Alkali Vapor Transport in Coal Conversion and Combustion Systems

34 Alkali Vapor Transport in Coal Conversion and Combustion Systems J. W. H A S T I E , E. R. P L A N T E , and D. W. B O N N E L L

Downloaded by UNIV LAVAL on July 11, 2014 | http://pubs.acs.org Publication Date: March 8, 1982 | doi: 10.1021/bk-1982-0179.ch034

Center for Materials Science, National Bureau of Standards, Washington, D C 20234

A l k a l i metal-containing vapor species are ubiquitous in coal conversion and combustion systems. These species originate from coal mineral and atmospheric impurities (organic and inorganic) and from ceramic construction mater ials. Alternatively, they are present as additives, such as with potassium seeding for MHD or with bulk glass as a p a r t i c l e absorbing medium, or with dolomite i n fluidized bed systems. A l k a l i vapor transport over representative slag, glass, and simple halide, hydroxide, and sulfate systems i s discussed i n relation to materials and process limitations in coal-supported energy systems. Problems associated with molecular-level vapor transport measurements are also considered. V a p o r s c o n t a i n i n g a l k a l i m e t a l s p e c i e s have d i v e r s e i m p l i c a t i o n s t o h i g h t e m p e r a t u r e p r o c e s s e s (I). P o t e n t i a l new a p p l i c a t i o n s o f a l k a l i e s i n combustion systems i n c l u d e - - t h e i r vapor phase c a t a l y t i c a c t i o n i n smoke r e d u c t i o n ( 2 , 3 ) , t h e i r l i q u i d p h a s e c a t a l y s i s o f c o a l g a s i f i c a t i o n ( 4 ) , and t h e i r r o l e as e l e c t r o n s o u r c e s f o r m a g n e t o h y d r o d y n a m i c (MHD) c o m b u s t i o n s y s tems ( 3 ) . I n most c o m b u s t i o n s y s t e m s , however, t h e i r p r e s e n c e i s undesirable. This i s p a r t i c u l a r l y t r u e i n f o s s i l energy systems. More e f f i c i e n t c o a l u t i l i z a t i o n c a n be r e a l i z e d w i t h combined power p l a n t c y c l e s . F o r i n s t a n c e , t h e p o s t c o m b u s t i o n g a s e s o f a c o n v e n t i o n a l c o m b u s t o r o r a n a d v a n c e d MHD s y s t e m c a n be f u r t h e r u t i l i z e d t o d r i v e a gas o r steam t u r b i n e . However, t h e s u s t a i n e d d u r a b i l i t y o f downstream t u r b i n e o r h e a t e x c h a n g e r components requires minimal transport o f corrosive f u e l i m p u r i t i e s . Control of mineral-derived i m p u r i t i e s i s also required f o r environmental protection. F o r t h e s p e c i a l c a s e o f open c y c l e - c o a l f i r e d MHD s y s t e m s , t h e thermodynamic a c t i v i t y o f p o t a s s i u m i s much h i g h e r i n t h e seeded c o m b u s t i o n gas ( p l a s m a ) t h a n i n common c o a l m i n e r a l s and s l a g s . T h i s r e s u l t s i n t h e l o s s o f plasma seed by s l a g a b s o r p t i o n and i s o f c r i t i c a l c o n c e r n t o t h e economic f e a s i b i l i t y o f MHD. 0097-6156/82/0179-0543$14.50/0 © 1982 American Chemical Society In Metal Bonding and Interactions in High Temperature Systems; Gole, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


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E m p i r i c a l e x p e r i e n c e w i t h c o n v e n t i o n a l c o a l - f i r e d power p l a n t s h a s i n d i c a t e d m i n e r a l s c o n t a i n i n g a l k a l i m e t a l (Na, K ) , s u l f u r - a n d c h l o r i n e - b e a r i n g s p e c i e s t o be t h e most a g g r e s s i v e f u e l components l e a d i n g t o f i r e - s i d e o r h o t c o r r o s i o n ( 5 ) . Species c o n t a i n i n g these elements appear t o a c t s y n e r g i s t i c a l l y i n degrading a l l o y o r ceramic m a t e r i a l s . The mechanisms b y w h i c h such s p e c i e s a r e r e l e a s e d from t h e i r m i n e r a l s o u r c e , t r a n s p o r t e d , and d e p o s i t e d a r e n o t known, t h o u g h t h e l i t e r a t u r e c o n t a i n s numerous s p e c u l a t i v e schemes ( s e e R e f . 3, p. 2 1 6 ) . Rational d e v e l o p m e n t o f new c o n t r o l s t r a t e g i e s , s u c h as gas c l e a n - u p o r the use o f f u e l a d d i t i v e s , r e q u i r e s a c l e a r understanding o f t h e r o l e p l a y e d b y t h e a c t i v e f u e l i m p u r i t i e s . F o r i n s t a n c e , new c o n t r o l s y s t e m s b a s e d on s c a v e n g i n g ( e . g . , a b s o r p t i o n o f a l k a l i by g l a s s o r o t h e r o x i d e m e d i a ) o r c h e m i c a l m o d i f i c a t i o n o f t h e a c t i v e i n o r g a n i c i m p u r i t i e s w i l l need as d e s i g n c r i t e r i a i n f o r m a t i o n , s u c h as s p e c i e s i d e n t i t y , c o n c e n t r a t i o n p r o f i l e s , dew p o i n t s , thermodynamic r e a c t i v i t y , n u c l e a t i o n and a b s o r p t i o n r a t e s and d i f f u s i v i t i e s . Such d a t a w i l l a l s o be p e r t i n e n t t o m i n i m i z a t i o n o f s e e d - s l a g i n t e r a c t i o n i n MHD s y s t e m s . P r e v i o u s a t t e m p t s t o d e f i n e t h e mode o f r e l e a s e and t r a n s p o r t o f f u e l i m p u r i t i e s have l a r g e l y b e e n u n s u c c e s s f u l , o w i n g m a i n l y t o a l a c k o f knowledge c o n c e r n i n g s p e c i e s i d e n t i t i e s . T h i s has r e s u l t e d f r o m t h e i n a b i l i t y o f m o l e c u l a r s p e c i f i c measurement t e c h n i q u e s t o f u n c t i o n u n d e r t h e combined a g g r e s s i v e conditions o f h i g h t e m p e r a t u r e , h i g h p r e s s u r e , and h i g h c h e m i c a l r e a c t i v i t y . We have d e v e l o p e d s e v e r a l new measurement t e c h n i q u e s i d e a l l y s u i t e d t o such c o n d i t i o n s . The f i r s t o f t h e s e t e c h n i q u e s i s a H i g h P r e s s u r e S a m p l i n g Mass S p e c t r o m e t r i c method f o r t h e s p a t i a l and t e m p o r a l a n a l y s i s o f f l a m e s c o n t a i n i n g i n o r g a n i c a d d i t i v e s ( 6 , 7). The s e c o n d method, known as T r a n s p i r a t i o n Mass S p e c t r o m e t r y (TMS) ( 8 ) , a l l o w s f o r t h e a n a l y s i s o f b u l k h e t e r o g e n e o u s s y s t e m s o v e r a w i d e r a n g e o f t e m p e r a t u r e , p r e s s u r e and c o n t r o l l e d gas composition. I n a d d i t i o n , t h e now c l a s s i c a l t e c h n i q u e o f K n u d s e n E f f u s i o n Mass S p e c t r o m e t r y (KMS) h a s b e e n m o d i f i e d t o a l l o w e x t e r n a l c o n t r o l o f ambient gases i n t h e r e a c t i o n c e l l ( 9 ) . S u p p l e m e n t a r y t o t h e s e methods a r e t h e a p p l i c a t i o n , i n o u r l a b o r a t o r y , o f c l a s s i c a l a n d n o v e l o p t i c a l s p e c t r o s c o p i c methods f o r i n s i t u measurement o f t e m p e r a t u r e , f l o w and c e r t a i n s i m p l e s p e c i e s c o n c e n t r a t i o n p r o f i l e s ( 7 ) . I n c o m b i n a t i o n , t h e s e measure ment t o o l s a l l o w f o r a d e t a i l e d f u n d a m e n t a l e x a m i n a t i o n o f t h e v a p o r i z a t i o n and t r a n s p o r t mechanisms o f c o a l m i n e r a l components i n a c o a l conversion o r combustion environment. As a l o n g - t e r m o b j e c t i v e , we a i m t o d e f i n e t h e mechanisms b y w h i c h i n o r g a n i c f u e l i m p u r i t i e s ( p a r t i c u l a r l y K, Na, C I , S, and h e a v y m e t a l s ) and a d d i t i v e s ( e . g . , K i n MHD) a r e r e l e a s e d t o o r removed f r o m t h e e n v i r o n m e n t , t r a n s p o r t e d i n a gas s t r e a m and deposited i n cooler or chemically less reactive regions. To meet t h i s o b j e c t i v e , we a r e a d d r e s s i n g t h e f o l l o w i n g b a s i c t a s k s : (a) Measurement o f s p e c i e s v a p o r i z a t i o n r a t e s and r e l a t e d t h e r m o dynamic f u n c t i o n s f o r w e l l - c h a r a c t e r i z e d s a l t , o x i d e , m i n e r a l ,

In Metal Bonding and Interactions in High Temperature Systems; Gole, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


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s l a g , g l a s s , and ash samples under c o n t r o l l e d gas c o n d i t i o n s and as a f u n c t i o n of temperature, time, gas composition and t o t a l pressure. (b) Development, from the b a s i c data derived from task ( a ) , together with a u x i l i a r y l i t e r a t u r e thermochemical data, of computer-based models f o r p r e d i c t i o n of r e l e a s e or r e t e n t i o n of a l k a l i and other i n o r g a n i c components under a c t u a l c o a l combustion, g a s i f i c a t i o n , or MHD c o n d i t i o n s . (c) V a l i d a t i o n of models developed from task (b) through comparisons with l a r g e - s c a l e t e s t data. This paper summarizes the status of t h i s a c t i v i t y . Emphasis i s given to systems showing unusual behavior, or where a d d i t i o n a l l i n e s of research are revealed. A more comprehensive p r e s e n t a t i o n of data f o r many of the systems considered here may be found i n the c i t e d references. Combustion Systems of I n t e r e s t A l k a l i , i n the form of Na and K-containing s p e c i e s , can lead to a dramatic r e d u c t i o n i n the d u r a b i l i t y of metal ( a l l o y ) and ceramic r e a c t o r components through a complex process known as hot corrosion (5). Examples of energy systems where t h i s process occurs i n c l u d e , c o a l - f i r e d u t i l i t y b o i l e r s , t u r b i n e s , g a s i f i e r s , MHD generators, and p r e s s u r i z e d f l u i d i z e d bed combustors (PFBC). In such cases, the a l k a l i enters the vapor phase by v a p o r i z a t i o n from c o a l minerals, dolomite (10) or limestone, as i n s u l f u r scrubbing processes, or from a i r - i n g e s t e d s a l t p a r t i c l e s (e.g., i n marine environments). Hardesty and Pohl (11) have r e c e n t l y reviewed the major problem areas and data l i m i t a t i o n s r e l a t i n g to the p r o p e r t i e s of c o a l mineral matter and ash. Even a minor amount of a l k a l i vapor t r a n s p o r t can be s i g n i f i cant, as revealed by the t u r b i n e t o l e r a n c e l e v e l of 0 . 0 2 ppm a l k a l i needed f o r c o r r o s i o n c o n t r o l i n p r e s s u r i z e d f l u i d i z e d bed combustors (120• I f we consider only the a l k a l i h a l i d e content of the dolomite component, t h i s tolerance l e v e l would require an a l k a l i - s c r u b b i n g e f f i c i e n c y of b e t t e r than 9 9 . 9 9 9 9 percent f o r PFBC. Even i f c o r r o s i o n ( a l k a l i ) r e s i s t a n t m a t e r i a l s were a v a i l able, u n c o n t r o l l e d a l k a l i vapor t r a n s p o r t would s t i l l lead to unmanageable deposits on cool downstream components. For instance, under t y p i c a l ^ c o a l g a s i f i e r c o n d i t i o n s , a species p a r t i a l pressure as low as 10 atm would lead to vapor t r a n s p o r t and d e p o s i t i o n i n metric ton q u a n t i t i e s on an annual b a s i s . The need f o r a b a s i c understanding of a l k a l i vapor t r a n s p o r t i n f o s s i l energy systems can be appreciated when we consider the d i v e r s i t y of c o n d i t i o n s such as temperature, pressure, chemical composition, and time s c a l e , present i n e x i s t i n g and developing f o s s i l f u e l technologies. Table I summarizes some t y p i c a l process conditions.


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Table I T y p i c a l Coal-Conversion and Combustion Systems Process

Temperature K

Pressure atm

Comments

e

Conventional Steam Plants 1000-1500

1

P r e s s u r i z e d F l u i d i z e d Bed Combustion 1200

2% excess 0 , 300 ppm S 0

1-10


2

3

Coal G a s i f i c a t i o n ^ Cogas

9 C

570-2250

3-6

entrained/slag

Koppers-Totzek^

1750-2100

0

entrained/slag, medium btu

Bi-gas

1200-1920

66-100

480-1750

0-0.3

Atgas

entrained/slag molten i r o n

Kellogg

1100-1480

27-80

molten s a l t

CO^ Acceptor

1100-1370

10-20

fluidized

(dolomite)

Hygas

920-1300

66-100

f l u i d i z e d , high b t u , high S

Synthane

370-1260

33-66

entrained/fluidized, high b t u

Lurgi

870-1260

20-33

f i x e d bed, low b t u

Magnetohydrodynamics Open c y c l e - hot walls

1-10

1500-3000

1% K seed, f u e l r i c h or s t o i c h i o m e t r i c

Comparable c o n d i t i o n s e x i s t i n ammonia p l a n t secondary reformers, e.g., 1060K, 13 to 20 atm, 28 percent H , 50 percent H 0, ,6.4 percent CO, and 4 percent C0 . A f t e r Crowley (14). For t e c h n o l o g i c a l s t a t u s , see Vorres (15) and Lenzer and ^Laurendeau (_16) . In commercial operation. l atm = 1.01325 x 10 kpascal. 2

2

2

e

2


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Coal M i n e r a l C h a r a c t e r i s t i c s T y p i c a l l y , c o a l contains about 10 wt.% mineral matter. There i s a recognized need f o r improved understanding of c o a l mineral transformations and slag-forming processes (13^). The chemical form of a l k a l i and halogen i n c o a l i s of considerable importance to the mode of a l k a l i vapor t r a n s p o r t . Analyses of Gluskoter and Ruch (17) suggest that halogen i s present i n two forms, NaCl and organic. A l s o , most of the potassium i s present i n a halogen-free h i g h l y bound form, such as f o r the mineral i l l i t e , or other potassium a l u m i n o - s i l i c a t e s . A number of c o a l combustion systems u t i l i z e limestone and dolomite a d d i t i o n s f o r s u l f u r removal. These m a t e r i a l s provide an a d d i t i o n a l source of alkali. A l k a l i (Na + K) contents of 0.05 to 1 wt.% are usual and the predominant mineral form i s the c h l o r i d e (18). Laboratory s i m u l a t i o n of c o a l combustion i n d i c a t e s s e v e r a l modes of mineral decomposition (19). Submicron-size p a r t i c l e s tend to be derived from v a p o r i z a t i o n with subsequent homogeneous and heterogeneous condensation. These p a r t i c l e s are r i c h i n s i l i c a (SiO vapor t r a n s p o r t ) but with l a r g e enhancements of t r a c e metals, i n c l u d i n g a l k a l i e s , Cd, As, and other heavy metals. Various mechanisms have been suggested concerning the combustion h i s t o r y of the a l k a l i components (20). The p r i n c i p a l a l t e r n a t i v e s are, that NaCl i s vaporized during combustion and i s not incorporated i n t o s i l i c a t e m i n e r a l s - - a t l e a s t i n the i n i t i a l combustion phase--or, that NaCl reacts with the ash thereby lowering the a l k a l i a c t i v i t y and hence the extent of vapor phase a l k a l i t r a n s p o r t . This l a t t e r statement remains q u a l i t a t i v e pending the determination of a l k a l i a c t i v i t i e s f o r c o a l minerals and slags i n combustion atmospheres. C o r r o s i o n by A l k a l i e s A l k a l i vapor t r a n s p o r t and d e p o s i t i o n i s a well-known, though p o o r l y understood, f a c t o r i n the c o r r o s i o n or f o u l i n g of a l l o y s and ceramics, both i n e s t a b l i s h e d and developing t e c h n o l o g i e s . Problem areas i n c l u d e o i l - f i r e d g l a s s melting operations (21), b l a s t furnaces, b o i l e r s , t u r b i n e s , c o a l g a s i f i c a t i o n (22), MHD (23, 24, 25) and c o a l - f i r e d p r e s s u r i z e d f l u i d i z e d beds (26). In general, the c o r r o s i v e e f f e c t s of a l k a l i deposits r e s u l t from the high s o l u b i l i t y of ceramic and oxide coatings (e.g., f o r a l l o y s ) i n molten a l k a l i s u l f a t e , carbonate, c h l o r i d e , or vanadate d e p o s i t s . This s o l u b i l i t y r e s u l t s from the high s t a b i l i t y of Na (or K ) - A l - s i l i c a t e s , or s i m i l a r oxide phases. Formation of these s i l i c a t e s , f o r i n s t a n c e , leads to a volume increase and l o s s of s t r u c t u r a l i n t e g r i t y i n ceramic m a t e r i a l s (Z7, 28). In a l l o y systems f l u x i n g can a l s o occur and t h i s r e s u l t s i n g r e a t l y increased o x i d a t i o n rates (5). Even when hot c o r r o s i o n i s not a problem, a l k a l i deposits can lead to f o u l i n g and thermal b a r r i e r effects. For i n s t a n c e , i n secondary naphtha reformers, oxide


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deposits containing Na 0 (21 wt.%), K 0 (3 wt.%), plus S i 0 , A 1 0 and CaO, lead to f o u l i n g of waste heat b o i l e r tubes (29). A l k a l i vapor transport and d e p o s i t i o n places severe l i m i t a t i o n s on ceramic m a t e r i a l s f o r MHD generator walls and e l e c t r o d e s . Here, the c o r r o s i v e a c t i o n of K S 0 and K C 0 - c o n t a i n i n g l i q u i d s appears to be the major problem. In the combined presence of potassium seed and coal s l a g , the rate of e l e c t r o c h e m i c a l corros i o n of MHD ceramic electrodes increases by two to three orders of magnitude. A l l o y c o r r o s i o n at intermediate temperatures (900 to 1200 K) a l s o can be r e l a t e d to formation of a l i q u i d N a S 0 phase, which prevents formation of a p r o t e c t i v e C r 0 s c a l e and g r e a t l y enhances c o r r o s i o n of Co- and Ni-Cr a l l o y s (30). The thermodynamics of c o r r o s i v e a l k a l i s a l t - o x i d e i n t e r a c t i o n i s not w e l l e s t a b l i s h e d . In an assessment of research needs f o r m a t e r i a l s i n coal conversion, the need f o r c a r b o n a t e - s i l i c a t e melt s t u d i e s , i n c l u d i n g a c t i v i t y and phase e q u i l i b r i u m measurements, was s t r e s s e d (31). The lack of thermodynamic data f o r fused s a l t s , and t h e i r reactions with oxides and a l l o y s leading to models of hot c o r r o s i o n , was also i n d i c a t e d . Hot c o r r o s i o n of Ni-base turbine a l l o y s by N a S 0 and K S 0 i s s t r o n g l y dependent on the Na 0 a c t i v i t y i n the s a l t (32). Further a c t i v i t y measurements of t h i s type are needed over a wider range of c o n d i t i o n s . Raymon and Sadler (22) have reviewed evidence f o r reactions i n v o l v i n g a l k a l i vapors and r e f r a c t o r y l i n i n g materials f o r coal g a s i f i e r s . They i n d i c a t e a p r e s s i n g need f o r studies of a l k a l i attack i n reducing atmospheres a t pressures up to 30 atm and temperatures to 1500 K. 2

2

2

2

3

2

4

2

3

2


2

2

4

3

4

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2

A l k a l i Vapor Transport

Modeling

Despite the incomplete state of a thermodynamic data base and l i m i t e d mechanistic i n s i g h t , s e v e r a l attempts to model a l k a l i vapor t r a n s p o r t i n r e a c t i v e atmospheres have been made. The increased s o p h i s t i c a t i o n of modeling e f f o r t s i n recent years i s demonstrated by the f o l l o w i n g examples: (a) Coal g a s i f i c a t i o n (33). (b) Glass furnace c o r r o s i o n (34). (c) P r e s s u r i z e d f l u i d i z e d bed combustion (35). In example (a), the gas composition was modeled assuming i d e a l s o l u t i o n phases and n e g l e c t i n g known complex vapor species, such as K S 0 , K C 0 and a l k a l i c h l o r i d e s . These serious l i m i t a t i o n s r e s u l t e d from the n o n - a v a i l a b i l i t y of oxide s o l u t i o n - a c t i v i t y data, accurate vapor species thermodynamic f u n c t i o n s , and the i n a b i l i t y of e x i s t i n g computer codes to handle non-ideal s o l u t i o n multiphase, multicomponent e q u i l i b r i u m computations. The more recent work of example (b) modeled Na vapor t r a n s port by i n c l u d i n g NaOH, NaCl and N a S 0 as vapor species, with the major u n c e r t a i n t i e s a r i s i n g from neglect of s o l u t i o n noni d e a l i t y and inaccurate thermodynamic functions f o r N a S 0 ( 8 ) . 2

4

2

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In example ( c ) , many of the l i m i t a t i o n s represented by (a) and (b) were r e s o l v e d ; the g r e a t e s t u n c e r t a i n t y r e s u l t e d from the h i g h l y approximate nature of the a l k a l i - s i l i c a t e a c t i v i t y data. For t h i s system, the data base requirements are more c r i t i c a l s i n c e , i n p r e s s u r i z e d f l u i d i z e d bed combustion, the a l k a l i t o l e r ance l e v e l s f o r downstream t u r b i n e o p e r a t i o n are of the order of 0.02 ppm. S p a c i l and Luthra (35) compared t h e i r thermochemical p r e d i c t i o n s with observed combustion gas stream a l k a l i concentrations. Here, N a S 0 , NaCl, NaOH, and the K-analogues were included as s i g n i f i c a n t molecular species i n the thermochemical model data base. A l b i t e and sanidine were assumed to represent the c o a l ash a l k a l i - g e t t e r s u b s t r a t e s . F a i r , but encouraging, agreement between c a l c u l a t e d and observed gas phase a l k a l i concent r a t i o n was obtained.


2

4

Experimental Methods The primary experimental methods used i n t h i s study are the Knudsen E f f u s i o n Mass Spectrometric (KMS) and T r a n s p i r a t i o n Mass Spectrometric (TMS) methods, as described elsewhere [KMS, (9); TMS, (8)]. Both are modulated molecular beam methods with phase s e n s i t i v e d e t e c t i o n , and they allow f o r accurate measurement of gaseous and condensible species c o n c e n t r a t i o n s . The b a s i c d i f f e r ences between the KMS and TMS methods are, the upper pressure -4 l i m i t s of 10 and one atm, r e s p e c t i v e l y , and the upper l i m i t gas residence times of about 0.04 and 20 sec, r e s p e c t i v e l y . Thus, the TMS method w i l l more c l o s e l y approach the e q u i l i b r i u m c o n d i t i o n f o r systems e x h i b i t i n g non-equilibrium behavior. As the TMS method i s s t i l l r e l a t i v e l y novel, a b r i e f d e s c r i p t i o n i s given here. The T r a n s p i r a t i o n Reactor. The TMS f a c i l i t y c o n s i s t s of a t r a n s p i r a t i o n r e a c t o r mounted i n one of two a v a i l a b l e m u l t i chambered (two or f o u r ) , d i f f e r e n t i a l l y pumped, vacuum systems with a quadrupole mass f i l t e r (cross-beam) l o c a t e d i n the high vacuum stage. E s s e n t i a l features of the r e a c t o r i n c l u d e : a sample container or boat, a boat c a r r i e r , a thermocouple f o r temperature measurement, a c a r r i e r gas i n l e t system, and a gas e x t r a c t i o n system or probe, as shown i n F i g u r e 1. The boat c a r r i e r allows f o r boat removal from the r e a c t o r without need f o r a complete disassembling of the t r a n s p i r a t i o n system. Molecular beam sonic probes are, t y p i c a l l y , c o n i c a l nozzles with design d e t a i l s determined by reasonably w e l l e s t a b l i s h e d gas dynamic criteria. However, f o r h i g h l y r e a c t i v e systems, we a l s o found i t d e s i r a b l e to develop a more robust c a p i l l a r y probe, at the p o s s i b l e expense of sampling f i d e l i t y . For the present study, a l l of these components were f a b r i c a t e d from platinum metal. The r e a c t o r i s u s u a l l y operated at t o t a l pressures of 0.2 to 1.0 atm, using N or Ar as a c a r r i e r gas and temperatures up to about 1700 K. 2


METAL

BONDING AND

INTERACTIONS


550


34.

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Alkali

Vapor

Transport

551

P a r t i a l Pressure Determination. For the KMS method, conversion of mass s p e c t r a l i o n i n t e n s i t i e s to species p a r t i a l pressures i s made through the b a s i c r e l a t i o n s h i p s , +

P. = k . I . T

(1)

11

I

and

GA.(2rfR/M.)V k, =

I

I

— ^ -l 4 -i

ac £ where, f o r species i ; P

i s p a r t i a l pressure,

ion i n t e n s i t y , T the temperature,


(2)

1+ t ( T ) * the

an instrument and

corresponding system

s e n s i t i v i t y constant, G a g r a v i m e t r i c f a c t o r , A^ an isotope abundance f a c t o r , R the gas constant, nu time, c the C l a u s i n g f a c t o r , a

the weight l o s s , t the

the o r i f i c e area, and H K

the

molecular weight (or an average value f o r Knudsen e f f u s i o n ) , as described elsewhere (9). In some cases, k^ f o r d i f f e r e n t s p e c i e s , but a r i s i n g from the same experimental c o n f i g u r a t i o n , can be i n t e r - r e l a t e d through known i o n i z a t i o n cross s e c t i o n s , Q., and t r a n s m i s s i o n / d e t e c t o r e f f i c i e n c i e s , S^, i . e . , k. oc (a.-S.-A.)" I

as discussed elsewhere (8).

i

The

l

1

(3)

l

degree of agreement f o r the

k/s

obtained from weight l o s s [expression (2)] or cross s e c t i o n data [expression (3)] provides a u s e f u l check on the i n t e r n a l c o n s i s tency of the p a r t i a l pressure data. R e l a t i o n (1) can a l s o be a p p l i e d to TMS data. The s e n s i t i v i t y f a c t o r k^ i s obtained by s e v e r a l independent methods, thereby p r o v i d i n g a good t e s t of i n t e r n a l c o n s i s t e n c y i n the data. b a s i c r e l a t i o n s h i p s are, k. =A.

R 5

A

t

/

£ l .

+

t

The

(4)

and k.

where, f o r species i and t r a n s p o r t gas j (N or A r ) ; n i s the number of moles of substrate t r a n s p o r t e d , V the volume of gas transported, At the t r a n s p o r t time i n t e r v a l , and S an instrument gas s c a t t e r i n g c o r r e c t i o n [S = 0.6 to 1.0; ( 8 ) ] . The reference or t r a n s p o r t gas s e n s i t i v i t y f a c t o r k. i s r e a d i l y obtained from 2


552

METAL BONDING AND

INTERACTIONS

equation (1), since the pressure of t r a n s p o r t gas i s known from an e x t e r n a l manometric determination. S i n g l e Component Systems In complex combustion systems, a l k a l i vapor t r a n s p o r t can occur as the metal or as molecular s p e c i e s , such as NaCl, ( N a C l ) , NaOH, (NaOH) , N a S 0 , NaSO (x = 2,3), NaPO (x = 2,3) and, 2

2

2

4

x

x

p o s s i b l y , other yet-to-be e s t a b l i s h e d p o s t u l a t e d s p e c i e s , such as N a C 0 and NaCl 2H 0. Potassium-containing systems show an analogous behavior. Species i d e n t i t i e s , and t h e i r b a s i c thermodynamic f u n c t i o n s , are u s u a l l y best e s t a b l i s h e d by v a p o r i z a t i o n studies over the s i n g l e component systems. Considerable u n c e r t a i n t y and l i t e r a t u r e disagreement has e x i s t e d f o r most a l k a l i c o n t a i n i n g systems. We have obtained new thermodynamic v a p o r i z a t i o n data f o r the most important systems i n t h i s category. #


2

3

2

NaCl(£) and N a S 0 ( £ ) V a p o r i z a t i o n . These systems are the best e s t a b l i s h e d and they serve, p r i m a r i l y , as t e s t cases f o r the TMS method. The p r i n c i p a l TMS r e s u l t s f o r l i q u i d NaCl may be summarized as f o l l o w s . A d d i t i o n a l d e t a i l may be found elsewhere (8). For the r e a c t i o n , 2

4

N a C l U ) = NaCl(g) log P

(atm) = 4.85

N a C 1

(± 0.3)

- 8820 (± 200)/T

AH

v

(1300) = 40.4

± 0.9

(42.7 ± 3.5)

kcal/mol, and

AS

v

(1300) = 22.2

± 1.4

(24.2 ± 0.5)

cal/deg mol

where T i s i n K e l v i n . These second-law data compare f a v o r a b l y with the JANAF (36) e v a l u a t i o n of previous l i t e r a t u r e data, i n d i c a t e d i n parentheses. Our second-law data, obtained by both TMS and KMS, f o r the d i m e r i z a t i o n r e a c t i o n , 2NaCl(g) =

(NaCl) (g) 2

give - AH

d

(1300) = 46.4

± 0.7

(47.1 ± 3.5)

kcal/mol, and

- AS

d

(1300) = 30.2

± 0.6

(29.7 ± 1.0)

cal/deg mol

which compare f a v o r a b l y with the JANAF (36) values, i n d i c a t e d i n parentheses. Thus, the v a p o r i z a t i o n data f o r t h i s system are well established. The N a S 0 ( £ ) system has h i s t o r i c a l l y been d i f f i c u l t to c h a r a c t e r i z e , due l a r g e l y to containment problems. However, from our TMS and KMS second-law data (8), and other recent l i t e r a t u r e 2

4


34.

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Alkali

Vapor

553

Transport

r e s u l t s , a q u a n t i t a t i v e thermodynamic d e s c r i p t i o n o f t h i s i s now p o s s i b l e . F o r t h e m a j o r v a p o r i z a t i o n p r o c e s s , N a S 0 ( £ ) = 2Na + S 0 + 0 2

4

2

2

AH

v

(1550) = 69.8 ± 3 (71.7

AS

v

(1550) = 27.8 ± 3 (29.7) c a l / d e g m o l

w i t h t h e JANAF (36) reaction,

data given

system

± 0.6) k c a l / m o l , and

i n parentheses.

For the

secondary

Na S0 U) = Na S0 (g) 4


2

2

4

our d a t a a r e c o n s i s t e n t w i t h t h o s e o f K o h l e t a l . ( 3 7 ) , who g i v e , AH

v

(1267) = 65.9 ± 3 k c a l / m o l , a n d

AS

v

(1267) = 22.3

± 2.8 c a l / d e g m o l

K S0 Vaporization. Potassium s u l f a t e i s a p o t e n t i a l l y impor t a n t c o n s t i t u e n t i n c o a l b u r n i n g s y s t e m s , b e i n g formed b y r e a c t i o n of a s h / s l a g potassium w i t h combustion S0 . The s o l u b i l i t y o f K S 0 (and N a S 0 ) i n c o a l a s h i s s m a l l , b u t r e a c t i o n c a n o c c u r t o f o r m K 0 (and N a 0 ) d i s s o l v e d i n t h e a s h w i t h r e l e a s e o f S 0 and 0 . However, t h e d e p o s i t i o n o f K S 0 (and N a S 0 ) i n h o t c o r r o s 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 K 0 (and N a 0 ) i s o n l y p a r t i a l l y removed by i n t e r a c t i o n w i t h c o a l a s h . T h i s c o u l d r e s u l t from slow k i n e t i c processes, competitive chemical reactions, transport o f the s u l f a t e as g a s e o u s m o l e c u l e s , o r b y a s e r i e s o f c h e m i c a l r e a c t i o n s i n v o l v i n g a l k a l i - c o n t a i n i n g gaseous m o l e c u l e s w h i c h l e a d t o a n e t t r a n s p o r t o f K S 0 (or N a S 0 ) across a temperature/concentration gradient. The t h e r m o d y n a m i c s o f K S 0 and N a S 0 c o n d e n s e d p h a s e s a r e g e n e r a l l y c o n s i d e r e d t o be w e l l known and a r e t a b u l a t e d b y JANAF ( 3 6 ) . Thermodynamic d a t a f o r t h e gaseous m o l e c u l e s a r e much l e s s c e r t a i n , however. U s i n g s e p a r a t e g r a v i m e t r i c Knudsen e f f u s i o n and KMS t e c h n i q u e s , we have o b t a i n e d d a t a f o r t h e h e a t o f v a p o r i z a t i o n t o f o r m K S 0 ( g ) , and d e t a i l e d r e s u l t s w i l l a p p e a r e l s e w h e r e ( 3 8 ) . From t h e r m a l d a t a , t h e d i s s o c i a t i o n p r e s s u r e , as r e p r e s e n t e d b y t h e reaction, 2

4

2

2

2

2

2

2

4

2

2

4

2

2

2

4

2

4

2

2

2

4

4

2

4

4

K S0 (s,£) = 2K(g) + S 0 ( g ) + 0 ( g ) 2

4

2

2

[1]

can be r e a d i l y c a l c u l a t e d . The o n l y o t h e r r e a c t i o n o f c o m p a r a b l e importance i s t h a t o f the s u b l i m a t i o n o r v a p o r i z a t i o n p r o c e s s ; K S0 (s,£) = K S 0 ( g ) 2

4

2

4


[2]

4

554

METAL

BONDING AND

INTERACTIONS

A number of a l t e r n a t i v e v a p o r i z a t i o n processes have p r e v i o u s l y been suggested f o r K S 0 . However, one or more of the p o s t u l a t e d r e a c t i o n products [ K 0 ( i , g ) , S 0 ( g ) ] have a s u f f i c i e n t l y p o s i t i v e free energy of formation so that the proposed a l t e r n a t e r e a c t i o n paths lead to an i n s i g n i f i c a n t p a r t i a l pressure of evaporation products compared to r e a c t i o n s [1] and [2]. Most of the previous K S 0 v a p o r i z a t i o n measurements are thought to be i n e r r o r because of container m a t e r i a l r e a c t i o n s or, p o s s i b l y , creep of l i q u i d K S 0 from the metal container ( u s u a l l y a platinum metal). In the present study, weight l o s s measurements of K S 0 ( £ ) were made using a thermobalance equipped with a Pt Knudsen c e l l . Combining these data with the known d i s s o c i a t i o n pressures f o r r e a c t i o n [1] leads to p a r t i a l pressure data f o r r e a c t i o n [2], as shown i n Figure 2. Complementary KMS data were a l s o obtained. For comparison we have also i n d i c a t e d , i n F i g u r e 2, smoothed data extrapolated from the t o r s i o n e f f u s i o n measurements of Lau, et a l . (39), and data from two mass e f f u s i o n measurements made by Efimova and Gorokhov (40). The absolute pressure data from these three independent recent i n v e s t i g a t i o n s i s i n unusually good agreement, but some s i g n i f i c a n t d i f f e r e n c e s s t i l l e x i s t (38). These pressure data are about 400 percent greater than the c o r r e s ponding values of F i c a l o r a et a l . (41). 2

4

2

2

4

2

4

3


2

4

K0H(£) and KC1(£) V a p o r i z a t i o n . The thermodynamic s t a b i l i t y of KOH i n the vapor phase can be obtained from the Second and T h i r d Law analyses of K0H(£) v a p o r i z a t i o n as the thermodynamic functions f o r the l i q u i d phase are reasonably w e l l e s t a b l i s h e d . JANAF (36) has evaluated the various d i s p a r a t e sets of KOH v a p o r i z a t i o n data but with considerable u n c e r t a i n t y . Much of the d i f f i c u l t y a s s o c i a t e d with o b t a i n i n g r e l i a b l e thermodynamic data f o r t h i s system a r i s e s from i t s r e a c t i v i t y with container m a t e r i a l s , the presence of carbonate impurity, and the coexistence of dimers and monomers. Previous studies have a l s o been hampered by decomp o s i t i o n to K and H 0. In the present work, using the TMS t e c h n i que, we have suppressed t h i s decomposition by a d d i t i o n of H 0 to the c a r r i e r gas. We have obtained extensive data f o r the K0H(£) and KC1(£) systems, which w i l l be presented i n a formal p u b l i c a t i o n e l s e where (42). Representative data f o r K0H(£) are presented here i n comparison with other recent r e s u l t s not considered by JANAF (36). Species p a r t i a l pressure data are summarized i n F i g u r e 3. Note that the KOH species data are i n good agreement with JANAF (36), as might be expected. However, there i s no agreement between workers regarding the (K0H) s p e c i e s , except that the r e l a t i v e amounts of dimer to monomer found i n the present study agree quite w e l l with the KMS r e s u l t s of Gusarov and Gorokhov (43). When the monomer and dimer p a r t i a l pressures are summed, the t o t a l pressures are about a f a c t o r of two greater than the JANAF (36) data. 2

2

2



10

4

/ T (K)

8.0

1250

TEMPERATURE

(K)

9.0

1110

2

4

Figure 2. Partial pressure mass effusion data for molecular K S0 over KzSOrfl) (3S). Key: O , present study; A , Ref. 39; and A , Ref. 40.

7.0

1428

(K)

2

Figure 3. Partial pressure data for KOH and (KOH) over liquid KOH, obtained by TMS. Key: , present study; , Ref. 43; and , Ref. 36.

104 / T (K)

TEMPERATURE


J-JJ

METAL

556

BONDING A N D INTERACTIONS

With regard to the KCl(J^) system, the KCl species data were found to be i n good agreement with JANAF (36). However, the dimer species (KC1) was found to have an appreciably greater enthalpy of formation than the JANAF (36) value. 2


Coal M i n e r a l Systems A l k a l i Benzoate/Carbonate V a p o r i z a t i o n . Part o f the a l k a l i content i n coal i s o r g a n i c a l l y bound and the benzoate s a l t s , NaCOO0 and KCOO0, have been s e l e c t e d to model a l k a l i release from such a state e.g., see Stewart e t a l . (44). For Na and K i n c o a l , t y p i c a l o r g a n i c / i n o r g a n i c d i s t r i b u t i o n r a t i o s l i e i n the range 2 to 9, and 0.02 to 0.1, r e s p e c t i v e l y . Using the TMS approach, we have obtained comprehensive data on the mechanism o f Na and K release from t h e i r benzoates and the r e l a t e d carbonatechar systems. The d e t a i l e d r e s u l t s w i l l be reported elsewhere (45). The p r i n c i p a l f i n d i n g s are as f o l l o w s . On heating to ^ 800 K, sodium and potassium benzoates decompose to y i e l d a carbonate plus char residue. At temperatures of ~ 1000 K, t h i s residue reacts to y i e l d Na i n the vapor phase according to the r e a c t i o n , N a C 0 ( c ) + 2C(c) = 2Na + 3C0 2

3

and s i m i l a r l y f o r potassium. Comparison of species p a r t i a l pressures with those of multicomponent e q u i l i b r i u m c a l c u l a t i o n s i n d i c a t e that t h i s r e a c t i o n i s a t , or near, e q u i l i b r i u m , a surp r i s i n g r e s u l t f o r what i s f o r m a l l y considered a s o l i d - s o l i d i n t e r a c t i o n at these r e l a t i v e l y low temperatures. Thus, the organic a l k a l i components of c o a l can be a s i g n i f i c a n t source o f a l k a l i i n the gas phase. Future studies are planned i n the presence of r e a c t i v e combustion gases, e.g., S 0 , H 0, C 0 , H , and HC1. 2

2

2

2

I l l i t e V a p o r i z a t i o n . The c l a y mineral i l l i t e i s f r e q u e n t l y found as a mineral c o n s t i t u e n t of c o a l and i s considered a major source of potassium vapor species i n combustion systems. A "Beaver's Bend i l l i t e " sample was used (provided by the Morgantown Energy Technology Center) with the f o l l o w i n g composition f o r the major components, i n wt.%: A l 0 ( 2 6 . 0 ) , F e 0 ( 4 . 4 ) , S i 0 ( 6 0 . 2 ) , K 0(7.4), Na 0(0.2), Mg0(2.1), and S(0.1). A water a n a l y s i s , c a r r i e d out by heating to 1300 K i n a i r , i n d i c a t e d a water content of 7.3 percent. I t should be noted that water i s present i n the i l l i t e s t r u c t u r e as OH groups and can be expelled only by heating to r e l a t i v e l y high temperatures. The melting behavior of dehydrated i l l i t e can be expected to be s i m i l a r to that f o r an approximately 30-60-10 wt.% composition i n the A l 0 - S i 0 - K 0 ternary system where melting begins a t about 1300 K. From t h i s analogy we can assume that over the 2

2

3

2

3

2

2

2

3

2

2


34.

HASTIE ET AL.

Alkali

Vapor

557

Transport

temperature range of the present experiments 1540 to 1950 K ) , i l l i t e (dehydrated) w i l l be present as a reasonably homogeneous liquid. During the i n i t i a l heating p e r i o d , s i g n i f i c a n t r e l e a s e of Na and K to the vapor phase was noted over the temperature i n t e r v a l 1300 K to 1500 K. The predominant vapor species were S 0 2

(y

atm), Na {y 10 atm), K {y 10" atm), and p o s s i b l y -6 NaOH (y 10 atm). The mass s p e c t r a l s i g n a l s f o r the l a t t e r species were l e s s c e r t a i n , being detected as s i g n a l / n o i s e r a t i o s of about two. These i n i t i a l data a l s o tended to show time dependence i n the form of pressure bursts and isothermal s i g n a l decay i n d i c a t i v e of sample inhomogeneity. A l s o during t h i s i n i t i a l p e r i o d , the 0 pressures were i n excess o f the r e a c t i o n [3] l e v e l owing to r e d u c t i o n o f F e 0 to F e 0 . F o l l o w i n g t h i s i n i t i a l v a p o r i z a t i o n phase the p r i n c i p a l r e a c t i o n s were, 10


2

2

K0

3

3

4

(soln.) = 2K(g)

2

+ 1/2 0 ( g ) 2

[3]

and Si0

2

( s o l n . ) = SiO(g) + 1/2 0 ( g ) 2

[4]

At the higher temperatures, F e 0 c o n t r i b u t e d to a small p a r t i a l pressure of F e ( g ) , of a magnitude s i m i l a r to the SiO(g) pressure. Representative v a p o r i z a t i o n data, obtained using the KMS and TMS methods, are given i n Figure 4 (see a l s o Table I I ) . A more d e t a i l e d account of t h i s study i s given elsewhere (46). Not a l l the 0 pressure data p o i n t s appear i n the f i g u r e f o r reasons of clarity. The 0 /K pressure r a t i o s v a r i e d from about 2 to 1/2 during the experiment. This behavior i s not c o n s i s t e n t with the v a p o r i z a t i o n s t o i c h i o m e t r y i n d i c a t e d by r e a c t i o n s [3] and [4]. Since the K pressure i s about 25 times g r e a t e r than f o r SiO, r e a c t i o n [4] i s a n e g l i g i b l e source of 0 , and most o f the 0 should r e s u l t from K 0 decomposition according to r e a c t i o n [3]. For t h i s to be t r u e , the 0 pressure should be only about 1/4 of the K pressure and have the same temperature dependence as K. The v a r i a t i o n of 0 /K pressure with time may be due to excess 0 d i s s o l v e d i n the i l l i t e or, more l i k e l y , to i r o n oxide decomposition. This time-dependent oxygen a c t i v i t y could a l s o account f o r the K pressure d i f f e r e n c e s between the KMS and TMS methods, as shown i n F i g u r e 4. 3

4

2

2

2

2

2

2

2

The

2

Soda-Lime-Silica System

Background. A commercially common s o d a - l i m e - s i l i c a g l a s s has been considered as an absorbing medium f o r removing f l y ash p a r t i c u l a t e s i n combustion gas streams (47). However, a p o s s i b l e l i m i t a t i o n with t h i s a p p l i c a t i o n i s the r e l e a s e of a l k a l i from the g l a s s i n t o the gas stream. Glass a l s o has some common features


METAL BONDING AND INTERACTIONS

558


TEMPERATURE K

5.0

6.0

7.0

104 / T (K) Figure 4. Partial pressure data for K, 0 , and SiO over illite. Key: A , TMS method with N atmosphere; • , O, KMS under vacuum vaporization conditions; and #, 0 with ticks indicating increasing (up) or decreasing (down) temperature chronology. 2

2

2



3

2

3

2

2

2

K 0-Al 0 -Si0

2

2

2

K 0-Al 0 -Si0

2

K 0-Si0

System

4

(KAlSi0 )

u

II

Probably not equilibrated because of slow condensed phase k i n e t i c s (59, 60).

16750

20923

20763

7.028

8.722

7.068

1200-1600

2

3

4

o

i

2

3

2

2

1800-2150

2

1750-2150

2

1600-1900

Liquid + 3Al 0 «2Si0 ,10.8[K 0]

2

A l ^ ,

Phases uncertain ^ 11.5[K 0]

3

6

1600-2000

2

2

«v 22[K 0]

Phases uncertain % 17[K 0]

2

K 0-9A1 0

Possibly K A l S i 0 ,

o

3

3

KAlSiO., K 0-9Al 0_, KAlSi-0. 4' 2 2 3 2 6

2

K 0.6Fe 0 -Fe 0

2

1400-1650

Phase boundary composition s l i g h t l y dependent on T. Data adjusted f o r constant K/0 (58). 12873

4.524

2

3

20693 19800 20424

6.350 4.667 4.464

8

Decreasing T-chronology used to minimize composition change.

Evidence of melt formation.

2

(57).

s i t i o n (56).

1150-1450

2

KFe0 -K 0.6Fe 0

2

15036

12873

3

3

- A1 0 ^

2

4.524

2

2

1100-1450

2

2

21453

Pressures higher than e a r l i e r data (56) because of o r i f i c e e f f e c t .

7.487

15624

Comments

5.489

C = 9.32

B

d,e

1500-1800

4.721

K-Pressure atm

1300-1800

0

KA10 + K 0-5A1 0 "

2

Temperature K

K 0.9A1 0

e

1300-1800

b

S o l u t i o n 44 t o * 5[K 0]

a

Phases ' '

Summary o f A l k a l i V a p o r i z a t i o n Data f o r K - C o n t a i n i n g Complex O x i d e and C o a l S l a g Systems

Table



2

3

2

Slags

3

3

2

3.8,

3

2

3.3,

3

3

2

[K 0]

7.4,

3

2

3

4.4,

2

iMgO] 2.1, [ S i 0 ] 6 0 . 2 .

2

12.5,

0.6,

3

[ A 1 0 ] 26.0, [ F e 0 ]

Illite:

2

2

[Fe 0 ]

[CaO] 1.8, [MgO] [ S i 0 ] 36.0.

2

[ A 1 0 ] 25.5,

2

S y n t h e t i c E a s t e r n Channel slag: [K 0] 23.6,

2

[CaO] 9.3, [MgO] [ S i 0 ] 34.8.

2

[ A 1 0 ] 24.6, [ F e 0 ] 5.3,

2

S y n t h e t i c Western Channel slag: [ K 0 ] 22.7,

2

2

[CaO]

1.0, ( S i 0 ] 46.8,

14.3,

[ N a 0 ] 0.5.

[MgO]

2

[Fe 0 J

2

R e a l MHD C h a n n e l S l a g (K ) : [ K 0 J 19.5, [ A 1 0 ] 1 2 . i f

2

3

S i m p l i f i e d Western K 0-Ca0-Al 0 -Si0

System

2

K 0

4

content

+ KAlSi0

Liquid

KAlSiO,

KAlSiO^

+ g l a s s y phase

+ g l a s s y phase

unidentified crystalline phase + s l a g

major + minor

higher

KAlSiO^

at

Solution

Phases

a,b,e

1550-2085

1600-1750

1500-1700

1600-1800

1400-1900

0

Continued

Temperature K

Table II. d,e

6.286

6.831

5.564

20642

19056

18472

16650

5.228 16794 C = 7.887 e x p - 2 , D = 1.741 e x p - 3 , E = 1.825 e x p - 2

K-Pressure atm


17-36,

2

3

3-14,

Standard

dev.

14-36,

2

[K 0]

(60-64).

4

14

(46).

3

2

[K 0].

18.9-17.6

Linear

(66).

9 (66).

por-

2

to [K 0]

o f AB, BC c u r v e s i n Figure

tion

2

corresponds (65).

23-3-22.1

2

[K 0]

~

Equation

2

I l l i n o i s n o . 6 c o a l , UTSI t e s t sample. I n i t i a l l y high press u r e o f K due t o p r e s e n c e o f K S0 and K C 0 (0.2 percent S).

have been o b t a i n e d

2

^ 30 p e r c e n t , maximum d e v . ~ 100 p e r c e n t . Additional data f o r 20[K O] compositions

2

[A1 0 ]

range,

[Si0 ]-balance.

[CaO]

Composition

Comments

3

2

5

H M > O H

2

o

>

o

o

w o

>

H

3

as o

HASTIE

Alkali

ET AL.

Vapor

561

Transport


3

ft

v a.

N

c

o

G 4J *J O

• 4»c •

O

r>-

V

•—•

>>co

\

NEUDORF t

KMS

\

KMS(1)

(3)\

I 7.0

6.0

8.0

10 / T (K) 4

Figure 5. Comparison of glass melt Na partial pressure data obtained by various workers for compositions (wt %) similar to Na 0(17), CaO(12), and Si0 (71). KMS(l), 17.0-16.7 and KMS(3), 15.6-13.3 wt %. TMS(l), 17.0 and TMS(2), 16.9 wt %. Key: #, Ref. 49; • , Ref. 51; A, Ref. 52; and O, Ref. 50. 2


2

564

METAL

BONDING AND

INTERACTIONS

elsewhere (48) and the data p o i n t s c l o s e l y followed the curves given i n Figure 5. The v e r t i c a l bar represents the maximum conceivable e r r o r that could have a r i s e n with the KMS (run 1) data. Several data sets from the l i t e r a t u r e are i n d i c a t e d f o r comparison. Neudorf and E l l i o t t (49) measured Na 0 a c t i v i t i e s i n the binary s i l i c a t e s o l u t i o n , as w e l l as the e f f e c t s of CaO on the Na 0 a c t i v i t y , using an emf method. We have extrapolated t h e i r data to our experimental conditions based on the e f f e c t s of Na 0 and CaO content on the Na 0 a c t i v i t y . The data p o i n t of Cable and Chaudhry (50) was obtained by a c l a s s i c a l t r a n s p i r a t i o n method under conditions where surface segregation e f f e c t s were negligible. S i m i l a r l y , the data p o i n t of Sanders e t a l . (51) represents a s t i r r e d - m e l t t r a n s p i r a t i o n experiment where surface d e p l e t i o n i s a l s o u n l i k e l y . The curve of Argent e t a l . (52) represents Knudsen e f f u s i o n mass spectrometric data (without beam modulation). 2

2


2

2

TMS Measurements. Glass v a p o r i z a t i o n i n a N atmosphere was monitored using the TMS technique. Representative N a - p a r t i a l pressure curves are given i n Figure 5. Note that these p a r t i a l pressures are more than an order of magnitude greater than those obtained by the KMS technique. I d e a l l y , both sets of data should c o i n c i d e . We b e l i e v e that the explanation f o r t h i s apparent discrepancy i s as f o l l o w s . Under the conditions of the KMS experiments, the rate of a l k a l i removal was about an order o f magnitude greater than f o r TMS. This may be seen by the c a l c u l a t e d composition change i n the r e s p e c t i v e glass samples, i . e . , KMS(3) f i n a l wt.% Na 0 = 13.3 and TMS(2) = 16.9 as compared with the i n i t i a l composition of 17.0 wt.%. During the i n i t i a l phase of each type of experiment, excessive amounts of C 0 and Na were released. Only when about one percent of the glass Na 0 was depleted d i d the excess C 0 and Na become n e g l i g i b l e i n the KMS experiments. As t h i s l e v e l of a l k a l i d e p l e t i o n was never reached during the TMS experiments, we b e l i e v e that these l a t t e r data correspond to the anomalously high Na pressures found i n the e a r l y phase o f the KMS experiments. These high a l k a l i pressures can be a t t r i b u t e d to the presence o f unreacted N a C 0 impurity i n the o r i g i n a l glass samples, even though care was taken to avoid t h i s i n the glass p r e p a r a t i o n . Residual carbonate impurity i s a common problem with glass e x p e r i mentation e.g., see Cable and Chaudhry (50). We c a l c u l a t e from the time-integrated C 0 and excess Na s i g nals that the i n i t i a l concentration o f impurity N a C 0 was 0.45 wt.%. From the r e l a t i v e amounts of Na, 0 , and C 0 released p r i o r to v a p o r i z a t i o n from the s i l i c a t e i t s e l f , two types of i m p u r i t y - r e l a t e d v a p o r i z a t i o n processes appear to be present, as represented by r e a c t i o n s [6] and [7]. Comparison of the TMS and KMS data i n d i c a t e s that the a c t i v i t y of Na 0 ( s o l u t i o n ) , produced by r e a c t i o n [7], i s s u b s t a n t i a l l y greater than that f o r the s i l i cate-bound Na 0 c h a r a c t e r i s t i c of the p r i s t i n e g l a s s . Apparently, 2

2

2

2

2

2

3

2

2

2

3

2

2

2


34.

HASTIE ET AL.

Alkali

Vapor

Transport

565

the Na 0 produced i n s i t u by carbonate decomposition i s not r e a d i l y incorporated i n t o the s i l i c a t e matrix, at l e a s t on the time s c a l e of the v a p o r i z a t i o n measurements. Formally, we can consider N a C 0 as a s o l u t e , i n a metastable glass s o l u t i o n , and with an a c t i v i t y defined by r e a c t i o n [6], as discussed i n the f o l l o w i n g s e c t i o n . A l t e r n a t i v e l y , one could argue that under the higher v a p o r i z a t i o n rate c o n d i t i o n s t y p i c a l of KMS, surface d e p l e t i o n of a l k a l i l e d to the r e l a t i v e l y low Na-pressures observed, e.g., see Cable and Chaudhry (50). However, no s i g n i f i c a n t isothermal time dependent v a p o r i z a t i o n was noted on the s e v e r a l minute time s c a l e of i n d i v i d u a l KMS measurements. A l s o , the KMS(l) pressures are greater than the s t i r r e d - m e l t data of Sanders et a l . , (51) which would not be the case i n a s u r f a c e - a l k a l i depleted system. 2


2

3

N a C 0 i n Glass. We have i n t e r p r e t e d the anomalously high a l k a l i vapor pressures of the TMS experiments described above i n terms of impurity N a C 0 decomposition i n a glass s o l u t i o n . By monitoring the r e l e a s e of C0 , and i n t e g r a t i n g over time, we have determined the mole f r a c t i o n s of N a C 0 present at the v a r i o u s measurement temperatures and times. Hence, from the observed p a r t i a l pressures f o r r e a c t i o n [6], and the corresponding reference s t a t e values (36), we can c a l c u l a t e N a C 0 a c t i v i t y c o e f f i c i e n t data, as shown i n F i g u r e 6. These data appear to be thermodynamically reasonable and tend to support the a l k a l i carbonate impurity i n t e r p r e t a t i o n of a l k a l i v a p o r i z a t i o n d i f f e r e n c e s between the KMS and TMS experiments (see above). We can l i k e w i s e argue that the data of Cable and Chaudhry (50), shown i n F i g u r e 5, a l s o appear to s u f f e r from t h i s impurity problem, even though they a l s o took precautions to e l i m i n a t e r e s i d u a l carbonate during the g l a s s synthesis process. Future s t u d i e s should be pursued under c o n t r o l l e d doping c o n d i t i o n s and i n atmospheres c o n t a i n i n g C 0 and 0 . The known s y n e r g i s t i c e f f e c t of C 0 on 0 - s o l u b i l i t y i n s i l i c a t e melts a t very high gas pressures has, i n f a c t , been i n t e r p r e t e d i n terms of N a C 0 formation i n s o l u t i o n . E f f e c t s of t h i s type could s i g n i f i c a n t l y enhance a l k a l i vapor t r a n s p o r t i n p r a c t i c a l combust i o n systems. 2

3

2

3

2

2

3

2

3

2

2

2

2

2

3

Complex Oxide Systems and Slags Background. As p a r t of a program of systematic s t u d i e s on potassium-containing s l a g systems, v a p o r i z a t i o n data have been obtained f o r a s e r i e s of s y n t h e t i c - b i n a r y , t e r n a r y , quarternary and sexternary oxide mixtures, as w e l l as an a c t u a l MHD c o a l - s l a g sample. V i r t u a l l y no experimental thermodynamic a c t i v i t y data e x i s t f o r these systems. Even f o r the most studied r e l a t i v e l y simple K 0 - S i 0 system, e x i s t i n g data i s extremely crude and incomplete. Comparison of our a c t i v i t y data f o r t h i s system with those reported by Charles (53), as based on the moist-atmosphere 2

2


METAL

BONDING AND

INTERACTIONS


566

Figure 6. Activity coefficient data (TMS) for Na C0 (0.45 wt %) in glass (see Fig. 5). O, run chronology of increasing temperature; and •, run chronology of decreasing temperature. 2

3


34.

HASTIE ET AL.

Alkali

Vapor

567

Transport

t r a n s p i r a t i o n data of Preston and Turner (54), i n d i c a t e s e v e r a l order-of-magnitude d i f f e r e n c e s , with the Charles's r e s u l t s being low. Likewise, the estimates of S p a c i l and Luthra (35), as based on the phase diagram, y i e l d a c t i v i t i e s one to two orders of magnitude lower than our experimental values [ f o r d e t a i l s , see (26)]. Table II contains a summary of s e l e c t e d potassium v a p o r i z a t i o n data f o r these systems. With a few noted exceptions, the oxygen p a r t i a l pressures coupled s t o i c h i o m e t r i c a l l y with potassium, i n keeping with r e a c t i o n [3]. Hence, K 0 ( £ ) a c t i v i t i e s can be derived, to a good to e x c e l l e n t approximation, using the c o r r e s ponding d i s s o c i a t i o n - p r e s s u r e data f o r pure l i q u i d K 0. That i s , 2

2


%o where

= ( / p

/

/2

2K

6

P

i s the d i s s o c i a t i o n constant (see r e a c t i o n [3]) f o r pure

l i q u i d K 0. We have derived a temperature dependent expression for K as f o l l o w s . From JANAF (36), K data f o r K 0 ( s ) are 2

p

2

a v a i l a b l e . Combining these r e s u l t s with the f u s i o n enthalpy and entropy data of Natola and Touzain (55) , and an estimated C f o r l i q u i d K 0 of 25 cal/deg mol, leads to the expression, ^ 2

log K

[K 0(£)] =

p

+ 11.7723

2

The potassium vapor pressure data were obtained under n e u t r a l conditions using the TMS technique. For most coal-conversion and combustion systems of i n t e r e s t , i r o n w i l l be present i n the s l a g as F e 0 . Hence the data reported i n Table II were obtained, f o r the most p a r t , at temperatures (and run times) where F e 0 (slag) had e s s e n t i a l l y converted to F e 0 ( s l a g ) . Evidence of t h i s reduction was conveniently e s t a b l i s h e d by monitoring the 0 pressure. These r e s u l t s can a l s o be a p p l i e d to systems where 0 sources other than r e a c t i o n [3] are present. In t h i s case, the K-pressures are converted to a c t i v i t i e s using equation (6). The a c t i v i t y data are then combined with the known e q u i l i b r i u m constants f o r r e a c t i o n [3], and the assigned 0 - p r e s s u r e s , to y i e l d the new K-pressure data. For r e l a t i v e l y low temperature o x i d i z i n g c o n d i t i o n s , where F e 0 (slag) may be present, we can reasonably assume that the present a l k a l i a c t i v i t y data w i l l be v a l i d . That i s , the various forms of i r o n oxide do not s i g n i f i c a n t l y a f f e c t the a l k a l i a c t i v i t y . Experimental d e t a i l s and an extended d i s c u s s i o n of these potassium vapor pressure r e s u l t s have e i t h e r appeared, or w i l l appear, elsewhere, as i n d i c a t e d i n Table I I . In general, the potassium v a p o r i z a t i o n data followed the expected C l a u s i u s Clapeyron behavior, as i n d i c a t e d i n Table I I . Exceptions to t h i s behavior could be traced to: 3

4

2

3

3

4

2

2

2

2

3


568

METAL

BONDING AND

INTERACTIONS


(a)

r e s i d u a l a l k a l i carbonate i m p u r i t i e s r e s u l t i n g e i t h e r from the sample synthesis method, or from condensation out of MHD plasmas with a c t u a l s l a g samples; (b) a l k a l i carbonate and s u l f a t e phases i n r e a l s l a g samples; (c) non-equilibrium e f f e c t s i n the condensed phase; (d) non-equilibrium between the condensed and vapor phase l e a d i n g to an unsaturated vapor; (e) changing phase boundaries due to incongruent v a p o r i z a t i o n ; ( f ) r a p i d l o s s of a l k a l i pressure with time (T c o n s t a n t ) , p o s s i b l y r e s u l t i n g from surface d e p l e t i o n i n h i g h l y viscous systems; and, (g) F e - c o n t r o l l e d redox r e a c t i o n s r e s u l t i n g i n changing oxygen and, hence, K - p a r t i a l pressure data with temperature and time. A systematic study of systems with a p r o g r e s s i v e increase i n the number of components was made i n order to i s o l a t e and q u a n t i f y such behavior. Some of these e x c e p t i o n a l cases are discussed i n the f o l l o w i n g s e c t i o n s . K 0-Al 0 -Si0 System. The K 0 - A l 0 - S i 0 system has the p o t e n t i a l f o r forming s e v e r a l s t a b l e or metastable phase assemblages i n which the K 0 a c t i v i t y i s f i x e d according to the phase r u l e (see Table I I ) . However, i n p r a c t i c e , we observe the K pressure (and, hence, K 0 a c t i v i t y ) to be dependent on the bulk K 0 c o n c e n t r a t i o n . This e f f e c t could r e s u l t from s e v e r a l none q u i l i b r i u m f a c t o r s i n c l u d i n g , slow condensed phase k i n e t i c s a t t r i b u t a b l e to the complex c r y s t a l chemistry, changing composition i n the p-alumina phase (which extends from K 0 / A 1 0 r a t i o s of 1/5 to 1/9), or to d i s s o l u t i o n of S i 0 i n the p-alumina phase. The vapor pressure equation i n Table I I , f o r the three phase region, i s based on the highest a l k a l i - p r e s s u r e s which were e f f e c t i v e l y independent of composition. Other evidence f o r non-equilibrium behavior i s shown i n Figure 7. In the previous experimental run, a l k a l i - p r e s s u r e data were obtained as a f u n c t i o n of i n c r e a s i n g temperature up to 1860 K. On decreasing the temperature, lower pressures were found than f o r the increased-temperature run, p a r t i c u l a r l y a t 1760 K and below. As shown i n F i g u r e 7, c o n t i n u a t i o n of t h i s experiment i n i t i a l l y produced low pressures over the AB i n t e r v a l . The temperature and K 0 c o n c e n t r a t i o n at which t h i s phenomenon was noted i s reasonably c o n s i s t e n t with the phase diagram (67) which shows e u t e c t i c melting at 1829 K and 22 wt.% K 0. We a t t r i b u t e t h i s l o s s of a l k a l i - v o l a t i l i t y to formation at the sample surface of a frozen e u t e c t i c melt which i s probably g l a s s y i n nature. F o r an e q u i l i b r i u m system, t h i s melt would r e c r y s t a l l i z e but f o r the present experimental c o n d i t i o n s there was probably i n s u f f i c i e n t time. The phase being depleted by v a p o r i z a t i o n i s K A l S i 0 ( k a l s i l i t e ) and the p r e s s u r e - l o s s r e s u l t s from the slow a l k a l i t r a n s f e r rate across the frozen e u t e c t i c which r e s u l t s i n p a r t i a l i s o l a t i o n of the remaining K A l S i 0 . Note, i n Figure 7, 2

2

3

2

2

2

3

2

2

2

2

2

2

3

2

2

2

4

4


HASTIE ET AL.

Alkali

Vapor

TEMPERATURE 1923


569

Transport

1724

5.0

(K) 1515

6.0 10

4

/ T (K)

Figure 7. Nonequilibrium vaporization effect in the K 0-Al 0 -Si0 system. represents pure KAlSiO,, phase. Run chronology for the partially decomposed KAlSiO system follows the temperature sequence ABC (KMS data). 2

2

3

2

h


570


the upward curvature of the AB i n t e r v a l with i n c r e a s i n g temperature. This unusual behavior probably r e s u l t s from remelting o f the e u t e c t i c barrier-phase with an increased rate of a l k a l i t r a n s p o r t through t h i s b a r r i e r together with i n c o r p o r a t i o n of a d d i t i o n a l K A l S i 0 i n the e u t e c t i c melt. The BC i n t e r v a l of Figure 7 represents v a p o r i z a t i o n from t h i s regenerated phase. This e f f e c t was noted only f o r experiments where a small amount of the K A l S i 0 phase remained and where a p o t a s s i u m - d e f i c i e n t surface glaze acted as a d i f f u s i o n b a r r i e r to v a p o r i z a t i o n . Representative vapor pressure data f o r "normal" behavior i n t h i s system are summarized i n Table I I . 4

4

K 0 - C a 0 - A l 0 - S i 0 System ( S i m p l i f i e d "Western" S l a g ) . These four component systems, designated as S i m p l i f i e d "Western" slags i n Table I I , are r e l a t i v e l y w e l l behaved i n terms of a l k a l i v a p o r i z a t i o n and are u s e f u l model systems f o r sub-bituminous b a s i c coal s l a g s . The data have been cast i n a n a l y t i c a l form, as summarized i n Table I I .


2

2

3

2

K 0 - C a 0 - M g 0 - A l 0 - F e 0 - S i 0 System (Synthetic MHD Channel Slags of "Eastern" and "Western" Coal Types). Literature compositional analyses of s e v e r a l hundred coal ash and a few MHD channel s l a g s , c o n t a i n i n g potassium seed, have been evaluated f o r the purpose of s e l e c t i n g r e p r e s e n t a t i v e compositions f o r modeling. Table II i n d i c a t e s the compositions s e l e c t e d as most representat i v e of MHD channel slags of "Eastern" and "Western" coal types. These slags are non-glassy with K A l S i 0 as the dominant c r y s t a l l i n e phase ( i n both s l a g types) and small amounts of K A l S i 0 and at l e a s t one u n i d e n t i f i e d phase ( f o r "Eastern" s l a g type o n l y ) . Thus, the bulk composition i s not as meaningful a v a r i a b l e as f o r homogeneous g l a s s y s l a g s . Vapor pressure measurements were made using a r e l a t i v e l y small e f f u s i o n o r i f i c e (0.34 mm diameter) to prevent vapor u n s a t u r a t i o n e f f e c t s found e a r l i e r on s i m i l a r mixtures but with l a r g e r o r i f i c e s (0.5 to 1.0 mm). Steady s t a t e pressures were not obtained u n t i l about ten percent of the K 0 content had been depleted by v a p o r i z a t i o n . Excess oxygen was vaporized during t h i s i n i t i a l experimental phase with r e d u c t i o n of FeO (x = 1.5 to 1.33). Following t h i s i n i t i a l r e d u c t i o n p e r i o d , the oxygen p a r t i a l pressures were, w i t h i n experimental e r r o r , what would be expected f o r K 0 d i s s o c i a t i o n ( r e a c t i o n [ 3 ] ) . 2

2

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2

3

2

4

2

6

2

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Figure 8 compares vapor pressure data f o r "Western" and "Eastern" s l a g s . This comparison c l e a r l y demonstrates a b a s i c d i f f e r e n c e between these s l a g types i n that "Eastern" slags require about twice as much K 0-content to achieve s i m i l a r a l k a l i pressures as f o r "Western" s l a g s . This e f f e c t r e s u l t s from the higher concentration of b a s i c CaO and MgO i n "Western" slags leading to a l e s s complex s i l i c a t e s t r u c t u r e i n the "Western" s l a g and, hence, a l e s s bound form of K 0. The v a p o r i z a t i o n behavior of the "Eastern" s l a g , i n the composition range 24 to 21 wt.% K 0, i s very d i f f e r e n t to t h a t 2

2

2


HASTIE ET AL.

Alkali

Vapor

571

Transport

TEMPERATURE (K) 1613

1515


1724

5.6

6.0

6.4

6.8

1 0 / T (K) 4

Figure 8. Selected K partial pressure data (KMS data) for synthetic "Western" (W) and "Eastern" (E) MHD-channel slags with K 0 compositions (wt %) of W , 19-17.6(0); W 12.9-10.8(A); and E, 23.3-22.1 ( ). "Eastern" slag data points omitted for clarity (66). 2

3>


2

572

METAL

BONDING AND

INTERACTIONS

for the "Western" s l a g as shown i n F i g u r e 9. Here, the vapor pressure curves show a s i g n i f i c a n t break at l o c a t i o n X, the p o s i t i o n of which depends on the amount of sample v a p o r i z e d . Note the s i g n i f i c a n t p o s i t i v e d e v i a t i o n of the observed K and 0 - p r e s s u r e s (XC i n t e r v a l ) from those obtained by e x t r a p o l a t i n g the higher temperature l i n e a r p o r t i o n of the r e s p e c t i v e curves. The curves l a b e l e d AB i n F i g u r e 9 were obtained p r i o r to the curves l a b e l e d BC. At temperatures above 1620 K, these two curve sets are seen to merge i n t o a s i n g l e l i n e a r p o r t i o n . This nonl i n e a r behavior i s a t t r i b u t e d to a changing oxygen p o t e n t i a l i n the s l a g . At the higher temperatures, the oxygen p a r t i a l pressures were greater than p r e d i c t e d by K 0 d i s s o c i a t i o n alone (react i o n [ 3 ] ) . However, at the lower temperatures (XC r e g i o n ) , l e s s oxygen was observed than expected f o r r e a c t i o n [3]. Apparently, FeO^ undergoes r e d u c t i o n at temperatures above 1620 K with release 2


2

of a d d i t i o n a l 0 (above the K 0 l e v e l ) . Below t h i s temperature, FeO i s o x i d i z e d by the 0 r e s u l t i n g from K 0 d i s s o c i a t i o n . I t 2

x

2

2

2

i s s i g n i f i c a n t that thermodynamic e q u i l i b r i u m i s maintained during t h i s redox process, as evidenced by a common K 0 a c t i v i t y over the AX and XC i n t e r v a l s (data not shown i n F i g . 9). For the lower FeO content "Western" slags t h i s e f f e c t was not observed. 2

x

Real MHD Channel Slag ( K ) . D e t a i l e d TMS and KMS studies were made of vapor t r a n s p o r t over a high l i q u i d u s temperature (^ 1700 K) potassium-enriched coal s l a g with i n i t i a l composition as i n d i c a t e d i n Table I I . This s l a g sample was obtained by combustion of I l l i n o i s No. 6 coal with a d d i t i o n a l potassium added to the combustor [see (65)]. Note that t h i s s l a g composition l i e s between those of the "Eastern" and "Western" c o a l - t y p e s . For i d e n t i f i c a t i o n purposes, t h i s s l a g i s given the d e s i g n a t i o n KJL. X-ray d i f f r a c t i o n data i n d i c a t e d that the bulk of the s l a g potassium was present as the compound K A l S i 0 . TMS a n a l y s i s i n d i c a t e d that about two percent of the s l a g potassium was present i n r e l a t i v e l y v o l a t i l e form, mainly K S 0 and K C 0 . t

4

2

4

2

3

I d e n t i t y of V o l a t i l e Species: The a s - r e c e i v e d potassiumenriched coal s l a g was subjected to a s e r i e s of heating c y c l e s (runs) i n n i t r o g e n c a r r i e r gas. During the i n i t i a l heating c y c l e , mass s p e c t r a l scans, obtained using the TMS technique, revealed many v o l a t i l e species i n a d d i t i o n to the expected K and Na species. The f o l l o w i n g species were p o s i t i v e l y i d e n t i f i e d : H 0, C0 , S0 , 0 , K, and Na. Some of the other l o w - i n t e n s i t y i o n s i g n a l s can be very t e n t a t i v e l y assigned to the species (some h y p o t h e t i c a l ) : KO or KOH, KS or KSH, S i S , SiSH, H S, H S 0 , and KSiO. From JANAF (36), we can expect to see KOH under these c o n d i t i o n s . Some of these more minor species may r e s u l t from s l a g o c c l u s i o n s and metastable phases and most l i k e l y do not represent an e q u i l i b r i u m release from the s l a g . F o l l o w i n g t h i s i n i t i a l heating c y c l e , the only s i g n i f i c a n t s l a g vapor species 2

2

2

2

2

2


4

34.

HASTIE ET AL.

Alkali

Vapor

Transport


TEMPERATURE K

10 /T(K) 4

Figure 9. Selected K and O partial pressure data (KMS) for a synthetic "Eastern" MHD-channel slag with composition (K>0 wt %) 23.3-22.8 (AB interval) and 22.8-22.1 (BC interval). Run chronology follows the temperature sequence ABC. Key: -O-, K data with increasing temperature run-chronology and K data with decreasing temperature run chronology. — A — , O data, with increasing temperature run chronology and — A — , O data with decreasing temperature run chronology.


573

METAL

574

BONDING AND

INTERACTIONS

were K and 0 , and these were present i n the approximate s t o i c h i ometric r a t i o expected f o r K 0 decomposition. 2

2

I n i t i a l Species P a r t i a l Pressure—Temperature Dependence: The i n i t i a l v o l a t i l e s showed a non-monotonic v a r i a t i o n of p a r t i a l pressure with temperature, as shown i n Figure 10. These v o l a t i l e s c o n s t i t u t e only a few percent of the t o t a l s l a g components and are not r e p r e s e n t a t i v e of the bulk s l a g composition. However, they do provide a s u f f i c i e n t l y high f l u x of a l k a l i (Na, K) and S0 to be a p o t e n t i a l source of c o r r o s i o n i n downstream MHD components. The high i n i t i a l p a r t i a l pressures of S0 , C 0 , K, and Na are i n d i c a t i v e of the presence of a l k a l i s u l f a t e and carbonate i n the s l a g . An a d d i t i o n a l c o n t r i b u t i o n to low temperature (T < 1300 K) a l k a l i release could r e s u l t from the high H 0 content leading to the formation of v o l a t i l e hydroxide species (KOH). However, no d e f i n i t i v e hydroxide s i g n a l s were observed. Note that a t T > 1400 K, the potassium pressures f a l l below those expected from KA10 , but that the S 0 , C 0 , and H 0 pressures are s t i l l r e l a t i v e l y high. Apparently, a t t h i s stage, the K produced by s u l f a t e and carbonate decomposition i s r e t a i n e d i n the bulk s l a g . A f t e r f u r t h e r heating, the sample was v i r t u a l l y depleted of Na, S 0 , and C 0 ; H 0 also continued to f a l l - o f f i n pressure to a n e g l i g i b l e l e v e l . Following t h i s i n i t i a l clean-up p e r i o d , the sample showed a more normal v a p o r i z a t i o n behavior and represent a t i v e data are summarized i n Table I I . 2

2

2


2

2

2

2

2

2

2

2

K 0 A c t i v i t y C o e f f i c i e n t s : Most of the b u l k - s l a g composition changes r e s u l t from K 0 d i s s o c i a t i v e v a p o r i z a t i o n . Therefore, a continuous monitoring of the K - p a r t i a l pressure (and 0 ) allows one to c a l c u l a t e the s l a g composition a t any stage of an experiment using r e l a t i o n s h i p s (1), (2), and (4) and the known i n i t i a l sample weight and composition. For both the TMS and KMS methods, the b a s i c experimental requirement f o r monitoring the bulk compos i t i o n i s the measurement of s i g n i f i c a n t species p a r t i a l pressures as a f u n c t i o n of time during an experimental run. An independent check on t h i s approach can be provided by chemical a n a l y s i s o f the sample remaining at the end of a run. In general, when a l l the s i g n i f i c a n t species are measured and the i o n i z a t i o n cross sections are known (though not necessary i n the present case), t h i s i n s i t u approach to monitoring composition changes provides a good mass balance at any stage of the experiment, as was shown for the NaCl and N a S 0 t e s t systems reported elsewhere (8). Since the mole f r a c t i o n of K 0 can be defined a t any stage of an experiment, i t i s ^ p o s s i b l e to convert K - p a r t i a l pressures to K 0 a c t i v i t y c o e f f i c i e n t s using equation [6]. By v a r y i n g the amount of K 0 present i n the s l a g during a v a p o r i z a t i o n run, we were able to f o l l o w the dependence of the K 0 "apparent" thermodynamic a c t i v i t y on temperature and composition. The term "apparent i s used to emphasize that the s l a g system may not always be i n a state of complete thermodynamic e q u i l i b r i u m . T y p i c a l data, 2

2

2

2

4

2

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34.

HASTIE ET AL.

Alkali

Vapor

575

Transport

TEMPERATURE


1620

6.0

(K)

1430

1260

S0

\

2

7.0

8.0

1 0 / T (K) 4

Figure 10. Partial pressure variation of initial volatiles (K O 19.5-19.1 wt %) as a function of temperature and time for the K slag (liquidus temperature ~ 1700 ± 30 K) using TMS approach (run 1). Conditions: 0.5 atm N , capillary probe. represents K-pressures over the K C0 and KA10 phases. 2

f

t

2

2

3

2


576

METAL

BONDING AND

INTERACTIONS

expressed i n a c t i v i t y c o e f f i c i e n t form [ y ( K 0 ) ] , are given i n Figure 11. Most of these data were obtained below the l i q u i d u s temperature. Good agreement was obtained between the TMS and KMS-based data a t r e l a t i v e l y high temperatures O 1600 K). From t h i s observation, we can conclude that the high temperature data represent thermodynamic e q u i l i b r i u m because of the g r e a t l y d i f f e r ent residence time scales i n v o l v e d , i . e . , TMS * 10 sec and KMS ^ 0 . 0 4 sec. The l a r g e s t experimental u n c e r t a i n t y i n comparing a c t i v i t y c o e f f i c i e n t data from the two d i f f e r e n t techniques (KMS and TMS) i s the accuracy of the K 0 mole f r a c t i o n , which i s probably u n c e r t a i n by ten percent i n each case. Comparison of these data with those f o r the K 0 - S i 0 system, f o r i n s t a n c e , a t 10 mole % K 0 i n each case and 1700 K, i n d i c a t e s a K s l a g a c t i v i t y c o e f f i c i e n t of about 30 times that f o r the b i n a r y system. This observation i s c o n s i s t e n t with the more b a s i c character of the K slag. Note i n Figure 11, the non-monotonic nature of the a c t i v i t y c o e f f i c i e n t curves. F o r normal non-ideal s o l u t i o n behavior, we would expect a l i n e a r monotonic r e l a t i o n s h i p with a negative slope r e p r e s e n t i n g a negative p a r t i a l molar enthalpy of s o l u t i o n for K 0 i n the s l a g . This type of behavior occurs f o r segments of each run (see F i g . 11), e.g., f o r run 1, up t o about 1430 K, and f o r run 2 between 1430 and 1630 K. However, the run 1 data are anomalous as they were obtained during the i n i t i a l heating p e r i o d when K S 0 and K C 0 decomposition was a s i g n i f i c a n t source of a d d i t i o n a l K. The r a p i d r e d u c t i o n i n v ( K 0 ) as the temperature i s increased beyond 1430 K r e s u l t s from the v i r t u a l l y complete d e p l e t i o n of these r e l a t i v e l y v o l a t i l e forms o f potassium. For runs 2 and 3, the i n i t i a l r e d u c t i o n of y ( K 0 ) with i n c r e a s i n g temperature i s b e l i e v e d to be due e i t h e r to d i f f u s i o n l i m i t e d ( i n s o l i d slag) K-transport to the s l a g surface or to changes i n the mode of 0 r e l e a s e from the s l a g , f o r i n s t a n c e , through F e 0 d i s s o c i a t i o n . The onset of i n c r e a s i n g y ( K 0 ) with temperature i s b e l i e v e d to a r i s e e i t h e r from an as-yet u n s p e c i f i e d physiochemical change i n the s l a g , l e a d i n g to a l e s s viscous (but s t i l l s o l i d ) form and increased d i f f u s i o n , or to d e p l e t i o n of secondary 0 sources. At higher temperatures, the bulk composition changes r a p i d l y (see mole f r a c t i o n s i n F i g . 11) and leads to a peaking i n v ( K 0 ) . We b e l i e v e that the data f o r runs 2 and 3, a t temperatures i n excess of 1450 K, represent an e q u i l i b r i u m v a p o r i z a t i o n condit i o n , p a r t i c u l a r l y as the KMS and TMS data are i n agreement f o r these c o n d i t i o n s . C l e a r l y , these unusual trends i n the y ( K 0 ) data i n d i c a t e the d i f f i c u l t y i n v o l v e d i n making a p r i o r i p r e d i c t i o n s of r e a l s l a g v a p o r i z a t i o n behavior. 2

2

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2

x

1

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2

Synthetic Low M e l t i n g Slag ( K ) . A lower l i q u i d u s temperature (^ 1480 K), l e s s viscous (as compared with the K sample) s y n t h e t i c s l a g was prepared f o r s t u d i e s analogous to those performed f o r the K system. I t was hoped that t h i s s l a g would not show the same anomalous a c t i v i t y behavior, a t lower temperatures, as f o r the Ki-MHD c o a l s l a g sample. 2

1

1


34.

HASTIE ET AL.

1700

Vapor

577

Transport

TEMPERATURE K 1500 1400 I I I

1300 I

1200 I


1900

Alkali

5.0

6.0

7.0

8.0

10 /TK 4

Figure 11. KMS data for variation of K,0 activity coefficient with temperature and composition for the K slag. The numbers, ranging from 0.154 to 0.08, refer to the mole fraction of K,0 (X) remaining in the sample at each measurement point. Runs 1-3 were carried out consecutively on the same sample. The •, data point at 1575 K (run 1) obtained by TMS with additional 0 present. t

2


578


Under f r e e v a p o r i z a t i o n (KMS) or N -atmosphere (TMS) condit i o n s , the predominant vapor species i n t h i s system are K and 0 , as shown i n Figure 12. The i n i t i a l excess of 0 present i n t h i s s l a g i s b e l i e v e d to r e s u l t from the f o l l o w i n g sources. First, the p r e p a r a t i o n procedure of melting and pouring i n room a i r may have l e d to oxygen absorption by the sample. Second, pressure bursts of K and 0 (and C0 ) were noted i n the i n i t i a l phase of the TMS experiments and p a r t i c u l a r l y near the l i q u i d u s temperature, e.g., at 1500 K i n Figure 12. This e f f e c t i s a t t r i b u t e d to K C 0 impurity. T h i r d , reduction of F e 0 to F e 0 , with release of excess 0 , i s favorable at these temperatures and, i n f a c t , has a s i m i l a r temperature dependence to that of the i n i t i a l 0 data shown i n Figure 12. Using the JANAF (36) thermochemical data f o r F e 0 and F e 0 , i t i s p o s s i b l e to p r e d i c t 0 p a r t i a l pressures f o r given condensed phase a c t i v i t i e s . On t h i s b a s i s , the i n i t i a l experimental data of Figure 12 at 1600 K, f o r instance, are c o n s i s t e n t with ^ 50 percent and 25 percent F e 0 reduction for the KMS and TMS experiments, r e s p e c t i v e l y . 2

2

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2

2

2

3

2

3

3

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2


3

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The KMS data were obtained using the i n t e g r a t e d i o n i n t e n s i t y weight loss method of pressure c a l i b r a t i o n [Eq. ( 2 ) ] , taking i n t o account the a d d i t i o n a l weight l o s s due to F e 0 reduction. C a l i b r a t i o n of the TMS data, on the other hand, was made using the r e l a t i v e i o n i z a t i o n cross s e c t i o n approach [Eq. ( 3 ) ] . The apparent d i f f e r e n c e between the KMS and TMS data, i n d i c a t e d i n Figure 12, i s r e l a t e d to the problem of a d d i t i o n a l sources of 0 already mentioned. That i s , the TMS data were obtained a t an e a r l i e r stage of the sample h i s t o r y , where the high 0 pressure depresses the K-pressure by the mass-action e f f e c t . In f a c t , i f the data are converted to K 0 a c t i v i t i e s , the KMS and TMS data are i n s a t i s f a c t o r y agreement. Such agreement i s good evidence of system thermodynamic e q u i l i b r i u m . At a l a t e r phase of the KMS experiments, the 0 /K pressures were of the c o r r e c t stoichiometry for K 0 ( £ ) decomposition. 2

3

2

2

2

2

2

A second type of time dependent phenomenon was observed f o r t h i s s l a g using the TMS method, as shown i n Figure 13. Once an isothermal c o n d i t i o n was achieved, the K-pressure decreased with time. This r e s u l t could be taken as evidence of surface d e p l e t i o n of K (and 0 ) from the sample, due to the bulk d i f f u s i o n rate being too small r e l a t i v e to the surface v a p o r i z a t i o n r a t e . A l s o , t h i s e f f e c t was found to be much l e s s pronounced a t higher temperatures where the d i f f u s i o n rates are higher. However, t h i s i n i t i a l i n t e r p r e t a t i o n (65) no longer seems reasonable. The order-ofmagnitude greater vapor t r a n s p o r t rates f o r KMS vs TMS experiments would i n d i c a t e lower apparent a c t i v i t i e s i n the former case due to surface d e p l e t i o n e f f e c t s . However, i n p r a c t i c e the KMS a c t i v i t i e s are somewhat higher. Hence, t h i s time dependent phenomenon i s a t t r i b u t e d to the combined e f f e c t s of K C 0 impurity decomp o s i t i o n , as noted p r e v i o u s l y f o r the analogous glass system, and, to FeO reduction, as noted with the "Eastern" s l a g data i n Figure 9. 2

2

3

x

That t h i s l a t t e r e f f e c t was more apparent i n the TMS, versus the


34.

HASTIE ET AL.

Alkali

Vapor

579

Transport


TEMPERATURE K

10

4

II

(K)

Figure 12. Vaporization of K and 0 from the K slag. Key: O, 0 (KMS); 0 (TMS); A , K (KMS); and A , K (TMS). Chronological order of data taken with increasing temperature except for ticked data points where the temperature was decreasing. 2

2

2

2


METAL

BONDING AND

INTERACTIONS


580


HASTIE

34.

ET AL.

Alkali

Vapor

Transport

581

KMS, experiments can be a t t r i b u t e d to the much higher t r a n s p o r t rates and s h o r t e r FeO r e d u c t i o n times f o r the l a t t e r case, x A p p l i c a t i o n of V a p o r i z a t i o n Data to Seed-Slag I n t e r a c t i o n . A key f a c t o r f o r s u c c e s s f u l MHD o p e r a t i o n i s the degree of i n t e r a c t i o n between plasma potassium seed and the s l a g medium. Using s l a g a c t i v i t y data from the present s t u d i e s , i t i s p o s s i b l e to p r e d i c t c o n d i t i o n s under which plasma seed w i l l be continuously depleted by s l a g a b s o r p t i o n of a l k a l i . Plante et a l . (56) presented s i m i l a r arguments e a r l i e r , based on t h e i r data f o r the b i n a r y oxide systems. A more d e f i n i t i v e a n a l y s i s can now be made from the present data on complex s y n t h e t i c and a c t u a l s l a g systems. The s t a b i l i t y of K 0 i n s l a g s o l u t i o n s can be r e a d i l y determined by comparison of the K 0 d i s s o c i a t i o n pressure-product 2


2

( P ^ * P Q ^ ) data i n the s l a g phase with that i n the plasma phase, 2

as represented i n F i g u r e 14. This d i s s o c i a t i o n pressure (DP) expression i s a convenient r e p r e s e n t a t i o n of s l a g a c t i v i t y data [ a c t i v i t y = DP, soln./DP, K 0 ( £ ) ] . The DP curves f o r the plasma phase were c a l c u l a t e d using a multicomponent e q u i l i b r i u m computer program, assuming s t o i c h i o m e t r i c combustion of CH^ ^ with a i r 2

+

(4/1 mole r a t i o N / 0 ) and KOH, K, KO, K , and e as vapor phase species. Pressures of ten and one atmosphere were chosen to be r e p r e s e n t a t i v e of the MHD combustor and the c h a n n e l - d i f f u s e r downstream seed recovery u n i t s , r e s p e c t i v e l y . Corresponding curves f o r the a c t u a l MHD s l a g ( K ^ and the S y n t h e t i c "Western" and S i m p l i f i e d "Western" s l a g s , were c a l c u l a t e d from the e x p e r i mental vapor pressure data given i n Table I I . Comparison of the DP curves f o r plasma and s l a g i n d i c a t e temperatures f o r K 0 s l a g s a t u r a t i o n i n the range of 2100 to 2300 K at 10 atm, and 1850 to 2050 K at one atm, depending on the s l a g type and composition. The approximately 200 K equivalence temperature d i f f e r e n c e between a c t u a l and model slags i s a t t r i b u t e d , p r i m a r i l y , to the low CaO c o n c e n t r a t i o n i n the former case. Slag a b s o r p t i o n of K 0 at the channel and d i f f u s e r surfaces depends s t r o n g l y on the plasma-surface i n t e r f a c e temperature. Experimentally, these i n t e r f a c e temperatures are d i f f i c u l t to measure though the data reported by S e l f (68) i n d i c a t e t h a t they are i n the r e g i o n of 2100 to 2300 K. At these temperatures, and at atmospheric p r e s s u r e , slags with at l e a s t 18 wt.% K 0 are p r e d i c t e d to be s t a b l e with respect to the plasma phase, as i n d i c a t e d i n F i g u r e 14. Such a p r e d i c t i o n i s i n accord with the observed 19.5 wt.% K 0 content of the a c t u a l MHD s l a g ( K ) . Conceivably, c a l c u l a t i o n s of t h i s type, together with a c t u a l s l a g analyses, could be used to i n f e r the a c t u a l i n t e r f a c e temperatures. 2

2

2

2

2

2

x

Note t h a t the DP equivalence temperatures i n F i g u r e 14 are s i g n i f i c a n t l y lower f o r the atmospheric c o n d i t i o n s c h a r a c t e r i s t i c of downstream u n i t s . A l s o , as the temperature decreases, the s l a g d i s s o c i a t i o n pressure-product decreases much f a s t e r than f o r the plasma, r e s u l t i n g i n a super-saturated a l k a l i vapor c o n c e n t r a t i o n


582

METAL

WALL

PLASMA2600

BONDING AND

INTERACTIONS

-) 1800

2200

T

4

l O " |-


S

w

10 ATM

CM

10"

5

10

10 '

-

10- 8

5.0

4.0 10

4

/ T (K)

Figure 14. Comparison of K 0 dissociation pressure-product data for the MHD plasma with 1 wt % K C0 seed ( ; and slag ( ; phases. Curve A, actual slag (Kj) with 14 wt % K 0 content; Curve B, synthetic "Western'' slag with 18 wt % K 0; and Curve C, quaternary simplified "Western" slag with 15 wt % K 0. 2

2

3

2

2

2


34.

HASTIE ET AL.

Alkali

Vapor

583

Transport

at the plasma-slag i n t e r f a c e . Hence, a d d i t i o n a l K 0 w i l l be absorbed by the s l a g . Below the dewpoint, t h i s seed w i l l tend to deposit on the s l a g surface i n a r e l a t i v e l y non-bound form, such as the a l k a l i s u l f a t e or carbonate, which i s d e s i r a b l e from a seed-recovery p o i n t of view. As the temperature decreases, the reduced s l a g species d i f f u s i o n rates and the increased tendency for phase s e p a r a t i o n to occur w i l l allow K S 0 and K C 0 deposits to remain a t , or near, the s l a g surface, as i s found i n p r a c t i c e . I n d i r e c t evidence of t h i s type of d e p o s i t i o n may be seen i n Figure 10. 2

2

4

2

3

S a l t - S l a g A l k a l i Exchange. The common d i s p o s i t i o n of a l k a l i i n c o a l minerals i s Na as NaCl and K as K 0--bound i n a lowa c t i v i t y s i l i c a t e phase. Thus, during c o a l conversion, Na i s expected to be released to the vapor phase more r e a d i l y than K. However, the p o s s i b i l i t y of NaCl-K 0 ( s l a g ) i n t e r a c t i o n to produce KCl-Na 0 (slag) could g r e a t l y enhance K-release to the vapor phase. A l s o , i n MHD s l a g s , where about 20 wt.% K 0 content i s p o s s i b l e , the problem of recovering t h i s l o s t seed could l i k e w i s e be resolved through replacement by NaCl. The f e a s i b i l i t y of such an exchange process was t e s t e d by a TMS monitoring of the vapor phase over the system, NaCl + K s l a g (19.4 wt.% K 0 ) . D e t a i l s of t h i s study w i l l be given elsewhere (69), but the main observations are as f o l l o w s . When a t h i n l a y e r of powdered NaCl was present on the surface of the K s l a g , a r a p i d exchange r e a c t i o n occurred near the melting p o i n t of NaCl, i . e . ,


2

2

2

2

x

2

1

NaCl(£) + K 0 2

(slag) = KC1(£) + Na 0 (slag) 2

This r e s u l t i s demonstrated i n F i g u r e 15, where the observed p a r t i a l pressures of NaCl and KCl are expressed i n thermodynamic a c t i v i t y form. Note the marked decrease i n NaCl a c t i v i t y and concomitant increase i n KCl a c t i v i t y j u s t above the m e l t i n g p o i n t of NaCl. A f t e r a h e a t i n g p e r i o d of about 50 min., the NaCl sample was v i r t u a l l y depleted, as was the KCl product. Insuffic i e n t s a l t was present i n the i n i t i a l mixture to convert a l l the a v a i l a b l e K 0 to K C l . However, 90 percent of the i n i t i a l NaCl was converted to Na 0 (slag) with s t o i c h i o m e t r i c r e l e a s e of K C l . About s i x percent of the a v a i l a b l e K 0 was converted to KCl vapor, and we expect that n e a r l y complete removal of K 0 from the s l a g would have been p o s s i b l e i f s u f f i c i e n t NaCl was present. The remaining ten percent NaCl was l o s t by v a p o r i z a t i o n b e f o r e , and during, the exchange process. During the isothermal, constant a c t i v i t y , phase of the exchange process (20 to 40 min. region of F i g . 15), a potassium vapor t r a n s p o r t enhancement f a c t o r o f , 2

2

2

2


METAL

584

TEMPERATURE 770

1074


f mp NaCl

BONDING AND

INTERACTI

(K). NON-LINEAR

1190 -

T CONSTANT

O

O

TIME (min) Figure 15. Thermodynamic activities (TMS data) for NaCl (-O-) and KCl (-%-) in the K slag-alkali exchange process. The indicated reference state partial pressures were obtained from Ref. 36. Key: P° NaCl = 3 X 10' atm; P° KCl — 5 X 10 atm. t

3

3


34.

HASTIE ET AL.

Alkali

Vapor

585

Transport

was observed. A l s o , during t h i s p e r i o d , the high KCl a c t i v i t y suggests formation of an e s s e n t i a l l y i d e a l s o l u t i o n of KCl-NaCl, as w e l l as the establishment of thermodynamic e q u i l i b r i u m . Note the near u n i t NaCl a c t i v i t y i n the i n i t i a l phase of the experiment (Fig. 15), which confirms the c a l i b r a t i o n f a c t o r s used to convert mass s p e c t r a l i o n i n t e n s i t i e s to p a r t i a l pressures and r e f l e c t s establishment of thermodynamic e q u i l i b r i u m . A d d i t i o n a l study of t h i s exchange process i s i n progress.


Heterogeneous

Reactive Gas Systems

Background. In the previous s e c t i o n s , we have considered a l k a l i vapor t r a n s p o r t from condensed phase systems i n the absence of e x t e r n a l i n f l u e n c e s , such as r e a c t i v e gases. However, some of the component gases of combustion systems, such as H 0, HC1, S0 , 0 , CO, and H , can be expected to s i g n i f i c a n t l y modify a l k a l i vapor t r a n s p o r t through mass a c t i o n e f f e c t s or formation of new molecular s p e c i e s . Some r e p r e s e n t a t i v e cases are considered as follows. 2

2

2

2

Na S0 -NaOH-H20 System. From a thermodynamic viewpoint, H 0 should react with N a S 0 to form NaOH vapor at high temperatures. However, s u r p r i s i n g l y , on the r e l a t i v e l y long time s c a l e of TMS measurements, no such r e a c t i o n was observed. When a small amount (0.25 to 1 percent) of condensed NaOH was mixed with the N a S 0 , r e a c t i o n with H 0 vapor was observed. A l s o , the a l k a l i vapor pressure enhancement was thermodynamically c o n s i s t e n t with the process, 2

4

2

2

4

2

4

2

H0

+ N a S 0 ( c ) = 2NaOH(g) + S 0

2

2

4

2

+ 1/2

0

2

D e t a i l s of t h i s study w i l l appear elsewhere (70). I l l i t e -H 0-H System. V a p o r i z a t i o n of potassium from the h i g h l y a c i d i c i l l i t e system, i n n e u t r a l atmospheres, i s expected to provide a r e l a t i v e l y i n s i g n i f i c a n t source of a l k a l i i n most coal combustion systems. However, i n the presence of r e a c t i v e combustion gases, such as H 0 and H , thermodynamic c o n s i d e r a t i o n s p r e d i c t a s i g n i f i c a n t KOH p a r t i a l pressure. In a d d i t i o n , an increase i n the K-pressure should r e s u l t from a r e d u c t i o n i n the 0 pressure, i n the presence of H . However, KMS experiments d i d not i n d i c a t e formation of KOH or a d d i t i o n a l K i n the presence of H gas. Thus, thermodynamic e q u i l i b r i u m does not appear to have been e s t a b l i s h e d i n t h i s heterogeneous system, even though the temperatures were s u f f i c i e n t l y high to have normally ensured a r a p i d approach to e q u i l i b r i u m . Further evidence of t h i s l a c k of thermodynamic e q u i l i b r i u m was provided by monitoring formation of SiO by H r e d u c t i o n of S i 0 , present e i t h e r as the s i l i c a t e i n i l l i t e or as pure s i l i c a . Figure 16 shows that SiO p r o d u c t i o n from both forms of S i 0 i s 2

2

2

2

2

2

2

2

2

2


586

METAL

BONDING

AND

INTERACTIONS


TEMPERATURE K

10

/T (K)

Figure 16. Silica activity function (KMS data). K refers to the reaction, Si0 (c) + H — SiO + H 0. Key: O, illite; quartz data with much larger sample surface area; and , Ref. 36 data for unit activity silica. p

2

2


2

HASTIE

34.

ET

AL.

Alkali

Vapor

Transport

587

about one to two orders of magnitude l e s s than the e q u i l i b r i u m values represented by the JANAF (36) curve. In the absence of H or H 0, the SiO and 0 pressures were reasonably c o n s i s t e n t with an e q u i l i b r i u m system. The k i n e t i c l i m i t a t i o n does not appear to r e s u l t from the heterogeneous r e a c t i o n process but, r a t h e r , from a l a c k of e q u i l i b rium f o r the homogeneous r e a c t i o n , 2

2

2

H0 2

as shown i n F i g u r e 17.

= H

When H

2

2

+ 1/2

was

0

[8]

2

the added reactant, K^ f o r

t h i s r e a c t i o n was found to be about two orders of magnitude greater than the accepted value. This discrepancy would r e s u l t i f a large f r a c t i o n of H e f f u s e d through the Knudsen c e l l without r e a c t i o n . However, when H 0 was the added reactant, K was a l s o


2

2

greater than the known value and by an order of magnitude. This case represents c l o s e r agreement over the system where H was added. Some of t h i s improvement may r e s u l t from the lower H 0 pressures used and/or the p o s s i b i l i t y f o r H 0 to undergo decompos i t i o n on the e f f u s i o n c e l l s u r f a c e s . To form H 0, however, H must undergo one or more c o l l i s i o n s with the oxide sample surface. The p o i n t s agreeing most c l o s e l y with the t h e o r e t i c a l H 0 d i s s o c i a t i o n curve i n Figure 17 were obtained by assuming that P„ = 2P , "2 U rather than using the measured P„ to c a l c u l a t e K . This assumpH2 P t i o n would be true f o r a s t a t i c system i n which H and 0 are formed only by decomposition of water. For Knudsen e f f u s i o n , the rate of escape i s i n v e r s e l y p r o p o r t i o n a l to the molecular weight and, at steady state c o n d i t i o n s , P„ = 1/2 P- . Under these H u dynamic c o n d i t i o n s , K^ would be 1/4 the s t a t i c value. Values of 2

2

2

2

2

2

n

2

2

2

2

2

K

c a l c u l a t e d on the b a s i s of t h i s assumption are not shown i n P Figure 17. The K values f o r the d i s s o c i a t i o n of H 0 using e i t h e r 2

of these two assumptions s c a t t e r w i t h i n a f a c t o r of two around the JANAF (36) curve over most of the temperature range. At the highest temperatures, the agreement i s l e s s s a t i s f a c t o r y because of excess 0 production from r e s i d u a l i l l i t e i n the e f f u s i o n c e l l . Values of K f o r water a d d i t i o n , where the observed P i s used to p ' H 2

TT

2

c a l c u l a t e K^, i n d i c a t e that the mass spectrometric method i s overestimating the H pressure by almost an order of magnitude. This e r r o r could r e s u l t from H 0 r e d u c t i o n on the furnace element and/ or the outer surface of the Knudsen c e l l . However, we have observed t h i s l a c k of e q u i l i b r i u m f o r the H 0 decomposition react i o n i n other systems and, more p a r t i c u l a r l y , with the TMS method, which has l e s s opportunity f o r furnace decomposition r e a c t i o n s , as w e l l as a much longer c h a r a c t e r i s t i c gas residence time. 2

2

2


METAL

BONDING AND

INTERACTIONS


588

Figure 17. Equilibrium constant for H,0 dissociation (KMS data). Key: O, data obtained during illite + H experiment; H 0 rather than H addition; P assumed equal to 2P for K calculation; and , Ref. 36 data. 2

2

02

2

p


H2

34.

HASTIE ET AL.

Alkali

Vapor

Transport

589

I f we consider the rate l i m i t i n g step i n the homogeneous H 0-0 -H system as, 2

2

2

0

2

+ H

=

2

20H

then the l i t e r a t u r e k i n e t i c data (71) i n d i c a t e r e a c t i o n times i n the approximate range of 10 to 100 sec f o r our experimental c o n d i t i o n s (y 1600 K). Thus, the TMS residence time range of 1 to 20 sec i s marginal f o r e q u i l i b r i u m i n the H 0 system, and the KMS system i s even l e s s favorable (as observed e x p e r i m e n t a l l y ) . However, at temperatures i n excess of 2000 K, t h i s l i m i t i n g rate i s s e v e r a l orders of magnitude f a s t e r , and e q u i l i b r i u m can be a t t a i n e d , as noted e a r l i e r (65). Evidence of non-equilibrium i n H 0-H -condensed phase systems was a l s o noted r e c e n t l y by Sasaki and B e l t o n (72). They observed 0 p a r t i a l pressures one to two orders of magnitude l e s s than the e q u i l i b r i u m values at a temperature of 1423 K over l i q u i d Cu. However, i n the presence of a Pt wire c a t a l y s t , they were able to c l o s e l y approach e q u i l i b r i u m at residence times not too d i f f e r e n t from those of the present mass spectrometric s t u d i e s . 2


2

2

2

Synthetic Slag ( K ) - H 0 - H System. In order to extend the vapor t r a n s p o r t c o n d i t i o n s i n s l a g systems to a reducing hydrous environment s i m i l a r to that present i n c o a l g a s i f i c a t i o n , a s e r i e s of TMS and KMS measurements were made using H or H 0 as the i n i t i a l reactant gas. With the TMS system, compositions of H -N -H 0 up to 10 v o l % H were a t t a i n e d p r i o r to hydrogen-induced c o r r o s i v e l o s s of the t r a n s p i r a t i o n r e a c t o r . 2

2

2

2

2

2

2

2

2

E f f e c t of H on K-Vaporization: As H was introduced to the s l a g system, the 0 c o n c e n t r a t i o n decreased and K and H 0 increased, as expected f o r the process, 2

2

2

2

K 0(slag) + H 2

2

= 2K + H 0

[9]

2

T y p i c a l TMS

data are given i n F i g u r e 18 where the H partial -4 -2 pressure was v a r i e d over the range 10 to 10 atm. Note the pronounced h y s t e r i s i s e f f e c t f o r increased versus decreased H and H 0-content. Though not shown here, t h i s e f f e c t i s a l s o present i n the K 0 a c t i v i t y data, as c a l c u l a t e d from the observed K and 0 - p r e s s u r e s . Hence the system i s not at thermodynamic e q u i l i b r i u m . From the e s t a b l i s h e d e q u i l i b r i u m constants f o r r e a c t i o n [3] with K 0 ( s l a g ) and K 0 (pure l i q u i d ) , together with the measured K 0 a c t i v i t y data, we c a l c u l a t e , 2

2

2

2

2

2

2

2

K

[9] = 209 at 1650

K

P The corresponding experimental value, obtained from the measured p a r t i a l pressures of K, H , and H 0, i s 2

2


METAL

BONDING AND

INTERACTIONS


590


34.

HASTIE ET

Alkali

AL.

Vapor

[9], obs. = 4.2

K

591

Transport

at 1650

K

Thus the system i s f a r from e q u i l i b r i u m . E f f e c t of H 0 on K-Vaporization: S i m i l a r TMS experiments were performed but with H 0 as the added reactant and a non-reducing atmosphere. An unexpected K-pressure dependence on H 0 was found, as shown i n Figure 19. No h y s t e r i s i s e f f e c t s were observed i n t h i s case. A s i m i l a r , though l e s s pronounced ( f a c t o r of four l e s s e f f e c t on K-pressure), H 0-induced K v a p o r i z a t i o n e f f e c t was noted i n the more a c i d i c and more viscous MHD (K ) s l a g sample (65). For the H 0-pressure and temperature c o n d i t i o n s used, KOH should have formed according to the process, 2

2

2

2

x


2

K0

(slag) + H 0

2

However, no KOH

was

2

observed

[10]

= 2K0H

i n the TMS

mass s p e c t r a .

We

also

+

e s t a b l i s h e d the K precursor as atomic K, from the pure KOH data and appearance p o t e n t i a l measurements. A higher temperature study, using the KMS method, d i d show the expected formation of KOH i n the presence of added H 0. However, the KOH-pressures were about an order of magnitude below p r e d i c t e d e q u i l i b r i u m values, even though the c o r r e c t H 0 pressure dependence was found, as shown i n F i g u r e 20. The apparently anomalous H 0-induced increase i n K-pressure can be explained as f o l l o w s . L i t e r a t u r e water s o l u b i l i t y data f o r aluminate and s i l i c a t e melts [e.g., see (73)] suggest s o l u b i l i t i e s of at l e a s t s e v e r a l hundred ppm f o r our experimental condit i o n s . Various acid-base r e a c t i o n mechanisms have been suggested to e x p l a i n water s o l u b i l i t y i n s i l i c a t e melts, as summarized by Turkdogan (74). For b a s i c melts, H 0 acts as an a c i d and enhances the s i l i c a t e network s t r u c t u r e , and v i c e - v e r s a f o r a c i d (high s i l i c a ) melts. Though apparently not p r e v i o u s l y recognized, these s t r u c t u r a l changes should be r e f l e c t e d i n the a l k a l i a c t i v i t y data. Thus, we can reasonably expect an a c t i v i t y increase when water i s incorporated i n t o the s i l i c a t e matrix of the r e l a t i v e l y b a s i c K s l a g . Reaction sets of the type, 2

2

2

2

2

H0 2

2

+ 0 "

(slag) = 2(0H~)

K0

(slag) = 2K + 1/2 0 ,

2

2

0

2

+ 2e =

and

2

20 "

would be c o n s i s t e n t with the observed one-half power dependence of log P on l o g P„ (see F i g . 19). These s t r u c t u r a l changes J\ HU should a l s o be r e f l e c t e d i n v i s c o s i t y data. The w a t e r - s o l u b i l i t y v i s c o s i t y enhancement e f f e c t s noted by Brower et a l . (75) f o r a s i m i l a r s l a g are c o n s i s t e n t with the present a c t i v i t y trends. 2



592

METAL

10~

10"3

PRESSURE

H 0 2

BONDING AND

INTERACTIONS

1CT

2

1

(ATM)

Figure 19. Isothermal (1610 K) dependence of K pressure on H 0 pressure for the K slag with added H 0. TMS conditions: N carrier gas pressure, 0.21 atm; and capillary nozzle. 2

2

2

2

cc Q.

6

10~ I

I

I

I

I

I I I I II

3

2

10"

10"

APPARENT PRESSURE H 0 (ATM) 2

Figure 20. Isothermal dependence of KOH pressure on H 0 apparent pressure for the K slag at 1794 K. KMS data with Pt-cell orifice diameter of 0.34 mm. Curve of slope 0.5 represents the theoretical pressure dependence for reaction 10. 2

2



34.

HASTIE ET AL.

Alkali

Vapor

Transport

593

For more b a s i c systems a decreased a l k a l i a c t i v i t y i s p o s s i b l e . The recent observations of Gray (76) , where water vapor decreased a l k a l i v a p o r i z a t i o n rates i n low s i l i c a g l a s s e s , could be i n t e r preted i n t h i s manner. We b e l i e v e that a s i m i l a r water vapor s o l u b i l i t y enhancement of a l k a l i vapor t r a n s p o r t i s p o s s i b l e i n s o d a - l i m e - s i l i c a g l a s s systems, and work i s i n progress to v e r i f y t h i s . Some of the d i s p a r i t i e s between v a r i o u s glass v a p o r i z a t i o n s t u d i e s may w e l l r e s u l t from v a r i a t i o n s i n water content and, hence, a l k a l i activities. The common explanation f o r water vapor enhanced a l k a l i vapor t r a n s p o r t over s i l i c a t e s has revolved around format i o n of v o l a t i l e NaOH (77) and KOH (53) s p e c i e s . However, no d i r e c t t e s t f o r the presence of these species has been made, and the p o s s i b i l i t y of water vapor enhancement of atomic Na and K t r a n s p o r t e x i s t s i n these systems. Mention should a l s o be made of the p o s s i b l e e f f e c t of H 0 d i s s o c i a t i o n ( i . e . , r e a c t i o n [8]) to y i e l d vapor phase H , which was suggested by Horn et a l . (78) as a f a c t o r i n ceramic degradation. However, i n the present system there i s an a d d i t i o n a l source of 0 and, at thermodynamic e q u i l i b r i u m , K should be H 0independent except f o r the noted H 0 - s o l u b i l i t y e f f e c t . 2

2

2

2

2

Glass-Combustion Gas System. C e r t a i n combustion gas components can promote a l k a l i vapor t r a n s p o r t i n glass systems. Such t r a n s p o r t i s important i n glass melting. A l s o , g l a s s had been suggested as a medium f o r trapping p a r t i c u l a t e m a t e r i a l i n combust i o n gas clean-up processes, such as f o r p r e s s u r i z e d f l u i d i z e d bed combustion (47). Using our experimental a c t i v i t y data f o r Na 0 i n g l a s s , we have modeled the e f f e c t of a t y p i c a l combustion gas mixture on a l k a l i v a p o r i z a t i o n (48). For t h i s purpose we have acquired, and adapted to our computers, a code known as SOLGASMIX (79) which i s unique i n i t s a b i l i t y to deal with non-ideal s o l u t i o n multicomponent heterogeneous e q u i l i b r i a . Previous attempts to model t h i s type of problem have been l i m i t e d to i d e a l s o l u t i o n assumptions (34). As i s demonstrated i n Table I I I , i f s o l u t i o n n o n - i d e a l i t y i s neglected, d r a s t i c e r r o r s r e s u l t i n the p r e d i c t i o n of a l k a l i vapor t r a n s p o r t processes. Table I I I and Figure 21 summarize the p r e d i c t e d a l k a l i species p a r t i a l pressures. The thermodynamic data base was constructed mainly from the JANAF (36) compilation. A d d i t i o n a l d e t a i l s of t h i s study w i l l be presented elsewhere. The p r i n c i p a l r e s u l t s of these c a l c u l a t i o n s can be summarized as f o l l o w s : (a) The presence of one percent halogen enhances a l k a l i t r a n s p o r t by more than two orders of magnitude at 1200 K, but only by a f a c t o r of four at 2000 K. (b) Only i n a N atmosphere i s atomic Na a s i g n i f i c a n t vapor species. (c) I f g l a s s i s t r e a t e d as an i d e a l s o l u t i o n , then halogen has only a s l i g h t enhancement e f f e c t on a l k a l i t r a n s p o r t , i . e . , NaOH i s the predominant s p e c i e s . 2

2


594

METAL

BONDING AND

INTERACTIONS

Table I I I E f f e c t of Atmosphere and S o l u t i o n Non-Ideality on A l k a l i Vapor Species D i s t r i b u t i o n Glass:

Na 0 (17 wt.%), CaO (12 wt.%), S i 0 2

I n i t i a l Gas Composition (mole p e r c e n t ) :


Total

(71 wt.%)

R y ) (4), 0 N

Temperature:

2

2

(5), C0

2

(76), S 0

2

2

(12),

(2), HC1 ( l )

1400 K

Pressure::

one atm

Non-Ideal Soln. N atm. 2

Species

Pressure (atm)

3.K-7)

Na

3

Ideal Soln. b Gas atm. Pressure (atm)

Non-Ideal Soln. 3

Gas atm.'

Pressure (atm)

2.2(-4)

1.0(-7)

NaOH

--

8.5(-2)

9.2(-5)

NaCl

--

9.8(-3)

8.8(-3)

--

7.K-4)

5.8(-4)

--

4.4(-6)

4.4(-6)

3.4(-8)

1.62C-1)

3.4(-8)

T o t a l Na

3.K-7)

9.6(-2)

l.K-2)

No Halogen

3.K-7)

8.5(-2)

1.0C-2)

Na Cl 2

2

Na S0

4

2

Na 0 A c t i v i t y 2

d

0

Computer n o t a t i o n , e.g., 3.1(-7) = 3.1 x 10 . 'Refers to mixed gas composition

given above.

I n f i n i t e supply of glass assumed and a c t i v i t y not a f f e c t e d by vaporization loss. ^Halogen content represents an upper l i m i t f o r combustion of a highhalogen c o a l i n a s a l t y (marine l o c a t i o n ) atmosphere. For typical low-halogen U. S. c o a l s , the c h l o r i d e c o n c e n t r a t i o n would be ^ 0 . 0 1 mol % and the NaCl pressures correspondingly l e s s .


34.

HASTIE ET AL.

Alkali

Vapor

595

Transport


T

800

1200

1600

TEMPERATURE

2000

(K)

Figure 21. Computer-calculated distribution oj alkali-containing vapor species as a junction oj temperature for the nonideal solution glass-combustion gas system specified in Table III.


METAL

596

BONDING AND

INTERACTIONS

(d)

Glass n o n - i d e a l i t y reduced a l k a l i t r a n s p o r t by an order of magnitude i n the presence of halogen but by three o r ders of magnitude i n a halogen-free r e a c t i v e atmosphere. (e) S u l f u r has a much lower e f f e c t than halogen, or H 0, on a l k a l i t r a n s p o r t and f o r the lower temperature range (< 1500 K) most of the s u l f u r i s removed from the gas stream due to N a S 0 ( £ ) formation. Note, however, ( i n F i g . 21) the non-monotonic production of N a S 0 vapor species with temperature, leading to s i g n i f i c a n t a l k a l i vapor t r a n s p o r t over the temperature i n t e r v a l 1300-2000 K. (f) At 1200 K, a l k a l i vapor t r a n s p o r t covers the range o f 7 to 1200 ppm, depending on the absence or presence of halogen, r e s p e c t i v e l y . From these r e s u l t s , i t i s c l e a r that a l k a l i release from glass under combustion gas atmospheres w i l l be a s i g n i f i c a n t source of -6 a l k a l i ( i . e . , > 10 atm) i n the combustion gas stream a t temperatures greater than 900 K, even f o r low-halogen combustion condit i o n s . We should s t r e s s that these p r e d i c t i o n s are s e n s i t i v e to the assumption that the gas stream a t t a i n s thermodynamic e q u i l i b rium with the glass s u b s t r a t e . Our experimental data on s u l f a t e and complex oxide systems i n d i c a t e s that such an e q u i l i b r i u m i s not always r e a d i l y a t t a i n e d . Incorporation of rate processes i n t o heterogeneous r e a c t i o n models must await f u r t h e r experiments to develop the necessary data base and mechanistic understanding. 2

2

4


2

4

Summary During the past few years, s u b s t a n t i a l progress has been made i n developing a thermodynamic data base and i n p r o v i d i n g mechanistic i n s i g h t i n t o the v a p o r i z a t i o n processes f o r a l k a l i metal s a l t , oxide, s i l i c a t e , and complex s l a g systems. Experimental techniques have been improved or newly developed which are s u i t able f o r thermodynamic s t u d i e s of complex, c o r r o s i v e a l k a l i - c o n t a i n i n g systems, i n c l u d i n g coal s l a g s . Computer codes are now a v a i l a b l e f o r thermodynamic c a l c u l a t i o n s of heterogeneous non-ideal s o l u t i o n multicomponent e q u i l i b r i u m systems, such as f o r combustion-coal s l a g i n t e r a c t i o n s . Vapor phase problems remaining i n c l u d e : (a) p o s s i b l e formation o f novel species a t very high gas pressures (10 to 100 atm); (b) l a c k o f mechanistic understanding and rate data f o r non-equilibrium heterogeneous systems; (c) nona v a i l a b i l i t y of computer codes f o r heterogeneous r a t e - l i m i t e d systems; and (d) development of d i a g n o s t i c methods and data f o r a c t u a l coal-conversion and combustion systems, i n c l u d i n g MHD channels, coal g a s i f i e r s , p r e s s u r i z e d f l u i d i z e d beds, and gas turbines.


34.

HAS TIE

ET AL.

Alkali

Vapor

Transport

597

Acknowledgments A s i g n i f i c a n t p o r t i o n of t h i s work was supported by the Department of Energy. Mr. A r t Sessoms provided valuable t e c h n i c a l a s s i s t a n c e . Valuable d i s c u s s i o n with our colleagues, Drs. W. Horton and L. Cook, i s a l s o acknowledged. Literature Cited

1. 2.


3. 4. 5. 6. 7. 8.

9.

Stwalley, W. C. and Koch, M. E. Opt. Eng. 1980, 19, 71. Haynes, B. S., Jander, H . , and Wagner, H. G. Symp. (Int.) Combust., 17th; The Comb. Inst. Pittsburgh, PA. 1978. p. 1365. Hastie, J . W. "High Temperature Vapors-Science and Technology", Academic Press: New York, NY, 1975. Gangwal, S. K. and Truesdale, R. S. Energy Res. 1980, 4, 113. Rapp, R. Ed. International Conference on High Temperature Corrosion, 1981. San Diego, CA, March 2-6, NACE. Proceedings in press. Hastie, J . W. Combust. Flame 1973, 21, 49. Hastie, J . W. and Bonnell, D. W. "Molecular Chemistry of Inhibited Combustion Systems", 1980. NBSIR 80-2169. Bonnell, D. W. and Hastie, J . W. "Transpiration Mass Spectrometry of High Temperature Vapors", in Characterization of High Temperature Vapors and Gases, Hastie, J . W., Ed. NBS-SP 561, US Government Printing Office: Washington, DC, 1979; p. 357. Plante, E. R. "Vapor Pressure Measurements of Potassium Over K O-Si Solutions by a Knudsen Effusion Mass Spectrometric Method", 1979; p. 265, ibid, (8). Yannopoulos, L. N., Toth, J . L . , and Pebler, A. Combust. Flame 1977, 30, 61. Hardesty, D. R. and Pohl, J . H. "The Combustion of Pulverized Coals--An Assessment of Research Needs", 1979, ibid (8). Spacil, H. S. and Luthra, K. L. J . Electrochem. Soc. 1979, 126, C134. Kolodney, M., Yernshalmi, J., Squires, A. M., and Harvey, R. D. Trans and J . Brit. Ceram. Soc. 1976, 75, 85. Crowley, M. S. Am. Ceram. Soc. Bull. 1975, 54, 1072. Vorres, K. S. Energy Res. 1980, 4, 109. Lenzer, C. R. and Laurendeau, M. N. "Gasification of Pulverized Coal Within Swirling Flows: An Interpretive Review", 1976. The Combustion Laboratory School of Mechanical Engineering, Purdue University, West Lafayette, Indiana 47907. Gluskoter, H. J . and Ruch, R. R. Fuel 1971, 50, 65. Shearer, J . A., Johnson, I., and Turner, C. B. Env. Sci. Tech. 1979, 13, 1113. Sarofim, A. F . , Howard, J . B., and Padia, A. S. Comb. Sci. Tech. 1977, 16, 187. 2

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4


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Transport

599

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71. 72. 73. 74. 75. 76. 77. 78. 79.

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2

RECEIVED September 10, 1981.


Alkali Vapor Transport in Coal Conversion and Combustion Systems

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