24 Slag Deposit Initiation Using a Drop-Tube Furnace M. F. Abbott and L. G. Austin 1
Mineral Processing Section, The Pennsylvania State University, University Park, PA 16802
A drop-tube furnace was designed and constructed for the purpose of simulating the time/temperature enviroment for p.c. combustion in a u t i l i t y furnace. The ash produced was impacted on oxidized boiler steel substrate at gas and metal temperatures similar to upper furnace waterwall tubes. Both fly ash and deposits were similar to those of a pilot-scale (7-9 kg/hr) combustor. Iron-rich slag droplets produced from pyrite-rich p.c. particles bonded strongly with the oxidized steel surface. These particle types were found at the base of ash deposits after removal of sintered and loose ash for both eastern and western coals. Adhesion of iron-rich droplets was a function of both flame and metal surface temperatures. Also, volatile species, i . e . , alkali and exchangeable cations influenced the "sticking" behavior of the iron-rich droplets. These trends are in qualitative agreement with previous sticking test results.
Coals c o n t a i n i n o r g a n i c m a t e r i a l , g e n e r a l l y c a l l e d m i n e r a l matter and when t h e c o a l i s burned t h i s m i n e r a l m a t t e r i s c o n v e r t e d t o ash. The management o f t h i s a s h c o n s t i t u t e s one o f t h e p r i n c i p a l design l i m i t a t i o n s f o r a pulverized coal ( p . c . ) - f i r e d e l e c t r i c u t i l i t y boiler. The i n v e s t i g a t i o n r e p o r t e d h e r e i s p a r t o f an ong o i n g r e s e a r c h program t o g a i n a b e t t e r u n d e r s t a n d i n g o f t h e i n i t i a t i o n o f s l a g d e p o s i t s on t h e upper w a l l s o f a b o i l e r f u r n a c e enclosure. T h i s paper r e p o r t s on t h e development o f a g a s - f i r e d v e r t i c a l m u f f l e tube ( d r o p - t u b e ) f u r n a c e as a new r e s e a r c h t o o l . The purpose o f t h e d r o p - t u b e f u r n a c e was t o s i m u l a t e u t i l i t y b o i l e r combustion and a s h f o r m i n g and d e p o s i t c o n d i t i o n s , i n a laboratory-scale device. I n p a r t i c u l a r , t h e f u r n a c e was used t o
1
Current address: Coal Research Division, Conoco, Inc., Library, PA 15129. 0097-6156/ 86/ 0301 -0325507.50/ 0 © 1986 American Chemical Society
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d e t e r m i n e which i n o r g a n i c c o n s t i t u e n t s i n c o a l gave ash p a r t i c l e s t h a t i n i t i a t e s l a g d e p o s i t s by a d h e r i n g t o r e l a t i v e l y c o l d ( e . g . , 300 t o 400°C) o x i d i z e d b o i l e r s t e e l s u r f a c e s . A previous laboratory t e s t , the s t i c k i n g t e s t ( 1 - 6 ) , l e d t o a number o f c o n c l u s i o n s conc e r n i n g the mechanism and c h e m i s t r y of m o l t e n s l a g drop a d h e s i o n . However, t h i s t e s t has s e v e r a l i n h e r e n t d i s a d v a n t a g e s . It required the use o f r e l a t i v e l y l a r g e m o l t e n a s h drops 4 mm i n d i a m e t e r , and t h e r e was no p r o o f t h a t the c o n c l u s i o n s c o u l d be a p p l i e d t o the s m a l l e r s i z e d r o p l e t s ( n o r m a l l y l e s s t h a n 50 ym) produced i n p . c f i r e d f u r n a c e s . A l s o , t h e l a r g e drops formed from a c o a l ash cont a i n e d a l l o f the c o n s t i t u e n t s o f t h e ash (the mean a s h c o m p o s i t i o n ) , which do not a c c u r a t e l y s i m u l a t e the v a r i e t y of m i n e r a l a s s o c i a t i o n s o c c u r r i n g on a p a r t i c l e - b y - p a r t i c l e b a s i s i n a p u l v e r i z e d c o a l (_7 ). Thus, the p r i n c i p a l r e q u i r e m e n t s o f the drop-tube f u r n a c e were t o produce c o a l f l y a s h p a r t i c l e s under the same t e m p e r a t u r e - t i m e environment h i s t o r y e x p e r i e n c e d i n f u l l - s c a l e b o i l e r combustion, w i t h i m p a c t i o n on an o x i d i z e d b o i l e r s t e e l s u b s t r a t e a t a c o n t r o l l e d temperature s i m u l a t i n g a f u r n a c e w a t e r w a l l . Drop-Tube Furnace
Description
S e v e r a l drop-tube t y p e f u r n a c e s were a l r e a d y i n e x i s t e n c e f o r cond u c t i n g r e s e a r c h on p.c. p y r o l y s i s , combustion and m i n e r a l m a t t e r b e h a v i o r (8-12). These were i n v e s t i g a t e d i n terms o f t h e i r s u i t a b i l i t y , advantages and d i s a d v a n t a g e s f o r s t u d y i n g t h e s l a g d e p o s i t i o n problem. I t was d e c i d e d t h a t the s i m p l e s t , l e a s t e x p e n s i v e and y e t most v e r s a t i l e a l t e r n a t i v e was a v e r t i c a l m u f f l e tube ( d r o p tube) f u r n a c e e x t e r n a l l y h e a t e d by f i r i n g w i t h n a t u r a l gas and a i r . The d r o p - t u b e f u r n a c e system i s shown i n F i g u r e 1. It consisted o f f o u r major component p a r t s : (1) a hot zone s e c t i o n ; (2) a p r e heat s e c t i o n and i n j e c t o r ; (3) t h e f l u i d i z e d - b e d p.c. f e e d i n g system; and (4) a w a t e r - c o o l e d ash c o l l e c t o r probe. The n e c e s s a r y f e a t u r e s i n c o r p o r a t e d i n t o the h o t zone d e s i g n were: maximum gas temperatures of the o r d e r o f 1500 t o 1750°C; p a r t i c l e r e s i d e n c e t i m e s between 1 and 2 seconds; and e x i t gas t e m p e r a t u r e s o f 1000 t o 1300°C. The p r e h e a t s e c t i o n and i n j e c t o r p r e h e a t e d the s e c o n d a r y a i r steam to about 1000°C and i n j e c t e d t h e c o l d p . c . - p r i m a r y a i r s t r e a m i n t o the hot combustion zone. The c o l l e c t o r probe impacted t h e a s h p a r t i c l e s onto a b o i l e r s t e e l s u r f a c e m a i n t a i n e d a t temperat u r e s between 250 and 450°C t o w i t h i n ± 5°C. A more d e t a i l e d s c h e m a t i c o f the d r o p - t u b e f u r n a c e h o t zone i s shown i n F i g u r e 2. I t was heated by t h r e e t a n g e n t i a l l y - f i r e d n o z z l e mix n a t u r a l g a s - a i r b u r n e r s (101NM, P y r o n i c s , I n c . , C l e v e l a n d , Ohio). These were s h o r t flame, h i g h c a p a c i t y b u r n e r u n i t s d e s i g n e d f o r v e r y wide turndown r a n g e s , g i v i n g a g r e a t d e a l of o p e r a t i n g f l e x i b i l i t y i n terms o f heat i n p u t . The temperature was c o n t r o l l e d by m e t e r i n g b o t h the t o t a l q u a n t i t y o f n a t u r a l gas and a i r ( b u r n e r n o z z l e gas p r e s s u r e ) , and the a i r - t o - f u e l r a t i o . The n a t u r a l gas f i r i n g r a t e a t an o p e r a t i n g temperature o f 1470 t o 1550°C was 1.5 SCFM o r 90,000 Btu/h (30,000 Btu/h f o r each o f the t h r e e burners). The o u t s i d e m u f f l e tube w a l l temperatures were measured a t t h r e e l o c a t i o n s by embedded P t / P t - 1 0 % Rh t y p e S thermocouples. The a n n u l a r w a l l temperatures were a l s o m o n i t o r e d i n the same m a t t e r (see F i g u r e 2 ) .
24.
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MINERAL MATTER AND ASH IN COAL
TOP VIEW ALUMINA FIBRE INSULATION 4" (10.2 cm) FIREBRICK
MOLDABLE ALUMINA FIBRE INSULATING BOARD 12" I.D.xl4.9 QD.x24' (305σηωχ368αηΟΰχ609βη), 99.8 % FUSED ALUMINA TUBE 2-1/2? LDx2-3/4"aD,x36" (835cm LD.x8.99cmQD.x9l.4cm)
AIR 1-9 AT SAME LEVEL AS EACH BURNER
N
EXHAUST
COAL /AIR STREAM FROM INJECTOR AND PREHEATER
SIDE VIEW
3 _J=-n -AIR -CH
A
1
\
3 TANGENTIALLYFIRED NOZZLE MIX BURNERS
WINDOW FOR VIEWING FLAME 0 1-6 THERMOCOUPLE POSITIONS ALL Pt-Pt/IORh TYPE S F i g u r e 2.
Drop-tube f u r n a c e h o t zone.
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D e t a i l s o f the p r e h e a t s e c t i o n and p.c. i n j e c t o r a r e shown i n F i g u r e 3. The h e a t i n g c o i l on the i n j e c t o r i s o l a t e d the c o l d i n j e c t o r from the p r e h e a t e d secondary a i r stream ( a p p r o x i m a t e l y 2.5 l i t e r s / m i n ) w h i c h e n t e r e d through two c h a n n e l s a t the t o p . The p r e h e a t e r c o i l and i n n e r w a l l temperatures were a l s o measured by type S thermocouples. The a l u m i n a honeycomb f l o w s t r a i g h t e n e r d i s t r i b u t e d the a i r i n s t r e a m l i n e s a c r o s s the m u f f l e tube c r o s s section. F l u i d i z e d bed f e e d e r s g e n e r a l l y g i v e more c o n s i s t e n t homogen eous p.c. f l o w f o r low f e e d r a t e s than o t h e r types o f f e e d e r s (13-15). The f l u i d bed f e e d e r used i n t h i s i n v e s t i g a t i o n i s shown s c h e m a t i c a l l y i n F i g u r e 4. The f e e d e r r e s t e d on a top l o a d i n g b a l a n c e i n o r d e r t o c o n t i n u o u s l y m o n i t o r the p.c. f e e d r a t e . It d e l i v e r e d between 0.15 and 0.3 grams o f c o a l w i t h a p p r o x i m a t e l y 0.5 l i t e r s of a i r p e r minute. The p.c. p a r t i c l e s i z e was kept between 60 and 325 mesh (250 t o 45 ym) t o i n s u r e c o n s i s t e n t performance. F i g u r e 5 i s a d e t a i l e d diagram o f the w a t e r - c o o l e d c o l l e c t o r probe. The c o n s t r i c t i o n (0.5 i n c h d i a m e t e r h o l e ) above the s t e e l s u b s t r a t e s u r f a c e was used t o a c c e l e r a t e the gas stream t o about 2 m/sec i n o r d e r t o impact the p a r t i c l e s on the s u r f a c e w i t h s u f f i c i e n t v e l o c i t y t o adhere. The s t e e l s u b s t r a t e s u r f a c e temperature was c o n t r o l l e d by a c o m b i n a t i o n o f t h r e e methods: v a r y i n g the sub s t r a t e t h i c h n e s s ; v a r y i n g the t h i c k n e s s o f the carbon s t e e l p l u g ; and v a r y i n g the c o o l a n t (water) f l o w r a t e . The s u r f a c e temperature was a g a i n measured by type S thermocouple as shown i n F i g u r e 5. Two b o i l e r s t e e l s were used f o r s u b s t r a t e m a t e r i a l s ; 1040 c a r b o n s t e e l and C r o l o y 1/2. Experimental
Procedures
Three P e n n s y l v a n i a b i t u m i n o u s c o a l s ( d e s i g n a t e d Keystone, Montour and T u n n e l t o n ) and a Decker, Montana, sub-bituminous c o a l were studied i n this investigation. The p r o x i m a t e and u l t i m a t e a n a l y s e s f o r t h e s e c o a l s were g i v e n i n T a b l e I. The m i n e r a l o g i c a l a n a l y s i s o f the low-temperature ash (LTA) from the P e n n s y l v a n i a c o a l s i s l i s t e d i n T a b l e I I and the e l e m e n t a l a n a l y s i s o f the ASTM h i g h temperature (ΗΤΑ) i s g i v e n i n T a b l e I I I . In a d d i t i o n , p u l v e r i z e r r e j e c t s from the Bruner I s l a n d Steam G e n e r a t i n g F a c i l i t y w h i c h burns the T u n n e l t o n c o a l (among o t h e r s ) were t e s t e d . The m i n e r a l o g i c a l and e l e m e n t a l a n a l y s e s o f t h i s m a t e r i a l a r e g i v e n i n T a b l e IV. T a b l e V g i v e s the m i n e r a l o g i c a l and e l e m e n t a l a n a l y s e s o f u n t r e a t e d and acid-washed Decker c o a l samples. The t h r e e P e n n s y l v a n i a c o a l s were burned a t t h r e e f u r n a c e temperatures. The o t h e r samples were a l l burned a t 1500 t o 1510°C f u r n a c e temperature. The p a r t i c l e s i z e d i s t r i b u t i o n o f the as r e c e i v e d c o a l samples was measured by the M i c r o t r a c l a s e r d i f f r a c t i o n a p p a r a t u s and s i z e a n a l y s e s f o r a l l samples a r e g i v e n i n T a b l e VI. The samples were s i e v e d t o remove any p a r t i c l e s s m a l l e r than 45 ym and l a r g e r than 250 ym. The f l u i d bed f e e d e r was l o a d e d w i t h a 20 t o 30 gram c o a l o r m i n e r a l sample. The f e e d e r was o p e r a t e d a t c o n s t a n t d i l u t i o n o r t r a n s p o r t a i r f l o w and the c o a l f l o w was v a r i e d by changing the bed p r e s s u r e (by e i t h e r i n c r e a s i n g the f l u i d i z i n g a i r f l o w o r d e c r e a s i n g the exhaust f l o w ) .
MINERAL MATTER AND ASH IN COAL
COAL/PRIMARY AIR FEED
-21/2"— (•.35 cm) F i g u r e 3.
Drop-tube f u r n a c e i n j e c t o r and p r e h e a t e r
section.
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Using a Drop-Tube
Furnace
EXHAUST AIR
DEFLECTOR PLATE
FOAM FILTERS
TO MANOMETER l-j"OOx Ι ^ Ί θ κ 5 GLASS TUBE (341 cm OD χ 3d65cmfDxC70cm)T Γ OD χ { § " ID χ 3 GLASS TUBE (2.54 cm00x2.3BcmlDx7.62cm) TRANSPORT AIR N
BED VIBRATOR Γ (2.54 em) SINTERED STEEL DISK FLUIDIZING AIR
P.C. FEED TO INJECTOR I mm ID OFFTAKE TUBE ALUMINUM SUPPORT STAND
F i g u r e 4.
Drop-tube f u r n a c e pc f l u i d bed f e e d e r .
332
MINERAL MATTER AND ASH IN COAL
li"(5.6lem) DIAMETER STEEL SUBSTRATE VARIABLE THICKNESS CARBON STEEL PLUG SUPPORT FINS FOR CENTER TUBE
DTF I l/2 ODxl7/l6 IDxe STAINLESS STEEL TUBE (341 cm00x3j6S em ID χ2032cm! N
N
WATER OUT -l/4"0D χ 7/32" ID x » STAINLESS STEEL TUBE (0.64 em 00 x 0.36 em ID χ 22.86 em ) WATER IN Figure probe.
5.
Drop-tube f u r n a c e w a t e r - c o o l e d a s h d e p o s i t
collector
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ABBOTT AND AUSTIN
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T a b l e I. Proximate and U l t i m a t e A n a l y s e s of T e s t ( a l l on a d r y b a s i s w i t h the e x c e p t i o n o f moisture determination) Coal
Montour
Keystone
Tunnleton
Furnace
333
Coals
Decker
Proximate Moisture V o l a t i t e Matter Ash F i x e d Carbon
1.0 25.9 16.0 58.1
0.8 29.9 18.1 51.9
0.6 26.4 21.9 51.9
22.0 43.8 4.5 51.7
Ultimate Carbon Hydrogen Nitrogen Sulfur Ash Oxygen
75.2 4.4 1.3 1.7 16.0 1.4
69.1 4.3 1.1 1.7 18.1 5.7
68.6 4.1 1.1 2.0 21.9 2.3
72.6 5.1 0.9 0.4 4.5 16.5
Table I I . Semi-Quantitative M i n e r a l o g i c a l A n a l y s i s o f Low-Temperature Ash (LTA) f o r the Pennsylvania Coals
Coal quartz pyrite calcite gypsum kaolinite illite feldspar
Montour
Keystone
25 15 5 n. d. 17 30 n.d.
25 10 n.d. 5 30 20 n.d.
Tunnelton 22 15 5 n.d. 18 30-40 5-10
n.d. = not d e t e c t e d i n s i g n i f i c a n t q u a n t i t y . Wt. % LTA 18.0 21.6 22.9
The f e e d e r and i n j e c t o r produced a t h i n p e n c i l - l i k e p . c . stream w h i c h p a s s e d down through the hot zone. The t o t a l combustion a i r s u p p l i e d was a p p r o x i m a t e l y 3 l i t e r s / m i n f o r the b i t u m i n o u s c o a l s , g i v i n g between 10 and 25 p e r c e n t e x c e s s a i r f o r p.c. f e e d r a t e s o f 0.24 t o 0.28 g/min. The f l o w and heat t r a n s f e r c o n d i t i o n s were modeled u s i n g the methods d e s c r i b e d by P i g f o r d (16) f o r c o n d i t i o n s o f superimposed n a t u r a l and f o r c e d c o n v e c t i o n a t v e r y low mass f l o w rates. P a r t i c l e r e s i d e n c e times were c a l c u l a t e d by summing the c e n t e r l i n e gas v e l o c i t y and t e r m i n a l v e l o c i t y u s i n g Stokes's law (17). The e r r o r i n t r o d u c e d u s i n g t h i s method s h o u l d never have exceeded 10 p e r c e n t , even when p y r i t e was t e s t e d and p a r t i c l e Reynold's numbers approached one. The r e s i d e n c e times thus c a l c u l a t e d were found t o be between one and two seconds.
MINERAL MATTER AND ASH IN COAL
T a b l e I I I . S p e c t r o c h e m i c a l A n a l y s i s o f ASTM H i g h Temperature A s h (ΗΤΑ) R e s i d u e s From t h e P e n n s y l v a n i a C o a l s , E x p r e s s e d as Weight P e r c e n t o f E q u i v a l e n t Oxides Coal Si0
2
A 1
2°3 Ti0 2
F e
2°3 CaO MgO Na 0 2
κο 2
S0
3
po 2
5
Totals Wt. % ΗΤΑ
Montour
Keystone
Tunnelton
51.7 25.6 1.3 14.1 2.4 0.9 0.2 2.4 1.5 0.4 100.5 15.3
54.1 25.9 1.3 9.7 1.8 1.0 0.3 2.9 1.1 0.4 98.5 18.3
50.3 26.8 1.3 11.0 2.5 1.0 0.4 2.9 2.3 0.4 98.9 20.1
T a b l e IV. S e m i - Q u a n t i t a t i v e M i n e r a l o g i c a l A n a l y s i s and S p e c t r o c h e m i c a l A n a l y s i s f o r LTA and ΗΤΑ o f P u l v e r i z e r R e j e c t s
M i n e r a l o g i c a l A n a l y s i s o f LTA Constituent quartz pyrite calcite kaolinite illite siderite iron sulfates LTA (wt.% as r e c e i v e d sample)
Wt. % LTA 15 30 2-5 10 30 2-5 5-10 79.6
S p e c t r o c h e m i c a l A n a l y s i s o f ΗΤΑ Constituent Si0 A1 0 Ti0 Fe 0 CaO MgO Na 0 2
2
3
2
2
3
2
κο 2
Total ΗΤΑ (wt.% as r e c e i v e d sample)
Wt. % ΗΤΑ 42.6 16.6 0.9 34.3 1.4 0.8 0.2 2.1 98.9 68.2
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T a b l e V. S e m i - Q u a n t i t a t i v e M i n e r a l o g i c a l A n a l y s i s and S p e c t r o c h e m i c a l A n a l y s e s f o r Two Decker C o a l Samples; U n t r e a t e d and Acid-Washed M i n e r a l o g i c a l A n a l y s i s o f LTA Constituent quartz pyrite calcite gypsum ( b a s s i n i t e ) kaolinite illite LTA (wt.% as r e c e i v e d c o a l )
untreated 20 5 5 30 11 n.d. 6.
S p e c t r o c h e m i c a l A n a l y s i s o f ΗΤΑ Constituent Si0
2
A 1
2°3 Ti0 2
F e
2°3 CaO MgO MnO Na 0 2
κ ο so 2
3
Total ΗΤΑ (wt.% as r e c e i v e d c o a l )
Wt. % LTA acid-washed
untreated
30 10 n.d. n.d. 16 n.d. 3.3 Wt. % ΗΤΑ acid-washed
28.0 16.5 1.2 6.9 19.0 3.5 0.03 7.7 0.5 15.8 99.1 4.2
53.0 28.5 2.7 9.2 4.1 0.7 0.01 0.4 0.6
99.2 2.2
Ash d e p o s i t s were c o l l e c t e d from an a c c e l e r a t e d gas s t r e a m impacted on a w a t e r - c o o l e d b o i l e r s t e e l s u b s t r a t e as shown i n t h e probe o f F i g u r e 5. The s a m p l i n g gas temperature was 1100°C and t h e 1040 c a r b o n s t e e l s u b s t r a t e was m a i n t a i n e d a t 340 ± 5°C f o r a l l t e s t s e x c e p t t h o s e w i t h t h e Decker c o a l ( s a m p l i n g gas temperature was 1000°C and C r o l o y 1/2 s u b s t r a t e temperature was 425 ± 5 ° C ) . The a s h d e p o s i t s where then c h a r a c t e r i z e d by m i c r o s c o p i c e x a m i n a t i o n , u s i n g o p t i c a l and 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 e s . Chemical charac t e r i s t i c s were then sometimes d e t e r m i n e d by energy d i s p e r s i v e x - r a y (EDX) f l u o r e s c e n c e equipment a s s o c i a t e d w i t h t h e s c a n n i n g e l e c t r o n microscope. Experimental Results The c e n t e r l i n e gas temperatures were measured by s h i e l d e d s u c t i o n pyrometry a t maximum w a l l t e m p e r a t u r e s r a n g i n g from 1450 t o 1650°C. A comparison o f gas and w a l l t e m p e r a t u r e s i s g i v e n i n F i g u r e 6. I n g e n e r a l , c e n t e r l i n e gas temperatures were about 30°C lower than t h e c o r r e s p o n d i n g w a l l temperatures a t t h e same p o s i t i o n on t h e m u f f l e tube. A d e p o s i t c o l l e c t i o n zone w i t h gas temperatures c o r r e s p o n d i n g t o upper f u r n a c e temperatures i n a u t i l i t y b o i l e r was i d e n t i f i e d
MINERAL MATTER AND ASH IN COAL
336
Table VI.
Size (ym)
500 355 250 176 125 88 62 44 31 22 16 11 7.8 5.5 3.9 2.8 2.0 1.4 1.0
Particle
Size Analysis
Keystone Coal
Montour Coal
Tunnelton Coal
100.00 98.06 96.61 93.78 88.94 81.34 75.67 65.72 59.35 53.17 45.81 38.12 30.79 24.28 18.82 14.36 10.85 8.12 6.05
100.00 99.65 98.82 96.54 92.12 86.59 79.06 68.83 58.05 47.83 38.67 30.78 24.21 18.86 14.59 11.28 8.59 6.56 4.99
100.00 99.86 99.17 95.30 87.42 75.81 68.41 56.12 45.80 37.18 30.06 24.23 19.47 15.61 12.50 9.99 7.97 6.36 5.07
(Sieve
Decker Coal
size) Brunner I s l a n d Pulverizer Tailings
99.70 99.21 97.93 94.29 88.68 77.76 64.47 57.81 43.52 32.76 24.67 18.57 13.98 10.52 7.92 5.92 4.49 3.38 2.55
S i z e a n a l y s i s f o r -44 ym c o a l was determined by M i c r o t r a c c o r r e c t i o n to equivalent sieve s i z e .
100.00 100.00 100.00 100.00 97.63 91.86 83.68 80.87 75.88 68.76 60.16 50.92 41.92 33.72 26.63 20.74 15.97 12.19 9.25 with
between 15 and 20 cm b e f o r e t h e tube e x i t . The c a r b o n c o n t e n t o f ash d e p o s i t s c o l l e c t e d a t t h i s p o s i t i o n i n d i c a t e d t h a t combustion was e s s e n t a i l l y complete ( g r e a t e r than 99.7%) i f t h e maximum h o t zone temperature was a t l e a s t 1420°C. The p h y s i c a l c h a r a c t e r i s t i c s o f t h e d e p o s i t c o l l e c t e d from a l l f o u r o f t h e t e s t c o a l s and t h e p u l v e r i z e r r e j e c t s were a l l s i m i l a r . F i g u r e 7 shows the t y p i c a l a s h d e p o s i t s t r u c t u r e on b o t h a macro and m i c r o s c a l e . The top p h o t o g r a p h ( F i g u r e 7A) shows t h e o u t e r s i n t e r p o r t i o n o f the a s h d e p o s i t c o l l e c t e d from the Decker c o a l on a C r o l o y 1/2 s t e e l s u b s t r a t e . The lower o p t i c a l m i c r o g r a p h ( F i g u r e 7B) shows t h e s t r o n g l y bonded m a t e r i a l on the s t e e l s u r f a c e a f t e r t h e l o o s e l y a d h e r i n g d e p o s i t has been brushed away. The opaque b l a c k d r o p l e t s were found t o be r i c h i n i r o n (85-100 w t . % ) , whereas the l i g h t t r a n s p a r e n t s p h e r e s were p r e d o m i n a n t l y a l u m i n o silicates. The d e p o s i t b u i l d - u p mechanism appeared t o be t h e same f o r a l l c o a l samples t e s t e d . A l l appeared t o o r i g i n a t e w i t h t h e r e l a t i v e l y strong bonding of i r o n - r i c h p a r t i c l e s to the s t e e l s u b s t r a t e oxide layer. I n a d d i t i o n t o t h e s e p a r t i c l e s , t h e r e was always a l a y e r o f v e r y f i n e p a r t i c l e s ( l e s s than 3 ym) c o v e r i n g t h e e n t i r e s u b s t r a t e s u r f a c e , which were n o t e a s i l y brushed o r blown from t h e s u r f a c e . A r e g i o n o f l o o s e l y bonded a s h p a r t i c l e s t h e n b u i l t upon the more strongly adherent d r o p l e t s . These p a r t i c l e s were p r i n c i p a l l y a l u m i n o - s i l i c a t e s , o f t e n c o n t a i n i n g some amounts o f most o f t h e
Slag Deposit initiation
ABBOTT AND AUSTIN
Using a Drop-Tube
DISTANCE FROM INJECTOR TIP 10 I
1700
20
Furnace
(in)
30
40
Τ
1500
DEPOSIT COLLECTION ZONE 1 0 0 0 - 1 2 0 0 *C
UJ
CO
μ ST
Pu re
S
sr
Φ
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CTJ
CJN
CO CN
CN CN
Cd
cd
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Furnace
MINERAL MATTER AND ASH IN COAL
F i g u r e 9. SEM p h o t o m i c r o g r a p h s o f i r o n - r i c h d e p o s i t base p a r t i c l e s c o l l e c t e d from t h e Keystone c o a l a t flame temperatures of: (A) 1470°C, and (B) 1500°C.
ABBOTT AND AUSTIN
Slag Deposit
Initiation
Using a Drop-Tube
Furnace
343
F i g u r e 10. SEM p h o t o m i c r o g r a p h o f i r o n - r i c h d e p o s i t base p a r t i c l e s c o l l e c t e d from the Keystone c o a l a t a flame temperature o f 1560°C.
MINERAL MATTER AND ASH IN COAL
F i g u r e 11. SEM photomicrographs o f i r o n - r i c h d e p o s i t base p a r t i c l e s c o l l e c t e d a t a flame temperature o f 1510°C from: (A) Montour c o a l , and (B) T u n n e l t o n c o a l .
0.24
340
feed rate g/min
0.24
p.c.
310
Substrate temperature °C
2.9
2.9
g
Total feed
Relative build-up rates mg/min
1.0 9.8
Deposit mass mg
12 118
22
2
Wt. % ASTM ΗΤΑ
Table VIII. D e p o s i t Furnace Mass and R e l a t i v e B u i l d - U p Rates f o r K e y s t o n e C o a l a t a F u r n a c e Temperature o f 1500°C a t Two D i f f e r e n t S u b s t r a t e S u r f a c e Temperatures ( D e p o s i t i o n Zone Gas Temperature, 1100°C; S u b s t r a t e , 1040 Carbon S t e e l ) .
346
MINERAL MATTER AND ASH IN COAL
T a b l e IX. D e p o s i t Mass and R e l a t i v e B u i l d - U p Rates f o r the P u l v e r i z e r R e j e c t s a t a F u r n a c e Temperature o f 1515°C ( D e p o s i t i o n Zone Gas Temperature, 1100°C; S u b s t r a t e , 1040 c a r b o n s t e e l ; s u r f a c e temperature, 324°C)
Description
Total deposit S i n t e r e d mass D e p o s i t base
D e p o s i t mass
359 319 40
Relative build-up rates mg/min 239 213 26
o f t h e base d e p o s i t p a r t i c l e s was v i r t u a l l y t h e same as t h o s e c o l l e c t e d from the t h r e e steam c o a l s . However, when the c o l l e c t o r probe was removed a f t e r so s h o r t a time p e r i o d , t h e r e was a d i s t i n c t s u l f u r odor, i n d i c a t i n g t h a t s u l f u r was e s c a p i n g from t h e d e p o s i t a t a r e l a t i v e l y r a p i d r a t e , even though i t was c o m p a r a t i v e l y c o o l i n temperature ( l e s s than 3 0 0 ° C ) . F i g u r e 12 shows a p e r p e n d i c u l a r view o f the d e p o s i t - s t e e l o x i d e i n t e r f a c e removed from the substrate surface. The average i n t e r f a c e c o m p o s i t i o n from x - r a y f l u o r e s c e n c e showed t h a t t h e r e was a s i g n i f i c a n t amount o f s u l f u r i n the d e p o s i t i n g p a r t i c l e s . A n a l y s i s o f the s u b s t r a t e s u r f a c e from which t h e d e p o s i t was removed showed r e g i o n s which c o n t a i n e d s u l f u r and sometimes t r a c e s o f p o t a s s i u m , c a l c i u m and s i l i c o n . Thus b o n d i n g o f t h e p a r t i c l e s t o t h e s u r f a c e appeared t o i n v o l v e c h e m i c a l t r a n s f e r from the p a r t i c l e t o the s t e e l s u b s t r a t e . T a b l e X g i v e s t h e r e l a t i v e b u i l d - u p r a t e s f o r two samples o f the Decker c o a l : an u n t r e a t e d sample, and a hydrogen exchanged sample i n which most o f the i o n exchange c a t i o n s i n c l u d i n g C a ^ and N a were r e p l a c e d w i t h H+" by a c i d washing. The d e p o s i t i o n r a t e s were somewhat l o w e r than t h e t h r e e P e n n s y l v a n i a c o a l s a t t h e same f u r n a c e temperature. The exchangeable c a t i o n s removed from the u n t r e a t e d sample appeared to p l a y a s i g n i f i c a n t r o l e i n d e p o s i t build-up. The s i n t e r e d m a t e r i a l (yellow-brown i n c o l o r ) c o l l e c t e d from t h e u n t r e a t e d c o a l was r e l a t i v e l y s t r o n g l y bonded t o g e t h e r , r e q u i r i n g a f o r c e o f 20 p s i t o b r e a k i t up. The s i n t e r ( c o r a l c o l o r e d ) from the acid-washed c o a l broke a p a r t w h i l e removing t h e s u b s t r a t e from t h e c o l l e c t o r probe. See F i g u r e s 7A and 13A f o r photos o f ash d e p o s i t s formed from u n t r e a t e d and acid-washed Decker c o a l samples, r e s p e c t i v e l y . The base l a y e r o f p a r t i c l e s from the Decker c o a l were t h e same i r o n - r i c h drops seen i n t h e P e n n s y l v a n i a c o a l d e p o s i t s (see Figure 7B). The c o n c e n t r a t i o n o f t h e s e p a r t i c l e s on the subs t r a t e s u r f a c e d e c r e a s e d when t h e c a t i o n s were removed from t h e coal. Compare F i g u r e s 7B w i t h 13B. The t o t a l p y r i t e concent r a t i o n i n t h e u n t r e a t e d and acid-washed c o a l samples were t h e same, 0.3 weight p e r c e n t i n b o t h i n s t a n c e s . Thus, i r o n - r i c h p a r t i c l e s may a l s o i n i t i a t e s l a g d e p o s i t s from Western c o a l s , even though t h e p y r i t e c o n c e n t r a t i o n i n the c o a l i s r e l a t i v e l y low. The c o n c e n t r a t i o n o f t h e exchangeable c a t i o n s appeared t o i n f l u e n c e the d e p o s i t i o n b e h a v i o r o f t h e i r o n - r i c h d r o p l e t s , a l t h o u g h the e x a c t mechanism o f t h i s e f f e c t i s unknown. +
+
ABBOTT AND AUSTIN
Slag Deposit
Initiation
Using a Drop-Tube
Furnace
F i g u r e 12. P e r p e n d i c u l a r view o f i r o n - r i c h d e p o s i t base o x i d e s c a l e i n t e r f a c e f o r d e p o s i t c o l l e c t e d from p u l v e r i z e r r e j e c t s (SEM p h o t o m i c r o g r a p h ) .
Untreated c o a l sample
Acid-washed c o a l sample
1.
2.
Test
p.c.
0.24
0.28
feed rate g/min 5.6
4.8
20
g
feed
Total
20
Time, min
25
82
Deposit mass mg
1.3
4.1
Relative build-up rates mg/min
D e p o s i t Mass and R e l a t i v e B u i l d - U p Rates f o r t h e Decker Samples a t a F u r n a c e Temperatrue o f 1500±5°C ( D e p o s i t i o n Zone Gas Temperature, 1050°C; S u b s t r a t e , C r o l o y 1/2; S u r f a c e Temperature, 4 2 5 ± 5 ° C ) .
T a b l e X.
24
34
Wt. % ASTM ΗΤΑ
ABBOTT AND AUSTIN
Slag Deposit initiation
Using a Drop-Tube
Furnace
F i g u r e 13. Ash d e p o s i t c o l l e c t e d from the acid-washed Cecker c o a l on C r o l o y 1/2 s t e e l s u b s t r a t e : (A) T o t a l d e p o s i t s t r u c t u r e on a m a c r o s c a l e , and (B) O p t i c a l p h o t o m i c r o g r a p h showing i r o n rich particles.
349
350
D i s c u s s i o n and
MINERAL MATTER AND
ASH IN COAL
Conclusions
The mechanism of d e p o s i t f o r m a t i o n f o r the t h r e e Pennsylvania steam c o a l s and the Decker (Montana) s u b - b i t u m i n o u s c o a l appeared t o be as f o l l o w s : (1) i r o n - r i c h m o l t e n s l a g drops formed from p y r i t e - r i c h m i n e r a l p a r t i c l e s i n the p.c. bonded t o the o x i d i z e d b o i l e r s t e e l s u b s t r a t e ; (2) c o n c u r r e n t l y a l a y e r o f f i n e p a r t i c l e s ( l e s s t h a n 3 ym) formed a t h i n l a y e r on the s t e e l coupon s u r f a c e ; (3) the i n i t i a l l a y e r of i r o n - r i c h drops t h e n p h y s i c a l l y t r a p p e d o r i n t e r a c t e d w i t h other a s h p a r t i c l e s r e a c h i n g the s u b s t a t e s u r f a c e , and the b u i l d - u p r a t e i n c r e a s e d as d e p o s i t i o n became l e s s d i s c r i m i n a t o r y w i t h r e s p e c t t o ash p a r t i c l e compostion o r s i z e ; (4) f i n a l l y , as the ash d e p o s i t grew and the temperature i n c r e a s e d f u r t h e r from the s u b s t r a t e s u r f a c e , s i n t e r i n g o c c u r r e d between d e p o s i t e d p a r t i c l e s u n t i l a semi-molten mass formed i n the o u t e r most r e g i o n . P r e f e r e n t i a l d e p o s i t i o n of i r o n - r i c h s l a g s has been s u g g e s t e d by o t h e r i n v e s t i g a t o r s (18,19) and was a l s o o b s e r v e d i n a s m a l l s c a l e p.c. t e s t combustor on an a i r - c o o l e d medium c a r b o n s t e e l probe (20). The f i n e p a r t i c l e l a y e r formed on the s u b s t r a t e s u r f a c e i s due t o c o n d e n s a t i o n on or t h e r m a l d i f f u s i o n t o the r e l a t i v e l y c o l d (300-350°C) s t e e l s u b s t r a t e (21). Its role i n d e p o s i t i n i t i a t i o n i s not y e t known. The weakest p o i n t i n the d e p o s i t o c c u r r e d i n the zone between the i n i t i a t i n g i r o n - r i c h p a r t i c l e s and the s i n t e r e d m a t e r i a l . This a l l o w e d removal o f the d e p o s i t down t o the i n i t i a l l a y e r by b r u s h ing with a f i n e b r i s t l e p a i n t brush or blowing with a high v e l o c i t y a i r j e t (25-30 m/sec). The f i n e p a r t i c l e l a y e r was removed by u s i n g d o u b l e - s i d e d s t i c k y tape. The s t r o n g l y bonded, i r o n - r i c h p a r t i c l e s had t o be s h e a r e d from the s u b s t r a t e s u r f a c e w i t h a razor blade. This suggests that conventional sootblowing techn i q u e s i n a u t i l i t y b o i l e r cannot remove a s t r o n g l y bonded i n i t i a t i n g l a y e r formed from s l a g d r o p l e t s bonded t o the s u r f a c e o r f i n e p a r t i c l e s d e p o s i t e d by c o n d e n s a t i o n o r t h e r m a l d i f f u s i o n and h e l d by Van der Waals f o r c e s . At h i g h e r flame temperatures the f l y ash p a r t i c l e s i z e appeared t o be r e d u c e d , presumably due t o a g r e a t e r degree o f b r e a k i n g up of i n d i v i d u a l b u r n i n g c o a l p a r t i c l e s , and the base o r i r o n - r i c h p a r t i c l e s and o v e r a l l d e p o s i t b u i d l - u p r a t e s were i n c r e a s e d . The Keystone c o a l i n p a r t i c u l a r gave drops which wet the s u r f a c e b e t t e r a t h i g h e r flame t e m p e r a t u r e s . The Keystone c o a l formed the most e x t e n s i v e d e p o s i t base of the t h r e e c o a l s a t the h i g h e s t flame temperature. The i r o n - r i c h p a r t i c l e s adhered to the o x i d i z e d s t e e l substrates a t temperatures as low as 310° C. S u r p r i s i n g l y , the same l a y e r o f i r o n - r i c h base d e p o s i t p a r t i c l e s was a l s o o b s e r v e d w i t h the low p y r i t e Decker c o a l . However, d e p o s i t i n i t i a t i o n and b u i l d - u p was r e d u c e d when the c o n c e n t r a t i o n of i o n - e x c h a n g e a b l e c a t i o n s i n the c o a l was r e d u c e d . S l a g d e p o s i t i n i t i a t i o n and b u i l d - u p were s e n s i t i v e t o b o t h flame and s t e e l s u b s t r a t e s u r f a c e t e m p e r a t u r e s , even though the gas temperature o f d e p o s i t i o n was h e l d c o n s t a n t . This suggests at l e a s t q u a l i t a t i v e agreement between r e s u l t s from the d r o p - t u b e f u r n a c e and t h o s e from the s t i c k i n g a p p a r a t u s (1,5). I t i s hypothes i z e d t h a t a h i g h e r flame ( m e l t i n g ) temperature g i v e s a more homo-
24.
ABBOTT AND
AUSTIN
Slag Deposit Initiation
Using a Drop-Tube
Furnace
351
geneous s l a g drop, which remains as a v i s c o u s , s t i c k y , s u p e r c o o l e d g l a s s a t d e p o s i t i o n t e m p e r a t u r e s , whereas i n h o m o g e n e i t i e s i n the m e l t a t lower temperatures a c t as n u c l e i t o g i v e more c r y s t a l l i z a t i o n on c o o l i n g . The h i g h c o n c e n t r a t i o n o f s u l f u r a t the d e p o s i t s t e e l i n t e r f a c e has been o b s e r v e d i n p.c. u t i l i t y b o i l e r s (22,23). The p r i n c i p a l q u e s t i o n s posed by the r e s u l t s of t h i s i n v e s t i g a t i o n were: (1) can a s h d e p o s i t s be formed i n the d r o p - t u b e f u r n a c e i n the absence o f i r o n - c o n t a i n i n g m i n e r a l s o r s l a g d r o p s ? ; (2) what r o l e , i f any, does the f i n e p a r t i c l e l a y e r p l a y i n d e p o s i t f o r m a t i o n ? ; (3) i s t h e r e any s i g n i f i c a n c e t o the h i g h s u l f u r con c e n t r a t i o n a t the d e p o s i t i n t e r f a c e and what i s the mechanism by w h i c h t h i s s u l f u r enrichment o c c u r s : does i t o r i g i n a t e from p y r i t e o r from c o n d e n s i n g s u l f a t e ? ; (4) what i n f l u e n c e do a l k a l i s have on s l a g d e p o s i t i n i t i a t i o n and b u i l d - u p ? To answer t h e s e q u e s t i o n s , i t i s p l a n n e d t o t e s t s y n t h e t i c c o a l / m i n e r a l m i x t u r e s of c o n t r o l l e d c o m p o s i t i o n p r e p a r e d by d i s p e r s i n g f i n e l y ground m i n e r a l s i n l i q u i d o r g a n i c polymer f o l l o w e d by s e t t i n g and s i z e r e d u c t i o n t o p.c. g r i n d . Acknowledgments T h i s s t u d y was funded by a g r a n t from the Department o f Energy, C o n t r a c t No. DE-FG22-80PC-30199. We would l i k e t o thank Babcock and W i l c o x A l l i a n c e R e s e a r c h C e n t e r ( A l l i a n c e , Ohio) f o r s u p p l y i n g the C r o l o y 1/2 s t e e l f o r the t e s t s u b s t r a t e s .
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6. 7. 8. 9. 10. 11. 12. 13.
Moza, A. K.; Austin, L. G. Fuel 1981, 60, 1057-64. Abbott, M. F.; Moza, A. K.; Austin, L. G. Fuel 1981, 60, 1065-72. Moza, A. K.; Austin, L. G. Fuel 1982, 61, 161-65. Abbott, M. F.; Austin, L. G. Fuel 1982, 61, 765-70. Abbott, M. F.; Conn, R. E.; Austin, L. G. "Studies on Slag Deposit Formation in Pulverized Coal Combustors. 5. The Effect of Flame Temperature, Thermal Cycling of the Steel Substrate, and Time on the Adhesion of Slag Drops to Oxidized Boiler Steels" Fuel, accepted. Abbott, M. F.; Austin, L. G. ibid 6. The Sticking Behavior of Slag from Three Pennsylvania Steam Coals" Fuel, accepted. Moza, A. K.; Strickler, D. W.; Austin, L. G., Proc. of Annual SEM Symp. Chicago, Il., 1979. Mims, C. Α.; Neville, M.; Quann, R. T.; Sarofim, A. F. Proc. from the National AIChE Mtg., Boston, Ma., Aug. 1979. Sarofim, A. F.; Howard, J. B.; Padia, A. S. Comb. Sci. and Tech., 1977, 16, 187-204. Winegartner, E. J.; Lin, C. J. ASME Paper No. 79-WA/Fu-1, Dec. 1979. Rees, D. "An Assessment of Pulverized Coal Fired Combustor Performance" DOE Combustion Contractors Mtg., Pittsburgh, Pa., March 1981. Moza, A. K. Fouling and Slagging Resulting from Impurities in Combustion Gases, Eng. Foundation, NY, NY, 1983, 231-44. Shiomoto, G. H.; Muzio, L. J.; Mansour, M. N. ibid, 363-73.
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14. Hamor, R. J.; Smith, I. W. Fuel 1971, 50, 394-404. 15. Altenkirch, R. E.; Peck, R. E.; Chen, L. S. Powder Tech. 1978, 20, 189-96. 16. Pigford, R. L. Chem. Eng. Prog. Symp. Series 17, 1955, 51, 79-92. 17. Bird, R. B.; Stewart, W. E.; Lightfoot, Ε. N. Transp. Phenomena John Wiley and Sons, Inc., 1950. 18. Borio, R. W.; Narcisco, R. R. J. of Eng. for Power 1979, 101(4), 500. 19. Bryers, R. W. J. of Eng. for Power 1979, 101, 506. 20. Austin, L. G.; Abbott, M. F.; Kinneman, W. P. ASME 83-JPGC -Pwr-42, Sept. 1983. 21. Rosner, D. E.; Gokoglu, S.; Israel, R. Fouling and Slagging Resulting from Impurities in Combustion Gases, 1983, Eng. Found., N.Y., N.Y., 235-56. 22. Hazard, H. R. Final Report, EPRI CS-1418, June 1980. 23. Fessler, R. R.; Skidmore, A. J.; Hazzard, H. R.; Dimmer, J. P. ASME Paper No. 79-WA/CD-1 1979. RECEIVED June 24, 1985
