• Basic schemes and working principle

    A coal based thermal power plant converts the chemical energy of the coal into electrical energy. This is achieved by raising the steam in the boilers, expanding it through the turbine and coupling the turbines to the generators which converts mechanical energy into electrical energy.

    In a coal based power plant coal is transported from coal mines to the power plant, usually by railway in wagons. This coal from the mines is of no uniform size. So it is taken to the Crusher house and crushed to a size of 20mm. From the crusher house the coal is either stored in dead storage (generally 40 days coal supply) which serves as coal supply in case of shortage of coal supply or to the live storage (1 day coal supply) in the raw coal bunker in the boiler house. Raw coal from the raw coal bunker is supplied to the coal mills (also known as pulverised fuel mill (16)) by a coal conveyor (14). The coal mills pulverizes the coal to 200 mesh size (i.e., 0.074 mm). The powdered coal from the coal mills is mixed with preheated air (24) driven by the forced draught fan (20). The hot air-fuel mixture is forced at high pressure into the boiler where it rapidly ignites. The temperature in fire ball is of the order of 1300 0C. The water tube boiler hanged from the top of the boiler convert water of a high purity into saturated steam (results when water is heated to the boiling point (sensible heating) and then vaporized with additional heat (latent heating)), and saturated steam is passed to the boiler drum.

    The saturated steam from the boiler drum is taken to the superheater for superheating (19) where its temperature and pressure increase rapidly to around 200 bars and 570°C, sufficient to make the tube walls glow a dull red. The superheated steam from the superheater is taken to the High Pressure Steam Turbine (HPT) (11), the first of a three-stage turbine process. In the HPT, the steam pressure is utilized to rotate the turbine and the resultant is rotational energy. From the HPT the out coming steam is taken to the reheater (21) in the boiler to increase its temperature as the steam becomes wet at the HPT outlet. After reheating, this steam is taken to the Intermediate Pressure Turbine (IPT) (9) and then to the Low Pressure Turbine (LPT) (6). The exiting steam of the LPT, now a little above its boiling point, is brought into thermal contact with cold water (pumped in from the cooling tower) in the condenser (8), where it condenses rapidly back into water, creating near vacuum-like conditions inside the condenser chest. The condensed water is then passed by a condensate pump (7) to a deaerator (12), then pumped by feedwater pump (28) and pre-warmed, first in a feed heater (13) powered by steam drawn from the high pressure set, and then in the economiser (23), before being returned to the boiler drum. The cooling water from the condensor is sprayed inside a cooling tower (1), creating a highly visible plume of water vapor, before being pumped back to the condenser in cooling water cycle. The turbine sets are coupled on the same shaft as the three-phase electrical generator (5) which generates an intermediate level voltage (typically 20-25 kV). This is stepped up by the transformer (4) to a voltage more suitable for transmission (typically 220-765 kV in India) and is sent out onto the three-phase transmission system (3). Exhaust gas from the boiler is drawn by the induced draft fan (26) through an electrostatic precipitator (25) and is then vented through the chimney stack (27).

    Components of a coal-fired thermal power station

    1. Cooling tower. 2. Cooling water pump. 3. Transmission line (3-phase). 4. Unit transformer (3-phase). 5. Electric generator (3-phase). 6. Low pressure turbine. 7. Condensate extraction pump. 8. Condenser. 9. Intermediate pressure turbine. 10. Steam governor valve. 11. High pressure turbine. 12. Deaerator. 13. Feed heater. 14. Coal conveyor. 15. Coal hopper. 16. Pulverised fuel mill. 17. Boiler drum. 18. Ash hopper. 19. Superheater. 20. Forced draught fan. 21. Reheater. 22. Air intake. 23. Economiser. 24. Air preheater. 25. Precipitator. 26. Induced draught fan. 27. Chimney stack. 28. Feed pump.

    Figure 1: Schematic diagram of thermal power plant

     

    Components of Coal Fired Thermal Power Station:

     

    A. Coal Handling Plant (CHP): The function of CHP is automatic feeding of coal to the boiler furnace.

     

    1. Fuel preparation system: In coal-fired power plants, the raw coal from the coal storage area is first crushed into small pieces and then conveyed to the coal feed hoppers at the boilers. The coal is next pulverized into a very fine powder, so that coal will undergo complete combustion during combustion process. Pulverizer is a mechanical device for the grinding of many different types of materials. For example, they are used to pulverize coal for combustion in the steam-generating furnaces of fossil fuel power plants.

     

    2. Dryers:  They are used in order to remove the excess moisture from coal mainly wetted during transport. As the presence of moisture will result in fall in efficiency due to incomplete combustion and also result in CO emission. 

     

    3. Magnetic separators: Coal which is brought may contain iron particles. These iron particles may result in wear and tear. The iron particles may include bolts, nuts, wire, fish plates etc. so these are unwanted and so are removed with the help of magnetic separators.

     

    The coal we finally get after these above process are transferred to the storage site. Storage of coal is primarily a matter of protection against the coal strikes, failure of the transportation system & general coal shortages. Storage permits some choice of the date of purchase, allowing the purchaser to take advantage of seasonal market conditions.

     

    There are two types of storage:

    a. Live Storage (boiler room storage): Storage, which supplies coal directly to the process and sufficient to meet 24 hour demand of the plant. It's also known as active storage.

     

    b. Dead storage: Storage, which guards against transportation. Mainly it is for longer period of time, and it is also mandatory to keep a backup of fuel for specified amount of days depending on the reputation of the company and its connectivity.

    There are many forms of storage some of which are –

    (i) Stacking the coal in heaps over available open ground areas.

    (ii) As in (i), But placed under cover or alternatively in bunkers.

    (iii) Allocating special areas and surrounding these with high reinforced concerted retaking walls.

     

    B. Boiler and auxiliaries

    A Boiler or steam generator essentially is a container into which water is converted into steam by the application of heat. Usually boilers are coal or oil fired. Thermal energy released by combustion of fuel is transferred to water, which vaporizes and gets converted into saturated steam at the desired temperature and pressure. The functions of a boiler thus can be stated as:-

    i) To convert chemical energy of the fuel into heat energy

    ii) To transfer this heat energy to water for evaporation as well to steam for superheating.

     

    The basic components of Boiler are:

    1) Boiler Furnace: A boiler furnace is a chamber in which fuel is burnt to liberate the heat energy. In addition, it provides support and enclosure for the combustion equipment i.e., burners. The boiler furnace walls are made of refractory materials such as fire clay, silica, kaolin etc. These materials have the property to resist change of shape, weight or physical properties at high temperatures. The refractory walls are made hollow and air is circulated through hollow space to keep the temperature of the furnace walls low. The recent development is to use water walls. These consist of plain tubes arranged side by side and on the inner face of the refractory walls. The tubes are connected to the upper and lower headers of the boiler. The boiler water is made to circulate through these tubes. The water walls absorb the radiant heat in the furnace which would otherwise heat up the furnace walls.
     

    2) Economiser: It is a device which heats the feed water on its way to boiler by deriving heat from the flue gases. This results in raising boiler efficiency, saving in fuel and reduced stresses in the boiler due to higher temperature of feed water. An economiser consists of a large number of closely spaced parallel steel tubes connected by headers of drums. The feed water flows through these tubes and the flue gases flow outside. A part of the heat of flue gases is transferred to feed water, thus raising the temperature of the latter. So by using the waste flue gas to heat the water, we are saving/economising extra coal usage. That is why this is called Economiser. At the same time Boiler Heat loss will be less, Boiler efficiency increases, most important we are reducing Global warming.
     

    3) Water Tube: A heat exchange devices that heat fluids, usually water, about the boiling point of liquid.
     

    4) Superheater: A superheater is a device which removes the last traces of moisture from saturated steam and converts it into superheated steam, where its temperature and pressure increase rapidly to around 200 bars and 570°C. It consists of a set of tubes through which wet or saturated dry steam flows and hot combustion gases pass around these tubes. By this way, the wet or saturated dry steam takes heat from the flue gases and become superheated. Superheated steam causes lesser erosion of turbine blades and reduces the consumption of fuel and water but there is a price to pay in increased maintenance costs. In most cases, the benefits outweighed the costs and superheaters were widely used. Superheaters are classified into three types according to mode of heat transfer from flue gases to steam viz: (a) Convection superheater (b) Radiant superheater (b) Combination superheater.
     

    a. Convective superheaters: In the convective superheaters, the superheaters are placed between or near the water tubes where the superheater tubes receive heat by convection from combustion gases. The  convective  superheater  is  shielded  away  from  the  furnace  and  the  flame and placed usually ahead of economiser. The convective superheaters are often termed as primary superheaters.
     

    b. Radiant superheaters: In the radiation superheaters, the superheaters are placed in the walls of the furnace of a steam boiler where the superheater tubes receive heat by direct radiation from fire and re-radiation from refractory walls. The radiant superheaters are often termed as secondary superheaters.      

                                     

    c. Combination superheaters: A combined superheater has both convection and radiation sections. The steam leaving the boiler drum first passes through the convective superheater and then through the radiant superheater.

     

    5) Reheater: They are the same as the superheaters but as their temperature are less than that of superheaters and their pressure is 20%-25% less than the super-heater, they can stand less quality material alloys. The function of reheater is to resuperheat the partly expanded steam from the HP turbine. This ensures that steam remains dry as far as possible through the last stage of the turbine.
     

    Figure: Boiler and auxiliaries

     

    C. Air Preheater

    After the flue gases leave economiser, some further heat can be extracted from flue gas by air preheater (APH) and used to heat incoming air for combustion. That is the heat carried out with the flue gases are further utilized for preheating the air before supplying to the combustion chamber and for supply of hot air for drying the coal in pulverized fuel systems so that satisfactory combustion of fuel takes place in the furnace.
     

    Figure: Air Preheater


    As a consequence, the flue gases are also sent to the flue gas stack (or chimney) at a lower temperature, allowing simplified design of the ducting and the flue gas stack. It also allows control over the temperature of gases leaving the stack (to meet emissions regulations, for example).

     

    D. Steam turbines

    Steam turbines have been used predominantly as prime mover in all thermal power stations. It converts steam energy into mechanical energy. The steam turbine uses pressurized steam from a boiler as the working fluid. The super-heated steam entering the turbine loses its pressure (enthalpy) moving through the blades of the rotors, and the rotors move the shaft to which they are connected. Steam turbines deliver power at a smooth, constant rate, and the thermal efficiency of a steam turbine is higher than that of a reciprocating engine but lower than gas turbine engines due to higher operating temperatures of the gas turbines (Gas turbines ~1500°C and steam turbines ~550°C).The steam turbines are of two types:-

     

    1. Impulse turbine: An impulse turbine is a type of steam turbine where the rotor derives its rotational force from the impact force, or the direct push of steam on the blades. The steam enters the impulse turbine through a fixed nozzle (stationary blades) to the turbine's bucket shaped rotor blades where the pressure exerted by the fixed nozzles causes the rotor to rotate and the velocity of the steam to reduce as it imparts its kinetic energy to the blades. The blades in turn change the direction of flow of the steam however its pressure remains constant as it passes through the rotor blades since the cross section of the chamber between the blades is constant. Impulse turbines are therefore also known as constant pressure turbines.

     

    2. Reaction turbine: A reaction turbine is a type of turbine that develops torque by reacting to the pressure or weight of a fluid; the operation of reaction turbines is described by Newton's third law of motion (action and reaction are equal and opposite). In a reaction turbine, unlike in an impulse turbine, the nozzles that discharge the working fluid are attached to the rotor. The acceleration of the fluid leaving the nozzles produces a reaction force on the pipes, causing the rotor to move in the opposite direction to that of the fluid. As the steam progresses through the nozzles, its velocity increases while at the same time its pressure decreases. Thus the pressure decreases in both the fixed and moving blades.

     

           Figure: Impulse Turbine          Figure: Reaction Turbine

     

    The turbine generator consists of a series of steam turbines interconnected to each other and a generator on a common shaft. There is a high pressure turbine at one end, followed by an intermediate pressure turbine, low pressure turbines, and the generator. The steam at high temperature (536 0C to 540 0C) and pressure (140 to 170 kg/cm2) is expanded in the turbine.

     

    E. Condenser 

    The condenser is a device which condenses the steam from the exhaust of the turbine into liquid. It serves two important functions. Firstly, it creates a very low pressure (as a empty region is created by liquidating steam) at the exhaust of turbine, thus permitting expansion of the steam in the prime mover to a very low pressure. This helps in converting heat energy of steam into mechanical energy in the prime mover. Secondly, the condensed steam can be used as feed water to the boiler. There are two types of condensers, namely:
     

    1. Surface Condenser: In a surface condenser, there is no direct contact between cooling water and exhausted steam. It consists of a series of horizontal tubes enclosed in a cast iron shell. The cooling water flows through the tubes and exhausted steam over the surface of the tubes. The steam gives up its heat to water and is itself condensed.

     

    Advantages of a surface condenser are as follows:

    a. The condensate can be used as boiler feed water.

    b. Cooling water of even poor quality can be used because the cooling water does not come in direct contact with steam.

    c. High vacuum (about 73.5 cm of Hg) can be obtained in the surface condenser. This increases the thermal efficiency of the plant.

    d. Less pumping power required.

    e. It is suitable for high capacity.

     

    Disadvantages of the surface condenser are as follows:

    a. The capital cost is more.

    b. The maintenance cost and running cost of this condenser is high.

    c. It is bulky and requires more space.
     

    2. Jet Condenser: In a jet condenser, cooling water and exhausted steam are mixed together. Therefore, the temperature of cooling water and condensate is the same when leaving the condenser.

    Advantages of a jet condenser are as follows:

    a. Capital cost, running cost and maintenance cost are low.

    b. It is not bulky and requires less floor area.

    c. Cooling water requirement is less.

     

    Disadvantages of the jet condenser are as follows:

    a. The condensate cannot be used as boiler feed water.

    b. Relatively less vacuum is created. This causes relatively poor thermal efficiency of the plant.

    c. High pumping power required.

    d. It is not suitable for high capacity.
     

    F. Boiler feed pump

    Boiler feed pump is a multi stage pump provided for pumping feed water to economiser. BFP is the biggest auxiliary equipment after Boiler and Turbine. It consumes about 4 to 5 % of total electricity generation.

     

    G. Cooling tower

    A cooling tower is a specialized heat exchanger, a semi-closed building-like structure, in which atmospheric air (the heat receiver) and hot water (the heat source) are brought into direct or indirect contact with each other in order to reduce the water's temperature. As this occurs, a small volume of water is evaporated, reducing the temperature of the water being circulated through the tower. The cooling towers are of four types:
     

    1. Natural Draft cooling tower: The cooling principle is the same (atmospheric cooling with wet technology), but the fan unit is missing. Here heat is removed from the cooling tower using a natural draft i.e., Warm moist air naturally rises due to the density different compared to the outside air. The natural draft cooling tower is the right choice for large power plants.
     

    2. Forced Draft cooling tower: In forced draft cooling tower, air is pushed by blowers located at the base of the air inlet face.
     

    3. Induced Draft cooling tower: Induced draft cooling towers have fans that are typically mounted on top of the unit and pull air through the fill media.
     

    4. Fan assisted natural draught cooling tower: A hybrid type that appears like a natural draft setup, though airflow is assisted by a fan.

     

    H. Fan or draught system

    In a boiler it is essential to supply a controlled amount of air to the furnace for effective combustion of fuel and to evacuate hot gases formed in the furnace through the various heat transfer area of the boiler. This can be done by using a chimney or mechanical device such as fans which acts as pump.

     

    1. Natural draught: When the required flow of air and flue gas through a boiler can be obtained by the stack (chimney) alone, the system is called natural draught. The draught produced by the chimney is due to the pressure difference of hot gases in the chimney and cold air outside the chimney. When the gas within the stack is hot, its specific weight will be less than the cool air outside; therefore there will be a pressure difference. This difference in the pressure will cause flow of gas from base of the stack to top of the stack.
     

    2. Mechanized draught: In a mechanical draught, the movement air and flue gas are due to the action of fan. There are 3 types of mechanized draught systems:
     

    a. Forced draught:  In this system a fan called forced draught fan (blower) is installed at the base of the boiler. This fan forces the atmospheric air through the boiler furnace and pushes out the hot gases from the furnace through superheater, reheater, economiser and air heater to stacks. This draught system is known as positive draught because the pressure of air throughout the system is above atmospheric pressure.
     

    b. Induced draught:  Here a fan called induced draught fan is provided at the outlet of boiler, more specifically just before the chimney. This fan sucks hot gases from the furnace through the superheaters, economiser, reheater and discharges gas into the chimney. ID fan will produce the pressure lower than the atmospheric pressure in the system or we may say that ID fan will produce the negative pressure in the furnace to remove the flue gases from furnace via electrostatic precipitators and to push the flue gases to chimney.
     

    c. Balanced draught: In this system both FD fan and ID fan are provided. The FD fan is utilized to draw control quantity of air from atmosphere and force the same into furnace. The ID fan sucks the product of combustion from furnace and discharges into chimney. The point where draught is zero is called balancing point.

     

    Figure: Balance draught system

     

    I. Ash handling system

    In Thermal Power Plants, coal is generally used as fuel and hence the ash is produced as the byproduct of Combustion. Ash generated in power plant is about 30-40% of total coal consumption and hence the system is required to handle Ash for its proper utilization or disposal. The disposal of ash from a large capacity power station is of same importance as ash is produced in large quantities. Ash handling is a major problem.

    a. Manual handling: While barrows are used for this. The ash is collected directly through the ash outlet door from the boiler into the container from manually.

    b. Mechanical handling: Mechanical equipment is used for ash disposal, mainly bucket elevator, belt conveyer. Ash generated is 20% in the form of bottom ash and next 80% through flue gases, so called Fly ash and collected in ESP.

    c. Electrostatic precipitator: An electrostatic precipitator (ESP), or electrostatic air cleaner is a dust collection device that removes particles from flue gas using the force of an induced electrostatic charge. The ESP has plate banks which are insulated from each other between which the flue gases are made to pass. The dust particles are ionized (negatively charged) and attracted by charged electrodes (positively charged). The electrodes are maintained at 60 kV. Hammering is done to the plates so that fly ash comes down and collects at the bottom. The fly ash in dry form is used in cement manufacture.

     

    J. Deaerator: A deaerator is a device that is widely used for the removal of oxygen and other dissolved gases from the feedwater to steam-generating boilers. In particular, dissolved oxygen in boiler feedwaters will cause serious corrosion damage in steam systems by attaching to the walls of metal piping and other metallic equipment and forming oxides (rust). Dissolved carbon dioxide combines with water to form carbonic acid that causes further corrosion.
     

    K. Electrical equipment: A modern power station contains various electrical equipments. However, the most important items are:

    1. Alternators: Each alternator is coupled to a steam turbine and converts mechanical energy of the turbine into electrical energy. The alternator may be hydrogen or air cooled. The necessary excitation is provided by means of main and pilot exciters directly coupled to the alternator shaft.
     

    2. Transformers: A generating station has different types of transformers, viz.,

    a. Main step-up transformers which step-up the generation voltage for transmission of power.

    b. Station transformers which are used for general service (e.g., lighting) in the power station.

    c. Auxiliary transformers which supply to individual unit-auxiliaries.
     

    3. Switchgear: It houses such equipment which locates the fault on the system and isolate the faulty part from the healthy section. It contains circuit breakers, relays, switches and other control devices.
     

    Advantages of coal based thermal Power Plant

    1. A portion of the steam generated can be used as a process steam in different industries.

    2. Steam engines and turbines can work under 25 % of overload continuously.

    3. Fuel used is cheaper.

    4. They form the backbone of grid as they provide stable output and are more reliable than renewable sources that tend to fluctuate.

    5. It requires less land per Megawatt with respect to Hydro, Solar, and Wind.

    6. Economical in initial cost compared to hydro plants and running costs are less compared to gas plants or diesel plants.

    7. Thermal plants can be placed near load centers unlike hydro and nuclear plants. Hence transmission of power losses can be minimized

     

    Disadvantages of coal based thermal Power Plant

    1. Maintenance costs are high.

    2. Long time required for erection and putting into action.

    3. A large quantity of water is required.

    4. Great difficulty experienced in coal handling and disposal of ash, nearly 20% to 30% of coal is rejected as Ash, which is a waste and needs to be dumped..

    5. Apart from CO2, other harmful gases like NOX and SOX are also generated lead to Acid Rain. Though with advanced combustion and other techniques these are lowered and also captured.

    6. Unavailability of good quality coal.

    7. Maximum of heat energy lost.

    8. Gestation period (period for commissioning of plant) takes long time

    9. Efficiency of thermal plant is quite less (30-35%)

    10. Operational cost of thermal plant is more costly compared to hydro and nuclear plant.

    11. Thermal power plant take a lot of time to start up ( generally 4–8 hrs) and don’t do well in cycling that is large up and down in power generation and mostly designed for constant load. Therefore any fluctuation is power demand can be detrimental to it life cycle.

     

    Efficiency of a Thermal Power Station

    A huge amount of heat is lost in various stages of the plant. Major part of heat is lost in the condenser. That is why the efficiency of thermal plants is quite low.

    1. Thermal Efficiency: The ratio of 'heat equivalent of mechanical energy transmitted to the turbine shaft' to the 'heat of coal combustion' is called as thermal efficiency. Thermal efficiency of modern thermal power stations is about 30%. It means, if 100 calories of heat are produced by coal combustion, the mechanical energy equivalent of 30 calories will be available at the turbine shaft.

    2. Overall Efficiency: The ratio of 'heat equivalent of electrical output' to the 'heat produce by coal combustion' is called as overall efficiency. The overall efficiency of a thermal plant is about 29% (slightly less than the thermal efficiency).

     

    Site Selection for Thermal Power Plant:

    1. Land requirement: The land for the thermal power plant should be large enough so that the present installation and future expansion of the plant can be easily done. Cost and bearing capacity of the land also plays important role while selecting a site for thermal power plant.

    2. Water supply: Site should be near to the river so that water required for the ash disposal, boiler feed water, cooling and circulating water for condensers should be available easily.

    3. Fuel supply: Plant should be near to the coal mines, because cost of transmission of electricity is less than cost of transportation of fuel.

    4. Ash disposal: Making ash ponds is a part of installation of the plant. Enough space should be there for ash ponds and water supply should available for it.

    5. Transport facilities: For fuel supply – road and railway links. Located near coal mines because if quality of coal is poor transportation of coal is costly.

    6. Environment requirement: Thermal power plant produces lot of pollution. Ash ponds may produce water and air pollution, smoke from chimneys produces air pollution. To avoid affects of pollution site should be far from populated area.