Thermal or Steam power plant
Operating Principle of thermal power plant
In thermal power station fuel is burn and the is used to convert water into steam at the required pressure and temperature. The fuel normally used is oil, coal or gas but coal is most commonly used. For efficient combustion enough air is supplied. The gaseous product of combustion gives most of their heat to the water boilers and super haters. Water in the boiled is converted into steam by the heat supplied by combustion of fuel in furnace.
The steam is super-heated in the super heater. The super steam is supplied to the turbine at the throttle valve (control valve). The steam expends and gives it potential energy to the turbine. The turbine converts this energy into mechanical energy and arrives at the electric generator. The electric generator converts mechanical energy into electrical energy after expansion through the turbine the steam goes through the condenser. Where the exhaust steam is condensed back into condensate water and is pumped back to the boiler by the condensate pump.
The same water is used repeatedly, some water may be lost by the leakage through steam traps various stages in pumping system. Therefore, some water is added in the feed to make up water.
Parts of thermal Power Plant:
The thermal power plant consists of following parts which are given below.
- Supper heater
- Steam turbine
- Electric generator
- Condensate pump
- Feed water heater
- Make up water system
- Feed water pump
The boiler is a device for producing steam under pressure. It converts water into steam at the required pressure of quality as nearly dry as possible. Boiler consist of a large drums in which a large number of tubes are provided. There are two types of boilers.
- Fire tube
- Water tube
In fire tube boilers the hot gases produce from the combustion of the fuel in furnace are led through the tubes after circulating through the entire tubing system inside the boiler. The gases leave the boiler water circulates around the tubes and takes heat from the tube continuously.
Water tube boiler
In this type the water occupies the interior of the tubes and the hot gasses pass over them. Generally, water tube boiler is used for electric power station.
After passing over the tubes and drum of the boiler the flue bases still has heat energy also the steam given out from the boiler is not completely dry and having some vapors in it. Therefore, to make use of the heat of flue gasses it is given to the super heater in order to make it completely dry to rise its pressure and temperature to a value as required by the steam turbine.
It is a device for converting heat energy of steam into mechanical energy. It consists of wheel to which are fix the number of blades. There are two types of steam turbines.
- Impulse Turbine
- Reaction turbine
In impulse turbine steam is expended from the initial high pressure to a lower pressure through a steam nozzle before it is delivered to the revolving blades during increase in the velocity of steam.
When steam is discharge through a nozzle there is a backward reaction which if the nozzle were free to move in the direction opposite to that of the steam gets. In the actual reaction turbine expansion take place in completely in the less attached to the revolving wheels.
The steam turbine drives AC generator. These are some time four poles machines with a speed of 1500rpm for 50 cycle per second frequency or 1800rpm for 60 cycle second frequency. But most of the time they are three phase type machines. All the high capacity generator are two poles with a speed of 3000rpm for 50 Hz or 3600rpm for 60 Hz frequency. A turbine generator consists of stator and rotor.
It is a device used for condensing the exhaust steam. Hence it produces vacuum and illuminates back pressure. There are two types of condensers.
- Jet Condenser
- Surface Condenser
it sucks condensate from the condenser and discharge it to the feed water heater.
Feed water heater
Before pumping into the boiler the feed water is heated in the feed water heater.
Make up water system
Some water may be loss during the steam circulation therefore water is added in the feed water system from makeup water system.
Feed water pump
Since the same water is used in the boiler repeatedly, feed water pump is used for this purpose.
After passing through the feed water pump, the feed water goes through the economizer and then into the boiler. In the economizer some more heat is added to the feed water from flue gases.
Uses of Thermal Power Plant
Private industrial plants
The industry requiring steam for process purposes can utilized their plant for power generation as well. These may be reciprocation engine driven unit of small capacity (2.25 MW) which are directly connect to the load of industry, it can be used up to 10MW capacity.
The size of set such operation lies between 10MW to 500MW.
Water Circulation System in Thermal Power Plant
Open Water Circulation System
In open water circulation system water for cooling the exhaust steam in condenser may be circulated from river or canal by pumps. After used this water is returned to the river or canal.
Close Water Circulating System
In this system water is obtained from a tube well and stored in a tank, from which it is pumped in to the condenser. The water produce heat from the exhaust steam and gets warm. This hot water is passed to a cooling tower were the water fall from height of 8 meters to 10 meters. Through the bottom of cooling tower. Water from the base of cooling tower is pumped to the condenser and the cycle is repeated.
Site Selection for Thermal Power Plant
The following factor should be considered while selecting a site of steam power plant.
- Prices of land should be reasonable and further extension should be possible in future.
- The soil strength should be enough to with stand the weight of building and heavy machinery
- Enough quantity of water should be available for condenser and cooling purpose.
- It should be away from the populated area to minimize the environmental pollution.
- There should be enough place for storing fuel and for disposable of ash.
- Facilities infrastructure should exist for transportation fuel and machinery to site.
- The cost of fuel should be less at the selected area. The station should be near the fuel station.
- The steam station should not be away from the load center in order to reduce the transmission cost and losses.
Advantages of Thermal Power Plant
- Less initial cost as compared to other power plants.
- It can be installed in any place.
- It requires less space as compared to the hydroelectric power plant.
- The cost of generation is less than that of the diesel power plant.
Disadvantages of Thermal Power Plant
- The pollution problem occurs due to steam power plant.
- It running cost is more then that of the hydroelectric power plant.
- More construction work is required for storing the coil and Furness oil.
- Transportation charges are also more then the other power plants.
Some Thermal Power Plants in Pakistan
- Goddu 1657MW
- Multan 260MW
- Faisalabad 247MW
- Hyderabad 86MW
- Jam Shoro 880MW