Simultaneous removal of NO and SO2 from exhaust gas by cyclic

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Article Cite This: Energy Fuels 2019, 33, 6591−6599

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Simultaneous Removal of NO and SO2 from Exhaust Gas by Cyclic Scrubbing and Online Supplementing pH-Buffered NaClO2 Solution Zhitao Han,*,†,§ Tian Lan,† Zhiwei Han,† Shaolong Yang,*,‡ Jingming Dong,† Deping Sun,† Zhijun Yan,† Xinxiang Pan,†,§ and Liguo Song† †

Marine Engineering College, Dalian Maritime University, 116026, Dalian 116024, China School of Naval Architecture and Ocean Engineering, Huazhong University of Science and Technology, Wuhan 430074, China § College of Ocean Engineering, Guangdong Ocean University, Zhanjiang 524088, China Downloaded via BUFFALO STATE on July 23, 2019 at 00:25:36 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

ABSTRACT: Experiments were conducted to simultaneously remove NO and SO2 from simulated exhaust gas by cyclic scrubbing and online supplementing pH-buffered NaClO2 solutions in a bench-scale reactor. The effects of key operating parameters on pollutant removal performance had been investigated. The results suggested that an extremely high oxidant utilization rate could be obtained when the oxidant supplementing molar rate approached the oxidant consumption molar rate. Though co-existing SO2 in flue gas would consume a certain amount of the oxidant during the absorption process, the intermediate products of SO32− ions generated from SO2 hydrolysis might be in favor of NO2 absorption to some extent. The introduction of ethanol in scrubbing solution could enhance NOx removal performance largely, which was possibly because of its inhibition effect on SO32− oxidation. The reaction mechanism and reaction products in scrubbing solutions were also discussed roughly through theoretical calculations and ion chromatography analysis.

1. INTRODUCTION At present, ships around the world emit a huge amount of gaseous pollutants into the atmosphere, so there is an imperious demand to simultaneously remove SOx and NOx with stringent regulations on marine emission coming into force.1 Though a lot of technologies have been developed and applied in stationary sources, most of them could not be adopted by vessels because of the limited space and resources on board. Up to date, more and more ship owners turn to wet exhaust gas cleaning systems (EGCSs) to reduce SO2 emissions. As for NOx emission abatement, selective catalytic reduction (SCR)2,3 and exhaust gas recirculation (EGR)4 exhibit enormous potential in marine application. However, it is very difficult to combine EGCS with SCR or EGR to remove SOx and NOx simultaneously from marine exhaust gas, because such a joint system requires much room, high cost, and complex operation.5 Thus, it is of great importance to develop a novel method to remove multipollutants from ship exhaust gas. Because the solubility of SO2 in water is good (maximum 35 m3 SO2 in 1 m3 water at 20 °C and 1 atm pressure), it is easy to remove SO2 and PMs with a high efficiency during the scrubbing process. It is known that NO accounts for more than 95% of NOx emitted from diesel engine. In order to improve the water solubility of NOx, a common way is to oxidize NO into higher valance NOx. During the past decades, some oxidants such as O3,6−8 KMnO4,9 H2O2,10−12 persulfate salts,13−15 NaClO,16−18 ClO2,19 and NaClO2,20−23 have been widely used to oxidize NO under wet scrubbing conditions. Among them, NaClO2 has been found to be one of the most promising oxidants due to its low toxicity and strong oxidative power.24−28 Similar to NaClO solution, available chlorine (or active chlorine) species are also the effective compositions in NaClO2 solution, which play a key role in the oxidation of © 2019 American Chemical Society

NOx. The active chlorine species in NaClO2 solution refer to HClO2, ClO2, and ClO2−. Their fractional ratios will be varied largely with solution pH. ClO2− will be in majority when NaClO2 is in a neutral or alkaline medium. At the moment, NaClO2 solution could not oxidize NO efficiently due to the low oxidative power of ClO2−. The fractional compositions of HClO2 and ClO2 will increase obviously with the decrease of solution pH. Especially in acidic ambient, NaClO2 solution will exhibit a very strong oxidative power, and it can oxidize NO efficiently. Therefore, solution pH is a critical factor when NaClO2 solution is used to remove NOx from the flue gas by scrubbing. However, most of the previous investigations were conducted without buffering NaClO2 solution pH in the wet scrubbing process. The solution pH would decrease obviously with the absorption of NOx in either cyclic or noncyclic scrubbing processes.29,30 It was reported for the first time that, Adewuyi et al. investigated the effect of buffering reagents on simultaneous removal performance of NO and SO2 in a labscale bubble column reactor using NaClO2 aqueous solution.23 The results showed that the buffered NaClO2 solution was determined to be more effective in absorbing NOx and SO2 and in controlling chlorine dioxide (ClO2) leakage. Considering that a cyclic spraying tower is usually adopted to remove gaseous pollutants from flue gas in practical applications, in which alkaline solutions might be supplemented continuously to adjust solution pH to be at a set value. In addition, the oxidant in solution is always at a relatively high concentration, which may be not in favor of obtaining high oxidant utilization. Here, we proposed to investigate the effects of various operating parameters on NOx and SO2 removal performance Received: April 9, 2019 Revised: June 3, 2019 Published: June 16, 2019 6591

DOI: 10.1021/acs.energyfuels.9b01106 Energy Fuels 2019, 33, 6591−6599

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meter and an ORP meter, respectively. During the cyclic scrubbing process, extra NaClO2 solution was supplemented into the scrubbing solution by a peristaltic pump (BT100-2J, Langer Pump Co.) with a flow of 0.73 mL·min−1. Gas concentrations of NO, NO2, SO2, O2, and CO2 were measured at the inlet and outlet ports of reactor by a gas analyzer (MGA 5, MRU). 2.3. Calculation of Removal Efficiency. Here, NOx concentration in simulated flue gas refers to the sum of NO and NO2 concentrations. NO, NOx, and SO2 removal efficiencies can be calculated as follows

by cyclic scrubbing NaClO2 solution in a lab-scale spraying reactor, in which buffering reagents were used to maintain solution pH during the whole scrubbing process. Besides, the oxidant was online supplemented into cyclic scrubbing solution at a fixed rate in order to maintain the oxidant concentration at a stable level. The results showed that a very high oxidant utilization rate could be achieved in such a scrubbing mode. Though the absorption of co-existing SO2 would consume some oxidants in solution, the intermediate product of SO32− might be beneficial to improve NO2 absorption. Especially when ethanol as an additive was introduced in cyclic scrubbing solution, it was helpful to reduce the outlet NO2 concentration obviously. The possible reaction mechanisms have also been discussed in detail.

η=

Cin − Cout × 100% Cin

(1)

where η represents the removal efficiency of NO or NOx or SO2, %; Cin is the concentration of the targeted pollutant in flue gas measured at the inlet port; Cout is the concentration of the targeted pollutant in flue gas measured at the outlet port.

2. EXPERIMENTAL SECTION 2.1. Materials. The reagents used in the experiments were of analytical reagent grade. NaClO2 powder of 80 wt % purity was purchased from Aladdin Company. Disodium hydrogen phosphate (Na2HPO4, 99.0%), critic acid (C6H8O7, 99.5%), and sodium citrate (Na3C6H5O7, 98.0%) were purchased from Sinopharm Company, and they were used as pH-buffering reagents. Five kinds of cylinder gases, N2, NO, SO2, O2, and CO2, were purchased from Dalian Special Gas Company, and they were used to prepare the simulated flue gas. 2.2. Experimental Setup and Procedure. Figure 1 shows the schematic diagram of the experimental apparatus, which consisted of

3. RESULTS AND DISCUSSION 3.1. Effect of NaClO2 Online Supplementary Rates. The effect of oxidant online injection rates on NOx and SO2 removal performance is investigated, and the results are shown in Figure 2. Basic experimental conditions are: 0.25 L of buffered NaClO2 solution was used for cyclic scrubbing of the flue gas. The solution pH and oxidant concentration were 6 and 1 × 10−2 mol·L−1, respectively. The flue gas flow was 1.5 L·min−1. Inlet NO and SO2 concentrations were 1000 and 300 ppm, respectively. NaClO2 supplementary rates were in the range of 0−5.48 × 10−5 mol·min−1.

Figure 1. Schematic diagram of the experimental setup (1−5 gas cylinders, 6−10 mass flow meters, 11 scrubbing reactor, 12 water bath, 13 NaClO2 cyclic scrubbing solution, 14 pH meter, 15 ORP meter, 16−17 peristaltic pumps, 18 NaClO2 supplementary solution, 19 drier, and 20 gas analyzer). the generation of simulated flue gas, the cyclic scrubbing system, and the detection of tail gas. Five mass flow meters were used to control cylinder gas flow rates. The typical gas concentrations were 200−1000 ppm for NO, 0−500 ppm for SO2, 0−10% for O2, 0−5% for CO2, and balanced by N2. The total flue gas flow was 1.5 L·min−1. The spraying column, made of polymeric methyl methacrylate, was installed with a fine-droplet spray nozzle (B1/4TT-SS+TG-SS0.4, Spraying System Co.) at the upper part and a gas distributor at the lower part. The height and inner diameter of the column were 30 and 6 cm, respectively. In each test, cyclic scrubbing solution was prepared by adding the NaClO2 powder into 0.25 L pH-buffered solution, and its temperature was maintained at 20 °C by a thermostatic water bath (F34, JULABO Labortechnick Gmbh). Cyclic scrubbing solution was supplied to the nozzle by a peristaltic pump (WT-600, Langer Pump Co.) with a flow of 0.35 L·min−1. The pH and ORP (oxidation− reduction potential) of scrubbing solutions were monitored by a pH

Figure 2. Changes of outlet NO and NO2 concentrations with scrubbing duration under various NaClO2 supplementary rates. 6592

DOI: 10.1021/acs.energyfuels.9b01106 Energy Fuels 2019, 33, 6591−6599

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Energy & Fuels Table 1. Calculation Result of Oxidant Supplementary Rates and Consumption Rates NaClO2 supplementary rate (mol·min−1)

NaClO2 consumption rate for converting NO into NO3− (mol·min−1)

NaClO2 consumption rate for oxidizing NO into NO2 (mol·min−1)

NaClO2 consumption rate for converting SO2 into SO42− (mol·min−1)

calculated total oxidant consumption rate (mol·min−1)

3.29 × 10−5 4.38 × 10−5

2.18 × 10−5 2.48 × 10−5

0.5 × 10−5 0.65 × 10−5

0.94 × 10−5 0.94 × 10−5

3.62 × 10−5 4.07 × 10−5

It could be seen from Figure 2 that the online supplementing oxidant would impose an obvious effect on the NOx absorption process. When the oxidant supplementary rate increased to 3.29 × 10−5 or 4.38 × 10−5 mol·min−1, NO and NO2 concentrations in the outlet gas changed very slightly with the scrubbing duration. It could be assumed that the oxidant supplementary rate approached to the oxidant consumption rate at the moment. Then, the reactions between targeted pollutants and oxidants could be analyzed roughly. The gas molar rates were calculated based on the data measured from 10th to 60th min during the cyclic scrubbing process. At first, it was supposed that SO2 in flue gas was removed completely through hydrolysis and NaClO2 oxidation, and then NaClO2 consumption molar rates for SO2 absorption could be calculated according to equations of (2−6). Similarly, NaClO2 consumption molar rates for oxidizing NO into NO2 and converting NO into NO3− could be calculated according to eqs 17 and 18, respectively. The calculation results are shown in Table 1. As shown in Table 1, when the NaClO2 supplementary molar rate was 3.29 × 10−5 mol·min−1, the total oxidant consumption rate was calculated to be 3.62 × 10−5 mol·min−1. The calculated value was a little higher than the oxidant supplementary molar rate. On the one hand, it implies that the NaClO2 oxidant has achieved an extremely high utilization by combining cyclic scrubbing with online supplementing. On the other hand, it suggests that there might be some other reaction pathways for the removal of NOx and SO2 except for the above-mentioned ones. It was thought that some SO32− ions might react with NO2 through eq 19.34−36 SO2 absorption reactions mainly take place at the surface of spraying droplets in the scrubbing reactor. Though the oxidative power of NaClO2 is strong, it is still possible for the hydrolysis product of SO32− to react with NO2 before being further oxidized into SO42−.

Under the above-mentioned experimental conditions, SO2 can attain 100% removal efficiency due to its high solubility in water and good reactivity with scrubbing solution. The reaction pathways for SO2 absorption are as follows20,23 SO2(g) ↔ SO2(l)

(2)

SO2(l) + H 2O → H 2SO3

(3)

H 2SO3 → H+ + HSO3−

(4)

HSO3− ↔ H+ + SO32 −

(5)

2SO32 − + ClO2− → 2SO4 2 − + Cl−

(6)

SO42−

Because SO2 will be oxidized into in the absorption process, it will consume some NaClO2 oxidant. As shown in Figure 2, when no extra NaClO2 was supplemented into the scrubbing solution, outlet NO concentration decreased quickly from 1005 to 333 ppm at the very beginning of the scrubbing process, while the outlet NO2 concentration increased sharply from 5 to 163 ppm. It illustrated that NO could be oxidized into NO2 efficiently by NaClO. During the scrubbing process, it is considered that NOx are mainly removed through the following reactions21,23,31−33 NO(g) ↔ NO(l) −

(7) −

2NO(l) + ClO2 → 2NO2(l) + Cl

(8)

NO2(l) ↔ NO2(g)

(9)

NO2(g) + NO(g) ↔ N2O3(g)

(10)

2NO2(g) ↔ N2O4(g)

(11)

N2O3(g) ↔ N2O3(l)

(12)

N2O4(g) ↔ N2O4(l)

(13)

N2O3(l) + H 2O → 2HNO2

(14)

N2O4(l) + H 2O ↔ HNO2 + HNO3

(15)

2NO2− + ClO2− → 2NO3− + Cl−

(16)

2SO32 − + 2NO2 → 2SO4 2 − + N2

When the oxidant supplementary molar rate increased from 3.29 × 10−5 to 4.38 × 10−5 mol·min−1, averaged NOx removal efficiency increased from 46.6 to 53.0%. The oxidative power of scrubbing solution was positively related with the oxidant supplementary molar rate, so it is reasonable to obtain a higher NOx removal efficiency with a higher oxidant supplementary molar rate. But the total oxidant consumption rate was calculated to be 4.07 × 10−5 mol·min−1, which was a little lower than the NaClO2 supplementary molar rate of 4.38 × 10−5 mol·min−1. The reason was ascribed to the enhanced oxidative power of scrubbing solution which would oxidize SO32− into SO42− effectively. 3.2. Effect of Buffering Solution pH. Cyclic scrubbing solutions used in the experiments were prepared by adding the oxidant powder into pH-buffered solution. The role of pHbuffering reagents in solution was used to maintain solution pH during the cyclic scrubbing process. The effect of buffering solution pH on NOx and SO2 removal performance was

Because NO is oxidized and converted to NO2 or NO3−, overall reactions of NO removal can be expressed as eqs 17 and 18. 2NO + NaClO2 → 2NO2 + NaCl

(17)

4NO + 3NaClO2 + 2H 2O → 4HNO3 + 3NaCl

(18)

(19)

When no extra NaClO2 was supplemented into the scrubbing solution, NaClO2 will be consumed gradually with the proceeding of cyclic scrubbing. Therefore, it was normal that the outlet NO concentration increased while the outlet NO2 concentration decreased slowly because of the reduction of the oxidant in cyclic scrubbing system. 6593

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Energy & Fuels studied, and the basic experimental conditions were: the volume of pH-buffered solution was 0.25 L and NaClO2 concentration was 1 × 10−2 mol·L−1. The pH of buffering solution was varied in the range of 2.4−8.0. Online supplementary molar rate of NaClO2 was 4.38 × 10−5 mol· min−1. Flue gas flow was 1.5 L·min−1, in which initial NO and SO2 concentrations were 1000 and 300 ppm, respectively. Figure 3 presents the changes of pH and ORP values of scrubbing solutions with cyclic scrubbing duration. It can be

divided into two steps: the first step is the hydrolysis of SO2 in water. Because the solubility of SO2 in water is good, SO2 will dissolve in water quickly, and hydrolyze to produce SO32−. The second step is the oxidation of SO32− into SO42− by oxidative species. Even when solution pH is higher than 7, the alkaline ambient is beneficial for SO2 absorption and hydrolysis. Also, then SO32− will be oxidized further into SO42− by active chlorine species in NaClO2 solution. Figure 4 presents the changes of outlet NOx concentrations with scrubbing duration under various buffering solution pH

Figure 3. Changes of pH and ORP values of scrubbing solutions with scrubbing duration under various initial buffering solution pH values.

Figure 4. Changes of NO and NO2 concentrations at the reactor outlet with scrubbing duration under various initial buffering solution pH values.

seen that scrubbing solution pH changed little after scrubbing for an hour. The buffer reagents have suppressed the decrease of solution pH effectively. As shown in Figure 3, the lower scrubbing solution pH is, the higher solution ORP is. It is known that solution ORP represents the relative degree of oxidizability or reducibility of the medium. Effective compositions in NaClO2 solution are available chlorine (or active chlorine) species, which mainly consist of HClO2, ClO2, and ClO2−. The oxidative power for these active species is in the order of HClO2 > ClO2 ≫ ClO2−.37 Normally, the fractional compositions of active chlorine species in NaClO2 solution vary largely with solution pH. With the decrease of solution pH from 8.0 to 2.4, more NaClO2 in solution will exist in forms of HClO2 and ClO2. Thus, cyclic scrubbing solution ORP increased obviously with the decrease of solution pH.38 As expected, a SO2 removal efficiency of 100% was reached during the whole scrubbing process. As mentioned above, the absorption of SO2 during the wet scrubbing process can be

values. It can be seen that buffering solution pH has exhibited a great influence on NO removal performance. Especially when scrubbing solution pH was 2.4 or 3.6, NO could be removed completely at the start of the scrubbing process. The majority of oxidants exist in forms of HClO2 and ClO2 in a strong acidic medium. They would oxidize NO into NO2 in a very efficient way. Here, the oxidant supplementary molar rate is kept the same, but oxidant consumption rates for scrubbing solutions with various pH values could be very different. When ClO2 in acidic solutions was relatively excess for NOx and SO2 oxidation, it might escape from the liquid phase, resulting in a loss of the oxidant. Then, NaClO2 concentration in cyclic scrubbing solution would decrease gradually, with the proceeding of cyclic scrubbing. As expected, there was an increasing trend for the outlet NO concentration after scrubbing for a period. Though the strong acidic medium is helpful to enhancing the oxidative power of scrubbing solution, 6594

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NOx concentrations changed slightly during the majority of the scrubbing process. After scrubbing for 60 min, the lower the inlet NO concentration was, the lower the outlet NOx concentrations were. For simple comparison, averaged values of outlet NOx concentrations and NOx removal efficiencies have been calculated for the whole scrubbing duration, and the results are presented in Figure 6. It showed that with inlet NO

excess ClO2 may escape from the liquid, resulting in a secondary pollution. Besides, it is easy to give rise to a severe corrosion problem. When scrubbing solution was neutral or weak alkaline, most of NaClO2 in solution exist in the form of ClO2−. The oxidative power of scrubbing solution was much weaker than those in acidic conditions. As shown in Figure 3, after scrubbing for 60 min, scrubbing solution pH decreased slightly from 8.0 to 7.5. Obviously, the neutral or alkaline medium has suppressed NO oxidation performance. After scrubbing for an hour, NO removal efficiency was about 60% for scrubbing solutions with buffered pH values of 7−8. 3.3. Effect of Inlet NO Concentration. Effect of inlet NO concentration on NOx and SO2 removal performance was investigated and the basic experimental conditions were: the volume of cyclic scrubbing solution was 0.25 L with a buffered pH of 6 and an initial NaClO2 concentration of 1 × 10−2 mol· L−1. Online supplementary molar rate of NaClO2 was 4.38 × 10−5 mol·min−1. Flue gas flow was 1.5 L·min−1 with an inlet SO2 concentration of 300 ppm. Inlet NO concentrations were varied in the range of 200−1000 ppm. In the experiments, all of SO2 had been removed completely. The changes of outlet NOx concentrations with scrubbing duration under various initial NO concentrations are shown in Figure 5. At the beginning of the cyclic scrubbing process, there was a sharp decrease of the outlet NO concentration and a quick increase of the outlet NO2 concentration. Then, outlet

Figure 6. Changes of NOx concentration at the reactor outlet with various initial NO concentrations in flue gas.

concentration increasing from 200 to 1000 ppm, averaged NOx removal efficiency increased from 36.5 to 53.1%. It was mainly ascribed to the enhanced gas-liquid mass transfer between NOx and oxidant during the scrubbing process. 3.4. Effect of Inlet SO2 Concentration. Figure 7 shows the changes of outlet NOx concentrations during scrubbing duration under various initial SO2 concentrations. In the tests, inlet SO2 concentrations were varied in the range of 0−500 ppm. It can be seen from Figure 7 that outlet NOx concentrations were kept at a relatively stable level during the cyclic scrubbing process. For comparison, averaged NOx concentrations at the reactor outlet and NOx removal efficiencies were calculated, and the results were shown in Table 2. With inlet SO2 concentration increasing from 0 to 500 ppm, averaged NO concentration at the outlet increased from 136 to 329 ppm, while averaged NO2 concentration at the outlet decreased from 235 to 157 ppm. It resulted in a decrease of averaged NOx removal efficiency from 62.9 to 51.4%. It demonstrated that when NaClO2 supplementary molar rate was kept constant, there was a competition between NOx and SO2 to react with oxidative species in solution. Similarly, NaClO2 consumption molar rates during the cyclic scrubbing process could be calculated according to reaction eqs 2−6, 17, 18, and the result is shown in Figure 8. With inlet SO2 concentration increasing from 0 to 500 ppm, the oxidant consumption molar rate increased from 3.68 × 10−5 to 4.46 × 10−5 mol·min−1. It implied that the increase of co-existing SO2 concentration in flue gas was helpful to improve oxidant utilization. Note that NaClO2 supplementary molar rate was maintained at 4.38 × 10−5 mol·min−1, which was a little lower than the calculated oxidant consumption molar rate corresponding to an inlet SO2 concentration of 500 ppm. It suggested that some other reaction pathways played an obvious role in NOx and SO2 absorption except for the above-mentioned ones for calculating oxidant consumption

Figure 5. Changes of NOx concentration at the reactor outlet with scrubbing duration under various initial NO concentrations in flue gas. 6595

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Figure 8. Calculated NaClO2 consumption rates during the cyclic scrubbing process under various inlet SO2 concentrations in flue gas.

Figure 7. Changes of NOx concentrations at the reactor outlet with scrubbing duration under various inlet SO2 concentrations in flue gas.

Table 2. Calculated Results of Averaged NOx Concentrations at the Reactor Outlet and NOx Removal Efficiencies for the Whole Scrubbing Process inlet SO2 concentration (ppm)

outlet NO concentration (ppm)

outlet NO2 concentration (ppm)

NOx removal efficiency (%)

0 100 200 300 400 500

136 169 229 255 308 329

235 226 209 202 167 157

62.9 60.5 56.2 54.3 52.5 51.4

molar rate. It is normal that with the increase of the inlet SO2 concentration in flue gas, more SO32− ions would be generated in the scrubbing process. A number of SO32− ions might react with NO2 at the interfaces between fine droplets and gas phase through eq 19 before being oxidized into SO42− by oxidant in liquid phase. 3.5. Effect of Ethanol Concentration in Solution. Here, various volumes of ethanol was added in cyclic scrubbing solutions to further investigate the effect of ethanol additive on SO2 on NOx removal performance, and the results are shown in Figure 9. The basic experimental conditions were that: the volume of cyclic scrubbing solution was 0.25 L with a buffered pH of 6 and an initial NaClO2 concentration of 1 × 10−2 mol· L−1. The online supplementary molar rate of NaClO2 was 4.38 × 10−5 mol·min−1. The flue gas flow was 1.5 L·min−1 with an inlet NO concentration of 1000 ppm and inlet SO 2

Figure 9. Changes of NOx concentration at the reactor outlet with scrubbing duration under various ethanol concentrations in scrubbing solution.

concentration of 300 ppm. Ethanol concentrations in scrubbing solution were varied in the range of 0−2.0 vol %. As expected, SO2 removal efficiencies were still 100% in the tests. It was interesting that the introduction of ethanol imposed an obvious influence on NOx removal. With the increase of ethanol concentration in scrubbing solution, there was a slight decrease in outlet NO concentration with the proceeding of the cyclic scrubbing process. After scrubbing solution with 0.8 vol % ethanol for 60 min, outlet NO 6596

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concentration in solution would decrease gradually, leading to the increase of outlet NO concentration and the decrease of outlet NO2 concentration. Note that during the whole cyclic scrubbing process, outlet O2 and CO2 concentrations are kept constant at 10 and 5%, respectively. The co-existing O2 would enhance NO oxidation, and it would also oxidize SO32− into SO42− through eq 21. But the existence of ethanol in scrubbing solution would effectively inhibit SO32− being oxidized by NaClO2 in solution and O2 in flue gas, and thus enhancing NO2 absorption to a large extent.

concentration decreased to 103 ppm, which was much lower than that for solution without ethanol. But it also showed an increase of the outlet NO concentration when further increasing ethanol concentration from 0.8 to 2.0 vol %. It implied that excess ethanol in solution might impose a negative effect on gas-liquid mass transfer. As shown in Figure 9b, with the introduction of ethanol in scrubbing solution, outlet NO2 concentration had been reduced greatly from ∼200 to ∼50 ppm. As an additive, ethanol would not react with oxidant or gas pollutants in the scrubbing process. But ethanol could be an excellent oxidation inhibitor and an absorption enhancer for NO2 absorption,39 which could inhibit SO32− being oxidized by oxidants in solution effectively. Thus, it was possible that ethanol could enhance NO2 absorption through the reaction eq 19, which was beneficial to improve oxidant utilization rate and NOx removal efficiency. 3.6. Effect of Co-Existing O2 and CO2. Experiments were conducted to preliminarily study the effect of co-existing O2 and CO2 in simulated flue gas on SO2 and NOx removal performance. The basic experimental conditions were: the volume of cyclic scrubbing solution was 0.25 L with a buffered pH of 6 and an initial NaClO2 concentration of 1 × 10−2 mol· L−1. Online supplementary molar rate of NaClO2 was 4.38 × 10−5 mol·min−1. The flue gas flow was 1.5 L·min−1, in which the inlet concentration of NO, NO2, SO2, O2, and CO2 were 1000, 93, 300 ppm, 10, and 5%, respectively. Ethanol concentrations in scrubbing solution were 0.8 vol %. The existence of NO2 in initial simulated flue gas was mainly due to the oxidation of NO by O2 during the preparation process. 2NO + O2 → 2NO2

2SO32 − + O2 → 2SO4 2 −

(21)

To better understand the process chemistry of NOx and SO2 absorption, the anions in cyclic scrubbing solutions were determined using ion chromatography. The results of ion chromatographic analysis were depicted in Figure 11. There

(20)

At the very beginning of the cyclic scrubbing process as shown in Figure 10, outlet NO and SO2 concentrations

Figure 11. Ion chromatograms of the absorption solution.

was no SO32− or NO2− found in scrubbing solution. The main products of liquid phase desulfurization and denitrification during the cyclic scrubbing process were SO42− and NO3−. Active chlorine species in NaClO2 solution had been reduced to Cl−. As shown in Figure 11, with the increase of cyclic scrubbing duration, the concentrations of SO42−, NO3− ,and Cl− anions in sample solution increased obviously. The existence of PO43− ions in sample solutions was mainly derived from the phosphate salt used to prepare pH-buffered solution. According to the experimental results and discussions above, the possible reaction pathways for NOx and SO2 removal by cyclic scrubbing and online-supplementing NaClO2 solution was described in Figure 12. It was worthy to be pointed out that though the oxidative power of NaClO2 Figure 10. Changes of NOx and SO2 concentrations at the reactor outlet with scrubbing duration.

decreased sharply to zero, while the outlet NO2 concentration increased from 93 to 190 ppm. After scrubbing for 90 min, outlet NO, NO2, and SO2 concentrations were 0, 134, and 0 ppm, respectively. At the moment, the corresponding NOx and SO2 removal efficiencies were 87.7 and 100%, respectively. Here, the complete removal of NO was ascribed to the highefficient oxidation by NaClO2 in solution and O2 in flue gas. As shown in Figure 10, online injection of NaClO2 was stopped after cyclic scrubbing for 90 min. It was common that further proceeding with the scrubbing process, the oxidant

Figure 12. Possible reaction pathways for the absorption of NO and SO2 during the cyclic scrubbing process. 6597

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Energy & Fuels solution was strong, it was still possible for SO32− ions to react with NO2 at the gas-liquid interfaces. It was beneficial to reduce oxidant consumption for practical applications.

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4. CONCLUSIONS In this study, a wet scrubbing process for simultaneous removal of NO and SO2 from flue gas by cyclic spraying and onlinesupplementing pH-buffered NaClO2 solution was proposed and systematically investigated. The results showed that SO2 could be absorbed completely because of its high solubility. The increase of the NaClO2 supplementary molar rate was beneficial to enhance the oxidative power of scrubbing solution, leading to a relatively high NO removal efficiency. With the decrease of buffered solution pH, NO removal performance could be enhanced obviously, which was ascribed to the increase of fractional compositions of HClO2 and ClO2 in NaClO2 solution. When inlet NO concentration increased from 200 to 1000 ppm, the averaged NOx removal efficiency increased from 36.5 to 53.1%. When NaClO2 supplementary molar rate was kept constant, NOx removal efficiency decreased obviously with the increase of the inlet SO 2 concentration. There was a competition between SO2 and NOx to react with NaClO2 in solution. The introduction of ethanol in scrubbing solution could improve NOx removal performance greatly. The reason is that ethanol was helpful in inhibiting the oxidation of SO32− into SO42− effectively, thus resulting in a number of SO32− ions to react with NO2. When 10% O2 and 5% CO2 co-existed in flue gas, removal efficiencies of NOx and SO2 reached 87.7 and 100%, respectively. The results demonstrated that it was a feasible method to remove NOx and SO2 simultaneously from the exhaust gas by cyclic scrubbing and online supplementing NaClO2 solution.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z.H.). *E-mail: [email protected] (S.Y.). ORCID

Zhitao Han: 0000-0001-5501-6067 Shaolong Yang: 0000-0003-2566-9803 Xinxiang Pan: 0000-0002-0251-5679 Liguo Song: 0000-0002-5677-5556 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (grants 51779024, 51809100, 51709029, and 51779023), and the Fundamental Research Funds for the Central Universities (grants 3132018249, 3132016337 and 2018KFYYXJJ015).

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DOI: 10.1021/acs.energyfuels.9b01106 Energy Fuels 2019, 33, 6591−6599

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DOI: 10.1021/acs.energyfuels.9b01106 Energy Fuels 2019, 33, 6591−6599