What is heat recovery steam generator (HRSG)?

Short Answer:

A Heat Recovery Steam Generator (HRSG) is a type of heat exchanger used in a combined cycle power plant to recover waste heat from the exhaust gases of a gas turbine. This recovered heat is then used to produce steam, which drives a steam turbine for additional electricity generation.

In simple words, an HRSG is a device that converts the unused heat energy from the gas turbine exhaust into useful steam energy. It helps to increase the overall efficiency of the power plant by utilizing waste heat instead of allowing it to escape into the atmosphere.

Detailed Explanation :

Heat Recovery Steam Generator (HRSG)

A Heat Recovery Steam Generator (HRSG) is one of the most important components of a combined cycle power plant, which combines a gas turbine and a steam turbine for efficient electricity generation. The HRSG is a heat exchanger that captures the waste heat from the gas turbine exhaust and uses it to convert feedwater into steam. This steam is then supplied to the steam turbine, which generates additional power without consuming extra fuel.

The HRSG thus plays a vital role in improving the thermal efficiency of the power plant by utilizing exhaust gases that would otherwise be wasted. By recovering and reusing this heat, the combined cycle power plant achieves an efficiency of around 55–65%, which is much higher than a simple gas turbine plant (35–40%).

Working Principle of Heat Recovery Steam Generator

The working of the HRSG is based on the principle of heat exchange between the hot exhaust gases from the gas turbine and the relatively cold feedwater. The process can be explained step-by-step as follows:

  1. Exhaust Gas Entry:
    • The hot gases (at around 500–600°C) coming out of the gas turbine enter the HRSG.
    • These gases flow through a series of tubes or heat exchanger surfaces inside the HRSG.
  2. Heat Transfer Process:
    • Feedwater is pumped through the HRSG in separate tubes or channels.
    • As the exhaust gases pass over these tubes, heat is transferred from the hot gases to the water.
    • The water absorbs this heat energy and starts converting into steam.
  3. Steam Generation:
    • The water first gets preheated in the economizer section, then boils in the evaporator section, and finally becomes superheated in the superheater section.
    • The generated steam is sent to the steam turbine for power generation.
  4. Exhaust Gas Exit:
    • After transferring its heat, the exhaust gas temperature drops significantly (to about 100–150°C).
    • The cooler gas is then released into the atmosphere through a stack.

This process allows efficient energy recovery and eliminates the need for additional fuel for steam production.

Main Components of HRSG

The HRSG is composed of several major parts that work together to extract and transfer heat efficiently. The main components include:

  1. Economizer:
    • This section preheats the feedwater before it enters the evaporator.
    • It uses the lowest-temperature exhaust gases for heating.
    • Preheating reduces the fuel requirement in the boiler and improves efficiency.
  2. Evaporator:
    • In this section, the preheated water is converted into saturated steam.
    • The medium-temperature exhaust gases provide sufficient heat to boil the water.
  3. Superheater:
    • The steam produced in the evaporator is further heated in the superheater to increase its temperature and energy level.
    • Superheated steam is then supplied to the steam turbine for expansion and power generation.
  4. Drum (in drum-type HRSG):
    • The drum acts as a separator that divides steam and water.
    • Steam moves to the superheater, while water is recirculated to the evaporator.
  5. Ducting and Stack:
    • Ducts guide exhaust gases through the HRSG and out to the stack.
    • The stack releases cooled gases safely into the atmosphere.
  6. Bypass System:
    • Some HRSGs are equipped with a bypass system that allows exhaust gases to skip the HRSG when steam generation is not required (useful for part-load operation).

Types of HRSG

HRSGs can be classified based on their design and operation:

  1. Once-Through HRSG:
    • Water passes only once through the HRSG and is converted directly into steam.
    • Suitable for high-pressure and high-efficiency applications.
  2. Drum-Type HRSG:
    • Contains a steam drum that separates steam and water.
    • Commonly used in large combined cycle plants.
  3. Horizontal HRSG:
    • Gas flow is horizontal, and water/steam tubes are arranged vertically.
    • Preferred due to easy maintenance and compact design.
  4. Vertical HRSG:
    • Gas flow is vertical, and water/steam tubes are arranged horizontally.
    • Suitable for plants with space limitations.

Advantages of Heat Recovery Steam Generator

  1. Improves Efficiency:
    • Recovers waste heat and converts it into useful steam energy, improving plant efficiency up to 65%.
  2. Reduces Fuel Consumption:
    • Since no extra fuel is needed for steam generation, fuel usage and costs decrease.
  3. Environmentally Friendly:
    • Reduces emission of greenhouse gases (CO₂, NOₓ) by using waste heat.
  4. Compact and Cost-Effective:
    • Combines two power generation systems in one, reducing overall plant size and cost.
  5. Reliability and Flexibility:
    • Can handle fluctuating load conditions and integrates well with both gas and steam turbines.

Limitations of HRSG

  1. High Initial Cost:
    • HRSG installation and integration require advanced materials and design, making it expensive.
  2. Complex Operation:
    • Requires precise temperature and flow control to avoid thermal stresses.
  3. Maintenance Requirements:
    • Fouling and corrosion in heat exchanger tubes can reduce efficiency and require periodic cleaning.
  4. Pressure Loss:
    • Small pressure drops occur due to gas and water flow resistance, slightly affecting performance.

Despite these challenges, the HRSG remains essential in modern combined cycle plants due to its high efficiency and environmental benefits.

Applications of HRSG

  • Combined Cycle Power Plants:
    Used to generate steam from gas turbine exhaust gases for additional power.
  • Cogeneration Plants:
    Produces both electricity and useful process steam.
  • Industrial Processes:
    Used for energy recovery in refineries, chemical plants, and manufacturing units.
Conclusion

In conclusion, a Heat Recovery Steam Generator (HRSG) is a critical component of a combined cycle power plant designed to recover waste heat from gas turbine exhaust gases and convert it into useful steam. This steam drives the steam turbine to generate additional electricity, improving the overall plant efficiency. The HRSG operates as a heat exchanger with sections like economizer, evaporator, and superheater, making it highly effective for energy conservation. Despite its complex design, the HRSG significantly reduces fuel use, operating costs, and environmental pollution, making it an indispensable part of modern power generation systems.