Cyclic Regeneration of Pyrolusite-Modified Activated Coke by

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Cyclic Regeneration of Pyrolusite-Modified Activated Coke by Blending Method for Flue Gas Desulfurization Lin Yang,† Xia Jiang,†,‡ Wenju Jiang,*,†,‡ Pengchen Wang,† and Yan Jin† †

College of Architecture and Environment, Sichuan University, Chengdu 610065, PR China National Engineering Research Center for Flue Gas Desulfurization, Chengdu 610065, PR China

‡

S Supporting Information *

ABSTRACT: The regeneration properties of a novel pyrolusite-modified activated coke (ACP) by blending method were investigated. Cyclic regeneration of ACP (ACP-Rn) shows the ACP was a good desulfurizer. The sample had the best desulfurization performance after five regeneration cycles (ACP-R5), and its sulfur capacity was 178 mg/g, 10.8% higher than that of ACP (161 mg/g). The SBET and pore volume were increased with the number of regeneration cycles, but they were not the limiting factors of the SO2 removal. The better surface functional groups, especially the basic sites, contributed more to the improvement of desulfurization performance of ACP-Rn. The loaded metals played an important role in SO2 removal. The accumulation of metal sulfate in ACP rapidly increased from the first to fifth regeneration cycles and then remained relatively stable during further reuse. The ACP regeneration process divided into three stages: drying, reaction, and pyrolysis. The wrapped metals relatively enhance the pyrolysis and acted as further surface modifications.

1. INTRODUCTION Activated coke (AC) has attracted considerable attention for its wide use in air pollution control in recent years, because it possesses good adsorption properties for SOx, NOx, toxins, and other pollutants based on physical-sorption and/or chemisorption.1−3 Among these processes, activated coke flue gas desulfurization (ACFGD) has been proven to be particularly promising and is widely used.4−6 The ACFGD techniques include two steps: SO2 removal and regeneration. In the desulfurization process, the SO2, H2O, and O2 in the flue gas first absorb on the AC surface, and then the SO2 is oxidized and reacted with H2O to form H2SO4.7−11 With the desulfurization process going on, the H2SO4 was stored and accumulated as byproduct in the pore structure and caused the desulfurization activity of AC to dramatically decrease. After that, the SO2 captured AC transfer to the regeneration step to recover its SO2 removal activity.12 Many methods have been developed for AC regeneration, in which thermal regeneration has proven to be much more effective. The used AC was put in a high-temperature and inert atmosphere (300−500 °C); the H2SO4 reacts with the carbon to release the SO2 and achieve the desulfurization activity recovery.9,11,13 The desorbed high concentration of SO2 in the regeneration process could be used to produce the sulfur acid or elemental sulfur. The thermal regeneration of the SO2 captured AC is a complicated process; it simultaneously contains the drying, pyrolysis, and gasification processes except the thermal desorption.14 Therefore, many factors, such as the material components and operational conditions, affect the regeneration properties of AC. In recent years, the transition metals, such as Fe,15,16 V,13 Ti,17 Mn,18 etc., were used as modifiers to enhance the SO2 removal of AC by changing the adsorption process to adsorption-catalysis. Impregnation and blending methods are the two most common methods to load the additive. In the © XXXX American Chemical Society

impregnation method, AC was immersed in an aqueous solution of metal salts and then dried and calcined in an inert atmosphere.15,19 However, the preparation process of the impregnation method is complex because it is a post-treatment process, and the deposition binding of the additive on the AC surface is relatively weak because the additive attached on only the AC surface.20 After regeneration at high temperature, the stability of the metal became so poor that the metal fell off because of the reaction between the carbon and sulfuric acid, which destroyed the surface features dramatically. Guo et al. prepared V2O5/AC by impregnation and studied its regeneration property, and the results showed that the desulfurization activity of V2O5/AC gradually reduced with the increase of regeneration cycles; the sulfur capacity reduced from 60 to 11 mg/g after eight adsorption−regeneration cycles.13 As an alternative, the blending method received widespread attention in recent years because it is relative simple. In this method, the additive is mixed with the carbon char directly and participates in the whole preparation process (including molding, carbonization, and activation). The preparation cost is very low and the loaded metals could achieve uniform distribution throughout the carbon matrix.20,21 In addition, it makes possible the addition of natural mineral and solid waste, which contain great amounts and many kinds of transition metals, for AC modification.24,25 The AC modified by blending method not only improves its desulfurization performance but also greatly reduces the preparation and modification cost. The low preparation cost and high activity means its application prospects are broader than those of the AC prepared by impregnation. Received: January 12, 2017 Revised: March 6, 2017 Published: March 8, 2017 A

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Energy & Fuels Table 1. The Proximate and Elemental Analysis of the Raw Materials (%) proximate analysis

elemental analysis

sample

Mad

Ad

Vd

FCd

Cad

Had

Nad

Sad

bituminous coal 1/3 coking coal

0.85 1.32

11.83 2.74

34.11 35.55

54.06 61.71

73.78 74.23

9.69 4.83

2.29 2.08

1.76 0.47

Figure 1. Sulfur capacity and breakthrough time of the ACP and regenerated ACP. was mixed thoroughly by a kneading machine. Subsequently, the mixture was molded by a vacuum hydraulic extruder with a high compression at 10 MPa to obtain the columnar coke (Φ 3.0 mm). In the activation process, the columnar coke was first carbonized in a rotary furnace at 600 °C for 60 min in N2 (400 mL/min) to remove the volatile substance, heated to the final temperature (900 °C), and activated by steam (MC:MH2O = 2:1) for 60 min. The heating rate of the whole process was 5 °C/min, and the prepared AC was labeled as ACP. 2.2. Characterization of Activated Coke. The textual properties evaluation of the samples were measured at −196 °C using ASAP 2460 analyzer (Micromeritics, United States) after the sample was degassed at 250 °C for 8 h. The specific surface areas (SBET) was calculated by the BET equation, assuming the nitrogen molecule surface area is 0.162 nm2.23 The total pore volume (Vtot) was estimated as the liquid volume of N2 adsorbed at a relative pressure of 0.995. The micropore (Vmic) volume and micropore area (Smic) were obtained by t-plot theory. The mesopore volume (Vmes) was determined by the Barrett− Joyner−Halenda model. The crystal structure of ACP was determined by X-ray diffraction with an X-Pert PRO MPD diffractometer (Panalytical, Netherlands) employing Cu Kα radiation at 30 kV and 20 mA and step-scanning over a 2θ range of 10−80°. The crystalline phases were identified by comparison with the reference data from the International Center for Diffraction Data (JCPDs). X-ray photoelectron spectroscopy characterization of the surface chemistry was carried out on a XSAM-800 spectrometer (Kratos, U.K.) with Al Kα radiation (1486.6 eV) under an ultrahigh vacuum, the source being operated at 12 kV and 15 mA. Energy calibration was done by recording the core level spectra of Au 4f7/2 (84.0 eV) and Ag 3d5/2 (368.30 eV). The high-resolution scans (0.1 eV) were obtained over the 278−294 eV (C 1s), 630−665 eV (Mn 2p), and 700−738 eV (Fe 2p) energy ranges. The chemical composition analysis (C, H, N, and S) of the ACP obtained at each operational stage was measured using a vario EL cube (Elementar, Germany). 2.3. Desulfurization and Regeneration. The desulfurization activity tests were carried out in a lab-scale, fixed-bed system; the reactor consisted of a glass tube with an internal diameter of 21 mm. The bed temperature and space velocity were 80 °C and 600 h−1,

It is important to highlight that not all the metal nanoparticles loaded by blending method dispersed on the surface of AC, there are some content metals were wrapped by the carbon matrix first. The regeneration process of AC not only completed the SO2 desorption but also exposed the new metals on the surface by the reaction between the carbon and H2SO4. The presence of metals and metal oxides in the carbon frame results in further surface modification of AC. These changes make the regenerated AC have a prominent desulfurization performance. Therefore, discussing the regeneration performance has great theoretical significance and practical value for widespread industrial application. Surprisingly, little attention has been devoted to investigating the regeneration properties of this novel kind of AC. In this study, the pyrolusite-modified activated coke (ACP) prepared by blending method was used to investigate its regeneration properties for flue gas desulfurization. The desulfurization performance, porosity, and surface chemistry of the regenerated ACP were carefully analyzed via Brunauer− Emmett−Teller (BET) analysis, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), EA, etc. In addition, the regeneration mechanism of the AC modified by the blending method was discussed systematically.

2. EXPERIMENTAL SECTION 2.1. Preparation of Activated Coke. In this study, both bituminous coal and 1/3 coking coal (from Shanxi province, PR China) as carbon materials, coal tar as main binder, and pyrolusite as modifier were used to prepare the pyrolusite-modified AC. The proximate and elemental analyses of the coals and coal tar are listed in Table 1. The coals were first crushed to 200-mesh and then mixed at weight ratio of 7:3 (bituminous coal to 1/3 coking coal). The pyrolusite powder (200-mesh), which was same as that used in our previous study,22 directly mixed with the coal powder at weight ratios of 4 wt %. After uniform mixing, the coal tar was added and the sample B

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Energy & Fuels respectively. The simulated flue gas compositions were 3000 ppm of SO2, 10% of water vapor, 8% of O2, and N2 as the balance gas. The inlet and outlet SO2 concentration were online monitored by a flue gas analyzer.The activity test was concluded when the outlet SO2 reached 10% (i.e., 300 ppmv). The sulfur capacity was determined by the integration of the SO2 breakthrough curve. The sulfur capacity of samples were calculated based on the following formula: ̀

SC =

∫0

t

3.2. Structure Properties. The N2 adsorption−desorption isotherms of the samples are depicted in Figure S1.The obvious N2 adsorption at low relative pressure indicates that the ACP and ACP-Rn are typically microporous materials.24 The somewhat feeble hysteresis loop clarifies that a certain content of mesopores existed.25 The textual properties listed in Table 2 Table 2. Textual Properties of the ACP and Regenerated ACP

nQ (C0 − Ct )10−6 dt 22.4m

where SC is sulfur capacity (mg/g), n the SO2 molecular weight, Q the gas flow (mL/min), t the working time, C 0 the inlet SO 2 concentration, Ct the outlet SO2 concentration at working time of t, and m the sample mass value. The thermal regeneration of the exhausted ACP was applied at a temperature of 500 °C (the heating rate is 5 °C/min) for 60 min, with a N2 flow rate of 100 mL/min. After that, the regenerated ACP was cooled to room temperature before the next SO2 removal operation. The regenerated ACPs were systematically named as ACP-Rn, where n is the number of regeneration cycles. The outlet gas in the regeneration process was analyzed online by Testo 350 Pro (Germany).

3. RESULTS AND DISCUSSION 3.1. Desulfurization Activity Test. The desulfurization performance of ACP-Rn expressed as sulfur capacity and breakthrough time are shown in Figure 1. It can be seen that the desulfurization activity of the ACP-Rn could be divided into two stages, the increasing stage from the 1st to the 5th regeneration cycles and the decreasing stage from the 6th to the 12th regeneration cycle. In the increasing stage, the desulfurization performance of ACP-Rn was fully recovered and even showed a certain degree of improvement as the number of regeneration cycles increased, except the ACP-R1. The sulfur capacity gradually increased and showed the highest sulfur capacity after five regeneration cycles (ACP-R5); the sulfur capacity of ACP-R5 was 178 mg/g, 10.80% higher than that of ACP. From the sixth regeneration cycle, the SO2 removal of ACP-Rn entered the declining phase, and its desulfurization activity linearly decreased. As Figure 1 shows, the ACP-R6 showed a remarkable decrease compared with ACP-R5, whose sulfur capacity reduced from 178 to 153 mg/g, lower than that of ACP. After that, the reduction of desulfurization performance became slow from the seventh to the ninth regeneration cycle. The sulfur capacity of ACP-R9 was 120 mg/g, which was 74.54% of the ACP. ACP-R10 showed another clear reduction of the sulfur capacity, which decreased from 120 to 76 mg/g, and further reduced to 59 mg/ g after 12 regeneration cycles; it was less than 40% of the ACP. Guo et al.13 studied thermal regeneration of V2O5/AC for flue gas desulfurization, which was prepared by pore volume impregnation and regenerated in Ar atmosphere (100 mL/ min) at 400 °C; the results showed that the desulfurization activity of V2O5/AC dramatically linearly reduced with the increase of regeneration cycles, and its sulfur capacity greatly reduced from 60 to 11 mg/g after eight regeneration cycles. In this study, the desulfurization performance of ACP increased from the first to the fifth regeneration cycles and maintained relatively better SO2 removal for further regeneration cycles even though the sulfur capacity linearly decreased. The better regeneration performance of ACP-Rn reveals that the blending method is better than impregnation to prepare a high-activity and low-cost AC desulfurizer.

sample

SBET (m2/g)

Smic (m2/g)

Vtot (cm3/g)

Vmic (cm3/g)

Vmes (cm3/g)

ACP ACP-R1 ACP-R2 ACP-R3 ACP-R4 ACP-R5 ACP-R6 ACP-R7 ACP-R8 ACP-R9 ACP-R10 ACP-R11 ACP-R12 ACP-R13 ACP-R14 ACP-R15 ACP-R16

503 474 521 537 548 574 626 635 642 645 669 717 701 707 709 701 693

404 379 423 424 433 455 468 462 458 449 461 457 469 453 427 368 379

0.286 0.280 0.310 0.321 0.325 0.341 0.356 0.366 0.366 0.371 0.377 0.402 0.390 0.391 0.395 0.398 0.387

0.199 0.187 0.208 0.209 0.213 0.224 0.236 0.230 0.225 0.225 0.231 0.229 0.233 0.225 0.211 0.181 0.183

0.067 0.074 0.080 0.087 0.089 0.092 0.095 0.098 0.096 0.096 0.094 0.105 0.101 0.103 0.114 0.137 0.127

show that the SBET of ACP-R1 decreased slightly, which could be due to the nonregenerable sulfur in the pore structure,13 which existed as metal sulfate and stored in ACP-R1. After that, the SBET linearly increased as the regeneration cycles increased and reached the highest SBET of 717 m2/g after 11 regeneration cycles (ACP-R11). The pore volume had a change similar to that of the SBET. These changes reveal that the regeneration of ACP acts as a further activation, which improved the pore structure of AC effectively.2,26 Furthermore, the pore size distribution of samples in Figure 2 shows that pores with the diameter larger than 1 nm were clearly increased, while pores with diameter less than 1 nm were not obviously changed. This means the pore volume improvement of ACP-Rn contributed to not only the expansion of existing pores but also the creation of new pores during the regeneration. An additional four cycles of regeneration of ACP-R12 were carried out to determine the influence of regeneration on its structural properties. The Vmes of the further regenerated sample apparently increased with the dramatical reduction of Vmic (Table 2); the Vtot also showed a clear reduction from the 15 regeneration (ACP-R15). These phenomena indicate that the pore structure of the ACP experienced a certain degree of damage, and micropores were destroyed to form more mesopores when the ACP was regenerated excessively. These results were well consistent with previous study, which reported that excessive activation could cause the collapse of the adjacent wall structures between the micropores and that more mesopores were formed.27,28 The literature reports that higher SBET and larger pore volume are favorable for SO2 removal because of the high dispersion of the active ingredient and better mass transfer.29−31 The desulfurization performance of the ACP-Rn, however, gradually decreased after the sixth regeneration cycle (Figure 1). This suggests that the textual C

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Figure 2. Pore size distribution plots of the ACP and regenerated ACP.

Figure 3. C 1s patterns of the ACP and regenerated ACP: (a) ACP; (b) ACP after desulfurization; (c) ACP-R1; (d) ACP-R1 after desulfurization; (e) ACP-R2.

quinone groups,33,34 and π−π* transition peaks,23,35 respectively. After flue gas desulfurization, as shown in Figure 3b,d, the energy peak around 290.00 eV disappeared, and the binding energy of the peak around 287.50 eV had a certain degree of increase. These results indicate that the surface functional groups clearly changed after being used for SO2 removal. The binding energy (BE) and relative contents (RCs) of ACP and ACP-Rn based on C 1s were calculated and are listed in Table 3. It can be seen that the RCs of the C−C group significantly increased, while the C−O and CO showed a clear reduction; the π−π* transition peak cannot be detected after desulfurization. As the regeneration cycles increased, the

properties were not the controlling factor for the desulfurization of ACP after six regeneration cycles. 3.3. Surface Functional Groups Characterization. The surface functional groups of ACP and ACP-Rn were characterized via XPS analysis, and the C 1s spectrum of the ACP, ACP-R1, and ACP-R2 before and after desulfurization are shown in Figure 3. It can be seen that the C 1s spectra of ACP (Figure 3a), ACP-R1 (Figure 3c), and ACP-R2 (Figure 3e) all gave four similar peaks at 284.62−284.66 eV, 285.73−285.96 eV, 287.23−287.68, and 289.57−289.96 eV, which corresponded to graphitizing carbon,19,32 C−O from phenolic etheric or alcoholic carbon,23,33 CO from carbonyl or D

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Energy & Fuels Table 3. Binding Energy (BE) and Relative Content (RC) of C 1s for ACP and Regenerated ACP C−C

a

C−O

π−π*

CO

samplea

BE (eV)

RC (%)

BE (eV)

RC (%)

BE (eV)

RC (%)

RC (%)

BE (eV)

ACP ACP(a) ACP-R1 ACP-R1(a) ACP-R2 ACP-R3 ACP-R4 ACP-R5 ACP-R6 ACP-R7 ACP-R8 ACP-R9 ACP-R10 ACP-R11 ACP-R12

284.62 284.67 284.63 284.68 284.66 284.61 284.63 284.58 284.64 284.61 284.60 284.62 284.63 284.61 284.62

65.40 75.60 63.67 75.07 63.89 63.80 64.64 65.94 65.89 67.66 67.90 68.10 68.38 68.67 68.85

285.73 285.77 285.92 285.98 285.96 285.85 285.89 285.88 285.77 285.85 285.86 285.74 285.74 285.74 285.74

23.78 19.14 23.84 19.19 23.55 22.02 21.84 20.92 22.82 22.25 23.53 23.93 24.31 24.49 24.61

287.23 287.67 287.62 288.29 287.55 287.69 287.75 287.86 287.57 287.62 287.67 287.75 287.75 287.75 287.75

6.17 5.27 6.67 5.74 7.48 9.05 8.88 8.60 7.52 6.58 5.18 4.59 4.09 3.97 3.83

289.57 − 289.82 − 289.96 289.97 290.11 290.22 290.2 289.66 289.78 289.91 289.91 289.91 289.91

4.65 − 5.82 − 5.08 5.13 4.64 4.54 3.77 3.51 3.39 3.38 3.22 2.87 2.71

ACP(a) and ACP-R1(a): the ACP and ACP-R1 after SO2 removal

Figure 4. XRD patterns of the ACP and regenerated ACP: (a) ACP, (b) ACP after desulfurization; (c) ACP-R1; (d) ACP-R1 after desulfurization; (e) ACP-R2.

We can see that the desulfurization performance of the ACPRn (Figure 1) has a change similar to that of the basic functional groups (i.e., CO and π−π* transition peaks in Table 3); its RCs significantly increased from the first to third regeneration cycles. ACP-R3 had the highest total RCs of the basic functional groups: 9.05% of CO and 5.13% of π−π* transition. In the fourth and fifth regeneration cycles, the content of CO and π−π* transition peaks slightly decreased, while 8.60% of CO and 4.54% of π−π* transition peaks of ACP-R5 were still higher than that of new ACP. However, ACP-R5 (Figure 1) had the best desulfurization performance with the highest sulfur capacity of 171 mg/g. This noted difference between the change of surface basic functional groups and desulfurization performance of ACP-Rn could be attributed to the increase of better textual properties (Table 2). The higher SBET of the sample made more active sites (basic functional groups and metals) distributed on the surface, and the larger pore volume was more favorable for mass transfer.

surface functional groups also showed specific changes. For the C−C group, the ACP-R1 exhibited a lower relative content and remained steady until the third regeneration cycle. After that, it gradually increased with the increase of regeneration cycles. For the C−O group, there was almost no change from the first to fifth regeneration cycles, but it gradually increased after six regeneration cycles. For the CO group, it slightly increased from the first to the fifth regeneration cycles and then showed a linear decrease with further reuse. Less than 4.00% of the C O was maintained after the 11th regeneration cycle. For the π−π* transition peaks, there was only a slight increase in the first three regeneration cycles, and it rapidly reduced after that. It was reported that the CO and π−π* transition peaks accounted for the basic nature of carbon surface,36,37 which were the active center of catalytic oxidation of SO2. That is why the CO and π−π* transition peaks significantly reduced after desulfurization. E

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Figure 5. Core-level spectra for Mn 2p3/2 (A) and Fe 2p3/2 (B) of the regenerated ACP: (a) ACP, (b) ACP after desulfurization, (c) ACP-R1, (d) ACP-R1 after desulfurization, and (e) ACP-R2.

The high-resolution Mn 2p and Fe 2p spectra are illustrated in Figure 5 to further clarify the metal phase on the surface of sample. As shown in Figure 5A, the Mn 2p3/2 region of the ACP sample consisted of two peaks at 640.15 and 641.86 eV (Figure 5A, spectrum a), which are assigned to Mn2+ of MnO.41 After flue gas desulfurization (Figure 5A, spectrum b), the binding energy increased to 642.27 eV, which corresponds to Mn2+ of MnSO4.42 After the regeneration process, for the ACPR1, the Mn 2p3/2 binding energy peak appearing at 641.85 eV also belongs to Mn2+ (Figure 5A, spectrum c). The slightly higher binding energy compared to that of ACP could be caused by the coexistence of the MnO and MnSO4, and this is well consistent with the XRD analysis. When the ACP-R1 was reused for desulfurization, the Mn 2p3/2 peak increased to a higher binding energy again (642.37 eV). However, there was no clear change detected in the Mn 2p3/2 on the surface of ACP-R2 (i.e., 642.13 eV). With the increase of the regeneration cycles, the MnSO4 accumulated, while the exposed new MnO gradually reduced. The changes of MnSO4 and MnO contents weakened the binding energy shift. Figure 5B shows the Fe 2p spectra of the samples. The Fe 2p3/2 spectrum of ACP showed two binding energy peaks at approximately 708.99 and 710.68 eV representing elemental Fe (ref 43) and Fe2+ of FeO,44 respectively. After SO2 removal, the Fe 2p3/2 binding energy peaks were shown to be around 711.15−711.63 eV, which represented Fe3+ (i.e., Fe2O3 and Fe2(SO4)3).19,45 These results indicate the Fe0 and Fe2+ were oxidized to a higher valence state in the desulfurization process, and there was no clear variation after it was regenerated. Previous study showed that the MnSO4 on the surface of AC creates a completely new liquid-phase catalysis.18 When the ACP was regenerated, the coexistence of MnO and MnSO4 formed the solid- and liquid-phase catalysis for desulfurization. With the increase of regeneration cycles, however, the MnO mostly converted to sulfate and the two-phase catalytic equilibrium was destroyed. The weakened catalysis coupled with the worse surface functional groups directly lead to the weakened desulfurization performance. To clarify the assumption, the ACP-R16 was regenerated at 900 °C for 60 min to decompose the MnSO4, termed ACP-RT900, and applied for SO2 removal. The desulfurization breakthrough curve in Figure

Furthermore, the attenuated surface acidity due to the lower RCs of C−O was also an important reason for the SO2 removal activity enhancement. After the sixth regeneration cycle, the content of basic functional groups of ACP-R6 (7.52% of CO and 3.77% of π−π* transition peaks) was lower than that of ACP and linearly reduced after further regeneration. The sulfur capacity also gradually decreased from the sixth regeneration cycle. When the sample cycled 12 times, ACP-R12 had only 3.83% of CO and 2.71% of π−π* transition peaks with the sulfur capacity of 59 mg/g, less than 40% of that of new ACP (161 mg/g). 3.4. Metal Phase Analysis. The surface metal chemistry of the ACP, ACP-R1, and ACP-R2 before and after SO2 removal was analyzed by XRD (Figure 4). For the ACP sample (Figure 4a), three diffraction peaks at 2θ = 34.96°, 40.90°, and 59.02° ascribed to MnO (JCPD 07-0230) and one peak at 2θ = 45.11° corresponding to Fe0 (JCPD 77-2438) were detected. After desulfurization, as Figure 4b shows, the peaks of MnO and Fe0 were weakened or even disappeared, while a new peak attributed to manganese sulfate (MnSO4) at 2θ = 25.36° (JCPD 65-3413) was detected. This means the MnO on the surface of ACP transformed to MnSO4 during the desulfurization process. That there was no ferrum sulfate detected could be due to its low content and poor crystallinity.38 When the used ACP was regenerated, however, the intensity of MnO and Fe were recovered relatively (Figure 4c) but gradually reduced with the increase of regeneration cycles (Figure 4e). In contrast, the peak belonging to MnSO4 became more intense. As we know, the decomposition temperature of MnSO4 is greater than 700 °C,39 which means the MnSO4 would accumulate gradually in the ACP with the number of desulfurization− regeneration cycles. For the ferrum sulfate (Fe2(SO4)3 and FeSO4), they also could not be decomposed at the final temperature (500 °C).40 Therefore, we can be sure that the recovery of MnO and Fe on the surface of ACP-Rn was due to the explosion of the new metals, which was wrapped in the carbon matrix previously. Certainly, it was the significant differences between the two loading methods, i.e., impregnation and blending, that was responsible for the uniform distribution of metal throughout the AC loaded by the blending method. F

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the good porosity of the prepared ACP exposed a large amount of the metals on the surface and also could help to explain the decrease of the carbon content and SBET of ACP-R1 (i.e., 5.49%, 29 cm3/g). For the ACP-R2 sample, the sulfur only slightly increased, i.e., 0.16%. This is attributed to the weak reaction in the first regeneration cycle, which restricted the exposure of new parts of the metal oxides. On the basis of the XRD and XPS analyses (Figures 4 and 5), it is known that the increased sulfur species in the sample is mainly attributed to the accumulation of the sulfate. The S 2p spectra of the regenerated samples are shown in Figure S3; the new BE peak at 164.10 eV of ACP-R1 and 163.55 eV of ACPR2 after SO2 removal correspond to the element sulfur,46 which illustrates that the SO42− and small portion of elemental sulfur coexisted on the surface of ACP after regeneration. This might be due to the reaction between the C, CO, and SO2 at high temperature.47 As the number of regeneration cycles increased, the MnO and Fe on the surface dramatically reduced; a higher ratio of the removed SO2 remained in the pore as H2SO4, which could desorbed effectively in the regeneration process. Furthermore, the reduced desulfurization performance shown in Figure 1 also indicates the increment of S species in ACP would become metastable with increase in the number of regeneration cycles. Figure 6 shows the outlet gas characterization of the SO2captured ACP during the regeneration process. The regeneration process of the SO2-captured ACP could be divided into three stages. The first stage was the temperature lower than 170 °C; the second stage was in the range at 170−385 °C; and temperature from 385 °C to the isothermal stage was the last stage. For the metal-modified AC, there are three kinds of sulfur species (i.e., adsorbed SO2, sulfur acid, and metal sulfate) coexist in the pore structure after desulfurization.9,15,48 During the regeneration process, the adsorbed SO2 could be desorbed easily when the temperature reach 150 °C; this stage was called desorption. When the temperature increased to 200 °C, the sulfur acid could react with the carbon to release the SO2, which was called reaction.2,11 With the temperature further increased, the sulfur acid was completely consumed and the metal sulfate decomposed to release SO2 when the temperature was high enough (i.e., the Fe2(SO4)3 at 550 °C, MnSO4 at 750

S2 shows that the ACP-RT900 had a desulfurization activity that was better than that of ACP-R16. The sulfur capacity was 47 mg/g, significanttly higher than the 37 mg/g of ACP-R16 (based on Figure S2). Therefore, the two-phase catalysis of MnSO4 (liquid-phase catalysis) and MnO (solid-phase catalysis) contributed to the improvement of the desulfurization performance of the regenerated ACP in the first to fifth regeneration cycles. 3.5. Regeneration Mechanism. The chemical analyses of samples were carried out to investigate the sulfur accumulation. As Table 4 shows, the ACP had 0.16% of sulfur, which is Table 4. Elemental Analysis of the ACP and Regenerated ACP (%) sample

C

H

N

S

ΔS

ACP ACP-R1 ACP-R2 ACP-R3 ACP-R4 ACP-R5 ACP-R6 ACP-R7 ACP-R8 ACP-R9 ACP-R10 ACP-R11 ACP-R12

67.56 62.07 61.09 58.79 56.17 54.28 50.61 49.85 48.39 47.66 47.23 46.76 46.25

1.27 1.32 1.45 1.54 1.50 1.66 1.63 1.63 1.75 1.73 1.71 1.74 1.72

0.36 0.34 0.35 0.36 0.33 0.37 0.38 0.39 0.41 0.41 0.40 0.41 0.43

0.16 2.56 2.72 3.71 4.28 4.46 4.64 4.80 4.95 5.06 5.12 5.18 5.21

− 2.40 0.16 0.99 0.57 0.18 0.18 0.16 0.15 0.11 0.06 0.06 0.03

attributed to the sulfur in raw materials (Table 1). As the number of regeneration cycles increased, the relative content of sulfur increased rapidly in the first few regeneration cycles and then remained relatively stable. The ACP-R1 sample had the maximum accumulation, and the increment was 2.40% (the relative content of sulfur increased from 0.16% to 2.56%). From the third regeneration cycle, the sulfur accumulation in each regeneration cycle decreased gradually; this was consistent with our previous study.18 For the ACP-R1, the maximum accumulation of sulfur could be due to the large amount of metals converted into sulfate. This phenomenon suggests that

Figure 6. Outlet gas components of the exhausted ACP during regeneration process. G

DOI: 10.1021/acs.energyfuels.7b00125 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels °C), which was called decomposition. In addition, as the temperature increased, some kinds of oxygen-containing functional groups were also decomposed during the regeneration at high temperature, which were labeled as pyrolysis and gasification. In the first stage, no CO, CO2, or SO2 was detected even though the temperature increased to 170 °C. This resuls indicates the amount of adsorbed SO2 was very low on the surface of ACP. In the second stage, high concentrations of CO, CO2, and SO2 were detected simultaneously and showed a similar change with the increase of temperature. The outlet concentration first profoundly intensified and then gradually decreased. The SO2 reached the maximum value at 295 °C; this is well consistent with the results of Yan et al.2 However, the CO (310 °C) and CO2 (320 °C) showed a hysteresis reaching the concentration vertex, which indicates the CO and CO2 were formed by not only the reaction process but also the functional group pyrolysis. In the third stage, the concentration of SO2 was very low and gradually reduced as the temperature increased. It could be due to the residuals in the previous stage. It worth noting that the CO and CO2 showed second increases and reached the new vertex at 435 °C and the final temperature (500 °C), respectively. The intense formation of CO and CO2 in the third stage is mainly attributed to the pyrolysis and gasification of oxygen-containing functional groups in the presence of metals in the carbon matrix, which serve as further surface modification. Furthermore, the CO and CO2 showed a low relativity, which means the formation of CO and CO2 were two independent processes. The whole regeneration process exhibits no decomposition because of the relatively low regeneration temperature. All of the potential reactions are as follows, in which C-(A) corresponds to the A-containing functional groups. SO2 (ads) → SO2 (g)

(1)

C + H 2SO4 → CO(g) + SO2 (g) + H 2O(g)

(2)

C + H 2SO4 → CO2 (g) + SO2 (g) + H 2O(g)

(3)

C‐(CO) → CO(g)

(4)

C‐(CO2 ) → CO2 (g)

(5)

C + SO2 → S2 + CO2 (g)

(6)

partially transformed to sulfate during the SO2 removal. The accumulation of sulfate in ACP rapidly increased from the first to the fifth regeneration cycles and then remained relatively stable upon further reuse. The outlet gas analysis of SO2captured ACP during the regeneration process found that the regeneration process could be divided into three stages: (1) drying process, (2) the reaction between carbon and H2SO4, and (3) the pyrolysis of oxygen-containing functional groups (CO- and CO2-containing functional groups decomposition). The presence of metals in the carbon matrix enhanced the pyrolysis and acts as a further surface modification of ACP.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b00125. N2 adsorption−desorption curves of the ACPs; desulfurization breakthrough curve of the ACP-R16 and ACPRT900; and S 2p spectra of the regenerated activated coke (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86-28-85467800. Fax: +86-28-85405613. ORCID

Wenju Jiang: 0000-0003-1327-7159 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (No. 51378324) and Ministry of Education of China (New Century Distinguished Young Scientist Supporting Plan, No. NCET-13-038)

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REFERENCES

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4. CONCLUSIONS The subject of this study was the investigation of the regeneration properties of a novel pyrolusite-modified activated coke generated by the blending method. Cyclic regeneration of ACP showed that the ACP was a good desulfurizer; its regeneration efficiency first increased and then demonstrated a linear decrease. After five regeneration cycles (ACP-R5), the sample had the highest sulfur capacity (178 mg/g), which was 10.80% higher than that of ACP (161 mg/g). The SBET and pore volume linearly increased with the increase of regeneration cycle even though the sulfur capacity reduced from the sixth regeneration cycle. This result indicate that the porosity and surface area were not the limiting factors of SO2 removal. The regeneration process improved the surface oxygen functional groups of ACP, especially the basic functional groups, which contributed more to the desulfurization performance. The loaded metals played an important role for SO2 removal and H

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DOI: 10.1021/acs.energyfuels.7b00125 Energy Fuels XXXX, XXX, XXX−XXX