effect of impurities on ignition and combustion

EFFECT OF IMPURITIES ON IGNITION AND COMBUSTION

CHARACTERISTICS OF CRUDE GLYCEROL AS A BY-

PRODUCT OF BIODIESEL MANUFACTURING

Hendrix Yulis Setyawan

This thesis is presented for the degree of Doctor of Philosophy of

The University of Western Australia

School of Engineering

Centre for Energy

2018

iii

ABSTRACT

Biodiesel manufacturing produces crude glycerol as a by-product, through

transesterification of fatty acids derived from vegetable oils and animal tallows. Crude

glycerol is produced in large quantities and has low market value, and can be

environmentally and financially burdensome for the biodiesel manufacturing industry.

A potential means of dealing with the large quantities of crude glycerol produced could

be direct combustion to generate heat and possibly power. This could serve as a simple

and effective means of addressing the problems, but little is known about the

combustion of this by-product. In particular, the effects of glycerol impurities such as

water, methanol, soap, and biodiesel on the ignition characteristics and combustion

performance of crude glycerol have rarely been reported.

The overall aim of this research was to investigate the fundamental ignition and

combustion characteristics of glycerol to establish a scientific basis for utilisation of

crude glycerol. The specific objectives were to (i) use a single droplet combustion

technique to study the ignition and combustion characteristics of crude glycerol

compared with those of pure glycerol, biodiesel, diesel, and ethanol; (ii) investigate the

effects of water, methanol, biodiesel, and soap on the ignition and combustion

characteristics of glycerol; and (iii) use Chemkin Pro-based kinetic modelling to further

explore the effects of methanol and biodiesel impurities on the combustion

characteristics of crude glycerol.

Ignition and combustion characteristics of crude glycerol were measured using a single

droplet combustion technique. This technique included use of a horizontal furnace, a

step motor, a CCD camera, a computer, and a flame emission spectrometer. The single

droplet combustion allowed approximation of ignition delay time, burnout time, and

burning rate of the droplet. The results were compared to those of pure glycerol,

iv

biodiesel, diesel and ethanol. Specific attention was paid to the effect of water,

methanol, biodiesel and soap. Chemical effects of the impurities were investigated using

Chemkin Pro-based kinetic modelling. Methanol and biodiesel were selected for

modelling work since these have well-known combustion mechanisms. Combinations

of the combustion mechanisms of glycerol-methanol and glycerol-biodiesel were

proposed separately.

Crude glycerol had a shorter ignition delay time, more rapid burnout, and a higher

burning rate than pure glycerol. These differences were attributed to impurities that

changed the physical and thermochemical properties of crude glycerol, such as

increasing the vapour pressure, reducing the latent heat, and decreasing the boiling

point. The ignition delay time and burnout time of pure glycerol were longer than that

of biodiesel, diesel, and ethanol, leading to a lower burning rate. This occurred because

glycerol has a low Cetane number, high boiling point, and high auto-ignition

temperature. Impurities decreased the burnout time and increased the burning rate for

crude glycerol when compared with pure glycerol, biodiesel, diesel, and ethanol.

An investigation into the effect of each of the major impurities on the ignition and

combustion characteristics of crude glycerol was then carried out on pure glycerol

mixed with water, methanol, biodiesel, or soap, respectively. The addition of water

increased the ignition delay time, and decreased the burnout time and burning rate of

glycerol due to the formation of steam bubbles and subsequent micro-explosions. Water

evaporated at the beginning of the combustion process to form steam and bubbles inside

the droplet. Due to the high heat capacity and latent heat of water, steam formation

delayed heating of the droplet, decreased droplet temperature, and slowed the ignition

process. However, the burnout time decreased due to the micro-explosions of steam

v

bubbles, resulting in the formation of many smaller droplets and consequently an

increased burning rate.

The addition of methanol enhanced the combustion performance of glycerol by

decreasing the ignition delay time and burnout time, and increasing the burning rate.

This effect became more pronounced as the amount of methanol increased. Micro-

explosion phenomena were also observed during combustion of the droplets, which in

turn improved the combustion performance of the glycerol.

The addition of biodiesel decreased the ignition delay time and burnout time, and

increased the burning rate. The flame colour changed during combustion, and the

ignition delay time of the mixture decreased due to the early ignition of biodiesel. A

reddish flame appeared at the beginning of the combustion process due to early ignition

of biodiesel. The flame colour became greenish towards the end of the process due to

glycerol combustion. The burnout time of the glycerol droplets decreased, and the

burning rate increased with increasing addition of biodiesel.

The addition of soap decreased the ignition delay time slightly, decreased the burnout

time, and increased the burning rate. The mixture exhibited two-stage combustion

behaviour: a glycerol-dominated combustion stage followed by a soap-dominated

combustion stage. The addition of soap decreased the ignition delay time slightly and

decreased the overall burnout time of the droplets. The burning rate increased with

increasing soap content due to sodium ions in the flames that promoted the ignition and

subsequent combustion of fuel vapours. The two-stage combustion resembled

evaporation behaviour of a binary mixture of two components with vastly differing

volatilities.

Kinetic modelling results showed that the addition of methanol or biodiesel decreased

the ignition delay time of glycerol. The ignition delay time decreased with increasing

vi

concentration of methanol or biodiesel. The reaction pathway analysis showed that the

ignition was promoted due to an early injection of OH radicals. The presence of O

atoms in the system contributed to the increase in rates of the key reactions. In the

glycerol-methanol mixture, O atoms were released from the formation of combustion

products H2O and CO2, while in the glycerol-biodiesel mixture, O atoms were released

from consumption of CO.

This research has resulted in significant new experimental data regarding the ignition

and combustion characteristics of crude glycerol. Understanding of the effects of the

main impurities has also improved, providing a sound basis for future development of

glycerol combustion theories and technological applications.

vii

TABLE OF CONTENTS

Abstract ............................................................................................................................... iii

Table of contents ................................................................................................................. vii

Acknowledgements ............................................................................................................. xii

Authorship declaration: co-authored publications .............................................................. xiii

List of figures ...................................................................................................................... xviii

List of tables ........................................................................................................................ xxiii

Chapter 1 Introduction .................................................................................................... 1

1.1 Background and motivation ......................................................................................... 1

1.2 Scope and aims ............................................................................................................ 2

1.3 Thesis outline ............................................................................................................... 3

Chapter 2 Literature review ............................................................................................ 6

2.1 Pure glycerol and crude glycerol ................................................................................. 6

2.1.1 Glycerol chemistry and properties ...................................................................... 6

2.1.2 Crude glycerol from biodiesel manufacturing .................................................... 7

2.1.3 Impurities in crude glycerol ................................................................................ 9

2.1.4 Purification of crude glycerol ............................................................................. 11

2.1.5 Market size and current use of glycerol.............................................................. 14

2.1.6 Limitations of glycerol utilisation ...................................................................... 15

2.2 Advance glycerol utilisation Technologies .................................................................. 16

2.2.1 Glycerol pyrolysis and steam reforming ............................................................ 16

2.2.2 Glycerol gasification........................................................................................... 16

2.2.3 Conversion of glycerol into high-value chemicals ............................................. 17

2.3 Combustion Fundamentals and glycerol combustion .................................................. 20

2.3.1 Fundamentals of liquid fuel combustion ............................................................. 20

viii

2.3.2 Crude glycerol as a fuel ...................................................................................... 25

2.3.3 Glycerol combustion in CI engines .................................................................... 29

2.3.4 Glycerol combustion using various burners for utility boilers ........................... 30

2.3.5 Glycerol in fuel blends ....................................................................................... 31

2.4 Summary and specific research objectives .................................................................. 31

Chapter 3 Research methodology, approach and techniques ...................................... 34

3.1 Overview of research strategies ................................................................................... 34

3.2 Single droplet combustion ........................................................................................... 35

3.2.1 Principles of single droplet combustion ............................................................. 35

3.2.2 Single droplet combustion apparatus.................................................................. 36

3.2.3 Analysis of ignition and combustion characteristics .......................................... 41

3.2.4 Limitations and errors......................................................................................... 49

3.3 Single droplet combustion of glycerol ......................................................................... 53

3.3.1 Comparison between ignition and combustion of crude glycerol and that of

biodiesel, diesel, and ethanol .............................................................................. 53

3.3.2 Effect of water on glycerol combustion ............................................................. 53

3.3.3 Effect of methanol on glycerol combustion ....................................................... 55

3.3.4 Effect of biodiesel on glycerol combustion ........................................................ 55

3.3.5 Effect of soap on glycerol combustion ............................................................... 55

3.4 Kinetic studies of mixtures of glycerol with methanol and biodiesel .......................... 56

3.4.1 Chemkin Pro and kinetic model ........................................................................ 57

3.4.2 Reaction mechanisms and kinetic simulations for glycerol-methanol

mixtures .............................................................................................................. 58

3.4.3 Reaction mechanisms and kinetic simulations for glycerol-biodiesel

mixtures .............................................................................................................. 59

ix

Chapter 4 Properties and combustion characteristics of crude glycerol .................... 61

4.1 Properties of crude glycerol ......................................................................................... 61

4.1.1 Water in crude glycerol ...................................................................................... 62

4.1.2 Methanol in crude glycerol ................................................................................. 63

4.1.3 Soap in crude glycerol ........................................................................................ 63

4.1.4 Biodiesel in crude glycerol ................................................................................. 64

4.2 Ignition and combustion characteristics: comparison of crude glycerol against

pure glycerol, petroleum diesel, biodiesel, and ethanol ............................................... 64

4.2.1 Combustion phenomena of crude glycerol ......................................................... 64

4.2.2 Ignition delay time ............................................................................................. 67

4.2.3 Burnout time ....................................................................................................... 69

4.2.4 Burning rate ........................................................................................................ 70

4.3 Summary ...................................................................................................................... 72

Chapter 5 Effect of water................................................................................................. 74

5.1 Properties of the glycerol-water mixtures .................................................................... 74

5.2 The role of water in combustion .................................................................................. 75

5.3 Single droplet combustion study of a pure glycerol-water mixture ............................. 75

5.3.1 Combustion phenomena of pure glycerol-water droplets................................... 75

5.3.2 Ignition delay time .............................................................................................. 77

5.3.3 Burnout time ....................................................................................................... 79

5.3.4 Burning rate ........................................................................................................ 80

5.4 Summary ...................................................................................................................... 81

Chapter 6 Effect of methanol ......................................................................................... 83

6.1 Properties of glycerol-methanol mixtures .................................................................... 83

6.2 Single droplet combustion study of a pure glycerol-methanol mixture ....................... 85

x

6.2.1 Combustion phenomena of pure glycerol-methanol droplets ............................ 85

6.2.2 Ignition delay time ............................................................................................. 87

6.2.3 Burnout time ....................................................................................................... 88

6.2.4 Burning rate ........................................................................................................ 89

6.3 Summary ...................................................................................................................... 91

Chapter 7 Effect of biodiesel ........................................................................................... 92

7.1 Single droplet combustion study of a pure glycerol-biodiesel mixture ....................... 92

7.1.1 Combustion phenomena of pure glycerol-biodiesel droplets ............................. 92


7.1.3 Burnout time ....................................................................................................... 97

7.1.4 Burning rate ........................................................................................................ 98

7.2 Summary ...................................................................................................................... 99

Chapter 8 Effect of soap .................................................................................................. 100

8.1 Single droplet combustion study of a pure glycerol-soap mixture .............................. 100

8.1.1 Ignition and combustion phenomena of pure glycerol-soap droplets ................ 100


8.1.3 Burnout time ....................................................................................................... 104

8.1.4 Burning rate ........................................................................................................ 105

8.2 Sodium ions in the flame of pure glycerol-soap droplets ............................................ 106

8.3 Sodium ions in the solid residue ................................................................................. 108

8.4 Summary ...................................................................................................................... 110

Chapter 9 Kinetic modelling of glycerol-methanol and glycerol-biodiesel

mixtures ........................................................................................................... 111

9.1 Effect of addition of methanol on glycerol combustion kinetics ................................. 112

9.2 Effect of addition of biodiesel on glycerol combustion kinetics ................................. 116

xi

9.3 Summary ...................................................................................................................... 120

Chapter 10 Evaluation and practical implications ...................................................... 122

10.1 Integration and evaluation of the effects of impurities in crude glycerol

combustion ................................................................................................................. 122

10.1.1 Impurities that enhance the ignition and combustion of glycerol .................. 122

10.1.2 Impurities that partly enhance the ignition and combustion of glycerol ....... 124

10.1.3 Damaging effects of soap on combustion ...................................................... 125

10.2 Effect of combined impurities in glycerol combustion ............................................. 126

10.3 Practical implication ................................................................................................. 129

10.4 Identification of the new gaps ................................................................................... 130

Chapter 11 Conclusions and recommendations ............................................................. 131

11.1 Conclusions ............................................................................................................... 131

11.2 Recommendations...................................................................................................... 133

References .......................................................................................................................... 135

xii

ACKNOWLEDGEMENTS

I am sincerely grateful to the many people who contributed to the completion of my

PhD thesis. I would like to express my sincere and utmost gratitude to my supervisor

and academic father, Professor Dongke Zhang, for giving me a precious opportunity to

undertake this PhD study at the Centre for Energy at The University of Western

Australia. It has been an honour to learn from the best combustion engineer in the

world. Professor Zhang inspired and guided me with great patience, provided me with

continuous support and encouragement, graciously shared his phenomenal experience,

knowledge and passion for science with me, and provided me with invaluable advice on

academic and life philosophy. His passion for science has set a wonderful role model

for me to follow in the future. Without his supervision, continuous support and

encouragement, I would not have reached where I am.

I am grateful for the postgraduate research scholarships provided by The Australia

Awards Scholarship and specially thank to Mrs. Debra Basanovic. I am also sincerely

thankful to my home university, Universitas Brawijaya - Indonesia for the time and

support provided.

I am sincerely grateful to my academic family at the Centre for Energy, especially my

co-supervisors, Dr Mingming Zhu and Dr Yang Zhang, and colleagues Mr Zhezi Zhang,

Dr Zhijian Wan, Ms Yii Leng Chan, and other postgraduate students at the Centre for

Energy. I am grateful to Dr. Jo Edmonston, and Dr. Krys Haqq for their support on the

English writing of my thesis. I express my deepest love and gratitude to my parents,

Bapak Tamsir and Ibu Triasmiaisyah, for their continuous love and spiritual support,

and my wife, Juwita Ratna Dewi, for her consistent love and support during my PhD

journey. I am thankful to the Indonesian community in Perth for making me feels at

home, especially Maroonah, Warneds, Aipssa, IBE, and also the Indovarsity photo club.

xiii

AUTHORSHIP DECLARATION: CO-AUTHORED PUBLICATION

This thesis contains work that has been published and/or prepared for publication.

Details of the work:

Setyawan, H. Y., Zhu, M., Zhang, Z. Zhang, D. 2016. Ignition and combustion

characteristics of single droplets of crude glycerol in comparison with pure glycerol,

petroleum diesel, biodiesel and ethanol. Energy, 113, 153–159.

Location in thesis: Chapter 4

Student contribution to work:

The work was supported by and a part of an ARC Linkage Project grant held by

Professor Dongke Zhang

The candidate planned and conducted experiments, and analysed results with

assistance from Mr Zhezi Zhang

The manuscript was drafted by the candidate under the supervision of Professor

Dongke Zhang, and critically reviewed by Professor Dongke Zhang and Dr

Mingming Zhu


Setyawan, H. Y., Zhu, M., Zhang, Z. Zhang, D. 2015. An Experimental Study of the

Effect of Water on Ignition and Combustion Characteristics of Single Droplets of

Glycerol. Proceedings of the 7th International Conference on Applied Energy, 2015,

Abu Dhabi.



The work was supported by an ARC Linkage Project grant held by Professor Dongke

Zhang

xiv

The candidate planned and carried out experiments, and analysed results with

assistance from Dr Mingming Zhu and Mr Zhezi Zhang

The candidate drafted the manuscript under the supervision of Professor Dongke

Zhang and Dr Mingming Zhu

The candidate worked closely with Professor Dongke Zhang, Mr Zhezi Zhang and

Dr Mingming Zhu to critically review the manuscript

The candidate presented the findings at the 7th International Conference on

Applied Energy



Effects of Water on Ignition and Combustion Characteristics of Single Droplets of

Glycerol. Energy Procedia, 75, 578–583.



The work was supported by an ARC Linkage Project grant held by Professor

Dongke Zhang


assistance from Dr Mingming Zhu and Mr Zhezi Zhang



The candidate worked closely with Professor Dongke Zhang, Mr Zhezi Zhang and

Dr Mingming Zhu to critically review the manuscript

xv


Setyawan, H. Y., Zhu, M., Zhou, W. Zhang, D. 2013. Effects of Methanol Addition

on Combustion Characteristics of Single Droplets of Glycerol. Proceedings of the

Australian Combustion Symposium, 2013, The University of Western Australia, pp.

332–335.




Dongke Zhang


assistance from Dr Mingming Zhu


Zhang, Dr Mingming Zhu, and Dr Wenxu Zhou

The candidate presented the findings at the Australian Combustion Symposium



Effects of Biodiesel on Ignition and Combustion Characteristics of Single Droplets of

Glycerol. Proceedings of The 11th Asia-Pacific Conference on Combustion, 2017.


Candidate contribution to work:


Dongke Zhang


xvi



Zhang

The candidate worked closely with Professor Dongke Zhang and Dr Mingming

Zhu to critically review the manuscript

The candidate presented the findings at the Asia-Pacific Conference on

Combustion


Setyawan, H. Y., Zhu, M., Zhang, Z. Zhang, D. An Experimental Study of the Effects

of Soap on Ignition and Combustion Characteristics of Single Droplets of Glycerol.

Proceedings of the Australian Combustion Symposium, 2015, University of

Melbourne, pp. 376 - 379.




Dongke Zhang


assistance from Mr Zhezi Zhang and Dr Mingming Zhu

The manuscript was drafted by the candidate under the supervision of Professor

Dongke Zhang, and critically reviewed by Professor Zhang and Dr Mingming Zhu

The candidate presented the findings at the Australian Combustion Symposium

xvii


Setyawan, H. Y., Zhu, M., Zhang, Z. Zhang, D. An Experimental Investigation into

the Effect of Soap on Ignition and Combustion Characteristics of Single Droplets of

Glycerol. Combustion Science and Technology, 189, 1540–1550.




Dongke Zhang



The candidate analysed results and discussed findings with guidance from

Professor Dongke Zhang and Dr Mingming Zhu



The candidate worked closely with Professor Dongke Zhang and Dr Mingming

Zhu to critically review the manuscript

Student, Principle Supervisor,

Hendrix Yulis Setyawan Dongke Zhang, FTSE

Date: 31 July 2018

xviii

LIST OF FIGURES

1.1 Thesis map ................................................................................................................... 5

2.1 Chemical structure of glycerol ..................................................................................... 6

2.2 Industrial applications of glycerol ............................................................................... 14

2.3 Alternatives pathways and secondary products of glycerol conversion to

chemicals...................................................................................................................... 19

2.4 Flue gas composition of glycerol combustion ............................................................. 26

2.5 Adiabatic flame temperatures of glycerol, biodiesel, diesel, methanol, and

methane ....................................................................................................................... 28

2.6 The 7 kW refractory burners (left) and 82 kW refractory-lined furnaces (right) ........ 30

3.1 Detailed experimental design and network .................................................................. 35

3.2 Schematic of single droplet combustion indicating the radial distribution of fuel

mass and flame ............................................................................................................. 37

3.3 Experimental set-up: (a) a schematic illustration of the single droplet combustion

apparatus; (b) A photograph of the experimental system. ........................................... 38

3.4 The schematic of the furnace heating system .............................................................. 38

3.5 The temperature gradient along the furnace ................................................................ 39

3.6 The difference between actual and measured temperature as a function of

measured gas temperature ............................................................................................ 39

3.7 Schematic representation of a burning droplet ............................................................ 41

3.8 Pixel greyscale intensity .............................................................................................. 42

3.9 Binary image of droplet ............................................................................................... 43

3.10 Image of binary droplet separated from fibre ............................................................ 44

3.11 Image of filament separated from droplet ................................................................ 44

3.12 Major and minor axis of the droplet .......................................................................... 45

xix

3.13 Micro-explosion of droplet with numbering for user input ....................................... 46

3.14 Typical combustion of a droplet ................................................................................ 46

3.15 The d2 behaviour of droplet vapourisation and combustion ...................................... 48

3.16 A schematic representation of single droplet combustion experimentation

apparatus equipped with flame emission spectrometer ............................................. 48

3.17 Shape irregularities from droplet swelling ................................................................ 51

3.18 The reflected lights occur on the droplet surface ....................................................... 52

3.19 Schematic of the ignition delay time as simulated using Chemkin Pro .................... 58

3.20 Validation of the ignition of the combined mechanism of glycerol-methanol

model ......................................................................................................................... 59

3.21 Validation of the ignition of the combined mechanism of glycerol-biodiesel

model ......................................................................................................................... 60

4.1 Typical time-sequenced images of burning droplets for crude glycerol (CG),

pure glycerol (PG), biodiesel (BD), diesel (DS) and ethanol (ET). ............................. 65

4.2 Temporal variation of the square of the normalised droplet diameter of fuels

during combustion ....................................................................................................... 66

4.3 The ignition delay times at different temperatures for droplets of different fuels .... 67

4.4 Burnout times of various fuels tested at different temperature .................................... 69

4.5 Comparison of the burning rates of crude glycerol, pure glycerol, biodiesel,

diesel, and ethanol ........................................................................................................ 71

5.1 Typical images of burning glycerol droplets with various water concentrations ........ 76

5.2 Temporal variations of the square of the normalised droplet diameters of

glycerol with different water concentrations ............................................................... 77

5.3 Ignition delay time of glycerol droplets with and without water addition, and

crude glycerol ............................................................................................................... 78

xx

5.4 Burnout time of glycerol droplets with addition of different concentrations of

water ............................................................................................................................. 79

5.5 Burning rates of glycerol droplets with addition of different concentrations of

water ............................................................................................................................. 80

6.1 The viscosity of the glycerol-methanol mixture at various temperatures .................... 84

6.2 The density of the glycerol-methanol mixture at various temperatures ...................... 85

6.3 Typical images of burning glycerol droplets with various methanol

concentrations: pure glycerol (PG), glycerol-methanol mixture (GM) ....................... 86

6.4 Measured temporal variation of the square of the normalised droplet diameter of

glycerol with different concentration of methanol (GM) ............................................ 87

6.5 The effect of the addition of methanol on the ignition delay time of glycerol

combustion ................................................................................................................... 88

6.6 Burnout time of glycerol with different concentration of methanol ............................ 89

6.7 Burning rates of glycerol with different concentration of methanol............................ 90

7.1 Typical time-sequenced images of burning glycerol droplets and pure glycerol-

biodiesel additions (GB) at 1023 K ............................................................................. 94

7.2 Temporal variations of the square of the normalised droplet diameters of pure

glycerol (PG) and glycerol-biodiesel (GB) .................................................................. 95

7.3 Effect of the addition of biodiesel on the ignition delay time of glycerol droplets .... 96

7.4 Burnout time of glycerol droplets at different concentration of biodiesel ................... 97

7.5 Effect of the biodiesel addition on burning rate of glycerol droplets .......................... 98

8.1 Typical time-sequenced images of burning glycerol droplets and pure glycerol-

soap droplets ................................................................................................................ 101

8.2 Normalised temporal evolution of the squared diameter for pure glycerol, GS5,

and pure soap at 1023K................................................................................................ 103

xxi

8.3 Ignition delay time of glycerol droplets with soap addition at different

concentrations at 1023K .............................................................................................. 104

8.4 Burnout time of glycerol droplets with soap addition at different concentrations

at 1023K ....................................................................................................................... 105

8.5 The burning rates of glycerol droplets with the addition of different amounts of

soap: (a) Stage 1 and (b) Stage 2.................................................................................. 106

8.6 Changes in signals of sodium in flames of droplets with the addition of various

concentrations of soap in glycerol ............................................................................... 107

8.7 The SEM images of the solid residue surface at various soap concentration .............. 109

9.1 Mole fractions of glycerol, methanol, and OH of combustion of glycerol-

methanol mixture ......................................................................................................... 112

9.2 Effect of various methanol addition on ignition delay time of pure glycerol-


9.3 Relative rates (indicated by the horizontal bar) of the consumption of pure

glycerol (PG) and glycerol-methanol (GM) by various elementary reactions at

residence time 0.1, initial temperature 1023 K and the equivalence ratio 1.0 ............. 114

9.4 Comparison of mole fractions of OH, H2O, CO2, O, CO and H of pure glycerol

(PG) and 40v% glycerol-methanol mixture (GM40) ................................................... 115

9.5 Mole fractions of glycerol, methanol, and OH of combustion of glycerol-


9.6 Effect of various biodiesel additions on ignition delay time of pure glycerol-

biodiesel mixture .......................................................................................................... 118

9.7 Relative rates (indicated by the horizontal bar) of the consumption of pure

glycerol (PG) and 10v% glycerol-biodiesel (GB) by various elementary

xxii

reactions at residence time 0.1s, initial temperature 1023K and the equivalence

ratio 1.0 ........................................................................................................................ 119

9.8 Comparison of mole fractions of OH, CO2, H2O, CO, O, and H of pure glycerol

(PG) and 10v% glycerol-biodiesel mixture (GB10) .................................................... 120

10.1 Fuel atomisation with addition of water in the fuel ................................................... 125

10.2 Ignition delay time of pure glycerol mixed with various concentration of

methanol and biodiesel with addition of: a) 5% water and 1% soap; b) 20v%

water and 1v% soap; c) 5v% water and 5v% soap .................................................... 127

10.3 Burnout time of pure glycerol mixed with various concentration of methanol

and biodiesel with addition of: a) 5% water and 1% soap; b) 20v% water and

1v% soap; c) 5v% water and 5v% soap ..................................................................... 128

10.4 Burning rate of pure glycerol mixed with various concentration of methanol

and biodiesel with addition of: a) 5% water and 1% soap; b) 20v% water and

1v% soap; c) 5v% water and 5v% soap ..................................................................... 129

xxiii

LIST OF TABLES

2.1 Comparison of biodiesel production technologies ...................................................... 9

2.2 Various properties of glycerol ..................................................................................... 11

2.3 Projection of biodiesel production 2014–2023 ............................................................ 13

2.4 Alternative technological applications and products of glycerol conversion .............. 18

2.5 Properties of crude glycerol and pure glycerol ............................................................ 27

3.1 Properties of silicon carbide fibre ................................................................................ 40

3.2 Thermodynamic properties of crude glycerol compared with those of other fuels ..... 54

3.3 Compositions of olive oil used in the present study .................................................... 56

3.4 Simulation condition .................................................................................................... 58

4.1 Properties of crude glycerol ......................................................................................... 62

5.1 Key physical and thermodynamic properties of glycerol, water, and glycerol-

water mixtures (GW) ................................................................................................... 74

6.1 Chemical components and physical properties of glycerol and methanol ................... 84

7.1 Thermodynamic properties of pure glycerol and biodiesel ......................................... 93

8.1 Ash and sodium in ash produced by combustion of various concentrations of

soap in glycerol ............................................................................................................ 109

10.1 Research on sodium related to combustion ............................................................... 126

1

Chapter 1 Introduction

1.1 Background and motivation

Biodiesel is an alternative liquid transport fuel derived from various sources of fatty

acids including vegetable oils and animal tallow [1]. Crude glycerol is a by-product of

the production of biodiesel and contains impurities from the manufacturing process. The

production of every 100 tonnes of biodiesel yields approximately 10 tonnes of crude

glycerol as a by-product. [2]. In 2015, ~40 million tonnes of biodiesel were produced

globally [3], generating ~4 million tonnes of crude glycerol, twice the amount the global

market could absorb. Remaining crude glycerol causes significant environmental and

economic challenges for the biodiesel manufacturing industry, particularly regarding

disposal and further utilisation [2].

A number of processes have been investigated for the effective utilisation of crude

glycerol, including pyrolysis, fermentation, and steam reforming. While these processes

have been used to convert crude glycerol into valuable products such as hydrogen [4],

acrolein [5], syngas [6], and surfactant [7], the processes can be expensive or the

product volume and value too low to be economically viable. Utilisation of crude

glycerol is likely to require large throughputs to be economically viable [8], and

biodiesel plants need to dispose of large quantities of glycerol. Therefore, combustion

for heat and power production (CHP) in biodiesel plants could serve to effectively

utilise excess crude glycerol.

The combustion of crude glycerol is inherently challenging due to the nature of glycerol

and the impurities in crude glycerol. By nature, glycerol has high viscosity, low energy

density, high auto-ignition temperature, and low heating value. Combustion is further

2

complicated by impurities introduced during biodiesel production, including water,

methanol, biodiesel, and soap [9], and these are costly to remove.

Pure glycerol has been tested for direct combustion using utility boilers and diesel

engines [8, 10]. In a heating boiler equipped with a high-swirl refractory burner, pure

glycerol has been burned and combusted efficiently [10]. A diesel engine generator

fuelled with pure glycerol has also generated continuous power [8]. In contrast to

combustion research of pure glycerol, studies of crude glycerol are limited. Combustion

of crude glycerol is challenging due to the presence of impurities, but the role of these

impurities in combustion is not clear [11]. Therefore, to utilise crude glycerol as a fuel,

a better understanding of the fundamental combustion characteristics is required,

particularly regarding the effects of the impurities in combustion.

1.2 Scope and aims

This research aimed to investigate the utilisation of crude glycerol as a fuel for diesel

engines. Combustion characteristics of crude glycerol were compared with those of pure

glycerol and other fuels using the single droplet combustion technique. The effects of a

range of impurities on crude glycerol combustion were also assessed.

Four significant impurities of crude glycerol, namely water, methanol, biodiesel, and

soap, were investigated. The single droplet combustion technique was used to determine

the effects of these impurities on ignition delay times, burnout times and burning rates

of glycerol droplets. Investigation of the effects of soap included exploration of sodium

released in the flame and in the deposit, using a flame spectrometer and SEM-EDS,

respectively. The kinetic effects of methanol and biodiesel in glycerol combustion were

also examined.

3

1.3 Thesis outline

There are a total of 11 chapters in this thesis. Figure 1.1 presents a schematic map of the

11 chapters in this thesis, and each chapter is outlined below:

Chapter 1 Glycerol production and utilisation are identified, providing reasoning for

the alternative solution via combustion for the abundance production of

crude glycerol. Scope, aims and thesis structure are defined.

Chapter 2 Existing knowledge of glycerol production, consumption, and utilisation is

reviewed and combustion of glycerol is introduced. Knowledge gaps and

specific objectives for the research are identified.

Chapter 3 Methodology employed to achieve the research objectives identified in

Chapter 2 is presented. The experimental procedure using the single droplet

combustion technique is described. Kinetic modelling of the glycerol-

methanol and glycerol-biodiesel mixtures is also presented.

Chapter 4 Comparisons between the ignition and combustion of crude glycerol, pure

glycerol, biodiesel, diesel, and ethanol are discussed. An investigation of

whether the ignition and combustion of crude glycerol differs from that of

pure glycerol and the other fuels, and whether impurities change the

combustion characteristics of crude glycerol, is presented.

Chapter 5 The effect of water on the ignition and combustion characteristics of the

single droplet of crude glycerol, including combustion phenomena, ignition

delay, burnout time, and burning rate, are investigated.

Chapter 6 The effects of methanol on the ignition and combustion characteristics of

the single droplet of crude glycerol, including combustion phenomena,

ignition delay, burnout time, and burning rate, are investigated.

4

Chapter 7 The effect of biodiesel on the ignition and combustion characteristics of the

single droplet of crude glycerol is investigated.

Chapter 8 The effect of soap on the ignition and combustion characteristics of the

single droplet of crude glycerol is investigated. Sodium released in the

flame and the solid deposit is discussed.

Chapter 9 Simulation of the kinetic effect of the addition of either methanol or

biodiesel on the ignition and combustion of crude glycerol is described.

Chapter 10 Results are evaluated against those in the literature and specific research

objectives. The implications of these findings in functional processes, along

with new knowledge gaps identified for future studies, are discussed.

Chapter 11 Conclusions and recommendations for future investigations are presented.

5

Figure 1.1 Thesis map

Chapter 1: Introduction

• Background and motivation O Crude glycerol: production, utilisation, and limitation O Combustion of crude glycerol as a simple means of utilisation• The scope of thesis O Glycerol combustion with emphasis on the effect of impurities: water, methanol, biodiesel, and soap• Overall aims• Thesis structure

Chapter 2: Literature Review

• Pure and crude glycerol: properties, impurities, production, utilisation• Glycerol utilization technologies• Combustion of liquid fuels, glycerol combustion• Crude glycerol as a fuel• Conclusions from literature review• Specific research objectives of this thesis work

Chapter 3: Methodology, Approach and Experimental Techniques

• An outline of research strategies• Single droplet combustion technique O Flame emission spectroscopy O SEM/EDS for investigation of ash from crude glycerol combustion• Kinetic Modelling• Data analysis O Error analysis

Chapter 4: Properties and Combustion Characteristics of Crude Glycerol

• Crude glycerol properties• Ignition characteristics and combustion behavior of single droplets, comparison with pure glycerol, petroleum diesel, biodiesel and ethanol• Summary

Chapter 5: Effects of Water

• Ignition and combustion mechanisms• Ignition delay time• Burnout time • Droplet size• Burning rate • Summary

Chapter 6: Effects of Methanol


Chapter 7: Effects of Biodiesel


Chapter 8: Effects of Soap

• Ignition and combustion mechanisms• Ignition delay time• Burnout time • Droplet size• Flame spectroscopy• Ash morphology • Burning rate • Summary

Chapter 9: Kinetic Modelling of Ignition of Glycerol-Methanol and Glycerol- Biodiesel Mixtures

• Kinetic modelling of ignition of mixtures of glycerol – methanol• Kinetic modelling of ignition of mixtures of glycerol – biodiesel• Summary

Chapter 10: Evaluation and Practical Implications

• Integration of findings from present thesis work• Evaluation against the literature• Evaluation against the specific objectives• Practical implications.• Identification of new gaps.

Chapter 11: Conclusions and Recommendations

• Summarise new, significant findings as conclusions of the thesis research• Recommendations for future work based on new gaps identified

6

Chapter 2 Literature review

In this chapter, the production, and physical, and thermodynamic properties of

glycerol, are reviewed. The characteristics of crude glycerol as a by-product of

biodiesel manufacturing, and the current status of glycerol utilisation, including use

for fuel and combustion purposes, are also reviewed. The characteristics and

mechanism of glycerol combustion are discussed, and technical and scientific issues

regarding combustion of crude glycerol, particularly the effects of impurities, are

also highlighted.

2.1 Pure glycerol and crude glycerol

2.1.1 Glycerol chemistry and properties

Glycerol is also known as 1,2,3-propanetriol or glycerine [12] and has the chemical

formula C3H8O3. Glycerol is a stable alcohol under most conditions, but can easily

react to form derivatives. Glycerol is a non-toxic, odourless, colourless viscous

liquid with a high boiling point, and is miscible with both water and alcohol [13]. At

low ambient temperatures, glycerol remains in liquid form rather than forming

crystals.

Figure 2.1 Chemical structure of glycerol.

Glycerol is an important intermediate chemical compound in the chemical industry.

Glycerol consists of three hydrophilic alcoholic hydroxyl groups (Figure 2.1), that

cause glycerol to be hygroscopic in nature [14], and able to easily absorb moisture.

7

In the aqueous phase, intramolecular hydrogen bonds and intermolecular solvation of

the hydroxyl group serve to stabilise the glycerol molecule [14].

2.1.2 Crude Glycerol from biodiesel manufacturing

Glycerol was first discovered in the late sixteenth century [15]. In 1779, Scheele, a

Swiss pharmacist, isolated glycerol when heating a mixture of litharge (PbO) with

olive oil [15]. In 1811, a French chemist, Chevrel, patented the first industrial

method for obtaining glycerol that involved reacting fatty material with lime and

alkali [15]. In 1836, another French scientist, Pelouze, determined the empirical

formula of glycerol [15]. The earliest method of glycerol production used microbial

fermentation, and involved manipulation and selection of particular strains of fungi

and algae [16], such as Saccharomyces cerevisiae [17-19], Candida glycerinogenes

[20], and Dunaliella tertiolecta [21]. More recently, biodiesel industries have

become the largest producers of glycerol. Transesterification, a process converting

oil and fat into biodiesel, produces glycerol as a by-product.

Transesterification is a reaction of oils or fats with alcohol to form esters and

glycerol (Reaction 2.1). Various oils and fats are known as biodiesel feedstock,

including palm oil, rapeseed oil, canola oil, soybean oil, crambe oil, waste cooking

oil, and animal fat [22, 23]. Feedstock quality, particularly the free fatty acid (FFA)

content, dictates the process and type of catalysts used [24]. The triglyceride

feedstock, stimulated by a catalyst, reacts with alcohol to produce biodiesel and

glycerol (Reaction 2.2).

RCOOR′ + ROH → RCOOR + R′OH .............................................................. (R 2.1)

C3H5(OCOR)3 + 3CH3OH Catalyst↔ 3COOR + C3H5(OH)3 ............................. (R 2.2)

8

Methanol or ethanol is often used for transesterification. In a stoichiometric reaction,

each triglyceride requires three moles of alcohol (Reaction 2.2) but an excess of

alcohol, up to 100%, is needed to accelerate the reaction and obtain a higher yield of

biodiesel [22].

Catalysts commonly used for transesterification include NaOH, KOH, and sodium

methoxide (NaOMe). The productivity of transesterification varies according to the

chosen catalyst [25]. Base catalysts are commonly used in transesterification since

these can achieve a higher reaction rate than the acid counterparts. The average yield

for biodiesel in a transesterification process is approximately 90%. The remaining

10% consists of the by-product, crude glycerol [2].

Many studies have investigated modifying the transesterification process to increase

the yield of biodiesel, decrease that of crude glycerol and simplify the purification

process. Novel catalysts and processes investigated include biocatalysts [26, 27] and

the supercritical methanol method, neither of which are economically viable.

Biocatalysts are expensive and have low reaction rates for enzymatic biodiesel

production [28]. The supercritical methanol method, that involves simultaneous

etherification of fatty acids and rapid transesterification, does not require a catalyst

[29] but does require a high methanol-to-triglyceride molar ratio of 42:1, and this is

not feasible in chemical industry [30]. The enzymatic and chemical processes of

biodiesel production are compared in Table 2.1.

Research into methods of biodiesel production, particularly regarding the

development of catalysts, is ongoing [24]. Transesterification using a homogeneous

base catalyst is a kinetically rapid and economically feasible method [31]. One

drawback of this method, however, is the production of low-grade glycerol (crude

glycerol), that contains impurities.

9

Table 2.1 Comparison of biodiesel production technologies

Parameter Chemical process Enzymatic process

Alkaline process Acid process

FFA content in the

raw material

Saponified

product

Converted to

biodiesel

Converted to

biodiesel

Water content in the

raw material

Soap formation Catalyst

deactivation

Not deleterious

Biodiesel yield (%) >90 >90 ±90

Reaction rate High Slower than

alkaline process

Low

Glycerol recovery Complex, low-

grade glycerol

Complex, low-

grade glycerol

Easy, high-grade

glycerol

Reaction

temperature (oC)

60–80 >100 20–50

Catalyst cost Cheap Cheap Expensive

Environmental

impact

High, waste water

treatment needed

High, wastewater

treatment needed

Low, wastewater

treatment not

needed

2.1.3 Impurities in crude glycerol

Crude glycerol contains impurities such as water, soap, excess of transesterification

catalyst (NaOH or KOH), methanol, and fatty acid methyl ester (FAME) are

commonly present [9]. These impurities come from the feedstock, reactants, method

used, and the secondary reaction that produces soap and water.

Different types of feedstock and transesterification method affect the composition of

crude glycerol [32, 33]. The effects of feedstock on biodiesel and crude glycerol

10

have shown, for example, that feedstocks with high water content can increase the

proportion of water in crude glycerol [28]. The batch, or continued transesterification

process, affects the ratio of alcohol-to-catalyst in the process. The type of catalyst

used in the transesterification process has significant effects on properties of crude

glycerol, e.g. NaOH or KOH can determine the metal content in crude glycerol.

NaOH contributes 1.2–1.8wt% of sodium-to-crude glycerol as excess catalyst [9].

Variations on transesterification methods can be used to increase the degree of

biodiesel conversion [22]. However, excess reactants (catalyst and alcohol) may be

stored in crude glycerol once transesterification has been completed. These reactants,

e.g. methanol, can be undesirable due to toxicity [34].

Other impurities are also formed in crude glycerol during transesterification. Water

is consistently present in crude glycerol because glycerol is highly hygroscopic.

Impurities can also be derived from elemental reactions during transesterification, as

in Reactions 2.3–2.5 [35]. Reaction 2.5, the transesterification process that uses an

alkaline catalyst, produces soap and water.

CH3OH + NaOH → CH3ONa + H2 ................................................................ (R 2.3)

C3H5(OCOR)3 + H2O → C3H5OH(OCOR)2 + RCOOH .................................. (R 2.4)

RCOOH + NaOH → RCHCOONa + H2O ......................................................... (R 2.5)

Table 2.2 shows impurities known to occur in crude glycerol. The glycerol content

can vary from 27%wt to 92%wt, depending on the feedstock and transesterification

process. At lower glycerol concentrations (<50%), impurities are mostly methanol,

biodiesel, water, and soap. The transesterification process can be improved by

recovering the methanol contained in crude glycerol for use in the next round of

transesterification.

11

Table 2.2 Various glycerol properties

Glycerol

(wt%)

Na and K

salt (wt%)

Methanol

(wt%)

Water

(wt%)

Soap

(wt%)

Ash

(wt%) Reference

27 1 8.6 4.1 20.5 2.7 [36]

33 1 12.6 6.5 26.1 2.8 [36]

48.7 n/a 22.7 25.6 3.0 n/a [37]

50 4–5 1.3 36 n/a n/a [38]

56.5–62.4 n/a 12.8–28.3 n/a 15.3–25.2 n/a [39]

62.5–76.6 n/a 23.4–37.5 n/a n/a 0.25–5.5 [33]

65 4–5 3 26 n/a n/a [40]

65 4–5 1 28 n/a n/a [41]

70 12 2 14 n/a n/a [42]

80 4 1 13.5 n/a n/a [43]

80.1 5.6 <0.2 12.9 1.2 n/a [42]

83.4 n/a n/a 11.6 1.3 2.7 [44]

85.0 4.5 1.5 6.0 3.0 n/a [42]

86 6.5 <0.2 n/a n/a n/a [45]

92 2–3 0.01 6 n/a n/a [46]

2.1.4 Purification of crude glycerol

Since 2006, crude glycerol has been produced on a large scale as the by-product of

biodiesel [47]. Global production of glycerol is expected to increase as biodiesel

production increases. The average production of biodiesel in 2011–2013 was

approximately 31 million tonnes (Table 2.3), with a concomitant production of over

3 million tonnes of crude glycerol. Biodiesel production is projected to increase to 49

12

million tonnes by 2023, and this will significantly increase crude glycerol

production.

Since crude glycerol is continually produced as a by-product of biodiesel,

exploration of the economic utilisation of this by-product in biodiesel production is

essential. Despite being produced in large quantities, crude glycerol from

commercial biodiesel production is poor in quality and requires purification for

further utilisation [48]. Crude glycerol can be purified using a number of methods

including distillation, chemical extraction, adsorption using activated carbon, and

ion- exchange [23].

Glycerol purification has been achieved by molecular distillation at pH 3.5,

producing 96.6% glycerol, 0.03% ash, 1% water, and 2.4% matter organic non-

glycerol (MONG) [29]. However, this method is not feasible as the cost per yield is

too expensive to be economically viable [29].

Chemical extraction [26] neutralises alkaline substances with a strong acid and

remaining esters are saponified with an alkali, producing approximately 86wt% of

glycerol [29]. This method is unfavourable due to the resulting chemical waste.

Glycerol purification using sewage sludge-derived activated carbons (ACs) has

produced 93wt% glycerol [49]. However, the reaction rate is low, requiring two

hours of adsorption time. Ion exchange has been used to separate sodium from

glycerol [50]. Salt needs to be removed before separation of the sodium.

While purification of crude glycerol has been achieved using a number of

techniques, none of these can be considered cost-effective at present [51].

13

Table 2.3 Projection of biodiesel production for 2014–2023 [52]

Country or Union of Countries

Production (× 103 Tonnes)

Growth (%)

Domestic use (× 103 Tonnes)

Growth (%)

Average 2011–2013

Projection 2023

Projection 2014–2023

Average 2011–2013

Projection 2023

Projection 2014–2023

Canada 384 759 -0.01 667 1050 2.4

USA 5167 8245 2.3 4666 7981 2.2

EU 12,905 19,978 5.1 16325 24008 4.7

Australia 827 934 1.1 827 934 1.1

South Africa 94 136 3.36 94 136 3.4

Mozambique 88 121 2.92 24 59 5.1

Tanzania 78 164 7.29 0 72 64.2

Argentina 3282 4595 3.29 1270 2194 3.8

Brazil 3455 4903 1.9 3460 4830 1.8

Colombia 760 1191 3.4 760 1190 3.4

Peru 117 220 6.3 342 426 2.3

India 365 919 10.0 504 1115 7.5

Indonesia 2247 4098 4.4 715 2306 7.0

Malaysia 200 1054 11.5 112 816 15.0

Philippines 196 528 8.1 196 528 8.1

Thailand 1036 1462 1.96 1036 1462 2.0

Turkey 15 34 8.3 15 35 8.5

Vietnam 30 123 10.4 30 122 10.4

TOTAL 31248 49464 4.0 31046 49263 4.1

14

2.1.5 Market size and current use of glycerol

Industries that use glycerol as raw material are shown in Figure 2.2. In 2009,

approximately 0.9 million tonnes of refined glycerol were produced in the global

market and chiefly used for pharmaceutical and cosmetic purposes, animal nutrition,

and glycol substitution. In the pharmaceutical industry, glycerol is used to increase

humidity and viscosity of liquid drugs. Glycerol can also act as a humectant and

moistener in skin and hair care products by drawing water from its surroundings.

Glycerol is non-toxic and therefore useful in the food industry as a lubricant, solvent,

sweetener, preservative, and also serves to keep adhesives and glues from drying too

quickly [23].

Figure 2.2 Industrial applications of glycerol [21].

In 2015, approximately 2 million tonnes [2] of glycerol were produced in the global

market. The global market for refined glycerine increased with an increase in

products derived from glycerol. Examples include conversion of acrylic acid to

acrolein, propylene glycol, and methanol through glycerol dehydration [2].

15

2.1.6 Limitations of glycerol utilisation

Factors limiting development of the glycerol industry include geographically diverse

locations of glycerol production, problems with glycerol purification, and market

saturation. Most glycerol resulting from biodiesel production is produced in

countries of the European Union, the USA, Argentina, Brazil, Indonesia, and

Thailand (Table 2.3). Given the low price and high transportation cost of glycerol,

transport of crude glycerol from biodiesel production plants to glycerol purification

industries is generally not economically viable. Glycerol is a viscous liquid and

therefore difficult to transport by pipeline, and hence more expensive to transport

than other liquid fuels, such as gasoline or diesel.

Another limiting factor in the industrial glycerol market is the requirement for

glycerol feedstock with a high level of purity. However, purification of crude

glycerol is expensive and not economically viable. Purification of crude glycerol is

also potentially detrimental to the environment through the production of chemical

waste as by-product and this further restricts the use of crude glycerol in industry.

In addition to the limitations of geographical locations and purity, market saturation

is also a problem for development of the glycerol production industry. The

production of crude glycerol increased to 3.1 million tonnes from 2003 to 2013, but

less than 2 million tonnes could be absorbed by the market [2]. As a result, the

excess glycerol caused a drop in the prices of both refined and crude glycerol [2]. In

2000, the price for refined glycerol was 4000 Euro/tonne, but fell dramatically to 450

Euro/tonne early in 2010. By mid-2014, glycerol had fallen even further to 224

Euro/tonne [2]. Since the utilisation of glycerol is limited by geographical locations

of production sites and requirement for purity, technological applications need to be

implemented for the utilisation of crude glycerol.

16

2.2 Advanced glycerol utilisation technologies

Many efforts have been made to use glycerol as a raw material for industrial

products [13, 22], and are described below.

2.2.1 Glycerol pyrolysis and steam reforming

Glycerol can be converted into syngas through pyrolysis. Research into this method

is developing rapidly and has resulted in kinetic modelling of glycerol pyrolysis [53],

and a better understanding of the pathways of glycerol dehydration via glycidol [54].

The development of catalytic glycerol processes also suggests that glycerol could be

a potential source for syngas production [55]. However, current technological

applications are limited to the laboratory and have not been tested in industry.

The steam reforming method has potential for converting glycerol into hydrogen.

The optimum processing temperature for this conversion has been determined by

thermodynamic analysis of hydrogen production from glycerol, using oxidative

steam reforming [56]. Preliminary studies of co-steam reforming on glycerol mixed

with biomass [57], and the effect of catalysts (Ni/CeZrO2/Al2O3) have also been

undertaken [58]. Hydrogen production from glycerol steam reforming has been

highly selective, resulting in 88% hydrogen and 99.7 v/v% purity [59].

2.2.2 Glycerol gasification

Gasification is an advanced, stable technological application of glycerol combustion,

and involves gasification of glycerol-biomass and glycerol-liquid fuel mixtures.

Gasification of a glycerol-biomass (hardwood) mixture has been used to produce

syngas [60]. Gasification of 20wt% glycerol absorbed into hardwood chips showed

insignificant effects on emissions (e.g. H2 and CO2), LHV, and particles, but as

expected, increased CO, CH4, and tar concentrations [60]. While these results

17

suggest that glycerol could be used as a substitute for hardwood chips in syngas

production, further work is required to determine how glycerol should be loaded into

the mixture.

Glycerol can be added to fuel to improve consumption and reduce emissions [61].

Processes that convert glycerol into a fuel additive are etherification, acetylation, and

acetalation. Etherification of glycerol with isobutylene produces glycerol ethers that

have a high octane number, improving the combustion performance of gasoline.

Etherification of glycerol using tertbutyl alcohol produces an oxygenated additive

suitable for diesel fuels [62]. Catalytic glycerol decomposition produces acetol and

1.2 propanediol, that have been used to decrease the formation of particulate matter

in diesel engines [63]. However, the use of glycerol as a fuel additive requires pure

glycerol.

The acetylation of glycerol generates diacetylglycerol and triacetylglycerol, both of

which are miscible with gasoline and diesel [64], and results in over 90wt% glycerol

conversion [64]. However, in the acetylation process, glycerol reacts with acetone

and formaldehyde in the presence of ethanol [65], suggesting that glycerol acetal can

only be blended with gasoline in the presence of ethanol.

2.2.3 Conversion of glycerol into high-value chemicals

Glycerol has been used to create a variety of valuable derivative products. Table 2.4

shows the conversion of glycerol to high value products using microbial and

chemical processes. Propanediol, used for the production of polyesters,

polycarbonates, and polyurethanes, has been produced from glycerol using

fermentation.

18

Table 2.4 Alternative technological applications and products of glycerol

conversion

Product name Process method/Nature Researchers

1,3-

propanediol

Continuous and batch microbial fermentations by

Clostridium butyricum and Klebsiella

pneumoniae.

Xiu et al. (2004)

Menzel et al. (1997)

Wang et al. (2001)

Hydrogen

Continuous microbial fermentation by

Enterobacter aerogenes HU-101. Ito et al. (2005)

Catalytic reforming functioning at moderate

temperatures and pressures. Wood (2002)

Steam reforming of glycerol Hirai et al. (2005)

Pyrolysis and steam gasification of glycerol. Valliyappan (2001)

Succinic Acid Microbial fermentation by Anaerobiospirillum

succiniciproducens. Lee et al. (2001)

1,2-

propanediol

Low-pressure hydrogenolysis. Dasari et al. (2005)

Selective hydrogenolysis with Raney nickel

catalyst with hydrogen.

Perosa and Tundo

(2005)

Dihydroxy-

acetone

Chemoselective catalytic oxidation with

platinum metals. Garcia et al. (1994)

Selective oxidation of glycerol with the

platinum-bismuth catalyst. Kimura (2001)

Microbial fermentation by Gluconobacter

oxidants in a semi-continuous process. Bauer et al. (2005)

Polyesters

The reaction of glycerol and adipic acid in the

presence of tin catalysts.

Stumbe and

Bruchmann (2004)

Polycondensation of oxalic acid and glycerol. Alkanis et al. (1976)

The reaction of glycerol and aliphatic

dicarboxylic acids. Nagata et al. (1996)

Polyglycerols Selective etherification of glycerol Clacens et al. (2002)

Polyhydroxy

alkanoates

Fermentation of hydrolysed liquid phase of

glycerol and whey permeate by the osmophilic

organism.

Koller et al. (2005)

19

Figure 2.3 Alternative pathways and secondary products of glycerol conversion to chemicals.

20

Figure 2.3 shows alternative pathways for the conversion of glycerol into valuable

chemicals. One of the products derived from glycerol is acrolein, that has the

potential to produce acrylic acid (essential monomers for polyester industries),

acrylic acid esters, absorber polymers, and detergents [66-68]. While the dehydration

of acrolein from glycerol has increased to nearly 96% [69], increasing the selectivity

of acrolein, by avoiding the formation of hydrogenated products or coke through

minimising secondary reactions, remains a challenge [68]. Among the catalysts used,

super acid [70] and smaller catalysts provide better yields [71, 72]. The optimal

temperature for acrolein production is approximately 300 ºC [73]. The addition of

oxygen has also been shown to reduce catalyst deactivation and enhance yield [74].

While efforts have been made to utilise glycerol using pyrolysis, steam reforming,

gasification, and chemical conversion, these technological applications often require

pure glycerol, are under development, or not economically viable and have limited

usefulness. Considering the increasing amount of crude glycerol produced, a

technological application that can be used directly, such as use as a liquid fuel for

generating heat and power in biodiesel plants, would be beneficial.

2.3 Combustion fundamentals and glycerol combustion

2.3.1 Fundamentals of liquid fuel combustion

Combustion is a physical and chemical process that occurs when fuel reacts with

oxygen at high temperatures to produce heat and combustion products [75]. In a

typical combustion process, the fuel (generally hydrocarbon) is oxidised to form

carbon dioxide (CO2) and water (H2O). However, since combustion occurs in air

rather than pure oxygen, the presence of nitrogen can induce the formation of NO×.

Also, since many fuels are more complex than simple hydrocarbon, unburned and

partially burned products can be produced.

21

In the combustion of liquid fuel, information on the fuel composition is generally

limited. Liquid fuel is a mixture of hydrocarbon species. The most common

properties of a liquid fuel can be determined by ultimate analysis and assessment of

the physical properties of the fuel. Ultimate analysis can determine the fractions of

carbon, hydrogen, sulphur, oxygen, nitrogen, and ash that constitute the fuel. Useful

physical properties include specific gravity, viscosity, flash point, and distillation

profiles. Selected important physiochemical properties of liquid fuels are described

below [75]:

Density is the mass-to-volume ratio of fuel at 15ºC and is useful for measuring

the volume of fuel delivered into a combustion chamber. Density is an important

measurement of the caloric value of liquid fuel injected into a combustion

chamber to provide the required output of power.

Caloric value is the amount of heat emitted by complete combustion of a fuel

and is measured in MJkg-1.

Specific heat is the amount of energy required to raise the temperature of 1 kg of

oil by 1ºC. Light oils have low specific heats and heavier oils have higher

specific heats.

Specific gravity is the ratio of fuel density to that of a reference substance,

normally water at 4ºC. Fuels with low specific gravity evaporate easily, and

have lighter fractions and burn more rapidly compared to fuels with high

specific gravity.

Viscosity is the internal resistance of a liquid to flow, depending on temperature.

Fuel with high viscosity is difficult to flow and requires a preheating system for

handling, storage, and satisfactory atomisation during combustion.

22

Vapour pressure is the pressure developed by a vapour in the condensed phases

(solid or liquid) in thermodynamic equilibrium at a given temperature in a

closed system. Types of molecules involved and temperature affect vapour

pressure. The intermolecular force between the molecules determines the vapour

pressure. If the bond is strong, vapour pressure is low and if the bond is weak,

vapour pressure is high. At high temperatures, molecules have enough energy

with which to escape from the liquid or solid, but at a lower temperature, fewer

molecules can escape.

Flashpoint is the temperature to which a fuel must be heated for the fume can be

ignited with an igniter.

Auto-ignition temperature is the minimum reactor/environmental temperature

required to ignite a vapour or gas in air without a flame or igniter.

The Cetane number measures the ignition delay of fuel, ranging from ignition of

methylnaphthalene (standard value of 0) to that of cetane (standard value of

100).

The heat of evaporation is the amount of energy that must be added to transform

a quantity of a liquid into a gas.

During combustion, liquid fuel is sprayed into the combustion chamber.

Atomisation, a process that separates the liquid fuel into droplets through external

force [76], increases the surface area of the fuel and this in turn increases the

evaporation rate and rate at which fuel blends with air [77]. The atomisation of liquid

fuel produces fuel droplets at various sizes, and the size and volatility of the droplets

govern the combustion. Droplet size is determined by fluid mechanical forces and

surface tension that pull and hold the liquid during atomisation [78].

23

Droplets require time to vapourise, in which a raised temperature must be introduced

and latent heat of vapourisation must be supplied to convert the liquid droplet into

the gas phase. Droplet lifetime is the time required for the droplets to vapourise and

varies according to the square of its initial radius. Long droplet lifetime may cause

incomplete combustion or unreacted fuel which leads to inefficiency of the

combustion. Maximum droplet size must therefore be controlled to minimise these

effects [79].

When fuel droplets are mixed with oxygen and injected into a combustion chamber

in the presence of heat, ignition occurs [80]. Ignition can be achieved by elevated

temperature (auto-ignition) or with an external igniter. Heat and pressure

subsequently increase and a flame develops [81]. Two important parameters before

and after ignition are ignition delay and burnout time.

2.3.1.1 Ignition delay

Ignition delay is the time between fuel injection and ignition and a key

physicochemical property of a combustible air mixture. During ignition delay, fuel

vapourises and mixes with air before ignition occurs. The delay links the pre-flame

process and the formation of flame.

The duration of ignition delay is determined by physical and chemical delays [81].

Physical delays occur during atomisation of the liquid fuel, vapourisation and mixing

with air. The quality of fuel atomisation is determined by the properties of the fuel,

such as density, viscosity, and surface tension. These properties are inversely

proportional to the Cetane number, H:C ratio, pour point, and specific combustion

heat [82]. Other physical factors that delay ignition include air temperature and

pressure, engine speed, and combustion chamber design.

24

Chemical delays are caused by slow chemical reactions at the beginning of

combustion [83]. Given that chemical reactions occur more rapidly at higher

temperatures, physical delays become longer than the chemical delays, resulting in

the mixing process determining the ignition delay. During ignition delay, radicals

and molecular products of combustion are formed, e.g water, hydrogen peroxide, and

carbon oxides [82]. Duration of ignition delay can therefore be determined based on

formation of these products. Fuel properties also affect ignition delay. The Cetane

number can be used to determine ignition quality of the fuel. The higher the Cetane

number, the shorter the delay of fuel ignition [81].

Ignition delay can determine the overall course of the combustion process. Prolonged

ignition delay increases fuel quantity, and the vapourised fuel injected into the

combustion chamber, and promotes more efficient mixture of fuel vapour and air,

resulting in explosive combustion followed by a rapid increase in pressure. However,

this type of combustion can cause a jet engine to fail or result in a knock in a diesel

engine. An ignition delay that is too short can lead to incomplete combustion,

formation of smoke, loss of power, and reduction of engine efficiency [82, 84].

2.3.1.2 Burnout time

Liquid fuels, particularly atomised droplets, experience burnout after

ignition. Burnout time is the time required for a droplet to burn from the

moment ignition begins to the end of combustion when the droplets are

completely evaporated and burnt. During combustion of a single droplet,

temperature of the heat absorbed by the droplet changes from the ambient

temperature to the temperature of the flame when the droplet is surrounded

by the flame, showing that the flame is the main heat source for evaporation

and combustion of the droplet [81]. Based on d2-law of droplet burning,

25

burnout time decreases with decreasing droplet size [81]. The smaller the

droplet, the more rapidly combustion occurs because there is less fuel to be

evaporated. Oxygen also affects burnout time, since the more oxygen

available, the shorter the burnout time [85].

2.3.1.3 Burning rate

The burning rate is a measure of the linear combustion rate of a fuel over time [81].

The rate is controlled by the interaction between the rate of loss of fuel mass and the

oxygen. In liquid combustion, mass refers to the droplet sprayed in the combustion

chamber. The loss of mass of the droplet is affected by the availability of oxygen

[86], that is determined by the rate of oxygen diffusion into the fuel surface, as

opposed to the chemical reaction between fuel and oxygen [87]. The burning rate is

also affected by thermal enhancement around the droplets. The higher the air

temperature in the combustion chamber, the higher the burning rate of the droplets

[81]. Fuel properties also affect the burning rate significantly. A simulation result

showed that the burning rate is most sensitive to liquid fuel density, vapour pressure,

and surface tension, due to effects of these properties on atomisation processes [88].

Overall, liquid fuel is advantageous due to its physical properties, but atomisation is

required for combustion. Atomisation can influence combustion through droplet

lifetime and uniformity of droplet size. Key parameters for studying the fundamental

aspects of liquid combustion include ignition delay time, burnout time, and burning

rate, all of which are useful for studying glycerol combustion.

2.3.2 Crude glycerol as a fuel

In the combustion stoichiometry of glycerol, glycerol and oxygen molecules are

broken down and rearranged to form molecules of carbon dioxide and water

(Reaction 2.6).

26

2C3H8O3 + 7(O2) → 6CO2 + 8H2O ............................................................... (R 2.6)

The presence of inert diluent nitrogen in the air would not change the chemistry, and

the coefficients are determined by considering atom conservation. Figure 2.4 shows

the calculated combustion product of glycerol considering the excess of oxygen. The

excess oxygen, related to percentage excess air, is the amount of oxygen that added

during combustion to ensure completion of combustion.

Figure 2.4 Flue gas composition of glycerol combustion.

Glycerol is a medium quality fuel for combustion in an engine [89]. Table 2.5 shows

that glycerol has low energy density (18 MJkg-1), one-third that of diesel (42.6 MJkg-

1) or gasoline (43 MJkg-1). Glycerol is difficult to pump and atomise due to high

viscosity, and tends to form larger droplets than diesel in practical combustion

systems [10]. Glycerol is difficult to ignite due to high auto-ignition temperature,

vapour pressure, and flash point. Despite these difficulties, pure glycerol has been

used as an alternative fuel for furnaces [90] and has been successfully burned in

modified compression ignition engines [11] and burners [91].

0 20 40 60 80 100

0

20

40

60

80

CO2

H2O N2

O2

Mas

s fra

ctio

n of

flue

gas

(% m

ole)

Excess of oxygen (%)

27

Table 2.5 Properties of crude glycerol and pure glycerol

Fuel property Crude glycerol Pure glycerol

Formula n/a C3H8O3

Molecular weight (gmole-1) n/a 92.09

Density (kgm-3) 1205 1261

High heating value (MJkg-1) 14.84 18

Boiling point (K) 403 563

Cetane number ~6.7 5

Heat of Vaporisation (MJkg-1) n/a 0.67

Viscosity (mPa.s) 1100-1300 1500

Auto-ignition temperature (K) 673 796

Flash point (K) 463 450

Specific heat capacity (Jmol-1kg-1) ~2.8 2.43

Thermal conductivity (Wm-1K-1) n/a 0.29

Adiabatic flame temperature (K) n/a 2201

Stoichiometric fuel/air ratio(w/w) n/a 5

Spalding number n/a 4

Combustion of glycerol at low temperatures (270–370ºC) produces the toxic

substance acrolein [11]. However, the amount of acrolein produced in glycerol

combustion is only slightly higher than that produced in natural gas combustion

(13.3ppb). Acrolein concentrations of the combustion of methylated glycerol,

demethylated glycerol, and technical glycerol are 16.5 ppb, <18 ppb, and 20.7 ppb,

respectively. The low acrolein concentration in glycerol combustion is due to the

high oxygen content of glycerol. Glycerol has 1.33 oxygen/carbon ratio, and the high

28

oxygen content suppressed soot formation, and therefore complete combustion can

be expected from glycerol.

Figure 2.5 shows the calculated adiabatic flame temperature of glycerol compared

with that of biodiesel, diesel, methanol, and methane. The maximum adiabatic

temperature that can be achieved by glycerol is lower than that for biodiesel or diesel

due to the low heating value.

Figure 2.5 Adiabatic flame temperatures of glycerol, biodiesel, diesel, methanol,

and methane.

Crude glycerol has the potential to be used as a fuel as it has average energy content

and is inexpensive. Using crude glycerol as fuel could be beneficial for generating

heat and power in biodiesel plants, but economic benefit as a fuel source for industry

is low due to the high cost of transportation. Using crude glycerol as a supporting

fuel for the biodiesel industry would optimise energy integration and save costs.

Crude glycerol is relatively inexpensive compared with other commonly used fuels

[51]. In 2011, the price of crude glycerol was 40–110 US$/tonne, ~20% of the price

of diesel (~$500 US$/tonne) [22]. Biodiesel production requires energy for

29

generating power and steam, and a portion of this energy could be provided by crude

glycerol combustion. The cost of steam generation for an 8000 tonnes/year biodiesel

plant is ~$30,000–$170,000 /year, depending on the feedstock and process used [22].

2.3.3 Glycerol combustion in compression ignition engines

Utilisation of glycerol in both spark and compression ignition engines has been

investigated. The low volatility and high viscosity of glycerol can cause serious

problems in spark ignition engines, but a mixture of glycerol and methanol can

improve performance in these engines [92].

Glycerol is commonly used in compression-ignition (CI) engines (diesel engines),

mostly as an additional fuel, such as oxyfuel and fatty acid glycerol formal ester

(FAGE). Oxyfuel derived from etherification of glycerol with tertbutyl alcohol and

isobutylene is a mixture predominantly comprising higher glycerol ethers [93].

Similarly, the reaction of glycerol with dimethoxymethane and triglyceride converts

glycerol into FAGE in the presence of an acid [93]. The oxyfuel and FAGE are

subsequently blended to form a combination diesel/biodiesel fuel.

Glycerol is an uncommon diesel engine fuel as it potentially causes clogging and

produces acrolein [11]. Stenhede (2008) believes that glycerol cannot be ignited or

burned in petrol or diesel engines [8]. However, more recent studies suggest that

pure glycerol can be successfully used in diesel engines by creating a unique engine

cycle [8]. In the proposed cycle, gas is used to preheat the glycerol, and once the

engine heats up, glycerol can be used exclusively as the fuel [8]. Resulting output of

emissions is low due to the high oxygen content of the glycerol [8]. By contrast, the

combustion of crude glycerol in a diesel engine produces fly ash, possibly derived

from impurities. Little information regarding these impurities is available in the

literature.

30

2.3.4 Glycerol combustion using various burners for utility boilers

Glycerol can be burned in burners or boilers as an exclusive or additional fuel for

power production. As an exclusive fuel, the glycerol flash point should preferably be

maintained below 373K to stabilise the combustion. As an additional fuel, glycerol

can be mixed with another fuel with a viscosity below 200 mm s-1 or alcohol at 3–

40wt%. In both cases, vapour pressure and flash point should be reduced [90].

Figure 2.6 shows the recently developed high-swirl refractory burner for glycerol

combustion [91]. The burner overcomes the challenges related to flame ignition and

glycerol stability during combustion [91] and therefore effectively burns pure

glycerol. Modification of the burner nozzle, notably the flow-blurring liquid fuel

injector, enables glycerol to be burned without the need for preheating [90].

Emission output from glycerol combustion using this burner is low, with low

amounts of NO× and CO, undetectable acrolein, and negligible amounts of

acetaldehyde [94].

Figure 2.6 The 7 kW refractory burner (left) and 82 kW refractory-lined furnace

(right) [91].

31

2.3.5 Glycerol in fuel blends

A fuel can be mixed with a liquid to create an alternative fuel to enhance fuel

performance, e.g. diesel has been blended with jatropha, karanja, mahua, linseed,

rubber seed, cottonseed, and neem oils [95]. Twenty percent of these vegetable oil

blends did not diminish engine performance [96]. Diesel has also been blended with

butanol, methanol, or ethanol to reduce emission [97].

Mixing glycerol with other fuels can improve the combustion characteristics of

glycerol [98]. Combustion of a pure glycerol-propanol mixture was clean, creating

little soot [99]. However, the addition of crude glycerol to fuel is impracticable due

to the resulting decomposition and polymerisation reactions [100]. Further research

into the use of crude glycerol blends as starters as supporting fuels is needed,

particularly regarding performance, emissions, and process optimisation. Research

directed towards improving burner design is also needed to meet the challenges

associated with glycerol combustion.

2.4 Summary and specific research objectives

An increase in biodiesel production has led to a concomitant increase in the

production of crude glycerol. Currently, biodiesel production relies on

transesterification with a homogeneous base catalyst since this process is

economically viable and kinetically efficient. However, this process produces low-

grade glycerol (crude glycerol) that contains impurities. The presence of these

impurities renders the crude glycerol unsuitable for use as feedstock in industrial

applications. Crude glycerol cannot be appropriately absorbed by the current market,

which requires refined glycerol. This has resulted in a drop in the price of glycerol

and environmental problems, e.g. poisonous impurities (methanol). Efforts to utilise

crude glycerol, such as pyrolysis, thermal cracking, gasification, and chemical

32

conversion may be expensive, produce unwanted secondary products, have low

product volume and value, or require large throughputs and may therefore not be

economically viable or practical.

Crude glycerol may potentially be used as cheap liquid fuel for heat and power

generation in biodiesel plants. Combustion of glycerol in compression ignition

engines and burners for boilers, and the blending of glycerol with other fuels, have

been investigated. Though glycerol can be used as a fuel, pure product is needed, and

the presence of impurities limits the utilisation of crude glycerol. Little is known

about the influence of these impurities on crude glycerol combustion, therefore

understanding the combustion characteristics of crude glycerol is essential if this by-

product is to be utilised as fuel in practical combustion systems.

The objective of this research is to investigate the effects of the main impurities,

including water, methanol, soap, and biodiesel, on the ignition and combustion

characteristics of crude glycerol. This research will provide a basis for the

development of technological applications for the combustion of crude glycerol and

therefore aims to:

Investigate the differences between the ignition and combustion characteristics

of crude glycerol and pure glycerol.

Compare combustion of crude glycerol with that of well-known fuels, e.g

biodiesel, diesel, and ethanol.

Systematically investigate the effects of the main impurities, including water,

methanol, soap, and biodiesel, on the ignition and combustion of crude glycerol.

Develop an understanding of the chemical kinetics of glycerol combustion in

both the presence and absence of impurities, using the biodiesel and methanol

models since these have both been widely investigated. It is anticipated that the

33

results of this research will enhance the understanding of the behaviour of the

impurities in the ignition and combustion of glycerol and provide a kinetic study

of combustion of a mixture of methanol-glycerol and biodiesel-glycerol.

34

Chapter 3 Research methodology, approach and

techniques

In this chapter, details of research methodologies, approaches and techniques

employed to achieve the objectives are provided, including descriptions of

experimental and modelling techniques, and experimental facilities.

3.1 Overview of research strategies

The experimental program focused on the effects of impurities on ignition and

combustion of a single droplet of glycerol and kinetic modelling of glycerol

impurities. Interrelated components of the research strategies employed in the

experimental study are represented schematically in Figure 3.1.

The presence of impurities differentiates crude glycerol from glycerol. Although

analysis of crude glycerol has provided information regarding inherent impurities,

evidence that these impurities influence combustion characteristics of glycerol is

lacking. To determine potential effects, a series of investigations was conducted on

crude glycerol combustion and comparisons were made with combustion of diesel,

biodiesel, and ethanol.

Analysis of crude glycerol showed the main impurities to be water, methanol,

biodiesel, and soap. The effects of each of these impurities were investigated further

using the single droplet combustion method, to determine the fundamental properties

of ignition and combustion characteristics of glycerol. The single droplet combustion

method is well-established method [101] but has limitations and these are discussed

below. The kinetic study provides a theoretical approach to confirm the experimental

35

data on ignition. The kinetic study was done for the well-established kinetic model,

including glycerol, methanol, and biodiesel.

Figure 3.1 Detailed experimental design and network

3.2 Single droplet combustion

3.2.1 Principles of single droplet combustion

Single droplet experimentation is a well-established method for studying the

fundamental aspects of combustion [102]. The technique evolved from transient

combustion to microgravity single droplet combustion to reduce buoyancy [103,

104]. This method has been applied to combustion of liquid/liquid fuel, i.e. single

ethanol/octane droplet [105], combustion of alkaline and kerosene [106], and

combustion of solid/liquid fuel [107].

Pure glycerol Crude glycerol

Glycerol utilisation

CombustionWater Methanol Biodiesel Soap

Mechanism of impurities effecting glycerol

combustion

Kinetic model Single droplet combustion

Glycerol, methanol, biodiesel combustion model

Kinetic of binary mixture of glycerol-methanol and glycerol-biodiesel

Kinetic of mixture of glycerol-methanol-biodiesel

Ignition and combustion mechanism

Ignition delay

Burnout time

Droplet size

Combustion rate

Determination of sodium in flame using flame spectrometry

Determination of solid residue

Crude glycerol properties

Crude glycerol impurities

Comparison of crude glycerol combustion to pure glycerol and other fuels

36

The single droplet combustion technique simulates the behaviour of a fuel droplet in

an engine during combustion by burning a single droplet in a controlled

environment. The technique was developed from observation of the phenomenon in

a combustion chamber, e.g., diesel engine, where fuel sprayed into the combustion

chamber formed small droplets that later reacted with oxygen. The assumption was

that the droplets behaved uniformly during combustion, and that investigation of a

single droplet would be beneficial for understanding the combustion properties of the

fuel.

The evaporation and combustion mechanism of a single droplet of fuel in a furnace

is illustrated in Figure 3.2 [81], showing that the droplet (Rd) evaporates radially

outwards. In the evaporation zone, the mass fraction of the droplet (YF,S) decreases

from the surface of the droplet to the reaction zone, and conversely, the fuel vapour

temperature increases from the droplet surface (Ts) to the flame temperature (Tf).

These phenomena occur because of the difference in temperature between the

droplet and the air inside the furnace. As the droplet heats up, the surface is

gradually evaporated, forming the evaporation zone. The vapours mix with air from

the air at a relatively high temperature. When conditions are ideal, ignition occurs,

and this is indicated by formation of the flame enveloping the droplet (Rf). Outside

the reaction zone (the flame), the oxygen mass fraction (Yo) gradually increases to

the level of air, and the temperature (Yf) decreases to furnace temperature.

3.2.2 Single droplet combustion apparatus

The different stages of single droplet combustion were recorded chronologically by

capturing images with a high-speed camera, and this enabled investigation of the

fundamental phenomena of combustion. Apparatus of the single droplet combustion

system included a furnace, a droplet suspension system, a step motor for delivering

37

the droplet, and a CCD camera equipped with a computer for capturing the images

(Figure 3.3).

Figure 3.2 Schematic a single droplet combustion indicating the radial distribution

of fuel mass and flame.

3.2.2.1 Furnace

The horizontal tube furnace, 600 mm long and 40 mm wide, was equipped with

heating elements wrapped around the tube and covered with fibreglass insulation. At

the centre of the furnace, a thermocouple was attached to the tube to estimate the

temperature. The digital temperature control device was placed on the furnace body

to regulate the heating element according to the thermocouple reading.

Figure 3.4 shows the construction of the furnace. The isothermal zone within the

tube was in the centre of the furnace. The isothermal zone was approximately 300

mm long, and this was long enough to cover the required area for droplet ignition

and combustion. Temperature decreased at the front and back of the tube due to loss

of heat to the environment.

39

Figure 3.6 shows the differences between the actual temperatures and measured

temperatures of the gas as a function of measured gas temperature. The temperature

difference is increase with furnace temperature. However, in the present research, the

temperature used was 1023K and has insignificant temperature different.

Figure 3.5 The temperature gradient of along the furnace

Figure 3.6 Differences between actual and measured temperatures as a function of

measured gas temperature, Tg is gas temperature and Tp is measured

temperature

0 500 1000 1500 2000 2500 30000

50

100

150

200

250

300

350

400

T g-T

p(K)

Tp(K)

41

capture images of the burning droplet to record changes in droplet size. Aided by a

step motor, the droplet was delivered to the centre of the furnace at a linear velocity

of 1 ms-1.

3.2.3 Analysis of ignition and combustion characteristics

Methods used to investigate combustion phenomena, droplet size, ignition delay

time, burnout time, and burning rate are described below.

3.2.3.1 Measurement of droplet size

Droplet size (ds) was measured using a backlit image technique that showed a clear

edge of the burning droplet during combustion. This technique is schematically

represented in Figure 3.7, where the burning droplet is supported by a silicon carbide

fibre. When a droplet (d0) is heated to the furnace set temperature (T), vapourisation

occurs, causing the vapour to diffuse and mix with air. Once the fuel mixed with

oxygen in the present of heat, ignition occurs, and a flame forms around the droplet.

Heat generated by the flame induces evaporation and the droplet continually

decreases in size [81].

Figure 3.7 Schematic representation of a burning droplet

The decrease in droplet size was measured from the droplet at the centre of the

furnace (to) to the end of combustion (tec), by calculating droplet size per frame from

42

the images captured during combustion. Droplet size can be calculated manually by

measuring the diameter of the droplet. However, since approximately 1000 useful

images are produced per experiment, manually plotting droplet diameter is time-

consuming and impractical. Therefore, a Matlab code was developed to measure

droplet size.

Figure 3.8 shows typical data collected by Matlab to measure droplet size. The

greyscale intensity shows the relevant peaks that correspond to the droplet and

flame. The equivalent droplet diameter was calculated using the following equation

[77]:

d = √(dmax)(dmin)23 ........................................................................................... (Eq. 3.1)

Following Eq. 3.1, the equivalent spherical diameter d was equated by measuring the

height of droplet 𝑑max and width of droplet 𝑑min.

The script analyses each frame of the images captured during the single droplet

combustion. The steps taken to approximate the droplet size are described below.

Figure 3.8 Pixel greyscale intensity

1. Selection of the area of the droplet

43

The captured image occasionally has inconsistent background colour, particularly in

the corners, and this may lead to a false reading being taken of the droplet and

background. However, the script allows users to select the droplet area, the relevant

area in which the droplet experiences combustion. When selecting this area,

choosing the working area provided in the script is important, as is reducing the error

of the calculated area.

2. Conversion of the selected area into greyscale

Each image was converted to greyscale and the threshold value technique was

applied to separate the droplet from the background, creating a binary image.

Threshold value refers to the intensity of the image. Figure 3.9 shows that the area

above a selected threshold value will be black, and areas below the value will be

white. Threshold value was set to 200, allowing droplet leeway from the surrounding

light. Modification of the threshold value provides for areas of higher or lower

intensities. When background exposure is low, threshold value has to drop below the

intensity of the droplet.

Figure 3.9 Binary image of droplet

3. Fibre removal

A shape detection tool was used for removing the fibre (Figure 3.10), leaving only

the droplet. By measuring the pixel of the remaining area, the droplet size could be

determined by using an equivalent circular diameter (ECD) and the inbuilt

44

MATLAB tools that measure the properties of the remaining regions in the binary

image.

Figure 3.10 Image of binary droplet separated from fibre

4. Measurement of fibre width

The width of the fibre, 0.142 mm, is a key to obtaining the equivalent unit of

conversion. Measurements found in pixels of the fibre can be converted to mm by

using the ratio of fibre width in pixels to width in mm. Figure 3.11 shows the

separated fibre.

Figure 3.11 Image of filament separated from droplet

5. Droplet size approximation

The droplet becomes ellipsoidal in shape on the fibre. Approximation of the

spherical diameter is shown in Equations 3.2 to 3.4.

Vsphere = Vellipsoid ............................................................................................... (Eq. 3.2)

43π (D

2)3= 43π (L

2× W2× H2) .................................................................................. (Eq. 3.3)

45

where L, W, and H are the length, width and height of the ellipsoid. The droplet

changes shape in the direction of the filament and gravity, therefore L and W are

equal and Equation 3.4 can be reduced to the following:

D = (L2H)13 ............................................................................................................ (Eq. 3.4)

The length and height of the droplet correspond to lengths of the minor and major

axes respectively (Figure 3.12). Droplet size in mm is converted from pixels to mm

using the ratio of the measured filament size (Equation 3.5).

DmmDpixels

= FilmmFilpixels

= 0.142Filpixels

................................................................................... (Eq. 3.5)

Figure 3.12 Major and minor axes of the droplet

6. Detection of the micro-explosion

In the hot air environment, the droplet may swell, experience a micro-explosion and

separate into multiple droplets (Figure 3.13). Since the code cannot distinguish

which droplet to calculate, the user is required to provide the corresponding number

for the droplet. Important properties of the region are subsequently measured and

stored.

46

Figure 3.13 Micro-explosion of the droplet with numbering for user input

3.2.3.2 Ignition delay time, burnout time and total combustion time

The single droplet combustion apparatus recorded the arrival of the droplet at the

centre of the furnace (to), the ignition (ti), and the end of combustion (tec). Figure

3.14 shows a post-process glycerol droplet at the centre of the furnace (a), an ignited

glycerol droplet surrounded by a flame (b), and the end of droplet combustion (c).

The approximation of ignition delay time (ti), burnout time (tb), and total combustion

time will be discussed in the next section.

Figure 3.14 Typical combustion of a droplet showing: a) glycerol droplet at the

centre of the furnace, (b) an ignited glycerol droplet surrounded with a

flame, (c) the end of droplet combustion

Ignition delay time, burnout time, and total combustion time of a burning droplet

were calculated from the images taken by the CCD camera that had an exposure time

1

2

47

of 1000 µs (60 fps) (Figure 3.14). Ignition delay time (ti) was the difference in time

from the moment the droplet arrived at the centre of the furnace to the moment the

flame first appeared. Burnout time (tb) was the time from ignition until completion of

the combustion process. Total combustion time (tc) was the period from the moment

the droplet entered the furnace until the combustion process was completed.

To investigate the ignition delay time of each mixture, a controlled starting point was

defined as the point at which the droplet first reached the centre of the furnace. The

frames captured until ignition were subsequently counted and converted to seconds

using the appropriate capture rate. The data were normalised against d02 to find an

ignition delay time in smm-2. The size of the droplet at t0 was used to normalise

ignition delay data. The normalised ignition delay time was calculated using

Equation 3.6.

Ignition delayNorm =Frames

Capture Rate×Do2 .............................................................. (Eq. 3.6)

3.2.3.3 Burning Rate

The images of backlit burning droplets were used to determine the burning rate.

Droplets were backlit by the halogen lamp, and image capturing speed was 200

frames per second during the combustion. Combustion time was determined from

the image frame rate. The burning rate (𝑘) was determined using the d2-Law of

droplet combustion by measuring the rate of droplet size reduction during the

combustion process [80]:

2( )sd dkdt

............................................................................................................. (Eq. 3.7)

48

Figure 3.15 The d2 behaviour of droplet vapourisation and combustion [81]

3.2.3.4 Flame emission spectroscopy

Flame emission spectroscopy, that uses single droplet combustion apparatus and a

spectrometer, quickly and efficiently determines the presence of sodium ions in the

flame (Figure 3.16). The furnace and droplet suspension system are described in the

previous section.

Figure 3.16 A schematic representation of single droplet combustion

experimentation apparatus equipped with flame emission spectrometer

The Black-Comet-XR spectrometer, produced by StellarNet, measured spectra in the

wavelength range from 400 to 750 nm and the resolution was 1.5 nm. Spectral

energy was transmitted to the monochromators through a fibre optic cable terminated

by a collimator. A 2048 pixel CCD sensor served as the detector of the spectrometer.

49

Sodium signals were recorded by the spectrometer from the arrival of the droplet at

the centre of the furnace until completion of combustion. Software used to control

the spectrometer was Spectra Wiz, with Episodic Data Capture (EDC) acquisition

mode. The EDC saved spectral over time, allowing some episodes/runs to be saved

continuously. The episodes were saved per 10 µs to ensure high precision spectral

imaging. At least ten experiments were carried out for each sample to ensure

repeatability.

3.2.3.5 SEM-EDS analysis

The SEM-EDS was used to analyse the solid deposit left on the tip of the silicon

fibre during single droplet combustion. The sample for SEM-EDS analysis was

prepared by loading a small quantity of the solid deposit onto an aluminium stud.

The sample was coated with 4 nm of platinum to reduce charging. The SEM-EDS

images were captured using a Zeiss 1555 VP-FESEM scanning electron microscope

operating at 15 kV.

3.2.4 Limitations and errors

Two main limitations of the experimental work include physical properties of the

single droplet combustion method and the potential error in the calculation.

3.2.4.1 Limitation of the single droplet combustion method

In a combustion chamber, i.e. diesel engine, fuel is sprayed to create smaller droplets

to increase the relative surface area. However, physical properties of the single

droplet combustion apparatus limit the ability of the apparatus to mimic the original

droplets in the chamber. Limiting physical properties include droplet shape and size,

the fibre, and the pressure. In the single droplet combustion method, droplet shape is

assumed to be spherical through neglecting buoyancy and gravitational force.

50

However, in the experiment, buoyancy occurred, and the droplet shape was oval, not

spherical.

Droplet size and velocity in the combustion chamber of the single droplet

combustion method present a physical limitation. Droplet size in the combustion

chamber is assumed to be constant and therefore in the single droplet combustion

method, a single droplet is collected for behaviour investigation. However, the

droplets undergoing atomisation vary in size. Droplet size was normalised in the

experimentation to reduce this limiting effect. During atomisation in an engine, the

droplet has a velocity inside the combustion chamber. However, in the single droplet

combustion method, the droplet was attached on the fibre.

The furnace is maintained at a normal pressure of 1 atm. This pressure may differ in

a diesel engine, but the condition is necessary to obtain the expected image history.

3.2.4.2 Potential errors in the droplet analysis script

The script is beneficial in allowing users to rapidly analyse images but some

limitations exist and these are described below.

1. Shape irregularity

The code assumes that the droplet is ellipsoidal but this is not always the case. The

heating effect of the air often changes the shape of the droplet, especially if the fuel

is a mixture of molecules with greatly different boiling points. If this is the case, the

liquid with the lowest boiling point will evaporate more readily, and if the liquid is

trapped inside the droplet for a short length of time, the droplet may ‘explode’, as per

the theory of micro-explosions. In this instance, the droplet undergoes a massive

swelling and sudden bursting (Figure 3.17).

51

Figure 3.17 Shape irregularities from droplet swelling

In this scenario, the droplet is clearly not ellipsoidal and derivation of the equivalent

spherical diameter is no longer valid, but this does not present a problem in

determining the burning rate or droplet behaviour. Shape irregularity typically occurs

before ignition, where the trend of the size rather than the exact value of the squared

diameter of the droplet is important, and is only marginally affected and can be read

easily from the displayed data. If shape irregularity does occur in the combustion

stage, then the d2 law of single droplet theory does not apply as the data are no

longer linear, and the burning rate cannot be determined, proving once again that the

trend, that is unaffected, is the only important aspect.

2. Reflected light

The light can reflect off the surface of the droplet and increase the intensity in that

area and hence the binary image does not recognise that part of the droplet. The three

possible outcomes (Figure 3.18) are described below.

a) The droplet is recognised as one droplet, the software fills in any holes, and the

final droplet measurements are accurate as the whole droplet is being measured.

b) The reflected light obscures the edge of the droplet, and a crescent shape is cut

out of the resultant measured shape, thus reducing the calculated droplet size.

1 2 3

4 5 6

52

c) The droplet is identified as two droplets, and the user is asked to identify which

droplet is being measured.

Figure 3.18 The reflected lights occur on the droplet surface where 1) the greyscale

image of the droplet, 2) the binary image from thresholding 3) the

binary image after filling techniques have eradicated holes; and 4) the

final measured droplet

As a result, the script measures the incorrect droplet size. However, this sometimes

reflects on the droplet size plot where there is a significant reduction in droplet size.

A manual test is required to verify the outlier data.

3. Error from filament width calculation

Calculation of the filament is important for determining droplet properties. The

filament calculation was made from the initial image that was in focus. Image

selection is important as all the properties are dependent on the measurement. Any

error resulting from an unstable or blurred image will be transferred to every droplet.

This value is used in the conversion of the measurements from pixels to mm using

Equation 3.5.

53

3.3 Single droplet combustion of glycerol

The ignition and combustion behaviour of glycerol including and excluding

impurities was studied using the single droplet combustion technique. Combustion

behaviour of crude glycerol was compared with that of pure glycerol, diesel,

biodiesel, and ethanol, to provide a clearer perspective of the usability of crude

glycerol in an engine since glycerol is not a common fuel. The effects of each of the

main impurities of crude glycerol were investigated separately, and related ignition

and combustion characteristics including ignition delay time, burnout time, and

burning rate, were investigated.

3.3.1 Comparison between ignition and combustion of crude glycerol and that

of biodiesel, diesel, and ethanol

Crude glycerol combustion was investigated and compared with that of pure

glycerol, biodiesel, diesel, and ethanol. Pure glycerol and ethanol were acquired

from Sigma-Aldrich (Perth Australia), crude glycerol and biodiesel were provided by

Wilmar International (Gresik, Indonesia), and petroleum diesel (Caltex No.2) was

obtained from a local service station. The properties of these fuels are shown in

Table 3.2.

The effects of the main impurities were studied using the single droplet combustion

technique. Crude glycerol contains 22.9–63v% free glycerol, 9.9–19.2v% methanol,

7.2–34.5v% water, 0–2.6wt% soap, 0–2.8wt% biodiesel, 0–7.0wt% glycerides, 0–

3.0wt% free fatty acids (FFA), and 2.7–3wt% ash [9]. Impurities investigated

include water, methanol, biodiesel, and soap.

54

Table 3.2 Thermodynamic properties of crude glycerol compared to those of other

fuels

Fuel property

Fuels

Crude

glycerol*

Pure

glycerol

Biodiese

l Diesel

Ethano

l

Density @293K (kgm-3) 1205 1261 878 820-

850 789

High heating value (MJkg-1) 14.84 18 42.2 44.8 29.7

Boiling point (K) 403 563 588–630 433–

633 350

Cetane number ~6.7 5 46–51.7 47.4–

63.9 12

Heat of vapourisation

(MJkg-1) n/a 0.67 0.30 0.34 0.92

Viscosity @313K (mPa.s) 1100–

1300 1500

3.36–

3.68

2.2–

2.6 1.14

Auto-ignition temperature

(K) 673 796 646 588 638

Flash point (K) 463 450 422 328 282

Specific heat capacity

(J mole-1 kg-1) ~2.8 2.43 2.2 1.8 2.72

Thermal conductivity

(Wm-1K-1) n/a 0.29 0.15 0.12 0.20

Adiabatic flame temperature

(K) n/a 2201 2291 2413 2078

Stoichiometric fuel/air ratio

(w/w) n/a 5 11 12 9

Spalding number n/a 4 9 8 3

Vapour pressure at 323K (Pa) n/a 0.03 0.047 135 54800

55

3.3.2 Effect of water on glycerol combustion

The effect of water on glycerol combustion was investigated by adding pure

glycerol, purchased from Sigma-Aldrich, with distilled water. The distilled water

was added to pure glycerol at 5–20% by volume. The addition of water to the pure

glycerol was based on the proportion of water occurring in crude glycerol.

3.3.3 Effect of methanol on glycerol combustion

The effects of methanol on droplet combustion were investigated by adding

methanol to pure glycerol. The pure glycerol and methanol were purchased from

Sigma-Aldrich. Methanol was added to glycerol at 5%, 10%, and 15% by volume

according to the quantity of methanol in crude glycerol, ranging from 2.2% to 12.6%

by volume [9]. A high proportion of methanol-to-glycerol was intentionally used to

enhance the effects of methanol in glycerol combustion due to difficulties of

capturing the dim flame characteristics of methanol using the camera.

3.3.4 Effect of biodiesel on glycerol combustion

Biodiesel was added to pure glycerol at 1–3% by volume, according to the quantity

of biodiesel occurring in crude glycerol. Since biodiesel and glycerol are immiscible,

Tween 40 was added to the mixture as an additional surfactant at 0.1% v/v in each of

the glycerol-biodiesel samples. The mixture was homogenised at 1500 rpm for 10

minutes. During the single droplet combustion tests, a stirrer which consistently runs

at 300 rpm was also used to maintain the homogenous nature of the pure glycerol-

biodiesel mixture sample.

3.3.5 Effect of soap on glycerol combustion

Sodium hydroxide (NaOH) is used as a base catalyst for biodiesel production and

results in the formation of soap in crude glycerol. In the trans-esterification process,

56

excess NaOH reacts with free fatty acid from oil or fat in the presence of water,

producing soap as an unwanted by-product. The content of soap in the crude glycerol

can be up to 20wt% [9].

Pure glycerol used in the experiment was purchased from Sigma-Aldrich, and soap

was made as follows: 5.08 g NaOH was dissolved in 15 g water and the mixture was

poured into a flask containing 8.5 g olive oil that was heated for 15 minutes to 60 º

C. The general formula of the soap was NaCOOR1, where R1 was the fatty acid from

the olive oil, the compositions of which are listed in Table 3.3. The soap was added

to the pure glycerol at concentrations of 1%, 3% and 5% by weight. The

corresponding sodium contents in the mixtures were 0.1, 0.3, and 0.5wt%,

respectively.

Table 3.3 The compositions of the olive oil used in the present study

Fatty acid in olive oil

Concentration (%)

Lauric acid C12 saturated 0

Myristic acid C14 saturated 0

Palmitic acid C16 saturated 11

Stearic acid C18 saturated 2

Oleic acid C18 monounsaturated 78

Linoleic acid C18 di-unsaturated 10

Linolenic acid C18 tri-unsaturated 0

3.4 Kinetic studies of mixtures of glycerol with methanol and

biodiesel

Limited availability of kinetic modelling narrowed investigation of the kinetics of

selected impurities on glycerol combustion to the glycerol-methanol and glycerol-

57

biodiesel mixtures. The chemical model was based on research from the literature

and modelling was performed using Chemkin Pro software.

3.4.1 Chemkin Pro and kinetic modelling

To understand the kinetic effects of methanol and biodiesel on the ignition and

combustion characteristics of glycerol, detailed kinetic modelling was undertaken,

based on information in the literature. Chemical kinetic modelling was performed

using Chemkin Pro, commercial software developed by ANSYS Release 15151 (18

January 2016) with licence number 428841.

Combustion is initiated by the production of radical species after a short delay, and

followed by reactant consumption, and an increase in temperature [109]. The OH

radical occurs in most of the significant reactions in the model. Therefore, the time at

which the production of OH radicals peaked was considered the ignition time. The

use of maximum production of OH radicals as ignition time is supported by research

[110].

The simulation of the model was based on the combustion condition in the single

droplet combustion (Table 3.4) and ignition delay was calculated. Ignition delay time

was considered the time required for the mixture to achieve the maximum

concentration of OH. Figure 3.19 is a schematic representation of the ignition delay

as simulated by Chemkin Pro. The model used for computing the ignition delay is

adiabatic model. The simulation also identified the dominant reaction rate, that was

calculated from the consumption of the fuel (methanol and biodiesel), and the

sensitivity analysis based on the temperature gradient. The sensitivity analysis

measured the reactions that had a significant impact on the process if the temperature

increased or decreased.

58

Table 3.4 The simulation condition

Simulation operating condition value

Temperature 1023 K

Pressure 1 atm

Simulation time 0.1 s

Equivalence ratio 1.0

Figure 3.19 Schematic representation of the ignition delay time as simulated by

Chemkin Pro

3.4.2 Reaction mechanisms and kinetic simulations for of glycerol-methanol

mixtures

The glycerol-methanol model was built from a combination of the glycerol kinetic

model with either the methanol or biodiesel kinetic models. The glycerol kinetic

mechanism [53] consists of over 300 species and 7000 reactions. The methanol

combustion mechanism consists of 32 species and 41 reactions [111]. The model

calculated the mole fraction evolution of the species involved in the ignition.

61

Chapter 4 Properties and combustion characteristics of

crude glycerol

The combustion of pure glycerol is inherently challenging due to the properties of

glycerol, such as the high density, viscosity, and auto-ignition temperature.

Combustion of crude glycerol is complicated by the presence of impurities, and these

are costly to remove. Properties of crude glycerol, particularly in the sample received

from Wilmar International, are discussed in this chapter. Ignition and combustion

characteristics of crude glycerol are compared with those of several well-studied

liquid fuels, including biodiesel, diesel, and ethanol. Combustion characteristics

investigated include ignition delay time, burnout time, and burning rate.

4.1 Properties of crude glycerol

Table 4.1 shows a comparison between the properties of crude glycerol from the

Wilmar International sample and vegetable oil [9]. The Wilmar International crude

glycerol sample contained nearly 70% pure glycerol, 23% water, and a small

proportion of other impurities.

The properties of crude glycerol vary according to the transesterification feedstocks

used and processes applied, as discussed in Chapter 2. The impurities can be divided

into two groups, including those from the reactants, catalyst, and by-products of

transesterification as well as side reactions. Transesterification requires oil, alcohol,

and a catalyst, the excess of which becomes impurities, including water from the oil.

Reactions occur between the catalyst (particularly the base catalyst) and oil in the

presence of water from soap.

62

Table 4.1 Properties of crude glycerol

Crude glycerol from

Wilmar International

Crude glycerol from

vegetable oil

Composition (wt%) (wt%)

Free glycerol 68.2 22.9–63

Water 22.6 6.5–28.7

Methanol 2.2 6.2–12.6

Soap 1.3 0–26.5

Fatty acid methyl ester

(FAME) 0.5 0–28.8

Free fatty acids (FFA) 1.8 0–3.0

Ash 3 2.7–3

Elemental Analysis

C 31.85 24–54

H 10.25 10–12

N 0.32 0.3–1.2

Na 0.76 1.1–1.9

P 0.11 0.03–0.2

4.1.1 Water in crude glycerol

Water in crude glycerol originates from oil feedstock and the biodiesel separation

process. Biodiesel manufacturers use cheap oils to reduce the cost of biodiesel

production [35]. The low quality raw materials, such as waste cooking oils, animal

fats and non-edible oils, contain high concentrations of water. Water is also added

during purification processes in biodiesel production to wash the biodiesel, removing

63

catalyst, soap, and traces of glycerol, and this water remains in the crude glycerol

[35].

4.1.2 Methanol in crude glycerol

Methanol is an alcohol that is commonly used in transesterification. Methanol reacts

with oils or fats to form alkyl esters (biodiesel) and glycerol with the addition of

catalyst. Large amounts of methanol are used in transesterification, resulting in high

concentrations in crude glycerol. A 6:1 alcohol-to-oil ratio for the batch process [23]

and a 9:1 ratio for the continuous process are considered optimal for

transesterification [30]. A high methanol-to-triglyceride molar ratio of 42:1 is

common in transesterification using the supercritical methanol method, a

simultaneous process of etherification of fatty acids and rapid transesterification in

the absence of a catalyst [30].

4.1.3 Soap in crude glycerol

Excess alkaline catalyst in transesterification reacts with free fatty acid from oil/fat

in the presence of water to produce soap [114]. The soap is stored in crude glycerol

as a by-product of biodiesel production, where the concentration can be up to 20wt%

[9].

Soap causes problems during separation of biodiesel from crude glycerol, by

decreasing the yield of biodiesel [115]. During separation, soap reduces interfacial

tension and stabilises the emulsion of the water-biodiesel mixture. This results in the

dispersion of glycerol molecules into biodiesel due to the miscibility of the water-

glycerol mixture [116].

64

4.1.4 Biodiesel in crude glycerol

Biodiesel is the main product of transesterification, and the production process is

described in detail in Section 2.3.2. Biodiesel is a renowned fuel substitute for diesel

that consists of straight chain fatty acids ranging from C16 to C18, depending on the

oil feedstock used.

Inadequate separation of biodiesel from crude glycerol during transesterification

results in the presence of unrecovered biodiesel in crude glycerol. Crude glycerol

contains 0–20% biodiesel, depending on the quality of the separation process.

Impurities in crude glycerol, including water, methanol, biodiesel, and soap,

originate from oil/fat feedstock and the transesterification process. Ignition and

combustion behaviour of crude glycerol was compared with that of pure glycerol, to

gain an understanding of the combustion of crude glycerol, and this is described in

the following section. Comparisons were also made with other well-known fuels,

including biodiesel, diesel, and ethanol.

4.2 Ignition and combustion characteristics: comparison of crude

glycerol against pure glycerol, petroleum diesel, biodiesel, and

ethanol

4.2.1 Combustion phenomena of crude glycerol

Figure 4.1 shows typical time-sequenced non-backlit images of the combustion of

droplets of crude glycerol and other fuels. The bright colour represents the flame.

Images from left to right show the combustion sequence, from the moment the

droplets arrived at the centre of the furnace until completion of combustion. At

certain times, particularly concerning pure glycerol and ethanol, the flame was too

65

weak to be visible. In these instances, images were post-processed to identify the

formation of the flame.

Figure 4.1 Typical time-sequenced images of burning droplets of crude glycerol

(CG), pure glycerol (PG), biodiesel (BD), diesel (DS), and ethanol (ET).

Image 1 shows the droplet at the centre of the furnace; Image 2 shows the droplet at

ignition; Images 3 and 4 show the droplets during combustion; and Image 5 shows

the end of combustion when the droplet has evaporated. Upon arrival at the centre of

the furnace (Image 1), droplet sizes were similar. Upon ignition (Image 2), flames

were formed around the droplets. The flame of crude glycerol was bright yellow-

greenish, suggesting that soot particles were formed. Conversely, the flame of pure

glycerol was dim, with fewer soot particles visible compared with the crude glycerol

68

Ignition delay time results from pre-combustion reactions in the fuel/air mixture,

comprising a physical delay (droplet heating, evaporation, molecular diffusion, and

mixing with air), and a chemical delay (subsequent chemical reaction of the gas

phase between fuel and oxygen) [119]. The ignition delay can also be correlated with

the Cetane number of the fuel, with higher Cetane number affording a shorter

ignition delay [120]. The Cetane number of pure glycerol is 5 (Table 3.2), which is

lower than that of biodiesel (51.7), diesel (47.4–63.9), and ethanol (12). The Cetane

number for crude glycerol (6.7) was slightly higher than that of pure glycerol. The

Cetane numbers of these fuels were consistent with the result of the ignition delay

times measured with the single droplet combustion method. Glycerol, with a low

Cetane number, required more time to ignite, whereas the other fuels ignited faster.

The ignition delay time of the crude glycerol was slightly longer than that of pure

glycerol, particularly at higher temperatures, possibly due to the influence of

impurities including water, methanol, sodium, and FAME. Water has the ability to

significantly delay the ignition time of glycerol droplets [85]. Water would evaporate

from the surface of the droplet before glycerol would evaporate, and this could be

attributed to the low boiling point. The water vapour (steam) diluted the glycerol/air

mixture in the gas phase. Therefore, more time would be required for the ignition to

occur when more water had accumulated in the boundary layer of the droplet,

resulting in a delay in the ignition of the glycerol droplets. Within the crude glycerol

droplets, water would slow the increase in temperature by increasing the heat

capacity of the droplets, since the heat capacity of water is 4200 Jkg-1K-1, and this is

much higher than that of glycerol (2400 Jkg-1K-1). Also, water in hydrocarbon fuels

is known to suppress the ignition [121]. Similarly, the water content in the crude

glycerol would delay the ignition, as observed in the experimentation. On the

70

was the shortest among the fuels tested. This suggests that the impurities in crude

glycerol changed the heat of vapourisation of glycerol, particularly biodiesel, which

has half the amount of heat of vapourisation of glycerol. A physical factor, such as

better atomisation due to the presence of water and methanol, may shorten the crude

glycerol burnout time.

4.2.4 Burning rate

Temporal variation curves of droplet size (Figure 4.2) were linearly fit to obtain

burning rates of the five fuels following d2-law. Theoretical burning rates (k) of the

droplets at 1023 K were also calculated, based on the classic d2-law [81]:

,

8ln(1 )g

cp g

k Bc

..................................................................................................... (Eq. 4.1)

,, ( )o c

p g s

Y QC T T

BH

............................................................................................ (Eg. 4.2)

where λg is the thermal conductivity of air, cp,g is the specific heat of the air/vapour

mixture, B is the Spalding number, ρ is the fuel density, Yo, is the ambient oxygen

mass fraction, σ is the stoichiometric oxidizer to fuel mass ratio, T is the ambient

temperature, Ts is the droplet surface temperature, QC is the heat of combustion of

the liquid fuel, and H is the effective latent heat of vaporization.Figure 4.5 shows

consistency between the theoretical and experimental burning rates of the five fuels,

and only small deviations observed. These deviations are expected due to

experimental errors, simplification of the model, complexity of droplet combustion

processes, and effects of the fibre [123]. Figure 4.5 shows that the burning rate of

pure glycerol was lower than those of diesel, biodiesel, and ethanol. With reference

to Equation 4.1, the burning rate was mainly influenced by the density and Spalding

transfer number of the fuel, since the thermal conductivity and heat capacity of the

72

methanol and water) have a higher latent heat of vapourisation than glycerol. Due to

the differences in latent heat, the temperature would increase more rapidly in

glycerol than in methanol and water. However, the vapour pressures of methanol and

water are higher than that of glycerol. Consequently, methanol and water would

vapourise more rapidly than glycerol at the surface of the droplet, leading to the high

burning rate of crude glycerol that was observed.

The other impurities, including FAME, sodium, and ash may also affect the burning

rate of glycerol. For example, the burning rate of FAME is slightly higher than that

of glycerol (Figure 4.5). The addition of sodium to glycerol may enhance the

propensity for soot formation, that will, in turn, affect the burning rate [124].

However, the effects of these impurities on the burning rate would be very limited

since these have very low concentrations in crude glycerol (Table 4.1).

4.3 Summary

The ignition behaviour and combustion characteristics of crude glycerol were

investigated. The ignition delay times, burnout times, and burning rates of single

droplets of crude glycerol, pure glycerol, petroleum diesel, biodiesel, and ethanol

were investigated experimentally and compared. The following conclusions can be

made:

The main impurities of crude glycerol are water, methanol, biodiesel, and soap.

For all the fuels investigated, ignition delay and burnout times decreased, while

the burning rates increased with increasing air temperature. At a given

temperature, ignition delay and burnout times decreased in descending order

from pure glycerol, through crude glycerol, ethanol, and biodiesel, to diesel.

73

Crude glycerol had the highest burning rate, while pure glycerol had the lowest

rate among the fuels studied. Impurities had a profound influence on the

combustion characteristics of crude glycerol.

Burnout times of droplets of the fuels tested decreased in descending order from

pure glycerol, through ethanol, biodiesel, and diesel, to crude glycerol.

Differences in the burnout times could be attributed to properties of the fuel,

particularly latent heat, boiling point, vapour pressure, and micro-explosion.

Impurities changed the combustion characteristics of crude glycerol when compared

with those of pure glycerol. However, the roles of each of the main impurities in

crude glycerol combustion require further investigation, and hence the effects of

water, methanol, biodiesel, and soap will be discussed in Chapters 5–8.

74

Chapter 5 Effect of water

Water is present in crude glycerol, not only because of the natural properties of the

oil or fat, but also from the elementary reactions of alcohol and the catalyst during

transesterification. In this chapter, the effect of water on the ignition and combustion

characteristics of glycerol are discussed.

5.1 Properties of glycerol-water mixtures

Glycerol has three hydrophilic alcoholic groups that form intramolecular and

intermolecular hydrogen bonds, and these are responsible for the solubility of

glycerol in water. In Table 5.1, the relevant physical and thermodynamic properties

of glycerol, water, and mixtures between these are listed. The density, heating value,

and boiling point of the mixture decreases with the addition of water, but thermal

conductivity increases with the addition of water. These variations in properties

changed the combustion behaviour of the water-glycerol mixture, and this will be

described in the next section.

Table 5.1 Key physical and thermodynamic properties of glycerol, water, and

glycerol-water mixtures (GW) [127]

Property Pure glycerol Pure water GW 10% GW 20%

Density (kg m-3) 1261 1000 1235 1208

High heating value (MJ kg-1) 18 0 16.2 14.4

Boiling point (K) 563 373 411 394

Heat of vapourisation (kJ mol-1) 91.7 43.99 n/a n/a

Specific heat capacity (J mol-1kg-1) 221.9 4.18 n/a n/a

Thermal conductivity (mW m-1K-1) 289 608 292 326

75

5.2 The role of water in combustion

Water is not only a by-product, but also actively involved in combustion. In classic

combustion theory, combustion of fuel produces carbon monoxide and water [81]. In

liquid fuel droplet combustion, water initiates and drives internal circulation in the

droplet by enhancing the droplet evaporation due to its lower volatility [125]. Water

could also diffuse back to the droplet surface and decrease the flame temperature,

causing the flame to be extinguished [126].

Water has been studied as a fuel blend of liquid fuel, particularly diesel. Water may

not contain energy that can directly contribute to combustion, but the addition of

water can reduce emissions [128]. Water significantly reduces nitrogen oxides (NO

and NO2), carbon monoxide (CO), black smoke, and particulate matter [128].

Adding 15% water to diesel reduces NO× emissions by up to 35% [129]. The

addition of water to a fuel also increases combustion efficiency. In a water-oil

emulsion, water reduces the interfacial tension, resulting in finer atomisation of the

fuel during injection [130]. Fine droplet dispersion during atomisation increases the

surface area of the droplet for contacting air.

5.3 Single droplet combustion study of a pure glycerol-water

mixture

5.3.1 Combustion phenomena of pure glycerol-water droplets

Figure 5.1 shows the time-sequenced images of burning glycerol droplets with and

without the addition of water at initial droplet sizes (t0) of approximately 1 mm.

Initially, the general ignition and combustion processes for the glycerol with and

without the addition of water were similar. Before ignition (ti), bubbles formed

inside the droplet, causing distortion and expansion of the droplets. The bubbles

77

Figure 5.2 illustrates temporal variations of the squares of the normalised droplet

diameters (d/d0)2 for glycerol droplets with various water concentrations after

ignition. Droplet size decreased linearly with time, implying that combustion

complied with the d2-law [81]. The droplet size reduction shown in Figure 5.2 is

caused by evaporation during combustion process. However, droplet size fluctuated,

and the intensity was stronger when the water concentration in the glycerol

increased. This fluctuation can be attributed to formation of bubbles and collapse of

the droplets (Figure 5.1).

Figure 5.2 Temporal variations of the square of the normalised droplet diameters of

glycerol with different water concentrations

5.3.2 Ignition delay time

Figure 5.3 shows the effect of the addition of water to the ignition delay time of the

glycerol droplets. Ignition delay time increased as more water was added to the

glycerol. Ignition delay time is influenced by droplet heating and evaporation,

78

molecular diffusion, and mixing with air and the subsequent chemical reactions in

the gas phase between fuel and oxygen.

Figure 5.3 Ignition delay time of glycerol droplets with and without the addition of

water, and crude glycerol

Water evaporated at the beginning of combustion before ignition, since the boiling

point is lower than that of glycerol. The water vapour can suppress the chemical

reaction rate of the hydrocarbon-air mixture as this absorbs heat under certain

conditions [121], leading to a longer ignition delay of the glycerol-water droplet

combustion. The endothermic thermal resistance of water vapour form an energy

barrier that prevents the penetration of heat to the surface of the droplet [121]. This

causes a delay in evaporation of the fuel, and later reduces the accumulation of fuel

vapour on the surface of the droplet.

Figure 5.3 shows that the ignition delay time of crude glycerol is similar to that of

pure glycerol. Crude glycerol is known to contain water, that would delay the

ignition. However, crude glycerol also contains other impurities, such as biodiesel

and methanol, that can accelerate the ignition.

PG GW5 GW10 GW15 GW20 CG0.0

0.5

1.0

1.5

2.0

2.5

Igni

tion

dela

y tim

e (s

)

Fuel

79

5.3.3 Burnout time

Figure 5.4 shows the burnout time of glycerol droplets with and without the addition

of water. The burnout time decreased with an increase in water concentration in the

glycerol droplets. However, reduction of the burnout time of the glycerol droplets

was less than 0.2 s from pure glycerol to 20% water in glycerol.

Figure 5.4 Burnout time of glycerol droplets with the addition of different

concentrations of water

The small reduction of burnout time of glycerol droplets with the addition of water

was due to the evaporation of water in the early stage of combustion. The

combustion mechanism of glycerol-water droplets, as explained in Section 5.3.1,

showed that most of the water evaporated prior to ignition, leaving glycerol as the

main component of the droplet. After ignition, the combustion was predominantly of

glycerol, with minimal water in the droplet.

The burnout time of crude glycerol is shorter than that of pure glycerol and water-

doped glycerol droplets. As discussed in Section 4.2.3, burnout time of crude

glycerol was shorter than that of pure glycerol and other fuels due to bubble


0.5

1.0

1.5

Bur

nout

tim

e (s

)

Fuel

80

formation and subsequent micro-explosion that assist atomisation of the crude

glycerol droplets.

5.3.4 Burning rate

Figure 5.5 shows the burning rates of glycerol droplets with different water

concentrations. According to the classic combustion theory of droplets [81], the

burning rate of fuel increases as the latent heat of evaporation, fuel boiling point, and

density decrease [81]. Table 5.1 shows that the addition of water to glycerol reduced

the boiling point, the heat of evaporation, and the density of glycerol. Addition of

water to the pure glycerol resulted in higher burning rates of the glycerol droplets,

and the burning rate increased as the water concentration increased. The addition of

water further enhanced the burning rates of the glycerol droplets by promoting

bubble formation and subsequent micro-explosion of the droplets during combustion

(Figure 5.1).

Figure 5.5 Burning rates of glycerol droplets with addition of different

concentrations of water


0.5

1.0

1.5

2.0

2.5

Bur

ning

rate

(mm

2 s-1)

Fuel

81

The addition of 5–20% of water increased the burning rates of the glycerol-water

droplets. Droplet size and burnout time affect the burning rate. At ignition, droplets

in the water-glycerol mixture decrease in size with the addition of water. This is due

to evaporation of water at the beginning of the combustion process, leaving smaller

glycerol droplet during ignition. The burnout time was also decreasing with water

addition.

The burning rate of crude glycerol is similar to that of the glycerol-water droplets at

10–20wt%. This is possibly related to the water concentration of the crude glycerol,

that is 22.6wt% (Table 4.1). Water comprises the highest proportion of impurities in

crude glycerol, and as such, may have a greater influence than the other impurities,

and this could possibly have affected the burning rate of crude glycerol.

5.4 Summary

The effect of water on the ignition and combustion characteristics of single droplets

of glycerol was investigated experimentally using the suspended single droplet

combustion technique. Water promoted the formation of bubbles within the droplets

before ignition. Bubbles led to the occurrence of micro-explosions that contributed to

the increase of the burning rates and the decrease of the burnout times of the glycerol

droplets. Water also increased the ignition delay time of the glycerol droplets.

The presence of water in crude glycerol can be disadvantageous or advantageous in

combustion. Water does not add energy to combustion; therefore, increasing the

proportion of water in crude glycerol reduces the total energy content of the fuel.

Water also increased the ignition delay time that could impair combustion

performance, such as in an engine. Conversely, the presence of water increased the

burning rate of glycerol and assisted atomisation due to bubble formation and micro-

explosion.

82

Further research of the utilisation of glycerol doped with water, particularly

regarding developing an understanding of the acceptable load of water in glycerol

burned in a diesel engine, is required. Such research will determine the trade-off of

ignition delays with burning rates of the glycerol droplets.

83

Chapter 6 Effect of methanol

As the simplest alcohol, methanol is a clean renewable fuel, and a common source of

energy [131]. Combustion of methanol has been widely investigated, and includes

aspects such as the mechanism of combustion [132], methanol extinction during

combustion [133], activation energy during extinction [126], and theories of

methanol droplet combustion [134]. The utilisation of methanol as a fuel blend has

been investigated, where methanol has been used as an additional oxygen additive in

diesel engines [135].

In this chapter, the effect of methanol in glycerol droplet combustion is discussed.

Methanol is used extensively in the transesterification process for producing

biodiesel, and this result in large quantities of methanol being dissolved in crude

glycerol.

6.1 Properties of glycerol-methanol mixtures

Table 6.1 shows a comparison between the properties of glycerol and methanol.

Glycerol is a thick liquid of high density and burns at a higher temperature than

methanol. Conversely, methanol is a light liquid easily burnt at low temperatures and

it has low density. Methanol and glycerol are miscible [136]. An investigation of

glycerol-methanol as a binary mixture shows that the viscosity and density of the

mixture are temperature dependent [137].

Figure 6.1 indicates that the viscosity of the mixture decreases with increasing

temperature. At a set temperature, the viscosity of the mixture decreases with the

addition of methanol due to the viscosity of glycerol being much higher than that of

methanol.

84

Table 6.1 Chemical components and physical properties of glycerol and methanol

Properties Glycerol Methanol

Molecular formula C3H8O3 CH4O

Molecular weight (g/mole) 92.1 32.04

Density (kg/m³) 1260 791.8

Boiling point (K) 563 327.7 °C

Latent heat of evaporation (kJ mol-1) 91.7 1.3

Figure 6.1 Viscosity of the glycerol-methanol mixture at various temperatures: ▴

90% glycerol; ■ 80% glycerol; □ 70% glycerol; Δ 60% glycerol; and ○

50% glycerol [137]

The density of the mixture followed the same pattern as shown in Figure 6.2.

Glycerol and methanol have different densities with maximum values of 1.260 kg m-

3 and 0.791 kg m-3, respectively. As expected, the density of the glycerol-methanol

mixture decreased with increasing temperature. At low temperatures, the density of

the mixture increased due to a motionless molecule that contributed to the compact

nature of the mix. At constant temperature, the density of the mixture decreased with

85

an increasing methanol proportion in the mixture. The specifications of glycerol and

methanol are listed in Table 6.1.

Figure 6.2 Density of the glycerol-methanol mixture at various temperatures: □ 90%

glycerol; ○ 80% glycerol; Δ 70% glycerol; ■ 60% glycerol; and ▴ 50%

glycerol [137]

6.2 Single droplet combustion study of a pure glycerol-methanol

mixture

6.2.1 Combustion phenomena of pure glycerol-methanol droplets


without the addition of methanol. Image 1 indicates the moment at which the

droplets arrived at the centre of the furnace; Image 2 shows bubble/expansion of the

droplets; Image 3 shows a micro-explosion; Image 4 shows ignition of the droplets;

and Images 5 and 6 show the course of burning droplets towards completion of

droplet combustion.

Glycerol droplets with methanol added were large upon arrival at the centre of the

furnace than the initial droplet size due to the expansion of the methanol during

insertion (Figure 6.3). Upon arrival at the centre of the furnace, the droplet was

86

expected to grow larger with a subsequent micro-explosion (Images 2 and 3). When

ignition occurred (Image 4), expansion of the droplet ceased, and flames formed

around the droplet. Combustion continued until completion (Images 5 and 6).

Figure 6.3 Typical images of burning glycerol droplets with various methanol

concentrations: pure glycerol (PG), and glycerol-methanol mixture (GM)

Figure 6.4 illustrates the temporal difference of the square of the normalised droplet

diameter (d/d0)2 for pure glycerol with and without the addition of methanol. There is

a variation of the normalised square of droplet diameters, particularly for the

glycerol-methanol mixtures. Variations in droplet size were not linear due to the

micro-explosion of the pure glycerol-methanol droplets. The d2-law was followed

approximately after an initial heating period.

The micro-explosion phenomena of pure glycerol-methanol droplets were similar to

those of the pure glycerol-water droplets, as described in the previous Chapter.

Methanol, that is more volatile and has a lower boiling point than that of pure

glycerol, was trapped within the glycerol droplets during the combustion process.

88

enough to reduce the ignition delay of crude glycerol, but more water was present in

the crude glycerol to delay the ignition. This was due to the low concentration of

methanol in crude glycerol, which was only 2.2wt%.

Figure 6.5 Effect of the addition of methanol on the ignition delay time of glycerol

combustion

6.2.3 Burnout time

Burnout time of glycerol decreased substantially with increasing addition of

methanol (Figure 6.6). Pure glycerol required 1.61 s to burn completely, while with a

5% addition of methanol, the burnout time was only 0.75 s, and the burnout time

decreased with increasing addition of methanol. Short burnout times of the

methanol-glycerol droplet mixtures compared to those of pure glycerol droplets

could be attributed to the micro-explosions, that later increase the burning rates.

During combustion of glycerol-methanol mixtures, micro-explosions occur, which

leads to the disintegration of droplets into smaller ones and therefore the total

burning time is reduced.

PG GM5 GM10 GM15 CG-0.5

0.0

0.5

1.0

1.5

Igni

tion

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)

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89

The burnout time of crude glycerol was shorter than that of pure glycerol and

similar to that of the glycerol-methanol mixture (Figure 6.6). The presence of

methanol contributed to the reduction of the burnout time of crude glycerol, with

respect to other impurities contained in the crude glycerol. Droplet expansions and

subsequent micro-explosions occurred in the crude glycerol mixture due to the

difference in the boiling points of the impurities, and this could assist atomisation of

the crude glycerol, leading to a shorter burnout time.

Figure 6.6 Burnout time of glycerol with the addition of different concentration of

methanol

6.2.4 Burning rate

The addition of methanol increased the burning rates of glycerol droplets (Figure

6.7), likely due to the micro-explosion and reduction of the latent heat of evaporation

of the fuel mixtures.

The more methanol added, the stronger the micro-explosion, and this reduced droplet

lifetime two to three times more rapidly [139]. According to the classic combustion

theory of droplets, the burning rate of a fuel increases as the latent heat of

PG GM5 GM10 GM15 CG-0.5

0.0

0.5

1.0

1.5

Bur

nout

tim

e (s

)

Fuel

90

evaporation and boiling point decrease. Methanol has a lower latent heat of

evaporation and boiling point than that of glycerol [140]. Methanol also has the

ability to increase the combustion rate by having a close flame position in droplet

combustion compared to other hydrocarbon fuels in similar combustion conditions

[141], that in turn provide more heat for the droplet from the flame.

Figure 6.7 Burning rates of glycerol with additions of different concentration of

methanol

The improved combustion process could be attributed to the following factors: the

lower viscosity of methanol reduces the overall viscosity of the fuel blend, leading to

a more rapid evaporation, better mixing of fuel vapour and air, and more complete

combustion. In addition, the high oxygen content of methanol helps the blended fuel

to burn more efficiently, thereby increasing the combustion efficiency.

In Figure 6.7, there is also a comparison between the burning rate of crude glycerol

with those of pure glycerol and the glycerol-methanol mixture. Though the

concentration of methanol in the crude glycerol is low, the presence of methanol,

PG GM5 GM10 GM15 CG0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Bur

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(mm

2 s-1)

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91

along with other impurities, can contribute to the increased of burning rate of crude

glycerol compared to that of pure glycerol.

6.3 Summary

Methanol enhanced the combustion performance of glycerol. Methanol decreased the

ignition delay and burnout times and increased the burning rate of glycerol. This

effect became more marked with increasing addition of methanol. The addition of

15% methanol to glycerol reduced the ignition delay and burnout times to one-fifth

and one-third, respectively, and increased the burning rate fivefold. The addition of

methanol to glycerol droplets resulted in micro-explosions during the combustion

process, and this in turn improved the combustion performance of glycerol.

92

Chapter 7 Effect of biodiesel

In this chapter, the effect of biodiesel on the ignition and combustion of a glycerol

droplet are discussed. As the desired product of transesterification, biodiesel is

separated from the by-product, crude glycerol, at the end of the manufacturing

process. However, a small quantity of biodiesel might be trapped in crude glycerol as

a result of an incomplete separation process [25]. As one of the main impurities,

biodiesel influences the combustion characteristics of crude glycerol.

7.1 Single droplet combustion study of a pure glycerol-biodiesel

mixture

The thermochemical properties of pure glycerol and biodiesel are listed in Table 7.1.

Biodiesel has a lower density, experiences a higher heating rate, and evaporates and

ignites more rapidly than that of pure glycerol. These properties can affect the

combustion characteristics of the mixture of pure glycerol and biodiesel.

7.1.1 Combustion phenomena of glycerol-biodiesel droplets


without the addition of biodiesel at the initial droplet size of approximately 1 mm. In

Figure 7.1, images from left to right show the droplets from the moment of arrival at

the centre of the furnace until completion of combustion. Image 1 indicates the

moment at which the droplets arrived at the centre of the furnace; Image 2 shows the

moments of droplet ignition; Images 3 and 4 show the course of burning droplets;

and Image 5 shows completion of droplet combustion.

Upon arrival at the centre of the furnace, glycerol droplets with and without biodiesel

added were the same size (Figure 7.1, Image 1). However, droplets of different sizes

93

were observed (Image 2), particularly the pure glycerol-biodiesel droplets. At

ignition (Image 3), flames formed around the droplets.

Table 7.1 Thermodynamic properties of pure glycerol and biodiesel [142]

Glycerol Biodiesel

Density at 293 K (kgm-3) 1261 878

High heating value (MJkg-1) 18 42.2

Boiling point (K) 563 491–715

Latent heat of vapourisation at 373 K (kJkg-1) 706 450

Vapour pressure at 298 K (Pa) 0.03 135

Cetane number 5 51.7

Viscosity at 293 K (mPa.s) 1500 3.36–3.68

Auto-ignition temperature (K) 796 646

Flash point (K) 450 422

Specific heat capacity (Jmol-1kg-1) 2.43 2.2

Thermal conductivity (Wm-1K-1) 0.2900 0.1490

The flame of the pure glycerol droplet was greenish, suggesting a clean combustion,

and a very small quantity of soot particles formed. Oxygen moiety in the glycerol

may promote combustion, resulting in little or no soot formation. At the end of

combustion of the pure glycerol, the fibre tip was free of any soot or ash deposit.

Conversely, in the combustion of glycerol droplets with various additions of

biodiesel, two flame colours were observed, a red flame at ignition and a green flame

at the later stage of combustion. For example, in the case of 1% biodiesel in a pure

glycerol droplet, ignition (Image 3) started with reddish flames. The red colour was

mainly due to the presence of biodiesel in the flame, and colour intensity increased

94

as the biodiesel concentration increased (Image 3: GB2 and GB3). The intensity of

the red flame decreased as time elapsed (Image 5: GB1, GB2, and GB3). The red

flame was replaced by a green flame, indicating the combustion of pure glycerol that

continued until completion of combustion of the droplet.

Figure 7.1 Typical time-sequenced images of burning glycerol droplets and

additions of pure glycerol-biodiesel (GB) at 1023 K

Upon heating, biodiesel evaporated first, followed by glycerol. Biodiesel is more

volatile than glycerol due to the high vapour pressure. The latent heat of evaporation

of biodiesel (250 kJ kg-1) is also much lower than that of glycerol (706 kJ kg-1).

When the droplet was exposed to hot air, biodiesel at the surface of the droplet was

expected to evaporate first. Subsequently, biodiesel trapped inside the droplets would

96


Figure 7.3 shows the effect of the addition of biodiesel on the ignition delay of

glycerol droplets. Ignition delay time decreased with increasing addition of biodiesel

to the glycerol. As previously described, ignition was initiated by the combustion of

biodiesel. Figure 7.1 shows that when the ignition criterion was met, biodiesel

burned at the beginning of droplet combustion. Biodiesel has a lower latent heat and

much higher Cetane number (51.70) than pure glycerol (5). A high Cetane number

implies more rapid ignition [143] and explains why the ignition delay time decreased

with increasing addition of biodiesel.

Figure 7.3 also shows the ignition delay time of crude glycerol compared with that of

pure glycerol and the glycerol-biodiesel mixture. Though the presence of biodiesel

can reduce the ignition delay of crude glycerol, this delay remains similar to that of

pure glycerol. This is due to the presence of other impurities and the low

concentration of biodiesel in the particular crude glycerol (0.5wt%).

Figure 7.3 Effect of the addition of biodiesel on the ignition delay time of glycerol

droplets

PG GB1 GB2 PGB3 CG0.0

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7.1.3 Burnout time

Figure 7.4 shows the burnout time of glycerol doped with different concentrations of

biodiesel. Burnout time decreased with increasing addition of biodiesel to the

glycerol droplets. This suggests that the addition of biodiesel increased the droplet

evaporation rate due to the decrease in the latent heat of evaporation. Table 7.1

shows that the latent heat of evaporation of biodiesel (450 kJ kg-1) is lower than that

of glycerol (706 kJ kg-1), suggesting that the addition of biodiesel decreased the

latent heat of the pure glycerol-biodiesel mixture.

Figure 7.4 Burnout time of glycerol droplets with additions of different

concentration of biodiesel

Figure 7.4 also shows the burnout time of crude glycerol, compared with that of pure

glycerol and the glycerol-biodiesel mixture. The burnout time of crude glycerol is

much lower than that of pure glycerol. Biodiesel, that can decrease the burnout time

of glycerol, clearly played a role.


0.5

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1.5

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98

7.1.4 Burning rate

Figure 7.5 shows the burning rates of glycerol droplets with the addition of different

biodiesel concentrations. The addition of biodiesel was expected to increase the

burning rates of the pure glycerol droplets. Biodiesel, by nature, has a higher burning

rate than that of glycerol. According to the classic combustion theory of droplets, the

burning rate of fuel increases as the latent heat of evaporation, boiling point, and

density decrease [81]. The addition of biodiesel in glycerol was postulated to reduce

the boiling point, heat of evaporation, and density of glycerol. As a result, higher

burning rates of glycerol droplets were achieved with the addition of biodiesel to the

pure glycerol, and the burning rate increased as the biodiesel concentration

increased.

.

Figure 7.5 Effect of the addition of biodiesel on the burning rate of glycerol droplets

Figure 7.5 also shows the burning rate of crude glycerol, compared with that of pure

glycerol and the glycerol-biodiesel mixture. The burning rate of crude glycerol was

much higher than that of pure glycerol, and biodiesel contributed to the

augmentation of the burning rate.


0.5

1.0

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99

7.2 Summary

The effect of biodiesel on the ignition and combustion characteristics of single

droplets of glycerol were investigated experimentally using the suspended single

droplet combustion technique. In the combustion of pure glycerol-biodiesel droplets,

biodiesel would preferentially evaporate first, leading to ignition of the droplet and

formation of a reddish flame. Subsequently, glycerol evaporated and burned with a

greenish flame. The presence of biodiesel decreased the ignition delay and burnout

times, and increased the burning rates of the glycerol droplets.

100

Chapter 8 Effect of soap

In this chapter, the effect of soap on the ignition and combustion of glycerol are

discussed. Ignition and combustion behaviour of pure glycerol and a mixture of pure

glycerol-soap were investigated using the single droplet combustion method. The

main component of soap is sodium. Sodium is also present in crude glycerol (Table

4.1) and therefore, the release of sodium in the flame and remaining sodium in the

combustion residue are also addressed.

8.1 Single droplet combustion study of a pure glycerol-soap

mixture

8.1.1 Ignition and combustion phenomena of pure glycerol-soap droplets

Figure 8.1 shows typical time-sequenced, non-backlit images of combustion of

droplets of glycerol with and without the addition of soap. The bright colour

represents the flame. Images of the droplets from left-to-right show the sequence of

changes from the moment at which the droplets arrived at the centre of the furnace

until completion of combustion. Upon arrival at the centre of the furnace, the

glycerol droplets with or without the addition of soap were the same size (Figure

8.1). Upon ignition, flames formed that surrounded the droplets. The flame of the

pure glycerol droplet was greenish, suggesting a clean combustion and only a very

small quantity of soot particles formed. Oxygen moiety in the glycerol could

promote combustion, resulting in the formation of no soot, or only a small quantity

of soot. At the end of the pure glycerol combustion, the fibre tip was clean and free

of any soot or ash deposit.

101

Figure 8.1 Typical time-sequenced images of burning glycerol droplets and pure

glycerol-soap droplets (GSx, where x is the soap concentration. Image 1

(t=0) indicates the moment when the droplets arrived at the centre of the

furnace; and Image 2 shows the moment of droplet ignition.

The combustion of glycerol droplets with the addition of soap produced soot and a

solid residue. For example, in the case of an addition of 1% soap to pure glycerol

(GS1), the ignition and combustion phenomena were initially similar to those of pure

glycerol, but the flame was a yellowish colour. The change of flame colour was

mainly due to the presence of sodium ions in the flame and this will be discussed in

the next section. No residue was attached to the fibre tip at the end of combustion

(Image 4). Combustion of 3% and 5% soap in pure glycerol started with yellowish

flames (Image 2) that lasted for a period of time (Images 2 and 3) and subsequently

dimmed and decreased in size (Image 4). After a very short time, the flame became

bright yellow flame (Images 5 and 6). At the end of the combustion, a small amount

of solid residue remained on the fibre tip (Image 7). The residue was likely the ash

102

formed mainly from sodium that had not vapourised and other metals that became

mineralised [10].

These phenomena can be explained by the well-known evaporation mechanism of a

binary mixture with sufficiently different volatilities [118]. The vapour pressures of

glycerol and soap differed significantly [144]. When the glycerol-soap mixture was

heated, the more volatile glycerol component evaporated and burned preferentially.

The less volatile soap component became concentrated in a boundary layer near the

droplet surface. Since the temperature of the droplet surface would remain close to

the boiling temperature of the mixture, and as the surface mass fraction of the soap

would increase, the temperature of the droplet surface would also increase. Towards

the end of the glycerol evaporation, there was a short period during which the droplet

temperature would increase rapidly, and the evaporation rate of glycerol was

extremely low, leading to the flame contraction. Subsequent flame growth was

characterised by evaporation of the soap. Combustion the glycerol droplets with the

addition of soap therefore comprised two stages, namely preferential evaporation and

combustion of glycerol, followed by evaporation and combustion of soap.

Figure 8.2 illustrates the temporal variation of the square of the normalised droplet

diameter (d/d0)2 for droplets of pure glycerol (PG), 5% soap in pure glycerol (GS5),

and pure soap (PS) at 1023 K. Droplet size was monitored from the moment that the

droplet reached the centre of the furnace until completion of combustion. Droplet

size remained almost constant for PG and PS droplets during the ignition delay time

and showed an approximately linear decrease in size after ignition, implying that

droplet combustion conformed to the d2-law [81].

103

Figure 8.2 Normalised temporal evolution of the squared diameter (d/d0)2 for

droplets of pure glycerol, 5wt% soap in glycerol (GS5), and pure soap at

1023 K

However, changes in the droplet size for GS5 resembled those of bi-component

mixtures with vastly differing vapour pressures [11], as discussed in the previous

section. The first segment of these curves represents the first stage during which the

glycerol was evaporated and combusted preferentially, while the second segment

represents the second stage during which the soap was evaporated and burned. Due

to the relatively low soap content, the second stage lasted very briefly.


Figure 8.3 illustrates the effect of the addition of soap on the ignition delay time of

glycerol droplets. Ignition delay times remained relatively stable with increasing

addition of soap. As discussed in the previous section, glycerol, that is more volatile

than soap, evaporates and burns at the beginning of the glycerol-soap droplet

combustion, while the soap is concentrated in the droplet surface and burns

afterwards. This suggests that before ignition, glycerol was the main fuel vapour that

104

subsequently oxidised and burned, explaining the similarity between the ignition

delay of pure glycerol and that of the glycerol-soap mixture.

Figure 8.3 Ignition delay time of glycerol droplets with the addition of different

concentrations of soap at 1023 K

8.1.3 Burnout time

Figure 8.4 shows the burnout time of glycerol droplets with and without the addition

of soap. Burnout time decreased with the addition of soap in the combustion of

glycerol droplets. In the combustion of pure glycerol-soap droplets, sodium was

found in the flame and combustion residue, as discussed in Section 8.2.

Sodium has been used as an additive in combustion using gasoline fuel. Sodium is

considered to accelerate combustion by the following: increasing oxidation of fuel

vapour; increasing flame temperature; achieving higher reaction rate; decreasing

valve burning; reducing emissions of hydrocarbon from the exhaust; producing a

cleaner combustion chamber and valve; decreasing varnish on pistons; decreasing

sludge and varnish in crankcase parts and valve covers; and obtaining lower surface

ignition [145].

PG GS1 GS3 GS5 CG0.0

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Igni

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Figure 8.4 also shows the burnout time of crude glycerol, compared with that of pure

glycerol and the glycerol-soap mixture. The burnout time of crude glycerol is shorter

than that of pure glycerol, with soap contributing to the reduction. Soap would

change the evaporation rates of glycerol and subsequent chemical reactions of the

fuel vapours due to the sodium content.

Figure 8.4 Burnout time of glycerol droplets with the addition of different

concentrations of soap at 1023 K

8.1.4 Burning rate

Figure 8.5 shows the effect of the addition of soap on the burning rates of glycerol

droplets at 1023 K. Since the droplets with soap additions exhibited two stages of

combustion, the burning rates of both stages were calculated and are shown in Figure

8.5a and b. Figure 8.5a shows the burning rate of pure glycerol and the first stage of

glycerol-soap combustion. It can be seen that the addition of soap increased the

burning rates of glycerol droplets. Figure 8.5b shows the burning rates of glycerol-

soap droplets in the second stage. The average burning rates of the residues of GS3

and GS5 were similar to that of pure soap, confirming that the combustion of

PG GS1 GS3 GS5 CG0.0

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106

glycerol-soap droplets in the second stage was characterised by the burning of soap.

It is also evident that the burning rate of soap was much higher than that of glycerol.

Figure 8.5 Burning rates of glycerol droplets with the addition of different

concentration of soap: (a) Stage 1; and (b) Stage 2

8.2 Sodium ions in the flame of pure glycerol-soap droplets

Figure 8.6 shows the emission spectra of the flames of glycerol droplets with the

addition of different concentrations of soap. The spectra showed all possible light

emissions ranging from 400 nm to 750 nm during the combustion process, and a

clear spectrum with a main feature near 689 nm corresponding with the atomic

107

emission lines of sodium [122]. The intensity of the sodium signal from the flames

of soap-doped glycerol droplets increased with increasing concentrations of soap,

showing that a droplet with a higher soap concentration released more sodium in the

flame. This finding is consistent with the combustion phenomenon discussed in

Section 8.1.1.

Figure 8.6 Changes in signals of sodium in flames of droplets with the addition of

various concentrations of soap in glycerol

Based on the evidence presented above, soap promoted glycerol combustion as

follows: soap started to decompose at ~500 K, that is lower than the boiling point of

glycerol (563 K) [146]; the difference between the boiling points caused the soap to

decompose while glycerol was burning; when the surface temperature reached the

decomposition temperature of soap, the soap decomposed, releasing sodium atoms in

the reaction zone, and this promoted the oxidation of the fuel vapour.

Sodium promotes a higher reaction rate and increases the flame temperature of the

droplet. The higher flame temperature enhances heat transfer to the droplet, leading

to a higher burning rate and shorter burnout time. The effect of sodium in increasing

108

the burning rate is consistent with the literature, that describes atomic sodium

promoting ignition and combustion of hydrocarbons [145].

8.3 Sodium ions in the solid residue

In Section 8.1, a description of how solid residue was found attached to the fibre

after the single combustion was completed, was provided. Since the solid residue

sample from the single droplet combustion was small, the residue of mixture of

glycerol-soap combustion was collected from pool-fire combustion, providing a

sufficient sample for further analysis. The solid residue investigation was carried out

using Scanning Electron Microscopy (SEM) coupled with an Energy Dispersive

Spectroscopy (EDS) for element identification.

Table 8.1 shows the ash and sodium concentrations recovered from the pool-fire

combustion of soap in glycerol. Ash was absent from combustion of pure glycerol,

but present in combustion of soap in the glycerol mixture, where ash production

increased with increasing concentrations of soap added. The addition of 5wt% soap

in the 50 ml mixture of soap in glycerol produced 1wt% ash, and the ash increased to

3.6wt% for combustion of 15wt% soap in glycerol. These results verified that the

quantity of ash produced was directly related to the quantity of soap in glycerol.

As determined by the EDS, dominant elements in the ash included oxygen, sodium,

and carbon (Table 8.1). Sodium accounted for 32–42wt% in ash at various soap

concentrations. However, compared to the amount of sodium in the soap, 78–85wt%

sodium was recovered from the ash, confirming that most of the sodium remained in

the solid residue/ash during combustion. The increase in soap concentration in

glycerol reduced the recovery of sodium in the ash, suggesting that at higher soap

concentrations, more sodium vapourised in the flame, and this was consistent with

the release of sodium in the flame (Figure 8.5).

109

Table 8.1 Ash and sodium in ash produced by combustion of various

concentrations of soap in glycerol

Soap in glycerol

(wt%)

Ash

(wt%)

Ash main component (wt%) Sodium recovery

(wt%)* O Na C

0 0 - - - -

5 1.0 47.3 41.2 9.2 85.2

10 2.0 45.4 42.0 11.1 82.8

15 3.6 41.4 32.8 20.0 78.2

* Approximate value of average sodium found in spot tests of SEM-EDS to sodium added to soap in glycerol mixture

Figure 8.7 SEM images of the surface of the solid residue at various concentrations

of soap

110

Figure 8.7 shows SEM images of an ash particle from 5–15% of soap in glycerol.

Morphology of the solid residue particles varied due to agglomeration, and the effect

increased with increasing concentrations of soap. The addition of 5% soap produced

porous material with relatively larger particles. The 10% addition of soap produced

smaller, more compact particles, and the addition of 15% soap resulted in a sintered

solid residue.

8.4 Summary

An experimental study on the effect of the addition of soap on the ignition and

combustion characteristics of glycerol droplets at air temperature 1023 K was

undertaken. The ignition delay time, burnout time, and burning rate of single

droplets of glycerol with and without the addition of soap were determined, along

with the sodium intensity in the flame. The ignition and combustion process of soap-

doped glycerol droplets occurred in a two-stage manner: in the first stage, glycerol

was evaporated and combusted preferentially; and in the second stage, soap was

evaporated and burned. The two-stage evaporation resembled the evaporation

behaviour of binary mixtures with vastly different volatilities. The addition of soap

to glycerol decreased the ignition delay time slightly and reduced the burnout time of

the droplets. The burning rate of glycerol increased with increasing soap content.

Sodium ions were detected in the flames of the soap in glycerol droplets, and this

promoted the ignition and subsequent combustion of the fuel vapours.

111

Chapter 9 Kinetic modelling of combustion of glycerol-

methanol and glycerol-biodiesel mixtures

In experimental studies of droplet combustion of pure glycerol- methanol and pure

glycerol-biodiesel mixtures, the addition of methanol or biodiesel decreased the

ignition time of glycerol. In this chapter, a kinetic study of the combustion of

methanol and biodiesel in pure glycerol is discussed, to address the reasons for the

decrease in ignition time of glycerol due to the addition of methanol or biodiesel.

Kinetic aspects of the combustion of methanol as a single fuel or fuel blend have

been widely investigated [147]. The early development mechanism of methanol

oxidation included the mechanism developed by Westbrook and Dryer (1979), that

was upgraded by Held and Dryer (1994), using shock-tube and a flow reactor. The

latest development, based on the work of Ranzi et al., (2014), was considered to be

sufficient for hydrocarbon and oxygenated fuel, and was used in the current research

[111]. The kinetic study of a blend of methanol with another fuel, such as gasoline

[148], is of interest to the researcher. However, little information is available

regarding the kinetic study of the glycerol-methanol blend.

The kinetic study of biodiesel combustion has also been extensively explored [149].

Direct modelling of biodiesel is not preferred, due to the complexity of this

modelling. Simple components have been used as surrogate molecules that match the

characteristics of biodiesel, such as methyl butanoate, methyl crotonate, and ethyl

propanoate. However, these simple components do not represent biodiesel well. A

kinetic mechanism for larger molecules, such as methyl stearate and methyl

decanoate, based on the work on the smaller molecules, was used in the current

study.

113

investigation of the effect of pure glycerol-methanol on single droplet combustion, as

discussed in Chapter 6.

Figure 9.2 Effect of additions of various concentrations of methanol on ignition

delay time of pure glycerol-methanol mixture

The main reactions contributing to the consumption of glycerol were identified to

understand the effect of the addition of methanol on the ignition of glycerol ignition.

In Figure 9.3, reaction rates of pure glycerol and the glycerol-methanol mixture are

sorted in decreasing order of relative rates, which defined as the sum of the rate of

production and rate of consumption (the width of the bar indicates the proportion of

the reaction rate), and compared with the chemical reactions involved in the

consumption of pure glycerol (PG) and a 40v% glycerol-methanol mixture (GM) at

ignition. Additional methanol was added to the 40v% glycerol to enhance the effects.

Based on the ignition delay plot in Figure 9.2, the ignition delay time of glycerol,

with the addition of 40v% or more methanol, was similar to the ignition delay time

of pure methanol.

OH radicals contributed to all consumption reactions of glycerol and the glycerol-

methanol mixture (Figure 9.3). Ignition has been reported to occur when OH radicals

0 20 40 60 80 1000.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

Igni

tion

dela

y tim

e (m

s)

Methanol concentration (%)

114

accumulate in the reaction mixture, where most of the radicals react with molecules

of fuel, producing water and heat, and accelerating the fuel oxidation rate [150].

Figure 9.3 Relative rates (indicated by horizontal bar) of the consumption of pure

glycerol (PG) and glycerol-methanol (GM) by various elementary

reactions at residence time 0.1, initial temperature 1023 K, and

equivalence ratio 1.0

Figure 9.4 shows the comparison of mole fractions of various species during ignition

of pure glycerol and the glycerol-methanol mixture. Before ignition occurred,

characterised by the instantaneous rise of OH, there was a shift of the peaks of

species of the glycerol-methanol mixture compared with those of pure glycerol.

The OH mole fraction increased significantly with the addition of 40v% methanol,

and the peak occurred five times earlier than that of glycerol. As with OH, the

addition of methanol also increased the mole fraction of CO2 and CO. The mole

fraction of H2O was twice that of pure glycerol, and a sharp early increase of O and

H was also observed. The addition of methanol therefore increased the production of

those species, and this contributed to decreasing the ignition delay of glycerol.

Reaction 9.1 was the controlling reaction of glycerol-methanol consumption based

on Figure 9.3 and Figure 9.4. In the combustion of pure glycerol, the rate of Reaction

116

OH radicals produced H2O and O. Reaction 9.3 had the highest reaction rate for CO2

formation, where CO reacted with OH, producing CO2 and H. H subsequently

reacted with O2 to produce OH and O, adding more O in the system (Reaction 9.4),

and this was the highest rate of reaction for the consumption of O2.

2OH ↔ H2O + O ............................................................................................. (R 9.2)

CO + OH ↔ CO2 + H ...................................................................................... (R 9.3)

O2 + H ↔ O+ OH ........................................................................................... (R 9.4)

Reactions 9.2 to 9.4 show that the production and consumption of OH releases O in

the system, that later reacts with glycerol (Reaction 9.1) to enhance the combustion

of the glycerol-methanol mixture (Figure 9.2).

9.2 Effect of addition of biodiesel on glycerol combustion kinetics

Figure 9.5 shows the mole fractions of glycerol, biodiesel and OH during

combustion of the glycerol-biodiesel mixture. The mole fraction of glycerol and

biodiesel decreased with increasing time, while the mole fraction of OH peaked once

glycerol and biodiesel had been completely degraded. This occurred when OH

peaked, similarly to the ignition of the glycerol-biodiesel mixture.

Figure 9.6 shows the ignition delay time of the pure glycerol-biodiesel mixture at

various concentrations at an initial temperature of 1023 K and equivalence ratio 1.0.

The addition of biodiesel decreased the ignition delay time of the mixture,

suggesting that adding biodiesel promoted ignition. The ignition delay time of the

glycerol-biodiesel mixture decreased significantly with the increasing addition of

biodiesel, and with the addition of biodiesel over 10v%, the ignition delay time was

close to that of pure biodiesel.

118

Figure 9.6 Effect of additions of various concentrations of biodiesel on ignition

delay time of pure glycerol-biodiesel mixture

Figure 9.7 shows that the addition of biodiesel increased the relative rates of

reactions of O with glycerol to produce OH, C3H6O2 and ACETOL, and the

reactions of H with glycerol to produce H2, OH and C3H6O2. The OH radical

contributed to all of the consumption reactions of glycerol and glycerol-biodiesel.

The increase of O and H (Reactions 9.5 to 9.7) added more OH radicals to the

system that later decreased the ignition time of the glycerol-biodiesel mixture,

highlighting these as key reactions in glycerol-biodiesel ignition.

O + GLYCEROL → 2OH + C3H6O2................................................................. (R 9.5)

O + GLYCEROL → 2OH + ACETOL ............................................................... (R 9.6)

H + GLYCEROL → H2 + OH + C3H6O2 ......................................................... (R 9.7)

0 20 40 60 80 1000.00

0.01

0.02

0.03

0.04

0.05

0.06

Ig

nitio

n de

lay

time

(s)

Biodiesel concentration (%)

119

Figure 9.7 Relative rates (indicated by the horizontal bar) of consumption of pure

glycerol (PG) and 10v% glycerol-biodiesel (GB) by various elementary

reactions at residence time 0.1, initial temperature 1023 K and the

equivalence ratio 1.0

Figure 9.8 shows the comparison of mole fractions of various species (OH, CO2,

H2O, CO, O and H) during the ignition of pure glycerol and the glycerol-biodiesel

mixture. The peaks of these species shift with the addition of biodiesel. The OH

mole fraction increased significantly with the addition of biodiesel before ignition

(Figure 9.8). As with OH, the addition of biodiesel also increased the mole fraction

of CO2, H2O, O, and H at the same time (0.02 s). This shows that the mole fraction

of these species increased during ignition. The mole fraction of CO increased early

in combustion, suggesting that the molecule might control the ignition.

Based on formation Reaction 9.8, the O increased due to the reactions of H with O2

to produce O and OH. Reaction 9.9 shows the formation of H, mainly from the

reaction of CO with O2 to produce CO2. This reveals that the addition of biodiesel to

glycerol can decrease ignition time of the mixture due to the presence of CO.

H + O2 ↔ O + OH ........................................................................................... (R 9.8)

CO + OH ↔ CO2 + H ....................................................................................... (R 9.9)

121

significant reactions for glycerol consumption during combustion. OH played an

important role during glycerol consumption in both glycerol-methanol and glycerol-

biodiesel mixtures. The presence of O in the system contributed to increasing

reaction rates of the key reactions. In the glycerol-methanol mixture, O was released

from the formation of combustion products H2O and CO2, while in the glycerol-

biodiesel mixture, O was released from the consumption of CO.

122

Chapter 10 Evaluation and practical implications

In this chapter, results are evaluated against objectives and implications are

discussed. Results are also compared with the literature, leading to the identification

of knowledge gaps and recommendations for future studies.

10.1 Integration and evaluation of effects of impurities in crude

glycerol combustion

Combustion behaviour of crude glycerol was investigated and compared with that of

pure glycerol, petroleum diesel, biodiesel, and ethanol. Impurities changed the

combustion characteristics of crude glycerol. The roles of each of the known

impurities in glycerol combustion required clarification. The major impurities of

crude glycerol include water, methanol, biodiesel, and soap.

Investigation of the combustion characteristics of glycerol doped with water,

methanol, biodiesel, or soap, provided a better understanding of the behaviour of

these impurities in the combustion of glycerol. These main impurities can be divided

into three groups, based on the effect on the ignition and combustion characteristics

of glycerol:

10.1.1 Impurities that enhance the ignition and combustion of glycerol

Methanol enhanced the combustion characteristics of glycerol by decreasing the

ignition delay and burnout time, and increasing the burning rate of glycerol droplets.

These effects increased with the addition of increasing concentrations of methanol.

The addition of methanol also promoted micro-explosions during the combustion

process, and this in turn improved the combustion performance of glycerol.

123

In the literature, methanol is commonly reported to be used as a fuel additive in

diesel. The addition of 10–15% methanol to diesel fuel increases the performance of

diesel engines, due to the lower viscosity and higher oxygen content. Methanol

reduces the viscosity of the blended fuel, and this improves the spray of fuel

droplets, increasing fuel evaporation, and improving the mixing of fuel and air. The

higher oxygen content of methanol increases combustion efficiency of the blended

fuel [151].

Although methanol improves the combustion characteristics of glycerol, the

concentration in crude glycerol is limited, originating only from excess

transesterification reactants. Furthermore, methanol is recovered and fed back to the

transesterification process to reduce biodiesel production costs [24], leaving low

methanol concentrations in crude glycerol.

The addition of biodiesel enhanced the ignition and combustion characteristics of

single droplets of glycerol. Two flame colours were observed during droplet

combustion due to early combustion of biodiesel, as discussed in Chapter 7.

Biodiesel also reduced ignition time, decreased burnout times, and increased burning

rates of the glycerol droplets. The addition of biodiesel to other fuel, e.g. diesel,

reduced the ignition delay time and added an extra oxygen atom to enhance

combustion of the mixture [152, 153].

However, adding biodiesel into crude glycerol to increase the combustion

characteristics of the mixture has to be viewed from the economic viability. In

transesterification, biodiesel is the product and crude glycerol is the waste. It requires

a careful economic analysis to use the production yield (biodiesel) for waste

treatment.

124

Methanol and biodiesel improve the ignition and combustion characteristics of crude

glycerol, but the improvement is limited by the low concentrations in crude glycerol.

The addition of more methanol or biodiesel to further assist the ignition and

combustion characteristic of crude glycerol may require thorough analysis.

10.1.2 Impurities that partly enhance the ignition and combustion of glycerol

Water improved the ignition and combustion characteristics of crude glycerol by

improving atomisation of the glycerol droplets. The addition of water promoted the

formation of bubbles within the droplets before ignition. These bubbles led to the

occurrence of micro-explosions, that contributed to decreasing burnout times and

increasing burning rates of the glycerol droplets. However, the addition of water also

increased ignition delay time of the glycerol droplets, but this was not favourable

during combustion. Water did not add any energy to the combustion and reduced the

energy content at high concentrations.

In combustion, water can be advantageous and disadvantageous. Figure 10.1 shows

typical fuel atomisation with the addition of water in a diesel engine. Water promotes

secondary atomisation by bubble formation, followed by micro-explosion, as found

in glycerol doped with water [154], and hence the addition of water can increase the

burning rate of fuels [85]. The addition of water in a fuel blend is also beneficial as

the water dilution effect increases the mixing of fuel and air, and reduces the fuel

rich region that in turn reduces soot formation due to the increase of OH radicals

[155].

The disadvantage of an increasing proportion of water in the crude glycerol is

reduction of the energy content of the fuel. Water increased the ignition delay time

that could reduce combustion performance. Therefore, further research on glycerol

doped with water is required, particularly regarding the proportion of water related to

125

the decrease of the ignition delay time. Future work would require developing an

understanding of the acceptable load of water in glycerol which would determine the

balance of a sufficient ignition delay and fast burning rate.

Figure 10.1 Fuel atomisation with the addition of water in fuel [76].

10.1.3 Damaging effects of soap on combustion

Soap changed the ignition and combustion characteristics of glycerol, mainly

because of the sodium content. In the experimentation, the ignition and combustion

of soap-doped glycerol droplets, which occurred in a two-stage manner, decreased

the ignition delay time slightly, and shortened burnout time of the droplets, and this

subsequently augmented the burning rate. The burning rate of glycerol increased

with increasing soap content due to the presence of sodium ions in the flames that

promoted the ignition and subsequent combustion of the fuel vapours.

The drawback of soap is that sodium forms solid residue near the completion of

combustion. This phenomenon is documented in the utilisation of crude glycerol in a

swirl burner [10, 91]. Sodium can impede the fuel injector in a diesel engine due to

the reaction with fuel additives in the presence of water [156].

126

Table 10.1 lists the investigations of sodium in liquid fuel, which are mainly

characterisation of sodium in combustion. Further research is required to determine

how to remove sodium from crude glycerol. Unlike solid fuel where washing could

reduce the sodium content [79], liquid fuel requires a different approach. In some

studies of the transesterification process, the sodium content was decreased by using

a non-metal catalyst, such as enzymatic catalyst. However, the enzymatic catalyst is

expensive and has a low reaction rate, and this is not suitable for industrial

application. Another catalytic method to reduce sodium content involves use of an

immobilised oxide of zinc and aluminium to avoid catalyst loss [157]. These

knowledge gaps represent interesting topics for further research.

Table 10.1 Research on sodium related to combustion

Research topic Reference

Sodium as combustion additive [145, 158]

Characteristics of sodium droplet combustion [159, 160]

Immobilised catalyst to reduce sodium content [157]

Determination of sodium in flame [124, 161, 162]

Collison of sodium atom with glycerol [163]

10.2 Effect of combined impurities in glycerol combustion

Investigation of the effects of individual impurities on the ignition and combustion

characteristics of glycerol was undertaken to provide clarity of how each impurity

affected the glycerol combustion. However, impurities are mixed in crude glycerol

and therefore the effects of combined impurities were investigated.

Mixed impurities were investigated using the factorial design for characterising the

interaction of the known factors. The experiment was designed using Design Expert

127

10.0.5 software that creates a fractional-factorial design for multilevel category

factors. A subset of all possible combinations of the factors of the categories was

chosen. Factors include the impurities water, methanol, biodiesel, and soap. An

amount of 10 ml glycerol was used as a base for the samples. Impurities added

corresponded to the impurities used for the current research: 5–20v% water, 5–15v%

methanol, 1–3v% biodiesel and 1–5v% soap. Based on the factorial design, there

were 45 combinations of samples. The single droplet combustion technique was used

to approximate the ignition delay time, burnout time, and burning rate.

Figure 10.2 shows the ignition delay of glycerol droplets mixed with impurities. The

addition of both methanol and biodiesel decreased the ignition delay of the glycerol

droplets. Comparison of Figure 10.2 (a) and (b) shows that the addition of water

from 5v% to 20v% increased the ignition delay time of the mixture, confirming

findings in the current research that water can delay ignition time. Conversely,

comparison of Figure 10.2 (a) and (c) shows that the addition of increasing

concentrations of soap to the mixture reduced the ignition time of the glycerol

droplets.

Figure 10.2 Ignition delay time of pure glycerol mixed with various concentrations

of methanol and biodiesel with addition of: a) 5% water and 1% soap;

b) 20v% water and 1v% soap; and c) 5v% water and 5v% soap

128

Figure 10.3 shows the burnout time of glycerol droplets mixed with the impurities.

The addition of biodiesel and methanol decreased the burnout time of the glycerol

droplets, and this effect became stronger with an increase in biodiesel concentration

in the mixture. Comparison of Figure 10.3 (a) and (b) shows that the addition of

water from 5v% to 20v% decreased the burnout time slightly. Similarly, comparison

of Figure 10.3 (a) and (c) shows that the increase of soap concentration from 1v% to

5v% decreased the ignition time of the glycerol droplets.

Figure 10.3 Burnout time of pure glycerol mixed with various concentrations of

methanol and biodiesel with addition of: a) 5% water and 1% soap; b)

20v% water and 1v% soap; and c) 5v% water and 5v% soap

As discussed in Chapters 5–8, the addition of impurities increased the burning rate of

the glycerol droplets. Figure 10.4 shows the burning rate of glycerol droplets mixed

with the impurities. The mixture of biodiesel and methanol increased the burning

rate of the glycerol droplets. This effect increased further with the increase of

concentrations of both methanol and biodiesel. Comparison of Figure 10.4 (a) and

(b) shows that the increase of water concentration from 5v% to 20v% was further

increased the burning rate. Figure 10.4 (a) and (c) show that the effect of soap was

more significant than that of water.

129

Figure 10.4 Burning rate of pure glycerol mixed with various concentrations of

methanol and biodiesel with addition of: a) 5% water and 1% soap; b)

20v% water and 1v% soap; and c) 5v% water and 5v% soap

10.3 Practical implications

The current research reveals how the main impurities (water, methanol, biodiesel,

and soap) change the ignition and combustion characteristics of glycerol. These

findings have implications in the practical utilisation of crude glycerol as a fuel in

biodiesel plants. Understanding the effects of water, methanol, biodiesel, and soap in

crude glycerol provides insight into how each of these impurities changes the

ignition and combustion characteristics of glycerol. This insight provides a basis for

a strategy for the utilisation of crude glycerol as a fuel by identifying effects of

impurities that enhance and cause problems in glycerol combustion. This will enable

the use of crude glycerol as a fuel and solve problems arising from crude glycerol as

a by-product of biodiesel manufacturing by converting the glycerol into an energy

source that will benefit the biodiesel industry and environment. However, further

research and improvements are possible and necessary.

130

10.4 Identification of the new gaps

Although the objectives of the current research were achieved, various knowledge

gaps were identified following evaluation of the results and these are described

below.

Water may promote atomisation but delays the ignition of glycerol. Further research

is required to determine the maximum water content in crude glycerol tolerable for

the glycerol to be used as a fuel.

Further research is required to overcome the effect of soap/sodium in crude glycerol

combustion. While other impurities contribute to the combustion of glycerol, soap

(sodium) is the main impurity causing a drawback in the utilisation of crude glycerol

as a fuel. Investigation into decreasing soap content during transesterification is

recommended.

Only methanol and biodiesel were investigated in the kinetic modelling. Kinetic

investigations of the effect of water and the more complex effect of soap have not

yet been undertaken. More importantly, kinetic modelling of a mixture of the

impurities is required to mimic crude glycerol combustion.

131

Chapter 11 Conclusions and recommendations

11.1 Conclusions

Water, methanol, biodiesel, and soap affected the ignition and combustion

characteristics of crude glycerol, and each of these impurities behaved differently

during the combustion of glycerol. Methanol and biodiesel improved the combustion

characteristics of glycerol. Water also enhanced the combustion characteristics of

glycerol but delayed the ignition, and soap had a detrimental effect on combustion of

glycerol due to the sodium content. Detailed conclusions are described below.

Crude glycerol had shorter ignition delay times, more rapid burnout, and higher

burning rates than pure glycerol due to the presence of impurities. The

impurities changed the physical and thermochemical properties of crude

glycerol, by increasing the vapour pressure, reducing the latent heat, and

decreasing the boiling point. Ignition delays of pure and crude glycerol were

longer than those of biodiesel, diesel, and ethanol, leading to lower burning

rates. This occurred because glycerol has a low Cetane number, high boiling

point, and high auto-ignition temperature. The impurities decreased the burnout

time and increased the burning rate of glycerol, particularly when compared

with pure glycerol, biodiesel, diesel, and ethanol.

The effect of each of the main impurities on the ignition and combustion

characteristics of crude glycerol were investigated using pure glycerol mixed

with water, methanol, biodiesel, or soap. The addition of water to single droplet

combustion of glycerol increased the ignition delay time, decreased the burnout

time, and increased the burning rate. Water slowed the ignition time by

evaporating to form steam at the beginning of combustion, and this delayed the

132

heating of the droplet, decreased the droplet temperature, and slowed the

ignition process. However, the burnout time decreased due to the formation of

bubbles inside the droplet, leading to the micro-explosions, and this resulted in

the formation of many smaller droplets, and consequently an increased burning

rate.

Methanol enhanced the combustion performance of glycerol by decreasing

ignition delay and burnout time, and increasing the burning rate of the glycerol.

The formation of bubbles and occurrence of micro-explosions during

combustion improved the combustion performance of glycerol.

Biodiesel reduced the ignition delay and burnout times, and increased the

burning rates of the glycerol droplets. A change in the flame colour was also

observed during combustion. A reddish flame identified at the beginning of

combustion resulted from the early ignition of biodiesel. The flame colour

became greenish towards the end of the process due to the combustion of

glycerol. The ignition delay time of the mixture decreased due to the early

ignition of biodiesel. Burnout time of the glycerol droplets decreased, and the

burning rate increased with increasing addition of biodiesel.

Soap did not significantly affect the ignition delay time, but decreased the

burnout time, and increased the burning rate of single droplets of glycerol. The

increasing concentration of soap also increased sodium intensity in the flame.

The addition of soap decreased the ignition delay and burnout time slightly, and

increased the burning rate, and the mixture had a two-stage combustion process:

a glycerol-dominated combustion stage, followed by a soap-dominated

combustion stage. The ignition delay and burnout time decreased due to the

release of sodium ions in the flames that promoted the ignition and subsequent

133

combustion of the fuel vapours. The increase of soap addition also increased the

sodium intensity in the flame. This subsequently increased the burning rate that

increased with increasing soap content. The two-stage combustion of the

mixture resembled the evaporation behaviour of a binary mixture of two

components with vastly different volatilities.

The kinetic modelling results showed that methanol and biodiesel decreased the

ignition delay time of glycerol. The ignition delay time decreased with

increasing concentrations of methanol or biodiesel. The reaction pathway

analysis showed that ignition was promoted by an early injection of OH radicals.

The reaction rate increased with the presence of O in the system, derived from

the formation of H2O and CO2 in glycerol-methanol mixture, and from the

consumption of CO in the glycerol-biodiesel mixture.

11.2 Recommendations

Although the overall objectives of the present research were achieved, various new

knowledge gaps were also identified following evaluation of the findings from the

current research, leading to the recommendations for future research described

below.

In the singe droplet combustion experiment, the buoyancy effect was not

considered in the present research. Further investigation using single droplet

combustion in microgravity environment to study the buoyancy effect is

required.

Effects of the impurities on glycerol were investigated individually. Interactions

among the impurities in the ignition and combustion of glycerol were examined

statistically but remain largely unexplored. Further investigation is required to

134

gain an understanding of the interactions among impurities in the ignition and

combustion of glycerol.

The present of sodium in the flame was detected from the highest peak of

sodium captured by spectrometer during combustion. However, the present

technique could not detect the amount of sodium at various combustion stages,

which is interesting for further study.

The sodium content in crude glycerol promoted the formation of solid residue,

and this could potentially cause engine damage. Further investigation regarding

the removal of sodium from crude glycerol is required.

Kinetic investigation included only glycerol-methanol and glycerol-biodiesel

kinetic models. Kinetic modelling of other impurities would enhance

understanding of the kinetics of crude glycerol.

Combustion of crude glycerol in actual burners or engines has not been studied.

Investigation on the suitable type of burners or engines, whether using the

existing design or new design is interesting.

Investigation on the effects the impurities of crude glycerol in actual combustors is

essential.

135

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