Loopseal Heat Exchanger for Low-Load Steam Superheating
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Solution Overview
Problem
Fluidized bed boilers face challenges in producing high-temperature steam at low loads due to reduced bed material temperature and corrosion issues when burning low-quality fuel, leading to turbine shutdowns and efficiency losses.
Innovation Solution
Incorporating a third heat exchanger heated by high-quality fuel in a loopseal heat exchanger to further heat steam, bypassing direct contact with corrosive flue gases, and utilizing a damper system to manage heat exchange during load variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If low-quality fuel is burned to produce steam, then fuel cost is reduced, but corrosion of heat transfer surfaces increases due to alkali and halogen condensation
Solution Approach 1:
The steam heating process is divided into multiple stages: first heating in a heat exchanger exposed to flue gas, then further heating in a second heat exchanger within the fluidized bed, and finally superheating in a third heat exchanger exposed to flue gas from high-quality fuel combustion. This segmentation allows each stage to operate under different thermal and chemical conditions, reducing overall corrosion while achieving high steam temperatures.
Solution Approach 2:
A third heat exchanger is introduced as an intermediary component between the second heat exchanger and the steam turbine. This third heat exchanger is exposed to flue gas from burning high-quality fuel, which has lower corrosive content. The third heat exchanger acts as a mediator that transfers heat from a low-corrosion environment to the steam without requiring the steam to directly contact the corrosive flue gas from low-quality fuel combustion.
2Loss of energy
If the load of the fluidized bed boiler decreases, then fuel consumption is reduced, but the bed material temperature becomes too low to sufficiently superheat steam for the steam turbine
Solution Approach 1:
The steam heating process is divided into multiple stages: first heating in a heat exchanger exposed to flue gas, then further heating in a second heat exchanger within the fluidized bed, and finally superheating in a third heat exchanger exposed to flue gas from high-quality fuel combustion. This segmentation allows each stage to operate under different thermal and chemical conditions, reducing overall corrosion while achieving high steam temperatures.
Solution Approach 2:
The first and second heat exchangers perform preliminary heating of the steam before it reaches the third heat exchanger. This preliminary action raises the steam temperature to a level where it can be efficiently superheated in the third heat exchanger, reducing the fuel consumption requirement for the final superheating stage while ensuring the steam reaches the required temperature for turbine operation.
3Use of energy by moving object
If a second heat exchanger is placed inside the fluidized bed to superheat steam, then heat transfer efficiency is improved, but at low loads the bed material temperature is insufficient to achieve required steam temperature
Solution Approach 1:
A third heat exchanger is introduced as an intermediary component between the second heat exchanger and the steam turbine. This third heat exchanger is exposed to flue gas from burning high-quality fuel, which has lower corrosive content. The third heat exchanger acts as a mediator that transfers heat from a low-corrosion environment to the steam without requiring the steam to directly contact the corrosive flue gas from low-quality fuel combustion.
Solution Approach 2:
The first and second heat exchangers perform preliminary heating of the steam before it reaches the third heat exchanger. This preliminary action raises the steam temperature to a level where it can be efficiently superheated in the third heat exchanger, reducing the fuel consumption requirement for the final superheating stage while ensuring the steam reaches the required temperature for turbine operation.
4Temperature
If excess fuel is burned at low load to maintain steam temperature, then steam temperature is maintained, but operational efficiency considerably decreases
Solution Approach 1:
The steam heating process is divided into multiple stages: first heating in a heat exchanger exposed to flue gas, then further heating in a second heat exchanger within the fluidized bed, and finally superheating in a third heat exchanger exposed to flue gas from high-quality fuel combustion. This segmentation allows each stage to operate under different thermal and chemical conditions, reducing overall corrosion while achieving high steam temperatures.
Solution Approach 2:
The third heat exchanger serves multiple functions: it superheats the steam to required temperatures, uses flue gas from high-quality fuel combustion (which has lower corrosive content), and can operate independently to maintain steam temperature without requiring excess fuel burning in the main combustor. This multi-functionality allows the system to maintain efficiency while achieving the required steam temperature.
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
Enables efficient production of high-temperature steam at low loads, preventing turbine shutdowns and maintaining efficiency by minimizing corrosion and reducing fuel consumption.
Implementation Method 1
recovering heat from the flue gas to the heat exchange medium in a third heat exchanger
Implementation Method 2
the heat transfer from the fluidized bed to the second heat exchanger is much better
Implementation Method 3
The heat required for the third heat exchanger can be supplied by burning second fuel
Data Source
AI summary
A method for heating a heat exchange medium in a fluidized bed boiler (100), the method comprising burning first fuel (165) in a first furnace (162) of the fluidized bed boiler (100) to produce first flue gas (163); recovering heat from the first flue gas (163) to a heat exchange medium using a first heat exchanger (310); conveying the heat exchange medium from the first heat exchanger (310) to a second heat exchanger (320), of which at least a part is arranged in contact with a fluidized bed of the fluidized bed boiler (100); burning second fuel (175) in a second furnace (172) of the fluidized bed boiler (100) to produce second flue gas (173); conveying the heat exchange medium from the second heat exchanger (320) to a third heat exchanger (330); and recovering heat from the second flue gas (173) to the heat exchange medium using the third heat exchanger (330). A fluidized bed boiler (100) for performing the method. A loopseal heat exchanger (400) that is, when installed in a loopseal of a circulating fluidized bed boiler, configured to burn second fuel (175) in a second furnace (172) of the loopseal heat exchanger (400) to produce second flue gas (173); convey the heat exchange medium from the second heat exchanger (320) to a third heat exchanger (330); and recover heat from the second flue gas (173) to the heat exchange medium using the third heat exchanger (330).