SYSTEFI - PCA - The Portland Cement Association - America ...cement.org/manufacture/SP027.pdf ·...

RECOr l r lENDED GUIDELINES FOR

COAL SYSTEFI SAFETY

Prepared by the Coal System Safety Committee Manufacturing Process Subcommi t t e e of the PCA General Technical Committee

Dr. 11. von Seebach, Chairman Pol ys ius Corporation W. Berry Full e r Company L. Cockrell , J r . Ideal Basic Indus t r ies J . Goodwin Centennial Engineering, Inc. Or. N. Maycock r l a r t i n Marietta Cement E. T h o r n , J r . Southwestern Portland Cement Co.

COAL SAFETY MANUAL DISCLAIMER

This publ icat ion i s intended f o r t he use o f professional personnel competent t o evaluate the s ign i f i cance and l i m i t a t i o n s of i t s con- t e n t s and who wil l accept r e s p o n s i b i l i t y f o r t he app l i ca t ion of t h e mater ia l i t conta ins . The Ad Hoc Committee, t h e Manufacturing Pro- cess Subcommittee and the Portland Cement Association disclaim any and a l l respons ib i l - i t y for app l i ca t ion of the s t a t e d p r inc ip l e s or the accuracy of the sources referenced.

In the event of any error i n t h i s publica- t i o n , t he l i a b i l i t y of the Ad Hoc Committee, t he Manufacturing Process Subcommittee and the Portland Cement Association sha l l be l imi t ed , i n t he aggregate , t o f i f t y d o l l a r s ($50) . The provisions o f t h i s paragraph sha l l apply t o both con t r ac t a n d negligence c l a ims .

INDEX

Page

Introduction Design Coal Sel ecti on Economics o f Coal Firing Systems

Operation Coal Mill Problem Case Histories Figures and Graphs V.D.I. Guidelines C i terature

(Cost Considerations)

Sec. 1 4 Sec. 2 5 Sec. 3 26 Sec. 4 38

Sec. 5 43 Sec. 6 49 Appendix A Appendix B Appendix C

ABBREVIATIONS

AS TM BM C ITC C E I I E E E I SA NEC N FC NFPA V D I ZKG TUV

American S o c i e t y f o r T e s t i n g and M a t e r i a l s U.S. Bureau o f Mines Cement I n d u s t r y Techn ica l Conference Chemical Eng ineer ing I n s t i t u t e I n s t i t u t e of E l e c t r o n i c and E l e c t r i c a l Engineers Ins t rumen t S o c i e t y o f America N a t i o n a l E l e c t r i c a l Code N a t i o n a l F i r e Codes N a t i o n a l F i r e P r o t e c t i o n A s s o c i a t i o n German Eng ineer ing S o c i e t y Zemment-Kalk-Gips (Cement-Lime-Gypsum, P e r i o d i c a l ) Technischer Ueberwachungs Vere in (Techn ica l C o n t r o l A s s o c i a t i o n ) s i m i l a r t o €PA (Envi ronmenta l P r o t e c t i o n Agency) and OSHA (Occupat ional S a f e t y and H e a l t h Adm i n i s t r a t i on )

1.0 INTRODUCTION

1.1 Background

The p r i c e o f energy i n t h e pas t decade has increased a t a r a t e which demanded a change i n the a t t i t u d e and a p p l i c a t i o n o f f u e l s f o r t h e p rocess ing i n d u s t r y . The use of o i l 3s a f u e l f o r cement process ing i n Nor th America was economical, and p rov ided a c lean, r e l a t i v e l y s imple f i r i n g system. i J h i l e an abundant supply o f coal was r e a d i l y a v a i l a b l e , t h e problems r e l a t e d t o t h e use o f i t i n cement process ing were n o t o f f s e t by a p r i c e d i f f e r e n c e f rom o i l u n t i l r e c e n t l y .

The chemical v a r i a t i o n s i n coal a re r e f l e c t e d i n i t s b u r n i n g c h a r a c t e r i s t i c s and sometimes, undes i rab l y , i n t h e c l i n k e r produced. The s i z e o f t h e coal p a r t i c l e s burned and t h e mo is tu re con ten t may a l t e r t he f lame p a t t e r n as w e l l as the temperature. Many o t h e r problems can a r i s e i n t h e s torage and hand l i ng o f bo th the coa l as received, and the ground product . The p u l v e r i z i n g , o r g r i n d i n g m i l l used t o prepare t h e coal f o r f i r i n g has i t s own se t o f p e c u l i a r aspects t o be considered f o r safe and e f f i c i e n t ope ra t i on .

I n September, 1981, an Ad Hoc Committee on Coal System S a f e t y was e s t a b l i s h e d by t h e Manufactur ing Process Committee o f the P o r t l a n d Cement Assoc ia t i on . coa l f i r i n g systems and s u p p l i e r s o f t h e equipment necessary f o r coal p r e p a r a t i o n a re bo th represented on t h e committee. With t h e i r combined e f f o r t s they have assembled i n t h i s paper a v a l u a b l e s e t o f recommended g u i d e l i n e s r e l a t i n g t o t h e des ign and use o f coal f i r i n g systems.

Cement producers p r e s e n t l y u s i n g

1.3 Purpose

The i n t e n t o f these g u i d e l i n e s i s t o p r o v i d e a base o f understanding f o r p o t e n t i a l and p resen t users o f coal f i r i n g systems. The goal i s t o assure safe, e f f i c i e n t , and economical des ign and a p p l i c a t i o n of these systems.

There a re s p e c i a l p recau t ions and c o n s i d e r a t i o n s which are recommended t o be taken i n designing, ope ra t i ng , and m a i n t a i n i n g a coal f i r i n g system. The p o t e n t i a l f o r f i r e and/or exp los ions connected w i t h coa l are d i f f e r e n t than w i t h o t h e r f u e l s . Knowing and understanding what t h e hazards a re w i l l r e s u l t i n a sa fe system, when t h e proper methods a re employed t o a l l e v i a t e them. An e x p l a n a t i o n o f accepted procedures, i n c l u d i n g t h e reasoning behind them, i s presented here.

One s e c t i o n o f t h e paper i s devoted t o coal s e l e c t i o n . The p h y s i c a l and chemical c h a r a c t e r i s t i c s o f a coal determine the sa fe o p e r a t i n g parameters f o r a p a r t i c u l a r system. I n t h i s

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sense, an e x i s t i n g system w i l l a l s o d i c t a t e t h e parameters w i t h i n which t h e coal used must remain, o r when des ign changes may be requ i red . The most economical use o f a system does n o t have a d i r e c t r e l a t i o n t o t h e p r i c e per t o n of t h e coa l .

2.0 D E S I G N

2 .1 General

2.1.1 Due t o t h e hazards i n v o l v e d and t h e p o t e n t i a l f o r damage t o personnel and equipment from i m p r o p e r l y designed systems, competent and exper ienced engineers should be i n v o l v e d i n t h e des ign of p u l v e r i z e d coa l f i r i n g systems f o r cement k i l n s . Approp r ia te des ign standards, i n c l u d i n g NFPA 85F, shou I d be used.

2.1.2 The s t r a t e g y f o r p r o t e c t i o n a g a i n s t e x p l o s i o n i n v o l v e s assu r ing t h e absence o f one o r more o f t h e e s s e n t i a l elements: a c r i t i c a l d u s t concen t ra t i on , s u f f i c i e n t a i r o r oxygen, and sources o f i g n i t i o n energy.

2.1.3 Concen t ra t i ons o f coa l d u s t above 40 grams pe r cub ic meter a re considered t o be p o t e n t i a l e x p l o s i o n hazards.

2.1.4 The amount o f oxygen r e q u i r e d t o suppor t combustion o r an e x p l o s i o n depends upon p h y s i c a l p r o p e r t i e s o f t h e coa l dus t such as p a r t i c l e s ize, v o l a t i l e content , ash content , and t h e tendency o f t h e o rgan ic p o r t i o n t o conver t t o combust ib le gases such as methane, carbon monoxide o r vapor ized v o l a t i l e s . The t h r e s h o l d oxygen c o n c e n t r a t i o n below which t h e system i s con- s ide red t o be i n e r t i s 12% b y volume o r l e s s (depending upon t h e coa l c h a r a c t e r i s t i c s ) .

2.1.5 The i g n i t i o n energy i s g e n e r a l l y p rov ided i n t h e f o r m o f heat. I n i t i a t i o n may occur due t o a spark, h o t c inde r , o r c o n t a c t w i t h a h o t sur face. The i g n i t i o n temperature f o r coal s o l i d s ranges from 190°C (374OF) t o 750°C (1382OF). I t should be noted, however, t h a t t h e i g n i t i o n temperature of gases o r i g i n a t i n g f rom t h e coal may be lower.

2.1.6 Cons ide r ing t h e genera l c r i t e r i a f o r t h e avoidance of exp los ion and f i r e hazards, an e f f o r t has been made t o e s t a b l i s h a s e t of des ign requi rements f o r a coal g r i n d i n g system. I n a cement p l a n t , h o t process gases f o r d r y i n g and conveying t h e p u l v e r i z e d coal may be ob ta ined f rom t h e c l i n k e r coo le r , t h e t e r t i a r y a i r duct , o r f rom t h e preheater , o r f rom a

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2.2

’ s e p a r a t e l y - f i r e d a i r h e a t e r . Hot a i r f ro in a c l i n k e r c o o l e r would have an oxygen c o n t e n t o f 21% b y volume co r respond ing t o a tmospher ic a i r . The p r e h e a t e r o f f - g a s e s would be expec ted t o c o n t a i n a p p r o x i m a t e l y 3.5% t o 6% oxygen b y volume. I f s u p p l i e d f r o m an a i r hea te r , t h e h o t gas would c o n s i s t o f combust ion gas p l u s a tmospher ic a i r hav ing a p p r o x i m a t e l y 18% oxygen b y volume.

Lump Coal S to rage and R e c l a i m i n g

2.2.1 The s t o r a g e and b l e n d i n g o f coa l can be as s i m p l e as r e c l a i m i n g f r o m an open p i l e w i t h a f r o n t - e n d l o a d e r i n t o a dump hopper t o a f u r t h e r c o a l s t o r a g e b i n or as complex as hav ing a c i r c u l a r coa l b l e n d i n g b u i l d - i n g w i t h l a r g e and expens ive s t a c k i n g and r e c l a i m i n g equipment ( F i g u r e s 1, 2 and 3 ) . However, t h e developments i n t h e f u t u r e due t o t h e requ i remen ts f r o m t h e k i l n system may tend towards complex systems and more expens ive u n i t s , i n o r d e r t o ach ieve a c o n s t a n t compos i t i on o f t h e f u e l .

2.2.2 W i t h i n a c o a l s t o c k p i l e t h e r e a r e o x i d a t i o n processes t a k i n g p l a c e comparable t o s low combust ion. The h e a t removed f r o m t h e p i l e p e r u n i t t i m e shou ld be g r e a t e r t han t h e heat produced b y t h i s o x i d a t i o n process . I f i t i s no t , t hen t h e tempera tu re o f t h e c o a l w i l l r i s e so t h a t t h e spontaneous i g n i t i o n tempera tu re may be reached i n a v e r y s h o r t t ime. The most r a p i d a b s o r p t i o n o f oxygen t a k e s p l a c e i n c o a l t h a t has j u s t newly been s t o c k p i l e d . Spontane- ous i g n i t i o n w i l l occu r m o s t l y i n t h e f i r s t few months o f s to rage .

2.2.3 Some o f t h e main f a c t o r s t h a t i n c r e a s e t h e r i s k o f spontaneous i g n i t i o n o f c o a l a re :

2.2.3.1 Coal which c o n t a i n s a m i x t u r e o f coarse and f i n e p a r t i c l e s which p e r m i t t h e access o f a i r t o t h e i n t e r i o r o f t h e p i l e .

2.2.3.2 F i n e g r a i n c o a l which has a l a r g e r s u r f a c e a rea f o r t h e a b s o r p t i o n o f oxygen.

2.2.3.3 Coal w i t h a h i g h v o l a t i l e c o n t e n t .

2.2.3.4 M o i s t c o a l w i th a h i g h p y r i t e con ten t .

2.2.4 Coal s t o c k p i l e s shou ld be des igned so t h a t t h e coa l i n t h e s t o c k p i l e i s u t i l i z e d as soon as p o s s i b l e . T h i s i s , o f course, n o t always p r a c t i c a l n o r econom-

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i c a l , and t h e r e f o r e , a d d i t i o n a l s a f e t y measures shou ld be taken f o r r e d u c i n g t h e f i r e hazards f o r coa l s t o c k p i l e s which w i l l s t o r e coa l f o r a l ong t ime. Two ( 2 ) d i f f e r e n t methods o f s t o c k p i l i n g can be used:

2.2.4.1 Compacted p i l e s .

2.2.4.2 L o o s e l y b u i l t s t o c k p i l e s w i t h on t h e h e i g h t .

2.2.5 F o r compact s t o c k p i l e s , t h e p i l e shou ld l a y e r s , each l a y e r of which i s compactel

i m i t a t i o n s

be b u i l t i n b e f o r e t h e

n e x t l a y e r i s depos i ted . each success ive l a y e r shou ld n o t be more than 2 f e e t (0.6 meters ) . Compacting can be done b y use o f a f ron t -end l o a d e r on t o p o f t h e p i l e ( r e f e r t o F i g u r e 3 ) . These p i l e s can be b u i l t up t o 100 f e e t (30 m e t e r s ) i n h e i g h t ; however, i t i s s t i l l w ise n o t t o s t o r e d i f f e r e n t t ypes o f coa l i n t h e same p i l e . The r e c l a i m i n g shou ld be done v e r t i c a l l y from t h e p i l e i n o r d e r t o m i n i m i z e t h e newly-formed s u r f a c e area.

The maximum t h i c k n e s s o f

2.2.6 Loose s t o c k p i l e s can be used f o r l a r g e c o a l w i t h o u t f i n e s ; however, some p r e c a u t i o n s shou ld be taken.

As w i t h compacted s t o c k p i l i n g , c o a l f rom d i f f e r e n t sources and o f d i f f e r e n t grades shou ld n o t be s tock - p i l e d t o g e t h e r because o f t h e d i s p a r i t y o f t h e c o a l s i z i ng .

2.2.7 S t o c k p i l e s shou ld be b u i l t i n h o r i z o n t a l l a y e r s . I f an uncovered s to rage i s used then t h e end o f t h e s t o c k p i l e shou ld be f a c i n g t h e d i r e c t i o n o f t h e p r e v a i l i n g wind.

The d isadvantage o f an uncovered s t o c k p i l e i s t h e p o s s i b i l i t y o f r a i n o r snow e n t e r i n g t h e p i l e and t h u s t h e r e c l a i m e d c o a l w i l l have a v a r y i n g m o i s t u r e c o n t e n t which w i l l have an adverse i n f l u e n c e on t h e g r i n d i n g and d r y i n g c i r c u i t .

2.2.8 The h e i g h t o f a l o o s e s t o c k p i l e shou ld be l i m i t e d and de termined i n r e g a r d t o t h e v o l a t i l e c o n t e n t o f t h e coa l , e.g., low t o medium v o l a t i l e s : 20-33 f e e t (6-10 me te rs ) ; h i g h v o l a t i l e s ; 13-26 f e e t (4-8 meters 1.

2.2.9 W i t h t h e l i m i t a t i o n s on t h e s t o c k p i l i n g o f c o a l as ment ioned above, a b l e n d i n g bed can be used i n t h e

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2.2.10

same f a s h i o n as f o r t h e b l e n d i n g o f t h e raw m a t e r i - a l s f o r t h e cement process; however, t h e f o l l o w i n g p o i n t s shou ld be cons ide red :

I t i s p r e f e r a b l e t o s e l e c t a s t o c k p i l i n g method t h a t w i l l reduce s e g r e g a t i o n o f t h e coa l a t t h e exposed edges o f t h e p i l e , e.g., t h e Chevron method w i l l produce s e g r e g a t i o n whereas t h e Windrow method w i 1 1 m i n i m i z e t h i s seg rega t ion . F i g u r e 4 shows t h e two ( 2 ) t y p e s of p i l e s i n c r o s s - s e c t i o n .

F o r b l e n d i n g o f t h e coa l , a r e c l a i m i n g method shou ld be used which r e c l a i m s as much as p o s s i b l e o f a l l o f t h e l a y e r s o f t h e p i l e : e.g., b y u s i n g a b r i d g e - mounted sc raper r e c l a imer.

The comb ina t ion s t a c k e r r e c l a i m e r , w h i l e n o t r e c l a i m i n g on t h e end f a c e o f t h e p i l e , has a number o f advantages. P lease r e f e r aga in t o F i g u r e 2. One p r ime advantage b e i n g t h e c o s t o f t h e u n i t i t s e l f f o r t h e dua l f u n c t i o n o f s t a c k i n g and r e c l a i m i n g , ano the r b e i n g t h a t t h e d u s t p rob lem i s m in im ized w i t h t h i s s t a c k i n g and r e c l a i m i n g method. The d i sadvan tage o f t h i s u n i t i s t h a t w i t h r e c l a i m i n g b e i n g done on t h e s i d e face o f t h e s t o c k p i l e more s u r f a c e area i s exposed than i f r e c l a i m i n g i s done on t h e end face . The b l e n d i n g r a t i o i s a l s o reduced w i t h t h i s t y p e o f r e c l a i m i n g . I t i s a l s o d i f f i c u l t t o c o n t r o l spontaneous combust ion w i t h compact ing and c o n t r o l o f t h e t o e o f t h e p i l e . The t o e o f t h e p i l e i s t h e l a s t t o be r e c l a i m e d and when f i r e s s t a r t i n t h i s l o o s e toe , i t i s d i f f i c u l t t o g e t a f r o n t end l o a d e r i n t o t h e p i l e t o e x t i n g u i s h t h e f i r e . Consequent ly, f o r medium and low v o l a t i l e coa ls , t h i s comb ina t ion s t a c k e r i s v e r y s u i t a b l e .

2.2.11 A l l c o a l b e l t conveyors f r o m t h e s t o c k p i l e t o t h e c o a l p u l v e r i z e r shou ld be p r o t e c t e d b y an au tomat i c s p r i n k l e r f i r e suppress ion system.

I n a d d i t i o n t o t h e f i r e p r o t e c t i o n , an i n f r a - r e d pyrometer can be i n s t a l l e d ove r t h e c o a l r e c l a i m b e l t t o p r e v e n t any h o t c o a l f r o m e n t e r i n g t h e c o a l p u l v e r i z e r f eed b i n . A sma l l q u a n t i t y o f h o t coa l may n o t t r i p t h e au tomat i c s p r i n k l e r b u t c o u l d be a source o f i g n i t i o n f o r t h e coa l i n t h e b i n , p u l v e r - i z e r o r d u s t c o l l e c t o r .

2.2.12 Rec la im b e l t systems shou ld i n c l u d e a magnet o r m e t a l d e t e c t o r t o exc lude tramp meta l f r o m t h e c r u s h e r and p u l v e r i z e r . T h i s tramp meta l c o u l d damage t h e c rusher o r p u l v e r i z e r , and may cause s u f f i c i e n t sparks t o i g n i t e a f i r e .

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2.3

The r e c l a i m system shou ld a l s o i n c l u d e a l a r g e open- i n g screen o r g r i z z l y t o remove any rags o r wood. These f o r e i g n m a t e r i a l s promote b r i d g i n g , coa l b u i l d u p s and u l t i m a t e l y spontaneous combustion.

2.2.13 Coal c rushers shou ld be i n s t a l l e d on t h e r e c l a i m system i n s t e a d o f t h e coa l s t a c k i n g system as f r e s h - l y crushed c o a l w i l l s e l f - h e a t f a s t e r t h a n aged c o a l .

2.3.14 I f a coa l f i r e occu rs i n a s t o c k p i l e , i t shou ld be dug o u t and e x t i n g u i s h e d . P u t t i n g o u t t h e f i r e i n p l a c e w i t h smal l amounts o f wa te r may cause subsequent problems as p a r t i a l l y wet coa l w i l l s e l f - hea t more r a p i d l y than d r y coa l . I t i s t h e r e f o r e i m p o r t a n t t h a t a l l s i des o f a coa l s t o c k p i l e be a c c e s s i b l e t o m o b i l e equipment.

Raw Coal B i n s

2.3.1 The raw c o a l b i n shou ld be c o n s t r u c t e d o f c o n c r e t e t o p r e v e n t o r s t e e l . Covered b i n s shou ld be vented

c o n c e n t r a t i o n o f C0 f rom accumula t ing .

2.3.2 A mass f l o w d i s c h a r g e ( n o n r a t h o l i n g ) des s t r o n g l y recommended.

B i n s shou ld be designed t o e l i m i n a t e s t a

2.3.3

gn i s

i c c o a l d e p o s i t s . p r e v e n t a r c h i ng o r p l u g g i ng.

The b i n o u t l e t shou ld be s i z e d amply t o

A l l b i n s must be designed so t h a t t h e y a r e t o t a l l y s e l f - c l e a n i n g and t h e r e i s no chance o f a pocket of c o a l b e i n g l e f t i n a b i n o r hopper. Hopper sur faces shou ld have a minimum s lope o f 60". I n t e r n a l s u r f a c e s shou ld be k e p t f r e e f r o m s t i f f e n e r s , weld s t r i p s , o r f l a n g e d s u r f a c e s where coa l c o u l d p i l e up. S t r e n g t h e n i n g vesse l w a l l s and areas around manholes and doors shou ld be done e x t e r n a l l y .

2.3.4 No a i r pads o r a i r l a n c e p o r t s shou ld be p r o v i d e d t o improve f l o w o f t h e coa l o r coa l d u s t i n b i n s . I f f l o w improvement i s r e q u i r e d i n a p u l v e r i z e d c o a l b i n , mushroom t y p e nozz les f o r a i r ( o r CO2 w i t h h i g h v o l a t i l e c o a l s ) may be used.

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2.3.5 H i g h and low l e v e l d e t e c t o r s shou ld be p r a v i d e d .

2.3.6 P r o v i s i o n s f o r f i r e f i g h t i n g - - t o p ha tch , wa te r con- n e c t i o n , C02 c o n n e c t i o n - - s h o u l d be inade.

An emergency d i s c h a r g e c h u t e shou ld be p r o v i d e d . 2.3.7

2 .4 Raw Coal Feed System

2 .4 .1 Raw c o a l f e e d e r s shou ld be s e l e c t e d t o p r o v i d e a r e l i a b l e u n i n t e r r u p t e d f l o w o f coa l t o t h e p u l v e r - i z e r .

2 .4 .2 A t ramp meta l removal a r d e t e c t i o n d e v i c e i s d e s i r a b l e t o p r e v e n t damage t o t h e p u l v e r i z e r o r s p a r k i n g i n t h e system. T h i s equipment n a y be l o c a t e d b e f o r e o r a f t e r t h e raw coa l b i n .

2 .4 .3 Cleanup conveyors (screw, d r a g ) shou ld be i n s t a l l e d t o p r e v e n t c o a l b u i l d u p be low f e e d be1 t conveyors .

2.4.4 Des ign o f s t r u c t u r e s shou ld be " c l e a n " t o m i n i m i z e p o i n t s where d u s t can a c c u m u l a t e . The a rea must be easy t o keep c lean.

2.5 P u l v e r i z i n g - D r y i n g - F i r i n g Arrangement

2 .5 .1 There a r e s e v e r a l b a s i c p o s s i b l e a r rangements f o r c o a l f i r i n g systems f o r cement k i l n s . V i r t u a l l y a l l a r rangements can be c h a r a c t e r i z e d as e i t h e r d i r e c t ( D ) , s e m i d i r e c t (SD), s e m i - i n d i r e c t ( S I ) , o r i n d i - r e c t ( I ) . These t y p i c a l ar rangements a r e shown i n F i g u r e s 8, 9, 10 and 11 r e s p e c t i v e l y .

2.5.2 The d i r e c t f i r i n g system ( F i g u r e 8) i s t h e s i m p l e s t and most s t r a i g h t - f o r w a r d system. A l l o f t h e gases wh ich f l o w t h r o u g h t h e p u l v e r i z e r c a r r y t h e p u l v e r - i z e d c o a l d i r e c t l y t o t h e k i l n . T h i s l a r g e volume o f p r i m a r y a i r a t a r e l a t i v e l y l ow t e m p e r a t u r e (170°F t y p i c a l l y ) m i n i m i z e s t h e use o f a v a i l a b l e h i g h tempera tu re secondary a i r f r o m t h e c l i n k e r c o o l e r . Consequent ly , t h e c l i n k e r c o o l e r v e n t gases a r e h i g h e r i n tempera tu re and t h e o v e r a l l k i l n system f u e l e f f i c i e n c y i s somewhat l ower t h a n i n o t h e r systems.

2.5 .3 I n s e m i d i r e c t f i r i n g systems ( F i g u r e 91, t h e voluinr. o f gases g o i n g d i r e c t l y t o t h e k i l n i s reduced. The c o a l m i l l d i s c h a r g e i s d i r e c t e d t o a c y c l o n e d i i c h separa tes t h e c o a l f ro in t h e gas s t ream. P a r t of t h e

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m i l l d i scha rge gases a r e used t o t r a n s p o r t t h e coa l t o t h e k i l n and t h e remainder o f t h e gases a r e r e c i r c u l a t e d t o t h e c o a l m i 11.

2.5.4 A s e m i - i n d i r e c t system ( F i g u r e 10) i s i d e n t i c a l t o an i n d i r e c t system, b u t w i t h o u t t h e smal l p u l v e r i z e d coa l b i n under t h e cyc lone. T h i s system, l i k e t h e d i r e c t and semi d i r e c t systems, i s o n l y good f o r one f i r i n g p o i n t which l i m i t s i t s a p p l i c a t i o n .

2.5.5 An i n d i r e c t f i r i n g system ( F i g u r e 11) has separa te p u l v e r i z i n g and f i r i n g c i r c u i t s . T h i s system i s o f t e n used where t h e r e i s a s i n g l e p u l v e r i z e r u n i t w i t h m u l t i p l e f i r i n g p o i n t s such as m u l t i p l e k i l n s o r a p r e c a l c i n e r k i l n . The s i g n i f i c a n t f e a t u r e s o f t h i s system a re ( a ) a l l o r most o f t h e p u l v e r i z e r d i s c h a r g e gases a r e vented t o atmosphere, u s u a l l y t h rough a bag t y p e d u s t c o l l e c t o r , ( b ) p u l v e r i z e d coa l i s s t o r e d i n b i n s f o r a s h o r t p e r i o d o f t i m e b e f o r e i t i s d e l i v e r e d t o t h e f i r i n g p o i n t ( s ) , ( c ) s h o r t m i l l i n t e r r u p t i o n s w i l l n o t a f f e c t k i l n o p e r a t i o n and ( d ) l owes t o v e r a l l system hea t consumption.

I t shou ld be no ted t h a t t h e p i c k u p p o i n t f o r t h e r e c i r c u l a t i n g gas l i n e may be moved f r o m b e f o r e t o a f t e r t h e bag d u s t c o l l e c t i o n . T h i s w i l l a v o i d gases laden w i t h f i n e coa l f r o m b e i n g i n t r o d u c e d i n t o t h e h o t gas stream.

S ince p u l v e r i z i n g and d r y i n g a r e separa ted f r o m c o a l f i r i n g , i n e r t ( l o w oxygen) k i l n exhaust gases a r e sometimes used as p u l v e r i z e r d r y i n g gases.

A low volume o f a i r i s used t o t r a n s p o r t t h e p u l v e r - i z e d coa l t o t h e f i r i n g p o i n t , a l l o w i n g maximum use o f h i g h tempera tu re secondary a i r f r o m t h e c l i n k e r coo l e r .

The d isadvantages o f t h i s system a r e t h e need t o s t o r e f i n e coa l i n a b i n and t h e n e c e s s i t y f o r d u s t c o l l e c t i o n equipment. Bo th i t ems a r e p o t e n t i a l f i r e and e x p l o s i o n hazards.

The s e m i d i r e c t , s e m i - i n d i r e c t and i n d i r e c t systems may l o s e p a r t o f t h e v o l a t i l e m a t t e r as a gas w h i l e d r y i n g and g r i n d i n g t h e coa l . On f r e s h h i g h v o l a - t i l e coa l , t h i s may be as much as 500 b t u / l b s . o f c o a l (see F i g u r e 21) . D i r e c t and s e m i d i r e c t systems do n o t ven t gas e x t e r n a l t o t h e system, and

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2.6

t h e r e f o r e a l l methane f rom coa l f l o w s t o t h e bu rne r (assuming t h a t r e c y c l e d gas f rom s e m i d i r e c t c y c l o n e does n o t v e n t i n m i l l i n l e t ) .

P u l v e r i z e r s

2.6.1 The dominant t y p e o f c o a l p u l v e r i z e r used f o r cement k i l n coa l f i r i n g i s t h e r o l l e r m i l l p u l v e r i z e r ( F i g u r e 6 ) . B a l l m i l l s ( F i g u r e 5 ) a r e sometimes used and a t t r i t i o n m i l l s have a l s o been used i n a few p l a n t s . General g u i d e l i n e s f o r a l l p u l v e r i z e r s can be s t a t e d as f o l l o w s :

2.6.2 The p u l v e r i z e r and a s s o c i a t e d equipment shou ld be o f s t u r d y c o n s t r u c t i o n and capable o f w i t h s t a n d i n g an e x p l o s i v e p r e s s u r e o f 50 p s i g f o r con ta inment o f p o s s i b l e i n t e r n a l e x p l o s i o n s (NFPA 85F, 2.6).

2.6.3 I n l e t e x p l o s i o n ven t may be prov ided.

2.6.4 The i n l e t t o t h e coa l m i l l shou ld be designed t o m i n i m i z e a i r leakage i n t o t h e c o a l m i l l .

2.6.5 P r o v i s i o n s shou ld be made t o i n e r t t h e p u l v e r i z e r i n t e r n a l atmosphere a f t e r a planned o r emergency shutdown. P o s s i b i l i t i e s i n c l u d e :

2.6.5.1 A d d i t i o n o f l i m e s t o n e dus t .

2.6.5.2 C02 ( o r o t h e r i n e r t gas) i n j e c t i o n .

2.6.5.3 Water i n j e c t i o n (emergency f i r e suppress ion o n l y ) .

2.6.6

2.6.7

The area under t h e v e r t i c a l mill t a b l e shou ld be swept f r e e o f coa l r e j e c t s .

The i n t e r n a l arrangement o f g r i n d i n g s u r f a c e s f o r r o l l e r ( r i n g ) m i l l s t h a t a re a v a i l a b l e a re shown on F i g u r e 7. The a i r swept b a l l m i l l s u s u a l l y have a d r y i n g compartment ahead o f t h e g r i n d i n g compart- rnent(s) .

2.7 D r y i n g Gas Systems

2.7.1 Almost a1 1 p u l v e r i z e d coa l systems i n c o r p o r a t e m o i s t u r e removal from t h e coa l as p a r t o f t h e p u l - v e r i z i n g system. The d r y i n g p o r t i o n o f t h e system r e q u i r e s s e r i o u s a t t e n t i o n i n d e s i g n s i n c e t h e p o s s i b i l i t y

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o f f i r e s and exp los ions s i g n i f i c a n t l y increases as t h e coal becomes f i n e r , d r y e r and h o t t e r .

High m o i s t u r e c o a l s r e q u i r e more a i r f o r d ry ing , o r t h e p u l v e r i z e r must be operated a t a h i g h i n l e t temperature. T h i s second a1 t e r n a t i v e i s dangerous and should be avoided. When a power outage occurs, o r coal feed stops, temperatures r i s e r a p i d l y b e f o r e dampers can r e a c t and cool t h e m i l l . Once a m i l l system i s s i zed f o r a g i v e n a i r f l o w , i t w i l l con- t i n u e t o operate a t about t h a t a i r f l o w , even i f t h e coal source i s changed t o a much d r y e r coa l . I n d i - r e c t f i r i n g i s more b e n e f i c i a l f o r p l a n t s us ing v e r y wet coal , o r where t h e coa l m i l l i s overs ized.

2.7.2 M o i s t u r e Content L i m i t . I t i s recommended t h a t p u l v e r i z e d coa l s u r f a c e m o i s t u r e con ten t be no l e s s than 1% a t any p o i n t i n t h e p u l v e r i z i n g - d r y i n g - f i r - i n g system. Western semi and subbi tuminous c o a l s w i t h h i g h i n h e r e n t mo is tu re . These a re o f t e n d r i e d and ground w i t h 4 - 8% r e t a i n e d mo is tu re .

The 1% l e v e l does n o t apply t o many

2.7.3 Temperature L i m i t . I t i s recommended t h a t t h e temp- e r a t u r e o f t h e p u l v e r i z e d coal /gas m i x t u r e be l i m i t - ed t o 150°F f o r i n d i r e c t systems and 175°F f o r d i r e c t systems i n t h e p u l v e r i z i n g - d r y i n g - f i r i n g system upstream o f t h e bu rne r pipe. (Re fe r t o F i g . 26) These temperatures do n o t app ly f o r " i n e r t " systems.

I t i s f u r t h e r recommended t h a t t h e d r y i n g gas i n l e t temperature be no h i g h e r than 500°F i f t h e d r y i n g gas oxygen con ten t i s 21% b y volume.

Systems o p e r a t i n g a t low dew p o i n t s (20°F t o 40°F above ambient) can avo id condensat ion b y o p e r a t i n g a t 40°F above dew p o i n t s . Systems o p e r a t i n g a t h i g h e r dew p o i n t s ( g r e a t e r t han 4OoF above ambient) should be operated a t 50"-60°F above dew p o i n t . Ductwork, f ans , baghouses and conveyors o f such systems should be i n s u l a t e d .

2.7.4 Source o f D r y i n g Gases

2.7.4.1 C l i n k e r Cooler

The most common source o f d r y i n g gases f o r cement k i l n coal p u l v e r i z i n g systems i s h o t a i r f rom the c l i n k e r c o o l e r . Common duc t

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t a k e o f f p o i n t s a re t h e f i r i n g hood ( n o t recommended), t h e c o o l e r hous ing and t h e c o o l e r ven t . S ince these gases c o n t a i n c l i n k e r dus t , t h e y a r e n o r m a l l y c leaned i n a c y c l o n e ( d u s t t r a p ) b e f o r e pass ing t o t h e p u l v e r i z e r t o p r e v e n t wear i n t h e p u l v e r - i z e r , and i g n i t i o n f rom h o t p a r t i c l e s e n t r a i n e d i n t h e gas stream.

The advantage o f u s i n g c o o l e r gases f o r d r y i n g i s t h a t t h e y are a r e a d i l y access i - b l e source o f hea t and have a low dew p o i n t . The d isadvantages a r e t h a t t h e y c o n t a i n 21% oxygen and t h a t h e a t (secondary a i r if tapped a t t h e f i r i n g hood) a v a i l a b l e f o r t h e k i l n i s wasted.

2.7.4.2 K i l n o r P rehea te r Exhaust Gases

K i l n exhaust gases can be used f o r coa l p u l v e r i z i n g - d r y i n g . They c o n t a i n a lower p e r c e n t o f oxygen and, t h e r e f o r e , a re s a f e r , p a r t i c u l a r l y i n i n d i r e c t , semi- i n d i r e c t and s e m i d i r e c t coa l f i r i n g systems. These gases a re a l s o "waste gases" and u s i n g them does n o t reduce k i l n f u e l e f f i c i e n c y .

However, these systems may be more complex and r e q u i r e more c a r e f u l des ign and opera- t i o n . Also, t h e r e may n o t be a source o f i n e r t gases a v a i l a b l e f o r s t a r t - u p , and many of t h e r e p o r t e d c o a l f i r e s have occu r red du r - i n g t h i s p e r i o d , e s p e c i a l l y f o l l o w i n g emergency shutdowns o f t h e system.

Based on NFPA 69 and t h e Bureau o f Mines Repor t o f I n v e s t i g a t i o n 6543, t h e maximum oxygen c o n c e n t r a t i o n t o p r e v e n t t h e spark i g n i t i o n of coa l d u s t i s 17% b y volume f o r b i t um inous coa l and 15% f o r sub-b i tuminous c o a l . T h i s i s based on u s i n g carbon d i o x - i d e as a d i l u e n t . I f n i t r o g e n i s used, t hese va lues would be 16% and 13%, respec- t i v e l y . S ince k i l n gas i s p r i m a r i l y a m i x t u r e of carbon d i o x i d e and n i t r o g e n , t h e maximum oxygen c o n t e n t m i g h t be 16% f o r b i tumi nous coa l and 14% f o r sub-b i tumi nous c o a l . These l e v e l s must be f u r t h e r reduced

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if h i g h i n t e n s i t y i g n i t i o n sources a re a n t i c i p a t e d ; b y two percentage p o i n t s f o r an e l e c t r i c a l arc and by s i x percentage p o i n t s f o r an open f lame. Therefore, t h e "safe" oxygen l e v e l cou ld be r e p o r t e d as anywhere f rom 8% t o 16%. However, a l e v e l o f 12% oxygen i s g e n e r a l l y accepted as be ing i n e r t .

2.7.4.3 A u x i l i a r y Heated Gases

Ambient a i r o r gases f rom t h e k i l n systems as desc r ibed above can be heated by a boos te r hea te r w i t h a separate f u e l supply- - u s u a l l y n a t u r a l gas o r o i l - - t o supply d r y i n g gases t o t h e p u l v e r i z e r . Such equipment can be used con t inuous ly , o r f o r s t a r t - u p on ly .

2.7.5 Hot Gas Ducts

Hot gas duc ts should be designed t o i n s u r e a r e l i a - b l e supply o f gases t o t h e p u l v e r i z e r . G u i d e l i n e s f o r t h e i r des ign i nc lude :

2.7.5.1 Prevent dus t b u i l d - u p i n duc ts b y p r o v i d i n g adequate s lopes (40" w i t h f l ow , 50"opposed t o f l o w ) .

2.7.5.2 H o r i z o n t a l d u c t runs s h a l l be used o n l y when gases have been adequate ly c leaned and minimum gas v e l o c i t i e s can be assured.

2.7.6 Tempering A i r Dampers

Coal p u l v e r i z i n g systems, o p e r a t i n g w i t h o u t an i n e r t h o t gas supply, u t i l i z e tempering a i r dampers f o r temperature c o n t r o l and emergency coo l ing. These dampers are ex t reme ly impor tan t t o i n s u r e sa fe oper- a t i n g temperatures and must ope ra te r a p i d l y .

2.7.6.1 Emergency c o o l i n g a i r dampers a re n o t recommended f o r systems us ing i n e r t gases f o r d r y i n g . Limestone d u s t i n e r t i z a t i o n o r water deluge may be used ins tead .

2.7.6.2 Tempering a i r dampers should be h i g h q u a l i - ty, r e l i a b l e and rugged i n des ign and capa- b l e o f e f f e c t i v e l y o p e r a t i n g r a p i d l y

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even a f t e r some warpage and wear occurs , t i g h t s e a l s a r e recommended.

2.7.6.3 Dampers shou ld be r e a d i l y a c c e s s i b l e f o r maintenance and l o c a t e d so t h a t h e a t and m a t e r i a l b u i l d - u p do n o t hamper t h e i r oper - a t i o n .

2.7.6.4 G r a v i t y ope ra ted ( c o u n t e r w e i g h t ) q u i c k c l o s i n g dampers enhance t h e s a f e t y of t h e system beyond normal temper ing a i r damp- e r s .

2.8 P u l v e r i z e d Coal P i p i n g and Va lves

2.8.1 P u l v e r i z e d c o a l p i p i n g and v a l v e s shou ld be des igned t o w i t h s t a n d an e x p l o s i v e p r e s s u r e o f 50 p s i g f o r con ta inment o f p o s s i b l e i n t e r n a l e x p l o s i o n and s h o u l d meet a l l r equ i remen ts d e f i n e d i n NFPA 85F, 2.6.

2.8.2 V e l o c i t i e s i n a l l p i p e s convey ing p u l v e r i z e d coa l must be above 4000 f t / r n i n (20 m/sec).

2.'8.3 S l o p i n g d u c t s a r e recommended, w i t h an ang le o f 70" up and 45" down i n t h e d i r e c t i o n o f gas f l o w . bow duc ts , a l t h o u g h b e n e f i c i a l i n m i n i m i z i n g p res - s u r e drop, shou ld be avo ided because o f t h e ze ro s lope a t t h e apex.

Ra in-

2.8.4 P o i n t s i n t h e system where d u s t i s p r e c i p i t a t e d o u t s h o u l d d i s c h a r g e i n t o s u i t a b l e hoppers w i t h c o n t i n - uous m a t e r i a1 removal .

2.8.5 A l l c o a l p i p i n g must have a smooth i n t e r i o r w i t h a l l j o i n t s b e i n g smooth f l a n g e d j o i n t s or coup led w i t h v i c t u a l i c t y p e c o u p l i n g s . The use o f D resse r t y p e o f s e m i f l e x i b l e s leeve t y p e c o u p l i n g s i s f o r b i d d e n because these can t r a p sma l l pocke ts o f "dead c o a l " t h a t may t r i g g e r an e x p l o s i o n .

2.8.6 The b u r n e r s f o r i n t r o d u c i n g t h e c o a l t o t h e k i l n w i t h a d i r e c t f i r e d system a re u s u a l l y s t r a i g h t p ipes , a l t h o u g h sometimes a l i m i t e d amount o f f l a m e shap ing i s a t tempted. Combinat ion b u r n e r s t o b u r n c o a l w i t h an a l t e r n a t e f u e l may be i n s t a l l e d . A b a r r i e r v a l v e shou ld be i n s t a l l e d i n t h e c o a l p i p i n g o f any comb ina t ion b u r n e r t o p r e v e n t f l a s h b a c k when t h e a l t e r n a t e f u e l i s b e i n g burned.

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2.8.7

Flame s a f e t y dev i ces a r e sometimes promoted b y i nsu rance companies, b u t these have n o t been proven r e l i a b l e f o r cement k i l n s . I f such dev i ces a re i n s t a l l e d , t h e y shou ld m o n i t o r t h e au tomat i c i g n i - t i o n f l ame i n s t e a d of t h e coa l f lame, and shou ld d i sconnec t f r o m t h e i n t e r l o c k system when t h e k i l n reaches t h e a u t o i g n i t i o n tempera tu re o f t h e c o a l .

The r a t e o f f l a m e p r o p a g a t i o n i n a c o a l a i r s t r e a m may be as h i g h as 4500 f t . / m i n . (23 m/s), depending on t h e t y p e of coa l . Be sure t h a t normal t i p ve lo - c i t i e s on bu rne r p ipes are a t l e a s t 6800 f t / m i n (34.5 m/s).

I n s u r e t h a t t h e system i s purged of c o a l p r i o r t o r e d u c i n g t h e n o z z l e v e l o c i t i e s below t h e s a f e l e v e l o f 6800 f t . / m i n .

2.9 P u l v e r i z e d Coal B i n s ( F i g . 16)

I n d i r e c t coa l f i r i n g systems which u t i l i z e p u l v e r i z e d c o a l b i n s shou ld be designed u s i n g t h e f o l l o w i n g g u i d e l i n e s :

2.9.1

2.9.2

2.9.3

2.9.4

2.9.5

2.9.6

2.9.7

P u l v e r i z e d c o a l b i n s shou ld be designed t o w i t h s t a n d an e x p l o s i v e p r e s s u r e of 50 p s i g f o r con ta inment o f a p o s s i b l e i n t e r n a l e x p l o s i o n (NFPA 85F, 2.6).

Mass f l o w ( n o n r a t h o l i n g ) d i s c h a r g e des ign i s recommended.

A l l s u r f a c e s o f hoppers shou ld be s loped a t a m i n i - mum o f 60" a t a l l p o i n t s . T h i s i n c l u d e s t h e i n l e t t r a n s i t i o n s t o equipment.

I n t e r n a l s u r f a c e s shou ld be k e p t f r e e f rom s t i f f e n - e rs , weld s t r i p s , o r f l a n g e d s u r f a c e s where c o a l c o u l d p i l e up. S t r e n g t h e n i n g vesse l w a l l s and areas around manholes and doors shou ld be done e x t e r n - a l ly.

A i r l o c k s on convey ing equipment f o r c o a l d u s t shou ld be g r o s s l y ove rs i zed .

Do n o t l o c a t e p u l v e r i z e d f u e l b i n s i n a h o t atmosphere, and p r e v e n t b i n w a l l s f r o m b e i n g heated b y t h e sun o r r a d i a n t h e a t f rom k i l n s . I n s u l a t i o n o f b i n w a l l s i s recommended.

P r o v i s i o n shou ld be made t o i n e r t t h e p u l v e r i z e d f u e l b i n a f t e r a planned o r emergency shutdown.

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P o s s i b i l i t i e s i n c l u d e :

2.9.7.1 A d d i t i o n o f l i m e s t o n e d u s t .

2.9.7.2 CO2 ( o r o t h e r i n e r t gas) i n j e c t i o n .

2.9.7.3 o n l y ! 1.

Water de luge (emergency f i r e suppress ion

2.9.8

2.9.9

B i n s shou ld be a i r t i g h t i n l e t and e x i t ,

P r o v i d e tempera tu re sensors t o d e t e c t and a la rm h i g h tempera tu res .

2.10 V e n t i n g and Dust C o l l e c t i o n Systems

2.10.1 I n d i r e c t and semi - i n d i r e c t c o a l f i r i ng systems v e n t a l l o r p a r t o f t h e gases f rom t h e p u l v e r i z i n g -

d r y i n g c i r c u i t t o atmosphere t h r o u g h a d u s t c o l l e c t o r . T h i s p a r t o f t h e system has a h i g h p o t e n t i a l f o r f i r e s and e x p l o s i o n s . I n d e s i g n i n g d u s t c o l l e c t o r s , i t g e n e r a l l y assures t h a t t h e u n i t i s p r o t e c t e d a g a i n s t f i r e and p r e s s u r e d i s t o r t i o n . I n t h a t c o n t e x t , i t makes no d i f f e r e n c e whether t h e gas t o be dedusted i s i n e r t o r n o n i n e r t , s i n c e oxygen-poor gases c o u l d become r a p i d l y oxygen-enr iched due t o t h e p e n e t r a t i o n of u n d e s i r e d f r e s h a i r , t h e r e b y becoming dangerous n o n i n e r t gases.

2.10.2 E x p l o s i o n V e n t i n g Des ign -- The u n i t s u b j e c t e d t o t h e maximal d e s i g n p r e s s u r e can be deformed b u t n o t des t royed.

E x p l o s i o n - P r o o f Des ign -- U n i t s e c t i o n s cannot even be deformed f o l 1 owi ng an exp l o s i on.

2.10.3 NFPA Standard 85F r e q u i r e s t h a t c o a l systems be des igned t o c o n t a i n t h e f o r c e o f a 50 p s i e x p l o s i o n , u n l e s s t h e system i s s t a r t e d and opera ted under an i n e r t atmosphere. T h i s i s n o t t h e same as e x p l o s i o n p r o o f , as a ma jo r e x p l o s i o n can c r e a t e f o r c e s s t r o n g e r than 50 p s i . I n t e r n a l components a re n o t r e q u i r e d t o be e x p l o s i o n r e s i s t a n t , and may be expec ted t o be damaged d u r i n g an e x p l o s i o n . The r e a s o n i n g beh ind t h e NFPA requ i remen t i s t o p r o t e c t a g a i n s t personne l i n j u r y and ma jo r s t r u c t u r a l damage.

Some o p e r a t o r s p r e f e r n o t t o adhere s t r i c t l y t o NFPA Standard 85F and depend on e x p l o s i o n ven ts . These v e n t s shou ld be des igned a c c o r d i n g t o t h e recommen-

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d a t i o n of NFPA 68. Nomograph D from t h a t document (Figure 1 7 ) shows the relation f o r coal dust on l ine S t . 1. This nomograph i s applicable if the length t o diameter ra t io of the explosion vent i s less than three. For long ducts, the volumes should be divid- ed into several small areas each having a length t o diameter ra t io of 3 or less. Refer t o attached Figures 18 and 19 f o r typical explosion vents (pressure re1 i ef Val ves 1.

3.10.4 Explosion vents should n o t open t o the inter ior of a coal pulverizing building, as the force of the explosion could disperse dust t h a t has set t led and cause a secondary explosion. They also should n o t be located where a worker might be in l ine with the vent during an explosion. These two requirements often mean t h a t a d u c t i s required t o continue the explosion vents t o the outside of the building. However, such ducts reduce the effectiveness of explosion vents, and should be made as short as possi b 1 e .

2.10.5 Coal Cyclone Collector Guidelines:

2.10.5.1 Coal cyclones should be designed t o with- stand an explosion pressure of 50 psig ( N F P A 85F, 2 . 6 ) .

2.10.5.2 A n explosion vent i s also recommended. I t should be sized in accordance with NFPA 68 and routed safely t o atmosphere.

2.10.5.3 Provide ample size discharge opening with an oversized discharge valve.

2.10.6 Fabric F i l t e r Dust Collector Guidelines:

2.10.6.1 Alternate No. 1: Design t o meet NFPA 85F Code for 50 psig for explosion containment.

Alternate No. 2: Design f o r a limited explo-sive pressure of approximately 17-18 psig with pressure venting by rupturable diaphragms or explosion flaps.

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2.10.6.2 P u l s e a i r c l e a n i n g i s recommended i n o r d e r t o b r i n g as l i t t l e f r e s h a i r as p o s s i b l e i n t o t h e f i l t e r .

2.10.6.3 The bags shou ld be made o f f i r e - r e t a r d a n t m a t e r i a l w i t h e l e c t r i c a l r e s i s t a n c e s l o w e r t h a n lo8 Ohm, so t h a t e l e c t r i c a l l o a d s are c a r r i e d o f f b y t h e f i l t e r hous- i n g . e l e c t r i c a l l y c o n d u c t i v e b y hav ing some m e t a l l i c t h r e a d s o r t a c k i n g on me ta l s t r i p s .

The t e x t i l e s u t i l i z e d can be made

2.10.6.4 R e f e r t o S e c t i o n 2.9 f o r Dus t C o l l e c t o r Hopper des ign which shou ld be s i m i l a r t o b i n hoppers.

2.10.6.5 Hanger b e a r i n g s shou ld be avo ided i n screw conveyors.

2.10.6.6 P r o v i d e i s o l a t i o n v a l v e s a t i n l e t and c o a l d i s c h a r g e .

d e t e c t and a la rm accumu la t i on o f p u l v e r - i zed

2.10.6.7 P r o v i d e l e v e l d e t e c t o r s i n a l l hoppers t o

2.10.6.8 P r o v i d e tempera tu re sensors i n hopper and c l e a n a i r o u t l e t t o d e t e c t and a la rm h i g h tempera tures .

2.10.6.9 Oxygen and/or carbon monoxide d e t e c t o r s a r e recommended. They shou ld be i n s t a l l - ed i n t h e o u t l e t d u c t and shou ld i n i t i a t e a la rms and f i r e e x t i n g u i s h i n g a t p r e s e t va lues .

2.10.6.10 P r o v i d e p r o t e c t i o n a g a i n s t e x t e r n a l h e a t r a d i a t i o n .

2.10.6.11 When s h u t t i n g t h e system down, remove a l l t h e rema in ing d u s t i n t h e f i l t r a t i o n system and t h e connected convey ing u n i t s b y keeping t h e d u s t conveyors r u n n i n g f o r an extended p e r i o d a f t e r t u r n i n g o f f t h e p l a n t .

2.10.6.12 P r o v i d e f o r i n e r t i n g t h e d u s t c o l l e c t o r a f t e r a p lanned o r emergency shutdown. P o s s i b i l i t i e s i n c l u d e : a d d i t i o n o f l i m e -

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stone dust, CO2 (or other inert gas) injection o r water injection (emergency f i r e suppression only).

2.10.6.13 Inertization i s made more effective by providing quick closing gates before a n d a f te r the dust collector ( isolat ion of the col lector) .

2.11 Venting and Dust Collection Systems - (VDI Guidelines Appendix B )

Refer t o the following sections of VDI 3673 "Pressure Release of Dust Explosions" w h i c h are attached:

Section Page

5 Types and maintenance of pressure release B-12 devices

5.1 Rupture disc devices B-12 5.2 Explosion valves and explosion discs B-14 5.3 Spring loaded release devices B-14

6 Design of pressure release openings B-15

7 Safe discharge of the pressure wave, flame B-18 and exhaust gases

7 .1 Open a i r plants 7.2 Plants in closed areas

B-18 B-18

7.3 Effect of blow-off pipes on the reduced B-18 explosion pressure

7.4 Design of blow-off pipes B-19

8 Pressure release of elongated vessels B-19

9 Pressure release o f piping sections B-20

10 Pressure release of vessels connected by B-21 piping

2.12 Fire Extinguishing

2.12.1 I t must be recognized t h a t coal dust i s inherently dangerous t o handle and store. a f i r e suppression system, either halogen or

The instal la t ion o f

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2.13

carbon d i o x i d e , i s recommended and shou ld be s e r i - o u s l y cons idered.

A l though au tomat i c s p r i n k l e r systems can be f a i r l y e f f e c t i v e a f t e r t h e d e t e c t i o n o f f i r e , t h e y shou ld n o t n e c e s s a r i l y be cons ide red t o p r o v i d e e a r l y d e t e c t i o n and suppression.

A de luge wa te r sp ray below t h e m i l l t a b l e i s e f f e c - t i v e i n c o o l i n g t h e m i l l and any accumulated coa l d u s t upon m i l l shutdown.

2.12.2

2.12.3

2.12.4 F o r f i r e e x t i n g u i s h i n g purposes, foam c a r t r i d g e s can be mounted i n t o t h e d u s t c o l l e c t o r c e i l i n g t o pump foam i n t o t h e f i l t e r u t i l i z i n g t h e wa te r i n j e c t i o n system.

A system t o i n e r t p o r t i o n s o f t h e i n d i r e c t f i r e d systems shou ld be p rov ided . The recomnended agent t o i n e r t t h e system i s carbon d i o x i d e o r coo led k i l n e x i t gases. P r o v i s i o n s shou ld be made t o i n e r t a l l b i n s c o n t a i n i n g p u l v e r i z e d coa l , and t h e d u s t c o l l e c t o r s . Qu ick a c t i n g i s o l a t i o n va l ves are use- f u l t o reduce t h e r e q u i r e d q u a n t i t y of i n e r t gas.

I f a coa l system i s t r i p p e d under l o a d and t h e c o a l cannot be conveyed away, t h e system shou ld be coo led and k e p t under an i n e r t atmosphere u n t i l t h e system can be r e s t a r t e d ; o r u n t i l t h e coa l c o o l s t o ambient t empera tu re and can s a f e l y be removed.

2.12.6 I n e r t dus ts , such as l i m e s t o n e o r raw meal, may be used t o reduce e x p l o s i o n hazard. A 60% concent ra - t i o n o f i n e r t m a t e r i a l i s r e q u i r e d t o p r e v e n t f l a m e p r o p a g a t i o n i n t h e presence o f minus 200 mesh coa l dus t . coa l d u s t , up t o 90% i n e r t m a t e r i a l i s r e q u i r e d t o p r e v e n t i g n i t i o n .

I n t h e presence o f h o t s u r f a c e s or g low ing

I n s t r u m e n t a t i o n

R e f e r t o F i g u r e s 12-15 f o r t y p i c a l i n s t r u m e n t a t i o n drawings f o r d i r e c t , s e m i d i r e c t , s e m i - i n d i r e c t and i n d i r e c t coa l f i r i n g systems which show t h e recommended minimum i n d i c a t i n g , manual c o n t r o l and au tomat i c c o n t r o l i ns t rumen ts . The a c t u a l i n s t r u m e n t s used may depend on system c o n d i t i o n s and owner p re fe rence .

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2.14 E l e c t r i c a l Equipment f o r P u l v e r i z e d Fue l Systems

2.14.1 A l l apparatus s h a l l be l i s t e d f o r Class 11, D i v i s i o n 11, Group F, exp los ion -p roo f , per re ference t o NEC 500-5( b ) .

Ref: NEC 500 (NFPA 70)

2.14.2 Where p u l v e r i z i n g systems a re comp le te l y d u s t - t i g h t and i n compliance w i t h t h e i n s t a l l a t i o n and o p e r a t i o n o f P u l v e r i z e d Fuel Systems NFPA 85F, t h e y s h a l l n o t be considered hazardous.

Ref: NFPA 8 5 ~ 2-6.4.2

2.14.3 E l e c t r i c a l equipment and w i r i n g (pushbut ton s t a t i o n , motors, l i g h t i n g f i x t u r e s , e t c . ) s h a l l be i n s t a l l e d per t h e N a t i o n a l E l e c t r i c a l Code NFPA 70, A r t i c l e 500 through 502 and l o c a l a p p l i c a b l e codes.

Ref: NEC 500-502 (NFPA 70)

2.15 I n s t r u m e n t a t i o n

2.15.1 Thermocouples w i t h thermowel ls f o r sensing t h e coa l - a i r stream temperature s h a l l be s e l e c t e d based on t h e maximum v e l o c i t y and i n s e r t i o n l eng th . The thermowel l m a t e r i a l s h a l l be s u i t a b l e f o r t h e temperature, ab ras ion and c o r r o s i v e process atmosphere, b u t must have a f a s t response t ime t o changes i n temperatures.

Ref: I S A MC96.1 Temperature Measuring Thermocoupl es

2.15.2 Pressure t r a n s m i t t e r f o r sensing t h e c o a l - a i r stream p ressu re s h a l l be s e l e c t e d based on t h e maximum temperature and t h e c o r r o s i v e process atmosphere.

Ref: Standard P r a c t i c e ( C E I )

2.16 S a f e t y I n t e r l o c k Systems

2.16.1 I n t e r l o c k s f o r p u l v e r i z e r s s h a l l be arranged t o t r i p under t h e f o l l o w i n g c o n d i t i o n s :

2.16.1.1 Loss o f p r i m a r y a i r f l o w t r i p s , raw f u e l f eeder of a f f e c t e d p u l v e r i z e r s ( t h i s may r e q u i r e t h e c l o s i n g o f burner l i n e va l ves and dampers 1.

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2.17

2.16.1.2 Failure of pulverizer t r ips raw fuel feeder.

2.16.1.3 Closure of a l l pulverizer discharge valves t r ips raw fuel feeder.

2.16.1.4 Loss o f fuel feed through the pulverizer energizer alarms, and blocks restart ing of fuel feed until feeder start-up conditions are reestabl i shed.

Starting Interlocks

2 . 1 7 . 1 Permissive sequential s tar t ing interlocks shall be arranged so t h a t af ter appropriate ki I n interlocks have been sat isf ied, the pulverizer can be started only in the following sequence:

2 .17 .1 .1 Ignitors f o r a l l o f the burners served by the pulverizer are in service and proven.

2 .17 .1 .2 S ta r t primary a i r fan or exhauster if driven from the pulverizer.

2.17.1.3 Establish minimum a i r f l o w .

2.17.1.4 S ta r t pulverizer.

2.17.1.5 S t a r t raw fuel feeder.

2.18 Alarm System

Ref: NFPA 85E 7-2.1

2.18.1 Required annunciated alarms shall be provided.

2.18.1.1 Ignition fuel atomizing stream or ai r -oi l low different ia l pressure.

2.18.1.2 Ignition fuel high-low pressure.

2.18.1.3 Pulverizer tripped.

2.18.1.4 Primary a i r fan tripped.

2.18.1.5 Coal stoppage t o pulverizer.

2.18.1.6 Coal-air in le t and out le t high temperature.

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2.18.1.7 Furnace h i g h pressure .

2.18.1.8 H igh f u r n a c e d r a f t .

2.18.1.9 Loss o f o p e r a t i n g F.D. fan.

2.18.1.10 Loss o f o p e r a t i n g I . D . fan .

2.18.1.11 Low f u r n a c e a i r f l o w .

2.18.1.12 Loss o f i n s t r u m e n t a t i o n power.

2.18.1.13 Loss o f c o n t r o l power.

2.18.1.14 Loss o f f lame.

2.18.2 Recommended A1 arms and M o n i t o r s

Ref: NFPA 8% 7-2.2

2.18.2.1 Furnace t e l e v i s i o n and c o o l i n g a i r .

2.18.2.2 I g n i t i o n f u e l s u p p l y low pressure .

2.18.2.3 Combust ib les.

2.18.2.4 H igh - low oxygen.

2.18.2.5 F l u e gas ana lyze r f a i l e d ( k i l n i n l e t o r k i l n / p r e h e a t e r o f f gas ana lyze r f a i l e d ) .

2.18.2.6 H igh - low a i r / f u e l r a t i o .

2.18.2.7 No l o a d on p u l v e r i z e r .

2.18.2.8 P u l v e r i z e r ove r load .

2.18.2.9 Burne r r e g i s t e r c losed .

2.18.2.10 Flame d e t e c t o r m a l f u n c t i o n .

2.18.2.11 Flame d e t e c t o r i n d i c a t i o n .

2.18.3 E l e c t r i c a l Equipment f o r Coal S to rage B ins , Bunkers and Hoppers

2.18.3.1 A l l appara tus s h a l l be l i s t e d C l a s s 11, D i v i s i o n 11, Group F, Exp los ion -Proo f .

Ref: NFPA 85F 2-6.4

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2.18.3.2

2.18.3.3

2.18.3.4

3.0 COAL SELECTION

Where p u l v e r i z i n g systems a r e c o m p l e t e l y d u s t - t i g h t and i n compl iance w i t h NFPA 85F, t h e y s h a l l n o t be cons ide red hazar - dous.

E l e c t r i c a l equipment and w i r i n g (push- b u t t o n s t a t i o n , motors, l i g h t i n g f i x - t u r e s , l e v e l dev ices , e t c . ) s h a l l be i n s t a l l e d p e r t h e N a t i o n a l E l e c t r i c a l Code NFPA 70, A r t i c l e 500 th rough 502.

Ref: NEC (NFPA 70)

I n t e r l o c k System

1. The f o l l o w i n g i n t e r l o c k s sha l l be p r o v i d e d :

a.

b.

P r e s e n t accumula t ion o f f lammable m i x t u r e s o f a i r and f u e l d u s t and/or combus t ib le gases w i t h i n t h e s to rage b i n , bunker and hopper.

Ref : NFPA 85F 2-6.5.1.2

H igh and lower l e v e l f u e l d e t e c t o r .

Ref. NFPA 85F 2-6.5.2.3

3.1 H e a t i n g Values

Coal i s composed c h i e f l y o f carbon, hydrogen, oxygen, n i t r o - gen, su lphur , and m i n e r a l m a t t e r ( o r ash).

The p rox ima te a n a l y s i s g i v e s t h e h e a t i n g v a l u e o f t h e c o a l and amounts o f v o l a t i l e m a t t e r , which r e l a t e s t o t h e combust ion p r o p e r t i e s o f t h e c o a l .

The v o l a t i l e m a t t e r does n o t e x i s t i n coa l as such, b u t r e s u l t s f rom thermal decompos i t ion when t h e coa l i s heated under c e r t a i n c o n d i t i o n s . I t c o n s i s t s m a i n l y o f hydrogen, carbon monoxide, carbon d i o x i d e , t a r vapors and wa te r vapors, p l u s minor amounts o f methane and o t h e r hydrocarbons. N o n v o l a t i l e m a t t e r o t h e r than ash i n c l u d e s a c e r t a i n amount of s o l i d , f i x e d c a r -

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bon r e s i d u e . s u b t r a c t i n g from 100% t h e sum o f t h e percentages o f mo is tu re , ash, and v o l a t i l e m a t t e r .

The percentage o f f i x e d carbon i s o b t a i n e d b y

The s u l p h u r m i n e r a l s occu r m a i n l y as p y r i t e s (FeS2). s u l p h u r i s p resen t i n o r g a n i c m a t t e r . o r l e s s u n i f o r m l y d i s t r i b u t e d , and cannot be removed w i t h o u t m a t e r i a l l y a l t e r i n g t h e coa l makeup and s t r u c t u r e . s u l p h u r ranges from 20% t o 40% of t h e t o t a l su lphu r i n coa l . I n genera l , 50% o f t h e s u l p h u r i n coa l i s p y r i t i c , p a r t i c u l a r - l y when t h e s u l p h u r c o n t e n t i s h igh .

A lso , Organ ic s u l p h u r i s more

Organ ic

The ash c o n t e n t o f semib i tuminous and b i t u m i n o u s coa l i s t y p i - c a l l y f r o m 3% t o 12%, b u t may be as h i g h as 25% t o 30%; t h e carbon p o r t i o n can range from 65% t o 90% and t h e hyg roscop ic water c o n t e n t can range from 1 t o 6%. For l i g n i t e , t h e ash c o n t e n t i s 9% t o 20%, t h e carbon p o r t i o n i s 50% t o 60%, and t h e hyg roscop ic wa te r c o n t e n t i s 15% t o 20%. ( A l l o f these ranges a r e t y p i c a l and a r e o f t e n exceeded.

There are, of course, v a r i o u s q u a l i t y c o a l s on t h e market, and some b e l i e v e t h a t the cheapest coa l i s t h e b e s t coa l , b u t f r o m t h e aspec t o f cement c h e m i s t r y and k i l n o p e r a t i o n , t h i s i s u n f o r t u n a t e l y n o t t h e case. F i g u r e 20 shows t h e r e l a t i v e p r i c e s f o r v a r i o u s c o a l s i n t h e e a s t e r n r e g i o n s o f t h e U.S. w i t h i n c r e a s i n g s u l p h u r c o n t e n t o f t h e c o a l s . N a t u r a l l y , t h e r a r e 1 ow su 1 phur me ta l 1 u r g i c a l c o a l s a r e c o n s i d e r a b l y more expens ive than t h e c o a l s w i t h 2.5 t o 4% su lphu r , which a re a v a i l a b l e i n l a r g e q u a n t i t i e s . B u t w i t h dec reas ing p r i c e and i n c r e a s i n g s u l p h u r c o n t e n t of t h e coa l , t h e ash c o n t e n t genera l l y inc reases a1 so. U n f o r t u n a t e l y , h i g h ash and su 1 phur c o n t e n t s a re g e n e r a l l y accompanied b y h i g h amounts o f o t h e r contaminants.

G e n e r a l l y , t h e h i g h e r t h e h e a t i n g v a l u e t h e l ower t h e ash and/or i n h e r e n t m o i s t u r e i n t h e c o a l . Fo r t h e b u r n i n g zone, t h e h i g h e r t h e BTU con ten t , t h e e a s i e r i t i s t o c o n t r o l burn- i n g zone c o n d i t i o n s .

S ince t h e c o s t / m i l l i o n b t u i s i m p o r t a n t , i t i s necessary t o c o n s i d e r t h e purchase p r i c e p l u s f r e i g h t . I f t h e use p o i n t i s a l o n g d i s t a n c e f r o m t h e c o a l source, t h e f r e i g h t c o s t may equal t h e mine p r i c e pe r t o n and a h i g h b t u coa l i s d i c t a t e d b y economics.

I f t h e b t u c o n t e n t i s h i g h due t o a low ash con ten t , t h e hand- l i n g and p rocess ing c o s t s w i l l be lower b y 0 t o $.40 p e r m i l - l i o n BTU (1981 $ ) . I f t h e b t u c o n t e n t i s low due t o mo is tu re , t h e use o f an i n d i r e c t o r s e m i - d i r e c t system may be d i c t a t e d due t o k i l n e f f i c i e n c y ; however, t h e h i g h m o i s t u r e aggravates

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3.2

system problems p e r t a i n i n g t o f i r e s and e x p l o s i o n s i n t h e i n d i r e c t systems.

M o i s t u r e Con ten t of Coals

3.2.1 M o i s t u r e c o n t e n t o f t h e c o a l i s e s p e c i a l l y i m p o r t a n t s i n c e i t n o t o n l y a f f e c t s t h e g r i n d a b i l i t y , b u t a l s o t h e per fo rmance o f t h e m i l l b y way of t h e d r y i n g c a p a c i t y o f t h e system. g i v e n t o t h e p o t e n t i a l f o r f i r e s i n t h e system and loss o f v o l a t i l e m a t t e r due t o tempera tu res neces- s a r y t o d r y h i g h m o i s t u r e c o a l s .

C o n s i d e r a t i o n must a l s o be

M o i s t u r e can occu r i n two ways: e i t h e r i n h e r e n t l y as an i m p u r i t y i n t h e c o a l , o r as s u r f a c e m o i s t u r e . As an i m p u r i t y , t h e m o i s t u r e c o n t e n t can range as h i g h as 45% f o r some western c o a l s . I t i s d e s i r a b l e t o remove most wa te r t o improve t h e f u e l v a l u e o f c o a l as f i r e d , and t o ach ieve b e t t e r b u r n i n g zone c o n t r o l ; b u t h i g h r e s i d u a l m o i s t u r e can s t i l l be a p r a c t i c a l s o l u t i o n as an a l t e r n a t e t o e x t e r n a l d r y - i n g o r e x t e r n a l v e n t i n g o f c o a l m o i s t u r e . N o r m a l l y a 1.0% r e s i d u a l m o i s t u r e i s d e s i r a b l e because t h i s r e s i d u a l m o i s t u r e a i d s combust ion b y c a t a l y s i s o f t h e combust ion r e a c t i o n s .

F o r economic reasons, d r y i n g o f c o a l i n a p u l v e r i z e r shou ld be ach ieved b y h o t waste gases from a p r e - h e a t e r o r c l i n k e r c o o l e r . A t t h e same t ime, t h e c o a l must n o t be a l l owed t o reach t h e c r i t i c a l i g n i t i o n tempera ture . T h i s i s o f p a r t i c u l a r concern i n systems where r e c y c l e d gases a r e used and h i g h coal m o i s t u r e s d i c t a t e t h e u t i l i z a t i o n o f e l e v a t e d gas tempera tu res a t t h e m i l l i n l e t t o accompl ish t h e r e q u i r e d d r y i n g .

3.2.2 D i r e c t F i r e d Systems

The h i g h e r t h e m o i s t u r e c o n t e n t o f t h e c o a l , t h e less t h e k i l n e f f i c i e n c y , and f o r sub-b i tuminous c o a l s o r l i g n i t e s w i t h 25%-30% m o i s t u r e , i t i s necessa ry t o have a v e r y h i g h m i l l i n l e t t empera tu re wh ich i n i t s e l f can be dangerous i n t h e even t o f a power i n t e r r u p t i o n . If t h e m i l l i n l e t t empera tu re o f a r o l l e r t y p e m i l l i s above 400°F and t h e m i l l s tops , t h e bowl o r r o l l e r t a b l e may be h o t enough t o v a p o r i z e some o f t h e v o l a t i l e m a t e r i a l and e x p l o s i v e vapors may be p resen t . The m o i s t u r e accompanying t h e p r i m a r y a i r may lower t h e f l ame tempera tu re

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below what i s needed t o make good c l i n k e r . (Example: I n an East Texas l i g n i t e w i t h 8500 b t u / l b ( d r y ) t h a t conta ined 28% i n h e r e n t mo is tu re , t h e maximum f lame temperature t h a t cou ld be read w i t h an o p t i c a l pyrometer i n a b o i l e r was o n l y 2360°F.) O f course, where h i g h f l ame temperature i s n o t r e q u i r - ed, r e t e n t i o n o f i n h e r e n t m o i s t u r e as f i r e d i s s t i l l poss i b 1 e.

3.2.3 I n d i r e c t o r Semi -d i rec t Systems

M o i s t u r e has l e s s e f f e c t on these systems than on a d i r e c t f i r e d system, b u t i t s t i l l can cause problems. The coa l must be d r i e d and t h i s r e q u i r e s heat r e s u l t i n g i n h i g h m i l l i n l e t temperatures t h a t can cause s i m i l a r problems, i n t h e event o f a coa l feed i n t e r r u p t i o n o r a power outage, as noted i n Sec t i on 3.2.2.

I n an i n e r t system, r e c y c l i n g of combustion gases can o f t e n be used i n l o w e r i n g t h e oxygen l e v e l i n t h e m i l l c i r c u i t . T h i s i s accomplished b y r e c i r c u - l a t i n g a p o r t i o n o f t h e m i l l e x i t gases and r e t u r n - i n g them t o t h e m i l l i n l e t as shown on F i g u r e 11. I t should be noted t h a t t h e s a f e t y o f t h e system may be f u r t h e r enhanced b y r e l o c a t i n g t h e r e c i r c u l a t i o n p i ckup f rom b e f o r e t o a f t e r t h e dus t c o l l e c t o r . S u f f i c i e n t f r e s h h o t gas i s i n t roduced t o t h e system t o p r o v i d e t h e d r y i n g c a p a c i t y r e q u i r e d . A c o r r e - sponding q u a n t i t y o f gas i s removed f rom t h e m i l l c i r c u i t through t h e d u s t c o l l e c t o r . Bu t i n t h i s t y p e o f c i r c u i t , t h e dew p o i n t must be c a r e f u l l y moni tored.

I n most a p p l i c a t i o n s , coa l g r i n d i n g systems opera te w i t h a f i x e d m i l l i n l e t temperature and cons tan t gas f l o w r a t e th rough t h e m i l l . Consequently, t h e most severe dew p o i n t c o n d i t i o n s occur when o p e r a t i n g a t t h e maximum coa l f eed r a t e and mo is tu re .

The dew p o i n t o f ven t gas versus m o i s t u r e c o n t e n t and pe rcen t r e c i r c u l a t i o n i s shown on F i g u r e 22.

To avoid condensat ion d i f f i c u l t i e s , t h e temperature o f t h e m i l l e x i t gases should be ma in ta ined a t a minimum o f 20°C (36°F) above t h e dew p o i n t . Thus, p a r t i c u l a r l y i n c o l d weather, e x t e r n a l i n s u l a t i o n i s requ i red ; i n some instances, e l e c t r i c a l h e a t i n g of d u c t work and b i n hoppers ( e s p e c i a l l y o f t h e d u s t c o l l e c t o r ) i s needed.

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The occu r rence o f condensa t ion i s d e t r i m e n t a l t o t h e s a f e and e f f i c i e n t o p e r a t i o n o f t h e system s i n c e i t may l e a d t o agg lomera t i on o f c o a l i n hoppers and b i n s o r b l i n d i n g o f f a b r i c d u s t c o l l e c t o r bags, ( s i n c e condensate may be i n t h e f o r m o f coa l t a r s ) .

G e n e r a l l y , a h i g h i n h e r e n t m o i s t u r e ( n o t f r e e mois- t u r e ) i n d i c a t e s a r e l a t i v e l y "new" ( o r low r a n k ) c o a l which, i n a l l p r o b a b i l i t y , w i l l be a gassy c o a l t h a t , when ground, a t a t empera tu re h i g h enough t o remove t h e m o i s t u r e , may f o r m wa te r gas o r v a p o r i z e d v o l a t i l e s , which r e s u l t s i n a low " f l a s h p o i n t " c o a l . I t may be advantageous t o a v o i d u s i n g a "gassy" c o a l i n an i n d i r e c t f i r e d system because of t h e loss o f t h e c o m b u s t i b l e gas. ( F i g . 21)

These concepts a r e b e s t demonst ra ted b y c a l c u - l a t i n g t h e p r i m a r y a i r q u a n t i t y f o r t h e d i r e c t and s e m i d i r e c t f i r i n g systems. S i n c e a l l t h e a i r t h a t i s passed th rough t h e g r i n d i n g m i l l i n a d i r e c t f i r i n g system i s used as p r i m a r y a i r , a h e a t ba lance around t h e m i l l shows t h e p e r c e n t - age p r i m a r y a i r necessary t o d r y a g i v e n mois - t u r e c o n t e n t . The r e s u l t s a r e shown i n F i g u r e 23 f o r two d i f f e r e n t m i l l i n l e t t empera tu res and s e v e r a l o u t l e t t emper tu res . A l s o shown as a d o t t e d h o r i z o n t a l l i n e i s t h e minimum a i r volume t h a t must be passed th rough t h e m i l l f o r pneu- m a t i c convey ing o f t h e c o a l p a r t i c l e s o u t o f t h e m i l l . The m i l l can o n l y be opera ted i n c o n d i - t i o n s t h a t l i e above t h i s l i n e , and t h e i m p o r t - ance o f m o i s t u r e c o n t e n t i s immed ia te l y appar- en t . F o r example, a p p r o x i m a t e l y 20% p r i m a r y a i r i s m in ima l under any c o n d i t i o n . W i th a m i l l i n l e t t empera tu re o f 6OO0F, and an o u t l e t temp- e r a t u r e of 175"F, s u f f i c i e n t h e a t i s s u p p l i e d t o t h e system t o d r y a feed c o a l w i t h a 14% s u r f a c e m o i s t u r e c o n t e n t t o a r e s i d u a l m o i s t u r e c o n t e n t o f 1.5%. T h i s p r i m a r y a i r volume q u a n t i t y r e q u i r e s an a d d i t i o n a l h e a t consumption o f 100,000 b t u / s t o f c l i n k e r based on a p p r o x i m a t e l y 10,000 a d d i t i o n a l b t u / s t of c l i n k e r f o r each pe rcen tage o f p r i m a r y a i r o v e r 10%. i n l e t m o i s t u r e i n c r e a s e s t o say 20%, t h e p r i m a r y a i r q u a n t i t y must be i n c r e a s e d t o 30% w i t h a co r respond ing a d d i t i o n a l h e a t consumption o f 200 x l o3 b t u / s t o f c l i n k e r . t h e d i r e c t system i s t h a t t h e f i r i n g system and t h e c o a l m o i s t u r e , must be o p e r a t e d a c c o r d i n g t o t h e

I f t h e

The m a j o r d i sadvan tage o f

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requi rements of t h e pneumatic t r a n s p o r t i n the system and t h e coa l mois ture, r a t h e r than i n accordance w i t h t h e requi rements of t h e k i l n .

The s e m i d i r e c t system o f f e r s t h e o p p o r t u n i t y t o operate i n t h e r e g i o n below t h e d o t t e d h o r i z o n t a l l i n e i n F i g u r e 23. The p r i m a r y a i r q u a n t i t y i n t h e k i l n does not, over a wide range, depend upon t h e heat r e q u i r e d t o d r y t h e coal . Therefore, t h e f ir- i n g system can be operated a t t h e optimum f o r t h e burner, i.e., a t 10% p r i m a r y a i r as shown i n F i g u r e 24. The balance of a i r i s r e c i r c u l a t e d t o the m i l l . The coal m o i s t u r e content , w i t h which t h e system i s operable w i t h 10% p r i m a r y a i r , ranges up t o 10%. Only above 10% feed m o i s t u r e does t h e p r i m a r y a i r have t o be increased t o remove enough m o i s t u r e from t h e system.

I n summary, t h e r e s u l t s show t h a t f o r coa l m o i s t u r e below 10%-12%, o n l y t h e s e m i d i r e c t system p rov ides p r imary a i r q u a n t i t i e s c l o s e t o optimum f i r i n g con- d i t i o n s , i.e., 10%-12% p r imary a i r . The d i r e c t system always r e q u i r e s a minimum o f 20% p r i m a r y a i r , r e s u l t i n g i n a c o n t i n u o u s l y h i g h e r f u e l consumption of t h e k i l n system. F o r coa l mo is tu res above 12%-15%, an i n d i r e c t system w i t h baghouse and s to rage b i n i s t h e b e s t s o l u t i o n f o r thermal e f f i c i e n c y . I n d i r e c t systemss a re a l s o most s u i t a b l e f o r m u l t i p l e p o i n t f i r i n g systems, f o r i n s t a n c e k i l n - p r e c a l c i n e r systems. T h i s i s t h e b e s t s o l u t i o n t o remove excess m o i s t u r e f rom t h e system whi 1 e m a i n t a i n i n g optimum p r imary a i r requi rements.

3.3 V o l a t i l e M a t t e r

Depending on t h e t y p e o f coal , a l l c o a l s undergo some degree o f v o l a t i l i z a t i o n d u r i n g t h e p r e p a r a t i o n o f t h e p u l v e r i z e d f u e l . Therefore, v o l a t i l e c o n s t i t u e n t s a re always l i k e l y t o be p resen t i n t h e a i r - c o a l m i x t u r e and some o f t h i s v o l a t i l e m a t t e r w i l l be vented t o atmosphere on i n d i r e c t o r semi- i n d i r e c t systems. Consequently, t h e i g n i t i o n energ ies can be expected t o be s i g n i f i c a n t l y l e s s than " t h e o r e t i c a l " as i n d i - cated b y t e s t r e s u l t s .

The most d e s i r a b l e range o f v o l a t i l e m a t t e r i n coa l i s between 18% t o 30%. Below 18% v o l a t i l e y t h e coal becomes s a f e r from an e x p l o s i o n s tandpo in t , b u t i f t h e k i l n ge ts c o l d o r dusty, t h e r e may be a " f lame o u t " which can be more dangerous. example o f a v e r y low v o l a t i l e " c o a l " i s a n t h r a c i t e o r p e t r o - leum coke. I n b u r n i n g e i t h e r a n t h r a c i t e o r pet ro leum coke, i t

An

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i s necessary t o f u r n i s h a h i g h v o l a t i l e f u e l t o i n c r e a s e t h e " f l a m m a b i l i t y " o f t h e coke i n o r d e r t o have i t b u r n i n t h e p o r t i o n o f t h e k i l n where needed.

These medium t o low v o l a t i l e c o a l s a r e recommended f o r use i n i n d i r e c t f i r e d systems because t h e i r " s e l f - h e a t i n g ' ' tendenc ies , when p u l v e r i z e d , a re much l e s s t h a n w i t h h i g h v o l a t i l e coa ls .

H igh v o l a t i l e coa ls , when p u l v e r i z e d , shou ld n o t be s t o r e d f o r more t h a n one hour u n l e s s t h e coa l i s below 140°F.

F i g u r e 26 shows how t h e maximum recommended p u l v e r i z e d c o a l s t o r a g e tempera tu re must be decreased as t h e v o l a t i l i t y i nc reases s i n c e t h e a i r - f u e l m i x t u r e becomes more l i k e l y t o i g n i t e as t h e m a t e r i a l i s d e v o l a t i l i z e d . Fo r a h i g h l y v o l a t i l e f u e l , t h e maximum recomended p u l v e r i z e d coa l s t o r a g e tempera ture i s l i m i t e d t o 13O0F-150"F. There fo re , t h e a i r must be coo led b y t h e m o i s t u r e i n t h e f u e l , o r t h e a i r must be quenched p r i o r t o e n t e r i n g t h e m i l l , wh ich g i v e s l e s s e f f i c i e n t u t i l i z a t i o n o f t h e k i l n waste heat .

Furthermore, low m i l l o u t l e t tempera tures a re r e q u i r e d a l s o i n an i n d i r e c t system o p e r a t i n g w i t h h i g h l y v o l a t i l e coa ls , because t h e p u l v e r i z e d coa l i s s t o r e d i n smal l i n t e r m e d i a t e b i n s p r i o r t o convey ing t o t h e b u r n e r o r bu rne rs . H igh v o l a t i l e , h i g h m o i s t u r e c o a l s u b j e c t e d t o 600°F gases f o r d r y i n g w i l l u s u a l l y v a p o r i z e some o f t h e v o l a t i l e c o n t e n t which w i l l t hen be vented t o atmosphere i n an i n d i r e c t o r p a r t i a l l y vented i n a s e m i d i r e c t system. Loss o f v o l a t i l e c o n t e n t i n t h i s t y p e o f system has been r e p o r t e d i n t h e range of 300-500 b t u / l b o f coa l . (See F i g u r e 21.)

3.4 S e l f - I g n i t i o n

ASTM D-2013-72 s t a t e s t h a t i n sample p r e p a r a t i o n f o r c a l o r i m e t e r t e s t s , t h e sample s h a l l be a i r d r i e d . If oven d r i e d , t h e maximum oven tempera tu re s h a l l n o t exceed 104"F, and i f a h i g h v o l a t i l e c o a l above ambient.

I g n i t i o n tempera tu re i s t h e tempera ture a t which an obse rve r sees t h e coa l f i r e . It cannot be e x a c t l y d e f i n e d and, t h e r e f o r e , can o n l y be used t o de te rm ine r e l a t i v e s p o n t a n e i t y o f t h e c o a l s t e s t e d . Normally,, t h e i g n i t i o n tempera tu re o f coa l i s between 370" and 1400°F.

The thermal i g n i t a b i l i t y d a t a f o r Pocahontas seam coa l a r e shown i n F i g u r e 27 ( v o l a t i l i t y a p p r o x i m a t e l y 16%).

F i g u r e 28 shows t h e thermal a u t o i g n i t i o n d a t a f o r P i t t s b u r g h seam coa l d u s t (36% v o l a t i l i t y ) . The d a t a p o i n t s a re t h e minimum c o n c e n t r a t i o n s t h a t i g n i t e a t a g i v e n temperature. The c o n c e n t r a t i o n s are t h e a c t u a l c o n c e n t r a t i o n s , namely t h e

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mass o f dust d i v i d e d by the chamber volume. f i n e n e s s o f t h e p u l v e r i z e d coa l i s dependent upon t h e v o l a t i l i t y con ten t because a coal w i l l i g n i t e more r e a d i l y and r a p i d l y as t h e v o l a t i l i t y increases.

The r e q u i r e d

The v o l a t i l i t y i s p a r t i c u l a r l y i m p o r t a n t because t h i s determines the tendency o f t h e coal d u s t t o explode and i s a l s o r e l a t e d t o t h e g r i n d a b i l i t y o f t h e coal .

The r e q u i r e d i g n i t i o n energy depends upon t h e t y p e o f coa l , and as m igh t be expected, t h e i g n i t i o n energy decreases as t h e atmosphere becomes l e s s i n e r t .

The s m a l l e r t h e dus t p a r t i c l e , t h e more r a p i d l y i t i s heated and brought t o t h e i g n i t i o n temperature.

The v o l a t i l e con ten t of t h e coa l g i v e s t h e b e s t i n d i c a t i o n o f t h e p o s s i b l e i g n i t i o n temperature.

I n F i g u r e 25, i t can be seen t h a t t h e i g n i t i o n temperature f o r d u s t l a y e r s and dus t c louds decreases as t h e v o l a t i l e m a t t e r of t h e carbonaceous dus t increases.

As t h e v o l a t i l e con ten t o f carbonaceous dus t increases, a lower l i m i t f o r t h e i g n i t i o n temperature o f 190°C (374°F) i s reached. The average c loud and l a y e r i g n i t i o n temperatures a re shown below:

AVERAGE CLOUD IGNITION AVERAGE LAYER I G N I T I O N FUEL TEMPERATURE "C ( O F ) TEMPERATURE "C ( O F )

B i t . Coal 617 (1143) 222 (432) L i g n i t e 443 ( 829) 205 (401) Coke 727 (1341) 349 (660)

F o r t h e l i m i t s of f l a m m a b i l i t y t h e r e i s a s i g n i f i c a n t oxygen dependence a t a l l 02 concen t ra t i ons .

O x i d a t i o n of coa l occurs a t any temperature whenever t h e coa l i s exposed t o oxygen. I n e f f e c t , combustion occurs even a t low temperatures, proceeding a t a slow r a t e , b u t producing t h e p roduc ts o f combustion such as CO2, CO, and H20. O x i d a t i o n o f coa l i s p r i m a r i l y a sur face phenomenon; consequent ly, t h e r a t e of o x i d a t i o n i s g r e a t e r as t h e s u r f a c e area increases.

R e l a t i o n between p a r t i c l e s i z e and l e a n l i m i t o f f l a m m a b i l i t y i s shown on F i g u r e 29.

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For P i t t s b u r g h seam coa l , t h e l i m i t i n a i r i s app rox ima te l y 130 mg/l, independent o f p a r t i c l e s i z e up t o d iamete rs of 40 um. For coa rse r dus ts , t h e l i m i t c o n c e n t r a t i o n i nc reases marked ly w i t h d iameter .

Coal f i r e s due t o spontaneous combustion, o r s e l f - h e a t i n g , g e n e r a l l y deve lop b y s low o x i d a t i o n i n t h e coa l seams o r j o b areas and occu r most f r e q u e n t l y w i t h low-ranked coa ls .

I t i s g e n e r a l l y b e l i e v e d t h a t t h e low- tempera ture s tage of o x i d a t i o n produces coa l -oxygen complexes such as carbony l and c a r b o x y l compounds, which upon f u r t h e r hea t ing , l i b e r a t e CO and C02.

Thus, t h e CO and C02 emiss ion r a t e s o r t h e co r respond ing 02 d e p l e t i o n r a t e s a re f r e q u e n t l y r e l i e d upon as a c r i t e r i o n o f coa l spontaneous combustion. The spontaneous combust ion of coa l i s a s p e c i a l case o f i n c i p i e n t combustion; CO i s r e l e a s e d a t low t empera tu re (amb ien t t o 100°C) even i n t h e absence o f an i n c i p i e n t f i r e , whereas smoke p a r t i c u l a t e s a re g e n e r a l l y formed a t h i g h e r tempera tures .

Another c r i t e r i a f o r spontaneous combust ion i s t h e tempera tu re o r tempera ture r i s e o f t h e r e a c t i n g c o a l . r i s e o f t h e r e a c t i n g mass i s i n d i c a t i v e o f s e l f - h e a t i n g . The s e l f - h e a t i n g r a t e s w i l l i n c r e a s e w i t h i n c r e a s i n g tempera tu re and r e s u l t i n i n c r e a s i n g CO and C02 emiss ion r a t e s . em iss ion measured i n a c losed system inc reases w i t h dec reas ing c o a l rank and i s g r e a t e s t f o r wes tern coa ls . The minimum s e l f - h e a t i n g tempera tu re f o r b i t um inous coa l i s between 150"- 220°F. I t shows t h a t t h e CO f o r m a t i o n i s a s t r o n g f u n c t i o n o f t h e i n t r i n s i c m o i s t u r e and oxygen (mo is tu re -ash f r e e ) c o n t e n t o f t h e c o a l .

Any tempera tu re

The CO

The s e l f - h e a t i n g tempera tures o f t h e c o a l s decrease w i t h dec reas ing rank, and a re l owes t when t h e c o a l s a r e p r e d r i e d and exposed t o m o i s t a i r .

3.5 F a c t o r s A f f e c t i n g Spontaneous H e a t i n g o f Coal

3.5.1 A i r F low Ra te

A i r f l o w s s u f f i c i e n t t o m a i n t a i n h i g h oxygen concen- t r a t i o n s a t t h e coa l su r face , b u t n o t enough a i r f l o w t o remove heat b y c o n v e c t i v e c o o l i n g , w i l l i n c r e a s e t h e tendency toward spontaneous h e a t i n g .

3.5.2 P a r t i c l e S i z e

P a r t i c l e s i z e has an i n v e r s e r e l a t i o n s h i p t o spontaneous h e a t i n g o f c o a l . The s m a l l e r t h e c o a l p a r t - i c l e , t h e g r e a t e r i s t h e exposed s u r f a c e area and

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t h e g r e a t e r i s t h e tendency toward spontaneous hea t - i ng .

3.5.3 Chanaes i n M o i s t u r e Conten t

The changes i n m o i s t u r e c o n t e n t a f f e c t t h e tendency o f spontaneous h e a t i n g o f coa l .

W e t t i n g o f c o a l i s an exo the rm ic process. When m o i s t a i r comes i n c o n t a c t w i t h d r y coa l t h e tempera ture of c o a l w i l l i nc rease . A t tempera tures below 210°F t h e hea t o f w e t t i n g i s g r e a t e r t han t h e hea t o f o x i d a t i o n .

3.5.4 Temperature

The r a t e of c o a l o x i d a t i o n i s a d i r e c t f u n c t i o n o f temperature; t h e h i g h e r t h e temperature, t h e f a s t e r t h e r a t e a t which c o a l r e a c t s w i t h oxygen.

3.5.5 Rank

As t h e rank o f c o a l decreases, t h e hazard o f spontaneous h e a t i n g i nc reases . Low rank f u e l s , such as l i g n i t e s and sub-b i tuminous coa ls , a re most s u s c e p t i b l e t o spontaneous h e a t i n g .

3.5.6 P v r i t e Conten t

G e n e r a l l y , t h e p y r i t e c o n c e n t r a t i o n must exceed 2% b e f o r e i t has a s i g n i f i c a n t e f f e c t .

P r e v e n t i o n o f spontaneous h e a t i n g i s dependent on t h e s i t u a t i o n where t h e danger occurs. However, t h e f o l l o w i n g genera l p r i n c i p l e s a r e o f use:

3.5.7 I f access o f a i r can be c o m p l e t e l y p revented , t h e r e can be no danger o f spontaneous hea t ing , b u t t h i s does p r e s e n t t h e p o s s i b i l i t y o f CO c o n c e n t r a t i o n .

3.5.8 There i s t h e obv ious genera l p r i n c i p l e t h a t ex t rane - ous sources o f hea t near t h e coa l b i n a r e t o be avoided.

The development o f spontaneous h e a t i n g i n c o a l i s a l e n g t h y process, u s u a l l y on t h e o r d e r o f two weeks, so t h a t i f t h e c o a l can be made t o t r a v e l t h rough t h e s i l o i n such a way t h a t i t i s coo l when i t e n t e r s t h e s i l o and none o f i t s t a y s i n t h e s i l o f o r more than a day, t h e r i s k o f spontaneous h e a t i n g w i l l be g r e a t l y reduced.

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Coal p u l v e r i z a t i o n r e q u i r e s s p e c i a l measures so as t o a v o i d e x p l o s i o n s . S t a r t i n g a t a d u s t c o n t e n t o f 45 g/m3 up t o an upper l i m i t o f 2000 t o 7000 g/m3, a m i x t u r e o f a i r and p u l v e r - i z e d c o a l i s s u s c e p t i b l e t o e x p l o s i o n . However, t hese f i g u r e s cannot be taken as e x a c t s i n c e t h e s i z e o f p a r t i c l e s i s an i m p o r t a n t c o n t r i b u t i n g f a c t o r . I t i s a l s o necessary t o t a k e i n t o c o n s i d e r a t i o n t h e c o n t e n t o f v o l a t i l e substances such t h a t t h e m i x i n g r a t i o o f c o a l d u s t t o a i r reaches i t s c r i t i c a l p o i n t i n accordance w i t h t h e e x i s t i n g c o n d i t i o n s .

3.6 Chemical P r o p e r t i e s ( R e f e r t o Tab les 1 -41

3.6.1 C o a l - - U l t i m a t e A n a l y s i s

S ince t h e p r i m a r y combus t ib le m a t e r i a l i n c o a l i s hydrogen and carbon, and on an a s h - f r e e b a s i s w i l l be 95% p l u s o f t h e m a t e r i a l , t h e ba lance i s s u l - phur, oxygen, and n i t r o g e n . The s p e c i f i c k i l n system and t h e raw m a t e r i a l s w i l l de te rm ine t h e amount o f s u l p h u r t h a t can be accepted w i t h o u t caus ing t r o u b l e . N i t r o g e n w i l l a f f e c t t h e NOx emiss ion f r o m t h e k i l n exhaus t system, b u t w i l l have no e f f e c t on t h e process i t s e l f .

3.6.2 Ash A n a l y s i s

A v e r y i m p o r t a n t c o n s i d e r a t i o n because t h e ash a f f e c t s t h e p r o d u c t a n a l y s i s , p r o d u c t per fo rmance and raw m a t e r i a l c o s t . I f a p l a n t i s b u y i n g a source o f s i l i c a , i r o n , o r a lumina, t h e i n c l u s i o n o f t h a t p a r t i c u l a r element i n t h e ash can be q u a n t i f i e d as an economic b e n e f i t ( o r p e n a l t y ) . I t i s impor- t a n t t o i n s u r e t h a t t h e pounds o f ash pe r m i l l i o n b t u ' s i s c o n s i s t e n t and t h a t t h e chemica l a n a l y s i s of t h e ash i s c o n s i s t e n t o r q u a l i t y c o n t r o l becomes i m p o s s i b l e .

3.7 P h y s i c a l P r o p e r t i e s

3.7.1 G r i n d a b i 1 i t y

U s u a l l y expressed as a Hardgrove number w i t h a l ower number r e p r e s e n t i n g a h a r d e r g r i n d i n g c o a l . Most c o a l s f a l l i n a Hardgrove g r i n d a b i l i t y range o f 40 t o 60 and w i t h i n t h i s range a r e u s u a l l y b e s t hand led b y r o l l e r - t y p e m i l l s .

I f a m i l l i s p r o p e r l y s i z e d f o r a 55 Hardgrove c o a l , i t w i l l be t o o smal l i f a 40 Hardgrove c o a l i s used. I t i s , t h e r e f o r e , w ise t o d e s i g n t h e p u l v e r i z i n g

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sytem f o r t h e l owes t Hardgrove number t h a t you may encounter.

3.7.2 Product S i ze

I f t h e coal i s t o be s t o r e d f o r any p e r i o d o f t ime, i t i s impor tan t t o m in im ize t h e amount o f d u s t ( s m a l l e r than a 100 mesh screen) o r coal p i l e f i r e s w i l l r e s u l t . As t h e su r face area o f t h e coa l t o be s t o r e d increases, so does t h e p o s s i b i l i t y o f spontaneous combustion.

The feed s i z e f o r a r o l l e r m i l l i s a f u n c t i o n o f t h e r o l l e r s i ze , and t h i s v a r i e s w i t h t h e s i z e o f t h e m i l l . I n most cases, g i v e n t h e same c a p a c i t y requirement, a r o l l e r m i l l can accept a coarser feed than a b a l l m i l l . I n some cases, a coarser feed i s demanded. I f a r o l l e r t y p e o f m i l l i s used, t h a t r e q u i r e s some p ieces o f coal up t o 2", then as a minimum, t h e r e should be a t l e a s t 40% o f t h e m a t e r i a l t h a t i s p l u s 1 / 2 " . If t h i s c r i t e r i a i s n o t met, then a b a l l m i l l system can p robab ly do a b e t t e r j o b than a r o l l e r m i l l .

3.7.3 Ash: P h y s i c a l

The i n t e r n a l c i r c u l a t i n g l o a d w i l l be p redominan t l y p y r i t e s and wear on coal m i l l i n t e r n a l s may become excessive. A lso , +50 mesh s i l i c a g r a i n s w i l l n o t r e a d i l y combine i n t h e k i l n . The s i l i c a i n t h e ash should be f i n e ( s m a l l e r than 30 micrometers) .

3 . 7 . 4 Ash S o f t e n i n g o r M e l t i n g Temperature

Coals w i t h low ash m e l t i n g temperatures (be low 2200°F) u s u a l l y cause c o a t i n g b u i l d u p s i n a smal l area so t h a t i t i s necessary t o p h y s i c a l l y remove t h e r e s u l t i n g r i n g s o r c o a t i n g b u i l d u p . m e l t i n g temper ture i s above 2500"F, " r i n g " b u i l d u p i s seldom caused by t h e coal ash.

I f t h e

A dilemma i s caused s i n c e t h e low s i l i c a ashes a re u s u a l l y low m e l t i n g p o i n t ashes, and n e a r l y always t h e h i g h m e l t i n g p o i n t ashes a re a l s o h i g h s i l i c a ashes.

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3.8 E f f e c t on C l i n k e r

4.0

The ash content , carbon, and water con ten t ranges w i d e l y i n coa ls .

Ash chemist ry , ash mineralogy, and su lphur con ten t can have a marked e f f e c t upon c l i n k e r q u a l i t y .

Some l o w su lphu r c o a l s may cause a s l i g h t r e d u c t i o n i n t h e l i m e s a t u r a t i o n f a c t o r i n raw meal t o c l i n k e r and inc rease t h e l i q u i d phase s l i g h t l y i n t h e c l i n k e r . Also, o n l y a min imal i nc rease i n SO3 con ten t f rom t h e raw meal t o t h e c l i n k e r r e s u l t s . Some of t h e h i g h su lphur coals, however, cause a g r e a t r e d u c t i o n i n t h e l i m e s a t u r a t i o n f a c t o r i n raw meal t o t h e c l i n k e r . A t t h e same t ime, t h e l i q u i d phase and t h e SO3 con ten t of t h e c l i n k e r increases. The decreased l i m e sa tu ra - t i o n o f t h e c l i n k e r w i l l cause lower i n i t i a l s t r e n g t h s o f t h e cement and an increased SO3 con ten t w i l l , i n most cases, decrease t h e s e t t i n g t ime. I t should a l so be noted t h a t t h e c h l o r i n e con- t e n t i n t h e coa l may reach a c r i t i c a l l e v e l which may make i t necessary t o operate a p rehea te r /p reca l c i ner k i 1 n system w i t h a bypass. T h i s would r e q u i r e a d d i t i o n a l c a p i t a l investment and a d d i t i o n a l f u e l consumption o f t h e system.

ECONOMICS OF COAL F I R I N G SYSTEMS (COST CONSIDERATIONS)

4.1 C a p i t a l Investment

R e f e r r i n g t o t h e d e f i n i t i o n s o f d i r e c t , s e m i d i r e c t and i n - d i r e c t coaJ systems, t h e f o l l o w i n g f i g u r e s f o r c a p i t a l i n v e s t - ment can be used as g u i d e l i n e s f o r s i n g l e p o i n t f i r i n g :

4.1.1 The t o t a l investment cos ts f o r a r o l l e r o r bowl m i l l system i n c l u d i n g b u i l d i n g s and foundat ions are 5% t o 12% lower than t h a t f o r a b a l l m i l l system.

4.1.2 The t o t a l investment c o s t s f o r a d i r e c t system a re 65%-70%; f o r a s e m i d i r e c t system 75%-80% of t h e t o t a l c o s t s f o r an i n d i r e c t system.

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4.2 Opera t i ng Costs

Opera t i ng cos ts f o r t h e d i f f e r e n t coa l systems a re d i f f e r e n t i n power consumption, assuming t h e same coal . Also, assuming s i n g l e p o i n t f i r i n g , i n o t h e r words n o t c o n s i d e r i n g a p r e c a l - c i n e r k i l n f o r t h i s purpose, t h e a d d i t i o n a l power consumption f o r an i n d i r e c t system i s 0.7 t o 1.0 kWh/st. power consumption f o r a s e m i d i r e c t system i s 0.3 t o 0.5 kWh/st. These a re a c t u a l o p e r a t i n g f i g u r e s which a re a l s o supported by the f i g u r e s pub l i shed b y Bush, K reke l and Schmidt .

The a d d i t i o n a l

These a d d i t i o n a l power c o s t s do n o t a p p l y f o r a two o r m u l t i - p o i n t f i r i n g system.

Another o p e r a t i n g c o n s i d e r a t i o n i s t h e c o s t f o r t h e coa l . I t i s g e n e r a l l y known t h a t t h e lowest p r i c e b t u i s n o t always t h e o v e r a l l economical optimum. F i g u r e 20 shows p r i c e s f o r eas te rn U.S. coals; t h e h i g h e r t h e s u l f u r con ten t t h e lower t h e p r i c e . Genera l l y , i t a l s o i m p l i e s t h a t t h e h i g h e r t h e s u l f u r con ten t t h e h i g h e r t h e ash con ten t .

The raw m a t e r i a l composi t ion has t o be c o r r e c t e d f o r t h e coa l ash, which ma o t h e r aspect cannot handle

Consequently, su I f u r r e d u c t su I f u r i ntake

add cos ts t o t h e raw meal p r e p a r a t i o n . The s, o f course, t h a t heat e f f i c i e n t k i l n systems u n l i m i t e d amounts o f s u l f u r .

i t may even be necessary t o ope ra te a bypass f o r on i n a p rehea te r o r p r e c a l c i n e r system i f t h e w i t h t h e coa l i s t o o h igh.

A p o i n t u s u a l l y n o t cons idered i n a n a l y s i s o f i n d i r e c t o r s e m i - i n d i r e c t coal systems v s . d i r e c t f i r i n g i s t h e p o t e n t i a l loss of heat va lue o f t h e v o l a t i l e m a t t e r t h rough ven t ing .

Yet another o p e r a t i n g c o n s i d e r a t i o n i s t h e average and maximum m o i s t u r e con ten t o f t h e coa l . I n a d i r e c t f i r i n g system a l l t h e h o t gases needed f o r d r y i n g t h e coa l must be used as p r i - mary a i r f o r t h e k i l n . F i g u r e 23 shows t h e p r i m a r y a i r quan- t i t i e s r e s u l t i n g f rom t h e coa l d r y i n g versus t h e coa l mois- t u re , w i t h 600°F and 480°F coal m i l l i n l e t temperatures and 250"F, 175°F and 130°F m i 11 v e n t temperatures as parameters. A l so shown i s t h e minimum a i r volume which must be passed through t h e m i l l t o p n e u m a t i c a l l y convey t h e coal . The m i l l can o n l y be operated i n c o n d i t i o n s which l i e above t h i s l i n e , and t h e importance o f m o i s t u r e i s immediate ly apparent. Approx imate ly 20% o f t h e s t o c h i o m e t r i c a i r i s minimal under any c o n d i t i o n as p r i m a r y a i r . F o r example, w i t h a m i l l i n l e t temperature of 600°F and a m i l l o u t l e t temperature o f 175°F s u f f i c i e n t heat i s s u p p l i e d t o t h e system t o d r y a feed coa l

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4.3

w i t h 14% m o i s t u r e c o n t e n t t o a r e s i d u a l m o i s t u r e c o n t e n t of 1.5%. The f i g u r e shows t h a t t h i s p r i m a r y a i r q u a n t i t y r e q u i r e s an a d d i t i o n a l hea t consumption o f 100,000 b t u / s t of c l i n k e r . These f i g u r e s a l s o demonst ra te t h e o p e r a t i o n a l d i s - advantage o f t h e d i r e c t f i r i n g system; t h e shape and l e n g t h o f f lame change w i t h coa l mo is tu re , and t h e k i l n i s opera ted acco rd ing t o t h e requ i remen t o f t h e coa l system.

I n a d d i t i o n t o t h i s o p e r a t i o n a l d isadvantage t h e a d d i t i o n a l hea t consumption i s cons ide rab le . A t a c o s t o f $2 pe r m i l l i o n b t u a t t h e bu rne r t i p t h e above g i v e n examples cause a d d i t i o n - a l f u e l c o s t s o f 20 t o 35 cen ts pe r s t o f c l i n k e r .

The n e x t f i g u r e ( F i g u r e 24) shows t h e same o p e r a t i n g parame- t e r s f o r a s e m i d i r e c t system, which can be opera ted i n t h e r e g i o n below t h e minimum a i r q u a n t i t y l i n e i n t h e p r e v i o u s f i g u r e . T h i s i s p o s s i b l e s i n c e t h e m i l l v e n t a i r i s p a r t l y r e c i r c u l a t e d and o n l y p a r t l y used as p r i m a r y a i r . The re fo re , t h e f i r i n g system can be opera ted a t t h e optimum f o r f l ame and k i l n c o n d i t i o n s , which i s 8%-12% p r i m a r y a i r . The c o a l mois- t u r e c o n t e n t w i t h which t h i s system i s ope rab le i s up t o 10%. Only above 10% f e e d m o i s t u r e o f t h e c o a l does t h e p r i m a r y a i r q u a n t i t y have t o be i nc reased t o remove enough m o i s t u r e f r o m t h e coal so t h a t i t does n o t p l u g t h e cyc lone.

I n summary: t h e d i r e c t f i r i n g system causes a d d i t i o n a l f u e l c o s t s i n comparison t o a s e m i d i r e c t o r an i n d i r e c t system opera ted w i th optimum burne r des ign and 10% p r i m a r y a i r . s e m i d i r e c t system o n l y causes a d d i t i o n a l f u e l c o s t s i f t h e c o a l m o i s t u r e exceeds 10%. These a d d i t i o n a l f u e l cos ts , which aga in a re i n l i n e w i t h t h e f i g u r e s g i v e n b y Bush, K r e k e l and Schmidt, more than o f f s e t t h e a d d i t i o n a l power c o s t s f o r t h e s e m i d i r e c t and t h e i n d i r e c t systems and a r e t o be a p p l i e d a g a i n s t t h e maintenance c o s t s and t h e h i g h e r c a p i t a l i nves tmen t c o s t s o f these systems.

The

D i r e c t F i r i n g System - F low Sheet (Refer t o F i g u r e 8)

4.3.1 E a u i m e n t L i s t

H o r i z o n t a l M i l l , 20 s t p h A i r Lock f o r H o r i z o n t a l N i l 1 Coal M i l l l P r i m a r y A i r Fan w i t h Wear R e s i s t a n t Impe l 1 e r Dust Trap f o r C l i n k e r Dust (Cyc lone Type) w i t h A i r Lock A i r Heater Complete w i t h P r i m a r y A i r Fan and O i l Pumps Double T - Tempering Damper w i t h A c t u a t o r on Hot A i r Duc t

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Manual B u t t e r f l y Damper f o r System Fan Water Spray System f o r Coal M i l l P u l v e r i z e d Coal Sampling Device E l e c t r i c Motors f o r t h e Complete System Chutes and Ducts f o r t h e Complete System, i n c l u d i n g

Expansion J o i n t s

4.4 I n d i r e c t F i r i n g System - Flow Sheet (Refer t o F i g u r e 11)

4.4.1 Equipment L i s t :

H o r i z o n t a l M i l l , 20 s tph A i r Lock f o r H o r i z o n t a l M i l l Coal M i l l Fan w i t h I n l e t Damper Cyclone w i t h R o t a r y A i r Lock and Exp los ion Vents Dust C o l l e c t o r w i t h Hopper, Screw Conveyor and

R o t a r y A i r Lock System Vent Fan w i t h I n l e t Damper B u t t e r f l y Damper w i t h A c t u a t o r f o r Recycle Gas C o n t r o l Flame A r r e s t o r Two ( 2 ) Q u i c k C l o s i n g Gates Water Spray System f o r Coal M i l l P u l v e r i z e d Coal Sampling Device B u t t e r f l y Damper w i t h Ac tua to r f o r M i l l I n l e t Gas Flow

E l e c t r i c Motors f o r Complete System Chutes and Ducts f o r Complete System, i n c l u d i n g

C o n t r o l

Expansion J o i n t s

4.4.2 O p t i o n a l : CO2 - System f o r I n e r t i z i n g M i l l ,

Exp los ion D e t e c t i o n System

4.5 When c o n s i d e r a t i o n must be g i v e n t o f i r i n g more than one p o i n t , t h e c o s t comparisons can change d r a m a t i c a l l y . The f o l l o w i n g i s an example o f equipment needed t o f i r e one 2650 s t j d a y p r e c a l c i n i n g p rehea te r k i l n r e q u i r i n g 14 t o n per hour of coa l t o t h e system.

4.5.1 D i r e c t F i r e d - Two I d e n t i c a l Systems:

Two r o l l e r m i l l s complete w i t h 25 s t feed b ins , weigh scales, a l l duc ts and dampers, fans, motors, swi t chgear i n s t r u m e n t a t i o n and bu rne r p ipes.

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4.5.2 Indirect Fired System:

One r o l l e r mill complete with 40 s t feed bin, weigh scale , a l l ducts, dampers, fans, motors, switchgear instrumentation, dust co l lec tors , 10 s t ground coal b i n , s p l i t t e r , two pumps, and a l l ground coal d i s - t r ibu t ion pipes and burners.

Indirect f i r e d system for the kiln ins ta l led - 4% lower capi ta l cost than d i r e c t f i r e d system.

The d i r e c t f i red systems may i ncrease cal cul ated fuel consumption between 60,000 btu/ton with 4% moisture coal and 180,000 btu/ton with 12% moisture coal. The two d i r e c t f i r e d mi 1 1 systems have 630 t o t a l connected horse power. The indirect f i r e d m i l l system has 810 connected hp , a difference of 180 h p or 134 kWh on 1.26 kWh/ton clinker. This favors the two d i rec t f i r e d mill system.

4.5.3

Vaporization of v o l a t i l e s - no loss on d i r e c t f i r e d mil l . Indirect f i red mill - a t 0% coal moisture and mill i n l e t temp. of 180°F may lose 50 btu/lb coal, on 1 2 % H20 may lose 600 btu/lb.

K i l n heat requirements required i f using a 12,000 b t u / l b ( H H V ) coal with 40% vola t i les .

Kiln requirements 2,850,000 btu/ton LHV = 2,950,000 btu/ton H H V

No loss of v o l a t i l e s require 245.8 l b coal / s t

50 btu/lb loss requires 246.8 lbs coa l / s t

600 btu/lb loss requires 258.8 lbs coa l / s t

cl inker

clinker

The loss of available heat due to l o s t v o l a t i l e s will be in the range of 0 t o (258.8 x 600) 155,000 btu/st of clinker.

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Summary

Two D i r e c t F i r e d M i l l s

I nd i r e c t System

40,000 b t u l o s s due Power c o s t d i f f .

20,000 b t u l o s s due

140,000 b t u loss due

t o e x t r a p r i m a r y a i r 12,500 b t u l o s s o f

v o l a t i l e s @ 4% H20

v o l a t i l e s @ 12% H20

t o 4% H20 155,000 b t u loss o f

t o 12% H20

Wi th 4% H20 t h e two d i r e c t f i r e d m i l l s may be a t a s l i g h t d isadvantage b u t a t 12% H20 t h e i n d i r e c t f i r e d m i l l may pay a p e n a l t y .

5.0 OPERATION

5.1 General

5.1.1 I t must be understood t h a t t h e d i f f e r e n c e s i n des ign and l a y o u t o f d i f f e r e n t systems make development o f one s tandard o p e r a t i n g procedure d i f f i c u l t .

5.1.2 I t i s asumed t h a t t h e i n d i r e c t f i r i n g systems w i l l u t i l i z e e i t h e r d u s t c o l l e c t o r s , cyc lones o r both.

5.1.3 A f a c t t h a t should be known t o a l l o p e r a t i n g person- n e l o f these i n d i r e c t f i r i n g systems f o l l o w s :

Three components must be p r e s e n t i n a coa l system s i m u l t a n e o u s l y f o r a f i r e o r e x p l o s i o n t o occur . They a r e combus t ib le m a t e r i a l , oxygen (12%+), and i g n i t i o n s e r v i c e .

E l i m i n a t e any one and a f i r e o r e x p l o s i o n w i l l n o t occur . Obv ious l y oxygen i s t h e e a s i e s t one t o remove and c o n t r o l e i t h e r b y use o f i n e r t process gases, o r i n t r o d u c t i o n of C02 o r o t h e r oxygen r e p l a c i n g gas.

5.1.4 Good housekeeping i n a l l areas f rom c o a l s to rage t o coa l g r i n d i n g and f i r i n g i s a must f o r success fu l f i r e p reven t ion , i n c l u d i n g no smoking. A l i g h t e d match dropped on coa l d u s t can r e s u l t i n combust ion and exp los ion .

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5.1.5 S a f e t y C o n s i d e r a t i o n

M i l l s can be heated e i t h e r b y i n e r t gases as p r o v i d e d f r o m t h e k i l n exhaust gases, b y gases from o t h e r h e a t i n g sources, o r c o o l e r o u t l e t a i r .

5.1.6 I n e r t O p e r a t i n g C o n d i t i o n s

A c o a l m i l l under i n e r t o p e r a t i n g c o n d i t i o n s i s one i n which t h e O2 c o n t e n t i n the m i l l o u t l e t gases w i l l n o t exceed 10%-12% b y volume ( d r y ) even d u r i n g s t a r t - u p o r shutdown.

A f u r t h e r requ i remen t f o r i n e r t o p e r a t i o n o f coa l g r i n d i n g system i s c o n s t a n t c o n t r o l o f t h e 02 c o n t e n t .

To assure r e l i a b l e o p e r t i o n , t h e 02 c o n t e n t shou ld be i n t h e range o f 9%-11% b y volume. Based upon p r e s e n t exper ience, such v a l u e s can o n l y be a t t a i n e d b y h e a t i n g t h e m i l l w i t h k i l n exhaust gas (02 con- t e n t l e s s t h a n 5%). When a t t a i n i n g t h e l i m i t v a l u e s (13% 02) t h e system has t o be a u t o m a t i c a l l y sw i t ched of f b y t h e "emergency s top . 'I

Ever: w i t h i n e r t o p e r a t i n g c o n d i t i o n s t h e c y c l o n e and d u s t c o l l e c t o r shou ld be equipped w i t h p r e s s u r e r e 1 i e f vents .

5.1.7 N o n i n e r t O p e r a t i n g C o n d i t i o n s

I f t h e s p e c i f i e d 02 v a l u e s (10%-12%) cannot be main- t a i n e d such a system has n o n i n e r t o p e r a t i n g c o n d i - t i o n s .

An i m p o r t a n t c o n s i d e r a t i o n f o r t h e des ign of a c o a l f i r i n g system i s e x p l o s i o n p r e v e n t i o n . I t appears more p r a c t i c a l t o p r e v e n t e x p l o s i o n s b y removing t h e source o f i g n i t i o n , and t o a l i m i t e d e x t e n t a d j u s t - i n g t h e oxygen, r a t h e r t h a n t o reduce d u s t concen- t r a t i o n i n t h e system.

P u l v e r i z e d c o a l h a n d l i n g systems a r e s t i l l s u b j e c t t o ambient a i r due t o i n f i l t r a t i o n , t h e r e f o r e , s p e c i a l p r e c a u t i o n s must be cons idered.

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5.2 W r i t t e n Procedures

5.3.1 S p e c i f i c w r i t t e n procedures shou ld be r e a d i l y a v a i l - a b l e t o a l l o p e r a t i n g personne l f o r system checkout, Marmup, s t a r t u p , o p e r a t i o n and shutdown ( i n c l u d i n g s h o r t term, l o n g term, and emergency).

5.2.2 The w r i t t e n procedures shou ld be m o d i f i e d immediate- l y when o p e r a t i o n a l changes are de termined t o be necessary.

5.2.3 There shou ld n o t be v a r i a t i o n s i n i n t e r p r e t a t i o n and a p p l i c a t i o n o f w r i t t e n procedures from one o p e r a t o r t o t h e nex t .

5.2.4 W r i t t e n procedures shou ld be rev iewed r e g u l a r l y w i t h a l l o p e r a t o r s t o p r e v e n t g radua l changes i n a c t u a l o p e r a t i n g p r a c t i c e s .

5.3 P r e p a r a t i o n f o r S t a r t u p

The f o l l o w i n g i t ems shou ld be checked:

5.3.1 I n e r t gas must be a v a i l a b l e .

5.3.2 No l e a k s shou ld e x i s t where a i r c o u l d g e t i n t o t h e system o r p u l v e r i z e d c o a l l e a k ou t .

5.3.3 Conveying dev i ces ( r o t a r y feeders ) a t t h e d i s c h a r g e o f d u s t c o l l e c t o r s o r cyc lones must be r u n n i n g and hoppers c lean.

5.3.4 There shou ld be no ev idence o f combust ion ( s m e l l ) i n c o a l s i l o s , coa l m i l l , d u s t c o l l e c t o r s , o r c o l l e c - t i o n b i n .

5.3.5 F i r e suppress ion ( C O 2 ) must be ready f o r use.

5.3.6 Feed system must be r e a d y t o f e e d coa l .

5.3.7 Feed system a i r l o c k must be o p e r a t i n g p r o p e r l y .

5.3.8 Doors o r f l a p p e r s on r e j e c t system ( i f a p p l i c a b l e ) must be c l o s e d and work ing p r o p e r l y .

p r o p e r l y f o r a i r f l o w , tempera ture , oxygen, and CO. 5.3.9 Dampers and a u t o m a t i c c o n t r o l l e r s must be work ing

5.3.10 An adequate s u p p l y o f compressed a i r f o r bag c lean - i n g ( i f used) must be a v a i l a b l e .

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5.3.11 I s o l a t i o n g a t e s on s e l e c t e d d u s t c o l l e c t o r s , o r m i l l i n l e t when i n s t a l l e d , must be open.

5.3.12 S t o p / s t a r t sequence i n t e r l o c k i n g i s i n "on" p o s i - t i o n .

5.4 S t a r t i n g Sequence

5.4.1 Checkout p rocedure s h o u l d have been s a t i s f i e d .

5.4.2 Adequa te l y warm-up m i 11 system.

5.4.3 E s t a b l i s h p rede te rm ined a i r f l o w .

5.4.4 E s t a b l i s h p rede te rm ined s t a r t - u p tempera ture .

5.4.5 E s t a b l i s h p rede te rm ined oxygen s e t p o i n t .

5.4.6 S t a r t m i l l .

5.4.7 S t a r t feed.

5.4.8 Set a l l parameters f o r au tomat i c c o n t r o l a t o p e r a t - i n g s e t p o i n t s .

5.5 Normal O p e r a t i on

5.5.1 S a f e t y i s enhanced i f a l l key parameters ( c o a l f e e d r a t e , oxygen l e v e l , system tempera ture , and a i r f l o w ) a r e c o n t r o l l e d a u t o m a t i c a l l y . Se t p o i n t s s h o u l d be de termined b y knowledge o f coa l b e i n g used and b y exper ience.

5.5.2 A l l o p e r a t i n g personne l shou ld a l s o have a thorough know1 edge o f methods f o r compl e t e manu a1 o p e r a t i o n o f t h e system i n t h e even t o f t h e f a i l u r e o f au to - m a t i c c o n t r o l s .

5.5.3 Adequate a la rms shou ld be p r o v i d e d t o a l e r t t h e o p e r a t o r when t h e s e t p o i n t s a r e e x c e s s i v e l y v i o - l ated.

5.5.4 M a i n t a i n p u l v e r i z e d c o a l s t o r a g e b i n s as near f u l l as p o s s i b l e t o maximize t h e f u e l - t o - a i r r a t i o i n t h e b i n .

5.5.5 F r e q u e n t l y v e r i f y t h a t d u s t c o l l e c t o r s o r c y c l o n e s a r e n o t b u i l d i n g up m a t e r i a l i n t h e i r hoppers.

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5.5.6 F r e q u e n t l y v e r i f y t h a t sequence i n t e r l o c k i n g has n o t been sw i t ched t o t h e " o f f " p o s i t i o n .

5.6 Normal Shutdown

5.6.1

5.6.2

5.6.3

5.6.4

5.6.5

5.6.6

5.6.7

5.6.8

5.6.9

5.6.10

Reduce m i l l e x i t t empera tu re t o below combust ion tempera tu re of c o a l b e i n g used.

Stop c o a l f eeder .

Stop m i l l a f t e r s h o r t g r i n d - o u t .

C lose h o t a i r dampers when necessary b a l a n c i n g t h e need f o r i n e r t gases a g a i n s t tempera ture .

Con t inue c o o l i n g system u s i n g a i r b l e e d as tempera- t u r e g e t s below c r i t i c a l l e v e l .

F o r s h o r t te rm shutdown, l e a v e c o a l c o l l e c t i o n b i n f u l l . Fo r l ong t e r m shutdown, b i n shou ld be t h o r o u g h l y emptied.

A l l d u s t c o l l e c t o r s and cyc lones shou ld be p h y s i c a l - l y checked t o i n s u r e t h a t t h e y a r e c lean .

The use of some CQ2 d u r i n g normal shutdown may prove advantageous t o p r e v e n t f i r e s .

Adding a sma l l amount o f raw k i l n d u s t or l i m e s t o n e t o c o a t t h e system w i t h noncombust ib les i s a good p r a c t i c e p r i o r t o m i l l shutdown.

When emergency shutdowns a r e necessary, a l l zones of t h e g r i n d i n g systems shou ld be i s o l a t e d and p e r i o d - i c a l l y i n e r t e d w i t h CQ2 i n j e c t i o n s .

5.7 C1 e a r i ng Procedures

5.7.1 P u l v e r i z e d c o a l m o i s t u r e shou ld be m a i n t a i n e d below 2% t o m i n i m i z e t h e tendency f o r p l u g g i n g i n c o l l e c - t i o n b i n s , cyc lones and d u s t c o l l e c t o r hoppers.

Compressed a i r shou ld n o t be used t o c l e a r b u i l d u p s of p u l v e r i z e d c o a l when t h e y occu r .

5.7.2

5.7.3 V i b r a t o r s used c a r e f u l l y w i t h d r y m a t e r i a l can be e f f e c t i v e .

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5.8 P u l v e r i z e d Fue l System F i r e s

5.8.1 Temperature sensors i n a l l a reas o f t h e i n d i r e c t c o a l g r i n d i n g system shou ld a u t o m a t i c a l l y cause i s o l a t i o n o f t h e a f f e c t e d area and a c t i v a t e C O 2 d i s c h a r g e when e i t h e r

e x c e s s i v e l y h i g h tempera ture i s reached o r ex t re rne ly r a p i d i n c r e a s e i n tempera ture i s de tec ted .

5.8.2 Water shou ld o n l y be used i n c o a l m i l l f i r e s w i t h c a r e as e x p l o s i v e gases can be formed, and a l s o because damage t o m i l l components c o u l d r e s u l t . (Some m i l l s have water de luge systems i n s t a l l e d t o f l o o d t h e a rea below t h e g r i n d i n g t a b l e . )

5.8.3 A system t o f l o o d t h e coa l m i l l w i t h raw k i l n d u s t o r l i m e s t o n e w i l l e x t i n g u i s h f i r e s .

5.9 Maintenance

5.9.1 Adequate r e g u l a r p r e v e n t i v e maintenance i s abso lu te - l y e s s e n t i a l t o i n s u r e s a f e r e l i a b l e performance o f a c o a l f i r i n g sytem w i t h i n des ign s p e c i f i c a t i o n s .

5.9.2 The systems must be c leaned o u t and i n s p e c t e d t h o r o u g h l y p r i o r t o maintenance.

5.9.3 The use o f we ld ing , c u t t i n g , and o t h e r hea t genera t - i n g equipment must be c a r e f u l l y mon i to red .

5.9.4 P roper housekeeping must aga in be emphasized i n connec t ion w i t h t h e above-ment io ined t o o l s as we1 1 as any o t h e r p o s s i b l e h e a t p roduc ing equipment, such as over - heated b e a r i ngs , e t c .

5.9.5 The c l e a n s i d e o f d u s t c o l l e c t o r bags i n a c o l l e c t o r shou ld be t h o r o u g h l y c l e a r e d o f p u l v e r i z e d coa l f o l l o w i n g rep lacement o f bags t h a t have h o l e s i n them.

5.9.6 R o t a r y a i r l o c k s shou ld be m a i n t a i n e d i n t o p cond i - t i o n .

5.9.7 A i r l e a k s i n t h e system shou ld be immed ia te l y r e p a i r e d .

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6.0 COAL M I L L PROBLEM CASE H I S T O R I E S

I n c o o p e r a t i o n w i t h cement p roducers t h a t u t i l i z e a coa l f i r i n g system i n t h e i r process, a summary o f problems r e s u l t i n g i n damage has been compi led. problem, remedy, and t y p e of system i s g i ven .

A b r i e f d e s c r i p t i o n o f t h e problem, t h e o r i g i n o f t h e

A s ev idenced b y t h e g u i d e l i n e s presented , c o n t i n u a l c o n s i d e r a t i o n i s b e i n g g i v e n t o s a f e t y i n t h e d e s i g n o f coa l f i r i n g systems. I n some cases a d d i t i o n a l m o n i t o r i n g o f t h e system p reven ted t h e reoccu r rence o f t h e problem encountered. I n most cases a change i n o p e r a t i n g procedures and c o o r d i n a t i o n o f maintenance a1 l e v i a t e d t h e problem.

I t shou ld be no ted t h a t n o t a l l p l a n t s w i t h c o a l f i r i n g systems have exper ienced problems. The l i s t i s i m p r e s s i v e enough, however, t h a t f u r t h e r and c o n t i n u i n g e d u c a t i o n o f personne l i n v o l v e d w i t h c o a l m i l l s i s i n d i c a t e d . The p o t e n t i a l f o r a f i r e o r e x p l o s i o n i n any p u l v e r i z i n g / f i r i n g system f o r coa l does e x i s t . I n r e v i e w i n g t h e f o l l o w i n g l i s t , t h e q u e s t i o n shou ld be asked "Does t h e p o t e n t i a l f o r t h i s p rob lem e x i s t i n my p l a n t ? " I f i t does, i t i s recommended t o a p p l y t h e remedy b e f o r e t h e p o t e n t i a l evo l ves i n t o a problem.

-49-

I tn 0 I

l y l w o f Coal sys l c l l l

3

I’rob 1 ctii

1 rr t l i I cc 1

c o o l e l * gas

Iri(lirct I

K i I i r 0 1 f t),\\es (prelicnter)

rxp los ions o c c u r r e d lw i cc i n tlic c y c l o n e , c a u s i n g tlariicigc Lo tluc t work arrtl

L x p l ( ~ s i o n o f d u s t c o l l e c l o r a i i t l tliic 1 w o r k

I i re i n c o a l m i l l

sys I CIIIS f a n

I ntl I I I?( 1

1’) dw,i I ttr P X i 1 g a s w i t h I ol i t l i I io i i i i i c j prior 10 c o a l 5 y5 l ( ’ l I 1

I h s I c o l l e c t o r bags cai tgI i1 o n f i r c i n n i c d l a l e l y a f t e r . iiii i i a l s l a r t u p

Critls I t i i r ~ r d o f f a l l bags In one I)a!Jllotrse

I

I -_I

I h r r i c t l a l l bags i n one d u s t c o l 1 cc lor

Cause

P x i i l o s i o n s o c c u r r c t l a t r c s l a r t a f l c r s y s l r i i i was s l \ u l rlowir on etwt !jciicy. P u l v c r i z c t l co(i1 l iat l f a l l c r i ~ h r o w ~ h r i o z r l c ring o f hot

t r i i 1 1 a i i t l siiioiilclci-ctl carising igr i i t l o n i ipoii r c s t a r t

I k Id incJ on d u c t work be low ( lust c o l l e c t o r w l r i l c systciii i 11 opera t i or1

G a s k e t s l e a k i ti!] on o i l t c r I r‘ciiiil) i r o n co i i ip i r Liiic!rit

LO rcB jcc tcd i i i a l e r i a l d o o r s a1 1 owPd oxygc’rl t o gc 1

The t l t rst c o l l e c l o r wcis p a s s i n g soiiie c o a l , so was i s o l a t e d . Soiiic l i i i i e e l a p s e d p r i o r l o i n s p e c t i o n when l l i c ev idence o f a s o r i o i i s f i r e was fo~r r l t l

I - a i l t i r e o f r o l i l t - y t l i sc l i a rge f e e d e r c a i i s i n y h i I t l i i p o f I t i i l v c r i z c t l c o i 1 1 aiitl a s l w 1 tlcri ng f i IT

D r r s t co l1c :c lo r i i i l c l . tlrrc.1 wa: n iodif i r t l l o c l i r i i i t i a t c p o s s i - h i 1 i t y o f c o a l aciiuirrilaI ion

lype of Coal systclll

Suspected cat ise w a s a l i ig l i coal mi 11 t i i sclta rge 1 einpe ra t 11 re

I r i d i r cc 1

Ctiarigetl npcra I i ncJ procctlut-c t o l iaiit coal i i i i l l tliscliarge teinperalirre 1 o 1/10"1 i i t t ~ x .

I'robleir

Uags I)ut-nctl i n oiie this t col 1 e c l o r

Ciitis h i r t ied o f f litany o f I-lic bags i n one dust c o l l c c t o r

Ilitrtwd rwst o f the bags in OIiC col 1 ec tor

c a II s c

Cxccss i ve tcinlwi,ii 1rit.c rlur I rig s l i u t down wl i e 11 o x y CJ en 1) ec aiiie ava1lal) le

f i l l f i r e s occiireed wi th in the f i r s t s i x siontlis o f I n f l l a l sta i - t i t i ) . llicrc I iavc I x e i i rio f i r e s in near ly l i v e riioiillicr. 111(! c o r r e c l l v c ac t ions 1 l s t e t l liave coiiil)Inetl t o s o l v e the f i r e probleiiis, r io t r icccssar i ly tletl Lo lltc sl )cc i Cic

Scini-direct

Coolei- gas

Coal f i r e i n pulvcr tzcd coal h l n on air i n d i r e c t coal f i r i n g system.

T i t.e arid exp los ion i n tlic n i i 11 atid systeiii f a i l

1'ir.e i t i klie coal iiiill rlust col 1 ( !C l o r

I

lligli iiioisturc c o a l requi red t i I glr III 1 1 1 i r i 1 el. I caipcra trrrcs w i l l 1 h i g h v o l a l i l e s caus ing 1)rob I(1111S i I I sys I Clll t lus 1 col 1 ec t o r

Sys teiii w a s vciiLetl 1 Iiroitgli sepa ra t e dust col l e t t o r , w h i c h i s iiow a l ) i l i ~ d o ~ ~ c t l

Zero sl)eed i i i t l i (;a tcd oil

col 1ect . ing screw arid l eve l i n d i c a t o r i r i dus! c o l l e c t o r hopper

I Ln N

I

P roll 1 eiii

Coa 1 s l a g l o burncr caused coiiil)ust.lbles arid e x p l o s i o n a t p o i n t o f bleed- i t i a i r o f k i l i i cxliausl gases

C o c i I s l a g to biirricr as a l low

tire 111 tlie IuleL d u e l t o the coal n i l 1 1

Two i n c i t l e n t s o f f i r e and one incident o f f i r e arid cxl)losiot~ i n the dust col 1 ec lors on a sciiii - ( I i rec 1 coa 1 f 1 r i n g sys taii

~~

Carisc

Dlockage arid r e s t a r t o f coal cycloiie roIar-y a i r - lock

I l r i d g tn(J o f coal al,ovC! rolat-y a i r' 1 o c k __ ~~ ~ ~

Coal b r r i l t l u l i iii llic i r i l c l t l u c l lo lhe Ill111 carisctl by too l o w o f a i i i i l l orrllcl gas ten,pera turc ant i incollsistcnt c o a l l l r l n l i t y ( '1; 1I2O & D T U ) _ _ ~ ~

Came 0 1 tlie f irt. was r i o l d e f i r i 1 l c l y rle tcr 1111 nctl h i 1 was rriost 1 ikely tlur l o spontaneous l q i i i I i o n nf (cia1

itr the (lust. c o l l c ~ c t o t - llopJ)eI o r l n a f l a l spnl i n the

col lector ifl lel d I J C 1 10 the dIJSt

Rciiictly

I r i s t a l l a t l o n o f I)C drive ori a l r lock ar id ~ w t l i f i c d t t o i r o f s tarli tig scqrictire

c

' l d t l l Ty lw ol Coal Sys leiii

1 3 S c w i - i i i t l i rcc 1

Coo? 0)' qdSCS

A l l i i c k f i l l . c t - cake o f coal accuiiirila 1.ed O H 1 . 1 1 ~ t l r i s 1. c:1,1 lcctor- Iwgs W l l C l l I ll1e pulse a i r c:ouq)re%sor I i i i l c t l . l l i c pulse a i r coiiiprcssor was t ~ s t.arI.ctJ arid l l i c accitrwi- laletl coal was ~ ) a s s e t l on l o l l ic k i 1 ti himr L l ~ r o w ~ l r ttie t-o tary a i r lock on the tlus I: co l l ec tor lioppcr. N o t a l l o r the c t ~ l i ! j t i i k c t l i n l h biir i i i i ig zoiic arid a stxontlary explosion occtit.t-c?tl in llic prelim lor.

I'rob 1 ell1

I xolos ions occiirrwl i n the p t elie<i I ei- arid exhaus 1 tliic 1 w o i k .

I Ul w I

I I slioul r l IJU no tccl l l ia t a 1 1 27 non-(1 i rec 1 coa 1 sys leriis i n opci-d t ion a 1 I tic I iiw o f 1Ii i s s lirtly were c o ~ s i (let 4, 1 , 1 1 1 o l l l y the a l ~ o v e oiillincd pr~ol)lc~iris wore rqorletl in 1 3 of I tic 27 syslrii is.

A P P E N D I X A

TRCINSPORT CONVEYORS

BARGE UNLOADINO t t ,

4-

STORCIQE B L E N D I NQ b

F I G U R E 1

M -7%

UNLOADING 8 RELOADING FACILITY

Y h Q ACI I L A O A D

E TRUCK L O A O O U T

B Q

RECLAl M ING

FIOURE 2

COMBINATION STACKER/RECLRIMER

U I NOROY CHEVRON

FIOURE 4

WINDROW AND CHEVRON PILES

O U T L E T n I R 0 COFIL D U S T

& COhL HOT n

I I I I - 1 I

F I G U R E 5

A I R SWEPT BALL M I L L

i

I

I I

FIGURE 6

COFlL R O L L E R MILL

BhLL R I N Q H I L L S

SPRINQ FORCE H I L L S

B

SPRINQ ROLLER H I L L S \ c

CENTRIFUOhL FORCE MILLS

CENTRIFUQhL BOLL M I L L I SUSPENDED ROLLER H I L L S

FILIURE 7

c

RING ROLL MILL

PAN GRINDER

RING MILLS ACCORDING T O DIN 24101

R A Y COAL FROM STORAGE -1 HIGH SENSOR LEVEL

LOU LEVEL 4 SLIDE SENSOR

SLIDE GATE-

EHERGENCY 4 CHUTE

ROTARY AIR LOCK

TO FIRING

DAMPER PO I NT

MILL SYSTEH FAN

HOT AIR U I v u r # 1 ROLLER HILL BLEED AIR OAUPER

FIGURE 8

DIRECT F I R E D SYSTEM

R A Y COAL FROM STORAGE

f 4 I G l i LEVEL SENSOR

I COAL

LOU LEVEL SENSOR

SLIDE GATE- N SL I DE GATE 7- T EMERGENCY --/

CHUTE J r l l

YElQH FEEDER

ROTARY RIR

r ROLLER M I L L I

H I L L SYSTEM GOS FLOW

I -

E

AN SENSOR

DAMPER

R O T A R Y I FEEDERS

TO F I R I N G POINT DfWPEA

PA I MARY f i I R FClN

?k-- DAMPERS

HOT A I R J

@ -

FIGURE 9

SEMI-DIRECT FIRED SYSTEM

RQY H I L L FROH STORQQE

RECIRCULQTINQ

I l lGH LEVEL SENSOR

COIIL

LOU LEVEL SENSOR

SLIDE QQTE- N I - S L I D E GfiTE EHEROENCYJ T

CHUTE

YE I 0

CHUTE

YEIQH FEEDER

ROTtlRY Q l R LOCK 't lRY Q l R LOCK

I ROLLER H I L L

E

H I L L SYSTEM

CYCLONE

- 1

TO F I R I N Q - POiNT PunP

EXHQUST

DQMPER

TO QTHOSPHERE

*-- DRHPERS

HOT QQSES J b z

FIGURE 10

SEMI-INOIRECT F I R E D S Y S T E M

R A Y H I L L FROH STOROQE

H I Q H LEVEL SENSOR

- S L I D E QC)T

YE I OH FEEDER

ROTARY A I R LOCK

ROLLER H I L L I

RECIRCULfiTINO L l N E

c

DUST COLLECTOR

/

CYCLONE

EXHAUST

DAMPER

TO FIRINQ - POINT PUMP & POINTS

DAMPERS ?

FIOURE I 1

INDIRECT FIRED S Y S T E M I

!4 - FILARtl C - CONTROL D - DIFFEAENTIOL F - FLOY/FEED H - HANO I - INDICATE 0 - OXYGEN P - PRESSURE Q - QUANTITY R - RECORO T - TEHPERATURE Y - WEIGHT

c

TO FIRIN6

I POINT

MIL1 SYSTEH FAN

FIGURE 12

SIMPLIFIEO C O N T R O L D I F I G R Q M EIRECT FIRED SYSTEM

A - ALARM C - CONTROL D - DIFFERENTIAL F - FLOY/FEED H - HAND I - INDICATE 0 - OXYGEN P - PRESSURE O - OUCINTITY R - RECORD T - TEHPERfiTURE Y - YEIGHT

R O L L E R H I L L t

FIOURE 13

SIMPLIFIED CONTROL DIAGRAM SEMI-DIRECT F I R E D SYSTEM

A - c - 0 - F - H - I - 0 - P - 0 - R - T - w -

ALARM CONTROL OIFFERENTIFIL FLOW/FEED HFIND I ND 1 CATE OXYGEN PRESSURE OUANTITY RECORD TEMPEAATURE WE I GHT

I ROLLER M I L L I

TO F I R I N G __f POINT

1 % I I I PUHP

. I

FIGURE 1 4

SIMPLIFIED CONTROL D I A G R A M SEMI-INDIRECT F I R E D S Y S T E M

TO FITMOSPHERE

I -___

Q - c - D - F - H - 1 - 0 - P - Q - R - 1 - v -

fiLnRn CONTROL DIFFERENTIf iL FLOW/FEED tlflND I N 0 1 CfiTE OXYGEN PRESSURE QUfiNTITY RECORD TEnPERfiTURE WE I GI11 r

ROLLER n i L L I

i

’! FIGURE 15

S I M P L I F I E D CONTROL D I A G R A M I N D I R E C T F I R E D SYSTEM

- TO COAL HILt

TO KILN

TO PRECQLCINER I

%L---------- -J

FIGURE 16

PULVERIZED C O A L FEEDING SYSTEM

i

10 I 0.1 1 10 100

F I G U R E i a

PRESSURE R E L I E F V A L V E S

fl - I R U P T U R E C I R P H R R G M

1-1 RETAINING C L I P I

0 INLET

GaS FLOW

aUTLET G A S FLOW

F I G U R E 19

PRESSURE RELIEF

I

0 LL 2ol 10

SULFUR CONTENT O F COQL I N X

FIGURE 20

RELATIVE C O S T OF COALS WITH INCREASING SULFUR CONTENT

TENNESSEE - KENTUCKY COALS - TRUCK DELIVERY

4 Suppl iers

Jan. Feb. Mar. Apr. May June Ju ly Aug .

. Sept. Oct . lev.

Dec.

Total

The following t a b l e represents monthly averages (numerical) of a l l coal received and burned in an operat ing cement p lan t . All samples were a i r dr ied.

Tons r e c ' d incoming t w c k Coal feeder Coal Surner Column B & C A a C Di f f

SamD 1 es samples Pipe Samples STU /1 b BTU/ 1 b BTU/lb 8TU/lb

a86 1 71 35 3093

15958 7134 6588 7709

10364 13243 10016 10031 8527

11990 12100 12000 12030 1 1500 12400 12500 12600 12348 11863 11 903 12102

10374 11461 11 381 1 1482 10844 12428 12060 12375 11 767 11320 12147 11932

10068 10698 lGOlO 11 368 11318 11321 11994 12031 11296 11097 11 554 11347

-306 -770 -1 371 -114

(t47U) -1 107 -66 -344 -471 -223 -593 -585 -

10766 i t o n s 12086 11 631 11175 -356 BTU/lb

Column A:

8TiJ determination made; feed b e l t , sample a i r d r ied and STU determination made; Samples taken each s h i f t from burner pipe BTU d e t e m i n a t i o n made on d a i l y composite. months r e s u l t s .

Samples taken f r a n each t ruck , 5 t rucks canposited and the Column 6: One sample d a i l y f r a n coal mi l l

Column C:

The numbers shown a r e numerical averages of the

Figure 21

u a I- 2

0 L

3 w 0

LL -

- - -

so

40

30

20

10

40

30

20

10

DEW POINT

, I I 1 1 1 1 1 t 1 1 1 1 1 1 J

1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 1 5

UOISTURE CONTENT X

FIGURE 22

D R Y I N G WITH PREHEATER GAS (THE flILL OUTLET IS 21000 A C F f l )

a a

a a

a

I

>

S

0.

w 0

+ 2 W 0

w Q

I

a

a

so

40

30

20

10

0

U I L L OUTLET

T E H P E R f i T U R E

f l INIf lUH A I R QUaNT I TY

-0 f l I L L INLET T E M P E R A T U R E - 60Q°F -- 4 8 O o F

I I 10 20 50

H O I S T U R E CONTENT, %

FIGURE 2 3

THE EFFECT OF OISTURE CONTENT UPON THE PRIMARY A I R AN0 KILN HEAT CONSUMPTION

I N A DIRECT FIRING SYSTEM

2 0 I- \ 3 c m n 0 c

z 0

c L.

a S 3 (I) z .O u I-

u x 4

2 0

+ 0

a

a

L1

I

a a

a I

a > U

2:

Q

w D a c 2 w u U W e

a

a .I

I

20

0

- - - - - - - -, n I u

OUTLET TEMPERATURE I

HOT GAS TEMPERATURE - 6OO0F -- 48OoF

THE

'0 10 20

fl.0 I STURE CONTENT 8 X

FIGURE 24

EFFECT OF M O I S T U R E C O N T E N T U P O N

z 0 zoo c \ 3 c m

n 0 -

I I- a W I

I

c 0 0 U

L1

THE PRIMARY A I R AND KILN H E A T CONSUMPTION

IN A SEMI-DIRECT F I R I N G SYSTEM

u u3 W w P W 0

2

w 3 t (r:

W L S w I-

a

L)

a

a

1000

800

600

400

200

01

DUST ClOUOS IN A I R

OUST LAYERS IN aIR

1 I I 1

20 40 60 80

VOLATILE UATTER, X

FIGURE 25

1600

1200

000

400

3

LL

2

W

I

a

a ? U

W 0 x w c

IGNITION TEMPERATURE VS. VOLATILE CONTENT

150

BITUMINOUS CUCILS

I

100 c LIGNITE

50 - -ANTHRAC I TE

LOU VOLATILE VOLATILE

BITUtlINOUS BITUtlINOUS

0 0 10 20 30 41

3: VOLRTILES

FIGURE 26

THE EFFECT OF V O L A T I L I T Y UPON THE MFIXIMUM M I L L OUTLET TEMPERATURE

u u) W W oz 0 W 0

z

W

3 t a W (L S W I-

u

a

a

1100

1000

900

800

700

600

500

400

I

KEY

0 1 5 p 0 22 pn t, 34 p n + -200 meah % -120 neah

THEORY --I.

' I ;le 1

:i I \* ' I

I

FLAflflASLE AND THERflALLY IGNITABLE

FLAflflABLE BUT \ ! THERflALLY NONIGNITASLE NON-

FLAflflClBLE I

I I I I I 50 100 150 200 250 300

CONCENTRATION, m-'

F I G U R E 27

THERMAL IGNITION DATA FOR POCAHONTAS COAL DUST OF VARYING PARTICLE SIZE (VOLATILE MATTER = 1 6 % )

1100

1000

900

800

700

600

500

400 0

1100

1000

0 900 v) W W

(3 a

800

I

W 5 700 I- U a W Q 5 600 c

CI

500

4000

FLAfltlCIBLE FINO THERflCILLY 16NITCIBLE

A A

I I

I I I

n

\

1 I! I I I I I

50 100 1 so 200 250 300

FLCItltlFIBLE BUT I

THERHFILLY NONIBNITFIBLE !

NON- FLFItlflABLE

FIGURE 28

T H E R M A L I G N I T I O N D A T A F O R P I T T S B U R G H C O A L DUST OF V A R Y I N G P A R T I C L E S I Z E ( V O L A T I L E M A T T E R X 3 6 % )

1100

1000

900

800

7 0 0

600

500

400 0

FLAMMABLE

NONFLAMMABLE

300

2 5 0

200

150

100

55

I I I I I I I 1 1 I I 1 I 1 I I l l 2 4 6 8 1 0 25 40 60 85 100 200

MEAN PARTICLE DIAMETER, pm

F I G U R E 2 9

P A R T I C L E S I Z E DEPENDENCE

A P P E N D I X B

T r a n s l a t i o n o f : German E n g i n e e r i n g S o c i e t y G u i d e l i n e VDI3673 Dated : June 1979 Regard ing : Pressu re r e l e a s e o f d u s t e x p l o s i o n s

T h i s G u i d e l i n e was p u b l i s h e d i n t h e Federa l R e g i s t e r f o r p u b l i c comment.

Tab le o f Conten ts Page

P r e l i m i n a r y Remarks

1. D e f i n i t i o n o f Terms

2. E x p l o s i o n C y c l e i n Vesse ls and Ducts i n c l u d i n g Vessels Connected b y P ipes

8-2

6- 4

B-10

3. P ressu re Release o f Equipment B - 1 1

4. P ressu re Release o f Rooms B - 1 1

5. Types and Maintenance o f P ressu re Release Dev ices

5 .1 Rup tu re D i s c Dev ices

B - 1 2

8-12

5.2 E x p l o s i o n Va lves and E x p l o s i o n D i s c s 8-14

5.3 S p r i n g Loaded Release Dev ices B- 14

6. Des ign o f P ressu re Re lease Openings B-15

7. Safe D ischarge o f t h e P ressu re Wave, Flame and Exhaust Gases B- 18

7.1 Open A i r P l a n t s 6- 15

7.2 P l a n t s i n Closed Areas B- 18

7.3 E f f e c t o f B l o w - o f f P ipes on t h e Reduced E x p l o s i o n P ressu re B-18

7.4 Des ign o f B l o w - o f f P ipes B-19

8. P ressu re Re lease o f E longa ted Vesse ls B- 19

9. P ressu re Release o f P i p i n g S e c t i o n s

10. P ressu re Release o f Vesse ls Connected b y P i p i n g

11. L i m i t s o f A p p l i c a b i l i t y o f P ressu re Release

12. D e t e r m i n a t i o n and A p p l i c a t i o n o f E x p l o s i o n S izes f o r Combust ib le Dusts

12.1 T e s t Process

12.2 C l a s s i f i c a t i o n o f Dus ts

B-20

B-21

B-21

B-23

6-23

B-24

German B i b l i o g r a p h y B-26

F i g u r e s & Graphs B-1

Preliminary Remarks

The German Engineering Association Commission for Maintaining Air Purity, specialists from scientific, economic, and administrative areas developed self monitoring guidelines which are applied largely in the legal realm as the basis for laws, regulations, and administrative guidelines in the area of maintaining air quality. As recognized technical rules, these guidelines provide information regarding the the status of science and technology in the various scientific realms which effect air quality. The guidelines are summarized in the manual "Maintaining Pure Air"; they provide information regarding:

The status of technical knowledge for processes and equipment to limit emissions, as well as emission values, for the discharge o f dusts and gas.

Processes and equipment for gas purification and dust separation, with speci a1 characteristic data and instructions for cost calculation an3 dust handling technology.

Dispersal processes in the atmosphere, particularly for computing tile relationships between emission and immission.

The effect of air contaminants on people, plants, animals and property, and the recommendation of maximal emission values.

Measurement procedures t o determine gaseous and dusty contaminants in emission and immission, cri%eria for their selection and the evaluation of the measured rssults.

The Guidelines are published in a preliminary draft form which i s then subjected, upon notification, in the Federal Register and in the technical literature, to public hearings. This assures that the often varying opinions of the various parties involved can be taken into account before final formulation (white format).

The above Guideline describes one o f the possible measures to reduce the effects of dust explosions, and provides instructions for the selection and sizing of pressure release mechanisms. guideline for the selection and sizing of such equipment is generally applicable. Insofar as additional safety requirements are specified in specialized literature for particular applications, they also have to be considered in planning. measure for equipment in which the dusts are moved or stored, as well as for equipment designed for air purification. guideline are powdery substances, which when mixed with air can form explosive compounds, i.e. powder and fine meals. Explosive materials as defined by the Explosive Law are subject to special regulations.

The method specified in this

The pressure release applies also as a safety

Dusts in the context of this

9-2

An exp los ion pressure re lease as pe r G u i d e l i n e V D I 3673 i s n o t t o be designed if, thereby, m a t e r i a l s o r compounds a re re leased which a re poisonous o r c a u s t i c i n t h e framework of Sec t i on 1, Paragraph 1 of t h e ASV ( 3 2 ) .

I f a pressure r e l e a s e i s ac t i va ted , i t can have damaging environmental consequences. r e g a r d i n g t h e l e g a l p r i o r i t y o f exp los ion p r o t e c t i o n versus emission p r o t e c t i o n . Th is g u i d e l i n e i s in tended t o supplement t h e s t a t e and a s s o c i a t i o n g u i d e l i n e s by t e c h n i c a l d e t a i l s .

It i s n o t t h e task o f t h i s q u i d e l i n e t o deal w i t h ma t te rs

T h i s G u i d e l i n e i s intended t o prov ide, t o t h e engineer concerned w i t h quest ions o f pressure re lease o f t h e equipment i n h i s p l a n t , t h e i n f o r m a t i o n r e q u i r e d t o handle such tasks. c o n d i t i o n s , n o t every case which a r i s e s i n t h e i n d u s t r i a l rea lm can be d e a l t w i t h . d e t a i l e d d iscuss ions, h e r e i n presented, a s u i t a b l e design s o l u t i o n f o r h i s p a r t i c u l a r task.

Given t h e m u l t i p l i c i t y o f o p e r a t i n g

Nevertheless, t h e p l a n n i n g engineer should be able t o f i n d among t h e

I n o rde r t o preserve a i r p u r i t y , and f o r genera l reasons o f p r o d u c t i o n e f f i c i e n c y , one neve r the less should s t r i v e , d e s p i t e t h e i n c l u s i o n o f pressure r e l e a s e devices, t o reduce i g n i t i o n p o i n t s and the reby l i m i t one o f t h e f a c t o r s gener a t i ng explos ions.

B- 3

Designation

Symbol U n i t

R

E xp 1 an a t i on

esponse Pressure

dynamic response pressure)

Nominal response pressure = s t a t i c venting pressure

P Cbarl s t a t

Pressure which, d u r i n g an explosion actually impacts on the pressure release device as i t i s activated. As a rule i t i s higher t h a n the s t a t i c response pressure (nominal response pressure) and depends upon the rate of pressure increase, the diameter of the release opening, the fastening of t o u g h membrane materials or, i n explosion valves, on their mass moment of iner t ia . Since the nomographs Figures 7a t h r o u g h 8c are based on experimental resul ts , the effect of the dynamic response pressure has already been incorporated i n them.

The average pressure a t which a ruptured disc or an explosion valve activates ( i . e . gas begins t o flow o u t of the vessel t h r o u g h the pressure release mechanism) i f the resulting rate of change i n pressure does not exceed 0.5 bar per second. In explosive rupture discs, i t i s the pressure a t w h i c h i g n i t i o n of the explosive mixture i s supposed t o occur.

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Designat ion

Symbo 1 U n i t

Type o f vessel

Cubic vessels

Elongated vessels

Rupture s a f e t y dev ices

Pressure i nc rease v e l o c i t y

Maximum p ressu re increase over t i m e equals maximum r a t e o f pressure r i s e

[-] max Cbar / s l

Pressure increase over t i m e equals r a t e o f pressure r i s e

[:] Cbar /s l

Expl anat on

Vessels i n which t h e r a t i o of l e n g t h t o diameter i s equal t o o r l e s s than 5:l.

Vessels i n which t h e r a t i o of l e n g t h t o diameter i s g r e a t e r than 5: l .

Devices c o n s i s t i n g o f a r u p t u r e element such as r u p t u r e d i s c s as w e l l as t h e incorpporated fas ten ings . The opening c l e a r e d by t h e d i s c remains open u n t i l a new r u p t u r e d i s c i s i n s t a l l e d .

The maximal va lue f o r t h e - p r e s s u r e i nc rease over t i m e i n t h e exp los ion o f a s p e c i f i e d dus t i n a c losed vessel a t op t ima l concen t ra t i on , F i g u r e 1.

I n c l i n e of t h e tangent a t t h e t u r n i n g p o i n t o f t h e r i s i n g l i n e of t h e pressure t i m e curve o f a s p e c i f i e d dust i n a c losed vessel a t a g i ven concen t ra t i on , F i g u r e 2.

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1. D e f i n i t i o n of Terms

Designat ion

Syrnbo 1 U n i t

Ven t ing

Pressure r e s i s t a n t vessels

Exp los ion r e s i s t a n t vessels

Vent ing pressure = reduced exp los ion pressure

P CbarJ r e d

Exp lana t ion

P r o t e c t i v e concept which l i m i t s t he exp los ion pressure by v e n t i n q unburned m i x t u r e and combustion gases through p r e s e t openings so t h a t t h e equipment i s no t damaged.

Vessels and equi pment b u i l t i n accordance w i t h t h e a p p l i c a b l e r e g u l a t i o n s and g u i d e l i n e s as pressure vessels.

Vessels, equipment and r e l a t e d p i p i n g designed t o w i ths tand t h e pressure wave a r i s i n g due t o an explos ion, up t o a s p e c i f i e d l e v e l w i t h o u t d i s r u p t i o n . However , any t ype of deformat ion can a r i se .

The c a l c u l a t i o n of exp los ion p r o o f vessels i s based upon t h e so c a l l e d exp los ion p r o o f r e s i s t a n c e formti la.

Pressure t o be expected, i n case o f an exp los ion , i n a vented room o r a vented u n i t .

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1. D e f i n i t i o n of Terms

Des igna t ion

Symbo 1 U n i t

Ven t ing area

Absolute v e n t i n g area

FCm21

S p e c i f i c v e n t i n g area ( v e n t r a t i o )

Maximum e x p l o s i o n pressure

p Cbar l max

E xp 1 0s i on c h a r act e r i s t i c s

Exp los ion va l ves

Exp lana t ion

The t o t a l e f f e c t i v e v e n t i n g area i n c l u d e d i n a vessel , which cou ld c o n s i s t o f numerous p a r t i a l surfaces. Non-ruptur ing vacuum supports and o t h e r components which impede t h e m a t e r i a l f l o w have t o be considered i n t h i s con tex t .

Q u o t i e n t formed between t h e abso lu te v e n t i n g su r face FCm21 and t h e vessel volume VCm31.

The h i g h e s t pressure va lue a r i s i n g i n a c losed vessel d u r i n g t h e exp los ion o f a d u s t / a i r m ix o f op t ima l concen t ra t i on , F i g u r e 1.

P Maximum exp los ion pressure max and maximum pressure increase over t i m e (dp/dt Imax.

S a f e t y devices opened by exp los ion pressure. I n c o n t r a s t t o r u p t u r e d i scs , exp los ion va lves can r e s e a l t h e v e n t i n g openings a f t e r be ing actuated .


Designation

Symbo 1 U n i t

Hartmann tube

K -value s t

[bar x m.s-lI

[bar x m x s - l l

Cubic law

1 [z] max v3 [z- -

= const. = K S t

1 3 x v

max

Exp l anat ion

Closed t e s t device of approximately 1 . 2 l i t e r content t o determine the explosion charac te r i s t ics of dust (see Section 12).

A specif ic process charac te r i s t ic value of d u s t t es t ing calculated from the cubic law. I n numerical terms i t equals the value o f the maximum p r ssure increase over time

12) . The K

p a r t i c l e s i z e , the p a r t i c l e s ize dis t r ibut ion and the surface s t ructure of the d u s t , the turbulence of the dust /a i r m i x , the ignit ion source, and other factors such as the shape of the vessel.

in the 1 m 5 vessel (see Section

value depends upon the s t

The volumetric dependency of the maximum pressure increase velocity. Due t o the relationship between vo 1 ume V and ( d p / d t Imax i nf ormat i on f o r the maximum pressure increase velocity w i t h o u t simultaneous indication o f the volume i s n o t adequate f o r an analysis of the explosion factors .

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1. Definition of Terms

Desi gnat i on

Symbo 1 Unit

Dust explosion classes

Initi a1 pressure

p Cbarl V

Explanation

Defined areas, which are limited by specified Kstvalues (Table 1).

Initial pressure prevailing when the ignition source commences.

I

B-9

2. Explosion cycle in vessels and ducts including vessels connected by pipes.

The s t a t e of knowledge r e g a r d i n g t h e spread o f f lames and o f p ressu re i nc rease d u r i n g exp los ions i s based on exper iments performed i n i d e a l i z e d systems and t h e r e s u l t i n g analyses. There a re two r e l a t i v e l y s imple l i m i t i n g cases r e g a r d i n g t h e spread of t h e f lames:

Flame p a t t e r n s i n cub ic vessels ,

Flame p a t t e r n s i n p i p i n g .

F o r s p h e r i c a l f l ame p a t t e r n s , i n which d u s t / a i r mixes a re always t u r b u l e n t i n c o n t r a s t t o g a s / a i r mixes, t h e f lame v e l o c i t y remains small i n c o n t r a s t t o t h e sound v e l o c i t y so t h a t no l o c a l i z e d p ressu re va r iances a r i s e i n t h e c losed vesse l . The f i n a l p ressu re can a t t a i n 8 - 12 t imes t h e i n i t i a l p ressu re w i t h these va lues even be ing exceeded i n t h e case o f c e r t a i n dusts.

I n p i p i n g , t h e f lame spreads more r a p i d l y as t h e p i p e l e n g t h increases. w i t h some dusts, p a r t i c u l a r l y dus ts w i t h an average o f h i g h K S t va lue de tona t ions can occur a f t e r a s t a r t up c y c l e d u r i n g which t h e f lame f r o n t advances a t u l t r a s o n i c speed. The p ressu re on t h e p i p e w a l l can the reby a t t a i n l o c a l and s h o r t term pressures approx ima te l y 30 t imes t h e i n i t i a l pressure. A t p i p e c e i l i n g f l anges and elbows, even h i g h e r p ressu re peaks can a r i se , s i n c e a t these p o i n t s e x p l o s i v e m i x t u r e s have been depos i ted b e f o r e t h e a r r i v a l o f t he f lame f r o n t .

The equipment used i n i n d u s t r y i s o f t e n a combinat ion o f vesse ls and p i p i n g . Examples are:

S i l o s , d r y i n g and g r i n d i n g systems w i t h downstream d u s t separa to rs

S u c t i o n equipment w i t h connected r e c u p e r a t i o n systems f o r dus ts

Combinations of storage, b lending, and f i l l i n g vesse ls w i t h p i p i n g .

A d u s t e x p l o s i o n which advances i n such systems, f rom one vessel t o another, can be more s e r i o u s and generate g r e a t e r pressures than a d u s t e x p l o s i o n i n a s i n g l e vessel (see S e c t i o n 10).

B-10

3. Pressure r e l e a s e of equipment

By p r o t e c t i v e vent ing, t h e development of an irnpermissably h i g h pressure due t o an i n t e r n a l dust exp los ion , i s t o be avoided by t h e t i m e l y r e l e a s e o f s p e c i f i e d openings.

The exp los ion pressure i s t he reby r e s t r i c t e d t o a va lue beneath t h e r e s i s t a n c e o f t h e equipment, b y r e l e a s i n g unburned mixes and combustion gases i n t o t h e open. i t s e l f , b u t o n l y i t s dangerous consequences. deal w i t h t h e subsequent f i r e s . us ing pressure r e l e a s e dev ices. (e.g. r u p t u r e d i s c s ) as w e l l as f o r repeated use (e.g. exp los ion va l ves ) .

The pressure r e l e a s e does n o t impede t h e exp los ion I n a d d i t i o n , one has t o

The p ressu re r e l e a s e i s brought about These cou ld be designed f o r s i n g l e use

The use o f pressure r e l e a s e dev ices assumes t h a t t h e equipment i s s i z e d f o r a s p e c i f i e d v e n t i n g pressure (reduced exp los ion pressure Pred). I n so doing, a l l t h e p a r t s conce ivab ly s u b j e c t t o t h e exp los ion p ressu re i n t h e equipment such as f i t t i n g s , f u e l view glasses, access and c leanout openings, as w e l l as p i p e connect ions e t c . a re t o be i nc luded i n t h e c a l c u l a t i o n s r e g a r d i n g pressure r e s i s t a n c e .

I f t h e pressure r e l e a s e i s n o t d i r e c t , b u t t h rough a d ischarge p i p e i n t o t h e open, then t h e reduced exp los ion pressure (pre s u b s t . a n t i a l l y increased i n t h e vesse.1 t o be p r o t e c ed so t h a t h i g h e r requirements have t o be s e t f o r t h e pressure r e s i s t a n c e o f t h e equipment. F u r t h e r d e t a i l s a re d iscussed i n Sect ion 7.3.

9 ) can be

Determin ing t h e permissable l oad b e a r i n g c a p a c i t y o f t h e equipment, g i ven i n t e r n a l pressure b u i l d up as w e l l as i t s pressure r e s i s t a n c e , has t o be done i n accordance w i t h t h e e x i s t i n g r e g u l a t i o n s . Analyses r e g a r d i n g t h e exp los ion r e s i s t a n c e o f vessels are presented i n (30 ) . I n case o f doubt, t h e r e s p o n s i b l e t e s t i n g assoc ia t i ons decide (e.g. t h e TUV 1.

4. Pressure r e l e a s e of rooms

The goal i s t o app ly t h e p ressu re r e l e a s e measures p r i m a r i l y f o r equipment. i n such rooms has t o be remote c o n t r o l l e d and any access i n t o t h e room d u r i n g o p e r a t i o n of t h e system has t o be excluded. The pressure re lease , i n t h i s case, serves o n l y t o p r o t e c t t h e remainder o f t h e b u i l d i n g . It can t a k e p lace, f o r example, us ing windows, e x t e r i o r w a l l s o r through t h e r o o f o f t h e b u i l d i n g (see a l s o Sect ion 4.6.2 i n t h e V D I Gu ide l i ne 2263. The no rma l l y low s t r e n g t h p a r t s o f t h e b u i l d i n g must be

Rooms, as w e l l , can be p r o t e c t e d by ven t ing . The equipment

B-I 1

t a k e n i n t o account . W i t h p r e s s u r e r e l e a s e d e v i c e s i n s t a l l e d i n t h e s i d e s o f rooms, f i r m r a i l i n g s must be p r e s e n t i n s i d e t h e rooms i n accordance w i t h t h e e x i s t i n g b u i l d i n g r e g u l a t i o n s t o p r o t e c t peop le f r o m f a l l i n g o u t .

Peop le and e s s e n t i a l p r o p e r t y must n o t be endangered.

N e i t h e r window g l a s s , n o r asbes tos cement, o r s i m i l a r m a t e r i a l s shou ld b e used f o r p r e s s u r e r e l e a s e areas of rooms because o f t h e i r s p l i n t e r i n g e f f e c t . M a t e r i a l s wh ich do n o t genera te l a r g e sharp f ragmen ts shou ld be g i v e n pre ference; if used on t h e ground f l o o r t h e shot e f f e c t must a l s o be t a k e n i n t o account.

5. Types and main tenance of p r e s s u r e r e l e a s e d e v i c e s

P r e s s u r e r e l e a s e d e v i c e s can be des igned as r u p t u r e d i s c s , e x p l o s i o n va l ves , e x p l o s i o n d i s c s , s p r i n g loaded p r e s s u r e r e l e a s i n g d e v i c e s o r o t h e r s a f e t y dev i ces .

One always has t o be c e r t a i n t h a t t h e p r e s s u r e r e l e a s e d e v i c e i s and remains t h e weakest p a r t o f t h e e n t i r e system. I t i s thus necessary t h a t a f t e r ma in tenance work i s completed a l l open ings a r e shu t . Adequate maintenance o f t h e p r e s s u r e r e l e a s e d e v i c e s i s i m p e r a t i v e .

An i n c r e a s e of t h e minimum s t a t i c p r e s s u r e o f response (e.g. due t o d i r t ) can j e o p a r d i z e t h e e x p l o s i o n p r o t e c t i o n o f t h e whole p l a n t . A r e d u c t i o n of t h e minimum s t a t i c p r e s s u r e o f response (e.g. due t o c o r r o s i o n o r m a t e r i a l f a t i g u e ) w i l l cause a stoppage o f t h e p l a n t b y p rematu re response.

5.1 R u p t u r e d i s c d e v i c e s

R u p t u r i n g s a f e t y d e v i c e s c o n t a i n r u p t u r e d i s c s which, when a c t i v a t e d , crumble, t e a r open, o r a r e r e l e a s e d as d u c t i l e membranes o f sma l l mass wh ich cannot do any damage when f l y i n g o f f . g e n e r a l l y r e q u i r e d . The reduced c r o s s s e c t i o n must a l s o be t a k e n i n t o account.

Fo r use i n n e g a t i v e p r e s s u r e ranges a vacuum suppor t i s

To p r e v e n t p remature response due t o m a t e r i a l f a t i g u e r u p t u r e d i s c s must be r e p l a c e d a f t e r a s p e c i f i e d o p e r a t i n g t i m e span. T h i s rep lacement c y c l e depends on t h e w o r k i n g p r e s s u r e and t h e number o f l o a d changes as w e l l as on t h e the rma l behav iou r ( s o f t e n i n g , b r i t t l e n e s s ) and on r e s i s t a n c e t o wear ( e r o s i o n , c o r r o s i o n ) . Data can be o b t a i n e d f r o m t h e manu fac tu re r .

5.1.1. R u p t u r e d i s c s made o f m a t e r i a l s n o t s u s c e p t i b l e t o shap ing

1 B-12 I

Ruptu re d i s c s made o f m a t e r i a l s wh ich cannot be shaped a r e m a i n l y made o f r e s i n impregnated g r a p h i t e . The range o f a p p l i c a t i o n i s between nominal w i d t h s o f 25 t o 600 mm w i t h minimum s t a t i c p ressu res o f response f r o m 1.1 b a r ( c o r r e s p o n d i n g t o about 0.1 above a tmospher ic p r e s s u r e ) and tempera tures f r o m -20 t o +130°C w i t h a good c o r r o s i o n r e s i s t a n c e . The accuracy o f response i s g e n e r a l l y w i t h i n - + 10% o f t h e excess p r e s s u r e o f response.

We recommend t h a t a t e s t c e r t i f i c a t e i s s u e d b y recogn ized e x p e r t s be o b t a i n e d (e.g. t h e TUV). These r u p t u r e d i s c s can t o l e r a t e l o a d changes up t o 75% of t h e excess p r e s s u r e o f response w i t h o u t a change of t h e accuracy of response. They a r e p r e d o m i n a t e l y used i n a p l a i n c i r c u l a r form.

5.1.2 Rupture d i s c s made o f workable material

Workable r u p t u r e d i s c s a r e made of p l a s t i c s , meta l , me ta l a l l o y s , o r any o t h e r s u i t a b l e m a t e r i a l . They a re used as c i r c u l a r p l a i n o r c i r c u l a r dome types , b u t r e c t a n g u l a r p l a i n t ypes a re a l s o used. The range o f a p p l i c a t i o n depends t o a l a r g e e x t e n t on t h e p r o p e r t i e s o f t h e membrane m a t e r i a l s used. Res is tance t o c o r r o s i o n can be i nc reased b y a p r o t e c t i v e f o i l cove r ing . I n t h i s case t h e p o s s i b l e i n c r e a s e o f t h e excess p ressu re o f response must be taken i n t o account. The accuracy o f response i s g e n e r a l l y w i t h i n + 10% o f t h e excess p ressu re o f response. We aga in recommend t h a t you o b t a i n a t e s t c e r t i f i c a t e ( see S e c t i o n 5.1.1).

( F o o t n o t e 2 - The excess p r e s s u r e response i s t h e p r e s s u r e above normal p r e s s u r e a t which t h e r u p t u r e d i s c r u p t u r e s .

I f f o i l s a r e used as r e l e a s e membranes, t h e e f f e c t o f t empera tu re on t h e i r s t r e n g t h must be taken i n t o c o n s i d e r a t i o n . Thermal e f f e c t s can l e a d t o s o f t e n i n g , low o u t s i d e tempera tures can cause b r i t t l e n e s s ( i . e . l e a d i n g t o changes o f t h e minimum s t a t i c response p r e s s u r e ) .

I t i s o f t e n found i n p r a c t i c e t h a t r u p t u r e d i s c s of d u c t i l e m a t e r i a l ( t h i s a p p l i e s p a r t i c u l a r l y t o l a r g e r e l e a s e openings and low s t a t i c response p ressu res ) d i s p l a y a m e c h a n i c a l l y u n s t a b l e b e h a v i o r even a t sma l l p ressu re f l u c t u a t i o n s . V i b r a t i o n s o r f l u t t e r f r e q u e n t l y appear even a t smal l p e r i o d i c p r e s s u r e f l u c t u a t i o n s (e.g. i n pneumatic conveyors) and s u b s t a n t i a l l y i m p a i r t h e s e r v i c e l i f e . I n t h i s case t h e t o t a l r e q u i r e d r e l e a s e area can be subd iv ided i n t o separa te areas, i f necessary

B-13

o t h e r des igns (e.g. e x p l o s i o n d i s c s o f clamped rubber ; see S e c t i o n 5 . 2 . 2 ) can be used.

B-14

1 d

5.1.3 Separately actuated rupture discs

The d i s r u p t i o n o f t hese r u p t u r e d i s c s does n o t t a k e p l a c e d i r e c t l y t h rough t h e e x p l o s i o n p r e s s u r e b u t b y d e t o n a t o r s t r i g g e r e d b y f l ame o r p r e s s u r e d e t e c t o r s .

5.2 Explosion valves and explosion discs

5.2.1

5.2.2

5.3 Spring

Explosion valves, Figure 3

These a r e des igned t o r e c l o s e t h e open ing a u t o m a t i c a l l y a f t e r r e l e a s e . Depending upon t h e i n t e n s i t y o f t h e e x p l o s i o n i n t h e vesse l t o be p r o t e c t e d , an e f f e c t i v e p r e s s u r e r e l e a s e can be impeded b y t h e mass o f t h e e x p l o s i o n v a l v e i t s e l f . E x p l o s i o n v a l v e s must t h e r e f o r e be as l i g h t as p o s s i b l e and be s u b m i t t e d t o a s tandard t e s t (e.g a t t h e BVS) f o r c o r r e c t f u n c t i o n i n g , a t wh ich t i m e t h e i r r e l e a s e a c t i o n can be measured i n comparison t o a r u p t u r e d i s c . C o r r o s i o n e f f e c t s , improper p a i n t work on moving p a r t s , as w e l l as i c e and snow loads , can b r i n g about an i n c r e a s e o f t h e response p ressu re . C o r r e c t f u n c t i o n i n g and f r e e movement must be checked a t f i x e d i n t e r v a l s.

Explosion discs of clamped rubber shapes, F igure 4

The e x p l o s i o n d i s c i s h e l d i n p l a c e b y a r u b b e r c lamp ing r e t a i n e r ( s i m i l a r t o t h e w i n d s h i e l d i n a c a r ) . The e x p l o s i o n f o r c e s t h e d i s c o u t o f t h e r e t a i n e r , t h e d i s c b e i n g p reven ted f r o m f l y i n g o f f b y w i r e s o f adequate s t r e n g t h o r b y suppor ts . D i s c s o f t h i s k i n d can a l s o be suppor ted i n a way s i m i l a r t o e x p l o s i o n v a l v e s and reused. The d i s c s may be used f o r response p ressu res up t o 0.2 b a r above atmosphere. C o r r o s i o n e f f e c t s , i c i n g , snow l o a d s o r b r i t t l e n e s s o f t h e r u b b e r clamp a rea can b r i n g about a change o f t h e response p ressu re .

loaded re lease devices

The r e l e a s e open ing i s sea led b y a f l a n g e w i t h t h e h e l p o f s p r i n g a c t i o n . The s ta tements i n S e c t i o n 5.2.1 w i t h r e g a r d t o t e s t i n g o f t y p e and c o r r e c t o p e r a t i o n a l s o a p p l y here.

6. Design o f pressure r e l e a s e openings

A p r e r e q u i s i t e f o r t he des ign o f a p ressu re r e l e a s e dev i ce i s t h e knowledge o f t h e development o f t h e e x p l o s i o n i n c losed vessels and o f t h e p a r t i c u l a r c h a r a c t e r i s t i c q u a n t i t i e s i n v o l ved : e x p l o s i on pressure pmax and maximum r a t e o f pressure r i s e (dp/dt),,, . The r e l a t i o n s h i p between t h e maximum r a t e o f pressure r i s e (dp/dt Imaxand t h e volume o f t h e vessel i s g i ven b y t h e cub ic law.

blmax x V1l3=K [ b a r x m x s - l l S t

I n t h i s equa t ion V i s t h e volume [m3] and KSt[bar x m x s-'] a s p e c i f i c d u s t c h a r a c t e r i s t i c which depends m a i n l y upon t h e t y p e o f dus t and t h e d i s t r i b u t i o n o f p a r t i c l e s i ze , t h e t u r b u l e n c e o f t h e d u s t / a i r m i x t u r e a t t h e t i m e o f i g i n i t i o n , and t h e type o f i g n i t i o n (see F i g u r e 6) .

The va lues f o r t h e dusts a r b i t r a r i l y s e l e c t e d i n F i g u r e 6 a re n o t t o be considered g e n e r a l l y a p p l i c a b l e and are mere l y used f o r purposes o f t h i s ex amp 1 e . The des ign can be based e i t h e r on t h e KSt va lue o r on t h e d u s t e x p l o s i o n c l a s s o f t h e dus t i nvo l ved .

Tab le 1 p resen ts a r e l a t i o n s h i p between t h e d u s t e x p l o s i o n c l a s s and t h e KSt va lue.

D e t a i l s r e g a r d i n g t h e d e t e r m i n a t i o n o f t h e d u s t e x p l o s i o n c l a s s and t h e KSt v a l u e can be found i n S e c t i o n 12.

8-15

T a b l e 1. R e l a t i o n s h i p between d u s t e x p l o s i o n c l a s s and KSt v a l u e

Dus t E x p l o s i o n C l a s s

S t 1

S t 2

S t 3

KStCbar x m x s - l l

>o t o 200

>200 t o 300

> 300

I n p r a c t i c e i t has t o be borne i n mind t h a t i t i s f r e q u e n t l y d i f f i c u l t t o assess t h e i n i t i a l c o n d i t i o n s f o r an e x p l o s i o n (e.g. w i t h r e g a r d t o t u r b u l e n c e , p a r t i c l e s i ze , d i s t r i b u t i o n and t y p e o f i g n i t i o n ) . I n t h e f o l l o w i n g nomographs you n o r m a l l y s t a r t f r o m t h e KSt v a l u e o r d u s t e x p l o s i o n c l a s s o f t h e minus 63 m i c r o n components o f a dus t .

F o r v e s s e l s w i t h o u t i n s e r t s , t h e d imens ions o f t h e p r e s s u r e r e l e a s e areas a r e n o r m a l l y based on t h e empty volume. I f i n s e r t s a r e p r e s e n t (e.g. f i l t e r hoses and f i l t e r bags), t h e volume can be deducted f r o m t h e volume of t h e v e s s e l . I t must be i n s u r e d , however, t h a t t h e r e l e a s e a c t i o n i s n o t impeded. F o r example, f i l t e r hoses must n o t cove r t h e r e l e a s e area. I n case of doubt, unh inde red d i s c h a r g e must be proved b y t e s t .

The f o l l o w i n g nomographs ( F i g u r e s 7 & 8 ) a s s i s t i n f i n d i n g t h e s i z e o f t h e r e q u i r e d p r e s s u r e r e l e a s e area A on t h e b a s i s o f t h e re leased p r e s s u r e (pred) and t h e s t a t i c v e n t i n g p r e s s u r e (ps ta t ) , o f t h e r e l e a s e dev ice , if t h e volume ( V ) o f t h e vesse l , t h e KSt v a l u e o f t h e d u s t o r i t s d u s t e x p l o s i o n c l a s s a r e known. The s t r e n g t h o f t h e vesse l has t o w i t h s t a n d t h e chosen r e l e a s e p r e s s u r e (see S e c t i o n 3 ) .

The nomographs a p p l y t o d u s t w i t h a maximum e x p l o s ' o n p r e s s u r e up t o 11 bar , i f t h e KSt v a l u e does n o t exceed 300 b a r x m x s - ' ( S t l , S t2 ) ; t h e y a l s o a p p l y t o d u s t s w i t h a maximum e x p l o s i o n p r e s s u r e up t o 13 b a r i f t h e K

S t v a l u e above 300 b a r m s - l ( S t 3 ) .

To de te rm ine t h e r e q u i r e d r e l e a s e area, s t a r t f rom t h e chosen s t a t i c v e n t i n g p r e s s u r e (ps ta t ) which i s f i x e d b y t h e o p e r a t i n g p ressu re o f t h e equipment, and f i n d t h e v a l u e o f t h e volume ( V ) on t h e absc i ssa of t h e r i g h t hand p a r t of t h e co r respond ing nomograph. From t h e r e f o l l o w t h e v e r t i c a l l i n e upwards as f a r as i t s i n t e r s e c t i o n w i t h t h e i s o b a r of t h e d e s i r e d v a l u e f o r t h e reduced e x p l o s i o n p ressu re (Predy v e n t i n g p r e s s u r e ) , which depends on t h e s t r e n g t h o f t h e equipment. From here, a l i n e i s drawn p a r a l l e l t o t h e absc i ssa u n t i l i t i n t e r s e c t s w i t h t h e s lope l i n e i n t h e l e f t hand p a r t o f t h e nomograph, wh ich connects a l l p o i n t s o f equa l K S t v a l u e o r equal d u s t e x p l o s i o n c l a s s . The v a l u e on t h e a b s c i s s a a t t h i s p o i n t o f i n t e r s e c t i o n i n d i c a t e s the r e q u i r e d p r e s s u r e r e l e a s e area f o r b u r s t i n g s a f e t y dev ices , wh ich can be subd iv ided if necessary i n t o s e v e r a l i n d i v i d u a l areas. I f e x p l o s i o n v a l v e s a re used, S e c t i o n 5.2.1 must be observed.

o f t h e r e l e a s e d e v i c e and o f t h e reduced e x p l o s i o n p r e s s u r e Pred can be P s t a t reached o n l y b y e x t r e m e l y l a r g e r e l e a s e openings, which a r e h a r d l y p o s s i b l e i n a c t u a l p r a c t i c e .

I f o t h e r s t a t i c v e n t i n g p ressu res than those g i v e n i n t h e nomographs a r e used, t h e r e q u i r e d r e l e a s e areas can be found b y l i n e a r e x t r a p o l a t i o n f r o m t h e va lues f o r v e n t i n g p ressu res f rom 1.1 b a r t o 1.2 b a r and 1.5 b a r up t o a v e n t i n g p ressu re of pstat equal t o o r l e s s than 2 ba r . The nomographs are based on a normal a tmospher ic p r e s s u r e o f 1 b a r b u t can be used w i t h o u t c o r r e c t i o n up t o o p e r a t i n g p ressu res o f 1.2 b a r . For h i g h e r o p e r a t i n g p ressu res t h e va lues g i v e n i n t h e nomographs f o r t h e reduced e x p l o s i o n p r e s s u r e must be i nc reased p r o p o r t i o n a t e l y s t a r t i n g f r o m t h e normal p ressu re . (31 ) .

The nornographs i n F i r g u r e 7a t o 7c a r e des igned f o r d u s t e x p l o s i o n c l a s s e s l i m i t e d b y c e r t a i n KSt va lues ( S t l t o S t3 ) . r e l e a s e areas f o r any o t h e r K S t va lues a mathemat ica l ad jus tment was made (27 ) . The r e s u l t i n g nomographs a r e g i v e n i n F i g u r e s 8a t o 8c. The d i f f e r e n c e s r e s u l t i n g f rom t h e r e l e a s e areas between t h e two nomographs a re deemed j u s t i f i a b l e f r o m t h e p o i n t o f v iew of s a f e t y (29) .

For t h e purpose o f f i n d i n g t h e

E x t e n s i v e i n v e s t i g a t i o n s have shown t h a t t h e d i s t r i b u t i o n range o f measured v a l u e s i nc reases toward h i g h e r va lues o f reduced e x p l o s i o n pressure . The reduced e x p l o s i o n p r e s s u r e was t h e r e f o r e l i m i t e d t o 3 b a r i n t h e nomographs. H i g h e r va lues shou ld be chosen o n l y i n c o l l a b o r a t i o n w i t h e x p e r t s .

F o r vesse l volumes above 30 m3 c e r t a i n r e d u c t i o n s of t h e s i z e o f r e l e a s e areas a re p o s s i b l e i n s p e c i a l cases (29;31).

B-17

I 7. Safe discharge of the pressure wave, flame and exhaust gases

When a pressure release i s activated one must always expect a discharge of b u r n i n g and unburned dust, w i t h extensive flame and pressure effects . The lower the s t a t i c response pressure the greater the spread of flames; depending on the volume o f the pressure relieved vessel, i t can reach lengths from 10 t o over 50 meter.

I t i s imperative t h a t personnel n o t be endangered by these phenomena. This must be taken i n t o consideration when planning and can be avoided best if the pressure release i s vented upwards. For th i s reason, the pressure release devices should be mounted on t o p o f the vessel t o be protected and directed upwards as f a r as possible. I f th is i s not possible, the pressure release openings should be mounted as h i g h as possible on the side of the vessel. allowance should be made t o insure an adequate distance i s maintained between the lower edge o f the opening and the t o p o f the dust pile with the vessel f i l ed t o maximum capacity under operation conditions.

The process o f pressure release produces considerable reaction forces, w h i c h can even cause the equipment t o overturn. This additional s t ra in must be taken into account when mounting equipment and pipelines.

Due t o the dange r of dust ejection,

7 . 1 Open air plants

For systems i n the open a i r (e.g. s i los , vessels, e tc . ) care must be taken t h a t the surroundings are not endangered by flames or pressure release elements f l y i n g off. Flammable materials (e.g. roof coverings) should no t be present i n the vicinity of discharge openings.

7.2 Plants in closed areas

If pressure release i s designed f o r equipment installed in closed rooms the protection of the rooms and of personnel employed in them demands t h a t the pressure release be conducted t h r o u g h a pipeline ( the so called blow-off pipe) in a non-dangerous direction t o the open a i r .

7.3 Effect o f blow-off pipes on the reduced explosion pressure

I f a blow-off pipe i s connected t o the equipment, the discharge of the s t i l l unburned dust/air mixture and o f combustion gases i s impeded in comparison t o f ree release. I n addition t o this , the blow-off pipe may already be f i l l ed with an explosive dust/air mixture a f te r the release has responded, and before the flames enter i t from the vessel t o be protected.

B-18

Impeding t h e d ischarge process, as w e l l as t h e exp los ion process, i n t h e b l o w - o f f p i p e leads t o an i nc rease o f t h e reduced e x p l o s i o n pressure. The increase, a r i s i n g f rom bo th e f f e c t s , depends on the l e n g t h o f t h e b low-o f f p i p e (31). which i n d i c a t e t h a t t h e i nc rease o f t h e reduced exp los ion pressure i n t h e vessel t o be p r o t e c t e d i n comparison t o f r e e d ischarge, becomes s m a l l e r as t h e l e n g t h o f t he blow-off p i p e i s reduced. However, a maximum inc rease of t h e reduced e x p l o s i o n pressure must be expected when the speed o f e x p l o s i o n i n the blow-off p i p e reaches o r exceeds t h e speed o f sound. T h i s can occur i n b l o w - o f f p ipes whose l e n g t h i s o f t h e o rde r o f 3 meter o r above.

Corresponding va lues can be ob ta ined f rom F i g u r e 9

I n accordance w i t h t h e expected i nc rease of t h e reduced e x p l o s i o n pressure, e i t h e r t h e r e l e a s e area, o r t h e r e s i s t a n c e t o p ressu re ( e x p l o s i o n r e s i s t a n c e s t r e n g t h ) o f t h e vessel t o be p r o t e c t e d must be increased.

If, therefore, va lues o f 1.2 t o 1.8 ba r a re chosen f o r f r e e r e l e a s e as rep resen ted i n t h e nomograph, t h e e x p l o s i o n - p r o o f i n g o f t h e vessel t o be p r o t e c t e d must be a t l e a s t 3 t o 4 b a r when l o n g e r b l o w - o f f p ipes a re used. On t h e o t h e r hand, vesse ls whose exp los ion -p roo f s t r e n g t h i s below 2 bar, can n o t be p r o t e c t e d p r a c t i c a l l y b y t h e desc r ibed method i n connect ion w i t h l onger b l o w - o f f p ipes ( r e l e a s e areas t o o l a r g e ) .

7.4 Design o f blow-off pipes

I n accordance w i t h S e c t i o n 7.3 b l o w - o f f p ipes should be k e p t as s h o r t as p o s s i b l e and i n s t a l l e d i n s t r a i g h t runs. They should have a t l e a s t t h e same c ross s e c t i o n as t h e r e l e a s e opening and t h e same exp los ion r e s i s t a n c e as t h e vessel t o be p ro tec ted . We recommend t h a t p ipes l o n g e r than 3 meter be made f o r nominal p ressu re ND6 and N D l O (above atmosphere). C i r c u l a r r e l e a s e duc ts should be used i n p re fe rence t o r e c t a n g u l a r ones f o r reasons of s t r e n g t h .

I f , f o r t h e purpose o f maintenance, an i n s p e c t i o n h o l e i s p rov ided i n t h e v i c i n i t y o f t h e r e l e a s e dev i ce t h e cover and l o c k must have t h e same s t r e n g t h as t h e b l o w - o f f p i p e ( b l o w - o f f d u c t ) .

To p reven t p e n e t r a t i o n b y r a i n and snow i n t o t h e b l o w - o f f p ipe, l i g h t weight covers (e.g. f o i l s , F i g u r e 10 o r d i s c s o f clamp shapes F i g u r e 11) are permissable; these must be thrown o f f a t v e r y low pressures (equal t o o r l e s s than 1.1 b a r ) .

8. Pressure release o f elongated vessels

Elongated vessels, as d e f i n e d i n these g u i d e l i n e s , a re vessels where t h e

I

6-19

r a t i o o f h e i g h t o r l e n g t h t o diameter i s g r e a t e r than 5 : l . w i t h a r e c t a n g u l a r cross s e c t i o n t h e e q u i v a l e n t d iameter serves as an approximate value. vehement than i n cub ic vessels s i n c e i t depends n o t o n l y on t h e t y p e o f dust , b u t , a d d i t i o n a l l y , on a x i a l f l ows , changes i n tu rbu lence and compression e f f e c t s .

For a vessel

I n such vessels the e x p l o s i o n c y c l e can be more

I n e longated vessels, i r r e s p e c t i v e of t h e t y p e o f dust and volume o f t h e vessel , pressure r e l e a s e must be prov ided along t h e whole roof o r face area r e s p e c t i v e l y , s i n c e t h i s may o the rw ise be t o r n o f f . A p a r t from t h a t , t h e r e l e a s e areas must n o t be sma l le r than those r e s u l t i n g f rom t h e nomographs. The f a c t t h a t a d d i t i o n a l r e l e a s e areas are n o t p e r m i s s i b l e a t t h e s ide o f l o n g vessels e n t a i l s c e r t a i n r e l a t i o n s h i p s between t h e diameter o f a vessel and i t s maximum h e i g h t o r l eng th , r e s p e c t i v e l y depending on t h e exp los ion c h a r a c t e r i s t i c s o f t h e d u s t and t h e s t r e n g t h of t h e vessel . These r e l a t i o n s h i p s a re shown i n F i g u r e s 12 and 1 3 f o r s tanding s i l o s w i t h c i r c u l a r base area used as an example (31 ) .

F i g u r e s 14 and 15 show a s p e c i a l pressure r e l e a s e system f o r l o n g vessels us ing t h e whole r o o f area. r e l e a s e areas ( p a r t i a l r e l e a s e ) which are covered b y p l a s t i c f o i l respond, F i g u r e 14, whereas i n a vehement exp los ion the whole cover t e a r s open i n t o segments, F i g u r e 15.

I n a weak exp los ion o n l y those

9. Pressure release o f piping sections

The statements made i n r e f e r e n c e t o e longated vessels a p p l y i n p a r t i c u l a r t o p i p i n g i n which the speed o f t h e exp los ion can inc rease u n t i l t h e process becomes almost a de tona t ion . I n t h i s case, a p ressu re r e l e a s e i s e f f e c t i v e o n l y i f r e l e a s e dev ices o f adequate s i z e are arranged a t s h o r t i n t e r v a l s (1 t o 2 meter) on t h e p i p e w a l l . S ince i n p r a c t i c e such arrangements can be used o n l y i n open a i r i n s t a l l a t i o n s , because of t h e i n t e n s i v e f lame discharge, i t i s more a p p r o p r i a t e t o c o n s t r u c t endangered p i p e l i n e s i n ND 10 and t o d ispense w i t h l a t e r a l r e 1 ease.

I f e s p e c i a l l y h i g h pressure peaks a re expected a t bends o r end f l a n g e s o f l ong p i p e s e c t i o n s ( f r o m about 20 mete r ) ( 2 5 ) these must be re leased i n a x i a l d i r e c t i o n w i t h o u t decrease o f cross sec t i on . T h i s can be c a r r i e d o u t e i t h e r b y r u p t u r e d i s c s o r o t h e r t e s t e d r e l e a s e dev ices, F i g u r e s 16 and 17.

One must cons ide r here t h a t t h e response of such an arrangement under e x p l o s i o n l o a d i n g leads t o an i nc rease of t he exp los ion speed, as a r e s u l t o f t h e f e e d i n g e f f e c t s descr ibed, and therefore, t o an i nc rease

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o f t h e e x p l o s i o n pressure. lower t h e response pressure i s . T h i s must t h e r e f o r e be chosen s u f f i c i e n t l y h i g h (1 .5 t o 3 b a r ) so as n o t t o promote t h e development o f d e t o n a t i o n l i k e processes as a r e s u l t o f t h e re lease .

This takes p lace a17 t h e more r a p i d l y t h e

Pressure r e l e a s e dev ices in tended t o c l o s e again a u t o m a t i c a l l y a f t e r response, F igu res 16 and 17, must be used o n l y a f t e r t h e i r c o r r e c t o p e r a t i o n has been proved i n e x p l o s i o n t e s t s .

10. Pressure release o f vessels connected by piping

The nomographs, F i g u r e s 7 and 8, can l e a d t o unders ized r e l e a s e areas i f a d i r e c t exp los ion i s t r a n s m i t t e d f rom one vessel i n t o another through a p ipe. Precompression, increased tu rbu lence , en larged area, f lame j e t i g n i t i o n can a l l l ead t o an increased e x p l o s i o n i n t e n s i t y .

I n such cases, t h e f o l l o w i n g p r o t e c t i v e measures a re s u i t a b l e g i v e n our p resen t s t a t e o f knowledge:

The r e l e a s e must be designed f o r a low s t a t i c response pressure (c1.2 b a r ) .

Fo r vessels o f equal s i z e ( V + l O % ) , each a re t o be vented i n accordance w i t h t h e nomographs:

F o r vesse ls o f d i f f e r e n t s izes, each i s t o be vented i n accordance w i t h t h e nomographs. In a d d i t i o n , a l l vesse ls have t o be designed f o r an e x p l o s i o n p ressu re r e s i s t a n c e o f 3 bar . I f no p o s s i b i l i t y e x i s t s o f v e n t i n g t h e sma l le r vessel , t h i s has t o be designed f o r an e x p l o s i o n p ressu re r e s i s t a n c e corresponding t o t h e maximum e x p l o s i o n pressure.

I f a dust dus t e x p l o s i o n i s , t r a n s m i t t e d f rom an e x p l o s i o n p ressu re r e s i s t a n t , nonvented, r e l a t i v e l y smal l app l i ance (e.g. a p u l v e r i z e r ) t o a l a r g e p i e c e o f equipment ( s i l o , cyclone, f i l t e r ) t h e l a r g e r equipment has t o be designed f o r an e x p l o s i o n p ressu re r e s i s t a n c e o f a t l e a s t 3 b a r and t h e c a l c u l a t e d r e l e a s e area must be doubled.

The nornographs a p p l y w i t h o u t r e s t r i c t i o n i f t h e f l ame propogat ion, and t h e r e f o r e f lame j e t i g n i t i o n , i s s a f e l y prevented (e.g. b y i n s t a l l i n g q u i c k s h u t - o f f dev ices o r e x t i n g u i s h e r b a r r i e r s on b o t h s ides of d e t e c t o r s i n t h e connect ing p i p i n g )

11. Limits of applicability of pressure

As i n a l l s a f e t y devices, a p p l i c a b i r e 1 ease devices.

release

i t y l i m i t s a l s o e x i s t f o r pressure

Based upon t h e exper imenta l and p r a c t i c a l exper ience gained t h u s f a r these l i m i t s a re as f o l l o w s :

8-21

Vesse ls and p i p i n g f o r po isonous o r a c i d m a t e r i a l s cannot be vented ( s e e P r e l i m i n a r y Remarks). The p r o t e c t i v e e f f e c t has t o be a t t a i n e d by o t h e r measures.

The a p p l i c a t i o n o f t h e nornographs ( F i g u r e s 7 and 8) i s r e s t r i c e d b y an upper KSt v a l u e o f 600 b a r x m x s -I. Even f o r h i g h e r KSt v a l u e s a p r e s s u r e v e n t i n g i s s t i l l p o s s i b l e ; however, t o s i z e t h e s e openings, a t e s t i n g agency such as s p e c i f i e d i n 12.2 s h o u l d be asked f o r c o n s u l t i n g adv ice .

The nomographs ( F i g u r e s 7 and 8 ) a p p l y f o r c u b i c a l vesse ls w i t h volumes of up t o a p p r o x i m a t e l y 1000 m3. One can assume, o f course, t h a t t h e r u l es app l i cab1 e t o t h e nomographs a p p l y a1 so f o r 1 a r g e r vesse ls . However, t h i s assumption has n o t y e t been e x p e r i m e n t a l l y demonstrated.

I f p r e s s u r e r e l e a s i n g i s n o t a p p l i c a b l e (e.g. if p r e s s u r e and f lame e f f e c t s i n t h e su r round ings o f t h e equipment t o be p r o t e c t e d have t o be avo ided) then o t h e r p r o t e c t i v e measures a r e r e q u i r e d , e.g.:

a> I n e r t i z i ng,

b ) P r e s s u r e r e s i s t a n t o r e x p l o s i o n p r o o f c o n s t r u c t i o n i n accordance w i t h t h e maximal e x p l o s i o n pressure ,

C ) E x p l o s i o n suppress ion , e x t i n g u i s h i n g media.

The measures s p e c i f i e d under b ) and c ) can be a p p l i e d i n c o n j u c t i o n w i t h i n t e r l o c k systems f o r t h e au tomat i c s w i t c h - o f f o f p l a n t s e c t i o n s o r t h e e n t i r e p l a n t . Measures o f t h i s t y p e a r e s p e c i f i e d a l s o i n t h e A s s o c i a t i o n ' s " E x p l o s i o n P r o t e c t i o n Guide1 i n e s " .

The nomographs a r e b y no means a p p l i c a b l e f o r d e t e r m i n i n g t h e s i z e o f r e l e a s e openings i n t h e case o f gas e x p l o s i o n danger, even g i v e n comparable e x p l o s i o n s t r e n g t h . Flammable gases r e q u i r e l a r g e r v e n t s u r f aces ( 3 1 1.

T h i s a p p l i e s i n t h e same way f o r mixes o f f lammable gases o r vapors and f lammable d u s t s b lended w i t h a i r ( h y b r i d m ixes ) .

Dus ts which, o f themselves, a r e n o t e x p l o s i v e can fo rm e x p l o s i v e m i x t u r e s g i v e n t h e presence o f even s l i g h t amounts o f combus t ib le gases o r vapors (e.g. monomere, s o l v e n t s , r e t o r t c o a l gas), even i f t h e c o n c e n t r a t i o n s o f each c o m b u s t i b l e component b y i t s e l f remains beneath

B-22

t h e lower e x p l o s i o n l i m i t . r e q u i r e s a d d i t i o n a l analyses.

I n these cases s i z i n g r e l e a s e openings

12. D e t e r m i n a t i o n and a p p l i c a t i o n o f e x p l o s i o n s i z e s f o r combus t ib le d u s t s

12.1 Tes t process

B a s i c a l l y , i n a l l t h e t e s t procedures descr ibed he re in , < 63 m ic ron components o f t h e d u s t are t o be t e s t e d when dry . I n s p e c i a l cases t h e dus t can a l s o be t e s t e d as d e l i v e r e d .

Based upon our present l e v e l o f knowledge, a c y l i n d r i c a l t e s t dev i ce (d=h) of 1 m3 content, F i g u r e 18, generates r e l i a b l e va lues f o r exp los ion c h a r a c t e r i s t i c s . I n s i d e t h e device, t h e d u s t i s ma in ta ined under an a i r p ressu re of app rox ima te l y 20 ba r i n a 5 l i t r e vessel w i t h a de tona to r actuated va lve. A f t e r opening t h e v a l v e t h e dus t passes through a p e r f o r a t e d p i p e ( h o l e d iameter 4 t o 6 mn) i n t o t h e e x p l o s i o n chamber and i s exploded t h e r e a f t e r a s p e c i f i c d e l a y t i m e span t,= 0.6 seconds.

Two py ro t e c h n i c a l i g n i t i o n p o i n t s w i t h a t o t a l energy o f app rox ima te l y 10 k J b r i n g about t h e i g n i t i o n . The d e l a y t ime corresponds t o a s p e c i f i c t u r b u l e n c e o f t h e d u s t / a i r mix a t t h e t i m e o f i g n i t i o n .

Maximum values f o r t h e e x p l o s i o n p ressu re a re then obtained, F i g u r e 19, i f t h e i g n i t i o n takes p l a c e immediate ly a f t e r emptying t h e dus t s torage vessel (tv= 0.6s). The t u r b u l e n c e generated by t h i s d e l a y corresponds t o a v e r y s p e c i f i c va lue f o r t h e maximum p ressu re increment over t ime o f d u s t explos ion, and thus corresponds a l s o t o a s p e c i f i c KSt value. D e c l i n i n g t u r b u l e n c e ( e x t e n s i o n o f t h e i g n i t i o n d e l a y tv>0.6s) r e s u l t s i n a d e c l i n e o f t h e e x p l o s i o n s t reng th . I nc reased t u r b u l e n c e ( r e d u c i n g t h e i g n i t i o n d e l a y tv<0.6s) r e s u l t s i n an i nc rease i n t h e e x p l o s i o n s t r e n g t h .

To determine t h e c h a r a c t e r i s t i c s pmax and ( d p / d t I m v a l ues 1, t e s t s encompass i ng a w i de range o f c o n c e n t r a t i o n s a re r e q u i r e d .

( o r t h e KSt

Such systemat ic t e s t s i n t e s t equipment o f t h e s p e c i f i e d s i z e a re v e r y expensive and can p r e s e n t l y be performed o n l y a t a few p laces (e.g. BAM o r BVS) .

8-23

To e s t i m a t e t h e range w i t h i n wh ich t h e KSt v a l u e o f a s p e c i f i e d d u s t f a l l s ( d u s t e x p l o s i o n c l a s s ) , t h e Hartmann dev ice , a r e a c t o r o f a p p r o x i m a t e l y 1.2 l i t r e con ten t , can be used. T h i s comes i n two des igns .

U s i n g t h e s e a l e d Hartmann dev ice , F i g u r e 20, i t i s p o s s i b l e t o de te rm ine t h e p r e s s u r e c y c l e o f a d u s t e x p l o s i o n ove r t ime .

The m o d i f i e d Hartmann dev ice , F i g u r e 21, c o n s i s t s o f a v e r t i c a l g l a s s p i p e equipped w i t h a f l a p cover. The d u s t t o be t e s t e d i s w h i r l e d upwards b y a i r i n j e c t i o n , as i s t h e case i n t h e sea led Hartmann d e v i c e (11). A c o n s t a n t spark serves f o r i g n i t i o n .

The d u s t l a i r m i x exp lodes t h e cover f l a p s open t o v a r y i n g p o s i t i o n s depending upon t h e s t r e n g t h o f t h e e x p l o s i o n . o f t h e open ing i s i n d i c a t e d d i g i t a l l y v i a i n d u c t i v e t r a n s m i t t e r s i n two s tages .

I f no e x p l o s i o n r e s u l t s i n t h e m o d i f i e d Hartmann d e v i c e w i t h a c o n s t a n t spark ( i n d i c a t o r r e a d i n g O ) , and no f l ame spreads, t hen t h e t e s t s must be r e p e a t e d u s i n g a s t r o n g e r source o f i g n i t i o n . As soon as f l ame appears, even i f i t does n o t l e a d t o a l i f t i n g of t h e cover, t h e d u s t i s t o be ass igned an i d i c a t o r r e a d i n g of 1. I f these t e s t s demonst ra te no r a i s i n g o f t h e cover, no r even any f lame, t h e n one can assume t h a t t h e d u s t can e i t h e r n o t be i g n i t e d , o r o n l y i g n i t e d u s i n g e x t e n s i v e energ ies . I f , however, based upon t h e chemica l c o m p o s i t i o n o f t h e dus t , o r due t o o t h e r l a b o r a t o r y r e s u l t s , a d u s t e x p l o s i o n appears p o s s i b l e , t hen f o r c e r t a i n e v a l u a t i o n o f a d d i t i o n a l t e s t s i n o t h e r equipment and even w i t h o t h e r sources o f i g n i t i o n shou ld be performed.

The ang le

12.2 C l a s s i f i c a t i o n o f dusts

The KSt va lue , and t h e a l l o c a t i o n a c c o r d i n g t o d u s t e x p l o s i o n c l a s s e s d e f i n e d i n T a b l e 1, a r e based upon t e s t s i n c u b i c a l v e s s e l s w i t h a volume o f 1 m3.

The e x p l o s i o n c h a r a c t e r i s t i c s used t o measure p r e s s u r e r e l e a s e mechanisms can a l s o be measured i n o t h e r dev i ces i f i t has been demonst ra ted on numerous d u s t s t h a t , w i t h i n t h e framew r k o f t e s t

a r e o b t a i n e d as i n t h e 1 m s v e s s e l . p r e c i s i on, t h e same resu 1 t s

To e s t i m a t e t h e d u s t e x p l o s openings, one can o f t e n use Hartmann p i p e t e s t s . Dus ts exper ience , g e n e r a l l y f a l l

on c l a s s f o r s i z i n g p r e s s u r e r e l e a s e t h e r e s u l t s o b t a i n e d i n m o d i f i e d ass igned t h e r e a d i n g 1, based on n t h e d u s t e x p l o s i o n c l a s s S t 1.

8-24

The i n d i c a t i o n " 2 " can correspond t o dus t exp los ion c lasses S t 1, S t 2 o r S t 3 w i t h those dus ts f a l l i n g on the border between S t 1 and S t 2 be ing assigned t h e 2 reading.

Dusts hav ing a d u s t e x p l o s i o n c l a s s of S t 3 cannot be d i f f e r e n t i a t e d i n the m o d i f i e d Hartmann dev i ce ( i n terms o f t h e i r read ings ) f rom dus ts o f e x p l o s i o n c l a s s S t 2. Under c e r t a i n c i rcumstances they can be recognized because o f a p a r t i c u l a r l y s t r o n g r e a c t i o n .

The pressure c y c l e measured i n t h e sealed Hartmann dev i ce over t i m e can o n l y be t r a n s f e r r e d t o l a r g e r vesse ls w i t h c e r t a i n r e s e r v a t i o n s (1). T h i s dev i ce has, however, been in t roduced i n t e r n a t i o n a l l y , and a l a r g e number o f dus ts have been t e s t e d i n i t whose dus t e x p l o s i o n behav io r can be thus compared.

A t t h e p resen t t ime, t h e f o l l o w i n g t e s t i n g f a c i l i t i e s a re a v a i l a b l e t o t e s t dusts , o r p r o v i d e i n f o r m a t i o n r e g a r d i n g t e s t s t h a t have a1 ready been performed:

Federa l I n s t i t u t e f o r M a t e r i a l T e s t i n g (BAM) Un te r den Eichen 87, 1000 B e r l i n 45

M i n i n g Tes t Center (BVS) B e y l i n g s t r . 65, 4600 Dortmund-Derne

Dust Research I n s t i t u t e o f t h e A s s o c i a t i o n o f C r a f t Assoc ia t i ons e.V. (STF) Langwartweg 103, 5300 Bonn

6-25

GERMAN BIBLIOGRAPHY

1. Bar tknecht , W. : Exp los ionen-Ab lau f und Schutzmassnahmen. B e r l i n , He ide l berg, New York: S p r i n g e r 1978.

2. Ba r t knech t , W . : A b l a u f von Sas- und Staubexp los ionen und deren Bekaempfung, S i c h e r e A r b e i t , F a c h z e i t s c h r i f t f u e r S i c h e r h e i t s t e c h n i k und i n d u s t r i e l l e Med iz in 27 (1974) N r . 1.

3. R i c h t l i n i e n f u e r d i e Vermeidung de r Gefahren du rch e x p l o s i b l e Atmosphaere m i t Beispielsammlung. Explosionsschutz-Richtlinien (EX-RL). Beru fsgenossenschaf t der chemischen I n d u s t r i e . D r u c k e r e i W in te r , He ide lbe rg .

4. F rey tag , H. H. : Handbuch de r Raumexplosionen. Weinheim: V e r l . , Chemie 1965.

5. Kuehnen, G . : B e u r t e i l u n g der E x p l o s i o n s g e f a h r b e i brennbarem Staub. Staub 27 (19671, S. 529.

6. Palmer, K.N.: Dust E x p l o s i o n s and F i r e s . London: Chapman and H a l l 1973.

7. Verhuetung von Staubbraenden und Staubexplosionen. Vo r t raege de r VDI-Tagung Nuernberg 1970. VDI-Ber. 165. Duesse ldor f : VDI-Ver l . 1971.

8. V D I 2263 Verhuetung von Staubbraenden und Staubexp los ionen.

9. Grewer. Th. : Zur Se lbs ten tzuendung von abgelagertem Staub. VDI-Ber. 165, S.9. Duesse ldo r f : VDI-Ver l . 1971.

10. Leuschke, G.: Ueber d i e Untersuchung b rennbare r Staeube a u f Brand- und Exp los ionsge fa ren . Stabu - R e i n h a l t u n g de r L u f t 26, (1966) S. 49.

11. L u e t o l f , J. : Appara turen f u e r d i e Bestimmung de r Explosionscharakter is t iken von brennbaren Staeuben. Stabu - R e i n h a l t u n g der L u f t 33 (19731, S. 259.

12. R a f t e r y , M. : E x p l o s i b i 1 i ty t e s t s o f i n d u s t r i a l dus ts . F i r e Research Techn ica l Paper N r . 21, Lofidon: 1968.

13. Ra f te ry , M.: Untersuchung von i n d u s t r i e l l e n Staeuben a u f E x p l o s i o n s f a e h i g k e i t . VDI-Ber. 165, S. 45. Duesse ldo r f : VDI-Ver l . 1971.

14. Schoenewald, I.: V e r e i n f a c h t e Methode z u r Berechnung der u n t e r e n Zuendgrenze von Staub/Luftgemischen. Staub - R e i n h a l t u n g de r L u f t 3 1 (1971), S. 376.

15. S c h o l l , E. W.: Exp los ionsve rsuche m i t Zuckers taub i n Entstaubungsanlagen e i n e r s t i l l g e l e g t e n Z u c k e r f a b r i k . Wilhelmshaven: Hug & Co. 1973.

16. S e l l e , H., u. J. Zehr: Exper imen ta lun te rsuchungen von Staubverbrennungsvorgaengen und i h r e Be t rach tung vom reak t ions ther rnodynamischen Standpunkt. VDI-Ber. 19, S. 73. Duessel d o r f : VDI-Ver l . 1957.

17. Zehr, J . : A n l e i t u n g zu den Berechnungen ueber d i e Zuendgrenzwerte und d i e maximalen Exp los ionsdruecke. VDI-Ber. 19, S. 63. Duesse ldor f : VDI -Ver l . 1957.

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GERMAN BIBLIOGRAPHY (Cont.)

18. Donat, C.: Auswahl und Bernessung von Druckentlastungseinrichtungen f u e r S taubexp los ionen. VDI-Ber. 165. Duesse ldo r f : V D I V e r l . 1971.

19. Donat, C: E i n s a t z von B e r s t s i c h e r u n g e n b e i langsamem und schne l lem Druckans t ieg . Chemie-1ng.-Techn. 45 (19731, S. 790.

20. Donat, C.: E x p l o s i o n s d r u c k e n t l a s t u n g m i t Be rs tsche iben und Exp los ionsk lappen. 2. I n t e r n a t i o n a l e s K o l l o q u i u m f u e r d i e Verhuetung von A r b e i t s u n f a e l l e n und B e r u f s k r a n k h e i t e n i n d e r chemischen I n d u s t r i e . F rank fur t /M. : 1973.

21. Gre in , W., u. C. Donat: Anwendung, Auswahl und Bemesung von Bers ts i che rungen . Techn. Ueberw. 8 (1967), S. 185.

22. H e i n r i c h , H. J. : B e i t r a g z u r Kenn tn i s des A b l a u f s d r u c k e n t l a s t e t e r S taubexp los ionen b e i Zuendung durch t u r b u l e n t e Flammen. S taub - R e i n h a l t u n g de r L u f t 32 (19721, S. 293.

23. Palmer, K.N.: Dus t E x p l o s i o n Ven t ing - A Reassessment o f t h e Data. F i r e Research Note N r . 830, August 1970.

24. S c h o l l , E. W.: Explosionsdruckentlastung von Behaetern und Rohren b e i Gas- und Staubexp l osionen. D ie Beru fsgenossenschaf t (1974) , S. 289.

25. B a r t k n e c t , W. : Sicherheitsmassnahmen gegen d i e u n g e h i n d e r t e Ausbre i tung von Exp los ionen und gegen Exp los ionsauswi rkungen i n Rohrs t recken. Moderne U n f a l l v e r n u e t u n g 11 (19671, S. 41.

26. R i t t e r , K.: B e t r i e b l i c h e Massnahmen z u r Verhuetung von Staubbraenden und Staubexp los ionen. VDI-Ber. 165, S. 20. Duesse ldor f : VDI-Ver l . 1971.

27. H e i n r i c h , H. J. : D r u c k e n t l a s t u n g b e i S taubexp los ionen. A r b e i t s s c h u t z (1974) , N r . 11, S. 314.

28. Ba r t knech t , W.: B e r i c h t ueber Versuche z u r Erprobung von Sicherheitmassnahmen gegen Exp los ionen i n W i r b e l s c h i c h t t r o c k e r n . Base l : C I B A Ge igy (Mai 1974).

29. Ba r t knech t , W . : B e r i c h t ueber Untersuchungen z u r Frage de r Explosionsdruckentlastung brennbare r Stauebe i n Behaetern. T e i l 1, Staub - R e i n h a l t u n g de r L u f t (1974) N r . 11, S. 381. T e i l 2, Staub - R e i n h a l t u n g de r L u f t (1974) N r . 12, S. 456.

30. Gre in , W . , u. C. Donat: Der e x p l o s i o n s d r u c k f e s t e Behael t e r . E i n e Schutzmassnahme gegen Gas- und Staubexp los ionen. ( I n V o r b e r e i t u n g ) .

31. V o r t r a e g e des Ko l loqu iums "Drucken t las tung von Staubexp los ionen" de r V D I -

32. A r b e i t s s t o f f v e r o r d n u n g . Verordnung ueber g e f a e h r l i c h e A r b e i t s s t o f f e (Arb S t o f f V),

Kommission R e i n h a l t u n g de r L u f t am 5. J u n i 1975 i n Duesse ldor f .

Neufassunng der Verordnung vom 8. September 1975, T e i l 1: Koeln. B e r l i n . Bonn. Muenchen C a r l Heymanns 1975.

B-27

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S P R I N G L O A D E D R E L I E F V A L V E S

Q bar z 0 .-

5 u)

-1

X w

a a

X .= S 1

0

HETHYLCELLULOSE OUST

POLYETHVLEHE DUST

300 I I

I I 1

3 0 0 ,urn 4G0 0 100 200

M E D I A N V A L U E M

F I G U R E 6

D E P E N D E N C E OF E X P L O S I O N C H A R A C T E R I S T I C F I G U R E S ON P A R T I C L E SIZE

__ _____ --

O N - - _ - PRESSURES - R D S O L U T E PRCSSURES Pc.d' 1 .2 b o r

I . 4 1 . 6 I .0 2.0 2.3 5 . 0

10 in* 1 0 . 1 1 10 100 rnJ 1000

R E L I E F SURFOCL f C O N T F I I N L R V O L U M L V

F I GIJliL /o

PJ0110GFiAPHS R E F T O D U S T EXPLOSION ( P s - 1 . 1 b a r )

O N - P R E S S U R E S - F I B S O L U T E PRESSURES pr . 1 . 4

I .6 1 . 8 2 . 0 2 . 7 3.0

100 10 1 Inc 0.1 1 10 100 m J 1000

R t L I E F SURFFiCL F C O N T A I N E R VOLUME V

d- b a r

F I G U R E 7 b

N O M O G R A P H S R E F . T O D U S ? ~ E X P L O S I O N ( P ~ t ~ t - 1 . 2 b a r )

.- ._ I____-__

P R E S S U R E S - 64BSOLUTE P R E S S U R E S D - “ a -

* r.0- 1.6 b o r

1 . a 2 . 0 2 . 5 3 . 0

100 m3 1000 100 10 1 m 7 0 . 1 I 10

fit L I f f S U R l f tLI f C O t I T Q I t r L R V O I U I l t V

F 1Gl INI : 7 c

NOMOGRAPHS R E F . T O D U S T EXPLOSION ( Ps t a t - 1.5 b a r )

n o P R E S S U R E S - A B S O L U T E PRESSURES r n

f i r L I C F s u f ? F o c E F C O N T A I r 4 E H V O L U M C v

1 00 bar ... I50 200 2 5 0 300 400 500 600

1 0 0 " 2 10 1 0.1

P?.< I . 2 I . J 1 . 4 I . Y 1 . 6 I .B 2 . 0 2.3 2.6 3.0

1 10 100 mJ 1000

1- b a r

n o r n P R C SSURES - FIBSOI.UTE PRESSURES

Pre' I . J 1 . 4 I . s 1 . 6 1 . e 2 . 0 2.3 2.6 3 . 0

10 100 rnJ 1000 rnz 10 I 0.1 1

~t c I E F Sufi1 nrr t C O N T A I N L f 4 V O I U t l k V

f I G U R C 8b

- ~ _ _ _

PRESSUIICS - A B S O L U T E P f i E S S U R E S

5 0 bar

PY.d t . 6 1 .0 2 . 0 2.3 2.6 3 . 0

b o r

100 m J 1000 100 m 2 10 1 0.1 1 10

nrt I F F SuRrocc F C O N 7 H I t . I E R VULUML V

f I b U H L 8c.

N O F l O G R A P I i S R E F . T O h S T V A L U E S ( [ ' S t a t - 1 .5 b a r )

- ~ . _ _ _ _ ~

QEDUCE9 E Y P L O S I C u P S E S S U S E UIThOUT 3LOU O J T P I P E

F I G J R E 9

E F F E C T OF BLOW O U T P I P E S ON R E D U C E D E X P L O S I O N P R E S S U R E

Pred I N V E S S E L S T O BE P R O T E C T E D

F I G U R E 10

F O I L C O V E R

I

A C L A M P E D G:SC

c

F I G i l R E 1 1

C O V E R I N G BY C L A M P I N G DISCS

t- .,- L

(3

W L

0 J

m

- w

30

m

20

1 0

D U S T EXP. CLFlSS S t l

4 2 0 0 b a r . m . s - l K S t

0 ' I 2 4 . 5 7

D U S T EXP. CLASS s t 2 200<KS,

(300 b a r . m . s - '

D U S T EXP. CLASS st3

> 5 0 0 b a r . m . s - ' K S t

H-SO 1"

2 4.5 7 2 4 . 5 m 7

SIL3 O I A f l E T E R 0

F I G U R E 1 2

PERMISSIBLE SILO HEIGHT WHEN USING NOMOGRAPHS

Pstat - 1 . 1 b a r AND P,,d - 1 .2 b a r

DUST EXP. OUST EXP. GUST EXP. C L A S S S t l C L A S S s t 2 C L A S S s t 3

(200 b a r . m . s-' ( 3 0 0 b a r rn. s-' >300 9or.rn.s ' - 3

K S t 2 0 0 < K S + % S t

0 ' I I I 2 4 . 5 7 2 4 .5 7 2 4 . 5 7 7

SILO O I A M E T E R 3

F I G L I R E I 3

P E R M I S S I B L E SILO HEIGHT WHEN USING N O M O G R A P H S

P S t a t - 1 , I b a r A N D P,.d-l .4 b a r

FIGURE 16 - COUNTERWEIGHTED EXPLOSION COVER

S C H E M O T I C

F I G U R E 1 7

F I N A L S P R I N G L O A D E D R E L I E F V A L V E

P E R F O R A T E 0 H A L F R I M

DUST S U P P L Y \ V E S S E L \

1 I G N I T I O N SOUSCE

II P R E S S U R E SENSCR 3 V

\ E x : : sns

F I G U R E 1 8

T E S T UNIT TO D E T E R M I N E D U S T EXPLOSIVENESS

10

a X W

II 3 S

X 0

0

E

Q -

W

300 b a r / s

- a a w

n s 3 3 m rv, - w x n

S a a

H E A V Y TilRBULENCE TURBULENCE

I

00

0 0 0.5 1 .o S 1.5

I G N I T I O N D E L A Y t v

F I G U R E 19

E F F E C T OF T U R B U L E N C E O N D U S T E X P L O S I O N C H A R A C T E R I S T I C S

- r PRESSUPE SENSCR

PRESSURE V E S S ( "HQRTMFINN P I P

ELECTRODES

ST SAMPLE

V C L V E

'EL E " I

@) 'b , I I , Fi

C 3 U P Q E S S E 3 A i i?

c 3 ~ : p Q P 7 r E N T

F I G U R E 2 0

C L O S E D H A R T M A N N - U N I T

F I G U R E 21 SPARE U N I T

M O D I F I E D H A R T M A N N - U N I T

A P P E N D I X C

LITERATURE

1.

2 .

3 .

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

D r . Jay Warshawsky, A lan K r e i s b e r g ( F u l l e r Company) "Coal F i r i n g o f P r e c a l c i n e r K i l n s "

D r . von Seebach, N. P. Weyinont, M. D u r r ( P o l y s i u s Company) "Some Design C r i t e r i a f o r Coal F i r i n g Systems f o r Cement K i l n s "

R. L. Musto "Coal F i r i n g of Cement Kilns"

F. Dobrowsky " P u l v e r i z e d Coal and S u b s t i t u t e Fue ls f o r t h e Cement I n d u s t r y "

L u t z Schne ider (K rupp-Po lys ius ) "A i r swep t B a l l M i l l s and R o l l e r M i l l s as V i a b l e A l t e r n a t i v e s f o r Coal G r i nd i ng I'

John Mann, and M. von Seebach ( P o l y s i u s Corp.) "Recent Developments u s i n g Low Grade Fuel f o r Pyro-Process ing of C emen t 'I

Char les W. Bush ( K a i s e r Eng ineers ) R. J. Kreke l R. J. Schmidt (Combustion Eng.) " I n d i r e c t Coal F i r i n g - The Way t o Go?"

Ann G. K i m " L a b o r a t o r y S tud ies on Spontaneous Hea t ing o f Coa l " (Bureau o f M i nes)

J. M. Kuchta, V . R . Rowe, D. S. Burgess "Spontaneous Combustion S u s c e p t i b i l i t y o f U.S. Coa ls " U.S. Bureau o f Mines Repor t 8474

R. S. Con t i , K. L. Cashdo l l a r , I. Liebman, M. H e r t z b e r g "Thermal I g n i t i o n o f Dust Clouds"

M a r t i n Her t zbe rg , J. Kenneth Richmond, and Kenneth C a s h d o l l a r (Bureau o f Mines)

" F l a m m a b i l i t y L i m i t s and t h e Ex t i ngu ishmen t o f E x p l o s i o n s i n Gases, Dus ts and T h e i r M i x t u r e s "

M a r t i n Her t zbe rg , Kenneth L. Cashdo l l a r , and Char les P. Lazza ra "The L i m i t s o f F l a m m a b i l i t y o f P u l v e r i z e d Coals and Other Dusts"

P i t t s b u r g h Research Center, U.S. Bureau o f Mines " L i m i t s o f F l a m m a b i l i t y o f P u l v e r i z e d Coa ls "

14. National F i r e Protect ion Association Pub. No. 68 NExpansion Venting" Pub . No. 70 "National E l e c t r i c a l Code" Pub . No. 85F "Pulverized Fuel Systems"

15. J . Nagy, H. Dorsett, J r . , A. Cooper "Explosibil i t y o f Carbonaceous Dusts." Bureau of Mines Report 6597

16. E . A . Scholl "Burning and Explosion Behavior of ?u lver ized Coal." ZKG 5/1981: 227-232 .

17. K. G . Fredenberg, K . von Wedel "Coal Grinding and Drying w i t h Ver t ica l Mill and I n e r t Gas C i r c u i t " ZKG 9/80: 446-551.

18. H. G . Dorset t , e t a1 "Laboratory Equipment and Test Procedures f o r Evaluating E x p l o s i b i l i t y o f Dusts." U.S. Bureau of Mines Report RI 5624.

19. W . R . Mihailovich, A. J. Kreisberg "Control l ing Coal Feed ga te t o Kiln and Flash Calciner ." IEEE CITC Conference, Vancouver, B C , May 1982.

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