NO Removal from Flue Gas Using Conventional Imidazolium-Based

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Environmental and Carbon Dioxide Issues

NO removal from flue gas using conventional imidazolium-based ionic liquids at high pressures Xiaoshan Li, Liqi Zhang, Liwei Li, Yi Hu, Ji Liu, Yongqing Xu, Cong Luo, and Chuguang Zheng Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00154 • Publication Date (Web): 16 Apr 2018 Downloaded from http://pubs.acs.org on April 16, 2018

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NO removal from flue gas using conventional imidazolium-based ionic liquids at high pressures Xiaoshan Li, Liqi Zhang*, Liwei Li, Yi Hu, Ji Liu, Yongqing Xu, Cong Luo*, Chuguang Zheng State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, Hubei Province P.R. China * E-mail: [email protected] (Zhang L.) [email protected] (Luo C.) Tel: +86 27 87542417. Fax: +86 27 87545526.

ABSTRACT

Nitrogen oxide (NO) significantly contributes to environmental pollution problems, such as photochemical smog and acid rain. NO absorption in ionic liquids was an attractive issue but little attention was paid to the removal of NO with low concentration from coal-fired flue gas. In this work, conventional ionic liquids were used as environmentally benign solvents with low cost compared to functional ionic liquids to separate NO from simulated multi flue gas mixtures. The concentration of NO is only 1000ppm. NO removal performance by conventional ionic liquids was investigation on a high pressure reaction system. Results showed that among four conventional imidazolium ionic liquids, [Bmim][OAc] and [Bmim][NO3] presented high efficiency for NO removal. NO removal performance is mainly determined by the oxidation rate of NO to NO2 in the presence of O2 and the absorption capacity of 1

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NO2 in ionic liquids. High pressure was favored to NO removal efficiency. Both the oxidation rate of NO and NO2 absorption in ionic liquids were dramatically increased at elevated pressures. This work also provides interesting possibilities for simultaneous removal of SO2 and NO from flue gas. High-efficient simultaneous removal of SO2 and NO using [Bmim][OAc] was achieved with the removal efficiency of 98.0% and 93.5%.

Keywords: Ionic Liquid; NO; SO2; Removal

1. Introduction The production of nitrogen oxide (NOx) from anthropogenic activities is mainly from the combustion of nitrogen-bearing fossil fuels, such as coals and oil. NOx emission from coal-fired power plants is mainly composed of NO and NO2. Above 90% of NOx is insoluble NO during the process of coal combustion.1 Since NOx is one of the main sources causing photochemical smog and acid rain,2 flue gas denitrification is necessary. Presently, significant efforts are being made to develop several technologies to control NOx emission. Post-combustion NO reduction was studied over several solid SCR (Selective Catalytic Reduction) catalysts and liquid absorption solutions, such as V2O5 or CeO2,3-4 free radicals,5 NaClO2,6 KMnO4,7 etc. Liquid phase oxidation and absorption of NO in absorption solutions including inorganic oxidants and free radicals is a promising option for its advantage of low cost and simultaneous removal of other pollutants.8 Similarly, Pfeiffer and co-workers9,10 suggested that the removal of low-oxidation state gas could be achieved by oxidation and subsequent sorption. Great efforts are still needed for liquid phase absorption of NO from flue gas. As environmentally benign solvents, ionic liquids (ILs) seems to be an attractive 2


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option for gas separation and purification.11-13 It has been extensively reported that ILs showed high solubility for several gases, such as CO2, SO2, CO and Hg.14-21 ILs can be directly used as absorbents without any water, which could reduce the regeneration energy consumption. Moreover, the saturated ILs can be regenerated and reused for many times without secondary pollution. Due to the negligible vapor pressure of ILs, the released gases from the stripper could achieve high purity without any contamination, resulting in low energy consumption and cost.22 Therefore, ILs were considered to be potential candidates in flue gas treatment. It is reported that the typical post−combustion flue gas from coal combustion contains approximately 10−15 vol% CO2, 10 vol% H2O, 0.05−0.2 vol% SO2, 0.15−0.25 vol% NOx and so on.23 Currently, there were several studies focused on the capture and separation of flue gas pollutants by chemical sorbents.24-26 Using ionic liquids for CO2 and SO2 absorption has been well investigated. Since CO2, SO2 and NOx were acidic gases, it is supposed to be feasible to use ILs for NOx removal. It has been reported that Supported Ionic Liquid Phase (SILP) absorbents were used to remove the flue gas components of NO, SO2 and CO2, respectively.27 Mossin and co−workers28 suggested that [Bmim][NO3] could serve as catalysts for the oxidation of NO to nitric acid by oxygen in the presence of water. Duan and co−workers29 used a series of task-specific ILs, caprolactam tetrabutyl ammonium halide, for pure NO and NO2 absorption. Results showed that NOx was physically absorbed in the ILs and the absorption capacity for NO2 was larger than NO. The physically absorbed NOx could be easily released at higher temperatures, allowing the ILs to be recycled without a loss of absorption capacity. Wang and co−workers30 reported that pure NO was largely captured by azole-based ionic liquid through multiple-site chemical absorption. These studies provided a novel application of ionic liquids in the field of 3


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NOx removal. However, previous reports mainly focused on the absorption of NO with high concentration, which greatly differed with the real flue gas atmosphere. Moreover, the used task-specific ILs with high absorption capacity for NO was synthesized complicatedly with high cost. The viscosities of task-specific ILs were generally high, resulting in unsatisfied gas-liquid mass transfer. Due to the low cost and viscosity and high availability of conventional ILs compared to task-specific ILs, four conventional imidazolium ILs were used as absorbents for removal of NOx. We put forward an efficient way to remove NO at real flue gas atmosphere condition using conventional ionic liquids, that is, oxidizing NO to NO2 in the presence of O2 at elevated pressures and then NO2 absorbed in ionic liquids. Effects of pressure and temperature on NO removal performance were investigated. Since the NOx concentration in flue gas is similar with SO2 and ILs also showed high SO2 removal efficiency, the simultaneous removal performance of SO2 and NOx in ionic liquid was also studied, providing a possibility for simultaneous flue gas desulfurization and denitrification. 2. Experimental Section 2.1 Experimental setup NO removal performance by ILs was measured on a high pressure reaction system. The experimental apparatus is presented in Figure 1, including a simulated flue gas system, a high pressure reactor and a flue gas analyzer system. In the simulated flue gas system, 1.0vol% NO/N2, 1.0vol% SO2/N2, O2 and balanced gas N2 with different flow rate were entered into the gas mixer. The flow rate of these gases was controlled by the mass flow meters to prepare the simulated flue gas atmospheres. The initial concentrations of O2, NO and SO2 in mixture gas were set at 5.5%, 1000 ppm and

4


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1500ppm, which was close to real flue gas atmosphere. In the high pressure reactor, the working pressure was regulated by a back pressure regulator and the working temperature was program-controlled. For each test, the high pressure reactor is preloaded with 30g ionic liquids. Four conventional imidazolium ILs, [Bmim][BF4], [Bmim][PF6], [Bmim][NO3] and [Bmim][OAc], with the purity over 99% were purchased from Lanzhou Greenchem ILs, LICP, CAS, China. These conventional ILs were the most common used ILs with low cost compared to the functional ionic liquids. After the pressure increased to a given value, the simulated flue gas was continuously bubbled into ILs. The concentrations of flue gas components in the outlet stream were on-line measured by a flue gas analyzer (Multilyzer STe M60, AFRISO).

Figure 1. Experimental apparatus of NO removal from flue gas by ionic liquids at high pressure

It should be noted that there are serval factors to evaluate the gas sorbents, such as absorption capacity, Henry constant and removal efficiency. For multi component gases, it is inappropriate to determine the absorption capacity and Henry constant for each gas. Moreover, the emission standard for power plants stipulates NOx emission concentration, which is directly related to removal efficiency. It is believed that removal efficiency is a more critical indicator for the investigated multi component 5


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gases containing NO with low concentration. Therefore, removal efficiency was used to evaluate NOx control in this work. The removal efficiencies, η[NO] and η[SO2], were defined as Equation (1) and (2) to evaluate the removal performance. The accumulation removal efficiency during 120min was calculated by integrating the real-time data of NOx and SO2 concentration over the time.

∫ η[ NO] =

t1

t0

t1

t1

Cin [ NO]dt − ∫ Cout [ NO]dt − ∫ Cout [ NO2 ]dt t0

t0

∫

t1

t0

∫ η[ SO ] =

t1

t0

2

× 100%

(1)

Cin [ NO]dt

t1

Cin [ SO2 ]dt − ∫ Cout [ SO2 ]dt t0

∫

t1

t0

×100%

(2)

Cin [ SO2 ]dt

where Cin[NO] and Cin[SO2] refer to the initial concentration of NO and SO2 in simulated flue gas; Cout[NO], Cout[NO2] and Cout[SO2] refer to the outlet concentration detected by the flue gas analyzer after absorption in ILs. 2.2 Characterization The thermal stabilities of four ILs were analyzed by Thermal Gravimetric Analysis (TGA). The thermal decomposition properties of ILs were characterized on the STA 449F3 NETZSCH thermogravimetric analyzer. The samples were heated to 600°C in N2 atmosphere and the heating rate was 10 °C/min. The absorption mechanism was analyzed by Fourier transform infrared spectroscopy (FTIR). ILs before and after absorption were characterized on a FTIR spectrometer (Bruker, VERTEX 70). 2.3 Quantum chemical calculation To investigate the interaction between ILs and NOx, a theoretical calculation based on DFT method were conducted through the Gaussian 09 program.31 Since the investigated four ILs have the same imidazolium cation, the calculations were mianly focused on the complexes of different anions with NO and NO2. All the possible initial configurations were considered, among which the configuration with the lowest 6


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energy was the final optimal structure according to the energy minimization method. Geometry optimization and vibrational frequency calculation were performed using the

density

functional

dispersion

correction

(DFT-D)

method32

at

the

B3LYP-G3/6-31+G(d,p) level. Moreover, the basis set superposition error (BSSE) was corrected using the counterpoise method33 when calculating the interaction energies. 3. Results and Discussion 3.1 NO removal performance by conventional ILs Since the real flue gas contains approximatively 5 vol% O2, the addition of O2 in the simulated gas stream was necessary. NO removal by four conventional ILs, [Bmim][BF4], [Bmim][PF6], [Bmim][NO3] and [Bmim][OAc], was studied at the atmosphere of 1000ppm NO/5.5% O2/N2. As presented in Figure 2, after the absorption and separation by these ILs, the outlet NO concentration was significantly reduced to less than 50ppm. The outlet NO2 concentration increased rapidly at the beginning 30min and then increased slowly until reaching stabilization. The reason for the decreased NO concentration and increased NO2 concentration is that a certain amount of NO was oxidized to NO2 in the presence of O2 under the pressure of 1.0MPa. However, it was obvious that the total outlet NOx concentration was much lower than the inlet value. This indicated that a large amount of NOx was absorbed in these conventional ionic liquids. Therefore, conventional ionic liquids could be used as NOx removal regent from real flue gas containing O2 at elevated pressure.

7


Energy & Fuels

1000 NO NO2

400

(a) [Bmim][BF4]

Outlet concentration (ppm)


500

300 200 100 0

(b) [Bmim][PF6]

NO NO2

800 600 400 200 0

0

20

40

60

80

100

120

0

20

40

t (min)

60

80

100

120

t (min)

500

500

(c) [Bmim][NO3]

NO NO2

400

300

200

100



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(d) [Bmim][OAc]

NO NO2

400

300

200

100

0

0 0

20

40

60

80

100

0

120

20

t (min)

40

60

80

100

120

t (min)

Figure 2 NO removal by 4 conventional ILs (a. [Bmim][BF4], b.[Bmim][PF6], c. [Bmim][NO3] and d. [Bmim][OAc]) at the atmosphere of 1000ppm NO/5.5% O2/N2 (80°C, 1.0MPa)

Figure 3 shows the NO removal efficiency of four conventional ILs at the atmosphere of 1000ppm NO/5.5% O2/N2 under 1.0 MPa. The NO removal efficiency followed the order: [Bmim][OAc] > [Bmim][NO3] > [Bmim][BF4] > [Bmim][PF6]. Since these ILs have the same cation [Bmim]+, the anion of ILs played an important role in NOx removal. Among four investigated conventional ILs, [Bmim][OAc] and [Bmim][NO3] presented good NO removal performance with the efficiency above 80%.

8


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NO removal efficiency (%)

100

80

[Bmim][OAc] [Bmim][NO3]

60

[Bmim][BF4]

40 [Bmim][PF6] 20

0

ILs

Figure 3 NO removal efficiency of 4 conventional ILs at the atmosphere of 1000ppm NO/5.5% O2/N2 (80°C, 1.0MPa)

In addition, the NO removal performance by these ILs at the atmosphere of 1000ppm NO/N2 without O2 was also studied at 1.0MPa. Figure 4 shows the outlet NO concentration during the absorption in four ILs. It can be seen that the outlet NO concentration rapidly increased to the inlet level at the beginning of absorption, indicating that these four ILs performed poorly in NO removal without the presence of O2 even at elevated pressure. Therefore, the presence of O2 in the simulated gas atmosphere is one of the key conditions for efficient NOx removal. Since O2 can oxidize the low reactive NO to NO2 at elevated pressures, it seemed that NO2 can be easily absorbed in ionic liquids than NO. Outlet NO Concentration (ppm)

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Energy & Fuels

1000 800 600

[Bmim][BF4] [Bmim][PF6]

400

[Bmim][NO3] [Bmim][OAc]

200 0 0

20

40

60

80

100

120

t (min)

Figure 4 NO removal by 4 ILs at the atmosphere of 1000ppm NO/N2 (80°C, 1.0MPa)

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The interaction between ILs and NOx was studied based on a DFT method. The optimal configurations of ILs−NO and ILs−NO2 calculated at B3LYP-G3/6-31+G(d,p) level were presented in Figure 5. It can be seen that the investigated ILs showed physical interaction with NO and NO2 and there is no transition state for these complexes. The interaction energies and absorption enthalpies of ILs−NO and ILs−NO2 were listed in Table 1. The interaction energies and absorption enthalpies also followed the order: [OAc]− > [NO3]− > [BF4] − > [PF6] −, which is in good agreement with the order of removal efficiency in Figure 3. In addition, the interaction energies and absorption enthalpies of ILs−NO2 were larger than those of ILs−NO, indicating that NO2 showed stronger interactions with ILs than NO. This is mainly because the acidity and reactivity of NO2 is higher than NO. Therefore, ILs prefer to interact with NO2 rather than NO. The weak interaction between ILs and NO may explain the unfavorable NO removal performance at the atmosphere without O2. It has been reported that a large amount of NO was oxidized to NO2 by O2 at high pressures.34 NO2 can be easily absorbed in ILs due to their strong interaction, lowering the outlet NOx concentration and resulting in enhanced NO removal performance. Thus, it can be concluded that NOx removal is mainly determined by the oxidation rate of NO to NO2 and NO2 absorption in ILs. The presence of O2 and high pressure are both required for NO removal by ionic liquids.

Figure 5 Optimal configurations of [BF4]−NO, [PF6]−NO, [NO3]−NO, [OAc]−NO (upper) and

10


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[BF4]−NO, [PF6]−NO, [NO3]−NO, [OAc]−NO (bottom) calculated at B3LYP-G3/6-31+G(d,p) level. Table 1 The interaction energies and absorption enthalpies of ILs−NO and ILs−NO2 obtained from quantum chemical calculation. ILs−NO

ILs−NO2

ILs ∆E (kJ/mol)

∆H(kJ/mol)

∆E(kJ/mol)

∆H(kJ/mol)

[BF4]−

−5.9

−9.1

−14.8

−18.7

[PF6]−

−4.3

−8.0

−13.2

−17.7

[NO3]−

−15.1

−19.0

−29.3

−33.8

[OAc]−

−31.7

−36.4

−55.6

−61.0

3.2 Effect of pressure on NO removal. Since the pressure played a major role in determining the NO oxidation, it is essential to investigate the NO removal performance by ILs under different pressures. Firstly, dry gas oxidation of NO to NO2 at the atmosphere of 1000ppm NO/5.5% O2/N2 was studied under pressure from 0.1 to 2.0Mpa. In the dry gas experiment, ILs were not loaded in the high pressure reactor. In Figure 6, the total NOx concentration was constant under different pressures. It can be found that NO concentration decreased and NO2 concentration increased with pressure. This is due to the fact that the increase in pressure enhanced the oxidation rate of NO to NO2. At pressure above 1.0MPa, NO and NO2 concentration remained unchanged because the upper limit of NO oxidation rate was reached. The investigation on NO removal performance using [Bmim][OAc] and [Bmim][NO3] under different pressures at the atmosphere of 1000ppm NO/5.5% O2/N2 was performed in the high pressure reaction system. In comparison with dry gas experiment, the total NOx concentration after passing through [Bmim][OAc] and [Bmim][NO3] largely decreased as presented in Figure 7. The difference in the NOx 11


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concentrations between dry gas experiment and gas-liquid absorption demonstrated that the absorption of NO in ILs was effective. Moreover, the outlet concentration of both NO and NO2 decreased largely with the increasing pressure. Effect of pressure on NO removal efficiency by [Bmim][OAc] and [Bmim][NO3] was presented in Figure 8. Obviously, the increase in pressure enhanced NO removal performance. At atmospheric pressure, the overwhelming majority of outlet gas stream is NO. High pressure not only promoted the oxidation rate of NO, but also improved NO2 absorption capacity in ILs. [Bmim][OAc] and [Bmim][NO3] presented high NO removal efficiency at elevated pressures above 1.0MPa. In comparison with dry gas oxidation outlet concentration, the NO concentration decreased only a little after absorption in ILs due to the weak interaction. However, the NO2 concentration after absorption in ILs was significantly reduced, indicating that NO2 absorption in ILs contributed the most to NOx removal performance under high pressure. Thus, the promotion of NO removal efficiency at elevated pressure was mainly attributed to the enhanced oxidation of NO and NO2 absorption capacity in ILs. 1200

100

1000

NO NO2

800

NO oxidation rate (%)


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Total NOx

600 400 200

80

60

40

20 0 0.0

0.5

1.0

1.5

0.0

2.0

0.5

1.0

1.5

2.0

Pressure (MPa)

Pressure (MPa)

Figure 6 Dry gas oxidation of NO to NO2 at the atmosphere of 1000ppm NO/5.5% O2/N2 under pressure from 0.1-2.0Mpa.

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1000

1000 (a) [Bmim][OAc]

(b) [Bmim][NO3]

NO NO2

800



Total NOx 600 400 200 0 0.0

0.5

1.0

1.5

NO NO2

800

Total NOx 600 400 200 0

2.0

0.0

Pressure (MPa)

0.5

1.0

1.5

2.0

Pressure (MPa)

Figure 7 NOx outlet concentration using [Bmim][OAc] and [Bmim][NO3] under different pressures at the atmosphere of 1000ppm NO/5.5% O2/N2. 100 Outlet concentration (ppm)

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Energy & Fuels

80 60 [Bmim][OAc] [Bmim][NO3]

40 20 0 0.0

0.5

1.0 1.5 Pressure (MPa)

2.0

Figure 8 Effect of pressure on NO removal efficiency by [Bmim][OAc] and [Bmim][NO3] at the atmosphere of 1000ppm NO/5.5% O2/N2.

3.3 Effect of temperature on NO removal. The thermal stabilities of four conventional ILs were investigated through TGA method, shown in Figure 9. [Bmim][NO3], [Bmim][BF4] and [Bmim][PF6] presented good thermal stabilities with the onset temperatures above 280 °C. Although the onset temperature was approximately 190 °C, [Bmim][OAc] started to decompose at 90 °C. The thermal decomposition of [Bmim][OAc] was also observed by Clough and co-workers.35 They suggested that the acetate-based ionic liquids decompose mainly through SN2-type nucleophilic substitution mechanisms. Thus, the operating temperature of NO removal using [Bmim][OAc] was supposed to be lower than 80 °C 13


Energy & Fuels

due to the low thermal stability. The effect of temperature on NO removal by [Bmim][OAc] and [Bmim][NO3] was studied at temperatures ranging from 25 to 80 °C, as shown in Figure 10. For [Bmim][OAc], the change of NO removal efficiency at temperatures from 25 to 80 °C was relatively small, which could remain above 80%. For [Bmim][NO3], temperature have a negative influence on the removal efficiency, but still above 70%. Nevertheless, in comparison with pressure, the temperature had a relatively small influence on NO removal. The denitrification system using ILs presented a comparatively wide range of operation temperature (25~80 °C), which can be well adapted to the real flue gas condition. 120 [Bmim][BF4]

Weight loss (wt%)

100

[Bmim][PF6] [Bmim][NO3]

80

[Bmim][OAc]

60 40 20 0 0

100

200

300

400

500

600

700

Temperature (oC)

Figure 9 TGA curves of 4 ILs (a) [BMIM][OAc]

100

NOx removal efficiency (%)

100

NOx removal efficiency (%)

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90 80 70 60 50 20

40

60

(b) [Bmim][NO3]

90 80 70 60 50 20

80

30

Temperature (oC)

40

50

60

70

80

Temperature (oC）

Figure 10 Effect of temperature on NO removal efficiency by (a) [Bmim][OAc] and (b) [Bmim][NO3] at the atmosphere of 1000ppm NO/5.5% O2/N2.

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3.4 Effect of H2O on NO removal. Since [Bmim][Ac] was highly hydrophilic, H2O vapor in the flue gas is easily absorbed into ILs. To investigate the effect of H2O on NO removal performance, a certain amount of water was added into ILs prior to experiment. The effect of H2O on NO removal by [Bmim][OAc] was studied at 25 °C and 1.0MPa, as shown in Figure 11. The outlet concentration of NO2 in wet condition was lower than that in dry condition because partial NO2 reacted with H2O. The presence of H2O could enhance the removal efficiency, but also lead to the formation of HNO3. Figure 12 shows FTIR spectrum of [Bmim][OAc] before and after treatment in water and 1000ppm NO/5.5% O2/N2. During NO removal in the presence of water, some new peaks appeared at 829 cm−1, 1337 cm−1 and 1716 cm−1. The appearance of characteristic bands at 829 cm−1 was assigned to the formed HNO3 due to the reaction of NO2 with H2O. The characteristic bands at 1337 cm−1 and 1716 cm−1 was assigned to NO3− and CH3COOH, indicating that [Bmim][OAc] reacting with HNO3 would produce [Bmim][NO3] and acetic acid. After the regeneration of saturated ILs, acetic acid could be removed and [Bmim][NO3] could continue to absorb NOx.

NOx outlet concentration (ppm)

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Energy & Fuels

NO NO2 NO-H2O NO2-H2O

400 300 200 100 0 0

100

200

300

400

500

t/min

Figure 11 The effect of H2O on NO removal by [Bmim][OAc] at the atmosphere of 1000ppm NO/5.5% O2/N2 (25°C, 1.0MPa).

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NO3_ CH3COO_ HNO3

CH3COOH after 48h

after 24h before

600

800

1000 1200 1400 1600 1800 2000 wavenumber (cm-1)

Figure 12 FTIR spectrum of (a) [Bmim][OAc] before absorption, (b) [Bmim][OAc] after treatment in water and 1000ppm NO/5.5% O2/N2 for 24h, (c) [Bmim][OAc] after treatment in water and 1000ppm NO/5.5% O2/N2 for 48h.

3.5 The reusability performance One of the advantages of ILs as promising absorbents is that ILs can be reused and recycled for many times. The saturated ILs can be regenerated by heating and vacuum treatment and the regeneration consumption is lower than that of other volatile solution. The reusability performance of [Bmim][OAc] for NOx removal has been investigated for two absorption-desorption cycles, as presented in Figure 13. NO absorption

is

conducted

at

1.0MPa

and

40°C

at

the

atmosphere

of

1.0%NO/5.5%O2/N2 and the desorption was performed at 80°C under vacuum. For the reusability experiment, NO with the concentration of 1.0% was used rather than 1000ppm in order to increase the partial pressure of NOx and promote the absorption rate. As can be seen, the absorption for 1.0% NO lasted for 90 hours to obtain equilibrium state. The absorbed NOx in [Bmim][OAc] can be easily stripped out by heating at 80 °C under vacuum. After the first absorption-desorption cycle, [Bmim][OAc] maintained 80% of the initial absorption capacity during the second absorption. Therefore, [Bmim][OAc] possessed a good reusability performance for 16


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NOx removal. 0.30

0.30

First cycle

Weight increase (g NOx/g IL)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Second cycle

0.25

0.25

0.20

0.20

0.15

0.15

0.10

0.10

0.05

0.05

0.00

0.00 0

20

40

60

80

100 120

0

20

40

60

80

100 120

time (h)

time (h)

Figure 13 The reusability performance of [Bmim][OAc] for NOx removal

3.6 Simultaneous removal of SO2 and NOx Simultaneous removal of SO2 and NOx is regarded as a valuable approach due to the high efficiency and low investment in coal-fired power plants.36-38 It has been reported that ILs can be used as absorbents to separate SO2 from gas mixtures.39,40 Since the concentration of SO2 is comparative to NOx in flue gas, the co-capture using ILs should be possible to achieved. Simultaneous removal of SO2 and NO by [Bmim][OAc] was investigated at 25°C and 1.0MPa. Although the effect of temperature on NO removal was slight at temperatures below 80°C, while the absorption capacity of SO2 in ILs remarkably decreased with the increasing temperature.39 Large amount of the absorbed SO2 in [Bmim][OAc] was released at high temperature. 40 Thus the low temperature at 25°C was the appropriate condition for efficient co-capture of SO2 and NO. Figure 14 shows the outlet concentration of flue gas components during the simultaneous removal of SO2 and NO by [Bmim][OAc] at the atmosphere of 1500ppm SO2/1000ppm NO/5.5% O2/N2. The outlet concentration of SO2 and NOx was quite lower than the initial concentration. Both SO2 and NOx were effectively 17


Energy & Fuels

separated from the flue gas mixtures using [Bmim][OAc]. The removal efficiency of SO2 was 98.0%, mainly due to the strong interaction between [Bmim][OAc] and SO2. As mentioned above, high-efficient NO removal was due to the high oxidation rate of NO and NO2 absorption in ILs. It is interesting that NO removal performance was enhanced with the efficiency increasing to 93.5% in the presence of SO2, meaning that SO2 can promote the removal of NO. It can be seen in Table 2 that the outlet concentration of NO was lower than NO2 at the atmosphere of without SO2. Conversely, NO concentration increased and NO2 concentration decreased in the presence of SO2. This is because SO2 reacts with NO2 to form NO and SO3 under high pressure.41 NO concentration (ppm)

200

10 5 0 0

100

200 300 t/min

400

160 120 80 40 0 0

500

100

200

300

400

500

t/min

200

200

160 120 80 40 0 0

100

200

300

400

500

NO2 concentration (ppm)

O2 concentration (%)

15

SO2 concentration (ppm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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160 120 80 40 0 0

100

t/min

200

300

400

500

t/min

Figure 14 Outlet concentration of flue gas during the simultaneous removal of SO2 and NO by [Bmim][OAc] at the atmosphere of 1500ppm SO2/1000ppm NO/5.5% O2/N2 (25°C, 1.0MPa).

The dry gas experiment without using ionic liquids also confirmed the reaction between SO2 and NO2 in Table 2. For the atmosphere of 1000ppm NO/1500ppm SO2/5.5% O2/N2 under 1.0MPa, the concentration of SO2 decreased from 1500 to 740 18


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ppm. By comparing the outlet gas concentration at the atmosphere of 1000ppm NO/5.5% O2/N2 and 1000ppm NO/1500ppm SO2/5.5% O2/N2, NO concentration increased a little while NO2 concentration decreased in the presence of SO2. However, the reaction between SO2 and NO2 was minor and insufficient. Table 2 Outlet concentration of SO2 and NOx with and without [Bmim][OAc] under 1.0 MPa. Outlet concentration Atmosphere

T/°C

ILs /g NO/ppm

NO2/ppm

SO2/ppm

NO/O2/N2a

25

0

30

1030

0

NO/SO2/O2/N2b

25

0

80

950

740

NO/O2/N2a

25

30

20

130

0

NO/SO2/O2/N2b

25

30

52

13

30

a refers to the atmosphere of 1000ppm NO/5.5% O2/N2; b refers to the atmosphere of 1000ppm NO/1500ppm SO2/5.5% O2/N2.

3.7 Removal mechanism. To investigate the removal mechanism, the FTIR spectroscopy was used to characterize the structural change of ILs before and after absorption. Figure 15 shows the FTIR spectra of fresh [Bmim][OAc], [Bmim][OAc] after NOx absorption and [Bmim][OAc] after NOx and SO2 absorption. The FTIR spectra of [Bmim][OAc] was similar to that of [Bmim][OAc] after absorption, indicating that NOx was physically absorbed in [Bmim][OAc] without any changes on the molecular structure. However, new peaks appeared on [Bmim][OAc] after NOx and SO2 absorption. Since the interaction between [Bmim][OAc] and NOx was physical, the new product can be attributed to the chemical interaction between [Bmim][OAc] and SO2. The appearance of characteristic bands at 1271 and 1109cm−1 can be assigned to the vibrational mode of [HSO3]−. In the investigation of Lee,40 the acetate anion reacts with SO2 in the presence of H2O to form [HSO3]− and acetate acid. The interaction between 19


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

[Bmim][OAc] and SO2 was also calculated based on the density functional theory. The interaction energy and absorption enthalpy of [Bmim][OAc]−SO2 was -114.8 and -124.2 kJ/mol which was much higher than that of [Bmim][OAc]−NOx, confirming the strong bonding of SO2 with [Bmim][OAc].

c

b a 600

800

1000 1200 1400 1600 1800 2000

Wavenumber /cm-1

Figure 15 FTIR spectra of (a) fresh ILs [Bmim][OAc], (b) [Bmim][OAc] after NOx absorption and (c) [Bmim][OAc] after NOx and SO2 absorption.

Thus combined with dry gas reactions under high pressure, all the possible reactions involved in ILs-NOx-SO2 system was presented in Figure 16. NOx and SO2 removal reactions were rather complex when water vapor was presented in the system, including gas phase reactions and gas-liquid absorption. In the dry gas phase reactions, the oxidation of NO to NO2 was enhanced at high pressures and the total NOx is mostly composed of the formed NO2 at pressures above 1.0MPa. The reaction of SO2 with NO2 was minor and partial NO2 was converted to insoluble NO when reacting with SO2. NO2 would react with water vapor in flue gas to form HNO3 in the gas phase. SO2 can be also oxidized to SO3 in the presence of O2 and then formed H2SO4. In the gas-liquid reactions, NO and NO2 were physically associated with ILs while SO2 with relatively high acidity was chemically bonded with ILs.

20


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Energy & Fuels

Figure 16 The reaction pathway of ILs-NOx-SO2 system at high pressure (ILs1, ILs2 and ILs3 refer to [Bmim][OAc], [Bmim][NO3] and [Bmim][HSO3]).

When H2O was presented in flue gas, the formed NO2 at high pressure would react with H2O to form HNO3 in the gas phase. [Bmim][OAc] could be converted to [Bmim][NO3] when [Bmim][OAc] contact with HNO3. For [Bmim][NO3], the formed HNO3 in ILs could be easily stripped out by a simple evaporation at the desorption condition. Then [Bmim][NO3] could continue to absorb NOx. It should be noted that [Bmim][OAc] may not be optimal ILs for NO removal, but these results should be sufficient to draw a conclusion that high-efficient removal of NO could be achieved by conventional ILs at elevated pressure. 4. Conclusions In summary, conventional ionic liquids were used as environmentally benign solvents to separate NOx from multi flue gas components at elevated pressures. NO could be high-efficiently removed through the dry gas oxidation and liquid phase absorption. Among the investigated four ILs, [Bmim][OAc] and [Bmim][NO3] showed high-effective removal for NOx. The removal of NO was mainly determined by the oxidation rate of NO and NO2 absorption capacity in ILs. Both the oxidation 21


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

rate of NO and NO2 absorption in ionic liquids were dramatically increased under high pressure. The most appropriate operating pressure was suggested to be 1.0MPa which can be easily achieved. In addition, NO removal efficiency was favored at low temperatures. In the simultaneous removal of SO2 and NO process, 98.0% SO2 and 93.5% NO were removed by [Bmim][OAc] under 1.0MPa and 25°C. NO and NO2 were physically associated with [Bmim][OAc] while SO2 with relatively high acidity was chemically bonded with [Bmim][OAc]. It should be noted that NO2 would react with water vapor to form HNO3 in the gas phase. [Bmim][OAc] could be converted to [Bmim][NO3] when [Bmim][OAc] contact with HNO3. For [Bmim][NO3], the formed HNO3 in ILs could be easily stripped out by a simple evaporation at the desorption condition. Therefore, high-efficient NOx removal could be achieved by conventional ILs with good reusability at elevated pressure.

22


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Energy & Fuels

ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (Zhang L.) [email protected] (Luo C.) Tel: +86 27 87542417. Fax: +86 27 87545526. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors are grateful for the financial support by the International Science & Technology Cooperation Program of China (2016YFE0102500) and the Fundamental Research Funds for the Central Universities (2016YXZD007). The authors also acknowledge the extended help from the Analytical and Testing Center of Huazhong University of Science and Technology.

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NO Removal from Flue Gas Using Conventional Imidazolium-Based

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