Gas Power Plants
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In gas power plants natural gas is used to generate electricity. It is the most efficient way of converting fossil fuels into electricity and the emission of green house gases is relatively low. It has few advantages over the steam power plants:
- It is very simple as it does not contain the boiler, condenser and waste disposal system
- It has an ability to start and take load quickly
Cross section of a Gas turbine
Simple Gas turbine plants
A simple gas turbine plant consists of the following components.
- Combustion chamber
- An air compressor
- A turbine
Apart from these components several auxiliary for lubrication, speed control, fuel supply and starting are also a part of a gas turbine plant.
Combustion chamber is an airtight chamber inside which the whole setup including the compressor and turbine is present. It has two openings, one for air intake to the compressor and other for the discharge of exhaust gases.
Air compressor is used to supply compressed air to the combustor where a continuous flame is maintained.
Turbine is used to convert the heat and pressure of the burned air to mechanical rotation. The turbine is coupled to the generator which converts the mechanical energy into electrical energy.
Compressed air from the compressor is delivered to the combustion chamber where the fuel oil or gas is also supplied in steady flow and a continuous flame is maintained. Inside the combustion chamber spark is produced. This spark makes the original ignition. The heated air in the combustion chamber expands when ignited. The burned gases at high pressure enter the turbine through a nozzle and produces high velocity. This kinetic energy of the air is delivered to the blades of the turbine. A part of this energy is used to drive the compressor and the remaining is used to drive the generator. The pressure of the gases drop as it flows through the turbine and the gases finally exhausts into the surrounding air at atmospheric pressure.
The efficiency of simple gas turbine is very low. There are three methods to increase of gas turbines, they are:
It is the process of recovering waste heat from the high temperature exhaust gases of a gas turbine. The device used for extracting the heat energy from the heated gas is called regenerator or heat exchanger. The effectiveness of a regenerator is transferring all the energy available in the exhaust gases back to the inlet air
Reheaters can be used in multistage turbines so that partially expanded high temperature gases can be expanded further to produce additional work. There is no need of reheaters in a one turbine plants. The usage of reheaters in a multistage system improves the efficiency of the system.
In a gas turbine plant the compressor consumes very high energy and therefore two or more compressors provided with intercoolers can be used. The power required to run the compressor could be reduced because of reduction in the volume of air cooled. The number of stages of compressor are decided by based on the cost and energy saving. Inter-cooling results in the enhancement of thermal efficiency, air rate and work ratio. Therefore overall size of the power plant is reduced for same capacity.
Types of Gas Power Plants
Open cycle Gas Power plants
Open cycle is the simplest form of gas turbine plants. In this plant atmospheric air is taken in, compressed, heated at constant pressure and expanded in the turbine. The gases coming out of the turbine exhausted back to the atmosphere.
Closed cycle Gas Power plants
In this type of gas turbine plants the air or suitable gas is given without directly burning it. The exhausted air from the turbine is circulated again and again.
Advantages of gas turbine power plants over steam power plant
- Gas turbine system is compact and requires less space compares to steam power plants of same capacity.
- Less installation time.
- No condenser maintenance is required.
- Easy to control.
- The lubrication system used in steam power plants is very simple.
- The fuel consumption is low during starting and shutting down.
- The exhaust is clean and there is no stack required.
- No water requirement.
- Higher efficiency
- The capital cost is comparatively smaller than steam power plants.