Operational Flexibility Enhancements of Combined Cycle Power Plants
Dr. Norbert Henkel, Erich Schmid and Edwin Gobrecht
Source: Siemens AG, Energy Sector
High operational flexibility – the ability of a power plant for fast start-up and to adjust load output fast and predictable to changing market requirements – is an essential prerequisite to ensure economic success in a liberalized market. The paper describes upgrade opportunities for combined cycle power plants, which were originally built as base load plants and are now due to changing market conditions and fuel prices forced to operate as peak load plants or as cycling plants with daily start-up.
Major factors limiting the load output of an existing combined cycle power plant are the allowed pressure and temperature transients of the steam turbine and the heat recovery steam generator waiting times to establish required steam chemistry conditions and warm-up times for the balance of plant and the main piping system. Those limitations also influence the fast start-up capability of the gas turbine by requiring waiting times compared to a simple cycle start-up.
The authors’ company provides solutions to address all these limitations. For example, the use of final stage heat recovery steam generator attemperators and associated controls to adjust steam temperatures to steam turbine requirements independent from gas turbine load; stress monitoring systems for the thick walled components in the steam turbine and the heat recovery steam generator with different start-up modes for flexible use of component life; optimized main steam line warm-up systems; condensate polishing systems and flexible steam purity requirements; to name only a few.
The steam turbine start-up is modified to allow an early roll off and fast loading – the so called “start-up on the fly”. Here, the steam turbine is rolled off with the very first “cold” steam produced in the heat recovery steam generator with full pressure and temperature transients. With these upgrades a start-up time of less than 40 minutes is possible for a 400 MW combined cycle power plant after an overnight shutdown.
Existing power plants must cope with the challenges of liberalized and deregulated markets. Further on, the compliance with increasing environmental requirements is gaining more importance. Looking closer to the requirements of a power plant in modern power markets, operational flexibility becomes a major topic.
Operational flexibility comprises:
Fast start-up and shutdown
Fast load changes and load ramps
High start-up reliability and load predictability
Frequency control and ancillary services.
Drivers for this demand are risks like fluctuating fuel and electricity prices. Additionally, a flexible plant opens up new business opportunities like utilizing hourly and seasonal market arbitrage, participation in ancillary energy markets or peak shaving. An operating profile optimized for these market circumstances increases the economic value of the plant. Depending on seasonal load and the dispatch rank of the plant, driven by competition and fuel prices, it is likely that the plant will be partly operated as base load unit and partly as cycling unit over its lifetime. Therefore, a plant needs advanced cycling capabilities and highest efficiency at base load.
Fig. 1: View of a SCC5-4000F single shaft combined cycle power plant.
Typically, combined cycle power plants built in the 1990s and early years of the new
millennium were designed as base load plants with focus on highest efficiency and low initial cost. Due to the significant increase in gas prices and changing market conditions in
deregulated markets many combined cycle power plants are now being operated in
intermediate or even daily cycling mode. The current plant and control design may not fully support these cycling conditions and the requirements on start-up performance and times. The authors’ company provides upgrade solutions for existing plants to meet these market requirements which are based on the latest and proven cycling plant developments for new units.
3 Plant Design for Fast Cycling
Most of the existing combined cycle power plants were initially designed for base load
operation due to low fuel prices in the nineties resulting in low electricity costs. Nowadays, many operating combined cycle plants are shifted to intermediate load and new plants are specified for cycling load regimes because of today’s high gas prices. Therefore, features for high operational flexibility like short start-up and shut-down times are emphasized by
3.1 Cycling Plant Design Features
As an answer to the changed market requirements, Siemens has developed a fast start-up concept and implemented it into the reference power plant design for new units. With this design, a reduction of the start-up time of more than 50% can be achieved after an overnight shutdown. Additionally, the start-up times after a weekend outage shutdown (64 hours) and an extended outage shutdown (more than 120 hours) were also significantly reduced.
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