The first step is the same as the simple cycle gas turbine plant. An open circuit gas turbine has a compressor, a combustor, and a turbine. For this type of cycle the input temperature to the turbine is very high. The output temperature of flue gases is also very high.
This high enough to provide heat for a second cycle, which uses steam as the working medium, i.e. thermal power station.
This air is drawn though the large air inlet section where it is cleaned, cooled, and controlled. Heavy-duty gas turbines are able to operate successfully in a wide variety of climates and environments due to inlet air filtration systems that are specifically designed to suit the plant location.
Under normal conditions the inlet system has the capability to process the air by removing contaminants to levels below those that are harmful to the compressor and turbine.
The purified air is then compressed and mixed with natural gas and ignited, which causes it to expand. The pressure created from the expansion spins the turbine blades, which are attached to a shaft and a generator, creating electricity.
In the second step the heat of the gas turbine’s exhaust is used to generate steam by passing it through a heat recovery steam generator (HRSG), with a live steam temperature between 420 and 580 °C.
Heat Recovery Steam Generator
In the Heat Recovery Steam Generator, highly purified water flows in tubes and the hot exhaust gases heat it to produce steam. 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 then discharged into the atmosphere.
The steam condensing and water system is the same as in the steam power plant.
Typical Size and Configuration of CCGT Plants
The combined-cycle system includes single-shaft and multi-shaft configurations. The single-shaft system consists of one gas turbine, one steam turbine, one generator, and one Heat Recovery Steam Generator (HRSG), with the gas turbine and steam turbine coupled to the single generator on a single shaft.
Multi-shaft systems have one or more gas turbine generators and HRSGs that supply steam through a common header to a separate single steam turbine generator. In terms of overall investment, a multi-shaft system is about 5% higher in costs.
The primary disadvantage of multiple stage combined-cycle power plants is that the number of steam turbines, condensers and condensate systems—and perhaps the cooling towers and circulating water systems—increases to match the number of gas turbines.
Efficiency of CCGT Plant
The steam turbine cycle produces roughly one-third of the power and the gas turbine cycle produces two-thirds of the power output of the CCPP. By combining both gas and steam cycles, high input temperatures and low output temperatures can be achieved. This adds efficiency to the cycles, because they are powered by the same fuel source.
To increase the power system efficiency it is necessary to optimize the HRSG, which serves as the critical link between the gas turbine cycle and the steam turbine cycle, with the objective of increasing the steam turbine output. HRSG performance has a large impact on the overall performance of the combined-cycle power plant.
The electrical efficiency of a combined-cycle power station may be as high as 58 percent when operating at continuous output with ideal conditions. As with single-cycle thermal units, combined-cycle units may also deliver low temperature heat energy for industrial processes, district heating, and other uses. This is called cogeneration and such power plants are often referred to as a Combined Heat and Power (CHP) plant.
The efficiency of CCPT is increased by Supplementary Firing and Blade Cooling. Supplementary firing is arranged at the HRSG. In the gas turbine a part of the compressed air flow bypasses and is used to cool the turbine blades. It is necessary to use part of the exhaust energy through gas to gas recuperation. Recuperation can further increase the plant efficiency, especially the when gas turbine is operated under partial load.
Fuels for CCPT Plants
The turbines used in Combined-Cycle Plants are commonly fueled with natural gas, and is more versatile than coal or oil and can be used in 90% of energy applications. Combined-Cycle plants are usually powered by natural gas, although, fuel oil, synthesis gas, or other fuels can be used.