22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Improvement of hydrogen production from glucose by radio-freqeuncy in-liquid
plasma at atmospheric pressure
F. Syahrial1,2, S. Nomura1, S. Mukasa1 and H. Toyota1
1
2
Graduate School of Science and Engineering, Ehime University, Ehime, Japan
Center of Advanced Research on Energy, Faculty of Mechanical Engineering, Universiti Teknikal Malaysia Melaka,
Malacca, Malaysia
Abstract: Radio-frequency (RF) in-liquid plasma was used to produce hydrogen from
glucose at atmospheric pressure. An improvement of the method was made by means of
applying counter electrode RF in-liquid plasma and liquid bubbling flow RF in-liquid
plasma in order to enhance the hydrogen production. These methods have resulted in a
significant enhancement of the hydrogen production rate, yield and production efficiency.
Keywords: Radio-frequency, In-liquid plasma, Hydrogen, Ultrasonic vibration, Glucose
1. Introduction
Advanced oxidation processes (AOPs) in liquid have
received rapid and intense attention for environmental and
biomedical application. Plasma in-liquid is generated due
to the electrical breakdown of a liquid when an electric
field within bubbles sufficiently increases to the point of
causing the breakdown [1]. The bubbles are very
important for the generation of plasma in liquid [2].
Plasma appears within a bubble generated from the
evaporation of the liquid by the heating process [3].
Plasma discharge in water has been demonstrated for
producing hydrogen peroxide [4], molecular oxygen and
hydrogen [5] and hydroxyl [6], hydroperoxyl, hydrogen,
oxygen and other reactive radicals [7], [8]. The physical
processes resulting from plasma discharge include
formation of bubbles [9], possibility of supercritical fluids
condition development, production of high temperature
high pressure, localized regions and the formation of
shock and acoustic waves [10]. Both the physical and
chemical effects from plasma discharge may be important
for promoting desired chemical reactions [11].
Plasma in liquid is favourable for many AOP processes
due to the reactive radicals produced and physical effects
which have been reported about previously regarding
synthesizing diamonds [12] and nanoparticles [13] as well
as production of hydrogen gas from methane hydrate [14].
In this study, radio frequency (RF) in-liquid plasma from
an ultrasonic transducer was used to decompose a
biomass to produce hydrogen using glucose as a model.
Counter electrode RF in-liquid plasma with ultrasonic
vibration and liquid bubbling flow RF in-liquid plasma
were applied as improvement methods in order to enhance
the production rate, hydrogen yield and production
efficiency.
2. Experimental apparatus
Fig. 1 shows the schematic diagram of the RF in-liquid
plasma and ultrasonic vibration apparatus for hydrogen
production. An electrode consisting of a 3 mm diameter
P-I-3-2
copper rod enveloped by a glass pipe (used as a dielectric
substance) was inserted from the bottom of the
polycarbonate reactor vessel. Plasma was generated with
the impedance and input power of 27.12 MHz RF
generator simultaneously adjusted with a matching box.
The in-liquid plasma was generated at the tip of the
electrode.
Fig. 1. Experimental apparatus
The ultrasonic transducers used for inducing ultrasonic
vibration were a 29 kHz horn-type ultrasonic transducer
was attached to the top of reactor and a 1.6 MHz
piezoelectric transducer to its side. The input power of the
ultrasonic transducers was 30 W.
For the counter electrode RF in-liquid plasma vibration
experiment, a horn-type ultrasonic transducer was used as
a counter electrode. Additionally, a motor operating at
8 V was used to circulate the solution within the reactor
during liquid bubbling flow experiment.
The glucose solution was 200 mL with a 20 wt% initial
concentration. The pressure of the reactor was reduced to
0.01-0.02 MPa by an aspirator in order to facilitate the
1
3. Results
3.1 Plasma emission spectrum
A typical optical emission spectrum of glucose
decomposition by RF in-liquid plasma is shown in Fig. 2.
The observed radical species include OH (281.1nm), H β
(486 nm), H α (656.3 nm) and O (777 and 845 nm).
Undoubtedly, the existence of active C species in the
emission spectrum as shown in the figure clarified that the
glucose solution was decomposed into precursors or
intermediaries in the gas products. The electron
temperature within the plasma was estimated to be
between 3300 and 4800 K [15].
Hα
10000
5000
OH
400
O
C
600
800
Wavelength (nm)
Fig. 2. Emission spectrum of plasma in glucose solution
3.2 Initial concentration
In previous work [16], the utilization of RF in-liquid
plasma with ultrasonic vibrations enhanced the hydrogen
production rate as shown in Fig. 3. As initial
concentration increases, the hydrogen production rate
increases with the greatest enhancement achieved at
approximately 11 and 30% for the highest initial
concentration when RF in-liquid plasma was generated
along with 29 kHz ultrasonic transducer and with 1.6
MHz piezoelectric transducer, respectively.
2
RF
RF+29kHz
RF+1.6MHz
10
0
0
10
20
Fig. 3. Hydrogen production rate
3.3 Liquid bubbling flow
By applying liquid bubbling flow to the RF in-liquid
plasma for decomposition of glucose, the hydrogen
production rate tends to increase over that of no bubbling
condition as shown in Fig. 4. The voltage of the motor
used for circulating the liquid flow was 8 V. Glucose
droplets formed in the flowing liquid provide a relatively
large contact surface area for the plasma, which affects
the chemical reactions of the radical species formed
within the water droplets. In addition, formation of
hydrogen peroxide, which contributes to hydroxyl
radicals formation, is dependent on the liquid flow [17],
[18].
O
Hβ
0
200
20
Initial concentration (wt%)
Hydrogen production rate (μmol/s)
Intensity (a.u.)
15000
Hydrogen production rate (μmol/s)
plasma generation by expelling air. The RF input power
of 150 W was calculated by subtraction of the reflected
power from the generated power. The reflected power
was constantly maintained at the possible lowest value.
The gas produced was collected by a gas tight glass
syringe. It was analysed using a gas chromatograph (GC14A Shimadzu) to determine the composition and
concentration of the product gas. Argon gas was used as
the carrier gas.
30
Flow
No flow
20
10
0
0
500
1000
Elapsed time (s)
Fig. 4. Hydrogen production rate as elapsed time
3.4 Counter electrode
A counter electrode was applied to the RF in-liquid
plasma using a 29 kHz horn-type ultrasonic transducer
and a 1.6 MHz piezoelectric transducer in order to
observe the enhancement of hydrogen production over
that of previous study [16]. A horn-type ultrasonic
transducer used as a counter electrode (CE) was placed 2
mm from the tip of the electrode. As shown in Fig. 5, the
P-I-3-2
22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
40
35
30
25
20
15
10
5
RF
RF+29kHz RF+1.6MHz Bubbling
CE :
CE :
Flow : RF RF+29kHz RF+1.6MHz
Fig. 7. Hydrogen production rate from various methods
20
0
0
500
1000
Fig. 5. Hydrogen production rate as elapsed time
3.5 Comparison of hydrogen production
Analysis of the gas produced indicated that apart from
hydrogen (H 2 ), oxygen, carbon monoxide (CO), methane
(CH 4 ), carbon dioxide (CO 2 ), acetylene (C 2 H 2 ) and
ethylene (C 2 H 4 ) gases were produced as determined by
the gas chromatograph as shown in Fig. 6.
Hydrogen yield (%)
45
0
40
Elapsed time (s)
100%
C2H4
90%
C2H2
80%
CO2
70%
CH4
60%
CO
50%
O2
H2
40%
30%
20%
Fig. 7 shows improvement of the hydrogen production
rate according to the methods used. As can be seen, there
is a gradual increase of hydrogen production rate until the
counter electrode is applied. The methods of counter
electrode RF in-liquid plasma with 29 kHz ultrasonic
vibration and 1.6 MHz piezoelectric transducer indicated
250% and 200% enhancement over that of RF in-liquid
plasma, respectively, which are the highest enhancement.
The relative enhancement of liquid bubbling flow RF inliquid plasma compared to RF in-liquid plasma alone was
90%.
2
1
0
RF+29kHz
RF+1.6MHz
Bubbling Flow : CE : RF+29kHz
RF
CE :
RF+1.6MHz
Fig. 8. Hydrogen efficiency enhancement
10%
0%
RF
RF+29kHz RF+1.6MHz Bubbling
Flow : RF
CE :
CE :
RF+29kHz RF+1.6MHz
Fig. 6. Gas yield produced from various methods
The hydrogen (H 2 ) yield increased as ultrasonic
vibrations, liquid bubbling flow through the solution and
counter electrode methods were applied to the RF in-
P-I-3-2
50
Hydrogen production rate (μmol/s)
60
RF : no counter electrode
RF+29kHz : 2mm
RF+1.6MHz : 2mm
liquid plasma. The highest hydrogen yields were 70%
when using counter electrode RF in-liquid plasma with
the 1.6 MHz piezoelectric transducer and liquid bubbling
flow RF in-liquid plasma method. Additionally,
production of the green-house gas carbon monoxide (CO)
tended to decrease with the improvement of
decomposition of glucose in solution brought on by these
methods. Methane (CH 4 ) and carbon dioxide (CO 2 )
showed no significant change. Relatively small yields of
acetylene (C 2 H 2 ) and ethylene (C 2 H 4 ) were also
indicated, but the amounts were so small as to be
negligible.
Enhancement (%)
Hydrogen production rate (μmol/s)
hydrogen production rate when using counter electrode
RF in-liquid plasma with a 29 kHz ultrasonic transducer
and counter electrode RF in-liquid plasma with a 1.6 MHz
piezoelectric transducer increased 130% and 100%,
respectively, compared to that of the RF in-liquid plasma
method without a counter electrode.
Plasma channels produced in the method using counter
electrode RF in-liquid plasma induced highly energetic
electrons along the plasma channels, strong ultraviolet
radiation and shockwaves. Thus, the water molecules
were more greatly excited and ionized which then cause
the distribution of radical species mainly around the
plasma channels [19]. A larger number of radical species
might more effectively decompose a larger number of
glucose molecules.
Fig. 8 shows the hydrogen efficiency enhancement
percentage of liquid bubbling flow RF in-liquid plasma
and counter electrode RF in-liquid plasma applied with
ultrasonic vibration over that of previous methods. The
hydrogen production efficiency is defined as output
energy versus input energy as shown in Eq. 1, where n H2
is the molar flows of hydrogen, which when multiplied by
3
the lower heating value (LHV H2 ) is the energy in the
outlet stream. The energy in the inlet stream is defined as
total energy of generated power of radio-frequency (E RF )
and power input of ultrasonic transducer (E US ).
ηH =
2
n H 2 LHVH 2
E RF + EUS
(1)
Obviously, the counter electrode RF in-liquid plasma
with 29 kHz ultrasonic transducer resulted in the highest
hydrogen production efficiency of approximately 1.6
followed by the method using counter electrode RF inliquid plasma with 1.6 MHz piezoelectric transducer
which was 1.3 due to their highest mole achievement.
Also note, the hydrogen production efficiency ratio of
liquid bubbling flow RF in-liquid plasma was 0.60.
4. Conclusion
The methods used in this study resulted in improvement
in higher production rate, hydrogen yield and hydrogen
production efficiency over previous method.
Liquid bubbling flow RF in-liquid plasma provided a
relatively large contact surface area with plasma affecting
the chemical reactions of the radical species formed in the
water droplet for higher hydrogen production. It was also
found that counter electrode RF in-liquid plasma with
ultrasonic vibration had a significant enhancement on the
hydrogen production due to highly energetic electrons in
plasma channels, strong ultraviolet radiation and
shockwaves.
These two improvement methods could be considered
as
promising
techniques
for
more
detailed
experimentation in the future.
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