Catalysts - Influence of the applied power on products distribution

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 labourers. Researches are focused on developments of
 Methane and carbon dioxide constitute the most plasma-catalyst reactors that have the decisive advantage
important greenhouse gases. On one hand, methane of working at low temperature and quickly starting, so
remains an abundant molecule as the principal component they can be easily adapted to the charge’s variations.
of natural gas, and also produced with carbon dioxide Despite the intense researches, the energetic costs of the
during the fermentation of organic wastes. On the other CH 4 -CO 2 conversion remain too high (35 eV per
hand, carbon dioxide is produced during the combustion molecule of CH 4 transformed) [5].
of fossil hydrocarbons in energy plants and transports The aim of this paper is to present the results obtained
contributing to global warming and climate changes. in DBD-surface discharge, containing catalysts ZnO-Cu
 As a result, the international institutions and and NiO.
governments plan to reduce drastically the emissions of
these components. One important European 2. Experimental setup
Community’s directive recommends the 20% reduction of The most commonly used DBD-reactor’s configuration
greenhouse gas emissions prior to 2020, while a second is the “volume discharge” (VD) that consists of a
one recommends achieving at least a 10% share of cylindrical dielectric tube forming reactor’s volume. In
renewable energy in the total gasoline and diesel this arrangement, transient streamers are randomly
consumed in transport by 2020 [1]. Biogas is intended to developed, perpendicular to the axis of flow circulation,
play a major role in the management of organic wastes thus a relatively small amount of molecules pass through
and to provide a significant share in energetic the streamers but enough to initiate and to propagate
independency of EU’s states. The theoretical potential of efficient chemical reactions. The second configuration,
the primary energy production from biogas in 2020 is 166 and almost rarely used, is the “surface discharge” (SD)
million tonne oil equivalent in EU [2, 3]. The actual that consists of a plane dielectric with two conducting
tendency is to use biogas to produce heat and electricity. electrodes applied directly on both opposite surfaces of
However, the production of liquid hydrocarbons from the dielectric. In this case, the charge transfer takes place
biogas constitutes an important challenge that provides an in distinct channels that appear parallel to the gas flow’s
effective source of second generation biofuels and direction on the dielectric surface. This configuration
chemicals, leading to substantial economies in imported leads to form a curtain of streamers through which pass
hydrocarbons [4]. It constitutes also a solution to energy the flux of chemical molecules. Gibalov and Pietsch give
storage; energetic density per volume of liquids an exhaustive description of surface discharges in [6].
hydrocarbons is much higher than that of gases. In this work, we developed a DBD surface discharge
 Dry reforming of methane is known since decades. It that can contain ceramic beads of 3 mm in diameter in the
consists of transformation of CH 4 in presence of CO 2 at space, which is adjacent to electrodes. The ceramic beads
high temperature (700 °C), over specific catalysts to were previously coated with catalysts. The dielectric is
produce syngas (H 2 +CO), which is a preferable feedstock made of quartz quality glass sheet 3mm in thickness,
to form long chain hydrocarbons via Fischer Tropsch’s 150 mm in length and 100 mm in width. Each electrode,
reaction. There exist a limited number of industrial plants made of aluminium (3 mm in thickness), includes
that proceed the dry reforming of methane. However, 3 branches. The space between two branches was filled
these units have a great inertia that doesn’t match with the with catalyst beads Fig 1.
specifications to process low quantities of biogas, Cylindrical alumina (3 mm in diameter, 5 mm in
particularly in the case of small farms with non-qualified length), and spherical alumina beads, 2 mm in diameter,

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provided by SASOL CO., coated with catalysts, were
used during these experiments. The coatings were
 × 100
 =
 2 + 4 
 
 = × 100
 4 

 Hydrocarbon yield Y h :
 
 ℎ =
 
Fig. 1. Photograph of DBD-surface discharge reactor 3. Results
containing catalyst beads. Experiments were carried out by applying 2800 V peak
 to peak on electrodes, while a flow of CH 4 /CO 2 /Ar was
performed in two ways (i) impregnation, (ii) fluidized processed through the reactor. For each experience, about
spray plasma (FSP) by using nitrate precursor solutions. 10 g of alumina beads coated with catalyst was used. Gas
The FSP method is a new technique based on the analysis was carried out by sampling every 5 minutes that
association of plasma reactivity with spray pyrolysis’ permitted to draw the evolution of conversion rates and
properties. This technique is described in a separate products’ selectivity as a function of the running time.
contribution in ISPC 2015. Three types of catalysts The duration of each experience was fixed at 60 minutes.
materials were tested: ZnO-Cu20%, applied by The evolution of the conversion rates of CH 4 and CO 2
impregnation method, ZnO-Cu10% and NiO by FSP as a function of running, depicted on the Fig. 2, shows a
method. rapid stabilisation after almost 10 min.
 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 0.6
 0.5
 Conversion rate

was passed through a condenser at 4 °C to condense CH4
liquid products. 0.4
 GC-MS was used to identify reaction’s products. CO2
 0.3
Chromatograph Varian 4900 GC calibrated, with
 0.2
calibration gas supplied by Air Liquid, was used as on
line routine analysis of gaseous products. The liquid 0.1
products, collected in the condenser, were analysed in a 0
Varian 3400 chromatograph previously calibrated. 0 20 40 60
 High-voltage AC, 30 kHz was applied on electrodes. Running time /min
High voltage probe used to measure the applied voltage
and Pearson 2100 current probe were connected to a
LeCroy 500 MHz oscilloscope.
 Fig. 2. Temporal evolution of CH 4 and CO 2 conversion
 The average applied power was computed by
 rates. Total flow 60 mL/min (CH 4 /CO 2 /Ar: 20/20/20).
numerical integration of voltage-current.
 ZnO-Cu20%/Al 2 O 3 .
 1 τ
 = ∫0 ( ) 3.1. Influence of the applied power
 τ
 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
where U: applied voltage, I: current intensity, and catalysts: ZnO-Cu20% coated by impregnation method,
τ: period. 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
Definitions mL/min). Results depicted on Fig. 3 shows that the
Conversion rate: conversion rate of CH 4 increases from 22% to 48 % while
 4 that of CO 2 grows from 12% to 25% when the applied
 4 = × 100
 4 power increases from 20 W to 55W. It is noticeable that
 2 CH 4 conversion rate grows faster than that of CO 2 . It is
 2 = × 100
 2 noteworthy to highlight that the conversion rates follow
Selectivity: the same values and the same evolution for the tested
 2 catalysts. In other words the conversion rates depend only
 2 = × 100
 2 × 4 on the injected power in our conditions.

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0.6
 CH4 NiO FSP 16 S C2H4 NiO SFP
 0.5 14
 Conversion rate

 CO2 NiO FSP 12 S C2H6 NiO SFP
 0.4

 sélectivité
 10
 0.3 CH4 ZnO-Cu20%
 8 S C2H4 ZnO-
 impr Cu20% imp
 0.2 CO2 ZnO-Cu20% 6 S C2H6 ZnO-
 impr 4 Cu20% impr
 0.1
 CH4 ZnO-Cu10% 2 S C2H4 ZnO-
 0 FSP 0 Cu10% FSP
 0 10 20 30 CO2 ZnO-Cu10% 0 10 20 30 S C2H6 ZnO-
 FSP puissance moyenne /watt Cu10% FSP
 applied power/watt

Fig. 3. Evolution of conversion rates of CH 4 and CO 2 as Fig. 5. Effect of applied power on the selectivity of C 2 H 6
a function of the applied power for 3 catalysts used (NiO and C 2 H 4 for the tested catalysts.
coated by Fluidized Spray Plasma (FSP) technique,
ZnO-Cu10% coated by FSP and ZnO-Cu20% coated by forms 90%wt of the total liquids. At least
impregnation technique). Total flow 120 mL/min 11 hydrocarbons were detected in the liquid phase:
(CH 4 /CO 2 /Ar: 40/40/40 mL/min). acetaldehyde, acetone, methanol, tert-butanol,
 isopropanol, ethanol, 1-propanol, 2-butanol, acetic acid,
 The influence of the applied power on the selectivity of propionic acid and butanoic acid.
H 2 and CO in the presence of catalysts is reported on The hydrocarbon yield is defined as the mass of a
Fig. 4. The selectivity of H 2 and CO, respectively 36% compound Hc i on the total mass of liquid hydrocarbons
and 46% remain almost constant with applied power and ΣHc i . The results obtained with three types of catalysts,
with all of the tested 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
 100 applied power was fixed at 21 W.
 S H2 NiO FSP It can be observed that the distribution of liquids
 90
 depends highly on the nature and the composition of the
 80
 S CO NiO FSP catalysts. The use of ZnO-Cu catalysts results in the
 70
 Selectivity /%

 formation of acetic acid as the major liquid hydrocarbon,
 60 whereas NiO directs the reaction to form methanol and
 S H2 ZnO-Cu20%
 50 imp ethanol as the major liquid compounds. It is noticeable
 40 that ZnO-Cu20%, coated by impregnation, leads more to
 S CO ZnO-Cu20%
 30 imp
 form acetic acid than ZnO-Cu10%, coated by FSP
 20 method.
 S H2 ZnO-Cu10% The influence of applied power on the evolution of
 10 FSP
 liquid hydrocarbon distribution is presented on Fig. 7.
 0 S CO ZnO-Cu10% We can notice that acetic acid’s formation is favoured
 0 10 20 30 FSP with the increase of the applied power for ZnO-Cu
 Applied power /W catalysts; at the same time the ethanol yield decreases
 with the power, while with NiO catalysts, the acetic acid
Fig. 4. Effect of applied power on the selectivity of H 2 yield remains almost constant against the power increase.
and CO for the tested catalysts. In this case as in that of ZnO-Cu the ethanol yield
 decreases with the power.
 The selectivity of C2 hydrocarbons as a function of the
applied power is reported on Fig. 5. The main C2 4. Conclusion
hydrocarbons detected are ethane and ethylene. The DBD surface discharge has been used successfully with
selectivity of ethane decreases from 15% to 9% and that alumina beads, coated with catalysts, for reforming CH 4
of ethylene varies between 1.5 and 4%. and CO 2 flows. The conversion rates, around 50% and
 30% were achieved respectively for CH 4 and CO 2 . The
3.2. Liquid hydrocarbon conversion rates of both compounds depend strictly on the
 The liquids produced during the reforming reactions applied power while the nature of the catalysts modifies
were collected by condensing them at 4 °C for an the distribution of products. This study demonstrated that
experience running’s duration of 30 min. The condensed more than 11 liquid hydrocarbons are formed during the
liquids collected in our conditions constitute more than dry reforming of methane at, around, the room
10%wt of the total CH 4 and CO 2 transformed. Water temperature. This work showed also that ZnO-Cu is

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0.4 0.7 ethanol yield ZnO-
 Cu20% imp

 liquid hydrocarbon yiels /
 0.35 0.6
 Liq Hc Yield, NiO FSP acetic ac yield ZnO-
 0.3 0.5 Cu20% imp
 0.4 ethanol yield, ZnO-

 %mass
 0.25
 Cu10% FSP
 0.3
 0.2 acetic ac yield, ZnO-
 0.2
 0.15 Cu10% FSP
 0.1 ethanol yield, NiO
 0.1 0 FSP
 0.05 0 20 40 acetic ac yield, NiO
 0 Applied power /W FSP

 Fig. 7. Liquid hydrocarbon yields as the function of the
 applied power.
 0.4
 clearly more oxidative than NiO catalysts.
 0.35 Liq Hc Yield, ZnO-Cu 10% FSP These results point out the fact that oxidative reactions
 are amplified at high power ranges. The main reason is
 0.3 the high production of oxidant species in the discharge.
 0.25
 5. Acknowledgments
 0.2 Acknowoledgements are due to Programme Energie
 CNRS-2009, to Commissariat Général à l’Investissement
 0.15 (CGI), to Agence National pour la Recherche (ANR) and
 0.1 to Université Sorbonne Paris Cité Research Program.

 0.05 6. References
 [1] 23.01.2008 - Proposal for a Directive of the
 0 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/
 0.6 Biogas_MarketHandbook.pdf
 [3] www.eurobserv-er.org/downloads.asp
 0.5 Liq Hc yield ZnO-Cu20% imp
 www.eurobserv-er.org/pdf/baro212biogas.pdf
 [4]www.ec.europa.eu/agriculture/bioenergy/index_en.htm
 0.4 [5] A. Bogaerts. in: Conference XXXII ICPIC.
 (Granada, Spain) (2013)
 0.3 [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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