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Mixing phenomena in fluidized beds diagnostics and observations · · 2017-02-21Mixing phenomena...
Transcript of Mixing phenomena in fluidized beds diagnostics and observations · · 2017-02-21Mixing phenomena...
![Page 1: Mixing phenomena in fluidized beds diagnostics and observations · · 2017-02-21Mixing phenomena in fluidized beds – diagnostics and observations Filip Johnsson, David Pallarès,](https://reader031.fdocuments.in/reader031/viewer/2022022010/5b01d2eb7f8b9a65618e43ea/html5/thumbnails/1.jpg)
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
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Chalmers University of Technology
Bubbling fluidized bed boiler (BFBC)
Circulating fluidized bed boiler (CFBC)
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Chalmers University of Technology
Dual bed systems – indirect gasifier
2-4 MW integrated in Chalmers 12 MW CFB
Gasifier
fuel
Combustor
fuel
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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
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Chalmers University of Technology
Oxyfuel in fluidized-bed combustion
CFB Technology Metso Power (Now Valmet)
4 MW CFB Oxyfuel
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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
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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
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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)
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Chalmers University of Technology
Fuel mixing
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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 =
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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
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Chalmers University of Technology
3D hot conditions (12 MWth CFB Chalmers)
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Chalmers University of Technology
2D cold tests
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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
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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
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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
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Chalmers University of Technology
2D cold tests
– velocity and bed-height dependency
Red symbols - narrow unit
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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)
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Chalmers University of Technology
3D cold conditions (Chalmers gasifier bed)
Batch of fuel
particles
Fuel inlet
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Chalmers University of Technology
3D cold conditions (Chalmers gasifier bed)
Fuel inlet
Batch of fuel
particles
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Chalmers University of Technology
3D cold conditions (Chalmers gasifier bed)
Single fuel
particle
Fuel inlet
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Chalmers University of Technology
u/umf = 5
u/umf = 7.5
3D cold conditions (Chalmers gasifier bed)
Single fuel
particle
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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
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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
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Chalmers University of Technology
3D cold tests – downscaled Chalmers gasifier bed
Dispersion of inert solids
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Chalmers University of Technology
3D cold tests – downscaled Chalmers gasifier bed -UV light tracing
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Chalmers University of Technology
3D cold tests – downscaled Chalmers gasifier bed -UV light tracing
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Chalmers University of Technology
3D cold downscaled test
– ability to provide quantitative results
850 ºC
Scaling
rules
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Camera mounted 45 degrees downwards
Approximation of the region which is visible with camera probe
3D hot conditions - Chalmers gasifier bed
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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
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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
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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
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Chalmers University of Technology
With baffle
Without baffle
In gasifier bed – fuel dispersion should be limited
- Insertion of baffles
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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
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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
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Chalmers University of Technology
Oxyfuel in fluidized-bed combustion
CFB Technology Metso Power (Now Valmet)
4 MW CFB Oxyfuel
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Chalmers University of Technology
4 MW Oxyfuel runs
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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)
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Extras
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Chalmers University of Technology
Chalmers CFB model
Example of ongoing work
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Chalmers University of Technology
Heat extraction
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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
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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
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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
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Chalmers University of Technology
0 5 10
Time [s]
-25000
-20000
-15000
-10000
-5000
0
5000
Imp
act
pre
ssu
re [
Pa
]
0 5 10
Time [s]
-25000
-20000
-15000
-10000
-5000
0
5000
Imp
act
pre
ssu
re [
Pa
]
0 5 10
Time [s]
-25000
-20000
-15000
-10000
-5000
0
5000
Impa
ct
pre
ssu
re [
Pa
]
0 5 10
Time [s]
-25000
-20000
-15000
-10000
-5000
0
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]
-25000
-20000
-15000
-10000
-5000
0
5000
Impa
ct
pre
ssu
re [
Pa]
0 5 10
Time [s]
-25000
-20000
-15000
-10000
-5000
0
5000
Impa
ct
pre
ssu
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.
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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)