Thermal Power Plant :
Power plant is a place where electricity is produced for the purpose of domestic and industrial use. Thermal powerplant is a power plant consists of components which converts heat energy into electrical energy. Heat energy is supplied in the form of superheated steam. Let discuss the thermal power plant working in detail.
Thermal Power Plant Working:
Thermal power plant is a place where electric power is generated by using a steam produced from high pressure boilers. In the boiler, water is converted into steam by using the heat of coal combustion. High pressure and high temperature steam produced in the process is made to hit the blades of steam turbine. The turbine shaft is further connected to electric generator which converts the mechanical energy of turbine into electrical energy.
Exhausted low pressure steam is then send to the condenser where steam is converted into water at constant pressure. Thermal power plant is also known as steam power plant and it works on the Rankin cycle.
Components of Thermal Power Plant:
Steam power plant consist of many components for the conversion of the heat of coal combustion into electrical power. It consist of boiler for the generation of steam and other equipment for proper working and efficiency. The basic components of thermal power plant is as follows:
Generator- generates electricity
- Super heater
Coal storage and ash handling plant :
In a thermal power plant, coal is brought to the power plant by road or rail and store in coal storage plant. The most essential part of power plant is to store the fuel(coal) safely in quiet large amount. It is important so that the plant can run smoothly not only in normal days but also during the transportation strike or coal shortage period. From the coal storage plant, it is brought to the coal handling plant where it is crushed into small pieces. The pulverized coal (fuel) is then supplied to the boiler with the help of belt conveyors.
The feed water on the other hand is purified before going to the boiler. Purification and softening of water is done in order to improve the efficiency of boiler and its equipment’s.
The pulverized coal (fuel) is fed to the boiler furnace. Hot air is drawn from the force draught fan is mixed with the fuel(coal) in boiler furnace which causes the combustion of coal. The combustion heat is then added to the boiler with the help of boiler furnace. In the boiler, combustion heat of the fuel is utilized to convert water into high pressure steam at saturated temperature.
During the combustion of coal, flue gases are produced in the boiler. Flue gases produced in the boiler have high temperature. If we do not use the heat of flue gases, it will lead to large amount of heat losses which reduces the boiler efficiency. Hence the heat of flue gases is utilized by passing the flue gases through super-heater, economizer and air preheater (can be seen in the above fig). The heat of flue gases is absorbed by air preheater to heat the air required for the combustion of fuel. Economizer absorbs the heat of flue gases in order to increase the temperature of water before it is fed to the boiler. After absorbing the heat from flue gases, they are exhausted off in the atmosphere through chimney via electrostatic precipitator.
The air required for the combustion of coal is drawn from the atmosphere by the forced draught fan. It is pass to the air preheater before supplied to the boiler furnace. Air preheaters extracts the heat from the flue gases and utilized that heat to increase the temperature of the air. In this way, air preheater helps in increasing the thermal efficiency.
An economizer is a feed water heater. High pressure water is pass through the economizer before supplied to the boiler. Economizer absorbs the heat from the flue gases to increase the feedwater temperature.
The steam produced in the boiler is at saturated temperature. In order to increase the temperature of steam, super heater section is installed in the boiler. Super-heater increases the temperature of steam to maximum allowable limit (550degree approx.) without increasing its pressure. It also removes the moisture content from the steam. Superheated steam from the super-heater is then supplied to the steam turbine through the main valve.
Superheated steam from the super heater is supplied to the steam turbine. Superheated steam expands and looses its energy in order to rotate the blades of turbine which in turn rotates the steam turbine.
The generated steam rotates the turbine blades which in turn rotates the turbine shaft. The steam turbine shaft is coupled with an alternator↗ which converts the mechanical energy of steam turbine into electrical energy.
Output steam from the steam turbine is at low pressure and low temperature since the energy is absorbed by the turbine. In order to bring it back to high pressure, we may use a compressor but compressing a fluid in the gaseous state requires a very high energy and it is also not very efficient. Hence it is better to convert the steam into liquid state and then increase the pressure. For this purpose, we use a condenser coupled with a feed water pump. Condenser heat exchangers are present below the low-pressure turbines.
The wet steam coming out of the low pressure turbine rejects heat in the condenser at a constant pressure to become saturated liquid(water). In a thermal power plant, cold water is continuously made to circulate through the condenser with the help of cooling towers. Steam rejects the heat to this cold water and converts into liquid state as a feedwater. In this way, low pressure and low temperature steam condenses in the condenser to form a feedwater.
Feed Water Pump:
To increase the pressure of feed water, feed water pump is used. Pump increases the pressure of feedwater and supply high pressure water to the economizer.
Electrostatic Precipitator :
Flue gases are produced in the boiler after the combustion of coal. Since flue gases contains suspended particles which cannot be exhausted directly into the atmosphere as per government instructions. Hence the main purpose of using electrostatic precipitator is to remove the fly ash from the flue gases. The principle is simple is that the fly ash stick to the plates of ESP which are negatively charged. Suspended particles are then collected at the bottom by vibrating the plates.
Advantages of Thermal Power Plant :
Initial cost of thermal power plant is low as compared to other generating stations.
The fuel (coal) used is quite cheap.
Cost of generation is less as compared to diesel power plants.
Disadvantages of Thermal Power Plant :
Since it produces large amount of smoke and fumes and pollutes the atmosphere. Hence pollution is the main disadvantage of thermal power plant.
Operating cost is high as compared to hydro power plants.
Efficiency Of A Thermal Power Station :
The overall efficiency of thermal power plant is quiet low due to the large amount of heat losses at the various stages of plant. A major portion of the heat lost occurs in the condenser. Efficiency of steam power plant can be stated as ;
Thermal efficiency =
Thermal efficiency of steam power plant is about 30%.This means that if 100 calories of heat is produced from coal combustion, then 30 calories of mechanical energy will be available at the turbine shaft.
Overall Efficiency :
The ratio of heat equivalent of electrical output to the heat produced due to coal combustion is called overall efficiency of steam power station.