CFD modelling of fluidised bed combustion plants
IEA Workshop: CFD aided design and other design tools for industrial biomass combustion plants Thursday, 6th June 2013 Marko Huttunen, Perttu Jukola VTT Technical Research Centre of Finland
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Outline
CFD Modelling Activities on Combustion at VTT CFD Modelling of Fluidised Bed Combustion Plants- An Overview
Modelling of Combustion in BFB furnaces
Modelling Approach Examples of Case Studies Some On-going and Planned Activities at VTT
Concluding Remarks
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CFD Modelling Activities on Combustion at VTT
CFD Applied to Combustion Modelling since 1984 Major Applications:
BFB Boiler Furnaces PF- Combustion
Boilers and Burners Coal, Peat, Co-firing, Oil shale
Grate Fired Furnaces
Recovery Boilers CFB Boiler Furnaces
Staff: 9 researchers
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CFD Modelling of Fluidised Bed Combustion Plants- An Overview (1/2)
Roughly 15 to 20 Boilers Analysed During Last Ten Years
Boiler Size Range: Fuel Power from 10 to 300 MW
Fuel and Fuel Mixtures: Fresh wood (chips, bark, forest residue) Peat Sludge REF
Furnace Models Employed as a Design Tool in Close Co-Operation with
Boiler Operators and Designers (mostly confidential contract work)
Furnace Models Developed Simultaneously with Practical Cases Implementation of new sub-models (mainly from literature) Fine-tuning of existing models Finding best practises (e.g. definition of BC’s,
combination of sub-models)
3’ry
2’ry
1’ry+FGR
SNCR
SNCR
Fuel
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CFD Modelling of Fluidised Bed Combustion Plants- An Overview (2/2)
Furnace Model Development Based on
Feedback from customer (plant data, practical observations) Furnace measurements, if available Predictions of more detailed models of separate sub-processes
Some Issues Considered by Modelling: Reduction of NOx by primary methods and SNCR Combustion efficiency (CO, UBC) Furnace availability (tendency for fouling, slagging, corrosion)
By in-direct methods (control of furnace gas temperature, solid fuel and gaseous conditions near furnace walls)
Bed behaviour (bed temperature) Heat transfer to water walls Criteria for incineration of wastes (2 sec. /850 ºC)
FLUENT as Solver
UDF’s + built-in sub-models of Fluent
3’ry
2’ry
1’ry+FGR
SNCR
SNCR
Fuel
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Modelling of Combustion in BFB furnaces- A Modelling Approach (1/2)
FREEBOARD Model: solve fluid and particle flow, combustion, heat transfer
BED Model: Solve heat and mass balance
Exchange of mass and heat
H(FGR) H(1’ry air)
Q (particle heat)
∆H (latent heat)
1. ∆H (char burning)
2. ∆H (volatiles burning)
Q (rad + convection)
Bed heat balance
DOM-radiation (built-in), Eddy dissipation (EDCM or EDC), 2-eqs. turbulence models (built-in),
Langrangian particles + UDF’s for particle conversion, 2(3) –step global schemes for main chemistry, NOx sub-model for BFB combustion
(global chemistry + mixing)
User-implemented or built-in sub-models of Fluent, if stated above
Sand, ∆H
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Particle Mixing and Conversion in Bed Non-Splash model
Particles ‘trapped’ as they reach bed surface Remaining moisture and volatiles of particles
released at or close to location of particle landing
Splash Model Track particles in splash zone and in freeboard
Bed (Mean) Temperature Fixed or Estimated
Fix Bed Temperature, Estimate Heat of
Combustion in Bed by Bed Heat Balance
Fix Heat of Combustion in Bed, Estimate Bed Temperature by Bed Heat Balance
Modelling of Combustion in BFB furnaces- A Modelling Approach (2/2)
Non-Splash
Splash
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0.92
0.94
0.96
0.98
1
1.02
1.04
1.06
1.08
Bed Temp
CASE #
Bio/Peat =70/30 (by heat, 175 MW,f )
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
00.20.40.60.8
11.21.41.6
NOx
CASE #
CO
Bio/Peat =70/30 (by heat, 175 MW,f ) CO, w-ppm,nose
NOx[mg/m3,n,6% o2]
0.88
0.9
0.92
0.94
0.96
0.98
1
1.02
0.88
0.9
0.92
0.94
0.96
0.98
1
1.02
FEGT, meanFEGT, max
CASE #
Bio/Peat =70/30 (by heat, 175 MW,f ) FEGT,maxFEGT,mean
Modelling of Combustion in BFB furnaces- Examples of Case Studies (1/5)
Reduce Emissions (CO, NOx) of Existing Boiler Design (175 MWf; Bio/Peat = 70% / 30 % of Fuel Power) Arrangement and Positioning of Air Nozzles, Nozzle Damper
Positioning and Air Staging are Considered Furnace Exit Gas Temperature [FEGT] at Nose Elevation
Monitored to Assess Risks for Upper Furnace Corrosion and Fouling Tendency
Bed Heats Monitored to Assess Risk of Bed Sintering
NEW DESIGN VS. EXISTING DESIGN:
Reduction of NOx: ~ 30 % Reduction of CO: ~ 30-70 %
Peaks of FEGT (at nose elevation) lowered due
to enhanced mixing
Rise of Bed Temperature a Concern
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400
500
600
700
800
900
1000
1100
1200
1300
1400
2BR 2FR 2FL
Tem
p [C
]
Rauma, Bark
Ave
Measured
Max
Min
400
500
600
700
800
900
1000
1100
1200
1300
1400
3RC
Tem
p [C
]
Rauma, Bark
Ave
Measured
Max
Min
400
500
600
700
800
900
1000
1100
1200
1300
1400
4RF 4FC 4LB
Tem
p [C
]
Rauma, Bark
Ave
Measured
Max
Min
400
500
600
700
800
900
1000
1100
1200
1300
1400
5RF 5RC 5FC 5LB
Tem
p [C
]
Rauma, Bark
Ave
Measured
Max
Min
400
500
600
700
800
900
1000
1100
1200
1300
1400
6RF 6RC 6LF
Tem
p [C
]
Rauma, Bark
Ave
Measured
Max
Min
Comparison of Measured and Predicted In-furnace Gas Temperature (107 MWf; Bark)
Good Agreement in Upper Furnace Unbalanced Temperature in Upper Furnace
(Cf. 6RF To 6RC And 6LF) Captured Also by the Model
Most Discrepancies for Gas Temperatures Exist at Lower Part of Furnace
Large Gradients of Temperature
in Lower Furnace Predicted by Model
Minor Changes at Expected Measuring Locations May Yield Better (or Worse) Agreement
Modelling of Combustion in BFB furnaces- Examples of Case Studies (2/5)
Part of the work has been carried out within the project ChemCom 2.0 (2008-2010)
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50
70
90
110
130
150
170
190
210
Incid
ent H
eat F
lux [
kW/m
2]
Rauma, Bark Simulated
Measured
Sim-Max
Sim-min
LHSW RW RHSW FW
Floor 2 +11.500 m (+13.000m)
Floor 3 +15.500 m (+16.000m)
Floor 4 +18.700 m (+19.500m)
Floor 5 +23.000 m (+23.800m)
Floor 6 +27.500 m (+29.000m)
Floor 2 +11.500 m (+13.000m)
Floor 3 +15.500 m (+16.000m)
Floor 4 +18.700 m (+19.500m)
Floor 5 +23.000 m (+23.800m)
Floor 6 +27.500 m (+29.000m)
Modelling of Combustion in BFB furnaces- Examples of Case Studies (3/5)
Predictions of Incident Radiation and Measured Values are Compared (107 MWf; Bark)
Most Intense Measured Heat Fluxes Exist at Floors 3 And 4, and This is also Realised by the Model
Part of the work has been carried out within the project ChemCom 2.0 (2008-2010)
Model, Surface Incident Radiation [kW/m2]
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1
10
100
1000
10000
100000
600
700
800
900
1000
1100
1200
1300
1400
1500
1600
10 15 20 25 30 35 40
CO Volume Fraction
In-Furnace Gas Temperature
furnace elevation [m]
RAUMA, Bark, Day 1
Tave [C]
Tmax [C]
COave (ppm)
Nose Secondary
Modelling of Combustion in BFB furnaces- Examples of Case Studies (4/5)
Furnace Profiles (107 MWf; Bark)
High Temperature Regions at Secondary Air Level, where Most Intense Combustion Takes Place
Peaks of In-Furnace Gas Temperatures ~ 1400-1500 C
Low Emission of CO Predicted by the Model Measured 25-30 v-ppm
Part of the work has been carried out within the project ChemCom 2.0 (2008-2010)
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REDUCE FURNACE SLAGGING Particle concentration near walls
Old New
Modelling of Combustion in BFB furnaces- Examples of Case Studies (5/5)
REDUCE SUPERHEATER /REHEATER SLAGGING Combustion rate of char in freeboard)
Old New
(178 MWf; Bio/Peat = 45% / 55 % of Fuel Power) (294 MWf; Bio/Peat = 30% / 70 % of Fuel Power)
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Incorporation of Ash Chemistry into CFD Ash Melting Behaviour, Slagging, Heat Transfer
Fuel Particles
Fragmentation, Mixing in Bed
Improved Understanding of Bed Processes Utilize multiphase CFD Simulations of Bed
Integration of CFD Furnace Model with Process Modelling
Linking of Furnace and Water/Steam Cycles
Transient Simulation e.g. Load Changes
Modelling of Combustion in BFB furnaces- Some On-going and Planned Activities at VTT
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Concluding Remarks Furnace Models Employed to Analyse Real Processes
Design of New Furnace Concepts Retrofitting
Simultaneous Development of Furnace Models Based on Experiences of Modelling of Real processes
Combustion Modelling Motivated Largely by Reduction of Emissions (IED) and Furnace Availability Issues European Patent Application ”Method for reducing nitrogen oxide
emissions and corrosion in a bubbling fluidized bed boiler and a bubbling fluidized bed boiler”, (Application No. EP12397524, Applicant Fortum OYJ)
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VTT creates business from technology
