22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Relation between filament density and CO 2 dissociation in CO 2 dielectric
barrier discharge
S. Ponduri1, O. Guaitella1,2, C.A. Douat1, M.C.M. van de Sanden1,3 and R. Engeln1
1
Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
2
LPP, Ecole Polytechnique, UPMC, Université Paris Sud XI, CNRS, Palaiseau, France
3
Dutch Institute for fundamental Energy Research (DIFFER), P.O. Box 6336, 5600 HH Eindhoven, The Netherlands
Abstract: In this work we report that the functional dependence of CO 2 dissociation, to
produce CO in a pin-pin configuration, on specific energy input is same as the one observed
in a planar DBD. In this work the relation has been shown to be valid up to a single
filament. By correlating the number of filaments and charge transferred in each filament
measured, a purely electrostatic explanation of the observed phenomenon is proposed.
Keywords: CO 2 dissociation, DBD, filament statistics
1. Introduction
To make renewable energy sources the main energy
supply agents, their intermittency problem needs to be
addressed. Chemical storage of energy from the
renewable energy sources, especially in hydrocarbons so
called solar fuels, offers a lot of promise in this regard
[1, 2]. If carbon dioxide can be used as the source of the
carbon to make these solar fuels, with a very energy
efficient process then such a process can also serve to stop
increase in CO 2 emissions -- another major environmental
problem.
Dissociation of CO 2 into CO can open up many
possibilities, as CO is already a major industrial feed
stock. However, thermodynamically this process is very
energy intensive. High energy efficiencies even up to
80% have been shown to be possible in 1980s, in
dissociating CO 2 to CO using non-thermal plasmas
especially microwaves [3]. Recently dielectric barrier
discharges (DBDs) have emerged as a very popular
sources of atmospheric pressure non-thermal plasmas;
their intrinsically transient nature helps in reducing
energy losses with low gas heating. DBDs can act as test
bed for developing new diagnostic techniques for
studying atmospheric plasmas. Also, DBDs are simple in
construction and operation and are already used in
industrial scale for ozone production among many other
applications.
Hence many recent works [4,5,6,7] appeared reporting
studies on dissociation of CO 2 in a DBD. Many of these
reports have shown that CO 2 dissociation into CO can be
described using specific energy input as the scaling
parameter and that it is independent of operational
parameters like pressure, voltage frequency etc., In the
works done so far it has not been shown if the microdischarges (or filaments), fundamental units of DBDs, act
as a collectively or independently in CO 2 fed DBDs. In
this work we show that such a universal scaling parameter
is valid even when there is just one filament.
P-II-4-11
Another key feature of dependence of CO 2 dissociation
on specific energy (E spec ) in a DBD is that it follows a
power law with exponent <1 ; E spec is defined in equation
1, where P is the average power transferred in one voltage
cycle and Φ is flow in standard litres per minute (slm). In
[5] and [6], loss process of CO by chemical conversion
have been proposed as responsible for such a functional
dependence.
𝑃
(1)
𝐸𝑠𝑠𝑠𝑠 =
𝛷
Here in this work we propose that electrostatic effect
has at least a partial contribution in appearance of the
above mentioned power law dependence. We show a
simple method of counting the filaments and correlating it
with CO produced. We make a distinction between the
first filament, occurring in a cycle at a given condition
and subsequent filaments, by measuring the charge
transferred during the filaments and also by fast optical
imaging.
2. Experimental setup
The experimental setup, DBD and the electrical
characterization set up has been shown using a schematic
in figure. In this work we have used infra-red absorption
spectroscopy using a quantum cascade laser a (QCL) and
Fourier Transform infrared spectroscopy (FT-IR) to
measure CO (not shown in the figure).
A sinusoidal voltage source, with adjustable impedance
so as to resonantly match to the impedance of the reactor,
has been used; maximum voltage up-to 25 kV (pk-pk) was
achieved and depending on the plasma condition,
frequencies ranged between 22.5 and 23 kHz.
The voltage on the high voltage electrode is measured
using a commercially available high voltage probe; The
experiments were carried out in a set up consisting of two
pin electrodes covered with glass. The plasma was
powered with a sinusoidal high voltage (15 – 25 kV peak
to peak) at a frequency of 22 kHz.
1
The electrodes were mounted inside a cylindrical glass
cell to maintain reproducible gas conditions which acts as
the experimental reactor. A gas flow at a flow of 400 –
700 standard cubic cm per minute (sccm) was used. The
pressure in the cell is between 200 and 800 mbar which is
established using a rotary pump at the end of the exhaust.
The experiments were performed in pure CO 2.
Fig. 2: Conversion efficiencies at 700 sccm as a
function of specific energy input. Different
pressures are shown by different colors and
shapes.
Fig. 1: Experimental set-up with electrical
diagnostics. R is rogowski coil, LC lissajous
capacitor, 1: High voltage probe 2: probe to
measure charge transferred.
The CO density in the effluent of the DBD has been
measured by an ex-situ IR absorption spectroscopy
method with a QCL. The exhaust of the gas is directed
into an external cavity, through which laser is passed
twice, by thereby achieving a total path length of 948 mm.
The intensity of laser light is then measured by a detector
and recorded by an oscilloscope. Additionally, an etalon
and a reference cell are used to select the suitable
wavelength and calibrate the wavelength range of 2212 –
2216 cm-1. Then the exhaust of the gas goes in a second
cell, where the CO density is measured using FT-IR
spectroscopy. FT-IR absorption spectroscopy is used as a
second diagnostic to corroborate the measurements made
by QCL spectroscopy. For quantification a similar
approach to used by Brehmer et al [5] has been used.
However, compared to 49 cm path-length sufficient for
larger concentrations they measured, it is changed to 7m
using a multi-pass cell so as to increase the sensitivity. As
mentioned in [5], the knowledge of instrumental
broadening is required for the analysis which is 0.15 cm-1.
In principle, only one spectroscopic method is sufficient
to analyse the densities of CO. However by using two
techniques, which require gas to pass different amount of
path length no chemical processes are active outside the
DBD reactor.
3. Results and discussion
We found that conversion efficiency (α) – defined using
equation (2) – followed the same trend line even at lowest
2
𝛼=
𝐶𝐶𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝
× 100
𝐶𝐶2 𝑖𝑖𝑖𝑖𝑖
possible power inputs i.e. only filament per half cycle, at
different pressures and flows (shown only for 700 sccm).
The results are shown in figure 2. However, most of the
measurements are done at higher filament density to have
reasonable values of α, far above the detection limits of
the spectroscopes.
(2)
To understand why the power relation holds even for such
low filament density, properties of individual filaments:
Number of filaments per half cycle (N avg ), charge
transferred per filament, and spatial characteristics have
been measured.
Such a measurement was made possible because of the
capability afforded by the reactor to resolve individual
filaments. A typical oscillogram for a typical condition is
shown in fig 3. The temporal coordinates of each filament
are made used to count the number of filaments and the
charge transferred by each filament was measured by
measure the change in charge measured by Lissajous
capacitor at the same were made at the same time
coordinate.
Fig.3: A typical current voltage plot showing
independent filaments. T refers time period
approximately 44µs.
N avg is plotted as a function of power (fig 4). It clearly
shows a type of power law dependence i.e it is
increasingly difficult to create new filaments with power.
P-II-4-11
It has been also found that the charge transferred also
increases with the power input. More importantly it has
been found that the charge transferred of the first filament
across the conditions remains constant while the charge
transferred by a second filament increases with power.
The charge transferred by the first filament will distort
the electric field in its vicinity on its deposition on
cathode. This distortion will make the next filament to
spread wider requiring it carry more charge. Such
spreading in filaments have been also reported elsewhere
e.g [8]. The increased charge required to create the new
filament hints at the fact the energy required to create new
filament should also increase.
Fig. 4: Navg at 700 sccm as a function of specific
energy input. Different pressures are shown by
different colors and shapes.
4.
Conclusion
In this work, by using a simple technique of filament
counting and charge measurements transported by
individual filaments, We have shown that there is a direct
link between the difficulty in creation of new filaments in
a DBD fed with pure CO 2 , after the first filament is
created and the power law relation observed between the
energy input and conversion efficiency.
P-II-4-11
References
[1] Lewis, N. S., & Nocera, D. G. (2006). Powering the
planet: Chemical challenges in solar energy
utilization. Proceedings of the National Academy of
Sciences,103(43), 15729-15735
[2] Z. Jiang, T. Xiao, V. L. Kuznetsov, and P. P. Edwards,
“Turning carbon dioxide into fuel.,” Philos. Trans. A.
Math.s Phys. Eng. Sci., vol. 368, no. 1923, pp. 3343–64,
Jul. 2010
[3] Rusanov, V. D., A. A. Fridman, and G. V. Sholin.
"The physics of a chemically active plasma with
nonequilibrium vibrational excitation of
molecules." Soviet Physics Uspekhi 24.6 (1981) 447
[4] Mei, Danhua, et al. "Plasma-assisted conversion of
CO 2 in a dielectric barrier discharge reactor:
understanding the effect of packing materials." Plasma
Sources Science and Technology 24.1 (2015) 015011
[5] Brehmer, F., et al. "CO and byproduct formation
during CO2 reduction in dielectric barrier
discharges." Journal of Applied Physics 116.12 (2014):
123303
[6] Aerts, Robby, Wesley Somers, and Annemie
Bogaerts. "Carbon Dioxide Splitting in a Dielectric
Barrier Discharge Plasma: A Combined Experimental and
Computational Study." ChemSusChem (2015)
[7] F. Brehmer, S. Welzel, M. C. M. van de Sanden, and
R. Engeln, “CO and byproduct formation during CO2
reduction in dielectric barrier discharges,” J. Appl. Phys.,
vol. 116, no. 12, p. 123303, Sep. 2014
[8] S. Paulussen, B. Verheyde, X. Tu, C. De Bie, T.
Martens, D. Petrovic, A. Bogaerts, and B. Sels,
“Conversion of carbon dioxide to value-added chemicals
in atmospheric pressure dielectric barrier discharges,”
Plasma Sources Sci. Technol., vol. 19, no. 3, p. 034015,
Jun. 2010
[9] Brandenburg, R., et al. "Axial and radial development
of microdischarges of barrier discharges in N 2 /O 2
mixtures at atmospheric pressure." Journal of Physics D:
Applied Physics 38.11 (2005) 1649
3