The effect of geometric modification on the performance of AC arc torch
K. T. KIM, D. G. KIM, J. O. LEE, D. H. LEE, M. HUR and Y. H. SONG
Environmental and Energy Systems Research Division, Korea Institute of Machinery & Materials
104 Sinseong-Ro, Yuseong-Gu, Daejeon 305-343, Korea
Abstract: Design approach for the development of plasma torch with long lifetime by less erosion of electrode
and volumetric generation of plasma has been carried. Plasma torch is based on the „rotating arc‟ design and the
effect of different geometry of ground electrodes or step, linear and diverge types were compared. The application
of the plasma torch is high temperature gasification process of waste to make synthesis gas and the process
requires long lasting electrode and volumetric plasma. Both the shape of the ground electrode end tip and length
affect generation of plasma volume. Comparison was done among selected geometries based on the size of the
plasma volume generated, thermal efficiency, changes in electrical characteristics or I-V relation and erosion of
electrode. In this study, diverge type was chosen to be best for the gasification process that was good at both
volumetric generation of plasma and long lasting electrode.
Keywords: plasma torch, gasification process, electrode lifetime, volumetric plasma
1. Introduction
Requirement for the plasma reactor for an efficient
gasification process can be listed as follows, 1)
enhancement of thermally active volume, 2)
enduring operation of electrode, 3) flexibility of
diverse working gas, 4) high efficiency and so on [1,
2]. Most of current plasma torches used in the waste
treatment are based on the DC torch technology and
can be classified as rod type cathode and hollow
type cathode as given in Figure 1. Rode type cathode
sustains discharge by thermionic emission of
electron in cathode and usually ThO2 embedded
tungsten is used to maximize the thermionic
emission. On the contrary, hollow type sustain arc
discharge by field emission and for this reason, Cu
that is good in thermal and electrical conductivity is
commonly used as electrode material.
Moreover, in view of application most important
difference between rod type and hollow type is on
working gas. In the case of rod type, working gas
containing oxygen results in generation of WO that
is formed by reaction of thermally dissociated
oxygen and tungsten. Tungsten has high melting
point of above 3,000 K but WO has rather low
melting point of below 2,000 K compared to
tungsten resulting in increased erosion of electrode.
In the case of hollow type, using Cu as an electrode
material, both Cu and CuO has melting point of
below 2,000 K and not better than tungsten.
However, arc point is not anchored or focused,
different to the case of rod type, and can be moved
both axial and radial direction, the drawbacks of
relatively low melting point can be overcome [3-5].
In this study, arc plasma reactor with rod type
electrode for waste gasification is developed as
shown in Figure 2. Discharge gas is N2 that has high
enthalpy and favorable for thermal decomposition of
wastes. AC driven discharge is adopted to avoid
imbalance of electrode erosion between cathode and
anode that is often observed in DC driven discharges.
Determining parameters of plasma volume are input
voltage, geometry of ground electrode. In this study,
3 different type of ground electrode are considered
for comparison. Step, linear and diverge are the three.
With the same applied power condition, comparison
of plasma flume and degree of electrode erosion has
been done among these types. All of the results
obtained are used for optimal design of the reactor.
Figure 1. Type of conventional DC arc plasma torches used for
hazardous waste treatment (rod and hollow types).
2. Experimental
2.1 Experimental apparatus
Experimental set-up consisted of a plasma reactor, a
power supply, flow controllers, and measurement
systems as schematically shown in Figure 2. The
reactor consists of an inner conical high voltage
electrode and an outer cylinder electrode. Initially an
arc will be ignited at the shortest distance between
electrodes (A-B), and owing to the convective flow
effect it stabilizes at the longest distance (C-D).
Elongated arc reactor can make longer arc string
than conventional DC arc torch resulting in higher
voltage condition and reduced current levels and all
of which are favorable for endurance of electrode.
AC power supply, which can provide up to 8 kV in
rms value with 5~20 kHz tunable frequency, was
used for plasma generation. The flow rates of gases
were controlled by mass flow controllers (Brooks,
5850E-series), and this mixture is supplied
tangentially into the top-section of the plasma
reactor. Electrical power delivered to the reactor is
measured by oscilloscope (Tektronix TDS5054B)
with 1000:1 high voltage probe and current probe
with amplifier that can measure up to 50 A
(Tektronix TCP 303 current probe and TCPA300
amplifier). Image of plasma flume is captured by
direct photography (Canon EOS 50D).
2.2 Test conditions
Once the available life of electrode is over, overhaul
of the process is required to change the electrode.
Thus increase of available life of the electrode is
essential factor of consideration in commercial
application as well as production of volumetric
generation of plasma flume.
Figure 2. Schematic of the experimental setup.
As shown in Figure 3, all three types or step, linear
and diverge shaped electrode are prepared and
operated for 20 hours with the same condition of
reactant flow rate of 70 L/min, input power of 10
kW. After the operation, each reactor is cut to cross
sections to compare the degree of erosion. Also, for
the comparison of reliability, voltage-current
characteristics of each reactor are recorded. Flume
image are used for the estimation of plasma volume
generation.
3. Results and discussion
3.1 Comparison of step and linear type
Determination of arc point in the ground electrode is
closely related with the axial velocity of working gas.
In the linear type, where the working gas flows
toward the exit of the reactor, where the Lorentz
force that is produced by cross product of gas
dynamic drag force induced by axial velocity and
magnetic field induced by arc current are balanced,
the arc point is determined. At this stage, arc current,
voltage, flow all has fluctuations of amplitude with
time, the arc point can be moved and this movement
prevents focusing of erosion.
On the contrary, in the step type reactor, the flow
confronts stagnant state at the point of step resulting
in high possibility of focusing of arc point. Figure
3(a) and (b) show designed shape of step and linear
types.
In the case of linear type, it is not expected to have
stagnant point. So, the arc string can have longer
length and to make the arc point located within the
reactor, length of the reactor is designed to have 2
times of that of step type (The diameter, D has the
same dimension in both the cases).
Figure 4 shows the comparison of voltage-current
characteristics during 20 hours of continuous
operation. All of the signals stabilized after initial
spikes. Initial peaks are considered to be originated
from the sensitive response of arc to non-eroded
surface characteristics and after moderate erosion of
electrode, the signals are stabilized. In the same
input power condition, linear type has high voltagelow current state compared to step type and that is
because linear type produces longer arc length.
Figure 5. Comparison of electrode erosion shape between step
and linear types.
Figure 6. Comparison of plasma jet flame (a) step type (b)
linear type.
Figure 3. Shapes of grounded electrodes (a) step, (b) linear and
(c) diverge types.
1.0
30
Voltage, Vrms [kV]
20
0.6
15
0.4
10
Linear(V)
Linear(I)
Step(V)
Step(I)
N2 flowrate: 70 L/min
Discharge power: 10 kW
0.2
0.0
0
4
8
12
16
Current, I [A]
25
0.8
5
0
20
Figure 5 shows cross cut of the electrodes after 20
hours of operation. Step type has focused erosion
point around step area but linear type has rather wide
distribution of arc point after the length of L and as a
result, no visible erosion is observed. Figure 6
compares the shape of plasma flume of the two cases.
Linear type has rather smaller volume of plasma
flume. With the results, linear type is evaluated to
be better in view of endurance, but step type has
better in generating reactive volume.
3.2 Comparison of linear and diverge
type
To overcome rather smaller plasma volume of linear
type, diverge type that has diverging part in linear
type reactor are proposed as shown in Figure 3(c).
Operating time [hr]
Figure 4. Comparison of voltage-current characteristics with
operating time between linear (circle) and step (triangle) types.
Erosion states after test of 20 hours are given in
Figure 7 and plasma flumes are given in Figure 8.
From figure 7, we can find that the diverge type still
have the merits of wide distribution of arc point and
Figure 8 shows that the effect of adding diverging
part is favorable for larger plasma volume. It is
suggested that more in detail optimization for
enlargement of plasma volume should be followed.
Figure 9 shows these optimization works to make
larger plasma volume. Among the tested conditions,
40mm of exit diameter shows best performance for
volumetric plasma generation and also reliability.
Figure 7. Electrode erosion shape of diverge type.
4. Conclusions
Optimization work for the development of plasma
reactor for waste gasification process is presented.
Step type shows best performance in generating
volumetric plasma and efficiency. However for the
focusing of arc point the type results in a severe
erosion of electrode. In the linear type, both axial
and tangential movement of arc point induces
distributed erosion and thus good endurance.
However, volumetric generation of plasma that is
essential for the gasification process is not
guaranteed. Finally, diverge type that has diverging
part at the end of linear type is proposed. Test results
of diverge type show that it has merits of both the
linear and step type. It has distributed arc point and
generates larger volume of plasma than linear type.
In succession, optimization of design in diverging
part is done. It is expected to have more efficient
gasification process with developed plasma reactor.
Acknowledgement
This work is supported by basic research program
(NK163B) of KIMM and research program of Small
& Medium Business Administration (AI1600).
References
Figure 8. Comparison of plasma jet flame (a) Linear type (b)
Diverge type.
Figure 9. Influence of diameter of diverge part on the plasma jet
flame.
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