Flue Gas Desulfurization - American Chemical Society

14 Energy Requirements for SO Absorption in Limestone Scrubbers Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on August 25, 2015 | http://pubs.acs.org Publication Date: July 1, 1982 | doi: 10.1021/bk-1982-0188.ch014

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ROBERT H. BORGWARDT U.S. Environmental Protection Agency, Industrial Environmental Research Laboratory, Utilities and Industrial Processes Division, Research Triangle Park, NC 27711 The energy needed to operate limestone scrubbers for flue gas desulfurization (FGD) has been estimated to be as much as 8 percent of the total energy produced by power plants. A major part of the energy demand is electrical power consumed by the fans and pumps of the SO absorber. This paper examines the net work input and gross specific electric demand of three types of limestone scrubbers, using data reported in the l i t erature on pressure drop and slurry recirculation rates for high-sulfur coal applications. It shows that the net work required for 90 percent SO removal is 70 ft-lb/cu ft of flue gas scrubbed in a turbulent contacting absorber (TCA) and 82 ft-lb/cu ft in a spray tower (gas volumes at 125°F and saturated). At 60 percent pump efficiency and 70 percent fan efficiency, the gross electric energy demand for 90 percent SO removal in the TCA and spray tower is 2.8 and 3.4 W/cfm, respectively. A spray tower will nevertheless require only 0.16 percent more of the total plant power production than a TCA scrubber. The data on a high-velocity cocurrent grid tower indicate that it will be more energy efficient than a counter -current spray tower, in addition to reducing the capital cost. Well designed FGD systems using no reheat should not require more than 1.3 percent of the total plant power production. Adipic acid, added at a concentration of 1400 ppm to the scrubbing liquor, can reduce the electric power demand of the absorber by 30 percent. The additive is most cost-effective, however, when used to increase the limestone utilization. New dual-alkali limestone scrubbers can reduce the total energy demand of a FGD system to less than that generated by the combustion of the sulfur in most high sulfur coals. 2

2

2

This chapter not subject to U.S. copyright. Published 1982 American Chemical Society. In Flue Gas Desulfurization; Hudson, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

FLUE

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on August 25, 2015 | http://pubs.acs.org Publication Date: July 1, 1982 | doi: 10.1021/bk-1982-0188.ch014

308

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

An a n a l y s i s of the energy and economic impacts of p o l l u t i o n c o n t r o l f o r c o a l - f i r e d power plants (_1) estimated that as much as 8 percent of the net power production would be r e q u i r e d to achieve 92 percent SO2 removal with limestone f l u e gas d e s u l f u r i z a t i o n (FGD) systems. Others (2^) have reported that 2 to 5 percent of the t o t a l power generating c a p a c i t y o f a b o i l e r u n i t i s g e n e r a l l y required to operate SO2 s c r u b b e r s — e x c l u s i v e o f the energy used to reheat the cooled f l u e g a s — a n d that the power p l a n t ' s net generating c a p a c i t y i s e f f e c t i v e l y "de-rated" by that amount. A study prepared by TVA (3) estimates that 3.5 percent o f the t o t a l power-plant energy input w i l l be r e q u i r e d to meet the current emission standard o f 1.2 l b SO2/M Btu, or 90 percent removal, f o r new p l a n t s burning high s u l f u r c o a l s . TVA s a n a l y s i s showed that the t o t a l energy demand of the FGD system i s d i v i d e d about e q u a l l y between the steam requirements f o r f l u e gas reheat and the e l e c t r i c i t y r e q u i r e d as "process energy." The process energy can be broken down i n t o four c a t e g o r i e s , each accounting f o r the approximate f r a c t i o n o f t o t a l e l e c t r i c demand i n d i c a t e d i n parentheses : 1

1) 2)

3)

4)

E l e c t r i c power r e q u i r e d by the scrubber fans and r e c y c l e pumps to e f f e c t SO2 absorption (60 p e r c e n t ) . E l e c t r i c power consumed by the scrubber fans to overcome the pressure drop i n the connecting ductwork and dampers, and to move the a i r leaked through the d u c t w o r k — p r i m a r i l y at the a i r preheater (24 p e r c e n t ) . E l e c t r i c power used f o r raw m a t e r i a l s handling and p r e p a r a t i o n ; e.g., limestone g r i n d i n g and waste d i s posal pumps (9 p e r c e n t ) . E l e c t r i c power needed to operate a n c i l l a r y scrubber equipment such as e f f l u e n t hold-tank mixers, s a t u r a t i o n spray pumps, and thickener rake (7 percent).

Since the energy r e q u i r e d f o r reheat i s as great as the sum o f a l l other energy demands, most of the e f f o r t to reduce FGD energy consumption has focused on the reheat problem. Bypass reheat has been proven the most e n e r g y - e f f i c i e n t technique f o r use at power plants burning low s u l f u r c o a l s . A limestone scrubber of t h i s type i s s a i d to operate on l e s s than 1.3 percent of p l a n t power product ion ( 4 ) . For most c o a l s , the 1979 EPA New Source Performance Standards w i l l e f f e c t i v e l y d i s a l l o w bypass reheat i n new power p l a n t s . Although reheat may provide a s l i g h t r e d u c t i o n i n ground l e v e l p o l l u t i o n C5) i n the immediate v i c i n i t y of a power p l a n t , i t s p r i n c i p a l f u n c t i o n i s not p o l l u t i o n c o n t r o l , but c o r rosion control. Some p l a n t s are a v o i d i n g reheat a l t o g e t h e r by using new stack l i n e r designs and more c o s t l y m a t e r i a l s o f c o n s t r u c t i o n , i n c l u d i n g Inconel 625, to p r o t e c t equipment downstream of the scrubber; Muela's survey of 103 e x i s t i n g and proposed u t i l i t y FGD systems i n the U.S. showed 20 using t h i s approach. A more innovative s o l u t i o n to the problem i s now being

In Flue Gas Desulfurization; Hudson, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


14.

BORGWARDT

Energy Requirements for S0

2

Absorption

309

used i n Japan and Europe: Ljungstrom type heat exchangers which reheat the scrubbed f l u e gas with the hot (320°F) scrubber i n l e t gas from the a i r preheater. Ando (6) reports a limestone FGD system that i s a c h i e v i n g 149°F of reheat by t h i s method. The r e l a t i v e magnitude o f the four c a t e g o r i e s of e l e c t r i c power demand w i l l vary with the type of scrubber and c o n f i g u r a t i o n of the FGD system. T y p i c a l l y , more than 80 percent of the process energy w i l l be consumed by the fans and r e c y c l e pumps ( c a t e g o r i e s 1 and 2). Most of t h i s process energy i s expended s p e c i f i c a l l y f o r SO2 absorption; i . e . , to overcome the pressure drop o f the absorption tower and to r e c i r c u l a t e the absorbing s l u r r y . Improvements i n scrubber design which reduce the power requirements o f the absorber can thus have a s i g n i f i c a n t impact on the t o t a l energy demand. For those systems using no reheat or heat exchange, the e l e c t r i c power r e q u i r e d f o r SO2 absorption w i l l comprise most o f the t o t a l energy input and large reductions are p o s s i b l e . A s e n s i t i v i t y a n a l y s i s o f operating v a r i a b l e s which a f f e c t the energy requirements o f limestone scrubbers (_7) i d e n t i f i e d pH as the most important f a c t o r r e l a t e d to the absorber. As a p r a c t i c a l matter, however, the operating pH i s f i x e d by c o n s t r a i n t s imposed by r e l i a b i l i t y and waste production so that i t does not o f f e r an e f f e c t i v e means o f reducing energy demand. I t i s known, f o r example, that the limestone s t o i c h i o m e t r i c r a t i o — w h i c h l a r g e l y determines pH—must be maintained below 1.17, corresponding to a pH of 5.6, to avoid mist e l i m i n a t o r f o u l i n g (8). A pH o f about 5.8 i s the maximum that can be used i n p r a c t i c e f o r closed loop operation with high s u l f u r c o a l s . Since t h i s pH corresponds to a s t o i c h i o m e t r i c r a t i o of 1.4 to 1.5, a s p e c i a l washing p r o t o c o l f o r the mist e l i m i n a t o r s must be s t r i c t l y adhered to f o r r e l i a b l e scrubber operation. The t r a d e o f f of absorption e f f i c i e n c y (and energy consumption) for r e l i a b i l i t y i s a l s o encouraged by the high cost of d i s p o s a l of the excess sludge produced when operating at high s t o i c h i o m e t r i c r a t i o s . An a l t e r n a t i v e to v a r y i n g pH i s to employ scrubber a d d i t i v e s which enhance mass t r a n s f e r , such as MgO or a d i p i c a c i d . These a d d i t i v e s have been shown to have a large e f f e c t on the l i q u i d - t o - g a s r a t i o r e q u i r e d to achieve a given SO2 removal e f f i c i e n c y and, i n the case of a d i p i c a c i d , to a l s o reduce the amount o f limestone needed. The o b j e c t i v e o f t h i s a n a l y s i s i s to e s t a b l i s h , by examination of a v a i l a b l e data on pressure drop and pumping r a t e s , the energy required for SO2 absorption (category 1) as a f u n c t i o n o f removal e f f i c i e n c y . The a v a i l a b l e data w i l l be compared f o r scrubbers of d i f f e r e n t types at a given removal e f f i c i e n c y and pH when operating with a c l o s e d water loop and f l u e gas from combustion o f high s u l f u r c o a l . Low s u l f u r c o a l s w i l l r e q u i r e l e s s energy or a given removal e f f i c i e n c y , since mass t r a n s f e r i s enhanced by lower i n l e t SO2 concentration. F i n a l l y , the e f f e c t on energy demand of a d d i t i v e s which a f f e c t SO2 mass t r a n s f e r w i l l be determined.


FLUE

310



Procedure The power r e q u i r e d to move f l u e gas through the scrubber, from the absorber i n l e t to the mist e l i m i n a t o r o u t l e t , was c a l c u l a t e d from pressure drop and gas flow-rate data reported i n the l i t e r a t u r e f o r a range o f s u p e r f i c i a l v e l o c i t i e s , l i q u i d / g a s r a t i o s , and i n t e r n a l scrubber packings. A fan of the wet inducedd r a f t type was assumed f o r each case, operating on saturated f l u e gas at 125°F. The gas-side power input was added to the power d e l i v e r e d through the s l u r r y r e c i r c u l a t i o n pumps which was c a l culated from the volumetric flow r a t e s and the minimum discharge pressures required f o r the given scrubbers. The t o t a l power input for SO2 absorption was thus determined as: Ρ where:

=

Q(AP)

+

L(H)

(1)

Ρ i s the t o t a l net mechanical power d e l i v e r e d to the scrubber, f t - l b / m i n ; Q i s the volumetric flow r a t e of f l u e gas out of the scrubber (saturated at 125°F), cu ft/min; ΔΡ i s the gas pressure drop across the absorber ( i n c l u d i n g the mist e l i m i n a t o r ) , l b / s q f t ; L i s the s l u r r y r e c i r c u l a t i o n r a t e through the ab sorber cu ft/min; and H i s the s l u r r y discharge head of the pumps, determined as: y

H where:

=

(H-H )p + t

P

+

n

p

(2)

L

h i s the height o f the absorber i n l e t n o z z l e s , f t ; h i s the height of the s l u r r y surface i n the e f f l u e n t hold tank, f t ; ρ i s the d e n s i t y o f the s l u r r y (64.8 lb/cu f t at 8 percent s o l i d s ) ; ρ i s the spray nozzle pressure drop, l b / s q f t ; and p^ i s the pressure drop i n the s l u r r y p i p i n g due to f r i c t i o n and a c c e l e r a t i o n , l b / s q f t . t

To make comparisons on a c o n s i s t e n t b a s i s , Ρ was d i v i d e d by the f l u e gas throughput to o b t a i n the work r e q u i r e d per cubic foot o f gas cleaned. T h i s net s p e c i f i c - w o r k input, i n f t - l b / c u f t , was used to compare the r e l a t i v e e f f i c i e n c i e s of d i f f e r e n t scrubber types f o r achieving a given SO2 removal. The gross e l e c t r i c energy demand, i n W/cfm of f l u e gas scrubbed, was obtained by d i v i d i n g the net work input by the e f f i c i e n c i e s of the fan and pump f o r converting e l e c t r i c a l energy to mechanical energy: 1 - · Α Λ Gross e l e c t r i c demand

0.0226 = ne

r

ΔΡ [— n

A

f

L(H) — J QiTp 1

+


(3)

14.

BORGWARDT


where:


2

η

311

Absorption

i s the e l e c t r i c a l e f f i c i e n c y o f the motors; i s the mechanical e f f i c i e n c y o f the fan; and η ρ i s the mechanical e f f i c i e n c y o f the pumps. e

Table I summarizes the p r i n c i p a l features o f the scrubber systems. Most o f the data are from the prototype t e s t f a c i l i t y operated f o r EPA by Bechtel N a t i o n a l , Inc. from 1972 through 1980 at the TVA Shawnee Power P l a n t . The pumping heads shown f o r the TCA ( t u r b u l e n t c o n t a c t i n g absorber) and spray tower i n Table I represent o p e r a t i o n with a 6 - f t clearance between the bottom scrubber flange and the surface of the s l u r r y i n the e f f l u e n t hold tank (EHT). Due to an o v e r s i z e d EHT used i n the prototypes, most t e s t s were a c t u a l l y conducted with the EHT only o n e - t h i r d f u l l and the pumping heads were 11 f t higher than those i n d i c a t e d . This free space i s assumed to be designed out of both scrubbers when the work input i s c a l c u l a t e d here. An equal and constant value of ρ = 1000 l b / s q f t was assumed i n a l l cases i n accordance with the recommended p r a c t i c e of maintaining 8 to 10 f t / s e c s l u r r y v e l o c i t y i n the r e c y c l e l i n e s . An important feature o f the three scrubbers, and a necessary c o n d i t i o n f o r meaningful comparisons between them, i s that the same limestone type and g r i n d was used i n a l l t e s t s . I t contained 95 percent CaC03 and l e s s than 3 percent MgC03; the g r i n d was 90 percent passing 325 mesh. The scrubber i n l e t S0£ concentrations were 2300 to 2800 ppm. The scrubbers operated closed-loop with c h l o r i d e l e v e l s averaging 3000 ppm i n the scrubbing l i q u o r . TABLE I. FEATURES OF PROTOTYPE SCRUBBERS

TCA Scrubber cross s e c t i o n , sq f t 32 EHT s l u r r y l e v e l t o : 33 Upper spray header, f t N/A Lower spray header, f t N/A Venturi, f t 5 Pressure drop across nozzles, p s i

Spray Tower

Cocurrent Scrubber

50

12.5

33 24 20 10

15, 41 31 N/A 10

Results and D i s c u s s i o n TCA Scrubber. Head (9) reported SO2 removal e f f i c i e n c i e s o f a limestone TCA operated at a feed s l u r r y pH o f 5.8 and three l e v e l s each of gas v e l o c i t y and s l u r r y r e c i r c u l a t i o n r a t e . Those data are p l o t t e d i n Figure 1 as a f u n c t i o n of the net work input, c a l c u l a t e d according to Equations (1) and (2). The p a r t i a l l y closed symbols denote t e s t s with three beds o f n i t r i l e foam spheres, each bed



312

FLUE


having 5 i n . s t a t i c depth. The open symbols denote t e s t s without spheres, i n which case the absorber i n t e r n a l s c o n s i s t e d only of the four bar g r i d s which normally support the TCA spheres. I t i s evident from Figure 1 that a net work input of 70 f t - l b / c u f t i s r e q u i r e d to remove 90 percent o f the scrubber i n l e t SO2. For 70 percent removal, 41 f t - l b / c u f t i s r e q u i r e d . The net work required to remove a given amount o f SO2 was independent of scrubber gas v e l o c i t y w i t h i n the range t e s t e d , 8.3 to 12.5 f t / s e c . Figure 1 a l s o shows that removal e f f i c i e n c y increases i n a continuous manner with work input f o r o p e r a t i o n with and without spheres. Thus, the spheres do not p e r c e p t i b l y increase the work requirement, but g r e a t l y improve the SO2 absorption f o r a given s l u r r y r e c i r c u l a t i o n r a t e . The a d d i t i o n a l pressure drop caused by the spheres was thus f u l l y e n e r g y - e f f e c t i v e at bed depths up to 5 i n . Tests with the bed depth increased to 7.5 i n . (by adding more spheres) were not energy e f f e c t i v e ; the SO2 removal obtained f o r a given work input was about 10 percent lower than the curve of Figure 1. Spray Tower. Figure 2 shows the SO2 removals reported by Head 09) and by Burbank and Wang(10) f o r a limestone spray tower, p l o t t e d as a f u n c t i o n o f the net work input. The feed l i q u o r pH was again 5.8 ( s t o i c h i o m e t r i c r a t i o = 1.4 to 1.5). Gas v e l o c i t i e s and s l u r r y r e c i r c u l a t i o n rates were s y s t e m a t i c a l l y v a r i e d from 5.4 to 9.4 f t / s e c , and 15 to 30 gpm/sq f t , r e s p e c t i v e l y . The data i n c l u d e scrubber c o n f i g u r a t i o n s with and without a v e n t u r i preceding the spray tower. A l l data are f o r s i n g l e - l o o p mode o f o p e r a t i o n ; i . e . , the v e n t u r i and spray tower were fed from a s i n g l e EHT. The pumping energy was c a l c u l a t e d f o r the spray tower assuming that two pumps are used f o r s l u r r y r e c i r c u l a t i o n : one f o r the two upper spray headers and one f o r the two lower headers. Equal flows to the upper two headers were assumed f o r l i q u i d r a t e s up to h a l f the maximum t o t a l flow; beyond h a l f maximum flow, the bottom two headers were assumed to r e c e i v e the remaining flow* The v e n t u r i , when used, was assumed to be provided with a t h i r d pump. The pressure drop across the spray nozzles was assumed constant at 10 psi. The spray tower data c o r r e l a t e with work input i n a manner s i m i l a r to the TCA; the only data not f o l l o w i n g that c o r r e l a t i o n were the v e n t u r i t e s t s at the highest gas v e l o c i t y , 9.4 f t / s e c , i n d i c a t i n g an a d d i t i o n a l 15 f t - l b / c u f t was needed to a t t a i n a given SO2 removal. One can conclude that the v e n t u r i i s not energy e f f i c i e n t at v e l o c i t i e s above 7.4 f t / s e c . Comparison o f Figures 1 and 2 shows that the t o t a l net work r e q u i r e d f o r 90 percent SO2 removal i n the spray tower, 82 f t - l b / c u f t , i s about 17 percent higher than the 70 f t - l b / c u f t r e q u i r e d by the TCA. Work i s d e l i v e r e d to the spray tower p r i m a r i l y through the s l u r r y pumps. The TCA scrubber d e l i v e r s most of the work through the fans due to i t s lower l i q u i d / g a s r a t i o and higher pressure drop. I f the mechanical e f f i c i e n c i e s o f the pumps and fans were



14.

BORGWARDT

TOWER INTERNALS

WITH SPHERES

/ 10

313

Energy Requirements for SOn Absorption

20

WITHOUT SPHERES

30

40

50

LIQUOR RATE, gpm/sq ft

GAS RATE, ft/sec

8.3

19

28

37

Δ

Δ

Δ

12.5

y

αΦ

Β β

8.3

Δ

Δ

m Φ

• ο

Δ Β

10.4

9

10.4 12.5

60

70

_

-

θ

80

90

NET WORK INPUT, ft-lb/cu ft

Figure 1.

S0

2

removal efficiency of TCA scrubber as a function of net work input, pH 5.8.



314

FLUE


equal, then the gross e l e c t r i c a l energy demand o f the two scrubber types would f o l l o w the same r e l a t i v e responses as Figures 1 and 2. Although the e f f i c i e n c y o f s l u r r y pumps i s g e n e r a l l y agreed to be about 60 percent, fan e f f i c i e n c i e s are higher—sometimes approaching 80 percent. As a r e s u l t , the gross e l e c t r i c demand w i l l be lower f o r scrubbers that d e l i v e r most of the work through the fans. Table 2 compares the same data as Figures 1 and 2 on the b a s i s o f gross e l e c t r i c demand c a l c u l a t e d by Equation (3) f o r various assumed fan e f f i c i e n c i e s . Whereas the spray tower i s l i t t l e a f f e c t e d by fan e f f i c i e n c y , the e l e c t r i c demand o f the TCA can be reduced s i g n i f i c a n t l y by the use o f more e f f i c i e n t fans. Cocurrent Flow. Henson (11) reported t e s t s with a 4 5 - f t high limestone scrubber i n which both the f l u e gas and the scrubbing s l u r r y entered the top o f the tower and moved downward i n cocurrent flow. This c o n f i g u r a t i o n i s capable of higher gas v e l o c i t i e s than the countercurrent scrubbers discussed above. The p r i n c i p a l advantage expected of t h i s design i s a r e d u c t i o n of c a p i t a l cost due to the smaller scrubber diameter f o r a given throughput. The scrubber height was v a r i e d i n those t e s t s as w e l l as gas v e l o c i t y , L/G, and tower i n t e r n a l s . The limestone s t o i c h i o m e t r i c r a t i o was again 1.4 mol/mol o f SO2 absorbed. When operated as a 4 5 - f t open spray tower and 18 f t / s e c gas v e l o c i t y , a s l u r r y r e c i r c u l a t i o n rate o f 192 gpm/sq f t was r e q u i r e d f o r 88 percent SO2 removal. Henson reported an increase i n pressure across the tower under these c o n d i t i o n s due to the energy t r a n s f e r r e d to the gas by the downward flowing s l u r r y . Using Equations (1) and (2) to c a l c u l a t e the net work input from these data, c r e d i t i n g the work recovered due to the 0.6-in. H2O pressure g a i n , y i e l d s a value of 119 f t lb/cu f t . Comparison o f t h i s value with Figure 2 shows that the open cocurrent spray tower i s l e s s energy e f f i c i e n t than the countercurrent type. Henson added s i x 1-1/4 i n . t h i c k g r i d s to the cocurrent tower and obtained 90 percent SO2 removal with a shorter ( 3 5 - f t ) tower and higher (27 f t / s e c ) gas v e l o c i t y . A p o s i t i v e 3.0-in. H2O pressure drop was observed i n t h i s case with the s l u r r y r e c i r c u l a t i o n r a t e again at 192 gpm/sq f t . The net work input was reduced to 91 f t - l b / c u f t due to the lower pumping head. Another t e s t with s i x 3-3/4 i n . g r i d s i n the 35-ft tower y i e l d e d 6.5-in. H2O pressure drop at 27 f t / s e c gas v e l o c i t y , but only 112 gpm/sq f t was needed to achieve 90 percent SO2 removal. The net work input c a l c u l a t e d f o r these c o n d i t i o n s i s 78 f t - l b / c u f t . A d d i t i o n o f g r i d s to the cocurrent tower thus reduced the work needed f o r 90 percent SO2 removal to a l e v e l comparable to the countercurrent scrubbers. Since a l a r g e part of the work d e l i v e r e d to the g r i d packed cocurrent scrubber i s through the fan, i t w i l l be capable o f operating with lower e l e c t r i c power demand than the countercurrent spray tower, as shown by the comparison i n Table I I . One can conclude from these comparisons that the reductions i n c a p i t a l costs expected from cocurrent h i g h - v e l o c i t y scrubbers can be r e a l i z e d without penalty i n energy demand.


14.

BORGWARDT


2

TABLE I I .

315

Absorption

GROSS ELECTRIC ENERGY INPUT FOR 90 PERCENT S 0 REMOVAL


2

Mechanical E f f i c i e n c y o f Fan, percent

Gross Energy

Input, W/cfm

a

Cocurrent Grid Tower

TCA

Spray Tower

60

3.0

3.45

3.27

70

2.75

3.41

3.07

80

2.51

3.33

2.92

a

Pump e f f i c i e n c y = 60 percent; 90 percent motor e f f i c i e n c y .

The gross e l e c t r i c energy demands of the three types o f limestone scrubbers are compared i n Table I I I as percentages o f the t o t a l plant power production. The comparison i s based on 2420 cfm per MW (saturated f l u e gas at 125°F, 1 atm) and 70 percent fan e f f i c i e n c y . In no case does the e l e c t r i c demand f o r 90 percent S 0 removal exceed 0.83 percent o f production. I t i s i n t e r e s t i n g to note that the d i f f e r e n t i a l between the highest demand (spray tower) and the lowest demand (TCA) amounts to only 0.16 percent o f the t o t a l plant power production. From t h i s viewpoint, the s i m p l i c i t y and increased r e l i a b i l i t y o f the spray tower are not expensive a t t r i b u t e s i n terms o f a d d i t i o n a l energy d r a i n . 2

TABLE I I I . ENERGY CONSUMED FOR 90 PERCENT S0 REMOVAL IN LIMESTONE SCRUBBERS 3

2

Scrubber

Type

Percentage of T o t a l Plant Power Production

Spray tower

0.825

Cocurrent g r i d tower

0.743

Turbulent c o n t a c t i n g absorber (TCA)

0.666

a

Wet fan, 70 percent e f f i c i e n t ; 60 percent pump e f f i c i e n c y .

b

C o a l heating value = 11,000 Btu/lb.

1



316

FLUE


A d i p i c A c i d . Any a d d i t i v e which enhances SO2 mass t r a n s f e r , e s p e c i a l l y at low scrubber pH, w i l l increase limestone u t i l ization. The use of a d i p i c a c i d as an a d d i t i v e to limestone scrubbes has been shown to improve both the SO2 removal e f f i c i e n c y and limestone u t i l i z a t i o n at any given set of scrubber operating conditions. As a r e s u l t , both SO2 removal e f f i c i e n c y and limestone u t i l i z a t i o n are increased. A lower work input i s a l s o required to achieve a given removal e f f i c i e n c y . Figure 3 shows the e f f e c t of 1400 ppm a d i p i c a c i d i n the scrubbing l i q u o r of a TCA system (_12) ; comparison with F i g u r e 1 shows a 30 percent r e d u c t i o n of the work input r e q u i r e d f o r 90 percent SO2 removal, from 70 to 48 f t - l b / c u f t . S i m i l a r p l o t s at other a d i p i c a c i d concentrations y i e l d e d the r e l a t i o n s h i p shown i n F i g u r e 4. A l l of the t e s t s are reported to have been made at a limestone s t o i c h i o m e t r i c r a t i o o f 1.2, which corresponds to a u t i l i z a t i o n o f 83 percent. Two options are a v a i l a b l e . The a d d i t i v e may be used to reduce the energy demand or i t can be used to increase limestone utilization. Table IV compares these options, using the 30 percent r e d u c t i o n i n gross e l e c t r i c demand as a b a s i s f o r the evaluation. Since 90 percent removal r e q u i r e s only 0.67 percent of the plant power production (237 kW-hr/ton of SO2 absorbed) without a d i p i c a c i d , a 30 percent r e d u c t i o n does not y i e l d s u f f i c i e n t savings i n e l e c t r i c power to cover the cost of the a d d i t i v e . I f , on the other hand, 2.4 percent or more o f the power production were r e q u i r e d by the absorber, the value of the power saved would exceed the cost o f the a d i p i c a c i d used, r e g a r d l e s s of c r e d i t s f o r any other cost r e d u c t i o n s . Table IV shows that the a l t e r n a t i v e approach i s most e f f e c t i v e — m a i n t a i n i n g a higher work input and operating with as l i t t l e limestone feed (and t h e r e f o r e as l i t t l e sludge production) as p o s s i b l e . By using the a d i p i c a c i d to reduce the s t o i c h i o m e t r i c r a t i o from 1.4 to 1.1, the value of the limestone saved and the reduced cost o f sludge d i s p o s a l defray the t o t a l cost of the a d i p i c a c i d plus one t h i r d of the e l e c t r i c power demand o f the absorber. The b a s i s f o r estimating the e f f e c t of a d i p i c a c i d on u t i l i z a t i o n i s seven runs made i n the Shawnee TCA at 1600 ppm c o n c e n t r a t i o n and net work input of 72 f t - l b / c u f t . These runs, Nos. 903-2A&B, 907-2L, 928-2A, 932-2A, and 934-2C&D, averaged 95 percent SO2 removal and 90 percent limestone u t i l i z a t i o n . The slope o f the curve of F i g u r e 1 was used to estimate the energy demand at 90 percent removal. Table IV a l s o shows that the a d i p i c a c i d makeup r a t e must be held below 15.4 l b / t o n o f SO2 absorbed i n order to recover the cost of the a d d i t i v e when operating at 90 percent limestone u t i l i z a t i o n . This l i m i t i s increased only s l i g h t l y , to 16.5 l b / t o n of SO2, at a 97 percent l e v e l o f limestone u t i l i z a t i o n . The makeup r a t e used i n Table IV was obtained from two independent Shawnee measurements. One measurement i s the average of eight runs (9342A to -2H) made without f l y ash and with 78 percent s o l i d s i n the discharged sludge; these runs r e q u i r e d an average makeup r a t e of 9.95 l b / t o n SO2. The second measurement was obtained from


BORGWARDT


100 ι

~l

I

I

I

ι

317

Absorption

2

ι

I

I

I

Γ


90

c ο

2

80

70

ο

ο Ο" CO

TOWER INTERNALS

60

50

J_

40 20

10

30

OXIDATION MODE NATURAL

WITH SPHERES

#

WITHOUT SPHERES

Q

FORCED

•

_L

40

50

80

70

60

N E T W O R K INPUT, ft-lb/cu ft

Figure 3.

S0

2

removal efficiency of TCA scrubber vs. net work input with 1400 ppm adipic acid in scrubbing liquor.

d ζ < Έ m

a

cc LU

I ο ο

ill

ο ζ ο I(J D

Û LU

ce

400

800

1200

1600

2000

2400

ADIPIC ACID CONCENTRATION, ppm

Figure 4.

Effect of adipic acid concentration on gross electric power demand for 90% S0 removal in TCA scrubber. 2


318

FLUE

TABLE IV.

Basis:


COMPARISON OF OPERATING ALTERNATIVES FOR ADIPIC ACID ADDITION One ton o f S O 2 absorbed i n TCA scrubber 90 percent S 0 removal e f f i c i e n c y F l y - a s h - f r e e sludge, 70 percent s o l i d s Natural o x i d a t i o n Wet fan, 70 percent e f f i c i e n t


2

Without A d i p i c Acid

With A d i p i c Acid

Limestone s t o i c h i o m e t r i c ratio

1.4

1.2

1.1

Dry

2.18

1.87

1.71

2.65

2.34

2.18

a

166

207

1400

1500

limestone feed, tons

Sludge produced; dry b a s i s

tons,

Power input, kW-hr

237

A d i p i c a c i d cone., ppm

0

A d i p i c a c i d makeup, l b

0

11.9

10.7

Value of limestone saved (@ $7/ton)

$2.66

$3.29

Cost r e d u c t i o n f o r sludge d i s p o s a l (@ $9.4/ton)

$2.91

$4.42

Value o f power saved

$2.06

$0.87

$7.63

$8.58

Cost o f a d i p i c a c i d added (@ $0.55/lb)

$6.5

$5.9

T o t a l saving - cost of a d d i t i v e

$1.1

$2.7

T o t a l saving i n operating costs

a

T o t a l cost of power f o r 90 percent S 0 absorption = 237 kW-hr χ $0.029/kW-hr = $6.87 2



14.

BORGWARDT


2

Absorption

319

R e l i a b i l i t y Run 932-2A, which was made with f l y ash, discharged a sludge c o n t a i n i n g 61 percent s o l i d s , and r e q u i r e d a makeup of 17.5 l b / t o n SO2. When these data are normalized to a f l y - a s h - f r e e b a s i s , at equal limestone u t i l i z a t i o n , and equal sludge moisture, they y i e l d the same makeup rate of 10.7 l b / t o n SO2 for a c o n c e n t r a t i o n of 1500 ppm a d i p i c a c i d i n the scrubbing l i q u o r . The l i m i t a t i o n on a d i p i c a c i d makeup r a t e l i k e w i s e implies a minimum acceptable f i l t r a t i o n e f f i c i e n c y for break-even oper a t i o n : on a f l y - a s h - f r e e b a s i s , the sludge must c o n t a i n more than 46 percent s o l i d s when operating at 90 percent limestone u t i l i z a t i o n and 1500 ppm a d i p i c a c i d . For both economic and e n v i r o n mental reasons, f i l t e r washing should be employed whenever a d d i t i v e s are used i n limestone scrubbers. Figure 3 a l s o shows that 86 percent SO2 removal can be a t t a i n e d without spheres i n the TCA scrubber when using 1400 ppm a d i p i c acid. By i n c r e a s i n g the work input by only 5 f t - l b / c u f t , 90 percent SO2 removal can be expected while e l i m i n a t i n g the main tenance and r e l i a b i l i t y problems a s s o c i a t e d with the spheres. The necessary work can be d e l i v e r e d by r a i s i n g the pumping r a t e or by i n s t a l l i n g a d d i t i o n a l g r i d s i n the tower. In view of the above d i s c u s s i o n , the gross e l e c t r i c demand would be minimized by the l a t t e r course. Fan L o c a t i o n . The fan can be l o c a t e d downstream from the scrubber, i n which case i t operates on saturated f l u e gas at 125°F (normally), or the fan can be located upstream, where i t operates on hot (ca. 300°F) gas c o n t a i n i n g about 8 percent moisture. The hot booster fan handles a greater gas volume and t h e r e f o r e r e q u i r e s more energy than the wet fan. A fan located a f t e r a reheater w i l l l i k e w i s e consume more energy than the wet fan because of the higher temperature ( c a . 175°F) and the s l i g h t a d d i t i o n a l pressure drop caused by the reheater. Figure 5 i s a r e p l o t of the data i n F i g u r e s 1 and 2 to show the gross e l e c t r i c demand of scrubbers using hot booster fans. The spray tower i s unaffected: most of the energy i s d e l i v e r e d through the pumps and the energy r e q u i r e d for 90 percent SO2 removal remains at 0.83 percent of plant power production. The advantage of the higher- Ρ TCA scrubber v i r t u a l l y disappears, however, i n c r e a s i n g from 0.67 percent with a wet fan to 0.73 percent with the hot booster fan. Scrubbers which are preceded by 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 f o r dry removal of f l y ash can use a i r f o i l - t y p e booster fans that provide maximum e f f i c i e n c y . Assuming that a hot fan of t h i s type-and having 80 percent mechanical e f f i c i e n c y — i s used with the TCA, the gross energy demand for 90 percent SO2 removal i s 2.80 W/cfm. Category-2 Demands. The energy consumed by the fan to overcome the pressure drop i n that part of the ductwork which i s d i r e c t l y a s s o c i a t e d with the scrubber i s chargeable to the FGD system. This i n c l u d e s the m o d u l a r i z a t i o n ducts and dampers and the reheater, i f used. I t a l s o i n c l u d e s the energy r e q u i r e d to move through the



320 FLUE GAS DESULFURIZATION


14.

BORGWARDT


2

321

Absorption


scrubber any a i r leaked i n v i a the preheater. With a 2-in. pressure drop i n the m o d u l a r i z a t i o n ducts and 10 percent a i r leakage, the t o t a l process energy r e q u i r e d by a spray-tower FGD system operating without reheat i s shown i n Table V. The energy demand f o r a n c i l l a r y motors and raw m a t e r i a l s handling was obtained from the t o t a l horsepower r a t i n g s o f equipment l i s t e d (13) f o r a 500-MW power p l a n t . TABLE V.

PROCESS ENERGY REQUIREMENTS FOR LIMESTONE FGD

Energy Demand, Percent of Plant Power Production

Component

Scrubber

(spray tower)

0.83

A n c i l l a r y motors and raw m a t e r i a l s handling

0.28

M o d u l a r i z a t i o n ducts and dampers

0.10

Leakage

0.08 Total

1.29

S u l f u r . The energy produced by the combustion o f the s u l f u r i n c o a l i s not an i n s i g n i f i c a n t c o n t r i b u t i o n to the t o t a l power generated, e s p e c i a l l y f o r high s u l f u r c o a l . Depending on the ash and water content, 1 l b o f 4-percent s u l f u r c o a l w i l l y i e l d 1.1 to 1.5 percent more energy than would be obtained i f the s u l f u r were removed p r i o r to combustion. I t i s s i g n i f i c a n t to note that the t o t a l process energy r e q u i r e d by FGD systems o f current design i s w i t h i n the same range. P o t e n t i a l for Improvement. Further reductions can be expected i n the energy r e q u i r e d by the newer generation o f limestone FGD systems. The Chiyoda 121 process, which i s already o f f e r e d commercially, e l i m i n a t e s scrubber r e c y c l e e n t i r e l y and reduces energy demands and maintenance problems a s s o c i a t e d with s l u r r y pumps. The gas-side pressure drop reported (14) f o r the Chiyoda j e t - b u b b l e r absorber i s 11.8 i n . H 0 at 90 percent S 0 removal. Disregarding the energy used f o r a i r compression ( f o r c e d o x i d a t i o n i s an i n t e g r a l part of that process) the corresponding net work input i s 61 f t - l b / c u f t . By i n c o r p o r a t i n g sludge s t a c k i n g with the forced o x i d a t i o n , i t a f f o r d s the prospect o f e l i m i n a t i n g the c l a r i f i c a t i o n and f i l t r a t i o n steps which would a l s o a f f e c t c a p i t a l 2

2


FLUE


322

GAS

DESULFURIZATION

c o s t s favorably. Another example i s the dual a l k a l i system developed by Arthur D. L i t t l e , Inc., which uses a c l e a r sodium s u l f i t e s o l u t i o n f o r SO2 absorption and limestone regeneration of the spent l i q u o r . P i l o t plant t e s t s of t h i s system by EPA (15) have v e r i f i e d that 90 percent SO2 removal can be achieved with a l i q u i d / g a s r a t i o of only 2.2 gal./Mcf and 7 i n . water pressure drop i n a s i e v e - t r a y absorber. In a d d i t i o n , over 95 percent limestone u t i l i z a t i o n i s obtained. The gross e l e c t r i c demand of the absorber c a l c u l a t e d f o r these c o n d i t i o n s by Equation (3), based on the same pump pressure as was used f o r the TCA c a l c u l a t i o n s , i s 1.4 W/cfm. This energy demand amounts to 0.33 percent of the p l a n t power production f o r the scrubber and 0.79 percent f o r the e n t i r e FGD system, using the same data for other demands as are i n d i c a t e d i n Table V. Since i t uses a c l e a r s o l u t i o n f o r SO2 absorption, t h i s limestone process v a r i a t i o n should be p a r t i c u l a r l y w e l l s u i t e d f o r use with the h i g h - v e l o c i t y cocurrent packed tower developed by TVA. Most important, however, i s the p o t e n t i a l f o r o p e r a t i o n with minimum sludge production and with a power demand that i s less than the energy generated by the combustion of the s u l f u r i n most high s u l f u r c o a l s . Conclusions. The a n a l y s i s leads to the f o l l o w i n g conclusions regarding limestone FGD scrubbers: • •

•

•

• •

•

•

The net work input r e q u i r e d f o r 90 percent SO2 removal i s 76+6 f t - l b / c u f t (125°F, saturated) o f f l u e gas scrubbed. Except f o r scrubbers using a v e n t u r i prescrubber, the s p e c i f i c work input r e q u i r e d f o r 90 percent SO2 removal i s independent o f gas v e l o c i t y to 27 f t / s e c . The gross e l e c t r i c energy demand of a spray tower i s g r e a t e r than that o f a TCA, mainly due to the higher e f f i c i e n c y of fans r e l a t i v e to s l u r r y pumps. The d i f f e r e n c e between the two scrubber types, however, amounts to only about 0.16 percent of the generated power. A 27 f t / s e c cocurrent packed tower can achieve 90 percent SO2 removal with l e s s energy input than a countercurrent spray tower. The TCA scrubber i s more e n e r g y - e f f i c i e n t than e i t h e r type of spray tower. In h i g h - s u l f u r c o a l a p p l i c a t i o n s , 90 percent SO2 removal can be a t t a i n e d with a t o t a l process energy of 1.3 percent of the plant power production. A d i p i c a c i d can reduce the e l e c t r i c demand f o r 90 percent SO2 removal i n a TCA scrubber by 30 percent at a c o n c e n t r a t i o n of 1400 ppm. I t sets a c e i l i n g on the power demand at 2.4 percent of p l a n t power production; at a higher demand, the value o f the power saved exceeds the cost of the a d d i t i v e . A d i p i c a c i d w i l l be most e f f e c t i v e as a means of improving limestone u t i l i z a t i o n ; p o t e n t i a l reductions i n sludge d i s posal and limestone makeup c o s t s are greater than the value of e l e c t r i c power savings.


14.

BORGWARDT


2

Absorption

323

Systems now under development can be expected to reduce the t o t a l process energy demands f o r limestone FGD below the energy generated from the combustion o f the s u l f u r i n most high s u l f u r c o a l s .

Literature Cited Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on August 25, 2015 | http://pubs.acs.org Publication Date: July 1, 1982 | doi: 10.1021/bk-1982-0188.ch014

1.

2.

Farber, P.S., Livengood, C.D., "Energy and Economic Impacts of Pollution Control Equipment for Coal-Fired Power Plants: An Assessment Model," 1979, presented at 72nd Annual Meeting of the Air Pollution Control Association, Cincinnati, Ohio. Ferrell, J.D., Stenby, E.W., "Interface Design Problems Between Coal Fired Boilers and SO Scrubbing Systems," 1977, Proceedings of the Second Pacific Chemical Engineering Conference; AIChE, Vol. I, 295-302. McGlamery, G.G., Tarkington, T.W., Tomlinson, S.V., "Eco nomic and Energy Requirements of Sulfur Oxides Control Processes," 1979, Proceedings: Symposium on Flue Gas Desulfurization--Las Vegas, Nevada, March 1979, Vol. I, EPA -600/7-79-167a (NTIS PB 80133168), 137-214. Richman, Μ., "Limestone FGD Operation at Martin Lake Steam Electric Station," Ibid., 613-28. Muela, C.A., Menzies, W.R., Brna, T.G., "Stack Gas Reheat-Energy and Environmental Aspects," 1979, Proceedings: Sym posium on Flue Gas Desulfurization--Las Vegas, Nevada, March 1979, Vol. II, EPA-600/7-79-167b (NTIS PB 80133176), 116178. Ando, J., "SO and NO Abatement for Coal-Fired Boilers in Japan," 1981, Proceedings: Symposium on Flue Gas Desulfurization - Houston, Texas, October 1980, Vol. I, EPA -600/9-81-019a, 85-109. Ruben, E.S., Nguyen, D.G., J. Air Pollut. Contr. Ass. 1978, 28, 12, 1207-12. Epstein, Μ., Head, H.N., Wang, S.C., Burbank, D.A., "Results of Mist Elimination and Alkali Utilization Testing at the EPA Alkali Scrubbing Test Facility," 1976, Proceedings: Symposium on Flue Gas Desulfurization, New Orleans, March 1976, Vol. I, EPA-600/2-76-136a (NTIS PB 255317), 145-204. Head, H.N., "EPA Alkali Scrubbing Test Facility: Advanced Program, Third Progress Report," 1977, EPA-600/7-77-105 (NTIS PB 274544), 6-5. Burbank, D.A., Wang, S.C., Progress report for EPA Contract 68-02-3114, Bechtel National, Inc., September 1980, 1-8. Henson, L.J., "TVA/EPRI Shawnee Cocurrent Scrubber Test Results," 1980, Proceedings: Fifth Industry Briefing on IERL-RTP Lime/Limestone Wet Scrubbing Test Programs, De cember 1979, EPA-600/9-80-032 (NTIS PB 80199813), 116-165. Burbank, D. A., Wang, S.C., Progress report for EPA Contract 68-02-3114, Bechtel National, Inc., August 1979, 1-9. 2

3.

4. 5.

6.

7. 8.

9. 10. 11.

12.

2

x


FLUE GAS DESULFURIZATION

324

13.


14.

15.

Stephenson, C.D., Torstrick, R.L., "Current Status of De velopment of the Shawnee Lime-Limestone Computer Program," 1979, Proceedings: Industry Briefing on EPA Lime/Limestone Wet Scrubbing Test Programs (August 1978), EPA-600/7-79-092 (NTIS PB 296517), 94-139. Idemura, H . , Kanai, T . , Yanagioka, Η., "Jet Bubbling Flue Gas Desulfurization Process," 1977, Proceedings of the Second Pacific Chemical Engineering Conference; AIChE, Vol. I, 365-70. Borgwardt, R.H., Dempsey, J . H . , Chang, S.C., "Dual Alkali Scrubbing of SO at IERL-RTP Pilot Plant," 1980, Progress Report No. 2, U.S. Environmental Protection Agency, Research Triangle Park, NC. 2

RECEIVED

November 20, 1981.


Flue Gas Desulfurization - American Chemical Society

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