Thermal power station
A thermal power station is a type of
The design of thermal power stations depends on the intended energy source:
Fuels such as
Types of thermal energy
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Almost all
Commercial
Cogeneration plants, often called combined heat and power (CHP) facilities, produce both electric power and heat for process heat or space heating, such as steam and hot water.
History
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The 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;[1] 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 almost all large central power stations.
The largest reciprocating engine-generator sets ever built were completed in 1901 for the
Thermal power generation efficiency
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The
The
Currently most 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. This, in turn, limits their thermodynamic efficiency to 30–32%. Some advanced reactor designs being studied, such as the
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
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[6] while the efficiency of a wind turbine is limited by Betz's law, to about 59.3%, and actual wind turbines show lower efficiency.
Electricity cost
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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. Those indirect costs belong to the broader concept of
Boiler and steam cycle
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In the nuclear plant field, steam generator refers to a specific type of large heat exchanger used in a pressurized water reactor (PWR) to thermally connect the primary (reactor plant) and secondary (steam plant) systems, which generates steam. In a boiling water reactor (BWR), no separate steam generator is used and water boils in the reactor core.
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.
A fossil fuel steam generator includes an economizer, a steam drum, and the furnace with its steam generating tubes and superheater coils. Necessary safety valves are located at suitable points to protect against excessive boiler pressure. The air and flue gas path equipment include: forced draft (FD) fan, air preheater (AP), boiler furnace, induced draft (ID) fan, fly ash collectors (electrostatic precipitator or baghouse), and the flue-gas stack.[7][8][9]
Feed water heating
The boiler feed water used in the
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).
The water is usually pressurized in two stages, and typically flows through a series of six or seven intermediate feed water heaters, heated up at each point with steam extracted from an appropriate extraction connection 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 flows through a
Boiler operation
The boiler is a rectangular furnace about 50 feet (15 m) on a side and 130 feet (40 m) tall. Its walls are made of a web of high pressure steel tubes about 2.3 inches (58 mm) in diameter.[citation needed]
Fuel such as 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 is ignited to rapidly burn, 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. As the water in the boiler circulates it absorbs heat and changes into steam. 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 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
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.[citation needed] 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.[citation needed]
Steam condensing
The condenser condenses the steam from the exhaust of the turbine into liquid to allow it to be pumped. If the condenser can be made cooler, the pressure of the exhaust steam is reduced and efficiency of the cycle increases.
The surface condenser is a
For best efficiency, the temperature in the condenser must be kept as low as practical in order to achieve the lowest possible pressure in the condensing steam. Since the condenser temperature can almost always be kept significantly below 100 °C where the vapor pressure of water is much less than atmospheric pressure, the condenser generally works under vacuum. Thus leaks of non-condensible air into the closed loop 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
of about −95 kPa (−28 inHg) relative to atmospheric pressure. The large decrease in volume that occurs when water vapor condenses to liquid creates the vacuum that generally increases 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 station's location (it may be possible to lower the temperature beyond the turbine limits during winter, causing excessive condensation in the turbine). Plants operating in hot climates may have 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 tower to reject waste heat to the atmosphere, or once-through cooling (OTC) water from a river, lake or ocean. In the United States, about two-thirds of power plants use OTC systems, which often have significant adverse environmental impacts. The impacts include thermal pollution and killing large numbers of fish and other aquatic species at cooling water intakes.[15][16]
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. This is done by pumping the warm water from the condenser through either natural draft, forced draft or induced draft
The condenser tubes are typically made
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 often tempered with cool 'raw' water to prevent thermal shock when discharged into that body of water.
Another form of condensing system is the air-cooled condenser. 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. Air-cooled condensers typically operate at a higher temperature than water-cooled versions. While saving water, the efficiency of the cycle is reduced (resulting in more carbon dioxide per megawatt-hour of electricity).
From the bottom of the condenser, powerful condensate pumps recycle the condensed steam (water) back to the water/steam cycle.
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 slightly below atmospheric pressure in the furnace to avoid leakage of combustion products from the boiler casing.
Steam turbine generator
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A steam turbine generator consists of a series of steam turbines interconnected to each other and a generator on a common shaft.
Steam turbine
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 shaft that connects to 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 feet (30 m) long. It is so heavy that it must be kept turning slowly even when shut down (at 3
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.[citation needed]
Turbo generator
The generator, typically about 30 feet (9 m) long and 12 feet (3.7 m) in diameter, contains a stationary
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
Stack gas path and cleanup
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As the combustion
Where required by law, the sulfur and nitrogen oxide
In the United States and a number of other countries, atmospheric dispersion modeling[18] 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.[19][20] 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.
Auxiliary systems
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Boiler make-up water treatment plant and storage
Since there is continuous withdrawal of steam and continuous return of
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
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
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[clarification needed] 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.[citation needed]
Generator cooling
While small generators may be cooled by air drawn through filters at the inlet, larger units generally require special cooling arrangements.
The hydrogen pressure inside the casing is maintained slightly higher than atmospheric pressure to avoid outside air ingress, and up to about two atmospheres pressure to improve heat transfer capacity. The hydrogen must be sealed against outward leakage where the shaft emerges from the casing. Mechanical seals around the shaft are installed with a very small annular gap to avoid rubbing between the shaft and the seals on smaller turbines, with labyrinth type seals on larger machines.. Seal oil is used to prevent the hydrogen gas leakage to atmosphere.
The generator also uses water cooling. Since the generator coils are at a potential of about 22
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
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
Circulating water system
To dissipate the thermal load of main turbine exhaust steam, condensate from gland steam condenser, and condensate from Low Pressure Heater by providing a continuous supply of cooling water to the main condenser thereby leading to 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.[21]
See also
- Boiler
- Biochar processing in thermal power stations
- Cogeneration
- Cooling tower
- Cost of electricity by source
- Energy crop
- Flue-gas stack
- Fossil fuel power station
- Geothermal power
- Integrated gasification combined cycle
- Iron powder
- List of largest power stations
- List of thermal power station failures
- Nuclear power
- Ocean thermal energy conversion
- Powership
- Power station
- Surface condenser
- Water-tube boiler
References
- ^ the early days of the power station industry. CUP Archive. 1940.
- ISBN 1-59691-677-X
- ^ "DOE – Fossil Energy: How Turbine Power Plants Work". Fossil.energy.gov. Archived from the original on May 27, 2010. Retrieved 2011-09-25.
- ^ a b John Zactruba, The Efficiency of Power Plants of Different Types, Brighthub Engineering. Retrieved 2019-04-24.
- ^ Global CCS Institute, 5. Efficiency in Thermal Power Generation, Energy efficiency technologies: overview report, 1 March 2014. Retrieved 2019-04-24.
- Pew Center on Global Climate Change, October 2009
- ISBN 978-0-08-040510-0.
- ^ ISBN 978-0-9634570-0-4.
- ^ ISBN 978-0-07-019435-9.)
{{cite book}}
: CS1 maint: multiple names: authors list (link - ^ Pressurized deaerators
- ^ "Evoqua Water Technologies" (PDF). www.usfilter.com.
- ^ a b Air Pollution Control Orientation Course from website of the Air Pollution Training Institute
- ^ a b Energy savings in steam systems Archived 2007-09-27 at the Wayback Machine Figure 3a, Layout of surface condenser (scroll to page 11 of 34 pdf pages)
- ^ Robert Thurston Kent (Editor in Chief) (1936). Kents' Mechanical Engineers' Handbook (Eleventh edition (Two volumes) ed.). John Wiley & Sons (Wiley Engineering Handbook Series).
{{cite book}}
:|author=
has generic name (help) - ^ Economic Analysis for the Final Section 316(b) Existing Facilities Rule (Report). Cooling Water Intakes. Washington, D.C.: U.S. Environmental Protection Agency (EPA). May 2014. pp. 1–3. EPA-821-R-14-001.
- ^ "Cooling Water Intakes". EPA. 2017-08-30.
- ^ Maulbetsch, John; Zammit, Kent (2003-05-06). "Cooling System Retrofit Costs" (PDF). Cooling Water Intakes. EPA. Archived from the original (PDF) on March 9, 2008. Retrieved 2006-09-10. EPA Workshop on Cooling Water Intake Technologies, Arlington, Virginia.
- ^ Guideline for Determination of Good Engineering Practice Stack Height (Technical Support Document for the Stack Height Regulations), Revised, 1985, EPA Publication No. EPA–450/4–80–023R, U.S. Environmental Protection Agency (NTIS No. PB 85–225241)
- ^ Lawson, Jr., R. E. and W. H. Snyder, 1983. Determination of Good Engineering Practice Stack Height: A Demonstration Study for a Power Plant, 1983, EPA Publication No. EPA–600/3–83–024. U.S. Environmental Protection Agency (NTIS No. PB 83–207407)
- doi:10.1038/nclimate2903.)
{{cite journal}}
: CS1 maint: multiple names: authors list (link
External links
- Thermal Power Plant:Indian Context
- Conventional coal-fired power plant
- Power station diagram
- Power Plant Reference Books
- Steam jet ejectors
- Steam jet ejector performance guidelines
- First on YouTubevideo lectures by S. Banerjee on "Thermal Power Plants"