Mineral Matter and Ash in Coal - American Chemical Society

13 Sulfur Solubility in Slags for Cyclone Coal Combustors David H. DeYoung

Downloaded by UNIV OF PITTSBURGH on May 4, 2017 | http://pubs.acs.org Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch013

Smelting Process Development Division, Alcoa Laboratories, New Kensington, PA 15068

The absorption of sulfur by coal slags has been investigated to evaluate in-situ desulfurization of gases in coal combustors. Slag compositions which consisted of coal ash and inorganic additives, and which had low fusion temperatures and high capacities for sulfur, were identified. The sulfide capacities [(% S) (P /P )½] of these slags were measured at 1000 to 1300°C, and at oxygen potentials ranging from 10 to 10 atm, respectively. Results for slags based on the FeO-Al O -SiO , CaO-Al O -SiO , and Na O-Al O -SiO systems showed that at a given basicity the sulfide capacities were ranked in the order FeO > CaO > Na O. These results were used to quantitatively evaluate desulfurization in a staged, slagging, cyclone combustor. slag

O2

S2

-13

-11

2

2

2

3

3

2

2

3

2

2

2

T h i s study was conducted t o s e l e c t p o t e n t i a l s l a g c o m p o s i t i o n s f o r use i n a s l a g g i n g , s t a g e d , c y c l o n e c o a l combustor, and t o obtain the necessary data t o evaluate the d e s u l f u r i z i n g a b i l i t y o f t h e combustor. The f i r s t s t a g e o f such a combustor would be o p e r a t e d q u i t e r e d u c i n g t o f a c i l i t a t e s u l f u r removal by a s l a g formed from t h e c o a l ash and i n o r g a n i c a d d i t i v e s (e.g., lime). A t a n g e n t i a l motion imparted t o t h e gas would throw ash, c o a l , and a d d i t i v e s t o t h e combustor w a l l where they would combine t o form a molten s l a g . T h i s s l a g , c o n t a i n i n g some d i s s o l v e d s u l f u r , would c o n t i n u a l l y d r a i n out o f a t a p h o l e at t h e e x i t end o f t h e h o r i z o n t a l l y - p l a c e d c y l i n d r i c a l combustor. Advantages o f t h i s t y p e o f combustor a r e removal o f some s u l f u r , low p a r t i c u l a t e e m i s s i o n s , and low N0 emissions. T h i s paper w i l l be d i v i d e d i n t o t h r e e p a r t s . F i r s t , the s e l e c t i o n o f s l a g c o m p o s i t i o n s w i l l be o u t l i n e d . Second, s u l f i d e c a p a c i t y measurements o f t h e s e s l a g s w i l l be d i s c u s s e d . Third, the d e s u l f u r i z i n g p o t e n t i a l of a slagging, cyclone combustor w i l l be e v a l u a t e d u s i n g t h e s e measurements. X

0097-6156/ 86/ 0301 -0170506.00/ 0 © 1986 American Chemical Society

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

DE YOUNG

Sulfur Solubility

Slag Composition

171

in Slags

Selection

The s t r a t e g y was f i r s t t o s e l e c t p o s s i b l e a d d i t i v e s , t h e n l o c a t e phase diagrams f o r systems o f major ash components p l u s a d d i t i v e s , and f i n a l l y , s e l e c t l o w - m e l t i n g e u t e c t i c c o m p o s i t i o n s as c a n d i d a t e s l a g s . A d d i t i v e s were chosen f o r t h e i r known a b i l i t y t o form l o w - m e l t i n g s i l i c a t e s ( e . g . , t h e a l k a l i s ) or f o r t h e i r known a b i l i t y f o r d e s u l f u r i z a t i o n ( e . g . , t h e a l k a l i n e e a r t h e l e m e n t s ) . An e a s t e r n c o a l was used f o r t e s t s o f a p i l o t combustor. I t s ash c o m p o s i t i o n , used t o c a l c u l a t e a d d i t i v e compositions, i s given i n Table I. Major components, S1O2, AI2O3, and F e 0 3 , account f o r a p p r o x i m a t e l y 80% o f t h e ash. T e r n a r y phase diagrams f o r t h e S i 0 2 ~ A l 2 0 3 ~ a d d i t i v e and S i 0 2 ~ F e O - a d d i t i v e systems were i n v e s t i g a t e d f o r p o s s i b l e s l a g compositions. U n l e s s o t h e r w i s e n o t e d , a l l phase diagrams were t a k e n from L e v i n , et a l (1-3) or Roth, et a l ( 4 ) . The s e l e c t e d c o m p o s i t i o n s which were t e s t e d a r e g i v e n i n T a b l e I I , as are estimated l i q u i d u s temperatures. Obviously, the l i q u i d u s temperature o f t h e s l a g c o n s i s t i n g o f c o a l ash and t h e a d d i t i v e w i l l be d i f f e r e n t from t h o s e g i v e n by t h e phase diagrams because o f t h e minor components o f t h e ash. However, t h e phase diagrams p r o v i d e r e a s o n a b l e i n i t i a l s e l e c t i o n s . A d d i t i v e c o m p o s i t i o n s and q u a n t i t i e s a r e g i v e n i n T a b l e I I I .


2

S u l f i d e C a p a c i t y Measurements S u l f i d e c a p a c i t i e s o f t h e s e l e c t e d s l a g c o m p o s i t i o n s were measured t o r a t e t h e s l a g s and t o p r o v i d e d a t a f o r e v a l u a t i o n of t h e o p e r a t i o n o f a combustor w i t h t h e s e s l a g s . Chemistry o f S u l f u r i n S l a g s . There has been c o n s i d e r a b l e r e s e a r c h on t h e c h e m i s t r y o f s u l f u r i n s l a g s r e p o r t e d i n t h e literature. Most was aimed toward u n d e r s t a n d i n g and improving t h e d e s u l f u r i z a t i o n o f i r o n and s t e e l . These s t u d i e s (5-7) have shown t h a t at h i g h oxygen p o t e n t i a l s s u l f u r d i s s o l v e s i n s l a g s as a s u l f a t e , and at low oxygen p o t e n t i a l s , t h e c o n d i t i o n r e l e v a n t t o t h e two-stage combustor, s u l f u r d i s s o l v e s as a sulfide. T h i s can be r e p r e s e n t e d by t h e r e a c t i o n , 2

S ( g ) + (o ~)=

1/2

1/2 0 ( g )

2

2

+ (s2-).

(1)

A q u a n t i t y c a l l e d t h e s u l f i d e c a p a c i t y (6) can be d e f i n e d a s :

=

(wt % S)

I 3 _ ]

1

/

2

(2)

where wt % S r e f e r s t o s u l f u r d i s s o l v e d i n t h e s l a g , and P Q and P s a r e t h e p a r t i a l p r e s s u r e s o f oxygen and s u l f u r i n t h e atmosphere w i t h which t h e s l a g i s e q u i l i b r a t e d . The s u l f i d e c a p a c i t y f o r many s l a g s has been found C5,6) t o be independent of s u l f u r and oxygen p o t e n t i a l s f o r wide r a n g e s , and t h e r e f o r e is a useful quantity for rating slags. One e x c e p t i o n r e l e v a n t t o 2

9


172

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

Table I .

Ash From L o v e r i d g e Seam (West V i r g i n i a ) C o a l

Component A1 0 Si0 Fe 0 CaO MgO Na 0 K 0 Ti0 2

Wt P e t . 18.4 44.5 15.9 4.56 1.08 1.10 1.11 1.20 9.02 0.33

3

2

2

3

2


2

2

so p o 3

2

Ash A n a l y s i s N o r m a l i z e d ( e x c l u d i n g s u l f u r and t a k i n g i r o n as FeO)

5

24.1 47.8 17.4 4.9 1.2 1.5 1.3 1.1 0.5

Table I I . Normalized _ _ _ _ _ _

Composition, S l a g No.

S i

°2

A 1

2°3

F

e

°

2-A-l 2-A-2 2-A-3 2-A-7 2-A-8 2-A-9 2-A-10

46.0 40.0 18.1 43.3 35.3 27.0 23.6

2-B-l 2-B-2 2-B-3 DSE-1 DSE-2

39.3 56.4 33.2 42.5 37.0

48.0 21.8 26.2 28.9 26.0

2-C-l

37.7

46.4

2-D-l 2-D-2 2-D-3

43.8 62.7 61.6

18.2 23.2 12.2

2-E-l

42.1

20.1

2-1-1

55.3

21.5

19.9 12.0 5.9 19.8 14.1 8.3 5.9

Compositions o f C a n d i d a t e Major Components Only

L i q u i d u s Temperature (°C) (Major

Wt. P e t . N

a 2

°

C

a

0

34.1 48.0 76.0 36.9 50.6 64.4 70.4

Components O n l y )

1205 1083 1148 1220 1200 1150 1155 12.7 21.8 40.6 28.6 37.0

1000 1050 900

15.7 37.9 14.0 26.2

13.1

1093 915 1063 732

37.8 10.0

Slags -

1265 990



2

75.5 10.9 43.8 46.3 6.3

0.61 0.93 0.40 1.60 0.43 0.99

2-C-l

2-D-l 2-D-2 2-D-3

2-E-l

2-1-1

49.1

75.8 81.5

24.2 18.5

0.69 0.97

DSE-1 DSE-2

37.4

89.1 56.2 53.7

34.2 96.1 81.1

2

65.7 3.9 18.9

3

0.69 0.30 1.20

77.1 73.0 92.6 87.2 92.6 92.6 86.1 86.1 86.1 86.1 73.5 86.7 86.1

2

2-B-l 2-B-2 2-B-3

3

3

93.7

24.5

2.5

2

5

3

2

14.7 % MgO 3.4 % C a F

2

2

Other

5.8 % C a F 5.8 % B 0 5.8 % P 0

A d d i t i v e C o m p o s i t i o n , Wt. P e t . Na C0 CaO Fe 0

13.5

2

A1 0

0.34 1.20 3.52 3.74 3.52 3.52 0.36 0.89 2.19 3.47 4.07 3.49 4.99

22.9 27.0 7.4 7.0 1.6 1.6 13.9 13.9 13.9 13.9 11.8 7.5 13.9

sio

A d d i t i v e Compositions For Candidate Slags

2-A-l 2-A-2 2-A-3 2-A-4 2-A-5 2-A-6 2-A-7 2-A-8 2-A-9 2-A-10 2-A-10b 2-A-ll 2-A-12

S l a g No.

A d d i t i v e Mass g/g Ash

Table I I I .


174



t h i s study i s t h a t , f o r s l a g s c o n t a i n i n g FeO, Cg i s expected t o change w i t h oxygen p o t e n t i a l as the r a t i o o f f e r r o u s t o f e r r i c i o n s i n t h e s l a g changes. A r e v i e w o f t h e l i t e r a t u r e (5-22) showed t h a t v i r t u a l l y a l l work on s u l f u r i n s l a g s was on systems r e l e v a n t t o t h e d e s u l f u r i z a t i o n o f i r o n and s t e e l and at temperatures r a n g i n g from 1400-1600°C. No d a t a were found f o r l o w - m e l t i n g s l a g s ( l i q u i d u s t e m p e r a t u r e s , a p p r o x i m a t e l y 1000-1100°C), and p a r t i c u l a r l y f o r t h e i r o n - a l k a l i - a l u m i n o s i l i c a t e s from which many o f t h e proposed c o m p o s i t i o n s a r e composed. T h e r e f o r e , e x p e r i m e n t a l measurements were n e c e s s a r y t o o b t a i n t h e d a t a needed f o r s e l e c t i o n o f s l a g s . E x p e r i m e n t a l Method. An e q u i l i b r a t i o n t e c h n i q u e was chosen t o measure t h e s u l f i d e c a p a c i t i e s o f t h e c a n d i d a t e s l a g s . Slag samples were e q u i l i b r a t e d w i t h a CO-C02"*S0 gas m i x t u r e h a v i n g f i x e d oxygen and s u l f u r p o t e n t i a l s , quenched t o room temperature, and a n a l y z e d f o r s u l f u r . S u l f i d e c a p a c i t i e s were t h e n c a l c u l a t e d from t h e s u l f u r c o n c e n t r a t i o n s u s i n g E q u a t i o n 2. T h i s t e c h n i q u e was chosen because i t i s a d i r e c t method, and because t h e oxygen and s u l f u r p o t e n t i a l s c o u l d be a c c u r a t e l y c o n t r o l l e d , and, i f n e c e s s a r y , t h e s e c o u l d be s e t t o match t h e a c t i v i t i e s f o r oxygen and s u l f u r which were a n t i c i pated i n t h e a c t u a l c o a l combustor. The apparatus used f o r s u l f i d e c a p a c i t y measurements i s shown s c h e m a t i c a l l y i n F i g u r e 1. S l a g s were p r e p a r e d by m i x i n g preweighed amounts o f a d d i t i v e s and c o a l a s h . The c o a l ash was o b t a i n e d from Bituminous C o a l R e s e a r c h , I n c . I t was p r e p a r e d by a s h i n g L o v e r i d g e Seam, West V i r g i n i a c o a l i n a i r at 750°C, f o l l o w e d by a r e d u c t i o n i n a 60%CO-40%CO2 gas at 1000°C, t h e n c o o l e d under nitrogen. The gas c o m p o s i t i o n s f o r each experiment were chosen t o o b t a i n as low an oxygen p o t e n t i a l as p o s s i b l e , w i t h o u t r e d u c i n g FeO t o Fe m e t a l . They were a l s o chosen t o o b t a i n as low a s u l f u r p o t e n t i a l as p o s s i b l e t o match a n t i c i p a t e d c o n d i t i o n s i n t h e a c t u a l combustor, yet l a r g e enough so t h a t they c o u l d be p r e p a r e d by m i x i n g g a s e s . The e q u i l i b r a t i o n time f o r s l a g samples was determined by p e r i o d i c a n a l y s e s o f t h e gas e x i t i n g the r e a c t o r . Quenched s l a g samples were a n a l y z e d f o r s u l f u r u s i n g a Leco t i t r a t o r , and were a n a l y z e d f o r S i , A l , Fe, Na, K, Ca, Mg, T i , and Ρ by atomic a b s o r p t i o n . 2

Results. T a b l e IV g i v e s t h e r e s u l t s f o r a l l s u l f i d e c a p a c i t y measurements. F i g u r e s 2 and 3 show t e r n a r y phase diagrams f o r s e l e c t e d systems on which t h e r e s u l t s are shown. Compositions shown were o b t a i n e d by t a k i n g t h e t h r e e major components from t h e s l a g a n a l y s e s and n o r m a l i z i n g t o 100%. The s u l f i d e c a p a c i t i e s a r e shown as a f u n c t i o n o f b a s i c i t y i n F i g u r e 4, which summarizes a l l r e s u l t s o f t h i s s t u d y . Molar b a s i c i t i e s (Emole f r a c t i o n bases/Emole f r a c t i o n a c i d s ) were c a l c u l a t e d from the s l a g analyses. I t was found t h a t a f t e r e q u i l i b r a t i o n w i t h t h e s u l f u r i z i n g gas, c e r t a i n s l a g s i n t h e F e O - A l 2 0 3 - S i 0 system c o n s i s t e d o f two i m m i s c i b l e l i q u i d s at 1100°C. One phase was a 2


13.

DEYOUNG

Sulfur

Solubility

175

in Slags

Radiation Shields


/Gas Out

Closed-End Alumina Tube

Stainless Steel Flange

'Alumina Reaction Tube Sample Tray

F i g u r e 1.

Reactor

used f o r s u l f i d e c a p a c i t y measurements.

F i g u r e 2. The FeO-Al 0 - S i 0 phase diagram (1) w i t h measured s u l f i d e c a p a c i t i e s i n d i c a t e d . Oxide phases i n e q u i l i b r i u m with m e t a l l i c iron. 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 1. C o p y r i g h t 1964 American Ceramic S o c i e t y . 3

2


176


T a b l e IV. Exp't. No.

S u l f i d e C a p a c i t y Measurements Run Time (h)

Τ (°C)


1100

24

1100

115

US l L aa gg Composition

γ

Wt % S

S

4.22 5.34

-4.31 -4.21

2-B-l 2-B-2 2-D-l 2-D-f 2-D-3

6.05 0.26 0.96 0.87 0.69

-4.16 -5.52 -4.96 -5.00 -5.10

DSE-1 2-A-l 2-A-2 2-A-3 2-A-4 2-C-l

4.31 4.80

-4.06 -4.01

22.5 23.5 7.91

-3.34 -3.32 -3.79

2-A-5? 2-A-6 2-A-7 2-A-8 2-A-9 2-A-10^ 2-A-ll 2-B-3

20.2 13.4 5.06 9.81 16.6 19.1

-3.38 -3.56 -3.99 -3.70 -3.47 -3.41

2.65

-4.27

2-Α-ί 2-A-4 2-A-5* 2-A-6 2-A-l(£ 2-A-ll? 2-A-12^ 2-A-3a 2-1-1

25.4 25.1 25.5 24.1 23.6 24.2 25.8 26.3 0.44

-5.04

2-A-3

2.67 1.97 3.10

-2.86 -3.00 -2.80

d

e

144.6

g

DSE-1

6

1100

-,

°

e

e

d

7

f

1000

168.75

b

b

K

b

11 §

1300

70.0

2-A-10 2-A-10b

-14 = 7.2 χ 10' '20 ^ Gas c o m p o s i t i o n : 70.2% C0X

—4 = 5.4 χ 10 .

s

b

D i d not m e l t .

c

9

2 2 9 t 6.°/ % nry C 0 -_0. . 2 5 % au S0 . X χ 1 10U ; ; Λ X - = 1.6 χ 1 θ " x = O6.8 .O Χ ι e °22 . d_ , e_ ff 2^2 Sample c r e p t o u t . Two phases. Gas c o m p o s i t i o n : 74.5% C0o

1 4

9

u

0

Z - J / o

4

n

r

2

25.3%

S

4

C0 -0.18% S 0 .

= 6.8 χ 10 ; X = 1.6 χ 1 θ " . 2 2 Gas composition: 66.3% CO-33.6% C0 -0.14% S 0 . Χ = 4.9 χ -11 -4 °2 10 ; X = 1.9 χ 10 9

z

0

1

c

u

b

g

6

o

2

o

Λ

2

c


DEYOUNG

Sulfur Solubility

in Slags

s.o

2


(17,3 ΐ5·!

2-B-3 Ο

charged composition

•

analyzed after experiment

F i g u r e 3. The F e 0 - N a 0 - S i 0 phase diagram (2) w i t h measured sulfide capacities indicated. Oxide phases i n e q u i l i b r i u m with m e t a l l i c iron. 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 1969 American Ceramic S o c i e t y . ?

I

-3

9

I I I I I I Τ = 1300X

^ w/MgO

h

w/CaO

w/B 0 2

3

CO

Ο

0> Ο

System · , Δ 2-A

FeO - A l 0

- Si0

2

FeO - N a 0 - S i 0

2

2

Ο 2-B A 2-C -

3

2

FeO - CaO - S i 0

2

5

I

I Q|

I I

I

I

• 2-D

Na 0 - A l 0

•

CaO - A l 0

2

2-E

2

2

I I I '

'

ι

« I

3

3

- Si0 - Si0

» '

2

2

J J

'

Molar basicity Figure 4. Measured s u l f i d e c a p a c i t i e s o f c a n d i d a t e s l a g s . B a s i c i t y c a l c u l a t e d from c h e m i c a l a n a l y s e s . P o i n t s denoted by W/P2O5 and W/B2O3, r e p r e s e n t s l a g s i n which 5% S i 0 was r e p l a c e d by 5% o f t h e s e o x i d e s . P o i n t denoted by w/CaO i s f o r s l a g t o which 5% C a F was added, but a l l f l u o r i n e was l o s t d u r i n g e x p e r i m e n t . P o i n t denoted by w/MgO f o r s l a g t o which 12% MgO was added. 2

2


178



glass. The o t h e r phase had a m e t a l l i c appearance and w i l l be r e f e r r e d t o as t h e "matte phase". The s u l f i d e c a p a c i t i e s f o r t h e s e s l a g s s h o u l d be c o n s i d e r e d as " a p p a r e n t " because t h e s u l f i d e c a p a c i t y i s d e f i n e d f o r a s i n g l e l i q u i d phase. The matte phase i n a l l o f t h e s e double-phased s l a g s c o n t a i n e d 27-31% S, w h i l e t h e g l a s s phase c o n t a i n e d from 0.2 t o 13% S. X-ray d i f f r a c t i o n a n a l y s e s showed t h e g l a s s phase t o be amorphous and t h e matte phase t o c o n t a i n FeS and FeS2* Discussion. S l a g c o m p o s i t i o n s 2 - A - l , 2-A-7, 2-B-2, 2 - C - l , 2-D-2, and 2 - E - l were c l o s e s t t o t h e c o a l ash c o m p o s i t i o n g i v e n i n T a b l e I , c o n t a i n i n g 25-38% a d d i t i v e . As seen from T a b l e IV, l o g Cg ranged from a p p r o x i m a t e l y -3.8 t o -5.5 at 1100°C. U s i n g t h e b a s i c i t y o f t h e ash c a l c u l a t e d from T a b l e I and t h e d a t a shown i n F i g u r e 4, t h e s u l f i d e c a p a c i t y f o r pure ash i s e s t i m a t e d t o be a p p r o x i m a t e l y l o g Cg = -5.2. This i s quite low as compared t o r e s u l t s o b t a i n e d f o r s l a g s c o n t a i n i n g s i g n i f i c a n t quantities of additives. As w i l l be demonstrated l a t e r i n t h e r e p o r t , s u l f u r c a p t u r e d by c o a l ash s l a g w i t h t h i s s u l f i d e c a p a c i t y would be i n s i g n i f i c a n t even at v e r y f a v o r a b l e c o n d i t i o n s - v e r y low oxygen p o t e n t i a l and low t e m p e r a t u r e . There i s a g e n e r a l c o r r e l a t i o n between s u l f i d e c a p a c i t y and b a s i c i t y f o r a g i v e n system, as shown i n F i g u r e 4. There i s a s h a r p drop i n s u l f i d e c a p a c i t y between b a s i c i t i e s o f 1.0 t o 0.5, which c o r r e s p o n d s t o t h e m e t a s i l i c a t e t o d i s i l i c a t e c o m p o s i t i o n s in a binary s i l i c a t e . F o r a g i v e n b a s i c i t y , systems 2-A (FeO) and 2-C (FeO, CaO) have s i g n i f i c a n t l y h i g h e r s u l f i d e c a p a c i t i e s t h a n systems 2-D (Na20) and 2-E (CaO), so t h a t f o r a g i v e n b a s i c i t y , FeO i s s u p e r i o r t o CaO and Na20 as an a d d i t i v e . T h i s i s not what would be expected c o n s i d e r i n g t h e s t a n d a r d f r e e e n e r g i e s o f f o r m a t i o n o f t h e s u l f i d e s and o x i d e s o f Fe, Ca, and Na. C o n s i d e r i n g standard f r e e energies f o r the formation of m e t a l s u l f i d e s from m e t a l o x i d e s , FeO and CaO s h o u l d be a p p r o x i m a t e l y e q u i v a l e n t d e s u l f u r i z e r s and N a 0 s h o u l d be superior. However, s l a g s a r e f a r from i d e a l s o l u t i o n s because o f t h e s t r o n g i n t e r a c t i o n s among s p e c i e s — p a r t i c u l a r l y with Si0 . T h i s i s why e x p e r i m e n t a l measurements o f s u l f i d e c a p a c i t i e s were needed. F r e e energy o f m i x i n g d a t a (24) f o r N a 0 , CaO, and FeO b i n a r y s i l i c a t e s show t h a t t h e c h e m i c a l i n t e r a c t i o n w i t h s i l i c a d e c r e a s e s i n t h e o r d e r N a 0 , CaO, FeO, and f o r a g i v e n b a s i c i t y , t h e a c t i v i t y o f t h e b a s i c o x i d e i n t h e s i l i c a t e s i n c r e a s e i n t h e o r d e r Na20, CaO, and FeO. On t h i s b a s i s , FeO s h o u l d be a b e t t e r d e s u l f u r i z e r t h a n CaO or N a 0 . T h i s i s c o n s i s t e n t with the present r e s u l t s . Not s u r p r i s i n g l y , t h e m e t a l o x i d e - s i l i c a i n t e r a c t i o n i s a major f a c t o r i n t h e d e s u l f u r i z a t i o n a b i l i t y of the s l a g . S e v e r a l m o d i f i c a t i o n s o f s l a g s based on t h e F e O - A l 0 3 - S i 0 2 system were t e s t e d t o determine i f a l e s s e x p e n s i v e a d d i t i v e c o u l d be s u b s t i t u t e d f o r some o f t h e i r o n o r i f a d d i t i v e s c o u l d be used t o reduce l i q u i d u s t e m p e r a t u r e s . F i g u r e 4 shows t h a t r e p l a c i n g a p o r t i o n o f t h e i r o n o x i d e i n s l a g s of the F e O - A l 0 3 - S i 0 system w i t h CaO (5 wt %) or MgO (12 wt %) had no e f f e c t on t h e s u l f i d e c a p a c i t i e s . Results f o r c o m p o s i t i o n 2 - C - l a l s o support t h i s c o n c l u s i o n , because f o r 2

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t h i s c o m p o s i t i o n a p p r o x i m a t e l y 14% o f t h e FeO o f an e q u i v a l e n t c o m p o s i t i o n i n t h e F e O - A l 0 3 - S i 0 2 system was r e p l a c e d by CaO, w i t h o n l y a s l i g h t d e c r e a s e i n s u l f i d e c a p a c i t y . Replacement o f p o r t i o n s o f t h e S 1 O 2 i n F e O - A l 2 0 3 - S i 0 s l a g s by B 0 3 or P 0 5 has no e f f e c t on s u l f i d e c a p a c i t i e s ; t h e r e f o r e , these a d d i t i v e s are p o t e n t i a l l y u s e f u l f o r reducing s l a g l i q u i d u s temperatures. F i g u r e 5 compares some r e s u l t s o f t h i s study t o t h o s e found i n t h e l i t e r a t u r e f o r s i m i l a r s l a g s at h i g h e r t e m p e r a t u r e s . These l i t e r a t u r e d a t a were a d j u s t e d t o a b a s i c i t y o f 1.65 u s i n g d a t a g i v e n i n F i g u r e 4. The d a t a from t h i s study a r e q u i t e consistent with the l i t e r a t u r e data. The l i n e a r i t y o f t h e s u l f i d e c a p a c i t y w i t h i n v e r s e temperature i s c o n s i s t e n t w i t h t h e theoretical relationship, 2

2


2

2

L J ^ i

=

- AH° /R * H^o/R - H? /* e

eS

(3)

Ο

where Δ Η 1/2 and

.

i s t h e s t a n d a r d e n t h a l p y change f o r t h e S (g)

+ FeO

HÎ?

and

2

λ

FeO

1/2

=

H*J

0 ( g ) + FeS 2

reaction, (4)

„

FeS

a r e t h e p a r t i a l molar e n t h a l p i e s o f m i x i n g o f FeO and FeS i n t h e slag. T h i s r e l a t i o n s h i p can be d e r i v e d from E q u a t i o n 2, t h e e q u i l i b r i u m c o n s t a n t f o r E q u a t i o n 4, and t h e G i b b s - H e l m h o l t z equation. Most o f t h e s l a g s t e s t e d were found t o have been p a r t i a l l y or c o m p l e t e l y melted at 1100°C. However, at 1000°C, a l l s l a g s from t h e F e O - A l 0 3 - S i 0 2 system were not m o l t e n . It is thought t h a t f o r t h e s e c o m p o s i t i o n s t h e e n t i r e a d d i t i v e r e a c t e d w i t h s u l f u r s p e c i e s i n t h e atmosphere w h i l e none r e a c t e d w i t h t h e S 1 O 2 or w i t h o t h e r components i n t h e ash. Hence, s u l f i d e c a p a c i t i e s measured from t h i s experiment are not t r u e s u l f i d e c a p a c i t i e s of the s l a g s . Another p o i n t which s u p p o r t s t h i s i s t h a t t h e "measured" s u l f i d e c a p a c i t i e s at 1000°C are g r e a t e r t h a n t h o s e at 1100°C, w h i l e F i g u r e 5 shows t h e o p p o s i t e t r e n d for r e s u l t s f o r molten s l a g s . A l s o , o t h e r l i t e r a t u r e d a t a show t h a t s u l f i d e c a p a c i t i e s g e n e r a l l y i n c r e a s e w i t h temperature. T h i s p o i n t s out an i n h e r e n t d i s a d v a n t a g e i n u s i n g a c o a l ash s l a g f o r d e s u l f u r i z a t i o n . When s i l i c a r e a c t s w i t h t h e d e s u l f u r i z i n g agent, e.g., l i m e , t h e e f f e c t i v e n e s s o f t h e d e s u l f u r i z i n g compound i s g r e a t l y r e d u c e d . Hence, i t i s d e s i r a b l e t o d e s i g n a d e s u l f u r i z i n g combustor i n which t h e ash does not r e a c t w i t h t h e d e s u l f u r i z i n g m a t e r i a l . 2

E v a l u a t i o n Of A P i l o t

Combustor

Calculations. The measured s u l f i d e c a p a c i t i e s were used t o e s t i m a t e s u l f u r e m i s s i o n s from a s t a g e d , s l a g g i n g , c y c l o n e combustor o p e r a t i n g c l o s e t o e q u i l i b r i u m , and t o determine t h e


180

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A N D A S H IN

COAL


e f f e c t s o f v a r i o u s o p e r a t i n g v a r i a b l e s on the s u l f u r r e m o v a l . To c a l c u l a t e s u l f u r e m i s s i o n s , an e q u a t i o n f o r g a s - s l a g c h e m i c a l e q u i l i b r i u m f o r s u l f u r ( E q u a t i o n 2) and a mass b a l a n c e f o r s u l f u r are s o l v e d s i m u l t a n e o u s l y . F i r s t , t h e e q u i l i b r i u m gas c o m p o s i t i o n s were c a l c u l a t e d f o r t h e combustion o f c o a l w i t h a i r f o r a range o f s u l f u r c o n c e n t r a t i o n s i n t h e c o a l . T h i s was done u s i n g A l c o a ' s C h e m i c a l E q u i l i b r i u m Computer Program ( 2 3 ) . Next, the c o n c e n t r a t i o n s o f s u l f u r i n t h e s l a g s f o r e q u i l i b r i u m w i t h t h e combustion gases were c a l c u l a t e d . F i n a l l y , t h e q u a n t i t y o f a d d i t i v e s needed t o o b t a i n t h e s e c o m p o s i t i o n s were c a l c u l a t e d from s u l f u r mass b a l a n c e s . Results. F i g u r e 6 shows an example o f t h e r e s u l t s f o r t h e s e c a l c u l a t i o n s , f o r combustion w i t h 55% o f s t o i c h i o m e t r i c a i r ( s t a g e 1) at 1100°C. T h i s i s an i s o t h e r m a l c a l c u l a t i o n , i . e . , b o t h s l a g and gas t e m p e r a t u r e s are assumed t o be 1100°C. O b v i o u s l y , t h i s cannot o c c u r i n p r a c t i c e , but t h e r e s u l t s o f t h e c a l c u l a t i o n s h o u l d p r o v i d e an upper bound f o r s u l f u r r e m o v a l . A r e a s o n a b l e g o a l f o r the s u l f u r c a p t u r e , c o n s i d e r i n g p r o j e c t i o n s o f f u t u r e EPA r e g u l a t i o n s , i s 70%. The s l a g mass can v a r y between 85 and 350 g/kg c o a l ( t h e upper l i m i t was e s t a b l i s h e d from a heat b a l a n c e f o r A l c o a ' s p i l o t combustor), so t h e n e c e s s a r y l o g Cg f o r a 70% s u l f u r removal i s between -2.75 and -3.3. A s u l f i d e c a p a c i t y of l o g C « -3.3 at 1100°C was o b t a i n e d f o r c e r t a i n s l a g s based on t h e FeO-Al203-Si02 system, e.g., c o m p o s i t i o n s 2-A-3 or 2-A-10. T h i s shows t h a t 70% s u l f u r removal i s t h e r m o d y n a m i c a l l y p o s s i b l e . As t h e combustion s t o i c h i o m e t r y i s d e c r e a s e d , t h e c u r v e s i n F i g u r e s 6 are r o t a t e d c o u n t e r c l o c k w i s e about t h e o r i g i n , i . e . , t h e s u l f u r removal i s i n c r e a s e d . An i n c r e a s e i n temperature w i l l have t h e o p p o s i t e e f f e c t . The c u r v e s are r o t a t e d c l o c k w i s e about t h e o r i g i n . However, f o r a p a r t i c u l a r s l a g c o m p o s i t i o n t h e s u l f i d e c a p a c i t y i n c r e a s e s w i t h t e m p e r a t u r e , as shown i n F i g u r e 5. The net r e s u l t o f t h e two o p p o s i n g e f f e c t s ( u s i n g t h e temperature b e h a v i o r shown i n F i g u r e 5) i s t h a t t h e s u l f u r removal d e c r e a s e s w i t h i n c r e a s i n g t e m p e r a t u r e . I n t h e range o f c o a l - s u l f u r c o n t e n t s i n v e s t i g a t e d , 2-6%, t h e f r a c t i o n o f s u l f u r removed by s l a g does not change w i t h s u l f u r content i n t h e coal. The t o t a l s u l f u r e m i t t e d i n c r e a s e s w i t h i n c r e a s i n g s u l f u r c o n c e n t r a t i o n i n t h e c o a l , but the s u l f u r removal by t h e s l a g also increases. s

A f i n a l p o i n t t o note r e g a r d i n g s u l f u r removal i s t h a t as the c o n c e n t r a t i o n o f hydrogen i n t h e combustion gases i s d e c r e a s e d , t h e s u l f u r removal by t h e s l a g w i l l i n c r e a s e . This i s due t o t h e h i g h s t a b i l i t y o f t h e h y d r o g e n - s u l f u r s p e c i e s , such as H S ( g ) , as compared t o t h e c a r b o n - s u l f u r s p e c i e s , such as COS. Thus, d r y i n g and c h a r r i n g o f c o a l would s i g n i f i c a n t l y i n c r e a s e t h e t h e o r e t i c a l removal o f s u l f u r by t h e s l a g . These c a l c u l a t i o n s assume g a s - s l a g e q u i l i b r i u m w i t h r e s p e c t to sulfur. T h i s i s p r o b a b l y o n l y approached at t h e g a s - s l a g s u r f a c e near t h e e x i t o f t h e f i r s t s t a g e . At t h e e n t r a n c e end o f t h e combustor, the c o n d i t i o n s would p r o b a b l y be more o x i d i z i n g t h a n c o n d i t i o n s c a l c u l a t e d from the o v e r a l l combustion s t o i c h i o m e t r y , ff and t h u s s u l f u r s o l u b i l i t y i n t h e s l a g would 2

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-T(°C) 1600

1400

I

I

1200 I I

I

I

1000 I

Molar basicity = 1.65 -

-2

—

• Present study • Fincham & Richardson.

-3


log C

8

= 2.55 - 8330/T

-4 I

I

I

I

5

I

I

I

I

I

I

6

I

I

I I

7

8

l/T x 1 0 ( K" ) 4

e

1

F i g u r e 5. Comparison o f r e s u l t s from t h i s study t o t h o s e o f Fincham and R i c h a r d s o n (6) f o r an i r o n s i l i c a t e w i t h molar b a s i c i t y o f 1.65· Data from Fincham and R i c h a r d s o n were f o r pure i r o n s i l i c a t e s w h i l e t h e s e from t h e p r e s e n t study c o n t a i n e d some c o a l a s h .

0 Ε 0

I

I

I

I

I

I

1—1

100 200 300 400 500 600 700 Slag mass, g/kg coal

F i g u r e 6. E q u i l i b r i u m s u l f u r removal by s l a g f o r a combustor o p e r a t i n g w i t h L o v e r i d g e Seam (West V i r g i n i a ) coal.



182


be l e s s t h a n t h a t c a l c u l a t e d . At some depth below t h e s l a g s u r f a c e near t h i s e n t r a n c e end o f t h e f i r s t s t a g e , t h e c o n d i t i o n s would be more r e d u c i n g t h a n t h o s e c a l c u l a t e d from t h e o v e r a l l combustion s t o i c h i o m e t r y . T h i s would r e s u l t i n increased s u l f u r s o l u b i l i t y . The a c t u a l combustion p r o c e s s and s u l f u r removal p r o c e s s e s a r e q u i t e complex, and t h e extent o f s u l f u r removal w i l l depend on t h e combustion k i n e t i c s . For example, c o n s i d e r two extreme s i t u a t i o n s . I n one, where most c o a l i s combusted a f t e r i t h i t s t h e s l a g g e d w a l l , s u l f u r removal s h o u l d be r e l a t i v e l y good. I n t h e o t h e r extreme, where a l l t h e c o a l i s combusted b e f o r e i t r e a c h e s t h e s l a g g e d w a l l , s u l f u r removal would be r e l a t i v e l y poor because i t would be dependent on mass t r a n s p o r t through t h e gas phase, and t h e gas has a r e l a t i v e l y short residence time. In summary, t h e k i n e t i c s o f t h e combustion p r o c e s s i s important w i t h r e g a r d t o s u l f u r removal. The k i n e t i c s must be c o n s i d e r e d e i t h e r by m o d e l l i n g or e x p e r i m e n t a t i o n b e f o r e a f i n a l judgment on d e s u l f u r i z a t i o n i n a s l a g g i n g , c y c l o n e combustor can be made. The r e s u l t s o f t h i s study show t h a t i t i s theoretically possible. Conclusions S u l f i d e c a p a c i t y measurements o f r e l a t i v e l y low m e l t i n g ( a p p r o x i m a t e l y 1100°C i n most c a s e s ) s l a g s based on t h e FeO-Al203-SiC>2, FeO-Na20-Si0 , FeO-CaO-Si0 , N a 2 0 - A l 0 3 - S i 0 , and C a O - A l 2 0 3 - S i 0 systems but composed o f c o a l ash + a d d i t i v e s , have shown t h a t t h e F e O - A l 0 3 - S i 0 - b a s e d s l a g s had t h e h i g h e s t s u l f i d e c a p a c i t i e s . For a given b a s i c i t y , t h e s u l f i d e c a p a c i t i e s c o u l d be ranked i n t h e f o l l o w i n g o r d e r : FeO-Al 03-Si0 > FeO-CaO-Si0 > FeO-Na20-Si02 > CaO-Al203-Si02 > N a 0 - A l 0 3 - S i 0 2 . The c h e m i c a l i n t e r a c t i o n o f t h e b a s i c o x i d e s w i t h s i l i c a appears t o be a dominant f a c t o r c o n t r o l l i n g t h e s u l f i d e c a p a c i t y . There was good c o r r e l a t i o n between s u l f i d e c a p a c i t y and s l a g b a s i c i t y , and s u l f i d e c a p a c i t i e s i n c r e a s e d with temperature. C a l c u l a t i o n s o f t h e e q u i l i b r i u m s u l f u r removal f o r a commercial combustor u s i n g t h e measured s u l f i d e c a p a c i t i e s , showed t h a t i t was t h e o r e t i c a l l y p o s s i b l e t o remove 70% o r more of t h e s u l f u r i n c o a l . The s u l f u r removal i n c r e a s e s w i t h d e c r e a s i n g temperature, d e c r e a s i n g combustion s t o i c h i o m e t r y i n t h e f i r s t s t a g e o f t h e b u r n e r , i n c r e a s i n g s l a g f l o w , and d e c r e a s i n g c o n t e n t o f hydrogen i n t h e f u e l . T h i s work showed t h a t a s l a g g i n g , c y c l o n e combustor can remove s u l f u r i n t o t h e s l a g , but k i n e t i c m o d e l l i n g and/or e x p e r i m e n t a t i o n i s needed t o prove whether o r not t h e concept w i l l work. 2

2

2

2

2

2

2

2

2

2

2

2

Acknowledgment T h i s work was sponsored by t h e U.S. Department o f Energy under C o n t r a c t No. DE-ACO7-78CS40037, " P u l v e r i z e d C o a l F i r i n g o f Aluminum M e l t i n g F u r n a c e s . "


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