Catalytic Oxidation and Adsorption of Elemental ... - ACS Publications

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Catalytic Oxidation and Adsorption of Elemental Mercury over CuCl2‑Impregnated Sorbents Wen Du,† Libao Yin,‡ Yuqun Zhuo,†,* Qisheng Xu,‡ Liang Zhang,† and Changhe Chen† †

Key Laboratory for Thermal Science and Power Engineering of Ministry of Education Thermal Engineering Department, Tsinghua University, Beijing 100084, People’s Republic of China ‡ Electric Power Research Institute of Guangdong Power Grid Corporation, No.8 Shuijungang, Dongfengdong Road, Guangzhou 510080, China S Supporting Information *

ABSTRACT: CuCl2-impregnated sorbents were employed to remove elemental mercury from flue gas. Three carriers including neutral Al2O3, artificial zeolite, and activated carbon have been investigated in this research. The performances of these prepared sorbents have been tested in a bench-scale fixed-bed reactor under different simulated flue gas atmospheres and temperatures (333−573 K). CuCl2-impregnated activated carbon showed the best adsorption ability. However, CuCl2-impregnated neutral Al2O3 and zeolite have demonstrated an adsorption rate similar to that of CuCl2-impregnated activated carbon in the early stage of the tests (5 min), and they achieved relatively high mercury oxidation efficiencies. These non-carbon sorbents could remarkably enhance the technoeconomical properties of mercury removal in coal-fired power plants and have great potentials in industrial application. The appropriate mercury capture temperature for these sorbents is 333−473 K. The possible mechanisms of elemental mercury oxidation have been discussed.

1. INTRODUCTION With the development of fossil fuels combustion and metallurgical industries, a large amount of mercury is released into the atmosphere every year, which brings major environmental issues.1 Mercury and its compounds have strong biological toxicities to people’s health. Methylmercury is the initiation substance of the Minamata disease in Japan. The coalfired power plant industry is one of the biggest anthropogenic mercury emission sources in the world. The total mercury emission from coal-fired power plants in the United States was 48 tons in 2000 and 12.06 tons from 25 power plants in the European Union (EU) in 2004.2 In December 2011, the United States Environmental Protection Agency (EPA) issued the first national standards for mercury pollution emitted from power plantsFinal Mercury and Air Toxics Standards (MATS).3 MATS regulates the mercury emission amount from major and area sources, including boilers and certain incinerators. On Dec. 20, 2012, the U.S. EPA finalized a specific set of adjustments to Clean Air Act standards. The mercury emission limitation in the 2012 final standards is 2.0−3.0 tons/year for the boilers that need to meet emission limits in the 2012 final standards.4 There are three forms of mercury in coal-fired flue gas: elemental mercury (Hg0), oxidized mercury (Hg2+), and particulate-bound mercury (Hgp).5 Hg0 is volatile and insoluble in water, so it can cause global air pollution by long-distance transportation in the atmosphere. Hg2+ is soluble in water and can be easily removed by wet flue gas desulfurization (WFGD) in power plants. HgP can be collected with fly ash by particulate control systems. In order to reduce atmospheric mercury emissions, Hg0 should be converted to Hg2+ or HgP to the largest extent. Using existing air pollution control devices has a limited mercury removal rate and is unable to reach the strict © 2013 American Chemical Society

environmental limitations in the future. Statistical results from an ICR (information collection request) in the U.S. have shown that the mercury removal rate for CS-ESP (cold-side electrostatic precipitator), HS-ESP (hot-side electrostatic precipitator), and FF (fabric filter) is 27%, 4%, and 58%, respectively.5 WFGD can remove up to 90% oxidized mercury, but it has no effect on elemental mercury. Selective catalytic reduction (SCR) has some effect on oxidation of elemental mercury.6 Sorbent injection combined with existing air pollution control devices has been considered as one of the most significant technologies in mercury removal. Though experiments and numerical simulations showed that activated carbon sorbents possess very high mercury removal rates,7,8 the application of activated carbon injection has been limited due to its high cost. In recent years, many researchers have been committed to developing non-carbon sorbents for economic reasons. A large amount of experiments demonstrated good effects by using mercury sorbents from non-carbon-based materials with high specific surface areas via active substances impregnation.9 A transition metal is commonly used as the active substance because of its unique electronic structure. Previous studies showed some kinds of transition metals performed good mercury removal effects and mercury catalytic oxidation capacities, such as copper (Cu), manganese (Mn), and iron (Fe).9−11 However, the effect of such an active substance has not been well-understood yet. This study researches CuCl2-impregnated sorbents in simulated flue gas. As a kind of transition metal, copper may Received: Revised: Accepted: Published: 582

May 20, 2013 December 13, 2013 December 16, 2013 December 16, 2013 dx.doi.org/10.1021/ie4016073 | Ind. Eng. Chem. Res. 2014, 53, 582−591

Industrial & Engineering Chemistry Research

Article

Figure 1. Schematic of the fixed-bed reactor.

by a PSA Cavkit 10.534 mercury generator. The water vapor was generated by an IAS Hovocal gas generator. The concentrations of Hg0 and Hg2+ in simulated flue gas were online monitored by Thermo CEMS. HgP absorbed by modified sorbents was analyzed by Lumex RA-915 M after each experiment. Lumex RA-915 M was calibrated by the Standard Reference Material 1632d (provided by the U.S. National Institute of Standards and Technology), and the error was within ±5%. The Hg generator and analyzer were calibrated before experiments. It has been verified that the readings of Thermo CEMS fluctuated within ±1% of the Hg0 generator set values. The total system error was within ±10%. The fixed-bed reactor was made of a 40 mm (inside diameter) quartz tube which could be heated by a temperaturecontrolled furnace. The composition and flow rates of simulated flue gas were accurately controlled (with the error within ±2%) by mass flow controllers (MFCs). Hg0 carried by N2 and simulated flue gas was first heated to the experiment temperature in the preheating zone of the fixed-bed reactor and then passed through the sorbent bed supported by a sintered quartz disc to complete the experiment. To monitor the reaction temperature, a thermocouple covered by quartz tube was inserted into the reaction zone. To avoid unnecessary Hg contamination, all of the tubes, joints, and valves with Hgcontaining gas passing through were made of either quartz or poly(tetrafluoroethylene). The tail gas was treated by an activated carbon trap before being released to the air. 2.3. Experimental Procedure and Data Analysis. Before each test, a new quartz fiber filter was placed on the sintered disc for the prevention of disc blockage by falling sorbent. A 0.5 g amount of impregnated sorbent was homogeneously mixed with 5 g of quartz sand before being placed on the filter. The bed height of sorbent/sand mixture was about 6 mm, and the residence time was 0.2 s. At the beginning of each test, the reactor was heated to the operating temperature (normally 413 K). During each test, the Hg0 stream was switched on only after the reaction conditions attained. The inlet Hg0 concentration was set at 20 μg/Nm3, and the total flow rate was controlled at

have special catalytic effects for mercury oxidation. Moreover, copper chloride contains a certain amount of chlorine ions, which are beneficial to mercury oxidation. The essential analysis has been carried out to reveal the possible mechanisms of elemental mercury oxidation and adsorption.

2. EXPERIMENTAL SECTION 2.1. Sorbent Preparation. Neutral Al2O3 (100−200 mesh), artificial zeolite (60−80 mesh), and activated carbon (provided by Indoor Environment Doctor Co., Ltd.) have been selected as the carriers in this study. These materials are with relatively large specific surface areas. Cupric chloride (CuCl2· 2H2O, provided by Sinopharm Chemical Reagent Co., Ltd.) solution of different concentrations (10 and 5 wt %) were impregnated onto the carriers. During the impregnation, 5 g of carrier was submerged onto 30 mL of CuCl2 solution in a 50 mL plugged conical flask, magnetically stirred for 3 h and allowed to stand for another 3 h. Then the prepared sorbents were filtered and rinsed 3 times by deionized water. The sorbents were then dried in a 378 K oven for 12 h. The rinsed water was collected and analyzed by inductively coupled plasma−atomic emission spectroscopy (ICP-AES) afterward to determine the quality of CuCl2 loaded onto the carriers. The loading of CuCl2 to sorbent is defined as loading =

(Moriginal CuCl2 − M remainingCuCl2) (Moriginal CuCl2 − M remainingCuCl2 + Mcarrier)

× 100% (1)

where Moriginal CuCl2 is the original mass of CuCl2 solutions (10 or 5 wt %), Mremaining CuCl2 is the mass of CuCl2 solutions that remained in rinsed water after impregnation, and Mcarrier is the mass of the carrier. 2.2. Apparatus. The schematic of the fixed-bed reactor system used for Hg0 oxidation and adsorption experiments is shown in Figure 1. The reactor system consists of four parts, Hg generation, simulated flue gas mixing, fixed-bed reactor, and tail gas treatment. The elemental mercury (Hg0) was generated 583

dx.doi.org/10.1021/ie4016073 | Ind. Eng. Chem. Res. 2014, 53, 582−591


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2 L/min. The experimental time normally lasted 1 h, during which time the Hg0 and Hg2+ concentrations in gas stream after bed were online measured by Thermo CEMS. Hg captured by sorbent (HgP) was determined by Lumex RA-915 M after the experiment. The total Hg0 removal rate (η) has been defined as η=

t ∑0 (c0

−

t ) c Hg 0

c0t

× 100%

Table 1. Characteristics of the Carriers and Impregnated Sorbents BET data

sample α-alumina 5% CuCl2-A 10% CuCl2-A artificial zeolite 5% CuCl2-Z 10% CuCl2-Z activated carbon 5% CuCl2-C 10% CuCl2-C

(2)

The total Hg0 adsorption rate (ηads) and oxidation rate (ηoxi) within a certain time are defined as follows: ηads =

t m Hg P

c0tv

× 100%

(3)

t

ηoxi =

t ∑0 c Hg 2+

c0t

× 100%

(4)

ctHg0

0

BET specific surface area (m2/g)

total pore vol (cm3/g)

BET av pore size (nm)

80.8 80.5 79.7 74.2 65.3 58.3 683.8

2.15 × 10−1 7.1 × 10−4 3.2 × 10−4 7.8 × 10−3 3.2 × 10−3 2.9 × 10−3 2.6 × 10−1

8.9 8.8 9.1 10.5 13.7 16.7 1.9

651.7 620.2

2.5 × 10−1 2.4 × 10−1

1.9 1.9

loading (%) 3.3 6.75 16.8 24.8

4.2 8.57

activated carbon sorbents were chosen to compare with impregnated sorbents in this study, i.e., Norit Darco FGD and Norit Darco LH (brominated). From the BET data in Table 1, it can be found that the specific surface areas of neutral Al2O3 and zeolite are much smaller than those of activated carbons. The physical adsorption properties of activated carbons are expected to be much better than those of noncarbon sorbents. Among all of the carriers, the loading of CuCl2 on zeolite is much more than those of neutral Al2O3 and activated carbon. Moreover, the loadings of CuCl2 increased with the increasing mass fraction of CuCl2 solution for all of the carriers. In order to understand the crystal form of CuCl2 loading on the carriers, XRD analysis was applied to the modified sorbents in this research. The XRD patterns for the carriers and modified sorbents were shown in Figure 2. After being impregnated, the XRD patterns of the carriers changed. There were significant variation for zeolite and activated carbon. As can be seen in

ctHg2+

where c0 is the inlet Hg concentration, and is the outlet Hg0 and Hg2+ concentration at the reaction time, respectively. mtHgP is the mercury mass in the used catalysts, and v is the flow rate. The mercury removal efficiency of each sorbent was measured by eqs 2−4 in the experimental data processing. 2.4. Charaterization of the Sorbents. In order to determine the properties of modified sorbents, characterization measurements were carried out. The surface area and pore size of the catalysts were tested on automatic nitrogen adsorption analyzer (Micrometritics ASAP2010). The surface area was measured by Brunauer−Emmett−Teller (BET) method, and the pore volume and pore size were tested by Barrett−Joyner− Halenda (BJH) method. X-ray diffraction (XRD) was performed to determine the crystals species in the catalysts, and the tests were carried out with a diffractometer (Bruker D8, Karlsruhe, Germany) using Cu Kα radiation. Transmission electron microscopy (TEM) was applied to investigate the microstructures of the sorbents with an electron microscope (TECNAI G2 20, 200 kV). X-ray photoelectron spectroscopy (XPS, 250Xi ESCA) was used to analyze the copper valence on the surface of the modified sorbents. The C1s line at 284.8 eV was taken as the reference for the binding energy calibration.

3. RESULTS AND DISCUSSIONS 3.1. Performance of the Fixed-Bed Reactor. Before each test, the reactor tube and downstream connections were rinsed by deionized water, 10% nitric acid, deionized water, and absolute ethanol in sequence (repeated 3 times) and then dried. During each test, 2 L/min nitrogen was passed through to check the air tightness of the whole system after the system was preheated. The mercury mass balance was calculated by eq 5, and it was in a reasonably acceptable range (90−105%) for each test. mercury mass balance =

t t t m Hg + m Hg 0 + m P Hg 2 +

c 0tv

× 100% (5)

3.2. Morphological Analysis of the Sorbents. The characteristics of the carriers and impregnated sorbents are listed in Table 1. In order to simplify the expressions, the carriers are correspondingly abbreviated as follows: A, neutral Al2O3; Z, artificial zeolite; C, activated carbon. Two commercial

Figure 2. XRD patterns for carriers and sorbents: (a) activated carbon, (b)10% CuCl2-C, (c) zeolite, (d) 10% CuCl2-Z, (e) neutral alumina, and (f) 10% CuCl2-A, (g) 10% CuCl2-A after test (413 K, 6 vol % O2, 12 vol % CO2, 5.12 vol % H2O, 20 ppm HCl, 500 ppm SO2, 300 ppm NO, and balanced by N2). 584



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Figure 2, CuCl2 mainly present in the crystal forms of Cu2Cl(OH)3 on the carriers. The XRD pattern of used sorbent (tested in the atmosphere of 10% CuCl2−Al2O3, 413 K, 6 vol % O2, 12 vol % CO2, 5.12 vol % H2O, 20 ppm HCl, 500 ppm SO2, 300 ppm NO, and balanced by N2) was also shown in Figure 2. The diffraction peaks of Cu2Cl(OH)3 became weaker, and the diffraction peaks of CuCl appeared in the XRD patterns of used sorbent. This phenomenon indicates that part of Cu2Cl(OH)3 converted to CuCl in the reaction of mercury removal. Figure 3 shows the TEM micrographs for CuCl2-impregnated neutral alumina. As can be seen from the result, the particle size of the carrier is about 5 nm. The particle sizes for the modified sorbents are smaller than the carrier, and the sorbents are well-dispersed. The decrease of particle size may be attributed to the suppression of particle growth by the introduction of Cu atoms into alumina crystal structure.12,13 The surface properties of 10% CuCl2−Al2O3 were studied using XPS analysis. The main and the satellite peaks of Cu 2p3/2 for the fresh and the tested sorbents (tested in 10% CuCl2− Al2O3, 413 K, 6 vol % O2, 12 vol % CO2, 5.12 vol % H2O, 20 ppm HCl, 500 ppm SO2, 300 ppm NO, and balanced by N2) are shown in Figure 4. The main peaks of Cu 2p3/2 are from 926 to 939 eV. CuCl2 on the sorbents are present mainly in the states of Cu+ and Cu2+ with the corresponding energy at 932.1−932.6 and 934−935.2 eV, respectively.14−19 The spectrum area ratio of Cu+ to Cu2+ is 0.19 for the fresh sorbent, while the ratio is 0.45 after the test in the simulated flue gas. Some amount of Cu2+ converted to Cu+ during the adsorption process, which is consistent with the XRD analysis. The Cu atomic ratio of the reacted sorbents is 6.1%, and the Cu ratio of the fresh sorbents is 5.88% tested by the XPS. Taking into account the experimental error, the effluent is considered to be free of Cu. The fixing rate of Cu between the incineration temperature (773−1373 K) is stable.20 Kameda et al. verified that Cu volatilized as CuCl at 1173 K in the presence of high HCl content.21 The boiling point of CuCl2 is 1266 K, while the reacting temperature is 413 K and the reaction time is always less than 1 min in industrial application. The Cu-based sorbents are therefore considered to be stable, and the employment of the modified sorbents will not bring Cu emission to flue gas or even the atmosphere. In this research, Hg 4f7/2 spectra of the sorbent is difficult to detect because the mercury content of absorbed sorbent is less than 0.01 wt %, which is much lower than the detection limit of our XPS analysis (1 wt %). However, Qiao et al.10 did suggest that the presence of Hg2+ could be verified by using XPS analysis. 3.3. Mercury Speciation. To investigate the mercury removal capacities of the modified sorbents, the total oxidized and adsorbed mercury within 1 h’s reaction time was studied first. The simulated flue gas composition was 20 ppm HCl complete gas (6 vol % O2, 12 vol % CO2, 5.12 vol % H2O, 20 ppm HCl, 500 ppm SO2, 300 ppm NO, and N2 balance). The testing temperature was kept at 413 K. The mercury speciation in the 1 h experiment is shown in Figure 5. Sorbents 10% CuCl2-C and 5% CuCl2-C performed the best adsorption capability and they adsorbed over 90% Hg0, which was similar to the results from previous research conducted on Cu-based sorbents.22 Sorbent 10% CuCl2-A demonstrated better adsorption and catalytic oxidation capability than other noncarbon sorbents.

Figure 3. TEM micrographs of the modified sorbents and carrier: (a) neutral Al2O3, (b) 5% CuCl2-A, and (c) 10% CuCl2-A.

The time-dependent breakthrough curves of elemental mercury for six modified sorbents under 20 ppm HCl complete gas conditions are shown in Figure 6. CuCl2-impregnated activated carbons retained high efficiencies of mercury removal rates, and breakthroughs have not been seen in the reaction. Sorbent 10% CuCl2-A demonstrated the best Hg0 removal rate among non-carbon sorbents in the 1 h tests, and the halfbreakthrough time was about 20 min. 3.4. The Effect of Flue Gas Components. 3.4.1. The Effect of HCl. Among all of the acid gases in coal-fired flue gas, HCl has the greatest impact on catalytic oxidation of elemental 585



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mercury. In order to understand the effect of HCl concentration on the performance of six modified sorbents, the following experiments were conducted in a complete flue gas atmosphere. The overall depletions of Hg0 in 1 h at various HCl concentrations are shown in Figure 7. With the increasing

Figure 4. XPS spectra of Cu 2p3/2 for (1) fresh 10% CuCl2-A and (2) 10% CuCl2-A with 50 μg/g of adsorbed mercury after test (413 K, 6 vol % O2, 12 vol % CO2, 5.12 vol % H2O, 20 ppm HCl, 500 ppm SO2, 300 ppm NO, and balanced by N2).

Figure 7. Total outlet Hg0 under different HCl concentrations in 1 h (413 K, 6 vol % O2, 12 vol % CO2, 5.12 vol % H2O, 10−100 ppm HCl, 500 ppm SO2, 300 ppm NO, and balanced by N2).

of the HCl concentration, the Hg0 removal efficiency was increased for all of the sorbents. HCl is conducive to Hg0 removal. Modified activated carbon demonstrated great mercury removal capability when there was HCl in the simulated flue gas regardless of the HCl concentration. The Hg0total removal capability of 10% CuCl2-A was better than other non-carbon sorbents in a 1 h experiment. Previous study showed HCl promoted the adsorption of mercury onto sorbents.23,24 The Deacon process25 may well explain the mechanism of the catalytic oxidation and adsorption of mercury in the presence of HCl:

Figure 5. Hg speciation in 1 h tests (413 K, 6 vol % O2, 12 vol % CO2, 5.12 vol % H2O, 20 ppm HCl, 500 ppm SO2, 300 ppm NO, and balanced by N2).

4HCl + O2 ↔ 2Cl 2 + 2H 2O

(6)

HCl has been noted to have little significance in mercury speciation,26 while Cl2 promotes the oxidation and chemisorption of mercury.26−30 Hg0 reacted with chlorine species to form Hg2+, and Hg2+ was easily absorbed on the surface of the sorbents, as shown in reactions 7−9. Hg + Cl ↔ HgCl(g)

(7)

HgCl + Cl ↔ HgCl2(g)

(8)

Hg + Cl 2 ↔ HgCl2(g)

(9)

As the HCl concentration increased, the chlorine concentration increased as well and Hg0 removal was enhanced. 3.4.2. The Effect of SO2. The conversion of Hg0 in the presence of different SO2 concentrations is shown in Figure 8. The removal efficiencies of Hg0 by CuCl2-impregnated activated carbons seemed to have little change when the concentration of SO2 was increased from 0 to 2000 ppm in simulated flue gas. In contrast, the total outlet Hg0 in 1 h by non-carbon sorbents seemed to be very sensitive to SO2. Griffin31 stated that SO2 might deplete Cl2 through the following reaction:

Figure 6. Breakthrough curves of elemental mercury for the modified sorbents (413 K, 6 vol % O2, 12 vol % CO2, 5.12 vol % H2O, 20 ppm HCl, 500 ppm SO2, 300 ppm NO, and balanced by N2).

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CuCl2-impregnated neutral Al2O3 in different atmospheres were shown in Figure 9. The basic gas components are 6 vol %

Figure 8. Total outlet Hg0 under different SO2 concentrations in the 1 h test (413 K, 6 vol % O2, 12 vol % CO2, 5.12 vol % H2O, 0−2000 ppm SO2, and balanced by N2).

Cl 2 + SO2 + H 2O ↔ 2HCl + SO3

(10)

HCl has been noted to have little significance in mercury speciation.26 The exhaust Deacon Cl2 may be the reason that SO2 inhibits mercury removal alone. For the tests of noncarbon sorbents, the emitted Hg0 increased with the increase of SO2 concentration, and the most remarkable increase was in the range of 0−1000 ppm SO2. When the concentration of SO2 reached 1000 ppm, there was no significant change in mercury oxidation rates and adsorption rates for non-carbon sorbents. This is mainly due to the following reactions occurring in the presence of high SO2 concentration:32 Hg + O2 + SO2 ↔ HgSO4 (s)

(11)

HgCl2 + SO2 + O2 ↔ HgSO4 (s) + Cl 2

(12)

Figure 9. Breakthrough curves of elemental mercury for 10% CuCl2− Al2O3under different simulated flue gases at 413 K.

O2, 12 vol % CO2, and 5.12 vol % H2O, and the reaction temperature is 413 K. For other modified sorbents, there were similar variations among different flue gas atmosphere. Among the tests, there was obvious inhibition effects on mercury removal in the presence of water vapor alone, which is consistent with previous research.33 Compared with activated carbon in the tests, the inhibition of water vapor to non-carbon sorbents was stronger. The Hg0 removal rate dropped about 20% in the 5.12% water vapor atmosphere compared to the nitrogen atmosphere for 10% CuCl2-Z and 5% CuCl2-A. From the results, it can be seen that oxygen reduced the Hg0 removal. As XRD results suggested, Cu2Cl(OH)3had been formed on the surface of the sorbents after modification and it decomposed to CuCl in the reaction. Without HCl, CuCl can react with O2 to form little Cl2:11

The inhibition effects of SO2 for non-carbon sorbents were much stronger than activated carbons. CuCl2-modified activated carbons have a high capacity of sulfur resistance, which gives them great potential for application in power plants with high SO2 concentration or without wet flue gas desulfurization systems. The reaction mechanism between SO2and modified activated carbon will be our further study. 3.4.3. The Effect of Other Components in Simulated Flue Gas. There are different flue gas components (SO2, NOx, HCl, CO2, O2, and H2O, etc) in coal-fired flue gas, and they will interact with Hg0 in different ways. Sulfur and chlorine have an important role in mercury transformation.24 In the presence of HCl, SO2 has a negative effect on the adsorption capacity of sorbents for both elemental mercury and oxidized mercury.32 Wilcox et al.34 noted SO2 and H2O may interact with the carbon surface and affect the Hg adsorption. Previous studies found out that the catalytic oxidation and adsorption efficiencies were inhibited for manganese-impregnated noncarbon sorbents in the presence of 500 ppm SO2.10,35 In this study, the effects of SO2, NO, H2O, O2, and CO2 over the impregnated sorbents on the conversion of Hg0 were investigated at 413 K. The Hg0 breakthrough curves of 10%

CuCl 2 + 0.5O2 ↔ CuO + Cl 2

(13)

The generated Cl is the key factor of mercury oxidation and adsorption in the basic flue gas atmosphere. There was also a certain amount of oxidized mercury in a 300 ppm NO atmosphere. NO could be easily transformed to NO2, and NO2 could react with elemental mercury in the following reaction:29,36 Hg + NO2 ↔ HgO(s, g) + NO

(14)

2Hg + 5NO2 ↔ Hg 2O(NO3)2 + NO

(15)

Compared to the complete atmosphere, it can be seen that NO and SO2 suppress mercury oxidation and adsorption and the inhibition of SO2 was stronger than NO. 3.5. The Effect of Temperature. Figure 10 shows the effect of temperature on the adsorption capacity of six modified 587



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shown that the highest Hg0 concentration appeared in the test under 413 K and the highest Hg2+concentration appeared in the test under 573 K. The oxidation of mercury was positively correlated with temperature, and the maximum oxidation rate can reach 83% at 573 K. This indicates the catalytic oxidation of elemental mercury by the modified sorbents. However the mercury adsorption amount showed a different variation. The data in this research support that chlorine has a significant effect on the chemisorption of mercury to the sorbents and mercury oxidation. The Deacon process is the key factor of chemisorption and oxidation of mercury in the presence of HCl, and copper, iron, and manganese salts are suitable catalysts for the Deacon process.25 If CuCl is the catalyst, the reactions are shown as follows:

Figure 10. Total adsorbed mercury under different temperature in 1 h tests (6 vol % O2, 12 vol % CO2, 5.12 vol % H2O, 20 ppm HCl, 500 ppm SO2, 300 ppm NO, and balanced by N2).

2CuCl + 0.5O2 ↔ Cu 2OCl 2

(16)

Cu 2OCl 2 + 2HCl ↔ 2CuCl 2 + 2H 2O

(17)

CuCl 2 + 0.5O2 ↔ CuO + Cl 2

(13)

The chlorine release step is endothermic and is favored at high temperatures. As temperature increases, the Hg0 catalytic oxidation rates increase as well. 3.6. Adsorption Capacity of Hg0 in the Early Stage of the Tests. In the actual mercury removal process in coal-fired power plants, the most important reactions always take place in the very early stage of the adsorption process. The contact time of the flue gas and mercury removal sorbent is usually less than 1 min. In order to study the reaction in the short period that sorbents actually function, 5 min experiments were conducted under 20 ppm HCl complete flue gas, and the results were illustrated in Figure 12. Most non-carbon sorbents had

sorbents. The tests lasted for 1 h under 333−573 K and 20 ppm HCl complete flue gas. When the temperature increased, the adsorption capacity of all the sorbents dropped significantly. For activated carbon impregnated sorbents, the total adsorbed Hg0 in 1 h dropped from 90% to 73% when the temperature rose from 333 to 473 K, and when the temperature reached 573 K, the adsorbed HgP was only 6.3%. Therefore, the suitable working temperature is expected to be 333−473 K. For noncarbon sorbents, the adsorption capacity also decreased when the reacting temperature increased. Moreover, the adsorption capacities completely disappeared when the temperature was above 473 K. It has been noticed that 10% CuCl2 solution impregnated zeolite had adsorption capacity of Hg0 similar to that of activated carbon when the temperature was 333 K. This was probably due to the strong physical adsorption capacity of zeolite and the catalytic effect of CuCl. The increasing reaction temperature would have a negative effect on physical adsorption of Hg0. Figure 11 shows the total outlet amount of Hg0 and Hg2+ of 10% CuCl2- zeolite after 1 h experiment under different temperatures. The results have

Figure 12. Mercury speciation in 5 min experiments (413 K, 6 vol % O2, 12 vol % CO2, 5.12 vol % H2O, 20 ppm HCl, 500 ppm SO2, 300 ppm NO, and balanced by N2).

adsorption capacity similar to modified activated carbon, and 10% CuCl2- zeolite had the best adsorption capacity among non-carbon sorbents. Taking into account that the cost of noncarbon sorbent is generally much lower than that of activated carbon, 10% CuCl2−alumina and 10% CuCl2−zoelite could be reasonable alternatives to activated carbon sorbents. The adsorption capacities of CuCl2 solution activated carbons were higher than Darco FGD and equally matched with Darco LH.

Figure 11. Total outlet Hg0 and Hg2+ of 10% CuCl2- zeolite in the 1 h test (6 vol % O2, 12 vol % CO2, 5.12 vol % H2O, 20 ppm HCl, 500 ppm SO2, 300 ppm NO, and balanced by N2). 588



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3.7. Kinetics for Catalytic Oxidation Reaction in 5 min Tests. The kinetics for catalytic oxidation reaction of elemental mercury and sorbents in 5 min tests can be written as eq 18.35 c Hg 0 θ t −ln = εKSc HCl c0 (18)

chlorination: 2CuCl + 1 2 O2 ↔ Cu 2OCl 2 Cu 2OCl 2 + 2HCl ↔ 2CuCl 2 + 2H 2O CuO + 2HCl ↔ CuCl 2 + H 2O

In the equation, KS is the surface reaction rate constant (m/ s) and θ is the dimensionless reaction order regarding HCl concentration. ε is the specific area per unit volume of the sorbent (m2/m3). In the experiment, the HCl concentration could be considered to be a constant because it was much higher than the concentration of Hg0. From the experiment results, the relationship of ln[−ln(cHg0/ c0)] and ln cHCl was linear and θ and εKS can be obtained from the experimental data. The derived kinetic parameters for six sorbents were listed in Table 2. The parameter εKS reflects the

CuCl 2 + 1 2 O2 ↔ CuO + Cl 2 HCl ↔ H + Cl

mercury oxidation and adsorption: Cl 2 ↔ Cl + Cl

Hg + Cl ↔ HgCl

Table 2. Kinetic Parameters for the Sorbentsa

Hg(g) + Cl 2 ↔ HgCl + Cl

sorbent

R2

θ

εKS (mg/m3s)

10% CuCl2-A 10% CuCl2-Z 10% CuCl2-C 5% CuCl2-A 5% CuCl2-Z 5% CuCl2-C LH FGD

0.9972 0.9878 0.9718 0.9680 0.9841 0.9981 0.9462 0.9778

0.25 0.09 0.08 0.30 0.27 0.07 0.24 0.25

3.70 7.10 7.87 2.17 3.23 8.27 5.16 4.72

HgCl + Cl 2 ↔ HgCl2 + Cl HgCl(g) ↔ HgCl(ads)

HgCl2(g) ↔ HgCl2(ads) HgCl(ads) + Cl ↔ HgCl2(ads)

3.9. Management of the Sorbents. Although the purpose of this research was to develop a low-cost but effective technology for mercury capture in flue gas, efforts must be taken to manage the mercury contamination in the used sorbents. The commonly employed techniques for hazardous waste remediation are solidification/stabilization (S/S), immobilization, vitrification, thermal desorption, soil washing, electro-remediation, phytostabilization, phytoextraction, and phytovolatilization.37−46 Solidification/stabilization processes are nondestructive methods to immobilize the hazardous constituents (Hg, Cu) in the used sorbents while reducing permeability. This method involves enclosing or physically binding contaminants within a stabilized mass (solidification).38 Cement, pozzolan, and organophilic clay can be used as a binder for the mercury sorbents.39,40 Cement-based S/S is very important as an option for remediating the sorbents due to its low cost. Nevertheless, the environmental stability of the mercury captured by Cu-based sorbent needs further investigation.

a

Conditions: 413 K, 6 vol % O2, 12 vol % CO2, 5.12 vol % H2O, 10− 100 ppm HCl, 500 ppm SO2, and 300 ppm NO

catalytic oxidation rates, and bigger εKS is favored for better sorbent performance in the early stage of mercury removal reaction. The parameter εKS of six sorbents and two commercial activated carbons were different but in the same order of magnitude. θ reflects the effect of HCl concentration on the sorbent. Increasing HCl concentration is more beneficial for mercury removal by the sorbent when it is bigger. 3.8. Mechanism for Catalytic Oxidation and Adsorption of the Sorbents. From all of the experimental data, the mechanisms for catalytic oxidation and adsorption of the sorbents can be derived. After the sorbent has been modified, the surface of the sorbents was covered with adsorption active sites, which would contribute to the capture of mercury.11 In the early stage of the reaction, the modified non-carbon sorbents had an adsorption rate similar to that of modified activated carbon. However, the total amount of adsorbed mercury for activated carbon was much higher than that for modified non-carbon sorbents. This is possibly due to the excellent physical adsorption capacities of activated carbon. The adsorption process on the surface of non-carbon sorbents might be quickly saturated and the adsorption rates decreased along with the desorption process. The key factor of mercury oxidation and adsorption could be explained by the Deacon process.25 The proposed process can be described as follows.

4. CONCLUSION CuCl2- neutral alumina, zeolite, and activated carbon can remove elemental mercury from flue gas in coal-fired power plants by catalytic oxidation and adsorption. HCl can promote the mercury removal process, while SO2 has a negative effect. The appropriate mercury removal temperature for the sorbents is 333−473 K. The adsorption capacity of modified non-carbon sorbents is very similar to that of activated carbon in the initial adsorption process. Over 80% Hg0 can be removed by CuCl2zeolite with 20 ppm HCl in 413 K in 5 min tests, while the adsorption rate is over 90% for 5% CuCl2- activated carbon. Considering the cost of mercury removal, CuCl2- alumina and zeolite may be good alternatives to activated carbon. These non-carbon sorbents could remarkably enhance the technoeconomical properties of mercury removal in coal-fired power plants and have great potential in industrial application.

decomposition of Cu2Cl(OH)3: 2Cu 2Cl(OH)3 ↔ 2CuCl + 2CuO + 3H 2O + 1 2 O2 589



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ASSOCIATED CONTENT

S Supporting Information *

Tables listing BET data for commercial activated carbon, detailed gas compositions for different experiments, and mercury adsorption rates of the sorbents in 5 min tests. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support from the China National Natural Science Foundation via Project No. 51376109 is greatly appreciated. REFERENCES

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