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 operations at the plant, 

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

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

Gavin personnel presented during the site visit 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 collected during the site visit and the presite visit phone call, which was held on August 8, 2008. 

Plant Overview 

• Gavin operates two generating units at the power plant. It is a baseload 

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

contains some general information for each unit. 

Unit 

Installation 

Date 

Capacity 

(MW) 

Particulate 

Control NOx Control SO2 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 burn eastern bituminous coal. The coal 

has a sulfur content of 3.5 to 4 percent. 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, Gavin 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 injection system for SO3 mitigation. Trona is 

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

heaters. Gavin stated that temperature control of the process is important 

to avoid scale development and plugging 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 SO2. 

Ash Handling 

• Gavin operates a dry fly ash handling system. 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 sluicing 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 fly 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 handling 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 percent) is discharged to the Ohio 

River. 

FGD System 

• Gavin operates magnesium-enhanced-lime 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, five 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 located above the perforated tray and the 

other is located below the tray. 

• Gavin adds emulsified sulfur to the FGD system to inhibit the oxidation of 

calcium sulfite to calcium sulfate. The emulsified sulfur reacts to form 

thiosulfate, an oxygen scavenger that removes oxygen from the system, 

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

added throughout the year. 

• Gavin receives Thiosorbic® Lime from their supplier via covered barges. 

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

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

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

oxide, which improves the scrubber chemistry and removal of SO2. 

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

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

(calcium hydroxide and magnesium 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 slurry 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 and 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 slurry to the absorber. The typical 

chlorides concentration in the scrubbers 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 absorbers 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-down, decreasing its level. 
Page 4 

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

five thickeners: two thickeners, operated 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 quicker 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 absorber cycling, and the unit load. 

• The blowdown from the absorbers is first transferred to a tank and then 

through a distribution box, where polymer 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 ratio 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 thickener underflow contains 28 to 32 percent solids 

and is pumped to the thickener underflow 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 may 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 rotates in the opposite 

direction of the centrifuge and pushes the solids back toward the inlet side 

of the centrifuge and out of the centrifuge. 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 contain 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. The calcium sulfate crystal is a flat 

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

crystal is typically larger and spherical. In this chemistry, minimizing 

calcium sulfate crystal formation is desirable; growing the sulfate crystals 

within the plate that composes the spherical 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 are removed from the FGD system in 

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

FGD solids are sent to the landfill 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 recycled 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 material 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 FGD waste. Gavin stated that the 

FGD byproduct is thixotropic, meaning that it stiffens when it is not 

moving, but becomes more fluid when handled. 

• Gavin stated that a contractor transfers 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 isolated 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 layer is to provide a porous layer to 

collect and drain the leachate 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 (to be located on top of Phases A, B, 

and C, respectively). 
Page 6 

• Gavin stated that stormwater is collected in ditches and flows into the 

leachate ponds. 

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

piping system into dedicated settling 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 the outlet of the settling ponds (inlet 

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

• Gavin stated that they monitor the 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 all three phases are discharged into 

Kryger Creek or Turkey Run, which flow into the Ohio River. 
Table 1. Site Visit Attendees 

Name Organization (Representing) Telephone E-maDon Anderson Ohio Power Company – Gavin dmanderson@aeDoug Workman Ohio Power Company – Gavin deworkman@aeTom Hart American Electric Power (614) 716-3260 tomlhart@aep.cTim Lohner American Electric Power (614) 716-1255 twlohner@aep.cColin Mckean American Electric Power ccmckean@aep.Chris Svoboda American Electric Power casvoboda@aepDonna Hill Southern Company. (205) 257-5234 dbhill@southernJosh Hall EPA/Office of Water (202) 566-1002 hall.josh@epa.gRon Jordan EPA/Office of Water (202) 566-1003 jordan.ronald@eDeb Bartram ERG (Contractor to EPA) (703) 633-1669 deborah.bartramKurt Blohm ERG (Contractor to EPA) (703) 633-1675 kurtis.blohm@eT.J. Finseth ERG (Contractor to EPA) (703) 994-7130 thomas.finseth@

 Limestone Limestone

– Inhibited Oxidation Inhibited Oxidation

 SO2 + CaCO3 + 1⁄2 H2O Æ CaSO3.1/2 H2O + CO2

(Calcium Sulfi (Calcium Sulfite)

– Oxidized Oxidized

 SO2 + 1/2O2 + CaCO3 +2H2O Æ CaSO4.2H2O + C

(Gypsum)

 Ammonia Ammonia

– 2SO2 + 2 NH3 + 2H2O ÅÆ 2NH4HSO3 + O2 

(Ammonium Bisulfite) (Ammonium Bisulfite)

NH4HSO4 + NH3 + H2O Æ (NH4)2SO4 + H2O

(Ammonium Sulfate) (Ammonium Sulfate)
SLAKING

CaO + H2O Æ Ca(OH)2 (519 BTU/LB )

(Calcium Oxide) (Calcium Hydroxide)

MgO + H2O Æ Mg(OH)2 (350 – 450 BTU/LB)

(Magnesium Oxide) (Magnesium Hydroxide)

SCRUBBING

MgSO3 + SO2 + H2O Æ Mg(HSO3)2 

(Magnesium sulfite) (Magnesium Bisulfite)

Mg(OH)2 + SO2 + H2O Æ MgSO3 + 2H2O

(Magnesium Hydroxide) (Magnesium sulfite)

Ca(OH)2 + SO2+ H2O Æ CaSO3.1/2 H2O + 1 1⁄2 H2O

(Calcium Hydroxide) (Calcium Sulfite)

REGENERATION

Mg(HSO3)2 + Ca(OH)2 Æ MgSO3 + CaSO3. 1⁄2 H2O + 1 1⁄2 H2O

(Manesium Bisulfite) 

Mg(OH)2 + Mg(HSO3)2 Æ 2 MgSO3 + 2 H2O
SO2

Mg

Mg(HSO3)2

CaO

H2O

Ca(OH)2

LIME

FLUE GAS

H2O

SLAKING

MgSO3 + H2O

CaSO3. 1⁄2 H2O + 1-1/2 H2O

SCRUBBING T

MgSO3 + CaSO3 .1⁄2 H2O + 1-1/2 H2O

2 MgSO3 + 2 H2O

LIME SLURRY

Mg(O

REGENERATION
Scrubber Targets for Achieving 

Scrubber Targets for Achieving 

Maintaining 98% SO2 Remo

Maintaining 98% SO2 Remo

Efficiency Efficiency

 pH 6.4 to 6.8 6.4 to 6.8 Sulfites Sulfites 7,000 to 9,000 mg/l 7,000 to 9,000 mg/l Magnesium Magnesium 7,000 to 9,000 mg/l 7,000 to 9,000 mg/l

 Loss of chemistry limited to filter cake. Loss of chemistry limited to filter cake.

– Filter cake moisture Filter cake moisture 

≤ 60

-wt%

– Absolute minimal loss of reclaim water to the 

Absolute minimal loss of reclaim water to the 

pond.

 Minimal use of fresh water Minimal use of fresh water

– Pump seal water Pump seal water– Slaking water Slaking water
MgSO3

SO2

H2SO3

MgSO3

Mg(HSO3)2

SO2 SO

H2

Mg(HSO3)2

SO2

COMPLETE CHEMICALREACTION
THICKENER

SURGE

TANK

ABSORBER GAS IN

461

LIME

238

ME

TK.

GAS OUT

959

ME WASH

600

QUENCH

520

ALTER.

SUPPLY

54

DUCK

POND

SEAL WATER

100.

ME WASH

546

MAKE

215

-UP

FIL

Blow down

1106

TUF

324

TOF

1022

IN FLOWS

GAS ----------- 461

LIME SLURRY- 238

DUCK POND -– 268

SURGE TANK ----44

SEAL WATER -- 100

1111

OUT FLOWS

GAS ---------- 959

CAKE & CHEM----153

1112

OUT FLOW

-44

Total

268

400 MW – 10% - 9 Duck Pond/91 Reclaim ME Blend

Chem. H20

9

Note: Values in RED are flow of water, corrected for SP. Gr. Blue values are total slurry flows

Conesville Conesville** 53 25% 98%

Mitchell Mitchell 71 29% 98%Gavin** Gavin** 28-32 25% 97%Mountaineer Mountaineer 180 -- 98%

SO2 

REMOVA

REMOVA

Tray 

Opening Opening

PLANT L/G

** Mag Chemistry ** Mag Chemistry
18 hrs. 18 hrs.

6 hrs

4 hrs 400 M

200 MW 44 

gpm

*

1080 

mins

=

47,500

70.0 K

70.0 K

START

22.5 K

117.5 K

134

gpm

*

360

mins

.

=

48,200

70.7 K

44

gpm

*

1080

mins

=

47, 500

23.2 K

69.3 K

116.8 K

134 gpm *

240

mins

.

=

32,200

84.6 K

55.4 K

44

gpm *

1080

mins

=

47, 500

132.1 K

7.9 K

9.5 hrs.

134

gp
SULFATE (gypsum) SULFATE (gypsum)
SULFITE (GOOD) SULFITE (GOOD)
Lime Slaking System Lime Slaking System 

- Gen

Gen

Process Description Process Description

The purpose of this system is to: The purpose of this system is to:

 Convert magnesium enhanced pebble li

Convert magnesium enhanced pebble li

the lime slurry reagent required for the s

the lime slurry reagent required for the s

dioxide SO dioxide SO

2 removal process. removal process.

 Prepare a lime slurry with 25% total sus

Prepare a lime slurry with 25% total sus

solids. solids.

 Store lime slurry and supply fresh slurry 

Store lime slurry and supply fresh slurry 

absorbers. absorbers.
Lime Slaking System Lime Slaking System 

- Gen

Gen

Process Description Process Description

 In the slaking process magnesium oxide a

In the slaking process magnesium oxide a

calcium oxide (major components of the lim

calcium oxide (major components of the li

react with water to form the hydroxide of th

react with water to form the hydroxide of th

compounds. This exothermic reaction prod

compounds. This exothermic reaction prod

calcium hydroxide (Ca(OH) calcium hydroxide (Ca(OH)

2

), and magnes

and magnes

hydroxide (Mg(OH) hydroxide (Mg(OH)

2).

CaO

+ H2O 

Æ Ca(OH)2 Ca(OH)2 

MgO

+ H2O 

Æ Mg(OH)2 Mg(OH)2
Ohio Power Company

Gavin Plant

Balance Diagram

Figure 2: Water

Resource Services

Water & Ecological Water & Ecological ## Outfall number

water usage.

Flows represent average

NOTE:

ALL FLOWS MEASURED IN

MILLION GALLONS PER DAY (MGD)

05.25.01 GV_water_bal.tif FGD Landfill Leachate Pond #3

Flyash Pond (rainfall) 

Cooling Tow

Blowdown

Low Volume 

Wastewate

Sewage Treatment 

Plant

Bottom Ash 

Pond

Recycle Scrubber Process Water

Scru

Water 

006 002 005 004

A

FGD Emergency 

Overflow Pond

Bottom Ash a

Pyrites Sluic

Fly Ash Transfer 

Building Sumps

Metal Cleaning 

Waste Tank

Coal Pile Run

Rainfall

Rainfall

Evaporation Evaporation

Stingy Run

009

FGD 

Landfill 

Leachate 

Pond #2

008

FGD 

Landfill 

Leachate 

Pond #1

Turkey Run

Kyger Creek

Ohio River

Intake Scree

Backwash

Reclaim Pond

1.4

0.119

0.142

0.109

8

8.39

0.17

18.92

0.52

5.76

5.76

0.01 0.04

0.36

8.48 0.01

007

0.52

0.01

27.39

0.02

0.01

0.17

0.11

11.52

7.24

001

Storm Water

Urea 

Mixing 

Skid Sump
Distribution 

Box

Make Up Water/Polymer

Thickeners (5)

Thickener overflows Reclaim T

Filter Feed 

Tank Thickener underflows

Centrifuge Centrifuge Overflow

Pug Mill

Fly Ash and Lime

Truck to Landfill

F

G

D

P

u

r

g

e

S

t

r

e

a

m

s

FGD Unit 1FGD Unit 2

Gavin Power Plant FGD 

Process Flow Diagram