23P015-O
Natural gas reforming method using an inverter plasma
インバータープラズマを用いた天然ガスの改質
Satoshi Sugimoto1, Yasushi Matsuda2

杉本敏司1, 松田恭司2

1Osaka University, School of engineering
2-1,Yamada-oka, Suita, Osaka 565-0871, Japan
大阪大学院工, 〒565-0871 大阪府吹田市山田丘2-1
2Sumitomo Metal Technology, Inc .
1 Higashimukoujima Nishino-Cho, Amagasaki 660-0891, Japan
住友金属テクノロジー(株)〒660-0856 尼崎市東向島西之町１番地

The characteristics of a natural gas reforming method for hydrogen production were investigated. Thereforming method combines a plasma process operated by an inverter power supply with a catalytic
reaction to improve the energetic efficiency for reforming. A simple process model is presented andexamined using a methane gas reforming experiment. The validity of the process model and thetemperature dependency of the catalyst activity are discussed with respect to the results.

1. Introduction
To reduce CO2 gas emission to the atmosphere,
it is hoped that natural gas, including a majority ofCH4, will be used in a reforming method that
decomposes CH4 into H2 gas and C-containing
components [1]. Reforming methods that employ
plasma discharge have advantages in that no CO2 is
produced through the reforming process itself andthe process plant can be manufactured on a compact
scale for application in locations such as gas
stations and farms. [2,3]. Improvement of thereforming energy efficiency, i.e., the ratio of the
combustive energy of produced H2 divided by theelectric power for plasma discharge, is required torealize practical application.
We propose a CH4 gas reforming method that
employs a plasma discharge generated using aninverter power supply with high electric power
efficiency. This method features a reformingprocess that involves the combination of the plasma
chemical reaction and a catalytic reaction toimprove the reforming energy efficiency.
In this article, we describe the process model forthe proposed method and the experimental
apparatus. We then present experimental results anddiscuss the characteristics of the reforming process.
2. Process Model
The simple process model for CH4 gas
reforming is shown in Fig. 1. The modeled process
is divided into stages I and II. In stage I, raw CH4
gas is supplied to the stage I area and is excited byelectrical plasma discharge. A portion of CH4 isdissociated as CH4 → CH3 + H. The excited gas,
which includes CH3 radicals, flows into the stage II
area and is exposed to a catalytic material coated onthe substrate. Pure carbon or hydrocarbon isdeposited on this substrate from the CH3 radicals.
The substrate is heated to TII to maintain activationof the catalyst.

Stage I

Stage II(discharge)(deposition)

WP
CH4(1-kI) CH4kI H

kI

k ICH3ＴII( kI(1-kII)CH3)(1-kI)CH43k Ik II H
k Ik II C k (1+3k )HIIIHeaterWH
SubstrateFig.1. Model for the proposed reforming process.
CH4 gas is introduced and excited at stage I. Carbon
from the excited gas is deposited on the heatedsubstrate at stage II. The output gas includes
produced H2 gas and unprocessed CH4 gas.

Here, the dissociation rate of CH4 at stage I andthat of CH3 radicals at stage II are defined as kI andkII, respectively. The H2 production rate is thus
expressed as k I(3 kII +1). In addition, the electrical
power consumed at stage I and the combustiveenergy of liberated H2 gas are defined as WP and WH,
respectively. The reforming energy efficiency is
calculated as WH/WP.

3. Experiment
The configuration of the experimental apparatus
used to examine the process model is shown in Fig.
2. A plasma is generated between upper and lower
parallel-plate electrodes. A substrate is placed on
the lower electrode with a heater. Stainless steel 304
alloy plate, which contains the catalytic component(Fe), was used as the substrate. When the substratetemperature reached TII under vacuum, CH4 gas wasintroduced into the reaction chamber up to 600 Pa
and the plasma discharge was maintained for afurther 40 min.
Gas partial pressures in the reaction chamber
were analyzed using a gas analyzer with aquadrupole mass-spectrometer and a differential
evacuation system. The mass of carbon depositedon the substrate was measured using an electronicbalance after processing. The morphology of thedeposited carbon was also observed using scanningelectric microscopy (SEM).
Substrate (SUS304)

Cu++00

Ch4

Plasma-Gas
FeederInverter Power
SupplyGas
Analyzer

SubstrateHeater(a)(b)
Fig.4. SEM images of substrate surfaces afterprocessing at substrate temperatures of (a) TII ≈ 350and (b) TII ≈ 450 °C.

Upper/LowerElectrode)

Reaction
Chamber

images of the substrates after processing. The
catalyst was activated in the temperature region
greater than ca. 450 °C and a larger amount ofcarbon was deposited.
In the lecture, we will describe the validity of theprocess model and the dependency of the reforming
energy efficiency on TII.

RotaryPump

Fig.2. Experimental apparatus configuration for theCH4 reforming process.

4. Results and Discussions
Typical time evolutions of the H2 and CH4 partialpressures in the reaction chamber are presented inFig. 3. The production of H2 gas using the proposed
plasma process was confirmed. Here, we focus on
the relation between carbon deposition and thesubstrate temperature, TII. Figure 4 shows SEM

Fig.3. Experimental apparatus configuration for
the CH4 reforming process.

Acknowledgments
This work was partially supported by
Grant-in-Aid for Scientific Research (22612002)
from the Japan Society for the Promotion ofScience.
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