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Combined Cycle Gas Turbines (CCGT): A Deep Dive into Efficiency and Innovation

Combined Cycle Gas Turbines (CCGT)

When it comes to modern power generation, Combined Cycle Gas Turbines (CCGT) stand out as a pinnacle of efficiency and engineering ingenuity.

These systems are revolutionizing how we produce electricity, blending the power of gas and steam turbines to maximize output while minimizing environmental impact.

Whether you’re an engineer, a plant manager, or simply curious about cutting-edge energy solutions, this article will take you through everything you need to know about CCGT technology—from its core principles to real-world applications and advanced solutions like those offered by TURBIVAP.

Let’s explore the fascinating world of CCGT plants, breaking down their operation, benefits, challenges, and the latest technological updates that keep them at the forefront of the energy sector.

What is a Combined Cycle Gas Turbine (CCGT)?

A Combined Cycle Gas Turbine (CCGT) is an advanced power generation system that integrates two thermodynamic cycles—the Brayton cycle (gas turbine) and the Rankine cycle (steam turbine)—to produce electricity with exceptional efficiency.

Unlike traditional single-cycle plants, which rely solely on a gas turbine or steam turbine, CCGT technology captures waste heat from the gas turbine exhaust and uses it to generate additional power via a steam turbine.

This dual-cycle approach can achieve thermal efficiencies of up to 60-64%, far surpassing the 33-43% efficiency of simple-cycle gas turbines.

At its core, a CCGT plant consists of a gas turbine, a heat recovery steam generator (HRSG), and a steam turbine, all working in harmony.

The gas turbine burns fuel (typically natural gas) to produce electricity, while the HRSG recovers the exhaust heat to create steam, which then drives the steam turbine for additional power generation.

This synergy makes CCGT systems a preferred choice for large-scale power plants aiming to optimize fuel use and reduce greenhouse gas emissions.

How Does a Combined Cycle Gas Turbine Work?

The operation of a Combined Cycle Gas Turbine is a marvel of thermodynamic efficiency. Here’s a step-by-step breakdown of how it functions:

Gas Turbine Operation (Brayton Cycle):
Air is compressed in the gas turbine’s compressor to a high pressure (typically 8-16 pressure ratio). This compressed air mixes with fuel in the combustion chamber, where it ignites, producing hot, high-pressure gases (up to 1,700°C). These gases expand through the turbine, spinning its blades to drive a generator and produce electricity. The exhaust gases, still hot at around 500-600°C, exit the turbine.

Heat Recovery Steam Generator (HRSG):
Instead of letting the exhaust heat dissipate, the CCGT system channels it into an HRSG. This sophisticated heat exchanger uses the residual heat to boil water and produce high-pressure steam. Depending on the plant design, the HRSG may operate at multiple pressure levels (e.g., triple-pressure with reheat) to maximize steam production efficiency.

Steam Turbine Operation (Rankine Cycle):
The steam from the HRSG flows into a steam turbine, where it expands and drives another generator to produce additional electricity. After expansion, the steam is condensed back into water and recycled into the HRSG, completing the cycle.

This combined cycle process ensures that nearly every bit of energy from the fuel is utilized, making CCGT plants a leader in energy efficiency.

Applicability of Combined Cycle Gas Turbines

The versatility of CCGT technology makes it applicable across various sectors. Here’s where it shines:

Large-Scale Power Generation:
Onshore CCGT plants are widely used for baseload power, providing a stable electricity supply to grids. Their high efficiency (up to 64% in optimal conditions) and lower CO2 emissions (around 0.34 tonnes/MWh compared to 0.52 tonnes/MWh for open-cycle gas turbines) make them ideal for utilities transitioning to cleaner energy.

Offshore Installations:
Offshore oil and gas platforms, use CCGT systems to power operations with efficiencies around 50%. These installations benefit from reduced fuel consumption in space-constrained environments.

Industrial Combined Heat and Power (CHP):
In industries requiring both electricity and heat (e.g., chemical plants), CCGT plants can operate in CHP mode, achieving overall efficiencies up to 90% by supplying steam for heating processes alongside power generation.

From urban power grids to remote offshore rigs, CCGT technology adapts to diverse energy demands with remarkable flexibility.

What is a Steam Turbine?

A steam turbines is a mechanical device that extracts thermal energy from pressurized steam and converts it into mechanical work.

This mechanical energy can then be used to drive electrical generators, pumps, or compressors. Steam turbines are widely regarded for their efficiency and adaptability, making them a cornerstone in power generation and industrial processes.

Steam turbines consist of a series of rotors and stators. Steam flows through these components, causing the rotor blades to spin and generate rotational energy.

Their versatility allows them to operate in various environments, from small-scale industrial applications to massive utility-scale power plants.

"A steam turbine is a mechanical device that extracts thermal energy from pressurized steam and converts it into mechanical work."

Principal Aspects of CCGT Systems

Several key aspects define Combined Cycle Gas Turbines and their performance:

Thermal Efficiency:
By integrating two cycles, CCGT plants achieve efficiencies of 50-64%, depending on turbine size, ambient conditions, and design (e.g., triple-pressure HRSG with reheat circuits). This is a significant leap over the 35-45% efficiency of open-cycle gas turbines (OCGT).

Fuel Flexibility:
While natural gas is the primary fuel, advancements like integrated gasification combined cycle (IGCC) allow CCGT systems to use coal or heavy oil, broadening their applicability.

Environmental Impact:
CCGT plants emit fewer pollutants (e.g., NOx, SOx) and CO2 compared to coal-fired plants, aligning with global decarbonization goals. However, they still produce emissions, driving research into carbon capture integration.

Operational Flexibility:
Gas turbines in CCGT setups offer rapid startup (as quick as 30 minutes), making them suitable for load-following to support intermittent renewables like wind and solar. However, efficiency drops at partial loads, a trade-off for this flexibility.

These aspects highlight why CCGT technology remains a cornerstone of modern energy systems.

Principle of Functioning: The Thermodynamics Behind CCGT

The Combined Cycle Gas Turbine operates on two fundamental thermodynamic cycles:

Brayton Cycle (Gas Turbine):
This cycle involves compression, combustion, and expansion. Air enters the compressor at ambient conditions (e.g., 15°C, 1 atm), is compressed to 8-16 times its initial pressure, and heated to 1,200-1,700°C during combustion. The resulting gas expands through the turbine, producing work. The efficiency of this cycle alone is typically 33-43%, limited by exhaust heat loss.
Rankine Cycle (Steam Turbine):
The HRSG captures the exhaust heat (500-600°C) to produce steam at pressures up to 120 bar. This steam expands through the steam turbine, generating additional work. The cycle’s efficiency depends on steam conditions (e.g., superheated steam at 540°C improves output).

The genius of CCGT technology lies in coupling these cycles. The HRSG acts as a bridge, converting waste heat into usable energy, pushing the overall system efficiency beyond what either cycle could achieve alone.

Advanced designs incorporate variable geometry (e.g., inlet guide vanes) and reheat circuits to optimize performance across load ranges.

Common Problems and Solutions in CCGT Systems

Despite their advantages, CCGT plants face challenges. Here’s a look at the most common issues and solutions:

Corrosion and Fatigue in HRSG Tubes:
- Problem: Offshore plants often experience tube failures due to poor water quality (e.g., high pH or oxygen levels) and maloperation, leading to corrosion and fatigue.
- Solution: Rigorous water treatment (e.g., deoxygenation, pH control) and adherence to operational procedures reduce risks. Advanced materials like stainless steel or Inconel enhance durability.

Efficiency Drop at Partial Load:
- Problem: Efficiency falls to 40-50% at partial loads, common when backing up renewables.
- Solution: Inlet guide vane control (IGVC) or fuel flow control (FFC) optimizes part-load performance. Simulation-based optimization, as studied by Haglind (2011), further refines strategies.

High Ambient Temperature Impact:
- Problem: In hot climates, air density decreases, reducing turbine output.
- Solution: Turbine Inlet Air Cooling (TIAC) technologies, like evaporative cooling or ice storage, boost performance by precooling combustion air.

Comparing Solutions:

Water Treatment vs. Material Upgrades: Water treatment is cost-effective but requires ongoing maintenance, while advanced materials offer longevity at a higher upfront cost.

IGVC vs. FFC: IGVC provides better regulation capacity, but FFC is simpler and cheaper for smaller plants.

TIAC Options: Evaporative cooling is affordable but less effective in humid climates; ice storage offers greater efficiency gains at higher capital costs.

Technological Updates Technological Updates in CCGT Systems

Recent advancements keep CCGT technology ahead of the curve:

Higher Turbine Inlet Temperatures: Research targeting 1,370°C (2,500°F) inlet temperatures (e.g., Siemens SGT5-8000H) pushes efficiencies to 60.75%.

Autonomous Operation: Mitsubishi’s T-Point 2 plant uses AI for fully autonomous CCGT management, reducing personnel needs and optimizing maintenance.

Carbon Capture Integration: Trials of integrated gasification combined cycle (IGCC) with carbon capture aim for near-zero emissions, expanding fuel options to coal and hydrogen.

These updates ensure CCGT plants remain relevant in a decarbonizing world.

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Steam Turbine

Key Features of Steam Turbines

High Efficiency: Steam turbines boast thermal efficiency rates of up to 50%.
Wide Range of Capacities: Models range from a few kilowatts to hundreds of megawatts.
Durability: Designed to operate continuously under high-pressure and high-temperature conditions.
Scalability: Suitable for small industrial plants to large power stations.
Environmentally Friendly Options: Compatible with renewable steam sources, such as geothermal or biomass plants.

Applications of Steam Turbines

Steam turbines is used in a variety of settings:

- Power Plants: Coal, natural gas, nuclear, and geothermal power stations.
- Oil and Gas Industry: Driving compressors in refineries and pipelines.
- Chemical and Pharmaceutical Plants: Supporting manufacturing processes.
- Desalination Plants: Powering systems that convert seawater into freshwater.

Principle of Operation Steam Turbines

The operation of a steam turbine is based on thermodynamic principles, particularly the Rankine cycle:

High-pressure steam enters the turbine through nozzles.
The steam expands and loses energy, causing the rotor blades to spin.
The rotational energy is transmitted to a shaft, which drives an external load, such as an electric generator.
Low-pressure steam exits the turbine, often utilized in heat recovery systems or condensed into water for reuse.

Comparing Steam Turbine Solutions

Criteria	Condensing Turbine	Backpressure Turbine
Primary Use	Power generation	Industrial heat and power
Efficiency	Higher thermal efficiency	Optimized for cogeneration
Steam Usage	Fully exhausted	Partially used
Cost	Higher upfront cost	More economical

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DIGITAL STEAM TURBINE COURSE ADVANCED MODULE

See the topics in the tab below. Total course load 25h.

*Includes the Essential Module

SUBJECTS / CLASSES

ESSENTIAL – Module 1

Start Here
Features and Tools
Course Presentation
Introduction
Application
Rankine Cycle – Transforming Heat into Work
Thermal Cycle Equipment
Types of Steam Turbines
Condensing Turbine
Backpressure Turbine
Types of Extraction
Speed Diagram
Principles of Action and Reaction
Stage / Action Turbine (Impulse)
Stage / Reaction Turbine
Curtis Type Stage
Rateau Type Stage

ESSENTIAL – Module 2

Single and Multistage Turbines
Direction of Rotation
LOA-LA / HP-LP / AP-BP Concept
Components
Components – Steam Chamber
Components – Emergency Valve
Components – Partializing Valve
Components – Casings
Components – Injector / Expander
Components – Rotor
Components – Vanes
Components – Belt
Components – Fixed Vanes (Reverser)
Components – Diaphragm
Components – Diaphragm Holder
Components – Vane Holder
Components – Radial Bearing
Components – Axial Bearing
Components – Bearing Housing
Components – Vapor Sealing System
Components – Vapor Sealing – Carbon Ring
Components – Vapor Sealing – Labyrinth Ring
Components – Vapor Sealing – Sealing Blades
Components – Oil Sealing
Components – Mechanical Speed Regulator
Components – Electronic Speed Regulator
Components – Actuator
Components – Coupling
Components – Reducer
Components – Slow Rotation
Components – Lubrication System
What is TRIP?
Condensation System
Condensation System – Devices
Certificate
Final Considerations and Certificate

ADVANCED – Module 1 – Introduction and Fundamentals

Start Here
Course Presentation
Introduction
Historical Context
Application
Waste To Energy – Energy Recovery Plant “URE”
Module 1 Handout

ADVANCED – Module 2 – Basic Thermodynamics for Steam Turbines

Thermodynamic Characteristics
Steam Thermal Plant Cycles
Rankine Cycle – Transforming Heat into Work
Thermal Cycle Analysis – Condensation Turbine (Efficiency Comparison)
Thermal Cycle Equipment
Practical Rankine Cycle
TS Diagram
Mollier Diagram
Rankine Cycle Variations
Reheat Cycle
Regenerative Cycle
Combined Cycle
Reuse of Thermal Energy
What is Steam?
Quiz 1
Module 2 Handout

ADVANCED – Module 3 -Classification of Steam Turbines

Classification of Steam Turbines
Types of Steam Turbines
Condensing Turbine
Backpressure Turbine
Pressure Reduction Turbine
Types of Extraction
Operating Principle
Speed Diagram
Principles of Action and Reaction
What is a Stage?
Action (Impulse) Stage / Turbine
Reaction Stage / Turbine
Curtis Type Stage
Rateau Type Stage
Quiz 2
Single and Multistage Turbines
Radial and Axial Turbines
Direction of Rotation
LOA-LA / HP-LP / AP-BP Concept
Applicable Standards
Internal Losses
Turbine Efficiency
Saturation Curve
Quiz 3
Module 3 Handbook

ADVANCED – Module 4 – Components Part 1

Components
Components – Steam Chamber
Components – “Quick Closing” Emergency Valve
Components – “Quick Closing” Emergency Valve Part 2
Components – Partializing Valve
Components – Casings
Torquing Turbine Casings – Hydraulic Tensioner
Sealant for Junction Plane (Housings) and Flanges
Components – Hood Spray System
Components – Injector / Expander
Components – Rotor
Rotor Components – Balance Piston
Rotor Components – Control Wheel (Wheel Chamber)
Rotor Components – Rigid and Flexible
Rotor Components – Runout
Rotor Components – Magnetization
Quiz 4

ADVANCED – Module 5 – Components Part 2
Automatically released after 7 days from the date of purchase.

Rotor Components – Alignment
Rotor Components – Mechanical Alignment
Rotor Components – Piano String Alignment
Rotor Components – Laser Alignment
Rotor Components – Catenary Curve
Rotor Components – Critical Speed
Rotor Components – Vibration
Rotor Components – Balancing
Quiz 5
Balancing – High Speed Balance
Components – Vanes
Components: Vanes: 3D Printing
Components – Belt
Components – Fixed Vanes (Reverser)
Components – Diaphragm
Components – Diaphragm Holder
Components – Vane Holder
Module 5 Handbook

ADVANCED – Module 6 – Components Part 3

Automatically released after 7 days from the date of purchase.

Components – Radial Bearing
Bearing Components – Operating Principle
Radial Bearing Components – Cylindrical
Radial Bearing Components – Pressure Dam
Radial Bearing Components – Multilobular
Radial Bearing Components – Tilting Pad
Radial Bearing Components – Oil Lift
Bearing – General Concepts
Bearing Components – Materials
Bearing Components – Temperature Monitoring
Components – Tilting Pad Bearing – Temperature Monitoring
Radial Bearing Components – Alignment
Radial Bearing Components – Clearance
Radial Bearing Components – Clearance Complement
Components – Radial Bearing – Clearance Check
Axial Expansion
Components – Axial Bearing
Quiz 6

ADVANCED – Module 7 – Components Part 4
Automatically released after 7 days from the date of your purchase

Components – Bearing Housing
Components – Vapor Sealing System
Components – Vapor Sealing – Carbon Ring
Components – Vapor Sealing – Ring Maze
Components – Vapor Sealing – Sealing Blades
Components – Oil Sealing
Turbine Speed Control – Overview
Components – Mechanical Speed Governor
Mechanical-Hydraulic Speed Governor – Deep Dive
Components – Electronic Speed Governor
Components – Actuator
Components – Coupling
Quiz 7
Components – Reducer
Components – Slow Rotation
Components – Thermal / Acoustic Insulation
Components – Base and Foundation

ADVANCED – Module 8 – Systems: Condensation, Control, Safety and Others
Automatically released after 7 days from the date of purchase.

Condensation System – Overview
Condensation System – Devices
What is TRIP?
Types of TRIP – Turbine Disarming
TRIP Disarming Standards and Criteria
Synchronization

ADVANCED – Module 9 – Lubrication System
Automatically released after 7 days from the date of purchase.

Lubrication System – Overview
Lubrication System – Consumers
Lubrication System – Hydraulic Unit
Oil Catcher Ring Applied in Single-Stage Turbines
Oil Contamination Level – NAS Indicator
Limit Concentration of Water in Oil
Flushing in Lubrication System

ADVANCED – Module 10 – Operation, Inspection and Maintenance
Automatically released after 7 days from the date of purchase.

Turbine Start-up and Stop Curves
Turbine Start-up Procedures
Main Materials
NDT – Non-Destructive Testing
Routine Operation and Maintenance
Failure Analysis and Solutions
Performance Analysis and Diagnosis
Assembly Clearances
Questionnaire 8
Certificate
Instructions for issuing your Certificate
Final Considerations and Certificate

Module – Learning About Varnish

Handout 4

Concept, formation, detection, correction and prevention of varnish formation
Introduction to varnish
Types of Turbine Oils
What is varnish?
Varnish Formation
Why the increasing trend in varnish formation? Varnish Presence Limits and Correction
Patent Metal Adhesion Module

Inspection Guide and Technical Standards
Introduction
Patent Metal Application Methods
Patent Metal Application Standards
Details ISO 4386 Part 2 Destructive Testing
Design for ISO 4386 Part 2 Laboratory Testing
Manufacturing Test Specimens for ISO 4386 Part 2 Laboratory Testing
Execution of Destructive Tensile and Compression Testing

Failure Analysis Modules

Steam Turbine Blade – Shear
Failure Analysis of Unexpected Steam Turbine Trips
Remote Diagnostic Analysis – Excessive Vibration Problem Solution
Rotor and Stator Blade Erosion Analysis
Vane Failure – Change in Opening of Partializing Valves
PED Bearing Resonance LOA
Introduction to SCC Failure Analysis in Steam Turbines
Failure Analysis Vibration Peaks Steam Turbines (Turbopumps)

505 Controller Module

Hardware Overview and Front Panel
Installation, Power and Startup Basics
Changing Modes and Languages
Installation, Power and Startup Basics
Navigation Screens
Wiring Connections for Analog and Speed Signals
Wiring Connections for Communication Links
Wiring Connections for Discrete Signals
Ethernet IP Address Settings

SST 800 Module

SST 800 Condensing and Backpressure Steam Turbine Details and Systems
Lubrication System – Hydraulic Unit
Lubrication System – Consumers
Control System
Condensing System
Steam System – Condensing Turbine
Steam System – Backpressure Turbine

SST 400 Module

SST 400 Condensing Steam Turbine Details and Systems
Lubrication System – Hydraulic Unit
Lubrication System – Consumers
Control System
Condensation System

FAQs About Combined Cycle Gas Turbines

What is the efficiency of a CCGT plant?
Typically 50-64%, depending on design and load conditions.
How does CCGT differ from OCGT?
CCGT adds a steam cycle to recover exhaust heat, boosting efficiency over OCGT’s 35-45%.
What fuels can CCGT use?
Primarily natural gas, but IGCC variants support coal or hydrogen.
Why does efficiency drop at partial load?
Reduced mass flow and heat recovery efficiency lower output below design capacity.
How can TIAC improve CCGT performance?
It cools inlet air, increasing density and turbine power output.
What are common maintenance issues?
HRSG tube corrosion and turbine blade wear are frequent, requiring vigilant monitoring.

"Steam flows through these components, causing the rotor blades to spin and generate rotational energy."

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Conclusion

Combined Cycle Gas Turbines (CCGT) represent the gold standard in efficient, flexible power generation.

By marrying gas and steam cycles, they deliver unmatched thermal efficiency, adaptability, and a reduced environmental footprint. While challenges like corrosion or partial-load inefficiencies persist, innovative solutions—from TIAC to AI-driven automation—keep CCGT technology at the cutting edge.

Whether you’re powering a city or an offshore platform, understanding and optimizing CCGT systems is key to success.

Ready to dive deeper? Explore TURBIVAP’s CCGT training programs and empower your team with the skills to excel in this dynamic field. Let’s harness the power of CCGT together!

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Ronaldo Pexe

Note: The opinions and information contained in this publication do not necessarily reflect the opinion of TURBIVAP.

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Combined Cycle Gas Turbines (CCGT)

Combined Cycle Gas Turbines (CCGT): A Deep Dive into Efficiency and Innovation

Sumário

Combined Cycle Gas Turbines (CCGT)

What is a Combined Cycle Gas Turbine (CCGT)?

How Does a Combined Cycle Gas Turbine Work?

Applicability of Combined Cycle Gas Turbines

What is a Steam Turbine?

Principal Aspects of CCGT Systems

Principle of Functioning: The Thermodynamics Behind CCGT