Method for operating a combined cycle power plant and corresponding combined cycle power plant

A preheating system for a heat recovery steam generator comprising a low-pressure economizer and a drum type evaporator, the preheating system comprising:

• a feed line configured to receive condensate from a steam turbine and direct feedwater to the low-pressure economizer of the heat recovery steam generator, the low-pressure economizer comprising a conduit extending between an inlet and an outlet of the low-pressure economizer, the low-pressure economizer comprising a hot source downstream from an intermediate source, and the intermediate source is disposed between the inlet and the outlet;
• a first control valve having a first position and a second position, wherein a first channel of the first control valve is fluidly coupled between the intermediate source of the low-pressure economizer and the feed line in when the first control valve is in the first position, a first line fluidly couples the first channel to the intermediate source between the inlet and the outlet, a second channel of the first control valve is fluidly coupled between the hot source of the low-pressure economizer and the feed line when the first control valve is in the second position, a second line fluidly couples the second channel to the hot source, and the low-pressure economizer is separately coupled to the feed line, the first line, and the second line;
• and
• a bypass comprising a second control valve and coupled between the outlet of the low-pressure economizer or the drum type evaporator and the feed line, wherein:
• if the temperature of the condensate is equal to or above a preset temperature, both the first control valve and the second control valve are switched off;
• or
• if the temperature of the condensate is lower than the preset temperature:
• the first control valve is turned to the first position to direct a first fluid from the intermediate source to heat the condensate;
• and
• if the temperature of the condensate is lower than the preset temperature after heating exchange between the first fluid from the intermediate source and the condensate:
• the first control valve is turned to the second position to direct a second fluid from the hot source to heat the condensate;
• and
• if the temperature of the condensate is lower than the preset temperature after heating exchange between the second fluid from the hot source and the condensate, the second control valve is opened to direct at least part of the condensate from the feed line to the outlet of the low-pressure economizer.

A system, comprising:

• a preheating system, comprising:
• a feed line configured to receive a condensate from a steam turbine and direct a feedwater to a low-pressure economizer of a heat recovery steam generator, wherein the low-pressure economizer comprises a conduit extending between an inlet and an outlet of the low-pressure economizer, the low-pressure economizer comprises a hot source downstream from an intermediate source, and the intermediate source is disposed between the inlet and the outlet;
• a first control valve having a first position and a second position, wherein a first channel of the first control valve is configured to be fluidly coupled between the intermediate source of the low-pressure economizer and the feed line when the first control valve is in the first position, a first line is configured to fluidly couple the first channel to the intermediate source between the inlet and the outlet, a second channel of the first control valve is configured to be fluidly coupled between the hot source of the low-pressure economizer and the feed line when the first control valve is in the second position, a second line is configured to fluidly couple the second channel to the hot source, and wherein the feed line, the first line, and the second line are configured to be separately coupled to the low-pressure economizer;
• and
• a bypass comprising a second control valve, wherein the bypass is configured to be coupled between the outlet of the low-pressure economizer or a drum type evaporator of the heat recovery steam generator and the feed line, wherein:
• if the temperature of the condensate is equal to or above a preset temperature, both the first control valve and the second control valve are switched off;
• or
• if the temperature of the condensate is lower than the preset temperature:
• the first control valve is turned to the first position to direct a first fluid from the intermediate source to heat the condensate;
• and
• if the temperature of the condensate is lower than the preset temperature after heating exchange between the first fluid from the intermediate source and the condensate:
• the first control valve is turned to the second position to direct a second fluid from the hot source to heat the condensate;
• and
• if the temperature of the condensate is lower than the preset temperature after heating exchange between the second fluid from the hot source and the condensate, the second control valve is opened to direct at least part of the condensate from the feed line to the outlet of the low-pressure economizer.