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Absorption of NO and Simultaneous Absorption of SO2/NO Using Vacuum Ultraviolet Light/Ultrasound/KHSO5 System Yangxian Liu, Wen Xu, Liang Zhao, Yan Wang, and Jun Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01274 • Publication Date (Web): 26 Sep 2017 Downloaded from http://pubs.acs.org on September 30, 2017

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Absorption of NO and Simultaneous Absorption of SO2/NO Using Vacuum Ultraviolet Light/Ultrasound/KHSO5 System a

Yangxian Liu* , Wen Xu a, Liang Zhaob, Yan Wanga, and Jun Zhangb a. School of Energy and Power Engineering, Jiangsu University, Zhenjiang, Jiangsu 212013, China b. Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, Southeast University, Nanjing, 210096, People’s Republic of China ABSTRACT: Absorption of NO from flue gas using vacuum ultraviolet light (VUV)-activated KHSO5 solution in the presence of ultrasound (US) in a VUV-US reactor was studied. The influencing factors, active species, products and mechanism of NO removal were investigated. The results indicate that 185 nm is the most effective light wavelength for NO removal. US enhances NO removal due to the enhancement of mass transfer and chemical reaction (low-frequency is more effective than high-frequency). NO removal efficiency increases at higher KHSO5 concentration, light intensity or ultrasonic power density. Solution pH and temperature have obvious double effect on NO removal. The key active species such as ozone, hydroxyl radicals and sulfate radicals were successfully captured. VUV/US/KHSO5 coupling system had the highest free radical yield and NO removal efficiency. NO removals by oxidations of free radicals and O·/O3 are the main removal routes. Simultaneous absorption of SO2/NO and potential applications of removal process were also discussed initially. Keywords: NO removal; Simultaneous absorption of SO2 and NO; Vacuum ultraviolet light (VUV)-activated KHSO5; Ultrasound (US); Free radicals.

1. Introduction SO2 and NOx are the precursors of photochemical smog, acid rain and fine aerosols, thus their emissions have great harm for human health and environment.1-2 Currently, alkali-based wet flue gas desulfurization (alkali-WFGD) process and NH3 selective catalytic reduction (NH3-SCR) denitrification process are widely used

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for SO2 and NOx removals in coal-fired boilers.1-2 This kind of hierarchical processing strategy for SO2 and NOx removals has complex systems.1,4 In China, there are a large number of small and medium-sized coal-fired boilers and industrial furnaces (over 800000).5 The hierarchical processing strategy for SO2 and NOx are not suitable for these small and medium-sized combustion equipments due to complex systems (some industrial furnaces can not even install SCR denitrification technique due to the harsh restriction of process or technology itself).5 Due to compact unit and low costs, simultaneous removal technology of SO2 and NOx was recognized as a kind of promising flue gas purification technology for small and medium-sized combustion equipments.4-5 Many SO2 and NOx simultaneous removal technologies have been developed, 6-22 and can be classified into dry and wet two categories. The dry method mainly includes plasma removal, catalytic oxidation, photochemical removal, ozone removal, adsorption removal, etc.7-14 The wet method mainly includes reduction-absorption, complex-absorption, conventional oxidation-absorption, free radical oxidation-absorption, etc.15-22 These techniques have shown good prospects in the laboratory or pilot stage, but they can not achieve large-scale applications yet because of one or more disadvantages in system reliability, investment and operating costs, secondary pollution, simultaneous removal efficiency of multi-pollutants, etc.4-6 With the increasing pressure in environmental protection, developing new flue gas simultaneous removal technologies has important significance. Ultraviolet light-activated H2O2 technologies have been used for removing SO2, NOx and Hg0 from flue gas,4,5,23 which has shown a good prospect due to several advantages in efficient simultaneous removal of multi-pollutants and environmentally friendly process (products are recyclable and removal process has no secondary pollution). However, our previous results4,5,23,24 demonstrate that the existing ultraviolet light-activated H2O2 removal technologies still have the following two major shortcomings, which hinder its industrial application: (1) it is well known that common industrial grade hydrogen peroxide solution contains water of 72.5%. The reaction products will be very dilute sulfuric acid and nitric acid mixed solution because adding

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hydrogen peroxide will inevitably bring in a lot of water at the same time. This will lead to huge costs and energy consumption in concentrating the products. (2) In actual water and coal-fired flue gas, various impurities, dust or dirt are widely present. These impurities, dust or dirt may attach to the surface of quartz tubes, and hinder the effective transmission of ultraviolet light, which will affect the system operation and security. Some results24-28 have recently shown that ultraviolet light can also activate KHSO5 (the main active ingredient of Oxone, with the chemical formula of 2KHSO5·KHSO4·K2SO4) to produce highly reactive species, -

including hydroxyl radicals (·OH) and sulfate radicals (SO4 ·) to degradate organic pollutants in wastewater, which shows good prospects. KHSO5 is a watersoluble, environmentally friendly, safe to handle and stable oxidant. Especially as a solid oxidant, it has a big advantage in concentrating the reaction products (when solid oxidizer is added, no water is carried), and is also much easier to store and transport than commercial H2O2 solution3,6,29. Adewuyi et al.3 developed the absorption-oxidation process of NO induced by aqueous solutions of KHSO5 in a bubble column reactor, and studied the process parameters and NO absorption rates. Recently, we reported the simultaneous removal of NO and SO2 using aqueous KHSO5 with synergic activation of Cu2+/Fe3+ and high temperature in an impinging stream reactor.6 However, in the present works, we find that the oxidizing ability of UV light-activated KHSO5 to NO is much stronger than that of KHSO5 alone or with activation of Cu2+/Fe3+ and high temperature, which can be seen from the Figure 10. Therefore, this result drives us to study removal of NO and SO2 using UV light-activated aqueous KHSO5. Ultrasonic technology has been widely used in the area of organic pollutants degradation, synthetic and cleaning industries.30-34 Considering the pollution and fouling problem of quartz tube surface of UV lamp, the authors propose to add ultrasonic cavitation in the KHSO5 solution to keep the quartz tube surface clean. As a coupling removal process, studying the additional effects of ultrasonic cavitation for removal process is very necessary (in fact, ultrasonic cavitation has been proved to have a significant impact on mass transfer and

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chemical reaction32-34). In addition, our previous results4,5,23,24 mostly used shortwave ultraviolet light of 254nm to produce ·OH to oxidize SO2, NOx and Hg0. However, shortwave ultraviolet light of 254nm can not excite O2 and H2O to produce reactive species O3, ·O and ·OH (both O2 and H2O are the main components of flue gas and solution, which are also good free radical sources). The present results from Figure 2 show that due to higher free radical yields and the generation of O3, vacuum ultraviolet light of 185 nm has higher NO removal efficiency than 254 nm. Based on the above several ideas, the authors propose a novel removal process of SO2 and NO2 from flue gas using vacuum ultraviolet light (VUV)/ultrasound (US)/KHSO5 coupling system, and investigate the fundamental issues in the removal process. Both SO2 and NO2 are easy to remove by solution absorption because they have very high solubility in water. As the main component of nitrogen oxides, NO often accounts for more than 90-95% of NOx in typical coal-fired flue gas, and is quite difficult to remove due to its low solubility in water. Therefore, the main purpose of this manuscript is to study the fundamental issues of NO removal, which are as follows: (1) to examine the feasibility of this new removal process in a VUV-US reactor; (2) to study the effects of key influencing factors (e.g., light wavelength, ultrasonic power density, light intensity, ultrasonic frequency, KHSO5 concentration, solution temperature and pH value) on NO removal; (3) to detect the main active species and products; (4) to reveal the main mechanism and pathways (removal route) of NO removal. In addition, the feasibility of simultaneous removal of NO and SO2, and industrial amplification ideas of device and technical route are also tested and proposed initially. These results will provide important theoretical guidance for the follow-up studies and the industrial applications of this new removal technology.

2. Experimental Section 2.1 Experimental device As described in Figure 1, the experimental device mainly includes a simulated flue gas blending device, a

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temperature regulating device, a vacuum ultraviolet light-ultrasound bubbling reactor (VUV-US reactor) and an analysis device, which is shown in Figure 1. The simulated flue gas blending device includes CO2, O2, SO2, NO, N2 steel cylinders (1-5), five flowmeters (6-10), a flue gas mixer (11) and two flue gas valves (12-13). The temperature regulating device includes a thermometer (14), an ultrasonic reactor with temperature-controlled (19), a glass cooling coil (20) and a refrigeration device (21); The VUV-US reactor includes a VUV lamp and quartz tube (16), a bubbling reactor (17) (8.5 cm i.d. and 40 cm length; Glass), a reactor lid (15), a flue gas bubbler (18) and an ultrasonic reactor (19); The analysis device includes a flue gas analyzer (22) and an exhaust absorption pipe (23).

(1-5) CO2, O2, SO2, NO, N2 Steel Cylinder; (6-10) Flowmeters; (11) Flue Gas Mixer; (12-13) Flue Gas Valves; (14) Thermometer; (15) Reactor Lid; (16) UV Lamp and Quartz Tube; (17) Bubbling Reactor; (18) Flue Gas Bubbler; (19) Ultrasonic reactor with temperature-controlled; (20) Glass cooling coil; (21) Refrigeration device; (22) Flue Gas Analyzer; (23) Exhaust Absorption Pipe Figure 1. Flow-process diagram of experiment device 2.2 Experimental method Containing-CO2/O2/NO/SO2/N2 simulated flue gas is produced by regulating the CO2, O2, SO2, NO, N2 steel cylinders and flowmeters. Import concentrations of CO2, O2, SO2 and NO in flue gas are first measured using the 5


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flue gas analyzer through the bypass line (the line with flue gas valve 13). 500mL of KHSO5 solution is produced by Oxone (2KHSO5 ·KHSO4·K2SO4) reagent (AR, Sinopharm Chemical Reagent Co., Ltd.) and deionized water. The pH value of solution is adjusted by combining use of HCl/NaOH solution and a pH meter. The prepared KHSO5 solution is added to the VUV-US reactor by opening the reactor lid. The temperature of KHSO5 solution is adjusted to the required values by the combining use of ultrasonic reactor with temperature-controlled, thermometer, glass cooling coil and refrigeration device. The containing-CO2/O2/NO/SO2/ N2 simulated flue gas begins to enter the VUV-US reactor to carry out a gas-liquid reaction. The export concentrations of NO and SO2 in tail gas are measured using the flue gas analyzer through the outlet of the reactor. Each measurement period is kept for 30min, and the average concentration within 30min is used as export concentrations of NO and SO2. The containing-NO/SO2 flue gas will be further purified using exhaust absorption pipe. 2.3 Detection and analysis methods The concentrations of SO2, NO and O2 in flue gas were measured by electrochemical sensors in flue gas analyzer (MRU-VARIO PLUS, Germany). The concentration of CO2 in flue gas was measured by non dispersive infrared sensors in flue gas analyzer (MRU-VARIO PLUS, Germany). The concentrations of O3 were measured by an ozone analyzer. The concentrations of anions such as NO2-, NO3-, SO32- and SO42- in reaction solution were measured by ion chromatography (Metrohm IC-883, Switzerland) under the following chromatographic conditions: anion dual 2 anion column, aluent (1.8 mmol/LNa2CO3 + 1.7 mmol/L NaHCO3 ), flow rate (1.0 mL/min), injection volume (50 µl ), column temperature (303 K), and automatic regeneration suppression system (H2O and 60 mmol H2SO4). The key active species such as sulfate radical and hydroxyl radical were captured by electron spin resonance (ESR) spectrometer (Bruker ESP-300) combining with 5,5-dimethy l-1- pyrrolidine N-oxide (DMPO) (>99%, Sigma) as a spin trap agent under the following setting conditions: X-band spectrometer, temperature of 298 K, microwave power of 10 mW, resonance frequency of 9.82 GHz, modulation

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amplitude of 0.1 mT, modulation frequency of 100 kHz, sweep width of 10 mT, sweep time of 180 s, time constant of 148 ms and receiver gain of 3.0×105. 2.4 Removal efficiency Removal efficiencies of NO and SO2 are calculated by the following equation (1):

Removal efficiency =

Cin − Cout × 100% Cin

(1)

Where Cin is import concentration of NO or SO2 in gas; Cout is export concentration of NO or SO2 in gas.

3. Results and discussions 3.1 Effects of key influencing factors on NO removal efficiency and optimization parameters 3.1.1 Selection of light wavelength and effects of wavelength on NO removal efficiency In the application and research area, vacuum ultraviolet light (VUV: central wavelength, 185nm), short-wave ultraviolet (UVC: central wavelength, 254nm), long-wave ultraviolet (UVA: central wavelength, 365nm) and visible light (wavelength range is about 400nm-760nm) are the most commonly used four light sources for various photochemical reactions.5 In this work, to select an efficient excitation light source, the yields of sulfate radicals and hydroxyl radicals in KHSO5 solution activated by radiation of light with four different wavelengths are determined by ESR spectrometer combining with 5,5-dimethy l-1- pyrrolidine N-oxide (DMPO). The results are shown in Figure 2 (b)-(e). It is found that compared to the other three light sources, the vacuum ultraviolet light (185 nm) has the highest yield of free radicals (the yields of free radicals are proportional to the peak heights). No obvious free radical signals are captured in KHSO5 solution under the radiation of visible light and long-wave ultraviolet (365 nm), showing that the visible light and long-wave ultraviolet (365 nm) lack of sufficient activation capacity for KHSO5.

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Figure 2. NO removal efficiencies (a) and free radical yields (b)-(e) under different light wavelengths. Basic experimental conditions: KHSO5 concentration, 0.50 mol/L (just for Figure (b)-(e)); Light intensity, 147 µW/cm2; Ultrasonic power density, 0 W/mL; Solution temperature, 298 K; Solution pH, 2.17; Flue gas flow, 600 mL/min; O2 concentration, 6.0%; SO2 concentration, 1500 ppm; NO concentration, 400 ppm; CO2 concentration, 12%. In order to further determine the suitable light source, the effects of light wavelengths on NO removal efficiency are also studied, and the results are shown in Figure 2(a). It can be seen that under 0.5 mol/L of KHSO5 concentrations, NO removal efficiencies are 67.4 %, 60.2%, 7.4% using VUV, UVC and UVA radiation, respectively. And under 0.02 mol/L of KHSO5 concentrations, NO removal efficiencies are 28.8%, 22.2%, 5.1% using VUV, UVC and UVA radiation, respectively. The results show that the vacuum ultraviolet light (185 nm) achieves the highest NO removal efficiency under the test conditions. In addition, as a comparison, NO removal efficiencies under visible light and black box were also tested. It is found that NO removal efficiencies are 8


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basically consistent with that of long-wave ultraviolet (365 nm). This result further shows that the long-wave ultraviolet (365 nm) and visible light almost has no activation capacity for KHSO5. The above phenomenon can be revealed by the following reasons. The results24-28 show that as the key active ingredient in Oxone (2KHSO5·KHSO4·K2SO4), KHSO5 contains the unstable peroxide bond, and thus can produce sulfate radical (SO4-·) and hydroxyl radical (·OH) by UV photolysis. The SO4-· can reacts with water molecules to regenerate ·OH. The relevant process can be described via the following reactions (2) and (3).24-28

HSO 5- + hv → SO -4 ⋅ + ⋅ OH

(2)

SO -4 ⋅ + H 2O → ⋅OH + HSO -4

(3)

As shown in Figure 2 and 9, we successfully detected the SO4-· and ·OH in solution using ESR spectrometer combining DMPO, which furthers confirmed the above speculation. Both SO4-· and ·OH have very strong oxidizing, and can oxidize NO to produce nitric acid by the following reactions (4)-(11).4-6,35-37

SO -4 ⋅ + NO → NO 2 + SO 3- ⋅

(4)

⋅ OH + NO → H + + NO −2

(5)

⋅ OH + NO -2 → NO3- + ⋅H

(6)

3NO 2 + H 2O → 2H + + 2 NO 3- + NO

(7)

NO 2 + ⋅OH → H + + NO3-

(8)

2SO -4 ⋅ + NO -2 + H 2O → NO3- + 2H + + 2SO 24 -

(9)

2NO 2 + HSO -5 + H 2O → 2NO 3- + 2H + + HSO -4

(10)

NO-2 + HSO -5 → NO3- + H + + SO 24 -

(11)

The energy of UV photon can be calculated by the Planck equation (12):5

ε = hv = h

c

λ

(12)

where ε is energy of photon, kJ; ν is ultraviolet frequency, 1/s; h is Planck constant, 6.626×10-34 J·s; c is

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speed of light, 2.998×108 m/s; λ is ultraviolet wavelength, 100-400nm. The equation (12) shows that the shorter the UV wavelength, the bigger the UV photon energy. The bigger UV photon energy can more easily destroy the peroxide bond in KHSO5 to produce more SO4-· and ·OH radicals. Besides, the results11,36 of the previous researchers show that vacuum ultraviolet light (185 nm) can also directly induce oxygen and water to produce oxygen atoms, ozone and ·OH, which can be expressed as the following equations (13)-(15). We successfully detected the ozone in vacuum ultraviolet light (185 nm) radiation system using an ozone analyzer (the ozone was not detected in the other light radiation systems), which furthers confirmed the following equations (13) and (14). 185nm O 2 + hv ← → ⋅ O + ⋅ O

(13)

O 2 + ⋅ O ↔ O3

(14)

185nm H 2O + hv ← → ⋅ OH + ⋅ H

(15)

Both oxygen atoms and ozone also have very strong oxidizing, and can oxidize NO in flue gas by the following reactions (Eq.s 16 and 17).11,36,38 The product or intermediate NO2 can be finally further removed by the above reactions (Eq.s 7, 8 and 10).

NO + ⋅O ↔ NO 2

(16)

NO + O3 ↔ NO 2 + O 2

(17)

Moreover, as shown in Figure 9(l), ·OH was also successfully detected in water using ESR spectrometer combining DMPO, which further confirmed the above equation (15). Therefore, the 185 nm is the most effective wavelength in this photochemical reaction. In the next works, the vacuum ultraviolet light (185 nm) is chosen as the excitation light source of photochemical reactions. 3.1.2 Effects of ultrasonic power density on NO and SO2 removal efficiencies Considering the pollution or scaling problems of quartz tube surface, the authors try to initiate ultrasonic

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cavitation in the KHSO5 solution using ultrasound. The shock waves and micro jets produced by ultrasonic cavitation can keep the surface of the quartz tubes clean.33,34 The ultrasonic power density (ultrasonic power) is a key process parameter for generation and strength of ultrasonic cavitation in solution.33,34 The effects of ultrasonic power density on NO removal were studied, and the results are shown in Figure 3(a). It can be observed that when ultrasonic power density increases from 0 to 0.033 W/mL, NO removal efficiency has a significant increase (66.1% to 77.1%). A lot of results30-34 have shown that ultrasonic cavitation can effectively strengthen mass transfer and chemical reaction process. To determine the strengthening mechanism of ultrasound to NO removal. The change of free radicals yield and mass transfer parameters in the presence and absence of ultrasound were measured by ESR technique and chemical methods.6 The results are shown in Figure 9(k) and Figure 3(b). It can be seen from the Figure 3(b), the key mass transfer parameters such as liquid phase mass transfer coefficient and gas-liquid specific interfacial area obviously increase with the increase of ultrasonic power density. Absorption process of NO in solution is a gas-liquid heterogeneous reaction process, thus an increase in mass transfer efficiency will promote removal of NO. In addition, it can be seen from the Figure 9(k), compared with no ultrasound in the Figure 9(a), the presence of ultrasound only very slightly increases the free radical yield. Therefore, the enhancement role of chemical reaction caused by ultrasonic cavitation to NO removal is existent, but may be minor in NO removal. The slight enhancement of chemical reaction caused by ultrasound can be revealed by the following reactions (Eq.s 18 and 19).30,32,39,40 Ultrasound HSO 5- ← → SO -4 ⋅ + ⋅ OH Ultrasound H 2O ← → ⋅ OH + H ⋅

(18) (19)

Based on the above results and discussions, we can infer that ultrasound may enhance NO removal through two aspects of chemistry and mass transfer, but the enhancement of mass transfer may play a leading role. In addition, we can see that when ultrasonic power density exceeds 0.02 W/mL, the change of NO removal efficiency

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becomes slight. Considering the energy consumption of removal system, 0.02 W/mL is chosen as the optimized value.

Figure 3. NO removal efficiencies and free radical yields under different ultrasonic power densities. Basic experimental conditions: Light wavelength, 185 nm; KHSO5 concentration, 0.5 mol/L (just for Figure (b)); Light intensity, 147 µW/cm2; Ultrasonic frequency, 28 kHz; Solution temperature, 298 K; Solution pH, 2.17; Flue gas flow, 600 mL/min; O2 concentration, 6.0%; SO2 concentration, 1500 ppm; NO concentration, 400 ppm; CO2 concentration, 12%.

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3.1.3 Effects of solution temperature on NO removal efficiency The effects of solution temperature on NO removal efficiency are tested, and the results are shown in Figure 4. It is observed that solution temperature shows a double impact on NO removal. Under 0.5 mol/L of KHSO5 concentration, when solution temperature increases from 298 K to 318 K, NO removal efficiency increases from 74.2% to 85.1%. However, when it further increases from 318 K to 348 K, NO removal efficiency decreases from 85.1% to 76.8%. Under 0.02 mol/L of KHSO5 concentration, as the solution temperature increases from 298 K to 328 K, NO removal efficiency increases from 48.8% to 56.1%. But when it further increases from 328 K to 348 K, the removal efficiency of NO decreases from 56.1% to 47.5%. The results6,37 indicate that solution temperature often has a double impact on gas absorption in solution. On the one hand, based on Arrhenius equation, increasing temperature often can increase chemical reaction rate,4-6 thereby promoting NO removal. Besides, many results show that high temperature can activate KHSO5 to produce SO4-· and ·OH by the following equation (20),6 which is also conducive to removal of NO. heat HSO 5-  → SO -4 ⋅ + ⋅ OH

(20)

It can be seen from the following Figure 9(b) that both SO4-· and ·OH are successfully captured in solution at high temperature (318 K), but were not detected by ESR at low temperature of (298 K) (Figure 9(a)), which proves the equation (20). On the other hand, increasing solution temperature usually will reduce the solubility coefficients of gas in water (for example, the solubility coefficients of NO in water are 1.91×10-8 mol/(L·Pa) at 298 K, 1.66×10-8 mol/ (L·Pa) at 308 K, 1.47×10-8 mol/(L·Pa) at 318 K, 1.36×10-8 mol/ (L·Pa) at 318 K, 1.28×10-8 mol/(L·Pa) at 338 K, 1.24×10-8 mol/(L·Pa) at 348 K).41,45,46 The Henry's Law can be described by the expression (21).41

C NO = H NO ⋅ p NO

(21)

where CNO is the solubility of NO in water, mol/L; HNO is the solubility coefficient of NO in water, mol/(L·Pa);

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pNO is the partial pressure of NO in gas body, Pa. It can be seen from the above expression (21) that an increase in solution temperature will reduce the solubility of NO in liquid phase, thereby suppressing NO removal. In summary, the effect of solution temperature on NO removal is complex. 318 K-328 K is the optimized temperature range for NO removal, which is close to the common operating temperatures of most wet flue gas desulfurization processes.

Figure 4. Effects of solution temperature on NO removal efficiency. Basic experimental conditions: Light wavelength, 185 nm; Light intensity, 147 µW/cm2; Ultrasonic power density, 0.02 W/mL; Ultrasonic frequency, 28 kHz; Solution pH, 2.17; Flue gas flow, 600 mL/min; O2 concentration, 6.0%; SO2 concentration, 1500 ppm; NO concentration, 400 ppm; CO2 concentration, 12%. 3.1.4 Effects of ultrasonic frequency on NO removal efficiency Ultrasonic frequency is a critical parameter for ultrasonic cavitation. The effects of ultrasonic frequency on NO removal were studied (two most common ultrasonic frequencies, 28 kHz and 40 kHz, were used to make a simple comparative study), and the results are shown in Figure 5. It is observed that NO removal efficiency under low-frequency is higher than that under high-frequency. Many studies have shown that compared to high-frequency ultrasound, low-frequency ultrasound will produce stronger cavitation effect,30,32 which will result in higher mass transfer efficiency and radical yield. Therefore, in the next works, the low-frequency ultrasound 14


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will be chosen as the sound source to investigate the other process parameters.

Figure 5. Effects of solution temperature on NO removal efficiency. Basic experimental conditions: Light wavelength, 185 nm; KHSO5 concentration, 0.50 mol/L; Light intensity, 147 µW/cm2; Ultrasonic power density, 0.02 W/mL; Solution temperature, 318 K; Solution pH, 2.17; Flue gas flow, 600 mL/min; O2 concentration, 6.0%; SO2 concentration, 1500 ppm; NO concentration, 400 ppm; CO2 concentration, 12%. 3.1.5 Effects of KHSO5 concentration on NO removal efficiency Figure 6 shows the effects of KHSO5 concentration on NO removal efficiency under different process prameters. The results show that in low KHSO5 concentration range, with increasing KHSO5 concentration, NO removal efficiencies greatly increase under different NO concentrations (400 ppm and 1000 ppm) and light intensities (57 µW/cm2 and 147 µW/cm2). It can be seen from the previous equations (2) and (3) that increasing KHSO5 concentration can increase the yields of SO4-· and ·OH in liquid phase, thereby enhancing NO removal. It can be seen from the following Figure 9(i) and 9(d) that with the increase of KHSO5 concentration, the yields of SO4-· and ·OH in solution obviously increase, which proves this above speculation.

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Besides, an increase in KHSO5 concentration can also enhance the oxidation rate between KHSO5 and NO, which can be described by the reactions (22) and (23) as follows.6

NO + HSO -5 → NO 2 + HSO -4

(22)

2NO + HSO 5- + H 2O → 2NO -2 + 2H + + HSO -4

(23)

However, in high concentration range (exceeding 0.5 mol/L), with further increasing KHSO5 concentration, the changes of NO removal efficiencies become extremely slight. As a gas-liquid heterogeneous reaction, removal process of NO is simultaneously dominated by mass transfer and chemical reaction. Hence, with the continuous enhancement of chemical reactions, the mass transfer process is likely to become a major controlling step of NO removal. Besides, with the increase of KHSO5 concentration, the following side reactions (Eq.s 24-28) with high reaction rates also will become more intense in solution, which can consume SO4-· and ·OH,20-22 thereby suppressing NO removal. Therefore, NO removal efficiency almost keeps unchanged with further increasing KHSO5 concentration in the high concentration range.

HSO5- + ⋅OH → SO5- ⋅ + H 2O

k = 1.7 × 107 M −1s −1

HSO 5- + SO -4 ⋅ → SO -5 ⋅ +SO 24 - + H +

k ≤ 1.0 × 105 M −1s −1

(24) (25)

⋅ OH + ⋅OH → H 2O 2

k = 5.3 × 109 M −1s −1

(26)

SO -4 ⋅ + SO -4 ⋅ → S2O82 -

k = 4.4 ± 0.4 × 108 M −1s −1

(27)

⋅ OH + SO -4 ⋅ → HSO 5-

k = 0.95 ± 0.08 × 1010 M −1s −1

(28)

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Figure 6. The effects of KHSO5 concentration on NO removal efficiency. Basic experimental conditions: Light wavelength, 185 nm; Light intensity, 147 µW/cm2 (just for Figure (a)); Ultrasonic power density, 0.02 W/mL; Ultrasonic frequency, 28 kHz; Solution temperature, 318 K; Solution pH, 2.17; Flue gas flow, 600 mL/min; O2 concentration, 6.0%; SO2 concentration, 1500 ppm; NO concentration, 400 ppm (just for Figure (b)); CO2 concentration, 12%. 3.1.6 Effects of light intensity on NO removal efficiency The number of ultraviolet photons and the energy consumption of removal system are closely related to the the light intensity of ultraviolet lamp.5 Thus the interrelation between light intensity and NO removal efficiency was studied under 0.5 mol/L and 0.02 mol/L of KHSO5 concentrations, and the results are in Figure 7. It is found

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that with increase of light intensity, NO removal efficiencies greatly increase under KHSO5 concentrations of 0.5 mol/L and 0.02mol/L. Based on the above equation (2), we can see that increasing UV light intensity can effectively increase the yields of SO4-· and ·OH by producing more ultraviolet light quantum,5 thereby enhancing NO removal. It can be seen from the following Figure 9(f), 9(d) and 9(g) that with the increase of UV light intensity, the yields of SO4-· and ·OH in solution greatly increase, which proves the above discussions. Nevertheless, when the UV light intensity exceeds 147 µW/cm 2 , the growth rate of NO removal efficiency becomes small. Synthetically considering removal efficiency and energy consumption, 147

µW/cm 2 is

determined as the optimal value of light intensity in this removal system.

Figure 7. The effects of light intensity on NO removal efficiency. Basic experimental conditions: Light wavelength, 185nm; Ultrasonic power density, 0.02 W/mL; Ultrasonic frequency, 28 kHz; Solution temperature, 318 K; Solution pH, 2.17; Flue gas flow, 600 mL/min; O2 concentration, 6.0%; SO2 concentration, 1500 ppm; NO concentration, 400 ppm; CO2 concentration, 12%. 3.1.7 Effects of solution pH on NO removal efficiency Figure 8 shows the effects of solution pH on NO removal efficiency. The results indicate that the solution pH has a double impact on NO removal under different KHSO5 concentrations and light intensities. When solution pH increases from 1.02 to 6.37, NO removal efficiency slightly increases from 84.8% to 88.5% and 57.5% to 18


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61.4% under KHSO5 concentrations of 0.50 mol/L and 0.02 mol/L, resprctively, and slightly increases from 86.6% to 90.3% and 69.1% to 75.8% under light intensities of 218 µW/cm2 and 57 µW/cm2, respectively. Many -

results 24-26,35 have demonstrated that SO4 · can react with OH- to generate ·OH in neutral and alkaline mediums, which can be expressed as the following equation (40). The oxidation ability of ·OH to NO is much greater than -

that of SO4 · to NO.5,35,37 Thus appropriate increasing solution pH can effectively promote removal of NO.

SO -4 ⋅ +OH - → SO 24 - + ⋅OH

(40)

However, when solution pH further increases from 6.37 to 11.88, NO removal efficiency greatly decreases from 88.5% to 59.5% and 61.4% to 39.8% under KHSO5 concentrations of 0.50 mol/L and 0.02 mol/L, respectively, and greatly decreases from 90.3% to 62.9% and 75.8% to 41.2% under light intensities of 218 µW/cm2 and 57µW/cm2, resprctively. The results5,6,35 of the previous researchers showed that ·OH was very unstable under strong alkaline conditions, and can be greatly consumed by the following reaction (Eq. 41) with very high reaction rate (the rate constant is up to 1.3 × 1010 M −1s −1 ). Thus too high solution pH will inhibit removal of NO.

⋅ OH + OH - → H 2 O + O - ⋅

(41)

As shown in Figure 9(d) and (e), we determined the yields of ·OH and SO4-· in solution at pH=2.17 and 11.88 by ESR technique. The results indicate that compared with those at pH=2.17, the yields of ·OH and SO4-· in solution at pH=11.88 decrease. Moreover, during the experiment, we also observed that the decomposition of KHSO5 (or Oxone) significantly occurred under strong alkaline conditions (pH=11.88) (many small bubbles were produced in solution), which is also not conducive to removal of NO.

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Figure 8. The effects of solution pH on NO removal efficiency. Basic experimental conditions: Light wavelength, 185 nm; KHSO5 concentration, 0.50 mol/L (just for Figure (b)); Light intensity, 147 µW/cm2 (just for Figure (a)); Ultrasonic power density, 0.02 W/mL; Ultrasonic frequency, 28 kHz; Solution temperature, 318 K; Flue gas flow, 600 mL/min; O2 concentration, 6.0%; SO2 concentration, 1500 ppm; NO concentration, 400 ppm; CO2 concentration, 12%. 3.2 Products and free radicals analysis In order to reveal the reaction pathways of NO removal in this removal system, the products of NO removal were analyzed, and the results are displayed in Table 1. The results show that there are a large number of sulfate ions in solutions. They may come from three parts: (a) Oxone (2KHSO5·KHSO4·K2SO4) contains sulfate itself; (b)

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The products of HSO5- decomposition from the above equations (2), (3), (18) and (20); (c) The oxidation products of SO2 and intermediates by the above equations (9)-(11), (25), (28) and (36)-(39). In addition, nitrate ions were also detected in solutions. They should be the oxidation products of NO. The key free radicals such as SO4-· and ·OH also were captured in solution using electron spin resonance (ESR) spectrometer combining with DMPO. We can see from the Figure 9 (b)-(g) and (i)-(l) that ESR spectrometer successfully captures free radical signals in different removal systems or under different experimental conditions. According to the literature data from the previous researchers, these hyperfine splitting constants of radical adducts ( a N = 13.7 G , aH = 10.2 G ,

aH = 1.45 G and aH = 0.76 G ) has a good consistency with the literature data ( a N = 13.8 G , aH = 10.1G , aH = 1.44 G and aH = 0.79 G ) .35,37,43 The results show that the radical adduct DMPO-SO4 is captured, which further proves that SO4-· is produced in solution. The hyperfine splitting constants ( a N = 15.1G and

a N = 14.8 G ) also has a good consistency with the literature data ( a N = 15.0 G and a N = 14.8 G ) .44-46 The results show that the radical adduct DMPO-OH is captured, further proving that ·OH is produced in solution. In fact, a seven-line peak marked out by three solid circles (representing SO4-·) and four rectangles (representing ·OH) is the typical mixed spectrum shape of ·OH and SO4-· radical adducts measured by ESR spectrometer.44-46 The measuring results show that both ·OH and SO4−· are generated in the removal system, which provide an important support for revealing the reaction pathways of NO removal in this removal system.

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Figure 9. Capture of SO4-· radicals and ·OH radicals by ESR spectrometer combining with DMPO. Basic experimental conditions: Light wavelength, 185 nm; Ultrasonic power density, 0.02 W/mL; Ultrasonic frequency, 28 kHz; Solution temperature, 318 K; Flue gas flow, 600 mL/min; O2 concentration, 6.0%; SO2 concentration, 1500 ppm; NO concentration, 400 ppm; CO2 concentration, 12%. Table 1. Determination of removal products in reaction solution. Time (10min)

SO42-

SO32-

NO3-

NO2-

Measured ion concentration (mg/L)

4.11 × 104

—

11.7

—

SO42-

SO32-

NO3-

NO2-

5.29 × 104

—

24.3

—

SO42-

SO32-

NO3-

NO2-

7.82 × 104

—

36.1

—

Time

(20min)

Measured ion concentration (mg/L) Time

(30min)

Measured ion concentration (mg/L)

3.3 Removal mechanism and reaction path To reveal the removal mechanism and reaction path of NO in this removal system, an experiment about comparison of different reaction systems is carried out, and the results are shown in Figure 10. The results show that NO removal efficiencies are only 1.2% and 2.9% in UV alone and UV+H2O systems, respectively, which indicate that the removal shares are very small, but the NO removal reactions occur in the two systems (UV alone 23


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and UV+H2O). Many results11,36 have shown that vacuum ultraviolet light can directly decompose NO. Besides, H2O can be also decomposed by vacuum ultraviolet light to produce ·OH to oxidize NO.11,36 The results in this study are consistent with those of other scholars.11,36 NO removal efficiency reaches 6.1% by oxidation of KHSO5 alone. Besides, as shown in the Figure 9 (a), no radical signals were captured in KHSO5 solution alone (298K). Based on the information, we can infer that NO removal by the oxidation of KHSO5 alone is one of the NO removal routes. However, compared with 6.1% of KHSO5 alone (298K), KHSO5 alone (318K) achieves removal efficiency of 13.7% for NO. As shown in the Figure 9 (b), the radical signals were captured in KHSO5 solution (318 K). Based on the two contrast, it can be inferred that NO removal by oxidation of ·OH and SO4-· produced by the thermal decomposition of KHSO5 (Eq. 20) is also one of the NO removal routes. Compared with 2.9% of UV + H2O, UV + H2O + O2 achieves removal efficiency of 9.9% for NO. Compared with 68.8% of UV + KHSO5, UV+ KHSO5 + O2 achieves removal efficiency of 77.1% for NO. Based on the contrast, it was found that O2 played a role in the NO removal process. Based on the successful capture of ozone, we infer that NO removal by the oxidation of O3 and ·O is also one of the NO removal routes. Compared with KHSO5 solution alone, the yields of ·OH and SO4-· have a great increase in UV + KHSO5, UV + KHSO5 + O2, UV + KHSO5 + US + O2 removal systems. Correspondingly, the removal efficiencies of NO also have a great increase in these removal systems. Based on the two contrast results, we can infer that NO removal by oxidation of ·OH and SO4-· produced by the photolysis of KHSO5 (Eq.s 2 and 3) is also one of the NO removal routes. As shown in Figure 9 (k), although the yields are very low, both ·OH and SO4-· are also detected in US + KHSO5 at low temperature of 298K. This result show that NO removal by oxidation of ·OH and SO4-· produced by the ultrasound decomposition of KHSO5 and/or water (Eq.s 18 and 19) is also one of the NO removal routes.

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The above results demonstrate that the removal mechanism and reaction paths of NO in this removal system are very complicated. In these removal routes, by comparing with the share of NO removal efficiencies in different removal systems, we find that NO removal by the oxidation of ·OH and SO4-· produced by the photolysis of KHSO5 (Eq.s 2 and 3) is the most important removal pathway. Removals of NO by the oxidation of O3/·O from UV photolysis of O2, oxidation of KHSO5 alone, and oxidation of ·OH and SO4-· produced by the thermal decomposition of KHSO5 are the three second important removal pathways. The other removal routes mentioned above only play a minor role for the removal of NO in this removal system. The above mechanism and routes for NO removal may be also represented by the following schematic 11.

Figure 10. Comparison of different reaction systems. Basic experimental conditions: Light wavelength, 185 nm; KHSO5 concentration, 0.50 mol/L; Light intensity, 147 µW/cm2; Ultrasonic power density, 0.02 W/mL; Ultrasonic frequency, 28 kHz; Solution pH, 2.17; Flue gas flow, 600 mL/min; O2 concentration, 6.0%; SO2 concentration, 1500 ppm; NO concentration, 400 ppm; CO2 concentration, 12%.

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Figure 11. Removal routes of NO using vacuum ultraviolet light (VUV)/ultrasound (US)/KHSO5 system

3.4 Simultaneous removal of NO and SO2 using vacuum ultraviolet light/ultrasound/KHSO5 system alone, and combining with common wet desulfurization processes Simultaneous removal of NO and SO2 in this removal system are investigated preliminarily, and the results are shown in Figure 12(a). We can see that this removal system has a good simultaneous removal performance for NO and SO2. The average efficiencies are 86.2%, 85.1% and 82.0% for NO within 15 min, 30 min and 160 min, respectively, and are up to 100% for SO2 within 160 min. Alkali-based wet flue gas desulfurization (WFGD) processes are widely applied due to the very high desulfurization efficiency, and the simple and reliable device.1,3 In China, CaCO3, ammonia and urea are the most common desulfurizers, and have been widely applied in almost all areas of flue gas purification. Nevertheless, they have very low NO removal efficiencies. As shown in Figure 12(b), all of CaCO3, ammonia and urea can achieve complete removal of SO2, but have very poor removal capacity for NO. The previous results in “3.1.7 Effects of solution pH on NO removal efficiency” find that compared with acidic conditions, neutral and weakly alkaline conditions are more conducive to removal of NO in this removal system. In order to test the feasibility of this new process strengthening NO removal in alkali-based WFGD (the

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purpose is to achieve the simultaneous removal of NO and SO2 in these existing alkali-based WFGD devices by an union or improvement), here the experiments on simultaneous removal of NO and SO2 in this removal system under the presence of urea, CaCO3 or ammonia were conducted. The results are shown in Figure 12(c). It can be observed that after combining with this removal system, urea, CaCO3 and ammonia (representing the three typical wet desulfurization processes) can still achieve 100% SO2 removal. In particular, it is worth noting that the removal efficiencies of NO in urea (95.2%), CaCO3 (81.4%) and ammonia (72.1%) have a great rise as compared to the original three processes. The results suggest that this new removal process may be used to simultaneously remove NO and SO2 from the coal-fired boilers and industrial furnaces by combining with or reforming these existing alkali-based WFGD processes. The related contents will be further studied in future works.

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Figure 12. Simultaneous removal of NO/SO2 by this new process alone (a), by several common alkali-based WFGD systems (b), and by this new process combining with several common alkali-based WFGD systems (c). Basic experimental conditions: Light wavelength, 185 nm; KHSO5 concentration, 0.50 mol/L; Light intensity, 147 µW/cm2; Ultrasonic power density, 0.02 W/mL; Ultrasonic frequency, 28 kHz; Solution temperature, 318 K; Solution pH, 2.17; Flue gas flow, 600 mL/min; O2 concentration, 6.0%; SO2 concentration, 1500 ppm; NO concentration, 400 ppm; CO2 concentration, 12%. 3.5 Potential amplification and application of this new removal process Simultaneous removal of NOx, SO2 and Hg0 from flue gas has a good development prospects in the area of small and medium-sized coal-fired boilers and kilns.8,23,37 Our previous works36 have shown that Hg0 from flue gas can be efficiently removed by vacuum ultraviolet light radiation in the presence of oxygen. The efficient simultaneous removal of NO and SO2 using this removal system (vacuum ultraviolet light/ultrasound/KHSO5) also has been verified in the present study. Therefore, simultaneous removal of NOx, SO2 and Hg0 using this removal system may be feasible in the future. The potential process flow and devices are described as follows. Combustion of fossil fuels in Boiler and various solid wastes in Industrial Furnace will produce containing-SO2/NOx/Hg0 flue gas. The produced containing-SO2/NOx/Hg0 flue gas sequentially enters Dust Collector and Heat Exchanger to remove dust, and reduce the flue gas temperature. The pretreated 28


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containing-SO2/NOx/Hg0 flue gas further enters VUV-US Coupled Reactor through Bubblers, and to initiate a gas-liquid reaction with oxidizing agents. NOx, SO2 and Hg0 are oxidized into HNO3, H2SO4 and Hg2+ in solutions by absorption-oxidation reactions. The Hg2+ in liquid phase can be separated in Mercury Separation Tower by addition of S2+. S2+ will react with Hg2+ to produce HgS precipitation, which can be recycled via a simple precipitation. HNO3 and H2SO4 mixed solution will be converted to recyclable sulfate and nitrate by adding alkaline substances in Neutralizing Tower, which can be used as industrial raw materials. In China, due to the large-scale application of biomass burning power generation, every year a lot of plant ash is produced. The plant ash has been divided into solid waste by environmental regulations in some areas of China.42 The plant ash often contains potassium carbonate (K2CO3). The highest concentration of K2CO3 in some straw ash even exceeds 20%.42 We propose to use boiler flue gas waste heat to extract K2CO3 from plant ash. The K2CO3 will be used to absorb HNO3 and H2SO4 through a neutralizing reaction to generate KNO3 and K2SO4. The containing-KNO3 and K2SO4 solution (it contains the new produced KNO3/K2SO4 and the original K2SO4 in Oxone) will be evaporated and crystallized (separated) in Evaporating, Crystallizing and Separation Tower using the waste heat from boilers, which does not need to provide additional energy/heat consumption. The produced water vapor will be recovered in Water Vapor Condensation Recovery Tower, and will be recycled via re-adding to Reagent Tower. The cleaned flue gas was discharged into atmosphere through Chimney. The above ideas may be imperfect at present, but it will provide a new option/proposal for post-processing of removal products.

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Figure 13. Process flow diagram of simultaneous removal of SO2, NOx and Hg0 from flue gas by vacuum ultraviolet light/ultrasound/KHSO5 technology.

4.Conclusions Removal process of NO and SO2 from flue gas using vacuum ultraviolet light/ultrasound/KHSO5 system in a VUV-US bubbling reactor was studied. The feasibility, influencing factors, active species, products, mechanism and route of NO removal were investigated. The results show that: (1) VUV (185 nm wavelength) is the most effective light source for NO removal in this removal system. US enhances NO removal due to the enhancement of mass transfer (major) and chemical reaction (minor), and ultrasound with low-frequency is more effective than that of high-frequency. (2) NO removal efficiency increases with increasing KHSO5 concentration, light intensity, ultrasonic power or oxygen concentration. Solution pH and temperature have double effect on NO removal. The key active species 30


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such as ozone, hydroxyl radicals and sulfate radicals were successfully captured. VUV/US/KHSO5 coupling system had the highest free radical yield and NO removal efficiency. (3) NO removal by the oxidation of ·OH and SO4-· produced by the photolysis of KHSO5 is the most important removal pathway. Removals of NO by the oxidation of O3/·O from UV photolysis of O2, oxidation of KHSO5 alone, and oxidation of ·OH and SO4-· produced by the thermal decomposition of KHSO5 are the three second important removal pathways. (4) The developed removal process has a good simultaneous removal performance for NO and SO2, and may be also used to simultaneously remove NO and SO2 by combining with or reforming existing alkali-based WFGD processes. The recycling of the removal products, and the potential amplification and application of this removal process were also discussed finally. AUTHOR INFORMATION Corresponding Author *Telephone/Fax: +86-0511-89720178. E- mail : [email protected] (Y.L.) Notes The authors declare no competing financial interest.

Acknowledgements This study was supported by National Natural Science Foundation of China (No.51576094; No.51206067), Jiangsu “Six Personnel Peak” Talent-Funded Projects (GDZB-014), China Postdoctoral Science Foundation (2017M610306), and Training Project of Jiangsu University Youth Backbone Teacher.

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