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- 150 -A STUDY OF FLY ASH REACTIONS AND DEWSIT FORMATION

I N A LARGE, CYCLONE-FIRED STEAM GENERATORA. A. Orning

U. S . Depar tment o f t he I n t e r i o r , Bureau o f M ines,P i t t s b u r gh Ene rgy Res ea rch Cen t e r , P i t t s b u rgh , Pa.

I n t r o d u c t i o nMany, i f n ot m os t, o f t h e p ro bl em s p e c u l i a r t o t h e u s e o f c o a l a s a f u e l a r er e l a t e d t o i t s mi nera l matter con t e n t . The a s h and s l a g p roduc t s caus e fou l i ng

and sometimes c o r r o s i o n o f h e a t t r a n s f e r s u r f a c e s , a r e h a r d t o remove when s l a gdeposi ts become massive, a r e d i f f i c u l t t o c o n t a i n when t h e s l a g i s f l u i d , a n d a r ec o s t ly t o remove as p a r t i c u l a t e m a t t e r i n t h e s t a c k g a s e s . The p ur po se o f t h i s 'pape r is t o d is c u s s t h e r e l e a s e of m i n e r al matter i n b u rn i ng c o a l a n d t o p r e s e n tda t a on r ea c t i ons o f a s h depo s i t s w i t h combust ion p roduc t gas es i n a l a rg e cyc l one -f i r e d s te am g e n e r a t o r . C o nc l us i on s w i l l be drawn t h a t mus t be i n t e rp r e t e d a sp e r t i n e n t t o t h e p a r t i c u l a r u n i t th ou gh b as ed i n p a r t on o b s e r v a ti o n s i n o t h e ru n i t s . The i n t e n t i s t o demons t r a t e t ha t va l uab l e i n fo rma t i on can be ga ined bys y s t e m a t i c s a mp li ng an d a n a l y s i s o f f l y a s h and a s h d e p o s i t s .The Release of Minera l Matter

11Ash, a s formed f rom t he mi ne ra l ma t t e r o f co a l i n t he ASTM Sta nda rd Method,-h a s a n a l y s e s a s i l l u s t r a t e d by s a m p le s 8 and 9 o f T a bl e I f o r two I l l i n o i s c o a l s .S e v e r a l t h i n g s a r e n o te w or t hy : 1) Carbonates are c o n v er t e d t o o x i d e s , 2) s u l f u ri s r e t a i n e d as s u l f a t e t o t h e e x t e n t t h a t s u l f a t e f o rm a t io n p r ec e de s c ar b o n at edecompos i t ion , and 3) t h e weigh t percen tag es o f ash components, rep res en t ed a s .ox i des , add app rox i ma t e ly t o 100%. When the ash i s formed under high-temperaturec o n d i t i o n s , as i n t h e s te am g e n e r a t o r f u r n ac e r a t h e r t h a n t h e t e m p e r at u r e -c o n t ro l l edm u f f l e f u r n a c e u se d f o r t h e ash d e t e r m i n a t i o n , a l e s s e r p r o p o r ti o n o f t h e s u l f u r i sr e t a i n e d a s s u l f a t e s and t he a l k a l i metals a re a t l e a s t p a r t i a l l y v o l a t i l i z e d .Vapor p r e s s u r e s f o r v a r i o u s a l k a l i m a t e r i a l s a r e g iv e n i n T a bl e 11. The f r e e a l k a -li m e t a l s are m os t v o l a t i l e , b u t e q u i l i b r i a f a v o r c o n v er s i on t o t h e h y dr o xi d es . Theo x i d e s sh o ul d be c o n s i d e r e d , a l t ho u g h e q u i l i b r i a a l s o f a v o r t h e i r co n v e rs i o n t o t h ehyd rox i des . L i t t l e i n f o r m a t i o n i s av a i l a b l e on t he k i ne t i c s o f s uch h igh- tempera -t u r e , g a s- ph as e r e a c t i o n s . A ci d- ba se r e a c t i o n s a r e f a s t i n water s o l u t i on bu t mayb e slow i n t h e g a s p h as e. H C 1 g a s seems r e l a t i v e l y i n e r t . A l k al i ne m i s t s havebeen observed to f a l l ou t o f t he plume from i nc i ne ra t i on o f o rgan i c wastes withh i g h NaCl con t en t . Thes e con s i d e ra t i on s s ugges t t ha t t he hyd rox i des a r e t h ep r o ba b l e mode of t r a n s p o r t o f a l k a l i metals i n the hot combus t ion gases .

TABLE I.- Analyses o f as h and s la g samples , weigh t perce n tNo. Locat ion SO3 Fe2O3 A1203 S i02 CaO MgO Na20 K20 T o t a l13456

. 789

S l a g t a n kReheat superheaterPr i mary s upe rhea t e rEconomizer hopperA i r h e a t e r h op pe rP r e c i p i t a t o r ho pp erCoal ash , ACoa l a s h , B

0 . 2 5 19.7 17.610.2 21.0 16.4

2 .2 18 .6 18 .35.0 20.6 17 .42.2 21.7 17.63 . 5 1 8 . 9 17 .67.5 28.2 15 .58.0 17.2 18.9

45.832 .547.143.245.445.835.744.5

8 .67 0 .9510.95 ,.80

5.96 .865.79 .757.61 .865.04 .907.68 .787.55 .91

2.16 1.99 97 .12.80 2.19 96. 82.47 2.04 97.52.56 2.13 97 .42.12 2.05 9 9 . 52.91 2.82 97.52.43 1.83 9 9 . 61.98 1. 83 100.9

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- 151 -TABLE 11.- Tem pe ra tu re s a t s e l e c t e d v a po r p re s s u r e s

T CComponent l m m 10 m 10 0 mmNa 439 549 701K 34 1 443 586NaOH 739 897 1111KOH 7 19 86 3 1064N a C l 865 1017 1220KC1 82 1 968 1164

I r o n s u l f i d e , F eS, a nd m e t a l l i c i r o n t e n d t o a p pe a r w i t h d e f i c i e n t a i r supp ly .T h i s i s aggravated in w e t bottom fur nac es when p y r i t e (FeS2) impinges on th s l ags u r f a c e . The i ro n i n c o a l i s p r e s e n t p r i m a r i l y a s py r i t e . Ba i ley and Elyx? haveshown that i ron tends t o s e g r e g a te i n t o s l a g an d i n t o f u r n a c e d e p o s i t s . The a u t h o rha s observed a fur nac e w i t h f r o n t w a l l b u rn er s a n d a p l a n a r e a t o o smal l t o p r e v en tflame impingement on the wall s u r f a c e s . I r o n c o n t e n t o f t h e w a l l s lag w a s overt h r e e times t h a t e x p e c t e d from t h e c o a l a s h a n a l y s i s .de f i c i en t i n the a r ea o f f l ame impingement and t h a t l i q u i d d r o p l e t s o f F eS w er ep r e f e r e n t i a l l y a dh er in g t o t h e w a l l , a l t h o u g h i n t e r m i t t e n t o x i d a t i o n g a v e a nox id ized s lag .

I t appeared t h a t oxygen was

F i r e s i d e c o r r o si o n a s s o c i at e d w i t h s e gr eg at ed d e p o s i t s h i g h i n a l k a l i - i r o ns u l f a t e s w ere f i r s t f oun d o n fu r n a c e w a l l t u b e s o f w e t b o t t o m E u r n a c e s . u /Presence of CO i n t h e l o c a l g a s s t r ea m was i n d i c a t i v e o f l o c a l o xy ge n d e f i c i e n c yand f l ame impingement wi th p r e f e r en t i a l adherence o f FeS d r op le t s and h igh lo ca lh e a t t r a n s fe r r a t e s . However, t h e s e g reg a t ed d e p o s i t s were f ound t o b e s u l f a t e sw it h d e t e c t a b l e s u l f i d e on l y a t t h e i n t e r f a c e b et we en the d e p o s i t and t h e t u b em e t a l . I n t e r m i t t e n t o x i di z i n g c o n d i t i o n s c o n v e r t s u l f i d e s t o s u l f a t e s . W ith t ub esu r fac e t empera tu res on the o rde r o f 400'C, th e complex a l ka l i - i ro n pyr osu l fa tesbecome s t ab le l i qu id s . While these l i q u i d s i n p a r t may b e c o r r o s i o n p ro d u c t s , t h eyi n t u r n a r e c a u s e f o r e l e c t r o l y t i c c o rr o 2i on i f n ot t h e a c t i v e c o r r o si v e a g e nt s .

Reac t ions be tween su l f u r ox i des and f l y ash have been the s u b j e c t o f s t u d y andspecu la t io n fo r many years .b o i l e r s had a l i t t l e c a t a l y t i c t i o n on t h e o x id a ti o n o f s u l f u r d i ox i de i n f l u eg a s . C ro s s l e y , P o l l and S w ee t tz ? s t u d i e d t h e s e r e a c t i o n s t o e x p l a i n f i n d i n g s oft h e B o i l e r A v a i l a b i l i t y Committee t h a t f i r i n g o f p u l v e r i z e d c o a l o ve r t h e ba ck e ndof m e ch an ic al s t o k e r s was e f f e c t i v e i n c o n t r o l l i n g a c i d i c d e p o s i t s i n t h e b o i l e r .The a p pe a ran c e o f t h e t y p i c a l a l ka l i - iro n -al u m in u m s u l f a t e s o n s u p e rh e a t e r t u b e sb rough t renewed i n t e r es t i n the rea c t i ons between f y ash and combus tion gases .The ext ens ive l i t e r a t u r e ha s been reviewed by Reid. -- / The de po si ts a r e c h a r a c t e r -ized by water s o l ub i l i t y , a low pH, h igh conte nt o f a l k a l i , iron, aluminum ands u l f a t e , and r e l a t i v e l y low c o n t am i n at i o n b y o t h e r c o n s t i t u e n t s o f f l y a s h . Thesedep os i t s under l i e t yp ic a l ash de po s i t s . They have an amorphous or m i c r o c r y s t a l l i n eappearance when cold but sometimes show s i gn s of having been l iqu id . They appearwhere gas t empera tu res , the t h ickn ess of t h e o v e r l y i n g as h d e p o s i t , a n d t h e steamc o n d i t i o n s are s uc h t h a t t h e r e s u l t i n g h e a t t r a n s f e r p ro du ce s t e mp e ra t ur e s n e a r t h etube su rf ac e approac hing 65OoC. The segr egat ed de po si t may be sep ar at ed from t hetube meta l by a poor ly c onso l i da ted ox ide sca le . The autho r has a l s o found sampleswhere l iqu id wet ted the metal tube and pene t ra te d under the ox i de sca le so as t obend i t away f ro m t h e t u b e s u r f a c e . R e l a t i v e l y low o c c l u s i o n o f f l y a s h i n t h es e g reg a t e d d e p o s i t i n d i c a t e s t h a t c omponents n o t s o l u b l e i n t h e d e p o s i t are mechan-i c a l l y e j e c t e d by c y c l i c me l t in g and s o l i d i f i c a t i o n .American bi tuminous co a l s , has a somewhat va r i ab le compos i t ion , p ar t i cu la r l y the

J o hn s to n &/ c o nc l ud e d t h a t f l y a s h f ro m s t o k e r - f i r e d

3

The r e s u l t i n g d e p o s i t , from

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- 152 -propo r t i on o f i r on and al uminum s u l f a t e s , bu t i s so c h a r a c t e r i s t i c a l l y d i f f e r e n tfro m th e o v e r ly i n g f l y a s h d e p o s i t s t h a t i t i s v i s u a l l y d e t e c t a b l e w i th o ut n eed f o rc h e m i c a l a n a l y s i s .

Occur rence o f t he s eg rega t ed d ep os i t unde r t yp i c a l a s h dep os i t s ha s causeds p e c u l a t i o n a s t o w h et h er t h e d e p o s i t was fo rm ed f i r s t and t h e f l y a sh d e po s i t e dl a t e r o r wh e th er t h e s e g r e g a t e d m a t e r i a l w a s s y n t h e s iz e d be ne a th t h e f l y a shd e p o s i t .

$7'y n t h e s i s o f t h e c om ple x s u l f a t e s r e q u i r e s a p a r t i a l p r e s s u r e of SO3 h i gt han t h a t found i n t he m a i n st re am of combustion gas es . Anderson and Goddard-conc lude t h a t t he SO2-SO3 rea c t i on r eaches e qu i l i b r i um wi t h i n t h e depo s i t and t h a ts o l u t i on phenomena a t ope r a t i ng t emperat u re s r educe t h e neces s ary p a r t i a l p re s s u reof SO3 below th a t exh ib i t ed by pure phases o f a lk a l i - i r on and a lka l i -a luminums u l f a t e s t h a t a pp e ar i n t h e d e p o s i t a f t e r c o ol in g .

While informat ion on r e a c t i o n s b etwe en s u l f u r o x i d e s and f l y a sh i s sketchy ,t h e re i s even l e s s 4 7 fo r m at io n o n r e a c t i o n s i n v o l v i ng t h e a l k a l i m e t a l s .Go ldberg and Orning- found t h a t t he a l k a l i con t e n t o f f l y a s h , s ub j ec t ed t o at e m p e r a t u r e g r a d i e n t i n a f l ue g as a tmosphere , migra ted towards a c o l d s u r f a c e a t375OC.beyond t h a t of t h e o r i g i n a l f l y a s h , t h e r e w a s i n s u f f i c i e n t a l k a l i f o r t h e sy nt he -s i s . S i n c e t h e a l k a l i c o mp on en ts of t h e m i n e r a l mat ter of c o a l are v o l a t i l e u n d e rcombus ti on co nd i t i o ns , it seemed d e s i r a b l e t o g a t h e r d a t a on t h e r e a b s o r p t i o n ofa l k a l i by f l y a s h and by d e p o s i t s a t v a r i o u s p o i n t s w i t h in a c o a l - f i r e d steamg e n e r a t o r . A s t u d y o f d e p o s i t s i n a cyc l one - f i r ed un i t seemed p re fe ra b l e becauset h e h i gh t empera tu re s w i t h i n t he cyc l one would a s s u re more compl e te vo l a t i l i za t i ono f the a l ka l i componen t s .

Weintraub,

The complex a l k a l i s u l f a t e s d i d no t 'appear . In t he absence o f a supply ,

The Steam GeneratorThe s team gene ra t o r c r o s s s e c t i on i s shown i n Fi g. 1. I t was r a t e d a t

3 ,290 ,000 lb of s t eam /hr , 3625 ps ig and 1005% superhe at wi th 575 ps ig and 1005%r e h e a t . It had 10 cyc lon es , 10 f t i n d i a m et e r , f i v e e ac h on t h e f r o n t and rearw a l l s . Sampl ing po i n t s a r e shown by t he c i r c l e d numeral s , excep t fo r t he e l e c t r o -s t a t i c p r e c i p i t a t o r h op pe r w hich i s not shown. The f u r n a c e w a l l t u b e s were f u l l ys tu d de d t o a l e v e l a b ou t 5 f t be lo w t h e g a s r e c i r c u l a t i o n p o r t s . A t h i n , g l a s s ys l ag coat was i n t a c t th rou ghou t th e s tudded zone fo l lo wing shu tdown. Slag shedding .had occurred on a l l upper wal l s u r f a c e a e x c e p t i n t h e c o rn e r s j u s t a bove t h e s tu d -d ing and o n the w a l l s u n de r some o f t h e g a s r e c i r c u l a t i o n p o r t s .

Ash and Slag SamplesTwo s out he rn I l l i n o i s b i t umi nous co a l s were used . Analyses o f th e ash and

s l ag samples a re shown i n Table I . Data on t he mole perce n t b a s i s , shown i n Table111, a re more r eve a l i ng . Unfo r t una t e l y , t h e two co a l s t h a t were used have somewhatd i f f e re n t a s h ana l y s es . The SO3 i s e q u i v a l en t t o a b o u t 70% of th e CaO fo r bothc o a l a s h e s , w hi le s u l f u r i n a s h a cc ou nt ed f o r 2 4. 6% o f t h e s u l f u r i n c o a l A and3 8. 5% o f t h e s u l f u r i n c o a l B . R e t e nt i on o f s u l f u r i n t h e c o a l a s h , as produced a t700' t o 75OoC by t he s ta nda rd method f o r ash c on te n t , i s r e l a t ed t o t he c a l c i t ec o n t e n t r a t h e r t h a n t o t h e s u l f u r c on t e n t o f t h e c o a l .

The compos i t ion of va r i ou s s l a g , dep os i t , and f ly -as h samples may be expectedt o de pe nd upon o p p o r t u n i t i e s f o r s e g r e g a t i o n , o n t e mp e ra t ur e s a t v a r i o u s p o i n t sa l ong t he gas f low pa t h , and on t he r ead s o rp t i on o f s u l f u r and a l k a l i componen ts .The temperatures were no t measured bu t were probab l y on t he o rd e r o f ove r 170OOCi n t h e Fyc1one,100O0C i n the secondary su per hea ter , lo ca t i on 2 , and 325OC a t t hea i r h e a t e r , l o c a t i on 6 .

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- 153 -TABLE 111.- Ash a n a l y s e s , m ole p e r c e n t , and p h y s i c a l c h a r a c t e r i s t i c s

9 No .. Location ColoraS la g t a nkA i r h e a t e rPr imary superhea te rEconomizer hopperRehea t superhea te rP r e c i p i t a t o r h o p p e rCoal ash, ACoal a sh , B

BL.BL .R.B.B.R.B.R.G.B.

bMag. SO31 0 . 23 2 .12 2 .22 4.90 10.22 3.4- 7 .6- 7 . 5

Fe2039 . 5

1 0 .48 .91 0 . 2

1 0 .59 . 2

1 4 .38 . 0

~

A12031 3 . 31 3 . 21 3 .91 3 . 41 2 . 91 3 . 41 2 . 313 .8

Si07 CaO MgO59.0 11.9 1 . 85 8 . 0 1 0 . 4 1 . 66 0 . 4 8 . 2 1 . 65 6. 7 8 . 2 1 . 543 .6 15 .7 1 .65 9 .4 7 . 0 1 . 748 .3 1 1 . 1 1 . 65 5 . 2 10.0 1 . 7

Na20 K202 .7 1 .62 .6 1 .73 . 1 1 . 73 . 3 1 .83 . 6 1 . 93 . 6 2 . 33 . 2 1 . 62 . 4 1 . 4

abS c a le of i nc r e a s in g m a gne t ic c ha r a c t e r .Color code: BL. , b l a c k ; R. B. , red-brown; G.B., gray-brown; B.R., b r i c k - r e d .

S u l f u r c o n t e n t o f t h e s l a g i s almost immeasurably low. The 0 . 2 mole of SO3c o r r e sponds to 0 . 1 w t p e r c e n t o f s u l f u r , w hi ch i s r e po r te d t o t h e n e a r e s t 0.1%.The sodium and potassium co nt en ts of the s l a g a re c om pa ra ble t o t hose o f t he c oa la s h e s .E i t h e r th e a l k a l i metals were n ot s u b s t a n t i a l l y v o l a t i l i z e d i n the c y c l o n e s o r theywere readsorbed as t h e s l a g fl ow ed a c r o s s t h e f u r n a c e f l o o r .

S l a g from t h e c y cl o n e s fl ow s a c r o s s t h e f u r na c e f l o o r t o t h e s l a g d r i p . -

Deposit samples were no t t a ke n f r om loc a t ion 2 , Fig. 1 ( l ea d t u b e o f t h e l a s tp l a t e n o f t h e se co n da ry s u p e r h e a t e r ) . V i s u a l l y i d e n t i f i a b l e complex a l k a l i - i r o n -aluminum sulfa te , covered by a t h i n s h e l l o f ash, was found on th ese tubes . Thea sh s h e l l had p a r t i a l l y shed on c o o l i n g o f the f u r n a c e . L a rg e s e c t i o n s c o u ld b eremoved i n t a c t w it h l i g h t f i n g e r p r e s s u r e o n t h e e d g e t o e xp os e t h e u n d e rl y i n gs u l f a t e d e po s it . The s h e l l w as so t h i n t h a t c l e a n s e p a r a t i o n f ro m t h e u n d er l y in gs u l f a t e d e p o s i t c o u ld n o t b e a s s u r e d f o r t h e p ur po s e o f c he m ic al a n a l y s i s .

Depos i t s a t l o c a t i o n 3 ( l e a d tube o f the r e h e a t s u p e r h e a t e r p l a t e n ) wereshaped l i k e a n t l e r s j u t t i n g i n t o t h e g as stream and were j o ine d by a t h i n s h e l la c r os s t he f r o n t o f t he tube . They were so h a r d t h a t t h e a u t h o r c o u ld n o t b r e akt h e m o f f by hand though they pro je c te d about 3 inches i n f r o n t of the t u b e . Thea n a l y s i s g av e a n e x c e p t i o n a l l y h i g h c o n t e n t o f SO3 and maximum va lu es f o r th ea l k a l i metals , e x c e p t K20 i n t h e p r e c i p i t a t o r h o pp er sa mp le . The s u r f a c e o f t h ede po s i t was dus ty , a nd f r a c tu r e su r f a c e s showed no ind ic a t io n o f f u s i on , sugge s t ingt h a t t h e a sh w a s ce men te d b y f o r ma t io n o f a l k a l i s u l f a t e s w i t h i n p o r e s o f t h ed e p o s i t .

The samples f r om loc a t ion 4 ( the top o f the pr imary superhea te r ) were ha rdnodules b lown f rom an unknown loc a t i on and to o la r ge t o pass be tween t he sup er -h e a t e r t u b e s . While thes e nodules were as ha rd a s sample No. 3 , t h e s u l f u r c o n t e ntwas n ot a s h igh . The u n d e r l y i n g t u b e s were f r e e o f d e p o s i t s .

The economizer hopper ( l oc at io n 5) was lo ca te d below th e downflow se ct io n ofc o n v ec t i v e h e a t t r a n s f e r t u b e s . The a i r h e a t e r h o p p e r was u n de r t h e f lo w r e v e r s a lduc t be twee n se c t io ns o f t h e a i r h e a t e r . The c l a s s i f y i n g a c t i o n o f ch an ge s i nf l o w d i r e c t i o n , compared t o e l e c t r o s t a t i c p r e c i p i t a t i o n , may be r e s p o n s i b l e f o r t h edi f fe r enc es in an a l ys is be tween samples from th e economizer hopper , t h e a i r h e a t e rh o p pe r , and t h e p r e c i p i t a t o r h o p pe r , l o c a t i o n s 5 , 6 , an d 7 . There i s a l s o a p ro -g r e s s i ve d r op in t e m pe r atu r e betwe en the se loc a t ion s .

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1,The sam ple a na l yse s a r e r em a r ka b ly c o ns i s t e n t e xc e p t f o r t h e v a r i a t i o n i n s o 3

c on te n t a nd a d i sp r opor t iona t ion be twe e n l i m e and s i l i c a i n s am ple No. 3. The ironc on t e n t o f t he hoppe r and de po s i t s a mple s i s be twe e n those o f the t w o c o a l a s h e s ,wh i l e t he silica c o n t e n t s , w i th t h e o n e e x c e p t i o n , are h i g h e r then thzt of e i t h e rc o a l a s h . iThe hopper and de po s i t samples showed remarkable d i f f e r en ce s in co lor and rpm a gn e t ic p r o p e r t i e s . T a b l e 111 g i ve s da t a fr om g r c s s a ppe a ra nc e . Mic rosc op icexamina t ion showed he te rogeneous ch ar ac te r . The s la g tank sample looked l i k e pul -v e r i z e d b l a c k g l a s s w i t h a few t ra ns lu ce nt wedges and a few m a gne t i c pa r t i c l e s ,presumably magne t i te . The br i ck- red ma t e r i a l f rom the r e h e a t supe r he a t e r t ube hada v e r y small number of mag ne t i te p a r t i c l e s . The economizer hopper and sup erhe a te rnodu le s am ples had in c r e a s i ng p r opor t ions o f m a gne t i t e mixed w i th th e b r i c k r e d .The a i r h e a t e r s a mp le a p p e ar e d a l m o s t a s b l a c k as th e s la g tap sample , and had al a r g e p o r t i o n o f m a g n e t i t e w i t h a s p r i n k l i n g of smal l r ed p a r t i c l e s on t h e s u r f ac eo f t h e l a r g e r b la ck p a r t i c l e s .po r t io n o f m a gne t i te w i th e nough wh i t e s (some opa le s c e n t be a ds ) a nd r e d s t o g iv e agray-brown appearance.were m a gne t ic , p re suma b ly m a gne t i t e . The a na lyse s f o r i r on are g i v e n i n terms ofFeqO3 without regard t o t h e a c t u a l s t a t e o f o x i da t io n .t o f e r r i c o x id e , and t h e c o l o r v a r i a t i o n s may be us ed t o i n f e r v a r i a t i o n s i n f e r r i co x i d e c o n t e n t .

I

1The pr e c ip i t a t o r hopper s am ple had a c ons ide r a b le -

A lm os t a l l b l ac k p a r t i c l e s , e x c ep t t ho s e f rom t h e s l a g t a n k,The b r i c k r e d c o lo r i s due

D i s c u s s i o nMost of the ash f rom cy clo ne combustion i s c o l l e c t e d i n t h e s l a g ta nk w i t h

a lm os t n e g l i g i b l e s u l f u r c o n t en t .l ong pe r i ods of t i m e a n d r e p r e s e n t a n e g l i g i b l e p a r t o f t h e m a t e r i a l ba l a nc e . Thef l y a sh h a s a s i g n i f i c a n t s u l f u r c o n te n t b u t r e p r e s e n t s a minor f r a c t i o n o f t h e 'm i n e r a l m a t t e r i n p u t . Most o f t h e s u l f u r p a s s e s t h ro u gh t h e s y st em a s S02, b u to x i d a t i o n t o SO3 a nd s u l f a t e f o r m a t i o n by r e a c t i o n w i t h o t h e r m i n e r a l mat ter com-p o n e n t s h a v e c o n s i d e r a b l e e f f e c t on d e p o s i t f o r m a t i o n .

D e p o s i t s wi th h igh su l f u r c on te n t a r e f ormed ove r

The behavior of i r o n i s o f p a r t i c u l a r i n t e r e s t . I t i s p r e se n t i n t he c o a la l m o s t e n t i r e l y as p y r i t e , o f t e n d i s t r ib u t e d as v e r y f i n e p a r t i c l e s . Rapid h e a t in gt o 22OOoC p ro d uc es l i q u i d d r o p l e t s a pp ro x im a ti n g F eS , f e r r o u s s u l f i d e .lo/se que nc e o f m e l t i ng , de c om pos i t i on , and ox ida t ion i s unknown. Larger p a r t i c l e s a r ep r ob a b ly t hr ow n o n t o t h e s l a g w i t h i n t h e c y c lo n e . S m a ll e r p a r t i c l e s are conver tedi n s u s pe n s io n t o f i n e l y d i v i d e d f e r r i c o x i de . The a u t h o r h as o bs er v ed a c yc l on e -f i r e d u n i t w i th a me ch an ic al b u t no e l e c t r o s t a t i c p r e c i p i t a t o r .was br ic k re d . Had i t be e n b l a c k i t would have been rated number 4 smoke. Thef i n e l y d i v i d e d f e r r i c o x i d e pr od u ce s a d u s t c o a t i n g on t u b e s u r f a c e s . A t a p p r o p r i -a t e t e m p e r a t u r e l e v e l s , o x i d a t i o n of S O 2 t o SO3 w i t h i n t h e d u s t l a y e r a nd r e a c t i o nw i t h a d s o r b e d a l k a l i me t a l s pr oduc es the c om plex a l ka l i - i r o n s u l f a t e s . Dependingupon t e m pe r atu r e s and t e m pe ra tu r e g r a d ie n t s , t he s e s u l f a t e s may a c t a s cementingag en ts or may accumulate a s a se g r e ga t e d pha se ne a r t he tube su r f a c e . C yc l i cm e l t i n g and f r e e z i n g t h e n c a u s es e j e c t i o n o f n o n- s o lu b l e p a r t i c u l a t e s a nda c c um ula t ion of t he s e g r e g a te d de p os i t .

The

The sta ck plume

A s y st e ma t ic v a r i a t i o n i n a l k a l i metal c o n t e n t was n o t a p p a r e n t u n t i l t h e d a t awere a r r a nge d i n the s e que nc e o f Ta b le 111. Data f o r t h e s l a g t a n k sample a r el i s t e d f i r s t . T ho se f o r t h e p r e c i p i t a t o r h op pe r sample and t h e c o a l as h es a r el i s t e d l a s t . I n te rv e ni n g d a t a a r e i n t he s=:quence o f i nc r e a s in g c on te n t o f f e r r i ci r o n a s i n f e r r e d fro m c o l o r . V a r i a t i o n f ro m t h e s e qu e nc e of p o s i t i o n s al o n g t h ef low pa th i s a r e s u l t o f s e g r e g a t i o n of p a r t i c u l a t e m a t t e r and t h e a c t i o n o f r e -t r a c t a b l e b l ow er s us ed p e r i o d i c a l l y t o blow l o o s e d e p o s i t s o f f t u b e s u r fa c e s .h i g h e s t c o n t en t of f e r r i c i r o n was found i n t h e h a rd d e p o s i t on t h e r e h e a t s u p e r-h e a t e r t u b e .

TheD us ty d e p o s i t s a r e blown o f f t h e t u b e s by t h e r e t r a c t a b l e b lo w e rs .

I

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I.Ii1I11I

- 155 -Depending upon agg lomera t ion , the se t end t o f a l l ou t i n the economizer hopper o rc a r r y through t o t h e e l e c t r o s t a t i c p r e c i p i t a t o r .a p pe a r s t o be f a vo re d by t h e f i n e l y d i v i d e d f e r r i c o xi de w i th some e f f e c t o ft e mp e rat u re on o p p o r t u n i t i e s fo r s u l f a t e fo rm a ti o n . Te m pe ra tu re s a re lowes t int he p r e c i p i t a t o r .d e p o s i t s on t h e s ec on da ry s u p e r h e a t e r , l o c a t i o n 2 , and t h e r e h e a t s u p e rh e a t e r ,l o c a t i o n 3.the unknown t imes r e q u i r e d f o r d e p o s i t f o rm a ti o n.d e p o s i t s i s n e g l i g i b l e i n co mp ar is on t o t o t a l f lo w o v e r t h e t i m e o f t h e i r f o r m at i on .R e l a t i v e l y h i g h c o n t e nt i n t h e p r e c i p i t a t o r s am pl e i s o f f s e t by t h e l a r g e r p o r t i o no f t h e t o t a l a s h c o l l e c t e d i n t h e s l a g ta n k. I mm ea sur ab ly l o we r c o n t e n t i n t h e s l a gtank sample i s s u f f i c i e n t t o p r o vi de m a t e r i a l b a la nc e. T h e d i s p r o p o r t i o n a t i o n o fMgO and S iO p, i n t h e d e p o s i t a t l o c a t i o n N o . 3, t h e r e h e a t s u p e r h e a t e r l e a d t u b e ,i s e x p la i n ab l e i n te rm s o f t h e r e s u l t s o f i n t e r m i t t e n t o p e r a t i o n o f r e t r a c t a b l eb lowers. Acid format ion and a l k a l i adso rp t i on ex tend th rough th e i n t e r f a c e b e-twe en t h e c em en te d d e p o s i t and t h e d u s t y o u t e r c o a t . B a s i c c o n s t i t u e n t s , a sa g a i n s t t h e a c i d i c s i l i c a p r e s e n t i n se g re g at e d p a r t i c l e s , a re p r e f e r e n t i a l l yr e t a i n e d w hen t h e d u s t c o a t i n g i s i n t e r m i t t e n t l y removed b y t h e b l o w e r a c t i o n .Reconrmendations f o r Fut ur e S tudy

A b so r pt i on o f t h e a l k a l i metals

Condi t ions are most f a v o ra b l e f o r r e a c t i o n w i t h SO3 w i t h i nMaterial b a l a n c e s a re o b sc u red by d i f f e r e n c e s b et we en t h e c o a l s and

High SO 3 c o n t e n t i n s e g r eg a te d

Data p r e s e n t e d h e r e a re f o r samples taken from a cyc lone-combus t ion - f i red -un i t .I t h a s o f t e n b ee n o bs er ve d t h a t t h e s e u n i t s p ro du ce r e d d is h f l y a s h r a t h e r t h a n t h et y p i c a l g r a y f l y a s h fr om p u l v e r i z e d -c o a l - f i r e d s y st e ms , i n d i c a t i n g t h e p r es e nc e off i n e l y d i v id e d f e r r i c ox id e. N e v e rt h e le s s , t h e d a t a i n d i c a t e t h e p o s s i b i l i t i e s f o rb e t t e r u n d e rs t a n di n g o f t h e r e a c t i o n s i n vo l ve d i n d e p o s i t f o rm at i o n . S u pp l em e nt a ld a t a on t empera tu res , gas composi t ions , and par t i cu la te samples t aken from the gasstream a t v a r i o u s p o i n t s a l o n g t h e g as f l ow p a t h would b e h e l p f u l . P re p a ra t i o n fo rtaking samples from l e s s a c c e s s i b l e p o s i t i o n s would a l s o be h e l p f u l .

Labora tory s tu d i es o f rea c t i ons between gases and minera l matter componentsbecome m e a ni n gfu l o n l y when i n t e rp re t e d i n r e l a t i o n t o t h e c o n d i t i o n s and m a t e r i a l st h a t e x i s t w i t h i n a c t u a l s t e am g e n e r a t o r s y st em s. L a bo ra to r y s t u d i e s s h ou ld be e n-c o ura g ed , b u t t h ey m us t b e co o rd in a t e d w i t h a c t u a l c o n d i t i o n s i n l a r g e steamg e n e r a t o r s .Conc l u s ions

1) Samples of f l y ash and de pos i t s were o b t a in e d f ro m v a r i o u s l o c a t i o n s i n al a rg e , c y c l o n e - f i r e d s te am g e n e ra t o r .

2 ) Wide v a r i a t i o n s i n c o l o r were d ue t o v a r i a t i o n s in f e r r i c o x i d e c o n te n t .3) V a r ia t i on s i n a l k a l i metal c o n t e n t were r e l a t ed q u a l i t a t i v e l y t o v a r i a t i on s

i n f e r r i c o xi de c o n t e n t a nd t e m p er a tu r es .4) Ty p i c a l s e g re g a t e d d e p o s i t s of complex a lka l i - i ron -a lumina s u l f a t e s werev i su a l1 . y i d e n t i f i e d w i t h i n t h e s e co n da ry s u p e rh e a t e r .5) A h a rd d e p o s i t on t h e r e h e a t e r s u p e rh e a t e r p rob a bl y w a s formed by t h e

c e me nt in g a c t i o n of s u l f a t e s form ed w i t h i n t h e p o re s o f a n o v e r l y i n g d u s t y d e p o s i tand i n c r em e nt a l b u i l du p of c em ent ed m a t e r i a l l e f t a f t e r e ac h c y c l e o f r e t r a c t a b l eb l o w e r o p e ra t i o n .

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- 156 -Refe rences-1 ASTM Standard Method of Labo ratory Sampling and An aly si s o f Coal and Coke,-/ B a i l e y , E.G., and Ely , F.G., T r . Am . SOC. Mech. Engrs . 3, 65-71 (1941).-1 Re id , W.T., Core y, R.C., and Cro ss, B . J . , Tr. Am. SOC. Mech. Engrs . 67,-1 Corey, R . C . , C r o s s , B . J . , and Reid, W.T., Tr. Am. SOC. Mech. Engrs . 67,-/ J o h n s t o n e , H.F., Univ. of I l l i n o i s Eng. E xp. S t a . B u l l . 2 2 8 , 1 9 3 1 , pp. 4 4 , 4 5 .-1 C r o s s l e y , H . E . , Poll, A . , S w e e t t , F., J . I n s t . Fuel 21, 207-13 (Apr. 1948).-1 Reid , W.T., Ex te r na l Cor ros ion and Depos i t s -Boi le r s and G a s T u r b i n e s , E l s e v i e r ,-1 Anderson, C.H . , and Goddard, C.W., J . I n s t . Fuel 6 l . 357 -64 (1968).-/ Weint raub , M., Gol dbe rg, S.A., and Orning, A.A. , T r . Am. Soc. Mech. Eng.,

D271-64.

279 -88 (1945).289 -302 (1945).

New York, 1971.

S e r i e s A, 83, 4 4 4 - 5 0 (Oc t . 1961).-01 H a l s t e a d , W.D . , and Raask, E . , J. I n s t . F u e l 6 2 , 344-49 (1969).

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-. 157 -

Secondary superheater

Figure 1 . - Sampling locations in the cyclone fired steam generator.Location 7 , the precipitator hopper i s not shown.

4

,