combined-cycle

name	combined-cycle
description	Combined cycle power plant — Brayton + Rankine coupling, HRSG design, bottoming cycle efficiency, F-class gas turbines, overall efficiency, part-load performance, CCGT, ASME PTC-22.
metadata	{"priority":7,"promptSignals":{"phrases":["combined cycle","CCGT","combined cycle power","HRSG","gas turbine steam turbine cycle","bottoming cycle"],"minScore":3}}

Combined Cycle Power Plant — Complete Skill

Cycle Configuration

Topping cycle: Brayton (gas turbine); air → compress → combust → expand → exhaust at 500–600°C Bottoming cycle: Rankine (steam turbine); HRSG recovers exhaust heat → steam → expand → condense Coupling: HRSG (Heat Recovery Steam Generator) thermally couples Brayton exhaust to Rankine

Combined cycle efficiency: η_CC = η_GT + (1 - η_GT) × η_bottoming × η_HRSG η_GT = 38–42% (modern F-class); η_bottoming = 35–40% (steam side); η_HRSG = 0.95 (heat recovery) η_CC = 0.40 + 0.60 × 0.37 × 0.95 ≈ 0.61 (61% LHV — typical for 1×1 F-class CCGT)

State-of-the-art H-class turbines: η_CC = 63–64% (GE HA, Siemens H, Mitsubishi J); TIT = 1600–1700°C

Gas Turbine Performance

Compressor pressure ratio: PR = 15–25 (F-class); up to 23 (GE 7HA.02) Turbine Inlet Temperature (TIT): 1400–1600°C (F-class); 1600–1700°C (H-class); limited by blade cooling/material

Brayton cycle efficiency (ideal): η_GT_ideal = 1 - T₁/T₂ = 1 - PR^(-(γ-1)/γ) [T₁ = inlet; T₂ = after compression] For PR = 18, γ = 1.4: η_ideal = 1 - 18^(-0.286) = 1 - 0.546 = 45.4% Actual: 38–42% after compressor/turbine isentropic losses and cooling

Exhaust temperature: T_exhaust = T_TIT / PR^((γ-1)/γ) × η_turbine ≈ 550–650°C (F-class) Higher T_exhaust → more heat to HRSG → better CC efficiency

HRSG (Heat Recovery Steam Generator)

Multi-Pressure HRSG

Triple-pressure (most common for large CCGT): High pressure (HP): 120–170 bar; superheated steam 540–580°C → HP turbine Intermediate pressure (IP): 20–40 bar; reheated steam 540–565°C → IP turbine Low pressure (LP): 3–6 bar; saturated or slightly superheated → LP turbine + condenser

Energy balance at each HRSG section: Q_HRSG = m_exhaust × c_p_gas × (T_in - T_out) [kW; m_exhaust from GT; c_p_gas ≈ 1.07 kJ/(kg·K)] Q_steam = m_steam × (h_superheat - h_feed) [kW; h from steam tables]

Pinch temperature: ΔT_pinch = T_gas at saturation exit - T_saturation of steam [minimum 15–20°C for each pressure level] Smaller pinch: more heat recovered; larger HRSG surface needed

Stack temperature: T_stack ≥ 80–100°C (no SCR); ≥ 110°C (with SCR ammonia injection to avoid ABS deposits) Acid dew point limit: ≥ 120°C if sulfur in fuel

Heat Transfer in HRSG

Overall UA: Q = U × A × LMTD [kW] U_evaporator ≈ 30–50 W/(m²·K) [boiler; tube outside/inside convection + thin wall conduction] U_superheater ≈ 15–25 W/(m²·K) [gas-to-gas steam] U_economizer ≈ 35–60 W/(m²·K) [gas-to-water]

Module areas: A = Q / (U × LMTD) [m²; sum for each section]

Steam Turbine (Bottoming Cycle)

Three-cylinder layout: HP turbine: inlet 120–170 bar, 540°C → exhaust 20–40 bar (to IP reheater) IP turbine: inlet 20–40 bar, 540°C → exhaust 3–8 bar LP turbine: inlet 3–8 bar, 180–200°C → condenser 50–80 mbar

Rankine efficiency: η_Rankine = W_turbine / Q_HRSG η_Rankine = 35–42% for modern triple-pressure reheat cycle

Condenser: Condensing pressure: 50–80 mbar abs (45–45°C saturation) for sea water cooling Air-cooled condenser: 80–120 mbar (higher exhaust losses; dry climate)

Part-Load Performance

At part load (gas turbine throttled): Lower mass flow + lower firing temperature → lower T_exhaust → HRSG produces less steam Inlet Guide Vanes (IGV): modulate GT air flow; maintain T_exhaust at reduced load → better part-load efficiency

HRSG bypass: not common; duct burner used instead to add heat when more steam needed

Duct burner: Supplemental firing in HRSG duct; heats gas above GT exhaust T Useful for grid peaking (adds steam output without new GT); η_duct_burn ≈ 80–85%

Combined Cycle Output

Net electrical output: W_net = W_GT + W_ST - W_auxiliary [MW] Auxiliary: BFP (boiler feedwater pump), cooling tower fans, condenser pumps, controls Auxiliary ≈ 1.5–3% of total output

Heat rate: HR = 3600 / η_CC [kJ/kWh]; HR_CC ≈ 5800–6000 kJ/kWh (for η ≈ 0.60–0.62) Heat rate HHV vs. LHV: natural gas HHV/LHV = 1.106; convert as needed

ASME PTC-22 Performance Testing

GT performance test: corrects measured output to ISO conditions (15°C, 60% RH, 1.013 bar) Corrections: temperature (1°C ≈ 0.5–0.9 MW power loss); humidity; inlet losses Acceptance tolerance: ±1% on output; ±0.5% on heat rate CC test: tests GT and ST separately + jointly; measure HRSG temperatures

Standards

Standard	Scope
ASME PTC-22	Gas turbine performance testing
ASME PTC-6	Steam turbine performance testing
ASME PTC-4.4	HRSG performance testing
IEC 60045-1	Steam turbines; requirements
ISO 2314	Gas turbines; acceptance tests

Output

Provide: GT model (F/H class), TIT [°C], pressure ratio, GT net power [MW] and efficiency η_GT [%], exhaust temperature [°C], steam levels (HP/IP/LP pressure [bar] and temperature [°C]), HRSG pinch temperature [°C], stack temperature [°C], steam mass flows [kg/s] at each level, steam turbine output W_ST [MW], CC net efficiency η_CC [%], CC net output W_net [MW], part-load performance strategy (IGV/duct burner), and applicable standard (ASME PTC-22, ASME PTC-6).