Chalmers University of Technology
Mixing phenomena in fluidized beds
– diagnostics and observations
Filip Johnsson, David Pallarès, Erik Sette
Department of Energy and Environment
Chalmers University of Technology, 412 96, Göteborg
The 68th IEA-FBC meeting
Beijing, China, 12-13 May, 2014
Chalmers University of Technology
Bubbling fluidized bed boiler (BFBC)
Circulating fluidized bed boiler (CFBC)
Chalmers University of Technology
Dual bed systems – indirect gasifier
2-4 MW integrated in Chalmers 12 MW CFB
Gasifier
fuel
Combustor
fuel
Chalmers University of Technology
Biomass gasification
Research and development at
Chalmers with associated
industries
Chalmers lab-reactor
Chalmers 2-4 MW
pilot plant
GoBiGas Phase 1
20 MW SNG demonstration plant
Göteborg Energi
GoBiGas Phase 2
80 MW SNG
Commercial plant
Göteborg Energi
2008 2012 2016
Chalmers University of Technology
Oxyfuel in fluidized-bed combustion
CFB Technology Metso Power (Now Valmet)
4 MW CFB Oxyfuel
Chalmers University of Technology
CFB & BFB characteristics • Group B solids
– CFB: Primary gas velocity > ut for major part of bed solids
– BFB: Primary gas velocity < ut for major part of bed solids
• Furnace height-to-width ratio < 10
– Large cross section, Lcharact up to 10 meters
• Dense bottom-region height << furnace width
• Main solids backmixing processes
– Bottom-region clustering/bubble flow (highly dynamic)
– Splash-zone solids cluster flow
– Furnace wall-layer backmixing (dispersed core region flow)
• Low solids recirculation flux
– CFB: Gs < 10 kg/m2 s (oxyfuel: higher Gs may be required)
– BFB: No external solids flux
Chalmers University of Technology
Back-mixing:
Bottom-region
clustering/bubble flow
Back-mixing:
Splash-zone solids
cluster flow
Furnace wall-layer backmixing
(dispersed core region flow)
Bottom region/bed Splash zone Transport zone
Chalmers University of Technology
CFB characteristics result in:
• Good vertical solids mixing
• Limited lateral solids mixing
– Fuel mixing is crucial
– Important to establish basis for modeling of fuel mixing from known parameters (gas velocity, gas and solid properties, bed and gas distributor geometry)
Chalmers University of Technology
Fuel mixing
Chalmers University of Technology
Da
Fuel maldistribution: consequences
Fuel concentr
atio
n
- Higher air-to-fuel ratio needed
- Lateral gas concentration gradients
- More fuel feeding ports needed
Fuel conversion (drying, devolatilization, combustion)
Fuel
mixing transport τ kinetics τ Da =
Chalmers University of Technology
Experimental observations - Chalmers
2D cold tests
3D cold down-scaled tests
3D hot tests
3D cold tests
Qualitative
Quantitative (?)
Quantitative
Qualitative
Chalmers University of Technology
3D hot conditions (12 MWth CFB Chalmers)
Chalmers University of Technology
2D cold tests
Chalmers University of Technology
Hb~ 0.33 m
u0=1.5 m/s
X [mm] X [cm]
y [cm] y [mm] Dh=0.93∙10-2 m2/s Dh=1.23∙10-2 m2/s
2D cold tests – wide vs narrow unit
Chalmers University of Technology
SCgradDdivt
C
Fuel “dispersion” is the sum of convection (dominating) and diffusion.
Dispersion coefficient can be determined using a diffusion analogue
Expressing fuel mixing
Chalmers University of Technology
CDt
C 2
Diffusion analogue only on macroscopic level
Diffusion analogue only
on macroscopic
(bubble path) level
in large cross sections
with homogeneous
nozzle distribution
Chalmers University of Technology
2D cold tests
– velocity and bed-height dependency
Red symbols - narrow unit
Chalmers University of Technology
u: 0.6 m/s, 1 m/s
H0: 0.4 m
Tracer particles: Wood chips, Bark pellets
Y
X Fuel
Camera
3D cold conditions (Chalmers gasifier bed)
Chalmers University of Technology
3D cold conditions (Chalmers gasifier bed)
Batch of fuel
particles
Fuel inlet
Chalmers University of Technology
3D cold conditions (Chalmers gasifier bed)
Fuel inlet
Batch of fuel
particles
Chalmers University of Technology
3D cold conditions (Chalmers gasifier bed)
Single fuel
particle
Fuel inlet
Chalmers University of Technology
u/umf = 5
u/umf = 7.5
3D cold conditions (Chalmers gasifier bed)
Single fuel
particle
Chalmers University of Technology
Bed geometry scaled by a factor 1/6
gL
u 2
0
f
s
f
ps du
0
f
f Lu
0
0u
G
s
s
rdistributo
bed
P
P
Parameter value
Length 1.8 m
Width 0.8 m
u0 0.32 m/s
ρs 2600 kg/m3
ρf 0.18 kg/m3
Parameter value
Length 0.3 m
Width 0.13 m
u0 0.14 m/s
ρs 15700 (8900) kg/m3
ρf 1.21 kg/m3
3D cold tests – downscaled unit -Chalmers gasifier bed
Chalmers University of Technology
Sy
CD
yx
CD
xt
C
3D cold tests – downscaled Chalmers gasifier bed
Dispersion of inert solids -fitting dispersion equation to outlet sampling of tracer
Chalmers University of Technology
3D cold tests – downscaled Chalmers gasifier bed
Dispersion of inert solids
Chalmers University of Technology
3D cold tests – downscaled Chalmers gasifier bed -UV light tracing
Chalmers University of Technology
3D cold tests – downscaled Chalmers gasifier bed -UV light tracing
Chalmers University of Technology
3D cold downscaled test
– ability to provide quantitative results
850 ºC
Scaling
rules
Camera mounted 45 degrees downwards
Approximation of the region which is visible with camera probe
3D hot conditions - Chalmers gasifier bed
Chalmers University of Technology
U=0.11 m/s U=0.19 m/s
3D hot conditions - Chalmers gasifier bed
Work in progress – development of camera probe
Chalmers University of Technology
Improvement under way
– to minimize/eliminate
- Deposits on lens
- Condensation on lens
- Camera resolution
- Camera adjustments
3D hot conditions - Chalmers gasifier bed
Work in progress – development of camera probe
Gasification: The possibility to increase residence time of fuel particles
From laboratory to pilot scale
Fuel
Steam
Fuel
Steam
Bed-material
Bed-material
Bed-material
Bed-material
Chalmers University of Technology
With baffle
Without baffle
In gasifier bed – fuel dispersion should be limited
- Insertion of baffles
In-bed tube bundles reduce bubble size
Air distributor
Air distributor
Andersson, Johnsson, Leckner, Proc Int. Conf. Fluidized Bed Combustion, 1989, BookNO-I0290A
Without tubes Sparse tube packing Dense tube packing
Gasification: Influence of tube bundles on fuel residence time (cross flow of solids)
Velocity field, u, induced by cross-flow of solids
Solids flow in and out of gasifier
Tube Bundle
Application of
scaling laws Fuel feed inlet
Cross section of gasifier
Chalmers University of Technology
Oxyfuel in fluidized-bed combustion
CFB Technology Metso Power (Now Valmet)
4 MW CFB Oxyfuel
Chalmers University of Technology
4 MW Oxyfuel runs
Chalmers University of Technology
Summary • Fuel mixing crucial for modeling CFB (BFB) performance
• Need for experimental data and measurement methods
• Measurements carried out so far indicate:
– Highly convective mixing process
– Fuel vortex structures related to bubble flow
– Possible to relate fuel mixing to bubble flow, i.e. to known parameters (which determine bubble flow)
– Dynamic modeling required
• Bed internals can be used to control mixing – application to indirect gasification in a dual-bed arrangement to enhance gas yield
• Active bed material can enhance mixing
• Need for continued development of fuel mixing measurement methods/technologies (2D and 3D)
Extras
Chalmers University of Technology
Chalmers CFB model
Example of ongoing work
Chalmers University of Technology
Heat extraction
Chalmers University of Technology
Optimization: heat transfer
Heat extraction panels
Test varying
- locations
- shapes
- functions (EV, SH)
and optimize by evaluating
the pdf of W/m2
Chalmers University of Technology
Supporting experiments – new cold CFB
To determine how solids flow/circulation is influenced by: - Tapered walls
- Internals
- 2y air or bottom FGR
- Furnace exit geometry
Chalmers University of Technology
Bottom-region clustering/bubble flow
• u0 > ut of major part of solids. Yet, a dense region can
be maintained
– Limited air-distributor pressure drop
– Velocity at distributor varies in time and over cross section
Primary gas distributor
u0 = 2.7 m/s
ut = 2.1 m/s
Chalmers University of Technology
0 5 10
Time [s]
-25000
-20000
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0
5000
Imp
act
pre
ssu
re [
Pa
]
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act
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ssu
re [
Pa
]
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ct
pre
ssu
re [
Pa
]
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5000
Impa
ct
pre
ssu
re [
Pa
]
ER, 50 mm from wall ER, 2550 mm from wall
L2f3, 50 mm from wallL2f3, 2000 mm from wall
36.7 m above air distributor
3.8 m aboveair distributor
0 5 10
Time [s]
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ct
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ct
pre
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re [
Pa]
17.7 m above air distributor
L5f, 50 mm from wall L5f, 2500 mm from wall
Front wall
Wall layer Core
Solids flux – Momentum measurements (235 MWe boiler)
Furnace wall-layer backmixing
Johnsson, et al.
Chalmers University of Technology
Bottom region
0 5 10 15 20 25 30 35 40HEIGHT ABOVE AIR DISTRIBUTOR, z [m]
0
2
4
6
8
10
PR
ES
SU
RE
DR
OP
, p
- p
exit [k
Pa
]
Chalmers 12 MWthTurow 235 MWe0 1 2 3
0
2
4
6
8
Cold unit (exploding bubble regime)Cold unit (transport condition)
Large boiler > 200 MWe
dp/dh b < 0.5
dp/dh b < 0.5
b = (- mf)/(1-mf)
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