A thermal power plant is the main source of the generation of electrical energy in almost the country. Sometimes thermal power plant is called a steam power plant. In a thermal power plant, the heat energy is converted into electrical energy. The heat energy is generated by the burning of fuel and this generated heat is used to heat water to generated steam. This generated steam is used to move the turbine. The mechanical energy generated in the turbine is used to move the alternator which is connected with the turbine. Thermal power plants require coal and an ample quantity of water for cooling.
Component of thermal Power Plant
Coal Handling Plant
The function of the coal handling plant is the automatic feeding of coal to the boiler. A grate at the bottom of the furnace holds the fuel bed. Coal is weighed and led to a hopper through a conveyor mechanism. From the hopper, it is fed to the grate through some from of stoker mechanism. The stoker may be an overfeed stoker depending on weather coal entry is above or below the air entry.
Pulverizing Plant
In modern thermal power plant coal is pulverized i.e ground to dust-like size and carried to the furnace in a stream of hot air. Pulverization means of exposing a large surface area to the action of oxygen and consequently helping the combustion.
Advantage of Pulverisation
- The rate of combustion can be easily controlled and change quickly to meet the varying load
- Automatic combustion control can be used
- The boiler can be started from a cold condition very quickly.
- A wide variety of coal can be used.
Draft System
During the combustion of coal in the combustion chamber, it required a sufficient amount of air and removal of exhaust gases. The circulation of air in the burning chamber is done by the pressure difference between the chamber pressure and atmospheric pressure.To provide a continuous flow of air it is required to maintain a pressure difference between boiler and atmosphere and it can be gain by two methods (a) Natural Draft (b) Mechanical Draft (Artificial Draft)
Natural Draft
Natural Draft is provided by using chimney and stack.there are two types of purposed served by the Chimney, first, it serves to provide fresh enough air to the boiler and then the product of combustion discharged to the atmosphere. Second, it delivers the product of combustion and flies ash to a high altitude so that air pollution is reduced. The pressure and temperature of gases inside the boiler are very high as compared to the pressure and temperature of the outer atmosphere.
Mechanical Draft
The mechanical draft is required for modern-day power plants that have very large volume of gases and where a single chimney can not provide enough draft this enough of air .therefore mechanical draft is necessary. In mechanical Draft, a chimney is installed but the movement of air is due to the action of a fan.
A mechanical draft needs additional capital investment and maintenance but it is necessary for modern large size of power plant
Boiler
A boiler or steam generator is a closed vessel in which water is converted into steam.it is one of the main components of the thermal power plant. While designing the boil it is taken under consideration that it will absorb the maximum amount of heat energy released on the combustion of fuel. The generated heat is transferred to the boiler by all the three modes of heat transfer i.e conduction, convection, and radiation. Boilers are classified into two type
Fire Tube Boiler
This boiler is a so-called fire tube boiler because the product of combustion passes trough the tubes surrounded by water. if the tubes are vertical then it is called a vertical tube boiler. if the tubes are horizontal then it is called a horizontal tube boiler. A fire-tube boiler is simple, compact, and rugged in construction. its initial cost is low.
Water Tube Boiler
In the water tubes boiler, water flows inside the tubes and hot gases flow outside the tube. the tubes are connected to a common water channel and to a steam outlet. Water tubes boilers are of three types depending on the designing of its boiler. Either it may be a vertical boiler, Horizontal boiler, or inclined Boiler. if its boiler is designed either vertical, horizontal, or inclined respectively.
Super Heater
The superheater is sometimes called reheater. its main function is to reheat the saturated heat. Superheated steam has more heat energy as compared to saturated steam at the same pressure. this extra heat provides more energy to the turbine and hence electrical power output is more. The function of the superheater is to remove last-stage moisture present in the saturated steam and raise the temperature of the saturated steam. the figure given below describes the working of the superheater.
A convection Superheater is kept somewhere in the gas stream and receives most of the heat by convection. A radiant superheater is placed in or near the furnace where it gets receives heat by radiation. A combination of convection and radiation superheater receives heat in both ways.
Steam Turbine
In a thermal power plant, a steam turbine works as an intermediate because it converts the heat energy of steam into mechanical energy and drives the generator. It works on the principle of continuity Equation which states that when a huge amount of fluid flow through a small opening attains a high velocity.
There are two types of steam turbines (a) Impulse Turbine (b) reaction Turbine
Impulse Turbine
In impulse turbine steam is expanded in the turbine nozzle and attains a high velocity. The steam jet impinges on the blades on the rotor periphery. The rotor may be a built-up rotor or integral rotor. A built-up rotor consists of a forged steel shaft on which a separate forged steel disc is shrunk and keyed. In an integral rotor, the wheels and shaft are formed from one solid forging.
Reaction Turbine
In a reaction turbines only partially expansion takes place in the nozzle and further expansion takes place as a stream flows over the rotor blade. The relative velocity of steam increases as it expands while passing over the blade.
Ash Handling Plant`
Ash is a waste material. In thermal power plant about most of the plant use coal as a fuel for heat generation. After burning of coal ash is produced as a waste material, so it becomes necessary to handle this ash. In modern-day, a power plant of 200MW produces 5000 tons of ash daily.out of this total ash about 25% of ash is collected at the bottom of the furnace and the remaining 75% ash is pulverized fuel ash.The small power plant uses some conveyor arrangement to carry ash to the dumpsite or proper management to load it to the truck or other vehicle for disposal at a different site. But in the case of a large power plant, it required an elaborate arrangement and separate system for the furnace bottom and fly ash. The furnace bottom ash is inert but fly ash is acidic because it consists of sulfur dioxide gas.
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