Fluidised-bed Boiler Combustion Control via Segmented Oxygen Zones
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Solution Overview
Problem
In fluidised-bed boilers, existing techniques face challenges in controlling combustion conditions and reducing nitrogen oxide emissions, particularly when using oxyfuel combustion, as the temperature and gas profiles become uncontrollable with varying fuel quality, leading to inefficient heat transfer and potential sintering of solid particles.
Innovation Solution
The method involves feeding combustion gases with different oxygen contents at multiple levels and zones within the furnace, allowing for the creation of oxidising and reducing zones to control temperature and nitrogen oxide reduction, thereby stabilizing the combustion process and reducing emissions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If oxyfuel combustion is used to reduce carbon dioxide emissions, then carbon dioxide recovery becomes simpler and more cost-effective, but temperature and gas profiles become uncontrollable with varying fuel quality
Solution Approach 1:
The furnace is divided into multiple combustion zones (primary zone at bottom, secondary zone above fuel feed height) with independent gas supply systems. Each zone can be controlled separately to maintain suitable temperatures and gas profiles despite varying fuel quality, while overall oxyfuel combustion is maintained for simplified CO2 recovery.
Solution Approach 2:
Different oxygen contents are supplied to different horizontal zones of the furnace. The primary zone receives one oxygen content while the secondary zone receives a different oxygen content, allowing local optimization of combustion conditions for each zone's specific requirements while maintaining overall oxyfuel combustion benefits.
2Object-generated harmful factors
If staged air supply is used to reduce nitrogen oxide emissions, then nitrogen oxide reduction is achieved, but temperature control becomes difficult leading to potential sintering of solid particles
Solution Approach 1:
The air supply system is segmented into primary air (at bottom) and secondary air (above fuel feed height) with independent control. This allows separate optimization: primary air is controlled to prevent sintering while secondary air is controlled to reduce nitrogen oxide formation, resolving the contradiction between temperature control and emission reduction.
Solution Approach 2:
Different oxygen contents are supplied to different horizontal zones. The primary zone receives oxygen content optimized for temperature control to prevent sintering, while the secondary zone receives oxygen content optimized for nitrogen oxide reduction, allowing both objectives to be achieved simultaneously.
3Productivity
If light fuel fractions are fed to the furnace, then fuel combustion is efficient, but gas and temperature profiles alter uncontrollably affecting nitrogen oxide emissions
Solution Approach 1:
The furnace is divided into multiple zones with independent gas supply and control systems. This segmentation allows the combustion process to be controlled locally even when light fuel fractions are used, preventing uncontrolled changes in gas and temperature profiles that would otherwise increase nitrogen oxide emissions.
Solution Approach 2:
The system includes control means that monitor and adjust gas supply to each zone based on actual combustion conditions. When light fuel fractions are introduced, the feedback control system automatically adjusts oxygen supply to maintain suitable temperature and gas profiles, preventing uncontrolled changes in emissions.
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 approach enhances the control of combustion conditions, reduces nitrogen oxide emissions, and maintains suitable temperatures for sulphur reduction, making the carbon dioxide recovery process more cost-effective and reducing the risk of secondary pollution in flue gas processing.
Implementation Method 1
oxygenous combustion gas is fed at two or more height levels... When fuel is combusted in a traditional way by means of air, flue gas contains a considerable quantity of nitrogen which originates from the air
Implementation Method 2
In fluidised bed combustion, the combustion occurs in solids suspension which is fluidised and circulated by means of a gas flow blown from below
Implementation Method 3
After water that has been carried along with fuel or developed in combustion reactions has been removed from flue gas by condensing
Implementation Method 4
the remaining carbon dioxide can be liquefied by cooling and compressing
Implementation Method 5
the remaining carbon dioxide can be liquefied by cooling and compressing
Data Source
Figure 1
Figure 2
Figure 3~7
AI summary
A method and an arrangement for optimising combustion conditions in a fluidised-bed boiler, in which combustion gas is fed at two or more height levels, the first of which is a primary level (P) which is located at the height of a furnace bottom and the second is a secondary level (S) which is located above fuel feed height (F), above which secondary level (S) there can be still other combustion gas feed levels (T,...). At least one combustion gas feed level (P, S, T,...) is fed at different points of the furnace (11) in its horizontal direction with combustion gases having different oxygen contents such that zones of different oxygen content can be formed in the horizontal direction of the furnace (11).