An Heat Recovery Steam Generator (HRSG) boosts plant power output, enhances Combined Cycle plant efficiency, and (therefore) reduce the amount of fuel needed per MW hour. The initial HRSG costs (CAPEX) drive long-term OPEX savings over the 20+ year useful life of HRSG systems.
For plant performance data of HRSGs when combined with different gas turbines, please see the following tables listing gas turbine and Combined Cycle plant configurations along with HRSG performance data when combined with best selling models from GE Vernova, Siemens, Mitsubishi (and more). CLICK HERE.
Heat Recovery Steam Generator (HRSG): Definition and Function
A Heat Recovery Steam Generator (HRSG) extracts thermal energy from high-temperature exhaust gases—commonly originating from gas turbines or industrial processes—and utilize this energy to generate steam. The produced steam is subsequently used to drive steam turbines for electrical power generation or other industrial applications. The HRSG serves as a critical component in combined cycle power plants (CCPPs) and cogeneration systems, enabling significant improvements in overall thermal efficiency by recovering waste heat that would otherwise be lost to the atmosphere.
Purpose and Advantages: HRSG Implementation costs
- Thermodynamic Efficiency and Environmental Sustainability
The principal rationale for integrating HRSGs within power generation systems lies in enhancing cycle efficiency. By recovering thermal energy from flue gases, HRSGs increase the overall energy utilization of the plant, reducing specific fuel consumption and greenhouse gas emissions. This aligns with global sustainability objectives and improves the energy return on investment (EROI).
- Economic Viability: HRSG costs vs benefits
Although HRSGs involve a significant capital expenditure (CAPEX), a lifecycle HRSG cost-benefit analysis justifies the investment due to prolonged operational lifespans (typically over 20 years), high reliability, and continuous duty cycles. The efficiency gains lead to reduced fuel usage per unit of output, translating into considerable OPEX savings and favorable payback periods.
Servicing is a continuous activity starting from commercial operation. Routine maintenance (meaning daily, weekly, monthly) can take place without shutting down the units. It is also typical to perform maintenance on HRSGs during overhaul outages when units are shut down.
Experts at John Cockerill suggest if preventive maintenance and scheduled maintenance are executed properly, no extra maintenance is needed. Also, John Cockerill can provide a permanent device to measure the stress and the damage on some items, such as with our in-house developed boiler stress evaluator (BSE). The BSE consists of a program that calculates on-line the boiler’s remaining life (calculating fatigue and creep) according to EN 12952-3 and EN 12952-4. This technology particularly suits the heavy cycling of combined cycle power plants.
John Cockerill can also monitor the lifetime of a boiler pressure parts by performing some activities of remaining life time assessment (RLTA), which is based on destructive or non-destructive tests.
- Operational Flexibility
HRSGs are engineered for integration with a variety of heat sources and industrial systems, offering broad applicability in sectors such as:
- Combined Cycle Power Generation
- District Heating Networks
- Oil Refineries and Petrochemical Complexes
- Pulp and Paper Mills
- Steel and Cement Manufacturing
They can be configured for single-pressure or multi-pressure operation depending on application-specific steam requirements.
Application in Combined Cycle Power Plants
In a combined cycle configuration, the HRSG is positioned downstream of a gas turbine . The gas turbine generates mechanical power and exhausts flue gases at temperatures ranging from 900°F to 1,100°F (482°C to 593°C). These gases are directed into the HRSG, where thermal energy is transferred to a series of heat exchanger components, producing steam that is then routed to a steam turbine generator for secondary power generation.
This two-stage process increases thermal efficiency from approximately 35–40% (simple cycle) to 55–62% (combined cycle), depending on specific design parameters and operating conditions.
HRSG Component Architecture
A conventional HRSG consists of multiple heat exchange sections organized according to pressure levels—Low Pressure (LP), Intermediate Pressure (IP), and High Pressure (HP)—each comprising the following subsystems:
- Economiser: Preheats feedwater by recovering residual heat from the lowest temperature flue gases.
- Steam Drum: Acts as a phase-separation vessel for the water/steam mixture; supplies saturated steam to the superheater and recirculates liquid to the evaporator.
- Evaporator: Facilitates phase transition (liquid to vapor) as water is heated by exhaust gas flow via thermal conduction through heat exchanger tubes.
- Superheater: Increases the temperature (sensible heat addition) of saturated steam to generate dry, superheated steam suitable for turbine inlet conditions.
These components are configured in a series arrangement, ensuring progressive thermal energy exchange from the flue gas stream to the working fluid.
Steam Generation Flow Path
The typical thermodynamic flow path through a HRSG can be summarized as:
- Feedwater Inlet → enters via the economiser at the coldest point.
- Economiser → preheats the water close to saturation temperature.
- Steam Drum → receives preheated water and initiates phase separation.
- Evaporator → converts water to a saturated steam/water mixture through heat absorption.
- Steam Drum (return) → separates saturated steam and recirculates residual water.
- Superheater → elevates steam temperature to turbine specifications (up to ~1,022°F or 550°C).
- Steam Outlet → directs superheated steam to turbine(s).
- Exhaust Gas Discharge → gases exit the HRSG at ~250–300°F (121–149°C), ensuring condensation does not occur in the exhaust stack.
Pressure-Level Configurations
- Single-Pressure HRSG: Utilizes one steam drum and one set of heat exchangers (economiser, evaporator, superheater). Common in smaller or simpler applications.
- Multi-Pressure HRSG: Employs multiple pressure levels (typically HP, IP, LP), with dedicated steam drums and heat exchange sections per level. Enhances thermal efficiency through staged heat recovery and reduced exergy loss. Standard in modern CCPPs.
Orientation Classifications
- Vertical HRSG: Features vertically upward or downward gas flow over horizontal tube bundles.
- Horizontal HRSG: Employs horizontal gas flow across vertically arranged heat exchange tubes.
The choice between vertical and horizontal configuration is influenced by factors such as spatial constraints, maintenance access, and structural considerations.
Key Operating Principles
| Process Step | Description |
| Heat Recovery | Inlet exhaust gases from gas turbines enter the HRSG at ~900–1,100°F (482–593°C). |
| Feedwater Preheating | Water is heated in the economiser, raising its temperature to near-saturation. |
| Phase Separation | Saturated water/steam mixture is routed through the steam drum, separating dry steam. |
| Steam Generation | Water circulates through evaporator tubes, undergoing phase transition. |
| Superheating | Steam is raised to turbine inlet temperatures via the superheater (~1,022°F or 550°C). |
| Mechanical-to-Electrical Conversion | Superheated steam drives turbines; mechanical work is converted to electricity via a generator. |
| Exhaust Gas Emission | Spent flue gases are discharged to the atmosphere, ensuring stack temperature remains above dew point. |
Conclusion
A Heat Recovery Steam Generator is a highly engineered, thermodynamically optimized system that serves as a cornerstone in modern high-efficiency power generation and industrial energy recovery. Its ability to convert waste heat into usable energy makes it indispensable in combined cycle power plants and various process industries seeking improved efficiency and reduced carbon footprint. Installed HRSG costs are far outweighed by overall operational savings, especially in hot weather conditions. .
Useful link:
- Looking for plant performance data of HRSGs combined with different gas turbines? See the following tables with plant configurations and performance specs, including information about the best selling models from GE Vernova, Siemens, Mitsubishi (and more). CLICK HERE.
HRSG Case Studies:
- HRSG with 4 x GE Vernova 9F gas turbines for Besyama III Combined Cycle plant in Iraq, rated at 1500 MW capacity.
- HRSG with Mitsubishi M701JAC gas turbine for Keppel Sapra Cogeneration plant in Singapore, rated at 600 MW
- 3-pressure level + reheat HRSG with a CO catalyst for 600 MW hydrogen-ready power plant using MItsubishi M701JAC gas turbine.
Additional Reading
To read about the World’s Largest Heat Recovery Steam Generator. CLICK HERE.
