Removal of NOx with Selective Catalytic Reduction ... - ACS Publications

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Removal of NOx with Selective Catalytic Reduction Based on Nonthermal Plasma Preoxidation Bin Guan, He Lin,* Qi Cheng, and Zhen Huang Key Laboratory for Power Machinery and Engineering of Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, China ABSTRACT: An extensive series of experiments have been conducted using a nonthermal plasma generated by dielectric barrier discharge (DBD) process combined with vanadium pentoxide catalyst to reduce the nitrogen oxides (NOx) from diesel engine exhaust over a broad reaction temperature (100500 °C). In this system, the effects of input voltage, propylene (C3H6) concentration, and sulfur content, etc. on the plasma facilitated (PF) selective catalytic reduction of NOx with NH3 were examined. In the presence of C3H6 as an additive, the oxidation of NO to NO2 is largely enhanced even with lower input voltages. The PF NH3SCR system enhanced the overall reaction and showed a remarkable improvement in NOx removal efficiency at temperatures of 100250 °C. The removal of NOx was found to be largely increased by the input voltage and the addition of propylene. Besides the small amount of nitrous oxide and the significant amount of carbon monoxide, aldehydes-type unregulated byproduct such as formaldehyde and acetaldehyde were also observed at the outlet of the DBD reactor, while formaldehyde and acetaldehyde could be almost completely removed in the NH3SCR reactor. The NOx conversion decreases at lower temperatures but increases at higher temperatures with SO2 concentration increases. The PF NH3SCR hybrid system can be used stably with several hundreds of ppm of SO2 in durability tests. Moreover, the presence of SO2 inhibits N2O formation at all employed reaction temperatures.

1. INTRODUCTION Removal of NOx from the diesel engine exhaust is one of the most challenging issues facing the engine engineers in the coming decade. In order to achieve compliance with the future stringent emission control legislation, a great deal of research work concerning lean NOx catalyst, lean NOx trap, selective catalytic reduction (SCR), etc., has been done to find sound solutions for the diesel NOx reduction. Among them, SCR is considered as the most promising one in the near future.15 The NH3SCR technology, which takes ammonia as reductant, has been widely applied to remove NOx from power plant flue gases since 1980s. Generally, NO constitutes a large part of NOx emission from diesel engines (more than 95%). The typical vanadium-based SCR catalyst is active in reducing NO to N2 within the temperatures range of 250450 °C and is less active when the reacting temperature is lower than 250 °C. In fact, the temperature of diesel exhaust may vary widely from 100550 °C, depending on the engine operating modes and loads. Thus, the low temperature performance of SCR must be improved for its practical application to the effective treatment of diesel exhausts,610 to this end, a new concept of the fast SCR has been advanced in recent years. It was found that the low temperature performance of socalled the standard SCR reaction 1 will be elevated largely by increasing the amount of NO2 in diesel exhausts, especially when the amount of NO and NO2 is equimolar and the fast SCR reaction 2 will take place at the highest reaction rate. Afterward, plenty of research on the fast SCR were conducted through preoxidation of NO to NO2 over Pt-based catalyst11,12 4NO þ 4NH3 þ O2 f 4N2 þ 6H2 O

ð1Þ

2NO þ 2NO2 þ 4NH3 f 4N2 þ 6H2 O

ð2Þ

Pt-based catalyst is high sulfur-sensitive and the diesel fuel with low sulfur content less than 50 ppm is required. However, in r 2011 American Chemical Society

many developing countries such as China, India, and Mexico, low sulfur diesel fuel is unavailable in the near future. In China, for example, the diesel fuel with sulfur content up to 500 ppm is used in most areas of the country, and this will continue for many years. One potential method to increase the portion of NO2 in NOx may be nonthermal plasma (NTP) technology due to its capability of efficiently selective partial oxidation of NO to NO2. The plasma facilitated SCR hybrid system has been proved to be an effective means for converting NOx to N2 even at low temperatures.19 The main advantages of plasma pretreatment for the reduction of NOx in the lean-burn engine exhaust are as follows: (1) The nonthermal plasma process can be operated within a wider temperature range, i.e. from room temperature to several hundred degrees Celsius. (2) The NO2 fraction can be controlled by simply adjusting the input electrical power of the nonthermal plasma reactor.79 So far, many research works have been done in terms of plasma assisted NH3SCR system;116 however, the effect of SO2 concentration on the DBD preoxidation NO process and the PF NH3SCR system had not been reported yet, and the formation of byproduct in this system still needs further investigation. This paper presents an experimental research on the synergistic effect of nonthermal plasma on selective catalytic reduction of NOx over V2O5WO3/TiO2 catalyst. A coaxial dielectric barrier discharge (DBD) reactor is utilized to preoxidize NO in front of the NH3SCR reactor. Effects of input voltage, C3H6 Received: September 26, 2010 Accepted: March 1, 2011 Revised: January 26, 2011 Published: March 21, 2011 5401

dx.doi.org/10.1021/ie1019744 | Ind. Eng. Chem. Res. 2011, 50, 5401–5413

Industrial & Engineering Chemistry Research

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Figure 1. Schematic diagram of the experimental setup for DBD facilitated NH3SCR hybrid system.

concentration, and sulfur content, etc. and the formation of various byproduct are examined.

2. EXPERIMENTAL SECTION The schematic diagram of the DBD facilitated NH3SCR hybrid system is presented in Figure 1. A corundum tube with an inner diameter of 20 mm was functioned as the dielectric material. A stainless steel screw rod with a diameter of 14 mm was placed along the axis of the corundum tube and was connected with the ground. The outer surface of the corundum tube was wrapped with stainless steel mesh connected to high voltage supply with the frequency of 50 Hz. The effective length of plasma discharge zone can be adjusted from 0 to 150 mm by changing the length of mesh. Two 22 nF parallel connection sampling capacitors connected to the DBD reactor in series were used for the discharge power measure. The DBD reactor was placed downstream of an electric tubular resistance furnace, in which the reacting gases can be heated up before entering the DBD reactor. The SCR reactor was placed downstream of the DBD reactor. The SCR catalyst used was a commercial V2O5WO3/TiO2 catalyst with the vanadium pentoxide content of 2% and the tungsten trioxide content of 9%. The catalyst was supported on a honeycomb monolith (400 cells per square inch). This honeycomb structure catalyst with the volume of 0.29 cm3 was inserted in the quartz tube with 9 mm inner diameter. The quartz catalytic reactor tube was installed in a pipe electric furnace whose temperature can vary from room temperature to 1000 °C. 2.1. Experimental Methods. The main components of the synthetic diesel engine exhaust gas stream containing NO, NH3, O2, C3H6, and SO2 balanced with N2 were prepared in a gas handing system and their composition was controlled by calibrated high-precision mass flow controllers (MFC) (Model D07-19B Sevenstar Instruments Inc.). Initial concentration of NOx (NO þ NO2) at the reactor inlet was varied from 50

ppm (parts per million, volumetric) to 600 ppm, the NH3 whose concentration was adjusted from 35 ppm to 900 ppm was injected as reducing agent at the outlet of DBD reactor. The flow rate of O2 was adjusted in the range of 010% (v/v). To investigate the effects C3H6 on the performance for oxidation of NO to NO2, the concentration of C3H6 added to the DBD reactor was changed from 50 ppm to 1200 ppm. In order to evaluate the SO2 tolerance of the PF NH3SCR system, 200800 ppm SO2 was added in the feed gas. The typical feed gas flow rate was 100 mL/min at STP (standard temperature and pressure). This gas flow rate corresponds to about 20770 h1 of space velocity in the SCR reactor on the basis of room temperature. 2.2. Measurement and Analysis Instruments System. The major gaseous components concentrations of NO, NO2, C3H6, NH3, CO, SO2, etc. were continuously quantified using an online high resolution Fourier Transform Infrared absorption spectrometer (FTIR, NICOLET 6700 Thermo Environmental Instruments Inc.) equipped with a heated (185 °C), low volume (200 mL), and 2 m-permanently aligned multiple path length absorption gas-measuring cell. The FTIR uses the OMNIC Quantpad software to compute the concentrations of components present in the sample gas from the absorbance spectrum. According to the wavenumbers and the absorbance of each infrared spectrum, we can do qualitative and quantitative analysis quickly and accurately for each gas-phase molecule. Quantitative analyses of the byproduct like nitrous oxide (N2O), formaldehyde (HCHO), and acetaldehyde (CH3CHO) were detected. Three ports for FTIR sampling were located before and after the DBD reactor and behind the SCR reactor. The voltage applied to the DBD reactor was measured with a 300 MHz digital phosphor oscilloscope (TDS 3034C, Tektronix) connected to a 1000:1 voltage divider. The current was obtained by measuring the voltage of a noninductive resistor of 50 Ω connected in series in the high voltage source. The discharge power was measured by checking the charge-voltage (Q-U) Lissajous.1722 Figure 2 (a) 5402

dx.doi.org/10.1021/ie1019744 |Ind. Eng. Chem. Res. 2011, 50, 5401–5413


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Figure 3. Effect of the input voltage on the NOx removal efficiency as a function of reaction temperature. Reaction conditions: NOx= 600 ppm, NH3= 600 ppm, O2= 6% (v/v), total gas flow rate = 100 mL/min, gas hourly space velocity (GHSV) = 20770 h1, and balance N2.

Figure 2. (a) Voltage waveforms measured at the discharging electrode and at the 44 nF capacitor. (b) Corresponding charge-voltage (Q-U) Lissajous figure.

shows an example of the voltage waveforms measured at the discharge electrode and the 44 nF equivalent capacitor, respectively, while Figure 2 (b) shows the typical Q-U plot at the corresponding voltage, i.e. the Lissajous figure. In order to clarify the indication of the ignited phases and off phases, the points 1, 2, 3, and 4 are marked on the ellipse shape charge-voltage Lissagous figure in Figure 2b. The figure corresponds to the variation of a discharge cycle, among which point 1 corresponds to the minimum voltage (valley value) in the voltage cycle. The DBD reactor (capacitance) is charging as the voltage increases, but the discharge has not yet occurred at this time. After the voltage reaches point 2 (the corresponding voltage is 0), the charge between two electrodes becomes high enough, the gas discharge occurs in the DBD reactor, the conduction charge increases dramatically at the same time, and then the discharge current is produced. The voltage continues increasing to point 3 (the peak value of voltage), and the amount of electric charge between electrodes is insufficient to sustain the discharge process, and then the applied voltage begins to reverse charge for two electrodes of the DBD reactor. The process of 341 is the reversed phase of the process 123. The discharge process operates according to 12341 in each cycle. The ellipse in Figure 2b represents the average results of many cycles. The area of the Q-U Lissajous figure in Figure 2 (b) conforms to the discharge energy per cycle at a certain voltage, and the average discharge power can be obtained by multiplying the discharge energy per cycle by ac frequency. The energy density, a widely used parameter in the field of nonthermal plasma, is defined as the ratio of the power dissipated in the DBD reactor to the total gas flow rate in standard conditions.

3. RESULTS AND DISCUSSION 3.1. Effect of the Input Voltage on the Performance of the Plasma Facilitated NH3SCR Hybrid System. Figure 3

presents the influence of input voltage on the NOx removal efficiency for the PF NH3SCR process as a function of the reaction temperature in the range of 100500 °C. It is shown in Figure 3 that the input voltage had significant effect on NOx reduction particularly at low temperatures and almost vanished at high temperatures, i.e. about 300450 °C. Reaction temperature had little effect on NOx removal efficiency when the input voltage was 6 KV; however, in the other input voltage, temperature had great effect on NOx reduction, when the temperature was low (100250 °C), NOx removal efficiency at different input voltage had large difference, this difference decreased with increasing the temperature. It was found from Figure 3 that without voltage input, i.e. the NH3SCR reactor worked alone, the NOx removal efficiency was very sensitive to the reaction temperatures since the NOx reduction was 70% and above at temperatures varied from 300 to 400 °C. Nevertheless, at low temperature conditions (reaction temperatures were below 250 °C), the SCR catalytic activity dropped sharply and nearly no activity for NOx removal was observed below 150 °C. The optimum operating reaction temperatures (activation temperatures) window for NH3SCR reactor alone is only in the range of 300400 °C, which is so narrow and must be extended drastically. These observations may be explained by the following considerations. The reduction of NOx within the active reaction temperatures window of the vanadium pentoxide catalyst we used is mainly due to the so-called the NO-SCR reaction 1, whose activation energy is as high as 73.5 KJ/mol. Therefore, the NO-SCR reaction can hardly occur at low temperatures, leading to extremely low NOx removal efficiency. Whereas, when the temperatures exceed 400 °C, side reactions 3, 4, and 5 will happen, causing the oxidation of reductant-ammonia and decreasing the efficiency of NOx removal15,17 2NH3 þ 2O2 f N2 O þ 3H2 O

ð3Þ

4NH3 þ 3O2 f 2N2 þ 6H2 O

ð4Þ

4NH3 þ 5O2 f 4NO þ 6H2 O

ð5Þ

When the DBD reactor was turned on (with the input voltage from 0 KV to 6.0 KV), the NOx removal efficiency at the reaction temperatures as low as 100250 °C was considerable. The NOx 5403



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Figure 4. Concentration profiles of NO, NO2, and NOx at the outlet of DBD reactor. Reaction conditions: NOx = 600 ppm, O2= 6% (v/v), total gas flow rate = 100 mL/min, and balance N2.

removal efficiency was significantly improved from only 12.11%, 12.88%, and 28.6% at reaction temperatures of 100 °C, 150 °C, and 200 °C without input voltage to 68.2%, 69.1%, and 73.5%, respectively, at corresponding reaction temperatures with 6.0 KV input. At temperatures above 300 °C, the NOx removal was only slightly enhanced as input voltage increased, meaning that the removal of NOx was insensitive to the input voltage. These behaviors might be attributed to the fact that when the DBD reactor was applied, the concentration of NO2 oxidized from NO increased with increasing input voltage as shown in Figure 4. The NOx removal efficiency increased sharply with increasing NO2/NO ratio from 0 to 1:1, which resulted from the increasing input voltage from 0 KV to 6.0 KV. The highest NOx reduction was observed at the input voltage of 6.0 KV at which the NO2/NO ratio was approximately 1:1, thereafter, the reduction of NOx decreased when the input voltage exceeded 6.0 KV and the NO2/NO ratio was more than 1. The reason is that when NO2 increased in the feed gas stream, the fast SCR reaction 2, which has a reaction rate about 10 times higher than that of the standard SCR reaction 1 at the temperature below 200 °C, can achieve, and the remaining NO reacts with ammonia according to the standard SCR reaction, promoting the reduction of NOx to a large extent at temperatures below 250 °C. For NO2 fractions between 50% and 100% at the moment of input voltage above 6.0 KV, that is the ‘‘NO2-rich’’ zone, the NO2 had a negative effect on NOx reduction. The decrease in NOx conversion is due to the fact that NO2 reacts slowly with ammonia according to the NO2SCR reaction 6 and nitrate formation reaction 7 at low temperatures.62 The results show that the presence of NO2 had a dramatic effect on the active of SCR performance, especially at low temperatures. The strong improvement of deNOx performance with increasing fraction of NO2 is evident 4NH3 þ 3NO2 f 3:5N2 þ 6H2 O

ð6Þ

2NH3 þ 2NO2 f N2 þ NH4 NO3 þ H2 O

ð7Þ

At temperatures above 350 °C, the rate of NOx conversion essentially does not depend on the NO2 content in the feed. This is due to the convergence of the reaction rate constants of reactions 1, 2, and 8. It is conceivable, however, that the contribution of these reactions is influenced by the thermodynamic equilibrium of reaction 8 temperatures34,35,63

Figure 5. Fourier transform infrared spectra showing the reduction of NOx before and after DBD processing. Reaction conditions: NOx = 600 ppm, NH3 = 600 ppm, O2 = 6% (v/v), total gas flow rate = 100 mL/ min, GHSV = 20770 h1, and balance N2, reaction temperature = 150 °C, input voltage = 6 KV.

Figure 6. Effect of C3H6 on the NOx removal efficiency as a function of reaction temperature. Reaction conditions: NOx = 600 ppm, NH3 = 600 ppm, O2 = 6% (v/v), total gas flow rate = 100 mL/min, GHSV = 20770 h1, and balance N2, input voltage = 3.5 KV.

NO2 a NO þ 0:5O2

ð8Þ

Figure 5 shows the Fourier transform infrared spectra (FTIR) which inform the DBD playing vita important role in removing NOx. The spectrum of the inlet gas is shown in the bottom (“inlet feed gas”) of Figure 5. Without the processing of plasma discharge, i.e. SCR catalyst only, the NOx reduction efficiency was extremely low at the reaction temperature as low as 150 °C, as shown in the “outlet of the SCR catalyst without DBD processing”. When the high voltage (6 KV) was applied to gas stream, part of NO was oxidized to NO2, as shown in the “after DBD processing”, the NOx reduction was still very low, almost the same amount of total NOx was left in the gas stream. When the gas stream was first passed through the DBD reactor, the oxidation of partial NO to NO2 was accomplished with plasma, and the NO2-containing feed gas then passed through the SCR reactor, the NOx removal efficiency was improved dramatically from 12.88% to 69.1% as shown in the top (“after DBD facilitated SCR”), implying that the fast SCR reaction 2 significantly contributes to the removal of NOx. 3.2. Effect of C3H6 Concentration on the Performance of the Plasma Facilitated NH3SCR Hybrid System. The effect of 5404



Figure 7. Effect of inlet C3H6 on the concentration variation of NO, NO2, NOx and oxidation efficiency of C3H6 in the plasma processing. Reaction conditions: NOx = 600 ppm, O2 = 6% (v/v), total gas flow rate = 100 mL/min, and balance N2, input voltage = 3.5 KV.

hydrocarbons on the plasma processing of feed gas and the performance of the PF NH3SCR hybrid system were further examined. Due to its superior efficiency of NO oxidation paralleled with economy in input energy, C3H6 was used as a representative hydrocarbon for an effective additive.4,17 Figure 6 demonstrates the effect of C3H6 concentration on NOx removal as a function of reaction temperatures at the input voltage of 3.5 KV. It is evident that C3H6 plays a vital important role in improving NOx reduction at lower temperatures. It can be seen from Figure 6 that the NOx removal efficiency is improved significantly at temperatures in the range of 100250 °C with increasing concentration of C3H6. However, at temperatures above 300 °C, the removal of NOx in dependence of temperature was insensitive to the inlet C3H6 concentration. It was found that in the absence of C3H6, the NOx removal efficiency was only 17.8%, while with the addition of 800 ppm C3H6, it obtained 58.8% of NOx reduction. Meanwhile, with the addition of C3H6, the NOx removal efficiency was higher than that in the absence of C3H6 at temperatures above 300 °C which may contribute to the excited species produced in the plasma process and enhanced the efficiency of catalyst furthermore. Figure 7 illustrates the profiles concentration of NO, NO2, NOx and oxidation efficiency of C3H6 as a function of initial C3H6 concentration at the input voltage of 3.5 KV at the outlet of the DBD reactor. The oxidation of NO to NO2 increased with the concentration of C3H6, and the strong dependence on C3H6 concentration was extended to more than 1000 ppm at the input voltage of 3.5 KV. It is evident that some C3H6 is required for effective oxidation of NO to NO2 and that C3H6 is a leading reaction accelerator, enhancing greatly NO to NO2 conversion performance as an essential step for the subsequent NH3SCR reaction, paralleled with economy in the input energy cost.17,24,61 As can be seen in Figure 7, there was hardly any NO2 production without C3H6 addition which led to extremely low NOx removal efficiency at lower reaction temperatures. In the absence of C3H6, the number of NO molecules oxidized to NO2 was determined by the number of O radicals, which was proportional to the energy density deposited into the plasma. Back conversion of NO2 to NO by the O radicals via reaction 8 decreased the oxidation efficiency. Under plasma discharge, C3H6 is added to the gas stream as an O 3 getter and is decomposed into useful intermediates such as

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methyl (CH3), methoxy (CH3O) radicals, and partial oxidation products of C3H6 conversion, including peroxyl radicals (RO2 3 ) and hydro-peroxy radicals (HO2 3 ).54,58 The radical responsible for the oxidation of NO to NO2 is no longer the O radical. It will be shown in the following chemical kinetics analysis that RO2 3 and HO2 3 are the radicals capable of accelerating the oxidation of NO to NO2 when the plasma processing is done in the presence of C3H6. On the other hand, peroxyl radicals and hydro-peroxy radicals allow a stable conversion of NO to NO2 without back reactions taking place according to wide accepted mechanisms. The number of HO2 3 and RO2 3 radicals produced in the plasma is a function of both the energy input to the plasma and the propylene concentration in the feed gas.50,54 The addition of C3H6 changes the oxidation reaction mechanism significantly. The first-hand contribution by nonthermal plasma discharge is the producing of O radical and then the breakdown reaction of propylene by O radical primarily to produce several fragments based on the following pathway reactions 911.12,16,29 These intermediate species further react with oxygen to form HO2 3 radicals and RO2 3 radicals in the reactions 1216.25,26,29 The HO2 3 radicals and RO2 3 radicals are strong oxidizing radicals capable of oxidizing NO to NO2 efficiently. The radicals convert NO to NO2 by changing the oxidation state as reactions 171922,27,34 C3 H6 þ O f C2 H5 þ HCO

ð9Þ

C3 H6 þ O f C2 H2 þ CH3 O þ H

ð10Þ

C3 H6 þ O f CH2 þ CH3 þ HCO

ð11Þ

HCO þ O2 f HO2 þ CO 1:0 T 1:41 k ¼ 4:82 10-11 exp cm3 molecule-1 s-1 298 RT ð12Þ C2 H5 þ O2 f C2 H5 O2 0:98 T 0:27 cm3 molecule-1 s-1 ð13Þ k ¼ 8:93 10-11 exp 298 RT CH3 þ O2 f CH3 O2 0:98 T 2:66 cm3 molecule-1 s-1 ð14Þ k ¼ 6:44 10-12 exp 298 RT CH3 O þ O2 f CH2 O þ HO2 1:0 T 1:65 cm3 molecule-1 s-1 ð15Þ k ¼ 1:68 10-11 exp 298 RT

H þ O2 þ M f HO2 þ M -1:30 T k ¼ 2:52 10-32 cm6 molecule-2 s-1 298 NO þ HO2 f NO2 þ OH 1:0 T 1:85 cm3 molecule-1 s-1 exp k ¼ 1:94 10-12 298 RT 5405

ð16Þ

ð17Þ



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Figure 8. The FTIR spectrum of main products by plasma facilitated NH3SCR hybrid system. Reaction conditions: NOx = 600 ppm, NH3 = 600 ppm, O2 = 6% (v/v), C3H6 = 400 ppm, GHSV = 20770 h1, and balance N2, energy density = 79 J/L.

NO þ C2 H5 O2 f NO2 þ CH3 CH2 O 1:0 T 2:74 cm3 molecule-1 s-1 exp k ¼ 3:11 10-12 298 RT NO þ CH3 O2 f NO2 þ CH3 O 1:0 T 2:33 cm3 molecule-1 s-1 k ¼ 3:01 10-12 exp 298 RT

Figure 9. CO formation at the outlet of the plasma facilitated SCR system as a function of energy density at different C3H6 concentration. Reaction conditions: NOx = 600 ppm, NH3 = 600 ppm, O2 = 6% (v/v), total gas flow rate = 100 mL/min, and balance N2, reaction temperature = 150 °C.

ð18Þ

C3 H6 þ O f Cx Hy Oz

ð20Þ

HCHO þ O f OH þ HCO 0:98 T 12:89 -9 exp cm3 molecule-1 s-1 k ¼ 3:01 10 298 RT

ð19Þ

ð21Þ 3.3. Formation of Byproducts in the Plasma Facilitated NH3SCR Hybrid System. The plasma facilitated NH3SCR

hybrid system is usually accompanied by the formation of various byproduct, either from incomplete oxidation of hydrocarbons, a partial oxidation process leading to the formation of carbon monoxide (CO), formaldehyde (HCHO), acetaldehyde (CH3CHO), and a large variety of other byproducts, such as methyl nitrate (CH3ONO2), formic acid (CH2O2), and nitrous acid (HONO), or from the insufficient selective reactions of N-containing species (HCN, N2O, N2O5, N2O3, etc.).19,32 Therefore, it is necessary to study these byproducts, especially the effects and mechanism on the formation, to reveal the essence of byproduct production and to optimize various parameters to reduce the emissions of byproducts as much as possible in order to avoid the secondary pollution. Figure 8 shows the FTIR spectrum which illustrates the hybrid system effect on the main byproduct formation at a reaction temperature of 150 °C and an energy deposition of 79 J/L. 3.3.1. CO Production. Figure 9 demonstrates the CO production measured at the outlet of the plasma facilitated NH3SCR hybrid system as a function of the energy density with C3H6 as one parameter. The CO production increased with increasing energy deposition in the DBD reactor. The rate of rise of NO concentration was more pronounced at high C3H6 concentration. CO production increased from a few ppm to 17 ppm when we added 100 ppm C3H6, and it reached 155 ppm at 600 ppm C3H6. These results provided a picture of a significant amount of CO as a result of the acceleration of C3H6 decomposition reaction with the energy density. The detailed reaction mechanism for the formation of CO is believed to result from the following reactions 202618,22,59

HCHO þ OH f HCO þ H2 O 0:98 T 5:86 -10 exp cm3 molecule-1 s-1 k ¼ 6:48 10 298 RT ð22Þ HCO þ O2 f CO þ HO2 1:0 T 1:41 k ¼ 4:82 10-11 exp cm3 molecule-1 s-1 298 RT ð23Þ HCO þ O f CO þ OH k ¼ 5:0 10-11 cm3 molecule-1 s-1

ð24Þ

HCO þ HCO f HCHO þ CO k ¼ 3:0 10-11 cm3 molecule-1 s-1

ð25Þ

CH3 CHO þ O2 f HCHO þ CO þ H2 O k ¼ 3:0 10-14 cm3 molecule-1 s-1

ð26Þ

Further studies are required to bring down the CO level to an allowable limit for practical applications of this system. Otherwise, the use of other additives capable of promoting the oxidation of NO to NO2 without evolving CO may be preventive methods against this problem. One candidate of chemical additive for this purpose may be ozone, hydrogen peroxide, or the postoxidation catalyst reactor.8 5406



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of the feed gas via EleyRideal SCR mechanism but also with small quantities of NO2 adsorbed species to produce H2O and N2O. The dual active site mechanism for N2O formation is presented in reactions 313447

Figure 10. N2O formation at the outlet of the plasma facilitated SCR system as a function of energy density at different reaction temperatures. Reaction conditions: NOx = 600 ppm, NH3 = 600 ppm, O2 = 6% (v/v), GHSV = 20770 h1, and balance N2.

3.3.2. N2O Production. The N2O formation as a function of the energy density at four different reaction temperatures is shown in Figure 10. It was observed that N2O formation increased as the temperature increases from 0 ppm at 150 °C to 12 ppm at 450 °C without energy input. This is because N2O can be formed by the gas phase reaction 27 between NH3 and NO,14,15 and by the direct oxidation of ammonia in the reaction 28 at high temperatures,25 meanwhile, the catalyst carrier TiO2 further oxidizes ammonia through the surface reaction 2949 4NH3 þ 4NO f 4N2 O þ 6H2 O

ð27Þ

2NH3 þ 4O2 f 2N2 O þ 6H2 O

ð28Þ

TiO2 þ xNH3 f 0:5xN2 O þ 1:5xH2 O þ TiO2-2x

ð29Þ

In high temperature above 400 °C, the major source of N2O formation is ammonia oxidation, which causes unnecessary ammonia consumption. On the other hand, the Lewis acid centers (V = O) formation in V2O5 catalysts is favored with the increasing temperature; the increase in N2O generation has been assigned to a higher presence of Lewis acid sites on the catalyst surface.47 An increase in N2O generation with increasing input energy density was also observed, which could be related to reaction 3021,23,58 NO2 þ N f N2 O þ O k ¼ 5:8 1:0 T 1:83 -12 10 exp cm3 molecule-1 s-1 298 RT

ð30Þ

Besides, NO2 forms from the NO oxidation during the plasma process, and Juan Antonio et al.47 described the strong adsorption of NO2 on Lewis acid sites (V = O) over titania supported vanadia catalysts as responsible for nitrate formation (NO3—) on the catalyst surface. Taking into account that the vanadium-titania catalysts have Lewis acid sites where quantities of NO2 present in the gas stream may be absorbed and give rise to nitrate species, which could react with ammonia adsorbed on the BrΦnsted acid sites, the adsorbed ammonia species are able to react not only with NO

3.3.3. Aldehydes-type Unregulated Byproducts Formation. Formaldehyde (HCHO) and acetaldehyde (CH3CHO) are determined as the major aldehydes-type unregulated byproduct constituents of partial oxidation of C3 H6 in the plasma process according to the selective absorption of gas on FTIR spectra. The concentration of HCHO and CH 3 CHO at the outlet of DBD reactor as a function of the energy density with the addition of different concentration C3H6 is presented in Figure 11. Using C 3H6 as an additive generating HCHO and CH3CHO in the DBD reactor, and this can be explained from two aspects: the first is that C3H6 can directly react with O radicals to produce HCHO via reaction (35) and the second is that several intermediate species of C3 H6 decomposition and fragments can further form HCHO and CH 3CHO easily via the following reactions (35)(47) listed in Table 1.5,8,10,11,14,17,25,44 It is observed from Figure 11 that the formation of HCHO and CH3CHO increased with increasing energy density and the addition of C3H6 concentration. It was found that significant amount of HCHO and CH3CHO as a result of C3H6 decomposition was emitted from the DBD reactor, especially at the high energy density input and the high concentration of C3H6 addition. Figure 12 presents the comparison for the production of HCHO and CH3CHO as a function of the C3H6 concentration at the outlet of the DBD reactor and at the outlet of the PF NH3SCR hybrid system. As can be seen from Figure 12, a considerable amount of HCHO and CH3CHO was observed at the outlet of the DBD reactor as mentioned above; however, it is interesting to note that they were almost completely removed when the feed gas was processed in the catalytic reactor, i.e. the concentration of HCHO and CH3CHO at the outlet of the plasma catalytic reactor was only a few ppm in the experimental condition. This may indicate that the HCHO and CH3CHO can take part in the NOx removal in NH3SCR reactions and that HCHO, CH3CHO, and many other excited species produced in 5407



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compounds, and other excited hydrocarbon radical intermediates DBD plasma

NO þ HCs þ O2 sf NO2 þ HC-productsðCx Hy Oz Þ ð48Þ where CxHyOz refers to partially oxidized hydrocarbons. NO, NO2 ; formaldehyde; and acetaldehyde; SCR catalyst

nitrogen-contained organic compounds sf N2 , CO2 , H2 O, CO, N2 O; and other byproduct SCR catalyst

NO þ NO2 þ NH3 þ Cx Hy Oz þ HCs sf N2 þ H2 O þ CO2 þ byproducts

ð49Þ

3.4. Effect of SO2 on the Plasma Facilitated NH3SCR Reaction. The effect of SO2 concentration on the NOx removal

Figure 11. Concentration of HCHO (a) and CH3CHO (b) formation at the outlet the DBD reactor as a function of energy density at different concentration of C3H6. Reaction conditions: NOx = 600 ppm, O2 = 6% (v/v), total gas flow rate = 100 mL/min, and balance N2, energy density = 65 J/L.

the plasma process may contribute to enhance the efficiency of the activity of the NH3SCR system. Consequently, the effect of HCHO, CH3CHO, and other excited species on the removal efficiency of NOx in the hybrid process is to be investigated next. Figure 13 illustrates the detailed pathways and the chemical kinetic reaction mechanism to NO oxidation and formation of CO, HCHO, and CH 3 CHO in the presence of C 3 H6 as additive. 17 According to the above experiment results, it confirms the contribution of DBD facilitated NH3SCR hybrid system. After the plasma pretreatment, NO is partially oxidized to NO2, on the other hand, parts of propylene oxidize to the more powerful reductants like aldehydes for downstream SCR of NOx over catalyst, and the key intermediates like nitrogen-contained organic compounds in connection with decomposition of propylene can be obtained at room temperature during the plasma process. In this sense, the plasma-assisted catalytic reduction of NOx is accomplished in essentially two steps, and the reaction mechanism for synergistic effect to reduce NOx can be explained schematically as follows. First, the plasma oxidizes NO to NO2 in the presence of propylene, and the propylene also partially oxidizes as reaction 48,15,19 and second, the SCR catalyst reduces NO, NO2 by selective reduction using ammonia as reaction 4932,41 DBD plasma

NO, O2, C3H6 sf NO, NO2, aldehydes (including formaldehyde and acetaldehyde), nitrogen-contained organic

efficiency in the PF NH3SCR reaction is shown in Figure 14 where NOx conversion decreased approximately 13.2% with increasing SO2 concentration at a lower temperature of 150 °C. However, it was almost constant at the temperature of 250 °C and increased slightly (23%) by SO2 at a higher temperature of 400 °C. It is evident that the presence of SO2 inhibited NH3SCR reactions at lower temperatures and improved reactions at higher temperatures. In addition, the presence of SO2 in the feed gas inhibited N2O formation and the formation of N2O decreased with increasing SO2 concentration at all the employed reaction temperatures.60 Figure 15 shows the effect of SO2 on NOx conversion in the PF NH3SCR hybrid system at temperature of 150 and 400 °C for durability tests. It can be seen that in the absence of SO2, the steady state NOx removal efficiency was 58.77% and 83.11% at reaction temperatures of 150 and 400 °C, respectively. When SO2 was introduced into the feed gas, the effect of SO2 was 2-fold. At a lower reaction temperature of 150 °C, the hybrid system activity decreased with the addition of SO2, and the deteriorated activity increased with increasing the concentration of SO2. Noticeably, SO2 had some effects on the catalytic activity, but the activity decreased slightly in the range of time on stream. On one hand, the effect of SO2 at lower temperatures may come from the chemiadsorption competition on the active site of the SCR catalyst between SO2 and the reactants, NH3 and NOx. On the other hand, SO2 containing in the feed gas can easily form instable sulfite ion, which reacted with the chemisorbed oxygen to form sulfate and led to the decrease of the active site of the catalyst and the activity of the hybrid system.60 At a higher reaction temperature of 400 °C, the hybrid system activity increased with the addition of SO2. The promoted effect of SO2 increased continuously, from a NOx conversion of 83.1% to 87.5%, 88.59%, and 89.9% in 12 h at SO2 concentration of 200 ppm, 400 ppm, and 600 ppm, respectively. The SO2 promotion may attribute to the formation of SO42- on the catalyst surface, which can increase the catalyst surface acidity to make NH3 adsorbed and to promote reactions between NH3 and NOx. It is possible that the increase in the surface acidity is due to the fact that sulfation promotes conversion of C3H6 to partially oxygenated species, but validation of such a mechanism would require further investigation.

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Table 1. Mechanisms of HCHO and CH3CHO Formation at 1 atm reaction rates expression (cm3 molecule-1 s-1)

reaction number

Mechanism of HCHO Formation C3H6 þ O f HCHO þ C2H4 CH3 þ O f HCHO þ H

/ 0:03 T 0:15 exp 298 RT 0:98 T 56:55 -12 exp k ¼ 1:06 10 298 RT k ¼ 3:0 10-11 1:0 T 1:65 k ¼ 1:68 10-11 exp 298 RT k ¼ 5:0 10-11 k ¼ 2:61 10-11 k ¼ 1:0 10-10 k ¼ 8:0 10-12 k ¼ 7:4 107

k ¼ 1:25 10-10

CH3 þ O2 f HCHO þ OH HCO þ HCO f HCHO þ CO CH2OH þ O2 f HCHO þ HO2 CH3O þ O2 f HCHO þ HO2 C2H5 þ O f HCHO þ CH3 CH3O þ CH3O f CH3OH þ HCHO CH2CHO þ O2 f HCHO þ CO þOH CH3CH(OH)CH2O f CH3CHOH þ HCHO C2H5 þ O f CH3CHO þ H HOC3H6O2 þ NO f CH2OH þ CH3CHO þ NO2 CH3CH2O þ O2 f CH3CHO þ HO2

Mechanism of CH3CHO Formation k ¼ 1:33 10-10 1:0 T 200 k ¼ 4:2 10-12 exp 298 RT 1:0 T 4:57 k ¼ 6:29 10-14 exp 298 RT

(35) (36) (37) (38) (39) (40) (41) (42) (43) (44) (45) (46) (47)

Figure 13. Schematic diagram of the main chemical reaction mechanism for the oxidation of NO in the presence of C3H6.

Figure 12. Comparison for the concentration of HCHO (a) and CH3CHO (b) formation at the outlet of DBD reactor and at the outlet of the plasma facilitated NH3SCR hybrid system at different concentration of C3H6. Reaction conditions: NOx = 600 ppm, O2 = 6% (v/v), total gas flow rate = 100 mL/min, GHSV: 20770 h1, and balance N2, reaction temperature = 150 °C, energy density = 65 J/L.

Figure 16 illustrates the variations of NO, NO2, retained SO2, and C3H6 concentrations at the outlet of the DBD reactor as a function of SO2 addition. In the plasma process,

the amount of NO oxidized to NO2 decreased with the increase of SO2 concentration. Meanwhile, the NOx level and the sum of NO and NO2 were kept almost constant regardless of the oxidation of NO to NO2 and the concentration of SO2. It is evident that there was a competition between the oxidation of NO and the oxidation of SO2, which suppressed the oxidation reaction of NO to NO2. The inhibition degree increased with increasing SO2 concentration. This may be one of the reasons explaining that NOx removal efficiency decreased with increasing SO2 concentration. The inhibition effect of SO2 can be understood from the oxidation of the SO2 reaction, which was competing with the oxidation of the NO reaction.24,61,64 5409



Figure 14. Effect of SO2 concentration on the NOx removal in the plasma facilitated NH3SCR reaction. Reaction conditions: NOx = 600 ppm, NH3 = 600 ppm, O2 = 6% (v/v), C3H6 = 100 ppm, GHSV = 20770 h1, and balance N2, energy density = 42 J/L.

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Figure 16. Concentration profile of NO, NO2, NOx, retained SO2 and C3H6 as function of SO2 concentration at the outlet of DBD reactor. Reaction conditions: NOx = 600 ppm, NH3 = 600 ppm, O2 = 6% (v/v), C3H6 = 100 ppm, total gas flow rate =100 mL/min, and balance N2, energy density = 36 J/L.

Figure 17. Concentration of SO2 before and after exposure to the plasma. Reaction conditions: NOx = 600 ppm, NH3 = 600 ppm, O2 = 6% (v/v), C3H6 = 100 ppm, total gas flow rate = 100 mL/min, and balance N2, energy density = 36 J/L.

Figure 15. Effect of different concentration of SO2 on the NOx removal efficiency in dependence of time in the plasma facilitated NH3SCR reaction: (a) reaction temperature of 150 °C and (b) reaction temperature of 400 °C. Reaction conditions: NOx = 600 ppm, NH3 = 600 ppm, O2 = 6% (v/v), C3H6 = 100 ppm, GHSV = 20770 h1, and balance N2, energy density = 36 J/L.

the catalyst at lower temperatures, and make the hybrid system more tolerant to the sulfur content. Through examining the competition between the oxidation of SO2 and the oxidation of NO, we found that the mechanism for nonthermal plasma is more selective toward the oxidation of NO compared with the oxidation of SO 2, especially in the presence of C3H6 in the feed gas. The reactions of formation of SO3 are 50, 51, and 52,31 and the back conversion of SO3 to SO2 is 53.45 The reactions of oxidation of NO to NO2 are 545723,61 O þ SO2 þ M f SO3 þ M k ¼ 1:40 10-33 cm6 molecule-2 s-1

ð50Þ

O2 þ SO2 þ M f SO3 þ O þ M

From Figure 17, it can be seen that the oxidation efficiency of SO2 to SO3 were 5.71%, 3.75%, 4.67%, and 3.87% at the inlet SO2 concentration of 200 ppm, 400 ppm, 600 ppm, and 800 ppm, respectively. This can decrease the formation of sulfuric acid and sulfates which would increase the particulates, poison the active sides on

k ¼ 2:16 10-16 cm3 molecule-1 s-1

ð51Þ

O3 þ SO2 þ M f SO3 þ O2 þ M k ¼ 1:0 10-22 cm3 molecule-1 s-1 5410

ð52Þ



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O þ SO3 þ M f SO2 þ O2 þ M -1 -1

ð53Þ

k ¼ 1:0 10-31 cm6 molecule-2 s-1

ð54Þ

k ¼ 4:57 10 cm molecule s -17

3

O þ NO þ M f NO2 þ M

O3 þ NO þ M f NO2 þ O2 þ M k ¼ 1:8 10-14 cm3 molecule-1 s-1

ð55Þ

O þ C3 H6 f HO2 3 þ RO2 3 k ¼ 4:96 10 cm molecule s -12

3

-1 -1

ð56Þ

HO2 3 ðRO2 3 Þ þ NO f NO2 þ OHðROÞ k ¼ 3:11 10-11 cm3 molecule-1 s-1

ð57Þ

where HO2 3 and RO2 3 are hydro-peroxyl radicals and peroxyl radicals, respectively. It is evident that the rate constant for the oxidation of NO reactions is much large than the one for the SO2 oxidation. Therefore, most O and O3 radicals will be scavenged by the oxidation of NO, and the level of SO2 oxidation will decrease sharply.

4. CONCLUSIONS 1) The combination of NH3SCR and the nonthermal plasma enhances the overall reaction and allows for an effective removal of NOx at the reaction temperatures as low as 100 °C. The oxidative potential of the nonthermal plasma in off-gases with excess oxygen results in an effective conversion of NO to NO2 with the sum of NO and NO2 almost constant and converted synergistically by NH3SCR to nitrogen with appropriate catalysts. 2) For the effective NO oxidation using the DBD reactor, it is necessary to add an additive such as C3H6 to the gas stream, especially to lower the electrical energy cost for oxidation of NO to NO2, which plays an important role in enhancing NOx removal at relatively low temperatures. 3) The C3H6 added to the gas mixture has at least four essential functions in the nonthermal plasma: first, it assists the gas-phase oxidation of NO to NO2 by electric discharge in excessive oxygen; second, it lowers the energy cost for this oxidation; third, the intermediate products produced by propylene and O radicals can further facilitate the NOx reduction; fourth, the C3H6 suppresses the oxidation of SO2 to SO3, making the hybrid system more sulfur tolerant. 4) The oxidation of NO increased with the increase of input voltage and the concentration of C3H6. Nevertheless, excess input voltage and propylene may lead to a great of byproduct emission such as N2O, CO, HCHO, and CH3CHO, and excessive energy consumption. 5) Two of the main byproducts, HCHO and CH3CHO produced in the DBD reactor, were decomposed almost completely in the PF NH3SCR reactor, indicating that HCHO and CH3CHO can make a difference in the NOx removal. 6) The presence of SO2 inhibits the NOx removal efficiency of the PF NH3SCR system at lower temperatures and improves NOx removal efficiency at higher temperatures.

Besides, the conversion of NOx is affected slightly in long tests. SO2 also inhibits N2O formation at all the employed reaction temperatures.

’ AUTHOR INFORMATION Corresponding Author

*Phone: þ86 21 34206540. Fax: þ86 21 34205553. E-mail: [email protected].

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