28 Mineral Matter Catalysis of Coal Conversion Thomas D . Padrick and Barry Granoff
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Sandia National Laboratories, Albuquerque, NM 87185
Since the 1920s, a variety of studies have investigated the effects of mineral matter on coal conversion processes. This interest accelerated greatly about 10 years ago. We have followed the development of this field from those investigations that screened the effects of various mineral types on coal conversion processes to those investigations that studied the detailed mechanism of a specific mineral on either coal gasification or coal liquefaction. As an outgrowth of this field of study, we have witnessed the development of both catalytic coal gasification and slurry phase catalytic coal liquefactions. We will review the field of mineral matter effects on coal processing and discuss the impact on the coal conversion industry.
In general, a c a t a l y s t c o n s i s t s of a support (alumina, s i l i c a , s i l i c a - a l u m i n a ) , a n a c t i v e c a t a l y t i c m e t a l and, i n some c a s e s , a promoter ( 1 ) . The s u p p o r t i s u s u a l l y a h i g h - s u r f a c e - a r e a (up t o s e v e r a l hundred square meters p e r gram) porous s o l i d . Gamma a l u m i n a , f o r example, has a s u r f a c e a r e a o f 100-300 n r V g . The s u p p o r t i s n o t n e c e s s a r i l y i n e r t and may p l a y a s i g n i f i c a n t r o l e i n c h e m i s o r p t i o n and o x i d a t i o n s t a t e c o n t r o l . The a c t i v e m e t a l , which may be d e p o s i t e d by s e v e r a l t e c h n i q u e s , i s h i g h l y d i s p e r s e d on t h e s u p p o r t . The promoter i s an a d d i t i v e t h a t c a n i n c r e a s e t h e a c t i v i t y o f t h e m e t a l and/or m a i n t a i n t h e p h y s i c a l c h a r a c t e r i s t i c s of the support. A t y p i c a l example i s Co-Mo/AI2O3, which i s used as a hydrodesulfurization catalyst. I n t h i s case, alumina i s t h e s u p p o r t , Mo i s t h e c a t a l y t i c a l l y a c t i v e m e t a l and Co i s t h e promoter. I n a c t u a l p r a c t i c e , t h e s e c a t a l y s t s must f i r s t be s u l f i d e d to convert the metals t o metal s u l f i d e s . I t i s believed that these metal s u l f i d e s a r e the c a t a l y t i c a l l y a c t i v e species f o r hydrodesulfurization (2). The need f o r c l e a n l i q u i d f u e l s from c o a l has l e d t o t h e development o f c a t a l y s t s t h a t had o r i g i n a l l y been used f o r 0097-6156/86/ 0301 -0410S06.00/ 0 © 1986 American Chemical Society
In Mineral Matter and Ash in Coal; Vorres, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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28.
PADRICK AND
GRANOFF
Mineral
Matter
Catalysis of Coal Conversion
411
p e t r o l e u m r e f i n i n g . In d i r e c t c o a l l i q u e f a c t i o n , p u l v e r i z e d c o a l i s mixed w i t h a c o a l - d e r i v e d s o l v e n t and t r e a t e d w i t h hydrogen a t h i g h t e m p e r a t u r e s (400-450°C) and p r e s s u r e s (1500-3000 p s i ) Ç 3 ) . At some s t a g e i n the c o n v e r s i o n p r o c e s s , h y d r o t r e a t i n g c a t a l y s t s a r e used t o improve the d i s t i l l a t e y i e l d , i n c r e a s e the hydrogen c o n t e n t and reduce the heteroatom (S, N, 0) c o n t e n t of the l i q u i d products. Since coals contain a l u m i n o s i l i c a t e s ( c l a y s ) , metal s u l f i d e s ( e . g . , p y r i t e ) and numerous m e t a l - c o n t a i n i n g s p e c i e s ( 4 ) , i t would seem r e a s o n a b l e t o propose t h a t a n a t u r a l l y o c c u r r i n g c a t a l y s t system e x i s t s w i t h i n the i n h e r e n t m i n e r a l m a t t e r I n c o a l . This c o n c e p t has been e x p l o r e d s i n c e the 1920's and s e v e r a l studies have f o c u s e d on the c a t a l y t i c e f f e c t s o f m i n e r a l m a t t e r on c o a l conversion (5). In r e c e n t y e a r s , we have w i t n e s s e d an i n c r e a s e i n the l e v e l o f c o a l r e s e a r c h and the development o f new c o a l u t i l i z a t i o n processes. In p a r a l l e l w i t h t h i s a c t i v i t y , t h e r e have been r e p o r t s on the e f f e c t s o f c o a l m i n e r a l s on c o a l l i q u e f a c t i o n , c o a l g a s i f i c a t i o n , i n - s i t u c o a l g a s i f i c a t i o n , and o t h e r a r e a s o f c o a l u t i l i z a t i o n . ( 5 - 8 ) . The terms m i n e r a l s , m i n e r a l m a t t e r , and a s h w i l l be used synonymously. An attempt w i l l not be made t o r e v i e w the e f f e c t s o f a l l c l a s s e s o f m i n e r a l s , but w i l l o n l y c o n s i d e r t h o s e m i n e r a l s which have shown a l a r g e e f f e c t on c o a l conversion processes.
Coal
Liquefaction
The Germans used c o a l l i q u e f a c t i o n on a commercial s c a l e from 1930 t o the end o f the second World War. They found t h a t a c a t a l y s t c o u l d enhance l i q u i d y i e l d s and h e l p remove h e t e r o a t o m s . The B e r g i u s p r o c e s s used an i r o n oxide-aluminum c a t a l y s t a t a 2-3% by c o a l weight c o n c e n t r a t i o n . I n r e c e n t y e a r s , i t has been r e a l i z e d t h a t m i n e r a l m a t t e r p l a y s an i m p o r t a n t r o l e i n c o a l l i q u e f a c t i o n ( 9 - 1 1 ) , s i m i l a r t o the r o l e of the added c a t a l y s t i n the B e r g i u s p r o c e s s . Several e x p e r i m e n t a l t e c h n i q u e s have been used t o s t u d y the e f f e c t s o f m i n e r a l s on c o a l l i q u e f a c t i o n and t o i d e n t i f y the s p e c i f i c c a t a l y t i c phase ( 1 2 ) . Most s t u d i e s (12-14) s t r o n g l y imply t h a t the i r o n s u l f i d e s a r e the most a c t i v e s p e c i e s , and the o t h e r m i n e r a l s appear to have l i t t l e e f f e c t on enhancement o f l i q u i d y i e l d or q u a l i t y . The s p e c i f i c r o l e o f p y r i t e (FeS2) as a c a t a l y s t has been under i n v e s t i g a t i o n s i n c e p y r i t e was i d e n t i f i e d as the most a c t i v e inherent mineral for coal l i q u e f a c t i o n . Under l i q u e f a c t i o n c o n d i t i o n s , FeS2 i s t r a n s f o r m e d i n t o a n o n s t o i c h i o m e t r i c i r o n s u l f i d e , F e i - x S (0
