Evolution of sulfur species on titanium ore modified activated coke

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Environmental and Carbon Dioxide Issues

Evolution of sulfur species on titanium ore modified activated coke during flue gas desulfurization Jin Yuan, Xia Jiang, Dong Chen, Wenju Jiang, and Jianjun Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01605 • Publication Date (Web): 03 Jul 2018 Downloaded from http://pubs.acs.org on July 10, 2018

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Energy & Fuels

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Evolution of sulfur species on titanium ore modified

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activated coke during flue gas desulfurization

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Jin Yuan, a Xia Jiang,* ab Dong Chen,a Wenju Jiangab and Jianjun Li* ab

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a

College of Architecture and Environment, Sichuan University, Chengdu 610065, China.

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b

National Engineering Research Center for Flue Gas Desulfurization, Sichuan University, Chengdu

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610065, China.

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*Corresponding author: Xia Jiang, Jianjun Li

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E-mail: [email protected] (Xia Jiang); [email protected] (Jianjun Li)

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Tel: +86-28-8540 7800; fax: +86-28-85405613

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1

ACS Paragon Plus Environment

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ABSTRACT: This study aimed to investigate the evolution of sulfur species on

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titanium ore, Fe2O3 and TiO2 modified activated coke (i.e. TOAC, FeAC and TiAC)

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during flue gas desulfurization process. The results showed that TOAC, FeAC and

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TiAC displayed better desulfurization performance than blank one, with the highest

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sulfur capacity for TOAC at 209.4 mg g-1. With desulfurization time, the ratios of the

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adsorbed-S and other-S on activated coke decreased gradually, while those of the

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water-soluble sulfate increased significantly. The water-soluble sulfate was the main

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desulfurization product, which accounted for 66.1%, 78.4% and 76.6% of total

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removed SO2 for FeAC, TiAC and TOAC at breakthrough time, respectively. The

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production of water-soluble sulfate could be related to the decrease of C=O group and

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the increase of C-O group. Meanwhile, the produced water-soluble sulfate covered the

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active sites (i.e. functional groups, TiO2 and Fe2O3) on activated coke, resulting in the

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decrease of the adsorption and oxidation of SO2. Higher sulfur capacity of TOAC

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could be attributed to the synergistic effects between Fe2O3 and TiO2 on TOAC. TiO2

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can serve as an oxygen carrier and promote the transfer of oxygen molecule to oxidize

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SO2, while Fe2O3 was transformed into Fe2(SO4)3.

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Keywords: Activated coke; Titanium ore; Desulfurization; Sulfur evolution

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1. INTRODUCTION

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The removal of sulfur dioxide (SO2) by activated coke (AC) was first developed in

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German in the 1960s. Currently, it is an increasingly preferred flue gas desulfurization

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method because of its many advantages: a small area take-up, generation of small

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amounts of by-products,1 conversion of SO2 into useful sulfur-containing products,2

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simultaneous removal of various pollutants (SO2, NOx, Hg and VOCs)3 and low

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consumption of energy and water.4 However, the SO2 capture capacity of common

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activated coke is relatively low, which severely limited its wide application.5 A large

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number of studies have reported that high SO2 capture capacity was achieved when 2


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AC were modified by the addition of transition metals, such as Fe,6, 7 Mn8, 9 and Ti,10,

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11

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mg g-1 for the Fe-modified AC. The sulfur capacity of TiO2 modified AC reached

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200.6 mg g-1, which was 1.4 times higher than that of blank one.10

etc. Guo, et al.

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reported that the SO2 capture capacity increased from 84 to 322

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Titanium ore is a kind of widely distributed natural ore containing abundant

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transition metals, such as Ti and Fe. When titanium ore is used as the additive to

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replace chemical agents for activated coke modification, the cost of desulfurizers

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could be significantly reduced. Moreover, different transition metals (Ti and Fe) in

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titanium ore may have a synergistic effect for higher desulfurization activity than

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single metal component.13 Recently, we found that the desulfurization performance

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can be significantly improved by adding titanium ore for the preparation of modified

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AC.11, 14 Wang, et al.14 reported that the sulfur capacity of modified AC with 1% of

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titanium ore increased to 203.3 mg·g-1 from 120.1 mg·g-1 for blank AC. This was

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ascribed to the presence of basic functional groups and active metals contained in

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titanium ore on AC, which promotes the adsorption and oxidation of SO2 into more

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stable sulfur species. Moreover, it was also reported that the synergistic effect

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between Fe and Ti in titanium ore was conducive to the development of proper

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physicochemical properties for SO2 removal.5, 11, 14

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During desulfurization process, different desulfurization products could be

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produced and accumulated on AC, which will influence the physicochemical

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properties of AC,15 resulting in the change of desulfurization activity. It was reported

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that sulfur species in common AC mainly existed as the adsorbed SO2 and sulfuric

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acid after desulfurization.16 However, for metals modified AC, sulfur species also

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existed as metal sulfates after desulfurization, owing to the chemical reactions

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between metals and sulfuric acid/SO2.17,

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slowly in SO2 removal process,5, 11, 14 possibly resulting from the evolution of surface

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chemical characteristics on AC, which could severely be influenced by sulfur species

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produced during desulfurization process. Due to the existence of several transition

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Natural ores modified AC deactivated

3


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metals in natural ores, it could be more complex for the evolution of sulfur species on

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ores modified AC with desulfurization time. Previous studies mainly focused on the

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change of surface chemical properties on AC after desulfurization breakthrough.

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However, it is not clear on the evolution of sulfur species and chemical properties on

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ores modified AC during the whole desulfurization process.

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Therefore, in this study, the evolution of sulfur species on titanium ore modified AC

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during desulfurization process was investigated. Meanwhile, through the comparison

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with Fe2O3 and TiO2 modified AC, i.e. its two main metal components, the role of

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sulfur species on AC was explored to provide a better understanding of the evolution

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of surface chemical characteristics and desulfurization performance during

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desulfurization process.

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2. EXPERIMENTAL SECTION

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2.1. Preparation of Activated Coke. Fe2O3, TiO2 and titanium ore modified ACs

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were prepared following our previous study.14 In a typical procedure, two kinds of raw

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coals, bituminous coal and coking coal, were firstly crushed and sieved to pass through

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200-mesh screen. Subsequently, bituminous coal and coking coal were mixed at the

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weight ratio of 7.0:3.0. Fe2O3, TiO2 or titanium ore powder which was also sieved to

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200 meshes was blended with the coals mixture at the ratio of 1 wt %. Distilled water

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and coal tar were used as the binder and then the resultant mixtures were pressed to

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columnar samples with 3 mm diameter in a vacuum extruder under 10 MPa pressure.

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Next, the columnar samples were heated to 543 K under air atmosphere for 40 min to

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pre-oxidation, 873 K under N2 condition for 1 h to remove the volatiles, and then

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activated at 1173 K under a certain gas flow ( M H 2O : M C = 0.5) for 40 min in a tube

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furnace. The heating rate was 5 K min-1. The activated samples were cooled to room

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temperature under N2 atmosphere. The as-prepared Fe2O3, TiO2 and titanium ore

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modified samples were labeled as FeAC, TiAC and TOAC, respectively. A blank 4


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sample was prepared by the carbonization and activation of coals mixture without the

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additives and designated as AC. TOAC, TOAC-2, TOAC-5, TOAC-10 and TOAC-BT

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represented the TOAC after being used to remove SO2 for 0, 2, 5, 10 h and in

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breakthrough time, respectively.

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2.2. Desulfurization Activity Test. A fixed-bed glass reactor with the inner

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diameter of 21 mm was used for desulfurization activity tests. The packed height of

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samples was 100 mm. The reactant gas mixture contained: 3000 ppmv SO2, 10% O2,

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10% H2O vapor and balanced with N2. And the gas flow rates were controlled by mass

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flow controller system. Moreover, the corresponding bed temperature was 353 K and

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the gas space velocity was 600 h-1. The inlet and outlet concentrations of SO2 were

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continuously monitored on-line by a flue gas analyzer (Gasboard-3000, China) with a

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detection limit of 1 ppmv. Once the outlet SO2 concentration was reached 10% of inlet

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concentration (i.e. 300 ppmv), the desulfurization test was stopped. The

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corresponding working time was regarded as the breakthrough time, and the sulfur

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capacity was computed by the SO2 breakthrough curve.

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2.3. Characterization of Samples. N2 adsorption on the samples at 77 K was

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measured using the surface area analyser (ASAP 2460, Micromeritics, USA). The

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specific surface areas (SBET), micropore volume (Vmic), mesopore volume (Vmes) and

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total pore volume (Vtot) were calculated from the N2 adsorption isotherms using the

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BET equation, t-plot method, BJH method and adsorption amount at the maximum

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relative pressure, respectively. The X-ray diffraction (XRD) patterns of the samples

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were detected by a power X-ray diffractometer (X’Pert PRO MMPD, Netherlands)

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using Cu Kα radiation (λ = 0.15406 nm) in the 2θ range of 10 ~ 80°. The crystalline

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phases were recognized through comparing with reference data from the International

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Center for Diffraction Data (JCPDs). The surface chemistry properties of the samples

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were determined by X-ray photoelectron spectroscopy (XPS), which performed on a

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spectrometer (XSAM–800, Kratos Co., UK) with Al Kα radiation source. And the core 5


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level spectra of Au 4f 7/2 (84.0 eV) and Ag 3d5/2 (368.3 eV) was recorded to calibrate

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binding energy (BE).

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2.4. Analytical Methods. After desulfurization, the used samples were firstly placed

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in a tube furnace to be heated to 423 K at a heating rate of 5 K min-1 and kept for 1 h

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under N2 condition. The exhaust gas passed through a solution containing excessive

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H2O2 (3%) and the formed H2SO4 was determined by titrating with 0.01 mol L-1 of

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NaOH solution.11

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Subsequently, the desorbed samples were transferred to a 500 mL of Erlenmeyer

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flask and 100 mL of deionizing and deoxidizing water was added. Afterwards, the

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flask was put in an ultrasonic washer to be ultrasonically cleaned at 353 K for 20 min.

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And then the washing solution was poured out and its pH was measured. The

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ultrasonically cleaning was repeated until the pH value of washing solution was

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constant. Finally, the washing liquid was collected and SO32- and SO42- amount was

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measured. The concentration of SO32- was determined by iodine titration method

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according to Chinese standard HJ/T 56-2000. The concentration of SO42- was

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determined by turbidimetric method according to American standard (ASTM

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D516-2007).

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3. RESULTS AND DISCUSSION

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3.1. Desulfurization Performance. The desulfurization performance of the

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prepared samples is presented in Fig. 1. Fe2O3, TiO2 and titanium ore modified

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samples displayed better desulfurization performance than blank AC, revealing that

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the addition of Fe2O3, TiO2 and titanium ore could markedly improve desulfurization

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activity of AC. For all the samples, the SO2 outlet concentrations gradually increased

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with working time after 10 h, showing that the samples exhibit a gradual deactivation

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process. This could be due to the evolution of surface chemical characteristics and the

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accumulation of desulfurization products.11,

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capacity of TOAC at breakthrough time was 209.4 mg g-1, which was 1.74 times higher

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As shown in Fig. 1 (b), the sulfur

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than that of blank AC. The breakthrough time of TOAC was considerably extended to

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24.1 h, which was 78.30% longer than that of blank one. It was also found that the

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sulfur capacity of TOAC was higher than those of TiAC and FeAC, which could be

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attributed to the synergistic effects between Fe2O3 and TiO2 in TOAC.11

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As shown in Table 1, compared with the blank one, Fe2O3 modified AC showed

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an obvious increase in the SBET, Vtot and Vmic. This was primarily attributed to the

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reaction between Fe2O3 and carbon matrix during activation process, which can

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improve the micropore structure of AC.19, 20 SBET and Vtot of TiO2 modified AC did

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not apparently change in comparison with blank AC. However, when the sample was

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prepared with the addition of titanium ore, there was an obvious decrease in SBET and

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Vtot, especially Vmes of AC. This indicates that titanium ore particles could enter into

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the mesopores of AC.

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As shown in Fig. 1, the desulfurization activity of TOAC was the highest among

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these samples. This indicates that the pore structure is not the key factor for affecting

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the desulfurization performance of the modified AC. The great improvement of

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desulfurization performance of TOAC might be ascribed to its surface chemical

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characteristic, i.e. the metal species in titanium ore and high content of active sites.2

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3.2. Evolution of Sulfur Species during Desulfurization Process. The sulfur

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species on activated coke were divided into adsorbed-S, water-soluble sulfate and

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other-S in this study. It was reported that the sulfur species obtained by temperature

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programmed desorption and ultrasonic washing included adsorbed SO2.1, 3, 21 In the

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presence of water vapor, a large amount of hydroxyl was produced on the surface of

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Fe2O3 due to its high hydrophilia 21, 22. And then, SO2 can easily bond to the hydroxyl

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and be transformed to H2SO3 during desulfurization process. H2SO3 in the AC could

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be easily decomposed into SO2 during thermal treatment process, and also can be

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dissolved in water when ultrasonic washing. Therefore, in this study, H2SO3 may also

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exist in the adsorbed-S. The amount of water-soluble sulfate was measured by the 7


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concentration of SO42- in both collected aqueous solution from the receiver flask and

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ultrasonic washing liquid of the used samples. The water-soluble sulfate mainly

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consisted of H2SO4 and metals sulfate which can be washed off from AC.23 The

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amount of other-S was obtained by sulfur mass balance calculation according to the

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sulfur capacity, adsorbed-S and water-soluble sulfate. Other-S was considered as other

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more stable sulfur species, water insoluble sulfate and the sulfate in the pores which is

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hard to be removed from the used samples by water washing.

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3.2.1. Adsorbed-S. The amounts of the adsorbed-S on the modified ACs within 2, 5,

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10 h and breakthrough time are shown in Fig. 2. For all samples, the contents of the

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adsorbed-S increased with desulfurization time. Within 10 h, the amount of the

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adsorbed-S followed the order of FeAC > TiAC > TOAC, which was the same order

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of SBET, Vtot, Vmes and Vmic (Table 1). A plenty of mesopores and micropores in the

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FeAC can offer a larger accessible area and volume for SO2 adsorption, a shorter SO2

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diffusion distance, and smaller resistance for SO2 transport through the framework.7, 21

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Thus, the FeAC can adsorb more SO2 compared with TiAC and TOAC. The result

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also indicated that the pore structure of activated coke is important for SO2 adsorption.

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From 10 h to breakthrough time, the amount of the adsorbed-S on FeAC remained

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nearly constant. This suggested that the dynamic equilibrium between the SO2

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adsorption and oxidation process was achieved after 10 h for FeAC. The result also

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indicated that SO2 was first adsorbed on the surface of samples and then oxidized to

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more stable species during desulfurization process. At breakthrough time, the amounts

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of the adsorbed-S on the TOAC and TiAC were higher than that of FeAC, which

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could be related to the active metal species for SO2 adsorption on the surface of the

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samples. In addition, it is also interesting to observe that the amount of adsorbed-S on

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TOAC was greater slightly than that on TiAC, which could be due to the synergistic

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effect between Ti and Fe in titanium ore for SO2 adsorption.11

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3.2.2. Water-Soluble Sulfate. The amounts of water-soluble sulfate on the samples 8


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within 2, 5, 10 h and breakthrough time are displayed in Fig. 2. For the three samples,

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the amounts of water-soluble sulfate increased with desulfurization time. This

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suggested that the adsorbed-S in the system was continuously oxidized into SO42-

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during desulfurization process. It can be seen that the amount of water-soluble sulfate

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followed the order: TOAC > TiAC > FeAC, through the whole desulfurization process.

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Titanium ore modified AC had the highest amount of water-soluble sulfate, which was

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attributed to the synergistic effect between Ti and Fe in the TOAC, resulting in the

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higher catalytic oxidation for SO2.5 In addition, the amount of generated water-soluble

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SO42- on the TiAC was higher than that of the FeAC. This could be related to TiO2

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crystallites supported on activated coke which had strong catalytic abilities for SO2

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oxidation.10, 11

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3.2.3. Other-S. XRD was performed to further clarify the existence of other-S. Fig.

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3 shows the XRD patterns of FeAC, TiAC and TOAC before and after desulfurization

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and desorption/water washing. From Fig. 3 (a), for all fresh samples, the diffraction

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peaks of SiO2 were observed. TiO2 peaks at 2θ = 25.5°, 32.9° and 56.1° (JCPDs

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75-1582) were detected on the surface of TiAC and TOAC, and Fe2O3 peaks were

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found on FeAC and TOAC at 2θ = 35.2°, 36.9° and 61.1° (JCPDs 52-1449). After

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desulfurization and desorption/water washing (Fig. 3 (b)), the new peaks attributed to

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CaSO4 at 2θ = 25.3°, 32.8° and 36.9° (JCPDS 74-1782) were clearly observed on the

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surface of all the modified samples. Moreover, new peaks attributed to Fe2(SO4)3 at 2θ

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= 25.5°, 35.3° and 40.9° (JCPDS 70-1819) appeared in TOAC and FeAC.

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The amounts of other-S on the modified ACs during desulfurization process are

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shown in Fig. 2. The amounts of other-S on FeAC, TiAC and TOC all increased with

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working time, with the order as follows: FeAC > TOAC > TiAC. The Fe2O3 modified

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AC had the highest content of other-S among the three samples. This could be

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ascribed to the highest content of Fe2O3 in the FeAC. Fe2O3 was able to be transferred

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into Fe2(SO4)3 through surface reaction, which was retained in the pores of the sample 9


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during desulfurization process and was not washed off by water easily. The results

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also suggested that TiO2 is a more suitable additive to modify activated coke

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compared with Fe2O3. H2SO4 was only produced during TiO2 modified AC

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desulfurization process, which is very easy to be water washed for regeneration of

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AC.

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3.2.4. Sulfur Balance. The evolution in the ratios of sulfur species with desulfurization

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time are depicted in Fig. 4. For all the samples, the ratios of water-soluble sulfate

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increased while those of adsorbed-S and other-S decreased with working time. At

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breakthrough time, the ratios of water-soluble sulfate on FeAC, TiAC and TOAC were

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66.1%, 78.4% and 76.6%, respectively. This showed that the major product of

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desulfurization by the modified samples were in the oxidized state of sulfur species.

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The ratio of other-S on Fe2O3 modified AC was 17.4% at breakthrough time, which

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was the highest among the three samples. This could be attributed to the Fe2(SO4)3

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deposited on the Fe2O3 modified AC (Fig. 3 (b)), which was hard to be removed by

240

water washing. For TiAC and TOAC, the ratios of other-S were only 3.2% and 7.1%

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at breakthrough time, respectively, while they showed higher ratios of water-soluble

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sulfate than FeAC. This suggests their good catalytic oxidation activity and

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regeneration potential. These are very important for the industrial application of

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activated coke for desulfurization process. The results demonstrated that titanium ore

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is an effective additive to prepare modified activated coke for SO2 removal.

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3.3. Evolution of Elements during Desulfurization Process. Fig. S1-S3 show the

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wide-scan XPS spectra for Fe2O3, TiO2 and titanium ore modified samples during

248

desulfurization process, respectively. The peak attributed to titanium (Ti 2p3) at 459.1

249

eV was observed on the surface of TOAC and TiAC. The low peak corresponded to

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iron (Fe 2p) was due to iron oxide and iron sulfate formed during desulfurization

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process. As desulfurization proceeded, a clearly new peak assigned to sulfur (S 2p) at

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169.4 eV was detected. 10


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The evolution of elements on FeAC, TiAC and TOAC during desulfurization

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process is displayed in Table 2. Before desulfurization, the relative oxygen content of

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TOAC was the highest among the three samples, indicating that the introduction of

256

titanium ore was beneficial to the development of oxygen containing functional

257

groups. The contents of oxygen on the surface of the three samples increased with

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exposure time. This could be related to the absorbed oxygen, water and

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desulfurization products (adsorbed SO2, H2SO4 and sulfates) on the surface of samples

260

during desulfurization process. Moreover, the ratios of oxygen to carbon on the three

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samples increased with desulfurization time, which implied that the surface acidic

262

sites could increase with desulfurization time.24

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During the whole desulfurization process, the relative contents of sulfur on the

264

surface of the samples were as follows: TOAC > TiAC > FeAC. This is corresponded

265

with the sulfur capacities of the three samples (Fig. 1). The change of sulfur species

266

on TOAC with desulfurization time are shown in Fig. S4. The peaks at around 169.4

267

eV were observed for all the samples, which are assigned to sulfur in the form of

268

sulfate (SO42-).25 There was no discernible peak corresponded to sulfite found. This

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was mainly attributed to the adsorbed-S on the surface of carbon, which could be

270

oxidized into sulfate easily during the sample preparation for XPS analysis. As shown

271

in Table 2, the relative contents of sulfur on the surface of the samples first increased

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slowly with desulfurization time, and then increased significantly especially after 10 h.

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Meanwhile, the desulfurization products deposited on the surface of the samples

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would occupy and cover the active sites, i.e. the surface groups and metal oxides,

275

leading to the decrease in desulfurization activity of activated coke.

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3.4. Evolution of Surface Functional Groups during Desulfurization Process.

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The C 1s XPS patterns of FeAC, TiAC and TOAC are displayed in Fig. S5-S7,

278

respectively. The C 1s complex spectrum of the samples were fitted into four peaks:

279

(1) the peak at 284.3 ± 0.3 eV was assigned to graphitic carbon, i.e., C-C carbon;26, 27

280

(2) the peak at 285.6 ± 0.2 eV was corresponded to C-O carbon;28, 29 (3) the peak at

281

287.0 ± 0.2 eV was due to the C=O carbon;27, 30 (4) the peak at 289.2 ± 0.3 eV was

282

attributed to carbonate or the π-π* transitions in aromatic rings.26,31

283

The corresponding relative contents of C 1s for the samples during 11


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desulfurization process are listed in Table 3. During the desulfurization process, the

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relative contents of C-O group for all samples increased with working time, while

286

those of C=O group decreased. This indicates that the oxygen-containing groups

287

might involve in the SO2 removal process. It was reported that the high content of

288

C-O group was not conductive to desulfurization because C-O group has acidic

289

characteristics.32 On the other hand, since the C=O group has Brønsted basic

290

properties,33 the desulfurization performance would decrease with the decrease of

291

C=O group contents.

292

There could be some reactions happened on carbon matrix in the presence of O2

293

in flue gas during desulfurization process (eq. (1) and (2)).34, 35 The formed SO3

294

could be most likely adsorbed on the C=O group because of its Brønsted basic

295

properties (negatively charged).21 It is reported that the C=O group mainly exists as

296

the quinone and chromene on the surface of AC.36 Thus, the role of the C=O group

297

in the SO2 removal process could be described as eq. (3).21 Meanwhile, water vapor

298

in flue gas would result in the formation of H2SO4 (eq. 4).

299

SO2(g) → SO2(ads)

(1)

300

SO2(ads) + O(ads) → SO3(ads)

(2)

301

(3)

302

(4)

303

Reactions (3) and (4) demonstrated that the C=O group could help to oxidize SO2

304

into H2SO4 in the presence of H2O and O2 while it would be transformed into the C-O

305

group. Therefore, it is assumed that the production of water-soluble sulfate could be

306

related to the decrease of C=O groups and the increase of C-O groups during the 12


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Energy & Fuels

307

desulfurization process (Table 3).

308

3.5. Evolution of Metals Phase during Desulfurization Process. XRD analysis

309

was conducted to clarify the evolution of metal phase on the modified samples during

310

desulfurization process, and the results are illustrated in Fig. 5. For all the samples, the

311

diffraction peaks of SiO2 at 2θ = 20.9°, 22.8°, 28.6° and 43.9° (JCPDS 78-1423) were

312

detected, which were derived from the coals used for the sample preparation. As shown

313

in Fig. 5 (a), the obvious peaks at 2θ = 20.8°, 28.9° and 60.9° were observed in FeAC,

314

which were identified as Fe2O3 (JCPDS 76-1821). Furthermore, the intensity of

315

diffraction peaks of Fe2O3 was weaken with desulfurization time, and finally

316

disappeared after 10 h. On the other hand, new peaks assigned to iron sulfate at 2θ =

317

24.1°, 35.3° and 48.7° (JCPDS 70-1819) appeared at 2 h and become noticeable with

318

further desulfurization. For TOAC, the similar evolution of iron species was observed

319

in Fig. 5 (c). The results indicated that Fe2O3 and Fe species in titanium ore were

320

involved in the SO2 removal process.

321

It was reported that the gas-phase SO2 cannot react with Fe2O3 particles

322

directly.22, 37 In the presence of water vapor, SO2 can easily bond to the surface of iron

323

oxide and form adsorbed sulfurous acid (H2SO3) due to the high hydrophilia of Fe2O3,

324

resulting in a large population of hydroxyl on the surface of Fe2O3 particles.38

325

Sulfurous acid was further transformed into H2SO4 through a series of heterogeneous

326

reactions in the presence of oxygen.24, 26 Since H2SO4 and Fe2O3 co-existed on the

327

surface of sample, it could be highly possible that Fe2O3 react with H2SO4 to form

328

Fe2(SO4)3.

329

As shown in Fig. 5 (b), the peaks at 2θ = 25.3°, 31.4° and 56.2° were observed in

330

TiAC, which were assigned to the characteristic peaks of TiO2 (JCPDS 75-1582). The

331

peaks of TiO2 were detected in the whole desulfurization process. However, the peaks

332

of Ti-containing sulfate were not observed. Similar phenomena were also observed in

333

titanium ore modified samples, as shown in Fig. 5 (c). The result suggests that TiO2 13


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Page 14 of 31

334

on activated coke remained stable during the desulfurization process. It was very

335

different from the Fe2O3-modified one, where Fe2O3 on activated coke was transferred

336

into Fe2(SO4)3 in the desulfurization process. This suggests that the role of TiO2 in

337

SO2 removal was different from that of Fe2O3. TiO2 may serve as an oxygen carrier

338

and promote oxygen transfer to catalytic oxidize SO2 into SO3 indirectly, due to the

339

high redox ability and variable electronic structure of TiO2 which can adsorb oxygen

340

molecules.39

341

3.6. Desulfurization Mechanism of Titanium Ore Modified Activated Coke.

342

Above results demonstrated that the addition of titanium ore on activated coke greatly

343

enhanced the desulfurization performance, which could be attributed to its unique

344

mechanism of SO2 removal. It could be assumed that SO2 gas can be bonded to the

345

surface of the titanium ore modified AC following several paths, as shown in Fig. 6:

346

(1) on the surface of carbon matrix, including Oxygen-containing functional group i.e.

347

C=O; (2) on the surface of TiO2; (3) on the surface of Fe2O3.

348

The pathway of SO2 removal by TiO2 may consist of four steps. First, SO2 is

349

adsorbed on the active sites adjacent to TiO2 on activated coke and oxygen is

350

adsorbed on the surface of TiO2. Second, SO2 can be oxidized to SO3 by oxygen

351

adsorbed on TiO2. Third, H2SO4 is formed in the presence of water vapor. Last, the

352

formed H2SO4 can be washed off from the surface of TiO2 to recover the active sites

353

by excessive amount of water. The total reaction could be expressed as:

354

SO2 +

1 TiO2 O2 + H2O   → H2SO4 2

(5)

355

Some of the generated H2SO4 might retain in the pores of activated coke, leading to

356

a decrease in the oxidation of SO2 to SO3, and thus the desulfurization activity

357

deceased (Fig. 1).

358

The removal of SO2 on the surface of Fe2O3 on the modified AC might occur by the

359

following procedure: (1) H2O is adsorbed on the surface of Fe2O3; (2) SO2 bonds to

360

H2O and form H2SO3 on the surface of Fe2O3; (3) H2SO3 is transformed into H2SO4 14


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Energy & Fuels

361

by the catalytic oxidation of Fe2O3 in the presence of oxygen; (4) Fe2O3 reacts with

362

H2SO4 into Fe2(SO4)3. The main reactions are likely as follows:

363

SO2 +

1 O2 + H2O Fe 2O 3 → H2SO4 2

(6)

364

3H 2 SO 4 + Fe 2 O 3 → Fe 2 (SO 4 ) 3 + 3H 2 O

(7)

365

The generated Fe2(SO4)3 would deposit on the surface and in the pores of TOAC

366

(Fig. 5). Their presence would lead to a decrease in the number of SO2 active

367

adsorption sites since iron in Fe2(SO4)3 can bond to carbon matrix via oxygenated

368

groups,40 which consequently cover the surface groups on activated coke. In addition,

369

the decrease of Fe2O3 contents on the samples with desulfurization time (Fig. 5) also

370

weakened the ability of SO2 adsorption and oxidation. Both the decrease of Fe2O3 and

371

the deposition of Fe2(SO4)3 would result in a decrease in desulfurization activity of

372

activated coke (Fig. 1).

373

4. CONCLUSIONS

374

The results demonstrate that titanium ore was an effective additive to prepare

375

modified activated coke for SO2 removal. Adsorbed-S, water-soluble sulfate and

376

other-S on activated coke all increased with desulfurization time, with water-soluble

377

sulfate as the main desulfurization product, which accounted for 66.1%, 78.4% and

378

76.6% of total removed SO2 for FeAC, TiAC and TOAC at breakthrough time,

379

respectively. XPS analysis showed that water-soluble sulfate accumulated on the

380

surface of samples first increased slowly with desulfurization time, and then increased

381

rapidly after 10 h. The production of water-soluble sulfate could be related to the

382

decreased content of C=O groups and increased content of C-O groups on activated

383

coke. TiO2 remained stable while Fe2O3 was transformed into Fe2(SO4)3 during flue

384

gas desulfurization. The deposition of desulfurization products (Fe2(SO4)3 and H2SO4)

385

consequently covered the actives sites on activated coke (i.e. functional groups, TiO2

386

and Fe2O3), resulting in the decrease of the adsorption and oxidation of SO2. 15


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387

ASSOCIATED CONTENT

388

Supporting Information

389

Fig. S1-S3 show the Wide-scan XPS spectra for FeAC, TiAC and TOAC during

390

desulfurization process, respectively. Fig. S4 presents the S 2p XPS spectra for TOAC

391

during desulfurization process. Fig. S5-S7 exhibit the C 1s XPS pattern of FeAC,

392

TiAC and TOAC during desulfurization process, respectively.

393

ACKNOWLEDGMENTS

394

This work is supported by National Nature Science Foundation of China (No.

395

51778383) and Science & Technology Department of Sichuan Province

396

(2018HH0096).

397

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398 399 400

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Jing, W.; Guo, Q.; Hou, Y.; Han, X.; Huang, Z. Study of SO2 oxidation over

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Liu, X.; Sun, F.; Qu, Z.; Gao, J.; Wu, S. The effect of functional groups on the SO2

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Fan, L.; Chen, J.; Guo, J.; Jiang, X.; Jiang, W. Influence of manganese, iron and pyrolusite

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Shu, S.; Guo, J.; Liu, X.; Wang, X.; Yin, H.; Luo, D. Effects of pore sizes and

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Üçer, A.; Uyanik, A.; Aygün, Ş. F. Adsorption of Cu(II), Cd(II), Zn(II), Mn(II) and Fe(III)

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Xiao, Y.; Liu, Q.; Liu, Z.; Huang, Z.; Guo, Y.; Yang, J. Roles of lattice oxygen in V2O5 and

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activated coke in SO2 removal over coke-supported V2O5 catalysts. Appl. Catal. B-Environ. 2008,

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82 (1), 114-119.

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Zhang, C.; Yang, D.; Jiang, X.; Jiang, W. Desulphurization performance of TiO2-modified

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activated carbon by a one-step carbonization-activation method. Environ. Technol. 2016, 37 (15),

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1895-905.

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Guo, J.; Fan, L.; Peng, J. F.; Chen, J.; Yin, H.; Jiang, W. Desulfurization activity of metal

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oxides blended into walnut shell based activated carbons. J. Chem. Technol. Biot. 2014, 89 (10),

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Guo, J.; Liu, X.; Luo, D.; Yin, H.; Li, J.; Chu, Y. Influence of calcination temperatures on

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the desulfurization performance of Fe supported activated carbons treated by HNO3. Ind. Eng.

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Chem. Res. 2015, 54 (4), 1261-1270.

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Carabineiro, S. A. C.; Ramos, A. M.; Vital, J.; Loureiro, J. M.; Órfão, J. J. M.; Fonseca, I.

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M. Adsorption of SO2 using vanadium and vanadium–copper supported on activated carbon. Catal.

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Wang, P.; Jiang, X.; Zhang, C.; Zhou, Q.; Li, J.; Jiang, W. Desulfurization and

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regeneration performance of titanium-ore-modified activated coke. Energy Fuels 2017, 31 (5),

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5266-5274.

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Gao, X.; Liu, S.; Zhang, Y.; Luo, Z.; Cen, K. Physicochemical properties of metal-doped

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activated carbons and relationship with their performance in the removal of SO2 and NO. J. Hazard.

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Mater. 2011, 188 (1-3), 58-66. 17


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Yang, L.; Jiang, X.; Yang, Z.; Jiang, W. J. Effect of MnSO4 on the removal of SO2 by

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Yang, L.; Huang, T.; Jiang, X.; Li, J.; Jiang, W. The effects of metal oxide blended

activated coke on flue gas desulphurization. RSC Adv. 2016, 6 (60), 55135-55143.

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on cypress-sawdust-based activated carbon by microwave-assisted H3PO4 mixed with Fe/Mn/Zn

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activation. Desalin. Water Treat. 2017, 100, 214-222.

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Arcibar-Orozco, J. A.; Rangel-Mendez, J. R.; Bandosz, T. J. Reactive adsorption of SO2

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on activated carbons with deposited iron nanoparticles. J. Hazard. Mater. 2013, 246-247 (4),

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300-309.

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Fu, H.; Wang, X.; Wu, H.; Yin, Y.; Chen, J. Heterogeneous uptake and oxidation of SO2

on iron oxides. J. Phys. Chem. C 2007, 111 (16), 6077-6085. (23)

Xu, X.; Huang, D.; Zhao, L.; Kan, Y.; Cao, X. Role of inherent inorganic constituents in

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SO2 sorption ability of biochars derived from three biomass wastes. Environ. Sci. Technol. 2016,

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50 (23), 12957-12965.

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effects of the surface oxidation of activated carbon, the solution pH and the temperature on

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Liu, H.; Li, G.; Hu, C. Selective ring C-H bonds activation of toluene over Fe/activated 18


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Jiménez-López, A.; Azevedo, D. C. S.; Cavalcante, C. L. Effects of textural and surface

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characteristics of microporous activated carbons on the methane adsorption capacity at high

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pressures. Appl.Surf. Sci. 2007, 253 (13), 5721-5725.

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Li, Z.; Wu, L.; Liu, H.; Lan, H.; Qu, J. Improvement of aqueous mercury adsorption on

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Lisovskii, A.; Semiat, R.; Aharoni, C. Adsorption of sulfur dioxide by active carbon

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treated by nitric acid: I. Effect of the treatment on adsorption of SO2 and extractability of the acid

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formed. Carbon 1997, 35 (10-11), 1639-1643.

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Yin, S.; He, S.; Ge, M. Reaction between sulfur dioxide and iron oxide cationic clusters.

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Energy & Fuels

500

Table Caption

501

Table 1. Textural Properties of Prepared Samples.

502

Table 2. Relative contents of C 1s, O 1s and S 2p on samples during desulfurization.

503

Table 3. Relative contents of C 1s of samples during desulfurization process.

504

21


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

505

Page 22 of 31

Table 1. Textural properties of prepared samples SBET

Vtot

Vmic

Vmeso

Dmean

(m2·g-1)

(cm3·g-1)

(cm3·g-1)

(cm3·g-1)

(nm)

AC

444

0.240

0.181

0.042

2.16

FeAC

531

0.284

0.221

0.044

2.14

TiAC

428

0.234

0.178

0.033

2.19

TOAC

379

0.187

0.173

0.006

1.97

Sample

506

Notes: SBET, BET surface area; Vtot, total pore volume; Vmeso, mesopore volume; Vmic,

507

micropore volume; Dmean, average pore diameter.

508

22


Page 23 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

509

Table 2. Relative contents of C 1s, O 1s and S 2p on samples during desulfurization. Sample

C (%)

O (%)

S (%)

O/C

S/C

FeAC

85.10

14.30

0.48

0.17

0.006

FeAC-2

81.21

15.38

0.64

0.19

0.008

FeAC-5

80.23

18.84

0.83

0.23

0.010

FeAC-10

79.58

19.41

0.94

0.24

0.012

FeAC-BT

73.62

23.48

2.85

0.32

0.039

TiAC

85.02

14.18

0.64

0.17

0.008

TiAC-2

73.97

23.82

2.10

0.32

0.028

TiAC-5

73.25

23.84

2.85

0.33

0.039

TiAC-10

69.74

26.26

3.89

0.38

0.056

TiAC-BT

57.77

35.63

6.21

0.62

0.110

TOAC

82.67

16.24

0.51

0.20

0.006

TOAC-2

74.40

23.38

2.06

0.31

0.028

TOAC-5

69.76

26.50

3.53

0.38

0.051

TOAC-10

63.83

30.96

5.09

0.48

0.086

TOAC-BT

55.56

38.45

7.06

0.69

0.130

510 511

23


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

512

Page 24 of 31

Table 3. Relative contents of C 1s of samples during desulfurization process. Samples

C-C

C-O

C=O

π-π*

FeAC

70.69

15.20

9.51

4.69

FeAC-2

65.25

18.73

9.43

6.59

FeAC-5

61.28

20.10

9.28

8.80

FeAC-10

60.32

21.31

9.10

9.07

FeAC-BT

60.48

21.11

9.05

9.36

TiAC

65.50

16.70

10.78

7.23

TiAC-2

64.50

17.10

10.40

8.08

TiAC-5

64.30

18.78

10.36

6.97

TiAC-10

63.46

19.81

9.70

7.30

TiAC-BT

62.43

21.01

9.36

7.21

TOAC

62.92

22.41

10.36

4.31

TOAC-2

60.80

23.97

9.70

5.53

TOAC-5

60.16

23.28

9.44

7.12

TOAC-10

59.66

22.43

9.30

8.61

TOAC-BT

58.79

24.14

9.11

7.96

513 514

24


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515

Figure Caption

516

Fig. 1. SO2 breakthrough curves (a) and sulfur capacities (b) for prepared samples.

517

Fig. 2. The amount of adsorbed-S (a), water-soluble sulfate (b) and other-S (c) on

518

FeAC, TiAC and TOAC during desulfurization process.

519

Fig. 3. XRD patterns of FeAC, TiAC and TOAC (fresh samples (a) and exhausted

520

samples after desorption treatment and ultrasonically cleaning (b)).

521

Fig. 4. The ratios of adsorbed-S, water-soluble sulfate and other-S on FeAC (a), TiAC

522

(b) and TOAC (c) during desulfurization process.

523

Fig. 5. XRD patterns of FeAC (a), TiAC (b) and TOAC (c) during desulfurization

524

process.

525

Fig. 6. Schematic diagram of SO2 removal on TOAC.

526

25


(a)

(b)

AC FeAC TiAC TOAC

200

209.4

100

0

200

175.5

120.1

122.9

AC

FeAC

100

0 5

527 528

Page 26 of 31

300 Sulfur capacity (mg g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

SO2 outlet concentration (ppm)

Energy & Fuels

10

15

20

25

Working time (h)

TiAC

TOAC

Sample

Fig. 1. SO2 breakthrough curves (a) and sulfur capacities (b) for prepared samples.

529

26


Page 27 of 31

0.60

(a)

FeAC

TiAC

TOAC

Mass (mmol g-1)

0.45

0.30

0.15

2.50

(b)

Mass (mmol g-1)

2.00 1.50 1.00 0.50 0.00

(c) 0.45 Mass (mmol g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0.30

0.15

0.00

530

2h

5h

10 h

BT

Working time

531

Fig. 2. The amount of adsorbed-S (a), water-soluble sulfate (b) and other-S (c) on

532

FeAC, TiAC and TOAC during desulfurization process.

533

27


Energy & Fuels

▼

(a)

▽ Fe2O3

● TiO2

□ CaS

★FeO

●

▼ SiO2

(b)

▼ ▼

▼ ▼

● □

● ▽▽

□

10

20

30

▽ ▽

◆

◇

◆ ◇

□

▼ ▼

●

TOAC

TOAC

◆

□

40

◆

●◇ ◆

TiAC

★

▼

●

TiAC

●

▼

▼ ▼

◆

● ▼◇ ▼

□

▼ ▼ ●

◇ Fe2(SO4)3 ◆CaSO4 ● TiO2 ▼ SiO2

◆ ● ◆

□

★

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 31

50

▽

60

◇

◇ ◆

FeAC

FeAC

70

10

20

30

40

50

60

70

2θ (°)

2θ (°)

534

Fig. 3. XRD patterns of FeAC, TiAC and TOAC (fresh samples (a) and exhausted

535

samples after desorption treatment and ultrasonically cleaning (b)).

536

28


80

Page 29 of 31

100 (a) Ratio (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(c)

(b)

80 60 40 20 0

2h

5h

10 h

BT

Working time

2h

5h

10 h

BT

Working time

Other-S

Adsorbed-S

2h

5h

10 h

BT

Working time Water-soluble sulfate

537 538

Fig. 4. The ratios of adsorbed-S, water-soluble sulfate and other-S on FeAC (a), TiAC

539

(b) and TOAC (c) during desulfurization process.

540

29


Energy & Fuels

▽ Fe2O3 ◇Fe2(SO4)2 ▼ SiO2

(a) ◇◆ ▼

◇ ▼ ◆

▽

▼ ▽

◇◆

▼ ◆

◆CaSO4 ● TiO2

◆

▼

◇

▽◆

▼

◇

▼

◇

▽◆

▽

▽

FeAC-2

▽

FeAC

◆

●

◆

▼

● ●

◆

▼ ●

◆

▼

●

●

● ▼

◆▼

●

◆▼

●

◆▼

●

◆▼

●

▼

●

TiAC-BT TiAC-10 TiAC-5 TiAC-2

● TiAC

◇ ◆ ▼

◆

▼ ● ◇ ◆ ▼ ▼

▼

● ▼▽

20

▼● 30

● TOAC-BT

● ▼◆ ▽

▼▽ ▼

▼

◇

▼◆ ●

◆ ▼◇

10

FeAC-5

● ▼

●

(c)

▽

▼

★

(b)

FeAC-10

▽

◆

▼ ▽

★ FeO FeAC-BT

★

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 31

▽ ▽

◇

●

◇

●

▽● ▽●

TOAC-10

TOAC-5

●

▽

TOAC-2

●

40

50

TOAC

60

70

80

2θ (°)

541 542

Fig. 5. XRD patterns of FeAC (a), TiAC (b) and TOAC (c) during desulfurization

543

process.

544

30


Page 31 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

545 546

Fig. 6. Schematic diagram of SO2 removal on TOAC.

31


Evolution of sulfur species on titanium ore modified activated coke

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