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The e f f e c t o f c o a l p a r t i c l e s i z e o n t h e p er fo rm an ceof a f l u i d i se d bed co a l combu stor

byM. B r i k c i - N i g a s s a , E.S. Garbet t and A.B. Hedley

S he ff ie ld Coal Research Unit , Department of ChemicalEngineer ing and Fuel Technology, Univ ers i t y o f S he f f i e l d

S h e f f i e l d S1 3 J D , United Kingdom.

In t ro d u c t io nThe technology of f l u i d i s e d bed c oa l combustion (FBC) and i t s advantages

o v e r co n v en t io n al c o a l b u rn in g sy st ems i s now w e l l e s t a b l i s h e d a n d i se x t e n s i v e ly r e p o r t e d i n t h e l i t e r a t u r e C 1, 2, 3, 4) . A common way of introducingc o a l t o t h e b ed i s v i a c o al f ee d p o i n t s i n t h e d i s t r i b u t o r p l a t e and f o r t h i smethod i t i s u s u a l t o u se c ru sh ed c o a l of p a r t i c l e s less then 6 mm. Problemsasso cia te d wi th t h i s method inc lu de the de terminat ion of t he co r r ec t number andsp ac in g o f f eed p o in t s , b lo ck ag es an d th e o b v io u s exp en se i n co a l p r ep a ra t i o n .Crushed coal i s used because i r . w a s t ho ug ht n e c e s s a r y t o k ee p t h e c o a l p a r t i c l es i z e s approx imate ly equal t o thos e making up th e bu lk o f bed and so maintain goodf l u i d i s a t i o n c h a r a c t e r i s t i c s . However, H ig hl ey e t a l . (5 ) showed th a t l a r ge coa lp a r t i c l e s (< 5 0 mm) co uld be b u rn t q u i t e ea s i ly i n an FBC wh i l s t a t t h e same timeovercoming the coal feed prob lems o u t l i ne d above by overbed feed ing . Also, usinguncrushed co a l a l lows the bed and f reebo ard he ig h t s t o be reduced (5) makingo bv io us s a v in g s i n c a p i t a l a nd r u n n i n g c o s t s . An i n c r e a se i n t h e s i ze of coalp a r t i c l e f e d t o t h e c om bu st or r e s u l t s i n a n i n c r e a s e i n t h e bed c arb on loading(6) which i nf lu en ce such impo rta nt phenomena as NO e m i s si o n s and e l u t r i a t i o n ( 7 ) .However, there i s l i t t l e i n fo rma t io n on th e e f f ec t on t he p erfo rma nce of an FBCdue t o a v a r i a t i o n o f p a r t i c l e s i z e i n t h e co a l f eed .rep or ts a s tudy of t he combust ion of monosized coa l f r ac t i on s fe d con t inuously tothe bed v ia an overbed feeder .and combust ion ef f i c i en cy a r e p res en ted . Measurements us ing crushed co a l( < 1.5 mm ) f ed p n eu mat i ca l ly to th e b ed are inc luded for compar ison .2. Exp erimenta l procedur e

This paper ' therefore ,Da ta showing th e e f f e c t o f co a l s i z e , ex cess a i r

T 'ne f luidised bed combustor shown schematical ly in Fig . 1, was 0.3 m squareThe bed which was 0.6 m deep, co ns is te d of sand of mean

F l u i d i s i n g a i r was in t ro duce d t o th e bed th rough a bubbleCrushed coal ( < 1.5 mm) was f ed p n eu mati ca l ly i n t o th e

Large

sec t io n an d 1.83 m h ig h an d co n s t ru c ted f ro m s t a i n l e s s s t e e l , t h e w a l l s b e i n gi n s u l a t e d w i t h kaowool.p a r t i c l e s i z e 600 pm.cap d i s t r i b u t o r p l a t e .bed f rom a se a l ed hopper v i a a c a l i b r a t e d r o t a r y v a l ve f e e d e r ( F ig . 1).co al (N.C.B. 501) pre vio usl y si ev ed t o give monosized fr ac t i on s (6 .3 , 9.5 and12.5 mm) was f e d by a v ib r a to ry f ee d e r f ro m a p r e ssu r i s ed ho pp er t o th e su r f ac eof th e bed.feed occurred when a screw feeder w a s i n i t i a l l y used.two g r id s which a l lo wed an y f i n es p r e se n t to f a l l t hro ug h .was achieved usi ng an overbed gas bur ne r which prehea ted th e bed t o 725 K beforeco a l was in j ec t ed .removed by a two-stage cyclone. S ol id s se pa ra te d by the cyclone s dropped i n t oca tchpots (Fig . 1). I n or d e r t o me as ur e t h e r a t e of e l u t r i a t i o n o f m a t e r i a ldurin g ste ady -st ate combustion, the car ry over f rom the combustor was di ve r t edi n t o a sep e ra te ca t ch p o t . The t emp era tur e o f t h e bed was c o n t ro l l ed by a cool ingc o i l immersed i n t he bed. The rmocou ples were lo ca ted in th r ee p o s i t i o n s i n th ebed: top , midd le and bo ttom and a ls o i n the f reebo ard (Fig . 1 ) . A l l bed andf reeb oard tempera tures and the coo lin g wate r temperature were recordedcont inuously on ch a r t r eco rd e r s .

A v ib ra t o ry f eed in g sys t em was ad op ted a f t e r d eg rad a t ion o f t h e co a lThe l a r ge coa l passed over

S t a r t up of th e bedP ar t i cu la t e ca r ry o v e r i n th e g aseou s combu st io n p ro du c t s was

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Table 1. Combust ion E f f i c i en cy a s a fu nc t ionof coa l s i z e and bed tempera tu re .

Temp (K)jCombustion efficiency@ 20% XSA12.5 nun9 3An inc rea sed carbon combus tion e f f i c i en cy i s ach ieved wi th the inc rease o fa i r up t o abo ut 2 0-2 5% , a f u r t h e r i n c r ea s e i n ex ces s a i r b eyo nd t h i s v a lu e d oesno t improve th e ca rbon combust ion e f f i c i en cy s i gn i f ic an t l y . The combust ione f f i c i e n c y i s o b se rv ed t o i n c r ea s e w i th bed t em p e ra tur e f o r t h e 6 . 3 and 9.5 nuncoa ls ( t ab le 1 ) . The 12.5 mm r e s u l t s show o n ly a s l i g h t s e n s i t i v i t y t o bedtemperature (1043 K - 1193 k, 20% exce ss a i r ) . The hi gh es t carbon combust ionef f ic i enc y of 95% i s ach ieved wi th the 9 .5 nun a t a bed temper ature of 1193 K( t a b l e 1 ) .4 . Discussion

The le ve l s of N O a t th e e x i t of an FBC may be expr ess ed as a sum of th er a t e s of formation and redu ct io n, witho ut sp ec if yi ng any mechanisms, as fo l lows

Rate ofate of Rate of Rate offormation - r ed u c t i o n + formation - r ed u c t i o ni n t h e i n t he i n t h e i n t hebed bed f reeboar d f reeboar d

A B C D

Rate of NOem it t ed a t =t h e f l u e 1)

i t i s c lear f rom F igs . 2 z d 3 t h a t A > B and D : f o r a l l t h e c o o ls used i nthes e exper iments . There a r e many exper imental da ta ava i l ab le which show th at NOredu cti on i n an FBC can ta ke pl ac e v ia NO-char re ac ti o ns (8,9,10 ) and s o t h el e v e l of NO r educ t ion may be expec ted to be p rop or t ion a l to the carbon load ing inth e bed , which i n tu rn i s p r o p o r t i o n a l t o t h e di am et er of t h e c o a l p a r t i c l e s i nth e feed. When la rg e co al i s f e d t o t h e be a t h e r a t e o f NO formation w i l l bes lower and lower than fo r c rushed coa l bu t the r a t e o f r educ t ion w i l l a l s o b elower even fo r l a rg er ca rbon load ing because o f the low carbon su r fac e a re a peru n i t mass.

This could ex pl ai n why, a s shown i n Figu res 2 and 3 , approx imatel y th e samelev e l s o f N O concen t r a t ion a re observed a t th e top of th e bed f o r bo th c rushedand l a r g e co a l s . T hese s i m i l a r l ev e l s of NO a l s o c o n t r a d i c t t h e s u gg e s ti o n ( 11)t h a t overbed feed ing of l a r ge co a l may incr eas e NO r e d u ct i o n a t t h e t o p of t h ebed due t o an increased carbon load ing i n th a t r eg ion . The NO co n cen t r a t i o n s i nt h e 0 . 3 m square FBC, t he re fo re , appear t o be independent of c o a l f e e d p o s i t i o nand s i ze o f coa l f ed .p lace i n the f r eeboard (F igs . 2 and 3 ) .occurs i n the r eg ion immedia te ly above t he bed where t he char conce n t ra t ion i shigh due t o sp lash ing . I t i s i n t h e f r e e bo a r d t he n t h a t t h e e f f e c t of c arb onload ing i n th e bed on NO r ed u c t i o n i s m os t ev id en t s i n ce t h e l ev e l o b se rv ed f o rthe 9 .5 mn co a l a r e l ow er t h an f o r t h e 6 . 3 m and crushed co al s . The NO l e v e l si n t h e f re e b o ar d a r e s e e n t o d e c r ea s ei n t he same manner a s t he s o l i ds popu la t i on dec reases ( 1 2 ) .

The major po rt io n of NO r ed u c t i o n r ep o r t ed h e r e , t ak e sI n p a r t i c u l a r t h e h i g h e s t r e d u ct i on r a t e

ex p o n en t i a l l y w i th h e ig h t ( F ig s. 2 and 3)

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Assuming t ha t la rge coa l p ar t i c le s do no t b reak when in t roduced to the bedthen i t would be ex p e ct e d t h a t t h e e l u t r i a t i o n r a t e w ould be s i g n i f i c a n t l yreduced compared t o when crushed coal i s fed to the bed. Fig ure 5 shows ar ed uc ti on i n th e e l u t r i a t i o n r a t e of c ar bo n a s t h e c o a l fe ed p a r t i c l e s i z ei n c r e a s e s b u t t h e d i f f e r e n c e i s n o t as gr ea t a s would be expected be ari ng i n mindt h a t t h e l a r ge c o a l i s a monosized feed and does not include any f ines.p a r t i c u l a r t he c arb on e l u t r i a t i o n r a t e f o r t h e 12.5 mm c o a l a lt ho ug h i n i t i a l l y(Tb = 1043 K) less t h an th a t measured f o r th e 9 .5 mm subs eque ntly becomes g re at erf o r Tb > 1093 K.p a r t i c u l a r s u f f e r s from breakage due t o th ermal shock when in tro duc ed t o the bed.This a ls o exp lai ns why th e N O c o n c e n t r at i o n s f o r t h i s c o a l f a l l b etw een t h o s emeasured for the 6.3 and 9.5 nun c o a ls ( F i g. 4 ) . P a r t i c l e a t t r i t i o n may a l s o bes i g n i f i c a n t w i t h i n th e bed ( 13,14,15) p a r t i c u l a r l y f o r t he l a r g e r c o al s . M er ri ckand Highley ( 16 ) d e r i v e an e x pr e s s io n f o r p a r t i c l e s i z e r e d u c t io n due t oa t t r i t i o n based on R i t t i ng er s Law of abras ion and showed t h a t the shr inkage ra tewas p r o p o r t io n a l t o t h e p a r t i c l e s i z e v i z :

I n

This may be exp la ined by the f a c t t h a t th is coa l (12.5 mm) i n

Thus t h e e l u t r i a t i o n r a t e f o r t h e l a r g e r c o a l s c ou ld be s i g n i f i c a n t l y e nh an ce ddue to a t t r i t i o n phenomena. The lo wes t e lu t r i a t io n r a t e s o b se rved (F ig . 7) a r ef o r t h e 9 . 5 mm co a l a t 1193 K and 20% XSA.tempera ture and excess a i r w i l l b e to in c r e ase t h e co mb us tion r a t e wi th aconsequent reduction i n th e amount of carbon thrown i n t o the free boa rd. Thus thee l u t r i a t i o n r a te s w i l l d e c r e a s e f o r an in cr ea se i n bo th Tb and XSA.can be seen i n Figs. 5 and 6 .

The e f f e c t o f in c r eas in g the bed

Th i s t r en d

5. Conc.lusionsThe measurements of n i t r i c ox ide concen t ra t io ns i n t he bed and f reeboard of

the 0 .3 m s qu ar e f l u i d i s e d b ed ha ve shown t h a t n i t r i c o x id e i s produced withinth e bed and a r e r edu ced i n th e f r eeb o a rd . E l u t r i a t i o n r a t e s and NOco n cen t r a tio n s measu red a t t h e ex i t of t h e f r eeb o a rd b o th d ec rease wi thi n c r e a s i ng c o a l p a r t i c l e s i z e up t o a s i z e of 9 .5 mm f o r most con dit ion s. Thecombu st io n of mo no sized co a l p a r t i c l e s i n th e f lu id i se d b ed h as h ig h l i g h t ed th ein te rdep en d en ce of e lu t r i a t io n r a t e , bed ca rb on co n ten t , c arbo n co n cen t r a t io n i nt h e f r e eb o a rd and n i t r i c o xi de e m i s s io n s . The r e s u l t s a l s o i n d i c a t e t h a t a noptimum o p e ra t in g co n d i t io n fo r t h i s p a r t i c u l a r f lu id i se d b ed co mb us to r may ex i s tfo r th e 9 .5 mm c oa l s i z e a t 2 0% XSA. However, f u r t h e r ex p e r imen ta l r e su l t s a r en ecessa ry , i n pa r t ic u l ar wi th resp ec t to t h e complex phenomena occur r ing i n thef reeboard reg ion .6 . Acknowledgements

This work fo rms p a r t of the ac t i v i t i e s of the Sh ef f i e l d Coal Research Uni tsponsored by S he ll Coal In te rn at io na l and the Natio nal Coal Board, to whom thea u th o rs a r e g r a t e f u l f o r f i n a n c i a l as s i s t a nc e .th o se of th e au th o r s and n o t n eces sa r i ly th ose o f th e sp o n so r s .7. Nomenclatured c o a l p a r t i c l e d i a .f ( d p )H t o t a l bed h e i g h t .K ab ras io n co n s tan t .U s u p e r f i c i a l f l u i d i s i n g ve l o c i ty .

The views expres sed here a r e

P f r a c t i o n of c o a l p a r t i c l e s i n bed s m a l l e r t ha n dP '

minimum f lu id is ing ve loc i ty .UmfY v e r t i ca l co -o rd in an ce .E dimension less he igh t ( = y/H).

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8. References1.

2.

3.4 .5.

6.

7.8.

9.10.11.

12.13.

14 .

15.

16.17.

Skinner , D .G . , Flu id ised dombust ion of c o a l , Mills & Boon, Monograph No.CE /3 , 1914.I ns t. of Fuel Symp. Se r. No. 1, Fl u i d i se d Combustion , 1975, Sess ions B, Cand D.Gibson, J. and Highley, J., J . Inst . Energy, 1979, 52, 51 .S i x t h I n t . Conf. on F l ui d i s e d Bed Combustion, At lan ta, Proceeding s, 1980.Highley, J . e t a l . , In s t . o f Fuel Symp. S e r . , No. 1, Fluidised Combustion,1975, Paper B3.Donsi, G. e t a l . , 1 7 t h Symp. on Combustion ( I n t . ) , Leeds, 1978.

Sa ro fi m, A.F. and Beer, J.M., 1 7t h Symp. o n Combustion (Int.), Leeds, 1978,189.Beer, J.M. e t a l . , 5 t h In t . Conf. on Fl u i d i se d Bed Combustion , WashingtonD.C., 1977.Furusawa, T. and Kuni i , K. , SOC. o f Chem. E ng., Jap an, 1977.Gulyur t lu , I . , Ph.D. T he si s, Un iv er ai ty of Sh ef f i e ld , 1 9 8 0 .Bachovchin, D.M., Beer, J . M . and S a r o f i n , A.F., A.1.Ch.E. Symp. Ser. , 1981,77 , N o . 205, 76.Lewis, W.K. e t a l . , Chem. Eng. Pro g. Symp. Ser ., 1962, 52, 38.D'Amore e t a l . , b t h in c. Conf. on F lu id is ed Bed Combustion, A tl an ta , 1980,1, 75 .Beer, J . M . e t a l . , I ns t . Energy Symp. Se r. , No. 4, London, 1980,Paper IV-5.Cowley, L.T. and Ro be rt s, P.T., F l u i d i s e d Comb. Conf., Cape Tom, 1981, 2_,443.Merr ick, D. and Highley, J., A.1.Ch.E. Symp. S e r . , 1974, 70, No. 137, 366.Brikci-Nigassa, M., Ph.D. Thesis, Un iv e r s i ty of Sh ef f i e ld , 1 9 8 2 .

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