22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Methane gas phase to liquid conversion via dielectric barrier discharge
C. Liu1,2, A. Fridman1, A. Rabinovich1 and D. Dobrynin1
1
2
A.J. Drexel Plasma institute, 08103 Camden, NJ, U.S.A.
Department of Electrical and Computer Engineering, Drexel University, 19104 Philadelphia, PA, U.S.A.
Abstract: In this paper, plasma-assisted direct liquefaction of methane phase is proposed.
Small scale DBD-based setup is built to examine the possibility of methane liquefaction in
presence of N 2 and liquid fuel (diesel). Preliminary results show increase of liquid fuel
volume, as well as mass change of fuel.
Keywords: methane liquefaction, atmospheric pressure dielectric barrier discharge
1. Introduction
Natural gas is fairly abundant in our environment. It
is found mostly in the earth’s crust, sea floor or other
remote locations. With the advent of natural gas drilling
and extracting technics, the production of natural gas
has increased over the years. Recent development of
shale gas in the United States and other countries has
provided an abundant supply of natural gas. This
increased supply has led to a dramatic drop in the price
of natural gas. In addition, an enormous amount of
natural gas has been wasted by the practice of flaring in
the petroleum industry whereby the natural gas from an
oil well is simply burnt. The World Bank estimates that
in excess of 140 billion cubic meters of natural gas was
flared in 2011 alone, polluting the atmosphere and
wasting approximately $50 billion worth of natural gas.
There is an increasing international pressure to end the
practice of flaring. One solution used by the petroleum
industry is re-injecting these massive quantities of
natural gas back into oil wells in lieu of flaring, albeit at
a substantial additional cost that does not offer a
substantial benefit to the company. In light of this, large
energy companies are actively looking for better ways
to convert the natural gas from oil wells to liquid fuels
that are more stable and easier to transport.
The traditional technology used to convert natural gas
into high-value oils and "drop-in" fuels involves
converting methane to syngas followed by
Fischer-Tropsch synthesis (FTS) [1]. This method
however is extremely capital intensive. The technology
uses a multi-stage process to break methane molecules
apart into carbon and hydrogen, then rebuilds synthetic
oil molecules from the carbon and hydrogen, and
finally, refines the synthetic oil into finished "drop-in"
synthetic fuels. The synthetic oils are made entirely of
converted methane molecules. The high cost of this
process however means it is economically viable only at
massive scales with abundant supplies of nearby cheap
natural gas.
Here we propose to employ a non-thermal plasma in
order to activate gaseous hydrocarbons such as methane
into a reactive state without cleaving the bonds of the
gaseous hydrocarbon molecules. The activated gaseous
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hydrocarbons are able to react with the longer chain
hydrocarbons in a liquid fuel thereby incorporating
components of the natural gas into liquid fuels.
2. Experimental setup
Two reactors were used in preliminary experiments;
the first reactor used for measurements of fuel volume
change is shown in Fig. 1. A parallel plate electrode
configuration is employed, with the high-voltage
electrode surface insulated via a 1 mm thick quartz disc.
Electrodes are fixed inside a stainless-steel chamber
filled with 100 ml of diesel. The gas mixtures were
bubbled through the gap from the bottom of the
chamber at rates of 0.7 slpm and 0.14 slpm. To ignite
the DBD discharge inside the gas bubbles, a nanosecond
pulsed power generator was used. The power supply
(FIDTech, GmBH) generated pulses with +15.5 kV
pulse amplitude in 50 Ohm coaxial cable (31 kV on the
high-voltage electrode due to pulse reflection), 10 ns
pulse duration (90% amplitude), 2 ns rise time and 3 ns
fall time [2].
Fig. 1. DBD reactor for volume measurement.
Experiments were performed with methane-nitrogen
and pure nitrogen feed gas environments. To quantify
the conversion of gas to liquid activity, volumetric
measurements were done on the liquid to record the
1
changes as a result of the process.
3. Results
Both experiments with methane/nitrogen mixture
(each of 0.07 slpm flow rate) and pure nitrogen (of
0.14 slpm flow rate) were carried out. Volume decrease
was recorded in both cases, however, it was less in the
experiment with methane-nitrogen mixture. The net
volume increase in the case of methane-nitrogen
mixture compared to Nitrogen is shown in Fig. 2.
As shown in Fig. 4 with discharge in pure nitrogen the
mass of diesel is decreasing but adding methane results
in less weight loss. The difference of weight loss
between two experiments is calculated as an equivalent
weight increase. In each experiment the weight of the
whole system is recorded as an average of a hundred
measurements to increase the accuracy.
Fig. 4. Comparison of weight loss with discharge in
different gases.
Fig. 2. Net volume change of diesel versus treatment
time.
To measure the corresponding effect as a change in
mass, a lighter reactor was built. The cross-section
experiment setup is illustrated in Fig. 3. The setup was
filled with 30 ml diesel. In this experiment, the
methane-nitrogen mixture was bubbled into the gap
with a flow rate of 0.5 slpm each, and in the case of
experiments with nitrogen the flow rate was set to
1.0 slpm. Applied voltage and frequency parameters are
the same for the different gases, and weight of the
whole system is measured every 10 minutes.
Fig. 3. Cross-section of DBD tube.
2
The amount of weight increase corresponds to a
conversion of 0.001 mol methane, with an energy cost
of 684 J (energy per pulse is ~0.5 mJ). Using the
relation in equation 1 we can calculate that the energy
cost of the process is 0.2 eV/molecule:
𝐶=
𝐸×𝑎
𝑛×𝑏
(1)
where C is energy cost in eV/molecule, E is the total
energy consumption in J; and n is the amount of
methane converted in terms of mole; a and b here are
constants, a is 6.25×1018 eV/J; b is 6.02×1023 mol-1.
Additional experiment with a dry ice condenser
connected to the gas outlet has been done to rule out the
effect of diesel evaporation. This experiment shows
that the difference of weight decrease in both type of
gases are not significant in the first 40 minuets, but start
to show higher weight difference later. Which indicates
that the process would require a certain temperature to
occur. The corresponding energy cost and conversion
rate of all experiments are calculated and presented in
Table 1.
4. Conclusion
The experiment data proves that converting methane
via non-thermal plasma is possible and the energy cost
is relatively low. However, conversion rate is still low
for economically large-scale production. Paramount to
optimizing the process is the development of a better
understanding of the chemical processes taking place in
the system. We are currently starting investigations
using gas chromatography and emission spectroscopy to
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Table 1. Energy cost and conversion calculation.
Experimental
setup
Measurement
Energy cost
eV/molecule
Conversion
rate
%
First setup
1. Low flow
rate (~0.1 slpm)
2. Small
discharge area
Equivalent mass
converted from
volume
difference
0.1
40
Second Setup
1. High flow
rate (~1 slpm)
2. Large
discharge area
Mass from
weight
measurements
0.2
2
Mass from
weight
measurements
(with
condenser）
0.2
2
trace the chemical processes occurring in these
configurations. Finally, we comment that optimizing
the setup with the inclusion of exhaust gas recirculation
and proper condensation equipment can help achieve
higher conversion rates.
5. References
[1] G. Henrici-Olivé and S. Olivé Angen. Chrm Int.
Ed. Engl., 15 136-141 (1976)
[2] C. Liu, et al. J. Phys. D: Appl. Phys., 47 252003
(2014)
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