Heat Recovery Steam Generator (HRSG)

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By: Eng/ Ahmed Algharaz

Introduction

A heat recovery steam generator or HRSG is an energy recovery heat exchanger that recovers heat from a hot gas stream. It produces steam that can be used in a process (cogeneration) or used to drive a steam turbine (combined cycle).

The exhaust gas temperature from a Gas Turbine is about 580 °C.

Higher exhaust temperatures will reduce the efficiency of the gas turbine.

Operation Mode

Combined Cycle Mode:-

Power is generated via a steam turbine generator.

Cogeneration Mode:-

Steam producer from the HRSG is mainly used for process application.

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The HRSG consist of three main parts :

1-evaporator

2-ecnomizer

3-super heater

And anther three parts:

4-reheaters

5-deaerators

6-preheaters

Parts of HRSG:

1. Evaporator

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HRSG evaporator or boiler sections act to vaporize water and produce steam in one component.

2. Ecnomizer

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The gas temperature leaving an evaporator varies from 150~ 320 deg c, depending upon the steam pressure being produced. If no other heat transfer component is installed downstream, this remaining energy is wasted.

3. Super heater

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While the evaporator produces dry-saturated steam, this is rarely acceptable for large steam turbines, and is frequently not the appropriate condition for process applications.

4. Reheaters

Reheaters are a heat transfer component similar to super heaters, and are employed in advanced multi-pressure power generation cycles.

They accept superheated or semi-saturated steam at a low pressure from a steam turbine after it’s first section of expansion, and re-superheat or “reheat” the steam back towards the original super heater's outlet temperature.

Accordingly, reheaters are generally interspersed among the super heater sections in the HRSG, so that the same outlet temperatures can be achieved.

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5. Deaerators

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All power plant cycles employ a deaerator to control oxygen levels in the feedwater. Heating steam is provided to strip oxygen from the condensate falling through the pressurized deaerator’s tray systems.

6. Preheaters

Preheaters are located at the coolest end of the HRSG gas path, and absorb energy from the gas stream to preheat liquids such as condensate, makeup water, water/glycol mixtures or proprietary heat exchange fluids.

The most common application is to preheat condensate prior to entry into the deaerator, which reduces the amount of deaeration steam required.

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Types of HRSG : 1 - Natural-circulation (NC)

2- Forced-circulation (FC)

3-A once-through (OT)

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Type of HRSG (Based On Heat Input )

Unfired

Fired Supplementary Fired

Exhaust Fired

Unfired HRSG

When the available GT exhaust energy, the consequential HRSG steam production, and the steam requirements are well balanced, an unfired HRSG can be selected.

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The exhaust gases from a typical GT include from 14% ~ 16% oxygen by volume, which makes it possible to locate a supplemental burner downstream of the gas turbine exhaust – frequently called a duct burner.

Fired HRSG

Unfired

SupplementaryFired

Exhaust Fired

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Types Of HRSG (Based on operation pressure)

Single Pressure

Multi Pressure

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Post combustion emission controls

The most common post-combustion process applied to HRSGs is Selective Catalytic Reduction or SCR. Most new combined-cycle plants where stringent emissions limits exist are equipped with both DLN combustors on the gas turbine, and an HRSG incorporating an SCR system.