Coal Desulfurization during Gaseous Treatment

22 Coal Desulfurization during Gaseous Treatment E D M U N D TAO KANG H U A N G * and A L L E N H. PULSIFER

Downloaded by UNIV LAVAL on June 28, 2014 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0064.ch022

Department of Chemical Engineering and Nuclear Engineering and Energy and Mineral Resources Research Institute, Iowa State University, Ames, IA 50011

Sulfur removal from coal during treatment with gaseous atmospheres at elevated temperatures is of interest both because such a treatment might serve as a basis for a coal desulfurization process (1,2,3) and because knowledge of the fate of sulfur when coal is heated has application during carbonization or gasification. Early interest in desulfurization under these conditions concentrated mainly on the production of metallurgical coke. The various approaches included pretreatment of the coal before carbonization, carbonization of the coal in various gases to remove sulfur compounds, or treatment of the coke to reduce its sulfur content. Only recently has interest turned to desulfurization of coal or char for use as a power plant fuel. Sulfur removal from coal during carbonization has been investigated both with and without the addition of reactive gases such as hydrogen and oxygen. During carbonization in the presence of only the gases derived from the coal, hydrogen sulfide is released along with small amounts of other volatile sulfur-containing compounds (4). The iron pyrite decomposes when heated and releases half its sulfur, while one-quarter to o n e - t h i r d o f the organic s u l f u r i s converted t o hydrogen s u l f i d e (_5,(>). The t o t a l amount o f s u l f u r removed f r o m c o a l d u r i n g c a r b o n i z a t i o n i s i n f l u e n c e d b y t h e c o a l r a n k and q u a n t i t y and composition of the mineral matter (7). Snow ( 8 ) h e a t e d c o a l i n v a r i o u s r e a c t i v e g a s e s and f o u n d h y d r o g e n t o be t h e most e f f e c t i v e d e s u l f u r i z i n g a g e n t o f t h e gases t e s t e d . A h i g h e r r a t e o f d e s u l f u r i z a t i o n and l o w e r c h a r s u l f u r c o n t e n t were r e p o r t e d when h y d r o g e n was u s e d as compared w i t h normal c a r b o n i z a t i o n , w i t h both q u a n t i t i e s being favored by i n c r e a s e d h y d r o g e n p r e s s u r e (9,_10). P y r i t e r e a c t s w i t h hydrogen i n two s t e p s , b e i n g f i r s t c o n v e r t e d t o f e r r o u s s u l f i d e and t h e n t o pure i r o n . Y e r g y e t a l . (Il) found t h a t t h e f i r s t r e a c t i o n b e g i n s a t 400°C w h i l e t h e s e c o n d r e a c t i o n o c c u r s a t a somewhat higher temperature. The c o n v e r s i o n o f t h e p y r i t e t o s u l f i d e a p p e a r s t o be f a i r l y r a p i d w h i l e t h e r e a c t i o n o f h y d r o g e n w i t h f e r r o u s s u l f i d e i s slow (12). The c o n v e r s i o n o f o r g a n i c s u l f u r 290

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

22.

HUANG

A N D PULSIFER

Desulfurization

during

Gaseous

Treatment

291

t o h y d r o g e n s u l f i d e was r e p o r t e d b y P o w e l l ( 1 3 ) n o t t o be s i g n i f i c a n t b e l o w 500°C b u t i n c r e a s e s g r e a t l y a s t h e t e m p e r a t u r e i s r a i s e d f r o m 500° t o 1000°C. H y d r o d e s u l f u r i z a t i o n o f c o a l i s s t r o n g l y i n h i b i t e d by t h e presence o f hydrogen s u l f i d e i n t h e gas phase (_12,14,15), a n d G r a y e t a l . ( 1 2 ) f o u n d t h a t s u l f u r removal i s r a p i d and l i m i t e d by t h e e q u i l i b r i u m between hydrogen s u l f i d e and t h e s u l f u r i n t h e c o a l . An a c i d l e a c h o f t h e c o a l removes i r o n a n d c a l c i u m s u l f i d e s , r e d u c i n g t h e b a c k r e a c t i o n o f c o a l w i t h h y d r o g e n s u l f i d e and s i g n i f i c a n t l y i m p r o v i n g t h e e f f e c t i v e n e s s o f h y d r o d e s u l f u r i z a t i o n (16,17). D e s u l f u r i z a t i o n o f coke and c o a l w i t h o x y g e n - c o n t a i n i n g gases has been t h e s u b j e c t o f s e v e r a l i n v e s t i g a t i o n s (18,19,20). M a i n l y p y r i t i c s u l f u r i s removed i n o x i d i z i n g a t m o s p h e r e s . S i n h a a n d W a l k e r (18) r e p o r t e d t h a t 907 o f t h e p y r i t i c s u l f u r was removed i n 10 m i n u t e s a t 450°C w i t h a 5-177 sample w e i g h t loss. However, B l o c k e t a l . (20) f o u n d i n e x p e r i m e n t s a t s i m i l a r c o n d i t i o n s t h a t l e s s p y r i t i c s u l f u r was removed w i t h a g r e a t e r weight loss. S e v e r a l i n v e s t i g a t o r s have r e p o r t e d t h a t some o f t h e i n o r g a n i c s u l f u r i n c o a l i s t r a n s f o r m e d i n t o o r g a n i c s u l f u r upon h e a t i n g (5,7,8,11). C e r n i c - S i m i c (7^) added r a d i o a c t i v e s u l f u r i n t h e f o r m o f i r o n p y r i t e s t o a c o a l sample a n d a f t e r c a r b o n i z a t i o n f o u n d t h a t p a r t o f t h e r a d i o a c t i v e s u l f u r was r e t a i n e d by t h e c o a l i n t h e f o r m o f o r g a n i c s u l f u r . The amount o f r a d i o a c t i v e s u l f u r r e t a i n e d as o r g a n i c s u l f u r i n c r e a s e d as c o a l rank decreased. I n t h e work r e p o r t e d h e r e , d e s u l f u r i z a t i o n o f b o t h a raw and d e a s h e d Iowa c o a l were i n v e s t i g a t e d i n t h r e e d i f f e r e n t g a s e s : n i t r o g e n , h y d r o g e n , and a 6% o x y g e n - 947 n i t r o g e n gas m i x t u r e . W i t h each g a s , b o t h t h e t e m p e r a t u r e and h o l d i n g time a t temperat u r e were v a r i e d . W i t h h y d r o g e n and n i t r o g e n , t h e t e m p e r a t u r e was v a r i e d b e t w e e n 300° a n d 700°C. The t e m p e r a t u r e r a n g e u s e d w i t h t h e o x y g e n - n i t r o g e n m i x t u r e was o n l y 350° - 455°C b e c a u s e o f t h e l a r g e w e i g h t l o s s a t h i g h e r t e m p e r a t u r e s c a u s e d b y comb u s t i o n o f t h e sample. H o l d i n g t i m e a t t e m p e r a t u r e was v a r i e d f r o m 0 t o 60 m i n u t e s , e x c e p t i n t h e o x i d i z i n g a t m o s p h e r e where t h e maximum h o l d i n g t i m e was 40 m i n u t e s .


o

0

e

Experimental The i n v e s t i g a t i o n was c a r r i e d o u t w i t h a R i g a k u CN8001 H type thermal a n a l y z e r which included a thermal g r a v i m e t r i c a n a l y z e r , a preprogrammed h e a t i n g u n i t a n d t e m p e r a t u r e c o n t r o l s y s t e m , and a d a t a r e c o r d i n g u n i t . The c o a l sample was c o n f i n e d i n a 10-mm d i a m e t e r p l a t i n u m p a n h a v i n g a d e p t h o f 5 mm. The pan was s u p p o r t e d on a b a l a n c e t h a t c o u l d d e t e c t a w e i g h t change o f l e s s t h a n 0. 1 mg. The sample h o l d e r was c o n t a i n e d i n a 20-mm d i a m e t e r q u a r t z t u b e w h i c h was c o n t i n u o u s l y p u r g e d w i t h g a s . To c a r r y o u t a r u n , about 300 mg o f c o a l (o> So =

S i n c e t h e i n o r g a n i c and 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 c h a r s p r o d u c e d f r o m b o t h t h e r a w and d e a s h e d c o a l s were d e t e r m i n e d a t


COAL

300

Table I I I .


Temp. (°C)

C a l c u l a t e d Parameters f o r S u l f u r Holding Time (min)

f

DESULFURIZATION

Reactions

Si

fi

-0.05 -0.06 -0.06 -0.05 -0.07 -0.05 -0.08 0.02 0.02 0.22 0.12 0. 20 0.23 0. 12 0. 13 0.19 0. 15 0. 14 0. 09 0.09 0. 10 0.17 0. 15 0. 19 0.29

0. 15 0. 42 0.52 0. 55 0.57 0. 58 0.58 0. 58 0. 61 0.58 0. 62 0.57 0.59 0. 62 0. 68 0.71 0. 75 0.73 0.76 0. 74 0. 83 0. 84 0.87 0. 88 0.90

2

Hydrogen atmosphere 300 400 400 400 400 400 400 500 500 500 500 500 500 600 600 600 600 600 600 700 700 700 700 700 700

0 0 10 20 30 40 60 0 10 20 30 40 60 0 10 20 30 40 60 0 10 20 30 40 60

0.10 0. 29 0. 34 0.41 0. 38 0. 42 0.45 0.51 0.51 0.43 0. 39 0. 34 0. 34 0. 49 0. 48 0.52 0.57 0.49 0.49 0.49 0. 47 0. 43 0.32 0. 25 0. 23


22.

HUANG

A N D

puLSiFER

Table I I I .


Nitrogen

during

Gaseous

Treatment

301

Continued Holding Time (min)

Γ emp.

Coal Desulfurization during Gaseous Treatment

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