From equipment selection to design optimization and project management, successful execution of combined cycle projects using advanced gas turbine technology presents many challenges. EPC contractors can learn from experience, explains Bechtel.

Justin Zachary, Ph.D., James Bauerschmidt, P.E., Christopher Loy, Eng., Bechtel Power Corporation, Frederick, Maryland, USA

ombined cycle power plants have become the cleanest and most efficient form of electrical power generation using fossil fuels. The combined cycle plant has evolved and all manufacturers have made strides in developing better and more cost-effective products such as gas turbines, steam turbines, heat recovery steam generators (HRSGs), balance of plant (BOP) and other equipment. A major reason for these improvements is the introduction of G and H gas turbine technologies, where an inseparable thermodynamic and physical link exists between the primary and secondary power generation systems by using steam instead of air in a closed loop to perform cooling.

However, to ensure successful and reliable operation, all of the above components must be integrated harmoniously. This presents a challenge for engineering, procurement and construction (EPC) contractors such as Bechtel. Bechtel’s experience shows how continuous effort and collaboration between equipment suppliers, contractors and owners can ensure that complex projects are executed successfully.

Over the years, Bechtel has executed more than 96 projects using 170 gas turbines and more than 96 steam turbines in combined cycle applications. In the last five years alone, Bechtel has executed 30 projects, installing and starting up 56 gas turbines and 40 steam turbines.

Gas turbines

In the last five years, Bechtel has been involved in the engineering, procurement, and construction phases of several projects using ‘FX’ (generic designation of the latest models of F class technology FA, FB, FD, etc.) and G class technology.

The equipment selection process includes a technology review to verify the quality control for the engineering and manufacturing process, and the performance data offered by the suppliers must be normalized and correlated with the performance of the same type of gas turbine achieved in other projects. The analysis should also include a comparison with other manufacturers’ competitive products.

One of the major issues related to the evaluation process of gas turbine performance in combined cycle plants is the requirement to evaluate, in addition to power and heat rate, the exhaust flow and temperature. All four parameters are heavily interdependent.

For example, a gas turbine that exceeds the guaranteed power output usually has better-than-expected component efficiency, and as a result, the exhaust temperature is lower. A lower turbine exhaust temperature has a negative impact on HRSG steam production and therefore a lower steam turbine output.


Figure 1. Comparison between guarantees and test results for gas turbine power output and heat rate
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It is virtually impossible for any given gas turbine to meet all four guarantees concurrently. When establishing the HRSG and steam turbine design conditions, it is good engineering practice to allow some variability for the gas turbine exhaust flow and temperature values. This way, the design could accommodate either shortfalls or better-than-guaranteed exhaust energy of the gas turbine. Therefore, more realistic and competitive values can be predicted for combined cycle performance.

Bechtel’s experience indicates that owners are still well advised to engage an experienced EPC contractor in purchase or reservation agreements for gas turbines. Such collaboration is crucial for projects involving advanced gas turbines. The EPC contractor can verify that the terms and conditions essential to managing project design, construction, and commissioning are adequately covered in these agreements. An EPC contractor with direct experience in this type of gas turbine can also work with the equipment supplier and the owner to ensure that the scope is complete and all interfaces are well defined. It can also add value to an owner’s reservation agreement by assuring adequate coverage of performance test tolerances and measurement uncertainty, and evaluating technology risk issues, for example.

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EPC contractors are responsible for the startup of a plant and are subject to penalties if successful, timely operation is not demonstrated. Therefore EPC contractors must evaluate the cost-effectiveness of the provisions that facilitate rapid startup. The problem becomes more critical for merchant power plants, where cycling and part-load operation are often required. Depending on the specific completion requirements and previous experience with a particular type of advanced gas turbine, the startup schedule must allow time for unforeseen events, for example:

  • Dual-fuel operation: advanced gas turbines generally use natural gas as the primary fuel. Dual-fuel capacity with oil as the alternative fuel is an option that attracts many owners but it adds complexity to already very complicated combustion systems and controls. A challenge is to achieve a switchover from one fuel type to another at a reasonably high power level. For some manufacturers, this activity took longer than expected, therefore affecting commissioning schedules.
  • Combustion system commissioning and tuning: to meet emissions requirements, advanced gas turbines operate with dry low NOx (DLN) combustion systems. The combustion system operation from diffusion mode at low loads to full premix mode at base load takes place in several complicated steps, requiring very close control of fuel flow and exhaust temperature. The process is sensitive to ambient conditions, combustion associated instabilities, and even manufacturing or assembly tolerances. Currently, each gas turbine is individually adjusted to meet the performance guarantees and emissions requirements without combustion oscillations. The execution schedule is extended to perform a water wash of the compressor prior to the tuning process and to install and remove temporary instrumentation for full-blown performance testing of the gas turbine. Because emissions limits must be met at all ambient conditions, adjustments made in the field might modify the performance correction curves for ambient temperature.

Steam turbines

In the last ten years, steam turbines in combined cycle applications have evolved from small 80 MW, two-pressure, non-reheat configurations to large, multiple-admission-pressure, reheat turbines in the 350 MW range.

Although steam turbine manufacturers typically employ a modular building block system with standardized components and turbine parts, the steam blade path is individually designed for each application to achieve high efficiency. Even small blade path improvements can translate into large savings in operation.

The steam turbine equipment selection process includes an independent technology assessment of the equipment’s operating history and quality control for the engineering and manufacturing processes. As with gas turbine performance, the steam turbine performance offered by the original equipment manufacturers for a specific project is normalized and reconciled with past performance of the same equipment in a similar configuration on other projects.


Figure 2. Steam turbine module efficiencies for duct-fired and unfired cases
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The steam turbine startup flexibility and commissioning time play a significant role in startup of the entire combined cycle plant. Due to higher fuel costs and market conditions, combined cycle plants are dispatched as intermediate-duty units rather than base load. Achieving a fast and reliable startup requires careful design and integration of the steam turbine, gas turbine, and BOP requirements. On one hand, the steam turbine supplier must provide flexible steam turbine startup parameters while maintaining reasonable constant life consumption, controlling low cycle fatigue by monitoring the maximum wall temperature differences and permissible ramp rates. On the other hand, an EPC contractor has to assist the process by optimizing the performance of the auxiliary equipment. Examples of such measures include means to improve the heat retention after shutdown, design of advanced water treatment systems capable of achieving steam purity quickly, provisions for additional warmup lines, and use of an auxiliary steam boiler to reach desired condenser vacuum more rapidly.

Some gas turbine-HRSG-steam turbine combinations are better suited for rapid cycling than others. It is not possible to accurately predict combined cycle power plant startup times from the startup curves for each piece of equipment. Each supplier will make assumptions on startup conditions that are most favourable for its individual piece of equipment. These assumptions are usually different, often conflicting, and sometimes incompatible.

In Bechtel’s design process, the overall startup integration is considered early in the project, and consistent requirements are provided at the bid phase for each supplier. Stringent steam temperature matching requirements are requested as these requirements vary between steam turbine suppliers and cannot always be met by every gas turbine-HRSG combination. The gas turbine operating at low loads has a limited capability to maintain high exhaust gas temperature, and this capability varies between manufacturers. Combining a gas turbine with limited ability to control exhaust gas temperatures at low loads with a steam turbine that has very strict temperature requirements at startup can lead to unacceptably long startup times. The HRSG may not be capable of accommodating the gas turbine and steam turbine requirements simultaneously without significant bypass capability.

HRSG

The HRSG, as part of the power island, must also match the aggressive developments implemented in gas turbines in terms of dramatic increases in their exhaust flow and temperatures. The new HRSG designs are required to cope with incremental jumps in gas turbine sizes and extensive duct firing and cycling operations.

To successfully meet the technical and economic challenges, many HRSG manufacturers offer the choice between a horizontal and a vertical flue gas path. An important lesson learned as an EPC contractor was to establish early on in the project the selection criteria for HRSG type – vertical or horizontal. Both types of HRSG designs have advantages and limitations.

The following comparison between vertical and horizontal arrangements assumes equal values for the pressure levels, steam parameters, output, and efficiency:

  • Plot plan area: based on the most recent layouts of inlet ducts, the difference in footprint does not exceed ten per cent, with the vertical generally requiring less space than the horizontal HRSG.
  • Forced circulation versus natural circulation: the vertical tube arrangement in a horizontal HRSG permits the fluid to flow unassisted by natural circulation. Vertical HRSGs have only recently abandoned the use of forced circulation pumps that impose a parasitic load and detract from cycle efficiency. Some manufacturers still require an auxiliary pump for startup, which could negatively affect HRSG reliability and maintenance cost.
  • Mode of operation: HRSG operation is defined as ‘base load’ or ‘cycling’. Most combined cycle plants in the US are operating in two-shift cycling mode with daily overnight shutdown and large load variations. This pattern, requiring 150 to 200 shutdowns per year and numerous load swings, raises the potential for damage to HRSG components from thermal and pressure cycling. In cycling operational mode, the mechanical design features can significantly affect HRSG reliability.

    The design of the horizontal HRSG, using top-supported harps, makes the assembly of tubes and headers more rigid than in the design of the vertical HRSG. The operating experience of the cycling units has identified numerous failures. However, the manufacturers have been sensitized to problems and continue to improve the design.

    Some of the problems are related to the interaction of different components. For example, as a result of the exhaust gas turbine rapid acceleration, many occurrences of low cycle fatigue have been observed, particularly in HP and IP superheaters, where the temperature increases by more than 300°C within five minutes. Corrosion-related problems are also typical for cycling operation. The design of the vertical HRSG offers an arrangement where the tubes can expand freely in their longitudinal axes without being blocked by headers. This makes the vertical HRSG somehow more reactive during cycling operation and allowing it to achieve a faster startup time.
  • Supplementary firing: many owners of combined cycle plants want the capability to boost power output at short notice using extensive supplemental firing that can increase the HRSG steam production output up to 100 per cent. Duct firing burners can be located at the HRSG inlet or inter-staged between the heat transfer surfaces. Intensive use of duct burners in horizontal boilers leads to corrosion and overheating of flame holders. In cases of extensive duct firing and/or increased gas turbine exhaust flow, a detailed analysis of the design through analytical and experimental models must be conducted to ensure proper temperature distribution through flow control vanes. The supplementary duct firing affects the length of the horizontal HRSG and the height of the vertical HRSG. Intensive duct firing affects drum sizing, thickness, and tube and liner material selection. These factors also influence HRSG startup times.

Bechtel has erected and commissioned more than 30 horizontal triple-pressure reheat HRSGs. An important aspect of the execution process was to provide technical specifications that were very detailed and properly designed for each boiler type. It was also paramount to develop performance test procedures ahead of time that could demonstrate how the equipment meets its contractual guarantees. To meet tight schedules, Bechtel also has developed in-house expertise capable of solving HRSG-related problems in the execution phase, particularly for commissioning, startup, and integrated control issues.

HRSG start-up

The initial commissioning and startup of an HRSG are critical steps that influence the entire plant operation. During this phase, many issues relating to water chemistry, cleaning process, initial control schemes, and thermal stress could lead to persistent operation and maintenance problems later on. For a horizontal HRSG, there is an increased risk for failure of casings and expansion joints, which are more numerous and larger than in a vertical HRSG.

During startup and purge of the horizontal HRSG, condensate is formed in the first stages of the superheater/reheater. In this setup, gravity and steam flow are two forces working in opposite directions, and due to limited drainage, the condensate remains for a longer time, blocking tubes and creating additional temperature difference and potential quench cooling.

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This is one of the main causes of low-cycle fatigue. Due to steep angles of the horizontal HRSG inlet duct, the purging air does not clear the upper portion of the unit, leaving potentially explosive gases with a lighter-than-air density to remain in the boiler. In the vertical setup, the potentially explosive gases will evacuate through the stack at the top of the HRSG.

Cascaded blowdown systems are not suited for rapid cleanup during commissioning and startup. The drive for higher efficiencies and better heat rates often tempts HRSG designers to recover blowdown energy by cascading HP drum blowdown to the IP drum. On advanced combined cycle plants based on G class gas turbine technology, the blowdown design sometimes goes a step further by cascading HRSG drum blowdown to kettle-boiler-type rotor air coolers. The ability to rapidly achieve steam chemistry during startup or commissioning through high boiler blowdown is compromised by the cascaded design. In modern combined cycle plant designs, HP drum flow rates can be 5 to 10 times higher than IP drum flow rates. Thus, high HP blowdown (3 to 5 per cent) will represent a significant fraction of the total IP flow and cannot be removed by any reasonable blowdown rate from the IP drum. This problem is even worse for heavily fired units or units with blowdown cascaded to kettle boilers. If cascaded blowdown is used during normal operation, separate startup blowdown connections are required from each drum direct to the blowdown tank. If these are not available, significant startup delays can be experienced during waits to achieve steam chemistry, especially after prolonged shutdowns.

Reference

Justin Zachary, Ph.D, James Bauerschmidt, P.E., and Christopher Loy, Eng. “Combined Cycle Power Plants Integration – Lessons Learned” Paper presented at POWER-GEN Europe 2005, Milan, Italy.