P O W E R

Technical Paper

FGD 

Upgrades

Techniques for

Improving FGD System Performance to

Achieve Ultra-High SO2 Removal Efficiencies
Abstract

With increasing pressure to economically reduce SO2 emissions over coming years, utilities need to derive as much benefit as possible from their existing assets. URS has developed a

number of technologies marketed under the trade name

OverScrubTM, which when implemented will allow utilities to

substantially reduce their SO2 emissions. SO2 removal efficiencies can typically be increased in a cost-effective manner

to well above 95 percent. In addition URS’ OverScrubTM can be

used to provide additional fuel flexibility by enabling a scrubber designed to treat flue gas from low sulfur coal to operate

effectively on flue gas from higher sulfur fuels. In addition, the

OverScrubTM technologies allows utilities currently using

expensive but highly reactive reagents such as sodium, lime,

and mag-lime to switch to a lower cost limestone chemistry.

Actual case studies are provided to illustrate the use of

Computational Fluid Dynamics (CFD) modeling to assess the

performance of scrubbers. The modeling shows the impact of

poor gas distribution on scrubber performance and provides a

review of available solutions.

Several case studies are presented providing a detailed economic analysis of the benefits of OverScrubTM. The analyses

demonstrate that using OverScrubTM to achieve high removal

efficiencies allows a utility to: generate excess emission credits, provides for additional fuel flexibility, makes it possible to

switch to a lower cost reagent, and/or significantly reduce the

auxiliary power consumption. These modifications can be very

profitable for coal fired power plants.

Techniques for 

Improving FGD System Performance to 

Achieve Ultra-High SO2 Removal Efficiencies

Dr. Jonas S. Klingspor

Project Manager

Greg N. Brown

Project Manager

URS Corporation

9400 Amberglen Boulevard

Austin, TX 78729
Introduction

Legislative initiatives such as the Clean Air Amendment of

1990, are gradually forcing utilities to reduce SO2 emissions

from coal-fired power plants. With the start of Phase II in

2000, the national emission cap was reduced to 9.1 million

tons per year (tpy). Currently, annual emissions are approximately 11.2 million tpy, and the shortfall is consuming

banked emission credits at a rate of 2.1 million tpy. At this

rate, banked emission credits will be consumed by 2005

although the market is likely to realize the effects of a tight

supply as early as 2003. In addition, the existing coal-fired

stations are operating at increasingly higher capacity factors.

The current power shortage is likely to lead to new coal-fired

generation in the coming years putting further pressure on

the SO2 emission credits market and on utilities to reduce

SO2 emissions.

The cycle time to install new FGD systems is at best 36

months indicating that a market for new FGD systems should

emerge shortly. However, the existing fleet of FGD system is

currently underutilized and could potentially contribute to a

future reduction in SO2 emissions. Currently, SO2 emissions

from the existing FGD fleet is about 1.4 million tpy. However,

the early vintage of FGD systems were designed for only

moderate SO2 removal efficiencies, leaving room for

improved technologies such as OverScrubTM. 

It is estimated that SO2 emissions from the existing FGD

fleet can be cut in half by applying various types of enhancement options such as URS’ OverScrubTM technologies. URS

has developed a set of technologies marketed under the

trade name OverScrubTM which can enable virtually every

scrubber type and design to greatly improve its capacity for

SO2 removal. Required modifications include both mechanical and scrubber chemistry changes. These modifications

typically have a 1 to 3 year payback of required capital.

The OverScrubTM technologies can also provide additional

fuel flexibility and/or significantly reduce auxiliary power

requirements. As demonstrated below, applying the

OverScrubTM technologies to reduce emissions makes it possible to provide fuel flexibility, switch to a lower cost

reagent, or reduce auxiliary power consumption. This will

reduce cost and improve profitability.

Overview of legislative and market drivers

Drivers that potentially could motivate utilities to reduce SO2

emissions fall basically in two categories, legislative and

financial. Current and future legislative initiatives targeting

SO2 emissions include the 1990 Clean Air Amendments,

known as Phase I and Phase II, future PM2.5 legislation, and

future mercury legislation. The PM2.5 legislation will likely

require installation of new scrubbers as well as a reduction

of emissions from existing scrubbers such as sulfate emissions that contribute to PM2.5 emissions. Scrubbers are also

efficient in controlling mercury emissions. Also, emissions

from new fossil fuel based power generation will have to be

accommodated within the existing national emissions cap.

Hence, emissions from new units must be offset by a reduction from existing units by either improving the performance

of existing scrubbers or by adding scrubbers to unscrubbed

units. 

A reduction in SO2 emissions can in many cases be accomplished in conjunction with a reduction in operating costs.

Hence, economic drivers exist, which may motivate utilities

to reduce SO2 emissions absent of any legislative drivers.

Some of the benefits include: the opportunity to generate

excess emissions credits for sale, reduction in auxiliary

power consumption, lower cost of reagent, sale of byproduct,

and reduced landfill costs.

EPA summarizes the goals of the 1990 clean air amendments as follows: 

“Title IV of the Clean Air Act set a goal of reducing annual

SO2 emissions by 10 million tons below 1980 levels. To

achieve these reductions, the law required a two-phase

tightening of the restrictions placed on fossil fuel-fired

power plants. 

Phase I began in 1995 and affected 263 units at 110

mostly coal-burning electric utility plants located in 21

eastern and midwestern states. An additional 182 units

joined Phase I of the program as substitution or compensating units, bringing the total of Phase I affected units to

445. Emissions data indicate that 1995 SO2 emissions at

these units nationwide were reduced by almost 40%

below their required level. 

OverScrubTM

1
Phase II, which began in the year

2000, tightened the annual emissions

limits imposed on these large, higher

emitting plants and also set restrictions on smaller, cleaner plants fired

by coal, oil, and gas, encompassing

over 2,000 units in all. The program

affects existing utility units serving

generators with an output capacity of

greater than 25 megawatts and all

new utility units.”1

SO2 emissions declined at a steady

pace from 1980 to 1995. Phase I of the

Clean Air amendment went into effect

1995 and while emissions were reduced

substantially from targeted utilities during

this period, SO2 emissions on a national level did not change

substantially. The first year of Phase II showed a significant

drop in SO2 emissions, however, not sufficient to bring emissions in parity with available SO2 emission allowances. SO2

emissions in 2000, the first year of Phase II, exceeded available allowances by about 2.1 million TPY indicating the possibility of a strong market for emission credits1 in future

years.

Under the guidelines of Phase I, utilities banked significant amounts of emission credits from 1995 to the end of

1999. These credits can be used to offset emissions at plants

exceeding their provided allowance cap. At the end of 1999,

utilities had banked about 11 million tons of SO2 emission

credits, which are available for use during any future year.

However, in 2000, the first year of Phase II, the deficit was

about 2.1 million tons of emission credits indicating that

banked allowances could be depleted as early as 2005.

However, it is likely that utilities will gradually reduce emissions in the coming years by reducing emissions from plants

equipped with scrubbers. This should in theory slow the

depletion of banked emission allowances. However the

increase in power production, either through increased

capacity factors or new production, is likely to offset any

gains from improved scrubber performance and could even

speed the depletion of banked emission credits. Hence, 

generating emission credits is a significant reason to

OverScrubTM.

SO2 emissions from the existing scrubber fleet was 1.4

million tons in 19991. The average SO2 removal efficiency for

the installed scrubber fleet is about 80 percent leaving room

for an additional reduction in SO2 emissions by improving the

performance of the installed scrubber fleet. Each scrubber

installation is unique and may be subject to a number of

issues which may limit the performance of the scrubber, e.g.

the stack flue cannot accommodate a saturated flue gas and

require flue gas bypass as the source for flue gas reheat. It

has been estimated that on an average, the removal efficiency of the installed scrubber fleet can be increased from

80 to 90 percent by applying various OverScrubTM technologies, potentially cutting emissions in half from scrubbed

units or by about 700 thousand TPY. At cost of $200/ton, 700

thousand tons of emission credits would be worth $140 million per year. Also, a reduction in SO2 emissions from the

existing scrubber fleet would slow the depletion of banked

allowances, thus delaying the time required for installation

new FGD systems.

URS’ OverScrubTM technologies can also be used to provide fuel flexibility, reduce auxiliary power consumption and

switch to a sodium, lime or mag-lime based scrubber system

to a lower cost limestone-based chemistry. Typically, one

absorber recycle pump can be taken out of service without

any adverse effect on SO2 emissions. Although one recycle

pump may seem insignificant, the increased generating

capacity from taking one recycle pump out of service represents significant value.

Techniques for Improving FGD System Performance to Achieve Ultra-High SO2 Removal Efficiencies

2

0

2

4

6

8

10

12

14

16

18

20

1980 1985 1990 1995 2000 2005 2010

US SO2 Quantities, 106 TPY

Emissions

Allowance Cap

Banked Allowances

Figure 1: SO2 Emissions, Allowance Cap, and Banked Allowances1
Overview of available OverScrubTM options

Options to reduce SO2 emissions from existing scrubbers

basically fall in two categories, slurry reactivity and mechanical design improvements. As each scrubber type and installation is somewhat unique, the optimum solution may be

different for each installation. Some of the more common

options available to reduce SO2 emissions are

listed in Table 1.

Before any improvement options should be

considered, the condition of the existing scrubber

system must be assessed. If a scrubber has operational problems such as poor gas distribution or

scaling and pluggage issues, it is likely that these

problems may be further aggravated by any

attempt to run the scrubber harder. Examples of

issues to review are shown in Table 2.

The issues highlighted in Table 2 must be considered as potential secondary effects of any

attempt to reduce SO2 emissions. Hence, any

OverScrubTM conversion must be preceded with a detailed

characterization of the existing operating conditions to

assess the possibility of any undesired secondary effects.

Also, the capacity of the existing additive preparation, oxidation air compressors, and product dewatering systems must

be reviewed.

OverScrubTM

3

Table 1. Example of available OverScrubTM options. 

Slurry Reactivity Mechanical Design Changes

Increase pH Increase L/G

Reduce chloride concentration Reduce bypass

Use buffer additives Liquid distribution rings

Increase dissolved sulfite concentration Tray

Eliminate blinding conditions Packing

Forced oxidation Spray coverage improvements 

Table 2. Potential secondary effects from additional SO2 removal.

Increased SO2 Removal Areas of Concern Potential Solutions

Change in oxidation Scaling Forced oxidation

Blinding Inhibited oxidation

Change in reagent utilization Reagent cost Optimization of additive prep.

Waste volume Mechanical improvements

ME performance Buffer additives

Change in solids properties Dewatering Control of oxidation

Water balance Eliminate crystal modifiers 
Liquid-Phase Alkalinity Improvement Options

The three main parameters available to increase the liquidphase alkalinity are the slurry pH, buffer additives, and dissolved sulfite concentration. Likely the easiest way to

increase the SO2 removal efficiency in any scrubber is to

increase the slurry pH as shown in Figure 2.

The SO2 removal typically responds very quickly to an

increase in pH. An increase in slurry pH is typically achieved

by increasing the reagent feed rate. In this case, the limestone utilization deteriorates quickly with an increase in pH.

A limestone utilization below 90 percent can lead to significant operational problems such as scaling and plugging of

the mist eliminator. A second path to increase the slurry pH is

to improve the grind size produced by the limestone mills.

This would allow operation at a higher pH while maintaining

the limestone utilization.

Organic acids have been widely applied in the scrubber

industry and are well known to improve the SO2 removal efficiency in wet scrubbers. Organic acids work by buffering the

pH at the gas liquid interface, by reducing the back pressure

of SO2 and by lowering the liquid-film resistance to mass

transfer. DBA (dibasic acid) and formate are the most commonly used additives mainly due to their relative low cost.

DBA is a mixture of succinic, glutaric and adipic acids and

buffers at pH 5.4 and 4.3. Formate buffers the slurry at a pH

of 3.8. Figure 3 shows the typical impact on SO2 removal

using of buffering additives.

The maximum practical level of buffer additives is limited

by several factors including cost and physical limits. Buffer

additives only enhance the liquid phase, and if a sufficient

concentration is used, the removal efficiency will be decided

by the gas-phase resistance only. 

Selection of buffer additive is typically a cost issue which

is a function of the cost of the additive and the consumption

rate. The consumption rate of buffer additives is a composite

of many factors including solution and non-solution losses

such as coprecipitation, chemical degradation, and vaporization losses. Solution losses can be minimized by operating

the scrubber system in a close loop configuration and by minimizing the liquid blowdown. They can be calculated from a

material balance. Non solution losses are basically a function of the slurry chemistry and must be estimated on a case

by case basis. 

Coprecipitation is the primary non-solution loss mechanism

in inhibited and natural oxidation systems. Coprecipitation is

not an issue in systems designed to produce gypsum.

Chemical degradation is the primary non-solution loss mechanism in forced-oxidation systems where organic acids are

oxidized and degraded through contact with oxidation air.

Vaporization losses are only applicable to formate based systems but can account for a significant fraction of non-solution losses. The absorber temperature is an important

variable deciding the amount of vaporization losses.

Techniques for Improving FGD System Performance to Achieve Ultra-High SO2 Removal Efficiencies

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60

70

80

90

100

5.6 5.8 6 6.2 6.4

pH

Removal Efficiency, percent

60

70

80

90

100Limestone Utilization, percent

Removal

Utilization

85.0

90.0

95.0

100.0

0 1000 2000 3000 4000

Additive Concentration

Removal Efficiency, percent

DBA FOX

DBA IOX

Formate FOX

Formate IOX

Figure 2: Impact on SO2 Removal and Limestone Utilization 

by Slurry pH2

Figure 3: Impact on SO2 Removal Efficiency by Buffer Additives2
Figure 4 shows non-solution loss rates for DBA and 

formate based chemistry in forced oxidized and in inhibited

oxidation systems.

Mechanical Improvement Options

A range of mechanical improvement options are available

that can provide a significant increase in SO2 removal. The

most common features include increasing the liquid-to-gas

ratio by increasing the absorber recycle pump flow, addition

of liquid distribution rings to minimize wall sneakage, and

addition of a new contact device such as a tray or packing.

Applicability of available mechanical improvement options

are site specific and depend on a range of factors such as

availability of space for a tray or packing and availability of

required fan power to overcome additional pressure drop. A

summary of the main mechanical improvement options is

shown in Table 3.

Liquid-to-Gas Ratio

The recycle flow rate from existing absorber recycle pumps

can be increased by reducing the pump head, increasing the

rotational speed of the pump impeller, or by simply replacing

an existing pump with a larger model. The pump head can

frequently be reduced by replacing high-pressure nozzles

potentially operating at 15 to 18 psi to low pressure nozzles

operating at 8-10 psi. The reduction in pump head will

increase the flow substantially in most cases but can also be

used to reduce the overall power consumption by slowing

down the pump. The impact on SO2 removal from an increase

in L/G is shown in Figure 5.

OverScrubTM

5

0

10

20

30

40

50

60

70

0 1000 2000 3000 4000 5000

Additive Conc., ppm

Non-solution Loss Rate, lb/ton SO2 removed

DBA - IOX

Formate - FOX

DBA - FOX

Formate - IOX

Figure 4: Non-Solution Loss Rates for DBA and Formate2

Table 3. Main features of mechanical improvement options.

Option Main Issue Comment

L/G Increased power consumption Effectiveness limited by wall sneakage

Increased pressure drop

Liquid Distribution Rings None Improved performance at no increase in power

consumption or pressure drop

Tray Increased pressure drop Improved gas distribution

Scaling potential

Packing Increased pressure drop Scaling potential 

60

70

80

90

100

0 500 1000 1500

DBA Concentration, ppm

 SO2 Removal, percent

66 L/G

133 L/G

Figure 5: Impact by L/G on SO2 Removal2
Liquid Distribution Rings

Open spray towers provide a balanced approach to efficient

gas/liquid contact, reliability, and draft loss expenditure.

However, inherent in any design using spray nozzles for introduction and distribution of the recycle slurry, is a region of

lower spray density close to the absorber walls. A reduction

in spray density is a result of recycle slurry being captured by

the absorber walls and a limitation in nozzle placement in the

wall region leading to a zone of low spray density.

Liquid distribution rings were developed to reintroduce

slurry running down the absorber walls to reduce the flue gas

sneakage. The liquid distribution rings are sloped downwards to re-inject wall slurry into the spray zone to increase

the spray density close to the wall. The liquid distribution

devices are placed between spray levels as shown in Figure 6.

Tests on full-scale scrubbers revealed a steep concentration gradient from the wall penetrating 3 to 5 feet into the

scrubber, as shown in Figure 7. The SO2 concentration profile

basically mimics the spray density gradient in the same

region. Hence, the flue gas is exposed to a region with lower

spray density providing a path of lower resistance. The result

is a region close to absorber wall which is characterized by a

higher than average gas velocity, lower than average spray

density (resulting in a lower than average liquid-to-gas ratio)

and reduced SO2 removal capacity. A five foot section in an

open spray tower 40 ft inch diameter, represents 35 percent

of the total cross sectional area. In a 60-foot diameter tower,

the sneakage area is 25 percent, still a significant percentage of the overall cross section.

As shown in Figure 8, the SO2 removal efficiency increases

significantly when liquid distribution rings are installed. The

increase in SO2 removal is achieved without a measurable

increase in gas-side-pressure drop. Figure 8 illustrates the

ability to operate with one spray level out of service after liquid distribution rings are installed while maintaining the

same or improved SO2 removal. This provides an opportunity

to save both pump and fan power as the pressure drop is

reduced.

Techniques for Improving FGD System Performance to Achieve Ultra-High SO2 Removal Efficiencies

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Figure 6: Liquid Distribution Rings

0

100

200

300

400

500

600

700

0 2 4 6 8 10

Distance from Absorber Wall, ft

SO 2 COncentration, ppm

Absorber A, 36 ft

Absorber B, 46 ft

Figure 7: Wall Sneakage in Open Spray Towers
Trays

Perforated trays have been applied successfully in lime and

limestone-based scrubbers and is a proven contact device for

calcium based systems. A tray provides additional SO2

removal by creating additional surface area obtained from

liquid holdup on the tray. 

The degree of improvement in SO2 removal by adding a

tray is very similar to adding liquid distribution rings, as

shown in Figure 8. However, the addition of a tray will significantly increase the gas-side pressure drop and the applicability of a tray is dependent on sufficient fan capacity being

available. Also, trays are subject to plugging and scaling if

the scrubber chemistry is not controlled carefully. Trays are

less attractive if the absorber gas velocity must be increased.

Finally, trays are particularly useful if space is limited or

when efficient spray header designs are not present. 

Reagent Optimization Options

A key element required to achieve improved SO2 removal

while controlling the operating cost is efficient reagent utilization. In the FGD industry, there has historically been a limited number of reagents used to effectively control SO2

emissions from coal-fired power plants. These include primarily limestone, lime, mag-lime, sodium and more recently

ammonia. The review below will focus on the impact on

scrubber efficiency and SO2 removal cost for calcium based

reagents.

Scrubber Chemistry

Each reagent provides distinct advantages and disadvantages for both retrofit and green field installations and to a

particular scrubber design, i.e. capital cost, reliability and

operating cost. Historically, plants have been locked into

using the same reagent for the life of the plant regardless of

changes in market conditions. The use of OverScrubTM technologies, however, provide an opportunity to switch from

higher cost reagents such as sodium, lime, and mag-lime to a

lower cost reagent such as limestone without requiring

extensive and costly modifications to the existing scrubber

system.

Typically, the cost of lime and mag-lime is substantially

higher than the cost of limestone. Table 4 shows the comparative cost for each reagent.

The reagent cost is likely the

most significant component in the

cost of controlling SO2 emissions.

Table 5 shows the impact of SO2

compliance cost on a $/ton of SO2

removed basis for the most common reagents.

As clearly illustrated in Table

5. limestone-based chemistry provides a tremendous annual cost

OverScrubTM

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75

80

85

90

95

100

1 2 3

Pumps in Service

SO2 Removal, percent

Conventional

With Tray

With LDR

Figure 8: Impact on SO2 Removal by Liquid Distribution Rings 

or a Tray2

Table 4. Typical cost of lime, mag-lime and limestone.

Lime, $/Ton Mag-Lime, $/Ton Limestone, $/Ton 

$60 $55 $10

Table 5. Impact of reagent selection on compliance cost, 500-MWe plant, 2/5% sulfur.

Lime Mag-Lime Limestone

SO2 Removed, TPY 85,500 85,500 85,500

Reagent Consumed, TPY 78,500 78,500 140,300

Cost, $/Y $4,700,000 $4,300,000 $1,400,000

Cost, $/Ton SO2 Removed $55.10 $50.50 $16.40
advantage. However, traditionally lime, and mag-lime based

scrubber designs have enjoyed a capital cost advantage,

mainly due to lower cost reagent preparation and absorber

systems. Historically the lower L/G and smaller size reaction

tank of lime and mag-lime based systems made it cost prohibited to convert these systems to a limestone- based

chemistry. OverScrubTM technologies can comfortably work

within the physical constraints of lime and mag-lime based

systems in a cost-effective manner.

The benefit of these technologies is that many of the existing absorber and auxiliary systems can be reused and modified to process limestone instead of sodium, lime, and

mag-lime at the same or reduced SO2 emission levels.

A cost-effective conversion is accomplished by applying

many of the enhancement options discussed above as well

as two proprietary technologies. The first is an unique

patented high efficiency oxidation device, which makes oxidation possible in the very shallow tanks typical of limebased scrubbers. This allows complete in-situ oxidation and

improved control of the process chemistry resulting in a preferred gypsum quality. The second device involves a low cost

unique limestone grinding system, currently installed at the

LG&E Mill Creek plant. This system provides a more cost

competitive method of grinding limestone when compared to

standard ball mill systems.

As shown in Table 5, the annual savings in reagent is

about $3.5 million for a 500-MWe plant burning 2.5% sulfur

coal including the sale of the byproduct gypsum. Typically the

capital need for these technologies is less than $10 million.

The savings result in a payback of less than three years,

offering an attractive Internal Rate of Return.

Reagent Utilization Optimization

A vast majority of the FGD systems using calcium based

chemistry already use limestone as their reagent. Through

the years URS has developed a series of technologies to optimize reagent utilization. This typically results in a 3 to 5 percent reduction in annual reagent consumption. Based on the

limestone consumption shown in Table 5, a 3 to 5 percent

reduction in limestone consumption provides annual savings

of $50,000 to $100,000 which in itself is not a substantial

number. However, without an optimization of the reagent utilization, any increase in SO2 removal will require an increase

in reagent consumption. By optimizing limestone consumption in conjunction with an increase in SO2 removal, the

increase in limestone consumption can be kept to a minimum

and frequently stays unchanged.

These technologies and experience were developed over a

number of years and in some cases require little additional

capital investment when combined with other OverScrubTM

technologies. Because of the proprietary nature of these

techniques and system enhancements it would be inappropriate to specify the exact methods involved. However, it is

safe to assume that reagent grind size, operational pH, and

other chemistry controls can be optimized simultaneously to

minimize reagent consumption.

Flue Gas Treatment Options

It was common for many of the early FGD systems to use a

portion of flue gas for reheat purposes by bypassing 10 to 20

percent of the flue gas around the scrubber system. This typically allowed the stack temperature to operate slightly

above saturation (5-15oF) and resulted in a detached plume.

The primary drivers for using bypass reheat was a reduction

in plume visibility, lower cost construction materials for the

stack, and elimination of stack rain. However, by reducing or

eliminating the scrubber bypass, utilities can very effectively

reduce SO2 emissions at minimal investment cost. This

requires the existing scrubbers to operate at higher flue gas

velocities potentially creating operational problems with

existing equipment such as the mist eliminator and stack.

Utilities are also increasing the generating capacity of

existing plants resulting in an increase in flue gas volume.

The performance of the existing scrubber system can become

a bottleneck potentially limiting the expected increase in

generating capacity.

Finally, flue gas maldistribution is common in the older vintage scrubbers leading to areas of high gas velocities which

can result in excess erosion, plugging, scaling, and mist eliminator carryover. At best, this results in increased operating

and maintenance costs but can also lead to unscheduled

down-time or derating of a unit.

Increased Flue Gas Flow 

Obviously, flue gas bypass has a significant impact on SO2

emissions. Typically, existing scrubbers can be made to

process higher flue gas volumes. SO2 emissions can be maintained or reduced by applying some of the enhancement

Techniques for Improving FGD System Performance to Achieve Ultra-High SO2 Removal Efficiencies

8
technologies discussed above. However, increased flue gas

velocities can lead to operational problems such as mist

eliminator scaling, pluggage, and carryover problems leading

to excessive stack rain. High velocity mist eliminators are

available for this application.

One of the OverScrubTM technologies includes the ability

to model the flue gas distribution through the scrubber system, the interaction between spray nozzles and other

obstructions and the ability to identify areas of flue gas

maldistribution. This tool, Computational Fluid Dynamics

(CFD), can be used to place flow correction devices, reorient

nozzles, remove obstructions, and improve gas distribution

ahead of the mist eliminator. 

Furthermore, CFD is capable of modeling duct and stack

condensation and can be used to place liquid collection

devices in the ductwork leading up to the stack and inside

the stack flue to eliminate stack rain. CFD modeling is an

important tool that can be used to enable existing scrubbers

to treat higher than design flue gas volumes. Experience indicates that each system is unique, with a unique set of economic drivers and different technical challenges leading to a

variety of potential solutions. The CFD tool allows for optimization of the OverScrubTM technologies for each installation relative to cost, reliability, and operational impact.

Case Studies

In recent months URS has employed CFD modeling to understand the gas liquid interaction in two very different applications. The results of the CFD simulations provide improved

understanding of the technical issues involved. In addition,

URS was able to develop cost effective modifications that

were confirmed through modeling to solve the identified gas

flow distribution problems that follows. 

Tampa Electric Big Ben Unit 4

Tampa Electric’s Big Bend Unit 4 absorbers were projected to

experience an accelerated rate of corrosion in the upper loop

of the dual loop absorbers. This situation was anticipated to

occur because of operation of the absorbers at an elevated

velocity and the method of initial gas quenching in the lower

loop. These combined factors could result in excess carryover

of high chloride liquor from the lower loop to the upper loop

causing a corrosive environment. Through CFD modeling,

shown in Figure 9, URS was able to demonstrate the impact

of nozzle placement and gas liquid interaction. URS simulated several different nozzle orientations before settling on

a recommended optimized orientation, which could significantly reduced the mal-distribution and carryover issue. 

OverScrubTM

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Figure 9: Tampa Electric Big Bend Station Unit 4 FGD System
Midwest Utility

Historically, this utility had poor gas distribution across the

mist eliminator that resulted in high carryover, excessive

scaling, and a limitation in gas flow through the absorber. It

had long been suspected that the primary cause of the limitations of the scrubber system was a poor flue gas distribution

ahead of the mist eliminator. CFD modeling demonstrated

that poor mist eliminator performance was a result of the

gas-to-liquid interaction earlier in the vessel. A proposed

change in nozzle configuration resulted in an improvement in

the gas-to-liquid contact and greatly improved the mist eliminator gas profile as shown in Figure 10.

Nozzle Configuration

As shown above, CFD modeling can be used to effectively

optimize existing nozzle configurations to improve SO2

removal, minimize pressure drop, improve mist eliminator

performance, and enhance the reliability of the absorber gas

path. All of these optimizations improve the economics of

SO2 capture. 

In addition, a unique nozzle configuration (patent pending)

for application in open spray towers has been developed.

This enhanced nozzle configuration further improves the

overall SO2 removal performance and reduces gas sneakage

along the absorber walls, a common drawback of countercurrent scrubbers. 

Application of this novel technology will enable some

installations, currently limited in flue gas volume, to effectively process additional flue gas by preferentially directing a

disproportionate amount of flue gas through the center of the

vessel. This redistribution of the flue gas in the zone of highest slurry density is predicted to improve SO2 removal efficiency but more importantly improve the gas distribution at

the mist eliminator, thus increasing the amount of flue gas a

scrubber can process. 

Economic Analysis

The application and therefore the value of the OverScrubTM

technologies is dependent on economic and environmental

drivers which typically are site specific. However, common

for all projects is a desire to reduce operating cost.

Depending on the economic drivers, a reduction in operating

cost can be achieved in several ways including: (a) generationing SO2 emissions credits; (b) fuel savings by switching to

a lower cost, higher sulfur fuel; (c) switching to a lower cost

reagent; and (d) reducing power consumption or any combination these options. All of these options can be combined

with a production of the ability to produce a wallboard grade

gypsum byproduct. The basis for the economic analysis is

shown in Table 6.

Techniques for Improving FGD System Performance to Achieve Ultra-High SO2 Removal Efficiencies

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Figure 10: Gas Profile through the Mist Eliminator, after Modifications (left) and Before Modifications (right) 
Case 1: Use of OverScrubTM to 

Provide Fuel Flexibility

A utility client has a scrubber initially designed for low-sulfur

coal. To off-set increasing fuel costs, the utility would like to

burn a lower-cost, higher sulfur fuel; but the existing scrubber design is not capable of handling the higher sulfur fuel.

The OverScrubTM technologies are applied to enable the

scrubber to operate efficiently at the higher sulfur loading. In

addition, the system is converted to produce wallboard grade

gypsum.

Based on the assumptions provided in Table 6, the economics of this conversion case is rewarding as shown in

Table 7. For a 500-MWe plant the annual savings are in

excess of $4 million and the payback is about 1 year.

Case 2: Use of OverScrubTM to Switch to 

Lower Cost Reagent

A utility has a scrubber that originally was designed to operate with mag-lime and is looking for an opportunity to reduce

scrubber related operating costs. The scrubber systems are

retrofitted with an in-situ forced oxidation system specifically designed to operate in shallow tanks and with URS SO2

removal enhancement technologies specifically designed to

operate in a low liquid-to-gas environment. The OverScrubTM

technologies allow the utility to use a lower cost reagent,

make wallboard grade gypsum, and maintain or reduce SO2

emissions.

A summary of the economic analysis is shown in Table 7.

Based on the assumptions provided in Table 6, the economics

of this case study are appealing. For a 500-MWe plant, the

annual savings are about $3.5 million and the payback is

about 3 years.

OverScrubTM

11

Table 6. Basis for economic analysis.

Case 1 Case 2 Case 3

High Sulfur Fuel Blend Lime to Limestone Switch Power Savings

SO2 Credits, Fuel Savings, SO2 Credits, Reagent

Gypsum Revenue Savings, Gypsum Revenue Power Savings

Base OverScrub Base Overscrub Base OverScrub

Coal Heating Value (Btu/lb) 8,526 12,000 12,500 12,500 8,526 8,526

Coal Sulfur Content (%) 0.34 2.40 3.00 3.00 0.34 0.34

SO2 Content, (lb-SO2/mmBTU) 0.80 4.00 4.80 4.80 0.80 0.80

Coal Cost ($/MM Btu) $1.40 $1.25 $1.30 $1.30 $1.05 $1.05

Alternative Fuel Cost ($/MM Btu) $0.65 $0.65 $0.65 $0.65 $0.65 $0.65

Pet Coke Cost ($/ton) $18.85 $18.85 $18.85 $18.85 $18.85 $18.85

Reagent Cost ($/ton) $10.50 $10.50 $55.00 $10.50 $12.00 $12.00

FGD Waste Disposal Cost ($/ton) ($1.00) ($1.00) ($2.00) ($1.00) $0.00 $0.00

FGD Waste Sale Price ($/ton) $0.00 $3.50 $0.00 $3.50 $1.00 $5.00

FGD Lime Stabilization Cost, $/ton $0.00 $0.00 $55.00 $0.00 $55.00 $0.00

Electricity Cost ($/MWhr) $30 $30 $30 $30 $50 $50

Allowance Value $175 $175 $175 $175 $175 $175
Case 3: Use of OverScrubTM for 

Reduction in Power Consumption

A utility is located in a geographical area where the availability of power is in short supply and the cost of electricity is

elevated. The utility would like to reduce the auxiliary power

consumption to increase the net electrical output from the

station. The utility has little incentive to reduce SO2 emissions but applies URS’ OverScrubTM technologies to take one

spray pump out of service per scrubber module. 

To allow the utility to maintain SO2 emissions at their current level with one spray pump less in service, the scrubber

system is retrofitted with URS’ liquid distribution rings. The

retrofit will reduce required pump power but will also reduce

the gas-side pressure drop providing savings in fan power. 

Based on the assumptions provided in Table 6, the economics of this conversion case are very attractive. For a 500-

MWe plant the annual savings are about $0.8 million and the

payback is less than 2 years. A summary of the economic

analysis is shown in Table 7. 

Techniques for Improving FGD System Performance to Achieve Ultra-High SO2 Removal Efficiencies

12

Table 7. Economics of OverScrubTM for Power Savings.

Case 1 Case 2 Case 3

High Sulfur Fuel Blend Lime to Limestone Switch Power Savings

SO2 Credits, Fuel Savings, SO2 Credits, Reagent

Gypsum Revenue Savings, Gypsum Revenue Power Savings

Base OverScrub Base Overscrub Base OverScrub

Capital Investment, $M $0 $4.3 $0 $10.5 $0 $1.5

Revenue

Fuel Savings, $M - $5.57 - - - -

Incremental SO2 Allowance, $M - $0.10 - $0.02 - -

Gypsum Sales, $M - $0.59 - $0.74 - -

Fly Ash Sales, $M $0.20 $0.21 $0.28 $0.28 - -

Total Revenue, $M $0.20 $6.48 $0.28 $1.03

Cost

Reagent Cost, $M $0.24 $1.30 $4.47 $1.55 - -

DBA Costs, $M - $0.55 - $0.66 - -

Fly Ash Disposal Costs, $M - - - - - -

Lime Stabilization Cost, $M - - $0.32 - - -

Waste Sludge Disposal Cost, $M $0.01 $0.01 $0.48 $0.04 - -

Incremental Power Costs, $M - $0.11 - $0.11 - $(0.74)

Incremental O&M Costs, $M - $0.15 - $0.15 - -

Total Cost, $M $0.25 $2.11 $5.27 $2.51 - $(0.74)

Revenue Net Costs, $M $(0.04) $4.37 $(4.99) $(1.47) - $0.74

Net Revenue Increase, $M $4.41 $3.52 $0.79

Capital Pay Back Period, Y 1.0 3.0 1.9
Summary

A range of economic and legislative drivers indicate that a

reduction in SO2 emissions from existing FGD systems will be

attractive in coming years. SO2 emission credits are predicted to be depleted as early as 2005 indicating that the utility industry will install new scrubbers to maintain the

national emission cap.

URS has developed a range of OverScrubTM technologies

that can be applied on the existing scrubber fleet to enhance

the scrubber operation and reduce SO2 emissions in a costeffective manner. The OverScrubTM technologies allow a

scrubber to operate well outside its design limits in a cost

effective and reliable manner. The enhanced scrubber capability can be utilized to reduce SO2 emissions but it can also

be applied to gain unrelated savings in operating cost.

Frequently, high sulfur fuels can provide significant fuel savings. Similarly, a limestone-based chemistry can offer significant savings over sodium, lime, or mag-lime based systems.

By applying the OverScrubTM technologies, existing scrubber

systems can be enabled to operate successfully on higher

sulfur fuels or switched to a limestone-based chemistry.

The analysis provided in this paper demonstrates that

achieving high removal efficiencies to generate excess emission credits, providing for additional fuel flexibility, switching

to a limestone-based chemistry, and or generating significant

auxiliary power savings can be a profitable undertaking.

References

1. www.epa.gov/airmarkets

2. High-Efficiency FGD Workshop, Sponsored by EPRI & DOE,

September 1994.

OverScrubTM

13
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and construction services worldwide with annual revenues exceeding $10 billion. URS has 

approximately 45,000 employees in a network of offices in major cities in the Americas, 

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for federal, state, and local governmental agencies, as well as Fortune 500 and other 

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