Design and characterization of the linear atmospheric pressure DC ARC plasma
source for flue gas treatment
V. Valinčius, V. Grigaitienė, P. Valatkevičius
Plasma Processing Laboratory, Lithuanian Energy Institute, Kaunas, Lithuania
Abstract: The generalized electric and thermal characteristics of the atmospheric pressure, a
linear direct current arc plasma torch have been presented. Results on investigation of voltagecurrent characteristics were generalized employing method of dynamic similarity theory and
presented for two-chamber plasma torch. The research has been carried out under different
flow rate and distribution of air into reaction chamber.
Keywords: Plasma, plasma torch, electric arc, plasma processing.
1. Introduction
The arc discharged plasma is an effective tool for many
types of application including surfaces treatment,
sputtering, etching, deposition of coatings and a wide
range of other processes. The thermal treatment of
hazardous substances including the processing of sludge
produced in the municipal water purification stations is
also not solved problem. In the process of incineration
new chemicals are created inside the furnace and
products of incomplete combustion may occur. Therefore
the increase the temperature and the activation of the
processing reaction in the thermal treatment system by
the employment of atmospheric pressure arc plasma is
welcomed. Electric arc plasma is also a promising
medium in the indirect environmental application: high
quality catalytic coatings can be formed employing
plasma spray technology.
Flue gas treatment may be combined by two thermal
methods such as combustion and plasma. However the
development of new environmentally friendly plasma
technology requires new and effective plasma sources.
Consequently the aim of present work is to design and
manufacture suitable direct current (DC) plasma torch
(PT) and analyze its operating and thermal
characteristics. The aim of this paper is also to get a
better understanding of phenomena in discharge channel,
the distribution of plasma jet characteristics after the
exhaust nozzle of PT.
The analysis of similarity of physical processes
undergoing in PT enabled to create general methods of
projection novel plasma sources [1, 2]. This has been
related with the consideration that despite the complexity
of the process, the number of working regimes
determinant parameters of PT is small and near to the
number of primary criterions. So, non-dimensional
equations have been indispensable for qualitative
characterization of physical processes in PT.
Consequently, the voltage-current characteristics (VCC)
have been established in the DC PT designed for
different purpose and operating in atmospheric pressure
air or nitrogen. This work is also devoted to study,
summarize and describe the VCC of PT.
The similarity theory is successfully applied for formation
of the criteria equations used in further investigations [3].
2. Methodology
Several configurations of linear dc plasma torches with
hot cathode and step-formed anode were considered. One
of them was PT 70 kW of power, with radial and
tangential injection designed especially for the production
of non-equilibrium plasma jet already described
elsewhere in [4]. The schematic presentation of the novel
developed PT is shown in Fig. 1. It consists of a cathode
junction with hafnium emitter, cathode-coupled section
for arcing 3, diffuser 5, insulation rings and step-formed
anode 7, 9 made of high purity copper. In the case when
magnetic stabilization of flow is necessary, the coil 8 is
useful.
While designing a new PG intended for heating the
material that is delivered into the reaction arc zone,
selecting their operating and cooling regimes, it is
necessary to know both average and local heat losses of
their elements that are operating under extreme
conditions. By the selection of design of PT the
preference has been given to the PT with neutrals, fixed
average arc length, and step-formed exit electrode. This
enabled to create the prosperity for arc shunting after a
step and ensure the stability of medium length of electric
arc in the wide range of gas flow and current variation.
The mentioned plasma source also differs from the
ordinary plasma sources with the slightly conical
expanded solid step formed anode. The step in the anode
also serves for reduction of the static pressure drop inside
the chamber channel and to fix the arc in the stable
position. The total length of PT is 0.25 m, the diameter of
a tight part of anode is 0.03 m, and the diameter of anode
after the step is 0.04 m. The diffuser part of the torch is
isolated from the anode and makes a neutral section
separated by the insulating rings of glass textolite. Each
ring contains a pair of blowholes for tangential air supply
(GN, G1 and G3) for the arc stabilization. The
experimental equipment for producing arc plasma consists
of the rectifier for power supply, gas supply, watercooling systems and airing devices. The linear, sectional,
two-chamber, high voltage (300-400 V) and limited
current (150-200 A) PT has been designed and
manufactured by the authors of the presented work.
The presented plasma torch was connected to the
plasma chemical reactor for flue gas treatment. Firstly
flue gas goes to an electrostatic precipitator, which
removes the major particulates. The gas then goes to the
plasma chemical reactor in which hazardous substances
are decomposed into simple compounds such as CO2,
H2O and synthetic gas.
The modified similarity theory has been used for the
analysis of operating and thermal characteristics and
results generalization [1, 2, 4]. Electric characteristics of
the linear, sectional step anode fixed PG are generalized
as follows [1, 4]:
Ud
I
I2
A
Gd
m
G
d
n
( pd ) k (l / d ) r .
(1)
PG performance and thermal characteristics can be
evaluated by its efficiency
indicating what part of
generated energy is transferred to gas:
GH (UI ) .
(2)
Generalization of the thermal characteristics of PG is
similar to generalization of the electric characteristics:
1
B
I2
Gd
m2
G
d
n2
pd
k2
l
d
r2
.
(3)
Here U is arc voltage, I is arc current, G is the total gas
flow rate, d2 is anode diameter, p is pressure. The value of
may be presented also as the Stanton number [1]:
1
4l
St ,
d
(4)
Fig. 1. The schematic presentation of linear 50 kW DC PT. 1– cathode junction with hafnium emitter; 2, 4, 6 –
insulating rings with gas injection; 3 – intermediate anode; 5 – neutrode; 7, 9 – step-formed anode; 8 – magnetic coil
3. Results and discussion
This research concludes that VCC of PT depend on the
following prevailing factors: i) arc chamber geometry; ii)
gas composition; iii) flow and injection to produce the
desired arc; iiii) radial and tangential injection of plasma
forming gas. The last factor was performed after an
experimental investigation at the constant and various
values of gas flow rate. In the present study when the
radial injection is not applied, VCC was observed as
decreasing in the range of current of 150 – 200 A (Fig. 2).
That appears as a result of decreasing electric field
intensity, linearly depending on the current. It was
determined that intensity of electric field and voltage drop
linear decreases with increasing of G in the range of (7–
10) 10-3 kg s-1 and G3,5=(5-8) 10-3 kg s-1. When the mixed
injection (radial and tangential) in different locations is
employed, the electric arc is strongly turbulized and there
appears a possibility to heat up much larger amount of gas
in the PT of reduced dimensions. Thus the voltage drop in
such plasma generator increases up to 70% and there
appears the possibility for better control of plasma
forming process. The characteristics of such plasma
source can be described employing the system of
differential equations of continuity, impulse conservation,
energy, state and Maxwell’s considering to Ohm’s law [4,
5, 6].
During the process when under tangential injection of
plasma forming flue gas is supplied inside the PG anode
the VCC becomes as slightly dropping or remains as
stabile. The influence of arc current, gas flow rate and
anode diameter on the VCC and thermal efficiency for
similar PT are described in [7]. Static PT characteristics
curves may be also slightly rising with increase of current
strength.
The criterion relationship was established for
generalized PT VCC and thermal characteristics (Fig. 3).
An analysis of the experimental data shows that the
density of heat flux to the PT elements is different and
makes (4.5–7)·106 W/m2 to the cathode, (1.8–4)·106 W/m2
to the arc ignition section, (2.5–5)·106 W/m2 to the
intermediate section, and (4–6.5)·106 W/m2 to the anode.
Under application and increment of the flow rate of
radial injection G3 from 0 to (12 10-3) kg s-1, the electric
field intensity increases very much, but the further its
increase is expedient only in the case of currents less than
150 A.
Besides high current levels the arc shunting takes place,
and the voltage become as decreasing. The shunting
comes into the play also during the air injection into the
middle of PG (G1), when G1 5 10-3 kg s-1. The maximum
effect has been reached after the injection of gases into
the entrance region of the anode (G3 5 10-3 kg s-1). The
effect of the radial injection location and intensity is
apparently visible in Fig. 3. Increasing G3 10-12% of the
total flow rate, the intensity of electric field increases as
well. Further increasing of G3 values has no effect to the
electric field.
The results obtained are presented on logarithmic axes.
As the pressure in the reaction chamber is near to
atmospheric, the value of the member pd is close to unity.
Effects of place of gas injection and of amount injected
upon CVC are evaluated through the parameter G1/G [5].
The CVCs of the PG investigated are described by the
following relationship:
Ud 2
I
1360
I2
Gd 2
0, 54
G
d2
0 ,14
. (5)
1
2
3
350
Ud 2
I
c
I2
Gd 2
m
0,5
d2
I
G1
G
0,11
.
(6)
The member G1/G evaluates the effect of gas injection
location and gas flow rate. At the significant values of arc
current when relatively small amount of gas is heating the
2
member I
in the case when
2.5 10 8 and
Gd 2
additional gas is not supplied into the anode, constants c
and m in (6) are 1.36·103 and 0.54, respectively.
Considering the above case, the total heat losses to the PT
walls are generalized and presented in Fig.5 which can be
described by the relation
1
5.5 10
3
0, 22
I2
Gd 2
0,12
I
d2
.
(7)
It was determined that properly distributed gas injection
increases current strength and potential difference, allows
control and regulation of the plasma torch parameters and
ensures operating stability. It helps to control the
parameters of plasma jet introduced into the chamber of
the combustion system and regulate properties of the
combustion product. So, the gas injection location and
flow rate has significant influence on the arc shape and
dimensions. Under the adjustment of the location and the
flow rate of gas injection it is possible to significantly
increase plasma torch power also change VCC character.
This allows increase the plasma flow temperature and the
efficiency.
-1
10
-2
9x10
8x10
-2
7x10
-2
6x10
-2
5x10
-2
4x10
-2
Ud2/I
Voltage,V
The flow rate and location of tangential injection has a
significant influence on the arc voltage. Under the
tangential injection voltage intensively increases when G1
increases till 8% of the total gas flow rate G and then the
U slowly increases till U=1.3U1. The tangential injection
into the anode entrance part is effective only in the case of
G1 ≤5%. Further increasing of G1 leads to the arc
shunting.
After the analysis of the experimental results, VCC
were generalized on the basis of evaluation of the gas
flow vortex in the reaction zone of arc discharged
chamber. The dependence in Fig. 4 VCC strongly
depends on several factors, which were established and
described by the equation (5). After generalization the
conclusive equation has been deduced to the expression
Ud
2 /I=1
,36
)10 3
(I 2
/Gd
2 ) -0,54
300
Current, A
250
170
I2/Gd2
10
180
190
200
210
8
220
Fig. 2. Plasma torch VCC with tangential air injection in
dependence on total gas flow rate (10-3 kg s-1)
respectively: 1 – 7.0, 2 – 8.0, 3 – 10.0
Fig. 3. Generalized VCC with radial air injection at gas
flow rate of (7.0 – 10.0) 10-3 kg s-1
.
0,75
(Ud2/I)10
3
Efficiency
30
0,74
20
2
(I /Gd2)10
200
300
-6
400
Fig. 4. Generalized plasma torch VCC
Current, A
0,73
180
190
200
210
220
Fig. 5. Generalized PG efficiency
For the present measurements over 100
experiments were carried out varying with the help of
resistors arc current strength and the air flow rate G1 and
G3 blown in. Some geometrical PG characteristics and
ranges of experiments carried out are summarized in
Table 1.
Table 1. Plasma source technical parameters
Power, P (kW)
33-78
Arc current, U (A)
175-245
Arc voltage, I
160-335
Cooling water flow rate, Gw (kg s-1)
0.16-0.18
Water temperature increment (deg):
plasma torch
15-23
cathode
1.1-1.53
ignition section
1.08-2.16
neutrode
anode
13.0-19.3
Source gas flow rate (kg s-1):
cathode, GN
0.54-1.0
neutrode, G1
anode, G3
1.85-7.6
Plasma jet average mass temperature (K) 3460-5200
Plasma jet velocity, w (m s-1)
350-1000
Efficiency, η
0.58-0.78
Reaction chamber diameter (10-3 m):
d1
4.0
d2
8.0
d3
12.0
4. Conclusions
The generalized electric and thermal characteristics of the
PT permit determination of plasma flow parameters
required for plasma treatment of combustion products and
selection of optimal operating modes.
Generalization of characteristics based on the principles
of dynamic similarity theory has been found to be
reasonably successful. The designed and manufactured
PT is suitable for the system of thermal treatment of flue
gas, hazardous substances and waste.
The intensity of convective heat transfer in the plasma
generator is directly proportional to the strength of
electric field.
Heat transfer is the most intense in the anode part of the
plasma generator due to the arc spot and convection. In
order to improve the efficiency of the plasma generator, it
is necessary to reduce heat transfer by radiation between
the electric arc and the walls of PG anode.
5. References
[1] M. F. Zhukov, I. M. Zasypkin, Thermal Plasma
Torches: Design, Characteristics, Applications, (2007).
[2], O. P. Solonenko, Thermal Plasma Torches and
Technologies (2000).
[3] P. Valatkevičius, V. Krušinskaitė, V. Valinčiūtė , V.
Valinčius, Surf. Coat. Technol. 173–174 (2003).
[4] V. Valinčius et al, Plasma Sources Sci. Technol., 13, 2
(2004).
[5] M. F. Zhukov, Principles of calculation of linearcircuit plasma generators, Novosibirsk,1979 (in Russian).
[6] O.Yas’ko, Pure appl.Chem., 62 (1990).
[7] V. Snapkauskienė V. Valinčius, P. Valatkevičius,
Heat transfer research., 40, 5 (2009).
Accnowledgement. This research was funded by a grant
(No.ATE-02/2012) from Research Council of Lithuania.