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
Mineral Matter and Ash in Coal Downloaded from pubs.acs.org by UNIV LAVAL on 05/15/16. For personal use only.
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
Mineral Matter and Ash in Coal Downloaded from pubs.acs.org by UNIV LAVAL on 05/15/16. For personal use only.
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
Mineral Matter and Ash in Coal Downloaded from pubs.acs.org by UNIV LAVAL on 05/15/16. For personal use only.
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
Mineral Matter and Ash in Coal Downloaded from pubs.acs.org by UNIV LAVAL on 05/15/16. For personal use only.
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
Mineral Matter and Ash in Coal Downloaded from pubs.acs.org by UNIV LAVAL on 05/15/16. For personal use only.
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
Mineral Matter and Ash in Coal Downloaded from pubs.acs.org by UNIV LAVAL on 05/15/16. For personal use only.
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
Mineral Matter and Ash in Coal Downloaded from pubs.acs.org by UNIV LAVAL on 05/15/16. For personal use only.
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.
M I N E R A L M A T T E R A N D A S H IN C O A L
Mineral Matter and Ash in Coal Downloaded from pubs.acs.org by UNIV LAVAL on 05/15/16. For personal use only.
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
Mineral Matter and Ash in Coal Downloaded from pubs.acs.org by UNIV LAVAL on 05/15/16. For personal use only.
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 -
M I N E R A L M A T T E R A N D A S H IN C O A L
Mineral Matter and Ash in Coal Downloaded from pubs.acs.org by UNIV LAVAL on 05/15/16. For personal use only.
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.
Mineral Matter and Ash in Coal Downloaded from pubs.acs.org by UNIV LAVAL on 05/15/16. For personal use only.
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 ) :
^
Mineral Matter and Ash in Coal Downloaded from pubs.acs.org by UNIV LAVAL on 05/15/16. For personal use only.
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
Mineral Matter and Ash in Coal Downloaded from pubs.acs.org by UNIV LAVAL on 05/15/16. For personal use only.
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 (