The goal of atmospheric fluidized bed combustion (AFBC) research at the .... points, and distributor plate performance which affect bed material agglom- eration.
BED AGGLOMERATES FORMED B Y ATMOSPHERIC FLUIDIZED BED COMBUSTION OF A NORTH D A K O T A L I G N I T E S t e v e n A . Benson, F r a n k R . K a r n e r , a n d Gerald M. Goblirsch Grand Forks Energy Technology Center U.S. D e p a r t m e n t o f E n e r g y G r a n d F o r k s , N o r t h Dakota 58202 David W. Brekke D e p a r t m e n t o f Geology U n i v e r s i t y o f N o r t h Dakota G r a n d F o r k s , N o r t h Dakota 58202 INTRODUCTION T h e goal o f atmospheric f l u i d i z e d b e d combustion ( A F B C ) research at the G r a n d Forks E n e r g y T e c h n o l o g y C e n t e r i s t o p r o v i d e a data base f o r design, operat i o n , process c o n t r o l , a n d emission c o n t r o l r e q u i r e m e n t s for l o w - r a n k coals. T h e application o f t h e AFBC process has t h e potential t o solve some o f t h e problems associated w i t h conventional combustion. These problems a r e .ash f o u l i n g o n h e a t e x c h a n g e surfaces, t h e expense a n d r e l i a b i l i t y of SO2 c o n t r o l devices such as s c r u b b e r s , a n d t h e system s e n s i t i v i t y t o f u e l variables (moisture, N a 2 0 concentration, etc.). These problems can b e r e d u c e d by t h e AFBC process f o r l o w - r a n k coals b e cause t h e alkaline c h a r a c t e r i s t i c s o f t h e ash o r s o r b e n t added d i r e c t l y t o t h e comb u s t i o n zone p r o v i d e s t h e s u l f u r r e t e n t i o n , w h i c h would eliminate o r r e d u c e t h e T h e temperatures in t h e combustion zone need f o r post combustion SO2 c o n t r o l s . a r e a t t h e r i g h t levels t o p r o v i d e maximum reaction o f SO2 a n d alkali t o f o r m solid a l k a l i sulfate waste. One problem w h i c h t h e f l u i d i z e d b e d combustion of l o w - r a n k coals seems t o e x h i b i t i s a t e n d e n c y t o w a r d t h e formation o f agglomerates o f t h e material which a r e used t o make u p t h e bed. Agglomerates i n t h i s case a r e d e f i n e d as a c l u s t e r o f i n d i v i d u a l b e d material p a r t i c l e s held t o g e t h e r b y a substance n o t y e t well understood, a n d manifest in m a n y d i f f e r i n g f o r m s . T h e u n d e r s t a n d i n g o f t h e mechanism o f formation o f these agglomerates i s v i t a l t o t h e i r control, a n d t h e r e f o r e t h e f u l l u t i l i z a t i o n o f l o w - r a n k coal in A F B C . Once formed these agglomerates w i l l t e n d t o decrease heat t r a n s f e r , a n d f l u i d i zation q u a l i t y r e s u l t i n g in p o o r combustion e f f i c i e n c y a n d loss o f c o n t r o l o f b e d o p e r ational parameters ( i . e . , excess a i r , temperature, e t c . ) . I n severe cases t h e format i o n of agglomerates can lead t o a f o r c e d p r e m a t u r e s h u t d o w n o f t h e system. While t h e a d d i t i o n o f limestone, o r calcium b e a r i n g materials i n t o t h e f l u i d bed, o r t h e f o r m i n g o f t h e b e d i t s e l f b y limestone p a r t i c l e s has shown a t e n d e n c y t o inhibit t h e formation o f agglomerates, agglomerates o f a severe n a t u r e h a v e been o b s e r v e d in a bed of limestone alone, o r limestone a n d sand p a r t i c l e m i x t u r e s while burning a h i g h sodium coal f o r a n e x t e n d e d p e r i o d o f time.
In general w i t h a h i g h sodium coal t h e agglomeration o f limestone b e d material i s d e p e n d e n t o n l y on t h e l e n g t h o f run time, i f t h e run i s l o n g enough agglomerat i o n in a limestone b e d w i l l o c c u r , a n d c a n b e as d e v a s t a t i n g as those which o c c u r w i t h a silica bed.
174
GFETC c o n s t r u c t e d a 0 . 2 square meter experimental A F B C . A detailed descripT h e combustor can b e opert i o n of t h e unit i s g i v e n by Goblirsch a n d o t h e r s (1). ated o v e r a wide r a n g e o f conditions as l i s t e d below:
--
A v e r a g e b e d temperature Superficial gas v e l o c i t y Excess a i r A s h reinjection (% Of p r i m a r y cyclone c a t c h )
700 t o 982OC 0.9-2.7 m/sec 10 t o 50%
---
-- 0
t o 100%.
T h e nominal coal feed r a t e i s 80 kg/hr a t 1 . 8 m/sec s u p e r f i c i a l gas v e l o c i t y and 20% excess a i r . T h i s paper discusses t h e performance o f q u a r t z o r limestone as a b e d material during t h e combusting o f high sodium N o r t h Dakota l i g n i t e . T h e l i g n i t e i s f r o m the Beulah mine o f Mercer C o u n t y , N o r t h Dakota. T h e composite coal a n d coal ash analy s i s i s summarized in Table 1. T h e l i g n i t e was p a r t i a l l y d r i e d b e f o r e t h i s series of tests; i t s as-mined moisture c o n t e n t was 3630, a n d i t s h e a t i n g v a l v e 15,000 J/g. T A B L E 1.
Ultimate Analysis Carbon Hydrogen Nitrogen Sulfur Ash Moisture Heating Valve (8372 B t u / l b )
T Y P I C A L COAL AND COAL ASH A N A L Y S I S OF HIGH Na BEULAH L I G N I T E
,
As Fired
52.65 4.59 0.75 1.33 9.7 20.0 19,459 J / g
Coal A s h A n a l y s i s , Si02
3
A1203 Fe203
Ti02 p205 CaOMgO Na20 K20
so3
%
o f Ash
15.8 12.1 9.9 0.8 1 .o 17.5 6.2 8.8
0.0 27.2
O t h e r i m p o r t a n t considerations are t h e operation o f t h e combustor a n d how operational parameters a f f e c t t h e performance of t h e b e d material, s u l f u r r e t e n t i o n on coal ash a n d b e d material, a n d heat t r a n s f e r . T h e most i m p o r t a n t operational parameters of t h e AFBC f o r t h e tests t o b e discussed h e r e a r e l i s t e d in Table 2. T A B L E 2.
AFBC OPERATIONAL PARAMETERS
Run Number Coal T y p e B e d Material A v e r a g e B e d T e m p e r a t u r e (OF) S u p e r f i c i a l Gas Velocity (M/sec) Excess A i r (%) Additive A s h Reinjection (%) Coal Feed Rate ( k g / h r )
21 81 Beulah Quartz 1467 1.8 25.49 None None 53
2281 Beulah Limestone 1460 2.0 22.65
*
100 57
2481 Beulah Limestone 1450 1.8 24.38
*
100 48
* A d d i t i o n o f supplemental b e d material t o maintain b e d d e p t h . T h e t e n d e n c y f o r t h e b e d t o agglomerate has been shown t h r o u g h extensive t e s t i n g t o depend on t h e following parameters: 1. 2.
Bed temperature ( h i g h e r t e m p e r a t u r e increases t e n d e n c y ) Coal sodium c o n t e n t (increased coal sodium c o n t e n t shows increased s e v e r i t y o f agglomeration)
175
3. 4. 5.
Bed material composition (high calcium c o n t e n t t e n d s t o delay, a n d decrease t h e s e v e r i t y o f agglomerates formed) A s h r e c y c l e (increased r e c y c l e o f ash t e n d s t o increase agglomeration tendency) T h e r e appears t o b e a b e d d e s i g n parameter such as position o f coal feed points, a n d d i s t r i b u t o r p l a t e performance w h i c h a f f e c t b e d material agglomeration.
T h e bed material u s e d in baseline run 2181 was 10 mesh q u a r t z sand. Upon s t a r t u p t h e b e d was sampled e v e r y e i g h t h o u r s f o r t h e d u r a t i o n o f t h e 69 h o u r run. A t t h e end o f t h e run t h e system was cooled a n d opened t o expose t h e i n s i d e o f the combustor. T h e b e d material was removed a n d several agglomerates were found, w h i c h v a r i e d in size a n d shape, w i t h t h e l a r g e s t h a v i n g a diameter o f 6 cm. These agglomerates were found b o t h f r e e f l o a t i n g in t h e b e d a n d attached t o t h e inside wall o f t h e combustor. T h e limestone bed material was t e s t e d in run 2281 u s i n g 100% ash reinjection f o r a d u r a t i o n o f 73 h o u r s . T h e b e d was sampled in t h e same manner as 2181. T h e formation o f agglomerates in t h e run was v e r y minimal w i t h n o major agglomerates f o u n d . O n t h e o t h e r h a n d , run 2481 w h i c h u s e d a limestone b e d r a n f o r 160 After h o u r s w i t h 100% ash r e i n j e c t i o n a n d h a d severe problems w i t h agglomeration. t h e run numerous agglomerates w e r e f o u n d loose in t h e bed. In addition, a large agglomerate was f o u n d o n t o p o f t h e d i s t r i b u t o r plate a t t h e bottom o f t h e combustor. T h e agglomerate h a d dimensions o f 30.5 cm X 30.5 cm X 12.5 cm; it weighed 10 kg a n d covered 30% o f t h e d i s t r i b u t o r plate. T h e bed material a n d agglomerates w e r e c h a r a c t e r i z e d b y polished thin section s t u d y , polarized l i g h t microscopy, a n d scanning electron microscopy/microprobe (SEM) - b o t h secondary e l e c t r o n (SEI) a n d b a c k s c a t t e r electron ( B E l ) images were used. B u l k samples were analyzed by x - r a y d i f f r a c t i o n a n d x - r a y fluorescence. T h e goals in c h a r a c t e r i z i n g t h e b e d material a n d agglomerates a r e t o i d e n t i f y the stages which lead t o agglomeration a n d p o s s i b l y p o s t u l a t e a mechanism o f t h e i r format i o n t o t h e r e b y determine methods a n d p r o c e d u r e s t o c o n t r o l t h e i r g r o w t h . RESULTS A N D DISCUSSION Q u a r t z B e d Agglomerates Agglomeration o f q u a r t z b e d material i s t y p i f i e d by run 2181 u t i l i z i n g h i g h - N a Beulah l i g n i t e a n d ash injection. Samples o f b e d material t a k e n a t v a r i o u s i n t e r v a l s during t h e run a r e i l l u s t r a t e d in F i g u r e s 1 t o 11 w i t h chemical analyses data given in T a b l e 3. T h e following f o u r stages can b e u s e d t o summarize t h e agglomeration process : Stage 1 .
I n i t i a l ash c o a t i n g .
I n i t i a l samples o f b e d material h a v e a f i n e coating, a b o u t 50 microns t h i c k , c o n s i s t i n g o f s u l f a t e d aluminosilicate p a r t i c l e s ( F i g u r e 1 ) . T h e coatings contain some coarser ash materials in t h e o u t e r p a r t s a n d t h e i n n e r p a r t s have p e n e t r a t e d t h e q u a r t z g r a i n s s l i g h t l y along g e n t l y c u r v e d o r c u s p a t e embayments. The quartz g r a i n s a r e e x t e n s i v e l y f r a c t u r e d , a p p a r e n t l y as a r e s u l t o f thermal stresses. Stage 2.
Thickened nodular coatings.
Longer b e d usage r e s u l t s in t h e development o f t h i c k e r ash coatings about 100 - 300 microns t h i c k w i t h n o d u l a r o u t e r surfaces r e s u l t i n g f r o m i n c o r p o r a t i o n o f l a r g e r ash p a r t i c l e s ( F i g u r e 2). S u l f a t i n g , shown b y l i g h t e r colored areas in t h e SEM p h o t o g r a p h s , i s common w i t h i n b o t h t h e f i n e r a n d coarser ash p a r t i c l e s o f t h e coating.
176
\:
Stage 3.
S u l f a t e d ash-cemented agglomerates.
I n t h i s stage t h e q u a r t z g r a i n s a r e loosely h e l d t o g e t h e r b y a cement of sulfated aluminosilicate ash ( F i g u r e 3 ) . Penetration of q u a r t z g r a i n s b y fine g r a i n e d ash is more extensive. Stage 4. 1
'
,'
,
Glass-cemented agglomerates.
In t h e f i n a l stage q u a r t z g r a i n s a r e b o n d e d b y s u l f a t e d ash w h i c h has p a r t l y melted a n d c r y s t a l l i z e d t h r o u g h reaction of t h e h o t ash a n d t h e q u a r t z grains. Resultant cooled agglomerates consist of q u a r t z g r a i n s o f t h e b e d material bonded b y a m i x t u r e o f sulfated ash a n d Ca-rich, S-poor glass ( F i g u r e 41, w i t h an intermediate reaction zone made u p of a n S-depleted, S i - e n r i c h e d ash p o r t i o n w i t h a f r i n g e of melilite o r a u g i t e c r y s t a l s p r o j e c t i n g i n t o t h e glass ( F i g u r e s 5 and 6). Some q u a r t z g r a i n s a r e p a r t l y melted a n d / o r r e c r y s t a l l i z e d t o c r i s t o b a l i t e o r o t h e r phases.
Limestone Bed Agglomerates Agglomeration of limestone b e d material appears t o b e d e p e n d e n t on ash deposit i o n a n d sulfation combined w i t h e x t e n s i v e reaction a n d d e t e r i o r a t i o n of t h e bed material. A s h b u i l d u p o n t h e g r a i n s a n d s u l f a t i n g i s comparable t o reactions t h a t o c c u r w i t h t h e q u a r t z b e d material. However, t h e limestone g r a i n s appear t o u n d e r g o t h e following reactions:
1. Loss o f CO, a n d conversion t o CaO w i t h a d d i t i o n o f S, Fe, Na a n d o t h e r elements. These reactions p r o d u c e concentric a l t e r a t i o n zones, h i g h Ca a n d S cont e n t s a n d t h e r e d d i s h color t h a t characterizes t y p i c a l g r a i n s ( F i g u r e 7 ) . 2. Continued reaction p r o d u c e s t h i c k e r s u l f a t e d ash coatings a n d more t h o r o u g h l y altered bed g r a i n s . 3. B e d g r a i n s d i s i n t e g r a t e e x t e n s i v e l y a n d become m i x e d w i t h ash coatings p r o d u c i n g a weakly bonded agglomerate consisting o f masses o f s u l f a t e d a s h a n d a l t e r e d limestone b e d g r a i n s and f r a g m e n t s ( F i g u r e 8 ) . T h e a l t e r e d limestone appears t o r e c r y s t a l l i z e t o coarse c r y s t a l s o f a n h y d r i t e in a f i n e - g r a i n e d m a t r i x cont a i n i n g a b u n d a n t Ca, S a n d Si ( F i g u r e 9). O t h e r phases, n o t y e t i d e n t i f i e d , o c c u r in t h e limestone agglomerates i n c l u d i n g c r y s t a l l i n e Fe-Ca oxides as shown in F i g u r e IO, a n d o t h e r i r o n - r i c h zones a n d coatings. Where q u a r t z a n d limestone b e d materials a r e combined m u t u a l i n t e r a c t i o n s p r o duce reaction zones on t h e q u a r t z c o n t a i n i n g secondary needles of an u n k n o w n calcium silicate mineral ( F i g u r e 1 1 ) . B u l k x - r a y d i f f r a c t i o n analyses were performed o n t h e b e d material agglomerates t o i d e n t i f y t h e phases p r e s e n t . T h e c r y s t a l i n e phases f o u n d in t h e q u a r t z bed agglomerates from run 2181 i n c l u d e q u a r t z , a member o f t h e series Ca2AI2SiO7Ca2Mg2Si07 w h i c h includes melilite, a n d CaS04. T h e major phases i d e n t i f i e d in t h e limestone bed agglomerate a r e CaS04, CaSi04 a n d CaO. T h i s d a t a s u p p o r t s t h e SEM microprobe data. X - r a y fluorescence analysis was p e r f o r m e d o n b e d material sampled c o n t i n u a l l y t h r o u g h o u t t h e run t o determine t h e changes in composition o f major ash c o n s t i t u ents. T h e most appreciable changes w h i c h o c c u r r e d in t h e q u a r t z b e d run were: SiO, decreased f r o m 95 t o 47% o f t h e b e d because o f d i l u t i o n ; SO3 increased t o 243, because of a d s o r p t i o n b y alkali c o n s t i t u e n t s o f t h e coal ash in t h e bed; CaO increased t o 11% and Na20 increased t o 7% o f t h e b e d . T h e CaO a n d NazO reacted w i t h t h e SO3 a n d adhered t o t h e q u a r t z g r a i n s o f t h e b e d . Al2O3, Fez03 a n d MgO remained r e l a t i v e l y constant t h r o u g h o u t t h e run a f t e r t h e i n i t i a l e i g h t h o u r s . T h e
1 77
components o f t h e limestone b e d o f run 2481 changed as follows: CaO decreased b y d i l u t i o n t o 37% b y t h e coal ash; SO3 increased t o 28% b y a d s o r p t i o n ; a n d Na20 increased t o 7.7% o f t h e bed. Si02, A1203, Fez03 a n d MgO remained constant t h r o u g h o u t t h e run a f t e r t h e i n i t i a l e i g h t h o u r s . T h e concentrations o f alumina a n d silica remain constant during t h e run i n d i c a t i n g t h a t t h e aluminosilicate c l a y p a r t i c l e s leave t h e b e d during combustion. O n t h e o t h e r h a n d , t h e calcium o x i d e a n d sodium o x i d e w h i c h l a r g e l y o r i g i n a t e from t h e o r g a n i c s t r u c t u r e o f t h e l i g n i t e a r e f r e e t o react w i t h t h e SO2 a n d b e d material increasing their concentration. TABLE OXIDE Si02 A1203
FeO MgO CaO Na20
so3 OXIDE Si02 A1203
FeO MgO CaO Na20
so3
3.
CHEMICAL A N A L Y S I S D A T A FOR FIGURE 1 T O 11. WT. %
A
B
C
D
E
F
G
10.76 11.70 4.82 5.71 18.11 12.70 34.45
12.45 11.20 0.55 1.I4 32.86 3.99 37.25
8.22 18.94 6.94 11.86 12.63 10.14 30.80
12.16 8.94 3.02 5.73 12.83 17.97 38.27
31 .EO 19.13 13.14 10.27 19.82 2.04 3.39
47.80 10.03 9.95 4.44 20.28 6.11 0.43
48.32 10.61 9.48 6.26 20.29 3.73 0.52
H
15.50 10.28 0.75 3.84 40.45 1.91 21.49
I
19.66 0.15 0.37 0.09 53.36 1.74 24.39
J
L
K
12.39 5.13 1.15 0.93 44.23 0.78 35.37
1.81 5.91 0.32 0.17 39.58 0.40 51.66
0.89 1.87 81.69 0.91 13.55 0.00 6.33
M
31.50 8.20 0.56 0.00 44.19 2.76 11.89
I
CONCLUSIONS
Agglomeration o f t h e b e d material can b e manifested i n many d i f f e r e n t ways depending on t h e chemical composition o f t h e bed. T h e elements w h i c h have a major e f f e c t are sodium, calcium and s u l f u r w h i c h r e a c t w i t h t h e b e d material possibly forming p o s s i b l y a molten phase. T h i s phase causes o t h e r ash c o n s t i t u e n t s t o adh e r e t o t h e b e d p a r t i c l e s . As t h i s phenomenon r e o c c u r s many times, agglomeration becomes more severe. REFERENCE
1.
Goblirsch, G.M., V a n d e r Molen, R . H . , and Hajicek, D . R . , AFBC T e s t i n g of N o r t h Dakota L i g n i t e , S i x t h I n t e r n a t i o n a l Fluidized Bed Conference, Atlanta, Georgia, A p r i l , 1980.
178
1
FIG. 1. I n i t i a l ash c o a t i n g on q u a r t z bed m a t e r i a l . SEM/BEI image. A n a l y s i s A ( T a b l e 3 ) . Run 2181, 40 h r s .
FIG. 3. Q u a r t z bed g r a i n s l o o s e l y cemented by s u l f a t e d a l u m i n o s i l i c a t e ash. SEM/BEI image. A n a l y s i s D g i v e n i n Table 3. Run 2181, 69 h r s .
173
I
F I G . 2. Thickened n o d u l a r ash c o a t i n g on q u a r t z bed m a t e r i a l . SEM/BEI image. A n a l y s i s B o f f i n e l i g h t s u l f a t e d ash and C o f d a r k e r i n t e r i o r o f c o a r s e r ash p a r t i c l e ( T a b l e 3 ) . Run 2181, 54 h r s .
FIG. 4. Quartz bed agglomerate bonded by a l t e r e d s u l f a t e d ash, bottom o f t h e d o t t e d boundary ( a n a l y s i s E); C a - r i c h , Spoor glass, t o p o f t h e dashed l i n e ( a n a l y s i s F ) ; and i n t e r m e d i a t e f r i n g e o f m e l i l i t e o r a u g i t e c r y s t a l s . SEM/BEI image. Run 2181, 69 h r s .
FIG. 5. T r a n s m i t t e d l i g h t image ( p a r t i a l l y crossed p o l a r s ) o f lower c e n t e r o f F i g u r e 4 showing l i g h t q u a r t z g r a i n , g r a y g l a s s , and c r y s t a l s o f m e l i l i t e o r a u g i t e c r y s t a l s p r o j e c t i n g i n t o g l a s s . Run 2181, G9 h r s .
F I G . 6. D e t a i l o f l o w e r c e n t e r o f Figure 5, showing d e n d r i t i c c r y s t a l form o f m e l i l i t e o r a u g i t e . Analysis G given i n Table 3. SEM/SEI image. Run 2181, 69 hrs.
FIG. 7 . C o n c e n t r i c a l t e r a t i o n zones and r e a c t e d l i m e s t o n e bed m a t e r i a l . A n a l y s i s H g i v e n i n T a b l e 3. SEM/BEI image. Run 2481, 169 h r s .
FIG. 8. Weakly bonded agglomerate o f s u l f a t e d ash and a l t e r e d l i m e s t o n e bed g r a i n s and fragments. A n a l y s i s I and J a r e g i v e n i n Table 3. SCM/BEI image. Run 2481, 169 hrs.
180
F I G 9. Limestone bed m a t e r i a l a l t e r e d t o coarse c r y s t a l s o f Cas04 i n a f i n e g r a i n e d m a t r i x of Ca, S , and S i . L i g h t area t o l e f t i s ash c o a t i n g . A n a l y s i s K g i v e n i n Table 3. SEM/BEI image. Run 2481, 169 h r s .
FIG. 10. Bladed c r y s t a l s o f Fe-Ca o x i d e s i n l i m e s t o n e agglomerate. Ash c o a t i n g t o r i g h t . A n a l y s i s L g i v e n i n Table 3. SEM/ BE1 image. Run 2481, 169 h r s .
F I G . 11. R e a c t i o n zone c o n s i s t i n g o f secondary needles o f an unknown C a - s i l i c a t e m i n e r a l on a q u a r t z g r a i n c o n t a i n e d i n t h e l i m e s t o n e bed. A n a l y s i s M g i v e n i n Table 3. SEM/BEI image. Run 2481, 169 hrs.
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