FINE PARTICLE SYNTHESIS IN TUBULAR FLAME SYSTEMS

概要

FINE PARTICLE SYNTHESIS IN TUBULAR FLAME SYSTEMS
（管状火炎システムによる微粒子合成）

A Thesis submitted to
The Chemical Engineering Program
Graduate School of Advanced Science and Engineering
Hiroshima University

presented by
TOMOYUKI HIRANO

In Partial Fulfillment of the Requirements for the Degree of
Doctor of Engineering

Hiroshima University
March 2022

Approved by

Professor Takashi Ogi
Adviser

　

F I N E PA R T I C L E S Y N T H E S I S I N
TUBULAR FLAME SYSTEMS

tomoyuki hirano
March 2022

ACKNOWLEDGMENTS

I would like to express my deepest gratitude to Prof. Takashi Ogi, Chemical Engineering Program, Graduate School of Advanced Science and Engineering, Hiroshima University, for his continuing guidance, encouragement, support, and patience throughout
the five years of my study in his laboratory.
I am grateful to Prof. Kikuo Okuyama for all their help and advice. Many thanks go
to Prof. Daisuke Shimokuri who introduced me to the field of combustion science.
I wish to thank Prof. Akihiro Yabuki and Prof. Kunihiro Fukui for their patience,
wisdom, and valuable comments and suggestions. My grateful thanks are also dedicated to Dr. Shuhei Nakakura, Dr. Annie Mufyda Rahmatica, and Dr. Kiet Le Anh Cao
for their great support, advice, and guidance during my research.
I am grateful to Mrs. Michiyo Tachibana, Mrs. Eka Lutfi Septiani, Mr. Hiroyuki
Murata, Mr. Hayato Horiuchi, Mr. Shuto Taniguchi, Mr. Hiromitsu Fukazawa, Mr.
Hikaru Osakada, Mr. Kazuki Kamikubo, Mr. Chikara Nishikawa, Ms. Yuki Matsuo,
Mr. Hisaaki Inaba, Mr. Kazuya Tasaka, Mr. Jun Kikkawa, Ms. Marin Nishida, Mr. Tue
Tri Nguyen, Mr. Yusuke Kitou, Mr. Youhei Toyoda, Mr. Phong Hoai Le, Mr. Takama
Tsuboi, Mr. Shogo Kaseda, Mr. Yasuhiko Kitamoto, Mr. Ryosuke Narui, Mr. Shunki
Yamashita, and all other members of the Thermal-Fluid Engineering Laboratory for
their day-to-day help and kindness.
The Japan Society for the Promotion of Science (JSPS), Hosokawa Powder Technology Foundation, and Kato Foundation for Promotion of Science are acknowledged for
the financial support of my research at Hiroshima University and my living allowance.
Finally, I would like to express my appreciation to my father Kazuyuki Hirano, my
mother Mayumi Hirano, and my brother Souta Hirano for their patience, support, and
love, without which it would have been very difficult to persevere through my many
years of school.

Tomoyuki Hirano
Higashi Hiroshima,
February 2022

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S U M M A RY

In this dissertation, we apply a tubular flame to the reaction field in gas-phase synthesis of fine particles. Flames have been used to fabricate various functional fine particles
and devices, and it is important to predict the temperature and gas concentration to
control particle characteristics with high energy efficiency even in the so-called “dirty”
gas phase generated by combustion. Tubular flame combustion, a new combustion
technology, has high thermal and aerodynamic stability, and the flame temperature
and gas composition can be controlled. In addition, the tubular structure is convenient
and can be easily integrated into various gas-phase processes. As a first step toward
the development of a particle synthesis process using tubular flame combustion, we
developed a new tubular flame burner, investigated the effects of various combustion
parameters on particle formation, and clarified the detailed flame structure by spectral
analysis of chemiluminescence. A brief description of each chapter in this dissertation
is given below.
Chapter 1 introduces the current research background for flame aerosol synthesis
of nanostructured particles. A review of gas-phase combustion synthesis and burner
types in previous research is also presented in this chapter.
In Chapter 2, we describe the development of a tubular flame burner for particle
synthesis and investigate the synthesis of tungsten oxide nanoparticles by efficient use
of combustion energy. When synthesizing fine particles using the flame-assisted spray
pyrolysis method—which is one of the flame aerosol synthesis methods—submicronsized particles are easily obtained owing to the size of the raw material droplets. However, by using a high-temperature tubular flame, energy can be supplied to the particles efficiently. As a result, the gasification of the particles is accelerated and they
renucleate in the gas phase, resulting in the formation of tungsten oxide nanoparticles
with a primary particle size of 5-20 nm.
Chapter 3, describes the successful preparation of tungsten metal nanoparticles using fuel-rich methane/air tubular flames due to the effect of reducing species in the
combustion gas. Because the tubular flame structure has high-temperature combustion
gas inside and low-temperature unburned gas outside, the produced particles are not
affected by the unburned gas and react in the combustion gas with a controlled composition and temperature. When the composition of the combustion gas was examined
under various equivalence ratio (𝜙) conditions, the oxygen concentration approached
zero for 𝜙 > 1.0, while the concentration of CO, a reducing species, increased significantly. Under the condition 𝜙 > 1.0, tungsten trioxide was synthesized as described in
the previous chapter. In addition to tungsten trioxide (WO3 ), the crystalline phases of
tungsten suboxide (WO2.72 ) and tungsten metal (W) were precipitated. Furthermore,
increasing the residence time of the particles in the tubular flame accelerated the reduction effect and caused the WO3 and WO2.72 phases to disappear, and only the W
phase was observed. The particle size decreased with increasing residence time, and
the primary particle size of the tungsten metal particles was 5–10 nm. It was shown

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that the oxidation state and particle size of the flame-made particles could be widely
controlled using the controlled reaction atmosphere of tubular flame combustion and
by adjusting the residence time.
In Chapter 4, a direct spray type tubular flame burner was developed and its flame
structure was analyzed to establish a particle synthesis system by liquid fuel combustion using a tubular flame burner. Liquid fuel (ethanol) was sprayed into the tubular
flame burner from the axial direction using a two-fluid nozzle capable of transporting
liquid at a high flow rate, and the characteristics of the resulting flame were evaluated.
When ethanol was sprayed onto the burner with a tubular flame, a uniform tubular
flame was observed. The flame appearance was observed while varying the overall
equivalence ratio, and it was shown that combustion was possible for a wide range of
equivalence ratios. Temperature measurements showed that the flame structure comprised high-temperature gas inside and low-temperature gas outside, and exhibited
the temperature distribution characteristics of tubular flames.
Furthermore, the detailed flame structure and the effect of tubular flame combustion
were investigated by measuring the intensity distribution of the chemiluminescence of
the flames. It was found that when the equivalence ratio of the tubular flame was
outside the combustible range, the base of the flame was lifted even when the overall
equivalence ratio was in the combustible range. In contrast, if the tubular flame was
in the combustible range, a stable flame could be formed from the burner base.
In Chapter 5, we summarize the results obtained in this study and detail the prospects
of the tubular flame system for particle synthesis.

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CONTENTS
1

overview: particle synthesis in flames
1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2 The flame aerosol process . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2.1 Vapor-fed aerosol flame synthesis (VAFS) . . . . . . . . . . . . . .
1.2.2 Liquid-fed aerosol flame synthesis (LAFS) . . . . . . . . . . . . .
1.3 Progress on flame aerosol synthesis . . . . . . . . . . . . . . . . . . . . .
1.3.1 Particle synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3.2 Direct deposition of flame-made particles . . . . . . . . . . . . . .
1.3.3 In situ diagnostics of flame synthesis . . . . . . . . . . . . . . . .
1.4 Combustors for flame aerosol synthesis . . . . . . . . . . . . . . . . . . .
1.4.1 Coflow diffusion flame burner . . . . . . . . . . . . . . . . . . . .
1.4.2 Spray flame burner . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.4.3 Flat flame burner . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.5 Tubular flames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.6 Objectives and outline of the dissertation . . . . . . . . . . . . . . . . . .
2 tubular flame combustion for nanoparticle production
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.1 Particle synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2 Tubular flame burner . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.3 Characterizations . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3 Results and Discussions . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.1 Effect of carrier gas flow rate on morphology and crystallinity of
WO3 particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.2 Formation mechanisms of WO3 nanoparticles by the tubular flame
method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.3 Comparison between tubular flame-made WO3 and premixed
Bunsen flame-made WO3 . . . . . . . . . . . . . . . . . . . . . . .
2.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3 utilization of inner reducing gas region of tubular flames
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.1 Materials . . . . . . . . . . . . . . . . . . . . . ...