Limestone Test Results at the EPA Alkali

13 Adipic Acid-Enhanced Lime/Limestone Test Results at the EPA Alkali Scrubbing Test Facility SHIH-CHUNG WANG and DEWEY A. BURBANK Bechtel Group, Inc., San Francisco, CA 94119

This paper summarizes the results of tests conducted from July 1978 through March 1981 at the EPA, 10-MW equivalent, lime/limestone wet-scrubbing FGD test facility, during which adipic acid as an additive was tested and shown to be a powerful scrubber additive for improving SO removal. The optimum concentration of adipic acid is only 700 to 1500 ppm at a scrubber inlet pH of 5.2 or higher. SO removal efficiencies in excess of 90 percent and reliable operation were demonstrated in four long term, limestone/adipic acid runs. Factorial tests were also conducted to characterize SO removal as a function of gas and slurry flow rates, pH, and adipic acid concentration. Intermediate duration optimization runs and favorable economics are also reported. 2

2

2

Introduction and Background This report describes the results of the Shawnee Lime and Limestone Wet Scrubbing Test Program conducted by EPA's Industrial Environmental Research Laboratory, Research Triangle Park, North Carolina (IERL-RTP). In this program, flue gas desulfurization (FGD) tests were conducted at the EPA 10 MW prototype Shawnee Test Facility located at the Tennessee Valley Authority (TVA) coalfired Shawnee Power Station near Paducah, Kentucky. Bechtel Group, Inc. of San Francisco was the major contractor and test director, and TVA was the constructor and facility operator. Results of the program before July 1978 have been reported elsewhere (1,2) . This report describes the results of adipic acid-enhanced lime and limestone testing at the Shawnee Test Facility from July 1978 through March 1981. It also summarizes earlier adipic acid additive test results from the IERL-RTP 0.1 MW pilot plant, which led to the testing at Shawnee. Also reported are preliminary results from the 100 MW full-scale demonstration being 0097-6156/82/018 8-0267$ 11.00/0 © 1982 American Chemical Society

268

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conducted at the Southwest Power P l a n t of S p r i n g f i e l d C i t y U t i l i t i e s , S p r i n g f i e l d , M i s s o u r i and from the 27 MW equivalent indust r i a l b o i l e r t e s t at Rickenbacker A i r Force Base. As the emission standards f o r s u l f u r d i o x i d e become i n c r e a s i n g l y s t r i n g e n t and the f o s s i l - f u e l e d u t i l i t i e s become hard pressed to meet these standards, there i s a strong i n c e n t i v e to improve the FGD system performance while minimizing the o p e r a t i n g costs. A primary o b j e c t i v e of the EPA a l k a l i wet scrubbing t e s t program during the l a s t s e v e r a l years has been to enhance SO2 removal and improve the r e l i a b i l i t y and economics of lime and limestone wet scrubbing systems by use of a d i p i c a c i d as a chemical additive. A d i p i c a c i d i s a six-carbon d i c a r b o x y l i c organic a c i d , H00C(CH2)4C00H, that b u f f e r s the pH i n the scrubber s l u r r y . In theory, any a c i d which i s intermediate i n s t r e n g t h between c a r bonic a c i d and s u l f u r o u s a c i d , and whose calcium s a l t i s reasonably s o l u b l e , may be employed. However, a d i p i c a c i d was s e l e c t e d because i t i s one of the most c o s t - e f f e c t i v e organic a c i d b u f f e r s on a molar b a s i s and i s commercially abundant. I t s main use i s as a raw m a t e r i a l i n the manufacturing of nylon. The b u f f e r i n g a c t i v i t y of a d i p i c a c i d l i m i t s the drop i n pH that normally occurs at the g a s - l i q u i d i n t e r f a c e during SO2 absorption, and the r e s u l t a n t higher c o n c e n t r a t i o n of SO2 at the i n t e r f a c e s i g n i f i c a n t l y a c c e l e r a t e s the l i q u i d - p h a s e mass t r a n s fer. The c a p a c i t y of the bulk l i q u o r f o r r e a c t i o n w i t h SO2 i s a l s o increased by the presence of calcium adipate i n s o l u t i o n . Thus, the SO2 absorption becomes l e s s dependent on the d i s s o l u t i o n r a t e of limestone or calcium s u l f i t e i n the absorber to provide the necessary a l k a l i n i t y . In the case of limestone scrubbing, i t l o g i c a l l y f o l l o w s that a given SO2 removal e f f i c i e n c y can be achieved at a lower limestone s t o i c h i o m e t r y , thereby improving scrubber r e l i a b i l i t y . A d i p i c a c i d degrades to lower molecular weight (C1-C5) monoc a r b o x y l i c a c i d s , p a r a f f i n i e hydrocarbons, and carbon d i o x i d e and water. The feed r a t e of a d i p i c a c i d v a r i e s depending on the opera t i n g c o n d i t i o n s , such as pH and o x i d a t i o n , which i n f l u e n c e the degree of degradation, but i s u s u a l l y l e s s than f i v e times the t h e o r e t i c a l requirement. Most of the c a r b o x y l i c a c i d degradation products, such as v a l e r i c a c i d , s t i l l o f f e r a proper pH b u f f e r range f o r e f f e c t i v e SO2 removal enhancement. At Shawnee, a l l reported concentrations of a d i p i c a c i d ( i n ppm) are, i n r e a l i t y , the concentrations of t o t a l c a r b o x y l i c a c i d expressed i n terms of "adipic acid." At the Shawnee Test F a c i l i t y , major emphasis has been placed on the use of a d i p i c a c i d i n conjunction w i t h f o r c e d o x i d a t i o n of calcium s u l f i t e to calcium s u l f a t e , s i n c e t h i s system r e s u l t s i n b e t t e r sludge dewatering p r o p e r t i e s and reduced waste s o l i d s d i s posal c o s t s . Furthermore, the more t i g h t l y c l o s e d l i q u o r loop,

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achievable as a r e s u l t o f forced o x i d a t i o n , reduces the a d i p i c a c i d makeup requirements. Advantages o f A d i p i c A c i d as a Scrubber A d d i t i v e A number of a t t r a c t i v e features of a d i p i c a c i d as a scrubber a d d i t i v e are presented below. Handling. A d i p i c a c i d i s non-toxic, non-hygroscopic, and u s u a l l y comes i n powder form. I t i s easy to handle and no hazards are encountered i n the usual a p p l i c a t i o n s other than p o s s i b l e dust explosions, which are t y p i c a l of any organic dust. At Shawnee, i t i s r o u t i n e l y dry-fed d i r e c t l y to the e f f l u e n t hold tank, a l though i t has been added to the f r e s h limestone s l u r r y makeup tank i n some i n s t a n c e s . B u f f e r Reaction Mechanism. The mechanism by which a d i p i c a c i d b u f f e r s the pH i s simple. I t r e a c t s w i t h lime or limestone i n the e f f l u e n t hold tank to form calcium adipate. In the absorber, calcium adipate r e a c t s w i t h absorbed S02(H2S03> to form CaS03 and simultaneously regenerates a d i p i c a c i d (the b u f f e r r e a c t i o n ) . The regenerated a d i p i c a c i d i s returned to the e f f l u ent hold tank f o r f u r t h e r r e a c t i o n w i t h lime o r limestone. With a s u f f i c i e n t l y high c o n c e n t r a t i o n of calcium adipate i n s o l u t i o n , u s u a l l y on the order of 10 m-moles/liter t o r e a c t w i t h the absorbed S02, the o v e r a l l r e a c t i o n r a t e i s no longer c o n t r o l l e d by the d i s s o l u t i o n r a t e of limestone o r calcium s u l f i t e . R e t r o f i t . Use of a d i p i c a c i d i n an e x i s t i n g lime or l i m e stone system does not r e q u i r e m o d i f i c a t i o n o f process flow conf i g u r a t i o n o r absorber design; t h e r e f o r e , i t i s p a r t i c u l a r l y s u i t e d f o r r e t r o f i t a p p l i c a t i o n s . The f a c t that i t may be added at any p o i n t i n the s l u r r y c i r c u i t provides a greater f l e x i b i l i t y i n the l o c a t i o n and i n s t a l l a t i o n o f a simple s o l i d s storage and feed system, a minimal c a p i t a l investment. Quantity and Concentration. Depending on the operating parameters, the degree o f degradation, and the t i g h t n e s s of the l i q u o r loop, the q u a n t i t y of a d i p i c a c i d r e q u i r e d i s q u i t e small i n r e l a t i o n t o the a l k a l i feed. At Shawnee, where a f i l t e r i s normally used as the f i n a l sludge dewatering device, the a d i p i c a c i d consumption r a t e i s u s u a l l y l e s s than 10 l b / t o n o f limestone fed to the system, and sometimes as low as 2 l b / t o n o f limestone. These values correspond t o only 0.6 t o 3.0 tons o f a d i p i c a c i d per day f o r a 500 MW p l a n t . A d i p i c a c i d has two pH b u f f e r p o i n t s . These are pH 4.5 and 5.5 i n the absence of c h l o r i d e i n the l i q u o r , and about 4 and 5 w i t h 5,000 to 7,000 ppm c h l o r i d e . To f u l l y u t i l i z e the b u f f e r c a p a c i t y o f a d i p i c a c i d , t h e r e f o r e , the s l u r r y pH should be kept above these v a l u e s . A t Shawnee, where the c h l o r i d e c o n c e n t r a t i o n

FLUE

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i s u s u a l l y a few thousand ppm, a s l u r r y pH above about 5.2 i s s u f f i c i e n t to keep a d i p i c a c i d f u l l y a c t i v e (or i o n i z e d ) . The optimum c o n c e n t r a t i o n range of a d i p i c a c i d at a pH above 5.2 i s only 700 to 1,500 ppm f o r 90 percent removal of approximately 2,500 ppm i n l e t S02. Higher concentrations would be r e q u i r e d at a lower pH to maintain equivalent b u f f e r c a p a c i t y i n the l i q u i d . I t should be noted that most of the degradation products of a d i p i c a c i d , such as v a l e r i c and g l u t a r i c a c i d , are a l s o e f f e c t i v e buffers. Limestone U t i l i z a t i o n . At a scrubber i n l e t pH of about 5.2, the corresponding limestone u t i l i z a t i o n i s normally 80 percent or higher f o r an a d i p i c acid-enhanced system, as compared to 65 to 70 percent i n unenhanced limestone systems at an equivalent SO2 removal. Thus the quantity of waste s o l i d s generated i s reduced i n an a d i p i c acid-enhanced system. Higher limestone u t i l i z a t i o n a l s o c o n t r i b u t e s to more r e l i a b l e scrubber o p e r a t i o n by reducing the f o u l i n g tendency. T h i s increased r e l i a b i l i t y i s a very a t t r a c t i v e f e a t u r e of a d i p i c acid-enhanced systems, s i n c e r e l i a b i l i t y problems have h i s t o r i c a l l y plagued limestone FGD. Operating pH. With proper pH c o n t r o l and s u f f i c i e n t l y high a d i p i c a c i d c o n c e n t r a t i o n ( s u f f i c i e n t b u f f e r c a p a c i t y ) , the scrubber performance i s more s t a b l e , and steady o u t l e t SO2 conc e n t r a t i o n s can be maintained, even with wide f l u c t u a t i o n s of i n l e t SO2 c o n c e n t r a t i o n s . With the lower o p e r a t i n g pH (about 4.6 to 5.4) i n an a d i p i c acid-enhanced limestone system, compared to the higher pH (about 5.5 to 5.8) u s u a l l y needed f o r an unenhanced limestone system, the system becomes more amenable to other process concepts and improvements. P o t e n t i a l advantages of low pH o p e r a t i o n are: •

•

•

•

• • •

Reduced a d i p i c a c i d consumption. A d i p i c a c i d degradation has been found to decrease w i t h decreasing pH E a s i e r f o r c e d o x i d a t i o n i n the scrubber s l u r r y loop or bleed stream, and a smaller a i r (and compressor energy) requirement P o t e n t i a l f o r e s s e n t i a l l y complete limestone u t i l i z a t i o n w i t h improved scrubber o p e r a t i n g reliability Reduced s e n s i t i v i t y of the system to l i m e stone type and g r i n d . F i n e g r i n d i n g of limestone i s probably not r e q u i r e d Lower s u l f i t e s c a l i n g p o t e n t i a l B e t t e r prospects ( s e n s i t i v i t y ) f o r automatic pH c o n t r o l Greater f l e x i b i l i t y f o r SO2 emission c o n t r o l . Higher s e n s i t i v i t y of SO2 removal at lower pH

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allows r a i s i n g pH to increase the a d i p i c a c i d b u f f e r c a p a c i t y and SO2 removal when needed A p p l i c a b i l i t y to l o w - s u l f u r subbituminous and l i g n i t e c o a l s c o n t a i n i n g a l k a l i n e ashes, which are e x t r a c t a b l e only a t low pH Lower costs due to a l l of the above f a c t o r s

Economics. Since limestone d i s s o l u t i o n i s not a r a t e c o n t r o l l i n g step i n SO2 absorption f o r an a d i p i c acid-enhanced limestone system, a d i p i c a c i d should promote use of l e s s expensive and l e s s energy-intensive limestone r a t h e r than lime. A d i p i c acid-enhanced limestone scrubbing has lower p r o j e c t e d c a p i t a l and operating costs than unenhanced limestone or MgOenhanced limestone scrubbing. T h i s i s due p r i m a r i l y to the reduced limestone consumption a t the lower o p e r a t i n g pH, the reduced g r i n d i n g c o s t , and the reduced quantity of waste sludge generated. Forced O x i d a t i o n . The mechanism by which a d i p i c a c i d promotes S02 removal i s not a f f e c t e d by forced o x i d a t i o n . Therefore, i t can be used w i t h both lime and limestone i n systems w i t h or without forced o x i d a t i o n . Since f o r c e d o x i d a t i o n converts s u l f i t e to s u l f a t e , i t has an adverse e f f e c t on SO2 removal i n an unenhanced lime system i n which s u l f i t e i s the major SO2 scrubbing s p e c i e s . T h i s i s a l s o true i n MgO-enhanced lime and limestone systems i n which the promotion of SO2 removal r e l i e s on an increased s u l f i t e - b i s u l f i t e b u f f e r . When a d i p i c a c i d i s used with lime, calcium adipate becomes a major b u f f e r species; t h e r e f o r e , both good S0 removal and s u l f i t e o x i d a t i o n can be achieved using within-scrubber-loop forced o x i d a t i o n . 2

C h l o r i d e E f f e c t . The e f f e c t i v e n e s s of a d i p i c a c i d i s not adversely a f f e c t e d by c h l o r i d e s , as i s the e f f e c t i v e n e s s of MgO i n an MgO-enhanced process. Tests a t the IERL-RTP p i l o t p l a n t showed that SO2 removal e f f i c i e n c y obtained w i t h 17,000 ppm c h l o r i d e i n the scrubbing l i q u o r was not s i g n i f i c a n t l y d i f f e r e n t from that obtained without c h l o r i d e under s i m i l a r l e v e l s of a d i p i c a c i d c o n c e n t r a t i o n . Thus, use o f a d i p i c a c i d i s e s p e c i a l l y a t t r a c t i v e f o r systems w i t h a very t i g h t l y c l o s e d l i q u o r loop. S o l i d s Dewatering. A d i p i c a c i d does not s i g n i f i c a n t l y a f f e c t the s e t t l i n g and f i l t r a t i o n p r o p e r t i e s o f o x i d i z e d or unoxidized s l u r r y s o l i d s , whereas magnesium does. T o t a l D i s s o l v e d S o l i d s . A d d i t i o n o f a d i p i c a c i d does n o t s i g n i f i c a n t l y i n c r e a s e the t o t a l d i s s o l v e d s o l i d s i n l i q u i d as does magnesium. High t o t a l d i s s o l v e d s o l i d s i n l i q u i d entrainment can increase p a r t i c u l a t e emissions and f o u l i n g tendencies of equipment downstream o f the scrubber.

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Test Programs The t h e o r e t i c a l b a s i s f o r the e f f e c t of a d i p i c a c i d on the performance of lime and limestone scrubbers was f i r s t developed i n d e t a i l by G. R o c h e l l e i n 1977 (3). In October 1977, EPA began an i n v e s t i g a t i o n of a d i p i c a c i d w i t h the 0.1 MW IERL-RTP p i l o t p l a n t to determine i t s e f f e c t i v e n e s s as an a d d i t i v e to limestone scrubbers f o r improving S02 removal e f f i c i e n c y (4). I n i t i a l r e s u l t s demonstrated, as p r e d i c t e d by R o c h e l l e , that a d i p i c a c i d was indeed an a t t r a c t i v e and powerful a d d i t i v e . Based on the f i n d i n g s at the IERL-RTP p i l o t p l a n t , a program was s e t up at the 10 MW Shawnee Test F a c i l i t y to develop commerc i a l l y usable design data f o r a d i p i c a c i d as a chemical a d d i t i v e . A c t u a l t e s t i n g at Shawnee began i n J u l y 1978, and l a s t e d through March 1981. Some of the s i g n i f i c a n t t e s t r e s u l t s during the f i r s t two-thirds of t h i s p e r i o d have been presented elsewhere Ç5,j3,^7) . The t e s t schedule f o r t h i s p e r i o d i s shown i n F i g u r e 1. T e s t s were conducted over a p e r i o d of 33 months, using both lime and limestone w i t h and without f o r c e d o x i d a t i o n . As can be seen, major emphasis was placed on limestone t e s t i n g w i t h forced oxidation. As p a r t of EPA s c o n t i n u i n g program of FGD technology t r a n s f e r , and to f u r t h e r demonstrate the e f f e c t i v e n e s s of a d i p i c a c i d and to encourage i t s use, EPA contracted w i t h Radian C o r p o r a t i o n i n the s p r i n g of 1980 to conduct a f u l l - s c a l e demonstration program of a d i p i c acid-enhanced limestone scrubbing ( 8 ) . The program, being conducted w i t h two 100 MW Turbulent Contact Absorbers (TCA) l o c a t e d at the S p r i n g f i e l d C i t y U t i l i t i e s ' Southwest S t a t i o n near S p r i n g f i e l d , M i s s o u r i , w i l l continue through September 1981. Some p r e l i m i n a r y t e s t r e s u l t s are included i n t h i s r e p o r t , as are data from the i n d u s t r i a l - s i z e d (27 MW) scrubber t e s t conducted by PEDCo Environmental, Inc. on the Bahco system at Rickenbacker A i r Force Base. 1

During some f a c t o r i a l t e s t s conducted at the Shawnee Test F a c i l i t y i n 1979, i t was n o t i c e d that the r a t e of a d i p i c a c i d a d d i t i o n r e q u i r e d to maintain a d e s i r e d c o n c e n t r a t i o n i n the scrubber l i q u o r was s u b s t a n t i a l l y reduced when the scrubber i n l e t pH was c o n t r o l l e d at 5.0 or lower. In order to v e r i f y the Shawnee f i n d i n g s , the EPA i n i t i a t e d s e v e r a l programs to study the a d i p i c a c i d degradation phenomenon. C o n t r a c t o r s i n v o l v e d i n c l u d e d U n i v e r s i t y of Texas at A u s t i n , Radian C o r p o r a t i o n , Acurex Corporat i o n , and the Research T r i a n g l e I n s t i t u t e . Programs were set up to i n v e s t i g a t e the e f f e c t s of pH, o x i d a t i o n , and c a t a l y s t s such as manganese and i r o n on a d i p i c a c i d degradation, to develop a n a l y t i c a l procedures and to i d e n t i f y the degradation products. Although the a d i p i c a c i d degradation mechanism i s complex, the p r i n c i p a l v a r i a b l e s a f f e c t i n g degradation and i t s major products have been i d e n t i f i e d . The r e s u l t s of these s t u d i e s are beyond the scope of t h i s r e p o r t and w i l l be reported s e p a r a t e l y .

100)

200)

simulation)

Note:

J

A

S Ο Ν MM

Boiler N o 10 Outage. Train 200 • on Bo.ler No. 9

D J Α Μ

J J A

1979

Figure 1.

:

ι

S O

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Boiler N o 10 Outage Tram 100 on B o n e r No. 9

F M

DOWA P r o c e s s •·•'··'··'··: Demonstration

D J

Shawnee adipic acid test schedule.

ι

A l l tests d i e w i t h a d i p i c a c i d a n d h i g h f l y ash l o a d i n g u n l e s s o t h e r w i s e n o t e d .

no a d i p i G acid (Glitsch G r i d packing)

Limestone without forced oxidation,

(Springfield

L i m e s t o n e , l o w f l y ash l o a d i n g

no adipic acid

Limestone with forced o x i d a t i o n ,

Limestone factorial

Limestone with forced oxidation

Lime with forced oxidation

no adipic acid

Lime without forced oxidation,

Lime w i t h o u t forced oxidation

Limestone without forced oxidation

T C A System (Train


Limestone factorial

L i m e s t o n e factorial, no adipic acid

Spray Tower System (Train

Limestone without forced oxidation

Limestone factorial

100)

simulation)

Lime and limestone (Venturi only)

no adipic acid ( W i n d o w s Creek

Limestone with forced oxidation,


Lime with forced oxidation

Venturi/Spray Tower System (Train

A d i p i c A c i d Test B l o c k

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Removed Venturi from Train 100

J J A S O N

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to

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GAS D E S U L F U R I Z A T I O N

Shawnee Test F a c i l i t y . T e s t s w i t h a d i p i c a c i d a t Shawnee have been conducted on two p a r a l l e l scrubber systems: a v e n t u r i / spray tower system ( T r a i n 100) and a TCA system ( T r a i n 200). Each system has i t s own s l u r r y handling and dewatering f a c i l i t i e s , and each i s designed to remove both S02 and p a r t i c u l a t e from a p p r o x i mately 10 MW equivalent of f l u e gas (up to 35,000 acfm at 300 F). The f l u e gas which normally contains 1,400 to 3,500 ppm by volume of S02, i s obtained e i t h e r upstream ( c o n t a i n i n g h i g h f l y ash l o a d ing of 2 to 7 g r a i n s / d r y s c f ) or downstream ( c o n t a i n i n g low f l y ash l o a d i n g of 0.2 to 0.6 grain/dry s c f ) from B o i l e r No. 10 part i c u l a t e removal equipment. In June 1980, the v e n t u r i scrubber was removed from T r a i n 100, a l l o w i n g operation with the spray tower only. P r i o r to the removal of the v e n t u r i scrubber, operation w i t h a true spray tower-only c o n f i g u r a t i o n was not p o s s i b l e without some i n t e r f e r ence from the v e n t u r i , even with i t s a d j u s t a b l e plug wide open and minimum s l u r r y flow f o r f l u e gas c o o l i n g . Q

Shawnee Test Blocks. T e s t s conducted a t the Shawnee T e s t F a c i l i t y can be c l a s s i f i e d i n t o blocks a c c o r d i n g to type of a l k a l i , f l y ash l o a d i n g i n the f l u e gas, a d i p i c a c i d a d d i t i o n , and f o r c e d o x i d a t i o n scheme. T a b l e 1 l i s t s the combinations of these v a r i a b l e s which have been t e s t e d at Shawnee, i n c l u d i n g f a c t o r i a l tests. Forced o x i d a t i o n i s achieved by a i r sparging of the s l u r r y i n an o x i d a t i o n tank, e i t h e r on the bleed stream t o the s o l i d s dewatering system or on the r e c i r c u l a t e d s l u r r y w i t h i n the scrubber s l u r r y loop. For a one-scrubber-loop f o r c e d o x i d a t i o n system, the s l u r r y e f f l u e n t from a l l scrubbers i n the system (e.g., the v e n t u r i scrubber and spray tower at Shawnee c o n s t i t u t e a twoscrubber system, and the spray tower alone or TCA, a one-scrubber system) are sent to a s i n g l e e f f l u e n t hold tank, which i s the o x i d a t i o n tank. F o r a two-loop f o r c e d o x i d a t i o n system, there a r e two scrubbers i n s e r i e s (e.g., v e n t u r i and spray tower at Shawnee) w i t h e f f l u e n t from each scrubber going to a separate tank; the e f f l u e n t hold tank f o r the upstream scrubber (with respect t o gas flow) i s the o x i d a t i o n tank. F o r e i t h e r one-loop or two-loop forced o x i d a t i o n systems, the o x i d a t i o n tank may be followed by a second tank, i n s e r i e s , to provide f u r t h e r limestone d i s s o l u t i o n and gypsum desupersaturation time p r i o r to r e c y c l e to the scrubber. Test R e s u l t s I t i s beyond the scope of t h i s r e p o r t to present a l l of the Shawnee t e s t r e s u l t s from the t e s t blocks l i s t e d i n Table 1. Therefore, only the t y p i c a l and important t e s t r e s u l t s a r e p r e sented below. R e s u l t s of long-term t e s t s (longer than one month) are i n c l u d e d . R e s u l t s from f a c t o r i a l or p a r t i a l f a c t o r i a l t e s t s , which normally l a s t e d a minimum of 12 hours i n c l u d i n g 5 to 7 hours

WANG AND BURBANK

Adipic Acid-Enhanced Lime I Limestone

Table 1 . Test Blocks C o n d u c t e d at Shawnee

Test Block

Alkali

Fly Ash

Adipic Acid

Oxidation

No. o f Tanks

Loading

Addition

Scheme

in Oxid. Loop

Venturi/Spray Tower System: 1

Lime

High

Yes

2-Loop

2

Lime

High

Yes

No

-

3 ^

Limestone

High

Yes

2-Loop

2

4

Limestone

High

Yes

2-Loop

1

5

Limestone

High

Yes

1-Loop

2

6

Limestone

High

Yes

1-Loop

1

7

Limestone

High

Yes

Bleed S t r e a m

8

Limestone

High

Yes

No

-

9^°)

Limestone

High

No

2-Loop

2

2

2

Spray T o w e r System: 1 0 ^

Limestone

High

Yes

1-Loop

11

Limestone

High

Yes

No

-

12

Limestone

High

No

1-Loop

2

13

Limestone

High

No

No

-

T C A System: 14

Lime

High

Yes

1-Loop

1

15

Lime

High

Yes

No

1 6 ^

Lime

High

No

No

-

17

Limestone

High

Yes

1-Loop

2

18

Limestone

High

Yes

1-Loop

1

19^)

Limestone

High

Yes

No

-

20

Limestone

High

No

1-Loop

1

21

Limestone

High

No

No

22^)

Limestone

Low

Yes

1-Loop

—

23^ )

Limestone

Low

Yes

No

_

d

(a) (b) (c) (d)

Includes long-term (greater than one month) tests. Widows Creek forced oxidation simulation tests. Glitsch Grid packing tests. Springfield adipic acid simulation tests.

1

276

FLUE

GAS

DESULFURIZATION

of steady s t a t e operation, are a l s o included as f i g u r e s to i l l u s t r a t e the e f f e c t s of pH and a d i p i c a c i d concentration on SO2 removal. A summary of i n i t i a l t e s t s at the IERL-RTP p i l o t plant and the p r e l i m i n a r y r e s u l t s from the f u l l - s c a l e TCA t e s t s at Springf i e l d are a l s o given. IERL-RTP P i l o t Plant Test R e s u l t s . The i n i t i a l t e s t i n g of a d i p i c a c i d as a scrubber a d d i t i v e was c a r r i e d out by EPA beginning i n October 1977 i n the 0.1 MW in-house p i l o t p l a n t located at IERL-RTP (4). A s i n g l e - l o o p limestone scrubber was used f o r t h i s purpose, operated w i t h forced o x i d a t i o n i n the scrubbing loop. In a d d i t i o n to e f f e c t s on SO2 removal and o x i d a t i o n e f f i c i e n c i e s , these t e s t s sought to determine whether a d i p i c a c i d caused any change i n the p r o p e r t i e s of the o x i d i z e d sludge. So that these p r o p e r t i e s could be c l e a r l y seen, the system was operated without f l y ash. C h l o r i d e was added as HC1 and c o n t r o l l e d at the high l e v e l s expected f o r t i g h t l y closed loop systems. The r e s u l t s of the t e s t s showed a d i p i c a c i d to be very e f f e c t i v e i n improving SO2 removal e f f i c i e n c y , even when o p e r a t i n g at c h l o r i d e l e v e l s as high as 17,000 ppm. A TCA scrubber, which removed 82 percent of the i n l e t SO2 without the a d d i t i v e , y i e l d e d 89 percent SO2 removal w i t h 700 ppm a d i p i c a c i d , 91 percent removal w i t h 1,000 ppm, and 93 percent removal w i t h 2,000 ppm a d i p i c a c i d . The limestone u t i l i z a t i o n was concurrently increased from 77 percent without the a d d i t i v e to 91 percent w i t h 1,600 ppm a d i p i c a c i d . The observed e f f e c t s thus confirmed the t h e o r e t i c a l expectations i n a l l r e s p e c t s . In a d d i t i o n , the t e s t s showed no serious i n t e f e r e n c e by a d i p i c a c i d on the performance of the o x i d i z e r , operating at pH 6.1. The q u a l i t y of the o x i d i z e d sludge was s i m i l a r to that obtained when operating without a d i p i c a c i d , although small d i f ferences were detected. For example, the f i l t e r e d sludge averaged 80 percent s o l i d s ( f o r 13 one-week t e s t s ) vs 84 percent s o l i d s f o r 11 t e s t s without the a d d i t i v e , when o p e r a t i n g at 97 to 99 percent o x i d a t i o n i n both cases. The s e t t l i n g r a t e of the s l u r r y ( f l y ash f r e e at 50°C) averaged 2.3 cm/min during the a d i p i c a c i d t e s t s and 3.4 cm/min without a d i p i c a c i d ; bulk s e t t l e d d e n s i t i e s averaged 1.0 and 1.2 gm solids/cm^ s l u r r y , r e s p e c t i v e l y . I t was concluded from these r e s u l t s that the l a r g e improvements i n sludge q u a l i t y that can be achieved by forced o x i d a t i o n are not compromised by the use of a d i p i c a c i d as a scrubber a d d i t i v e . T e s t s without forced o x i d a t i o n a l s o demonstrated the e f f i c a c y of a d i p i c a c i d . Operating a TCA scrubber w i t h 2,000 ppm a d i p i c a c i d and 6 inches H2O pressure drop, 92 percent SO2 removal was obtained at a limestone u t i l i z a t i o n l e v e l of 88 percent. By comp a r i s o n , only 75 percent SO2 removal would be expected i n the p i l o t p l a n t at these t e s t c o n d i t i o n s without the a d d i t i v e . At t h i s a d i p i c a c i d l e v e l , the unoxidized sludge f i l t e r e d to 49 percent s o l i d s ; at lower a d i p i c a c i d l e v e l s (1,500 ppm or l e s s ) , the

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f i l t e r a b i l i t y of the s l u r r y was the same as that obtained without a d d i t i v e s : 55 percent s o l i d s . During the t e s t i n g w i t h a d i p i c a c i d , the scrubbing l i q u o r had a n o t i c e a b l e odor, even though the a d d i t i v e feed d i d not. The odor has been i d e n t i f i e d as that of v a l e r i c a c i d , CH3(CH2)3 COOH, an intermediate product formed by s i d e r e a c t i o n s that degrade a d i p i c a c i d at scrubber operating c o n d i t i o n s . At Shawnee, t h i s odor was r a r e l y n o t i c e d and was not a problem. Limestone Long-Term Tests with Two Scrubber Loops and Forced Oxidation. The v e n t u r i / s p r a y tower system was modified f o r twoscrubber-loop operation w i t h f o r c e d o x i d a t i o n as shown i n F i g ure 2. Two tanks were used i n the o x i d a t i o n loop ( v e n t u r i l o o p ) ; a i r was i n j e c t e d to the f i r s t of these tanks through a simple 3-inch diameter pipe below the a g i t a t o r . A d i p i c a c i d was dry-fed to the spray tower e f f l u e n t hold tank. T h i s was accomplished by manually adding one-pound increments hourly to maintain s p e c i f i e d concentration, u s u a l l y t o t a l i n g only a few pounds per hour. A small screw feeder would serve the purpose i n a f u l l - s c a l e p l a n t . The main advantage of t h i s two-loop system, as f a r as forced o x i d a t i o n i s concerned, i s that i t permits operation of the f i r s t loop ( v e n t u r i loop) at lower pH f o r good o x i d a t i o n e f f i c i e n c y , while maintaining higher pH i n the second loop (spray tower loop) for good SO2 removal. T h i s c o n f i g u r a t i o n a l s o maximizes the limestone u t i l i z a t i o n . With a d i p i c acid-enhancement, however, some of these advantages can a l s o be obtained i n a s i n g l e - l o o p scrubber because an a d i p i c a c i d enhanced system can be operated at a lower pH (4.6 to 5 . 4 ) . Thus, both good o x i d a t i o n and good SO2 removal can be achieved without an independent f i r s t loop. Table 2 summarizes the r e s u l t s of two long-term t e s t s (exceeding one month), Runs 907-1A and 907-1B, w i t h a d i p i c a c i d a d d i t i o n and with a v a r i a b l e gas flow r a t e . The r e s u l t s of Run 901-1A, a base case run without a d d i t i v e and w i t h a constant spray tower gas v e l o c i t y of 9.4 f t / s e c , are a l s o included f o r comparison. Run 907-1A was a month-long a d i p i c acid-enhanced limestone run w i t h f o r c e d o x i d a t i o n , designed to demonstrate o p e r a t i o n a l r e l i a b i l i t y w i t h respect to s c a l i n g and plugging and to demons t r a t e the removal enhancement c a p a b i l i t y of the a d i p i c a c i d a d d i t i v e . T h i s run was c o n t r o l l e d at a nominal limestone stoichiometry of 1.7 (compared to 1.4 f o r the base case run, Run 901-1A) and 1,500 ppm a d i p i c a c i d i n the spray tower. V e n t u r i i n l e t pH was c o n t r o l l e d at a minimum of 4.5 by the o c c a s i o n a l a d d i t i o n of limestone to the v e n t u r i loop. F l u e gas flow r a t e was v a r i e d from 18,000 acfm to a maximum of 35,000 acfm (spray tower gas v e l o c i t y between 4.8 and 9.4 f t / sec) to f o l l o w the d a i l y b o i l e r load c y c l e , which normally f l u c t u ated between 100 and 150 MW. The a d j u s t a b l e v e n t u r i plug was f i x e d i n a p o s i t i o n such that the pressure drop across the v e n t u r i

Figure 2.

Desupersaturation Tank

Bleed t o Solids Dewatering System

Clarified L i q u o r f r o m Solids Dewatering System

M a k e u p Water

Flow diagram for adipic acid-enhanced scrubbing in the venturi/spray tower system with two scrubber loops and forced oxidation.

Reheat

F l u e Gas

δ

H

r

C

w C/5

Ο

w ο >

r

00

to

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Table 2. A d i p i c A c i d - E n h a n c e d L i m e s t o n e Tests o n t h e T w o - L o o p V e n t u r i / S p r a y Tower System w i t h Forced Oxidation

Run No.

901-1A

907-1A

907-1Β 1,666

Onstream hours

187

719

F l y ash l o a d i n g

High

High

High

0

2,360

2,180

A d i p i c acid c o n e , i n v e n t u r i , p p m A d i p i c acid c o n e , i n spray t o w e r , p p m (controlled) S p r a y t o w e r gas v e l o c i t y , f t / s e c

0

1,560

1,510

9.4

4.8-9.4

5.4-9.4

V e n t u r i liquid-to-gas ratio, gal/Mcf

21

21-42

21-37

Spray t o w e r liquid-to-gas ratio, gal/Mcf

57

57-111

57-100

15

15

15

5.9

6.1

5.9

V e n t u r i inlet p H (controlled)

4.50

4.65

4.65

Spray t o w e r inlet p H

5.45

5.45

5.35

9.0

3.0-9.6

3.5-9.2

V e n t u r i s l u r r y solids c o n e . , w t % ( c o n t r o l l e d ) S p r a y t o w e r s l u r r y solids c o n e . , w t %

V e n t u r i pressure d r o p , i n . O x i d a t i o n t a n k level, f t O x i d a t i o n t a n k residence t i m e , m i n

18

18

18

11.3

11.3

11.3

4.7

4.7

4.7

S p r a y t o w e r e f f l u e n t t a n k residence t i m e , m i n

14.7

14.7

14.7

Spray t o w e r limestone stoich. ratio (controlled)

1.36

1.77

1.70

57

97.5

97

2,800

2,350

2,500

98

98.5

98 1.9-3.3

D e s u p e r s a t u r a t i o n t a n k residence t i m e , m i n

Average p e r c e n t SO2

removal

Average i n l e t SO2 c o n c e n t r a t i o n , p p m Percent o x i d a t i o n o f s u l f i t e t o s u l f a t e Air stoichiometry, atoms oxygen/mole SO2 a b s o r b e d

2.30

2.0-3.85

Overall p e r c e n t l i m e s t o n e u t i l i z a t i o n

97

88

92

V e n t u r i inlet liquor gypsum saturation, %

95

110

105

Spray t o w e r inlet liquor gypsum saturation, %

95

105

110

F i l t e r cake solids c o n t e n t , w t %

85

87

85

280

FLUE

GAS

DESULFURIZATION

was 9 inches H2O at 35,000 acfm maximum gas r a t e . A c t u a l pressure drop ranged from 3.0 to 9.6 inches H2O. The s l u r r y r e c i r c u l a t i o n r a t e s to the v e n t u r i and spray tower were f i x e d at 600 gpm (L/G = 21 to 42 gal/Mcf) and 1,600 gpm (L/G = 57 to 111 gal/Mcf), r e s p e c t i v e l y . The o x i d a t i o n tank l e v e l was 18 f t and the a i r flow r a t e was held constant at 260 scfm. The run began on October 8, 1978 and terminated November 13, 1978. I t ran f o r 719 onstream hours (30 days) w i t h no unscheduled outages. The scrubber was down once f o r a scheduled 3-hour i n s p e c t i o n and again when the b o i l e r came down f o r 135 hours to i n s t a l l a new s t a t i o n power transformer. Average SO2 removal f o r Run 907-1A was 97.5 percent at 2,350 ppm average i n l e t SO2 concentration. The SO2 removal stayed w i t h i n a narrow range of 96 to 99 percent throughout almost the e n t i r e run. T h i s was a s i g n i f i c a n t improvement over the 57 percent SO2 removal f o r the base case run, Run 901-1A, at 9.4 f t / s e c spray tower gas v e l o c i t y under s i m i l a r c o n d i t i o n s . On October 19 and on October 27, SO2 removal dropped b r i e f l y to l e s s than 90 percent when the pump which s u p p l i e d the s l u r r y to the top two spray headers was brought offstream f o r repacking, and the spray tower s l u r r y flow r a t e was cut i n h a l f to 800 gpm. At the reduced s l u r r y r e c i r c u l a t i o n r a t e , SO2 removal was 82 to 87 percent. V e n t u r i and spray tower i n l e t pH averaged 4.65 and 5.45, r e s p e c t i v e l y . O v e r a l l limestone u t i l i z a t i o n was 88 percent and the spray tower limestone u t i l i z a t i o n was 56 percent, demonstrati n g the advantage of good limestone u t i l i z a t i o n i n a two-scrubberloop operation. Average a d i p i c a c i d concentrations were 2,360 ppm i n the v e n t u r i loop and 1,560 ppm i n the spray tower loop. S u l f i t e o x i d a t i o n i n the system bleed s l u r r y averaged 98.5 percent, with the a i r s t o i c h i o m e t r i c r a t i o v a r i e d between 2.0 and 3.85 atoms oxygen/mole SO2 absorbed. The f i l t e r cake s o l i d s content was 87 percent. The mist e l i m i n a t o r was c l e a n during the e n t i r e run. The system was f r e e of plugging and s c a l i n g and there was no i n c r e a s e i n s o l i d s or s c a l e deposits on the scrubber i n t e r n a l s during Run 907-1A. Following Run 907-1A, a second a d i p i c acid-enhanced limestone long-term run with forced o x i d a t i o n was made during which f l u e gas monitoring procedures were evaluated by EPA. T h i s run, Run 907-1B, was made under the same c o n d i t i o n s as Run 907-1A except that the gas flow r a t e was v a r i e d according to a " t y p i c a l " u t i l i t y b o i l e r load c y c l e r a t h e r than the a c t u a l Unit No. 10 b o i l e r load. Run 907-1B began on November 13, 1978 and terminated January 29, 1979. I t ran f o r 1,666 onstream hours (69 days) w i t h only 27 hours of s c r u b b e r - r e l a t e d outages. The scrubber was also out of s e r v i c e 146 hours when Unit 10 came down f o r replacement of a broken t u r b i n e t h r u s t bearing.

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Excluding b o i l e r outages and scheduled i n s p e c t i o n s , the combined Runs 907-1A and 907-lB operated f o r a p e r i o d of over 3 months w i t h an onstream f a c t o r of 98.9 percent. No d e p o s i t s whatsoever were observed i n the mist e l i m i n a t o r f o r the e n t i r e 3-month t e s t p e r i o d . On only one occasion d i d s o l i d s accumulation cause an outage; the cross-over l i n e c a r r y i n g s l u r r y e f f l u e n t from the v e n t u r i to the o x i d a t i o n tank plugged w i t h s o f t s o l i d s and had to be cleaned out. Because of problems a s s o c i a t e d w i t h converting the Shawnee v e n t u r i / s p r a y tower system to two-scrubber-loop opera t i o n , t h i s cross-over l i n e followed a tortuous path (see Figure 2). A p r o p e r l y designed system would not have t h i s problem. R e s u l t s of Run 907-lB were as good i n every respect as those of Run 907-1A. Average SO2 removal remained w i t h i n a narrow band of 95 to 99 percent. S02 removal dropped b r i e f l y ( t y p i c a l l y 30 minutes) below 90 percent f i v e times when one of the two spray tower r e c i r c u l a t i o n pumps was taken out of s e r v i c e f o r maintenance, e f f e c t i v e l y c u t t i n g the s l u r r y r e c i r c u l a t i o n r a t e i n h a l f . O v e r a l l limestone u t i l i z a t i o n during t h i s run was 92 percent. S u l f i t e o x i d a t i o n averaged 98 percent and the waste sludge f i l t e r cake q u a l i t y was e x c e l l e n t , having a s o l i d s content of 85 percent. SO2 emissions f o r Run 907-1A and 907-lB were c a l c u l a t e d based on an assumed c o a l heating v a l u e of 10,500 B t u / l b , on 100 percent s u l f u r overhead (none i n bottom ash), and on an assumed excess a i r of 30 percent. T h i s excess a i r r a t e r e s u l t e d i n about 700 ppm i n l e t SO2 per 1.0 weight percent s u l f u r i n c o a l f o r the above c o n d i t i o n s . The average SO2 emission f o r the e n t i r e 3-month opera t i n g p e r i o d was only 0.20 l b / 1 0 Btu. The highest 24-hour average SO2 emission during Run 907-1A was 0.37 l b / 1 0 Btu, and during Run 907-lB was 0.41 l b / 1 0 Btu. A m a t e r i a l balance c a l c u l a t i o n f o r the a d i p i c a c i d consumpt i o n was made f o r Run 907-lB. A c t u a l a d i p i c a c i d feed r a t e was 8.3 l b / t o n of limestone fed to the system, of which 1.8 l b / t o n were discharged w i t h the f i l t e r cake ( t h e o r e t i c a l requirement) and 6.5 l b / t o n were unaccounted f o r , g i v i n g an a c t u a l - t o - t h e o r e t i c a l consumption r a t i o of 4.6. 6

6

6

Limestone Long-Term Test w i t h One Scrubber Loop and Without Forced O x i d a t i o n . Perhaps the most s t r a i g h t f o r w a r d i l l u s t r a t i o n of the e f f e c t i v e n e s s of a d i p i c a c i d i s demonstrated by a long-term limestone t e s t conducted on the Shawnee TCA system, i n which the a d d i t i v e was introduced without any system m o d i f i c a t i o n s . Table 3 l i s t s the r e s u l t s of the long-term t e s t , Run 932-2A, w i t h a d i p i c a c i d enhancement. The r e s u l t s of a base case run, Run 926-2A, without the a d d i t i v e , are a l s o included i n the t a b l e for comparison. F i g u r e 3 d e p i c t s the simple s i n g l e - l o o p , one-tank c o n f i g u r a t i o n of the TCA system used f o r these runs. The TCA contained three beds of 1-7/8 i n c h diameter, 11.5-gram n i t r i l e foam spheres

FLUE

282


T a b l e 3. A d i p i c A c i d - E n h a n c e d Limestone Test o n the Single-Loop T C A System w i t h o u t Forced O x i d a t i o n Run No. Onstream hours

926-2A

192


High

A d i p i c acid c o n c e n t r a t i o n , p p m ( c o n t r o l l e d )

0 12.5 50 15 1.2

S c r u b b e r gas v e l o c i t y , f t / s e c Liguid-to-gas r a t i o , g a l / M c f S l u r r y solids c o n c e n t r a t i o n , w t % ( c o n t r o l l e d ) Limestone stoichiometric ratio (controlled) T o t a l s t a t i c bed h e i g h t , inches o f 11.5 g r a m n i t r i l e spheres E f f l u e n t h o l d t a n k residence t i m e , m i n Average p e r c e n t SO2

removal

Average i n l e t SO2 c o n c e n t r a t i o n , p p m SO2

make-per-pass, m - m o l e s / l i t e r

Percent o x i d a t i o n o f s u l f i t e t o s u l f a t e Scrubber inlet p H Percent l i m e s t o n e u t i l i z a t i o n Scrubber inlet liquor gypsum saturation, % C e n t r i f u g e cake solids c o n t e n t , w t % *Clarifier underflow solids content.

15 4.1 71 2,750 10.1 13 5.65 80 90 37*

932-2A

833 High

1,620 8.4-12.5 50-75 15 1.2 15 4.1 96 2,450 4-18 21 5.30 82 110 61

13.

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283

F l u e Gas

Reheat -

S7\ S7\

- 4 Makeup Water

ST\

A A A

5*" F l u e Gas

Oo δ °o°o

TCA

Clarified Liquor f r o m Solids Dewatering System

Alkali Slurry

Adipic Acid

Bleed t o Solids Dewatering System Effluent Hold Tank

Figure 3.

Flow diagram for adipic acid-enhanced scrubbing in the TCA system without forced oxidation.

FLUE

284

GAS

DESULFURIZATION

r e t a i n e d between bar g r i d s . Each bed contained 5 inches s t a t i c height of spheres. A d i p i c a c i d was manually fed by the operator to the e f f l u e n t hold tank. Run 932-2A was made to demonstrate both o p e r a t i o n a l r e l i a b i l i t y w i t h respect to s c a l i n g and plugging of the TCA and the SO2 removal enhancement c a p a b i l i t y of the a d i p i c a c i d a d d i t i v e . The run began on September 26, 1978 and terminated on November 2, 1978, f o r a t o t a l of 833 onstream hours (35 days). During the run, the scrubber was out of s e r v i c e f o r 48 hours due to a b o i l e r outage caused by a tube leak, 5 hours f o r a scheduled i n s p e c t i o n , and 8 hours f o r unscheduled outages to c l e a n and r e p a i r the scrubber induced-draft fan damper. E x c l u d i n g b o i l e r outages and scheduled i n s p e c t i o n s , Run 932-2A operated w i t h an onstream f a c t o r of 99.0 percent. As was t y p i c a l of a l l long-term runs, the scrubber was more r e l i a b l e than the b o i l e r . The run was c o n t r o l l e d at a nominal limestone s t o i c h i o m e t r i c r a t i o of 1.2 and 1,500 ppm a d i p i c a c i d c o n c e n t r a t i o n i n the s l u r r y l i q u o r . S l u r r y s o l i d s c o n c e n t r a t i o n was c o n t r o l l e d at 15 percent. The f l u e gas flow r a t e was v a r i e d between 20,000 and 30,000 acfm (8.4 to 12.5 f t / s e c s u p e r f i c i a l gas v e l o c i t y ) as the b o i l e r load f l u c t u a t e d between 100 and 150 MW. The s l u r r y r e c i r c u l a t i o n r a t e was f i x e d at 1,200 gpm (L/G = 50 to 75 gal/Mcf). The e f f l u e n t hold tank residence time was only 4.1 minutes. SO2 removal during Run 932-2A averaged 96 percent at an average i n l e t SO2 c o n c e n t r a t i o n of 2,450 ppm. Excluding the f i r s t few days of unsteady s t a t e operation, SO2 removal stayed w i t h i n a narrow range of 94 to 98 percent as the i n l e t SO2 c o n c e n t r a t i o n v a r i e d widely between 1,400 and 3,500 ppm. By c o n t r a s t , SO2 r e moval during the base case run without a d i p i c a c i d , Run 926-2A, averaged only 71 percent at 2,750 ppm average i n l e t SO2 concentrat i o n , and at constant 12.5 f t / s e c gas v e l o c i t y and 50 gal/Mcf liquid-to-gas ratio. SO2 emissions were c a l c u l a t e d f o r Run 932-2A on the same b a s i s as f o r the v e n t u r i / s p r a y tower runs, Runs 907-1A and 907-1B. Excluding the f i r s t few days of unsteady s t a t e operation, the SO2 emissions f o r the 27-day p e r i o d from October 6 through the end of the run on November 2, 1978, averaged only 0.26 lb/10° Btu. The highest 24-hour average SO2 emission during t h i s p e r i o d was only 0.44 l b / 1 0 Btu. The mist e l i m i n a t o r was completely clean at the end of Run 932-2A and the e n t i r e scrubber system f r e e of s c a l i n g and p l u g ging. Limestone u t i l i z a t i o n during the run averaged 82 percent. S o l i d s discharged from the c e n t r i f u g e averaged about 61 percent, which i s t y p i c a l of unoxidized limestone sludge. An a d i p i c a c i d m a t e r i a l balance c a l c u l a t i o n was made f o r a 21-day p e r i o d during Run 932-2A. The a c t u a l a d i p i c a c i d feed r a t e was 9.2 l b / t o n limestone feed, of which 4.2 l b / t o n were discharged w i t h the c e n t r i f u g e cake ( t h e o r e t i c a l requirement) and 5.0 l b / t o n were unaccounted l o s s , g i v i n g an a c t u a l - t o - t h e o r e t i c a l consumption 6

13.

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AND

BURBANK


285

r a t i o of 2.2. T h i s r a t i o was l e s s than the value of 4.6 f o r v e n t u r i / s p r a y tower Run 907-lB when o x i d a t i o n was f o r c e d , i n d i c a t ing that forced o x i d a t i o n promotes a d i p i c a c i d degradation. However, the a c t u a l feed r a t e of 9.2 l b / t o n limestone f o r Run 932-2A was higher than the 8.3 l b / t o n limestone f o r Run 907-lB, because of the higher moisture content i n the discharge cake f o r Run 932-2A without forced o x i d a t i o n . Thus, the net e f f e c t of f o r c e d o x i d a t i o n was to reduce the a d i p i c a c i d makeup requirements by approximately 10 percent. In summary, the o b j e c t i v e s of t h i s long-term t e s t were met. High removal was c o n s i s t e n t l y achieved at a good limestone u t i l i z a t i o n , and no f o u l i n g , s c a l i n g , or plugging occurred. Lime t e s t s w i t h One Scrubber Loop and Without Forced O x i d a t i o n . T e s t s w i t h a d i p i c a c i d i n lime scrubbing a l s o were impressive i n enhancing S02 removal, both on the v e n t u r i / s p r a y tower and TCA systems. Table 4 shows some t y p i c a l r e s u l t s of a d i p i c acid-enhanced lime t e s t s from the Shawnee TCA without forced o x i d a t i o n . The flow diagram f o r these t e s t s i s shown i n F i g u r e 3. A l l three runs l i s t e d i n Table 4 were operated under the same c o n d i t i o n s , except f o r the a d i p i c a c i d c o n c e n t r a t i o n . Run 978-2A was a base case t e s t without a d i p i c a c i d . F o r Runs 979-2A and 980-2A, a d i p i c a c i d concentration was c o n t r o l l e d at a nominal 600 ppm and 1,200 ppm, r e s p e c t i v e l y (615 ppm and 1,305 ppm a c t u a l ) . The scrubber i n l e t pH was c o n t r o l l e d at 7.0 f o r a l l runs. Average SO2 removal improved from 83 percent at 2,350 ppm average i n l e t S02 c o n c e n t r a t i o n f o r the base case run, to 93 percent SO2 removal at the higher i n l e t SO2 c o n c e n t r a t i o n of 2,900 ppm with 615 ppm a d i p i c a c i d , and to 97.5 percent removal at 2,750 ppm i n l e t SO2 w i t h 1,305 ppm a d i p i c a c i d . Thus, w i t h 600 to 1,300 ppm a d i p i c a c i d , SO2 removal improved by 10 to 15 percent over the base case removal of 83 percent at 50 gal/Mcf l i q u i d - t o gas r a t i o and 7.0 scrubber i n l e t pH. Lime Test w i t h One Scrubber Loop and Forced O x i d a t i o n . Within-scrubber-loop forced o x i d a t i o n i n a s i n g l e - l o o p scrubbing system would not be expected to give good SO2 removal f o r a lime scrubber because of the o x i d a t i o n of the major scrubbing s p e c i e s , s u l f i t e i o n , i n t o nonreactive s u l f a t e i o n . With a d i p i c a c i d a d d i t i o n , however, s a t i s f a c t o r y SO2 removal should be p o s s i b l e because calcium adipate becomes the major scrubbing s p e c i e s . In a d d i t i o n , the lower pH at which a l i m e / a d i p i c a c i d system operates should f a c i l i t a t e s u l f i t e oxidation. Table 5 l i s t s the t e s t r e s u l t s of such a lime run, Run 951-2E, using within-scrubber-loop forced o x i d a t i o n w i t h 1,330 ppm a d i p i c a c i d . The system c o n f i g u r a t i o n used f o r t h i s run was the same as that shown i n F i g u r e 3, except that the o x i d i z i n g a i r was i n j e c t e d i n t o the e f f l u e n t hold tank ( o x i d a t i o n tank). A s i n g l e

286

FLUE


Table 4. A d i p i c A c i d - E n h a n c e d L i m e Tests o n t h e S i n g l e - L o o p T C A S y s t e m w i t h o u t Forced Oxidation

Run No.

978-2A

Onstream hours

116

979-2A

980-2A

177

247


High

High


0 12.5 50 8 7.0

615 12.5 50 8 7.2

1,305 12.5 50 8 6.95

15 4.1 83 2,350 10.3 21 92 125

15 4.1 93 2,900 14.3 14 92 90

15 4.1 97.5 2,750 14.3 10 88 75

S c r u b b e r gas v e l o c i t y , f t / s e c Liguid-to-gas r a t i o , g a l / M c f S l u r r y solids c o n c e n t r a t i o n , w t % ( c o n t r o l l e d ) Scrubber inlet p H (controlled) T o t a l s t a t i c b e d h e i g h t , inches o f 11.5 g r a m n i t r i l e spheres E f f l u e n t h o l d t a n k residence t i m e , m i n Average p e r c e n t SO2 Average i n l e t SO2 SO2

removal

concentration, ppm


Percent o x i d a t i o n o f s u l f i t e t o s u l f a t e Percent l i m e u t i l i z a t i o n Scrubber inlet liquor gypsum saturation, %

High

Table 5. A d i p i c A c i d - E n h a n c e d Lime Test o n the Single-Loop T C A System w i t h Forced Oxidation

Run No.

951-2E

Onstream hours

103


High


1,330

S c r u b b e r gas v e l o c i t y , f t / s e c

12.5

Liquid-to-gas ratio, gal/Mcf

50

S l u r r y solids c o n c e n t r a t i o n , w t % ( c o n t r o l l e d ) Scrubber inlet p H (controlled)

8 5.0

T o t a l s t a t i c b e d h e i g h t , inches o f 11.5 g r a m n i t r i l e spheres

15

O x i d a t i o n t a n k level, f t

17

O x i d a t i o n t a n k residence t i m e , m i n Average p e r c e n t SO2 Average i n l e t SO2

removal

SO2 c o n c e n t r a t i o n ,

4.1 82

ppm


Percent o x i d a t i o n o f s u l f i t e t o s u l f a t e A i r s t o i c h i o m e t r y , a t o m s o x y g e n / m o l e SO2 a b s o r b e d Percent l i m e u t i l i z a t i o n

2,400 10.4 98 1.95 97

Scrubber inlet liquor g y p s u m saturation, %

105

S c r u b b e r i n l e t l i q u o r 3O3/HSO3 c o n c e n t r a t i o n , p p m

100

C e n t r i f u g e cake solids c o n t e n t , w t %

72

13.

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287

3-inch diameter pipe was used f o r t h i s purpose, w i t h the a i r d i s charging downwards at the center of the o x i d a t i o n tank 5 inches from the tank bottom. Run 951-2E was made w i t h a scrubber i n l e t pH of 5.0 (oxidat i o n tank pH). Higher pH i n c r e a s e s the calcium s u l f i t e s c a l i n g tendency and a l s o decreases o x i d a t i o n e f f i c i e n c y . Lower pH r e duces the calcium adipate b u f f e r c a p a c i t y and S02 removal efficiency. At 5.0 scrubber i n l e t pH and 50 gal/Mcf l i q u i d - t o - g a s r a t i o , s u l f i t e o x i d a t i o n averaged 98 percent and the SO2 removal was s a t i s f a c t o r y at 82 percent. I t should be noted that under the o p e r a t i n g c o n d i t i o n s chosen f o r Run 951-2E, the major SO2 scrubbing s p e c i e s was calcium a d i pate because there was l i t t l e s u l f i t e a v a i l a b l e , both i n l i q u o r or s o l i d s , normally the major scrubbing s p e c i e s i n an unenhanced lime system without forced o x i d a t i o n . Higher SO2 removal than the 82 percent i n Run 951-2E should be a c h i e v a b l e by simply r a i s ing the a d i p i c a c i d c o n c e n t r a t i o n beyond the 1,330 ppm t e s t e d . An SO2 removal of only 65 percent would be p r e d i c t e d under the same o p e r a t i n g c o n d i t i o n s as Run 951-2E, but without f o r c e d o x i d a t i o n and without a d i p i c a c i d a d d i t i o n . With forced oxidat i o n and without a d i p i c a c i d enhancement, the expected SO2 r e moval should be s i g n i f i c a n t l y lower than 65 percent. Limestone Long-Term T e s t w i t h One Scrubber Loop and Forced O x i d a t i o n . A one-scrubber-loop system has an inherent advantage over a two-scrubber-loop system i n i t s simple design and lower c a p i t a l and o p e r a t i n g c o s t s . I f a simple one-loop limestone (or lime) system i s operated w i t h a d i p i c a c i d , which o f f e r s the advantage of lower o p e r a t i n g pH, then both good SO2 removal and s u l f i t e o x i d a t i o n can be achieved w i t h minimum cost. T h i s was i l l u s t r a t e d i n a long-term a d i p i c acid-enhanced limestone run, Run 917-1A, conducted on the Shawnee spray tower system from December 26, 1980, t o March 13, 1981. F i g u r e 4 shows the flow diagram f o r t h i s long-term run w i t h f o r c e d o x i d a t i o n using two s e r i e s tanks i n the s l u r r y loop. O x i d a t i o n was forced i n the f i r s t tank w h i l e f r e s h limestone was added to the second. Use of two tanks i n s e r i e s i n a within-scrubber-loop forced o x i d a t i o n system has s e v e r a l advantages over a s i n g l e tank: •

•

•

Lower pH i n the f i r s t tank ( o x i d a t i o n tank), which r e c e i v e s the scrubber e f f l u e n t s l u r r y , gives b e t t e r o x i d a t i o n e f f i c i e n c y Limestone b l i n d i n g p o t e n t i a l by calcium s u l f i t e i s reduced because l i q u o r s u l f i t e i s o x i d i z e d i n the f i r s t tank b e f o r e f r e s h limestone i s added to the second tank Limestone u t i l i z a t i o n i s improved w i t h two tanks in series

Figure 4.

Bleed t o Solids Dewatering System

Compressed Air

Clarified L i q u o r f r o m Solids Dewatering System

M Makeup Water

Flow diagram for adipic acid-enhanced limestone scrubbing in the spray tower system with forced oxidation and two tanks.

Reheat

F l u e Gas

2

δ

Η

w c r

Ο

w

00 00

to

13.

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289

The second tank o f f e r s e x t r a time f o r gypsum desupersaturation and p r e c i p i t a t i o n The second tank provides a i r - f r e e s u c t i o n f o r s l u r r y r e c i r c u l a t i o n pumps

In any within-scrubber-loop f o r c e d o x i d a t i o n system, i r r e s p e c t i v e of whether i t i s a d d i t i v e promoted or not, the p o s s i b i l i t y e x i s t s f o r calcium s u l f i t e b l i n d i n g of limestone because the r e c i r c u l a t e d s l u r r y l a c k s the s o l i d CaS03 c r y s t a l seeds. Under t h i s environment, and i f the o x i d a t i o n i n t e n s i t y i s not s u f f i c i e n t l y high, l i q u o r s u l f i t e could b u i l d up to a l e v e l at which CaS03 begins to p r e c i p i t a t e on a l k a l i n e limestone part i c l e s , causing limestone b l i n d i n g , reduced d i s s o l u t i o n , and a pH drop. The problem i s u s u a l l y avoided by i n c r e a s i n g the a i r stoichiometry to prevent the buildup of s u l f i t e i n the l i q u o r . The p o t e n t i a l of limestone b l i n d i n g i s f u r t h e r reduced by the use of two tanks i n s e r i e s , as described above, to permit s u l f i t e o x i d a t i o n before limestone a d d i t i o n . Table 6 summarizes the important t e s t r e s u l t s of Run 917-1A. As i n the previous runs w i t h f o r c e d o x i d a t i o n , a i r was i n j e c t e d i n t o the o x i d a t i o n tank through a s i n g l e 3-inch diameter pipe. The system was onstream f o r 1,688 hours. During the run, the scrubber was out of s e r v i c e f o r 78 hours due to equipment problems and 84 hours due to b o i l e r outages. E x c l u d i n g b o i l e r outages, Run 917-1A operated with an onstream f a c t o r of 95.6 percent. The run was c o n t r o l l e d at a scrubber i n l e t pH of 5.0 to 5.1 and an a d i p i c a c i d c o n c e n t r a t i o n of 1,300 to 1,700 ppm to o b t a i n 90 percent or higher SO2 removal. The f l u e gas flow r a t e was v a r i e d between 20,000 and 35,000 acfm (5.4 to 9.4 f t / s e c superf i c i a l gas v e l o c i t y ) according to a " t y p i c a l d a i l y b o i l e r load c y c l e . " The s l u r r y flow r a t e was f i x e d at 2,400 gpm (L/G = 85 to 150 g a l / M c f ) . S l u r r y s o l i d s c o n c e n t r a t i o n was c o n t r o l l e d at 15 percent. SO2 removal d u r i n g the run averaged 93.4 percent at 2,660 ppm average i n l e t SO2 c o n c e n t r a t i o n . A t the low L/G of 85 g a l / Mcf, SO2 removal v a r i e d from 87 to 92 percent with 1,300 ppm a d i p i c a c i d , and from 90 to 93 percent w i t h 1,700 ppm a d i p i c a c i d . At the high L/G of 150 gal/Mcf, the removal was 97 to 99 percent. D a i l y average SO2 removal was 92 to 95 percent. Limestone u t i l i z a t i o n averaged 92.6 percent. S u l f i t e oxidat i o n was e x c e l l e n t at 99.8 percent and the f i l t e r cake s o l i d s content was high, averaging 86 percent. Gypsum s a t u r a t i o n i n the scrubber i n l e t l i q u o r was only 93 percent. The mist e l i m i n a t o r was completely clean a t the end of the run, and there was no evidence of plugging or s c a l i n g w i t h i n the spray tower. The a c t u a l a d i p i c a c i d consumption r a t e during Run 917-1A was only 5.4 l b / t o n of limestone feed, four times the t h e o r e t i c a l requirement.

FLUE

290

GAS

DESULFURIZATION

Table 6. A d i p i c Acid-Enhanced Limestone Test on the Single-Loop Spray T o w e r System w i t h Forced O x i d a t i o n and T w o Tanks Run No.

9Ί7-1Α

Onstream hours

1,688


High

A d i p i c acid c o n c e n t r a t i o n , p p m (controlled)

1,300-1,700

S c r u b b e r gas v e l o c i t y , f t / s e c

5.4-9.4

Liquid-to-gas ratio, gal/Mcf

85-150

S l u r r y solids c o n c e n t r a t i o n , w t % ( c o n t r o l l e d ) Scrubber inlet pH (controlled) O x i d a t i o n t a n k level, f t

18

O x i d a t i o n t a n k residence t i m e , m i n

2.8

E f f l u e n t h o l d t a n k residence t i m e , m i n Average p e r c e n t SO2 Average i n l e t SO2 SO2

8.3

removal

93.4

concentration, ppm

2,660


4.0-8.9

Percent o x i d a t i o n o f s u l f i t e t o s u l f a t e A i r s t o i c h i o m e t r y , a t o m s o x y g e n / m o l e SO2 Oxidation tank pH Percent l i m e s t o n e u t i l i z a t i o n

15 5.0-5.1

99.8 absorbed

1.4-2.4 4.9 92.6

Scrubber inlet liquor gypsum saturation, %

93

F i l t e r cake solids c o n t e n t , w t %

86

13.

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Limestone T e s t s w i t h Bleed Stream O x i d a t i o n . A major advantage of the bleed stream o x i d a t i o n i s i t s simple flow c o n f i g u r a tion. In o p e r a t i o n without forced o x i d a t i o n , the scrubber bleed stream would be sent d i r e c t l y to the s o l i d s dewatering system. To o x i d i z e t h i s bleed stream, i t i s necessary only to i n s t a l l an o x i d a t i o n tank and the a s s o c i a t e d a g i t a t o r and compressed a i r system anywhere between the e f f l u e n t h o l d tank and the s o l i d s dewatering area. Thus, the bleed stream o x i d a t i o n scheme i s part i c u l a r l y w e l l s u i t e d f o r r e t r o f i t when m o d i f i c a t i o n s of the e x i s t i n g scrubber system f o r within-scrubber-loop f o r c e d o x i d a t i o n are not p o s s i b l e due to p h y s i c a l c o n s t r a i n t s . Bleed stream o x i d a t i o n of unenhanced lime or limestone s l u r r y i s u s u a l l y not f e a s i b l e because the pH r i s e caused by the r e s i d u a l a l k a l i i n the o x i d a t i o n tank makes i t d i f f i c u l t to r e d i s s o l v e the s o l i d calcium s u l f i t e . With a d i p i c acid-enhanced limestone scrubbing, however, t h i s c o n s t r a i n t i s removed because of the low o p e r a t i n g pH and low r e s i d u a l a l k a l i i n the bleed s l u r r y . Thus, the o x i d a t i o n tank can be maintained a t a low pH f o r good s u l f i t e o x i d a t i o n , while a c h i e v i n g h i g h SO2 removal e f f i c i e n c y w i t h a s u f f i c i e n t l y high c o n c e n t r a t i o n of a d i p i c a c i d i n the scrubber liquor. Table 7 gives the r e s u l t s of a t y p i c a l bleed stream o x i d a t i o n t e s t , Run 915-1C, which was conducted w i t h a d i p i c acid-enhanced limestone on the v e n t u r i / s p r a y tower system. The e f f l u e n t s l u r r i e s from the v e n t u r i and the spray tower were discharged i n t o a common e f f l u e n t hold tank. The scrubber bleed stream was pumped from the e f f l u e n t hold tank to an o x i d a t i o n tank i n t o which a i r was i n j e c t e d through a 3-inch diameter p i p e . The f i n a l system bleed was withdrawn from the o x i d a t i o n tank and sent to the s o l i d s dewatering system. Good s u l f i t e o x i d a t i o n of 98 percent was achieved i n the o x i d a t i o n tank a t 4.8 pH and 1.8 a i r s t o i c h i o m e t r y . SO2 removal was h i g h a t 96 percent w i t h 4.8 scrubber i n l e t pH, 4,140 ppm a d i p i c a c i d , and 2,030 ppm i n l e t SO2 c o n c e n t r a t i o n . Degradation of a d i p i c a c i d was low, as expected with the low pH o p e r a t i o n . The a c t u a l - t o - t h e o r e t i c a l a d i p i c a c i d consumption r a t i o was only 1.26 f o r a r a t e o f 8.7 l b / t o n of limestone feed. The c e n t r i f u g e cake s o l i d s content was 79 percent. F a c t o r i a l Test R e s u l t s . F u l l or p a r t i a l f a c t o r i a l t e s t s have been conducted a t Shawnee, p r i m a r i l y to i n v e s t i g a t e the e f f e c t s of a d i p i c a c i d concentration and pH on SO2 removal. These t e s t s u s u a l l y l a s t e d 12 hours or longer, i n c l u d i n g at l e a s t 5 to 7 hours of steady-state o p e r a t i o n . Scrubber c o n f i g u r a t i o n s used were: v e n t u r i alone, spray tower alone, combined v e n t u r i and spray tower, and TCA. Limestone was used i n a l l scrubber c o n f i g u r a t i o n s . Lime was used only w i t h the v e n t u r i alone. Only the t y p i c a l r e s u l t s from the TCA and spray tower t e s t s a r e presented below to show the degree of e f f e c t of pH and a d i p i c a c i d c o n c e n t r a t i o n on SO2 removal.

292

FLUE

GAS

DESULFURIZATION

T a b l e 7. A d i p i c Acid-Enhanced Limestone Test on the V e n t u r i / S p r a y T o w e r System w i t h Bleed S t r e a m O x i d a t i o n Run No.

915-1C

Onstream hours

127


High

A d i p i c acid c o n c e n t r a t i o n , p p m (controlled)

4,140

S p r a y t o w e r gas v e l o c i t y , f t / s e c

9.4

V e n t u r i liquid-to-gas r a t i o , g a l / M c f

21

Spray t o w e r liquid-to-gas ratio, gal/Mcf

57

S l u r r y solids c o n c e n t r a t i o n , w t % ( c o n t r o l l e d ) Scrubber inlet p H (controlled) V e n t u r i pressure d r o p , i n . H2O

9

O x i d a t i o n t a n k level, f t

17

E f f l u e n t h o l d t a n k residence t i m e , m i n Average p e r c e n t SO2 Average i n l e t SO2

15 4.8

9.1

removal

96

concentration, ppm

2,030

Percent s u l f i t e o x i d a t i o n in e f f l u e n t h o l d t a n k

54

Percent s u l f i t e o x i d a t i o n in o x i d a t i o n t a n k

98

A i r s t o i c h i o m e t r y , a t o m s o x y g e n / m o l e SO2 Oxidation tank pH Percent l i m e s t o n e u t i l i z a t i o n

absorbed

1.8 4.8 88

Scrubber inlet liquor gypsum saturation, %

105

Oxidation tank liquor gypsum saturation, %

100

C e n t r i f u g e cake solids c o n t e n t , w t %

79

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F i g u r e s 5 through 7 show the r e s u l t s of p a r t i a l f a c t o r i a l limestone runs conducted on the TCA system using two tanks i n s e r i e s . Operating c o n d i t i o n s common to a l l runs were: F l y ash l o a d i n g : High (2 to 7 grains/dry s c f ) S l u r r y s o l i d s c o n c e n t r a t i o n : 15 percent O x i d a t i o n tank l e v e l (7 f t diameter): 18 f t E f f l u e n t hold tank l e v e l (20 f t diameter): 6.2 f t A i r flow to o x i d i z e r : 220 scfm ( f o r runs w i t h forced oxidation) F i g u r e 5 shows the S02 removal as a f u n c t i o n of a d i p i c a c i d concentration and s l u r r y flow r a t e f o r the TCA without spheres ( g r i d tower). With a c o n t r o l l e d limestone stoichiometry of 1.2 (5.6 to 6.1 scrubber i n l e t pH) and a s l u r r y flow r a t e of 37 gpm/ f t ^ , a d i p i c a c i d c o n c e n t r a t i o n greater than 2,000 ppm would be required to achieve 90 percent SO2 removal. F i g u r e 6 i s s i m i l a r to F i g u r e 5 except that the data used f o r F i g u r e 6 were obtained using 15 inches (5 inches per bed) of s t a t i c height of spheres i n the TCA. With a c o n t r o l l e d limestone stoichiometry of 1.2, 90 percent SO2 removal could be obtained w i t h 2,000 ppm a d i p i c a c i d and only 19 gpm/ft^ s l u r r y flow r a t e , or w i t h only 600 ppm a d i p i c a c i d at 28 gpm/ft^ s l u r r y flow r a t e . With 37 gpm/ft^, the r e q u i r e d a d i p i c a c i d concentration i s only 300 ppm to achieve 90 percent removal. Both F i g u r e s 5 and 6 show that, at 1.2 limestone stoichiometry (5.6 to 6.1 scrubber i n l e t pH), SO2 removal begins to "taper o f f " at about 600 ppm a d i p i c acid concentration. The e f f e c t s of scrubber i n l e t pH and a d i p i c a c i d concentrat i o n on SO2 removal i n the TCA are given i n F i g u r e 7. In comparing F i g u r e 7 with F i g u r e s 5 and 6 (high pH data) at the same s l u r r y flow r a t e of 28 gpm/ft^, the low pH curves of F i g u r e 7 have a n o t i c e a b l y steeper slope f o r a d i p i c a c i d c o n c e n t r a t i o n above 600 ppm than i s the case f o r the high pH data. At low pH, the a d i p i c a c i d i s p a r t i a l l y i n e f f e c t i v e because of a s i g n i f i c a n t amount of unionized a d i p i c a c i d . For example, F i g u r e 6 shows that at a scrubber i n l e t pH of 5.6 to 6.1 and 28 gpm/ft^, 75 percent SO2 removal can be achieved without a d i p i c a c i d . To achieve t h i s same 75 percent removal, F i g u r e 7 i n d i c a t e s that 600 ppm a d i p i c a c i d i s r e q u i r e d at 5.3 scrubber i n l e t pH and 1,700 ppm a t 4.6 i n l e t pH. Therefore, operation at a very low pH w i t h a d i p i c a c i d enhanced limestone i s not as a t t r a c t i v e as at the higher pH from the process standpoint, s i n c e a d i p i c a c i d i s only p a r t i a l l y u t i l i z e d f o r SO2 scrubbing, and the SO2 removal i s f a r more s e n s i t i v e to f l u c t u a t i o n s i n both pH and a d i p i c a c i d c o n c e n t r a t i o n . F i g u r e s 8 and 9 show the r e s u l t s of p a r t i a l f a c t o r i a l limestone runs made on the spray tower. Common o p e r a t i n g c o n d i t i o n s f o r these runs were:

294

FLUE

GAS

DESULFURIZATION

100

40

— I I 0

I

I

I

I

I

1

400

800

1200

1600

2000

2400

ADIPIC ACID CONCENTRATION,

ppm

Figure 5. Effect of adipic acid concentration and slurry flow rate on S0 removal in the TCA with four grids and without spheres. Key: A, 37 gpm/ft ; O, 28 gpm/ ft ; and 0, 19 gpm/ft . Inlet S0 , 1800-2800 ppm; gas velocity, 8.4-12.5 ft/s; scrubber inlet pH, 5.6-6.1 (limestone stoich., 1.2); height of spheres, 0 in. with forced oxidation. 2

2

2

2

2

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100

400

800

1200

1600

2000

2400

ADIPIC ACID CONCENTRATION, ppm

Figure 6. Effect of adipic acid concentration and slurry flow rate on S0 removal in the TCA with four grids and 15 in. of spheres. Key: V , pH 5.6; 0, pH 5.3; gpm/ft ; and 0, 19 gpm/ft . Inlet S0 , 1800-2800 ppm; gas velocity, 8.4-12.5 ft/s; scrubber inlet pH, 5.6-6.1 (limestone stoich., 1.2); height of spheres, 15 in. with and without forced oxidation. 2

2

2

2

FLUE

296

GAS

DESULFURIZATION

100

< > Ο

Έ LU

CC CM Ο CO

IZ LU Ο

ce

400

800

1200

1600

2400

2000


Figure 7. Effect of adipic acid concentration and scrubber inlet pH on S0 removal in the TCA with four grids and 15 in. of spheres. Key: V , pH 5.6; 0, pH 5.3; O pH 5.0; and Δ , pH 4.6. Inlet S0 1800-2800 ppm; gas velocity, 10.4 ft/s; slurry flow rate, 28 gpm/ft ; height of spheres, 15 in. with forced oxidation. 2

2)

f

2

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297

100

400

800

1200

1600

2400


Figure 8. Effect of adipic acid concentration and scrubber inlet pH on S0 re moval in the spray tower without forced oxidation. Key: Δ , pH 5.4; Ο, pH 5.0; and 0, pH 4.6. Inlet S0 2380-3000 ppm; gas velocity, 9.4 ft/s; and L/G, 85 gal/Mcf. 2

2)

298

FLUE

GAS

DESULFURIZATION

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F l y ash l o a d i n g : High (2 to 7 grains/dry s c f ) S l u r r y s o l i d s c o n c e n t r a t i o n : 15 percent Gas v e l o c i t y : 9.4 f t / s e c Liquid-to-gas r a t i o : 85 gal/Mcf (2,400 gpm) A i r flow to o x i d i z e r : 250 scfm ( f o r runs w i t h forced o x i d a t i o n ) For runs without f o r c e d o x i d a t i o n , a s i n g l e e f f l u e n t tank 20 f t i n diameter w i t h 8.5-ft tank l e v e l was used. For runs w i t h f o r c e d o x i d a t i o n , two tanks i n s e r i e s were used w i t h an o x i d a t i o n tank preceding the e f f l u e n t hold tank. The o x i d a t i o n tank was 8 f t i n diameter w i t h an 18-ft tank l e v e l . F i g u r e 8 gives the SO2 removal as a f u n c t i o n of a d i p i c a c i d c o n c e n t r a t i o n and spray tower i n l e t pH f o r runs made without f o r c e d o x i d a t i o n and at a constant l i q u i d - t o - g a s r a t i o of 85 g a l / Mcf. SO2 removal i s s e n s i t i v e to both pH and a d i p i c a c i d concent r a t i o n w i t h i n the ranges shown i n the f i g u r e . At a l i q u i d - t o gas r a t i o of 85 gal/Mcf, 90 percent SO2 removal could be achieved at 5.4 scrubber i n l e t pH and 1,200 ppm a d i p i c a c i d , or 5.0 i n l e t pH and 2,200 ppm a d i p i c a c i d . At 4.6 i n l e t pH, the r e q u i r e d a d i p i c a c i d c o n c e n t r a t i o n i s estimated to be i n excess of 3,000 ppm to y i e l d 90 percent SO2 removal. F i g u r e 9 shows the e f f e c t s of scrubber i n l e t pH and a d i p i c a c i d c o n c e n t r a t i o n on S02 removal f o r runs made w i t h forced o x i d a t i o n . As i n F i g u r e 8, the l i q u i d - t o - g a s r a t i o was h e l d constant at 85 gal/Mcf f o r the runs shown i n F i g u r e 9. By comparing the two f i g u r e s , i t i s seen that f o r c e d o x i d a t i o n d r a m a t i c a l l y improved the SO2 removal, e s p e c i a l l y at the scrubber i n l e t pH below about 5.0. For example, at 1,200 ppm a d i p i c a c i d concentrat i o n and without forced o x i d a t i o n , SO2 removals were 59.77, and 90 percent at scrubber i n l e t pH of 4.6, 5.0, and 5.4, respectively. The corresponding SO2 removals w i t h forced o x i d a t i o n were 87, 91, and 94 percent. The reason f o r improved SO2 removal, p a r t i c u l a r l y at low pH, i s that forced o x i d a t i o n e l i m i n a t e s b i s u l f i t e s p e c i e s , thereby reducing the SO2 vapor pressure at the g a s - l i q u i d i n t e r f a c e and improving the SO2 mass t r a n s f e r e f f i c i e n c y . T h i s mechanism of improved SO2 removal holds true when s u l f i t e i s not a major scrubbing species and the SO2 removal does not depend on the s u l f i t e b i s u l f i t e buffer. In the case of F i g u r e 8 and 9, calcium adipate i s the major scrubbing reagent. Therefore, i t would be advantageous to operate a low pH, a d i p i c acid-enhanced limestone or lime system w i t h w i t h i n scrubber-loop f o r c e d o x i d a t i o n which, i n a d d i t i o n to improved SO2 removal, r e q u i r e s low a d i p i c a c i d makeup, minimizes gypsum s c a l ing p o t e n t i a l , and produces a sludge w i t h good d i s p o s a l properties. Based on F i g u r e 9, 90 percent SO2 removal can be achieved at 5.0 i n l e t pH and only 1,100 ppm a d i p i c a c i d , or at 4.6 i n l e t pH w i t h 1,400 ppm a d i p i c a c i d .

300

FLUE

GAS

DESULFURIZATION

S p r i n g f i e l d F u l l - S c a l e Demonstration. In August and September 1980, the EPA, through i t s contractor, Radian Corporation, conducted the f i r s t demonstration of the commercial f e a s i b i l i t y of a d i p i c a c i d a d d i t i o n to a f u l l - s c a l e limestone scrubber (8). The host f a c i l i t y was the Southwest Power Plant of the C i t y U t i l i t i e s of S p r i n g f i e l d , M i s s o u r i . In that f a c i l i t y , 3.5 percent s u l f u r eastern c o a l i s burned i n two b o i l e r s with a t o t a l genera t i n g capacity of 200 MW. The f l u e gas from the 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 i s scrubbed by two p a r a l l e l 100 MW TCA s. During t h i s i n i t i a l two-month t e s t period, seven d i f f e r e n t sets of t e s t c o n d i t i o n s were examined. Table 8 presents the major r e s u l t s of two b a s e l i n e t e s t s without a d i p i c a c i d conducted on Modules S - l and S-2, and seven t e s t s with a d i p i c a c i d conducted on Module S - l . A l l t e s t s were without forced o x i d a t i o n . SO2 removal improved from 68 to 72 percent f o r the b a s e l i n e t e s t s to 91, 95, and 96 percent with 840, 1,040, and 1,650 ppm a d i p i c a c i d , r e s p e c t i v e l y , at the same scrubber i n l e t pH of 5.5. At 5.0 i n l e t pH, SO2 removal remained high at 84, 90, and 93 percent w i t h 1,250, 13,00, and 1,800 ppm a d i p i c a c i d , r e s p e c t i v e l y . At the lower pH of 5.0, the limestone u t i l i z a t i o n a l s o increased from 76 to 84 percent f o r the b a s e l i n e t e s t s (pH 5.5) to 84 to 97 percent. The r e s u l t s are c o n s i s t e n t w i t h the Shawnee f i n d i n g s . Furthermore, the SO2 removal obtained at S p r i n g f i e l d l a y w i t h i n 1 to 3 percentage p o i n t s of model p r e d i c t i o n based on the Shawnee data under s i m i l a r operating c o n d i t i o n s . I t should be noted that the odor associated w i t h the a d i p i c a c i d t e s t i n g also was not a problem at S p r i n g f i e l d . Following these i n i t i a l t e s t s , the scrubber system was shut down f o r scheduled maintenance. Subsequently, the demonstration continued with a d i p i c a c i d t e s t i n g , both w i t h and without forced oxidation. These r e s u l t s w i l l be reported separately by others. f

Rickenbacker I n d u s t r i a l B o i l e r Demonstration. In February, March, and A p r i l 1981, the EPA, through i t s contractor, PEDCo Environmental, Inc., conducted a d i p i c acid-enhanced limestone scrubber t e s t s on an i n d u s t r i a l - s i z e d system. The t e s t i n g was c a r r i e d out at the Rickenbacker A i r Force Base on a ResearchC o t t r e l l / B a h c o system rated at 55,000 scfm, or about 27 MW equival e n t . The t e s t s , conducted with c e r t i f i e d instrumentation, i n d i cated an SO2 removal e f f i c i e n c y increase from 55 percent without a d i p i c a c i d , to 90 to 95 percent with a d i p i c a c i d . T h i s improvement was achieved at a scrubber i n l e t pH of 5.0 and a d i p i c a c i d concentrations of between 2,000 and 2,500 ppm. More complete data w i l l be reported separately by others. Economics The economics of limestone scrubbing, with or without a d d i t i v e , have been p r o j e c t e d f o r forced o x i d a t i o n systems designed to

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Table 8. Results of Springfield Full-Scale Adipic Acid Demonstration Adipic Gas

Slurry

Inlet

Scrubber

Acid

Test

Flow*

so

Inlet

Cone.

Av. S 0 Removal

Limestone

Flow

Util.

Period

10 dscfm

gpm

ppm dry

PH

ppm

%

%

hrs,

S-1

171

13,500

2,410

5.5

0

68

76

497

S-2

163

13,500

2,410

5.5

0

72

84

408

840

91 95

82 75

122

94

50 74

91

44

Test

3

2

2

Baseline

Adipic Acid (S-1) 1 2

201 187

13,500

2,460

13,500

2,360

5.5 5.5

1,040

3

190

13,500

2,420

5.2

1,000

4

193

13,500

2,500

5.0

1,250

88 84

5

192

13,500

5.0

1,800

93

84

33

6 7

196 175

13,500

2,540 2,500

5.5

1,650

96

13,500

2,640

5.0

1,300

90

73 97

89 84

" S l u r r y contains a p p r o x i m a t e l y 10 wt % solids.

302

FLUE


achieve an average of 90 percent SO2 removal from h i g h - s u l f u r f l u e gas. The c a p i t a l investment and revenue requirements are c a l c u l a t e d using a Design/Economics Computer Program which was j o i n t l y developed by TVA and B e c h t e l under EPA sponsorship (9,10). For the purposes of t h i s r e p o r t , four cases were s t u d i e d , i n c l u d i n g a limestone case with MgO a d d i t i v e . The operating cond i t i o n s f o r these cases a r e presented i n Table 9. The evaluat i o n s were based on a 500 MW scrubbing f a c i l i t y i n c o r p o r a t i n g forced o x i d a t i o n , and operating on f l u e gas from a b o i l e r burning eastern c o a l c o n t a i n i n g 4 percent s u l f u r by weight. The cases evaluated were: Case 1 — A limestone base case without a d d i t i v e operated at r e l a t i v e l y high limestone s t o i c h i ometry and l i q u i d - t o - g a s - r a t i o to achieve 90 percent SO2 removal. I t should be noted that long-term r e l i a b i l i t y w i t h t h i s mode of operat i o n has not been demonstrated at Shawnee. Case 2 — A limestone case with MgO a d d i t i o n . O x i d a t i o n of the scrubber bleed stream was chosen because i n - l o o p o x i d a t i o n i s incompatible w i t h magnesium-enhanced scrubbing. As i n Case 1, long-term r e l i a b i l i t y has not been demonstrated at Shawnee f o r t h i s mode of o p e r a t i o n . Case 3 — A limestone case w i t h a d i p i c a c i d a d d i t i o n operated at h i g h pH. Although only 800 ppm a d i p i c a c i d i s r e q u i r e d to o b t a i n 90 percent SO2 removal, degradation of a d i p i c a c i d at high pH r e q u i r e s about f i v e times the t h e o r e t i c a l adipic acid addition rate. Case 4 — A limestone case w i t h a d i p i c a c i d a d d i t i o n operated a t low pH. For t h i s case, 2,000 ppm a d i p i c a c i d i s r e q u i r e d . However, the low pH operation r e q u i r e s only 1.4 times the t h e o r e t i c a l a d i p i c a c i d a d d i t i o n r a t e and 1.05 limestone stoichiometry. The r e s u l t s of the economic e v a l u a t i o n s are presented i n Tables 10 and 11. The c a p i t a l investment and the f i r s t - y e a r revenue requirement i n Table 10 i n c l u d e the dewatering equipment (thickener and f i l t e r ) but exclude the waste sludge ( f i l t e r cake) d i s p o s a l area. Table 11 l i s t s s e p a r a t e l y the f i r s t - y e a r revenue requirement f o r the waste sludge d i s p o s a l area. As shown i n Table 10, both the t o t a l c a p i t a l investment and the f i r s t - y e a r revenue requirement are the lowest f o r a d i p i c a c i d enhanced limestone scrubbing at low pH (Case 4). The t o t a l c a p i t a l investment i s reduced by 4.8 percent, and the f i r s t year revenue requirement reduced by 5.8 percent f o r the limestone/ a d i p i c acid/low pH case (Case 4), compared with the conventional

WANG

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Table 9. Conditions f o r Economie Analysis o f Limestone Scrubbing w i t h Forced O x i d a t i o n and w i t h o r w i t h o u t A d d i t i v e

Capacity:

500 MW

Coal:

4 w t % sulfur

Scrubber:

T C A w i t h 3 beds, 4 grids, a n d 5 inches

SO2 r e m o v a l e f f i c i e n c y :

90%

o f static h e i g h t o f spheres per b e d S u p e r f i c i a l gas v e l o c i t y :

12.5 f t / s e c

Number o f trains:

5, i n c l u d i n g o n e spare t r a i n

Solids d e w a t e r i n g :

T o 8 0 % solids b y t h i c k e n e r a n d r o t a r y

Onstream factor:

5,500 hr/yr

d r u m vacuum filter E f f l u e n t h o l d t a n k residence 5 min

time: O x i d a t i o n t a n k residence t i m e :

5 min

O x i d a t i o n t a n k level:

18 f t

A i r sparger pressure d r o p :

5 psi

O x i d a t i o n tank agitator H p :

0.002 brake Hp/gal

Solid sulfite o x i d a t i o n :

99%

Air stoichiometry:

1.7 l b - a t o m s 0 / l b m o l e SO2 a b s o r b e d

Number of tanks:

2 (effluent hold tank and o x i d a t i o n tank)

Alkali:

Limestone Case N o .

1

2

Additive

_

MgO

4

3 Adipic Acid

Adipic Acid

Additive concentration, p p m

—

5,500< >

800

A d d i t i v e rate, I b / h r

-

104

83.3< )

53.6< >

L/G, gal/Mcf

58

50

50

50

Limestone s t o i c h i o m e t r y , moles C a / m o l e SO2 a b s o r b e d

1.52

1.20

1.20

1.05

T C A inlet p H

5.8

5.4

5.6

4.8

1 loop, 2 tanks

bleed stream

1 loop, 2 tanks

1 loop, 2 tanks

Mode of oxidation

(a) Excess of molar equivalent of chloride. (b) Five times theoretical consumption. (c) 1.4 times theoretical consumption.

a

2,000 b

c

FLUE

304

GAS

DESULFURIZATION

Table 10. Results o f E c o n o m i c A n a l y s i s E x c l u d i n g Waste Sludge Disposal A r e a

Total Capital Investment Case N o .

$ M M (1982)

First Year Revenue R e q u i r e m e n t

$/kW

Cost F a c t o r

$ M M (1984)

Mills/kWh

Cost Factor

1

87.40

174.8

1.000

25.01

9.09

1.000

2

85.26

170.5

0.975

24.15

8.78

0.966

3

83.97

167.9

0.961

24.01

8.73

0.960

4

83.22

166.4

0.952

23.56

8.57

0.942

Revenue r e q u i r e m e n t includes 1 4 . 7 % a n n u a l capital charge. Raw m a t e r i a l costs ( 1 9 8 4 ) :

Limestone

— $8.5/ton

MgO

-

$460/ton

Adipic Acid

-

$1200/ton

Table 1 1 . Revenue R e q u i r e m e n t in Waste Sludge Disposal A r e a F i r s t Y e a r Revenue R e q u i r e m e n t , M i l l s / k W h ( 1 9 8 4 ) Filter Cake,

Total Excluding

Sludge

Case N o .

dry tons/hr

Sludge Disposal

Disposal

9.09

0.97

1

48.7

2

41.6

8.78

3

41.6

8.73

4

38.3

8.57

Total

Cost F a c t o r

10.06

1.000

0.83

9.61

0.955

0.83

9.56

0.950

0.77

9.34

0.928

Revenue r e q u i r e m e n t includes 1 4 . 7 % a n n u a l capital charges. Sludge disposal cost assumes $ 1 0 / d r y t o n , i n c l u d i n g 1 4 . 7 % a n n u a l c a p i t a l charge.

13.

WANG AND BURBANK

Adipic Acid-Enhanced

Lime/Limestone

305

limestone case (Case 1). The revenue requirement i n c l u d e s 14.7 percent annual c a p i t a l charge. T o t a l c a p i t a l investment and o p e r a t i n g costs f o r a d i p i c acid-enhanced limestone at high pH (Case 3) are higher than those f o r l i m e s t o n e / a d i p i c a c i d at low pH (Case 4), but are s t i l l lower than those f o r the conventional limestone (Case 1) or the limestone/MgO case (Case 2). T o t a l c a p i t a l investment i s lower by 3.9 percent, and the f i r s t - y e a r revenue requirement i s lower by 4.0 percent f o r Case 3, compared w i t h Case 1. Table 11 i l l u s t r a t e s the a d d i t i o n a l savings that r e s u l t from a d i p i c a c i d a d d i t i o n . Because of the lower pH o p e r a t i o n , and thus lower limestone consumption, the amount of waste s o l i d s produced i s lower f o r l i m e s t o n e / a d i p i c a c i d cases (Cases 3 and 4) than f o r a limestone case (Case 1). Assuming a l a n d f i l l d i s p o s a l cost of $10/dry ton, i n c l u d i n g 14.7 percent annual c a p i t a l charge, the f i r s t - y e a r revenue requirements f o r the sludge d i s p o s a l area a r e 0.97, 0.83, and 0.77 mills/kWh f o r Cases 1, 3, and 4, r e s p e c t i v e l y . Thus, the t o t a l f i r s t - y e a r revenue requirement, i n c l u d i n g the sludge d i s p o s a l area, i s 9.34 mills/kWh f o r Case 4, compared with 10.06 mills/kWh f o r Case 1. T h i s i s a r e d u c t i o n of 7.2 percent, compared w i t h 5.8 percent when the sludge d i s p o s a l cost i s not i n c l u d e d . These cost f i g u r e s are c i t e d as r e p r e s e n t a t i v e of t y p i c a l scenarios only, and some v a r i a t i o n from them would be normally expected. Moreover, the d i f f e r e n c e s i n t o t a l c a p i t a l investments and o p e r a t i n g costs between these cases are small. The p r i n c i p a l c o n c l u s i o n from these e v a l u a t i o n s i s that a d i p i c a c i d a d d i t i o n to a limestone scrubbing system decreases cost c o n s i s t e n t l y when compared on the same b a s i s . I t should be noted that a d i p i c a c i d use provides a l e v e l of f l e x i b i l i t y i n f u e l and reagent choice and c o n t r o l l e v e l not a v a i l a b l e w i t h other systems, and i n s i t e - s p e c i f i c cases, may prove to be much more economically advantageous than i n d i c a t e d above. Acknowledgment T h i s paper u t i l i z e s data that were acquired as part of the Environmental P r o t e c t i o n Agency's Shawnee Wet Scrubbing Test Program under Contract No. 68-02-3114. J . E. W i l l i a m s , R. H. Borgwardt, and J . D. Mobley were the EPA P r o j e c t O f f i c e r s .

Literature Cited 1.

Bechtel Corporation, "EPA Alkali Scrubbing Test Facility: Summary of Testing through October 1974," EPA-650/2-75-047, NTIS PB 244901, June 1975.

FLUE GAS DESULFURIZATION

306

2.

Bechtel National, Inc., "EPA Alkali Scrubbing Test Facility: Advanced Program, Final Report (October 1974-June 1978)," EPA-600/7-80-115, NTIS PB 80-204241, May 1980.

3.

Rochelle, G. T . ; King, C. J . Ind. Eng. Chem. Fundam. 1977, 16, 67-75.

4.

Borgwardt, R. H. Proceedings: Industry Briefing on EPA Lime/Limestone Wet Scrubbing Test Programs (August 1978), EPA-600/7-79-092, NTIS PB 296517, March 1979, pp. 1-9.

5.

Head, H. N.; Wang, S. C.; Rabb, D. T . ; Borgwardt, R. H.; Williams, J . E.; Maxwell, M. A. Proceedings: Symposium on Flue Gas Desulfurization — Las Vegas, Nevada, March 1979; Volume I, EPA-600/7-79-167a, NTIS PB 80-133168, July 1979, pp. 342-385.

6.

Burbank, D. Α.; Wang, S. C. Proceedings: The Fifth Industry Briefing on IERL-RTP Lime/Limestone Wet Scrubbing Test Pro grams (December 1979), EPA-600/9-80-032, NTIS PB 80-199813, July 1980, pp. 27-113.

7.

Burbank, D. Α.; Wang, S.C.; McKinsey, R. R.; Williams, J . E. Proceedings: Symposium on Flue Gas Desulfurization — Houston, October 1980; Volume 1, EPA-600/9-81-019a, April 1981, pp. 233-286.

8.

Radian Corporation, "Technical Note — Results of Baseline and Adipic Acid Testing at City Utilities Southwest Power Plant, August-September 1980," EPA Contract No. 68-02-3191, October 20, 1980.

9.

Stephenson, C. D.; Torstrick, R. L. Proceedings: The Fifth Industry Briefing on IERL-RTP Lime/Limestone Wet Scrubbing Test Programs (December 1979), EPA-600/9-80-032, NTIS PB 80-199813, July 1980, pp. 167-222.

10.

Anders, W. L.; Torstrick, R. L. "Computerized Shawnee Lime/Limestone Scrubbing Model Users Manual," EPA-600/881-008, TVA/OP/EDT-81/15, March 1981.

RECEIVED

December 21, 1981.

Limestone Test Results at the EPA Alkali

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