Removal of Elemental Mercury from Flue Gas by Thermally Activated

Article pubs.acs.org/est

Removal of Elemental Mercury from Flue Gas by Thermally Activated Ammonium Persulfate in A Bubble Column Reactor Yangxian Liu* and Qian Wang School of Energy and Power Engineering, Jiangsu University, Zhenjiang, Jiangsu 212013, China S Supporting Information *

ABSTRACT: In this article, a novel technique on removal of elemental mercury (Hg0) from flue gas by thermally activated ammonium persulfate ((NH4)2S2O8) has been developed for the first time. Some experiments were carried out in a bubble column reactor to evaluate the effects of process parameters on Hg0 removal. The mechanism and kinetics of Hg0 removal are also studied. The results show that the parameters, (NH4)2S2O8 concentration, activation temperature and solution pH, have significant impacts on Hg0 removal. The parameters, Hg0, SO2 and NO concentration, only have small effects on Hg0 removal. Hg0 is removed by oxidations of (NH4)2S2O8, sulfate and hydroxyl free radicals. When (NH4)2S2O8 concentration is more than 0.1 mol/L and solution pH is lower than 9.71, Hg0 removal by thermally activated (NH4)2S2O8 meets a pseudo-first-order fast reaction with respect to Hg0. However, when (NH4)2S2O8 concentration is less than 0.1 mol/L or solution pH is higher than 9.71, the removal process meets a moderate speed reaction with respect to Hg0. The above results indicate that this technique is a feasible method for emission control of Hg0 from flue gas.

1. INTRODUCTION Mercury has great harm for human health and environment due to its persistence, bioaccumulation and neurological toxicity. Coal combustion is considered as the largest source of anthropogenic mercury emission.1 Divalent mercury in flue gas can be effectively removed by wet scrubbing in existing wet flue gas desulfurization systems. However, Hg0 is difficult to remove due to its high volatility and low solubility in water. Therefore, studying effective Hg0 control methods has become one of the hot topics in environmental protection field.2 At present, adsorption and wet scrubbing are considered as two of the most promising Hg0 control technologies.1,2 Some results show that Hg0 can be effectively removed by a wide variety of oxidation technologies.3−12 However, so far, some technical problems such as development and application costs, safety and reliability and secondary pollution of reaction products, can not be effectively solved yet.1 Some other results show that Hg0 can be adsorbed by activated carbon and then be converted to particulate mercury, which can be captured by existing dust equipment. However, the high using cost hinders its large-scale applications.1 The other new adsorbents such as metal oxides, precious metals, activated coke, fly ash, calciumbased materials, molecular sieves and natural mineral materials have shown good development prospects, but because of the deficiencies in adsorbent’s stability and reliability and high cost, they are still unable to obtain large-scale applications.2 In summary, although many flue gas Hg0 control technologies have been developed, so far none of them is suitable for largescale commercial applications yet. © 2014 American Chemical Society

Thermally activated persulfate technology has been widely studied in degrading organic pollutants from wastewater due to having extremely strong oxidation ability, simple and secure process, and environmental protection.13 In the field of flue gas purification, Adewuyi et al. used thermally activated persulfate technology to simultaneously remove NO and SO2 by wet oxidation, and has obtained good results. This related process parameters, reaction products and kinetics were also studied.14,15 However, yet until now, the aqueous removal of Hg0 from flue gas using thermally activated persulfate technology is not reported yet. If thermally activated persulfate technology can also effectively remove Hg0 from flue gas, it may eventually be developed into an effective simultaneous removal technology of SO2, NOx and Hg0. In addition, existing studies14,15 mainly use Na2S2O8 as the thermally activated oxidant. However, the price of (NH4)2S2O8 is only the 60% of sodium persulfate, and the final decomposition product of (NH4)2S2O8 is mainly the ammonium sulfate, which can be made into agricultural fertilizers. Therefore, studying simultaneous removal of SO 2 , NO x and Hg0 by thermally activated (NH4)2S2O8 will have better economic and practical value. In the present work, the main influencing parameters, mechanism and kinetics of Hg0 removal by thermally activated (NH4)2S2O8 are studied in a bubble column reactor. The results will provide Received: Revised: Accepted: Published: 12181

April 23, 2014 August 18, 2014 September 24, 2014 September 24, 2014 dx.doi.org/10.1021/es501966h | Environ. Sci. Technol. 2014, 48, 12181−12189

Environmental Science & Technology

Article

2.3. Measurement Methods of NO2−, NO3−, SO32−, SO42−, NH4+, ·OH, SO4·−, Hg0 and Total Mercury in Solution. The NH4+ in solution was determined by using UV spectrophotometric method (Shimadzu UV2450). The NO2−, NO3−, SO32− and SO42− in solution were determined by IC (792 Basic IC). The ·OH and SO4·− were detected by ESR spectrometer (Bruker ESP-300) combining with 5,5-dimethy l1- pyrrolidine N-oxide (DMPO) (>99%, Sigma) as a spin trap agent. The concentrations of total mercury and Hg0 were determined by a fluorescence mercury analyzer, and the measurement method can reference the literature.1 2.4. Removal Efficiency. The concentrations of Hg0, SO2, NO measured by bypass line are used as inlet concentrations. The average concentrations within 30 min measured by reactor outlet are used as outlet concentrations. The removal efficiencies of Hg0, SO2, NO in gas can be calculated by the following eq 1: C − Cout Hg 0, SO2 , NO removal efficiency = in × 100(%) Cin

some theoretical guidance for industrial applications of simultaneous removal of Hg0, NO and SO2 by thermally activated (NH4)2S2O8.

2. EXPERIMENTAL SECTION 2.1. Experimental System. It can be seen in Figure 1, the experimental system mainly consists of flue gas preparation

Figure 1. Schematic diagram of experimental system.

(1)

Where Cin is inlet concentration of Hg0, SO2, NO in gas; Cout is outlet concentration of Hg0, SO2, NO in gas. 2.5. Hg0 Absorption Rate. Based on material balance of Hg0, Hg0 absorption rate can be calculated by eq 2:16

system, measuring system and bubble column reactor. Cylinder gases 1−4, including O2, NO, SO2, N2 (Purity, 99.99%), are used to make simulated flue gas and carrier gas. The mercury generator 13, including U-tube and mercury permeation tube (VICI Metronics, Poulsbo, WA), is used to produce Hg0 vapor. The bubble column reactor (High, 30 cm; Inside diameter, 8.5 cm) is made by Silicate Glass. The bubbler 18 is installed at 5 cm away from the bottom of bubble column reactor to distribute gas. Constant temperature water bath 21 (HH-42, Changzhou GuoHua Co., China) and mercury thermometer 15 are used to control solution temperature. Flue gas analyzer 19 (MRU-VARIO PLUS, Germany) and mercury analyzer 20 (QM201H, Suzhou Qingan Instrument Co., China) are used to determinate concentrations of SO2, NO, O2, and Hg0, respectively. The solution can be added to the bubble column reactor by opening the reactor cover 16. The exhaust gas can be further processed by exhaust absorption pipe 21. 2.2. Experimental Procedures. 0.8L/min of simulated flue gas was made by using the cylinders and the mercury generator. Flows and concentrations of gas were regulated by the rotameters. Inlet concentrations of SO2, NO, O2 and Hg0 were measured by flue gas analyzer and mercury analyzer through gas bypass line. 0.6L of ammonium persulfate solution was prepared by using ammonium persulfate reagents (Guoyao Chemical Reagent Co., AR, China) and deionized water. Solution pH was adjusted by adding HCl and NaOH, and was measured by an acidimeter (Kedida instrument Co., CT-6023, China). The prepared solution was added to the bubble column reactor by opening the reactor cover. Solution temperatures were adjusted to the required temperatures by combination use of constant temperature water bath and mercury thermometer. When solution temperature reached required value and kept stable, the valve 11 was closed and the valve 12 was opened. Then simulated flue gas entered the bubble column reactor to make a gas−liquid reaction. The outlet concentrations of Hg0, SO2, O2 and NO were measured by mercury analyzer and flue gas analyzer. Each experimental run was kept for 30 min. The concentrations of Hg0, SO2 and NO in reactor outlet were recorded once every 10 min. The average concentration within 30 min (not including zero value) was used as outlet concentrations of Hg0, SO2 and NO.

NHg 0 =

ηHg 0 ·C Hg 0,in·Q G × 10−3 60·MHg 0 ·aHg 0 ·VL

(2)

Where ηHg0 is removal efficiency of Hg , %; CHg0,in is inlet concentration of Hg0, μg/m3; QG is gas flow, L/min; MHg0 is molecular weight of Hg0, g/mol; aHg0 is specific interfacial area of Hg0, m−1; VLis solution volume, L. 2.6. Physical and Mass Transfer Parameters. The related physical and mass transfer parameters are measured by using the calculation and measurement methods which are described in Supporting Information (SI) 1 and the data are summarized in Table 1. 0

Table 1. Diffusion Coefficients, Solubility Coefficients and Mass Transfer Parameters DHg0,L × 109 m2/s

HHg0,L × 108 mol/L·Pa

kHg0,L × 104 m/s

kHg0,G × 106 mol/ s·m2·Pa

aHg0 m−1

3.54

9.64

3.93

2.21

41.35

3. RESULTS AND DISCUSSIONS 3.1. Effects of (NH4)2S2O8 Concentration. The effects of (NH4)2S2O8 concentration on Hg0 removal efficiency are shown in Figure 2 (a). It can be observed that Hg0 removal efficiency greatly increases from 0 to 99.6% when (NH4)2S2O8 concentration increases from 0 to 0.8 mol/L. However, when (NH4)2S2O8 concentration exceeds 0.2 mol/L, the growth rate of Hg0 removal efficiency gradually becomes smaller. The results show that as a strong oxidizing agent, (NH4)2S2O8 can oxidize and remove Hg0 from flue gas by the following reactions 3 and 4.17 H 2O

12182

(NH4)2 S2 O8 ⎯⎯⎯→ 2NH+4 + S2 O82 −

(3)

S2 O82 − + Hg 0 → Hg 2 + + 2SO24 −

(4)

dx.doi.org/10.1021/es501966h | Environ. Sci. Technol. 2014, 48, 12181−12189


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Figure 2. Effects of main influencing parameters on Hg0 removal efficiency: (a) (NH4)2S2O8 concentration; (b) activation temperature; (c) Hg0 inlet concentration; (d) Solution pH; (e) SO2 concentration; (f) NO concentration. Conditions: O2 concentration, 6.0%; SO2 concentration, 1500 ppm; NO concentration, 400 ppm; Hg0 concentration, 50 μg/m3; Activation temperature, 75 °C; Solution pH, 3.41; (NH4)2S2O8 concentration, 0.40 mol/L.

Besides, the results show that S2O82− can generate SO4−· by using high temperature activation according to the following reaction 5.13−15

·OH also has very strong oxidizing (its redox potential is up to 2.80 V) and can oxidize and remove Hg0 from flue gas by the following reactions 8 and 9.1,18

heat

S2 O82 ‐ ⎯⎯⎯→ 2SO−4 ·

(5)

SO4−· has very strong oxidizing because its redox potential is up to 2.65 V, and thus it can oxidize and remove Hg0 from flue gas by the following reaction 6.17 2SO−4 ·+Hg 0

→ Hg

2+

+

2SO24 −

(8)

·OH + Hg + → Hg 2 + + OH−

(9)

It can be inferred from the above reactions 3-9 that increasing (NH4)2S2O8 can produce more S2O82− SO4−· and ·OH, thereby being able to enhance removal of Hg0. However, when (NH4)2S2O8 concentration greatly increases, several side reactions such as the following eqs 10-16 and the selfdecomposition of (NH4)2S2O8 will also occur quickly in solution, which may result in the self-loss of S2O82−, SO4−· and

(6)

In addition, SO4−· can react with H2O to generate ·OH by the following reaction 7.14 SO−4 ·+ H 2O → · OH + HSO−4 k1 = 6.6 × 102s−1

·OH + Hg 0 → Hg + + OH−

(7) 12183



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Figure 3. ESR spectra of SO4−· and ·OH free radical adducts at 25 °C (a), 45 °C (b), 55 °C (c), and 75 °C (e). (Circles represent the DMPO-SO4 and triangles represent the DMPO-OH.).

Table 2. Reaction Products of SO2, NO and Hg0 Removals and Mass Balance for Total Mercury ion category

NH4+

measured concentration (mg/L)

1.38 × 10

4

SO42−

SO32−

NO3−

NO2−

total mercury

Hg0

316.33

0

5.06

0

1.5 (μg/L) 1.7 (μg/L) 11.76%

0

relative error (%)

SO−4 ·+ S2 O82 − → SO24 − + S2 O−8 · k 3 = 6.1 × 105M−1s−1

(11)

SO−4 ·+ SO−4 · → S2O82 −k4 = 4.0 × 108M−1s−1

(12)

· OH + · OH → H 2O2 k5 = 5.5 × 109M−1s−1

(13)

H 2O2 + · OH → HO2 ·+ H 2Ok6 = 2.7 × 107M−1s−1

(14)

HO2 ·+ HO2 · → H 2O2 + O2 k 7 = 8.3 × 105M−1s−1

(15)

HO2 ·+·OH → H 2O + O2 k 8 = 6.9 × 109M−1s−1

(16)

3.2. Effects of Activation Temperature. The effects of activation temperature on Hg0 removal efficiency are shown in Figure 2 (b). It can be seen that increasing activation temperature has an obvious positive impact on removal of Hg0. When activation temperature increases from 25 to 85 °C, Hg0 removal efficiency greatly increases from 44.8% to 82.8% under 0.4 mol/L of (NH4)2S2O8 concentration, and increases from 19.6% to 42.5% under 0.05 mol/L of (NH4)2S2O8 concentration, respectively. The results show that increasing temperature can increase chemical reaction rate,1 thus being able to enhance removal of Hg0 by S2O82− oxidation (eq 4). Additionally, it can be seen from the above reactions 5 and 7 that both of the yields of SO4−· and ·OH will increase by

Figure 4. Plot of log(CHg0, i) versus the log(NHg0).

·OH.1,2,13−15,17 Furthermore, with the enhancement of chemical reactions, the mass transfer step may also gradually begin to play an important role. Thus, the growth rate of Hg0 removal efficiency gradually becomes smaller with the further increase of (NH4)2S2O8 concentration. · OH + S2 O82 − → OH− + S2 O−8 · k 2 = 1.2 × 107M−1s−1 (10) 12184



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Table 3. Kinetic Parameters of Hg0 Removal by Thermally Activated Ammonium Persulfate projects parameters CS2O82− (mol/L)

0.002 0.004 0.008 0.012 0.025 0.05 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80

ηHg0 %

NHg0 × 1011 mol/m2·s

kHg0,L × 104 m/s

kov1 × 10−2 s−1

Ha

8.7 14.4 23.6 29.2 35.3 42.8 49.8 68.8 73.4 80.5 89.2 94.5 98.8 99.6

1.17 1.93 3.16 3.91 4.73 5.74 6.67 9.22 9.84 10.79 11.95 12.66 13.24 13.35

3.93 3.93 3.93 3.93 3.93 3.93 3.93 3.93 3.93 5.93 3.93 3.93 3.93 3.93

0.14 0.37 1.02 1.58 2.35 3.51 4.81 9.64 11.07 13.53 16.94 19.25 21.32 21.76

0.57 0.92 1.52 1.90 2.32 2.84 3.32 4.70 5.04 5.57 6.23 6.64 6.99 7.06

solution pH

1.88 2.85 3.41 4.52 7.15 8.28 9.71 11.28

98.8 97.8 80.5 75.3 72.3 66.3 62.3 31.9

13.24 13.11 10.79 10.09 9.69 8.88 8.35 4.27

3.93 3.93 3.93 3.93 3.93 5.93 3.93 3.93

21.32 20.79 13.49 11.70 10.66 8.93 7.82 1.90

6.99 6.90 5.56 5.18 4.94 4.52 4.23 2.09

CHg0 (μg/m3)

10 20 30 40 50 60 70 80

85.2 84.7 83.5 82.6 80.5 79.5 77.8 76.5

2.30 4.60 6.76 8.84 10.79 12.88 14.70 16.37

3.93 3.93 3.93 3.93 3.93 3.93 3.93 3.93

15.57 15.56 14.93 14.29 13.53 13.37 12.75 12.07

5.97 5.97 5.85 5.72 5.57 5.52 5.41 5.25

CNO (ppm)

0 400 800 1200 1600 2000 2400

81.3 80.5 79.8 79.7 79.2 78.8 78.3

10.89 10.79 10.69 10.68 10.61 10.56 10.49

3.93 3.93 3.93 3.93 3.93 3.93 3.93

13.84 13.53 13.33 13.20 13.03 12.91 12.74

5.63 5.57 5.53 5.52 5.46 5.44 5.40

CSO2 (ppm)

0 500 1000 1500 2000 2500

88.9 84.2 82.6 80.5 78.9 76.5

11.91 11.28 11.07 10.79 10.57 10.25

3.93 3.93 3.93 3.93 3.93 3.93

16.83 14.91 14.30 13.53 12.93 12.11

6.21 5.85 5.73 5.57 5.44 5.27

reasons to explain the phenomena. The first reason is that increasing Hg0 inlet concentration will increase the amount of Hg0 through the bubble column reactor per unit time, which will decrease the relative molar ratio of S2O82−, SO4−· and ·OH to Hg0, thereby being able to reduce removal efficiency of Hg0.2 The another reason is that according to two-film theory, increasing Hg0 inlet concentration will increase partial pressure of Hg0 in gas phase, and thus can enhance the mass transfer driving force of Hg0, thereby promoting removal of Hg0.2 However, from the above results, it can be inferred that the first

increasing activation temperature. Therefore, increasing activation temperature is conducive to removal of Hg0. 3.3. Effects of Hg0 Inlet Concentration. The effects of Hg0 inlet concentration on Hg0 removal efficiency are shown in Figure 2 (c). It can be seen that increasing Hg0 inlet concentration results in the decrease of Hg0 removal efficiency. For example, when Hg0 inlet concentration increases from 10 μg/m3 to 80 μg/m3, Hg0 removal efficiency decreases from 85.2% to 76.6% under 0.4 mol/L of (NH4)2S2O8 concentration, and decreases from 50.1% to 27.2% under 0.05 mol/L of (NH4)2S2O8 concentration, respectively. There are two main 12185



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Figure 5. Removal mechanism of Hg0 by thermally activated ammonium persulfate in bubble column reactor.

reason plays a leading role, which finally results in the increase of Hg0 removal efficiency. 3.4. Effects of Solution pH. The effects of solution pH on Hg0 removal efficiency are shown in Figure 2 (d). It can be seen that increasing solution pH leads to a great decrease in Hg0 removal efficiency. For example, when solution pH increases from 1.88 to 11.28, Hg0 removal efficiency decreases from 98.8% to 31.9% under 0.4 mol/L of (NH4)2S2O8 concentration, and decreases from 82.9% to 6.9% under 0.05 mol/L of (NH4)2S2O8 concentration, respectively. The effects of solution pH on removal of Hg0 can be attributed to the following reasons. The results show that SO4−· can react with OH‑ to produce ·OH by the following reaction 17.14,15,17 SO−4 ·+ OH− → · OH + SO24−k 9 = 6.5 × 107M−1s−1

(18)

The oxide radical, ·O‑, is known to react more slowly with the same substrate than ·OH, which will weaken removal of Hg0.1,14 Furthermore, S2O82− will accelerate self-decomposition, and its oxidizability will also significantly decrease in strong alkaline solutions, thereby being unfavorable for removal of Hg0. Thus, with the increase of solution pH, Hg0 removal efficiency greatly decreases. 3.5. Effects of SO2 Concentration. The effects of SO2 concentration on removal efficiency of Hg0 are shown in Figure 2 (e). It can be seen that when SO2 concentration increases from 0 to 2500 ppm, Hg0 removal efficiency decreases from 88.9% to 76.5% under 0.4 mol/L of (NH4)2S2O8 concentration, decreases from 56.8% to 33.8% under 0.05 mol/L of (NH4)2S2O8 concentration, and decreases from 34.3% to 3.1% under 0.008 mol/L of (NH4)2S2O8 concentration, respectively. SO2 can often react with and consume S2O82−, SO4−· and ·OH by the following reactions 19−26,1,2,14,15 thus the increase of SO2 concentration can decrease Hg0 removal efficiency. (19)

HSO−3 ↔ SO32 − + H+

(20)

HSO−3 + ·OH → SO−3 ·+ H 2Ok11 = 9.5 × 109M−1s−1

(21)

HSO−3 + S2 O82 − + H 2O → 3HSO−4

(23)

SO32 − + S2 O82 − + H 2O → 2HSO4 − + SO24 −

(24)

HSO−3

(25)

+

2SO−4 ·+H 2O

→

3HSO−4

(26)

3.6. Effects of NO Concentration. The effects of NO concentration on removal efficiency of Hg0 are shown in Figure 2 (f). It can be seen that when NO concentration increases from 0 to 2400 ppm, Hg0 removal efficiency decreases from 81.3% to 78.3% under 0.4 mol/L of (NH4)2S2O8 concentration, decreases from 41.6% to 36.3% under 0.05 mol/L of (NH4)2S2O8 concentration, and decreases from 15.3% to 6.8% under 0.008 mol/L of (NH4)2S2O8 concentration, respectively. NO can also consume S2O82−, SO4−· and ·OH by the following reactions 27-32,1,2,14,15 thus the increase of NO concentration can also decrease Hg0 removal efficiency.

(17)

SO2 + H 2O ↔ HSO−3 + H+

(22)

2SO−4 ·+SO32 − + H 2O → 2HSO4 − + SO24 −

However, at the same time, the following side reaction 18 with a very fast reaction rate also will quickly occur in solution, which can quickly consume ·OH free radicals.1,2 · OH + OH− ↔ · O− + H 2Ok10 = 1.3 × 1010M−1s−1

SO32 − + · OH → SO−3 ·+ OH−k12 = 5.5 × 109M−1s−1

S2 O82 − + NO + H 2O → NO2 + 2H+ + 2SO24 −

(27)

SO−4 ·+NO + H 2O → NO−2 + HSO−4 + H+

(28)

NO + · OH → H+ + NO‐2 k13 = 2.0 × 1010M−1s−1

(29)

S2 O82 − + NO−2 → NO2 + SO‐4 ·+SO24 −

(30)

SO‐4 ·+ NO‐2 → NO2 + SO24−k14 = 9.8 × 108M−1s−1

(31)

NO2 + ·OH → H+ + NO−3 k15 = 4.5 × 109M−1s−1

(32)

3.7. Free Radicals, Products and Material Balance. To study removal mechanism of Hg0 by thermally activated ammonium persulfate, SO4−· and ·OH in solution was captured by using ESR spectrometer and DMPO, and the results are shown in Figure 3. As shown in Figure 3 (a), ESR spectrometer is not able to capture a clear free radical signal at 25 °C. However, it can be seen from Figure 3 (b)−(d), under 45 °C, 55 and 75 °C, the hyperfine splitting constants of DMPO free radical adducts (obtained by simulation, aN = 13.9 G, aH = 10.2 G, aH = 1.45 G, and aH = 0.76 G) were in good agreement with the literature data aN = 13.8 G, aH = 10.1 G, aH = 1.44 G, and aH = 0.79 G.19,20 These are representative of SO4−· free radicals added to DMPO (DMPO-SO4). In addition, the typical fourline ESR spectra were also monitored. These were charac12186



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with respect to Hg0. Thus, the intrinsic rate equation of Hg0 removal by thermally activated (NH4)2S2O8 can be expressed into the following eq 34:

terized with the hyperfine splitting constants aN = 15.1 G and aH = 14.8 G, which were also in good agreement with the literature data aN = 15.0 G and aH = 14.8 G.19,20 They are representative of ·OH free radicals added to DMPO (DMPOOH). Thus, under 45, 55, and 75 °C, SO4−· and ·OH coexist in solution, and as the activation temperature increases, both of the yields of SO4−· and ·OH increase. The results can provide powerful supports for the above-mentioned free radical oxidation reactions caused by SO4−· and ·OH. Furthermore, the concentrations of NO2−, NO3−, SO32−, SO42−, NH4+, total mercury and Hg0 in solution were measured by IC, UV spectrophotometric method and fluorescence mercury analyzer, and the results are shown in Table 2. The results show that all of SO32−, NO2− and Hg0 can not be detected in solution. However, all of SO42−, NO3− and Hg2+ are detected in solution. The SO42− may derive from the final decomposition product of S2O82− and the final oxidation product of SO2, and the NO3− may derive from the final oxidation product of NO, which the results are consistent with the previous series of discussions. Additionally, to further definite the transfer paths of Hg element in flue gas, based on the determined results of total mercury concentration and Hg0 removal efficiency, the calculation of material balance for total mercury is made and the results are also shown in Table 2. It can be seen that the determined values of total mercury concentration are in good agreement with the predictive values, with a relative error of 11.76%, which further suggests that Hg0 is mainly removed by oxidation reaction, Hg2+ is the main oxidation product of Hg0 in solution. The error may derive from the measurement error of Hg0 in flue gas and the digestion process of mercury in solution. 3.8. Kinetics of Hg0 Removal. 3.8.1. Theoretical Fundamentals. The results have confirmed that Hg0 from flue gas can be removed by S2O82− aqueous oxidation.17 Furthermore, in thermally activated (NH4)2S2O8 solution, the successful capture of SO4−· and ·OH by ESR spectrometer show that removal of Hg0 by the oxidations of SO4−· and ·OH is also two of the reaction pathways of Hg0 removal. The analysis results of liquid products further show that Hg2+ is the final reaction product of Hg0 removal, and Hg0 is mainly removed by oxidation. Based on the above results, removal of Hg0 by thermally activated (NH4)2S2O8 mainly includes three reaction pathways: removal of Hg0 by ·OH oxidation, removal of Hg0 by SO4−· oxidation, and removal of Hg0 by S2O82− oxidation. Thus, total reaction of Hg0 removal can be expressed as the following eq 33:

m r Hg 0 = kovmC Hg 0 ,i

Where kovm is pseudo-mth-order reaction rate constant with respect to Hg0, s−1; CHg0,i is Hg0 interface concentration, mol/L; m is partial reaction order with respect to Hg0. According to two-film theory,22,23 under steady-state, Hg0 absorption rate meets the following eq 35: NHg 0 = kHg 0 , G(pHg 0, G − pHg 0, i ) = E ·kHg 0,L(C Hg 0, i − C Hg 0,L) (35)

Where NHg0 is Hg0 absorption rate, mol/m2·s; kHg0,G is gas phase mass transfer coefficient, mol/s·m2·Pa; PHg0,G is Hg0 partial pressure in gas phase body, Pa; PHg0,i is Hg0 partial pressure in phase interface, Pa; CHg0,L is Hg0 concentration in liquid phase body, mol/L; kHg0,L is liquid phase mass transfer coefficient, m/ s; E is chemical reaction enhancement factor. According to Henry law, the equilibrium relation 36 in gas− liquid phase interface can be met:22,23 C Hg 0, i = HHg 0,L·pHg 0, i

(36) 0

Where HHg0,L is solubility coefficient of Hg in liquid phase, mol/(L·Pa). The absorption rate eq 37 of Hg0 in gas−liquid two-phase can be obtained by jointly solving the above eqs 35 and 36: ⎛ ⎞ C Hg 0,L ⎞ ⎛ 1 1 ⎟⎟/⎜⎜ ⎟ NHg 0 = ⎜⎜pHg 0, G − + HHg 0,L ⎠ ⎝ kHg 0, G E ·HHg 0, L·kHg 0,L ⎟⎠ ⎝

(37)

The above results show that Hg0 is mainly removed by oxidations of SO4−·, ·OH and S2O82− in thermally activated ammonium persulfate system. The results indicate that the chemical reaction rate constants between most of pollutants and SO4−· or ·OH are up to 107−10 M−1 s−1,16,17 thus the chemical reaction rate of Hg0 removal by thermally activated ammonium persulfate may be very fast. To facilitate to identify kinetics region, here it is first supposed that removal of Hg0 by thermally activated ammonium persulfate is a fast reaction, i. e. the chemical reaction rate is larger than the mass transfer rate (This assumption will be verified in the back.). On basis of twofilm theory, for a fast reaction, as Hg0 have been completely consumed before entering liquid phase body, the concentration of Hg0 in liquid phase body is zero, that is, CHg0,L = 0. Thus, Hg0 absorption rate eq 37 can be further simplified to the following eq 38: pHg 0, G NHg 0 = (kHg 0, G)−1 + (E · HHg 0,L· kHg 0,L)−1 (38)

aHg 0 + bS2O82 ‐ + c·OH + dSO‐4 · activated temperature

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ e Hg 2 + + f other products

(34)

(33)

Where a, b, c, d, e, and f are the stoichiometric coefficients of reactants and products. The concentration of S2O82− (10−1 mol/L) is much larger than that of Hg0 (10−11 mol/L), thus according to classical excessive concentration method,21 the concentration of S2O82− can be approximately as a constant in a short experimental period. Besides, it is well-known that both of ·OH and SO4−· have very low concentrations because of very short lifetime, so both of ·OH and SO 4 − · concentrations can be also approximately regarded as constants according to the famous steady-state approximation method.21 Based on the above discussions, removal of Hg0 by thermally activated (NH4)2S2O8 can be regarded as an irreversible pseudo-mth-order reaction

For an irreversible pseudo-mth-order reaction, E can be expressed as the following eq 39:22,23 E=

Ha[(Ei − E)/(Ei − 1)]n /2 tanh{Ha[(Ei − E)/(Ei − 1)]n /2 }

(39)

Where Ei is instantaneous chemical reaction enhancement factor; Ha meets the following expression 40:22,23 ⎛ 1 ⎞ 2 m−1 ⎟⎟ Ha = ⎜⎜ kovm·DHg 0,L ·C Hg 0 ,i 0 k m 1 + ⎝ Hg ,L ⎠

{

12187

1/2

}

(40)



Article

Where DHg0,L and kHg0,L can be obtained by Table 1. The pseudo-first-order reaction rate constant, kov1, can be calculated by the above eqs 2 and 46. The results shown in Table 3 indicate that when concentration of (NH4)2S2O8 is more than 0.1 mol/L and solution pH is lower than 9.71 (In fact, in order to meet environmental protection requirements of at least more than 80% of Hg 0 removal efficiency, the concentration of (NH4)2S2O8 should keep at least more than 0.1 mol/L and solution pH also should keep at least lower than 9.71 in the actual industrial operation), all of Ha s are larger than 3.0, showing that removal of Hg 0 by thermally activated (NH4)2S2O8 is a fast reaction (This above assumption has been verified). Thus, removal of Hg0 from flue gas by thermally activated (NH4)2S2O8 in the bubble column reactor meets a pseudo-first-order fast reaction with respect to Hg0. According to two-film theory, for a fast reaction, chemical reaction process is completed in liquid film. Chemical reaction rate of Hg0 removal is larger than mass transfer rate, and the mass transfer process is the control step of Hg0 removal process. Based on the above eq 46, it can be seen that Hg0 absorption rate mainly depends on gas phase mass transfer coefficient, gas−liquid specific interfacial area and Hg0 partial pressure, but is independent of liquid phase mass transfer coefficient. So Hg0 absorption rate can be strengthened by increasing gas phase mass transfer coefficient, gas−liquid specific interfacial area and Hg0 partial pressure. Hg0 partial pressure usually associates with coal-fired types, which is not easy to change, thus increasing gas−liquid specific interfacial area (For example, choosing a reactor with a larger gas−liquid specific interfacial area) and gas phase mass transfer coefficient (For example, enhancing the agitation in gas phase body) may effectively enhance the absorption rate of Hg0 in thermally activated (NH4)2S2O8 system. In addition, when (NH4)2S2O8 concentration is less than 0.1 mol/L or solution pH is higher than 9.71, the removal process meets a moderate speed reaction with respect to Hg0 (3.0 > Ha > 0.03). At this time, the chemical reaction rate of Hg0 removal is roughly equivalent to the mass transfer rate, and both of the chemical reaction process and the mass transfer process are the control step of Hg0 removal process. Hg0 removal can be enhanced by simultaneously increasing the chemical reaction rate and the mass transfer rate. 3.9. Removal Mechanism of Hg0. Based on two-film theory22,23 and the above results about chemical reaction mechanism and mass transfer-reaction kinetics, although several other side reactions may also occur in liquid phase, the main removal mechanism of Hg0 by thermally activated (NH4)2S2O8 in the bubble column reactor may be presumably summarized as the following several parts: (1) Hg0 (g) in gas phase body will first reach gas−liquid interface by diffusion through gas film, and keep a gas−liquid equilibrium in gas−liquid interface; (2) Hg0 (l), which has been dissolved in liquid phase, will enter liquid film by diffusion through gas−liquid interface, and will react with S2O82−, SO4−· and ·OH from liquid phase body in a reaction surface; 3 In the reaction surface, a series of chemical reactions 3-9 of Hg0 removal will occur, and at same time, these side reactions 10-32 will also occur; 4 The potential gaseous products from reaction process will return gas phase body by diffusion through two-film. However, the liquid product such as Hg2+ will enter liquid phase body by diffusion through liquid film. In summary, removal mechanism of Hg0 from flue gas by thermally activated (NH4)2S2O8 in the bubble column reactor

Where Hatta coefficient, Ha, is defined as the ratio of chemical reaction rate to physical absorption rate in liquid film. According to two-film theory, when Ha > 3.0, the absorption process of gas in liquid phase is a fast reaction, and there is E = Ha.22,23 Thus, based on the above assumption of fast reaction, the absorption rate eq 38 of Hg0 can be further rewritten as the following expression 41: ⎛ ⎞−1 1 1 ⎟ NHg 0 = pHg 0, G ⎜⎜ + 1/2 ⎟ kHg 0 , G 0 (2/(m + 1) · k 0 · C m −01 ) H · D Hg , L ovm Hg , L Hg , i ⎝ ⎠

(41) 0

3.8.2. Reaction Order with Respect to Hg . To determine m value of reaction order with respect to Hg0, the above eq 41 is changed into the following eq 42: NHg 0·kHg 0, G HHg 0, L·(pHg 0, G ·kHg 0 , G

⎛ 2DHg 0, L ·kovm·C m −01 ⎞1/2 Hg , i ⎟ = ⎜⎜ ⎟ − NHg 0) ⎝ m+1 ⎠

(42)

The following eq 43 of Hg0 interface concentration can be obtained by jointly solving the above eqs 35 and 36: C Hg 0, i = HHg 0,L(pHg 0, G − NHg 0/kHg 0, G)

(43)

The following eq 44 can be obtained by jointly solving the above eqs 42 and 43: NHg 0

⎛ 2DHg 0, L ·kovm·C m +01 ⎞1/2 Hg , i ⎟ = ⎜⎜ ⎟ m 1 + ⎝ ⎠

(44)

The following eq 45 can be obtained by taking the logarithm on both sides of the above eq 44: log(NHg 0) =

1 ⎛ 2DHg 0,L ·kovm ⎞ m+1 ⎟+ log⎜ log(C Hg 0, i) 2 ⎝ m+1 ⎠ 2

(45)

Using the data in Figure 2(c) and Table 1, the value of CHg0,i can be calculated by using the above eq 43. Then the plot of log(CHg0, i) versus the log(NHg0) are made and the results are shown in Figure 4. It can be seen that there is an approximately linear relationship between log(CHg0, i) versus the log(NHg0) (the correlation coefficient is 0.999). The slope of fitted line, (m + 1)/2, is 0.964, and m = 0.93, taking the integer m = 1.0. The results show that the reaction is a pseudo-first-order reaction with respect to Hg0. So Hg0 absorption rate equation may be finally represented by the following expression 46: NHg 0

⎛ ⎞−1 1 1 ⎟ = pHg 0, G ⎜⎜ + HHg 0, L(kov1·DHg 0, L)1/2 ⎟⎠ ⎝ kHg 0, G

(46)

3.8.3. Kinetic Parameters. The mass transfer-reaction kinetic parameters of gas absorption in solution are the essential parameters for industrial design and amplification of reactor and numerical simulation of gas absorption process. Thus, the related kinetic parameters of Hg0 removal were determined, and the results are shown in Table 3. For a pseudo-first-order reaction for Hg0, the above expression 40 of Ha can be further simplified to the following discriminant 47:

Ha =

kov1·DHg 0, L kHg 0, L

(47) 12188



Article

(4) Granite, E. J.; Pennline, H. W. Photochemical removal of mercury from flue gas. Ind. Eng. Chem. Res. 2002, 41, 5470−547. (5) Granite, E. J.; King, W.; Stanko, D.; Pennline, H. W. The implications of mercury interactions with band-gap semiconductor oxides. Main Group Chem. 2008, 7, 227−237. (6) Granite, E. J.; Pennline, H. W.; Hoffman, J. S. Effects of photochemical formation of mercuric oxide. Ind. Eng. Chem. Res. 1999, 38, 5034−5037. (7) Wang, J. W.; Yang, J. L.; Liu, Z. Y. Gas-phase Elemental mercury capture by a V2O5/AC catalyst. Fuel Process. Technol. 2010, 91, 676− 680. (8) Martinez, A. I.; Deshpande, B. K. Kinetic Modeling of H2O2enhanced oxidation of flue gas elemental mercury. Fuel Process. Technol. 2007, 88, 982−987. (9) Ma, X. Y. Experimental Study on Removal Mercury from Flue Gas by Aqueous Solution; North China Electric Power University: Baoding, China, 2008 (10) Zhan, F. M.; Li, C. T.; Zeng, G. G.; Tao, S. S.; Xiao, Y. J. Experimental Study on oxidation of elemental mercury by UV/Fenton system. Chem. Eng. J. 2013, 232, 81−88. (11) Wang, Z. H.; Zhou, J. H.; Zhu, Y. Q.; Wen, Z. H. 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. (12) Jeong, J.; Jurng, J. Removal of gaseous elemental mercury by dielectric barrier discharge. Chemosphere 2007, 68, 2007−2010. (13) Gu, X. G.; Lu, S. G.; Li, L.; Qiu, Z. F.; Sui, Q.; Lin, K. F.; Luo, Q. S. Oxidation of 1,1,1-trichloroethane stimulated by thermally activated persulfate. Ind. Eng. Chem. Res. 2011, 50, 11029−11036. (14) Adewuyi, Y. G.; Sakyi, N. Y. Removal of nitric oxide by aqueous sodium persulfate simultaneously activated by temperature and Fe2+ in a lab-scale bubble reactor. Ind. Eng. Chem. Res. 2013, 52, 14687− 14697. (15) Adewuyi, Y. G.; Khan, M. A.; Sakyi, N. Y. Kinetics and modeling of the removal of nitric oxide by aqueous sodium persulfate simultaneously activated by temperature and Fe2+. Ind. Eng. Chem. Res. 2014, 53, 828−839. (16) Liu, Y. X.; Zhang, J. A study on kinetics of NO removal from simulated flue gas by wet UV/H2O2 advanced oxidation process. Energy Fuels 2011, 6, 1102−1107. (17) Ye, Q. F. Gaseous Mercury Absorption from Simulated Flue Gas; Zhejiang university: Hangzhou, 2006. (18) Goodsite, M. E.; Plane, J. M. C.; Skov, H. A theoretical study of the oxidation of Hg0 to HgBr2 in the troposphere. Environ. Sci. Technol. 2004, 38, 1772−1776. (19) Fang, G. D.; Dionysiou, D. D.; Wang, Yu.; Al-Abed, S. R.; Zhou, D. M. Sulfate radical-based degradation of polychlorinated biphenyls: Effects of chloride ion and reaction kinetics. J. Hazard. Mater. 2012, 227−228, 394−401. (20) Zamora, P. L.; Villamena, F. A. Theoretical and experimental studies of the spin trapping of inorganic radicals by 5,5-dimethyl-1pyrroline N-Oxide (DMPO). 3. Sulfur dioxide, sulfite, and sulfate radical anions. J. Phys. Chem. A 2012, 116, 7210−7218. (21) Xu, Y. Chemical Reaction Kinetics; Chemical Industry Press: Beijing, 2004. (22) Zhang, C. F. Gas-Liquid Reaction and Reactor; Chemical Industry Press: Beijing, 1985. (23) Danckwerts, P. V. Gas-Liquid Reactions; McGraw-Hill: New York, 1970. (24) Liu,Y. X. Study on Integrated Desulfurization and Denitrification by UV/H2O2 Advanced Oxidation Process; Southeast University: Nanjing,2010.

can be presumably described by the following Figure 5 and the above chemical reactions 3-32. 3.10. Application of this Process. Boiler flue gas contains large amounts of waste heat, and the temperature is often more than 120 °C.24 Thus, the heat which is used to activate persulfate can be completely provided by the waste heat from boiler. The thermally activated (NH4)2S2O8 process can simultaneously oxidize SO2, NOx and Hg0 from flue gas into the available (NH4)2SO4, NH4NO3 and Hg2+. The Hg2+ in mixed solution can be separated by adding S2+ to react with Hg2+ to produce HgS precipitation, which can be recycled by simple precipitation separation in a separation tower. The remaining (NH4)2SO4 and NH4NO3 mixed solution can be used to manufacture fertilizer such as (NH4)2SO4 and NH4NO3 by adding part of ammonia (neutralizing the H2SO4 and the HNO3 in mixed solution) after it is concentrated and crystallized by using flue gas waste heat. According to the statistics,24 there are about more than 650 000 widely used small and medium size coal-fired boilers, industrial furnaces, and even refuse incinerators only in China. Therefore, this technology has larger potential application prospects for simultaneous removal of multipollutants from these small and medium size combustion equipment that they are almost impossible to simultaneously install flue gas desulfurization, denitrification and mercury removal equipment separately due to the huge cost.

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ASSOCIATED CONTENT

* Supporting Information S

Calculation and measurement methods of physical and mass transfer parameters. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 86-0511-89720178; e-mail: liuyangxian1984@163. com. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by National Natural Science Foundation of China (No.51206067), Open Research Fund Program of Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education (Southeast University), Key Laboratory of Efficient & Clean Energy Utilization, College of Hunan Province (Changsha University of Science & Technology), Training Project of Jiangsu University Youth Backbone Teacher, New Teacher Fund for the Doctoral Program of Higher Education of China (No.20123227120016).

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REFERENCES

(1) Liu, Y. X.; Pan, J. F.; Wang, Q. Removal of Hg0 from containingSO2/NO flue gas by ultraviolet/H2O2 process in a novel photochemical reactor. AIChE J. 2014, 6, 2275−2285. (2) Liu, Y. X.; Zhang, J.; Pan, J. F. Photochemical oxidation removal of Hg0 from flue gas containing SO2/NO by an ultraviolet irradiation/ hydrogen peroxide (UV/H2O2) process. Energy Fuels 2014, 28, 2135− 2143. (3) Fang, P.; Cen, C. P.; Tang, Z. J. Experimental study on the oxidative absorption of Hg0 by KMnO4 solution. Chem. Eng. J. 2012, 102, 198−199. 12189


Removal of Elemental Mercury from Flue Gas by Thermally Activated

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