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ZSM5 catalysts assisted by plasma
Applied Thermal Engineering 130 (2018) 1224–1232
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
Effects of temperature on NOx removal with Mn-Cu/ZSM5 catalysts assisted by plasma Tao Wang a,b,⇑, Xinyu Zhang a, Jun Liu a, Hanzi Liu a, Yang Wang a, Baomin Sun a a b
Education Ministry Key Laboratory on Condition Monitoring and Control of Power Plant Equipment, North China Electric Power University, Beijing 102206, China Institute of Energy Environmental Science and Engineering, North China Electric Power University, Beijing 102206, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Removal of NOx by plasma-catalyst
system at different temperatures were studied. Catalyst addition dramatically improved NOx removal efficiency with plasma. 12Mn-12Cu/ZSM5 exhibited the best performance in NOx removal with plasma. Temperature affected the discharge and catalytic activity for NOx removal.
a r t i c l e
i n f o
Article history: Received 8 December 2016 Revised 22 November 2017 Accepted 22 November 2017 Available online 23 November 2017 Keywords: NOx removal Catalyst Mn-Cu/ZSM5 Plasma Temperature
a b s t r a c t Manganese-copper catalysts are well-known catalysts for selective catalytic reduction (SCR) of NOx. To evaluate the NOx removal in the combination system of catalysts and plasma, a series of Cu-Mn/ZSM-5 catalysts with varying amounts of manganese doping were synthesized by an incipient wetness impregnation method and used for SCR of NOx assisted by plasma at different temperatures. The results showed that the combination of catalyst and plasma promoted NO removal efficiency, the introduction of Mn on Cu/ZSM5 enhanced the catalyst activity, and 12Mn-12Cu/ZSM5 exhibited the best NO removal performance with about 90% NO removal efficiency at 25 °C. With a rise in temperature, the reduced electric field (E/N) increased, intensifying the discharge and generating more active radicals. In the absence of catalyst, an increase in the temperature had a negative effect on NO removal due to the reduction of the predominant oxidant O3. However, when the temperature increased in the combination system, NO removal efficiency decreased below 100 °C, while increased above 100 °C, which increased dramatically at low discharge power above 150 °C. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction In the past 30 years, China has rapidly developed, characterized by expanding industrialization, explosive growth in the number of ⇑ Corresponding author at: Education Ministry Key Laboratory on Condition Monitoring and Control of Power Plant Equipment, North China Electric Power University, Beijing 102206, China. E-mail address: [email protected] (T. Wang). https://doi.org/10.1016/j.applthermaleng.2017.11.113 1359-4311/Ó 2017 Elsevier Ltd. All rights reserved.
cars, and constant anthropogenic activity. These changes have resulted in large amounts of pollutants being disposed into the air [1,2]. From 2000 to 2010, energy consumption in China increased by 120%. Coal accounted for 70% primary energy consumption [3]. The vast consumption of coal was concentrated in the serious haze pollution areas. Nitrogen oxides are a primary component of haze, which could lead to the formation of photochemical smog, acid deposition [4]. SCR is the most promising
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MFC
t=1 mm
MFC
MFC
MFC
t=1.5 mm Catalyst N2
O2
Electrode RF
Quartz partition h=850 mm
PS
NO C1
NH3
DO
C2 Quartz tube
4 mm 20 mm RF: Resistance furnace; PS:Power supply DO: Digital oscilloscope; GA: Gas analyzer AA:Alkali absorption; MFC:Mass flow controller
GA
AA
Fig. 1. Schematic of the DBD plasma experiment.
alternative technology for reducing NOx and is a widely used NOx post-treatment technology for diesel and lean-burn vehicles [5,6]. Generally, among NOx emissions, NO is generally the major gas (95%) and the content of NO2 is less than 5%. Catalytic removal of NO at low temperature is possible when part of the NO is converted into NO2 in advance. Plasma is capable of oxidizing NO to NO2 in the presence of O2. Plasma combined with TiO2 [7], Pt/ BaO/Al2O3 [8], Ag/Al2O3 [9] and CuO/TiO2 [10] exhibits high deNOx activities in the low temperature range. In the past ten years, the low-temperature SCR catalysts have obtained extensive attention. In particular, some ZSM-5 zeolites modified by transition metals such as Cu and Fe are successfully used to remove NO due to their high thermal/hydrothermal stability, strong acidity and welldefined microporosity [11–14]. Among zeolite-based catalysts, the Cu/ZSM-5 has been considered as one of the best lowtemperature SCR catalysts [15,16]. Olsson [17] reported that the copper plays a decisive role in the catalytic activity of the Cu/zeolites. Liu et al. [18] reported that the high deNOx performance of the Cu/zeolites catalyst was due to the excellent redox property. Hanna et al. [19] compared the activity of H-, Na- and Cu-ZSM-5 and the result showed that the activity was greatly enhanced by the introduction of copper ions. Meanwhile, manganese oxides, because of their unique characteristics, exhibit strong redox abilities as there are various types of labile oxygen on their surfaces, resulting higher SCR activity at lower temperatures compared with other metal oxides [20,21]. Min et al. [22] indicated that Cu–Mn oxide catalysts showed the complete NOx conversion in a wide
range of reaction temperature from 323 to 473 K. Liu et al. [23] demonstrated that the good performance of Cu-Mn-oxide catalyst which was closely associated with more adsorbed oxygen species and high lattice oxygen mobility. Mohsen et al. [24] revealed that Cu–Mn binary oxide catalysts markedly contained the most Lewis acid sites, high surface area and pore volume leading to superior catalytic performance. However, few studies have investigated the combination of plasma and Mn-Cu/ZSM-5 for NOx removal. Therefore, we studied the synergetic mechanism of NOx removal by a combination of DBD and Mn-Cu/ZSM5 catalyst. In this work, we investigated the effect of Mn-Cu/ZSM5 with different Mn loadings on NOx removal in the dielectric barrier discharge reactor. The study firstly investigated NOx removal in the plasma reactor alone, and then the effect of the combined system on NOx removal was examined in the temperature range of 25– 300 °C. Also, the Boltzmann equation has been used to simulate the electron distribution function, focusing on synergetic mechanisms of NOx removal with plasma-catalyst system at various temperatures.
2. Experimental 2.1. Experimental setup As shown in Fig. 1. All gases were controlled by MFCs(mass flow controllers) and then the gases flowed into the reactor. An alkali
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Mn and 12 wt% Cu (12Mn-12Cu), and 16 wt% Mn and 12 wt% Cu (16Mn-12Cu). XRD (X-ray diffraction) patterns were tested by a X-ray diffractometer (Bruker D8 advance). 2.3. Removal efficiency calculation Removal efficiencies of NO and NOx are calculated by Eqs. (1) and (2):
gNO ¼
½NOinlet ½NOoutlet 100% ½NOinlet
gNOx ¼
ð1Þ
½NOinlet ½NOoutlet ½NO2 outlet 100% ½NOinlet
ð2Þ
3. Results and discussion Fig. 2. Effect of catalyst with different Mn loadings on NO removal efficiency.
absorption device was used to absorb exhaust gas. The plasmacatalyst reactor was kept in a resistance furnace to maintain the desired gas temperature. The NO and NO2 concentrations were obtained using a gas analyzer (TESTO Pro.350). The discharge power was calculated from a Lissajous figure captured by the digital-storage oscilloscope (Rigol DS1202CA). The gas contained 500 ppm (parts per million) NO, 500 ppm NH3, 6% O2, and N2 as the balance gas. The typical flow rate of the simulated gas was 2 L/min and the gas hourly space velocity (GHSV) was 30,000 h1. The voltage was decreased by 1000 times for voltage measurement, the frequency was equal to 10 kHz, and the measuring capacitance (Cm) was 0.14 lF. The reactor consisted of two dielectric tubes with a height of 850 mm (outer diameter: 20 mm; inner diameter: 17 mm), and a stainless steel rod (diameter: 4 mm) placed along the axis of the tube. The thicknesses of the outer and inner tubes were 1.5 and 1 mm, respectively. The outer tube was covered with copper wire mesh. 2.2. Catalyst preparation and characterization Mn-Cu/ZSM5 were produced by incipient wetness impregnation of ZSM5 with CuSO43H2O and Mn(NO3)2 solutions. The mixed solutions were dispersed via ultrasonic treatment for 2 h, dried overnight at 100 °C and then calcined in muffle furnace at 550 °C for 4 h. The Mn-Cu based ZSM5 catalyst contained different Mn loadings: 0 wt% Mn and 12 wt% Cu (12Cu), 4 wt% Mn and 12 wt% Cu (4Mn-12Cu), 8 wt% Mn and 12 wt% Cu (8Mn-12Cu), 12 wt%
3.1. NOx removal with catalyst The deNOx performances of the catalysts with different loadings of Mn (0, 4, 8, 12, and 16 wt%) are depicted in Fig. 2. All catalysts exhibit excellent performance in NOx removal, the addition of Mn on Cu/ZSM5 catalyst can promote the reduction of NO, and NO removal efficiency of 12Mn-12Cu catalyst can reach above 90%, showing the best deNOx activity. Meanwhile, A variety of Mn-based catalysts (shown in Table 1), showing the superior catalytic performance, were believe to be an excellent catalyst for the NO removal. For the low temperature NH3-SCR, Mn/Ce/TiW [25], MnCe(29.4)/TNTs [26], Mn-Fe/CeZr [27], Fe0.3Mn0.5Zr0.2 [28], Cu–MnOx/TiO2 [29] and MnOx–CuOx/TiO2 [30] achieved 90–100% NO conversion rate at 120–360 °C. In the combination system of catalysts and plasma, the previous works [31–34] revealed that different Mn-based catalysts showed 58.0–72.4% NO removal efficiency at 25–100 °C in plasma-catalyst system. However, in this study, 12Mn-12Cu/ZSM5 sample exhibited the best performance with about 90% NO removal efficiency at 25 °C. As shown in Fig. 3, NO2 concentration is halved with catalyst compared to that without catalyst. The gas mixture containing NO2 is a more efficient for NOx removal, because the adsorption capability of NO2 is larger than that of NO [35] and NO2 can participate in the fast SCR reaction. NO2 can react fast with NO as well as decompose easily into nitrogen as follows [36–38]:
NO þ NO2 þ 2NH3 ! 2N2 þ 3H2 O
ð3Þ
6NO2 þ 8NH3 ! 7N2 þ 12H2 O
ð4Þ
As shown in Fig. 4. The addition of NH3 did not affect the NO and NO2 concentrations under plasma. However, NO and NO2 concentrations clearly decreased with NH3 and 12Mn-12Cu under plasma.
Table 1 Mn-based catalysts in published literature.
*
Catalyst
Experimental condition
NO removal efficiency
Ref.
Mn/Ce/TiW MnCe(29.4)/TNTs Mn-Fe/CeZr Fe0.3Mn0.5Zr0.2 Cu–MnOx/TiO2 MnOx–CuOx/TiO2 Mn-Ce-Ni/TiO2 Mn-Co-Ox MnOx/TiO2–MWCNT Mn–Co-Ox 12Mn-12Cu/ZSM5
NH3-SCR NH3-SCR NH3-SCR NH3-SCR NH3-SCR NH3-SCR NH3-SCR NH3-SCR NH3-SCR NH3-SCR NH3-SCR
>90%* within 180–300 °C 92%* at 250 °C >90% in 150–300 °C 100% within 200–360 °C 100% at 180 °C 100% within 120–260 °C 58.0% at 50 °C 71% at 125 °C 72.4% at 25 °C 67.0% at 100 °C 90% at 25 °C
[25] [26] [27] [28] [29] [30] [31] [32] [33] [34] This study
The value represents the NOx removal efficiency.
assisted assisted assisted assisted assisted
by by by by by
plasma plasma plasma plasma plasma
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Thus, (R8) and (R9) become the predominant reactions for NO removal. When the catalyst is combined with plasma, the major reaction of SCR will be R1 and R10 [47–49]. The fast SCR reaction (R1) exhibits a reaction rate at least 10 times higher than that of the standard SCR reaction (R10) below 200 °C [50]. As Fig. 4(b) shows, NO2 concentration can reach above 120 ppm without catalyst, whereas it is less than 50 ppm with 12Mn-12Cu. This indicates that almost 70 ppm of NO2 participates in catalytic reaction.
4NH3 þ 4NO þ O2 ! 4N2 þ 6H2 O
ðR8Þ
3.2. XRD results
Fig. 3. Effect of catalyst with different Mn loadings on NO2 concentration.
As Fig. 5 show, when the discharge is 8 W, NO and NOx removal efficiencies are 56 and 35% with NH3, and change to 88% and 79% over 12Mn-12Cu. In the NO/O2/N2/NH3 system with plasma, the active electrons collide with N2 and O2 [39–42]:
e þ N2 ! N þ N þ e
ðR1Þ
e þ O2 ! O þ O þ e
ðR2Þ
e þ O2 ! O þ Oð1 DÞ þ e
ðR3Þ
O þ O2 ! O3
ðR4Þ
Compared to N2, the active electrons react more easily with O2 because the bond dissociation energies of O2 (498.36 kJ mol1) are much lower than those of N2 (945.33 kJ mol1). The obtained radicals O, O3, and N react with NO in the following reduction reactions [43–46]:
N þ NO ! N2 þ O
ðR5Þ
O þ NO ! NO2
ðR6Þ
O3 þ NO ! NO2 þ O2
ðR7Þ
Fig. 6 shows the XRD spectra of all catalysts. No peaks of crystalline phase of manganese oxides were observed on 12Cu, 4Mn-12Cu, 8Mn-12Cu, and 12Mn-12Cu. This demonstrates that well dispersed MnOx exists on the surface of 4Mn-12Cu, 8Mn12Cu, and 12Mn-12Cu catalysts [51] and a homogeneous coverage of the active phase forms as a monolayer [52]. The peaks corresponding to Mn2O3 appeared in 16Mn-12Cu, but only Mn2O3 could be observed to calcine MnOx catalyst up to 500 °C [53]. Qi [54] and Ettireddy [55] also reported that MnOx peak appeared at 15 wt% Mn loading. The presence of amorphous manganese oxide phase in catalysts may be one of the key factors for their excellent catalytic activity in deNOx performance [56,57]. CuO, which showed a peak at 35.5°, was overlapped by the peak of Cu1.4Mn1.6O4 at 35.8°. Copper is present nearly exclusively in the form of mononuclear ions and oligomeric species, which are known to be active in the DeNOx process [58–60]. 3.3. Influence of temperature on discharge characteristics Gas temperature can affect NOx removal efficiency in two major respects: electric field strength divided by the total gas density (E/N) and the reaction rate. Electrostatic field model was performed by finite element modeling according to a previous method [61]. Fig. 7(a) shows the cross-section of the DBD reactor. High voltage is 10 kV connected to the inner electrode, and the voltage of outer electrode is 0 V; the relative permittivities of gas and quartz are 1 and 3.75 respectively. The calculation model was set up based on the actual size of the reactor. The electrostatic field of a quarter of the reactor was studied since the reactor was symmetrical. As shown in Fig. 7(b), on the inner electrode, the E is the strongest and decreases along the radial direction. The total gas density (N) is calculated by Eq. (3) [62]:
Fig. 4. Effect of 12Mn-12Cu/ZSM5 catalyst on NO and NO2 concentration under plasma.
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Fig. 5. Effect of 12Mn-12Cu/ZSM5 catalyst on NO and NOx removal efficiency under plasma.
is high enough [66]. N2 dissociation is 6.04 1018 cm3 s1 at 100 Td, which increases to 1.02 1015 cm3 s1 at 300 Td. As shown in Fig. 8, the gas temperature affects the gas discharge power, when applied voltage keep constant, the discharge power increases with gas temperature. 3.4. NOx removal with different gas temperatures As shown in Fig. 9(a), High temperature leads to much lower deNOx activities compared to those at low temperature. When the discharge power was 10 W, the NO removal efficiency was 60, 57, 55, 51, 41, and 27% at temperatures of 25, 50, 100, 150, 200, and 300 °C, respectively. As stated previously, R8 and R9 are the major reactions for NO removal without catalyst. However, with the accumulation of generated NO2, repression reactions gradually intensify, showing a negative effect on NO removal [67,68]: Fig. 6. XRD patterns of the Mn-Cu/ZSM5 catalyst: (a) 12Cu, (b) 4Mn-12Cu, (c) 8Mn12Cu, (d) 12Mn-12Cu, (e) 16Mn-12Cu.
N¼
P kT
ðR9Þ
where k is the Boltzmann constant, T is the gas temperature, P is the gas pressure. When the gas temperature increases, N decreases, increasing E/N. Fig. 7(c) shows E/N in the reactor at various temperatures, E/N is a main factor affecting discharge in terms of occurrence and performance. The electric field is 113 Td at 25 °C on the inner electrode and increases to 220 Td at 300 °C. The distribution function of electron is calculated by Eq. (4) [63,64]:
@fðr; u; tÞ eE þ u rfðr; u; tÞ þ rv fðr; u; tÞ ¼ C½fðr; u; tÞc ; @t m
ðR10Þ
Keeping the pressure and temperature unchanged, resulting in N unchanged according to Eq. (3). So, E/N increases with an increasing in E. We use BOLSIG+ to calculate the Boltzmann equation in the N2/ NO/O2/NH3 system. As shown in Fig. 7(d). The electron mean energy is 2.356 eV at 100 Td and increases to 6.789 eV at 300 Td. The ratio of energetic electrons also increases with an increasing in E/N [42]. The amounts of active radicals (N and O) are influenced by the electron mean energy and density of energetic electrons [65], and molecular dissociation will be enhanced as the energy of electron
O þ NO2 ! NO þ O2
ðR11Þ
N þ O2 ! NO þ O
ðR12Þ
O þ N ! NO
ðR13Þ
The reaction of O3 consumption is strengthened dramatically when the temperature increases (see Table 2) [69,70]:
O2 þ O3 ! O2 þ O2 þ O
ðR14Þ
O3 þ O ! O2 þ O2
ðR15Þ
As discussed above, the quantities of active radicals can be enhanced by a higher temperature. However, the reaction rate of O and NO (R8) decreases from 3.02 1011 cm3 s1 at 25 °C to 1.85 1011 cm3 s1 at 300 °C (see Table 2). Moreover, the rates of R11-R13 (reactions of NO formation) are 6.25 1012 cm3 s1, 7.66 1017 cm3 s1, and 1.08 1032 cm6 s1, while they change to 9.99 1012 cm3 s1, 2.86 1014 cm3 s1, and 8.72 1033 cm6 s1, respectively, at 300 °C. The rates of R11 and R12 obviously increase, but that of R13 decreases. However, the effect of the decrease in R13 rate can be negligible since it is too small. On the other hand, the reaction rate of R6 decreases as the temperature increases, but those of R14 and R15 increase. Thus, when the temperature increases, the generation of O3 is reduced, but the decomposition of O3 is promoted significantly.
T. Wang et al. / Applied Thermal Engineering 130 (2018) 1224–1232
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Fig. 7. Calculated electric field and distribution function of electron at difference temperatures: (a) sectional drawing of the reactor, (b) distribution of radial electric field, (c) the E/N at different temperatures, (d) calculated dissociation rate coefficient and electron mean energy.
Fig. 8. Discharge power as a function of applied voltage.
In brief, the increase in temperature improves the formation of NO and serves as an obstacle to the oxidation of NO to NO2. As shown in Fig. 10(a), when the temperature increases, NO2 formation is reduced.
The effect of temperature on NOx removal with all catalysts assisted by plasma was studied, and the trend of NO removal efficiency remains consistent with all catalysts. Fig. 9(b) presents the effect of temperature on NO removal with 12Mn-12Cu/ZSM5 catalyst. When the discharge power was 10 W, NO removal efficiency decreased from 90% at 25 °C to 76% at 100 °C and then increased to 97% at 300 °C with 12Mn-12Cu. When the discharge power was 2 W, NO removal efficiency remained around 30% at 25, 50, and 100 °C and increased greatly at 150, 200, and 300 °C. NO removal efficiency decreases as the temperature increases with only plasma. However, the deNOx performance increases above 100 °C with plasma-catalyst system. Bröer [50] reported that the rate coefficients of catalytic NO reduction were 0.12, 1.2, 5.3, 8.1, and 12 s1 at 120, 160, 200, 225, and 250 °C with plasma combined with catalyst (V2O5-WO3/TiO2), indicating that an increased temperature can enhance NO removal between 100 and 250 °C, which is similar to this paper’s conclusion. Plasma reaction can generate NO2 in the N2/NO/O2/NH3 system, and the removal of NOx is mainly caused by reaction R1 around 100 °C [50,36]. As shown in Fig. 10(b), NO2 concentration decreases upon heating with plasma-catalyst system and decreases noticeably at 150 °C. When the temperature dropped below 200 °C, NO2 is very important for the NOx reduction. The rate of R1 is much faster than that of R10 [71]. R1 also consumes large amounts of NO2 below 200 °C. NO2 produced by plasma becomes more important for NOx removal.
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Fig. 9. Effect of temperature on NO removal efficiency, (a) no catalyst, (b) 12Mn-12Cu. Table 2 Major reactions and their rate coefficients. Reactions
No.
Rate coefficients (cm3 T s1/ cm6 s1) 25 °C
100 °C
e þ N2 ! N þ N þ e e þ O2 ! O þ O þ e e + O2 ? O + O(1D) + e O þ O2 ! O3
R3 R4 R5 R6
fðE=NÞ fðE=NÞ fðE=NÞ 4.45 1034
3.30 1034
N þ NO ! N2 þ O
R7
3.42 1011
3.86 1011
O þ NO ! NO2
R8
3.02 1011
2.84 1011
R9
14
14
O3 + NO ? NO2 + O2
2.01 10
Ref.
50 °C
12
3.04 10
12
O þ NO2 ! NO þ O2
R11
6.25 10
6.74 10
N þ O2 ! NO þ O
R12
7.66 1017
1.94 1016
O þ N ! NO
R13
1.08 1032
1.05 1032
O2 + O 3 ? O 2 + O 2 + O
R14
15
3.54 10
15
5.31 10
O3 þ O ! O 2 þ O 2
R15
7.95 1015
1.36 1014
2.04 1034 4.67 1011 2.55 1011 5.90 1014 7.61 1012 8.71 1016 9.89 1033 1.01 1014 3.19 1014
150 °C
1.41 1034 5.40 1011 2.32 1011 9.80 1014 8.34 1012 2.78 1015 9.47 1033 1.66 1014 6.14 1014
200 °C
1.06 1034 6.06 1011 2.14 1011 1.46 1013 8.97 1012 7.05 1015 9.16 1033 2.45 1014 1.03 1013
The rate coefficients were obtained from NIST Chemical Kinetics Database (the rate coefficients of R6 and R13 are cm6 s1).
Fig. 10. Effect of temperature on NO2 concentration, (a) no catalyst, (b) 12Mn-12Cu.
300 °C
6.88 1035
[39] [39] [40] [42]
7.18 1011
[44]
1.85 1011
[46]
2.64 10
13
[44]
9.99 10
12
[67]
2.86 1014
[67]
8.72 1033
[67]
14
[69]
2.20 1013
[70]
4.36 10
T. Wang et al. / Applied Thermal Engineering 130 (2018) 1224–1232
4. Conclusions The effect of the electric discharge and catalytic reaction on NO reduction with Mn-Cu/ZSM5 catalyst was examined at various temperatures. NO removal efficiency is about 60% with only plasma and increases to 90% with 12Mn-12Cu/ZSM5 catalyst. The pretreatment of the mixed gas by DBD plasmas generates a certain amount of NO2, which can enhance NO reduction via the fast SCR reaction. The introduction of Mn on Cu/ZSM5 enhances the catalytic property of catalyst, and 12Mn-12Cu/ZSM5 exhibits the best performance in NOx removal. An increase in temperature leads to a decrease in total gas density (N), but E/N increases; the electron mean energy at 300 Td is 2.9-fold higher compared to that at 100 Td. More energy is transferred from electrons to gas molecules during the collisions, leading to the generation of more active radicals. In the absence of catalyst, the increase in temperature acts as an obstacle to NO removal because O3 reduction, which is the predominant oxidant acting to remove NO, is hindered by the high temperature, and increased temperature improves NO formation. NO removal efficiency decreased as the temperature increased with plasma-catalyst system below 100 °C, while the deNOx performance increased above 100 °C. NO removal efficiency increased dramatically at low discharge power above 150 °C.
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This work was supported by National Natural Science Foundation of China (51706069).
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Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
Effects of temperature on NOx removal with Mn-Cu/ZSM5 catalysts assisted by plasma Tao Wang a,b,⇑, Xinyu Zhang a, Jun Liu a, Hanzi Liu a, Yang Wang a, Baomin Sun a a b
Education Ministry Key Laboratory on Condition Monitoring and Control of Power Plant Equipment, North China Electric Power University, Beijing 102206, China Institute of Energy Environmental Science and Engineering, North China Electric Power University, Beijing 102206, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Removal of NOx by plasma-catalyst
system at different temperatures were studied. Catalyst addition dramatically improved NOx removal efficiency with plasma. 12Mn-12Cu/ZSM5 exhibited the best performance in NOx removal with plasma. Temperature affected the discharge and catalytic activity for NOx removal.
a r t i c l e
i n f o
Article history: Received 8 December 2016 Revised 22 November 2017 Accepted 22 November 2017 Available online 23 November 2017 Keywords: NOx removal Catalyst Mn-Cu/ZSM5 Plasma Temperature
a b s t r a c t Manganese-copper catalysts are well-known catalysts for selective catalytic reduction (SCR) of NOx. To evaluate the NOx removal in the combination system of catalysts and plasma, a series of Cu-Mn/ZSM-5 catalysts with varying amounts of manganese doping were synthesized by an incipient wetness impregnation method and used for SCR of NOx assisted by plasma at different temperatures. The results showed that the combination of catalyst and plasma promoted NO removal efficiency, the introduction of Mn on Cu/ZSM5 enhanced the catalyst activity, and 12Mn-12Cu/ZSM5 exhibited the best NO removal performance with about 90% NO removal efficiency at 25 °C. With a rise in temperature, the reduced electric field (E/N) increased, intensifying the discharge and generating more active radicals. In the absence of catalyst, an increase in the temperature had a negative effect on NO removal due to the reduction of the predominant oxidant O3. However, when the temperature increased in the combination system, NO removal efficiency decreased below 100 °C, while increased above 100 °C, which increased dramatically at low discharge power above 150 °C. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction In the past 30 years, China has rapidly developed, characterized by expanding industrialization, explosive growth in the number of ⇑ Corresponding author at: Education Ministry Key Laboratory on Condition Monitoring and Control of Power Plant Equipment, North China Electric Power University, Beijing 102206, China. E-mail address: [email protected] (T. Wang). https://doi.org/10.1016/j.applthermaleng.2017.11.113 1359-4311/Ó 2017 Elsevier Ltd. All rights reserved.
cars, and constant anthropogenic activity. These changes have resulted in large amounts of pollutants being disposed into the air [1,2]. From 2000 to 2010, energy consumption in China increased by 120%. Coal accounted for 70% primary energy consumption [3]. The vast consumption of coal was concentrated in the serious haze pollution areas. Nitrogen oxides are a primary component of haze, which could lead to the formation of photochemical smog, acid deposition [4]. SCR is the most promising
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MFC
t=1 mm
MFC
MFC
MFC
t=1.5 mm Catalyst N2
O2
Electrode RF
Quartz partition h=850 mm
PS
NO C1
NH3
DO
C2 Quartz tube
4 mm 20 mm RF: Resistance furnace; PS:Power supply DO: Digital oscilloscope; GA: Gas analyzer AA:Alkali absorption; MFC:Mass flow controller
GA
AA
Fig. 1. Schematic of the DBD plasma experiment.
alternative technology for reducing NOx and is a widely used NOx post-treatment technology for diesel and lean-burn vehicles [5,6]. Generally, among NOx emissions, NO is generally the major gas (95%) and the content of NO2 is less than 5%. Catalytic removal of NO at low temperature is possible when part of the NO is converted into NO2 in advance. Plasma is capable of oxidizing NO to NO2 in the presence of O2. Plasma combined with TiO2 [7], Pt/ BaO/Al2O3 [8], Ag/Al2O3 [9] and CuO/TiO2 [10] exhibits high deNOx activities in the low temperature range. In the past ten years, the low-temperature SCR catalysts have obtained extensive attention. In particular, some ZSM-5 zeolites modified by transition metals such as Cu and Fe are successfully used to remove NO due to their high thermal/hydrothermal stability, strong acidity and welldefined microporosity [11–14]. Among zeolite-based catalysts, the Cu/ZSM-5 has been considered as one of the best lowtemperature SCR catalysts [15,16]. Olsson [17] reported that the copper plays a decisive role in the catalytic activity of the Cu/zeolites. Liu et al. [18] reported that the high deNOx performance of the Cu/zeolites catalyst was due to the excellent redox property. Hanna et al. [19] compared the activity of H-, Na- and Cu-ZSM-5 and the result showed that the activity was greatly enhanced by the introduction of copper ions. Meanwhile, manganese oxides, because of their unique characteristics, exhibit strong redox abilities as there are various types of labile oxygen on their surfaces, resulting higher SCR activity at lower temperatures compared with other metal oxides [20,21]. Min et al. [22] indicated that Cu–Mn oxide catalysts showed the complete NOx conversion in a wide
range of reaction temperature from 323 to 473 K. Liu et al. [23] demonstrated that the good performance of Cu-Mn-oxide catalyst which was closely associated with more adsorbed oxygen species and high lattice oxygen mobility. Mohsen et al. [24] revealed that Cu–Mn binary oxide catalysts markedly contained the most Lewis acid sites, high surface area and pore volume leading to superior catalytic performance. However, few studies have investigated the combination of plasma and Mn-Cu/ZSM-5 for NOx removal. Therefore, we studied the synergetic mechanism of NOx removal by a combination of DBD and Mn-Cu/ZSM5 catalyst. In this work, we investigated the effect of Mn-Cu/ZSM5 with different Mn loadings on NOx removal in the dielectric barrier discharge reactor. The study firstly investigated NOx removal in the plasma reactor alone, and then the effect of the combined system on NOx removal was examined in the temperature range of 25– 300 °C. Also, the Boltzmann equation has been used to simulate the electron distribution function, focusing on synergetic mechanisms of NOx removal with plasma-catalyst system at various temperatures.
2. Experimental 2.1. Experimental setup As shown in Fig. 1. All gases were controlled by MFCs(mass flow controllers) and then the gases flowed into the reactor. An alkali
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Mn and 12 wt% Cu (12Mn-12Cu), and 16 wt% Mn and 12 wt% Cu (16Mn-12Cu). XRD (X-ray diffraction) patterns were tested by a X-ray diffractometer (Bruker D8 advance). 2.3. Removal efficiency calculation Removal efficiencies of NO and NOx are calculated by Eqs. (1) and (2):
gNO ¼
½NOinlet ½NOoutlet 100% ½NOinlet
gNOx ¼
ð1Þ
½NOinlet ½NOoutlet ½NO2 outlet 100% ½NOinlet
ð2Þ
3. Results and discussion Fig. 2. Effect of catalyst with different Mn loadings on NO removal efficiency.
absorption device was used to absorb exhaust gas. The plasmacatalyst reactor was kept in a resistance furnace to maintain the desired gas temperature. The NO and NO2 concentrations were obtained using a gas analyzer (TESTO Pro.350). The discharge power was calculated from a Lissajous figure captured by the digital-storage oscilloscope (Rigol DS1202CA). The gas contained 500 ppm (parts per million) NO, 500 ppm NH3, 6% O2, and N2 as the balance gas. The typical flow rate of the simulated gas was 2 L/min and the gas hourly space velocity (GHSV) was 30,000 h1. The voltage was decreased by 1000 times for voltage measurement, the frequency was equal to 10 kHz, and the measuring capacitance (Cm) was 0.14 lF. The reactor consisted of two dielectric tubes with a height of 850 mm (outer diameter: 20 mm; inner diameter: 17 mm), and a stainless steel rod (diameter: 4 mm) placed along the axis of the tube. The thicknesses of the outer and inner tubes were 1.5 and 1 mm, respectively. The outer tube was covered with copper wire mesh. 2.2. Catalyst preparation and characterization Mn-Cu/ZSM5 were produced by incipient wetness impregnation of ZSM5 with CuSO43H2O and Mn(NO3)2 solutions. The mixed solutions were dispersed via ultrasonic treatment for 2 h, dried overnight at 100 °C and then calcined in muffle furnace at 550 °C for 4 h. The Mn-Cu based ZSM5 catalyst contained different Mn loadings: 0 wt% Mn and 12 wt% Cu (12Cu), 4 wt% Mn and 12 wt% Cu (4Mn-12Cu), 8 wt% Mn and 12 wt% Cu (8Mn-12Cu), 12 wt%
3.1. NOx removal with catalyst The deNOx performances of the catalysts with different loadings of Mn (0, 4, 8, 12, and 16 wt%) are depicted in Fig. 2. All catalysts exhibit excellent performance in NOx removal, the addition of Mn on Cu/ZSM5 catalyst can promote the reduction of NO, and NO removal efficiency of 12Mn-12Cu catalyst can reach above 90%, showing the best deNOx activity. Meanwhile, A variety of Mn-based catalysts (shown in Table 1), showing the superior catalytic performance, were believe to be an excellent catalyst for the NO removal. For the low temperature NH3-SCR, Mn/Ce/TiW [25], MnCe(29.4)/TNTs [26], Mn-Fe/CeZr [27], Fe0.3Mn0.5Zr0.2 [28], Cu–MnOx/TiO2 [29] and MnOx–CuOx/TiO2 [30] achieved 90–100% NO conversion rate at 120–360 °C. In the combination system of catalysts and plasma, the previous works [31–34] revealed that different Mn-based catalysts showed 58.0–72.4% NO removal efficiency at 25–100 °C in plasma-catalyst system. However, in this study, 12Mn-12Cu/ZSM5 sample exhibited the best performance with about 90% NO removal efficiency at 25 °C. As shown in Fig. 3, NO2 concentration is halved with catalyst compared to that without catalyst. The gas mixture containing NO2 is a more efficient for NOx removal, because the adsorption capability of NO2 is larger than that of NO [35] and NO2 can participate in the fast SCR reaction. NO2 can react fast with NO as well as decompose easily into nitrogen as follows [36–38]:
NO þ NO2 þ 2NH3 ! 2N2 þ 3H2 O
ð3Þ
6NO2 þ 8NH3 ! 7N2 þ 12H2 O
ð4Þ
As shown in Fig. 4. The addition of NH3 did not affect the NO and NO2 concentrations under plasma. However, NO and NO2 concentrations clearly decreased with NH3 and 12Mn-12Cu under plasma.
Table 1 Mn-based catalysts in published literature.
*
Catalyst
Experimental condition
NO removal efficiency
Ref.
Mn/Ce/TiW MnCe(29.4)/TNTs Mn-Fe/CeZr Fe0.3Mn0.5Zr0.2 Cu–MnOx/TiO2 MnOx–CuOx/TiO2 Mn-Ce-Ni/TiO2 Mn-Co-Ox MnOx/TiO2–MWCNT Mn–Co-Ox 12Mn-12Cu/ZSM5
NH3-SCR NH3-SCR NH3-SCR NH3-SCR NH3-SCR NH3-SCR NH3-SCR NH3-SCR NH3-SCR NH3-SCR NH3-SCR
>90%* within 180–300 °C 92%* at 250 °C >90% in 150–300 °C 100% within 200–360 °C 100% at 180 °C 100% within 120–260 °C 58.0% at 50 °C 71% at 125 °C 72.4% at 25 °C 67.0% at 100 °C 90% at 25 °C
[25] [26] [27] [28] [29] [30] [31] [32] [33] [34] This study
The value represents the NOx removal efficiency.
assisted assisted assisted assisted assisted
by by by by by
plasma plasma plasma plasma plasma
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Thus, (R8) and (R9) become the predominant reactions for NO removal. When the catalyst is combined with plasma, the major reaction of SCR will be R1 and R10 [47–49]. The fast SCR reaction (R1) exhibits a reaction rate at least 10 times higher than that of the standard SCR reaction (R10) below 200 °C [50]. As Fig. 4(b) shows, NO2 concentration can reach above 120 ppm without catalyst, whereas it is less than 50 ppm with 12Mn-12Cu. This indicates that almost 70 ppm of NO2 participates in catalytic reaction.
4NH3 þ 4NO þ O2 ! 4N2 þ 6H2 O
ðR8Þ
3.2. XRD results
Fig. 3. Effect of catalyst with different Mn loadings on NO2 concentration.
As Fig. 5 show, when the discharge is 8 W, NO and NOx removal efficiencies are 56 and 35% with NH3, and change to 88% and 79% over 12Mn-12Cu. In the NO/O2/N2/NH3 system with plasma, the active electrons collide with N2 and O2 [39–42]:
e þ N2 ! N þ N þ e
ðR1Þ
e þ O2 ! O þ O þ e
ðR2Þ
e þ O2 ! O þ Oð1 DÞ þ e
ðR3Þ
O þ O2 ! O3
ðR4Þ
Compared to N2, the active electrons react more easily with O2 because the bond dissociation energies of O2 (498.36 kJ mol1) are much lower than those of N2 (945.33 kJ mol1). The obtained radicals O, O3, and N react with NO in the following reduction reactions [43–46]:
N þ NO ! N2 þ O
ðR5Þ
O þ NO ! NO2
ðR6Þ
O3 þ NO ! NO2 þ O2
ðR7Þ
Fig. 6 shows the XRD spectra of all catalysts. No peaks of crystalline phase of manganese oxides were observed on 12Cu, 4Mn-12Cu, 8Mn-12Cu, and 12Mn-12Cu. This demonstrates that well dispersed MnOx exists on the surface of 4Mn-12Cu, 8Mn12Cu, and 12Mn-12Cu catalysts [51] and a homogeneous coverage of the active phase forms as a monolayer [52]. The peaks corresponding to Mn2O3 appeared in 16Mn-12Cu, but only Mn2O3 could be observed to calcine MnOx catalyst up to 500 °C [53]. Qi [54] and Ettireddy [55] also reported that MnOx peak appeared at 15 wt% Mn loading. The presence of amorphous manganese oxide phase in catalysts may be one of the key factors for their excellent catalytic activity in deNOx performance [56,57]. CuO, which showed a peak at 35.5°, was overlapped by the peak of Cu1.4Mn1.6O4 at 35.8°. Copper is present nearly exclusively in the form of mononuclear ions and oligomeric species, which are known to be active in the DeNOx process [58–60]. 3.3. Influence of temperature on discharge characteristics Gas temperature can affect NOx removal efficiency in two major respects: electric field strength divided by the total gas density (E/N) and the reaction rate. Electrostatic field model was performed by finite element modeling according to a previous method [61]. Fig. 7(a) shows the cross-section of the DBD reactor. High voltage is 10 kV connected to the inner electrode, and the voltage of outer electrode is 0 V; the relative permittivities of gas and quartz are 1 and 3.75 respectively. The calculation model was set up based on the actual size of the reactor. The electrostatic field of a quarter of the reactor was studied since the reactor was symmetrical. As shown in Fig. 7(b), on the inner electrode, the E is the strongest and decreases along the radial direction. The total gas density (N) is calculated by Eq. (3) [62]:
Fig. 4. Effect of 12Mn-12Cu/ZSM5 catalyst on NO and NO2 concentration under plasma.
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Fig. 5. Effect of 12Mn-12Cu/ZSM5 catalyst on NO and NOx removal efficiency under plasma.
is high enough [66]. N2 dissociation is 6.04 1018 cm3 s1 at 100 Td, which increases to 1.02 1015 cm3 s1 at 300 Td. As shown in Fig. 8, the gas temperature affects the gas discharge power, when applied voltage keep constant, the discharge power increases with gas temperature. 3.4. NOx removal with different gas temperatures As shown in Fig. 9(a), High temperature leads to much lower deNOx activities compared to those at low temperature. When the discharge power was 10 W, the NO removal efficiency was 60, 57, 55, 51, 41, and 27% at temperatures of 25, 50, 100, 150, 200, and 300 °C, respectively. As stated previously, R8 and R9 are the major reactions for NO removal without catalyst. However, with the accumulation of generated NO2, repression reactions gradually intensify, showing a negative effect on NO removal [67,68]: Fig. 6. XRD patterns of the Mn-Cu/ZSM5 catalyst: (a) 12Cu, (b) 4Mn-12Cu, (c) 8Mn12Cu, (d) 12Mn-12Cu, (e) 16Mn-12Cu.
N¼
P kT
ðR9Þ
where k is the Boltzmann constant, T is the gas temperature, P is the gas pressure. When the gas temperature increases, N decreases, increasing E/N. Fig. 7(c) shows E/N in the reactor at various temperatures, E/N is a main factor affecting discharge in terms of occurrence and performance. The electric field is 113 Td at 25 °C on the inner electrode and increases to 220 Td at 300 °C. The distribution function of electron is calculated by Eq. (4) [63,64]:
@fðr; u; tÞ eE þ u rfðr; u; tÞ þ rv fðr; u; tÞ ¼ C½fðr; u; tÞc ; @t m
ðR10Þ
Keeping the pressure and temperature unchanged, resulting in N unchanged according to Eq. (3). So, E/N increases with an increasing in E. We use BOLSIG+ to calculate the Boltzmann equation in the N2/ NO/O2/NH3 system. As shown in Fig. 7(d). The electron mean energy is 2.356 eV at 100 Td and increases to 6.789 eV at 300 Td. The ratio of energetic electrons also increases with an increasing in E/N [42]. The amounts of active radicals (N and O) are influenced by the electron mean energy and density of energetic electrons [65], and molecular dissociation will be enhanced as the energy of electron
O þ NO2 ! NO þ O2
ðR11Þ
N þ O2 ! NO þ O
ðR12Þ
O þ N ! NO
ðR13Þ
The reaction of O3 consumption is strengthened dramatically when the temperature increases (see Table 2) [69,70]:
O2 þ O3 ! O2 þ O2 þ O
ðR14Þ
O3 þ O ! O2 þ O2
ðR15Þ
As discussed above, the quantities of active radicals can be enhanced by a higher temperature. However, the reaction rate of O and NO (R8) decreases from 3.02 1011 cm3 s1 at 25 °C to 1.85 1011 cm3 s1 at 300 °C (see Table 2). Moreover, the rates of R11-R13 (reactions of NO formation) are 6.25 1012 cm3 s1, 7.66 1017 cm3 s1, and 1.08 1032 cm6 s1, while they change to 9.99 1012 cm3 s1, 2.86 1014 cm3 s1, and 8.72 1033 cm6 s1, respectively, at 300 °C. The rates of R11 and R12 obviously increase, but that of R13 decreases. However, the effect of the decrease in R13 rate can be negligible since it is too small. On the other hand, the reaction rate of R6 decreases as the temperature increases, but those of R14 and R15 increase. Thus, when the temperature increases, the generation of O3 is reduced, but the decomposition of O3 is promoted significantly.
T. Wang et al. / Applied Thermal Engineering 130 (2018) 1224–1232
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Fig. 7. Calculated electric field and distribution function of electron at difference temperatures: (a) sectional drawing of the reactor, (b) distribution of radial electric field, (c) the E/N at different temperatures, (d) calculated dissociation rate coefficient and electron mean energy.
Fig. 8. Discharge power as a function of applied voltage.
In brief, the increase in temperature improves the formation of NO and serves as an obstacle to the oxidation of NO to NO2. As shown in Fig. 10(a), when the temperature increases, NO2 formation is reduced.
The effect of temperature on NOx removal with all catalysts assisted by plasma was studied, and the trend of NO removal efficiency remains consistent with all catalysts. Fig. 9(b) presents the effect of temperature on NO removal with 12Mn-12Cu/ZSM5 catalyst. When the discharge power was 10 W, NO removal efficiency decreased from 90% at 25 °C to 76% at 100 °C and then increased to 97% at 300 °C with 12Mn-12Cu. When the discharge power was 2 W, NO removal efficiency remained around 30% at 25, 50, and 100 °C and increased greatly at 150, 200, and 300 °C. NO removal efficiency decreases as the temperature increases with only plasma. However, the deNOx performance increases above 100 °C with plasma-catalyst system. Bröer [50] reported that the rate coefficients of catalytic NO reduction were 0.12, 1.2, 5.3, 8.1, and 12 s1 at 120, 160, 200, 225, and 250 °C with plasma combined with catalyst (V2O5-WO3/TiO2), indicating that an increased temperature can enhance NO removal between 100 and 250 °C, which is similar to this paper’s conclusion. Plasma reaction can generate NO2 in the N2/NO/O2/NH3 system, and the removal of NOx is mainly caused by reaction R1 around 100 °C [50,36]. As shown in Fig. 10(b), NO2 concentration decreases upon heating with plasma-catalyst system and decreases noticeably at 150 °C. When the temperature dropped below 200 °C, NO2 is very important for the NOx reduction. The rate of R1 is much faster than that of R10 [71]. R1 also consumes large amounts of NO2 below 200 °C. NO2 produced by plasma becomes more important for NOx removal.
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Fig. 9. Effect of temperature on NO removal efficiency, (a) no catalyst, (b) 12Mn-12Cu. Table 2 Major reactions and their rate coefficients. Reactions
No.
Rate coefficients (cm3 T s1/ cm6 s1) 25 °C
100 °C
e þ N2 ! N þ N þ e e þ O2 ! O þ O þ e e + O2 ? O + O(1D) + e O þ O2 ! O3
R3 R4 R5 R6
fðE=NÞ fðE=NÞ fðE=NÞ 4.45 1034
3.30 1034
N þ NO ! N2 þ O
R7
3.42 1011
3.86 1011
O þ NO ! NO2
R8
3.02 1011
2.84 1011
R9
14
14
O3 + NO ? NO2 + O2
2.01 10
Ref.
50 °C
12
3.04 10
12
O þ NO2 ! NO þ O2
R11
6.25 10
6.74 10
N þ O2 ! NO þ O
R12
7.66 1017
1.94 1016
O þ N ! NO
R13
1.08 1032
1.05 1032
O2 + O 3 ? O 2 + O 2 + O
R14
15
3.54 10
15
5.31 10
O3 þ O ! O 2 þ O 2
R15
7.95 1015
1.36 1014
2.04 1034 4.67 1011 2.55 1011 5.90 1014 7.61 1012 8.71 1016 9.89 1033 1.01 1014 3.19 1014
150 °C
1.41 1034 5.40 1011 2.32 1011 9.80 1014 8.34 1012 2.78 1015 9.47 1033 1.66 1014 6.14 1014
200 °C
1.06 1034 6.06 1011 2.14 1011 1.46 1013 8.97 1012 7.05 1015 9.16 1033 2.45 1014 1.03 1013
The rate coefficients were obtained from NIST Chemical Kinetics Database (the rate coefficients of R6 and R13 are cm6 s1).
Fig. 10. Effect of temperature on NO2 concentration, (a) no catalyst, (b) 12Mn-12Cu.
300 °C
6.88 1035
[39] [39] [40] [42]
7.18 1011
[44]
1.85 1011
[46]
2.64 10
13
[44]
9.99 10
12
[67]
2.86 1014
[67]
8.72 1033
[67]
14
[69]
2.20 1013
[70]
4.36 10
T. Wang et al. / Applied Thermal Engineering 130 (2018) 1224–1232
4. Conclusions The effect of the electric discharge and catalytic reaction on NO reduction with Mn-Cu/ZSM5 catalyst was examined at various temperatures. NO removal efficiency is about 60% with only plasma and increases to 90% with 12Mn-12Cu/ZSM5 catalyst. The pretreatment of the mixed gas by DBD plasmas generates a certain amount of NO2, which can enhance NO reduction via the fast SCR reaction. The introduction of Mn on Cu/ZSM5 enhances the catalytic property of catalyst, and 12Mn-12Cu/ZSM5 exhibits the best performance in NOx removal. An increase in temperature leads to a decrease in total gas density (N), but E/N increases; the electron mean energy at 300 Td is 2.9-fold higher compared to that at 100 Td. More energy is transferred from electrons to gas molecules during the collisions, leading to the generation of more active radicals. In the absence of catalyst, the increase in temperature acts as an obstacle to NO removal because O3 reduction, which is the predominant oxidant acting to remove NO, is hindered by the high temperature, and increased temperature improves NO formation. NO removal efficiency decreased as the temperature increased with plasma-catalyst system below 100 °C, while the deNOx performance increased above 100 °C. NO removal efficiency increased dramatically at low discharge power above 150 °C.
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This work was supported by National Natural Science Foundation of China (51706069).
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