Thermal Power Plant or Thermal Energy (Chapter-2)

Energy conversion is the process of changing one form of energy into another, a fundamental capability that enables modern civilization to function. It can occur in various ways, from converting the kinetic energy of wind into mechanical power through windmills to transforming solar energy into electrical energy in solar panels. This transformation is essential not just for daily usage but also for harnessing and utilizing natural resources more efficiently. In the context of rural electrification, this process plays a critical role. By converting available local energy resources into electricity, rural communities can access a stable and reliable power supply. This not only improves the quality of life but also supports economic development by powering homes, schools, businesses, and healthcare facilities. Consequently, energy conversion facilitates the broader goal of rural electrification, demonstrating the interconnection between technological innovation and societal advancement.

1. Chapter-2 Thermal Power Plant Introduction: In Steam/Thermal power plants, the heat of combustion of fossil fuels is utilized by the boilers to raise steam at high pressure and temperature. The steam so produced is used in driving the steam turbines or sometimes steam engines couples to generators and thus in turn generating electrical energy. Steam power plants may be installed either to generate electrical energy only or generate electrical energy along with generation of steam for industrial purposes such as in paper mills, textile mills, sugar mills, Beer brewing process and food industry etc.

2. Figure: Schematic diagram of a Thermal power plant.

3. Major Components of a Thermal Power Plant Coal and ash handling arrangement Steam generating plant Steam turbine Alternator Feed water and Cooling arrangement The whole arrangement can be divided into the following stages for the sake of simplicity :

4. Cont.… 1. Coal and ash handling plant . From the coal storage plant, coal is delivered to the coal handling plant where it is pulverised ( i.e ., crushed into small pieces) in order to promoting rapid combustion process. The pulverised coal is fed to the boiler by belt conveyors. The coal is burnt in the boiler and the ash produced after the complete combustion of coal is removed to the ash handling plant and then delivered to the ash storage plant for disposal.

5. Cont.… 2 . Steam generating plant. The steam generating plant consists of a boiler for the production of steam and other auxiliary equipment for the utilisation of flue gases . i. Boiler A boiler (or steam generator) is a closed vessel in which the heat of combustion of coal inside it is utilised to convert water into steam at high temperature and pressure.

6. Cont.… The flue gases from the boiler make their journey through superheater , economiser, air pre-heater and are finally exhausted to atmosphere through the chimney. Major types of boilers are: (i) fire tube boiler and (ii) water tube boiler Generally water tube boilers are used for electric power stations. In this boiler, the water flows inside the tubes and hot gases flow outside the tube. Water tube boiler are classified as Vertical tube boiler Horizontal tube boiler Inclined tube boiler

7. Cont.… Figure :(a) inclined Water tube (b) vertical water tube (c) fire tube boilers (a) (b) (c)

8. Cont.… ii. Super heater : The function of the super heater is to remove the last trash of moisture from the saturated steam leaving the boiler tubes and also increases its temperature above the saturation temperature. For this purpose the heat of the combustion gases (Flue gases) from the furnace (boiler) is utilized. Super heated steam is that steam which contains more heat than the saturated steam at the same pressure. The additional heat provide more energy to the turbine hence power out put is more and hence increase the overall efficiency.

9. Cont.… Figure : A super heater

10. Cont.… iii. Economiser . An economiser is essentially a feed water heater and derives heat from the flue gases for this purpose. The feed water is fed to the economiser before supplying to the boiler. The economiser extracts a part of heat of flue gases to increase the feed water temperature . An increase of about 20% in boiler efficiency is achieved by providing both economizer and air pre-heaters. Economizer alone gives only 10-12% efficiency increase, causes saving in fuel consumption 5-15 %.

11. Cont.… iv. Air preheater An air preheater increases the temperature of the air supplied for coal burning by deriving heat from flue gases. Air is drawn from the atmosphere by a forced draught fan and is passed through air preheater before supplying to the boiler furnace. The air preheater extracts heat from flue gases and increases the temperature of air used for coal combustion.

12. Cont.… 3. Steam turbine. The dry and superheated steam from the superheater is fed to the steam turbine through main valve. The heat energy of steam when passing over the blades of turbine is converted into mechanical energy. After giving heat energy to the turbine, the steam is exhausted to the condenser which condenses the exhausted steam by means of cold water circulation. .

13. Cont.… Figure: Steam turbines.

14. Cont.… 4. Alternator. The steam turbine is coupled to an alternator. The alternator converts mechanical energy of turbine into electrical energy. The electrical output from the alternator is delivered to the bus bars through transformer, circuit breakers and isolators,

15. Cont.… 5. Feed water. The condensate from the condenser is used as feed water to the boiler . The feed water on its way to the boiler is heated by water heaters and economiser. This helps in raising the overall efficiency of the plant. 6. Cooling arrangement. In order to improve the efficiency of the plant, the steam exhausted from the turbine is condensed* by means of a condenser. The circulating water takes up the heat of the exhausted steam and itself becomes hot.

16. Cont.… This hot water coming out from the condenser is discharged at a suitable location down the river. In case the availability of water from the source of supply is not assured throughout the year, cooling towers are used. During the scarcity of water in the river, hot water from the condenser is passed on to the cooling towers where it is cooled . The cold water from the cooling tower is reused in the condenser.

17. Steam Cycle: Figure 2 shows a simplified version of the major components of a typical steam plant cycle. This is a simplified version and does not contain the exact detail that may be found at most power plants. However, for the purpose of understanding the basic operation of a power cycle, further detail is not necessary. The following are the processes that comprise the cycle: 1-2: Saturated steam from the steam generator is expanded in the high pressure (HP) turbine to provide shaft work out put at constant entropy. 2-3: The moist steam from the exit of the HP turbine is dried and superheated in the moisture separator reheate r(MSR ).

18. Cont.… 3-4: Super heated steam from the MSR is expanded in the low pressure (LP) turbine to provide shaft work output at constant entropy. 4-5: Steam exhaust from the turbine is condensed in the condenser in which heat is transferred to the cooling water under a constant vacuum condition. 5-6: The feed water is compressed as a liquid by the condensate and feed water pump and the feed water is preheated by the feed water heaters. 6-1: Heat is added to the working fluid in the steam generator under a constant pressure condition.

19. Cont.… Figure 2 : Typical Steam Plant Cycle

20. Cont.… The previous cycle can also be represented on a T-s diagram as was done with the ideal Carnot cycle. This is shown in Figure 3. Figure 3 : Steam Cycle (Ideal)

21. Fuels and Combustion: Fuels : We use mainly oil and coal, to heat the water to produce the steam to derive the turbine for huge power generation system (thermal power generation). These fuels (such as coal, oil and natural gas) are often referred to as fossil fuels. Fuels are majorly classified into Liquid fuels Solid fuels Gas fuels

22. Cont.… Liquid fuels: Liquid fuels like furnace oil and Low Sulphur Heavy Stock (LSHS) are predominantly used in industrial applications. Liquid fuels are Liquid diesel oil Furnace oil LSHS Kerosene

23. Cont.… Sulphur content for the furnace oil is in the order of 2 – 4% because the main disadvantages of sulphur is the risk of corrosion by sulphuric acid formed during and after combustion and condensation in cool parts of the chimney, air preheater and economizer. The gross calorific values of the liquid fuel are in the range of 10500 to 11100 Kcal/Kg. The ash value is in the range of 0.03 to 0.07%.

24. Cont.… Solid fuels: Solid fuels are nothing but a coal. Coal is classified into Anthracite (oldest coal from a geological perspective and it is a hard coal) Bituminous Lignite (youngest coal from a geological perspective and it is a soft coal) Coal is further classified into Semi Anthracite Semi Bituminous Sub Bituminous

25. Cont.… The common coals used in industry are Bituminous and Sub Bituminous coal. The gradation of coal is based on its calorific value and it is shown in table below. Grade Calorific Value in Kcal/Kg. A Exceeding 6200 B 5600 – 6200 C 4940 – 5600 D 4200 – 4940 E 3360 – 4200 F 2400 – 3360 G 1300 – 2400

26. Cont.… Gaseous fuels: These are most convenient because they require the least amount of handling and are used in the simplest and most maintenance free burner system. Gas is delivered “on tap” via a distribution network and it is suited for highly populated area or industrial density. Types of gaseous fuels are Fuels naturally found in nature (natural gas, methane gas from coal mines) Fuel gases made from solid fuel (gases derived from coal, waste and biomass) . Gases made from petroleum (Liquefied Petroleum Gas [LPG] and Refinery gases).

27. Combustion: Combustion refers to the rapid oxidation of fuel accompanied by the production of heat and light. Complete combustion of a fuel is possible only in the presence of an adequate supply of oxygen (O 2 ). Oxygen is one of the common elements on earth. That is 20.9% of oxygen is present in the air. Rapid fuel oxidation results in large amount of heat. Solid or liquid fuels must be changed to gas before they will burn. For this the heat is required to change the state (solid or liquid to gas ).

28. Cont.… Fuel gas will burn in their normal state if enough air is present. . The objective of good combustion is to release all of the heat in fuel. This is accomplished by controlling three T’s of Combustion which are Temperature – high enough to ignite Turbulence – intimate mixing of the fuel and oxygen Time – sufficient for complete combustion.

29. Gas Turbine: A gas turbine , also called a combustion turbine , is a type of internal combustion engine . It has an upstream rotating compressor coupled to a downstream turbine , and a combustion chamber in between and it is shown in Figure below.

30. Cont... The basic operation of the gas turbine is similar to that of the steam power plant except that air is used instead of water. Fresh atmospheric air flows through a compressor that brings it to higher pressure. Energy is then added by spraying fuel into the air and igniting it so the combustion generates a high-temperature flow. This high-temperature high-pressure gas enters a turbine, where it expands down to the exhaust pressure , producing a shaft work output in the process.

31. Cont.. The turbine shaft work is used to drive the compressor and other devices such as an electric generator that may be coupled to the shaft. The energy that is not used for shaft work comes out in the exhaust gases . Gas turbines are used to power aircraft , trains , ships , electrical generators , or even tanks .

32. Selection of site for thermal power plant Nearness to the load centre: The power plant should be as near as possible to the load centre to the centre of load . So that the transmission cost and losses are minimum. Water resources: For the construction and operating of power plant large volumes of water are required for the following reasons. To raise the steam in boiler. To raise the steam in boiler For cooling purpose such as in condensers As a carrying medium such as disposal of ash.

33. Cont.… Availability of Coal: Huge amount of coal is required for raising the steam. So, for power generation purposes, the steam power plants should be located near the coal mines to avoid the transport of coal & ash. Land Requirement: The land is required not only for setting up the plant but for other purposes also such as staff colony, coal storage, ash disposal etc. Transportation Facilities : The facilities must be available for transportation of heavy equipment and fuels e.g near railway station. Labour supplies: Skilled and unskilled laborers should be available at reasonable rates near the site of the plant.

34. Advantages And Disadvantages Of Thermal Power Plant Advantages: Less initial cost as compared to other generating stations. It requires less land as compared to hydro power plant. The fuel (i.e. coal) is cheaper. The cost of generation is lesser than that of diesel power plants. Disadvantages: It pollutes the atmosphere due to the production of large amount of smoke. This is one of the causes of global warming. The overall efficiency of a thermal power station is low (less than 30%). Requires long time for errection and put into action. Requirement of water in huge quantity.

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