Ind. Eng. Chem. Res. 2004, 43, 5077-5088
5077
N Atom Radicals and N2(A3∑u+) Found To Be Responsible for Nitrogen Oxides Conversion in Nonthermal Nitrogen Plasma Gui-Bing Zhao, Xudong Hu, Morris D. Argyle, and Maciej Radosz* Department of Chemical & Petroleum Engineering, University of Wyoming, Laramie, Wyoming 82071-3295
All species that are likely to be responsible for nitrogen oxides (N2O, NO, and NO2) conversion in nitrogen plasma are analyzed in detail through carefully designed systematic experiments and theoretical analysis. The effect of ppm-level CO2, CO, and 1% CO on N2O conversion reveals that the N2O conversion occurs mainly by interaction with N2(A3∑u+) excited species. The effect of 1% CO on the NO conversion suggests that only N atom radicals are predominantly involved in NO conversion. NO2 conversion, on the other hand, occurs by interaction with both N2(A3∑u+) and N atom radicals. Therefore, only two active species, N2(A3∑u+) and N atom radicals, are found to be responsible for nitrogen oxides conversion in nitrogen plasma. Introduction Application of short-duration high-voltage pulses to a wire-cylinder reactor producing nonthermal plasma induced by highly nonhomogeneous electric fields (corona discharge processes) has been extensively investigated1-5 and used for conversion of nitrogen oxides (NOx).6-16 When the wire electrode is positively charged, induced plasma channels (positive streamers) propagate from the wire anode to the cylinder cathode, and the discharge pulse itself is then called a positive corona discharge. The energetic electrons in the streamer can excite molecular nitrogen and produce many kinds of chemically active species, including metastable excited states (like N2(A3∑u+)), radicals (like N atoms), and cations (like N2+), depending on the electron energy. These active species may contribute to the conversion of nitrogen oxides. Despite the extensive previous research, substantial uncertainty remains about the mechanism of nitrogen oxides conversion in nonthermal nitrogen plasma. In particular, the contribution of the electronic excited states of N2 to N2O, NO, and NO2 conversion has not been fully recognized. (1) N2O conversion: Three mechanisms for N2O conversion have been proposed. First, N2O conversion may occur through interaction with the electronic excited states of the N radical, N(2D), as suggested by Hill et al.17 via the following reaction:
N(2D) + N2O f N2 + NO k ) 1.32 × 1012 cm3‚mol-1‚s-1 Second, N2O conversion may occur through interaction with the lowest energy electronic excited state of N2, N2(A3∑u+), as suggested by Thomas et al.18 and Golde: 19
3
+
N2(A Σu ) + N2O f 2N2 + O k ) 3.73 × 1012 cm3‚mol-1‚s-1 Third, N2O conversion may occur through interaction * Corresponding author. E-mail:
[email protected]. Tel: 307-766-2500. Fax: 307-766-6777.
with the cation N2+, as suggested by Willis et al.20 and Hu et al.21 because the charge-transfer reaction of N2+ and N2O has a large rate constant:22
N2+ + N2O f N2 + N2O+ k ) 3.61 × 1014 cm3‚mol-1‚s-1 The subsequent electron-ion recombination dissociation reaction has an even larger rate constant:23
N2O+ + e f N2 + O k ) 1.20 × 1017 cm3‚mol-1‚s-1 (2) NO conversion: The conversion mechanism of nitric oxide (NO) in nonthermal plasmas has been more extensively investigated than N2O. Two mechanisms for NO conversion have been presented. Most investigators10,12,13,15,24-27 have proposed that N atom radicals are responsible for NO conversion in a balance gas of nitrogen. By contrast, Fresnet et al.28,29 proposed that an excited electronic state of N2, N2(a′1∑u-), plays the main role in NO conversion kinetics, due to the large rate constant for the reaction of N2(a′1∑u-) and NO:30
N2(a′1Σu-) + NO f N2 + N + O k ) 2.17 × 1014 cm3‚mol-1‚s-1 (3) NO2 conversion: Hu et al.7 proposed that NO2 conversion occurs by reaction with N radicals:
N + NO2 f N2O + O k ) 1.81 × 1012 cm3‚mol-1‚s-1 N + NO2 f NO + NO k ) 1.38 × 1012 cm3‚mol-1‚s-1 A single positive streamer event is an ionization wave which propagates against the direction of the electron drift, with a typical velocity of 107-108 cm/s. The plasma channel of the streamer has a radius of ∼10-1-10-2 cm and contains ∼1014 electrons/cm3,31,32 with the result that the discharge lasts less than 100 ns in a cylindrical reactor that has a diameter on the order of 1 cm, as reported by Hu et al.21 The postdischarge period (the interval period between the discharges, typically, larger
10.1021/ie049795z CCC: $27.50 © 2004 American Chemical Society Published on Web 07/16/2004
5078 Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004 Table 1. Experimental Conditions flow rate (m3/s)
system
N2O Conversion 51.1 ppm N2O + N2 4.87 × 10-4 4.87 × 10-4 52.4 ppm N2O + 0.987% CO + N2 105 ppm N2O + N2 4.87 × 10-4 106 ppm N2O + 1.01% CO + N2 4.87 × 10-4 217 ppm N2O + N2 4.87 × 10-4 215 ppm N2O + 0.98% CO + N2 4.87 × 10-4 275.6 ppm N2O + N2 4.18 × 10-4 309 ppm N2O + 220 ppm CO + N2 4.18 × 10-4 275.6 ppm N2O + 550 ppm CO2 + N2 4.18 × 10-4
189.0 189.0 189.0 189.0 189.0 189.0 140.7 140.7 140.7
NO Conversion 614 ppm NO + N2 600 ppm NO + N2 595 ppm NO + 0.98% CO + N2 614 ppm NO + N2 597 ppm NO + 1.0% CO + N2
4.18 × 10-4 4.18 × 10-4 4.18 × 10-4 4.87 × 10-4 4.87 × 10-4
140.7 140.7 140.7 189.0 189.0
4.18 × 10-4
140.7
Figure 1. Potential energy of excited nitrogen species.
than 1 ms) is much longer and hence should play a critical role for the subsequent plasma chemical reactions. For ppm-level concentrations of nitrogen oxides in nitrogen, electrons mainly collide with the background gas, N2. Figure 1 shows the potential energy curves of N2.33 Presumably, all the active species shown in Figure 1 can be produced in the streamer, if the electrons have enough energy, and any of these species may contribute to NOx conversion. The goal of this work is to distinguish which species in the plasma are responsible for conversion of nitrogen oxides. Experimental Section The experimental apparatus and measurement procedures have been described in detail previously.7,21 In brief overview, the ten-tube pulsed corona discharge reactor (PCDR) consists of a high-voltage power supply, control unit, and the pulser/reactor assembly. The highvoltage supply controls the pulsed power delivered to the reactor. The pulser/reactor assembly contains the pulsed power generator and the pulsed corona discharge reaction chambers. The reactor has UV-grade quartz windows for diagnostics and plasma observation. In all of the experiments described in this work, only four of the ten tubes were wired for plasma generation. The corona power is calculated as the product of the pulse voltage (V) and current (I); the energy is the time integral of power (∫VI dt). The power consumed can also be calculated as the product of the input energy per pulse and the pulse frequency. The system design permits variation and measurement of the applied voltage and its frequency, reactor current and voltage, and discharge power and energy. The experimental conditions are listed in Table 1. The feed mixture, maintained at ambient temperature, around 300 K, was fed through the PCDR at two flow rates, 4.78 × 10-4 m3/s at 189.0 kPa and 4.11 × 10-4 m3/s at 140.7 kPa. Gas samples, collected from the discharge end of the PCDR in small stainless steel cylinders, were analyzed for stable species with a Spectrum 2000 Perkin-Elmer Fourier transform infrared (FTIR) spectrometer with a narrow band mercury cadmium telluride (MCT) detector, as reported previously.21 The measurement accuracy is (10%. All experimental data were reproducible within this error limit.
reactor pressure (kPa)
495 ppm NO2 + N2
NO2 Conversion
Corona-induced emission (optical emission) for the pure nitrogen was observed through the quartz window and was imaged onto the entrance slit of a 0.3-m focal length monochromator (SpctraPro-300i model). In our experiments, a monochromator grating blazed to select photons in the wavelength of 200-900 nm was used. An intensified charge coupled device (ICCD) camera, mounted onto the spectrometer, was used to measure the luminescence. Results and Discussion Active Species in N2 Plasma. As shown in Figure 1, only three electronic states of N atoms (4S, 2D, and 2P) are energetically accessible below the ionization energy of the N2 molecule. Investigations by Cosby34 and Walter et al.35 proved that dissociation of N2 to form N(2D) + N(4S) is the dominant dissociation mechanism, which suggests that N(2P) does not play a major role in NOx conversion. The actual electron energy in the pulsed corona discharge places limits on which active species may be produced by direct electron collision. The average electron energy in the pulsed corona discharge reactor is e10 eV24,36 and the peak energy is e20 eV,37 which implies that cations such as N2+ and their excited states cannot be produced in significant numbers by direct electron collision because the ionization energy of N2 is 15.6 eV, as shown in Figure 1. However, the ground state of N2+ may be populated by stepwise ionization processes, as discussed by Guerra et al.:38
N2 + e f e + N2(A3Σu+) N2(A3Σu+) + e f e + e + N2+ or by association ionization processes, as presented by Brunet and Rocca-Serra:39
N2(A3Σu+) + N2(a′1Σu-) f N2 + N2+ + e k ) 3.01 × 1013 cm3‚mol-1‚s-1 N2(a′1Σu-) + N2(a′1Σu-) f N2 + N2+ + e k ) 1.20 × 1014 cm3‚mol-1‚s-1 Therefore, the possible chemically active species formed
Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004 5079 Table 2. Rate Constant of Active Species Consumption by Radiation, Quenching, and NOx Conversion radiation
quenching
NOx reaction
excited state
kI (s-1)
reference
kq (cm3‚mol-1‚s-1)
reference
kr (cm3‚mol-1‚s-1)
N2
A3∑u+ B3Πg B′3∑ua′1∑ua1Πg W1∆u C3Πu E3∑g+
0.526 2 × 105 2.60 × 104 43.5 1.80 × 104 6.50 × 102 2.73 × 107 5.26 × 103
Lofthus and Krupenie33 Piper63 Lofthus and Krupenie33 Piper56 Marinelli et al.64 Fraser et al.65 Lofthus and Krupenie33 Lofthus and Krupenie33
1.81 × 106 1.81 × 1013 1.81 × 1013 1.14 × 1011 1.32 × 1013 6.02 × 1012 6.02 × 1012 6.02 × 1012
Kossyi et al.30 Piper59 Fresnet29 Piper56 Marinelli et al.60 Sa and Loureiro61 Simek et al.62 Simek et al.36
7.83 × 1012 2.41 × 1014 2.41 × 1014 2.17 × 1014 2.17 × 1014 2.17 × 1014 [-] [-]
Herron54 Young et al.55 Fresnet29 Piper56 Cenian et al.57 Fresnet29 [-] [-]
N2+
X
[-]
3.61 × 1013
Anicih22
Herron54 [-]
3.61 × 1013 1.87 × 1013
Herron54 Atkinson et al.58
molecule
2D
N
X
0 1.07 × 10-5 0
[-]
0 1.02 × 1010 0
Zipf66 [-]
reference
Table 3. Initial Selectivity of Active Species Consumption by Radiation, Quenching, and NOx Conversion molecule N2
N2+ N
active species A3∑u+ B3Πg B′3∑ua′1∑ua1Πg W1∆u C3Πu E3∑g+ X 2D X (4S)
kI (s-1)
kqCN2 (s-1)
krCNOx (s-1)
SI (%)
Sq (%)
Sr (%)
0.526 2 × 105 2.60 × 104 43.5 1.80 × 104 6.50 × 102 2.73 × 107 5.26 × 103 0 1.07 × 10-5 0
73.5 7.35 × 108 7.35 × 108 4.63 × 106 5.36 × 108 2.44 × 108 2.44 × 108 2.44 × 108 0 4.14 × 105 0
3.18 × 105 9.78 × 106 9.78 × 106 8.81 × 106 8.81 × 106 8.81 × 106 [-] [-] 1.47 × 107 1.47 × 106 7.59 × 105
∼0 0.027 ∼0 ∼0 ∼0 ∼0 10.1 ∼0 0 ∼0 0
∼0 98.7 98.7 34.4 98.4 96.5 89.9 100 0 22.0 0
∼100 1.3 1.3 65.6 1.6 3.5 0 0 100 78.0 100
from electron collision reactions in the pulse-on period are N2(A3∑u+), N2(B3Πg), N2(B′3∑u-), N2(a′1∑u-), N2(a1Πg), N2(W1∆u), N2(C3Πu), N2(E3∑g+), N2+, N(4S), and N(2D). They can be consumed by three parallel processes in the postdischarge period in the presence of NOx:
(1) Natural radiation accompanying optical emission: A* f A + hν
RI ) kI‚CA*
(2) Quenching by the background gas (N2): A* + N2 f products
Rq ) kq‚CA*‚CN2
(3) Reaction with NOx (conversion of NOx): A* + NOx f products
Rr ) kr‚CA*‚CNOx
In these chemical equations, A* represents any active species; kI, kq, and kr are the rate constants of radiation, quenching, and NOx conversion, respectively, and R is the reaction rate. The initial selectivity of these three parallel processes can be defined as
SI )
RI kI ) × 100% RI + Rq + Rr kI + kqCN2 + krCNOx (1a)
Sq )
kqCN2 Rq ) × 100% RI + Rq + Rr kI + kqCN2 + krCNOx (1b)
Sr )
krCNOx Rr ) × 100% RI + Rq + Rr kI + kqCN2 + krCNOx (1c)
where SI, Sq, and Sr are the initial selectivities to optical
Figure 2. Optical emission spectra in pure nitrogen.
emission reactions, quenching reactions, and NOx conversion reactions, respectively. Only the active species which contribute predominantly to NOx conversion (high Sr) need to be considered. Table 2 shows the rate constants for consumption of active species (A*) by radiation, quenching, and NOx conversion. For kr, the largest rate constant among the reactions of A* with NO, NO2, or N2O is shown in Table 2. For flue gas from power plants, the NOx content is less than 1000 ppm,40 with typical values of 200 ppm.41-43 Assuming the NOx content is 1000 ppm, concentrations of N2 and NOx at 1 atm and 300 K are 4.06 × 10-5 and 4.06 × 10-8 mol/cm3, respectively. With use of these concentrations in eqs 1a-1c, the initial selectivity for the three parallel processes is calculated and presented in Table 3. Most active species in Table 3 are consumed by quenching with the background N2 gas and by natural radiation, except for N2(A3∑u+), N2(a′1∑u-), N2+, N(4S), and N(2D), which are predicted to be strongly consumed through NOx conversion. The results of Table 3 also show that only two active species, N2(B3Πg) and N2(C3Πu), should contribute to optical emission. This prediction is confirmed in Figure 2, which shows the optical emission spectra of pure nitrogen.
5080 Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004 Table 4. Reactions and Rate Constants Relevant to NOx Conversion in Nitrogen Plasma chemical reaction N2 + e f N2(A3∑u+) + e N2 + e f N2(a′1∑u-) + e N2 + e f N(2D) + N(4S) + e N2(A3∑u+) + e f e + e + N2+ N2(A3∑u+) + N2(a′1∑u-) f N2 + N2+ + e N2(a′1∑u-) + N2(a′1∑u-) f N2 + N2+ + e
rate constant (cm3‚mol-1‚s-1)
3.01 × 1013 1.20 × 1014
source
no.
Brunet and Rocca-Serra39 Fresnet et al.29 Cosby34 Guerra et al.38 Brunet and Rocca-Serra39 Brunet and Rocca-Serra39
R1 R2 R3 R4 R5 R6
N2(A3∑u+) + N2 f N2 + N2 N2(A3∑u+) + O2 f N2 + 2O N2(A3∑u+) + O2 f N2O + O N2(A3∑u+) + O2 f N2 + O2 N2(A3∑u+) + O f N2 + O N2(A3∑u+) + N f N2 + N N2(A3∑u+) + NO f N2 + NO N2(A3∑u+) + N2O f 2N2 + O N2(A3∑u+) + NO2 f N2 + NO + O N2(A3∑u+) + CO f N2 + CO N2(A3∑u+) + CO2 f products
N2(A3∑u+) Reactions 1.81 × 106 1.51 × 1012 4.70 × 1010 7.77 × 1011 1.81 × 1013 2.71 × 1013 3.31 × 1013 3.73 × 1012 7.83 × 1012 9.63 × 1011 1.20 × 1010
Kossyi et al.30 Herron and Green67 Kossyi et al.30 Kossyi et al.30 Herron and Green67 Herron and Green67 Herron and Green67 Herron and Green67 Herron and Green67 Herron54 Herron54
R7 R8 R9 R10 R11 R12 R13 R14 R15 R16 R17
N2(a′1∑u-) + N2 f products N2(a′1∑u-) + O2 f products N2(a′1∑u-) + NO f N2 + N + O N2(a′1∑u-) + N2O f products N2(a′1∑u-) + CO f products N2(a′1∑u-) + CO2 f products
N2(a′1∑u-) Reactions 1.14 × 1011 1.69 × 1013 2.17 × 1014 1.02 × 1014 6.62 × 1013 1.51 × 1013
Piper56 Piper56 Kossyi et al. 30 Piper56 Piper56 Piper56
R18 R19 R20 R21 R22 R23
N(2D) + N2 f N + N2 N(2D) + O2 f NO + O N(2D) + O f N + O N(2D) + NO f N2 + O N(2D) + N2O f N2 + NO N(2D) + CO f products N(2D) + CO2 f NO + CO
N(2D) Reactions 1.32 × 1010 3.01 × 1012 8.43 × 1011 3.61 × 1013 1.32 × 1012 1.14 × 1012 2.17 × 1011
Herron and Green67 Herron and Green67 Herron54 Herron54 Herron54 Herron54 Herron54
R24 R25 R26 R27 R28 R29 R30
N + NO f N2 + O N + NO2 f N2O + O N + NO2 f 2NO N + NO2 f N2 + O2 N + NO2 f N2 + 2O N + O2 f NO + O N + O + M f NO + M (M ) N2, O2) N + N + M f N2 + M (M ) N2, O2)
N(4S) Reactions 1.87 × 1013 1.81 × 1012 1.38 × 1012 4.21 × 1011 5.48 × 1011 5.78 × 107 3.68 × 1015 [M] 1.59 × 1015 [M]
Atkinson et al.58 Atkinson et al.58 Kossyi et al.30 Kossyi et al.30 Kossyi et al.30 Atkinson et al.58 Kossyi et al.30 Kossyi et al.30
R31 R32 R33 R34 R35 R36 R37 R38
Sieck et al.37 Anicich22 Anicich22 Anicich22 Anicich22 Anicich22 Anicich22
R39 R40 R41 R42 R43 R44 R45
N2+ + NO f N2 + NO+ N2+ + N2O f N2 + N2O+ N2+ + CO f N2 + CO+ N2+ + CO2 f N2 + CO2+ N2+ + O2 f N2 + O2+ N2+ + O f N + NO+ N2+ + N f N2 + N+
N2+ Reactions 2.35 × 1014 3.61 × 1014 4.39 × 1013 4.82 × 1014 3.01 × 1013 8.43 × 1013
