22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Plasma temperature measurement of hydrogen RF plasma in microwaveassisted reactor for dechlorination of PCBs
K. Abe1, Y. Inada1, S. Yamaguchi1, A. Kumada1, K. Hidaka1, K. Amano2, K. Itoh2 and T. Oono2
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2
The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, Japan
Tokyo Electric Power Company R&D Center, 4-1 Egasaki-cho, Tsurumi-ku, Yokohama, Kanagawa, Japan
Abstract: In recent years, microwave-assisted process of PCBs (polychlorinated
biphenyls) dechlorination has been developed. The authors found the occurrence of
hydrogen plasmas in the process and pointed out the hydrogen radicals produced in plasmas
may have not a small influence on the dechlorination reaction. In this paper, plasma
temperature in the reactor is measured using the relative emission intensity method.
Keywords: PCBs, microwave, hydrogen plasma, electron temperature, RF plasma
1. Background
Polychlorinated biphenyls (PCBs) had been widely
used as an insulation media because of their good
insulation performance. Due to PCB’s toxicity, their
production is prohibited and their use must be abolished
by 2027 in Japan.
These days, effective declorination method has been
developed [1]. In this process, the mixed solution
composed of PCBs, insulating oil, isopropyl alcohol
(IPA) as a source of hydrogen supply, KOH as an alkaline
substance and Pd/C as a catalyst, is irradiated by
microwave (MW) of 2.45GHz under the nitrogen purge in
order not to ignite the produced hydrogen. The
temperature of the solution is monitored and kept to be
60 ℃ . Using this process, PCBs are declorinated and
turned into harmless biphenyls. Fig. 1 is the chemical
reaction equation of this process [1].
The authors recently reported the occurrence of
hydrogen plasmas including hydrogen radicals in the
dechlorination process [2]. Fig. 2 is the image of light
emission of hydrogen plasmas in the solution. Tiny
bubbles of H 2 are generated in the solution due to the
excessive hydrogen supply from IPA. And in such
bubbles, RF discharge occurs under MW irradiation. In
the previous research [3], plasma temperature was
measured using the relative emission intensity method. Its
value was about 6400-14000K, and the plasma are
composed of active species such as H and H+.
However, this experiment was conducted under the
laboratory scale condition where MW power density was
higher than practical-scale one to generate the hydrogen
plasmas. Therefore, it is needed to confirm whether
hydrogen plasmas are formed or not and its property
changes or not in the practical-scale reactor. In this paper,
new measuring device, which enables capturing twoimages synchronously, was developed and plasma
temperature was measured in the practical-scale reactor.
2. Principles
The preliminary light emission measurement showed
that the duration time of the hydrogen plasmas turned out
to be more than 100ms [3], while the time required to
establish the local thermal equilibrium (LTE) conditions
is roughly 1ns in the case of hydrogen thermal plasmas
[4]. Such longer duration time than the LTE requirement
indicates that the hydrogen plasmas in this study were in
the state of LTE.
Under the LTE conditions, Saha equation, GuldbergWaage equation and state equation are valid, and the
plasmas are electrically neutral. The particle composition
can be calculated as a function of electron temperature.
The each equation is displayed below in order.
N A+ N e
NA
=(
 Ei on 
2πme k BT 32 2 g A+
)
exp − A  ...................... (1)
2
h
gA
 k BT 
Fig. 2. Emission image in insulating oil [2]
Fig. 1. Chemical reaction equation
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1
 E di s 
2πm AmBk BT 32 g A g B
N AN B
)
exp − AB  .................... (2)
=(
2
N AB
g AB
m ABh
 k BT 
P = N t ot al RT ........................................................................ (3)
N H+ = N e ............................................................................. (4)
N is the density of each species, g is the partition
function, Eion is the ionization energy, Edis is the
dissociation energy.
Fig. 3 shows the calculated composition under 0.1MPa,
taking 4 species into account: H 2 , H, H+ and electron.
The plasma temperature of hydrogen plasmas in the LTE
state can also be calculated as a function of the ratio of
radiation intensity I α /I β , which are for H α (=656nm) and
H β (=486nm), respectively. The emitted light from
hydrogen plasmas is composed of line emission [5] and
continuum emission [6]. Continuum emission generally
originates through four processes: recombination,
bremsstrahlung, electron attachment, and electron-neutral
collisions. In this study, electron attachment and electronneutral collisions are ignored, because the preliminary
numerical calculation showed that the continuum
radiation associated with neutral particles was negligibly
small compared to the aforementioned ion-related factors.
Particle densities in the line emission, recombination and
bremsstrahlung terms were cited from Fig. 3. The plasma
temperature variation is plotted in Fig. 4 as a function of a
light intensity ratio of I α /I β .
3. Measurement system
Experimental setup is shown in Fig. 5. It is composed
of two parts, the MW reactor part and the optical part.
The MW reactor was filled with 2kg Pd/C catalyst and
16L solution. The solution was composed of 8L insulating
oil (JIS C2320 class 1–2, high-voltage insulation oil K,
ENEOS) and 8L IPA. The reactor was continuously
irradiated by MW from the magnetron under N 2 purge in
order not to ignite the hydrogen. Temperature of the
solution is monitored and kept to be 55℃ to 60℃ by MW
irradiation time.
Fig. 3. Composition of hydrogen plasma (0.1MPa) [3]
2
The optical part was composed of a beam splitter
module, an image intensifier (I.I., Hamamatsu Photonics
K.K., C-9016-03), and an intensified charge coupled
camera (ICCD, Andor Co., Ltd. DH734-18U-03).The
beam splitter module consist of a dichroic mirror and
interference filters (656nm, 486nm), whose wavelengths
are correspond to the emitted spectra of H α and H β .
With this beam splitter module, two images, each image
corresponds to the light-emission image through each
interference filter, can be measured simultaneously by one
ICCD camera. As shown in Fig. 4, the relationship
between plasma temperature and emitted light intensity
ratio I α /I β has steep gradient around I α /I β =0.56, so it is
hard to measure plasmas over 20000K.
4. Experiment Result
Fig. 6 is the image of light emission on Pd/C catalyst in
the reactor under MW irradiation, which was captured
using above-mentioned optical system. The output power
of the magnetron was 820W. The left half side is the
656nm emission image and the right half side is the
486nm emission image. The hydrogen plasma was
generated in a small H 2 bubble on Pd/C and the light
Fig. 4. Relationship between plasma temperature and
emitted light intensity ratio I α /I β [3]
Fig. 5. Experimental setup
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emitting spot from such plasma is recognized as marked
by a red circle in this figure. By taking the 656nm and
486nm light intensity ratio of the total spot area, the
plasma temperature was obtained. In this research,
median value was adopted as the representative value of
plasma temperature.
Fig. 7 shows the influence of MW power on the median
electron temperature. Median electron temperature hardly
seems to depend on the MW power, and its value is
approximately 8000K-10500K. In the laboratory-scale
reactor, the median plasma temperature was about 6400K14000K. There are small differences between the two.
Therefore, it is natural to say that the scale of the reactor
has little effect on the plasma temperature. As shown in
Fig. 3, in hydrogen plasmas over 8000K, most hydrogen
molecules are decomposed into hydrogen atoms, and
several percent of hydrogen atoms are ionized. It is
natural to say that such hydrogen radicals contribute to
PCBs dechlorination reaction denoted in Fig. 1.
Fig. 8 shows the influence of MW power on the number
of light emission events per 100 pictures. The number of
light emission events increased with MW power. In the
previous study with the laboratory-scale reactor [3], event
number was at least 20 times per 100 pictures. This
difference may be caused by the difference of the local
MW power in the reactor. For further quantitative
discussion, it is necessary to get the precise information
on MW power distribution and the temperature
distribution of the observed region in the reactor.
5. Conclusion
A new hydrogen-plasma temperature measuring device
is developed based on the relative emission intensity
method. With this device, hydrogen plasma temperature
in microwave-assisted practical-scale reactor for PCBs
dechlorination was measured. It turned out that hydrogen
plasma temperature in the reactor is about 8000K-10500K,
where the plasma is mainly composed of active species,
i. e., hydrogen radicals.
Compared with the hydrogen plasma in laboratory-scale
reactor, small differences were recognized in the plasma
temperature in spite of the fact that the number of plasma
decreased in practical-scale reactor. Due to such decrease
in number, less hydrogen radical seems to be generated.
This can affect dechlorination efficiency.
Fig. 6. Two-dimensional light intensity distribution for H
α and H β (MW power 820W)
Fig. 8. Influence of MW power on light emission event
number
Fig. 7. Influence of MW power on median electron
temperature
P-III-9-1
6. References
[1] K.Amano et al., Proceedings Book 10th International
Conference on Microwave and High Freqency Heating,
pp.60-63(2005)
[2] A. Kumada et al., Applied Physics Letters, 99,131503,
pp.1-3 (2011)
[3] Y. Inada et al., Applied Physics Letters, 105,174102,
pp.1-4 (2014)
[4] L. Spitzer, J Physics of Fully Ionized Gases, 2nd Rev.
Ed., Interscience Publishers , pp. 131-136 (1962)
[5] W. L. Wiese et al., J. Phys. Chem. Ref. Data 38, 565
(2009).
[6] F. Cabannes et al., Reactions Under Plasma
Conditions, Wiley Interscience, pp. 357-470 (1971)
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