# FGD Upgrades

Techniques for Improving FGD System Performance to Achieve Ultra-High SO 2

## Removal Efficiencies

### Abstract With increasing pressure to economically reduce SO 2 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 SO 2 emissions. SO 2 removal efficiencies can typically be increased in a cost-effective manner to well above 95 percent. In addition URS’ OverScrubTM can beused 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 removalefficiencies 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 SO 2 

# 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 SO 2 emissions from coal-fired power plants. With the start of Phase II in 2000, the national emission cap was reduced to 9.1 milliontons 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 SO 2 emission credits market and on utilities to reduce SO 2 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 SO 2 emissions. Currently, SO 2 emissions from the existing FGD fleet is about 1.4 million tpy. However, the early vintage of FGD systems were designed for only moderate SO 2 removal efficiencies, leaving room for improved technologies such as OverScrubTM.

It is estimated that SO 2 emissions from the existing FGDfleet 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 SO 2 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 theOverScrubTM 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 SO 2 emissions fall basically in two categories, legislative and financial. Current and future legislative initiatives targeting SO 2 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 reductionof 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 SO 2 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 SO 2 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 SO 2 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 unitsjoined 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 SO 2 emissions at these units nationwide were reduced by almost 40% below their required level. OverScrubTM

1Phase II, which began in the year 2000, tightened the annual emissions limits imposed on these large, higheremitting 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

SO 2 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, SO 2 emissions on a national level did not change substantially. The first year of Phase II showed a significant drop in SO 2 emissions, however, not sufficient to bring emissions in parity with available SO 2 emission allowances. SO 2emissions 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 SO 2 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. SO 2 emissions from the existing scrubber fleet was 1.4 million tons in 19991. The average SO 2 removal efficiency for the installed scrubber fleet is about 80 percent leaving room for an additional reduction in SO 2 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. Ithas been estimated that on an average, the removal efficiency of the installed scrubber fleet can be increased from80 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, 700thousand tons of emission credits would be worth $140 million per year. Also, a reduction in SO 2 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 SO 2 emissions. Although one recycle pump may seem insignificant, the increased generatingcapacity from taking one recycle pump out of service represents significant value. Techniques for Improving FGD System Performance to Achieve Ultra-High SO 2 Removal Efficiencies

2 0 2 4 6 8 10 12 14 16 18 20

1980 1985 1990 1995 2000 2005 2010US SO2Quantities, 106TPY Emissions Allowance Cap
Banked Allowances

Figure 1: SO 2 Emissions, Allowance Cap, and Banked Allowances 1

### Overview of available OverScrubTM options Options to reduce SO 2 emissions from existing scrubbersbasically 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 SO 2 emissions are listed in Table 1.

Before any improvement options should be considered, the condition of the existing scrubbersystem 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 SO 2 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 SO 2 removal.

Increased SO 2 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 modifiersLiquid-Phase Alkalinity Improvement OptionsThe 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 SO 2 removal efficiency in any scrubber is to increase the slurry pH as shown in Figure 2.

The SO 2 removal typically responds very quickly to an increase in pH. An increase in slurry pH is typically achievedby 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 SO 2 removal efficiency in wet scrubbers. Organic acids work by buffering the pH at the gas liquid interface, by reducing the back pressure of SO 2 and by lowering the liquid-film resistance to masstransfer. 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 SO 2 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 lossessuch as coprecipitation, chemical degradation, and vaporization losses. Solution losses can be minimized by operatingthe scrubber system in a close loop configuration and by minimizing the liquid blowdown. They can be calculated from amaterial 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 SO 2 Removal Efficiencies

4 60 70 80 90 100

5.6 5.8 6 6.2 6.4 pHRemoval 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 ConcentrationRemoval Efficiency, percent

DBA FOX
DBA IOX
Formate FOX
Formate IOX Figure 2: Impact on SO 2 Removal and Limestone Utilization by Slurry pH 2

Figure 3: Impact on SO 2 Removal Efficiency by Buffer Additives 2Figure 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 SO 2 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 SO 2 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., ppmNon-solution Loss Rate,lb/ton SO2removed

DBA - IOX
Formate - FOX
DBA - FOX
Formate - IOX

Figure 4: Non-Solution Loss Rates for DBA and Formate 2

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, ppmSO2Removal, percent

66 L/G 133 L/G

Figure 5: Impact by L/G on SO 2 Removal 2Liquid 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 gassneakage. 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 SO 2 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 SO 2 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 SO 2 removal efficiency increases significantly when liquid distribution rings are installed. The increase in SO 2 removal is achieved without a measurable increase in gas-side-pressure drop. Figure 8 illustrates theability to operate with one spray level out of service after liquid distribution rings are installed while maintaining the same or improved SO 2 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 SO 2 Removal Efficiencies

6 Figure 6: Liquid Distribution Rings

0 100 200 300 400 500 600 700

0 2 4 6 8 10 Distance from Absorber Wall, ftSO2COncentration, ppm Absorber A, 36 ft Absorber B, 46 ft

Figure 7: Wall Sneakage in Open Spray TowersTrays 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 SO 2 removal by creating additional surface area obtained from liquid holdup on the tray.

The degree of improvement in SO 2 removal by adding a tray is very similar to adding liquid distribution rings, asshown 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 SO 2 removalwhile 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 SO 2emissions 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 SO 2 removal cost for calcium based reagents. Scrubber ChemistryEach 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 ofchanges 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 substantiallyhigher 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 SO 2 emissions. Table 5 shows the impact of SO 2 compliance cost on a $/ton of SO 2removed basis for the most common reagents. As clearly illustrated in Table5. limestone-based chemistry provides a tremendous annual cost OverScrubTM

7 75 80 85 90 95 100

1 2 3 Pumps in ServiceSO2Removal, percent

Conventional
With Tray

With LDR

Figure 8: Impact on SO 2 Removal by Liquid Distribution Rings or a Tray 2

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
SO 2 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 SO 2 Removed $55.10 $50.50 $16.40advantage. 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 reactiontank 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 SO 2 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 uniquepatented high efficiency oxidation device, which makes oxidation possible in the very shallow tanks typical of limebased scrubbers. This allows complete in-situ oxidation andimproved 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. Throughthe 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 substantialnumber. However, without an optimization of the reagent utilization, any increase in SO 2 removal will require an increasein reagent consumption. By optimizing limestone consumption in conjunction with an increase in SO 2 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 thesetechniques 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 20percent 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 SO 2 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 whichcan 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 SO 2 emissions. Typically, existing scrubbers can be made to process higher flue gas volumes. SO 2 emissions can be maintained or reduced by applying some of the enhancement Techniques for Improving FGD System Performance to Achieve Ultra-High SO 2 Removal Efficiencies

8technologies 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 abilityto 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 scrubbersto 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 avariety of potential solutions. The CFD tool allows for optimization of the OverScrubTM technologies for each installation relative to cost, reliability, and operational impact.



### Case StudiesIn 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 impactof nozzle placement and gas liquid interaction. URS simulated several different nozzle orientations before settling ona recommended optimized orientation, which could significantly reduced the mal-distribution and carryover issue. OverScrubTM

9 Figure 9: Tampa Electric Big Bend Station Unit 4 FGD SystemMidwest 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. Ithad 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 inthe 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 SO 2 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 SO 2 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 SO 2 removal performance and reduces gas sneakagealong the absorber walls, a common drawback of countercurrent scrubbers. Application of this novel technology will enable someinstallations, 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 thevessel. This redistribution of the flue gas in the zone of highest slurry density is predicted to improve SO 2 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 operatingcost can be achieved in several ways including: (a) generationing SO 2 emissions credits; (b) fuel savings by switching to a lower cost, higher sulfur fuel; (c) switching to a lower costreagent; 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 SO 2 Removal Efficiencies

10 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 toburn 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 ReagentA 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 areretrofitted with an in-situ forced oxidation system specifically designed to operate in shallow tanks and with URS SO 2 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 SO 2 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 SO 2 Credits, Fuel Savings, SO 2 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

SO 2 Content, (lb-SO 2/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

### C ase 3: Use of OverScrubTM for Reduction in Power ConsumptionA 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 SO 2 emissions but applies URS’ OverScrubTM technologies to take one spray pump out of service per scrubber module. To allow the utility to maintain SO 2 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 SO 2 Removal Efficiencies

12 Table 7. Economics of OverScrub TMfor Power Savings.

Case 1 Case 2 Case 3 High Sulfur Fuel Blend Lime to Limestone Switch Power Savings SO 2 Credits, Fuel Savings, SO 2 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 SO 2 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 SO 2 emissions from existing FGD systems will be attractive in coming years. SO 2 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 SO 2 emissions in a costeffective manner. The OverScrubTM technologies allow a scrubber to operate well outside its design limits in a costeffective and reliable manner. The enhanced scrubber capability can be utilized to reduce SO 2 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 thatachieving 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

13URS is a full-service organization providing engineering, procurement, project management, 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, Asia-Pacific, the Middle East and Europe. Our broad-based expertise is a valued resource for federal, state, and local governmental agencies, as well as Fortune 500 and other private industry clients worldwide. The Engineering-News Recordhas ranked URS as one of the top three design firms for the past decade.

© 2010 URS Corporation 

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