Utilisateur:Oimabe/Thermal power station

Nantong Power Station, a coal-fired power station in Nantong, China.

Mohave Generating Station, a 1,580 MW thermal power station near Laughlin, Nevada, US, fuelled by coal.

Nuclear thermal power station in Bavaria, Germany.

Geothermal power station in Iceland.

Taichung Thermal Power Station, the world's largest coal-fired power station, in Taichung, Taiwan.

Une centrale thermique est une centrale électrique dans laquelle l'énergie thermique est convertie en énergie électrique. Dans la plupart des endroits du monde, la turbine fonctionne à la vapeur. L'eau est chauffée, se transforme en vapeur et fait tourner une turbine à vapeur qui entraîne un générateur électrique. Après avoir traversé la turbine, la vapeur est condensée dans un condenseur et recyclée à l'endroit où elle était chauffée; c'est ce qu'on appelle un cycle de Rankine. La plus grande variation dans la conception des centrales thermiques est due aux différentes sources de chaleur; les combustibles fossiles dominent ici, bien que l'énergie thermique nucléaire et l'énergie solaire soient également utilisées. Certaines centrales thermiques sont également conçues pour produire de l'énergie thermique à des fins industrielles, de Réseau de chaleur, ou de dessalement d'eau, en plus de générer de l'énergie électrique.

Types of thermal energy

Almost all coal, petroleum, nuclear, geothermal, solar thermal electric, and waste incineration plants, as well as many natural gas power stations are thermal. Natural gas is frequently combusted in gas turbines as well as boilers. The waste heat from a gas turbine, in the form of hot exhaust gas, can be used to raise steam, by passing this gas through a heat recovery steam generator (HRSG) the steam is then used to drive a steam turbine in a combined cycle plant that improves overall efficiency. Power stations burning coal, fuel oil, or natural gas are often called fossil fuel power stations. Some biomass-fueled thermal power stations have appeared also. Non-nuclear thermal power stations, particularly fossil-fueled plants, which do not use cogeneration are sometimes referred to as conventional power stations.

Commercial electric utility power stations are usually constructed on a large scale and designed for continuous operation. Virtually all Electric power stations use three-phase electrical generators to produce alternating current (AC) electric power at a frequency of 50 Hz or 60 Hz. Large companies or institutions may have their own power stations to supply heating or electricity to their facilities, especially if steam is created anyway for other purposes. Steam-driven power stations have been used to drive most ships in most of the 20th century until recently. Steam power stations are now only used in large nuclear naval ships. Shipboard power stations usually directly couple the turbine to the ship's propellers through gearboxes. Power stations in such ships also provide steam to smaller turbines driving electric generators to supply electricity. Nuclear marine propulsion is, with few exceptions, used only in naval vessels. There have been many turbo-electric ships in which a steam-driven turbine drives an electric generator which powers an electric motor for propulsion.

Cogeneration plants, often called combined heat and power (CH&P) facilities, produce both electric power and heat for process heat or space heating, such as steam and hot water.

History

The initially developed reciprocating steam engine has been used to produce mechanical power since the 18th Century, with notable improvements being made by James Watt. When the first commercially developed central electrical power stations were established in 1882 at Pearl Street Station in New York and Holborn Viaduct power station in London, reciprocating steam engines were used. The development of the steam turbine in 1884 provided larger and more efficient machine designs for central generating stations. By 1892 the turbine was considered a better alternative to reciprocating engines; turbines offered higher speeds, more compact machinery, and stable speed regulation allowing for parallel synchronous operation of generators on a common bus. After about 1905, turbines entirely replaced reciprocating engines in large central power stations.

The largest reciprocating engine-generator sets ever built were completed in 1901 for the Manhattan Elevated Railway. Each of seventeen units weighed about 500 tons and was rated 6000 kilowatts; a contemporary turbine set of similar rating would have weighed about 20% as much.

Thermal power generation efficiency

A Rankine cycle with a two-stage steam turbine and a single feed water heater.

The energy efficiency of a conventional thermal power station, considered salable energy produced as a percent of the heating value of the fuel consumed, is typically 33% to 48%.As with all heat engines, their efficiency is limited, and governed by the laws of thermodynamics. Other types of power stations are subject to different efficiency limitations, most hydropower stations in the United States are about 90 percent efficient in converting the energy of falling water into electricity while the efficiency of a wind turbine is limited by Betz's law, to about 59.3%.

As with all heat engines, their efficiency is limited, and governed by the laws of thermodynamics. Other types of power stations are subject to different efficiency limitations, most hydropower stations in the United States are about 90 percent efficient in converting the energy of falling water into electricity while the efficiency of a wind turbine is limited by Betz's law, to about 59.3%.

The energy of a thermal power station not utilized in power production must leave the plant in the form of heat to the environment. This waste heat can go through a condenser and be disposed of with cooling water or in cooling towers. Si la chaleur perdue est plutôt utilisée pour le chauffage urbain, cela s'appelle la cogénération. Une classe importante de centrales thermiques est associée aux installations de dessalement; Celles-ci se trouvent généralement dans les pays désertiques disposant d'une importante réserve de gaz naturel et, dans ces usines, la production d'eau douce et l'électricité sont des coproduits tout aussi importants.

The Carnot efficiency dictates that higher efficiencies can be attained by increasing the temperature of the steam. Sub-critical fossil fuel power stations can achieve 36–40% efficiency. Supercritical designs have efficiencies in 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 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, in order 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.

Electricity cost

The direct cost of electric energy produced by a thermal power station is the result of cost of fuel, capital cost for the plant, operator labour, maintenance, and such factors as ash handling and disposal. Indirect, social or environmental costs such as the economic value of environmental impacts, or environmental and health effects of the complete fuel cycle and plant decommissioning, are not usually assigned to generation costs for thermal stations in utility practice, but may form part of an environmental impact assessment.

Typical coal thermal power station

Typical diagram of a coal-fired thermal power station

1. Cooling tower	10. Steam control valve	19. Superheater
2. Cooling water pump	11. High pressure steam turbine	20. Forced draft fan
3. Transmission line (3-phase)	12. Deaerator	21. Reheater
4. Step-up transformer (3-phase)	13. Feedwater heater	22. Combustion air intake
5. Electrical generator (3-phase)	14. Coal conveyor	23. Economiser
6. Low pressure steam 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 pressure steam turbine	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 fly 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.

Boiler and steam cycle

Dans le domaine des centrales nucléaires, on entend par générateur de vapeur un type spécifique d'échangeur de chaleur de grande taille utilisé dans un réacteur à eau sous pression (PWR) pour connecter thermiquement les systèmes primaire (centrale) et secondaire (centrale à vapeur). Dans un réacteur nucléaire appelé réacteur à eau bouillante (BWR), l'eau est bouillie pour générer de la vapeur directement dans le réacteur lui-même et il n'y a pas d'unités appelées générateurs de vapeur.

In some industrial settings, there can also be steam-producing heat exchangers called heat recovery steam generators (HRSG) which utilize heat from some industrial process, most commonly utilizing hot exhaust from a gas turbine. The steam generating boiler has to produce steam at the high purity, pressure and temperature required for the steam turbine that drives the electrical generator.

Geothermal plants do not need boilers because they use naturally occurring steam sources. Heat exchangers may be used where the geothermal steam is very corrosive or contains excessive suspended solids.

Un générateur de vapeur à combustible fossile comprend un économiseur, un tambour à vapeur et le four avec ses tubes générateurs de vapeur et ses bobines de surchauffeur. Les soupapes de sécurité nécessaires sont situées à des endroits appropriés pour réduire la pression excessive de la chaudière. L'équipement du circuit d'air et de gaz de combustion comprend: ventilateur à tirage forcé, préchauffeur d'air, four à chaudière, ventilateur à tirage induit, collecteurs de cendres volantes (précipitateur électrostatique ou dépoussiéreur) et la cheminée.

Feed water heating and deaeration

L'eau d'alimentation de la chaudière utilisée dans la chaudière à vapeur est un moyen de transférer l'énergie thermique du combustible en combustion à l'énergie mécanique de la turbine à vapeur en rotation. L'eau d'alimentation totale est constituée d'eau condensée recyclée et d'eau d'appoint purifiée. Étant donné que les matériaux métalliques en contact sont soumis à la corrosion à des températures et des pressions élevées, l'eau d'appoint est hautement purifiée avant utilisation. Un système d'adoucisseurs d'eau et de déminéraliseurs à échange d'ions produit une eau si pure qu'elle devient par coïncidence un isolant électrique, avec une conductivité de l'ordre de 0,3 à 1,0 microsiemens par centimètre. L'eau d'appoint d'une centrale de 500 MWe équivaut peut-être à 120 gallons américains par minute (7,6 L / s) pour remplacer l'eau extraite des fûts de chaudière pour la gestion de la pureté de l'eau et pour compenser les petites pertes dues aux fuites de vapeur.

The feed water cycle begins with condensate water being pumped out of the condenser after traveling through the steam turbines. The condensate flow rate at full load in a 500 MW plant is about 6,000 US gallons per minute (400 L/s).

Diagram of boiler feed water deaerator (with vertical, domed aeration section and horizontal water storage section).

L'eau est pressurisée en deux étapes et traverse une série de six ou sept chauffe-eau intermédiaires, chauffés à chaque point avec de la vapeur extraite d'un conduit approprié sur les turbines et gagnant de la température à chaque étape. En général, au milieu de cette série de chauffe-eau d'alimentation, et avant la deuxième étape de pressurisation, le condensat et l'eau d'appoint s'écoulent dans un dégazeur. qui élimine l'air dissous de l'eau, purifie davantage et réduit sa corrosivité. L'eau peut être dosée après ce point avec de l'hydrazine, une substance chimique qui élimine l'oxygène restant dans l'eau à moins de 5 parties par milliard (ppb). Il est également dosé avec des agents de contrôle du pH tels que l'ammoniac ou la morpholine pour maintenir l'acidité résiduelle faible et donc non corrosive./

Fonctionnement de la chaudière/

La chaudière est un four rectangulaire d'environ 50 pieds (15 m) de côté et de 130 pieds (40 m) de hauteur. Ses parois sont constituées d'une bande de tubes en acier à haute pression d'un diamètre d'environ 2,3 pouces (58 mm)./

Le charbon pulvérisé est soufflé à l'air dans le four par des brûleurs situés aux quatre coins ou le long d'un mur ou de deux murs opposés. Il s'enflamme rapidement pour former une grande boule de feu au centre. Le rayonnement thermique de la boule de feu chauffe l'eau qui circule dans les tubes de la chaudière à proximité du périmètre de la chaudière. Le débit de circulation de l'eau dans la chaudière est trois à quatre fois supérieur au débit. Comme l'eau circule dans la chaudière, elle absorbe la chaleur et se transforme en vapeur. Il est séparé de l'eau à l'intérieur d'un tambour en haut du four. La vapeur saturée est introduite dans des tubes suspendus surchauffés qui sont suspendus dans la partie la plus chaude des gaz de combustion lorsqu'ils sortent du four. Ici, la vapeur est surchauffée à 540 ° C (1000 ° F) pour la préparer à la turbine./

Plants designed for lignite (brown coal) are increasingly used in locations as varied as Germany, Victoria, Australia, and North Dakota. Lignite is a much younger form of coal than black coal. It has a lower energy density than black coal and requires a much larger furnace for equivalent heat output. Such coals may contain up to 70% water and ash, yielding lower furnace temperatures and requiring larger induced-draft fans. The firing systems also differ from black coal and typically draw hot gas from the furnace-exit level and mix it with the incoming coal in fan-type mills that inject the pulverized coal and hot gas mixture into the boiler.

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 used to make superheated steam that is then used in a conventional water-steam generation cycle, as described in the gas turbine combined-cycle plants section.

Boiler furnace and steam drum

The water enters the boiler through a section in the convection pass called the economizer. From the economizer it passes to the steam drum and from there it goes through downcomers to inlet headers at the bottom of the water walls. From these headers the water rises through the water walls of the furnace where some of it is turned into steam and the mixture of water and steam then re-enters the steam drum. This process may be driven purely by natural circulation (because the water is the downcomers is denser than the water/steam mixture in the water walls) or assisted by pumps. In the steam drum, the water is returned to the downcomers and the steam is passed through a series of steam separators and dryers that remove water droplets from the steam. The dry steam then flows into the superheater coils.

The boiler furnace auxiliary equipment includes coal feed nozzles and igniter guns, soot blowers, water lancing, and observation ports (in the furnace walls) for observation of the furnace interior. Furnace explosions due to any accumulation of combustible gases after a trip-out are avoided by flushing out such gases from the combustion zone before igniting the coal.

The steam drum (as well as the superheater coils and headers) have air vents and drains needed for initial start up.

Superheater

Fossil fuel power stations often have a superheater section in the steam generating furnace.^{[réf. nécessaire]} The steam passes through drying equipment inside the steam drum on to the superheater, a set of tubes in the furnace. Here the steam picks up more energy from hot flue gases outside the tubing, and its temperature is now superheated above the saturation temperature. The superheated steam is then piped through the main steam lines to the valves before the high-pressure turbine.

Nuclear-powered steam plants do not have such sections but produce steam at essentially saturated conditions. Experimental nuclear plants were equipped with fossil-fired superheaters in an attempt to improve overall plant operating cost.^{[réf. nécessaire]}

Condensation à la vapeur/

Le condenseur condense la vapeur de l'échappement de la turbine en liquide pour permettre son pompage. Si le condenseur peut être refroidi, la pression de la vapeur d'échappement est réduite et le rendement du cycle augmente./

Schéma d'un condenseur de surface refroidi à l'eau typique.

Le condenseur de surface est un échangeur thermique à coque et à tube dans lequel circule de l'eau de refroidissement à travers les tubes. La vapeur d'échappement de la turbine basse pression pénètre dans la coque, où elle est refroidie et convertie en condensat (eau) en circulant sur les tubes, comme le montre le diagramme ci-contre. Ces condenseurs utilisent des éjecteurs de vapeur ou des échappements entraînés par un moteur rotatif pour éliminer en continu l'air et les gaz du côté vapeur afin de maintenir le vide.

Pour un meilleur rendement, la température dans le condenseur doit être maintenue aussi basse que possible afin d'obtenir la pression la plus faible possible dans la vapeur de condensation. Comme la température du condenseur peut presque toujours être maintenue sensiblement inférieure à 100 ° C lorsque la pression de vapeur de l'eau est très inférieure à la pression atmosphérique, le condenseur fonctionne généralement sous vide. Il faut donc empêcher les fuites d’air non condensable dans la boucle fermée.

En général, l'eau de refroidissement entraîne la condensation de la vapeur à une température d'environ 25 ° C (77 ° F) et crée une pression absolue dans le condenseur d'environ 2 à 7 kPa (0,59 à 2,07 inHg). 95 kPa (-28 inHg) par rapport à la pression atmosphérique. La diminution importante du volume qui se produit lorsque la vapeur d'eau se condense en liquide crée le vide qui aide à faire passer la vapeur et augmente l'efficacité des turbines.

Le facteur limitant est la température de l’eau de refroidissement, elle-même limitée par les conditions climatiques qui prévalent à l’endroit de la centrale (il peut être possible de baisser la température au-delà des limites de la turbine en hiver turbine). Les installations fonctionnant dans des climats chauds peuvent devoir réduire leur production si leur source d'eau de refroidissement du condenseur devient plus chaude; Malheureusement, cela coïncide généralement avec des périodes de forte demande en électricité pour la climatisation.

Le condenseur utilise généralement soit de l'eau de refroidissement en circulation provenant d'une tour de refroidissement pour rejeter la chaleur résiduelle dans l'atmosphère, soit une eau traversant une rivière, un lac ou un océan.

A Marley mechanical induced draft cooling tower

La chaleur absorbée par l'eau de refroidissement en circulation dans les tubes du condenseur doit également être éliminée pour maintenir la capacité de l'eau à refroidir au fur et à mesure de sa circulation. Cela se fait en pompant l'eau chaude du condenseur à travers des tours à tirage naturel, à tirage forcé ou à tirage induit (voir l'image ci-contre) qui réduisent la température de l'eau de 11 à 17 ° C (20 jusqu'à 30 ° F) —expulser la chaleur perdue dans l'atmosphère. Le débit de circulation de l'eau de refroidissement dans une unité de 500 MW est d'environ 14,2 m³ / s (500 ft³ / s ou 225 000 US gal / min) à pleine charge./

Les tubes du condenseur sont en laiton ou en acier inoxydable pour résister à la corrosion des deux côtés. Néanmoins, ils peuvent être encrassés à l'intérieur pendant le fonctionnement par des bactéries ou des algues dans l'eau de refroidissement ou par l'entartrage des minéraux, qui inhibent tous le transfert de chaleur et réduisent l'efficacité thermodynamique. De nombreuses usines comprennent un système de nettoyage automatique qui fait circuler les balles en caoutchouc éponge à travers les tubes pour les nettoyer sans avoir à retirer le système de la ligne.^{[réf. nécessaire]}/

L'eau de refroidissement utilisée pour condenser la vapeur dans le condenseur retourne à sa source sans avoir été modifiée autre que d'avoir été réchauffée. Si l'eau retourne à un plan d'eau local (plutôt qu'à une tour de refroidissement en circulation), elle est souvent trempée dans de l'eau froide «brute» pour empêcher un choc thermique lorsqu'elle est déversée dans cette masse d'eau./

Un autre type de système de condensation est le condenseur à air. Le processus est similaire à celui d'un radiateur et d'un ventilateur. La chaleur d'échappement de la section basse pression d'une turbine à vapeur traverse les tubes de condensation, les tubes sont généralement à ailettes et l'air ambiant traverse les ailettes à l'aide d'un grand ventilateur. La vapeur se condense en eau pour être réutilisée dans le cycle eau-vapeur. Les condenseurs à refroidissement par air fonctionnent généralement à une température supérieure à celle des versions à refroidissement par eau. Tout en économisant de l'eau, l'efficacité du cycle est réduite (entraînant plus de dioxyde de carbone par mégawattheure d'électricité)./

Depuis le bas du condenseur, de puissantes pompes à condensat recyclent la vapeur condensée (eau) vers le cycle eau / vapeur./

Reheater

Power station furnaces may have a reheater section containing tubes heated by hot flue gases outside the tubes. Exhaust steam from the high-pressure turbine is passed through these heated tubes to collect more energy before driving the intermediate and then low-pressure turbines.

Air path

External fans are provided to give sufficient air for combustion. The Primary air fan takes air from the atmosphere and, first warms the air in the air preheater for better economy. Primary air then passes through the coal pulverizers, and carries the coal dust to the burners for injection into the furnace. The Secondary air fan takes air from the atmosphere and, first warms the air in the air preheater for better economy. Secondary air is mixed with the coal/primary air flow in the burners.

The induced draft fan assists the FD fan by drawing out combustible gases from the furnace, maintaining a slightly negative pressure in the furnace to avoid leakage of combustion products from the boiler casing.

Steam turbine generator

Rotor of a modern steam turbine, used in a power station

The turbine generator consists of a series of steam turbines interconnected to each other and a generator on a common shaft. There is usually a high-pressure turbine at one end, followed by an intermediate-pressure turbine, and finally one, two, or three low-pressure turbines, and 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 may be over 200 metric tons and 100 pieds (30,48 m) long. It is so heavy that it must be kept turning slowly even when shut down (at 3 rpm) so that the shaft will not bow even slightly and become unbalanced. This is so important that it is 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 station, 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 to 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 directly to the long-bladed low-pressure turbines and finally exits to the condenser.

The generator, typically about 30 pieds (9,144 m) long and 12 pieds (3,6576 m) in diameter, contains a stationary stator and a spinning rotor, each containing miles of heavy copper conductor. There is generally no permanent 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 in a sealed chamber cooled with hydrogen gas, selected because it has the highest 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 in the chamber first displaced by carbon dioxide before filling with hydrogen. This ensures that a highly explosive hydrogen–oxygen environment is not 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 design of large turbines, since they are 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 work satisfactorily and safely. The steam turbine generator, being rotating equipment, generally has a heavy, large-diameter shaft. The shaft therefore requires not only supports but also has to be kept in position while running. To minimize the frictional resistance to the rotation, the shaft has a number of bearings. The bearing shells, in which the shaft rotates, are lined with a low-friction material like Babbitt metal. Oil lubrication is provided to further reduce the friction between shaft and bearing surface and to limit the heat generated.

Stack gas path and cleanup

As the combustion flue gas exits the boiler it is routed through a rotating flat basket of metal mesh which picks up heat and returns it to incoming fresh air as the basket rotates. This is called the air preheater. The gas exiting the boiler is laden with fly ash, which are tiny spherical ash particles. The flue gas contains nitrogen along with combustion products carbon dioxide, sulfur dioxide, and nitrogen oxides. The fly ash is removed by fabric bag filters in baghouses or electrostatic precipitators. Once removed, the fly ash byproduct can sometimes be used in the manufacturing of concrete. This cleaning up of flue gases, however, only occurs in plants that are fitted with the appropriate technology. Still, the majority of coal-fired power stations in the world do not have these facilities.^{[réf. nécessaire]} Legislation in Europe has been efficient to reduce flue gas pollution. Japan has been using flue gas cleaning technology for over 30 years and the US has been doing the same for over 25 years. China is now beginning to grapple with the pollution caused by coal-fired power stations.

Where required by law, the sulfur and nitrogen oxide pollutants are removed by stack gas scrubbers which use a pulverized limestone or other alkaline wet slurry to remove those pollutants from the exit stack gas. Other devices use catalysts to remove nitrous oxide compounds from the flue-gas stream. The gas travelling up the flue-gas stack may by this time have dropped to about 50 °C (120 °F). A typical flue-gas stack may be 150–180 metres (490–590 ft) tall to disperse the remaining flue gas components in the atmosphere. The tallest flue-gas stack in the world is 419.7 metres (1,377 ft) tall at the Ekibastuz GRES-2 Power Station in Kazakhstan.

In the United States and a number of other countries, atmospheric dispersion modeling studies are required to determine the flue-gas stack height needed to comply with the local air pollution regulations. The United States also requires the height of a flue-gas stack to comply with what is known as the "good engineering practice" (GEP) stack height. In the case of existing flue gas stacks that exceed the GEP stack height, any air pollution dispersion modeling studies for such stacks must use the GEP stack height rather than the actual stack height.

Fly ash collection

Fly ash is captured and removed 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 fly ash is periodically removed from the collection hoppers below the precipitators or bag filters. Generally, the fly ash is pneumatically transported to storage silos for subsequent transport by trucks or railroad cars.

Bottom ash collection and disposal

At the bottom of the furnace, there is a hopper for collection of bottom ash. This hopper is kept filled with water to quench the ash and clinkers falling down 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 used to discharge ash from municipal solid waste–fired boilers.

Auxiliary systems

Boiler make-up water treatment plant and storage

Since there is continuous withdrawal of steam and continuous return of condensate to the boiler, losses due to blowdown and leakages have to be made up to maintain a desired water level in the boiler steam drum. For this, continuous make-up water is added to the boiler water system. Impurities in the raw water input to the plant generally consist of calcium and magnesium salts which impart hardness to the water. Hardness in the make-up water to the boiler will form deposits on the tube water surfaces which will lead to overheating and failure of the tubes. Thus, the salts have to be removed from the water, and that is done by a water demineralising treatment plant (DM). A DM plant generally consists of cation, anion, and mixed bed exchangers. Any ions in the final water from this process consist essentially of hydrogen ions and hydroxide ions, which recombine to form pure water. Very pure DM water becomes highly corrosive once it absorbs oxygen from the atmosphere because of its very high affinity for oxygen.

The capacity of the DM plant is dictated by the type and quantity of salts in the raw water input. However, some storage is essential as the DM plant may be down for maintenance. For this purpose, a storage tank is installed from which DM water is continuously withdrawn for boiler make-up. The storage tank for DM water is made from materials not affected by corrosive water, such as PVC. The piping and valves are generally of stainless steel. Sometimes, a steam blanketing arrangement or stainless steel doughnut float is provided on top of the water in the tank to avoid contact with air. DM water make-up is generally added at the steam space of the surface condenser (i.e., the vacuum side). This arrangement not only sprays the water but also DM water gets deaerated, with the dissolved gases being removed by a de-aerator through an ejector attached to the condenser.

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 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 in the event that their main fuel supply (coal or oil) is interrupted. In such cases, separate gas burners are provided on the boiler furnaces.

Barring gear

Barring gear (or "turning gear") is the mechanism provided to rotate the turbine generator shaft at a very low speed after unit stoppages. Once the unit is "tripped" (i.e., the steam inlet valve is closed), the turbine coasts down towards standstill. When it stops completely, there is a tendency for the turbine shaft to deflect or bend if allowed to remain in one position too long. This is because the heat inside the turbine casing tends to concentrate in the top half of the casing, making the top half portion of the shaft hotter than the bottom half. The shaft therefore could warp or bend by millionths of inches.

This small shaft deflection, only detectable by eccentricity meters, would be enough to cause damaging vibrations to the entire steam turbine generator unit when it is restarted. The shaft is therefore automatically turned at low speed (about one percent rated speed) by the barring gear until it has cooled sufficiently to permit a complete stop.

Oil system

An auxiliary oil system pump is used to supply oil at the start-up of the steam turbine generator. It supplies the hydraulic oil system required for steam turbine's main inlet steam stop valve, the governing control valves, the bearing and seal oil systems, the relevant hydraulic relays and other mechanisms.

At a preset speed of the turbine during start-ups, a pump driven by the turbine main shaft takes over the functions of the auxiliary system.

Generator cooling

Bien que les petits générateurs puissent être refroidis par de l'air aspiré à travers des filtres à l'entrée, les plus grandes unités nécessitent généralement des dispositifs de refroidissement spéciaux. Le refroidissement à l'hydrogène, dans un boîtier étanche à l'huile, est utilisé car il présente le coefficient de transfert de chaleur le plus élevé connu de tous les gaz et sa faible viscosité, ce qui réduit les pertes dues au vent. Ce système nécessite une manipulation particulière au démarrage, l'air dans le boîtier du générateur étant tout d'abord déplacé par le dioxyde de carbone avant d'être rempli d'hydrogène. Cela garantit que l'hydrogène hautement inflammable ne se mélange pas à l'oxygène dans l'air./

La pression d'hydrogène à l'intérieur du boîtier est maintenue légèrement supérieure à la pression atmosphérique pour éviter l'entrée d'air extérieur. L’hydrogène doit être scellé contre les fuites vers l’extérieur du puits. Les joints mécaniques autour de l'arbre sont installés avec un très petit espace annulaire pour éviter le frottement entre l'arbre et les joints. L'huile de phoque est utilisée pour empêcher la fuite de gaz d'hydrogène dans l'atmosphère./

The generator also uses water cooling. Since the generator coils are at a potential of about 22 kV, an insulating barrier such as Teflon is used to interconnect the water line and the generator high-voltage windings. Demineralized water of low conductivity is used.

Generator high-voltage system

The generator voltage for modern utility-connected generators ranges from 11 kV in smaller units to 30 kV in larger units. The generator high-voltage leads are normally large aluminium channels because of their high current as compared to the cables used in smaller machines. They are enclosed in well-grounded aluminium bus ducts and are supported on suitable insulators. The generator high-voltage leads are connected to step-up transformers for connecting to a high-voltage electrical substation (usually in the range of 115 kV to 765 kV) for further transmission by the local power grid.

The necessary protection and metering devices are included for the high-voltage leads. Thus, the steam turbine generator and the transformer form one unit. Smaller units may share a common generator step-up transformer with individual circuit breakers to connect the generators to a common bus.

Monitoring and alarm system

Most of the power station operational controls are automatic. However, at times, manual intervention may be required. Thus, the plant is provided with monitors and alarm systems that alert the plant operators when certain operating parameters are seriously deviating from their normal range.

Battery-supplied emergency lighting and communication

A central battery system consisting of lead–acid cell units is provided to supply emergency electric power, when needed, to essential items such as the power station's control systems, communication systems, generator hydrogen seal system, turbine lube oil pumps, and emergency lighting. This is essential for a safe, damage-free shutdown of the units in an emergency situation.

Circulating water system

Pour dissiper la charge thermique de la vapeur d'échappement de la turbine principale, du condensat du condenseur de vapeur du gland et du condensat du chauffe-eau basse pression en fournissant de l'eau de refroidissement au condenseur principal, conduisant ainsi à la condensation./

The consumption of cooling water by inland power stations is estimated to reduce power availability for the majority of thermal power stations by 2040–2069.

Transport of coal fuel to site and to storage

Most thermal stations use coal as the main fuel. Raw coal is transported from coal mines to a power station site by trucks, barges, bulk cargo ships, or railway cars. Generally, when shipped by railway, the coal cars are sent as a full train of cars. The coal received at site may be of different sizes. The railway cars are unloaded at site by rotary dumpers or side tilt dumpers to tip over onto conveyor belts below. The coal is generally conveyed to crushers which crush the coal to about 3/4 pouces ( ) size. The crushed coal is then sent by belt conveyors to a storage pile. Normally, the crushed coal is compacted by bulldozers, as compacting of highly volatile coal avoids spontaneous ignition.^{[réf. nécessaire]}

The crushed coal is conveyed from the storage pile to silos or hoppers at the boilers by another belt conveyor system.

References

[[Catégorie:Conversion d'énergie]]