Effect of gas composition on dry reforming of methane in a dielectric barrier
discharge reactor
Xuming Zhang, Min Suk Cha*
Clean Combustion Research Center
King Abdullah University of Science and Technology
Thuwal 23955, Saudi Arabia
Abstract: In this work, we systematically investigated the effect of gas composition on dry reforming of methane in
a dielectric barrier discharge reactor. The effects of H 2O addition, O2 addition, CO2/CH4 ratio and CH4+CO2 portion
in the reactant are studied. In experiments, the background temperature is kept at 673 K, where DBD reactor shows
the best performance for CH4/CO2 mixture (CH4:CO2=1:1) reforming according to our previous study. The results
show that H2/CO ratio of the products can be controlled in the range of 0.2-2.3.
Keywords: dry reforming, plasma, carbon dioxide, methane, gas composition
1. Introduction
Dry reforming of methane to produce syngas, has
recently attracted particular interests for simultaneously
utilizing and reducing of two greenhouse gases,
methane and carbon dioxide [1]. However, the
application of conventional catalytic dry reforming
process is limited due to two major disadvantages:
requirement of high reaction temperature (typically >
1000 K) and rapid deactivation of catalysis due to coke
poisoning. Non-thermal plasma (NTP) based processes
have been proposed as alternative techniques for fuel
reforming. Its recognized merits, compared with the
conventional catalytic processes, include rapid start-up
and shutdown, easy maintenance, and moderate
operation conditions (atmospheric pressure and ambient
temperature) [2].
In NTP, dry reforming reactions are initiated by
electron impact dissociation reactions. During the
dissociation reactions, various kinds of radicals, such as
CHx (x=1-3), O and H are produced. H2 is mainly
produced from the dissociation of methane and H
radical recombination, whereas CO is mainly produced
from the dissociation of CO2 and CHx radical oxidation
and hydrocarbons are produced from the recombination
of CHx radicals. Considering the system performance in
terms of energy efficiency and product distribution,
various effects, such as NTP generation methods, gas
compositions, plasma processing techniques and
reaction temperature, have been studied [3-6]. Among
these, gas composition is considered as an essential
important parameter because it may affect the
reforming pathway through influencing the kinds and
concentrations of radicals, which in turn affect the
system performance.
Dielectric barrier discharge (DBD) is used as the
plasma source in current work. Comparing to other
NTP generation methods, DBD is a mature, stable and
reliable plasma source. Our recent work demonstrates
that the reforming pathways are different between the
low temperature DBD reactor and the high temperature
DBD reactor. The increase in background temperature
is favorable for both thermo-chemistry and
electron-induced chemistry, which in turn benefits the
optimization of the reforming performance [6, 7].
In the present work, we systematically investigate the
effect of gas compositions on dry reforming of methane
in the DBD reactor. The effects of H2O addition, O2
addition, CO2/CH4 ratio and CH4+CO2 portion in the
reactant are studied. In experiments, the background
temperature is kept at 673 K, where DBD reactor shows
the best performance for CH4/CO2 mixture
(CH4:CO2=1:1) reforming according to our previous
study.
2. Experimental Setup
Figure 1 shows a schematic diagram of experimental
setup. The non-thermal plasma is generated in a DBD
reactor (quartz tube) with an inner diameter of 20 mm
and a length of 45 mm. The high voltage electrode
consists of a stainless steel rod with a diameter of 17.5
mm, which is energized by an adjustable AC high
voltage source. A stainless steel mesh is wrapped
around the quartz tube serving as a ground electrode.
The reactor is located at the downstream of a quartz
tube with an inner diameter of 50 mm and a length of
450 mm, which is designed to stabilize a gas
temperature. A function generator (AFG 3021B,
Tektronix) together with a signal amplifier (20/30 A,
Trek) is used to generate AC high voltage. The applied
voltage is measured with a 1000:1 high voltage probe
(P 6015A, Tektronix), while the discharge current is
monitored with a current sensor (6595, Pearson). V-Q
Lissajous method is used to determine the discharge
power in the DBD. The charge Q is determined by
measuring the voltage across a capacitor (30 nF) using
a 10:1 voltage probe (TPP 1000, Tektronix). Such
capacitor is connected in series to the ground line of the
DBD reactor. The signals of applied voltage, discharge
current and charge are recorded using a digitizing
oscilloscope (DPO 4104B, Tektronix). The V-Q
Lissajous diagram is averaged over 216 scans. The
discharge power is calculated from the area of the
diagram by multiplying the frequency. A digital camera
(D700, Nikon Co., Ltd.) is used for recording the image
of the discharge.
Mass flow controllers (SLA5850, Brooks instrument)
are used to control the flow of individual gases. The
mixture is heated to 673 K and then maintained at a
constant temperature using the feedback-controlled
electrical heating furnace (O series, NDB). Detailed
test conditions are listed in Table 1.
flame ionization detector (FID) and two thermal
conductivity detectors (TCD). The FID channel is
configured to analyze the hydrocarbons from C1 to C5.
The first TCD channel (reference gas helium) is
configured to analyze CO2, CO and N2, while the
second TCD channel is dedicated to analyze H2. Before
experiments, the GC system is carefully calibrated
using reference gas mixtures.
To characterize the dry reforming process, the CH4 and
CO2 conversions are defined as:
(E1)
moles of CH 4 converted
CH 4 conversion(%)
CO 2 conversion(%)
moles of CH 4 input
100
(E2);
moles of CO 2 converted
100
moles of CO 2 input
the H2/CO molar ratio is defined as:
H2
CO
(E3).
moles of H 2 produced
moles of CO produced
3. Results & Discussions
1.
Figure 1 Schematic diagram of experimental setup.
Table 1 Experimental conditions.
CO2
(ml/min)
additive
(ml/min)
0
Varying CO2
amount
Adding O2
Adding H2O
Varying
CH4+CO2
amount
20
20
20
10
20
30
40
10
20
40
60
80
20
20
10
20
30
40
N2
(ml/min)
Discharge
Power
(w)
180
-
170
160
140
120
100
0
160
4
8
12
16
156
152
148
144
0
160
4
8
12
16
156
152
148
144
-
180
160
140
120
10
10
Applied voltage
100
10
5
Current
0
0
-5
Current (mA)
CH4
(ml/min)
Figure 2 shows the typical time resolved voltage and
current waveform at T = 673 K. For each cycle of
applied voltage, a large number of current spikes can be
observed. These spiky current forms correspond to
filamentary micro-discharges in the discharge gap.
They are randomly distributed over the dielectric
surface with around 10 ns duration. The number and
amplitude of current spikes are insensitive to the
varying of AC frequency. However, they increase with
the increase of the applied voltage.
Voltage (kV)
Experimental
Approach
3.1 Characteristics of discharge
-100
-10
10
0.0
0.2
0.4
0.6
Time (ms)
Figure 2 Typical voltage and current waveforms at Pdis
= 10 w, CH4:CO2:N2=1:1:8.
10
To measure the species composition of the product gas,
the reformed gas is dried by a homemade
ice-water-cold trap and analyzed using an online gas
chromatograph (HP 7890A, Agilent) equipped with one
Figure 3 shows the typical image of the dielectric
barrier discharge at T = 673 K. In appearance, the
filamentary micro-discharges are uniformly distributed
in the discharge gasp. The DBD intensity increases with
the discharge power. The characteristics of discharge at
T = 673 K is consistent with that at a room temperature
condition [8, 9]. The DBD at T = 673 K can be
Figure 3 Typical image of DBD at T = 673 K, Pdis=10
w, CH4:CO2:N2=1:1:8.
3.2 Reactants conversion
Figure 4 shows the effect of gas compositions on
methane conversion. Since the variation of discharge
power only affects reactant conversion and it cannot
change the reaction pathways to give different product
selectivities [6], the discharge power is maintained at
Pdis = 10 W in this study to focus on the effects of the
gas compositions. We noted that no reaction could be
observed without plasma due to the limitation in
residence time (not enough reaction time) inside the
reactor, though one may obtain calculated
thermodynamic equilibrium composition.
O2/CH4
25
CH4 Conversion (%)
0.8
0.4
20
CO2/CH4
0.2
H2O/CH4
CO2+CH4 (%)
CH4+OH
CH3+H2O
(R2)
However, the CO2 and H2O conversion generates too
low O radicals and OH radicals, respectively, to
significantly affect the CH4 conversion. On the other
hand, the electron density in DBD is dependent on
specific energy density (electrical energy deposition
per unit volume of reactant). Since the production of
electrons per discharge energy is limited, the increase in
CO2+CH4 portion leads to obvious decrease in the CH4
conversion. The limitation of electron production per
discharge energy also leads to the decrease in CO2
conversion when increasing CO2/CH4 ratio and
CO2+CH4 portion as shown in Fig.5. In addition, Fig.5
shows that the CO2 conversion drops from 5.4 to -1.9%
with increase of O2/CH4 ratio from 0 to 0.8. The
enrichment of O radicals is favorable for CH 4
conversion. However, the excess of O radicals can
significantly enhance backward reactions R3 and R4.
CO+O
CO+OH
CO2
(R3)
CO2+H
(R4)
10
CO2+CH4 (%)
8
CO2 Conversion (%)
characterized as a typical filamentary micro-discharge
mode.
10
CO2/CH4
6
4
O2/CH4
0
0.5 1.0
H2O/CH4
0 0.2 0.4 0.8
20
30
2.04.0
40
0.2
2
0.4 0.8
0
10
15
0.5 1.02.0 4.0
0 0.2 0.4 0.8
-2
20
0
10
30
40
CO2/CH4
O2/CH4
H2O/CH4 CO2+CH4 (%)
Figure 5 Effect of gas compositions on CO2 conversion
at T = 673 K, Pdis=10 w.
5
0
CO2/CH4
O2/CH4
H2O/CH4 CO2+CH4 (%)
Figure 4 Effect of gas compositions on CH4 conversion
at T = 673 K and Pdis=10 w.
With increase of O2/CH4 ratio from 0 to 0.8, CH4
conversion significantly increases from 11.5 to 21.4%.
In contrast, the variation of CO2/CH4 and H2O/CH4
ratios exert a slight effect on the CH4 conversion. These
results imply that the O2 conversion in DBD can
efficiently produce a large amount of O radicals for
further facilitate methane conversion through radical
chain reactions R1 and R2.
CH4+O
CH3+OH
(R1)
3.3 H2/CO Ratio
Figure 6 shows the results of H2/CO ratio by varying
gas compositions. Although the variation of CO2/CH4
and H2O/CH4 ratios do not significantly change the
CH4 conversion, they obviously affect product
selectivities. With increase of CO2/CH4 ratio from 0.5
to 4.0, O2/CH4 ratio from 0 to 0.8 and CH4+CO2 portion
from 10 to 40%, the H2/CO ratio decreases from 2.3 to
0.5, from 1.3 to 0.2 and from 1.5 to 1.1, respectively.
When increasing H2O/CH4 ratio from 0 to 0.8, however,
the H2/CO ratio increases from 1.3 to 1.7.
3
CO2/CH4
0.5
2
H2O/CH4
H2/CO
O2/CH4
1.0
CO2+CH4 (%)
0.8
0.4
0.2
0
10
0
20 30
40
1
2.0
4.0
0.2
0.4
0.8
0
CO2/CH4
O2/CH4
H2O/CH4 CO2+CH4 (%)
Figure 6 Effect of gas compositions on H2/CO ratio at
T = 673 K, Pdis=10 w.
Because of H2 consumption reactions R5 and R6, the
H2/CO ratios decrease with the increase of CO2/CH4
ratio and O2/CH4 ratio.
H2+O
H2+OH
OH+ H
H2O+H
(R5)
(R6)
With increase of CH4+CO2 portion, the backward
reactions, R3 and R4, could be restricted and the CO
production reaction R7 could be enhanced, which may
lead to the decrease of H2/CO ratio.
H+CO2
CO+OH
(R7)
The addition of H2O has two positive effects on H2
production: (1) producing H radicals through R8 and (2)
preventing H2 consumption reaction R6.
H2O+e OH+H
(R8)
4. Conclusion
This paper studied the effect of gas composition on dry
reforming of methane in a dielectric barrier discharge
reactor at background temperature of 673 K. The main
conclusions are summarized as follows:
(1) The dielectric barrier discharge at T=673 K is a
typical filamentary micro-discharge.
(2) The varying of O2/CH4 ratio and CO2+CH4 portion
not only change reactants conversion but also
affect products distribution.
(3) The varying of CO2/CH4 and H2O/CH4 ratio do not
significantly change the CH4 conversion, but they
obviously affect the products distribution.
However, detailed chemical kinetic studies are need.
Acknowledgement
This work is supported by KAUST.
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