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Handbook for Cogeneration and Combined Cycle Power Plants, Second Edition

Meherwan P. Boyce, P.E.
Meherwan P. Boyce, P.E.
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ASME Press
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The performance analysis of the new generation of gas turbines in combined cycle operation is complex and presents new problems, which have to be addressed. The new units operate at very high turbine firing temperatures. Thus, variation in this firing temperature significantly affects the performance and life of the components in the hot section of the turbine. The compressor pressure ratio is high, which leads to a very narrow operation margin, thus making the turbine very susceptible to compressor fouling. The turbines are also very sensitive to back pressure exerted on them by the heat-recovery steam generators. The pressure drop through the air filter also results in major deterioration of the performance of the turbine.

The performance of the combined cycle is also dependent on the steam-turbine performance. The steam turbine is dependent on the pressure, temperature, and flow generated in the heat-recovery steam generator, which, in turn, is dependent on the turbine firing temperature and the air mass flow through the gas turbine. It is obvious that the entire system is very intertwined and that deterioration of one component will lead to off-design operation of other components, which, in most cases, leads to overall drop in cycle efficiency. Thus, determining component performance and efficiency is the key to determining overall cycle efficiency.

If a life cycle analysis were conducted, the new costs of a plant are about 7% to 10% of the life cycle costs. Maintenance costs are approximately 15% to 20% of the life cycle costs. Operating costs, which essentially consist of energy costs, make up the remainder, between 70% and 80 % of the life cycle costs, of any major utility plant. Thus, performance evaluation of the turbine is one of the most important parameters in the operation of a plant.

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