1 

## Innovative Low-Load Supplemental Heating for the F. B. Culley Unit 3 SCR System

Paper # 127

Ron Bradford, John Minnette, and B. J. Reynolds Vectren Corporation

20 N.W. Fourth Street Evansville, Indiana 47741

Mark J. Chapin and James P. Walawender Black & Veatch Corporation

11401 Lamar Overland Park, Kansas 66211



### ABSTRACT

To maintain emissions compliance at low bo iler loads, many SCR systems have included provisions to maintain the flue gas above the minimum SCR operating temperature. Historically, SCR designers have empl oyed systems requiring some degree of

modifications to the boiler such as economizer gas-side bypass or a split economizer to raise the flue gas temperature at the SCR and allow the SCR to remain in service during low load operation. Typically, these alternativ es have had a very high capital cost to install due to the limited space available to retrofit the SCR sy stem and its auxiliaries.

An innovative application of gas-fired supplem ental heating system was commissioned in 2003 at Vectren’s F. B. Culley Station Unit 3. This unit is primarily base-loaded during the ozone season, and therefore low load supplemental heatin g will not be required for the majority of the operating hours. This design basis allows the higher cost of operation (when required) to be justified over the othe r alternatives because of the considerably

lower installed cost.

This alternative controls the flue gas temper ature entering the ammonia injection grid and the SCR reactor by adding supplemental heat from the combustion of natural gas with ambient air. The two flue gas streams ar e then thoroughly mi xed and the resulting temperature is controlled to achieve the mi nimum required SCR operating temperature.

Versatility and flexibility provided by this al ternative are proving very beneficial. This alternative provides an opport unity to develop startup pr ocesses which minimize NOx emissions. This method of supplemental heatin g can tolerate considerable more variation

in coal fuel sources than the tradi tional low load heating systems.

Installation of this system however does intr oduce some additional design challenges that are not present in the other alternatives, which required special attention during the2 project to ensure safe and reliable operation. In particular, special attention was given to the design of the downstream mixing devices, to the requirements of NFPA 85 and the development of the control software. This software was developed in-house by Vectren.

This system has been demonstrated to be able to elevate and maintain the flue gas temperature entering the SCR system. Additio nally, this system can be used to reduce the time required to warm the SCR reactor during startup. This paper presents an overview of the economics, design, and operating results for the Unit 3 low-load supplemental heating system.



### PLANT DESCRIPTION

F. B. Culley Station Unit 3 is owned and operated by Vectren and is located on the Ohio River near Evansville, Indiana. The un it began operation in 1973 and has a B&W dry bottom, opposed wall fired boiler. The uni t was converted to balanced draft and

retrofitted with low NOx burners in 1990. Un it 3 fires a variety of local and Illinois Basin coals ranging in sulfur content up to 51⁄2 percent. In addi tion to the low NOx burners, existing pollution contro l equipment at the time of the SCR retrofit included cold-side electrostatic precipitators (ESPs) and a wet limestone spray tower flue gas desulfurization system (scrubber).

Figure 1 shows the south side of the unit prior to the installation of the SCR system. Flue gas exited the economizer through two ducts; on e located on each side of the boiler, and flowed down into one Ljungstrom type air h eater per train. Downstream of the air heaters, each flue gas train passed through an ESP and induced draft (ID) fan before being combined with the flue gas from Un it 2 and passed to the flue gas scrubber.

The original chimney (which was abandoned af ter installation of th e flue gas scrubber)

was located between the ESPs and ID fans. Immediately south of th e scrubber inlet ducts

is the Ohio River.

The basis of design for the SCR system is shown in Table 1. This design basis was developed by Black & Veatch (B&V) and V ectren during the preliminary engineering phase of the project.3 Figure 1 South Side of F. B. Culley Stati on Before SCR System Installation

Table 1: Design Basis for the F. B. Culley Unit 3 SCR

Parameter Value Unit Size, MW 287 Required Operating Load Range, percent 40 to 100 Typical / Maximum Coal Sulf ur Content, percent 3.75 / 5.48

Uncontrolled NO

x Emissions (Typical), lb/MBtu 0.45 Required SCR Outlet NO

x Emissions, lb/MBtu 0.06 Required SCR Outlet Ammonia Slip, ppmvd at 3 percent O 2 2

SCR SO 2 to SO 3 Oxidation Rate, percent 0.5



### SCR SYSTEM DESCRIPTION

B&V provided engineering, procurement, and c onstruction management services as agent for Vectren. The design of the SCR and plant interfaces was established during preliminary engineering, which served as the ba sis for the execution phase of the project. During both preliminary engineering and proj ect execution phases, Vectren engineers and management worked closely with B&V to develop designs and solutions that were both cost-effective and ideally suite d for the particular unit.4 In designing the SCR reactor and ductwor k, B&V took advantage of the unique site characteristics to develop a compact and economical SCR system layout. Figures 2 and 3 show the plant arrangement in the SCR area after installati on of the SCR system. Figure

4 shows an isometric view of the SCR reactor.

Figure 2 SCR System Elevation5 Figure 3 SCR System Plan Arrangement

Figure 4 Isometric View of SCR System6 

### System Layout

The SCR system consists of a single reactor ho using up to four (4) la yers of catalyst. Flue gas exiting the boiler is directed through two new econo mizer outlet ducts which are

now located to the south of the boiler. On ce past the boiler support steel, the two ducts are combined into a single duct and flue ga s passes through flow correction devices and an ammonia injection grid (AIG) before entering the SCR reactor.

Demolition of the abandoned Unit 3 chimney provided a clear and open location for the installation of the SCR and afforded the oppor tunity to lower the SCR compared to the traditional system location above the unit air heat er(s). As a result, the top of the Unit 3

SCR is located at the economizer outlet elev ation and the SCR reac tor extends downward to grade. Flue gas exiting the SCR reactor turns vertically upward and is directed into two ducts, one serving each air heater. Figure 5 shows the completed installation.

An SCR bypass is included for catalyst prot ection during unit startup or when the SCR system is not required to be in operation. In addition to inlet/bypa ss diverter and outlet isolation dampers, high temperature guillotine dampers are installed at the inlet and outlet of the SCR reactor. Seal air systems are provided on the dampers to maintain reactor isolation. Secondary air is supplied as seal air to the inlet/bypass diverter and outlet isolation dampers and ambient air is supplie d as seal air to the guillotine dampers.7 Figure 5 Completed SCR System Installation



### SCR Catalyst

Homogenous honeycomb SCR catalyst was s upplied by CERAM-Frauenthal GmbH. The catalyst is composed of a porous metal oxide (catalyst support) and active metals which facilitate the reduction of nitrogen ox ides in the presence of ammonia. The catalyst support is TiO 2 and the active metals are V 2 O 5 and WO 3

.

Major catalyst design parameters are shown in Table 2.

Table 2: Catalyst Design for the F. B. Culley Unit 3 SCR

Parameter Value Catalyst Volume per layer, m 3 177 Pitch, mm > 6.0 Wall Thickness, mm 1.0 Catalyst Operating Temperature, o F 590 – 8508 

### Ammonia System

Nineteen percent aqueous ammonia is used as the reagent. Ammonia is received by truck and unloaded into a single, 35,000 gallon ammoni a storage tank that provides three days of onsite storage. Redundant fans supply dilu tion air to a gas-fired dilution air heater.

Heated dilution air flows in to a vaporization chamber where the required flow of ammonia is mixed and evaporated into the d ilution air. The air/ammonia mixture flows to a multi-zone adjustable ammonia injecti on grid. The grid is designed to ensure uniform mixing between the ammonia stream a nd main flue gas stream. This provides Vectren with the flexibility to field balance the NH 3 /NOx distributio n during initial commissioning and long-term operation. Each zone is equipped with a manually adjustable valve for regulating the flow of the ammonia/air mixture into the grid and a flow orifice which displays an indication of branch line flow on a digital indicator.



### Balance of Plant

In addition to erection of the SCR and relate d ductwork, the plant re quired several other modifications to complete the installati on. The project team also performed the following balance-of-plant modifications.

Air Heater

The existing Ljungstrom air heaters were convert ed from three-layer to two-layer design

in which the cold end consists of enamel co ated steel baskets. Enameling of the air heater’s cold end baskets not only provides co rrosion protection of the impacted area of the heat exchanger, but also significantly reduces the corros ion rate should water washing ever become necessary.

ID Fans

The ID fans required replacement to accommoda te the additional draf t losses of the SCR and the wide range of operating conditions and draft losses imposed by the addition of

SCR.

Chimney Demolition

As noted previously, the abandoned chimney was demolished to provide room for the

SCR installation.

ESP and Duct Stiffening

The existing design pressure of the ESPs a nd the ductwork downstream of the air heater

was not sufficient for the new modified de sign. These components were analyzed and

reinforced.9 Control System

SCR system control was implemented in th e existing Honeywell TPS DCS system. B&V provided the engineering desi gn and control narrative descriptions, and Vectren performed the control system programming and implementation.

Emissions Monitoring

In addition to NOx monitoring for the S CR, Vectren elected to install ammonia monitoring at the SCR outlet to provide a gr oss assessment of ammonia slip from the

SCR.



### LOW-LOAD SUPPLEMENTAL HEATING SYSTEM

As shown in Figure 6, the economizer exit temperature falls below the minimum SCR operating temperature at loads below approxima tely 200 MW. Furthermore, an analysis of the air heater design ch aracteristics and expected ope rating conditions leaving the boiler indicated that the flue gas temperature leaving the air heaters would fall below the calculated acid gas dew point with an air heater leakage of 10 percent.

450 475 500 525 550 575 600 625 650 675

0 50 100 150 200 250 300

Unit Load (MW)Economizer Exit Temperature (F)

Catalyst Minimum Operating Temperature

Figure 6 Economizer Exit temperature vs. Unit Load10 

### Evaluation of Alternative Methods of Supplemental Heating

Although the catalyst minimum operating temperat ure was established by the supplier as 590 F, the minimum design flue gas temperature entering the SCR was initially established at 605 F to allow margin for pot ential temperature mal-distribution during low load operation and to account for any di fference between the measured temperature and the minimum temperature within the duct. As operational experience becomes more known and predictable, this va lue will be adjusted as ap propriate. Since the normal operating economizer outlet flue gas temper ature drops below 590 F at loads below approximately 70 percent of full load, a method of maintaining this minimum temperature across the unit operating load ra nge was required. On previous SCR retrofit projects, this flue gas temperature control has been accomplished by bypassing a portion of the flue gas stream entering the boiler eco nomizer and mixing this flue gas with gas exiting the economizer, resulting in elevated flue gas temperatures at the SCR inlet. This flue gas bypass system has b een a relatively high capital cost system for the hightemperature ductwork, dampers and boiler wall pe netrations needed to provide the source of high temperature flue gas. Somewhat unique to the Culley Unit 3 site, footprint limitations and the minimal gap between th e primary superheater and the economizer would have required lowering the entire ec onomizer to accommodate economizer bypass. To avoid the high capital cost and maintenance costs of the bypass system, the use of natural gas fired burners was evaluated.

Operation of an economizer bypass system woul d result in lower boi ler efficiency and a higher coal fuel burn rate compared to the ex isting unit performance. Either the bypass system or the supplemental gas-fired heater system would be expected to require the same fuel heat input (MBtu/hr basis) to maintain the same S CR flue gas inlet temperature. Additional coal would be requir ed for the bypass system or natural gas woul d be required for the gas-fired heaters. Due to the use of higher cost fuel for the natural gas fired heaters, the operating cost of this system would be higher than that of the economizer bypass system by the difference between the cost of coal and the cost of natural gas on a
$/MBtu basis.

The operating load profile for Unit 3 indicate s that the majority of operating hours occur near full unit load conditions and fewer operating hours occur at reduced loads, where the normal economizer outlet flue gas temperatur e is below the minimum temperature for SCR operation. The unit generation records were reviewed, indicating that the unit operated above 70 percent unit load 90 percent of the time. Because of this high average unit load characteristic the lower capital co st gas-fired flue gas reheating system was evaluated to be economically advantage ous compared to the higher capital cost economizer bypass system.11 Table 3: Comparative Economics of Alternativ e Methods of Supplemental Heating

Economizer Bypass System Gas Fired

Duct Burner Capital Cost

Differential Maintenance Cost Differential Fuel Cost, Capitalized $ 1,020,000

-- Base Base (No Credit)

$ 450,000 Differential Evaluated Cost $570,000 Base

Although no credit was given in the analysis for the anticipated maintenance savings associated with the gas fired duct burner syst em, the comparative economics indicate that the gas fired duct burner system was expected to provide life-cycl e cost reductions in excess of $500,000.



### Additional Benefits Provided by Supplemental Heating

In addition to providing a cost-effective me thod of low-load suppl emental heating to support SCR system operation, the Unit 3 duct bur ner system provides several benefits to unit operations.

Boiler Operations

Use of the supplemental heating system doe s not impact normal boiler operations as would the traditional methods for supplementa l heating. Additionally, the duct burners

reduce the demand on the steam coil air he aters during cold weather compared to traditional methods for supplemental heating. Furthermore, the duct burners can lessen the impact of episodes of feedwater heater limitations and backpass fouling and can be individually controlled to balance gas temperature gradients at the economizer outlet, minimizing the energy input.

Unit Startup

The supplemental heating system provides the capability to raise SCR temperature earlier during startup compared to the traditional me thods for supplemental heating, allowing the SCR to be placed in service earl ier in the unit startup process.

SCR Operating Load Range

Since the heat source is inde pendent of furnace temperatur e, the supplemental heating system can maintain the required minimum SC R operating temperature at all loads down to minimum stable unit load. Additionally, because the multi-zone adjustable ammonia injection grid does not rely on flue gas velocity to develop the proper ammonia distribution within the duct, the Unit 3 S CR provides the capability to substantially

reduce NOx emissions at a ll unit operating loads.12 Acid Dew Point Corrosion Mitigation

The sulfur content of the coal for Unit 3 re sults in the potential fo r acid condensation at temperatures below approximately 280 F. Si nce the supplemental heating system raises the flue gas temperature downstream of the boile r exit, the air heater exit temperature is sustained during supplemental heating system operation. This prov ides the opportunity to maintain the air heater exit temperature above the acid dew point at all boiler loads, minimizing corrosion in downstream ductwork and equipment.



### Challenges in Implementing Supplemental Heating

Careful attention was paid to several issues regarding the gas-fired supplemental heating system during the design, startup, and initial operation of the Culley Unit 3 SCR system.

Flue Gas Mixing

The unique configuration of the Unit 3 SCR resulted in a much shorter length of

ductwork between the economizer outlet and th e SCR inlet than is typical of more common arrangements. In addition to in stalling the inlet/bypass and inlet isolation dampers and the AIG in this limited area, the arrangement required that the dispersion and mixing of the duct burner exit gases be a ccomplished as well. Special attention was paid to the mixing devices during the model st udy to ensure that the gas streams could be thoroughly mixed and gas temperatures main tained between the minimum and maximum catalyst operating temperatures. Model stud y results indicate that the temperature

distribution could be maintained within ± 30 F (±2.7 percent RMS) at the AIG and ±20 F (±1.4 percent RMS) at the first layer of catalyst, which were within the design objectives. Figure 7 shows a plan view of the airfoil type mixers that were implemented to distribute the supplemental heating gases within the flue gas duct.

Figure 7 Plan View of Duct Burner Mixing System13 Combustion Hazards

The use of gas-fired duct burners for s upplemental heating presented two operating scenarios that requir ed special attention.

The first operating scenario is the potential hazard created if a duc t burner fails to light off during startup and the burner management system failed to detect that failure and close the gas valve. Under this scenario, me thane would be injected into the main flue gas stream and carried downstream to other eq uipment, particularly the ESP. Detailed design analyses showed that there would be insufficient oxygen and insufficient methane to create a hazard, even shoul d both burners fail to light.

The second operating scenario is the possibili ty that the duct burne r and/or dilution air heater gas valves could leak methane gas into the SCR ductwork and reactor when the SCR is in bypass mode. To ensure that any potentially hazardous conditions in the SCR are safely neutralized, the duct burners and dilution air heater are not fired until the entire SCR ductwork and reactor are purged with flue gas by the ID fans and the dilution air heater and associated piping is purged with ambient air from the dilution air fans. Additionally, positive means fo r isolation of the duct burner natural gas supply is provided in order to maintain syst em purge credit during SCR bypass.

Commissioning Issues

In addition to ordinary mechanical and instrumentation commissioning problems which were resolved by the vendor, two issues were identified and solved during system startup.

Duct Burner Purge Normal startup of the duct burners requires purging the burners before light-off. It was found during system startup that the burner purge air flow would cool the economizer exit flue gas below the required minimum ammo nia injection temperature. The purge air flow was found to depress the gas temperat ure by 30 to 60 degree s, depending on unit load and ambient temperature. Operationa l modifications, including reduced volumetric flow rate for the purge and coordinati ng the fan start and damper opening, were implemented which successfully resolved the issue.

Duct Burner Fan Damper The duct burner fans were initially equipped with an air operated discharge damper. Since the SCR inlet ductwork operates under negative pressure , the quick-opening characteristic of the damper created a sudden inrush of ambient air which required compensation by the variable speed ID fans. The damper movement was also jerky, alternately binding and jumping during travel . Occasionally, the damper limit switch failed to make contact, causing the damper to quickly close just after opening. The elimination of the air inflow through the burne r resulted in furnace pressure fluctuations.14 The vendor’s air operator was replaced with a linear electric operator, which proved successful in resolving the issue.

Duct Burner Control

As initially conceived and installed, the duct burner operated with a constant air flow, and fuel gas flow was modulated to control gas exit temperature. Although this method of operation performed acceptably, system operating costs could be reduced by operating

the duct burner at maximum allowable discha rge temperature, thus reducing the volume

of air that must be heated.

After system startup, the burners were retr ofitted with modulating inlet guide vanes (IGVs) for the duct burner fans . In order to successfully implement this approach,

accurate and reliable measurement of both duct burner air flow and fuel flow was needed so that flame buoyancy can be maintained with in the desired range. Because an accurate air flow measurement was not available, the IGVs were characterized to establish a correlation between IGV position, fuel flow, a nd burner gas exit temperature. However, because the air density varied according to changes in ambient temperature and furnace pressure, this system was further refined by implementing a feedb ack loop based on the burner gas exit temperature.

Ash Accumulation

During initial operation of the SCR system wh en supplemental heating was not required, ash accumulation in the duct burner and fuel gas piping was noted. A purge system utilizing ambient air was impl emented but ultimately resulted in ash agglomeration in the burner from the moisture in the purge air. A method to potentially resolve the ash

accumulation would be to retrof it isolation dampers at the discharge connections of the

burners.



### OPERATING RESULTS

The F. B. Culley Station Unit 3 SCR was r eady for final commissioning and checkout in May/June 2003 as scheduled. System tuning was conducted in July 2003 and the system was put into commercial operation during fa ll 2003. Performance testing was conducted on November 5-7, 2003. The results of in itial tuning, performance testing, and SCR system load following capability are as follows.



### Initial Tuning

Prior to performance testing, the AIG was t uned to ensure acceptable NOx and ammonia distribution at the catalyst outlet. A fixe d tuning grid consisting of 24 sampling points

was installed at the outlet of the reactor to allow measurement of NOx across the entire

cross-section of the reactor. NOx was measur ed at each point in the testing grid. It15 should be noted that NOx was used to perf orm tuning instead of ammonia because NOx can be more easily and accurately measured.

A handheld NOx analyzer was used to perf orm the AIG tuning at both high and low unit loads. Figure 8 shows the results of the system tuning.

1 2 3 4

1 2 3 4 5 6 0 20 40 60 80 100NOx (ppm) at Tuning Grid

Figure 8 Results of Ammonia Injection Grid Tuning



### Performance Testing

Performance tests were conducted at full and lo w load. Table 4 shows the test results and compares them to the SCR system design requirements. This comparison shows that the SCR performance fulfilled the design basis requirements.

Table 4: Summary of Performance Test Results

Parameter Design Basis Full Load

Test Results Low Load

Test Results

SCR Inlet NO

x Emissions, lb/MBtu 0.48 0.49 0.38

SCR Outlet NO

x Emissions, lb/MBtu 0.06 0.059 0.054 Ammonia Slip, ppmvd at 3 percent

O 2 2 0.86 0.7316 Parameter Design Basis Full Load

Test Results Low Load

Test Results

SO 2 to SO 3 Oxidation Rate, percent 0.5 0.09 Inconclusive System Pressure Drop, in. w.g. 7.3 5.7 3.55



### Load Following Capability

As discussed previously, the operating flexibility afforded by the supplemental heating system increased the operating load range of the Unit 3 SCR compared to conventional approaches. Figure 9 shows the demonstrated results of the system to maintain SCR inlet temperature and NOx reduction capab ility during unit load reductions.

530.0 550.0 570.0 590.0 610.0 630.0 650.0

18:43 19:17 19:52 20:26 21:01 21:36 22:10 22:45 23:19 23:54 0:28 1:03 1:37

TimeTemperature (F)

0.0 100.0 200.0 300.0 400.0 500.0 600.0Unit Load (MW)NOx Concentration (ppm) SCR Inlet Temperature

Economizer Outlet Temperature

Unit Load

SCR Inlet NOx

SCR Outlet NOx

Figure 9 Load Following Operation

During the period of time shown on Figure 9, th e unit load was decreased from about 265

MW to 175 MW with intermedia te periods of operation at 225 MW and 200 MW. As the unit load decreased, the economizer outlet temperature decreased from approximately 640 F to 585 F, but the supplemental low-load heating system maintained the SCR inlet temperature at 620 ±3 F. During the same period, the SCR inlet NOx varied from 220 ppm to 135 ppm and outlet NOx was maintain ed between 10 ppm and 38 ppm, quickly stabilizing at 30 ppm after each load change.17 

### CONCLUSIONS

In addition to providing cost-effective and ec onomical supplemental low load heating, the application of gas-fired duct burners offers reduced retrofit requirements compared to conventional alternatives, increases unit operating flexibility, and can be used to mitigate the impact of acid condensation downstream of the air heater. This improved air heater outlet temperature control becomes more importa nt for high sulfur coal fueled units that are moving toward fabric filters to reduce particulate emissions.

The lessons learned during the Unit 3 project are being ap plied by Vectren and B&V to

the Warrick Station Unit 4 and A. B. Brow n Station Units 1 and 2 SCR projects. Additional refinements on subsequent app lications include the use of dedicated combustion air and tempering air fans to separate these functions and facilitate

modulation of burner air flow.

As a result of Vectren’s positive part icipation and coopera tive commitment to successfully implementing the gas-fired duct burner system for low-load supplemental heating, this concept was successfully mature d at the F. B. Culley Station Unit 3 SCR

installation.



### KEY WORDS

Black & Veatch Culley

Duct Burner Heating

Low Load

NOx Selective Catalytic Reduction (SCR)

Vectren