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O&M Feature: Maximize That Uprate: Don’t Overlook Balance of Plant Systems & Equipment

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:

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.

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 SOX 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:

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.

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:

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.

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