”Ari Vepsäläinen
LTY
Heterogeneous Mass Transfer Characteristics
in Char Combustion
at Fluidized Beds
Laboratory-to-Pilot
Fluidization Scaling Study
with Eulerian Multiphase CFD Model
Research program on sustainable energy ”SusEn”
Multiphase fluidized bed processes
in sustainable energy technologies
FIELD OF RESEARCH
Near Future Actions
I will start by presenting four themes for intruducing different concepts and separate sub-research fields..
1)
2)
3)
4)
5)
Cluster/Particle Scales
3 Fluidized Bed Models
Challenge of Sherwood – Heterogeneous mass transfer in fluidized Beds
Art of Scaling Fluid dynamics – Mr. Horio
Char combustion modelling approach and model applied here
−
A simple kinetic model, but interesting backgrounds..
Mid Term Execution Plan
.. Let’s continue by going getting familiar with FB Models and see what they can do and tell us about
heterogeneous characterisitcs of fluidized beds
A. Transient Eulerian CFD
B. Steady-state 1D 1-phase
C. Steady-state 1D 2-phase model
1.Cluster/Particle Scales &
2. FB Models
“..Local Bubble of thought..”
3. Challenge of Sherwood
.. Analysis:
1. How different Models consider heterogeneous Mass
transfer?
2. Usually expressed with dimensionless relationship with
Reynolds number
What is mass transfer resistance in FB?
• Scale: tests in lab scale
• Local / bed average
• Fluidization Mode

Versatility of available Models
• 1d, 1.5d, 2d, 3d
• Steady, quasi-steady, transient, dynamic
• Phases: dispersed / bubble & emulsion
• Included in reaction rate / mass transfer model
Lappeenrantalainen Ajatuskupla
4. Art of Scaling of Fluid dynamics
− Mr. Horio’s Approach [1]:
.. Analysis: How different Models consider?
1. Scale-up: tests in lab scale
2. Local / bed average phenomenon
3. Fluidization Mode
Lappeenrantalainen Ajatuskupla
4. Art of Scaling of Fluid dynamics
”Can we do it.. With CFD?”
.. Analysis: How different Models consider?
1. Scale-up: tests in lab scale
2. Local / bed average phenomenon
3. Fluidization Mode
Lappeenrantalainen Ajatuskupla
4. Art of Scaling of Fluid dynamics
”Can we do it.. With CFD?”
.. Analysis: How different Models consider?
1. Scale-up: tests in lab scale
2. Local / bed average phenomenon
3. Fluidization Mode
Lappeenrantalainen Ajatuskupla
4. Art of Scaling of Fluid dynamics
”How about.. Fluidization Regime Map”
.. Analysis: How different Models consider?
1. Scale-up: tests in lab scale
2. Local / bed average phenomenon
3. Fluidization Mode
5. Char Combustion Model
”at it’s simplest form”
− To be able to just consider, focus and analyze transient heterogeneous
mass limitation..
− .. Let us consider only ONE chemical reaction:
Combustion Regime
• Kinetics with Arrhenius:
• CFD: Kinetically controlled Suitable for fine porous coal char
• 1D Models: additional mass transfer resistances:
o Convection-Diffusion + intra-emulsion mixing
o Bubble–emulsion phase mass transfer
• Char particle
o One particle size: 0.4mm (lab) & 1.25mm (Pilot)
o Constant density: 1000kg/m3
“.. And then Models, your turn.., first..”
2D Eulerian CFD Model - transiently
Similar approach as presented by my colleague Shah et al. [22]
− Eulerian 3-phase CFD model
− The modeling approach: the kinetic theory
of granular flow.
− Shah et al. (2012): mesh and time step
effects on cluster resolution and solid
density profiles in pilot scale CFB riser.
2D Eulerian CFD Model
Governing equations
2D Eulerian CFD Model
Pilot, 3.5m/s, mesh: 0.5cm*0.5 cm
10 seconds of fluidization (~10 hours)
2D Eulerian CFD Model
“Focusing to dense bed region”
1.
2.
3.
Bubble size & rising velocity
Reaction regimes
Frequency of fluctuations..
Emulsion/Bubble
interchange
Extreme lateral gas velocity
gradients
 low velocity at dense regions,
high at bubbles
Oxygen-char contact
near grid
..Response from 2D
Eulerian CFD Model:
Char-Oxygen Contact Efficiency
”One Way of saying it.. Straigthforward way”
 Scaling of mass transfer limitation on
reaction rate
 Based on averaged CFD results

e.g. Oxygen continuity equation in 2phase model:
2D Eulerian CFD Model
.. Fluidization Characteristics
..Cluster frequency analysis..
Steady-state 1D 2-phase Model
Steady-state 1D 2-phase Model
− Phase characteristics
Steady-state 1D 2-phase Model
Steady-state 1D 2-phase Model
Phase mass transfer coefficient kl [1/s]
Pilot reactor - phase mass transfer analysis:
..last, but not least..
Steady-state 1D 1-phase Model
Steady-state 1D 1-phase Model
..giving Sherwood.. and showing dimensionless relationship with Reynolds number
Vepsäläinen et al. (2012)
”SHAH ANALOGY”
One extra possibility..
Dimensionless relationship between
Phase Drag
&
Heterogeneous Mass Transfer
 Can this kind of relationship be established?
 If YES, by just CFD modeling fluid dynamics, mass transfer could be estimated..
 Just for Fun?
Nice to know – Note: Combination of solid and gas phase momentum equation [Kallio]:
Let’s Sum it Up
Themes
1) Fluid dynamic Scaling (Horio) Lab-to-Pilot
−
interpreting test results based with proper understanding of fluid dynamics
2) Characteristics of FB Models:
−
Eulerian transient 2D CFD, Two steady-state 1D models: 1 - & 2-phase
3) Challenge of Sherwood Number
4) Eulerian CFD Validation:
−
Heterogeneous mass transfer can be predicted on certain accuracy with proper modelling
technique
Results
− 3 Ways of presenting mass transfer limitation in heterogeneous reactions
− 3 FB Models
− Lots of Modeling Experience..
.. ”Ja jottei homma näyttäisi täysin itse keksityltä” ..
References
ACKNOWLEDGEMENT
− The authors would like to acknowledge the financial support for this
research from the Academy of Finland under grant 124368.
− Special thanks also to Fortum Oyj.
− The author would like to express his thanks to Prof. Dr. Timo Hyppanen for
his valuable inspiration and contribution to this work.
Research program on sustainable energy ”SusEn”
Multiphase fluidized bed processes
in sustainable energy technologies
See you at..
.. 21th International FBC Conference