22G16
Reforming of Methane to Methanol by a Low-Pressure Steam Plasma
低圧水蒸気プラズマを用いたメタンガスのメタノールへの改質
Takayuki Tsuchiya, and Satoru Iizuka
土屋 貴之, ，飯塚 哲
Department of Electrical Engineering, Graduate School of Engineering Tohoku University
Aoba 6-6-05, Aramaki, Aoba-ku, Sendai 980-8579, Japan980-8579
6-6-05東北大学大学院工学研究科， 〒仙台市青葉区荒巻字青葉

In this study, we studied basic phenomena of a conversion of methane to methanol using a plasma without
using catalysis. The discharge took place under different discharge parameters such as voltage, gas flow
rate, gas-mixing ratio, where methane was mixed with steam at total gas pressure of 1 - 10 Torr. We
observed gaseous organic materials such as ethane, ethylene, acetylene, and methanol. The major productwas carbon monoxide and the concentration ratio of methanol among the gaseous materials containing
carbon was about 30 % under optimized condition. The control of dissociation of methane was a key factor
for methane conversion.

1. Introduction
In recent years, global warming has been aserious social problem. An increment of greenhousegas, such as CO2 and CH4, in the air is one of theimportant causes. It is thought that natural outflowof methane as methane hydrate from the seabedbrings about the global warming, resulting in anabnormal weather phenomenon and a raise of thesea level by dissolution of the ice in the ArcticOcean. The greenhouse effect of CH4 is 21 times asmuch as that of CO2. Therefore, the suppression ofmethane emission into the environment has been acrucial subject to reduce the global warming.
Methane, a main ingredient of natural gas, is in agas state at normal temperature. Thus, the handlingof methane is dangerous, and its long-distancetransportation costs much due to the construction ofpipelines. If we can convert methane to methanol ina liquid state, for example, it would be very
convenient and safety for the transportation andpreservation. Moreover, since methane contains
carbon and hydrogen, it will be also used for thesource of hydrogen gas and hydrogenerated organic
compounds such as ethane and acetylene, together
with other carbon materials such as graphene,carbon nanotube (CNT), diamond-like carbon(DLC), and diamond. These materials will beavailable as semiconductor, luminescence device,solar cell film, and hard film, and so on.
For these reasons, we have developed a plasma
system, in which simultaneous generation ofhydrogen, organic compound, and highly valuablecarbon materials can be produced from methane.
Previously, we examined the generation ofmethanol, as a result of plasma reaction in thecylindrical glass tube, where a mixed gas ofmethane and steam was supplied [1].

Simultaneously, generation of carbon monoxide CO
and polymerization compounds, such as ethane,ethylene and acetylene, were observed as abyproduct. However, these products could not bedisregarded. So, more efficient methanol generation
method should be developed. In case of plasma
decomposition with the mixed gas, superfluous
decomposition occurs for methane molecule, which
will result in a generation of polymerization
compounds and CO. Therefore, methane should not
be directly introduced into the steam plasma, butshould be made contact with steam plasma.
2. Experimental Setup
The outline of the experimental device is shownin Fig.1. The plasma is generated by a directcurrent power supply for a low-pressure glow

Fig.1: Schematic of experimental apparatus

discharge. The electrode is made of stainless, andcovered with an insulator. Here, coaxial small tubes
are adopted, along which reactive gas flows
between the electrode and small grass tubes. The
gas used was a mixture of methane and steam.
Low-pressure steam plasma, generated within asmall tube, makes contact with methane, and thenthey react each other. The plasma state during thereaction was analyzed by optical emission, and thegas components after the discharge were analyzed
by Q-mass or FT-IR. Generation efficiency of theproducts was controlled by changing the plasma
parameters, such as gas flow rate, electric power,pressure, and rearrangement of the dischargeequipment.
We expect reactions in the plasma as follows.
CH4 CH3＊+H＊(1)
H2O OH＊+H＊(2)
CH3＊+ OH＊ CH3OH
(3)

→→→

concentration moles. We also define adecomposition rate of methane as D = <CH4
dissociated> / <CH4 supplied>. From the results inFig. 2(b) we find S ~ 30 % which is much larger
than that in case of Fig. 2(a).
Variation of FT-IR intensity is shown in Fig. 3 as
0.35

methnolformaldehyde
CO0.3

3. Experimental result and discussion
The FTIR spectrum of the gas produced by theelectric discharge with a mixed gas of methane andsteam is shown in Fig.2(a). We observe a methanol(a).] 0.25.u
a[0.2yistn0.15
edn
I 0.1

0.05
01

methanol(b)ethanemethanolethaneethylen
CO0.3.]
u.
a[0.25yistnednI

71012Fig.4: Discharge current dependence.

production as shown by arrows, together with aproduction of ethane and carbon monoxide CO. On
the other hand, in case of the contact discharge ofsteam with methane, the generation of ethane isfairly suppressed, while the generation of methanol
is almost maintained. Generation of CO was stillobserved. Here, we define a selectivity of methanol
as S = <CH3OH> / <Materials contained carbon
produced by the discharge>, where < > denotes

0.35

5

Current [mA]

COFig.2 : FTIR spectra of the gases processed by(a) gas mixture discharge and (b) gas contact
discharge.

0.43a function of methane flow rate under a fixed steam
flow rate. Only methanol and CO are generated fora large steam/methane flow rate ratio. Whenmethane concentration is increased, generation ofmethanol is also increased. At the same time,
however, the reaction for polymerization compound
like ethane is promoted. Here, for the production ofethane, methyl radical will be required. Therefore, it
is considered that methyl radical is a key reactant
for the generation of methanol as expressed in theabove reactions (1)-(3). The production of methanol
is saturated for a large methane/steam ratio.
The discharge current dependency is shown inFig. 4. With an increase of the discharge current,
CO production increases monotonically. The
amount of methanol is also increased, but finallysaturated. At 12 mA, formaldehyde is generated,accompanied with a decrease of methanol
production. So, it is thought that formaldehyde ismade from methanol. Low current discharge isrequired for an efficient methanol generation.
4. Conclusion
It turns out that generation of methanol isefficient when the mixing ratio of methane/steam islow at the low power discharge. Polymerization
reaction also gives an effect for the methanol
production. Control of plasma and gas feeding is akey subject for an efficient methanol production.

0.2

0.15
0.1
0.05
0247

9

methan flow rate [sccm]

10Fig.3: Methane flow rate dependence.

Referenceth[1] T. Tsuchiya et al., 71 Autumn meeting, JapaneseSoc. Applied Physics, 17a-ZH-7 (2010).