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Decomposition Mechanism of Fluorinated Compounds in Water Plasmas Generated Under Atmospheric Pressure

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Decomposition Mechanism of Fluorinated Compounds in Water Plasmas Generated Under Atmospheric Pressure

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Decomposition mechanism of HFC-134a, HFC-32, and CF4 in water plasmas at atmospheric pressure has been investigated. The decomposition efficiency of 99.9% can be obtained up to 3.17molkWh−1 of the ratio of hydrofluorocarbon (HFC) feed rate to the arc power and 1.89molkWh−1 of the ratio of perfluorocarbon (PFC) feed rate to the arc power. The species such as H2, CO, CO2, CH4, and CF4 were detected from the effluent gas of both PFC and HFC decomposition. However, CH2F2 and CHF3 were observed only in the case of HFC decomposition. The HFC and PFC decomposition generate CH2F, CHFx (x:1–2), and CFy (y:1–3) radicals, then those radicals were subsequently oxidized by oxygen, leading to CO and CO2 generation in the excess oxygen condition. However, when there is insufficient oxygen available, those radicals were easily recombined with fluorine to form by-product such as CH2F2, CHF3, and CF4. KeywordsHydrofluorocarbons-Perfluorocarbons-Water plasmas-Thermal plasmas
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1 23
Plasma Chemistry and
Plasma Processing
ISSN 0272-4324
Volume 30
Number 6
Plasma Chem Plasma Process
(2010) 30:813-829
DOI 10.1007/s11090-010-9259-
y
Decomposition Mechanism of Fluorinated
Compounds in Water Plasmas Generated
Under Atmospheric Pressure
1 23
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ORIGINAL PAPER
Decomposition Mechanism of Fluorinated Compounds
in Water Plasmas Generated Under Atmospheric
Pressure
Narengerile
Hironori Saito
Takayuki Watanabe
Received: 15 May 2010 / Accepted: 5 October 2010 / Published online: 24 October 2010
Ó Springer Science+Business Media, LLC 2010
Abstract Decomposition mechanism of HFC-134a, HFC-32, and CF
4
in water plasmas
at atmospheric pressure has been investigated. The decomposition efficiency of 99.9% can
be obtained up to 3.17 mol kWh
-1
of the ratio of hydrofluorocarbon (HFC) feed rate to the
arc power and 1.89 mol kWh
-1
of the ratio of perfluorocarbon (PFC) feed rate to the arc
power. The species such as H
2
, CO, CO
2
,CH
4
, and CF
4
were detected from the effluent gas
of both PFC and HFC decomposition. However, CH
2
F
2
and CHF
3
were observed only in
the case of HFC decomposition. The HFC and PFC decomposition generate CH
2
F, CHF
x
(x:1–2)
, and CF
y (y:1–3)
radicals, then those radicals were subsequently oxidized by oxygen,
leading to CO and CO
2
generation in the excess oxygen condition. However, when there is
insufficient oxygen available, those radicals were easily recombined with fluorine to form
by-product such as CH
2
F
2
, CHF
3
, and CF
4
.
Keywords Hydrofluorocarbons Perfluorocarbons Water plasmas Thermal plasmas
Introduction
Industrial development is accompanied with increase of environmental deterioration such
as global warming which is a big problem for human, animals, and plants in recent years.
Perfluorocarbon (PFC) used in the semiconductor industry and hydrofluorocarbon (HFC)
used in refrigeration, collectively called fluorinated compounds, have been identified as
greenhouse gas due to their large global warming potential (GWP) and long atmospheric
lifetimes. In order to save the environment, the Kyoto protocol was adopted at the third
session of the conference of the parties in Kyoto, Japan in 1997. The Kyoto treaty commits
industrialized nations to reducing emissions of greenhouse gases including HFC and PFC
by years 2008–2012 to 5% below 1990 levels.
Narengerile H. Saito T. Watanabe (&)
Department of Environmental Chemistry and Engineering, Tokyo Institute of Technology, Yokohama
226-8502, Japan
e-mail: watanabe@chemenv.titech.ac.jp
123
Plasma Chem Plasma Process (2010) 30:813–829
DOI 10.1007/s11090-010-9259-y
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Conventional process for reducing emissions of fluorinated compounds, e.g. alternative
chemistry process, recovery and recycle, and process optimization, have proved to be
inadequate. To deal with the problem, processes for the destruction of PFC and HFC were
performed by means of abatement methods including combustion, catalytic destruction,
and plasma abatement. Combustion technologies that have historically been used for the
destruction of fluorinated compounds may fail to meet stringent environmental regulations
because of the generation of other hazardous substances in the effluent gas [1, 2]. The
catalytic process for controlling PFC emission involves passing the PFC-laden stream over
the catalyst [3, 4]. For the catalyst to be employed in a commercial PFC abatement process,
it must be both high reactive and durable, i.e. able to maintain a high decomposition
efficiency for an extended period of time. Plasma treatment has been widely studied and
applied to waste destruction, and has the advantages of high efficiency, flexibility, and
environmental compatibility.
Atmospheric nonthermal plasma process was attempted for the decomposition of highly
concentrated fluorinated compounds at low power levels. However, the nonthermal plasma
techniques showed some drawbacks such as low decomposition efficiency and generation of
harmful compounds if applied without catalyst. Jasinski et al. [5, 6] proposed the use of low-
power (\100 W) microwave discharges, operating in air plasma at atmospheric pressure, for
decomposition of highly concentrated fluorinated compounds. Their results showed that the
decomposition efficiencies were reached almost 100% for the fluorinated compounds such
as dichlorodifluoromethane (CFC-12), chlorodifluoromethane (HCFC-22), 1,1,1,2-tetraflu-
oroethane (HFC-134a), and trichlorofluoromethane (CFC-11) at a feed rate of 1.0 L min
-1
.
However, the electrical energy consumption was as low as 0.5 mol kWh
-1
. Furthermore,
the toxic compounds e.g. phosgene (COCl
2
) and carbonyl fluoride (COF
2
) were detected in
the effluent gas. Futamura and Annadurai [7] studied on the HFC decomposition by using
silent discharge plasma and surface discharge plasma in combination with MnO
2
and TiO
2
SiO
2
catalysts. Although these catalysts showed high activity for the HFC oxidation, their
irreversible deactivation was very severe due to fluorination of the catalysts.
Thermal plasmas can offer distinct advantages for waste treatment process; high
enthalpy to enhance the reaction rate, oxidation or reduction atmospheres in accordance
with required chemical reactions, and rapid quenching (10
5
–10
6
Ks
-1
) to produce
chemical non-equilibrium compositions. The PLASCON process developed by CSIRO in
Australia is one good example of thermal plasma waste treatment [810]. The arc was
generated by direct current (DC) discharge with the arc power from several kW up to
hundreds of kW. The plasma supporting gas was argon. This process can destroy a wide
range of fluorinated compounds including HFC, PFC, and chlorofluorocarbon (CFC), for
which decomposition efficiency was at levels above 99.99%. Ozawa et al. [11] studied on
decomposition of HFC-134a using water plasma. In order to obtain high decomposition
efficiency, they injected HFC-134a with a mixture of air and steam. When the steam and
air feed rate were set at 12 and 90 L min
-1
, respectively, the highest decomposition
efficiency of 99.99% was obtained at 30 A. Kim et al. [12] used a non-transferred DC arc
torch to decompose polychlorinated biphenyl (PCB) mixture waste using steam as the
plasma supporting gas. For the decomposition of PCB mixture waste containing 15%
trichlorobiphenyl (C
12
H
7
C
l3
), 12% tetrachlorobiphenyl (C
12
H
6
C
l4
), and 73% CCl
4
, the
decomposition efficiency of 99.999% had been obtained at feed rate of 19 g s
-1
(0.18 L min
-1
) at an arc power of 100 kW. Glocker et al. [13] decomposed the CFC by
using DC arc torch with steam as main plasma gas and argon as protecting gas in a power
range of 8-32 kW. However, the steam plasma process requires complicated system
including the heating-up of the steam feeding line for preventing from condensation.
814 Plasma Chem Plasma Process (2010) 30:813–829
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Hrabovsky et al. [14, 15] developed hybrid plasma torch with combined stabilization of
electric arc by water vortex and gas flow. The cathode section was stabilized by a vortex
gas flow. Plasma flows through the cathode nozzle into the torch where the arc column was
surrounded by a water vortex. The water vortex was formed in three cylindrical segments
with a tangential water injection as it is in water-stabilized torches. The torch was operated
at an arc power from 80 to 300 kW with an exit centerline plasma velocity from 2.0 to
6.5 km s
-1
. The system was mainly studied for pyrolysis and gasification of waste for
production of syngas because it can produce oxygen-hydrogen plasma with extremely high
enthalpies.
We developed a DC water plasma torch which with a cathode of hafnium embedded
into a copper rod and a nozzle-type copper anode in our previous work [16]. The hafnium
used as cathode material can overcome the erosion problems and achieve a long operating
time in oxidation atmospheric. The torch can generate stable 100%-water plasmas using
DC discharge at the arc power of 1.0 kW without any steam generator or gas supply
system. Using the water plasma produced by the plasma torch, we succeed in decomposing
liquid waste of alcohol solutions [17] and gaseous waste such as HFC and PFC [18, 19]. In
References [18, 19], the decomposition efficiency of 99.9% can be obtained up to
1.54 mol kWh
-1
of the ratio of HFC-134a feed rate to the arc power where the maximum
feed rate was 0.59 L min
-1
at an arc power of 0.91 kW.
The objective of this study is to decompose fluorinated compounds by water plasmas
generated under atmospheric pressure and to investigate the decomposition mechanism of
fluorinated compounds in the water plasmas. The experiments have been performed with
different types of fluorinated compounds: HFC-134a and difluoromethane (HFC-32), and
tetrafluoromethane (CF
4
). In the first part, the optimum decomposition conditions were
determined by calculating the thermodynamic equilibrium in order to forecast and design
the decomposition process. In the second part, HFC-134a, HFC-32, and CF
4
were
decomposed by water plasmas generated by DC discharge at atmospheric pressure. The
decomposition efficiency obtained for each compound were compared and discussed. The
influence of operational conditions such as feed rate, additional oxygen, and arc power
were examined to help explain the decomposition mechanism. Finally, the decomposition
mechanism of fluorinated compounds in the water plasmas was discussed from the point of
view of chemical kinetics.
Thermodynamic Consideration
Thermodynamic equilibrium was calculated from the minimization of Gibbs free energy of
the system with assumption that chemical equilibrium is attained, which requires that
chemical reaction rates are sufficiently larger than rates of convection and diffusion. The
chemical composition obtained from the calculation is probably not accurate. However, the
thermodynamic equilibrium is useful to assess the important species in fluorinated com-
pounds decomposition. In this study, the thermodynamic equilibrium for HFC-134a with or
without water was calculated by FACT (Centre for Research in Computational Thermo-
chemistry, Canada). FACT is a computer program, linked to a database, for finding the
chemical equilibrium of a gas mixture by minimizing the Gibbs free energy. The calcu-
lations were performed with atmospheric pressure condition.
The equilibrium composition of the pyrolyzed HFC-134a as a function of temperature
between 500 and 6,000 K is shown in Fig. 1. As can be seen, decomposed species such as
F, H, HF, C(g), CF, and C
2
were observed in the decomposition system at a temperature of
Plasma Chem Plasma Process (2010) 30:813–829 815
123
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6,000 K. CF, CF
2
, and CF
3
were generated in the largest quantities at 3,760, 2,900, and
2,690 K, respectively. The by-product of CF
4
and solid carbon appeared at room tem-
perature. It indicates that the HFC-134a was decomposed in high-temperature range, but
by-products such as CF
4
will be generated if the quenching step is not sufficiently fast.
Hence, optimization of the process is a prerequisite for HFC decomposition. The equi-
librium compositions of 1.0 mol HFC-134a with 2.0 mol H
2
O were calculated and shown
in Fig. 2. In this calculation, the ratio of HFC-134a to water was determined from the
stoichiometric coefficient of the following overall reaction (1).
CH
2
FCF
3
þ 2H
2
O ! 4HF þ H
2
þ 2CO ð1Þ
As shown in Fig. 2, CF, CF
2
,CF
3
, and further CF
4
can be eliminated by adding water. It
indicates that the unwanted by-products can be suppressed by adding water in the process
of HFC decomposition because CF, CF
2
, and CF
3
radicals are easily combined with F atom
or each other to generate PFC such as CF
4
or tetrafluoroethylene (TFE) in general.
Meanwhile, H
2
and CO were increased while C (s) was decreased.
The influence of H
2
OorO
2
supply on by-product formation during the HFC-134a
decomposition was studied by calculation of thermodynamic equilibrium. The total
amount of CF
y (y:1–4)
as a function of temperature is shown in Fig. 3. A large amount of
CF
y
was observed in the process of the HFC-134a thermal decomposition. With
increasing additional amount of O
2
, the total amount of CF
y
was greatly decreased over
2,400 K because CF
y
was oxidized by O
2
to generate CO in higher temperature range
([2,400 K). Comparatively, with an increasing amount of H
2
O, the total amount of CF
y
was decreased in lower temperature range (\3,000 K) because CF
y
was removed by H to
produce CH
4
and HF. It indicates that oxygen and hydrogen play an important role in
controlling the reformation of undesirable by-product when the decomposition of fluo-
rinated compounds.
10
-3
10
-2
10
-1
10
0
10
1
1000 2000 3000 4000 5000 6000
Mole [mol]
Temperature [K]
H
H
2
CF
2
C(s)
CF
4
HF
F
CF
C
3
C
2
C
2
H
CF
3
C(g)
Fig. 1 Equilibrium composition
of pyrolyzed 1.0 mol HFC-134a
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Experimental
The decomposition system, which is the same as described in [19], is presented in Fig. 4.
The system consists of a DC power supply, a water plasma torch, a water absorption
material (Stainless steel fibers SUS 316L) for leading water into the arc region, a reaction
tube for decomposition of PFC and HFC, and a neutralization vessel for absorption of
fluorine. The torch is a DC non-transferred arc plasma generator of coaxial design with a
10
-3
10
-2
10
-1
10
0
10
1
1000 2000 3000 4000 5000 6000
Mole [mol]
Temperature [K]
H
O
H
2
O
H
2
CO
C
CO
2
C(s)
CH
4
HF
F
Fig. 2 Equilibrium composition
of 2.0 mol H
2
O and 1.0 mol
HFC-134a
10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
2
2000 4000 6000 8000 10000
Total Amount of CF
y
[mol]
Temperature [K]
HFC-134a
HFC-134a + 1mol O
2
HFC-134a + 1mol H
2
O
HFC-134a + 2mol O
2
HFC-134a + 2mol H
2
O
Fig. 3 Total amount of CF
y (y:
1–4)
as a function of temperature
in different HFC-134a
decomposition system
Plasma Chem Plasma Process (2010) 30:813–829 817
123
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cathode of hafnium embedded into a copper rod and a nozzle type copper anode. The
diameter of the hafnium was 1.0 mm. After arc ignition, the large heat from arc was
conducted by the copper anode to the water absorption material which is excellent in
electrical and optical performance and set between the anode case and evaporator. When
water was introduced into the torch with a controlled feed rate, plasma is created by
heating and ionization of steam that is produced by evaporation of water from the water
absorption material. Then the PFC and HFC were injected through the injection hole
towards the center of the water plasmas at the position of 6 mm from the nozzle exit. The
plasma and the injected gas are expected to undergo complex reactions in the reaction tube.
The hot gaseous mixture produced was quickly quenched in 1.0 M NaOH solution. The
quenching step is important to suppress by-product formation in the waste treatment
process.
The effluent gas was analyzed by a gas chromatograph (GC-TCD, Shimadzu GC-8A)
equipped with a thermal conductivity detector, and a quadruple mass spectrometer (QMS,
Ametek Dycor Proline). The recovery of fluorine was measured by neutralizing the
solution in the vessel. The decomposition efficiency and the recovery of fluorine were
calculated according to the following equations, respectively:
Decomposition efficiency ð%Þ
¼ 1
Concentration of the feeding fluorinated
compound in the effluent gas
,
Concentration of the feeding
fluorinated compound
!" #
100%: ð2Þ
Recovery of fluorine ð%Þ¼
Absorbed fluorine in
NaOH solution ðmol)
,
Total feeding
fluorine ðmol)
"#
100%: ð3Þ
The flow rate of plasma is depended on steam characteristics such as vapor pressure and
specific heat because the torch used in this study was a steam-stabilized torch. Hence, the
water feed rate for production of the plasma gas was controlled within the range of
40–65 mg s
-1
. The system was operated at atmospheric pressure with the arc power from
0.8 to 1.2 kW and the voltage from 110 to 150 V.
HFC Decomposition
The decomposition of HFC-32 and HFC-134a were carried out using the proposed
decomposition system. The effect of the HFC-32 feed rate on the recovery of fluorine and
the decomposition efficiency is shown in Fig. 5. The HFC-32 feed rate was changed from
0.33 to 1.0 L min
-1
. The experiments were operated at atmospheric pressure with the arc
current of 7 A. As can be seen, the feed rate had small effect on the decomposition
Fig. 4 Schematic diagram of
PFC and HFC decomposition
system
818 Plasma Chem Plasma Process (2010) 30:813–829
123
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efficiency, but strongly affected the recovery of fluorine itself. The different between the
decomposition efficiency and the recovery of fluorine indicates the by-products formed in
the effluent gas. The composition of the effluent gas as a function of the feed rate is shown
in Fig. 6. The main components of the effluent gas were H
2
(49.3%), CO (27.7%), and CO
2
(10.1%) at feed rate of 1.0 L min
-1
. The insufficient oxygen leads to production of large
amount of CO and small amount of CO
2
if larger feed rates were applied.
The QMS spectra of the effluent gas produced by decomposing HFC-32 with different
feed rate are shown in Fig. 7. As shown in Fig. 7a, the intensities peak of 25(C
2
H
?
),
26(C
2
H
2
?
), 31(CF
?
), 32(CHF
?
), 33(CH
2
F
?
), 51(CHF
2
?
), and 52(CH
2
F
2
?
) were observed
0
20
40
60
80
100
0
20
40
60
80
100
0.0 0.2 0.4 0.6 0.8 1.0 1.2
Recovery of Fluorine
Decomposition Efficiency
Arc Current: 7 A
Recovery of Fluorine [%]
Decomposition Efficiency [%]
CH
2
F
2
Feed Rate [L min
-1
]
Fig. 5 Effect of feed rate on
fluorine recovery and HFC-32
decomposition efficiency
10
-4
10
-3
10
-2
10
-1
10
0
0.0 0.2 0.4 0.6 0.8 1.0 1.2
Mole Fraction [-]
CH
2
F
2
Feed Rate [L min
-1
]
H
2
CH
2
F
2
CH
4
CO
CO
2
Arc Current: 7 A
Fig. 6 Mole fraction of
compositions of effluent gas as a
function of HFC-32 feed rate
Plasma Chem Plasma Process (2010) 30:813–829 819
123
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at feed rate of 1.0 L min
-1
. Comparing the spectra to the data in NIST library [20], the
intensities peak of 51(CHF
2
?
) and 52(CH
2
F
2
?
) correspond to CHF
3
and undecomposed
CH
2
F
2
, respectively. However, those complex intensities peak disappeared during
decomposition of HFC-32 at a low feed rate of 0.33 L min
-1
, indicating a complete
decomposition, shown in Fig. 7b.
The effect of the HFC-134a feed rate on the decomposition efficiency and the recovery
of the fluorine is shown in Fig. 8. The arc current was set at 7 A. As can be seen, when the
feed rate had been increased from that of 0.215 to 1.0 L min
-1
, the HFC-134a decom-
position efficiency remained at 97%, while the recovery from the fluorine greatly
10
-13
10
-12
10
-11
10
-10
Intensity [a.u.]
m/z [-]
C
+
H
2
O
+
F
+
CO
2
+
CO
+
H
2
+
CH
2
F
2
: 0.333 L min
-1
Arc Current: 7 A
CH
3
+
(b)
01020304050607080
01020304050607080
10
-13
10
-12
10
-11
10
-10
Intensity [a.u.]
m/z [-]
C
+
H
2
O
+
F
+
CH
CH
2
F
+
CHF
2
+
CH
2
F
2
+
H
2
+
CHF
CF
+
CO
2
+
CO
+
O
+
or CH
4
+
CH
2
F
2
: 1.0 L min
-1
Arc current: 7 A
C
2
H
2
+
C
2
H
+
(a)
Fig. 7 Mass spectra of gaseous product after decomposition of HFC-32 with different feed rate
(a) 0.33 L min
-1
,(b) 1.0 L min
-1
0
20
40
60
80
100
0
20
40
60
80
100
0.0 0.2 0.4 0.6 0.8 1.0 1.2
Recovery of Fluorine
Decomposition Efficiency
Decomposition Efficiency [%]
Recovery of Fluorine [%]
CH
2
FCF
3
Feed Rate [L min
-1
]
Arc Current: 7 A
Fig. 8 Effect of feed rate on
fluorine recovery and HFC-134a
decomposition efficiency
820 Plasma Chem Plasma Process (2010) 30:813–829
123
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decreases. The composition of the effluent gas as a function of the feed rate is shown in
Fig. 9. As the HFC-134a feed rate increases, H
2
and CO
2
decrease while CO, CH
2
F
2
,CH
4
,
and undecomposed HFC-134a increase, because the plasma temperature as well as the
residence time in high temperature range was decreased by increasing feed rate. In addi-
tion, HFC feed rate increases carbon-containing species such as CF, CF
2
,CF
3
, CHF, and
CH
2
F in the water plasmas.
The composition of the effluent gas during decomposition of HFC-134a at arc current of
5 or 7 A was analyzed by QMS and shown in Fig. 10. In the both case, the HFC-134a feed
rate was controlled at 1.0 L min
-1
. As shown in Fig. 10a, the intensities peak of 2(H
2
2?
),
12(C
?
), 13(CH
?
), 14(CH
2
?
), 15(CH
3
?
), 16(O
?
), 18(H
2
O
?
), 19(F
?
), 26(C
2
H
2
?
),
28(CO
?
), 31(CF
?
), 32(CHF
?
), 33(CH
2
F
?
), 44(CO
2
?
), 50(CF
2
?
), 51(CHF
2
?
), 62(C
2
F
2
?
),
63(C
2
HF
2
?
), 69(CF
3
?
), 83(C
2
HF
4
?
) were observed at the arc current of 5 A. Comparing
the spectra to the data in NIST library, the intensities peak of 15(CH
3
?
), 26(C
2
H
2
?
),
33(CH
2
F
?
), 51(CHF
2
?
), and 69(CF
3
?
) correspond to CH
4
,C
2
H
2
,CH
2
F
2
, CHF
3
, and CF
4
,
respectively. The intensities peak of 63(C
2
HF
2
?
) and 83(C
2
HF
3
?
) were considered to be
derive from the undecomposed HFC-134a. When the arc current was increased from 5 to
7 A, the intensities peak of 31(CF
?
), 50(CF
2
?
), 63(C
2
HF
2
?
), 69(CF
3
?
), and 83(C
2
HF
3
?
)
disappeared, meanwhile the intensity peak of 33(CH
2
F
?
) was also decreased as shown in
Fig. 10b. Herein, the weak intensity peak of 33(CH
2
F
?
) correspond to CH
2
F
2
. The
unwanted by-products had been generated in the effluent gas even with a high arc current
of 7 A. In comparison to that of HFC-32, decomposition efficiency of the HFC-134a was
slightly larger than that of the HFC-32 decomposition because the HFC-134a included the
weak C–C single bond. The by-products of CH
2
F
2
and CHF
3
had been detected from the
effluent gas for both of the HFC-32 and HFC-134a decomposition, but CF
4
was only
detected in the case of the HFC-134a. Mole fractions of H
2
and CH
4
in the effluent gas
produced by HFC-32 decomposition were larger than that of HFC-134a.
10
-4
10
-3
10
-2
10
-1
10
0
0.0 0.2 0.4 0.6 0.8 1.0 1.2
Mole Faction [-]
CO
2
CO
H
2
CH
2
F
2
CH
4
Arc Current: 7 A
CH
2
FCF
3
CH
2
FCF
3
Feed Rate [L min
-1
]
Fig. 9 Mole fraction of
compositions of effluent gas as a
function of HFC-134a feed rate
Plasma Chem Plasma Process (2010) 30:813–829 821
123
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The effect of additional O
2
on the HFC-134a decomposition efficiency and the recovery
of fluorine is shown in Fig. 11. The HFC-134a with oxidation gas of O
2
was injected
through injection hole into the water plasmas. The HFC-134a feed rate remained at
0.515 L min
-1
while the O
2
feed rate was increased. The arc current was set at 7 A. As the
additional O
2
feed rate increases, the decomposition efficiency remained nearly constant
while the recovery of fluorine increases. The composition of the effluent gas as a function
of O
2
feed rate is presented in Fig. 12. The increasing additional O
2
feed rate lead to
increasing CO
2
and decreasing CO because CH
2
F, CHF, and CF
y
were completely oxi-
dized by the excess oxygen to generate CO
2
. When additional O
2
feed rate was
0.5 L min
-1
, the compositions of the effluent gas were mainly CO
2
(47.8%), H
2
(32.4%),
10
-13
10
-12
10
-11
10
-10
m/z [-]
CO
+
CO
2
+
C
+
Arc Current: 5 A
CH
2
FCF
3
: 1.0 L min
-1
H
2
O
+
F
+
O
+
CH
3
+
CH
2
F
+
C
2
HF
4
+
CF
3
+
C
2
HF
2
+
C
2
F
2
+
CHF
2
+
CF
2
+
C
2
H
2
+
Intensity [a.u.]
H
2
+
CH
2
+
CF
+
CH
+
CHF
+
(a)
0 20 40 60 80 100 120
0 20 40 60 80 100 120
10
-13
10
-12
10
-11
10
-10
Intensity [a.u.]
m/z [-]
CO
+
CO
2
+
H
2
+
Arc Current: 7 A
CH
2
FCF
3
: 1.0 L min
-1
H
2
O
+
F
+
O
+
CH
3
+
CH
2
F
+
C
2
H
2
+
C
+
(b)
Fig. 10 Mass spectra of gaseous product after decomposition of 1.0 L min
-1
HFC-134a (a) at 5 A; (b) at
7A
0
20
40
60
80
100
0
20
40
60
80
100
0.0 0.1 0.2 0.3 0.4 0.5 0.6
Recovery of Fluorine
Decomposition Efficiency
Decomposition Efficiency [%]
Recovery of Fluorine [%]
Feed Rate of O
2
[L min
-1
]
Arc Current: 7 A
CH
2
FCF
3
: 0.515 L min
-1
Fig. 11 Effect of O
2
addition on
fluorine recovery and HFC-134a
decomposition efficiency
822 Plasma Chem Plasma Process (2010) 30:813–829
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and CO (19.7%). The undecomposed HFC-134a and the by-product of CH
2
F
2
were sup-
pressed to level of 1%. Furthermore, CH
3
F and CF
4
were not detected from the effluent
gas.
PFC Decomposition
Decomposition of CF
4
by the water plasmas was tested because CF
4
is one of the most stable
gas among the PFC and its decomposition is extremely difficult. The effect of feed rate on the
decomposition efficiency and the recovery of fluorine are shown in Fig. 13. At the arc
10
-4
10
-3
10
-2
10
-1
10
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6
Mole Faction [-]
Feed Rate of O
2
[L min
-1
]
CO
2
CO
H
2
O
2
CH
2
F
2
CH
4
Arc Current: 7 A
CH
2
FCF
3
: 0.515 L min
-1
CH
2
FCF
3
Fig. 12 Mole fraction of
compositions of effluent gas as a
function of additional O
2
feed
rate in HFC-134a decomposition
system
0
20
40
60
80
100
0
20
40
60
80
100
0.0 0.2 0.4 0.6 0.8 1.0 1.2
Decomposition Efficiency
Recovery of Fluorine
Decomposition Efficiency [%]
Recovery of Fluorine [%]
CF
4
Feed Rate [L min
-1
]
Arc Current: 7 A
Fig. 13 Effect of feed rate on
fluorine recovery and CF
4
decomposition rate
Plasma Chem Plasma Process (2010) 30:813–829 823
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current of 7 A, both the CF
4
decomposition efficiency and the recovery of fluorine were
lower than 70% even at small feeding rate of 0.215 L min
-1
. As the feed rate increases, the
CF
4
decomposition efficiency decreases because high CF
4
feed rate means that less energy
will be available for decomposing each CF
4
molecular. The fact that there was no difference
found between the CF
4
decomposition efficiency and the recovery of fluorine indicates that
there is negligible formation of by-products containing fluorine. The composition of the
effluent gas as a function of the feed rate is shown in Fig. 14. The main compositions were
CO
2
(68.9%), CF
4
(24.9%), H
2
(4.5%), CO (1.1%), and O
2
(0.6%) at feed rate of
0.215 L min
-1
. No by-product was detected by GC-TCD except undecomposed CF
4
.
Compared with the HFC-32 decomposition, the mole concentration of CO-generating was
much smaller than in the case of CF
4
decomposition at the same feed rate supplied, because
the CF
4
decomposition efficiency as well as the carbon generation was very smaller. CO
2
decreases significantly with increasing of CF
4
feed rate. This decrease was due to increase of
the C-containing species owing to the large feed rate in the high temperature region.
The concentration of CO generated by decomposing CF
4
was much lower than that of
the HFC decomposition under the same decomposition conditions. The concentrations of
CO
2
and CO in the effluent gas provide important information of the decomposition
mechanism, because these concentrations are related to the reaction temperature and the
oxidation condition. CO is stable at higher temperatures than CO
2
. The higher CO con-
centration obtained for HFC indicting that the HFC decomposition at higher temperature
and the CO
2
reduction by the H present in HFC.
The decomposition rate as a function of the ratio of feed rate to the arc power is shown
in Fig. 15. In the case of without O
2
, the arc power of 0.98 kWh was remained constant
while the CF
4
feed rate increases. The decomposition efficiency was as low as 67%. Hence,
the experiment of CF
4
decomposition with O
2
was performed in order to improve the
decomposition efficiency. The CF
4
and the additional O
2
feed rate was remained at
0.215 L min
-1
while the arc power was increased. A CF
4
decomposition efficiency of 99%
can be obtained at an electrical energy consumption of 1.89 mol kWh
-1
.
10
-3
10
-2
10
-1
10
0
0.0 0.2 0.4 0.6 0.8 1.0 1.2
Mole Faction [-]
CF
4
Feed Rate [L min
-1
]
CO
2
CO
H
2
Arc Current: 7 A
CF
4
O
2
Fig. 14 Mole fraction of
compositions of effluent gas as a
function of CF
4
feed rate
824 Plasma Chem Plasma Process (2010) 30:813–829
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This result was compared with the work of Sun and Park [21]. They used DC non-
transferred arc torch to decompose CF
4
by argon plasma with additional gas such as H
2
and
O
2
at atmospheric pressure. The almost 100% decomposition efficiency was obtained at the
feed rate of 1.0 L min
-1
CF
4
with 15 L min
-1
Ar, 2 L min
-1
O
2
, and 4 L min
-1
H
2
. Their
results thus obtained that the CF
4
electrical energy consumption was 0.30 mol kWh
-1
.
The PLASCON system has been widely used in industrial waste decomposition process.
For instance, decomposition efficiency of 99.99% was obtained for CCl
2
F
2
at feed rate of
40 L min
-1
at an arc power of 15 kW [10]. In this case, the electrical energy consumption
was 6.5 mol kWh
-1
when the injection mixture gas ratio of CCl
2
F
2
to H
2
O was 1:1. By
comparison, the decomposition efficiency for the PLASCON system is much higher than
that of for the water plasma system because the electrical arc power of PLASCON system
was 15 times larger than that of the water plasma torch. The large arc power provides a
high plasma temperature source which leading to thorough dissociation of the decompo-
sition gas and the additional gas. In addition, the PLASCON system as specialize in
commerce, its design was enhanced energy conservation. Waste enters the torch at a
specially designed injection manifold and instantly mixes with the plasma. Also, in order to
prevent the formation of any undesired organic molecules, the hot gases exiting the flight
tube undergo rapid quenching by direct sprays of cool alkaline liquor. Compared with the
PLASCON system, the water plasma torch is extremely simple and small although its
decomposition efficiency is not ideal. For the application of water plasma torch in industry,
additional efforts should be needed to deal with improving decomposition efficiency by
means of modifying of the apparatus’s design or increasing arc power.
Discussion
Thermal plasma decomposition of fluorinated compounds strongly depends on their
chemical structures at a given plasma temperature. To summarize the experimental results,
0
20
40
60
80
100
0.0 0.5 1.0 1.5
CF
4
Without O
2
CF
4
With O
2
CF
4
Decomposition Efficiency [%]
Feed Rate / Arc Power [mol kWh
-1
]
Fig. 15 CF
4
decomposition
efficiency plotted as a function of
the ratio of CF
4
feed rate to arc
power
Plasma Chem Plasma Process (2010) 30:813–829 825
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higher decomposition efficiency obtained for HFC-134a were attributed to the low bond
energy of C–C bond (348 kJ mol
-1
) in contrast to those of C–H and C–F bonds. The
decomposition mechanism of HFC-134a, HFC-32, and CF
4
were discussed on the basis of
the experimental results and the chemical kinetics.
The distribution of plasma temperatures of the water plasma jet were described in our
previous work [18]. The average temperature of the water plasmas in the region just
downstream of the nozzle exit was measured by emission spectroscopy for free water
plasma jet. The average temperatures were determined from the Boltzmann plot from
hydrogen atoms. As the result, the average temperature was 4,000 K at a position of 6 mm
from the nozzle exit at an arc current of 7 A. Hence, the average decomposition tem-
perature for HFC and PFC were assumed 4,000 K for kinetics consideration.
The decomposition rates were calculated for HFC-134a, HFC-32, and CF
4
and listed in
Table 1. For calculating the reaction rate, we assumed that only the reactions R1, R2, R3,
and R4 run to completion within 6.0 mm of the nozzle exit, respectively. The concen-
trations of the HFC-134a, HFC-32, and CF
4
had been calculated by estimation of that: (1)
HFC-134a, HFC-32, and CF
4
are the ideal gaseous state (2) the feeding water dissociated
100%. The rate constant parameters were taken from the literatures [2224]. Owing to the
lack of reliable data at high temperature, most of these constants were used for tempera-
tures over the recommended range. Temperature dependent rate parameters were described
by the extended Arrhenius equation as the following:
k ¼ AT
n
expðE=RTÞð4Þ
The calculation results obtained that the reactions R1, R2, and R3 were important steps
for primarily decomposition of HFC-134a, HFC-32, and CF
4
, respectively, because of their
relatively high decomposition rate. Herein, it should be noted that the dissociation of HFC-
134a, HFC-32, or CF
4
molecular by electron attachment was considered negligible because
the decomposition gas was injected into the downstream region of the water plasmas.
As can be seen from Table 1, the decomposition rates of HFC-134a and HFC-32 were
1.57 9 10
5
and 2.56 9 10
3
mol cm
-3
s
-1
, respectively. Hence, the high decompostion
rates lead to complete decomposition of HFC-134a and HFC-32 in high temperature
region. In contrast, the CF
4
decomposition rate was just 5.76 9 10
-2
mol cm
-3
s
-1
. The
CF
4
decomposition needs long decomposition time because of the exceedingly slow
decomposition rate, result in low decomposition efficiency and generation of large amount
of CF
y (y:1–3)
radicals in low temperature range.
The by-product formation mechanism were studied from the comparative study of the
reaction rate constant due to unknown the decomposed species concentration which cannot
Table 1 Decomposition rates for first steps of HFC-134a, HFC-32, and CF
4
decomposition
No. Reactions Forward reaction Decomposition rate Re
AnE
R1 CH
2
FCF
3
? CH
2
F ? CF
3
7.9 9 10
16
0 386 1.57 9 10
5
[22]
R2 CH
2
F
2
? H ? CH
2
F ? HF 1.6 9 10
-11
2 148 2.56 9 10
3
[23]
R3 CF
4
? H ? CF
3
? HF 4.1 9 10
-11
1.6 173 5.76 9 10
-2
[23]
R4 CF
4
? CF
3
? F 3.39 9 10
-1
-4.64 122 6.06 9 10
-17
[24]
Rate coefficient of the forward reaction is k = AT
n
exp(-E/RT), where A is in s
-1
and cm
3
molecule
-1
s
-1
for 1 and 2 reactant species, respectively, the activation energy E is in kJ mol
-1
, R is the ideal gas constant,
and T is the temperature. Unit for decomposition rate is mol cm
-3
s
-1
826 Plasma Chem Plasma Process (2010) 30:813–829
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be measured experimentally. The reactions which have high possibilities in the thermal
plasmas are described in Table 2 involving 20 species and 17 reaction steps [10, 2437].
The rate reaction constants were calculated at a temperature of 2,000 K which is estimated
from thermodynamic calculation.
The experimental results of the CF
4
decomposition were showed that CHF
3
,CH
2
F
2
, and
CH
3
F had not been generated during the CF
4
decomposition, indicating CF
3
,CF
2
, and CF
radicals were not combined with H radical. For CF
4
,CF
3
,CF
2
, and CF, the reaction rate
constants of their reduction and oxidation (R2–R7) were extremely larger than that of their
recombination with H, O or each other. In addition, the produced hot gaseous were quickly
quenched during the experiment. Hence, all the reactions of their recombination with H, O
or each other were not listed Table 2. From the perspective of activation energy, reaction
(5) is initiated through an electrophonic addition step of H atom to trifluoromethyl radical.
Since the addition step is greatly exothermic, chemically activated energized trifluorom-
ethane (CHF
3
*) is formed. Subsequently, CHF
3
* is decomposed through HF elimination
(6). HF elimination (7) is superior to the F dissociation (8) for the activation energy for
reaction (7) is much lower than that of reaction (5).
CF
3
þ H ! CHF
3
ð5Þ
CHF
3
! CF
2
þ HF ð6Þ
CF
3
þ H ! CF
2
þ HF ð7Þ
CF
3
þ H ! CHF
2
þ F ð8Þ
For CF
2
removal, the reaction of (9) is less important because of its relatively small
reaction rate constant in comparison with reactions of (10) and (11).
Table 2 Kinetics parameters for fluorinated compounds decomposition by water plasmas
No. Reactions Forward reaction Re.
AnE(kJ)
R1 CH
2
F ? F ? CHF ? HF 8.3 9 10
-11
00[25]
R2 CF
3
? O ? COF
2
? F 2.6 9 10
-11
00[26]
R3 CF ? O ? CO ? F 3.9 9 10
-11
00[27]
R4 CF
2
? O ? CO ? 2F 3.9 9 10
-11
00[10]
R5 CF
3
? H ? CF
2
? HF 8.9 9 10
-11
00[26]
R6 CF
2
? H ? CF ? HF 6.6 9 10
-11
00[28]
R7 CF ? H ? CH ? F 6.1 9 10
-11
079[29]
R8 CF
2
? F ? M ? CF
3
? M 1.8 9 10
-24
-910[24]
R9 CF
3
? F ? M ? CF
4
? M 8.9 9 10
-28
-512[24]
R10 CF
2
? H
2
? CH
2
F
2
4.7 9 10
-12
0 142 [30]
R11 CF
3
? H
2
? CHF
3
? H 2.6 9 10
-11
071[31]
R12 CF
2
? O
2
? COF
2
? O 6.6 9 10
-13
072[32]
R13 COF
2
? H ? COF ? HF 7.5 9 10
-11
096[33]
R14 COF
2
? O ? CO
2
? F
2
2.2 9 10
-11
00[34]
R15 COF ? O ? CO
2
? F 1.0 9 10
-10
00[35]
R16 COF ? H ? CO ? HF 5.7 9 10
-11
0.5 0 [36]
R17 COF ? F ? CO ? F
2
2.9 9 10
-10
0.5 137 [37]
Plasma Chem Plasma Process (2010) 30:813–829 827
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CF
2
þ O
2
! COF
2
þ O k
1
¼ 6:6 10
13
molecule
1
cm
3
s
1
ð9Þ
CF
2
þ H
2
! CF þ HF k
2
¼ 1:6 10
12
molecule
1
cm
3
s
1
ð10Þ
CF
2
þ O
2
! CO þ2F k
3
¼ 3:8 10
11
molecule
1
cm
3
s
1
ð11Þ
Therefore, the chemically stable hydroflourocarbons and carbonyl fluoride products
were not detected from the CF
4
decomposition process. The low CF
4
decomposition
efficiency was considered due to the recombination of CF
3
with F (12).
CF
3
þ F ! CF
4
k
4
¼ 2:0 10
12
molecule
1
cm
3
s
1
ð12Þ
A relatively-low concentration of CH
2
F
2
, CHF
3
, and CF
4
was generated during
decomposition of HFC-134a, as shown in Fig. 10. It is proposed that they were formed by
the recombination of CH
2
F and CHF radicals with fluorine (13)–(15).
CH
2
F þ F ! CH
2
F
2
ð13Þ
CHF þ F ! CHF
2
ð14Þ
CHF
2
þ F ! CHF
3
ð15Þ
Comparing the results of QMS measurements show in Fig. 10a, b, it can be seen that
CHF
3
and CF
4
were generated only at low arc current. The amount of CH
2
F
2
generated
was much larger than that of CF
4
for the decomposition of HFC-134a by the water
plasmas. This means that the CH
2
F radical was much more stable and much more dom-
inant than CF
3
radical in the water plasmas. It can be explained by electron affinity. Carbon
atom is in the electron delocalization state in CF
3
radical compared with the carbon atom
of in CH
2
F radical, when three F atom which is the most electronegative atom pulls on the
bonding electrons. As a result, CH
x
F easily combines with F to generate by-product.
However, H will combine with F atom to improve the decomposition of CH
x
F and CF
y
.
Conclusions
A feasibility of decomposition process for fluorinated compounds such as HFC-134a,
HFC-32, and CF
4
were demonstrated by water plasmas generated by DC discharge. The
decomposition efficiency and the reduction unwanted by-product were improved by adding
oxygen as oxidation gas. The compositions in the effluent gas were mainly H
2
,CO
2
, CO,
and CH
4
in which the mole fraction of syngas of H
2
and CO was between that of 25–70 for
HFC decomposition and between that of 10–20 for PFC decomposition. By-products of
CH
2
F
2
, CHF
3
, and CF
4
were detected in trace amounts in HFC decomposition process;
however there is no by-product formed in PFC decomposition. The by-product formation
strongly depends on the availability of F atom to combine with the other decomposed
species. The decomposition mechanism as well as the by-product formation mechanism
was also studied.
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... (3) Injection after quenching in the direction of the inner reactor wall [116,122]. (4) Transmission of the cold plasma gas through an upstream water tank [108,123]. Oxidative processes require the presence of oxygen molecules or atoms. An alternative to steam would therefore be oxygen as plasma gas, but pure oxygen leads to corrosive effects on electrodes and other components of the torch and reactor. ...
... (3) Injection after quenching in the direction of the inner reactor wall [116,122]. (4) Transmission of the cold plasma gas through an upstream water tank [108,123]. (5) Passing the hot plasma gas through a downstream water tank [108,123]. ...
... (4) Transmission of the cold plasma gas through an upstream water tank [108,123]. (5) Passing the hot plasma gas through a downstream water tank [108,123]. ...
Article
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Plasma technology is already used in various applications such as surface treatment, surface coating, reforming of carbon dioxide and methane, removal of volatile organic compounds, odor abatement and disinfection, but treatment processes described in this context do not go beyond laboratory and pilot plant scale. Exemplary applications of both non-thermal plasma and thermal plasma should underline the feasibility of scale-up to industrial application. A non-thermal plasma in modular form was built, which is designed for up to 1000 m³∙h⁻¹ and was successfully practically tested in combination of non-thermal plasma (NTP), mineral adsorber and bio-scrubber for abatement of volatile organic components (VOCs), odorous substances and germs. Thermal plasmas are usually arc-heated plasmas, which are operated with different plasma gases such as nitrogen, oxygen, argon or air. In recent years steam plasmas were gradually established, adding liquid water as plasma gas. In the present system the plasma was directly operated with steam generated externally. Further progress of development of this system was described and critically evaluated towards performance data of an already commercially used water film-based system. Degradation rates of CF4 contaminated air of up to 100% where achieved in industrial scale.
... It was found that products of molecular fragmentation may easily recombine forming CF 4 [21,22]. Reacting agent preventing this recombination that also suppress soot or byproduct formation is water (steam) [22]. ...
... In spite of this low effectives, whilst the aim of this study was not directed to optimize abatement process, the abatement performance as expressed in terms of DRE is still comparable or better than reported in the literature (see e.g. [21,22]) and could be further increased (see below). ...
... [20] has reported abatement of CCl 2 F 2 mixed with oxygen and showed formation of by-product CF 4 . Narengerile and co-authors [21] showed several values of DRE for FPP from ca 0.6 to 1.4 mol/kWh corresponding to the the values of relative residual concentrations from ca 0.48 to 0.3 and feed rate 0.215 slm at 0.98 kW (Fig. 15 in [21]). They used small DC-plasma torch for generating steam plasma whereas substantial better performance (DRE of 99%) they achieved with additional oxygen for the same flow rate of CF 4 . ...
Article
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Perfluorinated compounds (PFCs) increasingly utilized in electronic manufacturing represent a potent source of global warming effect. Because of extremely high stability of PFCs only very high temperature is effective for their destruction. Thermal plasma offers higher destruction and removal efficiency as compared to conventional methods allowing to reach sufficiently high temperature as well as suitable conditions, including high enthalpy and reactive environment for destruction even of the most persistent PFCs. The aim pursued by this work is to apply water and gas stabilized DC-plasma torch for generating steam plasma for efficient abatement of the most persistent PFC, i.e., CF4, and to observe a dependence of destruction and removal efficiency on operational conditions, including concentration of CF4, input arc power of the plasma torch and an influence of an additional gas. The experiments were carried out at 20 kW and 40 kW of torch power in the concentration range 1–20% of CF4 in mixture with both nitrogen and argon and total feed rate 50 L/min in plasma chemical reactor. The mixture with argon exhibit considerably higher destruction efficiency than that with nitrogen. The highest destruction efficiency was attained in the mixture CF4/argon at 40 kW of torch power. Among other gases (CO2, O2, H2) added to CF4 the only hydrogen exhibited a positive effect to destruction performance. It was found an optimal feed rate of additional hydrogen corresponding to the maximum of destruction efficiency.
... The catalytic decomposition of CF 4 is primarily a hydrolysis reaction. Even when only plasma is used for CF 4 decomposition, H 2 O or H 2 is recommended for the fixation of HF from F atoms that are dissociated from CF 4 [41]. However, additives such as H 2 O require additional energy and utility costs in a scrubber. ...
Article
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CF4 is commonly used in semiconductor industries, and its removal requires a large amount of energy because it is a highly stable perfluorinated compound. In this study, a rotating arc-catalytic reactor that uses thermal plasma as the heat source for catalysts is introduced as an efficient method for CF4 removal. An AC rotating arc plasma was used with the catalyst for CF4 removal, and the plasma and catalyst were configured serially in this reactor. Destruction and removal efficiency (DRE) of CF4 and the energy efficiency of the removal process were experimentally investigated in the input power range of 2–3 kW and a steam-to-CF4 molar ratio of up to 20. The obtained results suggest that the DRE of CF4 was up to 98% and the energy efficiency was as high as 36.1 g/kWh, which is approximately 3.5 times higher than that obtained using a rotating arc reactor alone. In addition, the thermal efficiency was estimated to be approximately 67%, based on the temperatures of the inlet and outlet gases of the rotating arc-catalytic reactor. Thus, further improvement in the performance can be expected through subsequent thermal management and system optimization. These results confirm that using rotating arc-catalytic reactors is an energy-efficient strategy for application as scrubber systems to remove CF4.
... Over the years, TP plasma units have been commissioned to degrade ozone depleting substances, such as PLASCON system [4,11] developed in Australia, which uses DC plasma torch at several kW up to 150 kW of input torch power. Water stabilization is an advantage over gas-stabilized arc, which produces steam TP with significantly higher enthalpy and containing reactive radicals that improve the yield of plasma-chemical reactions and thus performance of destruction and removal efficiency (DRE) [12,13]. A device equipped with a plasma scrubber unit with a steam TP generated by a nitrogen-stabilized DC plasma torch was operated at the Nuclear Energy Research Institute in Taiwan for efficient abatement of nitrogen-diluted fluorinated gases [13,14]. ...
Article
This study presents a numerical model of the hybrid–stabilized argon–steam thermal DC plasma torch of a new design for generating an argon–steam plasma suitable for efficient abatement of persistent perfluorinated compounds (PFCs). The model includes the discharge region and the plasma jet flowing to the surrounding steam atmosphere contained in a plasma-chemical chamber. Compared to previous studies, the torch had a smaller nozzle diameter (5.3 mm) and a reduced input power (20-40 kW) and arc current (120-220 A). The outlet region for the plasma jet extends to 20 cm downstream of the exit nozzle. Fluid dynamic and thermal characteristics together with diffusion of argon, hydrogen and oxygen species, and distribution of plasma species in the discharge and the plasma jet are obtained for currents from 120 to 220 A. The results of the calculations show that the plasma jet exhibits high spatiotemporal fluctuations in the shear layer between the plasma jet and colder steam atmosphere. The most abundant species in the plasma jet are hydrogen and oxygen atoms near the jet center, and molecules of H2, O2 and OH in colder surrounding regions. Satisfactory agreement is obtained with measurements of the radial temperature and electron number density profiles near the jet center close to the nozzle exit.
... Incinerating PFAS compounds with hydrocarbon-rich fuel sources may improve destruction efficiency. Narengerile et al. (2010) calculated the destruction of hydrofluorocarbons in thermal plasma with and without water. In the presence of water, which supplied hydrogen and oxygen radicals, fluorocarbon by-products were eliminated. ...
Article
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Per‐ and polyfluoroalkyl substances (PFAS) are a recalcitrant group of chemicals and can be found throughout the environment. They often collect in wastewater systems with virtually no degradation prior to environmental discharge. Some PFAS partitions to solids captured in wastewater treatment which require further processing. Of all the commonly applied solids treatment technologies, incineration offers the only possibility to completely destroy PFAS. Little is known about the fate of PFAS through incineration, in particular, for the systems employed in wastewater treatment facilities. This review covers available research on the fate of PFAS through incineration systems with a focus on sewage sludge incinerators (SSI). This research indicates that at least some PFAS destruction will occur with incineration approaches used at wastewater treatment facilities. Furthermore, PFAS in flue gas, ash, or water streams used for incinerator pollution control may be undetectable. Future research involving full‐scale fate studies will provide insight on the efficacy of PFAS destruction through incineration and whether other compounds of concern are generated.
... The distinct advantages of thermal plasmas in the waste decomposition are high temperature and abundant radicals, which promote the decomposition and conversion of waste into environmentally benign materials in a compact system. Since the temperature of typical thermal plasma is over 10,000 K, thermal plasma can destroy any kind of non-degradable material preventing the generation of undesired chemical compounds such as furan, dioxin, sulfur oxides, nitrogen oxides in waste treatment processes (Heberlein and Murphy, 2008;Narengerile et al., 2010). ...
Article
Fluctuation characteristics of plasma jet flow in an innovative long DC arc system with ring-shaped anode were successfully clarified on the basis of the high-speed camera visualization. The long DC arc with long electrode gap distance more than 350 mm has been applied to gas decomposition due to its advantages such as large plasma volume and long residence time of treated gas. However, large heat loss at a conventional hemispherical-shaped anode was critical issue in the long DC arc system. Therefore, a ring-shaped anode was utilized to convert large energy loss at the anode into the plasma jet flow. Two kinds of the experiments were conducted. One was the estimation of energy balance in the long DC arc system. Calorimetric measurements were carried out. Another was the high-speed camera observation of the arc fluctuation and the plasma jet fluctuation. Results indicated that the 60% of heat loss at the conventional hemispherical-shaped anode was converted into the plasma jet flow when the ring-shaped anode was utilized. High-speed camera observation revealed that the plasma jet fluctuation with sharp FFT peak in the range of 25-500Hz was attributed to the arc fluctuation, which originated from the restrike phenomena of the anode spot. In contrast, results also suggested that the plasma jet fluctuation with broad FFT peaks in the range of 100-300Hz was attributed to the eddy formation due to the entrainment of ambient cold gas. To understand and control the fluctuation phenomena in the plasma jet enables to establish the innovative waste treatment by thermal plasmas.
... The increasing amount of electronic gas pollution has prompted innovations of new removal technologies. Two methods are currently and commercially available, combustion and plasma arc (Rittmeyer and Vehlow (1993); Lee et al. (1996); Lee et al. (2005); Gal et al. (2003)), which produce mostly another greenhouse gas, CO 2 , and its derivatives (COF 2 , CO, H 2 O), with HF (Xu et al. (2007); Gandhi and Mok (2012); Narengerile et al. (2010); Zhang et al. (2005)) depending on the carrier gas. Recently, catalytic combustion has attracted considerable interest towards a more practical level (Xu et al. (2011); ), particularly hydrolytic combustion because of its lower Gibbs free energy (150 kJ/mol). ...
Article
Remediation of electronic gas CF4 using commercially available technologies results in another kind of greenhouse gas and corrosive side products. This investigation aimed to develop CF4 removal at room temperature with formation of useful product by attempting an electrogenerated Cu1+[Ni2+(CN)4]1- mediator. The initial electrolysis of the bimetallic complex at the anodized Ti cathode demonstrated Cu1+[Ni2+(CN)4]1- formation, which was confirmed by additional electron spin resonance results. The degradation of CF4 followed mediated electrochemical reduction by electrogenerated Cu1+[Ni2+(CN)4]1-. The removal efficiency of CF4 of 95% was achieved by this electroscrubbing process at room temperature. The spectral results of online and offline Fourier transform infrared analyzer, either in gas or in solution phase, demonstrated that the product formed during the removal of CF4 by electrogenerated Cu1+[Ni2+(CN)4]1- by electroscrubbing was ethanol (CH3CH2OH), with a small amount of trifluoroethane (CF3CH3) intermediate.
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A lot of research on international convention-controlled halogenated gases (CHGs) has been carried out. However, few bibliometric analyses and literature reviews exist in this field. Based on 734 articles extracted from the Science Citation Index (SCI) Expanded database of the Web of Science, we provided the visualisation for the performance of contributors and trends in research content by using VOSviewer and Science of Science (Sci2). The results showed that the United States was the most productive country, followed by the United Kingdom and China. The National Oceanic and Atmospheric Administration had the largest number of publications, followed by the Massachusetts Institute of Technology (MIT) and the University of Bristol. In terms of disciplines, environmental science and meteorological and atmospheric science have contributed the most. By using cluster analysis of all keywords, four key research topics of CHGs were identified and reviewed: (1) emissions calculation, (2) physicochemical analysis of halocarbons, (3) evaluation of replacements, and (4) environmental impact. The change in research substances is closely related to the phase-out schedule of the Montreal Protocol. In terms of environmental impact, global warming has always been the most important research hotspot, whereas research on ozone-depleting substances and biological toxicity shows a gradually rising trend.
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This article proposes a numerical model to predict the abatement process of carbon tetrafluoride (CF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> ) in a reaction chamber with the assistance of nitrogen thermal plasma generated by a nontransferred and direct-current torch. The magneto-hydrodynamic equations, that is, the continuity, momentum, energy, and current continuity equations together with turbulence transport equations, are solved by an in-house parallelized finite volume code to obtain the thermal plasma flow jetting out the plasma torch. The thermal plasma is assumed in local thermal equilibrium, optically thin and electrically neutral. In the reaction chamber, a kinetics model is adopted to describe the transport phenomena as well as the chemical interactions among various species involving in the decomposition process. Fifty-six species and 235 chemical reactions are considered in the proposed full kinetic model. Three different models, namely a full kinetic one, a reduced one, and a simplified one, are compared for the prediction accuracy of the destruction and removal efficiency (DRE) along with the abatement products formed in the decomposition process. With the full kinetic model, the predicted DRE at three CF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> concentration conditions, that is, 7200, 12 600, and 16 000 ppm, has a good agreement with the experimental result indicating a nearly full decomposition except for a very small discrepancy less than 0.5% for the case of 16 000 ppm. The predicted abatement efficiency becomes saturated and approaches to a complete abatement provided the participated H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O outnumbers CF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> in moles. A dry abatement process is forecast with a destruction rate decline from 97% to 92% as the CF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> concentration grows from 7200 to 16 000 ppm. Although the reduced kinetic model along the simplified model using a compact set of chemical reactions underpredicts the DRE for the investigated CF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> concentration in the range of 1%–2%, they practically deliver an incomplete group of abatement products that might mislead a proper design of the scrubber behind the reaction chamber. The validation with the experimental measurement justifies the employment of the proposed full kinetic model capable of delivering an accurate abatement prediction of the detoxifying process of CF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> .
Article
The structure of two premised lean and stoichiometric H-2/O-2/Ar/CF3H flames and of a stoichiometric H-2/O-2/Ar/CF2HCl name has been established using a mass spectrometer coupled with molecular beam sampling. The main goal of this work is to study the first step of the CF3H - and the CF2HCl - consumption by reaction with radicals in flames. The rate coefficients of reactions (1) CF3H + H --> products and (2) CF3H + O --> CF3 + OH have been determined: k(1) = 1.16 10(14) exp(-8800/T) cm(3) mole(-1) s(-1) in the temperature range 960 - 1300 K and k(2) = 1.10 10(12) exp(-1600/T) cm(3) mole(-1) s(-1) in the temperature range 920 - 1150 K. In the lean flame 5% of CF3H is consumed by H. 83% by O and 12% by OH, while in the stoichiometric flame 40% is consumed by H, 42% by O and 18% by OH. The determination of the rate coefficient of the reaction (4) CF2HCl + H --> CF2H + HCl in the CF2HCl containing flame leads to the expression k(4) = 4.65 10(14) exp(-7730/T) cm(3)mole(-1)s(-1) in the temperature range from 930 to 1155 K. The rate coefficient of the reaction (7) CF2O + H --> CFO + HF determined in the burnt gases of the investigated flames is k(7) = 4.50 10(13) exp(-11500/T) cm(3)mole(-1)s(-1) between 1175 and 1190 K.
Article
The elementary reactions in the C-H–F-system [formula omitted] and [formula omitted] as well as [formula omitted] were studied in an isothermal flow reactor at room temperature and a nozzle reactor (T = 220 K) with mass spectrometric and laser induced detection devices. The rate constants k1(T) = 2.1·1013 (T/300K)2.4 cm3/mol·s, k2a (300K) = 5·1013 cm3/mol·s, k2b (300K) = 5·1O13 cm3/mol· s, K3 (300K) = 7.5·1O12 cm3/mol·s, and k4 (300K) = 6.8·1013 cm3/mol·s were determined. The products of the reaction (3) were found to be C2H4F2 and C2H3F + HF. The pathway leading to C2H3F + HF was detected as the main product channel (> 50%). The rate and product distribution of reaction (3) was determined to be independent of pressure in the range 1≤p (He)/mbar ≤ 7 and with quenching gases He, CO2 and SF6. In the reaction (4) the pathway (4 a) contributes 12%, and the atom abstraction (4 b) is with 88% the main pathway.
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Transition states for the H-abstraction, F-abstraction and substitution pathways of the reaction of H with fluoromethanes were characterized at the HF and MP2(FU) levels of theory with the 6–31G(d) basis set. The reaction barrier heights for these pathways were obtained from single point energy calculations using the Gaussian-2 and BAC-MP4 methods. These results were employed to calculate rate constants via transition state theory. The computed rate constants are in good accord with available experimental data, and are discussed in the context of the differing flame suppression chemistries of CH3F, CH2F2, CHF3 and CF4.
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A two-dimensional numerical model of the PLASCON™ plasma reactor is used to investigate the destruction of ozone-depleting substances in the reactor. The model includes electromagnetic, fluid dynamic and chemical kinetic phenomena. Calculated temperature, flow and species concentration fields within the plasma torch, the injection manifold and the reaction tube are presented for the case of the destruction of CFC-12 (CF2Cl2). Conversion of CFC-12 to CFC-13 (CF3Cl), a more stable ozone-depleting substance, is found to occur in the region close to the injection manifold, and to be unaffected by reaction tube geometry. CFC-13 is predicted to be the dominant ozone-depleting substance in the exhaust gas. The predictions of the model are found to be in good agreement with measurements of the exhaust gas composition.
Article
The burning velocities of fluoromethane (HFC-41), 1,2-difluoroethane (HFC-152), fluoroethane (HFC-161) and ethane were measured by the spherical-vessel (SV) method at room temperature and at initial pressures of 80–107 kPa over a wide range of HFC/air equivalence ratios (ϕ). The burning velocities were determined from the measured pressure increases by application of a spherical flame model. Schlieren photography was used to directly observe flame propagation behavior in a cylindrical vessel equipped with optical windows. The time evolution of the flame radii derived from the pressure increases agreed with the time evolution observed with the Schlieren technique. The maximum burning velocities of HFC-41, HFC-152, HFC-161 and ethane were 28.3 cm s−1 at ϕ = 1.01, 30.1 cm s−1 at ϕ = 1.07, 38.3 cm s−1 at ϕ = 1.07 and 40.9 cm s−1 at ϕ = 1.05, respectively. The maximum burning velocities for the HFCs, including previously reported C1 and C2 fluoroalkanes, decreased with increasing F-substitution rate (the ratio of the number of F atoms to the sum of the number of H and F atoms). The concentrations of chemical species in the flames were investigated by means of an equilibrium calculation, and the results suggested that the burning velocity was correlated with the concentrations of H and OH radicals that were not deactivated by F radicals in the flame. The results also suggested that the burning velocities were linearly related to the heats of combustion of the C1 and C2 fluoroalkanes.
Article
The shock tube technique coupled with H-atom atomic resonance absorption spectrometry has been used to study the reactions (1) CFâ + Hâ â CFâH + H and (2) CFâH + H â CFâ + Hâ over the temperature ranges 1168--1673 K and 1111--1550 K, respectively. The results can be represented by the Arrhenius expressions k⁠= 2.56 à 10⁻¹¹ exp(-8549K/T) and kâ = 6.13 à 10⁻¹¹ exp(-7364K/T), both in cm³ molecule⁻¹ s⁻¹. Equilibrium constants were calculated from the two Arrhenius expressions in the overlapping temperature range, and good agreement was obtained with the literature values. The rate constants for reaction 2 were converted into rate constants for reaction 1 using literature equilibrium constants. These data are indistinguishable from direct k⁠measurements, and an Arrhenius fit for the joint set is k⁠= 1.88 à 10⁻¹¹ exp(-8185K/T) cm³ molecule⁻¹ s⁻¹. The CFâ + Hâ â CFâH + H reaction was further modeled using conventional transition-state theory, which included ab initio electronic structure determinations of reactants, transition state, and products.
Article
The gas phase pyrolysis and oxidation of several low molecular weight fluorocarbons were investigated in single-pulse shock tubes, using vapor phase chromatography and mass spectral analyses to establish the nature and the relative amounts of all the products generated as a function of the reflected shock temperature and the initial composition. Dwell times were approximately 1 msec and quenched rates were about 1°∕μsec. Shocks were run in mixtures ranging from 0.5-3&percnt; of fluorocarbons in argon. Plots of product distributions as a function of reflected shock temperatures have been prepared for perfluorocyclobutane, butadiene, cyclopropane, propene, and ethylene. Several equilibrium constants for the interconversions of fluorocarbons were evaluated at a function of temperature, a mechanism of the pyrolysis reactions δ developed, and the corresponding rate constants estimated. Mixtures of perfluoro-ethylene in oxygen, highly diluted in argon, were shock-heated for periods of about 1.3 msec to temperatures in the range 1200-2000°K. The major products were F2CO, CO, CF4, C2F6 and very small amounts of C3F6 and CO2. On the basis of the product distribution analyses a plausible mechanism for the oxidation reaction has been developed.
Article
The literature on the plasma destruction of ozone depleting substances (ODS) such as CCl2F2 and CBrF3 is reviewed, and compared with more recent work on the decomposition of CCl2F2 and CBrClF2 in oxygen and steam. A comprehensive kinetic scheme for the decomposition of CBrClF2, which includes the decomposition of CCl2F2 and CBrF3, is presented. Simulations performed with this scheme, and experimental results, demonstrate the importance of allowing for the interconversion of ODS in the assessment of plasma destruction devices. Both experimental and modeling results show that the efficiency of operation of a practical plasma ODS destruction device can be quantified in terms of a throughput parameter, the feed to plasma power ratio (units mol (kWh)-1), or in terms of the thermochemical mixing temperature, Tm, of the plasma, ODS and oxidant. At low throughputs and high Tm, essentially complete destruction may be achieved, with below-ppm quantities of ODS remaining in the plasma exhaust gases. As throughput rises and Tm falls, a threshold is reached above which the ODS residual rises steeply towards the practical working limit set for ODS destruction by the Montreal Protocol (a destruction level of 99.99%). The assessment of this limit must include all ODS in the exhaust gases, weighted for ozone depleting potential. The use of steam, rather than oxygen, as the oxidizing gas gives superior destruction performance.
Article
Non-transferred DC steam (H2O) plasma working with 100kW was applied to minimize production of the toxic byproducts such as dioxins and furans of which formation is not avoidable in the conventional incineration. In the steam plasma process of polychlorinated biphenyl (PCB) mixture waste, content of combustible gas that can be used as gaseous fuel was about 30% based on wet gas. For the mixture of 27% PCB and 73% CCl4, total toxic equivalent concentration of PCDD/PCDF was about 0.056ng TEQ/Nm3. It is concluded that the steam plasma torch process was more effective for waste-to-energy and hazardous waste treatment than the air plasma torch process injected steam and the conventional incineration process.
Article
Tetrafluoromethane (CF4) decomposition by water plasma generated under atmospheric pressure was investigated by means of thermodynamic analyses and experiments. Thermodynamic equilibrium calculations were performed between 300 and 6000K at atmospheric pressure. Experimental results indicated that CF4 was completely decomposed by water plasma, and recovery of fluorine can be achieved more than 99%. Influence of factors such as arc current and additive flow rate of O2 on CF4 decomposition was determined. Furthermore, the decomposition mechanism of CF4 was investigated from chemical kinetics consideration. CFx (x: 1–4) was thermally decomposed above 4000K, oxidized in the temperature range of 4000–2400K, and removed by H radical at temperatures below 2400K.