CFB Boiler Load Control via Pre-combustion Char Reduction
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Solution Overview
Problem
Circulating fluidized bed (CFB) boilers face slow load change capabilities due to large bed char inventories, which hinder rapid steam generation and increase emissions, as existing methods for optimizing fuel and air feeding rates are inefficient or adverse to environmental emissions.
Innovation Solution
Increasing the oxygen/fuel ratio during load increases by temporarily elevating the oxygen content in the oxidant gas stream, specifically using a mixture of pure oxygen and recycled flue gas, to rapidly combust bed char inventory and enhance thermal power generation, thereby reducing bed char inventory and improving load change rates.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the feeding rates of fuel and oxidant are suddenly increased to new values corresponding to new stable load conditions, then the boiler load can be increased, but the concentration of residual oxygen in the flue gas temporarily increases and the bed char inventory increases slowly to a new equilibrium value, slowing down the load following capacity
Solution Approach 1:
The patent applies preliminary action by pre-combusting a portion of the bed char inventory before the actual load increase. This is achieved by feeding oxidant at a rate higher than the fuel feeding rate during a pre-combustion period, thereby reducing the bed char inventory in advance. When the load increase is then initiated, the system responds faster because there is less unburned char inventory that needs to be combusted to generate the additional thermal power.
Solution Approach 2:
The patent employs parameter changes by dynamically adjusting the oxidant feeding rate relative to the fuel feeding rate. During the pre-combustion phase, the oxidant-to-fuel ratio is increased above stoichiometric levels to accelerate char combustion. During the load increase phase, the oxidant rate is further optimized to maintain high combustion efficiency. This dynamic parameter adjustment enables faster load response while controlling emissions.
2Object-generated harmful factors
If the oxidant feeding rate is increased to maintain complete combustion and minimize emissions, then carbon monoxide and NOX emissions are minimized, but the bed char inventory increases and the load changing rate is reduced
Solution Approach 1:
The patent applies preliminary action by pre-combusting a portion of the bed char inventory before the actual load increase. This is achieved by feeding oxidant at a rate higher than the fuel feeding rate during a pre-combustion period, thereby reducing the bed char inventory in advance. When the load increase is then initiated, the system responds faster because there is less unburned char inventory that needs to be combusted to generate the additional thermal power.
Solution Approach 2:
The patent employs parameter changes by dynamically adjusting the oxidant feeding rate relative to the fuel feeding rate. During the pre-combustion phase, the oxidant-to-fuel ratio is increased above stoichiometric levels to accelerate char combustion. During the load increase phase, the oxidant rate is further optimized to maintain high combustion efficiency. This dynamic parameter adjustment enables faster load response while controlling emissions.
3Stability of the object's composition
If a large bed char inventory is maintained to ensure stable combustion, then combustion stability is improved, but the load changing capability is slowed down because the inventory takes time to reach equilibrium
Solution Approach 1:
The patent applies preliminary action by pre-combusting a portion of the bed char inventory before the actual load increase. This is achieved by feeding oxidant at a rate higher than the fuel feeding rate during a pre-combustion period, thereby reducing the bed char inventory in advance. When the load increase is then initiated, the system responds faster because there is less unburned char inventory that needs to be combusted to generate the additional thermal power.
Solution Approach 2:
The patent applies dynamics by making the oxidant feeding rate adjustable and time-dependent rather than constant. The control system dynamically modifies the oxidant-to-fuel ratio based on the operational phase (pre-combustion, load increase, or steady state), enabling the system to transition between different operational states more rapidly while maintaining combustion stability throughout the transitions.
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 method allows for faster load changes in CFB boilers by efficiently combusting unburned char, reducing emissions, and enabling rapid steam generation while minimizing the need for oversized air separation units and reducing auxiliary power consumption.
Implementation Method 1
combust the fuel at the first fuel feeding rate
Implementation Method 2
feeding a feed stream of oxygen at a first oxygen feeding rate into the furnace, so as to combust the fuel
Data Source
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AI summary
A method of controlling combustion in the furnace of a circulating fluidized bed boiler plant, the method comprising in first stable load conditions the steps of: feeding fuel at a first fuel feeding rate, proportional to the first load, into the furnace;feeding a feed stream of oxygen at a first oxygen feeding rate, adjusted to maintain a predetermined first oxygen/fuel ratio, into the furnace, so as to combust the fuel at the first fuel feeding rate, whereby a bed char inventory is being maintained in the furnace, and in conditions of increasing the load from the first stable load conditions, the steps of : feeding fuel at a second fuel feeding rate into the furnace, wherein the second fuel feeding rate is higher than the first fuel feeding rate, and feeding a feed stream of oxygen at a second oxygen feeding rate into the furnace, wherein in the conditions of increasing the load the oxygen/fuel ratio is higher than the first oxygen/fuel ratio, so as to combust the fuel at the second fuel feeding rate and to decrease the bed char inventory by combusting char of the bed char inventory.