- Introduction
A thermal power plant may be a power plant during which heat is converted to electrical power. In most of the places within the world, the turbine is steam-driven. Water is heated, turns into steam, and spins a turbine which drives an electrical generator. After it passes through the turbine, the steam is condensed during a condenser and recycled to where it had been heated; this is often referred to as a Rankine cycle. The greatest variation within the design of thermal power stations is thanks to the various heat sources; fuel dominates here, although nuclear heat and solar heat are also used. Some like better to use the term energy center because such facilities convert sorts of heat into electricity. Certain thermal power stations also are designed to supply heat for industrial purposes, or district heating, or desalination of water, additionally to generating electric power.
A thermal power generation plant or thermal power plant is the most conventional source of electrical power. The thermal power station is additionally referred to as a coal thermal power station and turbine power station. Before going into detail of this subject, we’ll attempt to understand the road diagram of the electrical power generation plant.
2. Theory of Thermal Power Station
The theory of thermal power plants or working of the thermal power plant is extremely simple. A power generation plant mainly consists of alternator runs with help of a turbine. The steam is obtained from high-pressure boilers. Generally in India, soft coal, lignite, and peat are used as fuel of the boiler. The soft coal is employed as boiler fuel has volatile matter from 8 to 33% and ash content 5 to 16%. To increase the thermal efficiency, the coal is employed within the boiler in powder form
3. Thermal power generation efficiency
A Rankine cycle with a two-stage turbine and one feed hot-water heater.
The energy efficiency of a standard thermal power plant, considered salable energy produced as a percent of the heating value of the fuel consumed, is usually 33% to 48%. As with all heat engines, their efficiency is restricted and governed by the laws of thermodynamics. Other sorts of power stations are subject to different efficiency limitations, most hydropower stations within us are about 90 percent efficient in converting the energy of falling water into electricity while the efficiency of a turbine is restricted by Betz’s law, to about 59.3%.
The energy of a thermal power plant not utilized in power production must leave the plant within the sort of heat to the environment. This waste heat can undergo a condenser and be disposed of with cooling water or in cooling towers. If the waste heat is instead utilized for district heating, it’s called cogeneration. An important class of thermal power plant is related to desalination facilities; these are typically found in desert countries with large supplies of gas and these plants, freshwater production and electricity are equally important co-products.
The Carnot efficiency dictates that higher efficiencies are often attained by increasing the temperature of the steam. Sub-critical fuel power stations can do 36–40% efficiency. Supercritical designs have efficiencies within the low to mid 40% range, with new “ultra critical” designs using pressures of 4400 psi (30.3 MPa) and multiple stage reheat reaching about 48% efficiency. Above the critical point for the water of 705 °F (374 °C) and 3212 psi (22.06 MPa), there is no phase transition from water to steam, but only a gradual decrease in density.
Currently, most of the nuclear power stations must operate below the temperatures and pressures that coal-fired plants do, to provide more conservative safety margins within the systems that remove heat from the nuclear fuel rods. This, in turn, limits their thermodynamic efficiency to 30–32%. Some advanced reactor designs being studied, such as the very-high-temperature reactor, Advanced Gas-cooled Reactor, and supercritical water reactor, would operate at temperatures and pressures similar to current coal plants, producing comparable thermodynamic efficiency.
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Typical coal thermal power
Typical diagram of a coal-fired thermal power plant
1. cooling system 10. Steam control valve 19. Superheater
2. Cooling pump 11. high turbine 20. Forced draft fan
3. cable (3-phase) 12. Deaerator 21. Reheater
4. transformer (3-phase) 13. Feedwater heater 22. Combustion air intake
5. Electrical generator (3-phase) 14. Coal conveyor 23. Economizer
6. low turbine 15. Coal hopper 24. Air preheater
7. Condensate pump 16. Coal pulverizer 25. Precipitator
8. Surface condenser 17. Boiler steam drum 26. Induced draft fan
9. Intermediate 18. Bottom ash hopper 27. Flue-gas stack
For units over about 200 MW capacity, redundancy of key components is provided by installing duplicates of the forced and induced draft fans, air preheaters, and ash collectors. On some units of about 60 MW, two boilers per unit may instead be provided. The list of coal power stations has the 200 largest power stations ranging in size from 2,000MW to 5,500MW.
5. Boiler and steam cycle
The steam generating boiler has got to produce steam at the high purity, pressure, and temperature required for the turbine that drives the electrical generator. Geothermal plants don’t need boilers because they use present steam sources. Heat exchangers could also be used where the geothermal steam is extremely corrosive or contains excessive suspended solids.
A fuel steam generator includes an economizer, a steam drum, and therefore the furnace with its steam-generating tubes and superheater coils. Necessary safety valves are located at suitable points to alleviate excessive boiler pressure. The air and flue gas path equipment include: forced draft (FD) fan, air preheater (AP), boiler furnace, induced draft (ID) fan, ash collectors (electrostatic precipitator or baghouse), and the flue-gas stack
6. Feedwater heating and deaeration
The boiler feedwater utilized in the boiler may be a means of transferring heat from the burning fuel to the energy of the spinning turbine. the entire feed water consists of recirculated condensate water and purified makeup water. Because the metallic materials it contacts are subject to corrosion at high temperatures and pressures, the makeup water is very purified before use.
The makeup water during a 500 MWe plant amounts to perhaps 120 US gallons per minute (7.6 L/s) to exchange water drawn faraway from the boiler drums for water purity management, and to also offset the tiny losses from steam leaks within the system.
The condensate flow at full load during a 500 MW plant is about 6,000 US gallons per minute (400 L/s).
Diagram of boiler feed water deaeration (with vertical, domed aeration section, and horizontal water storage section).
The water is pressurized in two stages, and flows through a series of six or seven intermediate feedwater heaters, heated up at each point with steam extracted from an appropriate duct on the turbines and gaining temperature at each stage. Typically, in the middle of this series of feedwater heaters, and before the second stage of pressurization, the condensate plus the makeup water flow through a deaerator that removes dissolved air from the water, further purifying and reducing its corrosiveness. The water may be dosed following this point with hydrazine, a chemical that removes the remaining oxygen in the water to below 5 parts per billion (ppb).[vague] It is also dosed with pH control agents such as ammonia or morpholine to keep the residual acidity low and thus non-corrosive.
7. Boiler operation
The boiler may be a rectangular furnace about 50 feet (15 m) on a side and 130 feet (40 m) tall. Its walls are made from the internet of high steel tubes about 2.3 inches (58 mm) in diameter. Pulverized coal is air-blown into the furnace through burners located at the four corners, or along one wall, or two opposite walls, and it’s ignited to rapidly burn, forming a large fireball at the center.
The water circulation rate within the boiler is three to fourfold the throughput. As the water within the boiler circulates it absorbs heat and changes into steam. It is separated from the water inside a drum at the highest of the furnace. The saturated steam is introduced into superheat pendant tubes that persevere the most well-liked part of the combustion gases as they exit the furnace. Here the steam is superheated to 1,000 °F (540 °C) to organize it for the turbine. Plants designed for lignite (brown coal) are increasingly utilized in locations as varied as Germany, Victoria, Australia, and North Dakota. Lignite may be a much younger sort of coal than black coal. It has a lower energy density than black coal and requires a way larger furnace for equivalent heat output.
The firing systems also differ from black coal and typically draw hot gas from the furnace-exit level and blend it with the incoming coal in fan-type mills that inject
Plants that use gas turbines to heat the water for conversion into steam use boilers known as heat recovery steam generators (HRSG). The exhaust heat from the gas turbines is employed to form superheated steam that’s then utilized in a standard water-steam generation cycle, as described within the turbine combined-cycle plants section.
8. Steam condensing
The condenser condenses the steam from the exhaust of the turbine into the liquid to allow it to be pumped. If the condenser can be made cooler, the pressure of the exhaust steam is reduced and the efficiency of the cycle increases.
Diagram of a typical water-cooled surface condenser.
The surface condenser may be a shell and tube device during which cooling water is circulated through the tubes.
Such condensers use steam ejectors or rotary motor-driven exhausts for continuous removal of air and gases from the steam side to take care of the vacuum. For best efficiency, the temperature within the condenser must be kept as low as practical to realize rock bottom possible pressure within the condensing steam. Since the condenser temperature can nearly always be kept significantly below 100 °C where the vapor pressure of water is far but air pressure, the condenser generally works under vacuum. Thus leaks of non-condensable air into the closed-loop system must be prevented. Typically the cooling water causes the steam to condense at a temperature of about 25 °C (77 °F) and that creates an absolute pressure in the condenser of about 2–7 kPa (0.59–2.07 inHg), i.e. a vacuum of about −95 kPa (−28 inHg) relative to air pressure.
The limiting factor is that the temperature of the cooling water which, in turn, is restricted by the prevailing average climate at the facility station’s location (it could also be possible to lower the temperature beyond the turbine limits during winter, causing excessive condensation within the turbine). Plants operating in hot climates may need to reduce output if their source of condenser cooling water becomes warmer; unfortunately, this usually coincides with periods of high electrical demand for air conditioning. The condenser generally uses either circulating cooling water from a cooling system to reject waste heat to the atmosphere, or once-through water from a river, lake, or ocean.
This is done by pumping the nice and cozy water from the condenser through either natural draft, forced draft, or induced draft cooling towers (as seen within the adjacent
The circulation flow of the cooling water during a 500 MW unit is about 14.2 m³/s (500 ft³/s or 225,000 US gal/min) at full load. The condenser tubes are made from brass or chrome steel to resist corrosion from either side. Nevertheless, they’ll become internally fouled during operation by bacteria or algae within the cooling water or by mineral scaling, all of which inhibit heat transfer and reduce thermodynamic efficiency. Many plants include an automatic cleaning system that circulates sponge rubber balls through the tubes to wash them clean without the necessity to require the system off-line.[citation needed] The cooling water wont to condense the steam within the condenser returns to its source without having been changed aside from having been warmed. If the water returns to an area water body (rather than a circulating cooling tower), it’s often tempered with cool ‘raw’ water to stop thermal shock when discharged into that body of water. Another sort of condensing system is that the air-cooled condenser. The process is analogous thereto of a radiator and fan. Exhaust heat from the low-pressure section of a turbine runs through the condensing tubes, the tubes are usually finned and ambient air is pushed through the fins with the help of a large fan. The steam condenses to water to be reused within the water-steam cycle. Air-cooled condensers typically operate at a better temperature than water-cooled versions. While saving water, the efficiency of the cycle is reduced (resulting in additional CO2 per megawatt-hour of electricity).
From the rock bottom of the condenser, powerful condensate pumps recycle the condensed steam (water) back to the water/steam cycle.
9. Steam turbine generator
The rotor of a modern steam turbine, used in a power station
The turbine generator consists of a series of steam turbines interconnected to every other and a generator on a standard shaft. There is usually a high-pressure turbine at one end, followed by an intermediate-pressure turbine, and eventually one, two, or three low-pressure turbines, and therefore the generator. As steam moves through the system and loses pressure and thermal energy, it expands in volume, requiring increasing diameter and longer blades at each succeeding stage to extract the remaining energy. The entire rotating mass could also be over 200 metric tons and 100 feet (30 m) long. It is so heavy that it must be kept turning slowly even when packing up (at 3 rpm) so that the shaft won’t bow even slightly and become unbalanced. This is so important that it’s one of only six functions of blackout emergency power batteries on-site. (The other five being emergency lighting, communication, station alarms, generator hydrogen seal system, and turbogenerator lube oil.)
For a typical late 20th-century power plant, superheated steam from the boiler is delivered through 14–16-inch (360–410 mm) diameter piping at 2,400 psi (17 MPa; 160 atm) and 1,000 °F (540 °C) to the high-pressure turbine, where it falls in pressure to 600 psi (4.1 MPa; 41 atm) and 600 °F (320 °C) in temperature through the stage. It exits via 24–26-inch (610–660 mm) diameter cold reheat lines and passes back into the boiler, where the steam is reheated in special reheat pendant tubes back to 1,000 °F (540 °C). The hot reheat steam is conducted to the intermediate pressure turbine, where it falls in both temperature and pressure and exits on to the long-bladed low-pressure turbines and eventually exits to the condenser.
The generator, typically about 30 feet (9 m) long and 12 feet (3.7 m) in diameter, contains a stationary stator and a spinning rotor, each containing miles of heavy copper conductor. There is generally no static magnet, thus preventing black starts. In operation it generates up to 21,000 amperes at 24,000 volts AC(504 MWe) as it spins at either 3,000 or 3,600 rpm, synchronized to the power grid. The rotor spins during a sealed chamber cooled with hydrogen gas, selected because it’s the very best known heat transfer coefficient of any gas and for its low viscosity, which reduces windage losses. This system requires special handling during startup, with air within the chamber first displaced by CO2 before filling with hydrogen. This ensures that a highly explosive hydrogen-oxygen environment isn’t created.
The power grid frequency is 60 Hz across North America and 50 Hz in Europe, Oceania, Asia (Korea and parts of Japan are notable exceptions), and parts of Africa. The desired frequency affects the planning of huge turbines since they’re highly optimized for one particular speed.
The electricity flows to a distribution yard where transformers increase the voltage for transmission to its destination.
The steam turbine-driven generators have auxiliary systems enabling them to figure satisfactorily and safely. The steam turbine generator, being rotating equipment, generally has a heavy, large-diameter shaft. The shaft, therefore, requires not only support but also has got to be kept in position while running. To minimize the frictional resistance to the rotation, the shaft features several bearings. The bearing shells, during which the shaft rotates, are lined with a low-friction material like Babbitt metal. Oil lubrication is provided to further reduce the friction between the shaft and bearing surface and to limit the warmth generated.
Fly ash is captured and far away from the flue gas by electrostatic precipitators or fabric bag filters (or sometimes both) located at the outlet of the furnace and before the induced draft fan. The ash is periodically faraway from the gathering hoppers below the precipitators or bag filters. Generally, the ash is pneumatically transported to storage silos for subsequent transport by trucks or railroad cars.
At rock bottom of the furnace, there’s a hopper for the collection of bottom ash. This hopper is kept filled with water to quench the ash and clinkers falling from the furnace. Arrangements are included to crush the clinkers and convey the crushed clinkers and bottom ash to a storage site. Ash extractors are wont to discharge ash from municipal solid waste–fired boilers.
10. Fuel preparation system
Conveyor system for moving coal (visible at far left) into a power station.
In coal-fired power stations, the raw feed 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. The pulverizers may be ball mills, rotating drum grinders, or other types of grinders.
Some power stations burn fuel oil rather than coal. The oil must be kept warm (above its pour point) in the fuel oil storage tanks to prevent the oil from congealing and becoming unpumpable. The oil is usually heated to about 100 °C before being pumped through the furnace fuel oil spray nozzles.
Boilers in some power stations use processed natural gas as their main fuel. Other power stations may use processed natural gas as auxiliary fuel if their main fuel supply (coal or oil) is interrupted. In such cases, separate gas burners are provided on the boiler furnaces.
11. Conclusion
In a coal-based power plant coal is transported from coal mines to the power plant by railway in wagons or a merry-go-round system. Coal is unloaded from the wagons to a moving underground conveyor belt. 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 coal supply bottleneck or to the live storage(8 hours coal supply) in the raw coal bunker in the boiler house. Raw coal from the raw coal bunker is supplied to the Coal Mills by a Raw Coal Feeder. The Coal Mills or pulverizer pulverizes the coal to 200 mesh size. The powdered coal from the coal mills is carried to the boiler in coal pipes by high-pressure hot air. The pulverized coal air mixture is burnt in the boiler in the combustion zone.
Generally, in modern boilers, a tangential firing system is used i.e. the coal nozzles/ guns form a tangent to a circle. The temperature in the fireball is of the order of 1300 deg.C. The boiler is a water tube boiler hanging from the top. Water is converted to steam in the boiler and steam is separated from water in the boiler Drum. The saturated steam from the boiler drum is taken to the Low-Temperature Superheater, Platen Superheater, and Final Superheater respectively for superheating. The superheated steam from the final superheater is taken to the High-Pressure Steam Turbine (HPT). In the HPT the steam pressure is utilized to rotate the turbine and the resultant is rotational energy. From the HPT the outcoming steam is taken to the Reheater 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) and then to the Low-Pressure Turbine (LPT). The outlet of the LPT is sent to the condenser for condensing back to water by a cooling water system. This condensed water is collected in the Hotwell and is again sent to the boiler in a closed cycle. The rotational energy imparted to the turbine by high-pressure steam is converted to electrical energy in the Generator.
