# Notes from Site Visit at Ohio Power Company’s Gavin Plant

on September 12, 2008

EPA conducted a site visit at Ohio Power Company’s Gavin Plant (Gavin) located in

Cheshire, OH on Friday, September 12, 2008 to learn about the operatio ns at the plant,

specifically the FGD solids dewatering and ha ndling operations. Table 1, located at the

end of this memorandum, identifies the site vi sit participants. The presentation that the

Gavin personnel presented during the site vi sit is provided as an attachment to the

memorandum. Also included as attachments to this memorandum are an overall process

flow diagram for the plant and a process flow diagram for the FGD solids dewatering and

handling operations.

This memorandum describes the information co llected during the si te visit and the presite visit phone call, which was held on August 8, 2008.

Plant Overview

• 

# Gavin operates two generating units at th e power plant. It is a baseload

plant with a total net generating cap acity of 2,600 MW. The table below

contains some general information for each unit.

Unit Installation

Date Capacity

(MW) Particulate

Control NOx Control SO 2 

# Control

1 1974 1,300 Cold-side

ESP SCR installed

in 1999 Wet FGD installed

in 1994

2 1975 1,300 Cold-side

ESP SCR installed

in 2000 Wet FGD installed

in 1995

• 

# Gavin stated that they primarily bur n eastern bituminous coal. The coal

has a sulfur content of 3.5 to 4 percen t. The coal has a chlorine content of

approximately 0.05 percent. About 15% of the coal burned at the plant is

Powder River Basin (PRB) coal.

• 

# Gavin has four coal pile runoff ponds, one located at each corner of the

coal pile. During the site visit, Gavi n staff measured pH of water in each

of the coal pile runoff ponds. The following table summarizes the data

collected during the site visit.

Pond Designation pH

Southwest Pond A (STA 4) 3.5

Southeast Pond B (Old DTE) 4.3

Northwest Pond C (STA 6) 3.7

Northeast Pond D (STA 9) 5.1

• 

# Gavin operates SCRs on both of the generating units. Gavin operates the

the SCRs year-round.

# Page 2 • 

# Gavin operates a trona in jection system for SO 3 

# mitigation. Trona is

injected into the flue gas upstream of the ESP and downstream of the air

heaters. Gavin stated that temperatur e control of the process is important

to avoid scale development and pl ugging downstream of the injection

points.

.

• 

# Gavin operates magnesium-enhanced -lime inhibited oxidation FGD

scrubbers on each of the generating units. The systems achieve 97%

removal of SO 2



# .

Ash Handling

• 

# Gavin operates a dry fly ash handling sy stem. The dry fly ash is mixed in a

pug mill with dewatered FGD solids hydrated lime to form a cementitious

material before being landfilled.

• 

# Gavin originally had a wet fly ash sl uicing system; however, they started

to handle the fly ash dry in the mid 1990’s when the FGD scrubbers were

installed so that they would have fl y ash available for combining with the

FGD solids in the pug mill. To facilitate the switch to dry fly ash handling,

the plant installed a pneumatic conveyance system using vacuum pumps,

filter separators, and blowers to move the fly ash to storage silos. [CBI

information has been removed from this document.]

• 

# Gavin stated that they have an old fly ash pond (approximately 300 acres)

which is no longer in operation. The pond collects rainfall and additionally

the pond receives acid mine drainage from an old coal mine. Gavin still

has a NPDES permit to discharge from the fly ash pond and the plant drips

caustic into the pond to control the pH.

• 

# Gavin operates a wet bottom ash hand ling system. The bottom ash sluice

is transferred to the bottom ash pond. The bottom ash pond also receives

metal cleaning waste, urea mixing skid sump water, coal pile runoff, fly

ash transfer building sump water, cooling tower blowdown, and low

volume waste water. The pond contains a baffle to prevent short

circuiting.

• 

# The bottom ash pond discharges to the reclaim pond. Most (70 percent) of

the reclaim pond water is reused as service water or scrubber system

makeup water. A smaller portion (30 pe rcent) is discharged to the Ohio

River.

FGD System

• 

# Gavin operates magnesium-enhanced-lim e inhibited oxidation tray tower

FGD scrubber systems on each of the generating units. A process flow

# Page 3 diagram for the FGD solids dewatering and handling operations is

included as an attachment to this memorandum.

• 

# Each unit has six scrubber modules, fi ve that are operating at a time and

one spare module. The modules are made of 317LM stainless steel.

• 

# Each scrubber module has two spray levels supplied by two recirculation

pumps. The first spray level is loca ted above the perforated tray and the

other is located below the tray.

• 

# Gavin adds emulsified sulfur to the FGD system to inhi bit the oxidation of

calcium sulfite to calcium sulfate. Th e emulsified sulfur reacts to form

thiosulfate, an oxygen scavenger th at removes oxygen from the system,

which prevents the oxidation of calcium sulfite. The emulsified sulfur is

added throughout the year.

• 

# Gavin receives Thiosorbic® Lime fro m their supplier vi a covered barges.

The lime is unloaded and conveyed to three silos each holding 19,000

tons. All of the lime used by the plan t is provided by two nearby mines.

The lime supplied contains a small percentage (3 to 5%) of magnesium

oxide, which improves the scrubbe r chemistry and removal of SO 2



# .

• 

# Gavin operates three ball mills, one for each unit and one common ball

mill that can be used with either uni t, to slake the lime into hydrated lime

(calcium hydroxide and magnesi um hydroxide). Gavin slakes

approximately 40 tons of lime per hour in the ball mills. Gavin adds 4

parts lime for one part water. Typically, two ball mills are in operation

with the third maintained as a spare.

• 

# From the slaker, the hydrated lime slu rry is sent to hydroclones to ensure

the particle size of the lime in the slurry. The large particles are returned

to the ball mill for further processing. The overflow from the hydroclones

is sent to the lime slurry storage a nd then added to the FGD scrubbers. The

final lime slurry is approximately 25 weight percent solids.

• 

# The pH in the FGD scrubbers ranges from 6.0 to 6.7 S.U. A decrease in

pH triggers the addition of lime slu rry to the absorber. The typical

chlorides concentration in the sc rubbers is 2,500 to 3,000 ppm, although it

can go as high as 6,000 ppm. The maximum allowable chlorides

concentration, as determined by metallurgical limitations, is

approximately 8,000 to 9,000 ppm. The typical solids concentration in the

scrubbers is 10 to 12 percent solids.

• 

# Key control parameters for the absorb ers are pH, liquid level and density.

As pH drops, lime slurry is added to the absorber. Clarified recycle water

is added to the absorber to maintain the level. When the density increases,

the absorber is blown-dow n, decreasing its level.

# Page 4 • 

# The FGD scrubber blowdown is pumped to the thickeners. Gavin operates

five thickeners: two thickeners, operate d in parallel, are dedicated to each

unit, and one common thickener that can service either unit. Gavin does

not commingle the FGD blowdown from the different units because they

do not want to blend the chemistries of the two systems, thereby having

fewer chemical adjustments and a qu icker return to normal operation.

Each thickener has a diameter of 200 feet. The “spare” thickener is

occasionally used for fines recovery.

• 

# The blowdown from the absorbers is based on absorber density. The

blowdown from the absorbers varies based on the coal, sulfur dioxide

removal in the absorber, the abso rber cycling, an d the unit load.

• 

# The blowdown from the absorbers is fi rst transferred to a tank and then

through a distribution box, where polym er is added to aid in settling.

Gavin stated that the settling time within the thickeners is a key parameter

which can hinder productivity if the ra tio of polymer and slurry is not

controlled properly.

• 

# The thickener overflow is transferred to the clarified recycle tanks (or

thickener overflow reclaim tanks). An important function of the reclaim

tanks is that they provide surge volume for the entire FGD system,

effectively smoothing out the effects of high and low operational periods.

The water in the thickener overflow tanks is returned to the absorbers.

• 

# Gavin stated that the th ickener underflow contains 28 to 32 percent solids

and is pumped to the thickener underf low tanks. These tanks also provide

surge volume for the centrifuges. There are two tanks per unit.

• 

# Gavin has 18 centrifuges, 9 for each unit. Gavin stated that the same

polymer added to the thickeners ma y be added to the centrifuges.

• 

# The centrifuges spin and push the solids to the outside walls of the

centrifuge. A rotating screw within the centrifuge rota tes in the opposite

direction of the cen trifuge and pushes the solids back toward the inlet side

of the centrifuge and out of the centr ifuge. The water stays towards the

inside of the screw and flows through holes located on the other end of the

centrifuge.

• 

# The liquid from the centrifuges, referred to as centrate, is returned to the

thickeners.

• 

# FGD solids from the centrifuges cont ain 42 to 47 percent solids. The

solids are mixed with lime and fly ash in a pug mill. Gavin stated that the

mixing of FGD solids, fly ash, and lime to create the cementitious material

for landfilling is analogous to the mixing of cement, sand, and lime to

create concrete.

# Page 5 • 

# Gavin showed EPA an image from a microscope comparing calcium

sulfite and calcium sulfate crystals. Th e calcium sulfate crystal is a flat

crystal which grows in the shape of a long rectangle. Th e calcium sulfite

crystal is typically larger and sphe rical. In this chemistry, minimizing

calcium sulfate crystal formation is desirable; growing the sulfate crystals

within the plate that composes the sp herical sulfite crystals tends to open

up the inter-plate spaces and make it more difficult to dewater.

• 

# Gavin stated that the FGD solids used in the generation of the

cementitious material contains approximatley 50 percent moisture.

Because a large amount of chlorides ar e removed from the FGD system in

this way, the plant does not need a sepa rate wastewater purge stream. The

FGD solids are sent to the landfil l in the cementitious material.

• 

# Gavin stated that they operate a FGD emergency overflow pond; however,

they don’t typically use the pond. If they do need to send water to the

pond, it will typically be r ecycled back to the scrubber. They also have

the option to transfer FGD water to the bottom ash pond.

Landfill

• 

# The FGD byproduct is a cementitious mate rial which is conveyed from the

pug mill over a distance of approximately 6,500 feet to the FGD product

landfill.

• 

# As the FGD waste is conveyed to the landfill area, it is temporarily

stacked using a radial stacker. The stacker pad has sufficient area to store

approximately one week’s worth of F GD waste. Gavin stated that the

FGD byproduct is thixotropic, meani ng that it stiffens when it is not

moving, but becomes more fluid when handled.

• 

# Gavin stated that a contractor tran sfers the FGD waste from the radial

stacker pile to the FGD product landfill. Gavin stated that they typically

let the product cure for three to four days before it is trucked and laid in

the landfill.

• 

# Gavin stated that there are three phases (A, B, and C) in its FGD

byproduct landfill. Each phase is isol ated from the groundwater by the

following layers: 1) 18 inch compacted clay layer (95 percent compaction

criteria); 2) 30 ml PVC liner; and 3) 12 inch sand/gravel layer. The

purpose of the 12 inch sand/gravel laye r is to provide a porous layer to

collect and drain the le achate into a system of underground pipes for

transfer to the leachate ponds.

• 

# Gavin stated that they will complete Phases A, B, and C with a temporary

cover and begin Phases D, E, and F (t o be located on top of Phases A, B,

and C, respectively).

# Page 6 • 

# Gavin stated that stormwater is colle cted in ditches and flows into the

leachate ponds.

• 

# Gavin stated that leachate from each phase flows from the underground

piping system into dedicated settli ng ponds. Each settling pond overflows

into its own clear water pond.

• 

# Gavin stated that the pH in the clearwater ponds ranges from 6.5 to 9.

Carbon dioxide may be bubbled into th e outlet of the settling ponds (inlet

of the clearwater ponds) as necessary to adjust the pH.

• 

# Gavin stated that they monitor th e pH and TSS within the clearwater

ponds. They are now beginning to monitor metals such as mercury. Gavin

stated that the effluent from the leachate ponds meets drinking water

standards.

• 

# The clearwater pond effluents from a ll three phases are discharged into

Kryger Creek or Turkey Run, which flow into the Ohio River.Table 1. Site Visit AttendeesNameOrganization (Representing)TelephoneE-mailDon AndersonOhio Power Company – Gavindmanderson@aep.comDoug WorkmanOhio Power Company – Gavindeworkman@aep.comTom HartAmerican Electric Power(614) 716-3260 tomlhart@aep.comTim LohnerAmerican Electric Power(614) 716-1255 twlohner@aep.comColin Mckean American Electric Power ccmckean@aep.comChris SvobodaAmerican Electric Powercasvoboda@aep.comDonna Hill Southern Company. (205) 257-5234 dbhill@southernco.comJosh Hall EPA/Office of Water (202) 566-1002 hall.josh@epa.govRon Jordan EPA/Office of Water(202) 566-1003 jordan.ronald@epa.govDeb Bartram ERG (Contractor to EPA)(703) 633-1669 deborah.bartram@erg.comKurt Blohm ERG (Contractor to EPA)(703) 633-1675 kurtis.blohm@erg.comT.J. Finseth ERG (Contractor to EPA) (703) 994-7130 thomas.finseth@erg.comLimestoneLimestone–Inhibited OxidationInhibited OxidationSO2+ CaCO3+1⁄2H2OÆCaSO3.1/2 H2O + CO2(Calcium Sulfi(Calcium Sulfite)–OxidizedOxidizedSO2+ 1/2O2+ CaCO3+2H2OÆCaSO4.2H2O + CO2(Gypsum)AmmoniaAmmonia–2SO2+ 2 NH3+ 2H2OÅÆ2NH4HSO3+ O2Æ(Ammonium Bisulfite)(Ammonium Bisulfite)NH4HSO4+ NH3+ H2OÆ(NH4)2SO4+ H2O(Ammonium Sulfate)(Ammonium Sulfate)SLAKINGCaO + H2OÆCa(OH)2(519 BTU/LB )(Calcium Oxide)(Calcium Hydroxide)MgO + H2OÆMg(OH)2(350 – 450 BTU/LB)(Magnesium Oxide)(Magnesium Hydroxide)SCRUBBINGMgSO3+ SO2+ H2OÆMg(HSO3)2(Magnesium sulfite)(Magnesium Bisulfite)Mg(OH)2+ SO2+ H2OÆMgSO3+ 2H2O(Magnesium Hydroxide)(Magnesium sulfite)Ca(OH)2+ SO2+ H2OÆCaSO3.1/2 H2O + 1 1⁄2 H2O(Calcium Hydroxide) (Calcium Sulfite)REGENERATIONMg(HSO3)2+ Ca(OH)2ÆMgSO3+ CaSO3.1⁄2 H2O + 1 1⁄2 H2O(Manesium Bisulfite)Mg(OH)2+ Mg(HSO3)2Æ2 MgSO3+ 2 H2OSO2MgOMg(HSO3)2CaOH2OCa(OH)2LIMEFLUE GASH2OSLAKINGMgSO3+ H2OCaSO3.1⁄2 H2O + 1-1/2 H2OTO: LANDFILLSCRUBBINGMgSO3+ CaSO3.1⁄2 H2O + 1-1/2 H2O2 MgSO3+ 2 H2OLIME SLURRYMg(OH)2REGENERATIONScrubber Targets for Achieving andScrubber Targets for Achieving andMaintaining 98% SO2 RemovalMaintaining 98% SO2 RemovalEfficiencyEfficiencypH6.4 to 6.86.4 to 6.8SulfitesSulfites7,000 to 9,000 mg/l7,000 to 9,000 mg/lMagnesiumMagnesium7,000 to 9,000 mg/l7,000 to 9,000 mg/lLoss of chemistry limited to filter cake.Loss of chemistry limited to filter cake.–Filter cake moistureFilter cake moisture≤60-wt%–Absolute minimal loss of reclaim water to the duckAbsolute minimal loss of reclaim water to the duckpond.Minimal use of fresh waterMinimal use of fresh water–Pump seal waterPump seal water–Slaking waterSlaking waterMgSO3SO2H2SO3MgSO3Mg(HSO3)2SO2SO2H2SO3Mg(HSO3)2SO2COMPLETECHEMICALREACTIONTHICKENERFILTERSURGETANKABSORBERGAS IN461LIME238METK.GAS OUT959ME WASH600 

# QUENCH



# 520CAKE144ALTER.SUPPLY54DUCKPONDSEAL WATER100.ME WASH546 

# MAKE

-UP



# 215FILTRATE135

Blow down 1106TUF324



# TOF



# 1022IN FLOWSGAS -----------461LIME SLURRY-238DUCK POND -–268SURGE TANK ----44SEAL WATER --1001111OUT FLOWSGAS ----------959CAKE & CHEM----1531112OUT FLOW-44Total268400 MW – 10% - 9 Duck Pond/91 Reclaim ME BlendChem. H209Note: Values in RED are flow of water, correctedfor SP. Gr. Blue values are total slurry flows98%25%53ConesvilleConesville**98%29%71MitchellMitchell97%25%28-32Gavin**Gavin**98%--180MountaineerMountaineerSO2REMOVALREMOVALTrayOpeningOpeningL/GPLANT** Mag Chemistry** Mag Chemistry18 hrs.18 hrs. 4 hrs

6 hrs400 MW200 MW 

# 4

4 g p

m * 1 08 0 m i n

s =47,50070.0 K70.0 KSTART22.5 K117.5 K 

# 1

3

4 g p

m * 3 6

0 m i n

s

. =48,20070.7 K 

# 4

4 g p

m * 1 0

8

0 m i n

s =47, 50023.2 K69.3 K116.8 K 

# 1

3

4

g

p

m * 2 4

0 m i n

s . =32,20084.6 K55.4 K 

# 4

4 g p

m * 1 0

8

0 m i n

s =47, 500132.1 K7.9 K9.5 hrs. 

# 1 3

4 g p

m * 5

7

0 m i n

s . =76,40084.3 K55.7 KSULFATE (gypsum)SULFATE (gypsum)SULFITE (GOOD)SULFITE (GOOD)Lime Slaking SystemLime Slaking System-GeneralGeneralProcess DescriptionProcess DescriptionThe purpose of this system is to:The purpose of this system is to:Convert magnesium enhanced pebble lime intoConvert magnesium enhanced pebble lime intothe lime slurry reagent required for the sulfurthe lime slurry reagent required for the sulfurdioxide SOdioxide SO2removal process.removal process.Prepare a lime slurry with 25% total suspendedPrepare a lime slurry with 25% total suspendedsolids.solids.Store lime slurry and supply fresh slurry to theStore lime slurry and supply fresh slurry to theabsorbers.absorbers.Lime Slaking SystemLime Slaking System-GeneralGeneralProcess DescriptionProcess DescriptionIn the slaking process magnesium oxide andIn the slaking process magnesium oxide andcalcium oxide (major components of the lime)calcium oxide (major components of the lime)react with water to form the hydroxide of thesereact with water to form the hydroxide of thesecompounds. This exothermic reaction producescompounds. This exothermic reaction producescalcium hydroxide (Ca(OH)calcium hydroxide (Ca(OH)2),and magnesiumand magnesiumhydroxide (Mg(OH)hydroxide (Mg(OH)2).CaO+H2OÆCa(OH)2Ca(OH)2MgO+H2OÆMg(OH)2Mg(OH)2

# Ohio Power Company



# Gavin Plant

Figure 2: Water

Balance Diagram Water & Ecological Water & Ecological

Resource Services ## Outfall number NOTE:

Flows represent average

water usage. ALL FLOWS MEASURED IN

MILLION GALLONS PER DAY (MGD)

05.25.01 GV_water_bal.tifFGDLandfillLeachatePond #3Flyash Pond (rainfall)Evaporation and DriftCooling TowerBlowdownLow VolumeWastewaterUnits 1 & 2Plant ServiceWaterSewage TreatmentPlantBottom AshPondRecycleScrubberProcess WaterScrubber SystemWater Loss Make-up006002005004AAFGD EmergencyOverflow PondBottom Ash andPyrites SluiceFly Ash TransferBuilding SumpsMetal CleaningWaste TankCoal Pile Run-offRainfall

Rainfall

EvaporationEvaporationEvaporation

S t i n g y R u n009FGDLandfillLeachatePond #2008FGDLandfillLeachatePond #1 

# T u

r k

e

y

R

u

n Kyger CreekOhio RiverRiver WaterIntakeIntake ScreenBackwash003Storm WaterReclaim Pond1.40.1190.1420.10928.520.520.421.907.2411.528.398.390.170.751.4714.903.5018.920.525.765.760.040.0143.250.360.360.018.480070.520.01

27.39

0.02

0.01 0.17

0.1111.527.24001Storm WaterUnits 1 & 2CoolingTowers 41.42UreaMixingSkid SumpDistributionBoxMake Up Water/PolymerThickeners (5)Thickener overflowsReclaim TankFilter FeedTankThickener underflowsCentrifugeCentrifuge OverflowPug MillFly Ash and LimeTruck to Landfill



# F G D P u r g e S t r e a m sFGD Unit 1FGD Unit 2Gavin Power Plant FGDProcess Flow Diagram