Heat Recovery Steam Generator (HRSG)

A Heat Recovery Steam Generator (HRSG)

• It is an energy recovery heat exchanger that recovers heat from the exhaust gases of a gas turbine or other combustion-based power generation processes to produce steam. The steam generated by the HRSG can be used for various purposes, such as power generation, industrial processes, or district heating.
• The basic principle of an HRSG involves capturing waste heat from the exhaust gases of a gas turbine, which would otherwise be lost to the atmosphere, and using it to produce steam. This is typically done in a series of heat exchangers where the hot exhaust gases transfer their heat to water flowing through the HRSG.

The HRSG consists of several key components:

1. Heat Exchangers: HRSGs have multiple heat exchangers or sections through which the exhaust gases pass and transfer heat to water. These sections may include the economizer, evaporator, and superheater.
2. Economizer: The section of the HRSG where water is preheated using the heat from the exhaust gases before it enters the boiler, thereby increasing the overall efficiency of the system.
3. Evaporator: The section where the preheated water is further heated to its boiling point, and it turns into steam. The evaporator section of the HRSG is where the water is converted into steam. The hot gases from the gas turbine or combustion engine pass through the evaporator, transferring their heat to the water inside the tubes. This process turns the water into steam, which is collected in the steam drum.
4. Superheater: The superheater is a separate section in the HRSG that further increases the temperature of the steam to achieve the desired temperature and specific enthalpy. Superheating the steam ensures that it remains in a gaseous state throughout its journey to the turbine, improving turbine efficiency and preventing moisture-related damage.
5. Feedwater Pumps: Feedwater pumps are used to supply water from the deaerator or the condensate storage tank to the HRSG. They increase the pressure of the feedwater to the required level for efficient operation of the HRSG.
6. Deaerator (Mechanical Oxygen scavenger): The deaerator is a device that removes oxygen and other dissolved gases from the feedwater to prevent corrosion in the HRSG. It heats the feedwater using Pegging steam and reduces its pressure, allowing the dissolved gases to escape.
7. Drums: Where the steam is collected and separated from the water.
8. Stack: The stack is the exhaust outlet for the HRSG. It releases the flue gases (after heat recovery) into the atmosphere at a safe height and away from the surrounding environment.

10. Chemical injection in an HRSG (Heat Recovery Steam Generator) is crucial for water treatment, foam, Scale and Deposit formation, corrosion control, and maintaining the overall health of the system. The specific chemicals used and their injection locations may vary depending on the HRSG design, water quality, and operating conditions. However, here are some common chemical injections and their typical locations in an HRSG:

a. Oxygen Scavenger: Oxygen scavengers are chemicals used to remove dissolved oxygen from the feedwater to prevent corrosion in the HRSG. They are usually injected at the deaerator or in the feedwater storage tank.

b. PH Adjustment: pH control is important to maintain proper water chemistry. Acid and alkaline chemicals are injected at different locations to adjust the pH of the feedwater. Acid injection is typically done at the deaerator or economizer inlet, while alkaline chemicals may be added in the feedwater or at various locations to balance the pH.

c. Phosphate Treatment: Phosphate-based chemicals are added to the feedwater to control pH and prevent scale formation on heat transfer surfaces. These chemicals are commonly injected into the feedwater system, typically at the deaerator or economizer inlet.

• It's essential to follow the manufacturer's recommendations and industry best practices for chemical injection in HRSGs to ensure safe and efficient operation. Regular water quality monitoring and adjustments to chemical dosages are necessary to maintain proper water chemistry and prevent issues like corrosion, scale formation, and biofouling. Additionally, local regulations and environmental considerations should be taken into account when designing and operating chemical injection systems in an HRSG.

The generated steam can be used to drive a steam turbine to produce additional electricity or provide heat for industrial processes or district heating. By utilizing this waste heat, HRSGs significantly increase the overall efficiency of a power plant, which is often referred to as a Combined Cycle Power Plant (CCPP). CCPPs combine the gas turbine's Brayton cycle and the steam turbine's Rankine cycle, maximizing energy conversion and reducing greenhouse gas emissions.

HRSGs play a vital role in the energy industry, especially in power plants, where they contribute to the sustainable and efficient use of fuel resources. Additionally, they have applications in various industrial processes where waste heat recovery can lead to cost savings and environmental benefits.

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