2 Prospects for Coal Conversion i n Canada N. BERKOWITZ
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Research Council of Alberta, 11315 87th Avenue, Edmonton, Alberta, Canada T 6 G 2C2
Since the late 1960s, and more especially since 1973, when OPEC policies reversed fossil fuel pricing patterns that had virtually eliminated i t as a major component of the Canadian energy economy, coal has not only regained substantial footholds in industrial fuel markets, but also attracted increasingly serious attention as a key resource from which, in future, more diverse energy demands could be met. The beginnings of this renaissance can be traced to the early 1960s, when electric u t i l i t i e s in Alberta and Saskatchewan found i t economically more advantageous to burn surface-mined coal in mine-mouth generating facilities than to fuel power stations in load-centres with natural gas; and by the early 1970s, these advantages, in conjunction with developing concerns about future natural gas prices, proved so persuasive that Alberta adopted coal-firing of new base-load thermal plants as provincial policy. But wider appreciation of benefits from greater reliance on coal came only with sharply escalating oil and gas prices, and with the recognition that known reserves of oil and gas in the Western Canadian sedimentary basin will not sustain demands for oil and natural gas beyond the late 1980s. These factors are now tending to accelerate re-entry of coal into some of its former traditional Western Canadian markets, and beginning to remove obstacles to the use of coal where coal costs are very much higher than in the prairie provinces: Nova Scotia, where steps are now being taken to reduce dependence on offshore fuel oils by greater utilization of indigeous underground-mined coal, is a case in point. Even i f current (per million btu) price differentials between coal and gas (or oil) do not widen much further, such direct substitution of coal for other hydrocarbon fuels is certain to become increasingly attractive - and may, indeed, prove imperative in the national interest. But in the long run equally important is that technological advances, coupled with the abundance and projected cost of Western Canadian coal, now make i t possible to contemplate large-scale conversion of coal into gaseous and/or This chapter not subject to U.S. Copyright. Published 1979 American Chemical Society. Tomlinson et al.; Chemistry for Energy ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
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CHEMISTRY FOR ENERGY
l i q u i d h y d r o c a r b o n s , and t h e r e b y augment p r o d u c t i o n from remote a r c t i c r e g i o n s a n d m a n u f a c t u r e o f " s y n t h e t i c s " w h i c h w i l l be needed t o o f f s e t d i m i n i s h i n g s u p p l i e s o f c o n v e n t i o n a l o i l and gas f r o m more " t r a d i t i o n a l " s o u r c e s . The C h e m i s t r y o f C o a l
Conversion
In c h e m i c a l t e r m s , t h e s i m p l e s t c o n v e r s i o n t e c h n i q u e i s t h e t r a n s f o r m a t i o n o f c o a l i n t o a c o m b u s t i b l e gas by gasification. i n i t s e a r l i e s t form - i n t r o d u c e d i n B r i t a i n c a . i860 by S i r W i l l i a m Siemens - t h i s i n v o l v e d g e n e r a t i o n o f a p r o d u c e r g a s , m a i n l y a m i x t u r e o f CO, C 0 and n i t r o g e n , by i n c o m p l e t e combust i o n o f c o a l i n a i r , i . e . , by
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2
C + 02
•
C0
£
. . .
(i)
C0
•
2C0
. . .
( i i )
f o l lowed by + C
2
The y i e l d a n d / o r h e a t v a l u e o f t h i s gas was l a t e r c o - g e n e r a t i o n o f a s o c a l l e d w a t e r gas v i a + H20
C
•
CO + H
improved
. . .
2
by
(iiî)
by a l t e r n a t i n g ( c y c l i c a l ) i n j e c t i o n o f a i r and s t e a m i n t o t h e b u r n i n g f u e l bed. In modern g a s i f i c a t i o n p r a c t i c e , p r i n c i p a l r e l i a n c e i s p l a c e d on t h e c a r b o n - s t e a m r e a c t i o n ( i i i ) w h i c h , d e p e n d i n g on t h e mode o f o p e r a t i o n o f t h e r e a c t o r , may be v a r i o u s l y accomp a n i e d by t h e " s h i f t " r e a c t i o n CO + H 2 0 as w e l l
as by c a r b o n
•
CO 2 +
•
CH^
H
2
2
and t h e r m a l c r a c k i n g o f p y r o l y t i c a l 1 y g e n e r a t e d f o r m a l l y r e p r e s e n t e d by
H
m n
(iv)
. . .
(v)
hydrogénation
C + 2H
C
. . .
"
H
+
^
volatile
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matter,
(
v
i
)
;
and p a r t i a l c o m b u s t i o n s e r v e s p r i m a r i l y as a s o u r c e o f p r o c e s s heat. In some e x p e r i m e n t a l s y s t e m s , c o m b u s t i o n i n t h e g a s i f i e r i s a c c o r d i n g l y r e p l a c e d by an e x t e r n a l l y g e n e r a t e d l i q u i d { ] ) o r s o l i d (2) h e a t - c a r r i e r w h i c h i s c i r c u l a t e d t h r o u g h t h e g a s i f i e r . Where s u c h e x t e r n a l h e a t - c a r r i e r s a r e e m p l o y e d , and t h e c o a l i s g a s i f i e d by i n j e c t i o n o f s t e a m o n l y , i t i s o b v i o u s l y immateri a l w h e t h e r a i r o r o x y g e n i s used f o r g e n e r a t i o n o f t h e h e a t
Tomlinson et al.; Chemistry for Energy ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
2.
BERKOWITZ
Coal
Conversion
in
13
Canada
source. B u t when p r o c e s s h e a t i s s u p p l i e d by p a r t i a l c o m b u s t i o n i n t h e g a s i f i e r , t h e c h o i c e between a i r and o x y g e n i s c r i t i c a l l y important: I f c o m b u s t i o n i s s u s t a i n e d w i t h a i r , t h e p r o d u c t gas w i l l c o n t a i n a l a r g e p r o p o r t i o n o f n i t r o g e n and c o n s e q u e n t l y o n l y have a h e a t v a l u e o f Ή 2 0 - 1 5 0 / b t u / s c f ( ^ ί . 5 - 5 . 5 MJ/m ) , w h i l e c o m b u s t i o n w i t h oxygen w i l l y i e l d a gas t h a t t y p i c a l l y c o n t a i n s ^80 v / v p e r c e n t CO + H a n d , d e p e n d i n g on t h e a c t u a l C0:H2 r a t i o , p o s s e s s e s a h e a t v a l u e o f 2 7 0 - 3 5 0 b t u / s c f (^10-13 M J / m ) . T h i s d i f f e r e n c e b e a r s d i r e c t l y and i m p o r t a n t l y o n how t h e p r o d u c t gas c a n be u s e d . Because o f t h e i m p r a c t i c a b i l i t y o f s e p a r a t i n g n i t r o g e n f r o m i t , t h e low-btu gas can o n l y by d e p l o y e d as an i n d u s t r i a l f u e l ; and s i n c e i t r e q u i r e s a l a r g e r c o m b u s t i o n s p a c e t h a n a r i c h e r g a s , t h e f a c i l i t i e s i n w h i c h i t c o u l d be used must be s p e c i f i c a l l y d e s i g n e d f o r i t . In c o n t r a s t , a medium-btu gas f r o m o x y g e n - b l o w n r e a c t o r s c a n be accommodated i n e x i s t i n g i n s t a l l a t i o n s w i t h o n l y m i n o r b u r n e r - t i p a d j u s t m e n t s ; and a f t e r c l e a n - u p ( t o remove CO2, H 2 S , COS, e t c . ) and c o r r e c t i o n o f t h e C0:H2 r a t i o t o t h e s t o i c h i o m e t r i c v a l u e s needed f o r d o w n s t r e a m p r o c e s s i n g , i t a l s o o f f e r s a p e t r o c h e m i c a l f e e d s t o c k ( o r "syngas") f u l l y e q u i v a l e n t t o t h o s e now most o f t e n made by p a r t i a l o x i d a t i o n ( o r " r e f o r m i n g " ) o f n a t u r a l gas o r n a p h t h a . A d j u s t m e n t o f t h e C0:H2 r a t i o i s e f f e c t e d by t h e s h i f t r e a c t i o n ( i v ) w h i c h p r o c e e d s o v e r a chromium-promoted i r o n c a t a l y s t a t 700-800°F ( 3 7 0 - 4 2 5 ° C ) o r o v e r a r e d u c e d c o p p e r / z i n c c a t a l y s t a t 375- *50°F ( 1 9 0 - 2 3 0 ° C ) ; and t h e f r a c t i o n o f c r u d e gas s e n t t h r o u g h t h e s h i f t r e a c t o r i s c a l c u l a t e d f r o m t h e i n i t i a l gas com p o s i t i o n and s p e c i f i c d o w n s t r e a m r e q u i r e m e n t s . The l a t t e r a r e îIlustrated by 3
2
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3
2
(a)
m e t h a n a t i o n , i . e . CO + 3 H 2 — • CHi+ + produce a h i g h - b t u " s u b s t i t u t e n a t u r a l w i t h 9^0+ b t u / s c f (^35+ M J / m ) ;
H2O, u s e d t o g a s " (SNG)
3
(b) m e t h a n o l (c)
synthesis,
i . e . CO + 2H2
• C H 3 O H ; and
ammonia s y n t h e s i s , i . e . N2 + 3 H 2 — • 2NH3, i n wh i c h c a s e a l l CO i s a b s t r a c t e d f r o m t h e s y n g a s .
For p r o d u c t i o n o f l i q u i d h y d r o c a r b o n s and o x y g e n a t e d compounds o t h e r t h a n m e t h a n o l , s h i f t i n g i s u s u a l l y c a r r i e d t o C O : H r a t i o s i n t h e r a n g e 1.8-2.4 and u s e i s made o f v a r i a n t s o f Fischer-Tropsch synthesis (3). Gas c l e a n i n g b e f o r e a n d / o r a f t e r s h i f t i n g i s a c c o m p l i s h e d by a b s o r b i n g a c i d gases i n , e.g., h o t aq. c a r b o n a t e , a q . m e t h y l ami n o - p r o p i o n i c a c i d , d i m e t h y l - a m i n o - a c e t i c a c i d , mono- o r d i e t h a n o l a m i n e , d i m e t h y l - e t h e r s o f p o l y e t h y l e n e g l y c o l o r methanol ( a t between - 1 8 ° and - 6 2 ° C ) . P r o p r i e t a r y t e c h n i q u e s e m p l o y i n g these ( o r o t h e r ) a d s o r b e n t s a r e b e i n g r o u t i n e l y used i n n a t u r a l gas p r o c e s s i n g and c a n r e d u c e r e s i d u a l c o n c e n t r a t i o n s o f CO2 a n d H2S t o w e l l b e l o w 20 ppm a n d 5 ppm r e s p e c t i v e l y . 2
Tomlinson et al.; Chemistry for Energy ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
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CHEMISTRY FOR ENERGY
The c h e m i s t r y o f coal liquefaction is less well understood; and how s t u d i e s o f t h i s m a t t e r a r e i n t e r p r e t e d depends t o some e x t e n t on how t h e " m o l e c u l a r " s t r u c t u r e o f c o a l i s p e r c e i v e d . What i s e v i d e n t i s t h a t i n t r o d u c t i o n o f s u f f i c i e n t h y d r o g e n i n t o c o a l t o r a i s e i t s a t o m i c H/C r a t i o f r o m M).65 t o >1.0 (in " p r i m a r y " c o a l l i q u i d s t h a t can be u p g r a d e d by common r e f i n e r y procedures) i s only p o s s i b l e in a r e l a t i v e l y narrow temperature r a n g e c e n t e r e d on 800°F (425°C). O n l y i n t h i s r a n g e does a c t i v e t h e r m a l d e c o m p o s i t i o n g e n e r a t e " f r e e r a d i c a l s " t h a t can be s t a b i l i z e d by h y d r o g e n a d d i t i o n b e f o r e t h e y r a n d o m l y r e p o l y m e r i z e o r crack to e x t i n c t i o n . At s u b s t a n t i a l l y lower temperatures, a d d i t i o n o f h y d r o g e n - e.g., by r e a c t i n g c o a l w i t h l i t h i u m i n e t h y 1 e n e d i a m i n e a t 90-100°C \k) - s u c c e e d s o n l y i n i n c r e a s i n g t h e s o l u b i l i t y o f t h e c o a l i n a m i n e - t y p e s o l v e n t s , e v e n though as many as 55 H atoms p e r 100 C atoms can be added i n t h i s manner; and a t t e m p e r a t u r e s much a b o v e 850°F (450°C), r a p i d c o n c u r r e n t c a r b o n i z a t i o n (and c o n s e q u e n t a r o m a t i z a t i o n ) o f t h e c o a l makes h y d r o génation p r o g r e s s i v e l y more d i f f i c u l t . C o n f i r m a t i o n t h a t c o n v e r s i o n o f c o a l i n t o l i q u i d s depends on l i m i t e d p y r o l y t i c d i s r u p t i o n o f c o a l " m o l e c u l e s " and on prompt s t a b i l i z a t i o n o f t h e r e s u l t a n t f r a g m e n t s by h y d r o g e n i s p r o v i d e d by l i q u e f a c t i o n i n a hydrogen-donor w h i c h a l l o w s such r e a c t i o n s as
H
H
H
H
K i n e t i c s t u d i e s (5) o f s u c h s y s t e m s i n d i c a t e t h a t t h e i n i t i a l s t a g e s o f l i q u e f a c t i o n i n v o l v e c o n v e r s i o n o f t h e c o a l i n t o a more o r l e s s c o m p l e t e l y p y r i d i n e - s o l u b l e s o l i d and t h e r e a f t e r i n t o a benzene-soluble material which i s gradually transformed i n t o a v i s c o u s l i q u i d a s i n c r e a s i n g amounts o f h y d r o g e n combine w i t h i t . T h i s p r o c e s s can be c a t a l y z e d b y , e.g., c o b a l t m o l y b d a t e , b u t p r o c e e d s r a p i d l y even i n t h e a b s e n c e o f c a t a l y s t s . A t 775°F (400°C), t o t a l p y - s o l u b i 1 i t y (and ^60 p e r c e n t s o l u b i l i t y i n benz e n e ) can be a t t a i n e d w i t h i n l e s s t h a n 10 m i n u t e s . A n o t a b l e f e a t u r e o f 1 i q u e f a c t i o n i n Η-donor s y s t e m s i s t h a t t h e e f f e c t i v e l i f e o f t h e d o n o r can be s u b s t a n t i a l l y p r o l o n g e d by c o n d u c t i n g the r e a c t i o n i n the presence o f m o l e c u l a r hydrogen. But i t i s n o t y e t c l e a r w h e t h e r th i s e f f e c t s terns f r o m d i r e c t hy drogénation o f t h e c o a l by H (and f r o m c o n s e q u e n t l o w e r demand on t h e d o n o r ) o r f r o m r e - h y d r o g e n a t i o n o f the d o n o r as i t i s s t r i pped o f a v a i l a b l e h y d r o g e n ; and n e i t h e r i s much known a b o u t 2
Tomlinson et al.; Chemistry for Energy ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
2.
BERKOWITZ
Coal
Conversion
in
15
Canada
how t h e c o a l decomposes o r a b o u t what t y p e s o f f r a g m e n t s a r e most amenable t o s t a b i l i z a t i o n by h y d r o g e n . U n c e r t a i n t y about the c h e m i s t r y o f l i q u e f a c t i o n h a s , however, not i n h i b i t e d development o f s e v e r a l "second g e n e r a t i o n " l i q u e f a c t i o n p r o c e s s e s t h a t depend on Η-transfer f r o m a d o n o r ; and a number o f s u c h p r o c e s s e s h a v e i n f a c t r e a c h e d a d v a n c e d s t a g e s o f development. By s i m p l i f y i n g o p e r a t i o n s , t h e s e o f f e r i m p o r t a n t t e c h n i c a l and e c o n o m i c a d v a n t a g e s o v e r c l a s s i c Bergius hydrogénation w h i c h has b e e n used i n Germany a n d B r i t a i n t o m a n u f a c t u r e s y n t h e t i c g a s o l i n e s , d i e s e l f u e l and h e a t i n g o i l s f r o m c o a l a n d c o a l t a r s i n t h e 1930s a n d t h r o u g h o u t W o r l d War I I . The B e r g i u s a p p r o a c h e n t a i l e d t w o - s t e p p r o c e s s i n g , w i t h a c o a l - o i l s l u r r y f i r s t being reacted w i t h H over iron oxide o r N H ^ C l - p r o m o t e d t i n c a t a l y s t s a t J»57-W5°C/25-70 MPa, and t h e r e s u l t i n g " m i d d l e o i l " ( b . p . 180-325°C) t h e n b e i n g u p g r a d e d by v a p o u r - p h a s e hydrogénation o v e r a t u n g s t e n s u l p h i d e c a t a l y s t ( 6 ) . The l i q u e f a c t i o n t e c h n i q u e s now b e i n g d e v e l o p e d r e s e m b l e t h i s f o r m o f hydrogénation i n r e t a i n i n g a t w o - s t e p s e q u e n c e , b u t a r e much more e n e r g y - e f f i c i e n t and a l s o r e t u r n b e t t e r y i e l d s t h r o u g h being less d r a s t i c . The f i r s t s t a g e t y p i c a l l y e n t a i l s r e a c t i o n o f c o a l w i t h H and a d o n o r - u s u a l l y a h y d r o g e n a t e d r e c y c l e o i l a t 3 7 0 - 4 5 0 ° C / 1 0 - l 8 MPa; a n d , i n some v e r s i o n s , t h i s s t a g e p r o v i d e s o p t i o n s f o r p r o d u c i n g solvent-refined coaly i.e.,a substantially m i n e r a l m a t t e r - a n d s u l p h u r - f r e e s o l i d f u e l w h i c h a l s o o f f e r s raw m a t e r i a l f o r m a n u f a c t u r e o f c a r b o n e l e c t r o d e s and o t h e r s p e c i a l t y products. In t h a t c a s e , h y d r o g e n t r a n s f e r t o t h e c o a l i s l i m i t e d to l e v e l s that a l l o w the coal to d i s s o l v e (or d i s p e r s e ) i n the d o n o r f l u i d , b u t do n o t i n d u c e c o n c u r r e n t l i q u e f a c t i o n . The d i s p e r s i o n i s t h e n f i l t e r e d , and t h e s o l u t e i s s e p a r a t e d f r o m s o l v e n t by p r e s s u r e - r e d u c t i o n , d i s t i l l a t i o n , p r e c i p i t a t i o n o r a combination o f these. The c h e m i s t r y o f a t h i r d g r o u p o f c o n v e r s i o n t e c h n i q u e s i . e . , partial conversion methods w h i c h s k i m h y d r o c a r b o n g a s e s a n d / o r l i q u i d s f r o m t h e c o a l and l e a v e a c h a r s u i t a b l e f o r u s e a s a b o i l e r f u e l o r g a s i f i c a t i o n f e e d s t o c k - i s , i f a n y t h i n g , even more s p e c u l a t i v e t h a n t h e c h e m i s t r y o f l i q u e f a c t i o n . E x c e p t f o r s u p e r c r i t i c a l gas e x t r a c t i o n ( s e e b e l o w ) , t h e s e techniques i n v o l v e very r a p i d h e a t i n g o f t h e coal t o temperatures a t w h i c h i t decomposes, a n d u t i l i z e t h e f a c t t h a t t h e c o a l w i l l t h e n g e n e r a t e an amount o f " v o l a t i l e m a t t e r " t h a t f a r e x c e e d s t h e n o m i n a l v o l a t i l e m a t t e r c o n t e n t d e t e r m i n e d by s t a n d a r d a n a l y t i c a l p r o c e d u r e s (])· Under optimum o p e r a t i n g c o n d i t i o n s , y i e l d s o f l i q u i d h y d r o c a r b o n s c a n t h e r e f o r e be p u s h e d much beyond t h o s e a c c r u i n g from c a r b o n i z a t i o n i n coke ovens o r ( c o a l ) - g a s r e t o r t s . For example, w h i l e c o n v e n t i o n a l c a r b o n i z a t i o n ( a t h e a t i n g r a t e s
