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)

4

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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