22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Microwave plasma reforming of heated air containing naphthalene
A. Pilatau1, H. de Souza Medeiros1, A.V. Gorbunov1, A.S. da Silva Sobrinho1, G. Petraconi Filho1 and O. Nozhenko2
1
2
ITA Technological Institute of Aeronautics, Sao Jose dos Campos,SP, Brazil
East Ukrainian Volodymyr Dahl National University, North-Donetzk, Ukraine
Abstract: In this research naphthalene conversion in a non-equilibrium 2.45 GHz
microwave (MW) plasma reformer was studied. These results show some improvement in
conversion efficiency (over 94% vs. 80% under air plasma gas application) over work [5] in
gliding arc for tar reforming.
Keywords: plasma, microwave reforming, tar
1. Introduction
Gasification can be gaining attention as one of a route
for alternative energy production, providing a reduction
on the carbonic gas emission and being respectful to
environment and techno-economically competitive.
However, thermo-chemical conversion of the fuel covers
different processes combustion, gasification and pyrolysis
with interleaved boundaries. Fuels which are produced in
gasification and can be used for internal combustion
engine, often need [1] of some type of modification to
replace the conventional diesel fuel, because the syngas
obtained from this process usually contains unacceptable
levels of detrimental impurities such as tar and
particulates [2]. Tar can cause operational problems in
downstream processes by blocking gas coolers, filter
elements and engine suction channels and can be lead to
seriously damages in the units. Therefore, before the gas
can be used, it is required to remove the tar and at least
part of dust.
Thus, tar reforming and destruction is appointed as a
key challenge for a successful commercial application of
gasification technologies. A variety of plasmas have been
studied to both reforming and destruction tars in many
discharges types including corona, gliding arc, and
microwave discharges [3]. However, the different ways to
tars decomposition require a variety of the energy
consumption to the removal process. Based on that, in this
paper a 2.45 GHz microwave (MW) plasma torch at
atmospheric pressure is applied to naphthalene destruction
using air as carrier gas. In addition, a model was used to
compare the results
2. Model component and reaction pathway
Naphthalene C 10 H 8 is a PAH (Polycyclic Aromatic
Hydrocarbon) that is very difficult to crack in comparison
to other tertiary tars; also it appears that it is often the
main species found in tertiary tars (in gasification, tertiary
aromatics are predominant). Tertiary aromatics (like
naphthalene) are most common [4], they have the highest
formation temperature (800-1000 oC), and high dew point
as well as they are the most stable and difficult to crack
catalytically. For these reasons, the naphthalene has been
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selected as a model compound representative of the
tertiary class tars.
For describing of the tar decomposition pathway is
present in [3,4], a common scheme including to cracking,
steam and dry reforming and partial oxidation. Indeed,
Standard Gibb’s Energy of the cracking reaction becomes
highly negative at the temperature encountered in the MW
plasma reactor which is depicted in Fig.1, what proves
that they have to be considered.
Fig. 1. Schematic diagram of the microwave plasma
torch equipment
Based on general scheme [3] and established reactions
[4] of naphthalene decomposition, it has been chosen the
simple reactions scheme showed in Table 1. The
presented pathway (Fig.2) has been made an attempt to
take into account influence of microwave plasma effect
on cracking molecules of naphthalene in the reactor Fig.1.
According to this scheme (Fig.2) and Table 1,
microwave plasma assists to hydrogen atom generation
under steam decomposition. It is accept that steam is
contained in air up to 1.45 % in weight. Obtained
hydrogen atoms joins together with assisting molecules M
of air (such as nitrogen, oxygen). Generated by the
process, hydrogen molecules are used for direct
naphthalene cracking as well as for steam reforming of
the tar. Oxygen takes part in oxidization of secondary
product. There is benzene which is generated from
naphthalene under the its dry reforming.
1
Table 1. Reactions and reaction rates used in the model (concentration.
kmol× m-3, rate in kmol m-3 s-1).
№
Reaction
Reaction rates
1
C 10 H 8 -= 10C +4H 2
k 1 =5.56×1015exp(-3.6×105/RT)
r 1 =k 1 [C 10 H 8 ]2[H 2 ]-0.7
2
C 10 H 8
+4H 2 O=C 6 H 6 +4CO+5H 2
k 2 =1.58×1012exp(-3.24×105/RT)
r 2 =k 2 [C 10 H 8 ]2[H 2 ]0.4
3
C 6 H 6 +5H 2 O=5CO+6H 2 +CH 4
k 3 =4.4×108exp(-2.2×105/RT)
r 3 =k 3 [C 6 H 6 ]
4
C 6 H 6 +7.5O 2 =6CO 2 +4H 2 O
k 4 =1.783×101exp(-1.255×105/RT)
r 4 =k 4 [C 6 H 6 ]-0.1[O 2 ]1.25
5
C 6 H 6 +3O 2 =6CO+3H 2
k 5 =1.58×1015exp(-2.026×105/RT)
r 5 =k 5 [C 6 H 6 ][O 2 ]
6
H 2 O+e =H+OH+e
k 6 =2.6×10-10exp(57491/T)
r 6 =k 6 [H 2 O]n e-
7
2H+M=H 2 +M
k 7 =5.4×109 T-1.3
r 7 =k 7 [H]2[M]
8
H 2 +M=2H+M
K 8 =2.23×1011 exp(48306/T)
r 8 =k 7 [H 2 ][M]
-
-
constant at 70 ºC. The experiments were carried out with
a fixed power supply at 1.5 KW and the carrier gas flow
rate at 20 L/min. Total incoming gas flow rate is Qin=1.29
m3/h; incoming gas composition is presented (% in vol) as
79,11%;
O2
=
19.4;
H 2 O=0,3%;
N2=
CO 2 =0.18%;C 10 H 8 .= 0.019% (30 g/m3).
Fig.3 Experimental setup of microwave plasma torch
The 2.45 GHz microwave plasma torch (Fig.4) reformer
utilized in this work has a maximum power of 3 KW and
source voltage of 5500 V from SAIREM. The rectangular
waveguide, short circuit movable piston, the stubs, and
the circulator are also from SAIREM. A 10 mm quartz
tube (ID) was used as a reactor where the microwave is
absorbed by the gas coming from the feeding system, on
swirl way, ionizing a fraction of molecules in this gas and
then generating the plasma torch.
Fig.2. Naphthalene cracking kinetic model.
The amount of air introduced in the MW plasma reactor
is enough for satisfying of requirements for complete
combustion and prevention of intensive soot formation in
outlet gas as well as plasma torch stability. Electron
temperature is assumed at the range of 4000-4500K,
although average gas temperature in the reactor has been
determined as 1000 -1300 oC.
3. Materials and methods
The experimental setup for the tar reforming includes
(Fig.3) a carrier gas (air) preheating system, a chamber
for the naphthalene heating and gas mixing, and a MW
plasma reformer. The preheated gas temperature in both
carrier gas system and naphthalene chamber are measured
by thermocouples and controlled by an electrical panel in
order to keep them constant. The gas line is also heated
and the temperature is carefully controlled to avoid tar
condensation and hence line blocking. The carrier gas and
gas line temperature was kept at 200 ºC and the
naphthalene temperature inside the chamber remains
2
Fig. 4. Microwave plasma torch ignited. (P=1.5 KW;
Air/C 10 H 8 =10.85).
4. Results and discussion
A complete validation of the model would require the
experimental knowledge of the gas leaving the reactor (in
terms of composition and temperature), which was not
possible. In order to partially validate the model,
experiments for measurement of naphthalene conversion
efficiency value based on [5], have been performed (Fig.
3) according to the scheme Fig. 1.
A comparison between the model and experimental data
is presented on the Fig. 5.
Based on the specially developed software simulation,
is evident that the average temperature of 1300oC in MW
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plasma torch is sufficient for reaching almost the
naphthalene conversion in the MW plasma reactor in
good agreement with the most detailed kinetic model
which is realized in CHEMLIN computation model [5].
Fig. 5. Comparison of experimental and calculating
results
For sensitivity analysis [4], two influences on
conversion efficiency have been checked. There are the
temperature of gas in MW-reactor and the ratio (in
weight) of air to naphthalene (equivalence ratio).
The temperature is one of the key influences on
naphthalene destruction in the MW plasma reactor. Its
dependence on conversion efficiency is depicted on the
Fig. 6.
Fig.7 Gas temperature variation as a function of the MW
power.
Besides, it is necessary to pointed that changing of
temperature can cause difficulties in practice, because
only way to do it, is by changing of MW power.
However, measurements indicate that the temperature
changes at a short range of 4000 – 4500 K (Fig. 7) in the
MW plasma reactor.
Thereupon, it is considered a second factor of influence
on conversion efficiency, the equivalence ratio (ER). The
dependence of conversion efficiency and also energy
efficiency value given in [5], is showed on the Fig. 8.
Increasing the equivalence ratio (ER) causes enhance of
amount of the steam in concerning air carrier gas, also it
appears that amount of generated atoms of hydrogen has
been raised and the reaction rate of naphthalene cracking
(reaction #1, Table 1) is decreased. However, the value of
specific energy input (SEI) [5] has been decreased.
Furthermore, if it has got assumption [5] that inlet gas
flow rate, at the point of concentration measurement, is
equals to the outlet gas flow rate, it means that the energy
efficiency according to [5] will be higher at bigger value
of ER.
Fig. 8. Energy and conversion efficiencies [5] of the
microwave plasma torch vs. equivalence ratio
Fig. 6. Influence of gas temperature on conversion
efficiency according to [5] of naphthalene.
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5. Conclusion
Naphthalene conversion was studied in a nonequilibrium MW plasma reformer. At tar concentration of
30 g/m3, over 94% of naphthalene conversion was
achieved at the benchmark specific energy input of 1,15
kW/m3 and energy efficiencies of 25 g/kWh. These
results show some improvement in conversion efficiency
(over 94% vs. 80% under air plasma gas application) over
work [5] in gliding arc for tar reforming. Thus, in
comparison to the gliding arc plasma reformer, MW
plasma reformer can be used in wide region of ER.
However, energy efficiency of MW plasma torch at
present work is less than 2,4 times than the value obtained
by gliding arc reformer. It means that to the MW plasma
tars reforming achieves the gliding arc performance in
terms of energy efficiency, increasing the operational
equivalence ratio more than level of ER=25 could be
helpful.
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6. Acknowledgments
The authors kindly thank the financial support provided
by FAPESP (Grant 2012/14568-6) and CAPES
(Processes # 88887.060497/2014-00).
7. References
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(2011)
[2] J.D. Martinez, Syngas production in down draft
biomass gasifier and its application in internal combustion
engines, Renew Energ, 38,1 (2012)
[3] Li Chunshan,. Tar property, analysis, reforming
Mechanism and model for biomass gasification an
overview, Renew Sust Energ Rev, 13,594 (2009)
[4] A. Fourcault,. Modelling of thermal removal of tars in
a high temperature stage fed by a plasma torch, Biomass
and bioenergy, 34,1363 (2010)
[5] Nunnally T, Gliding arc plasma oxidative steam
reforming of a simulated syngas containing naphthalene
and toluene. International Journal of Hydrogen Energy,
Int. J. Hydrogen Energy, 39,11976 (2014)
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