Removal of tars using an external magnetic field assisted-plasma torch
K. T. Kim1, D. G. Kim1, M. Hur1, S. A. Roh1, S. I. Keel1, K. Y. Kang1 and M. S. Cha2
1
Environmental and Energy Systems Research Division, Korea Institute of Machinery & Materials
104 Sinseong-Ro, Yuseong-Gu, Daejeon 305-343, Korea
2
Clean Combustion Research, Mechanical Engineering Program, King Abdullah University of Science and
Technology, Jeddah, Saudi Arabia
Abstract: The influence of external magnetic field on the discharge characteristics is investigated in a plasma
torch designed for the removal of tars ejected from a gasification reactor. Toluene (C7H8) is used as a simulation
gas for tars, and byproducts produced by passing plasmas are identified and quantified with a Fourier transform
infrared (FTIR) spectroscopy. The external magnetic field allows for a more uniform plasma generation in space
because of a faster rotation of arc column. As the power applied to the solenoid increases, the arc voltage is more
fluctuated with time, and as a result the arc power is greatly raised up without increasing arc current. In addition,
both the decomposition rate and energy efficiency for C7H8 are enhanced with the external magnetic field due to
an increasing arc power and more uniform plasmas in space.
Keywords: removal of tars, energy efficiency, plasma torch, external magnetic field
1. Introduction
Syngas production from waste is very attractive in
that useless and even hazardous materials are
converted into valuable fuels. We have developed a
waste gasification system, mainly consisting of an
oxy-fuel burner and a plasma torch, which are used
as a heat source for waste destruction and a reformer
for tar removal, respectively. In gasification by an
oxy-fuel burner, tars, caused by the non-uniform
temperature distribution in the reactor, are
byproducts which should be removed for high-purity
syngas production. The tar removal is, however,
known to be very difficult because tars produced
from gasification process have generally very strong
C-H bonds. To destroy tars and thus to produce
high-purity syngas from waste, a plasma torch is
placed downstream of the gasification reactor.
Plasma torches have widely used as a heat or
chemical source for melting, reforming and waste
treatment because of their high temperatures, high
energy densities, and abundant reactive species,
which can’t be provided by any other devices.
Disadvantage of plasma torches originates from the
use of electricity, most expensive form of energy. In
our system, an oxy-fuel burner is used as a main
heating source, whereas a plasma torch is used as a
reformer for tar removal. The combination of an
oxy-fuel burner with a plasma torch allows for
lessening a burden of operation cost of plasmas, and
this method is useful especially in large-scale
devices such as gasification process. This work aims
to demonstrate the feasibility of a plasma torch for
removing tars emitted from a gasification reactor,
especially focused on the effects of external
magnetic field.
2. Experimental
Figure 1 shows a schematic of experimental setup.
The plasma torch mainly consists of a conical
cathode, cylindrical anode, and a swirl chamber. The
cathode has a protruding part for an easier gas
breakdown. The smallest gap between the cathode
and the anode is 2 mm while the internal diameter of
anode is 50 mm. The swirl chamber induces a swirl
motion of discharge gas which contributes to the
improvement of discharge stability and the reduction
of electrode erosion.
Table 1. Gas conditions used to simulate the gas compositions
ejected from the gasification reactor.
3. Results and discussion
Figure 1. A schematic of experimental system.
To obtain more uniform plasmas in space and
thereby to enhance the decomposition rate of tars, a
solenoid is installed on the periphery on the anode.
The solenoid (80 mm in length and 100 turns)
induces the magnetic field in the axial direction, and
then Lorentz force, proportional to the arc current
and the magnetic field induced, acts in the azimuthal
direction. As a result, the arc column rotates with a
very high speed, expecting to improve plasma
uniformity especially in the radial direction. Two
DC power supplies are used for the plasma torch (10
kV - 1 A) and solenoid (4 V - 60 A).
H2, CO, and CO2 are the main gases ejected from
the gasification reactor, and small quantity of H2O
and tars is mixed with them. By consulting the
preliminary experiments, the gas conditions in Table
1 are selected to simulate the gas compositions
ejected from the gasification reactor. Toluene (C7H8)
is used as a simulation gas for tars, and the Fourier
transform infrared (FTIR) spectroscopy is used for
the evaluation of decomposition rate along with the
identification and quantification of byproducts
produced.
Figure 2 shows the front discharge images taken
by varying the power applied to the solenoid.
Without the external magnetic field, plasmas have
the region with very weak light intensities at some
distance away from the center. The region with weak
light intensities is surely to be inefficient to heat the
tars. As the power applied to solenoid increases, the
faster rotation of arc column is achieved in the radial
direction. As a result, the weak intensity zone
gradually disappears with the increase in power
applied to the solenoid.
Figure 2. Influence of external magnetic field on the discharge
images. The power applied to the solenoid is varied from 0 to 95
W.
The magnetic field induced by the solenoid
greatly influences the waveform of arc voltage as
plotted in Figure 3. The external magnetic field
enables the arc voltage to fluctuate with time, which
leads to the reduction of the period of low voltages.
This results in the increase in the arc power under
the same arc current. For example, when 60 W is
applied to the solenoid, the arc power of 0.7 kW
without the external magnetic field is increased by
100 % to 1.4 kW. This tendency would be
advantageous to the plasma torch, because the
electrode erosion is proportional to the arc current.
Tars are changed into the liquid state at low
temperatures and then hold together, and finally
form sludge. This sludge is very harmful to the aftertreatment system.
respectively, plotted as a function of specific energy
density (SED) with and without the power applied to
the solenoid. The influence of external magnetic
field on the energy efficiency seems to be quite
small. However, the decomposition rate for C7H8 is
greatly enhanced with the use of external magnetic
field. The external magnetic field allows for an
increase in power and an improvement in plasma
uniformity, which enable more destructions of C7H8
without increasing the current.
6000
Voltage [V]
4000
Gas flowrate: 90 L/min
Discharge power:
-Arc only: 0.7 kW
-Magnetic: 1.4 kW
Magnetic
2000
0
Arc only
-2000
-30
-20
-10
0
10
20
30
Time [msec]
Figure 3. Influence of external magnetic field on the arc voltage.
The power applied to the solenoid is fixed at 60 W.
Figure 4. FRIR spectra measured as a function of total gas flow
rate at a fixed input power of 1.4 kW and solenoid power of 60
W.
Figure 4 shows the typical spectra indicating the
decomposition of Toluene (C7H8) which is used as a
simulation gas for tars. It is seen that C7H8 is
decomposed into lower hydrocarbons such as CH4
and C2H2, which remain the gaseous state even at
low temperatures. As the total gas flow rate
increases, the decomposition rate for C7H8 decreases.
This means that a higher arc power is required to
destruct an increasing quantity of C7H8, and this
tendency is well consistent with the previous results
done in similar conditions. The influences of
external magnetic field on the decomposition of
C7H8 are investigated based on the spectra measured
with the FTIR spectroscopy.
Figures 5 and 6 show the decomposition rate and
decomposition energy efficiency for C7H8,
C7H8 decomposition rate [%]
80
Arc only
Magnetic
60
40
Gas flowrate: 70~110 L/min
Discharge power:
-Arc only: 0.7 kW
-Magnetic: 1.4 kW
70(L/min)
90
110
20
90
0
0.2
0.4
70
0.6
0.8
1.0
1.2
1.4
SED [kJ/L]
Figure 5. Influence of external magnetic field on the
decomposition rate for C7H8.
C7H8 decomposition energy efficiency [g/kWh]
waste treatment”, Plasma Sci. Techonol. 9 709
(2007).
10
Arc only
Magnetic
[3] J. F. Brilhac, B. Pateyron, G. Delluc, J. F.
Coudert, and P. Fauchais, “Study of the dynamic and
static behavior of dc vortex plasma torches: part I:
button type cathode”, Plasma Chem. Plasma
Process., 15 231 (1995)
8
110
6
90
90
70(L/min)
70
4
Gas flowrate: 70~110 L/min
Discharge power:
-Arc only: 0.7 kW
-Magnetic: 1.4 kW
2
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
SED [kJ/L]
Figure 6. Influence of external magnetic field on the energy
efficiency for C7H8 decomposition.
4. Conclusions
We demonstrate the feasibility of a plasma torch
assisted by the external magnetic field for the
removal of tars ejected from a gasification reactor.
An increase in the power applied to the solenoid
allows for a more uniform plasma generation in
space, and leads to the growth in arc power without
increasing arc current. The increase in arc power and
plasma uniformity, realized by the external magnetic
field, is beneficial to the destruction of C7H8, used as
a simulation gas for tars. The decomposition rate for
C7H8 is enhanced with the use of external magnetic
field.
Acknowledgement
This work was supported by Basic Research
Program (SC0810 and NK163B) of the Korea
Institute of Machinery & Materials.
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