Oxidative Absorption of Elemental Mercury from Flue Gas Using

Mar 11, 2019 - A novel oxidative absorption process of elemental mercury (Hg0) from flue gas using Mn2+- modified Fe3+/H2O2 wet scrubbing system (i.e...
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Oxidative Absorption of Elemental Mercury from Flue Gas Using a Modified Fenton-like Wet Scrubbing System Yan Wang,† Hui Xu,‡ and Yangxian Liu*,† †

School of Energy and Power Engineering and ‡Institute for Energy Research, Jiangsu University, Zhenjiang, Jiangsu 212013, P. R. China

Energy Fuels Downloaded from pubs.acs.org by UNIV AUTONOMA DE COAHUILA on 03/27/19. For personal use only.

S Supporting Information *

ABSTRACT: A novel oxidative absorption process of elemental mercury (Hg0) from flue gas using a Mn2+-modified Fe3+/ H2O2 wet scrubbing system (i.e., Mn2+-modified Fenton-like wet scrubbing system) was proposed. The influence of several technological parameters (concentration of Fe3+, H2O2, and Mn2+, solution pH value, solution temperature, and concentration of Hg0, NO, and SO2) on Hg0 absorption in a bubbling stirred reactor was investigated. On the basis of the detection of free radicals and reaction products, the routes and mechanism of Hg0 oxidative absorption were also proposed. Results revealed that adding Mn2+ greatly promoted Hg0 absorption using a Fenton-like wet scrubbing system. The enhancement role resulted from generating more •OH radicals (they were produced from the synergistic effect of Mn2+ and Fe3+ in a Mn2+-modified Fenton-like wet scrubbing system). Hg0 removal efficiency was elevated via increasing the concentration of Fe3+, Mn2+, or H2O2 and was decreased by increasing the concentration of Hg0, NO, or SO2. Increasing the solution temperature and pH value showed double influence on Hg0 absorption. Hg0 was oxidized by •OH and H2O2, and •OH played a key role. •OH was found to be produced from three sources: (1) Fe2+/H2O2 system, (2) Mn2+/H2O2 system, and (3) by the synergistic role of Fe3+ and Mn2+ in a Mn2+-modified Fenton-like wet scrubbing system.

1. INTRODUCTION Mercury has serious harm for human health and the environment. In all kinds of human activities, coal combustion is considered to be the leading pollution source of anthropogenic mercury emission.1,2 Among several common forms of mercury in coal combustion waste gas (typically including Hg2+, Hg0, Hgp, etc.), Hg0 has been recognized as the focus of the current mercury emission control because it is extremely hard to be captured owing to its extremely poor water solubility and high volatility at low temperature.3,4 In the last few decades, a great deal of Hg0 remediation technologies were developed. According to the basic principle of the removal process, Hg0 remediation technologies can be divided into adsorption, catalytic oxidation, chemical oxidation, etc.1−18 The Fenton-like wet oxidation system has been considered as a promising Hg0 remediation technology since it has good potential to achieve simultaneous removal of multipollutants, such as SO2, NO, Hg0, VOCs, H2S, etc., and has no secondary pollution.17,20−24 In the field of flue gas Hg0 remediation, Tan et al.19 preliminarily studied the technical feasibility of Hg0 oxidation removal using a Fenton-like wet scrubbing system. Lu et al.20 used Fenton-like wet scrubbing to oxidize Hg0 from flue gas in a bubbling reactor. Liu et al.21 further systematically studied the oxidation removal of Hg0 from simulated flue gas using a Fenton-like wet scrubbing system in a spraying reactor. However, they also found that the Fenton-like wet scrubbing system has low oxidation efficiency for Hg0 from gas due to low hydroxyl radical yield. Recently, related results reported that adding trace Mn2+ could greatly improve the yield of hydroxyl radical in a Fentonlike wet scrubbing system.24,25 The Mn2+-modified Fenton wet scrubbing system has shown good prospects because it has © XXXX American Chemical Society

stronger oxidizing ability than that of the Fenton-like wet scrubbing system.24,25 Furthermore, many industrial production processes can discharge Mn2+-containing wastewater.24 The “Hygienic Standard for Drinking Water” (GB5749-85) and “Comprehensive Wastewater Discharge Standard (GB 8978-1996)” promulgated by the Chinese Government stipulate that manganese contents in drinking water and wastewater must be less than 0.1 and 2.0 mg/L, respectively. This exploration on using containing-Mn2+ wastewater to enhance the oxidation removal of Hg0 from gas using Fentonlike wet scrubbing system will also be quite valuable. However, up to now, no relevant works on oxidative absorption of Hg0 from gas using a Mn2+-modified Fenton-like wet scrubbing system were reported. The purpose of the present work is to explore Hg0 oxidative absorption from simulated flue gas using a Mn2+-modified Fenton-like wet scrubbing system in a bubbling stirred reactor. The objectives of this study are follows: (1) to examine the technical feasibility of Hg0 oxidative absorption using a Mn2+-modified Fenton-like wet scrubbing system; (2) to study the effect of several technological parameters (concentrations of Fe3+, H2O2, and Mn2+, solution pH value, solution temperature, and concentrations of Hg0, NO, and SO2) on Hg0 oxidative absorption; (3) to speculate possible pathways and mechanism of Hg0 oxidative absorption in a Mn2+-modified Fenton-like wet scrubbing system. The results will provide a preliminary basis for the follow-up development of this new Hg 0 remediation process. Received: December 26, 2018 Revised: March 6, 2019 Published: March 11, 2019 A

DOI: 10.1021/acs.energyfuels.8b04487 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

2.2. Experimental Procedures. N2, NO, CO2, SO2, O2, and Hg0 mixture gases (1400 mL/min) were prepared using the cylinders gases and regulating the flow meters. The concentrations of NO, CO2, SO2, O2, and Hg0 in mixture gas were measured using a gas analyzer and Hg0 analyzer. Half a liter of Mn2+-modified Fenton-like reagent (Mn2+ /Fe3+/H2O2 mixed reagent) had been prepared using commercial analytical reagents (H2O2, MnCl2·4H2O, and FeCl3· 6H2O) and self-made deionized water. The initial pH value of the Mn2+-modified Fenton-like reagent was adjusted using an acidity meter. Half a liter of Mn2+-modified Fenton-like reagent was moved to the bubbling stirred reactor by opening the silica gel reactor lid. The temperature of the Mn2+-modified Fenton-like reagent was adjusted to the set values using the constant temperature water jacket (constant temperature water was circulated using a pump), a magnetic mixing propeller, and a high-precision thermocouple. N2, NO, CO2, SO2, O2, and Hg0 mixture gases began to enter the bubbling stirred reactor by switching the valves (gas−liquid reaction occurs). The outlet concentrations of Hg0 were measured via the Hg0 analyzer (QM 201H, Suzhou Qingan Instrument Equipment Co., Ltd., China). The concentrations of CO2, N2, NO, SO2, and O2 were measured via a flue gas analyzer (VARIO PLUS, Germany MRU). The operating time of every test was kept for 20 min since in the first 20 min, the removal efficiency of Hg0 is relatively stable. Hydroxyl radicals were detected using an electron spin resonance (ESR) spectrometer (Bruker ESP-300), with joint use of 5,5-dimethyl-1-pyrrolidine Noxide (DMPO, Sigma) as a spin trap agent for hydroxyl radicals. The oxidation products, such as SO32−/SO42− and NO3−/NO3−, in the residual liquid after the Hg0 oxidation were measured using ion chromatography (IC) (model: ICS-1600: Dionex). 2.3. Hg0 Removal Efficiency. Removal efficiency of Hg0 is calculated by the formula 1

2. EXPERIMENTAL SECTION 2.1. Experimental Installation. The experimental device mainly includes three subdevices: (1) an Hg0 mixture gas blending device with several cylinders gases (N2, CO2, NO, SO2, and O2), several flow meters, a gas mixer, and several valves; (2) a bubbling stirred reactor (length 30 cm and diameter 8.5 cm, made of glass) with a silica gel reactor lid, magnetic mixing propeller, a constant temperature water jacket, a high-precision thermocouple, a magnetic mixing propeller and a bubbler; the bubbling stirred reactor also contains a gas inlet, gas outlet, water inlet, and water outlet; (3) an analytical and aftertreatment device, including a flue gas analyzer, a Hg0 gas analyzer, and an absorption bottle. The related schematic diagram on the experimental system is described in Figure 1.

removal efficiency =

Cin − Cout × 100% Cin

(1)

where Cin represents the inlet concentration of Hg0 in the mixture gas and Cout represents the outlet concentration of Hg0 in the mixture gas.

Figure 1. Schematic diagram on the experimental system.

Figure 2. Influence of several technological parameters on Hg0 removal efficiency: (a) Fe3+ concentration; (b) Mn2+ concentration; (c) solution pH value; (d) solution temperature. Basic test conditions: Fe3+ concentration of 0.09 mol/L, Mn2+ concentration of 0.005 mol/L, solution pH value of 2.95, solution temperatures of 328 K, H2O2 concentrations of 1.1 mol/L, Hg0 concentration of 40 μg/m3, SO2 concentration of 1500 ppm, NO concentration of 300 ppm, CO2 concentration of 0%, O2 concentration of 6.0%, a stirring rate of 400 rpm. When studying the change of one parameter, the other parameters were kept unchanged and under basic test conditions. B

DOI: 10.1021/acs.energyfuels.8b04487 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 3. Influence of several technological parameters on Hg0 removal efficiency: (a) H2O2 concentration; (b) SO2 concentration; (c) Hg0 concentration; (d) NO concentration; (e) CO2 concentration; and (f) O2 concentration. Basic test conditions: Fe3+ concentration of 0.09 mol/L, Mn2+ concentration of 0.005 mol/L, solution pH value of 2.95, solution temperatures of 328 K, H2O2 concentrations of 1.1 mol/L, Hg0 concentration of 40 μg/m3, SO2 concentration of 1500 ppm, NO concentration of 300 ppm, CO2 concentration of 0%, O2 concentration of 6.0%, stirring rate of 400 rpm. When studying the change of one parameter, the other parameters were kept unchanged and under the basic test conditions.

3. RESULTS AND DISCUSSION 3.1. Influence of Fe3+ Concentration. Figure 2a displays the influence of Fe3+ concentration on the removal efficiency of Hg0 in a Mn2+-modified Fenton-like wet scrubbing system. It can be found that when the concentrations of Fe3+ increase from 0 to 0.18 mol/L, Hg0 removal efficiency greatly increases (e.g., from 0 to 98.6%). Related results of the previous researchers21−23 showed that Fe3+ could activate H2O2 to produce •OH, which is also called as a Fenton-like oxidation reaction system, through the following formulas 2 and 3. Fe3 + + H 2O2 → Fe2 + + HO2• + H+

(2)

Fe 2 + + H 2O2 → Fe3 + + •OH + OH−

(3)

Hg 0 + 2•OH → Hg(OH)2 → HgO + H 2O

(4)

Hg 0 + •OH → HgO + H•

(5)

On the basis of the above formulas 2−5, the rise of Fe3+ concentration can improve the yield of •OH, thereby being able to enhance the Hg0 oxidation removal. 3.2. Influence of Mn2+ Concentration on Hg0 Removal Efficiency. Figure 2b demonstrates the influence of Mn2+ concentration on Hg0 removal efficiency. It is observed that with the addition of Mn2+, Hg0 removal is markedly enhanced. For example, when Mn2+ concentration changes from 0 to 0.012 mol/L, Hg0 removal efficiency clearly increases from 61.8 to 95.5% in the Mn2+-modified Fenton-like wet scrubbing system. Related results24,25 of some scholars had proved that Mn2+ could effectively strengthen the Fenton-like process through improving the reduction rate of Fe3+ to Fe2+. The above eq 2 (reduction of Fe3+ to Fe2+) has been recognized as the leading rate controlling step of the whole Fenton-like process. Addition of Mn2+ can effectively enhance the cycling rate of Fe3+ and Fe2+ 4,25 and thus is able to efficiently enhance Hg0 oxidation removal in the Fenton-like wet scrubbing system. In addition, some results24,25 also indicated that in

Compared with H2O2, •OH has a much stronger oxidizability for Hg0 because its redox potential is up to 2.80 V (the redox potential of H2O2 is only 1.77 V). The produced •OH can efficiently remove and oxidize Hg0 from gas on the basis of the following formulas 4 and 5.21−23,26 In addition, the produced • H may further induce a series of side reactions, which are described in detail in the literature.27−30 C

DOI: 10.1021/acs.energyfuels.8b04487 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 4. (a) Comparative experiments of Hg0 removal efficiency in different systems; (b) ESR spectrums of •OH adducts in the Mn2+-modified Fenton-like wet scrubbing system; (c) comparative experiments of hydroxyl radical yield in different systems; (d) analysis of reaction products and by-products. Basic test conditions: Fe3+ concentration of 0.09 mol/L, Mn2+ concentration of 0.005 mol/L, solution pH value of 2.95, solution temperatures of 328 K, H2O2 concentrations of 1.1 mol/L, Hg0 concentration of 40 μg/m3, SO2 concentration of 1500 ppm, NO concentration of 300 ppm, CO2 concentration of 0%, O2 concentration of 6.0%, and a stirring rate of 400 rpm. When studying the change of one parameter, the other parameters were kept unchanged and under the basic test conditions.

3.4. Influence of Solution Temperature on Hg0 Removal Efficiency. Figure 2d exhibits the influence of solution temperature on Hg0 removal efficiency. It is seen that on raising the solution temperature from 298 to 348 K, the removal efficiency of Hg0 increases from 86.3 to 92.5% at first and then decreases from 92.5 to 88.2% in the Mn2+-modified Fenton-like wet scrubbing system (328 K is an optimized temperature). A great deal of results26,32−34 had confirmed that increasing temperature would increase the reaction rates of the chemical reaction equations, such as 2−5, thus being able to strengthen Hg0 oxidation. Nevertheless, many scholars26,32−34 also pointed out that increasing temperature of solution would also result in the decline of Hg0 solubility in the solution, thereby restraining the Hg0 oxidation. Hence, the change of temperature of solution exhibits a double influence on the Hg0 oxidation in the Mn2+-modified Fenton-like wet scrubbing system. 3.5. Influence of H2O2 Concentration on Hg0 Removal Efficiency. Figure 3a shows the influence of H 2 O 2 concentration on Hg0 removal efficiency. The reports indicated that when H2O2 concentration changed from 0 to 0.8 mol/L, the removal efficiency of Hg0 greatly increased from 0 to 90.8%. On the basis of the above formulas 2−5, raising the H2O2 concentration could elevate the •OH yield, thus improving Hg0 oxidation. Furthermore, a rise of H2O2 concentration also would effectively promote the Hg 0 oxidation removal via oxidation of H2O2, which can be depicted through eq 9.20,21,23

spite of having a lower activation efficiency, Mn2+ could also directly activate H2O2 to produce •OH to oxidize gaseous Hg0 (it is also called the Fenton-like system), which can be described via the eqs 6 and 7. Mn 2 + + H 2O2 → Mn 3 + + •OH + OH−

(6)

Mn 3 + + H 2O2 → Mn 2 + + HO2• + H+

(7)

3.3. Influence of Solution pH Value on Hg0 Removal Efficiency. Figure 2c shows the influence of solution pH value on Hg0 removal efficiency. The result demonstrates that in the range of low solution pH values, raising the solution pH value (from 0.95 to 2.82) slightly increases the Hg0 removal efficiency. It can be seen from the formula 2 that a rise in solution pH value (or OH− concentration) will promote the generation of Fe2+, which will elevate the yield of •OH, and thus strengthen the Hg0 oxidation. However, in the range of high solution pH values, raising the solution pH value (from 2.95 to 7.55) greatly decreases the Hg0 removal efficiency. Many results31−34 proved that •OH showed obvious instability in a strong alkaline medium since it would be consumed according to the side reaction 8, which contains a very high chemical reaction rate constant (k = 1.3 × 1010 M−1 s−1). Adding excessive OH− will markedly reduced the concentration of •OH, thereby hindering the oxidation removal of gaseous Hg0. •

OH + OH− → H 2O + O−•

(8)

Hg 0 + H 2O2 → H 2O + HgO

Moreover, many reports33,34 have indicated that in alkaline solutions, Fe 3+ and Mn2+ could be transformed into precipitates (e.g., Mn(OH)2 and Fe(OH)3), which will reduce the yield of •OH through inhibiting the reactions in eqs 2 and 3. Therefore, a high solution pH value greatly decreases Hg0 removal efficiency.

(9)

However, when H2O2 concentration further changed from 0.8 to 1.5 mol/L, Hg0 removal efficiency only slightly increased from 90.6 to 92.6%. Many reports35−40 had indicated that in the Fenton-like wet scrubbing system, adding excess H2O2 might bring about the consumption of •OH through the D

DOI: 10.1021/acs.energyfuels.8b04487 Energy Fuels XXXX, XXX, XXX−XXX

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the Fenton-like system (Fe3+/H2O2), the Hg0 removal efficiency greatly increased from 56.8 to 92.5%. The new increase in Hg0 removal efficiency (32.2% = [92.5% − (56.8− 6.6%) − (10.1−6.6%) − 6.6%]) had been found in the Mn2+modified Fenton-like wet scrubbing system. Related studies24,25,40 confirm that several transition metals and Fe3+ show a remarkable synergistic role in activating H2O2 to produce more •OH. In this work, Mn2+ might effectively promote the reduction rate of Fe3+ to Fe2+,24,25,40 being responsible for the enhancement role of Hg0 oxidation in the Mn2+-modified Fenton-like wet scrubbing system. According to the comparison of Hg0 removal efficiency and •OH yield in different systems, it can be inferred that Hg0 is mainly oxidized by •OH and H2O2 and •OH plays a key role. •OH is found to be produced from three sources: (1) Fe3+/H2O2 system; (2) Mn2+/H2O2 system; and (3) by the synergistic role of Fe3+ and Mn2+ in the Mn2+-modified Fenton-like wet scrubbing system. Furthermore, to further verify the results, the oxidation products of Hg0 in the liquid phase were measured and mass balance calculations (S, N, and Hg in SO2, NO, and Hg0, respectively) before and after the reaction were performed. The detection results of the liquid oxidation products indicated that Hg2+ was definitely detected in the residual solution after the oxidation removal reactions, which further verified the above oxidation reactions 4, 5, and 9 (the postprocessing process of oxidation products in solution and the application prospect of this technology can be found in the Supporting Information). Some amounts of sulfate and nitrate in the reaction solution were also detected, which may be the oxidation products of NO and SO2 from flue gas. The results of the mass balance calculations show that the relative errors between the theoretical values and the measured values are 14.3, 7.5, and 14.0% for SO42−, NO3−, and Hg2+, respectively (the theoretical values of SO42−, NO3−, and Hg2+ in reaction solutions can be calculated by element conservation and are 360 μg/L, 27.6 mg/L, and 2.07 μg/L, respectively). On the basis of the above discussions and results, the routes and the mechanism of Hg0 oxidation using the Mn2+-modified Fentonlike wet scrubbing system can be also described by the following schematic diagram in Figure 5.

following eqs 10−13 (all of them have very high reaction rates). Therefore, adding a high concentration of H2O2 could not obviously promote Hg0 oxidation removal. •

OH + H 2O2 → H 2O + HO2• k = 2.7 × 107 M−1 s−1 (10)

•

OH + •OH → H 2O2 k = 4.2 × 109 M−1 s−1

•

OH + HO2• → H 2O + O2 k = 1.0 × 1010 M−1 s−1

(11)

(12)

HO2• + HO2• → H 2O2 + O2 k = 3.4 × 107 M−1 s−1 (13)

3.6. Influence of the Main Components of Flue Gas on Hg0 Removal Efficiency. For different combustion processes and devices, fuels, and combustion conditions, the concentrations of the main components of flue gas in combustion waste gas will change significantly. Therefore, in the present work, the influence of the change of the five key components, such as Hg 0 , NO, O 2 , SO 2 , and CO 2 concentrations, on Hg0 removal was studied, which is shown in Figure 3b−f. The results showed that the changes of Hg0, NO, and CO2 concentrations had no significant influence on the Hg0 oxidation removal efficiency. However, with the rise of SO2 concentration (400−3500 ppm) and O2 concentration (0−12%), Hg0 removal efficiencies clearly reduce from 95.9 to 80.2% and slightly increase from 90.3 to 93.4%, respectively. Many studies35−37,41,42 have reported that SO2 has high activity and can quickly react with H2O2 and •OH, which would reduce the concentrations of H2O2 and •OH in the liquid phase, thus being disadvantageous for Hg0 oxidation. However, O2, which often acts as a capture intermediate of free radicals, can improve the effective utilization of free radicals, thus being helpful for Hg0 oxidation removal. 3.7. Comparative Study of Different Systems and the Routes and the Mechanism of Hg0 Oxidation. Figure 4a demonstrates the contrast results of Hg0 removal efficiencies under different systems. It can be seen from Figure 4a that the three systems of H2O, H2O/Fe3+, and H2O/Mn2+ have no removal capacities for Hg0. Accordingly, as indicated in Figure 4c, there are no signals of hydroxyl radicals in H2O, H2O/Fe3+, and H2O/Mn2+ systems. As indicated in Figure 4a, H2O2 reagent alone can achieve an Hg0 removal efficiency of 6.6%. As shown Figure 4c, no signals of hydroxyl radicals are found in H2O2 reagent alone. The results prove that this part of Hg0 may be oxidized by H2O2 alone, which can be described by eq 9. As seen in Figure 4a, Hg0 removal efficiencies achieve 10.1 and 56.8% in Mn2+/H2O2 and Fe3+/H2O2 systems, respectively. As indicated in Figure 4b,c, clear signals of hydroxyl radicals were found using an ESR spectrometer (DMPO as a trapping agent). The hyperfine splitting coefficients, including aN = 15.0 G and aH = 14.6 G, exhibit a quite satisfactory agreement with the recognized data (aN = 15.1 G and aH = 14.8 G),41−43 representing the hydroxyl radical adduct such as DMPO−OH. The result proved that •OH had been produced in Fe3+/H2O2 and Mn2+/H2O2 systems. These newly increased Hg0 that are removed (10.1−6.6 and 56.8−6.6%) are oxidized by •OH produced from Mn2+/H2O2 and Fe3+/H2O2 (they are defined as Fenton-like systems). Compared with the separate Fenton-like system (Fe3+/H2O2 or Mn2+/H2O2), when a small amount of Mn2+ was added to

Figure 5. Mechanism of gaseous Hg0 oxidation removal using the Mn2+-enhanced Fenton-like wet scrubbing system.

4. CONCLUSIONS Oxidation removal of Hg0 from flue gas using the Mn2+modified Fenton-like wet scrubbing system was studied in a bubbling stirred reactor. Results showed that adding Mn2+ greatly promoted the oxidation removal of Hg0 in the Fentonlike wet scrubbing system. The enhancement role was resulted from generating more •OH radicals (they were produced by the synergistic effect of Mn2+ and Fe3+ in the Mn2+-modified E

DOI: 10.1021/acs.energyfuels.8b04487 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Fenton-like wet scrubbing system). Hg0 removal efficiency was elevated via increasing concentration of Fe3+, Mn2+, or H2O2 and was decreased by increasing concentration of Hg0, NO, or SO2. Increasing the solution temperature and pH value showed double influence on Hg0 oxidation removal. Hg0 was oxidized by •OH and H2O2, and •OH played a key role. •OH was found to be produced from three sources: (1) Fe3+/H2O2 system, (2) Mn2+/H2O2 system, and (3) by the synergistic role of Fe3+ and Mn2+ in the Mn2+-modified Fenton-like wet scrubbing system.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.8b04487. Postprocessing of products; application prospect and economics of this technology (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86 0511 89720178. Fax: +86 0511 89720178. ORCID

Yangxian Liu: 0000-0001-9069-4007 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the National Natural Science Foundation of China (Nos. 51576094 and U1710108), Jiangsu “Six Personnel Peak” Talent-Funded Projects (GDZB-014).

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DOI: 10.1021/acs.energyfuels.8b04487 Energy Fuels XXXX, XXX, XXX−XXX