Industrial operations generate enormous amounts of thermal energy that is often simply vented into the atmosphere as waste heat. The challenge lies in capturing this low-grade, high-volume energy stream and converting it into a usable form. The Heat Recovery Steam Generator (HRSG) is the specialized industrial equipment designed to solve this problem, acting as a crucial energy recovery device.
Defining the Heat Recovery Steam Generator
A Heat Recovery Steam Generator is essentially a large, specialized heat exchanger that captures thermal energy from a hot gas stream and uses it to produce steam. This hot gas is typically the exhaust from a gas turbine, which can be as hot as 650°C (1200°F). Unlike a conventional boiler that burns fuel, the HRSG utilizes heat already present as a byproduct of another process.
The primary function of this device is to serve as a thermal link, converting the waste heat into high-pressure, high-temperature steam. By doing so, it vastly improves the thermal efficiency of the entire power generation or industrial process.
Core Components and Internal Structure
The internal structure of an HRSG is organized into distinct heat transfer sections, known as modules, which are banks of finned tubes. The largest of these sections are the economizer, the evaporator, and the superheater, often arranged in series within the exhaust gas path. The economizer is positioned to receive the coolest exhaust gas and preheats the incoming feedwater, raising its temperature closer to the boiling point.
The evaporator section is where the phase change occurs, converting water into saturated steam. It is connected to a steam drum, which separates the steam from the water to ensure only dry steam proceeds. The superheater is the final section, typically located closest to the incoming hot exhaust gas, where the saturated steam is heated well above its boiling point. In larger systems, an HRSG may feature multiple pressure levels—high, intermediate, and low—each with its own dedicated economizer, evaporator, and drum to maximize heat extraction.
The Thermodynamic Process of Steam Generation
The process begins as pressurized feedwater enters the HRSG at the economizer section, absorbing heat from the gas stream flowing in the opposite direction. This water is heated up to its saturation temperature, which is the boiling point corresponding to the drum pressure. The heated water then flows into the steam drum, where it is circulated through the evaporator tubes.
Within the evaporator, the continued absorption of heat causes the water to boil and transition into a steam-water mixture. This mixture returns to the steam drum, where the drier, less dense steam separates and rises to the top.
A key limiting factor in this heat transfer is the pinch point, the smallest temperature difference between the hot exhaust gas and the water/steam mixture in the evaporator. A smaller pinch point indicates more efficient heat recovery, but requires a significantly larger heat transfer surface area, increasing the HRSG’s cost and size.
Another thermodynamic design parameter is the approach temperature, defined as the difference between the saturation temperature and the water temperature leaving the economizer. Maintaining a positive approach temperature is necessary to prevent the water from prematurely boiling within the economizer tubes, which could lead to flow instability.
The steam that separates in the drum is initially saturated, meaning it is at its boiling temperature and still contains some moisture. It is then directed to the superheater, where it continues to absorb heat from the highest temperature exhaust gas. This process converts the saturated steam into superheated steam, which is dry and has a higher thermal energy content, making it suitable for driving turbines.
Primary Applications in Power Generation
The most common application for the HRSG is within a Combined Cycle Gas Turbine (CCGT) power plant configuration. In this setup, a gas turbine generates electricity in a primary cycle, and the HRSG captures the heat from the exhaust to generate steam. This steam then powers a separate steam turbine, which generates additional electricity, boosting the overall plant efficiency to levels that can exceed 60 percent.
HRSGs are also widely used in industrial settings for Cogeneration, also known as Combined Heat and Power (CHP) systems. Here, the recovered steam is often supplied directly to an adjacent industrial process, such as heating, chemical refining, or desalination, rather than being directed to a turbine. This dual use of energy for both power and process heat maximizes fuel utilization.