Long Life DC Arc Plasmatron with Nanocarbon Coating of Electrodes
V.E. Messerle*, A.B. Ustimenko**, V.G. Lukyashchenko*, V.Zh. Ushanov**, E.I. Karpenko***
* Research Enterprise “NTO Plasmotekhnika” Ltd., Almaty, Kazakhstan,
** Research Institute of Experimental and Theoretical Physics, Almaty, Kazakhstan,
*** Applied Centre of Plasma-Power Technologies of Russian J.S.Co. “UPS of Russia”, Gusinoozersk, Russia
**Corresponding author e-mail: [email protected]
Abstract: To improve fuel utilisation efficiency and to decrease pollutions
plasma torches are widely used in heat-and-power engineering, metallurgy and
chemical industry. Plasma torches ensure the competitiveness of the mentioned
industries. Nanocarbon coating can prolong the plasma torches life and provide
their competitiveness. At the same time, the plasma technology of nanocarbon
coating of the electrodes is of independent value for nanomaterials production.
Keywords: plasmatron, life time, electrod, nanocarbon coating
1. Introduction
Simplicity and reliability of the arc plasma
torches using cylindrical copper cathode and air
as plasma forming gas predestine their
application at heat and power engineering for
plasma aided coal combustion [1]. Life time of
these plasma torches electrodes is critical and
usually limited to 500 hours. Considered in this
paper the long life direct current (DC) arc
plasma torch has the electrodes life significantly
exceeded 1000 hours. To ensure the electrodes
long life the process of hydrocarbon gas
dissociation in the electric arc discharge is used.
In accordance to this method atoms and ions of
carbon from near-electrode plasma deposit on
the active surface of the electrodes and form
electrode carbon condensate which operates as
“actual” electrode [2]. To realize aforesaid the
construction of electro arc generator of air
plasma has been developed and tested. Fig.1
gives the photo of the plasma torch.
Butane-propane gas is supplied to the zone of
the arc conjunction to the copper water-cooled
electrodes (cathode and anode). Linked with the
arc in series, the magnetic coils guaranty
stabilization of the discharge on the electrodes.
Arisen from ionization positive carbon ions
deposit onto the electrodes surface under the
influence of near-cathode decline in potential
and form coating of the electrode condensate
[3]. This coating is a composite nano-carbon
material (graphite, single wall and multiwall
nanotubes, flakes) [4] and it is “actual” cathode,
deterioration of which is compensated by the
flow of carbon ions and atoms. The coating
thickness depends mainly on ratio of the flows
butane-propane gas and air and the arc current.
Note that a similar mechanism of carbon
deposition of positive ions on the cathode
surface is observed for the graphite cathode.
The results of complex physicochemical
investigation of phase, structure and element
compound of the material of carbon
nanostructural coating which is generated from
butane-propane gas on the cathode of the
plasma torch are the following.
2. Experimental results
Figure 1. Long life DC arc plasmatron.
Some measurements of the long life plasma
torch regimes are gathered in Table 1. In these
experiments the plasma forming gas (air)
consumption was 300 l/min. It is seen that when
power of the plasma torch was in interval 76 –
132 kW and butane-propane gas flow in range
of 0.4 – 1.5 l/min thermal efficiency of the
plasma torch (η) reached 90 %. At that mass
averaged temperature at the exit of the plasma
torch (T) increased from 3500 to 5000 K.
During the plasma torch operation a film of the
cathode condensate is formed in accordance
with the processes of butane-propane gas
molecules dissociation and carbon atoms
ionization (Fig. 2).
Table 1. The results of the experiments.
I,
U,
P, Qgas,
N
T, К
η
A
V kW l/min
1
200 380 76
0.4
0.9 3500
2
300 360 108
0.4 0.89 4100
3
400 330 132
0.4 0.88 5000
4
300 360 108
0.7 0.89 4200
5
300 380 114
0.7
0.9 4500
1.5
6
250 384 96
0.9 4000
Figure 2. Photo of graphite cathode inset before (on the
left side) and after (on the right side) deposition a
protective nanocarbon layer.
As it was mentioned arisen from ionization
positive carbon ions deposit onto the grahpite
cathode inset surface under the influence of
near-cathode decline in potential and form the
bulk film of the electrode condensate (Fig. 2 on
the right). This layer is continuously renewed
when the plasma torch is in operation and
butane-propane gas is feeding into a zone of
electric arc discharge. Study by electron
microscope Quanta 3D (USA) with energydispersive attachment showed (Fig. 3) that this
layer consists of carbon (97.76 %) and oxygen
(2.24 %). This cathode layer of condensate is an
electrically
conductive,
polycrystalline,
graphitic material. Specific electrical resistance
of the electrode condensate is less than
10-8 Ω·m.
Fig. 4 presents Raman spectrum of the cathode
condensate film. The spectrum was obtained
using a laser with wavelength of 473 nm (25
mW) on the confocal Raman spectrometer
NTEGRA SPECTRA (NT-MDT, Russia). In
the Raman spectrum along with the main bands
of nanocarbon graphene structures (D = 1368
cm-1, G = 1590 cm-1, 2D = 2741 cm-1) there are
bands of stretching vibrations of C = O at wave
number 1866 cm-1 and the valence band
vibrations of the O – H at wave number
3716 cm-1. The vibrational frequencies of C = O
and O – H are characteristic of the cyclic
anhydride groups, occurring at the boundary
defects of the graphene structure during the
oxidation. Thus, due to the Raman spectrum
becomes clear, where does and in what form
detected by electron microscopy oxygen
(2.24%) is present in the cathode deposit. Weak
bands at 2470, 2990 and 3273 cm-1 are
overtones or combined tones of the major
carbon bands D and G.
Now it is well known that physical-chemical
properties
of
graphene
nanostructures
(graphene, carbon nanotubes, flakes etc.) are
unique [4]. Their conductivity is better than
conductivity of all conventional conductors and
nanotubes can withstand current density 102-103
times more than metals. Also they have high
heat conductivity, they are very mechanically
firm, 100 times firmer than steel and they gain
the properties of semiconductors at their curling
or flexion. For long-life work of the electrodes
it is of especial importance that graphene
nanocarbon have high emission of electrons,
they are chemically inert at high electric field
intensity (107 - 108 V/m) and residual gas ions
bombardment. All these properties indicate that
graphene flakes, nanotubes and the composites
on their base are ideal materials to protect the
plasma torch electrodes and as a consequence to
prolong their life. Thus the cathode condensate
is produced in accordance with butane-propane
gas oxidative pyrolysis in conditions of high-
accuracy arc discharge with magnet focusing
without use of rare gas (argon or helium).
The electrode condensate was examined using,
scanning electron microscope (SEM) (Fig. 5)
and transmission electron microscope (TEM)
(Figs. 6 and 7). The basic mass of the carbon
sample (about 80 %) is represented as film and
band graphite particles collected to aggregates
of various size and density. Basal cleavage
spacing of these particles is a little more than
one of “ideal” graphite. It is d002 = 3.45-3.55 Ǻ,
in contrast to d002 =3.35 Ǻ of “ideal” graphite.
Width of the bands varies from 40 to 160 nm.
Sometimes film and band nanoparticles are
collected into stratified packages. Fig. 7
demonstrates that the last-named are gathered to
well-ordered nanostructures. Also Fig. 7 traced
the role of copper nanoparticles as a promoter
of the formation of carbon nanostructures [3].
The figure clearly observed nanocarbon
"flower" with a "stem" of the bundle of carbon
nanotubes, which is visible near the diamondshaped multi-layer copper nanoparticle with
rising from the surface of carbon nanotubes,
combined into a bundle as a kind of "8", length
of which reaches a micron or more.
Figure 3. General view of the sample of cathode carbon
deposit, and its energy-dispersive spectrum.
Figure 4. Raman-spectrum of the carbon condensate
from the plasma torch cathode obtained by confocal
Raman spectrometer NTEGRA SPECTRA (NT-MDT,
Russia): I - the intensity of Raman scattering (arbitrary
units); 1/cm - wave number.
Figure 5. SEM images of a sample of the electrode
condensate.
which consists mainly of graphene flakes,
glassy carbon, multi-walled carbon nanotubes
and other carbonic forms.
It is established that the carbon nanostructures
of the products of the plasma pyrolysis of
hydrocarbon gases may be formed on the
surface of copper electrodes, and on the surface
of graphite electrodes.
Life length of the electrode totals more than
1000 hours. The experiments confirmed
principal possibility for unlimited long-life of
the cathode filmed with carbon nanostructural
material.
Created long-life plasma torch was industrially
tested for plasma ignition of a pulverised coal
flame in Gusinoozersk thremal power plant,
Russia. The tests confirmed efficiency of the
plasma torch in conditions of a power plant.
References
Figure 6. TEM images of a sample of the electrode
condensate.
Figure 7. TEM image of a sample of the copper cathode
condensate.
3. Conclusion
On the base of the microscopy and the Ramanspectroscopy investigation, it can be concluded
that the electrode condensate is composite
carbonic stuff made of carbon nanoclusters
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