Removal of Elemental Mercury from Coal Pyrolysis Gas Using Fe–Ce

In this paper, Fe–Ce mixed oxides supported on lignite semi-coke were prepared by the hydrothermal impregnation method. The sorbents were employed t...
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Removal of Elemental Mercury from Coal Pyrolysis Gas Using Fe−Ce Oxides Supported on Lignite Semi-coke Modified by the Hydrothermal Impregnation Method Xiaoyang Zhang, Yong Dong,* Lin Cui, Donghai An, and Yuanyang Feng

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National Engineering Laboratory for Coal-Fired Pollutants Emission Reduction, Shandong University, Jinan, Shandong 250061, People’s Republic of China ABSTRACT: In this paper, Fe−Ce mixed oxides supported on lignite semi-coke were prepared by the hydrothermal impregnation method. The sorbents were employed to capture elemental mercury from coal pyrolysis gas. The influences of hydrothermal impregnation temperature, mole ratio of Fe/Ce, reaction temperature, and H2S concentration on Hg0 removal efficiency were investigated in a fixed-bed reactor. The physicochemical properties of the sorbent samples were characterized by X-ray diffraction, Brunauer−Emmett−Teller, scanning electron microscopy, and X-ray photoelectron spectroscopy (XPS). The results showed that the Fe/Ce-modified semi-coke sorbents, synthesized by hydrothermal impregnated at 200 °C, had larger specific surface areas and pore volumes than the original semi-coke. The sorbent with a Fe/Ce molar ratio of 0.4:0.2 exhibited the highest Hg0 removal efficiency of 83.5% at 150 °C. On the basis of the XPS characterization of the sorbent, a mechanism of Hg0 removal over the Fe0.4Ce0.2/SC200 sorbent was proposed, suggesting that H2S could react with Fe/Ce mixed to form active sulfur, with which Hg0 could react to form HgS. In addition, the stability and regeneration of the sorbent were also investigated. the-art technologies.18−20 Figure 1 present a schematic diagram of pre-combustion mercury control.21 In pre-

1. INTRODUCTION Mercury emissions from a variety of emission sources have been of great concern as a result of their severe air toxicity, long distance air transport, and great exposure risks to both humans and ecosystems,1 with coal-fired power plants having become the major emission sources in many countries around the world.2 In recent decades, coal has become the main energy supply in China, accounting for 25−40% of global anthropogenic Hg emissions annually.3 The latest emission standard of air pollutants, announced by the Chinese government in 2014, has set up the strictest ever Hg0 emission limits for thermal power plants, highlighting the great concern over mercury emissions and the need to develop cost-effective mercury control technologies. Typically, there are three forms of Hg species in coal-fired flue gas streams, including oxidized mercury (Hg 2+ ), particulate bound mercury (Hgp), and gaseous elemental mercury (Hg0). Hg2+ and Hgp can be easily captured by conventional air pollution control devices [e.g., wet flue gas desulfurization (WFGD), electrostatic precipitator (ESP), and fabric filter (FF)]. However, Hg0 in the gaseous phase is difficult to capture by the existing control devices because of its high volatility and low solubility.4 Many control methods have been investigated, such as adsorption,5−8 catalytic conversion,9−12 plasma,13−15 photocatalysis,16 etc. Among these control methods, activated carbon (AC) injection has been considered as being the most effective technology for the removal of Hg0 emissions from coal-fired flue gas. However, the high operational cost has limited its application.17 As a result, new mercury control technologies have continuously been investigated, with pre-combustion mercury control being considered as a viable technology having the potential to reduce the cost of mercury control compared to the state-of© XXXX American Chemical Society

Figure 1. Pyrolysis before combustion for Hg removal from raw coal.

combustion mercury removal, coal was first pyrolyzed or partially gasified to yield pyrolysis char and combustible gaseous fuel and mercury released into the gaseous phase was then removed before the combustion of the char and gaseous fuels. It was reported that over 90% of mercury present in raw coal can be released into the gaseous phase during pyrolysis or gasification, with elemental mercury accounting for about 60% of total mercury.21,22 In comparison to coal-fired flue gas, the fuel gas streams generated from coal pyrolysis or gasification are usually enriched with higher element mercury concentrations, giving rise to higher removal efficiency and lower operational cost. However, much less work has been performed on pre-combustion mercury removal and related Received: July 10, 2018 Revised: November 25, 2018 Published: November 28, 2018 A

DOI: 10.1021/acs.energyfuels.8b02358 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Proximate and Ultimate Analyses of the Semi-coke Sample proximate analysis (wt %)

ultimate analysis (wt %)

Mad

Aad

Vdaf

FCdaf

Cdaf

Hdaf

Odafa

Ndaf

St,daf

6.41

7.62

30.46

69.54

79.37

3.86

15.44

0.95

0.38

a

By difference.

Figure 2. Schematic of the Fe/Ce−SC sorbent preparation via hydrothermal impregnation.

Figure 3. Schematic diagram of the experimental system.

Hou et al.29 reported that Ce/Ti sorbents exhibit higher Hg0 removal efficiency compared to S-impregnated AC. Ma et al.30 investigated the performance of Fe−Ce mixed oxide nanoparticles for Hg0 removal from coal-fired flue gases. In this investigation, the performance of Fe−Ce hybrid oxide sorbents supported on lignite semi-coke, which were synthesized by a hydrothermal impregnation method, was investigated for mercury removal from simulated coal pyrolysis product gas streams and the effect of hydrothermal impregnation conditions, Fe/Ce molar ratios, adsorption temperatures, and H2S concentrations on mercury removal were investigated. To help reveal the mercury retention mechanisms, the physical and chemical properties of the prepared sorbent materials were also characterized by a range of tools, such as X-ray diffraction (XRD), Brunauer−Emmett− Teller (BET), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS).

capture mechanisms compared to post-combustion mercury control, as aforementioned, which forms the aim of this investigation. As is well-known, some disadvantages of AC adsorption have impeded further development of this technology. Therefore, developing more economic and effective alternative sorbents is of important scientific and practical significance. Lignite semicoke is a pyrolytic product of lignite at medium or low temperature (600−800 °C), highly abundant, and more costeffective than AC in China. With good mechanical strength, a large number of surface functional groups, and microporosity, it can be considered as an ideal catalyst or sorbent carrier. However, lignite semi-coke almost has poor catalytic and adsorption ability for Hg0. Physicochemical modification is a simple and effective way to improve the pore structure and mercury adsorption capacity. Iron-based sorbents have been widely used to capture element mercury from coal-derived fuel gases. Wu et al.23,24 reported that iron oxide sorbents had good performance in mercury removal at temperatures varying from 65 to 100 °C. It was found that H2S plays an important role when an ironoxide-based sorbent was used for mercury adsorption. Wang et al.25 indicated that H2S can greatly enhance mercury removal of iron-based sorbents, with its effect determined by the concentrations of H2S present in the systems. Cerium-based catalysts/sorbents were well-known for their excellent high oxygen storage capacity and abundant oxygen vacancies.26,27 Zhou et al.28 found that CeO2−TiO2 exhibited excellent catalytic performance for element mercury from syngas, while

2. EXPERIMENTAL SECTION 2.1. Preparation of Sorbents. Lignite semi-coke, produced in Yulin of Shaanxi Province, China, was obtained as the substrate for sorbent preparation. The semi-coke was first ground and sieved to yield the fraction of 250−380 μm for use in sorbent preparation. The proximate and ultimate analyses of the semi-coke sample were shown in Table 1. The lignite semi-coke was named SC. In this investigation, a hydrothermal impregnation methodology, as outlined in Figure 2, was used for the Fe−Ce sorbent preparation. In each preparation, calculated amounts of Fe(NO3)3·9H2O and Ce(NO3)3·6H2O were first dissolved in 50 mL of deionized water under stirring conditions. Then, 10 mL of SC was added to the prepared Fe(NO3)3 and B

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Cin and Cout were the Hg0 concentrations in the feed and effluent gases (μg/m3), respectively.

Ce(NO3)3 solution under stirring conditions, and the mixture was further stirred for 1 h before it was transferred to a Teflon-lined stainless-steel autoclave. The autoclave was then heated to different selected impregnation temperatures for 5 h before it was allowed to cool to ambient conditions. The prepared raw sorbent samples were then separated by filtration, washed by deionized water, and dried at 105 °C for 12 h. Finally, the sorbent samples were calcined at 500 °C for 5 h under a nitrogen atmosphere to obtain the final sorbents for testing. The samples were designated as FexCey/SCt, where x and y represented the molar concentrations (mol/L) of ferric nitrate and cerium nitrate in the precursor solution and t represented the temperature of hydrothermal impregnation, for example, Fe0.4Ce0.2/ SC200. The Fe/Ce molar concentration (mol/L) ratios of the precursor solutions varied from 0.6:0.2 to 0.1:0.2, while the hydrothermal impregnation temperatures investigated ranged from 180 to 210 °C, which gave rise to impregnation pressures from 9.89 to 18.82 atm. The self-generated hydrothermal pressure was regarded as the supersaturated water vapor pressure, which was found to be consistent with the calculated pressure by the Antoine equation.31,32 2.2. Characterization of the Sorbents. The pore structure was characterized by the physical adsorption of N2 at 77 K using an automatic surface analyzer (ASAP 2460, Micromeritics, Norcross, GA, U.S.A.). The XRD measurements were carried out with a D8 powder diffractometer (Bruker, Germany) to examine the crystallinity and dispersity of iron and cerium species on the SC carriers using Cu Kα radiation in the range of 10−80° (2θ) with a step size of 0.02°. The surface morphologies of the sorbents were obtained using fieldemission scanning electron microscopy (ZEISS, Germany). The surface atomic states of the sorbents were analyzed by a Kα X-ray photoelectron spectrometer (Kratos, U.K.) with an Al Kα X-ray source at room temperature. The observed spectra were adjusted with the C 1s binding energy (BE) value of 284.6 eV. 2.3. Performance Characterization of Mercury Sorbent Materials. The performances of sorbents for Hg0 removal were evaluated in a fixed-bed system, as shown in Figure 3, which consisted of four major components, including the simulated coal pyrolysis gas stream, temperature control, fixed-bed reactor, and gas detection or monitoring system. The simulated coal pyrolysis gas system was composed of four gas cylinders (N2, CO, and H2, all with a purity of 99.999%, and 0.2% H2S in N2), five mass flowmeters, a gas mixing chamber, and a Hg0 vapor permeation device. The temperature control system consisted of a thermostat drying oven and a heating tape to maintain a constant temperature. The fixed-bed reactor included a quartz reactor (inner diameter, 10 mm; length, 400 mm; quartz glass) and a silicone cover. The gas detection system consisted of a gas mercury analyzer [Hg continuous emission monitoring system (CEMS), Thermo Fisher Scientific] and a H2S analyzer (GC 7820, Lunan Analytical Instrument Co., Ltd.). 2.4. Mercury Adsorption Tests. In each experiment, the fixedbed reactor was first loaded with about 600 mg of selected sorbent (ca. 1.0 mL in volume) and the sample was placed vertically in a thermostatic oven with temperature control. All pipes, connecting joints, and containers were constructed of Teflon to reduce the adsorption of mercury on the surface of other materials. The flow rate controlled by flowmeters was set at 1000 mL/min, corresponding to a gas hourly space velocity (GHSV) of 60 000 h−1. The Hg0-permeating tube (VICI Metronics, Poulsbo, WA, U.S.A.) was settled to produce a constant release of 70 μg/m3 Hg0 vapor. The Hg0 concentration in the inlet and outlet of the reactor was monitored by CEMS. The duration time of each adsorption test was 150 min, and H2S was introduced into the reaction system after the experiment ran for 30 min. Each adsorption experiment was repeated 3 times, and their mean values were reported. The Hg0 removal efficiency (η) for each sorbent was defined as follows:

η (%) =

3. RESULTS AND DISCUSSION 3.1. Characterizations of the Sorbent Samples. 3.1.1. BET Analysis. The BET surface area, pore sizes, and pore volumes of samples were summarized in Table 2. SC has Table 2. Effect of the Hydrothermal Temperature on Structure Parameters of Samples sample

BET surface area (m2/g)

pore volume (cm3/g)

average pore diameter (nm)

SC SC/200 Fe0.4Ce0.2/SC180 Fe0.4Ce0.2/SC200 Fe0.4Ce0.2/SC210

35.54 81.41 271.77 279.56 271.42

0.024 0.065 0.136 0.145 0.141

2.80 2.61 2.01 2.07 2.07

the smallest BET surface area (35.54 m2/g) and the smallest pore volume (0.024 cm3/g) among all of the samples examined. The BET surface area and pore volume of semicoke increased after hydrothermal treatment. However, the BET surface area and pore volume of modified samples significantly increased after the hydrothermal impregnation. The results indicate that the SC substrate used appears to be somewhat activated under the hydrothermal conditions created during the hydrothermal impregnation treatment. Given the low hydrothermal temperature used, any activation reactions, such as gasification reactions of carbon with stream, were not expected to occur; therefore, the additional surface area and pore volume most likely resulted from the enhanced dissolution of the SC-contained minerals or other water solubles under the hydrothermal conditions. However, when the hydrothermal impregnation temperature reached 210 °C, the BET surface area and pore volume were found to decrease slightly. Thus, the optimal impregnation temperature was selected at 200 °C. The curves of the N2 adsorption−desorption isotherm and pore size distribution of samples were shown in panels a and b of Figure 4. The volume of N2 adsorbed of the SC substrate material was the lowest among the samples. The sorbents modified by hydrothermal impregnation had a much higher N2 adsorption volume, primarily because of the newly developed porosity as a result of the hydrothermal impregnation treatment. According to the International Union of Pure and Applied Chemistry (IUPAC) classification of absorption isotherms, the results indicated that the nitrogen adsorption−desorption isotherms of the modified samples belonged to type I isotherms. The micropores were the major pores of the sorbents. At the relative pressures below 0.05, N2 adsorption capacity showed a rapid growth trend. The limit adsorption capacity was contributed by the available micropore volume rather than the surface area.33 The pore size distribution curves of modified samples exhibited multimodal distributions, with the peaks centered at 0.6 and 1.2 nm. The Fe0.4Ce0.2/SC200 sorbent had the highest N2 adsorption capacity and the highest microporosity. 3.1.2. SEM Analysis. The SEM micrographs of SC, Fe0.4Ce0.2/SC180, Fe0.4Ce0.2/SC200, and Fe0.4Ce0.2/ SC210 samples were shown in Figure 5. It could be seen that SC had some protruding round particles on a smooth, non-porous, continuous film. However, other samples

Cin − Cout × 100 Cin C

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Figure 4. (a) N2 adsorption−desorption isotherm and (b) pore size distribution of SC, SC200, Fe0.4Ce0.2/SC180, Fe0.4Ce0.2/SC200, and Fe0.4Ce0.2/SC210.

Figure 5. SEM images of (a) SC, (b) Fe0.4Ce0.2/SC180, (c) Fe0.4Ce0.2/SC200, and (d) Fe0.4Ce0.2/SC210.

contained many small gaps and round particles, resulting from mixed Fe−Ce oxides. In comparison to Fe0.4Ce0.2/SC180 and Fe0.4Ce0.2/SC210, the Fe0.4Ce0.2/SC200 sample showed more wrinkles and higher microporosity, accounting for the increase of the BET surface area shown in Table 2. 3.1.3. XRD Analysis. The XRD characterization was employed to determine the crystal structure of the sorbent samples prepared. The powder XRD patterns of SC, Fe0.4Ce0.2/SC180, Fe0.4Ce0.2/SC200, and Fe0.4Ce0.2/ SC210 were shown in Figure 6a. In comparison to SC, both Fe2O3 and CeO2 were detected on the surface of the sorbents. It confirmed that Fe−Ce mixed oxides were successfully loaded onto SC. The XRD spectra of Fe0.2Ce0.2/SC200, Fe0.2Ce0.4/ SC200, Fe0.4Ce0.2/SC200, and Fe0.6Ce0.2/SC200 were shown in Figure 6b. All of the Fe−Ce mixed oxides had

exhibited diffraction peaks at 28.7°, 33.2°, 47.7°, and 56.5°, which were attributable to CeO2,28,29 whereas the diffraction peaks for Fe2O3 were detected at 35.7° and 62.4°. As shown in Figure 6b, as the iron/cerium mole ratio increased from 3:1 to 1:2, the intensity of the diffraction peaks of the active component of CeO2 was found to increase sharply, while the peak intensity of iron oxide decreased accordingly. 3.2. Hg0 Removal Performance. 3.2.1. Effect of the Hydrothermal Impregnation Temperature. To examine the effect of different impregnation temperatures on the performance of the sorbents for mercury removal, the mercury removal performances of different sorbent samples prepared at different impregnation temperatures were tested and the results were shown in Figure 7. The modification of Fe−Ce mixed oxides greatly increased the Hg0 removal capacity of SC. In the absence of H2S, elemental mercury removal efficiency was D

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Figure 6. XRD patterns of SC and sorbents prepared under different hydrothermal temperatures.

Figure 8. Effect of the Fe/Ce molar ratios in precursor solution on Hg0 removal efficiency. Evaluation conditions: H2 concentration, 10%; CO concentration, 20%; H2S concentration, 100 ppm; Hg0 concentration, 70 μg/m3; and reaction temperature, 150 °C.

Figure 7. Effect of the hydrothermal impregnation temperature on Hg0 removal efficiency. Evaluation conditions: H2 concentration, 10%; CO concentration, 20%; H2S concentration, 100 ppm; Hg0 concentration, 70 μg/m3; and reaction temperature, 150 °C.

from 1:2 to 2:1. Then, efficiency decrease from 83.5 to 71.3% when the Fe/Ce molar ratio further increased from 2:1 to 3:1. It was reported34 that iron oxide could react with H2S to produce active elemental sulfur, which could react with gaseous mercury to form HgS. In addition, there was a potential for Ce-based sorbents to be generated via a route that directly produces elemental sulfur, which was used to capture Hg0 in syngas.28 The Fe−Ce mixed oxide modified could provide new active sites on the surface of SC with the increase of loading ratios. However, an excessive loading value might have led to the blockage of the pore structure on the surface of SC. The experimental results indicated that the optimum ratio of Fe/Ce in the precursor was 0.4:0.2. 3.2.3. Effect of the Reaction Temperature on Hg0 Removal Efficiency. The Fe0.4Ce0.2/SC200 sorbent was selected to study the effect of the reaction temperature on Hg0 removal efficiency in the reaction temperature range of 120− 210 °C, and the results were illustrated in Figure 9. It could be seen that the average Hg0 removal efficiency increased as the gas temperature increased from 120 to 150 °C and then decreased significantly when the temperature further increased to 210 °C. At 150 °C, the Fe0.4Ce0.2/SC200 sorbent showed the highest Hg0 removal efficiency. The increase in the reaction temperature can promote the chemical reaction between Hg0,

greatly inhibited. The highest removal efficiencies of SC, SC/ 200, Fe0.4Ce0.2/SC180, Fe0.4Ce0.2/SC200, and Fe0.4Ce0.2/ SC210 were 20.25, 27.81, 38.9, 39.2, and 37.6%, respectively. When 100 ppm of H2S was introduced into the coal pyrolysis gas, the Hg0 removal efficiency increased rapidly. H2S significantly promoted mercury removal over the sorbent,29 except SC and SC/200. These beneficial Hg0 removal results were supported by the BET surface areas of the sorbents in Table 2 and the XRD analysis in Figure 6a. The pore structure provides the space to accommodate adsorbed mercury, while the active components of Fe2O3−CeO2 on the surface of sorbents provide the active sites for capturing and converting mercury. The Hg0 removal efficiency of the Fe0.4Ce0.2/SC200 sorbent remained above 83.5% for 85 min, which was much higher and more stable than the others. On the basis of the Hg0 removal performance of the sorbents, the Fe0.4Ce0.2/SC200 sorbent was chosen for subsequent research. 3.2.2. Effect of the Fe/Ce Molar Ratios. The effects of Fe/ Ce molar ratios on Hg0 removal efficiency were shown in Figure 8. The average Hg0 removal efficiency first increased from 70.6 to 83.5% with the increase of the Fe/Ce molar ratio E

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Figure 10. Effect of H2S concentrations on Hg0 removal efficiency. Evaluation conditions: H2 concentration, 10%; CO concentration, 20%; H2S concentration, 100/300/500 ppm; Hg0 concentration, 70 μg/m3; sorbent, Fe0.4Ce0.2/SC200; and reaction temperature, 150 °C.

Figure 9. Effect of the reaction temperature on Hg0 removal efficiency. Evaluation conditions: H2 concentration, 10%; CO concentration, 20%; H2S concentration, 100 ppm; Hg0 concentration, 70 μg/m3; and sorbent, Fe0.4Ce0.2/SC200.

mixed oxides, and H2S. However, the adsorption ability of the Fe0.4Ce0.2/SC200 sorbent decreased sharply at 180 and 210 °C. One major cause of this phenomenon might be the weakness of the bonding strength between Hg0 and active sulfur on the sorbent surface. Thermodynamic parameters for the reaction of sulfur and mercury were shown in Table 3. Another reason might be that active sulfur melted and was blown away by the pyrolysis gas at a high reaction temperature.35

panels a and b of Figure 11. There were two different oxygen peaks observed on the fresh and used sorbents. The binding energy peak around 529.5−530.0 eV was considered lattice oxygen (Oα).37 The binding energy peak around 531.0−531.6 eV was regarded as chemisorbed oxygen (Oβ).38 The ratio of Oα/Oβ was calculated and shown in Table 4. In comparison to the fresh sample, the content of Oα of the used sample increased from 42.15 to 52.65%. The Oα/Oβ ratio increased from 0.73 to 1.11. It was indicated that both Oα and Oβ participated in the mercury removal process. Part of active oxygen contributed to the formation of active surface sulfur, which was generated from H2S. According to the report of Zhou et al.,28 H2S was efficiency oxidized into active sulfur (S0) by active oxygen. Then, Hg0 could be easily captured by active sulfur (S0) to form HgS. Panels c and d of Figure 11 illustrated Fe 2p spectra of the Fe0.4Ce0.2/SC200 sorbent before and after mercury adsorption. Three peaks at 710.2, 711.5, and 719.5 eV were observed, which could be attributed to Fe2+ and Fe3+ in two different species.39 The results showed two mixed states of Fe2+ and Fe3+ existed in the sorbent. The iron-based sorbents could oxidize H2S into elemental sulfur selectively.24,25 However, as observed in Figure 11h, the peak of S 2p around 162.5 eV was the characteristic of S2− species.29 The results confirm that H2S was not only oxidized to elemental surfer but also reacted with iron oxide to produce FeS. The Ce 3d XPS spectra of the fresh and used sample were shown in panels e and f of Figure 11. They were fit with eight peaks, divided into two groups, Ce 3d3/2 states and Ce 3d5/2 states,40 marked as U and V. The peaks marked as U1 and V1 represented Ce3+, and the rest of the peaks represented Ce4+. In comparison to the fresh sorbent, the ratio of Ce4+/Ce3+ decreased from 1.86 to 1.24 in the sorbent after mercury adsorption. The results indicated that Ce4+ oxides were beneficial to the oxidation of H2S to S0, giving rise to the enhanced Hg0 removal efficiency. The XPS spectra of Hg 4f detected in the sorbent were shown in Figure 11g. The peak at 102.4 eV was considered as Si 2p,38 and the Hg 4f peak was observed at 101.8 eV. The S 2p XPS spectra was illustrated in Figure 11h, and it could be

Table 3. Thermodynamic Parameters for the Reaction of Sulfur and Mercury33 S(g) + Hg(g) → HgS(s) temperature (°C)

ΔG (kJ/mol)

K

120 160 200 240

−71.48 −66.84 −62.31 −58.10

3.130 × 109 1.146 × 108 7.55 × 106 6.38 × 105

3.2.4. Effect of the H2S Concentration on Hg0 Removal Efficiency. Zhang et al.36 have reported that the elemental mercury removal performance could be significantly improved in the presence of H2S. Figure 10 illustrated the effects of H2S concentrations on the Hg0 removal efficiency of the Fe0.4Ce0.2/SC200 sorbent. The results showed that the average Hg0 removal efficiency was maintained stable with the H2S concentration increasing from 100 to 300 ppm. As the H2S concentration further increased from 300 to 500 ppm, the average Hg0 removal slightly decreased from 79 to 70%. Excessive H2S could react with iron oxides on the surface of the sorbent to form thermally stable iron sulfides.24,25 The active sites might be covered by a large number of FeS, resulting in deactivation of the sorbent for mercury removal. 3.3. Analysis of the Mechanism of Hg0 Removal. To examine the mechanism of Hg0 removal, the chemical states of adsorbed elemental mercury were investigated by XPS. The XPS spectra of O 1s, Fe 2p, Ce 3d, Hg0 4f, and S 2p were shown in Figure 11. The XPS spectra of O 1s for the sorbent of Fe0.4Ce0.2/ SC200 before and after mercury adsorption were shown in F

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Figure 11. XPS spectra of fresh Fe0.4Ce0.2/SC200 and used Fe0.4Ce0.2/SC200 for O 1s, Fe 2p, Ce 3d, Hg 4f, and S 2p.

seen that two subpeaks were observed at 161.9 and 163.9 eV, corresponding to S2− and S0, respectively.29 The results indicated that Hg0 was captured by surface sulfur species. The above results tend to suggest the following possible Hg0 removal mechanism and pathways in the pyrolysis gas: G

H 2S(g) → H 2S(ads)

(1)

Hg 0(g) → Hg 0(ads)

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Energy & Fuels Table 4. Concentration of Different Oxygen in Fresh and Used Fe0.4Ce0.2/SC200 sample

Oα (%)

Oβ (%)

Oα/Oβ

fresh Fe0.4Ce0.2/SC200 used Fe0.4Ce0.2/SC200

42.15 52.65

57.85 43.35

0.73 1.11

Fe2O3 + H 2S → FeS + H 2O

(3)

Fe2O3 + H 2S → FeS + S0(ads) + H 2O

(4)

CeO2 ↔ Ce2O3 + O(ads)

(5)

H 2S(ads) + O(ads) → S0(ads) + H 2O

(6)

Hg 0(ads) + S0(ads) → HgS

(7) Figure 13. XRD patterns of fresh and used Fe0.4Ce0.2/SC200 sorbents.

3.4. Stability, Deactivation, and Regeneration of Sorbents. The stability test of the Fe0.4Ce0.2/SC200 sorbent for Hg0 removal was investigated, and the result was shown in Figure 12. The Hg0 removal efficiency decreased from 83.8 to

Table 5. Specific Surface Area, Pore Volume, and Pore Diameter of Fresh and Used Samples sorbent

BET surface area (m2/g)

pore volume (cm3/g)

pore dimeter (nm)

fresh Fe0.4Ce0.2/SC200 used Fe0.4Ce0.2/SC200

279.59 186.12

0.15 0.12

2.07 2.13

pore channels of the sorbent were partly blocked by solid particles (e.g., FeS, HgS, and/or S0). The decrease of the amount of active sites and the blocking of the pore channels by solid particles were considered to be the main causes of deactivation. On the basis of the analysis of deactivation, the regeneration of used Fe0.4Ce0.2/SC200 was carried out by heating the used sorbent at 400 °C for 30 min under N2 and 2% O2−N2 atmospheres, separately. The average Hg0 removal efficiency of Fe0.4Ce0.2/SC200 in 3 cycles was shown in Figure 14. It could be seen that the average mercury removal efficiency of regenerated sorbents decreased at different degrees. The weak Figure 12. Stability test for Hg0 removal efficiency. Evaluation conditions: H2 concentration, 10%; CO concentration, 20%; H2S concentration, 100 ppm; Hg0 concentration, 70 μg/m3; and reaction temperature, 150 °C.

72.5% during a 300 min continuous test. The results indicated that the sorbent achieved good chemisorption performance with a high level of mercury removal efficiency. It was necessary to analyze causes of deactivation of the sorbent and find measures for its regeneration. The XRD patterns and structural parameters of fresh and used Fe0.4Ce0.2/SC200 sorbents were shown in Figure 13 and Table 5. The XRD spectra of CeO2 on the used sorbent was similar to those on the fresh sorbent, which both showed the diffraction peaks of dispersed CeO2, and the spectra of CeO2 on the fresh sample showed sharper diffraction peaks. However, the peak (yield) of the active component Fe2O3 was higher on used Fe0.4Ce0.2/SC200 compared to the fresh Fe0.4Ce0.2/SC200 sorbent. This was because the diffraction peak of FeS coincided with that of Fe2O3. In addition, in comparison to the fresh sample, the BET surface area of the used sample decreased from 279.59 to 186.12 m2/g and the pore dimeter increased from 2.07 to 2.13 nm, indicating that

Figure 14. Regeneration performance of the sorbent for Hg0 removal efficiency. Conditions: H2 concentration, 10%; CO concentration, 20%; H2S concentration, 100 ppm; Hg0 concentration, 70 μg/m3; and reaction temperature, 150 °C. H

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oxides. The destruction of the pore structure and the decrease of the active sites were the major factors for deactivation of the sorbent. A weak oxidizing atmosphere was found to be more effective in rejuvenating the spent mercury sorbents.

oxidizing atmosphere was conductive to active adsorption site exposure and solid particle decomposition, which were covered by FeS, HgS, or S0. However, with the increase of the cycling time, the Hg0 removal efficiency decreased sharply. The crystalline structure and structural parameters of regenerated samples were shown in Figure 15 and Table 6.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-0531-88399372-602. E-mail: dongy@sdu. edu.cn. ORCID

Xiaoyang Zhang: 0000-0001-6546-5990 Yong Dong: 0000-0002-8530-3857 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Foundation of the State Key Laboratory of Coal Combustion (FSKLCCA1603) and the Key R&D Program of Shandong Province (2016CYJS10B02).

■

Figure 15. XRD patterns of fresh and regenerated samples.

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Table 6. Specific Surface Area, Pore Volume, and Pore Diameter of Regenerated Samples sorbent

BET surface area (m2/g)

pore volume (cm3/g)

pore dimeter (nm)

fresh Fe0.4Ce0.2/SC200 first regeneration second regeneration third regeneration

279.59 170.92 113.74 89.94

0.15 0.091 0.083 0.056

2.07 2.13 2.62 3.07

REFERENCES

With the increase of regeneration times, the diffraction peaks of metal oxide became wider and their intensity decreased obviously. The specific surface area of the sorbents decreased dramatically when the sorbents were regenerated after 3 times. It indicated that the destruction of the pore structure and the decrease of the amount of active sites during the process of regeneration resulted in a worse adsorption capacity of the regenerated sorbents.

4. CONCLUSION In this paper, Fe−Ce mixed oxide sorbents for mercury removal were prepared by the hydrothermal impregnation method with SC as the substrate or support material. The result showed that the hydrothermal impregnation significantly improved the specific surface area and pore volume of sorbent Fe0.4Ce0.2/SC200, at up to 279.56 m2/g and 0.145 cm3/g, respectively. The Hg0 removal performance of sorbent Fe0.4Ce0.2/SC200 was up to 83.5% for 85 min in the presence of H2S. The optimal Fe/Ce molar ratio and reaction temperature were 0.4:0.2 and 150 °C, respectively. H2S was found to play an important role in Hg0 removal, which is believed to be due to the formation of highly reactive elemental sulfur from the reaction of H2S with the metal oxides impregnated onto the sorbents. However, the presence of excessive H2S in the gas stream inhibited the Hg0 removal of the sorbent as a result of the excessive reaction of H2S with the active metal oxide, which can quickly exhaust the active metal I

DOI: 10.1021/acs.energyfuels.8b02358 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.8b02358 Energy Fuels XXXX, XXX, XXX−XXX