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85-423: In a fossil fuel power plant using a steam cycle for power generation, the primary heat source will be combustion of coal , oil , or natural gas . In some cases byproduct fuel such as the carbon monoxide rich offgasses of a coke battery can be burned to heat a boiler; biofuels such as bagasse , where economically available, can also be used. In a nuclear power plant, boilers called steam generators are heated by

170-467: A greenhouse gas which is a major contributor to global warming . The results of a recent study show that the net income available to shareholders of large companies could see a significant reduction from the greenhouse gas emissions liability related to only natural disasters in the United States from a single coal-fired power plant. However, as of 2015, no such cases have awarded damages in

255-448: A steam locomotive . This is because unavoidable temperature and/or pressure loss that occurs as steam travels from the boiler to the machinery will cause some condensation, resulting in liquid water being carried into the machinery. The water entrained in the steam may damage turbine blades or in the case of a reciprocating steam engine , may cause serious mechanical damage due to hydrostatic lock . Superheated steam boilers evaporate

340-405: A boiler is combustion of any of several fuels , such as wood , coal , oil , or natural gas . Electric steam boilers use resistance- or immersion-type heating elements. Nuclear fission is also used as a heat source for generating steam , either directly (BWR) or, in most cases, in specialised heat exchangers called "steam generators" (PWR). Heat recovery steam generators (HRSGs) use

425-630: A cooler cooling system. However, it may be used in cogeneration plants to heat buildings, produce hot water, or to heat materials on an industrial scale, such as in some oil refineries , plants, and chemical synthesis plants. Typical thermal efficiency for utility-scale electrical generators is around 37% for coal and oil-fired plants, and 56 – 60% (LEV) for combined-cycle gas-fired plants. Plants designed to achieve peak efficiency while operating at capacity will be less efficient when operating off-design (i.e. temperatures too low.) Practical fossil fuels stations operating as heat engines cannot exceed

510-404: A cooler medium must be equal or larger than the ratio of absolute temperatures of the cooling system (environment) and the heat source (combustion furnace). Raising the furnace temperature improves the efficiency but complicates the design, primarily by the selection of alloys used for construction, making the furnace more expensive. The waste heat cannot be converted into mechanical energy without

595-503: A factory or data center, or may also be operated in parallel with the local utility system to reduce peak power demand charge from the utility. Diesel engines can produce strong torque at relatively low rotational speeds, which is generally desirable when driving an alternator , but diesel fuel in long-term storage can be subject to problems resulting from water accumulation and chemical decomposition . Rarely used generator sets may correspondingly be installed as natural gas or LPG to minimize

680-491: A fine filter that collects the ash particles, electrostatic precipitators use an electric field to trap ash particles on high-voltage plates, and cyclone collectors use centrifugal force to trap particles to the walls. A recent study indicates that sulfur emissions from fossil fueled power stations in China may have caused a 10-year lull in global warming (1998-2008). Fossil-fuel power stations, particularly coal-fired plants, are

765-478: A fossil fuel power plant the chemical energy stored in fossil fuels such as coal , fuel oil , natural gas or oil shale and oxygen of the air is converted successively into thermal energy , mechanical energy and, finally, electrical energy . Each fossil fuel power plant is a complex, custom-designed system. Multiple generating units may be built at a single site for more efficient use of land , natural resources and labor . Most thermal power stations in

850-636: A major source of industrial wastewater . Wastewater streams include flue-gas desulfurization, fly ash, bottom ash and flue gas mercury control. Plants with air pollution controls such as wet scrubbers typically transfer the captured pollutants to the wastewater stream. Ash ponds , a type of surface impoundment, are a widely used treatment technology at coal-fired plants. These ponds use gravity to settle out large particulates (measured as total suspended solids ) from power plant wastewater. This technology does not treat dissolved pollutants. Power stations use additional technologies to control pollutants, depending on

935-408: A particular fuel. As an example, a new 1500 MW supercritical lignite-fueled power station running on average at half its capacity might have annual CO 2 emissions estimated as: = 1500MW x 0.5 x 100/40 x 101000 kg/TJ x 1year = 1500MJ/s x 0.5 x 2.5 x 0.101 kg/MJ x 365x24x60x60s = 1.5x10 x 5x10 x 2.5 x 1.01 x 3.1536x10 kg = 59.7 x10 kg = 5.97 Mt Thus the example power station

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1020-433: A serious impact on public health. Power plants remove particulate from the flue gas with the use of a bag house or electrostatic precipitator . Several newer plants that burn coal use a different process, Integrated Gasification Combined Cycle in which synthesis gas is made out of a reaction between coal and water. The synthesis gas is processed to remove most pollutants and then used initially to power gas turbines. Then

1105-442: A source of many serious injuries and property destruction due to poorly understood engineering principles. Thin and brittle metal shells can rupture, while poorly welded or riveted seams could open up, leading to a violent eruption of the pressurized steam. When water is converted to steam it expands to over 1,000 times its original volume and travels down steam pipes at over 100 kilometres per hour (62 mph). Because of this, steam

1190-523: A water-cooled furnace, which allows for higher supplemental firing and better overall efficiency. Some HRSGs include supplemental, or duct firing. These additional burners provide additional energy to the HRSG, which produces more steam and hence increases the output of the steam turbine . Generally, duct firing provides electrical output at lower capital cost. It is therefore often utilized for peaking operations. HRSGs can also have diverter valves to regulate

1275-523: Is a deceiving baseline for comparison: just the Chernobyl nuclear disaster released, in iodine-131 alone, an estimated 1.76 EBq. of radioactivity, a value one order of magnitude above this value for total emissions from all coal burned within a century, while the iodine-131, the major radioactive substance which comes out in accident situations, has a half life of just 8 days. Heat recovery steam generator A heat recovery steam generator ( HRSG )

1360-424: Is a relatively cheap fuel. Coal is an impure fuel and produces more greenhouse gas and pollution than an equivalent amount of petroleum or natural gas. For instance, the operation of a 1000-MWe coal-fired power plant results in a nuclear radiation dose of 490 person-rem/year, compared to 136 person-rem/year for an equivalent nuclear power plant, including uranium mining, reactor operation and waste disposal. Coal

1445-422: Is an energy recovery heat exchanger that recovers heat from a hot gas stream, such as a combustion turbine or other waste gas stream. It produces steam that can be used in a process ( cogeneration ) or used to drive a steam turbine ( combined cycle ). HRSGs consist of four major components: the economizer , evaporator , superheater and water preheater . The different components are put together to meet

1530-522: Is an efficient method of moving energy and heat around a site from a central boiler house to where it is needed, but without the right boiler feedwater treatment, a steam-raising plant will suffer from scale formation and corrosion. At best, this increases energy costs and can lead to poor quality steam, reduced efficiency, shorter plant life and unreliable operation. At worst, it can lead to catastrophic failure and loss of life. Collapsed or dislodged boiler tubes can also spray scalding-hot steam and smoke out of

1615-414: Is boiled the result is saturated steam , also referred to as "wet steam." Saturated steam, while mostly consisting of water vapor, carries some unevaporated water in the form of droplets. Saturated steam is useful for many purposes, such as cooking , heating and sanitation , but is not desirable when steam is expected to convey energy to machinery, such as a ship's propulsion system or the "motion" of

1700-419: Is burned that significant amounts of these substances are released. A 1,000 MW coal-burning power plant could have an uncontrolled release of as much as 5.2 metric tons per year of uranium (containing 74 pounds (34 kg) of uranium-235 ) and 12.8 metric tons per year of thorium. In comparison, a 1,000 MW nuclear plant will generate about 30 metric tons of high-level radioactive solid packed waste per year. It

1785-408: Is by simply using an induced draught fan (ID fan) which removes flue gases from the furnace and forces the exhaust gas up the stack. Almost all induced draught furnaces operate with a slightly negative pressure. Mechanical forced draught is provided by means of a fan forcing air into the combustion chamber. Air is often passed through an air heater; which, as the name suggests, heats the air going into

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1870-553: Is caused by the emission of nitrogen oxides and sulfur dioxide . These gases may be only mildly acidic themselves, yet when they react with the atmosphere, they create acidic compounds such as sulfurous acid , nitric acid and sulfuric acid which fall as rain, hence the term acid rain. In Europe and the US, stricter emission laws and decline in heavy industries have reduced the environmental hazards associated with this problem, leading to lower emissions after their peak in 1960s. In 2008,

1955-428: Is converted to steam in the boiler; additional heating stages may be included to superheat the steam. The hot steam is sent through controlling valves to a turbine. As the steam expands and cools, its energy is transferred to the turbine blades which turn a generator. The spent steam has very low pressure and energy content; this water vapor is fed through a condenser, which removes heat from the steam. The condensed water

2040-403: Is delivered by highway truck , rail , barge , collier ship or coal slurry pipeline . Generating stations adjacent to a mine may receive coal by conveyor belt or massive diesel-electric -drive trucks . Coal is usually prepared for use by crushing the rough coal to pieces less than 2 inches (5 cm) in size. Gas is a very common fuel and has mostly replaced coal in countries where gas

2125-507: Is estimated that during 1982, US coal burning released 155 times as much uncontrolled radioactivity into the atmosphere as the Three Mile Island incident . The collective radioactivity resulting from all coal burning worldwide between 1937 and 2040 is estimated to be 2,700,000 curies or 0.101 EBq. During normal operation, the effective dose equivalent from coal plants is 100 times that from nuclear plants. Normal operation however,

2210-421: Is estimated to emit about 6 megatonnes of carbon dioxide each year. The results of similar estimations are mapped by organisations such as Global Energy Monitor , Carbon Tracker and ElectricityMap. Alternatively it may be possible to measure CO 2 emissions (perhaps indirectly via another gas) from satellite observations. Another problem related to coal combustion is the emission of particulates that have

2295-448: Is generated at single pressure level whereas multi pressure HRSGs employ two (double pressure) or three (triple pressure) steam drums. As such triple pressure HRSGs consist of three sections: an LP (low pressure) section, a reheat/IP (intermediate pressure) section, and an HP (high pressure) section. Each section has a steam drum and an evaporator section where water is converted to steam. This steam then passes through superheaters to raise

2380-457: Is often used in superheater sections that will not be exposed to boiling water , and electrically-heated stainless steel shell boilers are allowed under the European "Pressure Equipment Directive" for production of steam for sterilizers and disinfectors. In live steam models , copper or brass is often used because it is more easily fabricated in smaller size boilers. Historically, copper

2465-609: Is referred to as a combined cycle power plant because it combines the Brayton cycle of the gas turbine with the Rankine cycle of the HRSG. The turbines are fueled either with natural gas or fuel oil. Diesel engine generator sets are often used for prime power in communities not connected to a widespread power grid. Emergency (standby) power systems may use reciprocating internal combustion engines operated by fuel oil or natural gas. Standby generators may serve as emergency power for

2550-464: Is still important as the fuel source for diesel engine power plants used especially in isolated communities not interconnected to a grid. Liquid fuels may also be used by gas turbine power plants, especially for peaking or emergency service. Of the three fossil fuel sources, oil has the advantages of easier transportation and handling than solid coal, and easier on-site storage than natural gas. Combined heat and power (CHP), also known as cogeneration ,

2635-502: Is still unknown as to which kinds of particulate matter pose the most harm, which makes it difficult to come up with adequate legislation for regulating particulate matter. There are several methods of helping to reduce the particulate matter emissions from coal-fired plants. Roughly 80% of the ash falls into an ash hopper, but the rest of the ash then gets carried into the atmosphere to become coal-fly ash. Methods of reducing these emissions of particulate matter include: The baghouse has

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2720-462: Is the " nameplate capacity " or the maximum allowed output of the plant, " capacity factor " or "load factor" is a measure of the amount of power that a plant produces compared with the amount it would produce if operated at its rated capacity nonstop, heat rate is thermal energy in/electrical energy out, emission intensity (also called emission factor ) is the CO 2 emitted per unit of heat generated for

2805-431: Is the once-through steam generator. In this design, the inlet feedwater follows a continuous path without segmented sections for economizers, evaporators, and superheaters. This provides a high degree of flexibility as the sections are allowed to grow or contract based on the heat load being received from the gas turbine. The absence of drums allows for quick changes in steam production and fewer variables to control, and

2890-536: Is the use of a thermal power station to provide both electric power and heat (the latter being used, for example, for district heating purposes). This technology is practiced not only for domestic heating (low temperature) but also for industrial process heat, which is often high temperature heat. Calculations show that Combined Heat and Power District Heating (CHPDH) is the cheapest method in reducing (but not eliminating) carbon emissions, if conventional fossil fuels remain to be burned. Thermal power plants are one of

2975-433: Is then pumped into the boiler to repeat the cycle. Emissions from the boiler include carbon dioxide, oxides of sulfur, and in the case of coal fly ash from non-combustible substances in the fuel. Waste heat from the condenser is transferred either to the air, or sometimes to a cooling pond, lake or river. One type of fossil fuel power plant uses a gas turbine in conjunction with a heat recovery steam generator (HRSG). It

3060-465: Is what remains after the coal has been combusted, so it consists of the incombustible materials that are found in the coal. The size and chemical composition of these particles affects the impacts on human health. Currently coarse (diameter greater than 2.5 μm) and fine (diameter between 0.1 μm and 2.5 μm) particles are regulated, but ultrafine particles (diameter less than 0.1 μm) are currently unregulated, yet they pose many dangers. Unfortunately much

3145-522: The Carnot cycle limit for conversion of heat energy into useful work. Fuel cells do not have the same thermodynamic limits as they are not heat engines. The efficiency of a fossil fuel plant may be expressed as its heat rate , expressed in BTU/kilowatthour or megajoules/kilowatthour. In a steam turbine power plant, fuel is burned in a furnace and the hot gasses flow through a boiler. Water

3230-621: The Carnot efficiency and therefore produce waste heat . Fossil fuel power stations provide most of the electrical energy used in the world. Some fossil-fired power stations are designed for continuous operation as baseload power plants , while others are used as peaker plants . However, starting from the 2010s, in many countries plants designed for baseload supply are being operated as dispatchable generation to balance increasing generation by variable renewable energy . By-products of fossil fuel power plant operation must be considered in their design and operation. Flue gas from combustion of

3315-828: The European Environment Agency (EEA) documented fuel-dependent emission factors based on actual emissions from power plants in the European Union . Electricity generation using carbon-based fuels is responsible for a large fraction of carbon dioxide (CO 2 ) emissions worldwide and for 34% of U.S. man-made carbon dioxide emissions in 2010. In the U.S. 70% of electricity is generated by combustion of fossil fuels. Coal contains more carbon than oil or natural gas fossil fuels, resulting in greater volumes of carbon dioxide emissions per unit of electricity generated. In 2010, coal contributed about 81% of CO 2 emissions from generation and contributed about 45% of

3400-416: The condenser . This results in slightly less fuel use and therefore less greenhouse gas production. The term "boiler" should not be used for a supercritical pressure steam generator, as no "boiling" occurs in this device. A fuel -heated boiler must provide air to oxidize its fuel. Early boilers provided this stream of air, or draught , through the natural action of convection in a chimney connected to

3485-460: The fossil fuels contains carbon dioxide and water vapor, as well as pollutants such as nitrogen oxides (NO x ), sulfur oxides (SO x ), and, for coal-fired plants, mercury , traces of other metals, and fly ash . Usually all of the carbon dioxide and some of the other pollution is discharged to the air. Solid waste ash from coal-fired boilers must also be removed. Fossil fueled power stations are major emitters of carbon dioxide (CO 2 ),

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3570-436: The greenhouse gas carbon dioxide within the atmosphere will "very likely" lead to higher average temperatures on a global scale ( global warming ). Concerns regarding the potential for such warming to change the global climate prompted IPCC recommendations calling for large cuts to CO 2 emissions worldwide. Emissions can be reduced with higher combustion temperatures, yielding more efficient production of electricity within

3655-450: The heat energy of combustion into mechanical energy , which then operates an electrical generator . The prime mover may be a steam turbine , a gas turbine or, in small plants, a reciprocating gas engine . All plants use the energy extracted from the expansion of a hot gas, either steam or combustion gases. Although different energy conversion methods exist, all thermal power station conversion methods have their efficiency limited by

3740-518: The United States. Per unit of electric energy, brown coal emits nearly twice as much CO 2 as natural gas, and black coal emits somewhat less than brown. As of 2019 , carbon capture and storage of emissions is not economically viable for fossil fuel power stations, and keeping global warming below 1.5 °C is still possible but only if no more fossil fuel power plants are built and some existing fossil fuel power plants are shut down early, together with other measures such as reforestation . In

3825-431: The air intake and firing chute, injuring the firemen who load the coal into the fire chamber. Extremely large boilers providing hundreds of horsepower to operate factories can potentially demolish entire buildings. A boiler that has a loss of feed water and is permitted to boil dry can be extremely dangerous. If feed water is then sent into the empty boiler, the small cascade of incoming water instantly boils on contact with

3910-476: The corresponding feedwater pressure must be even higher, demanding a more robust pump design. Another consideration is safety. High pressure, superheated steam can be extremely dangerous if it unintentionally escapes. To give the reader some perspective, the steam plants used in many U.S. Navy destroyers built during World War II operated at 600  psi (4,100  kPa ; 41  bar ) pressure and 850 degrees Fahrenheit (454 degrees Celsius) superheat. In

3995-423: The cycle. As of 2019 the price of emitting CO 2 to the atmosphere is much lower than the cost of adding carbon capture and storage (CCS) to fossil fuel power stations, so owners have not done so. The CO 2 emissions from a fossil fuel power station can be estimated with the following formula: CO 2 emissions = capacity x capacity factor x heat rate x emission intensity x time where "capacity"

4080-510: The electricity generated in the United States. In 2000, the carbon intensity (CO 2 emissions) of U.S. coal thermal combustion was 2249 lbs/MWh (1,029 kg/MWh) while the carbon intensity of U.S. oil thermal generation was 1672 lb/MWh (758 kg/MWh or 211 kg/ GJ ) and the carbon intensity of U.S. natural gas thermal production was 1135 lb/MWh (515 kg/MWh or 143 kg/GJ). The Intergovernmental Panel on Climate Change ( IPCC ) reports that increased quantities of

4165-422: The event of a major rupture of the system, an ever-present hazard in a warship during combat , the enormous energy release of escaping superheated steam, expanding to more than 1600 times its confined volume, would be equivalent to a cataclysmic explosion, whose effects would be exacerbated by the steam release occurring in a confined space, such as a ship's engine room . Also, small leaks that are not visible at

4250-400: The exhaust of the combustion chamber. Since the heated flue gas is less dense than the ambient air surrounding the boiler, the flue gas rises in the chimney, pulling denser, fresh air into the combustion chamber. Most modern boilers depend on mechanical draught rather than natural draught. This is because natural draught is subject to outside air conditions and temperature of flue gases leaving

4335-431: The first central stations used reciprocating steam engines to drive generators. As the size of the electrical load to be served grew, reciprocating units became too large and cumbersome to install economically. The steam turbine rapidly displaced all reciprocating engines in central station service. Coal is the most abundant fossil fuel on the planet, and widely used as the source of energy in thermal power stations and

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4420-596: The following configurations: To define and secure boilers safely, some professional specialized organizations such as the American Society of Mechanical Engineers (ASME) develop standards and regulation codes. For instance, the ASME Boiler and Pressure Vessel Code is a standard providing a wide range of rules and directives to ensure compliance of the boilers and other pressure vessels with safety, security and design standards. Historically, boilers were

4505-436: The forced draught fan allowing the furnace pressure to be maintained slightly below atmospheric. Fossil fuel power plant 2021 world electricity generation by source. Total generation was 28 petawatt-hours . A fossil fuel power station is a thermal power station which burns a fossil fuel , such as coal , oil , or natural gas , to produce electricity . Fossil fuel power stations have machinery to convert

4590-573: The fuel system maintenance requirements. Spark-ignition internal combustion engines operating on gasoline (petrol), propane , or LPG are commonly used as portable temporary power sources for construction work, emergency power, or recreational uses. Reciprocating external combustion engines such as the Stirling engine can be run on a variety of fossil fuels, as well as renewable fuels or industrial waste heat. Installations of Stirling engines for power production are relatively uncommon. Historically,

4675-450: The furnace in order to increase the overall efficiency of the boiler. Dampers are used to control the quantity of air admitted to the furnace. Forced draught furnaces usually have a positive pressure. Balanced draught is obtained through use of both induced and forced draught. This is more common with larger boilers where the flue gases have to travel a long distance through many boiler passes. The induced draught fan works in conjunction with

4760-434: The furnace, as well as the chimney height. All these factors make proper draught hard to attain and therefore make mechanical draught equipment much more reliable and economical. Types of draught can also be divided into induced draught , where exhaust gases are pulled out of the boiler; forced draught , where fresh air is pushed into the boiler; and balanced draught , where both effects are employed. Natural draught through

4845-411: The heat produced by nuclear fission. Where a large volume of hot gas is available from some process, a heat recovery steam generator or recovery boiler can use the heat to produce steam, with little or no extra fuel consumed; such a configuration is common in a combined cycle power plant where a gas turbine and a steam boiler are used. In all cases the combustion product waste gases are separate from

4930-501: The heat rejected from other processes such as gas turbine . There are two methods to measure the boiler efficiency in the ASME performance test code (PTC) for boilers ASME PTC 4 and for HRSG ASME PTC 4.4 and EN 12952-15 for water tube boilers: Direct method of boiler efficiency test is more usable or more common. where To measure the boiler efficiency in indirect method, parameter like these are needed: Boilers can be classified into

5015-460: The hot exhaust gases from the gas turbines are used to generate steam to power a steam turbine. The pollution levels of such plants are drastically lower than those of "classic" coal power plants. Particulate matter from coal-fired plants can be harmful and have negative health impacts. Studies have shown that exposure to particulate matter is related to an increase of respiratory and cardiac mortality. Particulate matter can irritate small airways in

5100-441: The inlet flow into the HRSG. This allows the gas turbine to continue to operate when there is no steam demand or if the HRSG needs to be taken offline. Emissions controls may also be located in the HRSG. Some may contain a selective catalytic reduction system to reduce nitrogen oxides (a large contributor to the formation of smog and acid rain) or a catalyst to remove carbon monoxide . The inclusion of an SCR dramatically affects

5185-528: The layout of the HRSG. NOx catalyst performs best in temperatures between 650 and 750 °F (343–399 °C). This usually means that the evaporator section of the HRSG will have to be split and the SCR placed in between the two sections. Some low-temperature NOx catalysts have recently come to market that allow for the SCR to be placed between the evaporator and economizer sections (350–500 °F [177–260 °C]). A specialized type of HRSG without boiler drums

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5270-459: The longevity of older wrought-iron boilers far superior to that of welded steel boilers. Cast iron may be used for the heating vessel of domestic water heaters. Although such heaters are usually termed "boilers" in some countries, their purpose is usually to produce hot water, not steam, and so they run at low pressure and try to avoid boiling. The brittleness of cast iron makes it impractical for high-pressure steam boilers. The source of heat for

5355-428: The lungs, which can lead to increased problems with asthma, chronic bronchitis, airway obstruction, and gas exchange. There are different types of particulate matter, depending on the chemical composition and size. The dominant form of particulate matter from coal-fired plants is coal fly ash , but secondary sulfate and nitrate also comprise a major portion of the particulate matter from coal-fired plants. Coal fly ash

5440-479: The main artificial sources of producing toxic gases and particulate matter . Fossil fuel power plants cause the emission of pollutants such as NO x , SO x , CO 2 , CO, PM, organic gases and polycyclic aromatic hydrocarbons. World organizations and international agencies, like the IEA, are concerned about the environmental impact of burning fossil fuels , and coal in particular. The combustion of coal contributes

5525-488: The most to acid rain and air pollution , and has been connected with global warming . Due to the chemical composition of coal there are difficulties in removing impurities from the solid fuel prior to its combustion. Modern day coal power plants pollute less than older designs due to new " scrubber " technologies that filter the exhaust air in smoke stacks. However, emission levels of various pollutants are still on average several times greater than natural gas power plants and

5610-640: The operating requirements of the unit. See the attached illustration of a Modular HRSG General Arrangement. Modular HRSGs can be categorized by a number of ways such as direction of exhaust gases flow or number of pressure levels. Based on the flow of exhaust gases, HRSGs are categorized into vertical and horizontal types. In horizontal type HRSGs, exhaust gas flows horizontally over vertical tubes whereas in vertical type HRSGs, exhaust gas flow vertically over horizontal tubes. Based on pressure levels, HRSGs can be categorized into single pressure and multi pressure. Single pressure HRSGs have only one steam drum and steam

5695-415: The overall energy efficiency of the steam plant (the combination of boiler, superheater, piping and machinery) generally will be improved enough to more than offset the increased fuel consumption. Superheater operation is similar to that of the coils on an air conditioning unit, although for a different purpose. The steam piping is directed through the flue gas path in the boiler furnace, an area in which

5780-614: The particular wastestream in the plant. These include dry ash handling, closed-loop ash recycling, chemical precipitation, biological treatment (such as an activated sludge process), membrane systems, and evaporation-crystallization systems. In 2015 EPA published a regulation pursuant to the Clean Water Act that requires US power plants to use one or more of these technologies. Technological advancements in ion exchange membranes and electrodialysis systems has enabled high efficiency treatment of flue-gas desulfurization wastewater to meet

5865-491: The point of leakage could be lethal if an individual were to step into the escaping steam's path. Hence designers endeavor to give the steam-handling components of the system as much strength as possible to maintain integrity. Special methods of coupling steam pipes together are used to prevent leaks, with very high pressure systems employing welded joints to avoided leakage problems with threaded or gasketed connections. Supercritical steam generators are frequently used for

5950-402: The production of electric power . They operate at supercritical pressure. In contrast to a "subcritical boiler", a supercritical steam generator operates at such a high pressure (over 3,200 psi or 22 MPa) that the physical turbulence that characterizes boiling ceases to occur; the fluid is neither liquid nor gas but a super-critical fluid. There is no generation of steam bubbles within

6035-530: The scrubbers transfer the captured pollutants to wastewater, which still requires treatment in order to avoid pollution of receiving water bodies. In these modern designs, pollution from coal-fired power plants comes from the emission of gases such as carbon dioxide, nitrogen oxides , and sulfur dioxide into the air, as well a significant volume of wastewater which may contain lead , mercury , cadmium and chromium , as well as arsenic , selenium and nitrogen compounds ( nitrates and nitrites ). Acid rain

6120-412: The steam to carry more energy. Although superheating adds more energy to the steam in the form of heat there is no effect on pressure, which is determined by the rate at which steam is drawn from the boiler and the pressure settings of the safety valves . The fuel consumption required to generate superheated steam is greater than that required to generate an equivalent volume of saturated steam. However,

6205-407: The steam within. The design of any superheated steam plant presents several engineering challenges due to the high working temperatures and pressures. One consideration is the introduction of feedwater to the boiler. The pump used to charge the boiler must be able to overcome the boiler's operating pressure, else water will not flow. As a superheated boiler is usually operated at high pressure,

6290-564: The superheated metal shell and leads to a violent explosion that cannot be controlled even by safety steam valves. Draining of the boiler can also happen if a leak occurs in the steam supply lines that is larger than the make-up water supply could replace. The Hartford Loop was invented in 1919 by the Hartford Steam Boiler Inspection and Insurance Company as a method to help prevent this condition from occurring, and thereby reduce their insurance claims. When water

6375-508: The temperature beyond the saturation point. The steam and water pressure parts of an HRSG are subjected to a wide range of degradation mechanisms, for example creep , thermal fatigue , creep-fatigue, mechanical fatigue, Flow Accelerated Corrosion (FAC), corrosion and corrosion fatigue, amongst others. Packaged HRSGs are designed to be shipped as a fully assembled unit from the factory. They can be used in waste heat or turbine (usually under 20 MW) applications. The packaged HRSG can have

6460-409: The temperature is typically between 1,300 and 1,600 degrees Celsius (2,372 and 2,912 degrees Fahrenheit). Some superheaters are radiant type, which as the name suggests, they absorb heat by radiation. Others are convection type, absorbing heat from a fluid. Some are a combination of the two types. Through either method, the extreme heat in the flue gas path will also heat the superheater steam piping and

6545-583: The updated EPA discharge limits. Coal is a sedimentary rock formed primarily from accumulated plant matter, and it includes many inorganic minerals and elements which were deposited along with organic material during its formation. As the rest of the Earth's crust , 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

6630-485: The use of a chimney is a type of induced draught; mechanical draught can be induced, forced or balanced. There are two types of mechanical induced draught. The first is through use of a steam jet. The steam jet oriented in the direction of flue gas flow induces flue gases into the stack and allows for a greater flue gas velocity increasing the overall draught in the furnace. This method was common on steam driven locomotives which could not have tall chimneys. The second method

6715-414: The water and then further heat the steam in a superheater , causing the discharged steam temperature to be substantially above the boiling temperature at the boiler's operating pressure. As the resulting " dry steam " is much hotter than needed to stay in the vaporous state it will not contain any significant unevaporated water. Also, higher steam pressure will be possible than with saturated steam, enabling

6800-406: The water, because the pressure is above the critical pressure point at which steam bubbles can form. As the fluid expands through the turbine stages, its thermodynamic state drops below the critical point as it does work turning the turbine which turns the electrical generator from which power is ultimately extracted. The fluid at that point may be a mix of steam and liquid droplets as it passes into

6885-399: The working fluid of the steam cycle, making these systems examples of external combustion engines . The pressure vessel of a boiler is usually made of steel (or alloy steel ), or historically of wrought iron . Stainless steel , especially of the austenitic types, is not used in wetted parts of boilers due to corrosion and stress corrosion cracking . However, ferritic stainless steel

6970-438: The world use fossil fuel, outnumbering nuclear , geothermal , biomass , or concentrated solar power plants. The second law of thermodynamics states that any closed-loop cycle can only convert a fraction of the heat produced during combustion into mechanical work . The rest of the heat, called waste heat , must be released into a cooler environment during the return portion of the cycle. The fraction of heat released into

7055-455: Was found in the late 20th century or early 21st century, such as the US and UK. Sometimes coal-fired steam plants are refitted to use natural gas to reduce net carbon dioxide emissions. Oil-fuelled plants may be converted to natural gas to lower operating cost. Heavy fuel oil was once a significant source of energy for electric power generation. After oil price increases of the 1970s, oil was displaced by coal and later natural gas. Distillate oil

7140-655: Was often obtained from specialist ironworks , such as those in the Cleator Moor (UK) area, noted for the high quality of their rolled plate , which was especially suitable for use in critical applications such as high-pressure boilers. In the 20th century , design practice moved towards the use of steel, with welded construction, which is stronger and cheaper, and can be fabricated more quickly and with less labour. Wrought iron boilers corrode far more slowly than their modern-day steel counterparts, and are less susceptible to localized pitting and stress-corrosion. That makes

7225-510: Was often used for fireboxes (particularly for steam locomotives ), because of its better formability and higher thermal conductivity; however, in more recent times, the high price of copper often makes this an uneconomic choice and cheaper substitutes (such as steel) are used instead. For much of the Victorian "age of steam", the only material used for boilermaking was the highest grade of wrought iron , with assembly by riveting . This iron

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