The use of CO2 as a feedstock gas in plasma gas conversion
Tom Martensa, Bert Verheydeb, Sabine Paulussenb, Ming Maoa and Annemie Bogaertsa
a) Department of Chemistry, University of Antwerp, Universiteitsplein 1, 2610, Belgium
b) VITO, Flemish institute for technological research, Materials Technology, Mol, Belgium
Abstract: In this paper we evaluate the energetic behavior of CO2 in a dielectric
barrier discharge using numerical simulations. In order to obtain a better
description of the transient behavior of such discharges a pulsed power is used.
The selectivities, conversions and the influence of the pulses on the discharge
and its chemistry are investigated.
Keywords: DBD, plasma, atmospheric pressure, greenhouse gases, gas
conversion
1. Introduction
CO2 is one of the most important greenhouse gases.
Due to its highly oxidized character and very low
reactivity it is very difficult to use it as a feedstock
gas. However, for certain processes in chemical
industry it is available in an almost pure form as a
waste gas and using highly advanced adsorbing
techniques it can be extracted from the air.
To overcome the inert character of CO2, gas
discharges are used to let the electrons activate the
molecules, so that the vibrational excitations
stimulate the dissociation and the electronic
excitations stimulate ionization. The atmospheric
pressure dielectric barrier discharge (DBD) was
chosen as the experimental setup. Such discharge
has a great ease of implementation. Operating at
atmospheric pressure avoids leakage problems due
to large pressure differences between the inside and
the outside and in practice such discharges can be
implemented just as a sophisticated tube.
The use of CO2 as a feedstock gas is recognized as a
very important scientific challenge for the present
and the future. In order to use CO 2 as a feedstock gas
it has to be split. Such a reaction would use a
harmful greenhouse gas and transforms it into CO,
which is a valuable feedstock gas for chemical
industry. In this paper we investigate the splitting of
CO2 in dielectric barrier discharge conditions.
Although the atomic and molecular plasma
chemistry can be very complex, the splitting of CO2
can be summarized as follows:
CO2 → CO + 1/2 O2
Thermodynamically, this reaction requires 283
kJ/mol or 2.94 eV/molecule at standard conditions.
It is this value to which the energy consumption will
be evaluated in this paper.
In order to obtain a detailed description of the CO 2
chemistry, a reaction set was constructed which
contains the vibrational and electronic excitations of
the CO2, CO and O2 molecules, because
experimental gas chromatography measurements
showed that these were the only species of major
importance in the plasma. Previous studies which
include these excited states usually only describe the
formation and quenching of these excited states [1],
while it is known that up to 97% of the discharge
power for a low-temperature plasma is going to the
vibrational excitation in a molecular plasma [2].
Therefore, in the present research we investigate the
influence of the vibrationally and electronically
excited states on the ongoing chemistry.
2. Description of the simulations
The experimental setup is a cylindrical dielectric
barrier discharge. The outer electrode is powered
and is formed by sputtering chromium over a length
of 90 mm on the outside of an alumina tube with an
external diameter of 30 mm and an inner diameter of
26 mm. The inner electrode is grounded. It is a
stainless steel tube with an outer diameter of 22 mm.
Therefore, the discharge gap amounts up to 2 mm.
Typically applied voltages of about 7 kV are used at
a frequency of 30 kHz. In the present work the
averaged applied power will always be 150 W. More
details on the setup were previously reported by
Paulussen et al. [3].
In order to describe the excited levels of CO2, CO
and O2 several excited levels have been grouped.
The purpose of the grouping is mainly to limit the
amount of chemical reactions in the model. This
number can quickly increase if every excited species
can react in the same way as its ground state. This
becomes important in a later stage when time
consumption becomes an issue, when this chemistry
will be used to simulate full geometries in twodimensional models
After critical evaluation of the available cross
sections of CO2, CO and O2 several groupings were
chosen. The different vibrational levels of CO2 have
been grouped in four different levels CO2v1, CO2v2,
CO2v3 and CO2v4. CO2v1 represents the first
bending mode (010), CO2v2 represents the sum of
the first symmetric stretch (100) with the second
bending mode (020), CO2v3 represents the first
asymmetric stretch mode (001) and CO2v4
represents the sum of the symmetric stretch (n00)
and bending (0n0) modes. The electronic excitation
of CO2 is described using two levels: i.e. CO2(1g)
and CO2(1u).
For CO all vibrational excitation is described using
one mode COv1 at a threshold of 1.01 eV. The
electronic excitations have been grouped in four
different levels. COe1 describes the excited level
CO(A3), COe2 describes the excited level
CO(A1), COe3 describes the sum of the triplet
levels CO(A3), CO(D3), CO(E3) and CO(B3)
and COe4 describes the sum of the singlet levels
CO(C1), CO(E1), CO(B1), CO(I1) and
CO(D1).
O2 is described using four vibrational states. O2v1
describes the first and second vibrational level. O2v2
describes the third and fourth vibrational level and
O2v3 describes the fifth and sixth vibrational level.
The electronic excitations are grouped into two
states. O2e1 groups the singlet states O2(a1) and
O2(b1), while O2e2 groups O2(B3) and the higher
triplet states. These excited states have the same
chemistry as their ground levels, except that the
vibrationally excited states have a stimulated
dissociation and the electronically excited states
have a stimulated ionization. The stimulation of the
dissociation is described using the theory explained
by A. Fridman [4]. The stimulation of the ionization
is described straightforwardly as a lowering of the
threshold energy.
The resulting reaction chemistry consists of 43
chemical species who engage in 476 chemical
reactions. This large number of chemical reactions is
not an issue for the used model, since a typical
calculation never takes more than 10 minutes. The
used model is a zero-dimensional plasma model
developed by R. Dorai, K. Hassouni and M. Kushner
[5]. It consists of two major parts. The first part is a
Boltzmann solver which uses the cross sections for
the electron induced reactions and constructs lookup tables for the reaction rate coefficients versus the
mean electron energy. These look-up tables are
refreshed every 0.05 s for a calculation describing
about 5 seconds. The second part is the gas phase
kinetics module which calculates the changes in
energy of electrons and gas and the changes in the
density of every species.
3. Results and discussion
For the validation of the chemistry a comparison is
being made with the validated calculation results of
A. Cenian and coworkers [6]. Their research
concerned a pure CO2 discharge, operating at 30
Torr at a temperature of 400 K with an applied
potential difference of 200 V between the electrodes
and a current density of 7.5 A/cm2. Figure 1 shows
their results for the most important species in the
discharge.
Figure 2. Calculated results for a pure CO2 discharge at 30 Torr
and 400 K. The large jump in the beginning is due to the
implementation of the applied power as a pulse of 0.23s
Figure 1. Calculated results from A. Cenian and coworkers [6]
for a pure CO2 discharge at 30 Torr and 400K.
In order to validate our chemical model the
developed chemistry is used at the same pressure,
temperature and energetic conditions as was done by
A. Cenian and coworkers [6]. The results for the
same species CO, O2, O3 and O are shown in figure
2. The calculated time on the bottom axis in figure 2
is different from the bottom axis in figure 1, because
the models work in a very different way. In our
model the energy deposition is implemented as a
pulse of 0.23s with a relaxation time of about 4s.
The average deposited energy in the reactor is
exactly the same. The vertical axis in figures 1 and 2
uses the same scale. It depicts the percentage of
fractional density of a species versus the total
density of chemical compounds in the discharge.
The calculated fractional CO and O2 densities shown
in figure 2 are the same as the results obtained by A.
Cenian and coworkers shown in figure 1. CO is
present for about 30% in the discharge and O2 for
about 17%. O3 has a fractional density of 0.05% in
the discharge, which is slightly lower than in figure
1 and O has a fractional density of 0.02% in the
discharge, which is slightly higher than in figure 1.
Since the differences in O3 and O densities with the
results of A. Cenian and coworkers [6] are very
small and the agreements in the CO and O2 densities
are very good, a first successful validation has been
made.
The focus of this research lies on the atmospheric
pressure gas conversion. Figure 3 shows the
calculation results when operating at atmospheric
pressure. The implemented power pulse is again
0.23 seconds with a time averaged power deposition
of 150 W.
The energy deposition for this setup leads to a CO2
conversion of 17.7%. Almost all the converted CO2
leads to the production of CO, since the carbon
selectivity to CO is 97.5%. The oxygen from the
converted CO2 is used in the productions of CO, O2
and O3, which have oxygen selectivities of 48.8%,
41.6% and 4.8% respectively.
If we consider that the 150 W of input power is
solely used for the reaction CO2 → CO + 0.5 O2,
each such splitting of CO2 requires 73.6 eV.
Thermodynamically the splitting of CO2 into CO and
O2 requires about 2.94 eV per CO2 molecule (see
above). This means that the input power was used
with an efficiency of about 4%, which is very low.
The use of only one discharge pulse to describe the
behavior of a DBD lies far from the experimental
situation where the many filaments cause many
small pulses. In order to improve our description of
the DBD, many blocks of narrow pulses were
implemented. The power input was implemented in
a similar way as was done by Gentile and Kushner
[7] where pulses of 10 ns of 3 mJ per cm3 per pulse
Figure 3. Calculated results for the most important species in a
pure CO2 discharge at 760 Torr and 300 K. The averaged power
deposition is 150 W.
were used. In order to obtain a similar power
deposition as in the experiment, in our work pulses
of 7.2 mJ per cm3 per pulse were implemented.
These pulses were grouped in a small train of 24
pulses distributed in a block of 0.017 s of which the
first 3 s contain the pulses, while the rest of the
block is relaxation time. This block is repeated until
a residence time of 4.5 s is obtained.
Figure 4 shows the calculated densities of the most
important species as a function of time. The train of
narrow pulses clearly leads to a higher conversion.
The conversion of CO2 is 42.4% of which all carbon
is converted into CO. The oxygen of the converted
CO2 is used for 50% for the production of CO, for
43.4% for the production of O2 and for 4.7% for the
production of O3.
Dividing the total energy deposition in the discharge
by the number of produced molecules of CO, an
energy consumption of 27.4 eV per molecule is
obtained. Since the minimal thermodynamic
consumption is 2.94 eV, the deposited power is used
with an efficiency of about 10.7%, which is
considerably higher.
The splitting of CO2 in the DBD appears to be a very
promising technique were non-storable electricity
can be used to convert a greenhouse gas into a
chemical feedstock gas.
Figure 4. Calculated results for the most important species in
the same discharge conditions as in figure 3. The difference lies
in the implementation of the power which is implemented here
as train of narrow pulses instead of one wide pulse.
We have shown that the use of several pulses
considerably increases conversion and selectivity.
The following steps in these investigations are
thorough validation and investigation of basic
influences on the discharge.
We would also like to thank prof. M. Kushner for
the advice and for helping us with the computational
tools for these investigations.
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