Introduction to Steam Power Plant
● Steam power plants are producing about half of the total power requirement in India. In a steam Power plant, Water is heated using thermal energy, turns into steam and spins a steam turbine turbines to produce mechanical energy is converted into electrical energy in a generator.
Definition of Rankine cycle
Energy and the steam is condensed in a condenser and recycled to where it was heated; this is known as a Rankine cycle.
● Steam power plants are suitable for large scale production of electrical power.Certain thermal power plants also are designed to produce heat energy for industrial purposes of district heating, or desalination of water, in addition to generating electrical power. Commercial electric utility power stations are usually constructed on a large scale and designed for continuous operation.
● Steam power plant uses fossil fuels to generate steam. The fuel may be in different form such as solid, liquid, pulverized or gaseous. The selection of the type of fuel depends on the availability of fuels and economical conditions. 1) Solid fuels: Coal is the major fuel used for thermal power plants to generate steam.
Advantages of Steam :
1. Steam can be raised quickly from water which is available in plenty.
2. It does not react much with materials of the equipment used in power plants.
3. It is stable at temperatures required in the plant.
Component and System of Thermal Power Plant :
1) Boiler and furnace
2) Coal or fuel handling system
3) Ash and dust handling system
4) Draught system
5) High pressure boiler
6) Steam turbine and Generator (prime-mover)
7) Condensers and cooling towers
8) Feed water purification plant
9) Different accessories like economiser, super-heater. Air- preheater etc. to increase the thermal efficiency of the plant.
General Layout of a Thermal Power Plant :
The general layout of the thermal power plant consist of mainly four circuits.
1. Coal and ash circuit
2. Air and gas circuit
3. Feed water and steam flow circuit
4. Cooling water circuit.
A thermal power station using steam as working fluid works basically on the Rankine cycle, Steam is generated in a boiler expanded in the prime mover and condensed in condenser and again fed into the boiler.
1. Coal and fuel circuit :
In this circuit, the coal from the storage is fed to the boiler through coal handling equipment for the generation of steam. or pulverized coal as the fuel. The fuel is required in large quantities, which necessitates a fuel handling system. Usually belt conveyors or bucket elevator are used for handling the fuel.
2. Ash handling System:
The ash produced by the combustion of coal in a steam power plant amounts to about 10 % to 20% of the fuel burnt. This should be removed from the furnace and disposed. This is done by a proper ash handling system. Depending upon the capacity, the system used may be mechanical, hydraulic, and pneumatic or steam jet type.
This selection is based on the quantity of ash to be handled and the type of boiler used.
3. Air and flue Gas circuit :
Atmospheric air is supplied for the combustion of the fuel in combustion chamber called furnace of the boiler either by force draught or induced draft which force the flue gases from the furnace pass over boiler tubes and super henter tubes and then the flue gases pass through economizer to heat the feed water, through the furnace boiler system. The dust from the air is removed before supplying to the combustion chamber. The exhaust gases carrying sufficient quantity of heat and are passed through the air-preheater where the exhaust gases reject heat to air. This pre-heated air is supplied to the furnace to aid the combustion of fuel. Then the exhaust gases passed through dust collector where most of the dust is removed and then exhausted to atmosphere through a chimney. It increases the rate of combustion so efficiency of plant increases.
4. Feed water and steam circuit :
It consists of feed pump, economizer, boiler drum, super heater, turbine condenser etc. Feed water is pumped to the economizer from the hot well. This water is preheated by the flue gases in the economizer. The steam generated in the boiler is passed through a super heater. It is superheated by the flue gases. The superheated steam is then expanded in steam turbine (prime-mover) to develop electrical power. The steam coming out of prime mover is condensed in the condenser and then re-circulated in the circuit. Some of the steam and water is lost while passing through different component. So feed water is supplied from external source and this feed water passed through purifying plant to reduce the dissolved salts and avoid the scaling of the boiler tubes or boiler shell.
5. Cooling water circuit :
The circuit includes a pump, condenser, cooling tower etc. the exhaust steam from the turbine is condensed in condenser The quantity of cooling water required to condensed the steam is large and it taken from river, lake or pond. In the condenser, cold water is circulated to condense thesteam into water. The steam is condensed by losing its latent hent to the circulating cold water. Thus the circulating water is heated. This hot water is then taken to a cooling tower, Incooling tower, the water is sprayed in the form of droplets through nozzles. The atmospheric air enters the cooling tower from the openings provided at the bottom of the tower. This air removes heat from water. Cooled water is collected in a pond known as cooling pond. This cold water is re-circulated through the pump. condenser and cooling tower. Thus the cycle is repeated again and again. Some amount of water may be lost during the circulation due to vaporization etc. Hence, make up water is added to the pond by means of a pump. This water is obtained from a river or lake. Such system is used when adequate supply of cooling water is available. This is known as open system.
6. Lubrication System :
The turbines and generators are coupled to each other and run at high speeds about 3000 rpm. A proper lubrication system is essential to keep the system running continuously. The lubrication system minimizes wear and tear but also provides a little cooling effect for the bearings of the system.
When the cooling water is less, then water coming out from the condenser is cooled by cooling pond or cooling tower. This cooling effected by partly evaporating the water.
Working of thermal power plant:
The steam is generated in the boiler of the thermal power plant using the heat of the fuel burned in the combustion chamber. This steam is passed through the steam turbine where, it expand and the part of its thermal energy is converted into mechanical energy. The turbine shaft is coupled with generator which convert mechanical energy into electrical energy (electric power). The steam coming out of steam turbine is condensed in condenser and the condenser is supplied back to the boiler with the help of feed pump with some make up feed water and cycle is repeated.
Advantages and Disadvantages of Thermal Power Plant :
Advantages :
1. Less setup, Repair and maintenance cost is low.
2. Life of the plant is about 25 to 30 years as compared to other power plant
3. Unskilled operators can operate the plant
4. The construction cost of plant decreases with the increase in unit capacity and less compare to nuclear plants.
5. It is not directly related to nature climate condition like hydro power plant.
6. There are no transmission losses since they are located near load centres.
7. Suitable for varying load conditions.
8. No harmful radio active wastes are produced as in the case of nuclear plant.
9. Less land area required.
Disadvantages:
1. Ash handling is a big problem as it creates large amo of ash
2. Fuel transportation, handling and storage charges are more
3. Cooling water required is more.
4. Storage space required for the fuel is more
5. For large units, the capital cost is more. Initial expenditure on structural materials, piping, storage mechanisms is more.
6. The amount of Cooling water required is more.
7. Operation cost is high compare to hydro and nuclear
8. Low cyclic efficiency around 35 to 45 percent.
9. It continuously generates smoke which contributes in increase air pollution.
Limitation :
It is important to limit such as SO2, and NOx and other particulate emissions in environment which cause acid rain by separation capacities commensurate with the size of the power plant.
Due to Acidic Formation has an impacts on soil and Bround water pollution. The soil quality, for example, can be adversely affected by dust sediment, particularly in the near vicinity of the plant. The seriousness of ground-level pollution depends on the heavy-metal content of the dust The fertility of the soil can be altered by acidic precipitation (acid rain, characterized mainly by the acid formers SO2, and NOx. Under unfavourable conditions, acidification can pass from the soil to both the ground water and surface waters
