Effect of Mineral Species on Oil Shale Char Combustion - American

31

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 4, 2015 | http://pubs.acs.org Publication Date: August 1, 1983 | doi: 10.1021/bk-1983-0230.ch031

Effect of Mineral Species on Oil Shale Char Combustion RALPH P. CAVALIERI and WILLIAM J. THOMSON Washington State University, Department of Chemical Engineering, Pullman, WA 99164-2710

Six oil-shale samples with differing mineral compositions were retorted identically and the resulting char was subjected to combustion kinetic studies using TGA techniques. Elemental compositions were obtained using x-ray fluorescence. On shales with high concentrations of carbonates, mineral changes were effected by allowing partial decarbonation and/or s i l i c a t i o n to take place prior to the combustion studies. Combustion results are compared in terms of simple kinetic expressions and are discussed in the context of the mineral species present during initial combustion. Intrinsic rate constants were found to vary from one shale to another by a factor of ten. Catalytic activity was attributed to alkaline earth oxides formed by mineral carbonate decomposition of nahcolite and calcite. I n order t o i n c r e a s e t h e energy e f f i c i e n c y o f aboveground o i l s h a l e processes, t h e carbonaceous r e s i d u e (char) r e m a i n i n g on r e t o r t e d o i l s h a l e ( s p e n t s h a l e ) w i l l e i t h e r be c o m b u s t e d U , 2 ) o r g a s i f i e d (3)· A l t h o u g h t h e r e i s no g r e a t d i f f i c u l t y i n c o m b u s t i n g t h e c h a r , i t i s i m p o r t a n t t h a t c o m b u s t i o n be c a r r i e d o u t i n a c o n t r o l l e d f a s h i o n . F a i l u r e t o do s o c a n r e s u l t i n h i g h t e m p e r a t u r e s (> 900 K) a n d t h e d e c o m p o s i t i o n o f m i n e r a l carbonates. These d e c o m p o s i t i o n r e a c t i o n s a r e n o t o n l y e n d o t h e r m i c (4) b u t some o f t h e p r o d u c t s have t h e p o t e n t i a l t o c a u s e e n v i r o n m e n t a l d i s p o s a l p r o b l e m s (5). C o n t r o l o f o i l s h a l e c h a r c o m b u s t i o n i s more e a s i l y managed i f t h e r e i s a k n o w l e d g e o f how t h e r a t e o f c o m b u s t i o n d e p e n d s o n O2 c o n c e n t r a t i o n a n d t e m p e r a t u r e . T h i s m o t i v a t i o n l e d t o an e a r l i e r s t u d y (6) o f t h e c o m b u s t i o n k i n e t i c s o f spent 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 w e s t e r n C o l o r a d o . That study

0097-6156/83/0230-0543$06.00/0 © 1983 A m e r i c a n C h e m i c a l S o c i e t y

In Geochemistry and Chemistry of Oil Shales; Miknis, F., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


544

GEOCHEMISTRY AND CHEMISTRY OF OIL SHALES

p r o v i d e d e v i d e n c e t h a t one o r more o f t h e m i n e r a l s p e c i e s present i n t h e s h a l e acted as an o x i d a t i o n catalyst. C o n s e q u e n t l y i t was d e c i d e d t o f o l l o w up o n t h a t i n v e s t i g a t i o n by e x a m i n i n g t h e c o m b u s t i o n a c t i v i t y o f o t h e r o i l s h a l e s ; s p e c i f i c a l l y , those w i t h d i f f e r i n g e l e m e n t a l and/or m i n e r a l compositions. Six o i l s h a l e samples were s e l e c t e d f o r e v a l u a t i o n and comparison: one f r o m t h e P a r a c h u t e C r e e k Member (PCM), one f r o m a d e e p c o r e s a m p l e i n t h e C-a t r a c t ( C - a ) , t w o f r o m t h e s a l i n e zone i n w e s t e r n C o l o r a d o (S-A & S-B), one f r o m t h e G e o k i n e t i c s s i t e i n e a s t e r n U t a h (GEOK) a n d one s a m p l e o f A n t r i m s h a l e f r o m M i c h i g a n (ANT). Experimental The e x p e r i m e n t a l t h e r m o g r a v i m e t r i c a n a l y s i s (TGA) e q u i p m e n t was e s s e n t i a l l y i d e n t i c a l t o t h a t d e s c r i b e d b y T h o m p s o n a n d T h o m s o n (_7). H o w e v e r , i n t h i s s t u d y a l a r g e r r e a c t o r w a s emp l o y e d ( 1 0 cm d i a m e t e r v s . 7.5 c m ) a n d t h e t w o t h e r m o c o u p l e s were p o s i t i o n e d d i f f e r e n t l y . I n t h i s c a s e one was p l a c e d 2 cm above t h e s h a l e a n d t h e o t h e r s o t h a t i t b a r e l y m i s s e d t o u c h i n g the s h a l e s a m p l e . The l a t t e r was used t o m o n i t o r t e m p e r a t u r e e x c u r s i o n s d u r i n g t h e i n i t i a l s t a g e o f c o m b u s t i o n . The t e m p e r a t u r e d i f f e r e n c e b e t w e e n t h e s u r f a c e o f t h e s h a l e a n d 2 cm a b o v e i t r a r e l y e x c e e d e d 10 K a n d t h e n o n l y f o r o n e t o t w o m i n u t e s . A l l o f t h e s h a l e samples were r e t o r t e d i n master batches a n d u n d e r i d e n t i c a l c o n d i t i o n s i n a 2.5 cm d i a m e t e r f i x e d b e d r e t o r t . A n i t r o g e n sweep g a s a t 100 s c c / m i n was e m p l o y e d a n d the t e m p e r a t u r e was e l e v a t e d a t a r a t e o f 5 K / m i n t o a maximum t e m p e r a t u r e o f 785 K a t w h i c h p o i n t i t was h e l d f o r 1 hour. T a b l e I shows the q u a n t i t y o f o i l c o l l e c t e d d u r i n g r e t o r t i n g , the percentage o f o r g a n i c carbon on t h e spent s h a l e and t h e p e r c e n t a g e o f some o f t h e more i m p o r t a n t e l e m e n t s ( o b t a i n e d b y x-ray fluorescence). Although there i s a wide v a r i a t i o n i n t h e o i l y i e l d s , we h a v e p r e v i o u s l y s h o w n ( 8 ) t h e r e t o b e n o e f f e c t on t h e c o m b u s t i o n a c t i v i t y o f s p e n t s h a l e . However, i t i s i n t e r e s t i n g t o n o t e t h a t t h e GEOK s p e n t s h a l e s a m p l e h a d n e a r l y t w i c e t h e o r g a n i c c a r b o n c o n t e n t o f t h e PCM s a m p l e e v e n t h o u g h the t w o h a d s i m i l a r o i l y i e l d s . C o m b u s t i o n t e s t s w e r e c a r r i e d o u t by h e a t i n g t h e s a m p l e t o the d e s i r e d t e m p e r a t u r e i n a h e l i u m a t m o s p h e r e a n d t h e n e x p o s i n g i t t o a p r e - s e l e c t e d 0 c o n c e n t r a t i o n by d i l u t i n g a i r w i t h h e l i u m . I n some c a s e s t h e s a m p l e s w e r e f i r s t s u b j e c t e d t o h i g h t e m p e r a t u r e s (800-1050 K) i n e i t h e r a h e l i u m o r C 0 a t m o s p h e r e i n o r d e r t o e f f e c t changes i n t h e m i n e r a l c o m p o s i t i o n s and t h e n cooled t o the desired combustion temperature. Combustion a c t i v i t i e s were e v a l u a t e d f o r 0 p a r t i a l p r e s s u r e s between 5 and 20 k P a a n d a t t e m p e r a t u r e s b e t w e e n 700 a n d 825 K. 2

2

2



STEAM

Figure 1. Schematic of thermogravimetric analysis equipment.

REACTOR/RETORT

LIQUID COLLECTION

CONDENSER

CAHN ELECTROBALANCE


TO G . C .

» TO VENT

1

5"

|

ft

O

on

o

H

o

r m 2 >

546


Table I .

C o m p o s i t i o n o f Spent Shale Wt


Sample G P T PCM C-a GEOK S-A S-B ANT

50 24 44 30 40 11

a

C

b

6.9 2.9 12.0 3.8 3.9 5.7

% Spent

Samples

Shale

Ca

Mg

Fe

Al

Na

10.2 12.3 15.2 8.4 1.0 0.7

3.4 3.5 4.3 3.6 1.0 1.2

2.8 2.5 3.1 3.1 5.6 5.4

5.0 4.3 4.8 6.3 10.9 8.3

2.6 2.0 2.3 2.8 0.4 0.4

K

Si

1.7 1.7 2.0 2.1 1.7 3.4

18.8 16.2 19.6 22.3 30.2 31.0

* Gallons per ton raw shale Organic carbon spent s h a l e

Analysis T h e u s e o f t h e r m o g r a v i m e t r i c a n a l y s i s (TGA) a p p a r a t u s t o o b t a i n k i n e t i c d a t a i n v o l v e s a s e r i e s o f t r a d e - o f f s . S i n c e we chose t o employ a u n i t which i s s i g n i f i c a n t l y l a r g e r than commercially a v a i l a b l e instruments ( i n order t o obtain accurate chromatographic data), i t was d i f f i c u l t t o a c h i e v e time i n v a r i a n t 0 c o n c e n t r a t i o n s f o r r u n s w i t h r e l a t i v e l y r a p i d comb u s t i o n r a t e s . The r e a c t o r c l o s e l y a p p r o x i m a t e d i d e a l backm i x i n g c o n d i t i o n s and consequently a dynamic m a t h e m a t i c a l model was u s e d t o d e s c r i b e t h e t i m e - v a r y i n g 0 c o n c e n t r a t i o n , t e m p e r a t u r e e x c u r s i o n s on t h e s h a l e s u r f a c e and t h e s i m u l t a n e o u s r e a c t i o n r a t e . K i n e t i c i n f o r m a t i o n was e x t r a c t e d f r o m t h e m o d e l by m a t c h i n g t h e c o m p u t a t i o n a l p r e d i c t i o n s t o t h e measured experimental data. B e c a u s e we u s e d r e l a t i v e l y l a r g e s h a l e l o a d i n g s ("1.5 g ) , we w e r e c a r e f u l t o s p r e a d t h e s h a l e o v e r t h e b a s k e t i n a r e l a t i v e l y t h i n l a y e r (-0.4 mm). Worst-case diffusional c a l c u l a t i o n s i n d i c a t e d t h a t d i f f u s i o n r e s i s t a n c e c o u l d be n e g l e c t e d a n d t h i s was v e r i f i e d when e x p e r i m e n t s w i t h h a l f l o a d i n g s g a v e t h e s a m e r e s u l t s a s t h o s e w i t h f u l l 1.5 g l o a d ings. A n o t h e r f a c t o r o f i m p o r t a n c e was t h e l i m i t a t i o n o f g a s s o l i d mass t r a n s f e r . A t t e m p t s w e r e made t o a v o i d t h i s p r o b l e m by u s i n g h i g h gas f l o w r a t e s . H o w e v e r , t h e maximum g a s f l o w r a t e s were d i c t a t e d by t h e s t a b i l i t y o f t h e t h e r m o g r a v i m e t r i c readings. I t was f o u n d t h a t t h i s f l o w r a t e was n o t s u f f i c i e n t t o e l i m i n a t e g a s - s o l i d mass t r a n s f e r r e s i s t a n c e f o r s h a l e s w i t h the h i g h e s t c o m b u s t i o n r a t e s . A g a i n , the m a t h e m a t i c a l model was u s e d i n t h e s e c a s e s t o c o m b i n e mass t r a n s f e r a n d k i n e t i c r a t e s i n order t o e x t r a c t the l a t t e r . To i l l u s t r a t e t h e s i t u a t i o n w h e r e g a s - s o l i d mass t r a n s f e r p l a y s a n i m p o r t a n t r o l e , F i g u r e 2 shows t h e c h a r c o m b u s t i o n d a t a 2

2


Oil Shale Char


CAVALIERI AND THOMSON

2

6

Combustion

10

14

18

ADJUSTED TIME, MIN Figure 2. Mass transfer effects (GEOK,

PQ - 10 kPa). 2

American Chemical Society Library 1155 16th St., N.W. In Geochemistry and Chemistry of Oil Shales; Miknis, F., et al.; Washington, D.C. 20036

ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


548

f o r t h e GEOK s a m p l e . N o t e t h a t t h e h i g h e r t e m p e r a t u r e d a t a do not f o l l o w a s t r a i g h t l i n e on a f i r s t o r d e r p l o t . The f a c t t h a t the l o w e s t t e m p e r a t u r e r u n d i d f o l l o w a s t r a i g h t l i n e and t h a t the l o w c o n v e r s i o n data are s i m i l a r a t a l l temperatures, i s a c l e a r i n d i c a t i o n o f mass t r a n s f e r p r o b l e m s . The d o w n w a r d t r e n d of t h e h i g h c o n v e r s i o n d a t a i s due t o t h e s h i f t i n g o f t h e c h a r r e a c t i o n o r d e r from z e r o (mass t r a n s f e r c o n t r o l l e d ) t o one (kinetic controlled). As shown i n o u r e a r l i e r work (6), assuming f i r s t order w i t h respect t o both 0 i char, the r a t e i s g i v e n by a n (


2

k P

Q

C° ( l - X ) k C° ( l - X ) c

where k i s t h e k i n e t i c r a t e c o n s t a n t , k i s the mass t r a n s f e r c o e f f i c i e n t , C ° i s t h e i n i t i a l c h a r c o n c e n t r a t i o n and X i s t h e f r a c t i o n char converted. T h u s a t l o w v a l u e s o f X, t h e r i g h t hand term i n t h e d e n o m i n a t o r i s d o m i n a n t and t h e r a t e i s z e r o order w i t h respect t o char. A t h i g h v a l u e s o f X, t h e d e n o m i n a t o r a p p r o a c h e s 1.0 and t h e r a t e becomes f i r s t o r d e r . The m a s s t r a n s f e r c o e f f i c i e n t w a s o b t a i n e d f r o m t h e l o w c o n v e r s i o n d a t a i n F i g u r e 2 and when i t was u s e d i n c o m b i n a t i o n w i t h t h e f i r s t o r d e r a s s u m p t i o n made above, e x c e l l e n t p r e d i c t i o n s o f t h e e x p e r i m e n t a l d a t a were o b t a i n e d f o r a l l t h e s h a l e samples i n c l u d i n g those which were k i n e t i c a l l y c o n t r o l l e d throughout. The m o d e l p r e d i c t i o n i s shown a l o n g w i t h t h e experimental data i n F i g u r e 2 f o r the high temperature run. m

c

Rate

Constants

As i n o u r e a r l i e r w o r k (6) t h e c o m b u s t i o n r e a c t i o n r a t e was f o u n d t o be f i r s t o r d e r w i t h r e s p e c t t o b o t h 0 and char content. Table I I l i s t s t h e apparent r a t e constants i n terms o f t h e p r e - e x p o n e n t i a l f a c t o r and t h e a c t i v a t i o n e n e r g y f o r a l l s i x s a m p l e s a s w e l l a s c o m p a r a t i v e v a l u e s a t 700 K. The S-A s a m p l e had t h e h i g h e s t a c t i v i t y and h a s h i g h c o n c e n t r a t i o n s o f t h e m i n e r a l s d a w s o n i t e and n a h c o l i t e . Although these m i n e r a l s w i l l have decomposed p r i o r t o c o m b u s t i o n , t h e d e c o m p o s i t i o n p r o d u c t s ( N a C 0 g , A l 0 g ) a r e p r e s e n t a n d , a s w i l l be shown, t h e r e i s s t r o n g evidence t o i n d i c a t e t h a t t h e sodium a c t s as a c a t a l y s t . I t i s also i n t e r e s t i n g that the a c t i v a t i o n energies a r e lowest f o r t h e t w o s h a l e s ( S - A a n d GEOK) w i t h t h e h i g h e s t r a t e c o n stants. I ti stempting to a t t r i b u t e this r e s u l t to c a t a l y t i c i n f l u e n c e s o f t h e m i n e r a l m a t t e r b u t , i n t h e c a s e o f t h e GEOK s a m p l e , t h e r e i s no s e p a r a t e e v i d e n c e t o s u g g e s t t h a t t h e m i n e r a l s p r e s e n t a t t h e s e l o w t e m p e r a t u r e s a r e , i n any way, catalytic. 2

2

2


31.

Oil Shale Char Combustion


Table

II.

Kinetic

k = koEXPJzEJ


Sample PCM C-a GEOK S-A S-B ANT a

ko

x 10"

.025 18500. .00067 .0031 .224 1.28

7

549

Parameters (kPa-min)" E

1

a

97.07 147.9 54.85 50.04 88.16 93.72

k

(700K) .0140 .0168 .054 .0571 .0059 .013

kj/mol

Char c o m b u s t i o n k i n e t i c s have been p r e v i o u s l y r e p o r t e d f o r A n t r i m s h a l e by Rostam-Abadi and M i c k e l s o n (9). I n that study t h e a u t h o r s r e p o r t e d t h a t t h e r a t e was s e c o n d o r d e r w i t h r e s p e c t to t h e char r e m a i n i n g and t h a t t h e r e was n o t i c e a b l e c h e m i s o r p t i o n o f 02« A t t e m p t s t o f i t o u r d a t a f o r t h e A n t r i m s h a l e t o a second order r a t e e x p r e s s i o n were u n s u c c e s s f u l and, i n a l l cases, t h edata appeared t o f o l l o w f i r s t order kinetics. A l t h o u g h we d i d n o t h a v e t h e p r e c i s i o n t o m e a s u r e 0 c h e m i s o r p tion, t h i s phenomenon i s c o n s i s t e n t w i t h o u r p r e v i o u s o b s e r v a t i o n s (6^) o f c a t a l y t i c a c t i v i t y i n t h o s e shales c o n t a i n i n g decomposed m i n e r a l carbonates. That i s , t h e c a t a l y t i c a c t i v i t y o f CaO w a s a t t r i b u t e d t o i t s a b i l i t y t o c h e m i s o r b 02* A s w i l l b e d i s c u s s e d i n m o r e d e t a i l b e l o w , t h e A n t r i m s h a l e sample d i d n o t c o n t a i n such carbonates and no c a t a l y t i c b e h a v i o r was observed. However, t h e magnitude o f t h e r a t e c o n s t a n t s r e p o r t e d by R o s t a m - A b a d i and M i c k e l s o n (9) a r e v e r y s i m i l a r t o those measured here. F i n a l l y i t s h o u l d be p o i n t e d o u t t h a t t h e p r e - e x p o n e n t i a l f a c t o r l i s t e d i n T a b l e I I f o r t h e PCM s a m p l e , d i f f e r s f r o m t h e v a l u e we r e p o r t e d e a r l i e r (6^). F r o m o u r m e a s u r e m e n t s o f t h e a c t u a l shale temperature, we have d i s c o v e r e d t h a t t h e m e a s u r e d temperatures i n t h ee a r l i e r study were i ne r r o r . The v a l u e s l i s t e d i n T a b l e I I a r e now c o n s i s t e n t w i t h t h e r e p o r t e d m e a s u r e m e n t s o f Sohn and K i m ( 1 0 ) . 2

Mineral Catalysis A l t h o u g h a d e t a i l e d x-ray d i f f r a c t i o n a n a l y s i s was n o t c o n d u c t e d a s p a r t o f t h i s s t u d y , some i n s i g h t i n t o t h e c a r b o n a t e m i n e r a l b e h a v i o r o f t h e s h a l e s c a n be o b t a i n e d b y a l l o w i n g t h e s h a l e s t o decompose i n a n i n e r t e n v i r o n m e n t . F i g u r e 3 shows t h e mass l o s s f o r t h e s p e n t s h a l e s a s t h e t e m p e r a t u r e was r a i s e d l i n e a r l y a t 2.7 ° C / m i n i n a h e l i u m p u r g e s t r e a m . I n t h e GEOK s a m p l e , o n l y one l a r g e mass l o s s i s a p p a r e n t and t h i s o c c u r s a t



550


— J

400

i

I

i

500

600

700

TEMPERATURE, «C Figure 3. Mass loss due to mineral decomposition. Conditions: helium purge, heating rate - 2.7 K/min).



31.


Oil Shale Char

Combustion

551

a b o u t 600 °C. T h i s i s a t t r i b u t e d t o d o l o m i t e / c a l c i t e d e c o m p o s i t i o n . T h e r e a r e t w o d i s t i n c t mass l o s s o c c u r r e n c e s w i t h t h e ANT s a m p l e ; one a t 500 °C a n d t h e o t h e r a t 625 °C. T h e s e a r e a t t r i buted t o c l a y m i n e r a l s which a r e t y p i c a l l y found i n devonian shale. The c o m p a r i s o n b e t w e e n §-A a n d S-B i s v e r y i n t e r e s t i n g s i n c e t h e r e a p p e a r s t o be much l e s s m i n e r a l d e c o m p o s i t i o n i n t h e S-B sample. T h e mass l o s s a t 480°C i n t h e S-A s a m p l e i s a t t r i b u t e d t o d a w s o n i t e d e c o m p o s i t i o n and t h e l o s s a t 620°C i s a t t r i buted t o c a l c i t e decomposition. T h e S-B s a m p l e i s l o w i n dawsonite w i t h i t s aluminum probably present i n s i l i c a t e s . A s c a n be s e e n f r o m t h e e l e m e n t a l C a m e a s u r e m e n t s l i s t e d i n T a b l e I , i t s h o u l d be l o w i n c a l c i t e a s w e l l . One o f t h e e x p e r i m e n t s w h i c h we c o n d u c t e d o n t h e PCM s a m p l e was t o t h e r m a l l y decompose t h e c a r b o n a t e m i n e r a l s ( d o l o m i t e a n d c a l c i t e ) t o t h e i r o x i d e s a t 900 K a c c o r d i n g t o r e a c t i o n s (1) a n d (2). CaMg ( C 0 ) 3

CaC0

3

^ C a O + MgO + 2 C 0

2

^

^

CaO + C 0

(1)

2

(2)

2

When t h e t e m p e r a t u r e w a s l o w e r e d t o 700 K and t h e s a m p l e e x p o s e d to 0 , t h e o b s e r v e d c o m b u s t i o n r a t e was t e n t i m e s h i g h e r t h a n when t h e c a r b o n a t e s were l e f t i n t a c t . By p r o c e s s o f e l i m i n a t i o n , t h e i n c r e a s e d a c t i v i t y was a t t r i b u t e d t o t h e p r e s e n c e o f CaO. I n o r d e r t o f u r t h e r i n v e s t i g a t e this phenomena, t h e same e x p e r i m e n t was c a r r i e d o u t w i t h t h e C-a a n d t h e ANT s a m p l e . T h e C-a s a m p l e w a s c h o s e n due t o t h e f a c t t h a t i t c o n t a i n e d more Ca t h a n char on a m o l a r b a s i s . On t h e o t h e r hand, t h e ANT s a m p l e had a v e r y l o w C a c o n t e n t . F i g u r e 4 shows t h e c o m p a r a t i v e r e s p o n s e s o f t h e r a w t h e r m o g r a v i m e t r i c r e a d i n g s when t h e decomposed s a m p l e s w e r e e x p o s e d t o 1 0 % 0 a t t i m e = 0. S i m i l a r b e h a v i o r w a s o b s e r v e d f o r t h e C - a and PCM s a m p l e s ; t h a t i s , t h e r a w w e i g h t i n c r e a s e d due t o t h e r e c a r b o n a t i o n o f CaO. 2

2

C + 0

C0

2

CaO + C 0

2

(3)

2

**CaC0

3

(4)

Since the combustion rate i s a t l e a s t as f a s t as the recarbonat i o n rate, the data i n F i g u r e 4 correspond t o a combustion r a t e i n c r e a s e o f about an o r d e r o f magnitude i n both samples. This a l s o l e a d s t o t h e c o n c l u s i o n t h a t t h e s o u r c e o f CaO d o e s n o t a p p e a r t o be i m p o r t a n t s i n c e m o s t o f i t i s p r o d u c e d from a n k e r i t i c d o l o m i t e i n t h e C-a s a m p l e a n d o v e r 3 0 % f r o m f r e e c a l c i t e i n t h e PCM sample. I t s h o u l d be p o i n t e d o u t t h a t t h e w e i g h t change i n PCM r e a c h e d a maximum w e i g h t a n d t h e n d e c r e a s e d d u e t o t h e t o t a l r e c a r b o n a t i o n o f t h e a v a i l a b l e CaO p r i o r t o



552


TIME, min Figure 4. Oxidation after thermalpretreatment. Conditions: 700 K, PQ - kPa). 2



31.


Oil Shale Char

553

Combustion

c o m p l e t e c o m b u s t i o n o f t h e c h a r . T h i s was n o t t h e c a s e f o r C-a w h i c h h a s m o r e CaO t h a n 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 n o t e t h a t t h e ANT s a m p l e , w h i c h h a s m i n i m a l C a , d i d n o t e x p e r i e n c e a weight g a i n d u r i n g combustion. I n f a c t , t h e combustion r a t e was i d e n t i c a l t o t h a t o b s e r v e d f o r ANT s a m p l e s w h i c h h a d n o t b e e n thermally pretreated. These r e s u l t s tend t o s u p p o r t t h e h y p o t h e s i s o f CaO a s a c o m b u s t i o n c a t a l y s t . A d d i t i o n a l e x p e r i m e n t s were a l s o r u n i n o r d e r t o examine the e f f e c t s o f w a t e r s o l u b l e m i n e r a l s p e c i e s o n t h e c o m b u s t i o n rate. F i g u r e 5 shows f i r s t o r d e r p l o t s f o r one S-B a n d t w o S-A s a m p l e s . A s p o i n t e d o u t e a r l i e r , t h e S-A a n d S-B s a m p l e s a r e s i m i l a r except f o r high concentrations o f n a h c o l i t e and dawsonite i n t h e former andi t i s t h i s sample w h i c h had t h e highest combustion a c t i v i t y (Table I I ) . Since i t i s possible t o w a t e r l e a c h s o d i u m m i n e r a l s , t h e S-A s a m p l e w a s s u b j e c t e d t o w a t e r washing p r i o r t o combustion. A f t e r d r y i n g , t h e sample was combusted under i d e n t i c a l c o n d i t i o n s and, a sc a n be seen f r o m F i g u r e 5, t h e c o m b u s t i o n r a t e f o r t h e w a t e r w a s h e d S-A s a m p l e was i d e n t i c a l t o t h a t o b s e r v e d w i t h t h e S-B s a m p l e . I n ordert o d e t e r m i n e t h e e l e m e n t s removed d u r i n g t h e w a t e r leaching p r o c e s s , t h e wash-water was a n a l y z e d using atomic absorption. T a b l e I I I shows t h e r e s u l t s o f t h e s e a n a l y s e s f o r t h e S-A s a m p l e a s w e l l a s f o r t h e S-B a n d GEOK s a m p l e s . A s e x p e c t e d , t h e S-A l e a c h a t e was e x t r e m e l y h i g h i n Na. The f a c t t h a t n e i t h e r C a n o r F e w a s l e a c h e d f r o m t h e S-A s a m p l e a n d t h a t t h e GEOK s a m p l e showed no change i n c o m b u s t i o n r a t e d e s p i t e s i m i l a r K v a l u e s p o i n t s t o t h e p r o b a b l e r o l e o f Na a s a n o x i d a t i o n c a t a l y s t . T h i s i s n o t t o o s u r p r i s i n g s i n c e i t i s w e l l known t h a t t h e Group I-a and, t o a l e s s e r e x t e n t , Group I l - a e l e m e n t s a r e good g a s i f i c a t i o n c a t a l y s t s (11). Table I I I .

E l e m e n t a l A n a l y s i s o f Leachate Water

Concentrations SHALE S-A S-B GEOK

Na 21.7 0.15 0.93

mg/g S p e n t S h a l e K

Ca

Fe

0.24 0.05 0.23

0.0 0.49 1.90

0.0 0.0 0.0

An a t t e m p t w a s a l s o made t o f o r m s i l i c a t e s b y a l l o w i n g t h e mineral carbonates t o react w i t h the s i l i c a present i n the shale to see i f c o m b u s t i o n a c t i v i t y would drop. U n f o r t u n a t e l y i t was necessary t o m a i n t a i n a C 0 atmosphere during s i l i c a t i o n ( t o p r e v e n t t h e more f a v o r a b l e f o r m a t i o n o f CaO) w h i c h p e r m i t t e d t h e char t o undergo C 0 g a s i f i c a t i o n . 2

2



554


I I

I 2

I 4

I 6

I 8

I 10

1 12

TIME, min Figure 5. Effect of sodium removal on char combustion. Conditions: 700 K, Po - 10 kPa). 2


31.

Oil Shale Char


C0

2

Combustion

555

+ C^2C0

As a r e s u l t t h e r e was n o t enough c h a r r e m a i n i n g combustion t e s t s .

(5) to

conduct


Conclusions On t h e b a s i s o f t h e s t u d i e s c o n d u c t e d h e r e , i t i s r e a d i l y apparent t h a t the presence o f m i n e r a l s c a n d r a s t i c a l l y a l t e r t h e r e a c t i v i t y o f t h e r e s i d u a l char on spent o i l s h a l e . More detailed quantitative studies o f them i n e r a l compositions a r e n e c e s s a r y i n o r d e r t o be a b l e t o a s s e s s t h e i r i m p o r t a n c e u n d e r t y p i c a l o i l s h a l e p r o c e s s i n g c o n d i t i o n s a n d w i l l be t h e s u b j e c t of f u t u r e m a n u s c r i p t s from t h i s l a b o r a t o r y . However, a t t h i s t i m e , t h e r e a r e s e v e r a l c o n c l u s i o n s w h i c h c a n be made. First, the combustion o f the char i n a l l s i x o f the s h a l e s f o l l o w e d f i r s t o r d e r k i n e t i c s w i t h r e s p e c t t o the oxygen p a r t i a l p r e s s u r e and t h e c h a r a v a i l a b l e . For thewestern shales this i s i n a g r e e m e n t w i t h p r e v i o u s w o r k s w h i c h s t u d i e d PCM a n d A n v i l P o i n t s s h a l e s , b u t i t does c o n f l i c t w i t h the r e s u l t s o f Rostam-Abadi and M i c k e l s o n (9) who r e p o r t e d s e c o n d o r d e r k i n e t i c s f o r A n t r i m shale. S e c o n d l y , we f o u n d t h a t CaO h a s a c a t a l y t i c e f f e c t o n c h a r c o m b u s t i o n , most l i k e l y due t o a c h e m i s o r p t i o n p r o c e s s . A n d f i n a l l y we f o u n d t h a t N a 0 , a s d e r i v e d f r o m t h e t h e r m a l d e c o m p o s i t i o n o f n a h c o l i t e , has a pronounced c a t a l y t i c e f f e c t on t h e c h a r c o m b u s t i o n r a t e o f s a l i n e zone s h a l e . 2

Acknowledgment T h i s w o r k w a s s u p p o r t e d b y t h e U. S. D e p a r t m e n t o f E n e r g y u n d e r C o n t r a c t No. DE-AM06-76RLO2221.

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

Hall, R. N. paper presented at National AIChE Meeting, Anaheim, CA., June 8, 1982. Griswold, C. and Duir, J. H., paper presented at National AIChE Meeting, Anaheim, CA., June 8, 1982. "Union Claims Boost in Shale-oil Technology", 1974, 24, 267. Campbell, J. H. Lawrence Livermore Laboratory, Rept. No. UCRL-52089 part 2, March 13, 1978. Mavis. J. D. and Rosain, R. M. paper presented at National AIChE Meeting, Detroit, MI., August 16-19, 1981. Soni, Y. and Thomson, W. J. I & EC Proc. Des. & Dev, 1979, 18, 661-7. Thompson L. G. and Thomson, W. J. in this SYMPOSIUM SERIES. Soni, Y. and Thomson, W. J. Proceedings of 11th Oil Shale Symposium, p. 364, April 1978.


556


9. Rostam-Abadi, M. and Mickelson, R. W. paper presented at National AICHE Meeting, Anaheim, CA, June 7-10 1982. 10. Sohn, H. Y. and Kim, S. K. I & EC Proc. Des. & Dev, 1980, 19, 550-5. 11. Wen, W. Y. Catal. Rev.-Sci. Eng. 1980, 22, 1.


RECEIVED April 18, 1983


Effect of Mineral Species on Oil Shale Char Combustion - American

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