In this article we will discuss about:- 1. Introduction to Gas Turbine Power Plant 2. Selection of Site for Gas Turbine Power Plant 3. Fuels 4. Operation and Control 5. Combined Steam and Gas Turbine Power Plant 6. Plant Layout 7. Comparison of Gas Turbine Power Plant with Diesel Engine and Steam Power Plants 8. Merits and Demerits of Gas Turbine Power Plant 9. Applications .
Introduction to Gas Turbine Power Plant:
Gas turbine electric power plant is a power plant in which a gas turbine is used as the prime mover for the generation of electrical energy.
In steam power plants, the products of combustion do not form the working medium. These are utilized to produce an intermediate fluid, called the steam, which is expanded in the turbine. In case the intermediate step of converting water to steam by means of gases is eliminated, the arrangement would be far simple and less wasteful. This principle is employed in gas turbine electric power plants where the gas is directly expanded in the turbine. The gas turbine is different from the diesel engine in this respect that the combustion region is external to the prime mover.
In a gas turbine, the working medium is either a mixture of combustion products and air or heated air at a certain pressure and a higher initial temperature. With respect to design, a gas turbine is quite similar to a steam turbine. In the turbine blading, working gas expands and the heat energy is converted first into the kinetic energy and then to the work of the turbine shaft rotation.
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The principle of a gas turbine power plant is that a turbo-compressor compresses working medium to a high pressure and then it is burnt in the combustion chamber where the combustion takes place at a constant pressure and temperature of the working medium is raised. This high pressure and high temperature working medium is then expanded in a gas turbine coupled to the generator (or alternator).
The overall efficiency of a gas turbine as a prime mover is limited due to the fact that a large portion of the power developed by the turbine is used in driving the compressor and also by the temperature safely attainable. Thus two factors that need improvement are the design of the compressor and design of turbine with suitable materials to withstand high temperatures. A lot of research work is in progress in this direction.
As a result of advances in the design of gas turbines, the advanced gas turbines with special features (high firing temperatures, advanced cooling systems, advanced materials to withstand higher temperatures and more efficient compressors with transonic blades) have been developed. These gas turbines have high efficiency (more than 60%) less emission of NO2, carbon monoxide and unburnt hydrocarbon; operate at higher temperature of about 1,400°C.
Aero derivative gas turbines, based on aircraft engine technology, are now widely employed in the field of generation of electrical power. The propulsion engines are modified as per needs of power generation. They do not need large lubricating oil reservoirs, coolers and pumps.
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The main advantages of such turbines are compact size, modular design and quick installation (6 to 9 months), fast starting (about 3 minutes), easier and quicker maintenance, lesser space requirement, high operating efficiency (fully-load as well as part load) and high availability (about 95%). However, such turbines are costlier (about 10-15% more than heavy duty gas turbines) and need clean fuel involving higher fuel treatment costs.
Natural gas fired (including LNG fired) power plants account for almost 20% of the world’s electrical power generation. These power plants use gas turbines or gas turbine based combined cycles. Gas turbines in the simple cycle mode (only gas turbines running), have an efficiency of 32 to 38%. The most important parameter that governs the efficiency is the maximum gas temperature possible. The latest gas turbine with technological advances in materials and aerodynamics has efficiencies up to 38%.
Selection of Site for Gas Turbine Power Plants:
The following factors should be considered while selecting a site for gas turbine power plant:
1. Distance from Load Centre:
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The site should be as near to the load centre as possible so that the transmission costs and losses are minimised.
2. Availability of Land:
The land should be available at cheap rate in order to keep the capital cost of the plant low.
3. Availability of Fuel:
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The fuel should be easily available and at reasonable rate.
4. Availability of Transportation Facilities:
The transportation facilities should be available.
5. Distance from Populated Area:
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The site should be away from thickly populated area because of noisy operation.
6. 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 Power Plants:
The most important requirement of the fuel used in a gas turbine is that its products of combustion should contain a minimum of matter which might get deposited on the turbine blades or the heat exchanger.
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 has a very high calorific value and is generally used for auxiliary power generation in oilfields.
Nowadays the gas turbine power plants use mainly natural gas and liquid petroleum fuels. Natural gas contains 80% methane and small fraction of other gases.
Operation and Control of Gas Turbine Power Plants:
Gas turbines are not self-starting and do not become self-sustaining until they attain around 50% speed and therefore, need means to crank the unit to firing speed and to assist acceleration after firing. Temperature control is also required to be provided to limit the maximum temperature in order to prevent damage to the turbine. Turbines that normally operate on bunker C oil must be started on diesel oil and transferred to bunker C at or above idle speed.
The starting and stopping of gas turbine power plant needs a certain procedure, which has to be properly controlled in correct sequence. This can be accomplished automatically. Operation of master switch initiates the starting or stopping sequence, which then proceeds automatically through the various steps required.
In starting of gas turbines, the following steps are required:
1. Cranking of unit to around 10% speeds to establish air flow through the combustion chamber.
2. Admitting correct amount of fuel, applying ignition and booster air to initiate combustion.
3. Accelerating of unit after firing assisted by the starting motor.
4. Disengagement of starting motor on attainment of self-sustained speed by the gas turbine.
5. Transfer of turbine from diesel oil to bunker C.
Stopping of gas turbine requires the following steps:
1. Transfer of turbine from bunker C to diesel oil and operation on diesel oil for sufficient time in order to remove the heavy bunker C from the fuel pump, fuel lines, and fuel nozzles.
2. Stoppage of the supply of fuel to the combustion chambers.
3. Rotating of unit after shutdown to allow proper cooling of various parts.
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 on the output shaft and regulates a valve in the fuel supply line by its action. In a closed cycle gas turbine power plant, the load is controlled by controlling the density of air in the closed circuit. This is obtained by passing air into or out of the circuit from reservoirs under pressure. The flow of fuel is also regulated simultaneously with the air density.
Combined Steam and Gas Turbine Power Plants:
The gas turbine power plants are mainly used for supplying peak loads in other types of power plants e.g., in steam and hydroelectric power plants. The heat content of gas turbine exhaust is quite substantial. The temperature of gas turbine exhaust is about 540°C and the oxygen content in this exhaust is roughly 16%. Essentially a combined gas turbine-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 combined cycles usually employed as given below:
1. Use of Exhaust Gases of Gas Turbine Power Plant for Heating of Feed Water:
The heat energy of exhaust gases from the gas turbine can be utilized in heat exchanger for raising the temperature of the feed water coming from the steam turbine condenser, before it is delivered to the steam boiler. In this arrangement, bleeding of steam from the steam turbine required for heating of feed water is avoided and the full steam supply to the steam turbine is available for expansion and producing mechanical power.
2. Use of Exhaust Gases from Gas Turbine as Combustion Air in Steam Boiler:
In this arrangement the gas turbine exhaust is used as preheated air for the boiler of the steam plant. The gas turbine exhaust has about 16% oxygen which is enough to support combustion in the boiler. The boiler can be supplied with supplementary fuel and air and would be larger than the conventional boiler. Plant heat rate can be raised by about 5% by using combined cycles.
3. Use of Gases from a Supercharged Boiler for Expansion in Gas Turbine:
In this arrangement, the high pressure air from the compressor is supplied to the steam boiler and the exhaust gases from the steam boiler (at about 700°C) are expanded in the gas turbine and then through an economiser to heat the feed water before leaving out through stack. The heat transfer rates in the steam boiler are increased due to the high density of air and therefore the weight of the boiler gets reduced by about 50%. Heat rate also gets improved by 7-8%. The capacity of the power plant is also increased and there is only a slight increase in the requirements of cooling water.
The advantages of combined cycle operation are enumerated as below:
1. Saving in exhaust heat of the gas turbine and, therefore, increase in its heat rate.
2. Reduction of stack emissions.
3. Reduction in space requirement in comparison with conventional generating units of a given capacity.
4. Reduction in requirements of condensing water by 60% as compared to a fossil fuelled plant of given capacity.
5. Reduction in starting time. The gas turbine can be brought to full load within 20 minutes from initial start giving around 60% of total plant output and subsequently the steam-turbo alternator can be synchronised and brought to full load giving 100% of plant output in less than an hour following an overnight shutdown.
The efficiency of a steam cycle can be increased by about 2%, with a combined cycle arrangement. Although there will be some loss of efficiency of the gas turbine owing to the back pressure created by the boiler furnace, there is still a useful overall improvement in cycle efficiency.
Capital costs per kW of these combined cycle plants are about 40% lower than those of conventional steam power plants.
Plant Layout of Gas Turbine Power Plant:
The plant layout of a gas turbine power plant has a significant effect on the performance of the plant and great care has therefore to be taken in the design and layout of the air as well as gas circuits, otherwise there will be unnecessary wastage of power in interconnecting ducts with a large number of sharp bends.
In a gas turbine power plant, turbine house accommodating the major portion of the plant and the auxiliaries is the main building.
The fuel oil storage tanks of capacity sufficient to operate the plant for about 100 hours are usually arranged outside but adjoining the turbine house. In some installations even heat exchangers are placed outside the main building. Intercoolers, combustion chambers, heat exchangers, waste heat boilers and interconnecting duct work occupies the major portion of the building. The rail siding is also provided for easy transport of fuel oil.
Comparison of Gas Turbine Power Plants with Diesel Engine and Steam Power Plants:
Comparison with Diesel Engine Power Plants:
The gas turbine power plants have numerous inherent advantages over diesel power plants such as smaller site area, lower cost of buildings, low initial cost, better heat rate, less water requirement, easier maintenance, reduced attendance charges, improved reliability, less vibrations, no cyclic variations etc. There are no problems of fuel injection and cylinder lubrication as with diesel engines. Possibilities of use of lower grades of fuel oil or low grade waste gases with gas turbines are also there.
The only drawback of gas turbine power plant in comparison to diesel power plants is higher fuel consumption (0.2 – 0.25 kg per kWh output) which is insignificant against their inherent advantages.
Comparison with Steam Power Plants:
Advantages of gas turbine power plants over steam power plants are lower capital cost, smaller site area, simple and cheap foundations and buildings, smaller installation period, fewer auxiliaries, no standby losses, lesser water requirement, short starting time and quick load sharing, simple control and possibility of partial or full automation, lesser operational personnel (about one-third), possibility of location near the load centre (even in cities and towns) etc.
Storage of fuel is much smaller and its handling is easy and as such problems of coal and ash handling are completely eliminated in case of open cycle gas turbine power plants using gas or liquid fuel. Moreover, the heat rate of a gas turbine is usually higher than that of a steam turbine. The efficiency of a gas turbine plant is almost the same as that of a steam power plant using liquid fuel.
The drawbacks of gas turbine power plants over steam power plants are:
1. The operating costs of gas turbine power plants are much higher than those of a steam power plant.
2. Gas turbines can be employed only in small sizes (about 50 MW or so).
3. Like steam turbines, the gas turbine is not readily reversible.
4. In case of a gas turbine power plant, the compressor has to be started before starting of turbine and the former requires power from some external source.
Merits and Demerits of Gas Turbine Power Plant:
The gas turbine power plant has got some inherent advantages such as simplicity of design and installation, high reliability, simple lubrication system, clean exhaust requiring no stack, compactness, low initial cost, no standby losses, requiring small building space and light foundations and requiring little cooling water.
The delivery and installation time for such power plants is much less than that for steam power plants. The gas turbine can be started quickly and can be put to share full load within a few minutes and efficiency can be improved considerably by using heat economy devices. Gas turbines may be operated by remote control and need little or no personnel while operating and while shutdown. Though the fuel costs in such power plants are usually higher than in other power plants, but maintenance costs are lower than for diesel power plants. Such plants have higher availability rate (about 90-95% as compared to about 80% for conventional steam power plants).
The gas turbine has got some limitations also, which probably will be overcome or minimised with more experience and research, such as inability of using coal or heavy residual petroleum as fuel, low net output (a greater part of power developed by the turbine being used in driving the compressor), low overall efficiency (the exhaust gases from the turbine contain sufficient heat), noisy operation, high specific fuel consumption and limited unit capacity.
The gas turbine, like steam turbine, is not readily reversible. In summer, some pre-cooling of the air to the compressor is desirable and possibly essential since at temperatures higher than 35°C the density of air may be so low that oxygen starvation may occur. The temperature of combustion chamber is quite high 1,650°C and therefore its life is comparatively reduced.
The unit has got starting problems as the compressor has to be started before starting of turbine and the former requires power from some external source. However, once the unit starts, the external power is not required as the turbine itself supplies the necessary motive power to the compressor.
Applications of Gas Turbine Power Plant:
1. Majority of such power plants are usually used for driving generators and supplying peak loads in other types of power plants (steam and hydro-electric power plants), because they can be started and loaded quickly. Other reasons for it are low initial costs and higher fuel costs.
2. Such a plant is used as a starting plant for driving auxiliaries in other power plants.
3. A large number of such plants are used as standby power plants.
4. Gas turbine power plants are also used as base load power plants where fuel oil or natural gas are cheap and easily available, water supply is scarce and load factor is very low, say 15-18%. Base load gas turbine power plants are likely to play, significant role in future.
5. Such plants can be used in parallel with tidal power plants.
6. These plants are used to operate as combination plants with conventional steam boilers.
Because of recent advances in gas turbines and their inherent advantages, the gas turbine power plants are finding wide spread use, even as base load plants. As on June 30, 2011, and as per Central Electricity Authority the total installed capacity of gas based power plant in India is 17,706.35 MW. This accounts for 10% of total installed capacity. GAIL is the main source of fuel for most of these plants.