By Anthony J. Munisteri, P.E., Peter M. Kelly, P.E. and George R. Kotynek, Sigma Energy Solutions

Competition in the electric power generation market demands that power plants operate efficiently and generate the maximum amount of power for which they were designed consistently and safely. In addition, the significant increase in the price of natural gas has caused a renewed interest in coal-fired power plants, both existing and new-build. Existing coal-fired plants are being retrofitted with emissions control equipment to meet stricter standards. This equipment consumes significant amounts of auxiliary power, which plant owners might like to regain by increasing the retrofitted units’ maximum output.

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Central station power plants have been designed with a significant amount of margin. Boosting a generating unit’s output by taking advantage of this design margin can provide plant owners with an available source of new power with attractive returns. Improvements in equipment efficiency and control system performance allow electric generating units to take advantage of the existing design margins.

In taking advantage of equipment design margins, it almost always is the case that one or more design limits will prevent full utilization. Usually, the best performance results are identified in finding improvements in the major energy conversion equipment such as boilers and steam turbines. With new computer-based analytical methods, steam generator and steam turbine manufacturers can identify significant power increase potentials. An evaluation of the power plant auxiliary systems is necessary to assess the full effect major system uprates have on plant operation. Balance-of-plant equipment may limit the potential improvement of major power uprates.

This article discusses the need to thoroughly examine the balance-of-plant equipment and systems as part of any power uprate study. Examples will be given from recent studies to show how much additional generated power can be obtained if the balance-of-plant equipment and systems are considered as part of the boiler and turbine uprate.

Power plant owners investigate restoring or increasing plant capacity for many reasons including:

• Recovery from degradation—In many cases older coal-fired power plants, which were scheduled for retirement, are now planned to run well into the future.
• Recovery from emissions control retrofits—The addition of control equipment to reduce plant emissions have significantly increased the auxiliary power consumption of fossil-fueled power plants.
• Design margin availability—Central station power plants have been designed with a significant amount of margin. The ability to increase a generating unit’s output by taking advantage of this margin provides an available source of new power with a minimum of capital cost.
• Competition—In those regions where coal-fired power plants are competitive because of high-priced natural gas generated power or higher-priced imported power, investment in capacity upgrades is attractive.
• Technology improvements—New-generation steam turbine inner cylinders and rotors equipped with high-efficiency nozzles and blades can generate more power using the same amount of steam flowing through the original turbine frame.

At a minimum, power plant owners want to restore the original net power output of these plants, with a preference to increase the net power output if it is economical to do so. Over the last 20 years, boiler and turbine manufacturers have improved their equipment’s performance and efficiency so that major uprates of existing equipment are possible.

To add steam capacity, boiler manufacturers can add heat transfer surface utilizing improved designs and materials, if necessary. Additional heat transfer surface cleaning equipment can be added such as steam sootblowers, water wall blowers and water cannons. In addition, more efficient burners and rotating classifiers for coal pulverizers can be added. Boiler designers can predict the effectiveness of these boiler modifications using computer modeling programs based on computational fluid dynamics.

Turbine manufacturers have designed highly efficient turbine blades and nozzles so that a turbine can generate more power at the same steam flow. Still more power can be generated if the associated boiler can produce additional steam. For existing turbines, the inner cylinders and rotors can be replaced by the newly designed components and the power outputs can be increased while reusing the outer cylinders. As a result, piping and foundation changes are not required.

These major equipment changes affect the balance-of-plant equipment and systems. For example, an increase in boiler steaming capacity requires a corresponding increase in feedwater flow. As a result, the boiler feed pumps and condensate pumps must be analyzed to determine if sufficient pumping capacity is available. The additional steam flow through the turbine increases extraction steam flow, heater drain flow and, finally, the low-pressure turbine exhaust steam flow to the condenser. The effects of these additional steam and water flows must be assessed. Upgrades may be required to realize the maximum potential associated with the boiler and turbine upgrades.

Approaches to Asset Optimization

Depending on a unit’s original design; past retrofits; past and present maintenance and operating practices; age and anticipated operating regime, various optimization approaches are available to the owner. These may include the following.

Thermal performance audits—These are used when a unit’s heat rate is significantly higher than the original design. Thermal performance audits are also useful when it is necessary to evaluate the improvement in a unit’s heat rate when planning the replacement of deteriorated equipment with new equipment of a different design and/or modifying the operating regime.

Bottleneck relief studies—Studies are necessary when a unit’s owner needs greater output and suspects existing limitations can be overcome by upgrading or replacing equipment. Economic analyses are performed to allow selection of the most viable alternatives.

Turbine upgrades—To gain additional capacity and enhance reliability and efficiency, entire turbine sections may be replaced. Reasons for retrofitting various sections of turbines may include the following.

• High-pressure turbines—The main reasons for retrofitting HP turbines with new, inner cylinders and rotors are to increase efficiency and power output. In addition, if the boiler steaming capacity can be increased, the design steam-swallowing capacity of the HP turbine will be increased for even greater power output. Other reasons for include reducing the risk of solid particle erosion by utilizing enhanced blade designs.
• Intermediate-pressure turbines—Reasons for retrofitting IP turbines are to increase steam flow capacity and power output, moderately increasing the efficiency and increasing the interval between overhauls.
• Low-pressure turbines—Replacing a shrunk-on rotor with a welded drum rotor equipped with new blades eliminates the risk of LP turbine rotor stress corrosion cracking. Greater LP turbine efficiency and reliability is achieved by utilizing free-standing “L-0” blades. These blades do not use lashing wires thus avoiding coupling interaction between adjacent blades. Blade/rotor interaction does not occur due to the stiff welded design of the drum rotor, allowing accurate tuning of the blades.

Boiler uprates—Uprating a boiler first requires a complete analysis of the existing operational issues and constraints, including emissions and Q-fired limits and identification of available solutions. Next, changes to the boiler main steam, reheat steam and feedwater flows, temperatures and pressures brought about by modifications to the steam turbine must be evaluated and potential solutions identified.

For example, installing a new and more efficient HP turbine inner cylinder and rotor will decrease the cold reheat steam temperature by 20 degrees Fahrenheit or more for the same main steam flow. The existing boiler reheater may have to be enlarged to maintain the design hot reheat steam temperature entering the IP turbine. Likewise, if a boiler has excess steaming capacity, then new, more efficient turbine inner cylinders with a greater steam flow capacity would need consideration. Finally, boiler efficiency improvements can be obtained by upgrading components with better designs and materials, optimizing boiler subsystem performance and eliminating conditions that reduce boiler efficiency between maintenance outages. This review needs to consider fuel preparation and transport, firing system design, furnace wall cleaning, convective heat transfer surface arrangement and cleaning, regenerative air heater upgrades and so on.

Boiler/turbine/balance of plant comprehensive study—Optimizing the heat rate and power output of electric generating units requires balancing the boiler, steam turbine-generator and balance of plant equipment. Balance of plant analysis identifies both the ability to support any planned boiler/turbine uprates as well as the equipment technology enhancements that could improve unit capacity and/or efficiency.

Balance-of-Plant Assessment Methodology

Utilizing a consistent methodology to assess the balance-of-plant equipment and systems for existing and uprated conditions is key to the overall success of an uprate program.

The balance-of-plant assessment methodology consists of several steps.

First are the preliminary discussions with the power plant personnel regarding information about the plant equipment and systems and equipment. During these discussions, balance-of-plant equipment nameplate data, specifications and data sheets, piping specifications, valve list and data sheets, P & IDs, system descriptions and so on are obtained for analysis.

Second, the unit is tested at turbine “throttle valves wide open” and at other loads as agreed upon to establish a base line for plant performance levels.

Third, balance-of-plant mechanical and electrical equipment and systems are analyzed to determine existing capacity and capability for operating at the uprated process conditions. The boiler and turbine computer models are calibrated based on the unit test performed at the site. These models are then used to predict the process conditions for the uprated boiler and/or turbine. The balance-of-plant equipment is then reviewed and analyzed to determine where bottlenecks exist and where efficiency improvements can be obtained. The unit’s electrical system is analyzed to determine adequacy of the existing electrical high voltage and auxiliary power systems and possible bottlenecks at the uprated conditions. When bottlenecks and potential efficiency improvements are identified, budgetary estimates of the recommended solutions are prepared to develop a prioritized ranking of improvement potentials. A rough draft of the report is then prepared for use at the joint project team design review meeting.

Fourth, the joint project team design review meeting is conducted with the client and all members of the major equipment and balance-of-plant assessment teams. Options are discussed and additional computer model runs are performed to define and select the optimum thermal performance envelope for the plant’s boiler-turbine complex. A final uprated plant configuration is developed and a final report is issued to serve as a guide for the plant uprating project.

Recent Asset Optimization Studies

An electric utility in the Midwest had committed to installing a SO_X reduction system and wanted to increase generation to offset additional auxiliary load. It planned to increase generation by retrofitting new IP/LP inner cylinders and rotors on its nominal 758 MW pulverized coal-fired unit. Concurrently, the utility asked for help in determining maximum unit uprate through a boiler/turbine study as well as a comprehensive review of the balance-of-plant equipment. This was aimed at identifying potential capacity and/or efficiency limitations associated with the increased generation and providing economical solutions to remove these constraints.

An iterative and integrated plant performance evaluation identified plant modifications to the boiler and balance- of-plant systems which, if implemented, could achieve a new gross generating capacity rating between 808 MW and 817 MW, depending upon seasonal ambient temperatures. Integrating the boiler and balance-of-plant systems into the overall analysis removed constraints to allow the IP/LP turbine retrofit to achieve its full potential, plus deliver an additional 30 MW under certain ambient and operating conditions.

The boiler had at least two limitations, for which the project team analyzed and developed solutions. First, firing rate and steam generation were curtailed when the flue gas temperature leaving the economizer exceeds 850 F. The reheat boiler performance program sized the additional primary superheater and economizer surface that would be needed to rectify this limitation and increase the boiler’s steaming capacity.

Second, the existing water lances were not keeping the furnace waterwalls clean enough to provide good control of furnace slagging and, thus, furnace outlet gas temperature. The boiler performance program recommended adding a heat flux monitoring system and water cannons.

Balance-of-plant systems were analyzed and several limitations to increased generation and plant efficiency were identified.

Midwestern Industrial Facility

Here, the client’s goal was to achieve an overall 100 MW increase in generating capacity from three of its four coal-fired units. This represented a nominal 25 percent increase from the original unit design basis and would offset auxiliary load increases that were expected from an emissions control retrofit. The evaluation examined the effect of firing different coals and an assessment of the industrial production facility’s energy needs and their impact on generating plant production.

A plant capacity evaluation generated options to increase each unit’s net generating capacity to 25 percent (without an HP/IP turbine retrofit) or 50 percent (with an HP/IP turbine retrofit and constrained by a new Environmental Protection Agency permit Q-fired limit). Initial efforts base-lined current operating performance boundaries by conducting a “valves wide open” performance test on one of the three identical turbines. Engineering studies optimized the interaction between the boiler and turbine to determine the required boiler modifications so that it could generate the necessary steam flow rate to realize the HP/IP turbine upgrade’s full potential. Using the retrofitted boiler/turbine process parameters, a detailed balance of plant analysis was performed to eliminate any constraints that would prevent the plant from attaining the predicted generating capacity increase.

The more significant boiler modifications required to support a gross turbine-generator output increase of around 40 percent from each of the three units include the following:

• Convert the existing electrostatic precipitators from hot side to cold side operation. To increase precipitator collection efficiency, the first two fields should be converted from transformer-rectifier to switched integrated rectifier operation.
• Upgrade the FD fans with new, larger rotors to fit within the existing housings together with new, larger motors to increase capacity.
• Repair regenerative air preheater seals to maintain leakage at 12 percent or less.
• Upgrade coal pulverizers by replacing existing static classifiers with dynamic classifiers to obtain better coal fineness and thus reduce unburned carbon in the fly ash. In addition, replace existing integral, table-type, volumetric coal feeders with posimetric, calculated-weight-type coal feeders.
• New, larger-capacity safety valves.
• New, larger-capacity boiler drum internals.
• Make pressure part design changes to reduce boiler back pass flue gas velocities.

The significant recommendations and modifications to the balance-of-plant systems to support the generation output increase of around 40 percent by each unit included the following.

• The two existing 100 percent (each) direct motor-driven boiler feed pumps needed replacement with larger boiler feed pumps and motors to reduce high auxiliary power usage when both pumps need to be operated in parallel to provide sufficient feedwater at higher loads.
• Because of significantly higher feedwater and condensate water flow velocities, a flow accelerated corrosion study was necessary during the project’s detailed engineering design phase. Heater drains needed to be included because of increased two-phase flow.
• Because the plant’s once-through condenser cooling water system used river water, an on-line condenser tube cleaning system was recommended to remove the sludge build-up that normally occurs and that requires periodic manual cleanings. Keeping the condenser tubes clean would consistently decrease condenser pressures and thus decrease heat rate. Cleaning also would increase generation capability, especially during the summer when river water temperatures approach 90 F.
• Replacement of the expansion joint in the extraction steam line to LP Heater No. 3 was necessary because of higher extraction steam pressures caused by the HP/IP turbine retrofit.
• One unit generator needed upgrading from non-direct field cooling to direct (air) field cooling to increase its generating capacity.

East Coast Electric Utility

The client is adding a flue gas desulphurization system to its two-unit, coal-fired, 1,300 MW electric generating station, causing auxiliary power to increase. To offset the additional power usage, the client asked for a study evaluating the benefit of upgrading the HP turbine on both units. This new HP turbine will allow the unit gross generation to be increased from the present 680 MW (nominal) to 720 MW (nominal) without any significant boiler modifications.

To maximize the HP turbine upgrade, the utility requested a balance-of-plant technical study be performed to identify what equipment will require replacement or upgrading. The most significant results of this analysis included:

• The HP feedwater heater, which was scheduled for replacement because of performance problems, would be replaced with one using stainless steel tubes per HEI recommendations and designed for the upgraded flow, pressure and temperature of the feedwater and extraction steam systems. The tube side velocity of the feedwater flowing through another HP feedwater heater (with the higher feedwater flow due to the HP turbine upgrade) was calculated to be 35.6 percent higher than the HEI standards for closed feedwater heaters. A proposed solution recommended that stainless steel tubes be considered for the HP feedwater heater.
• The plant experienced elevated condenser shell pressures during summer months, preventing maximum load from being achieved on hot days. Since the condenser’s heat removal duty was expected to increase with the upgrade, the problem was thought to be worse after the upgrade. A detailed condenser cooling water optimization study was ordered to determine the necessary steps to achieve the desired condenser pressure and to obtain maximum generation under the worst summer ambient temperature conditions.
• It is suspected that the performance of the fresh-water, closed-cooling-water-system cooling towers was affected by fouling of the cooling tower fills caused by ash deposited from a nearby fly ash silo. A detailed study was ordered to develop solutions.
• Three condensate pumps were found to need modification to operate at a higher head and desired condensate flow. In the alternative, consideration should be given to replacing the pumps with the revised design conditions required by the upgrade.
• Additional equipment was identified as presenting a potential bottleneck: main boiler feed pumps, induced draft fans, circulating water pumps, pulverizers and primary air fan motors.

These examples illustrate the need for the implementation of a balance-of-plant assessment methodology when upgrades to boilers and/or turbines are being considered.

Analytical techniques, with field testing confirmation, can be used to perform comparative studies on determining what upgrades will provide a cost-effective solution.

Combined with the modeling, knowledge of modern construction techniques and the latest offerings from equipment vendors allows developing an integrated plant upgrade solution with a desirable payback.

Authors: Anthony Munisteri is asset optimization director for Sigma Energy Solutions Inc., a unit of Alstom Power. He is a registered professional engineer with over 20 years of experience in the power industry and holds a BChE Degree from The Cooper Union and an MBA from Adelphi University. Peter Kelly is the chief electrical and I&C engineer for Sigma Energy Solutions. He is a registered professional engineer in four states with over 28 years of experience in the power industry and holds a BME Degree from Manhattan College and an MSME Degree from Columbia University. George Kotynek is a senior consulting engineer at Sigma Energy Solutions with over 40 years of Power Engineering experience. He is a professional engineer in Illinois and holds a BS in Mechanical Engineering from the Illinois Institute of Technology.