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Scan for outdated or missing drivers - takes under a minuteDriver Scan →Repair Windows errors before they cause bigger problemsFix Now →A combined-cycle gas power plant generates electricity twice from the same fuel stream: first with a gas turbine, then by using the turbine’s hot exhaust to make steam that drives a second turbine. The heat recovery steam generator (HRSG) connects the two stages. This arrangement captures useful energy that a simple-cycle plant would release in its exhaust, although it does not recover all of that energy and actual plant performance varies.
The two linked power cycles
A combined-cycle plant has a topping cycle and a bottoming cycle. The gas turbine is the topping cycle: it burns fuel and produces the first electrical output. The steam turbine is the bottoming cycle: it uses heat remaining in the gas-turbine exhaust to make steam and produce additional electricity. The cycles are linked by heat transfer in the HRSG, rather than by sending the gas turbine’s exhaust through the steam turbine.
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In a natural-gas combined-cycle plant, air is compressed, fuel is burned in that pressurized air, and the hot expanding gas turns turbine blades. The turbine drives a generator, while also supplying the mechanical power needed to run the compressor. The turbine exhaust still contains useful heat. It flows through the HRSG, where heat passes into water circuits and produces steam. That steam expands through a separate steam turbine connected to another generator. The cooled flue gas then leaves through a stack.
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The National Energy Technology Laboratory (NETL) describes the sequence as natural gas combusting in a gas turbine to drive a generator, followed by using the hot flue gas to raise steam in an HRSG for a steam Rankine bottoming cycle. The defining idea is therefore heat recovery between two power cycles: the exhaust is not itself a second fuel source, and the HRSG is not a second gas turbine.
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How the plant makes electricity, step by step
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Air enters the compressor
The compressor draws in air and raises its pressure before combustion. It is mechanically connected to the gas turbine, so part of the turbine’s power is used to keep the compressor operating. The generator’s output is the electrical power produced after the turbine supplies that internal mechanical demand and drives the generator.
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Fuel burns in the compressed air
The combustion system mixes fuel with compressed air and releases heat by burning the fuel. Natural gas is the most common fuel in NGCC, or natural-gas combined-cycle, plants. The hot combustion gas expands through the gas turbine. As the U.S. Department of Energy (DOE) explains, expanding hot gas spins the turbine’s rotating blades.
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The gas turbine drives a generator
The rotating turbine provides mechanical power to the compressor and to a generator, which produces the plant’s first electrical output. The exhaust leaving the turbine is cooler than the gas inside it, but it is still hot enough to be useful. A simple-cycle plant can generate power with the gas turbine alone; a combined-cycle plant adds equipment to put some of that remaining exhaust heat to work.
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The HRSG transfers exhaust heat to water
Gas-turbine exhaust passes through the heat recovery steam generator. The HRSG transfers heat from that gas into water and steam circuits, raising steam for the bottoming cycle. DOE summarizes its role directly: “A HRSG generates steam by capturing heat from the turbine exhaust.” The HRSG is a heat-recovery boiler system; it does not burn the turbine exhaust to create a second combustion-turbine stage.
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Steam produces a second electrical output
Steam from the HRSG expands through a steam turbine connected to a generator. Its energy turns the turbine and produces additional electricity. This second output is why the plant can generate more electricity from a given fuel input than a comparable simple-cycle arrangement that does not use exhaust heat for another power cycle.
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Steam condenses and returns to the cycle
In a conventional steam loop, steam leaving the steam turbine is condensed and water returns to the HRSG. A condenser and cooling system commonly support this return, but plant cooling arrangements differ; there is no single configuration established as universal. After heat recovery, the flue gas exits the plant through the stack.
What the major equipment does
| Equipment | Role in the process |
|---|---|
| Compressor | Pressurizes incoming air for combustion; it is driven by the gas turbine. |
| Combustion system | Mixes fuel and compressed air and releases heat through combustion. |
| Gas turbine and generator | Convert energy in expanding hot gas into mechanical power and the first electrical output; the turbine also drives the compressor. |
| Heat recovery steam generator (HRSG) | Transfers heat from gas-turbine exhaust to water and steam circuits. |
| Steam turbine and generator | Use steam produced in the HRSG to make additional electricity. |
| Condenser and cooling system | In a conventional steam loop, condense steam leaving the turbine so water can be returned; cooling layouts vary. |
| Stack | Releases the flue gas after it has passed through the heat-recovery process. |
The equipment has distinct jobs. The gas turbine uses combustion directly to make shaft power; the HRSG recovers heat; and the steam turbine converts steam energy into another shaft-power output. The Energy Information Administration (EIA) glossary defines combined cycle as using otherwise lost gas-turbine exhaust heat in a boiler or HRSG to generate steam for a steam turbine, a process it says increases generating-unit efficiency.
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Why combined cycle can use fuel more efficiently
A gas turbine’s exhaust carries heat after the first generator has produced electricity. In a simple-cycle arrangement, that exhaust heat is not used to drive a steam power cycle. In a combined-cycle arrangement, the HRSG transfers some of it to water, and the resulting steam drives a second turbine. The gain comes from extracting another electrical output from heat that would otherwise leave with the exhaust.
Heat recovery is not complete: not all the fuel’s energy becomes electricity, and not all exhaust energy is captured. Nor does the name “combined cycle” guarantee the same efficiency at every facility. Plant configuration and operating conditions matter, as does the basis used to express fuel energy. Efficiency comparisons should identify whether the fuel is measured on a higher-heating-value (HHV) or lower-heating-value (LHV) basis, and whether the number is a design rating or observed performance.
For context, DOE gives a 20–35% efficiency range for simple-cycle gas turbines in its gas-turbine explainer. That figure is for simple-cycle units; it is not a combined-cycle fleet average. NETL’s advanced combustion turbine program describes a research objective of greater than 65% combined-cycle efficiency on an LHV natural-gas benchmark, alongside load-following capabilities. That is a development target, not evidence that the installed fleet generally operates above 65%.
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Plant features and fleet statistics need context
Some combined-cycle plants include duct burners, which add energy to gas-turbine exhaust. They are a possible plant feature, not a defining requirement of every combined-cycle design. In a June 2022 U.S. fleet snapshot, EIA reported that about 75%—208 GW out of 278 GW—of U.S. combined-cycle capacity had duct burners. Those figures describe U.S. capacity at that time, not the share of plants worldwide or a universal design rule. See EIA’s explanation of duct burners in combined-cycle plants.
Utilization is also different from efficiency. EIA reported that the U.S. combined-cycle fleet’s capacity factor rose from 40% in 2008 to 57% in 2022. Capacity factor describes how much electricity a fleet generated over time relative to its potential output if it had run at full capacity continuously. It does not measure how efficiently fuel was converted into electricity. The comparison is a historical U.S. fleet statistic, not a performance figure for an individual plant. EIA’s 2023 analysis of combined-cycle utilization discusses this trend.
What the basic process does not tell you
The cycle explains how electricity is generated, but it does not establish the cost, emissions rate, water requirement, start-up time, or load-following performance of a particular plant. Those outcomes depend on the facility and operating conditions, so a general description cannot supply a defensible plant-specific figure. Comparisons of actual facilities need the same measures on the same basis, including net electrical efficiency and its HHV or LHV basis, output, operating flexibility, fuel and delivered fuel cost, cooling arrangement and water use, and emissions rates.
Natural-gas combustion produces flue gas. A carbon-capture system, where installed, would separate CO2 from a plant’s flue-gas stream; the existence of a capture research program does not mean that carbon capture is standard on combined-cycle plants. Determining whether a particular facility has capture equipment, and what its emissions or water use are, requires plant-specific documentation rather than inference from the combined-cycle label.
How to compare efficiency and operating claims
When a plant or technology is described as “more efficient,” first check what the percentage measures. Thermal efficiency relates electrical output to fuel energy input; capacity factor instead describes output over time compared with the amount a unit could have generated at continuous full capacity. A plant can have a high capacity factor without that number being its efficiency, and the U.S. fleet figures above should not be used to calculate the efficiency of a specific station.
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Then check the fuel-energy convention. HHV and LHV account differently for the energy associated with water vapor in the fuel’s combustion products, so an efficiency percentage on one basis cannot be compared directly with a percentage on the other without accounting for the difference. Also note whether the number is a design rating or measured operating performance, and whether it describes gross or net output. These labels are needed to make a like-for-like comparison rather than treating a single headline percentage as a universal result.
For an actual plant comparison, match the configuration and the conditions as well as the metric. Relevant items include the gas-turbine and steam-cycle arrangement, net output and efficiency basis, start-up and load-following characteristics, fuel type and delivered fuel cost, cooling method and water use, and emissions rates. A general description of the cycle or a fleet statistic cannot rank two named plants on those dimensions. Plant-specific technical or regulatory documents are needed for defensible figures.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What duct burners change—and what they do not
A duct burner adds fuel to the gas-turbine exhaust path, supplying additional heat before the exhaust gases pass through the HRSG. That can add energy for steam generation, but duct burners are not required for the combined-cycle principle: the basic arrangement recovers heat already present in the gas-turbine exhaust. The EIA figure—208 GW of 278 GW of U.S. combined-cycle capacity in 2022—counts capacity associated with duct burners, not a share of all worldwide plants and not a measure of how much extra electricity each burner produces.
Likewise, an advanced turbine program’s efficiency objective should be read as a research goal. NETL describes a target above 65% on an LHV natural-gas basis together with load-following capabilities. It does not establish the operating efficiency or flexibility of every existing plant, nor does the target by itself say how a given unit performs at part load.
Frequently Asked Questions
What is the difference between a simple-cycle and a combined-cycle gas plant?
A simple-cycle plant generates electricity with a gas turbine. A combined-cycle plant also routes the turbine’s hot exhaust through an HRSG to make steam for a separate steam turbine, creating a second electrical output from some of the exhaust heat.
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Does the HRSG burn fuel or turbine exhaust?
The HRSG transfers heat from gas-turbine exhaust to water and steam circuits. It is a heat-recovery boiler system, not a second combustion turbine. Some plants have duct burners that add energy to the exhaust path, but they are not a universal feature.
Do all combined-cycle plants have duct burners?
No. Duct burners are an optional feature. EIA reported that about 75% (208 GW of 278 GW) of U.S. combined-cycle capacity had them in 2022; that dated U.S. capacity figure is not a worldwide share.
Is capacity factor the same as efficiency?
No. Efficiency describes how fuel energy is converted into electricity. Capacity factor compares electricity generated over time with what a unit could generate at continuous full capacity. EIA’s reported rise in U.S. combined-cycle fleet capacity factor from 40% in 2008 to 57% in 2022 is a utilization trend, not an efficiency figure.
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No. Results vary with plant configuration and operating conditions, and comparisons need a stated HHV or LHV fuel basis and whether the figure is a design rating or measured performance. NETL’s greater-than-65% LHV objective is a research target, not an installed-fleet average.
Are emissions, water use, cost, and start-up time fixed by the combined-cycle design?
No plant-specific values for those measures follow from the cycle description alone. They depend on the facility and operating conditions; compare plants using operator or regulatory documentation and the same measures and basis.
Does a combined-cycle gas plant automatically capture carbon dioxide?
No. Natural-gas combustion produces flue gas, and carbon capture would require equipment to separate CO2 from that stream. Carbon capture should not be assumed unless documentation confirms it is installed at the particular plant.
The Bottom Line
A combined-cycle gas plant first generates electricity with a gas turbine, then uses some of its hot exhaust heat in an HRSG to make steam for a second turbine. That additional output can improve fuel use compared with simple cycle, but efficiency, capacity factor, emissions, water use, and operating flexibility are distinct measures that require plant-specific context.
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