gas turbine

American International University-Bangladesh (AIUB)Gas Turbine Power Station*

Introduction

Gas turbine power plant is a power plant in which a gas turbine is used as the prime-mover for the generation of electrical energy. In the gas turbine power plant, the products of the combustion are directly expanded in the turbine whereas in the steam power plant, the products of the combustion (which is used to produce steam) do not form the working medium for driving the turbine. The gas turbine is different from the diesel engine in this respect that the combustion region is external to the prime-mover.*

Advantages of Gas Turbine Power Plants

Gas turbine power plants offer some advantages over other power plants. These advantages can be summarized as :

The capital cost of a gas turbine plant is lower than that of a steam plant of the same size.It has no standby losses.It requires lesser area than a steam plant of the same capacity.It has less water requirements as compared to a steam plant.Its foundations and buildings are simple and cheap.They can be started and put on load more quickly than steam plants.A gas turbine plant can be located very near the load even in the cities and towns.*

Disadvantages of Gas Turbine Power Plants

However, the gas turbine power plants have the following demerits also:The operating costs of a gas turbine plant are very high as compared to other plants.It has low net output (a greater part of power developed by the turbine being used in driving the compressor).Its overall efficiency is low (the exhaust gases from the turbine contain sufficient heat).Its operation is noisy.It consumes high specific fuel.Its unit capacity is low (around 50 MW).*

Site Selection of Gas Turbine Power Plants

The following factors should be considered while selecting a site for gas turbine power plants:Distance from load centre: The site should be as near to the load centre as possible so that the transmission costs and losses are minimized.Availability of land: The land should be available at cheap rate in order to keep the capital cost of the plant low.Availability of fuel: The fuel should be easily available and at reasonable rate.Availability of transportation facilities: The transportation facilities should be available.Distance from populated area: The site should be away from thickly populated area because of noisy operation.Type of land: The land should be of high bearing capacity to withstand the load of the plant and also the vibrations transmitted to the foundations from compressors and turbines.*

Fuels for Gas Turbines

A variety of fuels solid, liquid and gaseous are available for use in gas turbines. Though coal and peat can be employed as gas turbine fuels but these are not used mainly because of coal handling and ash handling problems. The petroleum fuels such as kerosene, gas oil, diesel oil, residual oil, are quite suitable for use in gas turbines but their use is limited because of their higher costs. Natural gas which is mainly methane with high calorific value and liquid petroleum fuels are normally used in the gas turbine power plants now-a-days.*

Principle of Operation of Open Cycle Gas Turbine Power Plants

Figure 1: A Simple Gas Turbine Power Plant*


The air is sucked by the compressor from the atmosphere through the filter, which removes the dust from the air. The compressor compresses this air and supplies the pressurized air to the combustion chamber. Fuel is injected into the combustion chamber and burnt in the stream of air supplied by the compressor. The combustion raises the temperature of air under constant pressure. The hot pressurized gas mixtures 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 1000F.The alternator is coupled to the gas turbine, converts mechanical energy of the turbine into electrical energy.*


An electric motor used for starting the compressor (compressor is required to be started before starting the turbine) is mounted on the same shaft as that of the turbine. Once the unit starts, a part of the mechanical power output of the turbine drives the compressor and there is no need of motor now. Such plants are known as open cycle gas turbine power plants.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, as shown in the figure 2. 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 *


Figure 2: Gas Turbine Power Plant with Two Turbines*

Efficiency

The overall efficiency of the open cycle gas turbine power plants is very low since the 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 the above reasons, seldom exceeds 20%.*

Principle of Operation of Open Cycle Gas Turbine Power Plants with Regenerator, Intercooler and Reheater:

Figure: Open Cycle Gas Turbine Power Plant with Regenerator, Intercooler and Reheater*


The air is sucked by the compressor from the atmosphere through the filter, which removes the dust from the air.The low pressure compressor partly compresses this air. An intercooler cools the air at constant pressure and then supplies the air to the high pressure compressor.The highly compressed air is preheated by the heat of the exhaust gases using regenerator.The compressed and preheated air is then applied to the combustion chamber. Fuel is injected into the combustion chamber and burnt in the stream of air supplied by the compressor. The combustion raises the temperature of air under constant pressure. The hot pressurized gas mixtures are then passed through the gas turbines.*


Reheater increases the temperature of the partly expanded gas from the high pressure gas turbine by burning more fuel before admitting the air to the low pressure turbine.The alternator is coupled to the gas turbine, converts mechanical energy of the turbine into electrical energy.An electric motor used for starting the compressor is mounted on the same shaft as that of the turbine. Once the unit starts, a part of the mechanical power output of the turbine drives the compressor and there is no need of motor now. *

Improvement of thermal efficiency

The regenerator utilizes the heat of exhaust gases to heat the compressed air before it is sent to the combustion chamber, reduces the fuel consumption of the plant and improves the cycle thermal efficiency. However, for short time operation such as peak loads, the cost of regenerator may not justify its use in gas cycle. The intercooler is heat exchanger which cools the partly compressed air in order to reduce volume and increase density. By using intercooling, the total size of the compressor is reduced for the same output. Thus it reduces the internal power consumption of the compressor and increases the thermal efficiency. The rise in temperature in the reheater also increases the thermal efficiency of the plant. Thus using regenerator, intercooler and reheater, the thermal efficiency of a gas turbine power plant can be increased to around 30%. *

Principle of Operation of Closed Cycle Gas Turbine Power Plants with Regenerator, Intercooler and Reheater:

Figure: Closed Cycle Gas Turbine Power Plant with Regenerator, Intercooler and Reheater*


In closed cycle gas turbine power plant, fuel is not mixed with the working medium (helium, argon, hydrogen, neon etc.). The working medium is sucked by the low pressure compressor which partly compresses it. An intercooler cools the working medium at constant pressure and then supplies it to the high pressure compressor.The highly compressed working medium is preheated by the heat of the exhaust gases using regenerator. The compressed and preheated working medium is then applied to the combustion chamber, where it is heated externally.The combustion raises the temperature of the working medium under constant pressure. The hot pressurized working medium is then passed through the gas turbines.*


Reheater increases the temperature of the partly expanded working medium from the high pressure gas turbine by burning more fuel before admitting it to the low pressure turbine.The alternator is coupled to the gas turbine, converts mechanical energy of the turbine into electrical energy.From the regenerator, the working medium is passed through the gas cooler which cools working medium before admitting it to the low pressure compressor to start the cycle again.An electric motor used for starting the compressor is mounted on the same shaft as that of the turbine. Once the unit starts, a part of the mechanical power output of the turbine drives the compressor and there is no need of motor now.*

Advantages of closed cycle plants

The advantages of closed cycle gas turbine power plants areA working medium with more desirable properties than air can be used.As fuel is not mixing with the working medium, low graded fuel can be used.The risk of corrosion of turbine blades is absent since the turbine is free from the products of combustion.Maximum unit capacity may be increased by employing closed cycle, since in it, the gas is at relatively high pressure and therefore physical dimensions of the compressor and the turbine are reduced.In an open cycle plant, the incoming air has to be filtered. This is eventually eliminated in a closed cycle plant.However the above advantages are offset by additional complexity of plant because a closed cycle plant requires externally fired heater, gas cooler etc.*

Procedures for Starting and Stopping of Gas Turbine Power Plants:

In starting of gas turbine power plants, the following steps are required:Cranking of unit to around 10% speed to establish air flow through the combustion chamber.Admitting correct amount of superior fuel (diesel oil) and applying ignition to air to initiate combustion.Accelerating the unit by the starting motor.Disengagement of starting motor on attainment of self sustained speed by the gas turbine.Transfer of fuel from diesel oil to bunker C. *

Procedures for Starting and Stopping of Gas Turbine Power Plants:

Stopping of gas turbine power plants requires the following steps:Transfer of fuel from bunker C to diesel oil and operation of diesel oil for sufficient time in order to remove the heavy bunker C from the fuel pump, fuel lines and fuel nozzles.Stoppage of the supply of fuel to the combustion chamber.Rotating of unit after shut down to allow proper cooling of various parts.*

Load Controlling of Gas Turbine Power Plants:

In case of an open cycle gas turbine power plant, the regulation is obtained by controlling the flow of fuel to the combustion chamber. The governor is located at the output shaft and regulates a valve in the fuel supply lines by its action.In a closed cycle gas turbine power plant, the load is controlled by controlling the density of working medium in the closed circuit. This is obtained by passing working medium into or out of the circuit under pressure. The flow of fuel is also regulated simultaneously with the air density.*

Combination Gas Turbine Power Plants:

A combination of gas turbine and steam turbine cycle aims at improving the overall plant efficiency by using the heat of exhaust gases from the gas turbine as a heat source for a steam plant cycle. There are three arrangements of combinational cycles usually employed as given below:

Use of Exhaust Gases of Gas Turbine Power Plant for Heating of Feed WaterUse of Exhaust Gases from Gas Turbine as Combustion Air in Steam Boiler Use of Gases from Supercharged Boiler for Expansion in Gas Turbine*

Use of Exhaust Gases of Gas Turbine Power Plant for Heating of Feed Water

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Use of Exhaust Gases from Gas Turbine as Combustion Air in Steam Boiler

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Use of Gases from Supercharged Boiler for Expansion in Gas Turbine:

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Advantages of Combined Cycle Operation:

The advantages of combined cycle operation are enumerated as below:

Saving in exhaust heat of the gas turbine and, therefore, increased in its heat rate.Reduction of stack emissions.Reduction in space requirement in comparison with conventional generating units of a given capacity.Reduction in requirements of condensing waterThe starting time required for the steam turbine power plant can be reduced. *

Advantages and DisadvantagesGreat power-to-weight ratio compared to reciprocating engines. Smaller than their reciprocating counterparts of the same power. Lower emission levels

Expensive: high speeds and high operating temperatures designing and manufacturing gas turbines is a tough problem from both the engineering and materials standpointTend to use more fuel when they are idlingThey prefer a constant rather than a fluctuating load.

Emission in Gas TurbinesLower emission compared to all conventional methods (except nuclear)Regulations require further reduction in emission levels

Needs for Future Gas TurbinesPower GenerationFuel EconomyLow EmissionsAlternative fuelsMilitary AircraftsHigh ThrustLow WeightCommercial AircraftsLow emissionsHigh ThrustLow WeightFuel Economy

Half the size and twice the thrustDouble the size of the Aircraft and double the distance travelled with 50% less NOx

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