deflchryscne" perylenen CnH2n-20N2" C0-Cl-11H-cyclopenta[ghi]chrysene;" Co-picene;"C5-C6-hydroxyanthracenes" Co-C1-anthanthrenes" C0-dibenzopyrene;"C6-hydroxypyrene' 342' Co: C1,(l Cz,OC3-coronenes;Con-C,-dibenzochrysenes Cl-dibenzocyclopenta[cd] pyrene 392b Co-C3-benzocoronenes;C&-tribenzochrysenes Co-C2-tribenzopyrenes 3906 ? 376' 4426 ? ?
208
298' 322' 346'
Lower Part: Structures Inactive in the Reactions Cl-C2-naphthalenes" C4-C7-dihydroxybenzenesnand Co-C3-fluorenes' C1-C3-anthracenes" C2-4H-cyclopenta[deflphenanthrene Cl-C,-acenaphthenes" and C13H12NZn C2-C4-pyrenesn C2, C4,(1C5-chrysenes C2,(1C3,(1C4, C6-perylenes C3, C4, C5n-11H-cyclopenta[ghi]chrysenes C2-C5-anthanthrenes C2,(1C3, C4-dibenzopyrenes
"The proposed structure is in accord with the elemental composition formula that was determined by high resolution and accurate mass measurements. The structure shows only one of all possible isomers. Other isomers can be found el~ewhere.~Structures of the FI mass signels that are also correlated with maximum fluidity (see Table 11).
were detected. There are two reasons why it is so. One of them may be that some components are present in the same content in coals (see the difference spectrum in Figure for numerous FI mass signals of very low height) and for this reason no correlation can exist between the same intensities of their signals and a differentiated property in coals. The second reason may be that some reactive and unreactive components have the same molecular mass and give the same FI mass signal. When none of them dominates, the FI signal cannot be correlated. In summarizing the present results, the following statements can be made in response to the issue posed in the Introduction. Field ionization mass spectrometry and chemometric studies of a set of thermoplastic coals revealed components, arising from a coal or formed pyrolytically, that have an impact on its thermoplastic properties. There are two groups of such components that influence in opposite ways the temperatures of maximum fluidity and of resolidification as well as maximum fluidity. The higher the content of one of them the lower are the thermoplastic properties; it is concluded that components of this group undergo condensation and polymerization in a coal melt. The higher the content of the second group components the higher are the thermoplastic properties; this indicates that the components are inert in the reactions in question. The same two groups of components were involved in opposite way in hydrogen transfer when the coals were reacted with tetralin as H donor. This shows that components reactive (or inert) in thermally induced
condensation or polymerization are also reactive (or inert) towards the H donor. None of components shown by FI MS were correlated with softening temperature. This result is in accord with a conclusion (see part 1) that softening is a colligative property that indicates a phase transition due to increase of thermal mobility of the total coal system.
Conclusions In conclusions drawn from the present results (parts 1 and 2) as well as from other studies, the following view of chemistry of coal thermoplasticity can be presented. Softening, the first thermoplastic phenomenon observed on heating coals in -340-400 OC range, is a phase transition that brings about an increase of thermal-induced mobility of coal system. The energy demand of the transition is related to the softening temperature. The temperature is a colligative property; it depends on amounts and not on nature of the following system components: (a) content of molecular part (measurable by pyridineextract yields), since the energy demand for its thermalinduced mobility is lower compared with the demand of macromolecular part; (b) density of cross-linking of the macromolecular part (which can be measured by swelling) for the reason that the less cross-linked is the macromolecular part the lower is its energy demand for thermal mobility; (c) content of aliphatic and alicyclic structural units in coal material (which is directly related to coal hydrogen
Schulten et al.
108 Energy & Fuels, Vol. 6, No. 1, 1992
content) for the reason that the energy demand of these units is lower compared with aromatic structural units. Development of Fluidity and Maximum Fluidity. Further increase of temperature initiates reactions of thermal decomposition as well as reactions of polymerization and condensation; the reactions have an impact on all other thermoplastic properties which are their resultant effects. There are two aspects of relationship between thermal decomposition and reactions of condensation and polymerization. One of them is that some pyrolyzates are reactants in these reactions. The other aspect is that the reactions in question have opposite effects on the development of fluidity. There are two important routes of thermal decomposition of coal material; one of them is cleavage of "thermally weak" bonds such as C(sp3)-C(sp3) and C(sp3)-O bonds; the other is biomolecular mechanism of cyclohexadienyl radical mediated scission of strong bonds such as C(sp3)-C(sp2)b ~ n d s . ' ~ JThe ~ two mechanisms bring about a decrease of molecular weights of coal material. Moreover, the bimolecular mechanism results in formation of unsubstituted or less substituted aromatic ring systems compared with their substitution in original coal material. As it can be inferred from these mechanisms, two properties of coal material determine the extent of its decomposition. One of them is the amount and distribution of C(sp2)-C(sp3)bonds as well as C(sp3)-C(sp3)and C(sp3)-0 bonds in coal material. This amount and distribution directly exert influence on molecular sizes of thermal degradation products and hence, on fluidity of the system. The other property is the content of hydroaromatic structures that give rise to highly reactive intermediates which are cyclohexadienyl radicals, either by reaction of the hydroaromatics with aromatic structures or with other radicals.13J4 There is no direct method of determination of content of such hydroaromatics. However, it is likely that their content is related in some way to hydrogen-donor capabilities of coals that can be determined in reaction with anthra~ene.~fjJ~ Two other decisive properties have been inferred from the present study. One of them is the content of the species, either present in original coal or formed pyrolytically, that can undergo condensation reactions at relatively low temperature that is below a temperature of maximum fluidity. The species are unsubstituted or sparsely substituted hydrocarbons that contain at least five aromatic rings, such as picene, dibenzopyrene, coronene, and dibenzochrysene. Their condensation interferes with development of high fluidity due to thermal decomposition. The fourth property is a content of species that are inactive in polymerization and condensation reactions. Thus, it may be concluded that maximum fluidity that can be developed in (13)McMillen, D.F.;Malhotra, R.; Nigenda, S.E. Prep. Pap-Am. Chem. Soc., Diu. Fuel Chem. 1987,32(3),18e188. (14)McMillen, D.F.;Malhotra, R.; Hum, G. P.; Chang, S.-J.Energy Fuels 1987,1, 193-198. (15)Bermejo, J.; Canga, J. S.;Guillen, M. D.; Gayol, 0. M.; Blanco, C. G.Fuel Process. Technol. 1990,24, 157-162.
a coal depends on these four properties of coal material. Temperatures of Maximum Fluidity and Resolidification. TF(.max) is the first observable symptom showing that, from this temperature, molecular weights of plastic material increase and fluidity decreases as a result of the condensation and polymerization reactions. TRindicates that the system is already solid again. The two temperatures depend on the composition of products of thermal degradation of coal material i.e., on the total content of substances that are active in condensation and polymerization and those that are inert in these reactions. Thus, the higher the content of reactive species and the lower the content of unreactive ones, the lower are the temperatur- of maximum fluidity and of resolidifcation. The reactive species are various hydrocarbons representing 12 classes with respect of aromatic ring number and mode of their arrangement; they are unsubstituted and methyl aromatics that contain at least three aromatic rings; higher aromatics may have slightly more, that is up to three, alkyl carbon atoms. Some examples of the aromatics are 4Hcyclopenta [defl phenanthrene, pyrene, Co-C2-benzo[ghi]fluoranthenes, and perylene. Others are shown in the upper part of Table 111. The likely mechanism by which they react in a heated coal is condensation associated with elimination of hydrogen. Another group of reactive components consists of hydroxy compounds that contain from one to four aromatic rings and from none to C7alkyl substituents (see the upper part of Table 111). It is assumed that the hydroxy compounds undergo polymerization. It is believed that condensation and polymerization of the reactive species, if they are present in high content in a coal melt, may exert volume expansion of the system, similar to that observed during crystallization of water. The unreactive compounds (shown in the lower part of Table 111) are represented by C143 hydrocarbons that contain only two aromatic rings as well as hydrocarbons with three to six aromatic rings, however, with higher amount of alkyl substituents compared with their unreactive counterparts. As was shown in part 1 (see Introduction and Experimental Section), temperatures of maximum fluidity and resolidification depend on contents in coal of oxygen compounds and material that can be extracted by a solvent; the higher their content the lower are the temperatures. Similar effect is imposed by a content of exinite. All these relations simply indicate that these groups of compounds provide substances that are substrates in condensation or polymerization reactions. The presented view of chemistry of thermoplasticity forms a basis for modification procedure of thermoplastic properties of coals. It can also provide means of diagnosis of "dangerous" coals that are liable to damage coke over walls because of excessive pressure increase developed during carbonization. Acknowledgment. This work was supported by the Deutsche Forschungsgemeinschaft, Bonn-Bad Godesberg (Project Schu 416/15-1)and by the Polish Academy of Sciences. Registry No. Tetralin, 119-64-2.
