22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
CH 4 -CO 2 reforming in surface-discharge reactor containing ZnO-Cu and NiO
catalysts - Influence of the applied power on products distribution
M. Nikravech, A. Rahmani. C. Lazzaroni and K. Baba
LSPM–CNRS, Institut Galilée, Université Sorbonne Paris Cité, Paris 13, Av. J.B. Clément, FR-93430 Villetaneuse,
France
Abstract: Dry reforming of methane was carried out in DBD discharge reactor. The
influence of catalyst’s nature and the applied power on conversion rates of CH 4 and CO 2
are emphasized. The distribution of liquid hydrocarbons and their dependency on applied
power as well as on the nature of catalysts are highlighted.
Keywords:
CH 4 , CO 2 , biogas, reforming, catalysts, surface discharge, liquid
hydrocarbons
1. Introduction
Methane and carbon dioxide constitute the most
important greenhouse gases. On one hand, methane
remains an abundant molecule as the principal component
of natural gas, and also produced with carbon dioxide
during the fermentation of organic wastes. On the other
hand, carbon dioxide is produced during the combustion
of fossil hydrocarbons in energy plants and transports
contributing to global warming and climate changes.
As a result, the international institutions and
governments plan to reduce drastically the emissions of
these components.
One important European
Community’s directive recommends the 20% reduction of
greenhouse gas emissions prior to 2020, while a second
one recommends achieving at least a 10% share of
renewable energy in the total gasoline and diesel
consumed in transport by 2020 [1]. Biogas is intended to
play a major role in the management of organic wastes
and to provide a significant share in energetic
independency of EU’s states. The theoretical potential of
the primary energy production from biogas in 2020 is 166
million tonne oil equivalent in EU [2, 3]. The actual
tendency is to use biogas to produce heat and electricity.
However, the production of liquid hydrocarbons from
biogas constitutes an important challenge that provides an
effective source of second generation biofuels and
chemicals, leading to substantial economies in imported
hydrocarbons [4]. It constitutes also a solution to energy
storage; energetic density per volume of liquids
hydrocarbons is much higher than that of gases.
Dry reforming of methane is known since decades. It
consists of transformation of CH 4 in presence of CO 2 at
high temperature (700 °C), over specific catalysts to
produce syngas (H 2 +CO), which is a preferable feedstock
to form long chain hydrocarbons via Fischer Tropsch’s
reaction. There exist a limited number of industrial plants
that proceed the dry reforming of methane. However,
these units have a great inertia that doesn’t match with the
specifications to process low quantities of biogas,
particularly in the case of small farms with non-qualified
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labourers. Researches are focused on developments of
plasma-catalyst reactors that have the decisive advantage
of working at low temperature and quickly starting, so
they can be easily adapted to the charge’s variations.
Despite the intense researches, the energetic costs of the
CH 4 -CO 2 conversion remain too high (35 eV per
molecule of CH 4 transformed) [5].
The aim of this paper is to present the results obtained
in DBD-surface discharge, containing catalysts ZnO-Cu
and NiO.
2. Experimental setup
The most commonly used DBD-reactor’s configuration
is the “volume discharge” (VD) that consists of a
cylindrical dielectric tube forming reactor’s volume. In
this arrangement, transient streamers are randomly
developed, perpendicular to the axis of flow circulation,
thus a relatively small amount of molecules pass through
the streamers but enough to initiate and to propagate
efficient chemical reactions. The second configuration,
and almost rarely used, is the “surface discharge” (SD)
that consists of a plane dielectric with two conducting
electrodes applied directly on both opposite surfaces of
the dielectric. In this case, the charge transfer takes place
in distinct channels that appear parallel to the gas flow’s
direction on the dielectric surface. This configuration
leads to form a curtain of streamers through which pass
the flux of chemical molecules. Gibalov and Pietsch give
an exhaustive description of surface discharges in [6].
In this work, we developed a DBD surface discharge
that can contain ceramic beads of 3 mm in diameter in the
space, which is adjacent to electrodes. The ceramic beads
were previously coated with catalysts. The dielectric is
made of quartz quality glass sheet 3mm in thickness,
150 mm in length and 100 mm in width. Each electrode,
made of aluminium (3 mm in thickness), includes
3 branches. The space between two branches was filled
with catalyst beads Fig 1.
Cylindrical alumina (3 mm in diameter, 5 mm in
length), and spherical alumina beads, 2 mm in diameter,
1
provided by SASOL CO., coated with catalysts, were
used during these experiments. The coatings were
𝑆𝐶𝐶 =
performed in two ways (i) impregnation, (ii) fluidized
spray plasma (FSP) by using nitrate precursor solutions.
The FSP method is a new technique based on the
association of plasma reactivity with spray pyrolysis’
properties. This technique is described in a separate
contribution in ISPC 2015. Three types of catalysts
materials were tested: ZnO-Cu20%, applied by
impregnation method, ZnO-Cu10% and NiO by FSP
method.
The feedstock gas was prepared by mixing argon,
carbon dioxide and methane at several rates varying from
20 mL/min to 80 mL/min of each gas. The outlet flow
was passed through a condenser at 4 °C to condense
liquid products.
GC-MS was used to identify reaction’s products.
Chromatograph Varian 4900 GC calibrated, with
calibration gas supplied by Air Liquid, was used as on
line routine analysis of gaseous products. The liquid
products, collected in the condenser, were analysed in a
Varian 3400 chromatograph previously calibrated.
High-voltage AC, 30 kHz was applied on electrodes.
High voltage probe used to measure the applied voltage
and Pearson 2100 current probe were connected to a
LeCroy 500 MHz oscilloscope.
The average applied power 𝑝𝑚 was computed by
numerical integration of voltage-current.
1
τ
𝑝𝑚 = ∫0 𝑃(𝑡) 𝑑𝑑
τ
𝑃 = 𝑈. 𝐼
where U: applied voltage, I: current intensity, and
τ: period.
Definitions
Conversion rate:
𝑚𝑚𝑚𝑚 𝐶𝐶4 𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡
𝑥𝐶𝐶4 =
× 100
𝑖𝑖𝑖𝑖𝑖𝑖𝑖 𝑚𝑚𝑚𝑚 𝐶𝐶4
𝑚𝑚𝑚𝑚 𝐶𝐶2 𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡
𝑥𝐶𝐶2 =
× 100
𝑖𝑖𝑖𝑖𝑖𝑖𝑖 𝑚𝑚𝑚𝑚 𝐶𝐶2
Selectivity:
𝑚𝑚𝑚𝑚 𝐻2 𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝
𝑆𝐻2 =
× 100
2 × 𝑚𝑚𝑚𝑚 𝐶𝐶4 𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡
2
𝑆𝐶𝐶𝐶𝐶 =
𝑚𝑚𝑚𝑚 𝐶𝐶𝐶𝐶 𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝
× 100
𝑚𝑚𝑚𝑚 𝐶𝐶4 𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡
Hydrocarbon yield Y h :
𝑚𝑚𝑚𝑚 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 𝑖
𝑌ℎ𝑖 =
𝑚𝑚𝑚𝑚 𝑡𝑡𝑡𝑡𝑡 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝
3. Results
Experiments were carried out by applying 2800 V peak
to peak on electrodes, while a flow of CH 4 /CO 2 /Ar was
processed through the reactor. For each experience, about
10 g of alumina beads coated with catalyst was used. Gas
analysis was carried out by sampling every 5 minutes that
permitted to draw the evolution of conversion rates and
products’ selectivity as a function of the running time.
The duration of each experience was fixed at 60 minutes.
The evolution of the conversion rates of CH 4 and CO 2
as a function of running, depicted on the Fig. 2, shows a
rapid stabilisation after almost 10 min.
Conversion rate
Fig. 1. Photograph of DBD-surface discharge reactor
containing catalyst beads.
𝑚𝑚𝑚𝑒 𝐶𝐶 𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 × 100
𝑚𝑚𝑚𝑚 𝐶𝐶2 𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡 + 𝑚𝑚𝑚𝑚 𝐶𝐶4 𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡
0.6
0.5
0.4
0.3
0.2
0.1
0
CH4
CO2
0
20
40
60
Running time /min
Fig. 2. Temporal evolution of CH 4 and CO 2 conversion
rates. Total flow 60 mL/min (CH 4 /CO 2 /Ar: 20/20/20).
ZnO-Cu20%/Al 2 O 3 .
3.1. Influence of the applied power
The applied power was monitored by varying the high
voltage of AC generator. The influence of the power on
conversion rates of CH 4 and CO 2 was measured for three
catalysts: ZnO-Cu20% coated by impregnation method,
ZnO-Cu10% and NiO coated by FSP method. The total
flow was fixed at 120 mL/min (CH 4 /CO 2 /Ar: 40/40/40
mL/min). Results depicted on Fig. 3 shows that the
conversion rate of CH 4 increases from 22% to 48 % while
that of CO 2 grows from 12% to 25% when the applied
power increases from 20 W to 55W. It is noticeable that
CH 4 conversion rate grows faster than that of CO 2 . It is
noteworthy to highlight that the conversion rates follow
the same values and the same evolution for the tested
catalysts. In other words the conversion rates depend only
on the injected power in our conditions.
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CH4 NiO FSP
0.5
0.4
CO2 NiO FSP
0.3
CH4 ZnO-Cu20%
impr
CO2 ZnO-Cu20%
impr
CH4 ZnO-Cu10%
FSP
CO2 ZnO-Cu10%
FSP
0.2
0.1
0
0
10
20
applied power/watt
30
Fig. 3. Evolution of conversion rates of CH 4 and CO 2 as
a function of the applied power for 3 catalysts used (NiO
coated by Fluidized Spray Plasma (FSP) technique,
ZnO-Cu10% coated by FSP and ZnO-Cu20% coated by
impregnation technique).
Total flow 120 mL/min
(CH 4 /CO 2 /Ar: 40/40/40 mL/min).
Selectivity /%
The influence of the applied power on the selectivity of
H 2 and CO in the presence of catalysts is reported on
Fig. 4. The selectivity of H 2 and CO, respectively 36%
and 46% remain almost constant with applied power and
with all of the tested catalysts.
100
90
80
70
60
50
40
30
20
10
0
S H2 NiO FSP
S CO NiO FSP
S H2 ZnO-Cu20%
imp
S CO ZnO-Cu20%
imp
S H2 ZnO-Cu10%
FSP
0
10
20
Applied power /W
30
S CO ZnO-Cu10%
FSP
Fig. 4. Effect of applied power on the selectivity of H 2
and CO for the tested catalysts.
The selectivity of C2 hydrocarbons as a function of the
applied power is reported on Fig. 5. The main C2
hydrocarbons detected are ethane and ethylene. The
selectivity of ethane decreases from 15% to 9% and that
of ethylene varies between 1.5 and 4%.
3.2. Liquid hydrocarbon
The liquids produced during the reforming reactions
were collected by condensing them at 4 °C for an
experience running’s duration of 30 min. The condensed
liquids collected in our conditions constitute more than
10%wt of the total CH 4 and CO 2 transformed. Water
4
16
14
12
10
8
6
4
2
0
S C2H4 NiO SFP
S C2H6 NiO SFP
sélectivité
Conversion rate
0.6
0
10
20
30
puissance moyenne /watt
S C2H4 ZnOCu20% imp
S C2H6 ZnOCu20% impr
S C2H4 ZnOCu10% FSP
S C2H6 ZnOCu10% FSP
Fig. 5. Effect of applied power on the selectivity of C 2 H 6
and C 2 H 4 for the tested catalysts.
forms 90%wt of the total liquids.
At least
11 hydrocarbons were detected in the liquid phase:
acetaldehyde,
acetone,
methanol,
tert-butanol,
isopropanol, ethanol, 1-propanol, 2-butanol, acetic acid,
propionic acid and butanoic acid.
The hydrocarbon yield is defined as the mass of a
compound Hc i on the total mass of liquid hydrocarbons
ΣHc i . The results obtained with three types of catalysts,
are presented in Fig. 6. The total flow rate was fixed at
120 mL/min, (CH 4 /CO 2 /Ar: 40/40/40 mL/min). The
applied power was fixed at 21 W.
It can be observed that the distribution of liquids
depends highly on the nature and the composition of the
catalysts. The use of ZnO-Cu catalysts results in the
formation of acetic acid as the major liquid hydrocarbon,
whereas NiO directs the reaction to form methanol and
ethanol as the major liquid compounds. It is noticeable
that ZnO-Cu20%, coated by impregnation, leads more to
form acetic acid than ZnO-Cu10%, coated by FSP
method.
The influence of applied power on the evolution of
liquid hydrocarbon distribution is presented on Fig. 7.
We can notice that acetic acid’s formation is favoured
with the increase of the applied power for ZnO-Cu
catalysts; at the same time the ethanol yield decreases
with the power, while with NiO catalysts, the acetic acid
yield remains almost constant against the power increase.
In this case as in that of ZnO-Cu the ethanol yield
decreases with the power.
4. Conclusion
DBD surface discharge has been used successfully with
alumina beads, coated with catalysts, for reforming CH 4
and CO 2 flows. The conversion rates, around 50% and
30% were achieved respectively for CH 4 and CO 2 . The
conversion rates of both compounds depend strictly on the
applied power while the nature of the catalysts modifies
the distribution of products. This study demonstrated that
more than 11 liquid hydrocarbons are formed during the
dry reforming of methane at, around, the room
temperature. This work showed also that ZnO-Cu is
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0.35
0.3
Liq Hc Yield, NiO FSP
0.25
0.2
0.15
0.1
0.05
0
liquid hydrocarbon yiels /
%mass
0.4
ethanol yield ZnOCu20% imp
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
acetic ac yield ZnOCu20% imp
ethanol yield, ZnOCu10% FSP
acetic ac yield, ZnOCu10% FSP
ethanol yield, NiO
FSP
0
20
40
Applied power /W
acetic ac yield, NiO
FSP
Fig. 7. Liquid hydrocarbon yields as the function of the
applied power.
0.4
0.35
Liq Hc Yield, ZnO-Cu 10% FSP
0.3
0.25
5. Acknowledgments
Acknowoledgements are due to Programme Energie
CNRS-2009, to Commissariat Général à l’Investissement
(CGI), to Agence National pour la Recherche (ANR) and
to Université Sorbonne Paris Cité Research Program.
0.2
0.15
0.1
0.05
0
0.6
0.5
clearly more oxidative than NiO catalysts.
These results point out the fact that oxidative reactions
are amplified at high power ranges. The main reason is
the high production of oxidant species in the discharge.
Liq Hc yield ZnO-Cu20% imp
0.4
0.3
6. References
[1] 23.01.2008 - Proposal for a Directive of the
European Parliament and of the Council on the
promotion of the use of energy from renewable
sources. (Brussels: Belgium: Commission of the
European Communities) 2008/0016
[2]www.crossborderbioenergy.eu/fileadmin/crossborder/
Biogas_MarketHandbook.pdf
[3] www.eurobserv-er.org/downloads.asp
www.eurobserv-er.org/pdf/baro212biogas.pdf
[4]www.ec.europa.eu/agriculture/bioenergy/index_en.htm
[5] A. Bogaerts.
in: Conference XXXII ICPIC.
(Granada, Spain) (2013)
[6] V.I. Gibalov and G.J. Pietsch. J. Phys. D: Appl.
Phys., 33, 2618-2636 (2000)
0.2
0.1
0
Fig. 6. Distribution of the main liquid hydrocarbons for
the catalysts NiO, ZnO-Cu10% and ZnO-Cu20%.
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