Flame Vitrification and Sintering Characteristics of Silicate Ash

11 Flame Vitrification and Sintering Characteristics of Silicate Ash

Mineral Matter and Ash in Coal Downloaded from pubs.acs.org by UNIV LAVAL on 05/15/16. For personal use only.

Erich Raask Technical Planning and Research Division, Central Electricity Generating Board, Leatherhead, Surrey, United Kingdom

Silicate species constitute the bulk of the mineral matter in most coals, and the formation of boiler deposits depends largely on the physical and pyrochemical changes of the ash residue constituents. In this work the mode of occurrence of coal silicate minerals, and the flame induced vitrification and sodium initiated sintering mechanisms have been studied. The pulverized coal flame temperature is sufficiently high to vitrify the quartz particles. On cooling some devitrification occurs and the rate of sintering depends largely on the ratio of glassy phase to crystalline species in the ash. The flame volatile sodium captured by the vitrified silicate particles can initiate the coalescence of deposited ash by viscous flow and the rate of sintering is markedly increased by the alkali-metal dissolved in the glassy phase. The flame i m p r i n t e d c h a r a c t e r i s t i c s o f p u l v e r i z e d c o a l ash r e l e v a n t to b o i l e r s l a g g i n g , c o r r o s i o n and e r o s i o n have been d i s c u s s e d p r e v i o u s l y (1,2). S i l i c a t e m i n e r a l s c o n s t i t u t e between 60 and 90 p e r c e n t o f ash i n most c o a l s and b o i l e r d e p o s i t s a r e l a r g e l y made up from the s i l i c i o u s i m p u r i t y c o n s t i t u e n t s . T h i s work s e t s o u t f i r s t t o examine t h e mode o f o c c u r r e n c e o f the s i l i c a t e m i n e r a l s p e c i e s i n c o a l f o l l o w e d by a c h a r a c t e r i z a t i o n assessment o f the flame v i t r i f i e d and sodium e n r i c h e d s i l i c a t e ash p a r t i c l e s . The ash s i n t e r i n g s t u d i e s a r e l i m i t e d t o i n v e s t i g a t i o n s o f the r o l e o f sodium i n i n i t i a t i n g and s u s t a i n i n g the bond f o r m i n g r e a c t i o n s t o the f o r m a t i o n o f b o i l e r d e p o s i t s . Silica

( Q u a r t z ) and S i l i c a t e M i n e r a l

Species

i n Coal

The q u a r t z and a l u m i n o - s i l i c a t e s p e c i e s found i n most c o a l s c o n s t i t u t e t h e b u l k o f combustion ash r e s i d u e . The a l u m i n o - s i l i c a t e s i n c l u d e m u s c o v i t e and i l l i t e which c o n t a i n p o t a s s i u m , and k a o l i n i t e species (3-6). The s i l i c a (S1O2) and a l u m i n a (AI2O3) as d e t e r m i n e d by c h e m i c a l a n a l y s i s a r e p r e s e n t i n a l u m i n o - s i l i c a t e s on an average 0097-6156/ 86/ 0301 -0138S06.00/ 0 © 1986 American Chemical Society

11.

RAASK

Characteristics

of Silicate

139

Ash

w e i g h t r a t i o o f 1.5 t o 1 as r e p o r t e d by Dixon e t a l . ( 6 ) . The excess of s i l i c a r e p r e s e n t s the amount o f q u a r t z i n c o a l m i n e r a l m a t t e r :

(sio )

(sio ) - 1, 5 ( A 1

2

2

t

2

0 )

(1)

3

Where ( S i 0 ) q , ( S i 0 ) c and (A1 0^) denote r e s p e c t i v e l y the q u a r t z , t o t a l s i l i c a and alumina c o n t e n t s o f a s h . An approximate amount o f p o t a s s i u m a l u m i n o - s i l i c a t e s i n c o a l m i n e r a l m a t t e r can be o b t a i n e d from the p o t a s s i u m o x i d e ( K 0 ) c o n t e n t of a s h . The amount o f n o n - s i l i c a t e p o t a s s i u m s p e c i e s i s s m a l l i n most c o a l s and t h e s i l i c a t e m i n e r a l s c o n t a i n on average 11 p e r c e n t K 0 by w e i g h t ( 6 ) . Thus the p o t a s s i u m a l u m i n o - s i l i c a t e c o n t e n t o f c o a l mineral matter ( K ^ L - S I L ) w e i g h t p e r cent i s : 2

2

2

2


2

KO Κ

A L - S I L

c m

=

9

·

1

κ

(2)

2 °

where K 0 denotes the p o t a s s i u m o x i d e c o n t e n t o f a s h . The t o t a l amount o f s i l i c a t e m i n e r a l s e q u a l s a p p r o x i m a t e l y the sum o f S1O2, A 1 0 ^ and K 0 i n ash, and an e s t i m a t e o f k a o l i n i t e s p e c i e s i s thus g i v e n b y : 2

2

Kaolinite

2

=

(Si0

2

+ A1 0 2

3

+ K 0 ) - (Quartz + P o t . 2

Silicates) (3)

T a b l e I g i v e s the S i 0 , A 1 0 ^ and K 0 c o n t e n t s o f some US and B r i t i s h b i t u m i n o u s c o a l asnes (4,7) w h i c h were used t o c a l c u l a t e the approximate amounts o f q u a r t z , p o t a s s i u m a l u m i n o - s i l i c a t e and k a o l i n i t e species i n the mineral matter. 2

Table I .

2

2

E s t i m a t e d Amounts o f S i l i c a t e S p e c i e s i n Bituminous Coal M i n e r a l Matter Mineral species (weight per cent)

Ash c o n s t i t u e n t s (weight p e r cent of ash) Type o f c o a l Si0

o

A 1

2°3

KoO

Quartz

P o t . alum, silicates

Kaolinite

Low silica

British U.S.

31.1 29.2

18.1 14.2

1.2 1.5

3.9 7.9

10.9 13.6

26.2 23.6

Medium silica

British U.S.

46.5 46.6

22.8 27.8

2.8 1.1

12.3 4.9

25.5 10.0

34.3 60.6

High silica

British U.S.

55.5 56.5

30.0 32.2

2.7 2.6

10.5 8.0

24.5 23.6

53.2 59.7

T a b l e I shows t h a t the k a o l i n i t e s p e c i e s c o n s t i t u t e up t o 60 p e r c e n t o f the c o a l m i n e r a l m a t t e r . The amount o f p o t a s s i u m a l u m i n o - s i l i c a t e s , c h i e f l y m u s c o v i t e and i l l i t e i s between 10 and 25 p e r c e n t , and the q u a r t z c o n t e n t i s u s u a l l y below 12 p e r c e n t . The a l u m i n o - s i l i c a t e minerals contain frequently i r o n , calcium, magnesium and sodium as p a r t replacement f o r p o t a s s i u m and p a r t l y


140

M I N E R A L M A T T E R A N D A S H IN C O A L

i n c o r p o r a t e d i n the k a o l i n i t e s t r u c t u r e . A l s o , the s i l i c a t e m i n e r a l s o c c u r as h y d r a t e d s p e c i e s w i t h the i n h e r e n t water c o n t e n t o f between 2 to 5 p e r c e n t , thus the s i l i c i o u s m i n e r a l c o n t e n t s are l i k e l y to be about 5 p e r c e n t h i g h e r than those g i v e n i n T a b l e I . The s i l i c a and a l u m i n a c o n t e n t s o f the f i r s t two samples a r e e x c e p t i o n a l l y low f o r bituminous c o a l ashes. The u s u a l c o n c e n t r a t i o n range o f s i l i c a i s 35 t o 55 p e r c e n t and t h a t o f alumina i s 20 to 30 p e r c e n t , t h u s the a l u m i n o - s i l i c a t e s p e c i e s t o g e t h e r w i t h q u a r t z c o n s t i t u t e between 60 t o 90 per cent o f b i t u m i n u s c o a l m i n e r a l m a t t e r . The s i l i c a t e s p e c i e s o c c u r i n c o a l c h i e f l y as s e p a r a t e s t r a t a and l a r g e p a r t i c l e i n c l u s i o n s , and t h i s mode o f o c c u r r e n c e i s termed the " a d v e n t i t i o u s " m i n e r a l m a t t e r . F i g u r e l a shows a t y p i c a l sample o f the a d v e n t i t i o u s s i l i c a t e m i n e r a l p a r t i c l e s , d e n s i t y s e p a r a t e d from p u l v e r i z e d c o a l . The d e n s i t y s e p a r a t i o n t e c h n i q u e does n o t remove the s m a l l s i l i c a t e p a r t i c l e s , c h i e f l y a l u m i n o - s i l i c a t e s p e c i e s , the " i n h e r e n t " m i n e r a l matter, i n the c o a l substance ( F i g u r e l b ) . The ash c o n t e n t o f b i t u m i n o u s c o a l s d e l i v e r e d to u t i l i t y power s t a t i o n i s u s u a l l y between 10 and 25 p e r cent ( 4 , 8 ) . About 25 p e r c e n t o f the ash i s p r e s e n t i n the form o f i n h e r e n t m i n e r a l m a t t e r o f d i s p e r s e d s m a l l p a r t i c l e s and a l s o as m i n e r a l elements r e a c t e d w i t h the c o a l s u b s t a n c e . The m i n e r a l elements can be h e l d i n the c o a l s u b s t a n c e as o r g a n o - m e t a l l i c s a l t s , and a l s o as a r e s u l t o f m o l e c u l a r a d s o r p t i o n and c o - v a l e n t b o n d i n g . The m i n e r a l s p e c i e s d i s s o l v e d i n c o a l pore w a t e r , c h i e f l y c h l o r i d e s can a l s o be c o n s i d e r e d as p a r t o f the i n h e r e n t matter. The l i g n i t e s and sub-bituminous c o a l s can have a h i g h f r a c t i o n o f the m i n e r a l elements, c h i e f l y sodium, c a l c i u m and a l s o aluminium and i r o n c h e m i c a l l y combined i n the f u e l s u b s t a n c e (9,10). The c h e m i c a l r e a c t i v i t y and p o r o s i t y o f the f u e l m a t r i x decrease w i t h the i n c r e a s e o f c o a l age from l i g n i t e t o b i t u m i n o u s rank. The l o s s o f c a r b o x y l , h y d r o x y l and quinone b o n d i n g s i t e s i n the f u e l m a t r i x r e s u l t s i n a low " c h e m i c a l " m i n e r a l m a t t e r c o n t e n t of bituminous c o a l s . C h l o r i d e i n C o a l Pore and

Seam Water

C h l o r i d e m i n e r a l s a r e r a r e l y found i n c o a l i n the form o f s o l i d s p e c i e s because o f h i g h s o l u b i l i t y o f sodium, c a l c i u m and t r a c e m e t a l c h l o r i d e s i n coal s t r a t a waters. The " i n h e r e n t " water c o n t e n t o f c o a l i s r e l a t e d t o i t s p o r o s i t y and thus the m o i s t u r e c o n t e n t o f l i g n i t e d e p o s i t s can exceed 40 p e r cent d e c r e a s i n g to below 5 p e r cent i n f u l l y b i t u m i n o u s c o a l s ( 1 1 ) . C h l o r i d e s , c h i e f l y a s s o c i a t e d w i t h sodium and c a l c i u m c o n s t i t u t e the b u l k o f w a t e r - s o l u b l e m a t t e r i n B r i t i s h b i t u m i n o u s c o a l s ( 1 2 ) . S k i p s e y (13) has found t h a t the d i s t r i b u t i o n o f c h l o r i n e c o a l s was c l o s e l y r e l a t e d t o the s a l i n i t y o f mine w a t e r s . Hypersaline b r i n e s with concentrations of d i s s o l v e d s o l i d s up to 200 kg m"** o c c u r i n s e v e r a l o f the B r i t i s h C o a l f i e l d s . The mode o f f o r m a t i o n o f h y p e r s a l i n e b r i n e s has been d i s c u s s e d by the o s m o t i c f i l t r a t i o n through c l a y and s h a l e d e p o s i t s . The s a l i n i t y o f the b r i n e ground waters i n c r e a s e s w i t h depth and when they are i n c o n t a c t w i t h f u e l b e a r i n g s t r a t a , c o r r e s p o n d i n g l y more c h l o r i d e i s taken up by the f u e l . However, a c c o r d i n g t o S k i p s e y (13) the h i g h rank b i t u m i n o u s c o a l s because o f t h e i r low p o r o s i t y are unable t o take up l a r g e amounts o f the c h l o r i d e and a s s o c i a t e d c a t i o n s , and the c h l o r i n e c o n t e n t r a r e l y exceeds 0.2 p e r c e n t . The


11.

RAASK

Characteristics

of Silicate

141

Ash

c h l o r i n e c o n t e n t of low rank b i t u m i n o u s c o a l s can r e a c h one p e r cent and c o r r e s p o n d i n g l y the sodium f r a c t i o n a s s o c i a t e d w i t h c h l o r i n e w i l l amount up t o 0.4 p e r cent o f c o a l . That i s , the ash from a h i g h c h l o r i n e c o a l can c o n t a i n up t o 3 per cent o f flame v o l a t i l e sodium. The c h l o r i n e c o n t e n t o f l i g n i t e s and sub-bituminous c o a l s i s u s u a l l y low, below 0.1 p e r c e n t , and sodium i s h e l d c h i e f l y i n the f u e l s u b s t a n c e i n the form of organo-metal components (9,10). A l l c o a l s c o n t a i n some sodium combined i n the a l u m i n o - s i l i c a t e s p e c i e s which w i l l remain l a r g e l y i n v o l a t i l e i n the f l a m e . The r a t i o o f the s i l i c a t e sodium to n o n - s i l i c a t e sodium v a r i e s o v e r a wide r a n g e . The a l k a l i - m e t a l i s p r e s e n t c h i e f l y i n the s i l i c a t e s i n low c h l o r i n e bituminous c o a l s . In the h i g h c h l o r i n e b i t u m i n o u s c o a l s and i n many l i g n i t e s and sub-bituminous c o a l s i t i s p r e s e n t m a i n l y i n a flame v o l a t i l e form. Flame V i t r i f i c a t i o n o f S i l i c a

Minerals

A c h a r a c t e r i s t i c f e a t u r e of flame h e a t e d ash i s t h a t the p a r t i c l e s are s p h e r i c a l i n shape as shown i n F i g u r e 2. The t r a n s f o r m a t i o n o f the a n g u l a r s i l i c a t e m i n e r a l p a r t i c l e s i n p u l v e r i z e d c o a l to s p h e r i c a l p a r t i c l e ash i s a r e s u l t of the s u r f a c e t e n s i o n f o r c e a c t i n g on the v i t r i f i e d s p e c i e s . The s t r e s s ( f ) on a n o n - s p h e r i c a l s u r f a c e s e c t i o n of the p a r t i c l e i s : f

=

2γ/ρ

(4)

where γ i s the s u r f a c e t e n s i o n of g l a s s y s i l i c a t e and ρ i s the radius of curvature. I t i s e v i d e n t from E q u a t i o n (4) t h a t the s t r e s s 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 r a d i u s o f c u r v a t u r e and thus the s m a l l sharp-edged p a r t i c l e s are f i r s t to take a s p h e r i c a l form. F r e n k e l (15) has shown t h a t time ( t ) r e q u i r e d t o t r a n s f o r m an a n g u l a r p a r t i c l e to sphere i s g i v e n to f i r s t a p p r o x i m a t i o n by: -t/Z

r

=

r

/CN (5)

ζ

=

4irnr /γ ο

(6)

ο

where

and r i s the d i s t a n c e of a p o i n t on the o r i g i n a l s u r f a c e from the c e n t e r o f a sphere of e q u i v a l e n t volume h a v i n g r a d i u s r , η i s the v i s c o s i t y and γ i s the s u r f a c e t e n s i o n . E q u a t i o n (5) can be used t o c a l c u l a t e the approximate time r e q u i r e d f o r a p a r t i c l e to assume a s p h e r i c a l shape when the s u r f a c e t e n s i o n , v i s c o s i t y , s i z e and i n i t i a l shape o f p a r t i c l e are known. A l t e r n a t i v e l y , an e s t i m a t e of the v i s c o s i t y f o r the change t o take p l a c e , can be made when the r e s i d e n c e time o f p a r t i c l e s at a g i v e n temperature i s known. T a b l e I g i v e s the c a l c u l a t e d v a l u e s o f v i s c o s i t y when the time f o r the change i s one second. I t was assumed t h a t the t h i c k n e s s o f moving s u r f a c e l a y e r was about ten p e r cent o f the r a d i u s , and the s u r f a c e t e n s i o n o f f u s e d ash was t a k e n Q

-ι

to be

0.32

N m

'

as measured p r e v i o u s l y

(16).

142

MINERAL

Table I I .

MATTER

A N D A S H IN C O A L

Calculated V i s c o s i t i e s f o r Spheridization of D i f f e r e n t Size S i l i c a t e P a r t i c l e s

r a d i u s (μπι) Viscosity (N s m )

χ

7

2

5

x

1 Q

6

2

>

5

x

1 Q

5

2

.5 x 10

4

2.3 χ 1 0

3


Δ

T a b l e I I shows t h a t the s m a l l i r r e g u l a r l y shaped p a r t i c l e s t r a n s f o r m t o spheres i n c o a l flame when the v i s c o s i t y o f the m a t e r i a l i s s e v e r a l o r d e r s h i g h e r than t h a t r e q u i r e d f o r b u l k f l o w under g r a v i t y , which i s about 25 N s m~^. A l a b o r a t o r y t e c h n i q u e was used to d e t e r m i n e t h e minimum temperature a t w h i c h c o a l m i n e r a l s p e c i e s are t r a n s f o r m e d to s p h e r i c a l shapes ( 1 7 ) . P a r t i c l e s o f 10 t o 200 ym i n d i a m e t e r were i n t r o d u c e d i n t o a gas s t r e a m and then p a s s e d t h r o u g h a v e r t i c a l furnace. The temperature o f the f u r n a c e was v a r i e d from 1175 t o 2025 Κ and was measured by a r a d i a t i o n pyrometer and by thermocouples p l a c e d i n the f u r n a c e . The r e s i d e n c e time o f p a r t i c l e s i n the f u r n a c e was between 0.2 and 0.5 s e c . depending on the p a r t i c l e size. F i g u r e 3a shows a s u r f a c e - f u s e d s i l i c a t e p a r t i c l e h e a t e d t o a temperature some 25 Κ lower than t h a t r e q u i r e d f o r i t s s p h e r i d i z a t i o n . F i g u r e 3b shows a s p h e r i d i z e d p a r t i c l e h e a t e d i n the l a b o r a t o r y furnace. F i g u r e 4 shows the temperature range a t w h i c h the shape change o f d i f f e r e n t c o a l m i n e r a l p a r t i c l e s o c c u r r e d . The c h l o r i t e m i n e r a l c o n t a i n s some q u a r t z and the two s p e c i e s s p h e r i d i z e d a t markedly d i f f e r e n t temperatures as shown by c u r v e s D^ and D2. The t e m p e r a t u r e o f m i n e r a l p a r t i c l e s i n the p u l v e r i z e d c o a l flame exceeds 1800 Κ ( F i g u r e 5 ) , and i t i s t h e r e f o r e t o be e x p e c t e d t h a t a l l p a r t i c l e s w i t h the e x c e p t i o n o f l a r g e s i z e q u a r t z w i l l v i t r i f y and change t o s p h e r i c a l shapes. F i g u r e 6a shows a s u r f a c e - f u s e d b u t n o n - s p h e r i c a l q u a r t z p a r t i c l e found i n a sample o f f l y ash c a p t u r e d i n the e l e c t r o s t a t i c p r e c i p i t a t o r . O c c a s i o n a l l y e l o n g a t e d e l l i p o s o i d a l p a r t i c l e s o f a l u m i n o - s i l i c a t e s ( F i g u r e 6b) can be found i n the ash i n d i c a t i n g t h a t t h e h i g h temperature r e s i d e n c e time was too s h o r t f o r complete s p h e r i d i z a t i o n . However, the m a j o r i t y o f the ash p a r t i c l e s appear t o be s p h e r i c a l as shown i n F i g u r e 2. The s p h e r i c a l s i l i c a t e ash p a r t i c l e s , when viewed a t c l o s e - u p range appear t o h o s t a l a r g e number o f sub-micron p a r t i c l e s a t the surface (Figure 6c). The m i c r o i d s c o u l d be s i l i c a t e c r y s t a l l o i d s p r e c i p i t a t e d from the v i t r i f i e d phase o r s u l p h a t e fume p a r t i c l e s formed from the n o n - s i l i c a t e c o a l m i n e r a l s ( 1 8 ) . The l a t t e r a r e s o l u b l e i n a d i l u t e a c i d (HC1) s o l u t i o n and F i g u r e 6d shows the a c i d etched p a r t i c l e s . C l e a r l y , most o f the m i c r o i d p a r t i c l e s were d i s s o l v e d and the l e a c h s o l u t i o n c o n t a i n e d sodium and p o t a s s i u m sulphates. A n o t h e r d i a g n o s t i c t e s t f o r s i l i c a t e a s h i s to t r e a t the p a r t i c l e s w i t h h y d r o f l u o r i c (HF) a c i d s o l u t i o n (18-20). The a c i d w i l l d i s s o l v e the g l a s s y phase r e v e a l i n g s k e l e t o n s o f c r y s t a l l i n e s p e c i e s w h i c h may be i n the form o f m u l l i t e n e e d l e s ( F i g u r e 6e) o r q u a r t z c r y s t a l l o i d s ( F i g u r e 6 f ) . The r a t i o o f the g l a s s y phase t o c r y s t a l l i n e s p e c i e s v a r i e s from p a r t i c l e t o p a r t i c l e depending on


RAASK

Characteristics

of Silicate

Ash

F i g u r e 1. Mineral matter i n c o a l . inherent white p a r t i c l e s .

Figure 2.

Figure

3.

particles.

Surface

fused

(a) A d v e n t i t i o u s ;

Pulverized fuel

(a) and

(b)

ash.

spheoidized

(b)

silicate

144

M I N E R A L MATTER A N D A S H IN C O A L


2100

25

50

75

100

PARTICLE RADIUS, μιπ F i g u r e 4. S p h e r i c a l shape t r a n s f o r m a t i o n of g r a n u l a r m i n e r a l s i n h o t gas streams: A, i l l i t e ; B, m u s c o v i t e ; C, n a t i v e q u a r t z ; D, c h l o r i t e . 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 2. C o p y r i g h t 1985 I t e m i s p h e r e P u b l i s h i n g Corp.

11.

RAASK

Characteristics

145

of Silicate Ash


2000

UJ ce

ce UJ CL Σ UJ

ιοοο I -

10

20

TIME, s F i g u r e 5. Temperature/time p l o t f o r ash p a r t i c l e s i n a 500 MW pulverized coal f i r e d b o i l e r : 0.1 ym ( t o p curve t o 100 \\m (lower curve) s i z e s . A, combustion and heat exchange chambers; B, e l e c t r o s t a t i c p r e c i p i t a t o r s and chimney. 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 2. C o p y r i g h t 1985 Hemisphere P u b l i s h i n g Corp.



146

Figure 6. D i a g n o s t i c f e a t u r e s of flame heated ash. a, unfused q u a r t z p a r t i c l e ; b, e l o n g a t e d s i l i c a t e p a r t i c l e s ; c, m i c r o i d s on ash; d, a c i d c l e a n e d ash; e, m u l l i t e n e e d l e s i n ash; and f , quartz c r y s t a l l o i d s .

11.

RAASK

Characteristics

147

of Silicate Ash

the o r i g i n a l c o m p o s i t i o n o f the s i l i c a t e m i n e r a l s , the c a p t u r e o f v o l a t i l e sodium and the r a t e o f c o o l i n g o f f l u e gas borne a s h . The flame i m p r i n t e d c h a r a c t e r i s t i c s o f s i l i c a t e m i n e r a l s p e c i e s from the p o i n t o f view o f subsequent s i n t e r i n g a r e summarized i n T a b l e I I I . Table I I I .

Vitrification

Particle


Constituent species

Quartz Kaolinite Potassium alumino silicates

and R e c r y s t a l l i z a t i o n o f S i l i c a t e s

vitrification

Temperature range (K)

Extent

1700 t o 1900 1600 t o 1700 1400 t o 1600

Medium High High

Recrystallization tendency

Low High Low

Glass content

Medium Medium High

The r e l a t i v e amount o f c o a l m i n e r a l q u a r t z s u r v i v i n g i n the p u l v e r i z e d f u e l flame depends on the p a r t i c l e s i z e and temperature. In the i n t e n s e combustion o f c y c l o n e f i r e d b o i l e r s the flame temperature exceeds 2000 Κ and the q u a r t z p a r t i c l e s o f a l l s i z e s w i l l vitrify. Some q u a r t z p a r t i c l e s i n the c r y s t a l l i n e form w i l l s u r v i v e the flame t r e a t m e n t 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 and the ash may c o n t a i n 25 p e r c e n t o f the o r i g i n a l c o a l q u a r t z i n the c r y s t a l l i n e form ( 2 1 ) . The k a o l i n i t e m i n e r a l s p e c i e s i n c o a l c o n t a i n some sodium, c a l c i u m and i r o n i n the c r y s t a l l i n e s t r u c t u r e (6) and the p r e s e n c e of f l u x i n g m e t a l s enhances v i t r i f i c a t i o n o f the flame h e a t e d particles. The h i g h temperature c r y s t a l l i n e form o f k a o l i n i t e s p e c i e s i s m u l l i t e and the c h a r a c t e r i s t i c n e e d l e shapes o f m u l l i t e ( F i g u r e 6e) a r e f r e q u e n t l y found i n l a r g e , above 5 ym d i a m e t e r particles. The m u l l i t e n e e d l e c r y s t a l s i n ash a r e always embedded i n a g l a s s y phase o f the l a r g e p a r t i c l e s and i t appears t h a t the s m a l l , below 5 ym d i a m e t e r p a r t i c l e s o f the flame h e a t e d k a o l i n i t e s p e c i e s a r e n o t e x t e n s i v e l y r e c r y s t a l l i z e d on c o o l i n g . The c r y s t a l l i n e s p e c i e s o f i l l i t e and m u s c o v i t e a r e n o t found i n the flame h e a t e d ash and thus i t i s l i k e l y t h a t the p o t a s s i u m a l u m i n o s i l i c a t e s remain on c o o l i n g l a r g e l y i n the form o f g l a s s y p a r t i c l e s . The i n h e r e n t s i l i c a t e ash ( F i g u r e l b ) w i l l c o a l e s c e on combustion f i r s t to a s i n t e r e d m a t r i x i n s i d e the b u r n i n g c o a l p a r t i c l e and a l s o to s m a l l s l a g g l o b u l e s a t the s u r f a c e o f coke residue. F i g u r e 7a shows the s l a g g l o b u l e s on a coke p a r t i c l e s e p a r a t e d from p u l v e r i z e d c o a l ash and F i g u r e 7b shows a l a c e s k e l e t o n o f s i n t e r e d ash i n a n o t h e r coke p a r t i c l e r e v e a l e d a f t e r combustion a t 900 K. D u r i n g combustion o f the m i n e r a l r i c h c o a l p a r t i c l e s i n t h e p u l v e r i z e d f u e l flame, ash e n v e l o p e s may be c r e a t e d which can take the form o f censopheres as shown i n F i g u r e 7c and d. The gas b u b b l e e v o l u t i o n l e a d i n g t o cenosphere f o r m a t i o n (16,22) and f l y ash u s u a l l y c o n t a i n s between 0.1 and 2 p e r c e n t by w e i g h t o f the l i g h t w e i g h t a s h . The m i n e r a l r i c h c o a l p a r t i c l e s may l e a v e t h e combustion a s h r e s i d u e a l s o i n the form o f p l e r o s p h e r e ( s p h e r e s -



148

F i g u r e 7. C o a l e s c e n c e p r o d u c t s of i n h e r e n t ash i n flame. a, ash p a r t i c l e s on coke; b, ash s k e l e t o n i n coke; c, cenospheres; d, f r a c t u r e s cenosphere; and e, p l e r o s p h e r e .

11.

RAASK

Characteristics

of Silicate

149

Ash

i n s i d e - s p h e r e ) as shown i n F i g u r e 7e. The above examples show t h a t the i n h e r e n t s i l i c a ash p a r t i c l e s undergo e x t e n s i v e c o a l e s c e n c e by s i n t e r i n g and s l a g g i n g d u r i n g combustion o f the h o s t c o a l p a r t i c l e s However, the a d v e n t i t i o u s ash r e t a i n the p a r t i c l e i d e n t i t y i n the flame and the p r o c e s s e s o f s i n t e r i n g and s l a g g i n g take p l a c e a f t e r d e p o s i t i o n on b o i l e r tubes.


T r a n s f e r o f Flame V o l a t i l e Sodium to

Silicates

The c o a l sodium o r i g i n a l l y p r e s e n t as c h l o r i d e and o r g a n o - m e t a l l i c compounds i s r a p i d l y v o l a t i l i z e d i n the p u l v e r i z e d c o a l flame ( 2 3 ) . S u b s e q u e n t l y the v o l a t i l e s p e c i e s are p a r t l y d i s s o l v e d i n the s u r f a c e l a y e r o f flame h e a t e d s i l i c a t e p a r t i c l e s and p a r t l y s u l p h a t e d i n the f l u e gas ( 8 ) . The f o r m a t i o n o f sodium s u l p h a t e can p r o c e e d v i a two routes : Route 1 - In the F l u e

Gas

+ S0 + | 0J+2HC1 + N a S 0 — : vapour phase r e a c t i o n s

|2Na

+ H0 2

2X + /„ 2NaX O X T

v

2

2

v

Route 2 - At the S u r f a c e Ash

2X + 2NaX

V

a

P

o

2

u

r

Na S0 fume . •% particles 2

4

Particles

-, • + H qJ->2HCl + N a 0 ^ N a 0 + S 0

|2Na

4

2

phase r e a c t i o n s

2

Reactions

2

1°2 + | Na S0 2

a t ash

4

surface

Route 1 f o r g e n e s i s o f sodium s u l p h a t e fume can be d e s c r i b e d as the n o n - c a p t i v e f o r m a t i o n and r o u t e 2 as the c a p t i v e f o r m a t i o n . Some p o t a s s i u m s u l p h a t e can a l s o be formed v i a the two r o u t e s . P o t a s s i u m i s p r e s e n t i n c o a l c h i e f l y i n the form o f p o t a s s i u m a l u m i n o - s i l i c a t e s ( T a b l e I) and a l a r g e p a r t of the a l k a l i - m e t a l w i l l remain i n v o l a t i l e i n the flame h e a t e d s i l i c a t e p a r t i c l e s . Some 5 t o 20 p e r c e n t o f the p o t a s s i u m i s r e l e a s e d f o r s u l p h a t i o n (24) which takes p l a c e p a r t l y at the s u r f a c e o f the p a r e n t p a r t i c l e s (25) and p a r t l y v i a the v o l a t i l i z a t i o n r o u t e s as d e s c r i b e d above. However, sodium s u l p h a t e c o n t e n t o f f l y ash h e a t e d i n p u l v e r i z e d c o a l flame, and chimney s o l i d s i s always h i g h e r than t h a t of p o t a s s i u m s u l p h a t e . The d i s t r i b u t i o n o f the flame v o l a t i l e sodium between the ash s i l i c a t e and s u l p h a t e phases i s markedly i n f l u e n c e d by the temperature and r e s i d e n c e time o f the ash p a r t i c l e s i n the f l a m e . The h i g h temperature o f l a r g e b o i l e r flame reduces the v i s c o s i t y o f v i t r i f i e d s i l i c a t e p a r t i c l e s and as a r e s u l t a l a r g e f r a c t i o n o f the v o l a t i l e sodium i s d i s s o l v e d i n the s i l i c a t e phase. On average 60 p e r c e n t o f the sodium i s d i s s o l v e d i n the s i l i c a t e ash p a r t i c l e s ^ the remainder b e i n g p r e s e n t as s u l p h a t e fume p a r t i c l e s i n the f l u e gas ( 8 ) . The

Mechanism and Measurements o f Sodium Enhanced

Sintering

The f o r m a t i o n o f s i n t e r e d ash d e p o s i t s on b o i l e r tubes r e q u i r e s f i r s t a c l o s e , m o l e c u l a r d i s t a n c e c o n t a c t between the p a r t i c l e s f o l l o w e d by a growth o f p a r t i c l e - t o - p a r t i c l e b r i d g e s c h i e f l y by v i s c o u s f l o w . Sodium s u l p h a t e phase t o g e t h e r w i t h some p o t a s s i u m s u l p h a t e may p l a y

MINERAL

150

MATTER

A N D A S H IN C O A L

a s i g n i f i c a n t r o l e i n the i n i t i a l s t a g e o f s i n t e r i n g by b r i n g i n g t h e s i l i c a t e p a r t i c l e s t o g e t h e r as a r e s u l t o f s u r f a c e t e n s i o n . Sodium s u l p h a t e m e l t s a t 1157 Κ b u t mixed a l k a l i - m e t a l s u l p h a t e s can form a m o l t e n phase a t lower temperatures ( 2 6 ) . Once the c l o s e c o n t a c t between the s i l i c a t e p a r t i c l e s has been e s t a b l i s h e d a v i s c o u s f l o w o f the p a r t i c l e s u r f a c e l a y e r can commence and the s i n t e r bonds a r e e s t a b l i s h e d a c c o r d i n g t o E q u a t i o n 7, as d i s c u s s e d by F r e n k e l ( 1 5 ) :

^


r

m

2

h

i

(7) U

2nr

)

where χ i s the r a d i u s o f neck growth between the s p h e r i c a l p a r t i c l e s o f r a d i u s r , γ i s the s u r f a c e t e n s i o n , η i s the v i s c o s i t y o f f u s e d ash, and t i s the t i m e . The ( x / r ) ^ r a t i o can be t a k e n as a c r i t e r i o n o f the degree o f s i n t e r i n g , i . e . t h e s t r e n g t h o f b o i l e r d e p o s i t ( s ) d e v e l o p e d i n time t , t h a t i s :

s

and

the r a t e o f d e p o s i t s t r e n g t h development i s : ds_ dt

=

3ky 2nr

/q\ K

J

where k i s a c o n s t a n t . E q u a t i o n (9) shows t h a t the r a t e o f a s h s i n t e r i n g , i . e . the development o f c o h e s i v e s t r e n g t h o f a d e p o s i t m a t r i x i s p r o p o r t i o n a l to the s u r f a c e t e n s i o n and i n v e r s e l y p r o p o r t i o n a l t o the v i s c o s i t y . The s u r f a c e t e n s i o n and p a r t i c l e s i z e a r e n o t markedly changed by d i s s o l u t i o n o f sodium, i r o n o r c a l c i u m o x i d e s i n the g l a s s y phase o f s i l i c a t e a s h . However, t h e v i s c o s i t y i s markedly changed by the oxides. I n p a r t i c u l a r , an e n r i c h m e n t o f sodium i n the s u r f a c e l a y e r o f t h e s i l i c a t e ash p a r t i c l e s can l e a d t o a h i g h r a t e o f s i n t e r i n g . Some o f the flame v o l a t i l e sodium i s d i s s o l v e d i n the v i t r i f i e d s i l i c a t e a s h p a r t i c l e s b e f o r e d e p o s i t i o n and an a d d i t i o n a l amount o f sodium i s t r a n s f e r r e d from the s u l p h a t e t o s i l i c a t e phases d u r i n g sintering. The r e a c t i o n between sodium s u l p h a t e and s i l i c a t e s a t a s h s i n t e r i n g temperatures has been m o n i t o r e d by t h e r m o - g r a v i m e t r i c measurements. Some o f t h e r e s u l t s a r e g i v e n i n T a b l e IV. Table

IV.

Weight Loss o f S u l p h a t e

Loss i n i t i a t i o n temperature (K)

Na S0, 2 4 o

1425

v

CaSO^

Na S0, .. Kaolin

Na S0, 2 4 Ash

CaSO^

1085

1175

>1525

2

Sample

and S u l p h a t e / S i l i c a t e M i x t u r e s

o

Kaolin 1275

CaSO. 4 Ash 1275

Anhydrous s u l p h a t e samples and the s u l p h a t e / s i l i c a t e m i x t u r e s (50 p e r c e n t by w e i g h t s u l p h a t e ) were h e a t e d i n a i r a t the r a t e o f 6 Κ per m i n u t e .

11.

RAASK

The sulphate l o s s due

Characteristics


Ash

151

r e s u l t s i n T a b l e IV show the r e a c t i o n between sodium and k a o l i n commenced a t 1085 Κ as i n d i c a t e d by the weight t o r e l e a s e o f SO2 and SO3:

Al 0 .2Si0 2

of Silicate

3

2

+ xNa S0 2

4

-> x N a O . A l 0 . 2 S i 0 2

2

3

2

+ x ( S 0 , S0 )+ 2

3

(10)

A t y p i c a l b i t u m i n o u s c o a l ash r e q u i r e d a h i g h e r temperature o f 1175 Κ f o r the s u l p h a t e d e c o m p o s i t i o n r e a c t i o n , i n d i c a t i n g t h a t the ash s i l i c a t e s p e c i e s were l e s s r e a c t i v e than k a o l i n m i n e r a l o f s m a l l (

Flame Vitrification and Sintering Characteristics of Silicate Ash

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