Heat recovery steam generators
A heat recovery steam generator (HRSG) is a heat exchanger or series of heat exchangers that recovers heat from a hot gas stream and uses that heat to produce steam for driving steam turbines or as process steam in industrial facilities or as steam for district heating.[8]
An HRSG is an important part of a combined cycle power plant (CCPP)[9] or a cogeneration power plant.[10] In both of those types of power plants, the HRSG uses the hot flue gas at approximately 500 to 650 °C from a gas turbine to produce high-pressure steam. The steam produced by an HRSG in a gas turbine combined cycle power plant is used solely for generating electrical power. However, the steam produced by an HRSG in a cogeneration power plant is used partially for generating electrical power and partially for district heating or for process steam.
The combined cycle power plant, schematically depicted in Figure 8 below, is so named because it combines the Brayton cycle for the gas turbine and the Rankine cycle[11] for the steam turbines. About 60 percent of the overall electrical power generated in a CCPP is produced by an electrical generator driven by the gas turbine and about 40 percent is produced by another electrical generator driven by the high-pressure and low-pressure steam turbines. For large scale power plants, a typical CCPP might use sets consisting of a gas turbine driving a 400 MW electricity generator and steam turbines driving a 200 MW generator (for a total of 600 MW), and the power plant might have 2 or more such sets.
The primary component heat exchangers of an HRSG are the economizer, the evaporator and its associated steam drum and the superheater as shown in Figure 9 below. An HRSG may be in horizontal ducting with the hot gas flowing horizontally across vertical tubes as in Figure 9 or it may be in vertical ducting with the hot gas flowing vertically across horizontal tubes. In either horizontal or vertical HRSGs, there may be a single evaporator and steam drum or there may be two or three evaporators and steam drums producing steam at two or three different pressures. Figure 9 depicts an HRSG using two evaporators and steam drums to produce high pressure steam and low pressure steam, with each evaporator and steam drum having an associated economizer and superheater. In some cases, supplementary fuel firing may be provided in an additional section at the front end of the HRSG to provide additional heat and higher temperature gas. Figures 7A and 7B (just above) shows the actual physical appearance of horizontal HRSGs in a multi-unit combined cycle power plant.
There are a number of other HRSG applications. For example, some gas turbines are designed to burn liquid fuels (rather than fuel gas) such as petroleum naphtha or diesel oil[12] and others burn the syngas (synthetic gas) produced by coal gasification in an integrated gasification combined cycle plant commonly referred to as an IGCC plant. As another example, a combined cycle power plant may use a diesel engine rather than a gas turbine. In almost all such other applications, HSRGs are used to produce steam to be used for power generation.
Heat recovery steam generators
A heat recovery steam generator (HRSG) is a heat exchanger or series of heat exchangers that recovers heat from a hot gas stream and uses that heat to produce steam for driving steam turbines or as process steam in industrial facilities or as steam for district heating.[8]
An HRSG is an important part of a combined cycle power plant (CCPP)[9] or a cogeneration power plant.[10] In both of those types of power plants, the HRSG uses the hot flue gas at approximately 500 to 650 °C from a gas turbine to produce high-pressure steam. The steam produced by an HRSG in a gas turbine combined cycle power plant is used solely for generating electrical power. However, the steam produced by an HRSG in a cogeneration power plant is used partially for generating electrical power and partially for district heating or for process steam.
The combined cycle power plant, schematically depicted in Figure 8 below, is so named because it combines the Brayton cycle for the gas turbine and the Rankine cycle[11] for the steam turbines. About 60 percent of the overall electrical power generated in a CCPP is produced by an electrical generator driven by the gas turbine and about 40 percent is produced by another electrical generator driven by the high-pressure and low-pressure steam turbines. For large scale power plants, a typical CCPP might use sets consisting of a gas turbine driving a 400 MW electricity generator and steam turbines driving a 200 MW generator (for a total of 600 MW), and the power plant might have 2 or more such sets.
The primary component heat exchangers of an HRSG are the economizer, the evaporator and its associated steam drum and the superheater as shown in Figure 9 below. An HRSG may be in horizontal ducting with the hot gas flowing horizontally across vertical tubes as in Figure 9 or it may be in vertical ducting with the hot gas flowing vertically across horizontal tubes. In either horizontal or vertical HRSGs, there may be a single evaporator and steam drum or there may be two or three evaporators and steam drums producing steam at two or three different pressures. Figure 9 depicts an HRSG using two evaporators and steam drums to produce high pressure steam and low pressure steam, with each evaporator and steam drum having an associated economizer and superheater. In some cases, supplementary fuel firing may be provided in an additional section at the front end of the HRSG to provide additional heat and higher temperature gas. Figures 7A and 7B (just above) shows the actual physical appearance of horizontal HRSGs in a multi-unit combined cycle power plant.
There are a number of other HRSG applications. For example, some gas turbines are designed to burn liquid fuels (rather than fuel gas) such as petroleum naphtha or diesel oil[12] and others burn the syngas (synthetic gas) produced by coal gasification in an integrated gasification combined cycle plant commonly referred to as an IGCC plant. As another example, a combined cycle power plant may use a diesel engine rather than a gas turbine. In almost all such other applications, HSRGs are used to produce steam to be used for power generation.
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