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A heat recovery steam generator or HRSG is a heat exchanger that recovers heat from a hot gas stream. It produces steam that can be used in a process or used to drive a steam turbine. A common application for an HRSG is in a combined-cycle power station, where hot exhaust from a gas turbine
is fed to an HRSG to generate steam which in turn drives a steam
turbine. This combination produces electricity more efficiently than
either the gas turbine or steam turbine alone. Another application for
an HRSG is in diesel engine combined cycle power plants, where hot
exhaust from a diesel engine is fed to an HRSG to generate steam which
in turn drives a steam turbine. The HRSG is also an important component
in cogeneration plants. Cogeneration
plants typically have a higher overall efficiency in comparison to a
combined cycle plant. This is due to the loss of energy associated with
the steam turbine
HRSGs consist of three major components. They are the Evaporator, Superheater, and Economizer. The different components are put together to meet the operating requirements of the unit. See Modular HRSG GA.
Modular HRSGs can be categorized by 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 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 the temperature and pressure past the saturation point.
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 20MW) applications. The packagaged HRSG can have 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 utilised for peaking.
HRSGs can also have diverter valves to regulate in 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) and/or a catalyst to remove carbon monoxide.
The inclusion of an SCR dramatically effects the layout of the HRSG.
NOx catalyst performs best in temperatures between 650°F and 750°F.
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 allows for
the SCR to be placed between the Evaporator and Economizer sections
(350°F-500°F. Applications
- Heat recovery can be used extensively in energy projects.
- In the energy-rich Persian Gulf region, the steam from the HRSG is used for desalination plants.
- Universities are ideal candidates for HRSG applications. They can
use a gas turbine to produce high reliability electricity for campus
use. The HRSG can recover the heat from the gas turbine to produce
steam/hot water for district heating or cooling.
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