22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
CO 2 conversion in a cylindrical packed-bed dielectric barrier discharge
K.J. Nordheden1, A.M. Banerjee1,2, J. Billinger1, S.M. Stagg-Williams1, R.V. Chaudhari1 and B. Subramaniam1
1
Chemical and Petroleum Engineering, University of Kansas, Lawrence, KS, U.S.A.
2
Bhabha Atomic Research Centre, Mumbai, India
Abstract: The conversion of CO 2 into CO and O 2 in a quartz cylindrical packed-bed
dielectric reactor has been studied using CO 2 and Ar gas mixtures at near ambient
conditions with quartz wool, γ-Al 2 O 3 , and TiO 2 packing. The conversion was evaluated as
a function of flow rate, composition of feed, plasma power, and the type of dielectric
packing material. For a 20% CO 2 in argon mixture using TiO 2 packing, a 28.6%
conversion of CO 2 was observed (14.4W, 35 sccm total flow rate), with corresponding
measured yields of 26.1% CO and 12.8% O 2 .
Keywords: CO 2 conversion, dielectric barrier discharge, plasma
1. Introduction
The mixture of CO and O 2 obtained from the
conversion of CO 2 could be used for the synthesis of
industrially profitable chemicals provided it can be
accomplished economically. The use of dielectric barrier
discharge (DBD) reactors offers an attractive alternative
to traditional conversion processes since the energetic
electrons and active species in the plasma can stimulate
dissociation even when the bulk gas is near room
temperature and atmospheric pressure [1]. There have
been some previous reports of CO 2 splitting using various
cylindrical DBD plasma configurations [2-4]. Yu et al.
studied the effect of silica gel, quartz, α-Al 2 O 3 , γ-Al 2 O 3 ,
and CaTiO 3 dielectric pellets for packing [2]. With a
discharge length of 15 cm and discharge gap of 4 mm,
they achieved a maximum CO 2 conversion of 20% using
CaTiO 3 pellets at a power of ~35W and flow rate of 40
sccm. Mei et al. have found that the addition of BaTiO 3
packing significantly enhances the electric field and mean
electron energy of the discharge. The addition of glass
beads or BaTiO 3 packing resulted in increased CO 2
conversions of 35% and 75% respectively, over that of the
unpacked reactor [3]. With a discharge length of 6 cm, a
gap of 3 mm containing BaTiO 3 packing, a flow rate of
50 sccm and a discharge power of 50 W, they achieved a
maximum conversion of ~28%.
2. Experimental
The experiments were conducted in a cylindrical
packed-bed dielectric barrier discharge (DBD) reactor as
shown in Figure 1. The quartz outer tube is 0.75”
OD/0.55” ID and the inner tube is 0.25” OD, resulting in
an annular gap of about 0.15” (or about 3.8 mm). The
space between the two tubes was packed with the
dielectric pellets (20-40 mesh or 420-840 μm in diameter)
and held in place with quartz wool. The inner high
voltage electrode is a 1/8” stainless steel rod, and the
outer grounded electrode is a stainless steel mesh wrapped
around the outer tube. The length is 2.5 cm, resulting in
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an unpacked volume of about 3.0 cm3.
Fig. 1. Schematic of the DBD reactor.
A high-frequency AC power supply which can provide
a peak-to-peak voltage of 20 kV at a frequency of 20-60
kHz was used. The applied voltage and frequency was
measured with a high voltage probe (Tektronix P6015A)
and the voltage on an external capacitor between the
reactor and the ground electrode was measured to obtain
the charge generated in the discharge. All signals were
recorded on a four-channel digital oscilloscope (Agilent
DSO6104A). The charge was measured with an external
capacitor and the power supplied to the reactor was
calculated from the area of the charge-voltage (Q-V)
Lissajous figures.
The feed gases entered through an electrically isolated
1
tee and were controlled by mass flow controllers. The
effluent flow rate was monitored using an Agilent
ADM2000 flowmeter. Temperature was measured with
an optical pyrometer. Products of reaction were evaluated
for CO 2 decomposition using a SRI Instruments online
gas-chromatograph (GC) with both a thermal conductivity
detector (TCD) and a methanizer-equipped flame
ionization detector (FID). CO 2 conversions were
measured from the CO 2 concentration at the inlet and
outlet of the reactor and the CO and O 2 yield was
measured at the outlet of the reactor. The γ-Al 2 O 3
(Sigma Aldrich) and the TiO 2 (Aerolyst) powders were
first dry pressed and then sieved through 20-40 mesh.
Surface analysis of the packing materials was also
conducted with N 2 adsorption-desorption using a
Quantachrome BET (Brunauer-Emmett-Teller).
3. Results and Discussion
The conversion of CO 2 as a function of power with the
various packing materials is shown in Fig. 2. For
comparison, the conversion in the unpacked DBD reactor
is also shown. For all cases, the CO 2 conversion
increases with increasing power and the presence of
packing results in overall higher conversion rates. The
frequency was ~ 30 kHz, and the temperature varied from
50°C to 70°C. The highest conversion rates were
observed with the TiO 2 packing. Both γ-Al 2 O 3 and quartz
wool also showed an enhancement in conversion over an
unpacked reactor. For a 20% CO 2 in argon mixture using
TiO 2 packing, a 28.6% conversion of CO 2 was observed
(14.4W, 35 sccm total flow rate), with corresponding
measured yields of 26.1% CO and 12.8% O 2 . Within
experimental error, this indicates that the conversion is
specific only to the desired CO and O 2 reaction products.
No other products were observed in the GC.
play a role in the conversion, but that role is still not well
understood [2,3]. It is interesting to note, however, that
the dielectric constants are 3.8 for quartz, ~10 for
alumina, and >86 for TiO 2 [5], which indicates that our
work agrees with the previously observed trend of
enhanced conversion with increasing dielectric constant
of the packing. In addition, the effect of TiO 2 packing on
a N 2 DBD has been investigated by Tu et al. and their
work indicated a shift in the electron energy distribution
towards higher energy electrons as evidenced by an
increase in the vibrational temperature of N 2 [6].
4. Acknowledgement
This work was supported by the Center for
Environmentally Beneficial Catalysis (CEBC) and The
University of Kansas.
5. References
[1] A. Fridman. Plasma Chemistry. (New York:
Cambridge University Press) (2008)
[2] Q. Yu, M. Kong, T. Liu, J. Fei and X. Zheng.
Plasma Chem. Plasma Process., 32, 153 (2012)
[3] D. Mei, X. Zhu, Y.-L. He, J. D. Yan and X. Tu.
Plasma Sources Sci. Technol., 24, 015011 (2015)
[4] R. Aerts, W. Somers and A. Bogaerts.
ChemSusChem, 8, 702 (2015)
[5] K.F. Young and H.P.R. Frederikse. J. Phys. Chem.
Ref. Data, 2, 313 (1973)
[6] X. Tu, H.J. Gallon and J.C. Whitehead. J. Phys. D:
Appl. Phys., 44, 482003 (2011)
Fig. 2. Effect of the dielectric packing on the conversion
of CO 2 as a function of discharge power (20% CO 2 in
Ar, 35 sccm).
There has been some indication that the dielectric
constant of the packing material may influence the
physical characteristics of the discharge and consequently
2
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