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Chalmers University of Technology
Fuel mixing in fluidized beds
- experimental observations
Filip Johnsson, David Pallarès, Erik Sette
Department of Energy and Environment
Chalmers University of Technology, 412 96, Göteborg
64th IEA-FBC Meeting, Naples, June 3, 2012
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
Chalmers University of Technology
Bubbling fluidized bed boiler (BFBC)
Circulating fluidized bed boiler (CFBC)
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
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
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)
Chalmers University of Technology
Splash-zone solids cluster flow
Splash zone
Dense bed
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 [P
a]
0 5 10
Time [s]
-25000
-20000
-15000
-10000
-5000
0
5000
Impa
ct pre
ssu
re [P
a]
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
Impact
pre
ssu
re [
Pa]
0 5 10
Time [s]
-25000
-20000
-15000
-10000
-5000
0
5000
Impact
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.
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 concentration
Fuel conversion (drying, devolatilization, combustion)
Fuel
mixing transport τ kinetics τ
Da =
Da Da
Fuel distribution – determined by Da number
Maldistribution
- Low lateral mixing
- Large cross section
- High volatile fuel
- High char reactivity
Chalmers University of Technology
Continuous fuel feeding as a sum of time-delayed fuel batches
Fuel conversion fuel particle terminal velocity is reduced with time
2m
kg
Modeling fuel mixing and conversion
Chalmers University of Technology
Fuel mixing – experimental observations
• 2 Dimensional cold measurements
• 3 Dimensional clod measurements - large unit
• 3 Dimensional cold scaled measurements
• 3 Dimensional hot measurements
Chalmers University of Technology
General pattern – highly convective 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
Pallarès, D., Johnsson, F., 2010
3 D cold fuel mixing measurements (The Chalmers gasifier)
u: 0.6 m/s, 1 m/s
H0: 0.4 m
Tracer particles: Wood chips, Bark pellets
Y
X Fuel
Camera
3 D cold
Fuel (tracer)
Feed position
3 D cold
3 D cold fuel mixing measurements (The Chalmers gasifier)
Single wood
particle
24 cases
u/umf = 7.5
u/umf = 5
u/umf = 7.5
3 D cold fuel mixing measurements (The Chalmers gasifier)
Bed geometry scaled by a factor 1/6
gL
u 20
f
s
f
ps du
0
f
f Lu
0
0u
G
s
s
rdistributo
bed
P
P
Parameter Value
Length L (m)
Width W (m)
Superficial velocity (m/s)
ρs 2600 (kg/m3)
ρf 0.18 (kg/m3)
Parameter Value
Length L/6 (m)
Width W/6 (m)
Superficial Velocity (m/s)
ρs 15700 (8900) (kg/m3)
ρf 1.21 (kg/m3)
6
2
0U
0U
3 D cold fuel mixing measurements
Downscaled unit
Tracer measurement – UV light
U = 0.33 m/s. Fuel particle density 200 – 1200 k/m3
3 D cold fuel mixing measurements
Downscaled unit
0 200 400 600 800 1000 12000
0.1
0.2
0.3
0.4
0.5
0.6
0.7Specific Dispersion Coefficient, x direction
Time [s]
Dis
pers
ion c
oeff
icie
nt
of
one p
art
icle
[m
2/s
]
3 D cold fuel mixing measurements
Downscaled unit
Camera mounted 45 degrees downwards
Approximation of the region which is visible with camera probe
3 D hot fuel mixing measurements (The Chalmers gasifier)
u = 0.15 m/s, wood pellets, 800 C
u = 0.27 m/s, wood pellets, 800 C
Macroscopic pattern of fuel flow
Homogeneously-distributed ”mixing cells” can be seen as isotropic mixing
CDt
C 2
More detailed modeling required
Chalmers University of Technology
Summary
• Fuel mixing crucial for modeling CFB (BFB) perfromance
• 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
• Need for continued development of fuel mixing measurement methods/technologies (2D and 3D)
Chalmers University of Technology
Extras
Chalmers University of Technology
Fuel mixing in fluidized beds
- simulations
Filip Johnsson, David Pallarès
Department of Energy and Environment
Meisam Farzaneh, Srdjan Sasic
Department of Applied Mechanics
Chalmers University of Technology, 412 96, Göteborg
64th IEA-FBC Meeting, Naples, June 3, 2012
Chalmers University of Technology
Fuel mixing simulation
Eulerian-Eulerian-Lagrangian(E-E-L)
• Gas and inert particle phases are resolved using
Eulerian-Eulerian scheme
• Properties of mixture (gas and inert) such as density,
viscosity, … are obtained
• Fuel particle is tracked using Eulerian-Lagrangian
Chalmers University of Technology
Results of E-E-L
Fluidization velocity = 0.4 m/s
Bed dimension = 0.4 m * 1.0 m
Static bed height = 0.56 m
Plenum included
0.33 mm inert particles
12 mm fuel particle
Chalmers University of Technology
Fuel Particle concentration
Simulations and experiments
Simulation Measurements
Chalmers University of Technology
Fuel Particle concentration - simulations
Chalmers University of Technology
Fuel Particle concentration - simulations
Chalmers University of Technology
Importance of fuel mixing - test in a the Chalmers gasifier
Intro 2D Cold 3D Cold 3D Cold down-
scaled 3D Hot
Fuel mixing controls residence
time of fuel particles
Gasification reactions slow
(compared to combustion)
Long residence time required
22 HCOOHC
Fuel inlet
Solids
outlet
Bed material
inlet
Gas outlet
Chalmers University of Technology
dTP :10 mm u0: 0.4 m/s
ρTP : 985 kg/m3
RUN 1
mb: 1.5 kg
RUN 16
mb: 5.0 kg
Fuel mixing
0.059 m/s 0.431 m/s
Pallarès, D., Johnsson, F., 2010 2 D cold
Chalmers University of Technology
Dispersion of fuel particles
Intro 2D Cold 3D Cold 3D Cold down-
scaled 3D Hot
U = 0.36 m/s. Particle density corresponds to chipped wood.
Chalmers University of Technology
Model for fuel conversion Main assumptions
- Fuel particle approximated to an ideal geometry (∞-plane, ∞-cylinder, sphere)
- Quasi-steady state
- Convective term shown to be neglectable
Chalmers University of Technology
0
500
1000
1500
2000
2500
3000
3500
4000
2005 2010 2015 2020 2025 2030 2035 2040 2045 2050
Ele
ctr
icity g
enera
tion [
TW
h]
Year
Hydro
Nuclear
Lignite
Hard coal
Gas
biomass & waste
New Wind
New Biomass & waste
New Hard coal
New Gas
Hydro replacements
Hard coal CCS
Lignite CCS
Nuclear reinvestments
Wind
New Lignite
Role of thermal conversion → gas-solids flow: - Same level of importance
- From fossil fuel dominated to renewables and CCS
Johnsson, F., Odenberger, M., Energy Procedia 4 (2011) 5869–5876
Chalmers University of Technology
Bubbling fluidized bed boiler (BFBC)
Circulating fluidized bed boiler (CFBC)
Chalmers University of Technology
CFBC with and without EHE
+ 55,4 m
+ 2,3 m
External Heat Exchanger (EHE)
Chalmers University of Technology
Bottom bed definition
Time averaged pressure drop
Johnsson, et al. 1991
Chalmers University of Technology
Gas flow distribution in bottom bed
Dynamic in time and space
Pallarès, D., Johnsson, F., 2010
Chalmers University of Technology
FBC characteristics → Key features/problems • Ratio of mixing and fuel conversion
• Solids segregation
• Dynamics of mixing
200 m2 bed surface (235 MW CFB Turow)
Air distributor
Chalmers University of Technology
Back-mixing:
Splash-zone solids
cluster flow
Splash zone