How a Thermal power plant works (Coal Based)?

  

   Electricity is one of the basic needs for all the people in the world. It helps us to power the electrical and electronic devices we use daily. This electricity comes to our home, from a faraway power plant. A Power Plant is a place in which electrical energy is generated through some means. There are various means through which electricity is generated. One of the major power plants that do a major contribution in generating a large amount of electricity is the thermal power plant. More than half of the electricity needs in the world is supplied by thermal power plants. There are many types of thermal power plants, which use the heat energy of a particular medium to generate electricity. The major types of thermal power plants are,

• Coal-based thermal power plant
• Nuclear thermal power plant
• Geothermal power plant
• Solar thermal power plant

  All these power plants use water as a working fluid for generating electricity. Among all these power plants, the coal-based thermal power plant is widely used to generate a huge amount of electricity.   

    In this topic, let us look at, how electricity is generated by a coal-based thermal power plant by understanding its construction and working principles. The below diagram shows the construction and operation of a coal-based thermal power plant,

The layout of Coal based thermal power plant

    A coal-based thermal power plant generates electricity, by producing heat energy from the chemical energy of the coal. This heat energy is produced, by burning a large amount of coal in the boiler. The produced heat in the boiler is then used to boil the water (feed water) in the water channel. Now the water will get converted into steam which will be fed into a steam turbine (the heart of the thermal power plant). Now the steam turbine starts to rotate by using the heat energy of the steam thereby converting the heat energy into kinetic energy and then to mechanical energy. When an alternator is coupled to the steam turbine, it also starts to rotate, thus generating electricity. As this process seems to be simple, several processes are done in between to generate quality electricity. Lets us briefly discuss the operation:

    The coal-based thermal power plant usually operates in four cycles to generate electricity. The four different cycles of a coal-based thermal power plant are,

• Coal and ash circuit
• Air and flue gas circuit
• Water and steam circuit
• Cooling water circuit

Coal and ash Circuit:

Coal

    In this first cycle, the main ingredient of the thermal power plant (coal) is brought inside the power plant and supplied through the conveyors. The coal supplied in the conveyors will vary in size which would be from minute particles to huge rocks. To get a clean-burning inside the boiler, the coal must be pulverized (crushed) in a pulverizer, before feeding into the boiler. After the coal has been pulverized in a pulverizer, it is blown and burnt up in the boiler. Most thermal power plants use a water tube boiler where the water flows inside a tube surrounded by heat, to convert the water into steam. After all the coal is burnt in the boiler it will be get converted into ash. This ash is then moved to the ash handling circuit where the ash mixtures are processed. This processed ash is then disposed to the ash storage part for other uses.

 

Credit (Water tube Boiler): By Jooja - This file was derived from: Water tube boiler schematic.png:, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=97301264

Air and Flue gas circuit:

    In order for a thing to get burnt, three things are required, namely, fuel, oxygen and heat. So for the coal to get burnt in the boiler, the air is sucked in from the atmosphere and applied into the boiler through a fan. But before applying, into the boiler, the air must be filtered from the moisture content to obtain a clean-burning inside the boiler. So the air is passed into the air preheater before being applied into the boiler. The air preheater uses the power of flue gas to preheat the air, coming from the atmosphere. This flue gas is generated from the burning of coal inside the boiler.

Credit (Flue gas) :By © Raimond Spekking / CC BY-SA 4.0 (via Wikimedia Commons), CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=20598312

Water and steam circuit:

  During the burning process inside the boiler, a large amount of heat will be generated. The temperature range developed inside the boiler will be in the range of 1100°C to 1200°C. Water walls carrying water is made near to the boiler to get converted into steam. But before passing the water near to the boiler, it is passed from the economiser section, where the water is preheated by the power of flue gas, generated during the burning of the coal.  Also, the water supplied to the boiler must be deaerated, which means it must be free from dissolved gases. The dissolved gases occur in the feed water, by the sucking of atmospheric air during the turbine stage. These dissolved gases may stick to the water walls and reduce the lifetime of the water walls. To remove the dissolved gases from the feedwater, a deaerator is used.

Deaerator

Working of Deaerator

    The deaerator separates the dissolved gases in the water, by supplying hot steam in contact with the feed water. The main principle of the deaerator is; that the gas solubility in the water decreases when the temperature of the water increases. So when the hot steam comes in contact with the feed water of the boiler, the dissolved gases like oxygen comes out from the feed water as bubbles. These gases are then vented out. When the preheated, deaerated water is passed to the water walls in the boiler through the downcomer, the water gets converted into steam due to the extreme temperature in the boiler. This steam is then given to the steam drum, where the steam is collected. The purpose of the steam drum is to remove the tiny droplets of water molecules present in the steam. Now this steam will be in the order of high pressure and high temperature. The collected steam in the steam drum is again superheated in a superheater to increase the efficiency of the power plant (The higher the temperature of the steam, the faster the steam turbine rotates and higher the efficiency). Superheating of the steam also helps in reducing the moisture content, thus increasing the temperature of the steam. But the turbine blades cannot withstand temperatures more than 600°C. So superheating is done to an approximate amount of 550°C and fed to the steam turbine.

Superheater coils

    The steam turbine is a collection of many blades which is attached to the periphery of the stator and rotor of the turbine. The blades are made in airfoil and bucket shape to make the turbine rotate. The airfoil shape is similar to the aeroplane wings which creates a lift force when a forced-air comes in contact with it. The steam turbine in the thermal power plants is mainly classified into three stages, namely high-pressure turbine, intermediate pressure turbine and low-pressure turbine. The necessity of classifying the steam turbine is because of the expansion of the steam. At first, the steam produced from the boiler will be in the order of high pressure and high temperature. But as it flows over the turbine blades its temperature and pressure drop, thus decreasing from a higher point to a lower point. So to accommodate the different pressure of the steam, different stages of turbines are used. A detailed explanation of the classification and working of the steam turbine is explained in another post. Please visit the below article for reference,

https://sciencetopic03.blogspot.com/2021/12/working-principle-and-compounding-of.html

 

Credit (Steam turbine): Image by Les Chatfield from flickr.com

      But the efficiency of the power plant decreases if we use the same steam to get passed all over the turbines. So the temperature of the steam is increased slightly before passing to the next stage of the turbine. So after the steam passes over the intermediate turbine it is carried over to the reheater, where it is reheated to gain more energy. The difference between a superheater and a reheater is the pressure. The reheater usually offers a low-pressure steam due to its low heating power than the superheater; while the superheater offers more pressurized steam. Both types of heaters, gain heating power from the boiler. Now this reheated steam is passed over to the low-pressure turbine to absorb power. The shaft of all the types of turbines is connected with a single shaft and is coupled with the alternator to generate electricity.

 Cooling Water circuit

Condenser working

    After the steam has got all over the turbines, it exits through the low-pressure turbine, and it is reused again by attaining the same criteria as high temperature and high pressure. But now the steam is at low pressure and low temperature. To change the steam back to the high stage, a condenser is used; which first converts the steam into liquid. A compressor can be used to easily convert the steam back to its stage. But adding a compressor would not be an efficient way to increase the efficiency of the power plant. So at first, the steam is converted into liquid, by a condenser. The steam is condensed in the condenser by the supply of cold water through the tubes around the steam. By this process, the steam gets cooled and gets converted into a liquid stage. This process is called condensation. The cold water to the condenser is mainly supplied by a river. The cold water absorbs the heat from the steam and becomes hot water. This hot water is fed to the cooling tower for reuse.

Credit (Surface Condenser): By © Marie-Lan Nguyen / Wikimedia Commons, CC BY 2.5, https://commons.wikimedia.org/w/index.php?curid=19518434

   The cooling tower is a type of tower that is used for cooling the hot water coming from the condenser, thus feeding again into the condenser for condensing the steam. Generally, the type of cooling tower used in thermal power plants is the natural draft cooling tower, and the heat transfer method used is the wet cooling method (Wet cooling method operates on the principle of evaporative cooling).  This cooling tower generally consists of three main parts namely,

• Water Distribution Nozzle
• Cooling Fill
• Drift Eliminator 

Cooling tower working

    When the hot water from the condenser is passed into the cooling tower, it is distributed over the cooling fins by the water distribution nozzle. As the hot water is made to pass over several cooling fills, the temperature of the water is decreased. This is done by the factor of surface area. Surface area is a concept in which, when a medium is passed over a large area, the temperature of the medium is made to decrease quickly. The cooling of the water also causes evaporation of the water, so a little quantity of the water is lost during this process. This evaporated water vapour also consists of some droplets with it. To use all of the water from the water vapour, a drift eliminator is used. This drift eliminator consists of blades that tend to decrease the velocity of the water droplets by creating an impact on them. This impact causes the water droplets in the water vapour to get fall back into the tower. Atmospheric air naturally flows into the cooling tower and cools the water even more. A huge fan unit is also used at the top of the cooling tower, in the case of the induced draft cooling system. This system sucks the air from outside, thus increasing the cooling efficiency of the water. Now the cold water is stored in the cold water basin and sent back to the condenser. The evaporated water is then sent out to the atmosphere.

 

Cooling tower

    After the liquid conversion of the steam in the condenser takes place, it is passed inside a long tube through the feedwater pump. The pump generally used is a multi-stage centrifugal pump. This feed water pump increases the pressure of the liquid by pumping it. Thus we have achieved one criterion for the steam passing through the turbines (i.e., high pressure). This pump passes the water to the economiser, where the feed water is preheated by the flue gas, generated through the burning process of the coal. The preheated water is then passed into the water downpipe and through the water channel around the boiler to get heated again. This resultant steam is then stored in the steam drum, which had achieved the criteria as high pressure and high temperature. Again this steam is heated in a superheater for higher efficiency and is ready to be applied to the turbines for another cycle. This process is repeated again and again and the electricity is generated continuously.

Credit (Alternator) :By Peter Van den Bossche from Mechelen, Belgium - Generator, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=3240227

     The main objective of the thermal power plant is to generate electricity at a standard frequency. As the load connected to the system varies according to time, the speed of the alternator also changes frequently which alters the frequency of the supply. So the speed of the turbine also varies. For this purpose, a governor mechanism is used. If the speed of the turbine reduces, the governor mechanism allows more steam to get passed over the turbine blades to rotate it at a high speed, to generate more electricity; and if the load decreases, the turbine speed increases and the governor mechanism allows less steam to flow into the turbine blades to make it rotate slowly. In this way, we can achieve a clean energy output from the coal-based thermal power plant.