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Desulfurization and Regeneration Performance of Titanium Ore Modified Activated Coke Pengchen Wang, Xia Jiang, Chuanjun Zhang, Qiying Zhou, Jianjun Li, and Wenju Jiang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b03153 • Publication Date (Web): 24 Mar 2017 Downloaded from http://pubs.acs.org on April 1, 2017
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Energy & Fuels
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Desulfurization and Regeneration Performance
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of Titanium Ore Modified Activated Coke
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Pengchen Wang,a Xia Jiang,ab Chuanjun Zhang,a Qiying Zhou,a Jianjun Liab and Wenju
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Jiang*ab
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a. College of Architecture and Environment, Sichuan University, Chengdu 610065, P.R.
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China.
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b. National Engineering Research Center for Flue Gas Desulfurization, Chengdu 610065,
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P.R. China.
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*Corresponding author: Wenju Jiang
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E-mail:
[email protected] 11
Tel: +86-28-8540 7800, Fax: +86-28-8540 3016
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ABSTRACT： ：In this study, the modified activated coke (AC-Tit) was prepared from
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coals using titanium ore by blending method with one-step carbonization-activation
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process. The results show that the addition of titanium ore affect the physicochemical
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properties of AC-Tit. The AC-Tit samples have higher Vmic and Vmic/Vtot ratios, as well
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as the oxygen and C=O functional group. The metal oxides (i.e. FeTiO3, TiO2, Fe2O3 and
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Fe) were detected on the surface of samples, which attributed to the existence of Fe and
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Ti in the titanium ore. The titanium ore addition improved the desulfurization
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performance of the AC-Tit effectively, the highest sulfur capacity was 203.3 mg/g, which
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was much higher than that of blank one (120.1 mg/g). After desulfurization, some metal
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sulfates, i.e. Fe2(SO4)3 and CaSO4 were detected on the AC-Tit samples. When the
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sample was regenerated, its surface area (SBET) and Vmic did not change significantly, the
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relative content of C=O decreased, while the COOH increased evidently. In addition,
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metal sulfates on the AC-Tit samples were not decomposed during the regeneration
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process. The variation of surface chemical properties of the regenerated AC-Tit caused
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the reduction of desulfurization activity after regeneration.
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1. INTRODUCTION
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In recent years, a large amount of exhaust gases are poured into atmosphere, leading to a
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suitable condition for the heavy hazy pollution in China. The sulfur dioxide (SO2)
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generated from the fossil fuels combustion is the chief contamination, which is the main
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precursors of PM2.5.1 Activated coke (AC) has been proven to be an effective material to
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control the SO2 emission and other air pollutants ascribed to its high surface area,
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abundant pore structure, surface functional groups and high mechanical strength.2-4 2 ACS Paragon Plus Environment
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However, the common AC has the advantage of relatively low SO2 adsorption capacity,
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resulting in a high operating cost, which greatly limits its application in industry.5
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Numerous research have found that the SO2 removal ability of AC was effectively
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enhanced through surface properties modification by loading some transition metals, such
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as Ti, Fe and V.6-8 Generally, two modification methods (i.e. impregnation method and
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blending method) are used for loading transition metals on the AC. Blending method has
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the advantage of simple process compared with another method because the blending
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method is considered as one-step activation, which makes the additives distributed
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uniformly in the matrix.9 More important, the blending method is solid-solid mixing
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method, which suggests that natural ores and other solid wastes containing some
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transition metals are possibly used to modify AC.10 Therefore, the cost of the preparation
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process of modified AC will be significantly reduced.
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Previous study showed that the natural ore such as pyrolusite (mainly contain Mn and
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Fe) modified activated carbon had higher desulfurization performance than single metal
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oxide modified samples (i.e. Mn or Fe).11 Compared with single metal oxide modified
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adsorbent, bimetallic or multi-metallic adsorbents offer several advantages, such as low
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reaction temperature, high thermal stability, high catalytic activity and high sulfur
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capacity.12 It was reported that the sulfur capacity of Fe2O3 modified AC (113.9 mg/g)
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was 1.2 times of blank AC (97.0 mg/g).13 It was also reported that the sulfur capacity of
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TiO2 modified AC was 200.6 mg/g, while the sulfur capacity of blank AC was 149.6
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mg/g.14 Thus, some natural ores including Ti and Fe could be one kind of potential
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additives for the preparation of modified AC for flue gas desulfurization.
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Titanium ore is a kind of cheap and easily obtained natural ore, which mainly contain 3 ACS Paragon Plus Environment
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Ti, Fe and other trace metal elements.15 When the titanium ore was blended with carbon
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precursors to prepare activated carbon, the AC’s desulfurization performance was
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increased greatly.16 This could be attributed to the synergetic effects of Ti, Fe and other
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trace metals in titanium ore. However, the modified AC prepared by blending titanium
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ore and two different coals through carbonization-activation process were not concerned.
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It is possible that carbon precursors in coals can react with titanium ore drastically during
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carbonization and activation process, which might affect textural structures and surface
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properties of modified activated coke, resulting in a different desulfurization
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performance. Moreover, the regeneration performance of titanium ore modified AC is
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unclear, which is different from single metal oxides modified AC.
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Thus, in this study, titanium ore was used as the additive to modify activated coke
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using one-step carbonization-activation method by solid-solid blending method. The
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surface physicochemical property and desulfurization activity of the modified AC and
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regenerated sample were investigated.
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2. EXPERIMENTAL SECTION
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2.1. Materials. The materials used to prepare the activated coke were two kinds of coal,
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the coal tar and the titanium ore. The bituminous coal, which contains (wt%) C, 73.8; H,
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9.7; O, 14.1; N, 2.3 and S, 0.1; the coking coal, which contains (wt%) C, 74.2; H, 4.8; O,
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17.4; N, 2.1 and S, 1.5, both kinds of coals were bought from Shanxi province, P.R.
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China. The coal tar from Sichuan Cola Ind. Co., Ltd. mainly contains (wt%) C, 80.1; H,
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3.9; O, 14.6; N, 1.2 and S, 0.3, and the titanium ore contains (wt%): Fe, 21.7; Ti, 20.8;
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Mg, 2.9; Si, 2.8; Al, 1.5; Ca, 0.6; Mn, 0.4 and Na, 0.1. The materials were analysed by
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elemental (Euro EA 3000, Italy) and mineral analyses. 2.2. Preparation of Activated Coke. Two kinds of coal were first grinded and sieved
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to the size of 0.074 mm, and then mixed uniformly at the weight ratio of (wt%) 7:3
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(bituminous coal to coking coal). Titanium ore powder was directly blended with mixed
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coal powder at the ratio from 1 to 12 wt%. The molded columnar samples with 3 mm
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diameter were obtained in a vacuum extruder under high pressure at 10 MPa. The
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columnar samples were first carbonized at 600 °C under N2 atmosphere for 1 h, and then
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activated at 900 °C under steam atmosphere (MH2O:MC=1:2) for 40 min in a tube furnace
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(length of 127.0 cm and diameter of 9.5 cm). The samples were cooled down under N2
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atmosphere after activation. The prepared sample without the additive was named as
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“AC-0”, and the sample containing titanium ore was named as “AC-Titx”, with “x” as the
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blending ratio of the additive. The samples with Ti, Fe and the mixture of Ti and Fe were
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prepared using the same contents of Ti and Fe in the AC-Tit1 sample, which was named
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as “AC-Ti1”, “AC-Fe1” and “AC- TiFe1”, respectively.
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The weight loss (η) of samples was calculated through the equation as following:
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η=
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where m and m1 (g) are the sample mass before and after activation process,
m1 − m × 100% m
(1)
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respectively. The amount of raw materials of each experiment was 100 g and the weight
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losses of prepared samples caused by the organics and volatiles in raw materials during
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carbonization and activation process were 41.2, 40.7, 39.2, 37.1, 36.6% for AC-Tit1, AC-
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Tit3, AC-Tit6, AC-Tit9 and AC-Tit12, respectively, which were lower compared with
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AC-0 (42.8%).
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2.3. Desulfurization and Regeneration Experiment. The fixed-bed reactor system
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with continuous flow was used for desulfurization activity test. The mass of samples in
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glass tube reactor (diameter: 21 mm, loading height: 100 mm) were in the range of about
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22-26 g. The flue gas used in this experiment consists of 3000 ppmv of SO2, 10% of H2O
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vapor, 10% of O2 and balance gas of N2. The bed temperature of desulfurization
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experiment was 80 °C and gas space velocity was 600 h−1. Flue Gas Analyzer (Gasboard-
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3000, China) were used to continuously monitor the inlet and outlet SO2 concentrations,
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and when the SO2 outlet concentration of reached 300 ppmv (i.e. 10% of inlet
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concentration), the desulfurization test stopped. The corresponding working time of
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samples in desulfurization experiment was designated as breakthrough time, and the
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breakthrough sulfur adsorption amounts were computed through the SO2 adsorption
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curves.
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The kinetic study of SO2 adsorption was analysed using Bangham model represented
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as following:
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qe lg lg qe − qt
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k = lg + m lg t 2.303
(2)
where, qe and qt (mg/g) are the adsorption capacity in the equilibrium and “t” time, and k (min-1) and m are the rate constants of the Bangham model equation. After desulfurization experiment, the AC-Tit1 and AC-Tit3 were adopted for
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regeneration test. The used AC-Tit1 and AC-Tit3 samples were heated at 400 °C under
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N2 atmosphere for 1 h, and the regenerated samples were used for desulfurization test
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again. The desulfurization-regeneration experiment was performed to check the
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regeneration performance of the modified AC samples. The regenerated sample was
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named as “AC-Titx-Rn”, with “n” as the regeneration cycle (n=1, 2, 3). 2.4. Characterization of Activated Coke. N2 adsorption of samples was measured at
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77 K using the surface area analyzer (ASAP 2460, USA). The specific surface area
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(SBET), total pore volume (Vtot), mesopore volume (Vmes), micropore volume (Vmic) and
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pore size distribution were obtained from the N2 adsorption isotherms using the BET
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equation, N2 adsorption amount at the maximum relative pressure, BJH method, t-plot
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method and DFT method, respectively.
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The crystal structure of AC was analyzed by X-ray diffraction (XRD) using an X-ray
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diffractometer (X-Pert PRO MPD, NL). The crystalline phases were identified by
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comparing with the reference data from the International Center for Diffraction Data
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(ICDD). The surface chemistry was determined by an X-ray photoelectron spectroscopy
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(XPS) (XSAM-800, UK). Energy calibration was done by recording the core level
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spectra of Au 4f7/2 (84.0 eV) and Ag 3d5/2 (368.3 eV).
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Fourier-Transforms Infrared Spectroscopy (FTIR) characterization of samples was
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analyzed by the spectrometer (FTIR-6700, USA). The surface morphology and the
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distribution of Fe and Ti on the surface of samples was observed microscopically by
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scanning electron microscopy (SEM) (JSM-7500F, Japan) along with energy dispersive
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x-ray analysis (EDAX).
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3. RESULTS AND DISCUSSION
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3.1. Characterization of Prepared Activated Coke. 3.1.1. Textural Properties. The
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textural properties of the modified AC with different loading amount of titanium ore were
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shown in Table 1, which were calculated based on their N2 adsorption isotherms (Figure
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S1a). It is obviously that all AC samples exhibited high N2 adsorption capacity at low
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relative pressure stage (P/P01 wt%) also could confirm this conclusion because it had
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the same reduction with the SBET and Vtot. (Table 1 and Table 4).
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The C=O in activated coke also possibly influence the oxidation of SO2.32 Table 2
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shows that the content of O 1s and the ratio of O 1s/C 1s of AC-Tit increased evidently,
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compared with AC-0. Table 3 shows that the relative content of C=O of AC-Tit with low
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blending ratios (i.e. 1-3 wt%) was slightly higher than AC-0. As is reported the C=O has
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basicity, and was the active centre of catalytic oxidation of SO2, which could improve the
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removal of SO2. Moreover, electrons can be transferred to the active sites on carbon
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surface by C=O, which contributes to form the SO3 by oxidation reaction between SO2
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and oxygen species.16 In this study, the AC-Tit with low blending ratios (i.e. 1-3 wt%)
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showed higher C=O content than AC-0, and the C=O content decreased with the increase
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of titanium ore content. Thus, the higher content of C=O group on the modified AC could
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contribute to a higher sulfur capacity (Table 4).
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As shown in Figure 3a, the metal oxides, such as FeTiO3, Fe2O3, Fe and TiO2, were
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observed on the surface of AC-Tit. It was reported that FeTiO3 has the capability of
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catalytic oxidation due to its Fe2O3-like structure, in which the two Fe3+ ions were
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replaced by Fe2+ and Ti4+.16 This characteristics might be beneficial for the
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desulfurization of modified activated coke. Moreover, literatures also reported that Fe2O3
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and TiO2 play a vital role in SO2 removal process because it can change the oxidation
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state in desulfurization process.14, 16 For Fe2O3, it improves desulfurization process by 14 ACS Paragon Plus Environment
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oxidation and directly reaction with SO2 to form Fe2(SO4)3. Furthermore, the existence of
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TiO2 could help the diffusion of O2 molecules into pores and help improving the reaction
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between O2, SO2 and Fe2O3.28 Thus, the existence of the metals (i.e. Ti and Fe) in AC-Tit
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could greatly improve its desulfurization performance.
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3.3. Characterization of Activated Coke after Desulfurization. The FTIR of AC-Tit
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after desulfurization depicted in Figure 1b shows that the intensity of C-O increased and
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the intensity of C=O not obviously changed compared with AC-0. This demonstrates that
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the desulfurization process affects the AC’s surface oxygen-containing functional groups.
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New peaks detected at around 890-840 cm-1 and 680-660 cm-1 were classified to the out-
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of-plane vibration of C-H, the weak bands of out-of-plane and in-plane of carboxylic
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acids, respectively.32, 33 The new peak at near 590-600 cm-1 ascribed to the S=O and S–O
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suggesting the existence of SO42- or HSO4-.34 The intensity of the S=O and S–O of AC-
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Tit was stronger than AC-0, suggesting that the AC-Tit produced more sulfates than AC-
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0. Thus, the fact that SO2 can be catalysed and oxidized by the AC-Tit during
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desulfurization process was confirmed.
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As shown in Figure 3b, some peaks of FeTiO3 disappeared, especially with low
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blending ratio. No peak of Fe2O3 was observed, while the peaks of TiO2 still existed.
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Some new peaks of CaSO4 at 2θ=25.44°, 31.32° and 40.78° (JCPDs 37-0184) and
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Fe2(SO4)3 at 2θ=20.80° and 25.17° (JCPDs 42-0225) were observed. The results show
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that the reaction between CaS, FeTiO3, Fe2O3 and SO2 happened in the presence of
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oxygen and water vapor. For XPS analysis (Figure 4), the peaks of Ti 2p3/2 and Fe 2p3/2
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not evidently changed before and after desulfurization, which suggests that the states of
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Ti and Fe not changed during desulfurization process. The results fall in line with the 15 ACS Paragon Plus Environment
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XRD results (Figure 3b). It can be assumed that FeTiO3, TiO2 and Fe2O3 in the AC-Tit improve the
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desulfurization performance. According to the characteristics of AC-Tit before and after
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desulfurization, there might be two pathways for Fe2O3 involved to form metal sulfate
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(i.e. Fe2(SO4)3): (1) SO2 could be transformed into the sulfur acid at the presence of metal
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oxides (Fe2O3), oxygen-containing functional groups (C=O) and O2, and then directly
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reacted with metal ions to produce sulfate;14, 16 (2) Fe2O3 could directly react with SO2 in
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the presence of O2 and H2O to form corresponding sulfate (i.e. Fe2(SO4)3). In addition,
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the FeTiO3, TiO2 was detected and the Ti(SO4)2 was not detected on the AC-Tit, which
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indicates that the SO2 was oxidized into SO3 due to the catalytic effect of FeTiO3 and
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TiO2, instead of directly reacting with it.14
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3.4. Regeneration Performance of Activated Coke. Thermal regeneration
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performance of AC-Tit1 and AC-Tit3 was discussed, and the result shows that the SO2
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removal ability of the AC-Tit1 and AC-Tit3 decreased after thermal regeneration (Figure
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5). During the three desulfurization-regeneration cycles, the desulfurization capacity of
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the samples decreased gradually, with more evidently for the AC-Tit1 than AC-Tit3.
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With the increasing of regeneration cycles (Table 7), the C=O content of regenerated AC-
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Tit1 decreased more drastically than those of the AC-Tit3. After 3 times of regeneration,
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the C=O of AC-Tit1-R3 (4.54%) was much lower than that of AC-Tit3-R3 (5.87%).
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Thus, the decrease of C=O caused the drastically reduction of SO2 removal efficiency due
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to its key role in desulfurization.14
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The N2 adsorption isotherms of AC-Tit1 and AC-Tit3 not significantly changed before and after regeneration (Figure S3). All samples exhibited high N2 adsorption capacity at 16 ACS Paragon Plus Environment
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low relative pressure stage (P/P0
