Coal Desulfurization

12 Coal Desulfurization Test Plant Status—July 1977 L. J. VAN NICE, M. J. SANTY, E. P. KOUTSOUKOS, R. A. ORSINI, and R. A. MEYERS

Downloaded by IMPERIAL COLL LONDON on June 28, 2014 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0064.ch012

TRW

Systems and Energy, Redondo Beach, CA 90278

An 8-metric ton/day process test plant for chemical desulfurization of coal utilizing ferric sulfate leach solution, has just been built atTRW'sCapistrano Test Site in California. The plant, shown in Figures 1 and 2, was constructed under an Environmental Protection Agency sponsored project for the development of the Meyers process. Current plans call for plant shakedown followed by processing of 100-200 tons of American Electric Power Service Corporation's Martinka mine coal. The Meyers process and its applicability for directly desulfurizing raw run-of-mine coal were presented in the previous chapter and the process is described in connection with the broad field of chemical desulfurization in a newly issued book (1). We have very recently verified that the ferric sulfate leach solution, which has a specific gravity of 1.2-1.4, can be used to gravity separate coal into float and sink fractions in a manner very advantageous to the Meyers process. This float-sink approach was tested many years ago by A. Z. Yurovskii in the U.S.S.R but only published in the open literature very recently (2). Yurovskii subsequently treated the sink coal with a mixture of nitric acid and ferric sulfate to remove pyritic sulfur, not realizing, that the separation medium itself was sufficiently active to accomplish near total pyritic sulfur removal. I n f a c t , a p r a c t i c a l method f o r a c t u a l f l o a t - s i n k c l e a n i n g of c o a l i n a dense l i q u i d has l o n g been sought as an a l t e r n a t i v e to mechanical c l e a n i n g . Heavy l i q u i d s , s u c h a s z i n c c h l o r i d e water, o r c h l o r i n a t e d , brominated, o r f l u o r i n a t e d hydrocarbons a r e u s e f u l f o r p r e d i c t i o n o f y i e l d s w h i c h c a n t h e o r e t i c a l l y be obtained i n mechanical washing p l a n t s , but a r e i m p r a c t i c a l f o r a c t u a l p r o d u c t i o n b e c a u s e t h e y a r e e x p e n s i v e and a d d p o l l u t a n t s t o t h e c o a l , a t m o s p h e r e o r w a t e r t a b l e . We f i n d t h a t t h e d e n s i t y o f aqueous f e r r i c s u l f a t e l e a c h s o l u t i o n , a s u t i l i z e d i n t h e M e y e r s process, i s i d e a l for accomplishing a p r a c t i c a l gravity separation o f c o a l f o r s p e c i f i c g r a v i t i e s b e t w e e n 1.2 a n d 1.4. F o r a b o u t 4 0 % o f A p p a l a c h i a n c o a l , t h e f l o a t c o a l ( w h i c h i s o f t e n 40-60% b y w e i g h t o f t h e t o t a l ) a v e r a g e s 0.8-1.2 l b s S O / 1 0 b t u a n d n e e d s 6

2

153

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


COAL DESULFURIZATION

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In Coal Desulfurization; Wheelock, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977. Figure 2.

Test plant—view through tank farm


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no f u r t h e r p r o c e s s i n g t o meet s t a n d a r d s , w h i l e t h e s i n k c o a l s l u r r y c o n t a i n s most o f t h e c o a l p y r i t e . T h i s c o a l s l u r r y can be p r o c e s s e d t h r o u g h t h e M e y e r s p r o c e s s t o g i v e c o a l c o n t a i n i n g 1.21.5 l b s b t u . This approach a l l o w s p r o d u c t i o n o f c o a l which w i l l meet a i r p o l l u t i o n c o n t r o l s t a n d a r d s f o r b o t h New a n d E x i s t ing S t a t i o n a r y S o u r c e s , and reduces t h e c o s t o f t h e Meyers p r o c e s s t h r o u g h a l l o w i n g 40-60% o f t h e c o a l t o b y p a s s r e a c t o r , e l e m e n t a l s u l f u r e x t r a c t i o n and d r y e r u n i t s . We t e r m t h i s a p p r o a c h , G r a v i c h e m C l e a n i n g . Where e s t i m a t e d processing costs using u t i l i t y financed depreciation of c a p i t a l and i n c l u d i n g c o a l g r i n d i n g a n d c o m p a c t i o n w e r e f o r e c a s t s f o r t h e new G r a v i c h e m C l e a n i n g a p p r o a c h a r e $8.50/ton

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K i n e t i c s a n d Scheme

The p r o c e s s i s b a s e d o n t h e o x i d a t i o n o f c o a l p y r i t e w i t h f e r r i c s u l f a t e ( E q u a t i o n 1). The l e a c h i n g r e a c t i o n i s h i g h l y s e l e c t i v e t o p y r i t e w i t h 60% o f the p y r i t i c s u l f u r converted t o s u l f a t e s u l f u r and 40% t o e l e m e n t a l s u l f u r . The r e d u c e d f e r r i c i o n i s r e g e n e r a t e d b y o x y g e n o r a i r a c c o r d i n g t o E q u a t i o n s 2 o r 3. FeS

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R e g e n e r a t i o n c a n be p e r f o r m e d e i t h e r c o n c u r r e n t l y w i t h c o a l p y r i t e l e a c h i n g i n a s i n g l e o p e r a t i o n o r s e p a r a t e l y . The n e t e f f e c t o f t h e p r o c e s s i s t h e o x i d a t i o n o f p y r i t e w i t h oxygen to y i e l d r e c o v e r a b l e i r o n , s u l f a t e s u l f u r , a n d e l e m e n t a l s u l f u r . The f o r m o f p r o c e s s p r o d u c t s v a r i e s t o some e x t e n t w i t h t h e d e g r e e of r e g e n e r a t i o n . Thus, E q u a t i o n s 1 and 2 l e a d t o t h e o v e r a l l p r o cess c h e m i s t r y i n d i c a t e d by E q u a t i o n 4 p r o d u c i n g a m i x t u r e o f i r o n s u l f a t e s and e l e m e n t a l s u l f u r . E q u a t i o n s 1 and 3 y i e l d f e r r o u s s u l f a t e , s u l f u r i c a c i d , and e l e m e n t a l s u l f u r as i n d i c a t e d by E q u a t i o n 5. FeS

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Several options e x i s t i n product recovery. I r o n s u l f a t e s may be r e c o v e r e d a s p u r e s o l i d s b y s t e p w i s e e v a p o r a t i o n o f a s p e n t r e agent s l i p s t r e a m w i t h f e r r o u s s u l f a t e b e i n g recovered f i r s t b e cause o f i t s lower s o l u b i l i t y . A l t e r n a t e l y , f e r r o u s s u l f a t e may be r e c o v e r e d b y c r y s t a l l i z a t i o n a n d f e r r i c s u l f a t e o r s u l f u r i c



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Desulfurization

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a c i d removed b y l i m i n g s p e n t r e a g e n t o r s p e n t wash w a t e r s l i p streams. I r o n s u l f a t e s may b e s t o r e d a s s o l i d s f o r s a l e o r may be c o n v e r t e d e a s i l y t o h i g h l y i n s o l u b l e b a s i c i r o n s u l f a t e s (by a i r o x i d a t i o n ) o r c a l c i u m s u l f a t e (by low-temperature s o l i d phase r e a c t i o n ) f o r d i s p o s a l . E l e m e n t a l s u l f u r may b e r e c o v e r e d f r o m c o a l b y v a p o r i z a t i o n w i t h s t e a m o r b y vacuum, o r i t c a n b e l e a c h e d out w i t h o r g a n i c s o l v e n t s such as acetone depending on product m a r k e t a b i l i t y and p r o d u c t r e c o v e r y economics. Recovery economics may b e i n f l u e n c e d b y q u a n t i t y a n d c o n c e n t r a t i o n o f p r o d u c t i n t h e process e f f l u e n t streams which i n t u r n a r e i n f l u e n c e d by t h e p y r i t e c o n c e n t r a t i o n i n t h e c o a l and t h e d e s i r e d e x t e n t o f d e s u l f u r i z a tion. The p r o c e s s h a s b e e n e x t e n s i v e l y s t u d i e d a t b e n c h - s c a l e . Parameters i n v e s t i g a t e d include c o a l t o p - s i z e , reagent composition, s l u r r y c o n c e n t r a t i o n , r e a c t i o n t e m p e r a t u r e and p r e s s u r e , and r e a c t i o n time. A d d i t i o n a l i n v e s t i g a t i o n s c o m p l e t e d o r underway i n c l u d e c o n current coal leaching-reagent regeneration, product recovery, product s t a b i l i t y , and t h e e f f e c t o f c o a l p h y s i c a l c l e a n i n g on p r o c e s s p e r f o r m a n c e a n d e c o n o m i c s . The p r o c e s s scheme d e p i c t e d i n F i g u r e 3 i s based on t h e b e n c h - s c a l e t e s t i n g . Coal i s (1) Crushed t o t h e d e s i r e d s i z e f o r p r o c e s s i n g (2) Contacted w i t h h o t r e c y c l e d reagent i n the mixer (90°-100°C) (3) Leached o f p y r i t e i n t h e r e a c t o r ( s ) w i t h simultaneous or separate reagent regeneration (4) Washed w i t h h o t w a t e r (5) S t r i p p e d o f e l e m e n t a l s u l f u r , d r i e d and f i n a l l y cooled. The i r o n a n d s u l f a t e s u l f u r a r e r e c o v e r e d f r o m s p e n t r e a g e n t s l i p s t r e a m s p r i o r t o r e a g e n t r e c y c l e . F i g u r e 4 shows t y p i c a l d a t a o n p y r i t e r e m o v a l r a t e s f r o m A p p a l a c h i a n c o a l a s a f u n c t i o n o f temperature. D u r i n g s l u r r y m i x i n g a n d h e a t - u p , 10-20% o f t h e p y r i t e i s removed. Bench-scale data i n d i c a t e d that t h e p y r i t e l e a c h i n g r a t e from c o a l c a n be a d e q u a t e l y r e p r e s e n t e d by t h e e m p i r i c a l r a t e e x p r e s s i o n (Equation 6 ) . r. = -

=h

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(6)

where: = A^ exp (-E^/RT), W = w t % p y r i t e i n c o a l , Y = f e r r i c i o n - t o - t o t a l i r o n r a t i o i n t h e r e a c t o r r e a g e n t , a n d A^ and E^ a r e c o n s t a n t s f o r e a c h c o a l a n d p a r t i c l e s i z e a t l e a s t o v e r most o f the r e a c t i o n range. The l e a c h r a t e i s a f u n c t i o n o f c o a l t y p e . Pyrite extraction r a t e s vary c o n s i d e r a b l y , as d e t a i l e d i n a study o f the Meyers p r o c e s s a s a p p l i e d t o U.S. c o a l ( 3 ) , e.g., t h e r e was more t h a n one o r d e r o f magnitude d i f f e r e n c e between t h e f a s t e s t and s l o w e s t r e a c t i n g c o a l . The r e a g e n t r e g e n e r a t i o n r a t e i s g o v e r n e d b y t h e rate expression (Equation 7 ) .



COAL DESULFURIZATION

Figure 3.

Process flow schematic

REACTION TIME, HOURS

Figure 4.

Temperature effect on processing of 14 mesh top-size Lower Kittanning coal (33% w/w slurries)


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