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: Power plant: محطات القدرة موضوع يتضمن التوضيح ومواقع الكترونية

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    الاخوة الاعزاء لغرض الاشارة الى المشاركة رقم #5 والتي تحتوي على المخطط ورغبة منا في توضيح برج التبريد في محاطات القدرة ارتأيت ان ادرج لكم برج التبريد او برج التهوية حسب الترجمة لمصادرنا العربية /ومن خلال بحثنا لاحظنا ان هذه الموسوعة اكثر فائدة ومن خلال المنتدى الرائع نقدم الموضوع*
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    Cooling tower

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    Image 1: Natural draft wet cooling hyperboloid towers at Didcot Power Station, UK



    Image 2: A mechanical induced draft cooling tower


    Cooling towers are evaporative coolers used for cooling water or other working medium to near the ambient wet-bulb air temperature. Cooling towers use evaporation of water to reject heat from processes such as cooling the circulating water used in oil refineries, chemical plants, power plants and building cooling, for example. The towers vary in size from small roof-top units to very large hyperboloid structures (as in Image 1) that can be up to 200 metres tall and 100 metres in diameter, or rectangular structures (as in Image 2) that can be over 40 metres tall and 80 metres long. Smaller towers are normally factory-built, while larger ones are constructed on site********************************************** *

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    Classification by use
    Cooling towers can generally be classified by use into either HVAC (air-conditioning) or industrial duty.

    [edit] HVAC

    An HVAC cooling tower is a subcategory rejecting heat from a chiller. Water-cooled chillers are normally more energy efficient than air-cooled chillers due to heat rejection to tower water at near wet-bulb temperatures. Air-cooled chillers must reject heat to the dry-bulb temperature, and thus have a lower average reverse-Carnot cycle effectiveness. Large office buildings, hospitals, schools typically use one or more cooling towers as part of their air conditioning systems. Generally, industrial cooling towers are much larger than HVAC towers.
    HVAC use of a cooling tower pairs the cooling tower with a water-cooled chiller or water-cooled condenser. A ton of air-conditioning is the rejection of 12,000 Btu/hour (12,661 kJ/hour). The equivalent ton on the cooling tower side actually rejects about 15,000 Btu/hour (15,826 kJ/hour) due to the heat-equivalent of the energy needed to drive the chiller's compressor. This equivalent ton is defined as the heat rejection in cooling 3 U.S. gallons/minute (1,500 pound/hour) of water 10°F, which amounts to 15,000 Btu/hour, or a chiller coefficient-of-performance (COP) of 4.0. This COP is equivalent to an energy efficiency ratio (EER) of 13.65.

    [edit] Industrial

    Industrial cooling towers can be used to reject heat from various sources such as machinery or heated process material. The primary use of large, industrial cooling towers is to remove the heat absorbed in the circulating cooling water systems used in power plants, petroleum refineries, petrochemical plants, natural gas processing plants, food processing plants, semi-conductor plants, and other industrial facilities. The circulation rate of cooling water in a typical 700 MW coal-fired power plant with a cooling tower amounts to about 71,600 cubic metres an hour (315,000 U.S. gallons per minute)[1] and the circulating water requires a supply water make-up rate of perhaps 5 percent (i.e., 3,600 cubic metres an hour).
    If that same plant had no cooling tower and used once-through cooling water, it would require about 100,000 cubic metres an hour [2] and that amount of water would have to be continuously returned to the ocean, lake or river from which it was obtained and continuously re-supplied to the plant. Furthermore, discharging large amounts of hot water may raise the temperature of the receiving river or lake to an unacceptable level for the local ecosystem. A cooling tower serves to dissipate the heat into the atmosphere instead and wind and air diffusion spreads the heat over a much larger area than hot water can distribute heat in a body of water.
    Some coal-fired and nuclear power plants located in coastal areas do make use of once-through ocean water. But even there, the offshore discharge water outlet requires very careful design to avoid environmental problems.
    Petroleum refineries also have very large cooling tower systems. A typical large refinery processing 40,000 metric tonnes of crude oil per day (300,000 barrels per day) circulates about 80,000 cubic metres of water per hour through its cooling tower system.
    The world's tallest cooling tower is the 200 metre tall cooling tower of Niederaussem Power Station.
    الروابط فعالة

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    Heat transfer methods

    Image 3: Mechanical draft crossflow cooling tower used in an HVAC application


    With respect to the heat transfer mechanism employed, the main types are:
    • wet cooling towers or simply cooling towers operate on the principle of evaporation.
    • dry cooling towers operate by heat transmission through a surface that divides the working fluid from ambient air. They thus rely mainly on convection heat transfer to reject heat from the working fluid, rather than evaporation.
    • hybrids are also available.
    In a wet cooling tower, the warm water can be cooled to a temperature lower than the ambient air dry-bulb temperature, if the air is relatively dry. (see: dew point and psychrometrics). As air is drawn past a flow of water, the two flows attempt to equalize. The air, if not saturated, absorbs additional water vapor, leaving less heat in the remaining water flow.
    To achieve better performance (more cooling), a media called fill is used to increase the surface area between the air and water flows. Splash fill consists of material placed to interrupt the water flow causing splashing. Film fill is composed of thin sheets of material upon which the water flows. Both methods create increased surface area.

    [edit] Air flow generation methods

    With respect to drawing air through the tower, there are three types of cooling towers:
    • Natural draft, which utilizes buoyancy via a tall chimney. Warm, moist air naturally rises due to the density differential to the dry, cooler outside air. Counterintuitively, more moist air is less dense than drier air at the same temperature and pressure (this is counterintuitive because intuition tells us that the O2 and N2 molecules of air should be lighter than the H2O molecule of water vapor). This moist air buoyancy produces a current of air through the tower.
    • Mechanical draft, which uses power driven fan motors to force or draw air through the tower.
      • Induced draft: A mechanical draft tower with a fan at the discharge which pulls air through tower. The fan induces hot moist air out the discharge. This produces low entering and high exiting air velocities, reducing the possibility of recirculation in which discharged air flows back into the air intake. This fan/fill arrangement is also known as draw-through. (see Image 2, 3)
      • Forced draft: A mechanical draft tower with a blower type fan at the intake. The fan forces air into the tower, creating high entering and low exiting air velocities. The low exiting velocity is much more susceptible to recirculation. With the fan on the air intake, the fan is more susceptible to complications due to freezing conditions. Another disadvantage is that a forced draft design typically requires more motor horsepower than an equivalent induced draft design. The forced draft benefit is its ability to work with high static pressure. They can be installed in more confined spaces and even in some indoor situations. This fan/fill geometry is also known as blow-through. (see Image 4)

    Image 4: A forced draft cooling tower


    • Fan assisted natural draft. A hybrid type that appears like a natural draft though airflow is assisted by a fan.
    Hyperboloid (aka hyperbolic) cooling towers (Image 1) have become the design standard for all natural-draft cooling towers because of their structural strength and minimum usage of material. The hyperbolic form is popularly associated with nuclear power plants, due to media coverage at Three Mile Island. However, this association is misleading, as hyperbolic natural-draft cooling towers are often used at large coal-fired power plants as well.

    [edit] Categorization by air-to-water flow


    [edit] Crossflow

    Crossflow is a design in which the air flow is directed perpendicular to the water flow (see diagram below). Air flow enters one or more vertical faces of the cooling tower to meet the fill material. Water flows (perpendicular to the air) through the fill by gravity. The air continues through the fill and thus past the water flow into an open plenum area. A distribution or hot water basin consisting of a deep pan with holes or nozzles in the bottom is utilized in a crossflow tower. Gravity distributes the water through the nozzles uniformly across the fill material.


    [edit] Counterflow

    In a counterflow design the air flow is directly opposite of the water flow (see diagram below). Air flow first enters an open area beneath the fill media and is then drawn up vertically. The water is sprayed through pressurized nozzles and flows downward through the fill, opposite to the air flow.

    Common to both designs:
    • The interaction of the air and water flow allow a partial equalization and evaporation of water.
    • The air, now saturated with water vapor, is discharged from the cooling tower.
    • A collection or cold water basin is used to contain the water after its interaction with the air flow.
    Both crossflow and counterflow designs can be used in natural draft and mechanical draft cooling towers.

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    Cooling tower as a flue gas stack (Industrial chimney)

    At some modern power stations, equipped with flue gas purification like the Power Station Staudinger Grosskrotzenburg and the Power Station Rostock, the cooling tower is also used as a flue gas stack (industrial chimney). At plants without flue gas purification, this causes problems with corrosion.

    [edit] Wet cooling tower material balance

    Main article: Cooling tower system
    Quantitatively, the material balance around a wet, evaporative cooling tower system is governed by the operational variables of makeup flow rate, evaporation and windage losses, draw-off rate, and the concentration cycles:[3]

    M= Make-up water in m³/hrC= Circulating water in m³/hrD= Draw-off water in m³/hrE= Evaporated water in m³/hrW= Windage loss of water in m³/hrX= Concentration in ppmw (of any completely soluble salts … usually chlorides)XM= Concentration of chlorides in make-up water (M), in ppmwXC= Concentration of chlorides in circulating water (C), in ppmwCycles= Cycles of concentration = XC / XM (dimensionless)ppmw= parts per million by weight
    In the above sketch, water pumped from the tower basin is the cooling water routed through the process coolers and condensers in an industrial facility. The cool water absorbs heat from the hot process streams which need to be cooled or condensed, and the absorbed heat warms the circulating water (C). The warm water returns to the top of the cooling tower and trickles downward over the fill material inside the tower. As it trickles down, it contacts ambient air rising up through the tower either by natural draft or by forced draft using large fans in the tower. That contact causes a small amount of the water to be lost as windage (W) and some of the water (E) to evaporate. The heat required to evaporate the water is derived from the water itself, which cools the water back to the original basin water temperature and the water is then ready to recirculate. The evaporated water leaves its dissolved salts behind in the bulk of the water which has not been evaporated, thus raising the salt concentration in the circulating cooling water. To prevent the salt concentration of the water from becoming too high, a portion of the water is drawn off (D) for disposal. Fresh water makeup (M) is supplied to the tower basin to compensate for the loss of evaporated water, the windage loss water and the draw-off water.
    A water balance around the entire system is:
    M = E + D + W Since the evaporated water (E) has no salts, a chloride balance around the system is:
    M (XM) = D (XC) + W (XC) = XC (D + W) and, therefore:
    XC / XM = Cycles of concentration = M ÷ (D + W) = M ÷ (M – E) = 1 + [E ÷ (D + W)] From a simplified heat balance around the cooling tower:
    E = C · ΔT · cp ÷ HV where: HV= latent heat of vaporization of water = ca. 2260 kJ / kgΔT= water temperature difference from tower top to tower bottom, in °Ccp= specific heat of water = ca. 4.184 kJ / (kg°C)
    Windage (or drift) losses (W) from large-scale industrial cooling towers, in the absence of manufacturer's data, may be assumed to be:
    W = 0.3 to 1.0 percent of C for a natural draft cooling tower without windage drift eliminators W = 0.1 to 0.3 percent of C for an induced draft cooling tower without windage drift eliminators W = about 0.005 percent of C (or less) if the cooling tower has windage drift eliminators Cycles of concentration represents the accumulation of dissolved minerals in the recirculating cooling water. Draw-off (or blowdown) is used principally to control the buildup of these minerals.
    The chemistry of the makeup water including the amount of dissolved minerals can vary widely. Makeup waters low in dissolved minerals such as those from surface water supplies (lakes, rivers etc.) tend to be aggressive to metals (corrosive). Makeup waters from ground water supplies (wells) are usually higher in minerals and tend to be scaling (deposit minerals). Increasing the amount of minerals present in the water by cycling can make water less aggressive to piping however excessive levels of minerals can cause scaling problems.
    As the cycles of concentration increase the water may not be able to hold the minerals in solution. When the solubility of these minerals have been exceeded they can precipitate out as mineral solids and cause fouling and heat exchange problems in the cooling tower or the heat exchangers. The temperatures of the recirculating water, piping and heat exchange surfaces determine if and where minerals will precipitate from the recirculating water. Often a professional water treatment consultant will evaluate the makeup water and the operating conditions of the cooling tower and recommend an appropriate range for the cycles of concentration. The use of water treatment chemicals, pretreatment such as water softening, pH adjustment, and other techniques can affect the acceptable range of cycles of concentration.
    Concentration cycles in the majority of cooling towers usually range from 3 to 7. In the United States the majority of water supplies are well waters and have significant levels of dissolved solids. On the other hand one of the largest water supplies, New York City, has a surface supply quite low in minerals and cooling towers in that city are often allowed to concentrate to 7 or more cycles of concentration.
    Besides treating the circulating cooling water in large industrial cooling tower systems to minimize scaling and fouling, the water should be filtered and also be dosed with biocides and algaecides to prevent growths that could interfere with the continuous flow of the water.[3] For closed loop evaporative towers, corrosion inhibitors may be used, but caution should be taken to meet local environmental regulations as some inhibitors use chromates.
    Ambient conditions dictate the efficiency of any given tower due to the amount of water vapor the air is able to absorb and hold, as can be determined on a psychrometric chart

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  5. [15]
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    نرجوا ان نكون قد وفقنا في هذا الموضوع حول محطات القدرة الحرارية * وبامكانكم متابعة الروابط المتداخلة للدخول الى المواضيع ذات الارتباط * ورغبة منا في توضيح المحطات التي تعمل بالوقود الحجري نضيف الى موضوعنا هذا هذه المشاركة علما ان الرابط الخاص بهذه المشاركة موجود ضمن المشاركات ال 14# السابقة *مع كل التقدير لكل الاخوة الاعضاء اخوكم حسن العراقي
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    : Fossil fuel power plant

    Fossil fuel power plant

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    Mohave Generating Station, a 1,580 MW coal power plant near Laughlin, Nevada




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    A fossil fuel power plant is an energy conversion center that burns fossil fuels to produce electricity, designed on a large scale for continuous operation.

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    Basic concepts
    In a fossil fuel power plant the chemical energy stored in fossil fuels such as coal, fuel oil, natural gas or oil shale is converted successively into thermal energy, mechanical energy and, finally, electrical energy for continuous use and distribution across a wide geographic area. Almost all large fossil fuel power plants are steam-electric power plants, except for gas turbines and utility-sized reciprocating engines that may run on natural gas or diesel.
    The burning of fossil fuel is summarized in the following chemical reaction:
    and the simple word equation for this chemical reaction is:
    All fossil fuels generate carbon dioxide, when combusted. Chemical side reactions also take place, generating, among others, sulfur dioxide (predominantly in coal) and oxides of nitrogen. Each fossil fuel power plant is a highly complex, custom-designed system. Present construction costs, as of 2004, run to US$1,300 per kilowatt, or $650 million for a 500 MWe unit. Multiple generating units may be built at a single site for more efficient use of land, natural resources and labor.

    Coal Power Station in Tampa FL



    [edit] Fuel transport and delivery

    Coal is delivered by mass transport systems, truck, rail, barge or collier. A large coal train called a "unit train" may be two kilometers long, containing 100 cars with 100 tons of coal in each one, for a total load of 10,000 tons. A large plant under full load requires at least one coal delivery this size every day. Plants may get as many as three to five trains a day, especially in "peak season", during the summer months when power consumption is high. A large thermal power plant such as the one at Nanticoke Ontario stores several million tons of coal for winter use when the lakes are frozen.
    Modern unloaders use rotary dump devices, which eliminate problems with coal freezing in bottom dump cars. The unloader includes a train positioner arm that pulls the entire train to position each car over a coal hopper. The dumper clamps an individual car against a platform that swivels the car upside down to dump the coal. Swiveling couplers enable the entire operation to occur while the cars are still coupled together. Unloading a unit train takes about three hours.
    Shorter trains may use railcars with an "air-dump", which relies on air pressure from the engine plus a "hot shoe" on each car. This "hot shoe" when it comes into contact with a "hot rail" at the unloading trestle, shoots an electric charge through the air dump apparatus and causes the doors on the bottom of the car to open, dumping the coal through the opening in the trestle. Unloading one of these trains takes anywhere from an hour to an hour and a half. Older unloaders may still use manually operated bottom-dump rail cars and a "shaker" attached to dump the coal. Generating stations adjacent to a mine may receive coal by conveyor belt or massive diesel-electric-drive trucks.
    A collier (cargo ship carrying coal) may hold 40,000 tons of coal and takes several days to unload. Some colliers carry their own conveying equipment to unload their own bunkers; others depend on equipment at the plant. Colliers are large, seaworthy, self-powered ships. For transporting coal in calmer waters, such as rivers and lakes, flat-bottomed vessels called barges are often used. Barges are usually unpowered and must be moved by tugboats or towboats.
    For startup or auxiliary purposes, the plant may use fuel oil as well. Fuel oil can be delivered to plants by pipeline, tanker, tank car or truck. Oil is stored in vertical cylindrical steel tanks as large as 90,000 barrels (14,000 m³). The heavier no. 5 "bunker" and no. 6 fuels are steam-heated before pumping in cold climates.
    Plants fueled by natural gas are usually built adjacent to gas transport pipelines or have dedicated gas pipelines extended to them.

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    Fuel processing
    Coal is prepared for use by crushing the rough coal to pieces less than 2 inches (50 mm) in size. The coal is transported from the storage yard to in-plant storage silos by rubberized conveyer belts at rates up to 4,000 tons per hour. A 400 ton silo may feed each coal pulverizer (coal mill) at a rate of up to 60 tons per hour. Coal fed into the top of the pulverizer and ground to a powder, the consistency of face powder, is blown, with air, into the furnace. A 500 MWe plant will have six such pulverizers, five of which can supply coal to the furnace at 250 tons per hour under full load.

    [edit] Feedwater heating and deaeration

    The feedwater used in the steam boiler is a means of transferring heat energy from the burning fuel to the mechanical energy of the spinning steam turbine. The total feedwater consists of recirculated condensed steam, referred to as condensate, from the steam turbines plus purified makeup water. Because the metallic materials it contacts are subject to corrosion at high temperatures and pressures, the makeup water is highly purified before use. A system of water softeners and ion exchange demineralizers produces water so pure that it coincidentally becomes an electrical insulator, with conductivity in the range of 0.3–1.0 microsiemens per centimeter. The makeup water in a 500 MWe plant amounts to perhaps 20 US gallons per minute (1.25 L/s) to offset the small losses from steam leaks in the system.
    The feedwater cycle begins with condensate water being pumped out of the condenser after travelling through the steam turbines. The condensate flow rate at full load in a 500 MWe plant is about 6,000 US gallons per minute (0.38 m³/s).

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


    The water 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, the condensate plus the makeup water then flows through a deaerator[1][2] that removes dissolved air from the water, further purifying and reducing its corrosivity. 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). It is also dosed with pH control agents such as ammonia or morpholine to keep the residual acidity low and thus non-corrosive.

    [edit] Boiler operation


    Diagram of a steam power plant boiler.


    The boiler is a rectangular furnace about 50 ft (15 m) on a side and 130 ft (40 m) tall. Its walls are made of a web of high pressure steel tubes about 2.3 inches (60 mm) in diameter.
    Pulverized coal is air-blown into the furnace from fuel nozzles at the four corners and it rapidly burns, forming a large fireball at the center. The thermal radiation of the fireball heats the water that circulates through the boiler tubes near the boiler perimeter. The water circulation rate in the boiler is three to four times the throughput and is typically driven by pumps. As the water in the boiler circulates it absorbs heat and changes into steam at 700 °F (370 °C) and 3,200 psi (22.1 MPa). It is separated from the water inside a drum at the top of the furnace. The saturated steam is introduced into superheat pendant tubes that hang in the hottest part of the combustion gases as they exit the furnace. Here the steam is superheated to 1,000 °F (540 °C) to prepare it for the turbine.
    Plants designed for lignite (brown coal) are increasingly used in locations as varied as Germany, Victoria, 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 exhaust the pulverised 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 HRSGs, Heat Recovery Steam Generators. The exhaust (waste) heat from the gas turbines is used to make superheated steam that is then used in a conventional water-steam generation cycle.

    [edit] Steam turbine generator

    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, two low pressure turbines, and the generator. As steam moves through the system and loses pressure and temperature 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 tons and 100 ft (30 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 four functions of blackout emergency power batteries on site. They are emergency lighting, communication, station alarms and turbogenerator lube oil.
    Superheated steam from the boiler is delivered through 14–16 inch (350–400 mm) diameter piping to the high pressure turbine where it falls in pressure to 600 psi (4 MPa) and to 600 °F (315 °C) through the stage. It exits via 24–26 inch (600–650 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, 30 ft (9 m) long and 12 ft (3.7 m) diameter, contains a stationary stator and a spinning rotor, each containing miles of heavy copper conductor—no permanent magnets here. In operation it generates up to 21,000 amps 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 the highly flammable hydrogen does not mix with oxygen in the air.
    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 electricity flows to a distribution yard where transformers step the voltage up to 115, 230, 500 or 765 kV AC as needed for transmission to its destination.

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    [Steam condensing


    Diagram of a typical water-cooled surface condenser


    The lower is the pressure of the exhaust steam leaving the low pressure turbine, the more efficient is the train of turbine stages. The exhaust steam from the low pressure turbine is condensed in a shell and tube heat exchanger commonly referred to as a surface condenser. Cooling water circulates through the tubes in the condenser's shell and the low pressure exhaust steam is condensed by flowing over the tubes as shown in the adjacent diagram. Typically the cooling water causes the steam to condense at a temperature of about 32–38 °C (90–100 °F) and that creates an absolute pressure in the condenser of about 5–7 kPa (1.5–2.0 in Hg), a vacuum of about 95 kPa (28 mmHg) relative to atmospheric pressure. The condenser, in effect, creates the low pressure required to drag steam through and increase the efficiency of the turbines. The limiting factor is the temperature of the cooling water and that, in turn, is limited by the prevailing average climatic conditions at the power plant's location.
    From the bottom of the condenser, powerful pumps recycle the condensed steam (water) back to the feedwater heaters for reuse. The heat absorbed by the circulating cooling water in the condenser tubes must also be removed to maintain the ability of the water to cool as it circulates.

    A Marley mechanical induced draft cooling tower


    This is done by pumping the warm water from the condenser through either natural draft, forced draft or induced draft cooling towers (as seen in the image to the right) that reduce the temperature of the water, by 11–17°C (20–30 °F), by evaporation - expelling waste heat to the atmosphere. The circulation flow-rate of the cooling water in a 500 MWe unit is about 14.2 m3/s (225,000 US gal/minute) at full load.[3]
    The condenser tubes are made of brass or stainless steel to resist corrosion from either side. Nevertheless they may become internally fouled during operation by bacteria or algae in 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 scrub them clean without the need to take the system off-line.
    Another form of condensing system is the air-cooled condenser. While these systems are similar in operation to mechanical cooling towers, they typically are more environmentally acceptable forms of condensing steam. The process is similar to that of a radiator and fan. Exhaust heat from the low pressure section of a steam 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 in the water-steam cycle.
    The cooling water used to condense the steam in the condenser returns to its source without having been changed other than having been warmed. If the water returns to a local water body (rather than a circulating cooling tower), it is is tempered with cool 'raw' water to prevent thermal shock when discharged into that body of water.

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  9. [19]
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    [edit] Diagram


    Simplified thermal power station

    1. Cooling tower10. Steam control valve19. Superheater2. Cooling water pump11. High pressure steam turbine20. Forced draught (draft) fan3. Three-phase transmission line12. Deaerator21. Reheater4. Step-up Transformer13. Feedwater heater22. Combustion air intake5. Electrical generator14. Coal conveyor23. Economiser6. Low pressure steam turbine15. Coal hopper24. Air preheater7. Boiler feedwater pump16. Coal pulverizer25. Precipitator8. Surface condenser17. Boiler steam drum26. Induced draught (draft) fan9. Intermediate pressure steam turbine18. Bottom ash hopper27. Flue gas stack




    [edit] Stack gas path and cleanup

    see Flue gas emissions from fossil fuel combustion and Flue gas desulfurization for more details 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 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 plants in the world do not have these facilties.[citation needed] 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 plants.

    Flue gas stack at GRES-2 Power Station in Ekibastus, Kazakhstan


    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. The gas travelling up the flue gas stack may by this time only have a temperature of about 50 °C (120 °F). A typical flue gas stack may be 150–180 m (500–600 ft) tall to disperse the remaining flue gas components in the atmosphere. The tallest flue gas stack in the world is 420 m (1,375 ft) tall at the GRES-2 power plant in Ekibastusz, Kazakhstan.
    In the United States and a number of other countries, atmospheric dispersion modeling[4] studies are required to determine the flue gas stack height needed to comply with the local air pollution regulations. The United State also limits the maximum height of a flue gas stack to what is known as the "Good Engineering Practice (GEP)" stack height.[5][6] 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.

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  10. [20]
    حسن هادي
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    Supercritical steam plants
    Above the critical point for water of 705 °F (374 °C) and 3,212 psia (22.1 MPa), there is no phase transition from water to steam, but only a gradual decrease in density. Boiling does not occur and it is not possible to remove impurities via steam separation. In this case a new type of design is required for plants wishing to take advantage of increased thermodynamic efficiency available at the higher temperatures. These plants, also called once-through plants because boiler water does not circulate multiple times, require additional water purification steps to ensure that any impurities picked up during the cycle will be removed. This takes the form of high pressure ion exchange units called condensate polishers between the steam condenser and the feedwater heaters. Subcritical fossil fuel power plants can achieve 36–38% efficiency. Supercritical designs have efficiencies in the low to mid 40% range, with new "ultra critical" designs using pressures of 4,400 psia (30 MPa) and dual stage reheat reaching about 48% efficiency.
    Nuclear power plants must operate far below the temperatures and pressures than coal fired plants do. This limits their thermodynamic efficiency to the order of 34–37%. One proposed plant design, the Supercritical_water_reactor, operates at temperatures and pressures similar to current coal plants, producing comparable efficiency.

    [edit] Gas turbine combined-cycle plants

    An important class of fossil power plant uses a gas turbine, sometimes in conjunction with a steam boiler "bottoming" cycle. The efficiency of a combined cycle plant can approach 60% in large (500+ MWe) units. Such turbines are usually fueled with natural gas or diesel. While highly efficient and very quick to construct (a 1,000 MW plant may be completed in as little as two years from start of construction), the economics of such plants is heavily influenced by the volatile cost of natural gas. The combined cycle plants are designed in a variety of configurations composed of the number of gas turbines followed by the steam turbine. For example, a 3-1 combined cycle facility has three gas turbines tied to one steam turbine. The configurations range from (1-1),(2-1),(3-1),(4-1), (5-1), to (6-1).
    Simple-cycle gas turbine plants, without a steam cycle, are sometimes installed as emergency or peaking capacity; their thermal efficiency is much lower. The high running cost per hour is offset by the low capital cost and the intention to run such units only a few hundred hours per year.

    [edit] Environmental impacts

    Fossil fuel power contributes to acid rain, global warming, and air pollution (electricity generation is responsible for 38 percent of USA carbon dioxide emissions).[7] Acid rain is caused by the emission of nitrogen oxides and sulfur dioxide into the air. These themselves may be only mildly acidic, yet when it reacts with the atmosphere, it creates acidic compounds such as Carbolic acid, nitric acid and sulfuric acid that fall as rain, hence the term acid rain. In Europe and the USA, stricter emission laws have reduced the environmental hazards associated with this problem.
    Another danger related to coal combustion is the emission of fly ash, tiny solid particles that are dangerous for public health. (Natural gas plants emit virtually no fly ash) These can be filtered out of the stack gas, although this does not happen everywhere. The most modern plants that burn coal use a different process, in which synthesis gas is made out of a reaction between coal and water. This is purified of most pollutants and then used initially to power gas turbines, then the residual heat is used for a steam turbine. The pollution levels of such plants are drastically lower than those of "classical" coal power plants. However, all coal burning power plants emit carbon dioxide. Research has shown that increased concentration of carbon dioxide in the atmosphere is positively correlated with a rise in mean global temperature, also known as climate change.
    Coal also contains low levels of uranium, thorium, and other naturally-occurring radioactive isotopes whose release into the environment leads to radioactive contamination. While these substances are present as very small trace impurities, enough coal is burned that significant amounts of these substances are released. A 1,000 MW coal-burning power plant could release as much as 5.2 tons/year of uranium (containing 74 pounds of uranium-235) and 12.8 tons/year of thorium. The radioactive emission from this coal power plant is 100 times greater than a comparable nuclear power plant with the same electrical output; including processing output, the coal power plant's radiation output is over 3 times greater.[8].
    Trace amounts of mercury can exist in coal and other fossil fuels.[9] When these fuels burn, mercury vapor can be released and the mercury is a neurotoxic heavy metal which bioaccumulates in food chains and is especially harmful to aquatic ecosystems. According to the United States Department of Energy, the worldwide emission of mercury from both natural and human sources was 5,500 tons in 1995.[9] and coal-fired plants in the USA release an estimated 48 tons annually, which is less than 1 percent of the worldwide emissions.[9] The Environmental Working Group (a privately funded environmental advocacy organzation) alleges that coal-fired power plants are the largest emitters of mercury in the USA.[10]
    Alternatives to fossil fuel power plants include solar power and other renewable energies (see non-carbon economy).

    [edit] Clean coal

    Main article: clean coal
    Recent developments in the application of fossil fuels include the utilisation of clean coal. Here carbon dioxide produced from the combustion process can be stored in geological formations. This is particularly applicable to empty oil and gas deposits. A number of these carbon sequestration schemes are being planned, most notably for the North Sea.

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