A combined cycle gas turbine (CCGT) plant is one in which the principles of a gas turbine power plant and a steam power plant are combined. It involves a gas turbine generating heat, which in turn is converted into steam for the turbine in a downstream waste heat boiler. The gas turbine is connected to the generator that produces the electrical energy. A higher degree of efficiency – up to 60 percent – can be achieved by combining these two principles, with the fuel being used particularly efficiently.
Following expansion via the gas turbine, the flue gas still has a temperature of between 550 and 620°C. The hot flue gas is then used to generate electrical energy. For this purpose, it is passed through a heat recovery steam generator. This is a heat exchanger that transfers the energy present in the flue gas to the water or steam from the steam turbine process. Once almost all of the absorbed energy has been extracted from the steam, it must be recondensed in order to close the cycle. The condensate is then fed back to the heat recovery steam generator at high pressure via the condensate and feedwater pumps. From there, flue gas is discharged into the environment via the chimney.