cfd modelling of wet flue gas desulphurization (wfgd) unit...
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CFD Modelling of Wet Flue Gas Desulphurization (WFGD)
Unit: A New Era of Process System Control and Optimization
A. Arif , R. C. Everson, H. W. J. P. Neomagus
Emission Control
North-West University, Potchefstroom Campus, Potchefstroom 2520, South Africa
Outline
Purpose
Theoretical framework
Results and conclusion
Implications
Acknowledgement
1
2
3
4
5
Purpose
• Overall objective of research
– Comprehensive CFD model for WFGD unit of high capacity coal fired power
station to develop
» Simulator for examining operation and trouble shooting
» Simpler models for integrated system analysis for energy optimization
» Recommendation for process modifications (sorbent, composition,
control etc.)
• Objective of presentation
– CFD Model of an industrial WFGD to study
» Hydrodynamics of flue gas and slurry droplets
» Heat transfer between flue gas and slurry droplets
» Slurry droplets characteristics (size, interaction, distortion etc)
» Slurry droplets evaporation
» Slurry distribution (nozzle location)
Introduction
• Power industry has been challenged by environmental initiatives
• Emission controls on modern power stations account for 10-20 % of the capital
investment1
• Significant operation and maintenance cost
• Power generation represents the largest controllable source of SO2 emissions
• Dominating SO2 absorption technology in the world is wet flue gas
Desulphurisation (WFGD)
– Where SO2 of flue gas is scrubbed by slurry of lime stone in counter current operation
• For an estimation of SO2 removal, It is important to know2
– Exact flow characteristics
– Slurry droplets behavior
• Detailed modeling using computational fluid dynamics (CFD) platform
1Marocco, L., 2008. PhD Dissertation, Politecnico di Milano, Italy 2Bautsch, C., Fahlenkamp, H., 2006, IClass-2006, Kyoto, Japan
Introduction
• Open literature studies are limited to small scale / pilot plants, ignoring
– Droplet-wall interactions, distortion & size distribution
– Enhancement studies
– CO2 desorption & water condensation
– Natural oxidation of sulphite to sulphate
– Interphase chemistry with rate studies
5
South African power stations & WFGD
7
South African power stations & WFGD
WFGD's Absorber
Absorber geometry
Nozzle # 42
Slurry spray bank
CFD modeling
– Modeling approach = Euler-Lagrange
– Phase interaction = Two way coupling
– Turbulence model = k-ε turbulence model
– Nozzles = 1520 hollow cone point injectors
– Drag force = Liu dynamic drag coefficient model
– Droplet distortion = TAB distortion model
– Mist eliminator = Porous media with suitable pressure drop
– Droplet-wall interaction = Escape, Rebound and Bai-Gosman wall impingement model
– Droplet size distribution = Rosin Rammler particle size distribution model
– Domain discretization = Polyhedral and prism layer cells with surface remesher
– Evaporation = Quasi - steady state droplet evaporation model
– Absorption = Coupled specialized model
Rosin Rammler particle size
distribution model
Rosin-Rammler Exponent = 3.05
Rosin-Rammler Diameter = 2650 μm
Minimum Droplet Diameter = 268 μm
Maximum Droplet Diameter = 5100 μm
1 exp
q
ref
DF D
D
100exp
100log log
100log log log log log log
100log log log
q
ref
q
e
ref
ref
DR
D
D
R D
q D q D eR
q D CR
Bai- Gosman wall impingement
Incident Weber Number
Laplace Number
Boundary Temperature
Wall State (Wet or Dry)
Bai, C. and Gosman, A.D. 1996
The Liu dynamic drag coefficient is intended to account for the dependence of the
drag of a liquid droplet on its distortion under the action of aerodynamic forces.
As the distortion of the droplet increases, its shape is assumed to become a disk
whose axis is aligned with the relative velocity. This increases the drag on the droplet.
The Liu drag coefficient models this effect by noting that the high Reynolds number
limit of the drag coefficient of a disk is 1.54. It then assumes that the disk drag is
1.54/0.424 higher than the sphere drag at all Reynolds numbers, and that the drag of
intermediate shapes can be interpolated between those two extremes. So
The interpolation factor y is 0 for a sphere and 1 for a disk. It is identified as the TAB
distortion, and hence the Liu drag coefficient requires the TAB distortion model to
calculate this quantity.
Liu dynamic drag coefficient
Liu, 1993
TAB distortion model
The TAB Distortion model is used to calculate the distortion of liquid droplets under the
action of aerodynamic forces. It calculate the instantaneous displacement x of the
droplet equator from its equilibrium position .
Distortion Rate Surface Tension
Viscosity Weber Number
Damping Coefficient Stiffness Coefficient
Critical Weber Number
t
d k
crit
Where
y
We
C C
We
Baumgarten, 2006, O'Rourke, 1987
Modeling equations
Continuous Phase
ρ = Density of flue gas τ = Shear stress
u = Velocity of flue gas τR = Reynolds stress tensor
p = Pressure of flue gas ωA = Mass fraction of component A
g = Acceleration due to gravity kc = Thermal conductivity of flue gas
T = Temperature of flue gas DAB = Binary diffusion coefficient of A in B
Modeling equations
Dispersed Phase
After evaluation of flue gas velocity field, particles trajectories can
be computed. The equation of single parcel takes the following
forms
The above equations can be solved by stepwise integration over discrete
time steps, using the continuous phase flow properties at the current droplet
positions.
The evaluation of particles source terms allows the determination of gas
source terms.
Mass transfer source terms
Mass Source Term
• Mass transfer by evaporation of water from slurry to gas phase and absorption
of SO2 from gas phase to slurry droplet, ignoring water vapor condensation and
SO2 desorption.
2 2 2 2, , ,k mass k H O k SO H O SO
S S S m m
2 2 2 2, , ,H O H O g H O H O iN k P P
2 2 2 2 2, , ,SO SO tot SO SO SON k P H C
M. Gerbec, A. Stergarsek, R. Kocjancic, 1995. Computer Chem. Engg.
SO2 absorption and chemical reaction
2 2 2
2 2 2
2 2 2 2
2
2
2 2
2
,
,
o
,
,
...........................(1)
For 0 150 for Air-Water System
ln ln G.Maurer-1980
0
Then
SO i
SO i SO SO
SO SO
SO
SO
SO
SO G SO
d
SO G SO d
d d
d
d
SO G SO
d
N K P P
P H C
N K P H C
C
AH B T CT D
T
Initially
C
N K P
C
2 Surface Area Droplet Residence Time
Droplet Volume
SON Droplet
2
2
2
2
2
2
2
3
Mass transfer flux
Global mass transfer coefficient
Partial pressure of SO in gas bulk
System absolute pressure Mole fraction SO
Henery's ConstantSO
SO
G
SO
moleN
m s
moleK
m s pa
P pa
pa mH
mole
2, 23
Concentration of SO in liquid bulk
Droplet Temperature
SO d
d
moleC
m
T K
SO2 absorption and chemical reaction
2
2
2
2 2
2
2
2
2
2
,
,
0.5 0.33
,
,
1
23 1.75
, 21/3 1/3
, ,
1
1
10
2 0.55Re
,Re
1 19.86 10
SO
G
G SO d
d SO
d
d
d SO H O
d
G dd
f SO
f d d d
f f SO d
f SO
f SO
m f m SO
KH
k k
Dk
d
DConst
T
k RTdSh Sc
D P
v dSc
D
TM M
DP V V
2
2
2
2
, 2
Gas side mass transfer coefficient
Liquid side mass transfer coefficient
10 Enhancement Factor
(Binary liquid diffusion coefficient of SO in water)
Gas Constant
G
d
SO
d SO
mk
s
molek
m s pa
mD
s
JR
mole K
P
2
2
, 2
3
Gas pressure in absolute values
Gas Temperature
Droplet diameter
Binary gas diffusion coefficient of SO in air
Viscosity
Density
Volume
d
f SO
m
pa
T K
d m
mD
s
kgpa s
m s
kg
m
M MolecularWeight
V Molecular
SO2 absorption and chemical reaction
Soren Kill, Michael L. Michelsen and Kim Dam Johansen., 1998, Ind. Eng. Chem. Res.
SO2 absorption and chemical reaction
• Absorption (Spray Zone)
• Neutralization (Spray Zone)
• Eight nonlinear algebraic
equations and eight unknowns
dissolved species
concentration i.e SO2 (aq),
CO2(aq), H+, OH-, HSO3
-,
SO32-, HCO3
-, CO32-
SO2 absorption and chemical reaction
Marocco, L., 2008. PhD Dissertation, Politecnico di Milano, Italy
Results: Sensitivity analysis
0 50 100 150 200 250 300 350
0
1
2
3
4
5
6
7
8
Ve
locity (
m/s
)
Iterations
7.4 M Cell
3.2 M Cell
2.5 M Cell
0 50 100 150 200 250 300 350
-100
0
100
200
300
400
500
600
700
800
Pre
ssu
re D
rop
(kP
a)
Iterations
7.4 M Cell
3.2 M Cell
2.5 M Cell
0 50 100 150 200 250 300 350
6
7
8
9
10
11
L/G
(1
00
0*V
ol F
rac)
Iterations
7.4 M Cell
3.2 M Cell
2.5 M Cell
25
20 m
20 m 20 m
Results: Single nozzle dynamics
Results: Single nozzle dynamics
Results: Full absorber dynamics
27
Results: Velocity profile
28
10 cm upstream of ME
Results: Slurry droplets diameter
29
Results: Evaporation
30
Results: L/G (dm3 of slurry / m3 of gas)
31
Level 1 & 2
Level 2 & 3 Level 3 & 4
Axial Profile
Conclusion
• CFD analysis shows higher velocity close to the WFGD's wall which
results in low L/G ratio near the wall and higher L/G ratio in the middle of
the column.
• Concentrating more slurry nozzles near the wall, results in uniform L/G
profile across the column.
• The estimated values of pressure drop across ME and nozzle dispersion
of slurry are with close agreement with the manufacturer data.
• As the flue gas flows inside the WFGD cooling occur with an increase in
moisture content which is due to counter current interaction with the
slurry droplets, thereby exchange heat and mass transfer as a result of
this interaction.
• Saturation is reached very soon after the gas enters the tower, which is
very close to the real plant observations.
32
Future work
• Modeling of interphase mass transfer
– Natural oxidation of sulphite to sulphate due to the oxygen
content in the flue gas.
– Studies with South African sorbents
• Modeling of interphase chemical reactions
– Complex chemistry with rate studies
– Enhancement studies
• Modeling of pilot plant for validation
33
Implications
The modelling results can be used
• In the design phase of industrial full-scale WFGD and for the
retrofitting of existing plants with WFGD unit
• Improve the efficiency of WFGD absorber at coal-fired power
plants by revealing regions of poor gas/liquid contact and
identifying a solution to eliminate them.
• Recommendations for major modifications to accommodate
alternative sorbents, different gas compositions and new process
control strategies.
• Understand the complex multiphase process inside the column
and thereby will help to cope with trouble shooting and emergency
conditions.
34
Acknowledgement
Thank You For Your Attention
Any Questions ??