Numerical modeling of coal gasification
in a small scale circulating fluidized bed reactor
A. Klimanek, W. Adamczyk, A. Katelbach-Woźniak, A. Szlęk
Institute of Thermal Technology
Silesian University of Technology, Poland
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Motivation:
- Gasification of coal is one of the promissing `clean coal technologies’.
This includes gasification in CFBs
- Variety of modeling approaches can be used: from lumped models to
detailed DNS/DEM
- A few approaches are applicable to simulate medium and large scale
industrial facilities with detailed flow information in reasonable time
- Project realized within the program: Developing a technology of coal
gasification for high efficient production of fuels and electric power,
CzTB 5.2. funded by National Centre for Research and Development in
Poland
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Classification of multiphase models for fluidized beds
Length scale 1 μm 1 mm 0.1 m 10-50 m 1 m
Micro scale Meso scale Macro scale Lumped models
Unsteady
Steady and
quasi-steady
Correlation
models
0D
Global scale
Empirical and
semiempirical
1D/1.5D/3D
Euler-Lagrange
DEM/DPM-CFD,
2D/3D
Averaged CFD
2D/3D
Euler-Euler
CFD-TFM
2D/3D
Particle scale
DNS, LBM,
DEM/DPM 2D/3D
1 μs
1 ms
1 s
1 h-1 d
1 year
Reproduced from: K. Myohanen, T. Hyppanen, 2011
Euler-Lagrange Dense Discrete Phase Model - ANSYS Fluent
Averaged continuous phase (mainly RANS)
Dispersed phase tracked in Largrangian frame – single
particles or their groups (parcels)
Extended DPM for dense systems by means of KTGF –
calculated on Eulerian grid
PSD naturally taken into account
Grid independency for larger mesh sizes than TFM
(Cloete et al., 2010)
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5
Geometry of the model
coal inlet
char
recilculation
oxidizer
inlet
outlet
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Input data (measured at IChPW, Zabrze, Poland)
Air
gasification (1/W33)
Air/steam
gasification
(5/WW47)
Air mass flow rate, kg/h 193,0 233,0
Steam mass flow rate, kg/h - 18,3
Oxidizer temperature, ○C 15 227
Coal mass flow rate, kg/h 171,0 181,0
Char yield, kg/h 89,0 83,0
Coal inlet temperature, ○C 15 15
Excess air ratio 0,127 0,145
Coal LHV, MJ/kg 27,5 27,6
Moisture (ar), % 5,3 6,60
Volatiles (ar), % 34,1 27,72
Fixed carbon (ar), % 48,1 55,14
Ash (ar), % 12,5 10,54
Air gasification %
Air/steam gasification
%
C 68,1 68,8
H 4,1 4,0
N 1,1 1,2
O 8,1 8,3
S 0,6 0,6
A 12,5 10,5
M 5,5 6,6
Ultimate analysis (AR)
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Volatiles composition
Measured Mole fraction Species in the
simulation
Mole fraction
CH4 0,176 CH4 0,176
CO2 0,036 CO2 0,036
CO 0,077 CO 0,077
H2 0,385 H2 0,385
H2O 0,172 H2O 0,172
H2S 0,012 TAR 0,155
NH3 0,048
C2H6 0,043
C6H6 0,040
C10H8 0,012
Air gasification – input data
Coal gasification model
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Coal particle Devolatilization
Heating and
drying
Volatiles
Char
Homogeneous
reactions
Heterogeneous
reactions
Products
Products
Volatiles release and breakup
• Constant rate devolatilization model
• Fast artificial volumetric reaction of pseudo-species VOL breakup
VOL → 𝑎𝐶𝑂 + 𝑏𝐶𝑂2 + 𝑐𝐶𝐻4 + 𝑑𝐻2𝑂 + 𝑒𝐻2 + 𝑓𝑇𝐴𝑅
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Remaining homogeneous reactions
Tar combustion reaction (TAR) – Eddy dissipation model/finite rate
𝐶𝑂 + 0.5𝑂2 → 𝐶𝑂2
𝐻2 + 0.5𝑂2 → 𝐻2𝑂
𝐶𝐻4 + 2𝑂2 → 𝐶𝑂2 + 2𝐻2𝑂
CO, H2 and CH4 oxidation - Eddy dissipation model/finite rate
𝑇𝐴𝑅 + 𝑎𝑂2 → 𝑏𝐶𝑂2 + 𝑐𝐻2𝑂 + 𝑑𝑁2
𝐶𝑂 + 𝐻2𝑂 ↔ 𝐶𝑂2 + 𝐻2
Water gas shift (kinetics from Bustamante et al., 2004)
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Kinetics from Syamlal et al., 1992 Implemented via user defined
functions (UDF) mechanism
Char gasification – heterogeneous reactions,
multiple surface reactions model
Char oxidation
𝐶 + 0.5𝑂2 ↔ 𝐶𝑂
Bouduard reaction
𝐶 + 𝐶𝑂2 ↔ 2𝐶𝑂
Water gas reaction
𝐶 + 𝐻2𝑂 ↔ 𝐶𝑂 + 𝐻2
Methanation reaction
𝐶 + 2𝐻2 ↔ 𝐶𝐻4
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0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 1 2 3 4 5 6
Mas
s fr
acti
on
, kg
/kg
diameter, mm
Rosin-Rammler
eksperymentmeasured
Coal PSD
Input data
Mesh of the model
Calculations done for two mesh densities: 53.7 and 90.8 thousand elements
Caclulations procedure
• Calculations run unsteady
• Avareging started after obtaining pseudo steady state
• Time of averaging: 35 – 95 s
averaging time
Solids volume fraction
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Rys. 5. Chwilowe (rys. prawy) i średnie (rys. lewy) pole udziału objętościowego fazy rozproszonej
m/s
Gaseous phase mean (left) and instantaneous velocity (right)
m/s
Rys. 5. Chwilowe (rys. prawy) i średnie (rys. lewy) pole udziału objętościowego fazy rozproszonej
K
Gaseous phase mean (left) and instantaneous temperarture (right)
K
Rys. 5. Chwilowe (rys. prawy) i średnie (rys. lewy) pole udziału objętościowego fazy rozproszonej
Mass fractions of gasification products
CO CO2 H2 CH4
Rys. 5. Chwilowe (rys. prawy) i średnie (rys. lewy) pole udziału objętościowego fazy rozproszonej
Particle parcels
Particle residence time (left) and char mass fraction (right)
s
H2O mass fraction in gas ~ 7.0 %
H2O mass fraction in gas ~ 11.5 %
Comaprison of experimental and numerical results
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
N2 O2 H2 CO CO2 CH4 tar
mas
s fr
acti
on
in d
ry g
as
Air gasification
mesh 1
mesh 2
experiment
0.0
0.1
0.2
0.3
0.4
0.5
0.6
N2 O2 H2 CO CO2 CH4 tar
mas
s fr
acti
on
in d
ry g
as
Air/steam gasification
mesh 1
mesh 2
experiment
Results for air gasification
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Mass fraction of H2O in gas: 5.2%
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
N2 O2 H2 CO CO2 CH4 tar
mas
s fr
acti
on
in d
ry g
as
Gasification with air
experiment
simulation
simulation - modified wgs
Summary:
- Euler-Lagrange (DDPM) model applied for simulation of
coal gasification in a fluidized bed
- PSD included and coal surface reactions implemented
- ~ 4.5·106 particle parcels tracked with a time step 0.02 s
- Stiffeness of the reaction kinetics requires finer temporal
resolution
- Further validation of the results required for both air
and air/steam gasification
Acknowledgements:
The investigations have been supported by the National Centre for
Research and Development in Poland as a research project Developing a
technology of coal gasification for high efficient production of fuels and
electric power, CzTB 5.2.
www.zgazowaniewegla.agh.edu.pl www.ncbir.pl www.itc.polsl.pl www.polsl.pl
Thank you!
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