1

FGD Performance Capability 

Paper # 62 

Chuck Dene 

Electric Power Research Institute, 3420 Hillview Avenue, Palo Alto, CA 94304

Lesley A. Baker 

Washington Division of URS Corporation, P.O. Box 5888, Denver, CO 80217

Robert J. Keeth 

Washington Division of URS Corporation, P.O. Box 5888, Denver, CO 80217

ABSTRACT 

Utilities continue to specify FGD system performance at increasing SO2 removal efficiencies 

in response to permit requirements, local consent decrees and the potential need to install 

CO2 controls in the future. Environmental groups are also pressuring public utility 

commissions to push pulverized coal power plants to achieve emission levels similar to 

IGCC and NGCC. This paper summarizes FGD system design and operation upgrades that 

may be necessary to achieve 99+% SO2 removal on a continuous basis, resulting in single 

digit SO2 ppm emission rates. This paper will also address several multi-pollutant emission 

control technologies that can achieve or show promise of achieving very high levels of SO2

removal in addition to removal of NOx and/or mercury. 

INTRODUCTION 

The FGD market in the U.S. has been working at near capacity for the last few years. The 

regulatory environment for coal fired units has been more and more restrictive due to the 

perception of coal fired generating facilities as being dirty in comparison to gas fired plants 

and other renewable energy options. But coal still plays an integral part in the generating 

mix in the U.S. due to ever increasing demand for electricity. In response to public 

perception, the state and federal regulatory agencies continue to promulgate more restrictive 

legislation for coal fired plants, in some cases requiring 98-99% reduction of SO2 from new 

coal fired generating plants. The control of CO2 generated by new and existing coal plants 

will further push the SO2 reduction technologies to reduce their SO2 emission levels to the 

single digits to help reduce the operating costs associated with some of the CO2 removal 

systems. This paper provides a summary of the gains in FGD system performance that are 

now being offered in the U.S. utility market today, and describes some of the new 

technologies that may meet these requirements in the future. 
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EXPERIMENTAL METHODS 

Some of the major FGD systems suppliers serving the U.S. utility industry are listed below: 

• Siemens-Wheelabrator 

• Chiyoda 

• Babcock & Wilcox 

• Babcock Power 

• Alstom 

• Advatech/Mitsubishi 

• Hitachi America 

• Marsulex 

In 2006, it was estimated that these companies had combined sales for more than 90% of the 

wet FGD market in the United States. 

URS Washington Division contacted many of these major vendors of FGD equipment to 

discuss how they would plan to address the need for increased SO2 reduction efficiency for 

continuous, long-term operation of their FGD systems. Specific installations were identified 

that have already achieved 98% to 99% removal on a continuous basis. System upgrades that 

allowed these high levels of performance were also identified, including modifications that 

have been incorporated into recent installations, or could be used in existing FGD systems to 

achieve outlet emissions that approach single digit ppm levels. The information from the 

conference calls and supplier visits was combined with material from previous EPRI reports, 

conference proceedings, and public domain literature to complete the documentation for this 

paper. All of the information and data from these sources were compiled and are presented 

in multiple EPRI reports that were published in 2007-2008. 

Much of the discussion surrounding FGD systems focuses on wet systems as they have a 

distinct advantage over dry FGD systems in achieving 99+% SO2 removal. In addition to 

FGD processes, the report contains process descriptions and development information on 

multi-pollutant processes that have the capability or show promise to achieve 99+% SO2

removal. The discussion of these processes also includes their capabilities to remove NOx 

and mercury simultaneously. 

 

THE NEED FOR HIGHER SO2 REMOVAL EFFICIENCIES 

Power generation companies in the U.S. are facing an increasingly stringent regulatory 

landscape, not only from the EPA, which has implemented CAIR and visibility (and 

attempted mercury) regulations in the last few years, but also from states, some of which 

have established SO2 emission limits lower than the federal emissions standards. Most 

power producers have concluded that continuing public concerns regarding the overall 

impacts of coal-fired power generation will result in even more stringent limits of NOx, SO2, 
3

mercury, and primary particulates in the future. In addition, public pressure on regulators to 

mandate consideration of alternatives to pulverized coal (PC) fired generation because of 

their carbon footprint is on the rise. In response to public concern and pressure, future state 

and/or federal regulatory limits on SO2 emissions could require FGD process to reduce SO2

emission rates to single digit ppm levels. 

One of the alternatives to PC fired generation identified as the preferred method to achieve 

low emission rates by some environmental groups and regulatory agencies is Integrated 

Gasification Combined Cycle (IGCC). Although very few of these plants have been built, 

and many have recently been cancelled due to costs or regulatory uncertainty, they were 

becoming the emissions benchmark for coal-fired power generation technologies. As a 

result, utilities that have decided that PC-fired plants are more viable to meet increasing load 

demand because they are more cost-effective than IGCC designs may face regulatory 

requirements for new and retrofit FGD systems that can reduce sulfur emission to levels 

achieved by IGCC. Advocates of Natural Gas Combined Cycle (NGCC) are going one step 

further and lobbying to have these systems replace coal-generated power outright because 

NGCC emits virtually no SO2 and reduces green house gas (GHG) emissions. 

Green house gas emissions, especially CO2, play a role in the global warming theory. Public 

attention has driven federal and state regulators toward enactment of regulations to reduce 

CO2 emissions from new and existing power plants. New technologies are currently under 

development for pre- and post-combustion CO2 reduction, as well as modifications to the 

combustion process itself. Some of the post-combustion CO2 removal processes operate 

more efficiently and are more cost-effective when inlet SO2 concentrations are maintained at 

very low levels. 

All of these factors are leading utilities toward FGD processes that can achieve SO2 emission 

levels that approach the single digit ppm level when firing all types of coal in PC plants. To 

meet the challenges for near-zero SO2 emissions from PC plants, it is likely that FGD 

systems will have to incorporate design modifications to enhance SO2 reduction, and 

operating practices will have to be modified to achieve the lower SO2 outlet concentrations. 

ACHIEVING HIGHER SO2 REMOVAL EFFICIENCIES 

Utilities are already installing FGD systems with performance guarantees of 98+% SO2

removal. These systems incorporate some of the latest absorber design upgrades and 

modified operating conditions specified by the FGD original equipment manufacturers 

(OEMs). However, it should be noted that these newest installations are designed to meet the 

contract performance test requirements while the supplier staff are on the site, and typically 

the FGD system guarantees do not apply to the actual long-term operating performance. The 

operating procedures of plant staff will tend to be different from those used by the OEM 

staff, and changes in equipment performance as the system ages will tend to result in some 

reductions in the reduction efficiency capabilities of the FDG systems. Therefore, additional 
4

design upgrades may still be necessary to achieve continuous single digit SO2 emission rates 

during long term operation. 

Utilities will also need to consider adopting a different operating philosophy in order to 

achieve 99+% SO2 removal on a multi-day rolling average basis. To achieve these single 

digit SO2 levels, the utilities will need to place greater emphasis on monitoring FGD 

performance including augmenting operating staff with engineering staff who can 

appropriately respond to changes in process chemistry due to system upsets, and paying 

greater attention to the accuracy and performance of process instrumentation and equipment 

maintenance. In essence, U.S. utilities would need to adopt the Japanese FGD operating 

philosophy, which includes hands-on involvement of operating engineers on a day to day 

basis, regular preventative maintenance to instruments and operating components, and 

consistent vigilance by operating staff to ensure that the desired process chemistry is 

maintained within specified ranges so that the FGD performance can be maintained at it 

maximum capability. 

FGD System Design Modifications 

The following sections focus on the latest state-of-the-art FGD system design innovations 

that are allowing these systems to achieve SO2 reductions above 99% removal for high sulfur 

coals, and above 98.5% for low sulfur coals, resulting in SO2 emission rates that approach the 

single digit ppm level. It is assumed that the reader has a basic understanding of the FGD 

systems, including design and reaction chemistry, offered by the major FGD vendors that 

were mentioned earlier. Only brief descriptions of each process are given here, focusing on 

those elements in the design and operation of these systems that will contribute to increased 

SO2 removal. 

Chiyoda CT-121 Jet Bubbling Reactor (JBR) 

The CT-121 FGD process was developed more than 20 years ago by the Chiyoda 

Corporation in Japan. They have since licensed the technology to engineering and 

construction contractors throughout the world. The CT-121 process utilizes the same 

reaction chemistry as the wet Limestone with Forced Oxidation (LSFO) process. 

The major difference between the CT-121 process and other LSFO FGD systems is the Jet 

Bubbling Reactor that serves as the SO2 absorber, oxidizer, and reaction crystallizer in one 

vessel. Flue gas from the boiler is cooled using a water quench and then is forced down 

through a series of sparger tubes positioned in a tube sheet above the reaction tank and 

immersed in the limestone/gypsum slurry (see Figure 1). The gas flows down through the 

tubes and then bubbles upward through the absorber slurry to form a froth layer. The froth 

provides a gas-liquid contact zone to promote SO2 dissolution into the reaction tank slurry. 

Chiyoda has stated that the CT-121 process can achieve SO2 removal of more than 99%. 

They have some limited experience on very high SO2 inlet concentrations (4000 ppm), and 
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have demonstrated that achieving 99+% removal on these units is relatively easy. Chiyoda 

has also provided guarantees for 99+% removal on medium sulfur units, in particular the 

Shinko-Kobe Unit #2 with a design sulfur content of 0.93% S on these two coal fired units 

(700 MW). At this facility, Chiyoda guarantees greater than 99% SO2 reduction from an 

inlet flue gas stream containing 840 ppm SO2. The actual condition for the performance test 

was 320 ppm, and the Chiyoda system attained 99.8% SO2 reduction under those conditions. 

Figure 1: Chiyoda CT-121 Schematic Flow Diagram (Round Vessel) 

These high reduction efficiencies are achieved with an increase in pressure drop across the 

absorber (added power consumption) and operation at a higher slurry pH. The CT-121 

system operates at a higher pressure drop than an open spray tower design because the flue 

gas is bubbled through the slurry. This higher pressure drop, and resultant increase in ID fan 

power consumption is offset by the fact that the CT-121 process does not use large recycle 

pumps to feed slurry to the absorber vessel. In order to achieve higher reduction efficiencies, 

the CT-121 absorber can be run with a higher pressure drop by increasing the depth of the 

slurry through which the flue gas must flow to reach the top of the froth layer. This increases 

the contact time of the SO2 and the slurry, promoting greater removal, but it also increases 

the power consumption of the system. 

The latest CT-121 installations have been able to reduce their power consumption 15% by 

increasing the pH of the slurry by 1.0, thereby reducing the depth of the sparging tube 

immersion in the slurry. Chiyoda noted that high dissolved chloride concentrations can 

significantly suppress the pH in the absorber slurry, potentially reducing the SO2 removal. 

This effect is due to the high ionic strength caused by the chloride concentration, which 
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negatively impacts the transport of hydrogen ions to the surface of the limestone particles. 

This H+ ion transport is necessary for the limestone dissolution process. The CT-121 process 

is relatively unaffected by the chloride concentration. In a typical spray tower design it 

would be necessary to increase the dissolved alkalinity to maintain SO2 removal. This 

requires more un-dissolved limestone to be present, increasing the limestone feed rate and 

potentially the scaling potential of the slurry. 

Mitsubishi/Advatech Double Contact Flow Scrubber (DCFS) 

Mitsubishi Heavy Industries (MHI) has been providing emissions control systems for power 

plants for more than 40 years. The DCFS is the current state-of-the art for the MHI FGD 

technology and has been installed on power generation plants for the last 30 years. The 

Double Contact Flow Scrubber (DCFS) design is currently offered by MHI under the name 

Advatech in the North American markets. 

In low sulfur applications, flue gas enters the bottom of the absorber where the absorbent 

slurry is sprayed upward through large diameter (1.5”) nozzles creating a dense liquid-filled 

area. Slurry is supplied to a single spray header by multiple large recycle pumps. The flue 

gas contacts the slurry as it is sprayed out of the nozzle (co-current), and as the liquid falls 

back down towards the nozzles (counter-current). Therefore, the flue gas double contacts the 

limestone slurry. While the single tower design is mainly used for low sulfur applications up 

to 700 MW and 97% SO2 removal, efficiencies in excess of 99% have been maintained in 

high sulfur applications using a single DCFS module. A process flow diagram for this single 

tower arrangement is on the left in Figure 2. 

For larger units and those requiring higher removal efficiencies, as well as some higher sulfur 

fuel applications, MHI can add a second tower section so that the double-contact between the 

slurry and the flue gas occurs in two towers fed by a common reaction tank. The inlet tower 

is referred to as the co-current tower, and the outlet tower as the counter-current tower. A 

diagram of this dual tower application is shown on the right in Figure 2. The dual tower 

configuration has demonstrated removal efficiency capabilities of 99+%. One DCFS 

installation on a 2% oil-fired boiler has consistently been able to reach 99.9% SO2 reduction. 

One recent design enhancement for the MHI DCFS Process is the inclusion of a chemical 

oxygen demand (COD)/sulfite monitoring system that measures and controls the oxidation 

reduction potential (ORP) of the slurry. The ORP monitor maintains the slurry chemistry in 

the optimum range and indicates slurry conditions that could lead to aluminum fluoride 

blinding of the limestone particles in the slurry, allowing the operators to take action to stop 

the potential loss of reduction efficiency. 

Medium sulfur fuel applications of the DCFS system have now demonstrated continuous 

operation at greater than 99% SO2 removal efficiency. This commercial experience may 

translate to high efficiency commercial guarantees in the future. This level of performance 

can be achieved using either the single or twin tower design depending upon other operating 
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conditions at the plant. The DCFS design has achieved 99.9% efficiency with a single tower 

applied to a high inlet SO2 flue gas stream (2,670 ppm on 150 MW unit). The outlet 

concentration was measured at 3 ppm. This performance is stated to be possible due to the 

enhanced liquid-to-gas contact and high reagent activity in the 30 wt.% slurry. Similar 

performance was obtained at a second unit operating on a 2,600 ppm gas stream (99.6% SO2

reduction with design guarantee at 98.3% on 100 MW unit). 

Figure 2: MHI/Advatech Single and Dual Tower DCFS Diagram 

Alstom Open Spray Tower and FLOWPAC Scrubber 

The open spray tower was Alstom’s first FGD product offering with the first unit starting up 

in 1975. The absorbers generally have two or more spray levels, each with their own 

dedicated recycle pump. Design, selection and placement of spray nozzles at each spray 

level are critical to performance in high efficiency wet FGD systems. The spray nozzle 

configuration is determined by computer modeling in order to achieve the desired gas to 

liquid contact and scrubbing efficiency. 

One of the innovations added to the original spray tower design are wall rings. Wall rings 

are radial baffles that project inward from the spray tower wall and redirect any flue gas 

traveling up the tower wall back into the spray zone. These baffles have been shown to 

improve SO2 removal efficiency by 2-5% or alternatively to allow 10-20% reduction in 

liquid-to-gas ratio and achieve the same SO2 removal as without baffles. Additional 

advances in spray tower design and operation include the following: 

 Use of conservative design parameters in the areas of oxidation air stoichiometry and 

good air distribution has reduced the occurrence of oxidation deficiencies in the 

latest designs. 
8

 Maintaining 18-20% gypsum solids in the recycle slurry provides more surface for 

crystal growth in the slurry than is available on the internal surfaces. This helps to 

reduce scaling on internal surfaces and increase system reliability. Nuclear density 

meters have demonstrated their ability to tightly control suspended solids density in 

the recycle slurry. 

Alstom has guaranteed 98% removal for high efficiency applications. Several of Alstom’s 

operating spray towers have achieved 99+% removal for extended periods, while others have 

been able to achieve 2-3 ppm without the addition of any organic acid. The Navajo 

installation has maintained an outlet emission rate of 0.04 lbs/MMBtu, which is 

approximately equivalent to 17 ppm SO2. 

Alstom is using their commercial spray tower design at these high performance installations, 

incorporating wall rings and slurry spray patterns to eliminate the potential for flue gas 

sneakage past the sprays. Trays or additives should not be required to achieve these high 

levels of continuous performance in the future. To maintain constant 99+% reduction, it will 

probably be necessary to add another spray level, and increase the L/G to the absorber by 20-

25%. 

Alstom has recently developed a new scrubber design -- the FLOWPAC turbulent bed 

absorber. The chemical reactions occurring in the FLOWPAC absorber are essentially 

identical to those in all other LSFO systems. The primary difference between this process 

and standard open spray tower design is the unique absorber vessel used in the FLOWPAC 

system. Alstom has a full-scale commercial system operating at a 340 MW high sulfur oilfired unit located at the Karlshamm power plant in Sweden. This full scale unit achieves 

98% SO2 removal continuously when firing a 3.5 weight % sulfur oil. It is also reported to 

have achieved 99.5% SO2 removal when acetic acid is added to the slurry and the plant was 

burning 2.4% S fuel oil for a 3-month period. 

The first FLOWPAC absorber consisted of an absorber tank with a circumferential sieve tray 

installed at the top of the tank. The flue gas enters the absorber beneath the sieve tray. 

Recycled slurry pours over the sieve tray, and the flue gas bubbles up through the slurry 

forming a highly turbulent froth layer. The froth layer provides a large surface area of 

reagent and flue gas. Slurry flows through return pipes downward from the outside of the 

sieve trays to the bottom of the absorber tank where oxidation air is introduced. The 

oxidation air acts as the lifting method recycling the slurry from the bottom to the top of the 

reaction tank, thereby eliminating the need for slurry recycle pumps. Figure 3 provides a 

flow diagram of the FLOWPAC process. Alstom is testing a rectangular tank design on a 

pilot scale at the Karlshamn plant. 

The FLOWPAC absorber can change the depth of the bubble layer above the sieve tray to 

follow the plant load or changes in inlet SO2 concentration. This ability to change the depth 

above the sieve tray also provides an inherent capability to achieve higher SO2 reduction 
9

efficiencies, with the 99+% capability already possible and previously demonstrated. This 

increase in slurry depth comes with a higher absorber pressure drop.

Figure 3: Alstom FLOWPAC Turbulent Bed Absorber 

Alstom has recently begun to actively market the FLOWPAC absorber design around the 

world, and has introduced the concept to the U.S. utility market. To-date, there are no 

FLOWPAC installations or sales in the U.S. However, two have been sold overseas – one in 

Lithuania (3 x 150 MW) and one in Denmark (1 x 150 MW). These units are scheduled for 

startup in 2008 and 2009, respectively. 

Babcock & Wilcox Wet Lime and Limestone Forced Oxidation Tray Tower 

B&W has supplied FGD systems to the U.S. utility industry for decades. Their original focus 

was on the use of lime reagents in a tray tower design. Recent experience includes many 

installations of LSFO systems, again using a combination of trays and an open spray tower. 

B&W has also developed an oxidation systems for lime FGD systems, allowing the process 

to produce a gypsum product instead of a sulfite product that must be mixed with lime and 

flyash before disposal in a landfill. The typical B&W absorber uses a perforated tray and 

two absorber spray levels. The two spray levels include one under and one above the tray. 

The hot flue gas enters the absorbers and is cooled to saturation by absorber slurry sprayed 

from the spray level under the tray, as well as the slurry falling from the tray. The saturated 

flue gas flows through the absorber tray, which distributes the flue gas and slurry from the 

upper spray level across the absorber cross-section. As the gas passes through the tray 

perforations the increased gas velocity generates energetic bubbling thereby assuring liquidRaw Gas 

Absorbent 

slurry 

Compressed 

air 

Clean gas 

Demister 
10

gas contact. The flue gas then passes through the upper absorber spray zone, where absorber 

slurry spraying countercurrent to the flue gas flow completes sulfur dioxide removal. 

 Tests conducted on a LSFO FGD system operating at a coal-fired power plant 

showed that addition of a second perforated tray and operation at a higher L/G ratio 

increased the SO2 removal from 90% to 98% at slightly lower lime stoichiometry. 

 In a second test at the same installation, addition of the second perforated tray and 

increasing the pH by 0.5 increased the SO2 removal from 90% to 98%. 

As previously mentioned, B&W’s lime systems may warrant renewed interest, especially 

since magnesium enhanced lime (MEL) can achieve the same SO2 removal efficiencies as 

LSFO systems, but at much lower L/G ratios. Based on medium sulfur Appalachian coal, the 

B&W LSFO system was able to achieve 98% removal at an L/G ratio of 130, whereas MEL 

was able to achieve 98% removal at an L/G ratio of 40. When using the lime reagent, further 

increases in the recycle rate could lead to a system capable of 99+% SO2 reduction. There 

are 30 MEL installations in the U.S. ranging in size from 80 MW to 1400 MW. The new 

MEL oxidized process is used at seven of the units. This process allows for the SO2 reaction 

products to be oxidized from calcium sulfite to calcium sulfate. 

Babcock Power 

Babcock Power relies primarily on an open spray tower design using bi-directional sprays 

(slurry sprays upward and downward with respect to the flue gas flow). The tower has wall 

baffles to mitigate the chance of internal flue gas bypass. The geometry of each spray level 

varies and the direction of the spray from the spray nozzles is also designed to mitigate the 

chance of internal flue gas bypass. This LSFO process has the same process chemistry as the 

other LSFO processes. The following are recent design modifications and/or enhancements 

to the Babcock Power wet FGD process technology: 

 The absorber inlet is optimized to provide the best possible gas flow distribution – 

they no longer use a tangential gas inlet and the gas velocity through the scrubber has 

been increased by more than 20% to reduce the capital cost while improving the gasliquid contact efficiency. This is offset by an increase in the pressure drop across the 

absorber. 

 Babcock Power employs CFD modeling to optimize the design of their FGD 

absorbers. This modeling resulted in changes to the location of and the number of 

nozzles per spray levels in their current wet absorber designs. The nozzle design has 

also been optimized, using dual-direction nozzles that can spray both co-current and 

counter-current to the direction of the gas flow through the absorber vessel. These 

nozzles produce small droplets that increase the number of droplet collisions, 

producing additional spray pattern coverage. A staggered spray pattern is also used. 

 Nozzles are installed along the walls of the absorber vessel to spray countercurrent to 

the gas flow and minimize the potential for gas sneakage (wall effects) past the spray 

pattern. High spray flows along the absorber walls help to increase overall collection 
11

efficiency. Wall rings are also used to reduce the potential for gas slippage past the 

spray zones. 

 Babcock Power also offers a dual loop design for very high efficiency or high sulfur 

applications while reducing the overall power consumption. 

Babcock Power stated that in order to achieve the 99+% reduction efficiency, it is likely that 

it could be met by the addition of another spray level with its associated recycle pump. The 

L/G increase would correspond to the increase in pumping capacity. No internal packing, 

trays or additives should be necessary. Babcock is currently testing these design 

modifications to confirm single digit performance capability, to enable them to offer this 

level of performance commercially in the future. 

Siemens-Wheelabrator 

The Siemens-Wheelabrator (Siemens) FGD absorber design comes in various configurations, 

ranging from a typical open spray tower to the use of a sieve tray and spray headers. They 

have supplied systems based on lime and limestone reagents, and provided recent 

installations with performance guarantees up to 98% achieving outlet SO2 emission rates of 

20-25 ppm. Siemens did not provide any additional input regarding their future plans to meet 

the high efficiency FGD market at this time. 

Alternate FGD Technologies 

Other methods have been proposed to increase SO2 reduction efficiency across the FGD 

system. Some of these options are discussed in the following material. 

Dry Injection of Trona 

O’Brien and Gere, using trona supplied by FMC and Solvay, offers the trona injection 

technology for both SO2 and SO3 removal from coal-fired flue gas. Although these dry 

injection systems are not necessarily capable of achieving the 99+% reduction efficiency 

targeted in this analysis, they could provide some trim to the SO2 inlet concentrations 

entering the wet FGD systems, thereby reducing the load on the scrubber and possibly its 

outlet emissions concentration. Reduction rates of 40-90% have been demonstrated in 

previous tests, with the most economical reduction efficiency falling in a range of 50-60%. 

Recent advancements include the use of milled trona down to 15 micron particle size to 

improve reagent utilization rates, and injection at higher gas temperatures in front of the air 

preheater to provide the better performance. 

Dynawave/ReverseJet Scrubber 

Monsanto EnviroChem (now MECS) offers the Dynawave/Reverse Jet Scrubber technology 

for electric utility applications. This technology has seen most development and application 

for removal of SO2 from sulfur recovery units (SRUs) operating in refineries, achieving 

99.8% reductions. However, the inlet concentrations of SO2 for these units are very high, 
12

and the technology has not been applied to a coal-fired boiler in the U.S. to-date. Pilot 

testing has demonstrated the system’s capability to achieve less than 6 ppm SO2. The 

process relies on a reverse jet scrubber design; SO2 is captured by contacting the flue gas 

stream with an alkaline reagent in a froth zone region of intense turbulence with substantial 

back-mixing and a high rate of liquid surface renewal. This froth zone is generated in an 

inlet duct to the absorber where slurry is injected into the gas pass counter-current to the gas 

flow. These fountains provide intimate gas-liquid contact. The momentum of the gas causes 

the liquid to spread out and form a standing wave at the point where the liquid and gas 

momentums are balanced. Turbulent films of liquid occlude and envelop the particulate or 

gaseous contaminants, while also providing the gas-liquid contacting necessary for gas 

quenching and acid gas absorption. 

Until 1987, this technology was confidential and applied only within DuPont. In 1987, 

MECS realized the applicability of the froth scrubbing technology for FGD applications, and 

entered into a licensing agreement with DuPont. Since then, MECS has built more than 300 

DynaWave® systems for various industries around the world. 

Multi-Pollutant Systems 

The following material provides brief descriptions of multi-pollutant processes that 

simultaneously remove SO2, NOx, and mercury (Hg). These processes have the potential to 

remove high levels of SO2 and consequently are included in this report. Some of the 

processes have commercial-scale experience overseas, but most are only at the demonstration 

stage. These processes are of interest because of the benefit of being able to remove several 

pollutants, but it may be difficult for utilities to justify selection of one of these processes 

until they gain more extensive commercial experience. 

Powerspan ECOTM and ECO2TM 

Powerspan’s ECOTM process is designed for installation downstream of an existing 

particulate collection device. The process utilizes a dielectric barrier discharge reactor to 

oxidize pollutants in the flue gas stream, followed by a wet ammonia scrubber and wet ESP 

for removal of oxidized pollutants and byproducts. The technology produces a saleable 

ammonium sulfate fertilizer byproduct. A process flow diagram is shown in Figure 4. 

The ECOTM technology was pilot tested on a 1-MW equivalent slipstream of FirstEnergy’s 

R.E. Burger Plant in southern Ohio for four years and followed by the installation of a 50 

MW commercial demonstration unit at the same site. Emission results from the pilot tests 

and the commercial scale demonstration have shown similar performance, achieving 98% 

SO2 removal, 90% NOx removal, 85% Hg removal, and less than 0.01 lb/MMBtu emissions 

of fine particulate. Higher SO2 reduction efficiencies appear to be possible given the results 

of short term testing. Powerspan has entered into agreements to construct additional full 

scale installations in Ohio and as part of an ECO2 installation in North Dakota. 
13

Since the possible need for power plants to capture CO2 is one factor driving higher SO2 

removal efficiencies, it is worth mentioning Powerspan’s ECO2

TM Process, which is under 

development to absorb CO2 from power plant flue gas. The ECO2

TM process utilizes an 

aqueous solution of ammonia to absorb CO2 from the flue gas. The system is designed to be 

installed downstream, and work in conjunction with, the ECOTM technology. The CO2 is 

later released for sequestration, while the ammonia is recycled back to the system to 

minimize operating costs. 

Figure 4: ECOTM Process Flow Diagram 

Powerspan and the DOE’s National Energy Technology Laboratory (NETL) are working 

under a three-year cooperative research and development agreement (CRADA) to conduct 

tests on ECO2

TM. Initial results indicate up to 90% CO2 removal associated with the injection 

of aqua ammonia. Current plans are to eventually run a pilot test on a power plant near the 

end of the CRADA, at which point the process design and cost estimates will be finalized. 

Preliminary cost estimates predict that ECO2

TM will cost less than half of the current CO2

removal technologies that are commercially in use. 

J-Power ReACT 

The ReACT process utilizes a moving bed of activated coke (AC) with ammonia injection to 

simultaneously remove SO2, NOx, and mercury from the flue gas. Spent activated coke 

from the adsorption process is regenerated and recycled back to the adsorber, while SO2 rich 

gas is sent to a byproduct recovery unit and processed into a saleable sulfuric acid or gypsum 
14

byproduct. The ReACT system is currently being marketed worldwide by J-POWER 

EnTech Inc. J-Power, formerly known as Mitsui Electric Power Development Company, is a 

Japanese utility with 67 plants and 16,000 MW of capacity, approximately half of which is 

coal fired. 

The activated coke process was initially developed in Germany and Japan by Bergbau 

Forschung (BF) and licensed to Mitsui Mining Company. J-POWER has recently acquired 

the patents for the Mitsui-BF process and established J-POWER EnTech Inc. to market 

ReACT worldwide. 

The ReACT technology has been installed on 14 commercial units to date, including four 

coal-fired utility boilers and other industrial flue gas applications. All commercial 

installations of the technology are located abroad, with the majority located in Japan. The 

technology has been demonstrated successfully on large utility boilers, with a 350 MW 

installation in operation at J-Power’s Takehara Unit #2 since 1995, and a 600 MW 

installation in operation at Isogo #1 since 2002. J-POWER is currently constructing another 

600 MW unit at the same site, Isogo #2, scheduled for startup in July 2009. Figure 5 

contains a simplified process flow diagram for the ReACT process. 

Figure 5: ReACT Simplified Process Flow Diagram 

Flue Gas from

Existing ESP/FF 

Adsorption

Reactor

Regeneration

Tower

Flue Gas 

to Stack 

Coke Fines 

(to sale/disposal) 

Byproduct 

Processing 

Plant 

Sulfuric 

Acid 

Byproduct 

Make-up 

Activated 

Coke Silo 

Regen. 

Gas 

Furnace 

Natural 

Gas 

Ammonia 

Commercial installations located in Japan and Germany operate at 90-99% SO2 removal, 

with SO2 inlet concentrations as high as 1300 ppm SO2. NOx removal at existing 

installations ranges from 50-80% on coal-fired units, but is highly dependant on coal sulfur 

content. Sulfur dioxide in the flue gas reacts readily with injected ammonia, limiting the 

amount of NOx removal that can occur at an injection rate that limits ammonia slip. J-
15

POWER EnTech reports ammonia injection into the regenerator is necessary if more than 

50% NOx removal is required under higher SO2 concentrations. However, recent findings 

indicate that NOx removal reactions from reducing compounds on the surface of activated 

carbon allow removal of >50% NOx for gas streams with higher SO2 concentrations. Greater 

than 90% mercury removal has been demonstrated at the 600 MW Isogo #1 installation. 

At this point, the ReACT technology has not been demonstrated on high-sulfur coal 

applications. To simulate U.S. flue gas conditions, J-POWER has performed slipstream 

testing at Isogo #1 with sulfur content up to 2,000 ppm. Performance tests were completed at 

various conditions in order to optimize the design. J-POWER EnTech expects that there will 

be some modifications to the system design to adapt to flue gas from Eastern U.S. coals. 

They recently completed a slipstream test of the ReACT system at a utility site in Nevada. 

The ReACT technology is highly sensitive to coal sulfur content. The system was initially 

designed for low-sulfur (<1300 ppm) coals, and has not been optimized for operation with 

higher sulfur contents at this point. For high sulfur concentrations, the amount of activated 

coke fed to the adsorber increases substantially due to the need for more contact area to 

remove SO2 from the flue gas. This results in the need for additional adsorber and 

regenerator modules, and supplementary solids conveying equipment. Additionally, the 

amount of sulfuric acid byproduct increases with the amount of SO2 removed, resulting in 

increased sulfuric acid production plant costs. Demonstrated SO2 removal efficiency has 

approached 99% in some low sulfur coal commercial installations. 

Airborne 

The Airborne ProcessTM uses a sodium bicarbonate scrubbing technology combined with 

post-scrubbing oxidants to remove sulfur dioxide (SO2), sulfur trioxide (SO3), nitrous oxide 

(NOx), and mercury (Hg) from combustion flue gases, while producing a saleable fertilizer 

byproduct. Additionally, the process removes other heavy metals and toxics release 

inventory substances, such as sulfuric acid (H2SO4), hydrochloric acid (HCl), and 

hydrofluoric (HF) acid. A flow diagram of the process is provided in Figure 6. 

Flue gas desulfurization using sodium carbonate or bicarbonate has been previously 

identified as a highly effective process; there are over 3,200 MW of sodium scrubbing 

experience that has been in operation for over 25 years. However, sodium reagent costs are 

often economically prohibitive, and the sodium sulfite byproduct can pose a disposal 

problem. Sodium scrubbing has been in operation at PacifiCorp’s Jim Bridger Power Plant, 

Naughton Station in Wyoming, and at Nevada Power’s Gardner Station. Sodium scrubbing 

at these units is made economical by use of a waste product sodium material from a 

manufacturing process in Wyoming. 

Airborne Clean Energy (ACE) has developed a process where the sodium sulfite byproduct 

can be used to regenerate sodium bicarbonate reagent, while simultaneously producing an 
16

agricultural grade fertilizer. ACE is offering this technology in combination with oxidants to 

achieve additional NOx and Hg removal. 

Figure 6: Airborne Simplified Process Flow Diagram 

The regeneration component of the Airborne ProcessTM was successfully demonstrated at 

Airborne’s commercial sodium sulfate mine in Ormiston, Saskatchewan, Canada. The 18 

ton/day demonstration plant was initially built with the intention of upgrading sodium sulfate 

to commercial-grade sodium bicarbonate for sale. The demonstration plant at Ormiston 

produced a 98%+ sodium bicarbonate product and 99%+ ammonium sulfate from a sodium 

sulfate feed stream. Both sodium bicarbonate and ammonium sulfate were sold from the 

pilot plant. 

The first demonstration of the integrated technology was installed at LG&E’s Ghent 

Generating Station located in Carroll County, Kentucky. As a part of the DOE’s Clean Coal 

Power Initiative, the technology was installed on a 5 MW equivalent slipstream of Unit 2, 

and tested between January and July 2003. The tests demonstrated effective operation of the 

system including the reagent regeneration step. According to data provided by the vendor, 

the demonstration unit achieved 99.9% SO2 removal on a consistent basis. NOx removal was 

approximately 40% during normal operation, and increased up to 90% removal with the 

addition of an oxidant. Additional NOx removal could be obtained by the reduction of 

sodium bicarbonate particle size. Mercury removal during operation of the demonstration 

facility peaked at 60-80% removal. 
17

A commercial scale demonstration at the 300 MW Mustang Generating Station near Milan, 

NM was halted when Peabody Energy, HPD LLC was unable to secure an air permit from 

the New Mexico Government. ACE is installing their system at Saskatchewan Minerals and 

they are in negotiations with other clients in North America and China. 

CANSOLVTM Process 

 The CANSOLV system expands on the regenerable amine SO2 removal process by 

including CO2 removal in an integrated tower configuration. NOx and Hg removal 

capabilities, previously marketed, have been discontinued from CANSOLV’s product 

offerings. CANSOLV is focusing on marketing the SO2 and CO2 process configuration as 

shown in Figure 7. To better address the U.S. market, CANSOLV has teamed with Massey 

Energy and HG Engineering to form CANSOLV LLC to market their SO2 and CO2 process. 

Figure 7: CANSOLV Simplified Process Flow Diagram 

Prescrubber SO2 Lean-Rich

Exchanger

SO2 

Stripping 

Column

Electrolysis Salt 

SO2 Removal

Absorption

Water

Wash

Flue Gas

To Stack

Existing ESP/FF

Steam

Condensate

SO2 to Sulfuric

Acid Production

The CANSOLV System was initially developed as a SO2 scrubbing technology for industrial 

gas streams. CANSOLV’s SO2 system is currently operational at seven different commercial 

installations, with two more scheduled for startup at the end of 2007. Of the nine 

installations, four are in the U.S., two in Canada, and the others overseas. All of the 

installations operating or under construction are either at smelters or petrochemical plants. 

The inlet SO2 concentrations at these installations range from 800 ppmv to 11 volume % 

sulfur. The largest of these installations removes ~97% SO2 from a fluid catalytic cracking 

unit (FCCU) unit flue gas stream equivalent to 175 MW. At an acid gas installation, SO2 is 
18

reduced from 3,000 ppmv to 15 ppmv in the outlet flue gas (99.5 % removal). Three of the 

installations have been operating since 2002 and four have been operating since 2005/2006. 

CANSOLV’s SO2/NOx and CO2 technologies were tested at pilot-scale at the Saskatchewan 

Power Poplar River station in 2006. These tests showed positive results for the CO2 system, 

and less interesting results for the NOx and Hg process. As a result, CANSOLV has 

currently discontinued NOx control from its commercial offerings, but is actively pursuing 

CO2 control. They were competing for a large-scale CO2 capture system in Norway. 

Other Developing Multi-Pollutant Processes 

The following developing technologies are targeting 99% SO2 removal efficiency or higher: 

ISCA, Lextran, Consummator, Green Coal, Envirolution, Flue-Ace, PEA, EnviroScrub, and 

the IPR system. All of these systems are proceeding through initial stages of commercial 

development. 

COST IMPACTS 

It should be noted that the cost estimates provided herein are termed rough-order-ofmagnitude (ROM = ± 30% accuracy) and are dependent upon the various underlying 

assumptions, inclusions and exclusions utilized in developing them. Actual project costs will 

differ, and can be significantly affected by market pressures, other changes in the external 

environment, and the manner in which the project is implemented. Estimate accuracy ranges 

are only projections based upon cost estimating methods and are not a guarantee of actual 

project costs. The base case costs are generic and it should be understood that site-specific 

issues might result in wide variations in cost for these technologies. 

Increased SO2 Removal 

An EPRI study completed in 2006 identified the incremental cost of SO2 removal for one 

case based on the use of a wet limestone with forced oxidation FGD system (LSFO) in a 

spray tower absorber vessel. The system was installed on one 500 MW unit burning 

Appalachian coal (Pittsburgh # 8 coal with HHV of about 13,400 Btu/lb and 2.1 wt. % 

sulfur). Figure 8 provides the incremental cost of this LSFO case as the SO2 removal 

increases from 90% to 99%. 

From this figure, it is apparent that the incremental cost increases moderately from 92% to 

97%, and dramatically rises from 97% to 99%. To achieve these removal rates, the L/G ratio 

was increased for each percentage point increase in SO2 removal in this scenario. The 

impacts of increasing the L/G ratio include the following: need to purchase larger absorber 

circulation piping, more spray nozzles, larger and/or additional absorber circulation pumps, 

and an increase in the flue gas-side pressure drop resulting in increased ID fan power 

consumption, and repositioning of the spray header to accommodate the new spry pattern at 

the increased flow rates. 
19

Figure 8: Incremental Cost of Increasing SO2 Control 

Incremental Cost of SO2 Control for LSFO, Pitts #8 Coal - SI Units

$0

$500

$1,000

$1,500

$2,000

$2,500

$3,000

0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99 1

SO2 Removal, %

Levelized Control Cost of LSFO System, 

$/Incremental tonne SO2 Removed (Dec-2007 

$)

BASIS/DEFINITIONS:

1. Incremental Cost in $/tonne SO2 is Difference in Annual Cost divided by Annual 

Difference in SO2 Removed.

2. Levelized Costs Include TCR and All O&M Costs.

3. NOTE: SO2 Removal is Based on Design/ Performance Conditions.

4. 90% SO2 Removal is Base Case.

Low End of 

Range

Incremental Cost Impact of SO2 and CO2 Removal for Increasing FGD 

Removal Efficiency 

Although the bulk of this paper covers high efficiency SO2 removal, the looming possibility 

of regulations requiring fossil fuel-fired power plants to reduce CO2 emissions is relevant to 

the discussion. Some CO2 removal processes will directly impact the design and 

performance required from any FGD system operating upstream due to the low inlet SO2

concentration sometimes required by the CO2 reduction system. In many cases, sulfur 

dioxide needs to be in the low ppm range so that the CO2 removal processes can be a costeffective candidate for installation at a PC-fired power plant. An amine based system, which 

is widely used in the gas processing industry to remove CO2 from gas streams, is one 

available technology option for PC fired utilities to capture CO2 from their flue gas streams. 

Amine based systems require very low inlet SO2 concentrations, mainly to minimize the 

buildup of heat stable salts in the amine scrubbing loop. A thermal reclaimer is commonly 

included with an amine scrubbing system to remove these heat stable salts. This regeneration 

step requires energy, produces a waste product, and results in an increase in the amine 

degradation/makeup rate. 

An analysis was performed to examine the levelized cost for a PC power plant firing an 

Appalachian coal (2.6 wt% sulfur) with a LSFO FGD step and CO2 removal using an MEA 

(monoethanolamine) system. The plant size was assumed to be 500 MW, gross, excluding 

the power requirement for CO2 and SO2 removal systems. The levelized costs are presented 

in dollars per tonne of CO2 removed. 
20

The analysis examined the incremental cost for removing additional SO2 in increments from 

90% to 99% for the LSFO process. This included the incremental capital and operating costs 

associated with the FGD system at various control efficiencies, as well as the incremental 

operating costs for the amine scrubbing system as they related to variations in the inlet SO2

concentrations. These costs are primarily related to the operation of the thermal reclaimer 

including variations in steam usage, amine losses, and waste generation rates. All of these 

operating costs typically increase as the inlet SO2 concentration increases. 

The correlations for the increase in MEA consumption and increased reclaimer duty were 

taken from Simisky (1986). The cost for makeup of the MEA was assumed to be $2.11/lb. 

The steam cost was assumed to be $3.86/1000 lb and the amine waste disposal cost was 

based on $0.19/gallon. The results shown in Figure 9 demonstrate the impact that SO2 inlet 

concentration has on the incremental levelized cost for the sum of the FGD and CO2 removal 

systems. It should be noted that the analysis in Figure 9 does not include capital costs for the 

CO2 capture process nor does it include capital and operating costs for CO2 sequestration. 

Figure 9: Incremental Cost of SO2 Control for LSFO Plus Incremental Amine System 

Cost for CO2 Removal Based on Amine Inlet SO2, Appalachian Coal (2.6 wt% S) 

$0

$10

$20

$30

$40

$50

$60

$70

0 20 40 60 80 100 120 140 160

Inlet SO2 Concentration to Amine System, ppmv

Incremental Levelized Cost, $/tonne CO2 Removed 

(Jan-2008 $)

92%

95%

97%

98%

99%

% = SO2 removal percentage

Notes:

1. Incremental levelized cost is the difference starting at 90% 

SO2 removal then moving in increments up to 99% along with 

90% removal of CO2 from the amine system.

2. The incremental levelized cost for the amine system is that 

associated with the reclaimer (steam, amine loss, & waste 

generation) for higher inlet SO2 concentrations.

3. The incremental levelized cost of the additional sulfur 

removal inclues incremental TCR and O&M Costs for LSFO.

3. Cost in $/tonne CO2 is Levelized Annual Cost divided by 

Annual tonnes CO2 Removed.
21

It is clear that the lowest inlet SO2 concentration to the amine unit will result in the lowest 

additional cost for CO2 removal. The cost for amine losses is the largest contributor to the 

increase in levelized cost, followed by the additional energy cost associated with the steam 

requirement for thermal regeneration. Although the inlet SO2 concentration for the 

bituminous case is above single digit ppm levels, the evaluation emphasizes the need for 

FGD technologies to be able to maintain 99+% SO2 removal for the sake of operating a 

downstream CO2 removal system. 

SUMMARY 

In the foreseeable future it is possible that electric utilities may be required to reduce their 

SO2 emissions by 99+% due to the current regulatory environment and public pressure on 

regulatory agencies to control CO2 emissions in the future. Many of the major FGD 

suppliers in the U.S. have demonstrated SO2 removal efficiencies in the 98% - 99% range 

with only limited modifications to their absorber design and operating conditions. Several 

of the design modifications that the FGD vendors are employing to achieve these results 

include: installation of wall baffles or rings; addition of trays and spray levels; use of CFD 

modeling to improve the inlet gas distribution, spray nozzle configuration and slurry spray 

patterns to improve gas to liquid contact and limit the amount of gas sneakage. In a number 

of cases it may be necessary to use spare recirculation pumps and spray headers to increase 

liquid to gas (L/G) slurry recycle ratios by 20-30% to achieve increased SO2 removal. Other 

FGD absorber designs rely on adjusting some other operating characteristics such as 

increases in pH, increasing the froth layer contact depth, use of chemical additives, improved 

monitoring of process operating conditions, etc. To consistently maintain 99+% SO2

removal on a multi-day rolling average basis, greater emphasis will need to be placed on 

monitoring and maintaining FGD performance. This may require the utility industry to 

consider adopting a different operating philosophy; one that includes more hands-on 

involvement of engineering staff day-to-day and greater attention to instrument and 

equipment maintenance. 

Increased SO2 removal does not come without a cost. The incremental cost per ton of SO2

removed above 98% is exponential for a traditional LSFO system. If regulation of CO2

emissions is enacted, then not removing enough SO2 will only add to a utility’s operating 

costs. An amine based CO2 removal process, one of the few commercially proven and 

available CO2 removal processes at present, is more cost effective at lower inlet SO2

concentrations due to the negative impact that SO2 has on the operational chemistry of the 

process. 

Some of the latest process developments in the area of multi-pollutant control technologies 

may result in cost-effective alternatives that achieve high SO2 removal in conjunction with 

removal of NOx and mercury. Multi-pollutant technologies such as ECOTM, ReACT, 

Airborne, and CANSOLVTM show promise of achieving 99+% SO2 removal and continue to 

gain commercial experience, improve performance, and achieve higher reliability. As 
22

federal, state and local agencies demand additional reduction in SO2, NOx and mercury 

emissions, these integrated emission control systems may see more consideration due to 

operating advantages and cost competitive performance when compared to installation of 

separate control systems for each of the respective pollutants. 

REFERENCES 

1. Incremental Cost of Selected Emission Control Systems for PC Plants, EPRI EPP6623/C3368, September 2006 

2. State of the Art Emissions Control Systems, EPRI Report, 2006 

3. Alain, B., et. al. “New Scrubber Design for Wet FGD Applications”, Power-Gen 

Europe 2004. 

4. Simisky, P.L. “The Recovery of CO2 From Flue Gases”. American Institute of 

Chemical Engineers National Spring Meeting. New Orleans, Louisiana. April, 1986. 

5. Judd, Bill. Dow. Personnel correspondence. Calgary, Canada. November 2007. 

6. Testimony by Professor Gregory J. McRae of Massachusetts Institute of Technology 

on Clean Coal Technology – Science, Technology, and Innovation, United States 

Senate Committee on Commerce, Science, and Transportation, April 27, 2007 

7. Tracking New Coal-Fired Power Plants, Coal’s Resurgence in Electric Power 

Generation, DOE, National Energy Technology Laboratory, May 1, 2007