Pyridine extractable material from bituminous coal, its donor properties and its effect on plastic properties

Pyridine extractable material from bituminous coal, its donor properties its effect on plastic properties Koji Ouchi,

Satoru

Faculty of Engineering, (Received 8 December

Itoh,

Masataka

Makabe

and Hironori

and

ltoh

Hokkaido University, Sapporo 060, Japan 1987; revised 17 October 1988)

A Japanese bituminous coal (81.2%C) was extracted with pyridine at 2OCWOO”C to give an extraction yield of 50 wt% at 400°C. However, after quinoline extraction the same level of pyridine extraction (50 wt% yield) could be obtained at 300°C. This result was interpreted as being due to the intimate occlusion of extractable material in the network macromolecular structure or in the larger molecules. Room temperature pyridine extraction of methylated coal and the repeated pyridine extraction at 200°C reinforced this view Pyridine extracts at room temperature, 300°C and 400°C contained similar compounds. This was also taken as confirmation that the pyridine extracts are not thermal decomposition products, but the true extracts. Pyridine extraction residues obtained at room temperature, 200°C and 4WC, and the original coal were treated in naphthalene at 400°C for 15min and then extracted by pyridine. A total extraction yield of 50 wt% was obtained for all treatments, clearly indicating that the pyridine extract had no donor property. The implication of these results to the plastic phenomena of coal during heating is discussed. (Keywords:

bituminous coal; pyridine; solvolysis)

Understanding the plasticity that bituminous coals undergo during heating is a most important problem with regard to coke making, and has been the subject of study by numerous researchers. The many theories that have been proposed to explain the mechanism of the plastic phenomenon may be categorized as follows. Contribution

of y compound’-8

This theory assumes tacitly that the y compound melts first and then the higher molecular weight compounds dissolve in it so that finally the whole coal apparently shows plasticity. This theory is supported experimentally by the fact that solvent extraction of coal causes fluidity to decrease or to be removed completely. Ouchi et ~1.~ recently extended this theory by demonstrating that the quinoline extraction yield was almost proportional to the maximum Gieseler fluidity and they explained that the lower average molecular weight of coal suggested by a large solvent extractability, is the cause of the higher fluidity. Metaplast

theory”,”

The coal macromolecular structure is considered to decompose thermally when heated and the lower molecular weight compounds produced from it, termed metaplast, contribute to the plastic property of coal. Mathematical derivation could explain reasonably the effect of the heating rate on Gieseler curves1 ‘. Drydenl’ ascribed the increase in chloroform extraction yield after shock heating to thermal decomposition. However 0uchi13 suggested that the lower molecular weight compounds that are soluble in chloroform, could not be extracted completely because of their very intimate occlusion in the network structure of macromolecular compounds, and that relaxation of this network structure by shock heating facilitated extraction. It was shown13 COl6-2361/89/06073546$3.00 ( 1989 Butterworth & Co. (Publishers)

Ltd.

that the chloroform extraction yield of the pyridine extraction residue did not increase as a consequence of shock heating. Waters et ~1.‘~ also agreed with this view. Moreover, metaplast theory has a serious limitation in that after solvent extraction, plasticity decreases or is removed completely. If the plasticity is due to the thermal decomposition of macromolecules, even after extraction, it should still be plastic when heated. Neavel’ 5 attempted to explain this limitation by suggesting that the solvent soluble materials had donor properties and could stabilize the radicals produced from thermal decomposition. Larsen et ~1.‘~ produced experimental support for this. However, they used chloroform as the preliminary extraction solvent, which was usually retained in the coal structure (in our experiments, 1.5 wt% of chlorine was found in the residue), and chlorine has a tendency to accelerate the polymerization reaction. Their results also showed too small a reduction of extraction yield after treatment to ascribe this to the donor ability. Suggestions have been made17-19 that a typical solvent extraction method, e.g. Soxhlet extraction, cannot extract all of the extractable materials completely, and a large amount of extractable material is retained in the residue. This is similar to the view expressed by 0uchi13, but is still under discussion. If this is the case, the true amount of extractable material must be markedly greater than the value obtained by conventional Soxhlet extraction and the plastic phenomenon may be explained by the presence of significantly larger amounts of extractable lower molecular weight compounds than considered previously. This paper attempts to measure the true amount of extractable material, to determine its donor property and to explain the plastic property of coal on the basis of these results.

FUEL, 1989, Vol68,

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735

Pyridine extractable

material from bituminous

coal: K. Ouchi et al.

EXPERIMENTAL Sample coal

Vitrinite of Japanese Akabira bituminous coal, concentrated by float and sinks, was obtained in a yield of 63 wt%. This was ground to less than 149pm and stored under water. The ultimate analysis of the sample coal was C, 81.1; H, 5.9, N, 2.1; O(diff), 10.2wt% (daf). Another four coals, of which proximate analytical data are shown in Table 1, were also used as received. Pyridine extraction

Five g of the Akabira coal were extracted by 200ml of pyridine in a 350ml autoclave fitted with a magnetic stirrer, under 1 MPa nitrogen pressure for 2 h at 200, 300, 350, 370 and 400°C. The extraction yield was calculated from the weight of residue after correcting for the ash content. Quinoline extraction

Five g of Akabira coal were extracted with 200ml of purified quinoline using the same procedure as described for pyridine extraction and the residue was washed by quinoline, hydrochloric acid and water before drying. The quinoline solution was concentrated and poured into a large amount of diluted hydrochloric acid with violent stirring. The precipitate was filtered, washed with water and dried. The extract was then extracted by pyridine at room temperature under ultrasonic irradiation. The pyridine extract was also extracted by benzene similarly and its extract by n-hexane. Methylation of coal

Five g of Akabira coal were dispersed in 15ml of dimethyl sulphate of 70°C and 25 ml of 50 wt% KOH solution was added dropwise with stirring. The reaction was repeated four times. After reaction, hot water was added and the mixture filtered. The product was washed with water and dried. The methylated coal was extracted with pyridine at room temperature under ultrasonic irradiation as described above. The residue was again methylated and extracted. This procedure was repeated four times in total. Repeated pyridine extraction at 200°C

Akabira coal (20g) were extracted with 250ml of pyridine in a 350 ml shaking autoclave containing 15 steel balls, at 200°C under 5 MPa nitrogen atmosphere for l&20 h. After extraction, the product was filtered and washed by a small amount of pyridine and water. The extraction yield was calculated from the weight of residue. The residue was then extracted similarly and the extraction was repeated 7 times.

raphy into saturate, aromatic and polar fractions. Saturate and aromatic fractions were analysed by capillary g.c. using a 0.25mm i.d. x 25 m fused silica column coated with OV 101. Peaks were identified using g.c.-m.s. and comparison with standard samples. Examination of donor activity of extract

Akabira coal was extracted with pyridine at room temperature under ultrasonic irradiation and at 200°C or 400°C in an autoclave. All of the pyridine extraction residues and the original coal (each log) were heated at 400°C for 15 min under nitrogen with 40 g of naphthalene in an autoclave of 350ml capacity. After reaction, the products were extracted with pyridine at room temperature under ultrasonic irradiation. In addition, 20 wt% of pyridine extract obtained at room temperature was added to the original coal and this mixture was treated with naphthalene at 400°C for 15 min and extracted with pyridine using the procedure described previously. Quinoline, pyridine and benzene extraction of coals

The four coals listed in Table 1 were extracted with quinoline at 350°C for 1 h under 2 MPa nitrogen. The extracts were further extracted with pyridine and benzene at room temperature under ultrasonic irradiation. Thermogravimetric analysis

Thermogravimetric analysis of Akabira coal was carried out using a thermobalance connected to an infrared image furnace, the coal particle size being varied from 2.3-3.4mm to 0.2550.35 mm, under 66.7 Pa or normal pressure of nitrogen, and with heating rates of 10°C min-’ or l-160”Cmin- *. The sample amount was usually = 10mg.

RESULTS AND DISCUSSION Examination of true extract amount

The pyridine extraction yields at various temperatures are shown in Figure 2. Up to 350°C the yield increased gradually, then rapidly and finally attained 50 wt% yield at 400°C. The quinoline extraction yield (Figure I) also increased gradually up to 3OO”C,then 90 wt% yield was rapidly attained by 350°C. Pyridine extraction yield of quinoline extract followed a similar trend to that for the quinoline extraction yield up to 3OO”C,but then levelled off at about 50 wt%, which is almost the same as the simple pyridine extraction yield at 400°C although at a significantly lower temperature. Benzene and n-hexane extraction yield of the quinoline extract remained constant at about 10 wt% over the

Analysis of pyridine extract

Akabira coal was extracted with pyridine at room temperature under ultrasonic irradiation. The residue was then extracted with pyridine at 300°C and again at 400°C. The extracted solutions were concentrated under vacuum and poured into a large amount of dilute hydrochloric acid with violent agitation. The precipitates were filtered, washed with water and dried under vacuum. They were then extracted by n-hexane at room temperature under ultrasonic irradiation. The n-hexane extracts were fractionated by silica gel liquid chromatog-

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FUEL, 1989, Vol 68, June

Table 1 used

Proximate

analysis

and maximum

fluidity of bituminous

Proximate analysis (wt% d.b.)

coals

Coals

Ash

VM

FC

Max. fluidity ddpm

Drummond (USA) Goonyella (Australia) Daiyon (Australia) Neryungi (USSR)

7.1 8.6 1.6 9.7

32.1 24.9 31.1 18.4

60.2 66.5 54.7 71.9

6670 1629 703 57

Pyridine 100

0

I

200

I

250 TEMPERATURE

I

I

300

350

I

400

‘C

Figure 1 Pyridine and quinoline extraction yield and pyridine, benzene or n-hexane extraction yield of quinoline extract at various temperatures: c), pyridine extraction yield; 0, quinoline extraction yield; 0, pyridine extraction yield of quinoline extract; .A, benzene extraction yield of quinoline extract; and A, n-hexane extraction yield of quinoline extract

whole temperature range. This behaviour may be explained as follows. Quinoline is a very strong polar solvent and can relax the network structure or the entangling structure at high temperature. This leads to the complete extraction of pyridine-soluble material even at 300°C but pyridine itself cannot extract the soluble material completely below a temperature of 400°C. At temperatures < 3OO”C, quinoline extracted the components which were easily soluble in quinoline and this is also soluble in pyridine. At temperatures >3OO”C quinoline can dissolve the higher molecular weight materials, but pyridine cannot, and hence pyridine extraction levels off in this temperature range. It may be considered that quinoline or pyridine extraction at this high temperature results from thermal decomposition through splitting of covalent bonds, thereby lowering the molecular weight. If this was the case, the pyridine, benzene and n-hexane extraction yields of the quinoline extract would increase parallel to the quinoline extraction yield, and even the pyridine extraction itself (without preliminary quinoline extraction) should show 50wt% yield at 300°C. However, the results show that the pyridine extraction yield of the quinoline extract levelled off at 300°C and the benzene or n-hexane extraction yield remained constant. These results suggest that this extraction is not a result of thermal decomposition, but is a true extraction value. Benzene or n-hexane soluble compounds are so easily extracted that even at low temperatures, all the materials extractable by benzene and n-hexane were completely extracted into quinoline. To confirm this point further, the mild extraction with pyridine was carried out. First, methylated coal was extracted with pyridine. After three successive methylation experiments, OH absorption in the infrared spectra nearly disappeared, and the methylation was almost complete. Table2 gives the pyridine extraction yield of methylated coal. After four methylations, pyridine extraction yield at room temperature approached 50 wt% i.e. the same limiting value as above. Methylation breaks

extractable

material

from bituminous

coal: K. Ouchi

et al.

the hydrogen bonds or some type of donor-acceptor complex bonds, and this may contribute to the easy extraction by pyridine. This supports the hypothesis that the occluded soluble materials are retained in the network structure through hydrogen bonds or donor-acceptor bonds as suggested by Marzec2’. Following this, pyridine extraction was carried out at a lower temperature, e.g. 200°C. The result is shown in Figure2. After seven pyridine extractions at 200°C the cumulative extraction yield again approached 50 wt%. Extraction at 200°C is a mild enough extraction, nevertheless the pyridine extraction yield reached the same value. These results are firm evidence for the above conclusion. If the pyridine extraction at various temperatures is not a result of thermal decomposition, but the true extraction, each extract must contain similar types of compound. To confirm this, saturate and aromatic fractions of hexane solubles in pyridine extract at room temperature, 200°C and 400°C were analysed. The n-paraffins distribution in the saturate fractions is shown in Figure 3. All the extracts contain n-paraffins, and the maximum shifted to longer carbon side chain with the extraction temperature. If this is a thermal decomposition product, the oletin must be present in a fairly large amount, but the saturate fractions of the extracts contain no olefin. Moreover, for thermal decomposition the maximum position must shift to the shorter carbon chain side, but the result shows the opposite tendency. Therefore, these paraffins must be contained in the original coal. Figure4 shows the analytical result for the aromatic fractions. It can be seen that all the aromatic fractions contain similar compounds, although the amount is somewhat different. These results also confirm that these extracts are not thermal decomposition products, but the true extractable values. Mudanburi et ~1.~~ also suggested this from the analyses of pyridine extract and hydrogenation product.

Table 2

Pyridine

extraction

yield of methylated Cumulative (wt%J)

Methylation

!

:

s

pyridine

extraction

yield

30.8 38.5 41.8 46. I

2 3 4 -_____

z

coal

ol

0

L

20

A0

CUMULATIVE

60

I

60

REACTION

100

TIME

120

h

z

Figure2

Cumulative

pyridine

extraction

yield at 200’C

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wt%

extraction residue at 400°C). However, the total extraction yield did not change. To get further confirmation, 20 wt% of the room temperature pyridine extract was added to the original coal and the mixture was treated similarly in naphthalene at 400°C. Again the total pyridine extraction yield was 50 wt%. Those results definitely prove that pyridine extract has no donor property.

15.0

I

Room

Temperature

l&L

Relationship between solvent extractability

Gieseler

maximum

fluidity

and

As the metaplast theory is rejected, it is suggested that the lower molecular weight materials probably cause the plastic properties of coal. A linear relationship between

R.T.

0

15

20 Carbon

25 Number

Figure 3 n-Paraffin distribution in pyridine temperature, 300°C and 400°C (n-hexane extract)

Examination

b!nL 30

extracts

at

room

of donor ability of pyridine extract

As indicated in the introduction, the metaplast theory has a serious limitation, which Neavel” attempted to eliminate. LarsenI carried out experiments to confirm Neavel’s idea, but there is doubt about the validity of these results. To avoid this experimental doubt, an attempt was made to extract coal with pyridine at different temperatures, then the residues and the original coal were treated in naphthalene, and the products were extracted with pyridine. During this tretment, oxidation of extraction residue was carefully avoided. The results are given in Figure 5. The total pyridine extraction yield for the original coal, and room temperature, 200°C or 400°C pyridine extraction residues was nearly 50 wt% of pyridine extraction yield in all cases. It is noteworthy that this value coincides with those of the several extractions described above. The pyridine extraction yield at room temperature was 14.6 wt%, which is a large enough amount to lower the donor activity significantly, if it acts as donor. Therefore, the second extraction yield after reaction in naphthalene at 400°C must decrease to a large extent (if the extraction were complete, it would be zero, as in the case of pyridine

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of aromatic fractions of pyridine Figure 4 Gas chromatograms at room temperature, 300°C and 400°C (n-hexane extracts)

48.5

R.T. P.I.

2oooc

P.I.

extract

m before

g

w]4*.3

after reaction

19.5 /// P.s

46.5

P.S

added 4oo”c

I?s 55.0

P.I.

Original Coal

p/4 11

ps

9.6 I.

0

Extraction

-

I

50 yield

I

0

1.

10

(wt%)

yield of first pyridine extraction and that after Figure 5 Extraction naphthalene treatment at 400°C for 15 min: P.S., pyridine soluble; P.I., pyridine insoluble

Pyridine

_*2

/*

_.---LOG Figure 6 yield: 0,

2

3

\ MAXIMUM

FLUIDITY,

Relationship between log (maximum quinoline; 0, pyridine; A, benzene

ddpm)

fluidity) and extraction

Gieseler fluidity and quinoline extraction yield at 350°C was previously reported 9 for coals of similar carbon content. To give further experimental backing to this, the fluidity and solvent extraction yield were measured for the four coals listed in Table 1. The results, Figure 6, show a clear proportionality between log (maximum fluidity) and quinoline, pyridine or benzene extraction yield. Thus, readily soluble coals exhibit high fluidity. The coals of high quinoline extraction yield also have high pyridine and benzene extraction yield, that is to say those coals consist of relatively smaller molecules on average. These smaller molecules easily became plastic when heated and hence contributed to the higher fluidity.

extractable

material

from

bituminous

coal:

K. Ouchi

et al.

fluidity than under normal pressure. This has been observed and reported24. 2. It was reported that the reduction of coal particle size leads to a decrease in fluidity31-35. Figure7 shows the effect of particle size on the weight loss. Weight loss of 2.333.4mm coal clearly shifted to a higher temperature side than in the case of 0.25-0.35mm coal. This also indicates the accumulation of small molecules in the larger particles, probably because of the long diffusion time of vaporizable materials from the interior to the surface, leading again to high fluidity. 3. The plastic range is reported to shift to a higher temperature and the maximum fluidity increased with increasing heating rate”,’ 1,27-29. Figure 8a shows the effect of heating rate on the weight loss of Akabira coal. The curves of weight loss shifted to the higher temperature range with an increase in heating rate. In the case of a slow heating rate such as lL2”C min- ‘, the particles coagulate very weakly and readily collapse, although they became one strong block on rapid heating. A similar result was previously reported by Mochida et al.25 for three types of coal. Jiintgen26 also measured the effect of heating rate on the weight loss and product yields with similar results. Figure 8b shows the differences of weight loss between the curves of lowest heating rate and those of higher heating rate, which represent the accumulation of vaporizable small molecules in coal particles with increases in heating rate. The maximum values shifted to the higher temperature side and the weight loss increased with the increase of heating rate. The effects of heating rate on the Gieseler plastometer curves previously reported by van Krevelen’“*” and Chevallier2’ show similar trends to those in FigureSb, namely, the temperature of maximum fluidity shifted to the higher temperature side and the maximum fluidity increased with heating rate. van Krevelen interpreted this in terms of the metaplast theory. However, these similarities clearly show that the accumulation of small molecules in coal particles is the main cause of fluidity increase when the heating rate is increased, probably because of the diffusion of small molecules from the interior to the surface where they vaporize.

7hermograuimetric analysis

The phenomena observed in the thermogravimetric analysis as described below, give further evidence for the hypothesis proposed. 1. Under high pressure the fluidity generally increases and under vacuum it decreases or disappears29x30*31. Figure 7 shows the influence of reduced pressure, 66.7Pa, on the thermogravimetric analysis of Akabira coal. In comparison with the case of normal pressure of nitrogen, the weight loss begins to increase at a substantially lower temperature, namely the vaporization of small molecules starts earlier than is the case under normal pressure. Thus on heating, the amount of plastic substance in coal reduces more quickly before melting, leading to a decrease in fluidity or even its complete elimination, Although measurement under high pressure was not carried out, it is self-evident that the vaporization of small molecules is supressed under high pressure and they will accumulate in the coal particles, leading to the higher

i0

TEMPERATURE “C Figure 7 Influence of pressure and particle size on weight loss during heating. Heating rate: 10°C min- 1: -. -, 0.1 MPa, 2.3-3.4 mm; ----, 0.1 MPa, 0.25-0.35mm; -----, 66.7Pa. 0.2550.35mm

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ACKNOWLEDGEMENT The authors wish to express their sincere thanks for the support and samples supplied by NKK Ltd, and also for grant no. 63430016 from the Ministry of Education. REFERENCES

5 3E

b /y

6 I 8

\

$

9 10 II 12 13 14 15 II 0 g

0 250

\ 300

350

400

TEMPERATURE

Figure 8 a, Influence Particle size; 0250.35 l”Cmin~‘;----,2”Cmin-‘;

450

500

!

“C

of heating rate on weight loss during heating. mm; nitrogen pressure, 0.1 MPa: --, _ _ -,5”Cmin-I;-._--, lwCmin_‘;

-. .-, 2O’Cmin-‘; -. .-, 40”Cmin-I;--, 160”Cmin-‘. b, Accumulation of vaporizable materials in coal particles with increase I-2”Cmin-i; _--~, I-5”Cmin~‘; -._, of heating rate: ~, l-lO”Cmin-i; -. .-, l-20”Cmin-‘; -. .-, 140”Cmin-i; -_, 1-160”Cmin-1

16 17 18 19 20 21 22 23 24 25 26 21

28

CONCLUSION For the coal examined, the true amount of pyridine soluble material is significantly more abundant than the conventional extraction yield based on Soxhlet extraction. The extract has no donor property. The plastic property of Akabira coal is explained by the large amount of such soluble smaller molecular material. However, it is recognised that thermal decomposition and polymerization have a role at higher temperatures.

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29 30 31 32 33 34 35

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