Mineral Matter and Ash in Coal - American Chemical Society

19 Heat Transfer and Thermal Conductivity of Coal Slags

Downloaded by UNIV OF CALIFORNIA SAN FRANCISCO on February 18, 2015 | http://pubs.acs.org Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch019

K. C . Mills National Physical Laboratory, Teddington, Middlesex TW11 OLW, United Kingdom

The various factors affecting the complex heat transfer processes which occur in semi-transparent media like slags are discussed. This information has been used to evaluate the validity of the various experimental methods available for measuring the thermal conductivity of slags. A critical assessment of published experimental data for slags and magmas covering a wide composition range has been carried out. This review of thermal conductivity data showed that both the (Fe /Fe ) ratio and the crystallinity of the slag greatly influence the amount of heat transferred through the slag by radiation conduction. The heat transferred across the solid slag formed on the walls of the vessel has been shown to be dependent upon the thermal resistance of the slag/metal interface which, in turn, appears to be dependent upon the mineralogical constitution of the slag. 2+

3+

D u r i n g t h e g a s i f i c a t i o n o f c o a l , both molten and s o l i d s l a g s a r e formed i n the converter, and t h e heat transfer within the g a s i f i c a t i o n chamber i s governed t o a l a r g e e x t e n t by t h e t h e r m a l p r o p e r t i e s o f t h e s l a g phase. Thus i n o r d e r t o c a r r y o u t e i t h e r h e a t b a l a n c e o r m o d e l l i n g c a l c u l a t i o n s i t i s n e c e s s a r y t o have r e l i a b l e d a t a f o r t h e t h e r m a l p r o p e r t i e s o f b o t h s o l i d and l i q u i d c o a l s l a g s . However, the thermal transfer mechanisms i n high temperature p r o c e s s e s i n v o l v i n g s l a g s a r e e x c e e d i n g l y complex s i n c e h e a t c a n be t r a n s p o r t e d by c o n v e c t i o n , r a d i a t i o n and t h e r m a l c o n d u c t i o n . I n o r d e r t o s i m p l i f y t h e a n a l y s i s o f t h e combined c o n d u c t i v e - r a d i a t i v e t h e r m a l f l u x i t i s n e c e s s a r y t o r e s o r t t o one o f t h e v a r i o u s heat t r a n s f e r models a v a i l a b l e . The most w i d e l y - u s e d model i s the d i f f u s i o n approximation i n which t h e t h e r m a l f l u x i s used t o d e r i v e an e f f e c t i v e o r t o t a l t h e r m a l c o n d u c t i v i t y ( ^ ) which i s , i n t u r n , c o n s i d e r e d t o be made up o f c o n t r i b u t i o n s from ( i ) the thermal ("phonon") c o n d u c t i v i t y , k , ( i i ) r a d i a t i o n c o n d u c t i v i t y , k and e f f

c

This chapter not subject to U.S. copyright. Published 1986, American Chemical Society

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

R


19.

MILLS

Heat Transfer and Conductivity

of Coal Slags

257

( i i i ) electronic conductivity, k . Heat b a l a n c e c a l c u l a t i o n s must t a k e a c c o u n t o f each o f t h e s e t h % r m a l t r a n s p o r t mechanisms and t h i s is p a r t i c u l a r l y important as the boundary conditions in the e x p e r i m e n t a l d e t e r m i n a t i o n o f the t h e r m a l c o n d u c t i v i t y o f a s l a g may v a r y a p p r e c i a b l y from t h o s e a p p l y i n g t o the s l a g i n the g a s i f i e r . Thus i t i s n e c e s s a r y , where p o s s i b l e , t o determine the i n d i v i d u a l c o n t r i b u t i o n s t o the t o t a l h e a t f l u x f o r each c o n d i t i o n . In the p r e s e n t paper the f a c t o r s a f f e c t i n g the t h e r m a l c o n d u c t i v i t y o f s l a g s will be examined and the various methods available for the measurement o f t h e r m a l c o n d u c t i v i t i e s w i l l be assessed. Finally, e x p e r i m e n t a l d a t a f o r the t h e r m a l c o n d u c t i v i t y o f s l a g s , g l a s s e s and magmas w i l l be e v a l u a t e d t o p r o v i d e a r e l i a b l e d a t a base f o r the t h e r m a l c o n d u c t i v i t y o f s l a g s , and t o d e t e r m i n e the l i k e l y e f f e c t s o f v a r i a t i o n s i n c h e m i c a l c o m p o s i t i o n upon v a l u e s f o r c o a l s l a g s . Thermal c o n d u c t i o n mechanisms Thermal "phonon" c o n d u c t i v i t y (k ) . Heat i s t r a n s f e r r e d through a medium by phonons, which a r e q u a ^ a o f energy a s s o c i a t e d w i t h each mode o f v i b r a t i o n i n the sample. T h i s mode o f c o n d u c t i o n i s t h u s r e f e r r e d t o as t h e r m a l , phonon o r l a t t i c e c o n d u c t i o n . S c a t t e r i n g o f the phonons c a u s e s a d e c r e a s e i n the t h e r m a l c o n d u c t i v i t y which i s t h e r e f o r e s e n s i t i v e t o the s t r u c t u r e o f the sample. S c a t t e r i n g o f t h e phonons can o c c u r by c o l l i s i o n s o f the phonons w i t h one a n o t h e r , o r by impact w i t h g r a i n b o u n d a r i e s o r c r y s t a l i m p e r f e c t i o n s , such as p o r e s . Thus a l o w - d e n s i t y , h i g h l y - p o r o u s m a t e r i a l w i l l have a low t h e r m a l c o n d u c t i v i t y . In g l a s s y , n o n - c r y s t a l l i n e m a t e r i a l s i t has been s u g g e s t e d —* that thermal conductivity decreases as the d i s o r d e r i n g o f the s i l i c a t e network i n c r e a s e s . Radiation

conductivity

(k^.

Measurements o f t h e t h e r m a l c o n d u c t i v i t y o f g l a s s e s were found t o be dependent upon the t h i c k n e s s o f the specimens used, and t h i s i s shown i n F i g u r e 1. T h i s b e h a v i o u r was a s c r i b e d t o the c o n t r i b u t i o n from r a d i a t i o n c o n d u c t i v i t y , k , which can o c c u r i n s e m i - t r a n s p a r e n t media l i k e s l a g s and g l a s s e s . R a d i a t i o n c o n d u c t i v i t y o c c u r s by a mechanism involving absorption and emittance o f r a d i a n t energy by various s e c t i o n s through the medium. C o n s i d e r a t h i n s e c t i o n i n the s l a g , r a d i a n t energy absorbed by the s e c t i o n w i l l cause i t t o i n c r e a s e i n temperature and c o n s e q u e n t l y r a d i a n t h e a t w i l l be e m i t t e d t o c o o l e r s e c t i o n s . T h i s p r o c e s s can o c c u r c o n t i n u o u s l y t h r o u g h the medium and i t i s o b v i o u s t h a t the energy t r a n s f e r r e d i n t h i s way w i l l i n c r e a s e w i t h i n c r e a s i n g number o f s e c t i o n s ( i e . i n c r e a s i n g t h i c k n e s s ) until the p o i n t i s reached where k a t t a i n s a c o n s t a n t v a l u e . At t h i s p o i n t the s l a g i s s a i d t o be " o p t i c a l l y t h i c k , and this i s usually c o n s i d e r e d t o o c c u r when α d > 3 . 5 , where α and d a r e the a b s o r p t i o n c o e f f i c i e n t and t h i c k n e s s o f the s l a g , r e s p e c t i v e l y . At h i g h t e m p e r a t u r e , r a d i a t i o n c o n d u c t i o n can be the predominant mode o f h e a t t r a n s f e r , eg. i n g l a s s m a k i n g more than 90J o f the thermal transfer occurs by radiation conduction. The radiation c o n d u c t i v i t y can be c a l c u l a t e d f o r an o p t i c a l l y - t h i c k sample i f steady-state conditions apply and i f i t is assumed that the absorption c o e f f i c i e n t of the medium, α , i s independent of R

R

1 1


258

MINERAL MATTER AND ASH IN COAL

wavelength, λ , i e . grey-body c o n d i t i o n s o b t a i n . F o r t h e s e c o n d i t i o n s k can be c a l c u l a t e d by use o f E q u a t i o n 1, where σ and η a r e t h e Scefan-Boltzmann c o n s t a n t and r e f r a c t i v e i n d e x , r e s p e c t i v e l y .

_ "

k K

R

16 σ 3

2 η

τ

.

3 ( U

α

;

Values o f k cannot e a s i l y be c a l c u l a t e d f o r o p t i c a l l y - t h i n samples ( 3 . 5 ) . Hence i n c r e a s i n g α has the e f f e c t o f d e c r e a s i n g k„ and d e c r e a s i n g t h e depth a t which a s l a g becomes o p t i c a l l y t h i c k , i f a p a r t i c u l a r s l a g sample were o p t i c a l l y t h i n , t h e s e two f a c t o r s would operate in opposition to one another. The absorption c o e f f i c i e n t i s markedly dependent upon t h e amounts o f FeO and MnO p r e s e n t i n the s l a g w ; a l t h o u g h Fe 0^ a b s o r b s i n f r a - r e d r a d i a t i o n , i t s e f f e c t on α i s much l e s s pronounced than t h a t o f FeO. An e m p i r i c a l r u l e has been d e r i v e d ^ f o r g l a s s e s c o n t a i n i n g l e s s than 5% FeO, from which t h e a b s o r p t i o n c o e f f i c i e n t a t room temperature i s g i v e n by the r e l a t i o n s h i p , α = 11. (% FeO). A b a s i c assumption adopted i n d e r i v i n g E q u a t i o n 1 was t h a t α was independent o f wavelength; however, i n p r a c t i c e t h e s p e c t r a l a b s o r p t i o n c o e f f i c i e n t ( α ) v a r i e s w i t h t h e wavelength (λ) as shown i n F i g u r e 2 f o r a g l a s s c o n t a i n i n g c a . 5% F e O ^ . I t can be seen from t h i s f i g u r e t h a t t h e r e i s s t r o n g a b s o r p t i o n by FeO a t c a . 1 jam and by S i 0 a t c a . 4.4 um. At h i g h temperatures t h i s r e s t r i c t s a b s o r p t i o n by t h e s l a g t o a "window", i n t h e wavelength range 1-4.4 pm. However, even w i t h i n t h i s wavelength band t h e r e i s some v a r i a t i o n i n α and the a v e r a g e a b s o r p t i o n coefficient,α i s determined by w e i g h t i n g o f the α values. χ

2

λ

9

m

χ

The a v e r a g e a b s o r p t i o n c o e f f i c i e n t , α , can be a f f e c t e d by t e m p e r a t u r e i n two d i f f e r e n t ways. F i r s t l y , ftie a b s o r p t i o n spectrum, i e , ( α ) , can change markedly w i t h temperature and c o n s e q u e n t l y a l t e r t h e v a l u e o f α . S e c o n d l y , even i f t h e a b s o r p t i o n spectrum i s u n a f f e c t e d by t e m p e r a t u r e , α would c o n t i n u e t o be a f u n c t i o n o f temperature because t h e wavefength d i s t r i b u t i o n used i n d e r i v i n g α i s i t s e l f a f u n c t i o n o f t e m p e r a t u r e . T h i s can be seen i n F i g u r e 3 where t h e f r a c t i o n o f t o t a l energy e m i t t e d i n t h e "window" 1-4.4 p i c o n s t i t u t e s 61.1%, 79.5$ and 81.9% o f t h e t o t a l energy e m i t t e d a t 1073 K, 1573 Κ and 1773 K, r e s p e c t i v e l y . I n a s i m i l a r manner, the various α v a l u e s o f the spectrum will have t o be weighted d i f f e r e n t l y i n the c a l c u l a t i o n o f α f o r t h e t h r e e temperatures i n question. λ

χ

m

I t can be seen from F i g u r e 2 t h a t α increases with increasing temperature,, and s i m i l a r b e h a v i o u r has i e e n o b s e r v e d i n r o c k s and m i n e r a l s ^ ' ^ ^ . By c o n t r a s t , the a b s o r p t i o n c o e f f i c i e n t s (a ) o f amber g l a s s have been found t o d e c r e a s e 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 . ffi



MILLS


of Coal

Slags

F i g u r e 2. The wavelength dependence o f a g l a s s 67% S i 0 +16% N a 0 + 9% (FeO + F e 0 ) . 2

2

2

containing

3




1

2

3

4

5

6

λ/μπι

F i g u r e 3. Wavelength d i s t r i b u t i o n e m i s s i o n f o r a b l a c k body.

o f t h e s o u r c e energy


19.


MILLS

261

of Coal Slags

Extinction coefficient (E). In s o l i d s , radiant energy can be s c a t t e r e d by g r a i n b o u n d a r i e s , p o r e s and c r a c k s i n the m a t e r i a l . I n t h e s e c a s e s , i t i s n e c e s s a r y t o use the e x t i n c t i o n c o e f f i c i e n t (E) which i s g i v e n by the r e l a t i o n s h i p Ε = α + s, where s i s t h e scattering coefficient. E l e c t r o n i c c o n d u c t i v i t y (k ^ l * I t has been r e p o r t e d t h a t g l a s s e s which c o n t a i n s i g n i f i c a n t c o n c e n t r a t i o n s o f Fe i o n s behave i n a s i m i l a r manner t o s e m i - c o n d u c t o r s and hence t h e r m a l c o n d u c t i o n v i a c o n d u c t i o n e l e c t r o n s , h o l e s , e t c c o u l d be s i g n i f i c a n t , a c c o r d i n g t o F i n e e t a l ^ . L i t t l e i s known o f t h i s mechanism i n r e l a t i o n t o the h e a t t r a n s f e r i n s l a g s and c o n s e q u e n t l y the c o n t r i b u t i o n o f k . t o t h e measured t h e r m a l c o n d u c t i v i t i e s has been i g n o r e d i n t h i s r e v i e w .


j

T o t a l thermal c o n d u c t i v i t y ( k ) _ . In p r a c t i c e , the r a d i a t i o n and c o n d u c t i o n c o n t r i b u t i o n s t o trie h e a t f l u x (Q) a r e i n t e r a c t i v e , and t h e i n t e r p r e t a t i o n o f the combined c o n d u c t i v e - r a d i a t i v e heat t r a n s f e r i s complex. V a r i o u s models have t h e r e f o r e been proposed t o s i m p i f y t h e t h e o r y o f t h e heat t r a n s f e r p r o c e s s . One w i d e l y - u s e d model i s t h e d i f f u s i o n a p p r o x i m a t i o n which assumes t h a t the heat f l u x (Q) i s g i v e n by E q u a t i o n 2, where k i s t h e e f f e c t i v e thermal c o n d u c t i v i t y and i s d e f i n e d by E q u a t i o n 3 where χ i s the d i s t a n c e . G a r d o n ^ has p o i n t e d out t h a t t h i s model o n l y a p p l i e s s t r i c t l y when ( i ) k is s m a l l and ( i i ) α d > 8. f f

Q k

e f f

=

-k

=

k

c

(dT/dx)

e f f

+

k

(2) (3)

R

E x p e r i m e n t a l Methods f o r D e t e r m i n i n g Thermal C o n d u c t i v i t y The experimental methods available for measuring thermal c o n d u c t i v i t i e s a r e summarised below; more d e t a i l e d , r e v i e w s .of the experimental techniques are available e l s e w h e r e ^ ' — ' ϋ-' — . The techniques f o r the d e t e r m i n a t i o n o f thermal c o n d u c t i v i t y can be d i v i d e d i n t o t h r e e c l a s s e s : ( i ) s t e a d y - s t a t e methods, ( i i ) non-steady s t a t e methods, and ( i i i ) i n d i r e c t methods. The s t e a d y - s t a t e methods usually yield k v a l u e s and the non-steady s t a t e t e c h n i q u e s u s u a l l y produce t h e r m a l ûiffusivity (a ) v a l u e s , which can be c o n v e r t e d t o t h e r m a l c o n d u c t i v i t y v a l u e s by use o f E q u a t i o n 4, where Ρ and C a r e the d e n s i t y and heat c a p a c i t y r e s p e c t i v e l y o f the s l a g . p

k

=

a. C . Ρ

(4)

S t e a d y - s t a t e methods. These methods a l l y i e l d ^ . v a l u e s p r o v i d e d t h a t the specimen i s o p t i c a l l y t h i c k . In the l i n e a r h e a t - f l o w method two d i s c - s h a p e d specimens a r e p l a c e d on e i t h e r s i d e o f an electrically-heated plate and the temperature profiles across the samples are monitored by thermocouples s i t e d on b o t h f a c e s o f t h e specimens. The a p p a r a t u s i s w e l l i n s u l a t e d t o m i n i m i s e heat l o s s e s . In some v e r s i o n s o f t h i s method, the t o t a l heat f l u x p a s s i n g through t h e samples i s determined e f {


262


by c a l o r i m e t e r s i n c o n t a c t w i t h t h e specimens. When h i g h - t e m p e r a t u r e measurements a r e rénuired, t h i s t e c h n i q u e i s u s u a l l y o p e r a t e d a s a c o m p a r a t i v e method — . In t h e r a d i a l h e a t - f l o w method t h e specimen i s i n t h e shape o f a h o l l o w c y l i n d e r , which i s p o s i t i o n e d i n t h e a n n u l u s between two c o a x i a l c y l i n d e r s w i t h t h e i n t e r n a l c y l i n d e r a c t i n g a s a r a d i a l heat s o u r c e * - * ' . The temperature p r o f i l e a c r o s s t h e specimen i s determined by thermocouples p l a c e d on t h e i n s i d e w a l l s o f t h e two c y l i n d e r s . T h i s method r e q u i r e s a l a r g e i s o t h e r m a l zone i n t h e f u r n a c e , which i s d i f f i c u l t t o a c h i e v e a t h i g h t e m p e r a t u r e s . When t h i s t e c h n i q u e i s used f o r measurements on l i q u i d s , e r r o r s can o c c u r from c o n v e c t i v e heat t r a n s f e r .


1

Non-steady s t a t e methods. I n t h e r a d i a l wave method t h e s l a g i s p l a c e d i n a c y l i n d r i c a l c r u c i b l e s i t u a t e d i n t h e i s o t h e r m a l zone o f a f u r n a c e , and thermocouples a r e l o c a t e d on t h e w a l l s and a l o n g t h e g e o m e t r i c a x i s o f t h e c r u c i b l e ( t h e s l a g ) . The o u t s i d e w a l l o f t h e c r u c i b l e i s t h e n s u b j e c t e d t o v a r i a t i o n o f temperature and the v a r i a t i o n i n temperature o f t h e c e n t r a l thermocouple i s m o n i t o r e d . There i s a phase s h i f t between t h e i n p u t and o u t p u t which i s r e l a t e d t o t h e thermal diffusivity o f the s l a g . In the m o d i f i c a t i o n of this a p p a r a t u s used by E l l i o t t and co-workers ~ the p e r i o d i c v a r i a t i o n i n t e m p e r a t u r e i s produced i n a w i r e r u n n i n g a l o n g t h e c e n t r a l a x i s of t h e c y l i n d r i c a l c r u c i b l e , and t h e phase s h i f t i s measured by thermocouples l o c a t e d on t h e w a l l s o f t h e c r u c i b l e . The t h e r m a l d i f f u s i v i t y v a l u e s o b t a i n e d w i t h t h i s method may be v u l n e r a b l e t o e r r o r s a r i s i n g from c o n v e c t i v e heat t r a n s f e r . In t h e modulated beam method t h e specimen i s i n t h e form o f a d i s c , which i s m a i n t a i n e d a t a c o n s t a n t temperature, w h i l s t t h e f r o n t f a c e i s s u b j e c t e d t o a l a s e r beam which produces a p e r i o d i c v a r i a t i o n i n temperature o f c o n s t a n t f r e q u e n c y . The phase s h i f t between t h i s i n p u t and t h e s i g n a l from a temperature s e n s o r i n c o n t a c t w i t h t h e back f a c e i s d e t e r m i n e d . By c a r r y i n g o u t measurements a t two o r more f r e q u e n c i e s , S c h a t z and Simmons ~ were a b l e t o d e r i v e v a l u e s o f b o t h a and t h e e x t i n c t i o n c o e f f i c i e n t . I f t h i s method were a p p l i e d t o measurements i n l i q u i d s , i t t o o would be prone t o e r r o r s caused by convection. The l a s e r p u l s e m e t h o d ^ ' ffi when a p p l i e d t o s o l i d s uses a d i s c - s h a p e d s l a g specimen c o a t e d w i t h m e t a l l i c f i l m s on both p l a n a r s u r f a c e s . A l a s e r p u l s e i s d i r e c t e d on t o the f r o n t f a c e o f the specimen and t h e temperature o f t h e back face i s monitored c o n t i n u o u s l y . The maximum temperature i n c r e a s e o f t h e back face (ΔΤ^ ) u s u a l l y o c c u r s a f t e r c a . 10 seconds, and a , may be computed from t h e time t a k e n ( t ) f o r t h e back f a c e t o a t t a i n a temperature r i s e o f (0.5Δ T * ) , The method has a l s o been a p p l i e d t o measurements on l i q u i d s l a g s — ' — which were c o n t a i n e d i n A l 0. o r BN c r u c i b l e s . The major advantage o f t h i s t e c h n i q u e i s t h a t t h e s h o r t d u r a t i o n o f the experiment m i n i m i s e s t h e e r r o r s due t o c o n v e c t i o n . The major d i s a d v a n t a g e i s t h a t t h e maximum specimen t h i c k n e s s i s about 0.4 cm, and c o n s e q u e n t l y o p t i c a l l y - t h i c k c o n d i t i o n s o n l y a p p l y when t h e e x t i n c t i o n c o e f f i c i e n t i s g r e a t e r than 9 cm" . A second d i s a d v a n t a g e i s t h a t t h e l a s e r p u l s e method i s a t r a n s i e n t t e c h n i q u e and k cannot be c a l c u l a t e d by E q u a t i o n 1, which i s a p p l i c a b l e t o s t e a a y - s t a t e c o n d i t i o n s ; a t t h e p r e s e n t time no f o r m u l a e e x i s t f o r t h e c a l c u l a t i o n e f t {

m a

R


19.

MILLS


263

Slags

of k f o r t h i s method. Thus t h i s t e c h n i q u e i s most u s e f u l when a p p l i e d t o specimens which have ( i ) v e r y s m a l l v a l u e s o f « d ( i e . α d 45%) and (

D

ζ

>

m

Ν) -J Ν)


MILLS

of Coal Slags

ι

η

(α)

Ε £

1

Η—Φ—I

1

1

1

1

h

800

1000

1200


(b)

Ο

2 00

400

600

1 4 0 0 1600

T/°C

F i g u r e 10. (a) Thermal c o n d u c t i v i t y (b) t h e r m a l d i f f u s i v i t y o f coal slags; , Taylor; — — — , Vargaftik; upper and lower l i m i t s o f a v a l u e s r e p o r t e d by Gibby and B a t e s .

CaF

^Vv^iK-t

2

\\lFe0*Ca0*Si0

""2,..

!

Na 0*Si0 Continuous casting slogs 2

200

400

'.u^Coal

2

slags

2

Na 0*Si0 2

600

800

1000

1200

1400

2

1600

T/ C e

F i g u r e 11. The thermal c o n d u c t i v i t y o f v a r i o u s s l a g s , g l a s s e and m i n e r a l s : O , M g O - A l ^ ; T, S S U O - 2 S i 0 ; X, 2MgO*Si0 ; ^ CaF ~based slags; Na 0 + S i 0 ; , continuousc a s t i n g s l a g s ; and - · · - , c o a l s l a g s . 2

2

2

2


2


274

s m a l l . However k i n c r e a s e s d r a m a t i c a l l y w i t h temperature and even a s l a g w i t h a r e l a t i v e l y h i g h a b s o r p t i o n c o e f f i c i e n t o f 100 cm" would g i v e r i s e t o a c o n t r i b u t i o n o f k o f 0.4 Wm Κ a t 1800 K. H o w e v e r a s t h e a b s o r p t i o n c o e f f i c i e n t i s v e r y dependent upon t h e (Fe ) concentration the k value w i l l be dependent upon t h e various f a c t o r s a f f e c t i n g the t F e /Fe^ ) r a t i o i n the s l a g v i z . the ratio i n c r e a s e s with; ( i ) i n c r e a s i n g temperature; ( i i ) decreasing p(0 ); ( i i i ) i n c r e a s i n g SiOp and T i 0 c o n t e n t s and d e c r e a s i n g CaO, NapO and KpO c o n t e n t s i n t h e s l a g . T h i s r e v i e w has r e v e a l e d t h e u r g e n t neecr f o r a b s o r p t i o n c o e f f i c i e n t d a t a f o r c o a l s l a g s a t h i g h temperatures and f o r i n f o r m a t i o n r e l a t i n g t h e a b s o r p t i o n c o e f f i c i e n t t o t h e FeO c o n t e n t o f t h e s l a g . The h e a t t r a n s f e r p r o c e s s i n t h e c o a l g a s i f i e r can a l s o be a f f e c t e d by t h e l a y e r o f s l a g which l i n e s t h e w a l l s o f t h e g a s i f i e r . R e c e n t l y , G r i e v e s o n and B a g h a ^ — have developed a s i m p l e experiment f o r measuring t h e t h e r m a l f l u x (Q) i n v a r i o u s s l a g s used i n t h e c o n t i n u o u s c a s t i n g o f s t e e l . A w a t e r - c o o l e d , copper f i n g e r i s lowered i n t o a c r u c i b l e c o n t a i n i n g molten i r o n covered with a l a y e r o f s l a g and a l a y e r o f s o l i d i f i e d s l a g forms around t h e c o l d f i n g e r . The t h e r m a l f l u x i s determined by measuring t h e temperature r i s e o f t h e c o o l i n g water f l o w i n g through t h e copper f i n g e r . I t was found t h a t t h e heat f l u x was r e l a t e d t o ; ( i ) t h e t h i c k n e s s o f t h e s l a g l a y e r ; and ( i i ) the thermal r e s i s t a n c e o f the Cu/slag interface. The t h i c k n e s s o f t h e s l a g l a y e r i s , i n t u r n , dependent upon t h e v i s c o s i t y o f t h e s l a g and upon o t h e r f a c t o r s d e t e r m i n i n g t h e "melt back" o f t h e slag layer. Grieveson and B a g h a ^ — observed t h a t the thermal r e s i s t a n c e o f t h e C u / s l a g i n t e r f a c e appeared t o be r e l a t e d t o ; ( i ) t h e m i n e r a l o g i c a l c o n s t i t u t i o n o f t h e s l a g : and ( i i ) t h e s t r e n g t h o f t h e a d h e s i v e bond between t h e copper and t h e s l a g eg. t h e h e a t f l u x , Q, r e c o r d e d w i t h s l a g s from CaO.SiO phase f i e l d , which g i v e s good C u / s l a g a d h e s i o n , i s g r e a t e r than t h e Q r e c o r d e d f o r s l a g s from CaO.Al 0 .2SiO phase field with poor Cu/slag adhesion. Thus r e l a t i v e l y s i m p l e experiments l i k e t h e s e s i m u l a t i o n t e s t s c a n p r o v i d e a valuable insight into the f a c t o r s affecting heat transfer mechanisms o c c u r r i n g i n i n d u s t r i a l p r o c e s s e s . R

2+

+


2

Conclusions (i) E x p e r i m e n t a l d a t a f o r t h e t h e r m a l c o n d u c t i v i t i e s o f s l a g s must be c a r e f u l l y a n a l y s e d t o e s t a b l i s h t h e boundary c o n d i t i o n s o f the experiment ( e g * o p t i c a l t h i c k n e s s o f t h e specimen, magnitude o f k e t c ) . T h i s e v a l u a t i o n o f the d a t a a l l o w s one t o determine the s u i t a b i l i t y o f a s p e c i f i c thermal c o n d u c t i v i t y value f o r subsequent use i n heat b a l a n c e c a l c u l a t i o n s f o r the g a s i f i e r . (ii) The t h e r m a l c o n d u c t i v i t i e s o f c o a l s l a g s a r e n o t v e r y dependent upon t h e c h e m i c a l c o m p o s i t i o n o f t h e s l a g . ( i i i ) The t h e r m a l c o n d u c t i v i t y o f t h e s l a g i s dependent upon t h e degree o f c r y s t a l l i s a t i o n and c o n s e q u e n t l y upon t h e t h e r m a l h i s t o r y o f t h e specimen; t h e t h e r m a l c o n d u c t i v i t y o f the c r y s t a l l i n e phase i s g r e a t e r than t h a t o f t h e g l a s s y phase. (iv)

The r a d i a t i o n c o n d u c t i o n , k i s p r i n c i p a l l y determined by t h e magnitude o f t h e a b s o r p t i o n ( o r e x t i n c t i o n ) c o e f f i c i e n t . As t h e a b s o r p t i o n c o e f f i c i e n t o f t h e s l a g i s l a r g e l y dependent upon t h e (Fe ) c o n c e n t r a t i o n i n t h e s l a g , i t w i l l a l s o be dependent upon the f a c t o r s affecting t h e (Fe / F e ) ratio v i z . +

+


19.

MILLS


of Coal

Slags

275

temperature, p(0 ) and the SiO,,, CaO and Na O contents of the slag. * Experimental data are required f o r the absorption c o e f f i c i e n t s of coal slags at high temperatures so that the relationship between α and the FeO content can be established. Heat transfer i n the coal g a s i f i e r w i l l be p a r t i a l l y dependent upon the thermal resistance of the slag/wall interface and t h i s , i n turn, w i l l be dependent upon the mineralogical constitution of the slag adjacent to the wall. p

d

(v)


(vi)

Acknowledgements Valuable discussions with B. J. Keene, (National Physical Laboratory) Professor P. Grieveson (Imperial College) and Dr. R. Taylor (UMIST) are g r a t e f u l l y acknowledged.

References (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15)

(16)

Ammar, M.M; Gharib, S; Halawa, M M; El Badry, Κ; Ghoneim, Ν A; Batal, Ν A; E L, Batal, H A. J . Non-Cryst. Solids 1982, 53, 165. Steele, F.N; Douglas, R.W. Phys. Chem. Glasses 1965, 6, 246. Blazek, A; Endrys, J . Review of thermal conductivity data in glass, Part II Thermal conductivity at high temperatures published Intl. Commission on Glass, 1983. Fukao, Y, Mitzutani, H; Uyeda, S. Phys. Earth Planet Interiors 1968, 1, 57. Aronson, J.R; Belloti, L.H; Ecroad, S.W; Emslie, A.G; McConnel, R.K; Thuna, P.C von. J . Geophys. Res. 1970, 75, 3443. Schatz, J . F ; Simmons, G. J. Geophys. Res. 1972, 77, 6966. Fine, H.A; Engh, T; E l l i o t t , J . F . Metall. Trans B, 1976, 7B, 277. Gardon, R. paper presented at 2nd Intl. Thermal Conductivity Conference, Ottawa, 1962, 167. Gardon, R. Review of thermal conductivity data in glass, Part I Thermal conductivity at low and moderate temperatures published Intl. Commission on Glass, 1983. Touloukian, Y.S; Powell, R.W; Ho C.Y; Klemens, P.G. "Thermophysical Properties of Matter", volumes 1 (1970) and volume 10 (1973) published by IFI/Plenum, New York. Tye, R.P, "Thermal conductivity", volumes 1 and 2, 1969, published by Academic Press, New York. Kingery, W.D; Francl, J ; Coble, R.L; Vasilos, T. J . Amer. Ceram. Soc., 1954, 37, 107. Ogino, K; Nishiwaki, A; Yamamoto, K; Hama, S. Paper presented at Intl. Symp. Phys. Chem. Steelmaking, Toronto, 1982, III-33. Nauman, J ; Foo, G; E l l i o t t , J F. Extractive Metallurgy of Copper, Chapter 12, 237. Gibby, R.L; Bates, J . L . 10th Thermal Conductivity Conference, held Newton, Mass., Sept. 1970, IV-7,8. See also Bates, J . L . Final Report to National Science Foundation, Grant GI-44100 Properties of Molten coal slags relating to open cycle MHD, Dec. 1975. Taylor, R; Mills, K.C. Arch. f. Eisenhuttenw., 1982, 53, 55.



276

(31)


(17) Taylor, R; Edwards, R. cited by Mills, K.C and Grieveson, P, a paper presented at the Centenary Conference 1984, "Perspectives in Metallury" Sheffield University, July 1984. (18) Saito, A. Bull. Jap. Soc. Mech. Engr., 1980, 23, 1459. (19) de Castro, C.A.N; Li, S.F.Y; Maitland, G.C; Wakeham, W.A. in press Intl. J . Thermophys. (1984). (20) Mills, K.C; Powell, J.S; Bryant, J.W; Keene, B.J. Canad. Metall. Q., 1981, 20, 93. (21) Susa, M; Nagata, K; Goto, K.S. Trans. Iron Steel Inst. Japan, 1982 22, B42. (22) Mitchell, A; Wadier, J . F . Canad. Metall. Q, 1981, 20, 373. (23) Powell, J.S; Mills, K.C. Paper presented at the Ninth European Conference on Thermophysical Properties held in Manchester, September 1984. (24) Mills, K.C. Paper entitled "Estimation of physico-chemical properties of coal slags and ashes" to be presented at this conference. (25) Osinovskikh, L . L ; Kochetov, N.N; Bratchikov, S.G. Trudy Urals N-I Chern. Met., 1968 (8), 71. (26) Rudkin, R.L. Report US AF., ASD-TDR-62-24 II. (27) Keene, B.J; Mills, K.C. Arch f Eisenhuttenw. 1981, 52, 311. (28) Rafalovich, I.M; Denisova, I.A. Fiz. Khim Rasplav. Schlakov, 1970, 181. (29) Olusanya, A. "The Fundamental Properties of Continuous Casting Fluxes" PhD Thesis, Imperial College, London, 1983. (30) Mills, K.C; Grieveson, P; Olusanya, A; Bagha, S; Taylor, R. Paper entitled "Thermal and Physico-chemical properties of continuous-casting slags" to be presented at Ninth European Conference on Thermophysical Properties to be held in Manchester, Sept. 1984. Nagata, K; Goto, K.S. Private communication, Tokyo Inst. of Technology, 1984. (32) Powell, J.S; Mills, K.C. Unpublished thermal conductivity data on casting powders, National Physical Laboratory, 1984. (33) Ohmiya, S; Tacke, K.H; Sshwerdtfeger, K. Ironmaking and Steelmaking, 1983, 10, 24. (34) Varkaftik, N.B; Oleshchuk, O.N. Teploenergetika, 1955, 4, 13. (35) Ischenko, K.D. Met. Kobosokhim, 1970, (21), 82. (36) Kawada, K. Bull. Earthquake Res. Inst., 1966, 44, 1071. (37) Murase, T; McBirney, A.R. Science, 1970, 170, 16S. (38) Grieveson, P; Bagha, S cited by Mills, K.C; Grieveson, Ρ in a paper presented at the Centenary Conference 1984, "Perspectives in Metallurgy", Sheffield University, July 1984. RECEIVED June 17, 1985


Mineral Matter and Ash in Coal - American Chemical Society

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