Thermal Power Plant : Principle, Parts, Working, Advantages and Disadvantages

Basic Introduction or Principle:

We all are aware with the term "Generator". A device which converts mechanical energy into electrical energy is known as generator. This generator makes rotate with the help of some kind of external energy. When this energy extract from the energy of steam, the plant is known as steam power plant. 

A simple steam plant works on Rankine cycle. In the first step, water is feed into a boiler at a very high pressure by BFP (boiler feed pump). This high pressurized water is heated into a boiler which converts it into high pressurized super heated steam. This high energized steam passes through steam turbine (a mechanical device which converts flow energy of fluid into mechanical energy) and rotate it. Owing to extract full energy of steam, three stage turbines is used which is known as LPT (Low pressure turbine), IPT (intermediate pressure turbine) and HPT (High pressure turbine). The turbine shaft is connected to the generator rotor shaft which makes rotate the generator shaft and produce electricity. In this process the steam loses its energy. This low pressurized saturated steam further passes through condenser where it converts into water. This water further passes through BFP and boiler and completes the cycle. This cycle continuously run to produce electricity.

  

Now we describe each component of thermal power plant in detail.

Construction of steam power plant:

Any steam power plant can be grouped into following section.

Coal Handling Plant:

Coal Storage:

The place at which coal stored is known as coal storage. The coal initially received by mines is stored in proper place.

Bunker:

Coal from coal storage sends to bunkers. It is a container which is upper side of mill and used to continuously provide coal for mill machine. The minimum capacity of bunker is around 10 times of mill capacity.

Feeder:

Coal from the bunkers send to the feeder which provide coal to mill machine. The main reason to use feeder between bunkers and mill machine is that if we directly send coal to mill, it can damage the internal part of machine due to tones of pressure applied by the coal.

Mill Machine:

Coal does not directly used into boiler. The place where the coal is converted into pulverized form is known as mill machine. This pulverized coal sends to classifier from it.

Classifier:

Classifiers are used to separate pulverized and non-pulverized forms of coal. It sends pulverized coal to furnace and non-pulverized coal to mill machine.

Air Handling Plant:

PA Fan:

PA fan is primary air fan. This is used to transport pulverized coal to furnace. It also used to remove moisture contant from pulverized coal.

ID Fan:

ID fan means induced draft fan. This fan is used to suck the exhausted flue gases from the boiler and send it to atmosphere through chimney.

FD Fan:

FD means forced draft fan. It is used to provide air or we can say oxygen for proper burning of coal into the furnace. It provides hot air into the furnace.

Air Preheater:

It is a heat exchanger which transfer heat from exhausted flue gases to incoming PA and FD air. 

ESP (Electrostatic Precipitator):

This device is situated between Id fan and boiler exhaust and used to detect and block ash particles from flue gases and control the pollution being created by it. 

Chimney:

Chimney is used to create natural draft for exhausted flue gases. One chimney is used for two units. 

Boiler Section:

Economizer:

Economizer is the first component of boiler section. As the name implies, economizer is used to increase the efficiency of steam power plant. It is used to heat water upto saturation temperature. It extracts heat from exhausted flue gases and used it to heat water. It sends water to the boiler drum.  

Boiler:

Economizer sends water to the Boiler. Boiler is the main part of any thermal power plant. It is used to convert water into steam. In any steam power plant water tube boiler is used. It contains furnace inside the boiler shell. The Coal burns into this section. Drum is major part of steam power plant boiler. It is situated top of the boiler and used to separate water from steam. Steam from boiler section sends to super heaters.

Super heater:

The efficiency of thermal power plant is directly connected to the temperature of the steam. The boiler creates low temperature steam which is not so economical for any power plant. So a super heater is used to heat the steam again. The temperature of the steam is limited at 550 degree centigrade because the turbine material can’t sustain temperature above 600 degree centigrade. The steam from the super heater sends to high pressure turbine. 

Re heater:

When the steam expands into high pressure turbine, both its temperature and pressure get down. If this low temperature steam directly sends to IP turbine, it creates less power. To increase the power of the plant there is an arrangement  to send exhausted steam from HP turbine to Re-heater where it heated and get the initial temperature which is about 550 degree centigrade. 

Turbine Section:

High Pressure Turbine:

The steam from the super heater sends to HP turbine. All the three turbines are connected to same shaft which is further connected to the generator shaft. The HP turbine works around 150 Kg/cm2 pressure and 550 degree centigrade temperature. It is smallest among all turbines.

Intermediate Pressure Turbine:

As the name implies it works at intermediate pressure which is around 70 Kg/cm2. The steam from the Re-heater sends to the IP turbine at around 550 degree centigrade where it expands and generates power.

Low Pressure Turbine:

This is the main power generator. It generates around 40 percent of whole power. The steam from IP turbine directly sends to LP turbine where it expand and rotate the turbine. It is biggest part of turbine section.  

Extractor:

To increase the efficiency, some amount of steam is extracted from both HP section exhaust and LP section exhaust. This extracted steam is used to heat water before send to economizer.

Condenser Section:

Condenser:

In a thermal power plant, to complete the cyclic operation, we need to send water again to the economizer at high pressure. Steam exhausted from LP turbine is not in condensed form and it is not economical to compress the steam at a very high pressure around 150 Kg/cm2. So a device is needed which can condense the steam into water. This device is called Condenser. Condenser is also a heat exchanger in which the cold water runs into tubes and steam flow from shell. The cold water extracts heat from steam and convert it into water. Condenser works at vacuum pressure which is around -1 Kg/cm2. It is due to create pressure difference between LP turbine exhausted steam and condenser which is required for proper flow of steam in it. The condensed water sends to a container which is named as Hotwell.

Condensed Extractor Pumps (CEPs):

The condensed water extracted by the pumps and send to LPH.

LPH (Low Pressure Heater):

The water collected from the condenser is at low pressure. To increase the efficiency of the plant this water is heated by the extracted steam from LP turbine.

Deaerator:

The water collected from condenser has some dissolved gases like oxygen. This water can't directly send to boiler and turbines. it can cause of corrosion. To decrease the possibility of corrosion these dissolved gases should be removed from this water before converted it into steam. This work is done by Deaerator. Deaerator heat the water at some extant at which the dissolved gases converted into gases form and removed from water.

Feed Water Pump or Boiler Feed Pump:

BFP are centrifugal pumps which are used to increase the water pressure upto 150 Kg/cm2 and also send this water at economizer.

HPH (High Pressure Heater):

The pressurized water forced by BFP passes through high pressure heater which heats this water at some extent to increase the efficiency of plant. The steam extracted from HP exhaust is used to heat this water.

Cooling Tower:

Cooling tower is used to provide cold water for condenser tp condense steam into water.

Generator Section:

The turbine shaft is connected to the generator which rotates the rotor of the generator and produce electricity.

Exciter:

For the cold starting, the turbine does not rotate due to high inertia.  Exciter is used to rotate the turbine in the initial run.

Working of Steam Power Plant:

Now we have discussed about main components of thermal power plant and their uses. These all components work together to produce electricity. Its working can be summarized into following point. 

• In the starting of the plant, pulverized coal from mill machine supplied to the boiler furnace by PA fan. FD fan provides desire oxygen for proper burning.
• DM water is supplied by the Boiler feed pump to economizer.  This water achieves around 165 Kg per centimeter square pressure during this process.
• The economizer heats this water at the saturation point in ideal condition and sends it to the boiler drum.
• The water from the boiler drum sends to the circular ring which is situated at the bottom of the boiler. All the water wall tubes are connected to this circular ring.  
• Now the water flow through the water wall tube of boiler which one end is connected to the circular ring and other end is connected to the boiler drum. The water flow from bottom to up direction.
• This water converts into steam in water walls and send back to boiler drum. Boiler drum separates steam and water. 
• Now the steam from the boiler drum sends to the super heater which heat the steam around 550 degree centigrade.
• This superheated high pressurized steam passes through HP turbine where it expands and rotates the turbine. The pressure and temperature of steam drop down in HP turbine.
• This steam from the exhaust of HP turbine sends back to re-heater where it heat again and achieve the initial temperature around 550 degree centigrade. One thing note down that the steam get its initial temperature but the pressure of the steam is lower than the initial condition.
• Now this reheated intermediate pressure steam send to IP turbine where it expand again and drop its pressure and temperature down further. It rotates the IP turbine at the same speed as rotate HP turbine.
• This steam from the IP turbine sends directly to the LP turbine and allowed to expand there. The steam in LP turbine expands completely and done the maximum work.
• Now from the LP turbine outlet, steam send to the condenser. Condenser is situated just below the LP exhaust line. For cooling of steam in the condenser, cooling tower supplies cold water in the tubes situated in the condenser.
• The steam converts into water in the condenser and send to the LP heater. LP heaters extract heat from the exhaust of the LP turbine and use it to heat the feed water.
• After from LP heaters, the water sends to the deaerator where the dissolve gases of water removed. The water from deaerator sends to the BFP which send it to economizer through HP heater.

This water cycle repeats again and again and rotates the turbine continuously. The turbine further rotates the generator rotor and produces electricity.

Advantages and Disadvantages of Thermal Power Plant:

Advantages:

• Low setup and maintenance cost.
• It is not directly related to climate condition like hydro power plant.
• Large amount of coal available at earth. 
• Easy maintenance.
• Less land area required.
• It can be installed near load center which minimize transmission losses.
• It can be installed near coal mines which can minimize transportation cost of fuel. 

Disadvantages:

• Low cyclic efficiency around 35 to 45 percent.
• It continuously generates smoke which contributes in increase air pollution.
• It uses consumable fuel.
• Operation cost is high compare to hydro and nuclear power plant.
• It creates large amount of ash per hour so ash handling is quiet difficult. 
• Sometimes heated water directly drawn into river which can harm life cycle of water living.