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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?

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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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