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Overhauling Steam Turbine Controls
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Steam turbines are one of the most versatile and oldest prime mover technologies still in general production used to drive a generator or mechanical machinery. Power generation using steam turbines has been in use for about 100 years, when they replaced reciprocating steam engines due to their higher efficiencies and lower costs. Most of the electricity produced in the United States today is generated by conventional steam turbine power plants. The capacity of steam turbines can range from 50 kW to several hundred MWs for large utility power plants. Steam turbines are widely used for CHP applications in the U.S. and Europe.
Unlike gas turbine and reciprocating engine CHP systems where heat is a byproduct of power generation, steam turbines normally generate electricity as a byproduct of heat (steam) generation. A steam turbine is captive to a separate heat source and does not directly convert fuel to electric energy. The energy is transferred from the boiler to the turbine through high pressure steam that in turn powers the turbine and generator. This separation of functions enables steam turbines to operate with an enormous variety of fuels, varying clean natural gas to solid waste, including all types of coal, wood, wood waste, and agricultural byproducts (sugar cane bagasse, fruit pits and rice hulls). In CHP applications, steam at lower pressure is extracted from the steam turbine and used directly in a process or for district heating, or it can be converted to other forms of thermal energy including hot or chilled water.
Steam turbines offer a wide array of designs and complexity to match the desired application and/or performance specifications. Steam turbines for utility service may have several pressure casings and elaborate design features, all designed to maximize the efficiency of the power plant. For industrial applications, steam turbines are generally of simpler single casing design and less complicated for reliability and cost reasons. CHP can be adapted to both utility and Industrial steam turbine designs.
While steam turbines themselves are competitively priced compared to other prime movers, the costs of complete boiler/steam turbine CHP systems are relatively high on a per kW of capacity basis because of their low power to heat ratio; the costs of the boiler, fuel handling and overall steam systems; and the custom nature of most installations. Thus, steam turbines are well suited to edium- and large-scale industrial and institutional applications where inexpensive fuels, such as coal, biomass, various solid wastes and byproducts (e.g., wood chips, etc.), refinery residual oil, and refinery off gases are available. Because of the relatively high cost of the system, including boiler, fuel handling system, condenser, cooling tower, and stack gas cleanup, high annual capacity factors are required to enable a reasonable recovery of invested capital.
Industrial and CHP Applications
Steam turbine-based CHP systems are primarily used in industrial processes where solid or waste fuels are readily available for boiler use. In CHP applications, steam is extracted from the steam turbine and used directly in a process or for district heating, or it can be converted to other forms of thermal energy including hot water or chilled water. The turbine may drive an electric generator or equipment such as boiler feedwater pumps, process pumps, air compressors and refrigeration chillers. Turbines as industrial drivers are almost always a single casing machine, either single stage or multistage, condensing or non-condensing depending on steam conditions and the value of the steam. Steam turbines can operate at a single speed to drive an electric generator or operate over a speed range to drive a refrigeration compressor. For non-condensing applications, steam is exhausted from the turbine at a pressure and temperature sufficient for the CHP heating application.
Steam turbine systems are very commonly found in paper mills as there is usually a variety of waste fuels from hog fuel to black liquor recovery. Chemical plants are the next moset common industrial user of steam turbines followed by primary metals. There are a variety of other industrial applications including the food industry, particularly sugar mills. There are commercial applications as well. Many universities have coal powered CHP generating power with steam turbines. Some of these facilities are blending biomass to reduce their environmental impact.
Combined Cycle Power Plants
The trend in power plant design is the combined cycle, which incorporates a steam turbine in a bottoming cycle with a gas turbine. Steam generated in the heat recovery steam generator (HRSG) of the gas turbine is used to drive a steam turbine to yield additional electricity and improve cycle efficiency. An extraction-condensing type of steam turbine can be used in combined cycles and be designed for CHP applications. There are many large independent combined cycle power plants operating on natural gas that provide power to the electric grid and steam to one or more industrial customers.
District Heating Systems
There are many cities and college campuses that have steam district heating systems where adding a steam turbine between the boiler and the distribution system may be an attractive application. Often the boiler is capable of producing moderate-pressure steam but the distribution system needs only low pressure steam. In these cases, the steam turbine generates electricity using the higher pressure steam, and discharges low pressure steam into the distribution system.
About the Author:
The author of this article, Joseph Parker is hailed for his writing ability on various turbine related articles like API Turbines and steam turbines. He has a vast experience in mechanical field. For any query please visit www.nsterbo.in
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