Energy Consumption and Optimal Reactor Configuration for

1522

Energy & Fuels 2004, 18, 1522-1530

Energy Consumption and Optimal Reactor Configuration for Nonthermal Plasma Conversion of N2O in Nitrogen and N2O in Argon Gui-Bing Zhao,† Xudong Hu,† Ovid A. Plumb,‡ and Maciej Radosz*,† Department of Chemical & Petroleum Engineering and College of Engineering, University of Wyoming, Laramie, Wyoming 82071-3295 Received January 29, 2004. Revised Manuscript Received June 21, 2004

The analysis of experimental data, chemical reaction mechanisms, and kinetic modeling data is used to determine the power input and pulsed-corona-discharge reactor configuration that minimizes energy consumption for converting N2O in nitrogen and N2O in argon, which are model binaries reminiscent of more complex NOx in flue gas systems. Specifically, it is found that inseries reactors are much more energy efficient than a single reactor and more energy efficient than parallel reactors. For example, 12 reactors in series are needed to remove 90% of N2O if its initial concentration in nitrogen is about 200 ppm.

1. Introduction Nonthermal plasma processing is a promising technology for the conversion of NOx and SOx pollutants in flue-gas streams. Compared to thermal methods, nonthermal plasma techniques are more efficient because a majority of the electrical energy goes into the production of energetic electrons rather than into gas heating.1 The electrical energy supplied by the discharge is used preferentially to create energetic electrons, which are then used to produce radicals by dissociation and ionization of the carrier gas in which the pollutants are present. These radicals then decompose the pollutants. Compared to the selective catalytic reduction process, direct decomposition of NOx into nitrogen and oxygen using nonthermal plasma techniques has the advantages of relative simplicity, scalability, and lower capital cost as demonstrated by a study committee of MITI in Japan.2 Almost all of the currently tested catalysts in the selective catalytic reduction process suffer from such problems as easy catalyst deactivation, poor thermal and hydrothermal stability, and unsatisfactory activity.3 There are several issues that affect the practical application of nonthermal plasma processes including (1) energy cost, (2) byproduct emission, (3) power delivery method, and (4) reactor design. Different types of electrical discharge techniques (dc, ac, pulsed, and arc) have been exploited for facilitating discharge of nonthermal plasma with low energy consumption. Com-

pared to the other nonthermal plasma technologies using dc, ac, and arc discharge, pulsed corona discharge is energy efficient and is expected to be developed for dry DeNOx/DeSOx processes for utility power plant boilers.2 Recently, Hackam and Akiyama4 reviewed different electrical discharge techniques including dc, ac, pulsed, and arc discharge used in the conversion of the major polluting constituents, NOx and SOx, encountered in flue gases and exhaust emissions. The consistent conclusion is that nonthermal discharges using very fast rise and short duration pulses are likely to be promising technologies. To be competitive for remediation of diesel engine emissions, the energy cost for an idealized nonthermal DeNOx reactor should be lower than 10-20 eV/NO for concentrations ≈ 1000 ppm. This would correspond to an overall power consumption of lower than 5% of the total engine power.5,6 For pulsed-corona-discharge reactors (PCDRs), energy costs that have been reported vary considerably, for example, from 10 to 500 eV/molecule.6 The approaches to reducing electrical energy consumption can be divided into three categories. (1) The first category of approaches is activation of pollutant conversion reactions by chemical additives. Additives are introduced into the feed of flue gas reactors in order to enhance the conversion of the pollutants and neutralize nitric and sulfuric acids. One of the most widely used additives is ammonia.7-11 Other additives such as lime, methane, ethylene, propylene,

* Corresponding author. E-mail: [email protected]. Tel: 307-7662500. Fax: 307-766-6777. † Department of Chemical & Petroleum Engineering. ‡ College of Engineering. (1) Penetrante, B. M.; Hsiao, M. C.; Merritt, B. T.; Vogtlin, G. E.; Wallman, P. H. IEEE Trans. Plasma Sci. 1995, 23, 679-687. (2) Masuda, S. Report on novel dry DeNOx/DeSOx technology for cleaning combustion gases from utility thermal power plant boilers. In Nonthermal Plasma Techniques for Pollutuion Control; Penetrante, B. M., Schultheis, S. E., Eds.; Springer-Verlag: Berlin, 1993; Part B. (3) Luo, J.; Suib, S. L.; Marquez, M.; Hayashi, Y.; Matsumoto, H. J. Phys. Chem. A 1998, 102, 7954-7963.

(4) Hackam, R.; Akiyama, H. IEEE Trans. Dielectr. Electr. Insul. 2000, 7, 654-683. (5) Penetrante, B. M. Plasma Chemistry and Power Consumption in Non-thermal DeNOx. In Nonthermal Plasma Techniques for Pollution Control; Penetrante, B. M., Schultheis, S. E., Eds.; SpringerVerlag: Heidelberg, Germany, 1993; Part A. (6) Puchkarev, V.; Gundersen, M. Appl. Phys. Lett. 1997, 71, 33643366. (7) Chang, J. S.; Looy, P. C.; Nagai, K.; Yoshioka, T.; Aoki, S.; Maezawa, A. IEEE Trans. Ind. Appl. 1996, 32, 131-137. (8) Mok, Y. S.; Nam, I. S. IEEE Trans. Plasma Sci. 1999, 27, 11881196.

10.1021/ef049966c CCC: $27.50 © 2004 American Chemical Society Published on Web 08/05/2004

Conversion of N2O in Nitrogen and N2O in Argon

Energy & Fuels, Vol. 18, No. 5, 2004 1523

and hydrocarbons such as propanol, pentanol, and water vapor have been used in flue gas reactors. For example, ammonia, calcium hydroxide, and hydrated lime have been tested as additives to the flue gas by Dinelli et al.12 The test results have confirmed the physical feasibility of the process. Chung et al.13 have found that electrical energy consumption per converted NO molecule has a minimum value of 17 eV when pentanol is injected. When ethylene and propylene are injected, 30 and 22 eV of electrical energy are required for the conversion of a NO molecule. van Veldhuizen et al.14 found that NO conversion is strongly increased by the addition of SO2 or NH3. Mizuno et al.15 proposed that peroxide radicals, generated by reaction between atomic oxygen and ethylene, significantly enhance NOx conversion efficiency. The effect of water vapor on the conversion of NO has also been investigated by Mok et al.16 (2) The second category of approaches to reducing energy consumption is optimization of the electrical circuit for efficient energy transfer to the reactor, which refers to the efficiency of converting wall plug electrical power into power deposited by electrons into the plasma reactor. Chung et al.13 demonstrated that the ratio of the pulse-forming capacitance to reactor capacitance plays an important role in the energy-transfer efficiency to the reactor. However, we note that the capacitance of the reactor varies with the development of corona discharges. Rea and Yan17 proposed that the pulseforming capacitance should be at least 100 times the reactor capacitance for the generation of an intense streamer. Mok et al.,18 on the other hand, found that, when the pulse-forming capacitance was 5 times larger than the geometric capacitance of the reactor, the energy utilization efficiency was maximized. Recently, Chung et al.13 reported that the maximum energytransfer efficiency could be obtained when the pulseforming capacitance is 3.4 times larger than the reactor capacitance. Korzekwa et al.19 performed an energy balance analysis on the pulsed-corona-discharge system using a hydrogen-filled spark gap switch. More than 90% of the energy-transfer efficiency was obtained in their system. A theoretical analysis by Uhm and Lee20 indicated that the energy-transfer efficiency depends on the ratio of pulse-forming capacitance to reactor capacitance, geometry of the reactor, and normalized plasma mobility. The reactor capacitance plays a pivotal role in the circuit performance.

Table 1. Experimental Matrix

(9) Urashima, K.; Chang, J. S.; Ito, T. IEEE Trans. Ind. Appl. 1997, 33, 879-886. (10) Urashima, K.; Chang, J. S.; Park, J. Y.; Lee, D. C.; Chakrabarti, A.; Ito, T. IEEE Trans. Ind. Appl. 1998, 34, 934-939. (11) Park, J. Y.; Tomicic, I.; Round, G. F.; Chang, J. S. J. Phys. D: Appl. Phys. 1999, 32, 1006-1011. (12) Dinelli, G.; Civitano, L.; Rea, M. IEEE Trans. Ind. Appl. 1990, 26, 535-541. (13) Chung, J. W.; Cho, M. H.; Son, B. H.; Mok, Y. S.; Namkung, W. Plasma Chem. Plasma Process. 2000, 20, 495-509. (14) van Veldhuizen, E. M.; Zhou, L. M.; Rutgers, W. R. Plasma Chem. Plasma Process. 1998, 18, 91-111. (15) Mizuno, A.; Shimizu, K.; Chakrabarti, A.; Dascalescu, L.; Furuta, S. IEEE Trans. Ind. Appl. 1995, 31, 957-963. (16) Mok, Y. S.; Kim, J. H.; Nam, I. S.; Ham, S. W. Ind. Eng. Chem. Res. 2000, 39, 3938-3944. (17) Rea, M.; Yan, K. IEEE Trans. Ind. Appl. 1995, 31, 507511. (18) Mok, Y. S.; Ham, S. W.; Nam, I. S. Plasma Chem. Plasma Process. 1998, 18, 535-550. (19) Korzekwa, R. A.; Grothaus, M. G.; Hutcherson, R. K.; Roush, R. A.; Brown, R. Rev. Sci. Instrum. 1998, 69, 1886-1892. (20) Uhm, H. S.; Lee, W. M. Phys. Plasmas 1997, 4, 3117-3128.

reaction system

concn of N2O at the reactor inlet (ppm)

flow rate (m3/s)

reactor pressure (kPa)

N2O + N2

217 105 51.1 217 105

4.87 × 10-4 4.87 × 10-4 4.87 × 10-4 4.07 × 10-4 4.07 × 10-4

189.0 189.0 189.0 189.0 189.0

N2O + Ar

(3) The third category of approaches to reducing energy consumption is reactor configuration optimization. Puchkarev and Gundersen,6 Tas et al.,21 and Yan et al.22 recognized that the PCDR configuration is important for reducing energy cost in the conversion of NOx. However, no detailed studies of the reactor design, such as studies on the effect of the reactor diameter and length on energy consumption and on the effect of multiple in-parallel or in-series reactors on energy consumption, have been conducted. The goal of this work, therefore, is to understand the effect of the configuration of multiple PCDRs on energy consumption and to verify the results experimentally using two sets of reaction systems, N2O in N2 and N2O in Ar, which are model binaries reminiscent of more complex NOx in flue gas systems. The reason for selecting N2O, a potent greenhouse gas and stratospheric ozone depletion agent,23 is that, while the overall NOx emission decreases in fluidized-bed combustion, because of lower combustion temperatures (1000-1200 K), the N2O emission actually increases to 15-200 ppm, compared to 5 ppm observed for the pulverized coal combustion. 2. Experimental Section The PCDR used in this work consists of a high-voltage power supply and control unit and the pulser/reactor assembly.24 The high-voltage controller consists of electronic and gas controls required to regulate the high-voltage charging power supply as well as the pulsed power delivered to the reactor gas. The pulser/reactor assembly contains the pulsed power generator and the PCDR chambers. The reactor, consisting of 10 parallel reaction tubes, is fitted with UV-grade quartz windows for diagnostics and plasma observation. In all of the experiments described in this work, only 4 out of 10 tubes are wired for plasma generation. The corona power can be calculated from the product (VI) of the measured pulse voltage (V) and current (I); the energy is the time integral (∫VI dt) of power. The power consumed can also be calculated as the product of the input energy per pulse and the pulse frequency. The system design permits the variation and measurement of the applied voltage and its frequency, reactor current and voltage, and discharge power and energy. The test gases consist of mixtures of nitrous oxide (N2O) in nitrogen or argon. The test gas mixture, maintained at room temperature, flows through the PCDR at a constant flow rate. The power supply parameters were shown in previous work.24 The experimental test matrix is shown in Table 1. Gas samples, collected from the discharge end of the PCDR in small stainless steel cylinders, are analyzed for stable species by (21) Tas, M. A.; van Hardeveld, R.; van Veldhuizen, E. M. Plasma Chem. Plasma Process. 1997, 17, 371-391. (22) Yan, K.; Van Heesch, E. J. M.; Pemen, A. J. M.; Huijbrechts, P. A. H. J. Plasma Chem. Plasma Process. 2001, 21, 107-137. (23) Collings, M. E.; Mann, M. D.; Young, B. C. Energy Fuels 1993, 7, 554-558. (24) Hu, X.; Nicholas, J.; Zhang, J. J.; Linjewile, T. M.; de Filippis, P.; Agarwal, P. K. Fuel 2002, 81, 1259-1268.

1524 Energy & Fuels, Vol. 18, No. 5, 2004

Zhao et al.

means of a Spectrum 2000 Perkin-Elmer Fourier transform infrared spectrometer with a narrow-band mercury-cadmiumtelluride detector. Different experimental phenomena are observed for N2O conversion in nitrogen and argon. A blue glow with some shining points is observed in the region close to the central wire for N2O conversion in nitrogen at all power inputs. For N2O conversion in argon, all of the discharge volume seems to be filled with dispersed thin bright streamer filaments bridging the entire gap between the central wire anode and the cylinder cathode. Pulsing of the voltage and current is the source of energy for the gas. The corona power can be calculated from the product of the measured pulse voltage and current (VI). The energy is the time integral (∫VI dt) of power. For our experiments, the energy per pulse is 0.183 J. The power consumed, W, can be calculated as the product of the input energy per pulse and the pulse frequency. Energy consumption levels for the reaction are thus obtained. For the plasma conversion of N2O in N2, N2O is transformed into nitrogen and oxygen without the formation of detectable amounts of NO or NO2, as found by Futamura et al.25 and Zhao et al.26 Thus, conversion of N2O can be defined as

x)

C 0 - Cτ moles of converted N2O × 100% ) × 100% moles of N2O at the inlet C0 (1)

where C0 and Cτ are the concentration of N2O at the inlet and outlet of the reactor, respectively. Energy consumption in kilojoule per mole of converted N2O can be converted to energy consumption in electronvolts per converted N2O molecule as follows:

(2) The distribution function for the electron velocity is Maxwellian. The rate constant for the electron-impact reaction A + e f B can be expressed as

k[e] ) β

dC1

10 eV/N2O ) (6.022 × 1023)(1.602 × 10-19) 1.036 × 10-2 eV/N2O (2) Therefore, energy consumption for plasma conversion of N2O for the specific PCDR used in this study can be calculated as follows:

W × 10.36 eV/N2O 74.99FC0x

(3)

where 74.99 is the number of moles per reactor volume (m3) at 189.0 kPa and 30 °C (the reaction was done at 30 °C), W is the power input (J/s), F is the flow rate of the gas mixture (in m3/s), and C0 is the concentration of N2O at the inlet of the reactor in ppm (v/v).

3. Model of a PCDR The PCDR described above is modeled using a lumped kinetic model that describes the evolution of all species. This model, originally proposed by Hu et al.27 and modified by Zhao et al.,28 includes the following assumptions. (1) Electron-impact reactions occur during the entire residence time, during both the pulse-on period and the pulse-off period. (25) Futamura, S.; Zhang, A.; Yamamoto, T. IEEE Trans. Ind. Appl. 2000, 36, 1507-1514. (26) Zhao, G. B.; Hu, X.; Argyle, M. D.; Radosz, M. Ind. Eng. Chem. Res. 2004, 43, 5077-5088. (27) Hu, X.; Zhang, J.-J.; Mukhnahallipatna, S.; Hamann, J.; Biggs, M. J.; Agarwal, P. Fuel 2003, 82, 1675-1684. (28) Zhao, G. B.; Hu, X.; Yeung, M. C.; Plumb, O. A.; Radosz, M. Ind. Eng. Chem. Res. 2004, 43, 2315-2323.

(

exp -

RP W

)

M

)

dt dC2

∑φ r ) g (C ,C ,...,C ;R ,β ,...,R ,β ;W ) 1j j

1

1

2

m

dt

1

n

n

k

M

)

dt

dCm

1

j)1

∑φ r ) g (C ,C ,...,C ;R ,β ,...,R ,β ;W ) 2j j

2

1

2

m

j)1

‚‚‚ ‚‚‚ ‚‚‚ ‚‚‚ 3

0.75

(4)

where P is the system pressure, W is the power input, and β and R are constants of proportionality. Thus, the rate constant for electron-impact reactions is similar to the conventional Arrhenius rate expression with power input in lieu of temperature. The parameter β includes the influence of reactor and electrode geometry on the reaction rate. Its effect on the reaction rate will be similar to that of the frequency factor in the Arrhenius rate expression. The effect of parameter R on the reaction rate is similar to that of the activation energy. (3) Axial dispersion is negligible, and hence gas flow can be considered as plug flow. On the basis of these assumptions, a set of lumped model equations including electron-impact reactions in the pulse-on period and subsequent bulk reactions can be written as

1 kJ/mol of N2O )

EN )

xRP1 W

M

)

∑φ j)1

mjrj

1

1

n

n

k

}

k ) 1, 2, ..., L

) gm(C1,C2,...,Cm;R1,β1,...,Rn,βn;Wk)

(5)

where Ci is the concentration of species i (i ) 1, 2, ..., m); t is time; M is the total number of reactions occurring in the pulsed corona reactor including electronimpact reactions and subsequent bulk reactions; φij is the stoichiometric coefficient of the ith species in the jth reaction, and rj is the reaction rate of the jth reaction (j ) 1, 2, ..., M); n is the number of electron-impact reactions, and Rj and βj represent the model parameters for the jth electron-impact reaction as shown in eq 4; Wk (k ) 1, 2, ..., L) is the power input for the kth experiment. Each experiment corresponds to a different concentration distribution in the reactor due to different power input. The model parameters are determined from experimental data. An optimization method proposed by Zhao et al.28 is used to determine R and β for electron-impact reactions. 4. Results and Discussion 4.1. Reaction Mechanism. 4.1.1. N2O in N2. Relatively little research has been done on the mechanism of N2O conversion. Willis et al.29 and Hu et al.24 suggested that N2+ should be responsible for N2O conversion. Hill et al.30 proposed that the excited state of the N atom, N(2D), is significant to N2O conversion (29) Willis, C.; Boyd, A. W.; Bindner, P. E. Can. J. Chem. 1972, 50, 1557-1567. (30) Hill, R. D.; Rahmim, I.; Rinker, R. G. Ind. Eng. Chem. Res. 1988, 27, 1264-1269.



Table 2. List of Chemical Reactions for Modeling the System N2O in N2 chemical reaction N2 + e f N + N + e N2 + e f N2(A) + e N2(A) + N2O f 2N2 + O N2(A) + N f N2 + N N2(A) + O2 f N2 + 2O N2(A) + O2 f N2O + O N2(A) + O2 f N2 + O2 N2(A) + O f N2 + O N2(A) + NO f N2 + NO N2(A) + NO2 f N2 + NO + O N + N + N2 f N2 + N2 O + O + N2 f O2 + N2 N + O + N2 f NO + N2 N + NO f N2 + O O + NO + N2 f NO2 + N2 NO2 + N f N2O + O NO2 + N f N2 + O2 NO2 + N f N2 + 2O NO2 + N f 2NO NO2 + O f NO + O2

rate constant [cm3/(mol‚s)] 10-5

R ) 2.238; β ) 3.216 × R ) 6.910; β ) 4.997 × 10-6 3.73 × 1012 2.71 × 1013 1.51 × 1012 4.70 × 1010 7.77 × 1011 1.81 × 1013 3.31 × 1013 7.83 × 1012 1.59 × 1015 [N2] 1.10 × 1015 [N2] 3.68 × 1015 [N2] 1.87 × 1013 k0 ) 3.62 × 1016 [N2] k∞ ) 1.81 × 1013 Fc ) 0.85 1.81 × 1012 4.21 × 1011 5.48 × 1011 1.38 × 1012 5.85 × 1012

source

no.

this work this work Herron and Green40 Herron and Green40 Herron and Green40 Kossyi et al.41 Kossyi et al.41 Herron and Green40 Herron and Green40 Herron and Green40 Kossyi et al.41 Kossyi et al.41 Kossyi et al.41 Atkinson et al.42 Atkinson et al.43

R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15

Atkinson et al.42 Kossyi et al.41 Kossyi et al.41 Kossyi et al.41 Atkinson et al.43

R16 R17 R18 R19 R20

Figure 1. Comparison of experimental data and predicted results of the lumped model for the conversion of N2O in N2: (a) 217 ppm N2O in N2; (b) 105 ppm N2O in N2; (c) 51.1 ppm N2O in N2; (9) experimental data; (s) prediction with 20 reactions; (-‚‚-) prediction with 10 reactions (R1-R8, R11, and R12; overlapped with the solid line); (-‚-) prediction without reaction R8; (‚‚‚) prediction without reactions R5-R7; (- -) prediction without reaction R4.

at the corona discharge. Thomas et al.31 investigated electric quenching of N2(A), which is the lowest excited electronic state of N2, with an energy threshold of 6.2 eV and with a lifetime close to 2 s (Guerra et al.),32 by O2, NO, CO, N2O, and C2H4. The large yield of O atoms for N2(A) quenching by N2O indicated that excitation to a dissociative state of N2O is the principal outcome of the reaction with N2(A). The major path for dissociation of N2O to N2 + O by N2(A) was further verified by Golde.33 Therefore, N2(A) may play a central role in initiating many energy-transfer and chemical reactions.33-37 Recently, Zhao et al.26 investigated the conversion mechanism of N2O in nitrogen through a carefully designed set of experiments coupled with theoretical analysis. It was found that N2(A) is the only active species responsible for conversion of N2O in nitrogen. (31) Thomas, J. M.; Kaufman, F.; Golde, M. F. J. Chem. Phys. 1987, 86, 6885-6892. (32) Guerra, V.; Sa, P. A.; Loureiro, J. J. Phys. D: Appl. Phys. 2001, 34, 1745-1755. (33) Golde, M. F. Int. J. Chem. Kinet. 1988, 20, 75-92. (34) Morrill, J. S.; Benesch, W. M. J. Geophys. Res. 1996, 101, 261274. (35) Simek, M. Plasma Sources Sci. Technol. 2003, 12, 454-463. (36) Simek, M. Plasma Sources Sci. Technol. 2003, 12, 421-431. (37) Guerra, V.; Sa, P. A.; Loureiro, J. J. Phys. D: Appl. Phys. 2001, 34, 1-11.

On the basis of this prior work,26 20 reactions shown in Table 2 are selected to simulate N2O conversion in N2. The optimum model parameters for electron collision reactions R1 and R2 determined28 are shown in Table 2. These are determined using experimental data for the initial concentration of 217 ppm N2O in N2 (Figure 1a). The same model parameters are used for predicting N2O conversion at other initial concentrations. Parts b and c of Figure 1 show the experimental and calculated data (N2O concentration versus power input). The good agreement between the calculated and experimental data confirms the predictive capability of the lumped model. At the same time, validation of the reaction mechanisms considered in the conversion of N2O in N2 in this work is also verified. As shown in Table 2, many species such as N, O2, O, NO, and NO2 can consume N2(A). These species may decrease the conversion rate of N2O. Contribution of NO and NO2 to consumption of N2(A) is negligible because NO and NO2 were not detected in the experiments for conversion of N2O. Model simulation shows that concentrations of NO and NO2 are less than 0.03 ppm at all power levels. Deletion of reactions pertaining to NO and NO2 such as R9, R10, and R13-R20 does not affect the model predictions as shown in Figure 1. Figure 1


Zhao et al.

Table 3. List of Chemical Reactions for Modeling the System N2O in Ar rate constant [cm3/(mol‚s)]

chemical reaction Ar+

Ar + e f + 2e Ar+ + e f Ar Ar+ + N2O f N2 + O + Ar O + O + Ar f O2 + Ar

10-5

R ) 6.647; β ) 2.549 × R ) 7.470; β ) 1.105 × 105 1.99 × 1014 1.10 × 1015 [Ar]

source

no.

this work this work Shul et al.38 Kossyi et al.41

R21 R22 R23 R24

Figure 2. Comparison of experimental data and predicted results of the lumped model for the conversion of N2O in Ar: (a) 217 ppm N2O in Ar; (b) 105 ppm N2O in Ar; (9) experimental data; (s) prediction with four reactions; (- -) prediction without reaction R22.

also shows the effect of the deletion of reactions R4, R5R7, and R8, respectively. These results indicate that N atoms, formed from nitrogen discharge (R1), strongly retard the conversion of N2O through reaction R4 at high power input. O2 molecules, formed from N2O conversion (R3 and R12), also retard the conversion of N2O through reactions R5-R7. Reaction R8 has a weak effect in retarding N2O conversion through the O atom. 4.1.2. N2O in Ar. Because of the low concentration of N2O, it is unlikely that energetic electrons react directly with N2O. Unlike N2O conversion in nitrogen, strong streamer channel bridging between the central wire anode and the tube cathode may indicate a large number of argon cations formed for N2O conversion in argon. Therefore, the charge-transfer mechanisms may play a significant role in the conversion of N2O in argon. Specifically, it is hypothesized that argon is ionized in the reaction chamber and N2O+ ions are produced through the charge-transfer mechanism. The activated N2O+ ions, on collision with an electron, are assumed to decompose to nitrogen and oxygen. Because the charge-transfer reaction

Ar+ + N2O f Ar + N2O+ k ) 1.99 × 1014 cm3/(mol‚s) is relatively slow38 and the subsequent electron-ion recombination dissociation reaction

N2O+ + e f N2 + O

k ) 1.20 × 1017 cm3/(mol‚s)

is considerably faster,39 the combination of these series of reactions into one reaction

Ar+ + N2O f Ar + N2 + O k ) 1.99 × 1014 cm3/(mol‚s) is acceptable. Therefore, the four reactions shown in Table 3 are deemed to be responsible for the conversion

of N2O in argon. The model parameters for R21 and R22 (also shown in Table 3 for the argon system) are determined using an optimization method proposed by Zhao et al.28 from the experimental data at an initial concentration of 217 ppm N2O in Ar. Figure 2 shows the experimental data and predicted data (solid line). The good agreement between the experimental and predicted data confirm the predictive capability of the lumped model. Similarly, validation of the proposed reaction mechanisms considered in the conversion of N2O in argon is verified. While the Ar+ cation is used for N2O conversion in argon, electron collision reaction R22 also consumes Ar+. Figure 2 shows the calculated results without considering reaction R22. These results suggest that reaction R22 strongly retards the conversion of N2O in argon, especially at high power input. 4.2. Energy Consumption. Using eq 1, the experimental data and predicted results in Figure 1 can be converted to curves of N2O conversion in N2 with power input at different initial concentrations of N2O as shown in Figure 3a. Not surprisingly, N2O conversion increases with power input. Similarly, using eq 3, the experimental data and predicted results in Figure 1 can be converted to curves of energy consumption corresponding to different power inputs at different initial concentrations of N2O as shown in Figure 3b. Both energy consumption and N2O conversion from experimental measurements and model predictions are in good agree(38) Shul, R. J.; Upschulte, B. L.; Passarella, R.; Keesee, R. G.; Castleman, J. A. W. J. Phys. Chem. 1987, 91, 2556-2562. (39) Matzing, H. Chemical Kinetics of Flue Gas Cleaning by Irradiation with Electrons. In Advances in Chemical Physics; Prigogine, I., Rice, S. A., Eds.; John Wiley & Sons: New York, 1991; Vol. LXXX, pp 315-402. (40) Herron, J. T.; Green, D. S. Plasma Chem. Plasma Process. 2001, 21, 459-481. (41) Kossyi, I. A.; Kostinsky, A. Y.; Matveyev, A. A.; Silakov, V. P. Plasma Sources Sci. Technol. 1992, 1, 207-220. (42) Atkinson, R.; Baulch, D. L.; Cox, R. A.; Hampson, J. R. F.; Kerr, J. A.; Troe, J. J. Phys. Chem. Ref. Data 1989, 18, 881-1097. (43) Atkinson, R.; Baulch, D. L.; Cox, R. A.; Hampson, J. R. F.; Kerr, J. A.; Rossi, M. J.; Troe, J. J. Phys. Chem. Ref. Data 1997, 26, 13291499.



Figure 3. Energy consumption and conversion at different initial concentrations for the conversion of N2O in N2 at a gas flow rate of 4.87 × 10-4 m3/s: (a) N2O conversion vs power input; (b) energy consumption vs power input; (c) energy consumption vs N2O conversion. Experimental data at initial concentration: 217 ppm (0); 105 ppm (O); 51.1 ppm (4). Predicted results at initial concentration: 217 ppm (s); 105 ppm (- -); 51.1 ppm (-‚-).

Figure 4. Energy consumption and conversion at different initial concentrations for the conversion of N2O in Ar at a gas flow rate of 4.07 × 10-4 m3/s: (a) N2O conversion vs power input; (b) energy consumption vs power input; (c) energy consumption vs N2O conversion. Experimental data at initial concentration: 217 ppm (0); 105 ppm (O). Predicted results at initial concentration: 217 ppm (s); 105 ppm (- -).

power input. The following linear relationships can be found at high power inputs (>50 J/s) (Figure 3b): Figure 5. Schematic diagram of PCDRs in series.

ment at different initial concentrations of N2O in N2. This implies that we can use model calculations to examine the energy consumption. The following observations can be made from Figure 3, whether from model predictions or from experimental measurements in the pulsed corona reactor: (1) Conversion of N2O is more sensitive to the power input at low power inputs than at high power inputs (Figure 3a). (2) For each initial concentration of N2O in N2, an optimal power input corresponding to the lowest energy consumption per removed N2O molecule exists (Figure 3b). A similar conclusion can be drawn from the experimental data provided by Mizuno et al.15 for the conversion of NO in N2. Because it is recognized that the power input corresponds to the pulse frequency, the minimum value of the energy consumption in Figure 3b indicates that an optimal pulse frequency corresponding to minimal energy consumption exists for a specific initial concentration of N2O in a specific plasma reactor (this does not include the optimization of the electrical system). (3) As initial N2O concentration decreases, the energy consumption, per N2O molecule converted, increases at the same power input, as does the minimal energy consumption, as shown in Figure 3b. (4) At high power inputs, the energy consumption per N2O molecule converted is linearly proportional to the

Initial concentration of N2O: 217 ppm, EN (eV/N2O) ) 197.1 + 1.19 W (6) Initial concentration of N2O: 105 ppm, EN (eV/N2O) ) 212.2 + 2.46 W (7) Initial concentration of N2O: 51.1 ppm, EN (eV/N2O) ) 236.1 + 5.28 W (8) The slope increases with decreasing initial N2O concentration. (5) N2O conversion at minimum energy consumption decreases with increasing initial N2O concentration (Figure 3c). (6) It is essentially impossible to completely remove N2O in N2 by increasing the power input (also pulse frequency) using a single pulsed corona reactor (Figure 3c). The dominant active species to convert N2O is N2(A). However, as shown in Figure 1, N formed from reaction R1, O2 formed from reaction R12, and O formed from reaction R3, R5, and R6 quench chemically active species N2(A). Especially, at high power input, N, O2, and O strongly retard further N2O conversion. This is used to explain the flat sections of the curves in Figure 1. This also indicates that the promotion of N2O conversion through increasing power input in a single reactor is not an energy-efficient strategy. The combination of PCDRs is necessary to decrease the energy consumption.


Zhao et al.

Table 4. Optimal Operation Conditions of Twelve Pulsed Corona Reactors in Series with the Lowest Energy Consumption for the Conversion of 217 ppm N2O in N2 at a Gas Flow Rate of 4.87 × 10-4 m3/s reactor no.

power input (J/s)

concn at the inlet (ppm)

concn at the outlet (ppm)

single conversion (%)

accumulated conversion (%)

single energy cost (eV/N2O)

accumulated energy cost (eV/N2O)

1 2 3 4 5 6 7 8 9 10 11 12

36.77 35.43 34.06 32.73 31.46 30.29 29.24 28.34 27.58 26.95 26.45 26.07

217.0 181.7 150.5 123.5 100.5 81.20 65.30 52.38 42.02 33.80 27.34 22.30

181.7 150.5 123.5 100.5 81.20 65.30 52.38 42.02 33.80 27.34 22.30 18.40

16.27 17.15 17.95 18.64 19.20 19.59 19.78 19.78 19.56 19.11 18.42 17.51

16.27 30.63 43.08 53.69 62.58 69.91 75.86 80.64 84.43 87.40 89.72 91.52

290.9 317.7 352.3 397.1 455.6 532.1 632.4 764.1 937.5 1166 1467 1865

290.9 303.5 317.6 333.3 350.7 369.7 390.4 412.5 436.1 460.9 487.0 514.1

Table 5. Optimal Operation Conditions of Four Pulsed Corona Reactors in Series with the Lowest Energy Consumption for the Conversion of 217 ppm N2O in Ar at a Gas Flow Rate of 4.07 × 10-4 m3/s reactor no.

power input (J/s)

concn at the inlet (ppm)

concn at the outlet (ppm)

single conversion (%)

accumulated conversion (%)

single energy cost (eV/N2O)

accumulated energy cost (eV/N2O)

1 2 3 4

17.56 14.75 11.06 7.25

217.0 134.8 70.21 28.38

134.8 70.21 28.38 8.50

37.90 47.89 59.59 70.06

37.90 67.64 86.92 96.09

71.28 76.31 88.27 121.77

71.28 73.51 76.79 81.08

Similar results for energy consumption can be obtained for the conversion of N2O in Ar. Figure 4 shows the energy consumption and conversion of N2O in the balance of Ar from experimental data and model predictions. The good agreement between experimental measurements and model predictions is once again confirmed. For N2O reaction in argon, attempting to achieve high conversion through increasing the power input in a single reactor results in high energy consumption mainly from quenching of active species Ar+ by reaction R22. 4.3. Reactor Configuration. There are three possibilities to increase N2O conversion (for example, to achieve 90% conversion) in either N2 or Ar at the same total amount of feed gas (for example, 217 ppm N2O in N2 at a total flow rate of 4.87 × 10-4 m3/s and 217 ppm N2O in Ar at a total flow rate of 4.07 × 10-4 m3/s). The first possibility is to increase the power input in a single reactor. However, as discussed above, this approach results in high energy consumption. The predicted energy input for the concentration and flow included above for 90% conversion of N2O in N2 is 909 W, and the energy consumption is 1279 eV/N2O (Figure 3c). The required energy input for the concentration and flow included above for 90% conversion of N2O in Ar is 69 W, and the energy consumption is 118 eV/N2O (Figure 4c). The second possibility is to operate at the minimal energy consumption point in a single reactor or in multiple reactors. As shown in Figures 3 and 4, the conversion at the minimum energy consumption point (corresponding to a specific energy input) in a single reactor is limited. To achieve 90% conversion, one must operate multiple PCDRs in series as shown in Figure 5. In-series operation of multiple PCDRs results in different concentrations of feed composition to each reactor; the effluent of one reactor becomes the feed of the next reactor. For example, for 217 ppm N2O conversion in N2, the feed of the first reactor is N2O + N2 but the feed of the second and succeeding reactors is N2O

Figure 6. Schematic diagram of parallel PCDRs.

+ N2 + O2. For this example, calculated results shown in Table 4 suggest that a series of 12 PCDRs is needed to convert 90% N2O in N2, for which the total power input is 365 J/s. The total energy consumption is 514 eV/N2O, which is 40% of the energy consumption for the same N2O conversion using a single reactor. Results shown in Table 4 also suggest that the required power input at minimal energy consumption decreases with increasing reactor number, but the energy cost per reactor increases with increasing reactor number. This is because the N2O concentration decreases and the O2 concentration increases with the addition of each reactor. Table 5 shows results of a similar calculation for N2O conversion in Ar. A series of four PCDRs is necessary to convert 90% N2O in Ar (actually, at optimal operation conditions, the conversion of N2O is 96%). The total power input is 51 J/s, and the energy consumption is 81 eV/N2O. However, the energy consumption for 96% conversion of N2O in Ar using a single reactor is 171 eV/N2O (Figure 4c), which is twice as high as that of a series of four PCDRs operating at the minimum energy consumption condition. The third possibility to promote N2O conversion is to increase the residence time (decrease the gas flow rate), which one can accomplish with multiple in-parallel PCDRs, as shown in Figure 6. Figures 7 and 8 show the effect of the gas residence time and power input for N2O conversion in N2 and Ar, respectively. For any gas residence time, there always exists an optimal power



Figure 7. Effect of the residence time and power input on energy consumption and conversion of N2O in N2. (a) energy consumption vs power input and gas residence time; (b) N2O conversion vs power input and gas residence time.

Figure 8. Effect of the residence time and power input on energy consumption and conversion of N2O in Ar. (a) energy consumption vs power input and gas residence time; (b) N2O conversion vs power input and gas residence time.

input corresponding to minimum energy consumption. However, for any power input, a lower residence time corresponds to a lower energy consumption (Figures 7a and 8a). This implies that minimizing the residence time at an optimal power input will minimize the energy consumption. However, as shown in Figures 7b and 8b, a low residence time always corresponds to a low conversion. In other words, low residence times are good for energy consumption but bad for conversion. This means that from the point of energy consumption the in-parallel operation of multiple PCDRs offers no advantage. For example, for in-parallel operation of 12 PCDRs for N2O in N2 at a gas flow rate of 1/12 of 4.87 × 10-4 m3/s, the required energy input for 90% N2O conversion in N2 is 55 J/s per reactor, which means a total energy input of 660 J/s for 12 reactors. The corresponding energy consumption is 997 ev/N2O, which is almost twice as large as that for in-series operation of 12 PCDRs.

Similarly, the energy consumption for 96% conversion of N2O in Ar using four parallel PCDRs at the same total amount of feed gas is 84 eV/N2O, which is 4% higher than that using four in-series PCDRs. 5. Conclusions The lowest excited electronic state of N2, N2(A), is the only active species responsible for N2O conversion in N2. Quenching of N2(A) by N atoms formed from N2 discharge and by O2 and O atoms formed from N2O conversion suppresses N2O conversion at high power input in a single PCDR. On the other hand, Ar+ is the only active species responsible for N2O conversion in Ar. Quenching of Ar+ by electrons suppresses N2O conversion in Ar at high power input in a single PCDR. From both experimental measurements and model predictions, an optimal power input with the lowest


energy consumption for a single PCDR is found to be dependent on the initial concentration of N2O in N2 or Ar at the same gas residence time (the same gas flow rate). At constant power input, increasing the gas residence time (reducing the gas flow rate) leads to an increase in the energy consumption. Operation at lower residence time (high gas flow rate), therefore, favors lower energy consumption. By a comparison of the energy consumption of three possible reactor configurations (single reactor, in-series multiple reactors, and in-parallel multiple reactors), the most effective method for decreasing the energy consumption is found to be in-series multiple PCDRs. Acknowledgment. Dr. Pradeep K. Agarwal, who initiated this work, passed away in 2002. His influence is both recognized and remembered. Dr. Morris Argyle made comments that enhanced this manuscript. This work was funded by the National Science Foundation (Grants CTS-9810040 and CTS-0078700) and the Department of Defense (Contract ARO-DAAD19-01-10488). Matching support from the Research Office at the University of Wyoming is also gratefully acknowledged.

Zhao et al. Ci ) concentration of the ith component in the reactor, mol/ cm3 Cτ ) concentration of N2O at the outlet of the reactor, ppm EN ) energy consumption per removed N2O molecule, eV/N2O F ) gas flow rate, m3/s k ) rate constant, cm3/(mol‚s) L ) total number of experiments m ) total number of species M ) total number of reactions n ) total number of electron-impact reactions P ) system pressure, atm rj ) reaction rate of the jth reaction, mol/(cm3‚s) t ) time, s V ) voltage, V W ) power input, J/s Wk ) power input for the kth experiment, J/s x ) conversion Greek Letters R ) model parameter, J/(atm‚s) β ) model parameter, J-0.25‚s-0.75 τ ) residence time, s φij ) stoichiometric coefficient of the ith species in the jth reaction

Notation C0 ) concentration of N2O at the inlet of the reactor, ppm

EF049966C

Energy Consumption and Optimal Reactor Configuration for

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