ENGINEERING SOLUTION
Converting Open-Cycle Power Plants into Combined-Cycle Power Generation
Recovering Waste Heat to Generate More Power from Existing Assets
The Challenge
Many gas turbine and gas engine power plants operate in open-cycle configuration, where the hot exhaust gases from the prime mover are discharged to the atmosphere after electricity generation.
A substantial amount of thermal energy remains available in these exhaust gases.
For plant owners looking to improve efficiency, increase power output and reduce the carbon intensity of electricity generation, this represents an opportunity to extract additional value from an existing power-generation asset.
The engineering challenge is to recover this otherwise rejected heat and convert it into useful electrical power without fundamentally replacing the existing prime mover.
Our Engineering Solution
DK Engineers can evaluate the feasibility of converting a suitable open-cycle gas turbine or gas engine installation into a combined-cycle or waste-heat-recovery power-generation system.
The basic concept is:
- Existing Gas Turbine / Gas Engine
- Hot Exhaust Gas
- Heat Recovery Steam Generator (HRSG)
- Steam Generation
- Steam Turbine Generator (STG)
- Additional Electrical Power
Instead of allowing the exhaust heat to leave the plant unused, the HRSG recovers the thermal energy and uses it to generate steam. The steam is then expanded through an STG to generate additional electricity.
This enables the plant to obtain additional power from the same primary fuel input, subject to the actual operating conditions and system configuration.
From Open Cycle to Combined Cycle
Existing Open-Cycle Plant
- Fuel
- Gas Turbine / Gas Engine
- Generator
- Electricity
The remaining high-temperature exhaust is discharged through the exhaust system.
Proposed Combined-Cycle Configuration
- Fuel
- Gas Turbine / Gas Engine
- Generator
- Electricity
- Hot Exhaust
- HRSG
- Steam
- STG
- Additional Electricity
The existing generating equipment continues to serve as the primary power source, while a bottoming steam cycle utilises the available exhaust heat.
Why Consider a Retrofit?
A combined-cycle conversion can provide several potential benefits to existing plant owners.
1.Additional Power Generation
Recovering exhaust heat can generate additional electrical power through the steam turbine cycle.
The amount of additional power depends on:
- Gas turbine / engine capacity
- Exhaust gas flow
- Exhaust temperature
- Operating load profile
- Fuel characteristics
- Steam conditions
- HRSG configuration
- Cooling-system conditions
- Site ambient conditions
2.Improved Overall Efficiency
More useful electrical energy can be obtained from the fuel already being consumed by the primary generating equipment.
This can improve the plant's overall electrical efficiency compared with operating the prime mover alone in open-cycle mode.
3.Lower CO₂ Intensity
The HRSG and STG do not themselves capture CO₂.
However, generating more electricity from the same fuel input can reduce specific fuel consumption and CO₂ emissions per unit of electricity generated, depending on actual plant performance.
This can be particularly relevant for industries seeking to reduce their emissions intensity while continuing to utilise existing gas-fired generation assets.
4.Better Utilisation of Existing Assets
A retrofit can potentially utilise existing:
- Gas turbine / gas engine
- Generator
- Fuel system
- Electrical infrastructure
- Grid connection
- Plant utilities
- Control systems
- Site infrastructure
This creates an opportunity to enhance an existing power plant rather than developing an entirely new generating facility.
DK Engineers' Engineering Approach
A successful retrofit requires more than simply selecting an HRSG and steam turbine.
The existing plant must be studied as an integrated system.
01Existing Plant Assessment
DK Engineers can review the existing plant and operating data, including:
- Gas turbine / gas engine make and model
- Rated and actual operating capacity
- Exhaust gas flow
- Exhaust temperature
- Fuel consumption
- Operating hours and load profile
- Existing exhaust duct and stack arrangement
- Available space
- Existing electrical system
- Cooling-water availability
- DM / treated-water availability
- Process steam requirements
- Existing DCS and control philosophy
02Waste Heat Recovery Assessment
The available exhaust energy is evaluated to establish the potential for heat recovery.
The assessment can include:
- HRSG capacity
- Steam generation potential
- Steam pressure and temperature
- Economiser requirements
- Evaporator configuration
- Superheater requirements
- Single- or multi-pressure HRSG options
- Supplementary firing assessment, where applicable
- Exhaust back-pressure limitations
- Bypass-stack requirements
The objective is to determine how much of the available waste heat can be technically and economically converted into useful power.
03Steam Cycle Development
Based on the available heat source and required output, DK Engineers can develop the steam-cycle concept, including:
- Steam turbine selection
- Generator sizing
- Condensing or back-pressure configuration
- Condenser
- Vacuum system
- Cooling system
- Feedwater system
- Condensate system
- Steam bypass system
- Gland steam system
- Lube-oil system
- Governing and protection systems
Where the facility has a process steam requirement, the concept can also be evaluated as a combined heat and power (CHP) application.
Complete Plant Integration
The success of an open-cycle-to-combined-cycle conversion depends heavily on integration with the existing plant.
DK Engineers can support engineering across the major disciplines.
Mechanical & Piping
- HRSG integration
- Exhaust duct modification
- Bypass and diverter systems
- Steam piping
- Condensate piping
- Feedwater system
- Cooling-water system
- Pumps and auxiliaries
- Equipment layout
- Piping stress considerations
Electrical
- Generator integration
- HT/LT system integration
- Synchronisation
- Protection systems
- Transformer assessment
- Switchgear integration
- Load-flow considerations
- Short-circuit studies
- Grid interface
Instrumentation & Controls
- HRSG controls
- Steam turbine controls
- Gas turbine / engine interface
- DCS integration
- Interlocks and permissives
- Trip and protection philosophy
- Plant monitoring
- Performance monitoring
Civil & Structural
- HRSG support structures
- Equipment foundations
- Pipe racks
- Platforms and access
- Maintenance provisions
- Structural assessment of existing facilities
Supporting the Energy Transition
Improving the efficiency of existing power-generation assets can be an important part of the transition towards lower-carbon energy systems.
A combined-cycle conversion can provide a pathway in which an existing gas-based generating asset is progressively improved through:
- Efficiency Improvement
- Waste Heat Recovery
- Additional Power Generation
- Lower Specific Fuel Consumption
- Lower CO₂ Intensity
The improved efficiency can also be considered alongside future technologies such as:
- Carbon capture
- CO₂ compression and utilisation
- Renewable-energy integration
- Battery energy storage
- Solar-gas hybrid systems
- Flexible power-generation systems
Suitable Applications
The engineering concept can be evaluated for:
- Natural-gas-fired gas turbines
- Gas engine power plants
- Industrial captive power plants
- Distributed generation facilities
- Oil & gas installations
- Refineries and petrochemical facilities
- Chemical industries
- Steel and metal industries
- Cement plants
- Process industries
- Existing CHP installations
- Other facilities with significant high-temperature exhaust streams
The final configuration depends on the heat source, operating profile, site conditions, steam requirements and project economics.
DK Engineers – Engineering from Concept to Implementation
DK Engineers can support plant owners from the initial opportunity assessment through engineering and implementation support.
Our Potential Scope
- Plant & Data Review
- Understanding the existing power-generation system and operating conditions.
- Waste Heat Potential Assessment
- Determining recoverable thermal energy and potential additional generation.
- Pre-Feasibility Engineering
- Developing preliminary HRSG, STG and auxiliary-system configurations.
- Techno-Economic Evaluation
- Evaluating potential additional power generation, efficiency improvement and project economics.
- Basic & Detailed Engineering
- Mechanical, piping, electrical, instrumentation, civil and structural engineering.
- Equipment Integration
- Integration of HRSG, STG, condenser, auxiliaries and control systems with the existing plant.
- Project Engineering Support
- Vendor coordination, engineering review, inspection, site support and commissioning assistance.
- Performance Evaluation
- Assessment of actual system performance following implementation.
Engineering More Value from Existing Power Assets
The future of power generation is not only about adding new generating capacity.
It is also about extracting more useful energy from assets that are already operating.
For suitable gas turbine and gas engine installations, the exhaust that is currently released to the atmosphere can potentially become a valuable source of additional electrical power.
Recover the heat. Generate more power. Improve efficiency. Reduce emissions intensity.