TiO2 Catalytic Sorbent for Mercury Removal with

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Regenerable Ce−Mn/TiO2 Catalytic Sorbent for Mercury Removal with High Resistance to SO2 Xiang Wu, Yufeng Duan,* Na Li, Peng Hu, Ting Yao, Jialin Meng, Shaojun Ren, and Hongqi Wei

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Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing, 210096, China ABSTRACT: Mn/TiO2 (MT), Ce/TiO2 (CT), and Ce−Mn/TiO2 (CMT) for mercury removal were prepared by an impregnation method, and mercury adsorption tests were conducted in a fixed-bed reactor. Regeneration experiments were carried out in a thermal regeneration reactor, and the effects of temperature and atmosphere on the regenerability were investigated. Surface physicochemical characteristics of fresh, spent, and regenerated CMT were analyzed by means of N2 adsorption−desorption methods, scanning electron microscopy, and X-ray photoelectron spectroscopy. The results showed that CMT had a higher resistance to SO2 poisoning than MT and CT and maintained a high mercury removal capability within a wide range of SO2 concentrations. Optimal thermal regenerability of spent CMT was obtained after thermal desorption at 400 °C followed by N2 + 50% O2 for 2 h. Ten cycles of mercury adsorption−regeneration demonstrated that there was no significant change in mercury removal capacity relative to fresh catalytic sorbent after multiple regeneration cycles. The regeneration of CMT was mainly attributed to the decomposition of mercury compounds and the restoration of Mn4+, Ce4+, and the chemisorbed oxygen on the catalytic sorbent surface. The procedure for the centralized control of mercury emissions from the flue gas by CMT was also analyzed for industrial application.

1. INTRODUCTION Mercury is known as one of the four major pollutants derived from coal combustion and is extremely harmful to the environment and human health, which has caused considerable concern worldwide.1−3 Coal-fired power plants are the largest anthropogenic source of mercury emissions, with 38% coming from coal combustion.4,5 Therefore, it is significant to reduce the mercury emission from coal-fired power plants. Generally, mercury species exist as oxidized mercury (Hg2+) and particlebond mercury (Hgp) and can be removed using wet desulfurization and dust removal devices,6 respectively. However, elemental mercury (Hg0) is difficult to remove due to its low solubility and high volatility,7 which has become the focus of mercury pollution control. At present, activated carbon injection (ACI) is considered to be the commercialized control technology for flue gas mercury, but it is limited due to its high cost, dependence on the type of coal, inlet mercury concentration, and reaction temperature.8,9 Most importantly, the mercury sorbent in conventional ACI technology is not typically recycled. Therefore, searching for high-efficiency, lowcost, and renewable mercury removal sorbents has become a recent research focus. Oxides of transition metals have been identified as possible alternatives to activated carbon for mercury removal due to their high adsorption efficiencies and low cost.10,11 In recent years, a variety of transition-metal oxides have been developed for mercury removal, especially manganese-based sorbents or catalysts, such as Fe−Sn−MnOx,12 Ru−Mn−Ti,13 Mn/γFe2O3,14 MnOx/γ-Al2O3,15 and Mn−TiO2.16 Studies have shown that the adsorption and catalytic oxidation performance of manganese-based sorbents or catalysts are greatly affected by the SO2 component in flue gas.17−19 In view of the high sulfur content in coal-fired flue gas, it is particularly important to develop mercury removal sorbents with good SO2 resistance. © XXXX American Chemical Society

So far, CeO2 has been extensively reported as a good reservoir for oxygen, in which oxygen can be stored and released through the redox cycle of Ce4+/Ce3+ under oxidizing and reducing environments,20 respectively. It is often used to modify other elements. Xie et al. and Qu et al.21−23 prepared Zr-, Ce-, and Sn-modified manganese-based composite metal oxide sorbents for mercury removal, where the doping of Ce improved SO2 resistance. Under a flue gas atmosphere containing 500 ppm of SO2, the adsorption capacity of mercury rose to 5.0 mg/g compared to 1.0 mg/g before modification. Wang et al.24 prepared a MnOx−CeO2/γ-Al2O3 mercury removal sorbent by a coprecipitation method, and the efficiency of the mercury removal reached 80% under the condition of 400 ppm of SO2. Finally, Zhang et al.25 found that when 50 ppm of SO2 was introduced into the simulated flue gas, the mercury removal efficiency of a Zr−Mn sorbent was only 30%, while 70% was obtained after Ce modification. Therefore, Ce-modified manganese-based mercury removal sorbent was prepared in this work to investigate its resistance to SO2. In addition to the research on resistance to SO2, there is an urgent demand for the development and application of regenerable mercury sorbents for life cycle economy and environmental protection.26 Thermal desorption methods provide a sufficient amount of energy to break the adsorbent−adsorbate interactions across all the adsorption sites without degrading the adsorbates.27 Many studies have already shown that the thermal regeneration methods can be reliably used for recycling metal oxide sorbents. Yao et al.28 Received: March 30, 2019 Revised: July 23, 2019 Published: August 8, 2019 A

DOI: 10.1021/acs.energyfuels.9b00978 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 1. Schematic diagram of the fix-bed reactor system for mercury removal.

prepared α-MnO2 sorbent that was found to have a high recyclability of mercury capture and good durability for thermal regeneration at 500 °C for 2 h. Also, Xu et al.29 have investigated the reusability of LaMnO3 for Hg0 capture and found that the adsorption capacity did not decrease after five cyclic measurements with 4% O2 at 150 °C. Scala et al.30 have also found that a MnOx/γ-Al2O3 sorbent could be completely regenerated at a temperature as low as 500 °C after mercury uptake, and repeated cycles of mercury adsorption− desorption did not lead to any significant reduction of the sorbent capacity for mercury uptake. The previous research results outlined above all prove the reliability and practicality of thermal regeneration. Nevertheless, the effects of different thermal desorption temperatures and atmospheres on regenerability is ambiguous, and the cyclic mercury adsorption of manganese oxides based sorbents deserves in-depth investigation. In this work, Ce/TiO2 (CT), Mn/TiO2 (MT), and Ce− Mn/TiO2 (CMT) for mercury removal were prepared by an impregnation method and were used to investigate the SO2 resistance on a fixed-bed reactor. The regenerability of CMT and the optimum conditions needed for the catalytic sorbent regeneration were also investigated. The mechanisms of CMT deactivation and regeneration were discussed on the basis of the experimental and characterization results. In addition, the procedure for the centralized control of mercury emissions from the flue gas using CMT was analyzed for industrial application.

the Barrett−Joyner−Halenda (BJH) method. The surface morphologies of the sorbents were investigated by means of scanning electron microscopy (SEM) (Nova NanoSEM NPE218, FEI Co.). The binding energies of Mn 2p, O 1s, Ce 3d, S 2p, and Hg 4f were determined by X-ray photoelectron spectroscopy (XPS) (Thermo, ESCALAB 250) with Al Kα (hν = 1486.6 eV) as the excitation source. The C 1s line at 284.6 eV was taken as a reference for the binding energy calibration. 2.2. Mercury Adsorption Experiments. The mercury removal performance testing was conducted in a laboratory-scale fixed-bed reactor, and a schematic representation of the mercury adsorption system is shown in Figure 1. Mercury vapor was generated by a mercury permeation tube and carried by pure N2. Various gases, including N2, O2, and SO2, were introduced to the inlet of the gas mixer. The concentrations of mercury in the simulated flue gas located in the inlet and outlet of the reactor were monitored using a continuous mercury gas analyzer (VM 3000). The initial mercury concentration was 50 μg/m3, and 40 mg of catalytic sorbent was used for each experiment. The total flow rate of the tested gas was 1 L/min, and the flow rate of the N2 carrier gas and mercury vapor mixture was 200 mL/min. All gas flow rates were accurately controlled by mass flow controllers (MFC), and exhausted gas was cleaned by an activated carbon unit. When the concentration of mercury fluctuated within ±5% for more than 30 min, the gas was diverted to pass through the sorbent bed for the test. Two different calculation methods for mercury removal efficiency were used: instantaneous value (η i) and accumulation value (ηa ). The instantaneous mercury removal efficiency (ηi) is defined by the following equation ηi (%) =

2. EXPERIMENTAL SECTION

0 Cin0 − Cout

Cin0

× 100%

0 where Cin0 and Cout represented the instantaneous mercury concentration in the inlet and outlet of the reactor. The accumulation mercury removal efficiency (ηa) is defined by the following equation

2.1. Sorbent Preparation and Characterization. The CMT catalytic sorbent was prepared using the following impregnation method: 7.9 g of nanoscale commercial TiO2 (Degussa P25) powder was dispersed in 100 mL of distilled water, and then 4.6 mL of Mn(NO3)2 solution and 8.68 g of Ce(NO3)3·6H2O crystals were added to the solution, which was magnetically stirred at the room temperature for 10 h. After that, the samples were dried at 105 °C for 12 h in an oven and calcined at 300 °C for 6 h under air atmosphere, and then the samples were ground to 60−80 mesh and stored in a drying dish for use. MT and CT were prepared in the same way as above, but only Mn(NO3)2 solution or Ce(NO3)3·6H2O crystals were added in the first step. The specific surface areas and pore parameters of the sorbents were determined by a N2 adsorption−desorption method using a Micromeritics 2020 Instrument. The specific surface areas were measured by using the Brunauer−Emmett−Teller (BET) method, and the pore sizes and pore volumes were calculated on the basis of

t

ηa (%) =

t

0 ∑0 Cin0 − ∑0 Cout t

∑0 Cin0

× 100%

where ∑t0C0in and ∑t0C0out represented the accumulated mercury concentration in the inlet and outlet of the reactor. 2.3. Thermal Regeneration Experiments. The thermal regeneration reactor is shown in Figure 2. The experimental system consisted of the mass flow control system of gas, horizontal tube furnace, reaction tubes, and exhaust gas cleaning device. The samples were placed on a quartz boat and put at the center of the furnace with the heating rate of 10 °C/min. The carrier gases used in this study were pure N2, air, and N2 + 50% O2, and the flow rate was controlled B

DOI: 10.1021/acs.energyfuels.9b00978 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 2. Schematic of the heat regeneration reactor. at 2 L/min for 2 h. Each mercury removal test lasted for 4 h before regeneration.

3. RESULTS AND DISCUSSION 3.1. Mercury Removal Performance. 3.1.1. Effect of Reaction Temperature. Figure 3 shows the mercury removal Figure 4. Effect of SO2 on the mercury removal efficiency.

inactivation of Mn/Ce active sites.33 For CMT, the introduction of SO2 has no significant effect on its mercury removal performance, and it could still maintain more than 80% mercury removal efficiency in 120 min. Due to the higher affinity of Ce for sulfur, the doping of Ce onto the catalytic sorbent would have the advantage of reducing the contact between Mn and SO2 and reducing the coverage on Mn-based catalytic sorbent active sites by SO2.24 In other words, Ce could diminish the chance of the contact between Mn and SO2, protecting MnOx from SO2 poisoning. In addition, there was an interaction between Mn and Ce in the sorbent contributing to the mercury removal performance via the following reaction: Mn2O3 + 2CeO2 → 2MnO2 + Ce2O3.34 In order to further explore the resistance of CMT to SO2 poisoning, mercury removal performances were investigated under different SO2 concentrations, and the results are shown in Figure 5. It was shown that the mercury removal efficiency

Figure 3. Effect of reaction temperature on the mercury removal efficiency.

efficiencies over CT, MT, and CMT under different reaction temperatures. The tests were carried out in a simulated flue gas condition of N2 + 6% O2. It can be seen that CMT showed higher mercury removal efficiencies than CT and MT in the range of reaction temperatures considered. The mercury removal efficiencies of the three catalytic sorbents gradually increased with the increase of temperature in the range of 100−200 °C, while slightly decreased from 200 to 300 °C. Reaction rates can be accelerated with the increase of temperature; therefore, much more mercury could be removed at the higher reaction temperatures if the active species on the sorbents could react with gaseous mercury.20 However, part of the generated mercuric oxides may decompose into elemental mercury at high temperatures,31 resulting in the maximum mercury removal efficiencies of 91%, 58%, and 95% that were observed at 200 °C for MT, CT, and CMT, respectively. High activities of CMT at low temperatures (100−200 °C) allow it to be placed downstream of a particulate control system in a coal-fired power plant. 3.1.2. Effect of SO2. As shown in Figure 4, the mercury removal performances of CT, MT, and CMT were relatively stable when SO2 was not present. At the same time, MT and CMT maintained higher mercury removal rates than CT. When 1000 ppm of SO2 was introduced, the mercury removal efficiencies of CT and MT decreased continuously to 2% and 46% after 120 min, respectively. Results showed that SO2 has a serious inhibitory effect on the mercury removal performances of CT and MT, which is because of the competitive adsorption of SO2 and Hg0 on the active sites of the catalytic sorbent.32 Moreover, SO2 could react with the sorbent to form thermally stable manganese sulfate or cerium sulfate, resulting in the

Figure 5. Effect of SO2 concentration on the mercury removal efficiency of CMT.

of CMT reached 93.6% within 2 h under N2 + 6% O2 atmosphere. But when 500, 1000, 1500, and 2000 ppm of SO2 were introduced, the efficiencies of mercury removal decreased to 90.4%, 85.3%, 82.1%, and 80.1%, respectively. Though a continuous declining trend was observed as the concentration of SO2 increased, CMT still had a good mercury removal performance, indicating that it had high resistance to SO2 poisoning and could maintain high efficiency of mercury removal within a wide SO2 concentration range. 3.2. Mercury Temperature-Programmed Desorption (Hg-TPD). To further investigate the desorption performance of mercury from spent CMT under N2 + 6% O2 + 1000 ppm of C

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Energy & Fuels SO2 atmosphere, a Hg-TPD experiment was carried out, which is shown in Figure 6. It was observed that there were two peaks

Figure 7. Effect of regeneration temperature on the mercury removal efficiency in N2 + 50% O2 atmosphere.

Table 1. Specific Surface Area and Structure Parameters of Different Samples

Figure 6. Hg-TPD spectra of spent CMT.

during the desorption process, and the corresponding temperatures of these two peaks were about 300 and 550 °C, respectively. On the basis of results from prior researchers on the decomposition characteristics of a series of pure mercury compounds,35,36 the mercury compounds corresponding to these two peaks were HgO and HgSO4, respectively. Meanwhile, the content of HgO was apparently higher than that of HgSO4, which indicated that the mercury adsorbed on CMT mainly existed in the form of HgO, and only small amounts of HgSO4 were generated. Meanwhile, the Hg-TPD experiment indicated that the adsorbed mercury can be released using a thermal desorption method. This also showed that thermal desorption was a feasible and appropriate method for regeneration of CMT. According to the Hg-TPD profile, the thermal desorption temperature for CMT regeneration was set from 300 °C. 3.3. Regenerability. To evaluate the regenerability, spent CMT was taken out of the fixed-bed reactor after each mercury removal experiment and heated in the thermal regeneration reactor. The regenerated sorbent was then put back into the fixed-bed reactor to initiate the next cycle of mercury adsorption. In this work, the effects of the regeneration temperature and atmosphere on the mercury removal performance were investigated. 3.3.1. Effect of Regeneration Temperature. Regeneration temperature was one of the most important factors effecting the regenerability. Temperature not only effects the decomposition of mercury compounds adsorbed on the catalytic sorbent but also influences the morphology and crystalline structure. As shown in Figure 7, the optimum regeneration temperature was 400 °C. When the temperature was lower (300 °C), it affected the O2-driven restoration of deactivated sites on the catalytic sorbent, which resulted in a lower mercury removal efficiency. When the regeneration temperature was further increased to 500 °C, the mercury removal efficiency decreased, and the drop was even more obvious at 600 °C. At a high regeneration temperature, some agglomerations would occur. Additionally, poor mercury removal was caused by the gradual conversion from MnO2 to Mn2O3, the transformations of the crystalline phase of TiO2 from anatase to rutile, and the reduction of chemisorbed oxygen.37 As seen in Table 1, the specific surface area and total pore volume of CMT were increased, while the average pore size was decreased in comparison to that of pure TiO2, thus

samples

BET surface area (m2/g)

total pore volume (cm3/g)

av pore size (nm)

TiO2 CMT CMT-400 °C CMT-500 °C CMT-600 °C

50.384 55.116 53.568 46.549 34.213

0.102 0.187 0.152 0.104 0.066

17.713 13.552 16.354 20.334 27.086

indicating that manganese and cerium metal oxides have been successfully loaded on the surface of TiO2. The specific surface area and structure parameters of CMT-400 °C have no significant change compared with that of CMT, indicating that its physical structure was not damaged, which provided a guarantee for high mercury removal efficiency. For CMT-600 °C, the specific surface area decreased sharply to 34.213 m2/g, the total pore volume was only 0.066 cm3/g, and the average pore size increased to 27.086 nm, which showed that the high temperature caused serious agglomeration. Figure 8 presents the SEM images of pure TiO2, CMT-400 °C, CMT-500 °C, and CMT-600 °C. Compared with other samples, it was obvious that the particle size of CMT-600 °C was significantly increased, indicating that CMT-600 °C agglomerated due to high temperature, which was also consistent with the results of Table 1. 3.3.2. Effect of Regeneration Atmosphere. The effect of different regeneration atmospheres on the ability of mercury capture is shown in Figure 9. After regeneration in pure N2, the mercury removal efficiency of CMT was greatly reduced and it dropped to 47.7% from 91.8% after 240 min. In this case, though the preciously adsorbed mercury had been desorbed, the active sites like Mn4+ and Ce4+ that were consumed had not been supplemented. In comparison, the mercury removal performance was improved after regeneration in air. At 240 min, 57.8% of mercury removal efficiency was attained, but it was still lower than that of fresh CMT. The thermal regeneration experiment under N2 + 50% O2 atmosphere had the greatest effect. Compared with the original sorbent, the efficiency of mercury removal did not decrease significantly. 3.3.3. Stability of Regeneration Performance. To further investigate the reusability of CMT, four cycles of mercury adsorption−regeneration tests at 400 °C under N2 + 50% O2 for 2 h were performed. As shown in Figure 10, the average mercury removal efficiency of fresh CMT in 4 h was 79.3%, D

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Figure 8. SEM images of different samples.

continuous adsorption, where the equivalent of 10 adsorption−regeneration cycles occurred, was completed. As shown, the repeated adsorption−regeneration cycles did not decrease the effectiveness compared to fresh catalytic sorbent, which showed the good stability and reusability of CMT. 3.4. Surface Chemistry Changes of Sorbents. XPS technology was employed to explore the surface chemical composition of the sorbents. The Mn 2p, O 1s, Ce 3d, S 2p, and Hg 4f spectra of the fresh, spent, and regenerated CMT are shown in Figure 11, and the surface atomic ratios based on XPS results are summarized in Table 2. Figure 11a shows the XPS spectra of Mn 2p. The Mn 2p spectra was fitted with two different peaks, which were at binding energies of 642.93 eV (Mn4+) and 641.69 eV (Mn3+),38,39 respectively. Additionally, no apparent peak of Mn2+ was observed, indicating that Mn species of the catalytic sorbents mainly existed in the forms of MnO2 and Mn2O3. The decreased Mn4+ content of CMT-Hg was clearly apparent from Table 2. The ratio of Mn4+/(Mn4+ + Mn3+) changed from 0.589 to 0.427. This observation suggested that the Mn4+ gained electrons to form Mn3+ during mercury removal tests. After the thermal regeneration treatment, the ratio of Mn4+/(Mn4+ + Mn3+) increased to 0.568. This was attributed to the transformation of Mn2O3 to MnO2 during the thermal regeneration process. Many studies have shown that Mn4+ was favorable for mercury removal,15,18,21 which can explain why regenerated CMT maintained good mercury removal performance. As shown in Figure 11b for O 1s, two oxygen species were observed. The peaks at 529.7 eV were identified as lattice oxygen (denoted as Oα), such as O2−, whereas the other peak at 531.6 eV corresponded to surface chemisorbed oxygen (denoted as Oβ), such as O22− or O− from oxide defects or hydroxyl-like groups.40 Oβ was reported to be highly active in oxidation reactions because its mobility was higher than that of Oα, and the decrease of Oβ/(Oα + Oβ) resulted in the poor mercury capture capability of CMT.28,41 During the mercury removal experiment, the ratio of Oβ/(Oα + Oβ) decreased from

Figure 9. Effect of regeneration atmosphere on the mercury removal efficiency at 400 °C.

Figure 10. Effect of cycle times on the mercury removal efficiency.

while 78.5% efficiency was maintained over the regenerated CMT. Also, after four cycles, the mercury removal efficiency apparently still did not decrease compared with that of fresh CMT. In order to further study the effect of cycle times on the mercury removal ability, thermal regeneration after 40 h of E

DOI: 10.1021/acs.energyfuels.9b00978 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 11. XPS spectra of Mn 2p (a), O 1s (b), Ce 3d (c), S 2p (d), and Hg 4f (e) for samples: CMT-Hg, spent CMT; CMT-Hg-R, spent CMT after thermal regeneration at 400 °C in N2 + 50% O2; CMT-Hg-R-4, spent CMT after thermal regeneration at 400 °C in N2 + 50% O2 after four cycles.

“v4”, represented Ce4+. The other two subpeaks, which were marked as “u3” and “v3”, represented Ce3+.34 It can be observed that both Ce4+ and Ce3+ were present in CMT. The ratio of Ce4+/(Ce4+ + Ce3+) was 0.852, indicating that Ce4+ oxides were the primary species. Compared with CMT, the Ce4+/ (Ce4+ + Ce3+) value of CMT-Hg decreased to 0.791, which

0.407 to 0.207, but it rose to 0.354 after thermal regeneration, so the activity of CMT was restored. The XPS spectra of Ce 3d of the samples are shown in Figure 11c. The peaks, which were marked as “u” and “v”, represented Ce 3d3/2 and Ce 3d5/2 states, respectively. The six subpeaks, which were marked as “u1”, “u2”, “u4”, “v1”, “v2”, and F

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Energy & Fuels Table 2. Atomic Ratios of Mn, Ce, and O to the Total Concentration of Different Samples samples

Mn4+/(Mn4+ + Mn3+)

Ce4+/(Ce4+ + Ce3+)

Oβ/(Oβ + Oα)

CMT CMT-Hg CMT-Hg-R CMT-Hg-R-4

0.589 0.427 0.568 0.549

0.852 0.791 0.821 0.813

0.407 0.207 0.354 0.334

Figure 12. Schematic of regenerable CMT for mercury control from coal-fired power plants.

confirmed that part of the Ce4+ was reduced into Ce3+ in the mercury removal process. Also the ratio of Ce4+/(Ce4+ + Ce3+) increased to 0.821 after thermal regeneration, which was favorable for mercury removal.42 XPS spectra of S 2p and Hg 4f are also presented in Figure 11d,e. The S 2p peaks that appeared at 168.5 and 169.7 eV corresponded to the formation of surface SO32− and SO42− species,43 respectively. Compared to CMT-Hg, nearly no change was seen after regenerating. However, after four regeneration cycles, the mercury removal efficiency did not decrease significantly. It was possible that SO2 could only occupy partial active sites for mercury adsorption on the catalytic sorbent surface, while other active sites were still available for mercury removal.23 The peaks for Hg 4f at 100.8 and 104.8 eV can be attributed to HgO on the CMT-Hg surface.33,44 After thermal regeneration at 400 °C for 2 h, no significant peaks corresponding to mercury could be observed. This indicated that almost all adsorbed mercury had been released from the catalytic sorbent. 3.5. Mechanism of Spent Sorbent Regeneration. On the basis of the experimental and characterization results, a possible reaction mechanism for deactivation and regeneration of CMT was proposed. In the process of mercury removal, gaseous mercury was initially adsorbed on the catalytic sorbent surface and oxidized to Hg2+ along with the reduction of Mn4+/Ce4+ to Mn3+/Ce3+. The Hg2+ was finally adsorbed in the form of HgO by consuming oxygen. In addition, SO2 could interact with the catalytic sorbent to generate sulfate, which was then adsorbed on the active sites. After thermal desorption at 400 °C in N2 + 50% O2 atmosphere for 2 h, the mercury species adsorbed on the catalytic sorbent would be decomposed and released, leaving the active sites exposed. With the participation of O2, surface Mn4+, Ce4+, and chemisorbed oxygen could be recovered, and then the mercury adsorption activity was regenerated. 3.6. Application of Sorbent. The application of CMT for mercury removal is schematically shown in Figure 12. In order to separate the sorbents from dust, a two-stage fabric filter (FF) strategy was designed. The catalytic sorbents can be injected into the flue gas to remove mercury downstream of the first FF and then captured by the second one. The spent

CMT can be regenerated by thermal desorption and recycled by reinjecting into the flue gas after activation. The desorbed mercury from the regeneration process can be condensed and then collected in a container. In this case, the CMT can be regenerated and cycled, avoiding the risk of re-emissions mercury.

4. CONCLUSION This study investigated the SO2 resistance and regenerability of CMT for mercury removal. CMT is verified as having an excellent resistance to SO2 while high mercury removal capability is maintained within a wide range of SO 2 concentrations. When CMT is exposed to the flue gas with N2 + 6% O2 + 500−2000 ppm of SO2, an average mercury removal efficiency of more than 80% is obtained within 2 h. Optimal thermal regenerability is attained at 400 °C with N2 + 50% O2. After 10 adsorption−regeneration cycles, the mercury removal performance remains high compared to that of fresh catalytic sorbent, showing good stability and reusability. On the basis of the XPS characterization results, the regeneration of CMT is mainly due to the decomposition of mercury compounds with an increase of surface Mn4+, Ce4+, and chemisorbed oxygen, which are favorable for mercury removal. CMT as a regenerable catalytic sorbent with high SO2 resistance shows potential when prospectively applied to the control of mercury emissions from coal-fired flue gas.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel/Fax: +86-25-83795652. ORCID

Xiang Wu: 0000-0002-8319-2620 Yufeng Duan: 0000-0002-9015-2619 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by the National Key R&D Program of China (2016YFC0201105) and the National Natural Science Foundation of China (51576044, 51676041). G

DOI: 10.1021/acs.energyfuels.9b00978 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.9b00978 Energy Fuels XXXX, XXX, XXX−XXX