This document presents a thermal stresses based fragmentation model for pulverized coal particles during low temperature air plasma gasification. The model tracks temperature distributions within particles using a 1D radial approach and calculates thermal stresses to predict crack initiation and particle fragmentation. It was implemented using user-defined functions in ANSYS Fluent. Results show increased volatile release, CO production, and conversion rates when fragmentation is included. Future work will refine kinetic parameters, test different particle properties, and validate with experimental pilot plant data.

1. Development of a Thermal Stresses Based
Fragmentation Model for Pulverized Coal
Particles Gasification by Low Temperature
Air Thermal Plasma
25th May 2016
ANSYS Convergence Regional Conferences
City Hotel, Ljubljana, Slovenia
Belgrade University, Institute of Nuclear Sciences “Vinca” ,
Laboratory for Thermal Engineering and Energy, Belgrade,
Serbia
Rastko Jovanović, Dejan Cvetinović, Predrag Stefanović,
Predrag Škobalj, Zoran Marković

2. Introduction
Motivation
Model description
Results
 Conclusions
 Future work
Presentation outline
2/26

3. Introduction, Plasma fire support technology
3/26
Air-coal
mixture
channel
Plasma
generator Boiler furnace
Hot air plasma produced in plasma
generators is introduced into air – coal
mixture duct.
The plasma flame with high thermal
energy induces coal gasification and
partial char oxidation producing highly
reactive mixture
This highly combustible mixture is
easily ignited at the furnace entry
ensuring high flame stability and overall
increased combustion efficiency.

4. Introduction, Plasma coal gasification process
4/26
Specific phenomena
have to be taken into
account:
.Very high
temperatures
Complex reaction
mechanisms and
kinetics
Fragmentation

5. Motivation
5/26
It is necessary to demonstrate its advantages over conventional
systems
The main challenge is that the most CFD codes are suitable for p.
f. combustion simulation under conventional conditions.
Particle fragmentation is commonly neglected.
During plasma coal gasification, very high plasma temperature
induces strong particle thermal stresses.
These stresses lead to “thermal shock” and extensive particle
fragmentation.
Fragmentation intensifies devolatilisation (3-4 times) and
significantly accelerates char oxidation.
The main aims of this work are:
 development of fragmentation model based on calculations of the
thermal stresses inside pulverized coal particles
 Model implementation in ANSYS FLUENT combustion model using
User Defined Functions (UDFs).

6. Mathematical model, general features
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The reactive flow field was described in Eulerian manner.
The turbulence was modeled using the standard k − ε
turbulence model together with standard wall functions.
Radiation was modeled using Discrete Ordinates (DO)
model.
The gas radiation absorption coefficient is calculated as
function of characteristic cell-size and gas concentrations.
Gaseous reactions were modeled using finite rate/eddy
dissipation model.
Char reactions were modeled using kinetic rate/diffusion
limited model.
The pulverized coal particles combustion is modeled in a
Lagrangian reference frame.

7. Mathematical model, fragmentation modeling
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Physical
particles with
same properties
Computational
cell
Strength = 6
Strength = 4
Computational
particles − parcels
In the Lagrangian approach number of computational particles are chosen to
represent actual physical particles with a same characteristics.
In order to take into account number of physical particles in a single parcel
additional variable termed “strength” is used.

8. Mathematical model, fragmentation modeling
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9. Mathematical model, fragmentation modeling
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Standard model − zero dimensional model
TP
rP
vP
Tgas
radiation
convection

10. Mathematical model, fragmentation modeling
10/26
UDF based model − one dimensional model
rP,0 rP,max
i i+1i-1
rP,i-1
TP,i-1
rP,i+1
TP,i+1
rP,i
TP,i
Tgas
radiation
convection

11. Mathematical model, implementation
11/26
Governing transport equations
DPM equations
User FV code -> TP[i]
inert heating
devolatilization + fragmentation
(Dp,new, mP,new, ρP)
char combustion
σ1,3[i] ≥ σuu/N
σr[i], σt[i] = f(TP[i]) -> σ1[i], σ2[i], σ3[i]
inert heating
devolatilization
char combustion
yes no

12. Results, temperature distribution
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No fragmentation

13. Results, temperature distribution
13/26
5 fragments

14. Results, temperature distribution
14/26
8 fragments

15. Results, temperature distribution
15/26
10 fragments

16. Results, volatile species mass fraction
16/26
10 fragments
No fragmentation

17. Results, CO species mass fraction
17/26
10 fragments
No fragmentation

18. Results, volatile and char conversion
18/26

19. Fragmentation model based on fracture mechanics was developed and
implemented into ANSYS FLUENT solver with extensive use of UDFs
Model is able to predict crack time and location of crack initialization
inside combusting coal particle
Model was successfully applied to plasma supported coal particles
gasification
The obtained results show high influence of “thermal shock”
phenomenon on plasma gasification performance
Model will be further developed using trial and error procedure,
testing different kinetic constants, particle thermal and transport
properties, different particle sizes and different fragmentation
criteria
In the final stage it is expected to compare numerically predicted
data with experimental data on pilot plasma burner which is under
modernization in Laboratory for Thermal Engineering and Energy, Vinca
Institute of Nuclear Sciences, Serbia
Conclusions and future work
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20. THANK YOU FOR YOUR ATTENTION
Acknowledgments
The authors would like to acknowledge high appreciation
for the support and promotion of this work to the Public
Enterprise ”Electric power industry of Serbia”, Belgrade,
Serbia, and Ministry of Education and Science of Republic of
Serbia (Project No. III42010 and TR33050)