Oil Shale, Tar Sands, and Related Materials - American Chemical

8 Kinetics of Oil Shale Char Gasification 1

Downloaded by UNIV OF NEW SOUTH WALES on April 12, 2016 | http://pubs.acs.org Publication Date: September 3, 1981 | doi: 10.1021/bk-1981-0163.ch008

W. J.THOMSON ,M. A. GERBER, M. A. HATTER, and D. G. OAKES Department of Chemical Engineering, University of Idaho, Moscow, Idaho 83843

During o i l s h a l e r e t o r t i n g , whether i t be by i n - s i t u o r s u r face techniques, a c e r t a i n f r a c t i o n o f the o r g a n i c carbon i s l e f t behind on the r e t o r t e d s h a l e . T h i s "char" c o n t a i n s a s i g n i f i c a n t f r a c t i o n o f t h e a v a i l a b l e e n e r g y i n t h e raw s h a l e and can a c t u a l l y supply a l l the energy f o r the r e t o r t i n g process f o r s h a l e s assayed a t 20 g a l l o n s / t o n o r g r e a t e r ( l _ ) . T o r e c o v e r t h i s e n e r g y , t h e c h a r can b e b u r n e d i n a i r o r g a s i f i e d i n o x y g e n - s t e a m e n v i r o n m e n t s ; t h e l a t t e r i n o r d e r t o p r o d u c e a low t o medium BTU gas w h i c h can be b u r n e d e l s e w h e r e i n t h e p l a n t . C o n s e q u e n t l y we have been c o n d u c t i n g k i n e t i c s t u d i e s o f t h e r e a c t i o n s o f o i l s h a l e c h a r i n a n ong o i n g r e s e a r c h p r o g r a m . E a r l i e r we r e p o r t e d o n t h e r e s u l t s o f o u r o x i d a t i o n experiments[2) and h e r e we w i l l d i s c u s s o u r work w i t h CO2 and s t e a m g a s i f i c a t i o n o f t h e c h a r . I t s h o u l d be n o t e d t h a t t h e r e has been some p r e v i o u s l y publ i s h e d work d e a l i n g w i t h t h e s e r e a c t i o n s . S t u d i e s a t U n i o n O i l R e s e a r c h i n t h e e a r l y 1970*5 a p p e a r t o be among t h e f i r s t t o a t tempt t h e e x p l o i t a t i o n o f t h e s t e a m - c h a r r e a c t i o n and l e d t o t h e d e v e l o p m e n t o f t h e SGR r e t o r t i n g p r o c e s s ( 3 j . L a t e r , Burnham a t L a w r e n c e L i v e r m o r e L a b o r a t o r i e s ( 4 , 5 _ 6 ) c o n d u c t e d b o t h noni s o t h e r m a l and i s o t h e r m a l e x p e r i m e n t s and o b t a i n e d r e a c t i o n r a t e e x p r e s s i o n s f o r t h e r a t e o f c h a r c o n s u m p t i o n due t o b o t h r e a c tions 9

COo + C + 2C0 + C + CO + H 2

H0

They observed, a s d i d the e a r l i e r r e s e a r c h e r s a t Union O i l t h a t t h e w a t e r gas s h i f t r e a c t i o n CO + H 0 t CO2 + H 2

(3)

Current address: Department of Chemical Engineering, Washington State University, Pullman, WA 99164. 1

0097-6156/81/0163-0115 $05.00/0 ©

1981

American Chemical Society

Stauffer; Oil Shale, Tar Sands, and Related Materials ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

116

OIL SHALE, TAR SANDS, AND RELATED

MATERIALS

a l s o o c c u r r e d a t a r a p i d r a t e so t h a t t h e primary gaseous products were e s s e n t i a l l y H a n d C 0 . S i n c e we w i l l be c o m p a r i n g o u r c u r r e n t r e s u l t s w i t h t h o s e m e a s u r e d by Burnham a t L a w r e n c e L i v e r m o r e L a b o r a t o r i e s ( L L L ) , i t i s a p p r o p r i a t e t o r e p e a t them h e r e . Burnham was b e s t a b l e t o f i t h i s d a t a by a s s u m i n g t h e p r e s e n c e o f two s e p a r a t e c a r b o n s p e c i e s w h i c h r e a c t e d i n p a r a l l e l and s u g g e s t e d t h e f o l l o w i n g r a t e e x pression 2


r

C0

=

2

k

C

( l C!

2

+

k

C

2 c )

( p

2

C0

0.2 '

) 2

(4)

where C and C i n i t i a l l y r e p r e s e n t e d 7 5 % a n d 25%, r e s p e c t i v e l y , o f t h e t o t a l o r g a n i c c a r b o n . T h e C] s p e c i e s had a h i g h e r a c t i v a t i o n e n e r g y (205 k i l o j o u l e s / m o l e v s 134) and was t h u s more s e n s i t i v e t o increases i n temperature. T h e i r steam g a s i f i c a t i o n d a t a were a l s o f i t t o a p a r a l l e l r e a c t i o n r a t e e x p r e s s i o n c l

C 2

r

H 0 2

k

C

" 3 cJ

( P

H oi" 2

5 +

k C 4

(5)

C 2

b u t i n t h i s c a s e C -j a c c o u n t e d f o r 57% o f t h e i n i t i a l o r g a n i c c a r b o n and a g a i n had a h i g h e r a c t i v a t i o n e n e r g y (184 k i l o j o u l e s / mole v s 1 3 4 ) . The e m p i r i c a l n a t u r e o f t h e s e e x p r e s s i o n s i s a p p a r e n t and t h u s a m a j o r g o a l o f o u r work was t o a t t e m p t t o d e r i v e r a t e e x p r e s s i o n s more t y p i c a l o f what w o u l d be e x p e c t e d f o r c h a r g a s i f i c a t i o n r e a c t i o n s based on t h e c o a l l i t e r a t u r e ^ ) . Another goal was t o be a b l e t o p r e d i c t make-gas c o m p o s i t i o n s and t h u s a s e p a r a t e d e t e r m i n a t i o n o f t h e w a t e r - g a s s h i f t r e a c t i o n r a t e was a l s o undertaken. F i n a l l y , because o f evidence t h a t the i r o n p r e s e n t i n t h e s h a l e a c t e d t o c a t a l y z e t h e s h i f t r e a c t i o n , a number o f o x i d a t i o n / r e d u c t i o n e x p e r i m e n t s were r u n i n o r d e r t o a s s e s s t h e a b i l i t y o f t h e r e a c t i n g gases t o a f f e c t the o x i d a t i o n s t a t e o f the i r o n . S i n c e some o f t h e m i n e r a l s i n d i g e n o u s t o t h e s h a l e c a n a c t a s c a t a l y s t s , i t i s relevant t o l i s t the pertinent mineral reactions which can take p l a c e : c

CaMg(C0 ) CaC0 CaC0 + S i 0 CaFe(C0 ) 3

3

3

2

2

3

2

+ t + +

CaC0 + MgO + C 0 CaO + C 0 Silicates + C0 FeO + C a C 0 + C 0 3

2

2

3

2

2

(6) (7) (8) (9)

E q u a t i o n ( 6 ) , " d o l o m i t e d e c o m p o s i t i o n , " i s i r r e v e r s i b l e and takes p l a c e a t T >875K. E q u a t i o n ( 7 ) , " c a l c i t e d e c o m p o s i t i o n , " i s r e v e r s i b l e and c a n be p r e v e n t e d i f t h e r e i s a s u f f i c i e n t C 0 o v e r p r e s s u r e . E q u a t i o n ( 8 ) , " s i l i c a t i o n , " i s i r r e v e r s i b l e and t a k e s p l a c e a t h i g h e r t e m p e r a t u r e s (>1050K) p r o v i d e d t h a t c a l c i t e d e c o m p o s i t i o n i s p r e v e n t e d . E q u a t i o n (9) o c c u r s a t l o w e r temperat u r e s and i s s i g n i f i c a n t because the i r o n oxides t h a t r e s u l t can 2


8.

THOMSON E T AL.

Oil Shale

Char

Gasification

111

a c t a s c a t a l y s t s d e p e n d i n g on t h e v a l e n c e s t a t e o f i r o n , a n d t h i s can b e i n f l u e n c e d by t h e t e m p e r a t u r e a n d c o n c e n t r a t i o n o f t h e s u r rounding gases.


E x p e r i m e n t a l Equipment

and Procedures

Equipment. A l l o f t h e g a s i f i c a t i o n e x p e r i m e n t s were c o n d u c t e d w i t h t h e same a p p a r a t u s employed i n t h e e a r l i e r o x i d a t i o n work and h a s been d e s c r i b e d i n d e t a i l e l s e w h e r e ( 2 j . T h e t e c h n i q u e i n v o l v e d s i m u l t a n e o u s measurements o f mass l o s s (TGA) a n d e x i t g a s c o m p o s i t i o n s (gas c h r o m a t o g r a p h ) i n a v e s s e l w h i c h behaved a s a n i d e a l b a c k - m i x r e a c t o r . A l l e x p e r i m e n t s were r u n u n d e r i s o t h e r m a l c o n d i t i o n s . A s b e f o r e , powdered s h a l e samples (200 mesh) o f p r e v i o u s l y r e t o r t e d o i l s h a l e f r o m t h e P a r a c h u t e C r e e k member i n C o l o r a d o were s u s p e n d e d f r o m an e l e c t r o b a l a n c e a n d p l a c e d i n a f u r n a c e . I n t h i s way c o n t i n u o u s g r a v i m e t r i c r e a d i n g s were a v a i l a b l e t o m o n i t o r t h e c o n s u m p t i o n o f t h e c h a r . T h e o f f - g a s e s were a n a l y z e d on a C a r l e g a s c h r o m a t o g r a p h e q u i p p e d w i t h a C a r b o s i e v e B column. P r o c e d u r e s . S i n c e t h e c h a r r e a c t i o n s c a n be a c c o m p a n i e d b y m i n e r a l d e c o m p o s i t i o n r e a c t i o n s , some o f w h i c h a r e c a t a l y t i c , e v e r y a t t e m p t was made t o i s o l a t e t h e p e r t i n e n t r e a c t i o n s . O f course t h e r e i s always t h e p o s s i b i l i t y t h a t s i g n i f i c a n t i n t e r a c t i o n s w i l l be m i s s e d b y t h i s p r o c e d u r e a n d t h u s i t i s i m p o r t a n t t o s t a t e t h e p r o c e d u r e s w h i c h were employed. C0 - Gasification. The p r e v i o u s l y r e t o r t e d s h a l e (see (8) f o r d e t a i l s ) was f i r s t r a i s e d t o 900K i n a h e l i u m a t m o s p h e r e t o allow i r r e v e r s i b l e dolomite decomposition t o take place. F o r t h o s e e x p e r i m e n t s i n w h i c h CO2 was t h e o n l y s p e c i e s i n t h e f e e d g a s , t h e r a t e o f CO2 g a s i f i c a t i o n was f o l l o w e d by m o n i t o r i n g t h e r a t e o f p r o d u c t i o n o f CO; i . e . , f r o m G.C. m e a s u r e m e n t s . However t h i s was n o t a c c u r a t e i n t h o s e e x p e r i m e n t s w h i c h h a d CO i n t h e i n l e t g a s , a n d i n t h e s e c a s e s t h e r a t e was d e t e r m i n e d s o l e l y by g r a v i m e t r i c measurements. A f u r t h e r c o m p l i c a t i o n d u r i n g these e x p e r i m e n t s was t h e f a c t t h a t t h e p r e s e n c e o f even s m a l l q u a n t i t i e s o f CO r e t a r d e d t h e C 0 g a s i f i c a t i o n r a t e t o t h e p o i n t where s i l i c a t i o n r a t e s were on t h e same o r d e r o f m a g n i t u d e . Consequentl y t h e sample was p u r p o s e l y p r e t r e a t e d by a l l o w i n g c o m p l e t e s i l i c a t i o n t o t a k e p l a c e . T h i s was a c h i e v e d by e x p o s i n g t h e sample t o a 4 0 % C 0 - 3 0 % CO m i x a t H O O K f o r 8-12 h o u r s . T h e C 0 p r e v e n t e d c a l c i t e d e c o m p o s i t i o n ( e q u a t i o n ( 7 ) ) a n d t h e h i g h CO c o n c e n t r a t i o n r e t a r d e d C 0 g a s i f i c a t i o n s o t h a t o n l y 5-10% o f t h e c a r b o n was consumed d u r i n g t h e p r e t r e a t m e n t . 2

2

2

2

2

Steam Gasification.

Because o f t h e temperatures

required

f o r steam g a s i f i c a t i o n (>950K), s i g n i f i c a n t C 0 p r e s s u r e s w o u l d b e r e q u i r e d t o p r e v e n t c a l c i t e d e c o m p o s i t i o n . S i n c e we were a t t e m p t ing t o study t h e H 0 + C r e a c t i o n i n t h e absence o f C 0 g a s i f i c a t i o n , we d e c i d e d t o a l l o w t h e c a l c i t e t o decompose c o m p l e t e l y t o t h e o x i d e s . T h i s was done i n a h e l i u m purge s t r e a m a t 975K. T h e d e c o m p o s i t i o n t i m e and p u r g e r a t e were s u c h t h a t a t t h i s tempera2

2

2


118


MATERIALS

t u r e o n l y a b o u t 1 0 % o f t h e c a r b o n was consumed d u e t o CO2 g a s i f i c a t i o n . A l t h o u g h t h e k i n e t i c d a t a were t a k e n i n t h e p r e s e n c e o f o x i d e s , a f e w r u n s were a l s o c o n d u c t e d w i t h s h a l e samples subjected toacid leaching(9). Water Gas Shift Reaction (WGSR). T h e WGSR was s t u d i e d o v e r s h a l e samples w h i c h had been p r e v i o u s l y d e c h a r r e d a n d s i l i c a t e d . A f t e r d e - c h a r r i n g a t 700K i n a 1 0 % 0 s t r e a m , t h e sample was e x posed t o 4 0 % C 0 a t H O O K f o r 12 h o u r s . Upon c o m p l e t i o n o f s i l i c a t i o n t h e t e m p e r a t u r e was a d j u s t e d t o t h e d e s i r e d v a l u e a n d t h e s h a l e was e i t h e r o x i d i z e d ( i n a i r o r C 0 ) o r r e d u c e d ( i n H o r CO). T h i s was f o l l o w e d by WGSR e x p e r i m e n t s i n w h i c h v a r i o u s c o n c e n t r a t i o n s o f CO, H 0 , C 0 a n d H were f e d t o t h e r e a c t o r . V a r i a b l e s . C 0 g a s i f i c a t i o n d a t a were o b t a i n e d up t o P c o 100 kPa a n d a t t e m p e r a t u r e s between 975K a n d H O O K . Steam g a s i f i c a t i o n was s t u d i e d a t H 0 p r e s s u r e s between 15 a n d 75 kPa a n d a t t e m p e r a t u r e s between 975K a n d 1150K. T h e k i n e t i c s o f t h e WGSR were a l s o s t u d i e d o v e r t h i s same t e m p e r a t u r e r a n g e a n d w i t h v a r i ous f e e d g a s c o m p o s i t i o n s c o n s i s t i n g o f H 0 , CO, C 0 a n d H . I n o r d e r t o r e m a i n o n t h e l e f t hand s i d e o f t h e WGSR e q u i l i b r i u m , t h e maximum p r e s s u r e s o f C 0 a n d H were 30 kPa. T h e o i l s h a l e was f r o m t h e P a r a c h u t e C r e e k Member i n C o l o r a d o a n d a s s a y e d a t 50 g a l l o n s / t o n . O u r e a r l i e r w o r k ( 2 j h a d i n d i c a t e d t h a t t h e r e was no e f f e c t o f a s s a y o n t h e c h a r o x i d a t i o n k i n e t i c s . 2

2


2

2

2

2

2

=

2

2

2

2

2

2

2

2

Results C 0 G a s i f i c a t i o n . The r a t e e x p r e s s i o n which r e s u l t s from an analysis o r t h e u u g a s i f i c a t i o n data i s given i n equation (10). 2

2

r

C0

C

k 2

1 +

c

P

5 C0 K l

2

P

C 0 2

+ K P 2

( 1 0 ) C Q

where k, K? K 2

4

= 7.83 x 1 0 e x p (-184/RT) ( k P a - s e c ) " = 0.0495 ( k P a ) = 5.0 ( k P a ) "

1

1

1

T h i s f o r m o f t h e r a t e e x p r e s s i o n i s t y p i c a l o f t h a t used t o c o r r e l a t e much o f t h e e a r l y g a s i f i c a t i o n d a t a on c o a l . T h e p a r a m e t e r s t h e m s e l v e s were o b t a i n e d f r o m a. m u l t i p l e r e g r e s s i o n a n a l y s i s o f t h e r e c i p r o c a l o f e q u a t i o n ( 5 ) . An i n i t i a l v a l u e o f t h e a c t i v a t i o n e n e r g y was o b t a i n e d f r o m a power l a w f i t a n d was t h e n a d j u s t e d by t i r a l a n d e r r o r u n t i l i t was c o m p a t i b l e w i t h t h e b e s t f i t f r o m t h e r e g r e s s i o n . F i g u r e 1 g i v e s some i d e a o f t h e a b i l i t y o f t h e equation t o f i t t h e data over a wide C 0 pressure range. Of p a r t i c u l a r s i g n i f i c a n c e i s t h e f i t t o t h e data c o r r e s p o n d i n g t o t h e d a s h e d l i n e w h i c h were o b t a i n e d d u r i n g m i n e r a l d e c o m p o s i t i o n . T h a t i s , t h e o n l y s o u r c e o f C 0 was t h a t r e l e a s e d 2

2


Oil

Shale

Char

1

Gasification


THOMSON ET A L .

Figure

1.

Arrhenius plot for C0 gasification: 1.0 atm CO (^J; 0.5 atm (%); 0.1 atm C0 (A); 0.016 atm CO (0). 2

z

2

2


C0

2


120


MATERIALS

d u r i n g m i n e r a l d e c o m p o s i t i o n and, s i n c e t h e h e l i u m sweep gas r a t e was h i g h , t h i s r e s u l t e d i n v e r y low CO2 p r e s s u r e s . The s i m i l a r i t y between t h i s e x p r e s s i o n and r a t e e q u a t i o n s w h i c h have been d e r i v e d f r o m c o a l c h a r g a s i f i c a t i o n d a t a ( 7 j i n t h e a b s e n c e o f CO i s s t r i k i n g . A l t h o u g h t h e v a l u e o f Ki i s a l m o s t i d e n t i c a l t o t h e v a l u e r e p o r t e d by Smoot and P r a t h u ) f o r t h e CO2 g a s i f i c a t i o n o f c h a r , t h e v a l u e o f K2 f o r o i l s h a l e c h a r i s t e n t i m e s g r e a t e r . C o n s e q u e n t l y , even low p r e s s u r e s o f CO w i l l have a s t r o n g i n h i b i t i n g e f f e c t o n CO2 g a s i f i c a t i o n . D u r i n g t h e d e r i v a t i o n o f e q u a t i o n (10) we a l s o a t t e m p t e d t o f i t o u r d a t a t o t h e e x p r e s s i o n s u g g e s t e d by E r g u n and Menster(]_0) w h i c h i s s i m i l a r t o e q u a t i o n (10) e x c e p t t h a t K] Pc02 * ^2 C 0 t a k e n t o be much g r e a t e r t h a n 1.0. When t h i s was done, a v e r y p o o r f i t was o b t a i n e d and i t was c o n c l u d e d t h a t t h e i r e x p r e s s i o n i s n o t a p p r o p r i a t e f o r o i l s h a l e c h a r . I t i s a l s o i n t e r e s t i n g t o compare e q u a t i o n (10) t o t h e e x p r e s s i o n p r o p o s e d b y Burnham, e q u a t i o n (4). B e c a u s e o f t h e h i g h v a l u e o f K-|, t h e r e a c t i o n r a t e i s c e r t a i n l y f r a c t i o n a l o r d e r w i t h r e s p e c t t o CO2 and t h i s e x p l a i n s h i s r e a c t i o n o r d e r o f 0.2. T a b l e I shows a c o m p a r i s o n o f o u r i n i t i a l r a t e s w i t h h i s and, a s can be s e e n , e q u a t i o n (4) c o n s i s t e n t l y p r e d i c t s a r a t e which i s about f o u r times h i g h e r than ours. anc

P

a r e

TABLE I RATES OF C0 GASIFICATION 2

Rate x 10 T (K)

975 975 975 1100

PC0 (kPa) 2

10 10 40 10

4

1

(sec" )

p

CO (kPa) 0 1.0 0 0

T h i s Work R e f e r e n c e 5 E q . (10) Eq. (4) 0.75 0.17 1.0 9.8

3.1

-

4.1 53

Charcoal Ref. ( 7 ) 0.0076 0.0027 0.0268 0.19

Note a l s o t h a t even w i t h a low CO p r e s s u r e o f 1 kPa, t h e CO2 g a s i f i c a t i o n r a t e drops by a f a c t o r o f almost f i v e . R e c a l l t h a t Burnham p r o p o s e d two p a r a l l e l r e a c t i o n s , p r e s u m a b l y due t o two separate carbon species o f d i f f e r e n t a c t i v i t y . Since carbon s p e c i e s ' l ' i s more a c t i v e , one e x p l a n a t i o n c o u l d be t h e f a c t t h a t we l o s t 5-10% o f t h e c a r b o n d u r i n g p r e t r e a t m e n t and t h e m o s t a c t i v e c a r b o n w o u l d be e x p e c t e d t o g a s i f y u n d e r t h o s e m i l d c o n d i t i o n s . However t h i s i s s u b s t a n t i a l l y l e s s t h a n t h e 7 5 % a s s i g n e d t o s p e c i e s '1' b y Burnham and i n o u r o p i n i o n t h e d i f f e r e n c e s a r e more l i k e l y due t o t h e s t a t i s c a l a n a l y s e s o f r a t e data which are d i f f i c u l t t o measure. A t higher carbon c o n v e r s i o n s t h e two r a t e e x p r e s s i o n s a r e c l o s e r due t o t h e l o w e r a c t i v i t y o f c a r b o n s p e c i e s '2' i n e q u a t i o n (4).


THOMSON E T AL.

8.

Oil

Shale

Char

121

Gasification

Steam G a s i f i c a t i o n . The r a t e e x p r e s s i o n f o r steam g a s i f i c a t i o n i s given i n equation (11). r

k

H20 . C

+

1

C

P

6 H20

K P 0 3

(

h

+

H 2

n

)

\

where k K3 K


6

4

= 6.62 exp (-100.7/RT) ( k P a - s e c ) " = 0.20 exp (-17/RT) ( k P a ) ' = 0.15 ( k P a ) "

1

1

1

A g a i n t h e r e a c t i o n r a t e i s f i r s t o r d e r w i t h r e s p e c t t o c h a r and t h e r e m a i n i n g p a r a m e t e r s were d e t e r m i n e d as d e s c r i b e d a b o v e f o r C 0 g a s i f i c a t i o n . I t i s i n t e r e s t i n g t h a t t h e v a l u e o f Ko i s such t h a t i t i s e f f e c t i v e l y o n e - h a l f o r d e r w i t h r e s p e c t to H 0, c o n s i s t e n t w i t h t h e f i r s t o f t h e two r e a c t i o n r a t e e x p r e s s i o n s g i v e n by Burnham(j5). T a b l e I I shows a c o m p a r i s o n o f t h e r a t e s p r e d i c t e d by e q u a t i o n (11) w i t h t h o s e p r e d i c t e d by e q u a t i o n (5) and w i t h c h a r c o a l as r e p o r t e d by Smoot and P r a t h ( 7 ) . A g a i n Burnham*s r a t e expression p r e d i c t s higher rates but, i n t h i s case, the d i s c r e pancy i s a l m o s t a f a c t o r o f 20 a t t h e h i g h e r t e m p e r a t u r e . T h e l a t t e r i s due t o t h e v e r y h i g h a c t i v a t i o n e n e r g i e s r e p o r t e d b y Burnham w h i c h a r e 30-80% h i g h e r t h a n o u r v a l u e s . One p o s s i b l e e x p l a n a t i o n i s t h a t Burnham b a s e d h i s r a t e e x p r e s s i o n on t h e r a t e 2

2

TABLE I I RATES OF STEAM GASIFICATION Rate x 1 0 ( s e c ) 4

T (K)

975 975 975 1150

P^Q (kPa)

PH (k?a)

T h i s work Eq. (11)

30 30 70 30

0 10 0 0

4.64 2.50 5.6 26.5

- 1

Reference 6 Eq. (5) 15.8

-

19.5 474

Char Ref. (7) 0.75 0.21 1.33 53.3

o f H p r o d u c t i o n i n t h e make-gas. As m e n t i o n e d e a r l i e r , . t h e w a t e r gas s h i f t r e a c t i o n i s v e r y f a s t o v e r r e t o r t e d o i l s h a l e and t h i s w o u l d r e s u l t i n a d d i t i o n a l H t h a n t h a t p r o d u c e d by t h e s t e a m - c h a r r e a c t i o n . What i s more, t h e C 0 - c h a r r e a c t i o n w o u l d a l s o t a k e p l a c e u n d e r t h e s e c o n d i t i o n s and, i n t h e n o n - i s o t h e r m a l e x p e r i m e n t s employed by Burnham, i t i s d i f f i c u l t t o d i s t i n g u i s h between them. E q u a t i o n (11) o n t h e o t h e r hand, i s b a s e d s o l e l y on i n i t i a l r a t e d a t a so t h a t i t a p p l i e s s t r i c t l y t o t h e s t e a m - c h a r r e a c t i o n . In c o m p a r i n g o u r r e s u l t s t o t h o s e f o r t h e steam g a s i f i c a t i o n o f c h a r c o a l , i t i s seen t h a t t h e steam g a s i f i c a t i o n 2

2

2



122


MATERIALS

r a t e s f o r o i l s h a l e a r e 5-10 t i m e s h i g h e r a t t h e l o w e r t e m p e r a t u r e b u t l o w e r b y a f a c t o r o f two a t t h e h i g h e r t e m p e r a t u r e . It is i n t e r e s t i n g t h a t t h i s i s q u i t e d i f f e r e n t f r o m CO2 g a s i f i c a t i o n where t h e r a t e s f o r o i l s h a l e c h a r were a l m o s t two o r d e r o f magnitudes h i g h e r . R e c a l l t h a t e q u a t i o n (11) i s b a s e d o n d a t a c o l l e c t e d f o r steam g a s i f i c a t i o n i n t h e p r e s e n c e o f CaO; i . e . , a t h e r m a l l y d e c a r b o n a t e d s h a l e sample. F i g u r e 2 i s a f i r s t o r d e r p l o t which compares t h e r a t e o f c h a r c o n s u m p t i o n f o r samples w h i c h were l e a c h e d i n a c i d w i t h a t h e r m a l l y decarbonated sample under t h e same c o n d i t i o n s . T h e f a c t t h a t t h e l e a c h e d samples have s i g n i f i c a n t l y l o w e r r a t e s i s a p p a r e n t l y d u e t o changes i n d u c e d i n t h e mineral matrix as a r e s u l t o f t h e acid leaching. This i s b e t t e r u n d e r s t o o d when t h e r e s u l t s f o r g a s i f i c a t i o n i n t h e p r e s e n c e o f CO2-H0O m i x t u r e s a r e a n a l y z e d . F i g u r e s 3 a n d 4 show t h e p r e d i c t e d a n d e x p e r i m e n t a l r e s u l t s f o r two e x p e r i m e n t s c o n d u c t e d u n d e r s i m i l a r g a s c o m p o s i t i o n s b u t a t two d i f f e r e n t t e m p e r a t u r e s . The r e s u l t s shown i n F i g u r e 3 a r e a t a t e m p e r a t u r e o f 980K a n d , a t t h i s t e m p e r a t u r e , a CO2 p r e s s u r e o f 10 k P a i s s u f f i c i e n t t o prevent c a l c i t e decomposition. The p r e d i c t e d r a t e o f char cons u m p t i o n i s b a s e d o n a d y n a m i c m a t h e m a t i c a l model w h i c h i n c o r p o r a t e s e q u a t i o n s (10) a n d ( 1 1 ) . N o t e t h a t t h e a c t u a l r a t e i s much s l o w e r t h a n t h a t p r e d i c t e d by e q u a t i o n s (10) a n d ( 1 1 ) . T h e d a t a shown i n F i g u r e 4 were a l s o o b t a i n e d a t PCO2 = 10 kPa b u t a t a h i g h e r t e m p e r a t u r e o f 1040K. I n t h i s c a s e a p p r o x i m a t e l y o n e h a l f o f t h e a v a i l a b l e CaC03 decomposed t o CaO and, a s c a n b e seen, t h e p r e d i c t e d char consumption r a t e s a r e c l o s e t o those measured. Given these r e s u l t s and t h e lower r a t e s measured w i t h a c i d l e a c h e d s h a l e (where a l l t h e c a l c i u m i s r e m o v e d ) , i t i s a p p a r e n t t h a t CaO c a t a l y z e s t h e s t e a m - c h a r r e a c t i o n . T h i s o f c o u r s e i s no s u r p r i s e t o t h o s e f a m i l i a r w i t h t h e l i t e r a t u r e o n c o a l g a s i f i c a t i o n where a l k a l i n e e a r t h o x i d e s have been known t o c a t a l y z e t h e s e v e r y same r e a c t i o n s ( 1 1 ] 2 ) . W a t e r Gas S h i f t R e a c t i o n (WGSRjT AS d e s c r i b e d e a r l i e r , t h e WGSR was s t u d i e d o v e r d e c h a r r e d a n d t o t a l l y s i l i c a t e d s h a l e samp l e s . E a r l y i n t h e c o u r s e o f t h i s phase o f t h e s t u d y we d i s covered t h a t t h e i r o n p r e s e n t i n t h e s h a l e c o u l d be r e v e r s i b l y o x i d i z e d o r reduced, depending on t h e gas c o m p o s i t i o n and temperat u r e . A l t h o u g h t h e WGSR r a t e s were d e t e r m i n e d o n l y f o r samples w h i c h h a d been s u b j e c t e d t o r e d u c t i o n i n H2, a l i m i t e d number o f o x i d a t i o n / r e d u c t i o n e x p e r i m e n t s were a l s o c o n d u c t e d . A g a i n , t h e s e were a c c o m p l i s h e d i n t h e TGA a p p a r a t u s m e n t i o n e d above. T h e p r o c e d u r e was t o i n i t i a l l y r e d u c e t h e s a m p l e i n f l o w i n g H2 (100 kPa) p r i o r t o e x p o s i n g i t t o a n o x i d i z i n g a t m o s p h e r e (CO2 o r a i r ) and t o r e c o r d the weight gain as a f u n c t i o n o f time. Prior to r e d u c t i o n e x p e r i m e n t s t h e s a m p l e was c o m p l e t e l y o x i d i z e d i n 100 k P a a i r a n d t h e n e x p o s e d t o a r e d u c i n g a t m o s p h e r e (H2 o r C O ) . The o x i d a t i o n / r e d u c t i o n r a t e s were f o u n d t o be f i r s t o r d e r w i t h respect t o t h e gas c o n c e n t r a t i o n as well as t o t h e q u a n t i t y o f unconverted iron present i n t h e shale. Table III gives t h e values h a d

9


Oil

Shale

Char

Gasification


THOMSON E T AL.


124

MATERIALS



01

0

1

I

10

20

30

Time - Min Figure 4.

Mixed gasification-CaO present (?h o = 37 kPa, P o = 10 kPa, T = 1040 K) predicted ( ); experimental data (%). 2

C

2


8.

THOMSON E T AL.

Oil Shale

Char

125

Gasification

TABLE I I I FIRST ORDER RATE CONSTANTS FOR THE OXIDATION/REDUCTION OF IRON 1

Rate C o n s t a n t C0


Oxidation (T = 980K)

AIR

2

H

(sec" ) CO

2

0.07 3.18

Reduction (T = 1040K)

-

-

0.22

0.20

o f t h e f i r s t o r d e r r a t e c o n s t a n t s o b t a i n e d a t two d i f f e r e n t temp e r a t u r e s f o r o x i d a t i o n i n t h e p r e s e n c e o f 10 kPa o f a i r a n d CO2 and f o r r e d u c t i o n a t 10 kPa o f CO a n d H . As w o u l d be e x p e c t e d , t h e r a t e o f o x i d a t i o n i n a i r i s more t h a n an o r d e r o f m a g n i t u d e h i g h e r t h a n o x i d a t i o n i n CO2 whereas t h e CO a n d H r e d u c t i o n r a t e s are comparable. E q u a t i o n ( 1 2 ) i s a n e x p r e s s i o n f o r t h e WGSR r a t e . T h e d a t a were p u r p o s e l y f i t t o t h e e l e m e n t a r y r e a c t i o n r a t e e x p r e s s i o n g i v e n i n b r a c k e t s s o t h a t t h e r a t e w o u l d go t o z e r o a s e q u i l i b r i um was a p p r o a c h e d . 2

2

k

7 t C0 H 0 " V K E C 0 R

P

" k

1 h

W G S R

+

P

3

y

K

5

C0

= 4.16 X 1 0 ' e x p [ - 8 2 . 1 / R T ] =

.0278 ( k P a ) "

V

P

2

h

+ 2

R

2

H 0 2

2

moles/g-kPa -sec

1

K = .0492 ( k P a ) I t s h o u l d be n o t e d t h a t e q u a t i o n (12) p r e d i c t s a n i n h i b i t o r y e f f e c t o f C 0 on t h e r e a c t i o n r a t e . However j u s t t h e o p p o s i t e was o b s e r v e d f o r C 0 p r e s s u r e s l e s s t h a n 10 kPa a n d c o n s e q u e n t l y e q u a t i o n (12) i s o n l y v a l i d f o r Pco? 10 kPa C a t a l y t i c E f f e c t s . A l t h o u g h f t i s y e t t o b e p r o v e n , we b e l i e v e t h a t t h e anomalous b e h a v i o r o f C 0 on t h e WGSR r a t e i s d u e to i t s i n f l u e n c e on t h e o x i d a t i o n s t a t e o f i r o n . In f a c t d u r i n g t h e WGSR r a t e e x p e r i m e n t s we o b s e r v e d c h a n g e s i n t h e mass o f t h e s h a l e sample a s t h e g a s c o m p o s i t i o n was v a r i e d . C o n s e q u e n t l y a c o m p l e t e u n d e r s t a n d i n g o f t h e WGSR i s d e p e n d e n t o n a q u a n t i t a t i v e knowledge o f how t h e c a t a l y t i c a c t i v i t y o f i r o n v a r i e s w i t h i t s o x i d a t i o n s t a t e . Once t h i s i s known, r e a c t i o n r a t e e x p r e s s i o n s f o r t h e o x i d a t i o n / r e d u c t i o n o f i r o n c o u l d be c o m b i n e d w i t h WGSR r a t e e x p r e s s i o n s t o p r o v i d e a more a c c u r a t e p r e d i c t i o n o f t h e make-gas c o m p o s i t i o n s . 1

6

2

2

>

2



126

MATERIALS

The r o l e o f CaO a s a c a t a l y s t f o r t h e s t e a m - c h a r r e a c t i o n c a n be i n f e r r e d f r o m t h e r e s u l t s o b t a i n e d d u r i n g m i x e d CO2-H2O g a s i f i c a t i o n ( f i g u r e s 3 a n d 4 ) . I n many r e s p e c t s i t i s d e s i r a b l e t o p r e v e n t t h e d e c o m p o s i t i o n o f CaC03 t o CaO b e c a u s e o f t h e h i g h endothermic heats o f r e a c t i o n a s s o c i a t e d with t h i s r e a c t i o n . However, a s we have shown, t h e steam g a s i f i c a t i o n o f o i l s h a l e c h a r i s a b o u t t e n t i m e s s l o w e r when CaO i s n o t p r e s e n t . A b e t t e r e v a l u a t i o n o f t h e i m p o r t a n c e o f CaO r e q u i r e s a knowledge o f t h e d e p e n d e n c e o f t h e r e a c t i o n r a t e on t h e q u a n t i t y o f CaO p r e s e n t . Downloaded by UNIV OF NEW SOUTH WALES on April 12, 2016 | http://pubs.acs.org Publication Date: September 3, 1981 | doi: 10.1021/bk-1981-0163.ch008

Nomenclature C - char c o n c e n t r a t i o n , moles/g s h a l e k - rate constants K - a d s o r p t i o n c o n s t a n t s , kPa"' K[: - e q u i l i b r i u m c o n s t a n t Pi - p a r t i a l p r e s s u r e o f i , kPa r - r e a c t i o n r a t e s , moles char r e a c t e d / s - g s h a l e R - g a s c o n s t a n t , 0.008324 k j o u l e s / m o l e - ° K T - t e m p e r a t u r e , °K c

Abstract The kinetics of oil shale char gasification have been studied for Colorado oil shale from the Parachute Creek member. Reaction rate expressions similar to those previously reported for coal char were obtained f o r the H O-char, CO char and water gas s h i f t reactions. Evidence is presented to suggest that CaO, a product of mineral decomposition, catalyzes the H O-char reaction and that indigeneous iron catalyzes the water gas s h i f t reaction. The l a t t e r reaction proceeds rapidly so that the make-gas cons i s t s primarily of H and CO . 2

2-

2

2

2

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9.

Dockter, L., paper presented at 68th Annual Meeting of AIChE, Los Angeles, Ca., November 20, 1975. Soni, Y.; Thomson, W. J., I&EC PROC. DES. & DEV., 1979, 18, 661. OIL & GAS JOURNAL, June 17, 1974, p.26. Burnham, A. K., FUEL, 1979, 58, 285. Burnham, A. K., FUEL, 1979, 58, 713. Burnham, A. K., FUEL, 1979, 58, 719. Smoot, L . K.; Prath, D. T., ''Pulverized Coal Combustion and Gasification," PLENUM PRESS, N . Y . , 1979. Soni, Y.; Thomson, W. J., "PROC 11TH OIL SHALE SYMP," Colorado State Univ. Press, Golden, C O . , 1978. Thomson, W. J., paper presented at National Meeting of AIChE, Houston, T X . , March 30, 1979.


8.

THOMSON

ET

AL.

Oil

Shale

Char

Gasification

127

10. Ergun, S.; Menster, M., in "Chemistry and Physics of Carbon," Vol. 1, (ed. P. L. Walker, Jr.), Marcel Dekker, N.Y., 1965, p.203. 11. Lewis, W. K.; Gilliland, E. R.; Hipkin, H . , IND & ENG CHEM, 1953, 45, 1697. 12. Eakman, J . M.; Wesselhoft, J . J.; Dunkleman, J. J.; Vadovic, C. J., paper No. 63d, presented at 72nd Annual Meeting of AIChE, San Francisco, November 1979. January 19, 1981.


RECEIVED


Oil Shale, Tar Sands, and Related Materials - American Chemical

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