Combined Cycle Power Plant

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Power generation is most important to any country?s commerce and industry. As the power demand increases, more efficient power generation methods are developed. The increasing environmental concern mainly caused due to carbon emission also leading to emphasis on environmental friendly power generation techniques.

Combined cycle power generation is gaining popularity all over the world. In general working Combined Cycle efficiencies are greater than 50 percent on a lower heating value and Gross Output basis. Most combined cycle systems, especially the larger units, have peak, steady state efficiencies of 55 to 59 percent. In electric power generation a combined cycle is an assembly of heat engines that function in tandem off a common source of heat, converting it to mechanical energy, which in turn drives electrical generators.

But now the requirement to increase the efficiency of a Combined Cycle Power Plants has come up with the increase in demand of power. In this context, the methods to increase efficiency of a CCPP are discussed.


Basic Principle
The basic principle is that, the exhaust of one heat engine is provided as the heat source for another, thus producing more useful energy from the same heat, thus increasing the system's overall efficiency. This method works better because heat engines are only able to capitalize a portion of the energy their fuel generates (less than 50%).The reidual heat (e.g., hot exhaust fumes) from combustion is generally wasted. Combining two thermodynamic cycle results in improved overall efficiency, thus reducing fuel costs.
In a combined cycle power plant, or combined cycle gas turbine plant, a gas turbine generator generates electricity and energy in the exhaust is used to produce steam, which rotates a steam turbine to produce additional electricity. This last step increases the efficiency of electricity generation.
Now, to enhance the efficiency of a CCPP, various methods have been found out in such a short period of time after the implementation of the CCPP?s. Some of the methods are:

  • Quick Start-up: The greater the load change amount is, the faster the recommended additional startup timing. This tendency improves if the load change is quicker.
  • Inlet air temperature: Cooling or decreasing the inlet air temperature increases the power.
  • Use of HRSG: Optimization of Heat Recovery Steam Generators in combination with the use of gas turbine reheats and gas to gas recuperation increases the efficiency of the plant by 65 percent.
  • Pressure Stages: Increase in the number of pressure stages, also increases the efficiency of the plant.

All the above methods are fulfilled by using a new technology, which Siemens Power Generation has come up with, is the Complementary Fired Combined Cycle Power Plant.
All These methods will be discussed in detail later.


Basic layout
Power Plant General Layout

The whole site is separated into four part, they are main power block building area, switch gear equipment area, plant front area and auxiliary production area.
Main power block building area is arranged in the central area, it consists of the steam turbine house, gas turbine generation unit, gas turbine generation unit gas turbine generation unit and electrical building. The northern area of the plant is reserved for extension purposes. Switch gear equipment area consists of 220kV GIS and transformer yard. The rain water drainage pump house, switch gear equipment area and starting boiler house are arranged to the east to the main building area. Plant front area is arranged to the south of main building area, this area consists of the following items: production overall building, bathroom, mess hall, warehouse X, V and maintenance building. And gas regulator station is arranged to the east of the plant front area. Auxiliary production area is arranged in the western side of the pant, this area consists of combined water treatment station, industrial waste water treatment station, sewage water treatment, and chemical water treatment station and hydrogen generation plant.



Main features

Combined cycle power generation combines two cycles for the operation, the gas turbine cycle and steam turbine cycle. In the gas turbine power plant, compressed air and natural gas undergo combustion .The result is high pressure gas that drives the gas turbine which in produces electricity. Although it is fast in starting up and clean, the gas turbine power plant has a low thermo efficiency of about 25 to 30%. Most of the energy is wasted in the gas turbine exhaust. The combined cycle power generation makes use of the advantages of the high temperature (1100 to 1650?C) gas turbine cycle and the lower temperature (540 to650?C) steam turbine cycle. The hot exhaust gas in the gas turbine, instead of being released into atmosphere, is captured and diverted to the steam turbine where steam is heated by the exhaust to rotate the turbine.

A combined cycle power plant comprises of two main parts the gas turbine plant and the steam turbine plant. In the gas turbine plant the atmospheric air enters through the air compressor and into the combustion chamber where natural gas is added. Combustion takes place and the hot gas drives the turbine, which in turn rotates the generator and produces electricity. The hot flue gas from the gas turbine enters heat recovery boiler or heat recovery steam generator where it heats up the steam. The superheated steam is used to rotate the steam turbine which in turn drives the generator to generate electricity. The exit steam of the steam
Turbine goes into a condenser and then back to the heat exchanger where the cycle repeats. There are various categories of combined cycle power generation, some of them are combined cycle with supplementary firing, combined cycle with multi pressure steam cycle, combined cycle with feed water heating, combined cycle with regeneration and combined cycle for nuclear power plants.


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