Investigation of the destruction mechanism of toluene and ethylene in a
dielectric barrier discharge.
R. Aerts, A. Bogaerts
Department of Chemistry, University of Antwerp, Campus Drie Eiken
Universiteitsplein 1, BE-2610 Wilrijk-Antwerp, Belgium
E-mail: [email protected]
Abstract: The goal of this work is to investigate the mechanisms of destruction for toluene and ethylene in a
dielectric barrier discharge reactor with a global kinetic model. The effect of temperature, air humidity, pollutant
type and the effect of electron impact reactions is investigated. It was found that gas temperature and water
concentration have a major effect on the destruction efficiency of both pollutants. The addition of electron impact
reactions to the reaction mechanism of ethylene and toluene did not influence the destruction efficiency and the
CO2 selectivity. Further analysis of the reaction path showed that reactions with metastable nitrogen N2(A3∑u+)
are very important for the ring breaking process of toluene.
Keywords: dielectric barrier discharge, VOC, kinetic modeling, toluene, ethylene
1. Introduction
Non-thermal plasmas, such as a dielectric barrier
discharge (DBD), have been first developed and
applied to ozone generation; later they were
successfully applied to remove varieties of gaseous
pollutants, such as NOx, SOx, mercury, VOCs
(volatile organic compounds) and dioxins. Their
biggest potential is the removal of pollutants, such as
toluene and ethylene, from gas streams at
temperatures close to room temperature. This is not
the case for the commonly used pollution control
technologies, such as thermal and catalytic
oxidation, which are operated at temperatures above
400°C resulting in high energy costs [1].
In a DBD, the electrons, due to their light mass,
acquire much higher energy than other charged
particles, resulting in a mean electron temperature
typically ranging from 10,000 K to 100,000 K. Yet
the gas temperature stays more or less fixed at room
temperature. As a result the destruction of a VOC is
induced by the active species, i.e., electrons, but also
excited species, radicals and ions produced by
electron impact reactions. Although it has been
demonstrated that DBD reactors achieve acceptable
destruction efficiencies for a variety of applications,
the energy efficiency and CO2 selectivity still need
to be improved for further industrial application.
In the last years several modifications have been
presented as a solution for the limited energy
efficiency and CO2 selectivity. Among those, the
packed bed reactor and plasma catalysis have
received most attention. Although these
modifications show some promising results, still
almost nothing is known about the performance
enhancement in these modifications. Kinetic models
could provide information about the destruction
mechanism in such modifications and will help to
distinguish the different processes related to the
previously mentioned performance enhancement.
2. Description of model
The model used in this work is global_kin,
developed by M. Kushner and coworkers. It is a socalled zero dimensional kinetic model (0-d model).
The program consists of an off-line Boltzmann
solver module for the electron energy distribution, a
circuit module and a plasma chemistry module. A
full explanation of this model can be found in [2].
The focus of the presented calculations is on the
initiation of the destruction mechanism. Therefore, a
large number of different plasma species have to be
taken into account, including electrons, various ions,
molecules and radicals, which can react with each
other in various collision processes. The plasma
species and their reactions considered in the model
depend on the VOC being investigated and can be
found in the following references [3-5]. For the
destruction of toluene in a humid air plasma a
chemistry set is built with 95 species and 290
reactions. However, simulation of the complete
toluene destruction path in an air plasma would need
hundreds of species and several hundreds of
reactions causing instabilities in the model. For this
reason a simplified set with only a few byproducts is
assumed. This assumption can have a large impact
on the selectivity to CO/CO2 and therefore the
selectivity and carbon balance is only plotted once
for toluene. As indicated above the interest of this
research is to investigate the initiation of the process
and not to predict all end products. The second
chemistry set for ethylene is built with 98 species
and 630 reactions and describes the complete
destruction mechanism of ethylene in a humid air
plasma.
for CO and/or CO2, and the number of carbons (#C)
as follows:
(3.2)
(3.3)
A discharge in a DBD consist of a collection of
filaments or micro-streamers, each of which has a
short duration of 10 to 100 ns [5]. For this reason all
simulations were performed with an energetic pulse
of 100 ns and a power of 30 kW. The residence time
of the gas is also kept constant at 0.1 ms.
3. Results and discussion
First we will present some calculation results for the
effect of temperature on the VOC destruction. In
Figure 3.1 a graph is presented with the removal
efficiency (RE) as a function of the temperature for
both VOCs. The RE is calculated from the VOC
inlet concentration (VOCin) and the VOC outlet
concentration (VOCout):
(3.1)
It is observed that the RE increases with temperature
and complete removal is achieved at 600K for both
molecules. However, such temperature is much too
high for a low energy cost application. It is also clear
from Figure 3.2 that despite the increasing removal
efficiency the selectivity for CO/CO2 (SELCO2)
decreases at such high temperatures.
Figure 3.1. Effect of temperature on the RE for toluene and
ethylene, with a gas composition in mol fractions of 0.7875 N2,
0.1675 O2, 0.035 H2O and 0.001 toluene/ethylene.
Figure 3.2 presents the influence of temperature on
the carbon balance (CB) and the CO2 selectivity
(SELCO2), calculated with the outlet concentrations
Figure 3.2. Effect of temperature on the CB and the SELCO2 for
toluene and ethylene with a gas composition in mol fractions of
0.7875 N2, 0.1675 O2, 0.035 H2O and 0.001 toluene/ethylene.
It is seen that also the CB and the SELCO2 increase
with temperature. However, for ethylene a
maximum CO2 selectivity is reached at 500 K.
Indeed, the selectivity decreases because of the
elevated production of larger molecules as propene
and ethane. The low selectivity for both processes is
caused by the many byproducts formed in the DBD
and enforces the need for a catalyst in the reactor.
Another important parameter which influences the
removal efficiency is the air humidity. In figure 3.3
results are shown for different water contents in the
air, varying between 0 and 4%, at the same
conditions for both VOCs.The figure indicates a
positive effect for toluene and ethylene on the
removal efficiency but the effect is more strongly
observed for toluene. As indicated below the
destruction of toluene can be initiated by reactions
with OH-radicals, O radicals or O3.
1) C7H8 + O3 → products k ~ 1x10-21 cm³/s
2) C7H8 + O → products k ~ 8x10-14 cm³/s
3) C7H8 + OH → products k ~ 3x10-12 cm³/s
[7]
[4]
[4]
The reaction rate coefficient is the highest for OH
radicals and therefore the RE increases with higher
levels of moisture in the gas mixture. On the other
hand the simulations also predict a small decrease in
CO2 selectivity (±1%) and carbon balance (±2.5%)
upon increase of the air humidity.
Figure 3.3. Effect of water content on the RE for toluene and
ethylene with a O2/N2/VOC gas ratio 0.19/0.81/0.01 at 300 K.
Finally, we have investigated the effect of electron
impact reactions on the destruction mechanisms of
both ethylene and toluene. Therefore, expanded
reaction sets have been built with electron impact
reactions. For ethylene, electron impact dissociation
and dissociative ionization reactions with CH4, CH3,
CH2, CH, C, C2H6, C2H5, C2H4, C2H3, C2H2, C2H,
C2, C3H8, C3H7, C3H6 and C3H5 were added.
Although no effect on the RE or the CB is observed,
electron impact reactions do influence the total
process, as is obvious from the change of electron
density (ne) plotted in figure 3.6. This change of
electron density can influence the air chemistry and
also the destruction process indirectly. This indicates
that more simulations with different conditions have
to be made to completely rule out the influence of
electron impact reactions.
4E+12
E Without elektron impact
E With elektron impact
3E+12
ne (1/cm³)
The results in figure 3.4 show no contribution of the
electron impact reactions to the RE of ethylene in
dry air. Which emphasizes the fact that electron
impact reactions contribute in the destruction
process. The same conclusions can be made for
humid air mixtures.
Figure 3.5. Effect of electron impact reactions on the CB for a
range of ethylene concentrations in a dry air pulsed plasma with
a O2/N2 ratio of 0.1895/0.8095 at 300 K. EI indicates the
simulations with electron impact reactions.
2E+12
1E+12
0
2E-05
4E-05
6E-05
8E-05
0.0001
time (s)
Figure 3.6. Effect of electron impact reactions on the electron
density in a dry air pulsed plasma with a O2/N2 ratio of
0.1895/0.8095 and 100 ppm ethylene at 300 K. EI indicates the
simulation with electron impact reactions.
Figure 3.4. Effect of electron impact reactions on the RE for a
range of ethylene concentrations in a dry air pulsed plasma with
a O2/N2 ratio of 0.1895/0.8095 at 300K. EI indicates the
simulations with electron impact reactions.
The results also show no contribution of the electron
impact reactions to the selectivity of the process
which is indicated in figure 3.5 for the carbon
balance.
Not much cross section data are known about
aromatic molecules and their byproducts. However
some data about toluene is published [9]. The
following dissociative ionization electron impact
reactions were added to the chemistry set of toluene:
4) C7H8 + e → C7H8+ + 2e
5)
6)
7)
8)
9)
10)
C7H8 + e
C7H8 + e
C7H8 + e
C7H8 + e
C7H8 + e
C7H8 + e
→ C6H5CH2+ + H + 2e
→ C5H5+ + C2H3 + 2e
→ C5H5+ + C2H2 + H + 2e
→ C4H3+ + C3H5 + 2e
→ C5H3+ + C2H5 + 2e
→ C3H3+ + C2H3 + C2H2 + 2e
The results show no improvement in RE or in
selectivity when these electron impact reactions
were added to the reaction set, which is also reported
by G. Lombardi [3]. Indeed the added electron
impact reactions are just a small fraction of total
electron impact reactions in the actual process but
even with a few electron impact reactions a
difference in RE or selectivity should be noticeable.
These results rule out the direct contribution of
electron impact reactions as discussed with ethylene.
This indicates that other processes should be
considered in the ring breaking process of aromatic
molecules.
Based on the work of S. Suzuki [8] and N. BlinSimiand [4] the effect of metastable nitrogen
N2(A3∑u+) is investigated for the destruction of
toluene. Our simulations showed that the RE
decreased with 35 % if reactions with metastable
nitrogen N2(A3∑u+) were neglected. This showed
that the metastables play an important role in the
destruction mechanism and in the breaking of the
stable benzene ring.
4. Conclusions
A kinetic model is built for toluene and ethylene and
simulations indicate the importance of temperature
and air humidity on the removal efficiency of these
VOCs. It is observed that for increasing temperature
the removal efficiency increases at the cost of a
decreasing selectivity. Addition of water reveals the
same relations as for temperature however for
ethylene a maximum CO2 selectivity is reached at
500 K. Also the influence of electron impact
reactions is investigated for both molecules. The
results show that the actual electron impact reactions
do not contribute directly to the destruction of the
VOC. However other indirect effects, such as a
change of electron density, could influence the
destruction path. Other mechanisms like reactions
with metastables as N2(A3∑u+) are found to be very
important in the destruction of toluene and the
destruction of benzene rings. However, little is
known about the reaction rate coefficients of
metastables with the many byproducts in the
destruction of toluene. Therefore, future simulations
for other smaller well known molecules, such as
ethylene, will be performed.
Acknowledgements
We are also very grateful to M. Kushner and group
members from providing the global_kin code and
the useful advice in our work.
References
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