A Thermal Power Plant converts the heat energy of coal into electrical energy. Coal is burnt in a boiler which converts water into steam. The expansion of steam in turbine produces mechanical power which drives the alternator (generator) coupled to the turbine.
Thermal Power Plants contribute maximum to the generation of Power for any country. Thermal Power Plants constitute 75.43% of the total installed captive and non-captive power generation. In thermal generating stations coal, oil, natural gas etc. are employed as primary sources of energy.
GENERAL LAYOUT OF THERMAL POWER STATION
MAIN ANDAUXILIARY EQUIPMENTS
- Coal Handling Plant
- Pulverizing Plant
- Draft System
- Ash Handling Plant
- Cooling Towers and Ponds
- Feed Water Heater
- Superheater and Reheater
- Air Preheater
COAL HANDLING PLANT:
The function of coal handling plant is automatic feeding of coal to the boiler furnace. A thermal power plant burns enormous amounts of coal.
A 200MW plant may require around 2000 tons of coal daily.
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. Pulverized is a means of exposing a large surface area to the action of oxygen and consequently helping combustion.
The circulation of air is caused by a difference in pressure, known as Draft. Draft is a differential pressure b/w atmosphere and inside the boiler. It is necessary to cause the flow of gases through boiler setting. It may be natural draft OR mechanical draft
A boiler or steam generator is a closed vessel in which water under pressure, is converted into steam. It is one of the major components of a thermal power plant. Always designed to absorb maximum amount of heat released in the process of combustion. Boilers are of two types
- Fire tube boiler.
- Water tube boiler.
Superheater is a component of a steam-generating unit in which steam, after it has left the boiler drum, is heated above its saturation temperature. The amount of superheat added to the steam is influenced by the location, arrangement, and amount of super heater surface installed, as well as the rating of the boiler. The super heater may consist of one or more stages of tube banks arranged to effectively transfer heat from the products of combustion.
Reheater is also steam boiler component in which heat is added to this pressure steam, which has given up some of its energy in expansion through the highpressure turbine. The steam after reheating is used to rotate the second steam turbine where the heat is converted to mechanical energy. This mechanical energy is used to run the alternator, which is coupled to turbine, there by generating electrical energy.
A steam turbine converts heat energy of steam into mechanical energy and drives the generator. It uses the principle that steam when issuing from a small opening attains a high velocity. This velocity attained during expansion depends on the initial and final heat content of the steam. This difference b/w initial and final heat content represents the heat energy converted into kinetic energy. These are of two types
- Impulse turbine.
- Reaction turbine.
ASH HANDLING PLANT:
The percentage of ash in coal varies from 5% in good quality coal to about 40% in poor quality coal. Power plants generally use poor quality of coal, thus amount of ash produced by it is pretty large. A modern 2000MW plant produces about 5000 tons of ash daily. The stations use some conveyor arrangement to carry ash to dump sites directly or for carrying and loading it to trucks and wagons which transport it to the site of disposal.
These condensers are heat exchangers which convert steam from its gaseous to its liquid state, also known as phase transition.The purpose is to condense the outlet (or exhaust) steam from steam turbine to obtain maximum efficiency and also to get the condensed steam in the form of pure water, otherwise known as condensate, back to steam generator or (boiler) as boiler feed water.
COOLING TOWERS AND PONDS:
A condenser needs huge quantity of water to condense the steam. Typically a 2000MW plant needs about 1500MGallon of water. Most plants use a closed cooling system where warm water coming from condenser is cooled and reused. Cooling tower is a steel or concrete hyperbolic structure having a reservoir at the base for storage of cooled water.
FEED WATER HEATER:
Feed water heating improves overall plant efficiency. The dissolved oxygen and carbon dioxide which would otherwise cause boiler corrosion are removed in feed water heater. Thermal stresses due to cold water entering the boiler drum are avoided. Quantity of steam produced by the boiler is increased.
Flue gases coming out of the boiler carry lot of heat. An economizer extracts a part of this heat from flue gases and uses it for heating feed water. This use of economizer results in saving coal consumption and higher boiler efficiency.
After flue gases leave economizer, some further heat can be extracted from them and used to heat incoming heat. Cooling of flue gases by 20 degree centigrade increases the plant efficiency by 1%. Air preheaters may be of three types
- The fuel used is quite cheap.
- Less initial cost as compared to other generating plants.
- It can be installed at any place irrespective of the existence of coal.
- The coal can be easily transported to the site
- It requires less space as compared to Hydro power plants.
- Cost of generation is less than that of diesel power plants.
- It pollutes the atmosphere due to production of large amount of smoke and fumes.
- It is costlier in running cost as compared to Hydroelectric plants.
- Maintenance cost is more