24P117-P
Study on methane decomposition inplasma
プラズマ中でのメタン分解に関する研究
Kazunari Katayama, Satoshi Fukada, Masabumi Nishikawa
片山一成, 深田 智，西川正史
Interdisciplinary Graduate School of Engineering Sciences, Kyushu University,
6-10-1, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan
九州大学大学院総合理工学研究院 〒813-0041 福岡市東区箱崎6-10-1
In the fuel cycle system of a fusion reactor, tritium is extracted from exhaust gas and reused. Plasma
decomposition is one of techniques for extracting hydrogen from hydrocarbon. In order to evaluate a direct
decomposition of methane using helium RF plasma, a flow-type plasma reactor utilizing capacitively
coupled plasma was developed and direct decomposition of methane was investigated varying supplied RF
power and total gas pressure. The overall decomposition rate of methane in helium plasma increased
proportionally to the supplied RF power but decreased with increasing total pressure.

1. Introduction
Impurities in the exhaust gas from a vacuum
cumber of a fusion reactor are removed in a fuellean up system. Tritium contained in impurities
gases is extracted in a tritium recovery system.
Some amounts of hydrocarbon are included in theexhaust gas, when graphite materials are used in apart of plasma facing components. Especially, aconsiderable amount of hydrocarbon will bereleased in a discharge-cleaning to aim tritium
recovery from plasma facing wall. In tritium
handling facility, tritiated hydrocarbon isgenerally oxidized in a catalytic bed andrecovered in an adsorption bed as water vapor.
However, it is preferable that tritium is collected
as molecular, considering it is reused as a fuel.
Direct decomposition methods of methane withgettering materials [1-3] and Ni catalyst [4] havebeen investigated. In these ways, a regeneration
process using hydrogen or oxygen is needed
because decomposition performance deteriorates
by carbon deposits. Moreover, the heating for theimprovement of decomposition rate causes
permeation loss of tritium. Direct decompositionby plasma is one of candidate techniques fortritium recover from hydrocarbon. It is consideredthat the deterioration of decomposition
performance of plasma by carbon deposits issmall because the decomposition reaction iscaused by the collision of hydrocarbon withelectron in plasma. Therefore, it is expected thathydrocarbon is decomposed continuously for along time without a removal process of carbon
deposits. In the present study, decomposition
behavior of methane in helium RF plasma wasinvestigated varying supplied RF power and gas
pressure.

2. Experimental
A flow-type plasma reactor utilizing capacitively
coupled plasma was used for observing a direct
decomposition behavior of methane. Fig.1 shows aschematic diagram of the experimental apparatus.
Details of the size of the plasma reactor and theexperimental procedure have been mentioned in [5].
The observation of components in the outlet gas
was performed using a quadrupole mass
spectrometer.
Thedecomposition
reactionprogresses stepwise by the collision of methane andelectron in plasma. When the decomposition rate is
assumed to depend on first-order in methane
concentration and the reverse reaction can beignored, the following equation is given:
dCCH 4(1)Q  k decompCCH 4dv
where Q is the volumetric flow rate in plasma[m3/s], CCH4 is the methane concentration [mol/m3],
v is the micro-volume in plasma [m3], kdecomp is theoverall reaction rate constant [1/s]. When the total
volume of plasma is represented as V [m3], thefollowing equation is obtained by integration ofeq.(1).
Sample gas
Inlet
RF power

Matchingbox

Earthshield

Quadrupole mass
spectrometer

Mass flow
controller

Leak valve

Butterfly valve

Rotarypump

ElectrodePort 1
Port 2

Piranigauge

Port 3

Vacuum chamber

Plasma500mm

Fig.1 A schematic diagram of the experimental
apparatus.

1Q(2)

3. Results and discussions
Fig.2 and Fig.3 show the volumetric flow ratedependence on the decomposition rate at 150Pa and870Pa. The logarithm natural of methane
decomposition rate decreases linearly withincreasing the inverse of the volumetric flow rate.
This tendency was also observed in the experiment
at 370Pa. It was confirmed that the decomposition
rate has large pressure dependence. The overall
decomposition rates were obtained from thegradient of each line. These are compared in Fig.4,
where the values have been reported in [5] warealso shown. The decomposition rate is clearly
proportional to the supplied RF power. On the otherhand, it decreases with increasing total pressure.
These phenomena are speculated to be related to theelectron density and energy in plasma. The electron
density at around port 2 was measured by Langmuirprobe method. It was found that the electron density
linearly increases with increasing the supplied RFpower. The present authors considered that adecomposition rate decreases depending on totalpressure since an electron loses energy by thecollision with neutral particles when the pressure ishigh.
4. Summary
A flow-type plasma reactor utilizing
capacitively coupled plasma was developed anddirect decomposition of methane was investigatedvarying supplied RF power and gas pressure. The
overall decomposition rate of methane in helium
plasma increased proportionally to the suppliedRF power but decreased with increasing totalpressure.

10010W
20W
30W
40W
50W
60W

10-1

Total pressure:150Pa
CCH4,in=10040ppm
0200

400
31/Q [s/m ]600

Fig.2 Decomposition rate at 150Pa.
1001010W
20W
30W
40W
50W
60W

-1

Total pressure:870Pa
CCH4,in=10040ppm
400

600
31/Q [s/m ]800

Fig.3 Decomposition rate at 870Pa.

8Electrode length:26cm
CCH4,in:10040ppm150Pa

This work[5]250Pa

6References[1] K.Watanabe et al., Decomposition of methane on
Zr-Ni alloys, Fusion Eng. Des. 39-40 (1998)
1055-1060.
[2] Satoshi Fukada et al., Zr2Fe and Zr(Mn0.5Fe0.5)2
particle beds for tritium purification and impurity
removal in a fusion fuel cycle, Fusion Eng. Des.
49-50 (2000) 805-809.
[3] Takao Kawano et al., Methane decomposition
properties of ZrNi alloy in flowing gas system,
Fusion Eng. Des. 49-50 (2000) 863-872.

-33Acknowledgments
This work was supported by Grants-in-Aid forchallenging Exploratory Research (KAKENHI23656575)[4] Satoshi Fukada et la., Effects of temperature,oxygen-to-methane molar ratio and superficial gas
velocity on partial oxidation of methane forhydrogen production, Int. J. Hydrogen Energy 29(2004) 619-625.
[5] K. Katayama et al., Direct decomposition ofmethane using helium RF plasma, Fusion Eng.
Des. 85 (2010) 1381-1385.

CCH4,out/CCH4,in

CCH 4 ,in

 k decompV

CCH4,out/CCH4,in

CCH 4 ,outkdecompV [×10 m /s]

ln4370Pa370Pa
2870Pa950Pa001020304050RF power [W]

60

70

80

Fig.4 Dependence of the decomposition rate on thesupplied RF power and total pressure.