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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

2

O mixtures are usually converted into HNO

3

and/or NO

2

using different discharge approaches. In this study, a radio-frequency

discharge was successfully used to reduce NO mainly into N

2

at 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

2

O and HNO

2

, but neither HNO

3

nor NO

2

were 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

2

O

(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

2

formation 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

2

and/or HNO

3

, are commonly formed when converting the

NO/N

2

/O

2

/H

2

O mixtures

[6,9–11]

. Although NO

2

and HNO

3

are water soluble and can be removal with scrubbers, to reduce

NO into N

2

seems 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

2

in 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

2

and 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

2

to 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.

(2)

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

NO

were 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

NO

is much higher

with a close value of

I

N2

in the NO/N

2

mixtures than in the

NO/N

2

/O

2

mixtures (

Table 3

). Since O

2

is 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

2

mixtures. Moreover, the addition of oxygen

decreases the number of atomic N due to the destructive process

involving NO and O

2

molecules

[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

2

2nd

pos.

(E

th

= 11.1 eV),

N

2

metastable

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

2

or 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

2

will be dissociated by reacting with e, N or O to yield N

2

,

N

2

O, and NO

[22]

. Hence, almost no NO

2

was 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

2

at 30 W) and 4.2 g NO/kWh

(NO/N

2

/O

2

(6%)/H

2

O (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

2

for NO/N

2

/(O

2

)/(H

2

O) mixtures using the rf discharge

approach. The major product was N

2

, with several to tens ppm

of HNO

2

and N

2

O detected, but no HNO

3

and NO

2

. From the

observation of optical emission spectra, a large amount of N

2

and 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

2

or 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

2

O

(10%), NO/N

2

/O

2

(6%), NO/N

2

/O

2

(2%), and NO/N

2

mixtures,

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

2

should 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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數據

Fig. 4. A typical optical emission spectrum of NO/N 2 /O 2 (6%) mixtures at 120 W and 4 kPa.

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