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Numerical investigations of methane fueled oxy-fuel combustion model in a gas turbine combustor
1. Flow fields, temperature, and species distribution
2. Effect of CO2 recirculation By: M. A. NEMITALLAH
1, R. BEN-MANSOUR
2, and M. A. HABIB
3
1, 2, and 3Mechanical Engineering Department
KFUPM University
Dhahran 31261
SAUDI ARABIA
medhatahmed@kfupm.edu.sa
Abstract:-Oxyfuel combustion represents one of the major options for carbon capture technologies. The effect
of the amount of CO2 recirculated and the recirculation temperature on the combustion characteristics were
investigated numerically and the results were compared with the experimental work of two literature studies.
The two studies have been done on the same combustion chamber. The first study investigates a gas turbine
combustor for three swirling CH4/air diffusion flames at atmospheric pressure. In our work, the most stable
flame in the experimental work was taken in order to validate our code for the combustion of pure air only. The
second study describes experiments on partially premixed swirl stabilized oxyfuel flames carried out on the
same gas turbine combustor. The blends of O2/CO2 used in this study were 26% O2 (74% CO2), 30% O2, 34%
O2, 38% O2, in addition to atmospheric air that was added in this study to be our reference case. The results
showed a good agreements with the experimental data in case of the combustion with air for the first study.
Also in the second study for the oxy-fuel combustion, good agreements with the experimental data have been
obtained. The results showed that the flame speed will be reduced when substituting N2 by CO2 in the oxidizer.
This causes poor combustion performance and a modified distribution of temperature and species in the
combustion chamber. Nearly, the oxidizer mixtures between 34% and 38% O2 gives the same adiabatic
temperature as atmospheric air.
Keywords:- oxyfuel combustion, gas turbine combustor model
1 Introduction Global climate change is one of the greatest
challenges in the 21st century. The greenhouse gas
making the largest contribution to global climate
change from human activities is carbon dioxide
(CO2). CO2 emissions from the fossil fuel-based
large power plants are of main concern as they are
the largest sources of CO2 in the coming decades.
International Energy Agency forecasts have
indicated that some 38% of the world’s electricity
will still be generated from coal by 2020 [1]. For
decreasing greenhouse gas (mainly CO2) emissions,
several approaches have been evaluated and
reviewed for capturing CO2 in the utility industry,
namely Carbon Capture and Storage technology
(CCS), including pre-combustion capture, oxyfuel
combustion, and post-combustion capture. As a
promising CCS technology, oxyfuel combustion
can be used to existing and new power plants [2].
Combustion is made using pure oxygen (up to 97%
purity) together with a fraction of recycled flue gas
(RFG) which consists mainly of CO2 and H2O. The
application of oxyfuel combustion to the
Conventional boilers with modification including
addition of an air separation unit and a flue gas
recirculation system are more economically
acceptable. Compared to conventional air
combustion, oxyfuel combustion shows different
characteristics of heat transfer, ignition, char
burnout as well as NOx emission [3].
During oxyfuel combustion, a combination
of oxygen and recycled flue gases are used for
combustion of the fuel. The exhaust gases
consisting mainly of CO2 and H2O generated with a
concentration of CO2 ready for sequestration. The
recycled flue gases used to control flame
temperature and make up the volume of the missing
N2 to ensure there is enough gas to carry the heat
through the boiler. CO2 capture and storage by the
current technically viable options post-combustion
capture, pre-combustion capture and oxyfuel
combustion will impose a 7–10% efficiency penalty
Advances in Fluid Mechanics and Heat & Mass Transfer
ISBN: 978-1-61804-114-2 92
on the power generation process. The major
contributors to this efficiency penalty are oxygen
production and CO2 compression. The combustion
of fuel in a mixture of recirculated flue gas (RFG)
and oxygen, however, presents new challenges to
combustion specialists. Several experimental
investigations with oxy-firing pulverized coal
burners reported that flame temperature and
stability are strongly affected [3,4]. The substitution
of N2 with CO2 in the oxidizer will lead to a
reduction of the flame speed as reported by Zhu et
al. [5]. This causes poor combustion performance
and a modified distribution of temperature and
species in the combustion chamber. Liu et al. [6]
have performed numerical investigations on the
chemical effects of CO2. A comparison between
numerical and experimental data showed, that the
decrease in burning velocity for the oxyfuel
combustion can not entirely be described by only
considering the material properties of CO2.
Anderson et al. [7] have performed experiments on
a 100 kW test unit which facilitates O2/CO2
combustion with real flue gases recycle. The tests
comprise a reference test with air and two O2/CO2
test cases with different recycled feed gas mixture
concentrations of O2 (OF 21 @ 21 vol.% O2, 79
vol.% CO2 and OF 27 @ 27 vol.% O2, 73 vol.%
CO2). The results showed that the fuel burnout is
delayed for the OF 21 case compared to air-fired
conditions as a consequence of reduced temperature
levels. Instead, the OF 27 case results in more
similar combustion behavior as compared to the
reference conditions in terms of in-flame
temperature and gas concentration levels, but with
significantly increased flame radiation intensity. In
this work, we are focusing on investigating
numerically the effect of the amount of CO2
recirculated and the recirculation temperature on
the combustion characteristics. The results were
compared with the experimental work of two
studies. The most stable flame in the experimental
work by [8] was taken in order to validate our code
for the combustion of air only without comparing
with other O2/CO2 blends. The second study [9]
describes experiments on partially premixed swirl
stabilized oxyfuel flames carried out on the same
gas turbine combustor.
2 Numerical modeling The combustion chamber used in the two studies
that we are using here to validate our results is
shown in fig.1. The oxidizer mixture was supplied
through a central nozzle (diameter 15 mm) and an
annular nozzle (inside diameter of 17 mm and
outside diameter of 25 mm contoured to diameter
of 40 mm) as a co-swirling flow to the flame.
Between the two oxidizer flows the CH4 was fed
through a ring of 72 channels (0.5x l0.5 mm). The
exit plane of the fuel inlet and the inner oxidizer
nozzle is located 4.5 mm below the exit plane of the
outer air nozzle. The overall flow field of the flames
is characterized by a conically shaped inflow of
fresh gas, an inner recirculation zone (IRZ) and
outer recirculation zone (ORZ) as shown in Fig. 1.
In the shear layer formed between the inflow and
the IRZ, the mixing of hot combustion products
with fresh gas leads to a continuous ignition and
stabilization of the flame. The burner was mounted
in an optically accessible combustion chamber. The
chamber measures 85x85 mm (cross-section) and is
120 mm height. It consists of four planar quartz
windows supported at the corners by steel posts
(diameter 10 mm). The flow exits the chamber
through a rectangular to conical exhaust section.
Fig.1 Schematic diagram of the combustion
chamber used in the present study study [9].
For the first study [8], only the atmospheric
air was validated at thermal power of 34.9 Kw and
overall equivalence ratio of .65. The mass flow rate
of air used was 1095 gm/min and the mass flow
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rate of methane was 41.8 gm/min. The second
study [9] describes experiments on partially
premixed swirl stabilized oxyfuel flames carried out
on the same gas turbine model combustor. The
blends of O2/CO2 used in this study were 26% O2
(74% CO2), 30% O2, 34% O2, 38% O2, in addition
to atmospheric air was added in our work to be the
reference case to see the effect of CO2 recirculation
and increasing the amount of O2 on combustion
characteristics at thermal power of 22.7 Kw and an
equivalence ratio of 0.76. The mass flow rate of
fuel used was 27.24 gm/min. also the total mass
flow rates used of oxidizers were 615, 704.4, 602.7,
526.2, and 464.4 gm/min for atmospheric air, 26%
O2 (74% CO2), 30% O2, 34% O2, and 38% O2
respectively. The equations which govern the
conservation of mass, momentum and energy as
well as the equations for species transport may be
expressed in the following general form:
Φ+∂Φ∂
Γ∂∂
=
+Φ S ][
__________
ρφρρ∂∂
φjj
jj
j xxuU
x (1)
Where, Φ is the dependent variable, jU and u j are
the mean fluctuatuations of the velocity component
along the coordinate direction jx , __
ρ is the fluid
density, ΦΓ is the diffusion coefficient and ΦS is
the source term. The present modeling package
used in this work to predict the combustion
behavior utilizes the K-ε turbulence model. The
Reynolds stresses and turbulent scalar fluxes are
related to the gradients of the mean velocities and
scalar variable, respectively, via exchange
coefficients as follows:
− = +
−ρ µ
∂∂
∂
∂ρ δu u
U
x
U
xki j t
i
j
j
iij
2
3 (2)
j
jx
u∂∂
φρΦ
Γ=− Φ (3)
Where µt is the turbulent viscosity and ΦΓ is equal
to Φσµ /t . The turbulent viscosity is modeled as:
ερµ µ /k 2ct = (4)
Where, µc and Φσ are constants. The turbulent
viscosity is thus obtained from the solution of the
transport equations for K andε . RNG
(Renormalized group) turbulence model was used
to provide better results for vortical flows. The
eddy dissipation model that described turbulence-
chemistry interaction in non-premixed combustion
was utilized in the present package to provide the
production rate of species. In order to correctly
predict the temperature distribution in the furnace, a
radiative transfer equation (RTE) for an absorbing,
emitting and scattering medium was solved. Once
the radiative intensity is obtained, the gradient of
the radiative heat flux vector was found and
substituted into the enthalpy equation to account for
heat sources (or sinks) due to radiation. The
solution of the RTE was obtained using the discrete
ordinates (DO) radiation model. Fluent 6.2 [10] was
used to perform the calculations. As the combustion
chamber is symmetric, only quarter of the chamber
was modeled with a mesh of more than 1,000,000
finite volumes was used and the solution was
considered converged when the summation of the
residual in the governing equation summed at all
domain nodes was less than 0.01%. A one-step
chemical reaction model is used.
3 Results and discussions
3.1 For the First study In our work, the most stable flame in the
experimental work was taken in order to validate
our code for the case of air combustion only
without comparing with other O2/CO2 blends at
thermal power 34.9 kW and overall equivalence
ratios of 0.65. The two dimensional mean
temperature distributions is shown in Fig.2 for both
experimental results (right) and calculated one
using our code (left). As it is clear from the figure,
the calculated values are going in a good agreement
with the experimental values. It is also seen that the
temperatures of the flame is increasing with the
height this may due to the improved mixing
between fuel and air. To identify the differences
more clearly, Fig.3 shows the temperature
distributions through the vertical axial centre line.
At the start of the combustion, both the numerical
and experimental results exhibit a similar
temperature of T ≈ 1300 K. With increasing height,
T values increase strongly, reach a maximum at h
=5-15 mm, and decrease slowly afterward until a
height of 30mm. the temperature starts after that to
increase very slowly due to late combustion.
However you can see that the start of combustion
points in the two curves are not the same but this
may due to any missing details of the combustion
chamber geometry that are not mentioned in the
experimental work. Fig.4 shows the radial
temperature distribution at heights 5mm and 10
mm. The low-temperature regions at r ≈ 5–15 mm
reflect the inlet streams of fresh gas. However, at
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h=5 mm and especially for r>10 mm, the main
source of the elevated temperatures is mixing of hot
exhaust gas from the recirculation zones with fresh
gas. The increased temperature level at h=5 mm
enhances, of course, the reactivity of the gas
mixtures and, thus, the heat release and burnout [8].
This also can clearly be seen in the temperature
profiles at h=10 mm. The transition between the
inlet stream and the orz at r ≈20 mm is clearly
visible in the profile of the flame at h=5mm. It is
also obvious that for h=10 mm, the temperature
level in the outer recirculation zone (orz) is in
general lower than in the inner recirculation zone
(irz), which is due to the leaner mixtures and heat
loss to the wall in the orz. It is also clear from the
figure that the experimental and calculated are
going in a good agreement however at a height of 5
mm there is a little difference as in this area the
combustion starts and many reactions occurs in this
area. Finally for this part in our work, the mean
distributions of the mole fractions of H2O and O2
are presented in Fig.5. The shapes of the
distributions of the mole fraction of H2O, displayed
in Fig.5, resemble strongly that of temperature
distribution (see Fig.2). You can see that the larger
concentrations of H2O are near to the combustion
zone as the H2O is a product of combustion. For the
O2, the larger concentrations are at inlet and in the
orz and it decreases as the combustion proceeds. As
shown in Fig.5 also, the calculated mole fractions
for H2O and O2 are going in good agreements with
the experimental values. From these analyses, we
can say that our code is valid for using to predict
the combustion characteristics in case of oxyfuel
combustion that will be seen in the second part of
this work.
3.2 For the Second study The blends of O2/CO2 used in this study were 26%
O2 (74% CO2), 30% O2, 34% O2, 38% O2, in
addition to atmospheric air was added in our work
to be the reference case to see the effect of CO2
recirculation and increasing the amount of O2 on
combustion characteristics at thermal power of 22.7
Kw and an equivalence ratio of 0.76.
3.2.1 Effect of recirculated amount of CO2
From the previous analysis in this work, we
can use our code to predict the effect of CO2
recirculation on the combustion characteristics of
oxy-fuel as this point is not expressed in those two
studies. Fig.6 compares the radial temperature
profiles at the height of 60 mm for different
concentrations of O2/CO2 as compared to
atmospheric air. As shown in the figure, as the
amount of CO2 decrease (increasing the amount of
O2) in the mixture as the temperature increase. The
mixture with 26% O2 (74% CO2) gives the lower
temperature and 38% O2 gives the highest
temperature. This behavior may due to the
reduction in flame speed when substituting N2 by
CO2 in the oxidizer. This causes poor combustion
performance and a modified distribution of
temperature and species in the combustion
chamber.
The main reasons are obviously the larger
heat capacity of CO2 compared to N2 that leads to
the lower combustion temperature for the same
Fig.2 Two dimensional temperature contours (right
side: experimental results [8] and left part:
numerical values).
Fig.3 Temperature profiles through the vertical
centre line of the combustion chamber for the
present work and the experimental data of [8].
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Fig.4 radial profile of mean temperature at h=5mm
and h=10mm for both experimental data [8] and the
calculated values.
equivalence ratio and the lower laminar flame
speed. The lower burning velocity for oxy-
combustion of gaseous fuel theoretically can be
affected by the following features: (1) lower
thermal diffusivity of CO2, (2) higher molar heat
capacity of CO2, (3) chemical effects of CO2, and
(4) modified radiative heat transfer [11].
Fig.5 Two-dimensional distribution of the mean
H2O, and O2 mole fractions (for H2O the left
contours is the experimental data [8] and the right
ones are the calculated data; vice versa for O2)
Since the molar heat capacity affects the flame
temperature, its effect generally dominates. The
lower adiabatic flame temperature in oxy-
combustion can be increased by increasing the
oxygen concentration in the CO2 /O2 gas mixture,
thus reaching similar flame temperature levels as in
air combustion. We can see from the figure that in
order to get the same temperature profile as the
atmospheric air, the amount of O2 in a mixture of
O2/CO2 must lie between 34 to 38%. In case of
atmospheric air, the mixing between fuel and air
much better than the case of O2/CO2 blends. As
shown in Fig.7, the flame area for the combustion
with atmospheric air is much greater than that for
other cases and the high temperature region is near
to the burner. This means that the combustion starts
earlier in the case of air and the mixing is better.
So, the combustion of oxy-fuel is associated with
longer ignition delay period. The mole fraction of
H2O is a measure for the reaction progress as H2O
is a combustion product. This mean that as H2O is
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formed earlier as the combustion starts earlier.
Fig.8 shows the mole fraction of H20 for different
O2/CO2 concentrations as compared to atmospheric
air. As shown in the figure, the mixing and
combustion occur faster in case of air so H2O forms
earlier in larger uniform area.
Fig.6 radial temperature distribution at h=60 mm
above the burner for the flame with thermal power
of 22.7 kW and equivalence ratio of 0.76 for
different O2/CO2 concentrations.
3.2.2 Effect of temperature of recirculated CO2
The measurements in [9] work indicated that
variations of the temperature level of the inflow had
a significant influence on the heat release rate and
also on the overall temperature inside the
combustion chamber. As the CO2 is exhausted at
high temperature, we would like here to investigate
the effect of oxidizer mixture (O2 and CO2)
temperature on the combustion temperature inside
the combustion chamber.
Fig.7 Two dimension temperature distribution at
thermal power of 22.7 Kw and equivalence ratio of
0.76 for atmospheric air (left), 26% O2 (middle),
and 38% O2 (right).
As shown in Fig.9 at a height of 35mm, as the
inflow temperature increases as the combustion
temperature increases as expected due to better
mixing and shorter ignition delay. With regard to
gas turbine conditions, the flame temperature plays
an important role. Higher temperature implies
higher efficiency of the turbine, but this is manly
limited by the material used in the turbine
construction.
Fig.8 Two dimension mole fraction of H2O at
thermal power of 22.7 Kw and equivalence ratio of
0.76 for atmospheric air (left), 26% O2 (middle),
and 38% O2 (right).
Fig.9 Effect of inflow temperature on the
combustion temperature at h=35mm.
4. Conclusions This paper presents numerical a numerical
analysis for the effect of CO2 recirculation on the
combustion characteristics of oxy-fuel. Two
experimental studies working on the same
combustion chamber were used to validate our
results. The results showed a good agreements with
the experimental data in case of the combustion
with air for the first study. Also in the second study
for the oxy-fuel combustion, good agreements with
the experimental data have been obtained. The
results showed that the flame speed will be reduced
when substituting N2 by CO2 in the oxidizer. This
causes poor combustion performance and a
Advances in Fluid Mechanics and Heat & Mass Transfer
ISBN: 978-1-61804-114-2 97
modified distribution of temperature and species in
the combustion chamber. Nearly, the oxidizer
mixtures between 34% and 38% O2 gives the same
adiabatic temperature as atmospheric air.
Acknowledgements The authors wish to acknowledge the
support received from King Abdulaziz City for
Science and Technology (KACST) through the
science and technology unit at King Fahd
University of Petroleum and Minerals (KFUPM)
for funding this work through project No. 09-
ENE755-04.
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