13
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 13, 2015 | http://pubs.acs.org Publication Date: June 16, 1983 | doi: 10.1021/bk-1983-0223.ch013
High-Temperature Hydrogen Sulfide Removal Using a Regenerable Iron Oxide Sorbent S. S. TAMHANKAR and C. Y. WEN West Virginia University, Department of Chemical Engineering, Morgantown, WV 26506
A state-of-the-art review is presented of high temperature H S removal and sorbent regeneration using an iron oxide sorbent, with a particular emphasis on kinetic and mechanistic studies of the reactions involved. Selection criteria for a high temperature H S sorbent and the various sorbent regeneration options are f i r s t briefly discussed. Results are then presented on the kinetics of the various reactions involved in the absorption and the regeneration steps. These studies were conducted in a thermogravimetric analyzer (TGA). The weight change data obtained in the TGA was used in conjunction with x-ray and Mossbauer spectroscopic analyses of the solid samples to elucidate the mechanisms of the reactions. Conversion correlations are reported based on the grain model, which can be used for reactor design and for predicting reactor per formance. 2
2
S i g n i f i c a n t e f f o r t i s underway i n t h e U n i t e d S t a t e s t o d e v e l o p and c o m m e r c i a l i z e c o a l g a s i f i c a t i o n p r o c e s s e s f o r p r o d u c i n g gaseous f u e l s . One o f t h e m a j o r o b s t a c l e s i n t h e development o f such a p r o c e s s i s the p r e s e n c e o f u n d e s i r a b l e c o n t a m i n a n t s i n t h e p r o d u c t gas s t r e a m . The m a j o r c o n t a m i n a n t i n c o a l g a s i f i c a t i o n i s hydrogen s u l f i d e (H2S), which i s t o x i c , p o i s o n o u s t o d o w n s t r e a m c a t a l y s t s and e x t r e m e l y c o r r o s i v e i n nature. C o n t r o l o f H2S i n t h e f u e l gas t o a s a f e l e v e l i s therefore essential. The H S r e m o v a l r e q u i r e m e n t s a r e e v e n more c r i t i c a l when t h e f u e l gas i s u s e d i n c o m b i n e d c y c l e power generation or i n fuel c e l l s . C o n v e n t i o n a l methods o f H2S r e m o v a l use a c o l d s c r u b b i n g t e c h n i q u e , w h e r e i n t h e gas n e e d s t o b e c o o l e d t o n e a r room temperature. Although such techniques are e f f e c t i v e , they are a s s o c i a t e d w i t h a l o s s o f s e n s i b l e h e a t o f the g a s . The o t h e r 2
0097-6156/83/0223-0255$07.25/0 © 1983 A m e r i c a n Chemical Society
In Industrial Gas Separations; Whyte, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 13, 2015 | http://pubs.acs.org Publication Date: June 16, 1983 | doi: 10.1021/bk-1983-0223.ch013
256
I N D U S T R I A L GAS
SEPARATIONS
disadvantages o f c o l d scrubbing i n c l u d e r e s t r i c t i o n by a v a i l a b i l i t y o f water, sludge d i s p o s a l problem and condensation and mixi n g o f t a r s w i t h t h e scrubber water posing h a n d l i n g problems. I n v i e w o f t h e s e , a n d m a i n l y f o r t h e r m a l e f f i c i e n c y , r e m o v a l o f H2S at high temperatures i s very a t t r a c t i v e . Besides, fuel c e l l s and c o m b i n e d c y c l e power g e n e r a t i o n s y s t e m s u s e t h e h o t f u e l g a s directly. This f u r t h e r improves t h e o v e r a l l thermal e f f i c i e n c y . F i g u r e 1 shows a s c h e m a t i c o f t h e i n t e g r a t e d s y s t e m c o n f i g u r a tions for various applications. The f i g u r e a l s o i n c l u d e s t h e l i m i t v a l u e s o f H2S c o n c e n t r a t i o n f o r t h e s e a p p l i c a t i o n s . The s u b j e c t o f t h i s paper i s t h e b u l k s u l f u r r e m o v a l , whereby t h e c o n c e n t r a t i o n i s b r o u g h t down t o a b o u t 200 ppm. Sorbent S e l e c t i o n The c r i t e r i a u s e d i n s e l e c t i n g a s u i t a b l e s o r b e n t a r e : e f f i c i e n c y , s u l f u r capture c a p a c i t y , k i n e t i c s , c o s t , p h y s i c a l s t r e n g t h and r e g e n e r a b i l i t y . Sorbent e f f i c i e n c y i m p l i e s t h e maximum amount o f H2S t h a t c a n b e removed u n d e r a g i v e n s e t o f o p e r a t i n g c o n d i t i o n s , based on t h e thermodynamic e q u i l i b r i u m o f t h e s y s t e m . F i g u r e 2 ( 1 ) shows e q u i l i b r i u m H2S c o n v e r s i o n s i n t h e p r e s e n c e o f v a r i o u s s o r b e n t m a t e r i a l s i n r e l a t i o n t o t h e EPA s u l f u r emission standards. A t a l l t h e temperatures o f i n t e r e s t i r o n - b a s e d s o r b e n t s seem t o b e s a t i s f a c t o r y . Zinc oxide i s p r o b a b l y t h e b e s t i n t h i s r e s p e c t , and c a n i n f a c t be used f o r t h e f i n a l p o l i s h i n g s t e p t o r e d u c e H2S l e v e l t o l e s s t h a n 1 ppm. S u l f u r c a p t u r e c a p a c i t y o f a s o r b e n t depends on t h e s t o i c h i o m e t r y of the r e a c t i o n . I n t h e temperature range o f i n t e r e s t , i r o n o x i d e (Fe2Û3) c a p t u r e s a b o u t 44 w e i g h t % s u l f u r . An i m p o r t a n t a s s e t o f i r o n o x i d e a s a n H2S s o r b e n t i s i t s reactivity. The r e s u l t s o f a c o m p a r a t i v e s t u d y shown i n F i g u r e 3 ( 1 ) r e v e a l t h a t i r o n o x i d e i s b y f a r t h e most a c t i v e among the oxides considered. T h i s i s an i m p o r t a n t f a c t o r i n t h e c h o i c e o f Fe203 as a s o r b e n t . B a s e d on c r i t e r i a o f e f f i c i e n t H S r e m o v a l a b o v e 1000°F, p h y s i c a l s t r e n g t h , r e g e n e r a b i l i t y , sorbent l i f e and economic f e a s i b i l i t y , l a b o r a t o r y t e s t s (£,3) l e d t o t h e s e l e c t i o n o f i r o n o x i d e mixed w i t h f l y ash, i n extruded p e l l e t s , as a promising sulfur-removal sorbent. In a commercial s c a l e o p e r a t i o n p h y s i c a l s t r e n g t h o f the s o r b e n t p e l l e t s i s i m p o r t a n t , e s p e c i a l l y i f a moving o r a f l u i d i z e d - b e d r e a c t o r i s used. P e l l e t s o f i r o n o x i d e alone a r e n o t s t r o n g enough; a d d i t i o n o f a s u p p o r t m a t e r i a l i s necessary. I n so doing, the percentage o f i r o n oxide should be k e p t a s h i g h a s p o s s i b l e t o m a i n t a i n t h e o v e r a l l h i g h e f f i c i e n c y o f H2S r e m o v a l . E x t e n s i v e t e s t s were conducted a t METC ( 4 ) , a n d t h e b e s t s u p p o r t a n d a n o p t i m u m c o m p o s i t i o n w i t h i t were found. T h i s s o r b e n t c o n s i s t e d o f 45 w e i g h t % 2
In Industrial Gas Separations; Whyte, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
In Industrial Gas Separations; Whyte, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
HYDROCARBONS> IF ANY
PARTICULATE REMOVAL
1Û0ÛOC
REGENERATOR!
SORBENT
BULK S REMOVAL
2QQ
S
ELEMENTAL SULFUR RECOVERY
M
PPM
POWER GENERATION
COMBINED CYCLE
^ 1 PPM S
TRACE S REMOVAL
< 0.5
FUEL CELLS
S
F i g u r e 1. A s c h e m a t i c o f a n i n t e g r a t e d c o a l g a s i f i c a t i o n s y s t e m f o r f u e l c e l l o r combined-cycle-power-generation a p p l i c a t i o n s .
MPC
m -
HEAT RECOVERY
CONVENTIONAL USES
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 13, 2015 | http://pubs.acs.org Publication Date: June 16, 1983 | doi: 10.1021/bk-1983-0223.ch013
258
I N D U S T R I A L GAS
EPA fe/FeO
1200 K (1700°F) Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 13, 2015 | http://pubs.acs.org Publication Date: June 16, 1983 | doi: 10.1021/bk-1983-0223.ch013
SEPARATIONS
CaO
Cr 0 2
3
K CO
CO
Pb
ZnO
Ni MnO|
2
V
3
2°3
C
1
10
10"
10"
3
EPA E-
10'
No2s3
3
Pb
CO
Ni
Sn Bi
Na C0 2
10
-6
10
-7
IcaCO,
OQ 2
-5
Fe/FeO
1000 Κ (1340°F) Cr 0
10
4
ZnO
3
Cd
