A heat recovery steam generator, or HRSG, captures the hot exhaust gas leaving a gas turbine and uses that heat to produce steam. Instead of letting exhaust temperatures of 450°C to 650°C escape into the atmosphere, the HRSG routes that gas through a series of tube bundles filled with water, turning it into steam that can drive a second turbine or supply industrial processes. This dual use of a single fuel source is the foundation of combined cycle power plants, which achieve efficiency improvements of 10% to over 20% compared to running a gas turbine alone.
How an HRSG Produces Steam
The basic principle is straightforward: hot gas flows over tubes containing water, and the water absorbs enough heat to become steam. But the engineering inside an HRSG is more layered than a simple boiler. A typical unit contains three pressure stages, each with its own set of heat-absorbing tube bundles arranged in sequence along the gas path.
As exhaust gas enters the HRSG, it first hits the high-pressure section, where temperatures are highest. The gas then passes through intermediate-pressure and low-pressure sections, each extracting progressively cooler heat. This staged approach squeezes as much usable energy as possible from the exhaust before it exits through the stack. Within each pressure stage, the water goes through three phases: it’s preheated in an economizer, boiled into steam in an evaporator, and then heated further past its boiling point in a superheater. The superheated steam is what actually drives a steam turbine to generate additional electricity.
Horizontal vs. Vertical Configurations
HRSGs come in two main physical layouts, and the choice between them affects everything from plant footprint to maintenance access. In a horizontal HRSG, the exhaust gas flows horizontally through vertically oriented tube panels. This is the more common design in many parts of the world because it’s relatively easy to build and transport in modular sections. The tradeoff is that draining those vertical tubes requires a more complex branched system with shut-off valves and drain tanks installed below ground level.
A vertical HRSG flips the arrangement: the gas flows upward through horizontal tube coils stacked in a vertical duct. Draining is simpler here since water flows naturally downward through just two headers into a tank at ground level. Vertical units also distribute steam more evenly across the evaporator coils because of their parallel-flow pattern, while horizontal units use a cross-flow design that can create uneven steam distribution across the panels. An additional perk of the vertical layout is the “stack effect,” where the natural updraft of hot gas rising through the vertical duct actually reduces backpressure on the gas turbine, giving it a slight boost in power output. Vertical HRSGs tend to take up less ground space but require taller structures, making them a better fit for sites where land is limited but height restrictions are not a concern.
Duct Burners and Supplementary Firing
Most HRSGs can be equipped with duct burners, which are essentially rows of gas-fired burners installed inside the exhaust duct upstream of the tube bundles. These burners inject additional heat into the exhaust stream, increasing steam production when demand is high. The concept is called supplementary firing.
This feature is particularly useful during hot summer months. High ambient temperatures reduce the density of the air entering the gas turbine’s compressor, which lowers both gas turbine output and steam turbine performance. Firing the duct burners compensates for that seasonal drop. The burners are positioned at the hot end of the system, typically just before the high-pressure superheater, to maximize the additional steam flow where it matters most while keeping emissions low. The tradeoff is a slight decrease in overall plant efficiency since you’re burning extra fuel, but the increase in total power output often makes economic sense during peak demand periods when electricity prices are highest.
Where HRSGs Are Used
Combined cycle power plants are the primary application. A gas turbine generates electricity, its exhaust feeds the HRSG, and the resulting steam drives a second generator. This arrangement can push overall plant efficiency well above what either turbine could achieve on its own, all without burning additional fuel in the HRSG itself (unless duct burners are active).
Beyond power generation, HRSGs play a central role in cogeneration facilities, where the steam is split between electricity production and industrial heat supply. Refineries, chemical plants, and food processing operations use process steam for heating, drying, and chemical reactions. In some configurations, HRSG steam feeds desalination plants that convert seawater to drinking water. Inland oilfield cogeneration facilities are another common application, though these tend to suffer efficiency losses compared to coastal plants that can use wet cooling towers and have more options for directing waste heat to useful purposes.
Common Maintenance Challenges
The biggest long-term threat to HRSG reliability is a process called flow-accelerated corrosion, or FAC. This occurs when the protective oxide layer inside the steel tubes gradually dissolves, thinning the tube walls over time. The low-pressure evaporator section is especially vulnerable. In documented cases, tube walls have thinned to less than 1.0 mm in the upper and curved sections where turbulence is highest. Research has shown that the corrosion rate of the carbon steel commonly used in these tubes increases roughly fourteen-fold when water temperature rises from 110°C to 150°C under flowing conditions. At those higher temperatures, the steel also loses about a third of its strength.
Water chemistry plays a surprisingly large role. Phosphorus from water treatment chemicals can accumulate in the oxide layer on tube surfaces, destabilizing the protective coating and accelerating corrosion. Localized turbulence inside the tubes, particularly at bends and junctions, compounds the problem by physically stripping away weakened oxide. Preventing FAC requires careful attention to material selection (using alloys more resistant to this type of wear), monitoring water chemistry to avoid harmful chemical buildup, and designing tube layouts that minimize turbulent hotspots.
Managing Thermal Stress During Startup
HRSGs don’t respond well to rapid temperature changes. The steam drum, a large cylindrical vessel where water and steam separate, is the component most susceptible to thermal stress during startup. When a cold HRSG is suddenly exposed to hot exhaust gas, the inner surface of the drum heats up and expands faster than the outer surface, creating stress that can lead to cracking over hundreds of startup cycles.
Plant operators manage this through controlled startup procedures. One common approach uses a gas bypass damper that diverts part of the exhaust around the HRSG during the initial phase of startup, allowing the metal to warm gradually. Studies simulating different startup methods have shown that bypassing even a portion of the gas flow significantly lowers peak thermal stress in the drum. When the gas turbine and HRSG start up simultaneously, the stress is naturally lower because gas temperature and flow increase gradually rather than hitting the cold tubes all at once. In that scenario, only a small amount of gas bypass is needed to keep stress within safe limits. Whether operators use a step increase (raising gas flow in stages) or a ramp increase (raising it smoothly over time), the goal is the same: give the thick-walled components time to heat evenly.
Efficiency and Environmental Impact
Adding an HRSG to a gas turbine is one of the most effective ways to reduce carbon emissions per unit of electricity generated, because you’re producing more power from the same amount of fuel. Retrofit projects in the United States and Middle East have consistently demonstrated efficiency gains of 10% to over 20%, with corresponding emissions reductions and no increase in fuel consumption. For a power plant burning natural gas around the clock, that efficiency gain translates directly into less CO₂ per megawatt-hour.
The environmental case is straightforward: every unit of heat recovered by the HRSG is a unit that doesn’t need to come from burning additional fuel somewhere else. In cogeneration setups where the steam displaces a separate industrial boiler, the emissions savings are even greater because you’re eliminating an entire combustion source. This is why combined cycle plants with HRSGs have become the default choice for new natural gas power generation in most of the world.