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What is a heat recovery steam generator (HRSG)?
A Heat Recovery Steam Generator, commonly abbreviated as a HRSG, is a specialised piece of equipment designed to recover heat from hot gases. These hot gases often come from a gas turbine as exhaust (flue gases), or an industrial process that generates a lot of heat. The recovered heat is then used to boil water and produce steam, which can be utilised for power generation or other industrial processes.
Heat Recovery Steam Generators
Why do we need heat recovery steam generators?
Efficiency and Sustainability
One of the primary reasons for using a HRSG is to increase the efficiency of a system. For example, by recapturing waste heat from a combustion process, we reduce the amount of heat lost, and thus increase the system’s overall efficiency. An increase in plant efficiency correlates with reduced operational costs and reduced environmental impact.
Cost Savings
Whilst the initial capital investment in a HRSG might be large, the long-term efficiency gains make the investment cost-effective. Over the service life of the HRSG (potentially >20 years), it will pay for itself many times over. The high reliability of a HRSG also means that its operational time is high, this ensures a good return on investment.
Flexibility
HRSGs can be integrated into various industrial processes, offering flexibility in terms of application. Whether it's for power generation, district heating, or other industrial applications, a HRSG can increase system efficiencies considerably (because it recovers heat that would otherwise be lost).
HRSG Applications
A HRSG is typically installed downstream of a gas turbine (combustion turbine) or other combustion process within a power plant. For example, a combined cycle power plant (CCPP) utilises a gas turbine and HRSG installed in series. Within a CCPP, a gas turbine is used to generate electricity, whilst its exhaust gases are discharged to the HRSG, which is used to generate steam. With this setup, gas turbines are typically fired on natural gas although it is possible to use many other fuel types.
Steam from the HRSG is then used to drive a steam turbine, which also generates electricity. In industrial settings, HRSGs might also be found wherever there's a need to recover waste heat for steam production, such as in refineries or chemical plants.
Combined Cycle Power Plant Parts
What are the main parts of a HRSG?
Despite the size of most HRSGs, they have relatively few main parts and few systems. A typical HRSG will have a high-pressure, intermediate-pressure, and low-pressure steam system. Each system has an associated steam drum, economiser (economizer), evaporator, and superheater. Flow through the HRSG is from the economiser, to steam drum, to evaporator, and then to the superheater.
This flow occurs first in the low-pressure (LP), then intermediate-pressure (IP), and finally the high-pressure (HP) steam system. Each steam system also has a corresponding steam turbine i.e. high-pressure steam turbine, intermediate-pressure steam turbine, and low-pressure steam turbine.
The economiser, evaporator, and superheater are constructed from tubes so that they have a large contact surface area with the exhaust gases; this means they also have a large heat transfer capacity. It is best to think of these main parts as heat exchangers, as this is their primary function. Each of these three parts acts as a heat exchanger to produce steam (evaporator and superheater), or water/steam mixture (economiser).
Heat Recovery Steam Generator Parts
The four major components of a HRSG are listed below.
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Economiser – feedwater is fed first to the base of the economiser. The economiser preheats the feedwater. Boiler water is discharged from the economiser to the associated system steam drum. Preheating the feedwater increases the efficiency of the system by ensuring that water entering the steam drum is already warm (no risk of thermal shocking).
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Steam Drum – boiler water from the economiser is discharged to its respective steam drum. The steam drum separates steam and water. Steam rises to the top of the steam drum and is sent to the superheater. Water is discharged from the bottom of the steam drum to the evaporator. Boiler water is recirculated within the evaporator until it becomes steam.
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Evaporator – where the production of steam takes place. Water flows through tubes that are heated by hot exhaust gases. The water absorbs heat from the tubes as it travels through the evaporator, and this causes it to change phase/state to steam. Not all water changes phase to steam, thus it is a steam/water mixture (wet steam) that is discharged from the evaporator to the steam drum. The water that did not change state to steam, is recirculated again through the evaporator.
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Superheater – takes the steam produced in the evaporator and increases its temperature (and energy) even further, ensuring it's at the optimal condition for the steam turbine or industrial consumer. Superheaters add sensible heat to steam, they do not add latent heat because there is no change of phase at this stage (no change from water to steam).
How does a heat recovery steam generator work?
To understand how a HRSG works, it is best to study the below diagram.
HRSG Flow Path
Notice that water enters the HRSG at the coldest part (furthest from heat source) and is heated gradually as it progresses towards the heat source. Notice also that there is a standard flow pattern, which starts with the economiser, then the steam drum, evaporator, steam drum again, superheater, and finally to the steam turbines. If the HRSG has a HP, IP, and LP steam system, the flow path is the same, as each system has its own economiser, evaporator, and superheater.
The HRSG working principle is summarised below.
1. Heat Recovery – exhaust gases from a gas turbine or other heat source, typically at temperatures of 900°F to 1,100°F (482°C to 593°C), are directed into the HRSG.
2. Economiser Preheating – feedwater is preheated in the economiser. This process elevates the water temperature close to its boiling point, preparing it for the evaporator.
3. Steam Drum – water from the economiser is delivered to the steam drum, often also passing through a deaerator. Saturated steam is discharged from steam drums.
4. Evaporator Steam Generation – preheated water flows through the evaporator tubes and is heated by the hot exhaust gases. The heat exchanged results in the water boiling and changing state to steam. The temperature in the evaporator can range from 250°F to 600°F (121°C to 315°C), depending on the system pressure.
5. Superheating – generated steam from the evaporator is directed to the superheater. Steam within the superheater is exposed to hotter exhaust gases due to it being closer to the heat source. The superheater may raise the steam’s temperature up to 1,022°F (550°C), which is what is required by a typical power station high-pressure turbine. Steam turbines require superheated steam because of its high energy content and reduced moisture content (dry superheated steam is what is delivered to a steam turbine).
The amount of energy the steam contains corresponds to how much energy the steam turbine can extract, and consequently how much electrical power its generator can produce.
6. Steam Turbine Power Generation – dry superheated steam is discharged from the HRSG to one or more steam turbines. The steam turbine converts the heat energy of the steam to mechanical energy and passes this to a generator (both are installed on a common shaft).
The generator converts the mechanical energy into electrical power (electricity).
7. Exhaust Gas Discharge – after the exhaust gases have transferred most of their heat energy to the water and steam systems, they are discharged to the atmosphere at a temperature of between 250°F to 300°F (121°C to 149°C). It is important that the hot gas stream does not have an excessively low temperature because otherwise condensation may occur within the stack and a corrosive environment will be created.
Good to know – a ‘stack’ is similar to a ‘chimney’ although ‘stack’ is the more common term used in engineering.
Multi Pressure and Single Pressure HRSGs
If a HRSG operates at a single pressure level, it will have a single steam drum, single economiser section, single evaporator section, and single superheater section. If a HRSG operates at multiple pressure levels i.e. LP, IP, and HP pressure levels, it will also have multiple steam drums, economisers, evaporators, and superheaters. A HRSG system that operates at a single pressure level is referred to as a single pressure HRSG. A HRSG system that operates at multiple pressure levels is referred to as a multi pressure HRSG. Power plants use multi pressure HRSGs whilst single pressure HRSGs are more likely to be used for other industrial applications.
Vertical HRSGs and Horizontal HRSGs
It is possible to classify heat recovery steam generators based on their orientation:
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Vertical Type HRSG - exhaust gases flow vertically over horizontal tubes.
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Horizontal Type HRSG - exhaust gases flow horizontally over vertical tubes.