difference between heat recovery steam generators (hrsg) & combustion boiler
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How is the HRSG different from a fossil fuel fired boiler?
written by: johnzactruba edited by: Lamar Stonecypher updated: 7/13/2009
Heat Recovery Steam Generators (HRSG's) absorb heat from the exhaust of gas turbines to produce steam.
Functionally they produce steam like any other boiler but with some differences.
What is an HRSG?
Heat Recovery Steam Generator's (HRSG's) are waste heat boilers. The steam turbine or a downstream process uses
the steam. The term HRSG refers to the waste heat boiler in a Combined Cycle Power Plant. In its basic form these are
bundles of water or steam carrying tubes paced in the hot gas flow path. These recover the heat from the gas to
generate superheated steam, hence the name Heat Recovery Steam Generator.
The water steam circuit of an HRSG consists of an economizer, an evaporator, and a Super-heater placed in the flue
gas duct. The evaporator section consists of a drum to which the coils are connected to create the circulation.
The Differences
HRSG is only a heat transfer area. There is no furnace even though the sections like economizer, evaporator, and
super heaters are present. An exception is the supplementary-fired HRSG .
The exhaust gas temperature from a Gas Turbine is about 580 C. Higher exhaust temperatures will reduce the
efficiency of the gas turbine. This temperature head available for heat transfer is very low compared to a
conventional boiler. In a conventional boiler where fuel burning takes place, the temperature head available for
heat transfer is in the range of 1300 C.
Because of this low temperature head, proper positioning and apportioning of the heat transfer surfaces is very
important.
Two parameters are very critical in the design of the HRSG. They are the Approach point and Pinch Point. The
approach point is the difference between saturation and the water temperature leaving the economizer. The pinch
point is the difference between the gas temperature leaving the evaporator section and the saturation temperature.
These are not applicable in conventional boilers.
In conventional boilers the evaporation and super-heating takes place in a single pressure level. To get the
optimum heat transfer HRSG's in large Combined Cycle plants operate on a triple pressure format. Water-steam
conversion takes place in three different pressure levels in three independent circuits.
Unlike the power plant boilers, there are no air or gas handling fans. The flow of gases is due to the exhaust gas
pressure from the Gas turbine. Pressure drops in the gas path of a HRSG is at a minimum to avoid higher exhaust
gas pressure, which will affect the performance of the GT. This also eliminates the need of air preheaters.
The output of the HRSG is solely dependent on the performance and load of the gas turbine. Steam temperature
control is limited to the use of de-superheaters. Performance at part loads depends on the design of heat transfer
surfaces.
Most of the HRSG's are internally insulated so that expansion does not affect the external surfaces and the
structures. Conventional boilers on the other hand are top supported and free to expand so the insulation is on the
outside.
HRSG (HEAT RECOVERY STEAM GENERATORS)
HRSG's are boilers placed downstream of Gas turbines to absorb the exhaust heat and produce steam. This
equipment makes the combined cycle the most efficient power generation system available today. What are they? How
is it different? This series elaborates the different types and optimization tech
How is the HRSG different from a fossil fuel fired boiler?
What are the types of HRSG's?
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Dominic 3 years ago
Most HRSG's are built on site, as they are is most cases too large for transportation. Th
parameters that would be compared would be the same 5 parameters utilized when a bid
for a boiler's construction is considered. These are, and not necessarily in order, Cost,
Efficiency, Reliability, Serviceability and Safety.
Reply
Amir Farahani 5 years ago
which parameters is important when we compare two similar HRSGs from different
manufacturer offered for a project?
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