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Low temperature Conversion of Toluene to Methane using Dielectric Barrier Discharge
reactor.
Faisal Saleem*1, Kui Zhang1, Adam Harvey1
1 School of Engineering, Newcastle University Newcastle upon Tyne NE1 7RU
Keywords: Biomass gasification, Toluene, DBD (Dielectric barrier discharge) reactor, Non-
thermal plasma (NTP), Tar
Graphical Abstract
Abstract
A dielectric barrier discharge (DBD) non-thermal plasma reactor was used to convert toluene
(chosen as a tar analogue) into methane (> 90 %). This study showed that wall temperature and
plasma power played key roles in determining the product distribution. At ambient
temperature, solid formation was observed inside the reactor at all tested powers (50-85 W).
The maximum selectivity to lower hydrocarbons (C1-C6) was 63 % at these conditions (at
85W). Complete conversion of the toluene to lower hydrocarbons was observed at 75W and
above, when the surrounding temperature raised to 200 oC. At 50 W, selectivity increased from
35 % to 84 %, as temperature was increased from ambient to 200 oC. The selectivity to the
various lower hydrocarbons was strongly dependent upon power. Methane, for instance,
exhibited a maximum selectivity of >90 % at 85W and 200 oC, whereas at 50 W the other
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hydrocarbons (>C1) were >40% (along with methane). The selectivity to C2-C6 decreased with
increasing power, due to increased cracking into methane.
1. Introduction
The utilization of biomass resources has received significant attention in recent years, as an
emerging and green pathway of energy recovery [1]. However, the presence of tar in the
product gas can block pipes and foul heat exchangers and other equipment. Biomass tar is a
complex mixture of toluene, phenol, naphthalene, benzene and other aromatic compounds[2].
Biomass gasification tars, particulate matters, and volatile organic compounds (VOCs) can be
removed by using non-thermal plasma (NTP) technology [3-6]. In NTPs, the temperature of
the electrons is much higher than that of the background gas, meaning that this is a “non-
equilibrium plasma”[7]. The high energy electrons (1-10 eV) create reactive atmospheres
consisting of electrons, radicals and ions, by ionizing the background gas [8]. Tar compounds
can be removed either by direct impact of electrons or through the impact of generated reactive
species. However, a major drawback of NTPs is the production of soot/residues and toxic
organic compounds. It has proven very challenging to eradicate the production of these
hazardous products [9]. It has been observed that the risks associated with the newly formed
organic compounds were sometimes greater than that of the parent compound [9]. The
production of solid deposits was also widely reported [10-16]. It has been reported that for
the long-term stable operation of discharges, solid formation should be removed/prevented
[17]. The significant amount of soot formation was also reported during the cracking of
benzene [18]. Osman and Marc (2016) measured the pressure drop to investigate the
blocking inside the reactor with respect to time [15]. Significant increases in pressure drop
may produce leakages or cracks in the dielectric tubes. Black deposits have also been
observed inside the reactor and their formation increased when increasing flow rate [16].
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These by-products can be dangerous to human health and increase operational problems [19].
Therefore, it is very important to convert targeted compounds into non-toxic and environment
friendly products for successful industrial applications of the NTP technology.
2. Results and Discussion
The materials and experimental setup are reported in Saleem et al. (2019) [20]. Fig. 1 (a) shows
that the complete conversion of toluene was achievabel at all tested powers at ambient
temperature. It can be observed that the selectivity to lower hydrocarbons (C1-C6) increased
when increasing the power. This occurred because as the power increases, electrons become
more energetic, which may contribute to decompose aromatic ring to aliphatic
hydrocarbons. In addition, energetic electrons increase the number of reactive radicals by
ionizing the background gas [21]. It is probable that all these species (electrons, ions and
radicals) contribute to increasing the formation of lower hydrocarbons. However the
maximum selectivity to lower hydrocarbons was 63 % even at highest power (85 W) used.
A significant amount of carbon was missing due to residue formation inside the reactor. The
residue formation may occur due to agglomeration of intermediates (phenyl, benzyl and
methyl radicals), produced during the decomposition of the aromatic ring.
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Power (W)
50 60 70 80 90
%
0
20
40
60
80
100 Conversion of Toluene Selectivity to C1-C6
20 oC
(a)
Power (W)
50 60 70 80 90
%
0
20
40
60
80
100
Conversion of toluene Selectivity to C1-C6
(b)
(200 oC)
Fig.1. Effect of power on the selectivity to lower hydrocarbons at ambient and 200 oC; bars
represent standard deviation. Reaction conditions: concentration = 33 g/Nm3; flow rate 41
ml/min; residence time= 4.19 s; Specific input energy kJ/L= 73-123 kJ/L
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The effect of plasma input power on the selectivity to LHCs at 200 oC can be seen in Figure 1
(b). It shows that the selectivity to LHC (C1-C6) significantly increases at all tested powers as
the wall temperatures increase to 200 oC. At 50 W, the selectivity reaches >80 % and increases
to >98 % at 75 W. Hence, clearly, toluene was converted to lower hydrocarbons completely,
without the formation residues or heavy hydrocarbons.
It was reported before that the pyrolysis of toluene produced only benzene and methane at 750
oC, if hydrogen was used as a carrier gas. The maximum conversion of toluene observed was
15% even above 800 oC [22]. It was suggested that hydrogen could work as a scavenger for
methyl, benzyl and phenyl radicals. However pyrolysis of pure toluene in an inert gas produced
hydrogen and hydrocarbons ranging from methane to pyrene [23]. However, in the current
study, nearly complete conversion of toluene to C1-C5 was observed at 200 oC when using
plasma, and the product’s selectivity strongly depended upon plasma power. The methane was
observed as a major product (>90 %) at 85 W and 200 oC. This is possible due to the presence
of energetic electrons and H radicals, which contribute to conversion of aromatic rings to lower
hydrocarbons. It has been observed that increasing temperatures to 400 oC at 40 W increased
the selectivity (+99%) to lower hydrocarbons [20], but in this study similar results were
achieved at constant temperature by increasing the power. However, the product distributions
were substantially different. Previously, it has been seen that formation of benzene increases
with increasing temperature, with products including methane (60%), benzene (28 %) and C2
(9.97 %) [20].
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Power (W)
50 60 70 80 90
Selectivity (%)
0
20
40
60
80
100
CH4
C2 (C2H6+C2H4)
C3 (C3H6+ C3H8)
Butene
Butane
Pentene
Pentane
(a)
(20 oC)
Power (W)
50 60 70 80
Selectivty (%)
0
20
40
60
80
100
CH4
C2 (C2H6+C2H4)
C3 (C3H8+C3H6)
Butene
Butane
Pentene
Benzene
(b)
(200 oC)
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Fig.2 .Effect of power and temperature on selectivity of lower hydrocarbons; bars represent
standard deviation. Reaction conditions: concentration = 33 g/Nm3; flow rate 41 ml/min;
residence time= 4.23 s;
Fig. 2 (b) shows the individual selectivities to various lower hydrocarbons. It can be noted that
the methane exhibits the highest selectivity ( >90 %), whereas benzene is observed only at 200
oC, is always at low concentrations, and disappears entirely by 85 W. Therefore, it can be
concluded that the formation of toxic compounds and solid residue can be eliminated by
increasing the temperatures to 200 oC. From Figure 2 (a ) it can be seen that selectivity to C2-
C6 increases with increasing power. However, at 200 oC, selectivity started to decrease with
power. This is possibly due to hydrocracking of C2-C6 to methane. The main reaction channel
for C2-C5 hydrocarbons cracking involves the breakage of single and double carbon-carbon
bonds. The reactive H radicals react with theses fragment to produce methane. Therefore,
clearly plasma power at higher temperatures promotes the formation of methane instead of any
other hydrocarbons [4]. Therefore, the cracking reactions of C2-C5 increased with increasing
power at ambient temperature.
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Concentration (g/Nm3)
30 40 50 60 70 80
%
0
20
40
60
80
100
Conversion of toluene Selectivity
Fig.3. Effect of concentration on conversion and selectivity to lower hydrocarbons at 200
oC; bars represent standard deviation. Reaction conditions; flow rate 41 ml/min; residence
time= 4.19 s;
To investigate the performance of the DBD reactor at higher concentration, the toluene
concentration was increased to 74 g/Nm3 at elevated temperature. Fig. 3 shows that conversion
of toluene decreases with increasing the concentration. This is because the ratio of the reactive
species to unconverted toluene molecules decreases, hence a greater proportion of toluene
molecules escape from the discharge zone without conversion. As a result the overall removal
efficiency of toluene decreases.
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Concentration (g/Nm3)
30 40 50 60 70 80
Selectivity (%)
0
20
40
60
80
100
CH4
C2 (C2H6+C2H4)
C3 (C3H8+C3H6)
Butene
Butane
Pentene
Benzene
Fig.4. Effect of concentration on selectivity to Individual lower hydrocarbons; bars
represent standard deviation. Reaction conditions: concentration = 33-74 g/Nm3; flow rate
41 ml/min; power= 85 W; Temperature=200 oC residence time= 4.19 s;
Fig. 4 shows the effect of concentration on the product’s selectivity. The results show that the
selectivity to methane decreases when increasing the concentration, whereas selectivity to C2-
C6 increases. It has been observed that the formation of methane increased due to
hydrocracking conversion of C2-C6 lower hydrocarbons and aromatic compounds at constant
concentration [4]. However, in the current study the number of toluene molecules per unit
volume in the discharge zone increased, which ultimately reduced the relative amount of
reactive species. Therefore, the chances of effective collision between reactive species and
toluene molecules decreases. Hence, the hydrocracking conversion of >C1 hydrocarbons to
methane decreased and the selectivity to C2-C6 started to increase with increasing the
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concentrations. Moreover, the selectivity to benzene also significantly increased at higher
concentration due to radical substitution reactions instead of hydrocracking.
3. Conclusion
It was observed that the formation of heavy hydrocarbons from toluene in a non-thermal plasma
could be eliminated by increasing the power at elevated wall temperature (200 oC) in the
presence of a H2 carrier gas. Almost complete conversion of toluene to LHC (lower
hydrocarbons) was shown to be achievable by judicious choice of temperature and plasma
power.
The distribution of products depended upon temperature, power and concentration. At ambient
temperature, the selectivity of each lower hydrocarbon increased with increasing power. In all
cases undesirable formation and deposition of heavy hydrocarbons was observed. At 200 oC
and 85 W, selectivity to methane was over 90 %, with the remainder made up by lower
hydrocarbons. The selectivity to methane increased with increasing power at elevated
temperature, and decreased for C2-C6, due to hydrocracking reactions.
In summary, the synergistic effect of temperature and plasma input power can be used to
control the product distribution and elimination of solid residue formation for toluene streams.
Acknowledgement
The financial support provided to first author by University of Engineering and Technology
Lahore, Pakistan to conduct PhD research, and from Engineering and Physical Sciences
Research Council (EPSRC) Supergene Bioenergy Hub (EP/J017302/1) is gratefully
acknowledged
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