In this article we will discuss about the elements of a simple gas turbine power plant.
A simple gas turbine power plant essentially consists of the reaction type non-condensing turbine, a compressor mounted on the same shaft and coupled to the turbine, the combustion chamber, an alternator coupled to the turbine itself and auxiliaries such as starting motor, auxiliary lubrication pumps, oil system, fuel system and duct system etc.
The compressor used in the gas turbine power plant is usually of rotary type. The air is sucked in by the compressor from the atmosphere through the filter, which removes the dust from the air. The rotary blades of the compressor push the air between the stationary blades to raise its pressure to 4-5 atmospheres. Thus air is available at high pressure at the output of the compressor.
The combustion chamber consists of a vessel into which the air at high pressure from the compressor is fed. Fuel is injected into the combustion chamber and burnt in the stream of air supplied from the compressor. A part of the air is delivered ahead of the burning fuel in order to cool the very hot combustion products (at temperature of around 1,650°C) and bring them to a temperature around 800°C when these can be allowed to enter the turbine without causing damage to the first few rows of blades.
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The products of combustion comprising of mixture of gases at high temperature and pressure are passed through the gas turbine. These gases in passing over the turbine blades expand and thus result in motion of rotor and finally discharged to the atmosphere at a temperature of about 540°C.
Part of the power developed by the gases in passing through the turbine is used to run the compressor and auxiliaries and the rest is used to generate electrical energy driving the generator.
The alternator is coupled to the gas turbine, converts mechanical energy of the turbine into electrical energy and feeds it to the bus-bars through transformers, circuit breakers and isolators etc.
An electric motor used for starting the compressor (compressor is required to be started before starting of turbine) is mounted on the same shaft as that of the turbine. Once the unit starts, a part of mechanical power output of the turbine drives the compressor and there is no need of motor now. The motor is energised by the batteries.
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The gas turbine has to drive the compressor as well as the alternator. In some cases, these two functions of gas turbines are separated and two different turbines are employed—a high pressure turbine drives the compressor and a low pressure turbine drives the alternator. This arrangement has the advantage that the speed of the power turbine (the turbine driving the alternator) can be kept constant at synchronous speed while the speed of the turbine driving the compressor can be varied according to the output required.
The overall efficiency of such power plants called the open cycle plants is very low as a greater part (around 65%) of the power developed by the turbine is used in driving the compressor. The fact that the hot gases from the combustion chamber are cooled to a safe temperature before admitting it to the turbine also reduces the efficiency. The overall efficiency of the plant, owing to above reasons seldom exceeds 20%.
Merits and Demerits of Open Cycle Gas Turbine Power Plants:
Open cycle gas turbine power plants are simple in design, small in size (lower volume space per kW output), light in weight (lower specific weight), do not need any warm up time, possibility of great variety in the system arrangement and self-contained. The combustion chamber can be designed to burn any of the hydrocarbon fuels including solid fuels.
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However, the open cycle gas turbine power plants have drawbacks of higher air rate in comparison to other prime movers, sensitivity to changes in component efficiencies (due to dust or dirt deposited on the compressor blades, by carbon or other foreign deposits from combustion in the combustion chamber, turbine, and regenerator) and erosion and corrosion of turbine blades.