22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Study of catalytic properties of microwave-induced plasma in CO 2 utilization
reactions
J. Fernández de la Fuente1, G.S.J. Sturm1, A.I. Stankiewicz1 and G.D. Stefanidis1,2
1
2
Process & Energy Department, Delft University of Technology, Leeghwaterstraat 39, 2628 CB, Delft, the Netherlands
Department of Chemical Engineering, Katholieke Universiteit Leuven, Willem de Croylaan 46, 3001 Leuven, Belgium
Abstract: Two microwave-induced plasma reactors are being investigated. As first
approach, a small scale plasma reactor (100 - 300 ml/min) so-called Surfatron is studied to
gain insight into gas/plasma phase reactions. A parametric study of various key gas phase
reactions is being performed. Furthermore, an up-scaled plasma reactor (100 l/min) will be
utilized to understand the needs towards developing an industrial scale plasma reactor for
chemical synthesis. Primary results from the small scale plasma reactor are shown.
Keywords: microwave-induced plasma reactor, CO 2 utilization, chemical synthesis
1. Introduction
In the context of developing novel alternative energy
technologies, plasma processing applications are
investigated. The main objective is to study the catalytic
properties of microwave-induced plasma so that it can be
used as an alternative for heterogeneous catalysts, thus
avoiding its economic and operational problems, i.e.,
catalyst deactivation, expensive manufacturing processes.
In particular, plasma sources can provide both energy
(high temperature) and highly reactive species (electrons,
ions and radicals) simultaneously.
Hence, aimed
reactions can selectively and efficiently be activated [1].
Carbon dioxide (CO 2 ) utilization reactions are considered
as CO 2 is the main contributor (by 60%) to the total
greenhouse gas emissions. It can be utilized as carbon
source for production of valuable chemicals such as
methanol, DME and transportation fuels. Therefore,
relevant reactions include: reverse-water-gas-shift
reaction (RWGS) and dry reforming of methane. Other
reactions might be investigated as well, steam reforming
or partial oxidation of methane and N 2 hydrogenation for
ammonia production.
2. Experimental description
A solid-state microwave generator (MiniFlow 200S,
Fig. 1a) in combination with an electromagnetic surface
wave launcher (Surfatron, Fig. 1b) is used to ignite and
sustain plasma. The microwave generator is operated at
2.45 GHz with a maximum power of 200 W. As for
analytical techniques, mass spectroscopy is employed to
quantify product composition whereas optical emission
spectroscopy enables the characterization of plasma
parameters (electron temperature and density) as well as
gas temperature which are relevant for the performance of
plasma-assisted reactions.
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a)
b)
Fig. 1. a) Microwave generator, and b) Surfatron.
3. Modeling work
A two-dimensional axisymmetric model is presented.
A multiphysics approach is taken towards developing the
model, which includes gas flow, electromagnetic wave
propagation, electron/ion transport properties, heavy
species transport properties and heat transfer. Up to date,
the model is based on argon plasma. However, other
chemistries are being studied such as CO 2 plasma, H 2
plasma, CH 4 plasma and also gas mixture plasma.
A schematic representation of the computational domain
is shown in Fig. 2. In order to reduce complexity of the
model, an antenna parallel to the axes of symmetry is
considered. By comparing the EM field distribution of a
3-D geometry (perpendicular antenna) with the 2-D
axisymmetric approach, it is proven that this assumption
does not affect such distribution [3]. Key plasma
parameters as well as the temperature of heavy species
can be predicted by the model. Among the plasma
parameters, electron number density (1/m3) is presented in
Fig. 3. A maximum value of 4.4 x 1020 m-3 is reached in
front of the excitation surface in which a maximum
electric field strength (V/m) is also observed. The results
are in good agreement with previous microwave plasma
modeling work [3-5].
1
better yields [7], which may be adapted to our present
context.
cd
ab
Fig. 2. Schematic of computational domain.
Fig.4. CO 2 conversion versus H 2 /CO 2 feeding ratio at
different temperatures via conventional process [6].
5. Acknowledgment
This research is supported by Alternative Energy Forms
for Green Chemistry (ALTEREGO), an European
cooperation project funded within the 7th Framework
Programme.
Fig. 3. Electron density (1/m3) of the plasma.
4. Results and discussion
The RWGS reaction is successfully performed in a
non-catalytic microwave-induced plasma reactor. CO 2
conversion is found in the range of 50 - 75% (red dots,
Fig. 4).
Several advantages are identified when
comparing the non-catalytic plasma process to the thermal
counterpart, (1) compactness, it eliminates the need for
preheating reactants upstream the reactor; (2) clean
process, no by-products formation is noticed as compared
to previous studies [2], which in turn decreases the
recycle ratio and flow rate of the purge stream; (3)
simplicity and low price in terms of equipment; (4)
super-equilibrium conversions due to the catalytic
properties of plasma (excellent chemical medium); (5) no
need for solid catalyst avoiding aforementioned issues;
(6) milder operating conditions (non-thermal plasma), the
conventional process requires 973 K to reach the same
conversions (Fig. 4). Nevertheless, it is observed that the
input electrical energy is mainly dissipated into heat and
light energy instead of chemical energy. Note that the
system is not designed to operate at high energy
efficiencies, rather its purpose is to carry out fundamental
studies of gas/plasma-phase chemical reactions. Other
configurations at larger scale exist though achieving
2
6. References
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[3] M. Baeva, et al. “Modeling of microwave-induced
plasma in argon at atmospheric pressure”. Phys.
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[4] M. Jimenez-Diaz, et al. « A two-dimensional
Plasimo multiphysics model for the plasmaelectromagnetic interaction in surface wave
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[5] Y. Yang, et al. “Numerical study on microwavesustained argon discharge under atmospheric
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[6] M. De Falco, et al. “Methanol production from
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