Non-thermal plasma degradation of spent solvents and oils using a
gliding arc discharge reactor
Maria Prantsidou and J. Christopher Whitehead
School of Chemistry, University of Manchester, Manchester, M13 9PL, UK
Abstract: The treatment of spent oils and organic solvents from the nuclear industry which may be considered
as low level radioactive waste (LLW) represents an unsolved challenge in the clean-up of nuclear waste. One
of the common spent-oils found in nuclear waste is odourless kerosene. In this work, non-thermal
atmospheric plasma has been generated in an AC gliding arc discharge reactor, to treat odourless kerosene in
the vapour phase, as understanding the gas chemistry would be the key for future liquid treatment. The effect
of different carrier gas composition on the odourless kerosene destruction has been investigated using FTIR
spectroscopy and OES for quantitative and qualitative analysis.
Keywords: Gilding arc reactor, non-thermal plasma, odourless kerosene degradation
1. Introduction
Odourless kerosene (OK) can be found in
radioactive, organic liquid waste coming from
nuclear industries as it is used in hydraulics and
lubrication systems or as diluent in extraction
processes in reprocessing plants. The treatment of
this normally LLW presents an unsolved challenge
as conventional methods have disadvantages such
as large volume and high cost. Non-thermal
plasma treatment of this waste is a promising
method, simpler in construction and more
economic, as it can provide the same outcome as
incineration but at ambient temperature.
Gliding-arc discharge (GAD) is non-thermal
plasma but can be described as preserving the dual
character of thermal and non-thermal plasma
which makes its interactions with matter the same
as those of quenched plasma. [1] Its characteristics
make it an attractive tool for both scientists and
industry [2] and it was initially developed for gas
treatment applications, but it has also recently
been developed for liquid treatment [3].
In this work, non-thermal atmospheric plasma
decomposition of odourless kerosene in vapour
phase has been investigated using a gliding arc
discharge under different gas compositions. The
aim of this study focuses only on the vapour phase
destruction of odourless kerosene as it is an easier
first step to understand the gas chemistry which
would lead to an understanding for the future
liquid treatment. FTIR spectroscopy has been used
for qualitative and quantitative analysis of the
end-products. Comparison has been made between
different carrier gas streams such as N2, air and Ar
and the effect of humidity has also been studied.
Optical Emission Spectroscopy (OES) was used to
identify reactive species under the different
conditions.
2. Experimental set up
Non-thermal plasma was generated in a gliding arc
discharge reactor at low temperature and
atmospheric pressure. Figure 1 illustrates the
experimental configuration.
Figure 1. Schematic diagram of experimental configuration: 1) mass
flow controller, 2) bubbler with OK, 3) bubbler with water, 4) bypass
for experiments with no water, 5) AC gliding arc reactor, 6) gas FTIR
sample inlet, 7) optical emission spectrometer
The GAD reactor mainly consists of two stainless
steel diverging electrodes, 4 mm thick with
adjustable width, located under a feeding gas
nozzle of i.d. 1.5 mm. Reactions are performed in
a double walled jacketed Pyrex glass vessel of
~ 1 L capacity with water cooling at 25 °C . An
AC neon sign power supply provided a high
voltage of about 20 kVp-p at a frequency of 50 Hz.
The carrier gases used in these experiments were
N2, synthetic air (80% N2, 20% O2) and Ar with a
total flow of 5 L min-1 passing through bubblers
which contained odourless kerosene and water (for
the humidity investigation). The initial OK
concentration was about 700 ppm. On-line FTIR
spectroscopy (Shimadzu 8300) with a long path IR
cell (2.76 m) was used for the identification and
concentration determination of the gaseous
productions. Optical emission spectroscopy
measurements occurred in the integration of the
whole plasma plume by a Princeton Instrument
320PI CCD spectrograph with a 1200 g/mm
grating and 0.1 nm resolution in the range 200800nm.
The results obtained from OK conversion were
expressed as follows:
Ci − Co
% OK Conversion =
× 100
Co
where, Ci and Co are the input and output OK
concentrations, respectively.
Cproduct
% Product Selectivity =
×100
COKconverted
(CHCN + CCO + CCO 2)
% IC Selectivity =
× 100
COKconverted
(CC 2 H 4 + CC 2 H 2)
% C2 Selectivity =
× 100
COKconverted
where IC Selectivity is the selectivity of OK
conversion to inorganic carbon and C2 is the
selectivity to the unsaturated hydrocarbons
ethylene (C2H4) and acetylene (C2H2). Existence
of ethane (C2H6) could not be determined by FTIR,
as ethane absorption frequencies overlap with
those of odourless kerosene.
3. Results and Discussion
3.1 OES as diagnostics for the GAD under
different gas compositions.
In this work OK conversion using GAD was
applied introducing different gas streams of argon,
nitrogen and air, in both dry and humid conditions.
Changing the parameters causes observable
changes in the plasma plume characteristics like
length, colour and carbon deposition. OES was
used as an extra tool to identify the reactive
species under the different conditions which
caused these changes. Table 1 summarises the
reactive species that were identified in the plasma
plume in addition to the end-products identified in
the reaction gas outlet by FTIR spectroscopy.
Table 1. OES of gliding arc plasma, where (s), (m), (w) is strong,
medium and weak peak respectively
GAD
Plasma
N2
N2/C12
N2/C12/H2O
Ar
OES
Species
Spectral Region (nm)
N2 (s)
N2+(s)
N2 (w)
C2 (s)
CN (s)
315.9,337.1,353.6,357.7
391.4
315.90,337.15,353.66,357.69
516.4
386.1,387.1,388.3
N2 (m)
N2+(w)
C2 (s)
CN(s)
OH (w)
315,337,353,357
391.4
516.4
386.1,387.1,388.3
309
OK, HCN,
CO2, CO,
C2H4, C2H2
numerous
-
Ar (s)
Ar
C2 (s)
CH
Ar (s)
C2 (w)
OH (m)
CH (m)
H (s)
O (s)
N2 (s)
N2+(s)
O (s)
numerous
516.4
431.1
numerous
516.4
309
431.1
656.3
777
315.9,337.1,353.6,357.7
391.4
777
Air/C12
N2 (s)
N2+(s)
O (s)
OH (w)
315.9,337.1,353.6,357.7
391.4
777
309.01
Air/C12/H2O
N2 (s)
N2+(s)
O (s)
OH(s)
315.9,337.1,353.6,357.7
391.4
Ar/C12
Ar/C12/H2O
Air
777
309
FTIR
products
OK, HCN
C2H4,C2H2
C2H4, C2H2
CO, CO2,
C2H4, C2H2,
NO, NO2
OK, CO,
CO2,
NO, NO2
HNO2
oxygenated
products
OK, CO,
CO2,
NO, NO2
HNO2
oxygenated
products
It can be seen that the different gas mixtures leads
to different electronically-excited reactive species
which can be related to the different end-products.
Argon gives a very strong signal of C2 radicals but
the introduction of water leads to CO/CO2 and
oxygenated organic products. Similarly the
nitrogen GAD gives C2 but also a very strong
signal of CN which weakens with the introduction
of water. Air GAD is dominated by emission
from N2, N2+ and O while C2 and CN are not
detectible compared to the use of N2 and Ar.
3.2 Gas effect on the GAD odourless kerosene
conversion and products
OK non-thermal plasma conversion was studied
using Ar, N2, and air gliding arc discharge with the
maximum achievable input power for each gas
(Pmax(Ar) = 110 W , Pmax(N2) = 200 W, Pmax(air) =
200 W). The OK decomposition gaseous products
that were identified by FTIR were different under
different gases, as observed by others [4] and are
listed in Table 1. GAD in air results mainly in CO
(~2115 cm-1) but also CO2 (~2370 cm-1) and HCN
(~712 cm-1) as inorganic carbon and the major
organic products that were identified were mostly
C2H2 (~ 730cm-1) but also C2H4 (~949 cm-1). There
were also some other organic intermediates at the
region 1750-1670 cm-1 and ~1265 cm-1 which may
indicate the presence of oxygenated organic
products generally like carbonic acids, esters or
aldehydes but these could not be specified. In air,
there was formation of NO, NO2 and HNO2, which
are common in air discharges, but no N2O and
HNO3 was observed. The N2 GAD mineralises OK
to HCN and gives a high concentration of C2H2
and a smaller concentration of C2H4. Again, there
are some small peaks around the region 1600 1550 cm-1 that could correspond to primary or
secondary amines as other organic intermediates
but they could not be specifically determined.
Argon as the carrier gas degrades OK mainly to
C2H2 and C2H4 and no inorganic products are
observed. Figure 2 summarises the results of OK
conversion under the same conditions together
with the selectivity to inorganic carbon (IC) and
organic products of C2.
Figure 2. Odourless kerosene conversion in argon, nitrogen and air
GAD compared with the selectivity to inorganic carbon and C2 of
acetylene and ethylene
Nitrogen seems to be the most effective for OK
conversion under these conditions comparing to
Ar and air with about 39% conversion which
selectively led to 54 % of ethylene and acetylene
but only 8% of inorganic carbon. The air GAD
converted about 27% of the OK and resulted in
selectivity to inorganic carbon and C2 unsaturated
hydrocarbons of 8 % and 12% respectively. GAD
with Ar resulted in a poorer OK conversion of
12% with a high selectivity of 97% to C2
unsaturated hydrocarbons, but no inorganic carbon
(IC).
Figure 3 shows the selectivity of OK conversion to
the end-products for each gas. Generally, C2H2 is
produced with the highest selectivity: 93% in case
of argon which gives no inorganic carbon.
Nitrogen GAD conversion leads to 8% selectivity
for HCN and the use of air gives 8% for CO, 3%
and 1.5% for CO2 and HCN, respectively.
Figure 3. Effect of different GAD carrier gas to the conversion
selectivity of end-products
3.3 Humid gas effect on the GAD odourless
kerosene conversion and products
Water vapour is important in plasma processing as
it can be a major source of OH and HO2 radicals
which may accelerate the oxidation reactions. In
this work, the effect of humid argon, nitrogen and
air as carrier gases is investigated and compared to
the dry conditions. The water concentration was
about 1100 ppm in each case. Figure 4 shows that
water vapour increased the OK conversion in Ar
from 12% to 36% and from 26% to 41% for an air
discharge. In N2, the conversion was decreased
from 38% in dry conditions to 32% in humid gas.
It has been observed that reactions which are
controlled by the electron density are suppressed
by water vapour which preferentially consumes
electrons [5]. Accordingly, we could suggest that
this is the reason the humid nitrogen GAD reduces
the odourless kerosene conversion.
Figure 4. Effect of water vapour to the gliding arc discharge
conversion of odourless kerosene
The injection of water vapour to the system
changed the gaseous end-products distribution as
is shown in Table 1, and also the selectivity of
conversion to C2 molecules and inorganic carbon
as illustrated in Figure 5.
Figure 5. Effect of water vapour in the selectivity of GAD odourless
kerosene conversion to the inorganic carbon and C2H4 and C2H2
products
In the argon GAD, the water increases the OK
conversion rate introducing a 4% selectivity of IC,
although the C2 selectivity is dramatically
decreased from 97% to 12%. A smaller decrease
of C2 selectivity is also observed in humid
nitrogen and air.
5. Conclusions
Changing the gas composition causes observable
changes to the gliding arc plasma characteristics
and thus the OK conversion and end-products.
In dry conditions, the maximum OK conversion
achieved was 39% with nitrogen plasma. OK
processing in argon and nitrogen plasma is
controlled by the primary steps of electron impact
dissociation and electron-induced excitation
creating metastable Ar* and N2*. In air plasma,
formation of atomic oxygen initiates the OK
destruction, but also forms NOx.
In humid conditions, OK conversion is enhanced
in the argon and air plasmas by the OH radicals
whose presence is confirmed by OES providing
effective reactive destruction of OK. However, in
the humid nitrogen plasma, OK destruction is
reduced. It is suggested that water quenches the
excited N2* that controls OK processing. The OH*
emission seen in OES is much weaker for N2 than
for Ar and air. In nitrogen, reaction products
include HCN and CN* is observed in OES.
In absence of water, C2H2 and C2H4 are formed
with high selectivity in the argon and nitrogen
plasma. Whilst in air plasma, the selectivity for C2
is much lower due to the formation of both
inorganic and organic oxygenated reaction
products. There is a correlation between regarding
the observed selectivity of the C2 unsaturated
hydrocarbon products detected by FTIR and the
strength of excited C2* observed in OES.
In humid conditions, argon and air plasma there is
a small selectivity to C2, as the presence of OH can
lead to oxygenated products and this is reflected in
weak or absent C2* signals in OES. In humid
nitrogen, the weak OH* seen in OES suggests
possible OH formation but the selectivity to C2
molecules remains high and the C2* signal in
ORES remains strong.
6. Acknowledgements
Support from Nuclear Decommission Authority
(NDA) and EPSRC is gratefully acknowledged.
The author personally thanks Dr Xin Tu for his
valuable help and advice regarding OES and
Elizabeth Maingi for experimental assistance.
7. References
1. Fridman Alexander et al., Progress in Energy
and Combustion Science, 25(2) (1998).
2. Czernichowski A., Pure Appl. Chem., 66(6)
(1994).
3. Brisset J. L. et al., Ind. Eng. Chem. Res.,
47(16) (2008).
4. Yan J. et al., Plasma Chemistry and Plasma
Processing, 27(2) (2007).
5. Bo Zh. et al., Plasma Chemistry and Plasma
Processing, 27(5) (2007).