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CFD Simulation of an Industrial Wet Flue Gas Desulfurization Spray Tower: A Comprehensive Model with Special Attention Devoted to the Modeling of Absorption and Chemical Reactions

Authors:
Raymond C Everson, Arif Arif, Hein WJP Neomagus and Dawie J Branken
Coal Research Group, School of Chemical and Minerals Engineering, North-West University,
Potchefstroom Campus, Private Bag X6001,
Potchefstroom 2520, South Africa
CFD SIMULATION OF AN INDUSTRIAL WET FLUE GAS
DESULFURIZATION SPRAY TOWER: A COMPREHENSIVE MODEL
WITH SPECIAL ATTTENTION DEVOTED TO THE MODELING OF
ABSORPTION AND CHEMICAL REACTIONS
2016 International Pittsburgh Coal Conference
Cape Town, SOUTH AFRICA
August 8 12, 2016
Coal fired power plants in South Africa
2
CFPPs Name
Avg. remaining life
Decom. date SO2compliance
New Build Kusile 50+ Y
Medupi 50+ Y/6y of commissioning
Existing
Majuba 33 2046-2051 N
Kendal 26 2048-2053 N
Matimba 24 2047-2051 N
Lethabo 22 2045-2050 N
Tutuka 22 2045-2050 N
Duvha 17 2040-2044 N
Matla 16 2039-2043 N
Kriel 13 2036-2039 N
Arnot 08 2031-2039 N
Hendrina 08 2030-2036 N
Return to Service
Grootvlei 07 2021-2023 N
Camden 03 2025-2028 N
Komati -2 2024-2028 N
Source: Ebrahim M Patel (2012) Practical Considerations in the Implementation of Emissions Reduction Solutions at Eskom’s Coal
Fired Power Plant, 4th EU-SA Clean Coal Technologies Working Group Meeting Emperor’s Palace, Kempton Park, RSA.
Table 1: Status of coal fired power plants in South Africa
SO2 emissions regulations in South Africa
3
70 90 %
SO2removal
Fig. 1: SO2 emissions regulations in South Africa
Table 2: SO2emission regulations for new and existing plants in South Africa
Flue gas desulphurization technologies
4
Source: Andreas Poullikkas (2015) Review of Design, Operating, and Financial Considerations in Flue Gas Desulfurization Systems,
Energy Technology & Policy, 2:1, 92-103, DOI: 10.1080/23317000.2015.1064794
Parameters WFGD
(LSFO) Semi-Dry FGD (CSD) Dry FGD
(CFB)
DeSO
Xefficiency > 96 % 60-90 % 80-96 %
Reagent used
Limestone Lime Lime
Size constraints
(single
unit)
1000 MWe 300 MWe 400 MWe
Absorber/unit (>400
MWe) Single Multiple Multiple
Power consumption
1-2 % 0.5-1 % 0.5-2 %
Water consumption
0.21 l/kWh 0.14 l/kWh 0.14 l/kWh
Construction cost
(₵/kWh) 0.91-1.07 0.82-1.07 0.84-1.12
Table 3: Comparison of different FGD technologies
Process description
5
© ALSTOM
ESP
Furnace
Inlet Duct
WFGD
Source: Retrieved from internet on 24/11/2015.
Fig. 2: Process description of WFGD
Wet Flue Gas desulphurization
6© Eskom & ALSTOM
Fig. 3: Block diagram of WFGD absorber
Scope of presentation
A complete CFD (computational fluid dynamics) model for an
industrial scale flue gas desulphurisation (WFGD) spray tower
was developed and validated which included the hydrodynamics
of the gas and droplet phases as well as the mass and heat
transfer between the phases and the chemical reactions inside the
slurry droplets.
The model is capable of estimating the desulfurization
efficiency, pH, enhancement factor and the concentration of
chemical species in the slurry droplets.
The mass transfer is based on the two film theory and the
reactions in the droplets are assumed all to be at equilibrium.
7
Scope of presentation
A non-isothermal operation was modeled with all parameters
adjusted accordingly.
The model was validated against plant results obtained from an
industrial WFGD absorber which included the prediction of flue
gas velocity, temperature and desulfurization efficiency.
The dependence of the desulfurization efficiency on the
enhancement factor and slurry pH obtained from the chemical
model was found to be well aligned with the plant tests and
literature results.
8
Model description: Spray Tower
9
Fig. 4: Model description of WFGD spray tower
Model description: Continuous gas phase
and droplet phase
Table 4: Brief overview of modeling equations
Parameters
Equations
Continuous phase (flue gas)
Mass
 
 
/.
f f f m
t v S

 
Momentum
 
 
 
/ . . R
f f f f f f f f f v
v t v v p g S
 
   
Energy
Species
 
 
,
/ . .
f A f A f f AB A A m
t v D S
   
   
Dispersed phase (slurry droplets)
Droplet position
/
d d G
dx dt v v
Mass
/
dd
dm dt m
Momentum
 
/
d d D p vm g Others
m dv dt F F F F F    
Energy
 
/
d d d conv rad other
m c dT dt Q Q Q  
Species
22
d H O SO Others
m m m m  
Drag force
, , ,
0.5
D D f d p d slip d slip
F C A v v
Drag coefficient
 
 
2/ 3
,
d
24/ Re 1 1/ 6 Re Re 1000
0.424 Re 1000
d d d
D Sphere
C
,
(1 2.632 )
D Sphere
C C y
Evaporation
 
2
*ln 1
H O s
m g A B 
Droplet wall interaction
( ) ( ) ( )
II
d w t d w t n d w n
v v e v v e v v 
Droplet distortion
 
2/bd
y X C d
Droplet size distribution
 
 
1 exp / q
d d ref
F d d d

 


Mist eliminator (ME):
Pressure drop
 
f f f
p v v
 
 
10
Table 4: Brief overview of modeling equations
Chemical reactions occurring in overall
process
12
Table 5: Chemical reactions occurring in WFGD (Marocco & Inzoli 2009)
Full Spray Tower Dynamics
13
Fig. 5: Animation showing slurry-gas dynamics in the
spray tower
Droplets diameters
14
Fig. 6: Profile of slurry droplet distribution in the spray tower
Temperature prediction
15
..
Figure 7: Flue gas temperature axial profile, cross section profile at test Points 1o, 70o, 130o,
190o, 250oand 310oat absorbers outlet and comparison of results with plant tests
Velocity prediction
16
Figure 8: Flue gas velocity magnitude axial profile, cross section profile at test Points 10o, 70o, 130o,
190o, 250oand 310oat absorbers outlet and comparison of results with plant tests
DeSOx Efficiency
17
Figure 9: Desulfurization efficiency axial profile, cross section profile at test Points 10o, 70o, 130o,
190o, 250oand 310oat absorbers outlet and comparison of results with plant tests
Enhancement Factor and pH inside tower
18
Fig. 10: Profiles of enhancement factor and slurry pH in the spray tower
Evaporation
19
Fig. 11: Moisture profile in the spray tower
Evaporation
20
Fig. 12: Temperature profile in the spray tower
L/G (dm3of slurry / m3of gas) across absorber
21
Fig. 13: L/G profile in the spray tower
Conclusions
22
A diffusion-reaction model for SO2diffusion and reaction inside a slurry droplet
was successfully coupled with the hydrodynamics of a wet desulphurization
spray tower to provide an overall model. This was accomplished with a CFD
code with user coding using C language.
The slurry pH and enhancement studies can also be estimated with the
developed model.
The desulfurization efficiency was successfully estimated by the development of
chemical rate model and its introduction into STAR-CCM+ with the help of user
coding which was compiled in C language.
The slurry profile obtained from the model can directly be used as inlet
boundary conditions to model Reaction Tank of WFGD.
The developed model can easily be modified for any type of gas liquid
absorption process with Euler-Lagrange multiphase assumption.
Conclusions
The velocity profile at outlet of flue gas duct, and desulfurization efficiency, flue
gas temperature and flue gas velocity at outlet of absorber were in good agreement
with plant test with an approximate deviation of up to ±5-8 %.
With the inclusion of evaporation for the operating used it was found that the flue
gas is rapidly cooled at the inlet and that saturation was observed almost over the
entire the column. Thus isothermal conditions were established which agrees with
published results.
It is recommended that the results from this study be used for limestone dissolution
kinetics in the absorber zone, absorption-desorption equilibrium of CO2in addition
to that of only SO2, natural oxidation of sulfite to sulfate due to oxygen present in
the flue gas, reaction tank CFD modeling for both hydrodynamics and interphase
mass transfer, coupling of reaction tank model with absorber model, modeling and
simulations of flue gas inlet duct, reaction tank, absorber and flue gas stack in a
single model to represent the actual operation.
23
Thank you
Modeling of Hydrodynamics (Results later)
25
Parameters
Modeling Method
Modeling approach
Euler
-Lagrange (gas and droplet phases)
Phase interaction
Two way coupling
Turbulence model
k
-ε turbulence model
Nozzles
Dual flow hollow cone point injectors
Drag force
Liu dynamic drag coefficient model
Droplet
distortion & breakup
TAB
distortion and breakup model
Droplet Coalescence
No Time Counter (NTC)
and
O'Rourke algorithm
Mist eliminator
Porous media with suitable pressure drop
Droplet
-wall interaction
Bai
-Gosman wall impingement model
Droplet size distribution
Rosin Rammler particle size distribution model
Domain discretization
Polyhedral/prism layer/surface
re-mesher
Evaporation
Quasi
- steady state droplet evaporation model
Table 6: Model development of WFGD spray tower
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