Combined Cycle Power Plants – The Basics.

First step is the same as the simple cycle gas turbine plant. Burning of gas, the thrust rotating a gas turbine and the coupled generator produces Electricity. In the second step the hot gases leaving the gas turbine passes into boiler to produce steam. This boiler is called the ‘Heat Recovery Steam Generator (HRSG). The steam then rotates the steam turbine and coupled generator to produce Electricity. The hot gases leave the HRSG at around 140 degrees centigrade and are discharged into the atmosphere. The steam condensing, and water recycling system is the same as in the steam power plant. The attached animation shows the working of the CCPP.

First step is the same as the simple cycle gas turbine plant. Burning of gas, the thrust rotating a gas turbine and the coupled generator produces Electricity. In the second step the hot gases leaving the gas turbine passes into boiler to produce steam. This boiler is called the ‘Heat Recovery Steam Generator (HRSG). The steam then rotates the steam turbine and coupled generator to produce Electricity. The hot gases leave the HRSG at around 140 degrees centigrade and are discharged into the atmosphere. The steam condensing, and water recycling system is the same as in the steam power plant.

The attached scheme shows the working of the CCPP.

Roughly the steam turbine cycle produces one third of the power and gas turbine cycle produces two thirds of the power output of the CCPP. Normally there will be two generators, one driven by the gas turbine and one driven by the steam turbine. There are also systems with one generator connected through a single shaft to both the gas turbine and steam turbine.

Even though this system is having the best efficiency, it has limitations. The gas turbine can only use Natural gas or high grade oils like aviation or diesel fuel. Because of this the combined cycle can be operated only in locations where these fuels are available and cost effective.

Developments for gasification of coal and use in the gas turbine are in advanced stages. Once this is proven, Coal as the main fuel can also be used in the combined cycle power plant.