Thermal Power Plant

History

The thermal power plant is continuously developed since the 18th century. Initially, reciprocating engines were used to produce mechanical power by producing steam. In 1884, the steam turbine was introduced to increase efficiency, and finally, it was totally improved in 1905 by replacing an entire reciprocating system in the central power plant.

Thermal power plant

What is a thermal power plant?

A thermal power plant uses thermal energy from fuel to produce electric power. Normally coal is used as the source of thermal energy.

• This thermal energy is used to heat water and produce steam.
• Steam is used to pass through the turbine
• Turbine rotates
• The rotation of the turbine helps to produce power at the generator.

Due to the use of coal in thermal power plants, it is known as a coal-based thermal power plant or coal thermal power plant. In addition to that, due to the use of a steam, turbine arrangement, it is also known as a steam turbine power plant. So, what is the thermal power plant?

Overview of Thermal Power Plant

A typical Thermal Power Station Operates on a Cycle which is shown below.

The working fluid is water and steam. This is called the feed water and steam cycle. The ideal Thermodynamic Cycle to which the operation of a Thermal Power Station closely resembles is the Rankine Cycle.

In a steam boiler, the water is heated up by burning the fuel in the air in the furnace, and the function of the boiler is to give dry superheated steam at the required temperature. The steam so produced is used in driving the Steam Turbines.

This turbine is coupled to the synchronous generator (usually a three-phase Synchronous Alternator), which generates electrical energy.

Basic Principle of Thermal Power Plant

The steam turbine is the heart of the thermodynamic cycle, called the Rankine cycle. We have already learned that the Rankine cycle consists of main four components,

• Pump
• Boiler
• Turbine
• Condenser

In the thermal power plant, the working fluid is water which undergoes phase changes.

The Rankine cycle goes under the following process.

Process-1: Isentropic compression (Points 1-2)

• This process incurs the pumping of water.
• The pump absorbs energy, which is basically a work input.
• Water is transferred to the boiler which is normally kept at a high level, hence, a high head of pump with high work input is required.
• High pressure is developed in the water.
  

Process-2: Heat Addition in the boiler at Constant Pressure (Points 2-3)

• High-pressure water enters into the boiler and is heated through the furnace at constant pressure. 
• Due to heating, water is changed its phase from liquid to steam.
• The dry saturated vapor is formed.
• This is basically a heat input.

Process-3: Isentropic expansion (Points 3-4)

• In this process, expansion happens in the turbine as dry saturated steam enters & hits the blades. 
• This process is basically for power output.

Process-4: Heat rejection at constant pressure from the condenser (Points 4-5)

• In this process, wet steam or wet vapor enters to condenser from the turbine.
• Heat rejection happens in this stage at constant pressure.
• Steam is changed into saturated water.

Main Components of Thermal Power Plant:-

• Coal Handling System
• Boiler Feed pumps
• Boiler
• Turbine
• Condenser
• Condensate Extraction Pumps
• Circulating Pumps
• Cooling Tower
• Generator
• FD & ID Fans
• ESP
• Chimney
• Ash Handling System
• Water Treatment Plant

Coal Handling System:-

Fuel coal is crushed here by a crushing machine and led to the furnace of the boiler by a conveyer belt.

Boiler:- 

The boiler is the main component of the coal-based thermal power plant. There are different types of boilers like Water tube boilers and fire tube boilers. However, for example, purposes, we have considered a water tube boiler. 

It consists of:-

• Furnace
• Water Tube
• Boiler Drum 
• Water Drum
• Economizer
• Superheater
• Evaporator
• Water Preheater

Furnace & Water Tube:-

The furnace is the main heat-generating equipment in the boiler. The combustion of fuel happens in the Furnace.

• The water-tube boiler consists of the furnace. 
• Water flows through these tubes.
• Coal and Water burned in the furnace.
• The heat from the combustion heats the tube as well as the water inside the tube.
• Remember in the case of the fire tube boiler, the tube doesn't have water, it has hot gases instead of water.

Boiler Drum:-

A drum is provided in the boiler to store steam. Here, Hot water and Steam separated with the help of a steam separator.

• Separated steam is passing through the superheater.
• Separated water is going to the water drum.
• It stores the water.
• There is a continuous blowdown process to reduce the water TDS level.
• If the water level is cross the highest level, emergency blow down opts.

Mud or Water Drum:-

This water drum is connected to the main boiler drum, kept at the top of the boiler, through the water bundles.

• Solid or unwanted foreign particles are settled in the water drum.
• Based on the requirements, these settled particles are removed by blowdown.

Superheater:-

A superheater is a device used to convert saturated steam into superheated steam. Superheated means the temperature of the steam is further increased than the saturated steam.

• These are high-grade tubes.
• Mainly made of high-grade stainless steel or nickel alloy.
• Generally, it is placed between the radiant part and the convection part.
• There are three kinds of superheaters, used in the boiler.

1. Convection Superheater
2. Radiant Superheater
3. Conv-radiant Superheater

Economizer:-

In the furnace, due to the combustion of fuel, flue gas is produced. Economizer is used to recovering some amount of heat energy into the water.

• It works as a heat exchanger.
• It reduces energy losses and increases boiler efficiency.
• It is placed on the flue gas path before the air heater.

Preheater:-

Air is supplied to the furnace for combustion. Hence, it is the most preferable if it is pre-heated before supplying to the furnace.

• Preheater is used to pre-heat the air.
• It recovers the heat energy from the flue gas.
• Increase boiler efficiency.
• It drives the pulverizer or the mills.

LP & HP Heater:-

LP means low pressure and HP means high-pressure heater. To increase the efficiency of the boiler as well as the thermal power plant, condensate, water received from the condenser is heated.

• These all are basically heat exchangers.
• In the case of LP heater, water is heated by LP turbine and extracted steam.
• In the case of HP heater, water is heated by HP, and IP turbine extracted steam.
• Water is heated before entering the boiler and increases the overall performance.

Turbine:-

The turbine is one of the main parts of the thermal power plant. High-pressure steam from the boiler is passing through the turbine. Due to high pressure, exists a force on the turbine blades and the turbine rotates.

• Steam from the boiler passes through the turbine by a steam pipe.
• Steam at the outlet of the boiler is having high pressure.
• It exerts forces on the vanes of the turbine blades.
• The turbine blades rotate.
• Turbine speed increases drastically based on high pressure.
• The heat energy of the steam is converted into mechanical energy, as it rotates the turbine.
• The turbine produces work as the steam expands in it.
• The turbine has three parts, HP, IP & LP.
• The shaft of the turbine is connected to a generator.

Condenser

A condenser means a heat exchanger, which is used to exchange heat between the turbine exhaust and the cooling water. Exhaust from the turbine is cooled in the exchanger and cooling water takes the heat. It has water many tubes and heat exchange happens based on the types of cooling towers.

• The turbine exhaust which is steam comes below its boiling temperature in the condenser.
• Due to low boiling temperature, steam changed its phase into water.
• The vacuum is created inside the condenser, during this phase change.
• Due to the vacuum, steam changes into the water very rapidly.
• The water which is collected at the bottom of the cooling tower is called condensate.
• This condensate is transferred to the boiler through pumps.

Condenser

Deaerator

Before supplying the water from the condenser to the boiler, it is very much necessary to remove all the impurities from the water.

• It removes all dissolved gasses, oxygen, etc.
• No corrosive gases in the feed water
• No corrosion.

Deaerator

Boiler Feed Pumps

The boiler feed pump is used to supply water from the deaerator to the boiler drum. It is having a high head to feed the water up to the boiler drum.

Cooling Tower

The circulating water picks up the heat from the turbine exhaust and becomes heated. Hence, this heated cooling water needs to be cool to complete the cycle.

• Heat in the circulating water is rejected in the cooling tower.
• There will be around 2%evaporation loss happens in the cooling tower.

• There are circulating pumps are used to transfer the water in a closed cycle, from the condenser to the cooling tower.

Circulating Water (CW) Pumps

These pumps are transferring water from the cooling tower to the condenser.

Generator

The generator has a magnetic field, it has a starter, and a rotor. When the steam turbine rotates the generator shaft, it creates a magnetic field and produces electricity.

FD Fan & ID Fans

For burning in the combustion of the furnace, fuel is injected. However, the air is required for burning. The fan is pushed the supply air into the combustion chamber of the boiler and continues the entire combustion process. This fan is called a Forced Draft fan or shortly FD fan.

After combustion, flue gas is produced and the same needs to be taken out of the boiler. This is done by another fan, called ID fan or induced draft fan. Apart from these, there are other kinds of fans, used in the system, like,

• PA fan or Primary air fans,
• SA Fan or Secondary air fans
• Gas recirculation fans.

Electrostatic Precipitator (ESP)

An electrostatic precipitator (ESP) is another very important to filtrate the air. Due to the combustion of fuel, flue gas consists of ash particles, dust, smoke, foreign particles, etc. which needs to be removed to reduce the air pollution.

It creates a strong electric field to remove all unwanted particles from the flue gas.

Chimney

After ESP, or after the removal of all foreign particles, these flue gases are released to the atmosphere through a chimney.

Ash Handling Plant

Each thermal power plant has an ash handling plant. Once the coal is burned, ash is produced, hence, a separate plant is dedicated for the ash handling system.

Water supply from a river or canal

 A thermal power plant is required for different circuits, like

• Boiler water circuits
• Condensate water circuits
• Cooling water circuits.

For all the circuits, a large quantity of water is required and a continuous water source is required. River or canal water is used for the power plant.

Scheme Diagram of Power Plant

• Steam strikes turbine blades, lose their high pressure and blades rotate.
• Exhaust steam condenses in the condenser.
• This condensed water is supplied to an LP heater i.e. low-pressure heater as well as an HP heater i.e. high pressure to increase the temperature of this feed water.
• The turbine is connected to the generator, and power is generated.
• Scheme Diagram of Power Plant.

• Steam strikes turbine blades, lose their high pressure and blades rotate.
• Exhaust steam condenses in the condenser.
• This condensed water is supplied to an LP heater i.e. low-pressure heater as well as an HP heater i.e. high pressure to increase the temperature of this feed water.
• The turbine is connected to the generator, and power is generated.

Thermal Power Plant Working Diagram & Principle

The entire working principle is divided into a few parts for better understanding,

Coal Handling System

• Coal is the fuel that is burnt to get thermal energy. Each thermal power plant has its own coal-handling and ash-handling plant.
• Coal is collected from the wagon tippler and fed it to the crusher house through a conveyer belt.
• Crusher's house is having a vibrating screen.
• It crushes the coal and makes it granular.
• There may be a primary and secondary crusher house.
• These crusher houses make the coal into a fine powder
• Coarser coals are separated.
• The fine powder is fed to the boiler hopper through a conveyor belt and finally, it is fed to the furnace of the boiler.

Coal Handling in Thermal Plant

Ash Handling System

• After fuel combustion in the furnace, ash is collected through the bottom hopper.
• Fly ash is collected at the bottom of the ESP hoppers.

Ash Handling in Thermal Plant

Boiler Combustion

• Fuel is injected into the boiler.
• Spark is created and combustion starts.
• FD fan is used to supply air into the furnace.
• ID fan is used to release the flue gas from the boiler to the atmosphere through the chimney.

Boiler Water Circuit

• The boiler has water tubes all around the furnace.
• There will be two drums in the boiler, one is at the top i.e. boiler drum and another one is at the bottom, i.e. mud drum
• Both drums are connected to water tubes, a combination of riser and downcomers.
• Water-tube is heated by the furnace, hence, water is also heated.
• Hot water rises up to the boiler drum.
• Cold water is coming down to the furnace wall and continue the circulation process.
• Gradually the water in the boiler drum becomes hot water and starts to form steam.
• The steam becomes saturated and it again passes through the superheater to make superheated steam.
• Economizer, preheater, and reheater are used to increase the efficiency of the boiler.
• This superheated steam is fed to the turbine.

Coal Fired Boiler

Turbine Circuit

• Superheated steam is high-pressure steam and when it fed to the turbine, it strikes on the turbine blades.
• Pressure energy is changed into mechanical energy.
• Blades start rorating.
• There are H.P, I.P & L.P Turbines, and steam is provided H.P Turbine first.
• The turbine rotates with high speed.
• Turbine exhaust is supplied to the condenser.

HP-IP-LP Turbines

Condenser Circuit

• In the condenser, exhaust steam from the turbine is cooled.
• It is basically a heat exchanger.
• This cooling is by a separate cooling water circuit, which is connected to cooling towers.
• Steam comes below its boiling temperature.
• Phase change happens, that is from steam to liquid.
• Basically, condensate forms at the bottom of the condenser.
• Condensate from the condenser is supplied to the boiler through a deaerator.
• This circulation is done by condensate circulation pumps.
• Water is purified in the deaerator, purified means dissolved gasses are removed.
• This water then fed to the boiler through boiler feed pumps.

Cooling Tower Circuit

• The condenser is cooled by a cooling tower.
• Hot water from the condenser is circulated to the cooling towers.
• Cooled in the cooling towers, with the heat exchange between air and water by means of fans.
• Cooled water comes back to the condenser.
• Take the heat from the turbine exhaust and become heated.
• Continues the circuits.
• Water circulation happens by CW (Circulating Water) Pumps.

Generator Circuit

• When the turbine rotates, the shaft also rotates.
• This shaft is connected to a generator.
• The generator is having a starter along with the shaft or rotor.
• Due to the rotation of the rotor high magnetic field is created in the generator.
• Electricity is produced.
• It is stored in the powerhouse through the grid.

Water Treatment Plant

• In a thermal power plant, huge water is required in all the water circuits, at the beginning.
• Later on, due to water losses, in the various system like cooling towers, other losses, make-up water is required.
• This water is treated and demineralized water.
• There are several steps to produce this water which is used to feed as make-up.
• Water treatment consists of a raw water treatment plant, clarifiers, a pretreatment plant, RO plant, storage tanks, etc.

Advantages of Thermal Power Stations:

1. Economical for low initial cost other than any generating plant.
2. Land required less than a hydropower plant.
3. Since coal is the primary fuel and its cost is quite cheap than petrol/diesel so the generation cost is economical.
4. Maintenance is easier.
5. The thermal power plant can be installed in any location where transportation and bulk of water are available.

Disadvantages of Thermal Power Plants

1. The running cost for a thermal power station is comparatively high due to fuel, maintenance, etc.
2. A large amount of smoke causes air pollution. The thermal power station is responsible for Global warming.
3. The heated water that comes from a thermal power plant has an adverse effect on the aquatic life in the water and disturbs the ecology.
4. The overall efficiency of the thermal power plants is low less than 30%.

How coal-fired thermal power station works?