Coal Ash Deposition in Boilers - ACS Symposium Series (ACS

21 Coal Ash Deposition in Boilers R. W. Borio and A . A. Levasseur

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on March 10, 2018 | https://pubs.acs.org Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch021

Combustion Engineering, Inc., Windsor, CT 06095

There is a need for improved mineral matter behavior predictive techniques for coal fired boilers. This paper presents an assessment of older, traditional methods; i t presents some of the newer, improved methods; and i t identifies some of the remaining areas of uncertainty. Traditional ASTM techniques do not always provide accurate predictive information on mineral matter behavior. No one test can adequately describe coal ash behavior; a combination of tests, each designed to focus on a particular aspect of ash behavior represents a logical approach. Techniques for assessing potential slagging due to pyrites and fouling due to alkalies are described. The use of SEM as a promising new tool for accurate characterization of mineral matter is suggested. A major area of uncertainty is the location and extent of ash deposition in a commercial boiler as ascertained from bench scale results.

The management of coal ash in utility boilers continues to be one of the most important fuel property considerations in the design and operation of commercial boilers. The behavior of mineral matter in coal can significantly influence furnace sizing, heat transfer surface placement, and convection pass tube spacing. Ironically, many of the more reactive, low rank U. S. coals must have larger furnaces than the less reactive higher rank coals. This is strictly a requirement based on the mineral matter behavior; Figure 1 illustrates this point. Given the same mineral matter behavior the more reactive, lower rank coals would require less residence time and therefore smaller furnace volumes than the less reactive, higher rank coals. A l t h o u g h p u l v e r i z e d c o a l has been f i r e d f o r more t h a n 50 y e a r s and much i s known about combustion b e h a v i o r t h e r e a r e s t i l l a number of b o i l e r s e x p e r i e n c i n g o p e r a t i o n a l problems from c o a l ash e f f e c t s . A s h - r e l a t e d problems a r e one o f t h e p r i m a r y causes o f u n s c h e d u l e d outages, u n i t d e r a t i n g and u n a v a i l a b i l i t y . Because o f v a r i a b i l i t y i n a g i v e n c o a l seam and s i n c e many b o i l e r o p e r a t o r s may e x p e r i e n c e changes i n t h e i r c o a l s u p p l y d u r i n g t h e l i f e o f a b o i l e r , o p e r a t i o n a l problems caused by changes i n c o a l a s h p r o p e r t i e s can s i g n i f i c a n t l y a f f e c t b o i l e r performance. Not o n l y must t h e i n i t i a l b o i l e r d e s i g n be c o r r e c t l y d e t e r m i n e d based on t h e s p e c i f i c a t i o n c o a l but r e l i a b l e judgements must be made r e g a r d i n g t h e s u i t a b i l i t y 0097-6156/ 86/ 0301 -0288506.00/ 0 © 1986 American Chemical Society

Vorres; Mineral Matter and Ash in Coal ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

21.

BORIO AND LEVASSEUR

Coal Ash Deposition

of o t h e r c a n d i d a t e c o a l s and t h e i r e f f e c t l i f e t i m e of the b o i l e r .

in Boilers

289

on o p e r a t i o n d u r i n g t h e


The i n c r e a s e d emphasis on c o a l usage i n t h i s c o u n t r y and, i n d e e d , the s i g n i f i c a n t e f f o r t underway t o c o n s i d e r c o a l water m i x t u r e s as p o s s i b l e o i l s u b s t i t u t e s i n o i l - d e s i g n e d b o i l e r s u n d e r s c o r e s the need t o improve t h e p r e d i c t i o n o f m i n e r a l matter b e h a v i o r i n a b o i l e r environment. C o a l i s a v e r y heterogeneous, complex m a t e r i a l which produces h e t e r ogeneous, complex p r o d u c t s d u r i n g combustion. S i n c e , d u r i n g comb u s t i o n o f p u l v e r i z e d c o a l , c o a l p a r t i c l e s o f v a r i o u s o r g a n i c and m i n e r a l matter c o m p o s i t i o n s c a n behave i n c o m p l e t e l y d i f f e r e n t manners, p r e d i c t i o n s based upon t h e o v e r a l l o r average c o m p o s i t i o n may be m i s l e a d i n g . L i k e many o f the c u r r e n t l y - u s e d ASTM c o a l a n a l y s e s , t h e method f o r d e t e r m i n i n g a s h f u s i b i l i t y temperatures was d e v e l o p e d when s t o k e r f i r i n g was a predominant c o a l f i r i n g t e c h n i q u e ; t h e m e t h o d o l o g i e s and c o n d i t i o n s employed d u r i n g many of t h e ASTM t e s t s r e f l e c t t h i s . I t i s not s u r p r i s i n g t h a t t h e usef u l n e s s o f some ASTM t e s t r e s u l t s may be l i m i t e d when used f o r a p u l v e r i z e d coal f i r i n g a p p l i c a t i o n . In recent years researchers have developed m e t h o d o l o g i e s f o r c h a r a c t e r i z i n g c o a l a s h b e h a v i o r t h a t b e t t e r r e f l e c t t h e fundamental mechanisms c o n t r o l l i n g b e h a v i o r and more c l o s e l y s i m u l a t e t h e c o n d i t i o n s t h a t e x i s t i n a p u l v e r i z e d coal fired boiler. C l e a r l y t h e r e i s a need f o r improved t e c h n i q u e s f o r p r e d i c t i n g the b e h a v i o r o f m i n e r a l m a t t e r . T h i s paper w i l l p r o v i d e a statement of t h e a s h d e p o s i t i o n problem 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 ; i t w i l l p r e s e n t an assessment o f t h e o l d e r , t r a d i t i o n a l methods f o r p r e d i c t i n g m i n e r a l matter b e h a v i o r ; and i t w i l l a d d r e s s some of t h e newer t e c h n i q u e s t h a t have been suggested as b e t t e r ways of c h a r a c t e r i z i n g c o a l a s h b e h a v i o r . A d d i t i o n a l l y some a r e a s o f u n c e r t a i n t y w i l l be i d e n t i f i e d which r e q u i r e t h e development o f better p r e d i c t i v e techniques. STATEMENT OF THE PROBLEM The presence o f a s h d e p o s i t s and f l y a s h can c r e a t e t h e f o l l o w i n g problems i n a b o i l e r : 1. 2. 3. 4. 5.

Reduced heat t r a n s f e r Impedance o f gas f l o w P h y s i c a l damage t o p r e s s u r e p a r t s C o r r o s i o n of pressure parts E r o s i o n of pressure parts

These problems can r e s u l t i n reduced g e n e r a t i n g c a p a c i t y , unsche d u l e d outages, reduced a v a i l a b i l i t y , and c o s t l y m o d i f i c a t i o n s .

Ash which d e p o s i t s on b o i l e r w a l l s i n t h e r a d i a n t s e c t i o n o f a f u r n a c e i s g e n e r a l l y r e f e r r e d t o as s l a g . Ash d e p o s i t i o n on conv e c t i o n tube s e c t i o n s downstream o f t h e f u r n a c e r a d i a n t zone i s



290

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t y p i c a l l y r e f e r r e d t o as f o u l i n g . Ash s l a g g i n g and f o u l i n g can r e s u l t i n problems l i s t e d i n items 1 t h r o u g h 4. Item 5, e r o s i o n , i s the r e s u l t of impingement of a b r a s i v e ash on p r e s s u r e p a r t s . O f t e n c o a l ash d e p o s i t e f f e c t s are i n t e r - r e l a t e d . For example, s l a g g i n g w i l l r e s t r i c t w a t e r w a l l heat a b s o r p t i o n changing the temperature d i s t r i b u t i o n i n the b o i l e r which i n t u r n i n f l u e n c e s the n a t u r e and q u a n t i t y of ash d e p o s i t i o n i n downstream c o n v e c t i v e s e c t i o n s . Ash d e p o s i t s accumulated on c o n v e c t i o n tubes can reduce the c r o s s - s e c t i o n a l f l o w area i n c r e a s i n g f a n r e q u i r e m e n t s and a l s o c r e a t i n g h i g h e r l o c a l gas v e l o c i t i e s which a c c e l e r a t e f l y ash e r o s i o n . I n - s i t u d e p o s i t r e a c t i o n s can produce l i q u i d phase components which are i n s t r u m e n t a l i n tube c o r r o s i o n . One of the most common m a n i f e s t a t i o n s of a d e p o s i t i o n problem i s reduced heat t r a n s f e r i n the r a d i a n t zone of a f u r n a c e . Decreased heat t r a n s f e r due t o a r e d u c t i o n i n s u r f a c e a b s o r p t i v i t y i s a r e s u l t of the c o m b i n a t i o n of r a d i a t i v e p r o p e r t i e s of the d e p o s i t ( e m i s s i v i t y / a b s o r p t i v i t y ) and t h e r m a l r e s i s t a n c e ( c o n d u c t i v i t y ) of a d e p o s i t . Thermal r e s i s t a n c e ( t h e r m a l c o n d u c t i v i t y and d e p o s i t o v e r a l l t h i c k n e s s ) i s u s u a l l y more s i g n i f i c a n t because of i t s e f f e c t on a b s o r b i n g s u r f a c e t e m p e r a t u r e . P r e v i o u s work has i n d i c a t e d t h a t the p h y s i c a l s t a t e of the d e p o s i t can have a s i g n i f i c a n t e f f e c t on the r a d i a t i v e p r o p e r t i e s , s p e c i f i c a l l y m o l t e n d e p o s i t s show h i g h e r e m i s s i v i t i e s / a b s o r p t i v i t i e s than s i n t e r e d or powdery d e p o s i t s ( 1 ) . A l t h o u g h t h i n , m o l t e n d e p o s i t s are l e s s troublesome from a heat t r a n s f e r a s p e c t than t h i c k , s i n t e r e d d e p o s i t s , m o l t e n d e p o s i t s are u s u a l l y more d i f f i c u l t t o remove and cause f r o z e n d e p o s i t s t o c o l l e c t i n the lower r e a c h e s of the f u r n a c e where p h y s i c a l removal t h e n becomes a problem f o r the w a l l b l o w e r s . Impedance to gas f l o w i s the r e s u l t of heavy f o u l i n g on tubes i n the c o n v e c t i v e s e c t i o n . Problems of t h i s type are most l i k e l y t o o c c u r w i t h c o a l s h a v i n g h i g h sodium c o n t e n t s , such as those found i n low rank c o a l d e p o s i t s i n Western U.S. seams. Hard, bonded d e p o s i t s can o c c u r which are r e s i s t a n t t o removal by the r e t r a c t a b l e sootblowers. P h y s i c a l damage t o p r e s s u r e p a r t s can o c c u r i f l a r g e d e p o s i t s accumulate i n the upper f u r n a c e and become d i s l o d g e d or blown o f f and drop onto the s l o p e s of the lower f u r n a c e . Such d e p o s i t s are u s u a l l y c h a r a c t e r i z e d by t h e i r r e l a t i v e l y h i g h bonding s t r e n g t h s and t h e i r h e a v i l y s i n t e r e d s t r u c t u r e . F i r e s i d e c o r r o s i o n can o c c u r on both w a t e r w a l l and s u p e r h e a t e r tube s u r f a c e s . Normal s u l f a t e s and p y r o s u l f a t e s are f r e q u e n t l y the cause of w a t e r w a l l c o r r o s i o n , a l t h o u g h r e d u c i n g c o n d i t i o n s can a l s o cause d e p l e t i o n of p r o t e c t i v e o x i d e c o a t i n g s on tube s u r f a c e s . On h i g h e r temperature m e t a l s u r f a c e s , (superheaters/ r e h e a t e r s ) a l k a l i - i r o n - t r i s u l f a t e s are o f t e n the cause of c o r r o s i o n . C h l o r i n e can a l s o be a c o n t r i b u t i n g f a c t o r toward s u p e r h e a t e r m e t a l c o r r o s i o n i f s u l f u r c o n t e n t i s low. W h i l e exact mechanisms can


21.

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LEVASSEUR

Coal Ash Deposition

in

be argued t h e r e have been examples of both l i q u i d phase and phase c o r r o s i o n when c h l o r i d e s have been p r e s e n t ( 2 ) .


291

Boilers gas

E r o s i o n of c o n v e c t i v e pass tubes, w h i l e not a f u n c t i o n of d e p o s i t s , i s caused by the a b r a s i v e components i n f l y a s h . F l y a s h s i z e and shape, ash p a r t i c l e c o m p o s i t i o n and c o n c e n t r a t i o n , and l o c a l gas v e l o c i t i e s p l a y important r o l e s c o n c e r n i n g e r o s i o n phenomenon. Recent work has shown t h a t q u a r t z p a r t i c l e s above a c e r t a i n p a r t i c l e s i z e a r e v e r y i n f l u e n t i a l i n the e r o s i o n p r o c e s s and t h a t f u r n a c e temperature h i s t o r y p l a y s an important r o l e i n d e t e r m i n i n g e r o s i v e c h a r a c t e r i s t i c s of the p a r t i c l e s ( 3 , 4 ) . FUNDAMENTAL CONSIDERATIONS IN ASH

DEPOSITION

The c o a l ash d e p o s i t i o n p r o c e s s i n v o l v e s numerous a s p e c t s of c o a l combustion and m i n e r a l t r a n s f o r m a t i o n / r e a c t i o n . The f o l l o w i n g a l l p l a y a r o l e i n the f o r m a t i o n of ash and the d e p o s i t i o n p r o c e s s . Coal Organic P r o p e r t i e s Coal M i n e r a l Matter P r o p e r t i e s Combustion K i n e t i c s M i n e r a l T r a n s f o r m a t i o n and Decomposition F l u i d Dynamics Ash T r a n s p o r t Phenomena V a p o r i z a t i o n and C o n d e n s a t i o n of Ash S p e c i e s D e p o s i t C h e m i s t r y - S p e c i e M i g r a t i o n and R e a c t i o n Heat T r a n s f e r To and From the D e p o s i t D e s p i t e c o n s i d e r a b l e r e s e a r c h i n t h e s e a r e a s , t h e r e are gaps i n our fundamental u n d e r s t a n d i n g of the mechanisms r e s p o n s i b l e f o r m i n e r a l matter b e h a v i o r . The importance of f u r n a c e o p e r a t i n g cond i t i o n s on the combined r e s u l t s of each of the above a r e a s must be s t r e s s e d . F o r a g i v e n c o a l c o m p o s i t i o n , f u r n a c e temperatures and r e s i d e n c e times g e n e r a l l y d i c t a t e the p h y s i c a l and c h e m i c a l t r a n s f o r m a t i o n s which o c c u r . The ash f o r m a t i o n p r o c e s s i s p r i m a r i l y dependent on the time/temperature h i s t o r y of the c o a l p a r t i c l e . The r e s u l t a n t p h y s i c a l p r o p e r t i e s of a g i v e n ash p a r t i c l e g e n e r a l l y determine whether i t w i l l adhere t o heat t r a n s f e r s u r f a c e s . L o c a l s t o i c h i o m e t r i e s can a l s o i n f l u e n c e the t r a n s f o r m a t i o n p r o c e s s and t h e r e b y the p h y s i c a l c h a r a c t e r i s t i c s of ash p a r t i c l e s ; i r o n - b e a r i n g p a r t i c l e s are a prime example of t h i s . Aerodynamics can p l a y a r o l e i n the ash d e p o s i t i o n p r o c e s s i n a l l f u r n a c e s r e g a r d l e s s of the type of f i r i n g ; r e c e n t i n t e r e s t i n m i c r o f i n e g r i n d i n g of c o a l i s t e s t i m o n y t o t h i s f a c t . I t has been p o s t u l a t e d t h a t s m a l l e r ash p a r t i c l e s w i l l f o l l o w gas s t r e a m l i n e s and be l e s s l i k e l y t o s t r i k e heat t r a n s f e r s u r f a c e s . T h i s i s a l o g i c a l h y p o t h e s i s f o r those ash p a r t i c l e s t h a t cause d e p o s i t i o n due t o an impact mechanism. In a d d i t i o n t o p a r t i c l e s i z e , p a r t i c l e d e n s i t y and shape a l s o a f f e c t aerodynamic b e h a v i o r . Due t o the d i f f e r e n c e i n drag f o r c e s , molten, s p h e r i c a l p a r t i c l e s w i l l be l e s s l i k e l y t o f o l l o w gas s t r e a m l i n e s than a n g u l a r or i r r e g u l a r p a r t i c l e s of the same mass.


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Most c o a l ash w i l l r e s u l t i n d e p o s i t s which i n c r e a s e i n s e v e r i t y w i t h i n c r e a s i n g gas temperature. T h i s i s not a l i n e a r r e l a t i o n s h i p , as i l l u s t r a t e d i n F i g u r e 2, but r a t h e r a p r o g r e s s i v e l y more s e v e r e ash d e p o s i t c o n d i t i o n w i t h i n c r e a s i n g gas temperature ( 4 ) . B o i l e r s are n o r m a l l y d e s i g n e d so t h a t c l e a n a b l e , s i n t e r e d d e p o s i t s w i l l be formed. T h i s i s a r e a s o n a b l e compromise between a v e r y l a r g e , e c o n o m i c a l l y u n c o m p e t i t i v e b o i l e r t h a t may produce v e r y dry, d u s t y d e p o s i t s and a v e r y s m a l l , h i g h l y l o a d e d b o i l e r t h a t would produce molten, r u n n i n g ash d e p o s i t s . One d e s i g n o b j e c t i v e i s t o determine how s m a l l a f u r n a c e can be, f o r a g i v e n MW o u t p u t , and s t i l l r e s u l t i n d e p o s i t s t h a t are c l e a n a b l e w i t h c o n v e n t i o n a l s o o t b l o w i n g equipment. Because of the c o m p l e x i t y of the ash f o r m a t i o n and ash d e p o s i t i o n p r o c e s s , i t seems l o g i c a l t o d e a l f i r s t w i t h those key c o a l cons t i t u e n t s most r e s p o n s i b l e f o r ash d e p o s i t i o n . The i r o n and sodium c o n t e n t s of an ash have t y p i c a l l y been c o n s i d e r e d key c o n s t i t u e n t s . T e c h n i q u e s have been d e v e l o p e d t o determine how t h e s e key c o n s t i t u e n t s a r e c o n t a i n e d i n the c o a l , i . e . , the p a r t i c u l a r m i n e r a l forms t h a t a r e p r e s e n t or the g r a i n s i z e o f the c o n s t i t u e n t i n q u e s t i o n . O b v i o u s l y the remainder of the m i n e r a l m a t t e r has an e f f e c t , but depending on the c o n c e n t r a t i o n and form i n which i r o n and/or sodium c o n s t i t u e n t s a r e p r e s e n t , the r e m a i n i n g m i n e r a l m a t t e r o f t e n has second o r d e r e f f e c t s . As p r e v i o u s l y d i s c u s s e d , ash f o r m a t i o n and the r e s u l t i n g ash s i z e d i s t r i b u t i o n i s dependent on s e v e r a l f a c t o r s i n c l u d i n g i n i t i a l c o a l s i z e , c o a l burning c h a r a c t e r i s t i c s , mineral content, mineral g r a i n s i z e , v o l a t i l e ash s p e c i e s , m e l t i n g b e h a v i o r of the m i n e r a l matter, and temperature h i s t o r y of the p a r t i c l e . G e n e r a l l y , c o a l c o n t a i n i n g lower m e l t i n g m i n e r a l m a t t e r has a g r e a t e r p o t e n t i a l f o r ash a g g l o m e r a t i o n as the p a r t i c l e burns and y i e l d s fewer ash p a r t i c l e s per c o a l p a r t i c l e . T h i s r e s u l t s i n c o a r s e r ash p a r t i c l e s than those o f c o a l w i t h h i g h e r m e l t i n g ash. O b v i o u s l y t h i n g s l i k e ash q u a n t i t y , and m i n e r a l g r a i n s i z e s c o u l d i n f l u e n c e t h i s hypot h e s i s . The way a c o a l p a r t i c l e burns may a l s o i n f l u e n c e the number of ash p a r t i c l e s g e n e r a t e d , i . e . , a s h r i n k i n g sphere b u r n i n g mode may produce a d i f f e r e n t r e s u l t from a c o n s t a n t d i a m e t e r , d e c r e a s i n g d e n s i t y mode of b u r n i n g ; s w e l l i n g c o a l s may behave d i f f e r e n t l y than n o n - s w e l l i n g c o a l s . In r e f l e c t i n g on the above d i s c u s s i o n , i t becomes apparent t h a t one cannot c o m p l e t e l y d i v o r c e the p r e d i c t i v e t e c h n i q u e s employed, from the p a r t i c u l a r c o a l b u r n i n g a p p l i c a t i o n . P u l v e r i z e d c o a l f i r i n g w i l l r e q u i r e a s e n s i t i v i t y t o d i f f e r e n t c o n d i t i o n s than s t o k e r f i r i n g , or a s l a g g i n g combustor. F a i l u r e t o address the s p e c i f i c c o n d i t i o n s i n h e r e n t i n each type of f i r i n g system w i l l l e a d t o lower r e s o l u t i o n i n one's p r e d i c t i v e a b i l i t i e s than d e s i r e d .


21.

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1.07H

1.05H^_

BITUMINOUS

293

r

1.17h

1.15h

h

Boilers

1.16Wx1.08D

Π

fl

in

MIDWESTERN BITUMINOUS SUB-BITUMINOUS

WILCOX SEAM

YEGUA-JACKSON SEAM

F i g u r e 1. E f f e c t of C o a l p r o p e r t i e s on f u r n a c e s i z e . w i t h p e r m i s s i o n from r e f e r e n c e 11. C o p y r i g h t 1978 C-E

Reproduced Publication.

COALC

PHYSICAL STATE OF DEPOSITS

/COALB

POWDER SINTER-FRIABLE FUSED-MOLTEN

1200

1400

NORTHERN PLAINS LIGNITE

T E X A S LIGNITE

COAL A

1600

1800

2000

2200

2400

FLUE GAS TEMPERATURE (°F) F i g u r e 2. S u p e r h e a t e r d e p o s i t b u i l d u p v s . f l u e gas temperature. D e p o s i t c o l l e c t e d i n an 8-hour p e r i o d . D e p o s i t weight normalized: d e p o s i t weight/Cash i n p u t χ s u r f a c e a r e a of c o l l e c t i o n ) . Reproduced w i t h p e r m i s s i o n from r e f e r e n c e 5. Copyright 1977 C-E P u b l i c a t i o n .


294

MINERAL MATTER AND

ASSESSMENT OF

ASH IN COAL

TRADITIONAL PREDICTIVE METHODS


ASTM measurements such as ash f u s i b i l i t y (D1857) have formed the b a s i s f o r t r a d i t i o n a l ash b e h a v i o r p r e d i c t i v e t e c h n i q u e s . These b e n c h - s c a l e t e s t s p r o v i d e r e l a t i v e i n f o r m a t i o n on a f u e l which i s used i n a comparative f a s h i o n w i t h s i m i l a r d a t a on f u e l s of known b e h a v i o r . U n f o r t u n a t e l y , t h e s e commonly used t e s t s do not always p r o v i d e s u f f i c i e n t i n f o r m a t i o n t o permit a c c u r a t e comparison. The f u s i b i l i t y temperature measurement t e c h n i q u e attempts t o r e c o g n i z e the f a c t t h a t m i n e r a l m a t t e r i s made up of a m i x t u r e of compounds each h a v i n g t h e i r own m e l t i n g p o i n t . As a cone of ash i s heated some of the compounds melt b e f o r e o t h e r s and a m i x t u r e of melted and unmelted m a t e r i a l r e s u l t s . The s t r u c t u r a l i n t e g r i t y or d e f o r m a t i o n of the t r a d i t i o n a l ash cone changes w i t h i n c r e a s i n g temperature as more of the m i n e r a l s m e l t . However, more r e c e n t r e s u l t s i n d i c a t e t h a t s i g n i f i c a n t m e l t i n g / s i n t e r i n g can occur b e f o r e i n i t i a l d e f o r m a t i o n i s observed ( 5 ) . The f a c t t h a t the time/tempera t u r e h i s t o r y of l a b o r a t o r y ash i s q u i t e d i f f e r e n t from c o n d i t i o n s e x p e r i e n c e d i n the b o i l e r can r e s u l t i n d i f f e r e n c e s i n m e l t i n g b e h a v i o r . In a d d i t i o n , the ash used i n t h i s t e c h n i q u e may not r e p r e s e n t the c o m p o s i t i o n of ash d e p o s i t s t h a t a c t u a l l y s t i c k t o the tube s u r f a c e s . O f t e n t h e r e i s a major d i s c r e p a n c y between the c o m p o s i t i o n of a s - f i r e d ash and t h a t which i s found as d e p o s i t s (See T a b l e I ) . T h i s i s a major c r i t i c i s m of the ash f u s i b i l i t y t e s t . The d i s c r e p a n c i e s between f u s i b i l i t y temperature p r e d i c t i o n s and a c t u a l s l a g g i n g performance i s u s u a l l y g r e a t e r on ashes t h a t may l o o k r e a s o n a b l y good based on f u s i b i l i t y temperature r e s u l t s . One can u s u a l l y assume, w i t h r e a s o n a b l e c o n f i d e n c e , t h a t the m e l t i n g temperature of the w a t e r w a l l d e p o s i t s w i l l be no h i g h e r than ASTM f u s i b i l i t y temperatures; but d e p o s i t m e l t i n g temperatures can be and are o f t e n lower than ASTM m e l t i n g t e m p e r a t u r e s . T h i s i s because s e l e c t i v e d e p o s i t i o n of lower m e l t i n g c o n s t i t u e n t s can and does o c c u r w i t h a r e s u l t i n g enrichment of lower m e l t i n g m a t e r i a l i n the d e p o s i t . Ash v i s c o s i t y measurements s u f f e r s i m i l a r c r i t i c i s m w i t h r e s p e c t to the f u s i b i l i t y measurements. These t e s t s are conducted on l a b o r a t o r y ash and on a composite ash sample. V i s c o s i t y measurements are l e s s s u b j e c t i v e and more d e f i n i t i v e than f l u i d temperature d e t e r m i n a t i o n f o r the assessment of ash f l o w c h a r a c t e r i s t i c s . However, t h e s e measurements r e f l e c t the p r o p e r t i e s of a s o l u t i o n or l i q u i d and p r e c i p i t a t i n g c r y s t a l s (pending t e m p e r a t u r e ) , of a l l the ash c o n s t i t u e n t s and may not be r e p r e s e n t a t i v e of s l a g d e p o s i t p r o p e r t i e s 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 . During p u l v e r i z e d c o a l f i r i n g , a s e v e r e problem may a l r e a d y e x i s t b e f o r e s l a g d e p o s i t s r e a c h the f l u i d / r u n n i n g s t a t e . G e n e r a l l y , o n l y a s m a l l q u a n t i t y of l i q u i d phase m a t e r i a l e x i s t s i n d e p o s i t s and i t i s the p a r t i c l e t o - p a r t i c l e s u r f a c e bonding which i s most i m p o r t a n t . Much use i s made of the ash c o m p o s i t i o n which i s n o r m a l l y a comp i l a t i o n of the major elements i n c o a l ash e x p r e s s e d as the o x i d e form. From t h i s c o m p i l a t i o n of elements, e x p r e s s e d as o x i d e s , judgements a r e o f t e n made based on the q u a n t i t y of c e r t a i n key con-


21.

BORIO AND

Coal Ash Deposition

LEVASSEUR


TABLE

in

295

Boilers

I

ENRICHMENT OF IRON IN B O I L E R WALL DEPOSITS COMPARISON OF COMPOSITION OF ASH DEPOSITS AND A S - F I R E D COAL ASHES

1 UNIT SAMPLE

Ash

AS-FIRED COAL ASH

2

3

WATERWALL DEPOSIT

AS F I R E D COAL ASH

WATERWALL A S - F I R E D DEPOSIT COAL ASH

WATERWALL DEPOSIT

47.0

33.3

50.2

55.1

49.7

41.8

Composition

Si0

2

A1 0

3

26.7

18.0

16.9

14.6

16.5

15.8

Fe 0

3

14.6

43.5

5.9

18.3

12.0

28.5

CaO

2.2

1.2

12.8

7.2

6.5

9.0

MgO

0.7

0.5

3.5

2.0

0.9

0.9

Na 0 2

0.4

0.2

0.6

0.5

1.1

0.6

K 0

2.3

1.6

0.8

0.6

1.5

0.9

1.3

0.8

0.9

0.8

1.1

0.7

1.1

0.5

12.0

0.1

2.0

0.2

2

2

2

Ti0 so

3

2



296

MINERAL

MATTER

A N D A S H IN

COAL

s t i t u t e n t s l i k e i r o n and sodium. B a s e / a c i d r a t i o s are computed and used as i n d i c a t o r s of ash b e h a v i o r ; n o r m a l l y lower m e l t i n g ashes f a l l i n the 0.4 t o 0.6 range. I t has been shown t h a t b a s e / a c i d r a t i o s g e n e r a l l y c o r r e l a t e w i t h ash s o f t e n i n g t e m p e r a t u r e s , so a l t h o u g h b a s e / a c i d r a t i o s have h e l p e d e x p l a i n why ash s o f t e n i n g temperatures v a r i e d , i t has not improved p r e d i c t i v e c a p a b i l i t i e s i n the a u t h o r s ' o p i n i o n . Other r a t i o s such as Fe/Ca and S i / A l have been used as i n d i c a t o r s of ash d e p o s i t b e h a v i o r . R a t i o s l i k e t h e s e have h e l p e d t o e x p l a i n d e p o s i t c h a r a c t e r i s t i c s , but t h e i r use as a prime p r e d i c t i v e t o o l i s q u e s t i o n a b l e e s p e c i a l l y s i n c e t h e s e r a t i o s do not take i n t o account s e l e c t i v e d e p o s i t i o n nor do they c o n s i d e r the t o t a l q u a n t i t i e s of the c o n s t i t u e n t s p r e s e n t . An Fe/Ca r a t i o of 2 c o u l d r e s u l t from weight p e r c e n t r a t i o s of 6/3 or 30/15; the l a t t e r numbers would g e n e r a l l y i n d i c a t e a f a r worse s i t u a t i o n than the former, but r a t i o s don't show t h i s . Many s l a g g i n g and f o u l i n g i n d i c e s are based upon c e r t a i n ash cons t i t u e n t r a t i o s and c o r r e c t e d u s i n g such f a c t o r s as g e o g r a p h i c a l a r e a , s u l f u r c o n t e n t , sodium c o n t e n t , e t c . One commonly used s l a g g i n g i n d e x uses B a s e / A c i d r a t i o and s u l f u r c o n t e n t . F a c t o r i n g i n s u l f u r c o n t e n t i s l i k e l y t o improve the s e n s i t i v i t y of t h i s i n d e x t o the i n f l u e n c e of p y r i t e on s l a g g i n g . (As p r e v i o u s l y d i s c u s s e d , i r o n - r i c h m i n e r a l s o f t e n p l a y an i m p o r t a n t r o l e i n s l a g g i n g . ) Howe v e r , the use of such " c o r r e c t i o n " f a c t o r s i s o f t e n a crude subs t i t u t e f o r more d e t a i l e d knowledge of the fundamental f u e l p r o p e r t i e s . A n o t h e r example of t h i s i s the use of c h l o r i n e c o n t e n t i n a c o a l as a f o u l i n g i n d e x . T h i s can be v a l i d i f the c h l o r i n e i s p r e s e n t as N a C l ( t h e r e b y i n d i c a t i n g the c o n c e n t r a t i o n of sodium which i s i n an a c t i v e form) and t h a t the sodium w i l l , i n f a c t , cause the f o u l i n g . C h l o r i n e p r e s e n t i n o t h e r forms may or may not a d v e r s e l y a f f e c t f o u l i n g . S i n t e r i n g s t r e n g t h t e s t s have been used as an i n d i c a t i o n of f o u l i n g p o t e n t i a l . Assuming t h a t c o r r e c t ash c o m p o s i t i o n s have been r e p r e s e n t e d (which i s l e s s of a problem i n the c o n v e c t i o n s e c t i o n than i n the r a d i a n t s e c t i o n ) w o r t h w h i l e i n f o r m a t i o n may be o b t a i n e d r e l a t i v e t o a t i m e / t e m p e r a t u r e v s . bonding s t r e n g t h r e l a t i o n s h i p . In o r d e r f o r s i n t e r i n g t e s t s t o a c c u r a t e l y p r e d i c t a c t u a l b e h a v i o r i t i s n e c e s s a r y t h a t t e s t s be conducted w i t h ash produced under r e p r e s e n t a t i v e furnace c o n d i t i o n s (time-temperature h i s t o r y ) . F o u l i n g b e h a v i o r i s o f t e n g r e a t l y i n f l u e n c e d by sodium r e a c t i o n s . Sodium which v a p o r i z e s i n the f u r n a c e can condense i n downstream c o n v e c t i o n s e c t i o n s t h e r e b y c o n c e n t r a t i n g on f l y a s h s u r f a c e s . P a r t i c l e s u r f a c e r e a c t i o n s are p r i m a r i l y r e s p o n s i b l e f o r c o n v e c t i o n deposit bonding. In summary, t r a d i t i o n a l methods f o r p r e d i c t i o n of ash d e p o s i t c h a r a c t e r i s t i c s a r e h e a v i l y based upon ash c h e m i s t r y . These c o n v e n t i o n a l a n a l y s e s do not p r o v i d e d e f i n i t i v e i n f o r m a t i o n c o n c e r n i n g the m i n e r a l forms p r e s e n t i n the c o a l s and the d i s t r i b u t i o n of i n o r g a n i c s p e c i e s w i t h i n the c o a l m a t r i x . Such i n f o r m a t i o n can be e x t r e m e l y i m p o r t a n t i n e x t r a p o l a t i n g p r e v i o u s e x p e r i e n c e , s i n c e the n a t u r e i n which the i n o r g a n i c c o n s t i t u e n t s are c o n t a i n e d i n the c o a l can be the d e t e r m i n i n g f a c t o r i n t h e i r b e h a v i o r d u r i n g the ash depos i t i o n process.


21.

BORIO AND

LEVASSEUR


ASSESSMENT OF NEW

Coal Ash Deposition

in

Boilers

297

PREDICTIVE TECHNIQUES

G e n e r a l l y s p e a k i n g the newer bench s c a l e p r e d i c t i v e t e c h n i q u e s are f a r more s e n s i t i v e to the c o n d i t i o n s t h a t e x i s t i n commercial f u r n a c e s than the o l d e r p r e d i c t i v e methods. S e l e c t i v e d e p o s i t i o n , f o r example, has been r e c o g n i z e d as a phenomenon which cannot be i g n o r e d . More a t t e n t i o n i s b e i n g p a i d t o fundamentals of the ash f o r m a t i o n and d e p o s i t i o n p r o c e s s e s . New t o o l s , such as Scanning E l e c t r o n M i c r o s c o p y (SEM), a r e b e i n g c o n s i d e r e d as ways t o improve p r e d i c t i v e c a p a b i l i t i e s . Other, more s p e c i a l i z e d bench s c a l e apparat u s e s are b e i n g developed t o s i m u l a t e important a s p e c t s of comm e r c i a l c o n d i t i o n s and p r o v i d e q u a n t i t a t i v e i n f o r m a t i o n on p a r a meters t h a t i n f l u e n c e bonding s t r e n g t h . Recent work has shown t h a t p u l v e r i z e d c o a l , i f s e p a r a t e d by g r a v i t y f r a c t i o n a t i o n , can y i e l d important i n f o r m a t i o n r e l a t i v e t o s l a g g i n g p o t e n t i a l due t o the i r o n c o n t e n t . ( 6 , 7 ) . R e s u l t s of t h i s work have shown t h a t the percentage of i r o n i n the heavy f r a c t i o n s c o r r e l a t e s v e r y w e l l t o the s l a g g i n g b e h a v i o r i n commercial b o i l e r s . (See F i g u r e 3 ) . T h i s t e c h n i q u e appears t o i d e n t i f y the p r o p o r t i o n of r e l a t i v e l y pure p y r i t e s p a r t i c l e s t h a t are g e n e r a t e d i n the p u l v e r i z e d c o a l f e e d and t h a t a r e c a p a b l e of m e l t i n g a t r e l a t i v e l y low temperatures and t h a t would account f o r enrichment of i r o n i n lower f u r n a c e w a t e r w a l l d e p o s i t s . A method f o r measuring a c t i v e a l k a l i e s has been d e v e l o p e d as a means f o r improved p r e d i c t i o n of f o u l i n g p o t e n t i a l ( 8 ) . P r e v i o u s wisdom h e l d t h a t f o u l i n g p o t e n t i a l was d i r e c t l y r e l a t e d t o the t o t a l sodium c o n t e n t . Much of t h i s e a r l y work was done on low rank c o a l s i n which case i t was not uncommon f o r a l l of the a l k a l i e s t o be p r e s e n t as an a c t i v e sodium form ( 9 ) . However, t h e r e were many o c c a s i o n s where the f o u l i n g p o t e n t i a l was not a d e q u a t e l y p r e d i c t e d by the t o t a l sodium c o n t e n t . ( T a b l e I I p r o v i d e s some examples of anomalous f o u l i n g b e h a v i o r . ) The mechanism p o s t u l a t e d f o r s o d i u m - r e l a t e d f o u l i n g was one of a v a p o r i z a t i o n / c o n d e n s a t i o n mechanism. Simple forms of sodium compounds r e s u l t e d i n the v a p o r i z a t i o n of sodium i n the r a d i a n t zone of the f u r n a c e where peak temperatures are g e n e r a t e d . Subsequent c o n d e n s a t i o n of the sodium on the r e l a t i v e l y c o o l tube s u r f a c e s e f f e c t e d a p r o c e s s f o r d e p o s i t i o n of sodium. Sodium i s a known, e f f e c t i v e f l u x i n g agent t h a t can c r e a t e hard, bonded d e p o s i t s . The r e f e r e n c e d method r e l i e d on the use of weak a c i d t o p r e f e r e n t i a l l y l e a c h out sodium from s i m p l e compounds l i k e N a C l and/or o r g a n i c a l l y - b o u n d a l k a l i as would be p r e s e n t i n many of the lower rank c o a l s . T h i s method g i v e s r e s u l t s t h a t c o r r e l a t e w e l l w i t h f i e l d performance on c o a l s h a v i n g s i g n i f i c a n t sodium c o n t e n t s (See T a b l e I I ) . The use of new a n a l y t i c a l t e c h n i q u e s promises to g i v e r e s u l t s t h a t a l l o w m i n e r a l m a t t e r t o be i d e n t i f i e d a c c o r d i n g t o c o m p o s i t i o n , m i n e r a l form, d i s t r i b u t i o n w i t h i n the c o a l m a t r i x , and g r a i n s i z e .



MINERAL MATTER AND ASH IN COAL


21.

BORIO A N D LEVASSEUR

Coal Ash Deposition

in

Boilers

299

TABLE I I ANALYTICAL DATA ON U . S . COALS


Rank Region

Lignite N.Dakota

(Dry B a s i s ) Volatile Ash

H.T F.T Ash Composition S i 0 (%) 2

A1 0 2

Lignite Texas (Yegua)

Lignite Texas (Wilcox)

hvBb Utah

hvAB Pen η

Lignite Texas (Wilcox)

. . 44.4 .. 46.0 9.6

42.4 52.0 5.6

39.6 26.9 33.5

41.0 39.5 19.5

41.5 48.3 10.2

32.5 54.0 13.5

38.1 33.0 28.9

10640

12130

7750

9710

12870

13200

8420

.. .. .. ..

2130 2180 2190 2200

1980 2020 2060 2170

1940 2200 2430 2610

2150 2250 2340 2530

2190 2270 2390 2620

2370 2510 2560 2660

2210 2300 2420 2620

..

20.0

33.9

62.1

52.3

52.5

51.1

57.9

9.1

11.4

15.1

17.4

18.9

30.7

21.8 3.9

HHV ( B t u / l b ) . . . . (Dry B a s i s ) Ash F u s i b i l i t y I.D.(°F)

Sub Β Montana

3

..

10.3

10.8

3.5

5.3

1.1

10.0

..

22.4

21.0

6.2

9.4

13.2

1.6

7.1

6.4

2.7

0.7

3.2

1.3

0.9

2.1

Na 0

5.0

5.8

3.6

0.9

3.8

0.4

0.9

κο

0.5

1.6

2.1

1.2

0.9

1.7

0.8

0.4

0.7

0.9

1.2

1.2

2.0

1.1

21.9

12.0

6.1

9.6

6.2

1.4

4.4

5.8

3.6

0.9

3.8

0.4

0.7

1.9

1.2

0.9

1.7

0.8

0.15

0.16

2°3 CaO Fe

MgO 2

2

TiO S 0

..

3

T o t a l A l k a l i , %, Ash Basis 5.0

Na 0 2

0.5

κο 2

1.6

S o l u b l e A l k a l i , I », Ash Basis 5.58

Na 0 2

κο 2

3.88

0.71

1.49

-

0.44

0.04

0.08

111%

108%

-

-

6.45

-

0.05

R e l a t i v e Soluble A l k a l i , % 112%

Na 0 2

κο 2

F o u l i n g Potentia"1

Severe

High

High

79%

39%

3%

9%

Moderate

38%

-

Moderate Low


23% 6% Low


300


Techniques such as c o m p u t e r - c o n t r o l l e d s c a n n i n g e l e c t r o n m i c r o s c o p y (CCSEM), s c a n n i n g t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y (STEM), and X-ray d i f f r a c t i o n can be used t o c h a r a c t e r i z e t h e s e p r o p e r t i e s on an i n d i v i d u a l p a r t i c l e by p a r t i c l e b a s i s . New s p e c t r o s c o p i e s such as extended X-ray a b s o r p t i o n f i n e s t r u c t u r e s p e c t r o s c o p y (EXAFS), and e l e c t r o n energy l o s s s p e c t r o s c o p y , (EELS) a r e c a p a b l e o f d e t e r m i n i n g t h e e l e c t r o n i c bonding s t r u c t u r e and l o c a l atomic environment f o r o r g a n i c a l l y a s s o c i a t e d i n o r g a n i c s l i k e c a l c i u m , sodium and s u l f u r . Other new t e c h n i q u e s such as F o u r i e r t r a n s f o r m i n f r a r e d spectroscopy (FTIR), e l e c t r o n microprobe, e l e c t r o n spectroscopy f o r c h e m i c a l a n a l y s i s (ESCA), e t c . a l l p r o v i d e methods o f i m p r o v i n g p r e s e n t c a p a b i l i t i e s . By development and a p p l i c a t i o n o f t h e s e t e c h n i q u e s a much b e t t e r fundamental assessment o f c o a l m i n e r a l matter b e h a v i o r i s p o s s i b l e . The a u t h o r s b e l i e v e t h e s e r e s u l t s , coupled w i t h t h o s e o f o t h e r e x i s t i n g methods, c a n make a s i g n i f i c a n t improvement t o p r e d i c t i v e c a p a b i l i t i e s . AREAS OF UNCERTAINTY P r e d i c t i o n o f a s h d e p o s i t c h a r a c t e r i s t i c s based s o l e l y on benchs c a l e f u e l p r o p e r t i e s always r e q u i r e s s u b s t a n t i a l judgement and a l l o w s o n l y a c e r t a i n l e v e l o f c o n f i d e n c e . As d i s c u s s e d , the a s h d e p o s i t i o n p r o c e s s i s so complex t h a t d e t a i l e d m o d e l l i n g o f comm e r c i a l systems based on fundamental d a t a i s p r e s e n t l y u n r e a l i s t i c . However, c u r r e n t t e c h n i q u e s c a n p r o v i d e r e l a t i v e d a t a which i n most c a s e s i s s u f f i c i e n t t o make a c c u r a t e assessment o f s l a g g i n g and f o u l i n g p o t e n t i a l s r e l a t i v e t o o t h e r f u e l s . There remain many a r e a s o f u n c e r t a i n t y where e x p e r i e n c e d judgements must f i l l t h e gap between good l a b o r a t o r y r e s u l t s / p r e d i c t i o n s and b o i l e r d e s i g n d e c i s i o n s . One o f t h e s e a r e a s c o n c e r n s t h e e x t e n t of d e p o s i t coverage i n a b o i l e r . Though t h e f u e l s r e s e a r c h e r may a d e q u a t e l y c h a r a c t e r i z e a g i v e n c o a l a s h i n terms o f i t s p o t e n t i a l d e p o s i t e f f e c t s , he i s o f t e n a t a l o s s t o a d e q u a t e l y d e s c r i b e t h e e x t e n t o f coverage o f d e p o s i t s i n t h e b o i l e r . I t i s n e c e s s a r y t o a c c u r a t e l y describe furnace c o n d i t i o n s i n order to assess r e s u l t i n g d e p o s i t s i n p a r t i c u l a r b o i l e r r e g i o n s . Though some good work i s underway i n t h i s a r e a , t h e q u e s t i o n o f bonding s t r e n g t h and c l e a n a b i l i t y remains a problem as f a r as i t s p r e d i c t i o n from bench s c a l e t e s t s . Good c o r r e l a t i o n s have been d e v e l o p e d between i r o n c o n t e n t of heavy g r a v i t y f r a c t i o n s and s l a g g i n g p o t e n t i a l , but t h e r e does not e x i s t a bench s c a l e t e c h n i q u e t h a t can s i m u l a t e what t h e l o c a l ash d e p o s i t c o m p o s i t i o n s h a l l be when burned i n a commercial b o i l e r . It i s p o s s i b l e to i n c r e a s e the l e v e l of confidence i n p r e d i c t i o n of d e p o s i t e f f e c t s by c o n d u c t i n g p i l o t - s c a l e combustion s t u d i e s i n t e s t r i g s which s i m u l a t e t h e c o n d i t i o n s p r e s e n t i n commercial b o i l e r s . Combustion t e s t i n g a l l o w s e v a l u a t i o n o f t h e a s h f o r m a t i o n and d e p o s i t i o n p r o c e s s and p e r m i t s d e t a i l e d c h a r a c t e r i z a t i o n o f the d e p o s i t s g e n e r a t e d . R e s u l t s can a l l o w d e t e r m i n a t i o n o f d e p o s i t c h a r a c t e r i s t i c s as a f u n c t i o n o f fundamental b o i l e r d e s i g n p a r ameters ( s u c h as gas temperature, v e l o c i t y , e t c . ) . Combustion t e s t


21.

BORIO AND

LEVASSEUR

Coal Ash Deposition

in

Boilers

301


r i g s a l s o s e r v e as v a l u a b l e t o o l s f o r assessment of f u e l s w i t h v e r y u n u s u a l p r o p e r t i e s and can s i g n i f i c a n t l y reduce u n c e r t a i n t i e s i n e x t r a p o l a t i o n of t h e i r b e h a v i o r from p a s t e x p e r i e n c e . Whenever t e s t r e s u l t s a r e a s s e s s e d and used t o e s t a b l i s h b o i l e r d e s i g n parameters, the r e p r e s e n t a t i v e n e s s of the t e s t sample must be c a r e f u l l y c o n s i d e r e d . The degree of v a r i a b i l i t y i n the c o a l d e p o s i t and the impact of t h i s v a r i a b i l i t y on b o i l e r performance must be e v a l u a t e d . Judgements are a l e o r e q u i r e d on the e f f e c t s of more g r a d u a l changes t h a t c o u l d o c c u r i n c o a l p r o p e r t i e s as a f u n c t i o n of time and l o c a t i o n i n the c o a l seam. In summary, i t can be s t a t e d t h a t w h i l e s i g n i f i c a n t p r o g r e s s has been made i n p r e d i c t i n g ash b e h a v i o r , the ash f o r m a t i o n and depo s i t i o n p r o c e s s i s not f u l l y u n d e r s t o o d . T r a d i t i o n a l ASTM a n a l y s e s do not always p r o v i d e i n f o r m a t i o n t h a t can be used t o make p r e d i c t i v e judgements at the c o n f i d e n c e l e v e l s d e s i r e d . Newer t e c h n i q u e s have been d e v e l o p e d and are b e i n g d e v e l o p e d t h a t are more s e n s i t i v e t o the c o n d i t i o n s t h a t e x i s t i n the b o i l e r environment, and t h a t r e c o g n i z e the h e t e r o g e n e i t y of the i n o r g a n i c c o n s t i t u e n t s i n the c o a l m a t r i x . There appears t o be a r e c o g n i t i o n t h a t no one t e s t can a d e q u a t e l y d e s c r i b e c o a l ash b e h a v i o r ; a c o m b i n a t i o n of t e s t s , each d e s i g n e d t o f o c u s on a p a r t i c u l a r a s p e c t of ash b e h a v i o r seems t o be a l o g i c a l approach. Based on the r e s u l t s from many of t h e s e newer t e s t s , on c o a l s t h a t a r e p r e s e n t l y b e i n g burned i n e x i s t i n g u n i t s , the a u t h o r s f e e l c e r t a i n t h a t s i g n i f i c a n t im provements have been made i n p r e d i c t i n g ash b e h a v i o r .

REFERENCES 1.

G. J. Goetz, Ν. Y. Nsakala, and R.W. Borio, "Development of Method for Determining Emissivities and Absorptivities of Coal Ash Deposits," paper presented at the 1978 Winter Annual ASME Meeting, Dec. 1978, TIS-5890.

2.

R. W. Borio, et.al., "The Control of High Temperature Fireside Corrosion in Utility Coal-Fired Boilers," OCR R&D Report No. 41 (1969).

3.

F. Raask, "Flame Imprinted Characteristics of Ash Relevant to Boiler Slagging Corrosion and Erosion," presented at the 1981 Joint Power Generation Conference, ASME Paper No. 81-JPGC-Fu-1.

4.

W. P. Bauver, J. D. Bianca, J. D. Fishburn, and J. G. McGowan, "Characterization of Erosion of Heat Transfer Tubes in Coal -Fired Power Plants," paper to be presented at 1984 Joint Power Generation Conference, Toronto, Canada, September 1984.

5.

R. W. Borio, G. J. Goetz, and A. A. Levasseur, "Slagging and Fouling Properties of Coal Ash Deposits as Determined in a Laboratory Test Facility," paper presented at the ASME Winter Annual Meeting, December 1977, Combustion Engineering publi cation TIS-5155.



302


6.

G. P.Huffman and F.E. Higgins, "Investigations of Partial Ash Melting by Phase Analysis of Quenched Samples," presented at 1981 Engineering Foundation Conference-Fouling and Slagging Resulting From Impurities in Combustion Gases, July 1981, Henniker, NH.

7.

R. W. Borio, R. R. Narciso, Jr., "The Use of Gravity Fractionation Techniques for Assessing Slagging and Fouling Potential of Coal Ash," paper presented at the ASME Winter Annual Meeting, December 10-15, 1978, San Francisco, CA; available as Combustion Engineering publications TIS-5823.

8.

R. W. Bryers, "The Physical and Chemical Characteristics of Pyrites and Their Influence on Fireside Problems in Steam Generators, "ASME Paper No. 75-WA/CD-2, 1976.

9.

G. L. Hale, A. A. Levasseur, A. L. Tyler and R. P.Hensel, "The Alkali Metals in Coal: A Study of Their Nature and Their Impact on Ash Fouling," paper presented at Coal Technology '80, November 1980, TIS-6645.

10.

G. H. Gronhovd, W. Beckering, and P. H. Tlufte, "Study of Factors Affecting Ash Deposition from Lignite and Other Coals," an ASME publication presented at the ASME Winter Annual Meeting, November 16-20, 1969, Los Angeles, CA.

11.

R. P. Hensel, D. A. Harris, "Properties of Solid Fuels and Their Impact on Boiler Design and Performance," TIS-5889, September 10-13, 1978.

12.

W. H. Pollock, G. J. Goetz, E. Park, "Advancing the Art of Boiler Design by Combining Operating Experience and Advanced Coal Evaluation Techniques," Presented at American Power Conference, April 18-20, 1985.

RECEIVED October 10, 1985


Coal Ash Deposition in Boilers - ACS Symposium Series (ACS

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