Transformations and destruction of nitrogen oxides—NO, NO2 and N2O—in a pulsed corona discharge reactor☆

Fuel 82 (2003) 1675–1684 www.fuelfirst.com

Transformations and destruction of nitrogen oxides—NO, NO2 and N2O—in a pulsed corona discharge reactorq Xudong Hua,*, Ji-Jun Zhanga, Suresh Mukhnahallipatnab, Jerry Hamannb, Mark J. Biggsa,c, Pradeep Agarwala,1 a

Department of Chemical and Petroleum Engineering, University of Wyoming, Laramie, WY 82071, USA b Department of Electrical Engineering, University of Wyoming, Laramie, WY 82071, USA c Department of Chemical Engineering, The University of Edinburgh, Edinburgh EH9 3JL, Scotland, UK

Received 21 June 2002; revised 20 December 2002; accepted 20 December 2002; available online 24 April 2003

Abstract There has been an increasing recent research interest in the removal of NOx from combustion gases using electrical discharges, especially pulsed corona discharge reactors. The major issues in development of this technology are (a) the energy consumption required to achieve the desired pollutant reduction; and (b) the formation of undesirable byproducts. In this study, the transformations and destruction of nitrogen oxides—NO, NO2 and N2O—were investigated in a pulsed corona discharge reactor. Gas mixtures—NO in N2, N2O in N2, NO2 in N2 and NO – N2O –NO2 in N2—were allowed to flow through the reactor with initial concentrations, flow rates and energy input as operating variables. The reactor effluent gas stream was analyzed for N2O, NO, NO2, by means of an FTIR spectrometer. In some experiments, oxygen was measured using a gas chromatograph. Reaction mechanisms were proposed for the transformations and destruction of the different nitrogen oxides within a unified model structure. The corresponding reaction rates were integrated into a simple reactor model for the pulsed corona discharge reactor. The reactor model brings forth the coupling between reaction rates, electrical discharge parameters, and fluid flow within the reactor. It was recognized that the electron-impact dissociation of the background gas N2 leads to both ionic and radical product species. In fact, ionic reactions were found responsible for N2O destruction. Radical reactions were dominant in the transformation and destruction of NO and NO2. However, decomposition of Nþ 2 ions also leads to indirect production of N radicals; this appears to be a less-power intensive route for NO destruction though longer residence times may be necessary. In addition, the decomposition of Nþ 2 ions limits the N2O destruction that can be achieved. Comparison with our experimental data, as well as data in the literature, was very encouraging. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: Pulsed corona reactor; Non-thermal plasma; NO removal; N2O removal; Electrical discharge

1. Introduction and background Exhaust gases from the combustion of fossil fuels—in industrial boilers, power generation, as well as automotive applications—have been linked to several environmental pollution problems. The link between acid rain and SOx, generated from fuel sulfur, was established many years ago. The harmful contributions of NOx to acid rain, and photochemical smog have been recognized more recently. * Corresponding author. Tel.: þ 1-307-766-6312; fax: þ1-307-766-6777. E-mail address: [email protected] (X. Hu). q Published first on the web via Fuelfirst.com—http://www.fuelfirst.com 1 Deceased, September 21, 2002.

Control of these emissions has now become an international issue, with considerable pressures for the adoption of increasingly more stringent emission standards. Control strategies for NOx—defined most often to include as NO and N2O—can be divided into two major categories—combustion modifications; and post-combustion flue gas treating processes [1,2]. To reduce NOx, combustion modifications seek to decrease (a) the oxygen level at the peak temperature; and (b) peak temperature and residence time in the combustion zone [1 – 3]. However, combustion modifications may not be sufficient to achieve the very stringent emission standards in place already in many densely populated regions such as Japan, Germany, and states such as California in USA. Post-combustion

0016-2361/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0016-2361(03)00079-6

1676

X. Hu et al. / Fuel 82 (2003) 1675–1684

[NO]

Nomenclature b D½C ½e E0 E EC EY I kB ki me MW n [NO]0

cross-section parameter (m2) NOx destroyed (ppm), Eq. (1) electron concentration (molecules m23) threshold energy of free electrons (J) kinetic energy of free electrons (J) energy cost, Eq. (1) (eV molecule21) energy yield, Eq. (2) (kJ m23) current (A) Boltzmann constant (1.38 £ 10223 J K21) reaction rate constants electron mass (9.11 £ 10228 g) molecular weight reaction order initial concentration of NO (ppm molecules m23)

or

flue gas treatment processes, though substantially more expensive, have to be adopted. Current commercial practice involves the injection of ammonia into the flue gas stream to chemically reduce NOx usually in the presence of a catalyst. Secondary pollution by the reducing agent, fouling and loss of expensive catalyst are some of the problems encountered [4]. Another option considered promising [2] is catalytic oxidation to convert NO to NO2 which is more soluble in caustic solutions. Alternatively, injection of ozone would lead to the conversion of both NO and NO2 to N2O5 which is even more soluble in caustic solutions. However, these technologies appear to be transferring pollutants from one process stream to the other, as treatment/disposal of the spent caustic stream will, eventually, be necessary. Recently, fluidized bed combustion (FBC) has emerged as an environmentally attractive technology due to its low operating temperature, typically between 1025 and 1200 K. The low operating temperature results in lower NOx emission. Unfortunately, lower combustion temperature gives rise to a higher N2O emission, ranging from 15 to 200 ppm compared to the pulverized coal combustion boilers at 5 ppm. This raises concern over increased emission levels of N2O—a potent greenhouse gas and stratospheric ozone depletion agent [5]. We are currently investigating the use of electrical discharges, specifically pulsed corona discharge reactors, for treating nitrogen oxides—NO, NO2 and N2O—in exhaust gases from stationary and mobile power generation sources. The major issues [6] in development of this technology are: (a) the energy consumption required to achieve the desired pollutant reduction; and (b) the formation of undesirable byproducts. The pulsed corona discharge reactor appears to be energy efficient [7]—in comparison with other non-thermal plasma technologies, including those based on ac/dc corona or silent discharges— and recent economic evaluation by MITI in Japan concludes that this technology deserves to be developed for dry

P SIE Te V V VR W a b s t i j

concentration of NO at the outlet of reactor (ppm or molecules m23) pressure in the plasma zone (atm) specific input energy (J L21) electron temperature (K) volumetric flow rate (m3 s21) voltage (v) reactor volume (m3) power input to the reactor (W) constant (J atm21 s21) constant (J21) collision cross section for a reaction (m2) residence time (s) species index reaction index

deNOx/deSOx processes for utility thermal power plant boilers [8]. More recently, the effects of reactor and electrode geometry [9,10], catalysts [11,13] and additives—such as ammonia [10,14], hydrocarbons [11,12, 14– 17] and moisture, [12,14] among others [16,17]—on system performance have been investigated. Preliminary results from industrial experiments have also been reported [18] for NOx removal from the flue gas generated by power plants. Results on the corona discharge processing of exhaust gases from gasoline and diesel engines have also been reported recently [19,20]. Destruction of N2O was also recently demonstrated in our previous paper [21]. An exhaustive review on the experimental data on the use of electrical discharges, including corona discharge systems, for air pollution control has been published recently [22]. There are considerable difficulties in developing a system model for such reactors. A principal issue is the very large number of ionic and radical reactions. For example, Penetrante et al. [23] considered—for the case of NO removal in an atmosphere of nitrogen—a total of 287 reactions and 37 (charged and neutral) species. Inclusion of carbon oxides, moisture and other species would increase the number of reactions [24] to in excess of 750. With concomitant increase in number of species and, in turn, the increased number of coupled differential equations must be solved. Then, the coupling of reaction kinetics with electrical discharge parameters must be included. Under the continuum assumption, the electric field can be calculated from the applied voltage difference and the specified reactor geometry using Poisson’s equation. The electric field, in turn, can be used to estimate the electron energy distribution function using the Boltzmann equation. The electron energy distribution function may be used to estimate the reaction (ionization) rates between the electrons and the neutral gas species. These reaction terms can then be included in the continuity equations for the reactive species in the system [15,25]. Under the best of

X. Hu et al. / Fuel 82 (2003) 1675–1684

conditions, a system of stiff coupled partial differential equations is encountered. The major difficulty remains in the accurate treatment of the non-equilibrium charged particles in strong electric fields involving severe gradients. Identification of appropriate boundary conditions, too, remains difficult [26]. The detailed modeling of such systems, then, is difficult. On the other hand, simple models that bring out the essential features of reaction, discharge parameters, and fluid flow within the reactor are also not available. Several empirical measures that describe energy consumption and conversion efficiency are popular. For example [13,22], Energy Cost; EC ðin eV molecule21 Þ ¼ Energy Yield; EY ðin gðkW h21 ÞÞ ¼

SIE £ 250 D½C

D½CMW £ 0:15 SIE

ð1Þ ð2Þ

SIE is specific input energy, defined as the ratio of discharge energy per unit volume; in Eqs. (1) and (2) the units are kJ m23. D½C represents NO (or NOx) removed; in Eqs. (1) and (2) the units are ppm. MW is the molecular weight of the specie(s) under consideration. The factors of 250 and 0.15 are conversion factors at 20 8C and 1 atm. Published data suggest that 30 , EC , 75 eV molecule21 for NO removal [13,22] though Penetrante et al. [23] report a value as high as EC ¼ 238 eV molecule21 for NO removal in nitrogen. Also, experimental data in the literature lead to 2 , EY , 13 g (kW h21) for NOx removal [13]. The dependence of these macroscopic and empirical measures on the operating variables—initial concentration, among others—remains unclear [22]; the wide spread in values underscores the limited value of such measures for equipment design and economic evaluation. In this study, the transformations and destruction of nitrogen oxides—NO, NO2 and N2O, with N2 as the balance gas—are investigated experimentally in a pulsed corona discharge reactor. In addition, a reduced reaction set is proposed for the transformations and destruction of the different nitrogen oxides within a unified model structure. The reaction rates—for this reduced reaction set—is integrated into a simple flow model for the pulsed corona discharge reactor. The reactor model brings forth the coupling between reaction rates, electrical discharge parameters, and fluid flow within the reactor. Comparison with our experimental data, as well as data in the literature, is very encouraging. The utility of the empirical measures SIE, EC and EY is also discussed.

2. Experimental methods 2.1. Experimental apparatus The pulsed corona reactor (PCR) consists of a highvoltage power supply and control unit, and the pulser/reactor

1677

assembly. 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 pulsed corona discharge reaction chambers. These two sub-units are connected by a high voltage cable for charging the capacitors in the pulsed power system and by high-pressure gas lines for controlling the voltage delivered to the reactor. Electrical and switch gas supplies are connected to the control unit; the reactor gas supply and exhaust lines are connected directly to the reactor. The reactor consists of ten parallel reaction tubes; the length of each tube is 91.4 cm, with inner diameter of 2.3 cm. Some of these tubes are fitted with UV-grade quartz windows for diagnostics and plasma observation. In all the experiments described in this work, only four out of ten tubes were wired for plasma generation. The design permits the varying and measurement of applied voltage and its frequency, reactor current and voltage, and discharge power and energy. The high voltage supply in the control cabinet charges discrete capacitors located inside the pulser subassembly. Once the voltage on the capacitors is sufficiently high, a high-pressure spark gap (hydrogen) switch located in the pulser closes, connecting the capacitors to the reactor anodes. The high voltage applied to the reactor anodes causes the breakdown of gases flowing through the reactor, creating plasma. The energy from the capacitors is then discharged very quickly into the plasma. Because the average electron energy or temperature is much higher than that of the bulk gas molecules, the plasma is referred to as non-thermal or non-equilibrium. High voltage pluses produce short-lived micro-discharges that preferentially accelerate the electrons without imparting significant energy to the ions. In addition, since most of the applied energy goes to accelerating the electrons rather than the massive ions, larger reactor volumes are possible because the high-energy electrons are capable of filling larger volumes [7]. Pulsing of the voltage and current allows energy to be deposited in the gas in a highly concentrated form. The corona power can be calculated from the product (VI) of the measuredÐpulse voltage and current. The energy is the time integral ð VI dtÞ of power. Typical waveforms have been presented earlier [21]. The power consumed can also be calculated as the product of the input energy per pulse, and the pulse frequency. 2.2. Experimental procedure The test gases consisted of mixtures of various types of nitrogen oxides in nitrogen. The test gas mixture, kept at room temperature, flowed through the PCR at various flow rates. The experimental test conditions are summarized in Table 1. Gas samples, collected from the discharge end of the PCR in small stainless steel cylinders, were analyzed for stable species by means of a PE Spectrum 2000 FTIR

1678

X. Hu et al. / Fuel 82 (2003) 1675–1684

Table 1 Experimental conditions Reference

Initial Flow rate (m3 s21) concentration (ppm)

NO

Present work Present work Present work Present work Penetrante et al. [23] Penetrante et al. [23] Mok et al. [14]

207 410 600 614 100 (373 K) 100 (573 K) 211

6.29 £ 1024 6.29 £ 1024 4.72 £ 1024 6.29 £ 1024 1.47 £ 1023 2.27 £ 1023 1.67 £ 1024

N2 O

Present work Present work Present work Present work

51 96 105 217

6.29 £ 1024 4.72 £ 1024 6.29 £ 1024 6.29 £ 1024

NO2 þ O2

Present work

NO2: 495 O2: 800

4.72 £ 1024

NO: 493

4.72 £ 1024

Present work NO þ NO2 þ N2O þ O2

NO2: 311 N2O: 306 O2: 358

Ci denotes the concentration of the ith species, tð¼ VR =vÞ represents the residence time in the reactor of volume VR for the gas flowing at a volumetric flow rate of v; and fij represent the stoichiometric coefficients of the ith species in the jth reaction with reaction rate Rij : Over the years, several fundamental investigations on reaction kinetics have been reported on the radical and ion—molecule chemistry of nitrogen oxides—several comprehensive tabulations are available [24,27 –30]. The major difficulty in reactor systems involving electrical discharges appears to be the representation of electron – molecule reaction kinetics. These rates, in principle, must depend on the concentration of electrons as well as their energy distribution. For simplicity, assuming that the Maxwellian distribution can be used to represent electron velocities, the rate constant for electron-impact reactions can be expressed as:        8kB Te 1=2 1 2 ð1 2E k¼ EsðEÞexp dE ð4Þ kB Te kB Te pme E0 Assume also, following Huang and Suib [31], that the electron temperature is proportional to the power input, kB T e ¼

Spectrometer with a narrow band MCT detector. The spectral resolution was set at 0.5 cm21. The gas cell used for analysis was a Foxboro LV7 variable path-length multi-pass low volume (223 ml) cell. The path-length could be varied from 0.25 to 7.25 m; in the experiments described, the pathlength was set at 7.25 m. The instrument was calibrated for N2O, NO, and NO2 using certified concentration gas cylinders from US Airgas. Oxygen concentration was measured, in some experiments, using an HP 5890 Series II gas chromatograph equipped with a TCD (thermal conductivity detector), an HP 3396 Series II integrator, and a 6 ft packed CTR I column.

E0 W aP

ð5Þ

E0 is the threshold energy for ionization, P is the system pressure, W is the power input and a21 is a constant of proportionality. The collision cross-section for reaction can be represented [32] as:

sðEÞ ¼ 0 for E , E0 " # E0 for E $ E0 sðEÞ ¼ b 1 2 E

ð6Þ

Use of Eq. (4) with Eqs. (5) and (6) yields k1 / W 0:5 expð2aP=WÞ

ð7Þ

Finally, let us assume that 3. Mathematical model

½e / W n

3.1. Model equations Plug flow of gas, and other heavier charged species, is assumed through the cylindrical isothermal reactor with gas flow being coaxial with the electrodes. That there is no appreciable heating of the gas was verified through measurement of the temperature of the exit gas. The electrons, on the other hand, are assumed to be uniformly distributed in space (that is, well-mixed) since flow of electrons is primarily perpendicular to the direction of gas flow. Conservation equations for individual species can, therefore, be written [15] as: X dCi ¼ fij Rij dt j

ð3Þ

ð8Þ

where n must be determined from comparison with experimental data. In our previous paper [21], based on experimental data on the destruction of N2O, it was recommended that n ¼ 0:25: Thus, k½e ¼ bW 0:75 expð2aP=WÞ

ð9Þ

The form of Eq. (9) is similar to the temperature dependence in the conventional Arrhenius rate expression with input power in lieu of temperature. The parameter b includes the influence of reactor and electrode geometry on reaction rate. Its effect on the reaction rate will be similar to that of the frequency factor in the Arrhenius rate expression. The parameter a includes the threshold ionization potential, and its effect on reaction rate would be similar to that of the activation energy. Thus, lower a

X. Hu et al. / Fuel 82 (2003) 1675–1684

1679

Table 2 Reactions, and rates of reaction for transformation of nitrogen oxides in nitrogen plasma No.

Chemical reactions

1.

N2 þ e ! Nþ 2 þ 2e

2.

N2 þ e ! 2N þ e

3.

Nþ 2 þ e ! 2N

4.

N2Oþ þ e ! N2 þ O

5. 6. 7. 8. 9. 10.

N þ NO ! N2 þ O NO þ O ! NO2 O þ O ! O2 NO2 þ N ! N2O þ O NO2 þ O ! NO þ O2 þ Nþ 2 þ N2O ! N2O þ N2

Rate constant (cm3 mol21 s21)

a

b

5 3 5 20 20 5 5 20 20 2.04 £ 1013 1.40 £ 1012 1.02 £ 1011 1.81 £ 1012 5.84 £ 1012 3.01 £ 1014 1.81 £ 1014

1.26 £ 1026 7.00 £ 1027 8.19 £ 1027 2.00 £ 1026 1.30 £ 1026 104 650 102 65

Reference

Present work Mok et al. [14] Penetrante et al. [23] Present work, Mok et al. [14] present work, Mok et al. [14] Penetrante et al. [23] Present work, Mok et al. [14] Penetrante et al. [23] Willis and Boyd [29] Atkinson et al. [28] Willis and Boyd [29] Atkinson et al. [28] Atkinson et al. [28] McDaniel et al. [30] Matzing [24]

For electron– molecule reactions, kj ½e ¼ bj W 0:75 expð2aj P=WÞ:

would lead to reaction at lower input power levels, and reactions with higher a would be preferred at higher input power levels. A MATLAB code was written to solve the system of coupled ordinary differential equations. The reaction set found satisfactory for our data, and other data in the literature, and the corresponding reaction rates for the transformation of different nitrogen oxides are provided in Table 2. 3.2. Reaction system The minimum reaction set for modeling the transformations of the nitrogen oxides in nitrogen was developed by first considering the behavior of NO in N2; several investigators, for example [6,14,15,17,23,33 – 35], have examined this system. There appears to be some consensus [14,15,23] that this system involves, at least, reactions (2), (5) – (9) in Table 2. Thus, electron-impact leads to the formation of N radicals through reaction (2). These radicals combine with NO providing the primary mechanism for NO destruction, reaction (5). The O radicals formed in reaction (5) may combine together to form O2 through reaction (7), or with NO leading to the formation of byproduct NO2, reaction (6). This NO2, in turn, can be reduced by N radicals to form byproduct N2O through reaction (8), or oxidized by O radicals to form secondary NO, reaction (9). Since the rates for reactions (5) –(9) are available in the literature, aj and bj for the rate of electron-impact reaction (2) can be obtained through comparison with data. This, in fact, has been the generic approach in previous investigations

[14,15], where the rates for the electron-impact reaction(s) were treated as adjustable parameters; however, extrapolation to other systems remains difficult. However, the reaction set identified thus far is not complete since no mechanism has been included for the destruction of N2O, which involves, in our opinion [21], its interaction with Nþ 2 ionic specie. The importance of including the reaction for the production of ionic specie has been discussed by several authors [25,34,36], even for NO destruction. It then becomes necessary to include the ionization reaction (1) in Table 2 for the formation of the Nþ 2 ionic specie. This ionic specie, in turn, may interact with electrons to form N radicals, reaction (3)—this reaction, in fact, lowers the rate for charge transfer reaction (10) and permits the presence of N2O in nitrogen containing exit gas streams even at higher input power levels in accordance with experimental data discussed in Section 4. There is also another important consequence for NO destruction that will be discussed in context of simulation of the data of Mok et al. [14] in Section 4. Finally, N2Oþ ions—formed through the charge transfer reaction (10)—undergo electron impact dissociation according to reaction (4). The number of parameters—aj and bj s (j ¼ 1 to 4) for the electron-impact reactions—to be determined has increased. However, these can be obtained through consideration of data on the behavior of NO in N2, and N2O in N2 systems. The predictive ability was then tested through application of the model equations to other data in the literature, and to other systems like NO2 in N2, and mixtures of nitrogen oxides—NO, N2O and NO2—in N2.

1680

X. Hu et al. / Fuel 82 (2003) 1675–1684

4. Results and discussion 4.1. NO in N2 The variation in NO concentration in the exit gas stream as a function of input power, and also as a function of SIE (in J L21), is shown in Figs. 1 – 4. Several initial concentrations of NO—ranging from 207 to 614 ppm— and two flow rates—4.72 £ 1024 and 6.29 £ 1024 m3 s21were used Table 1. The experimental data show that complete removal of NO is possible. Both NO2 and N2O are formed as byproducts. NO2 reaches a maximum at an intermediate input power level; eventually, it too is destroyed as power level is increased. The formation of byproduct NO2 with a maximum at an intermediate input power level has been measured in other investigations [23, 34]. On the other hand, stabilization with increasing input power level has also been reported [14]. A small amount of N2O is formed and its concentration increases at higher power levels. Though the formation of N2O has been postulated mechanistically, and the increasing concentration with increase in input energy predicted through calculation [15], the measurements reported here are thought to be new. Figs. 1 –4 show that the model calculations are in reasonable agreement with experimental data. For NO and NO2, the discrepancy between experimental data and model simulation is within 20%. The predicted N2O is generally a lower than the experimental measurement, particularly for higher power levels and for lower initial concentration of NO. At least one contributing factor may be uncertainty in the rate of reaction for þ Nþ 2 þ N2 O ! N2 O þ N2 :

Two rate constants, differing by 40%, have been reported in the literature Table 2. Calculations shown here are based on the slower charge transfer reaction rate from Matzing [24].

Fig. 1. Comparison of model calculation with experiment—207 ppm NO in N2, flow rate ¼ 6.29 £ 1024 m3 s21. NO: simulation (—), data (X); NO2: simulation (– – –), data (O); N2O: simulation (· · ·), data (B).

Fig. 2. Comparison of model calculation with experiment—410 ppm NO in N2, flow rate ¼ 6.29 £ 1024 m3 s21. NO: simulation (—), data (X); NO2: simulation ( – – – ), data (O); N2O: simulation (· · ·), data (B).

Now consider the applicability of the model and the reaction set identified for other data on NO destruction reported in the literature. Consider first the data reported by Mok et al. [14] using a reactor 0.07 m in diameter, and 3 m in length. The slow flow—10 L min21—allowed a far greater residence time, t ø 70 s; in comparison with our experiments where the residence time was of the order of 2 – 3 s. A comparison between model predictions and their experiment data for NO and NO2 is shown in Fig. 5. The agreement is very good. aj s and bj s—which include the effect of electrode geometry and pulse generation circuitry—have to be adjusted as different electrode geometry and pulse generation exist between Mok’s and our experiment. The power consumption in the work of Mok et al. [14] is very low. Calculations revealed that reaction (2), representing break-up of nitrogen to form N radicals, did not occur at all since the aj value is high for the power levels under consideration. This would argue against the radical mechanism for NO destruction. On the other hand, Nþ 2 formation did occur since the aj value in reaction (1) is much lower. Of course, Nþ 2 ions, in turn, will dissociate to

Fig. 3. Comparison of model calculation with experiment—600 ppm NO in N2, flow rate ¼ 4.72 £ 1024 m3 s21. NO: simulation (—), data (X); NO2: simulation ( – – – ), data (O); N2O: simulation (· · ·), data (B).

X. Hu et al. / Fuel 82 (2003) 1675–1684

1681

Fig. 4. Comparison of model calculation with experiment—614 ppm NO in N2, flow rate ¼ 6.29 £ 1024 m3 s21. NO: simulation (—), data (X); NO2: simulation ( – – –), data (O); N2O: simulation (· · ·), data (B).

form N radicals that are responsible for NO destruction. Calculations omitting reaction (2) are also shown in Fig. 5 to substantiate this discussion. This result has two important consequences. First, it confirms the necessity of including ionic reactions in NO destruction mechanisms [25,34]. Second, the indirect production of N radicals for NO destruction through the less power-intensive route of Nþ 2 dissociation poses interesting practical implications. Of course, residence time may have to be increased. Consider next the data reported by Penetrante et al. [23] using a reactor 0.06 m in diameter and 0.30 m in length. The reactor was operated at 373 and 573 K, leading to with residence times of 0.573 and 0.373 s, respectively, with the flow rate maintained at 65 slpm for both temperatures. Model calculations are compared with these data in Fig. 6a and b. In both cases have been adjusted to 65% of those from our experiment. This is not surprising—Penetrante et al. [23] reported an energy cost ø 240 eV molecule (NO)21 far higher than reported in the literature [13,14,22],

Fig. 6. (a) Comparison of model calculation with experimental data of Penetrante et al. [23]—100 ppm NO in N2, bulk temperature ¼ 100 8C, flow rate ¼ 1.47 £ 1024 m3 s21. NO: data (X, O); original simulation: ( – – –, -·-·-); modified simulation: (—, · · ·). (b) Comparison of model calculation with experimental data of Penetrante et al. [23]—100 ppm NO in N2, bulk temperature ¼ 300 8C, flow rate ¼ 2.27 £ 1024 m3 s21. NO: data (X, O); original simulation: ( – – –, -·-·-); modified simulation: (—, · · ·).

and in the present work. However, with adjusted values of bj ; the agreement is excellent at both temperatures. 4.2. N2O in N2

Fig. 5. Comparison of model prediction with experimental data of Mok et al. [14]—211 ppm NO in N2, flow rate ¼ 1.67 £ 1024 m3 s21.

Several different initial concentrations of N2O—ranging from 51 to 217 ppm—and two flow rates—4.72 £ 1024 and 6.29 £ 1024 m3 s21—were used. In Fig. 7, model calculations are compared with experimental data for an initial concentration of nominally 100 ppm using a flow rate of 4.72 £ 1024 m3 s21. Additional calculations are included to show that if reaction (3)—corresponding to the breakdown of the ionic specie Nþ 2 into radicals—is not included in the reaction set, the depletion of N2O is achieved at considerably lower input power levels. In an earlier investigation [21], experiments suggested that N2O depletion in argon was far more energy-efficient. Clearly, one reason is that Arþ ions are neutralized only by transferring charge to N2O

1682

X. Hu et al. / Fuel 82 (2003) 1675–1684

Fig. 7. Comparison of model calculation with experiment—96.6 ppm N2O in N2, flow rate ¼ 4.72 £ 1024 m3 s21. Data: (X); original simulation: (—); simulation without reaction 3: ( – – –).

(leading to subsequent N2O decomposition through electron impact dissociation of N2Oþ), whereas Nþ 2 ions will dissociate into radicals that do not contribute to N2O decomposition. Further, as shown in Fig. 8, model calculations compare very well with experimental data for three initial concentrations using a flow rate of 6.29 £ 1024 m3 s21. In summary, the destruction role of the radical and ionic species in the destruction of different nitrogen oxides is illustrated in Fig. 9. 4.3. NO2 in N2 Further verification of the model’s predictive ability was sought through comparison with the measured behavior of a gas containing an initial concentration of 495 ppm NO2 in N2. Its decomposition, using a flow rate of 4.72 £ 1024 m3 s21, led to the formation of N2O as well as NO Fig. 10. Also shown in this figure are predictions for different nitrogen oxides; the agreement with experiment is very good. In this experiment,

Fig. 8. Comparison of model prediction with experiment—N2O in N2, flow rate ¼ 6.29 £ 1024 m3 s21. Initial N2O concentration at 51.1 ppm: data (X), simulation (· · ·); at 105 ppm: data (O), simulation ( – – – ); at 217 ppm: data (B), simulation (—).

Fig. 9. Role of ionic and radical species in the destruction of different nitrogen oxides.

oxygen concentration was also measured using a gas chromatograph; the calculations, somewhat higher, certainly follow the measured trend very well. 4.4. NO –N2O–NO2 in N2 Finally, as another verification of the model’s predictive ability, experiments were performed with a gas mixture containing all three nitrogen oxides—that is, 311 ppm NO2, 306 ppm N2O, and 493 ppm NO. Comaprison between model predictions and experimental data—for a flow rate of 4.72 £ 1024 m3 s21—is shown in Fig. 11. Once again, very good agreement is seen for the nitrogen oxides, as well as oxygen. 4.5. Global reaction kinetics The success of the model suggests that the concepts developed herein be used to examine the validity of

Fig. 10. Comparison of model prediction with experiment—495 ppm NO2 þ 800 ppm O2 in N2, flow rate ¼ 4.72 £ 1024 m3 s21. NO: simulation (—), data (O); NO2: simulation (– – –), data (X); N2O: simulation (· · ·), data (B); O2: simulation (-·-·-), data (V).

X. Hu et al. / Fuel 82 (2003) 1675–1684

Fig. 11. Comparison of model prediction with experiment—493 ppm NO þ 311 ppm NO2 þ 306 ppm N2 O þ 358 ppm O2 in N2, flow rate ¼ 4.72 £ 1024 m3 s21. NO: simulation (—), data (O); NO2: simulation ( – – – ), data (X); N2O: simulation (· · ·), data (B); O2: simulation (-·-·-), data (V).

the macroscopic empirical measures such as SIE, EC and EY : The rate-controlling step for the NO and possibly NOx) removal in electrical discharge systems is likely to be the electron impact reaction(s); thus, it may be convenient to write the overall global reaction mechanism and the corresponding rate as NO þ e ! Products d½NO ¼ 2k½e½NOn dt

ð10Þ

where n is the global reaction order. Using the approximation-based on Eq. (9)—that k½e / W 0:75 ; integration leads to: 2 ½NO12n Þ / W 0:75 t ð½NO12n 0

ð11Þ

[NO]0 is the initial (or inlet) concentration of NO and [NO] is its concentration in the exit gas. From trial and error using experimental data discussed in previous sections, it was found that n ø 0:25: This is in agreement with Luo et al. [17] who proposed this global order based on their data on higher concentrations of NO with helium as the background 3/4 3=4 gas. A plot of ([NO]3/4 t is shown in 0 2 [NO] ) versus W Fig. 12. It is clear that our data, for different initial concentrations, fall on one single line on this plot. Also, the data of Penetrante et al. [23] form another straight line with a steeper slope. Finally, the data of Mok et al. [14] form another straight line with a much smaller slope, indicative of larger residence time. This suggests that n ¼ 0:25 provides a simple global reaction order that may be more convenient (in comparison with detailed reaction schemes) to use in reactor design. The slope, however, must be determined independently for different electrode geometries, and other electrical discharge parameters. On the other hand, the global reaction order of n ¼ 0:25 also indicates why the conventional EC and EY measures are not necessarily adequate. A little thought indicates that the definitions of

1683

Fig. 12. Plot of [NO]0.75 –[NO]0.75 versus W 0:75 t: Experimental data: X, 0 Penetrante et al. [23]; *, Mok et al. [14]; V, 207 ppm NO in N2 (flow rate ¼ 6.29 £ 1024 m3s21); B, 410 ppm NO in N2 (flow rate ¼ 6.29 £ 1024 m3s21); O, 614 ppm NO in N2 (flow rate ¼ 6.29 £ 1024 m3s21); þ , 600 ppm NO in N2 (flow rate ¼ 4.72 £ 1024 m3s21).

these parameters are based on the assumption that n ¼ 0; only then will the energy cost be constant independent of the initial concentration. For positive non-zero reaction orders, the energy cost must increase as concentration decreases. The term on the right hand side of Eq. (11) suggests why SIE may not be a complete measure for scale-up. Assume for argument that the power dependence of the electronimpact reaction rate is linearly dependent on input power (k½e / W instead of k½e / W 0:75 ); then the right hand side of Eq. (11) can be rewritten the product of SIE and VR : Thus, information on reactor volume must necessarily complement plots that present the variation of concentration as a function of SIE.

5. Conclusions The transformations and destruction removal of nitrogen oxides, with nitrogen as the bulk gas, was investigated in a pulsed corona reactor. Gas mixtures containing nitrogen oxides—NO, N2O, and NO2 and their mixture—were allowed to flow in the reactor with initial concentrations, flow rates and energy input as operating variables. The reactor effluent gas stream was analyzed for N2O, NO, NO2, by means of an FTIR spectrometer. In some experiments, oxygen was measured using a gas chromatograph. The investigation leads to the following major conclusions: † N2O decomposes to form nitrogen and oxygen. The mechanism for its destruction involves Nþ 2 ions produced by primary ionization of nitrogen, followed by the charge transfer reaction to form N2Oþ, and finally electronimpact dissociation. The extent of destruction is, however, limited by the conversion of Nþ 2 ions to N radicals. This is thought to be the reason why destruction of N2O in argon [21] proceeds far more readily than in nitrogen.

1684

X. Hu et al. / Fuel 82 (2003) 1675–1684

† The destruction of NO and NO2 involves N radicals. These radicals may arise from direct electron impact dissociation of nitrogen. Alternatively, they may form from the electron-assisted conversion of Nþ 2 ions. Inclusion of both routes is necessary in the detailed modeling. In fact, the indirect route provides an interesting, less power-intensive, method for NO destruction though longer residence times may be necessary. † A simple mathematical model has been developed. The gas molecules are assumed to be in plug flow, whereas the spatial density of electrons is assumed to be uniform. Assuming Maxwellian distribution for electron velocities, the reaction rates for electron impact reactions were modeled in a form analogous to the conventional Arrhenius reaction rate with input power replacing temperature. A minimum set of reactions, and corresponding electron-impact reaction rates, were identified using experimental data on NO in N2 and N2O in N2 systems. This model was able to predict the behavior of the decomposition of NO2, and a gas mixture containing NO, NO2 and N2O in our reactor. The model calculations also compare favorably with data in the literature on the destruction of NO in nitrogen. However, more work is needed, especially to improve the N2O simulation. † Investigation of global kinetics leads to the conclusion that NO decomposition can be described by an overall reaction order of n ¼ 0:25: The definitions of macroscopic measures like energy cost, energy yield are more consistent with an assumption that n ¼ 0: For positive non-zero reaction orders, the energy cost must increase as concentration decreases. Data presented in terms of specific input energy (SIE) must be viewed in conjunction with reactor volume to be useful for scale-up.

Acknowledgements The authors are grateful to Ron Borgialli and Frank Garen in the departmental workshop, and Drs Temi Linjewile, Ashley Hull and Paolo De Filippis for their assistance in setting up the experimental apparatus. Financial support from the National Science Foundation (CTS-9810040; CTS-0078700); and the Department of Defense (ARO-DAAD19-01-1-0488) made the project possible. Dr Farley Fisher’s help and guidance at many occasions as the project matured and took off is acknowledged. Matching support from the Research Office at the University of Wyoming is also gratefully acknowledged.

References [1] Bradford M, Grover R, Paul P. Chem Engng Prog 2002;98(3):42. [2] Bradford M, Grover R, Paul P. Chem Engng Prog 2002;98(4):38. [3] Bosch H, Janssen FJJG. Catal Today 1988;2:369.

[4] Sedman CB, Ando J. Japanese Activities in SO2 and NOx Control. EPA-Report 600/D-87/369, 1987 [5] Collings ME, Mann MD, Young BC. Energy Fuels 1993;7:554. [6] Penetrante BM. In: Penetrante BM, Schultheis SE, editors. Nonthermal plasma techniques for pollution control. NATO ASI Series, vol. G 34. Berlin: Springer-Verlag; 1993. p. 65. Part A. [7] Eliasson B, Kogelschatz U. IEEE Trans Plasma Sci 1991;19:1063. [8] Masuda S. In: Penetrante BM, Schultheis SE, editors. Non-thermal plasma techniques for pollution control. NATO ASI Series, vol. G 34. Berlin: Springer-Verlag; 1993. p. 199. Part B. [9] Vogtlin GE, Penetrante BM. In: Penetrante BM, Schultheis SE, editors. Non-thermal plasma techniques for pollution control. NATO ASI Series, vol. G 34. Berlin: Springer-Verlag; 1993. p. 187. Part B. [10] Chang J-S. In: Penetrante BM, Schultheis SE, editors. Non-thermal plasma techniques for pollution control. NATO ASI Series, vol. G 34. Berlin: Springer-Verlag; 1993. p. 1. Part A. [11] Oda T, Kato T, Takahashi T, Shimizu K. Conference record of the 1996 IEEE industry applications conference. Thirty-First IAS Annual Meeting 1996;1803. [12] Mizuno A, Shimizu K, Yanagihara K, Kinoshita K, Tsunoda K, Kim HH, Katsura S. Conference record of the 1996 IEEE industry applications conference. Thirty-First IAS Annual Meeting 1996;1808. [13] Kim HH, Takashima K, Katsura S, Mizuno A. J Phys D: Appl Phys 2001;34:604. [14] Mok YS, Kim JH, Nam I-K, Ham SW. Ind Engng Chem Res 2000;39: 3938. [15] Sathiamoorthy G, Kalyana S, Finney WC, Clark RJ, Locke BR. Ind Engng Chem Res 1999;38:1844. [16] Chung J-W, Cho M-H, Son B-H, Mok Y-S, Namkung W. Plasma Chem Plasma Process 2000;20(4):495. [17] Luo J, Suib SL, Marquez M, Hayashi Y, Matsumoto H. J Phys Chem A 1998;102:7954. [18] Civitano L. In: Penetrante BM, Schultheis SE, editors. Non-thermal plasma techniques for pollution control. NATO ASI Series, vol. G 34.; 1993. p. 103. Part B. [19] Fujii K, Higashi M, Suzuki N. In: Penetrante BM, Schultheis SE, editors. Non-thermal plasma techniques for pollution control. NATO ASI Series, vol. G 34.; 1993. p. 257. Part B. [20] Vogtlin GE, Merritt BT, Hsiao MC, Wallman PH, Penetrante BM. US Patent 5711147, 1998 [21] Hu X, Nicholas J, Zhang J-J, Linjewile TM, de Filippis P, Agarwal PK. Fuel 2003; in press. [22] Hackam R, Akiyama H. IEEE Trans Dielectr Electr Insul 2000;7(5): 654. [23] Penetrante BM, Hsiao MC, Merritt BT, Vogtlin GE, Wallman PH. IEEE Trans Plasma Sci 1995;23:679. [24] Matzing H. In: Prigogine I, Rice S, editors. Advances in chemical physics, vol. LXXX. New York: Wiley; 1991. p. 315. [25] Mukkavilli S, Lee CK, Varghese K, Tavlarides LL. IEEE Trans Plasma Sci 1988;16:652. [26] Graves DB, Jensen KF. IEEE Trans Plasma Sci 1986;14:78. [27] Lowke JJ, Morrow R. IEEE Trans Plasma Sci 1995;23:661. [28] Atkinson R, Baulch DL, Cos RA, Hampson RF, Kerr JA, Troe J. J Phys Chem Ref Data 1989;18:881. [29] Willis C, Boyd AW. Int J Radiat Phys Chem 1976;8:71. [30] McDaniel EW, Cermak V, Dalgarno A, Ferguson EE, Friedman L. Ion-Molecule Reactions. Wiley-Interscience, p. 326 [31] Huang J, Suib SL. J Phys Chem 1993;97:9403. [32] Butt JB. Reaction kinetics and reactor design, Second ed. Marcel Dekker; 2000. p. 118. [33] Suzuki N, Nishimura K, Tokunaga O, Washino M. J Nucl Sci Technol 1978;15(8):597. [34] Tokunaga O, Suzuki N. Radiat Phys Chem 1984;24(1):145. [35] Kuehn DG, Khalafalla SE, Chanin LM. US Bureau Mines Rep 1975; 8063. [36] Sieck LW, Gorden R, Ausloos P, Lias SG, Fielf F. Radiat Res 1973; 56:441.