Flue Gas Heat Exchanger Loop for CO2 Capture Icing Prevention
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Solution Overview
Problem
Fossil fuel power generation systems using pressurized fluidized bed combustion face challenges in maintaining sufficient gas temperatures for efficient carbon dioxide capture, as expansion in gas expanders can lead to icing and acidic condensation, which existing solutions like the Benfield process cannot address without compromising operating conditions.
Innovation Solution
A system with a heat recovery steam generator and dual drive air compressor, including a gas expander, utilizes a closed heat exchanger loop to maintain flue gas temperature, ensuring it remains above icing conditions and compatible with carbon dioxide removal units, while conditioning the gas to remove particulates and sulfur dioxide, allowing for the use of lower-cost metal matrix filters.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the flue gas temperature is reduced to meet Benfield process requirements, then carbon dioxide capture becomes effective, but icing and acidic condensation occur in the gas expander and air emission stack
Solution Approach 1:
The system divides the flue gas treatment into separate stages: first cooling in the heat recovery steam generator to meet Benfield process requirements, then reheating in the gas expander inlet heat exchanger to prevent icing and condensation. This segmentation allows each stage to optimize for its specific function without compromising the other.
Solution Approach 2:
The system performs preliminary cooling of the flue gas in the heat recovery steam generator before it enters the Benfield process, ensuring the temperature is suitable for carbon dioxide capture. Then, it performs preliminary heating in the gas expander inlet heat exchanger to prevent harmful condensation and icing before the gas enters the expander and stack.
2Reliability
If the flue gas temperature is maintained higher to avoid icing and acidic condensation, then system reliability improves, but carbon dioxide capture efficiency decreases
Solution Approach 1:
The system separates the temperature control functions into distinct stages: initial cooling for carbon dioxide capture efficiency, then reheating for system reliability. This allows the Benfield process to operate at optimal lower temperatures while the gas expander and stack operate at safer higher temperatures.
Solution Approach 2:
The system dynamically changes the temperature parameter of the flue gas at different points in the process. The temperature is reduced to 120-200°C for the Benfield process to maximize carbon dioxide capture, then increased to above 250°F in the gas expander inlet to prevent icing and acidic condensation, optimizing both capture efficiency and system reliability.
3Productivity
If conventional cooling methods are used to prepare flue gas for carbon dioxide removal, then the Benfield process can operate, but the gas temperature drops below safe operating limits for the gas expander
Solution Approach 1:
The system performs preliminary cooling in the heat recovery steam generator to enable the Benfield process to operate effectively. Then it performs a second preliminary heating action in the gas expander inlet heat exchanger to raise the temperature to safe operating levels before the gas enters the expander, ensuring both carbon dioxide removal capability and safe operating temperature are achieved.
Solution Approach 2:
The gas expander inlet heat exchanger acts as an intermediary device between the Benfield process and the gas expander. It receives cooled flue gas from the Benfield process and heats it using external heat sources, mediating the temperature transition to ensure the gas enters the expander at the appropriate temperature for safe and efficient operation.
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 configuration maintains high flue gas temperatures, preventing icing and acidic condensation, and ensures compatibility with carbon dioxide capture processes, improving operational efficiency and reducing costs by using lower-cost filters and maintaining system integrity during start-up and service periods.
Implementation Method 1
a heat recovery steam generator that generates steam in response to feed water in combination with exhaust gasses from the pressurized fluidized bed combustion unit
Implementation Method 2
A steam turbine generator can generate electrical power in response to steam that is supplied from the heat recovery steam generator
Implementation Method 3
The air compressor also can have a second drive that may be a gas expander that receives flue gas and that is mechanically coupled to the air compressor
Implementation Method 4
The first heat exchanger and the second heat exchanger also may each have respective input ports and output ports with the input port of the first heat exchanger connected to the output port of the second heat exchanger and the output port of the first heat exchanger connected to the output port of the second heat exchanger such that a closed flow path is constructed through the input ports and output ports of the first and second heat exchangers. The circulation of thermal fluid through the closed flow path may convey heat from flue gas passing through the first heat exchanger to flue gas passing through the second heat exchanger
Data Source
AI summary
In a power generation facility (10) wherein a fluidized bed combustion unit (12) produces steam to power a steam turbine generator (32), a heat recovery steam generator (20) produces steam for the steam turbine generator. Electrical power from the steam turbine generator is conducted to a motor (40) that drives and air compressor (36). The air compressor provides pressurized air back to the fluidized bed combustion unit (12) to promote fuel combustion. Flue gas from the heat recovery steam generator is selectively conducted to a CO2 capture unit (18) and then to a gas expander (42) that assists the motor in driving the air compressor (36). A heat exchanger (46) that is upstream of the CO2 Capture Unit and a heat exchanger (56) that is downstream of the CO2 Capture Unit and upstream of the air expander have thermal fluid sides that are connected in a closed circuit. The heat exchangers (46 and 56) convey heat away from the CO2 Capture Unit and provide heat to flue gas flowing to the gas expander to avoid icing conditions in the gas expander and acid condensation in the air emission stack.