numerical simulation of heavy fuel oil combustion
TRANSCRIPT
NUMERICAL SIMULATION OF HEAVY FUEL OIL COMBUSTION CHARACTERISTICS AND NOX EMISSIONS IN CALCINER IN CEMENT INDUSTRY
Eng. Hisham Aboelsaod Prof.Essam E. Khalil
Cairo University, Cairo Egypt Cairo University, Cairo Egypt
ABSTRACT: The aim of the present study is to numerically investigate
the combustion characteristics of Heavy Fuel Oil (HFO)
and NOx emissions inside a calciner used in cement
industry. The calciner is a furnace placed before the rotary
Kiln its main objectives are the reduction of CO2
emissions and air pollutions while enhancing the cement
quality through separating the calcination and clinkering
processes. In order to conduct the present investigations
the calciner at CEMEX Egypt Cement Company was
considered and real dimensions and operating conditions
were applied. The combustion model was based on the
conserved scalar (mixture fraction) and prescribed
Probability Density Function (PDF) approach. The (RNG)
k-ε turbulence model has been used. The HFO droplet
trajectories were predicted by solving the momentum
equations for the droplets using Lagrangian treatment. The
radiation heat transfer equation was solved using P1
method. The formation of thermal NOx from molecular
nitrogen was modeled according to the extended
Zeldovich mechanism. The effects of varying the burner's
swirl number and viscosity grade on the combustion
performance of HFO and the resulting NOx emissions
were considered. The burner's swirl number influences the
mixing rate of air and fuel. A small swirl number ≤ 0.6 is
not desired as it elongates the flame; increases flue gases
temperatures and increases the NOx emissions inside the
calciner. A swirl number ≥ 0.6 is found optimal for good
combustion characteristics and NOx emissions
concentration. Meanwhile, it was found that the HFO
viscosity has a significant effect on the injection velocity
and must be considered as a function of temperature
during the analysis as this will significantly affects the
combustion characteristics.
Keywords: Combustion, Heavy Fuel Oil, Viscosity, Swirl
number effect, Nitrogen Oxides.
INTRODUCTION The main physico-chemical processes taking place in the
calciner are combustion and the strongly endothermic
calcination reaction of the raw materials CaCO3, the raw
material consist mainly of pulverized calcium carbonate
and silicon dioxide, passes through heating/drying at
temperatures from 100ºC to 500ºC the moisture evaporates
and after begin heated to the appropriate temperature, it
enters the calciner together with the fuel and the hot
gaseous from tertiary and kiln chamber, Figure 1. The
combustion heat released by the fuel causes calcination of
the raw material according to the reaction:
CaCO3 ------------- CaO + CO2 + 178 kj/mol
Combustion of HFO in Calciner
HFO combustion in calciner is non-premixed Liquid spray
combustion where a nozzle is used to atomize liquid fuel
into fine droplets injected in hot gaseous environment in
which it is mixed with the oxidizer. For calciner
combustion applications, the simple swirl nozzle is one of
the most common pressure atomizing nozzles. Transport,
dispersion, evaporation and combustion of liquid fuel
droplets and sprays are investigated in [1-3]. A few papers
conducted numerical and experimental studies of heavy
fuel oil spray combustion in cylindrical furnaces [4-6].
Barreiros et al. [4] investigated the effect of the burner
Proceedings of the ASME 2015 Power Conference POWER2015
June 28-July 2, 2015, San Diego, California
POWER2015-49569
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geometry and inlet velocities on the final products
temperatures, velocities and concentration inside the
furnace. Byrnes et al. [5] carried out an experimental study
on the same furnace to predict the gas temperatures, CO2,
CO, O2 and NO concentrations as well as particulate
emissions for five flames with different air excess ratios,
swirl numbers, primary air excess ratios and atomizer cup
speeds. Wu et al. [6] investigated the contribution of the
number, location, type and firing mode of fuel oil
atomizers on NO emissions reduction in an industrial
burner with heavy fuel oil combustion and highly
preheated air. Furuhata et al. [7] modeled heavy fuel oil
spray flame stabilized by a baffle plate. They obtained
good results for flow field under isothermal conditions. In
computational fluid dynamics (CFD) studies, Physical
model performance depends critically on the accuracy of
fuel property values, and the effectiveness of CFD
modelling depends on the precision of fuel thermophysical
properties. Kyriakides et al. [8] developed a model to
investigate the effect of HFO thermo physical properties
and compared the results of their model against
experimental data of Chryssakis et al. [9]. Recent
computational studies utilizing a simple representation of
HFO composition have considered a two-component
approach, with the two components representing a residual
(heavy) and a cutter (light) part, both characterized by
vapor pressures which are increasing with temperature,
and constant or temperature-dependent values for dynamic
viscosity, density, latent heat, specific heat capacity and
surface tension [10], [11]. Kontoulis et al. [12] presented
a good study with a new detailed model for calculating
IFO thermophysical properties that had been developed in
the model for different IFO qualities, the model builds on
the assumption that marine HFO is a heavy petroleum
fraction of undefined composition. The effect of injection
fuel temperature on the penetration tip investigated by
considering a five temperature values, while the
penetration tip is decreasing with fuel initial temperature,
higher penetration with the cold spray due to its minimal
breakup, and illustrating a minimal breakup and
evaporation of the cold spray affected on the spray
distribution including Sauter Mean Diameter (SMD)
values.
The motivation of the present study is to improve the
combustion of HFO inside the calciner in cement industry
through investigating:
1- The effect of HFO viscosity grade on the injection
velocity and the resulting combustion characteristics.
Two different viscosity conditions were considered.
Constant viscosity with its value fixed at 1.7×10-5
kg/m.s and variable viscosity with temperature as
stated by [12], Figures 1 and 2.
2- Effect of swirl number variation on the resulting
mixing between droplets and oxidizer inside the
calciner and the resulting improvement on the
combustion.
Numerical Model The calciner furnace, Fig.3, was investigated numerically
in the present study. The height of the furnace is 25m and
its volume is 150m3. Exhaust gases from kiln chamber and
atmospheric air from tertiary duct enter the Calciner. Two
burner nozzles 6 mm diameter, Figs. 4 and 5, are placed in
the first 8 m height from the calciner bottom to spray the
HFO inside calciner. Burner #1 injects the HFO in the
opposite (Z) axis direction while burner #2 injects the
HFO in the opposite (X) axis direction. The injection
characteristics are reported in Table 1. 3D model for the
calciner and the mesh was generated using (ANSYS 14.0
package), grid approximately 6500000 cells.
Figure 1: Computed dynamic viscosity of HFO qualities
versus temperature. A corresponding curve for C14H30 is
also used [12]
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Figure 4: Front view for the burner nozzle (6 mm)
Figure 2: Computed vapor pressure of HFO qualities versus
temperature. A corresponding curve for C14H30 was also
included used [12]
Figure 3: Calciner geometry and mesh
Figure 5: Side elevation of fuel distributer tangential slots
Tabel 1: Injection characteristics
MODELING FUEL OIL SPRAY COMBUSTION
In non-premixed combustion, fuel and oxidizer enter the
reaction zone in distinct streams. The present modeling of
mixture fraction [13,14] with the method of Probability
Density Function PDF with mixture fraction requires the
solution of transport equations for one or two conservative
scalar properties. The effect of turbulence is also
Inlet Boundary
Conditions
Burner #1
“ cone “
Burner #2
“ cone"
X- position, mm -100 794.09
Y-position, mm 4000 7200
Z-position, mm 609.09 -237
Axial velocity, m/s 40 & 80 40 & 80
Z-axis -1 --------
X-axis -------- -1
Cone angle 30 30
Swirl Number 0.1, 0.3, 0.6
and 2
0.1, 0.3, 0.6
and 2
Radius “cone”, mm 3 3
Fuel flow rate, kg/s 0.75 0.75
Prticle Diameter, µm 500 500
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considered. The method of mixture fraction with
Probability Density Function PDF has been developed
specifically for turbulent chemically reacting flow
simulations. The chemical reaction is determined by
turbulent mixing. The mixture fraction is as :
0
0
kkF
kk
zz
zzf
(1)
The value of ƒ is calculated from the solution of a time
averaged transport equation
m
ii
i
i
i
i
Sx
f
xf
xf
t
)()(
Simultaneously with the solution of Eq. (2), a conservative
equation for the variance of mixture fraction, 2f
describing the interaction between chemistry and
turbulence, is solved:
2
2
222 )()(
fk
Cx
fC
x
f
xf
xf
t
d
i
ig
it
i
i
i
i
The turbulence flow of the gas phase in the furnace is
governed by the equations for the conservation of mass,
momentum and energy, The Renormalization Group (RNG)
k-ε model include the swirling effects were applied.
Radiative heat transfer in the furnace is solved using P-1
radiation model for exchange between gas and particulates.
Fuel droplets evaporate relatively quickly and influence
radiative heat transfer only in the near-burner region. The
weighted sum-of-gray-gases (WSGGM) model is used for
absorption coefficient of the gas phase.
Heavy fuel oil droplets
droplet size distributions were assumed uniform droplet
size 500 µm, injected from the cone atomizer with angle 30º
and nozzel diameter 6 mm figur 5, The particle trajectories
are calculated from their corresponding motion equation:
i
p
p
D fgiUpUFdt
dUp
)(
Where the subscript “p” denotes particle, ρp , up is the
droplet density and velocity respectively, For spherical
particles, u and ρ is the gas phase velocity and density
respectively. The right hand side are the drag force per unit
of droplet mass and force of gravity on the droplet,
respectively.
Particle Heat transfer Model
To predict the vaporization from a discrete phase droplet,
when the temperature of the droplet reaches the
vaporization temperature Tvap, and continues until the
droplet reaches the boiling point Tbp, or until the droplet
volatile fraction is completely consumed
0,0, )1( pvp
bppvap
mfm
TTT
The effect of the convection flow of the evaporating from
the droplet surface to the bulk gas phase (Stefan flow), and
the vapor flux from the equation below becomes a source
of species I in the gas phase species transport equation.
)( ,, isici CCkN
Where Ni is the molar flux of vapor into the gas phase
(kmol/m2-s), Kc is convective mass transfer coefficient
(m/s), Ci,s and Ci,∞ stand for the vapor concentration at the
deroplet surface (kmol/m3) and in the bulk gas (kmol/m
3).
The boiling point Tpb and the latent heat hfg are defined as
constant property inputs for the droplet particle materials.
in the evaporation process, as the particle change its
temperature, the latent heat will vary according to:
bp
p
bp
p
T
T
pp
T
T
bpfggpfg dTchdTch ...
Include the droplet temperature effects on the latent heat by
selecting the “temperature dependent latent heat “option in
the discrete phase model. The droplet temperature is
updated according to a heat balance that relates the sensible
heat change in the droplet to the convective and latent heat
transfer between the droplet and the continuous phase:
)()(44
pRppfg
p
pp
p
pp TAhdt
dmTThA
dt
dTcm
Where CP is the droplet heat capacity (j/kg-k), Tp is the
droplet temperature (k), h is the convective heat transfer
coefficient (W/m2-k), T∞ the temperature of continuous
phase, dmp/dt is rate of evaporation (kg/s), hfg is latent heat (
j/kg), εp is particle emissivity (dimensionless), σ is stefan-
boltzmann constant (5.67×10-8
W/m2-k
4), θR is radiation
temperature. Radiation heat transfer to the particle is
included using the “particle radiation interaction“ When the
vaporization rate is computed by the convection/diffusion
controlled model, the convection heat transfer coefficient
“h” is calculated with a modified Nu number as follows:
)PrRe6.02()1ln( 3/12/1
d
p
B
B
k
hdNu
Where dp is the particle diameter, K∞ is the thermal
conductivity of the contiuous phase (w/m-k), Pr is prandtl
number of the continuous phase (CPu/K∞), BT is the
spalding heat transfer and is equal to the spalding mass
transfer number Bm. To predict the convective boiling of a
discrete phase droplet when the temperature of the droplet
has reached the boiling temperature Tbp, and while the mass
of the droplet exceed the nonvolatile fraction, ( 1-ƒv,0 ):
Tp ≥ Tbp and mp > ( 1-ƒv,0 ) mp,0 (10)
When the droplet temperature reaches the boiling point, a
boiling rate equation is applied, the mass transfer from the
(5)
(6)
(7)
(8)
(9)
(3)
(2)
(4)
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droplet to the gas phase is computed from the following
equation
)()(44
pRppppfg
pTATThAh
dt
dm (11)
vaporization and the boiling point temperature are 400 K
and 589 K, respectively.
3.3 Defining the dynamic viscosity and vapour pressure
In the present work for the CFD model HFO combustion
we depend on the data output from Kontoulis et al. [12].
For setting the dynamic viscosity and vapor pressure as
function with the temperature. From the Figures 2 and 3,
the CFD model HFO combustion for the main
thermophysical fuel properties were selected, and constant
values for the enthalpy, latent heat at vaporization
temperature 400 K were used. The numerical model for
combustion of heavy fuel oil is verified against data
collected from the practical consumption in the Calciner for
24 hours clinker production at one kiln line for cement
production. Boundary conditions for the fuel and the
oxidizer stream as well as the kiln line operating conditions
are reported in Table 2.
Table 2: Fuel Content by species in mass fraction.
RESULTS AND DISCUSSION HFO combustion in calciner is investigated in 3D model
with two setting for dynamic viscosity and injection
velocity. Constant viscosity value 1.7×10-5
kg/m.s with
injection velocity 40 m/s and viscosity function as
temperature start value 0.02 kg/m.s at the vaporization
temperature 400 K [12], and injection velocity 80 m/s and
40 m/s. Spray combustion of HFO is modeled using cone
angle 30º, particles diameter 500 µm, and the swirl number
is varied as 0.1, 0.3, 0.6 and 2.
The influence of viscosity grade on the HFO combustion
characteristics.
Figure 6 compare the combustion characteristics for the
two cases viscosity variation for swirl number 0.1.
Temperature distribution plane Y and Z at the center burner
#1 (a, c), and plane Y and X at the center burner #2 (b, d).
It is obvious from the figure 6 (c), the delay time of the
combustion starting especially with spray fuel oil injected
from burner #1 for the viscosity function as temperature.
This delay is a result of the injection of the fuel with high
viscosity at the vaporization temperature. The high
viscosity value at the evaporation temperature and its
variation with temperature as [12] adds a delay time before
the start of the combustion process to change the phase of
the fuel from liquid phase to vapor phase. In the constant
viscosity case figure 6 (a), the fuel is defined to be injected
in the vapor phase directly. Also, from the figure 6 (c, d) it
is observable that only small portion of the injected fuel is
combusted while the rest is combusted in the area
surrounding burner #2. Also, it is observable that the
maximum temperature inside the calciner, 2200K, is higher
than the corresponding maximum temperature in the case
of constant viscosity which was 2000K only. This is a
result of the delay in the combustion of burner #1 and the
concentration of the combustion in the area surrounding
burner #2. This in turn maximizes the heat released in this
region. Close to Burner #2, the temperature is higher than
that around burner #1 and the evaporation process takes
place more rabidly.
Composition
(mass fraction)
Heavy fuel oil
composition
Oxidizer
[Tertiary Duct]
Secandary Oxidizer
[kiln Chamber]
Carbon % 86.71 Hydrogen % 10.78
Nitrogen % 0. 21 Oxygen % 0. 21
Sulfure % 2.622
CO2 - - 0.12 O2 - 0.733 0.05
N2 - 0.233 0.83 Heavy fuel oil properties
Lower heating value, MJ-kg-1 4.231
Specific heat J/kg 2100 Molecular weight 430
Boundary Condition Inlet Temperature, K 400 1123 1300
Inlet Velocity, m/s 29 22 Mass flow rate, kg/s 1.5 69 42.46
Area, m2
1.93 2.378
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a) Burner #1, const viscosity b) Burner #2, const viscosity
c) Burner #1, viscosity Function temperature d) Burner #2, viscosity Function temperature
Figure 6: Temperature distribution in the plane of burner #1 and burner #2 axis for two cases,oK, at constant viscosity (a,b)
and variation of viscosity with temperature (c,d) for S = 0.1
The influence of the swirl number on combustion
characteristics.
High swirl number (S) strenghtens the circulation zone and
enhances mixing. By comparing Figure 6 it is observiable
that the circulation zone is narrow with S= 0.1 which leads
to elongated flame. Meanwhile, with S = 0.6, Fig.7, the
circulation zone is wider than that observed with S = 0.1
and the mixing is better. The latter leads to improved
temperature distribution inside the Calciner with S = 0.6
case. And disapperance of the flame shape when compared
with the cases of S= 0.1 and 0.3.
The influence of viscosity grade on droplet trajectory
penetration.
At constant viscosity, an injection velocity of 40 m/s was
considered Figure 8a, then 80 m/s as in Figure 8b. Injecting
the HFO with high viscosity grade affected the penetration
depth inside the calciner as is clear from Figure 8 and 9 for
burners # 2 for S = 0.1 and 0.6 respectively. The change in
the penetration depth affects the combustion characteristics
as it affects the interaction between the fuel and the
oxidizer streams. This is confirms that the change in the
viscosity grade has a significant effect on the HFO
penetration depth.
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a) Burner #1, viscosity Function temperature b) Burner #2, viscosity Function temperature
Figure 7: Temperature distribution in plane of burner #1 and #2 axis, oK, variable viscosity with temperature for S = 0.6
a) Burner #1, Injection Velocity 40 m/s b) Burner #2, Injection Velocity 80 m/s
Figure 8: Contours of particle injection velocity burner 1& 2, for two cases,m/s,
constant viscosity (a,b) and variation viscosity with temperature (c,d), S = 0.1
a) Injection velocity 40 m/s b) Injection velocity 80 m/s c) Injection velocity 40 m/s
Figure 9: Contours of injection velocity burner #2,m/s, constant viscosity
(a), variable viscosity with temperature (b) & (c), for swirl number 0.6
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CO concentration comparision, Center line axis of
burner #1
Figures 10 and 11, are the CO concentration variation with
axial position for burner #1. The figures support the
previous findings presented for the influence of the
viscosity grade variation on the heavy fuel oil spray
combustion charactarestics and the influences of the swirl
numbers on enhances the fuel mixing rate with the oxidizer,
Fig. 10 show the CO concentration comparision for the
case constant viscosity with injection velocity 40 m/s, and
the case viscosity variabe as temperature with injection
velocity 80 m/s, for swirl number 0.1, 0.3, 0.6 and 2. For
constant viscosity the spray combustion is defined to be
injected the domain approximatly at the vapor phase, so the
CO concentration started earleir, from the position burner
#1 injection position X= -609.09 mm. For the case variable
viscosity with temperature, the delay for CO concentration
is a result of the injection of the fuel with a high viscosity
value at vaporization temperature.
Figure 10: CO concentration across axial center burner #1, for case viscosity constant with injection velocity 40 m/s and
case viscosity variable with temperature, for S= 0.1, 0.3, 0.6 and 2.
Figure 11 is the CO concentration with axial position for
burner #1 at two different injection velocities,40 and 80
m/s, for viscosity variable with temperature. In general,
increasing the injection velocity enhances the penetration of
the injected fuel and hence the mixing with the oxdizer.
This is clear from comparing the CO concentration in the
two cases at different swirl numbers. It is observale from
the figure that the CO concentration is higher in the case of
low injection velocity especially at high swirling numbers,
S= 0.6 and 2.0 the CO concentration is very high close to
the burner location and then it reduces suddenly with axial
distance from the burner. This is a result of the low rate of
evaporation of the HFO and the reduction in the oxdizer
quantity. For swirl number 0.1 and 0.3 the CO
concentration is reduced, compared to the two cases of 0.6
and 2.0, because of the enhanced penetration and hence the
availability of more oxdizer quantities. Figure 12, show the
plane temperature distribution for the two axial burner #1
and #2 with injection velocity 40 m/s, for S= 0.3, the figure
support the previous finding presented for the CO
concentration above as Figure 11.
NO emissions concentration
The heavy fuel oil in present study contains 0.21% of
nitrogen, as shown in Table 2. Figure 13, are the thermal
NO emission rate along the centerlines of burners #1, show
the thermal NO rate for the combustion simulation for the
viscosity variable temperature with two injection velocity
40m/s and 80m/s, for considered S = 0.1, 0.3, 0.6 and 2.
Accoring to the Figure 13 for the thermal NO around the
center burner #1, it is clear the high rate NO formation for
the case injection velocity 40 m/s resulting by the high
temperature at the region front burner #1, and then increase
gradually to the center of the furnace resulting the precence
of sufficient quantities of oxygen and nitrogen affected by
the low penetration depth of the injection dropet fuel and
weak mixing with the oxidizer. Figure 12a for temperature
plane burner #1 enhancing our analysis. For the case
injection velocity 80m/s, the apsence of the high
temperature at the region front burner #1, and the good
mixing rate between the oxidizer and the fuel affected to
decrease the NO thermal rate.
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Figure 11: CO concentration across axial center burner #1, for case viscosity variable with temperature, with injection
velocity 40 m/s and 80 m/s, for S= 0.1, 0.3, 0.6 and 2.
a)Burner 2, Injection Velocity 40 m/s b)Burner 1, Injection Velocity 40 m/s
Figure 12: Temperature distribution in the plane of burner #1 and burner #2 axis,oK ,
for case variation viscosity with temperature, for S = 0.3
Figure 13: Rate of thermal NO across axial center burner #1, for case viscosity variable with temperature, with injection
velocity 40 m/s and 80 m/s, for S= 0.1, 0.3, 0.6 and 2.
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CONCLUSIONS A numerical Computational Fluid Dynamics model was
developed using the ANSYS 14 software. The effect of
Heavy Fuel Oil (HFO) viscosity and swirl number on the
combustion characteristics inside the Calciner was
investigated. The HFO used throughout this study is
Bunker C6. The numerical model is based on the solution of
Navier-stockes equation for the gas flow, and on dynamics
model with the influence of turbulence simulated by a two-
equation (k-ε) model.
The results of the simulations conducted can be
summarized in the following points.
1. The HFO viscosity must be considered as a function
of temperature during the analysis because it
significantly affects the combustion characteristics.
2. The viscosity grade affects the HFO penetration depth
inside the calciner, increasing the HFO viscosity
reduces the penetration depth for the same injection
velocity. This in turn affects the CO and NO
emissions distribution along the burners axes.
3. Based on the present study results it could be
concluded than A swirl number of 0.6 with cone angle
of 30 degrees results in highest combustion
improvements.
4. The CO and NO concentrations inside the Calciner are
highly dependent on the swirl number and HFO
injection velocity.
NOMENCLATURE
𝐶𝜀1, 𝐶𝜀2 turbulence model constant in the 𝜀 –equation
𝐶𝜇 constant in the definition of turbulent viscosity
𝐶𝑃 D
specific heat at constant pressure [J/Kg K]
molecular diffusion coefficient
f Mixture Fraction
ℎ𝑠 Sensible enthalpy, kJ/kg
I number of species fluctuation intensity
k turbulent Kinetic energy, m2/s2
𝑙𝑚 turbulent mixing length, m
P pressure[N/ m2]
𝑄𝑗 ratio of the heat released from reaction j
r radius of inlet fuel and air zone
Sϕ source term
t time, s
T temperature(K)
To. reference temperature
U mean velocity components, m/s
u Fluctuating velocity components. m/s
U axial velocity, m/s
V velocity magnitude, m/s
W Velocity component in y direction, m/s
𝑊j reaction rate of the reaction j
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