22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Hydrogenation of CO 2 to CH 4 using a low-pressure cross-field pulse discharge
with hydrogen
K. Arita and S. Iizuka
Department of Electrical Engineering, Graduate School of Engineering, Tohoku University, Aoba 6-6-05, Aramaki,
Aoba-ku, JP-980-8579 Sendai, Japan
Abstract: Conversion of CO 2 to CH 4 , using a low-pressure square-pulse cross-field
discharge, was investigated by changing the discharge parameters such as applied voltage,
gas flow rate, and so on. Carbon dioxide was reduced by hydrogen. The discharge took
place across the magnetic field inside a glass tube. Preferable improvements of CO 2
decomposition, CH 4 selectivity, and energy efficiency were established.
Keywords: carbon dioxide, methane, cross-field pulse discharge
1. Introduction
Carbon dioxide CO 2 is one of man-made greenhouse
gases that are emitted by combustion of fossil fuels, such
as coal, oil, and natural gas. Carbon dioxide is emitted
from many power plant for generating electricity, power
vehicles, heat homes, cook food and much more.
However, fossil fuels are essentially a non-renewable
energy source. Within the next 100 years it is widely
believed that the cost of finding and extracting new
underground resources will be much more expensive for
everyday use. It might be also serious that CO 2 would
cause global warming by absorbing and emitting radiation
within the infrared range.
Therefore, the reduction of emission of carbon dioxide
and the reduction of consumption of fossil fuels are
crucial subject that must be settled urgently.
The purpose of this study is to investigate fundamental
process of reduction of carbon dioxide to generate
beneficial and reusable organic materials like methane
and methanol by using low-pressure discharges [1, 2].
Here, we define decomposition ratio of CO 2 as α (%) (=
[CO 2 decomposed] / [CO 2 supplied]) ， methane
selectivity as β (%) (= [CH 4 produced] / [all carbon
species produced]). Here, [a] denotes amount of a. These
values show how much carbon in CO 2 has been converted
to methane.
The energy efficiency of methane
production, defined as γ (L/kWh) (= [CH 4 produced in
litter] / [energy consumed for the discharge]), is also an
important factor to realize a suitable system for producing
methane in high efficiency.
2. Experimental apparatus
Fig. 1 shows a schematic of the experimental apparatus.
Mixed gas of carbon dioxide and hydrogen was fed to the
discharge chamber made of glass tube. The gas-mixing
ratio and the total flow rate were controlled by mass flow
controllers, independently. Total pressure was fixed at
200 Pa. Here, we employed a square-pulse voltage that
was supplied to a small electrode. Repetition frequency
of the square pulse was variable from 10 kHz to 100 kHz.
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The gas passing through the discharge region was
evacuated by a rotary pomp. Fourier transform infrared
spectroscopy (FTIR) was employed to analyse the gas
species before and after the discharge.
B E
S
N
S
N
Fig. 1. Schematic of experimental setup.
In order to proceed a cross field discharge, two kinds of
permanent magnets were employed. One was a ring
magnet made of samarium cobalt, the magnetic strength
of which was 0.4 T at the end surface. The other one was
a rod magnet made of samarium cobalt, the magnetic
strength of which was also 0.4 T at the end surface.
Inside the ring magnet a powered electrode was inserted
for the discharge as shown in Fig. 1. The grounded
electrode was a stainless tube, which surrounded the rod
magnet. Since the magnetic field and the electric field
supplied by the powered electrode could intersect with an
oblique angle each other, we could perform a cross field
discharge.
From this configuration we could investigate how the
cross field discharge gave an effect on the hydrogenation
of CO 2 to CH 4 .
3. Experimental results and discussions
In this experiment we fixed applied voltage at 1.4 kV
and pulse repetition frequency at 1.25 kHz. Flow rate of
CO 2 was also fixed at 1 sccm. The change of gas species
measured by FTIR showed that the main product was CO
through the whole experiment. Here, CO might come
from the following dissociation reaction by the electron
impact; e + CO 2 → CO + O. Hydrogenated species was
only CH 4 , and the other CHOH and CH 3 OH were not
detected or were negligibly small. We could not detect
1
25
w: B field
25
20
15
w/o: B field
10
5
0
5
0
10
15
H2 [sccm]
20
25
Fig. 3. Variation of CH 4 selectivity β as a function of H 2
flow rate.
γ = αβΓ/P in .
w/o: B field
10
5
0
0
5
10
15
20
25
H2 [sccm]
Fig. 2. Variation of CO 2 decomposition ratio α as a
function of H 2 flow rate.
In our system, as shown in Fig. 2, α was roughly
independent of the hydrogen flow rate at the fixed input
electric power. However, β has a strong dependence on
H 2 flow rate in the regime less than 10 sccm, as shown in
Fig. 3. In the lower H 2 flow rate regime β varied almost
in proportional to H 2 flow rate. The difference between
the cases with and without the magnetic field was not
much, although we could get a larger value of β in the
range less than 10 sccm in the cross field discharge. This
result was preferable because we did not need much
hydrogen for the conversion of CO 2 to CH 4 . The
maximum of β attained about 27 % when the cross field
discharge was employed.
In the lower H 2 flow rate regime the reaction was
determined by the amount of H 2 supplied. According to
the following reaction the production of CH 4 was
increased with an increase in H 2 flow rate:
CO 2 + 4H 2 = CH 4 + 2H 2 O (gas)
(1).
However, since CO 2 flow rate was fixed, the CH 4
production would be saturated even when H 2 was
supplied much more than CO 2 . Taking account of the
simultaneous CO production it was reasonable that
optimum amount of H 2 for the CH 4 production exceeded
(2).
Here, Γ is input flow rate of CO 2 and P in is input electric
power for the discharge. The input power was usually
less than 30 W. In Fig. 4 it was found that γ in the case
with magnetic field raised almost 3 times as much as
those without magnetic field. The maximum value of γ
attained about 0.46 L/kWh when H 2 flow rate was about
10 sccm.
0.50
0.40
γ [L/kWh]
15
α [%]
30
Finally, we evaluated energy efficiency γ, which could
be derived from the following relation:
w: B field
20
2
stoichiometry value of 4 sccm. On the other hand, when
H 2 was much more increased, the transit time of H 2 gas
flow within the discharge region was decreased, which
might result in a decrease of CH 4 production.
β [%]
other organic materials such as ethane, ethylene, and
acetylene. But, the production of steam H 2 O was
detected. Therefore, it was shown that methane was
actually only a hydrogenated product from CO 2 .
Therefore, in this case, methane selectivity β could be
simply expressed by β = [CH 4 ] / ([CH 4 ]+[CO]).
First, the dependences of α on hydrogen flow rate were
examined before and after the magnets were installed. As
shown in Fig. 2 the CO 2 decomposition ratio α increased
with H 2 flow rate in a range less than 5 sccm. Then, α
was almost saturated and attained the maximum of α =
about 16%. However, when the magnetic field was
introduced, α was more raised up compared to the case
without the magnetic field. However, total dependence
on H 2 flow rate was almost similar to that without the
magnetic field, although the optimum H 2 flow rate was
shifted to the larger value.
w: B field
0.30
0.20
w/o: B field
0.10
0.00
0
10
20
30
H2 [sccm]
Fig. 4. Variation of energy efficiency γ as a function of
H 2 flow rate.
It should be also noted that γ in our discharge system
was one order of magnitude higher than that of
conventional high-pressure dielectric-barrier discharge
(DBD). The energy efficiency in the case of DBD was
reported to be 0.024 L/kWh, where α = 12.4%, β = 3.2%,
and Γ = 500 sccm for the input power of 500 W [3].
Therefore, it was found that energy efficiency in our case
was about 20 times as much as that in the DBD case.
4. Conclusion
Methane was produced from carbon dioxide in a lowpressure square-pulse discharge with hydrogen. Methane
was only organic species produced from CO 2 . Only CO
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was detected as non-organic by-product. We found that
the decomposition ratio α of CO 2 , methane selectivity β,
and energy efficiency γ were superior to those of the
magnetic-field free case. Therefore, the low-pressure
cross field discharge was effective for an efficient
conversion of CO 2 to CH 4 .
5. References
[1] M. Kano, G. Satoh and S. Iizuka. Plasma Chem.
Plasma Process., 32, 177 (2012)
[2] T. Tsuchiya and S Iizuka. J. Environm. Engng.
Technol., 2, 35 (2013)
[3] B. Eliasson, et al. Ind. Eng. Chem. Res., 37, 3350
(1998)
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