Modeling of fuel flow in fluidized
beds
Markku Nikku, Payman Jalali, Omid
Joneydi
Content of the presentation
−
−
−
−
Introduction to fluidized beds
Modeling of fluidized beds
Fuel flow and modeling at LUT
Conclusions
Metso Power
Introduction to fluidized beds
−
−
−
−
Fluidization is a multiphase phenomena with it’s own characteristics
”A bed” of solid material set to ”a fluid-like state” with a fluid flow
Properties of the bed affected by the flow rate of the fluid
Fluidized beds widely used in energy production with gas fluidization (air,
oxygen+fluegas)
− Packed fluidized beds
− grate firing/gasifiers/reactors
− packed bed reactors
− Bubbling fluidized beds
− BFB boilers/gasifiers/reactors
− external heat exchanger
− Circulating fluidized beds
− CFB boilers/gasifiers/reactors
− looping processes
− Pneumatic transport
− pneumatic conveyers
Modeling of fluidized beds
− Interconnected “puzzle” of hydrodynamics, chemical reactions and heat
transfer in various scales
− Computational fluid dynamics (CFD) simulations of fluidized beds have to
consider
− Fluctuating multiphase flow with (millions of) particles
− Interaction between phases and between particles
− Reactions and heat transfer
− Other issues
− Compromises have to be made in order to model commercial scale units
− Level of details vs. computational costs
Micro-scale
1 year
1 h...1 d
Meso-scale
Steady state
Quasi steady
Averaged CFD
2D/3D
Empirical and
semi-empirical
models
1D/1.5D/3D
Transient
Time scale
Macro-scale
Lumped
scale
Correlation
models
0D
1s
Eulerian-Eulerian
continuum models
CFD / TFM
2D/3D
1 ms
1 µs
Particle scale
DNS,LBM,DEM/DPM Lagrangian-Eulerian
DEM/DPM-CFD,DSMC
2D/3D
2D/3D
1 µm
1 mm
0.1 m
1m
Global
10...50 m
Space scale
Myöhänen, K. and Hyppänen, T. 2011. A Three-Dimensional Model Frame for Modelling Combustion and Gasification in Circulating Fluidized
Bed Furnaces, International Journal of Chemical Reactor Engineering 9(1)
Approaches to modeling and scale-up
Bench scale
Pilot scale
Boiler scale
EXPERIMENTAL
SCALES
1D-MODEL
flue gas to stack
v
MODELS AND
DESIGN TOOLS
Model
analyses
Vo
lati
le,
mo
istu
r
re
ele
bu
stio
n+1
n-1
ase
n-2
dYCO
dt
CO combustion
Mixing
Ch
ar
com
n
b exp( A / T )(d / d ref ) n
kef YCO
k ef
1 /(1 / kCO
rc
dmc
dt
m)
n
3
2
n
kmc X O2
Models for
phenomena
Secondary air
1
Primary air
1-D process
models
3-D process
models
Fuel flow and modeling
−
Fuel flow and mixing determine where the reactions take place,
directly resulting to
− fuel concentration
− changes in gas species
CFD
− temperature field…
− Three dimensional solution required
− Process characteristics and
behavior determined by fuel flow
− Flue flow can be modeled in large
3D Semi-empirical
scale in CFD with
and time− Single fuel particles – Lagrangian
averaged CFD
− As continuous phase – Eulerian
− Still many large scale fluidized bed furnace models are semi-empirical as ”pure”
CFD would be too computationally expensive
− Many use dispersion/diffusion type of mixing for fuel due to steady state
nature of the simulations
− Following slides presents some research done in LUT Energy/Laboratory of
modeling of energy system
Fuel flow:
Particle scale with DEM approach
− Single particle force balance
solved for each dicrete particle
− Every collision between particles
recorded and affecting to the
particle movement
− Very detailed results but
simulations computationally
heavy
− Study of drag force between the
fuel and bed particles
Large fuel particle (red) in a bed of small
particles. Force on the fuel particle recorded
as function of time.
Fuel flow:
Particle scale with DEM approach
Jalali, P., Nikku, M., Hyppänen, T. 2013. Particle-Cloud Drag Force in Dilute Particle Systems: Discrete Element Method versus Eulerian
Simulations, Industrial & Engineering Chemistry Research 52
Fuel flow:
Meso scale CFD
− CFD (3 phases, Eulerian approach) used
to model transient mixing of fuel from
the feeding without reactions
− 3 mesh sizes used and compared
− Time and space averaging
− Finding corrections to the drag force from
fine mesh to be applied with coarser
meshes
0s
5s
10s
15s
20s
25s
30s
35s
40s
Omid Joneydi. 2013. CFD Modeling of Circulating Fluidized Bed: Effect of Volume Averaging on Solid-Solid Drag Force. Master’s Thesis. LUT.
Fuel flow – 3D large scale modeling
− Existing 3D semi-empirical,
steady state, holistic furnace
model
− Convective solution for fuel
momentum equation added
− Target profile + dispersion
type solution in the old model
− Dispersion still used in the
modeling of mixing
− Improves modeling of biofuels
which are significantly lighter than
coal and bed material
− Model developed and validated in
ERA-NET Biomodelling project
Nikku, M., Myöhänen, K., Ritvanen, J. Hyppänen, T. 2014. Modeling of fuel flow in circulating fluidized bed furnaces. 11 th International
conference on fluidized bed technology, 14th-17th May 2014 Beijing. In review.
Nikku, M., Myöhänen, K., Hyppänen, T., 2012. ERA-NET Biomodelling - final presentation of LUT, 24th September 2012, Vienna.
Conclusions
− Fluidized beds and fuel flow modeling
− Several scales
− Varying level of details
− Practical applicability to different cases
− Micro scale models help to understand the phenomena involved
− Meso scale models bring insight when several different phenomena are
combined, might be applicable to laboratory and pilot fluidized beds
− Macro scale models are combining all the related phenomena and
simplifying the physics and chemistry in order to achieve reasonable
compational times
− As the computational power increases, more detailed models can be utilized
in modeling of commercial scale fluidized bed applications
Thank you for your
attention!
Questions, comments… ?