Steam Injection in Oxycombustion CFB Boiler Solids Handling
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Solution Overview
Problem
In oxycombustion CFB boilers, the presence of CaO in solid materials can lead to recarbonation back to CaCO3 when using recycled flue gas for fluidization, causing adverse behavior in the solids handling system, including potential bed agglomeration and loss of fluidizing gas.
Innovation Solution
Introducing a predetermined amount of steam into the external solid material handling system to control the CO2 partial pressure below the equilibrium for recarbonation, minimizing recarbonation of CaO and thereby reducing the formation of CaCO3 and associated issues like bed agglomeration and fluidizing gas loss.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If recirculating flue gas is used as fluidizing gas in the external solid material handling system, then the fluidization efficiency is improved and heat transfer is enhanced, but CaO recarbonates back to CaCO3 causing bed agglomeration and loss of fluidizing gas
Solution Approach 1:
The invention changes the chemical composition parameters of the fluidizing gas by introducing steam to modify the CO2 partial pressure. This parameter change shifts the equilibrium conditions to prevent CaO recarbonation while maintaining fluidization effectiveness. The steam addition alters the gas-phase chemistry without compromising the fluidization hydrodynamics.
Solution Approach 2:
The invention converts the harmful recarbonation reaction into a beneficial process by using the CO2 that would otherwise cause agglomeration. The introduced steam reacts with CO2 to form carbonic acid or bicarbonate species that prevent direct CaO recarbonation, while the CO2 is ultimately utilized in the boiler combustion process. The potential harm of CO2-induced recarbonation is transformed into a controlled chemical environment that protects the solids handling system.
2Stability of the object's composition
If steam is introduced into the external solid material handling system to prevent recarbonation, then CaO stability is improved and bed agglomeration is prevented, but the system complexity and energy consumption increase
Solution Approach 1:
The introduced steam serves multiple functions simultaneously: it prevents CaO recarbonation by modifying CO2 partial pressure, provides additional fluidization assistance, and can contribute to heat transfer enhancement. This multi-functionality justifies the added system complexity by delivering multiple benefits from a single intervention.
Solution Approach 2:
The steam introduction system is designed to be self-regulating where the steam itself creates the chemical environment needed to prevent recarbonation. The system uses its own byproducts (the steam-atmodified gas composition) to protect against the harmful effect, reducing the need for additional control mechanisms and external interventions.
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 ensures reliable operation of the external solid material handling system by preventing excessive CaCO3 formation and maintaining efficient fluidization, thereby avoiding bed agglomeration and fluidizing gas loss.
Implementation Method 1
a predetermined amount of steam is controlled so that CO2 partial pressure in the fluidizing medium is maintained below the equilibrium for recarbonation of CaO at the prevailing temperature in the external solid material handling system
Implementation Method 2
fluidizing the solid material in the solid material handling system by introducing fluidizing medium comprising recirculating flue gas into the external solid material handling system
Implementation Method 3
CaO has a tendency to recarbonating back to CaCO3 at certain circumstances
Data Source
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AI summary
The invention relates to a method of operating an oxycombustion CFB boiler (10) comprising a furnace (12) having a grid (26) at its bottom section, a solid material separator (16) connected to the upper part of the furnace and an external solid material handling system (20,21), the method comprising steps of arranging a circulating fluidized bed in the oxycombustion CFB boiler (10), introducing an oxidant gas into the CFB boiler through the grid (26) as fluidizing gas, the fluidizing gas com- prising recirculating flue gas, introducing fuel material (22) into the circulating fluidized bed, introducing sulphur reducing agent (24) comprising CaCO3 into the circulating fluidized bed, circulating solid material out of the furnace providing an external circulation of solid material by the external solid material handling system, fluidizing the solid material in the external solid material handling system by introducing fluidizing medium comprising recirculating flue gas into the external solid material handling system. In the method,a predetermined amount of steam (44) is introduced into the external solid material handling system as a component of the fluidization medium for fluidizing the solid material replacing recirculating flue gas in the fluidizing medium.