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Hydrogen transfer from tetralin and decalin to high-temperature coal tars. Relation with carbon deposit formation
FUEL PROCESSING TECHNOLOGY ELSEVIER
Fuel Processing Technology 48 (I 996) 73-8 I
Hydrogen transfer from tetralin and decalin to high-temperature coal tars. Relation with carbon deposit formation J. Pajak a, V. Krebs b, J.F. Mar&h6 b, G. Furdin bT* aInstitute of Coul b Luhoraroire
Chemisrry, Polish Acudemy ofScience.s
de Chimie du Glide
44-101
Gliwice, Poland
Mine’rul, URA 1.5%CNRS, Univrrsirh H. Poincare’ B.P. 239-54506
Vandoeuvre
le.7 Nancy Cedex. Frunce
Received 27 October 1995; accepted 5 March 1996
Abstract Studies of pyrolysis of a coal charge containing various quantities of water from 0.8 to IS wt.% in a two-stage reactor were performed. The rate of formation of solid carbon deposits was followed and some properties of high-temperature tars were investigated. These tars differ in elemental composition and hexane solubility. Hydrogen acceptor abilities were evaluated by reaction with tetralin at 320°C and with decalin at 400°C. Hydrogen transfer from decalin to several model organic compounds was also studied. The rate of carbon deposit formation and hydrogen acceptor abilities of tars are different for various quantities of water in the starting coal charge. The relation between the hydrogen transfer properties of tars expressed as the ratio of decalin hydrogen transfer (DHT) to tetralin hydrogen transfer (THT) and the rate of carbon deposit formation seems to indicate the importance of hydrogen transfer during secondary pyrolysis. Keywords:
Coal tar; Craking; Carbon deposit; Pyrolysis; Hydrogen transfer
1. Introduction The formation
of carbon
deposits
in coke
ovens
leads
to complications
in oven
and therefore much work has been done to understand the mechanism of such a phenomenon [ 1- 121.The results of recent works indicate that carbon deposits are mostly formed by condensation reactions during secondary pyrolysis of coal tars [ 131. Consequently, it seems worthwhile to search for the relationship between coal tar
maintenance
* Corresponding author. 0378-3820/%/$15.00
Copyright 0 1996 Elsevier
PII SO378-3820(96)01020-X
Science
B.V. All rights reserved.
composition and carbon deposition. Some studies [ 141 have indicated that the amount of water in a coal charge changes the rate of carbon deposit formation. Therefore, it seems strongly possible that the change of water content in the coal charge intluences the properties of tars. The aim of the present investigation was to search for differences which can be related to carbon deposit formation. A property that might be related to susceptibility of tars to condense and form carbon deposits is hydrogen donor/acceptor ability since it is recognized that the progress of high-temperature reactions of coal products often depends on hydrogen transfer reactions. However, there is a lack of established methods for measuring hydrogen transfer abilities of coal products. Most often tetralin has been used as a hydrogen donor reagent for coal [15-231 and was recently applied to estimate hydrogen acceptor ability of some coal products such as extracts, residues and products obtained at various stages of transformation of coal during the cokefaction [24-261. Therefore, we used tetralin as a hydrogen donor reagent to compare various high-temperature tars with regard to their hydrogen acceptance. However, in order to avoid decomposition of tetralin, the reaction temperature cannot exceed 340°C’. 9,10-Dihydroanthracene was used as a hydrogen donor to measure the hydrogen acceptor ability of coal pitches [27]. However, it is also known that above 350°C 9, IO-dihydroanthracene readily undergoes disproportionation to anthracene and 1,2,3,4tetrahydroanthracene. Another interesting compound that was used as a hydrogen donor compound for coal and could be used for coal tars is decalin. Decalin does not undergo significant disproportionation below 420°C and thus allows studies of hydrogen transfer at higher temperatures than tetralin. Moreover, it was found that the pathway by which the reaction between decalin and coal proceeds might be different from that between tetralin and coal [28,29]. Therefore, we have also undertaken here studies of the hydrogen transfer reaction from decalin to coal tars. Studies of model organic compounds under sirnilar conditions were also performed in order to find the most probable components of tar that accept hydrogen from decalin. Finally, we looked for a relation between hydrogen acceptor ability of tar and its aptitude to form carbon deposits.
2. Experimental The McClure coal used in this study has the following characteristic data (wt.%, daf basis): C-88.7; H-5.1; O-3.9; N-1.7; S-0.8; VM (dry basis)-26.9; ash content is 6.3. In order to follow the influence of the water content on carbon deposition, the water content of the coal charge was varied from 0.8 to 15 wt.%. Details of the coal carbonization system may be found elsewhere [13]. Briefly, a coal sample of 1 kg was pyrolyzed in a refractory steel two-stage reactor. The carbonization reactor was heated with a tubular furnace from ambient temperature to 900°C at the rate of 3”C/min. The volatile matter produced passed through the second stage cracking reactor in which a constant temperature of 900°C was maintained by means of the second tubular furnace. The weight of carbon deposit formed on a silica tube suspended in the cracking reactor was continuously measured by means of a Mettler AM 100
J. Pujuk et ul./
Furl
Processing
Technology
48
f 1996)
73-81
75
thermobalance. The system was equipped with two Liebig’s condensers and a 25000 V electrostatic trap to retain tars. Toluene was used to recover tars from the carbonization system; it was next evaporated in a rotary evaporator and then in a vacuum dryer. Reactions of coal tars with tetralin were carried out at 320°C for 30 hours in sealed glass tubes (rnass:volume ratio 1: I, usually 70 mg: 70 ~1) placed in a furnace electrically controlled within i- 1°C. Reactions of tars with cis-decalin were carried out at 400°C in glass tubes (massvolume ratio 1: 1, usually 80 mg:80 ~1) placed in the same oven. Reaction products were extracted with hexane and analyzed by gas chromatography-mass spectrometry (Hewlett Packard 5890 GC-5971 MSD). Reactions of model organic compounds with decalin were carried out in a similar way with the molar ratio 1:3. Hexane soluble fractions of tars were obtained by several extractions of the tar sample with hexane in an ultrasonic bath. After filtration and solvent evaporation, hexane solubles were calculated from the difference in tar weight. Organic reactants were purchased, mainly from Aldrich. Hexane was distilled before use. Elemental analyses of tars were performed by the Analytical Service of CNRS in Vernaison. France.
3. Results and discussion The results of elemental analysis of tars are shown in Table 1. They indicate significant differences among tars, particularly in oxygen content and in C/H ratio. The results suggest that the changes of water content in coal charge influence the formation of oxygen-containing structures in tars and their degree of condensation. Indeed, the amount of oxygen reaches its maximum (5.5 wt.%) with addition of about 10% water to coal charge and then decreases whereas low H/C atomic ratios, indicating a higher degree of condensation, are found for tars obtained with very low (0.8 wt.%) or very high (15 wt.%) content of water in coal charge. The transfer of hydrogen from tetralin to a tar was followed by measuring the conversion of tetralin into naphthalene during the reaction. However, a complication was encountered due to the fact, that in contrast to coal or some coal products used previously as acceptors of hydrogen from tetralin, the present coal tars contain large
Table I Elemental
analysis of tars (wt.%)
Moisture content in coal charge (wt.%) 0.8
7.8 10.0 15.0
C
H
0
N
s
93.0 89.2 84.6 87.5
3.6 4.4 4.4 4. I
2.3 I .7 5.5 5.2
1.1 2.0 I .7 1.6
0.7 1.2 1.5 0.9
Table 2 Hydrogen transfer from tetralin (THT)
(in mz H/l000
mg of sample) and hexane solubihty of tar obtamed for
coal charges containing various quantities of water Moisture content in coal charse (wt.%)
THT
I
Hexane solubility (wt.%)
7
0.8
2.
7.x
2.2
II
IO
2.4
IX
I5
2.3
17
quantities of naphthalene which was found during GC/MS studies. The problem was resolved since neither naphthalene nor phenanthrene, which is also found in the present with tetralin at 320°C. GC analysis allows calculating the tars, react naphthalene/phenanthrene ratio in a tar. After reaction with tetralin, the quantity of naphthalene originating from the tar could be estimated by using the previous result of naphthalene/phenanthrene analysis. Consequently, subtraction of the quantity of naphthalene from tars from the total amount of naphthalene after reaction gives the amount of naphthalene formed. The number of milligrams of hydrogen transferred to 1000 mg of tar sample from tetralin (THT) can be calculated as follows: THT = (4000/132)(Tx/M)
(1) where T is the amount of tetralin (mg) in the starting reaction mixture, M is the quantity of tar sample (mg) and x is the molar fraction of naphthalene formed during the reaction. For the experimental conditions used in this study, THT can be calculated using Eq. (2): THT = 0.3 X (2) where X is expressed in wt.% of naphthalene formed during the reaction. The results of studies of hydrogen acceptor reactivity from tetralin are presented in Table 2. The acceptance of hydrogen by tars obtained from coal charges containing various amounts of water is rather similar though a slow increase of THT with the coal moisture content might be postulated. In order to measure the transfer of hydrogen from decalin to a coal tar it was necessary to follow the formation of naphthalene and tetralin during the reaction. The problem of naphthalene in the original tar (before reaction) was resolved in the same way as in the case of tetralin since neither naphthalene nor phenanthrene react with decalin at 4OO”C, which was proved by the studies of model compounds. During the blank experiments with pure decalin the formation of naphthalene was not observed. Stoechiometry indicates that during hydrogen transfer from decalin to an acceptor 10 g of hydrogen are transferred when 138 g of decalin are converted to 128 g of naphthalene. When tetralin is formed, only 6 g of hydrogen are transferred to form 132 g of tetralin. In the experimental conditions of this work, the amount of hydrogen transferred to coal tar from decalin (DHT) can be calculated using Eq. (3): DHT = 0.67N + 0.4T (3) where DHT is the amount of hydrogen in milligrams transferred to 1000 mg of tar and N and T are the respective amounts of naphthalene and tetralin (in wt.%) formed from decalin during reaction.
J. Pujuk et ul./
Fuel Processing
Technology
48 (1996) 73-81
77
Table 3 Hydrogen transfer from decalin (DHT) (in mg H/ 1000 mg of sample) to tars and carbon deposition various water quantities in coal charge Moisture content in coal charge (wt.%)
DHT
DHT/THT
Carbon deposition
0.8 7.8 10.0 15.0
4.5 5.1 4.0 4.4
2.04 2.43
0.82 0.34 1.06 0.84
I.66 1.83
rate for
rate (km/min)
The preliminary experiments with coal tars have shown that after 50 hours the reaction is practically complete. Table 3 presents the results of hydrogen transfer from decalin (DHT) to coal tars at 400°C during 50 hours, this parameter being a measure of specific hydrogen acceptor ability of the tars, together with the results of carbon deposition studies during the pyrolysis. These results indicate that the addition of water to coal charge changes the DHT ability of resulting high-temperature tars, but no direct relation is obvious. The comparison between reactivity of tars with tetralin and with decalin reveals some differences. First, THT is much lower than DHT for all samples. This can be partly explained by the difference in reaction temperature. It seems possible that some structures, which are not yet active at the temperature of 320°C react at the higher temperature of 400°C. However, the increase of reactivity is not similar and varies among tars. If we calculate the DHT/THT ratio, it is the highest, 2.43, for the tar obtained from the coal charge containing 7.8% water and the lowest, 1.66, for the tar obtained from coal containing 10% water. The differences in DHT/THT ratio indicate that apart from the structures that accept hydrogen from both donors, tetralin and decalin, the tars studied contain the structures which seem to react only with decalin. Review of studies on hydrogen transfer from tetralin and decalin to coal and to organic model compounds reveals the great diversity of mechanisms proposed. For the case of the coal/tetralin reaction, for example, several reaction pathways, such as radical homolysis [151, ionic hydride transfer [30] and pericyclic transfer of two hydrogens [22,31] have been proposed. In the case of coal/decalin reaction, apparent ionic hydride transfer was postulated, but the radical process was invoked to explain reactions of some organic model compounds, particularly those containing oxygen [28,29]. One of the reason for complications during studies of complex substances like coal might be that various reaction mechanisms operate simultaneously. The particular reaction pathway depends on both reagents, donor and acceptor, and even for the same donor it is different when acceptors are different - different mechanism of reaction of tetralin with 1,2-diphenylethane [32] and with 1,3_diphenylpropane [33,34] may serve as an example. It also seems that decalin reacts by different pathways than tetralin with certain aromatic hydrocarbons, such as anthracene [28,35]. In order to look for the most probable components of tars, that accept hydrogen from decalin some studies at 400°C for several compounds were made and the results are presented in Table 4. These results also confirm the validity of our method of DHT studies for tars, for which hydrogenation cannot be directly measured. Naphthalene and
Table 4 Reaction of some organic compounds with decalin at 400°C Compound
Reaction
Ma,jor identified peaks a
time (hours) naphthalene
SO
phenanthrene
50
anthracene
SO
carbazole
90
acenaphthylene
90
pyrene
90
9,10-dihydroanthracene
22
I ,2..?,4-tetrahydro-anthracene n, t
49
acenaphthene
82
_
hexahydroacenaphthylene
9
ethylnaphthalene
8
n, t 4,Sdihydropyrene
5
n, chrysene
90
fluoranthene
90
Estimated yields as Ok of starting compound
t
I ,2,3,lOb-tetrahydrofluoranthene t
3
n,
a n,
t: naphthalene, tetraiin.
tetralin are produced only when hydrogenation of a model compound takes place. In a case when hydrogenation of model compound is not observed, neither naphthalene nor tetralin are formed; only cis-trans isomerisation of decalin is observed with a trace of butylcyclohexane. It also has to be noted that acenaphthylene probably undergoes partial polymerization in our reaction conditions, as indicated by the poor solubility of reaction products in hexane. The results of model compound studies show the highest reactivity of acenaphthylene and anthracene as acceptors of hydrogen from decalin. However, the GC studies of the hexane soluble fraction of tars showed that the sum of these two components in tars varies between 1.1-2.5 wt.%. For 25 mg of anthracene and 975 mg of inert substance the DHT would be 0.56 mg/lOOO mg, if anthracene was converted to tetrahydroanthracene. For 25 mg of acenaphthylene and 975 mg of inert substance it would be 0.33 mg/lOOO mg, if all acenaphthylene was hydrogenated into acenaphthene. These calculations indicate that the contribution of pure acenaphtylene or anthracene into total DHT of tar is not significant. Moreover, all other major tar components that were detected by GC/MS analysis: naphthalene, phenanthrene, pyrene, chrysene, fluoranthene and carbazole are completely unreactive or only weakly reactive with decalin at 400°C. It also has to be noted that the total amount of hexane soluble fraction (and only part of that fraction passes GC column) varies among the studied tars between 7-18 wt.%. Therefore, it must be concluded that the majority of hydrogen accepting centers in high-temperature tars belongs to hexane insoluble heavy fractions of tars. However, coming back to anthracene and acenaphthylene, it does not seem impossible that their oligomers or compounds with anthracene and acenaphthylene fragments comprise a significant por-
.I. Pujuk rt (II./ Fuel Procrssing Technology 48 (1996) 73-81
79
tion of the heavy fraction of tars providin g good hydrogen acceptor reactivity from decalin. Yet in tars there are probably many other hydrogen accepting components, practically insoluble in hexane, that might react with decalin at 400°C and a compound like anthraquinone [28] may serve as an example. Taking into consideration the results of carbon deposit formation and the DHT/THT ratio, one can see that the lower the DHT/THT ratio for high-temperature tar the more carbon deposit is formed during secondary pyrolysis. This relation might suggest important differences occurring during secondary cracking of primary tars, leading to some differences in composition of secondary tars and to various amounts of carbon deposits. Increasing the DHT/THT ratio indicates increasing amounts of structures in tars that accept hydrogen specifically from decalin-aliphatic hydrocarbon. A hypothesis may be proposed to explain the relationship between the DHT/THT ratio and carbon deposition. During secondary pyrolysis of primary volatiles, consisting of many parallel and competing reactions, inter- and intra-molecular transfer of hydrogen occurs. This transfer plays a significant role, quenching reactive products of the cracking of primary tars, known to contain aliphatic and alkylaromatic components. Smaller amounts of appropriate acceptor/donor structures might favorize other than transfer of hydrogen reaction pathways, particularly those leading to product condensation and subsequently to carbon deposit formation. Finally, it seems that since changes in water content in coal charge result in changes in high-temperature tars and in the rate of carbon deposition, they probably change the composition of primary tars which are the precursors of secondary tars and carbon deposits. Comparative studies of primary tars obtained from coal charge containing various quantities of water will be the subject of future work.
4. Conclusion High-temperature pyrolysis of coal charges containing different quantities of water leads to the differences in rates of carbon deposit formation and to some differences in tar properties. These high-temperature tars differ in elemental composition and hexane solubility; they also exhibit various hydrogen acceptor abilities as measured by reaction with tetralin and decalin. The relation between the hydrogen transfer ability of tars expressed by decalin/tetralin hydrogen transfer ability and the rate of carbon deposit formation was found. It seems that the decrease in hydrogen acceptor ability indicates the increase of processes leading to condensation and carbon deposition during secondary cracking of tars.
Acknowledgements This work was financially supported by the Groupement Scientifique: Pyrolyse du Charbon (ECOTECH-CNRS). The authors want to thank the Region Lorraine for its financial participation in the purchase of the GC/MS apparatus.
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On the mechanism of hydrogen transfer from decalin to coal. Pressure effect and kinetic isotope effect. Energy and Fuels, I (I 987) 363-366. J. Pajak, Hydrogen transfer from decalin to coal macerals. Fuel Processing Technology, 27 (1991) 203-213. D.S. Ross and J.E. Blessing, Alcohols as H-donor media in coal conversion. I. Base-promoted H-donation to coal by isopropyl alcohol. Fuel, 58 (1979) 433. P.S. Virk, D.H. Bass, C.P. Eppig and D.J. Ekpenyong, Am. Chem. Sot. Div. Fuel. Chem. Preprints. 24 (1979) 144. K.R. Brower, Kinetics and pressure effect in the thermolysis of liquid dibenzyl. J. Org. Chem., 45 (1980) 1004. K.E. Gilbert and J.J. Gajewski, Coal liquefaction model studies: free radical chain decomposition of diphenylpropane, dibenzylether and phenylether via B-scission reactions. J. Org. Chem., 47 (1982) 4899-4902. M.L. Poustma and C.W. Dyer, Thermolysis of model compounds for coal. 3. Radical chain decomposition of I ,3-diphenylpropane and I,Cdiphenylbutane. J. Org. Chem., 47 (1982) 4903-4914. H.H. King and L.M. Stock, Aspects of the chemistry of donor solvent coal dissolution. The hydrogen-deuterium exchange reactions of tetralin-d,? with Illinois no. 6 coal, coal products and related compounds. Fuel, 61 (1982) 257-264.
Fuel Processing Technology 48 (I 996) 73-8 I
Hydrogen transfer from tetralin and decalin to high-temperature coal tars. Relation with carbon deposit formation J. Pajak a, V. Krebs b, J.F. Mar&h6 b, G. Furdin bT* aInstitute of Coul b Luhoraroire
Chemisrry, Polish Acudemy ofScience.s
de Chimie du Glide
44-101
Gliwice, Poland
Mine’rul, URA 1.5%CNRS, Univrrsirh H. Poincare’ B.P. 239-54506
Vandoeuvre
le.7 Nancy Cedex. Frunce
Received 27 October 1995; accepted 5 March 1996
Abstract Studies of pyrolysis of a coal charge containing various quantities of water from 0.8 to IS wt.% in a two-stage reactor were performed. The rate of formation of solid carbon deposits was followed and some properties of high-temperature tars were investigated. These tars differ in elemental composition and hexane solubility. Hydrogen acceptor abilities were evaluated by reaction with tetralin at 320°C and with decalin at 400°C. Hydrogen transfer from decalin to several model organic compounds was also studied. The rate of carbon deposit formation and hydrogen acceptor abilities of tars are different for various quantities of water in the starting coal charge. The relation between the hydrogen transfer properties of tars expressed as the ratio of decalin hydrogen transfer (DHT) to tetralin hydrogen transfer (THT) and the rate of carbon deposit formation seems to indicate the importance of hydrogen transfer during secondary pyrolysis. Keywords:
Coal tar; Craking; Carbon deposit; Pyrolysis; Hydrogen transfer
1. Introduction The formation
of carbon
deposits
in coke
ovens
leads
to complications
in oven
and therefore much work has been done to understand the mechanism of such a phenomenon [ 1- 121.The results of recent works indicate that carbon deposits are mostly formed by condensation reactions during secondary pyrolysis of coal tars [ 131. Consequently, it seems worthwhile to search for the relationship between coal tar
maintenance
* Corresponding author. 0378-3820/%/$15.00
Copyright 0 1996 Elsevier
PII SO378-3820(96)01020-X
Science
B.V. All rights reserved.
composition and carbon deposition. Some studies [ 141 have indicated that the amount of water in a coal charge changes the rate of carbon deposit formation. Therefore, it seems strongly possible that the change of water content in the coal charge intluences the properties of tars. The aim of the present investigation was to search for differences which can be related to carbon deposit formation. A property that might be related to susceptibility of tars to condense and form carbon deposits is hydrogen donor/acceptor ability since it is recognized that the progress of high-temperature reactions of coal products often depends on hydrogen transfer reactions. However, there is a lack of established methods for measuring hydrogen transfer abilities of coal products. Most often tetralin has been used as a hydrogen donor reagent for coal [15-231 and was recently applied to estimate hydrogen acceptor ability of some coal products such as extracts, residues and products obtained at various stages of transformation of coal during the cokefaction [24-261. Therefore, we used tetralin as a hydrogen donor reagent to compare various high-temperature tars with regard to their hydrogen acceptance. However, in order to avoid decomposition of tetralin, the reaction temperature cannot exceed 340°C’. 9,10-Dihydroanthracene was used as a hydrogen donor to measure the hydrogen acceptor ability of coal pitches [27]. However, it is also known that above 350°C 9, IO-dihydroanthracene readily undergoes disproportionation to anthracene and 1,2,3,4tetrahydroanthracene. Another interesting compound that was used as a hydrogen donor compound for coal and could be used for coal tars is decalin. Decalin does not undergo significant disproportionation below 420°C and thus allows studies of hydrogen transfer at higher temperatures than tetralin. Moreover, it was found that the pathway by which the reaction between decalin and coal proceeds might be different from that between tetralin and coal [28,29]. Therefore, we have also undertaken here studies of the hydrogen transfer reaction from decalin to coal tars. Studies of model organic compounds under sirnilar conditions were also performed in order to find the most probable components of tar that accept hydrogen from decalin. Finally, we looked for a relation between hydrogen acceptor ability of tar and its aptitude to form carbon deposits.
2. Experimental The McClure coal used in this study has the following characteristic data (wt.%, daf basis): C-88.7; H-5.1; O-3.9; N-1.7; S-0.8; VM (dry basis)-26.9; ash content is 6.3. In order to follow the influence of the water content on carbon deposition, the water content of the coal charge was varied from 0.8 to 15 wt.%. Details of the coal carbonization system may be found elsewhere [13]. Briefly, a coal sample of 1 kg was pyrolyzed in a refractory steel two-stage reactor. The carbonization reactor was heated with a tubular furnace from ambient temperature to 900°C at the rate of 3”C/min. The volatile matter produced passed through the second stage cracking reactor in which a constant temperature of 900°C was maintained by means of the second tubular furnace. The weight of carbon deposit formed on a silica tube suspended in the cracking reactor was continuously measured by means of a Mettler AM 100
J. Pujuk et ul./
Furl
Processing
Technology
48
f 1996)
73-81
75
thermobalance. The system was equipped with two Liebig’s condensers and a 25000 V electrostatic trap to retain tars. Toluene was used to recover tars from the carbonization system; it was next evaporated in a rotary evaporator and then in a vacuum dryer. Reactions of coal tars with tetralin were carried out at 320°C for 30 hours in sealed glass tubes (rnass:volume ratio 1: I, usually 70 mg: 70 ~1) placed in a furnace electrically controlled within i- 1°C. Reactions of tars with cis-decalin were carried out at 400°C in glass tubes (massvolume ratio 1: 1, usually 80 mg:80 ~1) placed in the same oven. Reaction products were extracted with hexane and analyzed by gas chromatography-mass spectrometry (Hewlett Packard 5890 GC-5971 MSD). Reactions of model organic compounds with decalin were carried out in a similar way with the molar ratio 1:3. Hexane soluble fractions of tars were obtained by several extractions of the tar sample with hexane in an ultrasonic bath. After filtration and solvent evaporation, hexane solubles were calculated from the difference in tar weight. Organic reactants were purchased, mainly from Aldrich. Hexane was distilled before use. Elemental analyses of tars were performed by the Analytical Service of CNRS in Vernaison. France.
3. Results and discussion The results of elemental analysis of tars are shown in Table 1. They indicate significant differences among tars, particularly in oxygen content and in C/H ratio. The results suggest that the changes of water content in coal charge influence the formation of oxygen-containing structures in tars and their degree of condensation. Indeed, the amount of oxygen reaches its maximum (5.5 wt.%) with addition of about 10% water to coal charge and then decreases whereas low H/C atomic ratios, indicating a higher degree of condensation, are found for tars obtained with very low (0.8 wt.%) or very high (15 wt.%) content of water in coal charge. The transfer of hydrogen from tetralin to a tar was followed by measuring the conversion of tetralin into naphthalene during the reaction. However, a complication was encountered due to the fact, that in contrast to coal or some coal products used previously as acceptors of hydrogen from tetralin, the present coal tars contain large
Table I Elemental
analysis of tars (wt.%)
Moisture content in coal charge (wt.%) 0.8
7.8 10.0 15.0
C
H
0
N
s
93.0 89.2 84.6 87.5
3.6 4.4 4.4 4. I
2.3 I .7 5.5 5.2
1.1 2.0 I .7 1.6
0.7 1.2 1.5 0.9
Table 2 Hydrogen transfer from tetralin (THT)
(in mz H/l000
mg of sample) and hexane solubihty of tar obtamed for
coal charges containing various quantities of water Moisture content in coal charse (wt.%)
THT
I
Hexane solubility (wt.%)
7
0.8
2.
7.x
2.2
II
IO
2.4
IX
I5
2.3
17
quantities of naphthalene which was found during GC/MS studies. The problem was resolved since neither naphthalene nor phenanthrene, which is also found in the present with tetralin at 320°C. GC analysis allows calculating the tars, react naphthalene/phenanthrene ratio in a tar. After reaction with tetralin, the quantity of naphthalene originating from the tar could be estimated by using the previous result of naphthalene/phenanthrene analysis. Consequently, subtraction of the quantity of naphthalene from tars from the total amount of naphthalene after reaction gives the amount of naphthalene formed. The number of milligrams of hydrogen transferred to 1000 mg of tar sample from tetralin (THT) can be calculated as follows: THT = (4000/132)(Tx/M)
(1) where T is the amount of tetralin (mg) in the starting reaction mixture, M is the quantity of tar sample (mg) and x is the molar fraction of naphthalene formed during the reaction. For the experimental conditions used in this study, THT can be calculated using Eq. (2): THT = 0.3 X (2) where X is expressed in wt.% of naphthalene formed during the reaction. The results of studies of hydrogen acceptor reactivity from tetralin are presented in Table 2. The acceptance of hydrogen by tars obtained from coal charges containing various amounts of water is rather similar though a slow increase of THT with the coal moisture content might be postulated. In order to measure the transfer of hydrogen from decalin to a coal tar it was necessary to follow the formation of naphthalene and tetralin during the reaction. The problem of naphthalene in the original tar (before reaction) was resolved in the same way as in the case of tetralin since neither naphthalene nor phenanthrene react with decalin at 4OO”C, which was proved by the studies of model compounds. During the blank experiments with pure decalin the formation of naphthalene was not observed. Stoechiometry indicates that during hydrogen transfer from decalin to an acceptor 10 g of hydrogen are transferred when 138 g of decalin are converted to 128 g of naphthalene. When tetralin is formed, only 6 g of hydrogen are transferred to form 132 g of tetralin. In the experimental conditions of this work, the amount of hydrogen transferred to coal tar from decalin (DHT) can be calculated using Eq. (3): DHT = 0.67N + 0.4T (3) where DHT is the amount of hydrogen in milligrams transferred to 1000 mg of tar and N and T are the respective amounts of naphthalene and tetralin (in wt.%) formed from decalin during reaction.
J. Pujuk et ul./
Fuel Processing
Technology
48 (1996) 73-81
77
Table 3 Hydrogen transfer from decalin (DHT) (in mg H/ 1000 mg of sample) to tars and carbon deposition various water quantities in coal charge Moisture content in coal charge (wt.%)
DHT
DHT/THT
Carbon deposition
0.8 7.8 10.0 15.0
4.5 5.1 4.0 4.4
2.04 2.43
0.82 0.34 1.06 0.84
I.66 1.83
rate for
rate (km/min)
The preliminary experiments with coal tars have shown that after 50 hours the reaction is practically complete. Table 3 presents the results of hydrogen transfer from decalin (DHT) to coal tars at 400°C during 50 hours, this parameter being a measure of specific hydrogen acceptor ability of the tars, together with the results of carbon deposition studies during the pyrolysis. These results indicate that the addition of water to coal charge changes the DHT ability of resulting high-temperature tars, but no direct relation is obvious. The comparison between reactivity of tars with tetralin and with decalin reveals some differences. First, THT is much lower than DHT for all samples. This can be partly explained by the difference in reaction temperature. It seems possible that some structures, which are not yet active at the temperature of 320°C react at the higher temperature of 400°C. However, the increase of reactivity is not similar and varies among tars. If we calculate the DHT/THT ratio, it is the highest, 2.43, for the tar obtained from the coal charge containing 7.8% water and the lowest, 1.66, for the tar obtained from coal containing 10% water. The differences in DHT/THT ratio indicate that apart from the structures that accept hydrogen from both donors, tetralin and decalin, the tars studied contain the structures which seem to react only with decalin. Review of studies on hydrogen transfer from tetralin and decalin to coal and to organic model compounds reveals the great diversity of mechanisms proposed. For the case of the coal/tetralin reaction, for example, several reaction pathways, such as radical homolysis [151, ionic hydride transfer [30] and pericyclic transfer of two hydrogens [22,31] have been proposed. In the case of coal/decalin reaction, apparent ionic hydride transfer was postulated, but the radical process was invoked to explain reactions of some organic model compounds, particularly those containing oxygen [28,29]. One of the reason for complications during studies of complex substances like coal might be that various reaction mechanisms operate simultaneously. The particular reaction pathway depends on both reagents, donor and acceptor, and even for the same donor it is different when acceptors are different - different mechanism of reaction of tetralin with 1,2-diphenylethane [32] and with 1,3_diphenylpropane [33,34] may serve as an example. It also seems that decalin reacts by different pathways than tetralin with certain aromatic hydrocarbons, such as anthracene [28,35]. In order to look for the most probable components of tars, that accept hydrogen from decalin some studies at 400°C for several compounds were made and the results are presented in Table 4. These results also confirm the validity of our method of DHT studies for tars, for which hydrogenation cannot be directly measured. Naphthalene and
Table 4 Reaction of some organic compounds with decalin at 400°C Compound
Reaction
Ma,jor identified peaks a
time (hours) naphthalene
SO
phenanthrene
50
anthracene
SO
carbazole
90
acenaphthylene
90
pyrene
90
9,10-dihydroanthracene
22
I ,2..?,4-tetrahydro-anthracene n, t
49
acenaphthene
82
_
hexahydroacenaphthylene
9
ethylnaphthalene
8
n, t 4,Sdihydropyrene
5
n, chrysene
90
fluoranthene
90
Estimated yields as Ok of starting compound
t
I ,2,3,lOb-tetrahydrofluoranthene t
3
n,
a n,
t: naphthalene, tetraiin.
tetralin are produced only when hydrogenation of a model compound takes place. In a case when hydrogenation of model compound is not observed, neither naphthalene nor tetralin are formed; only cis-trans isomerisation of decalin is observed with a trace of butylcyclohexane. It also has to be noted that acenaphthylene probably undergoes partial polymerization in our reaction conditions, as indicated by the poor solubility of reaction products in hexane. The results of model compound studies show the highest reactivity of acenaphthylene and anthracene as acceptors of hydrogen from decalin. However, the GC studies of the hexane soluble fraction of tars showed that the sum of these two components in tars varies between 1.1-2.5 wt.%. For 25 mg of anthracene and 975 mg of inert substance the DHT would be 0.56 mg/lOOO mg, if anthracene was converted to tetrahydroanthracene. For 25 mg of acenaphthylene and 975 mg of inert substance it would be 0.33 mg/lOOO mg, if all acenaphthylene was hydrogenated into acenaphthene. These calculations indicate that the contribution of pure acenaphtylene or anthracene into total DHT of tar is not significant. Moreover, all other major tar components that were detected by GC/MS analysis: naphthalene, phenanthrene, pyrene, chrysene, fluoranthene and carbazole are completely unreactive or only weakly reactive with decalin at 400°C. It also has to be noted that the total amount of hexane soluble fraction (and only part of that fraction passes GC column) varies among the studied tars between 7-18 wt.%. Therefore, it must be concluded that the majority of hydrogen accepting centers in high-temperature tars belongs to hexane insoluble heavy fractions of tars. However, coming back to anthracene and acenaphthylene, it does not seem impossible that their oligomers or compounds with anthracene and acenaphthylene fragments comprise a significant por-
.I. Pujuk rt (II./ Fuel Procrssing Technology 48 (1996) 73-81
79
tion of the heavy fraction of tars providin g good hydrogen acceptor reactivity from decalin. Yet in tars there are probably many other hydrogen accepting components, practically insoluble in hexane, that might react with decalin at 400°C and a compound like anthraquinone [28] may serve as an example. Taking into consideration the results of carbon deposit formation and the DHT/THT ratio, one can see that the lower the DHT/THT ratio for high-temperature tar the more carbon deposit is formed during secondary pyrolysis. This relation might suggest important differences occurring during secondary cracking of primary tars, leading to some differences in composition of secondary tars and to various amounts of carbon deposits. Increasing the DHT/THT ratio indicates increasing amounts of structures in tars that accept hydrogen specifically from decalin-aliphatic hydrocarbon. A hypothesis may be proposed to explain the relationship between the DHT/THT ratio and carbon deposition. During secondary pyrolysis of primary volatiles, consisting of many parallel and competing reactions, inter- and intra-molecular transfer of hydrogen occurs. This transfer plays a significant role, quenching reactive products of the cracking of primary tars, known to contain aliphatic and alkylaromatic components. Smaller amounts of appropriate acceptor/donor structures might favorize other than transfer of hydrogen reaction pathways, particularly those leading to product condensation and subsequently to carbon deposit formation. Finally, it seems that since changes in water content in coal charge result in changes in high-temperature tars and in the rate of carbon deposition, they probably change the composition of primary tars which are the precursors of secondary tars and carbon deposits. Comparative studies of primary tars obtained from coal charge containing various quantities of water will be the subject of future work.
4. Conclusion High-temperature pyrolysis of coal charges containing different quantities of water leads to the differences in rates of carbon deposit formation and to some differences in tar properties. These high-temperature tars differ in elemental composition and hexane solubility; they also exhibit various hydrogen acceptor abilities as measured by reaction with tetralin and decalin. The relation between the hydrogen transfer ability of tars expressed by decalin/tetralin hydrogen transfer ability and the rate of carbon deposit formation was found. It seems that the decrease in hydrogen acceptor ability indicates the increase of processes leading to condensation and carbon deposition during secondary cracking of tars.
Acknowledgements This work was financially supported by the Groupement Scientifique: Pyrolyse du Charbon (ECOTECH-CNRS). The authors want to thank the Region Lorraine for its financial participation in the purchase of the GC/MS apparatus.
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