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Elemental Mercury Capture from Flue Gas by a Supported Ionic Liquid Phase Adsorbent Xiaoshan Li, Liqi Zhang,* Dong Zhou, Wenqian Liu, Xinyang Zhu, Yongqing Xu, Ying Zheng, and Chuguang Zheng State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China S Supporting Information *

ABSTRACT: As promising functional materials, ionic liquids have been widely used in flue gas purification and separation. Previous studies reported that ionic liquid based mixtures can be used for mercury removal due to their high economic efficiency and environmental friendliness. In comparison with ionic liquid based mixtures, a supported ionic liquid phase (SILP) adsorbent shows significantly improved mercury removal performance because of its greatly enhanced gas/liquid interfacial area. In this study, a novel supported task-specific ionic liquid phase adsorbent [C4mim][FeCl4]−SiO2 was prepared for high efficiency elemental mercury capture from flue gas. The Hg0 removal performance was investigated in a bench-scale fixed bed reactor, and the reaction mechanism was proposed based on the temperature programmed desorption (TPD) test. The results showed that Hg0 removal was mainly due to the oxidation by [C4mim][FeCl4]. The addition of [C4mim][FeCl4] on SiO2 in a certain composition range enhanced the Hg0 removal performance, and the optimized ionic liquid loading is 30%. The Hg0 removal by the adsorbent was favored at higher temperatures. TPD results showed that the mercury compound formed on the adsorbent was HgCl2. The Hg0 removal mechanism involves combined physisorption and chemisorption, which consists of Hg0 oxidation to HgCl2 by the ionic liquid and physical adsorption of HgCl2 on the porous adsorbent. Another promising class of materials for Hg0 removal are room-temperature ionic liquids (ILs). It has been extensively reported that ILs have been widely used in flue gas purification and separation, including the removal of CO2,17 SO2,18 NOx,19 H2S,20 etc. ILs are promising functional materials due to their advantages such as excellent thermal and chemical stabilities, high solubilities, and negligible vapor pressures. It is feasible to design task-specific ILs for oxidation of Hg0 due to their tunable structures. Recently, the use of IL-based mixtures (ILs + other solvents) for Hg0 removal has been reported. Barnea and co-workers21 synthesized task-specific ILs [Cnmim][XI2] by blending 1-alkyl3-methylimidazolium chloride ILs [Cnmim][X] with I2 for oxidation of Hg0 to HgI2. Neat ILs [Cnmim][X] performed poorly in Hg0 removal. Iodine was the key factor for oxidation of Hg0. In addition, Cheng and co-workers22,23 attempted to use ILs/H2O2 and Fe-based ILs/H2O mixtures for Hg0 removal from flue gas. The absorption behavior of the ILs/H2O2 mixtures was similar to that of [Cnmim][XI2], but H2O2 served as an effective oxidant for converting Hg0 to Hg2+. Fe-based ILs/H2O mixtures achieved a high Hg0 absorption capacity and possessed excellent reusability at the temperature of 80 °C. However, the evaporation of H2O from the mixtures is a significant obstacle preventing application of the Fe-based ILs/ H2O mixtures for Hg0 capture from high temperature flue gas. In addition, the high viscosities of ILs lead to low gas diffusion coefficients. Therefore, poor thermal stability, the mass loss of

1. INTRODUCTION Mercury remains of great concern due to its significantly toxic effect on human health, wildlife, and the environment. Both natural and anthropogenic activities can emit mercury, among which mercury emissions from anthropogenic sources are responsible for approximately 30−55% of the total atmospheric mercury emissions.1 Coal-fired power plants are the main atmospheric mercury emission source, especially in China, India, and the United States.2,3 It is well accepted that the main forms of mercury present in flue gas are gaseous elemental mercury Hg0, oxidized mercury Hg2+, and particulate-bound mercury Hgp.4 Among them, the oxidized mercury Hg2+ and particulate-bound mercury Hgp can be effectively captured by processes using air pollution control devices (APCD) such as wet flue gas desulfurization (WFGD), fabric filtration (FF), and electrostatic precipitation (ESP).5 However, the removal efficiency of elemental mercury, Hg0, is quite low due to its high volatility, stability, and low solubility in water. 6 Consequently, elemental mercury removal has captured a great deal of attention and several elemental mercury control technologies have been developed in recent years. Converting elemental mercury, Hg0, to the oxidized form, Hg2+, has become a significant focus in the development of appropriate mercury control technologies because the oxidized mercury is more water-soluble and better adsorbed on particles.7 Using solid adsorbents for mercury removal is currently a popular research topic. Several solid adsorbents have been studied for elemental mercury removal, such as carbonbased sorbents,8,9 chemically treated carbons,10−12 nanostructured chelating agents,13,14 metal oxides,15,16 etc. However, it is still necessary to develop new solid adsorbents for highly effective and recyclable mercury removal. © 2016 American Chemical Society

Received: August 5, 2016 Revised: November 6, 2016 Published: November 30, 2016 714

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Energy & Fuels other solvents, and the high viscosity of ILs are considered as serious obstacles to the application of IL-based mixtures. To avoid the drawbacks of IL-based mixtures, the supported ionic liquid phase (SILP) adsorbent seems to be a promising approach. A thin layer of IL covers the pore surface of a porous medium, resulting in an SILP sorbent, thus greatly enhancing the gas/liquid interfacial area which increases the absorption rate and efficiency. SILP adsorbents have been tried for capturing acidic gases from flue gas.24,25 In our previous work, an SILP adsorbent demonstrated high thermal stability and dramatically improved the acidic gas absorption kinetics and desorption efficiency.26 There has been little research on SILP sorbents for Hg0 removal so far. A previous study described six conventional ILs coated on a nanostructured chelating adsorbent for Hg0 and HgCl2 capture from flue gas.27 However, there was no evidence of oxidation of Hg0 by these adsorbents. The authors proposed that the capture of Hg0 was due to weak physisorption, resulting in a relatively low Hg0 removal efficiency. It can be predicted that the Hg0 removal performance could be improved by using task-specific ILs that can chemically react with Hg0. Therefore, in this study, supported task-specific ionic liquid phase adsorbent based on silica was adopted as a novel adsorbent to improve Hg0 removal efficiency. The adsorbent possesses the beneficial properties of both high-surface-area SiO2 and the strong oxidizing ability of the Fe-containing magnetic ionic liquid [C4mim][FeCl4]. [C4mim][FeCl4] serves as an additive to enhance mercury oxidation for high efficiency Hg0 removal, while the high-surface-area SiO2 enhances the contact area and improves the mass transfer between Hg0 and [C4mim][FeCl4]. To the best of our knowledge, the supported task-specific ionic liquid phase adsorbent for Hg0 removal and the reaction mechanism has not been reported. Therefore, this work focused on evaluating the Hg0 removal performance of the SILP adsorbent over a range of temperatures. Moreover, the effect of ionic liquid loadings and the flue gas components on the Hg0 removal efficiency were studied. In addition, using data from the temperature programmed desorption (TPD) experiment, a Hg0 capture mechanism was proposed. The TPD experiment was used to clarify the characteristics of the mercury species and identify the mercury compounds adsorbed on the sorbent. The reaction products could be inferred through the TPD experiment.

Figure 1. 1H NMR spectrum of the synthesized ionic liquid [C4mim][FeCl4].

Table 1. Elemental Analysis Data of [C4mim][FeCl4] [C4mim][FeCl4]-exptl [C4mim][FeCl4]-theor

%C

%H

%N

28.69 28.52

4.69 4.49

8.21 8.32

Raman spectroscopy. FTIR spectra were acquired with a Bruker VERTEX70 spectrometer. Raman spectra were obtained through a confocal LabRAM HR800 spectrometer. The SILP adsorbents, [C4mim][FeCl4]−SiO2, were prepared by a wet impregnation process, which is similar to our previous work.26 The prepared samples were characterized with a thermal gravimetric analyzer (TGA) and the isothermal adsorption method (BET). The thermal stabilities of IL and SILP adsorbents were tested using an NETZSCH STA 449F3 instrument. Samples of 10 ± 0.1 mg were heated to 600 °C at 10 °C/min with a N2 gas flow of 100 mL/min. The porosity was characterized through N2 adsorption and desorption experiment on a Micromeritics ASAP 2020 accelerated surface area and porosimetry system. UV−vis spectra of two samples, [C4mim][FeCl4] before and after Hg0 removal, were detected on a Shimadzu UV-2550 UV−visible spectrometer using acetonitrile as a solvent. 2.2. Experimental Apparatus and Procedures. The Hg0 removal performance of the prepared SILP adsorbents was tested through a bench-scale fixed bed reactor, seen in Figure 2. The Hg0 dynamic absorption measurement apparatus consisted of a simulated flue gas system, a mercury generator device, a fixed bed reactor, and a mercury concentration measurement system. The simulated flue gas including N2, O2, SO2, NO, and H2O was introduced into the gas mixer. In the mercury generator device, Hg0 vapor was generated by a mercury penetration tube (VICI, Metronics Inc., Santa Clara, CA) carried by N2. The temperature of the mercury generator was controlled by the thermostatic bath. The Hg0 concentration in the gas stream was measured online by a continuous VM-3000 mercury vapor monitor based on atomic absorption spectrometry with an accuracy of 0.1 μg/m3. The absorption experiment was performed at atmospheric pressure in the temperature range 80−160 °C. For each test, approximately 0.1 g of SILP adsorbent was placed in a vertical quartz reactor with an adsorbent bed height of approximately 1 mm. The gas space velocity and the residence time were approximately 3.9 × 106 h−1 and 0.0009 s, respectively. The instantaneous Hg0 removal efficiency ηi was defined by eq 1. The accumulation Hg0 removal efficiency ηa during the 120 min exposure time was calculated by integrating the real-time data of the Hg0 concentration over the time, which could be defined by eq 2 to evaluate the Hg0 removal performance.

2. EXPERIMENTAL SECTION 2.1. Preparation and Characterization of Adsorbent. 1-Butyl3-methylimidazolium tetrachloroferrate [C4mim][FeCl4] ionic liquid was synthesized following the method provided by Wang.28 1H NMR and elemental analysis were performed to characterize the purity of the synthesized IL. The 1H NMR spectroscopy of the synthesized [C 4 mim][FeCl 4 ] was collected on a Bruker AV400 NMR spectrometer using DMSO as a solvent. The result in Figure 1 is in consistent with Kim et al.’s result.29 The spectrum displayed a series of broad peaks associated with the imidazolium ring. Cruz et al. have found that [C4mim][FeCl4] is paramagnetic and the magnetic moment value is 5.8 μB/Fe.30 ILs with paramagnetic properties would disturb the magnetic field, resulting in broad peaks and difficulty in shimming. The 1H NMR results indicated that the synthesized IL was pure without any impurity. The C, H, and N contents of [C4mim][FeCl4], measured by a Vario MAX cube elemental analyzer from Elementar Analysensysteme GmbH, are shown in Table 1. The experimental values were in good agreement with the theoretical values, confirming the high purity of the synthesized IL. In addition, the molecular structure of the prepared IL was characterized by Fourier transform infrared (FTIR) spectroscopy and

⎛ C⎞ ηi = ⎜1 − ⎟ × 100% C0 ⎠ ⎝ 715

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Figure 2. Schematic diagram of the Hg0 dynamic absorption apparatus. t ⎛ ∫t 2 C dt ⎞⎟ ⎜ 1 ηa = ⎜1 − t ⎟ × 100% ⎜ ∫t 2 C0 dt ⎟⎠ ⎝ 1

(2)

0

where C0 refers to the initial Hg concentration when the simulated gas enters the bypass and C represents the Hg0 concentration of the outlet stream after adsorption in the fixed bed reactor. 2.3. TPD Experiment. The TPD experiments were performed to measure the thermal stability of the adsorbed mercury compounds formed on the adsorbents. Based on the different decomposition temperatures, the mercury species formed on the adsorbent can be identified. Before each TPD test, 0.1 g of [C4mim][FeCl4]−SiO2-30% adsorbent was treated with Hg0-containing N2 gas stream at 160 °C. The Hg-exposed SILP adsorbent was purged with pure N2 at room temperature until the Hg0 concentration fell to an insignificant level, in order to avoid the influence of physically adsorbed Hg0. Then the sample was heated from room temperature to 500 °C with a heating rate of 5 °C/min. The mercury compounds can decompose to Hg0 at high temperatures. The concentration of the desorbed Hg0 during the TPD run was monitored in real time.

Figure 4. FTIR spectra of [C4mim][FeCl4] and [C4mim][FeCl4]− SiO2-30%.

30%. In Figure 4a, the bands at 3153, 1572, 1462, 1165, 833, and 744 cm−1 were associated with different vibrational modes of the imidazole ring in the [C4mim]+ cation.23 In addition the C−H stretching frequencies at 2962 and 2871 cm−1 were also observed, which belonged to the butyl group attached to the imidazole ring.32 Thus, the Raman and FTIR spectra confirmed the structure of the prepared [C4mim][FeCl4]. In Figure 4b, a strong characteristic band at 1102 cm−1 was observed, which was assigned to the stretching vibrations of Si−O−Si of SiO2.33 In addition, the above absorption bands for the [C4mim]+ cation were also present in the FTIR spectrum of the [C4mim][FeCl4]−SiO2-30% adsorbent, which demonstrated that the Fe-containing IL was physically coated on the silica without any structural changes. The low intensity of the peaks for the [C4mim]+ cation was due to the relatively low concentration of [C4mim][FeCl4] compared with SiO2. A new hydroxyl group of H2O at approximately 3441 cm−1 appeared in the FTIR spectrum of [C4mim][FeCl4]−SiO230% because of the hygroscopic property of the adsorbent. 3.1.2. Pore Structure Characteristics. The pore structure was identified as one of the most important factors affecting adsorption performance. The pore structure characteristics of SILP adsorbents with various [C4mim][FeCl4] loadings can be seen in Figures 5 and 6. SiO2 displayed a high specific surface area and a high pore volume of 394.6 m2/g and 0.842 cm3/g, respectively. It was obvious that the pore size distribution of the adsorbent decreased with the loading of ionic liquid (Figure 5).

3. RESULTS AND DISCUSSION 3.1. Characterization of Adsorbent. 3.1.1. Raman and FTIR Characterization. The Raman spectrum of Fe-containing ionic liquid [C4mim][FeCl4] is presented in Figure 3. The strong absorption peaks at 328 and 106 cm−1 were assigned to the [FeCl4]− anion, corresponding well with the values for [FeCl4]− reported by Wang28 and Sitze.31 Figure 4 shows the FTIR spectra of [C4mim][FeCl4] and [C4mim][FeCl4]−SiO2-

Figure 3. Raman spectrum of [C4mim][FeCl4]. 716

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Thus, 350 °C was the upper limit for the operation temperature for [C4mim][FeCl4]−SiO2. When the temperature was increased to 600 °C, the [C4mim][FeCl4]−SiO2 particles had increasing weight loss with increasing loading of ILs. It should be noted that the weight loss was not equal to the ionic liquid loadings since [C4mim][FeCl4] was not completely decomposed at 600 °C. Neat [C4mim][FeCl4] had 35% residual weight left at 600 °C. Assuming a weight loss of only 65% at 600 °C, the samples with loadings of 10, 30, and 50% should have theoretical weight losses of 6.5, 19.5, and 32.5%, respectively, which were close to the actual values. Compared with the IL mixtures, the SILP adsorbents have good thermal stability and a wide operating temperature range, which can be applied to remove mercury from coal-fired flue gas. 3.2. Hg0 Removal Performance. The dynamic Hg0 removal performance of 0.1 g of bare SiO2, 10 g of neat [C4mim][FeCl4], and 0.1 g of SILP adsorbent at 160 °C in 120 min was investigated, as presented in Figure 8. Bare SiO2

Figure 5. Pore size distribution of [C4mim][FeCl4]−SiO2.

Figure 6. Surface area and pore volume of [C4mim][FeCl4]−SiO2.

The specific surface area and pore volume of [C4mim][FeCl4]−SiO2 also decreased with increasing ionic liquid loadings (Figure 6). The pore volume and surface area data demonstrates that ionic liquid was loaded into the pore channels of SiO2. One thing to note is that the pores of the SILP adsorbent with 50% loading were fully blocked with [C4mim][FeCl4]. The theoretical maximum loading of the SILP adsorbent was calculated to be approximately 45%, which was obtained from the density of [C4mim][FeCl4] and the pore volume of SiO2. 3.1.3. Thermal Stabilities. The thermal stabilities of the prepared samples [C4mim][FeCl4]−SiO2 with different loadings were tested by TGA. It can be seen in Figure 7 that these ionic liquid impregnated particles showed high thermal stabilities and started to decompose at approximately 350 °C.

Figure 8. Breakthrough curve of Hg0 with [C4mim][FeCl4]−SiO230% and SiO2.

displayed negligible removal of Hg0. This is because Hg0 adsorption on bare SiO2 was a weak physical adsorption process. Neat [C4mim][FeCl4] presented Hg0 removal efficiency of 78.6%. In comparison with Cheng’s work,23 the Hg0 removal efficiency was lower. This is because the usage of IL (10 g of IL) and gas flow rate (400 mL/min) in our work was less than that in Cheng’s work (40 g of IL/25 g of H2O, 800 mL/min). It is interesting to note that the SILP adsorbent demonstrated highly effective Hg0 capture in comparison with neat [C4mim][FeCl4]. Fe-containing IL with a large gas−liquid interfacial area contributed the most to Hg0 removal performance. It should be noted that the dynamic Hg0 removal experiment was conducted only for 120 min and the Hg0containing gas transferred to the bypass after 120 min. It could be predicted that breakthrough for the [C4mim][FeCl4]−SiO2 adsorbent would have required a much longer time. A summary of Hg0 removal performance by different types of IL-containing sorbents is given in Table 2. Pure ILs showed unsatisfactory Hg0 removal performance due to the weak physical interaction with Hg0.22 Although IL-based mixtures can achieve a high Hg0 removal efficiency, the large consumption of the sorbents would greatly reduce their competitiveness.21−23 Since the flue gas temperature is approximately 120−160 °C, the absorption temperature of IL-based mixtures is relatively low. In comparison, SILP adsorbents with much less ILs could achieve high Hg0 removal efficiency due to the large gas−liquid interface which increases

Figure 7. TGA curves of [C4mim][FeCl4]−SiO2. 717

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Energy & Fuels Table 2. Summation of Hg0 Removal Efficiency by IL-Containing Sorbents sorbent pure ILs

IL-based mixtures

SILP

TBAC [C4mim]CF3SO3 [C4mim]Cl TBAC/H2O2 [C4mim]CF3SO3/H2O2 [C4mim]Cl/H2O2 [C4mim]Cl/I2 [C4mim]Br/I2 [C4mim]Cl/H2O [C4mim]Br/H2O [C4mim][FeBrCl3]/H2O [C4mim][FeCl4]/H2O [C4mim][FeCl4]−SiO2-30%

usage

temp (°C)

efficiency (%)

ref

30 g 30 g 30 g 15 g of IL/15 g of H2O2 15 g of IL/15 g of H2O2 15 g of IL/15 g of H2O2 2.9 g of IL/(2.2−9.4 mg of I2) 2.6 g of IL/(2.2−9.4 mg of I2) 40 g of IL/25 g of H2O 40 g of IL/25 g of H2O 40 g of IL/25 g of H2O 40 g of IL/25 g of H2O 0.1 g

50 50 50 50 50 50 25 25 80 80 80 80 160

2.3 0.5 1.1 45.1 25.3 95.3 90.5−99.8 79.9−99.8 8 8 92 95.5 97.6

22 22 22 22 22 22 21 21 23 23 23 23 this work

the absorption rate and efficiency. Moreover, the high thermal stability of SILP adsorbents resulted in a wide operating temperature range close to the flue gas temperature. It can be concluded that the SILP sorbents have the advantages of a high Hg0 removal efficiency, low sorbent consumption, and an appropriate absorption temperature, thus lowering the equipment footprint and cost. 3.2.1. Effect of IL Loadings. Generally, mercury adsorption on porous materials is affected by many factors, such as the surface area, surface chemistry, etc. It can be seen from the results of the pore structure characterization that [C4mim][FeCl4] loadings had a great influence on the surface area of [C4mim][FeCl4]−SiO2 adsorbent. The effect of ionic liquid [C4mim][FeCl4] loadings on the Hg0 removal efficiency at 160 °C is presented in Figure 9. [C4mim][FeCl4]−SiO2 with a 10%

capture, the addition of [C4mim][FeCl4] on SiO2 in a certain range enhanced the Hg0 removal performance. Increasing the [C4mim][FeCl4] loading to 30% resulted in a high and stable Hg0 removal efficiency of 97.6%. However, the removal efficiency of [C4mim][FeCl4]−SiO2 adsorbent decreased when the loading was increased to 50%, which resulted from the decreased porosity at high loadings. Ionic liquid blocking the pore channel limited the mass transfer between Hg0 and [C4mim][FeCl4]−SiO2. Consequently, 30% is deemed to be the appropriate ionic liquid loading. 3.2.2. Effect of Reaction Temperature. The Hg0 removal performance using 0.1 g of [C4mim][FeCl4]−SiO2-30% adsorbent was studied as a function of temperature over the range 80−160 °C. As shown in Figure 10, the accumulation

Figure 10. Effect of reaction temperature on Hg0 removal efficiency. Figure 9. Effect of [C4mim][FeCl4] loadings on Hg0 removal efficiency.

Hg0 removal efficiency increased with the increasing temperature. It is obvious that Hg0 adsorption was favored at higher temperatures, revealing that Hg0 removal by the adsorbent was an endothermic process. Although low temperature suppressed the Hg removal activities, the efficiency remained greater than 90%. The results demonstrated that the [C4mim][FeCl4]−SiO2 adsorbent was effective over a wide range of temperatures. It is well accepted that Hg0 capture from flue gas by a sorbent can occur via the amalgamation, absorption, and adsorption (physisorption and/or chemisorption) processes.7 Among them, amalgamation and physisorption are processes that benefit from lower temperature. For virgin activated carbon, increasing the temperature had a negative effect on Hg0 removal since the physically adsorbed Hg0 could be easily desorbed at high temperature.34 In contrast, the chemisorption and chemical reaction processes perform better at higher

loading could capture Hg0 effectively at the beginning of the exposure, which could be attributed to the high efficiency of the gas−liquid mass transfer. The results of pore structure characterization showed that [C4mim][FeCl4]−SiO2-10% still retained high specific surface area and appreciable porosity, allowing the ILs to fully contact and interact with the Hg0containing flue gas. However, the instantaneous removal efficiency significantly decreased from 98.3 to 87.6% after 120 min. For the same weight of adsorbent, [C4mim][FeCl4]− SiO2-10% contains only 10% ILs, meaning there are fewer active adsorption sites for Hg0. Thus, the instantaneous removal efficiency of [C4mim][FeCl4]−SiO2-10% declined rapidly with time. Since the Fe-containing ILs contributed the most to Hg0 718

DOI: 10.1021/acs.energyfuels.6b01956 Energy Fuels 2017, 31, 714−723

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Energy & Fuels temperature. Therefore, it could be inferred that there is a chemical reaction between [C4mim][FeCl4]−SiO2 and Hg0. 3.2.3. Effect of Flue Gas Components. The effect of flue gas components including O2, SO2, NO, and H2O on the Hg0 removal performance by [C4mim][FeCl4]−SiO2-30% was investigated at 160 °C. Figure 11 presents the effect of O2 on

Figure 13. Effect of NO on mercury removal by [C4mim][FeCl4]− SiO2-30%.

Hg0 removal, 1000 ppm SO2 and 500 ppm NO were intermittently added into the baseline gases 6% O2/N2, as presented in Figure 14. At first, a Hg0 removal efficiency of 98% Figure 11. Effect of O2 on mercury removal by [C4mim][FeCl4]− SiO2-30%.

mercury removal by [C4mim][FeCl4]−SiO2-30%. Without the presence of O2, the accumulation Hg0 removal efficiency was approximately 90.3%. With the addition of 3 and 6% O2, the accumulation Hg0 removal efficiency was significantly improved to 95.5 and 97.6%, respectively. Therefore, the presence of O2 in flue gas was beneficial for Hg0 removal. The effect of SO2 on mercury removal by [C4mim][FeCl4]− SiO2-30% is shown in Figure 12. It is obvious that Hg0 capture Figure 14. Effect of SO2 and NO on Hg0 removal.

was achieved for the baseline gases 6% O2/N2. Adding 1000 ppm SO2 adversely affected the Hg0 removal efficiency, with the accumulation efficiency decreasing to 89%. Subsequently, it is interesting to note that the Hg0 removal efficiency recovered to the original level when SO2 was removed from the feed. Yang and co-workers36 studied the effect of SO2 on Hg0 removal by CuCl2 modified magnetospheres catalyst from fly ash and found that the activity of the catalyst could not be returned to the original level when the addition of SO2 was shut off. They suggested that the chemical interaction of SO2 with the catalyst resulted in changes in the chemical characteristics of the catalyst surface and partial deactivation of the Hg0 removal activity. In our cases, there is only physical interaction between SO2 and the adsorbent. It can be seen in Figure 15 that the adsorbent showed unsatisfactory SO2 removal performance during the mercury capture process, indicating a physical adsorption process. The SO2 could be easily and completely desorbed under a purge of the baseline gases O2/N2. Thus, the active adsorption site occupied by SO2 could be regenerated and the ability for Hg0 removal could be recovered. In addition, adding 500 ppm NO hardly affected the Hg0 removal performance, suggesting that NO did not cause any obvious changes in the surface chemistry of the adsorbent. Finally, the adsorbent maintained the original high and stable Hg0 removal efficiency when the gas stream was switched to the baseline feed of 6% O2/N2. Therefore, the presence of SO2 influenced Hg0 removal the most and the effect of NO was minimal.

Figure 12. Effect of SO2 on mercury removal by [C4mim][FeCl4]− SiO2-30%.

was greatly affected by SO2. The accumulation Hg0 removal efficiency decreased with the increasing SO2 concentration. The detrimental effect of SO2 may result from the competition between SO2 and Hg0. SO2 occupied some of the adsorption active sites, reducing the Hg0 removal efficiency. In addition, in the presence of 600 ppm SO2, the addition of 6% O2 enhanced Hg0 removal efficiency, which was consistent with the effect of O2 alone. In addition, Figure 13 shows the effect of NO on mercury removal by [C4mim][FeCl4]−SiO2-30%. Adding 500 ppm NO barely affected the performance of mercury removal, indicating that the adsorbent had excellent resistance to NO. It is reported that results obtained using an atmosphere without SO2 and NOx may be misleading for combustion systems.35 To deeply understand the effect of SO2 and NO on 719

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Figure 15. SO2 removal by [C4mim][FeCl4]−SiO2-30%.

Figure 17. UV−vis spectra of [C4mim][FeCl4] before and after Hg0 removal in acetonitrile.

H2O has been reported to present an inhibitive effect on Hg0 removal efficiency.37,38 In this study, the effect of H2O in flue gas on Hg0 removal performance was also measured. To avoid the interference of other flue gas components, only N2 was used in the gas stream. Before entering the fixed bed reactor, N2 was bubbled through a water bubbler at the constant temperatures of 45 and 60 °C, providing about 9.6 and 19.9 vol % H2O into the gas stream. The water content was calculated based on the saturated vapor pressure, which is obtained from NIST Refprop software. The gas pipes were wrapped with heating belts to prevent vapor condensation. The procedure for Hg0 removal is similar to that without water in the flue gas. The effect of H2O on Hg0 removal performance is presented in Figure 16. It can

the resulting liquid increased with the absorption time. Thus, the most likely products should be Cl-containing inorganic compounds, HgCl2, and [C4mim][FeCl3]. Since HgCl2 is regarded as the main mercury compound formed on the adsorbent, the TPD experiment was performed to confirm the mercury species. The TPD technique is a useful method for the clarification of the characteristics of surface mercury species and mercury compounds.41 During the temperature-programmed process, HgCl2 was evaporated, came into contact and interacted with the diluents (SiO2), and then decomposed to Hg0.42 The TPD results of the Hg0 treated adsorbents under the atmospheres of N2 are given in Figure 18. A large and wide

Figure 18. TPD curves of Hg0 treated adsorbents.

Figure 16. Effect of H2O on mercury removal by [C4mim][FeCl4]− SiO2-30%.

decomposition peak appeared with the peak temperature of approximately 390 °C, indicating that there was a new mercury compound formed on the adsorbent. The TPD curve of the Hg0 treated adsorbents with the adsorption time of 2 h showed a higher peak and larger integral area than that of 1 h. This was because the concentration of the mercury compound increased with longer adsorption time. Considering all the elements in the adsorbent, the mercury compound corresponding to the decomposition peak at 390 °C was likely to be HgCl2. To confirm the HgCl2 product, the decomposition behavior of pure HgCl2 mixed with [C4mim][FeCl4] and SiO2 was investigated by the TPD experiment. For pure HgCl2 mixed with [C4mim][FeCl4] and SiO2, HgCl2 was evaporated to Hg0 with a high and wide decomposition peak at about 380 °C (Figure 19). The decomposition peak at 390 °C of the SILP adsorbents after Hg0 adsorption appeared to be consistent with the decomposition peak at 380 °C of pure HgCl2, meaning the presence of HgCl2 in the SILP adsorbents after Hg0 adsorption. Therefore, it could be inferred that the mercury compound formed on the adsorbent should be HgCl2.

be seen that H2O exhibited a negative influence on Hg0 removal over the SILP adsorbents. When 9.6 and 19.9 vol % H2O were introduced to the gas stream, the accumulation removal efficiency decreased to 83.4 and 79.7%, lower than that under the atmosphere without H2O. The detrimental effect may be due to the competitive adsorption of H2O on active sites. Occupation of active sites hindered mercury adsorption. 3.3. Reaction Mechanism. In the above study, we explored the effects of the ionic liquid loading, reaction temperature, and flue gas components on Hg0 removal performance, and speculated on the most possible reaction mechanism, namely the oxidation of Hg0 by [C4mim][FeCl4]. UV−vis spectra of two samples, [C4mim][FeCl4] before and after Hg0 removal, are presented in Figure 17. The observed bands at 360, 311, and 240 nm are assigned to Fe−Cl in anion.39,40 The spectrum of fresh IL is similar to that of IL after Hg0 removal, indicating that Hg was not involved in the formation of IL anion. Cheng23 also suggested that the IL anion was converted to [Fe2+Cl3]− since the concentration of Fe2+ in 720

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Article

Energy & Fuels

[C4mim][FeCl4] was converted to [C4mim][FeCl3] during the oxidation of Hg0. In the presence of O2 in the simulated gas, [C4mim][FeCl3] could be partly oxidized and regenerated to [C4mim][FeCl4].23 Thus, the regeneration of the SILP adsorbent could be realized. It is proposed that the SILP adsorbent could be regenerated by treating the products by pure O2. After Hg0 adsorption, the SILP adsorbent mixed with water is treated by pure O2 at high temperature for several hours. Then the recycled SILP adsorbent could be obtained by removing the water in the air-dry oven at 120 °C. Further studies are in progress to develop a better method for the regeneration of the SILP adsorbent. Based on reaction 4, the reaction enthalpy was calculated through the Gaussian 09 program.43 Geometry optimization and vibrational frequency calculations of the reactants and products were carried out at the B3PW91/GENECP level. The detailed results are presented in the Supporting Information. The metallic (Hg, Fe) and nonmetallic (C, H, N, Cl) elements were calculated using the basis sets of CEP-31G and 6-311+ +G(d,p), respectively. The enthalpies of these molecules and the reaction enthalpy are shown in Table 3. Interestingly, the positive reaction enthalpy of 94.7 kJ/mol confirmed the endothermic process, which agrees with the observed effect of temperature. High temperature promoted the oxidation process of Hg0. Since the [C4mim][FeCl4]−SiO2 adsorbent exhibited high thermal stability up to 350 °C, high flue gas temperature was no longer the limiting factor when the adsorbent was applied to capture mercury from a coal-fired power plants. Using one of the several mercury oxidation enhancement additives such as the bromine, chlorine, and sulfur species, previously known adsorbents achieved a high adsorption performance at low temperature ( 120 °C > 80 °C. Although low temperature suppressed Hg0 removal activities, the adsorbent still achieved a removal efficiency greater than 90%, which indicates that the adsorbent was effective over a wide range of temperatures. 3. Flue gas components greatly affected the Hg0 removal. The presence of O2 in flue gas was beneficial to Hg0 removal, possibly because O2 could regenerate the active adsorption sites. The presence of SO2 negatively influenced Hg0 removal, and the effect of NO was minimal. H2O showed an inhibitive effect on Hg0 removal efficiency due to the competitive adsorption of H2O on active sites. 4. TPD results indicated that the mercury compound formed on the adsorbent should be HgCl2 . Hg0 removal by [C4mim][FeCl4]−SiO2 could be attributed to a combined physisorption and chemisorption. Hg0 was chemically adsorbed and converted to HgCl2 when reacting with the main active oxidant [C4mim][FeCl4]. Then, the product HgCl2 was physically adsorbed on the porous adsorbent.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b01956. Optimized structures of [C4mim][FeCl4] and [C4mim][FeCl3], Cartesian coordinates of optimized geometries, and vibrational frequency calculations for the reactants and products (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86 27 87542417. Fax: +86 27 87545526. ORCID

Liqi Zhang: 0000-0002-6421-680X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the National Natural Science Foundation of China (51076056), the International Science & Technology Cooperation Program of China (2016YFE0102500), and the Foundation of State Key Laboratory of Coal Combustion. The authors also acknowledge the extended help from the Analytical and Testing Center of Huazhong University of Science and Technology.

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DOI: 10.1021/acs.energyfuels.6b01956 Energy Fuels 2017, 31, 714−723