## High Efficiency Double Contact Flow Scrubberfor the U.S. FGD Market

Dr. Jonas S. Klingspor URS Corporation 9400 Amberglen Blvd

Austin, TX 78790 Kiyoshi Okazoe,

Tetsu Ushiku Mitsubishi Heavy Industries Ltd. Yokohama, Japan

George Munson 

## TVA 101 Market Street Chattanooga, TN 37402



## Abstract Recent awards in the U.S. have highlighted the advantages ofthe MHI "double contact flow scrubber" or DCFS FGD system which has been applied to large-scale units during the latteryears ofthe 1990's. This paper provides a detailed description ofthe DCFS FGD system including operating data from recent installations.
The DCFS design features are discussed in detail including the single headerstage fountain spray design, including the single and twin towerdesign. The operating experience of recent installations is reviewed including case studies for selected systems.
Recent operating experience 

### is reviewed in detail. In particular, the paper highlights design requirements to achieve 

### 802 removal efficiencies as high as 99.9 percent on high sulfur coals withoutthe use of buffer additives, particular removal efficiencies as high as 90 percent, while produdng wallboard grade gypsum and maintaining 100 percent system availability in operation forthe duration ofover2 to 4 years.
The DCFS design allows utilities to use a high degree ofstandardized equipment on a single installation. Modularization ofthe DCFS design extends the standardization features to multiple plants regardless ofdifferences in absorber size. The standardization and modularization features ofthe DFCS design are discussed in detail.
The Paradise DCFS system currently in the engineering phase is discussed in detail.



### 1

## Background MHI is the world's leadirg supplierof FGD systems,w ith over 60,000 MNe installed on 154 boilers in 14 countries. The first MHI FGD system was installed in 1964. MHI'swet FGD system has evolved to avery simple, reliable and highly efficient single loop, Double Contact Flow Scrubber. During recentyears, prominent technology improvements have included the single stage DCFS spray headerdesign, air rotary spargerfor combined slurry mixing and gypsum oxidation, and jet air spargerfor gypsum oxidation without use ofoxidation compressors. This paper presents an overview ofthe MHI Double Contact Flow Scrubber, recent operating experience, and provides an overview ofthe scrubber design forthe TVA Paradise FGD system currently in the engineering phase.



## DCFS Features The double contact flow scrubber comes in two design configurations; single tower and twin tower as shown in Figure 1. The single tower design is typically used on low-tomedium sulfur coals and S02 removal efficiencies upto 97 percent. The twin tower design is typically used on medium-ta-high sulfurcoals and S02 removal efficiencies above 97 percento rto achieve very high particulate removal efficiencies. This paperwill mainlyfocus on the twin tower design.

Figure 1. Single and Twin Tower DCFS system



### Twin TowerDCFS As the flue gas enters the top ofthe first tower, the wet dry interface is located nearly 8 feet into the top partofthe vessel. This interface iswashed routinely with fresh make-up waterto minimize build-up offly ash entrained in the flue gas. The flue gas encounters the top ofthe fountain spray as the gas flows counter-a.ment to the spray in the first of the twin towers. The recycle slurry is spouted upwards in fountain-like spray by multiple single-stage nozzles installed on a single spray header located at the lowersection of the first and second towers. This fountain or liquid column in the DCFS contacts the flue gas as itp roceeds counter-current to the liquid spray and again co-current as liquid and gas flow downward together. This "Double Cootact" providesfor intimate contactf or



### 2absorption of802, excellent utilization ofthe limestone reagent, and a very high level of removal ofincoming fly ash.
As the gas leaves the first tower, ittraversesthe top ofslurry in the reaction tank before entering the second tower. In the second tower, the flue gas passes co-current to the flow ofthe fountain spray and counter-currentto falling droplets ofslurry. This additional second tower and "double contact" design provides the additional gas-liquid contact such thatthe resulting 802 removal efficiency can be as high as 99.9%. The gas velocity in the first tower is typically between 15to 

### 30 fps while the velocity in the second tower is typically between 14to 

### 20 fps. This makes the absorbertowervery compact and cost effective.
The absorbertower is equipped with a singlelevel spray header in each tower. Lowpressure silicon carbide nozzles are used to provide afountain-like spray reaching about 15 

### ft to 

### 30ft in height. The recycle slurry exits the spray nozzle much like a liquid rod that gradually disintegrates into very large spray droplets as the slurry decelerates and is pulled back into the recycle tank by gravity. The fountain-type spray headerdesign provides avery high degree ofgas-ta-Iiquid contact and a high degree ofsurface renewal that improvesthe recycle slurry's neutralization capacity. The flue gas also contacts the slurrytwice as the liquid exits the nozzles and ultimately returns to the reaction tank. A picture ofthe fountain spray is shown in Figure 2.
The spray headers are connected to a single recycle header pipethat in turn is connected to the recycle pumps. The spray nozzle has no internals,p rovide maximum free passage, and avery low pressure drop as it is essentially an open pipe. The nozzle is made from a ceramic material and, because ofthe low pressure drop, experiences essentially no wear overthe life ofthe FGD system. The MHI spray nozzle is shown in Figure 3. The spray from the spray nozzle is introduced parallel to the absorberwalls and hence, wall erosion or headerto hecdererosion is not a problem with this design. Also, as the spray is directed awayfrom the inlet duct, no buildup in the inlet duct is experienced. Figure 2. MHI full-scale spray header test rig

TI Figure 3. MHI low pressure drop, maximum free passage spray nozzle

A pump suction deflection/screening plate is located in close proximityto the pump suction and spans the entire side ofthe reaction tank. The screening plate is installed to



### 3prevent airthat is sparged into the reaction tankfrom being entrained into the recycle pumps and recycle piping. Each pump is connected to a common recycle header and is isolated on the suction and discharge sides ofthe pump with knife-gate or butterflyvalves. Each pump,g ear box, and motor is identical providing a high degree ofstandardization. The first series of pumps are connected to a common manifold which feeds the single spray header in the 2nd tower. The last series of pumps are connected to a common manifold thatfeeds the 1st tower. A spare pump is available to conned to the 1st or2nd recycle header. A typical arrangement ofthe recycle pump bay is shown in Figure 4.

The reaction tank operates at 30 percent Figure 4. The DCFS recycle pump bay solids which promotes gypsum crystal growth and significantly reduces gypsum scaling. Also, the high level ofsuspended solids in the reaction tank provides elevated levels of limestonewhich promotes S02 removal and makes the system more tolerantto swings in inlet**S~** concentration.
The twin tower design has the added advantage ofusing top-mounted agitators,w hich also double as air spargers. This proprietary design, called theAir Rotary Spargeror "ARS,n is highly efficient in terms of mixing and oxidation. The ARS oxidation and agitation system is shown in Figure 5.
In addition to superioragitation, top-mounted agitators are inherently leakfree as the penetration through the absorber shell is above the slurry level in the reaction tank. TheARS is also used to distributethe oxidation air. COlTlJressed air is distributed to the horizontal arms of the agitator and sparged intothe recycle slurry. This approach is highly efficient and requires a lower quantity ofaircompared to side entry agitators or afixed grid sparger system. Hence, the power spent on agitation is more than offset bythe reduction in power consumed bythe oxidation air blcmers. Figure 5. Air rotary spargerfor agitation Afterthe flue gas exits the sprayzone ofthe and forced oxidation second tower, it passes through atwo-stage vertical flow mist eliminator. A 9 ft slurry and gas disengagementzone is provided between the top ofthe sprayzone and the face ofthe first-stage mist eliminator. The provided disengagementzone, coupledwith the very large droplets produced bythe fountain-type spray headers, results in avery low liquid loading to the mist eliminator, even at elevated gas velocities.



### 4As the DCFS FGD system operates at 30 percent suspended solids, the system does not require a primary dewatering system. The absorber bleed pumps can feed directlyto a belt-filter or drum filterfor dewatering to desired moisture level. This efficient design eliminatesthe primary dewatering step reducing the ca;t and complexity ofthe overall system.


### Single TowerDCFS The single tower DCFS system is very similarto twin towerwith a few exceptions. First, this towerdesign is typically applied on lowto medium sulfur coals and for S02 removal efficiencies at or below 97 percent. In the single tower design, the flue gas enters the absorber module from the side through atraditional absorber inletdesign and exits the absorberatthe top ofthe module as shown in Figure 1. A single fountain type spray header is located immediately above the inlet duct. No buildup in the inlet duct is experienced asthe fountain spray is directed awayfrom the inlet duct and parallel to the absorberwalls. The Recycle slurry pumps configuration is identical to the twin tower design. Agitation is achieved with either side entry agitators or by using thejet air sparger system. The single tower design has a fixed front to back distance ofabout 30 

### ft which makes side entry agitators very effective. An alternative to side entry agitators is the Jet Air Sparger (JAS) system which use a small portion ofthe recycle slurry flowto inject oxidation airthrough a eductor configuration. The eductors are located atthe same place side entry agitators would be located. Hence, the JAS system can provide efficient oxidation and agitation withoutthe use ofagitators and the need ofoxidation compressors. JAS systems operate successfully at several installations.



## Standardization and Modularization

The MHI DCFS FGD system is specifically designed to simplify design, engineering, and construction, and to maximize the portion ofthe system that can be prefabricated and brought to the site in large pieces:

• The absorberwalls are comprised ofstandardized panels thata re joined together.

• The front-ta-back distance is constantfor absorber sizes largerthan 

### 200 MWe up to an absorbersize as large as 1,200 

### rvrNe.

• All recycle pumps systems are identical, including flow rate, discharge pressure, gear boxes, and electrical motor.

• All recycle pump isolation valves are identical.

• All top mounted (ARS) orside-mounted agitators are identical.

• All spray headers and spray nozzles are identical.
The design ofthe MHI DCFS system is highly standardized, and the size ofthe absorber tower only expands in one drection to accommodate differentv olumes offlue gas, as shown in Figure 6. Therefore, the DCFS design, in addition to standardization, lends



### 5itselfto adopting a modulardesign approach. This approach is idealfor system-wide application when multiple size boilers are involved. By using a modular approach and multiple absorber sizes in steps of200 MWe, the cost ofthe FGD retrofit project can be reduced substantially. All FGD module sizes considered would be identical exceptforthe width ofthe absorber module. The depth and the heightwill be the same; the pump size, headersize, nozzle size, agitator size, and air compressorsize will all be the same - making 

### it possible for plantsto share spare parts or have a common spare parts facility. Finally, in the MHI FGD design, all recycle pumps, gearboxes, and motors are identical since the DCFS design only uses one spray headerat one elevation, exposing all recycle pumps to the same pump head pressure. 

# i

i

\1 

## --J

-

# ~,: ,

- _0" Figure 6. DCFS modularization Approach
For atypical system wide applications ofthe DCFS system, a base absorberofe.g. 300 MWe in size is selected and add-on modules of200 MWe in size used to provide module sizes of 500,700,900, and 1100 MWe. As shown in Table 1, the DCFS system can provide avery high degree ofstandardization on multiple absorber sizes including these different sized moduleswill use the same size, model, and type ofequipment, including recycle pumps, gear boxes, motors,a gitators, blowers, spray headers, spray nozzles and mist eliminator, etc.

Table 1 Standardization and Modularization capability ofthe DCFS system

I - 

# . ,- -- ,>- ,'. I**~,** I

- .. -

- - '-

. 

## ._-

-- - - .. -. - -.

- 

## -- -

-**-~** - . -- - ."]:

.....4'_·_

- . -**~-~-** .-. ..:....- -:::..._

.....

.. < --- . -**:......--=.~-~-** Recycle Pumps Same Same Different Same Recycle Pumps GearBoxes Different Same Different Same Recycle Pump Motors Different Same Different Same Recycle Pump Isolation Valves Same Same Different Same Agitators Same Same Different Same Agitator Motors Same Same Different Same Recycle Headers Same Same Different Same Recycle Nozzles Same Same Different Same Mist Eliminator Same Same Same Same Mist EliminatorWash Nozzles Same same Different Same



### 6Installed FGD capacity -60,000 MWe
FGD Orders Last10'ttlars -30,000 MWe Highest502 QJarCl1teed Removal 99.sor. Highest502 Removal w/OAdditIVes 99.9Or. Highest502 ConC9l'ltJatJoo 7,800ppm largest SingleAbsorber 1,OSOMWe LongostTIme berw-een OUtages 4yrs Spare Modules InstaUed 

### None HighestAvailability, SIngle Module**lcm,/~** 

### The modularization approach makes it possible to benefitfrom standardized equipment across afleeto f FGD system basically independentof absorbersize. Hence, a shared or a common spare parts inventory is feasible. It is also feasible

### to minimize the number of absorber modules required can be reduced bytreating two boilers with one absorber module. For example, a 178 MW and a 348 MW module can 

### be treated in one 600 MW absorber module as is currentlydone atthe Bailly Station in Indiana.


## Development History of DCFS After completion offundamental tests using 15,000 m3

### N/h pilotfacility at MHI Hiroshima R&D Center, the DCFS was subjected tojoint Verification tests with Chubu Electric Power Co. and Chugoku Electric Power Co. using actual coal flue gas in 15,000 m3

### N/h and 300,000 m3

### NIh facilities respectively. Subsequently, counter-current DCFS was selected forthe 136 MW heavy oil-fired NO.2 unitof Kashima-minami Joint power station in 1993followed by a co-current DCFS system for coal-fired 175 MW NO.1 unit ofChugoku Electric's Shimonoseki power station in 1994. A large scale co-current DCFS system was installed in 1998 atthe 1

### ,OOOMW Misumi Powerstation ofChugoku Electric Power Co., followed by a 2000 1,050

### MWtwin tower DCFS system atthe EPDCTachibanawan station and a 700MWtwin tower DCFS system at Shikoku Electric Power Co. Tachibanawan A high performancetwin tower DCFS system was installed in 1998 atthe 149 MW KOA oil refinery in Osaka. This system provides a**S~** 

### removal efficiency of99.9 percent on high sulfurfuel oil. More than 20,000 

### MW of DCFS systems have been sold and/or installed sinCE the introduction ofthe DCFS system in 1993.


## Experience MHI has experiencewith a Table 2. Overview of MHI FGD Experience wide range ofoperating conditions as shown in Table 2. It is noteworthythat MHI has never installed a sp:lre operating module at any of its 155 installations. However, most of the MHI systems operate a very high availability levels. The experience on hgh sulfur coals is very extensive with the highest S02 concentration experienced at7,800 ppm. The experience v.Ath large single modules and high S02 removal efficiencies is also extensive.



### 7

### Large Single Modules MHI has experiencewith the single absorber modules starting at a moderate size of 150 MWe to the largest absorbers in theworld at 1,050 MWe. The vast majority ofour FGD systems are based on a single absorber module, regardless ofsize. All systems operate without installed spare module capacity.
The experiencewith different size absorbers is less ofan issue with the MHI design. The depth and height ofthe MHI absorber are basically independentof absorber size. To accommodate a larger flue gas volume, the absorbertower is made wider, keeping all critical dimensions impacting gas/liquid contact unchanged from projectto project The experience IMth large single modules is shown in Table 3.


### High S02 Removal Efficiency The fountain spray design generates very effective gaslliquid contact, and the DCFS FGD design is capable ofvery high S02 removal efficiencies. The DCFS system has proven performance on all ranges ofsulfur and very high S02 removal performance upto 99.9%. Table 4 provides a partial listing of FGD plants operating at an S02 removal efficiency of 98% or higher.
The highestS O2removal guaranteed by MHI was 99.8% on a sulfur loading of 2,200 ppm S02. During guarantee testing, this unit recorded an S02 removal efficiency of 99.9% or 2 ppm S02 in the outlet duct. MHI has 50 FGD plants operating at an S02 removal efficiency of 95% or higher, 25 plants above 96%, 12 plants above 97%, 10 plants above 98%, and 4 plants above 99%. Table 3. Experience IMth Large Absorber Modules

Elldric_, De.elopmem Co lid 1.050'1 B82 44 950 (T_,Japan) Tohakw EIICIriC PowerCo. Inc:. 1,000·1 910 70 92.3 (HanunocIl.l.J8pan) SohmaJointThermal 1000-1 1.015 102 90.0 Power. LId (Sohma. Japan) Chugoku EJec:trjc Powe, Co LOOO·1 921 91 90.2 lnc.(\,IlIun'Uapan)

___'Co, 700>1 1015 91 91,0 Inc. (IoIalsuln. Japan) Ctwllu EIodt1cpowtlrCo" Inc 700>1 803 51 93,6 (Heldnan, Japan) Chubu EIec:anc PowerCo_, Inc. 100>11 803 51 93.6 (HeUlan Japan) Chullu EJec:trjcPowe' CO, Inc. 700-1 803 51 93,6 (H-n, Japan) EJedtidlyGenet8llng AutllOf1ly0' 700·1 1.733 79 95,4 ThaiaOO (RalcMIlurt. Thailand) Eledr1c:IIy Gene",1Ing AUlhort!y 01 700-1 1,733 79 954 Th81800 (RakNIlurt, Thailand) 

# --- 700-1 840 46 945 Co .IJll.(T--, Japan) ENE\. (8- Sud. 14. ftlly) eeO·l 1.610 81 950
ENE\. (8_1 Sud, 113. lilly) 660-1 1,810 81 95,0

Table 4. Experience IMth High S02 Removal Efficiencies

, . 

### . '



## - I

l'"

- ...- - 

## -- -

- -**~---**

1 2003 COSMOOtLCo.LlJl. lY_ 223><' 3.433 17 99,5 

# ...)

1 2004 KOAOlLCo.. Ud- (Uanll

...) 1<&811 4.087 25 994

I 1997 ENEL (SUds, 113, Italy) 240'1 5.740 8' 98,6

1994 ENEL (SUcis.-:2, Italy) 240'1 5,740 81 986

19i3 ENS. (sua.. 1f1. Italy) 

## I 240" 5,740 81 986

2004

# -_P

....-

lWxl 2,917 50 983 RIIIr*1oeo... Ud. (MlWoran_**JIO~)** 2003 Kalhini Northern Elec::tl1e POMlr 300<1/2 5.686 117 98.0 Co. (l(aal'/ma-KllafJ, Jopon) 2002 K-..I BedricPowerCo

.Inc 600rl 554 18 970**(Gobo03.~)** 1975 Tolin. Ud. (e*ne

....1 1.740 53 970

1977 CIugoku _-Co. 400'1 1,845 53 96.8 Inc (5I1Im<ln_kl, Japan)
1964 NKKCorp. 3,070 102 967 -)



### 8'00 '00 100

'00

100 100



# ... 95.3 95.. 95.0 "$

"'2 eolHncI**Co..~** eo.I.f'Ir.cli &w 95.0**~SGulh"",~r 1~7t** **.:.o·~a~" ~-""I\J**  1***Ir~:==:-'Co,** ,1llX).1 

### Table 5. Availability of single module DCFS Systems 

### High SystemAvailability Historically, the fundamental shortcoming of FGD systems has been their availability and the resulting impact
on powergeneration. This shortcoming was particularly evident in the late '70s and early '80s. The Clean AirAct Phase I FGD systems had addressed mosto f the earlier problems and improved the availability records to be in the high 90s. However, most Phase 1 FGD systems included a spare absorber module.
The drive in the domesticJapanese market has been to higherand higher availability, up to and including 100% online time. The DCFS technologywas successfully developed to meetthe market need for ultra high availability using asingle absorber module. Table 5 showsthe DCFS availability record ofselected DCFS installations. The availability data shown on Table 5 arethe cumulative availability overthe operational period (life of system) thus far. The simplicity ofthe DCFS design, combined with extensive experience and lessons learned, makes it possible to design the DCFS system to achieve 100% availabilitywith a single absorber.



## Case Studies

EPDC
- 1,050MWSingle Module

### DCFS System The FGD system at EPDC's Tachibanawan station is the largest capacity absorber in theworld, treating the entire flue gas volume ofthe 1,050 MW boiler in a single module. The absorber is the twin-tower Double Contact Flow Scrubber (DCFS) type including a co-currentf lowtower and a counter-currentflowtower. The absorber was designed to complywith extremely stringent particulate and S02 regulations. Particulate emissions are regularly below 

### 5mg/Nm3

### and SO:1 removal efficiency above 95 percent. The FGD process control system is designed to automatically control the numberof Figure 7. EPDC Tachibanawan 1,050 pumps in service depending on boiler MW Single module DCFS system load and coal sulfur levels. Since the inlet



### 9Figure 8. KOA high efficiency DCFS system S02 concentration is lowerthan the designed value, the absorber is automatically being operated with a reduced numberofslurry recirculation pumps, resulting in reduced power consumption. Th system produce wallboard grade gypsum which is sold locally. The FGD plant has been in commercial operation fortwo years and has achieved cumulative availability of 

### 100 percent. 

### KOA

- 

### 99.9% 

### S02 Removal The FGD plant atthe KOA refinery is a high sulfurtwin tower DCFS application that has achieved the industry's highest desulfurization performance of

### 99.9% exceeding the guaranteed S02 removal efficiency of

### 99.8 percentw ithout using performance additives. The FGD system hasthe equivalent size of 

### 149 MNe and treats flue gasfrom avacuum residue boiler. The S02 concentration ahead of the FGD system is typically about

### 2,000 ppm while the**S~** concentration in the stack is below

### 2 ppm. In addition to emitting very lowS02 emissions, particulate emissions are typically below 

### 1.3 mg/Nm3

. In the past, such a high desulfurization capability has been considered impossiblewithoutthe use of additives. It is achievablewith the twin tower DCFS design, due to the excellent gas/liquid contact and the absence of gas sneakage along the absorberwalls. The unitwent into operation in 

### 1998 and has achieved a cumulative availability of 

### 100 percent since startup.



## North American Activities MHI in partnership with URS through a JointVenture company named Advatech is marketing MHI's DCFS FGD system in North America. Advatech is currently providing design and implementation services forthe Tennessee Valley Authority for up to five FGD systems. The first FGD system will be installed atthe Paradise station on the Unit 

### 3 boiler. This system will be put in operation in 

### 2005 to 

### 2006time frame. Also, a conversion of theWidows Creek Unit 8 FGD system to atwin tower DCFS design is currently underway. The Widows Creek DCFS system will be operational in December of

### 2003. A more detailed description ofthe Paradise project is provided below. 

### Paradise Unit3 

### FGD System Advatech has been awarded the FGD system forthe Paradise Unit 3 station which is a 

### 1,050 MNe pulverized coal-fired boiloerfiring high sulfur eastern bituminous coal. Advatech will provide EPCservices forthe entire scope induding limestone preparation, absorber island, fans, ductwork, stack and balance of plant.



### 10The FGD system will based on the MHI DCFS twin tower design and will utilize a single absorbertowerwhich will provide a S02 removal efficiency of 98 percent. A pictorial of the FGD system layout is shown in Figure 

### 9. The FGD inlet is symmetrically divided to take flue gas from each ofthe ESP trains. New boosterfans are provided and are close coupled with each ofthe absorber inlet ducts

Figure 

### 9. Layout ofthe new FGD system at Paradise Unit 3 As the gas leavesthe first tower, it traverses the top of slurry in the reaction tank before entering the second tower. In the second tower, the flue gas passes cocurrentto the flow ofthe fountain spray and countercurrentto falling drops of slurry. The flue gas encountersthe top ofthe fountain spray as the gas flows counter-currentto the spray in the first ofthe twin towers. The recycle slurry is spouted upwards in fountain-like spray by multiple single-stage nozzles installed on a single spray header located atthe lower section ofthe first and second towers. The nature ofthis design makes it impossible to get recycle slurry intothe absorber inletductworkor boosterfans, which has been a common problem with spray and tray towers in recentyears. This fountain or liquid column in the DCFS contacts the flue gas as itp roceeds countercurrent to the liquid spray and again co-current as liquid and gas flow downward together. This "Double Contact" provides for intimate contactfor absorption of S02, excellent utilization ofthe limestone reagent, and a very high level of removal of incoming fly ash.

The recycle pump bay is located in fronto f the second tower. The FGD system will require 11 slurry recycle pumps rated at48,000 gpm. Four recycle pumps discharge into a common external recycle headerwhich provide slurry forthe first absorbertower which operates at about21 fps gas velocity. Six recycle pumps discharge into a second common recycle headerwhich provides slurry to a the second absorbertower. The second toweroperates at about 15 fps gas velocity. The 11 th pump is located between the two recycle headersand provide spare capacityto either external header.
Each ofthe twin towers is equipped with a single level spray header. Low-pressure silicon carbide nozzles are used to provide afountain-like spray reaching about 16 to 20 

### ft in height. The recycle slurry exits the spray nozzle much like a liquid rod that gradually disintegrates into very large spray droplets as the slurry decelerates and is pulled back into the recycle tank by gravityThe fountain-type spray headerdesign provides avery high degree ofgas-to-Iiquid contacta nd a high degree ofsurface renewal that improves

11the recycle slurry's neutralization capacity. The flue gas also contacts the slurrytwice as the liquid exits the nozzles and ultimately returns to the reaction tank. The low-pressure drop nozzle has no internals, a maximum free passage, and avery low-pressure drop. The nozzle is made from silicon carbide and, because ofthe lowpressure drop, experiences essentially no wear.
The single module, twin tower design hasthe added advantage of using top-mounted agitators,w hich also double as air spargers. This proprietary design, called the Air Rated Spargeror "ARS,n is highly efficient in terms of mixing and oxidation. The newsystem includesthree top-mounted "ARS,n agitators. Adequate agitation is critical to scale-free operation. The ARS is also used to distribute the oxidation air. Compressed air is distributed to the horizontal arms ofthe agitatorand sparged into the recycle slurry. This approach 

### is highly efficient and requires a lowerquantity ofair compared to side entry agitators or a fixed grid sparger system. Hence, the power spent on agitation is offset by the reduction 

### in power consumed bythe oxidation air blovvers. Two operating compressors and one spare air compressor provide oxidation air.
A pump suction deflection/screening plate is located in close proximityto the pump suction and spans the entire side ofthe reaction tank. The screening plate is installed to prevent airthat is sparged into the reaction tankfrom being entrained into the recycle pumps and recycle piping.
Afterthe flue gas exits the spray zone ofthe second tower, it passes through atwostage vertical flow mist eliminator. The provided design is based on using a high velocity, high efficiency chevron type of mist eliminator. An 8 ft disengagementzone is provided between the top ofthe spray zone and the face ofthe first-stage mist eliminator. The provided disengagementz one, coupled with the large droplets produced bythe fountaintype spray headers, results in avery low liquid loading to the mist eliminator, even at elevated gas velocities.

A fixed grid wash system is provided to wash the leading and trailing edges ofthe first mist eliminator and the leading edge ofthe second mist eliminator.
The proposed system is designed to operate in an open loop configuration and does not include any dewatering system otherthan reaction tank bleed pumps thatw ill pump reaction tank slurryto a pond. The water in the pond is not reclaimed or reused in the process.
A wet stackwill be provided to discharge the clean flue gases to the atmosphere.

A new limestone grinding system will be installed which will provide a limestone grind of 90% lessthan 325 mesh.



### 12

## Summary MHI introduced the DCFS system in the early '90s and, 

### to date, more than 20,000 MW has been installed and/or sold. The DCFS system was specifically designed to provide absolute reliability as demonstrated bythe numerous installations operating at 100 percent availability with a single a absorber module. The absorbertower is equipped with a single spray headerwhich by design does notg enerate any wall erosion from spray impacting on the walls or any headerto headererosion. The fountain spray provides avery efficient gas/liquid contactwhich is also demonstrated bythe very high particulate removal provided bythe system. The experiencewith high sulfur coals, very high removal efficiencies, high availability with a single absorber module is considerable. The firstg reenfield DCFS system in the U.S. is being installed atthe TVA Paradise station which will go into operation in 2006. A second system is being installed atTVA's Widows Creekstation through a rebuild ofthe existing FGD system to the DCFS design. The Widows Creek DCFS system will be operation in late 2003.

13