Desulfurization of Coals by High-Intensity High-Gradient Magnetic

10 Desulfurization of Coals by High-Intensity High-Gradient Magnetic Separation: Conceptual Process Design and Cost

Downloaded by HARVARD UNIV on June 28, 2014 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0064.ch010

Estimation

C. J. LIN and Y. A. LIU Magnetochemical Engineering Laboratory, Department of Chemical Engineering, Auburn University, Auburn, AL 36830 It is well known that the main difficulty in increasing the utilization of coal in the United States lies in the pollution problem, and emission levels of sulfur oxides and ash particles from coal burning facilities must be reduced to meet stringent environmental standards. Although the particulate emission standards can generally be met by using electrostatic precipitators, there apparently exists no accepted technology for controlling sulfur oxide emissions in flue gases (1). Thus, there has been a growing effort recently to develop effective and economical alternatives to flue gas desulfurization. One of the most attractive alternatives is the precombustion cleaning of coal. Several new physical and chemical methods for removing sulfur and ash from coal prior to its combustion have already been proposed and are currently under intensive further development (2). An important physical method for cleaning coal that appears to hold much promise is the well established magnetic separation technique. Previous experimental investigations have indicated clearly that most of the mineral impurities in coal which contribute to the pyritic sulfur, the sulfate sulfur, and the ash content are paramagnetic. These sulfur-bearing and ash-forming minerals, if sufficiently liberated as discrete particles, can be separated from the pulverized diamagnetic coal by magnetic means (3-8). Indeed, the technical feasibility of the magnetic cleaning of coal has been demonstrated in a number of previous studies, with substantial amounts of sulfur and ash removal reported (9,10). D u r i n g t h e past few y e a r s , the magnetic c l e a n i n g o f c o a l has b e e n g i v e n new i m p e t u s w i t h t h e i n t r o d u c t i o n o f a new l e v e l o f magnetic separation technology, t h e h i g h - i n t e n s i t y h i g h - g r a d i e n t m a g n e t i c s e p a r a t i o n (HGMS). The HGMS t e c h n o l o g y was d e v e l o p e d a r o u n d 1969 f o r t h e wet s e p a r a t i o n o f f e e b l y m a g n e t i c c o n t a m i n a n t s f r o m k a o l i n c l a y ( 1 1 - 1 7 ) . A t y p i c a l HGMS u n i t f o r t h i s wet a p p l i c a t i o n i s shown s c h e m a t i c a l l y a s F i g u r e l a . The e l e c t r o m a g n e t s t r u c t u r e c o n s i s t s o f e n e r g i z i n g c o i l s and a s u r r o u n d i n g i r o n enclosure. The c o i l s i n t u r n e n c l o s e a c y l i n d r i c a l , h i g h l y magn e t i z a b l e w o r k i n g volume p a c k e d w i t h f i n e s t r a n d s o f s t r o n g l y

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


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f e r r o m a g n e t i c p a c k i n g m a t e r i a l such a s f e r r i t i c s t a i n l e s s s t e e l wool. W i t h t h i s d e s i g n , a s t r o n g f i e l d i n t e n s i t y o f 20 kOe c a n be g e n e r a t e d and d i s t r i b u t e d u n i f o r m l y t h r o u g h o u t t h e w o r k i n g v o l u m e . F u r t h e r m o r e , by p l a c i n g i n t h e u n i f o r m m a g n e t i c f i e l d t h e f e r r o m a g n e t i c p a c k i n g m a t e r i a l s w h i c h i n c r e a s e and d i s tort the f i e l d i nt h e i r v i c i n i t y , large f i e l d gradients of the o r d e r o f 1-10 kOe/pmcan be p r o d u c e d . I n t h e wet b e n e f i c i a t i o n o f k a o l i n c l a y , t h e HGMS u n i t i s u s e d i n a b a t c h o r c y c l i c a l l y o p e r a ted p r o c e s s l i k e a f i l t e r . The k a o l i n f e e d c o n t a i n i n g t h e f e e b l y m a g n e t i c c o n t a m i n a n t s i n l o w c o n c e n t r a t i o n s i s pumped t h r o u g h t h e s t a i n l e s s s t e e l wool p a c k i n g o r m a t r i x o f t h e s e p a r a t o r from t h e b o t t o m w h i l e t h e magnet i s on. The m a g n e t i c m a t e r i a l s (mags) a r e c a p t u r e d and r e t a i n e d i n s i d e t h e s e p a r a t o r m a t r i x , and t h e n o n m a g n e t i c components ( t a i l s ) p a s s t h r o u g h t h e s e p a r a t o r m a t r i x and a r e c o l l e c t e d a s t h e b e n e f i c i a t e d p r o d u c t s f r o m t h e t o p o f t h e magnet. A f t e r some t i m e o f o p e r a t i o n , t h e s e p a r a t o r m a t r i x i s f i l l e d t o i t s l o a d i n g c a p a c i t y . The f e e d i s t h e n s t o p p e d , and the s e p a r a t o r m a t r i x i s r i n s e d w i t h water. F i n a l l y , t h e magnet i s t u r n e d o f f , and t h e mags r e t a i n e d i n s i d e t h e s e p a r a t o r m a t r i x a r e backwashed w i t h w a t e r and c o l l e c t e d . The w h o l e p r o c e d u r e i s repeated i n a c y c l i c f a s h i o n . I n general, i f t h i s batch process i s u s e d i n o t h e r wet a p p l i c a t i o n s where t h e m a g n e t i c m a t e r i a l s o c c u p y a l a r g e f r a c t i o n o f t h e f e e d s t r e a m , t h e down t i m e f o r b a c k w a s h i n g w i l l be c o n s i d e r a b l e , p o s s i b l y n e c e s s i t a t i n g t h e u s e o f one o r more b a c k - u p s e p a r a t o r s . To overcome t h i s p r o b l e m w h i c h i s inherent i n batch operations, a continuous process using a m o v i n g m a t r i x HGMS u n i t , c a l l e d t h e C a r o u s e l s e p a r a t o r , h a s been p r o p o s e d ( 1 2 , 15-17) a s shown s c h e m a t i c a l l y i n F i g u r e l b . A numb e r o f p i l o t - s c a l e s t u d i e s o f t h e wet b e n e f i c i a t i o n o f k a o l i n c l a y and i r o n o r e s u s i n g t h e C a r o u s e l s e p a r a t o r have b e e n r e p o r t e d (17). B e c a u s e o f t h e v e r y l o w c o s t s and t h e o u t s t a n d i n g t e c h n i c a l p e r f o r m a n c e o f HGMS d e m o n s t r a t e d i n t h e k a o l i n a p p l i c a t i o n , HGMS was r e c e n t l y a d a p t e d i n a b e n c h - s c a l e e x p l o r a t o r y s t u d y (8) t o remove s u l f u r and a s h f r o m a f i n e l y p u l v e r i z e d B r a z i l i a n c o a l suspended i n w a t e r . S i n c e t h e n , o t h e r i n v e s t i g a t o r s have u t i l i zed p i l o t - s c a l e HGMS u n i t s t o d e s u l f u r i z e and d e a s h w a t e r s l u r r i e s o f some E a s t e r n U.S. c o a l s . F o r i n s t a n c e , r e s u l t s o f p i l o t - s c a l e s t u d i e s that demonstrated t h e t e c h n i c a l f e a s i b i l i t y o f t h e m a g n e t i c s e p a r a t i o n o f s u l f u r and a s h f r o m w a t e r s l u r r i e s o f p u l v e r i z e d I l l i n o i s No. 6, I n d i a n a No. 5 and No. 6, K e n t u c k y No. 9/14, P e n n s y l v a n i a Upper F r e e p o r t and Lower K i t t a n n i n g c o a l s h a v e been r e p o r t e d ( 4 - 7 , 1 8 , 1 9 ) . I n p a r t i c u l a r , the q u a n t i t a t i v e e f f e c t s o f major s e p a r a t i o n v a r i a b l e s such as f i e l d i n t e n s i t y , r e s i d e n c e t i m e , e t c . , o n t h e g r a d e , r e c o v e r y and c o n c e n t r a t i o n breakthrough f o r the magnetic s e p a r a t i o n o f s u l f u r and a s h f r o m a w a t e r s l u r r y o f p u l v e r i z e d I l l i n o i s No. 6 c o a l have been e s t a b l i s h e d e x p e r i m e n t a l l y , and c a n be p r e d i c t e d s a t i s f a c t o r i l y by a n e w l y d e v e l o p e d m a t h e m a t i c a l m o d e l f o r HGMS ( 4 , 5 , 7 , 1 3 , 1 8 ) . D e p e n d i n g upon t h e t y p e s o f c o a l and t h e s e p a r a t i o n


L I N A N D Liu

High-Intensity and -Gradient Magnetic Separation


CLEANED

TAILINGS OUT

Figure 1.

(a) (above) Cyclic high-gradient magnetic separator; (b) (below) carousel high-gradient magnetic separator



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c o n d i t i o n s u s e d , t h e e x i s t i n g b e n c h - a n d p i l o t - s c a l e r e s u l t s have shown t h a t t h e u s e o f s i n g l e - p a s s HGMS was e f f e c t i v e i n r e d u c i n g t h e t o t a l s u l f u r by 4 0 - 5 5 % , t h e a s h by 35-45%, and t h e p y r i t i c s u l f u r b y 80-90%, w h i l e a c h i e v i n g a maximum r e c o v e r y o f a b o u t 95% ( 1 0 , 1 8 ) . These a v a i l a b l e r e s u l t s have i n d i c a t e d a l s o t h a t b o t h t h e g r a d e and r e c o v e r y o f t h e s e p a r a t i o n c a n be enhanced g e n e r a l l y w i t h the u s e o f a l a r g e r s e p a r a t o r m a t r i x o r by t h e recycle of thet a i l products. Further detailed review of the r e p o r t e d r e s u l t s on t h e m a g n e t i c c l e a n i n g o f p u l v e r i z e d c o a l s i n w a t e r s l u r r i e s c a n be found i n t h e l i t e r a t u r e ( 9 , 1 0 , 1 8 ) . These p u b l i s h e d d a t a and o t h e r r e c e n t a n a l y s e s (2,10,18,20^24) have i n d i c a t e d t h a t a s i g n i f i c a n t p o r t i o n o f t h e U.S. c o a l r e s e r v e s , l o w enough i n o r g a n i c s u l f u r , c a n be m a g n e t i c a l l y c l e a n e d f o r u s e as a n e n v i r o n m e n t a l l y a c c e p t a b l e , l o w s u l f u r f u e l . i t has been e s t i m a t e d t h a t 100 m i l l i o n s h o r t t o n s o f U.S. c o a l s p e r y e a r may be m a g n e t i c a l l y c l e a n e d . T h i s amounts t o o v e r 1 7 % o f t h e t o t a l U.S. p r o d u c t i o n p e r y e a r ( 1 0 ) . A l t h o u g h t h e e x i s t i n g d a t a have n o t y e t e s t a b l i s h e d t o t a l d e a s h i n g b y m a g n e t i c means, t h e r e i s some i n d i c a t i o n t h a t b y o p t i m i z i n g t h e s e p a r a t i o n c o n d i t i o n s , and e n h a n c i n g t h e m a g n e t i s m o f a s h - f o r m i n g m i n e r a l s , e t c . , t h e e f f e c t i v e n e s s o f m a g n e t i c s e p a r a t i o n o f a s h f r o m c o a l c a n be i m p r o v e d (10) . R e c e n t s t u d i e s (4,5,9,10,25-27) have a l s o s u g g e s t e d t h a t c o a l c l e a n i n g b y HGMS c o u l d s e r v e a s a s i g n i f i c a n t a d j u n c t t o coal l i q u e f a c t i o n processes. In particular, thetechnical feasib i l i t y o f a d a p t i n g HGMS a s a n e f f e c t i v e a l t e r n a t i v e t o convent i o n a l p r e c o a t f i l t r a t i o n i n t h e s o l v e n t r e f i n e d c o a l (SRC) p r o c e s s h a s a l r e a d y b e e n d e m o n s t r a t e d by a b e n c h - s c a l e , e x p l o r a t o r y s t u d y done a t H y d r o c a r b o n R e s e a r c h , I n c . ( H R I ) . HGMS e f f e c t i v e l y removed up t o 9 0 % o f t h e i n o r g a n i c s u l f u r f r o m t h e l i q u e f i e d SRC f i l t e r f e e d s l u r r y o f I l l i n o i s No. 6 c o a l , and a b o u t h a l f o f t h e e x p e r i m e n t a l r u n s c o n d u c t e d by HRI i n d i c a t e d o v e r 8 7 % i n o r g a n i c s u l f u r removal (6,15,26). I n g e n e r a l , t h e work done by HRI showed t h a t HGMS was l e s s e f f e c t i v e i n a s h r e m o v a l b u t d i d remove 25-35% o f t h e a s h . Q u i t e r e c e n t l y , a p i l o t - s c a l e HGMS s y s t e m t o remove m i n e r a l r e s i d u e f r o m l i q u e f i e d c o a l h a s b e e n d e s i g n e d and c o n s t r u c t e d by t h e a u t h o r s ( 5 , 2 5 ) . Typical results f r o m e x p e r i m e n t s c o n d u c t e d w i t h t h e l i q u e f i e d SRC f i l t e r f e e d s l u r r y o f K e n t u c k y No. 9/14 and I l l i n o i s M o n t e r e y c o a l s have been q u i t e e n c o u r a g i n g , i n d i c a t i n g t h a t HGMS c o u l d r e d u c e t h e t o t a l s u l f u r , a s h and p y r i t i c s u l f u r c o n t e n t s by a s much a s 7 0 , 76, and 95%, r e s p e c t i v e l y . A v a i l a b l e d a t a from the above benchand p i l o t - s c a l e i n v e s t i g a t i o n have showed a l s o t h a t a n even g r e a t e r d e a s h i n g o f t h e l i q u e f i e d SRC f i l t e r f e e d c a n be a c h i e ved b y i m p r o v e d s e p a r a t i o n c o n d i t i o n s . A d e t a i l e d d i s c u s s i o n o f t h e s e r e s u l t s a l o n g w i t h t h e i r t e c h n i c a l i m p l i c a t i o n s c a n be found i n the l i t e r a t u r e (10,25,27). Furthermore, c l o s e examinat i o n o f t h e i n h e r e n t p h y s i c a l and c h e m i c a l c h a r a c t e r i s t i c s o f the hydrogenated products p r i o r t o t h e f i l t r a t i o n step i n the SRC and o t h e r r e l a t e d l i q u e f a c t i o n p r o c e s s e s i n d i c a t e s t h a t HGMS



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may be d e v e l o p e d a s a p r a c t i c a l m i n e r a l r e s i d u e s e p a r a t i o n method. Hydrogénation w i l l g e n e r a l l y r e d u c e a m a j o r p o r t i o n o f t h e p y r i t i c s u l f u r t o t h e h i g h l y m a g n e t i c p y r r h o t i t e ; and t h e s u l f u r b e a r i n g and a s h - f o r m i n g m i n e r a l s t e n d t o be l i b e r a t e d more e a s i l y f r o m t h e d i s s o l v e d o r g a n i c components i n t h e f i l t e r f e e d s l u r r y t h a n f r o m p u l v e r i z e d c o a l suspended i n w a t e r . A l s o , t h e t y p i c a l mean p a r t i c l e s i z e o f t h e f i l t e r f e e d sample i s o f t e n l e s s t h a n 5 m i c r o n s , w h i c h d i c t a t e s t h e u s e o f s e p a r a t i o n methods l i k e HGMS c a p a b l e o f h a n d l i n g m i c r o n - s i z e m a t e r i a l s . A l l o f these f a c t o r s seem t o s u g g e s t a v e r y s i g n i f i c a n t p o t e n t i a l f o r u t i l i z i n g HGMS t o remove m i n e r a l r e s i d u e s f r o m l i q u e f i e d c o a l . F o r c e r t a i n t y p e s o f c o a l , e v e n w i t h o u t f u r t h e r improvement i n t h e m a g n e t i c r e m o v a l o f a s h , m a g n e t i c a l l y c l e a n e d SRC w o u l d be a c c e p t a b l e f o r u s e a s a f e e d t o b o i l e r s w h i c h a l r e a d y have e l e c t r o s t a t i c p r e c i p i t a t o r s (28). T h i s f o l l o w s because t h e c o s t o f s o l i d - l i q u i d s e p a r a t i o n i n c o a l l i q u e f a c t i o n i s g e n e r a l l y subs t a n t i a l , and m o d e r a t e l y l o w - a s h SRC s h o u l d be l e s s e x p e n s i v e (27-29). Indeed, a p r e l i m i n a r y c o s t e s t i m a t e f o r the magnetic d e s u l f u r i z a t i o n o f l i q u e f i e d c o a l b a s e d on t h e l a b o r a t o r y d a t a o b t a i n e d by HRI seems t o s u p p o r t t h i s o b s e r v a t i o n ( 2 7 ) . The p r e c e d i n g d i s c u s s i o n h a s i n d i c a t e d t h a t t h e s c i e n t i f i c and t e c h n i c a l f e a s i b i l i t y o f m a g n e t i c d e s u l f u r i z a t i o n o f b o t h c o a l / w a t e r s l u r r y and l i q u e f i e d c o a l h a s b e e n w e l l established. R e c e n t l y , s e v e r a l e s t i m a t e s o f the c o s t o f m a g n e t i c d e s u l f u r i z a t i o n have b e e n r e p o r t e d i n t h e l i t e r a t u r e ( 8 , 1 4 , 1 9 , 2 2 , 2 7 ) . B e c a u s e o f t h e s i m p l i f y i n g a s s u m p t i o n s i n v o l v e d , as w e l l a s t h e t e c h n i c a l p e r f o r m a n c e s p e c i f i e d and t h e e s t i m a t i o n methods u s e d , t h e r e s u l t s seem t o be somewhat a p p r o x i m a t e i n n a t u r e . In this p a p e r , t h e l a t e s t d a t a f r o m p i l o t - s c a l e s t u d i e s o f s u l f u r and a s h r e m o v a l f r o m b o t h l i q u e f i e d c o a l and c o a l / w a t e r s l u r r y by HGMS a r e used t o d e s i g n c o n c e p t u a l p r o c e s s e s f o r m a g n e t i c d e s u l furization of coal. Estimates o f magnetic d e s u l f u r i z a t i o n chara c t e r i s t i c s and c o n c e p t u a l p r o c e s s r e q u i r e m e n t s a s w e l l a s i n s t a l l a t i o n and p r o c e s s i n g c o s t s a r e p r e s e n t e d . In particular, t h e e x t e n t t o w h i c h t h e p r o c e s s i n g c o n d i t i o n s c a n a f f e c t magnet i c d e s u l f u r i z a t i o n c o s t s i s examined. F i n a l l y , t h e r e s u l t s a r e compared w i t h o t h e r a p p r o a c h e s f o r the d e s u l f u r i z a t i o n o f coal. Magnetic D e s u l f u r i z a t i o n

o f Coal/Water S l u r r y :

P r o c e s s and C o s t s

A c o n c e p t u a l p r o c e s s f o r the magnetic d e s u l f u r i z a t i o n o f p u l v e r i z e d c o a l suspended i n w a t e r by HGMS i s shown s c h e m a t i c a l l y i n F i g u r e 2. A c o a l s l u r r y o f a f i x e d c o n c e n t r a t i o n i s p r e p a r e d f i r s t by m i x i n g known amounts o f p u l v e r i z e d c o a l , w a t e r , and a dispersant (wetting agent) l i k e Alconox. The HGMS u n i t used h e r e i s t h e l a r g e s t c o m m e r c i a l u n i t now i n u s e f o r p r o d u c i n g h i g h q u a l i t y paper c o a t i n g c l a y s . I t i s operated at a f i x e d f i e l d i n t e n s i t y o f 20 kOe g e n e r a t e d i n a n open volume 7 f t i n d i a m e t e r and 20 i n . l o n g . A s t a i n l e s s s t e e l w o o l s e p a r a t o r m a t r i x h a v i n g


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94% v o i d s i s p l a c e d i n t h e open v o l u m e . The c o a l s l u r r y i s pumped t h r o u g h t h e e n e r g i z e d s e p a r a t o r m a t r i x a t a f i x e d r e s i dence time ( f l o w v e l o c i t y ) u n t i l the m a t r i x reaches i t s l o a d i n g capacity. A f t e r r i n s i n g w i t h w a t e r , t h e mags a r e s e n t t o a s e t t l i n g pond o r a c l a r i f i e r t o r e c o v e r w a t e r f o r r e - u s e . The t a i l s a r e c o l l e c t e d , d e w a t e r e d , and d r i e d . The s p e c i f i c a t i o n s o f p r o c e s s s t r e a m s f o r a t y p i c a l c a s e (Case Β i n T a b l e I ) a r e a l s o i n c l u d e d i n F i g u r e 2, and t h e d e t a i l e d o p e r a t i n g c o n d i t i o n s for such a case are g i v e n i n the Appendix. By r e m o v i n g 80-90% o f t h e p y r i t i c s u l f u r m a g n e t i c a l l y and a c h i e v i n g 85-90% r e c o v e r y as was d e m o n s t r a t e d by t h e r e s u l t s o f r e p o r t e d s t u d i e s of magnetic d e s u l f u r i z a t i o n of p u l v e r i z e d c o a l s i n w a t e r s l u r r i e s ( 4 - 1 0 , 1 3 , 1 8 , 1 9 ) , t h e p r o c e s s c a n be u s e d to c l e a n a b o u t o n e - f i f t h o f t h e r e c o v e r a b l e U.S, c o a l s w i t h a low o r g a n i c s u l f u r c o n t e n t o f 0.7-0.9 wt % t o p r o d u c e a n e n v i r o n mentally acceptable f u e l . A d e t a i l e d documentation of the r e s e r v e s and p r o d u c t i o n o f U.S. c o a l s w h i c h may be m a g n e t i c a l l y c l e a n a b l e t o 1 wt% t o t a l s u l f u r a c c o r d i n g t o t h e seam, d i s t r i c t and c o u n t y i n e a c h s t a t e , a l o n g w i t h t h e t o t a l and o r g a n i c s u l f u r c o n t e n t s c a n be f o u n d i n t h e l i t e r a t u r e ( 2 1 ) . H e r e , a r e a s o n a b l e r a n g e o f add-on c o s t s ( e x c l u d i n g t h o s e f o r g r i n d i n g , d e w a t e r i n g and d r y i n g and r e f u s e d i s p o s a l ) i s e s t i mated f o r t h e wet m a g n e t i c c l e a n i n g o f c o a l s l u r r i e s w h i c h have d e s u l f u r i z a t i o n c h a r a c t e r i s t i c s s i m i l a r to those r e p o r t e d i n the r e c e n t s t u d i e s ( 4 - 1 0 , 1 3 , 1 8 , 1 9 ) . The method used t o e s t i m a t e t h e c o s t s o f m a g n e t i c d e s u l f u r i z a t i o n i s b a s e d on t h e t e c h n i q u e u s e d by t h e F e d e r a l Power C o m m i s s i o n S y n t h e t i c G a s - C o a l F o r c e t o e s t i m a t e t h e c o s t o f s y n t h e s i s gas (29, 3 4 ) . The i n v e s t o r c a p i t a l i z a t i o n method u s e d i n t h i s a p p r o a c h i s t h e d i s c o u n t c a s h f l o w (DCF) f i n a n c i n g method w i t h assumed DCF r a t e s o f r e t u r n s u c h as 15% a f t e r t a x e s . T h i s method e s s e n t i a l l y d e t e r m i n e s t h e a n n u a l r e v e n u e d u r i n g t h e p l a n t l i f e w h i c h w i l l g e n e r a t e a DCF equal t o the t o t a l c a p i t a l i n v e s t m e n t f o r the p l a n t . Several m a j o r a s s u m p t i o n s a r e i n c l u d e d i n t h e method ( 2 9 , 3 4 ) : (1) The p l a n t l i f e i s assumed t o be 20 y e a r s w i t h no c a s h v a l u e a t t h e end o f l i f e . (2) A s t r a i g h t - l i n e method i s u s e d t o c a l c u l a t e t h e a n n u a l d e p r e ciation. (3) O p e r a t i n g c o s t s and w o r k i n g c a p i t a l r e q u i r e m e n t s a r e assumed to be c o n s t a n t d u r i n g t h e c o n s t r u c t i o n p e r i o d , and 100% e q u i t y c a p i t a l i s assumed. (4) T o t a l p l a n t i n v e s t m e n t , r e t u r n on i n v e s t m e n t d u r i n g t h e p l a n t l i f e , and w o r k i n g c a p i t a l a r e t r e a t e d as c a p i t a l c o s t s i n year zero (the year ending w i t h the c o m p l e t i o n of s t a r t - u p operations). (5) S t a r t - u p c o s t s a r e t r e a t e d as an e x p e n s e i n y e a r z e r o . (6) 48% f e d e r a l income t a x i s assumed. Based on t h e s e a s s u m p t i o n s , e q u a t i o n s f o r c a l c u l a t i n g t h e u n i t c o s t s ($ p e r t o n o f c o a l p r o c e s s e d ) a r e s u g g e s t e d by t h e p u b l i s h e d documents ( 2 9 , 3 4 ) . They a r e summarized i n t h e A p p e n d i x , i n w h i c h



10.

L I N A N D Liu


t h e d e t a i l e d o p e r a t i n g c o n d i t i o n s and e s t i m a t e d c o s t s f o r a t y p i c a l example shown i n F i g u r e 2 a r e a l s o g i v e n . The c o s t s o f m a j o r i n s t a l l e d equipment and t h e u n i t c o s t s l i s t e d i n t h e A p p e n d i x a r e based o n v a l u e s a s o f June 1976. For instance, the costs of pump and t a n k u s e d were e s t i m a t e d f i r s t a c c o r d i n g t o R e f . 3^5 and t h e n b r o u g h t up t o d a t e by m u l t i p l y i n g by t h e CE p l a n t c o s t index r a t i o o f (205/113.6), w h i l e the c o s t o f t h e i n s t a l l e d HGMS u n i t w i t h a s e p a r a t o r m a t r i x 7 f t i n d i a m e t e r and 20 i n . l o n g was e s t i m a t e d t o be $1.936 m i l l i o n ( 2 6 ) . The e s t i m a t e d c a p i t a l i n v e s t m e n t and u n i t c o s t s f o r f o u r t y p i c a l c a s e s , d e s i g n a t e d a s A-D, a r e summarized i n T a b l e I . S l u r r y v e l o c i t i e s o f 2.61 and 4.0 cm/sec, s l u r r y c o n c e n t r a t i o n s o f 15,25 and 35 wt%, a s w e l l a s s e p a r a t i o n d u t y c y c l e s o f 59,077.9% a r e c o n s i d e r e d . These s e p a r a t i o n c o n d i t i o n s i l l u s t r a t e c l e a r l y t h e e f f e c t s o f s l u r r y v e l o c i t y and c o n c e n t r a t i o n a s w e l l as s e p a r a t i o n d u t y c y c l e . F o r i n s t a n c e , c o m p a r i s o n o f c a s e s A t o C shows t h a t a t t h e same s l u r r y v e l o c i t y and s i m i l a r magnet i c d e s u l f u r i z a t i o n c h a r a c t e r i s t i c s the h i g h e r t h e s l u r r y conc e n t r a t i o n , t h e c h e a p e r w i l l be t h e i n v e s t m e n t and u n i t c o s t s . W h i l e t h i s o b s e r v a t i o n i s t o be e x p e c t e d , t h e r e h a v e been p i l o t s c a l e t e s t s which i n d i c a t e that i n c r e a s i n g the s l u r r y concent r a t i o n o f p u l v e r i z e d I l l i n o i s No. 6 c o a l f r o m 2.57 t o 28.4 w t % d i d n o t a p p r e c i a b l y change t h e g r a d e and r e c o v e r y o f t h e s e p a r a tion. F u r t h e r e f f e c t s o f p r o c e s s i n g c o n d i t i o n s , and o t h e r c o s t f a c t o r s on the u n i t c o s t s a r e i l l u s t r a t e d i n T a b l e I I . I t i s s e e n f r o m t h e t a b l e t h a t b y d o u b l i n g t h e amount o f c o a l p r o c e s s e d p e r c y c l e r e l a t i v e t o a f i x e d amount o f s t a i n l e s s s t e e l w o o l p a c k e d i n t h e s e p a r a t o r m a t r i x , t h e u n i t c o s t c a n be r e d u c e d by a b o u t 15%. T h i s r e s u l t shows t h e i m p o r t a n c e o f t h e s e p a r a t o r m a t r i x l o a d i n g c h a r a c t e r i s t i c s on t h e c o s t s o f m a g n e t i c d e s u l furization. Another f a c t o r w h i c h a f f e c t s the u n i t c o s t s cons i d e r a b l y i s the washing time r e q u i r e d i n a complete s e p a r a t i o n cycle. T h i s i s s e e n by c o m p a r i n g i t e m s 4 and 6 i n T a b l e I I . I n p a r t i c u l a r , t h e computed r e s u l t s i n d i c a t e t h a t d o u b l i n g t h e amount o f wash w a t e r r e q u i r e d o n l y l e a d s t o n e g l i g i b l e i n c r e a s e ( 0 . 2 7 0.60%) i n u n i t c o s t s . However, i f b o t h t h e amount o f wash w a t e r and t h e w a s h i n g t i m e a r e d o u b l e d , t h e u n i t c o s t s a r e i n c r e a s e d by a b o u t 15%. The above o b s e r v a t i o n s c l e a r l y s u g g e s t a n i m p o r t a n t economic i n c e n t i v e f o r f u r t h e r p i l o t - s c a l e i n v e s t i g a t i o n o f t h e s e p a r a t o r m a t r i x l o a d i n g and w a s h i n g c h a r a c t e r i s t i c s i n the magnetic d e s u l f u r i z a t i o n o f coal/water s l u r r y . F i n a l l y , item 7 o f T a b l e I I shows t h a t l a b o r c o s t seems t o be a s i g n i f i c a n t f r a c t i o n o f the u n i t c o s t . F o r t u n a t e l y , i t i s not expected that t h e l a b o r r e q u i r e m e n t w i l l be d o u b l e d i n a c t u a l c o m m e r c i a l p r a c t i c e from the nominal case i n Table I . T h i s f o l l o w s because e x i s t i n g experience i n the commercial c l e a n i n g o f k a o l i n c l a y s by HGMS i n d i c a t e s t h a t t h e l a b o r r e q u i r e m e n t s f o r b o t h o p e r a t i o n and m a i n t e n a n c e a r e m i n i m a l ( 1 1 , 2 2 ) . In Table I I I , the r e p o r t e d c o s t s o f magnetic d e s u l f u r i z a t i o n e x p r e s s e d i n t e r m s o f t h e c a p i t a l and u n i t c o s t s ($ p e r t o n o f


127

128

COAL

Table

1. 2. 3. 4. 5. 6.


7.

I.

Cost o f D e s u l f u r i z a t i o n o f Coal/Water S l u r r y by HGMS U s i n g S e p a r a t o r M a t r i x o f 7 - F t D i a m e t e r and 2 0 - I n L e n g t h

S l u r r y v e l o c i t y (cm/sec) S l u r r y c o n c e n t r a t i o n (wt %) Coal feed rate (ton/hr) C y c l e t i m e (min) D u t y c y c l e (%) Tons o f c o a l p r o c e s s e d per c y c l e U n i t c o s t s ($ p e r t o n coal processed) U

8.

Hi 5 C a p i t a l investment per ton c o a l p r o c e s s e d p e r y e a r ($)

Table I I .

DESULFURIZATION

Case A

Case Β

Case C

Case D

2.61 15 44.77 9.00 77.9

2.61 25 66.13 6.10 67.4

2.61 35 83.07 4.85 59.0

4.0 25 89.61 4.50 59.6

6.7

6.7

6.7

6.7

2.083

2.369

1.109

1.067

1.802

1.063

0.858

0.829

3.676

2.412

1.967

1.880

6.93

4.69

3.73

3.53

S e n s i t i v i t y A n a l y s i s o f U n i t C o s t s ($ P e r Ton C o a l P r o c e s s e d ) o f D e s u l f u r i z a t i o n o f C o a l / W a t e r S l u r r y by HGMS U $

1. B a s i s : 2.61 cm/sec, 25 wt % s l u r r y , and o t h e r c o n d i t i o n s i n t h e A p p e n d i x and T a b l e I 2. Amount o f c o a l p r o c e s s e d p e r c y c l e doubled 3. 25% r e d u c t i o n i n c a p i t a l investment 4. Amount o f wash w a t e r r e q u i r e d doubled (washing time un changed) 5. C o s t o f w a t e r i n c r e a s e d 5/3 t i m e s (5C/1000 g a l ) 6. B o t h amounts o f wash w a t e r and w a s h i n g t i m e d o u b l e d 7. L a b o r r e q u i r e m e n t d o u b l e d

—ο

% Change

$

% Change

1. 0628

0.00

2.4117

0.00

0. 9004

-15.28

2.0341

-15.66

0. 9389

-11.66

1.9506

-19.12

1. 0691

+0.60

2.4181

+0.27

1. 0835

+1.95

2.4324

+0.86

1. 2256 1. 3587

+15.32 +27.82

2.7883 2.7077

+15.62 +19.12



10.

L I N A N D Liu

High-Intensity

and -Gradient

Magnetic

Separation

129

c o a l p r o c e s s e d ) a r e compared w i t h t h e r e s u l t s o f t h i s s t u d y . The c o s t s g i v e n by M u r r a y (19) were b a s e d o n t h e e x i s t i n g c o s t e s t i mate f o r k a o l i n b e n e f i c i a t i o n by HGMS g i v e n i n R e f . 1 1 . F o r a r e s i d e n c e t i m e o f 0.5 m i n , t h e c o a l f e e d r a t e t o a c o m m e r c i a l HGMS u n i t w i t h a s e p a r a t o r m a t r i x 7 f t i n d i a m e t e r and 20 i n . l o n g was s e t a t 100 t o n s p e r h r by M u r r a y . T h i s r a t e a p p e a r s t o be h i g h e r t h a n t h a t e x p e c t e d i n c o m m e r c i a l p r a c t i c e . I n a d d i t i o n , t h e c o s t s o f l a b o r and m a i n t e n a n c e p e r HGMS u n i t were e s t i m a t e d by M u r r a y t o be $1 and $2 p e r h r , r e s p e c t i v e l y . These c o s t s a l s o a p p e a r t o be l o w e r t h a n t h o s e r e p o r t e d i n R e f . _11. C o n s e q u e n t l y , t h e c o s t s e s t i m a t e d by M u r r a y shown i n T a b l e I I I e s p e c i a l l y t h e u n i t c o s t U^, ($0.37 p e r t o n p r o c e s s e d ) , a r e b e l i e v e d t o be l o w e r t h a n t h e a c t u a l c o s t s . N e x t , w h i l e t h e c o s t s e s t i m a t e d by Oder (22) seem t o be r e l a t i v e l y c o m p a r a b l e to those obtained i n t h i s study, i t i s d i f f i c u l t t o i d e n t i f y c l e a r l y t h e d i f f e r e n c e s between the two e s t i m a t e s . T h i s f o l l o w s because s p e c i f i c d e t a i l s r e g a r d i n g the c o s t s o f major i n s t a l l e d e q u i p m e n t , c y c l e t i m e , a n d w a s h i n g t i m e , e t c . , were n o t r e p o r t e d i n R e f . _22. F i n a l l y , t h e c o s t s e s t i m a t e d by T r i n d a d e (8) a r e a l s o b e l i e v e d t o be l o w e r t h a n t h e a c t u a l c o s t s . I n t h e c o s t e s t i m a t e b y T r i n d a d e , a C a r o u s e l s e p a r a t o r was u s e d a s a b a s i s f o r t h e e s t i m a t i o n , a l t h o u g h t h e r e h a v e n o t y e t been a n y t e s t data r e p o r t e d on the magnetic d e s u l f u r i z a t i o n o f c o a l / w a t e r s l u r r y u s i n g a C a r o u s e l s e p a r a t o r . O n l y t h e s e p a r a t o r c o s t was i n c l u d e d i n t h e c a p i t a l c o s t i n t h e a n a l y s i s b y T r i n d a d e , and i t was a b o u t o n e - h a l f o f t h e c o s t o f i n s t a l l i n g a n e q u i v a l e n t c y c l i c HGMS u n i t . This l e d t o the r e l a t i v e l y low c a p i t a l investment p e r t o n o f c o a l p r o c e s s e d ( $ 0 . 8 2 - $ l . 6 4 ) e s t i m a t e d by T r i n d a d e as shown i n T a b l e I I I . The c o s t e s t i m a t i o n method u s e d by T r i n dade g e n e r a l l y l e a d s t o l o w e r u n i t c o s t s . F o r i n s t a n c e , b y u s i n g T r i n d a d e s method, t h e u n i t c o s t Up o b t a i n e d i n t h i s work f o r a s l u r r y v e l o c i t y o f 2.61 cm/sec shown i n T a b l e I I I w i l l be r e d u c e d f r o m $1.06 t o $0.85 p e r t o n o f c o a l p r o c e s s e d . 1

A c o m p a r i s o n o f a p p r o x i m a t e e s t i m a t e d c a p i t a l and u n i t c o s t s of d i f f e r e n t p y r i t i c s u l f u r removal p r o c e s s e s c u r r e n t l y under a c t i v e d e v e l o p m e n t (2,30,31,33) i s g i v e n i n T a b l e IV. With t h e e x c e p t i o n o f t h e MAGNEX p r o c e s s ( 3 1 ) , a l l t h e methods l i s t e d i n T a b l e I V a r e wet p r o c e s s e s and r e q u i r e g r i n d i n g o f f e e d c o a l , thus r e q u i r i n g r e l a t i v e l y comparable g r i n d i n g , dewatering, and d r y i n g c o s t s . T h i s t a b l e i n d i c a t e s t h a t t h e c o s t s o f wet m a g n e t i c d e s u l f u r i z a t i o n by HGMS a r e a t t r a c t i v e when compared w i t h those o f o t h e r approaches, even a f t e r adding the n e c e s s a r y c o s t s o f g r i n d i n g , d e w a t e r i n g and d r y i n g . However, t h e above c o m p a r i s o n i s o n l y a n a p p r o x i m a t e one, b e c a u s e o f t h e d i f f e r e n c e i n t h e methods u s e d t o e s t i m a t e t h e c o s t s and i n t h e d e s u l f u r i z a t i o n c h a r a c t e r i s t i c s r e p o r t e d , e t c . Based on t h e a v a i l a b l e c o s t i n f o r m a t i o n f o r these p y r i t i c s u l f u r removal p r o c e s s e s (3,30,31, 33), i t i s n o t y e t p o s s i b l e t o c a r r y out a r i g o r o u s comparison. Magnetic

Desulfurization of Liquefied Coal:

P r o c e s s and C o s t s


130

COAL

Table I I I .

Comparison

of Estimated Costs of D e s u l f u r i z a t i o n of Coal/Water

Murray(l_9)

1. 2. 3.

Slurry velocity (cm/sec) Slurry concentration (wt %) U n i t c o s t s , $ per ton c o a l processed


4.

C a p i t a l investment per ton c o a l processed per year ($)

T a b l e IV.

Oder(_22)

Trindade(8)

]1.7-3.4

2

2. .02

2. 3. A.

5.

25

1 .06 .

0..83

2..47-0.93

2,.41

1..88

4,.69

3..53

5..95-2.28

Comparison of U n i t Costs of

MAGNEX-Hazen Research, Inc.(31) Froth f l o t a t i o n Bureau of Mines(31) Meyers-TRW Systems and Energy Ledgeraont oxygen leaching-Kennecott Copper Corp.(30) HGMS-This work, see T a b l e I

4..0

2,.61

0..70-0.25. 0.39-0.84 0.22-0.45

1.64

0.82

E s t i m a t e d Approximate C a p i t a l and D i f f e r e n t P y r i t e Removal P r o c e s s e s

Process

1.

4

S l u r r y by HGMS

T h i s Work

30

0..37

i!o

DESULFURIZATION

hi

Capital

Investment^

5.83

7.05

4.17

2.77

4.47

5.71 13.80 (leaching 11.30 (leaching

6.0-14.0 comparable to Meyers 0.83-1.06

only)

3.53-A.69

1.88-2.41

U n i t c o s t s expressed i n $ per ton c o a l p r o c e s s e d . C a p i t a l investment expressed i n $ per ton c o a l p r o c e s s e d

only)

per

year.



10.

L I N A N D Liu

High-Intensity

and -Gradient

Magnetic

Separation

131

A f l o w d i a g r a m o f t h e c o n c e p t u a l p r o c e s s t o remove t h e m i n e r a l r e s i d u e f r o m l i q u e f i e d SRC by HGMS i s shown i n F i g u r e 3. The HGMS u n i t u s e d h e r e i s t h e same c o m m e r c i a l s e p a r a t o r used t o d e s u l f u r i z e the coal/water s l u r r y . The m a g n e t i c d e s u l f u r i z a t i o n o f l i q u e f i e d SRC i s t o be c o n d u c t e d a t e l e v a t e d t e m p e r a t u r e t o reduce the v i s c o s i t y o f the c o a l s l u r r y . Furthermore, the packed s t a i n l e s s s t e e l w o o l m a t r i x i s a l s o t o be h e a t e d t o t h e d e s i r e d s e p a r a t i o n t e m p e r a t u r e d u r i n g o p e r a t i o n . The e l e v a t e d t e m p e r a t u r e o f the m a t r i x w i l l prevent the c o a l s l u r r y from c o n g e a l i n g and p l u g g i n g t h e m a t r i x . I t i s a l s o necessary t o i n s u l a t e the h e a t e d p o r t i o n o f t h e m a t r i x f r o m t h e magnet w i n d i n g s . The i n s u l a t e d m a t r i x i s f u r t h e r s u r r o u n d e d by a w a t e r j a c k e t . These p r o v i s i o n s f o r h e a t i n g , i n s u l a t i n g , and c o o l i n g t h e s e p a r a t o r m a t r i x s l i g h t l y reduce the a c t u a l working volume o f t h e s e p a r a t o r m a t r i x and i t s d i a m e t e r f r o m 7.0 t o 6.83 f t . I n a c t u a l s e p a r a t i o n r u n s , t h e u n f i l t e r e d l i q u e f i e d SRC i s pumped t h r o u g h t h e energized separator a t a constant flow rate u n t i l the separator matrix reaches i t s l o a d i n g l i m i t . A f t e r r i n s i n g w i t h a process g e n e r a t e d s o l v e n t , t h e m a t r i x i s backwashed w i t h t h e same s o l v e n t w i t h t h e magnet d e - e n e r g i z e d . The mags a r e s e n t t o a h y d r o c l o n e separator. The o v e r f l o w f r o m t h e h y d r o c l o n e i s r e c y c l e d b a c k t o t h e wash s o l v e n t t a n k f o r r e - u s e ; w h i l e the u n d e r f l o w i s s e n t t o an e v a p o r a t o r t o r e c o v e r t h e s o l v e n t , and t h e r e s i d u a l s o l i d s a r e packed f o r o t h e r u s e s . The t a i l s f r o m t h e s e p a r a t o r a r e s e n t t o a vacuum c o l u m n t o r e c o v e r t h e s o l v e n t f o r p r o c e s s r e c y c l e and t h e vacuum b o t t o m i s s e n t t o a p r o d u c t c o o l e r t o p r o d u c e t h e s o l i d i f i e d SRC. The c o n c e p t u a l p r o c e s s i s d e s i g n e d t o a c h i e v e t h e same e x t e n t o f i n o r g a n i c s u l f u r r e m o v a l and m a t r i x l o a d i n g o b s e r v e d by HRI f o r s l u r r y v e l o c i t i e s o f 0.25-14.0 cm/sec ( 2 6 ) . The s p e c i f i c magnetic d e s u l f u r i z a t i o n c h a r a c t e r i s t i c s Corresponding t o t h e s e s l u r r y v e l o c i t i e s a r e summarized a s t h e f i r s t t h r e e rows o f T a b l e V. A c c o r d i n g t o a s u r v e y o f 455 U.S. c o a l s a m p l e s c o n d u c t e d by t h e B u r e a u o f M i n e s , t h e a v e r a g e t o t a l and i n o r g a n i c s u l f u r c o n t e n t s o f t h e s e s a m p l e s were 3.02 and 1.91 wt%, r e s p e c t i v e l y ( 2 0 ) . T h u s , i f t h e hydrogénation s t e p i n t h e SRC a n d o t h e r r e l a t e d l i q u e f a c t i o n p r o c e s s e s c a n remove 7 0 % o f t h e o r g a n i c s u l f u r , a r e d u c t i o n i n t h e i n o r g a n i c s u l f u r c o n t e n t by a b o u t 6 7 % a f t e r hydrogénation w i l l be s u f f i c i e n t t o p r o d u c e SRC w i t h a n e m i s s i o n l e v e l s m a l l e r t h a n 1.20 l b S 0 2 / m i l l i o n B t u , a s s u m i n g t h a t t h e SRC h a s a h e a t i n g v a l u e o f 16,000 B t u / l b . The same method o f c o s t e s t i m a t i o n summarized i n t h e Append i x e x c e p t f o r r e p l a c i n g t h e d i s p e r s a n t by steam w i t h a n o m i n a l c o s t o f $2/1000 l b was u s e d t o e s t i m a t e c a p i t a l i n v e s t m e n t and u n i t c o s t s o f the c o n c e p t u a l p r o c e s s . The r e s u l t s a r e p r e s e n t e d i n T a b l e V. H e r e , t h e c o s t o f m a j o r i n s t a l l e d equipment s u c h a s t h e HGMS u n i t , wash s o l v e n t t a n k , f e e d s u r g e t a n k , f e e d pump, f l u s h pump and e v a p o r a t o r , e t c . , h a s b e e n i n c l u d e d . I n T a b l e V I , t h e e f f e c t o f steam p r i c e on t h e u n i t c o s t U o f m a g n e t i c desulfurization of liquefied coal i s i l l u s t r a t e d . Doubling the Q


132

C O A L DESULFURIZATION

Water Supply 1.653 million gal/day Dispersant 5.3 Lb/Hr


Feed Ball Coal — 66.13 TPH Mill

Rinse Water 720 gal/cycle

794 6PM 25 Mixing Wt%j Tank Slurry 10,000 gal

Wash Water 1440 gal/cycle

HGMS 7'D, 20" L. 1480 2.61 cm/sec Velocity 6PM 480 gal. Canister Pump Volume 096 Tons S S. Wool 94% Void Volume 3846 GPM/Ft

Water Tails Vacuum Filter

2

Cycle Time 6.1 min. Duty Cycle 674%

Mags

Refute 9.92 TPH -

Figure 2.

Thermal Dryer

Clarifier or Settling Pond

Clean Dry Coal 56.21 TPH

Desulfurization of coal/water slurry by HGMS

Wash Oil Wash Oil Tank 2300 Gallons Rinse Oil 686 gal/cycle

Wash Oil 1372 gal/cycle Recycle Solvent

HGMS 7'D, 20" L. 2928 5.42 cm/sec Velocity GPM 457.23 gal. Canister Volume Pump 0.915 Tons S.S. Wool 94% Void Volume

Surge SRC Tank Filter Fee

Desulfurization of Coals by High-Intensity High-Gradient Magnetic

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