18th ifrf, germany, experimental investigation of oxy fuel combustion of cng flames stabilized over...

INTERNATIONAL FLAME RESEARCH FOUNDATION

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IFRF Members’ Conference – Flexible and clean fuel conversion to industry

Freising, Germany, 1, 2, 3 June 2015 – Paper n. 25

EXPERIMENTAL INVESTIGATION OF OXY-FUEL COMBUSTION OF CNG

FLAMES STABILIZED OVER A PERFORATED-PLATE BURNER

S. Samir Rashwan1, [email protected], A. H. Ibrahim

2, [email protected],

T. Wazier Abou-Arab3, [email protected]

CAIRO UNIVERSITY, Egypt

ABSTRACT

Carbon dioxide emissions are considered one of the most important factors that cause global warming.

For this reason, it is suggested to burn fuels using an oxidizer mixture consisting of oxygen and carbon

dioxide rather than the conventional burning in air. This combustion technique ensures that the resulting

products consist mainly of carbon dioxide and water vapor which facilitates capturing and sequestration

of the carbon dioxide gas and thus eliminates its release into the atmosphere. One of the main challenges

that faces this technique is the need to ensure the flame stability in a wide range of operating conditions

because of the adverse effects of adding carbon dioxide to the oxidizer mixture on chemical kinetics. This

study experimentally examines some of the conditions that must be met to ensure stable flame operation

when burning compressed natural gas in a controlled mixture of oxygen and carbon dioxide stabilized

over a perforated-plate burner at different equivalence ratios and at different oxygen fractions. Two sets

of experiments are carried-out in this study. In the first set, the study identifies the range of equivalence

ratios at constant oxygen fractions necessary for stable flame operation. In the second set, the study

identifies the range of oxygen fraction at constant equivalence ratios necessary for stable flame operation.

The study also documents the visual flame length and color and identifies the extinction mechanism

outside the flammability limits for both sets. The first set of experiments reveals that, in the range of

Reynolds number considered and at an oxygen fraction of 36%, stable flame operation is observed with

flammability limits that are approximately 80% of the corresponding air-fuel combustion under the same

Reynolds number. This ratio decreases as the oxygen fraction decreases. The visual flame length is longer

than that of air-fuel combustion by approximately 15%. Extinction occurs by blow-off outside the upper

and lower flammability limits. The second set of experiments reveals that for the burner and the range of

Reynolds numbers considered, operation below an oxygen fraction of 29% is not possible at all

equivalence ratios considered (namely 0.7, 0.85, 1 and 1.15) while the upper flammability limit is a

function of the equivalence ratio. Extinction occurs by blow-off below the lower flammability limit, and

by flash-back above the upper flammability limit.

Keywords: Oxy-fuel combustion, CNG/O2/CO2 combustion, flammability limit, extinction mechanism,

visual flame appearance and perforated-plate burner.

1. INTRODUCTION:

The average temperature of the Earth’s surface has increased by about 0.8 °C over the past 100 years,

with about 0.6 °C occurring just over the past three decades1. Scientists are 95-100% certain that this

increase in temperature is primarily caused by increasing concentrations of greenhouse gases produced by

anthropogenic activities such as the burning of fossil fuels and deforestation1.

Carbon dioxide (CO2) is the primary greenhouse gas emitted through human activities. In 2011,

CO2 accounted for about 84% of all U.S. greenhouse gas emissions from anthropogenic activities2.

The continued increase in the atmospheric concentration of carbon dioxide due to anthropogenic

emissions is predicted to lead to significant changes in global climate. For example, evaporation will

increase as the climate warms, which will increase average global precipitation. Soil moisture is likely to

decline in many regions, and intense rainstorms are likely to become more frequent. Worldwide effects of

global warming have been discussed3,4. Some of the impacts on Egypt in particular are reported in 5

,6.

The dramatic effects of global warming on the entire planet have caused many scientists to think of

methods to limit the release of greenhouse gases, with CO2 at the heart of them. Oxy-fuel combustion is

one of the viable methods to facilitate CO2 capturing for sequestration.

mailto:[email protected]



http://en.wikipedia.org/wiki/Fossil_fuel

http://en.wikipedia.org/wiki/Deforestation

http://en.wikipedia.org/wiki/Deforestation


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In this combustion technology, N2 is removed from air using an air separation unit. The remaining gases

which are mainly O2 and some impurities such as argon are used as oxidizers. This makes the flue gases

consist mainly of CO2 and H2O. In order to prevent excessively high temperatures if the fuel is burnt in

O2 alone, some of the CO2 form the flue gases is recirculated and mixed with the oxidizers. Then, H2O is

condensed from the flue gas allowing capture and possible reuse of the condensation heat. This

technology makes the flue gases consist primarily of CO2 and thus facilitates its capture and sequestration,

which in turn eliminates the release of CO2 into the atmosphere. This technology also reduces the flue gas

volume and mass significantly, with corresponding benefits in reduced heat losses and reduced size of

flue gas treatment equipment. The reduction in flue gas volume increases the concentration of pollutants,

making separation easier. Additionally, because N2 from air is eliminated from the reactants, thermal NOx

emissions are eliminated.

The oxy-fuel combustion concept can be described as combustion using substitute air in which N2 is

replaced with CO2. However, the combustion characteristics and radiative heat transfer in oxy-fuel

combustion differ from those of air-fuel combustion due to significant differences in the physical

properties of CO2 and N2. The replacement of N2 by CO2 in the oxidizer mixture impacts the flame in four

issues: changes in mixture density, volumetric heat capacity and adiabatic flame temperature, transport

properties (thermal conductivity, mass diffusivity, and dynamic viscosity), chemical kinetic rates and

radiative heat transfer7.

CO2 has higher density than N2, which affects gas volume, flame shape and pressure drop: the density of

air is 0.43 kg/m3 at 800 K while the densities of CO2/O2 mixture are 0.62, 0.61, 0.60 at oxygen fractions

of 29%, 32%, 36%, respectively. The higher density of CO2 also leads to higher volumetric heat capacity

of CO2/O2 mixtures with respect to air: at 800 K, the volumetric heat capacity of air is 480 J/m3K while

those of CO2/O2 mixtures at oxygen fractions of 29%, 32%, 36% are 480, 705, 700 and 690 J/m3K

respectively. The high volumetric heat capacity of CO2/O2 mixtures with respect to air directly reduces

the temperature level and causes less flame speeds and lower flame stability.

In addition to the changes in densities and in volumetric heat capacities, flame speed also is affected by

gas transport properties. Table 1 shows the thermal conductivity and the dynamic viscosity of N2 and CO2

gases at different temperatures as well as the mass diffusivity of O2 in both N2 and CO2.

N2 CO2

T (C) 25 500 1000 25 500 1000

K(W/mK) 25.5 52.9 74.2 16.9 53.1 81.9

*105 (Pa.S) 1.77 3.42 4.61 1.5 3.3 4.7

D*105 (m

2/s) 2.04 10.3 23.7 1.5 8.1 18.8

Table1: Gas transport properties for N2 and CO2 at 1 Atm7

The table shows that while the thermal conductivity and the viscosity of CO2 are very close to those of

N2, the mass diffusivity of oxygen in CO2 is approximately 20% lower than in N2.

Other factors causing lower flame speeds in CO2-diluted flames with respect to N2-diluted flames include

that CO2 causes adverse kinetic interactions by competing with other reactions requiring the H radical and

that the CO2 in the reacting gases leads to additional preheating, through radiative absorption of heat

emitted from product gases, thus decreasing the flame speed.

In conclusion, the laminar burning velocity is significantly lower for oxy-fuel combustion compared to

air-fuel combustion because of the adverse effects of adding CO2 to the oxidizer mixture. These adverse

effects cause lower flame speed, weaker and longer flames88


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Consequently, combustion in CO2-diluted systems at the same equivalence ratio as N2-diluted systems

requires oxygen levels higher than 21% in order to achieve comparable flame temperatures necessary to

sustain flame operation7. For example, Ditaranto and Hals9 found that as far as stability is concerned,

oxy-fuel combustion requires at least 30% oxygen to perform in a manner comparable to air-fuel

combustion.

The extinction mechanisms under premixed conditions were studied by Alberto et al.7 They studied

blow-off measurements for both oxy-fuel combustion (CH4/O2/CO2) and air-fuel combustion (CH4/air)

using a premixed swirl combustor. They showed that operating the CO2 diluted system significantly

contracts the operability boundaries, due to the slower kinetics of this system relative to burning in air.

The effects of oxygen fraction on oxy-fuel operation were described by Kutne et al.10. They described

experiments on partially-premixed swirl-stabilized oxy-fuel flames carried-out in a gas turbine model

combustor at atmospheric pressure. Their results showed a strong effect of the oxygen fraction on the

combustion behavior in contrast to the equivalence ratio which has a relatively smaller effect. Operation

with oxygen fractions smaller than 22% was not possible even at stoichiometric conditions. They reported

that as the oxygen fraction increases, the flame can be operated stably for much leaner conditions.

Hu et al.11 studied the effects of equivalence ratios (0.8-1.2) and oxygen concentrations (25%-35%) and

dilutions (N2, CO2) on the laminar flame speeds of CH4/O2/CO2 mixtures in atmospheric conditions. They

reported that the laminar flame speeds of the premixed oxy-methane mixture reach a maximum at the

stoichiometric ratio of 1 while it decreases gradually on either side. The laminar flame speeds increased

with the increase of the oxygen fraction with a quadratic function relationship between the flame

velocities and the O2 concentration. Compared with N2, the high concentration of CO2 decreased the flame

speeds and the measured flame speed using CO2 as a diluent was about one-fifth of that using N2 as a

diluent.

Islam et al. 12-13 studied the effect of blockage ratio on the stability of CNG/CO2/O2 mixture and

compared the results with baseline CNG/air as well as with enriched air.

As CH4/O2/CO2 flames are known to be characterized by slower chemical kinetics than methane-air

flames and as such flame stability is more problematic, this work experimentally investigates the

flammability limits over the range of equivalence ratios (0.23-1.9) and oxygen fractions (29%-36%) and

flow Reynolds number (1100-2000). The results are compared with baseline CNG/air flames. The work

also studies the visual flame length and appearance.

In this work, the oxidizer-to-fuel ratio is defined as the ratio of the mass flow rate of the oxidizer (O2/CO2

mixture in oxy-fuel combustion or air in air-fuel combustion) to the mass flow rate of the fuel. The

equivalence ratio, , is defined as the ratio of the stoichiometric oxidizer-to-fuel ratio to the used oxidizer-

to-fuel ratio. The oxygen fraction is defined as the volumetric ratio of O2 in the O2/CO2 mixture. The

degree of premixing is indicated through the L/D ratio, stating the ratio between the duct length measured

from the point of introducing the combustible mixture to the burner and the duct diameter. The flow

Reynolds number is estimated based on the mass flow rate of the combustible mixture, the combustible

mixture kinematic viscosity and the equivalent diameter of the burner holes.

Two sets of experiments are carried-out in this study. The first set of experiments identifies the range of

equivalence ratios for stable flame operation under constant oxygen fraction conditions. In this set,

several oxygen fractions are considered, namely 29%, 32% and 36%.The second set of experiments

identifies the range of oxygen fraction at constant equivalence ratios (namely, 0.71, 0.85, 1.0 and 1.15)

for stable flame operation. The visual flame length and the extinction mechanisms are observed for both

sets.


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2. EXPERIMENTAL SETUP AND PROCEDURE:

The flow diagram of the experimental oxy-fuel combustion experiments is shown on the left part of figure

1. The diagram describes the air, O2, CO2, CNG lines, the oxidizer mixer, the burner, the confinement and

the exhaust section. Each gas line contains a control valve to control the gas flow rate, a rotameter to

measure the volumetric flow rate, a thermocouple and a pressure gage to measure the gas temperature and

pressure, respectively.

Air is supplied from a reciprocating compressor with a 0.5 m3 storage tank and then is fed to the oxidizer

mixer. Oxygen (99.5% purity) is supplied from two oxygen bottles (120 bar) and then is fed to the oxidizer

mixer. Carbon dioxide (99.5% purity) is supplied from a bottle (70 bar) and then is fed to the oxidizer

mixer.

The oxidizer mixer is designed to achieve complete mixing between the O2 and CO2 gases before their

introduction to the burner. Both gases enter a 1.25 cm diameter, 3.85 m pipe length (L/d = 308). The pipe

exit is fed to the burner. A perforated plate burner (3 mm thick, 30-mm outer diameter, with 22 holes 4-

mm each and made of steel) is used in this work and its schematic is shown on the right part of figure 1.

Multi recirculation zones occur downstream of every hole, providing a region in which the transfer of

mass and energy from the burnt gases to the un-burnt ones is improved and thus enhancing the flame

stability. This type of burner is preferred over a large size single-hole burner since it provides better flame

stability14.

The O2/CO2 mixture is introduced normal to the CNG fuel at a distance of L/D =7 before the ignition

point, making the resulting flames neither purely diffusion nor fully premixed, and hence are considered

partially premixed. Six flame arrestors are used along the path of the combustible mixture to avoid

explosion of the partially-premixed combustible mixture in case of flash back.

A confinement (steel cylinder with 0.5 m length and 15 cm diameter) is used to ensure confined flame

operation. The confinement ratio, (D Confinement/D Burner)2 is 25 indicating that wall effects are

insignificants15. The confinement is followed by an exhaust section (1 m long and 15 cm in diameter) to

prevent air leakage into the system. The exhaust section ends with a cone (20 cm and 40-mm exit diameter)

to eliminate air entrainment into the combustion zone.

A rectangular sight glass (50 mm x 500 mm, mounted on the outer surface of the confinement and sealed

to prevent leakage) is used to allow optical access for capturing digital images of the flames. The visual

flame appearance under different conditions is documented using a digital camera (Canon EOS D1100, 14

Mega pixel resolution, 1/8-s exposure time)

In all experiments, the system is purged first by air flow, and then the oxidizer mixture is introduced

followed by the fuel. A pilot flame is used to provide a source of ignition.


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Figure 1: Experimental setup of air-fuel and oxy-fuel combustion experiments (left) and Schematic

of the burner setup (right)

3. RESULTS AND DISCUSSION

The results are presented in two separate sets. The first set presents results at constant oxygen fraction and

variable equivalence ratio. The second set presents results at constant equivalence ratio and different

oxygen fractions. The results are presented in terms of upper and lower flammability limits and visual

flame lengths. The flame appearances (inner and outer cones) are documented. The extinction

mechanisms are identified.

The Reynolds number is in the range of 1000-1900 and 1160-1900 for the first and second sets,

respectively. The burner heat capacity (based on a calorific value of 42 MJ/m3)

is in the range 0.3 - 3 kW

and 1.4 to 2.8 kW for the first and second sets, respectively

3.1. Results at constant oxygen fraction and variable equivalence ratio:

In this set, three oxygen fractions are used, namely 29%, 32% and 36%. Flammability limits are identified

by fixing the oxidizer mass flow rate and equivalence ratio at some stable value, then the fuel mass flow

rate is slowly turned down at constant oxygen fraction until the lower flammability limit. Then, the fuel

flow rate is slowly increased at constant oxygen fraction until the upper flammability limit. As such,

extinction is obtained by changing flame temperature and equivalence ratio at a nearly constant nozzle

exit velocity and constant oxygen fraction. Figure 2 shows the flammability limits for this set of

experiments, including comparison with baseline air-fuel combustion.


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Figure 2: Flammability limits in oxy-fuel combustion and comparison with air-fuel combustion

The results show that air-fuel combustion enjoys larger stability limits under all oxygen fractions

considered. However, using an O2/CO2 mixture with an oxygen fraction of 36% achieves about 79% -

82% of that of air-fuel combustion in the range of Reynolds number considered. Burners specially

designed for enhanced stability may improve this ratio.

Extinction on both sides occur by blow-off, indicating that the flame speed at equivalence ratios beyond

the flammability limits is less than the velocity of the combustible mixture, causing the flame to blow off.

The visual flame length is obtained by digital photography allowing studying its dependence on

equivalence ratios at different oxygen fractions under oxy-fuel and air-fuel conditions. The results show

that the visual flame length generally decreases as the equivalence ratio decreases (owing to less fuel

input) in all cases with flames burnt in O2/CO2 mixtures being longer than almost all those burned in air at

all oxygen fractions considered, due to the slower chemical kinetics when CO2 is added. Under oxy-

combustion conditions, the visual flame length generally decreases as the oxygen fraction increases, due

to enhanced chemical kinetics at higher oxygen fractions.

There are several conical premixed flames formed as inner cones downstream of the perforated-plate

holes and are surrounded by an outer cone. This work presents the visual appearance of both the inner and

outer cones.

The outer and inner cones of the different flames observed are shown in figures 3 and 4, respectively. The

digital images show that the flame length is almost the same between equivalence ratios 1.2 to 0.9, and

then decreases until an equivalence ratio of 0.4 before extinction mechanism occurs.

0.0

0.5

1.0

1.5

2.0

2.5

1,000 1,200 1,400 1,600 1,800 2,000

Equ

ival

ence

Rat

io

Reynolds Number

Air

36%

32%

29%

36 %

32 %

29 %

Air


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Figure 3: Visual flame length at constant oxygen fractions and comparison with air-fuel

combustion

The flame at a higher equivalence ratio consists of large outer and inner cones, while decreasing

the fuel flow rate the inner cones length begins to decrease gradually and the outer cone disappears

gradually then the flame becomes weak and blow off occurs. This is because of insufficient fuel and

because of the burning velocity is lower than the flow velocity.

Figure 4: Visual flame appearance (outer-cone) in the range of equivalence ratios of 1.2 to 0.5. The

oxygen fraction is 36%

0

5

10

15

20

25

0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2

Flam

e Le

ngt

h c

m

Equivalence Ratio

OF=29% OF=32% OF=36% Air

3 cm


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Figure 5: Visual flame appearance (inner cones) in the range of equivalence ratios of 1.2 to 0.5.

The oxygen fraction is 36% and the Reynolds number is 1500

3.2. Results at constant equivalence ratio and variable oxygen fraction

In this set, the flammability limits are obtained by fixing the oxidizer flow rate and the equivalence ratio

at some stable value. Then, the oxygen fraction is slowly turned down while the CO2 fraction is slowly

increased at a constant equivalence ratio until the lower flammability limit. The upper flammability limit

is determined similarly. This ensures that extinction is obtained by changing flame temperature and

oxygen fraction at a nearly constant nozzle exit velocity and constant equivalence ratio.

Extinction beyond the upper flammability limits occurs by flash back indicating that the flame speed at

large oxygen fractions become higher than the velocity of the combustible mixture, causing flash back.

The oxygen fraction corresponding to the upper flammability limit depends on the equivalence ratio. The

lower flammability limits occurs by blow-off indicating that the flame speed at oxygen fractions

below28%, at all equivalence ratios considered (0.7 – 1.15), is less than the velocity of the combustible

mixture, causing the flame to blow off.


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Figure 6: Flammability limits in oxy-fuel combustion at constant equivalence ratios

Figure 7: Visual flame length at constant equivalence ratio (0.85)

The outer and inner cones of the different flames observed in this set are shown in figures 7 and

8, respectively. The digital images show that the flame length is almost the same at the same

oxidizer mass flow rates, equivalence ratio and different oxygen fractions, its observed that

increasing the oxygen fraction the inner cones decreases its length and the outer cones becomes

thinner and brighter.

25%

30%

35%

40%

45%

50%

55%

1,000 1,200 1,400 1,600 1,800 2,000

Oxy

gen

Fra

ctio

n O

F%

Reynolds Number Re

Ø = 1.15

Ø = 0.70

Ø = 1

Ø = 0.85

Ø = 0.7-1.15

10

14

18

22

25 30 35 40 45

Flam

e le

ngt

h

Oxygen Fraction


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It is also observed that the flammability limits increase as the oxidizer mass flow rate increases, the flame

length slightly decreases and the flame color changes from bluish to white as the oxygen fraction

increases.

Figure 8: Visual flame appearance (outer-cone) for oxygen fractions of 29% and 36% and

comparison with air. The equivalence ratio is one and the Reynolds number is 1500

Figure 9: Visual flame appearance (inner-cone) for oxygen fractions of 29% and 36% and

comparison with air. The equivalence ratio is one and the Reynolds number is 1500

In conclusion, as the restrictions on the release of CO2 and other greenhouse gases into the

atmosphere tightens, the need for carbon capture and sequestration increases. The current work


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presents data on the flame stability and visual flame length and appearance under different oxy-

fuel combustion conditions.

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18th ifrf, germany, experimental investigation of oxy fuel combustion of cng flames stabilized over...

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