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Predictive model of decontamination efficiency of gaseous pollutant by non-equilibrium plasma
Journal of Electrostatics 68 (2010) 390e393
Contents lists available at ScienceDirect
Journal of Electrostatics journal homepage: www.elsevier.com/locate/elstat
Predictive model of decontamination efficiency of gaseous pollutant by non-equilibrium plasmaq Zhan-Guo Li*, Zhen Hu, Hai-Ling Xi, Peng Cao Research Institute of Chemical Defense, Beijing 102205, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 February 2009 Received in revised form 18 March 2010 Accepted 26 May 2010 Available online 9 June 2010
During non-equilibrium plasma (NEP) reactions, chemical bonds of pollutant compound are broken by energy generated in the reactor so that the pollutants are decontaminated. In this study, the energy conversion factor (Ef) is defined as the ratio of the dissociation energy of chemical bonds destroyed in NEP reaction system to the energy inputted in plasma reactor. The energy conversion factor of chemical bonds (Ef,i), SeH, CeCl, CeS, CeH, CeC etc., were determined by decontamination experiments of H2S and 2-CEES in plasma reactor. Based on the Ef,is, the predictive model of NEP decontamination efficiency of gaseous pollutant was developed and applied to predict decontamination efficiency of CH3CH2SH, in which all Ef,is of chemical bonds are known as described above. It was shown by the decontamination experiment of CH3CH2SH that the predictive value was well agreed to experimental data. Therefore, the model can be used to predict decontamination efficiency of those pollutants in which all Ef,is of chemical bonds have been determined. An improved model is also produced by the analysis of predictive error. Ó 2010 Elsevier B.V. All rights reserved.
Keywords: Non-equilibrium plasma Decontamination efficiency Gaseous pollutant Predictive model
1. Introduction Non-equilibrium plasma (NEP) processing has very strong chemical reactivity and has attracted more and more attention in the area of gaseous pollutants decontamination [1e3]. There have been a great deal of researches both experimental and theoretical on the decontamination of VOCs and NOx using NEP, especially on the general plasma chemistry process and kinetic models [4,5]. In NEP process, the fast electrons created by the discharge mechanism mainly initiate the chemical reactions [6]. Chang’s group [7] analyzed the generation process of plasma in the positive corona and negative corona, and discussed the generation, recombination, and loss processes of reactive species. They [8] also suggested that radical species could be generated through the dissociative recombination of the positive and negative species via formations of various kinds of ions from ion molecular reactions. Oda [9] suggested that the oxidation ability of the plasma is very strong, but the key points for its practical usage were reliability of the process and energy efficiency of the plasma. To improve the energy efficiency of the plasma process, combination with the adsorption and catalysts is feasible [10,11].
q Original version presented at ICESP’2008, 11th International Conference on Electrostatic Precipitation, 20e24 October 2008, Hangzhou, China. * Corresponding author. Tel.: þ86 10 66758313; fax: þ86 10 69768181. E-mail address: [email protected] (Z.-G. Li). 0304-3886/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.elstat.2010.05.010
In the study of plasma reaction kinetics, Choi’s group [12] developed a first order reaction kinetic model for the decomposition rate relative to the VOC concentration and studied the decomposition mechanism. It was reported [5,13,14] that the concentration of reactive particles was directly proportional to the energy input in reactor. Therefore, Mok’s group [15] assumed that the decomposition rate would be directly proportional to the concentration of the pollutant and the discharge power, and developed a kinetic equation for the plasma reactor. In order to evaluate the decomposition performance of NEP system, many parameters were used, such as the selectivity and conversion of pollutant to CO and CO2, the energy efficiency [16,17]. Many previous studies [18,19] also focus on the energy cost (EC; eV per removed molecule), the G-value (removed molecules per 100 eV input) and the energy yield (EY; g-removed per kWh input), which have been used to evaluate the performance of the discharge plasma systems. However, practical performance evaluation of the plasma process is not yet conducted. Major difficulties to evaluate the performance of NTP process using parameters mentioned above, are varied parameters with different pollutant. However, there will be much more laborious work if kinetic data are obtained from experiments for every single organic pollutant. Futamura’s group [20] reported that the decomposition behavior of VOCs is related to their chemical structures. From authors’ previous work [21], it was also found that the reaction rate was not only related to discharge parameters, but also to molecular structure of pollutant, during studying the
Z.-G. Li et al. / Journal of Electrostatics 68 (2010) 390e393
It is well known that NEP chemical reaction is an energy conversion process that active particles collide with pollutant molecules. It has been found that the lower chemical bond dissociation energy of the pollutant is, the higher the decontamination efficiency is, in a plasma system [15,22,23], which showed that decontamination efficiency of pollutant is closely related to its dissociation energy of chemical bonds. For example, when corona plasma reactor is used to treat H2S and 2-CEES (2-chloroethyl ethyl sulfide, CH3CH2SCH2CH2Cl), the reactions are described as below: Plasma
H2 S ! SO2 þ H2 O Plasma
ClCH2 CH2 SCH2 CH3 ! SO2 þ H2 O þ CO2 þ HCl The chemical bonds and their dissociation energy in the above reactions are listed in Table 1 [24]. In the reaction process, the initial mol concentration of pollutant is C0 (mol/m3), gas flow rate is Q (m3/h), average power input in plasma reactor is PT (W), and decontamination experimental efficiency is h. In another words, the chemical bonds of C0hQ mol/h pollutant were destroyed by energy of PT watt. Considering the consumption in the energy transferring process, PT must be more than the sum of dissociation energy of these chemical bonds. Here, the ratio of the total dissociation energy to PT is defined as energy conversion factor, symbolize as Ef:
Ef ¼
C0 $h$Q $
P
ni $Eb;i
PT
mol$eV=ðW$hÞ
(1)
Where, Ef-energy conversion factor of pollutant molecules, mol$eV/ (W$h); C0-initial mol concentration of pollutant, mol/m3; hdecontamination efficiency, %; Q-gas flow rate, m3/h; ni-hemical bond number with the same dissociation energy in pollutant molecule; Eb,i-dissociation energy of chemical bond i, eV; PTaverage power input in plasma reactor, W.
Table 1 dissociation energy of chemical bonds.
Ef0 ;i ¼ P
Eb;i ni $Eb;i
Pollutant
Chemical bond
ni
Eb,i (eV)
SeH CeS CH2eCH3 eSCHeH CH2CH2eH
2 2 1 4 3
3.95 3.14 3.56 4.04 4.36
Chemical bond eCeCl CH2eCH2Cl CHCleH
ni 1 1 2
Ef ;i ¼ 26:8
(2)
C0 $h$Q $Eb;i PT
(3)
Therefore, Ef,i means the part of average energy input in plasma reactor which is used to destroy chemical bond i of pollutant molecules. In order to calculated Ef,i using the data from some experiments, the equation (3) can be converted to following form
C0 $h ¼
Ef ;i P T 26:8Eb;i Q
(4)
k is defined as below
k ¼
Ef ;i 26:8Eb;i
(5)
Then, C0$h and PT/Q are variables for a pollutant in different experimental condition. In the decontamination experiments of H2S and 2-CEES [25], decontamination efficiency h is varied under different conditions, such as C0, PT and Q. C0$h depend on PT/Q, and k is the slope of the line, as shown in Fig. 1. From equation (5), Ef,HS can be obtained:
Ef ;HS Ef ;HS ¼ ¼ 2:24 105 26:8Eb;HS 26:8 3:95
(6)
Ef ;HS ¼ 2:37 103
(7)
Similarly, Ef,CeS, Ef,CeCl, Ef,CeC and Ef,CeH, are also obtained by substituting the dissociation energy of chemical bonds in 2-CEES molecule in equation (8):
Ef ;i ¼ 2:19 105 26:8Eb;i
(8)
We can take the obtained Ef,i values as the standard values, symbolized as E0f,i, and the dissociation energy of corresponding chemical bond is symbolized as E0b,i, shown in Table 2.
0.012 H2S 0.010
-5
C 0 • = 2.24×10 (P T /Q ) + 0.0033 2
2-CEES R = 0.9897
0.008 0.006 -5
C 0 • = 2.19×10 (P T /Q ) - 0.0008 2
R = 0.9912
Eb,i (eV) 3.65 3.85 4.21
C0 $h$Q $Eb;i mol$eV=ðW$hÞ PT
If energy units in the equations above are substituted by joule, i.e, 1 W$h ¼ 3.6 103 J and 1 eV ¼ 96.48 kJ/mol, the energy conversion factor is dimensionless and a coefficient a ¼ 26.8 is generated. The equation (2) is converted to
0.004 H2S 2-CEES
Ef ¼
3
2. Energy conversion factor of chemical bonds in NEP decontamination reaction
Molecule 2-CEES has several types of chemical bonds and all chemical bonds were destroyed, the energy conversion factor Éf,i of chemical bond i can be calculated by the following equation:
C0 • (mol/m )
decontamination kinetics model of NEP for gaseous pollutant treatment. It is well known that each organic pollutant has several special chemical bonds, and the types of chemical bonds are far less than the types of organic pollutants. Therefore, if the mathematical connection of decontamination efficiency, discharge parameter and dissociation energy of chemical bond can be revealed, it will be able to develop a theoretical model to predict the decontamination efficiency of the pollutant. This work will be significant for the plasma application. In this study, the quantitative relationship of decontamination efficiency, discharge power and dissociation energy of chemical bonds of pollutant was investigated. A predictive model of decontamination efficiency for NEP equipment was proposed and validated by comparing the experimental results with the predictive data. The improved methods for the model were also discussed.
391
0.002 150
170
190
210
230
250
270
290
310
330
3
P T /Q (W•h/m ) Fig. 1. Calculation for Ef,i values of chemical bonds in H2S and 2-CEES molecules.
392
Z.-G. Li et al. / Journal of Electrostatics 68 (2010) 390e393
Table 2 E0f,i values of chemical bonds. Chemical bond
E0b,i (eV)
E0f,i
SeH CeS CeCl CeC CeH
3.95 3.14 3.65 3.56 4.36
2.37 1.84 2.14 2.10 2.56
103 103 103 103 103
3. Predictive model of decontamination efficiency and its improvement 3.1. Predictive model of decontamination efficiency The decontamination efficiency of other pollutants having the chemical bonds, of which Ef,is obtained as described above, can be predicted in different discharge conditions, since the Ef,i value of chemical bond is constant in same type of discharge plasma mode. The predictive equation is written as:
P 8 ðni $Ef ;i Þ$PT 100%; If 26:8C $Q $ P >1 > < ðni $Eb;i Þ 0 P P h¼ ni $Ef ;i $PT ni $Ef ;i $PT > : 100%; If 1 P P 26:8C0 $Q $ ni $Eb;i 26:8C0 $Q $ ni $Eb;i (9) Relationship among decontamination efficiency, discharge power and dissociation energy of chemical bond is described in equation (9). The parameters in equation (9) are discharge power, gas flow rate, initial molar concentration and dissociation energy of chemical bond and so on. The model is independent of discharge mode or plasma reactor structure, so it can be generally used. But its precision and applicability are needed to be validated and improved in further more experiments. The necessary validation and improvement methods for the model are presented as follows.
3.2. Validation for the predictive model In order to validate the precision of the model, a new scale-up pulsed corona discharge reactor was designed to decontaminate ethyl-mercaptan (CH3CH2SH) and 2-CEES. As shown in Fig. 2, the reactor is in a stainless steel sting-to-plate structure with a size of 1000 mm 500 mm 100 mm, and the discharge electrodes are groups of stings (20 mm long) welded on the stainless steel bars with a dimension of 4 mm 2 mm. The gap distance between adjacent bars is 50 mm and the gap distance between adjacent stings on the same bar is 40 mm. Positive pulse voltage is generated by a capacitor bank and a rotating spark gap. The power supply is designed to produce various type of pulse discharge. The maximum peak voltage is 60 kV and the maximum pulse frequency is 200 Hz. The pulse rise time is less than 50 nano second. In the experiment, polluted gas from gaseous sample generator was introduced into plasma reactor to decontaminate. The detailed experimental
method can be consulted in literature [26]. The operation parameters and experiment results are listed in Table 4, conveniently for comparing with predictive data. Since the dissociation energy of chemical bond will vary with its surroundings in the molecule, its Ef,i must be modified. Therefore, a dissociation energy coefficient (s) is defined:
s¼
Eb Eb0
(10)
For example, the dissociation energy of HeS is 3.79 eV in CH3CH2SH, but E0b,HeS is 3.95 eV from Table 2, so sHeS ¼ 3.79/ 3.95 ¼ 0.9595. According to E0f,i values in Table 2 and Eb,i of chemical bonds in CH3CH2SH molecule, Ef,i values of chemical bonds in CH3CH2SH molecule can be calculated by
Ef ;i ¼ si $Ef0;i
(11)
The calculated data are listed in Table 3. Substituting the data of Table 3 and Table 2 in equation (9), the predictive decontamination efficiency of CH3CH2SH [21,27] and 2-CEES under special discharge conditions are obtained, as shown in Table 4. From Table 4, it can be found that the predictive data are agreed to experimental results under the experimental condition of low initial molar concentration (C0) of pollutant or gas flow rate (Q). However, the predictive error is higher with increased C0 or Q, which can be assumed as below: 1) The E0f,i values in Table 2 are average actual values as the E0f,i values are obtained from experimental data of 2-CEES composed of some different chemical bonds. When the E0f,i values are calculated simply by the proportion of dissociation energy of all chemical bonds, the reactive difference among the chemical bonds is ignored. Therefore, the E0f,i values are needed to be corrected. 2) The predictive model is assumed as all chemical bonds of decontaminated molecules of pollutant have been completely destroyed to produce CO2, H2O, SO2 and HCl. But pollutant may be decomposed to other organic compounds, which might give a contribution to the decontamination efficiency that even only one chemical bond is destroyed. Therefore, the test decontamination efficiency is likely higher than the predictive data because only a part of chemical bonds were destroyed when increasing initial concentration or gas flow rate.
3.3. Improvement for the predictive model According to the above analysis on predictive error, two approaches to improve the model are proposed: 1) Developing database of E0f,i of chemical bonds Table 3 Ef,i values of chemical bonds in CH3CH2SH molecule.
Fig. 2. Schematic diagram of the scale-up pulsed corona reactor.
Chemical bond
ni
Eb,i (eV)
si
Ef,i
SeH CeS CeC SCeH CH2eH
1 1 1 2 3 P
3.79 3.19 3.58 4.07 4.36
0.9595 1.016 1.006 0.9335 1.000 P (ni$Ef,i) ¼ 1.87
2.27 1.87 2.10 2.39 2.56
C2H5SH
(ni$Ebi) ¼ 31.78
102
103 103 103 103 103
Z.-G. Li et al. / Journal of Electrostatics 68 (2010) 390e393
Acknowledgments
Table 4 Comparison of predicting results and experimental data. Pollutant CH3CH2SH
2-CEES
C0 (mol/m3) 5.65 2.03 1.98 2.00 2.03 2.34 2.57 2.90
104 103 103 103 103 103 103 103
393
Q (m3/h)
PT (W)
h(Test)
h(Predict)
(%)
(%)
Predicting error (%)
4.0 1.2 2.0 2.8 1.2 1.2 1.2 1.2
96 96 112 112 112 112 112 112
98.2 97.3 74.0 53.8 98.9 96.8 93.1 86.1
93.3 86.5 62.1 43.9 100 87.5 79.53 70.5
5.0 11.1 16.1 18.4 1.1 9.6 14.6 18.1
We are grateful for the financial support by High-tech Research and Development Program of China (No. 2007AA06A408).
References
According to data calculated by quantum chemistry, the dissociation energy of the same chemical bonds varies with its surroundings in the molecule. While dissociation energy of chemical bonds slightly changes, a large change of reaction rate would occur. Therefore, it is necessary to determine and calculate each E0f,i of chemical bonds surrounded by other bonds in molecule, and then a database of E0f,i of chemical bonds could be generated. The database will provide more precise E0f,is which can be chosen to be used in equation (9) so that the precision of decontamination efficiency prediction can be enhanced. 2) Improvement for predictive model The predictive data is less than the experimental data due to all P P chemical bonds are assumed to be destroyed and (ni$Ebi), (ni$Ef,i), were used in equation (9). However, it can be used as the lowest limit of decontamination efficiency, as min (h). If substituting the minimal dissociation energy of chemical bond in equation (9), the highest limit of decontamination efficiency will be obtained, as max(h). In this way, the predictive model can be rewritten as follows:
P 8 ðni $Ef ;i Þ$PT > > >1 100%;If 26:8C $Q $P > < ðni $Eb;i Þ 0 P P minðhÞ¼ ni $Ef ;i $PT ni $Ef ;i $PT > > > 100%;If 1 P P : 26:8C0 $Q $ ni $Eb;i 26:8C0 $Q $ ni $Eb;i (12) 8 Ef ;i $PT > > < 100%; If 26:8C0 $Q $minðEb;i Þ >1 maxðhÞ¼ Ef ;i $PT Ef ;i $PT > > : 26:8C $Q $minE 100%; If 26:8C $Q $minE 1 0 0 b;i b;i (13) According to Eqs. (12) and (13), the range of decontamination efficiency of a pollutant by a NEP equipment: min (h) h max (h) can be calculated and predicted. 4. Conclusions The predictive model of decontamination efficiency is developed based on analyzing the relationship of discharge power, molecular structure of pollutant and decontamination efficiency. The energy conversion factor (Ef,i) of chemical bonds, SeH, CeS, CeCl, CeC and CeH, is obtained from the decontamination experimental data. The Ef,i values are used to estimate the decontamination efficiency of CH3CH2SH and 2-CEES, and the results show that the predictive values are very close to experimental data, therefore, the model is reasonable and applicable. It will be expected that the model can play an important role in predicting decontamination efficiency of pollutants.
[1] Y. Wu, J. Li, N.H. Wang, et al., Industrial experiments on desulfurization of flue gases by pulsed corona induced plasma chemical process. Journal of Electrostatics 57 (2003) 233e241. [2] J.S. Chang, K. Urashima, Y.X. Tong, et al., Simultaneous removal of NOx and SO2 from coal boiler flue gases by DC corona discharge ammonia radical shower systems: pilot plant tests. Journal of Electrostatics 57 (2003) 313e323. [3] Y.H. Lee, W.S. Jung, Y.R. Choi, et al., Application of pulsed corona induced plasma chemical process to an industrial incinerator. Environmental Science & Technology 37 (2003) 2563e2567. [4] Y.S. Mok, S.W. Ham, I. Nam, Mathematical analysis of positive pulsed corona discharge process employed for removal of nitrogen oxides. IEEE Transaction on Plasma Science 26 (1998) 1566e1574. [5] W. Sun, B. Pashaie, S.K. Dhali, Non-thermal plasma remediation of SO2/NO using a dielectric-barrier discharge. Journal of Applied Physics 79 (1996) 3438e3444. [6] B. Eliasson, U. Kogelschatz, Nonequilibrium volume plasma chemical processing. IEEE Transaction on Plasma Science 19 (1991) 1063e1077. [7] J.S. Chang, P.A. Lawless, T. Yamamoto, Corona discharge processes. IEEE Transactions on Plasma Science 19 (1991) 1152e1166. [8] I. Maezono, J.S. Chang, Reduction of CO2 from combustion gases by dc corona torchers. IEEE Transactions on Industry Applications 26 (1990) 651e655. [9] T. Oda, Non-thermal plasma processing for environmental protection: decomposition of dilute VOCs in air. Journal of Electrostatics 57 (2003) 293e311. [10] T. Oda, T. Takahahshi, K. Yamaji, Nonthermal plasma processing for dilute VOCs decomposition. IEEE Transactions on Industry Applications 38 (2002) 873e878. [11] Y.F. Guo, D.Q. Ye, K.F. Chen, Toluene removal characteristics by a superimposed wire-plate dielectric barrier discharge plasma reactor. Journal of Environmental Science 18 (2006) 276e280. [12] Y.S. Choi, Y.H. Song, S.J. Kim, et al., A study on the toluene decomposition using an adsorptive dielectric discharge plasma. HWAHAK KONGHAK 38 (2000) 423e428. [13] A.D. Koutsospyros, S. M Yin, C. Christodoulatos, et al., Plasma chemical degradation of volatile organic compounds (VOC) in a capillary discharge plasma reactor. IEEE Transaction on Plasma Science 33 (2005) 42e49. [14] D. Helfritch, Pulsed corona discharge for hydrogen sulfide decomposition. IEEE Transactions on Industry Applications 29 (5) (1993) 882e886. [15] Y.S. Mok, C.M. Nam, M.H. Cho, Decontamination of volatile organic compounds and nitric oxide by non thermal plasma discharge process. IEEE Transaction on Plasma Science 30 (2002) 408e415. [16] I. Antonius, J.W. Choi, H. Lee, et al., Decomposition of CCl4 and CHCl3 on gliding arc plasma. Journal of Environmental Sciences 18 (2006) 83e89. [17] U. Roland, F. Holzer, F.D. Kopinke, Improved oxidation of air pollutants in a non-thermal plasma. Catalysis Today 73 (2002) 315e323. [18] H.H. Kim, G. Prieto, K. Takashima, et al., Performance evaluation of discharge plasma process for gaseous pollutant removal. Journal of Electrostatics 55 (2002) 25e41. [19] J.S. Chang, Physics and chemistry of plasma pollution control technology. Plasma Sources Science and Technology 17 (2008) 045004. [20] S. Futamura, A.H. Zhang, T. Yamamoto, The dependence of nonthermal plasma behavior of VOCs on their chemical structures. Journal of Electrostatics 42 (1997) 51e62. [21] Z.G. Li, Z. Hu, Z.N. Liu, et al., A nonequilibrium plasma decomposition reaction kinetics model of gaseous pollutant. Journal of Chemical Industry and Engineering (China) 60 (2009) 1466e1470. [22] Z.G. Li, Z. Hu, X.L. Sun, et al. Decontamination of Mustard Gas by Nonequilibrium plasma. In: The 9th Symposium on Protection against Chemical and Biological Warfare Agents, Gothenburg, Sweden. 2007. [23] X.F. Yan, Z. Hu, Comparative analysis of experiment treating benzene and CEES by pulse corona plasma. Plasma Science & Technology 7 (2005) 2953e2954. [24] Y.R. Luo, Handbook of Bond Dissociation Energy. Science Press, Beijing, China, 2005. [25] Z.G. Li, Z. Hu, Y. Li, et al. Degradation of H2S by Pulsed Corona Discharge and Dielectric Barrier Discharge. In: The 16th International Conference on Gas Discharges and their Applications, Xi’an, China. 2006. [26] X.F. Yan, Z. Hu, Experiment and analysis on the treatment of gaseous benzene using pulsed corona discharge plasma. Plasma Science & Technology 6 (2004) 2241e2246. [27] Z.G. Li, Z. Hu, Y. Li, et al., Removal of ethyl mercaptan by nonequilibrium plasma. Chemical Industry and Engineering Progress 28 (2009) 698e702 (Chinese).
Contents lists available at ScienceDirect
Journal of Electrostatics journal homepage: www.elsevier.com/locate/elstat
Predictive model of decontamination efficiency of gaseous pollutant by non-equilibrium plasmaq Zhan-Guo Li*, Zhen Hu, Hai-Ling Xi, Peng Cao Research Institute of Chemical Defense, Beijing 102205, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 February 2009 Received in revised form 18 March 2010 Accepted 26 May 2010 Available online 9 June 2010
During non-equilibrium plasma (NEP) reactions, chemical bonds of pollutant compound are broken by energy generated in the reactor so that the pollutants are decontaminated. In this study, the energy conversion factor (Ef) is defined as the ratio of the dissociation energy of chemical bonds destroyed in NEP reaction system to the energy inputted in plasma reactor. The energy conversion factor of chemical bonds (Ef,i), SeH, CeCl, CeS, CeH, CeC etc., were determined by decontamination experiments of H2S and 2-CEES in plasma reactor. Based on the Ef,is, the predictive model of NEP decontamination efficiency of gaseous pollutant was developed and applied to predict decontamination efficiency of CH3CH2SH, in which all Ef,is of chemical bonds are known as described above. It was shown by the decontamination experiment of CH3CH2SH that the predictive value was well agreed to experimental data. Therefore, the model can be used to predict decontamination efficiency of those pollutants in which all Ef,is of chemical bonds have been determined. An improved model is also produced by the analysis of predictive error. Ó 2010 Elsevier B.V. All rights reserved.
Keywords: Non-equilibrium plasma Decontamination efficiency Gaseous pollutant Predictive model
1. Introduction Non-equilibrium plasma (NEP) processing has very strong chemical reactivity and has attracted more and more attention in the area of gaseous pollutants decontamination [1e3]. There have been a great deal of researches both experimental and theoretical on the decontamination of VOCs and NOx using NEP, especially on the general plasma chemistry process and kinetic models [4,5]. In NEP process, the fast electrons created by the discharge mechanism mainly initiate the chemical reactions [6]. Chang’s group [7] analyzed the generation process of plasma in the positive corona and negative corona, and discussed the generation, recombination, and loss processes of reactive species. They [8] also suggested that radical species could be generated through the dissociative recombination of the positive and negative species via formations of various kinds of ions from ion molecular reactions. Oda [9] suggested that the oxidation ability of the plasma is very strong, but the key points for its practical usage were reliability of the process and energy efficiency of the plasma. To improve the energy efficiency of the plasma process, combination with the adsorption and catalysts is feasible [10,11].
q Original version presented at ICESP’2008, 11th International Conference on Electrostatic Precipitation, 20e24 October 2008, Hangzhou, China. * Corresponding author. Tel.: þ86 10 66758313; fax: þ86 10 69768181. E-mail address: [email protected] (Z.-G. Li). 0304-3886/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.elstat.2010.05.010
In the study of plasma reaction kinetics, Choi’s group [12] developed a first order reaction kinetic model for the decomposition rate relative to the VOC concentration and studied the decomposition mechanism. It was reported [5,13,14] that the concentration of reactive particles was directly proportional to the energy input in reactor. Therefore, Mok’s group [15] assumed that the decomposition rate would be directly proportional to the concentration of the pollutant and the discharge power, and developed a kinetic equation for the plasma reactor. In order to evaluate the decomposition performance of NEP system, many parameters were used, such as the selectivity and conversion of pollutant to CO and CO2, the energy efficiency [16,17]. Many previous studies [18,19] also focus on the energy cost (EC; eV per removed molecule), the G-value (removed molecules per 100 eV input) and the energy yield (EY; g-removed per kWh input), which have been used to evaluate the performance of the discharge plasma systems. However, practical performance evaluation of the plasma process is not yet conducted. Major difficulties to evaluate the performance of NTP process using parameters mentioned above, are varied parameters with different pollutant. However, there will be much more laborious work if kinetic data are obtained from experiments for every single organic pollutant. Futamura’s group [20] reported that the decomposition behavior of VOCs is related to their chemical structures. From authors’ previous work [21], it was also found that the reaction rate was not only related to discharge parameters, but also to molecular structure of pollutant, during studying the
Z.-G. Li et al. / Journal of Electrostatics 68 (2010) 390e393
It is well known that NEP chemical reaction is an energy conversion process that active particles collide with pollutant molecules. It has been found that the lower chemical bond dissociation energy of the pollutant is, the higher the decontamination efficiency is, in a plasma system [15,22,23], which showed that decontamination efficiency of pollutant is closely related to its dissociation energy of chemical bonds. For example, when corona plasma reactor is used to treat H2S and 2-CEES (2-chloroethyl ethyl sulfide, CH3CH2SCH2CH2Cl), the reactions are described as below: Plasma
H2 S ! SO2 þ H2 O Plasma
ClCH2 CH2 SCH2 CH3 ! SO2 þ H2 O þ CO2 þ HCl The chemical bonds and their dissociation energy in the above reactions are listed in Table 1 [24]. In the reaction process, the initial mol concentration of pollutant is C0 (mol/m3), gas flow rate is Q (m3/h), average power input in plasma reactor is PT (W), and decontamination experimental efficiency is h. In another words, the chemical bonds of C0hQ mol/h pollutant were destroyed by energy of PT watt. Considering the consumption in the energy transferring process, PT must be more than the sum of dissociation energy of these chemical bonds. Here, the ratio of the total dissociation energy to PT is defined as energy conversion factor, symbolize as Ef:
Ef ¼
C0 $h$Q $
P
ni $Eb;i
PT
mol$eV=ðW$hÞ
(1)
Where, Ef-energy conversion factor of pollutant molecules, mol$eV/ (W$h); C0-initial mol concentration of pollutant, mol/m3; hdecontamination efficiency, %; Q-gas flow rate, m3/h; ni-hemical bond number with the same dissociation energy in pollutant molecule; Eb,i-dissociation energy of chemical bond i, eV; PTaverage power input in plasma reactor, W.
Table 1 dissociation energy of chemical bonds.
Ef0 ;i ¼ P
Eb;i ni $Eb;i
Pollutant
Chemical bond
ni
Eb,i (eV)
SeH CeS CH2eCH3 eSCHeH CH2CH2eH
2 2 1 4 3
3.95 3.14 3.56 4.04 4.36
Chemical bond eCeCl CH2eCH2Cl CHCleH
ni 1 1 2
Ef ;i ¼ 26:8
(2)
C0 $h$Q $Eb;i PT
(3)
Therefore, Ef,i means the part of average energy input in plasma reactor which is used to destroy chemical bond i of pollutant molecules. In order to calculated Ef,i using the data from some experiments, the equation (3) can be converted to following form
C0 $h ¼
Ef ;i P T 26:8Eb;i Q
(4)
k is defined as below
k ¼
Ef ;i 26:8Eb;i
(5)
Then, C0$h and PT/Q are variables for a pollutant in different experimental condition. In the decontamination experiments of H2S and 2-CEES [25], decontamination efficiency h is varied under different conditions, such as C0, PT and Q. C0$h depend on PT/Q, and k is the slope of the line, as shown in Fig. 1. From equation (5), Ef,HS can be obtained:
Ef ;HS Ef ;HS ¼ ¼ 2:24 105 26:8Eb;HS 26:8 3:95
(6)
Ef ;HS ¼ 2:37 103
(7)
Similarly, Ef,CeS, Ef,CeCl, Ef,CeC and Ef,CeH, are also obtained by substituting the dissociation energy of chemical bonds in 2-CEES molecule in equation (8):
Ef ;i ¼ 2:19 105 26:8Eb;i
(8)
We can take the obtained Ef,i values as the standard values, symbolized as E0f,i, and the dissociation energy of corresponding chemical bond is symbolized as E0b,i, shown in Table 2.
0.012 H2S 0.010
-5
C 0 • = 2.24×10 (P T /Q ) + 0.0033 2
2-CEES R = 0.9897
0.008 0.006 -5
C 0 • = 2.19×10 (P T /Q ) - 0.0008 2
R = 0.9912
Eb,i (eV) 3.65 3.85 4.21
C0 $h$Q $Eb;i mol$eV=ðW$hÞ PT
If energy units in the equations above are substituted by joule, i.e, 1 W$h ¼ 3.6 103 J and 1 eV ¼ 96.48 kJ/mol, the energy conversion factor is dimensionless and a coefficient a ¼ 26.8 is generated. The equation (2) is converted to
0.004 H2S 2-CEES
Ef ¼
3
2. Energy conversion factor of chemical bonds in NEP decontamination reaction
Molecule 2-CEES has several types of chemical bonds and all chemical bonds were destroyed, the energy conversion factor Éf,i of chemical bond i can be calculated by the following equation:
C0 • (mol/m )
decontamination kinetics model of NEP for gaseous pollutant treatment. It is well known that each organic pollutant has several special chemical bonds, and the types of chemical bonds are far less than the types of organic pollutants. Therefore, if the mathematical connection of decontamination efficiency, discharge parameter and dissociation energy of chemical bond can be revealed, it will be able to develop a theoretical model to predict the decontamination efficiency of the pollutant. This work will be significant for the plasma application. In this study, the quantitative relationship of decontamination efficiency, discharge power and dissociation energy of chemical bonds of pollutant was investigated. A predictive model of decontamination efficiency for NEP equipment was proposed and validated by comparing the experimental results with the predictive data. The improved methods for the model were also discussed.
391
0.002 150
170
190
210
230
250
270
290
310
330
3
P T /Q (W•h/m ) Fig. 1. Calculation for Ef,i values of chemical bonds in H2S and 2-CEES molecules.
392
Z.-G. Li et al. / Journal of Electrostatics 68 (2010) 390e393
Table 2 E0f,i values of chemical bonds. Chemical bond
E0b,i (eV)
E0f,i
SeH CeS CeCl CeC CeH
3.95 3.14 3.65 3.56 4.36
2.37 1.84 2.14 2.10 2.56
103 103 103 103 103
3. Predictive model of decontamination efficiency and its improvement 3.1. Predictive model of decontamination efficiency The decontamination efficiency of other pollutants having the chemical bonds, of which Ef,is obtained as described above, can be predicted in different discharge conditions, since the Ef,i value of chemical bond is constant in same type of discharge plasma mode. The predictive equation is written as:
P 8 ðni $Ef ;i Þ$PT 100%; If 26:8C $Q $ P >1 > < ðni $Eb;i Þ 0 P P h¼ ni $Ef ;i $PT ni $Ef ;i $PT > : 100%; If 1 P P 26:8C0 $Q $ ni $Eb;i 26:8C0 $Q $ ni $Eb;i (9) Relationship among decontamination efficiency, discharge power and dissociation energy of chemical bond is described in equation (9). The parameters in equation (9) are discharge power, gas flow rate, initial molar concentration and dissociation energy of chemical bond and so on. The model is independent of discharge mode or plasma reactor structure, so it can be generally used. But its precision and applicability are needed to be validated and improved in further more experiments. The necessary validation and improvement methods for the model are presented as follows.
3.2. Validation for the predictive model In order to validate the precision of the model, a new scale-up pulsed corona discharge reactor was designed to decontaminate ethyl-mercaptan (CH3CH2SH) and 2-CEES. As shown in Fig. 2, the reactor is in a stainless steel sting-to-plate structure with a size of 1000 mm 500 mm 100 mm, and the discharge electrodes are groups of stings (20 mm long) welded on the stainless steel bars with a dimension of 4 mm 2 mm. The gap distance between adjacent bars is 50 mm and the gap distance between adjacent stings on the same bar is 40 mm. Positive pulse voltage is generated by a capacitor bank and a rotating spark gap. The power supply is designed to produce various type of pulse discharge. The maximum peak voltage is 60 kV and the maximum pulse frequency is 200 Hz. The pulse rise time is less than 50 nano second. In the experiment, polluted gas from gaseous sample generator was introduced into plasma reactor to decontaminate. The detailed experimental
method can be consulted in literature [26]. The operation parameters and experiment results are listed in Table 4, conveniently for comparing with predictive data. Since the dissociation energy of chemical bond will vary with its surroundings in the molecule, its Ef,i must be modified. Therefore, a dissociation energy coefficient (s) is defined:
s¼
Eb Eb0
(10)
For example, the dissociation energy of HeS is 3.79 eV in CH3CH2SH, but E0b,HeS is 3.95 eV from Table 2, so sHeS ¼ 3.79/ 3.95 ¼ 0.9595. According to E0f,i values in Table 2 and Eb,i of chemical bonds in CH3CH2SH molecule, Ef,i values of chemical bonds in CH3CH2SH molecule can be calculated by
Ef ;i ¼ si $Ef0;i
(11)
The calculated data are listed in Table 3. Substituting the data of Table 3 and Table 2 in equation (9), the predictive decontamination efficiency of CH3CH2SH [21,27] and 2-CEES under special discharge conditions are obtained, as shown in Table 4. From Table 4, it can be found that the predictive data are agreed to experimental results under the experimental condition of low initial molar concentration (C0) of pollutant or gas flow rate (Q). However, the predictive error is higher with increased C0 or Q, which can be assumed as below: 1) The E0f,i values in Table 2 are average actual values as the E0f,i values are obtained from experimental data of 2-CEES composed of some different chemical bonds. When the E0f,i values are calculated simply by the proportion of dissociation energy of all chemical bonds, the reactive difference among the chemical bonds is ignored. Therefore, the E0f,i values are needed to be corrected. 2) The predictive model is assumed as all chemical bonds of decontaminated molecules of pollutant have been completely destroyed to produce CO2, H2O, SO2 and HCl. But pollutant may be decomposed to other organic compounds, which might give a contribution to the decontamination efficiency that even only one chemical bond is destroyed. Therefore, the test decontamination efficiency is likely higher than the predictive data because only a part of chemical bonds were destroyed when increasing initial concentration or gas flow rate.
3.3. Improvement for the predictive model According to the above analysis on predictive error, two approaches to improve the model are proposed: 1) Developing database of E0f,i of chemical bonds Table 3 Ef,i values of chemical bonds in CH3CH2SH molecule.
Fig. 2. Schematic diagram of the scale-up pulsed corona reactor.
Chemical bond
ni
Eb,i (eV)
si
Ef,i
SeH CeS CeC SCeH CH2eH
1 1 1 2 3 P
3.79 3.19 3.58 4.07 4.36
0.9595 1.016 1.006 0.9335 1.000 P (ni$Ef,i) ¼ 1.87
2.27 1.87 2.10 2.39 2.56
C2H5SH
(ni$Ebi) ¼ 31.78
102
103 103 103 103 103
Z.-G. Li et al. / Journal of Electrostatics 68 (2010) 390e393
Acknowledgments
Table 4 Comparison of predicting results and experimental data. Pollutant CH3CH2SH
2-CEES
C0 (mol/m3) 5.65 2.03 1.98 2.00 2.03 2.34 2.57 2.90
104 103 103 103 103 103 103 103
393
Q (m3/h)
PT (W)
h(Test)
h(Predict)
(%)
(%)
Predicting error (%)
4.0 1.2 2.0 2.8 1.2 1.2 1.2 1.2
96 96 112 112 112 112 112 112
98.2 97.3 74.0 53.8 98.9 96.8 93.1 86.1
93.3 86.5 62.1 43.9 100 87.5 79.53 70.5
5.0 11.1 16.1 18.4 1.1 9.6 14.6 18.1
We are grateful for the financial support by High-tech Research and Development Program of China (No. 2007AA06A408).
References
According to data calculated by quantum chemistry, the dissociation energy of the same chemical bonds varies with its surroundings in the molecule. While dissociation energy of chemical bonds slightly changes, a large change of reaction rate would occur. Therefore, it is necessary to determine and calculate each E0f,i of chemical bonds surrounded by other bonds in molecule, and then a database of E0f,i of chemical bonds could be generated. The database will provide more precise E0f,is which can be chosen to be used in equation (9) so that the precision of decontamination efficiency prediction can be enhanced. 2) Improvement for predictive model The predictive data is less than the experimental data due to all P P chemical bonds are assumed to be destroyed and (ni$Ebi), (ni$Ef,i), were used in equation (9). However, it can be used as the lowest limit of decontamination efficiency, as min (h). If substituting the minimal dissociation energy of chemical bond in equation (9), the highest limit of decontamination efficiency will be obtained, as max(h). In this way, the predictive model can be rewritten as follows:
P 8 ðni $Ef ;i Þ$PT > > >1 100%;If 26:8C $Q $P > < ðni $Eb;i Þ 0 P P minðhÞ¼ ni $Ef ;i $PT ni $Ef ;i $PT > > > 100%;If 1 P P : 26:8C0 $Q $ ni $Eb;i 26:8C0 $Q $ ni $Eb;i (12) 8 Ef ;i $PT > > < 100%; If 26:8C0 $Q $minðEb;i Þ >1 maxðhÞ¼ Ef ;i $PT Ef ;i $PT > > : 26:8C $Q $minE 100%; If 26:8C $Q $minE 1 0 0 b;i b;i (13) According to Eqs. (12) and (13), the range of decontamination efficiency of a pollutant by a NEP equipment: min (h) h max (h) can be calculated and predicted. 4. Conclusions The predictive model of decontamination efficiency is developed based on analyzing the relationship of discharge power, molecular structure of pollutant and decontamination efficiency. The energy conversion factor (Ef,i) of chemical bonds, SeH, CeS, CeCl, CeC and CeH, is obtained from the decontamination experimental data. The Ef,i values are used to estimate the decontamination efficiency of CH3CH2SH and 2-CEES, and the results show that the predictive values are very close to experimental data, therefore, the model is reasonable and applicable. It will be expected that the model can play an important role in predicting decontamination efficiency of pollutants.
[1] Y. Wu, J. Li, N.H. Wang, et al., Industrial experiments on desulfurization of flue gases by pulsed corona induced plasma chemical process. Journal of Electrostatics 57 (2003) 233e241. [2] J.S. Chang, K. Urashima, Y.X. Tong, et al., Simultaneous removal of NOx and SO2 from coal boiler flue gases by DC corona discharge ammonia radical shower systems: pilot plant tests. Journal of Electrostatics 57 (2003) 313e323. [3] Y.H. Lee, W.S. Jung, Y.R. Choi, et al., Application of pulsed corona induced plasma chemical process to an industrial incinerator. Environmental Science & Technology 37 (2003) 2563e2567. [4] Y.S. Mok, S.W. Ham, I. Nam, Mathematical analysis of positive pulsed corona discharge process employed for removal of nitrogen oxides. IEEE Transaction on Plasma Science 26 (1998) 1566e1574. [5] W. Sun, B. Pashaie, S.K. Dhali, Non-thermal plasma remediation of SO2/NO using a dielectric-barrier discharge. Journal of Applied Physics 79 (1996) 3438e3444. [6] B. Eliasson, U. Kogelschatz, Nonequilibrium volume plasma chemical processing. IEEE Transaction on Plasma Science 19 (1991) 1063e1077. [7] J.S. Chang, P.A. Lawless, T. Yamamoto, Corona discharge processes. IEEE Transactions on Plasma Science 19 (1991) 1152e1166. [8] I. Maezono, J.S. Chang, Reduction of CO2 from combustion gases by dc corona torchers. IEEE Transactions on Industry Applications 26 (1990) 651e655. [9] T. Oda, Non-thermal plasma processing for environmental protection: decomposition of dilute VOCs in air. Journal of Electrostatics 57 (2003) 293e311. [10] T. Oda, T. Takahahshi, K. Yamaji, Nonthermal plasma processing for dilute VOCs decomposition. IEEE Transactions on Industry Applications 38 (2002) 873e878. [11] Y.F. Guo, D.Q. Ye, K.F. Chen, Toluene removal characteristics by a superimposed wire-plate dielectric barrier discharge plasma reactor. Journal of Environmental Science 18 (2006) 276e280. [12] Y.S. Choi, Y.H. Song, S.J. Kim, et al., A study on the toluene decomposition using an adsorptive dielectric discharge plasma. HWAHAK KONGHAK 38 (2000) 423e428. [13] A.D. Koutsospyros, S. M Yin, C. Christodoulatos, et al., Plasma chemical degradation of volatile organic compounds (VOC) in a capillary discharge plasma reactor. IEEE Transaction on Plasma Science 33 (2005) 42e49. [14] D. Helfritch, Pulsed corona discharge for hydrogen sulfide decomposition. IEEE Transactions on Industry Applications 29 (5) (1993) 882e886. [15] Y.S. Mok, C.M. Nam, M.H. Cho, Decontamination of volatile organic compounds and nitric oxide by non thermal plasma discharge process. IEEE Transaction on Plasma Science 30 (2002) 408e415. [16] I. Antonius, J.W. Choi, H. Lee, et al., Decomposition of CCl4 and CHCl3 on gliding arc plasma. Journal of Environmental Sciences 18 (2006) 83e89. [17] U. Roland, F. Holzer, F.D. Kopinke, Improved oxidation of air pollutants in a non-thermal plasma. Catalysis Today 73 (2002) 315e323. [18] H.H. Kim, G. Prieto, K. Takashima, et al., Performance evaluation of discharge plasma process for gaseous pollutant removal. Journal of Electrostatics 55 (2002) 25e41. [19] J.S. Chang, Physics and chemistry of plasma pollution control technology. Plasma Sources Science and Technology 17 (2008) 045004. [20] S. Futamura, A.H. Zhang, T. Yamamoto, The dependence of nonthermal plasma behavior of VOCs on their chemical structures. Journal of Electrostatics 42 (1997) 51e62. [21] Z.G. Li, Z. Hu, Z.N. Liu, et al., A nonequilibrium plasma decomposition reaction kinetics model of gaseous pollutant. Journal of Chemical Industry and Engineering (China) 60 (2009) 1466e1470. [22] Z.G. Li, Z. Hu, X.L. Sun, et al. Decontamination of Mustard Gas by Nonequilibrium plasma. In: The 9th Symposium on Protection against Chemical and Biological Warfare Agents, Gothenburg, Sweden. 2007. [23] X.F. Yan, Z. Hu, Comparative analysis of experiment treating benzene and CEES by pulse corona plasma. Plasma Science & Technology 7 (2005) 2953e2954. [24] Y.R. Luo, Handbook of Bond Dissociation Energy. Science Press, Beijing, China, 2005. [25] Z.G. Li, Z. Hu, Y. Li, et al. Degradation of H2S by Pulsed Corona Discharge and Dielectric Barrier Discharge. In: The 16th International Conference on Gas Discharges and their Applications, Xi’an, China. 2006. [26] X.F. Yan, Z. Hu, Experiment and analysis on the treatment of gaseous benzene using pulsed corona discharge plasma. Plasma Science & Technology 6 (2004) 2241e2246. [27] Z.G. Li, Z. Hu, Y. Li, et al., Removal of ethyl mercaptan by nonequilibrium plasma. Chemical Industry and Engineering Progress 28 (2009) 698e702 (Chinese).