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Power Generation
HRSG (Heat Recovery Steam Generator) Simulator
A gas turbine throws away exhaust at 600°C. A combined-cycle plant (CCGT) catches that heat in an HRSG, raises steam and spins a second turbine. Move the pressure configuration, pinch point and feedwater temperature on the left and watch the recovered heat, steam flow, steam-turbine output and combined-cycle efficiency update in real time.
Parameters
GT exhaust flow
kg/s
Mass flow rate of gas turbine exhaust
GT exhaust temperature
°C
Gas temperature at HRSG inlet (modern H-class: 640-660°C)
HRSG pressure configuration
Number of pressure levels — more levels means higher recovery but higher cost
HP steam pressure
bar
IP steam pressure
bar
LP steam pressure
bar
Feedwater temperature
°C
Feedwater temperature leaving the deaerator and entering the economizer
Pinch point ΔT
°C
Minimum gas-to-steam temperature difference at the evaporator inlet
GT exhaust enters from the left and leaves through the stack on the right. Feedwater enters at bottom-right, is heated through the economizer (green) → evaporator (orange) → superheater (red), and leaves at top-left as HP steam toward the steam turbine.
T-Q diagram (gas-side vs steam-side profile)
Efficiency by configuration (single / dual / triple+reheat)
Q_rec: recovered heat (kW), ṁ_exh: GT exhaust mass flow (kg/s), c_p: exhaust specific heat (≈1.05 kJ/kg·K), T_in / T_stack: HRSG inlet and stack temperatures (°C). η_GT, η_HRSG and η_ST are the GT, HRSG and steam-cycle efficiencies, and η_CC is the resulting combined-cycle efficiency.
Simplified steam-generation and stack-temperature expressions. h_steam ≈ 3300 kJ/kg (HP superheated steam enthalpy), h_fw ≈ 251 kJ/kg (60°C feedwater), and k_conf is the pressure-configuration multiplier (single 1.0 / dual 1.15 / triple+reheat 1.25).
HRSG (Heat Recovery Steam Generator) — Combined-Cycle Power Generation
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I keep hearing "combined cycle" for modern power plants. I get that it uses both a gas turbine and a steam turbine, but why does stacking them like that raise the efficiency?
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Good question. A gas turbine burns at maybe 1500°C, but the exhaust still leaves around 600°C. Dump that straight up the stack and you waste more than half your energy. So the trick is "since that hot exhaust is sitting right there, let's boil water with it and spin a steam turbine too" — that is combined-cycle gas turbine, CCGT. The box that boils water from the exhaust is exactly the HRSG (Heat Recovery Steam Generator). GT alone gets 40%, but GT + HRSG + ST together hit 62%. That's why CCGT dominates modern LNG-fired power generation worldwide.
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What's inside an HRSG? How is it different from a regular boiler?
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A regular boiler burns fuel and uses a flame to heat water. An HRSG has no flame. The hot GT exhaust just flows down a long duct, and inside that duct sit three banks of finned tubes — economizer (feedwater heating), evaporator (boiling) and superheater (raising the steam temperature) — soaking heat out of the gas as it passes. Try switching "HRSG pressure configuration" on the left. Single-pressure has just one HP (high-pressure) level. Dual-pressure adds an LP (low-pressure) circuit. Triple-pressure with reheat has HP + IP + LP plus an IP reheat. Each extra pressure level squeezes more heat out: recovery efficiency climbs from about 80% to 88% to 93%.
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Then why not just always pick triple-pressure with reheat? Why do single-pressure plants still exist?
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Pure cost. Triple-pressure with reheat triples the number of circuits, so boilers, piping, valves and steam-turbine stages all multiply. Capex jumps about 1.5×. So the industry splits: small industrial cogen (under ~60 MW) is single-pressure, mid-size units (100-200 MW) go dual, and large utility LNG plants (300 MW+) use triple-pressure with reheat. Only big units that run many hours per year can pay back the extra capex through fuel savings.
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There's a "pinch point" parameter. Making it smaller raises efficiency, but what exactly is it?
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The pinch point is the headline concept in HRSG design. It's the place at the evaporator inlet where the temperature gap between the exhaust gas and the steam side is smallest. Shrink ΔT from 10°C to 5°C and you recover more heat — stack temperature falls. The catch is that the required heat-transfer surface explodes as ΔT shrinks. To go to 5°C you need more than double the finned-tube area, and cost scales the same way. So ΔT = 8-12°C is the economic sweet spot. The tool defaults to 10°C for that reason. Squeeze the pinch on the T-Q diagram and you'll see the gas line and steam line run almost parallel — heat transfer is "tight".
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So it's an area trade-off. One last question — how high have modern plants pushed combined-cycle efficiency?
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Since around 2020, Mitsubishi Heavy Industries' M501JAC, GE's H-class and Siemens' HL-class have all hit CC efficiency above 64%. That requires pushing GT inlet temperature to 1650°C, raising exhaust to roughly 660°C, and pairing it with a triple-pressure reheat HRSG that wrings out every last bit of heat. Going past 65% is thermodynamically tough; from here on, the gains come from optimizing every subsystem (deaerator heating, condenser vacuum, …). Try this tool with GT exhaust 660°C, triple-pressure with reheat and pinch 8°C — you should see 63-64%. That's the leading edge of CCGT today.
Frequently Asked Questions
An HRSG (Heat Recovery Steam Generator) is a boiler that uses the 500-700°C exhaust of a gas turbine (GT) as its heat source to raise steam for a steam turbine (ST). A standalone GT delivers only 35-42% efficiency, but combining it with a steam bottoming cycle through an HRSG (combined-cycle gas turbine, CCGT) lifts plant efficiency to 55-62% and above 64% in modern H/HL-class units. The HRSG contains three sections — economizer (feedwater preheat), evaporator and superheater — and adding more pressure levels (single, dual, triple-pressure with reheat) extracts more heat from the exhaust gas.
Single-pressure is the simplest: only an HP (high-pressure) level, leaving stack temperature relatively high and recovery efficiency around 80%. Dual-pressure adds an LP (low-pressure) level, pulling stack temperature down for 88-92% recovery. Triple-pressure with reheat (HP + IP + LP plus IP reheat) reaches the highest recovery (93-95%) but requires far more heat-transfer surface and equipment, raising cost. As a rule of thumb, single-pressure suits GTs up to ~60 MW, dual-pressure suits the 100-200 MW class, and 300 MW+ utility-scale CCGT plants use triple-pressure with reheat.
The pinch point is the location at the evaporator inlet where the temperature difference between the gas side and the steam side is smallest. It is the single most important parameter for HRSG economics: a small ΔT (e.g. 5°C) recovers more heat, but the required heat-transfer area grows roughly inversely with ΔT, so cost balloons. A large ΔT (e.g. 20°C) raises stack temperature and wastes heat. In real plants the economic optimum sits at ΔT = 8-12°C; this tool defaults to 10°C. Plants with high fuel cost and long running hours go smaller; peakers go larger.
Combined efficiency is η_CC = η_GT + (1 − η_GT)·η_HRSG·η_ST. η_GT is the standalone gas-turbine efficiency (40-42% for modern units), η_HRSG is the fraction of exhaust heat recovered on the steam side (80-95%) and η_ST is the steam-cycle efficiency (35-45%). With η_GT=0.40, η_HRSG=0.88 and η_ST=0.42 you get η_CC = 0.40 + 0.60·0.88·0.42 ≈ 0.62 (62%). Pushing GT efficiency higher lowers the exhaust temperature entering the HRSG, leaving less for the steam side — so for the latest 1600°C-class GTs, careful HRSG optimization becomes even more important.
Real-World Applications
Large-scale LNG-fired power plants: Modern 1000-1500 MW LNG-fired stations operated by major utilities use one to three trains of GT + HRSG + ST. Plants based on Mitsubishi Heavy Industries' M501JAC, GE's 9HA.02 or Siemens' SGT5-9000HL achieve combined-cycle efficiencies above 64% with triple-pressure reheat HRSGs. Examples include JERA's Kawasaki Unit 2 (1500 MW, 61% efficiency) and Kansai Electric's Himeji No. 2 (2919 MW, 60%).
Industrial cogeneration: Steel mills, chemical plants and pulp & paper mills use small- to mid-size HRSGs in cogeneration that supplies both on-site power and process steam. A 10-60 MW industrial gas turbine paired with a single-pressure HRSG can reach overall thermal utilization above 80% by delivering electricity plus process steam. Here the HRSG steam pressure is sized to match plant steam demand, so the optimum differs from pure power generation.
Offshore oil & gas platforms: Space-constrained North Sea and Middle East offshore platforms favour compact horizontal HRSGs (shorter than vertical types). They supply electricity and water-injection steam from the same exhaust gas, minimizing footprint while maximizing operating efficiency. Marine duty drives the choice of corrosion-resistant tube materials such as SUS316L or Incoloy 825.
Grid balancing and peaking duty: As renewables (solar, wind) penetrate the grid, CCGT plays a key flexible-generation role. Once-through steam generators (OTSG) are often chosen because they start much faster than conventional drum-type HRSGs — full load within ~30 minutes from a cold start. Combined with high efficiency, this fast-start capability has made modern CCGT the workhorse of balancing markets in the US (PJM) and Europe.
Common Misconceptions and Pitfalls
The most common pitfall is "smaller pinch point ΔT is always better". Going from 10°C to 5°C does recover a few percent more heat, but the heat-transfer surface required scales roughly inversely with ΔT — the finned-tube area for a 5°C design is more than double that of a 10°C design, and cost scales the same way. Small-ΔT designs are also more prone to flow instability and evaporator overshoot at part load, making operation tighter. The economic sweet spot is ΔT = 8-12°C, set from fuel price and running hours. Try comparing ΔT = 5°C and 20°C in this tool: the gap in real-world feasibility is much larger than the gap in recovered heat alone.
Next, "you can always push stack temperature lower". In theory, cooling stack gas down to feedwater temperature would mean 100% heat recovery, but reality has a floor. When LNG combustion products cool below their water dew point (around 50°C), water vapour condenses; combined with trace sulphur in the fuel it produces low-temperature acid-dew corrosion. So stack temperature is typically held above 90-120°C. The expression T_stack = T_fw + 0.7·ΔT_pinch used here is a rule-of-thumb fit; real plants enforce the corrosion floor separately. With 60°C feedwater and ΔT=10°C the formula gives 67°C, but a real plant would set the stack to 100°C or more for safety.
Finally, "better GT efficiency means a better CCGT by the same amount" is a mistake. Look at η_CC = η_GT + (1 − η_GT)·η_HRSG·η_ST: as η_GT rises, (1 − η_GT) shrinks, leaving less waste heat for the HRSG and ST to recover. Pushing GT efficiency from 40% to 42% only lifts CC efficiency from 62.2% to 63.0% — a gain of just 0.8 points. Worse, higher-efficiency GTs put out slightly lower exhaust temperatures (600°C → 580°C), so the steam side gets a colder source. The 64%+ figures from H/HL-class plants come from optimizing GT, HRSG and ST together; simply swapping in a newer GT will not deliver as much as expected.
How to Use
Enter gas turbine exhaust flow rate (kg/s) and temperature (°C) from your GT datasheet—typical range 400–650 kg/s at 580–620°C for a 150 MW unit.
Set high-pressure (HP) and intermediate-pressure (IP) steam drum pressures in bar; common values are 120 bar (HP) and 30 bar (IP) for CCGT plants.
The simulator calculates stack temperature (typically 90–110°C), recovered heat duty (MW), steam mass flow through each pressure level, steam turbine output (MW), and combined-cycle thermal efficiency (%).
Worked Example
A 150 MW gas turbine exhausts 420 kg/s at 610°C. HRSG is configured with HP steam at 120 bar and IP steam at 30 bar. The pinch point (minimum temperature approach) is set to 15°C. Simulator outputs: stack temperature 95°C, recovered heat 185 MW, HP steam generation 48 kg/s, IP steam generation 38 kg/s, steam turbine output 65 MW, and combined-cycle efficiency 58.2%. Total CCGT output is 215 MW (GT 150 MW + ST 65 MW).
Practical Notes
Pinch point (evaporator approach) of 15–20°C governs HRSG size and cost; tighter pinch demands larger surface area but improves cycle efficiency by 1–2 percentage points.
Stack temperature below 100°C risks acid condensation in flue ducts; check dew point of exhaust gas (typically 55–65°C for natural gas combustion) and maintain margin.
Three-pressure HRSG (HP/IP/LP) recovers 3–5% more heat than two-pressure; LP steam is often recycled as feedwater heater or sent to absorption cooling.
Fouling on gas side reduces effective heat transfer by 5–10% over 3 years; plan cleaning outages and account for duty loss in long-term performance contracts.