Desulfurization and Sulfidation of Coal and Coal Char - ACS

18 Desulfurization and Sulfidation of Coal and Coal Char

G. J. W. KOR U. S. Steel Corp. Research Laboratory, Monroeville, PA 15146

Downloaded by UNIV LAVAL on June 28, 2014 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0064.ch018

Desulfurization of Coal and Coal Char at Various Temperatures and Pressures This work was undertaken to obtain a better understanding of the desulfurization of Illinois No. 6 coal and of char derived therefrom. In particular, the effects of temperature, pressure, and methane content of the gas on the rate of sulfur removal and the final sulfur content of the product were studied. The desulfurization of high-sulfur coals, cokes, and chars has been the subject of many investigations in the past, those pertaining to the desulfurization of coke going back as far as the 1850s. In recent times the subject has gained importance because of the necessity of using large reserves of high-sulfur coal and of reducing the emission of sulfur-bearing gases in plants using coal or coke. Experimental. Illinois No. 6 coal or char derived therefrom was used in all experiments; the size of the particles was between 12 and 18 mesh (average particle diameter ~1.3 mm). The coal was dried for 24 hr at 110°C before it was used. Two types of char were prepared by treating the dried coals in for 3 hr at 600° or 800°C. The total sulfur content and the amounts of the various forms of sulfur present in the coal and the char (prepared at 600°C) are given in Table I. The d e s u l f u r i z a t i o n e x p e r i m e n t s , u s i n g e i t h e r d r i e d c o a l o r one o f t h e c h a r s , w e r e done i n R^, He, CH^, a n d m i x t u r e s o f H2 and CH^ f o r up t o 3 hr a t 600° a n d 800°C a t p r e s s u r e s up t o 10 atm, F o r e a c h e x p e r i m e n t 100-250 mg o f s a m p l e c o n t a i n e d i n a p l a t i n u m o r n i c k e l b a s k e t was s u s p e n d e d i n t h e h o t zone o f a r e s i s t a n c e furnace. The g a s f l o w r a t e was 0.5 L ( S T P ) / m i n i n a l l c a s e s . The s a m p l e s w e r e l o w e r e d i n t o a n d p u l l e d o u t o f t h e h o t zone a s q u i c k l y a s p o s s i b l e u n d e r a f l o w o f He. A t t h e e n d o f a n e x p e r i m e n t t h e e n t i r e s a m p l e was a n a l y z e d f o r t o t a l s u l f u r b y t h e c o m b u s t i o n method (_1) . 221

In Coal Desulfurization; Wheelock, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

222

COAL

DESULFURIZATION

I n s e l e c t e d c a s e s s u r f a c e a r e a measurements on p a r t i a l l y d e s u l f u r i z e d s a m p l e s w e r e made u s i n g t h e BET method. Use was a l s o made o f e l e c t r o n - p r o b e a n a l y s i s and o p t i c a l m i c r o s c o p y , e s p e c i a l l y where t h e f o r m o f t h e s u l f u r was o f i n t e r e s t . 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 f o r t h e d e s u l f u r i z a t i o n of d r i e d c o a l i n H , CH^, and He a t 600° and 800°C a r e shown i n F i g u r e s 1 and 2. I n a l l c a s e s s u l f u r was r a p i d l y l o s t t h e f i r s t 30 m i n . T h i s i n i t i a l r a p i d d e s u l f u r i z a t i o n i s c a u s e d p a r t l y by t h e r e d u c t i o n o f p y r i t e ( F e S ) t o p y r r h o t i t e (FeS) and p a r t l y by the l o s s of l e s s s t a b l e o r g a n i c s u l f u r . The c o a l o r i g i n a l l y c o n t a i n e d 0.52% s u l f u r a s p y r i t e ( T a b l e I ) , p r e s e n t as p a r t i c l e s w i t h an a v e r a g e d i a m e t e r 1-50 y. Typical p y r i t e p a r t i c l e s , a s o b s e r v e d i n t h e d r y c o a l , a r e shown i n F i g u r e 3a and 3b. E l e c t r o n - p r o b e a n a l y s i s showed t h a t t h e s e p a r t i c l e s have a c o m p o s i t i o n a p p r o a c h i n g t h a t o f F e S ( F i g u r e 3c). 2


2

2

The p a r t i a l p r e s s u r e o f s u l f u r i n e q u i l i b r i u m w i t h F e S and FeS i s 1 atm a t 690°C (_2) . T h e r e f o r e , some d e c o m p o s i t i o n o r p y r i t e i n t o p y r r h o t i t e i s e x p e c t e d i n an i n e r t a t m o s p h e r e a t 600°C. T h i s i s shown i n F i g u r e 3d, 3e, and 3 f f o r a c o a l w h i c h was t r e a t e d f o r 10 m i n i n He a t 600°C. The c o m p o s i t i o n o f t h e l a r g e p o r o u s p a r t i c l e i n F i g u r e 3d was c l o s e t o t h a t o f p y r r h o t i t e , as shown b y F i g u r e 3e, w h e r e a s t h e two s m a l l e r p a r t i c l e s i n F i g u r e 3d h a d a c o m p o s i t i o n b e t w e e n p y r i t e and p y r r h o t i t e (Figure 3 f ) . F i g u r e 3g i s a m i c r o g r a p h o f c h a r p r e p a r e d a t 600°C, t h e c h e m i c a l a n a l y s i s o f w h i c h i s shown i n T a b l e I . T h i s c h a r was the s t a r t i n g m a t e r i a l f o r subsequent d e s u l f u r i z a t i o n experiments. The p o r o u s p a r t i c l e s i n t h e c e n t e r o f t h e m i c r o g r a p h a r e p y r r h o t i t e f o r m e d by t h e c o m p l e t e r e d u c t i o n o f p y r i t e , as e v i d e n c e d b y t h e x - r a y s p e c t r u m i n F i g u r e 3h. This observation i s i n keeping w i t h t h e c h e m i c a l a n a l y s i s i n T a b l e I w h i c h showed t h a t no p y r i t i c s u l f u r was p r e s e n t i n t h i s c h a r . 2

The o b s e r v e d g a s i f i c a t i o n i n CH^ i s t h e same a t 600° and a t 800°C w h e r e a s i n He o r H t h e g a s i f i c a t i o n i s a b o u t 20% h i g h e r a t 800°C t h a n a t 600°C. I t i s g e n e r a l l y a c c e p t e d t h a t c o a l i s c a r b o n i z e d i n two s t a g e s ( 3 ) . I n t h e t e m p e r a t u r e r a n g e 350° 550°C, t h e s o - c a l l e d p r i m a r y d e v o l a t i l i z a t i o n ( n o t i n v o l v i n g CH^) t a k e s p l a c e . The s e c o n d a r y g a s i f i c a t i o n , i n v o l v i n g m a i n l y t h e r e l e a s e o f CH^ and H , b e g i n s a t a b o u t 700°C. I n t h e p r e s e n c e o f CH^, s e c o n d a r y g a s i f i c a t i o n i s t h e r e f o r e i n h i b i t e d a t 800°C and i n v o l v e s m a i n l y t h e p r i m a r y d e v o l a t i l i z a t i o n e q u a l to t h a t o b s e r v e d a t 600°C 2

2

A l t h o u g h two t y p e s o f c h a r were u s e d i n t h e d e s u l f u r i z a t i o n experiments, only the experimental r e s u l t s p e r t a i n i n g to the



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and Sulfidation

of Coal and Coal

Char

223

Figure 1. Desulfurization and gasification of dried Illinois No. 6 coal in H , CH and He at 600°C and 1 atm 2

h

Figure 2. Desulfurization and gasification of dried Illinois No. 6 coal in H , CH and He at 800°Candlatm 2

h>


224

COAL

DESULFURIZATION

char prepared a t 600°C are presented here. The r e s u l t s o b t a i n e d f o r t h e c h a r p r e p a r e d a t 8 0 0 ° C a r e s i m i l a r and w i l l n o t be presented i n d e t a i l . The e f f e c t o f p r e s s u r e and c o m p o s i t i o n o f t h e I ^ - C H ^ m i x t u r e and t e m p e r a t u r e on t h e d e s u l f u r i z a t i o n r a t e o f c h a r p r e p a r e d a t 6 0 0 ° C i s shown i n F i g u r e s 4, 5 and 6. Lower s u l f u r c o n t e n t s and h i g h e r d e s u l f u r i z a t i o n r a t e s a r e f a v o r e d by a n i n c r e a s e i n P h and t e m p e r a t u r e . The p r e s e n c e o f CH^ i n h i b i t s d e s u l f u r i z a t i o n .


2

I n gas m i x t u r e s c o n t a i n i n g more t h a n 2 5 % CH^, g a s i f i c a t i o n ceased a f t e r about 30 min o f r e a c t i o n time t o a p l a t e a u ( F i g u r e 5). The g a s i f i c a t i o n c o r r e s p o n d i n g t o t h i s p l a t e a u i s shown as a f u n c t i o n o f t h e p e r c e n t a g e o f CH^ i n t h e gas i n F i g u r e 7. For the char prepared a t 600°C, g a s i f i c a t i o n d u r i n g d e s u l f u r i z a t i o n a t 8 0 0 ° C ( T S ) d e c r e a s e d c o n s i d e r a b l y w i t h i n c r e a s i n g CH percentage i n t h e g a s . G a s i f i c a t i o n f o r the c h a r prepared a t 800°C o n l y s l i g h t l y depended on t h e amount o f CH, i n t h e g a s , i r r e s p e c t i v e of the temperature of d e s u l f u r i z a t i o n . As s e e n f r o m t h e s e r e s u l t s , d e s u l f u r i z a t i o n o f c o a l c h a r t a k e s p l a c e i n two d i s t i n c t s t a g e s . The f i r s t s t a g e shows a s i m u l t a n e o u s r a p i d d e s u l f u r i z a t i o n and g a s i f i c a t i o n . D u r i n g t h e s e c o n d s t a g e s u l f u r i s removed more s l o w l y , p r a c t i c a l l y i n d e p e n d e n t o f t h e e x t e n t o f any f u r t h e r g a s i f i c a t i o n . The observed i n i t i a l r a p i d l o s s of s u l f u r together w i t h the i n i t i a l r a p i d g a s i f i c a t i o n s u g g e s t s t h a t a r e l a t i o n s h i p may e x i s t b e t w e e n t h e i n i t i a l f r a c t i o n a l r e m o v a l o f s u l f u r , (hS/S )^ and t h a t o f Q

9

c a r b o n , (àC/C )^. Q

The d a t a i n F i g u r e 8 show t h e r e l a t i v e s u l f u r r e m o v a l a f t e r a b o u t 1 5 min r e a c t i o n t i m e as a f u n c t i o n o f t h e r e l a t i v e c a r b o n l o s s i n c u r r e d d u r i n g t h i s t i m e . A s i m i l a r r e l a t i o n s h i p was observed f o r the char p r e p a r e d a t 800°C. I t i s seen from F i g u r e 8 t h a t f o r each d e s u l f u r i z a t i o n temperature (TS) the d a t a p o i n t s corresponding to various experimental conditions (e.g., t o t a l p r e s s u r e , CH* c o n t e n t ) f o r m a c u r v e , i n d i c a t i n g a r e l a t i o n s h i p b e t w e e n (Δ5/5 0 ) ± and (àC/CQ)±. F u r t h e r e x a m i n a t i o n o f t h e d a t a i n F i g u r e 8 shows t h a t t h e same f u n c t i o n a l r e l a t i o n s h i p b e t w e e n (LS/S )^ and (hC/CQ)^ e x i s t s , i r r e s p e c t i v e of the temperature a t which d e s u l f u r i z a t i o n t o o k p l a c e . T h i s i s shown i n F i g u r e 9 where t h e o p e n c i r c l e s r e p r e s e n t a l l t h e d a t a p o i n t s i n F i g u r e 8. A l s o i n F i g u r e 9 are the r e s u l t s o b t a i n e d f o r t h e char p r e p a r e d a t 800°C, which show a s i m i l a r r e l a t i o n s h i p b e t w e e n (hS/SQ)^ and ( A C / C q ) ^ ; however, the s l o p e i s s t e e p e r than observed f o r the char p r e p a r e d at 600°C. The h i g h e r t h e c h a r p r e p a r a t i o n t e m p e r a t u r e , t h e more g a s i f i c a t i o n and d e s u l f u r i z a t i o n t a k e s p l a c e d u r i n g c h a r r i n g . Q


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Desulfurization

and Sulfidation

of Coal and Coal


18.

Char

225

Figure 3. Micrographs and electron probe analysis for coal and char, (top) Original dry coal; (middle) coal, treated in He at 600°C for 10 min; (bottom) char (3 hr in H at 600°C). 2

υ •

1

2

3

-

ce < * 2

-

0.6

OC

5

0.4

Q 1 atm

Z>

(Λ 0.2

^

5 atm

Figure 4. Desulfurization and gasi fication of Illinois char (prepared at 600°C) in H at indicated pres sures at800°C 2


COAL

DESULFURIZATION


226


18.

KOR

Desulfurization

and

0

50 CH


Sulfidation

4

of Coal and Coal

0

100

50

IN G A S , %

CH

4

Char

100

IN G A S , %

Figure 7. Final level of gasification as a function of the amount of CH in H -CH mixtures for two types of char, during desulfurization at 600° and 800°C (Ύ ) Jf

2

i

8

0

4

8

12

16

20

uc/c)„ % 0

Figure 8. Relation between initial loss of sulfur and car bon (after 15 min reaction time) for a char prepared at 600°C and subsequently desulfurized under indicated conditions at Ύ — 600° or 800°C β


227

C O A L DESULFURIZATION

228

T h e r e f o r e , i t s h o u l d n o t b e c o n c l u d e d t h a t i t i s g e n e r a l l y more advantageous t o use a c h a r , prepared a t a h i g h e r temperature, f o r subsequent d e s u l f u r i z a t i o n . A l s o i n c l u d e d i n F i g u r e 9 a r e t h e data from Jones e t a l . ( 4 ) who d e s u l f u r i z e d a c h a r d e r i v e d f r o m I l l i n o i s No. 6 c o a l , p r e p a r e d a t 870°C. The d e s u l f u r i z a t i o n t e m p e r a t u r e v a r i e d b e t w e e n 704° a n d 1010°C, t h e p r e s s u r e b e t w e e n 1 a n d ~8 a t m , a n d t h e y u s e d Η a s w e l l a s e q u i m o l a r m i x t u r e s o f H a n d CH^. A l t h o u g h t h e i r d a t a show some s c a t t e r , i t i s c o n c l u d e d t h a t t h e r e i s a r e l a t i o n s h i p b e t w e e n (AS/S ) a n d (àC/C ) . The d a t a o f B a t c h e l o r e t a l . ( 5 ) , who u s e d a c h a r p r e p a r e d a t 500°C f r o m a P i t t s b u r g h seam c o a l , a r e a l s o shown i n F i g u r e 9. D e s u l f u r i z a t i o n t o o k p l a c e i n H - H S m i x t u r e s ( p b e t w e e n 1 a n d 11 atm) a t t e m p e r a t u r e s v a r y i n g b e t w e e n 650° a n d 880°C. A l s o f o r t h e s e d a t a , a r e l a t i o n s h i p b e t w e e n (hS/So)^ a n d (AC/C^)^ i s o b s e r v e d . Thus t h e f u n c t i o n a l r e l a t i o n s h i p b e t w e e n (AS/S )± a n d (kC/CQ). depends o n l y o n t h e t e m p e r a t u r e a t w h i c h t h e c n a r was p r e p a r e d . Subsequent d e s u l f u r i z a t i o n o f a g i v e n char can o n l y be a c h i e v e d a t t h e expense o f l o s s i n carbon, the e x t e n t o f which i s determined by t h e a p p r o p r i a t e f u n c t i o n a l r e l a t i o n s h i p d e p i c t e d i n F i g u r e 9. A f t e r t h e i n i t i a l r a p i d drop i n s u l f u r c o n t e n t o f t h e c h a r , a more g r a d u a l d e c r e a s e i s o b s e r v e d ( F i g u r e s 4, 5, and 6 ) . Assuming t h a t f o r t h i s stage o f the p r o c e s s , the d e s u l f u r i z a t i o n r e a c t i o n may b e d e s c r i b e d b y a f i r s t - o r d e r r e a c t i o n r e l a t i v e t o the s u l f u r content o f t h e c h a r , then 2

2


Q

2

i

Q

2

±

H

(1) where S i s t h e c o n c e n t r a t i o n o f s u l f u r a t t i m e £ and.fega r a t e constant. Integration o f Equation 1 gives:

l o g y - = - fc £ Q

(2)

where 5 i s t h e s u l f u r c o n c e n t r a t i o n a f t e r 15 m i n o f r e a c t i o n t i m e , aîter w h i c h d e s u l f u r i z a t i o n p r o c e e d s more g r a d u a l l y . B e c a u s e o f t h e s c a t t e r i n t h e e x p e r i m e n t a l r e s u l t s , i t was n o t n e c e s s a r y t o t r e a t t h e r e s u l t s o b t a i n e d f o r t h e two c h a r s separately. F i g u r e 10, d e p i c t i n g t h e f i r s t - o r d e r p l o t s , t h e r e f o r e r e p r e s e n t s t h e a v e r a g e s f o r b o t h t y p e s o f c h a r . L o g (S/S ) i s a l i n e a r f u n c t i o n o f time w i t h i n t h e s c a t t e r o f t h e d a t a . Q


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Desulfurization


18.

0

and Sulfidation

4

of Coal and Coal

'2

8

Char

;6

!^C/C ),, % 0

Figure 9. General relationship between initial loss of sulfur and carbon for two types of char, showing that the data for various experimental conditions for a given char are represented by the same function

TIME,

hours

Figure 10. Desulfurization of char in mixtures of H and CH at 5 atm and indicated temperatures, as a first-order reaction 2

k


229

230

COAL

DESULFURIZATION


The r a t e c o n s t a n t fc^, o b t a i n e d f r o m t h e s l o p e s o f t h e l i n e s i n F i g u r e 1 0 , i s shown i n F i g u r e 11A a s a f u n c t i o n o f t h e c o n centration of H i n the l^-CH^ mixture f o r the d e s u l f u r i z a t i o n e x p e r i m e n t s a t 5 a t m a t 600° a n d 800°C. A l t h o u g h t h e e q u i l i b r i u m c o n c e n t r a t i o n s o f CH i n H^-CH m i x t u r e s a t 5 a t m a r e 5 6 % a n d 16% a t 600° a n d 8 0 0 ° C , r e s p e c t i v e l y ( 2 ) , no m e a s u r a b l e w e i g h t i n c r e a s e o f t h e c h a r was r e c o r d e d a f t e r t r e a t m e n t i n CH, a t 800°C. T h i s i n d i c a t e s t h a t CH^ d i d n o t d i s s o c i a t e t o a n y m e a s u r a b l e e x t e n t under t h e p r e s e n t e x p e r i m e n t a l c o n d i t i o n s . Therefore, i t may b e assumed t h a t t h e p a r t i a l p r e s s u r e o f H^ p r e v a i l i n g d u r i n g t h e d e s u l f u r i z a t i o n e x p e r i m e n t s i n H^-CH^ m i x t u r e s was t h e same as t h a t i n t h e i n g o i n g m i x t u r e . On t h i s b a s i s , F i g u r e 11B was p l o t t e d , s u p p l e m e n t e d w i t h some d a t a o b t a i n e d f r o m d e s u l f u r i z a t i o n e x p e r i m e n t s i n 1 0 0 % H^ a t 1 a n d 5 a t m . The r a t e c o n s t a n t p e r t a i n i n g t o t h e s e c o n d s t a g e o f d e s u l f u r i z a t i o n i n 1 0 0 % H i s t h e same a s t h a t i n " " 4 m i x t u r e s , although i n 100% H g a s i f i c a t i o n continues i n t h e s e c o n d s t a g e ( F i g u r e s 4, 5 a n d 6 ) . T h i s s u g g e s t s t h a t d e s u l f u r i z a t i o n and g a s i f i c a t i o n a r e i n t e r r e l a t e d i n t h e i n i t i a l stages o n l y ; i n t h e second stage t h e d e s u l f u r i z a t i o n i s independent o f gasification. H

C H

2

2

To e x p l a i n t h e s e o b s e r v a t i o n s , i t i s s u g g e s t e d t h a t t h e i n i t i a l l o s s o f c a r b o n — w h i c h i s accompanied by a simultaneous l o s s o f ( m a i n l y o r g a n i c a l l y b o u n d ) s u l f u r — c r e a t e s new p o r e s p r o v i d i n g b e t t e r a c c e s s f o r t h e r e d u c i n g gas t o t h e p y r r h o t i t e p a r t i c l e s embedded i n t h e c h a r . T h i s i s s u p p o r t e d by t h e observed change i n s u r f a c e a r e a o f t h e c h a r d u r i n g d e s u l f u r i z a t i o n . The i n i t i a l s u r f a c e a r e a o f c h a r p r e p a r e d a t 600°C i s a b o u t 2 m /g. The change i n s u r f a c e a r e a i s most p r o n o u n c e d d u r i n g t h e f i r s t h o u r o f d e s u l f u r i z a t i o n , p a r t i c u l a r l y a t 800°C ( F i g u r e 1 2 ) . The pore s u r f a c e a r e a i n c r e a s e s w i t h i n c r e a s i n g temperature and p r e s s u r e a n d , h e n c e , s o does t h e amount o f g a s i f i c a t i o n . 2

A c h a r , p r e p a r e d a t 600°C a n d s u b s e q u e n t l y d e s u l f u r i z e d f o r 2 h r i n 5-atm H^ a t 800°C, was a n a l y z e d f o r t h e v a r i o u s f o r m s o f s u l f u r present i n the product. The a n a l y s i s showed t h a t o f t h e t o t a l s u l f u r c o n t e n t o f 0.16%, a b o u t 0.11% was p y r r h o t i t e a n d a b o u t 0.05% was o r g a n i c s u l f u r . The m i c r o g r a p h i n F i g u r e 1 3 a f o r t h i s p a r t i a l l y d e s u l f u r i z e d p r o d u c t shows t h r e e t y p e s o f particles: b r i g h t , g r e y i s h c o l o r e d , a n d two-phase p a r t i c l e s p a r t l y b r i g h t and p a r t l y grey. The c o m p o s i t i o n o f t h e g r e y i s h p a r t i c l e s v a r i e s somewhat b u t n o m i n a l l y a p p r o a c h e s t h a t o f p y r r h o t i t e ( F i g u r e 13b a n d 1 3 c ) . The b r i g h t p a r t i c l e s a r e i r o n ( F i g u r e 13d) f o r m e d b y t h e c o m p l e t e r e d u c t i o n o f p y r r h o t i t e .


KOR

Desulfurization


H

2

and Sulfidation

IN INGOING H - C H 2

4

231

of Coal and Coal Char

MIXTURE, %

Figure 11. First-order rate constants for desulfurization at indicated temperatures shown as a function of (A) H2 content of gas mixture (5 atm) and (B) partial pressure of H (solid symbols represent data for 100% H) 2

PH '

a

t

m

2


2



232

Figure 13. Optical micrograph and electron analysis of char, treated in p „ = 5 atm, 800° C for 2 hr, showing the presence of pyrrhotite and reduced iron 2


18.

KOR

Desulfurization

and

Sulfidation

of Coal and Coal

Char

233


I t i s c o n c l u d e d f r o m t h e s e o b s e r v a t i o n s t h a t most o f t h e s u l f u r i s i n t h e form o f p y r r h o t i t e d u r i n g t h e l a t e r stages o f desulfurization. The o v e r a l l r a t e o f d e s u l f u r i z a t i o n d u r i n g t h i s stage i s thus expected t o be mainly governed by t h e slow r e d u c t i o n ( 6 ) o f p y r r h o t i t e i n Κ^· Conclusions. I t was f o u n d t h a t t h e d e s u l f u r i z a t i o n o f c o a l char i n mixtures o f a n d CH t a k e s p l a c e i n two d i s t i n c t s t a g e s . In t h e f i r s t stage r a p i d d e s u l f u r i z a t i o n i s accompanied by gasification. These two p r o c e s s e s w e r e shown t o b e i n t e r r e l a t e d , the r e l a t i o n s h i p depending on t h e char p r e p a r a t i o n temperature only. The s e c o n d s t a g e o f d e s u l f u r i z a t i o n p r o c e e d s a t a much slower r a t e and i s b e i n g c o n t r o l l e d by t h e slow r e d u c t i o n o f pyrrhotite to iron. S u l f i d a t i o n o f C o a l Char a n d S y n t h e t i c C h a r s D e s u l f u r i z a t i o n o f c o a l and c o a l char i n hydrogen r e s u l t s i n e v o l u t i o n o f H^S. D e p e n d i n g o n t h e p r o c e s s , t h e H S i s e i t h e r e n t i r e l y o r p a r t l y removed a n d r e c i r c u l a t e d . The w o r k , d e s c r i b e d i n P a r t I I , was u n d e r t a k e n t o o b t a i n a b e t t e r u n d e r s t a n d i n g o f t h e i n t e r a c t i o n b e t w e e n c h a r s and g a s m i x t u r e s c o n t a i n i n g H^S. A l i t e r a t u r e s u r v e y i n d i c a t e d t h a t no i n v e s t i g a t i o n s h a v e b e e n made o f t h e s u l f i d a t i o n o f c a r b o n a c e o u s m a t e r i a l s , i n c l u d i n g c h a r s , i n gas m i x t u r e s o f H^ a n d H S s u c h t h a t t h e s u l f u r p o t e n t i a l was s y s t e m a t i c a l l y v a r i e d . Experimental. C h a r f r o m two d i f f e r e n t s o u r c e s , p r e p a r e d u n d e r a v a r i e t y o f c o n d i t i o n s , was u s e d i n t h i s w o r k . The p r e p a r a t i o n c o n d i t i o n s a r e summarized i n T a b l e I I . The p r e p a r a t i o n o f c h a r f r o m I l l i n o i s No. 6 c o a l was d e s c r i b e d i n t h e f i r s t s e c t i o n o f t h i s chapter under 'Experimental". C h a r f r o m a s h - f r e e f i l t e r p a p e r ( 0 . 0 0 8 % a s h ) was p r e p a r e d b y c h a r r i n g t h e paper, contained i n a h i g h - p u r i t y alumina boat, i n an a t m o s p h e r e o f d r y He a t 600° o r 900°C f o r 3 h r . A f t e r t h e p a p e r was c h a r r e d , t h e b o a t was p u l l e d t o t h e c o o l e n d o f t h e r e a c t i o n tube where i t s l o w l y c o o l e d . S u b s e q u e n t l y , t h e c h a r was q u i c k l y t r a n s f e r r e d t o a d e s i c c a t o r w h e r e i t was s t o c k e d . The c h a r s t h a t w e r e f u r t h e r t r e a t e d i n He a t 1250° a n d 1500°c f o r 24 a n d 9 6 h r r e s p e c t i v e l y , w e r e t a k e n f r o m t h i s s t o c k . t

The s u l f i d a t i o n e x p e r i m e n t s w e r e done i n a v e r t i c a l f u r n a c e w i t h t h e H^H^S m i x t u r e e n t e r i n g a t t h e b o t t o m o f t h e r e a c t i o n tube. The H ^ h ^ S - r a t i o i n t h e gas was a d j u s t e d b y u s i n g t h e u s u a l arrangement o f c a p i l l a r y f l o w meters. I n most o f t h e e x p e r i m e n t s a g a s f l o w r a t e o f 30 cm3(STP)/min was u s e d .


234

COAL

DESULFURIZATION


F o r e a c h s u l f i d a t i o n e x p e r i m e n t a b o u t 100 mg o f c h a r was p l a c e d i n an alumina t r a y . I n most c a s e s t h e s u l f i d i z i n g t r e a t ment was 1 h r a f t e r w h i c h t h e s a m p l e was q u i c k l y p u l l e d up t o t h e c o o l t o p o f t h e r e a c t i o n t u b e w h i l e t h e He was k e p t f l o w i n g . The c o o l e d s a m p l e was t h e n t r a n s f e r r e d t o a d e s i c c a t o r . Sulfur i n t h e c h a r was a n a l y z e d by t h e c o m b u s t i o n method ( 1 ) . I n some s e l e c t e d c a s e s a b o u t 750 mg o f c h a r was s u l f i d i z e d , and a f t e r w a r d s a n a l y z e d f o r o x y g e n by t h e n e u t r o n - a c t i v a t i o n method. The s u r f a c e a r e a s o f t h e c h a r s u s e d w e r e d e t e r m i n e d by t h e BET method. R e s u l t s and D i s c u s s i o n . The p a r t i a l l y d e s u l f u r i z e d c o a l c h a r ( T a b l e I I ) was s u l f i d i z e d a t 600°, 800°, and 900°C i n H -H S c o n t a i n i n g 0-100% ^ S . The r e s u l t s a r e g i v e n i n F i g u r e 14, I n which the s u l f u r content of the char a f t e r l h r r e a c t i o n time i s p l o t t e d v s . t h e p e r c e n t a g e o f H^S i n t h e gas a t t h e e x p e r i m e n t a l t e m p e r a t u r e , d e n o t e d by ( % H ^ S ) . S i m i l a r r e s u l t s w e r e o b t a i n e d after sulfidation for 4 hr. T

I n c a l c u l a t i n g ^ H ^ S ) ^ f r o m t h e p e r c e n t a g e o f H^S i n t h e i n g o i n g gas ( a t room t e m p e r a t u r e ) , due a l l o w a n c e s h o u l d be made f o r t h e p a r t i a l d i s s o c i a t i o n o f H^S a t h i g h e r t e m p e r a t u r e s . The e q u i l i b r i u m H S p e r c e n t a g e o f t h e gas a t t h e r e a c t i o n t e m p e r a t u r e was c a l c u l a t e d f r o m t h e a v a i l a b l e t h e r m o d y n a m i c d a t a ( 2 ) . The p r e s e n t r e s u l t s may be compared w i t h t h o s e o b t a i n e d by P o l a n s k y e t a l . (_7) who t r e a t e d c o k e i n H^S-N^ m i x t u r e s c o n t a i n i n g 4.2 and 8.8% H^S. T h e i r r e s u l t s show no p r o n o u n c e d d i f f e r e n c e i n t h e e x t e n t o f s u l f u r a b s o r p t i o n a t 800° and a t 900°C. Of s p e c i a l i n t e r e s t i s t h e a b s o r p t i o n o f s u l f u r a t 900°C i n H -H^S m i x t u r e s , c o n t a i n i n g l e s s t h a n 2% H^S, by p r e v i o u s l y d e s u l f u r i z e d c o a l char (from I l l i n o i s c o a l ) . This c o a l char was p r o d u c e d b y d e s u l f u r i z a t i o n o f a c h a r t h a t o r i g i n a l l y c o n t a i n e d 0.13% s u l f u r as p y r r h o t i t e ; t h i s i s e q u i v a l e n t t o about 0.23% i r o n . Upon s u l f i d a t i o n o f t h i s d e s u l f u r i z e d c o a l c h a r , p y r r h o t i t e i s e x p e c t e d t o f o r m when t h e s u l f u r p o t e n t i a l i s s u f f i c i e n t l y h i g h . T h i s i s i l l u s t r a t e d i n F i g u r e 15, where a s u d d e n r i s e i n t h e amount o f s u l f u r a b s o r b e d i s o b s e r v e d when (^2 \ agreement w i t h t h e v a l u e c a l c u l a t e d from the thermodynamic d a t a f o r t h e i r o n / p y r r h o t i t e e q u i l i b r i u m ^ ) a t 900°C. S u l f u r c o n t e n t i n c r e a s e s a b o u t 0.20% a t t h e " b r e a k p o i n t " i n t h e a b s o r p t i o n c u r v e , i n good a g r e e m e n t w i t h t h e e s t i m a t e d amount o f i r o n p r e s e n t i n t h e c h a r . S

>

0 , 3 e

T

h

i

s

i s

i n

g

o

o

d

The s u l f u r a b s o r p t i o n c u r v e s , d e p i c t e d i n F i g u r e 14, h a v e t h e g e n e r a l c h a r a c t e r o f a b s o r p t i o n i s o t h e r m s . However, p r o p e r i n t e r p r e t a t i o n o f t h e s e r e s u l t s i s h i n d e r e d by t h e p r e s e n c e o f



18.

KOR

Desulfurization and Sulfidation of Coal and Coal Char

Figure 14. Sulfidation of previously desulfurized char from Illinois No. 6 coal at indicated temperatures

τ

)£L 0

,

0 4

,

0 8

,

1.2

(% H S ) . ?

T

balance

1.6

H

I 2

2

Figure 15. Sulfidation of previously desulfurized char at 900°C in H -H S mixtures for low percentages of HS t

2

2


235

236

COAL

DESULFURIZATION

i m p u r i t i e s i n t h e c o a l c h a r . I t was t h e r e f o r e d e c i d e d t o s t u d y t h e s u l f i d a t i o n o f e s s e n t i a l l y i m p u r i t y - f r e e c a r b o n s and s y n t h e t i c chars. S u l f i d a t i o n o f S y n t h e t i c C a r b o n s and C h a r s . Granulated s a m p l e s o f h i g h - p u r i t y e l e c t r o d e g r a p h i t e and p y r o l i t i c g r a p h i t e were e q u i l i b r a t e d w i t h a 50% 50% H S m i x t u r e a t 1000°C f o r 1.5 h r ; t h e r e was no d e t e c t a b l e s u l f u r a b s o r p t i o n i n e i t h e r f o r m of g r a p h i t e . I n a n o t h e r e x p e r i m e n t s a m p l e s o f e l e c t r o d e g r a p h i t e and p y r i t e w e r e p l a c e d i n s e p a r a t e p a r t s o f an e v a c u a t e d s i l i c a c a p s u l e and a n n e a l e d f o r 20 hr at 650°. This corresponds to a p a r t i a l p r e s s u r e o f s u l f u r v a p o r o f a b o u t 0.14 atm, as e s t i m a t e d f r o m t h e t h e r m o d y n a m i c d a t a f o r t h e p y r r h o t i t e / p y r i t e system(_2) . A f t e r t h i s t r e a t m e n t , no s u l f u r was d e t e c t e d i n t h e g r a p h i t e . The p r e s e n t o b s e r v a t i o n s g e n e r a l l y agree with those of W i b a u t and v a n d e r Kam(jS) who f o u n d t h a t e v e n a t s u l f u r p r e s s u r e s above a t m o s p h e r i c , no d e t e c t a b l e s u l f u r was a b s o r b e d by e i t h e r diamond powder o r C e y l o n g r a p h i t e . The r e s u l t s o b t a i n e d f o r t h e s y n t h e t i c c h a r s a r e g i v e n i n F i g u r e s 16 and 17 f o r 600° and 900°C, r e s p e c t i v e l y . I n most e x p e r i m e n t s t h e r e a c t i o n t i m e was l h r ; h o w e v e r , some s a m p l e s w e r e s u l f i d i z e d f o r up t o 3hr,„ These s a m p l e s a b s o r b e d e s s e n t i a l l y t h e same amount o f s u l f u r as t h o s e s u l f i d i z e d f o r l h r . M o r e o v e r , e q u i l i b r i u m c o u l d be r e a c h e d f r o m b o t h s i d e s . For example, f i l t e r - p a p e r c h a r ( p r e p a r e d a t 900°C) w h i c h was f i r s t s u l f i d i z e d i n a 5 0 % Η , 5 0 % fi^S m i x t u r e a t 600°C t o a f i n a l s u l f u r c o n t e n t of ~ 1 % c o u l d s u b s e q u e n t l y be p a r t i a l l y d e s u l f u r i z e d i n a 90% H^, 10% R^S m i x t u r e t o y i e l d a f i n a l s u l f u r c o n t e n t o f 0.4%. T h i s i s e s s e n t i a l l y t h e same as t h e s u l f u r c o n t e n t a f t e r d i r e c t s u l f i d a t i o n o f t h e c h a r i n t h e same gas m i x t u r e . Similar o b s e r v a t i o n s w e r e made a t 900°C, i n d i c a t i n g t h a t t h e a b s o r b e d s u l f u r i s i n e q u i l i b r i u m w i t h t h e gas and t h a t t h e p r o c e s s o f s u l f u r uptake i s r e v e r s i b l e .


2

The s u r f a c e areas of the c h a r s ( T a b l e I I ) are i n d i c a t e d i n F i g u r e s 16 and 17. A c h a r w i t h a l a r g e r s u r f a c e a r e a h a s , i n g e n e r a l , a l a r g e r c a p a c i t y f o r s u l f u r a b s o r p t i o n . The r e s u l t s o b t a i n e d f o r c o a l c h a r a r e a l s o shown i n F i g u r e s 16 and 17 f o r easy comparison. C o a l c h a r and f i l t e r - p a p e r c h a r ( p r e p a r e d a t 600° and 900°C) h a v e a b o u t t h e same s u r f a c e a r e a s and a b s o r b s i m i l a r amounts o f s u l f u r . The r e s u l t s o f x - r a y a n a l y s i s o f t h e v a r i o u s c h a r s u s e d i n t h i s i n v e s t i g a t i o n w e r e compared i n a q u a l i t a t i v e way w i t h t h e d a t a r e p o r t e d b y T u r k d o g a n e t a l . 0 9 ) a s shown i n T a b l e I I I t o g e t h e r w i t h e s t i m a t e d mean c r y s t a l l i t e s i z e s . I t was mentioned



KOR

237


18.

ι 8 •

ι

y

ι

2

S (m /g)

CHAR Fp (600"O

330

Δ Fp (900'C)

272

V COAL (800'C)

400

•ΤΗ

/δ.

0

20

40

60

(% H S ) , balance Η , 2

T

I 80

1 100

Figure 17. Sulfidation at 900°C in H H S mixtures of various chars, having in dicated pore surface area (S) 2

2



238 Table I .

Forms o f S u l f u r ( i n Wt.%) i n D r i e d C o a l and D e r i v e d C h a r ( 3 h r , 600°C)

Form o f S u l f u r Pyrite Sulfate Sulfide Organic Total


Coal

Dried Coal 53 12 005 27 93

Char

0.005 0.13 0.61 0.75

Table I I . Source, Conditions of P r e p a r a t i o n , I n i t i a l S u l f u r ( S ^ ) a n d Oxygen ( 0 ^ ) C o n t e n t s , and S u r f a c e A r e a (SA) of t h e V a r i o u s Chars S t u d i e d

Preparation Conditions

Source Illinois

Coal

5 a t m H , 800°C f o r 3 h r 2

A s h - f r e e F i l t e r 1 atm Paper (0.008% 1 atm 3h) 1 atm 1 atm 1 atm 1 atm

Table I I I .

paper paper paper paper

600°C, 3 h r 900°C, 3 h r 600°C, 3 h r + 1250°C, 24 h r 600°C, 3hr+ 1500°C, 96h

2

S

S A ( m / g ) i ( % ) °i(%) 400

0.05

—

330 272

0.00 0.00

4.0 1.4

30

0.00

0.34

3

0.00

0.14

Q u a l i t a t i v e Comparison o f t h e C r y s t a l l i n i t y o f F i l t e r - P a p e r C h a r s Used i n T h i s Work W i t h t h e C r y s t a l l i n i t y a n d Mean C r y s t a l l i t e S i z e o f Some Carbons I n v e s t i g a t e d by Turkdogan e t a l . ( 9 )

Type o f C h a r and P r e p a r a t i o n

Filter Filter Filter Filter

He, He, He, He, He, He,

P r o p e r t i e s o f t h e Char

(600)( (900)j (1250) (1500)

1

C r y s t a l l i n i t y f r o m T u r k d o g a n s Work Q u a l i t a t i v e Comparison Mean C r y s t a l l i t e Size From Xbetween coconut c h a r c o a l and " v i t r e o u s " carbon approaching v i t r e o u s carbon same a s v i t r e o u s c a r b o n

-10 - -16


-16 -16


18.

KOR

Desulfurization

and

Sulfidation

of Coal and Coal

Char

239

b e f o r e t h a t g r a p h i t i z e d e l e c t r o d e g r a p h i t e , w h i c h h a s a mean c r y s t a l l i t e s i z e o f 500 Â, d i d n o t a b s o r b s u l f u r w h e r e a s t h e n o n g r a p h i t i z e d c h a r s d i d . Thus, i t i s c o n c l u d e d from t h e s e r e s u l t s t h a t t h e a b i l i t y o f a g i v e n char o r carbon t o absorb s u l f u r i s determined f i r s t by i t s s t a t e o f c r y s t a l l i n i t y . I n p o o r l y g r a p h i t i z e d o r n o n g r a p h i t i z e d c a r b o n s , t h e amount o f absorbed s u l f u r i n c r e a s e s w i t h i n c r e a s i n g pore s u r f a c e a r e a . In view o f t h e s t r o n g a f f i n i t y between s u l f u r and oxygen, an a t t e m p t was made t o i n v e s t i g a t e t h e e f f e c t o f o x y g e n on t h e s u l f u r a b s o r p t i o n b y c h a r . The i n i t i a l o x y g e n c o n t e n t o f s y n t h e t i c c h a r s i s shown a s a f u n c t i o n o f t h e s u r f a c e a r e a i n F i g u r e 18. I t i s seen t h a t t h e i n i t i a l oxygen c o n c e n t r a t i o n i s a s t r o n g f u n c t i o n o f t h e s u r f a c e a r e a and hence t h e temperature a t w h i c h t h e c h a r was p r e p a r e d ( T a b l e I I ) . To s t u d y t h e change i n o x y g e n c o n c e n t r a t i o n a f t e r s u l f i d a t i o n o f t h e c h a r s , a s e r i e s o f s p e c i a l e x p e r i m e n t s was c o n d u c t e d ; t h e r e s u l t s a r e s u m m a r i z e d i n T a b l e I V . The o x y g e n c o n t e n t a f t e r · s u l f i d a t i o n was i n d e p e n d e n t , w i t h i n t h e a n a l y t i c a l e r r o r , o f t h e ratio ( p ^ s / P l ^ T S increase i nthe sulfidation temperature r e s u l t e d i n a lower oxygen c o n t e n t i n t h e c h a r , p a r t i c u l a r l y i f t h e char had a l a r g e r surface area (Figure 18). In a l l cases i n v e s t i g a t e d , t h e chars w i t h l a r g e s u r f a c e a r e a s c o n t a i n e d more o x y g e n . B e c a u s e o f t h e i n t e r d e p e n d e n c e of s u r f a c e area and oxygen c o n t e n t , i t i s d i f f i c u l t t o s e p a r a t e t h e i r e f f e c t s on t h e c a p a c i t y o f a g i v e n char f o r s u l f u r a b s o r p t i o n . However, some i n d i c a t i o n o f t h e i n f l u e n c e o f o x y g e n on t h e s u l f u r a b s o r p t i o n may b e o b t a i n e d f r o m t h e r e s u l t s shown i n Table IV. F o r i n s t a n c e , the s u r f a c e areas o f f i l t e r - p a p e r c h a r p r e p a r e d a t 600° a n d 900°C w e r e 330 a n d 272 m /g, r e s p e c t i v e l y , a d i f f e r e n c e o f a b o u t 20%. A f t e r s u l f i d a t i o n a t 600°C i n a g a s of h i g h s u l f u r p o t e n t i a l , t h e oxygen c o n t e n t s o f these c h a r s d i f f e r e d b y a b o u t a f a c t o r o f two, y e t t h e d i f f e r e n c e i n s u l f u r a b s o r b e d was n o t more t h a n a b o u t 10%. These f i n d i n g s i n d i c a t e t h a t t h e i n f l u e n c e o f o x y g e n on t h e s u l f u r a b s o r p t i o n i s p r o b a b l y secondary. I n t h i s c o n t e x t t h e w o r k o f Hofmann a n d O h l e r i c h ( l O ) s h o u l d be m e n t i o n e d . They t r e a t e d s u g a r c h a r c o a l i n d r y 0^ a t a b o u t 500°C t o o b t a i n a c h a r c o n t a i n i n g a b o u t 1 0 % o x y g e n a s s u r f a c e complexes. Upon s u l f i d a t i o n o f t h i s o x y g e n a t e d c h a r i n at 600°C, t h e y f o u n d t h a t t h e amount o f s u l f u r t a k e n up was e q u a l t o t h a t a b s o r b e d b y a c h a r w h i c h was n o t p r e v i o u s l y a c t i v a t e d i n o x y g e n . Hofmann a n d O h l e r i c h c o n c l u d e d , a s d i d Hofmann a n d N o b b e ( l l ) , t h a t t h e amount o f s u l f u r a b s o r b e d b y c h a r depends o n l y on i t s s u r f a c e a r e a . i

n

t

n

e

a s

A

n

2


COAL DESULFURIZATION


240

Some o f t h e f i l t e r - p a p e r c h a r s u s e d i n t h i s w o r k w e r e a n a l y z e d f o r hydrogen and n i t r o g e n . The r e s u l t s a r e summarized i n T a b l e V, w h i c h shows t h a t t h e m a j o r i m p u r i t i e s i n f i l t e r - p a p e r c h a r a r e oxygen a n d h y d r o g e n . The o x y g e n and h y d r o g e n c o n t e n t s decrease w i t h i n c r e a s i n g temperature o f char p r e p a r a t i o n w h i l e the n i t r o g e n content remains e s s e n t i a l l y c o n s t a n t . The shape o f t h e c u r v e s i n F i g u r e 1 9 , i n w h i c h t h e amount o f s u l f u r i n some s y n t h e t i c c h a r s i s shown a s a f u n c t i o n o f (pjj^s/PHo^T' s t r o n g l y s u g g e s t s a b s o r p t i v e b e h a v i o r . Hayward a n d T r a p n e l l t l 2 ) g i v e examples o f t y p i c a l a b s o r p t i o n i s o t h e r m s a n d note that chemisorption normally gives r i s e to isotherms of t h i s g e n e r a l form. A l t h o u g h a t r e a t m e n t o f t h e p r e s e n t r e s u l t s i n terms o f i d e a l i z e d a b s o r p t i o n i s o t h e r m s i s open t o c r i t i c i s m , a n a t t e m p t w i l l n e v e r t h e l e s s b e made t o t r e a t t h e r e s u l t s a c c o r d i n g l y . I t w i l l b e shown t h a t s u c h a t r e a t m e n t l e a d s t o r e s u l t s w h i c h may b e considered reasonable. A s s u m i n g t h a t t h e c h e m i s o r b e d s u l f u r forms a n i d e a l monolayer and t h a t e a c h c h e m i s o r b e d s p e c i e s o c c u p i e s a s i n g l e s i t e , a p p l i c a t i o n o f t h e i d e a l Langmuir i s o t h e r m g i v e s ( 1 2 ) :

-

θ

8(1

- θ)

(1)

where α i s t h e a c t i v i t y o f t h e c h e m i s o r b e d s p e c i e s , θ t h e f r a c t i o n a l coverage, and Β a temperature-dependent parameter containing the heat of chemisorption of s u l f u r . The f r a c t i o n a l c o v e r a g e θ = ν/υ , w h e r e ν i s t h e v o l u m e o f c h e m i s o r b e d s u l f u r (STP) p e r gram o ? c h a r a n d ν t h e v o l u m e g i v i n g a c o m p l e t e m o n o l a y e r o f s u l f u r o n t h e s u r f a c e o f t h e c h a r . The s u r f a c e a r e a , S nrVg, i s r e l a t e d t o ν by t h e f o l l o w i n g e x p r e s s i o n : m

where Ν i s A v o g a d r o ' s number a n d A t h e c r o s s - s e c t i o n a l a r e a o f an a b s o r b e d s p e c i e s i n A^. The v o l u m e , v , o f t h e c h e m i s o r b e d s u l f u r i s o b t a i n e d f r o m t h e measured s u l f u r c o n c e n t r a t i o n a s follows: (3)


18.

KOR

Desulfurization


Table IV.

and

Sulfidation

of Coal and Coal

Char

241

Oxygen and S u l f u r C o n t e n t s o f F i l t e r - P a p e r C h a r s a f t e r S u l f i d a t i o n a t 600° a n d 900°C f o r 1 h r i n Gases o f V a r i o u s S u l f u r P o t e n t i a l

Sulfidation Temperature C O

Preparation Temperature

600

600

4.0

0.05 61.5

1.7 1.6

0.80 9.50

900

1.4

0.05 61.5

0.8 0.8

0.20 8.40

/ Pn H ~ S \ &

α

m

900

CO

Λ

V

ί ν Λ

% )

/ 2 \

P H

^

\

,

,

S u l f i d i z e d Char

2 /τ

%S

%0

1250

0.34

0.05

0.2

0.03

1500

0.14

0.05 61.5

0.1 0.1

0.02 0.05

600

4.0

0.05 7.6

0.7 0.6

0.90 7.50

900

1.4

0.05 7.6

0.9 0.6

0.40 5.80

0.14

0.05 7.6

0.1 0.1

0.01 0.25

1500

a,

0^ » i n i t i a l o x y g e n c o n t e n t o f t h e c h a r .

T a b l e V.

Char Preparation Temperature(°C) 600 900 1250

C h e m i c a l A n a l y s i s o f F i l t e r - P a p e r Char i n R e l a t i o n t o P r e p a r a t i o n Temperature

Carbon 93.9 97.0 99.6

Composition(wt.%) Hydrogen Oxygen 4.0 1.4 0.34

1.9 0.6 0.2

Nitrogen 0.02 0.06 0.04




242

Figure 18. Oxygen concentration in synthetic chars as a function of surface area: initial, and after sulfidation at 600° and 900°C

Figure 19. Sulfidation of various chars at 600° and 900°C, showing curves typical for chemisorption


18.

KOR


S u b s t i t u t i n g f o r t h e s u l f u r a c t i v i tyy θ = v/v , a n d m a k i n g u s e o f E q u a t i o n s 2 and e x p r e s s i o n i s o b t a i n e d f r o m E q u a t i o n 1: a


( p

H

S

/ P

H

1.87

I

Β

+

( P

=

243

2

3, t h e f o l loowwi!n g

/ P

H S H > 2 2 -

(4)

T

The e x p e r i m e n t a l r e s u l t s p l o t t e d i n a c c o r d a n c e w i t h E q u a t i o n 4 a r e g i v e n i n F i g u r e s 20 a n d 21 f o r 600° a n d 900°C, r e s p e c t i v e l y . The s l o p e o f e a c h l i n e s h o u l d b e p r o p o r t i o n a l t o 1/S; i n f a c t , t h i s i s shown t o b e t h e c a s e i n F i g u r e 22, i n w h i c h t h e l o g (slope) i s d e p i c t e d as a l i n e a r f u n c t i o n o f l o g S w i t h a t h e o r e t i c a l slope of -1. From t h e i n t e r c e p t o f t h i s l i n e w i t h t h e o r d i n a t e t h e v a l u e o f A, t h e c r o s s - s e c t i o n a l a r e a o f a c h e m i s o r b e d s p e c i e s , i s c a l c u l a t e d t o b e 17 A . T h i s v a l u e i s Ço b e compared w i t h c r o s s - s e c t i o n a l a r e a s r a n g i n g f r o m 10 t o 50 A as estimated from p h y s i c a l a b s o r p t i o n data f o r a v a r i e t y o f gases(12). A c c o r d i n g t o E q u a t i o n 4, t h e i n t e r c e p t s o f t h e l i n e s i n F i g u r e s 20 a n d 21 w i t h t h e o r d i n a t e s h o u l d b e p r o p o r t i o n a l t o 1/S. T h i s i s shown t o b e t h e c a s e i n F i g u r e 2 3 , i n w h i c h l o g ( i n t e r c e p t ) i s d e p i c t e d as a f u n c t i o n o f l o g S w i t h a t h e o r e t i c a l slope of -1. From t h e i n t e r c e p t s o f b o t h l i n e s w i t h t h e o r d i n a t e , t o g e t h e r w i t h t h e p r e v i o u s l y d e t e r m i n e d v a l u e o f A, t h e t e m p e r ature-dependent parameter Β i s obtained. This parameter i s p r o p o r t i o n a l t o e ^ / ^ , where q i s t h e h e a t o f c h e m i s o r p t i o n o f s u l f u r on char. From t h e t e m p e r a t u r e dependence o f 8, t h e v a l u e o f q i s e s t i m a t e d t o b e a b o u t -10 k c a l / m o l e , a r e a s o n a b l e v a l u e when compared w i t h t h e h e a t s o f c h e m i s o r p t i o n o f o t h e r g a s e s o n c a r b o n , a s l i s t e d b y Hayward a n d T r a p n e l l ( 1 2 ) . The f o r e g o i n g a n a l y s i s , a l t h o u g h o f n e c e s s i t y o v e r s i m p l i f i e d , shows t h a t t h e a b s o r p t i o n o f s u l f u r b y s y n t h e t i c c h a r s i s most l i k e l y mainly governed by c h e m i s o r p t i o n . I t i s thus expected that the s u r f a c e area i s an important parameter i n determining whether a given char o r carbon i s a b l e t o r e t a i n s i g n i f i c a n t quantities of sulfur. T h i s i s i n agreement w i t h e a r l i e r w o r k by Hofmann a n d N o b b e ( l l ) a n d b y P o l a n s k y e t a l . ( 7 ) . However, t h e s u r f a c e a r e a i s n o t t h e o n l y p a r a m e t e r t o b e considered. Forinstance, electrode graphite i nthe unoxidized s t a t e h a s a s u r f a c e a r e a o f 1 m 2 / g ( 9 ) , a p p r o x i m a t e l y t h e same as f i l t e r - p a p e r c h a r p r e p a r e d a t 1500°C ( T a b l e I I ) . Y e t , no t a k e - u p o f s u l f u r b y e l e c t r o d e g r a p h i t e was o b s e r v e d a f t e r s u l f i d a t i o n i n 5 0 % H - 5 0 % H_S a t 1000°C. The mean c r y s t a l l i t e 2

2



244


Figure 20. Langmuir isotherms show ing chemisorption at 600°C on chars having indicated surface areas

J20

10

I

L40

30

50

(pH S/p 2

1

1

s = 3 m /g

_

2

.

Ο _

°

s

-

—

-

S

30

> Figure 21. Langmuir isotherms show ing sulfur chemisorption at 900°C on chars having indicated surface areas

272

S

-o-o—S—°

1

1

(pH S/pH ) 2

^/c,

?

T


18.

KOR

Desulfurization

and Sulfidation

of Coal and Coal


τ

J

,

0

1

I

I

,M

0.8

ν

Char

r

I 6

1

1

i.O

14

1 25

LOG S

Figure 22. Slope of chemisorption isotherms at 600° and 900°C as a function of the surface area of the char

0

0.4

0.8

1./

i.6

2.0

LOG S

Figure 23. Intercept of chemisorption iso therms with ordinate as a function of surface area of the char, sulfidized at indicated tem peratures


245

246



s i z e , together w i t h t h e x-ray a n a l y s i s , o f e l e c t r o d e graphite(9) shows t h a t i t s n a t u r e i s g r a p h i t i c a n d t h e r e f o r e more o r d e r e d . B l a y d e n and P a t r i c k ( 1 3 ) concluded f r o m t h e i r work t h a t s o - c a l l e d d i s o r d e r e d c a r b o n s w i t h s m a l l c a r b o n l a y e r s ( e q u i v a l e n t t o mean c r y s t a l l i t e s i z e ) a n d many d e f e c t s a r e b e t t e r a b l e t o a b s o r b s u l f u r t h a n t h e more c r y s t a l l i n e and g r a p h i t i c c a r b o n s . The present f i n d i n g s are consistent with t h i s viewpoint. Conclusions. I n t h e absence o f i m p u r i t i e s such as i r o n , t h e a b i l i t y of c h a r s o r c a r b o n s t o a b s o r b s i g n i f i c a n t amounts o f s u l f u r i n a s u l f i d i z i n g gas s u c h a s a H^-H^S m i x t u r e depends on t h e s t a t e o f c r y s t a l l i n i t y o f t h e c a r b o n a c e o u s m a t e r i a l . The s u l f u r absorption decreases w i t h i n c r e a s i n g c r y s t a l l i t e s i z e . ο

I n g e n e r a l , c a r b o n s h a v i n g a mean c r y s t a l l i t e s i z e o f a b o u t 15 A o r l e s s a b s o r b s i g n i f i c a n t amounts o f s u l f u r when t r e a t e d i n Η -H^S m i x t u r e s . F o r carbons w i t h a given c r y s t a l l i t e s i z e , the h i g h e r t h e p o r e s u r f a c e a r e a t h e h i g h e r i s t h e amount o f s u l f u r absorbed. S u l f u r a b s o r p t i o n i n h i g h - p u r i t y chars, obtained from a s h f r e e f i l t e r paper, i n c r e a s e d w i t h i n c r e a s i n g s u l f u r a c t i v i t y and pore surface area o f t h e char. This i s i n accord with the L a n g m u i r r e l a t i o n f o r c h e m i s o r p t i o n on s i n g l e s i t e s i n an i d e a l monolayer. I t i s concluded from t h e present experimental r e s u l t s t h a t s u l f u r i s n o t accommodated i n t h e t h r e e - d i m e n s i o n a l l a t t i c e o f t h e c a r b o n b u t i s c h e m i s o r b e d on t h e s u r f a c e . However, s u c h c h e m i s o r p t i o n t a k e s p l a c e o n l y on t h e p o r e w a l l s o f n o n g r a p h i t i c ( p o o r l y c r y s t a l l i n e ) c a r b o n s , o f w h i c h c h a r s a r e good e x a m p l e s . Acknowledgment The a u t h o r w i s h e s t o t h a n k J . R. C a u l e y o f t h i s for h i s a s s i s t a n c e w i t h the experiments.

laboratory

Litreature Cited

1. "Sampling and Analysis of Coal, Coke, and By-Products," pp. 84-86, Carnegie Steel Corp., Pittsburgh, 1929. 2. Kubaschewski, O., Evans, E. L., Alcock, C. B., "Metallurgical Thermochemistry," 4th ed., Pergamon, London, 1967. 3. van Krevelen, D. W., "Coal," Elsevier, Amsterdam, 1961. 4. Jones, J. F., Schmidt, M.R., Sacks, M.E., Chen, Y.C., Gray, C.A., Office of Coal Research, R&D Rept. 11, Suppl. to Final Rept., Jan. 9, 1967.



18. KOR Desulfurization and Sulfidation of Coal and Coal Char

247

5. Batchelor, J. D., Gorin, E., Zielke, C. W., J. Ind. Eng. Chem., (1960) 52, 161-8. 6. Kor, G. J. W., Met. Trans. (1974) 5, 339-43. 7. Polansky, T. S., Knapp, E. C., Kinney, C. R., J. Inst. Fuel, (1961) 34, 245-6. 8. Wibaut, J. P., van der Kam, E. J., Rec. Trav. Chim. Pays-Bas, (1930) 49, 121-37. 9. Turkdogan, E. T., Olsson, R. G., Vinters, J. V., Carbon, (1970) 8, 545-64. 10. Hofmann, U., Ohlerich, G., Angew. Chem., (1950) 62, 16-21. 11. Hoffman, U., Nobbe, P., Ber. Dtsch. Chem. Ges., (1932) 65, 1821-30. 12. Hayward, D. O., Trapnell, B. M. W., "Chemisorption," 2nd ed., Butterworths, Washington, D. C., 1964. 13. Blayden, H. E., Patrick, J. W., Carbon, (1967) 5, 533-44.


Desulfurization and Sulfidation of Coal and Coal Char - ACS

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