English

• Mercury (Hg) pollution with high toxicity, biological accumulation, migration capacity, and durability has drawn increasing global attention^[1]. The stationary combustion of fossil fuels and biomass is responsible for approximately 24% of global Hg emissions, with coal as the largest contributor^[2]. In China, coal combusted in the power plants accounts for approximately 50% of total coal consumption^[3]. With the announcement of Minamata Convention, the control of Hg emissions in the coal-fired power plants has become a serious concern in China^[4]. In 2011, the Chinese Ministry of Ecological Environment set the emission limit of Hg as 0.03 mg/m^3[5], which exacts the development of cost-effective and highly efficient technical scheme for Hg removal.

  Hg in coal-fired flue gas occurs mainly in the form of elemental (Hg⁰), particulate (Hg^p), and oxidation state (Hg²⁺) mercury^[6]. Current air pollution control devices (APCDs), such as selective catalytic reduction system, dust collectors, and desulfurization devices can synergistically remove most Hg^p and Hg²⁺, but have negligible effects on Hg⁰ removal^[7]. Accordingly, a number of sorbents developed for the removal of gaseous Hg from coal flue gas aimed at converting Hg⁰ to Hg²⁺ or Hg^p; Hg²⁺ and Hg^p could then be removed by the existing air pollution control devices^[8-12]. Among these sorbents, activated carbon is the most widely used trapper for Hg capture and corresponding activated carbon injection (ACI) technology has been commercially used in coal power plants. Nonetheless, ACI is expensive and has adverse effects on the further reutilization of fly ash due to the high carbon contents^[13]. In contrast, non-carbon-based adsorbents have many advantages including large surface area, high mercury removal efficiency^{[14, 15]}, high thermal stability^[16], numerous sources^[17], and little adverse effects on the fly ash products.

  The fluid catalytic cracking (FCC) catalyst has been widely used in oil refineries. In 2018, the global demand for refining catalysts was 831 kiloton, of which the FCC catalyst accounts for 65% and the residual FCC catalysts, i.e. the so-called spent fluid catalytic cracking (SFCC) catalyst, holds for 16%^[18]. The disposal of huge amounts of waste solid FCC catalyst generated from the oil refineries is still a great challenge^{[19, 20]}. Ferella et al^[21] found that the modified spent FCC catalysts were efficient in metal adsorption in a synthetic wastewater solution. Yuan et al^[22] reported that Fe₃O₄-modified spent catalyst was a suitable adsorbent for phosphate removal. The SFCC catalyst is similar to zeolite in the structure, containing silicon-oxygen and aluminum-oxygen tetrahedrons that are bridged by oxygen atoms in the center. Some void cages formed in three dimensions create the zeolite pore channel system, whilst the exchangeable cations in the framework play the roles for charge neutrality. The high porosity and large surface area of zeolite provide the FCC catalyst with the capacity in adsorption and catalysis^[23]. All these features make the FCC catalyst a potential sorbent to capture gaseous Hg in the flue gas of industrial boilers^[24].

  In this work, the spent fluid catalytic cracking (SFCC) catalysts were activated by an internal instant vaporization (IIV) method^[25]. The blocked catalyst channels can be effectively unchoked by the IIV method with low boiling organic solvent without impairing the internal channel structure and the activated SFCC catalysts were then used in the removal of Hg⁰ from a simulated flue gas in a fixed bed reactor. The effect of various operation parameters such as the SFCC activation conditions, adsorption temperature, and flue gas components on the Hg⁰ removal efficiency was investigated and a possible mechanism of mercury adsorption on the activated SFCC catalysts was then proposed. This should be rather meaningful as an idea of waste control by waste.

  1.   Experimental

  1.1   Materials and characterization

  About 10 g of the SFCC catalyst samples provided by Guangxi Tiandong Oil Chemical Factory Co. LTD. of China was calcined at 25, 120, 240, 360, 480 and 600 ℃ for 4 h, cooled to 25 ℃, and then ground into 140 mesh for the activity test. Depending on the calcination temperature, corresponding activated SFCC catalyst samples are labeled as SFCC, FCC-120, FCC-240, FCC-360, FCC-480 and FCC-600, respectively.

  To activate the SFCC catalyst by the internal instant vaporization (IIV) method with organic solvents, 10 g of the FCC-120 sample was placed in a 250 mL beaker containing 10 mL of methanol, ethanol, acetone, or petroleum ether. The beaker was then sealed with a membrane and kept at 25 ℃ and atmospheric pressure for 2 h. Subsequently, 50 mL of boiling deionized water was poured into the beaker, kept at 100 ℃ for 5 min. After that, the beaker was placed in a drying oven at 120 ℃ until a constant weight was achieved; the samples were then cooled to room temperature, and grounded into 140 mesh. Depending on the solvent used for IIV, the activated SFCC catalysts obtained with the solvent of methanol, ethanol, acetone, and petroleum ether are labeled as FCC-M, FCC-E, FCC-A and FCC-P, respectively.

  The surface area and pore diameter distribution of the activated and non-activated SFCC catalysts were measured on an ASAP2020 surface area analyzer of the Micromeritics Company, United States. The specific surface area and pore volume were calculated through the Brunauer-Emmett-Teller (BET) method and the Barrett, Joyner, and Halenda (BJH) model, respectively. Powder X-ray diffraction patterns (PXRDs) were measured on a Bruker AXS/ D8 ADVANCE powder diffractometer using Cu Kα radiation (40 kV and 20 mA), in the range of 10° to 80°. The microstructure was analyzed by Zeiss SUPRATM55 scanning electron microscopy (SEM). Thermogravimetric analysis (TG) was carried out on a Shimadzu DTG-60 AH instrument up to 850 ℃ with a heating rate of 10 ℃/min in air. X-ray photoelectron spectroscopy (XPS) was performed on a PHI Quantera II (Ulvac-Phi) spectrometer with Al Kα as the excitation source, transmission power of 150 W, and photoelectron energy of 200 eV; the C 1s line at 284.6 eV was taken as the reference for the binding energy calibration.

  1.2   Set-up for Hg⁰ adsorption

  The Hg⁰ adsorption test over the activated and non-activated SFCC catalysts was performed in a bench-scale fixed-bed adsorption test system, which included mainly an elemental mercury generator, a simulated flue gas generator, an adsorption fixed-bed reactor, a flue gas on-line analysis system, and an exhaust gas treatment system, as illustrated in Figure 1.

  Figure 1

  

  Figure 1.  Schematic diagram of the fixed-bed adsorption evaluation device

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  The simulated flue gas consists of N₂, CO₂, O₂, SO₂ and NO, regulated by several mass flow controllers. The overall gas flow rate is 1 L/min and detailed composition of the mixed gas is given in Table 1. The reactor was heated with a tubular furnace. The Hg⁰ generator included mainly an elemental mercury permeation tube (VICI Metronics Inc.), a U-shaped quartz tube, and a super thermostat. To yield a stable concentration of Hg⁰ (20.00±1.00) μg/m³, N₂ was used as the carrier gas. Before the test, the mercury permeation tube was bypassed to calibrate the signal baseline. After that, the carrier gas was then diverted into the mercury permeation tube to generate a stable mercury concentration. Subsequently, the gaseous stream with a stable concentration of Hg⁰ was diverted into the packed fixed-bed reactor and the breakthrough curves were monitored online using a mercury analyzer (Lumex RA-915M, Russia). The reactor was packed with 1 g of SFCC catalyst and the adsorption was performed at a space velocity of 75000 h^-1 and lasted for 2 h. Quartz wool was used as a support to prevent the loss of SFCC catalyst. Potassium dichromate solution and sodium hydroxide solution were used to address the exhaust gas emissions.

  Table 1

  

  Table 1.  Experimental conditions for the Hg⁰ adsorption tests

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  Set	Adsorbent	Activation conditions		Flue gas composition	Adsorption temp t/℃
  		calc. temp /℃	solvent
  I	SFCC, activated SFCC	120-600	-	6% O2, 12% CO2, N2	120
  II	activated SFCC	120	methanol, ethanol, acetone, petroleum ether	6% O2, 12% CO2, N2	120
  III	activated SFCC	120	ethanol	6% O2, 12% CO2, N2	80-300
  IV	activated SFCC	120	ethanol	0-10%O2, 12% CO2, N2	120
  V	activated SFCC	120	ethanol	6% O2, 12% CO2, 0.06%-0.1% SO2, N2	120
  VI	activated SFCC	120	ethanol	6% O2, 12% CO2, 0.01%-0.06% NO, N2	120

  Several preliminary tests were conducted to investigate the effect of activation and adsorption conditions on the removal efficiency of Hg⁰. The detailed operation conditions of the six sets of adsorption tests are summarized in Table 1. The removal efficiency of Hg⁰ from the flue gas was calculated as follows:

  $ \eta=\left(\left[\mathrm{Hg}^{0}\right]_{\mathrm{in}}-\left[\mathrm{Hg}^{0}\right]_{\mathrm{out}}\right) /\left[\mathrm{Hg}^{0}\right]_{\mathrm{in}} \times 100 \% $

  (1)

  where [Hg⁰]_in and [Hg⁰]_out represent the Hg⁰ concentrations in the stream at the inlet and outlet of the adsorption bed, respectively.

  2.   Results and discussion

  2.1   Characterization of the activated and non-activated SFCC catalysts

  As shown by the SEM images in Figure 2, the SFCC catalysts have a spherical appearance with a rough surface with some bumps and collapses as well as porous structure, which facilitate the adsorption of gaseous mercury. High temperature and organic solvent activation have insignificant influence on the morphology of the SFCC catalysts.

  Figure 2

  

  Figure 2.  SEM images of the activated and non-activated SFCC catalysts: ((a), (b)) SFCC; ((c), (d)) FCC-360; and ((e), (f)) FCC-M

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  As given in Table 2, the surface area and average pore diameter of the SFCC catalysts are approximately 74.9 m²/g and 3.181 nm, respectively. However, the surface areas of the activated SFCC catalysts are in the range of 76.5-101.2 m²/g and the pore diameter in the range of 3.382-4.862 nm, both are larger than those of the original SFCC catalysts. Moreover, it seems that the surface area of the activated SFCC catalysts increases with the increase of the activation temperature, probably attributing to the removal of more physically adsorbed water and other volatile matters at higher temperature. Furthermore, the carbonaceous matters deposited on the catalyst surface can also be burned out at high temperature, as confirmed by the results of thermogravimetric/differential thermal analysis (TG/DTA) analysis. As shown in Figure 3, the weight loss of the SFCC catalysts during the calcination can be divided into three stages, viz., 2.116% at 36.00-135.09 ℃, 2.615% at 135.09-474.52 ℃, and 0.491% at 762.5-840.00 ℃, which are probably ascribed to the removal of the physically adsorbed water, combustion of the deposited carbon, and aluminum stripping on the skeleton of the molecular sieves, respectively^[26].

  Table 2

  

  Table 2.  Textural properties of the activated and non-activated SFCC catalysts

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  Adsorbent	Surface area A/(m2·g-1)	Pore volume v/(cm3·g-1)	Pore diameter d/nm
  SFCC	74.9	0.09	3.181
  FCC-120	78.5	0.11	3.382
  FCC-360	90.4	0.11	3.795
  FCC-600	101.2	0.11	4.862
  FCC-M	87.3	0.10	3.804
  FCC-E	83.7	0.10	3.396
  FCC-A	77.9	0.09	3.400
  FCC-P	76.5	0.09	3.389

  Figure 3

  

  Figure 3.  TG/DTA curves of the SFCC catalyst in the air atmosphere (a) and N₂ atmosphere (b)

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  Table 3 illustrates that the SFCC catalyst consists of O, Al, Si, Ca, as well as a small amount of transition metals such as Ni, Fe, Ce and V. Meanwhile, XPS was also employed to analyze the states of surface elements on FCC-120, in order to probe the active species and the reaction pathways for mercury adsorption on the SFCC catalysts, as shown in Figure 4. The XPS survey indicates that O, Al, Si, C, Ca, Ni, Fe and Ce are present on the surface of FCC-120 (Figure 4(a)). The peaks in the Fe 2p region at 711.0 and 709.9 eV are attributed to Fe³⁺ (Figure 4(b)), whereas the peaks at 712.8, 719.1 and 724.8 eV correspond to Fe^III-OH, Fe³⁺ cations and Fe²⁺, respectively^{[27, 28]}. It indicates that iron mainly appears in the form of Fe³⁺ on surface of FCC-120. The peaks of u′ for Ce 3d_3/2 and v′ for Ce 3d_5/2 can be attributed to Ce³⁺, whereas other peaks in the Ce 3d region to Ce⁴⁺ (Figure 4(c))^[29]. In contrast, the peak at 529.6 eV for O 1s is ascribed to lattice oxygen, whereas the peak at 532.6 eV to surface oxygen (Figure 4(d))^[30].

  Table 3

  

  Table 3.  Chemical composition of the SFCC catalysts determined by XRF

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  Component	Percentage	Component	Percentage	Component	Percentage
  Al2O3	46.397	Cs2O	1.115	V2O5	0.169
  SiO2	43.147	Na2O	0.782	CeO2	0.166
  CaO	2.012	P2O5	0.714	Sb2O3	0.121
  SO3	1.546	TiO2	0.302	ZnO	0.093
  NiO	1.416	MgO	0.254	La2O3	0.089
  Fe2O3	1.330	K2O	0.177	else	0.163

  Figure 4

  

  Figure 4.  XPS spectra of the FCC-120 catalyst: (a) survey of elements; (b) Fe 2p; (c) Ce 3d; and (d) O 1s

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  The crystal structure and phase of the SFCC catalysts were characterized by XRD, as shown in Figure 5. Obviously, the activation process has little influence on the crystal structure of the SFCC catalysts; the activated SFCC catalysts still display the characteristic diffraction peaks of Y zeolite, Al₂O₃ and ZSM-5 zeolite^{[10, 31]}.

  Figure 5

  

  Figure 5.  XRD patterns of various catalyst samples: a: SFCC; b: FCC-360; c: FCC-600; d: FCC-M; e: FCC-E; f: FCC-A; g: FCC-P

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  2.2   Hg⁰ adsorption performance

  2.2.1   Impact of activation conditions

  As shown in Figure 6, the initial Hg⁰ removal efficiency of the SFCC catalysts is in the range of 17.77%-48.97% after the activation at high temperature. However, the Hg⁰ removal efficiency decreases with the adsorption time, as more active sites on the catalyst surface were covered by Hg with the extension of adsorption time, which may reduce the adsorption capacity of the SFCC catalyst^[32].

  Figure 6

  

  Figure 6.  Effect of the calcination temperature on the Hg⁰ removal efficiency of activated SFCC catalysts; the operation conditions of set I specified in Table 1 are used

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  Figure 6 also indicates that the initial Hg⁰ removal efficiency of the SFCC catalysts increases first and then decreases with the increase of the activation temperature. The SFCC catalyst activated at 360 ℃ displays the highest Hg⁰ removal efficiency (48.97%), whereas the Hg⁰ removal efficiency is only 17.77% over the catalyst activated at 600 ℃, probably due to the aluminum stripping at high temperature from the skeleton of the molecular sieve in the SFCC catalysts^[25].

  Meanwhile, the solvent used to activate the SFCC adsorbents by IIV also has a significant influence on the Hg⁰ removal efficiency, as shown in Figure 7. Methanol and ethanol as the solvent for activation give a higher initial Hg⁰ removal efficiency, 53.06% and 46.65%, respectively. The Hg⁰ removal efficiency over the FCC-E and FCC-M catalysts decreases with the adsorption time and becomes stable after the adsorption for 120 min. Although the SFCC adsorbents show higher adsorption efficiency for Hg⁰ after the treatment with either methanol or ethanol, ethanol is probably more popular as an organic solvent to activate the SFCC catalysts due to its less toxicity.

  Figure 7

  

  Figure 7.  Effect of the activation organic solvent on the Hg⁰ removal efficiency of the activated SFCC catalysts; the operation conditions of Set II specified in Table 1 are used

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  2.2.2   Impact of adsorption temperature

  As shown in Figure 8, the initial Hg⁰ removal efficiency on FCC-E is 47.35% at 80 ℃, decreases to 18% after adsorption for 30 min and keeps at the level of about 15% after adsorption for 90 min. In contrast, the initial Hg⁰ removal efficiency at 120-300 ℃ increases with the extension of the adsorption time. After adsorption for 15 min, the adsorption of Hg⁰ reaches equilibrium and the Hg⁰ removal efficiency on FCC-E then becomes stable with the adsorption time. Meanwhile, it seems that the Hg⁰ equilibrium removal efficiency over FCC-E increases with an increase in the adsorption temperature, indicating that the removal of Hg⁰ from the simulated flue gas by the SFCC catalysts is mainly contributed by chemical adsorption at high temperature, in comparison to that by the physical adsorption at low temperature.

  Figure 8

  

  Figure 8.  Effect of different adsorption temperature on the Hg⁰ removal efficiency of FCC-E; the operation conditions of Set III specified in Table 1 are used

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  2.2.3   Impact of flue gas composition

  A stable Hg⁰ removal efficiency of FCC-E is achieved after adsorption for about 90 min, as shown in Figure 9; however, the stable Hg⁰ removal efficiency increases with the increase of O₂ concentration in the flue gas. In the absence of O₂, the Hg⁰ removal efficiency of FCC-E is 5.34%; it increases to 33.25% in the presence of 10% O₂, about 6 times of that under anaerobic adsorption condition. That is, the adsorption of Hg⁰ on the activated SFCC catalysts can be greatly promoted by O₂ in the flue gas, as O₂ in the flue gas can be adsorbed on the activated SFCC catalysts to supplement the lattice oxygen^{[12, 33, 34]}.

  Figure 9

  

  Figure 9.  Effect of O₂ concentration in the flue gas on the Hg⁰ removal efficiency of FCC-E; the operation conditions of Set IV specified in Table 1 are used

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  Figure 10 shows that the Hg⁰ removal efficiency of FCC-E is only 15% when the flue gas contains 0.06% SO₂ but no O₂, higher than that (5.34%) in the inert atmosphere. It means that SO₂ may also promote the removal of Hg⁰ from the flue gas in the absence of oxygen. In the presence of O₂, the stable Hg⁰ removal efficiency of FCC-E increases with the increase of SO₂ concentration from 0.06% to 0.10%, possibly due to the fact that SO₂ is oxidized to SO₃ by O₂ and SO₃ reacts with Hg⁰ to form mercury sulfate and mercurous sulfate^[6]. However, when the SO₂ concentration increases to 0.18%, the stable Hg⁰ removal efficiency of FCC-E does not increase anymore with the rise in the SO₂ concentration. It was reported that there is a competitive adsorption between SO₂ and Hg^{0[27, 35, 36]}, which brings on a balance point for the effect of SO₂ concentration; that is, the promoting effect by mercury oxidation with SO₃ originated from SO₂ is counteracted by the inhibitive effect by the competitive adsorption of SO₂ with mercury and subsequent mercury oxidation^[37].

  Figure 10

  

  Figure 10.  Effect of SO₂ concentration in the flue gas on the Hg⁰ removal efficiency of FCC-E; the operation conditions of Set V specified in Table 1 are used

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  The effect of NO concentration in the flue gas on the Hg⁰ removal efficiency of FCC-E NO was also inspected, as shown in Figure 11. Obviously, NO has an evident promoting effect on the mercury removal. In the presence of NO, the mercury removal efficiency decreases first and then increases with the operation time, suggesting that the physical adsorption of mercury is changed to chemical adsorption with the extension of adsorption time, as more NO is adsorbed on the surface of the SFCC, forming more active sites conducive to mercury removal. Moreover, Figure 11 also shows that when the NO concentration is 0.03%, the initial Hg⁰ removal efficiency of the FCC-E in the absence of O₂ is lower than that in the presence of O₂ and the Hg⁰ removal efficiency increases significantly with the rise of NO concentration in the presence of oxygen. This is probably ascribed to the numerous hydroxyl radicals on the surface of the activated SFCC catalysts, which can oxidize NO into NO₂^* and then react with adsorbed Hg⁰ (ads) and O₂ into Hg(NO₃)₂, leading to a significant increase of the Hg⁰ removal efficiency^{[33, 38-41]}. This further demonstrates that Hg⁰ adsorption on the spent FCC catalysts is mainly contributed by the chemical adsorption.

  Figure 11

  

  Figure 11.  Effect of NO concentration in the flue gas on the Hg⁰ removal efficiency of FCC-E; the operation conditions of Set VI specified in Table 1 are used

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  To confirm the mechanism of oxidation-promoted mercury removal, a temperature program desorption (TPD) test was conducted to observe the mercury compounds on the adsorbent after the adsorption test in the presence of NO. N₂ was chosen here as the carrier gas and the adsorbent was heated from 50 to 600 ℃ at the heating rate of 5 ℃/min. The Hg⁰-TPD profile is shown in Figure 12. The release of Hg⁰ appears at approximately 145 and 240 ℃. The peak at 145 ℃ corresponds to the physically adsorbed Hg⁰. The boiling point of the pure Hg(NO₃)₂ is approximately 180 ℃; it decomposes to HgO and further to Hg⁰, with the increase of the temperature^[42]. In addition, the mercury species, heating rate, vapor pressure etc. can also influence the decomposition characteristics of the mercury-containing compounds. According to literature, the single peak at 240 ℃ could be assigned to the desorption of Hg from Hg(NO₃)₂.

  Figure 12

  

  Figure 12.  TPD profile of the SFCC-E adsorbent under N₂ after mercury adsorption in the presence of NO and O₂ at 120 ℃

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  In the presence of NO and O₂, the NO₂^* species is formed between NO and the surface or lattice oxygen over the FCC-E catalyst. The mechanism can be summarized as:

  $ \begin{array}{l} \mathrm{NO}(\mathrm{g})+\mathrm{O}^{*}(\mathrm{ads}) \rightarrow \mathrm{NO}_{2}{ }^{*}(\mathrm{ads}) \end{array} $

  (2)

  $ \mathrm{O}_{2}(\mathrm{~g}) \rightarrow \mathrm{O}^{*}(\text { ads }) $

  (3)

  $ \mathrm{Hg}(\mathrm{g}) \rightarrow \mathrm{Hg}^{*}(\text { ads }) $

  (4)

  $ \mathrm{Hg}^{*}(\mathrm{ads})+\mathrm{O}^{*}(\mathrm{ads})+\mathrm{NO}_{2}^{*}(\mathrm{ads}) \rightarrow \\ \mathrm{Hg}\left(\mathrm{NO}_{3}\right)_{2}(\mathrm{ads}) $

  (5)

  Current results illustrate that the Hg⁰ removal efficiency of the activated SFCC catalysts, prepared with ethanol as solvent and calcined at 120 ℃, is close to 100% in the presence of 0.06% NO, 12% CO₂, 6% oxygen and balanced N₂. The Hg⁰ removal efficiency remains stable after adsorption for 120 min. This indicates that the SFCC catalysts are suitable for the removal of Hg⁰ by adsorption from the coal-fired flue gas after proper activation.

  3.   Conclusions

  The spent fluid catalytic cracking (SFCC) catalysts were activated by an "internal instant vaporization (IIV)" method and used in the removal of Hg⁰ from a simulated flue gas in a fixed bed reactor; the effect of various operation parameters such as the SFCC activation conditions, adsorption temperature, and flue gas components on the Hg⁰ removal efficiency was investigated.

  The results indicate that the blocked catalyst channels can be effectively unchoked by the IIV method with the organic solvent without impairing the internal channel structure. The SFCC catalyst activated with methanol or ethanol performs adequately in terms of Hg⁰ removal, whilst the calcination temperature also has a great influence on the activation of the SFCC catalyst. O₂ in the flue gas favors the Hg⁰ removal, whilst NO facilitates the oxidation of mercury and displays a positive effect for mercury removal in the presence of O₂, accompanying with the formation of N-containing active species on the FCC-E surface. A relatively higher adsorption temperature is beneficial to the Hg⁰ removal on the activated SFCC catalyst, indicating that the chemical adsorption plays an important role. The presence of O₂ and NO promote the Hg⁰ adsorption on the activated SFCC catalyst, suggesting that the Hg⁰ adsorption on the spent FCC catalysts is contributed mainly by the catalytic oxidation. SO₂ in the flue gas, depending on its concentration, may exert the effect of catalytic adsorption or competitive adsorption on the Hg⁰ removal. Approximately 100% Hg⁰ can be removed in the stream of 6% O₂, 12% CO₂ and 0.06% NO at 120 ℃ by using the activated SFCC catalyst with ethanol as an organic solvent and calcined at 120 ℃. It suggests that the SFCC catalysts after proper activation are potential adsorbents for the removal of Hg⁰ from the coal-fired flue gas.

  
  本文的英文电子版由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18725813).
  
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  Figure 1  Schematic diagram of the fixed-bed adsorption evaluation device

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  Figure 2  SEM images of the activated and non-activated SFCC catalysts: ((a), (b)) SFCC; ((c), (d)) FCC-360; and ((e), (f)) FCC-M

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  Figure 3  TG/DTA curves of the SFCC catalyst in the air atmosphere (a) and N₂ atmosphere (b)

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  Figure 4  XPS spectra of the FCC-120 catalyst: (a) survey of elements; (b) Fe 2p; (c) Ce 3d; and (d) O 1s

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  Figure 5  XRD patterns of various catalyst samples: a: SFCC; b: FCC-360; c: FCC-600; d: FCC-M; e: FCC-E; f: FCC-A; g: FCC-P

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  Figure 6  Effect of the calcination temperature on the Hg⁰ removal efficiency of activated SFCC catalysts; the operation conditions of set I specified in Table 1 are used

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  Figure 7  Effect of the activation organic solvent on the Hg⁰ removal efficiency of the activated SFCC catalysts; the operation conditions of Set II specified in Table 1 are used

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  Figure 8  Effect of different adsorption temperature on the Hg⁰ removal efficiency of FCC-E; the operation conditions of Set III specified in Table 1 are used

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  Figure 9  Effect of O₂ concentration in the flue gas on the Hg⁰ removal efficiency of FCC-E; the operation conditions of Set IV specified in Table 1 are used

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  Figure 10  Effect of SO₂ concentration in the flue gas on the Hg⁰ removal efficiency of FCC-E; the operation conditions of Set V specified in Table 1 are used

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  Figure 11  Effect of NO concentration in the flue gas on the Hg⁰ removal efficiency of FCC-E; the operation conditions of Set VI specified in Table 1 are used

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  Figure 12  TPD profile of the SFCC-E adsorbent under N₂ after mercury adsorption in the presence of NO and O₂ at 120 ℃

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  Table 1.  Experimental conditions for the Hg⁰ adsorption tests

  Set	Adsorbent	Activation conditions		Flue gas composition	Adsorption temp t/℃
  		calc. temp /℃	solvent
  I	SFCC, activated SFCC	120-600	-	6% O2, 12% CO2, N2	120
  II	activated SFCC	120	methanol, ethanol, acetone, petroleum ether	6% O2, 12% CO2, N2	120
  III	activated SFCC	120	ethanol	6% O2, 12% CO2, N2	80-300
  IV	activated SFCC	120	ethanol	0-10%O2, 12% CO2, N2	120
  V	activated SFCC	120	ethanol	6% O2, 12% CO2, 0.06%-0.1% SO2, N2	120
  VI	activated SFCC	120	ethanol	6% O2, 12% CO2, 0.01%-0.06% NO, N2	120

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  Table 2.  Textural properties of the activated and non-activated SFCC catalysts

  Adsorbent	Surface area A/(m2·g-1)	Pore volume v/(cm3·g-1)	Pore diameter d/nm
  SFCC	74.9	0.09	3.181
  FCC-120	78.5	0.11	3.382
  FCC-360	90.4	0.11	3.795
  FCC-600	101.2	0.11	4.862
  FCC-M	87.3	0.10	3.804
  FCC-E	83.7	0.10	3.396
  FCC-A	77.9	0.09	3.400
  FCC-P	76.5	0.09	3.389

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  Table 3.  Chemical composition of the SFCC catalysts determined by XRF

  Component	Percentage	Component	Percentage	Component	Percentage
  Al2O3	46.397	Cs2O	1.115	V2O5	0.169
  SiO2	43.147	Na2O	0.782	CeO2	0.166
  CaO	2.012	P2O5	0.714	Sb2O3	0.121
  SO3	1.546	TiO2	0.302	ZnO	0.093
  NiO	1.416	MgO	0.254	La2O3	0.089
  Fe2O3	1.330	K2O	0.177	else	0.163

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