minerals found in coals for char gasification in air, C02 and H2. .... M. and Anderson, R. A., in "Chemistry and Physics of. Carbon" (P. L. Walker, Jr., Ed.), Vol. 4, p.
CATALYSIS OF CHAR GASIFICATION BY MINERALS A. Tomita, 0. P. Mahajan and P. L . Walker, Jr. M a t e r i a l S c i e n c e s Department, P e n n s y l v a n i a S t a t e U n i v e r s i t y U n i v e r s i t y P a r k , P e n n s y l v a n i a 16802 INTRODUCTION It is w e l l known t h a t most m i n e r a l s a c t more or less t o c a t a l y z e c a r b o n I n c o a l s , t h e r e a r e two broad c l a s s e s of i n o r g a n i c i m p u r i t i e s gasification (1,Z). t h e d i s c r e t e m i n e r a l m a t t e r , which i s u s u a l l y p r e s e n t i n p a r t i c l e s g r e a t e r than about 1 pm i n s i z e , and t h e minor and t r a c e e l e m e n t s , which a r e more o r less a s s o c i a t e d w i t h t h e o r g a n i c p h a s e o r m i n e r a l phase i n a h i g h l y d i s p e r s e d s t a t e . Because of c u r r e n t i n t e r e s t i n p r o d u c i n g low and h i g h BTU g a s e s by c o a l g a s i f i c a t i o n , i t i s d e s i r a b l e t o know t h e e x t e n t t o which i n o r g a n i c i m p u r i t i e s i n c h a r s (as a r e s u l t of b e i n g p r e s e n t i n t h e o r i g i n a l c o a l s ) i n c r e a s e s g a s i f i c a t i o n r a t e s . The aim of t h e p r e s e n t s t u d y i s t o i n v e s t i g a t e p o s s i b l e c a t a l y t i c a c t i v i t y of major m i n e r a l s found i n c o a l s f o r c h a r g a s i f i c a t i o n i n a i r , C02 and H2. S i n c e c o a l s and, hence, c h a r s are i n v a r i a b l y a s s o c i a t e d w i t h many i n o r g a n i c i m p u r i t i e s , a h i g h p u r i t y polymer char having a s t r u c t u r e s i m i l a r t o c o a l - d e r i v e d c h a r s h a s been used i n t h e p r e s e n t study.
--
EXPERIMENTAL Char P r e p a r a t i o n The h i g h p u r i t y c h a r w a s o b t a i n e d by c a r b o n i z i n g Saran ( a copolymer of PVDC and PVC i n a r a t i o of about 9 : l ) a t 900°C i n t h e manner d e s c r i b e d p r e v i o u s l y ( 3 ) . The c h a r w a s ground and t h e 200x325 mesh f r a c t i o n (44x74 pm p a r t i c l e s i z e ) h e a t e d i n N i n a f l u i d i z e d bed r e a c t o r up t o 1000°C a t a r a t e of 10°C/ min. Soak time a t 1000"C2waf 2 h r . The N2 (77'K) and C 0 2 (298'K) s u r f a c e a r e a s of t h e c h a r w e r e 900 and 1110 m / g . M i n e r a l s The f o l l o w i n g m i n e r a l s were used: k a o l i n i t e , i l l i t e , c a l c i t e , d o l o m i t e , gypsum, q u a r t z , r u t i l e , p y r i t e and s i d e r i t e . The m i n e r a l s were o b t a i n e d from Wards N a t u r a l Science E s t a b l i s h m e n t , I n c . , N e w York. P r e p a r a t i o n of Char-Mineral M i x t u r e s V a r i o u s char-mineral m i x t u r e s c o n t a i n i n g 52, by w e i g h t , m i n e r a l s w e r e p r e p a r e d by mixing t h e 200x325 mesh f r a c t i o n of t h e m i n e r a l s w i t h t h e same s i z e f r a c t i o n o f Saran c h a r . The m i x t u r e s were blended f o r 4 h r i n an e l e c t r i c r o t a t i n g device. Procedure R e a c t i v i t i e s of v a r i o u s c h a r - m i n e r a l m i x t u r e s were determined i n a i r (1 a t m ) a t 55OoC, CO (1 atm) a t 96OoC and H (20.4 atm) a t 94OOC. I n each c a s e , an 2 2 e l e c t r o b a l a n c e was used t o m o n i t o r weight changes o c c u r r i n g d u r i n g t h e g a s i f i c a t i o n r e a c t i o n . D e t a i l s o f t h e e x p e r i m e n t a l procedure have been d e s c r i b e d p r e v i o u s l y ( 4 , 5 ) . B r i e f l y , a s m a l l amount of t h e m i x t u r e c o n t a i n e d i n a p l a t i n u m pan w a s h e a t e d i n an i n e r t (N2 o r He) atmosphere t o t h e r e a c t i o n t e m p e r a t u r e a t a r a t e of 20°C/min. The sample was m a i n t a i n e d i s o t h e r m a l l y a t t h e r e a c t i o n t e m p e r a t u r e f o r 1 5 min t o e n s u r e t h e r m a l s t a b i l i t y . Following t h i s , t h e i n e r t atmosphere was r e p l a c e d by t h e r e a c t a n t gas. Weight changes o c c u r r i n g d u r i n g g a s i f i c a t i o n were t h e n determined continuously
.
R e a c t i v i t y Measurement I n t h e p r e s e n t s t u d y burn-off v s . t i m e c u r v e s had a g r a d u a l i n d u c t i o n p e r i o d followed by a r e g i o n i n which burn-off i n c r e a s e d w i t h t i m e . A t h i g h e r l e v e l s of carbon b u r n - o f f , t h e rates s t a r t e d t o d e c r e a s e . I n s t a n t a n e o u s r d a c t i v i t y (R ) a t d i f f e r e n t r e a c t i o n times was c a l c u l a t e d from t h e f o l l o w i n g equation: R
t
= L .dsJ w
t
dt
where wt and dw/dt a r e t h e weight of t h e u n r e a c t e d c h a r on a dry-ash-free
4
(daf)
-
basis and slope of the burn-off VS. time plot at time t, respectively. For reactions in CO and H2, the reactivity parameter R , which represents the constant rate in the regfon where the kinetics follow a firsf order rate expression with respect to amount of unreacted char, has been reported. For reaction in air, R increased monotonically with conversion so that R values cannot be given. TheEefore, the reactivity parameter for the reaction i$ air has been calculated from the following equation: R
o
=!Z
w
. -dtdw
where w is the starting weight of char on a daf basis and dw/dt is the maximum rectiliiear rate of weight l o s s . RESULTS AND DISCUSSION Reactivity parameters for various char-mineral mixtures in air, CO and H are 2 2 listed in Table 1. Reactivities could be duplicated within +2% in al1,cases. TABLE 1 REACTIVITY RESULTS Reactivity, mg hr-' R Mineral none siderite pyrite dolomite illite quartz gypsum rutile kaolinite calcite Reactivity in Air Values of R for Furthermore, instantaneous rat& at found to be.essentially the same as indicate that there is little or no reactivity in air.
in
mg-'
Rc in
Air
c02
"2
1.1
1.9 1.8 1.3 2.0 1.8 2.0 1.5 2.0 1.6 1.7
2.3 5.6 3.9 2.0 1.4 2.1 1.6
1.0 1.1 1.0
1.0 1.0 1.1 1.2 1.0
1.6
1.2 0.6
various samples vary between 1.0 and 1.2. a given conversion for different mixtures were for the Saran char itself. These results thus catalytic effect of the minerals studied on char
Most of the minerals investigated are known to undergo chemical and/or structural changes upon heating to higher temperatures. In order to see if such changes modify activity of the various minerals towards char gasification in air, a few char-mineral mixtures prior to making reactivity measurements at 55OoC were heated to 900'C in an inert atmosphere. The reactivities following heat treatment at 900°C were found to be essentially the same as when the heat treatment temperature was 550°C. Thus, the chemical o r structural formof the minerals investigated have no detectable catalytic effect on char gasification in air. It is seen (Table 1) that pyrite, gypsum, kaolinite and calcite Reactivit in CO inhibit tie reacgion in CO ; the effect is most pronounced for pyrite. It is noteworthy that although both 8-CO and C-0 gasification reactions involve an inter2 mediate oxygen transfer step ( ! , 2 ) , the aforementioned minerals inhibit the reaction in CO2 whereas they have no such effect in 0 2 . This shows high catalytic specificity of these minerals for gasification reactions in different oxidizing atmospheres. 5
The observed i n h i b i t i v e e f f e c t o f p y r i t e may b e due t o t h e f o l l o w i n g . Carbon Before CO i s desorbed i n t o t h e monoxide i s produced d u r i n g t h e C-C02 r e a c t i o n . gaseous p h a s e , i t h a s a two-dimensional m o b i l i t y on t h e s u r f a c e . During t h i s m o b i l i t y , CO may c o l l i d e w i t h p y r i t e p a r t i c l e s p r o d u c i n g COS which i n t u r n may d i s s o c i a t e a t t h e r e a c t i o n t e m p e r a t u r e i n t o CO and S. T h i s s u l f u r may be chemis o r b e d on t h e c h a r s u r f a c e . I t i s known t h a t s u l f u r and oxygen a r e chemisorbed e s s e n t i a l l y a t t h e same a c t i v e c a r b o n s i t e s ( 6 ) . S i n c e t h e f i r s t s t e p i n t h e overa l l g a s i f i c a t i o n p r o c e s s i s t h e d i s s o c i a t i v e c h e m i s o r p t i o n of t h e r e a c t a n t gas a t t h e a c t i v e c a r b o n s i t e s , i t i s o b v i o u s t h a t f o l l o w i n g c h e m i s o r p t i o n of s u l f u r t h e e x t e n t of oxygen c h e m i s o r p t i o n a n d , h e n c e , g a s i f i c a t i o n rate w i l l be r e t a r d e d . Reactivit i n H Most o f t h e m i n e r a l s have a n e f f e c t , e i t h e r c a t a l y t i c o r inhibitiv:, on $he h y d r o g a s i f i c a t i o n r e a c t i o n . S i d e r i t e and p y r i t e show l a r g e catalytic activity. Both t h e s e m i n e r a l s c o n t a i n i r o n as a major component. X-ray d i f f r a c t i o n s t u d i e s revealed t h a t d u r i n g t h e h y d r o g a s i f i c a t i o n r e a c t i o n both these m i n e r a l s a r e reduced t o e l e m e n t a l i r o n . It i s w e l l known t h a t m e t a l l i c i r o n I l l i t e , gypsum, r u t i l e , k a o l i n i t e and c a l c i t e have an c a t a l y z e s t h e C-H2 r e a c t i o n . The i n h i b i t i v e e f f e c t i s most i n h i b i t i v e e f f e c t on t h e h y d r o g a s i f i c a t i o n r e a c t i o n . pronounced f o r c a l c i t e . Although i t i s d i f f i c u l t t o e x p l a i n u n e q u i v o c a l l y t h e i n h i b i t i v e e f f e c t of v a r i o u s m i n e r a l s , w e s u g g e s t t h e f o l l o w i n g r e a s o n f o r t h e ob'served b e h a v i o r . Most i n o r g a n i c i m p u r i t i e s c a t a l y z e c a r b o n g a s i f i c a t i o n by p r o v i d i n g a c t i v e s i t e s a t which d i s s o c i a t i o n of r e a c t a n t molecules can r e a d i l y o c c u r (2). The d i s s o c i a t e d s p e c i e s t h e n d i f f u s e a c r o s s t h e s u b s t r a t e s u r f a c e and r e a c t w i t h t h e a c t i v e c a r b o n atoms. It is s u g g e s t e d t h a t i n t h e c a s e o f t h e m i n e r a l s which have a n i n h i b i t i n g e f f e c t on c h a r r e a c t i v i t y t h e r e v e r s e of molecular That i s , H2 i s d i s s o c i a t e d a t a c t i v e s i t e s hydrogen d i s s o c i a t i o n is predominant. on t h e c h a r s u r f a c e p r o d u c i n g hydrogen a t o m . Some hydrogen atom recombination on t h e s u r f a c e o f t h e m i n e r a l s r e s u l t s i n a r e d u c t i o n i n t h e r a t e of c h a r hydrogasification. ACKNOWLEDGEMENTS T h i s r e s e a r c h w a s s u p p o r t e d by ERDA on C o n t r a c t E(49-18)-2030. REFERENCES 1.
Walker, P . L . , Jr., Rusinko, F. J . and A u s t i n , L . G . , Vol. 11, p. 133, Academic P r e s s , New York, 1959.
2.
Walker, P. L . , Jr., S h e l e f , M. and Anderson, R. A . , i n "Chemistry and P h y s i c s o f Carbon" (P. L . Walker, Jr., E d . ) , Vol. 4 , p. 287, Marcel Dekker, New York, 1965.
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4.
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5.
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6.
P u r i , B. R . , i n "Chemistry and P h y s i c s o f Carbon" Vol. 6 , p. 1 9 1 , Marcel Dekker, New York, 1970.
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