Petroleum-Coke Calcining Technology - ACS Symposium Series

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Petroleum-Coke Calcining Technology H.H.Brandt Cal Carb, Inc., 27 River Ridge Road, Lake Charles,LA70605

The oldest and newest petroleum coke calciners employ rotary kilns to thermally upgrade green coke, thus rendering i t suitable for use by the amorphous carbon and graphite industries. In response to environmental pressures and rising fuel costs, modern coke calcining facilities are much cleaner and much more thermally efficient. Despite numerous attractive features of the newer rotary hearth calcining technology, most practi tioners still favor the well-proven rotary kiln concept. Basic Calcining Process P e t r o l e u m coke c a l c i n i n g i s a p r o c e s s whereby green o r raw p e t r o l e u m coke i s t h e r m a l l y upgraded t o remove associated m o i s t u r e and v o l a t i l e c o m b u s t i b l e m a t t e r (VCM) and t o o t h e r w i s e improve c r i t i c a l p h y s i c a l p r o p e r t i e s , e . g . , e l e c t r i c a l c o n d u c t i v i t y and r e a l d e n s i t y CO. The c a l c i n i n g p r o c e s s i s e s s e n t i a l l y a t i m e - t e m p e r a t u r e f u n c t i o n ; t h e most i m p o r t a n t v a r i a b l e s t o c o n t r o l a r e h e a t i n g r a t e , VCM t o a i r r a t i o and f i n a l t e m p e r a t u r e . To a t t a i n t h e c a l c i n e d coke p r o p e r t i e s n e c e s s a r y f o r i t s end use by t h e amorphous carbon o r g r a p h i t e i n d u s t r i e s , t h e coke must be heat t r e a t e d t o temperatures of 1200-1350°C ( 2 2 0 0 - 2 5 0 0 ° F ) , o r h i g h e r , t o r e f i n e i t s c r y s t a l l i n e structure. The e v e n t u a l q u a l i t y o f t h e c a l c i n e d coke i s d i r e c t l y r e l a t e d t o t h e p a r t i c u l a r c h a r a c t e r i s t i c s / q u a l i t y of the green coke f e d t o the c a l c i n e r . W h i l e c a l c i n a t i o n cannot improve upon c e r t a i n q u a l i t y l i m i t s i n h e r e n t i n t h e green coke, p o t e n t i a l q u a l i t y can be l o s t by improper c a l c i n i n g , e . g . , by u s i n g i n c o r r e c t h e a t i n g r a t e s a n d / o r atmospheric c o n d i t i o n s . The o l d e s t and newest coke c a l c i n e r s i n N o r t h America (which accounts f o r almost 75% of t h e f r e e w o r l d c a p a c i t y ) a r e r o t a r y k i l n s s i m i l a r t o those employed by t h e cement i n d u s t r y . The o r i g i n a l v e r s i o n s were q u i t e u n s o p h i s t i c a t e d , c o n s i s t i n g merely of l o n g , i n c l i n e d , r e f r a c t o r y l i n e d , s t e e l c y l i n d e r s which were a t t a c h e d by a b r e e c h i n g t o a s t a c k a t t h e upper end w h i c h p r o v i d e d t h e n e c e s s a r y 0097-6156/86/0303-0172$06.00/0 © 1986 American Chemical Society

Bacha et al.; Petroleum-Derived Carbons ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 14, 2016 | http://pubs.acs.org Publication Date: April 14, 1986 | doi: 10.1021/bk-1986-0303.ch012

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Petroleum-Coke Calcining Technology

d r a f t and t o a gas or o i l f e d burner at the lower end w h i c h p r o v i d e d the heat ( F i g u r e 1 ) . I n t h i s c o u n t e r c u r r e n t system, the green coke i s f e d i n t o the k i l n at the e l e v a t e d end and the process heat e n t e r s at the lower end. As the green coke moves d o w n h i l l due t o the r o t a t i o n of the k i l n , i t s temperature i n c r e a s e s . D u r i n g the j o u r n e y , m o i s t u r e e v o l v e s i n the h e a t - u p zone (up to about 400°C), d e v o l a t i l i z a t i o n occurs i n a second zone between about 400°C and 800-1000°C, and d e n s i f i c a t i o n t a k e s p l a c e i n the f i n a l zone (up t o about 1350°C). The t o t a l r e s i d e n c e t i m e , w h i c h i s c o n t r o l l e d by the r o t a t i o n a l speed of k i l n , can be anywhere between 45 and 90 minutes (or even l o n g e r i n some s p e c i a l c a s e s ) . When the hot ( 1200-1350°C) c a l c i n e d coke l e a v e s the k i l n , i t i s t r a n s f e r r e d to a r o t a r y cooler. T h i s c o n s i s t s of a s t e e l c y l i n d e r , u s u a l l y l i n e d w i t h r e f r a c t o r y at the feed end, t h a t i s s l i g h t l y i n c l i n e d t o i n d u c e downward t r a v e l as i t i s r o t a t e d . I n the c o o l e r the hot coke i s quenched by water sprayed from a number of n o z z l e s ; e x i t temperature i s c o n t r o l l e d at about 150°C (300°F) t o a s s u r e a moisture-free product. T h i s b a s i c process has a v e r y poor energy e f f i c i e n c y , p r i m a r i l y because of the v e r y h i g h temperature of the exhaust gases ( i n c l u d i n g uncombusted v o l a t i l e s ) l e a v i n g the system. The s t a c k gas temperature can exceed 1260°C (2300°F). I n the days of u n l i m i t e d s u p p l i e s of cheap f u e l , t h i s t h e r m a l i n e f f i c i e n c y was l a r g e l y i g n o r e d . Also i g n o r e d was the f a c t t h a t the t y p i c a l c a l c i n e r s t a c k e m i t t e d s e v e r a l tons of coke p a r t i c u l a t e s per h o u r , as p l a n t s were l o c a t e d i n areas where such e m i s s i o n s went u n n o t i c e d or were t o l e r a t e d . Modern R o t a r y K i l n C a l c i n e r s Changing times have d i c t a t e d major m o d i f i c a t i o n s of the basic c a l c i n i n g process. S k y r o c k e t i n g energy c o s t s and e n v i r o n m e n t a l considerations a r e undoubtedly the two most i m p o r t a n t factors p r o v i d i n g the impetus f o r the e v o l u t i o n of p e t r o l e u m coke c a l c i n i n g technology. Except f o r the r o t a r y k i l n i t s e l f , which now r e p r e s e n t s o n l y a f r a c t i o n of the t o t a l c a p i t a l c o s t , t o d a y s modern coke c a l c i n i n g plants bear little resemblance to the original facilities (Figure 2). Even the k i l n i t s e l f has undergone major changes, not t h e l e a s t of w h i c h i s i n c r e a s e d c a p a c i t y . A t y p i c a l modern k i l n , measuring a p p r o x i m a t e l y 200 f t ( l e n g t h ) χ 10.5 f t ( i n s i d e s h e l l d i a m e t e r ) , i s a b l e t o produce i n excess of 250,000 s h o r t tons annually compared to about 50,000 tons per year for its predecessors. To improve energy e f f i c i e n c y , refractories with superior Κ f a c t o r s are used i n l i n i n g the k i l n , thus r e d u c i n g r a d i a n t heat losses. M o r e o v e r , k i l n mounted blowers now i n j e c t combustion a i r i n t o the k i l n s i n the zone where the v o l a t i l e s e v o l v e from the c o k e , thus p e r m i t t i n g u t i l i z a t i o n of the Btu content i n these p r e v i o u s l y wasted g a s e s . I n modern c a l c i n e r s , most of the energy r e q u i r e d i s o b t a i n e d by b u r n i n g the coke v o l a t i l e s and f i n e p a r t i c u l a t e matter i n the k i l n . I n some i n s t a n c e s , r o t a r y k i l n s equipped w i t h k i l n mounted blowers a c t u a l l y o p e r a t e w i t h o u t e x t e r n a l f u e l (except f o r start-up). When these u n i t s are a l s o equipped w i t h i n c i n e r a t o r s ( t o combust the unburned v o l a t i l e s and e m i t t e d coke f i n e s ) and waste f



F i g u r e 1.

Diagram of o r i g i n a l r o t a r y k i l n

calciner.


C/3

Ο

CO

α n >

< m

2

m

D

Ο r m a

70

H

m

5


W A S T E HEAT BOILER

Figure 2.

INCINERATOR

S

Diagram of modern r o t a r y k i l n

KILN

AIR FANS

calciner.

COOLER


HY

DUST COLLECTOR

PETROLEUM-DERIVED CARBONS

176 heat b o i l e r s , they today's calcining utilities.

a c t u a l l y become net energy p r o d u c e r s . Some p l a n t s even generate power and s e l l it

of to


Other C a l c i n e r s Other l e s s common methods of c a l c i n i n g p e t r o l e u m coke are a l s o or have been p r a c t i c e d . I n e l e c t r i c c a l c i n i n g , p r a c t i c e d where an abundant s u p p l y of cheap e l e c t r i c a l power i s a v a i l a b l e , the r e q u i r e d temperature i s a c h i e v e d by e l e c t r i c a l a r c i n g and r e s i s t a n c e h e a t i n g ; such c a l c i n e r s employ e i t h e r b a t c h or continuous v e r t i c a l s h a f t kilns. I n d i r e c t - f i r e d v e r t i c a l shaft k i l n s , by-product coking ovens, as w e l l as bee h i v e coke ovens have a l s o been u s e d . Shaped carbon b a k i n g f u r n a c e s , when they employ green coke as p a c k i n g m a t e r i a l , can a l s o be c o n s i d e r e d t o be c a l c i n e r s . None of these systems, however, have consistently accounted for significant c a l c i n i n g tonnage and t h e r e f o r e are mentioned o n l y b r i e f l y . Rotary Hearth C a l c i n e r I n 1967, a new concept i n p e t r o l e u m coke c a l c i n i n g reached the commercial stage when a r o t a r y h e a r t h c a l c i n e r was p l a c e d o n - s t r e a m i n Burghausen, West Germany. T h i s p a t e n t e d p r o c e s s ( 2 , 3 ) , the h e a r t of w h i c h i s a s i n g l e , r o t a t i n g h o r i z o n t a l h e a r t h , was j o i n t l y developed by the Marathon O i l Company and the Wise C o a l and Coke Company. The green coke i s f e d i n at an outermost p o s i t i o n and t r a v e l s around the h e a r t h i n a s e r i e s of c o n c e n t r i c c i r c l e s , coke movement b e i n g a c h i e v e d by r o t a t i o n of the h e a r t h ( F i g u r e 3). R a b b l e s , or b l a d e s , suspended from the roof f o r c e the coke i n w a r d and s i m u l t a n e o u s l y t u r n the bed, thereby u n i f o r m l y e x p o s i n g the coke b e i n g c a l c i n e d t o the burners l o c a t e d i n the r o o f . The burners use f u e l o n l y f o r s t a r t - u p ; once e q u i l i b r i u m c o n d i t i o n s are a c h i e v e d , the v o l a t i l e s e v o l v i n g from the coke are p r o v i d e d w i t h combustion a i r v i a the burner p o r t s and the c a l c i n i n g p r o c e s s i s selfsustaining. I n the s u b j e c t p r o c e s s , a s o a k i n g p i t l o c a t e d a t t h e c e n t e r of the r o t a t i n g h e a r t h h o l d s the coke "at t e m p e r a t u r e " f o r a p e r i o d of time n e c e s s a r y t o a c h i e v e the d e s i r e d product p r o p e r t i e s ( F i g u r e 4 ) . The s o a k i n g p i t a l s o a c t s as a surge b i n and a gas s e a l to p r e v e n t a i r leakage i n t o the h e a r t h s e c t i o n . From the s o a k i n g p i t , the hot coke i s d i s c h a r g e d i n t o a c o o l e r f o r e i t h e r d i r e c t or i n d i r e c t w a t e r quench. The e n t i r e p r o c e s s system i s t i e d i n t o a waste heat b o i l e r t o f u r t h e r improve i t s t h e r m a l e f f i c i e n c y . Among the advantages claimed f o r t h i s technology a r e : • • • • •

P a r t i c u l a t e - f r e e exhaust gas E x c e l l e n t thermal e f f i c i e n c y S i m p l i c i t y of o p e r a t i o n Low r e f r a c t o r y maintenance Improved carbon r e c o v e r y

The l a s t c l a i m has c e r t a i n l y been v e r i f i e d , but i t has some negative aspects. The h i g h e r carbon y i e l d s are r e a l i z e d because the green coke f i n e s i n t r o d u c e d t o the c a l c i n e r e x i t as calcined product.


BRANDT

Petroleum-Coke Calcining Technology


12.

F i g u r e 4.

Cross s e c t i o n of r o t a r y h e a r t h c a l c i n e r .


111


178

PETROLEUM-DERIVED CARBONS

S i n c e most c a l c i n e d coke customers have r a t h e r s t r i n g e n t l i m i t s on fines content, t h i s "advantage" becomes a d i s t i n c t disadvantage. The s o l u t i o n t o t h i s problem i s t o f e e d fewer f i n e s t o the u n i t ; t h i s can be a c h i e v e d by v e r y c a r e f u l coke drum c u t t i n g and green coke h a n d l i n g . A l t e r n a t i v e l y , the green coke or the c a l c i n e d coke can be screened t o remove the f i n e s e i t h e r p r i o r t o or after c a l c i n i n g ; e i t h e r of these s o l u t i o n s , however, would r e q u i r e c a r e f u l scrutiny of associated costs and a l t e r n a t i v e o u t l e t s for the separated f i n e s . The o t h e r advantages c l a i m e d are not unanimously a c c e p t e d by the i n d u s t r y . I t i s s i g n i f i c a n t t o note t h a t s i n c e 1967, a dozen or more new c o n v e n t i o n a l k i l n s f o r coke c a l c i n i n g have been p l a c e d o n s t r e a m i n N o r t h A m e r i c a and o n l y two r o t a r y h e a r t h s . The o v e r s e a s r e c o r d i s s l i g h t l y more f a v o r a b l e t o the newcomer. M o r e o v e r , the o n l y companies a d o p t i n g t h i s new t e c h n o l o g y are new e n t r a n t s t o the coke c a l c i n i n g i n d u s t r y ; none of the e x i s t i n g c a l c i n e r s w h i c h employ c o n v e n t i o n a l k i l n s have c o n v e r t e d to the new t e c h n o l o g y . Is t h i s because the coke c a l c i n i n g i n d u s t r y i s o v e r l y c o n s e r v a t i v e or has the e v o l u t i o n of the c o n v e n t i o n a l system met the c h a l l e n g e of t h i s newcomer? I t w i l l p r o b a b l y be a l o n g time b e f o r e t h i s q u e s t i o n i s answered.

Literature Cited 1. 2. 3.

Martin, S. W. "Petroleum Coke" in "Petroleum Products Handbook"; Guthrie, V. Β., Ed.; McGraw-Hill Book Co.: New York, 1960. Allred, V. Dean "Rotary Hearth Calcining of Petroleum Coke"; presented at the meeting of the American Institute of Mining and Metallurgical Engineers (AIME), Paper No. A71-26. Reis, T. "About Coke — And Where the Sulfur Went," Chemtech, June 1977, pp. 366-373.

RECEIVED March 21, 1985


Petroleum-Coke Calcining Technology - ACS Symposium Series

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