How is the HRSG different from a fossil fuel fired boiler?

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.

• 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.