Characterization of tar, char and gas from pyrolysis of coal asphaltenes

Fuel Vol. 77, No. 9/10, pp. 1099-l 105. 1998 0 1998 Elsevier Science Ltd. All rights reserved

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Characterization of tar, char and gas from pyrolysis of coal asphaltenes Ying Liuar*, Werner Hodekb and Karl H. van Heekb aPulp and Paper Research Centre, Department of Chemistry, McGill University, 3420 University Street, Montreal, Quebec H2A 2A7, Canada bDMT-Gesellschaft fiir Forschung und Priifung mbH, Institute for Cokemaking and Fuel Technology, Franz-Fischer-Weg 67, D-45307 Essen, Germany

(Received

3 September

7996; revised 27 August

7997)

Asphaltenes generated by a multiple precipitation procedure were found to contain n-alkanes from Cl4 to C31, which show an odd over even preference. This result further indicates that alkyl chains or alkylene bridges appear mostly as substituents in the coal macromolecular network. Comparison of the elemental analysis and surface area of pyrolysis chars shows that the surface areas correlate well with the heteroatom content of chars and that of the original coals as well. However, the gasification reactivity in carbon dioxide of ash-free chars cannot be interpreted in terms of their surface area. During the pyrolysis of asphaltenes, chars were formed via the mesophase route. The polarized light optical microscopy of chars confirmed that fluidity of the carbonization system is a major factor controlling the optical texture of resultant chars. Pyrolysis of asphaltenes showed that most of the gaseous components are formed by the cleavage of weak bonds like aryl-alkyl-ethers and strong bonds, mainly methylene and biarylether bridges between the aromatic moieties and methyl groups on the aromatic rings in asphaltene structure. 0 1998 Elsevier Science Ltd. All rights reserved (Keywords: characterization;

asphaltenes; pyrolysis)

It is generally agreed that coal physical structure is composed of a macromolecular cross-linked network and trapped molecular compounds1-3. Based on observed slow and rapid proton relaxation rates in n.m.r. experiments, the macromolecular network is an immobile phase and host to a number of relatively small molecules called the mobile phase. In recent years, it has been strongly discussed whether the long chain molecules in the mobile phase exist independent on the network or associated with fragments of the macromolecular network3-5. Some experimental evidence suggests that these low molecular weight polymethylene compounds would be an integral part of the macromolecular network356-s. In previous research it has been shown the toluene soluble, n-hexane insoluble asphaltenes derived from donor solvent extraction of coal are considered representative of the original structural units in the coal macromolecular component9-13. The present paper describes the non-isothermal pyrolysis of coal asphaltenes with low heating rates in a thermobalance combined with as chromatographic on-line analysis of fi product gases”, . Tars and chars produced have been characterized in detail. It was hoped to obtain structural information about the original coal to provide a better understanding of the pyrolysis behaviour of asphaltenes. EXPERIMENTAL

different rank were used as raw coals. Two of these Chinese coals were obtained from the same Fengfeng district in Hebei province. Their properties are given in Table 1. Each coal was ground to 90-106 pm and dried in vacuum at 110°C for 24 h before use. Asphaltenes 400 g coal and 1000 ml tetralin were heated in an autoclave to 415°C. Once this temperature was reached, the product was cooled immediately. The tetralin insoluble residue was extracted with pyridine. The pyridine insoluble part was removed by filtration. The filtrate and the tetralin solution were distilled at 120°C under 18 Pa. The destillation residue was extracted with toluene. The toluene insoluble residue (preasphaltenes) was filtered off and dried in a vacuum oven (13 Pa) at 80°C for 24 h. The filtered toluene solution was reduced to 120 ml on a rotary evaporator and then dropped in 20 times such volume of nhexane to precipitate the asphaltenes. After being filtered off, the precipitated asphaltenes were dried in a vacuum oven (13 Pa) at 60°C for 24 h, then weighed and stored in sealed containers in the dark. Dissolution and precipitation of the crude asphaltenes were repeated until the n-hexane solution remained nearly colorless. It was usually necessary to repeat the dissolutionprecipitation cycle five times to achieve a clean initial asphaltene.

Coal samples A high volatile bituminous Germany (seam Chriemhilt)

coal from the Ruhr region of and four Chinese coals of

*To whom correspondenceshould be addressed

Pyrolysis Pyrolysis of asphaltenes was carried out in a thermobalance. The pyrolysis equipment, pyrolysis procedure and conditions have been described in detail elsewhere85’2.

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Characterization of tar, char and gas: Y, Liu et al.

Table 1 Coal

Analytical data of raw coals

samples

Fushun

Chriemhilt

Fengjiao

Liulin

Fengsou

Maceral composition and mineral matter (vol. %) Vitrinite

66

66

65

Exinite

18

14

1

Inertinite

13

16

Minerals

3

4

0.53

0.80

Water

3.4

2.4

Ash (dry basis)

6.6

7.3

11.0

Vitrinite reflectance

59

54

31

38

40

3

3

6

1.14

1.25

1.50

(%)

Hm

Proximate analysis (wt%) 0.1

0.1

0.8

10.0

10.9

Volatile matter (daf)

48.9

38.3

21.8

25.0

17.8

Fixed carbon (daf)

52.5

63.0

14.4

17.0

83.4

C

19.9

83.4

88.8

90.3

90.6

H

6.3

5.6

5.2

5.0

4.6

11.2

9.3

4.2

3.5

3.1

N

1.5

1.4

2.0

1.7

2.0

S

0.4

1.7

0.6

0.6

0.4

H/C

0.95

Ultimate analysis (wt%, dafl

0 (by dir.“)

0 + N + S/C (%)

0.81

12.3

10.6

0.70

0.66

0.61

5.1

4.8

4.6

“Directly determined

Fractionation All tars were fractionated into four chemical classes on a neutral alumina columned. The four chemical classes (aliphatic hydrocarbons, neutral polycyclic aromatic compounds, and two fractions of increasing polarity) were obtained by the elution with the following solvents: 20 ml hexane, 50 ml benzene, 70 ml chloroform (containing 0,75% ethanol) and 50 ml THF (containing 10% ethanol) solution, respectively.

where Am(AT), m(T) and At represent the weight loss (daf) in the temperature interval (AT), the weight (daf) at the average temperature (7) in the temperature interval (AZ) and the time interval during the weight loss (Am(AZ)), respectively. Polarized light optical microscopy The polarized light optical micrographs of chars were obtained by the method described elsewhere15.

Gas chromatography Samples were injected on to a 50 m WCOT fused silica capillary column at a 2O:l split ratio. The injector temperature was 300°C and the oven temperature programmed from 50 to 300°C at 3°C mm’, holding for 30 min at 300°C. Helium was used as carrier gas. Surface areas of chars Carbon dioxide adsorption isotherms of the chars from asphaltene pyrolysis were measured at 20°C by a gravimetric method. The corresponding surface areas of chars were then evaluated by carbon dioxide isotherm measured and the BET equation. Measurement of rate of gasi$cation Non-isothermal thermogravimetric analysis was carried out at a carbon dioxide partial pressure of 10 Pa to measure the dynamic change in conversion rate with time. A char sample of 50-70 mg in a sample holder was heated from 600 to 1100°C at a rate of 10 K mm-t. The weight change of the sample and the temperature were recorded continuously throughout the experiment. The reaction rate of char was evaluated on the basis of: Am(AT) r” = ----.lOO m(T).

1100

At

[% mm-‘]

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RESULTS

AND DISCUSSION

Tars According to the gas chromatograms of aliphatic fractions of tars from pyrolysis of asphaltenes derived from coals studied, these fractions consist mainly of a homologous n-alkane series in the range C12-C3 1. Undoubtedly, these chains are derived directly from the original asphaltene structure, for it is conceivable that long alkyl chains will be formed by the pyrolysis of branched or ring structures. The most likely occurrence of such chains is as alkyl substituents on aromatic nuclei. In addition, the presence of long chain n-alkyl groups in asphaltene structures is consistent with the results obtained by n.m.r. measurements on asphaltenes previously15. Asphaltenes from coal hydroliquefaction have been considered as part of the macromolecular network due to the multiple precipitation procedure for obtaining a clean initial asphaltene. Thus, these results indicate that alkyl chains and alkylene bridges appear mostly as substituents in the macromolecular network. Figure 1 shows as an example the carbon distribution in these n-alkanes of the tar aliphatic fraction from pyrolysis of Chriemhilt coal asphaltenes. The odd over even carbon preference can be observed. This reflects the

Characterization

of tar, char and gas: Y. Liu et al.

1 0.8 c

Carbon Number

Figure 1

Distribution

of the n-alkanes of the aliphatic fraction of tar from the pyrolysis of the asphaltenes

derived from Chriemhilt coal

Fushun

Chriemhilt

(2) Acenaphthylene (1) Biphenyl (5) Phenanthrene (4) 1-Methylfluorene (7) 4.5-Dimethylphenanthene (8) Fluoranthene (11)2,3-Benzofluoanthene (I 0) 2.3-Benzofluorene

Figure 2

Distribution

(3) Fluorene (6) 2-Methylanthracene (9) Pyrene (12) Coronene

of the PAH fractions of tar from the pyrolysis of the asphaltenes

origin of the plant materials of coal, since a similar odd over even hydrocarbon preference has also been shown in living plar& Analysis of gas chromatograms of the corresponding neutral aromatic fractions shows the presence alkylnaphthalenes, biphenyl, fluorenes, benzofluorenes, phenanthrenes, anthracenes, pyrene and other two- to four-ring aromatic compounds. The distribution of these polycyclic

derived from coals with different rank

aromatic hydrocarbons from all pyrolysis tars is shown in Figure 2. It can be seen that as rank increases, a noticeable shift to higher boiling products is apparent and the distribution of these products tends to be quite similar, indicating that, with maturation of coal, the amount and size of condensed aromatic rings increase and the product distribution favours the more thermally mature compounds.

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Characterization of tar, char and gas: Y. Liu et al.

Table 2

Elemental analysis and BET surface area of chars from pyrolysis of asphakenes

Sample

Elemental analyses (daf, wt%) N Odir.

H

C

obtained from coals used

S

WC Ratio

0 + N + S/C Ratio (%)

BET (m* g-‘)

Fushun

90.2

3.04

2.3

1.63

0.25

0.40

3.5

247.0

Chriemhilt

94.2

3.04

1.6

1.59

0.32

0.39

2.8

234.1

Fengjiao

93.5

3.05

1.4

1.50

0.58

0.39

2.7

225.5

Liulin

93.1

3.02

1.3

1.53

0.44

0.38

2.6

226.5

Fengsou

94.2

3.06

1.0

1.46

0.68

0.38

2.4

219.4

Fushun

1’92ipk .

Fengjiao

‘0 k =b 0.96

z

Liulin

0.48 1.44

0.00 900

Fengsou

950

1000

1050

1100

Temperature [“Cl

Figure 3 Gasification reactivity of chars by the pyrolysis of asphaltenes derived from hydroliquefaction of coals with different rank in CO2 atmosphere

loo -

L7

80 -

F

.

60-

a ? 40 -

Isotrope Figure 4

Optical texture of chars from the pyrolysis of asphaltenes derived from coals with different rank

Chars The properties and reactivity of chars from coal depend on various factors, including the parent coal rank, the ash content and the char formation conditions’7-‘9. The char from pyrolysis of asphaltenes as representative of parent coal structural unit contains no ash as catalyst during coal conversion. In this case, the relationship between the original coal structure and the char properties can be inferred directly from the investigation of this char. Table 2 shows the elemental analysis and BET surface area of the chars obtained from the asphaltene pyrolysis. The surface areas of the chars were found to be proportional to ratios of char heteroatom to char carbon atom as well as H/C ratio of chars. The char with the highest (0 + N + S)/C and H/C ratios has the greatest BET surface area and the reverse is also true. Compared with relative heteroatom and hydrogen

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Anisotrope

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of corresponding original coals summarized in Table 1, it can also be concluded that the BET areas of pyrolysis chars from asphaltenes are related to heteroatom and hydrogen contents of original coals. The constant values of the conversion rates of chars in CO2 gasification are plotted against the temperatures in Figure 3. The order of conversion rates of chars at the temperature range above 900°C were found as follows: Fushun > Fengjiao = Fengsou > Liulin * Chriemhilt, which does not correlate with the BET surface areas of the chars. Since there is no catalytic effect of mineral matter within the ash of the normal chars from coal, the deviations may be attributed to the molecular structure of parent materials*’ , rather than heteroatom content of original coals. The reactivity of chars obtained by the pyrolysis of asphaltenes from Fengjiao and Fengsou coals of the same

Characterization

of tar, char and gas: Y. Liu

et al.

W : asphaltenes

0 300

400

500

600 T

Mesoohase

700

800

[“Cl

30

carbon monoxide 1 Networking Anisotropic

Figure 5

\

1

Isotropic char

char

Carbonization

i

of asphaltenes

01 300

I 400

I 500

I 700

I 600 T

I 800

I 900

11 00

WI

Figure 7 Methane and carbon monoxide formation pyrolysis of asphaltenes from coals with different rank

during

(Fengfeng district, in Hebei) but different rank was found to be almost similar. This observation may further prove that coals from the same origin contain the same basic structures independent of rank. All chars studied and a char obtained by pyrolysis of Chriemhilt coal asphaltenes in vacuum (lo- Pa) were examined by polarized light optical microscopy (Figure 4). Interestingly, the normal chars are distinguished by a large variety of anisotropic forms, while the vacuum char is almost optically isotropic. Such optical texture of chars can be explained by mesophase formation in the carbonization system. As seen in Figure 5, the polycyclic aromatic and heteroaromatic hydrocarbons in asphaltenes undergo polymerization reactions during pyrolysis to form large pericondensed aromatic hydrocarbons. As these molecules enlarge (molecular weight more than 1000 dalton), an interaction vertical to the aromatic levels becomes strong enough to create liquid crystals (mesophase), resulting in optical anisotropic forms. If the produced low molecules are rapidly destillized from the carbonization system via vacuum, fluidity of the system decreases and no anisotropic carbon is created. The results confirm that the mesophase and fluidity are major factors controlling optical texture of resultant chars”.

origin

T [“Cl

0

I

I

I

I

200

400

600

800

T

I

1.000

[“Cl

Figure 6 Product formation during pyrolysis of the asphaltenes from Fushun coal (VM 48.9 wt%, daf, 3 K min-‘, N2)

Gases Pyrolysis of asphaltenes from Fushun coal. The integral and differential curves of different gaseous components (CH4,

C2H4,

C2H6,

C3HdC3fb

Hz,

co,

co2

ad

H20>,

measured in a thermobalance combined with on-line GC

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Characterization of tar, char and gas: Y. Liu et al.

Table 3

The amount of methane and carbon monoxide formation during pyrolysis of asphaltenes from coals studied Fushun

Chriemhilt

Fengjiao

Liulin

Fengsou

(34

31.34

41.17

47.34

50.17

45.16

co

10.43

6.04

5.26

5.59

5.30

mg g-’ (da0

at a low heating rate, are shown for the Fushun coal asphaltenes as an example in Figure 6.

It can be seen that below 400°C there is no gas formation, except for the small peak of carbon dioxide, probably due to the existence of intramolecular carboxylic acid anhydrides. The reaction step occurs at 400-6OO”C, where all gases are formed simultaneously. Three remarkable peaks can be observed. They can be attributed to different functional groups in asphaltenes of different thermal stability. By comparing the pyrolysis of asphaltenes with that of model substances, whose structures are well known and correspond to structural elements existing in coal, an explanation of these peaks can be provided22-24. The first reaction results from cleavage of aryl-methylethers. Methane is formed and so are other hydrocarbons by radical recombination. Some of the retained phenoxy radicals are decomposed and carbon monoxide split off. Carbon dioxide is formed by the Boudouard reaction. The second step is caused by cleavage of the strong bonds, e.g. methylene and biarylether bridges between aromatic units. Meanwhile, methane, carbon monoxide and carbon dioxide are formed simultaneously. The methane peak at 580°C originates from decomposition of methyl group attached to the aromatic units. If the temperature region is higher than 600°C the breakdown of the asphaltene structure is completed and char formed. The char still contains some oxygen containing structures, mainly aromatic heterocyclic structures, decomposed with the formation of carbon monoxide and methane.

cross-links between aromatic clusters since asphaltenes are an integral part of the macromolecular network. Total surface areas of the chars from pyrolysis of asphaltenes derived from coal hydroliquefaction are positively proportional to the heteroatom content in the chars, and the heteroatom content in the parent coals, but are not a relevant parameter for the reactivity of chars. Independent of rank, the basic structural units of coals from the same district are similar. Optical anisotropic structure of asphaltene chars can be explained by mesophase formed during pyrolysis. Fluidity of the carbonization system is a major factor controlling the char optical texture via the mesophase route. Asphaltene pyrolysis shows that most of the gaseous components come from cleavage of strong bonds, mainly methylene and biarylether bridges between the aromatic entities and methyl groups on the aromatic rings in asphaltene structure. The weak bonds like aryl-alkylethers are found only in asphaltenes resulting from low rank sub-bituminous Fushun coal. ACKNOWLEDGEMENTS This work owes great thanks to the Bundesministerium fur Forschung und Technologie of the Federal Republic of Germany, project no. 0326215 B.

REFERENCES Given, P. H., Marzec, A., Barton, W. A., Lynch, L. J. and Gerstein, B. C., Fuel, 1986, 65, 155. Given, P. H. and Marzec, A., Fuel, 1988,67, 242. Derbyshire, F., Marzec, A., Schulten, H. -R., Wilson, M. A., Davis, A., Tekely, P., Delpuech, J. -J., Jurkiewicz, A., Bronnimann, C. E., Wind, R. A., Maciel, G. E., Narayan, R., BartJe, K. and Snape, C., Fuel, 1989,68, 1091.

Comparison of pyrolysis of asphaltenes from all Great differences are seen in the shape of the coals.

curve for Fushun coal asphaltene pyrolysis in comparison with that of other coal asphaltene pyrolysis (Figure 7). In the temperature between 400 and 6OO”C, Fushun coal asphaltenes show three methane formation peaks due to cleavage of aryl-alkyl-ethers, methylene and biarylether bridges and methyl groups on the aromatic units, respectively, while only one peak in this region is observed during pyrolysis of other asphaltenes. This may result mainly from the cleavage of methylene and biarylether bridges between aromatic units. Interestingly, during pyrolysis of Fengjiao and Fengsou coal asphaltenes the height and temperature range of the peak maximum of methane and carbon monoxide evolutions are found similar. As seen in Table 3, the methane and carbon monoxide formed during pyrolysis of these two materials is almost equal in amount, while methane from asphaltenes of other coals increases and carbon monoxide decreases as coal rank increases. It can be concluded that basic structural units of Fengfeng coals of different rank are similar. This shows an excellent agreement with previous results’.

4 5 6

7 8 9 10

11 12

13

CONCLUSIONS The ‘clean’ asphaltenes were found to contain n-alkanes from Cl4 to C31, showing an odd over even preference. This result implies that these long alkyl chains are present as pendant side chains on the macromolecular network and

1104

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14 15

16

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Gorbaty, M. L., Fuel, 1994, 73, 1819. Carlson, R. E., Critchfield, S., Vorkink, W. P., Dong, J. Z., Pugmire, R. J., Lee, M. L., Zhang, Y., Shabtai, J. and Battle, K. D., Fuel, 1992, 71, 19. Nelson, P. F., Fuel, 1987, 66, 1264. Grigson, S. J. W., Kemp, W., Ludgate, P. R. and Steedman, W., Fuel, 1983, 62, 695. Hodek, W., Kraemer, M. and Jtintgen, H., Fuel Proc. Technol., 1990,24,417. Hodek, W. and Kijlling G., in Proceedings, 1983 Intemational Conference on Coal Science, Pittsburgh. Elsevier, 1983, p. 771. Kraemer, M., PhD thesis, University GHS, Essen, Germany, 1988. Steedman, W., Fuel Proc. Technol., 1985, 10, 209. Hodek, W., Spezielle analytische Elfahrungen und Aspekte bei der Untersuchung von Asphaltolen und Asphaltenen. DGMK Forschungsbericht no. 304, Verlag Chemie, Ham-

burg, Germany, 1982. Later, D. W., Lee, M. L., Bartle, K. D., Kong, R. C. and Vassilaros, D. L., Anal. Chem., 1981, 53, 1612. Liu, Y., PhD thesis, University GHS. Essen, Germany, 1996. Brooks, J. D. and Smith, J. W., Geochim. Cosmochim. Acta, 1967,31,2389.

Characterization

17

18 19 20

van Heek, K. H. and Mtihlen H. -J., in Fundamental Issues in Control of Carbon Gas$cation Reactivity, eds J. Lahaye and P. Ehrburger. Kluwer Academic, Dordrecht, Netherlands, 1991, p. 1. Chitsora, C. T., Miihlen, H. J., van Heek, K. H. and Jtintgen, H., Fuel Proc. Technol., 1987, 15, 17. Miura, K., Hashimoto, K. and Silveston, P. L., Fuel, 1989, 68, 1461. Stanczyk, K., Erdiil und Kohle-Erdgas-Petrochemie. 1993, 46(5), 199.

21 22 23

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Marsh, H. and Clarke, P. E., Erdiil und Kohle-ErdgasPetrochemie, 1986, 39(3), 113. van Heek, K. H. and Hodek, W., Fuel, 1994,73, 886. Hodek, W., Kirschstein, J., Diirkopf, H., Wahlers, W. and van Heek, K. H., Erdiil Erdgas Kohle, 1990, 106, 223. Hodek, W., Kirschstein, J. and van Heek, K. H., Fuel, 1991, 70, 424.

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