22 Deposit Constituent Phase Separation and Adhesion Erich Raask
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Technical Planning and Research Division, Central Electricity Generating Board, Leatherhead, Surrey, United Kingdom
The i n i t i a l deposit material on coal fired boiler tubes consists largely of silicate, sulphate and iron oxide particles. The fused silicates and molten sulphates form immiscible phases at high temperatures first on the micro-scale in individual particles and subsequently as separate layers in the deposit. The adhesion of ash deposit constituents to boiler tubes starts with the small particle retention as a result of the van der Waals, electrostatic and liquid film surface tension forces. Subsequently a strong bond will develop between the oxidized metal surface and iron saturated layer of ash deposit. The p u l v e r i z e d c o a l f i r e d b o i l e r s a t e l e c t r i c i t y u t i l i t y power s t a t i o n s a r e d e s i g n e d f o r " d r y " ash o p e r a t i o n where the b u l k o f m i n e r a l m a t t e r r e s i d u e i s removed i n t h e e l e c t r i c a l p r e c i p i t a t o r s i n the form o f p a r t i c u l a t e a s h . However, i t i s i n e v i t a b l e t h a t some d e p o s i t s o f s i n t e r e d ash and s e m i - f u s e d s l a g form on the h e a t exchange tubes and between 20 and 30 p e r c e n t o f c o a l a s h i s d i s c h a r g e d from the combustion chamber as c l i n k e r . The h i g h temperature c y c l o n e f i r e d b o i l e r s a r e d e s i g n e d f o r "wet" ash o p e r a t i o n and up t o 80 p e r c e n t o f c o a l ash i s d i s c h a r g e d from the f u r n a c e as molten s l a g . The b u i l d - u p o f s i n t e r e d a s h and f u s e d s l a g depends c h i e f l y on the r a t e o f a s h p a r t i c l e i m p a c t i o n and the a d h e s i v e c h a r a c t e r i s t i c s of t h e c o l l e c t i n g s u r f a c e . The i n i t i a l d e p o s i t on the h e a t exchange tubes i n p u l v e r i z e d c o a l f i r e d b o i l e r s c o n s i s t s o f a s h p a r t i c l e s o f d i a m e t e r r a n g i n g from l e s s than 0.1 ym t o 100 ym. S u b s e q u e n t l y the d e p o s i t e d ash may be r e - e n t r a i n e d i n t h e f l u e gas o r i t may form f i r s t a s i n t e r e d m a t r i x and l a t e r a f u s e d s l a g d e p o s i t c h i e f l y by v i s c o u s f l o w . F o r the d e p o s i t f o r m a t i o n the ash p a r t i c l e s must be f i r s t h e l d a t t h e c o l l e c t i n g s u r f a c e and s u b s e q u e n t l y the d e p o s i t m a t r i x bonded t o the b o i l e r tubes by a d h e s i v e f o r c e s s u f f i c i e n t l y s t r o n g to overcome t h e g r a v i t a t i o n a l p u l l , b o i l e r v i b r a t i o n and e v e n t u a l l y the s o o t b l o w e r j e t i m p a c t i o n . T h i s work s e t s o u t t o examine the a d h e s i v e c h a r a c t e r i s t i c s o f d i f f e r e n t c o n s t i t u e n t s o f the flame h e a t e d a s h and the f o r m a t i o n o f s i n t e r e d d e p o s i t s and s l a g bonded t o the heat exchange t u b e s . 0097-6156/ 86/ 0301 -0303506.00/ 0 © 1986 American Chemical Society
Vorres; Mineral Matter and Ash in Coal ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
304
MINERAL MATTER AND ASH IN COAL
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I n i t i a l Deposit
Constituents
The m i n e r a l m a t t e r i n c o a l c o n s i s t s c h i e f l y o f s i l i c a t e , s u l p h i d e , c a r b o n a t e s p e c i e s , and c h l o r i d e s and o r g a n o - m e t a l l i c compounds a s s o c i a t e d w i t h t h e f u e l substance ( 1 , 2 ) . The s i l i c a t e m i n e r a l p a r t i c l e s v i t r i f y p a r t i a l l y o r c o m p l e t e l y , i n the p u l v e r i z e d c o a l flame ( 3 ) , and thus the s i l i c a t e ash f r a c t i o n o f the i n i t i a l d e p o s i t c o n s i s t s o f p a r t i c l e s o f v a r i a b l e amounts o f a g l a s s y phase and c r y s t a l l i n e species (4). The s u l p h i d e , c a r b o n a t e , c h l o r i d e and organo-metal s p e c i e s d i s s o c i a t e and o x i d i z e i n the f l a m e . The o x i d e s may remain as d i s c r e t e p a r t i c l e s , c h i e f l y i r o n o x i d e ( m a g n e t i t e ) , can d i s s o l v e i n the g l a s s y phase o f s i l i c a t e s , and a f r a c t i o n o f c a l c i u m and sodium o x i d e s a r e s u l p h a t e d C5). Thus the i n i t i a l d e p o s i t m a t e r i a l w i l l c o n t a i n some c a l c i u m , sodium and p o t a s s i u m s u l p h a t e . The l a t t e r o r i g i n a t e s from the r e l e a s e o f p o t a s s i u m i n t h e flame h e a t e d a l u m i n o silicate particles (6). The r e l a t i v e c o n c e n t r a t i o n s o f flame heated ash c o n s t i t u e n t s , namely s i l i c a t e s , i r o n o x i d e and s u l p h a t e , can be e s t i m a t e d from the ash a n a l y s i s . However, the c o m p o s i t i o n o f the i n i t i a l d e p o s i t m a t e r i a l can be markedly d i f f e r e n t as a r e s u l t o f s e l e c t i v e deposition. I n p a r t i c u l a r , the d e p o s i t m a t e r i a l can be e n r i c h e d i n s u l p h a t e as shown i n F i g u r e 1. The r e l a t i v e c o n c e n t r a t i o n s o f d i f f e r e n t d e p o s i t c o n s t i t u e n t s were o b t a i n e d by a n a l y s i n g the m a t e r i a l on a c o o l e d m e t a l tube probe i n s e r t e d i n b o i l e r f l u e gas f o r s h o r t , 2 to 15 minute, d u r a t i o n (_7) . The s u l p h a t e c o n t e n t o f the f l u e gas borne a s h and probe d e p o s i t s i n a c y c l o n e f i r e d b o i l e r was h i g h e r t h a n t h a t i n the p u l v e r i z e d c o a l f i r e d b o i l e r ash and d e p o s i t s . T h i s was because i n c y c l o n e b o i l e r s the b u l k o f s i l i c a t e ash i s d i s c h a r g e d as m o l t e n s l a g b u t the r e s i d u a l ash i s r e l a t i v e l y r i c h i n sulphate. The r a t e o f a l k a l i - m e t a l s u l p h a t e d e p o s i t i o n w i l l d e c r e a s e when the temperature o f c o l l e c t i n g t a r g e t s u r f a c e exceeds 1075 Κ as shown i n F i g u r e 2. The d e c r e a s e i n the d e p o s i t i o n o f a l k a l i - m e t a l s u l p h a t e s i s r e l a t e d t o t h e c o n c e n t r a t i o n o f the v o l a t i l e a l k a l i m e t a l s i n the f l u e gas and the s a t u r a t i o n vapour p r e s s u r e o f sodium and p o t a s s i u m s u l p h a t e s ( 8 ) . The i n i t i a l d e p o s i t on c o o l e d s u r f a c e s c o n t a i n s a s m a l l amount o f c h l o r i d e as shown i n F i g u r e 2. In a r e d u c i n g atmosphere the d e p o s i t m a t e r i a l may c o n t a i n i r o n s u l p h i d e (FeS) formed on d i s s o c i a t i o n o f c o a l p y r i t e m i n e r a l . This i s l i k e l y to o c c u r on the combustion chamber w a l l tubes n e a r the b u r n e r s where the r e a c t i o n time i s s h o r t , below one second, f o r o x i d a t i o n o f FeS r e s i d u e t o the o x i d e . I t has been s u g g e s t e d t h a t c a l c i u m s u l p h i d e (CaS) may a l s o be p r e s e n t i n the ash m a t e r i a l d e p o s i t e d from a r e d u c i n g atmosphere gas stream as a r e s u l t o f s u l p h i d a t i o n o f calcium oxide ( 9 ) . Thermal S t a b i l i t y o f S u l p h a t e s
and I m m i s c i b i l i t y w i t h
Silicates
B i t u m i n o u s c o a l s u s u a l l y l e a v e a h i g h l y s i l i c i o u s a s h on combustion. T h a t i s , f u s e d a l u m i n o - s i l i c a t e s c o n s t i t u t e an a c i d i c media a t h i g h temperatures t h a t i s capable o f a b s o r b i n g l a r g e q u a n t i t i e s o f b a s i c m e t a l s i n the form o f o x i d e s , c h i e f l y those o f sodium, c a l c i u m and
Vorres; Mineral Matter and Ash in Coal ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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22.
Deposit
RAASK
A - COAL MINERALS
Constituent
Phase Separation
Β - GAS BORNE ASH
and
C - PROBE DEPOSIT
Adhesion
Dl D2 OUTER AND INNER LAYERS OF BOILER DEPOSIT
F i g u r e 1. Ash c o m p o s i t i o n changes on r o u t e from m i n e r a l m a t t e r to b o i l e r d e p o s i t s ^insoluble silicates; ^soluble silicates (ffljpyrites; uiron oxides; Qcarbonates ; Hchlorides; • sulphates.
5.0
< cc
2.5
L_
ζ ο ο CL LU
PROBE TEMPERATURE, K
F i g u r e 2. D e p o s i t i o n o f s u l p h a t e and c h l o r i d e i n c y c l o n e f i r e d b o i l e r , 0.28 p e r cent c h l o r i n e i n c o a l . A, NaSO^; B, K2SO4; C, NaCl.
Vorres; Mineral Matter and Ash in Coal ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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MINERAL MATTER AND ASH IN COAL
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magnesium. At lower temperatures the c o r r e s p o n d i n g s u l p h a t e s are t h e r m o d y n a m i c a l l y more s t a b l e i n the p r e s e n c e of s u l p h u r gases. The e q u i l i b r i u m d i s t r i b u t i o n o f a l k a l i n e o x i d e s between m o l t e n s u l p h a t e s and f u s e d s i l i c a t e s a t d i f f e r e n t temperatures can be c a l c u l a t e d from the a p p r o p r i a t e thermodynamic d a t a . However, the r e s i d e n c e time o f the flame borne m i n e r a l s p e c i e s b e f o r e d e p o s i t i o n i s s h o r t and the a l k a l i - m e t a l d i s t r i b u t i o n does n o t r e a c h the e q u i l i b r i u m s t a t e . The f u s e d s i l i c a t e p a r t i c l e s w i l l absorb the flame v o l a t i l i z e d sodium to the depth o f about 0.05 μπι ( 1 0 ) , and the remainder i s c o n v e r t e d t o s u l p h a t e p a r t l y i n the f l u e gas and p a r t l y a t the s u r f a c e o f ash p a r t i c l e s . The d i s t r i b u t i o n o f sodium i n the s i l i c a t e and s u l p h a t e phases can be e x p r e s s e d i n a form: m
sul
where m m
m
=
m
ο
- kw
.- + m , sil sul
=
(1) m ο
(2)
a n
sil> sul d % denote the amount o f sodium i n s i l i c a t e and s u l p h a t e f r a c t i o n s , and the t o t a l sodium i n ash r e s p e c t i v e l y ; k i s a c o n s t a n t and w i s the ash c o n t e n t o f c o a l . When the r a t i o o f sodium t o ash i n c o a l i s below 1 to 100 (10) and the b u l k o f sodium i s c a p t u r e d by the s i l i c a t e p a r t i c l e s E q u a t i o n (2) reduces t o :
and c o n s e q u e n t l y the amount o f sodium a v a i l a b l e f o r the f o r m a t i o n o f sulphate i s small. The m o l t e n sodium s u l p h a t e / s o d i u m s i l i c a t e system o f c o m p o s i t i o n Na2S04:Na20-Si02 has one l i q u i d phase a t 1475 K, but as the p r o p o r t i o n o f s i l i c a i n c r e a s e s , the melt s e p a r a t e s i n t o two l a y e r s (11,12). The change from the m i s c i b l e t o i m m i s c i b l e phase o f the system has been e x p l a i n e d by a l t e r a t i o n s i n the s i l i c a t e s t r u c t u r e as the r a t i o o f Na20 to S1O2 d e c r e a s e s . In more b a s i c , l e s s v i s c o u s m e l t s , the s i l i c a t e i o n s e x i s t i n the form o f S i 0 ^ ~ t e t r a h e d r a which have the same m o b i l i t y as s u l p h a t e ions,, and thus homogeneity o f the system i s to be e x p e c t e d . As the s i l i c a c o n t e n t i s i n c r e a s e d the c o m p l e x i t y o f the s i l i c a t e s t r u c t u r e reaches a p o i n t where the s i l i c a anions become r e l a t i v e l y immobile f o r a s e p a r a t i o n of s u l p h a t e from s i l i c a t o take p l a c e . The m i s c i b i l i t y o f the c o r r e s p o n d i n g p o t a s s i u m s u l p h a t e - s i l i c a t e system has been s t u d i e d by the u s u a l c r u c i b l e method as w e l l as by a t e c h n i q u e o f a h a n g i n g d r o p l e t ( 1 3 ) . The d r o p l e t s o f p o t a s s i u m s u l p h a t e / s i l i c a t e m i x t u r e s , 3 mm i n d i a m e t e r , were suspended from 0.5 mm p l a t i n u m w i r e w h i c h had a s e m i - s p h e r i c a l head 1.5 mm i n diameter. S e p a r a t i o n o f the s i l i c a t e ( i n t e r n a l ) and s u l p h a t e phases i n the d r o p l e t s can be o b s e r v e d d i r e c t l y i n the L e i t z h e a t i n g m i c r o s c o p e which i s used, i n i t s c o n v e n t i o n a l mode o f o p e r a t i o n , to a s s e s s the f u s i o n c h a r a c t e r i s t i c s o f c o a l ashes ( 1 4 ) . F i g u r e 3 shows the two phase s e p a r a t i o n o f 2K2SO4-K2O-2.IS1O2 system a t 1575 K, h a v i n g the t r a n s p a r e n t envelope o f s u l p h a t e through which the p l a t i n u m w i r e head (top) and g l o b u l e o f m o l t e n s i l i c a t e (bottom) can be seen. As the temperature was i n c r e a s e d t o 1725 Κ the two phases became m i s c i b l e because o f the i n c r e a s e d s o l u b i l i t y o f s u l p h a t e i n the s i l i c a t e melt a t the h i g h e r temperature.
Vorres; Mineral Matter and Ash in Coal ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
22.
Deposit
RAASK
Constituent
Phase Separation
and
Adhesion
307
The K2SO4-K2O-S1O2 phase diagram i s d e p i c t e d i n F i g u r e 4 which shows t h a t the system i s m i s c i b l e at 1575 Κ when the molar r a t i o of K2O t o S1O2 i s above 0.5. As i n the c o r r e s p o n d i n g sodium s u l p h a t e / sodium s i l i c a t e system, l e s s b a s i c m e l t s s e p a r a t e i n t o two i m m i s i c i b l e liquids. T h i s i s the case w i t h most c o a l ash s l a g s where the molar r a t i o of b a s i c o x i d e s (sum o f Na20, K2O, CaO and MgO) to S1O2 i s w e l l below 0.5. E x c e p t i o n s to t h i s are the sodium and c a l c i u m r i c h ashes o f some l i g n i t e and non-bituminous c o a l s , which can have s u f f i c i e n t amounts o f a l k a l i s to form a s i n g l e phase melt o f m i s c i b l e s u l p h a t e s and s i l i c a t e s a t h i g h t e m p e r a t u r e s .
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A d h e s i o n by Van
Per Waals and
E l e c t r o s t a t i c Forces
The ash p a r t i c l e s d e p o s i t e d on b o i l e r tubes are i n i t i a l l y h e l d i n p l a c e by s u r f a c e f o r c e s , i . e . van der Waals and e l e c t r o s t a t i c a t t r a c t i o n f o r c e s . Van der Waals f o r c e s become i m p o r t a n t when m o l e c u l e s or s o l i d s u r f a c e s are b r o u g h t c l o s e t o g e t h e r w i t h o u t a chemical i n t e r a c t i o n taking p l a c e . F o r a h e m i s p h e r i c a l p a r t i c l e of r a d i u s ( r ) h e l d at a d i s t a n c e o f n e a r e s t approach (h) from a p l a n e , the r e s u l t a n t f o r c e (F) i s g i v e n by: F
=
— y 6h
(4)
Z
where A i s the Hamaker c o n s t a n t (15). E q u a t i o n 4 a p p l i e s o v e r s h o r t d i s t a n c e s , up to 150 A (1.5 χ 10"8 m) and f o r l o n g e r d i s t a n c e s the " r e t a r d e d " van der Waals f o r c e s decay r a p i d l y ( 1 6 ) . An e q u a t i o n b a s e d on the d i e l e c t r i c p r o p e r t i e s of s o l i d s f o r the r e t a r d e d van der Waals f o r c e s ( F ) between a sphere of r a d i u s ( r ) at the d i s t a n c e (h) from a f l a t s u r f a c e i s ( 1 7 ) : T
F
