Carbon Dioxide Reforming of Methane Using DC Corona Discharge Plasma Reaction

Article abstract of DOI:10.1021/jp037008q

Carbon dioxide reforming of methane via dc corona discharge plasma reaction at atmospheric pressure has been investigated. The effects of the CH ₄CO₂ ratio in the feed, flow rate, discharge power, and corona types have been systematically studied. The results show that the molar ratio of H₂ to CO in the products strong depends on the molar ratio of CH₄ to CO₂ in the feed. The discharge power, flow rate, and corona types have slight influence on the syngas composition. When the CH₄/CO₂ ratio is 1/2, the syngas of lower H₂/CO ratio at about 0.56 is obtained, which is a potential feedstock for synthesis of liquid hydrocarbons. The conversions of methane and carbon dioxide increase with increasing the discharge power and decrease with increasing the flow rate. The conversions of reactants via positive corona are generally higher than that via negative corona, but the ratio of H₂/CO in the products is the other way round. Besides syngas and water, other products including various hydrocarbons and oxygenates are detected by a quadrupole mass spectrometer. There is visible coke mainly depositing on the cathode when the CH ₄/CO₂ ratio is higher than 2/1. We propose that the coke mainly formed via methane decomposition during the reaction.

Full text of DOI:10.1021/jp037008q

J. Phys. Chem. A 2004, 108, 1687-1693
1687
Carbon Dioxide Reforming of Methane Using DC Corona Discharge Plasma Reaction
†
‡
,†
†
†
Ming-wei Li, Gen-hui Xu, Yi-ling Tian,* Li Chen, and Hua-feng Fu
Department of Chemistry and School of Chemical Engineering, Tianjin UniVersity, Tianjin 300072, P. R. China
ReceiVed: October 7, 2003; In Final Form: January 8, 2004
Carbon dioxide reforming of methane via dc corona discharge plasma reaction at atmospheric pressure has
been investigated. The effects of the CH
have been systematically studied. The results show that the molar ratio of H
depends on the molar ratio of CH to CO in the feed. The discharge power, flow rate, and corona types have
slight influence on the syngas composition. When the CH /CO ratio is 1/2, the syngas of lower H /CO ratio
4
/CO
2
ratio in the feed, flow rate, discharge power, and corona types
2
to CO in the products strong
4
2
4
2
2
at about 0.56 is obtained, which is a potential feedstock for synthesis of liquid hydrocarbons. The conversions
of methane and carbon dioxide increase with increasing the discharge power and decrease with increasing
the flow rate. The conversions of reactants via positive corona are generally higher than that via negative
corona, but the ratio of H
including various hydrocarbons and oxygenates are detected by a quadrupole mass spectrometer. There is
visible coke mainly depositing on the cathode when the CH /CO ratio is higher than 2/1. We propose that
the coke mainly formed via methane decomposition during the reaction.
2
/CO in the products is the other way round. Besides syngas and water, other products
4
2
Introduction
the CH4/CO2 ratio in the feed, such as the reaction
CH + 2CO f H + 3CO + H O
During the past decades, there has been increasing concern
over the emission of CO2 and CH4 which contributes most of
the human-related global warming. The chemical method of
utilization of greenhouse gases, CO2 reforming of CH4, not only
eliminates them but also yields lower H2/CO molar ratio syngas
(5)
4
2
2
2
where the H2/CO ratio is 1/3. Consequently, if the H2/CO ratio
less than 1/1 is desired for the production of liquid hydrocarbon,
then CO2 reforming is preferable.
(i.e., a mixture of H2 and CO) which is a preferable feedstock
1
,2
for the Fischer-Tropsch synthesis of liquid hydrocarbons.
CO2 reforming using conventional catalytic methods, how-
ever, often has two serious problems. It is an intensively
endothermic reaction (∆H ) 247 kJ/mol) consuming much
energy, and the catalysts used in CO2 reforming are inclined to
deactivate due to coke deposition on the catalysts surface.2
Thermodynamic calculations suggested that the coke formation
could be avoided at higher temperature (e.g., 1073 K) and with
Thus, light gases flared in remote oil producing areas or in
industry during petroleum processing can be used with CO2
wasted in flue gas or in natural gas fields to produce valuable
products.
-7
The overall reaction stoichiometry for the production of
alkanes using syngas, if there occurs the water-gas shift reaction
simultaneously, is
2
8,9
CH4/CO2 ratio lower than unity. However, the higher reaction
temperature and higher energy consumption is a disadvantage
for the application of the process in industry. Additionally, a
higher reaction temperature increases the coke deposition via
CO disproportionation, which is exothermic.
(
n + 1)H + 2nCO f C H + nCO2
(1)
2
n
2n+2
It requires that the H2/CO ratio in the feed equals (n + 1)/(2n),
which is between 1/2 and 1/1. Up to now, the principal routes
for the conversion of methane to syngas include steam reforming
reaction 2), partial oxidation (reaction 3), and CO2 reforming
reaction 4).
Now, nonequilibrium plasma technology offers an alternative
method for chemical reactions whereby electricity provides the
(
(
10,11
reaction energy for endothermic process.
Nonequilibrium
plasma is far from thermodynamic equilibrium; i.e., within
nonequilibrium plasma free electrons have much higher energy
than ions and neutral particles. Thus, nonequilibrium plasma
usually has comparatively low gas temperature close to room
temperature and high-energy conversion rates. Nonequilibrium
plasma can be generated by different kinds of gas discharge,
including glow discharge, microwave discharge, dielectric
barrier discharge (DBD), corona discharge, etc. From a stand-
point of industry, mainly DBD and corona discharge have
chances to be applied to handle large gas volume because they
could be generated at near to or higher than atmospheric pressure
at lower temperature.
CH + H O f 3H + CO
(2)
4
2
2
1
CH + / O f 2H + CO
(3)
(4)
4
2
2
2
CH + CO f 2H + 2CO
4
2
2
Stoichiometrically, CO2 reforming produces syngas having the
lowest H2/CO ratio equaling 1/1. Moreover, the H2/CO ratio in
the products may be relatively easily controlled by adjusting
†
Department of Chemistry.
School of Chemical Engineering.
Corresponding author: Tel +86-22-27406140; Fax +86-22-27403475;
e-mail tianyiling@eyou.com.
The characteristic of DBD is that a dielectric layer covers
one or both of the electrodes, and an ac high electric field is
applied to generate gas discharge. It has been reported that DBD
‡
*
1
0.1021/jp037008q CCC: $27.50 © 2004 American Chemical Society
Published on Web 02/13/2004

1688 J. Phys. Chem. A, Vol. 108, No. 10, 2004
Li et al.
2
10) during the reaction. The discharge power is measured by
electronically integrating the product of voltage and current.
The effects of the CH4/CO2 ratio in the feed, discharge power,
flow rate of feed, and types of corona (i.e., positive corona or
negative corona) on the reaction were investigated. Under each
set of conditions, 30 min was allowed for stabilization before
quantitative analysis.
Products Analysis and Calculations. The products were
analyzed by an on-line quadrupole mass spectrometer (QMS)
(Balzers MSC 200) with a Faraday cup detector. The measure-
ment range of the QMS is between 0 and 200 amu (i.e., atomic
mass unit). The chemicals in the effluent were detected by
monitoring the signals of their main peaks, which are propor-
tional to their partial pressure. The main peaks of CO2, CH4,
H2, and CO are at 44, 16, 2, and 28 amu, respectively.
The reactor effluent was introduced into a cool trap to remove
liquid products, and then the gas products were quantitatively
analyzed by an on-line gas chromatograph equipped with a
carbon molecular sieve packed column and a thermal conductiv-
ity detector in argon carrier gas. The concentrations of CO2,
CH4, H2, and CO were determined by external standard
calibrations.
2 4
Figure 1. Schematic diagram of the process of CO reforming of CH
via corona discharge plasma: A, high-voltage dc source; B, wire
electrode; C, plate electrode; D, quartz tube; E, quadrupole mass
spectrometer; F, cool trap; G, gas chromatograph; H, flow meter; I,
corona discharge.
is effective in the activation of CH4 and CO2. In the process of
CO2 reforming via DBD, various products have been produced
The conversions (X) of CH4 and CO2, selectivities (S) of H2
and CO, and balance calculation (B) of carbon are defined as
1
2-17
15,16
include not only syngas
but also higher hydrocarbons
and oxygenates,1 implying that hydrocarbons and other organic
compound may be synthesized directly from CH4 and CO2 at
appropriate conditions via nonequilibrium plasma reaction.
Corona discharge could be initiated using a pair of inhomo-
geneous electrodes by a dc high electric field. In contrast with
DBD, an advantage of corona discharge is that it is relatively
easily to be established. Corona discharge has had many
applications in industry, such as reduction of NOx and SOx in
7
X(CH ) (%) ) (moles of CH before reaction -
4
4
moles of CH after reaction)/moles of CH4
4
before reaction × 100%
X(CO ) (%) ) (moles of CO before reaction -
2
2
2
moles of CO after reaction)/moles of CO2
flue gas, destruction of toxic compounds, and generation of
before reaction × 100%
ozone.1
8-20
It has been reported that corona discharge was used
21
22
for oxidative coupling of methane and decomposition of CO2.
S(H ) (%) ) 0.5 × moles of H produced/
2
2
The objective of this present study is to investigate the
characteristics of corona discharge plasma reaction influencing
on CO2 reforming. The effects of reaction conditions, including
the mixing ratio of CH4/CO2 in the feed, discharge power, flow
rate, and corona types, were studied. The formation mechanisms
of coke and syngas were analyzed, and the dependence of energy
efficiency of the dc corona plasma reaction on experimental
parameters has also been discussed.
(
moles of CH before reaction -
4
moles of CH after reaction) × 100%
4
S(CO) (%) ) moles of CO produced/
(moles of CH before reaction -
moles of CH after reaction +
4
4
moles of CO before reaction -
2
Experimental Section
moles of CO after reaction) × 100%
2
Experimental Apparatus. The schematic diagram of the
experiment is shown in Figure 1. One quartz tubular reactor
with an i.d. of 13.2 mm) consisting of a wire-plate stainless
B(C) (%) ) [1 - (moles of CH after reaction +
(
4
steel electrode configuration was used in this investigation. The
upper wire electrode was positioned with its top 10 mm above
the plate electrode. The plate electrode was always grounded
moles of CO after reaction + moles of CO formed)/
2
(
moles of CH before reaction +
4
moles of CO before reaction)] × 100%
(i.e., its potential is 0 V), and the wire electrode was at either
2
positive potential (called positive corona) or negative potential
(called negative corona).
The energy efficiency (E) of the reaction is defined as
The reactants, CH4 (>99.9%) and CO2 (>99.5%) in varied
ratio of CH4/CO2, were well mixed and then flowed through
the reactor at room temperature and atmospheric pressure. A
dc power supply with a high-voltage transformer was used to
initiate the corona discharge. The discharge voltage and
discharge current were measured with a voltage dial setting and
a current dial setting, respectively, and they were calibrated by
a high-voltage probe (Tektronix P6015) and a current probe
E (%) ) [moles of CO produced × ∆H (CO) -
f
(
moles of CH before reaction -
4
moles of CH after reaction) × ∆H (CH ) -
4
f
4
(moles of CO before reaction -
2
moles of CO after reaction) × ∆H
2
f
(
CO )]/electric energy consumption × 100%
(Tektronix CT-2) with a digital oscilloscope (Tektronix TDS
2

Carbon Dioxide Reforming of Methane
J. Phys. Chem. A, Vol. 108, No. 10, 2004 1689
Figure 3. QMS spectra of the reactor effluents before reaction (a)
4 2
and after reaction (b). Flow rate, 60 mL/min; CH /CO ratio, 2/1;
discharge power, 45 W; corona type, positive corona.
When CO2 reforming via positive corona, as shown in Figure
a, with increasing the CH4/CO2 ratio from 1/5 to 2/1, the
2
conversion of CO2 increases from 49.8% to 69.9%, and the
conversion of CH4 decreases from 91.9% to 72.6% simulta-
neously. The conversion of CH4 is always higher than that of
CO2. Figure 2b clearly shows that increasing the CH4/CO2 ratio
in the feed induces an increase of the H2/CO ratio in the
products. With the CH4/CO2 ratio increasing from 0.2 to 2.0,
the H2/CO ratio increases correspondingly from 0.21 to 2.02.
2 4
Figure 2. Effects of the mixing ratio on CO reforming of CH via
positive corona (s) and negative corona (- - -): (a) conversions and
b) H /CO ratio. Flow rate, 60 mL/min; discharge power, 45 W.
(
2
TABLE 1: Effects of CH
4
/CO
2 2
Ratio in Feeds on CO
a
Reforming of CH
4
CH /CO ratio
4
2
S(H
2
)/%
S(CO)/%
B(C)/%
As listed in Table 1, the selectivity of H2 increases propor-
tionally with the increasing CH4 concentration in the feed. It
increases from 39.4% and reaches a maximum of 96.7% at CH4/
CO2 ) 2/1. At the same time, the selectivity of CO decreases
from 98.9% to 64.5%. The balance calculation of carbon may
be used to estimate the yield of byproducts except syngas. The
values of B(C) indicate that the carbon-containing products
except CO increase sharply when the CH4/CO2 ratio is higher
than unity, implying that higher hydrocarbons, coke, or both of
them increase as the CH4/CO2 ratio increasing.
Positive Corona
1
1
1
1
1
2
/5
39.4
44.1
55.3
64.8
84.0
96.7
98.9
98.5
98.2
97.1
87.6
64.5
0.66
0.95
1.21
2.10
9.27
25.4
/4
/3
/2
/1
/1
Negative Corona
1
1
1
1
1
2
/5
/4
/3
/2
/1
/1
40.5
45.6
56.9
73.0
92.5
99.1
97.3
97.2
94.4
93.5
83.3
58.4
1.37
1.55
3.22
3.79
9.93
25.7
Similar changes of the conversions and the H2/CO ratio
happen within negative corona. But the conversions of CH4 and
CO2 via negative corona are obviously lower than that via
positive corona. During the same tested range, the conversion
of CO2 increases from 44.9% to 54.2%, and the conversion of
CH4 decreases from 82.1% to 65.5%. However, the H2/CO ratio
via negative corona increases from 0.22 to 2.41 during the CH4/
CO2 ratio increasing from 0.2 to 2.0, which is a little higher
than that via positive corona.
a
Flow rate, 60 mL/min; discharge power, 45 W.
where ∆Hf is the heat of formation for the corresponding CO,
CH4, or CO2. The heat of formation of hydrocarbons and coke
in the reaction is excluded in the calculation. The electric energy
consumption is calculated from the discharge power and the
reaction time.
The above experiments reveal that the H2/CO ratio in the
products depends strongly on the CH /CO ratio in the feed.
When the CH4/CO2 ratio in the feed is higher than 2/1, there
is obvious coke depositing on the cathode during the reaction.
The images of the coke were taken by a scanning electron
microscope (SEM) (Philips XL30ESEM) and a transmission
electron microscope (TEM) (JEOL JEM-100S). The C, H
microanalysis of the coke is performed by an analyzer (CHN-
O-Rapid) of Foss Heraeus Analysensysteme GmbH.
4
2
The ratio of H /CO ratio to CH /CO ratio is a little higher than
2
4
2
1. The results are benefit to control the H2/CO ratio in syngas
through adjusting the CH4/CO2 ratio in the feed. When the CH4/
CO2 ratio equals 1/2, the H2/CO is approximately 0.56, which
is a desirable feed for synthesis of long chain hydrocarbons.
In addition to syngas, there are some other chemicals also
be found by QMS (see Figure 3). The reactor effluent not only
consists of CH4 (16 amu), CO2 (44 amu), CO (28 amu), H2 (2
amu), C2H2 (26 amu), C2H4 (28 amu), C2H6 (30 amu), and a
lot of water (18 amu) but also consists of C3-C6 chain
hydrocarbons, a little of benzene (78 amu), toluene (91 amu),
etc.
Results and Discussion
Effects of the CH4/CO2 Ratio. To better understand the
influence of the feed gas composition on the reaction under
corona discharge plasma, we performed experiments by varying
the CH4/CO2 ratio from 1/5 to 2/1. A total feed flow rate of 60
mL/min and a discharge power of 45 W were applied during
the reaction. Figure 2 and Table 1 reflect the experimental
results.
Effects of the Corona Types. As discussed above, the
conversions of CH and CO via positive corona are obviously
4
2
higher than that via negative corona, but the H /CO ratio in the
2
products is the other way round. Similar situation occurred when

1690 J. Phys. Chem. A, Vol. 108, No. 10, 2004
Li et al.
TABLE 2: Effects of Discharge Power on CO
2
Reforming of
a
CH
4
discharge power/W
2 2
H /CO ratio S(H )/% S(CO)/% B(C)/%
2
3
4
5
6
7
6
5
4
3
0.62
0.59
0.53
0.54
0.54
65.8
67.5
64.8
68.3
69.4
91.7
95.4
97.1
98.5
97.1
5.07
2.99
2.10
1.17
2.39
a
4 2
Flow rate, 60 mL/min; CH /CO ratio, 1/2; corona type, positive
corona.
Figure 4. Effects of the discharge power on the conversions of
reactants via CO reforming of CH . Flow rate, 60 mL/min; CH /CO
ratio, 1/2; corona type, positive corona.
2
4
4
2
either the discharge power or the flow rate was changed. This
is related with the different characteristics between positive
corona and negative corona.
Positive corona and negative corona have different generation
2
0,23
mechanisms.
Negative corona generally propagates by
impact ionization of gas molecules, and its active volume is
confined to the near-electrode region. However, positive corona
depends more on photoionization for its propagation besides
the impact ionization mechanism. Positive corona forms when
the positive ion density is large enough to extend the region
into the interelectrode gap. This process builds by photoion-
ization, with the positive ion head moving in front of a nearly
neutral column. Accordingly, positive corona has an active
volume much larger than negative corona, and its electron
energy is also higher than that of the negative corona. It ensures
that CH4 and CO2 generally have higher conversions via positive
corona than that via negative corona. At the same time, the
selectivity of CO via positive corona is higher than that via
negative corona, but the selectivity of H2 is the other way round.
As a result, the ratio of H2/CO in the products via positive
corona is lower that that via negative corona. This mechanism
will be further explained later.
Figure 5. Effects of the flow rate on the conversions of reactants via
CO
2
reforming of CH
4
. CH
4
/CO
2
ratio, 1/2; discharge power, 45 W;
corona type, positive corona.
a
2 4
TABLE 3: Effects of Flow Rate on CO Reforming of CH
flow rate/mL/min /CO ratio S(H
H
2
2
)/% S(CO)/% B(C)/%
3
45
60
7
9
0
0.53
0.54
0.53
0.59
0.59
71.4
71.6
64.8
70.1
67.0
96.9
96.5
97.1
99.7
97.7
2.71
2.99
2.10
0.22
1.41
5
0
a
4 2
CH /CO ratio, 1/2; discharge power, 45 W; corona type, positive
corona.
CO change within narrow ranges from 64.8% to 69.43% and
from 91.7% to 98.5%, respectively.
Following experimental results were obtained using positive
corona, and the CH4/CO2 ratio in the feed equaling 1/2 was
used to produce syngas with lower H2/CO ratio.
The balance calculation of carbon reflects that there is a small
quantity of hydrocarbons or oxygenates forming during the
reaction. There are little benzene (78 amu) and toluene (91 amu)
detected by QMS. The amount of toluene decreases during the
discharge power increasing, and the amount of benzene increases
simultaneously. One explanation is that the dissociation energy
of toluene (3.94 eV) is lower than that of benzene (4.74 eV),
so toluene is easily decomposed and more stable benzene forms
when discharge power increases.
Effects of the Flow Rate. The influence of the flow rate on
the conversion of CH4 and CO2 is depicted in Figure 5. In a
mixture of CH4/CO2 ratio of 1/2 and discharge power of 45 W,
increasing flow rate decreases the conversions of CH4 and CO2.
Table 3 shows the effects of the flow rate on the reaction. A
change in the feed flow rate does not signally affect the H2/CO
ratio. When the flow rate increases from 30 to 90 mL/min, the
values of H2/CO ratio increase from 0.53 to 0.59. Increasing
flow rate results in little change for the selectivities of CO
(between 96.9% and 99.7%) and H2 (between 64.8% and
71.6%).
Effects of the Discharge Power. In a mixture of CH4/CO2
ratio of 1/2 and flow rate of 60 mL/min, the effects of discharge
power variation were investigated.
As depicted in Figure 4, the conversions of CH4 and CO2
increase from 78.3% to 94.1% and from 52.1% to 77.9% with
the discharge power increasing from 27 to 63 W, respectively.
The conversion of CH4 is always higher than that of CO2.
Comparing these conversions with the equilibrium conver-
24
sions, we find that the conversion of CH4 realized here equals
the equilibrium conversion of CH4 at the temperature between
9
00 and 1000 K, and the conversion of CO2 equals the
equilibrium conversion of CO2 at the temperature between 900
and 1400 K. This reflects the characteristic of nonequilibrium
plasma reaction.
As shown in Table 2, the main component in the products is
syngas, i.e., H2 and CO. The H2/CO ratio is approximately 0.56
and hardly depends on the discharge power. The H2/CO ratio
indicates that the products is not according to the stoichiometry
of reaction CH4 + 2CO2 f H2 + 3CO + H2O, which has a
H2/CO ratio equaling 1/3, implying that there are other products
formed besides syngas and water. The selectivities of H2 and
Coke Deposition. In CO reforming, different from steam
2
reforming, the deposited coke due to CO disproportionation
(reaction 6) and CH4 decomposition (reaction 7) cannot be

Carbon Dioxide Reforming of Methane
J. Phys. Chem. A, Vol. 108, No. 10, 2004 1691
volatilized by the reaction with steam (reaction 8).
2
CO f C + CO2
(6)
(7)
(8)
CH f C + 2H2
4
C + H O f CO + H2
2
As a result, catalyst deactivation due to coke deposition is a
serious problem for the application of catalysis method. Both
thermodynamic calculations25 and experimental observations4,5
indicated that CO disproportionation occurred on the catalyst
surface is the main contributor of the coke deposition, and there
often is serious coke deposition when CH4/CO2 ratio is close
4
-6
to unity.
During CH4 reforming via corona plasma method, however,
no coke formed evidently at low CH4/CO2 ratio. Interestingly,
when the CH4/CO2 ratio is higher than 2/1, it is found that coke
mainly deposited on the cathode, hardly on the anode. Figure 6
shows the SEM and TEM images of the coke depositing on the
tip of wire cathode during negative corona discharge. As shown
in Figure 6a, the coke has a treelike shape, which is similar to
the depicted image of negative corona.2 Figure 6b shows the
top of the coke. There are loose structures attaching to the
surface of the coke top. TEM image (Figure 6c) shows that the
coke consists of many tiny granules having dimensions of
approximately 50 nm.
0,23
In this corona plasma method, we suggest that coke mainly
formed via CH4 decomposition, not CO disproportionation. The
dissociation energy of CO possessing of 11.1 eV is higher than
that of dissociation energy of CHx (x ) 1-4), which is lower
than 5 eV. Compare with CHx, CO2 is more difficult dissociated
via the collision of electrons. The previous analysis of QMS
indicated that there was no coke formed during CO2 decompos-
ing under corona discharge.22 Moreover, our research showed
that carbon nanotubes can form in low-temperature corona
plasma as H2/CH4 ) 10/1.26 Therefore, CH4 decomposition may
be the main contributor of the coke deposition within nonequi-
librium plasma.
There exist two possible methods for coke formation. Both
of them are initiated by the decomposed of CH4 through
electron-collision dissociation
CH + e f CH + H + e
(9)
4
3
Figure 6. SEM images (a and b) and TEM image (c) of the coke
deposited on the cathode (i.e., the wire electrode) during negative corona
One method is that coke mainly forms via the continue
dehydrogenation of methyl radicals. The other method is that
coke mainly forms via dehydrogenation of aromatics formed
by polymerization of hydrocarbon such as trimerization of
acetylene. For the latter situation, the coke should have a lower
C/H ratio than the former. A literature reported that there was
solid (CH)n deposited on the cathode during methane decom-
4 2
discharge. CH /CO ratio, 4/1; flow rate, 60 mL/min; discharge power,
4
5 W; reaction time, 10 min.
Reaction Mechanism. It has been accepted by most re-
searchers that free radical processes are the main mechanisms
29,30
in nonequilibrium plasma reaction.
Here, the mechanism for
CO reforming via corona plasma reaction can simplified as
2
27
posing in glow discharge, and it is generally believed that coke
forms via dehydrogenation of aromatics in thermal coupling of
follows.
For CO reforming via the plasma method, the important
2
2
8
methane.
initial step is dissociation of CH and CO by electron collision
4
2
Experimentally, according to element microanalysis, the C/H
ratio in coke is 85.6/4.4, indicating the coke mainly consists of
carbon. Therefore, we suppose that the coke should mainly form
via dehydrogenation of methyl radical. In addition, we think
that some carbon-containing species are electropositive and are
easily ionized within dc corona plasma. These ions moved and
deposited onto the cathode under the influence of dc electric
field. Then they form tiny and loose carbonaceous deposition
rather than (CH)n polymer, but the accurate mechanism needs
further studying.
4 2
(reactions 9 and 10). CH and CO have dissociation energies
of 4.5 and 5.5 eV, respectively, which lie well within the electron
1
1
energy range in corona discharge.
CO + e f CO + O + e
(10)
2
Expect for the dissociation of carbon dioxide, CO could form
via reaction
C + O f CO
(11)

1692 J. Phys. Chem. A, Vol. 108, No. 10, 2004
Li et al.
Here, active oxygen atoms play an oxidant and keep coke from
depositing during methane decomposition. Oxygen atoms could
also react with other species and form many kinds of radical,
such as OH radical forming from the reaction
O + H f OH
(12)
As a result, besides molecules and ions, there exists a wide
variety of active free radicals, including CH3, CH2, CH, H, C2H5,
C2H3, C2H, OH, O, and others.
H2, H2O, and hydrocarbons, such as C2H2, could form via
the recombination of radicals
H + H f H2
(13)
(14)
(15)
Figure 7. Effects of the CH
rate, 60 mL/min; discharge power, 45 W; corona type, positive corona.
4
/CO
2
ratio on the energy efficiency. Flow
H + OH f H O
2
CH + CH f C H2
2
They could also form via the radicals reacting with each other
or with molecules, for example
H + CH f H + CH
(16)
(17)
(18)
4
2
3
H + CH f H + CH
3
2
2
H + OH f H + H O
2
2
Experimentally, the H2/CO ratio in the products mainly
depends on the CH4/CO2 ratio in the feed. The electron energy
within the tested range has a slight effect. Higher electron energy
results in a lower H2/CO ratio. It is supposed that higher electron
energy enables more CO2 molecules to be dissociated by
electron collision (reaction 10) and then results in higher yield
of CO and O. The active oxygen atoms readily react with
hydrogen atoms (reaction 12) and reduce the concentration of
hydrogen atoms. As a result, the selectivity of H2 decreases
because H2 could form via recombination of hydrogen atoms
Figure 8. Effects of the discharge power on the energy efficiency.
Flow rate, 60 mL/min; CH /CO ratio, 1/2; corona type, positive corona.
4 2
(reaction 13). Accordingly, a lower H2/CO ratio in the products
is the result of the changes of selectivities of CO and H2. This
may be the reason why the H2/CO ratio in the products via
positive corona is lower than that via negative corona.
Theoretically, methanol has almost twice the energy content
per volume in comparison to that of liquid hydrogen. The
literature has reported that methanol could be synthesized
31,32
Figure 9. Effects of the flow rate on the energy efficiency. CH
ratio, 1/2; discharge power, 45 W; corona type, positive corona.
4 2
/CO
through mixtures of CO2 and H2 using DBD.
Within the
reactor of CO2 reforming, there are large amounts of CO2 and
H2 which might be directly converted into methanol at ap-
propriate conditions. It has been reported that methanol formed
is 1/2 and the total flow rate is 60 mL/min, E decreases from
15.6% to 9.8% as the discharge power increases from 27 to 63
W. Although the total amount of reactants converted increases,
E decreases because more electric energy was consumed. The
effects of the flow rate on the energy efficiency are shown in
Figure 9. Increasing the flow rate increases the energy efficiency,
and the maximum energy efficiency is 13.5% in the tested range.
Although the conversions of both reactants decreases, the total
amount of reactants converted increases and more electric energy
is converted to chemical energy stored in the products.
Therefore, appropriate ratio of CH4/CO2 in the feed, relatively
low power discharge, and relatively large flow rate are of benefit
to higher energy efficiency.
As mentioned above, although the nonequilibrium plasma
method operates at lower reaction temperature, which is an
advantage over conventional catalytic methods, its energy
efficiency is still low. Most of the electric energy is converted
to heat energy, light energy, and others. Much work needs doing
to fill the large energy efficiency gap for the plasma process
33
via DBD using zeolites as catalysts. However, in our plasma
system, no significant methanol was detected by QMS, which
is consistent with the report of Gesser et al.12 It can be explained
that the dissociation energy (4.1 eV) of CH3O-H is relatively
low, so the newly formed methanol is in an excited stage and
will be more inclined to further react with other species to
produce additional hydrocarbons or oxygenates.
Energy Efficiency. The energy efficiency, E, reflects the
efficiency of converting electric energy to chemical energy
stored in the products including H2 and CO. The energy
efficiency for the reaction at different CH4/CO2 ratios is shown
in Figure 7. E is in the range 7.6-12.3%. As the CH4/CO2 ratio
increases, E increases initially and then decreases as the CH4/
CO2 ratio further increases. E reaches a maximum at CH4/CO2
)
1/3. This is due to the formation of a maximum amount of
CO at CH4/CO2 ) 1/3, and CO has a higher heat of formation
than hydrogen. As shown in Figure 8, when the CH4/CO2 ratio

Carbon Dioxide Reforming of Methane
J. Phys. Chem. A, Vol. 108, No. 10, 2004 1693
before it becomes a competitive alternative to conventional
catalytic methods.
(10) Fridman, A. A.; Rusanov, V. D. Pure Appl. Chem. 1994, 66, 1267.
(
11) Eliasson, B.; Kogelschatz, U. IEEE Trans. Plasma Sci. 1991, 19,
063.
12) Gesser, H. D.; Hunter, N. R.; Probawono, D. Plasma Chem. Plasma
Proc. 1998, 18, 241.
13) Huang, A.; Xia, G.; Wang, J.; Suib, S. L.; Hayashi, Y.; Matsumoto,
1
(
Conclusions
(
This investigation offers an alternative corona plasma method
for CO2 reforming of CH4, which is operated at atmospheric
pressure. Experiments confirm that corona plasma reaction can
lead to high conversions of methane and carbon dioxide, and
the ratio of H2/CO in the products strongly depends on the CH4/
CO2 ratio in the feed. The conversions of reactants via positive
corona are higher than that via negative corona. Higher electron
energy within corona discharge is beneficial to higher conver-
sions of CH4 and CO2 and producing syngas with lower H2/
CO ratio. Loose coke is found depositing mainly on the cathode
when the CH4/CO2 ratio in the feed is higher than 2/1. We
propose that the coke mainly forms via methane decomposition
during the reaction, which is different from the mechanism of
coke forming via CO disproportionation in catalytic methods
received by most researchers. Except for syngas, there were
various hydrocarbons and oxygenates forming simultaneously.
It is estimated that more than 10% of consumed electric energy
could be converted to chemical energy stored in the products
during the plasma reaction.
References and Notes
(
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(
(
(