Combined-Cycle Generation Process and its working Principle

In the combines cycle power plants we use heat engines that’s work on same source of fuel, converts in to mechanical then into electrical by moving the electrical generator. In this phenomena we use exhaust of one engine used as a fuel of second which seems to be more useful fuel and enhance the overall efficiency of the system. This system is cost effective because we use fuel for heat engine and the waste (e.g., hot exhaust fumes)   produced from the combustion of the fuel used by combining the two thermodynamic cycles so that the overall efficiency improved.

 Brayton cycle is batter for the stationary plants and Rankine cycle commonly used for steam power plant.

Combined cycle power plant consist of following main component.

1:  Gas turbine.

2:  steam turbine.

3:  heat recovery steam generator (HRSG)

Gas turbine

The Gas Turbine essentially comprises a multi stage axial flow compressor section with movable inlet guide vane (IGV), two vertical silo type combustion chambers with several burners, and multi stage axial flow turbine section. The compressor draws in air from the atmosphere, compresses it and

then feeds the compressed air into the combustion chambers. The fuel (natural gas or diesel oil) is injected into the combustion chambers where it is burned together with compressed air. The resulting hot compressed combustion chambers gas flows from the combustion chambers into the turbine section an expands while it drives the turbine. The mechanical enginery of the GT is utilized to drive an air cooled generator as well as compressor section of the turbine. The grid is fed by the generator. The expanded exhaust gases from the GT are fed either to the bypass stack or into the HRSG for further use. The energy given up to the turbine comes from the reduction in the temperature and pressure of the exhaust gas. All these processes make up the Brayton cycle.

GAS TURBINE PRINCIPLE

 A gas turbine works on the basic principle of the Brayton cycle. A Brayton cycle consists of three main components: 

1: A gas compressor 

2:A mixing chamber 

3: An expander/Energy Extractor IDEAL BRATON CYCLE STEPS: 

1-2.Isentropic process - 2-3.Isobaric process - 3-4.Isentropic process - 4-1.Isobaric process

Steam Turbine

The ST is of dual casing condensing type and is operated in the sliding pressure mode from 100 % to 60 % output and in fixed pressure mode below this range. The turbine comprises a combined HP (type D30-

16) and LP section (type N30-25) in which the thermal energy of the steam generated by the HRSGs is converted to mechanical energy in order to drive an air cooled generator to produce electric power. The exhaust steam is condensed in a conventional condenser located behind the exhaust side to the LP-turbine and cooled by the circulating cooling water system which transfers the heat to the atmosphere via a wet cooling tower

Steam turbine principle

Steam turbine works on the principle of Rankine cycle. It has four basic components.

1.     Boiler

2.   Turbine

3.   Condenser

4.   Condenser feed pump

Heat Recovery Steam Generator

Hrsg is heart of combined cycle power plant and a critical link between gas turbine and steam turbine. Hrsg is an energy recovery heat exchanger that recovers heat from hot exhaust gases(1050oF) from gas turbine. It produces steam that can be used to drive the steam turbine in combined cycle power plant.