Diversion System for Low Emission Start Converter
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Solution Overview
Problem
Combined cycle power plants face challenges in reducing start-up emissions of NOx and CO due to the latency in activation of emission control systems, which are often located downstream of heat exchanger elements, leading to high emissions during start-up and operational constraints.
Innovation Solution
A diversion system that reroutes exhaust gas from a gas turbine engine through a stationary emission converter positioned upstream of the heat recovery steam generator (HRSG) during start-up, allowing the converter to reach operating temperatures quickly and then switches to the primary exhaust path once the HRSG is heated, eliminating the need for complex and potentially leak-prone mechanisms.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If emission control systems are positioned downstream of heat exchanger elements in the HRSG, then the systems can treat exhaust gas during normal operation, but the systems cannot reach operating temperature quickly during start-up due to heat sink effects
Solution Approach 1:
The exhaust gas flow path is segmented into two separate paths: a first path through the HRSG for normal operation, and a second path through the emission control system for start-up conditions. This segmentation allows each path to be optimized independently, enabling the emission control system to receive hot exhaust gas directly from the gas turbine without being blocked by the thermal mass of the HRSG.
Solution Approach 2:
The system dynamically switches between two operational modes based on temperature conditions. During start-up, when exhaust gas temperature is below the threshold, the emission control system is activated in the second path. Once the HRSG and exhaust gas reach sufficient temperature, the system transitions to the first path through the HRSG. This dynamic switching resolves the contradiction by adapting the system configuration to operational conditions.
2Reliability
If moveable emission converters are used to selectively insert into the exhaust path during start-up, then emissions can be controlled during low load, but the mechanisms become complex and difficult to seal
Solution Approach 1:
Instead of moving the emission converter into the exhaust path when needed, the system inverts the approach by providing a separate, dedicated second exhaust path that leads directly through the emission control system. The stationary emission converter remains permanently installed in this alternative path, eliminating the need for complex insertion and withdrawal mechanisms while maintaining effective emission control during start-up conditions.
Solution Approach 2:
The patent introduces flow control elements (dampers or valves) as intermediary components that regulate exhaust gas flow between the two paths. These flow control elements are simpler to implement and seal compared to moving the emission converter itself, as they only need to adjust flow distribution rather than create tight seals around a large moving component.
3Object-generated harmful factors
If the gas turbine load is quickly raised from startup to reduce emissions, then start-up emissions are reduced, but additional equipment and complex control systems are required
Solution Approach 1:
The patent extracts the emission control function from the main exhaust path through the HRSG and places it in a separate, dedicated second path. This extraction allows emission control to occur independently of the HRSG thermal state, enabling effective emissions reduction during start-up without requiring complex coordination between load management and emission control systems.
Solution Approach 2:
The system uses the existing hot exhaust gas from the gas turbine to heat and activate the emission control system during start-up, rather than requiring external heating or complex control interventions. The flow control elements automatically direct exhaust gas through the appropriate path based on temperature conditions, enabling self-regulating emission control without additional complex control systems.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution reduces start-up emissions efficiently and safely, avoiding mechanical and operational issues associated with moving emission converters, while maintaining performance at high load conditions by keeping the emission converter stationary and using simple damper mechanisms for flow control.
Implementation Method 1
a selective catalytic reduction (SCR) system can convert NOx in the exhaust gas to nitrogen and water by causing the exhaust gas to react with a reducing agent
Implementation Method 2
the exhaust gas can be passed through a CO catalyst system to oxidize CO from the exhaust gas into carbon dioxide (CO2), as well as oxidizing volatile organic compounds (VOCs)
Implementation Method 3
the exhaust gas can be passed through a CO catalyst system to oxidize CO from the exhaust gas into carbon dioxide (CO2), as well as oxidizing volatile organic compounds (VOCs). However, during low load conditions of a combined cycle power plant, for example, the SCR system and the CO catalyst system may not be active because they may not attain the operating temperature of the emission controls systems
Data Source
AI summary
An emission reduction system for a combined cycle power plant including a gas turbine and heat recovery steam generator (HRSG) can comprise a stationary emission converter in fluid communication with and disposed upstream of the HRSG, and a diversion system operably coupled to an exhaust passage of the gas turbine, the exhaust passage defining an exhaust path for exhaust gas of the gas turbine through the heat recovery steam generator, the diversion system operable to define a primary exhaust path excluding the stationary emission converter and a start-up exhaust path including the stationary emission converter.