Spain, the fifth-largest electricity market in the European Union, expects annual demand growth of about 3.5% over the next five years. At the same time, the country is committed to a 20% reduction in its CO2 emissions by 2012. Meanwhile, Spain’s 100% merchant power market means that producers need maximum flexibility from their plants.
In 2005, HC Energía—a combination natural gas and electric utility that became part of Portugal’s EDP Group the prior year—awarded Alstom a contract to build a 400-MW combined-cycle plant adjacent to the utility’s existing Castejon 1 unit.
An important requirement for HC Energía was operational flexibility—the ability to operate in baseload or part-load mode or intermittently, with fast start-up and shutdown times. Such flexibility is especially important in the region of Navarra, where Castejon is located. Navarra now has several wind farms whose intermittent output must be backed up by fossil-fueled capacity—but not by pure baseload plants, as is the case in the U.S. Spain uses combined-cycle plants in the region to take up the slack. Accordingly, they must be able to operate at partial load when a lot of wind power is being produced but also be capable of ramping up to full load quickly when wind speeds fall.
Leveraging the GT design
Castejon 2 (Figure 1) is powered by a single combined-cycle system that Alstom calls the KA26-1 because one of its two prime movers is the company’s GT26 gas turbine (GT). The other is an Alstom STF15c steam turbine—a floor-mounted, two-casing, reheat unit that shares a shaft with the gas turbine.
1. Overachiever. High availability and reliability, as well as predictable O&M costs, were key design requirements for Castejon 2. Courtesy: Alstom Power
The third major subsystem of the KA26-1 is a heat-recovery steam generator (HRSG) that links the two turbines in a conventional way. Hot exhaust gases leaving the gas turbine at over 1,100F fire the HRSG, a triple-pressure, natural circulation unit with a horizontal internal arrangement.
However, it is the design of the GT-26 that makes the KA26-1 more fuel-efficient when it operates at less than full load. Each of the turbine’s two individually controlled combustor chambers has three variable guide vanes that can be manipulated to optimize the air flow through them for maximum efficiency and minimum emissions. Moving the vanes during low-load operation makes it possible to reduce air mass flow to 60% of the full-load level while maintaining the turbine’s exhaust temperature at the nominal level. Doing so ensures that the thermodynamic quality of overall combined-cycle combustion remains nearly constant. As a result, the system efficiency of the KA26-1 at 50% load, for example, is about 12% higher than that of a conventional combined-cycle power plant.
Castejon 2 uses the latest version of the GT26, which sports a higher power output and an improved operational range in part-load service. The KA26-1 has a net capacity of 424 MW and a net plant efficiency of 58.5% at ISO conditions.
The GT26 gives HC Energía fuel flexibility as well as operational flexibility. Normally, Castejon 2 runs on natural gas, but it can also automatically switch to oil—the backup fuel—if gas supplies are curtailed. The turbine can burn gas of various qualities, within certain parameters.
The shaft shared by the gas and steam turbines also drives Alstom’s hydrogen-cooled TOPGAS generator. During start-ups, the steam turbine is disconnected from the generator by activating a self-synchronizing clutch.
The final notable feature of Castejon 2’s design is an inlet fogging system that increases the GT26’s output by as much as 10 MW when ambient temperature is high and the plant requires additional cooling (Figure 2).
2. Versatile cooling system. Castejon 2’s cooling tower can operate either in wet mode or in hybrid (wet/dry) mode to eliminate troublesome plumes. Courtesy: Alstom Power
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