Energy Fuels 2010, 24, 5418–5425 Published on Web 08/13/2010
: DOI:10.1021/ef100451x
An Investigation on the Principal Paths to Plasma Oxidation of Propylene and NO He Lin, Bin Guan,* Qi Cheng, and Zhen Huang Key Laboratory for Power Machinery and Engineering of Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, China Received April 10, 2010. Revised Manuscript Received July 22, 2010
Hydrocarbons plasma oxidation combined with fast selective catalytic reduction is a promising technology for treating the diesel exhausts with high sulfur content. In this study, propylene (C3H6) oxidation in a nonthermal plasma process and its effects on NO transition were investigated. The experimental results and the results from simulation based on a simplified model indicated that the binding reactions of O radicals ionizing from O2 with C3H6 are the trigger reactions, from which carbonyl (HCO) and ethyl radicals (C2H5) are produced and play crucial roles in C3H6 and NO oxidation. It was found that carbon monoxide (CO), carbon dioxide (CO2), formaldehyde (HCHO), acetaldehyde (CH3CHO), formic acid (HCOOH), etc. are the final oxidation products of C3H6 oxidation in the absence of NO. With the presence of NO, peroxy (CH3O2 and C2H5O2), and hydroperoxy radical (HO2) that come from C3H6 oxidation are able to oxidize NO to NO2 efficiently and, meanwhile, induce the reactions that would lead to the formation of many kinds of byproducts such as nitrous oxide (N2O), nitrous acid (HONO), nitric acid (HONO2), methyl nitrite (CH3ONO), and methyl nitrate (CH3ONO2).
oxidation have great effects on NO oxidation.6-13 In the past few years, intensive research has been done on the plasma oxidation of hydrocarbons including alkyl, alkene, alcohols, aldehydes, and many kinds of aromatic and halogen compounds.14-20 It was shown that the volatile organic compounds (VOC) such as C3H6, propane, and isopropyl alcohol can be oxidized to CO and CO2, accompanied by substantive byproducts such as acetone, aldehydes, and formic acid.18-23 A great deal of literature reported that the combinations of C3H6 with O radicals originating from O2 ionization led to the formation of intermediate compounds with the chemical structure of CxHyOz (x=1-3, y=2-6, and z=0-1), and these binding reactions might be the trigger reactions for C3H6 oxidation. The generated intermediates were further oxidized to the peroxy radicals (RO2) and hydroperoxy (HO2), which are able to oxidize NO to NO2 efficiently.4,6-10,13 The interaction of C3H6 and O radicals is extremely complex, and the possible reaction
1. Introduction Fast NH3-selective catalytic reduction (SCR) technology based on vanadium catalyst is an effective approach to improve NOx reduction efficiency under low temperature conditions of diesel engines.1-3 The precondition to achieve fast SCR is to oxidize part of NO to NO2 in the upstream of the SCR reactor. It was found that in a nonthermal plasma process,4 the rate of NO oxidation is 2 orders of magnitude larger than that of SO2 oxidation. Therefore, plasma oxidation of NO is more tolerant to sulfur content as compared to NO oxidation over noble catalysts.1-5 Plasma oxidation of NO from diesel engine exhausts involving numerous other components such as O2 and unburned hydrocarbons (UHC) is a complex process, in which ionization of O2 molecules and the subsequent HC *To whom correspondence should be addressed. Telephone: þ86 21 34206379. Fax: þ86 21 34205553. E-mail:
[email protected]. (1) Broer, S.; Hammer, T. Appl. Catal., B 2000, 28, 101–111. (2) Tronconia, E.; Nova, I.; Ciardelli, C.; Chatterjee, D.; Weible, M. J. Catal. 2007, 245, 1–10. (3) Hoard, J. SAE Tech. Pap. Ser. 2001-01-0185, 2001. (4) Penetrante, B. M.; Brusasco, R. M.; Merritt, B. T.; Vogtlin, G. E. SAE Tech. Pap. Ser. 1999-01-3687, 1999. (5) Chae, J.-O. J. Electrostat. 2003, 57, 251–262. (6) Shin, H.-H.; Yoon, W.-S. Plasma Chem. Plasma Process. 2003, 23, 681–704. (7) Dorai, R. Modeling of plasma remediation of NOx using global kinetics models accounting for hydrocarbons. Master Thesis; University of Illinois at Urbana;Champaign: Urbana, IL, 2000. (8) Lin, H.; Huang, Z.; Shangguan, W. F.; Peng, X. P. Proc. Combust. Inst. 2007, 31, 3335–3342. (9) Khacef, A.; Cormier, J. M.; Pouvesle, J. M. J. Phys. D: Appl. Phys. 2002, 35, 1491–1498. (10) Lomnardi, G.; Blin-Simiand, N.; Jorand, F.; Magne, L. Plasma Chem. Plasma Process. 2007, 27, 414–445. (11) Filimonova, E. A.; Kim, Y. H.; Hong, S. H.; Song, Y.-H. J. Phys. D: Appl. Phys. 2002, 35, 2795–2807. r 2010 American Chemical Society
(12) Mok, Y. S.; Ham, S. W. Chem. Eng. Sci. 1998, 53, 1667–1678. (13) Shin, H.-H.; Yoon, W.-S. SAE Tech. Pap. Ser. 2000-01-2969, 2000. (14) Mok, Y. S.; Nam, I.-S. Chem. Eng. Technol. 1996, 22, 527–532. (15) Orlandini, I.; Riedel, U. Catal. Today 2004, 89, 83–88. (16) Hill, S. L.; Kim, H.-H.; Futamura, S; Whitehead, J. C. J. Phys. Chem. A 2008, 112, 3953–3958. (17) Magureanu, M.; Mandache, N. B. Appl. Catal., B 2005, 73, 12–20. (18) Lee, H. M. Ph.D. Thesis; National Central University: Chung-Li 320, Taiwan, 2001 (in Chinese). (19) Jarrige, J.; Vervisch, P. J. Appl. Phys. 2006, 99, 1–10. (20) Mok, Y. S.; Kim, J. H.; Nam, I. S.; Ham, S. W. Ind. Eng. Chem. Res. 2000, 39, 3938–3944. (21) Tasing, W.; Herroy, J. T. J. Phys. Chem. Ref. Data 1991, 20, 609– 663. (22) Hoard, J. W.; Wallington, T. J.; Bretz, R. L.; Malkin, A.; Dorai, R.; Kushner, M. Inc. Int. J. Chem. Kinet. 2003, 35, 231–238. (23) Filimonova, E. A.; Kim, Y. H.; Hong, S. H.; Song, Y.-H. J. Phys. D: Appl. Phys. 2002, 35, 2795–2807.
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Figure 1. Schematic diagram of the experimental setup for the oxidation of propylene and NO in nonthermal plasma.
paths that have been reported are as follows:15-27 C3 H6 þ O f C3 H6 O
ð1Þ
C3 H6 þ O f CH3 CH2 CHO
ð2Þ
•
•
C3 H6 þ O f C H2 CHO þ C H3 •
•
ð3Þ
C3 H6 þ O f C2 H5 þ H C O
ð4Þ
C3 H6 þ O f HCHO þ C2 H4
ð5Þ
•
C3 H6 þ O f CH3 CO þ C H3 •
investigated by checking the oxidation products of C3H6 and NO with gas phase Fourier transform infrared (FTIR) technology and by simulating the process with a simplified chemical kinetic model. The purpose of this study is to reveal the main pathway of C3H6 reacting with O radical in the plasma process and the dominant reactions in NO conversion. 2. Experimental Section 2.1. Experimental System. The schematic diagram of the experimental setup for the oxidation of propylene and NO in the nonthermal plasma system, which was composed of the gas supply system, the nonthermal plasma reactor, and the analysis and measurement system, is presented in Figure 1. The main component of the experimental setup is a dielectric barrier discharge (DBD) reactor, in which a corundum tube (inner diameter of 20 mm and outer diameter of 25 mm) acted as the dielectric material and a stainless steel screw rod (diameter of 14 mm) along the axis of the corundum tube acted as the ground electrode. The out surface of the corundum tube was wrapped with stainless steel mesh that was connected with ac high voltage. The effective length of plasma discharge can be adjusted from 0 to 150 mm. The DBD reactor was energized by an ac high voltage power with the frequency of 50 Hz. The discharge power deposited into the DBD reactor was measured with the charge-voltage (Q-U) Lissajous figure, which can be obtained by checking the damped discharge voltage and the charge in the sampling capacitors with a digital oscilloscope. The synthetic diesel engine exhaust gas stream containing NO, O2, and C3H6 (balanced with N2) was prepared with cylinder gases whose flow rates were controlled by mass flow controllers (MFC). The typical total feed gas flow rate was 100 mL/min (298 K and 1 atm). The initial concentration of NO was 600 ppm, and the flow rate of O2 was adjusted in the range of 0-20% (v/v). The gaseous concentrations of NO, NO2, N2O, CO, CO2, HCHO, CH3CHO, HONO, HONO2, and CH3ONO2 were quantitatively analyzed online using FTIR spectroscopy (Nicolet 6700), which can achieve ppm detection sensitivity, and HCOOH and CH3ONO were qualitatively analyzed by FTIR. 2.2. Modeling Method. The kinetic modeling of plasma oxidation of C3H6 and NO was performed using a spatially homogeneous 0-dimension plasma perfectly stirred reactor in the
ð6Þ
C3 H6 þ O f CH2 CO þ C H3 þ H
ð7Þ
C3 H6 þ O f CH3 CHCO þ H þ H
ð8Þ
C3 H6 þ O f C3 H5 þ OH
ð9Þ
So far, experimental results have been quite accurate in terms of the total rate constant of C3H6 reacting with O. However, the kinetic parameter of individual reaction mentioned above and the contribution of each reaction pathway to the total rate constant21-23 are still inconclusive. Consequently, the results of the simulation based on the overall rate constant are not consistent with the experimental results, especially with regard to the formation of byproduct. In this research, plasma oxidation of C3H6;the typical UHC in diesel engine exhaust and its effects on NO oxidation are (24) Tsang, W. J. Phys. Chem. Ref. Data 1991, 20, 221–273. (25) Orlandini, I.; Riedel, U. Combust. Theory Modell. 2001, 5, 447– 462. (26) Martin, A. R.; Shawcross, J. T.; Whitehead, J. C. J. Phys. D: Appl. Phys. 2004, 37, 42–49. (27) Sathiamoorthy, G.; Kalyana, S.; Finney, W. C. Ind. Eng. Chem. Res. 1999, 38, 1844–1855.
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Figure 3. Concentration profiles of C3H6, CO, CO2, HCHO, and CH3CHO. Initiation conditions: [C3H6] = 600 ppm, [O2] = 10%, Q = 100 mL/min, and T = 25 °C.
Figure 2. FTIR spectra of C3H6 transition with O2 concentration. Initial conditions: [C3H6] = 600 ppm, gas flow rate Q = 100 mL/min, inlet gas temperature T = 25 °C, and ED = 132 J/L.
CHEMKIN-V4.1. A Boltzmann equation solver “ELENDIF”28 determined the electron energy distribution function (EEDF) from which electron impact rate coefficients can be obtained. Inputs into the code are the gas composition, E/n, and electron-molecule collision cross-sections. The chemical kinetic model takes into account 84 species (atoms, molecules, radicals, ions, vibration, and electronic excited states) and 421 elementary reactions, which involve 403 gas phase chemical reactions and 18 electron collision reactions. Kinetic data for reactions and mechanistic parameters are mainly taken from NIST, LLNL, Reaction Design, and Sandia National Laboratories,29-34 while the data for the thermodynamic properties of chemical compounds are obtained from CHEMKIN Thermodynamic Database together with Prof. Burcat’s Thermodynamic Data.35-37 For the DBD engaged at a normal atmospheric pressure, magnitudes of the reduced field (E/N) generally range from 40 to 200 Td.
Figure 4. Mass balance rate of C element in C3H6 oxidation at different energy densities. Initiation conditions: [C3H6] = 600 ppm, [O2] =10%, Q = 100 mL/min, and T = 25 °C.
of C3H6 at a fixed power deposition. Figure 2 also demonstrates that C3H6 can be oxidized to various products such as CO (2033-2238 cm-1), CO2 (2272-2394 cm-1), HCHO (17461825 cm-1), CH3CHO (1670-1716 cm-1), HCOOH (10511144 cm-1), etc., whose absorbance peak areas are in proportion to the conversion rate of C3H6 in the plasma process. Figure 3 shows the influence of the ED on C3H6 concentration. The variation of the major products from C3H6 oxidation is also presented. As the ED increases, C3H6 decreases dramatically and is completely transformed to other products at the ED of 174 J/L. It is observed that the increasing ED promotes oxidization of C3H6 to CO and CO2, whose concentrations reach 228 and 150 ppm, respectively at the ED of 174 J/L. The concentrations of HCHO and CH3CHO increase rapidly as the ED increases to 174 J/L and then decrease moderately as the ED increases further. The highest concentrations of HCHO and CH3CHO are 214 and 203 ppm, respectively. It is interesting to note that the ED point at which HCHO and CH3CHO begin to decrease is the right point at which C3H6 is entirely oxidized. Figure 4 shows the mass balance rate of C element Fc (the ratio of the measurable C atoms number to the initial C atoms number) that was calculated according to the C-containing compounds measured in Figure 3. The mass balance rate of C element Fc is defined as the following equation:
3. Results and Discussion 3.1. C3H6 Transition in C3H6/O2/N2 in the Plasma Process. The transition of C3H6 in the plasma process was investigated by feeding a C3H6 (600 ppm)/O2/N2 mixture into the reactor. Figure 2 shows the FTIR spectra of the C3H6 transition in the gas mixtures with 2 and 6% O2 at an energy density (ED) of 132 J/L. The FTIR spectra allow identification of two main prominent absorbance peaks of C3H6 (2954 and 914 cm-1) in the IR absorption bands ranging from 400 to 4000 cm-1. It can be seen from Figure 2 that the absorbance peak in 914 cm-1 becomes obviously weak and the 2954 cm-1 peak also decreases to a certain extent when the O2 content increases from 2 to 6%. The molecule of C3H6 has an unsaturated CdC double bond and reacts with O radicals ionizing from O2 to form active oxides intermediates very easily.6 Therefore, increasing the O2 content properly could raise O radical quantities and improve conversion (28) Morgan, W. L. Comput. Phys. Commun. 1990, 58, 127–154. (29) Matzing, H. Adv. Chem. Phys. 1991, 80, 315–402. (30) Mallard, W. G. NIST Chemical Kinetic Database, Version 6.01; NIST Standard Reference Data: Gaithersburg, MD, 1994. (31) Atkinson, R.; Aschman, S. M.; Pitts, J. N.; Winer, A. M. Int. J. Chem. Kinet. 1984, 16, 679–706. (32) Atkinson, R. J. Phys. Chem. Ref. Data 1997, 26, 215–290. (33) Smith, G. P.; Golden, D. M.; Frenklach, M.; Moriatry, N. W. Available at http//www.me.berkeley.edu/gri_mech/. (34) Robert, J. K.; Fran, M. R.; Meeks, E. CHEMKIN_III ChemkinII A fortran chemical kinetics package for the analysis of gas phase chemical kinetics and plasma kinetics. SAND 96-8216, Sandia National Laboratory, 1996. (35) Reaction Design, The chemkin thermodynamic database. CHEMKIN Collection, Release 3.6; 2000. (36) Sharon, G. L.; Bartmess, J. E. Gas-phase ion thermochemistry. Available at http://webbook.nist.gov/chemistry/ion/#IE. (37) Burcat, A. Ideal Gas Thermodynamic Data in Polynomial form for Combustion and Air Pollution Use. Available at http://garfield. chem.elte.hu/Burcat/burcat.html.
F c ð%Þ ¼
number of the measurable C atoms 100 number of the initial C atoms
where the initial C atom number is the C atoms in initial C 3H6 . It is observed that Fc is 100% at the ED of 60 J/L and decreases dramatically to 56% until the ED achieves 260 J/L, implying that part of C3H6 (about 44%) is converted to other C-containing products in addition to CO, CO2, HCHO, and CH3CHO. Then, the C balance rate Fc maintains almost the same level when the ED exceeds 260 J/L. The FTIR spectra in 5420
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Table 1. Mechanisms of CO, CO2, HCHO, and CH3CHO Formation at 1 atm reaction rates expression (cm3 molecule-1 s-1)
reaction number
mechanism of CO formation C3 H5 O2 þ O2 f HCHO þ OH þ HCO þ HCO k ¼ 3:01 10 - 9
HCHO þ O f OH þ HCO HCHO þ OH f H2 O þ HCO
k ¼ 6:48 10 - 10
T 298
0:98
exp - 12:89 RT
0:98
1:0
T 298
exp - 5:86 RT
exp 1:41 RT
(10) (11) (12)
HCO þ O2 f CO þ HO2
k ¼ 4:82 10 - 11
HCO þ O f CO þ OH
k = 5.0 10-11
(14)
HCO þ OH f CO þ H2 O
k = 5.0 10-11
(15)
HCO þ HCO f CO þ HCHO
k = 3.0 10-11
(16)
HCO þ HCO f CO þ CO þ H2
k = 5.0 10-12
(17)
CH3 O þ HCO f CH3 OH þ CO
k = 1.5 10-10
(18)
CH2 CHO þ O2 f HCHO þ CO þ OH
k = 3.0 10-14
(19)
T 298
mechanism of CO2 formation 1:55 T k ¼ 3:55 10 - 14 298 exp 3:34 RT
CO þ OH f CO2 þ H
k ¼ 4:2 10 - 12
CO þ O2 f CO2 þ O
T 298
CO þ HO2 f OH þ CO2
k ¼ 1:66 10 - 13
HCO þ O2 f CO2 þ OH
k = 8.5 10-11
CO þ O3 f CO2 þ O2
k = 4.0 10-25
T 298
1:0
exp - 41:82 RT
(20) (21) (22)
(24)
k ¼ 2:61 10 - 11
HCO þ O f CO2 þ H
k = 5.0 10-11
HCO þ HO2 f CO2 þ OH þ H
k = 5.0 10-11 k ¼ 1:7 10 - 33
exp - 200 RT
(23)
CH3 O þ CO f CH3 þ CO2
CO þ O þ M f CO2 þ M
1:0
(13)
T 298
1:0
exp - 49:39 RT
(25) (26) (27)
1:0
T 298
a exp - 12:55 RT
(28)
mechanism of CH3CHO formation k = 1.33 10-10
C2 H5 þ O f CH3 CHO þ H
(29)
mechanism of HCHO formation - 0:03 T k ¼ 1:25 10 - 10 298 exp - 0:15 RT
CH3 þ O f HCHO þ H
(30)
C2 H5 þ O f HCHO þ CH3
k = 2.61 10-11
(31)
CH3 O þ CH3 O f CH3 OH þ HCHO
k = 1.0 10-10
(32)
CH2 CHO þ O2 f HCHO þ CO þ OH
k = 8.0 10-12
CH2 OH þ O2 f HCHO þ HO2
k ¼ 1:68 10 - 11
CH3 CHðOHÞCH2 O f CH3 CHOH þ HCHO
k = 7.4 107
a
6
Units are cm molecule
(33)
T 298
1:0
exp - 1:65 RT
(34) (35)
-2 -1
s .
Figure 2 indicate that part of the C atoms can transfer to HCOOH during C3H6 oxidation in the plasma process.9 Besides, a small part of C3H6 might be converted to other C-containing products, such as CH4, C2H2, C2H4, CH3OH, CH2CO, CH3COOH, CH3CHCO, C2H3CHO, CH3O2H, and C2H5O2H. These products are difficult to be measured by the FTIR used in this study but contribute to the loss of C atoms.9,15-18
3.2. Mechanism Analysis of C3H6 Transition in Plasma. The oxidation of C3H6 by O radical can produce a large number of intermediates, such as C3H6O, CH2CHO, CH3, C2H5, C3H5, CH3CO, and OH. These intermediates can further react with O2, O, and HO2 radicals to form stable oxidation products like CO, CO2, HCHO, and CH3CHO.13-18,22-26 The main reactions that may occur are listed in Table 1. 5421
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Figure 6. Concentration profiles of C3H6, NO, and NO2. Initiation conditions: [NO] = 600 ppm, [O2] = 10%, Q = 100 mL/min, T = 25 °C, and ED = 150 J/L.
Figure 5. Profiles of species selectivity in C3H6 conversion. Initiation conditions: [C3H6] = 600 ppm, [O2] = 10%, Q = 100 mL/min, and T = 25 °C.
because the dominant reaction (reaction 23) in CO2 formation has the same reactants (HCO and O2) and the same rate constant as that of reaction 13, which dominates CO formation. As the ED increases, the selectivities of CH3CHO and HCHO decrease; on the contrary, the selectivities of CO and CO2 increase. The increase of the selectivities of CO and CO2 is mainly due to the fact that the increasing ED promotes reaction 4, which produces more HCO and accelerates reactions 13 and 23. On the other hand, it is noted that the increase of HCO will accompany the increase of C2H5, which contributes to the formation of CH3CHO (as the ED ranging from 60 to 180 J/L in Figure 3). However, the reactants in reaction 29 are the secondary products with the concentration far less than that of O2; therefore, the CH3CHO production rate in reaction 29 is lower than the production rate of CO and CO2 from reactions 13 and 23, respectively. Similarly, as a result of a secondary reaction, the selectivity of HCHO decreases correspondingly as the selectivities of CO and CO2 increase. The experimental results also suggest that the increase of ED may not promote reaction 5 to improve the selectivity of HCHO directly. Hence, it is reasonable to infer that reaction 5 is neither the main source of HCHO formation nor the main reaction of C3H6 oxidation. 3.3. Oxidation of C3H6 in C3H6/NO/O2/N2 Mixture and Its Effects on NO Oxidation. Oxidation of C3H6 and NO was performed by feeding C3H6/NO/O2/N2 mixtures with different initial C3H6 concentrations to the DBD reactor. Figure 6 displays the profiles of plasma oxidation of C3H6 and NO at a fixed ED of 150 J/L. It can be observed in Figure 6 that the oxidation efficiency of C3H6 decreases as the initial C3H6 concentration increases. There is only 109 ppm NO (about 18%) oxidized to NO2 in the absence of C3H6. Under this condition, NO is oxidized by O radical and O3 through reactions 36 and 37.6
To date, which one among reactions 1-9 plays the dominant role in C3H6 oxidation and which reactions in Table 1 lead to the formation of CH3CHO, HCHO, CO, and CO2 are unclear. For this purpose, the selectivity for C3H6 conversion was calculated according to the experimental results and is shown in Figure 5. The selectivity of C-containing compound is defined as the fraction of total C atoms converted from C3H6 into C-containing compound, which is calculated from following equation: C-containing compound selectivity ð%Þ ¼
ðnumber of C in productÞðmoles of product producedÞ 100 3ðmoles of C3 H6 convertedÞ
It is shown that the selectivity of C3H6 to CH3CHO is 53% at the ED of 60 J/L, which indicates that the reaction 29 is the dominant reaction in C3H6 conversion. In this sense, reaction 4 that leads to the formation of ethyl radicals (C2H5) should account for the highest branching ratios among all kinds of reaction paths between C3H6 and O radical at this ED. The selectivity of HCHO achieves 30% in the same conditions. The possible paths that may produce HCHO include (A) direct oxidation of C3H6 (reaction 5); (B) reaction 31, in which O radical reacts with C2H5 coming from reaction 4; (C) reaction 30 and reaction 33, in which CH3 and CH2CHO are produced in reactions 3 and 6; and (D) reactions 32, 34, and 35, which play minor roles since they are more indirect. In conclusion, the main channels for HCHO formation may be paths A-C. Figure 5 shows that the selectivities of CO and CO2 are far less than that of HCHO; therefore, reaction 33 has a limited effect on HCHO formation since reaction 33 will produce one CO while producing one HCHO. From Table 1, we see that the reactions (13-19) that produce CO will involve the secondary oxidation product of HCO or CH2CHO. Among these reactions, reaction 13 may be the dominant one due to the following reasons: (A) HCO is the direct production of reaction 4, which dominates C3H6 oxidation, while CH2CHO is formed by reaction 3, which is a less dominant reaction in C3H6 oxidation; therefore, reaction 19 is not the dominant reaction to form CO. (B) The concentration of O2 is much higher than that of O, OH, CH3O, and HCO radicals; consequently, reaction 13 has the maximal probability among reactions 13-18 from the point of view of molecule collision. Likewise, CO2 is primarily produced by reactions 21 and 23, between which the latter is the dominant one due to the fact that CO in reaction 21 mainly comes from HCO conversion as well as that the rate constant of reaction 23 is 1 order of magnitude larger than that of reaction 21. Moreover, Figure 5 also shows that the selectivity of CO is close to that of CO2. This is
NO þ O f NO2
ð36Þ
NO þ O3 f NO2 þ O2
ð37Þ
The rate constants of above reactions are as low as 1.68 10-31(T/298)-1.17 exp(-1.74/RT) cm6 molecule-1 s-1 and 8.3 10-10(T/298)1.0 exp(-10.06/RT) cm3 molecule-1 s-1, respectively. The reverse reaction in which NO2 is converted back to NO should be considered under this condition. This reaction reduces the rate of NO oxidation.7 NO2 þ O f NO þ O2
ð38Þ
From the variation curve of NOx concentration shown in the top right corner of Figure 6, we observe that the total concentration of NO and NO2 is only 566 ppm in the absence 5422
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of C3H6, implying that part of NOx (about 34 ppm) is converted to other N-containing compounds. Likewise, the concentration of NOx is always less than the initial concentration (600 ppm) in the presence of C3H6. N-Containing compounds will be analyzed later. The results in Figure 6 show that the proportion of NO oxidation to NO2 increases as the C3H6 concentration increases. When 800 ppm C3H6 is added in the reaction system, 49% of NO is oxidized to NO2, which almost leads to an ideal NO/NO2 ratio of 1:1, an optimized ratio in the fast NH3-SCR reaction. The concentration of NO2 exceeds that of NO when the C3H6 concentration is over 800 ppm. With the addition of C3H6 to the plasma process, several kinds of strong oxidizing species such as peroxy radicals (CH3O2 and C2H5O2) and hydroperoxy radical (HO2) might be produced. These radicals can efficiently oxidize NO to NO2.32,33 The mechanisms for the formation of these intermediates and the subsequent oxidation reactions are as follows:
Figure 7. FTIR spectra of C3H6 and NO transition with the ED. Initiation conditions: [C3H6] = 600 ppm, [NO] = 600 ppm, [O2] = 10%, Q = 100 mL/min, and T = 25 °C.
C2 H5 þ O2 f C2 H5 O2 k ¼ 8:93 10 - 11
CH3 þ O2 f CH3 O2
T 298
0:98
k ¼ 6:44 10 - 12
0:27 exp RT
T 298
0:98
NO2 (1550-1653 cm-1), N2O (2155-2270 cm-1), N2O5 (637-701 cm-1), HONO (723-775 cm-1), and HONO2 (813-916 cm-1) are produced due to the presence of NO. Apparently, part of NOx is converted to N2O, N2O5, CH3ONO2, HONO, and HONO2. The changes of the species concentrations with the ED can be observed from the changes of the absorbance peaks shown in Figure 7. C3H6 is oxidized partially, and part of NO is oxidized to NO2 at the ED of 78 J/L. When the ED increases to 132 J/L, a great deal of C3H6 is oxidized, and more CO, CO2, HCHO, and CH3CHO are produced accompanied with the forming of CH3ONO2, HONO, and HONO2. In the plasma process, NO and NO2 can react with hydroxyl radicals (OH) (which are created by plasma oxidation of C3H6 in reaction 9) to form HONO and HONO2 according to reactions of 45 and 46.16 NO þ OH f HONO ð45Þ
ð39Þ
2:66 exp RT
ð40Þ HCO þ O2 f CO þ HO2 1:0 T 13:0 - 10 k ¼ 6:14 10 exp 298 RT C2 H5 O2 þ NO f C2 H5 O þ NO2 1:0 T 2:74 - 12 k ¼ 3:11 10 exp 298 RT CH3 O2 þ NO f CH3 O þ NO2 1:0 T 2:33 k ¼ 3:01 10 - 12 exp 298 RT HO2 þ NO f OH þ NO2 1:0 T 1:85 - 12 k ¼ 1:94 10 exp 298 RT
ð41Þ
ð42Þ
NO2 þ OH f HONO2
ð46Þ
The formation of CH3ONO2 is mainly due to an additional reaction of NO2 with methoxy radical (CH3O) in the presence of a third body M.15
ð43Þ
CH3 O þ NO2 þ M f CH3 ONO2 þ M
ð47Þ
It should be pointed out that methyl radical (CH3), produced by the oxidation of C3H6 (reaction 3 and 6), is the precursor of CH3O. From the FTIR spectra, we notice that HCOOH, O3, and N2O5 are detected at 252 J/L. The peak of NO2 corresponding to 252 J/L is much smaller than that to 132 J/L, which suggests that the NO2 concentration declines sharply. The effects of ED on NO oxidation are presented in Figure 8. The oxidation products of NO that can be measured quantificationally in this study mainly include NO2, N2O, HONO, HONO2, and CH3ONO2. It is shown that the NO oxidation rate increases sharply with the increasing ED; at low energy densities, most of NO is converted to NO2; and the concentrations of NO and NO2 almost achieve the same level at the ED of 84 J/L. When the ED reaches 138 J/L, NO is completely oxidized, and then, NO2 would be converted to more and more N2O, HONO, HONO2, and CH3ONO2 as the ED continues increasing.
ð44Þ
It is shown that C2H5, HCO, and CH3 radicals play critical roles in NO oxidation in the reactions mentioned above. C2H5 and HCO radicals originate from the reaction of C3H6 with O radical (reaction 4), while CH3 radical comes from reactions 3, 6, and 7. According to the former analysis of Figure 6, the N balance is unavailable based on the concentrations of NO and NO2, and other N-containing compounds should account for the N loss during NO oxidation in the plasma process. Figure 7 presents the FTIR spectra of the products from C3H6 and NO oxidation at different energy densities. In addition to CO, CO2, HCHO, CH3CHO, and HCOOH that form in the C3H6/O2/N2 system (shown in Figures 2 and 3), lots of N-containing compounds, such as CH3ONO2 (1246-1351 and 3507-3580 cm-1), NO (1784-1944 cm-1), 5423
Energy Fuels 2010, 24, 5418–5425
: DOI:10.1021/ef100451x
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It has been shown ahead that reaction 4 dominates the C3H6 oxidation, which suggests that the productivity of C2H5 or HCO radical is higher that of CH3 radical. In comparison with the kinetics, the rate constants of O2 with C2H5 and with HCO are 2 and 1 order of magnitude larger than that of O2 with CH3. Moreover, the rate constants of NO with the radicals from C2H5, HCO, and CH3 oxidation are of the same order of magnitude; therefore, the contribution of CH3 and its oxidation products to the oxidation of NO is less important. By analyzing the experimental results of C3H6 and NO oxidation in the plasma process, we speculate that the C2H5 and HCO radicals generated from the reaction of C3H6 with O radical dominate the C3H6 oxidation and play a leading role in NO oxidation. In the absence of NO, C2H5 and HCO will further react with O and OH radicals to form CO, CO2, HCHO, and CH3CHO, etc.; alternatively, C2H5 and HCO will react with O2 to form active radicals that can oxidize NO to NO2 efficiently. It should be pointed out that other paths of C3H6 with O radical and their derivative reactions also
affect C3H6 and NO oxidation but to a lesser extent. The dominant mechanism of C3H6 and NO oxidation in nonthermal plasma is illuminated in Figure 9. 3.5. Chemical Kinetics Modeling of C3H6 and NO Oxidation. Chemical kinetics modeling is performed by using reaction 4 (rate coefficient is 1.12510-12 cm3 molecule-1 s-1) as the trigger reaction and by adopting the subsequent reactions including the dominant reactions that form CO, CO2, and NO2, as well as other subordinate reactions relating to reaction 4. Modeling is performed for the mixtures of C3H6 (600 ppm)/O2 (10%)/N2 (balanced) and C3H6 (600 ppm)/ NO (600 ppm)/O2 (10%)/N2 (balanced). The total gas flow rate is 100 mL/min, the inlet temperature of the feed gas is 298 K, and the operating pressure is 101325 Pa. Figure 10 illustrates the comparison of experimental and simulation results of C3H6, CO, and CO2. The C3H6 concentrations in simulation are higher than the ones of experiments at every ED, indicating that C3H6 might be converted to other compounds by other reactions besides reaction 4. However, the average deviation of experimental and simulation results is 14.3% in the whole range of the ED shown in Figure 10, which implies that reaction 4 is indeed the principal reaction since it achieves 85.7% of branching ratios among the C3H6 oxidation reactions. The simulation concentrations of CO and CO2 are lower than the experimental results due to the fact that less C3H6 is oxidized in simulation. The average relative errors between the simulation and the experimental results are 12.2 and 6.4%, respectively. A comparison of NO and NO2 profiles in simulation and in experiments is presented in Figure 11. It is found that the overall NO in simulation is higher than that in experiments because
Figure 8. Profiles of NOx and N-containing compounds. Initiation conditions: [C3H6] = 600 ppm, [NO] = 600 ppm, [O2] = 10%, Q = 100 mL/min, and T = 25 °C.
Figure 10. Comparison of simulation and experimental results of C3H6, CO, and CO2. Initiation conditions: [C3H6] = 600 ppm, [O2] = 10%, Q = 100 mL/min, and T = 25 °C.
3.4. Discussion on the Mechanism of C3H6 Oxidation Affecting on NO Oxidation. From the experimental results, the addition of C3H6 improves the conversion of NO into NO2. The possible promotion paths might be as follows: (1) C3H6 is oxidized by O radical to produce C2H5 and HCO. These intermediates can react with O2 to form C2H5O2 and HO2, respectively, which again oxidize NO to NO2 (reactions 42 and 44). (2) CH3 radical originated from the oxidation of C3H6 by O radical reacts with O2 to produce CH3O2, which oxidizes NO to NO2.
Figure 9. Schematic diagram of the dominant mechanism in C3H6 and NO oxidation.
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Energy Fuels 2010, 24, 5418–5425
: DOI:10.1021/ef100451x
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4. Conclusions In this study, plasma oxidation of C3H6 and NO was investigated in a DBD reactor. The species concentrations after plasma process were analyzed using a FTIR technique and were compared with the simulation results based on a simplified molding of C3H6 and NO oxidation. The following main conclusions are drawn from the experimental and simulation results. (1) The reactions of C3H6 with O radicals are the trigger reactions for C3H6 oxidation in the NTP process, among which the reactions that bring about the formation of C2H5 and HCO are the dominating reactions. (2) The peroxy radical (C2H5O2) and hydroperoxy radical (HO2) produced by C3H6 oxidation in the plasma process facilitate the oxidation of NO to NO2 efficiently and play dominant roles in NO oxidation. (3) Many kinds of oxidation products are formed in the plasma oxidation of C3H6, including CO, CO2, HCHO, CH3CHO, HCOOH, N2O, O3, etc. With the presence of NO, some N-containing byproducts such as N2O5, HONO, HONO2, CH3ONO, and CH3ONO2 are produced. (4) Results of the simplified model that takes the dominant reaction as the trigger reaction are close to the experimental results. The subordinate reactions in C3H6 oxidation that are not taken into account can be responsible for the relative error between simulation and experimental results.
Figure 11. Comparison of simulation and experimental results of NO and NO2. Initiation conditions: [C3H6] = 600 ppm, [NO] = 600 ppm, [O2]=10%, Q=100 mL/min, and T = 25 °C.
the simulation is performed with a reduced mechanism in that some NO oxidation by the strongly oxidative radicals such as CH3O2 and HO2 will be neglected. The average relative error of NO is only 3.3% by calculation, which suggests that HCO and C2H5 produced by reaction 4 and their oxides play essential roles in oxidizing NO. The variation tendency of the simulation NO2 is the same as that of NO2 in experiments, while the relative error is relatively large (20.3%). In the reduced model, CH3 is the secondary product only coming from the reaction of C2H5 with O (reaction ); in the real plasma process, however, considerable amounts of CH3 radicals originate from reactions of C3H6 with O radical (reactions 3, 6, and 7). Therefore, the contribution of CH3O2 (the oxidation product of CH3) under the real condition is greater than that in simulation, and this may be the main reason that can explain why the relative error is so large.
Acknowledgment. This work was supported by the Yuchai Machinery Limited Company.
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