Fuel Processing Technology, 8 (1984) 253--258 253 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

STRUCTURE OF COKES FROM COKING COAL VITRITES

STEFAN JASIEI~IKO and IRENA GERUS--PIASECKA

Instytut Chemii i Technologii Nafty i Wegla, Politechnika WrocJawska, 50-344 WrocJaw (Poland)

(Received August 16th, 1983; accepted September 21th, 1983)

ABSTRACT

The coking process of vitrites and thermobitumens separated from vitrites was examin- ed; structural X-ray and microscopic examinations of the cokes obtained were carried out. A correlation between reflectance distribution of vitrites and microscopic structure of their cokes was found.

An increase in the structural ordering of the cokes from vitrites, passing from cokes of gas coal to cokes of orthocoking coals, is observed. It is accompanied by an increase of the optical anisotropy of the resultant cokes; this anisotropy first appears in coke from gas-coaking coal.

The cokes from the thermobitumens are lower ordered than the cokes from parent vitrites but all these cokes are partially or entirely optically anisotropic.

Total removal of the thermobitumens from coals deprives the cokes from the residues after the extraction of any optical anisotropy.

INTRODUCTION

The thermal stability of coal, the rate of decomposition and the character of structural changes of organic coal substances, as well as the properties of the products obtained depend on the type of coal, its petrographic composi- tion, its chemical properties and the temperature of heat treatment [1--4].

In the coals occupying a middle position in rank, relatively small fragments of macromolecules are able to split off. They do not volatilize but are col- lected in the intermolecular space and plastify the coal system [2]. Baked and swollen cokes are obtained from these coals.

The cokes obtained from medium-rank coals show the shortest interplanar spacing, d002, and the largest height of crystallites [3, 5--9]. These cokes are characterized by the appearance of optical anisotropy. The following types of optical anisotropy appear in the cokes obtained from the coals examined: fine-grained, coarse-grained and shape anisotropy of different sizes [6, 10].

The formation of structures exhibiting optical anisotropy is related to the transition of coal through a plastic state [11--16]. In the plastic state, a homogeneous process of evolution of lameUar macromolecules, called liquid crystals, from an isotropic liquid takes place. Initially, the liquid crystals

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have the form of parallel-set spheres. The spheres increase their dimensions with increasing temperature and then undergo coalescence forming semi- coke. The structure formed in the semicoke during the coalescence of liquid crystals undergoes only slight modification during further thermal treatment [12].

The formation and growth of liquid crystals is easiest in coking coals. Lower coalified coals have a low degree of aromatization and a high content of oxygen, partly forming cross-linkages; therefore, not all particles can form liquid crystals. The increase of liquid crystals removes suitable particles from their surroundings, leaving isotropic liquid. Cokes obtained from low-rank coals are characterized by the appearance of isolated anisotropic areas inside an isotropic ground mass. Dimensions and quantity of the anisotropic frac- tion increase with increasing coal rank.

The coking coals show high aromaticity and high homogeneity, as well as a suitable mobility of the basic structural unit core, and their plastic phase is characterized by high fluidity, which is favourable for the increase of liquid crystals.

In the higher-rank coals, the fluidity of the system is too low to allow the growth of liquid crystals [13, 14].

In this paper the main aim was a determination of the basic differences in the structure of cokes obtained from different types of coking coal vitrites and a determination of the correlation between the reflectance distribution of vitrinites and the structure of cokes.

EXPERIMENTAL

The vitrites separated from gas coal, gas-coking coal and orthocoking coals 535 and 433 {their characteristics were given previously [17] ) were carbon- ized in a Gray--King apparatus to 1273 K at 5 K min -1, with a soak period of 30 min. The thermobitumens were carbonized in a quartz boat heated in the quartz retort of the Gray--King apparatus under argon to the same temperature and with the same soak period.

The yield of the products (for thermobitumens only the yield of coke) was determined, and structural microscopic analysis and X-ray investigation of the resultant cokes were carried out.

Microscopic examination was carried out using a Zetopan 5 microscope on polished briquettes containing coke pieces set in epoxy resin. Quantitative estimations of optical anisotropy based on 1000 point-counts were made. The following classification of optical anisotropy was used: • isotropic (I) • fine-grained anisotropy (Fg) • coarse-grained anisotropy (Cg) • shape anisotropy (S) • leaf anisotropy (L) • fibrous anisotropy (F) • bulk anisotropy (B).

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In the s t ruc tu ra l X-ray examina t ions , the d i f f r a c t o m e t r i c m e t h o d was used. The m e a s u r e m e n t s were o b t a i n e d wi th a D r o n - l , 5 d i f f r a c t o m e t e r . I n t e r p l a n a r spacing, d002, as well as he ight , Lc, and d iamete r , La, o f crystal- lites were d e t e r m i n e d .

RESULTS AND DISCUSSION

Coking process o f vitrites and thermobi tumens

The yield o f cok ing p r o d u c t s o b t a i n e d f r o m cok ing coal vi t r i tes changes charac ter i s t ica l ly wi th the advance o f the coa l i f ica t ion process . The yie ld o f cokes increases, while the y ie ld o f t a r and d e c o m p o s i t i o n w a t e r decreases (Table 1). The yie ld o f cokes ob t a i ned f r o m c h l o r o f o r m ex t r ac t s is low fo r lower - rank p a r e n t vi t r i tes and increases wi th the r ank o f the pa r en t vitri tes. The re is also an increase in the yie ld o f cokes o b t a i n e d f r o m e x t r a c t i o n residues (Table 1).

TABLE 1

Yields of coking products of vitrites from coking coals and of group components separat- ed by thermosolvolysis

Sample Yield of products (mass %)

Coke Tar Decomposition water

Vitrite from Vitrite 68.8 12.8 5.3 gas coal Preheated vitrite 70.7 8.3 4.5

Extract 32.4 -- -- Residue 63.2 12.7 5.9

Vitrite from Vitrite 70.1 12.1 4.1 gas-coking Preheated vitrite 74.2 7.5 3.6 coal Extract 40.3 - -

Residue 73.4 5.8 4.9

Vitrite from Vitrite 76.3 7.1 3.2 orthocoking Preheated vitrite 78.5 5.7 2.8 535 coal Extract 53.1 - -

Residue 78.2 6.0 2.9

Vitrite from Vitrite 80.6 4.7 1.9 orthocoking Preheated vitrite 82.8 3.2 1.7 433 coal Extract 56.0 - -

Residue 83.7 5.3 2.8

X-ray examinations

T h e r e is a progress ive increase in the s t ruc tu ra l o rder ing o f the resu l t an t cokes o f vi t r i tes as the r ank o f the p a r e n t vi t r i tes increases; the in te rp lana r spacing decreases , t he height and d i a m e t e r o f the crysta l l i tes increase (Table 2).

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Cokes from chloroform extracts* are characterized by a lower degree of ordering than the cokes from parent vitrites (their values of d002 are higher, and Lc and La (except La for the cokes of the extracts of the lower-rank parent vitrites) are lower than for the cokes of the parent vitrites) (Table 2).

There is, however, a distinct increase in the structural ordering of the cokes from the extracts with increasing rank of the parent vitrites. There is a similar dependence for cokes from the extraction residues.

TABLE 2

Structural parameters of the cokes from coking coal vitrites and from extracts obtained from vitrites by thermosolvolysis

Sample d00 ~ L c L a (nm) (nm) (nm)

Vitrite from Coke from vitrite 0.370 1.23 3.31 gas coal Coke from extract 0.372 1.08 3.67

Coke from residue 0.370 1.04 3.84

Vitrite from Coke from vitrite 0.360 1.51 3.90 gas-coking Coke from extract 0.363 1.38 3.93 coal Coke from residue 0.360 1.20 3.66

Vitrite from Coke from vitrite 0.354 1.67 4.38 orthocoking Coke from extract 0.359 1.52 4.00 535 coal Coke from residue 0.359 1.35 4.29

Vitrite from Coke from vitrite 0.350 1.76 4.63 orthocoking Coke from extract 0.358 1.59 4.48 433 coal Coke from residue 0.358 1.39 4.31

Structural microscopic examinations

Coke from gas coal vitrite is optically isotropic. In cokes from gas-coking coal part of the compact substance shows fine-grained anisotropy. All the compact substance of the cokes from orthocoking coals is optically anis(> tropic. In the coke of the vitrite from orthocoking coal 433 higher forms of optical anisotropy appear: shape anisotropy and also bulk anisotropy (Table 3).

All cokes from chloroform extracts show partial (coke from extract of gas coal vitrite) or complete optical anisotropy. In the coke from the extract of gas-coking coal vitrite mainly fine-grained anisotropy is found; in the cokes from extracts of orthocoking coal vitrites shape anisotropy (535 coal) and fibrous anisotropy (433 coal) appear (Table 3).

Total removal of the thermobitumens from the coal system deprives the cokes from the extraction residues of any optical anisotropy.

*The structures of the cokes from the thermobitumens obtained from these vitrites by selective thermosolvolysis are presented in Ref. [18].

2 5 7

T A B L E 3

Content of anisotropic components in the cokes from c o k i n g c o a l v i t r i t e s a n d f r o m e x t r a c t s o b t a i n e d

f r o m v i t r i t e s b y t h e r m o s o l v o l y s i $ ~%)

S a m p l e I s o t r o p i c F i n e - C o a r s e - S h a p e F i b r o u s B u l k

I g r a i n e d g r a i n e d a n i s o - a n i s o - a n i s o -

F g C g tropy tropy tropy S F B

F r o m gas coal C o k e f r o m v i t r i t e 1 0 0 . . . . .

C o k e f r o m e x t r a c t 9 6 4 . . . .

F r o m gas-coking C o k e f r o m v i t r i t e 8 6 1 4 . . . .

coal C o k e f r o m e x t r a c t - - 9 5 5 - - - - - -

F r o m ortho- C o k e f r o m v i t r l t e - - 9 0 1 0 - - - - - -

cok ing 535 C o k e f r o m e x t r a c t - - - - 6 1 3 9 - - - - coal

F r o m ortho- C o k e f r o m v i t r l t e - - 5 6 2 3 6 - - 6

cok ing 433 C o k e f r o m e x t r a c t - - - - - - 8 7 1 3 - -

coal

DISCUSSION

Cokes obtained from gas coal, gas-coking coal and orthocoking coal vitrites differ in their properties and structure according to the type of parent vitrites.

The coke from gas coal vitrite consists of two phases, each corresponding to a vitrite with a different degree of coalification. The baked and porous coke was formed by the carbonization of higher coalified components, while the coke without pores was formed by the carbonization of lower coalified substance. This confirms the results of the investigations of the parent vitrites, especially the determination of the reflectance of the vitrinites: a high heterogeneity of the vitrite from the gas coal is shown by the reflec- tance measurements. In the both cases, however, the aromatic core of the basic structural units is too small, too mobile, and the distances between the layers of aromatic lameUas too large to allow the formation of a system of suitably oriented units which would give an anisotropic coke. Thus, the coke substance is optically isotropic.

In the coke from gas-coking coal vitrite only part of the compact coke substance shows optical anisotropy. Thus, in spite of the increase of the degree of condensation and aromatization and of packing of the aromatic lamellas, the core of the basic structural unit of the gas-coking coal vitrite is still too small and has low stability. The plastic mass formed during the coking process does not impregnate the whole coal system and so only part of coking substance is optically anisotropic.

Cokes from vitrites of orthocoking coals are optically anisotropic. For the coke from orthocoking coal 535 vitrite, mainly fine-grained anisotropy of different intensity is found, while for the coke from orthocoking coal 433 vitrite areas of the structure show many types of optical anisotropy from fine-grained to bulk type. Fine-grained anisotropy predominated. This is

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caused by the large dimensions of the aromatic cores, by their restricted but still sufficient mobility and high thermal stability and by the relatively small distances between aromatic lamellas.

The changes in the optical textures of the cokes examined are accompanied by characteristic changes of the structural parameters of these cokes. The coke from gas coal vitrite shows the lowest degree of structural ordering (interplanar spacing, d002, height, Lc, and diameter, La, of crystallites). Cokes obtained from orthocoking coal vitrite are characterized by the highest degree of structural ordering.

The type of coke from chloroform extract and its structure depends on the rank of the parent vitrite. The cokes from the examined extracts are strongly caked, swollen and show optical anisotropy. They show an increase in the structural ordering (i.e., decrease of d002 and increase of Lc and La) with increase of the rank of the parent vitrites.

The values of the structural parameters of the cokes from the extraction residues also increase with increase in the rank of the parent vitrites.

The increase in amount of the anisotropic areas and the appearance of a new anisotropic form in the cokes from extracts is caused by the develop- ment of a suitable viscosity during the plastic stage, depending on quanti ty and thermal stability of the aromatic core of the coal system, i.e., on the type of the parent vitrites.

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