That's the job of a heat recovery steam generator, or HRSG. This article walks through what an HRSG does, how the heat path works from inlet to stack, the main components and design choices, where these units show up across power and process plants, and what matters most when specifying or maintaining one.
Depending on the plant, an HRSG can support extra power generation, cogeneration, district heating, or process steam. The right configuration depends entirely on what the facility needs that steam to do.
Key Takeaways
- An HRSG captures gas-turbine or process exhaust heat and turns it into steam instead of wasting it up the stack
- Combined-cycle plants pair a gas turbine with an HRSG and steam turbine to push efficiency past 60% LHV, according to EPA's CHP catalog
- Drum-type, once-through, horizontal, and vertical designs each suit different cycling and footprint needs
- Steam-temperature control after the HRSG protects piping, turbines, and seals from damage
- Specifying an HRSG well requires exhaust data, steam requirements, and site constraints up front
What Is an HRSG?
An HRSG is heat-recovery equipment. It pulls thermal energy from hot gas, most often gas-turbine exhaust, and transfers it to feedwater to produce steam. Unlike a standalone boiler, it doesn't rely on its own furnace as the primary heat source.
In a combined-cycle power plant, the sequence runs in order:
- The gas turbine burns fuel and generates electricity first
- Hot exhaust leaves the turbine instead of going straight to a stack
- The HRSG captures that exhaust heat and produces steam
- The steam drives a second turbine, or supplies an industrial process
This pairing is what separates a combined-cycle plant from a simple-cycle one. The gas turbine alone recovers roughly 60-70% of inlet fuel energy, according to EPA's CHP catalog. The rest typically leaves as exhaust heat, which is exactly what the HRSG exists to recover.
How an HRSG Differs From a Conventional Boiler
A fired boiler burns fuel directly to make steam. An HRSG, by contrast, depends on an upstream heat source it doesn't control.
| Factor | Conventional boiler | HRSG |
|---|---|---|
| Heat source | Dedicated furnace/burners | Gas-turbine exhaust (primarily) |
| Exhaust oxygen | ~3% (emissions reference basis) | ~15%, which supports duct firing |
| Control dependency | Independent | Tied to turbine load and exhaust conditions |
| Firing | Always fired | Often unfired, optionally duct-fired |
That ~15% oxygen figure matters because it's high enough to support combustion if a plant adds supplementary firing, something a conventional boiler's leaner exhaust stream doesn't offer in the same way, per EPA's CHP catalog.
HRSGs aren't limited to utility combined-cycle plants, either. They show up in cogeneration, refineries, chemical plants, pulp and paper facilities, and waste-to-energy systems, anywhere hot exhaust coincides with a genuine steam demand.
How Does an HRSG Work?
The energy path through an HRSG follows a straightforward sequence, even though the equipment inside is complex.
Fuel and air combust in the gas turbine, producing shaft power first. The hot exhaust, often still carrying substantial thermal energy, then enters the HRSG. Inside, heat moves through tube walls into feedwater. That feedwater becomes steam, and the now-cooled exhaust gas exits through the stack.
Two Separate Flow Paths
Inside the unit, two flows never physically mix:
- Gas-side flow: Exhaust gas moves over the outside of heat-transfer tubes, giving up thermal energy as it travels through the unit
- Water/steam-side flow: Feedwater moves inside those same tubes, progressing through preheating, evaporation, and superheating stages
The water/steam path moves through three stages, from cooler gas zones toward hotter ones:
- Economizer: Preheats incoming feedwater using lower-temperature exhaust
- Evaporator: Boils that water into a steam-water mixture
- Superheater: Raises steam temperature further before delivery to the steam turbine or process header
In drum-type HRSGs, the steam drum sits at the center of this process. It separates water from steam, supports circulation back into the evaporator, and delivers separated steam onward to the superheater, according to Endress+Hauser's technical guide.
What Happens to the Steam
Once steam leaves the HRSG, it does one of two things:
- Drives a steam turbine to generate additional electricity (the combined-cycle arrangement)
- Supplies process heating, district energy, or other industrial loads directly
Supplementary or Duct Firing
Some plants need more steam than unfired turbine exhaust can provide. Supplementary firing, also called duct firing, adds burners in the HRSG's duct section to raise exhaust temperature before it reaches the heat-transfer surfaces.
This isn't a minor tweak. According to EPA's CHP catalog, duct firing can push exhaust temperature entering the HRSG to as much as 2,800°F and increase steam output by more than four times compared to unfired operation. Incremental steam-production efficiency above an unfired HRSG is often 85% or more on an HHV basis when firing natural gas. That figure describes steam production efficiency, not whole-plant electrical output.
Duct firing works because gas-turbine exhaust still contains enough oxygen to support combustion, something a conventional boiler's exhaust typically can't do at the same level.
What Are the Main HRSG Components and Design Types?
An HRSG is a series of tube-based heat exchangers. Surface area, gas-side flow distribution, pressure drop, and tube material all affect how well the unit performs and how long it lasts under cycling conditions.
Primary Components
- Economizer: Preheats incoming feedwater using lower-temperature exhaust, reducing the evaporator's thermal workload
- Evaporator: Transfers enough heat to boil water into a steam-water mixture
- Steam drum: Separates water from steam and supports circulation in drum-type arrangements
- Superheater: Raises separated steam's temperature before it reaches the turbine or process header
- Reheater: Reheats steam returning from a steam turbine, when the cycle design calls for it
GE's combined-cycle product documentation lays out this exact progression: economizer, evaporator, superheater on the water/steam side, with exhaust gas giving up heat in the opposite direction, according to GE's product guide.
Single-Pressure vs. Multi-Pressure Designs
Single-pressure HRSGs are simpler, with fewer components and less piping. Multi-pressure designs, with low-, intermediate-, and high-pressure circuits, recover more heat from the exhaust stream but add equipment, controls, and maintenance complexity.
EPRI notes that double-pressure, non-reheat HRSGs are less efficient than triple-pressure cycles. The added pressure levels bring their own equipment and operational tradeoffs, per EPRI's assessment.
Drum-Type vs. Once-Through
| Factor | Drum-type | Once-through |
|---|---|---|
| Circulation | Natural or assisted, via steam drum | No drum; continuous tube path |
| Startup behavior | Drum thermal mass affects response | Designed for faster cycling response |
| Water-level control | Drum level is a primary control point | No drum level to manage |
| Water chemistry sensitivity | Established, well-understood limits | Different sensitivity profile |
ALSTOM's design documentation describes once-through units as eliminating the thermally sluggish high-pressure drum. The design targets cycling service, though the available source doesn't quantify a specific startup-time advantage, according to ALSTOM's launch announcement.
Neither design wins in every scenario. Choice depends on the plant's cycling frequency and water-treatment capabilities.
Horizontal vs. Vertical Arrangements
Horizontal-gas-flow units route exhaust horizontally through vertical tube bundles. Vertical-gas-flow units do the reverse. Mitsubishi Power offers both configurations, and the choice often comes down to:
- Site footprint and available height
- Structural support requirements
- Access for maintenance and tube inspection
- Lifting and crane constraints during construction
EPRI's limited evaluation found the vertical design's systemic performance behavior very similar to the horizontal design, so orientation is more a site-fit decision than a performance one, per EPRI's assessment.
Unfired vs. Supplementary-Fired
Whether to add duct firing comes down to a few project-specific questions:
- How much does steam demand vary across operating conditions?
- Is additional fuel reliably available at the site?
- What do emissions requirements allow for the duct-firing arrangement?
- Does the operating profile justify the added complexity and fuel cost?
Where Are HRSGs Used and What Benefits Do They Provide?
HRSGs show up anywhere hot exhaust and steam demand coincide. Common applications include:
- Combined-cycle power plants generating utility-scale electricity
- Industrial cogeneration and combined heat and power (CHP) systems
- Refineries supplying both steam and electricity to process units
- Chemical facilities needing process steam alongside power
- Pulp and paper plants with substantial steam loads
- Waste-to-energy and landfill-gas power projects
- District heating installations
One documented example comes from EPA permitting records for the Air Liquide Bayou Cogeneration Plant in Pasadena, Texas. Four power blocks each pair a gas turbine with a duct-fired HRSG to produce steam for sale, according to EPA's permit filing.
That facility has no steam turbine. It's a straightforward illustration of CHP, not a combined-cycle power arrangement.
The Real Benefits
- Recovers fuel energy that would otherwise leave through the stack
- Delivers additional electricity or usable process steam
- Reduces the need for a separate, independently fired steam generator
- Lowers rejected heat compared with simple-cycle operation alone
When an HRSG Doesn't Pay Off
An HRSG isn't automatically a good investment just because exhaust heat exists. Value depends on:
- Available exhaust heat matching actual steam demand
- Required steam pressure and temperature aligning with the heat source
- Operating hours and steam demand profile supporting the capital cost
- Water-treatment capability and cost
- The market value of the electricity or process heat produced
Poor heat-transfer performance, fouling, tube damage, steam demand mismatch, or inadequate controls can all erode the expected value of an otherwise well-designed unit.
What Should You Consider When Specifying and Maintaining an HRSG?
Getting an HRSG specification right starts with giving the supplier accurate data, not just a general steam requirement.
Minimum Design Information to Provide
- Heat source details: Gas-turbine model, exhaust flow, temperature, composition, and load range
- Steam requirements: Flow, pressure, temperature, quality, and end use
- Feedwater conditions: Pressure, temperature, chemistry, and treatment arrangements
- Operating profile: Startup frequency, cycling pattern, minimum load, ramping needs, supplementary-firing requirements
- Site constraints: Elevation, ambient conditions, footprint, access, transport route, crane capacity, maintenance clearances
Why Steam-Temperature Control Matters
Steam leaving the HRSG doesn't always arrive at a stable, usable temperature on its own. Excessive or fluctuating steam temperature downstream can damage:
- Piping and steam traps
- Rotating seals on turbines
- Turbine components themselves
- Downstream process equipment relying on consistent steam conditions
This is where desuperheaters and attemperators come in. These devices inject a controlled amount of water into the steam stream to bring temperature down to the target range.
Correct sizing, spray quality, mixing distance, responsive controls, and accurate downstream temperature measurement all matter. POWER Magazine's technical coverage of desuperheater design emphasizes that primary and secondary atomization quality directly affects whether the water fully evaporates before it reaches piping walls, per POWER Magazine's design guide.
This is squarely where B-Tech Valve LLC operates. The company designs and sells American-made, application-sized desuperheater and attemperator valves for power and process plants, including:
- LPD (Large Pipe Desuperheater): A probe-type, variable-nozzle, fully actuated control valve and desuperheater combined in one assembly, built for steam pipes 6 inches and larger
- SPAD (Small Pipe Attemperator Desuperheater): A spray-ring, fixed-nozzle design paired with a separate temperature-control valve, built for steam pipes 4 inches and smaller
Both lines are offered in ASME B16.34 pressure classes 150 through 2700, rated for temperatures up to 1,200°F. Verify current specifications against B-Tech Valve's product literature before finalizing a design, and reach out directly for application-specific guidance.
Reliability Checklist
A quick reference for ongoing HRSG maintenance should cover:
- Economizer and evaporator tube condition (corrosion, erosion, thinning)
- Superheater and reheater overheating risk
- Gas-side fouling buildup
- Water chemistry against established limits
- Drum-level control accuracy
- Relief-device function and testing status
- Control-valve fail-safe positions
- Thermal cycling history and inspection records
EPRI's nondestructive examination guidelines cover inspection methods and tube locations most likely to show developing damage before it causes a failure, according to EPRI's inspection guidelines. Water chemistry limits for feedwater, boiler water, and steam are addressed in IAPWS's volatile treatment guidance, which plant chemists use to set and verify operating targets.
Periodically compare current piping and instrumentation diagrams against the as-installed system. Confirm no valve can unintentionally block a required flow path, and verify overpressure protection and bypass arrangements remain functional and tested.
ASME PTC 4.4 governs HRSG performance testing, and ASME BPVC Section I covers power boiler construction requirements relevant to pressure-part work.
Conclusion
An HRSG recovers heat that would otherwise disappear up an exhaust stack and turns it into steam with real economic value, for extra power generation, cogeneration, district heating, or industrial process use.
There's no single "correct" HRSG configuration. The right choice depends on exhaust conditions, steam demand, operating profile, site constraints, water chemistry, and how the plant plans to maintain the equipment over its service life.
If your plant is evaluating steam-temperature control, or you need a replacement desuperheater or attemperator valve, B-Tech Valve LLC can provide application-focused guidance and product selection. Reach the team at 609-321-2205 or Sales@B-TechValves.com.
Frequently Asked Questions
How does a heat recovery steam generator (HRSG) work?
An HRSG transfers heat from gas-turbine or process exhaust to feedwater flowing inside tubes, converting it into steam. That steam then drives a steam turbine for added power or supplies industrial process heat.
What is a heat recovery steam generator (HRSG)?
An HRSG is heat-recovery equipment that produces steam using hot exhaust gas rather than a dedicated furnace. It's most commonly installed downstream of a gas turbine in combined-cycle and cogeneration systems.
What are the key differences between a boiler and an HRSG?
A boiler burns fuel directly as its primary heat source, while an HRSG depends on an upstream exhaust source it doesn't control. HRSGs are often unfired but can add supplementary duct firing, since turbine exhaust typically runs around 15% oxygen, enough to support additional combustion.
What is the difference between OCGT and CCGT?
An open-cycle gas turbine (OCGT) generates power without a bottoming steam cycle, reaching over 40% LHV efficiency. A combined-cycle gas turbine (CCGT) adds an HRSG and steam turbine, recovering exhaust heat to exceed 60% LHV, according to EPA's CHP catalog.
What is the efficiency of a heat recovery steam generator?
HRSG efficiency depends on exhaust temperature and flow, steam conditions, pressure-level configuration, ambient conditions, fouling, and operating mode. There's no single universal figure; it varies by installation and design.
How much does an HRSG cost?
Cost varies widely based on steam capacity, pressure levels, firing arrangement, materials, emissions equipment, and whether it's new construction or a retrofit. Request a project-specific quotation rather than relying on a generic price estimate.
