Comparision of Evolution of Heteroatoms from Coal and Petroleum

31 Comparision o f Evolution o f Heteroatoms from Coal

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and Petroleum Based Electrode Cokes GEOFFREY M. KIMBER and M. DAVID GRAY National Coal Board, Research Establishment, Cheltenham, England

The National Coal Board, England, are presently developing a process for the production of electrode coke from coal utilising solvent extraction under non-hydrogenating conditions. Potential markets include both the graphite and carbon electrode industries which supply arc steel and aluminium smelters and use, at present, varying grades of petroleum coke. In order to assess the quality of the electrode coke from coal, it was necessary for the National Coal Board to build up background knowledge not only by using the empirical tests developed for petroleum coke, which may not necessarily be relevant, but also by investigating more fundamental aspects. This paper reports results of investigations into reactions which occur when calcined electrode cokes, both coal extract and petroleum based, are heated to graphitising temperatures. It shows that although there are differences between the types of coke, there are potential advantages in using a coal-based coke. Experimental The coal extract was made from a bituminous British coal by solvent extraction by anthracene oil at around kOO C for about 1 hour. The resulting coal digest was filtered and solvent recovered from the filtrate; coking at about 500 C under atmospheric pressure followed the solvent recovery. Much research on these stages has been done at the Coal Research Establishment, some of which has been described by Gray and Owen O). A petroleum thermal tar (a feedstock for premium electrode coke manufacture) was subjected to a similar coking treatment (giving a J/k% green coke yield). The resultant green cokes were calcined at 1300 C prior to kinetic studies at temperatures up to 3000 C. In this study the necessary heatinr; and cooling rates were achieved by using the apparatus shown in Figure 1. This consists of a graphite susceptor, weighing when loaded less than 100g, insulated with carbon black and inductively heated using a 444

Deviney and O'Grady; Petroleum Derived Carbons ACS Symposium Series; American Chemical Society: Washington, DC, 1976.


UMBER

Evolution of Hetewatoms

A N D GRAY

Silica window (For temperature

measurement)

Vent •Argon

*Syndanio'

Sand faced silica Coke

samples

Ο

Induction w o r k coil (Water cooled copper t u b e )

Ο

Ο Graphite-*

—«Carbon

susceptor

—

Carbon

Argon

black wool

'Syndanio' Scale

?

2

9

4

0

Figure 1. Heat treatment furnace


446

PETROLEUM DERIVED CARBONS

^50 MHZ •Radyne generator. The input power t o the susceptor was v a r i e d manually, up to a maximum of 8 kW, t o give r a t e s of heating of 100°C min"* up t o 1200°C and 50°C min" thereafter; the residence time at the heat treatment temperature (HTT) was normally 10 min. T h e c o o l i n g r a t e wag about 100°C min" from 25ΟΟ C reducing t o 50 C min at 1500 C. For each t e s t the susceptor was loaded with about ten i n d i v i d u a l l y i d e n t i f i e d and weighed p i e c e s of coke, i n c l u d i n g one p i e c e of the ' c o n t r o l petroleum coke so as to avoid the inherent d i f f i c u l t i e s i n reproducing heat treatments at e l e v a t e d temperatures. A f t e r heat treatment the coke p i e c e s were reweighed and prepared f o r elemental analyses and i n the case of e x t r a c t cokes, where sample s i z e permitted, f o r determination of d e n s i t y by immersion i n water. A l l samples were subjected to X-ray d i f f r a c t i o n t e s t s u s i n g a G u i n i e r de Wolff quadruple f o c u s s i n g camera with h i g h l y c r y s t a l l i n e quartz as a standard. Mean c r y s t a l l i t e h e i g h t s and diameters (L and L r e s p e c t i v e l y ) were c a l c u l a t e d from the measured h a l f peak widtês. 1

1

Q


1

R e s u l t s and D i s c u s s i o n The r e s u l t s of these t e s t s are shown i n Table I and F i g u r e s 2 and 3 . Sulphur. Although the sulphur contents of the two types of c a l c i n e d coke are the same, c o a l e x t r a c t cokes evolve t h e i r sulphur at a lower temperature, l o s i n g over 9g# a f t e r 10 min» at 2500 C; i n c o n t r a s t , a f t e r 10 min. at 3000 C, 23% of a petroleum coke's sulphur s t i l l remains. T h i s i n d i c a t e s t h a t the sulphur i s r e l a t i v e l y l e s s s t r o n g l y bound i n the c o a l e x t r a c t coke s t r u c t u r e and thus, when being evolved, i s l i k e l y to cause l e s s d i s r u p t i o n t o the s t r u c t u r e . I t i s perhaps not s u r p r i s i n g t h e r e f o r e t h a t i t i s p o s s i b l e to g r a p h i t i s e a carbon a r t e f a c t made from c o a l e x t r a c t coke without needing any a d d i t i v e t o prevent p u f f i n g which normally occurs when petroleum cokes are used. Nitrogen. The n i t r o g e n content of the c o a l e x t r a c t coke i s the h i g h e r , but most of i t i s evolved before 2000 C i s reached when the l e v e l i s the same as f o r petroleum coke. Weight Loss. I t w i l l be seen from Table I t h a t during heat treatment c o a l e x t r a c t cokes lose more mass than petroleum coke. I f these r e s u l t s are viewed i n c o n j u n c t i o n with the heteroatom e v o l u t i o n i t w i l l be seen from F i g u r e 2 t h a t whereas petroleum coke l o s e s only i t s heteroatoms (^N + =AW) the c o a l e x t r a c t coke evolves on average about 1-J atoms of carbon with every heteroatom ( A N + Δ S - AW/2) r e g a r d l e s s of whether n i t r o g e n , at 1300-1750 C, or sulphur, at 2000-2500°C, i s the major hetero atom being d r i v e n o f f . Whether t h i s i s because, i n the r e l e a s e


Evolution of Heteroatoms

D M B E R A N D GRAY

447

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Mean Devia tion

Analyses of Heat-treated Cokes [ on dry ash free basis Weight Loss above 1300 C

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Heat Treatment


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448



Total weight loss %> ( Δ ν ν ) Figure 2.

Coal

I

extract

ι

1300

ι

Weight loss from calcined cokes

coke

χ

ι

1500 1700 1900 Heat treatment

Figure 3.

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2100 2300 temperature

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Effect of heat treatment on crystallite dimensions


31. KiMBER

A N D GRAY

Evolution of Heteroatoms

449

of n i t r o g e n or sulphur, carbon t o carbon bonds are being broken r a t h e r than carbon t o heteroatom ones, has not been p o s i t i v e l y i d e n t i f i e d but there i s no doubt that the c o a l e x t r a c t heteroatoms are bound d i f f e r e n t l y i n the coke s t r u c t u r e t o those o f petroleum coke. I t i s o f i n t e r e s t t o note that 100 min. at 17^0°C are equivalent t o 10 min. at about 1880 C i n respect o f weight l o s s . Although t h i s i s the e a r l y stages o f g r a p h i t i s a t i o n , i t i s equivalent t o an a c t i v a t i o n energy o f 1^0 k c a l s mole- , a value not i n c o n f l i c t with values given by Hutcheon ( 2 ) .


1

Density. Water displacement d e n s i t y (Table I ) o f c o a l e x t r a c t cokes i n c r e a s e s p r o g r e s s i v e l y from 2.084 mg.mm"3 a f t e r heating t o 1300 C ( w i t h i n the normal s p e c i f i c a t i o n f o r a premium grade e l e c t r o d e coke) t o almost 2.20 mg.mnf^ a f t e r heat treatment at 2480 C. The bulk d e n s i t y o f the c o a l e x t r a c t cokes was s i m i l a r t o that of the petroleum coke used as standard. The dimensional s t a b i l i t y on heating t o 2500 C was good, i . e . a volume change o f l e s s than C r y s t a l l i t e dimensions. Although i t has r e c e n t l y been reported by s e v e r a l i n v e s t i g a t o r s , i n c l u d i n g Evans e t a l working on c o a l e x t r a c t coke heat t r e a t e d t o 2^00 C, that high r e s o l u t i o n e l e c t r o n microscopy shows the existence o f g r a p h i t i c c r y s t a l l i t e s an order o f magnitude l a r g e r than the average c a l c u l a t e d from x-ray d i f f r a c t o g r a m s , t h i s does not discount the use o f the much simpler x-ray technique f o r comparative s t u d i e s such as r e p o r t e d here. I t w i l l be seen from F i g u r e 3 that although petroleum coke always e x h i b i t s a l a r g e r c r y s t a l l i t e s i z e , t h g d i f f e r e n c e s are h a r d l y s i g n i f i c a n t , r e d u c i n g from 10% a t 1300 C t o a mere 3% a t 2500 C. Thus, no a l t e r a t i o n i n c u r r e n t g r a p h i t i s i n g procedure would appear t o be necessary i f a change were made t o c o a l e x t r a c t coke based a r t i f i c i a l g r a p h i t e s . Of the o r i g i n a l c a l c i n e d cokes, the c o a l e x t r a c t based m a t e r i a l had the l a r g e r i n t e r l a y e r spacing (see F i g u r e 3)» i n d i c a t i n g a wider divergence from true g r a p h i t i c spacing probably due t o i t s higher heteroatom content. However, i t w i l l be noted that by 2500 C the p o s i t i o n s are reversed with the petroleum based m a t e r i a l e x h i b i t i n g both the l a r g e r i n t e r l a y e r spacing and heteroatom content. Conclusions Sulphur e v o l u t i o n from c o a l e x t r a c t cokes takes place with l e s s d i s r u p t i o n t o the carbon s t r u c t u r e than i n petroleum cokes; p u f f i n g i n h i b i t o r s are u n l i k e l y t o be needed i n a r t i f i c i a l graphite production u s i n g c o a l e x t r a c t cokes. In a d d i t i o n t o i d e n t i f y i n g t h i s processing advantage the study has shown that c o a l e x t r a c t coke has the p o t e n t i a l necessary t o make a good


450


electrode in that it graphitises as well as petroleum coke» Acknowledgments


The National Coal Board is thanked for permission to publish this paper. Any views expressed are those of the authors and not necessarily those of the Board. Abstract A route for making, from coal, a high-purity coke suitable for the manufacture of both graphite and carbon electrodes has been developed. This process for electrode coke involves the solvent extraction of coal and is part of a wider National Coal Board programme into the beneficiation of coal which includes, besides electrode coke, hydrocarbon products, chemicals and carbon fibres. This paper describes the heat treatment of cokes in the range 1300-3000°C and shows that the heteroatoms in coal -based extract cokes are bound to the carbon structure in a different manner to those in petroleum coke. By measuring the growth of graphitic crystallites by X-ray diffraction, it is also shown that the rates of graphitisation are similar for both types of coke. Literature cited

1. Gray, M.D. and Owen, J. "Liquid Solvent Extraction of Coal - Influence of Process Conditions on Extract Properties", presented at Round Table Discussion "Chemical and Physical Valorisation of Coal", Rome, September, (1973). 2. Hutcheon, J.M. "Modern Aspects of Graphite Technology", 18 Academic press London,(1970). 3. Evans, E.L., Jenkins, J.L. and Thomas, J.M., Carbon (1972), 10, 637-42.


Comparision of Evolution of Heteroatoms from Coal and Petroleum

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