Combined cycle

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Heat" "On the Equilibrium of Heterogeneous Substances" "Reflections on the Motive Power of Fire" Timelines of: Thermodynamics · Heat engines Art: Maxwell's thermodynamic surface Education: Entropy as energy dispersal Scientists Daniel Bernoulli Sadi Carnot Benoît Paul Émile Clapeyron Rudolf Clausius Hermann von Helmholtz Constantin Carathéodory James Prescott Joule Julius Robert von Mayer William Rankine John Smeaton Georg Ernst Stahl Benjamin Thompson William Thomson, 1st Baron Kelvin John James Waterston v · d · e A combined cycle is an assembly of engines that convert heat into mechanical energy, which in turn usually drives electrical generators. The principle is that the exhaust of one heat engine is used as the heat source for another, increasing the system's overall efficiency. This works because heat engines are only able to use a portion of the energy their fuel generates (usually less than 50%). The remaining heat (e.g., hot exhaust fumes) from combustion is generally wasted. Combining two or more thermodynamic cycles results in improved overall efficiency, reducing fuel costs. In stationary power plants, a successful, common combination is the Brayton cycle (in the form of a turbine burning natural gas or synthesis gas from coal) and the Rankine cycle (in the form of a steam power plant). Multiple stage turbine or steam cylinders are also common. Historically successful combined cycles have used hot cycles with mercury vapor turbines, magnetohydrodynamic generators or molten carbonate fuel cells, with steam plants for the low temperature bottoming cycle. Bottoming cycles operating from a steam condenser's heat are theoretically possible, but uneconomical because of the very large, expensive equipment needed to extract energy from the small temperature differences between condensing steam and outside air or water. However, it is common in cold climates (such as Finland) to drive community heating systems from a power plant's condenser heat. Such cogeneration systems can yield theoretical efficiencies above 95%. In automotive and aeronautical engines, turbines have been driven from the exhausts of Otto, Diesel, and Crower cycles. These are called turbo-compound engines. Aside from turbochargers, they have failed commercially because their mechanical complexity and weight are less economical than multistage turbines. Stirling engines are also a good theoretical fit for this application. In a combined cycle power plant (CCPP), or combined cycle gas turbine (CCGT) plant, a gas turbine generator generates electricity and the waste heat is used to make steam to generate additional electricity via a steam turbine; this last step enhances the efficiency of electricity generation. Most new gas power plants in North America and Europe are of this type. In a thermal power plant, high-temperature heat as input to the power plant, usually from burning of fuel, is converted to electricity as one of the outputs and low-temperature heat as another output. As a rule, in order to achieve high efficiency, the temperature difference between the input and output heat levels should be as high as possible (see Carnot efficiency). This is achieved by combining the Rankine (steam) and Brayton (gas) thermodynamic cycles. Such an arrangement used for marine propulsion is called combined gas (turbine) and steam (turbine) (COGAS). Contents 1 Design principle 1.1 Typical size of CCGT plants 1.2 Efficiency of CCGT plants 1.3 Supplementary firing and blade cooling 2 Fuel for combined cycle power plants 2.1 Configuration of CCGT plants 3 Integrated gasification combined cycle (IGCC) 4 Automotive use 5 Aeromotive use 6 See also 7 References 8 External links Design principle Working principle of a combined cycle power plant In a thermal power station water is the working medium. High pressure steam requires strong, bulky components. High temperatures require expensive alloys made from nickel or cobalt, rather than inexpensive steel. These alloys limit practical steam temperatures to 655 °C while the lower temperature of a steam plant is fixed by the boiling point of water. With these limits, a steam plant has a fixed upper efficiency of 35 to 42%. An open circuit gas turbine cycle has a compressor, a combustor and a turbine. For gas turbines the amount of metal that must withstand the high temperatures and pressures is small, and lower quantities of expensive materials can be used. In this type of cycle, the input temperature to the turbine (the firing temperature), is relatively high (900 to 1,400 °C). The output temperature of the flue gas is also high (450 to 650 °C). This is therefore high enough to provide heat for a second cycle which uses steam as the working fluid; (a Rankine cycle). In a combined cycle power plant, the heat of the gas turbine's exhaust is used to generate steam by passing it through a heat recovery steam generator (HRSG) with a live steam temperature between 420 and 580 °C. The condenser of the Rankine cycle is usually cooled by water from a lake, river, sea or cooling towers. This temperature can be as low as 15 °C In an automotive powerplant, an Otto, Diesel, Atkinson or similar engine would provide one part of the cycle and the waste heat would power a Rankine cycle steam or Stirling engine, which could either power ancillaries (such as the alternator) or be connected to the crankshaft by a turbo compounding system. Typical size of CCGT plants For large scale power generation a typical set would be a 400 MW gas turbine coupled to a 200 MW steam turbine giving 600 MW. A typical power station might comprise of between 2 and 6 such sets. Plant size is important in the cost of the plant. The larger the plant, less will be the initial cost per kilowatt, efficiency increases with plant size. Large combined cycle power plants are combinations of single shaft combined cycle power plants or a multiple shaft combined cycle power plants. In multi shaft combined cycle power plants only one gas turbine, one heat recovery steam generator(HRSG), one steam turbine. Single shaft combined cycle power plants are categorized as with and without clutch. If the desired plant output is higher than can be produced by a single gas turbine plant,other possible arrangements in Multi shaft combined power plants may include two gas turbines, two heat recovery steam generators(HRSG), and one steam turbine, which is common in plants above 300MW. Similarly in Single shaft combined cycle power plants, two or three chains of single shaft arrangement. For instance two 1500 MW plants in Europe have five single shaft combined cycle power plant trains, and a major 2800 MW plant in China has eight single shaft combined cycle power plant trains. The primary disadvantage of Multiple stage combined cycle power plant is that the number of steam turbines, condensers and condensate systems-and perhaps the cooling towers and circulating water systems increases to match the number of gas turbines. For Multiple shaft combined cycle power plant there is only one steam turbine, condenser and rest of the heat sink for up to three gas turbines, only their size increases. Having only one large steam turbine and heat sink results in low cost because of economics of scale. Further a large steam turbine also allows the use of high pressure and efficient steam cycle. Thus the overall plant size and the associated number of gas turbines required have a major impact on whether Single shaft combined cycle power plant or Multiple shaft combined cycle power plant is more economical. Gas turbines of about 150 MW size are already in operation manufactured by at least four separate groups-General Electric and its licensees, Alstom, Siemens, and Westinghouse/Mitsubishi. These groups are also developing, testing and/or marketing gas turbine sizes of about 200 MW. Combined cycle units are made up of one or more such gas turbines, each with a waste heat steam generator arranged to supply steam to a single steam tubine, thus formatting a combined cycle block or unit. Typical Combined cycle block sizes offered by three major manufacturers (Alstom, General Electric and Siemens) are roughly in the range of 50 MW to 500 MW and costs are about $600/kW. Efficiency of CCGT plants (When talking about the efficiency of heat engines and power stations the convention should be stated ie HHV (aka Gross Heating Value etc) or LCV (AKA Net Heating value) AND whether Gross output (at the generator terminals) or Net Output (at the power station fence) are being considered. The two are of course separate but both must be stated. Failure to do so causes endless confusion.) In general in service Combined Cycle efficiencies are over 50 percent on an on a lower heating value and Gross Output basis. Most combined cycle units, especially the larger units, have peak, steady state efficiency efficiencies of 55 - 59%. Research aimed at 1370°C (2500°F) turbine inlet temperature has led to even more efficient combined cycles and 60 percent efficiency has been reached for at least one combined cycle unit, (e.g. the combined cycle unit of Baglan Bay, a GE H-technology gas turbine with a NEM 3 pressure reheat boiler. Utilising steam from the HRSG to cool the turbine blades). Other GT manufacturers also claim to have broken the 60% efficiency for combined cycle (e.g. Siemens). By combining both gas and steam cycles, high input temperatures and low output temperatures can be achieved. The efficiency of the cycles add, because they are powered by the same fuel source. So, a combined cycle plant has a thermodynamic cycle that operates between the gas-turbine's high firing temperature and the waste heat temperature from the condensers of the steam cycle. This large range means that the Carnot efficiency of the cycle is high. The actual efficiency, while lower than this, is still higher than that of either plant on its own.1 The actual efficiency achievable is a complex area.2 The electric efficiency of a combined cycle power station, calculated as electric energy produced as a percent of the lower heating value of the fuel consumed, may be as high as 58 percent when operating new, ie unaged, and at continuous output which are ideal conditions. As with single cycle thermal units, combined cycle units may also deliver low temperature heat energy for industrial processes, district heating and other uses. This is called cogeneration and such power plants are often referred to as a Combined Heat and Power (CHP) plant. Supplementary firing and blade cooling Supplementary firing may be used in combined cycles (in the HRSG) raising exhaust temperatures from 600°C (GT exhaust) to 800 or even 1000°C. Using supplemental firing will however not raise the combined cycle efficiency for most combined cycles. Only for one pressure boilers it may raise the efficiency if fired up to approximately 700- 750°C. For multiple pressure boiler however supplemental firing is often used to improved peak power production of the unit, or to enable large steam production to compensate failing of e.g. a second unit. Maximum supplementary firing refers to the maximum fuel that can be fired with the oxygen available in the gas turbine exhaust. The steam cycle is conventional with reheat and regeneration. Hot gas turbine exhaust is used as the combustion air. Regenerative air preheater is not required. A fresh air fan which makes it possible to operate the steam plant even when the gas turbine is not in operation,increases the availability of the unit. The use of large supplementary firing in Combined Cycle Systems with high gas turbine inlet temperatures causes the efficiency to drop. For this reason the Combined Cycle Plants with maximum supplementary firing are only of minimal importance today, in comparison to simple Combined Cycle installations. However, they have two advantages that is a) coal can be burned in the steam generator as the supplementary fuel, b) has very good part load efficiency. The HRSG can be designed with supplementary firing of fuel after the gas turbine in order to increase the quantity or temperature of the steam generated. Without supplementary firing, the efficiency of the combined cycle power plant is higher, but supplementary firing lets the plant respond to fluctuations of electrical load. Supplementary burners are also called duct burners. More fuel is sometimes added to the turbine's exhaust. This is possible because the turbine exhaust gas (flue gas) still contains some oxygen. Temperature limits at the gas turbine inlet force the turbine to use excess air, above the optimal stoichiometric ratio to burn the fuel. Often in gas turbine designs part of the compressed air flow bypasses the burner and is used to cool the turbine blades. Supplementary firing raises the temperature of the exhaust gas from 800 to 900 degree Celsius. Relatively high flue gas temperature raises the condition of steam (84 bar, 525 degree Celsius) thereby improving the efficiency of steam cycle. Fuel for combined cycle power plants The turbines used in Combined Cycle Plants are commonly fuelled with natural gas , which is found in abundant reserves on every continent.citation needed Natural gas is becoming the fuel of choice for private investors and consumers because it is more versatile than coal or oil and can be used in 90% of energy applications .Chile which once depended on hydropower for 70% of its electricity supply, is now boosting its gas supplies to reduce reliance on its drought afflicted hydro dams .Similarly China is tapping its gas reserves to reduce reliance on coal, which is currently burned to generate 80% of the country’s electric supply. Where the extension of a gas pipeline is impractical or cannot be economically justified, electricity needs in remote areas can be met with small scale Combined Cycle Plants, using renewable fuels. Instead of natural gas, Combined Cycle Plants can be filled with biogas derived from agricultural and forestry waste, which is often readily available in rural areas. Combined cycle plants are usually powered by natural gas, although fuel oil, synthesis gas or other fuels can be used. The supplementary fuel may be natural gas, fuel oil, or coal. Biofuels can also be used. Integrated solar combined cycle power stations combine the energy harvested from solar radiation with another fuel to cut fuel costs and environmental impact. The first such system to come online is Yazd power plant, Iran34 and more are under construction at Hassi R'mel, Algeria and Ain Beni Mathar, Morocco.5 Next generation nuclear power plants are also on the drawing board which will take advantage of the higher temperature range made available by the Brayton top cycle, as well as the increase in thermal efficiency offered by a Rankine bottoming cycle. Low-Grade Fuel for Turbines: Gas turbines burn mainly natural gas and light oil. Crude oil, residual, and some distillates contain corrosive components and as such require fuel treatment equipment. In addition, ash deposits from these fuels result in gas turbine debating’s of up to 15 percent they may still be economically attractive fuels however, particularly in combined-cycle plants. Sodium and potassium are removed from residual, crude and heavy distillates by a water washing procedure. A simpler and less expensive purification system will do the same job for light crude and light distillates. A magnesium additive system may also be needed to reduce the corrosive effects if vanadium is present. Fuels requiring such treatment must have a separate fuel-treatment plant and a system of accurate fuel monitoring to assure reliable, low-maintenance operation of gas turbines. Configuration of CCGT plants The combined-cycle system includes single-shaft and multi-shaft configurations. The single-shaft system consists of one gas turbine, one steam turbine, one generator and one Heat Recovery Steam Generator (HRSG), with the gas turbine and steam turbine coupled to the single generator in a tandem arrangement on a single shaft. Key advantages of the single-shaft arrangement are operating simplicity, smaller footprint, and lower startup cost. Single-shaft arrangements, however, will tend to have less flexibility and equivalent reliability than multi-shaft blocks. Additional operational flexibility is provided with a steam turbine which can be disconnected, using an synchro-self-shifting (SSS) Clutch,6 for start up or for simple cycle operation of the gas turbine. Multi-shaft systems have one or more gas turbine-generators and HRSGs that supply steam through a common header to a separate single steam turbine-generator. In terms of overall investment a multi-shaft system is about 5% higher in costs. Single- and multiple-pressure non-reheat steam cycles are applied to combined-cycle systems equipped with gas turbines having rating point exhaust gas temperatures of approximately 540 °C or less. Selection of a single- or multiple-pressure steam cycle for a specific application is determined by economic evaluation which considers plant installed cost, fuel cost and quality, plant duty cycle, and operating and maintenance cost. Multiple-pressure reheat steam cycles are applied to combined-cycle systems with gas turbines having rating point exhaust gas temperatures of approximately 600 °C. The most efficient power generation cycles are those with unfired HRSGs with modular pre-engineered components. These unfired steam cycles are also the lowest in cost. Supplementary-fired combined-cycle systems are provided for specific application. The primary regions of interest for cogeneration combined-cycle systems are those with unfired and supplementary fired steam cycles. These systems provide a wide range of thermal energy to electric power ratio and represent the range of thermal energy capability and power generation covered by the product line for thermal energy and power systems. by Engr. Bilal Pervez Integrated gasification combined cycle (IGCC) An integrated gasification combined cycle, or IGCC, is a power plant using synthetic gas (syngas). Syngas can be produced from a number of sources, including coal and fermentation of biomass. Automotive use Combined cycles have traditionally only been used in large power plants. BMW, however, has proposed that automobiles use exhaust heat to drive steam turbines.7 This can even be connected to the car or truck's cooling system to save space and weight, but also to provide a condenser in the same location as the radiator and preheating of the water using heat from the engine block. However, stirling engines can also be used if light weight is a priority (such as in a sports car or racing application), because they use air rather than water as the working fluid. It may be possible to use the pistons in a reciprocating engine for both combustion and steam expansion like in the Crower six stroke.8 Aeromotive use Some versions of the Wright R-3350 were produced as turbo-compound engines. Three turbines driven by exhaust gases, known as power recovery turbines, provided nearly 600 hp at takeoff. These turbines added power to the engine crankshaft through bevel gears and fluid couplings.9 See also Heat recovery steam generator Hydrogen-cooled turbogenerator Mercury vapour turbine Relative cost of electricity generated by different sources cogeneration COGAS References ^ "Efficiency by the Numbers" by Lee S. Langston ^ http://www.claverton-energy.com/the-difference-between-lcv-and-hcv-or-lower-and-higher-heating-value-or-net-and-gross-is-clearly-understood-by-all-energy-engineers-there-is-no-right-or-wrong-definition.html ^ http://payvand.com/news/07/apr/1132.html ^ http://www.industcards.com/cc-iran.htm ^ Abener has begun construction of the world's largest ISCC plant in Morocco ^ http://www.sssclutch.com/howitworks/100-2SSSPrinciples.pdf ^ "BMW Turbosteamer gets hot and goes" by John Neff, AutoBlog, December 9, 2005 ^ "Inside Bruce Crower’s Six-Stroke Engine" By Pete Lyons, AutoWeek, February 23, 2006 ^ Goleta Air and Space Museum: 2002 Camarillo EAA Fly-in External links Hunstown: Ireland's most efficient power plant @ Siemens Power Generation website ABB Power Generation website @ ABB_Group Natural Gas Combined-cycle Gas Turbine Power Plants Northwest Power Planning Council, New Resource Characterization for the Fifth Power Plan, August 2002 Combined cycle solar power v · d · eThermodynamic cycles External combustion cycles Without phase change (Hot air engines) Barton · Bell Coleman · Brayton/Joule (externally heated) · Carnot · Ericsson · Ported constant volume[1] · Stirling · Stirling (Pseudo / Adiabatic) · Stoddard With phase change Kalina · Rankine (Organic Rankine) · Regenerative · Two-phased Stirling[2] Internal combustion cycles Atkinson · Brayton/Joule · Diesel · Expander · Gas-generator · Homogeneous Charge Compression Ignition · Lenoir · Miller · Otto · Pressure-fed · Staged combustion Mixed cycles Combined · HEHC · Mixed/Dual Refrigeration cycles Hampson-Linde · Kleemenko · Linde dual-pressure · Pulse tube · Regenerative cooling · Transcritical · Vapor absorption · Vapor-compression · Siemens · Vuilleumier Uncategorized Claude [3] · Claude dual-pressure  · Fickett-Jacobs · Gifford-McMahon [4] · Hirn · Humphrey · Heylandt · Scuderi v · d · eElectricity generation Concepts Availability factor · Baseload · Black start · Capacity factor · Demand factor · Demand management · EROEI · Grid storage · Intermittency · Load following · Peak demand · Repowering · Spark spread Sources Nonrenewable Coal · Fossil fuel power plant · Natural gas · Petroleum · Nuclear · Oil shale Renewable Biomass · Geothermal · Hydro · Marine · Solar · Wind Technology AC power · Cogeneration · Combined cycle · Cooling tower · Induction generator · Micro CHP · Microgeneration · Rankine cycle · Virtual power plant Distribution Demand response · Distributed generation · Dynamic demand · Electricity distribution · Electrical grid · High-voltage direct current · Load control · Pumped hydro · Negawatts · Smart grid · Substation · Super grid · TSO · Transmission tower · Utility pole Policies Carbon offset · Coal phase out · Ecotax · Energy subsidies · Feed-in tariff · Net metering · Pigovian tax · Renewable Energy Certificates · Renewable energy payments · Renewable energy policy Categories: Electric power distribution · Electricity economics · Power station technology · Portals: Energy · Sustainable development