Removal of Trace Contaminants from the Air

In the late 60's when standards for sulfur content in fuel or emission limits were being promulgated the Potomac. Electric. Power Company (PEPCO) revi...
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DONALD A. ERDMAN Downloaded by UNIV OF MISSOURI COLUMBIA on October 26, 2017 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0017.ch009

Potomac Electric Power Co., 1900 Pennsylvania Ave., Washington, D. C. 20006

In the late 60's when standards for sulfur content in fuel or emission limits were being promulgated the Potomac Electric Power Company (PEPCO) reviewed available fuel supplies and reached the conclusion that sufficient low sulfur fuel was not available to provide a reliable s u p p l y . PEPCO presented this position to the State of Maryland where our three largest base load stations are located. PEPCO stated that we would actively pursue stack gas desulfurization if we could continue to burn higher sulfur fuel w h i l e developing desulfurization. Maryland accepted this proposal. We reviewed the various concepts for stack gas desulfuri­ zation and concluded that if at all possible the selected p r o ­ cess should be a recovery system. This was not based so much on anticipating a profit from the sale of elemental sulfur or sulfuric acid, as on the lack of sufficient disposal areas near our plant sites to dispose of the wastes from a throw-away s y s ­ tem. For instance, a lime throw-away system on 4 units at Dickerson would generate about 800,000 tons of sludge per y e a r . A recovery system will produce about 180,000 tons of sulfuric acid per year. If a large percentage of power plants elect to produce sulfuric acid then disposal of acid can become a prob­ lem. In agreement with the State, our first venture was a joint undertaking with Baltimore Gas & Electric i n a Wellman-Lord process to be installed at B G & E's Crane Station. This p r o c ­ ess went into operation in April, 1969 and by early 1970 it was determined that the process as then constituted was not suita­ ble for our use on a coal fired power boiler. This was not the same process as the one currently being promoted by Wellman-Lord which has demonstrated success on oil fired units. In late 1970 PEPCO decided to proceed w i t h the installation of a Chemico-Basic Mag-Ox scrubber on our Dickerson Unit No. 3. In 1971 the State held hearings on our delay in scrubber instal­ lation. Our participation with Baltimore Gas & Electric was reviewed. As a result of these hearings Maryland, in September, 106

Deitz; Removal of Trace Contaminants from the Air ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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1971, i s s u e d a c o m p l i a n c e o r d e r t h a t a l l o w e d u s t o b u r n f u e l w i t h up t o 2-1/4% s u l f u r p r o v i d e d we p r o c e e d e d w i t h t h e C h e m i c o B a s i c i n s t a l l a t i o n w i t h a t a r g e t i n - s e r v i c e date o f June, 1973. A c t u a l s t a r t - u p was September 1 3 , 1 9 7 3 , j u s t o v e r t h r e e months late. T h i s d e l a y was a t t r i b u t e d i n p a r t t o d e s i g n changes b a s e d on C h e m i c o s e x p e r i e n c e a t B o s t o n E d i s o n w i t h a s i m i l a r u n i t o n an o i l f i r e d b o i l e r and o n d e l a y s i n e q u i p m e n t d e l i v e r y . The s y s t e m c o n s i s t s o f a 2 - s t a g e s c r u b b e r i n s t a l l e d on o u r U n i t 3 and s i z e d t o t a k e h a l f t h e f l u e g a s - 295,000 ACFM - equi v a l e n t t o a p p r o x i m a t e l y 100 MW. The f i r s t s t a g e o f t h e s c r u b b i s f o r p a r t i c u l a t e r e m o v a l and t h e s e c o n d s t a g e f o r s u l f u r o x i d e removal. The s y s t e m i s d e s i g n e d t o t a k e f l u e g a s e i t h e r ahead o f o r a f t e r t h e e x i s t i n g e l e c t r o s t a t i c p r e c i p i t a t o r s . The s c r u b b i n g system i s i n p a r a l l e l w i t h t h e e x i s t i n g p l a n t I.D.fans, w i t h a wet I.D. f a n f o l l o w i n g t h e s c r u b b e r w i t h t h i s a r r a n g e m e n t l o s s o f t h e s c r u b b e r s y s t e m does n o t c a u s e a n y l o s s o f l o a d . The f l u e gas e n t e r s t h e f i r s t s t a g e s c r u b b e r where i t i s s a t u r a t e d and c o o l e d f r o m 250 F t o 120 F. A b l e e d s t r e a m f r o m the f i r s t stage r e c y c l e f l o w c a r r i e s f l y a s h over t o t h e t h i c k e n e r s f o r u l t i m a t e d i s p o s a l t o a s e t t l i n g pond. The c l e a n e d and s a t u r a t e d f l u e gas t h e n e n t e r s t h e a b s o r b e r t h r o u g h a f i x e d t h r o a t v e n t u r i where i t i s c o n t a c t e d w i t h t h e r e c y l i n g MgO s o l ution. The g a s t h e n goes t o t h e wet I . D. f a n , t h e n t h r o u g h m i s t e l i m i n a t o r s t o t h e s t a c k where i t m i x e s w i t h t h e u n s c r u b b ed g a s f r o m t h e o t h e r h a l f o f U n i t 3. As magnesium s u l f i t e c r y s t a l s b u i l d up i n t h e s l u r r y s y s t e m a b l e e d s t r e a m i s s e n t t o the c e n t r i f u g e . The c e n t r i f u g e c a k e i s s e n t t h r o u g h t h e d r y e r and t h e d r y magnesium s u l f i t e s t o r e d i n a s i l o f o r u l t i m a t e shipping t o the reprocessing plant. The m o t h e r l i q u o r f r o m t h e c e n t r i f u g e i s s e n t b a c k t o t h e a b s o r b e r and t h i s l i q u o r i s a l s o u s e d t o s l a k e t h e MgO. F r o m September 13 t o J a n u a r y 14 t h e s c r u b b e r was a v a i l a b l e f o r about 27% o f t h e time v e r s u s a b o i l e r a v a i l a b i l i t y i n exc e s s o f 9 5 % . The p r o b l e m s d u r i n g t h i s i n i t i a l o p e r a t i o n were v a r i o u s , c o r r o s i o n l e a k s due t o n o n - s p e c i f i e d m a t e r i a l i n e x pansion j o i n t s , bearing f a i l u r e s i n the s u l f i t e handling e q u i p m e n t , e t c . The m a j o r p r o b l e m t h o u g h was i n s l a k i n g t h e MgO. T h e r e were c o n t i n u a l p r o b l e m s w i t h p l u g g i n g i n t h e MgO m i x t a n k and s u c t i o n l i n e s t o t h e MgO make-up pumps. I n J a n u a r y t h e b o i l e r was t a k e n o u t o f s e r v i c e f o r i t s annual maintenance. D u r i n g t h i s outage a thorough i n s p e c t i o n o f t h e s y s t e m was made a f t e r some 700 h o u r s o p e r a t i o n . B a s i c a l l y , t h e s y s t e m was i n good s h a p e . T h e r e was a b s o l u t e l y n o s c a l i n g o r b u i l d - u p i n e i t h e r s c r u b b i n g s t a g e o r on t h e I.D. f a n o r d e m i s t e r s . However, i n t h e f i r s t s t a g e where t h e o p e r a t i n g pH i s l e s s t h a n t w o , we h a d c o r r o s i o n o f n u t s , b o l t s , h a n g e r r o d s , b e l l o w s , and t h e v e s s e l i t s e l f . I t was d e t e r m i n e d t h a t t h e c o r r o d e d p a r t s were o f n o n - s p e c i f i e d m a t e r i a l . T h i s emphasizes t h e need t o m a i n t a i n q u a l i t y c o n t r o l o v e r t h e i n s t a l l a t i o n t o a s c e r t a i n t h a t when a s p e c i f i c g r a d e o f s t a i n l e s s s t e e l i s

Downloaded by UNIV OF MISSOURI COLUMBIA on October 26, 2017 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0017.ch009

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Deitz; Removal of Trace Contaminants from the Air ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

Downloaded by UNIV OF MISSOURI COLUMBIA on October 26, 2017 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0017.ch009

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s p e c i f i e d t h a t t h a t grade i s i n f a c t i n s t a l l e d . The c o r r o s i o n o f t h e v e s s e l o c c u r r e d where t h e p r o t e c t i v e f l a k e g l a s s l i n i n g was p e n e t r a t e d . The p r o b l e m h e r e was i n some p a r t due t o i m p r o p e r a p p l i c a t i o n a n d , i n p a r t , due t o c o n s t r u c t i o n damage a f t e r t h e l i n i n g was i n s t a l l e d . T h i s was a v e r y s m a l l p e r c e n t a g e o f the t o t a l f l a k e g l a s s l i n i n g . D u r i n g t h i s o u t a g e Chemico m o d i f i e d t h e MgO f e e d s y s t e m w i t h t h e a d d i t i o n o f a p r e - m i x t a n k . The s y s t e m was f i n a l l y r e a d y f o r s e r v i c e and s t a r t e d up on A p r i l 15. The i n t e n t was t o o p e r a t e t o v e r i f y t h e m o d i f i c a t i o n s and t h e n s h u t down t o s e t up and o p e r a t e f o r p e r f o r m a n c e t e s t i n g . The p r e - m i x t a n k i m p r o v e d s l a k i n g b u t n o t t o an a c c e p t a b l e s t a n d a r d f o r l o n g - t e r m o p e r a t i o n . We d e c i d e d t h e end o f A p r i l t h a t t h e s y s t e m c o u l d be o p e r a t e d f o r p e r f o r m a n c e t e s t i n g . We s h u t down, c h e c k e d i n v e n t o r y o f MgO and s t o r a g e s p a c e i n t h e magnesium s u l f i t e s i l o , and f o u n d we d i d n o t h a v e enough MgO t o r u n t h e t e s t n o r s t o r a g e s p a c e f o r sulfite. B o s t o n E d i s o n was u s i n g t h e r e p r o c e s s i n g p l a n t w h i c h i s l o c a t e d a t an E s s e x s u l f u r i c a c i d p l a n t i n R u m f o r d , Rhode I s l a n d . PEPCO w o u l d n o t h a v e a c c e s s t o t h i s f a c i l i t y u n t i l J u l y 1, 197^·. I n c o n j u n c t i o n w i t h C h e m i c o , B a s i c , E s s e x and EPA, we a r e embarked on a s i x - m o n t h t e s t and d e m o n s t r a t i o n p e r i o d t o r u n f r o m J u l y t o December, 1974. Due t o l a c k o f MgO we d i d n o t o p e r a t e in July. Chemico d e c i d e d t o t r y and i m p r o v e t h e MgO s l a k i n g by r e p l a c i n g the pre-mix tank w i t h a " s o l i d s l i q u i d m i x i n g e d u c t o r . " T h i s e d u c t o r was u n s a t i s f a c t o r y and a f t e r one d a y ' s use was r e p l a c e d w i t h t h e p r e - m i x t a n k . The p r e - m i x t a n k had b e e n m o d i f i e d by r e l o c a t i n g b a f f l e s t o i m p r o v e a g i t a t i o n . Operation w i t h t h e s e m o d i f i c a t i o n s h a s been s a t i s f a c t o r y . V i r g i n MgO was - r e c e i v e d and o p e r a t i o n resumed A u g u s t 1 s t . P r e l i m i n a r y t e s t s i n d i c a t e d an S O 2 r e m o v a l e f f i c i e n c y i n t h e 707 r a n g e . Chemico f e l t t h a t t h e p r e s s u r e d r o p a c r o s s t h e a b s o r b e r w o u l d h a v e t o be i n c r e a s e d t o o b t a i n t h e d e s i g n 907 r e m o v a l . These m o d i f i c a t i o n s were made and t h e i n d i c a t e d r e m o v a l e f f i c i e n c y on v i r g i n MgO i s i n e x c e s s o f 907 . Our f i r s t r e c y c l e MgO was r e c e i v e d and i n t r o d u c e d i n t o t h e s y s t e m on A u g u s t 16. T h i s n e c e s s i s t a t e d some c h a n g e s i n o p e r a t i n g p r o c e d u r e s , s u c h as t e m p e r a t u r e i n t h e MgO t a n k and d r y e r o p e r a t i n g t e m p e r a t u r e . We a r e i n t o t h e t e s t p r o gram and a l t h o u g h r e s u l t s a r e n o t c u r r e n t l y a v a i l a b l e , i t l o o k s encouraging. Where does PEPCO s t a n d a f t e r a l m o s t s i x y e a r s and a p p r o x i m a t e l y $9,000,000? L a s t J a n u a r y when we a g r e e d t o g i v e t h i s p a p e r we b e l i e v e d we w o u l d h a v e more o f t h e a n s w e r s by now. Due t o d e l a y s , one o f t h e m a i n d e l a y s b e i n g a c c e s s t o t h e r e p r o c e s s i n g f a c i l i t y , we a r e c u r r e n t l y i n t h e m i d d l e o f t h e p e r f o r m a n c e o p t i m i z a t i o n and r e l i a b i l i t y t e s t i n g . The s y s t e m l o o k s p r o m i s i n g . To d a t e , t h e r e has been no p r o b l e m t o r u l e out t h e t e c h n i c a l f e a s i b i l i t y o f t h e MgO p r o c e s s . The a r e a t h a t s t i l l n e e d s v e r i f i c a t i o n i s t h e c y c l i n g o f MgO t h r o u g h t h e s c r u b b e r and calciner for several cycles. T h i s i s necessary to assure t h a t the o

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Deitz; Removal of Trace Contaminants from the Air ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

Downloaded by UNIV OF MISSOURI COLUMBIA on October 26, 2017 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0017.ch009

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MgO c a n be r e u s e d and t h a t i m p u r i t i e s t h a t may b u i l d up from f l y a s h c a r r y o v e r do n o t i n t e r f e r e w i t h t h e p r o d u c t i o n o f a marke t a b l e s u l f u r i c a c i d . T h i s d a t a i s a l s o needed t o d e t e r m i n e the amount o f w a s t e s t r e a m s , i f any, and a l s o t o d e t e r m i n e s y s t e m economics. PEPCO has a C e r t i f i c a t e o f C o n v e n i e n c e and N e c e s s i t y f r o m t h e S t a t e o f M a r y l a n d t o i n s t a l l an 800 MW a d d i t i o n a t D i c k e r son. A c o n d i t i o n o f t h i s c e r t i f i c a t e r e q u i r e s s t a c k gas d e s u l f u r i z a t i o n on t h e e x i s t i n g t h r e e u n i t s and t h e new u n i t . The a l l o w a b l e F e d e r a l g r o u n d l e v e l c o n c e n t r a t i o n f o r S O 2 i s .030 PPM on an a n n u a l b a s i s o r .14 on a 2 4 - h o u r b a s i s . M a r y l a n d has t a k e n t h e p o s i t i o n t h a t one s o u r c e s h o u l d n o t be a l l o w e d t o c o n t r i b u t e up t o t h e maximum. They h a v e t h e r e f o r e l i m i t e d o u r c o n t r i b u t i o n t o o n e - h a l f the a l l o w a b l e ground l e v e l c o n c e n t r a t i o n , o r a p p r o x i m a t e l y o n e - f o u r t h t h e EPA h e a l t h r e l a t e d s t a n d a r d . I m i g h t add h e r e t h a t as p a r t o f o u r c o m p l i a n c e o r d e r , we a r e r e q u i r e d by t h e S t a t e t o m a i n t a i n a t e a c h p l a n t a s e v e n d a y s s u p p l y o f f u e l c o n t a i n i n g l e s s t h a n 1% S. T h i s f u e l i s t o be u s e d , when o r d e r e d by t h e S t a t e , i n t h e e v e n t o f a r e a p o l l u t i o n p r o b l e m s . The S t a t e has n o t f o u n d i t n e c e s s a r y t o o r d e r us t o s w i t c h t o t h e low S f u e l . T h i s seems t o i n d i c a t e t h a t SO^ i s not a major c o n t r i b u t o r t o p o l l u t i o n i n our area. We do n o t a t t h i s t i m e h a v e a d e f i n i t i v e e s t i m a t e o f t h e cost to the customer of meeting these requirements. However, o u r r o u g h e s t i m a t e s a r e t h a t i f we i n s t a l l e d s c r u b b e r s a t o u r t h r e e M a r y l a n d p l a n t s , i t c o u l d r e s u l t i n a b o u t a 20% i n c r e a s e i n the c o s t of e l e c t r i c i t y . One o f t h e b i g d e b a t e s i s a r e s c r u b b e r s r e l i a b l e ? The t h r e e g e n e r a t i n g u n i t s a t D i c k e r s o n h a v e d e m o n s t r a t e d an a v a i l a b i l i t y o f 90%, o r b e t t e r , s i n c e t h e i r i n s t a l l a t i o n . A scrubber s y s t e m s h o u l d be c a p a b l e o f m a t c h i n g b o i l e r a v a i l a b i l i t y . The a v a i l a b i l i t y o f t h e D i c k e r s o n s c r u b b e r c o n t i n u e s t o show i m p r o vement. I t was 4 3 . 5 % f o r t h e month o f A u g u s t and f r o m A u g u s t 13 t o t h e end o f t h e month t h e a v a i l a b i l i t y was 57%. A u g u s t 13 i s s e l e c t e d as t h a t was t h e s t a r t - u p d a t e a f t e r t h e l a s t m o d i f i c a t i o n - i n c r e a s i n g pressure drop across the absorber t h r o a t . PEPCO c o n t i n u e s t o a n a l y z e t h e s t a t u s o f v a r i o u s s t a c k gas d e s u l f u r i z a t i o n s y s t e m s , t h e a v a i l a b i l i t y o f low s u l f u r f u e l and any o t h e r method f o r m e e t i n g o u r o b i l i g a t i o n t o p r o d u c e r e l i a b l e power w i t h a m i n i m a l a d v e r s e i m p a c t on t h e e n v i r o n m e n t . 1

Deitz; Removal of Trace Contaminants from the Air ACS Symposium Series; American Chemical Society: Washington, DC, 1975.