Modelling of NO and HC removal by non-thermal plasmas

Transcription

1 INSTITUTE OF PHYSICS PUBLISHING Combust. Theory Modelling 5 (2001) COMBUSTION THEORY AND MODELLING PII: S (01)17004-X Modelling of NO and HC removal by non-thermal plasmas Igor Orlandini and Uwe Riedel 1 Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, Universität Heidelberg, Im Neuenheimer Feld 368, D Heidelberg, Germany riedel@iwr.uni-heidelberg.de Received 11 September 2000 Published 15 August 2001 Online at stacks.iop.org/ctm/5/447 Abstract Public concern about the environmental situation creates strong restrictions on the purity of exhaust gases in various polluting industrial enterprises. A promising method of gas cleaning is the use of pulsed corona discharges. In these devices, short pulses at high frequency allow one to increase the energy input and radical production for subsequent transformation of toxic pollutants (NO x,so 2,C x H y, etc). For a better understanding and optimization of the possible applications of this technique it is necessary to develop models for the underlying physical and chemical processes which are responsible for the removal of pollutants. In this work, we have theoretically investigated the removal of NO and the effect of ethane and propene on NO removal by non-thermal plasma discharges at low temperature (373 K) and at atmospheric pressure. The model takes into account the production of radicals after every discharge and the subsequent removal of NO and HC by radicals. Reaction flow analysis reveals the characteristic differences of NO reactions in the presence of unburned alkanes and alkenes. The results of our numerical simulations show good agreement with experimental data published in the literature. Nomenclature t Time (s) E Electric field (V cm 1 ) ε Electrical energy density (J l 1 ) η i Production efficiency of species i (molecule J 1 ) v i Electron drift velocity (cm s 1 ) n Concentration (molecules cm 3 ) 1 Author to whom correspondence should be addressed /01/ $ IOP Publishing Ltd Printed in the UK 447

2 448 I Orlandini and U Riedel ρ Density (kg m 3 ) m Mass (kg) w i Mass fraction of the species i ω i Rate of formation of the species i (mol cm 3 s 1 ) M i Molar mass of the species i (kg mol 1 ) h i Specific enthalpy of the species i (J kg 1 ) C p Molar heat capacity at the constant pressure (J K 1 mol 1 ) p Pressure (bar) T Temperature (K) ν Discharge frequency (s 1 ) R Gas constant (J K 1 mol 1 ) [S i ] Concentration of the species i (molecule l 1 ) k r Rate of electron-impact reaction (cm 3 molecules 1 s 1 ) q e Electron charge ( C) 1. Introduction Nitric oxides (including NO and NO 2 ) emissions are a major environmental problem because of their negative influence on human health and vegetation. The transportation industry, mostly motor vehicles, contributes with about 40% of the total NO x load. Here, first of all, diesel engines are a major source for those pollutants [12]. The catalytic converters used with great success in spark-ignited gasoline engines (the reduction rates are typically up to 90%) cannot be used on diesel engines because of the excessive amount of oxygen in the exhaust gas. Therefore, new federal regulations in the European Union on limiting NO x emissions from diesel vehicles enhanced the need to study alternative cleaning technologies. One possibility, which has been investigated intensely in the past two decades is the treatment of exhaust gases with non-thermal corona discharges. A very intense discharge is produced by applying a sharp rising narrow high-voltage pulse between the electrodes. In the discharge, the electrons are highly accelerated while the gas molecules remain at approximately ambient temperature. The collisions of energetic electrons with neutral gas molecules cause different processes such as ionization, dissociation or excitation of the carrier gas molecules [9]. The key for the usage of plasmas as a method for gas purification are the different radicals: they react slowly with the carrier gas and are available for the degradation of the trace components. In the early stage of investigation of this technique it was already clear that in the gas mixtures containing over 10% of O 2, the main path of NO removal is oxidation to NO 2 [16]. However, in a recent publication by Westbrook [29], the author achieved over 90% reduction of NO x to the desired product N 2 by the combination of a plasma reactor and a catalyst. Although NO 2 can be dealt with in heterogeneous catalytic reactions, the energy consumption for the oxidation of NO to NO 2 needs to be reduced for a commercialization of the process. There are several important parameters for the decomposition of NO: initial NO concentration, composition and temperature of the carrier gas and the value of the reduced electric field (E/n). In order to choose the optimal operating conditions for a nonthermal plasma process, it is of great importance to understand the chemical mechanisms responsible for the decomposition of pollutant molecules in a plasma. In the past few years, simulation of plasma processing using detailed chemistry has become an important tool for investigating this problem. The influence of different parameters (concentration, gas

3 Modelling of NO and HC removal by non-thermal plasmas 449 temperature, unburned hydrocarbons) has been examined theoretically and experimentally [10, 17, 21, 23]. In an earlier paper [21] ethene is studied as a model-fuel for unburned hydrocarbons in the exhaust gas. This paper focuses on propene and ethane in humid air as unburned hydrocarbon because sensitivity studies and reaction flow analysis reveal that the characteristic reaction paths of NO in the system significantly differ for alkanes and alkenes. The aim of this work is to explain the main chemical processes responsible for the removal of NO and HC by pulsed corona discharges in the gas phase using a detailed chemical mechanism, and to validate the proposed model by a comparison with experimental data. Beyond this specific application, another aim of this paper is to demonstrate how different sub-models need to be coupled together to describe and develop new industrial applications in an area where detailed chemical models are important. Four different gas mixtures have been examined: (a) an NO/N 2 mixture; (b) NO and propene in dry air; (c) NO and propene in humid air; and (d) NO and ethane in humid air. The effect of pulsed discharges is included in the model by considering the (re-)production of radicals after every pulse and consecutive removal of NO by the radicals. The production of O, OH, H, N, O( 1 D), O 2 (a 1 g ) and N 2 (A 3 u + ) in the discharge is taken into account. The results of our simulations are in good agreement with experimental data published in the literature. 2. Radical production The overall process of gas purification can be separated into three phases: (a) the discharge phase corresponds to the formation of primary radicals and electronically excited species by electron molecule interactions; (b) a post-discharge metastable-quenching phase; and (c) a post-discharge radical utilization phase, in which the secondary radicals (O 3,HO 2 ) are formed and NO and HC reaction occurs. For typical operating conditions, the discharge phase is in the order of a few nanoseconds, the metastable-quenching phase is in the order of tens of nanoseconds, and the radical utilization phase is in the order of microseconds or longer [8]. The fact that these processes occur on different time scales allows us to treat the radical production and radical utilization as two independent processes. In our model, we include electron reactions with the carrier gas molecules (N 2,O 2, H 2 O) only. Since the mole fractions of NO and unburned hydrocarbons are in the order of a few hundred ppm, the fraction of discharge power deposited in those species will be small in any case. We do not include ionization processes in our model. Investigations by different authors have shown that the ionization processes of background molecules at given conditions have an extremely low influence on the removal of nitric oxides and unburned hydrocarbons and that only dissociation and excitation reactions contribute to the formation of the primary active species which are responsible for plasma remediation of toxic molecules [11, 14, 24]. The following assumptions are made: only O, OH, H, N radicals and electronically excited species: O( 1 D), N( 2 D), N( 2 P), O 2 (a 1 g ) and N 2 (A 3 u + ) are produced by the discharge; changes in temperature are assumed to be homogeneous in the discharge streamer; the radicals are homogeneously distributed in the discharge streamer.

4 450 I Orlandini and U Riedel 2.1. Estimation of radical concentrations In order to simulate the chemical kinetics of plasma induced processes it is necessary to know the concentrations of the active species within the discharge volume. The method for estimation of radical concentrations, which is the starting point for modelling of the post-discharge phase in the volume-averaged approximation can be found in the publications of Rosocha et al [25] and Mok et al [18] and only a brief summary is given here. The initiating electron reactions taken into account if humid air is the carrier gas are [4, 8, 26]: N 2 + e k 1 N2 (A 3 u + ) + e (1) N 2 + e k 2 N+N+e (2) N 2 + e k 3 N( 2 D) +N+e (3) N 2 + e k 4 N( 2 P) +N+e (4) O 2 + e k 5 O2 (a 1 g ) + e (5) O 2 + e k 6 O+O( 1 D) + e (6) O 2 + e k 7 O+O+e (7) H 2 O+e k 8 OH + H + e. (8) A production efficiency for a given active species (radical or electronically excited molecule) can be expressed as [18, 25]: η i = [S i] ε. (9) For O radicals, the equation is: η O = (2k 7 + k 6 ) [O 2 ] q e v e (E/n) n. (10) Similar relations can be derived for other species. The radical concentrations for model calculations are obtained by multiplying the radical production efficiencies by the energy dissipated for streamer propagation (usually given in J l 1 ). The electron drift velocity for mixtures containing large relative concentrations of nitrogen (c N2 > 50% vol ) and at E/n > 10 Td can be described by a semi-empirical relation as the function of the reduced electric field E/n (in Td) [18, 27]: ( v e cm s 1 ) = (E/n) 0.8. (11) A comparison between experimentally measured [7] and calculated (equation (11)) electron drift velocities in air can be seen in figure 1. In discharge processing, the rate coefficients for electron-impact reactions strongly depend on the mean electron energy in the discharge plasma. Figure 2 shows calculated radical production efficiencies with air as a carrier gas calculated from equation (9). Rates of electron reactions are calculated using the Boltzmann equation solver ELENDIF [19]. Input into the code are the gas composition, E/n and electron molecule collision cross sections. The mean electron energy in most electrical discharge reactors operating at atmospheric pressure is typically 3 6 ev [22]. In this range, a large fraction of the input power goes into

5 Modelling of NO and HC removal by non-thermal plasmas 451 Figure 1. Comparison between calculated (equation (11)) and measured [7] electron drift velocity in air (79% N 2, 21% O 2 ). Figure 2. Radical production efficiencies of O, O( 1 D) and N radicals in air (79% N 2, 21% O 2 ), calculated from equation (9). the formation of O and O( 1 D) radicals although N 2 is the major component of the carrier gas (see figure 2). The main reason for this behaviour is that the dissociation energy of N 2 is much higher than that of O 2 (9.2 ev compared with 4.8 ev), and consequently, very high mean electron energies are required for the production of significant concentrations of N radicals.

6 452 I Orlandini and U Riedel Figure 3. Removal of NO in a pulsed plasma reactor as a function of transferred energy. Carrier gas, 100% N 2 ; additive, 100 ppm NO. T = 373 K, p = 1 bar. Points, experiments [22]; curves, simulation. 3. Rate equation model In a volume-averaged approach, the state of the system is completely specified by the mass fractions of each species in the system, the temperature and the pressure. These properties change due to chemical reactions and heat release. Thus, we obtain the following conservation equations: conservation of the overall mass m of the mixture: (ρv) t = m t conservation of the species mass fraction w i of species i: = 0 (12) w i t M i ω i ρ = 0 (13) conservation of energy: T t 1 ρc p p t + 1 ρc p h i M i ω i = 0. (14) To close this system of ordinary differential equations the ideal gas law i p = ρrt i w i M i (15) is applied. All numerical simulations reported in this work are performed with the well stirred reactor code HOMRUN [15]. The system of conservation equations is solved with the implicit extrapolation method LIMEX [6]. As a solution, we obtain the concentration of all species in the system and the temperature as a function of time.

7 Modelling of NO and HC removal by non-thermal plasmas Reaction mechanism The detailed reaction mechanism used in this work is built in a hierarchical manner. The core of the mechanism represents the H 2 /O 2 submechanism, which was extended by adding further reactions for C and N species. The whole reaction scheme consists of three main parts: (a) the hydrocarbon mechanism, which was originally developed and validated for flame conditions is based on the work of Baulch et al [2]; (b) a reaction mechanism, describing the formation of NO x at high-temperature conditions, developed and validated by Klaus [13]; (c) reactions involving electronically excited species. In our reaction scheme, we adopted the Arrhenius parameters for the low-temperature range according to literature values and added new reaction paths and new chemical species which are important at given conditions (temperature of the exhaust gas). The rate coefficients used in this work are compiled from different sources and are applicable over the temperature and pressure range encompassed by the experiments. Essential new chemical species are different partially oxidated hydrocarbons and electronically excited species: O( 1 D), O 2 (a 1 g ),N( 2 D), N( 2 P) and N 2 (A 3 u + ). These species have a direct influence on both the evolution of certain radicals and the NO and HC transformation. The most important reactions that enhance NO and HC removal involving metastable species are [1]: N( 2 D) +O 2 NO + O (16) O( 1 D) +H 2 O OH + OH (17) N 2 (A 3 u + ) +H 2O N 2 +OH+H. (18) 5. Results 5.1. NO/N 2 mixture As a part of the validation of our model, the first mixture examined is 100 ppm NO in N 2 at 373 K. The comparison between numerical simulations and experimental results is presented in figure 3. Experimental data are available only for NO evolution. Additionally, the results for NO 2 and N 2 O are presented. In [22] a model in agreement with the experimental observation has already been presented. However, we decided to re-examine this case because there are very few well documented experiments in this area and validation is an integral part of model development. The chemistry of this simple system can be explained as follows: electron-impact dissociation of N 2 produces nitrogen radicals (reaction (3)), which can very selectively reduce NO to the desired product N 2 : NO+N N 2 +O. (19) Electronically excited nitrogen molecules N 2 (A 3 u + ) are also formed in the discharge (reaction (1)). However, in the absence of oxygen and water vapour, N 2 (A 3 u + ) molecules are relaxed in collisions with N 2 and have no influence on NO removal. Numerical simulations indicate an increase of the NO 2 concentration in the initial phase of the process. The O radical produced in reaction (19), reacts with NO to produce NO 2 (M is a third collision partner): NO+O+M NO 2 +M. (20)

8 454 I Orlandini and U Riedel NO 2 can be reduced again to form N 2 O: NO 2 +N N 2 O+O. (21) However, the concentration of O radicals is almost identical to the concentration of N radicals produced by the discharge (almost all N radicals produced by the discharge are consumed in reaction (19)). At this condition, only a small amount of NO 2 and N 2 O are formed Effect of HC on the removal of NO The experimental results used in this section are from two different sources. Penetrante et al [23] investigated the influence of propene on NO removal from a gas consisting of 90% N 2 and 10% O 2 at 373 K and at atmospheric pressure. The concentrations of NO and C 3 H 6 are 500 and 1000 ppm, respectively. The second source of experiments is the work of Bröer [3]. Here, the carrier gas is composed of 72% N 2, 18% O 2 and 10% H 2 O. The initial concentrations of NO and unburned hydrocarbons are 500 ppm, the temperature and pressure are 373 K and 1 bar, respectively. In both cases, the authors measure the concentration of different species as the function of energy transferred from the supply source to the gas per unit volume. When oxygen is present in the mixture, the paths of NO removals are notably changing to that described in the previous section: in the discharge, O and O( 1 D) radicals are formed rather than N radicals. Additionally, NO conversion is facilitated by the presence of unburned hydrocarbons. Thus, oxidation products (NO 2, HNO 2 and HNO 3 ) are governing the removal of NO. We also investigated the evolution of C species by plasma processing. We found for both experiments that at the energy inputs examined (40 J l 1 in the experiment of Penetrante et al [23] and 120 J l 1 in the experiment of Bröer [3]), the hydrocarbons are not fully oxidated to CO 2 and H 2 O. The main products are carbon monoxide, formaldehyde and acetaldehyde. In the remainder of the paper we will try to elucidate the role of these two important hydrocarbons on the NO removal by plasma processing NO/C 3 H 6 /dry air mixture. Propene is a very important intermediate formed during the combustion of many hydrocarbon fuels (octane, pentane, butane, propane) and its oxidation has been frequently examined in combustion environments [5]. Consequently, a good knowledge of its oxidation mechanism is needed. Figure 4 shows the species evolution by plasma processing of 500 ppm NO and 1000 ppm C 3 H 6 in 90% N 2 and 10% O 2. The results of our simulations are in good agreement with the experimental data. We can observe that the concentration of NO in both, experiment and calculation decreases almost proportionally with the growth of NO 2. Compared with figure 3, there is practically no NO x reduction to N 2 observed. The reasons for this have been explained earlier. At the early stage of the process, O radicals react with propene rather than with NO (reaction (20)). The following reactions are included in the mechanism: C 3 H 6 +O C 2 H 4 +CH 2 O (22) C 3 H 6 +O C 2 H 5 + CHO (23) C 3 H 6 +O CH 3 CO+CH 3. (24) Different fragments formed in reactions (22) (24) further promote the oxidation of NO to form

9 Modelling of NO and HC removal by non-thermal plasmas 455 Figure 4. Removal of NO and C 3 H 6 in a pulsed plasma reactor as a function of transferred energy. Carrier gas: 90% N 2, 10% O 2. Additive: 500 ppm NO, 1000 ppm C 3 H 6, T = 373 K, p = 1 bar. Points, experiments [23]; curves, simulation. NO 2. The most important reactions are: NO C 2 H 5 +O 2 C 2 H 5 O 2 C 2 H 5 O+NO 2 (25) NO CH 3 +O 2 CH 3 O 2 CH 3 O+NO 2 (26) CH 3 O+O 2 CH 2 O+HO 2 (27) CH 2 OH + O 2 CH 2 O+HO 2 (28) NO+HO 2 NO 2 +OH. (29) In the later stage OH radicals formed in reaction (29) are important for the oxidation of C 3 H 6 and NO (see the next section). In the absence of water, the electronically excited oxygen radical O( 1 D), formed in reaction (6) is rapidly quenched to form a ground state O radical: O( 1 D) +M O+M. (30) Reaction (29) is the dominant path for NO removal in the system. The main sources of HO 2 radicals are reactions (27) and (28) NO/C 3 H 6 /humid air mixture. Figure 5(a) shows the comparison of our simulations with the experimental results [3] for N species. We find a good agreement for NO and HNO 3, while the simulation shows excess of NO 2 compared with the experiment. In [3], the author suggests that 25% of the initial NO x has been reduced to N 2. However, this statement cannot be verified in our simulations and is in contradiction to the experimental results [23] presented in figure 4. In the case when the carrier gas is humid air, the mechanism of propene removal changes in the sense that OH radicals are also of importance in the initiating stage of the process. The main source of OH radicals is reaction (17), rather than dissociation of water by the electron impact (reaction (8)).

10 456 I Orlandini and U Riedel Figure 5. Removal of NO and C 3 H 6 in a pulsed plasma reactor as a function of transferred energy. Carrier gas: 72% N 2, 18% O 2, 10% H 2 O Additive: 500 ppm NO, 500 ppm C 3 H 6, T = 373 K, p = 1 bar. (a) Comparison between experiments [3] (points) and simulations (curves) for N species. (b) Results of simulations for C species. Opposite to the system investigated in figure 4, significant amounts of nitrous and nitric acid are formed: NO+OH+M HNO 2 + M (31) NO 2 +OH+M HNO 3 +M. (32) The main reaction consuming propene at conditions examined in figure 4 is the electrophilic addition of OH radicals to the π-bond system of propene. OH addition can occur on either side of the double bond producing highly energetic radical adducts: C 3 H 6 +OH C 3 H 6 OH. (33) C 3 H 6 OH symbolizes both possible isomers, which are distinguished individually in the

11 Modelling of NO and HC removal by non-thermal plasmas 457 mechanism. Reaction (33) shows a negative temperature dependence, and is found to be a rate limiting step for NO removal. In the presence of O 2, oxidation of adducts formed in reaction (33) proceeds via the formation of two different β-hydroxypropylperoxy radicals: HOCH 2 ĊHCH 3 +O 2 HOCH 2 CH(O 2 )CH 3 (34) H 2 ĊCH(OH)CH 3 +O 2 O 2 CH 2 CH(OH)CH 3. (35) Peroxy species formed in reactions (34) and (35) can dissociate back to reactants or decompose to acetaldehyde, formaldehyde and OH [30]. Therefore, in the presence of NO the main consumption pathway is conversion to β-hydroxyalkoxy radicals and NO 2 : O 2 C 3 H 6 OH + NO OC 3 H 6 OH + NO 2. (36) The resulting alkoxy radicals can undergo reaction with O 2, isomerize or decompose. However, previous studies [20, 28] indicate that decomposition leading to the formation of aldehydes is the dominant pathway for the consumption of β-hydroxyalkoxy radicals: HOCH 2 CH(O)CH 3 CH 3 CHO+CH 2 OH (37) OCH 2 CH(OH)CH 3 CH 3 CHOH + CH 2 O. (38) This reaction sequence is important due to the production of precursors which are responsible for HO 2 formation: CH 3 CHOH + O 2 CH 3 CHO+HO 2. (39) CH 2 OH species are consumed in reaction (28). According to our calculations, reactions (29) and (36) are the dominant pathways for NO removal at conditions investigated in figure 5. Regeneration of OH radicals is responsible for autocatalysis of the process. Main removal pathways for C 3 H 6 and NO are summarized in figure NO/C 2 H 6 /humid air mixture. The effect of ethane on the removal of NO at plasma conditions is presented in figure 7. The fact that ethane is a saturated hydrocarbon tends to decrease the initial reactivity compared with alkenes such as propene due to the higher bond strength. Compared with propene, in this case there is also no possibility for the rapid addition of radicals on the double bond. This results in the higher input energy required for the NO and C 2 H 6 removal compared with the system with propene (cf figures 5 and 7). Thus, the main initiating reactions for ethane is H abstraction through OH and O radicals to form C 2 H 5 : C 2 H 6 +OH C 2 H 5 +H 2 O (40) C 2 H 6 +O C 2 H 5 +OH. (41) C 2 H 5 reacts with the molecular oxygen to produce peroxy-radical (C 2 H 5 O 2 ), which enhances the oxidation of NO to form NO 2 and alkoxy radical C 2 H 5 O (equation (25)). The possible competitive reaction: C 2 H 5 +O 2 C 2 H 4 +HO 2 (42) does not contribute significantly to the formation of C 2 H 4 at the conditions examined in figure 7. C 2 H 5 O reacts with O 2 and OH in the H abstraction reaction or decomposes to formaldehyde and methyl-radical: C 2 H 5 O+O 2, OH CH 3 CHO+HO 2, H 2 O (43) C 2 H 5 O CH 2 O+CH 3. (44)

12 458 I Orlandini and U Riedel Figure 6. Dominant reaction pathways for plasma removal of C 3 H 6 and NO. (See figure 5 for conditions.) In this system, NO is mainly directly oxidated by O radicals (reaction (20)). Therefore, due to the lower reactivity of ethane compared with propene, formation of ozone and its subsequent reaction with NO becomes more apparent: O 2 +O+M O 3 + M (45) O 3 +NO NO 2 +O 2. (46) The results of reaction flow analysis for conditions examined in figure 7 are presented in figure 8.

13 Modelling of NO and HC removal by non-thermal plasmas 459 Figure 7. Removal of NO and C 2 H 6 in a pulsed plasma reactor as a function of transferred energy. Carrier gas: 72% N 2, 18% O 2, 10% H 2 O Additive: 500 ppm NO, 500 ppm C 2 H 6, T = 373 K, p = 1 bar. (a) Comparison between experiments [3] (points) and simulations (curves) for N species. (b) Results of simulations for C species. 6. Summary and conclusion In this paper, numerical simulations applying a detailed chemical reaction scheme are used to explain the processes responsible for the removal of NO by plasma processing. For this purpose, a detailed reaction mechanism has been developed and validated by comparison with experimental data. Four different mixtures have been investigated: (a) an NO/N 2 mixture; (b) NO and propene in dry air; (c) NO and propene in humid air; and (d) NO and ethane in humid air. All the systems are investigated at normal pressure and T = 373 K. Our simulations show that NO can be very efficiently reduced to N 2 only in the mixture containing pure nitrogen. However, in systems containing oxygen, the main pathway for NO removal is oxidation to NO 2. Although the later can be converted to N 2 and O 2 using

14 460 I Orlandini and U Riedel Figure 8. Dominant reaction pathways for plasma removal of C 2 H 6 and NO. (See figure 7 for conditions). heterogeneous catalytic methods, the energy input for NO oxidation needs to be optimized to reduce power consumption. Comparing the behaviour of two unburned hydrocarbons (propene and ethane) on oxidation of NO, we find that at the same conditions propene decreases the energy for NO conversion much better than ethane: at the input energy of 40 J l 1 only oxidation products are

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