Steam and gas turbines

A combined cycle power plant or gas and steam power station is a power plant in which the waste heat from a gas turbine power station is used in a steam power plant. The gases given off by the turbine serve as a heat source for a downstream waste heat boiler, which in turn acts as a steam generator for the steam turbine. A liquid is evaporated on the one side. The speed of the steam is high enough to drive a turbine. The steam then condenses and is pumped back into the cycle as a liquid with reduced volume. There is a large pressure difference between the evaporator and the condenser which stimulates the steam flow in the turbine.

Electricity is generated in the combined cycle power plant with one to four gas turbines and one steam turbine, based either on a multi-shaft system, where the turbines each drive one generator, or on a single-shaft system where one gas turbine drives the generator with the steam turbine (which can be uncoupled with a synchronous self-shifting clutch). The hot gases given off by the gas turbines are used in a waste heat boiler to generate steam. The steam is then expanded by means of a conventional steam turbine process (where the pressure and temperature increase).

If, in addition to using the heat given off by the gas turbine, there is an increase in the steam output and therefore in the electrical power of the steam turbine through additional firing of the steam boiler, this is referred to as a combined process. Gaseous or liquid fuels like natural gas, biogas or heating oil are used to operate the gas turbine. Other fuels can also be used to operate the burner in the steam boiler.

The gas turbine (gas expansion turbine) consists of an upstream compressor and an intermediate combustion chamber. The steam turbine consists of a heat source, a steam generator, a turbine with a generator and a unit for cooling the hot steam.