Photochemical Oxidation Removal of Hg0 from Flue Gas Containing

Photochemical Oxidation Removal of Hg0 from Flue Gas Containing .... A novel low-cost method for Hg0 removal from flue gas by visible-light-driven BiO...
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Photochemical Oxidation Removal of Hg0 from Flue Gas Containing SO2/NO by an Ultraviolet Irradiation/Hydrogen Peroxide (UV/H2O2) Process Yangxian Liu,*,† Jun Zhang,‡ and Jianfeng Pan† †

School of Energy and Power Engineering, Jiangsu University, Zhenjiang, Jiangsu 212013, People’s Republic of China Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, Southeast University, Nanjing, 210096, People’s Republic of China

‡

ABSTRACT: In the present study, the process of photochemical oxidation removal of Hg0 from a flue gas containing SO2/NO, using ultraviolet irradiation and hydrogen peroxide (UV/H2O2), was investigated in a photochemical gas−liquid reactor. The effects of some important parameters, such as H2O2 concentration, UV radiated power per unit solution, solution pH, Hg0 concentration, solution temperature, flue gas flow, SO2 concentration, NO concentration, and O2 concentration on the removal of Hg0 using the UV/H2O2 process were investigated. The simultaneous removal of Hg0, NO, and SO2 via the UV/H2O2 and UV/H2O2/Ca(OH)2 processes is studied preliminarily. In addition, the removal mechanism of Hg0 via the UV/H2O2 process is also proposed. The results show that O2 and UV radiation have a significant role in promoting the removal of Hg0. The removal of Hg0 is significantly promoted by adding a low concentration of H2O2, but it is inhibited by adding a high concentration of H2O2. The removal efficiency of Hg0 is significantly reduced as the Hg0 concentration, flue gas flow, or solution pH is increased, but it is only slightly reduced with increased solution temperature, SO2 concentration, or NO concentration. Hg0, NO, and SO2 can be simultaneously removed effectively using UV/H2O2/Ca(OH)2 processes. Hg2+ is the final reaction product of Hg0 removal, and Hg0 is mainly removed by the oxidations of H2O2, ·OH, ·O, and O3, as well as UV photoexcitation.

1. INTRODUCTION Because of its persistence, bioaccumulation, and neurological toxicity, mercury has received increasing attention as a pollutant of serious concern.1 Worldwide mercury emissions from human activities are estimated to be 1000−6000 t/yr, which account for 30%−55% of global atmospheric mercury emissions.1,2 Coal combustion is considered as the largest source of anthropogenic mercury emission.3,4 Mercury in coal-fired flue gas often presents as elemental mercury (Hg0), divalent mercury (Hg2+), and particulate mercury (Hgp). Hgp can be captured by existing dedusting systems. Hg2+ can be easily removed by wet scrubbing in existing wet flue gas desulfurization systems. However, because Hg0 has high volatility and low solubility in water, it is difficult to remove via existing desulfurization and dedusting systems. Once Hg0 enters the atmosphere, it can remain aloft for several months and even several years, which may result in a global mercury pollution through atmospheric transportation.3,4 Therefore, the study of effective Hg0 removal methods has become one of the hot topics in the field of environmental protection. Recently, in all gaseous mercury removal methods, wet scrubbing and adsorption are considered to be two of the most promising mercury removal technologies.1−4 Some studies3,4 have shown that Hg2+ and Hg0 can be adsorbed by using activated carbon or the other adsorbents, and then are converted to Hgp, which can be captured by existing dedusting equipment. Unfortunately, however, despite activated carbon having excellent adsorption performance for Hg0, the high cost of its use hinders widespread applications. The other new adsorbents such as precious metals, metal oxides, fly ash, activated coke, calcium-based materials, molecular sieves, and © 2014 American Chemical Society

natural mineral materials have shown potential development prospects, but because of the deficiencies in high application cost, as well as the stability and reliability of the adsorbent, they are still unable to obtain widespread commercial applications.3,4 Some studies5−15 have shown that the use of wet chemical oxidation processes or wet flue gas desulfurization systems combined with oxidizing additives (KMnO4, K2S2O8, NaClO2, NaClO, H2O2, Fenton and chlorine dioxide) and dry pretreatment oxidation technologies (catalytic oxidation, plasma oxidation, ozonation, photocatalytic oxidation, and photochemical oxidation) can achieve the simultaneous removal of mercury, SO2, and NOx, which is considered as a very promising technology. However, so far, some technical problems such as secondary pollution of reaction products, development and application costs, and safety and reliability, cannot be effectively solved yet.1−4 In summary, many flue gas mercury removal technologies have been studied, but, so far, none of them are suitable yet for large-scale commercial applications. Because of its extremely strong oxidation ability, environmental protection, and the simple and secure nature of the process, the ultraviolet radiation/hydrogen peroxide (UV/ H2O2) process has been widely studied and applied in degrading organic pollutants from industrial wastewater.16 In recent years, some results17−20 have shown that the UV/H2O2 process can simultaneously oxidize SO2 and NOx from flue gas into available sulfuric acid and nitric acid by wet scrubbing. In Received: August 25, 2013 Revised: January 1, 2014 Published: February 10, 2014 2135

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these works,17−20 the related process parameters were optimized and the reaction mechanism and kinetics were also investigated. However, so far, the removal of Hg0 from flue gas using the UV/H2O2 process by wet scrubbing has not been reported yet. If the UV/H2O2 process can also effectively remove Hg0 in flue gas, it is expected that it may eventually be developed into an effective technology for the simultaneous removal of SO2, NOx, and mercury from flue gas. In the present study, the photochemical oxidation removal of Hg0, SO2, and NO from flue gas via the UV/H2O2 process was investigated in a photochemical gas−liquid reactor. The effects of some important parameters, such as H2O2 concentration, UV radiated power per unit solution, solution pH, Hg0 concentration, solution temperature, flue gas flow, SO2 concentration, NO concentration, and O2 concentration, on the removal of Hg0 via the UV/H2O2 process were investigated. In addition, the simultaneous removal of Hg0, NO, and SO2 by UV/H2O2 and UV/H2O2/Ca(OH)2 processes were studied preliminarily. The reaction mechanism of Hg0 removal via the UV/H 2 O 2 process was also proposed, based on the determination of mercury removal products, the calculation of mercury material balance, and the identification and capture of intermediate active substances such as ·OH and O3. The results will provide some theoretical guidance for the follow-up studies and industrial applications of this technology.

plug, a quartz tube, a UV lamp, and a gas distributor. The bubble column reactor (height = 30 cm; inside diameter (ID) = 8.5 cm) is composed of Plexiglas. The gas distributor is installed at the bottom of bubble column reactor, to distribute the gas. The UV lamp (wavelength of λ = 253.7 nm) is placed in the bubble column reactor through the centerline to provide the excitation light source. In order to avoid the interference of a homogeneous photochemical reaction in the gas phase, the portion of the UV lamp above the liquid level is covered by black light-absorbing material. A constant-temperature water bath (Model HH-42, Changzhou GuoHua Co., China) with a circulating pump is used to control the solution temperature. The solution temperature is measured by the mercury thermometer. The flue gas analyzers (Model FGA-4100, Fuoshan Fuofen Instrument Co., China; Model MRU-VARIO PLUS, Germany) and the flue gas mercury analyzer (Model QM201H, Suzhou Qingan Instrument Co., China) are used to determinate the concentrations of SO2, NO, O2, and Hg0, respectively. 2.2. Experimental Procedures. A H2O2 solution (300 mL) was made using 30% commercial H2O2 solution (analytical reagent (AR) grade, Guoyao Chemical Reagent Co., China) and deionized water. The solution pH value was adjusted by adding HCl and NaOH and was determined using an acidimeter (Model CT-6023, Kedida Instrument Co., China). Simulated flue gas was made by opening the cylinders and the mercury generator. Flows of simulated flue gas and concentrations of gas components were regulated by the rotameters. Inlet concentrations of SO2, NO, O2, and Hg0 were measured by the flue gas mercury analyzer and the flue gas analyzer through the gas bypass line. The solution temperatures were adjusted to the required temperatures via the combined use of the constant temperature water bath and the mercury thermometer. When the solution temperature was kept stable, one valve (denoted as “13” in Figure 1) was closed, and another valve (denoted as “12” in Figure 1) was opened. The simulated flue gas then entered the photochemical reactor to create a gas−liquid reaction after the UV lamp was turned on. The output power of UV lamp can be adjusted by using UV lamps with different powers (the shape and wavelength are the same). The outlet concentrations of Hg0, SO2, and NO were measured by using the flue gas mercury analyzer and the flue gas analyzer. Each experimental period was maintained for 64 min. The concentrations of Hg0, SO2, and NO in the reactor outlet were recorded once every 8 min. [Note that the used flue gas mercury analyzer takes a cycle measurement mode of 1 min of sampling, 3 min of measurement, and 4 min of cleaning (the gold amalgamation method), thus obtaining every value often requires 8 min.] The average concentration of eight instantaneous values was used as outlet concentrations of pollutants. In addition, in order to analyze the removal pathways of Hg0, the concentrations of total mercury and Hg0 in reaction solutions were measured using a fluorescence mercury analyzer. The possible active intermediates (O3 and ·OH) were also identified and captured by the ozone test paper and the UV detector in the liquid chromatography−mass spectrometry (LC-MS) equipment. 2.3. Analytical Methods. 2.3.1. Determination of Total Mercury and Hg0 Concentrations. When the concentration of Hg0 in solution was measured, 20 mL of the solution was quickly transferred into a sealed reaction bottle, which was maintained at 323 K in a constanttemperature water bath without any pretreatments. At the same time, this measurement was carried out instantly and quickly. Because of the high volatility of Hg0, the possible Hg0 vapor would gasify and was directly blown into the fluorescence mercury analyzer by bubbling and carrying of N2. However, unlike the Hg0, when the concentration of total mercury was determined, in order to make possible solid mercury convert to soluble divalent mercury as completely as possible, 20 mL of the solution was first digested by adding 10% H2SO4−10% KMnO4 solutions (mass fraction) in an Erlenmeyer flask and then heated to a temperature of 353 K in a constant-temperature water bath for 1 h. Before analysis, in order to reduce the redundant KMnO4, several drops of hydroxylamine hydrochloride (10 wt %) were added into the Erlenmeyer flask until the color of solution turned clear. After holding for 10 min, 5 mL of sample was transferred into the reaction bottle, and 1 mL of a SnCl2 solution (10 wt %) was quickly added in order to

2. EXPERIMENTAL SECTION 2.1. Experimental System. As shown in Figure 1, the experimental system consists of an analytical system, a photochemical

Figure 1. Schematic diagram of experimental system. Legend: (1) SO2 cylinder; (2) NO cylinder; (3) O2 cylinder; (4) N2 cylinder; (5−10) rotameters; (11) gas mixing tank; (12, 13) gas valves; (14) flue gas analyzer; (15) flue gas mercury analyzer; (16) mercury thermometer; (17) quartz tube; (18) UV lamp; (19) rubber plug; (20) jacket heat exchanger; (21) bubble column reactor; (22) gas distributor; (23) circulating pump; (24) constant-temperature water bath; and (25) mercury generator. reactor, and a simulated flue gas system. Cylinder gases, including SO2, NO, O2, and N2 (purity = 99.99%), are used to make simulated flue gas and carrier gas. A mercury generator, including a U-tube and a mercury permeation tube (VICI Metronics, USA), is used to produce Hg0 vapor. The photochemical reactor consists of a bubble column reactor, a jacket heat exchanger, a mercury thermometer, a rubber 2136

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reduce Hg2+ to Hg0. Then the highly volatile Hg0 vapor produced was carried by N2 and flushed into the mercury analyzer to determine the concentration of Hg0.7,8 The average value of three measurements is used as the final measured value. 2.3.2. Determination of ·OH Free Radicals in the Liquid Phase. Because of its high reactivity and short life span, the ·OH radical is generally difficult to detect directly by instruments. Presently, the commonly used method involves the addition of a capturing agent to react with ·OH and to product intermediates that are relatively more stable, thereby indirectly detecting the presence of ·OH. In the present work, salicylic acid (SA) is used to capture ·OH via a hydroxylation reaction, which can produce relatively more-stable hydroxylated products, 2,3-DHBA and 2,5-DHBA. The presence of 2,3-DHBA and 2,5-DHBA, each of which having a relatively longer life than ·OH, can be determined by the UV detector of the LC-MS equipment. The relevant principles can be described by the following reaction:21

73.2% with the further increase of H2O2 concentration from 0.75 mol/L to 1.50 mol/L. Some results show that 1 mol of H2O2 can decompose and produce 2 mol of ·OH free radicals,17,18 which can oxidize and remove Hg0, according to reactions 4 and 5.22,23 H 2O2 + ℏv → 2·OH 0

Hg + 2·OH → Hg(OH)2

0

0

(2)

(3)

0

where mHg is the removal mass of Hg (in units of μg), ηHg0 the removal efficiency of Hg0, Cin,Hg0 the inlet concentration of Hg0 (in units of μg/m3), Q the flue gas flow (in units of L/min), and t the running time per each experiment (in minutes). 0

H 2O2 + ·OH → HO2 · + H 2O

(6)

·OH + ·OH → H 2O2

(7)

HO2 · + HO2 · → H 2O2 + O2

(8)

·OH + HO2 · → H 2O + O2

(9)

Therefore, adding excess H2O2 is not conducive to the removal of Hg0. Conditions. The experimental conditions are as follows: O2 concentration, 6.0%; SO2 concentration, 1000 ppm; NO concentration, 400 ppm; Hg0 concentration, 30 μg/m3; flue gas flow, 1.2 L/min; solution temperature, 20 °C; solution pH, 3.88; and UV radiated power per unit solution, 0.015 W/mL. 3.2. Effects of Radiated Power Per Unit Solution. As shown in Figure 3, the removal efficiency of Hg0 is only 10.8%

where Cin is the inlet concentration of pollutants and Cout is the outlet concentration of pollutants. Hg0 removal mass can be calculated using eq 3: mHg = ηHg C in,Hg Qt

(5)

The reaction product Hg(OH)2 can be captured through dissolution in solutions. From reaction 4, it can be inferred that an increase in H2O2 concentration can increase the yield of · OH free radicals, thereby being able to enhance the removal of Hg0. However, because H2O2 also has reducibility when it experiences stronger oxidants, and it often can become a scavenger of ·OH free radicals, so adding a high concentration of H2O2 may cause the following several side reactions (see reactions 6−9).17,18

2.4. Removal Efficiency and Removal Mass. The concentration of pollutants measured by the bypass line is used as the inlet concentrations of pollutants. The average concentration of eight instantaneous concentrations measured by the reactor outlet is used as the outlet concentrations of pollutants. The removal efficiency is calculated using the following equation: C − Cout removal efficiency (%) = in × 100 C in

(4)

0

3. RESULTS AND DISCUSSIONS 3.1. Effect of H2O2 Concentration. The effect of H2O2 concentration on Hg0 removal efficiency is shown in Figure 2. It can be observed that Hg0 removal efficiency increases from 61.9% to 80.6% when the H2O2 concentration increases from 0 to 0.75 mol/L; however, the efficiency decerases from 80.6% to

Figure 3. Effect of UV energy density per unit solution on Hg0 removal efficiency and Hg0 removal mass.

without the radiation of UV, suggesting that the oxidation capacity of H2O2 to Hg0 is very limited in a bubbling reactor. When the UV is added, Hg0 removal efficiency sharply increases. For example, when UV radiated power per unit solution increases from 0 to 0.015 W/mL, Hg0 removal efficiency increases from 10.8% to 80.6%. However, with the further increase of UV radiated power per unit solution from 0.015 W/mL to 0.025 W/mL, the removal efficiency of Hg0 increases only slightly, from 80.6% to 81.9%.

Figure 2. Effect of H2O2 concentration on Hg0 removal efficiency and Hg0 removal mass. 2137

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There are several explanations for the above experimental results. On one hand, as mentioned above, under UV radiation, H2O2 can release ·OH free radicals to oxidize and remove Hg0.14,17,18,22,23 In addition, several studies have found that, under the radiation of UV, O2 can produce ·O free radicals and O3, according to reactions 10 and 11.14,15,24,26 O2 + ℏv → ·O + ·O

(10)

·O + O2 → O3

(11)

(In fact, during the experiment, the active substance, O3, has been identified by using ozone test paper.) Both of O3 and ·O free radical have strong oxidizing property and can oxidize and remove Hg0 via reactions 12 and 13.11,12,14,28 Hg 0 + O3 → HgO + O2

(12)

Hg 0 + ·O → HgO

(13)

Figure 4. Effect of solution temperature on Hg0 removal efficiency and Hg0 removal mass.

strengthening the removal of Hg0. However, in this present study, the activation energy of free radical reactions is so low31 that increasing the solution temperature cannot play a significant role in enhancing the removal of Hg0. However, increasing the solution temperature will result in the reduction of Hg0 solubility in solution,32 thereby decreasing removal efficiency of Hg0. In this work, as the solution temperature increases, the Hg0 removal efficiency slightly decreases, suggesting that the back negative factor plays a major role. Conditions. The experimental conditions were as follows: O2 concentration, 6.0%; SO2 concentration, 1000 ppm; NO concentration, 400 ppm; Hg0 concentration, 30 μg/m3; flue gas flow, 1.2 L/min; solution pH, 3.88; H2O2 concentration, 0.75 mol/L; and UV energy density per unit solution, 0.015 W/mL. 3.4. Effect of Solution pH. The effect of solution pH on Hg0 removal efficiency is shown in Figure 5. It can be observed

Furthermore, under UV irradiation, O3 can react with H2O to produce H2O2 via reaction 14:27 O3 + H 2O + ℏv → H 2O2 + O2

(14)

The product H2O2 can re-release ·OH free radicals to oxidize and remove Hg0 via reactions 4 and 5 (mentioned previously). Some results have proved that, under the UV radiation, Hg0 can also directly react with H2O to produce HgO, according to the following photoexcitation reaction:4,28 Hg 0 + H 2O + ℏv → HgO + H 2

(15)

The reaction product HgO can be captured through precipitation in water or dissolution in acidic solutions. Reactions 4, 10, 14, and 15 suggest that an enhancement in UV radiation can produce more-effective UV photons, ·OH, ·O, and O3 and, thus, is able to enhance the removal of Hg0. However, when the UV radiated power per unit solution exceeds a certain value, several side reactions, such as reactions 6−9(given previously) and the following reactions (reactions 16−19), may also simultaneously occur in solutions,18,19,27,29,30 resulting in the self-loss of H2O2, ·OH, ·O, and O3. O3 + ·OH → HO2 · + O2

(16)

·O + ·O → O2

(17)

·O + O3 → O2 + O2

(18)

·OH + ·O → O2 + ·H

(19)

Thus, with further increases in the UV radiated power per unit solution, Hg0 removal efficiency experiences only a very slight increase. Conditions. The experimental conditions were as follows: O2 concentration, 6.0%; SO2 concentration, 1000 ppm; NO concentration, 400 ppm; Hg0 concentration, 30 μg/m3; flue gas flow, 1.2 L/min; solution temperature, 20 °C; solution pH, 3.88; and H2O2 concentration, 0.75 mol/L. 3.3. Effect of Solution Temperature. The effect of solution temperature on the removal efficiency of Hg0 was studied, and the results are shown in Figure 4. It can be seen that the solution temperature has a small impact on the removal of Hg0. For example, when the solution temperature increases from 20 °C to 60 °C, the removal efficiency of Hg0 only decreases from 80.6% to 77.4%. Some results indicate that increasing the solution temperature can increase the chemical reaction rate by lowering the activation energy,31 thus

Figure 5. Effect of solution pH on Hg0 removal efficiency and Hg0 removal mass.

that, when the solution pH increases from 1.28 to 10.06, the Hg0 removal efficiency sharply decreases, from 84.3% to 46.1%. The effects of solution pH on Hg0 removal include the following reasons. Some results indicate that H2O2 in solutions can produce the HO2− species, which is a scavenger of ·OH free radicals via the following hydrolysis reaction:17,21 H 2O2 ↔ HO2− + H+

(20)

Reaction 20 will result in an self-consumption of H2O2, being nonconducive to the oxidation removal of H2O2 to Hg0, according to reaction 21:6 2138

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H 2O2 + Hg 0 → HgO + H 2O

body (in units of Pa), and pHg0,i the Hg0 partial pressure in phase interface (also given in units of Pa). From eq 24, it can be inferred that increasing the Hg0 concentration (Hg0 partial pressure) can raise the mass-transfer driving force of Hg0, thereby promoting the removal of Hg0. Here, the front negative factor plays a major role; thus, the Hg0 removal efficiency decreases as the Hg0 concentration increases. Conditions. The experimental conditions were as follows: O2 concentration, 6.0%; SO2 concentration, 1000 ppm; NO concentration, 400 ppm; flue gas flow, 1.2 L/min; solution temperature, 20 °C; solution pH, 3.88; H2O2 concentration, 0.75 mol/L; and UV energy density per unit solution, 0.015 W/ mL. 3.6. Effect of NO Concentration. The effect of NO concentration on Hg0 removal efficiency is shown in Figure 7.

(21)

Besides, the hydrolysis product HO2− can also consume ·OH free radicals through reaction 22, thereby reducing the removal efficiency of Hg0.17,21 ·OH + HO2− → H 2O + O2−·

(22) −

Increasing the solution pH can increase the OH concentration in solution, thus it can infer from reaction 20 that OH− will increase the yield of HO2− by absorbing H+, according to the acid−base neutralization reaction described by reaction 23, thereby further reducing the utilization of H2O2 and ·OH free radicals. H+ + OH− → H 2O

(23)

17,21

Besides, several results show that, compared to alkaline conditions, H2O2 has stronger oxidizing property under acidic conditions. In summary, with the increase of solution pH, Hg0 removal efficiency greatly decreases. Conditions. The experimental conditions were as follows: O2 concentration, 6.0%; SO2 concentration, 1000 ppm; NO concentration, 400 ppm; Hg0 concentration, 30 μg/m3; flue gas flow, 1.2 L/min; solution temperature, 20 °C; H 2 O 2 concentration, 0.75 mol/L; and UV energy density per unit solution, 0.015 W/mL. 3.5. Effect of Hg0 Concentration. The effect of Hg0 concentration on Hg0 removal efficiency was studied, and the results are shown in Figure 6. It can be seen that, with

Figure 7. Effect of NO concentration on Hg0 removal efficiency and Hg0 removal mass.

It can be seen that, when the NO concentration increases from 0 to 2000 ppm, the Hg0 removal efficiency decreases from 81.2% to 76.8%. Some results show that NO can consume ·OH, ·O, O3, and H2O2 via reactions 25−32,17−21,29,30 so increasing the NO concentration can decrease the removal efficiency of Hg0.

Figure 6. Effect of Hg0 concentration on Hg0 removal efficiency and Hg0 removal mass.

increasing Hg0 concentration, Hg0 removal efficiency has a marked decrease. When the Hg0 concentration increases from 5 μg/m3 to 60 μg/m3, the Hg0 removal efficiency decreases from 90.8% to 66.2%. On one hand, the results show that increasing the concentration of pollutant will increase the amount of pollutant through the reactor per unit time,17 which can decrease the relative molar ratio of ·OH, ·O, O3, H2O2, and UV photons to Hg0, thereby being able to reduce the removal efficiency of Hg0. On the other hand, according to the two-film theory,32 the absorption rate of Hg0 can be expressed by the following gas absorption rate equation: NHg 0 = k Hg 0,G(pHg 0 − pHg 0, i )

NO + ·OH → HNO2

(25)

HNO2 + ·OH → NO2 + H 2O

(26)

NO + ·O → NO2

(27)

NO + O3 → NO2 + O2

(28)

HNO2 + H 2O2 → HNO3 + H 2O

(29)

NO2 + ·OH → HNO3

(30)

2NO2 + H 2O2 → 2HNO3

(31)

2NO + 3H 2O2 → 2HNO3 + 2H 2O

(32)

Conditions. The experimental conditions were as follows: O2 concentration, 6.0%; SO2 concentration, 1000 ppm; Hg0 concentration, 30 μg/m3; flue gas flow, 1.2 L/min; solution temperature, 20 °C; solution pH, 3.88; H2O2 concentration, 0.75 mol/L; and UV energy density per unit solution, 0.015 W/ mL. 3.7. Effect of SO2 Concentration. The effect of SO2 concentration on Hg0 removal efficiency is shown in Figure 8. It can be seen that, when the SO2 concentration increases from 0 to 2500 ppm, Hg0 removal efficiency slightly decreases from 82.6% to 80.2%. On the basis of reactions

(24) 0

where NHg0 is the absorption rate of Hg (in units of mol/(m2 s)), kHg0,G the gas-phase mass-transfer coefficient of Hg0 (in units of mol/(s m2 Pa)), pHg0 the Hg0 partial pressure in the gas 2139

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previously described reactions 10−13.14,24−26,28 Thus, increasing the O2 concentration can increase the Hg0 removal efficiency. However, when the O2 concentration further increases from 6.0% to 12.0%, Hg0 removal efficiency only increases from 80.6% to 82.1%, showing that O2 concentration may have remained in excess after exceeding 6.0%. Conditions. The experimental conditions were as follows: SO2 concentration, 1000 ppm; NO concentration, 400 ppm; Hg0 concentration, 30 μg/m3; flue gas flow, 1.2 L/min; solution temperature, 20 °C; solution pH, 3.88; H2O2 concentration, 0.75 mol/L; and UV energy density per unit solution, 0.015 W/ mL. 3.9. Effects of Flue Gas Flow on Hg0 Removal Efficiency. The effect of flue gas flow on Hg0 removal efficiency is shown in Figure 10. It can be observed that flue gas

Figure 8. Effect of SO2 concentration on Hg0 removal efficiency and Hg0 removal mass.

33−40,17−21,29,30 SO2 can also consume ·OH, ·O, O3, and H2O2; therefore, as the SO2 concentration increases, the Hg0 removal efficiency decreases. SO2 + H 2O ↔ HSO3− + H+

(33)

HSO3−

(34)

HSO3− SO3

2−

↔ SO3

2−

+H

+ ·OH →

+ ·OH →

+

·SO3−

·SO3−

+ H 2O

+ OH

−

(35) (36)

HSO3− + H 2O2 → SO4 2 − + H+ + H 2O

(37)

SO32 − + H 2O2 → SO4 2 − + H 2O

(38)

HSO3−

+ ·O → SO4 · + ·H

(39)

SO32 − + O3 → SO4 2 − + O2

(40)

−

Figure 10. Effect of flue gas flow on Hg0 removal efficiency and Hg0 removal mass.

flow has a clear impact on Hg0 removal efficiency. As the flue gas flow increases from 0.4 L/min to 2.0 L/min, the Hg0 removal efficiency greatly decreases, from 89.2% to 71.2%. The effects of flue gas flow on Hg0 removal efficiency can be explained as follows. Increasing flue gas flow can increase the gas−liquid mass-transfer rate by strengthening the disturbance in the gas−liquid two-phase mixture,32 thereby increasing the absorption rate of Hg0. However, increasing flue gas flow greatly decreases the residence time (or reaction time) of Hg0 in the reactor. In addition, the amount of Hg0 through the reactor per unit time also greatly increases, which can decrease the relative molar ratio of ·OH, ·O, O3, and H2O2 to Hg0, thereby further reducing the removal efficiency of Hg0. In this study, the back negative factors play a leading role, so the removal efficiency of Hg0 decreases as the flue gas flow increases. Conditions. The experimental conditions were as follows: O2 concentration, 6.0%; SO2 concentration, 1000 ppm; NO concentration, 400 ppm; Hg0 concentration, 30 μg/m3; solution temperature, 20 °C; solution pH, 3.88; H 2O 2 concentration, 0.75 mol/L; and UV energy density per unit solution, 0.015 W/mL. 3.10. Effects of the Aforementioned Influencing Factors on the Removal Mass of Hg0. Figures 2−5 and Figures 7−9 show that the change in trends of Hg0 removal mass are consistent with that of Hg0 removal efficiency, relative to the change in Hg0 concentration, solution temperature, solution pH, H2O2 concentration, NO concentration, SO2 concentration, O2 concentration, and UV energy density per unit solution. However, one should note that, unlike the other

Conditions. The experimental conditions were as follows: O2 concentration, 6.0%; NO concentration, 400 ppm; Hg0 concentration, 30 μg/m3; flue gas flow, 1.2 L/min; solution temperature, 20 °C; solution pH, 3.88; H2O2 concentration, 0.75 mol/L; and UV energy density per unit solution, 0.015 W/ mL. 3.8. Effects of O2 Concentration. The effect of O2 concentration on Hg0 removal efficiency was studied, and the results are shown in Figure 9. When O2 concentration increases from 0 to 6.0%, Hg0 removal efficiency greatly increases from 56.6% to 80.6%. Under the radiation of UV, O2 can produce strong oxidizing agents O3 and ·O to oxidize Hg0 by the

Figure 9. Effect of O2 concentration on Hg0 removal efficiency and Hg0 removal mass. 2140

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measured by the UV detector of LC-MS, which show that ·OH has been produced in the solution and the removal process of Hg0 via the UV/H2O2 process is caused by the free radical chain reactions. Furthermore, the concentrations of total mercury and Hg0 in reaction solutions were also determined, and the results are shown in Table 1. The results in Table 1 indicate that Hg0 can not be measured, but the total mercury has been determined in the reaction solution.

previously discussed parameters, the Hg0 removal efficiency greatly decreases, but the total removal mass of Hg0 greatly increases as the Hg0 concentration and flue gas flow each increase (see Figures 6 and 10). For example, the total mass of Hg0 removal greatly increases from 0.35 μg to 3.05 μg when the Hg0 concentration increases from 200 ppm to 1200 ppm and from 0.68 μg to 3.73 μg when the flue gas flow increases from 0.4 L/min to 2.0 L/min; these represent increases of 871.4% and 548.5%, respectively. 3.11. Comparison of Different Reaction Systems. The comparative experiments in different reaction systems were carried out, and the results are shown in Figure 11. It can be

Table 1. Removal Products and Material Balance for Different Mercury Forms (Reaction Time = 64 min) Value parameter

Hg0

total mercury in solution

measured concentration calculated concentration relative error

0 μg/L

5.46 ± 0.15 μg/L 6.19 μg/L 11.79%

On the other hand, based on the measured results of total mercury concentration, the material balance for mercury is calculated and the results are also shown in Table 1. It can be seen that the measured values of total mercury concentration in solution are in good agreement with the calculated values, with a relative error of 11.79%. The results show that Hg2+ is the final removal product of Hg0 in solution, and Hg0 is mainly removed by oxidation reaction. The errors of total mercury concentration in solution between the determined values and the calculated values may come from the digestion process. Conditions. The experimental conditions were as follows: SA concentration, 0.05 mol/L; O2 concentration, 6.0%; SO2 concentration, 1000 ppm; NO concentration, 400 ppm; Hg0 concentration, 30 μg/m3; flue gas flow, 1.2 L/min; solution temperature, 20 °C; solution pH, 3.88; H2O2 concentration, 0.75 mol/L; and UV energy density per unit solution, 0.015 W/ mL. 3.13. Removal Mechanism of Hg0. The results of the comparative experiments in Figure 11 have shown that Hg0 can achieve a removal efficiency of 11.2% in O2/H2O2, but even a small amount of Hg0 is removed in O2/H2O, showing that the removal of Hg0, via the oxidation of H2O2, occurs in the reaction process. Furthermore, a Hg0 amount of 32.1% is removed by UV/H2O, showing that the removal of Hg0 by the photoexcitation reaction of Hg0 with H2O is one of the Hg0 removal pathways. When O2 is added, Hg0 removal efficiencies clearly increase in the UV/H2O and UV/H2O2 processes. For example, Hg0 can achieve removal efficiencies of 61.1% and 80.6% in the UV/H2O/O2 and UV/H2O2/O2, which are much

Figure 11. Comparison of different reaction systems.

seen that Hg0 can achieve the removal efficiencies of 11.2%, 32.1%, 56.6%, 61.1%, and 80.6% in O2/H2O2, UV/H2O, UV/ H2O2, UV/H2O/O2, and UV/H2O2/O2, respectively, but even little Hg0 is removed in O2/H2O, showing that all of UV, H2O, H2O2 and O2 play an important role in removal of Hg0 and have a significant synergistic effect on each other. Conditions. The experimental conditions were as follows: O2 concentration, 6.0%; SO2 concentration, 1000 ppm; NO concentration, 400 ppm; Hg0 concentration, 30 μg/m3; flue gas flow, 1.2 L/min; solution temperature, 20 °C; solution pH, 3.88; H2O2 concentration, 0.75 mol/L; and UV energy density per unit solution, 0.015 W/mL. 3.12. ·OH Free Radicals, Removal Products, and Material Balance. In order to study the removal mechanism of Hg0, on one hand, the ·OH species in solutions was captured by the combination use of LC-MS and SA, and the results are shown in Figure 12. It can be seen that both of the hydroxylated products (2,3-DHBA and 2,5-DHBA) can be

Figure 12. Determination of hydroxylated products 2,3-DHBA and 2,5-DHBA by the UV detector in LC-MS. 2141

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higher than those in the UV/H2O process (32.1%) and the UV/H2O2 process (56.6%), respectively. In addition, the presence of O3 also further proves that the removals of Hg0 via the oxidation of O3 and ·O free radicals also may be a part of the removal reaction pathways. When H2O2 is added into the UV/H2O and UV/H2O/O2 processes, Hg0 can achieve removal efficiencies of 56.6% and 80.6% in UV/H2O2 and UV/H2O2/O2, respectively, which are higher than those in the UV/H2O process (32.1%) and the UV/H2O/O2 process (61.1%). The successful capture of·OH also further shows that the removal of Hg0 via the oxidation of · OH may be also one of the removal reaction pathways. In addition, the measured results of mercury removal products indicate that Hg2+ is the final removal product of Hg0 in solution, and Hg0 is mainly removed via the oxidation reaction in the UV/H2O2 process in the presence of O2, which can further prove that a variety of oxidation reactions and the aforementioned photoexcitation reactions may have occurred in the reaction process. In summary, based on the above results, although several other side reactions may also occur in solution, the main reaction pathways of Hg0 removal by UV/H2O2 process in the presence of O2 can be presumably concluded as follows:

Figure 13. Simultaneous removal efficiencies of Hg0, NO, and SO2 via the UV/H2O2 process, relative to run time.

Conditions. The experimental conditions were as follows: O2 concentration, 6.0%; SO2 concentration, 1000 ppm; NO concentration, 400 ppm; Hg0 concentration, 30 μg/m3; flue gas flow, 1.2 L/min; solution temperature, 20 °C; solution pH, 3.88; H2O2 concentration, 0.75 mol/L; and UV energy density per unit solution, 0.015 W/mL. 3.15. Simultaneous Removal of Hg0, NO, and SO2 via the UV/H2O2/Ca(OH)2 Process. In the past few decades, alkali-based wet flue gas desulfurization processes have been widely used, because the process is simple and has high removal efficiency.21 Our previous results have found that comparisons under acidic, neutral, and alkaline conditions are more conducive to the removal of NO via the UV/H2O2 process.17,21 Taking into account the wide applications of calcium-based absorbents in wet flue gas desulfurization processes, simultaneous removal of Hg0, NO and SO2 by UV/H2O2 process under enhancement of Ca(OH)2 (UV/H2O2/Ca(OH)2 process) was studied preliminarily, and the results are shown in Figure 14. It can be seen that the simultaneous removal

(a) The ·OH, ·O, and O3 species are produced through UV photolysis of O2 and H2O2, via reactions 4, 10, and 11; (b) The removals of Hg0 by the oxidations of ·OH, ·O, and O3 via reactions 5, 12, and 13; (c) The removal of Hg0 by the photoexcitation reaction of Hg0 with H2O via reaction 15; (d) The removal of Hg0 by the direct oxidation of H2O2 via reaction 21; and (e) The termination of free radical chain reactions via reactions 6−9 and 16−19. 3.14. Simultaneous Removal of Hg0, NO and SO2 via the UV/H2O2 Process. Currently, NOx and SO2 from flue gas are mainly controlled by simultaneously installing flue gas desulfurization and denitrification equipments, such as calciumbased wet flue gas desulfurization (Ca-WFGD) and ammoniaselective catalytic reduction denitration (NH3-SCR).17,18 In addition, taking into account the requirement of mercury removal in the future, if the coal-fired boiler continues to add mercury removal systems independently, most companies will not be able to bear such a large economic burden. Simultaneous removal of NOx, SO2, and Hg in a reactor can effectively reduce the complexity of systems and the investment and operating costs.21 Therefore, studying new technologies and new theories concerning the simultaneous removal of NOx, SO2 and Hg has become one of the hot issues in the field of environmental protection.21 In the present study, the simultaneous removal of Hg0, NO, and SO2 via the UV/H2O2 process was preliminarily studied, and the results are shown in Figure 13. It can be seen that when Hg0, NO, and SO2 were simultaneously removed via the UV/ H2O2 process, the simultaneous removal efficiencies of Hg0, NO, and SO2 were 81.2%, 41.8%, and 100%, respectively. The results show that, in the simultaneous removal process, Hg0 and SO2 can be effectively removed via the UV/H2O2 process. However, NO cannot be efficiently removed using the UV/ H2O2 process, because of its low solubility in water. Therefore, a search for effective measures to strengthen the removal of NO in the UV/H2O2 process is very necessary.

Figure 14. Simultaneous removal efficiencies of Hg0, NO, and SO2 via the UV/H2O2/Ca(OH)2 process, relative to run time.

efficiencies of Hg0, NO, and SO2 are 72.5%, 81.8%, and 100%, respectively, in the UV/H2O2/Ca(OH)2 process. These results show that the removal of NO is greatly improved, and the removal of Hg0 is only slightly reduced by the addition of Ca(OH)2. The above studies further suggest that the UV/H2O2 process may be used to simultaneously remove Hg0, NO, and SO2 from flue gas by combining it with calcium-based wet flue gas desulfurization processes, which has been applied commercially in the coal-fired boilers. The related contents will be further studied in subsequent works. 2142

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(10) Lu, D.; Anthony, E. J.; Tan, Y. W.; Dureau, R. Fuel 2007, 86, 2789−2797. (11) Wang, Z. H.; Zhou, J. H.; Zhu, Y. Q.; Wen, Z. C. Fuel Process. Technol. 2007, 88, 817−823. (12) Jeong, J. Y.; Jurng, J. S. Chemosphere 2007, 68, 2007−2010. (13) Wang, J. W.; Yang, J. L.; Liu, Z. Y. Fuel Process. Technol. 2010, 91, 676−680. (14) Granite, E. J.; Pennline, H. W. Ind. Eng. Chem. Res. 2002, 41, 5470−5476. (15) Li, Y.; Murphy, P.; Wu, C. Y. Fuel Process. Technol. 2008, 89, 567−573. (16) Liu, Y. X. J. Chem. Ind. Eng. 2010, 3, 33−38. (17) Liu, Y. X.; Zhang, J. Ind. Eng. Chem. Res. 2011, 50, 3836−3841. (18) Liu, Y. X.; Zhang, J.; Sheng, C. D. Chem. Eng. J. 2010, 162, 1006−1011. (19) Liu, Y. X.; Zhang, J. Energy Fuels 2011, 6, 1102−1107. (20) Liu, Y. X.; Zhang, J. Chem. Eng. J. 2011, 2, 106−111. (21) Liu, Y. X. Study on Integrated Desulfurization and Denitrification by UV/H2O2 Advanced Oxidation Process. PhD Thesis, Southeast University: Nanjing, PRC, 2011; pp 62−63. (22) Goodsite, M. E.; Plane, J. M. C.; Skov, H. Environ. Sci. Technol. 2004, 38, 1772−1776. (23) Chen, Z. Y.; Mannava, D. P.; Mathur, V. K. Ind. Eng. Chem. Res. 2006, 45, 6050−6055. (24) McLarnon, C. R.; Granite, E. J.; Pennline, H. W. Fuel Process. Technol. 2005, 87, 85−89. (25) Sathiamoorthy, G.; Kalyana, S.; Finney, W. C.; Clark, R. J.; Locke, B. R. Ind. Eng. Chem. Res. 1999, 38, 1844−1855. (26) Chaabouni, H.; Schriver-Mazzuoli, L.; Schriver, A. J. Phys. Chem. A 2000, 104, 3498−3507. (27) Garoma, T.; Gurol, M. D. Environ. Sci. Technol. 2004, 38, 5246− 5252. (28) Jia, L.; Dureau, R.; Ko, V.; Anthony, E. J. Energy Fuels 2010, 24, 4351−4356. (29) Owusu, S. O.; Adewuyi, Y. G. Ind. Eng. Chem. Res. 2006, 45, 4475−4485. (30) Adewuyi, Y. G.; Owusu, S. O. J. Phys. Chem. A 2006, 110, 11098−11107. (31) Xu, Y. Chemical reaction kinetics; Chemical Industry Press: Beijing, 2004; pp 39−45. (32) Zhang, C. F. Gas-liquid reaction and reactor; Chemical Industry Press: Beijing, 1985; pp 22−29.

Conditions. The experimental conditions were as follows: O2 concentration, 6.0%; SO2 concentration, 1000 ppm; NO concentration, 400 ppm; Hg0 concentration, 30 μg/m3; flue gas flow, 1.2 L/min; solution temperature, 20 °C; solution pH, 3.88; H2O2 concentration, 0.75 mol/L; Ca(OH)2 concentration, 0.005 mol/L; and UV energy density per unit solution, 0.015 W/mL.

4. CONCLUSIONS In this work, photochemical oxidation removal of Hg0, SO2, and NO from flue gas via an ultraviolet irradiation/hydrogen peroxide (UV/H2O2) process was investigated in a photochemical gas−liquid reactor. The results show that O2 and UV radiation have a significant role in promoting the removal of Hg0. The removal of Hg0 is significantly promoted by adding low concentration of H2O2, but it is inhibited by the addition of a high concentration of H2O2. The removal efficiency of Hg0 significantly decreases as the Hg0 concentration, flue gas flow, or solution pH increases, but decreases only slightly as the solution temperature, SO2 concentration, or NO concentration increases. Hg0, NO, and SO2 can be simultaneously effectively removed using the UV/H2O2/Ca(OH)2 processes. Hg2+ is the final reaction product of Hg0 removal, and Hg0 is mainly removed by the oxidations of H2O2, ·OH, ·O, and O3, as well as UV photoexcitation.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 0511 89720178. Fax: +86 0511 89720178. E-mail: [email protected]. 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), New Teacher Fund for the Doctoral Program of Higher Education of China (No. 20123227120016), Key Laboratory of Efficient & Clean Energy Utilization, College of Hunan Province (Changsha University of Science & Technology) (No. 2013NGQ003), and China Postdoctoral Science Foundation (No. 2013M531281).

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