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Gas Combustion in Shallow Fluidised Beds - Broughton

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  • 8/12/2019 Gas Combustion in Shallow Fluidised Beds - Broughton

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    G A S C O M B U S T I O N I N S H A L L O W F L U I D I S E D B E D S

    J BROUGHTONDepartment of Mechanical Engineering, The University of Aston in Birmingham,Gosta Green, Birmingham B4 7ET Great Britain)

    SUMMARYC o m b u s t io n o f p r e m i x e d g a s a n d a i r i n s h a l lo w f l u i d i s e d b e d s i s d e s c r ib e d a n d s o me o fthe l imi t s s e t by gas ve loc i ty an d par t ic le s i ze are d i scussed .

    There i s a lower l im i t o f gas ve loc i ty be low w hich hea t t rans fer back to the d i s t r ibu torp la te causes excess ive preh eat l ead ing to pre - ign i t ion a nd to t empera tures w e l l abovethe equ i l ibr ium leve l . Th is cond i t ion can cause par t ic le s in ter ing an d , a t wo rs t ,d i s t r ibu tor fa i lure . A graph show ing a so lu tion o f the pro blem i s presen ted .

    I t i s shown tha t there is a lower l imi t to par t ic le s i ze , w hich depends upon the par t ic ledens i ty , be low w hich s tab le com bus t ion wi l l no t occur . Th is l imi t i s s e t when enough o ft h e g as ~a ir m i x t u r e b y p a s s e s t h e b e d w i t h o u t b u r n in g t o p r e v e n t t h e b e d f r o m r e a c h in gc o mb u s t i o n t e mp e r a t u re s . T h e e x p e r i m e n t a l o b s e r v a t io n s a r e e x p l a i n e d i n t e r ms o f th et w o - p h a se t h e o r y o f f l u i d i s a t i o n b y p o s t u l a t i n g t h a t t h e f u e l w h i ch p a s se s t h r o u g h t h edense pha se o f the f lu id i sa t ion i s to ta l ly burn t , whi le tha t pass ing up through the bed inbubbles e i ther does no t reac t or reac t s too la te fo r i t s comb us t ion hea t to be t rans fer redto the bed.

    A q u a n t i t a ti v e mo d e l b a s e d u po n t h e se t w o a d - h o c a ss u mp t i o n s i s s h o w n t o p r o v i d ereasonable agree me nt w i th the observed resu l ts and poss ib le re f inemen ts o f the s impl i f i edtheory , to make i t more r igorous ly based , are ind ica ted .

    NOM ENCLATUREC p S p e c i fi c h e a t k J / k g KC ~ M e a n s p ec if ic h e a t o v e r a t e m p e r a t u r e r a n g e k J / k g KD n B u b b l e d i a m e t e r md r P a r t i c l e d i a m e t e r /~ mF R a d i a t i o n v i e w f a c t o r

    61Applied Energy (1) (197 5)- - Applied Science Publishers Ltd, England, 1975Printed in Great Britain

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    62 J. BROUGHTON

    fu F r a c t i o n o f f ue l u n b u r n tho H e a t t r a n s f e r c o e f fi c ie n t W / m KL B e d d e p t h mL, ,~ Bed de p th a t Uml mQ V o l u m e t r ic b u b b l e d e n s e p h a s e e x c h a n g e m 3 / sQ s H e a t f lu x t o d i s t r i b u t o r s u r f a c e W / m zT n B e d t e m p e r a t u r e KT ig F u e l i g n i t io n t e m p e r a t u r e KT o I n i ti a l g a s t e m p e r a t u r e KT s D i s t r i b u t o r s u r fa c e t e m p e r a t u r e KU 8 B u b b l e v e l o c i ty m / su S u p e r f i c i a l g a s v e l o c i t y m / su c C r i t ic a l g a s v e l o c i t y f o r f l a s h b a c k m / su m l M i n i m u m f lu i d is i n g v e l o c i t y m / sV 8 B u b b l e v o l u m e m 3X E x c h a n g e f a c t o r~B B u b b l e h o l d - u pP o G a s d e n s it y a t T o k g / m 3

    S t e fa n - B o l tz m a n n c o n s ta n t W / m 2 K 4

    1 N T R O D U C T I O N

    T h e l a s t d e c a d e h a s s e e n a la r g e d e v e l o p m e n t e f f o r t i n t h e f ie ld o f c o a l c o m b u s t i o ni n fl u id i s e d b e d s , b o t h o n p o w e r s t a t i o n a n d p a c k a g e b o i l e r sc a le s . ~,2 S i g n i f ic a n td e v e l o p m e n t s h a v e c o m e f r o m B r it ai n , 3 t h e U S A , 4 F r a n c e , 5 G e r m a n y , 6 C z e c h o -s l o v a k i a , 7 t h e U S S R 8 a n d A u s t r a li a . 9 A l a rg e n u m b e r o f c o a l t y p e s h a v e b e e nb u r n e d w i t h li tt le p r e t r e a t m e n t o t h e r t h a n s o m e cr u s h in g , a n d s u l p h u r r e t e n t i o n b yt h e a d d i t i o n o f l im e s t o n e t o t h e c o m b u s t i o n b e d s h a s b e e n s u c c e s s fu l lyd e m o n s t r a t e d . T h e d e v e l o p m e n t s h a v e b e e n r a p i d a n d h a v e o f t e n o u t s t r i p p e d t h es c ie n ce o f f l u i d i sa t i o n . F o r e x a m p l e , f u el m o v e m e n t a w a y f r o m i n j e c t io n p o i n t s h a so n l y r e c en t ly b e e n a d e q u a t e l y s o lv e d 1 , a n d t h e m e c h a n i s m s d e t e r m i n i n g c o a lc o m b u s t i o n h a v e o n l y r e c e n t l y b e e n s t u d i e d i n d e ta i l. ~

    T h i s e m p h a s i s o n c o a l c o m b u s t i o n h a s t e n d e d t o o v e r s h a d o w t h e m a n y o t h e rp o s s i b le e n g i n e e r in g a p p l i c a ti o n s o f f lu id b e d c o m b u s t i o n , t h e c o m b u s t i o n o f g as e sa n d o i l s h a v i n g b e e n l a r g e l y n e g l e c t e d .

    T h e i n t e r e s t i n o i l c o m b u s t i o n h a s c e n t r e d a r o u n d g a s i f i c a t i o n a n d M o s s h a sd e s c r i b e d a p r o c e s s i n w h i c h o il is s im u l t a n e o u s l y d e s u l p h u r i s e d a n d g a s if ie d , l zE l l io t t h a s d i s c u s se d s o m e o f th e a c t u a l a n d p o s s i b l e a p p l i c a t i o n s o f g a s - f ir e dc o m b u s t o r s o n a s m a l l s c a le , 13 s u c h a s c o n t r o l l e d a t m o s p h e r e m e t a l l u r g i c a lf u r n a c e s , d o m e s t i c c e n t r a l h e a t i n g u n i ts , r a d i a n t g a s fi re s a n d i n g a s t u r b i n e c y c l e s.

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    GAS COMBUSTION IN SHALLOW FLUIDISED BEDS 63T h e r e h a v e a l s o b e e n a n u m b e r o f d e v e l o p m e n t s in w a s t e i n c i n e r a ti o n i n f lu i di se db e d s . 14 T h i s p r o c e s s i s n o w w e l l e s t a b l i s h e d a s a m e a n s o f d i s p o s a l f o r o t h e r w i s ed i ff ic u lt t o p r o c e s s m a t e r i a l s a l t h o u g h v e r y f ew o f t h e d e s ig n s t o d a t e h a v e i n -c o r p o r a t e d h e a t r e c o v e r y . 15

    T a m a l e t h a s d e s c r i b e d a n o n - p r e m i x e d g a s - f i r e d c o m b u s t o r i n w h i c b t h e d i s -t r i b u t o r p l a t e is d e s i g n e d i n su c h a w a y a s t o o v e r c o m e t h e p r o b l e m o f p o o r g a sm i x i n g i n fl u id i s ed b e d s . 16 T h i s d e s i g n r e m o v e s t h e m i x i n g p r o b l e m r e p o r t e d b yZ a b r o d s k y a n d A n t o n i s h in w h o f o u n d t h a t b e d d e p t h s o f a l m o s t 150 m m w e ren e e d e d f o r c o m b u s t i o n i n a t e s t b u r n e r w h e r e t h e g a s e s w e r e n o t p r e m i x e d , w h i l el es s t h a n 4 0 m m s u ff ic e d w h e n t h e g a s e s w e r e p r e m i x e d . 17 H o w a r d r e p o r t s s o m el im i ts o f c o m b u s t i o n f o r p r o p a n e / o x y g e n / n i t r o g e n m i x t u r e s i n fl u id i se d b e d s 18 a n dB a s k a k o v e t a l . r e p o r t o n t h e u s e o f c a t a l y s t s t o i n c r e a s e t h e r i c h l i m i t, a s is n e e d e di n c a r b u r i s i n g f u r n a c e s . 19

    T h e w o r k d e s c r i b e d i n t h i s p a p e r c o n c e r n s p r e m i x e d g a s e o u s c o m b u s t i o n i ns h a l lo w f l ui di se d b e d s a n d w a s c o n s id e r e d i m p o r t a n t b o t h o n t h e o r e t ic a l a n dp r a c t i c a l g r o u n d s . T h e o r e t i c a l l y b e c a u s e i t i s i n t e r e s t i n g t o s p e c u l a t e h o w s h a l l o wc a n a fl u id i se d b e d c o m b u s t o r b e , w h a t i s t h e c o m b u s t i o n m e c h a n i s m , a n d w h a t a r ei ts l i m i t a t i o n s ? P r a c t i c a ll y b e c a u s e t h e e c o n o m i c s o f f lu i d is e d b e d c o m b u s t i o ni n d ic a t e t h a t s h a l l o w b e d s s h o u l d b e u s ed w h e r e v e r p o ss i b le to m i n im i s e p u m p i n gc o s t s , th i s b e i n g a m a j o r c o n s i d e r a t i o n f o r a ll sc a le s o f e q u i p m e n t 13. T h e b e d d e p t hr e q u i r e d c a n b e s e t b y c o m b u s t i o n e f fi ci en c y o r b y p a r t i c u l a r p r o c e s s n e e d s , s u c h a sc o v e r i n g t h e o b j e c t u n d e r t r e a t m e n t a s i n a m e t a l l u r g i c a l f u r n a c e o r c o v e r i n g t h et u b e s i n t h e f l u i d i s e d b e d b o i l e r . T h e l a t t e r c o n s i d e r a t i o n w a s c e r t a i n l y a f a c t o r i ne a r l y c o a l -f i re d f lu i d is e d b e d b o i l e r d e s i g n s b u t m a y n o l o n g e r h o l d i f f in n e d t u b e sa r e u s e d a n d c o u l d w e ll b e e li m i n a t e d e n t i r e ly b y u s i n g t h e s o l id s c i r c u l a t io nt e c h n i q u e s d e s c r i b e d b y E l l io t t . 13

    GENERAL CONSIDERATIONS

    T h e r e is a w i d e r a n g e o f v a r i a b l e s t o b e c o n s i d e r e d d u r i n g t h e d e s i g n o f fl u id i s ed b e db o i l e rs . F o r e x a m p l e , p a r t i c le s iz e a n d t y p e , b e d d e p t h , g a s fl o w s a n d t h e d e s ig n o fd i s t r i b u t o r , f r e e b o a r d a n d w i n d b o x c a n a ll h a v e i m p o r t a n t e ff e c ts o n t h e e ff ic ie n c y,t h e s t a r t - u p c h a r a c t e r i s ti c s a n d t h e r e l ia b i li ty o f th e f in a l d e s ig n . L e s s c o m m o nv a r i a b l e s a r e t h e o p e r a t i n g p r e s s u r e a n d , w h e n u s i n g c e n t r i f u g a l f o r c e o p p o s e db e d s , t h e n u m b e r o f g r a v i t ie s a c ti n g . T h i s p r e s e n t w o r k c o n c e r n s t h e e f fe c ts o fp a r t i c l e s iz e , g a s v e l o c i t y a n d d i s t r i b u t o r d e s i g n , a s th e s e f a c t o r s u s u a l l y d e t e r m i n et h e m i n i m u m b e d d e p t h r e q u ir e d f o r c o m b u s t i o n a n d t h e siz e a n d c o s t o f t h e sy s te m .

    W h i l e t h e c o m b u s t i o n p r o c e s s e s i n f lu i d is e d b e d c o m b u s t i o n a r e q u i t e d i f f e r e n tf r o m t h o s e i n s i m p l e g a s b u r n e r s , t h e r e i s a q u a l i t a t i v e s i m i l a r i t y b e t w e e n t h ep r o c e s s e s . I n t h e d i s t r i b u t o r a n d in t h e f lu i d is e d z o n e j u s t a b o v e t h e d i s t r i b u t o r

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    64 J. BROUG ttTONp l a t e , th e r e i s a re g i o n o f p r e h e a t . T h e l e n g t h o f t h is z o n e d e p e n d s o n a n u m b e r o ff a c t o r s s u c h a s p a r t i c l e s i z e , g a s d i s t r i b u t o r d e s i g n , g a s v e l o c i t y , b e d t e m p e r a t u r e ,t h e g r o s s c i r c u l a t i o n o f so l i d s in th e b e d a n d t h e f u e l p r o p e r t i e s .

    T h e p r e h e a t z o n e , w h i c h e n d s w h e n t h e g a s / a i r m i x t u r e r e a c h e s c o m b u s t i o nt e m p e r a t u r e , is f o l l o w e d b y th e r e a c t i o n z o n e w h i c h is u s u a l l y q u i t e s h o r t . T h ef i n al p o s t - c o m b u s t i o n r e g i o n i s u s u a l l y c h a r a c t e r i s e d b y i s o t h e r m a l c o n d i t i o n s . I ft h e l e n g t h o f t h e r e a c t i o n z o n e p l u s th e p r e h e a t z o n e a p p r o a c h e s a v a l u e e q u a l t o t h eb e d d i a m e t e r , t h e r e a c t io n m a y b e c o n t r o l l e d b y th e s l u g g in g p r o p e r t i e s o f t h e b e da n d s h o u l d t h e b u b b l e d i a m e t e r e x c e e d o n e - t h ir d o f th e b e d d i a m e t e r , th e r e s u lt sm u s t b e t r e a te d w i t h g r e a t c i r c u m s p e c t io n . A s c o m b u s t i o n o f g a s / a i r m i x t u r e s w a sk n o w n t o b e c o m p l e t e w i th i n 5 0 m m a b o v e t h e d i s t r ib u t o r p l a te , th e b e d d i a m e t e ro f 1 0 0 m m u s e d i n m o s t o f t h e e x p e r i m e n t a l w o r k r e p o r t e d h e r e i n w a s s u ff ic i en t lyl a r g e f o r s l u g g i n g t o b e i m p o s s i b l e .

    D I S T R I B U T O R P L A T E S

    T w o t y p e s o f p r o b l e m o c c u r in t h e d e s ig n o f d i s t r i b u t o r p l a t e s f o r f l u id i s ed b e dc o m b u s t o r s . F i r s t l y , g a s m a l d i s t r i b u t i o n c a n h a v e s e r i o u s c o n s e q u e n c e s s u c h a sg r o s s o v e r h e a t i n g a n d s i n te r in g o f t h e p a r t ic l e s i n th e b e d . T h i s p r o b l e m c a n b eo v e r c o m e b y t h e u s u a l m e t h o d o f e m p l o y i n g a p r es s u r e d r o p t h r o u g h t h e d is t r ib u t o rw h i c h i s c o m p a r a b l e w i t h t he p r e s s u re d r o p t h r o u g h t h e b e d , t h u s e n s u r i n g th a t t h ed i s t r ib u t o r is h o m o g e n e o u s .

    T h e s e c o n d p r o b l e m is t h a t o f h e a t t r a n s f e r b a c k t o t h e d i s tr i b u t o r b y r a d i a t i o nf r o m t h e b e d t o t h e p l a t e , c o u p l e d w i t h s o l i d s / p l a t e h e a t t r a n s f e r w h i c h c a u s e s t h et e m p e r a t u r e o f t h e d i s t r i b u t o r t o r i se . T h i s is p a r t i c u l a r l y e v i d e n t i n e x p e r i m e n t sw h e r e a t t e m p t s a r e m a d e t o r u n w i t h lo w m a s s t h r o u g h p u t s o f n e a r - s t o i c h i o m e t r i cm i x t u r e s .

    I n t h e s e c as e s , e s p e c i a l ly w h e n p o r o u s d i s t r i b u t o r p l a t e s a r e u s ed , t h e p l a t e s c a nh e a t u p t o s u c h a le v el t h a t c o m b u s t i o n s ta r ts t o o c c u r w i th i n t h e p l a t e s - - w h i c h t h e na r e s u b j e c t t o g r o s s o v e r h e a t i n g - - a n d p l a t e f a i lu r e is i n e v it a b le . T h u s , t h e r e i s al o w e r c r i t ic a l v a l u e o f fl o w b e l o w w h i c h i t is im p o s s i b l e t o o p e r a t e . T h i s c r i ti c a lv a l u e c a n b e s t b e d e t e rm i n e d e x p e r im e n t a l ly . D i s t r i b u t o r o v e r h e a t i n g c a n a l so o c c u ri n a n o t h e r w i s e a c c e p t a b l e d e s ig n if, a s m e n t i o n e d a b o v e , a p o r t i o n o f t h e d i s t r i b u t o rh a s a l o w e r t h a n n o r m a l f l o w . T h i s t h e n c a u s e s l o c a l o v e r h e a t i n g , s i n t e r i n g a n dfa i l u re .

    T h e a b o v e t w o p r o b l e m s a r e f a i r l y e a s i l y o v e r c o m e i n l a b o r a t o r y a p p a r a t u sw h e r e h i g h p r e s s u re a i r f r o m t h e l a b o r a t o r y c o m p r e s s e d a i r s u p p l y i s u s e d a n d c a nb e r e g u l a te d t o g i ve c o n s t a n t m a s s fl o w o f ai r, b u t a m u c h m o r e s e v e r e p r o b l e mo c c u r s w h e n u s i n g a l o w p r e s s u r e b l o w e r t o s u p p l y t h e a i r . T h i s i s b e c a u s e t h ep r e s s u r e d r o p / f l o w c h a r a c t e r i s ti c s o f t h e d i s t r i b u t o r p l a t e w il l v a r y w i t h th et e m p e r a t u r e o f t h e b e d d u e t o th e h e a t t r a n s f e r b a c k t o t h e p l at e c a u s in g t h e g a s / a i r

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    GAS COMBUSTION IN SHALLOW FLUIDISED BEDS 65mixture volume flow to increase. Thus, unless the pressure drop/flow characteristicsof the distributor are matched to those of the blower, progressive deterioration ofconditions will occur. By careful attention to design, these problems can be over-come even when using distributor pressure drops of less than 200 N/m 2.13

    When using pierced plate distributors, the heat soak-back problem is less evidentbecause the non-fluidised areas between the individual holes act as heat insulation,but flash-back through the holes themselves can occur. It has been found that suchcond itions do not occur provided a free area of less tha n 4 per cent is used. 2

    EXPERIMENTAL WORKApparatusThe apparatus used is shown diagrammatically in Fig. 1. The fuel gas, usuallymethane, is mixed with air before a flame-trap and passed into the windbox of thereactor. The reactor used in most experiments was a 100 mm inside diameter mild

    A I RFUEL

    O A S S A M P L .O L . .E

    1T EM P E RA TU R E M E A S U R E M E N T l C O O L E R

    O T A M E R S M A O M ET E R S

    Fig. 1. Flow diagram of the experimental equipment.

    steel tube 300 mm in length, although many other designs and sizes have been usedin other tests. ~3 The outside of the tube was lagged with Trimor ceramic blanket.The bed temperature was measured by a movable chromel/alumel thermocoupleand the temperature was controlled by an annular cooling tube whose area ofimmers ion in tile bed could be easily varied to give fine control o f the heat removalrate.

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    66 J. BROUGttTONThe flue gases were analysed using a Fison's mini half-gas chromatograph. The

    sampling device used was an expansion nozzle, from which the gases were passedthrough a drying tube to collecting loops for subsequent analysis. The chromato-graph contained a silica gel and a molecular sieve column which, by using helium asthe carrier gas, enabled H2, N2, 02 , CO, CO2 and CH 4 to be separated. Themaximum erro r in analysis was estimated by using calibrated samples to be less than10 per cent for any of these constituents. Oxides (NO + NO2) to be estimated to4- 5 ppm.

    The pressure drop across the bed and that across the distributor plate weremeasured by water manometers. Two types of distributor plate were used, porousceramic tiles and pierced plates of free area less than 4 per cent.P a r t i c l e s

    A wide range of particle sizes and types have been used in the tests: silica sand,quartz sand, alumina, magnesia, zirconia , limestone and coal ash of various sizesbetween 90 and 1500 itm. Provided that the ratio air supplied/stoichiometric air (~)

    T A B L E 1PARTICLES USED AND THEIR PROPERTIESParticle Densi ty Mean size umy u,,yK g / m 3 ~ tm N T P m / s T > 1 0 0 0 KS i l i c a 2 5 6 0 3 1 0 0 . 1 21 0 . 0 3 9s a n d 3 8 6 0 . 1 9 4 0 - 0 6 04 2 0 0 . 2 2 0 0 - 07 15 4 7 0 . 6 7 0 0 . 1 2 I7 7 7 0 . 8 4 0 . 2 4 5Z i r c o n i a 6 0 0 0 1 I 0 0 . 0 3 1 0 . 0 1 01 9 5 0 . 0 8 5 0 . 0 3 3

    is greater than 0.8, catalysis appears to be unimportant: therefore untreated sandand zircon ia were used in the majority of tests. The proper ties of these particles aregiven in Table I.

    E X P E R I M E N T A L P R O C E D U R E

    Particles were poured into the reactor to the desired bed depth, combustion wasinstigated as described by Elliott and Virr, 21 and the bed tempera ture was controlledat a selected value by altering the area of cooling surface immersed in the bed. Thebed depths used ranged from 20 mm to 100 mm and the temperatures from 1000 Kto 1400 K. The lower temperature limit was set by combustion stability as themethane and pr opa ne used in the experiments do not react well below about 1000 K.

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    GAS COMBUSTION IN SHALLOW FLUIDISED BEDS 6 7T h e u p p e r l i m i t w a s i m p o s e d b y t h e s te e l w a l l b u t i t w a s e x t e n d e d a s f ar a s p r a c t i c a b l eb y r e m o v i n g p a r t o f t h e la g g i n g a t h i g h t e m p e r a t u r e s i n o r d e r t h a t t h e w a l l c o u l dc o o l i t s e l f m a i n l y b y r a d i a t i o n .

    GENERAL OBSERVATIONSB y v a r y i n g t h e b e d d e p t h a t c o n s t a n t v a l u e s o f e a n d g a s v e l o c it y , i t w a sd e m o n s t r a t e d t h a t p r o v i d e d ~ l a y w i t h i n t h e r a n g e 0 -8 < ~ < 2 , t h e r e a c t io n w a sn o t d e p e n d e n t u p o n a n y o f t h e r a t e s o f r e a c t i o n . W h e n t h e b e d te m p e r a t u r ew a s l es s t h a n 1 10 0 K t h e c o m b u s t i o n t e n d e d t o b e a c c o m p a n i e d b y a r a p i d s e ri es o f

    . 2 -

    t 2 0w ' l

    g 2

    V I I I I 0 1 -4 1 .8( I

    Fig. 2. Flue gas analysis at di fferent ai r to fuel rat ios (~) . Th e l ines represen t the theoret icalequi l i b r ium com posi t i on a t 1273 K.

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    68 J. BROUGHTONsmall detonations and small flashes of gas-phase combustion could be observedbelow the bed surface. This effect was also strongly dependent upon particle size,being much less pron oun ced when the mean particle size was greater than 600 mthan with smaller particles.

    Stable combustion, even with bed depths of 100 ram, could not be attained whenusing silica sand particles smaller than 300 I t m . However, with zirconia combustionwas obtained with particles of mean diameter 110 pm.

    The oxides of nitrogen levels obta ined were always less than 10 ppm, 5 ppmbeing common. Analysis of gas samples taken from the bed indicated that com-bustion was substantially complete above 20 mm above the distributor plate whenparticles larger than 600/~m and the porous plate type of dist ributor were used.When pierced-plate distributors were used the bed depth required for completecombustion was about 50 mm.

    The flue gases were analysed using a bed depth of 80 mm and 777 pm sand to testthe appro ach to the rmod ynam ic equilibrium over the range of~ at a bed temperatureof 1273 K for superficial gas velocities of 0.2, 0.4 and 0.6 m/s. Th at equilibrium wasobtained is apparent from Fig. 2.

    THE THERMAL ENTRY ZONE

    The thermal entry zone is defined here as being that height of bed above thedistributor plate at which the temperature reaches the main bed temperature. Thiszone can be illustrated by reference to Fig. 3 where the entry zone can be seen toconsist of a preheating zone and a reaction zone where the temperature-can exceedthe bulk bed temperature. The thermal entry zone depends upon many parameters,so only general comments are possible at this time. As shown in Fig. 4, thetemperature fluctuations were low in the preheat zone but became very apparent atthe height where the temperature approached that of the bed. This suggests thatlittle reaction occurs in the preheat zone. The length of the latter appeared todepend mainly on the bed temperature and the gas velocity when porous distributorswere used, since these parameters tend to determine how much heat is transferredback to the plate and hence how much preheat could be supplied to the gases beforeentry to the bed.

    In the case of the pierced plate distributors, the preheat process is much influencedby the orifice spacing and consequently it is less easy to generalise about the effectsof gas velocity and bed temperature. 2 o, 22

    When the excess gas velocity was low, i . e . u - u,,z < 0.20 m/s, the temperatu remaximum usually occurred in the entry zone, probably as a consequence of thereduced solids mixing which occurs at these low bubbling rates. A further effect oflower velocities was that combustion tended to spread back into the plate whichcould lead to the plate sintering as described previously.

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    GAS COMBUSTION IN SHALLOW FLUIDISED BEDS 69

    12cx>-

    I - -

    I OOO-

    Fig. 3 .

    o~ =1.1

    ~ o 4 b 6 0H E I GH T A B O V E D IS T R I B U T O R m m

    T y p i c a l v e r t i c a l - p l a n e t e m p e r a t u r e p r o f i l e s .

    1140~

    I I O C

    I O 8 C O 4 0R A D I A L D I S T A N C E m m

    F i g . 4. T y p i c a l r a d i a l - p l a n e t e m p e r a t u r e p r o f i le s s h o w i n g f lu c t t l a t io n s a t v a r i o u s h e i g h t s a b o v e ap i e r c e d p l a t e d i s t r i b u t o r .

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    70 J. BROUGHTONBurning back

    A s p l a t e s t a b i l i t y i s a c r u c i a l c o n s i d e r a t i o n a n d b u r n - b a c k i n t o t h e p l a t e s l e a d st o t h e i r f a i l u r e , s o m e c r i t e r i a f o r s t a b l e p l a t e o p e r a t i o n w e r e n e e d e d . B u r n - b a c kc o u l d b e e a s il y d e t e c t e d b y o b s e r v i n g t h e p r e s s u r e d r o p a c r o s s t h e d i s t r i b u t o r p l a t ef o r , w h e n c o m b u s t i o n s p r e a d s i n t o t h e p l a t e , t h e p l a t e t e m p e r a t u r e r i s e s a n dc o n s e q u e n t l y t h e p r e s s u r e d r o p r i s e s r a p i d l y . A l s o , t h e b e d t e m p e r a t u r e t e n d s t or is e s u d d e n ly a n d a n y c o m b u s t i o n n o i se c e as e s.

    T h e p r o c e d u r e u s e d i n i n v e st ig a t in g b u r n - b a c k w a s t o s e le c t a b e d t e m p e r a t u r ew i t h t h e b e d b u b b l i n g v i g o r o u s l y , t h e n t o n o t e t h e d i s t r i b u t o r ' s a p p r o x i m a t es u r f a c e t e m p e r a t u r e b y a t h e r m o c o u p l e r e s ti n g o n th e s u r fa c e . S a n d o f m e a n s i ze4 2 0 / a m w a s u s ed . T h e a i r w a s r e d u c e d b y a s m a l l a m o u n t a n d t h e fu e l f lo w a lt e r e dt o m a i n t a i n t h e b e d t e m p e r a t u r e a t t h e d e s ir e d v a lu e , th e n t h e s u r fa c e t e m p e r a t u r ew a s n o t e d . T h e p r e s s u r e d r o p a c r o s s t h e p l a t e w a s a l s o m o n i t o r e d . W h e n b o t hm e a s u r e m e n t s b e c a m e s t e a d y t h e b e d w a s f u ll y in e q u i li b r iu m . T h e t i m e r e q u ir e df o r e q u i l ib r i u m t o b e o b t a i n e d w a s o f t h e o r d e r o f t e n m i n u t e s. T h i s p r o c e d u r e o fr e d u c i n g t h e t o t a l fl o w w a s c o n t i n u e d u n t il e i t h e r th e p l a t e s ' p r e s s u r e d r o p i n c r e a s e ds u d d e n l y o r t h e s u r f a c e t e m p e r a t u r e e x c e ed e d t h e b e d t e m p e r a t u r e a s e ac h o f th e s e

    5

    C

    lII 1 2 O O 1 3 O O

    BED TEMPER ATURE KFig. 5. Critical gas velocities for flashback into a por ou s plate dist ribu tor. A: eqn. (4) forT~ ~ T~o; B: eqn. (4) for T, 1000 K; C: eqn . (6) T, T,,; D: eqn. (6) T~ ~ 1000 K.

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    GAS COM BUSTION IN SHALLOW FLUIDISED BEDS 71i m p l i e d t h a t c o m b u s t i o n h a d s p r e a d b a c k i n t o th e p l a te . A t th i s p o i n t t h e fu e l w a si m m e d i a t e l y s h u t o f f i n o r d e r n o t t o d e s t r o y t h e p l a te . T h e h i g h e s t p l a t e s u r fa c et e m p e r a t u r e m e a s u r e d w i t h o u t f l a s h b a c k o c c u r r i n g w a s 6 50 C . H o w e v e r , t r u es u r fa c e t e m p e r a t u r e c o u l d n o t b e e s t im a t e d r e a d i ly a s t h e t h e r m o c o u p l e i n te r f e re dw i t h l o c a l fl o w s . T h e s e v a l u e s a r e o f l i tt l e i m p o r t a n c e s i n c e w h a t i s r e q u i r e d is t h ev a l u e o f g a s v e l o c i t y a t w h i c h f l a s h b a c k o c c u r s f o r a g i v e n b e d t e m p e r a t u r e . T h ed a t a o b t a i n e d f o r fiv e b e d t e m p e r a t u r e s i s s h o w n i n F i g. 5 , a n d i t is c l e a r t h a t b u r n -b a c k o n l y o c c u r s a t l o w s u p e r f i c i a l g a s v e l o c it i e s .

    T h i s o b s e r v a t i o n i s o f g re a t im p o r t a n c e w h e n a t t e m p t i n g t o d e s i g n a r a d i a n tf lu i d i se d b e d f o r r o o m h e a t i n g h a v in g a m a x i m u m r a d i a n t o u t p u t o f a b o u t 100k W / m 2, a s a h o t g a s v e l o c i t y o f 0 .2 5 m / s e c is u s e d f o r t h e m a x i m u m o u t p u t a n d t h ish a s t o b e r e d u c e d t o t u r n t h e o u t p u t d o w n , e.g. a t 4 0 k W / m 2 a h o t g a s v e l o c i t y o fa b o u t 0- 08 m / s w o u l d b e d e s i r e d a t a t e m p e r a t u r e o f 1 00 0 K a n d f l a s h b a c k w o u l db e a n t i c i p a t e d . I n d e e d , t h i s p r o b l e m o f f l a s h b a c k a t l o w v e lo c i t i e s i m p o s e s l i m i t a -t i o n s o n a l l c o m b u s t o r d e s i g n s.

    THE EFFECT OF PARTICLE SIZET h e e f fe c t o f p a r t i c l e s iz e w a s e x a m i n e d b y e x p e r i m e n t s i n w h i c h t h e b e d t e m p e r a t u r ew a s c o n t r o l l e d a t 1 10 0 K b y v a r y i n g t h e a m o u n t o f c o o l i n g s u rf a c e i m m e r s e d i n t h eb e d a n d m e a s u r i n g t h e f r a c t i o n o f t h e fu e l g a s w h i c h b y - p a s s e d t h e b e d . A s th em e a n p a r t i c l e d i a m e t e r w a s r e d u c e d m o r e f ue l b y - p a s s e d th e b e d w i t h o u t b e i n gb u r n e d , r e s u l t in g f i n a ll y in a l i m i t in g p a r t i c le s iz e b e lo w w h i c h c o m b u s t i o n w a s n o ts t a b l e . T h r e e b e d d e p t h s , t h r e e v a l u e s o f th e g a s v e l o c i t y a n d f o u r p a r t i c l e s iz e sw e r e u s e d a n d t h e re s u l ts a r e g i v en in T a b l e 2 . T h i s t e c h n i q u e h a d a n u m b e r o f

    T A B L E 2FUEL BY-PASS RESULTS

    Static bedParticle Dp u-u,,y depth 40 60 MeanFtm m/s (ram) 20Sil ica 310 0.10 0.17 0.36 0.240.20 0.28 0.21 0.29 0.2770.30 0-36 0.28 0.3386 0.10 0.21 0.16 0.160.20 0.14 0.10 0.15 0.1770.30 0-26 0-19 0.22420 0-10 0-01 0.08 0.040-20 0.11 0.03 0.00 0.0530-30 0.09 O- I 0 0-02547 0-10 0.02 0-00 0.00

    0-20 0.01 0.02 0-000-30 0-00 0.04 0.02Zir co nia 110 0-10 0.42 0.38 0-38 0.393195 0.10 0.34 0.36 0.22 0.307

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    72 J. BROUGHTONs h o r t c o m i n g s , e s p e c i a ll y w i t h r e g a r d t o t h e e f f e ct o f t h e d i s t r i b u t o r p l a t e a t d i f fe r e n tg a s v e l o c i t i e s . I n a d d i t i o n , r e p r o d u c i b i l i t y w a s f a r f r o m s a t i s f a c t o r y , v a r i a t i o n s a sl a r g e a s 1 0 p e r c e n t h a v i n g b e e n f o u n d i n s o m e c a se s . T h i s v a r i a t i o n a p p e a r e d t o b ed e p e n d e n t u p o n t h e p r e v i o u s h i s t o r y o f t h e b e d , p a r ti c u l a rl y w h e t h e r t h e f lo w w a si n c r e a s e d o r d e c r e a s e d f r o m i ts p r e v i o u s v a l u e w h i c h i n d i c a t e s t h a t t r a n s i e n tp h e n o m e n a m a y b e i m p o r t a n t . T h e e x p l a n a t i o n o f t h e f a c t t h a t t h e s te a d y - s t a tev a l u e s d e p e n d u p o n p r e v i o u s o p e r a t i n g h i s t o r y c o u l d l i e i n t h e i n t e r a c t i o n b e t w e e nt h e b u l k o f t h e b e d a n d t h e e n t r y z o n e , in t h a t t h e l o n g e r t h e p r e h e a t z o n e , t h es h o r t e r m u s t b e t h e r e a c t i o n z o n e a t a n y v e l o c i t y , h e n c e t h e g r e a t e r t h e f u e l b y - p a s sf r a c t i o n . T h i s e f f ec t i s e q u i v a l e n t t o a l t e ri n g t h e b e d d e p t h a t a n y g i v e n f l o w r a t e a n di s m o r e p r o n o u n c e d w i t h s m a l l t h a n w i t h l a r g e p a r t i c l e s .

    W h e n t h e b e d d e p t h w a s g r e a t e r t h a n 8 0 m m , th e b u b b l e b e h a v i o u r w a s m o r e l ik es l ug g i n g t h a n f re e b u b b l i n g in t h a t t h e m e a n b u b b l e s iz e e x c e e d e d 3 0 m m a n d t h ep r e s s u r e d r o p t e n d e d t o o s c i l l a t e v i g o r o u s l y .

    W h e n s il ic a s a n d o f m e a n p a r t ic l e s iz e le ss t h a n 2 4 6 p m w a s u s e d , s t a b l e c o m -b u s t i o n c o u l d n e v e r b e a c h i e v e d i n t h e b e d ( a l t h o u g h s t a b l e c o m b u s t i o n c o u l de x i s t o n t h e s u r f a c e ) . W i t h z i r c o n i a t h i s s iz e w a s 9 5 p m .

    D I S C U S S I O NT h e e n t r y z o n eF i g u r e s 3 a n d 4 s h o w t h a t t h e b e d i s i s o t h e r m a l a b o v e t h e e n t r y z o n e , a p a r t f r o mn e a r t h e u p p e r s u r f a c e . I n a d d i t i o n , t e m p e r a t u r e f l u c t u a t i o n s a n d v a r i a t i o n s a r ev e r y lo w a b o v e t h is z o n e . T h e a b s e n c e o f t e m p e r a t u r e g r a d i e n ts d o e s n o t m e a nt h a t a l l c o m b u s t i o n h a s f in i sh e d i n t h e e n t r y z o n e , o n l y t h a t t h e r e a r e n o r e g i o n s o fh i g h l o c a l c o m b u s t i o n i n t e n s i t y . T h i s i s d u e t o t h e b e d ' s h i g h p a r t i c l e c i r c u l a t i o nr a t e s w h i c h m a k e t e m p e r a t u r e g r a d i e n t s v e r y u n c o m m o n . I n d e e d , r e a c t i o n o c c u r st h r o u g h o u t t h e b e d i n s o m e c a s e s , n o t a b l y w h e n u s i n g s m a l l p a r t i c l e s .

    W h e n t h e m e a n p a r t i c le s iz e e x c e e d s 6 0 0 /~ m t h e r e a c t i o n z o n e l e n g t h t e n d s t o b es m a l l a n d t h e m i n i m u m b e d d e p t h i s s e t m a i n l y b y th e p r e h e a t l e n g t h . T h u s , i t i sc o n v e n i e n t t o t h i n k o f th e e n t r y z o n e a s b e i n g a r e g i o n o f p r e h e a t a n d i g n i t i o n ,a l t h o u g h a t p r e s e n t t h e p r e d i c t io n o f t h is l e n g th m u s t r e m a i n e m p i r i c a l b e c a u s e o ft h e n u m b e r o f p a r a m e t e r s w h i c h i n f lu e n c e it , e s p e c i al ly th e i n t e r a c t i o n b e t w e e n t h eb e d a n d t h e d i s t r i b u t o r p l a t e .

    F u r t h e r s t u d ie s o f th e e n t r y z o n e a r e r e q u i re d a s t h is z o n e c a n b e a d e t e r m i n i n gf a c t o r i n b e d d e s i g n a n d i n f l u e n c e s b u r n i n g b a c k i n t o t h e d i s t r i b u t o r p l a t e w h i c hs e ts a li m i t o n t h e p r a c t i c a l r a n g e o f o p e r a t i o n . I t is i n t e re s t in g t o n o t e t h a t B a s k a k o ve t a l . h a v e e x p l o i t e d t h is b u r n - b a c k e f f e c t in a p a c k e d f lu i d is e d b e d t y p e o f d i s t ri b -u t o r , b u t i n t h i s c a s e m u c h h i g h e r t e m p e r a t u r e s w i l l o c c u r i n t h e p a c k e d z o n e a n dt h e a d v a n t a g e s o f lo w o x i d e s o f n i tr o g e n p r o d u c t i o n a r e r e d u ce d .

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    G A S C O M B U S T I O N I N S H A L L O W F L U I D I S E D B E D S 73B u r n - b a c k

    A s i m p l e a d h o c r e l a t i o n s h i p b e t w e e n t h e b e d t e m p e r a t u r e a n d t h e g a s v el o c it y a tw h i c h f l a s h b a c k o c c u r s c a n b e p r e d i c t e d b y a s s u m i n g t h a t b u r n - b a c k o c c u r s o n c et h e p l i a t e - s u r f a c e t e m p e r a t u r e e x c e e d s t h e f u e l - g a s e s i g n i t i o n t e m p e r a t u r e .

    A s s u m i n g t h a t t h e g a s a t t h e d i s t r i b u t o r s u r f a c e i s a t t h e s u r f a c e t e m p e r a t u r e , as i m p l . e h e a t b a l a n c e g i v e s :

    fo u c C p d T = Q ~ (1)0p o u c C p ( T s - T o ) = Q s (2)

    w h e r e Q s, t h e h e a t t r a n s f e r r e d b a c k f r o m t h e b e d t o t h e p l a t e , m u s t b e e s t im a t e d .T h e r e a r e t w o e s t i m a t i o n a p p r o a c h e s , t h e f i r s t a s s u m i n g p u r e l y r a d i a t i v e h e a t i n g ,t h e s e c o n d a s s u m i n g h e a t t r a n s f e r a s t o s u r f a c e s i n t h e b e d . T h e r a d i a t i v e s o l u t i o ng i v e s :

    Q ~ = r a (TB 4 - - Ts 4) (3)F ( T~ 4 - Ts 4) (4)

    uc p o C p ( T ~ - T o)F o r r a d i a t i o n t o i m m e r s e d s u r fa c e s , a ra n g e o f em i s s iv i ty v a lu e s b e t w e e n 0 . 59 a n d0 .9 5 h a v e b e e n o b t a i n e d . T h e v a l u e o f t h e b e d e m i s s iv i t y i n t h e d i s t r i b u t o r z o n em u s t b e a t t e n u a t e d b y t h e c o o l e r p a r t i c l e s i m m e d i a t e l y a b o v e t h e p l a t e ( i . e . t h ep l a t e c a n n o t ' s e e ' t h e p a rt ic l e s in t h e b u l k o f th e b e d a p a r t f r o m a t b u b b l e f o r m a t i o np o i n t s ) s o t h e l o w e r v a l u e i s t a k e n a s m o r e r e p r e s e n t a t i v e o f t h is c a se . T h e e m i s s i v i t yo f th e p l a t e u s e d is a p p r o x i m a t e l y 0 .4 , s o t h a t t h e g e o m e t r i c a l v i e w - f a c t o r , F ,b e c o m e s 0 .2 4 . T h e c o n d u c t i v e s o l u t i o n g i v e s:

    Q s = ho ( TB - - Ts ) W / m z ( 5)F o r h e a t t r a n s f e r a t a n i m m e r s e d s u r f a c e u s i n g p a r t i c l e s o f th e s i ze u s e d h e r e : z

    h o - - 2 0 0 W / m / KF r o m e q n . ( 2 ) :

    h o ( T u - - L ) (6)uc p o C p ( T s - T o )

    F o r b u r n - b a c k , t h e su r f a c e t e m p e r a t u r e m u s t ex c e e d th e i g n it io n t e m p e r a t u r e o f t h eg a s . T h u s , f o r t h e m e t h a n e / a i r s y s t e m , T s > 925 K .

    P u t t i n g T s = Tio w e g e t t h e t h e o r e t i c a l l i n e s s h o w n o n F i g . 5 a n d i t i s c l e a rt h a t t h e c r i t ic a l v e l o c i ti e s a r e s i g n if i c an t ly l o w e r t h a n t h o s e p r e d i c te d b y t h e t h e o r ya n d a g r e e m e n t w i t h T s = 1 00 0 K i s m u c h b e t te r . T h i s is t o b e e x p e c t e d a s w h e nT~ = Tio, t h e p l a t e b e l o w t h e s u r f a c e i s c o o l e r s o t h a t a n y t e n d e n c y t o f l a s h b a c kw i l l b e q u e n c h e d . T h e d a t a f a l l b e t w e e n t h e v a l u e s p r e d i c t e d b y t h e t w o s i m p l e

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    74 J. BROUGHTONm o d e l s f o r T s = 1 00 0 K a n d t h i s s i m p l e a p p r o a c h a p p e a r s t o b e a d e q u a t e , a l t h o u g hn o t r i g o r o u s l y c h e c k e d , f o r d e s i g n p u r p o s e s .M o r e r i g o r o u s t h e o r e ti c a l a n a ly s e s o f t h e p r o b l e m h a v e b e e n d e v e l o p e d , b u t n op r e c i s e t h e o r e t i c a l s o l u t i o n c a n b e e x p e c t e d t o a c c o u n t f o r t h e i n t e r a c t i o n b e t w e e nt h e p l a t e a n d t h e b e d , w h i c h u l t im a t e l y d e t e r m i n e s Q ~, u n t i l m o r e i s k n o w n a b o u ti n i t i a l b u b b l e f o r m a t i o n a n d m o t i o n .

    A P P L I C A T I O N O F T H E T W O - P H A S E T H E O R Y O F F L U I D IS A T I O N

    I t is th e r e a c t i o n w h i c h t a k e s p l a c e in t h e i s o t h e r m a l b e d s e c t i o n a b o v e t h e e n t r yz o n e w h i c h d e t e r m i n e s b o t h t h e m i n i m u m b e d d e p t h r e q u i r e d f o r c o m p l e t ec o m b u s t i o n a n d t h e e f fe c t o f p a r ti c l e si ze o n t h e c o m b u s t i o n . I n o r d e r t o d e a l w i t ht h is i s o t h e r m a l z o n e , i t is in s t r u c t iv e t o c o n s i d e r o n e o f t h e t h e o r i e s o f f l u id i s e d b e db e h a v i o u r . T h e s im p l e tw o - p h a s e t h e o r y o f fl u id i s at io n is th e m o s t c o m m o n l y u s e da p p r o a c h a n d h a s b e e n s u c c e ss f u ll y a p p l i e d t o q u i t e c o m p l i c a t e d r e a c t i o n s y s te m s .T h e tw o - p h a s e t h e o r y a s s u m e s t h a t a l l o f t h e g as i n e x c e s s o f t h a t r e q u i r e d t of l u id i s e t h e b e d p a s se s t h r o u g h t h e b e d i n b u b b l e s , a n d i t is th e e x c h a n g e o f ga sb e t w e e n t h e b u b b l e p h a s e a n d t h e d e n s e p h a s e w h i c h d e te r m i n e s t h e e x t e n t o fr e a c t i o n . 2 3T h e r e a r e a v a r i e ty o f m o d e l s b a s e d u p o n t h e t w o - p h a s e t h e o r y w h i c h d e s c ri b et h e i n t e r c h a n g e o f g a s b e t w e e n t h e p h a s e s a n d t h e m o t i o n o f so l id s i n t h e b e d , b u tm o s t o f t h e s e m o d e l s a p p l y in s y s te m s i n w h i c h t h e b u b b l e c o a l e sc e n c e z o n eo c c u p i e s o n l y a sm a l l p o r t i o n o f t h e b e d w i t h t h e r e m a i n i n g , m u c h l o n g e r , z o n e o ft h e b e d b e i n g o n e o f m o r e o r l e ss s t e a d y b u b b l i n g . H e n c e , m o s t d e e p b e d p r o c e s s e sc a n b e d e s c ri b e d in t hi s m a n n e r . H o w e v e r , to d a t e , th e t w o - p h a s e t h e o r y h a s n o tb e e n a p p l i e d t o s h a l l o w b e d s y s t e m s w h i c h c o n s i s t a l m o s t e n t i r e ly o f a b u b b l ec o a l e s c e n c e z o n e , n o r t o k i n e t i c a ll y f a s t r e a c t io n s w h e r e m o s t o f t h e r e a c t i o n t a k e sp l a c e i n t h e f i r s t f e w m i l l im e t r e s o f b e d .

    T h e a p p l i c a t i o n o f t h e t w o - p h a s e t h e o r y t o s h a l l o w b e d p r o c e s s e s is s e v e r el yl im i t ed b y t h e l a c k o f u n d e r s t a n d i n g o f b u b b l e f o r m a t i o n i n s h a ll o w b e d s , h o wb u b b l e c o a l e s c e n c e o c c u r s in s w a r m s a n d t h e v e l o c i t y / d i a m e t e r r e l a t i o n s h i p f o rb u b b l e s i n s w a r m s . T h u s , t h e p r e l i m i n a r y a n a l y s e s o f sh a l l o w b e d p r o c e s s e s m u s ti n v o k e m a n y a p p r o x i m a t i o n s . H o w e v e r , e x p e r i m e n t a l o b s e r v a t io n s s h o w t h a t f r e eb u b b l i n g c a n b e o b t a i n e d i n b e d s le ss t h a n 5 m m d e e p o n p o r o u s p l at es a n d e v e n ag r e a t l y s im p l i fi e d a n a ly s i s u s in g t h e t w o - p h a s e t h e o r y c a n e x p l a i n m a n y o f t h eo b s e r v a t i o n s r e p o r t e d i n t h e p r e v i o u s s e c t i o n .

    F o l l o w i n g t h e a n a l y s is o f D a v i d s o n a n d H a r r i s o n z 4 f o r a v e r y fa s t r e a c t io no c c u r r i n g i n t h e d e n s e p h a s e o f a d e e p f l u id i s ed b e d , i t i s a s s u m e d t h a t c o m b u s t i o nt a k e s p l a c e o n l y w h e n t h e f u e l is in t h e d e n s e p h a s e a n d t h u s t h e f r a c t i o n o f f u e lp a s s in g t h r o u g h t h e b e d u n b u r n e d , f u , c a n b e w r i tt e n :

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    G A S C O M B U S T I O N I N S H A L L O W F L U ID I S E D B ED S 75

    f , , ( u - u . : ) e - X- ( 7 )Uw h e r e X i s a b u b b l e p h a s e t o d e n s e p h a s e e x c h a n g e f a c t o r d e f i n e d b y

    i L Q d Lx = (8 )o U B V eQ is t h e v o l u m e t r i c g a s e x c h a n g e r a t e b e t w e e n a b u b b l e a n d t h e d e n s e p h a s e , U e ist h e v e l o c i t y o f a b u b b l e o f v o l u m e V n.

    T h e a s s u m p t i o n o f d e n s e p h a s e r e a c t i o n a p p e a r s c o r r e c t si nc e o t h e rw i s e t h e r ew o u l d b e l a r g e t e m p e r a t u r e f l u c t u a t i o n s a s g a s b u b b l e s i g n i t e d , g i v i n g r i s e t o m u c hh i g h e r N O ~ le v el s t h a n o b s e r v e d a n d , m o r e o v e r , th e g a s in t h e b u b b l e s w o u l d h a v et o i g n it e b e l o w t h e l ea n l i m i t o f c o m b u s t i o n . T h e p r o b l e m i n u s i n g t h es e e q u a t i o n sl ie s m a i n l y in o b t a i n i n g r e a s o n a b l e v a l u e s f o r U n a n d D e a s f u n c t i o n s o f h e i g h ta b o v e t he p l at e. B u b b l e s in s w a r m s d o n o t a p p e a r t o o b e y th e D a v i e s - T a y l o r l aw o fb u b b l e m o t i o n , 14 s o U n a n d D n m u s t b e e v a l u a t e d s e p a r a t e l y . I n t h i s f i r s t a p p r o x -i m a t i o n f a i r l y b o l d a s s u m p t i o n s a r e r e q u i r e d .

    ( 1) A m e a n b u b b l e s iz e i s a s s u m e d .(2 ) T h e b u b b l e v e l o ci ty j s ta k e n f r o m t h e w o r k o f G o d d a r d a n d R i c h a rd s o n Z 5

    t o b e 0 .3 5 m / s a n d i n d e p e n d e n t o f s iz e . T h i s v a l u e i s se l e ct e d a s i t i s t h e o n l ya v a i l a b le e x p e r i m e n t a l m e a s u r e m e n t . T h u s :

    U . = 0 - 3 5 m / s ( 9 )(3 ) T h e e ff ec t o f b u b b l e c o a l e s c e n c e o n b u b b l e p h a s e t o d e n s e p h a s e g a s e x c h a n g e

    is a s s u m e d t o b e n e g l i g ib l e ; t h a t is , t h e o n l y i m p o r t a n t e f f e c t o f c o a l e s c e n c eis t h a t o f in c r e a s in g t h e b u b b l e ' s d i a m e t e r .

    ( 4 ) T h e v o l u m e t r i c e x c h a n g e r a t e i s t a k e n t o b e t h e s a m e a s t h a t f r o m i s o l a t e d ,l a r g e b u b b l e s w i t h n e g l i g i b l e b u l k d i f f u s i o n . T h u s :

    T h e r e f o r e :Q = ~ r r u , . : D e 2 m 3 / s (10)X = ( L 9 U , , : d L) o 2 U n D n

    9 u , . f L , , :- 0 7 n 1 - ~ e )= 12 86 u , , : L m : ( 11)

    ( 1 - t e ) D eT h e p r e d i c t io n o f D e i s n o t f e a s i b le a t p r e s e n t a n d a ' r e a s o n a b l e ' e x p e r i m e n t a le s t i m a t e i s t h a t t h e m e a n s iz e is o f t h e o r d e r o f t h e b e d h e i g h t :

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    76 J. BROUGHTON

    Therefore12.86 u ,~:, ~ - ( 1 2 )( 1 - - e a )

    The bubble hold-up reaches 0.4 at ( u - u , . : ) greater than 0.2 m/s.Therefore:

    X = u , , : x 21.43 (13)Thus, this simple model suggests that u , , : and ( u - u , . : ) should determine the gasby-passing. Shortcomings of this model are that the by-passing fraction is in-dependent of height above the distributor--which is not the case in practice, but isa reasonable first approximation--and that the integral does not use the correct

    8

    \ A D I A B A T I C L I M I T. i ~ C O N I A

    Z -4.YZ

    -2 \

    Fig. 6.

    ~ 2 0 0 4 0 0d p p m

    Com pariso n of experimental fuel by-pass fractions with the theoretical values for u-u,,:between 0.1 and 0-3 m/s.

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    G A S C O M B U S T I O N I N S H A L L O W F L U I D I S ED B E D S 77f o r m o f b u b b l e s i z e v a r i a t i o n w i t h h e i g h t ( a s th i s is n o t k n o w n ) i n it s e v a l u a t i o n .A l t e r n a t iv e b u b b l e g r o w t h h y p o t h e s e s w o u l d p r o v i d e d i ff e r e n t c o n c l u si o n s .H o w e v e r , t h e a i m h e r e is t o e x p l a i n t h e o b s e r v e d e f f e c t o f p a r t i cl e d i a m e t e r : t h is isa c h i e v e d a s s h o w n b y F i g . 6 w h e r e t h e t h e o r e t i c a l r e s u lt s o b t a i n e d f r o m e q n s . (7 ) a n d( 1 3 ) a r e c o m p a r e d w i t h t h e r e s u l ts o f T a b l e 2 .

    T h i s m o d e l a l s o h e l p s t o e x p l a i n t h e l a c k o f c o n s i s t e n c y in t h e e f f e ct s o f b e d d e p t ha n d g a s v e l o c i t y i n T a b l e 2 a s it is urns w h i c h p r i m a r i l y d e t e rm i n e s t h e fu e l b y -p a s s in g f r a c ti o n . A m u c h b e t t e r a g r e e m e n t c o u l d b e o b t a in e d b y p u t t i n g D n = L m l 2 i n e q n . (1 2 ) a n d , b y s u i ta b l y ' a d j u s t i n g ' D s , a m e t h o d o f p r e d i c t i n g t h e f u e l

    b y - p a s s f o r p a r t ic l e s o t h e r t h a n t h o s e t e s t ed c a n b e e s ta b l i sh e d . T h e m o d e l t e n d i n gt o o v e r p r e d i c t c o u l d a l s o b e a r e s u lt o f c o m b u s t i o n t a k i n g p l a c e i n t h e l e a n p h a s er e g i o n , o r c lo u d , o f p ar t ic l e s a b o v e t h e b e d w h i c h w o u l d l e a d t o m o r e c o m p l e t ec o m b u s t i o n .

    A n a p p r o x i m a t e v a l u e f o r t h e s m a l l e s t p a r t i c l e s i z e a t w h i c h s t a b l e c o m b u s t i o na t a t e m p e r a t u r e g r e a t e r t h a n 1 00 0 K c a n b e s u s t a in e d i s g i v e n b y a s s u m i n g t h a t a tl e as t 3 7 p e r c e n t o f t h e f u e l m u s t b u r n , w h i c h is t h e t h e r m o d y n a m i c a l l y i m p o s e dm i n i m u m . T h i s p r e d i c t s t h a t t h e s m a l l e st s il ic a s a n d p a r t i c l e w h i c h c a n b e u s e d i s2 2 0 / ~ m w h i l e f o r z i r c o n i a th is~ s i ze is 1 4 0 / ~m . T h e s e p r e d i c t i o n s a r e i n r e a s o n a b l ea g r e e m e n t w i t h t h e o b s e r v a t i o n s r e p o r t e d .

    F i g u r e 2 s h o w s t h a t p r o v i d e d a ll th e f u e l is b u r n e d , t h e g a se s l ea v i n g th e b e d a r ea t t h e r m o d y n a m i c e q u i l ib r i u m a t t h e b e d t e m p e r a t u r e a n d t h i s c o n c l u s i o n h a s b e e ns u b s t a n t i a t e d b y R u s s i a n w o r k e r s w h o u s e d d e e p e r b e d s in th e i r te s ts . T h e c o m -p o s i t i o n s g i v e n in F i g . 2 h a v e b e e n c o n v e r t e d t o a w e t - g a s b as i s b y u se o f a t o m i cm a s s b a l a n c e s .

    N O I S E

    T h e n o i s e e m i s s i o n f r o m a f l u i d i s e d b e d c o m b u s t o r c a n b e t r e a t e d q u a l i t a t i v e l y i nt e r m s o f t h e t w o - p h a s e t h e o r y , i n t h a t g a s p h a s e r e a c t i o n w o u l d b e e x p e c t e d i f t h ee x c h a n g e r a t e s w e re l o w c o m p a r e d w i t h t h e g as p h a s e r e a c t i o n r a te . H o w e v e r , f e wd a t a a r e a v a i la b l e c o n c e r n i n g t h e f a c t o r s w h i c h i n f u e n c e n o i s e e m i ss io n s , a l t h o u g hi t i s r e p o r t e d t h a t t h e c o m b u s t i o n n o i s e d e c r e a s e s o n c e d e n s e p h a s e c o m b u s t i o nb e c o m e s e s t ab l is h e d . A s m a ll a m o u n t o f b u b b l e p h a se c o m b u s t i o n c o u l d e x p l a i nt h e o b s e r v a t i o n t h a t t h e n i t ri c o x i d e le v e ls a r e h i g h e r t h a n s h o u l d b e f o r m e d a tt h e s e t e m p e r a t u r e s a s t h e n t h e g a s t e m p e r a t u r e s w i t h i n t h e b u b b l e s c o u l d l o c a l l ya t t a i n a s u f f ic i e n tl y h i g h v a l u e f o r s o m e n i t r o g e n p l u s o x y g e n r e a c t i o n t o t a k e p l a c ea n d t h i s w o u l d b e f r o z e n i n t h e d e n s e p h a s e .

    C O N C L U S I O N S

    ( i ) G a s c o m b u s t i o n i n s h a l lo w f l u id i se d b e d s i s o f c o n s i d e r a b l e p r a c t i c a li n t e r e s t s i n c e i t c a n l e a d t o h i g h c o m b u s t i o n i n t e n s i t i e s a t r e l a t i v e l y l o w , e v e n a n d

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    controlled temperatures. This ensures low oxides of nitrogen emission and has theadvantage of exceptionally high convective and radiative heat transfer.

    (2) The combustion of pre-mixed gas and air is safe provided that the gasvelocity is high enough. This prevents the distributor plate becoming overheated,which could lead to a bu rn-ba ck condit ion.

    (3) The bu bbl in g mech ani sm in shallow fluidised beds gives rise to a gas by-passmec han ism which, unde r some circumstances, leads to loss of com bus tio n withinthe bed. The majo r factor in this mech ani sm is particle size and it is shown that forparticles of the density of silica sand, there is a lower limit of size at about 250microns below which combustion is unstable. For denser zircon sands, this lowersize' limit reduces to ab ou t 110 microns . With large particles co mb us ti on is excellent,even with dilute gas/air mixtures.

    (4) A simple varia nt of the two-ph ase theory of fluidisation has been developedto account for the effects of particle size on combustion. The agreement withexperimental data was q uanti ta t i vely reasonable and shows that the two-phaseth eo ry --w hi ch is well established for deep be ds- -c an be usefully applied to shallowbeds.

    (5) Other major design parameters such as particle size, bed depth, distributordesign, gas velocity and bed temperature have been studied and a number of thelimitations of the process are discussed.

    REFERENCES

    1. D. F. WILLIAMSand J. MCLAREN,Combustion efficiency, sulphur retention and heat transferin fluidized bed combustors, J. Inst. Fuel (August 1969), pp. 303-12.2. S. J. WRIGHT,R. HICKMANand H. C. KETLEY,Heat transfer in fluidized beds of wide sizespectrum and high temperatures. Br. Chem. Eng., 15 (12), (1970), pp. 1-7.3. J. HIGHLEY (Ed.), Combustion o f coal influidized beds, Proc. Syrup. C.R.E. 23 May, 1968.4. Third international symposium on fluidized bed combustion, Hueston Woods, Ohio, 1972.5. A. A. GODEL. Fluidized bed combustion of coal, Revue Gkn. Therm., 5 (4) (1966), pp. 349-55.6. L. REH. Fluidized bed combustion, Chemie-lng-Tech, 40(11) (1968), pp. 509-15.7. P. NOVOTNV. Fluidized bed combustion of high ash coals, S.N.T.L. Tech. Dig., 7(12) (1965),pp. 883-91.8. A. P. BASKAKOV, . V. KIRNOSand V. 1. SVETLAKOV.The preparation of non-oxidisingatmos-pheres by fluidized bed combustion, Gazov. Prom. 13(1 l), (1968), pp. 25-9.9. P. L. WATERSand K. Mc. G. BOWLING. Fuel processing in fluidized beds, Brit. Chem. Eng. 13(8), (1968), pp. 1127-33.10. J. HIGHLEY, E. A. ROGERSand R. DRYBURGH.The measurement of particle mixing rates in afive foot i.d. bed. Fluidized Combustion Section, C.R.E. 0969). Report No. 23.I I. M. A. AVEDESIAN,Combustion of char in fluidized beds. PhD Thesis, Cambridge University,1 9 7 212. G. Moss, J. W. T. CRAIG and D. TISDALL,The fluidized bed desulphurising gasifier,A. 1. Ch.E .Syrup. Set. No. 126, 68 (1972), pp. 277-82.13. D. E. ELLIOTT, Exploiting fluidized bed combustion. Second Int. Syrup. on Fluidized BedCombustion, Hueston Woods, Ohio, pp. 0/2/1--0/2/10, 1970.14. G. G. COPLAND, Industrial waste disposal by fluid bed oxidation, ,4.1. Ch.E. Syrup. Set. No.122, 68 (1972), pp. 63-72.15. J. E. HANWAY,The use of fluidized bed technology in pollution control, A. 1. Ch.E. Symp.Set. No. 105, 66 (1970), lap. 253 62.

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    1 6. M . TA MA LE T, A p p l i c a t i o n s o f f lu i d i z e d b e d h e a t t r a n s f e r i n t h e m e t a l l u r g i c a l i n d u s t r i e s . Syrup.Chem. Eng. in Iron and Stee l Ind. , I . Ch. E. , Swa nse a , 1968 , pp . 105- 24 .17. S. S. ZABRODSKYa n d N . V . AN TO NIS HIN , T h e c o m b u s t i o n o f l i q u i d s a n d g a s e s i n f lu i d i z e d b e d s ,Inzh-Fi z -Zh . Nauk , B.S.S.R. 5(2) , (1972) , p . 10.1 8. W . B . H O W A R D, F l a m a b i l i t y l i m i t s i n f l u id i z e d b e d c o m b u s t i o n , Loss Fret , , 4 (1969), pp . 6 - 14 .19. A. P . BASKAKOV,V. I . K I RNOS a n d V . I . SVE TL AKOV, P r e pa r a t i on o f no n- o x id i s ing m e dia byf lu i di z ed b e d c o m b u s t i o n , Gasov . Prom, 1 3 ( l 1 ), 0 9 6 8 ) , p p . 2 5 - 2 9 .20. J. BROUGHTON, Com bust ion in f luidi zed beds, P h D T h e s is , T h e U n i v e r s i t y o f N e w c a s t l e u p o nTyne , 1972.2 1 . D . E . E LL IO TT a n d M . J . V I RR , S m a l l - s c a l e a p p l i c a t i o n s o f f l u id i z e d b e d c o m b u s t i o n a n d h e a tt r a n s f e r , Third Int . Syrup. on Fluidized Bed Combust ion, H u e s t o n W o o d s , O h i o , 1 97 2.22. W . M. S. KASSlM, Flo wb ack o f solids through distributor plate s of ga s f luidized beds, P h D T h e s is ,U n i v e r s i t y o f A s t o n i n B i r m i n g h a m , 1 97 2.23. D. KUNII and O. LEVENSPIEL, Fluidizat ion engineering, J o h n W i l e y & S o n s , N e w Y o r k , 1 9 69 .24. J. F . DAVIDSON an d D . HARRISON, Fluidizedpart ic les , C a m b r i d g e U n i v e r s i t y P r e s s , 1 9 6 3 .2 5 . K . G OD D AR D a n d J . F . R IC HA RD SO N, B u b b l e v e l o c it i e s a n d b e d e x p a n s i o n s i n f r e e ly b u b b l i n gb e d s , Ch. Eng. Sc i . 24 (1969) , pp . 663- 70 .


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