Combined Cycle Power Plants and Heat Recovery Generation System

7/27/2019 Combined Cycle Power Plants and Heat Recovery Generation System

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Combined Cycle Power Plants

First step is the same as the simple cycle gas turbine plant. Burning of gas, the thrust rotating a gas turbineand the coupled generator produces Electricity. In the second step the hot gases leaving the gas turbine passes

into boiler to produce steam. This boiler is called the Heat Recovery Steam Generator (HRSG). The steam

then rotates the steam turbine and coupled generator to produce Electricity. The hot gases leave the HRSG ataround 140 degrees centigrade and are discharged into the atmosphere. The steam condensing, and water

recycling system is the same as in the steam power plant. The attached animation shows the working of the

CCPP.

First step is the same as the simple cycle gas turbine plant. Burning of gas, the thrust rotating a gas turbineand the coupled generator produces Electricity. In the second step the hot gases leaving the gas turbine passes

into boiler to produce steam. This boiler is called the Heat Recovery Steam Generator (HRSG). The steam

then rotates the steam turbine and coupled generator to produce Electricity. The hot gases leave the HRSG at

around 140 degrees centigrade and are discharged into the atmosphere. The steam condensing, and water

recycling system is the same as in the steam power plant.

The attached scheme shows the working of the CCPP.

Roughly the steam turbine cycle produces one third of the power and gas turbine cycle produces two thirds ofthe power output of the CCPP. Normally there will be two generators, one driven by the gas turbine and one

driven by the steam turbine. There are also systems with one generator connected through a single shaft to

both the gas turbine and steam turbine.

Even though this system is having the best efficiency, it has limitations. The gas turbine can only use Natural

gas or high grade oils like aviation or diesel fuel. Because of this the combined cycle can be operated only in

locations where these fuels are available and cost effective.

Developments for gasification of coal and use in the gas turbine are in advanced stages. Once this is proven

Coal as the main fuel can also be used in the combined cycle power plant.


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

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 temperatureswill 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 transfersurfaces is very important.

Two parameters are very critical in the design of the HRSG. They are the Approach point andPinch 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 tothe 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 turbineSteam temperature control is limited to the use of de-superheaters. Performance at part loads

depends on the design of heat transfer surfaces.


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Most of the HRSG's are internally insulated so that expansion does not affect the externalsurfaces 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

What are the types of HRSG's?Heat Recovery Steam Generators (HRSG's) are a critical part of a Combined Cycle Power Plant. As

with any other product or process, there are different types. Here we take a look at the different types.

The main application of the (Heat Recovery Steam Generators) HRSG is in Combined Cycle powerplants. In these plants the power generation from the Rankine cycle part, ie. the steam turbine, is

around one third of the total power generated . HRSG's produce the steam for this.

Classification of HRSGs is on application, design, or operation. Some of the types are described

below.

Fired and UnfiredOne way is to classify it based on the heat input.

Normally HRSG's do not have any additional heat input. The performance and output of the HRSG is

dependent on the exhaust heat of the gas turbine. At part loads, this leads to reduced output from the

HRSG's. In addition, ambient conditions also affect the Gas Turbine performance. This could affect

the downstream process were the steam is used. To avoid such situations supplemental firing of oil or

gas takes place. Even though this may not be an efficient process, it avoids costly production

disturbances.

Supplemental Firing takes place in burners in the gas duct at HRSG inlet. Oil or gas is the

supplementary fuel. Since the flue gas at exhaust of a Gas Turbine is high in Oxygen content

additional air is not required for combustion. This eliminates the need of Forced draft or Induced

Draft fans.

Vertical and Horizontal TypesAnother classification is on the construction or design of the HRSG. Based on the gas flow it can be

vertical or horizontal.

Vertical types have gas flow vertically upward with coils placed horizontally. Horizontal types have gas flows horizontal with coils placed vertically.

From the performance and cost point of view both are the same. More than the technical issues it is a

proprietary design of individual manufacturers or client preferences. Some of the differences are :


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Horizontal types require a 30 % larger footprint area. More expansion joints are required in horizontal units. Structural requirements are higher in vertical types. Horizontal types are more difficult for maintenance and inspections. Overall cost may be same in both types.

Multiple Pressure Operation.Yet another classification is on the operation pressure.

Smaller HRSG units operate on single pressure. The water to steam conversion takes place inone single pressure circuit. This is similar to conventional combustion boilers.

In larger units, for optimizing the performance of the HRSG, steam generation takes place inmulti pressure circuits. The current optimum is to use three pressure levels. (Details in the next

article)

Combined Cycle Power Plants and Heat Recovery Generation System

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