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OAR Box 1828 

Prepped by Ollie Stewart 

Document Number: 

292) IV-A-2 

Docket Number: 

A-83-27 

Printed 11/12/2008 11:12:58 AM Sheet 292 of 325 
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A-83-27 

^ IV-A-2 

EPA-600/7-7B-030D 

March 1978 

FLUE GAS DESUFURIZATION SYSTEMS: 

DESIGN AND OPERATING CONSIDERATIONS 

Volume II. Technical Report 

by 

C. C. Leivo 

Bechtel Corporation 

50 Beale Street 

San Francisco, California 94119 

Contract No. 68-02-2616 

Program Element No. EHE624 RADIAN LIBRARY, 

DURHAM, N.C. 

EPA Project Officers: 

John E. Williams and Kenneth R. Durkee 

Industrial Environmental Research Laboratory Emission Standards and Engineering Division 

' Office of Energy, Minerals, and Industry Office of Air Quality Planning and Standards 

Research Triangle Park, N.C. 27711 Research Triangle Park, N.C. 27711 

Prepared for 

U.S. ENVIRONMENTAL PROTECTION AGENCY 

Office of Research and Development 

and Office of Air and Waste Management 

Washington, D.C. 20460 
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Section 4 

IMPLICATIONS OF REQUIRING 90 PERCENT OR GREATER 

S02 REMOVAL FOR NEW FGD INSTALLATIONS 

This section discusses the changes in a demonstrated FGD system design 

of, for example, 80 percent SO2 removal efficiency to obtain a capability 

of 90 percent or more. In summary, these design changes could involve: 

Increase in absorbent circulation rates 

Increase in scrubbing (contacting) stages or in contacting intensity (scrubber-type) 

More stringent requirements for uniform flow 

distribution 

Elimination of gas by-pass if used for reheat 

Increases in absorbent concentrations which may in 

turn require residence time or other changes to control enhanced scaling tendencies 

If the increased SO- removal requirement is also accompanied by an 

increase in coal sulfur content, this change will intensify all of 

the above requirements. Other fuel related factors such as coal 

heating value, ash content and composition, must also be accommodated 

and with increasing difficulty as removal requirement rises. 

LIME AND LIMESTONE SCRUBBING 

This section presents data to illustrate the effects of process operating parameters on SO- removal efficiency for the lime and limestone 

scrubbing process. Most of the data are taken from test results at the 

EPA Alkali Scrubbing Test Facility located near Paducah, Kentucky 

(Ref. 1). The test facility is integrated into the flue gas ductwork 

of a 160-MW coal-fired boiler at the TVA Shawnee Power Station. Two 

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parallel wet scrubber systems are in operation: a venturi/spray 

tower system and a turbulent contact absorber (TCA) system. Each 

system can treat approximately 10 MW (equivalent to 33,000 nm-Vhr, 

or 30,000 acfm at 300°F) of flue gas containing 1,500 to 4,500 ppm 

of S02-

The bulk of the data in this section is for limestone scrubbing on the 

TCA system. Additional data are presented in Appendix A for the TCA 

with the lime and the venturi and spray tower with lime and limestone 

operation. The Shawnee data are the result of short-term factorial 

tests 6 to 8 hours in length and long-term tests about one week in 

length. The short-term tests were aimed at determining SO2 absorption 

efficiencies without regard necessarily for possible long-term scaling 

conditions that might be associated with the process conditions chosen. 

Therefore, these particular data are not to be interpreted as evidence 

of long-term reliable operation. However, during other portions of 

the Shawnee program adequate demonstrations have been made that these 

FGD systems can be operated for extended periods without scaling 

problems. 

Approximate equations and nomographs for predicting SO2 absorption 

efficiency for the Shawnee scrubbers are presented in Appendix B. 

For the TCA Shawnee data in this section, the SO2 removal curves 

have been predicted by Equation B-4. 

Effects of Process Variables on SO2 Absorption Efficiency. 

The effects of the following variables on SO2 absorption efficiency 

are summarized in this section: 

• Inlet SO2 concentration 

• Liquid-to-gas ratio 

• Gas velocity 

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Scrubber inlet pH 

Scrubber internals 

Magnesium addition 

Number of scrubbing stages 

Lime versus limestone 

Inlet SO? Concentration. Figure 4-1 shows the effect of inlet SO2 concentration on SO2 removal for fixed design and operating conditions. 

Data have shown (Refs. 2, 3, 4) that in lime and limestone scrubbing 

over a wide range of inlet SO2 concentrations, greater removal efficiency is realized for low inlet SO2 concentrations than for high, for 

fixed design and operating conditions. This happens because the actual 

quantity of SO2 removed per volume of gas treated goes down, reducing 

the loading on the absorbing liquor. 

For high SO2 concentrations, more SO2 is actually absorbed by the liquor. 

For example, an absorber with 80 percent reduction from a gas containing 3,000 ppm SO2 might absorb 90 percent from a 1,000 ppm gas, all 

things being equal. But in absolute terms, 2,400 ppm would actually 

have been removed from the higher SO2 case, and only 900 ppm from the 

lower. 

It was at one time thought that as inlet SO2 concentration decreased, 

the driving force for absorption would decrease and high removal efficiencies would be more difficult. Clearly, as equilibrium SO2 back 

pressure over the bulk liquid approaches the SO2 gas concentration, 

this must pose a problem. In practice, however, the SO- back pressure 

over fresh lime and limestone scrubbing slurries is very low (less than 

1 ppm), and scrubbers can be designed so that the low S0„ concentration 

scrubbed gas exiting the scrubber is contacted by the freshest slurry 

(counter current or cross flows). For these reasons, removal efficiency greater than 95 percent has been achieved during tests from 

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100 -1 T 1 1 1 

O EPA PILOT TCA 

SPHERE HEIGHT -- 7INCHES/8ED. 3 BEDS 

LIQUID TO GAS RATIO = 85 gal/Mcf 

TCA GAS VELOCITY = 7.5 ft/sec 

O 0 TVA PILOT SPRAY TOWER 

LIQUID TO GAS RATIO = 85 gal/Mcf 

95 --

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75 - -

70 

LIMESTONE SCRUBBING 

1,000 2.000 3.000 

INLET S02 CONC, ppm 

4,000 5,000 

(Ref. 1) 

Figure 4-1. Effect of Inlet SO2 Concentration on S0 2 Removal 

Efficiency for Fixed Design and Operating Conditions 

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flue gas that averaged only 200 ppm at the inlet, as shown in Figure 4-2 

for a TCA system operated at the Southern California Edison Mohave Station (Ref. 5). The outlet SO- concentration in this case is less than 

10 ppm. It is important to recognize that to obtain equivalent efficiency for 3,000 ppm SO2 gas, an FGD system would need to absorb and 

process about 15 times as much SO2 (2,850 vs 190 ppm). Achievement of 

comparable efficiencies at the high loading corresponding to 3,000 ppm 

SO2 gas cannot be considered to have been demonstrated by the Mohave 

example. The gas S02 concentration, or coal sulfur content, must always 

be accounted for when extrapolating the performance of different installations. 

LiquidTto-Gas Ratio. Figure 4-3 illustrates the effect of L/G on S0_ 

rempval efficiency in a TCA for limestone scrubbing. Similar effects 

are observed for lime and in the venturi and spray tower scrubbers as 

shown in Appendix A. Liquid-to-gas ratio is a major design variable 

in scrubbing systems. As the liquid pumping rate is increased, additional alkali is added to the scrubber and the gas-liquid interfacial 

area increases, resulting in greater absorption efficiency. 

In most scrubber types, the benefits of increasing L/G for fixed 

scrubber condition can be designed into the system up to the point 

where problems such as flooding or nonuniform gas and liquor distributions occur. It can be seen from the way the curves flatten out 

at high L/G in Figure 4-3 that the incremental increase in pumping 

to improve the absorption from, say, 80 to 90 percent (an increase 

from about 50 to 80 L/G at a pH of 5.8) would be significantly greater 

than that required to improve it from 70 to 80 percent (an increase 

from about 35 to 50 L/G). Higher L/G could also be obtained by operating at lower gas velocity, at the expense of increased scrubber size. 

An increase in the design L/G for an FGD system requires the use of 

larger pumps, pipes, and spray headers. Scrubber recirculation tanks 

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93-I 

92—I 

9 H 

TCA 4 STAGES 

LIMESTONE 

90-

INLETS02

; 

GAS RATE 

200 PPM 

450,000 SCFM (Redrawn from Reference 5) 

T" 1 -

20 T T 60 

LIQUID TO GAS RATIO, gal/Mcf 

Figure 4-2. Effect of Liquid-to-Gas Ratio on SO- Removal Efficiency 

with Low Sulfur Coal at the Mohave Power Station 

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must be larger to maintain design residence time for the increased L/G 

and to provide residence time for the increased S02 absorption. The 

energy penalty for increased L/G is shown in Figure 2-3. At higher 

L/G's, gas pressure drop may also increase in some types of scrubbers, 

causing additional penalty as shown in Figure 2-2. 

Higher L/G's may tend to reduce scaling potential in the scrubber by 

reducing the SO- absorbed per gallon of absorbent. 

Scrubber Gas Velocity. Figure 4-4 shows the effect of gas velocity on 

SO- removal efficiency in a TCA. The small velocity effect at a 

fixed slurry flow rate represents a trade-off between factors tending 

to increase efficiency with velocity (e.g., superior sphere agitation 

and increased liquid-gas contact) and the corresponding decrease in 

L/G. On the other hand, a scrubber such as the spray tower (Figure 

A-14) will have higher efficiencies at lower gas velocities, at a fixed 

slurry flow rate. In the spray tower, there are no internals and little 

liquid hold-up, so that the decrease in L/G with increasing gas velocity is not offset by other factors. 

For cost reasons, scrubbers are designed to operate at maximum practical gas velocities, thereby keeping vessel diameters minimal. These 

maximum velocities are dictated by gas-liquid distribution characteristics of the scrubber and by the maximum allowable slurry entrainment 

that the mist eliminator can handle. Designers generally achieve the 

required L/G by increasing the slurry pumping rate rather than by decreasing the gas velocity. Designing for low gas velocities introduces 

larger and costlier scrubber systems (modules). 

In specifying high removal efficiency, it should be kept in mind that 

some very efficient scrubbers (including certain tray towers) have 

poor modular turndown characteristics, so that absorption efficiency 

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