Note: Descriptions are shown in the official language in which they were submitted.
CA 02621749 2008-02-19
DECOMPOSITION OF NATURAL GAS OR
METHANE USING COLD ARC DISCHARGE
FIELD OF THE INVENTION
This invention relates to a method and an apparatus for the decomposition of
natural gas or methane into its gaseous constituents and carbon using cold arc
discharge.
More particularly, the invention relates to the use of a cold arc discharge in
order to
achieve the decomposition of natural gas or methane into carbon in the form of
solid
particles and gaseous products consisting mainly of hydrogen and acetylene in
admixture
with unreacted methane.
BACKGROUND OF THE INVENTION
Arc discharge has been known for a long time. A sustained arc will generate
hot
plasma. However, by controlling the arc, one can generate a cold arc discharge
producing
a very dense plasma, but with considerably less heat. In the past, a small
pulse width
using a flyback transformer produced such dense plasma, but it was a rather
difficult and
expensive way of generating a "cold arc".
In Canadian patent application No. 2,353,752 there is disclosed a method and
an
apparatus for producing hydrogen and carbon from natural gas or methane using
a barrier
discharge non-thermal plasma, which is also called "cold plasma". Such process
requires, however, the use of precisely designed and formulated ceramic
materials as the
dielectric barrier. Such materials may be difficult and expensive to obtain or
produce.
There is, therefore, a need for a method and an apparatus that would transform
natural
gas or methane primarily into hydrogen and carbon without requiring such
materials.
CA 02621749 2008-02-19
SUMMARY OF THE INVENTION
It has been surprisingly found that hydrogen and carbon can be produced from
natural gas or methane by using cold arc discharge instead of dielectric
barrier discharge.
It should be noted that the chemical reaction mechanism remains similar in
both the
dielectric barrier discharge (DBD) and cold arc discharge (CAD), namely the
general
chemical reaction is:
CH4(g) 4 C(s) + H2(g)
However, the microdischarges in DBD are specifically produced so as not to
fall
into an arcing mode, whereas in CAD arcs of controlled time duration and
frequency are
produced to carry out the reaction.
A cold arc discharge may be defined as an intermittent arc discharge that
makes it
possible for the reactor to operate at relatively low temperatures, typically
below 200 C.
On the other hand, the hot arc discharge produces a continuous plasma arc
which
generates temperatures within the reactor in the range of 1700 C to 4000 C and
higher,
since the temperature of the hot plasma is in excess of 4000 K.
The present applicant has found that it is possible to generate a cold arc
discharge
by limiting the lifetime of an arc during a discharge by using a capacitor in
series with a
high voltage electrode while using a pulsating high voltage discharge, and
further that
the cold arc so produced is well suited for promoting the reaction mentioned
above,
namely dissociation of natural gas or methane into its components. It should
be noted
that the resulting products contain in addition to hydrogen and carbon, a
small but
measurable amount of acetylene and trace amounts of some other hydrocarbons
that are
usually produced in such reactions.
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The resulting gas flow exiting from the reactor is then normally passed
through a
solids collector, such as a HEPA (high efficiency particle arrester) to
collect the typically
nanoscale solid carbon particles that occur as a result of the process and the
remaining
gaseous mixture comprising essentially unreacted natural gas, hydrogen and
acetylene
proceeds to equipment that measures the gas composition and collects or
further
processes that gaseous mixture.
Thus, in essence, the method of the present invention for decomposing natural
gas or methane into its gaseous constituents and carbon comprises passing a
flow of
natural gas or methane through a reaction zone and generating a cold arc
discharge in
said reaction zone adapted to decompose the natural gas or methane into its
gaseous
constituents and carbon in the form of solid particles.
In a more specific embodiment, the method comprises:
(a) passing a flow of natural gas or methane through a reaction zone having a
ground electrode on one side and a wall of dielectric material on the other,
and having a high voltage (HV) electrode projecting through the wall of
dielectric material, said HV electrode having a capacitor in series
therewith; and
(b) imparting a pulsating high voltage discharge through said capacitor and
into said HV electrode so as to produce a cold arc discharge in the
reaction zone between said HV electrode and the ground electrode, which
decomposes the natural gas or methane into its gaseous constituents and
carbon in the form of solid particles.
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In order to make the system more efficient, a plurality of HV electrodes with
capacitors in series therewith may be installed in the reaction zone and
arranged in such a
manner as to produce a high degree of conversion. The wall of dielectric
material
through which the HV electrode or electrodes are projected need not be of any
special
material, and may be, for example, made of plastic that would resist
temperatures of the
order of 200 C - 400 C. If desired, however, walls of ceramic material may
also be used.
In another embodiment, the method comprises:
(a) passing a flow of natural gas or methane through a reaction zone formed
between an electrode connected in series with a capacitor which is
connected to the ground through a dielectric element on one side and a
high voltage (HV) electrode on the other side; and
(b) connecting a pulsating high voltage source to the HV electrode, so as to
generate a cold arc discharge in the reaction zone between the HV
electrode and the capacitor-connected electrode, said cold arc
decomposing the natural gas or methane into its gaseous constituents and
carbon in the form of solid particles.
In this embodiment, a plurality of capacitor-connected electrodes may also be
installed in the reaction zone. This may be done by mounting them on a central
cylindrical shaft made of dielectric material and surrounding this shaft with
the HV
electrode in the form of a tubular metal wall which is connected to the HV
source.
The basic apparatus in accordance with the present invention comprises a
reactor
with a reaction zone, means for passing a flow of natural gas or methane
through said
reaction zone and means for generating a cold arc discharge or a plurality of
cold arc
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discharges in said reaction zone such as to convert the natural gas or methane
into its
gaseous constituents and carbon.
In one non-limitative embodiment, the reactor has a tubular shape with a
rotatable
cylindrical ground electrode made of a conducting material, such as aluminum
or steel,
in the middle and with an outer wall of dielectric material, such as a plastic
or ceramic,
through which a plurality of HV electrodes extend into the reaction zone
formed in the
space between the ground electrode and the outer wall. Each of the HV
electrodes is
provided with a capacitor in series and is connected through the capacitor to
a pulsating
high voltage source adapted to produce a high frequency cold arc discharge
between
each HV electrode and the ground electrode. This enables conversion of the
natural gas
or methane introduced through an inlet port or ports and flowing through the
reaction
zone, into its gaseous constituents and carbon. The ground electrode is
normally rotated
during the operation of the reactor so that carbon would not form conducting
bridges
between the electrodes. Rotating speeds between 60 RPM and 200 RPM may be
used.
Outside of the reactor, means are provided to separate the solid particles of
carbon from
the gaseous products and to analyse the latter to determine the gaseous
constituents.
In another embodiment, the reactor again has a tubular shape, but with a
rotatable
cylinder of dielectric material in the middle, in which are embedded a
plurality of
electrodes extending outwardly from the cylindrical surface. Each such
electrode is
provided with a capacitor in series and is connected to a common ground. This
is
surrounded by a metal tube acting as a high voltage electrode. The reaction
zone is
formed between the capacitor-connected electrodes and the HV tube which can be
made
of any conducting metal, such as aluminum or steel. As in the previous
embodiment,
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means are provided for passing a flow of natural gas or methane through the
reaction
zone while connecting the conductive metal tube to a pulsating HV source. In
this
manner, arc discharges are generated in the reaction zone between the metal
tube,
namely the HV electrode, and the rotating capacitor-connected electrodes.
Thus, when
AC high potential is applied to a number of capacitor-connected electrodes
through the
common HV electrode (metal tube), the arc discharge occurs homogeneously at
each
electrode area. Because the capacitor-connected electrodes are mounted on the
central
shaft and are rotating, the arc discharges are also rotating, which increases
the reaction
area and, therefore, the decomposition efficiency.
It should be noted that the International PCT Publication WO 2004/061929 Al
describes a plasma generator for producing ozone which uses a pair of
electrodes without
interposition of a dielectric body, and a capacitor in series therewith. It
also requires a
second capacitor in parallel with the electrodes for supplementing the power
supply.
Furthermore, it is indicated that the capacitor in series with the electrodes
is an arc-
suppression capacitor producing an arc-suppressing discharge between the
electrodes,
thereby generating plasma. Moreover, according to this prior art disclosure,
the electrode
unit is composed of a floating electrode, and insulator arranged around the
floating
electrode and a ground electrode arranged around the insulator. This prior art
system is
therefore clearly different from the one of the present invention and is used
for a
completely different purpose.
The present invention will now be further described with reference to the
appended drawings which illustrate various embodiments of the invention.
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BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic diagram of an embodiment of the system in accordance
with
the present invention for producing a cold arc discharge;
Fig. 2A is a schematic diagram illustrating a positive discharge taking place
before a cold arc-generating discharge;
Fig. 2B is a schematic diagram illustrating the positive discharge taking
place
during the cold arc-generating discharge;
Fig. 2C is a schematic diagram illustrating the positive discharge taking
place at
the end of the cold arc-generating discharge;
Figs. 3A and 3B are graphs showing voltage waveforms produced at nodes
illustrated in Figs. 2A, 2B and 2C , respectively;
Fig. 4 is a cross-sectional side view of one embodiment of an apparatus
suitable
for the purposes of the present invention;
Fig. 5 is a cross-sectional plan view of the apparatus shown in Fig. 4;
Fig. 6 is a partial cross-sectional side view of another embodiment of an
apparatus suitable for the purposes of the present invention;
Fig. 7 is a cross-sectional plan view of the apparatus shown in Fig. 6;
Fig. 8 is a plan view of another embodiment of the reactor suitable for the
purposes of the present invention;
Fig. 9 is a cross-sectional side view of the reactor shown in Fig. 8;
Fig. 10 is a plan view of a still further embodiment of the reactor suitable
for the
purposes of the present invention;
Fig. 11 is a cross sectional side view of the reactor shown in Fig. 10;
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Fig. 12 is a schematic diagram of a further embodiment of the present
invention
using a bi-polar source and a rectifier with a positive discharge to produce a
cold arc;
Fig. 13 is a schematic diagram similar to that of Fig. 12 but adapted for a
negative discharge;
Fig. 14 is a schematic diagram of another embodiment of the system suitable
for
producing a cold arc discharge in accordance with the present invention;
Fig. 15 is a schematic diagram of a system such as shown in Fig. 14, but
illustrating use of a plurality of capacitor-connected electrodes and a
rotating arc
discharge;
Fig. 16 is a cross-sectional side view of an apparatus using a system such as
shown in Fig. 15;
Fig. 17 is a cross-sectional plan view of an apparatus shown in Fig. 16;
Fig. 18 is a flow diagram illustrating the process of the present invention;
Fig. 19 is a schematic of a circuit used to generate a cold arc discharge
according
to a further embodiment of the invention.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
In the drawings in which the same elements are designated by the same
reference
numbers, Fig. 1 illustrates the basic set-up in accordance with one embodiment
of the
present invention. It provides a reaction zone between the ground electrode 20
and a wall
of dielectric material 22. Through this wall 22 of dielectric material, a high
voltage
electrode 24 is projected which has a capacitor 26 in series therewith, and is
connected
through this capacitor 26 to a high voltage source 28.
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An exemplary high voltage source 28 is a high voltage line generating a
pulsating
high voltage discharge. The pulsating high voltage discharge is bi-polar;
i.e., it generates
both positive and negative pulses. A schematic for an exemplary high voltage
source 28
is depicted in Fig. 19. In Fig. 19, a microcontroller interface 50 is coupled
to electronic
switches 52 that produce the pulsating voltage 54 whose voltage is stepped up
by a
transformer 56 to the voltage levels that are useful for producing cold arc
discharges
when applied across the electrodes 20, 24. The output of the transformer 56 is
coupled
directly to the capacitor 26 which, in turn, is coupled to the electrodes 24,
20.
The capacitor 26 blocks the current during the discharge and produces a cold
arc
discharge in the reaction zone between the HV electrode 24 and the ground
electrode 20.
This cold arc discharge is adapted to decompose the natural gas or methane
flowing
through the reaction zone into its gaseous constituents and solid carbon
particles, which
is achieved by controlling the frequency and duration of the cold arc
discharge. The
frequency of the arc is controlled by a microcontroller. There is no specific
limit to the
frequency that can be used, but at a certain point, the cold arc discharge
will transform
into thermal plasma and this should be avoided. Frequencies in the range of
about 1 kHz
to about 20 kHz are quite suitable. As far as the arc duration is concerned,
it is mainly a
function of the capacitor, although the power supply impedance will also have
an effect
on the arc duration. The arc duration may be determined by the current
waveform during
a discharge. A small capacitor will give very short current pulses, while a
large capacitor
will give longer current pulses. For example, a capacitor of 1000 pF gives a
current pulse
width of approximately 3 ps, whereas a capacitor of 100 pF gives a current
pulse width
of approximately 40 ns. Capacitors from about 100 pF to about 3000 pF have
been found
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quite suitable for the purposes of the present invention, although this range
is not
limitative. Exemplary time constants that can be used in the circuit range
from about 0.1
s to about 0.2 s. It should also be noted that the size of the gap has an
effect on the
current pulse duration because the discharge impedance is higher. Generally,
pulse width
durations between about 40 ns and about 3 s are satisfactory and produce good
results
in conjunction with frequencies of about 1 kHz to about 12 kHz, although again
they are
not limitative for the purposes of the present invention.
Figs. 2A, 2B and 2C show the principle of a typical cold arc discharge in
accordance with the present invention. In Fig. 2A the high voltage source 28,
which
generates pulsating voltage, is connected through the capacitor 26 to the HV
electrode
24. It is assumed herein that the capacitor 26 is discharged and a positive
pulse is
starting. If it is not discharged, there will be a potential difference
between the two
capacitor leads as seen in, for example, Fig. 3A waveforms 2 and 4. When the
voltage
pulse starts to build up, the voltage across the electrodes 24 and 20 also
starts to build up.
The capacitor 26 is not charging since no current is flowing in the circuit,
other than a
small current due to leakage and ionization.
The polarity of the capacitor 26 has been set as shown in Fig. 2A, even though
the capacitor was not charged. The waveforms 2 and 4 in Figs. 3A and 3B show
the
voltage of the capacitor 26 with reference to ground. When the voltage after
the capacitor
26 reaches the breakdown voltage, an arc discharge starts to occur and both
voltages of
the capacitor 26 collapse. The current increases very fast, because the gap
between the
high voltage electrode and the ground electrode is conductive and the
capacitor 26 is
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charging. Waveform 1 in Figs. 3A and 3B show the current spikes. These spikes
have
risetime in the low nanoseconds.
During the arc discharge, illustrated in Fig. 2B, the voltage drop across the
gap
between the electrodes 24 and 20 is small; therefore the positive side of the
capacitor 26
will change to negative, forcing the capacitor 26 to fully charge. When fully
charged, the
polarity of the capacitor 26 will be completely reversed and current will stop
flowing.
The arc will extinguish and the gap will no longer be conductive. At this
point, the
polarities of the capacitor 26 and of the gap will be as shown in Fig. 2C.
Since no current
(or very little) is flowing at this stage in the circuit, the voltage will
rise again until the
end of the pulse. In this case, the pulse width supplied by the pulses was 10
s as
illustrated by the bottommost waveform, the "Control" waveform, in Figs. 3A
and 3B.
The "Control" waveform is illustrative of the pulsating voltage 54 prior to
its voltage
being stepped up by the transformer 56. A similar procedure will happen for a
negative
pulse as illustrated in Fig. 3B. Risetime and falltime are characterized by
the load and the
power supply.
In Figs. 3A and 3B the top line indicates where the pulse trigger T is
happening.
In both Figs. 3A and 3B it is almost in the middle, namely at 49.20%, as
indicated at the
bottom.
The numbers shown on the left side of the graph in both Figs. 3A and 3B are
the
scope's active channels, namely:
1 - plasma current
2 - high voltage before the capacitor in reference to ground - transformer
voltage
3 - high voltage after the capacitor in reference to ground - gap voltage
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In Fig. 3A, the maximum peak-to-peak voltage reading for channel 2 is 9.40 kV,
and the maximum peak-to-peak voltage reading for channel 4 is 11.6 kV. In Fig.
3B, the
maximum peak-to-peak voltage reading for channel 2 is 10.4 kV, and the maximum
peak-to-peak voltage reading for channel 4 is 10.0 kV, as indicated.
At the bottom of both Figs. 3A and 3B there are indicated the scales for each
channel and its units. Thus, 10.OA 0 represents 10 ampere per division. 92
symbol means
that this channel has the input set to 50 ohms. Other channels are at 1 MS2.
The time scale along the horizontal axis of the waveforms depicted in Figs. 3A
and 3B is 10.0 s per division.
In Fig. 3A, "A Ch2 - 7.2kV" means that the scope is triggering on channel 2
and
the trigger level is set to 7.2kV. The `A' in front means the trigger is in
"AUTO" mode.
It triggers on the rising edge of the signal. In Fig. 3B, the trigger level is
set to -6.70 kV.
Capacitance, gap discharge size and voltage level have an effect on conversion
levels. Increasing capacitance has increased conversion levels, but at the
same time it
increases the production of heat. By increasing the capacitance, it takes more
time to
charge the capacitor, therefore the arc is sustained longer and more power
from the
source is needed. Gap distance also increases the conversion since the plasma
volume is
increased. As a consequence to gap increase, the voltage will also be
increased because
of higher potential needed to create breakdown.
It should be noted that the breakdown voltage is mainly a function of gap and
frequency. For example, at a frequency of 8 kHz, the initial breakdown voltage
can drop
to about 6 kV and sustain a discharge with a gap of 5 mm and a capacitor of
100 pF. In
another example, a continuous discharge (not initial) is obtained at 11kV with
a capacitor
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of 500 pF at the same frequency of 8 kHz. For the purposes of the present
invention,
gaps can be between about 2.5 mm and about 20 mm with initial voltages between
about
15 kV and about 25 kV depending on the gap and frequency used. The duration of
each
pulse can be between about 10% to about 50% of the period of the input
voltage. For
example, assume an input voltage having a frequency of 20 kHz that transitions
between
OV and positive (during a positive pulse) and negative (during a negative
pulse) values
V,,,. and Vmin, respectively. This translates to an input voltage having a
period of 50 s.
If using a pulse of 10% duration, the input voltage would be at Vm. for 5 s
and then
return to 0 V for 20 s, following which the input voltage would transition to
Vmin and
hold for 5 s and then return to 0 V for another 20 s. The pulse duration
will depend
on, for example, the selected frequency and on the voltage source 28's ability
to generate
the desired voltage waveform.
Fig. 4 and Fig. 5 show an embodiment of a reactor 30 that is suitable for the
purposes of the present invention. It has a tubular configuration with a
rotary ground
electrode 20 and a plastic wall 22 through which four rows of HV electrodes 24
are
mounted in a triangular arrangement shown in Fig. 5. The high voltage bus 25
for the
HV electrodes 24 is provided in the middle of the four rows on the outside of
the plastic
wall 22. Pulsating high voltage discharge from a suitable source (not shown)
is supplied
through the bus 25 and through the capacitors 26 to the HV electrodes 24
thereby
generating cold arc discharges in the reaction zone between the ground
electrode 20 and
the HV electrodes 24, while natural gas or methane (indicated as NG INPUT) is
fed at
the input end 31 and flows through the reaction zone where it is transformed
into
products of the reaction which exit at the output end 32.
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The configuration shown in Fig. 6 and Fig. 7 further provides a positive
pressure
chamber 34 through which natural gas or methane is fed into the reaction zone
via
additional ports 31A and 31B. This is in addition to the flow via input 31.
The chamber
34 may be formed by an enclosure 36 which may be made of any suitable material
that is
not easily damaged by arcs. For this reason, one suitable material is a
ceramic material,
but some plastics would also be suitable. This arrangement helps to reduce or
eliminate
carbon buildup on the surface of the plastic wall 22 since, due to the
positive pressure in
the chamber 34 with reference to the reaction zone, carbon particles formed
during the
process cannot enter said chamber 34 and be deposited on the wall 22, thereby
preventing shorts between HV electrodes 24. Moreover, the flow of natural gas
coming
out of the hole 36 from within chamber 34 should be suitably controlled since
the time
the natural gas spends in the reaction zone affects the conversion rate of the
reaction.
Generally, the flow of CH4 passing through the reaction zone may be anywhere
between
1 m3/hr and 250 m3/hr depending on the number of electrodes used. Although in
this
embodiment only one set of HV electrodes 24 is shown, arranged in a triangular
configuration, several rows of such electrodes could be provided in a similar
fashion as
shown in Figs. 4 and 5. All such electrodes are, of course, provided with
capacitors in
series and are connected to a source of pulsating high voltage as previously
explained.
Fig. 8 and Fig. 9 show another arrangement of HV electrodes 24 within a
tubular
reactor with a rotatable ground electrode 20 in the middle. In this
arrangement, there are
four rows of 12 HV electrodes per row mounted in the plastic wa1122. Although
this is
not shown in Figs. 8 and 9, each HV electrode 24 is connected to a capacitor
in series
and to a source of pulsating high voltage in a similar manner as shown in
Figs. 4 and 5.
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A still further embodiment is illustrated in Fig. 10 and Fig. 11 where the HV
electrodes 24A and 24B are positioned in four rows in the wall 22, but in a
staggered
arrangement. The ground electrode 20 is again provided in the center and can
be
rotatable. This staggered arrangement of the HV electrodes helps to avoid
arcing
between the HV electrodes. Again, each HV electrode is connected to a
capacitor in
series and a source of pulsating high voltage generally as shown in Figs. 4
and 5.
Further embodiments of the invention are illustrated in Fig. 12 and Fig. 13.
In
previous embodiments, the high voltage source produced a standard type of bi-
polar
pulsating high voltage discharge. With such bi-polar discharge, a buildup of
carbon may
be formed on the plastic wall causing conductive paths between the HV
electrodes and
thereby reducing the effectiveness of some electrodes. Thus, it may be
preferable that
carbon be attracted only to the ground electrode which, being rotatable, will
make it
more difficult to form a heavy build up of carbon, but rather will allow it to
pass with the
gas toward the exit. On the other hand, with some arrangements, it may be
preferable
that carbon be attracted toward the outer plastic wall and repelled from the
ground
electrode. These conditions may be obtained by providing a unipolar pulsating
high
voltage discharge to the HV electrode which is either positive or negative.
As shown in Fig. 12, the unipolar positive discharge is provided by using a
high
voltage rectifier 38A after the capacitor 26 to which the high voltage
pulsating current is
supplied from a standard bi-polar source 28. From this rectifier, a positive
discharge is
imparted to the HV electrode 24 which produces a cold arc in the reaction zone
between
the ground electrode 20 and the plastic wall 22. This cold arc decomposes CH4
mainly
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into carbon and hydrogen as already described previously and provides a
condition
whereby carbon will be attracted only to the ground electrode 20.
On the other hand, as shown in Fig. 13, a high voltage rectifier 38B may be
used
after the capacitor 26 to impart a negative unipolar discharge to the HV
electrode 24,
leading to a cold arc discharge in the reaction zone that will produce the
attraction of
carbon toward the plastic wall 22, if this is found desirable.
In a further embodiment of the present invention, illustrated in Figs. 14 to
17, the
high voltage source is connected to a HV metal electrode and the capacitor-
connected
electrodes are connected to the ground through a dielectric element which can
be
rotatable. The dielectric element 23 is used to physically support the
electrode 24 and to
separate electrode 24 from the ground electrode 20 as well. Thus, Fig. 14
illustrates the
general principle of this alternative system, showing the pulsating high
voltage source 28
which is connected to a HV metal electrode 21 and the electrode 24, in series
with the
capacitor 26, is connected through the dielectric element 23 to the ground.
The reaction
zone is formed between the HV metal electrode 21 and the electrode 24 which is
connected through the capacitor 26 to the ground.
Fig. 15 illustrates the use of a plurality of electrodes 24 each connected to
its own
capacitor CAP1, CAP2, CAP3...CAPn and each in turn connected to the common
ground
after the capacitor. The voltage source 28 is connected to the common HV metal
electrode 21 and provides an AC high potential to the capacitor-connected
electrodes 24.
The cold arc discharge occurs homogeneously at each electrode 24 in the
reaction zone
between these electrodes 24 and the common HV electrode 21 which is a metal
tube
surrounding the dielectric rotating shaft 23. This is done while CH4 (herein
identified as
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NG) flows through the reaction zone and is converted into its constituents as
already
described previously.
A further, more detailed design of this system is illustrated in Figs. 16 and
17
which show respectively a longitudinal and a plan section of the apparatus.
Thus, the
electrodes 24 are embedded in the central shaft 23 made of dielectric
material, such as
plastic or ceramic, and are connected to capacitors 26 in series with the
electrodes 24
which in turn are connected to a common ground. The dielectric shaft 23 is
rotatable and
thus the electrodes 24 are rotating therewith when in operation. The pulsating
high
voltage source (not shown in these figures) is connected to the metal tube 21
representing a common HV electrode and thereby providing an AC high potential
to the
electrodes 24 with resulting cold arc discharge in the reaction zone between
the rotating
electrodes 24 and the surrounding HV electrode 21. The NG is input at the top
of the
reaction zone and flows therethrough while being decomposed into its gaseous
components and carbon that are withdrawn as products output.
Apart from the change in configuration set out in Figs. 14 to 17, all the
other
features and conditions remain essentially the same.
EXAMPLE OF OPERATION
A non-limitative example of the method of operation of the system of the
present
invention is now described in conjunction with Fig. 18. As shown in this
figure, natural
gas or methane (CH4) is fed through a flow measuring device MFCI and through
line 1
into the top of the reactor 40. In this example, the flow rate was 17 L/min.
Nitrogen is
fed through a flow measuring device MFC2 and line 3 to provide an inert
atmosphere
around the reactor for safety reasons. Other inert gases or a transformer oil
can also be
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used to isolate the central core of the reactor from the ambient and thereby
avoid sparks.
The reactor 40 has an internal construction such as illustrated in Figs. 4 and
5, in which
there are provided 12 HV electrodes positioned 3 per plane in four planes over
the height
of the reactor. The ground electrode 20 in the center is rotated by motor M at
a speed of
200 RPM. The ground electrode 20 made of aluminum supplies the ground for the
electric current that is passed through the HV electrodes during the
operation. Each HV
electrode is adjusted so as to have a gap of 5 mm between the tip of the HV
electrode and
the surface of the ground electrode. A 100 pico-farad capacitor was connected
in series
to each electrode and in turn connected to a central bus bar as shown in Figs.
4 and 5. A
voltage of 22 kV at a frequency of 8 kHz was then applied to the capacitors
and the
resulting power of 800 W produced a cold arc discharge in the gap between each
HV
electrode and the rotating ground electrode.
The process resulting from this operation has converted natural gas or methane
into its two main constituents, according to the following reaction:
CH4 4 C(S) + H2
Various other complementary reactions that also occur during the process give
trace amounts of other compounds. However, these unwanted compounds are very
minute with the exception of acetylene (C2H2) which is produced in a
measurable
amount. The resulting gas is then passed from the bottom part of the reactor
40 via line 2
through a HEPA (high efficiency particle arrester) 42 to collect the solid
carbon particles
that proceed via line 4 to a vessel 44. The remaining mixture of H2 and C2H2
as well as
unreacted CH4 proceed via line 5 to equipment 46 that measures the gas
composition. In
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CA 02621749 2008-02-19
the present example, the concentration of gases in the product stream was as
follows: H2
= 17.48 mol%; C2H2 = 3.07 mol% and CH4 = 79.45 mol%
Although the gas portion resulting from the above process could further be
treated to separate hydrogen from the other two gases in the mixture, it
should be noted
that the resulting gas is essentially hythane which is a gas consisting of
about 80%
methane and 20% hydrogen, and is a product in its own right since it can be
used as a
fuel for internal combustion engines. For example, U.S. Patent No. 5,516,967
discloses a
direct conversion of methane to hythane by subjecting methane to a controlled
oxidation
with water vapor at a temperature of 400 to 500 C and at a pressure from 1 to
5
atmospheres, in the presence of a particular catalyst. In the present process,
hythane is
also produced directly but using cold arc discharge instead of controlled
oxidation. In the
example given above, 0.9 kilowatts were used to produce 1 cubic meter of
hythane
which included a minor proportion of acetylene.
It should be understood that various modifications that would be obvious to
those
skilled in the art can be made to the method and apparatus of the present
invention
without departing from the following claims.
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