Split, Partial Oxidation and Mixed Absorption: A Novel Process for

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Split, partial oxidation, and mixed absorption: A novel process for synergistic removal of multiple pollutants from simulated flue gas Ping Fang, Zijun Tang, Xiongbo Chen, Jianhang Huang, Dingsheng Chen, Zhixiong Tang, and Chaoping Cen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b05029 • Publication Date (Web): 03 Apr 2017 Downloaded from http://pubs.acs.org on April 8, 2017

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Industrial & Engineering Chemistry Research

Split, partial oxidation, and mixed absorption: A novel process for synergistic removal of multiple pollutants from simulated flue gas Ping Fang, Zijun Tang, Xiongbo Chen, Jianhang Huang, Dingsheng Chen, Zhixiong Tang, Chaoping Cen*

South China Institute of Environmental Sciences, Ministry of Environmental Protection, 510655 Guangzhou, PR China

ABSTRACT: In this paper, a new method of split, partial oxidation, and mixed absorption for the synergistic removal of SO2, NOx, and Hg0 is proposed. Factors that affect the multi-pollutant removal, such as flue gas split ratio, NaClO2 concentration, initial pH and temperature of NaClO2 solution in the oxidation reactor, SO2，NO，O2 and CO2 concentrations, gas flow rate, and species of alkali absorbent in the absorption reactor are investigated, with a special focus on NOx removal. Results show that SO2 and Hg0 are removed quite efficiently and are slightly affected by reaction conditions, while NOx removal is seriously affected by the above factors except O2 and CO2 concentrations. Under the best experimental conditions, the average removal efficiencies of SO2, NOx, and Hg0 reach more than 99%, 82%, and 95%, respectively. Meanwhile, the mechanism of multi-pollutant removal is deduced based on

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related literature, experimental phenomena, and reaction products. The novel process has enormous potential in the removal of multi-pollutant from flue gas.

KEYWORDS: Novel process; NOx; SO2; Hg0; Synergistic removal; Removal mechanism

1. INTRODUCTION

Air pollution problems, such as acid rain, haze, and photochemical smog, have been occurring frequently in China in recent years. SO2 and NOx are classified as major contributors to air pollution because they are precursors of ground-level ozone and fine particles, as well as contributors to acid rain, haze, and smog formation.1,2 A large amount of Hg0 from coal combustion is also emitted into the atmosphere every year. Controlling Hg0 pollution has received increasing attention because of the high volatility, high toxicity, persistence, and bioaccumulation of Hg0.3 Power plant boilers, industrial boilers, and industrial furnaces are considered the major sources of anthropogenic SO2, NOx, and Hg0 emissions. At present, the combined process of selective catalytic reduction + activated carbon injection system + electrostatic / bag filter + wet flue gas desulfurization (WFGD) is commonly used to control the pollution of power plant flue gas in China.4 However, very low SO2, NOx, and Hg0 emission values that can meet the most stringent emission standards can be achieved. The complex system, high cost, and large area required by this integrated system have made it unsuitable for treating flue gas from industrial boilers and furnaces in China. Dust removal and WFGD technology are widely used to control dust and SO2 from the flue gas of industrial boilers and furnaces. However, no cost-effective control technologies for Hg0 and NOx have


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been established. Hence, developing an advanced and cost-effective control technology for the synergistic removal of Hg0, NOx, and SO2 is an urgent concern.

The cooperative control system for multi-pollutant has the advantages of high pollutant removal efficiency and low investment. Recently, various multi-pollutant cooperative control technologies have been extensively studied, including non-thermal plasma,5,6 pulsed corona discharge,7 catalysis oxidation,8,9 advanced absorption process,10,11 and carbon-based material adsorption technology.12 WFGD technology is currently in wide use in China, and the advanced absorption process based on WFGD technology is considered the most promising method to control multi-pollutant in flue gas. At present, advanced oxidation absorption technology is studied more than other absorption technologies. The solubilities of NO and Hg0 in water are extremely low; therefore, converting insoluble NO and Hg0 to soluble NO2 and Hg2+ is the key to the oxidation absorption process. Many reagents, such as K2S2O8,13 KMnO4,11,14 K2FeO4,15 NaClO2,10,16 O3,17 H2O2,18 and ClO2,19 have been tested for multi-pollutant control. Of this group, KMnO4 and NaClO2 are effective reagents for the oxidation and absorption of NOx, SO2, and Hg0. Fang et al.11 used a complex absorbent containing urea and KMnO4 to simultaneously remove SO2, NO and Hg0 from flue gas. Under the optimal conditions, the removal efficiencies of SO2, NO and Hg0 could reach 98.78%, 53.05% and 99.21%, respectively. Zhao et al.20 used a vaporized complex oxidant consisting of H2O2 and NaClO2 to oxidize Hg0 and NO and then used Ca(OH)2 solution to absorb the oxidation products. Under the concentration ratio of H2O2 to NaClO2 of 4:0.1 mol/L and other optimal experimental conditions, the average removal efficiencies of SO2, NO, and Hg0 could


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reach 100%, 87%, and 92%, respectively. Amin et al.21 utilized 0.2 mol/L NaClO2 solution to simultaneously remove NOx and SO2, and 81% of the NO and 100% of the SO2 were simultaneously removed under optimal experimental conditions. However, KMnO4 and NaClO2 consumptions are large in the above techniques, and the industrial application of these technologies is limited because of technical, economic, and other problems.

References16,20 indicate that NO can be completely oxidized to NO2 by NaClO2; if NO2 cannot be effectively absorbed, then its escape will not only reduce NOx removal efficiency but also cause more serious pollution. Scholars22,23 found that equimolar amounts of NO and NO2 can react to form N2O3; NO2 can also be converted to N2O4, and N2O3 and N2O4 can easily be absorbed by an alkali solution (eq 1- eq 4).

NO + NO → N O

(1)

2NO → N O

(2)

N O + H O → 2HNO

(3)

N O + H O → HNO + HNO

(4)

Thus, if we divide the flue gas into two parts, oxidizing NO to NO2 in one part of the flue gas with NaClO2 solution to form the NO2-containing flue gas, then the NO2-containing flue gas can be mixed with the other NO-containing original flue gas to form the N2O3-containing mixed flue gas. An alkali solution can then be used to remove multi-pollutant in the mixed flue gas. This novel process can improve the economic feasibility and multi-pollutant removal


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efficiencies of the NaClO2 absorption method. It can also make the technology more suitable for industrial applications. Therefore, this research aims to evaluate the feasibility of synergistic removal of SO2, NOx, and Hg0 using the new process. Moreover, factors affecting the synergistic removal of multi-pollutant were investigated, with special focus on the NOx removal, and the removal mechanism of SO2, NOx, and Hg0 by this process was hypothesized.

2. EXPERIMENTAL SETUP

The reagents used in the experiments, such as NaOH (96.00%, AR), H2SO4 (98.00%, AR), and NaClO2 (80.00%, AR), were obtained from Guangzhou Chemical Reagent Factory. The experimental apparatus is composed of a simulated flue gas generation system, an oxidation– absorption system, and an online detection system (Figure 1). The oxidation and absorption reactors with a diameter of 10 cm and a height of 25 cm were all made of borosilicate glass, which were heated by a temperature-controlled heating belt. The absorption reactor was filled with ceramic raschig rings with a diameter of 12.5 mm, a length of 12.5 mm, and a filling height of 5 cm. The oxidation reactor was an empty column. The pipe used in the experimental apparatus was made with Teflon, and heated with a heating belt to 120 °C to avoid condensation of Hg0 inside the pipe.


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Figure 1. Schematic diagram of the experimental apparatus. SO2, NO, O2, CO2, and N2 gases (Gas Co., Ltd. of Zhuo Zheng, Guangzhou) were supplied by cylinders, and their flow were controlled by mass flow controllers. NO, SO2, O2, CO2, and N2 were mixed in pipe mixer 1 to form mixed gas A. Mixed gas B (Hg0 vapor) was produced from a mercury osmotic tube (40±2 ng/min at 50 °C, VICI Metronics Co., USA) inside a U-shaped glass tube that was heated in a thermostatic water bath (HH-601, Changzhou Huanyu Jintan Scientific Instrument Factory) with 250 mL/min of N2 used as carrier gas. Next, mixed gases A and B were mixed in pipe mixer 2 to form the simulated flue gas. Total flue gas flow (Q) was maintained at 2 L/min. As depicted in Figure 1, the flue gas was divided into two parts (Q1 and Q2) in each typical experiment. Q1 was sent to pass through the oxidation reactor, and then NO was oxidized to NO2 by the NaClO2 solution (1 L) to form the NO2-containing flue


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gas. Q2 was sent directly into the gas mixing pipe, mixed with Q1 (NO2-containing flue gas), and then sent into the absorption reactor. Finally, multiple pollutants were effectively absorbed by the NaOH solution (1 L). The flow ratios of Q1 and Q2 were adjusted by mass flow controllers. In this study, the concentration ranges of SO2, NO, Hg0, O2, CO2 in the simulated flue gas were 0–3432 mg/m3, 268–938 mg/m3, 30–32 µg/m3, 1–15% (v/v), and 0–20% (v/v) respectively. Each experiment lasted 2 h. O2, CO2, NO, NO2, and SO2 concentrations were determined by an ECOM-J2KN Flue Gas Analyzer (RBR Company, Germany), and Hg0 concentration was measured with an RA-915M Mercury Online Analyzer (Lumex LTD. Company, Russia).

The solution pH was measured with an MP511 pH Detector (Shanghai Precision Instruments Co., Ltd.). NO2−, NO3−, SO42−, Cl−, ClO2−, and ClO3− concentrations were measured with an ion chromatography system (Metrohm 883, Switzerland). SO32− concentration was measured by spectrophotometry,22 and Hg2+ concentration was determined with an RA-915M Mercury Online Analyzer.

In this study, a sequence of experiments was carried out in order to evaluate the influences of different operational parameters on multi-pollutant removal efficiencies. The experimental conditions of the individual experiments were shown in Table 1.

Table 1. Experimental conditions of the individual experiments No. 1

Experiment Synergistic removal of SO2, NOx and Hg0 by the existing process

Experimental Conditions 0

Q=2 L/min, [Hg ]=31.2 µg/m3, [NO]=670 mg/m3, [SO2]=2860 mg/m3, O2=7%, [NaClO2]=0.08 wt.%, [NaOH]=5 wt.%, Tabsorption=40 ◦C


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2

Synergistic removal of SO2, NOx and Hg0 by the new process

3

Effect of flue gas split ratio

4

Effect of NaClO2 concentration Effect of initial pH of NaClO2 solution Effect of temperature of NaClO2 solution

5 6 7

Effect of SO2 concentration

8

Effect of NO concentration

9

Effect of O2 concentration

10

Effect of CO2 concentration

11

Effect of gas flow rate

12

Effect of species of alkali absorbent on NOx removal efficiency

13

Parallel tests

14

NaClO2 consumption rate

15

Product analysis

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Q=2 L/min, Q1:Q2=0.75:1, [Hg0]=31.8 µg/m3, [NO]=670 mg/m3, [SO2]=2860 mg/m3, O2=7%, [NaClO2]=0.08 wt.%, [NaOH]=5 wt.% , Toxidation=40 ◦C, Tabsorption=40 ◦C Q=2 L/min, [Hg0]=32 µg/m3, [NO]=670 mg/m3, [SO2]=2860 mg/m3, O2=7%, [NaClO2]=0.08 wt.%, [NaOH]=5 wt.%, Toxidation=40 ◦C, Tabsorption=40 ◦C Q=2 L/min, Q1:Q2=0.75:1, [Hg0]=30.6 µg/m3, [NO]=670 mg/m3, [SO2]=2860 mg/m3, O2=7%, [NaOH]=5 wt.%, Toxidation=40 ◦C, Tabsorption=40 ◦C Q=2 L/min, Q1:Q2=0.75:1, [Hg0]=31.2 µg/m3, [NO]=670 mg/m3, [SO2]=2860 mg/m3, O2=7%, [NaClO2]=0.08 wt.%, [NaOH]=5 wt.%, Toxidation=40 ◦C, Tabsorption = 40 ◦C Q=2 L/min, Q1:Q2=0.75:1, [Hg0]=30.6 µg/m3, [NO]=670 mg/m3, [SO2]=2860 mg/m3, O2=7%, [NaClO2]=0.08 wt.%, [NaOH]=5 wt.%, Tabsorption=40 ◦C Q=2 L/min, Q1:Q2=0.75:1, [Hg0]=31.8 µg/m3, [NO]=670 mg/m3, O2=7%, [NaClO2]=0.08 wt.%, [NaOH]=5 wt.%, Toxidation=40 ◦C, Tabsorption=40 ◦C Q=2 L/min, Q1:Q2=0.75:1, [Hg0]=31.5 µg/m3, [SO2]=2860 mg/m3, O2=7%, [NaClO2] =0.08 wt.%, [NaOH]=5 wt.%, Toxidation=40 ◦C, Tabsorption=40 ◦C Q=2 L/min, Q1:Q2=0.75:1, [Hg0]=30.7 µg/m3, [SO2]=2860 mg/m3, [NO]=670 mg/m3, [NaClO2] =0.08 wt.%, [NaOH]=5 wt.%, Toxidation=40 ◦C, Tabsorption=40 ◦C Q=2 L/min, Q1:Q2=0.75:1, [Hg0]=30.7 µg/m3, [SO2]=2860 mg/m3, [NO]=670 mg/m3, O2=7%, [NaClO2] =0.08 wt.%, [NaOH]=5 wt.%, Toxidation=40 ◦C, Tabsorption=40 ◦C Q1:Q2=0.75:1, [Hg0]=31.2 µg/m3, [SO2]=2860 mg/m3, [NO]=670 mg/m3, O2=7%, [NaClO2] =0.08 wt.%, [NaOH]=5 wt.%, Toxidation=40 ◦C, Tabsorption=40 ◦C Q=2 L/min, Q1:Q2=0.75:1, [Hg0]=32 µg/m3, [NO]=670 mg/m3, [SO2]=2860 mg/m3, O2=7%, [NaClO2]=0.08 wt.%, [NaOH]=5 wt.%, [urea]=5 wt.%, [CaCO3]= 20 wt.%, Toxidation=40 ◦C, Tabsorption=40 ◦C Q=2 L/min, Q1:Q2=0.75:1, [Hg0]=31.8 µg/m3, [NO]=670 mg/m3, [SO2]=2860 mg/m3, O2=7%, [NaClO2]=0.08 wt.%, [NaOH]=5 wt.%, Toxidation=40 ◦C, Tabsorption= 40 ◦C Q=2 L/min, Q1:Q2=0.75:1, [Hg0]=31.8 µg/m3, [NO]=670 mg/m3, [SO2]=2860 mg/m3, O2=7%, [NaClO2]=0.08 wt.%, [NaOH]=5 wt.%, Toxidation=40 ◦C, Tabsorption= 40 ◦C Q=2 L/min, Q1:Q2=0.75:1, [Hg0]=31.8 µg/m3, [NO]=670 mg/m3 , [SO2]=2860 mg/m3, O2=7%, [NaClO2]=0.08 wt.%, [NaOH]=5 wt.%, Toxidation=40 ◦C, Tabsorption= 40 ◦C

The NO, SO2, and Hg0 removal efficiencies were calculated by the following equation:

η=

× 100%

(5)

where η is the Hg0, NOx, or SO2 removal efficiency; and Cinlet and Coutlet are the inlet and outlet concentrations of Hg0, NOx, and SO2, respectively.

3. RESULTS AND DISCUSSION

3.1. Synergistic removal of SO2, NOx, and Hg0 by the existing process. In this experiment, the oxidation reactor was not used, and all the flue gas was sent directly into the absorption reactor. The effect of NaClO2+NaOH solution on SO2, NO, and Hg0 removal is


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shown in Figure 2. The results indicate that SO2 and Hg0 could be absorbed quite efficiently, and 100% of the SO2 and 97.46% of the Hg0 were synergistically removed. The NOx average removal efficiency was lower than that of SO2 and Hg0 at only 47.29%. However, compared with our previous work,11 the removal efficiencies of NOx and Hg0 using NaClO2+NaOH solution were much better than those using only the alkali absorption solution, in which the NO and Hg0 removal efficiencies were 7.98% and 15.65% respectively. A large amount of NO2 was also detected in the outlet flue gas. As depicted in Figure 3, the NO2 concentration increased rapidly and then almost stabilized as the reaction proceeded, and the average emission concentration of NO2 was 352 mg/m3 throughout the experiment. The main reason for the NO2 escape is that the gas–liquid contact time is too short, and the NO2 molecules do not have sufficient time to react with the solution. Thus, the result shows that NO can be effectively oxidized to NO2 by the NaClO2 solution, but the effective absorption of NO2 is a problem in the existing process.

Figure 2. Synergistic removal of SO2, NOx and Hg0 by the existing process.


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Figure 3. Effect of the existing process on NO and NO2 emissions. 3.2. Synergistic removal of SO2, NOx, and Hg0 by the new process. In this experiment, the total flue gas flow (Q) was kept at 2 L/min, and 42.5% of the total amount of flue gas (Q1=0.85 L/min) was introduced into the oxidation reactor. The remaining flue gas (Q2=1.15 L/min) was sent directly into the gas mixing pipe, and the flue gas split ratio (Q1:Q2) was approximately 0.75:1. Figure 4 shows the effect of the new process on the synergistic removal of multi-pollutant.

Figure 4. Synergistic removal of SO2, NOx and Hg0 by the new process.


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Figure 5. Effect of the new process on NO and NO2 emissions. As displayed in Figure 4, the new process has a good ability to remove SO2 and Hg0; their removal efficiencies were more than 99% and 95% respectively. Interestingly, the new process can effectively enhance the NOx removal compared with the existing process (Figure 2). Figure 4 shows that at the beginning of the reaction, the NOx removal efficiency increased rapidly with the increase of reaction time, and then the increasing trend slowed down until the NOx removal was stable. Finally, 85.5% NOx removal efficiency was achieved. As Figure 5 illustrates, the NO emission concentration in the outlet gas initially decreased rapidly with the increase of reaction time, then tended to be relatively stable (approximately 13.4 mg/m3), and at the later stage of the reaction, the NO emission concentration gradually increased. Meanwhile, the NO2 emission concentration in the outlet gas was maintained at approximately 61.5 mg/m3. Compared with the existing process, the new process can achieve high SO2, NOx, and Hg0 removal efficiencies and can better control the exhaust NO2 emissions.

3.3. Effect of flue gas split ratio. As can be seen from eq 1 and eq 3, to achieve high NOx


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removal, a 1:1 molar ratio of NO to NO2 must be controlled. Thus, controlling the flue gas split ratio is crucial. The effect of this ratio on the SO2, NOx, and Hg0 removal efficiencies is shown in Figure 6. The results indicate that the flue gas split ratio had little effect on SO2 removal; all SO2 removal efficiencies were maintained at 100% with the change of flue gas split ratio. However, the Hg0 and NOx removal efficiencies were greatly affected by the flue gas split ratio. As depicted in Figure 6, Hg0 and NOx removal efficiencies increased rapidly as the flue gas split ratio increased. When the flue gas split ratio varied from 0.5 to 0.75, the Hg0 and NOx removal efficiencies rapidly increased from 75.25% to 97.68% and from 69.68% to 84.61% respectively. When the flue gas split ratio further increased, the removal efficiencies of Hg0 and NOx decreased. For example, when the split ratio was 2:1, the NOx and Hg0 removal were reduced to only 14.48% and 56.62% when the reaction was continued for 100 min. Furthermore, the final average removal efficiencies were only 59.07% and 88.57% respectively. Research shows that NO2 can be used as a gas oxidant and can directly oxidize Hg0 into Hg2+.24,25 Thus, NO2 plays an important role in the removal of Hg0 in the mixing tube and in the absorption reactor. When the split ratio was low, the amount of NO2 produced and emitted from the oxidation reactor was less, the molar ratio of NO to NO2 in the mixing pipe deviated from 1:1, resulting in a poor NOx removal, and the lower oxidation and removal of Hg0. However, the higher the flue gas split ratio, the faster the consumption rate of NaClO2 and the shorter the effective removal time of pollutants.

In addition, it was found that the NO concentration in outlet gas was higher than the NO2 concentration when the flue gas split ratio was low, which was contrary to the outcome when


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the flue gas split ratio was high. With pollutant removal and economic costs considered, the optimal split ratio range of 0.7:1–1:1 for the new process and the optimal split ratio of 0.75:1 were selected.

When the flue gas split ratio remained unchanged at 0.75:1, the SO2 and Hg0 removal efficiencies remained constant at 99%–100% and 95%–100% respectively in all tests. However, the NOx removal was affected by many factors. Therefore, we focused on NOx removal in the next series of experiments.

Figure 6. Effect of flue gas split ratio on the removal efficiencies of multi-pollutant. 3.4. Effect of NaClO2 concentration. NaClO2 concentration has a significant effect on NOx absorption. As shown in Figure 7, when NaClO2 concentration in the oxidation reactor changed from 0.016 wt.% to 0.04 wt.%, NOx removal efficiencies sharply increased from 39.32% to 76.98% and then gradually increased. NOx removal efficiencies were maintained between 82% and 85% at the NaClO2 concentration range of 0.064–0.08 wt.%, and thereafter, decreased with the increase of NaClO2 concentration between 0.08 wt.% and 0.16 wt.%.


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When the NaClO2 concentration was low, the consumption rate of NaClO2 was faster, and the amount of NO2 produced by the oxidation reactor was less. When the NaClO2 concentration was high, NaClO2 played an inhibition role in the NO2 absorption,26 leading to the escape of a large amount of NO2 from the oxidation reactor. In both cases, the molar ratio of NO to NO2 in the mixing pipe deviated from 1:1, resulting in poor NOx removal. During the experiment, it was found that when the NaClO2 concentration was low, the NO concentration in the exhaust was higher than that of NO2 concentration; the effect was contrary when the NaClO2 concentration was high. A heavy, pungent odor similar to the smell of disinfectant was also detected during the test when the NaClO2 concentration was more than 0.08 wt.%. The literature23,26,27 reported that ClO2 and Cl2 produced by the decomposition of NaClO2 were not completely absorbed by the absorption liquid and were released into the air. Thus, when pollutant removal and economic costs were considered, the optimum range of NaClO2 concentration was from 0.06%–0.08 wt.%.

Figure 7. Effect of NaClO2 concentration on NOx removal efficiencies. 3.5. Effect of the initial pH of NaClO2 solution. pH has a significant influence on the


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stability and oxidation potential of NaClO2 in the solution. Thus, it is the key factor for multi-pollutants synergistic removal. Figure 8 shows the effect of the initial pH of the solution on NOx removal. In this study, the initial pH of NaClO2 solution at a concentration of 0.08 wt.% was 10. Figure 8 shows that the average removal efficiencies of NOx increased slowly from 73.32% to 82.61% in the pH range of 2–10, but the initial NOx removal increased with the decreasing solution pH. The oxidation of NaClO2 increased with the decrease of initial pH, while NaClO2 could decompose to produce more ClO2 at a low pH,23 leading to increased NO2 production and emission. Thus, the molar ratio of NO to NO2 in the mixing pipe would deviate from 1:1, resulting in poor NOx removal and shorter effective removal time under the low pH conditions. In addition, the color of the NaClO2 solution changed from colorless to yellowish green and then gradually deepened with the decrease of the initial pH. This outcome indicated that large amounts of ClO2 might be formed from the decomposition of chlorite under low pH conditions. When the initial pH of the solution exceeded 10, the NOx removal decreased with the increasing pH. This observation is attributed to the weak oxidation of NaClO2 under strong alkaline conditions.28 As Figure 8 illustrates, when the initial pH was 12, the NOx removal efficiencies initially increased gradually with the decrease of the solution pH, and then decreased with the reduction of NaClO2 in the solution, finally reaching the NOx average removal efficiency of only 68.00%. In additon, the research found that NO concentration increased and NO2 concentration decreased in the outlet gas with the increase in pH. The NOx concentration in the outlet gas initially decreased and then increased with the increasing pH. Overall, in this set of


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experiments, the denitration efficiencies could be stable at 76%–83% when the pH was 6–10. Therefore, the optimal initial pH range of the NaClO2 solution in the new process was selected as 6.0–10.0, and the best pH was determined as 10.

Figure 8. Effect of initial pH of NaClO2 solution on NOx removal efficiencies. 3.6. Effect of temperature of NaClO2 solution. Figure 9 presents the effect of temperature of the NaClO2 solution on NOx removal. The NOx removal decreased with the increase of solution temperature. NOx removal efficiencies were 89.57%, 84.43%, 80.06%, 78.93%, 77.02%, and 72.32% when the solution temperatures were 25, 40, 50, 55, 60, and 70 °C, respectively. Meanwhile, the study found NO2 and NOx concentrations increased with the increase of reaction temperature in the exhaust, but the NO concentration almost remained stable when reaction temperature varied from 25 °C to 70 °C. This observation was attributed to the accelerated diffusion rate of NO in the NaClO2 solution with the increase of reaction temperature. Moreover, the high temperature could promote ClO2 generation, resulting in an increase in NO oxidation rate and the amount of NO2 generation. Meanwhile, a high reaction temperature will promote increased NO2 emission from the oxidation reactor. A certain


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amount of ClO2 likewise emits into the mixing pipe, and NO is further oxidized to NO2 by the ClO2 in the mixing pipe, resulting in an increasing NO2/NO concentration ratio. Thus, a large amount of NO2 leaves the absorption reactor without absorption because of the constant gas– liquid contact time, and the NO removal efficiency decreased with the increase of the solution temperature. Generally, the temperature of the circulating absorption liquid of the wet desulfurization system is approximately 40 °C - 50 °C. The temperature of the oxidation and absorption solutions will be lower than the solution temperature of WFGD because of the special operation mode of the new process. Considering the actual situation, the temperatures of the oxidation and absorption solutions were all selected as 40 °C in this study.

Figure 9. Effect of temperature of NaClO2 solution on NOx removal efficiencies. 3.7. Effect of SO2 concentration. Figure 10 displays the effect of SO2 concentration on the NOx removal in the range of 0–3432 mg/m3. As shown in Figure 10, the NOx removal is significantly affected by SO2 concentration. When no SO2 was detected in the flue gas, NOx removal efficiency was only 43.98%, and the NO average concentration of 302.8 mg/m3 was considerably higher than that of NO2, which was 90.2 mg/m3 in the outlet gas. Then NOx


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removal efficiency increased rapidly with the increasing SO2 concentration; for example, when SO2 concentration changed from 0 mg/m3 to 572 mg/m3, the NOx removal efficiency sharply increased from 43.98% to 68.08%, while the NO concentration decreased and the NO2 concentration increased in the outlet gas. This observation is attributed to the increase in oxidation of NaClO2 with the decrease of solution pH. When no SO2 was detected in the flue gas, the pH of the NaClO2 solution decreased slowly, resulting in lower NO oxidation rate, NO2 production, and NOx removal efficiency. When the SO2 concentration was further increased, the NOx removal initially increased and then decreased. When SO2 concentration varied from 2860 mg/m3 to 3432 mg/m3, NOx removal efficiency sharply decreased from 84.43% to 70.14%. Meanwhile, the higher the SO2 concentration, the shorter the effective removal time of NOx. The main reason for this outcome was that the further increase of SO2 concentration caused severe competitive reactions of NO and SO2 for the limited oxidant. A high concentration of SO2 can consume large amounts of NaClO2 and inhibit the oxidation of NO and the formation of NO2, resulting in a decrease of NOx removal. Results show that SO2 can promote NOx removal, but a high concentration of SO2 will increase reagent consumption and decrease NOx removal.


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Figure 10. Effect of SO2 concentration on NOx removal efficiencies. 3.8. Effect of NO concentration. The effect of NO concentration on NOx removal is shown in Figure 11. Results show that the NO concentration has a certain effect on NOx removal. As Figure 11 illustrates, the NOx removal efficiency decreased slowly from 85.85% to 82.61% with the increase of NO concentration from 268 mg/m3 to 670 mg/m3, and then sharply decreased from 82.61% to 73.27% with the increase of NO concentration from 670 mg/m3 to 938 mg/m3. The NaClO2 consumption rate increased with increasing NO concentration, leading to a decrease in NOx removal.

Figure 11. Effect of NO concentration on NOx removal efficiencies. 3.9. Effect of O2 and CO2 concentration. The influence of O2 concentration on NOx


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removal was investigated by varying the O2 concentration from 0 to 15% (v/v) and results are shown in Figure 12. Results indicate that NOx removal efficiency was slightly affected by O2 concentration. When the O2 concentration was 1%, 5%, 7%, 10% and 15%, the NOx removal efficiency was 83.23%, 82.07%, 84.61%, 83.59% and 84.16%, respectively. This is because the oxidation of O2 was so weak in the experimental conditions compared with NaClO2, so the change of O2 concentration had little effect on the NOx removal. Figure 13 shows the influence of CO2 concentration on the NOx removal. Results suggest that the NOx removal efficiency was slightly affected by CO2 concentration. When the CO2 concentration varied from 0 to 20% (v/v), the NOx removal efficiency was 84.61%, 83.39% and 85.59%, respectively. CO2 does not have oxidizing ability, so it can not improve the NOx removal efficiency. However, dissolved CO2 can increase the acidity of the solution and enhance the oxidation of the NaClO2 solution, resulting in an increase in the NOx removal efficiency at the initial stage of the reaction.

Figure 12. Effect of O2 concentration on NOx removal efficiencies.


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Figure 13. Effect of CO2 concentration on NOx removal efficiencies. 3.10. Effect of gas flow rate. Figure 14 displays the effect of the gas flow rate on the NOx removal in the range from 1 to 3 L/min. Results show that the removal efficiency of NOx was significantly affected by gas flow rate. When the gas flow rate changed from 1 to 2 L/min, NO removal efficiency increased from 77.00% to 84.61%, then sharply decreased from 84.61% to 75.13% with an increase of gas flow rate between 2 and 3 L/min. Results also show that Hg0 removal efficiency were affected by gas flow rate. When the gas flow rate was 1 L/min, 2 L/min and 3 L/min, the Hg0 removal efficiency was 90.43%, 97.68% and 93.16%, respectively. When the gas flow rate was low, the amount of NO2 produced and emitted from the oxidation reactor was less. When the gas flow rate was high, the consumption rate of NaClO2 was sharply increased, meanwhile the high concentration of SO2 in the simulated flue gas could consume a lot of NaClO2. The above reasons leaded to the molar ratio of NO to NO2 in the mixing pipe deviated from 1:1, resulting in poor NOx and Hg0 removal.


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Figure 14. Effect of gas flow rate on NOx removal efficiencies. 3.11. Effect of absorbent solution. In this study, NaOH solution (5%, w/v), urea solution (5%, w/v), and CaCO3 solution (20%, w/v) were used to absorb multi-pollutants. The results are shown in Figures 15 and 16. Results show that the absorption capacities of the three kinds of absorbents are in the order of NaOH > CaCO3 > urea, with NOx average removal efficiencies of 85.7%, 78.94%, and 74.66%; and Hg0 average removal efficiencies of 96.1%, 92.1%, and 87.1%, respectively. This outcome is attributed to the pH of the absorption solution, which plays an important role in NOx and Hg0 removal. The initial pH values of NaOH, CaCO3, and urea solution were 13.83, 9.5, and 8.5, respectively. The flue gas entering the absorption reactor mainly contained SO2, NO, NO2, N2O3, N2O4, Hg0 and Hg2+; Therefore, the higher the pH, the higher the pollutant removal efficiencies. The NaOH solution clearly appeared to have better absorbent properties than the others because it had high initial pH.


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Figure 15. Effect of species of alkali absorbent on NOx removal efficiencies.

Figure 16. Effect of species of alkali absorbent on Hg0 removal efficiencies. 3.12. Parallel tests. The parallel tests were carried out under the optimal conditions (Table 1), and the results are shown in Table 2. As exhibited in Table 2, SO2, NOx, and Hg0 removal efficiencies have good reproducibility. The results indicate that this new process has high removal efficiency and stability for multi-pollutants. In addition, due to the low concentration and consumption of sodium chlorite during the absorption process, the new process will be running at a lower cost. Meanwhile, using coagulation, sedimentation and other techniques to treat the waste absorption solution, which can effectively avoid the waste absorption solution


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on the environment. So, the new process can offer an attractive method for controlling SO2, NO and Hg0 simultaneously.

Table 2. Results of parallel tests. NO. Average

Standard deviation

96.74

96.04

0.574

99.71

99.42

99.50

0.465

82.94

86.88

84.14

1.956

1

2

3

4

Average removal efficiencies of Hg0, %

95.35

96.13

95.92

Average removal efficiencies of SO2, %

99.38

99.50

Average removal efficiencies of NOx, %

84.19

82.55

4. POLLUTANT REMOVAL MECHANISM

4.1. NaClO2 consumption rate. The NaClO2 consumption rates in the new process and in the existing process were tested. During the experiments, the ClO2− concentration in the NaClO2 solution was sampled and analyzed every 5–20 min, and the results are shown in Table 3. In comparison with the existing process, the NaClO2 consumption rate in the new process was significantly reduced. Therefore, the new process was more economical in removing multi-pollutants in the flue gas. Figure 17 shows the NaClO2 consumption rate curve of the new process. The consumption rate equation of NaClO2 is as follows:

Y = 18.9824 + 0.4926 × t

(6)

The instantaneous concentration equation of NaClO2 in the solution can be obtained as follows:

C! = 0.8102"# − 0.004962 × t × "#

(7)


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where Y is the NaClO2 concentration rate, %; t is the reaction time, min; Ca is the instantaneous concentration of [ClO2−], mg/L; and C0 is the initial concentration of [ClO2−], mg/L.

Table 3. Consumption of NaClO2 in different processes. New process 0 min

5 min

20 min

40 min

60 min

80 min

100 min

120 min

659.76

527.15

445.14

401.40

354.69

292.34

194.37

135.18

0.00

20.10

32.53

39.16

46.24

55.69

70.54

79.51

Instantaneous concentration of ClO2-, mg/L Consumption rate, %

Existing process

Instantaneous concentration of ClO2-, mg/L Consumption rate, %

0 min

5 min

20 min

40 min

60 min

80 min

100 min

120 min

645.15

637.60

481.41

299.67

141.16

12.39

0

0

0.00

1.17

25.38

53.55

78.12

98.08

100

100

Figure 17. NaClO2 consumption rate curve of the new process. 4.2. NO2 emission from the oxidation reactor. SO2, NO, NO2, and Hg0 emissions in the outlet flue gas of the oxidation reactor are shown in Figure 18. As the reaction progressed, the NO concentration rapidly decreased. After 10 min, the NO concentration decreased to nearly


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0 mg/m3 and was maintained until the end of the experiment. NO2 concentration increased rapidly at first, exceeding 1025 mg/m3 after the first 35 min and was maintained until the first 80 min (indicating that NO was completely oxidized to NO2). Then NO2 concentration decreased slowly as the reaction progressed and reached 651.9 mg/m3 at the end of the experiment. This result is attributed to the rapid drop of the pH of the NaClO2 solution as the reaction proceeded. The oxidation of NaClO2 solution gradually increased, and thus more NO could be oxidized to NO2. Meanwhile, NO2 easily escaped from the solution under acidic conditions, leading to the increase of NO2 concentration. However, as the reaction proceeded, the NaClO2 concentration in the solution decreased, whereas the solubility of NO2 increased under the strongly acidic condition,29 resulting in the decrease of NO2 concentration.

Figure 18. Multi-pollutant emissions in the outlet flue gas of the oxidation reactor. As shown in Figure 18, initially, the SO2 concentration decreased rapidly to 0 mg/m3 and was maintained for approximately 30 min. Next, the concentration of SO2 increased at first and then decreased as the reaction proceeded. The solution was alkaline and had good removal effect on SO2 at the beginning of the reaction. The solubility of SO2 then decreased


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with the decrease of solution pH, resulting in the increase of SO2 concentration. However, SO2 solubility increased with the increase of nitrate concentration in the acidic solution (eq. 8), leading to a decrease of SO2 concentration.

2HNO + 3SO + 2H O → 2NO + 3H SO

（8）

In addition, the Hg0 concentration in the outlet flue gas decreased drastically as the reaction proceeded. Its concentration fluctuated between 0.1 and 1.3 µg/m3 during the whole experiment, and the average concentration of Hg0 was 0.57 µg/m3. The results show that NaClO2 solution has good oxidative effect on Hg0.

4.3. Product analysis. As listed in Table 3, only ClO2- and Cl- in the oxidation solution and no anions in the absorption solution were detected at the beginning of the reaction. At the end of the experiment, NO3−, SO42−, ClO2−, Cl−, and ClO3− were detected in the oxidation solution. Cl− concentration significantly increased, and ClO2− concentration significantly decreased compared with their initial concentrations; this result is attributed to the reaction of NaClO2 with NO and SO2, which produced large amounts of NO3−, SO42−, and Cl−. As the NaClO2 solution was the oxidizing solution, no NO2− and SO32− were detected in the solution. During the reaction, a certain amount of ClO3− was produced. The following reactions (eqs. 9 and 10) can be used to explain the phenomena:

2ClO + 2NaOH + 2NaClO → 3NaClO + NaCl + H O

(9)

2ClO + H O + 5NaClO → 5NaClO + 2HCl

(10)


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For the absorption solution, higher concentrations of NO2−, SO42−, and Cl− were detected, while a small amount of NO3− and SO32− were detected at the end of the experiment. As listed in Table 4, SO32− concentration was very low compared with that of SO42−. This result is attributed to the oxidation of SO32− to SO42− by the dissolved O2 in the solution. Meanwhile, SO32− could react with NO2 to produce SO42−.26 N2O3 could also react with H2O to produce NO2−, and part of the NO2− was oxidized to NO3− by the dissolved O2. In addition, N2O4 could react with water to produce NO2− and NO3−. SO32− and NO2− could exist in the alkaline solution compared with the oxidation solution. Cl− was detected in the absorption solution because of the following: on one hand, flue gas from the oxidation reactor brought a small amount of oxidation solution into the absorption reactor; on the other hand, ClO2 and Cl2 generated from the oxidation reactor flowed into the absorption reactor and finally reacted with other substances to generate Cl−.

Table 4. Analysis of ionic components in solutions. NO2-

NO3-

SO32-

SO42-

Cl-

ClO2-

ClO3-

Hg2+

Oxidation solution (NaClO2)

Initial

—

—

—

—

112.43

640.79

—

—

End

—

24.39

—

430.77

308.33

199.97

72.42

2.94

Absorption solution (NaOH)

Initial

—

—

—

—

—

—

—

—

End

136.72

46.41

13.71

612.36

67.94

—

—

4.21

Hg2+ was likewise detected in the oxidation and absorption solutions with concentrations of 2.94 and 4.21 µg/L respectively. The average concentration of Hg0 in the flue gas was 31.8 µg/m3. The Hg0 removal efficiency was calculated to be 93.68%, which was close to the actual Hg0 removal efficiency of 96.13%.


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4.4. Pollutant removal process and mechanism. Based on experimental phenomena, product analysis, and the literature,26,27,29,30 the removal process and mechanism of SO2, NOx, and Hg0 using the new technology was discussed, and the results are shown in Table 5. As listed in Table 5, SO2 is mainly converted to SO42− via acid–base neutralization reaction and oxidation. NOx is mainly converted to NO2− and NO3− via hydrolysis of N2O3 and N2O4, and the Hg0 is mainly converted to Hg2+ via oxidation. Finally, SO2, NOx, and Hg0 are removed efficiently by the new process.

Table 5. Removal process and mechanism. Desulfurization

Denitration

Hg0 removal

A

*+ (-) ↔ *+ (0) 2*+ (0) + "0+ (12) + 23 +(0) → 43 4 (12) + 2*+ (12) + "0 (12) 5*+ (0) + 2"0+ (0) + 63 +(0) → 1234 (12) + 5*+ (12) + 2"0 (12) 3*+ (0) + 25+ (12) + 23 +(0) → 25+(0) + 3*+ (12) + 43 4 (12)

5+(-) ↔ 5+(0) 45+(0) + 3"0+ (12) + 4+3 (12) → 45+ (12) + 3"0 (12) + 23 +(0) 25+(0) + "0+ (12) → 25+ (0) + "0 (12) 55+(0) + 2"0+ (0) + 3 +(0) → 55+ (0) + 23"0(12) 45+ (0) + "0+(12) + 4+3 (12) → 45+ (12) + "0 (12) + 23 +(0) 55+ (0) + "0+ (0) + 33 +(0) → 55+ (12) + "0 (12) + 63 4 (12) 5+ (0) ↔ 5+ (-)

3-# (-) ↔ 3-# (0) 23-# (0) + "0+ (12) → 23-+ ↓ +"0 (12) 23-# (0) + "0+ (12) + 43 4 (12) → 23-4 (12) + "0 (12) + 23 +(0) 53-# (0) + 2"0+ (0) + 83 4 (12) → 53-4 (12) + 2"0 (12) + 43 +(0) 3-4 (12) + *+ (12) → 3-*+ ↓

B

5*+ (-) + 2"0+ (-) + 63 +(0) → 5*+ (12) + 2"0 (12) + 123 4 (12) *+ (-) + "0 (-) + 23 +(0) → *+ (12) + 2"0 (12) + 43 4 (12)

55+(-) + 2"0+ (-) + 3 +(0) → 55+ (-) + 23"0(12) 5+(-) + "0 (-) + 3 +(0) → 5+ (-) + 23"0(12) 25+(-) + + (-) → 25+ (-) 5+(-) + 5+ (-) ↔ 5 + (-) 25+ (-) ↔ 5 + (-)

3-# (-) + 5+ (-) → 3-+ ↓ +5+(-) 53-# (-) + 2"0+ (-) + 43 +(0) → 53-4 (12) + 2"0 (12) + 8+3 (12) 3-# (-) + "0 (-) → 3-4 (12) + 2"0 (12)

*+ (-) ↔ *+ (0) *+ (0) + 3 +(0) ↔ *+ ∙ 3 +(0) *+ ∙ 3 +(0) ↔ 3 4 (12) + 3*+ (12) 3*+ (12) ↔ 3 4 (12) + *+ (12) 23*+ (12) + + (0) + 2+3 (12) → 23 +(0) + 2*+ (12) 2*+ (12) + + (0) → 2*+ (12)

5 + (-) ↔ 5 + (0) 5 + (-) ↔ 5 + (0) 5+ (-) ↔ 5+ (0) 25+ (0) + 2+3 (12) → 5+ (12) + 5+ (12) + 3 +(0) 5 + (0) + 2+3 (12) → 5+ (12) + 5+ (12) + 3 +(0) 5 + (0) + 2+3 (12) → 25+ (12) + 3 +(0) 35+ (0) + 3 +(0) → 23 4 (12) + 25+(12) + 5+(0) 25+ (0) + *+ (12) + 3 +(0) → 23 4 (12) + 25+(12) + *+ (12)

3-# (-) ↔ 3-# (0) 23-# (0) + + (0) + 23 +(0) → 23-(+3) ↓ 3-4 (12) + 2+3 (12) → 3-(+3) ↓ 3-4 (12) + *+ (12) → 3-*+ ↓

C

When the pH of the solution in the oxidation reactor decreases, the following chemical reactions will be occured, and the generated ClO2 and Cl2 will promote the SO2, NOx and Hg0 removal. 4 (12) 3 + "0+ (12) → 3"0+ (0) 83"0+ (0) → 6"0+ (0) + "0 (0) + 43 +(0) 43 4(12) + 5"0+ (12) → 4"0+ (0) + 23 +(0) + "0 (12) 4"0+ (12) + 23 4 (12) → "0 (12) + 2"0+ (0) + "0+ (12) + 3 +(0) 2"0+ (0) + 3 +(0) + 5"0+ (12) → 5"0+ (12) + 2"0 (12) "0+ (0) ↔ "0+ (-) "0 (0) ↔ "0 (-)

A, Main chemical reactions in the oxidation reactor; B, Main chemical reactions in the mixing pipe; C, Main chemical reactions in the absorption reactor.


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5. CONCLUSIONS

In this study, a novel process of split, partial oxidation, and mixed absorption for the synergistic removal of multiple pollutants was proposed. NaClO2 and NaOH were used respectively as an oxidizing reagent and absorption reagent in the new process. In view of the high removal efficiencies of SO2 and Hg0, the effects of different experimental conditions on NOx removal were mainly studied in this paper. Results indicate that flue gas split ratio, NaClO2 concentration, initial pH of NaClO2 solution, temperature of oxidation solution, SO2 and NO concentrations, and species of alkali absorbent all had important effects on the synergistic removal of multi-pollutant. Finally, the optimal experimental conditions were determined, and the average removal efficiencies were greater than 99%, 82%, and 95% for SO2, NOx, and Hg0, respectively. The removal mechanism of SO2, NOx, and Hg0 was deduced based on literature, experimental phenomena, and reaction products. The proposed method can effectively remove multi-pollutant, and thus has the potential to be applied in the future.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT


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This work was supported by the National Science and Technology Support Project (2014BAC21B04), the Guangdong Natural Science Foundation (2015A030310344), the Project of Science and Technology Program of Guangdong Province (2014A020216015, 2015A020220008, 2015B020215008 and 2016B020241002), and the Pearl River S&T Nova Program of Guangzhou (201610010150).

REFERENCES

(1) Lu, X. C.; Yao, T.; Li, Y.; Fung, J. C. H.; Lau, A. K. H. Source apportionment and health effect of NOx over the Pearl River Delta region in southern China. Environ. Pollut. 2016, 212, 135-146.

(2) You, C.; Xu, X. Coal combustion and its pollution control in China. Energy 2010, 35, 4467-4472.

(3) Fang, P.; Cen, C.P.; Tang, Z.J. Experimental study on the oxidative absorption of Hg0 by KMnO4 solution. Chem. Eng. J. 2012, 198-199, 95-102.

(4) Ma, Z. Z.; Deng, J. G.; Li, Z.; Li, Q.; Zhao, P.; Wang, L. G.; Sun, Y. Z.; Zheng, H. X.; Pan, L.; Zhao, S.; Jiang, J. K.; Wang, S. X.; Duan, L. Characteristics of NOx emission from Chinese coal-fired power plants equipped with new technologies. Atmos. Environ. 2016, 131, 164-170.

(5) Nasonova, A.; Pham, H. C.; Kim, D. J.; Kim, K. S. NO and SO2 removal in non-thermal plasma reactor packed with glass beads-TiO2 thin film coated by PCVD process. Chem. Eng. J. 2010, 156, 557-561.


31


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

Page 32 of 36

(6) Ma, S. M.; Zhao, Y. C.; Yang, J. P.; Zhang, S. B.; Zhang, J. Y.; Zheng, C. G. Research progress of pollutants removal from coal-fired flue gas using non-thermal plasma, Renew. Sust.

Energ. Re. 2017, 67, 791-810.

(7) Xu, F.; Li, Z. H.; Cao, W.; Wang, P.; Wei, B.; Gao, X.; Fang, M. X.; Cen, K. F.; Simultaneous oxidation of NO, SO2 and Hg0 from flue gas by pulsed corona discharge. J.

Environ. Sci. 2009, 21, 328-332.

(8) Su, C.; Ran, X.; Hu, J.; Shao, C. Photocatalytic process of simultaneous desulfurization and denitrification of flue gas by TiO2-polyacrylonitrile nanofibers. Environ. Sci. Technol. 2013, 47, 11562-11568.

(9) Huang, X. M.; Ding, J.; Zhong, Q. Catalytic decomposition of H2O2 over Fe-based catalysts for simultaneous removal of NOx and SO2. Appl. Surf. Sci. 2015, 326, 66-72.

(10) Zhao, Y.; Hao, R. L.; Yuan, B.; Jiang, J. J. Simultaneous removal of SO2, NO and Hg0 through an integrative process utilizing a cost-effective complex oxidant, J. Hazard. Mater. 2016, 301, 74-83.

(11) Fang, P.; Cen, C. P.; Wang, X. M.; Tang, Z. J.; Tang, Z. X.; Chen, D. S. Simultaneous removal of SO2, NO and Hg0 by wet scrubbing using urea+KMnO4 solution. Fuel Process.

Technol. 2013, 106, 645-653.

(12) Liu, Q.; Liu, Z. Carbon supported vanadia for multi-pollutants removal from flue gas,

Fuel 2013, 108, 149-158.


32

Page 33 of 36

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


(13) Xu, X. H.; Ye, Q. F.; Tang, T. M.; Wang, D. H. Hg0 oxidative absorption by K2S2O8 solution catalyzed by Ag+ and Cu2+. J. Hazard. Mater. 2008, 158, 410-416.

(14) Chu, H.; Chien, T. W.; Li, S. Y. Simultaneous absorption of SO2 and NO from flue gas with KMnO4/NaOH solutions. Sci. Total Environ. 2001, 275, 127-135.

(15) Zhao, Y.; Han, Y. H.; Guo, T. X.; Ma, T. Z. Simultaneous removal of SO2, NO and Hg0 from flue gas by ferrate(VI) solution. Energy. 2014, 67, 652-658.

(16) Hutson, N. D.; Krzyzynska, R.; Srivastava, R. K. Simultaneous removal of SO2, NOX, and Hg from coal flue gas using a NaClO2-enhanced wet scrubber. Ind. Eng. Chem. Res. 2008, 47, 5825-5831.

(17) Wang, Z. H.; Zhou, J. H.; Zhu, Y. Q.; Wen, Z. C.; Liu, J. Z.; Cen, K. F.; Simultaneous removal of NOx, SO2 and Hg in nitrogen flow in a narrow reactor by ozone injection: Experimental results. Fuel Process. Technol. 2007, 88, 817-823.

(18) Liu, Y.X.; Zhang, J.; Sheng, C.D.; Zhang, Y.C.; Zhao, L. Simultaneous removal of NO and SO2 from coal-fired flue gas by UV/H2O2 advanced oxidation process. Chem. Eng. J. 2010, 162, 1006-1011.

(19) Jin, D. S.; Deshwal, B. R.; Park, Y. S.; Lee, H. K. Simultaneous removal of SO2 and NO by wet scrubbing using aqueous chlorine dioxide solution. J. Hazard. Mater. 2006, 135, 412-417.


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

Page 34 of 36

(20) Zhao, Y.; Hao, R. L.; Yuan, B.; Jiang, J. J. Simultaneous removal of SO2, NO and Hg0 through an integrative process utilizing a cost-effective complex oxidant. J. Hazard. Mater. 2016, 301, 74-83.

(21) Amin, P.; Esmaeel, J.; Habib, A.; Sassan, D.; Study on simultaneous removal of NOx and SO2 with NaClO2 in a novel swirl wet system. Ind. Eng. Chem. Res. 2011, 50 () 8278-8284.

(22) Fang, P.; Cen, C. P.; Tang, Z. X.; Zhong, P. Y.; Chen, D. S.; Chen, Z. H. Simultaneous removal of SO2 and NOx by wet scrubbing using urea solution. Chem. Eng. J. 2011, 168, 52-59.

(23) Wei, J. C.; Luo, Y. B.; Yu, P.; Cai, B.; Tan, H. Z. Removal of NO from flue gas by wet scrubbing with NaClO2/(NH2)2CO solutions. J. Ind. Eng. Chem. 2009, 15, 16-22.

(24) Niksa, S. Helble, J. J.; Fujiwara, N. Kinetic modeling of homogeneous mercury oxidation: the importance of NO and H2O in predicting oxidation in coal derived systems.

Environ. Sci. Technol. 2001, 35, 3701-3706.

(25) Seneviratne, H. R.; Charpenteau, C.; George, A.; Millan, M.; Dugwell, D. R.; Kandiyoti, R. Ranking low cost sorbents for mercury capture from simulated flue gases. Energ. Fuel 2007, 21, 3249-3258.

(26) Hao, R. L.; Zhang, Y. Y.; Wang, Z. Y.; Li, Y. P.; Yuan, B.; Mao, X. Z.; Zhao, Y. An advanced wet method for simultaneous removal of SO2 and NO from coal-fired flue gas by


34

Page 35 of 36

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


utilizing a complex absorbent. Chem. Eng. J. 2017, 307, 562-571.

(27) Ye, C. B.; Tu, X. H.; Li, C. H.; Pei, X. D.; Chen, W. H. Conditions and mechanism of denitrification of NaClO2 aqueous solution. Chem. React. Eng. Techno. 2016, 32, 176-182. (in Chinese)

(28) Zhao, Y.; Guo, T. X.; Chen, Z. Y.; Du, Y. R. Simultaneous removal of SO2 and NO using M/NaClO2 complex absorbent. Chem. Eng. J. 2010, 160, 42-47.

(29) Geng, J.; Wang, X. X.; Yuan, G.; Zhang, F.; Zhang, Z. B. Investigation of NO2 absorption in water or dilute nitric acid. J. Nanjing U. Sci. Techno. 2013, 37, 164-168. (in Chinese)

(30) Chien, T. W.; Chu, H. Removal of SO2 and NO from flue gas by wet scrubbing using an aqueous NaClO2 solution. J. Hazard. Mater. 2000, 80, 43-57.


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Table of content (TOC)


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Split, Partial Oxidation and Mixed Absorption: A Novel Process for

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