Journal of Hazardous Materials 143 (2007) 409–414
Reducing nitric oxide into nitrogen via a radio-frequency discharge
Cheng-Hsien Tsai
a
,
∗
, Hsi-Hsien Yang
b
, Chih-Ju G. Jou
c
, How Ming Lee
d
aDepartment of Chemical and Material Engineering, National Kaohsiung University of Applied Sciences, 415 Chien-Kung Road,
Kaohsiung 807, Taiwan
bDepartment of Environmental Engineering and Management, Chaoyang University of Technology, Taichung, Taiwan cDepartment of Safety, Health and Environmental Engineering, National Kaohsiung First University of Science
and Technology, Kaohsiung, Taiwan
dPhysics Division, Institute of Nuclear Energy Research, Taoyuan, Taiwan
Received 4 May 2006; received in revised form 15 September 2006; accepted 16 September 2006 Available online 20 September 2006
Abstract
NO/N
2/O
2/H
2O mixtures are usually converted into HNO
3and/or NO
2using different discharge approaches. In this study, a radio-frequency
discharge was successfully used to reduce NO mainly into N
2at a low pressure (4 kPa). The influences of experimental parameters, including
carrier gas, inlet concentration of NO, O
2, steam, and applied power, are discussed. At least 95.7% of the total N atoms converted from NO into N
2.
Other traces of byproducts were N
2O and HNO
2, but neither HNO
3nor NO
2were detected. In addition, conversion of NO apparently increased
with elevated applied power or decreased inlet concentration of O
2, reaching 92.8% and 74.2% for the NO/N
2/O
2(2%) and NO/N
2/O
2(6%)/H
2O
(10%) mixtures, respectively, at 120 W. In addition, from the optical emission spectra, a large amount of N
2(first positive band and second positive
band) and NO (␥ system) were observed, and the important reactions for NO removal and N
2formation are proposed.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Nitric oxide; rf discharge; Reduction; Nitrogen
1. Introduction
NOx, the precursor of photochemistry products and acid
smog, is mainly emitted from industrial processes or mobile
sources. NOx is harmful to the human health and the
ecosys-tem, damages vegetation and degrades or corrodes materials
through acid deposition
[1]
. Many emissions control strategies
have been implemented, such as selective catalytic reduction,
selective noncatalytic reduction, and direct thermal
decompo-sition. However, the typical operating temperature of these is
relatively high, ranging between 300 and 1000
◦C
[2,3]
.
Hence, nonthermal plasma approaches, which can be free
of high temperature control and avoid the problems of catalyst
poisons and deactivities, have been developed to remove NO.
Several kinds of plasma technologies have been demonstrated
that can be operated at room temperature and be proceeded in
dry or wet process, such as dielectric barrier discharge, corona
discharge, and microwave discharge
[4–8]
.
∗Corresponding author. Tel.: +886 7 3814526x5110; fax: +886 7 3830674.
E-mail address:chtsai@cc.kuas.edu.tw(C.-H. Tsai).
However, higher oxidized nitrogen compounds, such as NO
2and/or HNO
3, are commonly formed when converting the
NO/N
2/O
2/H
2O mixtures
[6,9–11]
. Although NO
2and HNO
3are water soluble and can be removal with scrubbers, to reduce
NO into N
2seems a better choice because it does not need
further treatment of acidic wastewater and sludge. Hence, a
dry, single-stage, noncatalytic radio-frequency (rf) discharge
approach, which has previously not been tried to remove NO, is
successfully demonstrated to reduce NO mainly into N
2in this
study. The 13.56 MHz rf plasma is commonly used in industry
for IC manufacture and surface modification, as well as being
used to recover sulfur from SO
2and convert methane into syngas
[12,13]
.
In addition, high concentrations of NOx are yielded from
nitric acid plants, the high ovens of heat treating automotive
catalysts, and automobile exhaust gases. The NOx can reach a
concentration of 2000 or up to 10,000 ppm
[3,14]
, which is fed
into the rf discharge reactor in this study.
In the discharge zone, electrons interact with N
2, NO, and
O
2to generate initiating species for the overall reaction chains
[15]
. In addition to electron-impact dissociation and ionization
reactions, Penning ionization and charge-transfer processes take
0304-3894/$ – see front matter © 2006 Elsevier B.V. All rights reserved.C.-H. Tsai et al. / Journal of Hazardous Materials 143 (2007) 409–414 413
Fig. 4. A typical optical emission spectrum of NO/N2/O2 (6%) mixtures at
120 W and 4 kPa.
accompanied with a higher
η
NOwere found at 120 W than at
30 W (
Table 3
). This is because with an increased power supply
a higher plasma density provides more energetic electrons,
excit-ing more N
2, NO and O
2, which then increases the probability
of NO collision and removal.
When at the same power supply (120 W) I
NOis much higher
with a close value of
I
N2in the NO/N
2mixtures than in the
NO/N
2/O
2mixtures (
Table 3
). Since O
2is a very efficient
quencher
[32]
that results in some electronically excited species,
ionic molecular nitrogen, and atomic N or O cannot be identified
in the NO/N
2/O
2mixtures. Moreover, the addition of oxygen
decreases the number of atomic N due to the destructive process
involving NO and O
2molecules
[32]
, leading to a decrease in
the reaction rate of N + NO
→ N
2+ O and a decrease in
η
NO.
In rf discharge, the activated species, including N
22nd
pos.
(E
th= 11.1 eV),
N
2metastable
state
(N
2(A),
E
th= 6.22 eV) and atomic N (E
th= 9.76 eV) were obtained
directly via the electron impact. Subsequently, NO
␥-band
(NO(A)) was excited by the collisions with N
2(A) through
NO(X) + N
2(A)
→ NO(A) + N
2(X). Compared with the former
energy levels of energetic species, NO can be removed by
col-liding with electrons, excited N
2or N, due to the lower threshold
energy (E
th= 6.50 eV) of NO dissociation. Hence, reactions
(1)
played important roles in NO removal, as well as reaction
(1)
to reduce NO to N
2. As for the oxidation of NO via reaction
(2)
with a lower reaction rate constant, it yields NO
2, however,
NO
2will be dissociated by reacting with e, N or O to yield N
2,
N
2O, and NO
[22]
. Hence, almost no NO
2was detected in the
rf discharge.
Table 3
Relative intensities of INOandIN2in different discharge mixtures
Mixtures Power (W) INO IN2 NO conversion (%)
NO/N2/O2(6%) 30 1.0 1.0 44.8
120 2.1 7.8 77.6
NO/N2 120 3.5 7.5 97.2
Finally, the energy efficiency of the de-NO rf plasma that
represents the NO removed by a unit electrical power (kWh) is
calculated based on deposited power
[33]
, excluding the losses
of electrodes heating and power transformation/delivery (the
losses is about 50% of applied power). The energy
efficien-cies are 5.6 g NO/kWh (NO/N
2at 30 W) and 4.2 g NO/kWh
(NO/N
2/O
2(6%)/H
2O (10%) at 30 W), as well as are lower
than the typical values of energy efficiencies (g NOx/kWh) in
various plasmas, such as 3.7 for microwave, 3.8 for dc plasma,
17 for combined DBD scrubber system, 19 for electron beam,
25 for pulse corona, and 28 for packed-bed DBD
[33–38]
.
Hence, the performance of this rf conversion plasma approach
should be improved for elevating the energy efficiency via the
adjustment of operating parameters, such as the increase of
total inlet flow rate in the future.
4. Conclusion
As much as 95.7–100% of the N atoms converted from NO
into N
2for NO/N
2/(O
2)/(H
2O) mixtures using the rf discharge
approach. The major product was N
2, with several to tens ppm
of HNO
2and N
2O detected, but no HNO
3and NO
2. From the
observation of optical emission spectra, a large amount of N
2and NO were excited, suggesting the NO + N
2→ N
2+ N + O
reaction and then the NO + N
→ N
2+ O reaction play important
roles in the removal of NO and the formation of N
2. In
addi-tion, operating the discharge at a higher power, a lower inlet
molar fraction of O
2or NO, or the addition of steam, could
lead to a higher conversion of NO. NO conversion reached
74.2%, 77.6%, 92.8%, and 97.2% for the NO/N
2/O
2(6%)/H
2O
(10%), NO/N
2/O
2(6%), NO/N
2/O
2(2%), and NO/N
2mixtures,
respectively, at 120 W. The results revealed that the gaseous
com-positions significantly influenced the NO conversion, while they
had only a small effect on the fraction of total N atoms
con-verted from NO into N
2. However, the rf discharge approach is
not practical in this stage due to its lower operating pressure and
lower energy efficiency. The further research for plasma process
operating at atmospheric pressure with a higher flow rate to
con-vert NO mainly into N
2should be proceeded. In addition, the
other atmospheric pressure, lower electron temperature and gas
temperature plasmas are also in developing, such as a
continue-wave microcontinue-wave or low frequency ac plasma for converting NO
mainly into N
2.
Acknowledgement
We would like to thank the National Science Council in
Tai-wan for financially supporting this research work (Grant No.
NSC-92-2211-E-151-002).
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