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

Advances in Fluid Mechanics and Heat & Mass Transfer

ISBN: 978-1-61804-114-2 93

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

Advances in Fluid Mechanics and Heat & Mass Transfer

ISBN: 978-1-61804-114-2 94

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].

Advances in Fluid Mechanics and Heat & Mass Transfer

ISBN: 978-1-61804-114-2 95

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

Advances in Fluid Mechanics and Heat & Mass Transfer

ISBN: 978-1-61804-114-2 96

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.

References:

[1] R. Priddle, IEA World Energy Outlook, Paris,

1998.

[2] B.J.P. Buhre, L.K. Elliott, C.D. Sheng, R.P.

Gupta,T.F. Wall, Prog., Oxy-fuel combustion

technology for coal-fired power generation, Energy

Combust. Sci. Vol.31, 2005, 283–307.

[3] T.F. Wall, Combustion processes for carbon

capture, Proceedings of the Combustion Institute,

Vol.31, 2007, 31–47.

[4] E. Croiset, K.V. Thambimuthu, NOx and SO2

emissions from O2/CO2 recycle coal combustion,

Fuel, Vol.80, 2001, 2117–2121.

[5] D.L. Zhu, F.N. Egolfopoulos, C.K. Law,

Propagation and extinction of stretched premixed

flames, Symposium (International) on Combustion,

Vol. 21, Issue 1, 1988, 1419-1426.

[6] F. Liu, H. Guo, G.J. Smallwood, The chemical

effect of CO2 replacement of N2 in air on the

burning velocity of CH4 and H2 premixed flames,

Combust. Flame, Vol.133 (4), 2003, 495–497.

[7] Klas Andersson, Filip Johnsson, Flame and

radiation characteristics of gas-fired O2/CO2

combustion, Fuel, Vol. 86, 2007, 656–668.

[8] P. Weigand, W. Meier, X.R. Duan, W. Stricker,

M. Aigner, Investigations of swirl flames in a gas

turbine model combustor 1. Flow field, structures,

temperature, and species distributions Combust,

Flame, Vol. 144, 2006, 205–224.

[9] Peter Kutne, Bhavin K. Kapadia, Wolfgang

Meier, Manfred Aigner, Experimental analysis of

the combustion behaviour of oxyfuel flames in a

gas turbine model combustor, Proc. Combust. Inst. ,

2010, doi: 10.1016/ j.proci.2010.07.008.

[10] Fluent 6.2 user's guide. Fluent inc., Centre

Research Park, 10 Cavendish court, Lebanon,

NH03766, USA. , 2003

[11] Y. Tan, M.A. Douglas, K.V. Thambimuthu,

CO2 capture using oxygen enhanced combustion

strategies for natural gas power plants, Fuel, Vol.

81 (8), 2002, 1007–1016.

Advances in Fluid Mechanics and Heat & Mass Transfer

ISBN: 978-1-61804-114-2 98