Study on a Novel Oxidation Process for Removing Arsenic from Flue

Article pubs.acs.org/EF

Study on a Novel Oxidation Process for Removing Arsenic from Flue Gas Yi Zhao,* Wen Qiu, Chunyan Yang, and Jianan Wang School of Environmental Science and Engineering, North China Electric Power University, Baoding 071003, P. R. China ABSTRACT: The experiments of arsenic removal from flue gas were investigated at a lab-scale bubbling reactor with H2O2 solution to develop a novel oxidation process. The effects of various experimental parameters, such as H2O2 concentration, initial solution pH, reaction temperature, and flue gas components (O2, CO2, SO2, and NO) were systematically investigated, and the average removal efficiency was found to be 92.6%, under the obtained optimal conditions, in which H2O2 concentration was 0.2 mol/L, solution pH was 5.0, and reaction temperature was 50 °C. Meanwhile, the mechanism of the reaction was proposed based on characterizations of the removed products by high performance liquid chromatography coupled to hydride generation atomic fluorescence spectrometry, atomic fluorescence spectrometry as well as a review of the literature. combustion flue gas due to their strong oxidizing power. However, these oxidants either have lower economical efficiencies or may release several hazardous byproducts that can adversely affect the environment.13 In recent years, H2O2 has also been used to remove SO2 and NOx from flue gas due to its environmental friendliness and lower price,14−17 but its weak oxidizability makes it difficult to effectively and completely oxidize SO2 and NOx, severely restricting its industrialized application. Because As2O3 is more oxidized by H2O2 than NO, it is suitable for developing an As removal process from flue gas. In the present study, experiments to remove arsenic from flue gas were carried out at a bubbling reactor with H2O2 solution. The effects of various parameters, including H2O2 concentration, initial solution pH, reaction temperature, and flue gas components (O2, CO2, SO2, and NO), on the removal were investigated experimentally. In addition, a possible reaction mechanism was proposed, according to the characterizations of the removed products. These findings may be able to provide a reference for As removal from flue gas of power station boilers and industrial boilers by injecting H2O2 solution in flue gas desulfurization (FGD) system.

1. INTRODUCTION Arsenic is a hazardous air pollutant that poses a harmful impact on human health and the ecosystem.1,2 Coal-fired power plants are considered one of the major anthropogenic sources of arsenic emission.3 The effective removal of arsenic from coal-fired power plant flue gas is of great social, environmental, and economic significance. Research showed4 that, for the real status of flue gas under an oxidization atmosphere, AsO(g) was the only existing form of As when the temperature was higher than 1000 K; As2O3(g), As2O5(s), and AsO(g) coexisted in the temperature range of 650−1000 K; and moreover, As2O3(g) and As2O5(s) were the main existing forms in the temperature ranges of 750− 800 K and 650−700 K, respectively. It is important that As2O3(g), due to its toxicity and low solubility in water, is difficult to capture using existing air pollution control devices.5,6 Hence, the conversion and absorption of As2O3(g) are the research emphasis of arsenic pollution control.7,8 Several processes based on the adsorption materials including calcium-based materials, activated carbon, metal oxides, fly ashes, etc.,9−15 have been developed to remove arsenic. López-Antón et al.9 demonstrated that activated carbons exhibited an excellent arsenic adsorption capacity, and the difference of removal efficiency depended mainly on the speciation of arsenic in flue gases. Li et al.10 carried out an experiment of the simultaneous removal of SO2 and As2O3 from coal-fired flue gas using calcium oxide and found that the removal efficiency of arsenic increased in the reaction temperature range of 400−1000 °C. López-Antón et al.11 reported that arsenic could be removed by fly ashes and that the removal efficiency was mainly affected by the characteristics of the ashes and processing conditions. Baltrus et al.12 used palladium(Pd)-alumina(Al) sorbents to remove arsenic from flue gas and found that Pd had poor long-term performance for the adsorption of arsenic. Nevertheless, none of these methods demonstrated a clear advantage in both conversion efficiency and economy. For these reasons, the oxidation method appears to be the most promising approach. Moreover, the use of the oxidation method for arsenic absorption has rarely been reported in the literature. Various oxidants such as sodium chlorite, permanganate, ozone, etc., are often used to remove the gaseous pollutant in coal © XXXX American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Materials. All gases in experiments were obtained in compressed gas steel cylinders (North Special Gas Co., Ltd., China). All reagents used were analytical grade (AR) (Sinopharm Chemical Reagent Co., Ltd., China); all solutions were produced using deionized water obtained from a Millipore water purification system (Millipore Corp., Bedford, MA, USA), with the specific resistivity of >18.25 MΩ/ cm. Arsenic(III) stock standard solution (1000 mg L−1) was prepared by dissolving NaAsO2 (Sinopharm Chemical Reagent Co., Ltd., China) in 5% (v/v) HCl solution. Standard solutions of As(III) (0.1 μg·L−1 to 100 mg L−1) were made daily by appropriate dilution of the As(III) stock solution with 5% (v/v) HCl solution and maintained in plastic flasks at 4 °C when not in use in order to avoid metal adsorption on glass surfaces. Received: July 13, 2016 Revised: December 14, 2016 Published: December 15, 2016 A

DOI: 10.1021/acs.energyfuels.6b01724 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

where η is the removal efficiency; Cin is the inlet concentration, mg/m3 for arsenic; Cout is the outlet concentration with the reactor containing H2O2 solution, mg/m3 for arsenic. 2.3. Analytical Methods. Approximately 5 mL of each sample solution was mixed with 2.5 mL of 12 mol/L HCl and 5 mL of reducing agent (100 g/L thiourea and ascorbic acid) in a reaction vessel. The volume was filled up to 50 mL with deionized water and allowed to react for 30 min to reduce As(V) to As(III) that was measured by atomic fluorescence spectrometry (AFS, 933 type, Beijing Titian Company, China). The instrumental conditions for As determination are shown in Table 1. The inlet after Figure 1, 9) and outlet (before Figure 1, 15)

Sodium tetrahydroborate(III), used as a reducing solution, was prepared daily by dissolving NaBH4 pellets in water and stabilizing with a 0.5% (m/v) NaOH solution to reduce its rate of decomposition. Thiourea and ascorbic acid were prepared by dissolving the powders in water to yield a 100 g/L solution. All glassware and plastic bottles were treated in a solution of 10% (v/v) nitric acid for 24 h and then subsequently washed with deionized water. 2.2. Experimental Apparatus and Procedure. Figure 1 shows the schematic diagram of experimental equipment including a flue gas

Table 1. Continuous Flow HG-AFS Operating Conditions for As Determination As atomic fluorescence spectrometer parameters wavelength (nm) current (mA) delay time (s) analysis time (s) continuous flow hydride generation [HCl] (% (v/v)) [NaBH4] (% (w/v)) sample/HCl flow rate (mL·min−1) sample/NaBH4 flow rate (mL·min−1) Ar flow rate (mL·min−1)

simulation system, an absorption reactor, a flue gas analysis system, and a tail gas absorption system. A bubble reactor with 250 mL of effective volume and 15.5 cm of height was used as the absorption device. Arsenic vapor was generated from a tube type resistance furnace containing As2O3 (Figure 1, 7) (SK-2-13, Yong Guang Ming Medical Instrument Company, Beijing) with 0.2 L/min as the carrier gas. Total gas flow was maintained at 1.0 L/min. The method used for arsenic sampling was the EPA Method 29 recommended by the U.S. EPA,18,19 in which the oxidized arsenic escaped from H2O2 solution was sampled isokinetically through a heated sampling system to the impinger train in an ice bath containing a solution of 5% nitric acid in 10% hydrogen peroxide. During the experiments, N2 was measured by mass flow controllers (Figure 1, 6) and mixed with arsenic vapor in a buffer bottle (Figure 1, 9). This was performed to ensure that the arsenic was diluted to the desired concentrations by the N2, from which the simulated flue gas was formed. For a coal-fired power plant, the actual flue gas consists chiefly of N2, CO2, O2, SO2, NOx, etc., in which the concentrations of CO2 and O2 are generally 10−15% and 5−6%, respectively, and those of SO2 and NOx will change with coal species and combustion conditions of the boiler. Hence, the examined concentration ranges of the gases were set as 0−3500 mg/m3 for SO2, 0−300 mg/m3 for NO, 0−10% for O2, and 0−16% for CO2. The oxidation and absorption reactions occurred when the gas mixture entered the bubble reactor containing H2O2 solution (Figure 1, 13) that was adjusted by 0.2 M NaOH or 0.1 M H2SO4 solution before each experiment and tested by a pH meter (type PHS3C, Leici, Shanghai, China). For each experiment, 5 parallel tests were performed, and the duration of each test was 20 min. The removal efficiencies were calculated according to the inlet and outlet concentrations of arsenic via eq 1 C in − Cout × 100% C in

5 1.0 1.0 1.0 300

concentrations of flue gas components such as CO2, NO, O2, SO2 were detected using the flue gas analyzer (Figure 1, 18). The inlet (after Figure 1, 15) concentration of arsenic vapor was detected using atomic fluorescence spectrometry (AFS, 933 type, Beijing Titian Company, China) when H2O2 solution was replaced by water in the reactor, and the outlet (after Figure 1, 15) one was detected when H2O2 solution was added in the reactor. The removal products of arsenic before and after the oxidation reactions in the H2O2 solutions were characterized by a hydride generation-atomic fluorescence spectrometry (AFS-610A, Beijing Ruili Analysis Instrument Company, Beijing) coupled with a high performance liquid chromatograph (HPLC, P680, DIONEX Company, USA).

Figure 1. Schematic diagram of the experimental apparatus. 1−5 - N2, NO, SO2, CO2, O2 gas cylinders; 6 - flowmeters; 7 - tube type resistance furnace; 8 - temperature controller; 9 - buffer bottle; 10 - ice bath; 11 5% (v/v) HNO3−10% (v/v) H2O2 solution; 12 - thermostatic water bath; 13 - bubble reactor; 14 - 1 mL injector; 15 - ice bath; 16 - 5% (v/v) HNO3−10% (v/v) H2O2 solution; 17 - dryer; 18 - flue gas analyzer.

η=

197.3 70 15 35

3. RESULTS AND DISCUSSION 3.1. Stability Testing of Arsenic Vapor Evaporation. In order to ensure the stabilities generation of arsenic and experiments, all vessels used were made of borosilicate glass and the pipe lines used in the reaction system were made of Teflon and heated to 120 °C by heater bands. The inlet concentration of arsenic in the simulated flue gas was detected in duplicate, and the results are shown in Table 2. Relative standard deviation (RSD) of each group’s parallel tests was 0.0025, demonstrating that arsenic vapor evaporation was stable in 20 min. 3.2. Effect of H2O2 Concentration on the Arsenic Removal. As shown in Figure 2, the arsenic removal efficiencies increase rapidly when the concentrations of H2O2 solution increase from 0 to 0.2 mol/L. Thereafter, the removal efficiencies remain constant. This law revealed that the optimal concentration of H2O2 solution was 0.2 mol/L to the most efficient removal of arsenic. The effect of H2O2 concentration on the efficiency of arsenic removal can be explained by the following: Increasing H2O2 concentration can increase the oxidation capacity of the solution and thereby promote the arsenic removal efficiency.20 Second, adding high concentration H2O2 may cause the side reactions (eqs 2 and 3),21,22 resulting in a reduction in the amount of

(1) B

DOI: 10.1021/acs.energyfuels.6b01724 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 2. Concentrations of Arsenic in the Flue Gas (μg/m3) numbers

1

2

3

4

5

6

average

RSD

concentrations

420

420

426

415

425

427

427

0.0025

Figure 3. Effects of pH on arsenic removal efficiency. Arsenic concentration, 427 μg/m3; H2O2 concentration, 0.2 mol/L; reaction temperature, 50 °C; flue gas velocity, 1.0 L/min.

Figure 2. Effects of H2O2 concentration on arsenic removal efficiency. Arsenic concentration, 427 μg/m3; reaction temperature, 50 °C; solution pH, 5.0; flue gas velocity, 1.0 L/min.

oxidizing species,23 which is not conducive to arsenic removal. The removal efficiency change demonstrated the balance of the advantages and disadvantages of increasing H2O2 concentration in the H2O2 concentration range of 0.2−0.8 mol/L. HO• + HO• → H 2O2 •

(2) •

H 2O2 + HO → HO2 + H 2O

(3)

3.3. Effect of H2O2 Solution pH on the Arsenic Removal. The experiments for arsenic removal were carried out under varying pH conditions ranging from 0.5 to 10.0. The results are shown in Figure 3. As seen in the figure, arsenic removal efficiencies are found to be slightly affected by solution pH in the solution pH range of 0.5−10.0. Liu et al. reported that low solution pH could promote the decomposition of H2O2 and increase ·OH concentration,24 which was in favor of arsenic removal. However, increasing the solution pH could increase OH− concentration and enhance the yield of HO2−, which would consume ·OH, thereby arsenic removal efficiency reducing, as shown in eqs 4 and 5.25 However, the conclusions mentioned above do not agree with our experimental results, speculating indirectly that H2O2 might be the main oxide species for arsenic removing. Meanwhile, by estimating, the reaction product, AsO43− was gradually converted as H3AsO4 as the solution pH decreased from 0.5 to 10. According to the practical working situation FGD system, the solution pH was selected as 5.0.

H 2O2 → H+ + HO−2 •

HO +

HO−2

→ H 2O +

Figure 4. Effects of reaction temperature on arsenic removal efficiency. Arsenic concentration, 427 μg/m3; H2O2 concentration, 0.2 mol/L; solution pH, 5.0; flue gas velocity, 1.0 L/min.

the range of 30−50 °C. The highest removal efficiency was obtained at 50 °C, beyond which, the removal efficiency decreased rapidly. Accordingly, the optimal reaction temperature was identified as 50 °C. Findings reported by Hu and Liu26,27 indicated that increasing the solution temperature was able to improve the chemical reaction rate. However, H2O2 was easily decomposed when the temperature was too high (eq 6),28 resulting in a decrease in its oxidation capacity.

(4)

O−2

(5)

3.4. Effect of reaction Temperature on the Arsenic Removal. The effect of solution temperature on the removal of arsenic was studied, and the results are shown in Figure 4. When the temperature was below 50 °C, we found that the removal efficiencies of arsenic increased with reaction temperature within

H 2O2 → H 2O + O2 C

(6) DOI: 10.1021/acs.energyfuels.6b01724 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels 3.5. Effect of SO2 Concentration on the Arsenic Removal. The effect of SO2 concentration on the removal of arsenic is shown in Figure 5. It can be seen that the removal

Figure 6. Effects of NO concentration on arsenic removal efficiency. Arsenic concentration, 427 μg/m3; H2O2 concentration, 0.2 mol/L; solution pH, 5.0; flue gas velocity, 1.0 L/min.

mg/m3 for the limited oxidants, removal of arsenic for the limited oxidants, thereby being able to inhibit arsenic removal. However, the removal efficiency increased when NO concentration increased further to 200 mg/m3. The enhancement for arsenic removal can be speculated by that NO2 produced by the oxidation of NO may catalyze arsenic oxidation and can also directly oxidize arsenic,34 resulting in the improvement of the removal efficiency of arsenic. Meanwhile, the outlet concentration of NO under different inlet concentrations was measured. The results showed that the outlet concentrations of NO were 74, 154, and 231 mg/m3 with respect to 96, 200, and 300 mg/m3, indicating that that the absorption efficiency of NO was only 23% and remained basically unchanged in the NO concentration range of 96−300 mg/m3 because possibly of low water solubility of NO.

Figure 5. Effects of SO2 concentration on arsenic removal efficiency. Arsenic concentration, 427 μg/m3; H2O2 concentration, 0.2 mol/L; solution pH, 5.0; flue gas velocity, 1.0 L/min.

efficiencies of arsenic decrease as SO2 concentrations increase from 0 to 250 mg/m3. It was reported29,30 that SO2 could react with H2O2 and ·OH to be oxidized into H2SO4 (eqs 7−9), which meant that lower SO2 concentration seemed to go against the removal of arsenic. However, the removal efficiencies increased when SO2 concentration increased from 250 to 500 mg/m3, and the removal efficiencies were basically stable as SO2 concentrations increased continuously. The promotion action of SO2 for removing arsenic might be attributed to the formation of As2(SO3)5 (eq 10) that was more stable than H3AsO4. Besides, eq 10 also means that an increase of SO2 concentration is able to promote the mass transfer driving force of As2O3, thereby enhancing the absorption of As2O3. By determining, the outlet concentrations of SO2 were 35, 70, 150, 260, 375, and 600 mg/ m3 with respect to the inlet concentrations of 500, 1000, 1500, 000, 2500, 3000, and 3500 mg/m3, which meant that the absorption efficiency of SO2 decreased as its concentration increased and the competing reaction between SO2 and arsenic might occur for limiting oxidant. SO2 + H 2O → SO32 − + 2H+

(7)

SO32 − + •OH → •SO−3 + OH−

(8)

SO32 − + H 2O2 → SO24 − + H 2O

(9)

2As(V) + 5SO32 − → As 2 (SO3)5

(10)

NO + •OH → HNO2

(11)

2NO + 3H 2O2 → 2HNO3 + 2H 2O

(12)

HNO2 + H 2O2 → HNO3 + H 2O

(13)

HNO2 + •OH → HNO3 + •H

(14)

3.7. Effect of O2 Concentration on the Arsenic Removal. The variations of removal efficiencies for arsenic removal as a function of the O2 concentration are shown in Figure 7. It can be seen that, when O2 concentration increases from 0% to 2.0%, arsenic removal efficiencies increase from 92.5% to 99.0%, meaning that O2 may react with arsenic by the following reactions (eqs 15 and 16). However, when O2 concentrations increase from 2.0% to 10.0%, arsenic removal efficiencies decrease from 99.0% to 87.4%. Some results35,36 showed that O2 was an effective capture intermediate of ·OH, thus decreasing the arsenic removal efficiency.

3.6. Effect of NO Concentration on the Arsenic Removal. It can be seen from Figure 6 that the removal efficiencies of arsenic decrease from 92.55% to 72.4% when NO concentrations increase from 0 to 96 mg/m3. Previous works31−33 showed that NO could consume H2O2 and ·OH (eqs 11−14), indicating that the competing reaction between NO and arsenic might exist in the NO concentration of 0 to 96

AsO33 − + O2 → AsO34−

(15)

As 2 O3 + O2 → As 2 O5

(16)

3.8. Effect of CO2 Concentration on the Arsenic Removal. As shown in Figure 8, arsenic removal efficiencies D

DOI: 10.1021/acs.energyfuels.6b01724 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 3. Parallel Tests of Removal of Arsenic numbers removal efficiency (%)

1

2

3

4

5

average

standard error

92.2

92.8

92.0

93.1

92.9

92.6

0.4

4. REACTION MECHANISM In order to further understand the underlying reaction mechanism for the removal of arsenic by H2O2 solution, five scan blank samples and five reaction product samples were taken from the bubble reactor, in which the scan blank samples were the fresh H2O2 solutions before reaction, and the reaction product samples were the spent H2O2 solution after absorbing arsenic. As seen in Table 4 and Figure 9, there is no As(V) in the Table 4. Concentrations of Pentavalent Arsenic in the Spent Solution (μg/L) Figure 7. Effects of O2 concentration on arsenic removal efficiency. Arsenic concentration, 427 μg/m3; H2O2 concentration, 0.2 mol/L; solution pH, 5.0; flue gas velocity, 1.0 L/min.

numbers

1

2

3

4

5

average

scan blank samples

0 8.277

0 8.179

0 8.283

0 8.191

0 8.391

0 8.264

Figure 9. Chromatogram of arsenic species in the spent (II) H2O2 solution and fresh (I) H2O2 solution.

scan blank samples, whereas As(V) is found in the reaction product samples with an average concentration of 8.264 μg/L. In fact, by estimating, the total concentration of As3+ (arsenic) entering the reactor was 8.54 μg/L when the reactor of effective volume was 250 mL, duration of each test was 20 min, flue gas velocity was 1.0 L/min, and 5 parallel tests were performed for each experiment. By calculating, the concentration of As5+ was 7.9 μg/L in the reactor according to the average removal efficiency of 92.6% known from section 3.9. This data was basically close to the measured value of 8.264 μg/L and matched basically with the removal efficiency of 92.6%. The experimental results demonstrated that As2O3 was oxidized into to As(V) during the reaction by the H2O2 solution. With regard to electrochemistry, the standard electrode potential of H2O2 (1.776 V) is clearly higher than that of AsO43−/As2O3 (1.270 V), As2O5/As2O3 (0.721 V), HAsO42−/ As2O3 (0.901 V), and H2AsO4−/As2O3 (0.687 V), which indicates that As2O3 can be oxidized by H2O2. In summary, the possible reaction paths of arsenic removal are described as follows:

Figure 8. Effects of CO2 concentration on arsenic removal efficiency. Arsenic concentration, 427 μg/m3; H2O2 concentration, 0.2 mol/L; solution pH, 5.0; flue gas velocity, 1.0 L/min.

decrease from 92.6% to 78.8% when CO2 concentrations increase from 0% to 15%. Theoretically, CO2 as an inert gas will not react with the oxidant; however, the mass transfer resistance between gaseous arsenic and oxidant solution should be increased due to the dissolution of higher CO2 concentration in the solution, which will result in the decrease of the removal efficiency of arsenic. 3.9. Parallel Tests of Removal of Arsenic. The arsenic removal experiments were carried out with the H 2 O 2 concentration at 0.2 mol/L, reaction temperature at 50 °C, and pH of 5.0. An arsenic removal efficiency of 92.6% was obtained when the arsenic concentration in the flue gas was 427 μg/m3. The experimental data shown in Table 3 indicate that the H2O2 solution can achieve high efficiency in removing arsenic from flue gas.

As 2 O3 + 2H 2O2 + H 2O → 2AsO34− + 6H+ E

(17)

DOI: 10.1021/acs.energyfuels.6b01724 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels As 2 O3 + 2H 2O2 → As 2 O5 + 2H 2O

(18)

As 2 O3 + 2H 2O2 + H 2O → 2HAsO24 − + 4H+

(19)

As 2 O3 + 2H 2O2 + H 2O → 2H 2AsO−4 + 2H+

(20)

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On the basis of previous studies, volatilized arsenic was presumed to form gaseous As4O6 in the postcombustion flue gas.37,38 According to the analyses of the removal products of arsenic and the comparison of their standard electrode potentials, the reaction mechanism of arsenic removal is inferred as follows: As4 O6 + 4H 2O2 (aq) + 2H 2O(l) → 4H3AsO4 (aq)

(21)

5. CONCLUSION H2O2 solution demonstrated a strong oxidation capacity for arsenic, by which a removal efficiency of 92.6% for arsenic was obtained under the optimized experimental conditions, including a H2O2 concentration of 0.2 mol/L, reaction temperature of 50 °C, solution pH of 5.0, and flue gas flow of 1.0 L/min (1 atm, 50 °C). The analyses of removal products by high performance liquid chromatography coupled to hydride generation atomic fluorescence spectrometry (HPLC-HG-AFS) and AFS, and the comparison of the standard electrode potentials between H2O2 and arsenic, were used to elucidate the underlying reaction mechanism of arsenic removal. The results indicated that As2O3 could be oxidized to As(V) by H2O2 solution.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86-0312-7522343. Fax: +86-0312-7522192. E-mail: [email protected]. ORCID

Yi Zhao: 0000-0001-9974-0348 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors appreciate the financial support by a grant from the key project of the National Major Research and Development Program of China (No. 2016YFC0203700), the National Science-technology Support Plan of China (No. 2014BAC23B04-06), the Beijing Major Scientific and Technological Achievement Transformation Project of China (No. Z151100002815012), Special Scientific Research Fund of Public Welfare Profession of China. (No. 201309018), the Science & Technology Planing Project of Hebei (No. 15273706D), and Technology development projects of Sanhe Power Plant (SH[2015]-QT44).

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DOI: 10.1021/acs.energyfuels.6b01724 Energy Fuels XXXX, XXX, XXX−XXX