on Nonthermal-Plasma Reactions of Nitrogen Oxides in N

Gui-Bing Zhao, Xudong Hu, Morris D. Argyle, and Maciej Radosz*. Department of Chemical & Petroleum Engineering, University of Wyoming, Laramie, ...
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Ind. Eng. Chem. Res. 2005, 44, 3935-3946

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Effect of CO2 on Nonthermal-Plasma Reactions of Nitrogen Oxides in N2. 2. Percent-Level Concentrations Gui-Bing Zhao, Xudong Hu, Morris D. Argyle, and Maciej Radosz* Department of Chemical & Petroleum Engineering, University of Wyoming, Laramie, Wyoming 82071-3295

Both NOx conversion and CO2 conversion decrease with increasing percent-level CO2 concentration in nonthermal nitrogen plasma. The rate constants of electron collision reactions of both N2 and CO2 decrease with increasing CO2 concentration because electronegative CO2 reduces electron concentrations in the reactor due to the electron attachment process. The rate constant of CO2 dissociation through electron collision is 1-2 orders of magnitude higher than that of N2 dissociation because of low dissociation energy of CO2. Model data for reactor outlet NOx and COx concentrations agree well with experimental data. The effect of CO2 on NO, NO2, N2O, and CO concentrations can be explained on the basis of the proposed reaction mechanism and kinetic modeling. Introduction The removal of nitrogen oxides (NOx) from combustion exhaust streams has become an important international technology issue because of the key role NOx play in many global environmental problems, such as acid rain, photochemical smog formation, and the greenhouse effect. Pulsed corona discharge reactors (PCDR) are actively studied for conversion of pollutants such as NOx from the combustion of fossil fuels. Previous investigations have shown that NO is readily converted to benign products (N2 and O2, with 98.5% NOx conversion) in nonthermal N2 plasma1-6 and energy efficiency can be increased by configuring multiple reactors in-series7 with multiple parallel tubes in each reactor,8 and operating at low reactor pressure.9 However, experiments directly applying pulsed corona discharge technology to exhaust streams from an industrial incinerator and diesel engine showed that only 70% NOx conversion was reached,10,11 which is insufficient to meet increasingly stringent worldwide emission standards. Gas analysis from flue gas and diesel engine exhaust stream showed that real combustion exhaust gas contains up to 1000 ppm NO, saturated moisture, 6-14% O2, 5-8% CO2, and the balance N2.11,12 The effect of such gas components on NOx conversion must therefore be investigated to establish detailed mechanisms for NOx conversion in their presence. Previously, we investigated the effect of 2-14% O2 on NOx conversion13 and the effect of ppm-level CO2 concentration on NOx conversion.14 We found that 600 ppm CO2 addition does not affect NOx conversion. The effect of CO2 on NO conversion in different background gases has also been investigated by other research groups. Luo et al.15 found that the NO conversion decreased when 5.5% CO2 was added to NO/He gas mixtures. Kanazawa et al.16 investigated NOx conversion from simulated flue gases (NO/O2/ CO2/N2) and found that 10% CO2 caused a lower NO removal efficiency. Gentile and Kushner17 and Chang et al.18 drew similar conclusions by investigating NO conversion in a mixture of NO/O2/H2O/CO2/N2. Aritoshi * To whom correspondence should be addressed. Tel: 307766-2500. Fax: 307-766-6777. E-mail: [email protected].

et al.19 investigated the effect of percent-level CO2 concentration on the NOx conversion and found that the NOx reducing ability is markedly decreased with a 10% CO2 concentration. Yan et al.,20 Gasparik et al.,21 and Hensel et al.22 investigated the effect of CO2 added to NO/O2/N2 gas mixtures and found that NO removal was promoted because the type of discharge changed from glow to streamer as the CO2 concentration increased. Under streamer discharge conditions, the consistent conclusion is that CO2 inhibits NOx conversion. Despite the previous studies on the effect of CO2 on NO conversion, many issues are still unclear. First, the effect of CO2 on NO conversion cannot be clearly examined when O2 is present. Most investigations16-18,20-22 on the effect of CO2 on NOx conversion were conducted in the presence of O2. Both O2 and CO2 are electronegative gases,18,23 but O2 has a lower dissociation energy (5.16 eV/O2) than CO2 (5.51 eV/CO2).24 When O2 is present, the relative influence of O2 and CO2 on NOx conversion is difficult to distinguish because oxygen is also a strong inhibitor of NOx conversion (see ref 13 and references therein). Second, the effect of percent-level CO2 on the rates of electron collision reactions is not established. Kanazawa et al.16 found that the existence of CO2 reduced the discharge current because of the electron attachment process associated with the large electron affinity of this electronegative species. Presumably, the presence of CO2 could decrease the rate of electron collision reactions in the pulsed corona reactor. However, there are no studies to explore this issue. The third issue is the CO2 dissociation mechanism. Gentile and Kushner17 proposed that two sources are responsible for CO2 dissociation. One is the reaction with N radicals (N(4S) + CO2 f NO + CO), while the other is direct electron collision reaction (e + CO2 f CO + O). However, Aritoshi et al.19 proposed that only radical species of nitrogen, such as N(4S) and N(2D), and the excited electronic states of molecular N2, including N2(a′1∑u-) and N2(C3Πu), are responsible for CO2 dissociation, but they did not consider the direct electron collision dissociation of CO2. Recently, Zhao et al.14 found that about 18% of formed NO(A), which is the first excited state of NO, contributes to CO2 conversion. The

10.1021/ie048905z CCC: $30.25 © 2005 American Chemical Society Published on Web 04/21/2005

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Table 1. Experiment Matrix at 217 kPa system 574 ppm NO + 33.8 ppm NO2 + 600 ppm CO2 + N2 602 ppm NO + 0.987% CO2 + N2 604 ppm NO + 2.91% CO2 + N2 606 ppm NO + 4.83% CO2 + N2 580 ppm NO + 8.03% CO2 + N2

flow rate (m3‚s-1)

residence time (s)

1.64 × 10-4

23.0

1.73 × 10-4 2.58 × 10-4 1.64 × 10-4 2.34 × 10-4 1.77 × 10-4 2.59 × 10-4 1.67 × 10-4 2.50 × 10-4

21.5 14.4 22.8 15.9 21.1 14.4 22.4 14.9

fourth issue is the formation and conversion of byproducts, especially, CO and N2O. The goal of this work is to investigate the effect of percent-level CO2 concentrations (about 1-8%) on NOx conversion in nonthermal N2 plasma by both experiment and modeling, to explore the effect of CO2 on the rate of electron collision reactions, and to propose reaction mechanisms of NOx and CO2 conversion and byproduct formation in CO2/N2 plasma mixtures. Experimental Section The experimental setup has been discussed in detail previously.13,14 The experimental matrix is shown in Table 1. The reactant gases are 574 ppm NO and 33.8 ppm NO2 with 600 ppm CO2 in ultra high purity (UHP) N2, 602 ppm NO with 0.987% CO2 in UHP N2, 604 ppm NO with 2.91% CO2 in UHP N2, 606 ppm NO with 4.83% CO2 in UHP N2, and 580 ppm NO with 8.03% CO2 in UHP N2 (US Airgas). The test gas mixture is introduced into the PCDR at ambient temperature, around 300 K, and at different flow rates (1.64 × 10-4 to 2.59 × 10-4 m3‚s-1 at reactor inlet conditions). The pressure in the PCDR is maintained at 217 kPa by control valves on the outlet gas lines. The concentration of O2 and CO2 at the outlet of the PCDR is analyzed using a Hewlett-Packard 5890 series II Gas Chromatograph (GC) with a thermal conductivity detector (TCD) and an Alltech CTR I column (outer tube with 6 ft × 1/4 in. packing of activated molecular sieve and inner tube with 6 ft × 1/8 in. packing of porous polymer mixture). Relative errors for duplicate GC analyses are (5%. The outlet gas is collected in 300 mL stainless steel cylinders and analyzed for stable nitrogen oxides and CO using a Spectrum 2000 Perkin-Elmer Fourier transform infrared spectrometer (FTIR) with a narrow-band mercury cadmium telluride (MCT) detector. As found before,1 FTIR peaks of N2O occur in two regions. One region between 1245 and 1320 cm-1 is much less sensitive to N2O concentration than the region between 2175 and 2260 cm-1.1 The N2O band between 2175 and 2260 contains two subsections, one located between 2175 and 2223.8 cm-1, which overlaps the second CO band, while the other is between 2223.8 and 2260 cm-1. At ppm-level CO2 concentrations, the CO2 FTIR peaks do not overlap the N2O band between 2223.8 and 2260 cm-1.14 However, at percentlevel CO2 concentrations, the CO2 FTIR peaks extend to the N2O band between 2223.8 and 2260 cm-1 as shown in Figure 1 for a typical reactor outlet for 602 ppm NO and 0.987% CO2 in N2 with and without electrical discharge. Therefore, only the N2O band between 1245 and 1320 cm-1 can be used to quantify N2O concentration when high CO2 concentrations are

Figure 1. FTIR spectra showing the product distribution for 0.987% CO2 + 602 ppm NO in N2 at the flow rate of 1.73 × 10-4 m3‚s-1: (a) no discharge (0 Hz); (b) electrical discharge on (300 Hz).

present. However, due to the low signal from the N2O band between 1245 and 1320 cm-1, N2O FTIR measurement accuracy decreases from the previously reported (10%1,14 to (30% in this work. The FTIR accuracy for the other species, such as NO, NO2, and CO, does not change and remains (10%.1,14 The PCDR described previously14 is used in this work. The energy delivered to the reactor per pulse is calculated from 1/2CVc2,9,13 where C is the pulse-forming capacitance (800 pF) and Vc is the constant charge voltage (19.0 ( 1 kV) in the pulse-forming capacitance before discharge. The power consumed, W (J‚s-1), is calculated as the product of the input energy per pulse and the pulse frequency. The specific energy input, Es (kJ‚m-3), is defined as

Es )

W 1000‚u

(1)

where u is the gas flow rate (m3‚s-1). The total NOx conversion is calculated instead of NO conversion because total NOx conversion reflects the degree of direct decomposition of NOx into N2 and O2. NOx and CO2 conversion are defined as follows:

XNOx )

Ci,NO + Ci,NO2 - Co,NO - Co,NO2 - 2 × Co,N2O Ci,NO + Ci,NO2

×

100% (2) XCO2 )

Ci,CO2 - Co,CO2 Ci,CO2

× 100%

(3)

where Ci is the concentration of the given species at the reactor inlet (ppm), Co is the concentration at the reactor outlet (ppm), XNOx is the total conversion of NOx to N2, and XCO2 is the conversion of CO2 to CO. The pulsed corona discharge reactor described above is estimated using a lumped kinetic model that describes the evolution of all species, reported elsewhere.2 Results and Discussion Experimental Observations. Figure 1a shows the FTIR spectrum of the reaction mixture (602 ppm NO with 0.987% CO2 in N2) with no discharge, while Figure 1b shows the FTIR spectrum of the reaction mixture

Ind. Eng. Chem. Res., Vol. 44, No. 11, 2005 3937

Figure 2. (a) NOx conversion and (b) CO2 conversion as a function of specific energy 9: 574 ppm NO + 33.8 ppm NO2 + 600 ppm CO2 in N2 at the flow rate of 1.64 × 10-4 m3‚s-1; O: 602 ppm NO + 0.987% CO2 in N2 at the flow rate of 1.73 × 10-4 m3‚s-1; 4: 604 ppm NO + 2.91% CO2 in N2 at the flow rate of 1.64 × 10-4 m3‚s-1; 3: 606 ppm NO + 4.83% CO2 in N2 at the flow rate of 1.77 × 10-4 m3‚s-1; ×: 580 ppm NO + 8.03% CO2 in N2 at the flow rate of 1.67 × 10-4 m3‚s-1.

with discharge, both at a flow rate of 1.73 × 10-4 m3‚s-1. CO is detected at a pulse frequency of 300 Hz (Figure 1b), which indicates that CO2 is dissociated. As observed during NOx conversion in N2 with low CO2 concentration,14 NO, NO2, and N2O are the only NOx species detected. The same products are detected in all experiments shown in Table 1. Figures 2a and 2b show the NOx conversion and CO2 conversion, respectively, as a function of specific energy input at different CO2 concentrations. With 600 ppm CO2 addition to 574 ppm NO and 33.8 ppm NO2 in N2, the NOx conversion initially increases linearly with increasing specific energy input before reaching a constant value at 98.5 ( 0.5% (filled squares in Figure 2), as observed during NOx conversion without CO2 addition.8 CO2 conversion initially increases linearly with increasing specific energy input and then starts to decrease slowly for specific energy inputs of about 250 kJ‚m-3 and higher, which is the point of complete NO conversion. This trend of CO2 conversion with ppmlevel CO2 addition has been previously explained on the basis of both modeling and experiment.14 With 0.987% CO2 addition to 602 ppm NO in N2, the NOx conversion initially increases with increasing specific energy input until reaching a constant value at 90 ( 2% (open circles in Figure 2), which is less than the constant value of NOx conversion observed with 600 ppm CO2 addition. Similar to 600 ppm CO2,14 only N2O is detected once NOx conversion reaches a constant value (90 ( 2%) for 0.987% CO2. However, for CO2 concentrations of 0.987% and higher, CO2 conversion continuously increases with increasing specific energy input and does not display the maximum previously observed for 600 ppm CO2 when NOx conversion reaches a constant value.14 With further increases in CO2 concentration, both NOx conversion and CO2 conversion decrease at the same specific energy input, as shown in Figure 2. Electron Collision Reactions. The collisions of electrons initiated by the electrical discharge with gas molecules produce chemically active species, such as radicals, excited electronic states and ions, which contribute to NOx and COx (CO and CO2) formation and conversion.25 A selectivity analysis1 on the basis of the inlet concentrations is used to identify the chemically active species produced by electron collision reactions that are important in NOx and COx formation and conversion. For this analysis, electron interactions with NOx are not considered because NOx concentrations are always low (less than 1000 ppm).2,25 Previous investi-

gations1,26-28 have shown that most charged particles, such as N2+ and CO2+, are neutralized within 50 ns,14 which is much shorter than the time interval during which the bulk reactions of interest occur (on the order of µs),5 indicating that reactions with ions do not contribute to CO2 dissociation and NOx conversion. Therefore, reactions with ions are not considered in this analysis. As reported recently,1 the possible chemically active species formed by electron collisions with nitrogen include N2(A3∑u+), N2(B3∏g), N2(B′3∑u-), N2(a′1∑u-), N2(a1∏g), N2(W1∆u), N2(C3∏u), N2(E3∑g+), N(4S), and N(2D). The selectivity analysis of systematic experiments performed in the absence of CO2 showed that the active species which play an important role in NOx conversion are N2(A3∑ u+) and N(4S).1 Other active species are mainly quenched to the ground state by the nitrogen background gas. However, at percent-level CO2 concentrations, the situation may be different because active species contribute to CO2 dissociation and NO formation through the following reactions (at 300 K):29-34

N2(A3Σu+) + CO2 f N2 + CO + O k ) 1.20 × 1010 cm3‚mol-1‚s-1 N2(B3Πg) + CO2 f N2 + CO + O k ) 1.20 × 1014 cm3‚mol-1‚s-1 N2(C3Πu) + CO2 f N2 + CO + O k ) 6.38 × 1012 cm3‚mol-1‚s-1 N2(a′1Σu-) + CO2 f N2 + CO + O k ) 1.51 × 1013 cm3‚mol-1‚s-1 N2(a1Πg) + CO2 f N2 + CO + O k ) 7.83 × 1013 cm3‚mol-1‚s-1 N(2D) + CO2 f NO + CO k ) 2.17 × 1011 cm3‚mol-1‚s-1 N(4S) + CO2 f NO + CO k ) 6.62 × 106 cm3‚mol-1‚s-1 Therefore, the chemically active species produced by electron collision reactions with nitrogen may be consumed by four parallel processes: (1) natural radiation accompanying optical emission, (2) quenching by the N2 background gas, (3) dissociative reaction with CO2, and (4) reaction with NOx (conversion of NOx). As reported recently,14 the selectivity of these four parallel processes can be defined as

SI )

Sq )

RI ) RI + Rq + Rd + Rr kI × 100% (4a) kI + kqCN2 + kdCCO2 + krCNOx Rq ) RI + Rq + Rd + Rr kqCN2 kI + kqCN2 + kdCCO2 + krCNOx

× 100% (4b)

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Rd Sd ) ) RI + Rq + Rd + Rr kqCCO2 kI + kqCN2 + kdCCO2 + krCNOx Sr )

Rr ) RI + Rq + Rd + Rr krCNOx kI + kqCN2 + kdCCO2 + krCNOx

× 100% (4c)

× 100% (4d)

where SI, Sq, Sd, and Sr are the selectivities of the radiative emission reactions, quenching reactions, CO2 dissociation, and NOx conversion reactions, respectively; Ri is the reaction rate of reaction type i; kI, kq, kd, and kr are the rate constants of radiation, quenching, CO2 dissociation, and NOx conversion, respectively; and C is the initial mole concentration. Such a selectivity analysis is an effective method to examine the significance of the active species. Only the active species that significantly contribute to formation and conversion of NOx and COx need to be considered. In this work, the highest initial NOx concentration in the presence of CO2 is about 600 ppm, while the CO2 concentration is varied from 0.987% (mol/mol) to 8.03% in balance N2. At 217 kPa and 300 K, the concentrations of 600 ppm NOx, 0.987% CO2, and balance N2 are 5.21 × 10-8 mol/cm3, 8.59 × 10-6 mol/cm3, and 8.61 × 10-5 mol/cm3, respectively. Substituting these concentrations and the rate constants for consumption of active species by radiation, quenching, NOx conversion (summarized in our previous work1), and CO2 dissociation in eqs 4a4d yields the selectivities for the four parallel processes presented in Table 2. These results indicate that all of the electronic excited states of molecular nitrogen except N2(A3∑u+) and N2(a′1∑u-) are predominantly quenched, if they are formed in nonthermal plasma. N2(A3∑u+) contributes predominantly to NOx conversion and N2(a′1∑u-) contributes almost evenly to quenching, CO2 dissociation, and NOx conversion. However, our previous work has shown that N2(a′1∑u-) does not contribute to NOx conversion in nonthermal N2 plasma.1 We infer that N2(a′1∑u-) is not formed in the nonthermal plasma, and hence it does not contribute to CO2 conversion. Among N radicals, Table 2 also shows that N(2D) contributes predominantly to quenching and NOx conversion reactions and N(4S) contributes predominantly to NOx conversion reactions. Neither N(2D) nor N(4S) contribute to CO2 dissociation. CO2 can be converted by three mechanisms: (1) reaction with NO(A), the first excited state of NO, as found previously;14 (2) reaction with the active species of nitrogen produced from electron collision reaction with N2, or (3) direct electron collision dissociation of CO2. The results in Table 2 show that the active species of nitrogen do not contribute to CO2 dissociation. The results in Figure 2b show that CO2 conversion continuously increases with increasing specific energy input even if NO has been completely converted (i.e., no NO(A) is present). Therefore, direct electron collision dissociation of CO2 must occur when 0.987% CO2 is present in nonthermal N2 plasma. Table 3 shows selectivities for the four parallel processes at the initial concentration of 600 ppm NO with 8.03% CO2 in N2. All of the electronic excited states of molecular nitrogen except N2(A3∑u+) contribute to

quenching and CO2 dissociation. N2(A3∑u+) contributes predominantly to NOx conversion. At this high CO2 concentration, N(2D) contributes to CO2 dissociation and NOx conversion, while N(4S) contributes to NOx conversion reactions. Based on the data in Tables 2 and 3, the contribution of active species of nitrogen to CO2 dissociation increases with increasing CO2 concentration. Similar to electron collisions with nitrogen, many active species, including excited electronic states and dissociation products, may be produced when electrons collide with molecular CO2. Although the CO2 molecule has many excited electronic states (3Σu+, 3∆u, 3Σu-, 1Σu-, 1∆ ,, 1Σ +, 3Π , 1Π ),35-39 all of them have a large u u u u excitation energy compared to dissociation energy of CO2 (for example, the lowest excited state of CO2, 3Σu+, has an excitation energy of about 7.35-8.65 eV,35,37 which is 50% larger than dissociation energy of 5.51 eV24). Therefore, all of the excited electronic states of CO2 tend to dissociate, which is supported by investigations of Polak et al.40 and Wu and Johnson.41 Polak et al.40 found that the natural lifetime of excited states of CO2 is less than 1 µs because of efficient dissociation of these states. Wu and Johnson41 found that lifetime of the excited states 1∆u and 1Πu is about 0.3 ps. The strongest absorption band of the CO2 spectra, 1Σu, was not observed in their experiments because the dissociation rate of this state is too fast. When Itikawa42 reviewed the cross section data of electron collisions with CO2, his consistent conclusion was that the CO2 molecule typically dissociates into neutral fragments once it is excited. Therefore, we infer that the excited states of CO2 do not contribute to NOx conversion and instead contribute to CO2 dissociation. Mechanism and Kinetics. The previous discussion shows that electron collision reactions with CO2 occur even at the lowest CO2 concentration examined in this study (0.987%) and predominantly result in CO2 dissociation. Therefore, only the following electron collision reaction is considered for molecular CO2:

e + CO2 f CO + O + e

(R1)

As the earlier discussion on electron collision reactions with nitrogen has shown (Tables 2 and 3), N2(B3∏g), N2(a′1∑u-), N2(a1∏g), and N(2D) can contribute to CO2 dissociation at high CO2 concentration. N2(A3∑u+), N(2D), and N(4S) contribute predominantly to NOx conversion. Therefore, all electron collision reactions1 resulting in the formation of these active species are considered:

e + N2 f N(4S) + N(4S) + e

(E1)

e + N2 f N(2D) + N(2D) + e

(E2)

e + N2 f N2(A3Σu+) + e

(E3)

e + N2 f N2(B3Πg) + e

(E4)

e + N2 f N2(a′1Σu-) + e

(E5)

e + N2 f N2(a1Πg) + e

(E6)

The rate constant of an electron collision reaction is a function of electron energy distribution and the cross sections of electronic excitation and molecular dissociation.43 However, the electron energy distribution in the

Ind. Eng. Chem. Res., Vol. 44, No. 11, 2005 3939 Table 2. Selectivity of Consumption of Nitrogen Active Species by Radiation, Quenching, CO2 Dissociation, and NOx Conversion at 600 ppm NOx + 0.987% CO2 in N2 kI (s-1)

active species N2(A3∑u+) N2(B3∏g) N2(B′3∑u-) N2(a′1∑u-) N2(a1∏g) N2(W1∆u) N2(C3∏u) N2(E3∑g+) N(2D)

0.526 2 × 105 2.60 × 104 43.5 1.80 × 104 6.50 × 102 2.73 × 107 5.26 × 103 1.07 × 10-5 0

N(4S)

kqCN2 (s-1)

kdCCO2 (s-1)

krCNOx (s-1)

SI (%)

Sq (%)

Sd (%)

Sr (%)

155 1.55 × 109 1.55 × 109 9.79 × 106 1.13 × 109 5.17 × 108 5.17 × 108 5.17 × 108 8.76 × 105 0

1.03 × 1.03 × 108

2.01 × 106 1.26 × 107 1.26 × 107 1.13 × 107 1.13 × 107 1.13 × 107

∼0 ∼0 ∼0 ∼0 ∼0 ∼0 5.0 ∼0 ∼0 0

∼0 93.1 99.2 28.8 93.5 97.9 94.0 100 29.8 0

∼0.5 6.2

∼99.5 0.7 0.8 33.2 0.9 2.1

104

1.29 × 107 6.71 × 107 5.47 × 106 1.86 × 105 5.67

1.88 × 106 9.74 × 105

38.0 5.6 1.0 6.3 ∼0

63.9 100

Table 3. Selectivity of Consumption of Nitrogen Active Species by Radiation, Quenching, CO2 Dissociation, and NOx Conversion at 600 ppm NOx + 8.03% CO2 in N2 active species

kI (s-1)

kqCN2 (s-1)

kdCCO2 (s-1)

krCNOx (s-1)

SI (%)

Sq (%)

Sd (%)

Sr (%)

N2(A3∑u+) N2(B3∏g) N2(B′3∑u-) N2(a′1∑u-) N2(a1∏g) N2(W1∆u) N2(C3∏u) N2(E3∑g+) N(2D) N(4S)

0.526 2 × 105 2.60 × 104 43.5 1.80 × 104 6.50 × 102 2.73 × 107 5.26 × 103 1.07 × 10-5 0

144 1.44 × 109 1.44 × 109 9.09 × 106 1.05 × 109 4.80 × 108 4.80 × 108 4.80 × 108 8.14 × 105 0

8.38 × 104 8.38 × 108

2.01 × 106 1.26 × 107 1.26 × 107 1.13 × 107 1.13 × 107 1.13 × 107

∼0 ∼0 ∼0 ∼0 ∼0 ∼0 4.9 ∼0 ∼0 0

∼0 62.9 99.1 7.3 65.3 97.7 87.0 100 19.4 0

∼4.0 36.6

∼96.0 0.5 0.9 9.0 0.7 2.3

1.05 × 108 5.46 × 108 4.45 × 107 1.51 × 106 46.1

plasma is complicated and hence not measurable because the electric field is time-dependent and strongly nonuniform due to strong space-charge field effects.44 Further, there are large discrepancies (at least 20-25%) in the reported values of the cross sections of electronic excitation and molecular dissociation.42 Therefore, a calculation for all of these rate constants of electron collision reactions with N2 is not feasible. In our kinetic model,2 two parameters describe the rate constant of each electron collision reaction, R and β, as shown in the following equation:

xRP1 W

k[e] ) β

0.75

(

exp -

RP W

)

(5)

where [e] is the electron concentration, P is the system pressure, and W is the power input.This expression, based on a Maxwellian distribution function for the electron velocity, semiempirically describes the rate of electron collision reactions through a pseudo-first-order rate constant by combining the true rate constant with the electron concentration.2 This implies that 12 model parameters would be needed to describe the six electron collision reactions with N2 (E1-E6), which are too many parameters to produce meaningful results. However, the net effect of electron collision reactions E4-E6 is the dissociation of CO2 because all electronic excited states of molecular nitrogen predominantly contribute to CO2 dissociation or are selectively quenched, as shown in Tables 2 and 3. Thus, a reasonable simplifying assumption is to model the net result of electron collision reactions E4-E6 as the single-electron collision reaction R1. The contribution of all molecular nitrogen electronic excited states to CO2 dissociation is presumably at least 1 order of magnitude lower than that of direct electron collision reactions with CO2 because the dissociation energy per CO2 molecule, 5.51 eV/CO2, is at least 0.5 eV less than the critical electronic excitation energy of all N2 electronic excited states (Figure 1 in ref 1). In addition, N(2D) was found13,45 to have the same yield as N(4S) during electron collision reactions with N2. This equal yield of N(2D) and N(4S) produces

1.88 × 106 9.74 × 105

83.7 34.0 8.1 35.9 ∼0

44.7 100

accurate predictions of NOx conversion in N2 with and without 600 ppm CO2 addition.14 Therefore, both electron collision reactions E1 and E2 can be combined into one electron collision reaction:

e + N2 f N(2D) + N(4S) + e

(R2)

Therefore, only three electron collision reactions (R1-R3 in Table 4) are considered for modeling NOx conversion in N2 with percent-level CO2 concentrations. Numerous series and parallel reactions among active species, N2, CO2, CO, and NOx, are possible following the electron collision reactions. For example, Penetrante et al.3 used 287 reactions to simulate NOx evolution in a very simple system, NO in N2. In the present analysis, a total of 48 reactions (shown in Table 4) are selected to simulate NOx and COx evolution based on an approximate selectivity analysis to determine the controlling reactions assuming that the slowest reaction among series reactions is the controlling step, while the fastest reaction among parallel reactions is the controlling step. These 48 reactions for NOx and COx evolution were analyzed for the NO/N2/CO2 reaction system. For the NO in the N2/CO2 system, six model parameters (Ri and βi) must be determined for the three electron collision reactions (R1-R3). There are 14 components (N(2D), N(4S), N2(A3∑u+), CO2, CO, CO(a), O, NO, NO(A), NO2, N2O, NO3, O2, N2) in this reaction system, as shown in Table 4. Therefore, there are 14 equations for each of the nine power inputs, which leads to a total of 126 equations used to determine the six parameters (Ri and βi, i ) 1-3) by the previously reported optimization method2 for each experimental condition, such as 0.987% CO2 and 602 ppm NO in N2 at a flow rate of 1.73 × 10-4 m3‚s-1. The concentration of N2 at the outlet of the reactor can be obtained using the nitrogen material balance. Figure 3a shows NO, NO2, N2O, O2, CO, and CO2 concentrations calculated for the experiment used in fitting Ri and βi. The calculated curves in Figure 3a reasonably represent the experimental data because these data have been used for fitting. The same Ri and βi values are used without

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Table 4. List of Probable Chemical Reactions of NOx in N2/CO2 chemical reaction

rate constant (cm3‚mol-1‚s-1) β1

k1 )

e + N2 f N(2D) + N(4S) + e

k2 )

e + N2 f N2(A3∑u+) + e

k3 )

N(4S) + NO f N2 + O N(4S) + NO2 f N2O + O N(4S) + NO2 f N2 + O2 N(4S) + NO2 f N2 + 2O N(4S) + NO2 f 2NO N(4S) + O2 f NO + O N(4S) + CO2 f NO + CO N(4S) + N(4S) + N2 f N2 + N2 N(4S) + O + N2 f NO + N2 O + NO f NO2 O + NO2 f NO3 O + NO2 f NO + O2 O + NO3 f NO2 + O2 O + CO + N2 f CO2 + N2 O + CO + CO2 f CO2 + CO2 O + O + N2 f O2 + N2 N2(A3∑u+) + NO f N2 + NO(A) N2(A3∑u+) + NO2 f N2 + NO + O N2(A3∑u+) + N2O f N2 + N2 + O 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(4S) f N2 + N(4S) N2(A3∑u+) + CO f N2 + CO(a) N2(A3∑u+) + CO2 f N2 + CO + O N2(A3∑u+) + N2 f N2 + N2 N(2D) + N2 f N(4S) + N2 N(2D) + O2 f NO + O N(2D) + NO f N2 + O N(2D) + N2O f NO + N2 N(2D) + CO2 f NO + CO CO(a) + N2 f CO + N2 CO(a) + CO2 f 2CO + O NO(A) + NO f NO + NO NO(A) + NO2 f NO + NO2 NO(A) + N2O f NO + N2O NO(A) + O2 f NO + 2O NO(A) + N2 f NO + N2 NO(A) + CO2 f NO + CO + O NO(A) + CO2 f NO + CO2 NO(A) + CO f NO + CO NO(A) f NO NO + NO3 f NO2 + NO2 2NO + O2 f 2NO2

1.87 × 1013 1.81 × 1012 4.21 × 1011 5.48 × 1011 1.38 × 1012 5.91 × 107 6.62 × 106 1.59 × 1015 [N2] 3.68 × 1015 [N2] 2.45 × 1012 2.03 × 1012 5.85 × 1012 1.02 × 1013 3.88 × 1012[N2] 2.25 × 1012[CO2] 1.10 × 1015[N2] 3.85 × 1013 7.83 × 1012 3.73 × 1012 1.51 × 1012 4.70 × 1010 7.77 × 1011 1.81 × 1013 2.71 × 1013 9.63 × 1011 1.20 × 1010 1.81 × 106 1.02 × 1010 3.13 × 1012 3.61 × 1013 1.32 × 1012 2.17 × 1011 5.42 × 1012 1.02 × 1013 1.69 × 1014 3.23 × 1014 2.84 × 1014 9.09 × 1013 4.70 × 1010 3.89 × 1013 1.77 × 1014 2.44 × 1013 4.59 × 106 1.57 × 1013 7.25 × 109

[e] β2 [e] β3 [e]

( ( (

) ) )

R 1P 1 0.75 exp W R1P W R 2P 1 0.75 exp W R2P W R 3P 1 0.75 W exp R3P W

x x x

e + CO2 f CO + O + e

further fitting for predicting the concentrations obtained at the other gas flow rate of 2.58 × 10-4 m3‚s-1. The results are presented in Figure 4a. These results suggest that a single set of Ri and βj values derived from a single set of experimental data can predict the PCDR product compositions obtained at other gas flow rates for the set of 48 reactions shown in Table 4. Similarly, Figures 3b-d show the experimental and correlated data used in fitting Ri and βi for 2.91% CO2 with 604 ppm NO in N2 at a flow rate of 1.64 × 10-4 m3‚s-1, 4.83% CO2 with 606 ppm NO in N2 at a flow rate of 1.77 × 10-4 m3‚s-1, and 8.03% CO2 with 580 ppm NO in N2 at a flow rate of 1.67 × 10-4 m3‚s-1, respectively. Figures 4b-d show the experimental and predicted data at the other gas flow rates using the same Ri and βi values as those obtained from correlations presented in Figures 3b-d, respectively. The good agreement between experimental and predicted data suggests that the reaction mechanism in Table 4

source

no.

this work

R1

this work

R2

this work

R3

Atkinson et al.47 Atkinson et al.47 Kossyi et al.48 Kossyi et al.48 Kossyi et al.48 Fernandez et al.34 Fernandez et al.34 Kossyi et al. 48 Kossyi et al. 48 Atkinson et al.49 Atkinson et al.49 Atkinson et al.49 Atkinson et al.49 Tsang and Hampson50 Slanger et al.51 Kossyi et al.48 Herron29 Herron29 Herron29 Herron and Green52 Kossyi et al.48 Kossyi et al.48 Herron and Green52 Herron and Green52 Herron29 Herron29 Herron29 Herron29 Herron29 Herron29 Herron29 Herron29 Schofield53 Schofield53 Zhang and Crosley54 Paul et al.55 Akagi et al.56 Akagi et al.56 Imajo et al.57 Zhao et al.14 Zhao et al.14 Paul et al.55 Imajo et al.57 Atkinson et al.49 Atkinson et al.49

R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15 R16 R17 R18 R19 R20 R21 R22 R23 R24 R25 R26 R27 R28 R29 R30 R31 R32 R33 R34 R35 R36 R37 R38 R39 R40 R41 R42 R43 R44 R45 R46 R47 R48

realistically reflects NOx evolution and COx evolution in N2/CO2 plasma. The model parameters obtained for different CO2 concentrations for the three electron collision reactions (R1-R3) are shown in Figure 5. For each electron collision reaction, Ri remains constant at about 5.0 and independent of CO2 concentration (Figure 5a). However, βj decays with CO2 concentration as a power function (Figure 5b-d). Applying a least-squares regression analysis to these data, we find that βj is inversely proportional to the mole fraction of CO2 raised to a different power for each electron collision reaction:

For R1: β1 ) 2.00 × 10-5‚xCO2-0.510

(6a)

For R2: β2 ) 1.91 × 10-6‚xCO2-0.0744

(6b)

For R3: β3 ) 1.84 × 10-7‚xCO2-0.608

(6c)

Ind. Eng. Chem. Res., Vol. 44, No. 11, 2005 3941

Figure 3. Experimental and correlated data for varying CO2 content. Experimental data (9): NO; (b): NO2; (2): N2O; (1): CO; ([): O2; (f): CO2. Calculated data (s).

where xCO2 is the mole fraction of CO2. Figure 6a shows the rate constants of electron collision reactions R1-R3 (calculated from eq 5) as a function of CO2 concentration at 60 W power input. The rate constants decrease with increasing CO2 concentration because CO2 is electronegative16,18,23 and hence reduces the discharge current by capturing electrons. This electron

attachment process reduces the electron concentration during discharge, as observed by Kanazawa et al.,16 which results in the decreasing rate constant values observed in Figure 6a. Thus, all model parameters β for electron collision reactions R1-R3 are found to be CO2 concentration dependent because the electron concentration in eq 5 is dependent on CO2 concentration.

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Figure 4. Experimental and predicted data for varying CO2 content. Experimental data (9): NO; (b): NO2; (2): N2O; (1): CO; ([): O2; (f): CO2. Calculated data (s).

Figure 6b shows the ratio of the rate constant of CO2 dissociation caused by electron collision to the sum of the rate constants of N2 dissociation caused by electron collision as a function of CO2 mole fraction at 60 W power input. The total N2 dissociation rate constant is 1-2 orders of magnitude lower than that for CO2 dissociation. Hokazono et al.46 theoretically estimated the rate constant of electron collision dissociation of CO2 and N2 and found similar results for

this ratio, which supports our lumped model analysis. The dissociation rate constant of CO2 is much higher than that of N2 because the dissociation energy of CO2 (5.51 eV) is far lower than that of N2 (9.8 eV), as discussed earlier.24 This rate constant ratio decreases with increasing CO2 mole fraction because β2 is a weak function of CO2 mole fraction, as shown in eq 6b by its exponent, which is negative and close to zero.

Ind. Eng. Chem. Res., Vol. 44, No. 11, 2005 3943

Figure 5. Model parameters as a function of CO2 concentration: (a) Model parameter R for electron collision reactions R1-R3; (b) model parameter β for electron collision reaction R1; (c) R2; (d) R3. Experimental data: (9) R1, (b) R2, (2) R3, (s) regression results; (b): β ) 2.00 × 10-5 xCO2-0.510; (c): β ) 1.91 × 10-6 xCO2-0.0744, (d): β ) 1.84 × 10-7 xCO2-0.608.

Figure 6. (a) Rate constants of electron collision reactions R1R3 as a function of CO2 concentration at 60 W power input, (0): R1; (O): R2; (4): R3; (b) Ratio of rate constant of electron dissociation of CO2 and N2 as a function of CO2 concentration.

Substituting βj from eqs 6a-c in eq 5, the expressions for the rate of electron collision reactions R1-R3 become

R1 ) 2.00 × 10-5 xCO2-0.490

(

)

R1P P 1 0.75 W exp R1P W RT

x

(7a)

x

1 0.75 W R2P R2P P exp (7b) W RT

R2 ) 1.91 × 10-6 xCO2-0.0744 (1 - xCO2)

(

)

x ( )

1 0.75 W R3P R3P P exp (7c) W RT

R3 ) 1.84 × 10-7 xCO2-0.608 (1 - xCO2)

where Ri is the rate of reaction i, R is the gas constant, and T is the gas temperature. These equations indicate that the rate of the e-CO2 collision reaction (R1) increases with increasing CO2 concentration, while the rate of the e-N2 collision reactions (R2 and R3) decreases with increasing CO2 concentration. Figure 7a shows

Figure 7. (a) Rates of electron collision reactions R1-R3 as a function of CO2 concentration at 60 W power input; (b) Selectivity of electron collision reactions R1-R3 as a function of CO2 concentration. (0): R1; (O): R2; (4): R3.

these rate trends for the three electron collision reactions as a function of CO2 concentration. The formation rate of the O radical determines the oxidation rate of NO to NO2 (R13). Without CO2 addition, the O radical is predominantly produced from R4, as discussed previously.8,14 With CO2 addition, the O radical is predominantly formed from direct electron collision with CO2 (reaction R1) plus reaction R4. Therefore, the formation rate of the O radical increases with increasing CO2 concentration because the rate of the e-CO2 collision reaction (R1) increases with increasing CO2 concentration (as shown in Figure 7a) The selectivities for the three parallel electron collision reactions (R1-R3) are shown in Figure 7b. The selectivity of R1 increases with increasing CO2 concentration, while the selectivity of R2 and R3 decreases with increasing CO2 concentration, which is consistent with the results presented in Figure 7a. Effect of CO2 on NOx and CO2 Conversion. The model prediction results shown in Figure 4 suggest that the reaction mechanism of NO conversion in N2/CO2 plasma (shown in Table 4) with the lumped model parameters shown in Figure 5 captures the effect of CO2 concentration on the evolution of NOx and COx. The same model parameters are used to generate Figures 8a-d, which show the calculated concentrations of NO,

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Figure 8. Concentrations of NO (a), NO2 (b), N2O (c), and CO (d) as a function of CO2 mole fraction for inlet concentration of 600 ppm NO at the residence time of 20 s. Calculated data for power input of s (10 W), - - - (40 W), ‚‚‚‚‚‚ (80 W), - ‚ - (120 W), - ‚‚ - (160 W), and - - - (200 W).

NO2, N2O, and CO as a function of CO2 mole fraction for 600 ppm NO, a residence time of 20 s, and different power inputs. At the same CO2 concentration, NO concentration decreases with increasing power input until NO is almost completely converted (Figure 8a). NO2 concentration initially increases with increasing power input, reaches a maximum value, and decreases with increasing power input until NO2 is almost completely converted (Figure 8b). These model predictions for both NO and NO2 concentrations are consistent with the experimental data shown in Figures 3 and 4 and have been explained previously.8 At CO2 mole fractions less than about 0.015, N2O concentration, like NO2 concentration, initially increases with increasing power input, passes through a maximum, and then decreases with further increases in power input. Again, this behavior is consistent with the experimental data shown in Figure 3a. However, at CO2 mole fractions larger than about 0.015, N2O concentration monotonically increases with increasing power input (Figure 8c). The only path for N2O formation is reaction R5, involving the reaction of N(4S) and NO2, but there are two paths for N2O conversion: reaction with N2(A3∑u+) (R22) and reaction with N(2D) (R34). The contribution of N(2D) to N2O conversion is negligible because the rates of the parallel reactions of N(2D) (R31-R33 and R35) are ∼30 times larger than R34 at the prevailing reactant concentration. Therefore, only N2(A3∑u+) appears to be responsible for N2O conversion. At CO2 mole fractions less than about 0.015, before NO is completely converted, N2(A3∑u+) contributes predominantly to excite NO (R20) and not to convert N2O (R22) because the rate constant of R20 is over 10 times greater than that of R22. As a result, N2O concentration initially increases with increasing power input until NO is almost completely converted, after which N2(A3∑u+) contributes mainly to N2O conversion (R22), O2 dissociation (R23), CO excitation (R28), and CO2 dissociation (R29). At low CO2 concentration, the rate of N2(A3∑u+) consumption by R23, R28, and R29 does not completely depress reaction R22. Therefore, after reaching a maximum, N2O concentration decreases with increasing power input. However, concentrations of O2,

CO, and CO2 all increase with increasing CO2 concentration because CO2 conversion is always small (less than 6%, as shown in Figure 2b). When the CO2 mole fraction is higher than ∼0.015, the contribution of N2(A3∑u+) to N2O conversion (R22) is negligible due to competing reactions R23, R28, and R29 at the prevailing concentration of O2, CO, and CO2. Further, when NO concentration reaches a minimum at nearly complete conversion, the contribution of N(2D) to NO conversion is negligible. The contribution of N(2D) to NO formation (R35) increases with increasing CO2 concentration, as found from the selectivity analysis. Therefore, at CO2 mole fractions larger than 0.015, NO concentration weakly increases with power input after passing through the minimum due to R35 and R32, which increases as the O2 concentration increases. As a result, NO2 concentration increases by R13, leading to further increases in N2O concentration by R5. At the same power input, concentrations of NO and NO2 at the reactor outlet increase with increasing CO2 concentration (Figures 8a and 8b). The main active species responsible for NO conversion are N(4S) and N(2D) through reactions R4 and R33. The results presented in Figure 7a are consistent with weakly decreasing formation rates of both N(4S) and N(2D) with increasing CO2 concentration, which results in increasing NO concentration with increasing CO2 concentration. NO2 is formed mainly through reaction of NO and O (R13), while the main active species responsible for NO2 conversion is N(4S) (R5-R8). Therefore, the formation rates of both O radical and NO increase and the formation rate of N(4S) weakly decreases with increasing CO2 concentration, which causes NO2 concentration to increase with increasing CO2 concentration. At low power inputs (below ∼100 W), NO cannot be completely converted, while at high power inputs (above ∼100 W), NO is nearly completely converted at low CO2 concentrations. At high CO2 concentrations, NO is not completely converted at any power input because the formation rates of both N(4S) and N(2D) decrease with increasing CO2 concentration. The critical CO2 concentration, corresponding to the transition from complete to partial conversion of NO with increasing CO2 concentration at a given power input, increases with increasing power input (points A, B, and C, marked in Figures 8a at power inputs of 120, 160, and 200 W, respectively). The same critical CO2 concentrations correspond to the transition from complete to partial conversion of NO2 (A, B, and C, marked in Figure 8b at 120, 160, and 200 W, respectively) because NO2 is formed mainly from NO oxidation (R13). At low power inputs (below 100 W in Figure 8c), outlet N2O concentration decreases with increasing CO2 concentration, while at high power inputs (above 100 W in Figure 8c), N2O initially increases, and having reached a maximum value, it decreases with increasing CO2 concentration (points A, B, and C at 120, 160, and 200 W in Figure 8c, respectively, which correspond to the critical CO2 concentration shown in Figures 8a and 8b). N(4S) is the only active species responsible for N2O formation through reaction R5. However, N(4S) is consumed by converting NO in reaction R4. At low power inputs, NO cannot be completely converted and both NO and NO2 concentrations increase with increasing CO2 concentration, as shown in Figures 8a and 8b. The formation rate of NO increases faster than that of NO2. Therefore, the formation rate of N(4S) decreases

Ind. Eng. Chem. Res., Vol. 44, No. 11, 2005 3945

with increasing CO2 concentration and more N(4S) is consumed to convert NO through R4 instead of forming N2O through R5. Therefore, outlet N2O concentration decreases with increasing CO2 concentration at low power input. At high power input (e.g., 160 W), NO can be completely converted at CO2 mole fractions less than about 0.047 (point B shown in Figure 8a). As a result, the N2O concentrations reach maxima at the same CO2 concentrations at which NO concentrations begin to increase (points A, B, and C in Figures 8c and 8a). At the same CO2 concentration, CO concentration increases with increasing power input (Figure 8d) because the rate of electron collision reactions with CO2 increases with power input (eq 7a). At the same power input, outlet CO concentration increases with increasing CO2 concentration (Figure 8d), which is consistent with the results of Figure 7a. However, the rate of CO formation as a function of CO2 mole fraction changes and becomes smaller at the point of essentially complete NO conversion (corresponding to points A, B, and C in Figure 8d). Before NO is completely converted, N2(A3∑u+) contributes predominantly to NO excitation because the reaction rate of R20 dominates the other reactions of N2(A3∑u+) (R21-R30) at the prevailing reactant concentrations. Comparing the rates of NO(A) reactions (R38-R42, R45, R46) at the prevailing CO2 concentration, we conclude that the formed NO(A) contributes predominantly to CO2 conversion (R43) and quenching (R44). Therefore, the net effect of N2(A3∑u+) is to contribute to CO2 conversion through reaction with NO(A) before NO is completely converted. After NO is completely converted, N2(A3∑u+) contributes predominantly to CO excitation (R28) and CO2 dissociation (R29), but comparison of the reaction rates at the prevailing reactant conditions shows that the net result is that N2(A3∑u+) is mainly quenched to the ground state by reaction with CO(a) (R36) rather than CO2 dissociation. Thus, the formation rate of CO is higher before than after the point of essentially complete NO conversion. As a result, the change in slope of the CO concentrations occurs at the same CO2 concentration at which NO concentration begins to increase (points A, B, and C in Figures 8d and 8a). In real flue gases, CO2, O2, and H2O are major components (>2%) in addition to the background gas of N2. They have dissociation energies of 5.51 eV/CO2, 5.16 eV/O2, and 5.16 eV/H2O, which are all ∼ half of the dissociation energy of N2 (9.8 eV/N2). This causes electrons to preferentially react with O2, CO2, and H2O to produce oxidizing radicals such as O, OH, O3, and HO2, which contribute to NO to NO2 oxidization. This work and our previous investigation13 showed that the rate constants of electron collision reaction with CO2 and O2 are at least 1 order of magnitude higher than the rate constant of electron collision dissociation of N2. Therefore, complete NO conversion to N2 and O2 is more difficult in real flue gases that include O2, CO2, and H2O in addition to N2, as noted previously.16-19 Conclusions Both NOx conversion and CO2 conversion decrease with increasing percent-level CO2 concentration (∼18%) in nonthermal nitrogen plasma. A selectivity analysis shows that three electron collision reactions are enough to describe the NOx and COx concentrations at the reactor outlet. Model data for NOx and COx outlet concentration agree with experimental data. The rate

constants of electron collision reactions of both N2 and CO2 decrease with increasing CO2 concentration because electronegative CO2 reduces electron concentrations in the reactor due to the electron attachment process. The rate constant of CO2 dissociation by electron collision reaction is 1 to 2 orders of magnitude higher than that of N2 dissociation because of the low CO2 dissociation energy (5.51 eV/CO2). The effect of CO2 on the evolution of NO, NO2, N2O, and CO can be explained in detail on the basis of the proposed reaction mechanism and kinetic modeling. Acknowledgment This work was funded by the National Science Foundation (CTS-9810040; CTS-0078700) and the Department of Defense (ARO-DAAD19-01-1-0488). The matching support was provided by the Research Office, University of Wyoming. The authors gratefully acknowledge experimental assistance provided by Mr. S. V. B. Janardhan Garikipati, Dr. S. Legowski, and Mr. R. Borgialli. Literature Cited (1) Zhao, G.-B.; Hu, X.; Argyle, M. D.; Radosz, M. N Atom Radicals and N2(A3Σu+) Found to be Responsible for Nitrogen Oxides Conversion in Nonthermal Nitrogen Plasma. Ind. Eng. Chem. Res. 2004, 43, 5077-5088. (2) Zhao, G.-B.; Hu, X.; Yeung, M. C.; Plumb, O. A.; Radosz, M. Nonthermal Plasma Reactions of Dilute Nitrogen Oxide Mixtures: NOx in Nitrogen. Ind. Eng. Chem. Res. 2004, 43, 23152323. (3) Penetrante, B. M.; Hsiao, M. C.; Merritt, B. T.; Vogtlin, G. E.; Wallman, P. H., Comparison of Electrical Discharge Techniques for Nonthermal Plasma Processing of NO in N2. IEEE Trans. Plasma Sci. 1995, 23, 679-687. (4) Sathiamoorthy, G.; Kalyana, S.; Finney, W. C.; Clark, R. J.; Locke, B. R. Chemical Reaction Kinetics and Reactor Modeling of NOx Removal in a Pulsed Streamer Corona Discharge Reactor. Ind. Eng. Chem. Res. 1999, 38, 1844-1855. (5) Herron, J. T., Modeling Studies of the Formation and Destruction of NO in Pulsed Barrier Discharges in Nitrogen and Air. Plasma Chem. Plasma Process. 2001, 21, 581-609. (6) Yan, K.; Kanazawa, S.; Ohkubo, T.; Nomoto, Y., Oxidation and Reduction Processes During NOx Removal with CoronaInduced Nonthermal Plasma. Plasma Chem. Plasma Process. 1999, 19, 421-443. (7) Zhao, G.-B.; Hu, X.; Plumb, O. A.; Radosz, M. Energy Consumption and Optimal Reactor Configuration for Nonthermal Plasma Conversion of N2O in Nitrogen and N2O in Argon. Energy Fuels 2004, 18, 1522-1530. (8) Zhao, G.-B.; Garikipati, S. V. B. J.; Hu, X.; Argyle, M. D.; Radosz, M. Effect of Reactor Configuration on Nitric Oxide Conversion in Nitrogen Plasma. AIChE J 2005, 51, early view at http://www3.interscience.wiley.com/cgi-bin/jtoc/107061889/. (9) Zhao, G.-B.; Garikipati, S. V. B. J.; Hu, X.; Argyle, M. D.; Radosz, M. The Effect of Gas Pressure on NO Conversion Energy Efficiency in Nonthermal Nitrogen Plasma. Chem. Eng. Sci. 2005, 60, 1927-1937. (10) Lee, Y.-H.; Jung, W.-S.; Choi, Y.-R.; Oh, J.-S.; Jang, S.-D.; Son, Y.-G.; Cho, M.-H.; Namkung, W.; Koh, D.-J.; Mok, Y.-S.; Chung, J.-W. Application of Pulsed Corona Induced Plasma Chemical Process to an Industrial Incinerator. Environ. Sci. Technol. 2003, 37, 2563-2567. (11) Zhao, G.-B.; Hu, X.; Radosz, M. Final Report - Investigations of a Pulsed Corona Reactor System Towards Remediation of Diesel Engine Exhaust, Prepared for The Army Research Office; DADD19-01-1-048; Department of Chemical and Petroleum Engineering, University of Wyoming: Laramie, Aug 2004; pp 1-52. (12) van Veldhuizen, E. M.; Rutgers, W. R.; Bityurin, V. A. Energy efficiency of NO removal by pulsed corona discharges. Plasma Chem. Plasma Process. 1996, 16, 227-247. (13) Zhao, G.-B.; Garikipati, S. V. B. J.; Hu, X.; Argyle, M. D.; Radosz, M. Effect of Oxygen on Nonthermal Plasma Reactions of

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Received for review November 12, 2004 Revised manuscript received March 7, 2005 Accepted March 8, 2005 IE048905Z