Simultaneous Oxidation and Absorption of NOx and SO2 in an

Feb 23, 2016 - Simultaneous Oxidation and Absorption of NOx and SO2 in an Integrated O3 Oxidation/Wet Atomizing System ... *Telephone: +82-32-874-3785...
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Simultaneous Oxidation and Absorption of NOx and SO2 in an Integrated O3 Oxidation/Wet Atomizing System Hyung Jun Yoon, Hyun-Woo Park, and Dong-Wha Park* Department of Chemistry and Chemical Engineering and Regional Innovation Center for Environmental Technology of Thermal Plasma (RIC-ETTP), Inha University, 100 Inha-ro, Nam-gu, Incheon 402-751, Republic of Korea ABSTRACT: The simultaneous removal of NOx and SO2 was demonstrated by an integrated O3 oxidation/wet atomizing system. The dielectric barrier discharge with silicone rubber dielectrics was used for generating O3 to oxidize NO and SO2. The O3 yield and O3 energy yield were evaluated to be 13.3 g/h and 53.7 g/kWh, respectively. NO was effectively oxidized by O3, while SO2 was not significantly oxidized because the reaction rate of NO oxidation was much higher than that of SO2. The highest oxidation efficiencies of NO and SO2 were 97 and 8%, respectively. NOx and SO2 were absorbed as aqueous ions with a mist of H2O2 solution supplied by an ultrasonic humidifier. The total removal efficiencies of NOx and SO2 were 88.8 and 100%, respectively. NO and SO2 were oxidized to NO2, HNO3, N2O5, and SO3 by O3. These species were absorbed to form NO3−, HSO3−, and SO42− by reactions with the H2O2 solution.

1. INTRODUCTION Nitric oxides (NOx) and sulfur dioxide (SO2) gases cause serious problems to both human health and the environment.1,2 These air pollutants are generally exhausted during the combustion of fossil fuels by incinerators, automobiles, and thermal power plants.1−3 Technologies for the simultaneous treatment of NOx and SO2 in flue gas have been researched via non-thermal plasma,4−6 photocatalytic oxidation,7,8 and wet processing9−13 methods. The non-thermal plasma process directly oxidizes and decomposes gaseous NOx and SO2 by reaction with abundant radicals and high-energy electrons at a high rate.5 However, the method has technological problems, such as extremely high power consumptions of 70−780 eV/ molecules when the chemical bonds of the air pollutant are directly broken in the plasma reactor.13 Furthermore, the oxidation efficiency of NO is insufficient because of the conversion of NO2 to NO, referred to as a back reaction. The emission of additional nitric oxides generated in the plasma reactor also decreases the energy efficiency.4,6 Some researchers have focused on the photocatalytic oxidation method, which has received attention for the oxidation of NOx and SO2 gases. Chunyan et al. achieved the simultaneous removal of NO and SO2 using a photocatalytic reactor with the help of a TiO2−polyacrylonitrile (PAN).8 While the photocatalytic reactor achieved the highest removal efficiencies of 71.2% for NO and 99.3% for SO2, limitations, such as high operational costs and technical problems, for scaleup restrict it from application to large-scale industrial processes within a short time.3 Meanwhile, the wet process is one of the most promising methods for the large-scale removal of NOx and SO2.11 In this process, the NOx and SO2 gases absorb and oxidize through gas−liquid reactions with various chemical absorbents. Although high removal efficiencies of NO2 and SO2 were achieved with the wet scrubber system, the absorption of NO gas was difficult.5 This was caused by the solubility of NO into solution being much lower than those of NO2 and SO2.10 Therefore, NO gas must be oxidized to high-order N species © XXXX American Chemical Society

with higher solubility, such as NO2, NO3, HNO3, and N2O5. In previous works, strong oxidants, such as NaClO2, H2O2, ferrate (VI), KMnO4, Na2S2O8, and O3, have been used to oxidize NO gas.9−17 Notably, NO is oxidized homogeneously by O3 through a gas-phase reaction rather than a gas−liquid reaction. The homogeneous reaction in the gas phase has many advantages, such as high selectivity, oxidation efficiency, and reaction rate [k = 2.59 × 109 exp(−3.176/RT)], without the need for additional catalysts.12,18,19 By this gas-phase reaction, NO gas can be directly oxidized to NO2 and even N2O5, which is even more effectively absorbed into solution than NO2, by controlling the O3/NO molar ratio.17 Consequently, the simultaneous oxidation of NO and SO2 by O3 has been explored in multiple previous works.19−21 Dielectric barrier discharge (DBD), which has a high electron temperature range of 104−105 K at atmospheric pressure, is one of the most effective methods for O3 generation.22 However, the durability of the dielectric material in the DBD system must be very high to steadily produce O3 gas. Typically, dielectric materials of quartz, alumina, ceramics, and polymers are used in DBD reactors.23 These suffer damage from thermal shock by the collision of highly energetic electrons. This causes the heating of the dielectric surface, resulting in a decreased O3 yield by the thermal decomposition of O3.12,24 Moreover, stress can cause cracking of the dielectric material, leading to arc ignition on the cracked dielectric surface. In previous works, various methods were suggested to solve these problems. Muhammad et al. proposed the use of a mica sheet for the dielectric material, because it is both flexible and non-fragile.24 The thickness of the mica sheet was less than 0.5 mm, and it showed a much lower initiation voltage than glass or ceramic. This configuration provided low power consumption and reduced the stress on the dielectric material. Wei et al.25 and Received: December 15, 2015 Revised: February 20, 2016

A

DOI: 10.1021/acs.energyfuels.5b02924 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Park et al.26 applied water-cooling systems to prevent the thermal decomposition of O3 by cooling the heated electrodes and dielectric material. In the present work, silicone rubber was proposed for the dielectric layers of the DBD reactor. Silicone rubber has a good resistance to O3, ultraviolet (UV), and heat, a low relative permittivity (εr = 2.5), and a high dielectric strength of 20 kV/ mm.27 The electrodes were electrically isolated by surrounding the silicone rubber in both sides. This configuration was facilitated to generate lower levels of power dissipation than other DBD reactors composed of a single dielectric layer; it also prevented electrode corrosion by O3 gas.28 Furthermore, it can be designed to various configurations compared to conventional DBD reactors as a result of the flexibility of the silicon rubber dielectrics and electrodes. A conventional DBD reactor for ozone generation usually combined with a water-cooling system to synthesis O3 gas steadily. However, a novel DBD reactor was used to generate O3 without an additional cooling system. The effects of the specific energy density and the O2 content of the plasma-forming gas on the O3 yield and O3 energy yield were evaluated to determine the optimal operating conditions of the proposed DBD reactor. The simultaneous oxidation of NO and SO2 was performed with O3 produced from the DBD reactor. The effects of the molar ratio of O3/NO and the gas residence time in the gas-phase reactor on the oxidation efficiencies of NO and SO2 were investigated. The simultaneous absorption of NOx and SO2 in the wet atomizing reactor using a H2O2 solution was examined with respect to the variation of the molar flow rate of H2O2. The H2O2 substance is the most suitable oxidant from the view of cost and environmental impact for the simultaneous absorption of NO and SO2. Furthermore, the wet atomizing reactor can reduce the gas−liquid contact time between liquid droplets and gaseous pollutants.11 For this reasons, it could be used for the simultaneous absorption of NOx and SO2 at a short gas−liquid contact time. The concentrations of various nitrogen oxides and aqueous ions were measured to investigate the reaction pathways for the simultaneous oxidation and absorption of NOx and SO2 in each step of the process.

Figure 1. (a) Schematic of the DBD reactor, (b) cross-sectional view of the reactor, and (c) images of surface discharge when air and O2 are used as plasma-forming gases. flow rate of the plasma-forming gas from 37.5 to 16.7 L/min at a fixed plasma input power of 250 W. The O3 concentration was measured by an UV analyzer (OZM-5000G2, Okitrotech, Japan) at the end of the DBD reactor. The UV analyzer could detect a range of O 3 concentrations up to 400 g/m3 at a resolution of 0.1 g/m3. The O3 yield (OY) and O3 energy yield (EY) were calculated using the following equations to determine the optimal operating conditions of the DBD reactor: OY (g/h) = CO3Q

EY (g/kWh) =

OY Pin

(1) (2)

3

where CO3 (g/m ) is the outlet concentration of O3 produced from the DBD reactor, Q (m3/h) is the flow rate of the plasma-forming gas, and Pin (kW) is the electric input power of the plasma. 2.2. Integrated O3 Oxidation/Wet Atomizing System. Figure 2 shows a schematic of the integrated O3 oxidation/wet atomizing system for the simultaneous oxidation and absorption of NOx and SO2. The proposed system is composed of a gas supply part, an O3

2. EXPERIMENTAL SECTION 2.1. DBD Reactor for O3 Generation. Figure 1a presents the detailed configuration of the DBD reactor employed to generate O3. Panels b and c of Figure 1 present a cross-sectional view of the DBD reactor and images of the DBD discharge when air and O2 were used as plasma-forming gases, respectively. Copper wires with diameters of 2 mm were used as high-voltage electrodes. These were circularly surrounded by silicone rubber with a thickness of 2.5 mm. Two silicone rubber-coated electrodes were attached together with an electrode gap distance of less than 1 mm; these were inserted into a polytetrafluoroethylene (PTFE) tube with an inner diameter of 14 mm and a length of 300 mm. PTFE, which is well-known as having good dielectric strength, low relative permittivity, and good resistance of O3, can prevent an electrical safety accident. Inside the PTFE tube, surface discharge was generated between two silicone rubber surfaces at atmospheric pressure. A high-voltage alternating current (AC) power supply (30 kV, Plasma Technology, Korea) was used to generate the surface discharge; it provided a high voltage of 23 kV and a low current of 10.8 mA at a frequency of 20 kHz as a result of the high input voltage. A mixture of O2 and N2 gases was used as the plasma-forming gas. The flow rates of O2 and N2 were controlled by a mass flow controller (TSC-220, Korea Instrument T&C, Korea) to vary the O2 content of the plasma-forming gas from 10 to 100%. The specific energy density (SED) was changed from 400 to 900 J/L by varying the

Figure 2. Schematic diagram of the integrated O3 oxidation/wet atomizing system for the simultaneous treatment of NOx and SO2. B

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Energy & Fuels generation system, a gas-phase reactor, and a wet atomizing reactor. All reactors at each part were completely sealed to prevent damages of human health and equipment from the strong oxidant of H2O2 solution and corrosive gases, such as NOx, SO2, and O3. The flow rates of NO and SO2 were controlled by a mass flow controller (TSC220, Korea Instrument T&C, Korea). The concentrations of NO and SO2 were fixed at 350 and 800 ppmv, respectively, at the total gas flow rate of 150 L/min. The NO and SO2 gases were first diluted with N2 gas in the gas mixing chamber; this mixture of gases was introduced into the gas-phase reactor with O3 gas. The O3 concentration was changed from 0 to 650 ppmv to vary the O3/NO molar ratio from 0 to 1.8. PTFE tubing with an inner diameter of 14 mm was used for the gas-phase reactor, with length varying from 0.1 to 15 m to vary the gas residence time from 0.01 to 1 s. The wet atomizing reactor was designed as a cylindrical plastic tube with an inner diameter of 74 mm and a fixed length of 1.7 m to set the gas−liquid contact time at 3 s. A solution of H2O2 (30%, Daejung Reagents, Korea) was used for the absorption of NOx and SO2 gases in the wet atomizing reactor. Aqueous H2O2 was uniformly mixed with water using a stirrer to prepare the absorbent solution, and 4 L of the mixture was added to the ultrasonic humidifier (NH-5, Hwajeun Engineering, Korea). The ultrasonic humidifier generated fine mists with a droplet size distribution of 1−5 μm; the mist was injected into the wet atomizing reactor. The pH value and molar concentration of H2O2 were changed from 7.25 to 6.78 and from 0 to 0.7 mol/L to vary the molar flow rate from 0 to 35 mmol/min at a fixed solution flow rate of 3 L/h. The integrated O3 oxidation/wet atomizing system were operated at 295 K and atmospheric pressure. The residual gas exhausted from the O3 oxidation/wet atomizing system was carefully vented in a fume hood. The inlet and outlet concentrations of NO and NO2 were analyzed by a flue gas analyzer (MK 9000, RBR, Germany) with a detection range of up to 2000 ppmv at a resolution of 1 ppmv. The SO2 concentration and the presence of N2O5 and HNO3 were measured by Fourier transform infrared (FTIR) spectroscopy (IG-2000, Otuska Electronics, Japan). The detection wavenumber range of FTIR spectroscopy was from 700 to 5000 cm−1 at a resolution of 0.5−32 cm−1 using a 9.71 cm gas cell. In addition, the ion concentrations of NO2−, NO3−, and SO42− were analyzed by ion chromatography (ICS3000, Dionex, Sunnyvale, CA) at a resolution of 0.01 ppm. The oxidation (ηO) and total removal efficiencies (ηR) were calculated by eqs 3 and 4, respectively C C* ηO (%) = in out × 100 C in

(3)

C inCout × 100 C in

(4)

ηR (%) =

Figure 3. O3 energy yield and O3 yield for different specific energy densities of the DBD reactor at a fixed O2 content of 100% for the plasma-forming gas.

e* + O2 → 2O + e

(5)

O + O2 + M → O3* + M → O3 + M

(6)

In contrast, the O3 energy yield and O3 yield are slightly decreased by increasing the SED to a level exceeding 550 J/L. It was confirmed that the temperature of plasma-forming gas was slightly increased with the increase of energy density. The O3 gas decomposes and converts to O2 at a relatively high temperature of the plasma-forming gas via reactions 7−9. It means that the ozone decomposition reactions become dominant as a result of the enhanced reaction rate at higher gas temperatures.30,31 O + O + M → O2 + M

(7)

O + O3 → 2O2

(8)

2O3 + M → 3O2 + M

(9)

The O3 energy yield and O3 yield were also affected by the gas residence time in the DBD reactor. In the present work, the gas residence time in the reactor was changed from 0.166 to 0.074 s as the flow rate of the plasma-forming gas was varied from 16.7 to 37.5 L/min. Moreover, the SED was varied from 400 to 900 J/L with the decrease of the gas flow rate. Therefore, the gas residence time was quite short to produce a sufficient amount of O3 at SEDs below 550 J/L, because of the high gas flow rate. At SEDs above 550 J/L, O3 decomposition was enhanced by longer gas residence times, leading to decreased O3 energy yields and O3 yields. As a result, the optimal operating conditions to generate O3 were set to 550 J/L for SED and 0.1 s for the gas residence time in the DBD reactor. The highest O3 yield and O3 energy yield at these parameters were confirmed as 13.3 g/h and 53.7 g/kWh, respectively. Figure 4 presents the effects of the O2 content of the plasmaforming gas on the O3 energy yield and O3 yield at the fixed SED of 550 J/L. The O3 energy yield and O3 yield linearly increase from 10.5 to 53.7 g/kWh and from 2.6 to 13.3 g/h, respectively, with an increasing O2 content. In contrast, the conversion efficiency of O3 also affected the O2 content of plasma-forming gas, and it shows a different tendency compared to the O3 energy yield and O3 yield. The conversion efficiency of O3 based on the initial content of O2 was evaluated by following eq 10.

where Cin (ppmv) denotes the inlet concentrations of NO, NO2, and * (ppmv) and Cout (ppmv) are the outlet concentrations of SO2, Cout NO, NO2, and SO2 measured at the ends of the gas-phase reactor and the wet atomizing reactor, respectively.

3. RESULTS AND DISCUSSION 3.1. O3 Generation by the DBD Reactor. Figure 3 shows the O3 yield and O3 energy yield with changes in the SED from 400 to 900 J/L when the plasma-forming gas was pure O2. The O3 energy yield and O3 yield improve remarkably from 26.1 to 53.7 g/kWh and from 6.48 to 13.3 g/h, respectively, when the SED is increased from 400 to 550 J/L. The O3 generation reactions are enhanced by the higher energy density of the plasma. The O2 gas is excited to a high-energy state of O radicals by energetic electrons in the surface discharge region. These O radicals reacted with other O2 molecules and converted to O3 via reactions 5 and 6, known to be the main reactions for O3 formation.23,29 In these reactions, “e*” and “e” represent high- and low-energy electrons, respectively, and “M” represents a third-party collision component, such as O2 or N2. C

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Figure 4. O3 energy yield and O3 yield for different O2 contents of the plasma-forming gas at a fixed SED of 550 J/L.

conversion efficiency (%) =

Figure 5. O3 energy yield and O3 yield for different operation times of the DBD reactor at a fixed SED of 550 J/L and O2 content of 100%.

synthesized O3 (g/h) × 100 initial O2 (g/h)

proposed DBD reactor, the simultaneous oxidation of gaseous NO and SO2 with O3 was experimentally investigated. 3.2. Simultaneous Oxidation of NO and SO2 in the Gas-Phase Reactor. The effect of the gas residence time in the gas-phase reactor on the oxidation efficiencies of NO and SO2 was evaluated at a fixed O3/NO molar ratio of 1.8. The gas residence time was not shown to significantly affect the oxidation efficiencies of NO and SO2 in the range of gas residence times from 0.01 to 1 s. The oxidation efficiencies of NO and SO2 were slightly increased from 92.8 to 94.8% and from 1.3 to 9.7%, respectively, with the increase of the gas residence time in this range. The oxidation of NO to NO2 proceeded with an extremely high rate via reaction 17, whereas SO2 gas oxidized slowly by exposure to O3 because of the high energy barrier of reaction 18.17,32 The residence time in the gasphase reactor was set at 0.1 s.

(10)

The O3 conversion efficiency linearly decreased from 1.07 to 0.57% with increasing the oxygen content of plasma-forming gas from 10 to 100%. It means that O3 gas more efficiently synthesized at low O2 content than that of high O2 content. Because a high O2 content of plasma-forming gas leads to an increased concentration of O radicals in the plasma region through reaction 5 and also accelerates ozone decomposition reactions as reactions 7−9. In the regime of a high N2 content, the N2 molecules reacted with a high kinetic energy of electrons in the plasma region via reactions 11 and 12. These reactions dissipated the energetic electrons and created excited N2 species, such as N*, N, and N2*, where “N*” and “N” represent high and low energies of N atoms, respectively, and “N2*” represents the excited state of the N2 molecule.30 e* + N2 → e + N + N*

(11)

e* + N2 → e + N2*

(12)

NO + O3 → NO2 + O2 k = 1.8 × 10−14 cm 3 mol−1 s−1 SO2 + O3 → SO3 + O2

The partially excited N2 species reacted with O2 and converted to nitric oxides, such as NO, N2O, and NO2 in the plasma region via reactions 13−16. Furthermore, these reactions caused a decrease of both the O3 energy yield and O3 yield by the consumption of energetic electrons and O2 species.4,32 N* + O2 → NO + O

(13)

2N* + O2 → 2NO

(14)

N2* + O2 → N2O + O

(15)

O + NO + M → NO2 + M

(16)

(17)

k = 1.8 × 10−24 cm 3 mol−1 s−1

(18)

Figure 6 presents the concentrations of NO, NO2, and SO2 and the absorbance intensities of HNO3 and N2O5 with changes in the O3/NO molar ratio. The concentration of NO is linearly decreased with the increase of the O3/NO molar ratio from 0 to 1.0, while the NO2 concentration is increased. This indicates that the NO gas is oxidized to NO2 according to reaction 17. The NOx (NO + NO2) concentration is slightly increased from 350 to 404 ppmv by increasing the O3/NO molar ratio to 1.0, because additional NOx gases are generated from the DBD reactor via reactions 13−16 when a mixture of N2 and O2 is used as the plasma-forming gas. Meanwhile, HNO3 and N2O5 are not detected when the O3/NO molar ratio is lower than 1.0. While the absorbance intensities of HNO3 and N2O5 increase when the O3/NO molar ratio is higher than 1.0, the NO2 concentration is decreased from 373 to 93 ppmv. This indicates that NO2 is more thoroughly oxidized to N2O5 at higher O3/ NO molar ratios, via reactions 19 and 20. Because the reaction rate of reaction 20 was much higher than that of reaction 19, gaseous NO3 is spontaneously converted to N2O5 when NO2 existed.33 Furthermore, HNO3 is produced from N2O5 by the presence of moisture in the air via reaction 21.20 In the case of

The O3 yield and O3 energy yield with different operating times of the DBD reactor at a fixed SED of 550 J/L and pure O2 as the plasma-forming gas are shown in Figure 5. The average O3 yield and O3 energy yield are measured to be 13.3 g/h and 53.7 g/kWh, respectively, for 2 h of operation time. The O3 gas is observed to be steadily produced from the proposed DBD reactor without a water-cooling system. Therefore, it has a possibility that silicone rubber is an appropriate dielectric material for an O3 generator that operates for extended periods without a cooling system and with reliable O3 production efficiency. After the evaluation of the O3 generation in the D

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Figure 6. Concentrations of NO, NO2, and SO2 and the absorbance intensities of N2O5 and HNO3 in the exhaust gas from the gas-phase reactor for different O3/NO molar ratios.

Figure 7. Concentrations of NO, NO2, and SO2 and the absorbance intensities of N2O5 and HNO3 in the exhaust gas from the wet atomizing reactor for different H2O2 molar flow rates.

SO2 oxidation, its concentration is slightly decreased from 800 to 735 ppmv by increasing the O3/NO molar ratio from 0 to 1.8, because the oxidation rate of SO2 by O3 is much lower than that of NO, as shown in reactions 17 and 18. Therefore, NO gas is effectively oxidized to NO2 as well as HNO3 and N2O5 when the O3/NO molar ratio is higher than 1.0. The highest oxidation efficiencies of NO and SO2 of 97 and 8% are achieved at the O3/NO molar ratio of 1.8.

and N2O5 with changes in the H2O2 molar flow rate. The absorbance intensities of HNO3 and N2O5 are remarkably reduced at the molar flow rate of H2O2 of 0 mmol/min. N2O5 reacts with water mist and is converted to HNO3 via reaction 21, and HNO3 is easily absorbed into the solution.18 However, the concentrations of NO and NO2 are slightly decreased at the H2O2 molar flow rate of 0 mmol/min, because the solubility of NOx is much lower than those of gaseous HNO3 and N2O5. Therefore, NO and NO2 must be fully oxidized to high-ordered nitrogen species, such as HNO3 and N2O5, with a large amount of O3 to be absorbed into water without the presence of H2O2. The removal efficiency of NOx increased from 71.4 to 88.8% when the H2O2 molar flow rate is increased from 0 to 35 mmol/min. This is attributed to the absorption of NO and NO2 gases into NO2− and NO3− ions via reactions 30−32.34 NO2 gas converts to NO2− by reacting with HSO3− and SO42− ions via reactions 33 and 34.35 In addition, NOx gases oxidize and are converted to NO3− ions by the mist of H2O2 solution via reactions 35 and 36.36

NO2 + O3 → NO3 + O2

(19)

NO2 + NO3 → N2O5

(20)

H 2O + N2O5 → 2HNO3

(21)

Other high-order nitrogen species, such as N2O3 and N2O4, are also generated in the gas-phase reactor via reactions 22 and 23. However, N2O3 and N2O4 have extremely short lifetimes (0.0009 and 0.0025 s, respectively) compared to N2O5 (300 s) with respect to kinetic arguments about thermal decomposition under typical conditions (295 K and 3.33 hPa).31 These species are easily oxidized to N2O5 with O3 via reactions 24 and 25 or efficiently converted into NO or NO2.20 Although HNO2 could be generated from NO, NO2, N2O3, and N2O4 via reactions 26−28, it is easily converted to HNO3 by reaction 29.20

3NO2 + H 2O → NO + 2NO3− + 2H+ −

2NO2 + H 2O → NO2 +

NO3−

+ 2H

−

(30)

+

(31)

+

NO2 + NO2 → N2O4

(22)

NO2 + NO + H 2O → 2NO2 + 2H

NO + NO2 → N2O3

(23)

2NO2 + SO32 − + H 2O → 2NO2− + SO4 2 − + 2H+

N2O3 + O3 → N2O4 + O2

(24)

N2O4 + O3 → N2O5 + O2

(25)

(32)

(33)

2NO2 +

HSO3−

−

+ H 2O → 2NO2 + SO4

2−

+ 3H

+

(34)

H 2O + N2O3 → HNO2 + HNO2

(26)

2NO + 3H 2O2 → 2H +

H 2O + N2O4 → HNO3 + HNO2

(27)

2NO2 + H 2O2 → 2H+ + 2NO3−

H 2O + NO + NO2 → 2HNO2

(28)

HNO2 + O3 → HNO3 + O2

(29)

+

2NO3−

+ 2H 2O

(35) (36)

The concentration of SO2 is sharply decreased from 735 to 420 ppmv at the H2O2 molar flow rate of 0 mmol/min. SO2 gas is easily absorbed to form HSO3− ion by water mist in the gas− liquid interface via reaction 37. SO2 gas is perfectly absorbed into the solution at molar flow rates of H2O2 above 5 mmol/ min, because SO2 can additionally absorb to form the SO42− ion with H2O2 solution via reaction 38.14 Therefore, the SO2 removal efficiency is enhanced when H2O2 is used as the absorbent.

3.3. Simultaneous Absorption of NOx and SO2 in the Wet Atomizing Reactor. The simultaneous absorption of the gaseous pollutants exhausted from the gas-phase reactor was examined at a fixed gas−liquid contact time in the wet atomizing reactor of 3 s. Figure 7 shows the concentrations of NO, NO2, and SO2 and the absorbance intensities of HNO3 E

DOI: 10.1021/acs.energyfuels.5b02924 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels SO2 + H 2O → H+ + HSO3−

(37)

SO2 + H 2O2 → 2H+ + SO4 2 −

(38)

Table 1. Anion Concentrations in the Absorption Solution for Different Molar Flow Rates of H2O2 at the O3/NO Molar Ratio of 1.8

From the data in Figure 7, it is confirmed that the molar flow rate of H2O2 exerts a markedly greater impact on the absorption of SO2 than that of NOx. This is because relatively highly soluble SO2 gas is more effectively absorbed than NOx gases. Therefore, the evolution of harmful gaseous NOx and SO 2 as well as HNO 3 and N 2 O 5 can be controlled simultaneously by absorption by a mist of H2O2 solution in the wet atomizing reactor. 3.4. Reaction Pathways for Simultaneous Oxidation and Absorption of NOx and SO2. Spectra a and b in Figure 8

molar flow rate of H2O2 (mmol/min) NO2− concentration (ppm) NO3− concentration (ppm) SO42− concentration (ppm)

0

5

15

25

35

0

0

0

0

0

3754

3809

4251

4448

4532

630

6874

10528

12086

12265

absorption of NO and NO2, is converted to NO3− at pH conditions under 5 via reaction 39.35 3NO2− + 2H+ → NO3− + 2NO + H 2O −

NO2 + H 2O2 →

NO3−

+ H 2O

(39) (40)

HNO3 → H+ + NO3−

(41)

The concentration is also increased when the molar flow rate of H2O2 is increased from 0 to 35 mmol/min. However, the concentration of SO42− is much lower than that of the NO3− concentration at the H2O2 molar flow rate of 0 mmol/ min. This is because SO2 gas is mostly converted to the HSO3− ion by the water mist without H2O2 in the gas−liquid interface. In contrast, the concentration of SO42− significantly increases from 6874 to 12 265 ppm with an increase of the H2O2 molar flow rate to 35 mmol/min. This indicates that SO32− and HSO3− are oxidized to SO42− by the H2O2 solution in the liquid phase via reactions 42 and 43.37 SO42−

Figure 8. In situ infrared (IR) spectra of NO and SO2 exhausted from different experimental conditions: exhaust gas from the gas-phase reactor at the O3/NO molar ratio of (a) 0.6 and (b) 1.8 and from the wet atomizing reactor at the H2O2 molar flow rate of (c) 25 mmol/ min.

SO32 − + H 2O2 → SO4 2 − + H 2O

(42)

HSO3− + H 2O2 → HSO4 − + H 2O

(43)

On the basis of the measurements of the gas compositions and ion concentrations, reaction pathways for the simultaneous oxidation and absorption of NOx and SO2 in the integrated O3 oxidation/wet atomizing system are proposed, as shown in Figure 9. In the gas-phase reactor, NO gas oxidizes more rapidly to NO2, HNO3, and N2O5 by O3 gas than SO2 gas oxidizes to SO3. Other nitrogen species, such as N2O3, N2O4, and HNO2, can be produced; however, these are easily converted to N2O5 and HNO3. A small amount of SO2 gas is oxidized to SO3 in the gas-phase reactor. In the wet atomizing reactor, NO and NO2 are absorbed to form NO2− and NO3− ions with the mist of the H2O2 solution; in the gas−liquid interface, the NO2− ion is easily oxidized to the NO3− ion. N2O5 and HNO3 are converted mainly to NO3−. SO2 gas is more effectively absorbed to form HSO3− with water mist than NOx gas. HSO3− is mainly oxidized to SO42− with H2O2 in the liquid phase. 3.5. Comparison of the Proposed System with Various Treatment Processes. A performance comparison of the O3 oxidation/wet atomizing system and other methods for the simultaneous removal of NOx and SO2 is presented in Table 2. According to the previous studies, a novel removal process, such as vacuum UV (VUV) irradiation, can decomposes NOx and SO2 over 90 and 95%, respectively.1 While additional chemicals were not required for high removal efficiencies in the VUV method, it is difficult to reduce gas residence time in the reactor as a result of a low production of reactive oxygen species with a short irradiation time. Typically,

show the exhaust gas from the gas-phase reactor at the O3/NO molar ratios of 0.6 and 1.8, respectively, while spectrum c in Figure 8 shows the exhaust gas from the wet atomizing reactor at the H2O2 molar flow rate of 25 mmol/min. Although peaks for NO2 and SO2 are identified in spectrum a in Figure 8, various other peaks appear in spectrum b in Figure 8. The intensity of the NO2 peak declines in spectrum b in Figure 8 compared to that in spectrum a in Figure 8, and the peaks for HNO3 (887 and 1326 cm−1), N2O5 (743, 1250, and 1720 cm−1), and O3 (1055 cm−1) are newly appeared in spectrum b in Figure 8.20 In spectrum c in Figure 8, only the NO2 peak is identified after the gas−liquid reactions in the wet atomizing reactor. Consequently, it is concluded that NO gas was oxidized to NO2, HNO3, and N2O5 by O3 in the gas-phase reactor and that NOx and SO2 were subsequently absorbed to form aqueous ions with the H2O2 solution mist in the wet atomizing reactor. Table 1 presents the anion concentrations in the absorption solutions for different molar flow rates of H2O2. The concentration of NO3− is increased from 3754 to 4532 ppm with increasing molar flow rates of H2O2. NOx gas first absorbs to form NO2− and NO3− in the gas−liquid interface. Then, NO2− oxidizes and converts to NO3− in the liquid phase via reactions 39 and 40. In addition, HNO3 is converted to NO3− via reaction 41. The NO2− ion is not detected at any H2O2 molar flow rate because the NO2− ion, which evolves by the F

DOI: 10.1021/acs.energyfuels.5b02924 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 9. Proposed reaction pathways for the simultaneous treatment of NOx and SO2 in the integrated O3 oxidation/wet atomizing system.

Table 2. Comparison of the Performance of the O3 Oxidation/Wet Atomizing System and Other Methods for Simultaneous Removal of NOx and SO2 Gases process type

reaction source

total gas flow rate (Nm3/h)

VUV irradiation

CO2, water vapor, UV light

0.03

wetted-wall column

NaClO2 solution

2.7

wet scrubber

urea/NaClO2 solution

0.06

catalytic oxidation/absorption

H2O2/hemitite, ammonia solution

0.015

pyrolusite slurry combined with gas-phase oxidation

ozone, pyrolusite

0.9

plasma/absorption hybrid system

air plasma, (NH4)2SO4 solution with S2O32−

0.36

wet scrubber/electrochemical cell system

HNO3, Ag(I)NO3

0.3

magnesia slurry combined with ozone oxidation

ozone, MgO slurry

0.48

bubbling reactor combined with the gas phase

ozone, ammonia solution

0.95

O3 oxidation/wet atomizing system

ozone, H2O2

9

initial concentration (mg/m3)

maximum removal efficiency (%)

NOx: 350 SO2: 800 NO: 850 SO2: 640 NOx: 1250 SO2: 2000 NOx: 550 SO2: 17000 NO: 750 SO2: 2000 NO: 120 SO2: 525 NO: 400 SO2: 400 NO: 200 SO2: 500 NO: 200 SO2: 2000 NO: 350 SO2: 800

NOx: 90 SO2: 95 NOx: 67 SO2: 100 NOx: 93 SO2: 100 NOx: 80 SO2: 98 NOx: 82 SO2: 90 NOx: 71 SO2: 100 NOx: 87 SO2: 100 NOx: 72 SO2: 100 NOx: 90 SO2: 99 NOx: 89 SO2: 100

gas residence time (s) 240

reference 1

5

39

140

40

317

37

20

2

5

38

19

10

26

35

7.4

41

3.1

present study

Among them, Shaopeng et al. studied the simultaneous removal of SO2 and NOx with a bubble column combined with O3 oxidation using ammonia solution.41 In this process, the high NOx and SO2 removal efficiencies of 90 and 99% were achieved with a relatively short gas residence time of 7.4 s compared to other combined methods. However, a long gas residence time was still required for high removal efficiencies compared to the present work. In the proposed system, the gas residence time was more reduced with reliable removal efficiencies. This is because the reaction surface at a gas−liquid interface was significantly improved according to the decrease of the liquid droplet size using an ultrasonic humidifier. For this reason, NOx

the wet process is well-known for a simple method to remove NOx and SO2 simultaneously. However, the gas−liquid contact time between gaseous pollutants and liquid droplets was quite long to obtain high removal efficiency of NO because of a low solubility into solution.11 Therefore, some researchers have developed the wet process combined with various methods, such as non-thermal plasma, electrochemical cell, catalysts, and ozone oxidation, for oxidation of NO to high-ordered nitrogen species.2,10,35,37,38 These methods convert insoluble NO to soluble NO2 or high-order nitrogen species, such as HNO3 and N2O5. Therefore, NOx gases could be effectively removed with a short gas residence time in the wet process combined system. G

DOI: 10.1021/acs.energyfuels.5b02924 Energy Fuels XXXX, XXX, XXX−XXX

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ETTP) at Inha University, Korea, designated by the Ministry of Trade, Industry and Energy (MOTIE).

and SO2 gases were more rapidly absorbed into solution in the wet atomizing reactor compared to the bubbling reactor. As a result, gaseous of NOx and SO2 could be treated faster than that of other combined methods in the O3 oxidation/wet atomizing system. In terms of energy consumption, the specific energy consumption (SEC) was also calculated, which means energy consumption per unit volume of pollutants gas at the optimal operating conditions. In the case of the proposed ozone oxidation/wet atomizing system, the energy consumption for the simultaneous removal of NOx and SO2 was calculated to be 100 J/L when the throughput and gas residence time were 9 Nm3/h and 3.1 s, respectively. On the other hand, the SEC values were evaluated to 2325 and 378 J/L in the case of a plasma/absorption hybrid system and a bubbling reactor combined gas-phase system, respectively, at the optimal experimental conditions. While the magnesia slurry combined ozone oxidation was calculated to 80 J/L of SEC, it took a relatively high gas residence time of 26 s. In this result, the proposed system can achieve high removal efficiency at a relatively low energy consumption and a short gas residence time compared to other ozone oxidation methods.

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4. CONCLUSION The simultaneous treatment of NOx and SO2 was investigated by an integrated O3 oxidation/wet atomizing system. To produce O3 gas, a novel DBD reactor composed of siliconerubber-coated electrodes was designed. The O3 yield and O3 energy yield were affected by the specific energy density and the O2 content of the plasma-forming gas. The highest O3 yield and O3 energy yield were evaluated to be 13.3 g/h and 53.7 g/ kWh, respectively, at the SED of 550 J/L. In the gas-phase reactor, NO gas was mainly oxidized to NO2 at O3/NO molar ratios below 1.0. When the O3/NO molar ratio was increased above 1.0, NOx gas was converted to HNO3 and N2O5. SO2 gas was not significantly oxidized because the reaction rate between SO2 and O3 was much lower than that of NO oxidation. The highest oxidation efficiencies of NO and SO2 of 97 and 8%, respectively, were achieved at the O3/NO molar ratio of 1.8. The exhausted gas from the gas-phase reactor consisted of NO, NO2, HNO3, N2O5, SO2, and SO3. These gases were absorbed to form aqueous ions, such as NO3−, HSO3−, and SO42−, with the mist of H2O2 solution in the wet atomizing reactor. HNO3 and N2O5 were easily absorbed into water without H2O2. Therefore, NO and NO2 must first be converted to HNO3 and N2O5 by O3 to efficiently remove NOx via the wet atomizing reactor. SO2 gas was more rapidly absorbed than NOx gas because the solubility of SO2 is much higher than that of NOx. The highest removal efficiencies of NOx and SO2 of 88.8 and 100% were achieved when the H2O2 molar flow rate was 35 mmol/min.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +82-32-874-3785. Fax: +82-32-872-0959. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Regional Innovation Center for Environmental Technology of Thermal Plasma (RICH

DOI: 10.1021/acs.energyfuels.5b02924 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.5b02924 Energy Fuels XXXX, XXX, XXX−XXX