26 Influence of Thermal Properties of Wall Deposits on Performance of Pulverized Fuel Fired Boiler Combustion Chambers W. Richter, R. Payne, and M. P. Heap
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Energy and Environmental Research Corporation, Irvine, CA 92718
The properties which determine heat transfer through a deposit layer of given thickness are thermal conduc t i v i t y , emissivity, and absorptivity. These proper ties vary with deposit temperature, thermal history, and chemical composition. Parametric studies and calculations for existing boilers were carried out to show the sensitivity of overall furnace performance, l o c a l temperature, and heat flux distributions to these properties in large p . f . fired furnaces. The property values used cover the range of recent experi mental studies. Calculations for actual boilers were carried out with a comprehensive 3-D Monte Carlo type heat transfer model. Some predictions are compared to full-scale boiler measurements. The calculations show that the effective conduction coefficient (k/Δs) of wall deposits strongly influences furnace exit temperatures. eff
The b u i l d u p o f a s h d e p o s i t l a y e r s on tube w a l l s and s u p e r h e a t e r s i n d r y bottom p . f . b o i l e r combustion chambers does n o t o n l y d e t e r i o r a t e f u r n a c e a n d o v e r a l l b o i l e r e f f i c i e n c y , but a l s o i n c r e a s e s t h e tem p e r a t u r e l e v e l i n f u r n a c e and c o n v e c t i v e passages and a g g r a v a t e s e x i s t i n g d e p o s i t problems. T h i s can f i n a l l y lead t o expensive out a g e s when d e p o s i t f o r m a t i o n c a n n o t be c o n t r o l l e d by s o o t b l o w i n g alone. S i n c e e r r o r s i n f u r n a c e d e s i g n w i t h r e s p e c t t o s l a g g i n g and f o u l i n g o r i n c o r r e c t e s t i m a t e s o f t h e impact o f f u e l c o n v e r s i o n on d e p o s i t f o r m a t i o n a r e so c o s t l y i n l a r g e b o i l e r s , t h e r e i s c o n s i d e r a b l e f i n a n c i a l i n c e n t i v e t o d e v e l o p a n a l y t i c a l methods i n o r d e r t o p r e d i c t f u r n a c e performance f o r a wide range o f c o a l types and oper ating conditions. I t i s c l e a r t h a t such methods must take q u a n t i t a t i v e l y i n t o a c c o u n t , among o t h e r t h i n g s , t h e t h e r m a l p r o p e r t i e s o f a s h d e p o s i t s , i . e . t h e r m a l c o n d u c t i v i t y , e m i s s i v i t y , and a b s o r p tivity. The c u r r e n t paper p r e s e n t s r e s u l t s from v a r i o u s s t u d i e s c a r r i e d o u t by t h e a u t h o r s t o show t h e i n f l u e n c e o f t h e r m a l p r o p e r t i e s o f a s h d e p o s i t s on p e r f o r m a n c e o f l a r g e p . f . f i r e d b o i l e r f u r n a c e s . The paper i s d i v i d e d i n t o t h r e e s e c t i o n s . I n the f i r s t s e c t i o n , key 0097-6156/ 86/ 0301 -0375506.00/ 0 © 1986 American Chemical Society
Vorres; Mineral Matter and Ash in Coal ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
376
MINERAL MATTER AND ASH IN COAL
p a r a m e t e r s o f o v e r a l l f u r n a c e performance a r e i d e n t i f i e d w i t h the h e l p o f a s e n s i t i v i t y s t u d y and r e l a t e d to the ash d e p o s i t prop erties. The second s e c t i o n summarizes e x p e r i m e n t a l d a t a of t h e r m a l p r o p e r t i e s o f d e p o s i t s and d e f i n e s t h e r a n g e o f v a l u e s p r o b a b l y o c c u r r i n g i n b o i l e r combustion chambers. In the t h i r d section, t y p i c a l p r o p e r t y v a l u e s a r e used i n c o m b i n a t i o n w i t h a s o p h i s t i c a t e d 3-D h e a t t r a n s f e r model i n o r d e r to demonstrate the e f f e c t of e x i s t i n g d e p o s i t l a y e r s on l o c a l t e m p e r a t u r e and h e a t f l u x d i s t r i b u t i o n and performance o f p a r t i c u l a r b o i l e r f u r n a c e s .
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Parametric
Study of O v e r a l l Furnace
Performance
The p a r a m e t r i c s t u d i e s w e r e c a r r i e d o u t i n o r d e r t o i d e n t i f y the most i m p o r t a n t p a r a m e t e r s i n f l u e n c i n g o v e r a l l f u r n a c e heat a b s o r p tion. These parameters a r e : ο A d i a b a t i c Flame Temperature ο F i r i n g Density ο Total Emissivity ο Temperature of Heat S i n k S u r f a c e s ο Flow and Heat R e l e a s e P a t t e r n s F i g u r e 1 shows how these q u a n t i t i e s a r e r e l a t e d i n a complex manner to e a c h o t h e r , to f u e l c h a r a c t e r i s t i c s , to f u r n a c e o p e r a t i n g c o n d i t i o n s , and to w a l l d e p o s i t s . Some o f t h e r e l a t i o n s h i p s i n t h i s f i g u r e w e r e i n v e s t i g a t e d u t i l i z i n g a s i m p l e w e l l - s t i r r e d f u r n a c e m o d e l (I) which assumed t r a n s p o r t of gray r a d i a t i o n . This w e l l - s t i r r e d analysis predicted q u a l i t a t i v e l y t h e d e p e n d e n c e o f f u r n a c e e f f i c i e n c i e s r i f and e x i t temperature T , both r e l a t e d by E q u a t i o n 1 e x
T
M C 0
n
f
- ι -
— -
p
ex IT 0
(T
e x
-
T ) 0
(l)
:
Qo on f u r n a c e d e s i g n and o p e r a t i n g c o n d i t i o n s . An important d e s i g n parameter i s the f u r n a c e h e i g h t L r e q u i r e d to o b t a i n a d e s i r e d e f f i c i e n c y o r e x i t t e m p e r a t u r e (1 m of f u r n a c e h e i g h t c o r r e s p o n d s a p p r o x i m a t e l y to $500,000). I t was found t h a t the h e i g h t L depends c o n s i d e r a b l y on the c h a r a c t e r i s t i c s o f the w a l l d e p o s i t s , e s p e c i a l l y in large furnaces. T h i s i s shown i n F i g u r e s 2 and 3 i n which the efficiency i s p l o t t e d over the h e i g h t L w i t h s u r f a c e temperature T o f d e p o s i t s and s u r f a c e e m i s s i v i t i e s e as p a r a m e t e r s . The c a l c u l a t i o n s were c a r r i e d out f o r a r e c t a n g u l a r f u r n a c e box of w i d t h L/3. O t h e r i n p u t parameters a r e l i s t e d i n the f i g u r e s . The s t r o n g impact of w a l l temperatures f o r l a r g e r s i z e s ( F i g u r e 2) i s due t o t h e f a c t t h a t l a r g e f u r n a c e v o l u m e s a p p r o a c h b l a c k r a d i a t o r s and volume e m i s s i v i t i e s cannot be i n c r e a s e d anymore by i n c r e a s e i n s i z e . F o r f u r n a c e s o p e r a t e d w i t h the same t h e r m a l i n p u t a t low e f f i c i e n c i e s , the presence of w a l l d e p o s i t s r e q u i r e s o n l y a moderate increase i n size. A r e d u c t i o n of s u r f a c e e m i s s i v i t i e s from 1 ( c l e a n " s o o t y " w a l l s ) t o 0.4 w h i c h i s t h e l o w e s t range r e p o r t e d f o r ash d e p o s i t s a l s o c a u s e s a drop of r\f o r r e q u i r e s an i n c r e a s e i n h e i g h t i n o r d e r t o m a i n t a i n rif ( F i g u r e 3 ) . The s i z e r e q u i r e d v a r i e s n o n l i n e a r l y w
w
Vorres; Mineral Matter and Ash in Coal ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
26.
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RICHTER ET AL.
Adiabatic
Performance
377
Load
Input .
Density • Furnace Geometry Fuel Composition Combustion Conditions
Heat S i n k Area •
Downloaded by FUDAN UNIV on January 10, 2017 | http://pubs.acs.org Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch026
Chambers
Heating Value Moisture Content Ash Content Excess A i r A i r & Fuel Preheat
Flame Temperature -
Thermal Firing
of Boiler Combustion
E m i s s i v i t y eg of Combustion Products •
Total
Emissivity · I Emissivity e I s o r p t i v i ty a ι Heat Sink
w
w
Abof Wall
^Therm. C o n d u c t i v i t y •
Temperature of ^ Heat Sink Surface
Net Heat k
•
Flux
D e n s i t yf
