### Site Visit Notes: Duke Energy Carolinas’ Marshall Steam Station (March 13, 2008)

EPA conducted a site visit at Duke Energy Ca rolinas’ Marshall Steam Station located in Terrell, NC on Thursday, March 13, 2008 to le arn about the operations at the plant, specifically the FGD wastewater treatment system. Table 1, located at the end of this summary, identifies the site visit participants.

This summary describes the information collect ed during the site visit and the pre-site visit phone call, which was held on February 1, 2008.

Plant Overview

• Marshall operates four gene rating units at the power pl ant. It is a baseload plant with a total generating capac ity of 2,090 MW. The table below contains some general information for each unit.

Unit Installation

Date Capacity (MW) Particulate

Control NOx Control SO 2 Control

1 1965 385 Cold-side

ESP SNCR Wet FGD – 5/2007 (shared with Unit 2)

2 1966 385 Cold-side

ESP SNCR Wet FGD – 5/2007 (shared with Unit 1)

3 1969 660 Cold-side

ESP SNCR, but currently installing a

new SCR Wet FGD – 3/2007

4 1970 660 Cold-side

ESP SNCR Wet FGD – 10/2006

• All four units operate cold-side E SPs. Marshall recently installed new

ESPs for Units 3 and 4 to increase th e removal of fly ash before the FGD system so the gypsum would have a purity level sufficient for wallboard manufacturing.

• All four of the units currently ope rate SNCRs; however, Marshall is currently installing a SCR on Unit 3. Ma rshall is converting to the SCR to

achieve better removals of NO

x . Marshall is also conducting a study to

determine whether to install a SCR on Unit 4.

• Marshall is operating three wet FGD scr ubbers to scrub the flue gas for all

four units. Marshall’s first FGD scrubber was installed on Unit 4 in

October 2006. Marshall then instal led a scrubber for Unit 3 in March 2007, and installed a scrubber to scrub the flue gas from both Units 1 and 2 in May 2007.2 • Marshall burns eastern b ituminous coal in all four units. The average percent sulfur of the coal is 1.25 percen t. Marshall stated that before the FGD scrubbers were installed, the pl ant was burning coal with a sulfur

content of 2.0 lbs SO 2 /MBTU. Marshall blends coal on site to achieve a

sulfur content of 2.4 lbs SO 2 /MBTU. Marshall stated that the maximum design sulfur content for the FGD scrubbers is 2.8 lbs SO 2 /MBTU.

• Marshall stated that the majority of the coal they use is from Central Appalachia, but some of the coal is from Northern Appalachia. Marshall stated that they receive coal from about 40 different sources.

• Marshall burns approximately 5.5 million tons of coal per year. Marshall stores a 30-day supply of coal on site.

• Marshall operates once through cooling systems for all four units using water from Lake Norman. The average intake flow rate for the plant is approximately 1 million gallons per minute for all four units combined.

Ash Basin

• Marshall stated that they have the capability to wet sluice the fly ash; however, the plant primarily uses a dry fly ash handling system.

• Marshall stated that the dry fly ash generated by the pl ant contains too

much unburned carbon to sell for benefici al reuse. Therefore, Marshall disposes of the fly ash in an on-site st ructural fill. Marshall estimated that they generate 500,000 to 600,000 tons of fly ash per year.

• Marshall stated that the wet fly ash ha ndling system is used when there are equipment issues or after maintenan ce outages. Marshall stated that the duration of the wet fly ash sluici ng can be up to a couple of days, depending on the situation, but it is used very minimally.

• Marshall operates a wet bottom ash sluice system. The wet bottom ash sluice, as well as any wet fly ash sluice are sent to the ash basin.

• Marshall stated that the ash basin is approximately 5-6 meters deep at the deepest point.

• Marshall stated that they recently dredged some of the ash in the basin by moving the ash to the more in active portions of the basin.

• Marshall has two concrete basins that are adjacent to the confines of the

ash basin. Marshall directs the bottom ash sluice to one of the concrete basins during the day. The solids in th e sluice water settle to the bottom of

the concrete basin and the wastewater overflows to the ash basin. The next day, Marshall directs the bottom as h sluice to the other concrete basin

and allows the solids in the first conc rete basin to dry. Later in the day,3 Marshall removes the solids from the first concrete basin. Marshall repeats this process every day.

• Marshall has an on-site cont ractor that manages the sale of the bottom ash.

The contractor takes the dried bottom ash and sends it through a sieve that sorts the bottom ash by size. The contra ctor then sells the bottom ash for beneficial reuse (cinde r block manufacturing).

• The pyrites generated from the pulverizing process ar e sluiced to different basins (not the concrete basins) adja cent to the confines of the ash basin, and the plant treats the pyrites solids in the same manner as the bottom ash: allowing the solids to settle, de canting the overflow water into the ash basin, and removing the solids. Marsha ll stated that they are working to set up a coal/pyrite separation proces s that will separate the unburned carbon in the pyrites and allow the plant to reuse it in the boiler.

• The ash basin discharges though NPDE S permitted outfall 002 to Lake Norman ( i.e., the Catawba River).

FGD Scrubber

• Marshall is currently oper ating three FGD scrubbers that scrub the flue gas for all four units (one of the FGD scr ubbers scrubs the flue gas for Units 1 and 2).

• Marshall started operation of the firs t scrubber in October 2006; the last scrubber started operation in May 2007.

• Each of the FGD scrubbers has a sp ray tower design and has four spray levels; therefore, each scrubber has four recirculation pumps.

• Marshall operates ball mills to crus h limestone to make the limestone slurry that is used as th e sorbent in the scrubber.

• Marshall stated that they are not currently using DBA in the FGD scrubbers, but they have the capability and plan on using it in the future (note: its use has been delayed) . Marshall did add DBA during the performance testing of the scrubbe rs, but stopped using it immediately

after the test. Marshall stated that th ey want to achieve a 97% removal of

SO 2 , but currently they are achieving 95% removal.

• Marshall stated that the design maximu m chloride level in the scrubber is 12,000 ppm. Marshall stated that they plan to operate the system at <10,000 ppm chlorides.

• Marshall has two sets of hydroclones for each of the FGD scrubbers. In addition, there are two vacuum belt f ilters for the FGD operation. Marshall

stated that the blowdown from the sc rubbers to the primary hydroclones is4 intermittent, about four hours of every six hours. The overflow from the primary hydroclones is sent to a ho lding tank. The primary hydroclone underflow is sent to a feed tank, then to the vacuum filter belts to dewater the gypsum.

• The primary hydroclone overflow in th e holding tank is transferred to the secondary hydroclone for additional solids removal. The underflow from the secondary hydroclones is returned to the FGD scrubbers. The overflow from the secondary hydroclone s is sent to another holding tank (purge tank) and then sent to the F GD wastewater treatment system. The purge to the FGD wastewater treatmen t system is intermittent at a flow rate of approximately 440 gpm (maxim um design purge flow rate is 538 gpm). Marshall stated that the TSS le vels in the purge stream are usually between 6,000 and 8,000 mg/L. The T SS design level is 16,000 mg/L.

• Marshall washes the gypsum with service water as it is applied to the

vacuum filter belts. Th e filter belts dry the gypsum to less than 10 percent

moisture. The wash water and filtrate from the vacuum filter belts are sent

back to the absorbers.

• Marshall estimates that they produce 1,200 to 1,400 tons of gypsum per day. The gypsum is transferred to an outdoor storage area and is eventually hauled to a National G ypsum wallboard plant located near Charlotte. Any gypsum that is consid ered off-specification is sent to a new, lined on-site landfill.

FGD Wastewater Treatment System

• The Marshall FGD wastewater treatm ent system consists of a solids removal process followed by constructed wetlands. The targeted effluent quality for the wetlands effluent (out fall 004) is 0.5 ug/L mercury and 200 ug/L selenium. The wetlands effluent is sent to the ash basin; the ash

basin effluent is outfall 002.

• Figure 1, at the end of this memorandu m, presents a process flow diagram of the solids removal process.

• An equalization tank is the first component of the FGD wastewater treatment system. The equaliza tion tank receives the FGD purge wastewater from the secondary hydroc lones and return water from the filtrate sump (which is mainly the clarifier filter press filtrate).

• Marshall stated that the return flow from the filtrate sump is intermittent. Marshall operates 2-3 filter press cy cles daily. The contribution from the filtrate sump is 39,600 gpd.5 • The FGD wastewater flows continuously from the equalization tank to the

clarifier. Marshall stated that if th e equalization tank level is low, the flow rate will automatically decrease so it can flow continuously.

• Marshall stated that a cationic polymer is added to the wastewater as it is flowing to the clarifier. Marshall st ated that they changed the polymer from the anionic polymer previously used because a bench test showed that a cationic polymer was more eff ective. In addition, Marshall stated that the anionic polymer was in the dr y form and had to be blended on site, whereas the cationic polymer is in liquid form and already preblended.

• The solids removed from the clarif ier, containing approximately 30 percent solids, are sent to a sludge holding tank. From the holding tank, the sludge is sent to one of three filter presses. The solid cake formed by the filter press is transferred to the lined on-site landfill that also receives the off-specification gypsum. The filtrate from the filter press is sent to a sump and then transferred to the equa lization tank at the beginning of the FGD wastewater treatment system.

• The overflow from the clarifier is tr ansferred to the wetlands supply tank, where Marshall adds service water to reduce the temperature and the chlorides level. Marshall stated that the TSS design specification for the clarifier overflow was <1,000 ppm; howev er the results are typically less than 100 ppm.

• Marshall stated that they were initially adding service water to the FGD

wastewater at a ratio of 0.85:1 (service water: FGD wastewater), but

increased the ratio to 1:1 or 1.2:1 beca use the high concentrations of boron in the wastewater were impacting the plants. Marshall stated that the

boron levels need to be less than 50 mg/L. Also, the plants need the wastewater to have less than 4,000 ppm chlorides and temperature less

than 95F.

• The wastewater from the wetlands s upply tank is pumped 1.5 miles to the constructed wetlands treatment system . Figure 2, at the end of this memorandum, presents a process flow diagram of the constructed wetland treatment system. Marshall stated that they were initially interested in the constructed wetlands as a treatment option for the FGD wastewater because other technologies did not appear to be as effective.

• The first stage of the constructed we tlands consists of two equalization tanks which allow further cooling of the wastewater prior to entering the wetlands. The wastewater is then spl it to enter three identical constructed wetland treatment trains in parallel. The first and second cell of each train contain bulrush, then the wastewater flows over cascading rocks (Cell 3)

and enters the fourth cell containi ng cattails. The effluent from the constructed wetlands system discharges to the ash basin.6 • Marshall stated that they began transf erring wastewater to the wetlands in

December 2006. Marshall stated that in itially they had issues with the levels of solids in the system because the polymer feed was not operating properly causing more solids to enter th e constructed wetlands than they had expected.

• Marshall stated that the wetlands system is not yet optimized. The system is reportedly achieving good removal of mercury, but is not treating the selenium as well as they had planned. Plant staff stated that selenium reduction across the wetlands is approxi mately 10-20 percent. Marshall is working with the vendors to improve the selenium removal. Marshall stated that they are considering making adjustments to the amount of organic material in the system a nd increasing the water level in the wetlands, which creates an anoxic environment.

• Marshall stated that the system is probably about 2 years away from maturity. Marshall stated that some of the cells still have open space available and they are considering a dding more plants. Marshall stated

that the bulrush in the first two ce lls have not been producing as much organic material as they had expect ed and they may switch to cattails because they are expected to produce more organic material. Marshall is also considering adding a dditional organic material to the cells, such as mushroom compost. In July 2008, a p ilot test will be conducted using

nutrient addition to one of the trai ns. The study will last for several

months.

• Marshall is also working with EPRI on a pilot test for a vertical flow constructed wetland system. The vertic al flow system is similar to the current constructed wetland system, ex cept the plant would need to install leachate piping underneath the current system to allow water to be

removed from the bottom of the constructed wetland. Marshall stated that the study is also looking into using a different source of organic material (mushroom compost).

• Marshall stated that the constructe d wetlands are designed for a 30-year life span. Marshall stated that the wetlands are lined at the bottom with a geosynthetic clay liner. The soil is on t op of the clay. At the end of the 30 year life, Marshall plans to seal off the wetlands.

• EPA asked if there is any potential to generate methyl mercury in the system. Marshall stated that as long as the oxidation-redu ction potential in the system is above a certain level, then the mercury will not convert to methyl mercury. Marshall also stated that most of the mercury in the FGD purge stream is being removed in the clarifier; therefore, the level of mercury in the wetlands is low. At the ash basin discharge, mercury is typically <5 ppt throughout the year.7 • Marshall stated that they are not achie ving nitrification/denitrification in

the wetlands. The amount of nitr ate/nitrite and ammonia entering the system are typically equal to the amount leaving the system.

• Marshall stated that they are consid ering adding ferric chloride to the purge tank or equalization tank of th e solids removal process (or add a reaction tank to the treatment system) to help increase the selenium removals across the FGD wastewater treatment system.

FGD Wastewater Treatment Data

• EPA asked if the plant has 2007 data fo r the wastewater treatment system. Marshall stated that they have collect ed samples of the ash basin effluent and the constructed wetlands effluent. In addition, Marshall has collected samples of the FGD purge stream. Mars hall stated that most of the metals data are for total metals, but they do have some results for dissolved analyses.

• EPA stated that they would like to see the data that Marshall has available.Table 1. Site Visit AttendeesNameOrganization (Representing)TelephoneE-mailWilliam Bare Duke Energy (704) 622-0907 wjbare@duke-energy.comDonna Burrell Duke Energy (828)478-7820 dlburrel@duke-energy.comBobbie Chambers Duke Energy (828) 478-7647 bpchambers@duke-energy.comBill Kennedy Duke Energy (980) 721-7626 wkennedy@duke-energy.comRon Lewis Duke Energy (980) 373-5710 relewis1@duke-energy.comDebbie Nispel Duke Energy (317)838-1957 dnispel@duke-energy.comJay Perkins Duke Energy (704)875-5348 jperkins@duke-energy.comDave Styer Duke Energy (828)478-7506 dwstyer@duke-energy.comRobert Wylie Duke Energy (704) 382-4669 rrwylie@duke-energy.comDonna Hill Southern Company (205) 257-5234 dbhill@southernco.comKarrie-Jo Shell EPA/Region 4 (404)562-9308 shell.karrie-jo@epa.govJosh Hall EPA/Office of Water (202) 566-1002 hall.josh@epa.govRon Jordan EPA/Office of Water(202) 566-1003 jordan.ronald@epa.govT.J. Finseth ERG (Contractor to EPA) (703) 994-7130 thomas.finseth@erg.com9Figure 1. Process Flow Diagram of Solids RemovalPortion of FGD Wastewater Treatment System.10Figure2. Process Flow Diagram of Constructed Wetland Portion of FGD Wastewater Treatment System