Modeling of the bed inventory in CFB boilers - ProcessEng · In collaboration with Metso Power...
Transcript of Modeling of the bed inventory in CFB boilers - ProcessEng · In collaboration with Metso Power...
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Chalmers University of Technology
Modeling of the bed inventory in CFB boilers
David Pallarès
Dept. of Energy and EnvironmentChalmers University of Technology
In collaboration with Metso Power Project: ”An overall CFB model”
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Chalmers University of Technology
Modeling of the bed inventory in CFB boilers
Background Solids inventory control Modeling Results
• Background
• Solids inventory control
• Modeling
• Results
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Chalmers University of Technology
Overall CFB model - concept
Concentration fields (gases & solids)
Mass flux fields (gases & solids)
Temperature field
Heat flux field
• Design tool• Knowledge source• Training tool
Inputs
Outputs
Background Solids inventory control Modeling Results
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Chalmers University of Technology
Overall CFB model - modules
Background Solids inventory control Modeling Results
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Chalmers University of Technology
• Solids properties (PSDunit, density, sphericity)
Overall CFB model - inputs
• Geometry of the circulating loop• Operational conditions
Fuel flows (rate, location, composition, temperature, fuel frag.)
Air flows (rate, location, composition, temperature)
Furnace pressure dropSteam data
Influences the fluiddynamics stronglyOften difficult to guess or measure
Background Solids inventory control Modeling Results
Needs to be modeled
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Chalmers University of Technology
Solids inventory
Background Solids inventory control Modeling Results
The bed solids inventory (or at least a representative part of it) is usually meant to consist of fuel ash (+sorbent).
Without acting on the bed solids inventory, it would tend to zero or (mostlikely) to fill up the riser, depending on:
• PSD • Operational conditions• Cyclone performance
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Chalmers University of Technology
Control strategies
∆priser
∆pbottom
- Addition of fine makeup material
Low ∆pbottom
High ∆pbottom
- Addition of coarse makeup material - Removal of fine bed material
Background Solids inventory control Modeling Results
Usual measures to control the bed solids inventory
- Addition of coarse makeup material
- Removal of bottom bed material
Low ∆priser
High ∆priser
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Chalmers University of Technology
Control strategies
Background Solids inventory control Modeling Results
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Chalmers University of Technology
Solids attrition
Background Solids inventory control Modeling Results
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Chalmers University of Technology
Control strategies
Background Solids inventory control Modeling Results
t
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Chalmers University of Technology
Transient modeling
Operational strategy is needed as input and influences the results
Background Solids inventory control Modeling Results
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Chalmers University of Technology
Pseudo-steady stateWith control strategies of sudden nature, a steady state is never reached.However, a pseudo-steady state is finally reached in which a pattern of countermeasures is repeated at a constant frequency. The main fluiddynamical parameters keep oscillating slightly around their time-averaged values.
Background Solids inventory control Modeling Results
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Chalmers University of Technology
Transient modeling
Background Solids inventory control Modeling Results
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Chalmers University of Technology
Solids mixing - furnaceCluster & disperse phases (Johnsson and Leckner)
Backflow effect - Correlation
Background Solids inventory control Modeling Results
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Chalmers University of Technology
Pressure balance on circulating loop
Pseal - Pdc
mdc
Population balance on circulating loop
Solids mixing – return leg
Hdc
Background Solids inventory control Modeling Results
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Chalmers University of Technology
Results – 4 parameters studied
16 cases studied
Background Solids inventory control Modeling Results
No sorbent
Chalmers CFB boiler (12 MWth)
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Chalmers University of Technology
Results – Control strategies used∆priser -relatedCountermeasures applied if experimental value deviates >5% from nominal value
∆priser,exp > ∆priser,nom : Bottom bed material removal
∆priser,exp < ∆priser,nom : Coarse worn-out material addition
∆pbottom –relatedCountermeasures applied if experimental α=∆pbottom/∆priser is outside range 0.40-0.82
α < 0.40 : Coarse worn-out material addition and bottom bed material removal
α > 0.82 : Fine worn-out material addition and bottom bed material removal (no removal from seal available at Chalmers boiler)
Background Solids inventory control Modeling Results
(∆pbottom measured between h=0.135 and h=1.635 m)
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Chalmers University of Technology
Bed material modelingHigh-efficiency cyclone (slow attrition, xash,fuel )
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Chalmers University of Technology
Bed material modelingHigh-efficiency cyclone (slow attrition, xash,fuel )
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Chalmers University of Technology
Conclusions (ηcycl ↑)
- In all runs, relatively similar pseudo-steady state values for:
PSDunit, Hb, Fs,net
- Limited influence of the attrition rate also on all other variables
- Fstack,Fclass α xash,fuel Fstack/Fclass~constant
- High xash,fuel leads to sooner pseudosteady states
Background Solids inventory control Modeling Results
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Chalmers University of Technology
Bed material modelingLow-efficiency cyclone, no ∆pbot control (slow attrition, xash,fuel↓)
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Chalmers University of Technology
Bed material modelingLow-efficiency cyclone, ∆pbot control (slow attrition, xash,fuel↑)
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Chalmers University of Technology
Bed material modelingLow-efficiency cyclone, ∆pbot control (slow attrition, xash,fuel↑)
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Chalmers University of Technology
- For all cases
Higher attrition rates imply increased need of makeup material (and thereby changes in the fluiddynamics).
- Without ∆pbot control
The unit tends very slowly to a pseudo-steady state in which all bed material is formed by coarse, non-circulating ash. Only fines from attriting ash are entrained and go to stack.
- With ∆pbot control
The PSDs of added materials govern the pseudo-steadystate. This influence increases as xash,fuel decreases.
Conclusions (ηcycl ↓)
Background Solids inventory control Modeling Results
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Chalmers University of Technology
- A model for the solids bed inventory is built. It has a dynamical approach and provides PSDunit as well as solids flows within the CFB loop and in/out from the CFB unit (bottom/seal removal, stack, makeup).
- Cyclone performance is the most influential parameter and governs how other parameters influence the results
- Attrition rate influence increases as cyclone separation efficiency decreases
- Resolution and accuracy for finest sizes (ie ηcycl , PSDplateau) is crucial
Background Solids inventory control Modeling Results
Conclusions
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Chalmers University of Technology
- The model will be within short tested against experimental data from large-scales CFB boilers
Background Solids inventory control Modeling Results
Further work
Thank you for your attention!Questions?
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Chalmers University of Technology
Bed material modeling
• mfuel , xash,fuel
• Ash attrition pattern
• PSD of coarse and fine material
• ηcyclone (d), ηclassifier (d)
• Control strategy
•
Inputs needed
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Chalmers University of Technology
Bed material modeling
• mfuel , xash,fuel
• Ash attrition pattern
• PSD of coarse and fine material
• ηcyclone (d), ηclassifier (d)
• Control strategy
•
Inputs needed
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Chalmers University of Technology
Bed material modeling
• mfuel , xash,fuel
• Ash attrition pattern
• PSD of coarse and fine material
• ηcyclone (d), ηclassifier (d)
• Control strategy
•
Inputs needed
0
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Chalmers University of Technology
Bed material modeling
• mfuel , xash,fuel
• Ash attrition pattern
• PSD of coarse and fine material
• ηcyclone (d), ηclassifier (d)
• Control strategy
•
Inputs needed
∆priser,exp 5% deviation tolerance
α [0.40,0.82]