Experimental Study for CNG Flame Properties Burnt In O2-Co2 Mixture as Compared to Air and Enriched Air

EXPERIMENTAL STUDY FOR CNG FLAME

PROPERTIES BURNT IN O2/CO2 MIXTURE AS

COMPARED TO AIR AND ENRICHED AIR

by

Islam Ahmed El-Sayed Ramadan

A Thesis Submitted to the

Faculty of Engineering at Cairo University

in Partial Fulfillment of the

Requirements for the Degree of

MASTER OF SCIENCE

in

MECHANICAL POWER ENGINEERING

FACULTY OF ENGINEERING, CAIRO UNIVERSITY

GIZA, EGYPT

2011




by






MASTER OF SCIENCE

in


Under the Supervision of

Prof. Dr. Tharwat wazier Abou-Arab

Dr. Abdelmaged Hafez Ibrahim Essawey

Mechanical Power Engineering Department Faculty of Engineering

Cairo University


GIZA, EGYPT

2011




by






MASTER OF SCIENCE

in


Approved by the

Examining Committee

Prof. Dr. Hafez Elsalamawy, Member

Prof. Dr. Hindawi Salem, Member

Prof. Dr. Tharwat wazier Abou-Arab, Thesis Main Advisor


GIZA, EGYPT

2011

I

TABLE OF CONTENTS

LIST OF FIGURES ...................................................................................................... IV

LIST OF TABLES........................................................................................................ IX

LIST OF SYMBOLS AND ABBREVIATIONS .......................................................... X

ACKNOWLEDGEMENT ............................................................................................ XI

ABSTRACT ............................................................................................................... XII

1 INTRODUCTION .................................................................................................. 2

2 LITERATURE REVIEW ....................................................................................... 7

2.1 Oxy-Combustion: .............................................................................................. 7

2.2 Summary for the Three Carbon Capture Techniques: .................................... 21

2.3 Bluff-body Flame Stability: ............................................................................ 22

2.4 Objective of This Work: ................................................................................. 23

3 TEST RIG AND INSTRUMENTATION ............................................................ 26

3.1 Burners: ........................................................................................................... 27

3.2 Combustor and Exhaust System: .................................................................... 28

3.3 Fuel Supply System: ....................................................................................... 32

3.4 Oxidizer Supply System: ................................................................................ 32

3.5 Instrumentation: .............................................................................................. 33

3.5.1 Flow Meters.............................................................................................. 33

3.5.2 Pressure Gauges and Thermocouples: ..................................................... 36

3.5.3 Gas Analyzer: ........................................................................................... 36

3.6 Uncertainty and Error Analysis: ..................................................................... 37

3.7 Experimental Program: ................................................................................... 38

3.7.1 The Extinction limit: ................................................................................ 38

3.7.2 Flame Appearance: ................................................................................... 38

3.7.3 Flame Emissions: ..................................................................................... 39

3.7.4 System Compatibility: .............................................................................. 39

4 RESULTS AND DISCUSSION ........................................................................... 42

4.1 Introduction: .................................................................................................... 42

4.2 The Extinction Limits: .................................................................................... 43

4.2.1 Air: ........................................................................................................... 43

II

4.2.2 Enriched air (Air/O2): ............................................................................... 46

4.2.3 (CO2/O2) oxidizer: .................................................................................... 51

4.3 Flame Appearance: ......................................................................................... 59

4.4 Flame Emissions: ............................................................................................ 65

4.4.1 CNG/Air flame emissions: ....................................................................... 67

4.4.2 CNG/Air/O2 flame emissions: .................................................................. 77

5 CONCLUSTION AND RECOMMENDATION FOR FUTURE WORK .......... 97

5.1 Conclusion: ..................................................................................................... 97

5.2 Recommendations for Future Work: .............................................................. 98

REFERENCES ............................................................................................................. 99

APPENDIX (A) .......................................................................................................... 104

A.1. Flow Meters Calibration Certificates: ....................................................... 104

A.1.1. Air and CO2 rotameter (1): ................................................................. 104

A.1.2. Air and CO2 rotameter (2): ................................................................. 107

A.1.3. Fuel rotameter: .................................................................................... 107

A.1.4. O2 rotameter (1): ................................................................................. 108

A.1.5. O2 rotameter (2): ................................................................................. 109

A.2. Pressure Gauges Calibration Certificates: ................................................. 110

A.2.1. Air and CO2 pressure gauge: .............................................................. 110

A.2.2. O2 pressure gauge: .............................................................................. 110

A.2.3. Fuel pressure gauge: ........................................................................... 111

A.2.4. Oxidizer pressure gauge: .................................................................... 111

A.3. Thermocouples Calibration Certificates: ................................................... 112

A.3.1. Air and CO2 thermocouple: ................................................................ 112

A.3.2. O2 thermocouple: ................................................................................ 112

A.3.3. Fuel thermocouple: ............................................................................. 113

A.3.4. Oxidizer thermocouple: ...................................................................... 113

A.4. Gas Analyzer Calibration Certificate: ....................................................... 114

APPENDIX (B) .......................................................................................................... 117

Correction for Rotameter Reading ......................................................................... 117

APPENDIX (C) .......................................................................................................... 120

III

Uncertainty Calculations ........................................................................................ 120

C.1. Uncertainty Evaluation for Measured Parameters ..................................... 120

C.1.1. Type A evaluation ............................................................................... 120

C.1.2. Type B evaluation ............................................................................... 121

C.1.3. Combined and expanded uncertainty .................................................. 121

C.2. Uncertainty Evaluation for Calculated Parameters ................................... 122

C.2.1. Uncertainty evaluation for mass flow rates ��. � ............................... 122

C.2.2. Uncertainty evaluation for equivalence ratio (Ø) ............................... 123

C.2.3. Uncertainty evaluation for flow momentum (M): .............................. 123

C.2.4. Uncertainty evaluation for emission index: ........................................ 124

IV

LIST OF FIGURES

Figure 1.1: Adiabatic flame temperature versus oxidizer composition, for an adiabatic

equilibrium stoichiometric CH4 flame ............................................................................ 5

Figure 2.1: The block diagram of (NGCC) with CO2 capture ....................................... 7

Figure 2.2: The variation of centerline temperature with distance from burner tip for

different oxidizers ........................................................................................................... 9

Figure 2.3: The variation of centerline O2 concentration with distance from burner tip

for different oxidizers. .................................................................................................... 9

Figure 2.4: The variation of centerline CO2 concentration with distance from burner

tip for different oxidizers ................................................................................................ 9

Figure 2.5: The variation of centerline NO concentration with distance from burner tip

for different oxidizers. .................................................................................................. 10

Figure 2.6: The variation of centerline CO concentration with distance from burner tip

for different oxidizers. .................................................................................................. 10

Figure 2.7: An instability pattern diagram for CH4/CO2/O2 flame (Ø = 0.9).............. 11

Figure 2.8 : The minimum and maximum gas temperature for three different flames as

a function of axial distance from burner tip for different Oxygen fractions ................ 12

Figure 2.9: The flame radiation intensity for two different flames as a function of

radial distance from center line .................................................................................... 13

Figure 2.10: The variation of NO emissions with flue gas ratio .................................. 14

Figure 2.11: The measured and calculated values for the concentration of NO as a

function of axial distance from burner (using Air as an oxidizer). .............................. 15

Figure 2.12: The measured and calculated values for the concentration of NO as a

function of axial distance from burner (using O2/CO2 as an oxidizer) ........................ 15

Figure 2.13: The temperature variations with height of combustor for different flames

...................................................................................................................................... 15

Figure 2.14: The nitric oxide concentration with height of combustor for different

flames ............................................................................................................................ 15

V

Figure 2.15: The variation of maximum and mean and exit temperatures with flue gas

recycle ratio. ................................................................................................................. 16

Figure 2.16: The variation of oscillation frequency at the root and mid of flame with

flue gas recycle ratio[. ................................................................................................... 17

Figure 2.17: Comparison of experimental blow-off point and prediction based on

CH4/Air data for CH4/O2/CO2 flames at Ø=1.0. ........................................................... 19

Figure 2.18: comparison of calculated and measured data for CO concentration as a

function of flame temperature at different equivalence ratio. ...................................... 20

Figure 2.19: comparison of calculated and measured data for O2 concentration: O2

levels are represented as function of flame temperature at different equivalence ratio.

...................................................................................................................................... 20

Figure 2.20: Combustion regime diagram with varied central-fuel and annular-air

momentum at different Blockage ratio (BR). ............................................................... 23

Figure 3.1: Flow Diagram ............................................................................................ 26

Figure 3.2: Test rig. ...................................................................................................... 27

Figure 3.3: Burner. ........................................................................................................ 29

Figure 3.4: Combustor .................................................................................................. 30

Figure 3.5: Exhaust section .......................................................................................... 31

Figure 3.6: Oxidizer Mixer. .......................................................................................... 34

Figure 3.7: CNG/ Air flame emission at different equivalence ratio with blockage ratio

of 0.36. .......................................................................................................................... 39

Figure 4.1: The extinction limits of CNG/Air flame at four different blockage ratios.45

Figure 4.2: The extinction limits for CNG/Air/O2 flame with two different fuel

momenta (FM) at blockage ratio of 0.36. ..................................................................... 47


momenta (FM) at blockage ratio of 0.5. ....................................................................... 48

Figure 4.4: The extinction limits for CNG/Air/O2 flame with constant fuel momenta

(FM) at blockage ratio of 0.667. ................................................................................... 49

VI


momenta (FM) at blockage ratio of 0.82. ..................................................................... 50

Figure 4.6: The extinction limits for CNG/CO2/O2 flame at two different oxygen

fractions (OF), compared to CNG/Air flame at blockage ratio of 0.36. ...................... 53


fractions (OF), compared to CNG/Air flame at blockage ratio of 0.5. ........................ 54


fractions (OF), compared to CNG/Air flame at blockage ratio of 0.667. .................... 55


fractions (OF), compared to CNG/Air flame at blockage ratio of 0.82. ...................... 56

Figure 4.10: The Extinction limit for CNG/CO2/O2 flame with oxygen fraction of 32%

vol. at four different blockage ratios. ........................................................................... 57

Figure 4.11: The Extinction limit for CNG/CO2/O2 flame with oxygen fraction of 28%

vol. at four different blockage ratios. ........................................................................... 58

Figure 4.12: CNG/Air flames with a BR of 0.36 and a fuel momentum of 4.7E-5 N.

The air momenta of cases A, B and C are 0.00047, 0.001884 and 0.00912 N,

respectively. The flame lengths of cases A, B and C are 27.9 cm, 24 cm and 4 cm,

respectively. .................................................................................................................. 60

Figure 4.13: CNG/Air/O2 flames with a BR of 0.36, OF=28% vol. and a fuel

momentum of 4.7E-5 N. The air momenta of cases A, B and C are 0.000539,

0.002177 and 0.008834 N, respectively. The flame lengths of cases A, B and C are

15.4 cm, 11.8 cm and 5.3 cm, respectively. ................................................................. 61


momentum of 4.7E-5 N. The air momenta of cases A, B and C are 0.0006, 0.002434

and 0.009855 N, respectively. The flame lengths of cases A, B and C are 11.7 cm, 9.5

cm and 5 cm, respectively. ........................................................................................... 62

Figure 4.15: CNG/CO2/O2 flames with a BR of 0.36, OF=28% vol. and a fuel


and 0.005932 N, respectively. The flame lengths of cases A, B and C are 21.3 cm,

14.6 cm and 6 cm, respectively. ................................................................................... 63



VII

and 0.00654 N, respectively. The flame lengths of cases A, B and C are 18.3 cm, 11.5

cm and 5 cm, respectively. ........................................................................................... 64

Figure 4.17: Center line emissions of CNG/Air flame at a distance equals to 500 mm

from burner tip and blockage ratio of 0.36. .................................................................. 69

Figure 4.18: Radial emissions of CNG/Air flame at a distance equals to 500 mm from

burner tip, blockage ratio of 0.36 and equivalence ratio (Ø) of 0.9. ............................ 70


from burner tip and blockage ratio of 0.5. .................................................................... 71


burner tip, blockage ratio of 0.5 and equivalence ratio (Ø) of 0.9. .............................. 72


from burner tip and blockage ratio of 0.667. ................................................................ 73


burner tip, blockage ratio of 0.667 and equivalence ratio (Ø) of 0.9. .......................... 74


from burner tip and blockage ratio of 0.82. .................................................................. 75


burner tip, blockage ratio of 0.82 and equivalence ratio (Ø) of 0.9. ............................ 76

Figure 4.25: Center line emissions of CNG/Air/O2 flame with oxygen fraction of 32%

at a distance equals to 500 mm from burner tip and blockage ratio of 0.36. ............... 80

Figure 4.26: Radial emissions of CNG/Air/O2 flame with oxygen fraction of 32% at a

distance equals to 500 mm from burner tip, blockage ratio of 0.36 and equivalence

ratio (Ø) of 0.9. ............................................................................................................. 81





ratio (Ø) of 0.9. ............................................................................................................. 83


at a distance equals to 500 mm from burner tip and blockage ratio of 0.5. ................. 84

VIII


distance equals to 500 mm from burner tip, blockage ratio of 0.5 and equivalence ratio

(Ø) of 0.9. ..................................................................................................................... 85


at a distance equals to 500 mm from burner tip and blockage ratio of 0.5. ................. 86


distance equals to 500 mm from burner tip, blockage ratio of 0.5 and equivalence ratio

(Ø) of 0.9. ..................................................................................................................... 87


at a distance equals to 500 mm from burner tip and blockage ratio of 0.667. ............. 88



ratio (Ø) of 0.9. ............................................................................................................. 89


at a distance equals to 500 mm from burner tip and blockage ratio of 0.667. ............. 90



ratio (Ø) of 0.9. ............................................................................................................. 91





ratio (Ø) of 0.9. ............................................................................................................. 93





ratio (Ø) of 0.9. ............................................................................................................. 95

IX

LIST OF TABLES

Table 3-1: Specifications of Air and CO2 rotameters. .................................................. 35

Table 3-2: Specifications of fuel rotameter .................................................................. 35

Table 3-3: Specifications of O2 rotameter .................................................................... 35

Table 3-4: Specifications of pressure gauges ............................................................... 36

Table 3-5: Specifications of flow thermocouples ......................................................... 36

Table 3-6: Specifications of flue gas analyzer ............................................................. 37

Table 4-1: Gas transport properties for N2, CO2 at 1atm. ........................................... 52

X

LIST OF SYMBOLS AND ABBREVIATIONS

Alphabetic symbols

ATR Auto-Thermal Reformer

BR Blockage Ratio

CNG Compressed Natural Gas

CTF Combustion Test Facility

FGR Flue Gas Recirculation

FM Rate of Change of Fuel Momentum

IGCC Integrated Gasification Combined Cycle

LEL Relative Lower Explosive Limit

LELNG Lower Explosive Limit of natural gas

LPS Low Pressure Steam

MDEA Methyl Di-Ethanol Amine

NGCC Natural Gas Combined Cycle

OF Oxygen Fraction

PC Pulverized Coal

Q Volume flow rate

SG Synthetic Gas

SMOC Steam Moderated Oxy-Combustion

Greek symbols

Ø Equivalence ratio

λ Excess air factor

XI

ACKNOWLEDGEMENT

I am in a debt gratitude to my advisors, Prof. Tharwat Abou-Arab and Dr.

Abdelmaged, for their guidance, support and patience throughout this research. Their

trust and high expectations pushed me not only to finish this thesis but towards a new

level of experience.

I am also grateful to my colleagues in measurements and calibration laboratory for

their support and providing me with measurement instrumentations to complete this

work.

I am also grateful to all technicians in mechanical power department for their help.

Special thanks to Tarek for his support and help in machining many parts of my

experiment. I am also thankful to Magdi for his help in running experiments late in the

night.

I would like also to thank my teachers and advisors in Cairo University for their

support and encouragement.

My wholehearted gratitude is to my family for their support, love, prayers and

sacrifices. Their continuous encouragement throughout the years gave me the strength

to reach my goals.

XII

ABSTRACT

Concerns about global warming have encouraged interest in hydrocarbon combustion

techniques that allow easy capture and sequestration of carbon dioxide. One method

of achieving this objective is through the use of post-combustion CO2 capture and

sequestration. In this work, Compressed Natural Gas (CNG) flames were burned in

confined diffusion situation in oxy-combustion environment with different O2/CO2

mixtures. The flames were stabilized on a bluff-body burner and different blockage

ratios were studied. The appearance, stability and emissions of these flames are

compared to combustion in air and in enriched air. Three different regions were

observed, namely like-jet flames, central-jet dominated flames and recirculation zone

flames, depending on the ratio between oxidizer and fuel momenta. The flame color

changed from yellow in air, to blue with yellow tips in oxy-combustion to bright white

in the enriched air. The flame length was the highest in air, then lower in oxy-

combustion cases and the lowest in the enriched air cases. CNG flames burned in

enriched air have higher flame stability than those burned in air, which in turn have

higher stability than those burned in oxy-combustion. The maximum hydrocarbon

emission index was observed at blockage ratio of 0.82 and the hydrocarbon emission

index decreases with the increase in oxygen fraction in CNG/enriched air flame. Also

the CO2 emission index in CNG/enriched air flame was higher than in CNG/Air

flame.

1

CHAPTER (1)

INTRODUCETION

2

CHAPTER (1)

1 INTRODUCTION

Increasing concerns about climate change and the increasing in the penalties of

carbon dioxide emissions (Europe and Japan Face $46 Billion Global-Warming

Penalty)[1] encourage the interest in zero-CO2 emission hydrocarbon

combustion techniques that can accommodate carbon dioxide capture and

sequestration to reduce the effects of carbon dioxide as a green house gas

which causes global warming. There are three main techniques used for this

purpose namely:

• Post-combustion treatment.

• Pre-combustion treatment.

• Oxy-combustion.

The work presented in this thesis is on oxy-combustion. Post and pre-

combustion techniques are overviewed first.

In the post-combustion treatment, the flue gases leaving the combustion

process are cooled then fed into CO2 absorber, where they pass through an

absorbing solution (like amine) that contains a chemical that captures CO2. The

CO2 is then removed from the absorbing solution by steam, allowing the

absorber to be reused.

In this technique there are many researches, modifications and developments

such as Peeters, et al.[2] who presented a techno-economic analysis of natural

gas combined cycles with post-combustion CO2 absorption. Drage, et al.[3]

developed adsorbent techniques for post combustion CO2 capture.

3

Kothandaraman, et al.[4] compared different solvents for CO2 capture using the

ASPEN RateSep framework. The results indicate that the potassium carbonate

system is found to be particularly useful for CO2 capture from pressurized

combustion power plants and from integrated gasification combined cycle

(IGCC) applications.

There are two main problems for this techniques, the first one is the corrosion

of absorbers and degradation of amine in absorber due to presence of oxygen in

flue gases, so this application cannot be used in gas turbine. The second

problem is the amine emissions to air that reach to 40 160 tons/year [5].

The second technique is the pre-combustion treatment where the carbon in the

fuel is separated, or removed, before the combustion process. Instead of

burning coal or natural gas, the fuel can be converted to hydrogen and

carbon prior to combustion. The carbon can then be captured and stored, while

the hydrogen is burned to produce power.

In this technique there are many researches, modifications and developments

such as Matteo, et al.[6] investigated the thermodynamic and engineering

aspects of natural gas combined cycle (NGCC) with auto-thermal reformer

(ATR) and CO2 absorption by methyl di-ethanol amine (MDEA). The results

indicate that the net plant efficiency is 50.65%, about 8% points less than the

reference combined cyclewithoutCO2 capture, where a carbon capture ratio of

91.6% was used for the base case. Also, Seong and Seoa [7] presented a study

for enhancing CO2 separation for pre-combustion capture with hydrate

formation in silica gel pore structure.

The problem of this technique is the safety precautions which must be taken

into consideration while using hydrogen as a fuel; also, the concerns

surrounding the storage of hydrogen are a major issue [8].

The third technique, which is the focus of this work, is the oxy-combustion

technique where the oxidizer used is not air, but rather a mixture of Oxygen

4

and Carbon dioxide. The flue gases contain mainly Carbon dioxide and water

vapor, which is removed by condensation, leaving the carbon dioxide alone.

This technique facilitates the capture of the carbon dioxide for future

sequestration. Without this process, the flue gases contain a significant portion

of Nitrogen, making the separation of Carbon dioxide costly and impractical.

Another advantage of this technique is the elimination of thermal NOx.

However, the stability and emissions (particularly CO) of the flame burning in

a mixture of Oxygen and Carbon Dioxide is an issue that needs to be studied.

There are many differences between oxy-combustion and air-combustion as

Gibbs, et al.[9]

mentioned. These include that the temperature in oxy-

combustion is higher than in air-combustion; the ignition temperature in oxy-

combustion is lower than in air-combustion and the burning velocity in oxy-

combustion is higher than in air-combustion.

Since oxy-combustion system produces high temperature in comparison with

air-combustion and this high temperature may cause failure in material used in

combustion system, diluents as CO2 or N2 or H2O have been used to reduce the

temperature of oxy-combustion system. Charles [10] presented the effect of

oxygen concentration in oxidizer on adiabatic flame temperature as shown in

Figure 1.1.

This work studies the effects of blockage ratio, Oxygen fraction and

equivalence ratio with three different oxidizers (air, enriched air and CO2/O2).

The study investigated the flame appearance, stability, and emissions for all

oxidizers, with the exception of the emissions in the oxy-combustion case since

commercial gas analyzers are not designed to operate in flue gases containing

high percentages of CO2.

5

Figure 1.1: Adiabatic flame temperature versus oxidizer composition, for an adiabatic equilibrium stoichiometric CH4 flame[10].

6

CHAPTER (2)

LITERATURE REVIEW

7

CHAPTER (2)

2 LITERATURE REVIEW

The following chapter summarizes some of the previous experimental and

theoretical work which was done in the oxy-combustion technique.

2.1 Oxy-Combustion:

One of the first attempts in utilizing oxy-combustion with a dilutant was made

by YULIN, et al. [11] when they presented a study for natural gas combined

cycle (NGCC) with CO2 capture utilizing mixture of O2 (purity 99.7%) and

recycled CO2 from the flue gases as an oxidizer. The block diagram of their

experiment is shown in Figure 2.1.

Figure 2.1: The block diagram of (NGCC) with CO2 capture [11].

The results indicate that the gas turbine efficiency increases by 2.9% and the

net thermal efficiency decreases by 9% due to carbon capture process and air

separation process, this plant will generate 210 MW of electricity and 51 tones

of process steam per hour, and will also produce saleable products, including

8

9878 tones of nitrogen, 162 tones of argon and 2102 tones of liquid carbon

dioxide per day.

GOLOMB and YULIN [12] compared the efficiencies of two combined cycles,

one fuelled by natural gas (NG) and another one fuelled by synthetic gas (SG)

obtained from a coal gasification process. Both cycles utilize oxygen with a

purity of (99.5%) and carbon dioxide obtained from flue gas as an oxidizer.

Their results indicate that the gross efficiency of natural gas-fuelled cycle is

decreased by 16.6% due to CO2 capture and the gross efficiency of synthetic

gas fuelled cycle is decreased by 23.4% due to coal gasification process and

carbon capture process.

Yewwn, et al.[13]

compared the structure of three diffusion flames utilize

Natural gas (NG) as a fuel with three different oxidizers:

Oxidizer A: Air (21% vol. O2 and 79% vol. N2)

Oxidizer B: (28% vol. O2 and 72% vol. CO2).

Oxidizer C: Enriched Air (28% vol. O2 and 72% vol. N2).

The results as shown in Figure 2.2, Figure 2.3, Figure 2.4, Figure 2.5, and

Figure 2.6 indicate that the temperature of enriched air flame is higher than

other flames, the concentration of O2 and CO2 in O2/CO2 flame are higher than

other flames and the concentration of NO and CO in O2/CO2 flame are lower

than other flames.

9

Figure 2.2: The variation of centerline temperature with distance from burner tip

for different oxidizers[13].

Figure 2.3: The variation of

centerline O2 concentration with

distance from burner tip for different

oxidizers[13].

Figure 2.4: The variation of centerline CO2

concentration with distance from burner tip

for different oxidizers[13].

10

Figure 2.5: The variation of

centerline NO concentration with

distance from burner tip for different

oxidizers [13].

Figure 2.6: The variation of centerline CO

concentration with distance from burner tip

for different oxidizers [13].

Hals, et al. [14] presented an experimental study on combustion instability in a

sudden expansion premixed CH4/CO2/O2 flame.

Their results shown in Figure 2.7 indicate that the combustion stability is

dependent on oxygen enrichment. Flame sustainability in the combustor can

only be obtained with a minimum of 30% vol. of oxygen in oxidizer (CO2/O2)

to perform a comparable manner with an air-combustion. Their results indicate

that flash back is a main concern, especially at high Oxygen fractions (OF).

11

Figure 2.7: An instability pattern diagram for CH4/CO2/O2 flame (Ø = 0.9)[14]-.

Where:

Region I: Concerns flames that are sustained in the combustor, but reveal a

large-scale unsteady pattern generated by the shear layer.

Region II: Increasing the oxygen enrichment up to 40% leads to a drastic

change in the flame stabilization, which make it possible to counteract the

influence of the shear layer on the flow characteristics.

Region III: Is a transitional region between region II and region IV.

Region IV: Increase of O2 concentration at constant Re leads to a regime of

flashback.

Johnsson, et al. [15] measured in-furnace gas concentration, temperature and

total radiation profiles for three different flames:

1- C3H8/AIR flame (reference flame).

12

2-Two C3H8/O2/CO2 flames with different concentrations of O2 in oxidizer:

a- OF 21: 21 vol.% O2 and 79 vol.% CO2.

b- OF 27: 27 vol.% O2 and 73 vol.% CO2.

As shown in Figure 2.8 the temperature levels of the OF 21 flame are

significantly lower than in the air flame while the temperature levels of the OF

27 flame are approximately the same in air flame.

Figure 2.8 : The minimum and maximum gas temperature for three different

flames as a function of axial distance from burner tip for different Oxygen

fractions[15].

As shown in Figure 2.9 the flame radiation intensity increases with up to 30%

for the OF 27 flame compared to air flame.

13

Figure 2.9: The flame radiation intensity for two different flames as a function

of radial distance from center line[15].

HoKeun, et al. [16] experimentally investigated the NO emission characteristics

of oxy-fuel combustion with flue gas recirculation (FGR) or with CO2 addition

utilizing natural gas as a fuel.

The results as shown in Figure 2.10 indicate that the NO emission is reduced

by increasing the flue gas ratio or increasing CO2 ratio.

14

Figure 2.10: The variation of NO emissions with flue gas ratio[16].

Jyh-Cherng, et al. [17] investigated the emission characteristics of CO2, SO2 and

NOx in the flue gas of coal combustion by varying the compositions and

concentrations of feed gas (O2/CO2/N2) and the ratios of recycled flue gas. The

results indicate that the maximum concentration of CO2 in O2/CO2 combustion

system is 95% as the feed gas is 30% O2/70% CO2. By O2/CO2 combustion

technology, higher concentration of SO2 is produced as the feed gas is 30%

O2/70% CO2 or 40% O2/60% CO2, while higher concentration of fuel NOx is

produced as the feed gas is 20% O2/80% CO2 or 50% O2/50% CO2.

Klas, et al. [18] experimentally and numerically compared the amount of NO

emitted per unit of energy supplied from combustion of lignite utilizing two

different oxidizers: Air and OF 25(25 vol.% O2 and 75 vol.% CO2).

As shown in Figure 2.11 and Figure 2.12 during O2/CO2 firing, the amount of

NO emitted per unit of energy supplied is lower than the emission at air-firing

case.

15

Figure 2.11: The measured and

calculated values for the concentration

of NO as a function of axial distance

from burner (using Air as an oxidizer)

[18].

Figure 2.12: The measured and calculated values for the concentration of NO as a function of axial distance from burner (using O2/CO2 as an oxidizer) [18].

Guo-neng, et al. [19] compared the structure of different diffusion flames utilize

CH4 as a fuel and two different oxidizer (O2/CO2 oxidizer and O2/N2 oxidizer)

with different oxygen concentration (25%-45% by volume).

The results as shown in Figure 2.13 and Figure 2.14 show that the temperature

of O2/CO2 flame are higher than temperature of O2/N2 flame and the nitric

oxide concentration in the O2/CO2 flame is too small comparing to O2/N2 flame.

Figure 2.13: The temperature variations with height of combustor for different flames[19].

Figure 2.14: The nitric oxide concentration

with height of combustor for different flames[19].

16

John, et al. [20] investigated the impact of oxy-fuel combustion on diffusion

flame characteristics through the application of digital imaging and image

processing techniques. The characteristic parameters of the flame were derived

from flame images that were captured using a vision-based flame monitoring

system.

The experimental furnace used for this work was 0.5 MWth combustion test

facility (CTF). The CTF has a refractory lined combustion chamber with an

inner cross-section of 0.8 m x 0.8 m and approximately 4 m long. A water

jacket layer was fitted to the outside of the chamber to remove the input

energy.

As shown in Figure 2.15, the temperature of the flame is effectively controlled

by the flue gas recycle ratio. The flame temperature decreases with the recycle

ratio.

Figure 2.15: The variation of maximum and mean and exit temperatures

with flue gas recycle ratio[20].

17

As shown in Figure 2.16, the flame oscillation frequency decreases with the

recycle ratio, indicating that a high recycle ratio has an adverse effect on the

flame stability.

Figure 2.16: The variation of oscillation frequency at the root and mid of flame

with flue gas recycle ratio[20].

Pyong, et al. [21] evaluated power generation characteristics, economics, and

CO2 reduction effects of a proposed CO2-capturing repowering system that

utilizes low pressure steam (LPS) to increase generated power and to capture

generated CO2 based on the oxy-fuel combustion method.

The results indicated that the CO2 emission amount can be reduced by 21.8%

with 2.41% degradation of the net power generation efficiency, from 56.2% to

53.8%.

Sivaji and Sreenivas [22] investigated numerically the effects of steam

moderated oxy-combustion (SMOC) on the flame stability and emissions; in

addition an exergy analysis was implemented on lignite-fired power plant.

18

In this work the fuel was pure methane (CH4) and oxidizer was (O2/H2O)

mixture. The fuel was fed at 300 K and oxidizer mixture at 393 K at a pressure

of one atmosphere.

The results showed an oxygen concentration of 36% by mass is necessary for

stable combustion. An exergy analysis of a lignite-fired power plant operating

on the proposed method (SMOC with CO2 sequestration) showed that an

optimized, heat-integrated green field plant will have higher gross thermal

efficiency but the power required for air separation and CO2 sequestration leads

a net efficiency penalty of 8%.

Alberto, et al. [23] experimentally and theoretically investigated the flow

velocity limit for blow-off for three different premixed flames namely:

1- CH4/O2/CO2 flame.

2- CH4/O2/N2 flame.

3- CH4/AIR flame.

These experiments were performed utilizing 45o swirler/annular nozzle burner

fitted to a cylindrical combustor. The fuel is injected 150 cm upstream of the

combustor to achieve a premixed condition. The combustor consists of a quartz

tube with a 115 and 120 mm inner and outer diameter, respectively, Two quartz

tube lengths were tested: 304.8 mm (12 inches) and 508 mm (20 inches).

The results as shown in Figure 2.17 below indicates that operating the CO2

diluted system significantly contracts operability boundaries. Both the

experimental data and predictions show that the CO2 diluted system will blow

off at a flame temperature about 300K hotter than the air system at a given

nozzle exit velocity.

19

Figure 2.17: Comparison of experimental blow-off point and prediction based

on CH4/Air data for CH4/O2/CO2 flames at Ø=1.0[23].

Alberto, et al. [24] experimentally measured the concentrations of CO and O2

emissions for two different premixed flames namely:

1- CH4/O2/CO2 flame.

2- CH4/AIR flame.

These experiments were performed utilizing swirl stabilized annular nozzle

burner fitted in a cylindrical combustor.

They found that Equilibrium CO emissions in CH4/O2/CO2 flame are higher

than in CH4/AIR flame. In addition, at a fixed residence time, CO emissions are

also higher. As shown in Figure 2.18 and Figure 2.19 the Equilibrium CO and

O2 emissions are an exponentially increasing function of flame temperature.

20

CO and O2 emissions are minimized by operating at low stoichoimetry, flame

temperature and high pressure.

Figure 2.18: comparison of calculated and measured data for CO concentration

as a function of flame temperature at different equivalence ratio[24].

Figure 2.19: comparison of calculated and measured data for O2 concentration:

O2 levels are represented as function of flame temperature at different

equivalence ratio[24].

21

2.2 Summary for the Three Carbon Capture Techniques:

Mohamed, et al. [25] presented a summary of technical-economic studies for

three main methods for the capture of CO2: pre-combustion, post-combustion

and oxy-combustion.

These studies conducted on the following three systems:

1- An Integrated Gasification Combined Cycle (IGCC).

2- A conventional combustion of Pulverized Coal (PC).

3- Natural Gas Combined Cycle (NGCC).

The results show that pre-combustion capture by physical absorption

(methanol) should be used for IGCC, oxy-combustion should be used for PC

and post-combustion capture (amines) should be used for NGCC.

Amann, et al. [26] presented a study which aims to assess by simulation the

energy and environmental performances of natural gas combined cycle

(NGCC) with CO2 capture utilizes O2 and recycled CO2 from flue gases as an

oxidizer and comparing this system with a conventional (NGCC) with post-

combustion CO2 capture based on chemical absorption.

The results show that the O2/CO2 cycle with 85% CO2 recovery rate and an

oxygen purity of 90 mol. % the net electrical efficiency reaches 51.3% which

corresponds to an efficiency loss of 8.2% points relatively to the base case

(without carbon capture) and The quantity of CO2 avoided is about 280

g/kW.h.

For post-combustion CO2 capture system the net electrical efficiency of power

plant decreases about 10% points.

22

2.3 Bluff-body Flame Stability:

Because flame stability is a main concern in this work, and since the bluff-body

is used for flame stabilization, this section is devoted to bluff-body flame

stability.

The bluff body is a basic device to stabilize double concentric jet diffusion

flames in industrial burners, providing excellent turbulent mixing

characteristics, improvement in flame stability, and ease of combustion control

[27].

Kang and Yang [28] experimentally investigated the effect of blockage ratio on

the flame structure of bluff-body stabilized flame. The results as shown in

Figure 2.20 represent the effect of fuel and air momentum on the combustion

regime for different blockage ratio (BR) where the combustion regimes are

classified as follows:

Regime I: recirculation zone flame.

Regime II: central-jet dominated flame.

Regime III: jet-like flame.

Regime IV: intermittent central-jet dominated flame which is called the

partially quenched flame.

Regime V: lift-off flame.

23

Figure 2.20: Combustion regime diagram with varied central-fuel and annular-

air momentum at different Blockage ratio (BR) [28].

2.4 Objective of This Work:

In view of the available literature, it is obvious that there are three main

technologies used for carbon capture and storage (CCS), oxy-combustion

technique is one of the latest technologies in carbon capture, it eliminates the

CO2 and NOx emissions but it faces many challenges like flame stability and

emissions. This work focuses on the oxy-combustion technology, where the

oxidizer is a mixture of O2 and CO2. The work studies the effects of blockage

ratio, Oxygen fraction and equivalence ratio with three different oxidizers (air,

enriched air and CO2/O2). The study investigated the flame appearance,

stability, and emissions for all oxidizers, with the exception of the emissions in

24

the oxy-combustion case since commercial gas analyzers are not designed to

operate in flue gases containing high percentages of CO2.

25

CHAPTER (3)

TEST RIG AND INSTRUMENTATION

26

CHAPTER (3)

3 TEST RIG AND INSTRUMENTATION

The flow diagram of the experiment is shown in Figure 3.1 and a picture for the

test rig is shown in Figure 3.2. The test rig consists of a main burner, pilot

flame burner with purging system, confinement, oxidizer supply system, fuel

supply system, flow controllers and measuring devices.

Figure 3.1: Flow Diagram

27

Figure 3.2: Test rig.

3.1 Burners:

The main burner is a concentric flow, bluff body, variable blockage ratio, gas

fired burner shown in Figure 3.3. Two concentric tubes form the body of the

burner. The inner tube is stainless steel with inner diameter of 5mm and 0.5

mm thickness and fuel is supplied through the 2.5mm copper nozzle which

mounted on the end of inner tube, the outer tube is an iron with inner diameter

(D1) of 30mm and 0.5mm thickness at which oxidizer is supplied. A Perforated

plate is mounted between the two concentric tubes to ensure from concentricity

of two tubes.

Stability of the flame is achieved using bluff body. Bluff body in this study is a

disc that mounted on the outer diameter of fuel nozzle and there are four discs

with the following outer diameters (D2): 18mm, 21.2mm, 24.5mm and 27.2mm

28

which are corresponding to blockage ratios, BR, (D2/D1)2 of 0.36, 0.5, 0.66 and

0.82 respectively.

A pilot flame burner is used only for initiating the combustion in the main

burner to prevent accumulation of gases inside the confinement and hence

prevent any explosions. The pilot flame consists of two concentric tubes, fuel

passes through inner tube and the air passes through outer tube.

The air that passes through the outer tube of pilot flame burner is used only to

purge the confinement from any gases before initiating pilot flame burner.

3.2 Combustor and Exhaust System:

The combustor as shown in Figure 3.4 is an iron cylinder with 150mm diameter

and 500mm length; there are 11 holes along the surface of confinement at

successive distance of 50mm in the axial direction. These holes are used for

inserting thermocouple or gas analyzer probe. A rectangular sight glass (50mm

x 500mm) is mounted on the outer surface of the confinement and sealed to

prevent leakage; this sight glass is used to facilitate capturing photos for

flames.

An exhaust section is shown in Figure 3.5 is fitted at the end combustor with

length of 1000mm and 150mm in diameter. The exhaust section was ended by

a cone with 40mm exit diameter to eliminate air entrainment.

29

Figure 3.3: Burner.

30

Figure 3.4: Combustor

31

Figure 3.5: Exhaust section

32

3.3 Fuel Supply System:

A compressed natural gas (CNG) bottle (200 bar) is used in all runs for fuel

supply. A pressure regulator is mounted at the exit of natural gas bottle then the

fuel passes through needle valve followed by a rotameter to measure flow rate.

Pressure and temperature are measured at the exit of rotameter using pressure

gauge and thermocouple respectively. Finally the fuel is fed to the burner.

3.4 Oxidizer Supply System:

There are three gases (Air, Oxygen and Carbon dioxide) are mixed in different

ways to supply the following three categories of oxidizers to the burner:

• Oxidizer 1: Air.

• Oxidizer 2: Enriched air (Air+O2).

• Oxidizer 3: CO2+O2.

Air supply system consists of reciprocating compressor with large tank. Air

supplied from this tank passes through two ball valves followed by needle

valve. Air flow rate is measured using a rotameter. Pressure and temperature of

air are measured at the exit of rotameter using pressure gauge and

thermocouple respectively. Air is then fed to the oxidizer mixer.

Oxygen supply system consists of oxygen bottle (120 bar and 7m3) with 99.5%

purity. The supplied oxygen passes through a pressure regulator which is

followed by a rotameter to measure flow rate. The pressure and temperature of

oxygen are measured at the exit of rotameter using pressure gauge and

thermocouple respectively. Finally the oxygen is fed to the oxidizer mixer.

Carbon dioxide supply system consists of carbon dioxide bottle (70bar and

25kg) with 99.5% purity. The supplied carbon dioxide passes through the

regulator followed by needle valve. It passes through a rotameter to measure

flow rate. The pressure and temperature were measured at the exit of rotameter

using pressure gauge and thermocouple respectively. The carbon dioxide is fed

to the oxidizer mixer.

33

Oxidizer mixer as shown in Figure 3.6 consists of a pipe of ½" diameter and

2.5m long, this pipe has two inlets and one exit, before the exit there are two

perforated plates to enhance mixing process and both pressure and temperature

of oxidizer were measured at the end of mixer before delivering to burner.

3.5 Instrumentation:

3.5.1 Flow Meters

All rotameters are calibrated at NIS (national institute for standards) with an air

at standard temperature and pressure, see appendix (A) for calibration

certificates, hence, the rotameter reading taken were corrected as mentioned in

appendix (B) to be used for measuring natural gas, carbon dioxide and oxygen.

Refer to appendix (C) for the uncertainty calculations of the measured fuel and

oxidizers flow rates and equivalence ratio. Two rotameters of different ranges

were used to measure the flow rate of both air and carbon dioxide, the

specifications of these rotameters are given in Table 3-1 for. However, only

one rotameter was enough to measure the fuel flow rate, and the specifications

of this rotameter are given in Table 3-2. Two rotameter were used to measure

oxygen flow rate, refer to Table 3-3 for the specifications of these rotameters.

34

Figure 3.6: Oxidizer Mixer.

35

Table 3-1: Specifications of Air and CO2 rotameters.

Rotameter (1) Rotameter (2)

Measuring range 50-400 SCFH air 20-200 SCFH air

Resolution 10 SCFH air 5 SCFH air

Accuracy

±5% reading up to

300 SCFH

±11% reading up to

400 SCFH

±5% reading

Manufacturer Dwyer Instruments,

Inc

Dwyer Instruments,

Inc

Calibration

certificate

See appendix Error!

Reference source not

found.

See appendix A.1.2

Table 3-2: Specifications of fuel rotameter

Measuring range 0-10 SCFH air

resolution 0.2 SCFH air

Accuracy ±10% reading

manufacturer Dwyer Instruments, Inc

Calibration certificate See appendix A.1.3

Table 3-3: Specifications of O2 rotameter

Rotameter (1) Rotameter (2)

Measuring range 0- 50 liter/min Air 0- 10.6 liter/min Air

Accuracy ±10% reading ±1% reading

Resolution 0.4 liter/min 0.7 liter/min

Manufacturer Omega engineering fl-1448-c Brooks r-6-15-a

Calibration certificate See appendix A.1.4 See appendix A.1.5

36

3.5.2 Pressure Gauges and Thermocouples:

All pressure gauges and thermocouples were calibrated at MCL (Measurements

and Calibration Laboratory - faculty of engineering - Cairo University), see

appendix (A) for calibration certificates, Refer to appendix (C) for the

uncertainty calculations of the measured pressures and temperatures. Four

identical pressure gauges and thermocouples were used to measure the

pressures and temperatures of air, carbon dioxide, oxygen, fuel, mixed

oxidizer. Refer to Table 3-4 and Table 3-5 for pressure gauges and

thermocouples specifications respectively.

Table 3-4: Specifications of pressure gauges

Measuring range 0 - 4.1 kg/cm2 (g)

resolution 0.1 kg/cm2

Accuracy ±2.5% reading

Manufacturer Empeo

Calibration certificate See appendix A.2

Table 3-5: Specifications of flow thermocouples

Type J (iron–constantan)

Measuring range −40 to +750 °C

Wire diameter 600µm

Accuracy ±1.5% up to 50oC

Manufacturer Omega Engineering. Inc.


3.5.3 Gas Analyzer:

The emission concentrations were measured using IMR-2800A gas analyzer

Refer to appendix (C) for the uncertainty calculations of the measured emission

concentrations; refer to Table 3-6 for the specifications of gas analyzer.

37

Table 3-6: Specifications of flue gas analyzer

Sensors specifications

Sensor Type Range Resolution Accuracy

O2 Electrochemical 0-21 % vol. 0.1 % 0.1 %

CO Infrared 0-15% vol. 0.01 % ± 4 % rel

CO2 Infrared 0-20% vol. 0.01 % ± 4 % rel

Hydrocarbon Infrared 0-100% vol.

(LEL) 0.01 % ± 4 % rel

Flue gas

temperature

NiCr-Ni

thermocouple

-

20°C/1200°C 1 °C ± 2 %

Probe specifications

material Stainless steel

Length 250 mm

diameter 10 mm


Sampling process

Sample process is a continuous sampling where the flue gas sample is taken

by a built-in pump through the probe and passes over different sensors that

give electric output signals of which vary according to the values of the

concentrations measured. The results are displayed on the LCD screen or

printed on paper.

3.6 Uncertainty and Error Analysis:

As this study was performed on small amounts of fuel and oxidizers, so the

uncertainty calculations and error analysis are important issues.

There are two types of errors in measurements namely: systematic errors and

random errors. Systematic errors arise from errors in measuring instruments

and cannot be eliminated except by calibration (see calibration certificates in

Appendix A). Random errors arise from random variations including human

38

errors, random electronics fluctuations in instruments and they usually follow a

certain statistical distribution.

The uncertainty budgets for all measurements in this work were: Repeatability,

resolution of instrument and uncertainty from calibration certificate and drift.

Appendix (C) presents full details about uncertainty calculations.

3.7 Experimental Program:

The objective of this study is to investigate the extinction limits, emissions and

appearance of CNG flames with three different oxidizers in a concentric flow,

bluff-body burner. In order to accomplish this objective, the following

experimental program was performed.

3.7.1 The Extinction limit:

The extinction limit studied in this work was defined as the fuel to oxidizer

momentum ratio at which flame extinction occurs, when the oxidizer flow rate

is gradually increased for the same fuel flow rate, blockage ratio, and oxygen

fraction. The flow momentum was calculated from flow mass flow rate

multiplied by mean flow velocity at the exit of the flow stream.

The effect of changing blockage ratio, as the main parameter representing the

burner geometry, was investigated through four replaceable discs of different

diameters. The disc diameters were 18mm, 21.2mm, 24.5mm and 27.2mm

which were corresponding to blockage ratios, BR, of 0.36, 0.5, 0.66 and 0.82

respectively.

3.7.2 Flame Appearance:

The flame appearance studied in this work was defined as the flame length and

flame color.

A digital camera was used to capture photos for flames through the glass

window in the combustor. In order to determine the flame length a white mark

with defined length was fixed on the glass window to be the scale of captured

photos.

39

3.7.3 Flame Emissions:

The flame emission was measured using a gas analyzer at a cross section far

from burner tip with distance of 450 mm vertically; this cross section was

selected to minimize the stratification in the cross section and to ensure that the

reactions was ended at this cross section. The emission was measured at

different equivalence ratios which are calculated from fuel and oxidizer flow

rates. Also the emission was measured at different radial distanced in this cross

section to determine if there was any stratification. The emission readings were

taken when the value of measured parameters was nearly constant.

3.7.4 System Compatibility:

The following measurements were taken after the system was running for the

first time to ensure that all measurement devices are working correct without

any problems.

Figure 3.7: CNG/ Air flame emission at different equivalence ratio with

blockage ratio of 0.36.

The measurements for emissions in Figure 3.7 showed that the maximum CO2

is obtained at equivalence ratio ranged from 0.9 to 1.1 and the unburned fuel

represented in (% LELNG) was zero at equivalence ratio of 0.9, hence, these

-5

0

5

10

15

20

25

30

35

0

1

2

3

4

5

6

7

8

9

10

0.6 0.8 1 1.2

Pe

rce

nts

ge

of

LEL N

G(%

)

O2

, C

O2

(%

vo

l.)

Equivalence ratio (PHI)

O2

CO2

LEL

40

results indicate that the flow rates of fuel and oxidizer and the emission

analyses are correctly measured.

41

CHAPTER (4)

RESULTS AND DISCUSSION

42

CHAPTER (4)

4 RESULTS AND DISCUSSION

4.1 Introduction:

The aim of this work is to study the effect of different oxidizers and burner

geometry on the stability, appearance and emissions of Natural Gas (NG)

flames using different oxidizing environment. To achieve this goal, a

concentric flow bluff-body burner was designed to facilitate different blockage

ratio of burner, and the test rig was designed to facilitate different oxidizer

mixtures to be supplied to the burner.

The experimental program was planned to provide data to study the extinction

limits, appearance and emissions of different flames. This chapter discusses the

analysis and interpretation of the data obtained. A brief outline of the chapter

contents is given in the following few lines.

1. The Extinction limits and how it is affected by the different oxidizer and

different blockage ratio.

2. The effect of different oxidizer mixtures on the flame appearance (i.e. flame

length and flame color).

3. The flame emissions and how it is affected by the different oxidizer and

different blockage ratio.

43

4.2 The Extinction Limits:

The extinction limit studied in this work was defined as the fuel to oxidizer

momentum ratio at which flame extinction occurs, when the oxidizer flow rate

is gradually increased for the same fuel flow rate, blockage ratio, and oxygen

fraction. The flow momentum was calculated from flow mass flow rate

multiplied by mean flow velocity at the exit for flow stream.

The extinction limit in this work was studied for three different oxidizer

mixtures at different oxygen fraction for four different blockage ratios. These

oxidizers were:

• Air.

• Enriched Air (Air/O2) at different oxygen fractions (OF) ranged from

21%vol. to 24%vol. with 1%vol. step.

• (CO2/O2) at different oxygen fractions (OF) {OF=28% and OF=32%}.

4.2.1 Air:

In this section the extinction limit of CNG/Air flame will be studied and

analyzed. To determine the upper extinction limit of this flame the fuel flow

rate was kept constant at a certain value and the air flow rate was increased

gradually till the extinction occurs. To determine the lower extinction limit of

this flame the fuel flow rate was kept constant and the air flow rate was

decrease gradually till the extinction occurs.

These results were obtained for four different blockage ratios namely (0.36,

0.5, 0.667, and 0.82).

As shown in Figure 4.1 as fuel momentum increased the extinction limit was

increased, also as the blockage ratio increased the extinction limit was

increased till the blockage ratio of 0.667 beyond this value the stability limits

was decreased. This phenomenon occurred because as the blockage ratio was

increased the inner recirculation zone was increased, and hence this increases

the mixing process and enhance the flame stability. For high blockage ratio

44

(0.82) the distance between fuel and air streams were increased, hence the

mixing process between air and fuel were decreased which has adverse effect

on the flame stability.

The lower extinction limits were represented as a bold line on x-axis, because

the flame was extinct when the air flow rate is too small ,nearly was ceased,

this happened at all fuel flow rate.

45

Figure 4.1: The extinction limits of CNG/Air flame at four different blockage

ratios.

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

0.045

0.050

0.00000 0.00010 0.00020 0.00030 0.00040 0.00050

Ra

te o

f C

ha

ng

e o

f A

ir M

om

entu

m (

N)

Rate of Change of Fuel Momentum (N)

BR=0.36 BR=0.5 Br=0.667 BR=0.82

Stable Flame Zone

Extinct Flame

46

4.2.2 Enriched air (Air/O2):

In this section the extinction limits of CNG/Air/O2 flame at different oxygen

fraction (%vol.) is studied for different blockage ratios. Oxygen fraction in this

case was defined as the total oxygen flow rate �� 0.21� �� divided by

total oxidizer flow rate �� . �� . ��

To determine the upper extinction limits of the flame the fuel flow rate was

kept constant and the oxidizer flow rate was increased gradually with constant

oxygen fraction till the extinction occurred.

As shown in Figure 4.2, Figure 4.3, Figure 4.4, and Figure 4.5 for constant fuel

momentum the increase in the oxygen fraction increases the extinction limit

and as the fuel momentum was increased the extinction limit was increased. In

Figure 4.4 one fuel momentum is used because this test rig cannot supply

enough air flow rate to study higher fuel momentum.

By comparing the previous figures we can find that the best stability was

obtained at blockage ratio of 0.667 because as the blockage ratio increased the

inner recirculation zone was increased, hence, the mixing between fuel and

oxidizer was enhanced which has an effect on flame stability but when the

blockage ratio was too high the distance between the fuel and oxidizer streams

was increased, so the mixing is decreased and the extinction limit was

decreased.

The lower extinction limits were represented as a bold line on x-axis, because

the flame was extinct when the oxidizer flow rate is too small ,nearly was

ceased and this happened at all fuel flow rate.

47


momenta (FM) at blockage ratio of 0.36.

0.010

0.015

0.020

0.025

0.030

0.035

20.0 21.0 22.0 23.0 24.0 25.0

Ra

te o

f C

ha

ng

e of

Ox

idiz

er M

om

entu

m (

N)

Oxygen Fraction (% vol.)

FM=0.0000115N FM=0.0000471N

Stable Flame Zone

Extinct Flame zone

48



0.010

0.015

0.020

0.025

0.030

0.035

20.0 21.0 22.0 23.0 24.0 25.0

Ra

te o

f C

ha

ng

e of

Ox

idiz

er M

om

entu

m (

N)

Oxygen Fraction (% vol.)

FM=0.0000115 N FM=0.0000471N

Stable Flame Zone

Extinct Flame zone

49

Figure 4.4: The extinction limits for CNG/Air/O2 flame with constant fuel


0.01

0.015

0.02

0.025

0.03

0.035

20 21 22 23 24 25

Rate

of

Ch

an

ge

of

Ox

idiz

er M

om

entu

m (

N)

Oxygen Fraction (%vol.)

FM=0.0000115N

Stable Flame zone

Extinct Flame zone

50



0.010

0.015

0.020

0.025

0.030

0.035

20.0 21.0 22.0 23.0 24.0 25.0

Rate

of

Ch

an

ge

of

Ox

idiz

er M

om

entu

m (

N)

Oxygen fraction (%vol.)

FM=0.0000115 N FM=0.0000471N

Stable Flame Zone

Extinct Flame zone

51

4.2.3 (CO2/O2) oxidizer:

In this section the extinction limits of CNG/CO2/O2 flame at two different

oxygen fractions (28%vol. and 32%vol.) will be studied for different blockage

ratios. These two values of oxygen fractions were determined because Hals, et

al. [14] obtained a stable flame at oxygen fraction of 30% vol. In this research a

stable flame was obtained at oxygen fraction of 28% vol.

Oxygen fraction in this case was defined as the total oxygen flow rate �� divided by total oxidizer flow rate �� . ��

To determine the upper extinction limits of the flame the fuel flow rate was

kept constant and the oxidizer flow rate was increased gradually with constant

oxygen fraction till the extinction occurred.

As shown in Figure 4.6, Figure 4.7, and Figure 4.8 the extinction limit

increases as fuel momentum increases, also the CNG/CO2/O2 with oxygen

fraction 32% has higher extinction limit than flame with oxygen fraction of

28%. Also CNG/Air flame has higher extinction limit than that of

CNG/CO2/O2 flames this happened due to the effects of CO2 in oxidizer

mixture. CO2 as diluent impacts on flame in four ways [23], through changes in

1) mixture specific heat and adiabatic flame temperature, 2) transport

properties (thermal conductivity, mass diffusivity, viscosity) 3) chemical

kinetic rates, and 4) radiative heat transfer.

At 2000 K, the heat capacity of CO2 is about 1.7 times larger than that of N2;

consequently, combustion in CO2 at comparable flame temperatures and at the

same equivalence ratio as N2 diluted systems requires higher reactant oxygen

levels, in turn influencing the mixtures kinetic characteristics.

52

Diluent gas transport properties have important influences upon quantities such

as flame speed. Table 4-1 compares values of thermal conductivity, λ, binary

mass diffusivity (with O2), D, and dynamic viscosity, µ, for N2 and CO2

Table 4-1: Gas transport properties for N2, CO2 at 1atm.

N2 CO2

T (OC) 25 500 1000 25 500 1000

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

D*105 (m2/s) 2.04 10.3 23.7 1.5 8.1 18.8

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

As can be observed, thermal conductivity and viscosity of CO2 are very close

to that of N2, but the mass diffusivity of oxygen in CO2 is approximately 20%

lower than in N2. Compared to N2 diluted flames, CO2 does not act as passive

diluents in the fuel, but interact kinetically. The effect of CO2 dilution is

apparently due to its competing with other reactions requiring the H radical.

�� 2 �

The CO2 in the reacting gases leads to additional preheating, through radiative

absorption of heat emitted from product gases, thus decreasing the flame speed.

As shown in Figure 4.9 the extinction limit increases as fuel momentum

increases, also the CNG/CO2/O2 with oxygen fraction 32% has higher

extinction limit than flame with oxygen fraction of 28% and CNG/Air flame.

As shown in Figure 4.10, and Figure 4.11 the increase in blockage ratio leads

to increase extinction limits, hence, the higher extinction limit was obtained at

blockage ratio of 0.82 which differed from CNG/Air flame where the higher

extinction limit was obtained at blockage ratio of 0.667, this happened due to

the difference in properties of these oxidizers like density, thermal conductivity

and specific heat.

53

Figure 4.6: The extinction limits for CNG/CO2/O2 flame at two different

oxygen fractions (OF), compared to CNG/Air flame at blockage ratio of 0.36.

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

0.045

0.050

0.00000 0.00010 0.00020 0.00030 0.00040 0.00050

Ra

te o

f C

ha

ng

e o

f O

xid

izer

Mom

entu

m (

N)


Air OF=28% OF=32%

Stable Flame Zone

Extinct Flame

54



0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

0.045

0.050

0.00000 0.00010 0.00020 0.00030 0.00040 0.00050

Ra

te o

f C

ha

ng

e o

f O

xid

izer

Mo

men

tum

(N

)


Air OF=28% OF=32%

Stable Flame Zone

Extinct Flame zone

55



0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

0.045

0.050

0.00000 0.00010 0.00020 0.00030 0.00040

Ra

te o

f C

ha

ng

e of

Ox

idiz

er M

om

entu

m (

N)


Air OF=28% OF=32%

Stable Flame Zone

Extinct Flame zone

56



0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

0.045

0.050

0.00000 0.00010 0.00020 0.00030 0.00040

Ra

te o

f C

ha

ng

e o

f O

xid

izer

Mo

men

tum

(N

)


Air OF=28% OF=32%

Stable Flame Zone

Extinct Flame zone

57

Figure 4.10: The Extinction limit for CNG/CO2/O2 flame with oxygen fraction

of 32% vol. at four different blockage ratios.

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0.05

0 0.00005 0.0001 0.00015 0.0002 0.00025

Ra

te o

f C

ha

ng

e o

f O

xid

izer

Mom

entu

m (

N)


BR=0.36 BR=0.5 BR=0.667 BR=0.82

Stable Flame Zone

Extinct Flame zone

58

Figure 4.11: The Extinction limit for CNG/CO2/O2 flame with oxygen fraction

of 28% vol. at four different blockage ratios.

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0.05

0 0.0001 0.0002 0.0003 0.0004

Ra

te o

f C

han

ge

of

Ox

idiz

er M

om

entu

m (

N)

Rate of Change of Fuel momentum (N)

BR=0.36 BR=0.5 BR=0.667 BR=0.82

Stable Flame Zone

Extinct Flame zone

59

4.3 Flame Appearance:

The flame appearance studied in this work was defined as the flame length and

flame color. To determine flame color and length a rectangular glass window

was fitted at the combustor to facilitate image capturing. Also a white mark

was stuck on the glass window to determine the scale of image this mark had

an actual length of 16mm.

As shown in Figure 4.12 the CNG/Air flames have a yellow color due to the

burning of carbon atoms. At low air to fuel momentum ratio the fuel jet

penetrates the inner recirculation zone and form a like-jet flame as shown in

case (A). At intermediate air to fuel momentum ratio the fuel momentum still

able to penetrate recirculation zone but part of fuel was held behind the

recirculation zone and form a central-jet dominated flame, as shown in case

(B). At high air to fuel momentum ratio the fuel was held behind the

recirculation zone and form a recirculation-zone flame as shown in case (C),

the same results was observed by Kang and Yang [28].

As shown in Figure 4.13, and Figure 4.14 the CNG/Air/O2 flames have a bright

white color due to presence of excess oxygen that leads to high flame

temperature and hence a bright white flame is observed. Theses flames changed

from like-jet flame at low oxidizer to fuel momentum to central-jet dominated

flame at intermediate oxidizer to fuel momentum and finally a recirculation-

zone flame was obtained at high oxidizer to fuel momentum ratio.

As shown in Figure 4.15, and Figure 4.16 the CNG/CO2/O2 flame at low

oxygen fraction (i.e. 28%vol.) has a blue color due to high level of H radical,

and at high oxygen fraction (i.e. 32% vol.) the flame changed from white to

blue as the oxidizer momentum increased due to decrease in flame temperature

and the increase in concentration of H radical in flame. Theses flames changed

from like-jet flame at low oxidizer to fuel momentum to central-jet dominated

flame at intermediate oxidizer to fuel momentum and finally a recirculation-

zone flame was obtained at high oxidizer to fuel momentum ratio.

60

Figure 4.12: CNG/Air flames with a BR of 0.36 and a fuel momentum of

4.7E-5 N. The air momenta of cases A, B and C are 0.00047, 0.001884 and

0.00912 N, respectively. The flame lengths of cases A, B and C are 27.9 cm, 24

cm and 4 cm, respectively.

61



0.002177 and 0.008834 N, respectively. The flame lengths of cases A, B and C

are 15.4 cm, 11.8 cm and 5.3 cm, respectively.

62




are 11.7 cm, 9.5 cm and 5 cm, respectively.

63





64





From previous figures we can find that the CNG/Air flames have a higher

length than CNG/CO2/O2 flames and the shortest flames were a CNG/Air/O2

flames. Flame color changed from yellow in CNG/Air flame to blue in

CNG/CO2/O2 flame and bright white in CNG/Air/O2 flame.

65

4.4 Flame Emissions:

In this section the emissions of CNG/Air flame and CNG/Air/O2 flame will be

studied. The emissions were measured using a gas analyzer which measures the

carbon dioxide concentration (CO2) and Lower Explosive Limit (LEL) with

Infrared cells and also measures oxygen concentration (O2) with an

electrochemical cell. The emissions were represented as emissions index that

defined as the mass flow rate of measured emission divided by the mass flow

rate of fuel.

!"#$$#%& #&'!( )*+,�--�./0*12+3 4 � �55,67 � 89: � ; � <+=>?2-@A"12+3A

Where: �55,: The concentration of measured emission (ppm). 67: The molecular weight of measured gas. 8: The pressure of exhaust at measurement point (kpa). 9:: The universal gas constant (8314.4 J/kg.k). ;: The temperature of exhaust at measurement point (k). <+=>?2-@A : The volumetric mass flow rate of exhaust (m3/s). "12+3A : The mass flow rate of fuel (kg/s).

The sample of emissions was taken at a distance equals to 500mm from burner

tip at center line of combustor, to ensure that the cross section was not stratified

the measurements for radial profile of emissions were taken.

The sample was draught using an internal pump, where the sample passes

through a water trap to prevent water from entering to analyzer, then passes

through particulate filter to prevent soot from entering to analyzer and finally

the sample enters the analyzer to be analyzed and gives the value of emission

concentration.

66

The emissions of CNG/CO2/O2 flame were not measured because the CO2

concentration of this flame was too high (i.e. higher than 90% vol. dry gas

analysis) and the range of CO2 cell was 20% vol., so the gas analyzer was not

appropriate to measure in this range of CO2 concentration.

The solution for the problem of emission measurements in CNG/CO2/O2 flame

can be eliminated by using an accurate dilution system to dilute the sample

with an inert gas to enable measuring concentration.

67

4.4.1 CNG/Air flame emissions:

As shown in the following figures it is observed that the emission index of

oxygen is decreased as the equivalence is increased. The emission index of

hydrocarbon (HC) is increased as the equivalence is increased and its value

became zero at equivalence ratio of 0.9 due to excess air used in combustion.

The emission index of CO2 is nearly constant because the emission index of

CO2, theoretically, must be constant for lean and stoichiometric combustion

(λ≥1) as shown in the following combustion equation the number of moles of

CO2 depends on the completeness of combustion which occurs in lean and

stoichiometric combustion.

�/�, C DE FGH �I� J. KL M��N EOI� F� P�I �C Q �� DE FGHI� J. KL C DE FGH M� From radial profiles of measured emissions it was observed that the value of

hydrocarbon emission index is constant which indicates that this cross section

is not stratified. The values of oxygen and carbon dioxide emission indexes are

increased near to walls because the oxidizer exit in the burner is nearer to wall

than fuel exit.

As shown in Figure 4.17 it is observed that the value of CO2 emission index is

nearly constant and equals to 1050gCO2/kgfuel. The maximum value of

hydrocarbon emission index is about 85gHC/kgfuel at equivalence ratio of 1.1.

The minimum value of O2 emission index is about 500gO2/kgfuel at equivalence

ratio ranged from 0.9 to 1.1.






68











Form previous values it is observed that the emissions of blockage ratios of

0.36, 0.5, and 0.667 were nearly the same but at blockage ratio of 0.82 the

maximum value of O2, HC, and CO2 emissions indexes are too high in

comparison with other blockage ratios because at high blockage ratio the

distance between the fuel and air streams is too large which prevent these two

streams from mixing that effect on the emission of this flame.

69

Figure 4.17: Center line emissions of CNG/Air flame at a distance equals to

500 mm from burner tip and blockage ratio of 0.36.

-10

40

90

140

190

0

200

400

600

800

1000

1200

1400

1600

1800

0.6 0.7 0.8 0.9 1 1.1 1.2

emis

sio

n i

nd

ex (

gH

C/k

gfu

el)

emis

sio

n i

nd

ex {

g (

O2

or

CO

2)/

kg

fuel}

Equivalence ratio (Ø)

O2

CO2

HC

70

Figure 4.18: Radial emissions of CNG/Air flame at a distance equals to 500

mm from burner tip, blockage ratio of 0.36 and equivalence ratio (Ø) of 0.9.

-10

40

90

140

190

0

200

400

600

800

1000

1200

1400

1600

1800

0 10 20 30 40 50 60 70

emis

sio

n i

nd

ex (

gH

C/k

gfu

el)

emis

sio

n i

nd

ex {

g (

O2

or

CO

2)/

kg

fuel}

Distance from center (mm)

O2

CO2

HC

71



-10

40

90

140

190

0

200

400

600

800

1000

1200

1400

1600

1800

0.6 0.7 0.8 0.9 1 1.1 1.2

emis

sio

n i

nd

ex (

gH

C/k

gfu

el)

emis

sio

n i

nd

ex {

g (

O2

or

CO

2)/

kg

fuel}


O2

CO2

HC

72



-10

40

90

140

190

0

200

400

600

800

1000

1200

1400

1600

1800

0 10 20 30 40 50 60 70

emis

sio

n i

nd

ex (

gH

C/k

gfu

el)

emis

sio

n i

nd

ex {

g (

O2

or

CO

2)/

kg

fuel}


O2

CO2

HC

73



-10

40

90

140

190

0

200

400

600

800

1000

1200

1400

1600

1800

0.6 0.7 0.8 0.9 1 1.1 1.2

emis

sio

n i

nd

ex (

gH

C/k

gfu

el)

emis

sio

n i

nd

ex {

g (

O2

or

CO

2)/

kg

fuel}


O2

CO2

HC

74



-10

40

90

140

190

0

200

400

600

800

1000

1200

1400

1600

1800

0 10 20 30 40 50 60 70

emis

sio

n i

nd

ex (

gH

C/k

gfu

el)

emis

sio

n i

nd

ex {

g (

O2

or

CO

2)/

kg

fuel}


O2

CO2

HC

75



-10

40

90

140

190

0

200

400

600

800

1000

1200

1400

1600

1800

0.6 0.7 0.8 0.9 1 1.1 1.2

emis

sio

n i

nd

ex (

gH

C/k

gfu

el)

emis

sio

n i

nd

ex {

g (

O2

or

CO

2)/

kg

fuel}


O2

CO2

HC

76



-10

40

90

140

190

0

200

400

600

800

1000

1200

1400

1600

1800

0 10 20 30 40 50 60 70

emis

sio

n i

nd

ex (

gH

C/k

gfu

el)

emis

sio

n i

nd

ex {

g (

O2

or

CO

2)/

kg

fuel}


O2

CO2

HC

77

4.4.2 CNG/Air/O2 flame emissions:

As shown in the following figures it is observed that the emission index of

oxygen was decreased as the equivalence is increased, also the emission index

of O2 is lower than emission index of O2 when air is used in combustion for the

same blockage ratio and equivalence ratio. The emission index of hydrocarbon

(HC) is increased as the equivalence is increased and its value became zero at

equivalence ratio of 0.9 due to excess air used in combustion, also the emission

index of hydrocarbon is lower than emission index of hydrocarbon when air is

used in combustion for the same blockage ratio and equivalence ratio. The

emission index of CO2 is nearly constant and higher than the CO2 emission

index when air is used as an oxidizer, for the same blockage ratio and

equivalence ratio, because the oxygen concentration used in combustion

process is higher than air. While the oxygen fraction is increased the emission

indexes of O2 and hydrocarbon are decreased and the emission index of CO2 is

increased.

From radial profiles of measured emissions it is observed that the value of

hydrocarbon emission index is constant which indicates that this cross section

is not stratified. The values of oxygen and carbon dioxide emission indexes are

increased near to walls because the oxidizer exit in the burner is nearer to wall

than fuel exit.









78




























79






Form previous values it is observed that the emissions of blockage ratios of

0.36, 0.5, and 0.667 were nearly the same but at blockage ratio of 0.82 the

maximum value of O2, HC, and CO2 emissions indexes are too high in

comparison with other blockage ratios because at high blockage ratio the

distance between the fuel and air streams is too large which prevent these two

streams from mixing that effect on the emission of this flame. Also as the

oxygen fraction is increased the hydrocarbon emission index is deceased and

CO2 concentration is increased.

80

Figure 4.25: Center line emissions of CNG/Air/O2 flame with oxygen fraction

of 32% at a distance equals to 500 mm from burner tip and blockage ratio of

0.36.

-10

40

90

140

190

0

200

400

600

800

1000

1200

1400

1600

1800

0.6 0.7 0.8 0.9 1 1.1 1.2

emis

sio

n i

nd

ex (

gH

C/k

gfu

el)

emis

sio

n i

nd

ex {

g (

O2

or

CO

2)/

kg

fuel}


O2

CO2

HC

81

Figure 4.26: Radial emissions of CNG/Air/O2 flame with oxygen fraction of

32% at a distance equals to 500 mm from burner tip, blockage ratio of 0.36 and

equivalence ratio (Ø) of 0.9.

-10

40

90

140

190

0

200

400

600

800

1000

1200

1400

1600

1800

0 10 20 30 40 50 60 70

emis

sio

n i

nd

ex (

gH

C/k

gfu

el)

emis

sio

n i

nd

ex {

g (

O2

or

CO

2)/

kg

fuel}


O2

CO2

HC

82



0.36.

-10

40

90

140

190

0

200

400

600

800

1000

1200

1400

1600

1800

0.6 0.7 0.8 0.9 1 1.1 1.2

emis

sio

n i

nd

ex (

gH

C/k

gfu

el)

emis

sio

n i

nd

ex {

g (

O2

or

CO

2)/

kg

fuel}


O2

CO2

HC

83




-10

40

90

140

190

0

200

400

600

800

1000

1200

1400

1600

1800

0 10 20 30 40 50 60 70

emis

sio

n i

nd

ex (

gH

C/k

gfu

el)

emis

sio

n i

nd

ex {

g (

O2

or

CO

2)/

kg

fuel}


O2

CO2

HC

84



0.5.

-10

40

90

140

190

0

200

400

600

800

1000

1200

1400

1600

1800

0.6 0.7 0.8 0.9 1 1.1 1.2

emis

sio

n i

nd

ex (

gH

C/k

gfu

el)

emis

sio

n i

nd

ex {

g (

O2

or

CO

2)/

kg

fuel}


O2

CO2

HC

85




-10

40

90

140

190

0

200

400

600

800

1000

1200

1400

1600

1800

0 10 20 30 40 50 60 70

emis

sio

n i

nd

ex (

gH

C/k

gfu

el)

emis

sio

n i

nd

ex {

g (

O2

or

CO

2)/

kg

fuel}


O2

CO2

HC

86



0.5.

-10

40

90

140

190

0

200

400

600

800

1000

1200

1400

1600

1800

0.6 0.7 0.8 0.9 1 1.1 1.2

emis

sio

n i

nd

ex (

gH

C/k

gfu

el)

emis

sio

n i

nd

ex {

g (

O2

or

CO

2)/

kg

fuel}


O2

CO2

HC

87




-10

40

90

140

190

0

200

400

600

800

1000

1200

1400

1600

1800

0 10 20 30 40 50 60 70

emis

sio

n i

nd

ex (

gH

C/k

gfu

el)

emis

sio

n i

nd

ex {

g (

O2

or

CO

2)/

kg

fuel}


O2

CO2

HC

88



0.667.

-10

40

90

140

190

0

200

400

600

800

1000

1200

1400

1600

1800

0.6 0.7 0.8 0.9 1 1.1 1.2

emis

sio

n i

nd

ex (

gH

C/k

gfu

el)

emis

sio

n i

nd

ex {

g (

O2

or

CO

2)/

kg

fuel}


O2

CO2

HC

89


32% at a distance equals to 500 mm from burner tip, blockage ratio of 0.667

and equivalence ratio (Ø) of 0.9.

-10

40

90

140

190

0

200

400

600

800

1000

1200

1400

1600

1800

0 10 20 30 40 50 60 70

emis

sio

n i

nd

ex (

gH

C/k

gfu

el)

emis

sio

n i

nd

ex {

g (

O2

or

CO

2)/

kg

fuel}


O2

CO2

HC

90



0.667.

-10

40

90

140

190

0

200

400

600

800

1000

1200

1400

1600

1800

0.6 0.7 0.8 0.9 1 1.1 1.2

emis

sio

n i

nd

ex (

gH

C/k

gfu

el)

emis

sio

n i

nd

ex {

g (

O2

or

CO

2)/

kg

fuel}


O2

CO2

HC

91


28% at a distance equals to 500 mm from burner tip, blockage ratio of 0.667

and equivalence ratio (Ø) of 0.9.

-10

40

90

140

190

0

200

400

600

800

1000

1200

1400

1600

1800

0 10 20 30 40 50 60 70

emis

sio

n i

nd

ex (

gH

C/k

gfu

el)

emis

sio

n i

nd

ex {

g (

O2

or

CO

2)/

kg

fuel}


O2

CO2

HC

92



0.82.

-10

40

90

140

190

0

200

400

600

800

1000

1200

1400

1600

1800

0.6 0.7 0.8 0.9 1 1.1 1.2

emis

sio

n i

nd

ex (

gH

C/k

gfu

el)

emis

sio

n i

nd

ex {

g (

O2

or

CO

2)/

kg

fuel}


O2

CO2

HC

93




-10

40

90

140

190

0

200

400

600

800

1000

1200

1400

1600

1800

0 10 20 30 40 50 60 70

emis

sio

n i

nd

ex (

gH

C/k

gfu

el)

emis

sio

n i

nd

ex {

g (

O2

or

CO

2)/

kg

fuel}


O2

CO2

HC

94



0.82.

-10

40

90

140

190

0

200

400

600

800

1000

1200

1400

1600

1800

0.6 0.7 0.8 0.9 1 1.1 1.2

emis

sio

n i

nd

ex (

gH

C/k

gfu

el)

emis

sio

n i

nd

ex {

g (

O2

or

CO

2)/

kg

fuel}


O2

CO2

HC

95




-10

40

90

140

190

0

200

400

600

800

1000

1200

1400

1600

1800

0 10 20 30 40 50 60 70em

issi

on

in

dex

(g

HC

/kg

fuel)

emis

sio

n i

nd

ex {

g (

O2

or

CO

2)/

kg

fuel}


O2

CO2

HC

96

CHAPTER (5)

CONCLUSION AND RECOMMENDATION FOR

FUTURE WORK

97

CHAPTER (5)

5 CONCLUSTION AND RECOMMENDATION

FOR FUTURE WORK

5.1 Conclusion:

The flame appearance, stability and emissions of confined diffusion CNG

flames in CO2/O2 environment have been investigated. The obtained results

were compared with that of air and in enriched air (Air/O2). The effects of

blockage ratios on flame stability, appearance and emissions were also

investigated.

• The flame appearance falls in three different regimes, based on the

air-to-fuel momentum ratio.

• The flame color is changed from yellow to bright white to blue with

yellow tips in air, enriched air and oxy-combustion, respectively.

• The flame length was the highest in air, shorter in oxy-combustion

and the shortest in enriched air.

• The flame stability was the best in enriched air, lower in air and the

lowest in oxy-combustion. In general, the stability has been shown

to be improved as the fuel momentum increases (due to enhanced

mixing) or as the oxygen mole fraction increases (due to improved

chemical kinetics).

• The best flame stability is observed in air and in enriched air at a

blockage ratio of 0.67 while it occurred at a blockage ratio of 0.82 in

the (CO2/O2) cases.

• The maximum hydrocarbon emission index observed at blockage

ratio of 0.82 due to the increase in distance between oxidizer and

fuel stream. Also the CO2 emission index was nearly constant up to

98

equivalence ratio of 1 and the maximum hydrocarbon emission

index observed at equivalence ratio of 1.1.

5.2 Recommendations for Future Work:

• The characteristics of premixed CNG/CO2/O2 flame and the effect

of degree of premixing on flame characteristics can be studied. This

study needs high flow rates of the mixture of the fuel and the

oxidizer to overcome the problem of flash-back also. It also needs a

high level of safety to avoid any explosion.

• The effect of different shapes of bluff-body on the CNG/CO2/O2

flame characteristics can be studied.

• The heat release of CNG/CO2/O2 flames can be studied.

• The emissions of CNG/CO2/O2 flames can be measured by using a

gas analyzer with a dilution system to overcome the problem of high

concentration of CO2 emissions.

• The effect of swirling on the CNG/CO2/O2 flame characteristics can

be studied to overcome the problem of pressure drop resulting from

using bluff-body.

• The effect of other diluents like H2O on the oxy-flames

characteristics can be studied.

99

REFERENCES

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103

APPENDIX (A)

104

APPENDIX (A)

Calibration certificates

A.1. Flow Meters Calibration Certificates:

A.1.1. Air and CO2 rotameter (1):

105

106

107

A.1.2. Air and CO2 rotameter (2):

A.1.3. Fuel rotameter:

108

A.1.4. O2 rotameter (1):

109

A.1.5. O2 rotameter (2):

110

A.2. Pressure Gauges Calibration Certificates:

A.2.1. Air and CO2 pressure gauge:

A.2.2. O2 pressure gauge:

111

A.2.3. Fuel pressure gauge:

A.2.4. Oxidizer pressure gauge:

112

A.3. Thermocouples Calibration Certificates:

A.3.1. Air and CO2 thermocouple:

A.3.2. O2 thermocouple:

113

A.3.3. Fuel thermocouple:

A.3.4. Oxidizer thermocouple:

114

A.4. Gas Analyzer Calibration Certificate:

115

116

APPENDIX (B)

117

APPENDIX (B)

Correction for Rotameter Reading

As shown in APPENDIX ( all Rotameters were calibrated with Air at

atmospheric conditions, so to use these rotameters in measuring flow rates of

other fluids like carbon dioxide, natural gas, and oxygen, the reading of

rotameters must be corrected. This correction mainly due to change in changes

in molecular weight and density of fluid.

Principle of rotameter

The main principle of rotameter is the balance between forces the effect on

float, these forces are weight of float�RS�, drag force�TU�, and buoyancy

force�TV�. �W �X � Y�

��Z[\]�^_[\]�^� ��W Z[\]�^`[\abc d � Z[\abc`[\abc d

��Z[\]�^_[\]�^� ��W � �Z[\abc Q Z[\]�^�`[\abc d

_[\]�^� � � � �Z[\abc Q Z[\]�^�Z[\]�^ � `[\abc d ��W

_[\]�^� � eafgcbfc � �Z[\abc Q Z[\]�^�Z[\]�^

Since the value of float density is too high in comparison with fluid density and

the density of float is constant, so the equation tends to:

_[\]�^� h �Z[\]�^

�[\]�^. h �iZ[\]�^

118

�[\]�^. � ��. � j Z��Z[\]�^

So all readings for other fluids must be corrected at shown in previous

equation.

119

APPENDIX (C)

120

APPENDIX (C)

Uncertainty Calculations

C.1. Uncertainty Evaluation for Measured Parameters

This section represents the method of evaluating uncertainty of measured

quantities, this method depends on M3003 (The Expression of Uncertainty and

Confidence in Measurement) [29].

Uncertainty of measurement may be defined as the range over which a

measured value is expected to lie with a given probability.

There are two types of uncertainty; first, if an uncertainty is evaluated by

statistical analysis of a series of observations, it is known as a Type A

evaluation, second, Type B evaluations can apply to both random error and

bias. The distinguishing feature is that the calculation of the uncertainty

component is not based on a statistical analysis of data and the successful

identification and evaluation of Type B components depends on a detailed

knowledge of the measurement process and the experience of the person

making the measurements.

C.1.1. Type A evaluation

A Type A evaluation will normally be used to obtain a value for the

repeatability of a measurement process and calculated as following:

k � l/mn√&

Where, n: is the number of samples. l/mn: is the standard deviation of measurement.

l/mn � p 1& Q 1q�r� Q r:�s/�tn

Where, r�: is the value of measured quantity. r:: is the mean value of repeated measurements.

r: � rn rnrn u r/&

121

C.1.2. Type B evaluation

The components of Type B uncertainty are the following:

• The reported uncertainty from calibration certificate k� .

• The resolution of measurement device kv.

kv � w!xy#! 9!$%z{|#%&2

C.1.3. Combined and expanded uncertainty

Combined uncertainty �}��is the summation in quadrature of all uncertainties.

}� � ~k s k��s k�vs Where, k�� s because this source of uncertainty has a normal probability

distribution.

k�v � ��√� because this source of uncertainty has a rectangular probability

distribution.

The expanded (reported) uncertainty is calculated as following: k� � � � }�

Where, �: is the converge factor.

The following table represents the average values of uncertainty for measured

parameters in this study.

parameter unit Percentage of

Expanded uncertainty (%)

Temperature (t) OC 5

Pressure (P) Kg/cm2 2

Flow rate (Q) SCFH 6

emissions

LEL %vol. 10

CO2 %vol. 5

O2 %vol. 5

"The reported expanded uncertainty is based on a standard uncertainty

multiplied by a coverage factor k = 2, providing a coverage probability of

approximately 95%."

122

C.2. Uncertainty Evaluation for Calculated Parameters

This section represents the method of evaluating uncertainty of calculated

parameters; this method depends on propagation of uncertainties [30].

Suppose that x, …., z are measured parameters with uncertainties δx, …., δz

and the parameter (q) is function of (x, …., z) then the uncertainty of calculated

parameter (δq) is:

δq � j�∂q∂x δx�s u �∂q∂z δz�s C.2.1. Uncertainty evaluation for mass flow rates ��.�

". � 8 � 6�9: � �273 |� � � Where, P: the fluid pressure. 6�: the mlecular weight of �luid. 9:: Universal gas constant.

t: the fluid temperature.

Q: fluid flow rate.

The molecular weight of fluid is constant so, the mass flow rate tends to:

". � y%&$|�&| � 8�273 |� � � So, the uncertainty of mass flow rate (δ".) is:

δ". � j�∂".∂P δP�s �∂".∂t δt�s �∂".∂Q δQ�s Where, δP: the uncertainty of measured pressure. δt: the uncertainty of measured temperature. δQ: the uncertainty of measured �low rate. The percentage of uncertainty�δ".". %� � j�δPP %�s �δtt %�s �δQQ %�s

123

The average percentage of uncertainty of mass flow rates is 9%.

C.2.2. Uncertainty evaluation for equivalence ratio (Ø)

Ø � "12+3."�=�¨�©+�. � ªT-@.�«>�.,+@��« Where, "1. : The fuel mass flow rate.

"?. : The oxidizer mass flow rate. ªT-@.�«>�.,+@��«: The stoichiometric Air to fuel ratio.

The uncertainty of equivalence ratio (¬Ø) is:

δØ � j� ∂Ø∂"12+3. δ"12+3. �s � ∂Ø∂"�=�¨�©+�. δ"�=�¨�©+�. �s Where, δ"12+3. : The uncertainty of fuel mass flow rate.

δ".=�¨�©+�. : The uncertainty of oxidizer mass flow rate.

The stoichiometric Air to fuel ratio (ªT-@.�«>�.,+@��«� is constant, so the

percentage uncertainty of equivalence ratio (ØØ %) tends to:

�¬ØØ %� � j�δ"12+3."12+3. %�s �δ"�=�¨�©+�."�=�¨�©+�. %�s The average percentage of uncertainty of equivalence ratio is 13%.

C.2.3. Uncertainty evaluation for flow momentum (M):

6 � ". � �ª

Where, ".: The fluid flow rate.

Q: The fluid flow rate.

A: The cross sectional Area.

We can assume that the cross section area is constant.

124

Then, the uncertainty of fluid momentum (¬6) is:

δ6 � j�∂6∂". δ".�s �∂6∂� δ��s Where, δ".: The uncertainty of fluid mass flow rate. δ�: The uncertainty of fluid flow rate.

The percentage uncertainty of fluid momentum (®® %) tends to:

�¬66 %� � j�δ".". %�s �δ�� %�s The average percentage of uncertainty of fluid momentum is 11%.

C.2.4. Uncertainty evaluation for emission index:

!"#$$#%& #&'!( )*+,�--�./0*12+3 4 � �55,67 � 89: � ; � <+=>?2-@A"12+3A

Where: �55,: The concentration of measured emission (ppm). 67: The molecular weight of measured gas. 8: The pressure of exhaust at measurement point (kpa). 9:: The universal gas constant (8314.4 J/kg.k). ;: The temperature of exhaust at measurement point (k). <+=>?2-@A : The volumetric mass flow rate of exhaust (m3/s). "12+3A : The mass flow rate of fuel (kg/s).

Thus, The percentage uncertainty of emission index (+�+� %) tends to:

125

¯¬!#!# %° � j�δP8 %�s �δ<A<A %�s δ"12+3A"12+3A %�s The average percentage of uncertainty of emission index is 11%.

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