Department of Physics, University Abou Bekr Belkaid, Tlemcen, Algeria

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1 Chemical Analysis of NO x Removal Under Different Reduced Electric Fields A. HADDOUCHE, M. LEMERINI Department of Physics, University Abou Bekr Belkaid, Tlemcen, Algeria Abstract This work presents a chemical kinetic analysis of different species involved in nitrogen-oxygen mixed gas induced by stationary corona discharge at room temperature and atmospheric pressure. This study takes into account twenty different chemical species participating in one hundred and seventy selected chemical reactions. The reaction rate coefficients are taken from the literature, and the density is analyzed by the continuity equation without the diffusion term. A large number of investigations considered the removal of NO x showing the effects of N, O and O 3 radicals. The aim of the present simulation is to complete these studies by analysing various plasma species under different reduced electric fields in the range of Td (1 Td=10 21 V m 2 ). In particular, we analyze the time evolution of depopulation ( s) of NO x. We have found that the depopulation rate of NO and NO 2 is substantially affected by the rise of reduced electric field as it grows from 100 Td to 200 Td. This allows us to ascertain the important role played by the reduced electric field. Keywords: chemical kinetic, corona discharge, nitrogen oxide, reduced electric field PACS: Dg, Ya DOI: / /17/7/11 (Some figures may appear in colour only in the online journal) 1 Introduction It is well known that gas discharge plasmas are able to initiate chemical reactions in normally inert gas mixtures [1]. These effects can also have a direct impact on the targeted applications such as electron beam processes which were particularly studied for treatment of gaseous effluents polluted by nitrogen oxides (NO x ) and/or sulphur (SO x ) [2 4], ozone production [5 8], medical applications [9 13] and surface treatment [14,15]. Several previous studies have already shown the effect of non-equilibrium discharges gas dynamics at atmospheric pressure [16 20]. These effects can be dynamic (collective movement of drift) and/or thermal (correlated movements) by nature. The charged particles, particularly the ions, will transfer a part of their derivative movement to the neutral ones. The motion of a fluid induced by an electric discharge is referred to as an electric wind or ion wind. It was demonstrated long ago [21], and widely studied in the case of a corona discharge between a tip and a plane [18,22,23]. Generally, the charged particles created in the interelectrode space are accelerated by the electric field E. All electrical proprieties depend on the reduced field E/N, where N is the density of neutral gas and E the electric field [24,25], and increase the internal energy of the neutral gas. At discharge, the gas temperature around the tip can quickly reach 800 K to 1000 K [16,26]. During the phase of post-discharge, the vibrational energy reservoir gradually relaxes causing slight warming of the ionized channel and a local reduction in the gas density. The chemical reactivity of the neutral gas mixture enables transformation of the toxic molecule into harmless particles (such as N 2 O or N) or to create acids (such as the nitric acid) inside the plasma. These acids can be transformed into salt (by addition of a base) [20,27 29]. In the present study, we simulate - for different values of reduced electric field (100 Td, 200 Td) - the time evolution of neutral species (molecules N 2, O 2, O 3 atoms N, O, and nitrogen oxides NO, NO 2, N 2 O, N 2 O 5, NO 3 ), positive ions (NO +, N + 2, O+ 2 ) negative ions (O 4, O 3, NO 3 ), metastable species (N(2 D), O( 1 D), N 2 (A3)) and electrons in the mixture (N 2 : 80% and O 2 : 20%). This simulation must consider various effects induced by the passage of a corona discharge in a mixed gas. For the sake of simplification, we assume that the gas has no convective movement gradients and the pressure remains constant. 2 Mathematical model The mathematical model used in the present work consists of a system of equations that takes into account 589

2 the variation of the density and the chemical kinetics of the environment. We developed a zero order numerical code to resolve the transport equations for neutral and charged particles. The algorithm is based on the time integration of the system of equations under consideration. 2.1 Equations of density conservation The equation for the density of species i in the mixture can be written as follows: n i t = S i (T ), i [1,..., i s ]. (1) Here n i is the density of species i, t is the rate of change n i and S i (T ) is the source term corresponding to the gain or loss of species i due to the chemical reactions. The total density n of the gas is given by the ideal gas law: P = nk b T, (2) n i where P represents the pressure, k b Boltzmann constant and T the absolute temperature. Fig. 1 shows the time evolution of the density of NO for various values of reduced electric field. We notice that for 100 Td the density quickly decreases until 10 8 s then it stabilizes till the end, while there is a significant reduction between t= s and s for the other values of Td. We remark also that the reduction is faster with increasing reduced electric field, and is very important until t = 10 6 s. This is mainly due to the competition between two reactions R1 and R2, where NO can react with the oxidizing radicals to form NO 2 and O 2 via R2 reaction, and can also be converted into N 2 by the reaction R19 with the reducing radicals N. Finally, NO generation (R10, R17 and R18) largely depends on the radical concentration of N and NO 2. In the beginning, the NO generation is not significant because the N radical generated reacts mostly with NO x and the N concentration remains low. 2.2 Chemical kinetics The reactivity of the gas is taken into account in the source term S i (T ) of the density conservation Eq. (1). In the case where chemical reactions involve two bodies S i (T ) are given by the relation: S i (T ) = α ±K α (T ) (n q n p ) α, (3) K α (T ) is the coefficient of the chemical reaction number α and (n q n p ) is the product of densities of species p and q interacting in response to the reaction α. Positive and negative signs in Eq. (3) correspond respectively to creation or disappearance of species i. In the case of three bodies reactions, the term source is given by the product of the three densities. The coefficient K α (T ) satisfies Arrhenius formula: K α (T ) = A exp ( θ/t ), (4) where A is the a constant factor and θ is the activation energy of the reaction and T the absolute temperature of the species involved in the warm rain that has left the chemical reaction α. Fig.1 Time evolution of the NO density in mixture N 2/O 2 at atmospheric pressure and room temperature for different reduced electric fields [ Td] Fig. 2 shows the time evolution of NO 2 density at various values of reduced electric field. We observe, in the beginning from 10 9 s to 10 7 s, a little rise of the density followed by a significant reduction especially for three values of the reduced electric field (160 Td, 180 Td and 200 Td). We note also that the NO 2 generation decreases with the increase of thereduced electric field. The main reactions responsible for this reduction are: R2, R3, R4 and the destruction is mainly due to the reaction R5, R6. 3 Results and discussions We consider synthetic air at atmospheric pressure and room temperature. Our study deals with twenty chemical species among neutral species (molecules N 2, O 2, O 3 atoms N, O, and oxides NO, NO 2, N 2 O, N 2 O 5, NO 3 ), positive ions (NO +, N + 2, O+ 2 ), negative ions (O 4, O 3, NO 3 ), metastable species (N(2 D), O( 1 D), N 2 (A3)) and electrons. These different species react following 170 selected chemical reactions. Fig.2 Time evolution of the NO 2 density in the mixture 590

3 A. HADDOUCHE et al.: Chemical Analysis of NO x Removal Under Different Reduced Electric Fields Fig. 3 shows the time evolution of NO 3 density under various values of reduced electric field. We remark that the evolution of NO 3 density is almost similar to NO 2, while the NO 3 creation takes place from 10 7 s for all reduced electric field values. The main reactions responsible for the creation at the beginning are R7, R8 and R3 and for the reduction are mainly due to the reactions R9 and R10. Fig.3 Time evolution of the NO 3 density in the mixture. Fig. 4 shows the time evolution of N 2 O 5 density at various values of reduced electric field. Generally, the creation is observed for all reduced electric field values excepted for 100 Td from 10 4 s to 10 3 s. According to Table 1, we can say that the N 2 O 5 is converted from NO 2, NO 3, N 2 and O 2 species via three reaction: R24, R25 and R26. We note also that in the beginning, the generation of N 2 O 5 is faster than its disappearance into NO 2 and NO 3 via two reactions R21 and R23. The N 2 O 5 concentration decreases slowly because the N 2 O 5 decomposition reaction rate (R3 and R21 in Table 1) is not so fast compared to the generation and disappearance rates of NO, NO 2 and NO 3. Later, as the NO 2 and NO 3 concentration decreases the N 2 O 5 density becomes stable. Fig. 5 shows the temporal evolution of O 3 density for 100 Td to 200 Td. The O 3 generation (R11-R12 in Table 1) largely depends on the radical concentration of O. So, the O 3 generation is not significant because the O radical concentration remains low. We note that only for 100 Td there is a reduction whereas we have a creation for all other values due to the responsible reactions R11, R12 and R13. Table 1. The main plasma reactions included in the kinetic model and their rate constants. (Rate coefficients are in units of cm 3 molecule 1 s 1 for bimolecular reactions; cm 6 molecule 2 s 1 for trimolecular reactions. T is the absolute temperature in Kelvin) Reactions Rate constants References R1 O 3 + NO NO 2 + O 2 k 1 = [27] R2 NO + O 3 O 2 + NO 2 k 2 = [28] R3 N 2O 5 + O 2 NO 2 + NO 3 + O 2 k 3 = [20] R4 NO O 2 NO2 + O2 k4 = [20] R5 O 3 + NO 2 NO 3 + O 2 k 5 = [27] R6 N + NO 2 N 2 + O 2 k 6 = [27] R7 NO 2 + O 3 O 2 + NO 3 k 7 = exp( 2450/T ) [27] R8 NO 3 + O N 2 NO 3 + O 2 + N 2 k 8 = (300/T ) 0.5 [27] R9 NO 3 + NO 3 O 2 + NO 2 + NO 2 k 9 = [20] R10 NO 2 + NO 3 NO + NO 2 + O 2 k 10 = [28] R11 O + O 2 + N 2 O 3 + N 2 k 11 = [20] R12 O + O 2 + O 2 O 3 + O 2 k 12 = [20] R13 O 3 + NO2 NO 2 + O3 k13 = [27] R14 N + O + N 2 NO + N 2 k 14 = T 0.5 [27] R15 N + O + O 2 NO + O 2 k 15 = T 0.5 [27] R16 N + N + N 2 N 2 + N 2 k 16 = [28] R17 N + O 2 O + NO k 17 = [20] R18 N + NO 2 NO + NO k 18 = [27] R19 N + NO O + N 2 k 19 = [27] R20 NO 2 + O 3 O 2 + NO 3 k 20 = exp( 2450/T ) [27] R21 NO 2 + N2O5 NO 3 + NO3 + NO k21 = [27] R22 NO + NO 3 NO 2 + NO 2 k 22 = [20] R23 O 3 + N 2 O + O 2 + N 2 k 23 = [28] R24 NO 2 + NO 3 + N 2 N 2O 5 + N 2 k 24 = [28] R25 NO 2 + NO 3 + O 2 N 2O 5 + O 2 k 25 = [28] R26 NO 2 + NO 3 N 2O 5 k 26 = [20] 591

4 study concerns six values of the reduced electric fields (100 Td, 120 Td, 140 Td, 160 Td, 180 Td, 200 Td). Under these conditions radicals were added (O, N and O 3 ) that significantly affect the reduction of nitrous oxides. Indeed this analysis lets us better understand the chemical kinetics of NO x. Fig.4 Time evolution of the N 2O 5 density in the mixture. Fig.5 Time evolution of the O 3 density in the mixture In Fig. 6 and Fig. 7 we have shown the time evolution of primary radical density N and O respectively. Density (cm -3 ) Td 120 Td 140 Td 160 Td 180 Td 200 Td Time (s) Fig.7 Time evolution of the O density in the mixture Fig. 8 shows the rate evolution of the species NO change under the same conditions as above: a. It is noted for low values of the reduced electric field 100 Td and 120 Td an average reduction of 45% caused by R17 and R18, while for high values ( Td) we observed that the rate of reduction reached 65% on average, due to reactions R2 and R19. Generally, the fractions of the energy transferred from charged to neutral particles via elastic and inelastic processes are given from a Boltzmann equation solutio [22,8]. So, the fraction of energy lost during elastic, excitation and ionization processes in the same gas mixture depends on the electric reduced field. For example, at low values of E/N< 120 Td, the energy loss is due to rotational and vibration collisions, whereas for E/N >120 Td the energy loss is mainly due to electronic excitation and ionization collisions [20]. b. There is also a significant influence on the time reduction. For example at 200 Td the time reduction is achieved at about 10 7 s while for 120 Td it takes 10 4 s. Fig.6 Time evolution of the N density in the mixture It is noted that the behaviour of these two radicals is the same, and these radicals are also the origin of NO formation according to the reaction of R14 and R15. In this section we will calculate the rate variation: (N 0 N)/N 0 where N 0 represents the initial density and N the density values between 10 9 s and 10 3 s, for NO, NO 2, NO 3, N 2 O 5, N, O and O 3 species. This Fig.8 Time evolution of rate variation of NO species in for 592

5 A. HADDOUCHE et al.: Chemical Analysis of NO x Removal Under Different Reduced Electric Fields Fig. 9 shows the rate of change of NO 2 density, where we observe: a. a creation followed by consumption for all reduced electric field values, b. the rate production, for low values of Td (100 Td, 120 Td), reached 20% at the beginning but it exceeded 70% for time up to 10 7 s, c. unlike low values, the rate production for high values of Td ( Td) reached 50% - 80% at the beginning, d. and, unlike other species, we observe that the reduction time is almost the same for all values of the reduced electric field. mainly because of reaction R23, while for all other values we see creations more or less rapid, depending on reduced electric field due to the reactions of R11 and R12. Fig.11 Time evolution of rate variation of N 2O 5 species in for Fig.9 Time evolution of rate variation of NO 2 species in for Figs. 10 and 11 show respectively the rate of change of NO 3 and N 2 O 5. Firstly, for NO 3 it is noted that the behaviour is the same for all values of the reduced electric field. In the beginning, R20 and R21 are dominant, implying a rapid increase of density number and these reactions are offset by the destruction reactions R9 and R22 which indicates a more or less rapid decrease in a short time. Fig.12 Time evolution of rate variation of O 3 species in for 4 Conclusions Fig.10 Time evolution of rate variation of NO 3 species in for For N 2 O 5 species we note two behaviours: a. at low values we have the same variation as the NO 3 curves due to the reactions R3 and R26, b. and, at high values, we notice stabilization. Fig. 12 shows the rate variation of the ozone for the same conditions as above. It is clearly observed that there is a significant reduction of 80% only for 100 Td In the literature, it has generally been emphasized that certain radicals influence the NO or NO 2 removal. In this work, the results obtained show the significant role played by the high reduced electric field. a. The time evolution of the NO x density shows two types of evolution, which depends strongly on the increase of reduced electric field: (1). a reduction of dominant species in the mixture N 2 /O 2 such as NO, NO 2 and NO 3. (2). a creation of other species such as O 3 and N 2 O 5. b. In fact, the simulation results show that the reduced electric field influences the NO x removal. We obtained 43% for 100 Td against 60% for 200 Td. References 1 Raizer Y P. 1991, J. Phys. D: Appl. Phys., 37: Hammer T. 2002, Plasma Sources Sci. Technol., 11: A

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