Nonthermal-Plasma Reactions of Dilute Nitrogen Oxide Mixtures: NOx

Compared to N2 plasma, which produces a weak streamer glow discharge and a small temperature increase along the reactor, Ar plasma produces a strong ...
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Ind. Eng. Chem. Res. 2004, 43, 7456-7464

Nonthermal-Plasma Reactions of Dilute Nitrogen Oxide Mixtures: NOx-in-Argon and NOx + CO-in-Argon Xudong Hu, Gui-Bing Zhao, Ji-Jun Zhang, Linna Wang, and Maciej Radosz* Department of Chemical & Petroleum Engineering, University of Wyoming, Laramie, Wyoming 82071-3295

Analysis of conversion mechanisms for NO and N2O in Ar plasma suggests that NO is converted through the reaction Ar+ + NO + e- f Ar + N + O, whereas N2O is converted through the reaction Ar+ + N2O + e- f Ar + N2 + O. A time-averaged lumped model developed on the basis of this analysis matches the experimental data. CO inhibits N2O conversion but not NO conversion. However, parts-per-million levels of CO affect neither N2O nor NO conversion. Compared to N2 plasma, which produces a weak streamer glow discharge and a small temperature increase along the reactor, Ar plasma produces a strong streamer discharge and a small temperature decrease along the reactor. Introduction Flue gas streams contain parts-per-million levels of pollutants, such as NOx and SOx, that ideally are removed or converted to benign species prior to discharge. One way to convert NOx to nitrogen and oxygen is to expose the flue gas stream to electric discharges capable of generating radicals, ions, and excited molecules, which, in turn, activate the pollutants and convert them to benign stable species. Such a reactive mixture containing radicals, ions, and excited molecules in an otherwise neutral gas is referred to as plasma. If a potential difference is applied to plasma, the electric field will impart energy to the charged particles. The electrons, because of their small mass, will be immediately accelerated to a higher degree between the collisions than the heavier ions. If the pressure is low or the electric field is high or both, the electrons and the ions will, on average, have a kinetic energy higher than the energy corresponding to the random motion of the molecules. In plasma in such a state, usually referred to as nonequilibrium plasma, the highly energetic electrons are capable of ionizing and dissociating the neutral species at high rates even though the bulk gas temperature is quite low. Thus, it is said that such “cold nonequilibrium” discharges are capable of hightemperature chemistry at low temperatures. If, on the other hand, the pressure is so high that the charged particles do not move far between the collisions or the electric field is weak or both, the kinetic energy of the charged particles is not significantly different from the kinetic energy of the neutral species. Such plasma is called equilibrium plasma. In this work, we use a reactor in which nonthermal, nonequilibrium plasma is generated by a pulsed corona reactor (PCR). A PCR converts dilute NO, NO2, and N2O in nitrogen into the environmentally benign gases N2 and O2.1-4 The electrons generated in a PCR collide with the carrier gas and create chemically active species that initiate NOx reactions. Although much experimental and modeling work has been done on NO and N2O conversion in a diatomic background gas, such as N2, little work has been done on the conversion of NOx in a single* To whom correspondence should be addressed. E-mail: [email protected]. Tel.: 307-766-2500. Fax: 307-766-6777.

atom background gas, such as Ar,5,6 which produces fewer active species. Thus, using argon as a background gas might facilitate understanding of the chemical reactions in a corona discharge reactor, especially electron-molecule impact reactions. An additional justification for exploring argon plasma is that it has found a practical application in the treatment of flue gas by a radical injection technique as reported by Chang et al.12 Their results showed a very high rate of NOx destruction (85%) in the combustion exhaust gas. Ohkubo et al.25 used a small amount of Ar introduced into the flue gas stream through the corona discharging zone. Their results showed that the corona discharge characteristics and modes are significantly influenced by argon. Such work requires kinetic models. An example of a kinetic model is a time-averaged lumped model, initially proposed by Hu et al.6 and then improved by Zhao et al.,7 that was found to represent the NO, NO2, and N2O concentration evolution in nitrogen. The primary goal of this work is to extend this model to another type of carrier gas, such as argon, also studied by Maier,5 and to test it on new experimental data taken in this work. The secondary goal of this work is to understand the effect of another minor component, such as CO, as an example of a flue gas component that can alter the NOx conversion.2,9 The NOx conversion study in this paper refers to either NO or N2O conversion. Experiment The experimental apparatus and measurement procedures were described in detail previously.8 In a brief overview, the four-tube PCR used in this work consists of a high-voltage power supply, a control unit, and a pulser/reactor assembly. The high-voltage supply controls the pulsed power delivered to the reactor. The pulser/reactor assembly contains the pulsed power generator and the pulsed corona discharge tubes. The reactor has UV-grade quartz windows for diagnostics and plasma observation. In all of the experiments described in this work, only four of 10 tubes are 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

10.1021/ie0495731 CCC: $27.50 © 2004 American Chemical Society Published on Web 10/12/2004

Ind. Eng. Chem. Res., Vol. 43, No. 23, 2004 7457 Table 1. Experimental Conditions initial concentration (ppm)

flow rate (m3‚s-1)

reactor pressure (kPa)

pure N2 3.45 × 10-4

140.7

pure Ar 3.45 × 10-4

140.7

619 570 414 314 570 619

NO in Ar 3.45 × 10-4 3.45 × 10-4 3.45 × 10-4 3.45 × 10-4 4.01 × 10-4 4.01 × 10-4

140.7 140.7 140.7 140.7 189.0 189.0

290.3 217 105 47.6 217 47.6

N2O in Ar 3.45 × 10-4 3.45 × 10-4 3.45 × 10-4 3.45 × 10-4 4.01 × 10-4 4.01 × 10-4

140.7 140.7 140.7 140.7 189.0 189.0

NO + CO in Ar 3.45 × 10-4 3.45 × 10-4 3.45 × 10-4 4.01 × 10-4 4.01 × 10-4

140.7 140.7 140.7 189.0 189.0

N2O + CO in Ar 293 (N2O) + 197 (CO) 3.45 × 10-4 292 (N2O) + 305 (CO) 3.45 × 10-4 273 (N2O) + 389 (CO) 3.45 × 10-4 215 (N2O) + 10100 (CO) 4.01 × 10-4 50.7 (N2O) + 10100 (CO) 4.01 × 10-4

140.7 140.7 140.7 189.0 189.0

411 (NO) + 203 (CO) 396 (NO) + 435 (CO) 426 (NO) + 619 (CO) 575 (NO) + 10100 (CO) 652 (NO) + 10100 (CO)

of the input energy per pulse and the pulse frequency. The reactor is not thermally insulated. The experimental conditions are summarized in Table 1. The feed gas mixture of parts-per-million-level nitrogen oxides in argon of certified composition was purchased from US Airgas Company. This feed gas mixture, maintained at ambient temperature, around 300 K, was fed through the PCR at two flow rates, 3.45 × 10-4 m3‚s-1 at 140.7 kPa and 4.01 × 10-4 m3‚s-1 at 189.0 kPa. The feed samples and the reactor effluent samples, 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.8 It takes about 2.5 min to reach a steady state. All the experiments listed in Table 1 were conducted in 3 min. The measurement accuracy is within (10%. All experimental data were found to be reproducible within this error limit. Results and Discussion Figure 1 presents the reactor discharge voltage for pure Ar, NO in Ar, and N2O in Ar at the reactor pressure of 140.7 kPa with a flow rate of 3.45 × 10-4 m3‚s-1. Table 2 presents pulse peak, pulse width, and rising rate of discharge voltages for the above three systems. The pulse peak voltage is about 20 kV, and the pulse width is very short, about 20 ns. The reported pulse width is the full width at half-maximum. The experimental and computational results are discussed in the following sections. 1. Reaction Selection. A detailed PCR time-averaged lumped kinetic model was described by Zhao et al.7 In brief, the PCR reactions are divided into two categories: electron-molecule reactions and bulk molecule-molecule reactions.

Figure 1. Waveforms of discharge voltage for different reaction systems (reactor pressure ) 140.7 kPa, flow rate ) 3.45 × 10-4 m3‚s-1). Top: Pure Ar at 50 Hz. Middle: 619 ppm NO in Ar at 50 Hz. Bottom: 217 ppm N2O in Ar at 50 Hz. Table 2. Discharge Voltage for Different Reaction Systems at 50 Hz gas system

pulse peak (kV)

pulse widtha (ns)

rising rate (kV/ns)

pure Ar NO in Ar N2O in Ar

17 20 23

10 20 20

3.2 4.4 4.4

a

Full width at half-maximum.

For the electron-molecule reactions, the rate constant k, including the electron concentration denoted by [e-], was given by7

k[e-] )

β W0.75 exp(-RP/W) 0.5 (RP)

(1)

where W is the power input to the reactor and P is the

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Table 3. List of Chemical Reactions Considered for NO Conversion in Argon rate constant (cm3‚mol-1‚s-1)

chemical reaction 1

Ar + e- f Ar+ + 2e-

2

Ar+ + e- f Ar

3 4 5 6

Ar+ +

7 8 9 10 11 12 13 14

e-

NO + f Ar + N + O Ar+ + NO2 + e- f Ar + N + 2O Ar+ + N2O + e- f Ar + N2 + O NO + O + Ar f NO2 + Ar N + NO f N2 + O O + O + Ar f O2 + Ar NO2 + O f NO + O2 NO2 + N f N2O + O NO2 + N f N2 + O2 NO2 + N f N2 + 2O NO2 + N f 2NO N + O + Ar fNO + Ar

reactor pressure. Both W and P were measured experimentally, and R and β were determined by fitting the experimental data. It is important to identify the electron-molecule reactions in the PCR because they initiate the subsequent bulk chain reactions. Ar*, the excited state of Ar, and its ion form of Ar+ might contribute to the initiation reactions. Su et al.10 reported that the electron-collision reaction of Ar + e- f Ar* + e- has a rate constant of 4.6 × 1013 cm3‚mol-1‚s-1 in comparison with the electroncollision reaction of Ar + e- f Ar+ + 2e-, which has a rate constant of 3.6 × 1015 cm3‚mol-1‚s-1. The reaction rate constant for the formation of Ar+ is 2 orders of magnitude higher than that for the formation of Ar*. Also, Luo et al.11 observed Ar+ emission in their PCR using an optical emission technique. Additionally, Chang et al.12 employed Ar to minimize the generation of negative ions in the corona discharge because of its easy ionization to form Ar+. Argon has a very small dielectric strength of 0.18.24 Krasnoperrov and Kristopa26 proposed a similar charge-transfer mechanism for nonthermal plasma in oxygen. These analyses reveal that Ar+ is a primary species during electron-impact reactions with argon, i.e., Ar + e- f Ar+ + 2e-. In addition, the reverse reaction of Ar+ + e- f Ar occurs because of its extremely high reaction rate constant with an order of magnitude of 1016 cm3‚mol-1‚s-1.10 This finding could also be interpreted from chemistry as positively charged Ar+ and negatively charged electrons attracting each other. On the basis of the above analysis, the two electron-molecule reactions

Ar + e- f Ar+ + 2eAr+ + e- f Ar were selected. For nonelectron bulk reactions involving radicals, ions, molecules, and their excited states, only 12 bulk reactions for NO in Ar and 2 bulk reactions for N2O in Ar were selected for the modeling calculation as shown in Tables 3 and 4, respectively. This selection is based on our prior analysis for criteria in selecting bulk reactions:7 a high rate of the chemical reaction and a high concentration of reactants. Those with low concentrations or low reaction rates do not appear in Tables 3 and 4 because they contribute little to the various species concentration evolutions.

ref

R ) 2.50 β ) 6.46 × 10-6 R ) 5.05 β ) 1.83 × 105 1.63 × 1014 2.60 × 1014 1.99 × 1014 k0 ) 3.62 × 1016 [Ar] k∞ ) 1.81 × 1013 Fc ) 0.85 1.87 × 1013 1.10 × 1015 [Ar] 5.84 × 1012 1.81 × 1011 4.21 × 1011 5.48 × 1011 1.38 × 1012 3.68 × 1015[Ar]

present work present work Shul et al.18 Shul et al.18 Shul et al.18 Atkinson et al.20 Atkinson et al.20 Kossyi et al.15 Atkinson et al.20 Atkinson et al.20 Kossyi et al.15 Kossyi et al.15 Kossyi et al.15 Kossyi et al.15

Table 4. List of Chemical Reactions Considered for N2O Conversion in Argon rate constant (cm3‚mol-1‚s-1)

chemical reaction 1

Ar + e- f Ar+ + 2e-

2

Ar+ + e- f Ar

3 4

Ar+ +

N 2O + f Ar + N2 + O O + O + Arf O2 + Ar e-

R ) 6.33 β ) 2.26 × 10-6 R ) 7.30 β ) 2.26 × 105 1.99 × 1014 1.10 × 1015[Ar]

ref present work present work Shul et al.18 Kossyi et al.15

It must be noted that reaction 6 in Table 3 is a threebody reaction whose pseudo-second-order rate constant depends on temperature and pressure (concentration of background gas) as follows7

k ) k∞

(

)

k0/k∞ 2 10(log Fc)/{1+[log(k0/k∞)/(0.75-1.27 log Fc)] } 1 + k0/k∞ (2)

Here, k0 is the pseudo-second-order rate constant at a low-pressure limit, and k∞ is the second-order rate constant at a high-pressure limit. Fc is function of temperature and is equal to 0.85 at 300K. 2. NO in Ar. The dissociation of NO does not occur by direct electron impact because the probability of such an impact is low for low NO concentrations, such as our case of less than 800 ppm. Instead, NO dissociates by colliding with electronically excited background gas atoms. The previous section analyzed the initialization electron-collision reactions, i.e., argon undergoing the ionization reaction Ar + e- f Ar+ + 2e- in the reactor. This section focuses on the subsequent bulk reactions. Ar+ experiences electron-transfer reaction with NO: Ar+ + NO f Ar + NO+. The electron-impact reaction of NO+ + e- f N + O is finally responsible for the conversion of NO. The activated of NO+ ions, upon collision with an electron, are assumed to decompose to nitrogen. Charge-transfer reactions, such as these

Ar+ + NO f Ar + NO+ k ) 1.63 × 1014 cm3‚mol-1‚s-1 Ar+ + N2O f Ar + N2O+ k ) 1.99 × 1014 cm3‚mol-1‚s-1 Ar+ + NO2 f Ar + NO+ + O k ) 2.60 × 1014 cm3‚mol-1‚s-1

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Figure 2. Comparison of model calculation with experiment: NO in Ar (reactor pressure ) 140.7 kPa, flow rate ) 3.45 × 10-4 m3‚s-1).

Figure 3. FTIR absorbance A versus wavenumber (cm-1): NO in Ar (reactor pressure ) 140.7 kPa, flow rate ) 3.45 × 10-4 m3‚s-1). Top: FTIR spectrum for 619 ppm NO in Ar for feed gas. Bottom: FTIR spectrum for 619 ppm NO in Ar at 200-Hz PCR repitition rate.

are relatively slow.18 By contrast, electron-ion recombination dissociation reactions, such as these

NO+ + e- f N + O 16

3

-1

k ) 2.16 × 10 cm ‚mol ‚s

-1

N2O+ + e- f N2 + O

Figure 4. Comparison of model calculation with experiment: N2O in Ar.

accepted following reactions in the reactor are14-16

k ) 1.20 × 1017 cm3‚mol-1‚s-1 are relatively fast.19 The slower reactions control series reactions. Therefore, we can combine the above series reactions into reactions 3-5 in Table 3 and reaction 3 in Table 4. Tas et al.,13 who used helium as a background gas, and Tokunaga and Suzuki14 reported a similar reaction mechanism. As soon as N and O radicals form, the most

N + NO f N2 + O NO + O f NO2 NO2 + N f N2O + O O + O f O2 Table 3 lists the 12 subsequent bulk reactions for NO in Ar.

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Figure 5. FTIR absorbance A versus wavenumber (cm-1): N2O in Ar (reactor pressure ) 140.7 kPa, flow rate ) 3.45 × 10-4 m3‚s-1). Top: FTIR spectrum for 217 ppm NO in Ar for feed gas. Bottom: FTIR spectrum for 217 ppm NO in Ar at 200-Hz PCR repitition rate.

We tested four initial concentrations of NO, ranging from 314 to 619 ppm, at the flow rate of 3.45 × 10-4 m3‚s-1, and the results are shown in Figure 2. The data on the variation of species concentration at the outlet of the corona reactor are plotted versus power consumed, which can be calculated as the product of the energy per pulse and the pulse frequency.6 At lower power levels, ∼10-15% of the NO in the feed gas is converted to NO2; however, at higher power levels, NO2 is barely observed, and NO continues to decline. We used the experimental results for 619 ppm NO in Ar at a flow rate of 3.45 × 10-4 m3‚s-1 to fit the model

parameters R and β, as shown in Table 3. The model parameters were determined through an optimization method presented by Zhao et al.7 These parameters were then used to predict the experimental concentrations of all species at other operating conditions, such as the initial concentrations at the reactor inlet and the power input. Figure 2 shows how closely the model predictions correlate with the experimental measurements. In addition, Figure 3 presents a sample FTIR spectrum for 619 ppm NO in Ar. Whereas NO and NO2 are detected, N2O is not detected (2143.80-2030.07 cm-1). We expect that the experimentally observed formation of NO2 is the result of the reaction NO + O f NO2 whereas the N2O formation is the result of the reaction NO2 + N f N2O + O. N2O is not detected in the FTIR spectrum because the reaction rate constant of NO2 + N f N2O + O is 100 times lower than that of N + NO f N2 + O (see Table 3) and the concentration of NO2 is also much lower than that of NO in the reactor. 3. N2O in Ar. Similarly, the dissociation of N2O does not occur by direct electron impact because the probability of such an impact is low for low N2O concentrations, such as our case of less than 300 ppm. Instead, N2O dissociates by colliding with electronically excited background gas atoms. In the previous section, Ar+ was determined to be the major active species initiating the subsequent bulk reactions. For the bulk reactions, Ar+ undergoes an electron-transfer reaction with N2O and forms N2O+ through the reaction Ar+ + N2O f Ar + N2O+. The electron-impact reaction of N2O+ + e- f N2 + O is finally responsible for the conversion of N2O. We determined two bulk reactions for N2O in Ar, as listed in Table 4.

Figure 6. Comparison of NOx and CO in Ar with NOx in Ar (parts-per-million level of CO, reactor pressure ) 140.7 kPa, flow rate ) 3.45 × 10-4 m3‚s-1 for both experimental data and model calculation data). Top: Comparison of NO and CO in Ar with NO in Ar. Bottom: Comparison of N2O and CO in Ar with N2O in Ar.

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Four initial concentrations of N2O, ranging from 47.6 to 290.3 ppm at the flow rate of 3.45 × 10-4 m3‚s-1, were flowed through the reactor under different power inputs. We used the experimental results for 290.3 ppm N2O in Ar at a flow rate of 3.45 × 10-4 m3‚s-1 to fit the model parameters R and β, as shown in Table 4. The model parameters were determined through an optimization method presented by Zhao et al.7 These parameters were then used to predict the experimental concentration of all species at other operation conditions, such as the initial concentrations at the reactor inlet and the power input. Figure 4 shows how well the model predictions correlate with the experimental measurements. Unlike NO conversion in Ar (Figure 2), where NO is not completely converted even at high power inputs, N2O is almost completely converted when the power inputs are high (Figure 4). The mechanism of N2O conversion is similar to that of NO conversion in Ar except that no N radical formation occurs in the N2O-in-Ar system. As a result, we expect that neither NO2 nor NO is formed because of the absence of N radicals. The FTIR analysis results of 217 ppm N2O in Ar shown in Figure 5 support this analysis. We see only a characteristic peak of N2O and no other nitrogen oxide peaks. 4. Effect of CO on NO or N2O Conversion in Argon. When parts-per-million levels of CO are added, electrons cannot directly collide with CO as analyzed in the systems of dilute NO in Ar and N2O in Ar. Figure 6 shows the experimental results for various dilute initial concentrations of NO + CO and N2O + CO in Ar at the flow rate of 3.45 × 10-4 m3‚s-1 under different power inputs. The symbols are the experimental data, and the curves are the computational results based on Tables 3 and 4. The same values of model parameters R and β in Tables 3 and 4 were used to calculate the curves. In all of these experiments, the CO concentration was fairly low, ranging from 203 to 619 ppm. Figure 6 shows that low concentrations of added CO exhibit no effect on NO or N2O conversion in Ar. In addition to examining dilute CO addition, we conducted a set of experiments with a much higher concentration of CO to study the effect on NOx conversion. Figure 7 compares the experimental data for dilute NO in Ar with 1.01% CO addition to the gas mixture of NO in Ar at the flow rate of 4.01 × 10-4 m3‚s-1. Figure 7 illustrates that a 1.01% CO addition into the gas stream of NO in Ar shows no effect on NO conversion. Figure 8 compares the experimental data for the dilute N2O in Ar system with those for a 1.01% CO addition to the gas mixture of N2O in Ar at the flow rate of 4.01 × 10-4 m3‚s-1. Interestingly, Figure 8 reveals that a 1.01% CO addition into the gas stream of N2O in Ar shows a strong inhibition on N2O conversion. The observation that a small amount of added CO (parts per million) has no effect on either NO or N2O conversion in Ar but a large amount of added CO (1%) shows a strong inhibition on N2O conversion but no effect on NO conversion requires explanation. When CO is added, the charge-transfer reactions listed in Table 5 are expected to occur. Both Ar+ and CO+ contribute to NO and N2O conversion as a result of reactions 1 and 3 and 4 and 5, respectively. The chemical reaction rate is the product of the rate constant and the concentration of each reactant. Because the CO concentration is close to the tested NO or N2O concentration and the reaction rate constant of reaction 1 is 1

Figure 7. Comparison of NO and CO in Ar with NO in Ar (percentage level of CO).

Figure 8. Effect of percentage level of CO on N2O conversion.

order of magnitude higher than that of reaction 2 when several hundred parts per million of CO are added, the reaction rate of reaction 1 is about 1 order of magnitude

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Table 5. Charge-Transfer Reactions Occurring upon CO Addition chemical reaction 1 2 3 4 5

Ar+ + NO f NO+ + Ar Ar+ + CO f CO+ + Ar CO+ + NO f NO+ + CO Ar+ + N2O f N2O+ + Ar CO+ + N2O f N2O+ + CO

rate constant (cm3‚mol-1‚s-1)

ref

1.62 × 1014 2.65 × 1013 2.53 × 1014 1.99 × 1014 1.81 × 1014

Shul et al.18 Anicich17 Anicich17 Shul et al.18 Matzing19

higher than that of reaction 2. The dominant species responsible for NO or N2O conversion upon CO addition remains Ar+, not CO+. This is why no effect on NO or N2O conversion in Ar is observed when a small amount of CO is added. Although the rate constant of reaction 2 is 1 order of magnitude lower than that of reaction 1 with a 1.01% concentration of CO, the reaction rate of reaction 2 is much higher than that of reaction 1 because the CO concentration is 2 orders of magnitude higher than the NO and N2O concentrations. NO or N2O conversion is now primarily due to CO+ instead of Ar+. We also predict that a 1.01% CO addition will produce CO+ from direct electron collision with CO. Because the reaction rate constants of reactions 1 and 3 are very close, CO+ has nearly the same ability to convert NO as Ar+. Thus, switching the NO conversion mechanism from Ar+ species to CO+ species does not make any difference with respect to NO conversion. Similarly, with a high concentration of CO, the Ar+ produced in reaction 1 is quickly consumed by CO in reaction 2 to produce CO+, which shifts the N2O conversion mechanism from Ar+ to CO+. The reaction rate constant for reaction 5 is not readily available in the literature except for the value reported by Matzing.19 Our experimental data and their analysis suggest that, because CO+ is less effective than Ar+ in converting N2O, Matzing’s rate constant for reaction 5, 1.81 × 1014 cm3‚mol-1‚s-1, is probably too high. This is not surprising because the reported charge-transfer rate constants can be subject to (50% error or more, even

for heavily studied reactions.17 This issue, on the other hand, calls for more work on alternative mechanisms, for example, a mechanism through one or more of the argon excited states Ar*, rather than through Ar+, or a combined mechanism through both Ar* and Ar+. In summary, a small concentration of added CO (parts per million) has no effect on NO or N2O conversion in argon. A high concentration of added CO (1%) shows no effect on NO conversion but does show a strong adverse effect on N2O conversion in argon. 5. Comparison between Background Gas of N2 or Ar in the Reactor. Several prominent differences are observed experimentally between N2 and Ar as background carrier gases in the reactor. The typical performance waveforms of the reactor using these two background gases are presented in Figure 9. The signal in Ar plasma produces an oscillation characteristic with diminishing amplitude, and the signal in N2 plasma produces a monoexponential decay. Visual and auditory observations reveal that the discharge phenomenon occurs very differently in these two gas systems. A much louder noise can be heard in the background gas of Ar than in that of N2. In N2 plasma, the discharge is a strong glow discharge with a weak streamer discharge, whereas in Ar plasma, the discharge shows a very strong streamer discharge. In N2 plasma, the discharge is dark blue, whereas in Ar plasma, the discharge is bright white. In N2 plasma, as we analyzed elsewhere,23 N radical and N2(A3∑u+) are the primary species produced by electron-collision reactions with a nitrogen carrier gas. In Ar plasma, Ar+ is the primary species produced by electron-collision reactions with argon. Apparently, Ar+ is a better electric conductor than neutral species such as N radical or N2(A3∑u+). A large number of Ar+ ions present in Ar plasma produces a more vigorous discharge streamer. This effect can also be detected from the electronic waveforms of these two plasmas. In nitrogen plasma, the peak current is less than 350 A, and in argon plasma, the peak current reaches more than 800 A, as

Figure 9. Reactor electronic signals at 100 Hz for the PCR: (a) Ar, (b) N2 (reactor pressure ) 140.7 kPa, flow rate ) 3.45 × 10-4 m3‚s-1). Top: Discharge current. Bottom: Discharge voltage.

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input (>80 W), with more electrons generated, the reaction of Ar+ + e- f Ar, which consumes Ar+, might become dominant and, therefore, decrease the NO conversion rate. We do not observe a similar decreasing of the NO conversion rate in N2 plasma because N radical increases along with power. Conclusions

Figure 10. Comparison of NO conversion in Ar and in N2 (reactor pressure ) 140.7 kPa, flow rate ) 3.45 × 10-4 m3‚s-1).

shown in Figure 9. This phenomenon is consistent with Chang’s12 finding that Ar injection into the flue gas plasma caused the current to increase and thus enhanced the NO removal efficiency. The corona activity in Ar plasma is more pronounced in the reactor inlet side than the outlet side, as observed experimentally through window ports located 10 cm from the reactor inlet point, in the middle of the reactor, and 10 cm from the reactor outlet point. Thermocouples mounted in the reactor wall on the inlet and outlet sides, with no contact with plasma, do not exhibit interference with the plasma. The inlet and outlet temperatures were measured to be 30 and 23 °C, respectively, for 619 ppm NO in Ar at a 3.45 × 10-4 m3‚s-1 flow rate, a 140.7 kPa reactor pressure, and 400-Hz PCR operation frequency. The discharge activity in the nitrogen plasma, however, is just the opposite. A more vigorous discharge occurs at the outlet side rather than at the inlet side. In addition, the outlet temperature is significantly higher than the inlet temperature, 33 vs 23 °C, respectively, for 614 ppm NO in N2 at a 4.11 × 10-4 m3‚s-1 flow rate, a 140.7 kPa reactor pressure, and 400-Hz PCR operation frequency. The reason is not clear. One possible explanation is that Ar+ might be generated through the reaction of Ar + e- f Ar+ + 2e- in the inlet side of the reactor in Ar plasma and might subsequently vanish gradually because of the reverse reaction of Ar+ + e- f Ar occurring along the reactor, leading to the descending temperature profile there. Finally, a comparison of different background gases using the lumped model for NO conversion is provided in Figure 10, where the NO concentration at the reactor outlet versus power input is plotted for two initial concentrations of NO (400 and 600 ppm). The same residence time and reactor pressure (140.7 kPa) were used. We used the same model parameter values of R and β for the calculation of NO in N2 , based on the work of Zhao et al.7 We observe that NO conversion is faster at the low power input when Ar is used as a background gas but slower at the higher power input. Ar+ has a greater NO conversion ability compared with N radical when we compare the rate constants of the following two reactions: Ar+ + NO f NO+ + Ar (k ) 1.63 × 1014 cm3‚mol-1‚s-1) and N + NO f N2 + O (k ) 1.87 × 1013 cm3‚mol-1‚s-1). Thus, we observe faster NO decomposition at a low power input. However, at the high power

1. The conversion of NO or N2O in Ar occurs because of the formation of Ar+ in the corona plasma region. NO is converted through the reaction Ar+ + NO + e- f Ar + N + O, whereas N2O is converted through the reaction Ar+ + N2O + e- f Ar + N2 + O. A lumped timeaveraged mathematical model based on these reactions agrees well with the experimental data. 2. A small concentration of added CO (ppm) has no effect on NO or N2O conversion in Ar. A high concentration of added CO (1%) shows no effect on NO conversion but a strong adverse effect on N2O conversion in Ar. 3. N2 produces a weak streamer discharge, whereas Ar shows a strong streamer discharge in the reactor. In Ar plasma, the temperature at the reactor inlet side is higher than that at the outlet side. In N2 plasma, the temperature at the reactor inlet side is lower than that at the outlet side. Acknowledgment Dr. Pradeep K. Agarwal, who initiated this work, passed away in 2002. His influence and contributions are recognized and remembered. This work was funded by the National Science Foundation (CTS 9810040, CTS 0078700) and the U.S. Department of Defense (ARODAAD19-01-1-0488). Nomenclature R ) model parameter constant (J‚atm-1‚s-1) β ) model parameter constant (J-0.25‚s-0.75) [e-] ) electron concentration (mol‚cm-3) Fc ) temperature-dependent factor equal to 0.85 at 300 K I ) current (A) k ) reaction rate constant (cm3‚mol-1‚s-1) k0 ) pseudo-second-order rate constant at a low-pressure limit (cm3‚mol-1‚s-1) k∞ ) second-order rate constant at a high-pressure limit (cm3‚mol-1‚s-1) P ) pressure in the plasma zone (atm) V ) voltage (kV) W ) power input to the reactor (W)

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Received for review May 19, 2004 Revised manuscript received July 27, 2004 Accepted August 27, 2004 IE0495731