Mineral Matter and Ash in Coal - American Chemical Society

25

Mineral Matter and Ash in Coal Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 01/22/16. For personal use only.

Influence of Segregated Mineral Matter in Coal on Slagging Richard W. Bryers Foster Wheeler Development Corporation, Livingston, NJ 07039

The mineral content of any given rank of coal is a key factor in sizing and designing a steam generator or reactor. The mineral content becomes even more important as the premium solid fuels are consumed, leaving reserves with continually increasing mineral concentrations and lower quality ash. The problem of dealing with lower quality ash in coal is compounded by the increase in size of steam generators and refinements imposed by economic constraints. Empirical indices, based on coal ash chemistry and ASTM ash fusion temperatures or viscosities, are presently used to rank coals according to the fireside behavior of the mineral matter. Unfortunately, the indices are only marginally satisfactory, as they do not relate to operating or design parameters and frequently are based on a coal ash chemistry quite different from that deposited on the furnace wall. Frequently, different coals with identical ash chemistry produce decidedly different slagging conditions in steam generators of identical design operated in the same mode. Variations in composition of the slag, when compared with the coal ash, suggest specific minerals are being selectively deposited on furnace walls depending upon their specific gravity, size, composition, and physicochemical properties. It is quite apparent there is a need for a better understanding of the impact of mineral composition, its size, and its association with other minerals and carbonaceous matter on fireside deposits. S l a g g i n g and F o u l i n g The m i n e r a l s i n c o a l a r e c o n v e r t e d t o a s h d u r i n g combustion. The p o r t i o n o f t h e f l y ash i m p a c t i n g on heat t r a n s f e r s u r f a c e s , which i s r e t a i n e d as d e p o s i t s , depends on t h e p a r t i c l e s i z e , i t s c h e m i s t r y and i t s p h y s i c o c h e m i c a l b e h a v i o r d u r i n g combustion i n t h e steam gene r a t o r f u r n a c e , as w e l l as subsequent c o o l i n g i n t h e c o n v e c t i v e heat 0097-6156/ 86/ 0301 -0353506.50/ 0 © 1986 American Chemical Society


354

MINERAL MATTER AND ASH IN COAL

recovery section. The type o f d e p o s i t formed f a l l s i n t o one o f two c a t e g o r i e s — f u r n a c e s l a g , o r bonded c o n v e c t i v e heat t r a n s f e r depos its. S l a g i s d e f i n e d as a m o l t e n ash d e p o s i t e d on f u r n a c e w a l l s i n zones s u b j e c t e d t o r a d i a n t heat t r a n s f e r , as shown i n F i g u r e 1. O c c a s i o n a l l y , when c o a l m o i s t u r e l e v e l s approach 50 t o 70 p e r c e n t and the flame temperature becomes e x c e p t i o n a l l y low, bonded d e p o s i t s i n s t e a d o f s l a g form on f u r n a c e w a l l s . F o u l i n g o c c u r s i n the convec t i v e heat t r a n s f e r zones. The p r o d u c t o f f o u l i n g i s a bonded d e p o s i t c o n s i s t i n g o f an aggregate o f dry p a r t i c u l a t e m a t t e r bound t o g e t h e r by a m o l t e n phase t h a t has wetted a d j a c e n t p a r t i c u l a t e and subse q u e n t l y become f r o z e n . The bonded d e p o s i t i s f r e q u e n t l y i n i t i a t e d by a melt on the tube s i d e l a y e r s o f d e p o s i t formed by the condensa t i o n o f s u b s t a n c e s , such as a l k a l i s u l f a t e , w i t h low vapor p r e s s u r e s and low m e l t i n g temperatures. Bonding o r s i n t e r i n g may a l s o o c c u r as a r e s u l t o f v i s c o u s o r p l a s t i c f l o w between p a r t i c l e s , a gas phase r e a c t i o n , s o l u t i o n and p r e c i p i t a t i o n , o r d i f f u s i o n t r a n s f e r between adjacent p a r t i c l e s . On o c c a s i o n , when the f u r n a c e e x p e r i e n c e s a temperature e x c u r s i o n , s l a g g i n g w i l l o c c u r i n the h i g h temperature gas p a s s e s . Mineral Matter

i n Coal

M i n e r a l s o c c u r r i n g i n c o a l , which are r e s p o n s i b l e f o r f i r e s i d e depos i t s , may be c l a s s i f i e d i n t o f i v e main groups. These i n c l u d e s h a l e , c l a y , s u l f u r , and c a r b o n a t e s . The f i f t h group i n c l u d e s a c c e s s o r y m i n e r a l s such as q u a r t z and minor c o n s t i t u e n t s l i k e the f e l d s p a r s [1]. S h a l e , u s u a l l y the r e s u l t o f the c o n s o l i d a t i o n o f mud, silt, and c l a y , c o n s i s t s o f many m i n e r a l s i n c l u d i n g i l l i t e and m u s c o v i t e - these are forms o f m i c a . K a o l i n i t e i s the most common c l a y m a t e r i a l [1]. The s u l f u r minerals include p y r i t e s with some m a r c a s i t e . M a r c a s i t e has the same c h e m i c a l c o m p o s i t i o n as p y r i t e s but a d i f f e r ent m i n e r a l o g i c a l s t r u c t u r e . S u l f u r i s a l s o p r e s e n t as o r g a n i c mat ter and o c c a s i o n a l l y as sulfate. The l a t t e r u s u a l l y occurs i n weathered c o a l such as i n o u t c r o p s . The amount o f s u l f a t e s u l f u r i n c o a l i s g e n e r a l l y l e s s than 0.01 p e r c e n t . G e n e r a l l y , 60 p e r c e n t o f the s u l f u r i n c o a l o c c u r s as p y r i t e , p a r t i c u l a r l y when the s u l f u r c o n c e n t r a t i o n i s low. At h i g h e r concen t r a t i o n s i t may run as much as 70 t o 90 p e r c e n t . The m i n e r a l p y r i t e o c c u r s i n c o a l i n d i s c r e t e p a r t i c l e s i n a wide v a r i e t y of shapes and s i z e s . The p r i n c i p a l forms are [ 1 - 6 ] :

ο ο ο

ο ο

Rounded masses c a l l e d s u l f u r b a l l s or n o d u l e s an i n c h o r more i n s i z e . Lens-shaped masses which are thought to be f l a t t e n e d s u l f u r balls. V e r t i c a l , i n c l i n e d v e i n s or f i s s u r e s f i l l e d with p y r i t e r a n g i n g i n t h i c k n e s s from t h i n f l a k e s up t o s e v e r a l i n c h e s thick. S m a l l , d i s c o n t i n u o u s v e i n l e t s o f p y r i t e s , a number o f which sometimes r a d i a t e from a common c e n t e r , S m a l l p a r t i c l e s , 2μ, o r v e i n l e t s d i s s e m i n a t e d i n the c o a l . M i c r o s c o p i c p y r i t e o c c u r s i n f i v e b a s i c morphology t y p e s :

F i g u r e 1. P i l o t Plant Slagging to a F u l l - S c a l e Steam G e n e r a t o r

FULL SCALE STEAM GENERATOR

and

Fouling

Combustor

Compared

PILOT SCALE PLANT


356



(a) framboids; (b) i s o l a t e d , well-defined crystals; ( c ) n o n s p h e r i c a l aggregates o f w e l l - d e f i n e d c r y s t a l s ; (d) i r r e g u l a r shapes; and ( e ) f r a c t u r e d f i l l i n g s [ 7 ] . The term, framboid, i s d e r i v e d from the F r e n c h word f o r r a s p b e r r y and thus r e f e r s t o n a t u r a l l y o c c u r r i n g s p h e r o i d a l c l u s t e r s o f hundreds o f c u b i c o r o c t a h e d r o n a l c r y s t a l s o f p y r i t e s [ 9 ] . All c o a l s c o n t a i n some o f the t h i r d and f i f t h forms o f p y r i t e s , and some c o a l s c o n t a i n a l l f i v e o f t h e p r i n c i p a l forms [6, 8,9]. The carbonates are mainly c a l c i t e , dolomite, or s i d e r i t e . The o c c u r r e n c e o f c a l c i t e i s f r e q u e n t l y b i m o d a l . Some c a l c i t e o c c u r s as i n h e r e n t a s h , w h i l e o t h e r c a l c i t e appears as t h i n l a y e r s i n c l e a t s and f i s s u r e s . I r o n can be p r e s e n t i n s m a l l q u a n t i t i e s as h e m a t i t e , a n k o r i t e , and i n some o f the c l a y m i n e r a l s such as i l l i t e . In a d d i t i o n t o the more common m i n e r a l s , s i l i c a i s p r e s e n t sometimes as sand p a r t i c l e s o r q u a r t z . The a l k a l i e s a r e sometimes found as c h l o r i d e s o r as s u l f a t e s but p r o b a b l y most o f t e n as f e l d s p a r s , t y p i c a l l y o r t h o c l a s e and a l b i t e . I n the case o f l i g n i t e s , u n l i k e bituminous and subbituminous, sodium i s n o t p r e s e n t as a m i n e r a l but i s p r o b a b l y d i s t r i b u t e d throughout the l i g n i t e as the sodium s a l t o f a h y d r o x y l group o r a c a r b o x y l i c a c i d group i n humic a c i d . C a l c i u m , l i k e sodium, i s bound o r g a n i c a l l y t o humic a c i d . Therefore, i t too i s u n i f o r m l y d i s t r i b u t e d i n the sample [ 1 0 ] . The term, " m i n e r a l m a t t e r " , u s u a l l y a p p l i e s t o a l l i n o r g a n i c , noncarbonaceous m a t e r i a l i n the c o a l and i n c l u d e s those i n o r g a n i c elements which may o c c u r i n o r g a n i c c o m b i n a t i o n . P h y s i c a l l y , the i n o r g a n i c m a t t e r can be d i v i d e d i n t o two g r o u p s - - i n h e r e n t m i n e r a l m a t t e r and e x t r a n e o u s m i n e r a l m a t t e r . Inherent m i n e r a l matter o r i g i n a t e s as p a r t o f the growing p l a n t l i f e from which c o a l was formed. Under the c i r c u m s t a n c e s , i t has a u n i f o r m d i s t r i b u t i o n w i t h i n the coal. I n h e r e n t m i n e r a l m a t t e r seldom exceeds 2 t o 3 p e r c e n t o f the coal [12]. Extraneous m i n e r a l matter g e n e r a l l y c o n s i s t s o f l a r g e b i t s and p i e c e s o f i n o r g a n i c m a t e r i a l t y p i c a l o f the s u r r o u n d i n g g e o l o g y . In some cases the extraneous m a t t e r i s so f i n e l y d i v i d e d and u n i f o r m l y d i s p e r s e d w i t h i n the c o a l i t behaves as i n h e r e n t m i n e r a l matter. C o a l p r e p a r a t i o n can s e p a r a t e some o f the extraneous a s h from the c o a l s u b s t a n c e , but i t seldoms removes any o f the i n h e r e n t mine r a l matter. The p h y s i c a l d i f f e r e n c e s between i n h e r e n t and extraneous ash are important n o t o n l y t o those i n t e r e s t e d i n c l e a n i n g c o a l b u t a l s o to those concerned w i t h the f i r e s i d e b e h a v i o r o f c o a l a s h . I n h e r e n t m a t e r i a l i s so i n t i m a t e l y mixed w i t h c o a l t h a t i t s thermal h i s t o r y i s l i n k e d t o the combustion o f the c o a l p a r t i c l e i n which i t i s contained. T h e r e f o r e , i t w i l l most l i k e l y r e a c h a temperature i n excess o f the gas i n the immediate s u r r o u n d i n g s . The c l o s e p r o x i m i t y o f each s p e c i e s w i t h e v e r y o t h e r s p e c i e s p e r m i t s c h e m i c a l r e a c t i o n and p h y s i c a l changes t o o c c u r so r a p i d l y t h a t the subsequent ash particles formed w i l l behave as a s i n g l e m a t e r i a l whose c o m p o s i t i o n i s d e f i n e d by the m i x t u r e o f m i n e r a l s c o n t a i n e d w i t h i n the c o a l particle. The atmosphere under which the i n d i v i d u a l t r a n s f o r m a t i o n s take p l a c e w i l l , no doubt, approach a r e d u c i n g environment. Figure 2 i l l u s t r a t e s a model o f the c o a l and m i n e r a l m a t t e r as f e d t o the combustor and the f a t e o f t h e m i n e r a l s a f t e r combustion [ 1 3 ] .

BRYERS

Segregated Mineral

Matter

influence

HEAT

on

T R A N S F E R


/ / / / / /

H E A T AND

Slagging

T R A N S F E R

t

/(

SURFACES

f

t

/ / '

/

SURFACES

REFRACTORY

F i g u r e 2. Fate of M i n e r a l M a t t e r i n C o a l During Combustion, as Proposed by Dr. S a r o f i m . 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 23. C o p y r i g h t 1982 The Combustion I n s t i t u t e .


358


E x t r a n e o u s m a t e r i a l s can behave as d i s c r e t e m i n e r a l p a r t i c l e s comprised o f a s i n g l e s p e c i e s o r a m u l t i p l i c i t y o f s p e c i e s . As a l r e a d y i n d i c a t e d , a p o r t i o n o f t h i s m a t e r i a l may be so finely d i v i d e d i t can behave as i n h e r e n t m i n e r a l m a t t e r . D u r i n g combustion the l a r g e r p a r t i c l e s respond i n d i v i d u a l l y t o the r i s i n g temperature of the environment. In the absence o f carbon o r o t h e r e x o t h e r m i c r e a c t a n t s , the p a r t i c l e s h o u l d always be a t a temperature somewhat l e s s than the l o c a l gas temperature. However, the p a r t i c l e s may be s u b j e c t e d t o e i t h e r r e d u c i n g o r o x i d i z i n g c o n d i t i o n s . As each p a r t i c l e r i s e s i n temperature, i t l o s e s water o f h y d r a t i o n , e v o l v e s gas, becomes o x i d i z e d o r reduced, and e v e n t u a l l y s i n t e r s o r m e l t s , depending on i t s p a r t i c u l a r c o m p o s i t i o n o r temperature l e v e l . I t i s e v i d e n t , then, t h a t t h e r e can be a g r e a t d i f f e r e n c e i n the f i n a l s t a t e o f each p a r t i c l e , depending upon i t s c o m p o s i t i o n and whether i t i s i n h e r e n t o r e x t r a n e o u s ash. F i g u r e 3 summarizes the phase t r a n s f o r m a t i o n s which pure m i n e r a l m a t t e r commonly found i n c o a l undergoes d u r i n g h e a t i n g [12-18]. S i n c e t h i s d a t a was d e v e l oped p r i m a r i l y by m i n e r a l o g i s t s p e r f o r m i n g d i f f e r e n t i a l thermal a n a l y s i s under a i r a t slow h e a t i n g r a t e s , i t must be used o n l y as a g u i d e l i n e f o r p r e d i c t i n g the t h e r m a l b e h a v i o r o f m i n e r a l s i n c o a l . Thermal s h o c k i n g these m i n e r a l s i n the presence o f carbon and o t h e r m i n e r a l forms a t v e r y h i g h t e m p e r a t u r e s , no doubt, w i l l a l t e r some of t h e s e t r a n s f o r m a t i o n s and may d e f e r o t h e r s u n t i l postcombustion d e p o s i t i o n on heat t r a n s f e r s u r f a c e s . C l a y s and S h a l e . The m e l t i n g temperatures o f most pure m i n e r a l s are i n the v i c i n i t y o f o r g r e a t l y exceed the maximum flame temperat u r e e n c o u n t e r e d d u r i n g combustion. T h e r e f o r e , the f u s e d s p h e r o i d a l f l y ash, g e n e r a t e d from the m i n e r a l m a t t e r i n c o a l , p r i m a r i l y forms as the r e s u l t o f the f l u x i n g a c t i o n between pure m i n e r a l s c o n t a i n e d w i t h i n each p a r t i c l e . I l l i t e and b i o t i t e appear t o be an e x c e p t i o n . Both m i n e r a l s c o n t a i n s m a l l c o n c e n t r a t i o n s o f i r o n and p o t a s s i u m and form a g l a s s y phase a t 950°C and 1100°C, r e s p e c t i v e l y . Depending upon i t s f l u i d i t y , t h i s g l a s s y phase c o u l d be r e s p o n s i b l e f o r s u r f a c e d e f o r m a t i o n a t a r e l a t i v e l y low temperature and p r o v i d e the n e c e s s a r y sticking p o t e n t i a l t o p r e v e n t r e e n t r a i n m e n t upon c o n t a c t i n g heat transfer surfaces. Low-temperature ash o f a g r a v i t y f r a c t i o n cont a i n i n g i l l i t e was h e a t e d i n a t h e r m a l a n a l y z e r under a i r t o 600 and 1000°C, r e s p e c t i v e l y , and compared t o the low-temperature ash of a g r a v i t y f r a c t i o n v o i d o f i l l i t e . The s c a n n i n g e l e c t r o n photom i c r o g r a p h s , a p p e a r i n g i n F i g u r e 4, i n d i c a t e the m i n e r a l s c o n t a i n i n g i l l i t e d i d , i n d e e d , show s i g n s o f the f o r m a t i o n o f a m e l t . Quartz. The i n h e r e n t s i l i c a r e t a i n e d i n the char as q u a r t z o r s i l i c a r e l e a s e d from k a o l i n i t e and i l l i t e a t low temperatures ( i . e . , 950°C) i s p a r t i a l l y reduced t o s i l i c a monoxide. U n l i k e s i l i c a , which b o i l s at 2230°C, s i l i c a monoxide m e l t s a t 1420°C and b o i l s a t 2600°C [ 1 9 ] . The vapor p r e s s u r e o f s i l i c o n i s l o w - - i n the range o f temperatures e x p e r i e n c e d d u r i n g combustion. Honig r e p o r t s v a l u e s r a n g i n g from 0.01m Hg a t 1157°C t o lm Hg p r e s s u r e a t 1852°C [19-21]. The p r e s e n c e of o t h e r m i n e r a l m a t t e r and carbonaceous m a t e r i a l appears t o a l t e r vapor p r e s s u r e s u b s t a n t i a l l y . When a m i x t u r e o f a l u m i n o s i l i c a t e and g r a p h i t e was h e a t e d , the v o l a t i l i z a t i o n o f s i l i c o n monoxide began at about 1150°C and reached a maximum a t 1400°C. Mackowsky r e p o r t s t h a t v o l a t i l i z a t i o n o f s i l i c o n monoxide s t a r t s a t about 1649°C i n

500 °C , , — 2 ^ 0

925°C

3

—^Si0

2

(

,—^Si0

2

3

2

2

(cristobaUte)

* Si0

e

I400°C

' I300°C

II00°C

I050°C

class**

amorphous

2

zsfa )| X

mul (3AI 0,.

from middle layers of lattice.

2

3

3t

•S, S0

4

2

2

e

970 C Fe

Fe (partial melt)

—«-S

I liquid glass]

700 C e

I500°C I glass «• olivinel

y Temperatures soneatlve to mineral and carbon impurities^

Fe 0 liquid I600°C

e

525 C*

2

FeS, Fe (S0 ) Fe0| (partial melt)

475°C" —«»S,S0

JE 1 IQxidizinq

I Pyrites |

"Formed from alkali and S i O , from thel outer layers of lattice.

I200°C

2

3

2Si0 )

2

(3AI 0 -

4SÎ0Î)

2

mullite

(Κ 0·ΑΙ 0,· 2

leucite

~H glass formed h

(MaOFejOj)

4

(Fe 0 ) OR

(SiOj) 3

spinel

j lattice destroyed at IIOO°Cr quartz

' F o r m e d from oluminum and magnésium

I liquid qloss

(AljOj-MgO)

spinel*

e

950 C

iMuscovitTl

2

Q

3

)

e

SiO* liquid 1723 C

950° C

inversion

Temperature

I liquid glass |

I500°C

(AI

|only glass + corundum

I400°C

Ialumina or spinel I

I050°C

lattice destroyed at: 940°-980°C

F i g u r e 3. Phase T r a n s f o r m a t i o n of Some M i n e r a l M a t t e r Commonly Found i n C o a l . 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 19. C o p y r i g h t 1984 VGB.

CaO liquid 2570 C

1000° to II00°C

I Calcite |

Other members in kaolinite group, Ί (Le. liveeite and meta halloysito) are thought to undergo similar changes J

[

May not be completely liquid until 1800°C.

I

2

[mullite ^ A l g O ^ 2Si0 ) |

o

H400 C

fi-1 mullite-type phoseÎAfeOs · SiO^f

II00°C

2

1 silicon spinel (2Al 0 · 3Si0 )|

(clay lattice destroyed) ,

2

1 metakaolinite ( Α Ι 0 · 2Si02)|

Illite


i I"

ο a

t

I.

J?

73 ·< m 73

CO

F i g u r e 4. SEM P h o t o m i c r o g r a p h s S h o w i n g t h e I m p a c t o f I l l i t e o n t h e T h e r m a l B e h a v i o r o f Low T e m p e r a t u r e A s h . Reproduced w i t h p e r m i s s i o n f r o m r e f e r e n c e 19. C o p y r i g h t 1984 VGB.


n

Ο >

ζ

OC

>

D

>

H m

r

>

ζ m

s

25.

BRYERS

Segregated Mineral

Matter

Influence

on

Slagging

361

the p r e s e n c e o f c a r b o n a t e s and c l a y s and r e a c h e s a maximum a t 1704°C [22]. In the p r e s e n c e o f p y r i t e o r m e t a l l i c i r o n , v o l a t i l i z a t i o n b e g i n s a t about 1560°C and c o n t i n u e s a t a r a p i d r a t e as the tempera t u r e r i s e s u n t i l p r a c t i c a l l y a l l the s i l i c a i n the m i n e r a l i s v o l a tilized. S a r o f i m has shown t h a t about 1.5 t o 2.0 p e r c e n t o f the ASTM ash i n b i t u m i n o u s c o a l s v o l a t i l i z e [ 2 3 ] . A p p r o x i m a t e l y 35 t o 40 p e r c e n t o f the v o l a t i l i z e d m a t e r i a l was s i l i c a . The next l a r g e s t component was iron. Extraneous q u a r t z appears t o be relatively innocuous u n l e s s contaminated by F e 0 , CaO, o r K 0 .


2

3

2

Pyrites. The d e c o m p o s i t i o n o f p y r i t e has been examined by numerous investigators under oxidizing, neutral, or reducing environments [1,11,16,17]. TGA, r a t h e r than DTA, has been used by most i n v e s t i g a t o r s ( F i g u r e 5 ) . A c q u i s i t i o n of r e p r e s e n t a t i v e data i s d i f f i c u l t , as the d e c o m p o s i t i o n p r o c e s s i s complex and s e n s i t i v e to many v a r i a b l e s i n c l u d i n g the c h e m i c a l c o m p o s i t i o n of p y r i t e s , i t s g r a i n s i z e , i t s o r i g i n , and the p r e s e n c e of a d v e n t i t i o u s i m p u r i t i e s , the c o m p o s i t i o n of the l o c a l environment, and d i f f u s i o n r a t e s through s u l f a t e d l a y e r s Under o x i d i z i n g c o n d i t i o n s i t i s b e l i e v e d t h a t p y r i t e s decompose d i r e c t l y t o an i r o n o x i d e and S 0 o r S 0 , o r i r o n s u l f a t e and S 0 , depending upon the f i n a l temperature l e v e l . Under r e d u c i n g c o n d i t i o n s p y r r h o t i t e and e i t h e r s u l f u r o r hydrogen s u l f i d e form. Com plete reduction results i n e l e m e n t a l i r o n and c a r b o n disulfide. There i s a l s o a p o s s i b i l i t y t h a t p y r r h o t i t e may form under o x i d i z i n g c o n d i t i o n s as an i n t e r m e d i a t e s t e p i n the p r e s e n c e o f s u f f i c i e n t a d v e n t i t i o u s carbon. Pure p y r i t e s i g n i t e a t about 500°C i n the t h e r mal a n a l y z e r a t 20°C/min and burn out by 550°C i n a s i n g l e - s t e p p r o c e s s , as shown i n F i g u r e 6. Pure p y r i t e s do i g n i t e as r e a d i l y as b i t u m i n o u s c o a l ; however, the burnout time i s comparable. Although p y r r h o t i t e i g n i t e s r e a d i l y , i t s r e q u i r e s as much time as a n t h r a c i t e to complete combustion. Pyrites containing small quantities of a d v e n t i t i o u s c a r b o n , as might be found i n the -1.80+2.85 g r a v i t y f r a c t i o n , appear t o form p y r r h o t i t e d e f e r r i n g burnout u n t i l 800°C. W i t h i n the combustor the problem i s compounded by the f a c t that p y r i t e p a r t i c l e s do not s h r i n k d u r i n g the combustion p r o c e s s as do c o a l p a r t i c l e s , and hence t h e i r burnout time i s extended. The burn out time o f p a r t i c l e s i n e x c e s s o f 40μ appears t o exceed the r e s i dence time a v a i l a b l e i n most combustors. 2

3

2

TGA r e v e a l s d e c o m p o s i t i o n r a t e s but t e l l s l i t t l e about the p h y s i c a l s t a t e d u r i n g the combustion p r o c e s s . Phase diagrams f o r the Fe-S-0 and FeS-FeO system, r e p r e s e n t i n g t r a n s i t o r y s t a t e s a t the p a r t i c l e s u r f a c e , imply the f o r m a t i o n o f a temporary melt a t low t e m p e r a t u r e s . SEM p h o t o m i c r o g r a p h s , a p p e a r i n g i n F i g u r e 6, o f pure p y r i t e s h e a t e d t o 600°C, 800°C, and 1000°C under r e d u c i n g c o n d i t i o n s , c l e a r l y r e v e a l the f o r m a t i o n o f a melt a t temperatures as low a t 600°C. Large p a r t i c l e s o f p a r t i a l l y - s p e n t p y r i t e s , which may be m o l t e n on c o n t a c t w i t h the heat t r a n s f e r s u r f a c e , complete the o x i d a t i o n p r o c e s s i n s i t u , forming a s o l i d f u s e d d e p o s i t w i t h a v e r y h i g h m e l t i n g temperature. An e x a m i n a t i o n o f the t h e r m a l b e h a v i o r o f the m i n e r a l m a t t e r i n c o a l i n d i c a t e s the m i n e r a l o r i g i n o f the elements found i n c o a l ash and t h e i r j u x t a p o s i t i o n w i t h r e g a r d t o each o t h e r , as w e l l as o t h e r m i n e r a l forms, and determines t h e i r p h y s i c a l f a t e d u r i n g com bustion. As i n d i c a t e d i n F i g u r e 2, the p h y s i c a l s t a t e ( i . e . , vapor or s o l i d ) and the s i z e o f s o l i d i f i e d ash w i l l determine the mode


362 MINERAL MATTER AND ASH IN COAL


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