Applied Thermal Engineering 90 (2015) 831e837
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Applied Thermal Engineering
journal homepage: www.elsevier.com/locate/apthermeng
Research paper
CFD modeling of carbon combustion and electrode radiation in an
electric arc furnace
Cemil Yigit a, Gokhan Coskun a, *, Ekrem Buyukkaya a, Ufuk Durmaz a, Hasan R. Güven b
a
b
Department of Mechanical Engineering, Sakarya University, Sakarya 54187, Turkey
_
_
Department of Mechanical Engineering, Istanbul
University, Istanbul
54452, Turkey
h i g h l i g h t s
3D-CFD of carbon combustion and electrode radiation were performed for an EAF.
Particles surface and gas phase reactions used to predict coal particle combustion.
Realistic temperature distribution obtained for slag surface.
Temperature, CO and CO2 distribution of carbon combustion visually obtained.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 March 2015
Accepted 23 July 2015
Available online 4 August 2015
This paper, concentrates on a three-dimensional (3D) computational fluid-dynamics (CFD) model for coal
combustion and electrode radiation inside an electric-arc furnace (EAF). Simulation of the melting
process includes combustion reactions of coal particles and radiation from electrodes. Particle surface
and gas phase reactions were used to predict injected coal particle combustion. The predicted temperature distributions are realistic because of the inclusion of combustion reactions and radiation models
together. The CFD model provided detail information for the coal particles combustion and radiation
interactions phoneme inside the electric-arc furnace. The cooling process of the EAF walls were
considered in the CFD model hence heat losses from the walls have been found higher than the earlier
studies. Results showed that CFD simulation can efficiently be used to develop and investigate EAF in
design phase.
© 2015 Elsevier Ltd. All rights reserved.
Keywords:
Electric arc furnace
CFD
Combustion
Radiation
Heat transfer
1. Introduction
Today, energy is an expensive and valuable resource for the
heavy industry operations. Approaching to the maximum efficiency
is one of the critical targets for the industrial sustainability.
Generally electric arc furnaces consume high levels of energy to run
the steelmaking processes that uses the coal and the electricity as
the main resources of the energy. In order to increase the efficiency
of these types of huge processes expensive experimental investigations using trial-error methods are required. Recently
developed computational resources can solve interpenetrating
physical phenomenon such as the solid particles combustion and
the evaporation as well as the turbulence and the radiation
together. CFD is an advanced simulation tool with a capability to
* Corresponding author.
E-mail address: gcoskun@sakarya.edu.tr (G. Coskun).
http://dx.doi.org/10.1016/j.applthermaleng.2015.07.066
1359-4311/© 2015 Elsevier Ltd. All rights reserved.
comprehensive model for the industrial applications such as EAF. In
this study a commercial CFD software ANSYS FLUENT 14.0 [1] is
used to develop a simulation methodology on EAF. One of the
important targets to create an extensive CFD model of EAF is to
investigate the combustion phoneme of injected coal particles,
electric arc radiation and slag surface temperature distribution in
order to analyze and verify the accuracy of simulations. In addition,
the CFD model provided detailed information on the spatial distribution of the jet velocity, combustion species distributions and
heat output from the EAF walls.
A number of reference studies have been carried out on combustion for the EAF. A comprehensive CFD model on post combustion reactions in the EAF has been studied by Li and Fruehan [2].
The authors simulated the radiation heat transfer, post combustion
reactions, and de-post combustion reaction which is reacted between CO2 and carbon in the liquid metal or the electrodes. In the
light of this information, coal combustion reactions have been
added and solved with using three gas phases and three char
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C. Yigit et al. / Applied Thermal Engineering 90 (2015) 831e837
oxidation reactions in the current work. Radiation heat transfer
from the electric-arc has significant effect to melt the metal
mixture in the EAF. There are several reference studies to compute
the radiation on heat transfer using CFD. Guo and Irons [3,4]
simulated the EAF radiation using CFD in order to introduce the
energy distribution of the radiation heat transfer. They studied the
radiation energy distribution for the sidewall refractory, watercooled side panels and the furnace roof. Based on Guo's study
some assumptions on heat transfer coefficients, radiation emissivity of the walls and the electric-arc surfaces were applied in the
current model.
Electric arc furnaces are widely used to melt scrap metal in the
steelmaking industry. Taking this into account, the studies on energy efficiency are of a great significance. Therefore, this study aims
to develop a comprehensive three dimensional CFD model for the
coal combustion and the radiation heat transfer process. In this
model, the location or quantity of the injectors, the injection angle,
chamber dimensions of the EAF, the oxygen or the coal contents
and radiation rates of the electrodes etc. can vary to reach the
targeted temperature on the melt surface. On that sense the model
provides flexibility on design parameters. Such a flexible CFD model
can provide optimization the combustion and the electrode radiation to decrease the energy consumption in an EAF. In this study, an
EAF produced by CVS Technologies for scrap melting process has
been modeled.
Table 1
EAF parameters.
Internal height (mm)
Internal furnace radius (mm)
Internal slag surface radius (mm)
Electrode radius (mm)
Coal injector diameter (mm)
Oxygen injector diameter inner e outer (mm)
Injector angle e Three
Oxygen flow rate (kg/s/injector)
Coal flow rate (kg/s/injector)
3505.25
3600
3080
305
25.4
68.14e45.4
45 downwards
1.2955
0.333
2. CFD model
2.1. Geometry
The EAF parameters which are given in Table 1 were used to
create the 3-D model for the CFD simulation. Boundary conditions
and angles on x, y coordinate of injectors were given particularly in
Fig. 1. Injector 1, 2 and 3 were positioned on þ X axis
25 , 25 , 102 respectively. All injectors located 1.07 m above
from the slag surface and aiming 45 downwards. Coal particles and
O2 were sent into the furnace through the injectors as shown in
bottom-right of Fig. 1. One of the main purposes of the model is to
obtain upper surface temperature distribution of the melt layer. In
order to simplify the model, bottom surface of the computational
domain was accepted as a slag surface instead of modeling the
overall melt volume. Computational domain borders also include
inner surfaces of the furnace walls, upper and side walls of the
chimney. Combustion products and the other unburned gasses
leave the computational domain from the exhaust surface of the
EAF. Bottom surfaces of three electrodes generate electric arc and
the radiate energy with the constant heat flux.
2.2. Computational mesh
Reliability of the CFD analysis results is directly related to quality
of the mesh structure. The skewness is an important parameter to
understand the grid structure quality therefore great attention has
been given to the skewness. Triangle prism mesh structures were
used to create small computational domains in the model. The
boundary layer mesh and weight factors are also used to create
some critical zones for the computational accuracy. The sufficient
grid refinement at the slag surface, arc generated surfaces of the
electrodes and the injectors are necessary to capture the surface
interactions and provide computational stability. The mesh structure of the EAF model, mesh inflation at the slag surface and dense
grids at the injectors can be seen in Fig. 2.
Sensitivity of different grid densities, with the total number of
cells ranging about 560,000e1,130,000 is tested for the same
Exhaus
Electrodes
Furnace
Wall
Injector 1
Injector 2
Upper
Bath
Injector 3
Oxygen
Lower Bath
Slag surface
Fig. 1. 3-D model of the EAF.
Carbon
C. Yigit et al. / Applied Thermal Engineering 90 (2015) 831e837
833
Fig. 2. EAF mesh structure (left). Dense mesh structures for injector area (right-up) and boundary layer approach above the slag surface (right-down).
model. The results indicated that a sufficiently fine grid is achieved
with 733,000 cells. The average temperature on the slag surface no
longer changed by increasing the grid size as indicated in Table 2.
reaction [5,6] into account, CO applied as 0.021455 kg/m3s as an
inlet for the slag surface.
2.4. Numerical model of the EAF
2.3. Boundary conditions
The standard wall function was applied to each wall boundaries.
873 K and 773 K static temperatures applied for the lower and the
upper bath walls respectively. Considering the cooling process,
static temperature was applied as 393 K to the furnace wall.
1.15e þ 08 W/m2 heat flux was applied at the bottom surface of the
electrodes in order to model 100 MW electricity. To obtain morerealistic thermal boundary conditions convection to the outer
ambient and conduction on the melt layer were considered in the
model by activation convection and conduction heat transfer
equations at the slag surface boundary. Convection heat transfer
coefficient was used as 30 W/m2K, thermal conductivity was used
as 0.5 W/mK. The radiation internal emissivity was applied to the
each wall in the model. The internal emissivity as the boundary
condition was used as 0.7 for the slag surface, the lower and the
upper bath walls and for the granite electrode walls was 0.85 [3].
As shown in Figs. 1 and 2, the outer tube feeds the oxygen
enriched air and defines as velocity inlet boundary condition. A
constant velocity of the oxygen enriched air is 480 m/s and oxygen
mass fraction is 0.3871. Initial temperature of the air was set as
1500 K to activate the combustion reactions. Inner tube was defined
as an injection model which feeds the coal particles. Velocity and
temperature of the coal particles are 0.7591 m/s and 300 K
respectively. The CO gas emerges from the slag surface to inside the
furnace therefore the slag surface is treated as an inlet boundary
condition. In a typical heat in the EAF, about 1.5 kmol of CO is
generated for per ton of steel [2]. Taking CO produced from FeO
Table 2
Average results from slag surface of different grid densities.
Number of mesh Radiation
Static
elements
temperature (K) temperature
(K)
Mass fraction C <s>
of mvvol
concentration
564,657
733,962
1,129,940
0.14124
0.119
0.1186
2045
2045
2048
1880
1882
1884
0.00026661
0.0002215
0.000216
The model of EAF includes different types of mathematical
methods as the turbulent flows, combustion reactions and the radiation heat transfer, etc. Solving all these equations from the
beginning of the solution for the initial conditions simultaneously
is almost impossible for converging. In order to resolve this problem equations were activated progressively. In the beginning, flow
analysis run for the sufficient iterations, later combustion equations
and finally radiation equations were activated. The pressure velocity coupling is achieved through the COUPLED e algorithm.
The realizable keε model was applied in the model to solve the
turbulence which shows a good agreement with experimental
observations of the gas flows with the coal particles combustion
according to literature [7]. Previous studies on the liquid fuels and
the coal combustion modeling [8e10] were showed that discrete
phase model (DPM) which has applied in current study, is a useful
method to simulate the particle trajectory. While the coal particle
phase is treated with an Eulerian description needs lower CPU then
Lagrangian approach without any considerable differences [11].
Therefore Eulerian approach was considered in the current model.
The particle diameter distribution of the coal was modeled using
Rosin-Rammer method with spread parameter of 4.52 and 10
groups. The minimum, the maximum and the mean particle diameters were chosen as respectively 70e-06 m, 200e-06 m and
134e-06 m. Discrete random walk model was enabled for stochastic
tracking of coal particles.
From the literature on CFD simulation of the EAF, Scheepers
et al. [12] showed P1 radiation model in the Fluent is able to solve
the radiation effectively. Thus the P1 radiation model has been
chosen in order to solve the radiation originated from the electrodes and combustion reactions in the current model.
The conservation of the mass is solved for each relevant
chemical species. Reaction rates are calculated using the Finite-Rate
Eddy-Dissipation model [13] considering that the rate-controlling
mechanism which may be either the chemical kinetics or the turbulent mixing. In this way, the effect that local micro-mixing may
have in the determination of the rate of reaction [14].
In the CFD code, the coal is simply classified as a single species
due to the quantities of the fixed carbon and volatile matter [1].
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Table 3
Gas phase reactions.
Number
Reaction
A
Ea (j/kgmol)
1
2
3
mvvol þ 1:706O2 /CO2 þ 1:543H2 O
H2 þ 0; 5O2 /H2 O
CO þ 0; 5O2 /CO2
2.119eþ11
1eþ15
2.027eþ08
1eþ08
Several classifications for the coal can be found in the FLUENT
depending on property of the coal density, the specific heat and
component fractions etc. In this study the volatile matter (VM)
break up was assumed as a single hypothetic hydrocarbon
component consist of Carbon (C), Hydrogen (H), and Oxygen (O).
After injecting the coal particles into the computational volume,
volatile matters in the coal particles are initially converted to a
pseudo gas phase species with using the constant rate devolatilization model [15].
Three-step volumetric reactions with six species including the
volatiles matter combustion were applied to the model. These gas
phase reactions deals with the local chemical equilibrium are
calculated by Arrhenius type chemical kinetics. Three-step gas
phase reactions were listed in Table 3 including Arrhenius Reaction
Rate and Activation Energy.
When component of the volatiles matter is completely spread
out from the particle surfaces, solid phase reactions begin [16].
Assuming all solid matter composition is carbon in the model,
complex combustion mechanism of the char can be represented by
a simple three heterogeneous reactions to reduce the computational time. Surface reactions, as listed in Table 4, are solved in the
CFD code by using first-order global Arrhenius kinetics [7] and the
multiple surface reactions model [1]. Sum of the carbon consumption rates from each individual surface reaction equals to the
total carbon consumption rate at the computational volume [17].
3. Results and discussion
3.1. The flow field inside the EAF
Fig. 3(a) shows the velocity profiles on a plane intersecting the
center of the injector 1. Velocity data plotted in the Fig. 3(b) is
obtained from a straight line (demonstrated in the Fig. 3(a) with a
dash line) from the slag surface to the injector. Values range from a
maximum from 349 m/s to zero for the O2 stream and outer flow.
The speed of the jet decreases as it expands and mixes with the coal
particles and gases. Swirls and high temperature combustion effects expand the velocity gradients to the larger volumes. Jet reaches the melt surface about 75 m/s.
3.2. The thermal field inside the EAF
The temperature profiles inside the EAF are presented for a
plane intersecting from the center of the injectors. The combustion
temperature profiles of injector 1, 2 and 3 are shown in Fig. 4. The
temperature at the jet flow after injector 1 and 3 is distributed
between 2270 and 3200 K. Injector 2 which is located between two
Table 4
Char oxidation model of the coal particles.
4
5
6
A
3.3. Mass fraction of species inside the EAF
The gas and the solid particle concentrations in the EAF along a
line which is created from the slag surface (0 m) to the center of
injector 1 exit (1.2 m) are given in Fig. 8. Some of the solid coal
particles are evaporated rapidly as soon as they leave the injectors
while the surface reactions are started by the rest of them. The
surface reactions occur at 0.5 m away from the injector's exit
because of injector's design. Rapidly evaporated coal particles react
with the oxygen at high temperatures. As a result of the combustion
reactions, the mass fraction of the carbon dioxide increases to 0.35
while most of the O2 was consumed. The mass fraction of the
evaporated coal particles (the gas phase coal) dramatically decreases about 0.3 m away from the injector due to high temperature
combustion.
The carbon monoxide is one of the most important species for
the combustion reactions in the EAF. CO consists of two different
sources which are combustion of the coal particles and diffusion
from the slag surface. Some of the CO composed by the gas phase
and char reactions of the coal and CO2 species. The other source is
CO diffusion of the foamy slag coal layer at the slag surface [6]. CO
diffusion is originated from the injected coal and O2 among to the
metal and the slag interface. Fig. 9 shows CO and CO2 mass fraction
distribution around the injector 1. Formation of CO from combustion reactions is lower than the slag surface diffusion as indicated in
Fig. 9 (left). Additionally the beginning of the decline in the mass
fraction of CO2 right after high temperature region showing the
char oxidation (reaction 5 in Table 4) build up in the accumulated
coal particles above the slag surface.
3.4. Validation
Heterogeneous particle surface reactions
Number Reaction
injectors, the maximum combustion temperature at the jet flow is
investigated as 3000 K. The minimum temperature values are obtained at the near furnace walls as 692 K because of the heat
transfer effects from the cooled furnace walls. The maximum
combustion temperatures at the injector exits are continuously
decreased to around 2200 K on the slag surface.
The radiation temperature distribution which is identified on
the bottom surface of the electrode can be seen in Fig. 5. The radiation temperature range is around 150 K inside the furnace
chamber.
Probability Density Function (PDF) and the temperature distribution on the slag surface were introduced in Fig. 6. According to
PDF figure the slag surface static temperature variation is between
1860 K and 1970 K and the temperature of the slag surface mostly
accumulated around 1880 K which is enough to maintain melt
temperature at around 1800 K [18].
Fig. 7 shows the heat transfer distribution by the radiation and
the convection to the EAF walls, the slag surface and the chimney.
The heat generated by the radiation and the combustion are
100.75 MW and 5 MW, respectively. The heat transferred by the
radiation to the EAF's wall and the roof are 65,190 and 26,068 MW
respectively. The figure also shows that 8964 MW thermal energy is
transferred by the radiation to the metal slag surface. Besides that
the heat loss from the chimney via off gas is only 2348 MW. The
heat loss from the convection was very low in percentage compared
with the radiation heat transfer therefore a detailed diagram that
depicts the convection heat transfer is given in Fig. 7 (right).
Ea (j/kgmol) Diffusion rate constant
0.002 7.9eþ07
C < s > þ 0; 5O2 /CO
C < s > þ CO2 /2CO
C < s > þ H2 O/H2 þ CO
5e-12
In the current study the coal combustion was simulated by using
same reactions with Zhang et al. [7] which was validated with their
experimental results. To compare of these two studies proportional
gas temperature differences were obtained from belong figures.
Totally five different data points were selected. The highest
C. Yigit et al. / Applied Thermal Engineering 90 (2015) 831e837
835
Fig. 3. Velocity (m/s) magnitude for a plane intersecting the injector 1; (a) Velocity distribution profile and (b) Velocity data graph.
Fig. 4. Temperature (K) profile for a plane intersecting the injector 1, 2 and 3.
Fig. 5. Radiation temperature (K) profile for a plane intersecting one of the electrodes.
temperature in the core of the jet marked as point 3 of the injector 1
(Fig. 4) is 3200 K. The nearest ambient temperature to the jet flame
marked as point 1 is about 1960 K. The average temperature of jet
flame marked as point 5 is about 2580 K. Points 2 and 4 are
geometrically middle points. Each data points were divided by the
maximum temperature value in order to obtain proportional gas
temperature differences. Same procedure was applied for Figure 17
in Ref. [7]. The graphical comparison consisting of proportional gas
temperature differences from the simulation results was given in
Fig. 10. Simulation results in the temperature field are compatible
with each other as understand from the figure.
Li and Fruehan obtained distribution of the heat output from the
CFD simulation and they gave the graphical result in Ref. [2]. They
obtained 12% of the total energy transferred to the metal by the
radiation while in current study is about % 9. This situation shows
that just about 10% of the total energy can be transferred to the
metal. On the other hand reference [2] obtained 45% of the total
energy transferred to the wall by the radiation while in current
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C. Yigit et al. / Applied Thermal Engineering 90 (2015) 831e837
Fig. 6. Static temperature (K) and PDF distribution profile on the slag surface.
Fig. 7. Heat output obtained from convection, radiation and off-gas.
study is about 65%. This difference is due to applying lower and
varying thermal conditions on the furnace walls. Already applying
different wall temperatures to the EAF walls for more-realistic
thermal boundary conditions was aforementioned in Ref. [2]. Also
Istadi and Bindar [19] focused on cooler design of an EAF using CFD
modeling. They obtained cooled furnace wall temperature distribution around 423 K which is almost equal to the current model's
boundary condition. Furthermore geometry of the EAF and combustion model differences also caused energy distribution variations on other boundaries between current and reference models
[2].
4. Conclusion
Fig. 8. Coal gas, coal particles, carbon monoxide, carbon dioxide and oxygen at the
center of the injector 1 jet.
Experimental analysis of factors such as the combustion, the
chemical reactions and the temperature distribution are effected by
the radiation in a complicated manner. This is primary importance
on the efficiency of an Electric Arc Furnace. This is also extraordinary expensive. Detailed examination of the processes that
occurred in the existing furnace is very significant to develop and
become more efficient.
C. Yigit et al. / Applied Thermal Engineering 90 (2015) 831e837
837
Fig. 9. CO (left) and CO2 (right) mass fraction in combustion field on a plane intersection the injector 1.
Acknowledgements
Proportional Gas Temperature
Differences
1.2
Model
1
This work was supported by the CVS Technologies at GebzeKocaeli, TURKEY. We would like to thank the CVS for their support.
Zhang et al. [7]
0.8
0.6
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0.2
0
0
1
2
3
4
5
6
Data Point
Fig. 10. Proportional gas temperature differences for each data point. Comparison of
the current simulation and Zahng et al. [7].
In this study, the thermal effects of the coal combustion and the
electrode based arc radiation are modeled for a commercial EAF
using the state of art simulation technique of CFD. Core temperature occurred with the combustion in the jet and the temperature
distribution on the slag surface has been analyzed in detail. The
variation of the CO and CO2 which are occurred by the combustion
reactions and diffusion from the slag surface are given numerically
and graphically. The effect of the radiation on the temperature
distribution inside the EAF and on the slag surface is examined. The
total and the average temperature distribution on the slag surface is
around 1770 K. It is important to keep scrap metal at the fluid phase
for the better quality. The average temperature at the slag surface is
1880 K while the minimum temperature is 1770 K. This temperature distribution meets the requirements.
The developed model can help us to increase the thermal efficiency of an EAF by testing at the design level. It was determined
that the model is fast, reliable, low cost and able to give detailed
results. This model gives us an economic alternative against the
costly experimental studies and also gives realistic and detailed
results against the zero dimensional calculations which have the
less consistency.
It is seen that the major heat transfer mechanism is the radiation
predictably. As a result of the study the heat transferred to the
metal slag surface is very poor in comparison with the heat
transferred to the EAF walls and the roof. The radiation heat loss
from the EAF's wall and the roof is considerable.