21st International Symposium on Plasma Chemistry (ISPC 21) Sunday 4 August – Friday 9 August 2013 Cairns Convention Centre, Queensland, Australia Non-thermal plasma enhanced greenhouse gas conversion:
Advanced C1-chemistry
Tomohiro Nozaki1, Shota Moriyama2, Ken Okazaki2, Shodai Abe2, Kota Kawai2
1
2
Department of Mechanical Sciences & Engineering, Tokyo Institute of Technology, Tokyo, Japan
Department of Mechanical & Control Engineering, Tokyo Institute of Technology, Tokyo, Japan
Abstract: Direct conversion of methane to synthetic fuels was investigated using a micro-channel non-thermal plasma reactor. Non-thermal plasma was generated in a narrow-gap
co-axial dielectric barrier discharge configuration, thereby enhancing interaction between
non-thermal plasma and the reactor wall, producing a non-equilibrium product distribution
with syngas and methanol at high yields. H2 selectivity was increased by narrowing the gap
from 1.0 mm to 0.5 mm, producing syngas with H2/CO = 0.40.8 and CO selectivity of
3070%. H2 was used as the reaction sensitizer to produce hydrogenated reactive oxygen species such as H2O2, OH and HOO. A trace amount of H2 is sufficient to generate H2O2, which
is particularly important to increase selectivity for CO and CH3OH at low temperatures.
However, H2 was oxidized rather preferentially by oxidative radical species, therefore the net
H2 selectivity decreased markedly by H2 sensitizer. Correspondingly, CH4 conversion was decreased because O2 is consumed through H2 oxidation. Suppression of H2 oxidation is crucial
issue to reach higher methane conversion efficiency.
Keywords: C1-chemistry, hydrogen, methanol, gas-to-liquid
1. Introduction
Figure 1 schematically shows methane conversion
pathways to syngas (H2 and CO) and methanol. In an earlier study, we investigated direct conversion of methane to
methanol (R5) at low temperatures (5 C and 300 C)
using a micro-chemical non-thermal plasma reactor (microplasma reactor) [1–3]. Direct methanol synthesis is
beneficial because it obviates energy-intensive,
high-temperature syngas manufacturing processes [4,5].
Currently, syngas is produced via multi-step processes
shown as R1 and R2, where total combustion of initial
feedstock and steam/CO2 reforming is combined. Syngas
is then converted into liquid fuels such as methanol (R3).
Methane steam reforming is an equilibrium-limited step
that requires high-temperature thermal energy (ca. 1000
C). Multi-step, high-temperature methane conversion
processing is well established. No room exists for drastic
improvement of energy and material conversion efficiency and CO2 mitigation. Therefore, a new reaction control
technology such as photocatalysis, biochemical reaction,
and plasma catalysis have drawn keen attention as
next-generation green technologies [6–11]. Although solar
photocatalysis is regarded as the ultimate solution, CH4
conversion and production of useful chemicals are in
quantities too small to be industrially viable. We emphasize that non-thermal plasma is a viable and scalable solution as a next-generation CH4 conversion technology. The
energy necessary to generate non-thermal plasma can be
compensated by an appropriate combination of renewable
energy such as wind and photovoltaic power generation
system in which the efficiency and energy supply infra-
structure are developing rapidly worldwide [12]. Moreover, recent advances in plasma technology enable plasma-enhanced electrochemical reactions such as CO2 reduction [13], wastewater treatment [14] and in-liquid nanomaterial synthesis [15,16]. In those applications, the
concept of microplasma plays an important role [17,18] in
which we believe that existing solar photocatalysis and
electrochemical conversion of methane can be implemented by combining non-thermal plasma technology.
If syngas is manufactured via direct route (R4) with
moderate conditions, it is also economically more attractive than existing multi-step, high-temperature processes.
In fact, in an earlier study, not only CH3OH, but also
syngas was synthesized via a direct route at low temperatures (5 C and 300 C) and atmospheric pressure using a
microplasma reactor [3]. This paper presents direct synthesis of syngas and methanol using a newly developed
narrow-gap co-axial dielectric barrier discharge (DBD)
reactor. Such a configuration portends industry scale
ozone generators that can readily accommodate
large-scale gas treatment in the same manner [19]. The
narrow-gap configuration is extremely important to enhance interaction between non-thermal plasma and the
reactor wall, enabling non-equilibrium product distribution with syngas and methanol at high yields. In addition,
reaction enhancement by a trace amount of H2 sensitizer
is discussed.
Conventional thermochemical reaction (> 1000 C)
• Indirect syngas production
Full combustion CH4 + 2O2  CO2 + 2H2O
R1
21st International Symposium on Plasma Chemistry (ISPC 21) Sunday 4 August – Friday 9 August 2013 Cairns Convention Centre, Queensland, Australia Steam reforming CH4 + H2O  CO + 3H2
Dry reforming
CH4 + CO2  2CO + 2H2
• Syngas-to-methanol CO + 2H2  CH3OH
Plasma catalysis (< 300 C)
• Direct syngas production
CH4 + 0.5O2  CO + 2H2
• Direct methanol synthesis CH4 + 0.5O2  CH3OH
R2
R3
R4
R5
(20300 C)
CH4/O2
CO/H2
ing energy equations with constant power density, assuming uniform and steady state heat generation over the
plasma reactor [21,22]. Nevertheless, such a simplified
model does not provide reasonable gas temperature because of non-uniform heat generation and transient heat
and mass transport in DBD [20,23]. In this sense, the
aluminum electrode temperature was monitored instead of
the local gas temperature measurement by optical emission spectroscopy [24].
CH3OH
(20300 C)
Heat
Total
(catalytic reaction)
combustion
Reforming
(Steam/CO2 ~ 1000 C)
CO2/H2O
Fig. 1 Direct and indirect methane conversion routes.
Plasma-enhanced direct route is highlighted by red
arrows.
2. Experimental
Figure 2 schematically depicts a narrow-gap co-axial
DBD reactor. A high voltage sine wave (10 kHz) was applied between the aluminum rod and external metallic
electrode. The total gas flow rates were 100 and 150 cm3
min-1. The O2/CH4 ratios were 0.5 and 1.0. The respective
applied peak voltages were 6 and 7 kV. The discharge
power was measured by drawing Lissajous figures,
showing approximately 12 W for 6 kV and 20 W for 7 kV.
For this purpose, a capacitor (0.036 F) was inserted into
the circuit (Figure 2). Based on the previous work, the
narrow-gap configuration is extremely important for realizing non-equilibrium activation of gas molecules and
quenching of non-complete oxidation products simultaneously: these are paradoxical conditions in conventional
thermochemical processes [1]. Moreover, the co-axial
configuration resembles that of an industry-scale ozone
generator; the given reactor is readily scalable compared
to a thin glass tube reactor (i.d. 1.5 mm) used in earlier
studies [1–3]. To enhance the interaction between DBD
and reactor wall, the discharge gap was set to 0.5 mm and
1.0 mm. Heat produced by DBD and methane partial oxidation increased the aluminum electrode temperature to
ca. 100 C. In this temperature range, CH3OOH, HCOOH,
and HCHO are oxidized into syngas; CH3OH and syngas
became the main product in this condition [3]. The aluminum rod temperature is expected to be almost uniform
from the inlet to the outlet of the reactor because of high
thermal conductivity. However, the gas temperature
should increase monotonically downstream [20]. The average gas temperature in DBD is often estimated by solv-
Fig. 2 Narrow-gap co-axial DBD reactor.
3. Syngas production in narrow-gap DBD
Figure 3(a) portrays the H2/CO ratio shown against CH4
conversion. Although the residence time is halved by reducing the gap from 1.0 mm to 0.5 mm, CH4 conversion
was influenced only slightly by the gap length. One possible inference is that the mean electron energy might
increase at a 0.5 mm gap reactor which enhanced CH4
conversion at shorter residence time. The mean electric
field strength for 1-mm-gap DBD reactor was estimated
as 2030 kV/cm, while that for 0.5 mm-gap reactor was
3040 kV/cm, resulting in a higher radical production rate.
The H2/CO ratio is clearly enhanced by narrowing the gap.
The gas temperature would increase at higher power density, which preferentially oxidizes H2 rather than CO and
CH4, yielding a lower H2/CO ratio. Nevertheless, a higher
H2/CO ratio was obtained at a 0.5 mm gap. A water gas
shift (WGS) reaction (R6) [3] seems not to occur because
CO2 selectivity did not increase at the 0.5 mm gap.
H2O + CO  CO2 + H2
R6
It is noteworthy that, in a non-thermal plasma reactor,
reaction selectivity is a function not only of temperature
and pressure, but also of the reactor geometry. In other
words, the mean electron energy is a particularly important characteristic of plasma catalysis. Figures 3(b) and
3(c) respectively portray CO selectivity and CH3OH/CO
21st International Symposium on Plasma Chemistry (ISPC 21) Sunday 4 August – Friday 9 August 2013 Cairns Convention Centre, Queensland, Australia ratio. The CH3OH selectivity is determined by the initial
O2/CH4 ratio. The narrow-gap reactor does not enhance
CH3OH selectivity. The product selectivity such as Figure
3(b) is rather scattered because of the intrinsic nature of
DBD which consists of numbers of transient discharges
with nanosecond current pulses. However, a clear relationship exists by which CO selectivity increases continuously with increasing methane conversion.
However, based on numerical modeling [26], the electron impact dissociation of CH4 is the dominant CH4 activation pathway at low temperature and R7 is not as important as we might expect, probably because R7 is an
endothermic reaction and the thermal energy (high temperature situation) is also needed to compensate the reaction enthalpy. Atomic oxygen is expected to play an important role at elevated temperatures, but the results more
closely resemble the total combustion of methane, which
contradicts low-temperature plasma catalysis. However,
OH radical is apparently a promising oxidative agent because hydrogen activation proceeds via an exothermic
reaction (R8).
CH4 + OH  CH3 + H2O
(a) H2/CO ratio
(b) CO selectivity
(c) CH3OH/CO ratio
H = -14 kJ/mol
R8
To promote OH production, a trace amount of H2 was
injected as a reaction sensitizer. H2 is partially oxidized,
producing oxidative agents such as H2O2, HOO, and OH,
as shown in Figure 4. H2O2 is decomposed further either
by heat (pyrolysis) or radical species, producing OH radicals. However, R10 is not favored because this reaction
alternatively consumes reactive species. Actually, H2O2
pyrolysis (R11) is desired because H2O2 decomposes
spontaneously into two OH, and does not consume radicals. H2O2 pyrolysis becomes predominant when the
temperature becomes higher than 100 C [27].
H2O2 + O  OH + HOO
R9
< 100 C
H2O2 + H  OH + H2
R 10
< 100 C
H2O2  OH + OH
R 11
Pyrolysis; > 100 C
Figure 5 shows the product selectivity and CH4 conversion with respect to the amount of H2 sensitizer. H2O2
increased by a factor of two, indicating that H2O2 is synthesized efficiently by a trace amount of H2. Selectivity
for CO, CO2, and CH3OH increased with H2 sensitizer.
Correspondingly, FTIR analysis revealed that selectivity
for HCHO and HCOOH decreased remarkably.
Fig. 3 H2/CO and CH3OH/CO ratios and CO selectivity under various operating conditions. O2/CH4
ratios are 0.5 and 1.0. The respective discharge
gaps are 0.5 and 1.0.
4 H2 as a reaction sensitizer
Atomic oxygen is regarded as a key species to initiate CH4 partial oxidation, especially at low temperature
CH4 conversion [25].
CH4 + O  CH3 + OH
H = 3.2 kJ/mol
R7
Fig. 4 H2 oxidation routes and OH, HOO, and H2O2
reaction networks.
21st International Symposium on Plasma Chemistry (ISPC 21) Sunday 4 August – Friday 9 August 2013 Cairns Convention Centre, Queensland, Australia Fig. 5 CH4 conversion and product selectivity. Conditions: O2/CH4 = 1.0, gap = 0.5 mm, total gas flow rate
= 100 cm3 min-1.
Figure 5(b) shows selectivity for H2; CH4-oriented H2
was calculated by subtracting H2 sensitizer from the overall H2 concentration. H2 selectivity decreased monotonically with H2 sensitizer, and became almost zero at H2 =
20 cm3 min-1. The result clearly indicates that OH and
HOO preferentially oxidize H2. In other words, O2 is
consumed selectively by H2 oxidation. Accordingly, CH4
conversion decreased. It is important to note that H2 content in the reaction field invariably increases with CH4
conversion. That H2 also serves as a sensitizer, providing
oxidative agents such as H2O2, OH, and HOO. Consequently, CH4 conversion and H2 selectivity are expected
to slow as CH4 conversion increases. A tradeoff relation
exists between CH4 conversion and H2 selectivity.
5. Concluding remarks
Direct methane conversion to synthetic fuels was
demonstrated using a narrow-gap DBD reactor. Narrowing the gap from 1.0 to 0.5 mm produced syngas with a
higher H2/CO ratio at low temperature (ca. 100 C). A
trace amount of H2 produces hydrogenated reactive oxygen species such as OH, HOO, and H2O2 which in turn
increases syngas and methanol selectivity. However, H2 is
oxidized rather selectively, resulting in a lower H2/CO
ratio. For further increase in syngas and methanol yields,
one-pass CH4 conversion must be suppressed below 30–
40% and unreacted CH4 should be recycled after product
separation. Alternatively, pursuing an appropriate combination of non-thermal plasma and catalysts/ electrochemical system is a more attractive and promising approach
because an effective plasma-surface interaction is highly
expected to occur in a narrow-gap DBD, where surface-to-volume ratio of plasma reactor is increased.
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