Effects of the diluting mixture and of the temperature on the light
hydrocarbons treatment using a DBD reactor
Olivier Aubry and Jean-Marie Cormier
GREMI, Polytech'Orléans, CNRS - Université d'Orléans (UMR6606), Orléans (France)
Corresponding author: [email protected]
Abstract: The conversion of light hydrocarbons in low concentrations by using a
Non-Thermal Plasma (NTP) reactor in air at atmospheric pressure is studied in
this paper. In previous studies, we have shown that a heating highly promotes the
conversion rate of C2H6 or C3H8, in dry air or in wet air, with a simultaneous
decrease of the energy densities. Thus, a decrease of the total energy cost to
convert the pollutant is obtained. In this work, a Dielectric Barrier Discharge
(DBD) reactor is used and is settled in a furnace which allows a heating from
300K to 600K. The aim of this paper is to present experimental results on the
ethane treatment in various inlet mixtures conditions. The effects of the added
%O2 and the influence of the humidity in the inlet gas on the ethane conversion
rates and the main produced species are studied. The efficiency of the conversion
highly depends on the energy density injected in the plasma. The added %O2 and
the water vapor have important effects on the produced species.
Keywords: Non-thermal plasma, VOC, heating, ethane conversion
1. Introduction
2. Experimental set-up
The VOCs' treatment by non-thermal plasmas at
atmospheric pressure is extensively studied many
years ago. Several NTP reactors types can be used
(coronas, Dielectric Barrier Discharges…). The
plasma processes can be also coupled to others
technologies (catalysts, photocatalysts). Moreover,
the treatments of the VOC's can be realized in
various inlet mixtures (in N2, in air) [1-7]. To
improve the information about the DBD process, the
conversion of ethane in various inlet mixtures
conditions is studied. The effects of several
parameters on C2H6 treatment have been studied as
the amounts of O2 (from 0 to 20%) in N2+C2H6
mixtures and the humidity in the mixtures. These
experiments have been performed at 300K and 600K
to observe the influence of the temperature. This
latter has been already presented for the pollutants
treatment in air mixtures [7-9]. An increase of the
temperature all around the DBD reactor leads to a
high decreasing of the injected power with a propane
conversion rate close to 100% from 12W to 5W
between 300K and 800K, respectively. Thus, it is
possible to highly decrease the energetic cost.
The experimental wire to cylinder type plasma
reactor used in this work has been already described
[7-9]. To study th effects of the heating, the DBD
reactor is settled in an oven (300 ≤ T(K) ≤ 600). The
inlet mixtures injected in the DBD are C2H6 (about
1050 ppmv) in N2 or in N2+%O2 (from 1 to 20%).
The flow rates of C2H6, N2 and O2 are adjusted by
using mass flow controllers (Brooks 5850TR). The
total flow rate, Q, is maintained at 1 NL.min-1. To
observe the effects of the water vapour (wet inlet gas
conditions), N2 is injected in a water tank. The outlet
tank wet gas is added to C2H6 and O2 at the inlet of
the DBD reactor. The Energy density (Ed) injected
in the plasma reactor is expressed from Ed=(Ep.f)/Q
where, Ep is the discharge pulse energy, f is the
pulse repetition rate. In this work, to vary Ed, the
pulse repetition rate is varied from 15 to 200 Hz.
Chemical analyses are performed on Fourier
Transform Infra Red (FTIR, Nicolet Magna-IR 550
series II); C2H6, CO2, CO, C2H2 and CH4 are
quantified. By-products such as formaldehyde,
acetaldehyde, formic acid can been also detected.
NOx can be also detected and quantified.
3. Results
3.1. O2 effects on the propane conversion
a) In dry inlet mixtures, T = 300K
In figure 1, the C2H6 conversion rates are displayed
as functions of the energy density (Ed) and the
added %O2 in the inlet mixture. %O2 varies from 0%
(C2H6+pure N2) to 20% (C2H6 + air).
100
0% O2
90
1% O2
conversion rate (%)
80
5% O2
70
10% O2
60
15% O2
20% O2
50
For each studied mixture, one can note that the
conversion rate increases when Ed rises too. In
comparison to the dry conditions, higher conversion
rates are obtained for all the energy densities: a
conversion rate of 90% can be obtained for
Ed > 950 J.L-1 when %O2 is upper than 5% in the
inlet mixture.
In the wet N2 (0% O2) mixture, the conversion rate
highly increases compared to the dry N2 one. C2H6
conversion rates in wet N2 correspond to the
conversion rate ones when ethane is diluted in dry
N2+1%O2 or in dry N2+5% O2 mixtures (fig. 3).
90
40
80
30
70
10
0
0
200
400
600
Ed (J.L-1)
800
1000
1200
Figure 1. Ethane conversion rate vs. Ed in dry gas (T=300K).
conversion rate (%)
20
60
50
40
30
1% O2
5% O2
20
0% O2 + H2O
10
A rise of Ed leads to an increase of the conversion
rate is observed for each inlet mixture condition. The
conversion rates also highly depend on the inlet %O2
when Ed increases. For Ed ≥ 750 J.L-1, low %O2
lead to lower conversion rates. A conversion rate
close to 90% can be obtained when ethane is treated
in 20%O2 + N2. On the other hand, if C2H6 is treated
in pure N2, the conversion rate is divided by 2 for the
highest Ed. We can observe that for low %O2 (1 or
5%) the conversion rate is about the same for
Ed<750 J.L-1.
0
0
200
400
600
Ed (J.L-1)
800
1000
1200
Figure 3. Ethane conversion rates in wet N2 gas compared to
dry N2+1%O2 or dry N2+5% O2 gas (T= 300K).
3.2. Influence of the temperature
The effects of the temperature on the conversion rate
have been shown for the ethane and propane
treatments in air [7-9]. A rise of the temperature
(600K) implies a decrease of the necessary Ed to
obtain a high conversion rate (fig. 4).
100
90
80
70
conversion rate (%)
b) In wet inlet mixtures, T = 300K
In previous papers [7], the effects of the humidity in
air have been studied for the ethane and propane
conversions. In figure 2, we report the ethane
conversion rate as a function of the energy density
for various %O2 in the inlet gas.
60
50
40
20
10
90
80
80
conversion rate (%)
90
conversion rate (%)
100
60
50
0% O2 + H2O
40
0% O2
1% O2
5% O2
10% O2
15% O2
20% O2
0
100
70
a
30
70
60
50
40
0% O2 + H2O
b
1% O2 + H2O
1% O2 + H2O
30
30
5% O2 + H2O
20
15% O2 + H2O
20
10% O2 + H2O
10
20% O2 + H2O
15% O2 + H2O
10
0
20% O2 + H2O
0
0
0
200
400
600
Ed (J.L-1)
800
1000
1200
Figure 2. Ethane conversion rate vs. Ed in wet gas (T=300K).
200
400
600
Ed (J.L-1)
800
1000
1200
Figure 4. Conversion rates vs. Ed a) in dry gas; b) in wet gas
(T=600K).
1400
0% O2
1200
4
CO (ppm)
0
900
0% O2
800
1% O2
0% O2 + H2O
1% O2 + H2O
5% O2 + H2O
10% O2 + H2O
15% O2 + H2O
20% O2 + H2O
c
5% O2
700
10% O2
600
15% O2
500
d
20% O2
400
300
200
100
0
200
400
600
Ed (J.L-1)
800
1000
1200
0
200
400
600
800
Ed (J.L-1)
1000
1200
Figure 6. CO and CO2 concentrations vs. Ed (T=300K) in dry
gas (a and c) and in wet gas (b and d).
In the figures 7 are reported the concentrations of
CO and CO2 as functions of Ed at 600K and for
various %O2 added to the inlet mixture.
2000
b. 10% O2 + H2O 300K
CO (ppm)
Absorbance (a.u.)
20% O2 + H2O
1000
1400
0% O2
1% O2
5% O2
10% O2
1200
15% O2
20% O2
a
1600
c. 10% O2 600K
15% O2 + H2O
200
0
4
b
400
1800
0
10% O2 + H2O
600
a. 10% O2 300K
2
5% O2 + H2O
10% O2
20% O2
2
4
1% O2 + H2O
5% O2
15% O2
800
0
3.3. Produced species
In O2+N2 mixtures, the main carbon produced
species are CO and CO2 at 300K or 600K. Others
oxygenated carbon species can be also detected in
function of Ed, temperature or %O2. For example,
CH3CHO can be detected (fig. 5).
0% O2 + H2O
a
1% O2
1000
CO2 (ppm)
In the figure 4a, we observe that an increase of the
added %O2 in the dry inlet gas leads to a rise of the
conversion rate at a given Ed. A conversion rate
higher than 90% is obtained from about 300 J.L-1
when O2 is injected in the inlet mixture. In the dry
pure N2 mixture, an increase of the temperature
leads to a high rise of the conversion rate, multiplied
by 2, in comparison to the C2H6 conversion at
300K.In the wet inlet gas (fig. 4b), the conversion
rate is increased too in comparison to the results
obtained at 300K in the dry gas mixtures (fig. 4a).
At 600K, the conversion rate is weakly increased
when O2 is added to the inlet mixtures and highly
increased in N2 gas without O2.
b
0% O2 + H2O
1% O2 + H2O
5% O2 + H2O
10% O2 + H2O
15% O2 + H2O
20% O2 + H2O
1000
800
2
600
0
4
d. 10% O2 + H2O 600K
400
2
200
0
0
1300
1200
1100
1000
Wavenumber (cm-1)
900
800
2500
0% O2
1% O2
5% O2
10% O2
15% O2
20% O2
Figure 5. FTIR spectra (Ed=750 J.L-1).
2000
1500
CO2 (ppm)
The concentrations of CO and CO2, [CO] and [CO2]
respectively, at 300K in the dry gas or in the wet gas
are displayed in figures 6. [CO] and [CO2] increase
when Ed increases whatever added %O2. For
Ed < 800 J.L-1, the concentrations of CO are about at
the same level from dry or wet N2+O2 inlet mixtures.
For Ed > 800 J.L-1, the humidity in the inlet mixtures
implies differences on [CO]. In the wet mixtures,
[CO] are lower than in the dry gas. When C2H6 is
treated in pure N2, CO is not detected. In wet N2, CO
is detected, mainly for Ed upper than 600 J.L-1. In
wet or dry N2, CO2 is not detected. In the O2+N2
mixtures, CO2 is promoted when Ed increases. The
concentrations of CO2 are higher in the wet
conditions than in the dry ones whatever the added
%O2 in all the studied Ed range.
c
d
0% O2 + H2O
1% O2 + H2O
5% O2 + H2O
10% O2 + H2O
15% O2 + H2O
20% O2 + H2O
1000
500
0
0
200
400
600
800
Ed (J.L-1)
1000
1200
0
200
400
600
800
Ed (J.L-1)
1000
1200
Figure 7. CO and CO2 concentrations vs. Ed (T=600K) in dry
gas (a and c) and in wet gas (b and d).
We can note that the CO concentration profiles are
not similar to ones obtained at 300K. Indeed, when
Ed increases, the concentration of CO increases until
a maximum, [CO]max, then decreases. [CO]max is
achieved at about 300 J.L-1 in the dry gas or in the
wet gas for all the %O2 range. In the dry inlet gas,
[CO]max is about at the same value (1200 ppm). In
the wet inlet gas, [CO]max increases in comparison to
the results obtained from the dry gas. The added
1.2
1
carbon balance
0.8
0.6
0% O2
1% O2
10% O2
20% O2
0% O2 + H2O
1% O2 + H2O
10% O2 + H2O
20% O2 + H2O
0.4
0.2
0
0
200
400
600
Ed (J.L-1)
800
1000
1200
Figure 8. Carbon balances as functions of Ed (T=300K).
At 300K, when O2 is injected in the inlet mixtures
(from 1% to 20%), the carbon balance is completed
with only CO and CO2 for Ed ≥ 200 J.L-1. When the
balance carbon is not completed, species as CH2OH,
CH3CHO can be produced (fig.4). Nevertheless,
these species are not the main carbon products. In N2
without O2, CO and CO2 are minority produced
species. One of the main produced species is CH4
(fig. 9).The CH4 selectivity is expressed by:
S CH 4 (%) =
[CH 4 ]
× 100
2. ([C 2 H 6 ] i - [C 2 H 6 ] f )
(1)
CH4 is mainly produced when the temperature
increases and in the wet inlet gas. CH4 is not
detected for the inlet gas without O2.
100
0% O2 300 K
90
0%O2 + H2O 300 K
selectivty CH4 (%)
%O2 effects on [CO] are mainly observed for the
decrease of the CO concentration when Ed rises. An
increase of the inlet %O2 leads to a lower [CO]
decrease. The CO2 concentrations highly depend on
the energy density injected in the plasma. For the
highest Ed, the injected carbon is almost entirely
converted to CO2. An increase of the inlet %O2 leads
to an increase of [CO2], which corresponding to the
decreasing of CO in the same time. In the wet gas,
[CO2] is increased in comparison to the results
obtained in the dry gas at given Ed and %O2. One
can note that a very low concentration of CO2
appears in wet N2 without O2.
From these results, it appears that others carbon
species than CO and CO2 are produced when ethane
is treated at low Ed. This is well established from the
figure 8 where the carbon balances, Cf/Ci, are
displayed.
Cf=[CO]+[CO2]+2×[C2H6]f and Ci=2×[C2H6]i with
the concentrations of CO, CO2, non-consumed C2H6
and inlet C2H6, respectively.
80
0% O2 600K
70
0% O2 + H2O 600K
60
50
40
30
20
10
0
0
200
400
600
Ed (J.L-1)
800
1000
1200
Figure 9. CH4 selectivity vs. Ed.
Conclusion
The efficiency of the VOCs' treatment by using a
DBD can be improved by adjusting the added %O2
and the humidity in the inlet mixture. The heating all
around the DBD reactor is also an important
parameter to increase the conversion rate and modify
the concentrations of the produced species. The Oatoms produced from the O2 decomposition in the
plasma seem to be promoted when the inlet %O2
increases: a rise of the conversion rates can be
observed for high Ed (>1000 J.L-1) in the dry gases
and 800 J.L-1 in the wet gas mixtures. An increase of
the humidity in the inlet gas leads to a rise of the
conversion rate whatever %O2 in N2. This could be
due to a rise of OH and O-atoms amounts from the
H2O decomposition in the plasma. To heat leads to a
promotion of the C2H6 decomposition chemical
kinetics. The main produced species are CO and
CO2. The effects of the added %O2 in the inlet gas
on [CO] and [CO2] highly depend on the
temperature and the moisture conditions. In all the
studied cases CO2 become the main produced
species for the highest Ed.
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