Red

Article pubs.acs.org/EF

In Situ Preparation and Regeneration Behaviors of Zinc Oxide/Red Clay Desulfurization Sorbents Yu Feng, Jie Mi,* Mengmeng Wu, Jv Shangguan, and Huiling Fan The Affiliation Key Laboratory of Coal Science and Technology of Shanxi Province and Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, Shanxi, P. R. China ABSTRACT: High-temperature gas desulfurization is an efficient and environmentally friendly process for syngas cleanup. The present study investigated the in situ preparation of a zinc oxide/red clay desulfurization sorbent and its desulfurization− regeneration behavior in O2/N2 atmospheres. The effects of regeneration temperature, space velocity, and O2 concentration on the regeneration behaviors of ZnO/red clay sorbents were also investigated. The surface and structural properties of the sorbents before and after regeneration were characterized by thermogravimetric−differential scanning calorimetry, X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, elemental mapping, and N2 adsorption−desorption analyses. According to the results, the highest regeneration rate was achieved under the conditions of 6 vol % O2 and a space velocity of 3000 h−1 at 650 °C. After four cycles of desulfurization−regeneration experiments, the breakthrough time of the regenerated sorbents reduced by 18.7% and some breakages and cracks were also observed in the regenerated sorbents, although there was only a slight change in their mechanical strength. The characterization data indicate that the number of surface Zn atoms decreased in the regeneration process, which adversely affects the sorbent’s adsorption efficiency for acidic H2S. After regeneration, the results of N2 adsorption and SEM indicated the deteriorated structure of the sorbent. In addition, the XPS and XRD results indicated the presence of sulfur-containing compounds which also contributed to the lowered desulfurization activity of the sorbent.

1. INTRODUCTION The integrated gasification combined cycle (IGCC) is one of the most important coal-based poly-generation systems which has high power generation efficiency and environmentally friendly performance.1 However, a significant problem in the coal gasification process is the presence of sulfur-containing contaminants, such as H2S and small amounts of COS and CS2. Thus, it is necessary to clean up and further process the gas produced from coal gasification to avoid catalyst poisoning in subsequent conversion steps and to comply with environmental laws and regulations for protecting the public health. Hightemperature gas desulfurization represents an attractive solution to simplify syngas treatments and to improve the process efficiency, potentially reducing the final cost.1−14 Westmoreland and Harrison conducted extensive thermodynamic screening tests and found that 28 metal oxides showed good desulfurization potentials in hot coal gas. About 11 elements of these metal oxides (Fe, Zn, Mo, Mn, V, Ca, Sr, Ba, Co, Cu, and W) showed considerable thermodynamic feasibility as well as regeneration capacity.3 In particular, zinc oxide (ZnO) is an attractive option because of its favorable desulfurization thermodynamics and high affinity for H2S. ZnO can reduce the concentration of H2S to single-digit ppm levels; therefore, it has been used as a versatile sorbent to remove sulfur-based impurities from coal-derived gas or syngas.2,15−23 Previous studies have demonstrated that desulfurization reactivity is directly related to the available surface area or pore volume of the sorbent, and sintering during hightemperature regeneration causes loss of surface area and subsequent loss of reactivity.2,16 Gupta et al.24 carried out 100 desulfurization−regeneration cycles with zinc ferrite sorbent and reported that the loss in capacity of sorbent was likely due © XXXX American Chemical Society

to a combination of the following two factors: (1) loss in reactivity caused by changes in the chemical and physical characteristics of the sorbent, and (2) loss of sorbent in the reactor caused by elutriation. In previous studies by Efthimadis et al.,25 it was found that a highly porous structure of the sorbent had a positive effect on the desulfurization reaction. Another similar study26 also showed that the texture parameters of the desulfurization sorbent strongly affect the diffusion of reaction gas, the utilization efficiency of the desulfurization sorbent, and the rate of the desulfurization reaction. Since mass transfer and diffusion of reactants occur during the entire desulfurization and regeneration process, the texture of the desulfurization agent has a significant influence on the diffusion and desulfurization performance. Wei et al.27 found that the porous structure of zeolites with hierarchical pores can efficiently promote mass transfer in the reaction. Tian et al.28 reported that molecular sieves with a multistage pore structure showed better activity in the desulfurization reaction compared to molecular sieves with a narrow pore size distribution. The conventional methods of preparing desulfurization sorbents include the impregnation method,15,18 thermal oxidation,16 and solid-state method.19,20,22 However, these methods have many drawbacks which greatly limit the desulfurization performance of the sorbent. During the desulfurization process, the O atoms (0.140 nm) are replaced by S atoms (0.184 nm), so the formation of sulfide and sulfate compounds leads to damage of the internal structure and loss Received: August 24, 2016 Revised: November 8, 2016 Published: November 15, 2016 A

DOI: 10.1021/acs.energyfuels.6b02127 Energy Fuels XXXX, XXX, XXX−XXX

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In this paper, the in situ preparation of ZnO for application as a sorbent in high-temperature coal gas desulfurization has been reported. ZnO/red clay sorbent was prepared, and its desulfurization reactivity and reusability in H2S and N2 mixed atmospheres were investigated through desulfurization−regeneration cycles. In order to achieve the maximum desulfurization performance, regeneration conditions of the sorbent in the presence of O2 were also optimized. To understand the changes in surface and structural properties of the sorbents after regeneration, the sorbent precursor, fresh sorbent, and the thrice-regenerated sorbent were characterized by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and nitrogen adsorption−desorption. The information obtained by this study will be useful for designing a sulfur trap to clean up the output gas from the coal gasification process.

in surface area and pore volume of the sorbents as there is a large molar volume difference between ZnO or ZnS (15 and 24 cm3 mol−1, respectively) and ZnSO4 (46 cm3 mol−1). Unlike the conventional methods, the in situ preparation method shown in Figure 1 involves processing of the as-prepared

Figure 1. Schematic figure of the in situ preparation of desulfurization sorbent. (The purple, green, and red dots stand for Zn2+, S2−, and O2−, respectively.)

2. EXPERIMENTAL SECTION 2.1. Preparation of Zinc Oxide/Red Clay Sorbent. As shown in Figure 2, the ZnS powder was prepared by a homogeneous precipitation method. Aqueous solutions of Zn(Ac)2 and Na2S2O3 were freshly prepared separately before each experiment. These two solutions were mixed in a 1:1 volume ratio in a boiling flask, and then glacial acetic acid was added to adjust the pH value of the mixture to pH = 3.2. The surfactant, cetyl trimethyl ammonium bromide (0.2 mg/mL), was added, and then the boiling flask was placed in a thermostat bath (90 °C) while stirring continuously for the entire reaction. The ZnS precipitate was formed, which was then filtered by a frit funnel and thoroughly washed with water and absolute ethyl alcohol. The solid was dried at 90 °C for 2 h in a vacuum drying oven to give the desired ZnS powder. The ZnS powder (30 wt %) and red clay (SiO2:Al2O3:Fe2O3:MgO: K2O:CaO:Na2O = 66.08:18.07:0.08:0.1:13.5:0.28:1.3) were thoroughly mixed with distilled water to give a homogeneous mixture. Then, the mixture was extruded into cylinders (3 × 3 mm) and dried at 100 °C for 2 h, and the resulting solid was named as the ZnS precursor sorbent (ZP). The in situ regeneration tests were carried out using a vertical fixed-bed quartz tube reactor. About 5 g of the ZnS precursor was weighed into a quartz tube, which was then placed in a constant temperature zone. The ZP sorbent was heated in N2 with O2 in a certain proportion. The sorbent obtained after the in situ regeneration process was called fresh sorbent (ZF), and the sorbent obtained after three successive desulfurization−regeneration cycles was named as the third regenerated sorbent (ZR). The SO2 content of the outlet gas was analyzed using iodometry. The formula for calculating the concentration of SO2 is as follows

sorbent with a further in situ oxidative regeneration step upon the extruded mixture of metal sulfide and binder. This method is based on the space-occupying mechanism of metal sulfide, in which a larger molar volume crystal can reserve a large internal space for metal oxides with a smaller molar volume. Therefore, this approach can prevent physical pulverization of the sorbent due to replacement of the oxygen ions by the sulfur ions (oxides↔sulfides) and subsequent molecular volume expansion of the sorbent in the desulfurization reaction. By regulating the internal structure and optimizing the textural parameters of the desulfurization sorbent, this in situ preparation method can eliminate the adverse effects of repeated contraction−expansion of the microstructure during the desulfurization−regeneration cycles and improve the reactant and product diffusion efficiency. Thus, the reusability behavior of the sorbent would be improved, leading to higher desulfurization efficiency. In order to satisfy the industrial demand, favorable regeneration performance and good reusability are expected in the hot coal gas desulfurization sorbent. Up to now, the regeneration of desulfurization sorbents using O2, H2O, and SO2 has been extensively studied.2,8,16,20,29 Regeneration of the sulfurized sorbent in an O2 atmosphere is fast and inexpensive, and moreover, oxygen vacancies on the metal oxide surface can be repaired to enhance the desulfurization performance.30 Since the regeneration of sorbents is time-consuming, the desulfurization−regeneration process would be more efficient if the regeneration rate could be made faster. Previous studies have shown that the regeneration rate is faster in O2 compared to steam, SO2, or mixtures of steam and O2,30,31 as it takes a long time for the sorbents to regenerate completely in these other atmospheres.32 Thus, it is essential to study how to maximize the advantages of using O2 as a feed gas for fast and low-cost regeneration of sorbents.

SO2 (g/m 3) =

C × V1 × 64 × 1000 V

(1)

where V is the gas volume, L; V1 is the volume of iodine standard solution, mL; C is the concentration of iodine standard solution, mol/ L; and 64 is the molecular mass of SO2, g/mol. The regeneration rate (Xr) is defined as the following

Xr =

Crb − Cra × 100% Crb

(2)

Figure 2. Flowchart of the in situ preparation of desulfurization sorbent. B

DOI: 10.1021/acs.energyfuels.6b02127 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels where Crb and Cra represent the sulfur content of the sorbents before and after regeneration, %, respectively. 2.2. Desulfurization Experiments. In Figure 3, the desulfurization tests were conducted in a quartz tube, and 10 g of sorbents was

a rapid mass loss of 4.39% was observed in the temperature range from 630.9 to 688.1 °C, accompanied by the peak at 667.5 °C in the DSC curve. This is consistent with the theoretical value of 4.58%, corresponding to the mass loss of zinc sulfide which reacts completely with oxygen to produce zinc oxide in the regeneration process. When the temperature was higher than 710 °C, the TG curve became a horizontal line, indicating that the regeneration reaction had ended. 2.4. Analyses of the Characteristics of the Sorbents. The morphology of the sorbent was analyzed by SEM (MAIA3 TESCAN). The element distribution and content was detected by elemental mapping (MAIA3 TESCAN). The crystal structure of the sample was investigated by an X-ray diffraction device (JSM-6360 LV) using Cu Kα radiation. The analysis of pore structure was conducted on a Porosity Analyzer (Micromeritics TriStar-3000). The surface area was calculated based on Brunauer−Emmett−Teller (BET) theory. The XPS spectra of the sorbent was obtained using a PHI5000C spectrometer equipped with an Al Kα source operating at 250 W and 93.9 eV passed energy. The binding energy was calibrated by the C 1s peak at 284.6 eV. The content of sulfur in the sorbent was measured by a KZDL-8F-type fast-smart sulfur instrument made by Hebi Xianke Coal Instrument Co., Ltd.

Figure 3. Schematic figure of the desulfurization and regeneration processes of sorbents (1-cylinder; 2-valve; 3-flow meter; 4-water bath; 5-furnace; 6-thermocouple; 7-temperature controller; 8-inlet gas concentration detection; 9-absorption solution; 10-gas concentration detection; 11-desulfurization sorbent; 12-quartz tube; 13-tail gas).

3. RESULTS AND DISCUSSION 3.1. Effect of Temperature on the in Situ Regeneration Process. The effect of temperature on the regeneration performance was studied by varying the temperature from 550 to 700 °C, with 6 vol % O2 (N2 balance) at the space velocity of 3000 h−1. Figure 5 shows the concentrations of SO2 in the

placed in the middle of the quartz tube with silica wool to scatter the airflow. The sorbent was heated with N2, up to 300 °C at the heating rate of 10 °C/min, and H2S was added at 500 °C. The gas consisted of 2000−3000 ppm of H2S with N2 balance, and all the sulfidation tests were carried out at a space velocity of 3000 h−1. The content of H2S gas was analyzed every 30 min using a gas chromatograph (GC 2000, The Southwest Research & Design Institute of Chemical Industry) with a flame photometric detector. The test was terminated when the desulfurization efficiency was less than 80%. The corresponding adsorption time was defined as the breakthrough time. 2.3. Thermal Analysis of in Situ Regeneration Process. Regeneration temperature directly affects the structural properties and desulfurization activity of the sorbents. A low regeneration temperature is always accompanied by a low regeneration rate, and the sulfate formed during the regeneration is difficult to decompose completely. However, when the regeneration reaction temperature is too high, it is likely to cause sintering of the active component and the structure of the desulfurization sorbent will be destroyed.31 Thermal analysis was performed on the samples in order to investigate the influence of the regeneration temperature on the desulfurization agent. Figure 4 shows the TG−DSC curves of the in situ regeneration process with programmed temperature at a heating rate of 10 °C· min−1 with 6 vol % O2. The small, but distinct, inflection point in the TG curve can be attributed to loss of water that was adsorbed on the sample, which corresponded to the endothermic peak in the DSC curve with the maximum at 97.1 °C. With the increase in temperature,

Figure 5. Concentrations of SO2 in outlet gas and regeneration rates as a function of time at different temperatures.

outlet gas during regeneration over the temperature range of 550−700 °C. It can be seen that the concentration of SO2 in the outlet gas gradually decreased with increasing regeneration time, and finally it was almost zero when regeneration was nearly completed. It can be observed that the regeneration ability of the sorbent is extremely poor at 550 °C, requiring 12 h to be regenerated completely. With increase in the regeneration temperature, the regeneration time reduced and the concentrations of SO2 among the peak values gradually increased and reached the maximum concentration at the temperature of 700 °C (10.1 g/m3). From the viewpoint of kinetics, both reaction rate constant and effective diffusion coefficient increased with rising temperatures and, subsequently, the regeneration rate and the concentration of SO2 increased.30,32 As shown in Figure 5, with the increase in temperature, the regeneration rate of the sorbent first showed an upward trend until it reached a maximum of 86.2% at the temperature of 650 °C. However, the regeneration rate then decreased when the temperature was further increased up to

Figure 4. TG−DSC curves of in situ regeneration process. C

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Figure 6. SEM graphs of sorbents regenerated at different temperatures (a−d stand for sorbents regenerated at 550, 600, 650, and 700 °C, respectively.).

700 °C. This could be because the desulfurization sorbent undergoes a very exothermic reaction in the oxygen atmosphere during the regeneration process. Thus, higher regeneration temperatures can easily cause sintering of the sorbent, resulting in a significant lowering of the regeneration performance.29 As observed in Figure 6, due to the aggregation and coalescence of the ZnO particles after high-temperature calcination, the sorbent regenerated at 700 °C has a larger average particle size and a denser structure. These observations are consistent with the N2 adsorption results presented in Table 1, showing Table 1. N2 Adsorption−Desorption of Sorbents Regenerated at Various Conditions number

sorbent

SBET (m2/g)

V (mm3/g)

1 2 3 4 5 6 7 8 9 10

Z-550-6-3k Z-600-6-3k Z-650-6-3k Z-700-6-3k Z-650-2-3k Z-650-4-3k Z-650-8-3k Z-650-6-2k Z-650-6-4k Z-650-6-5k

7.94 9.23 9.97 1.96 9.32 8.11 2.51 9.91 7.02 3.78

0.16 0.17 0.19 0.09 0.18 0.15 0.09 0.20 0.14 0.10

Figure 7. Concentrations of SO2 in the outlet gas and regeneration rates at different O2 concentrations.

concentration of O2 enhances the surface O2 density on the desulfurization sorbent and improves the regeneration conversion. The chemical reaction kinetics also showed that the increased oxygen concentration could accelerate the reaction rate, thus making the regeneration time shorter. However, Figure 7 (inset) shows that the increase in oxygen concentration resulted in the regeneration rate of the sorbent first increasing sharply and then decreasing. When the concentration of O2 was 6%, the regeneration rate reached a maximum of 86.2%, and then the regeneration rate dropped when the concentration of O2 was 8%. As observed in Table 1, the surface area and pore volume of sorbent regenerated with 8% O2 showed an obvious reduction compared to those of the other three samples, which was further confirmed by the SEM graphs in Figure 8. The regeneration reaction and the mass transfer might be limited due to sintering of the inner structure of the sorbent regenerated with 8 vol % O2, which is likely responsible for the decrease in the regeneration conversion. Figure 8 presents the SEM graphs of sorbents regenerated with 2% and 4% O2. It can be seen that the lower concentration of O2 generates a smaller particle size but a denser surface structure, implying that the regeneration reaction was incomplete. Thus, from the above overall analysis, it can be seen that the optimum proportion of O2 is 6 vol % within the experimental conditions. 3.3. Effect of Space Velocity on the Regenerability of Sorbents. When regenerated in the oxygen atmosphere, a low space velocity means a slower regeneration rate, which requires a longer time for the sorbent to be regenerated completely. However, the regeneration reaction is an exothermic process and a higher airspeed will lead to the sintering of sorbent. Therefore, a suitable value of space velocity needs to be determined.33,34 In Figure 9, it can be seen that the regeneration rate of sorbents increased initially with the increase of space velocity and reached the maximum of

reductions in surface area and pore volume of the sorbents at different temperatures. The sorbent regenerated at 600 °C possessed the highest surface area. However, the completely regenerated sorbent required a long reaction time at 600 °C and also showed a lower regeneration conversion. Thus, considering all the above results and analyses, 650 °C was selected as the most suitable regeneration temperature. 3.2. Effect of O2 Partial Concentration on Regenerability. The regeneration reaction between O2 and zinc sulfide is exothermic; thus a higher O2 concentration not only leads to the release of a large amount of heat and temperature increase in the sorbent bed but also promotes the formation of sulfate.32 On the other hand, when the oxygen concentration is too low, the regeneration rate is low along with a prolonged regeneration time. Therefore, the effect of O 2 partial concentration in reactant gas on the regeneration performance was evaluated at the temperature of 650 °C and space velocity of 3000 h−1, with O2 concentration ranging from 2 to 8 vol %. As can be seen from Figure 7, the SO2 concentration in the outlet gas increased with increase in the oxygen concentration, and the regeneration time was also shortened. When the O2 concentration was 2%, the regeneration time was as long as 9 h. When the O 2 concentrations were 4% and 6%, the corresponding regeneration times were around 7 h. However, when the O2 concentration was 8%, the regeneration time was the shortest at 3 h. It can be inferred that the increasing D

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Figure 8. SEM graphs of sorbents regenerated at different concentrations of O2 (a−d stand for sorbents regenerated with the O2 concentration of 2, 4, 6, and 8 vol %, respectively.).

Figure 9. Changes of regeneration rates and SO2 concentrations in outlet gas at different space velocities with time.

Figure 11. Breakthrough curves and sulfur capacities of sorbents for four sulfurization cycles.

86.2% when the space velocity was 3000 h−1. Then, with further increase in the space velocity to over 3000 h−1, structural sintering and reduction in both surface area and pore volume were observed as shown in Table 1, resulting in a drop in the regeneration rate. It was also found that the regeneration reaction could not be completed at a very high reactant gas space velocity since a higher space velocity reduces the contact time between the reactant gas and the sorbent bed layer. Figure 10 shows the SEM graphs of sorbents regenerated with different regeneration airspeeds. It can be seen that the regeneration reaction is incomplete for the sorbent regenerated under a space velocity of 2000 h−1. On the basis of the above analyses, a space velocity of 3000 h−1 was selected for the subsequent experiments. 3.4. Reusability Evaluation of Zinc Oxide Sorbent. To investigate the reusability of the zinc oxide sorbent, four successive sulfurization−regeneration cycles were conducted in a simulated coal gas and regenerated (6 vol % O2, N2 balance) at 650 °C with a space velocity of 3000 h−1. Desulfurization breakthrough curves and breakthrough times for the four successive cycles are shown in Figure 11. As can be seen in Figure 11, the fresh sorbent has the longest desulfurization time of about 480 min. However, the breakthrough times of the

sorbents after desulfurization−regeneration cycles were lowered to 445, 420, and 390 min, respectively. The breakthrough time of the fourth regenerated sorbent was 18.17% lower compared to the fresh sorbent. On one hand, with the increasing number of cycles, the internal pore structure of the desulfurization sorbent was partially destroyed due to sintering during the high-temperature regeneration. Also, the specific surface area and the pore volume reduced, resulting in shorter breakthrough times. On the other hand, it is also possible that part of the active sites were covered by the zinc sulfate formed in the regeneration process, which inhibited the adsorption of H2S in the desulfurization reaction.2,32 In addition, during the desulfurization−regeneration cycles, some breakage and cracks were observed in the zinc oxide sorbent after the fourth regeneration, although there was hardly any change in the mechanical strength. 3.5. XRD Patterns of Sorbents in Desulfurization− Regeneration Cycles. The XRD spectra of the regenerated sorbents and fresh sorbent are shown in Figure 12. Strong diffraction peaks corresponding to SiO2 [PDF# 46-1045] and ZnO [PDF# 36-1451] were observed for all the sorbents.22 However, compared to the XRD patterns of fresh sorbent, the

Figure 10. SEM graphs of sorbents regenerated with different space velocities (a−d stand for sorbents regenerated at space velocity of 2000, 3000, 4000, and 5000 h−1, respectively.). E

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Figure 12. XRD patterns of fresh and three regenerated sorbents.

Figure 13. Zn 2p XPS spectra of fresh and regenerated sorbents.

peaks of regenerated sorbents became sharper, indicating that the particle sizes of the active component became larger due to the high regeneration temperature. The (101) peak of ZnO crystallite at about 2θ = 36° was chosen to estimate the crystallite size using Scherrer’s equations.22 The calculated grain sizes of ZnO in the fresh and regenerated sorbents were 36.3, 62.8, 62.9, and 63.1 nm, respectively. The enlarged particle size would result in smaller surface area and blocking of the pore structure, which are detrimental to the mass transfer and the reactions in sulfidation−regeneration cycles. These observations are further supported by the results of SEM and N2 adsorption in Table 2. However, the diffraction peaks at 2θ =

Table 3. Results of Element Contents from XPS Analyses elements

Zn

O

ZF ZR

18.86 16.39

49.7 53.06

desulfurization is the reaction of H2S with the surface ZnO of sorbents. Higher content of elements on the surface of sorbents favors the reaction of active components with H2S. As shown in Figures 14 and 15, after three regenerations, the binding energy

Table 2. Surface Areas, Pore Volumes, and Pore Size Distributions of ZP, ZF, and ZR Sorbents samples

SBET (m2/g)

pore volume (cm3/g)

average pore size (nm)

ZP ZF ZR

10.98 15.19 12.51

0.041 0.047 0.044

11.53 16.21 19.93

25.4° that were attributed to ZnSO4 [PDF# 32-1477 and 080491] and peaks at θ = 28.1° that belonged to ZnS [PDF#391363] appeared in XRD patterns due to the incomplete regeneration of the sorbents. 35 Moreover, the species mentioned above were also found in the fresh sorbent though there were only trace amounts existed inferred from the XRD patterns. This may account for its declined regeneration conversion and desulfurization performance. 3.6. XPS Analyses of Sorbents in Desulfurization− Regeneration Cycles. XPS analyses were conducted to investigate the surface chemical states of Zn, O, and S on the outer surface of ZF and ZR sorbents. As shown in Figure 13, XPS spectra of the sorbent after three regenerations showed similar symmetrical peak shapes but showed different particle sizes compared to that of fresh sorbent. From the surface 2p3/2 Zn spectra of fresh and regenerated sorbents, the binding energy for both was found to be approximately 1021.2 eV, which can be assigned to Zn2+.15,17,19 This indicates that the chemical environment of the Zn element on the surface of sorbents did not change even after repeated desulfurization and regeneration. Table 3 indicates that the content of zinc on the sorbent surface was decreased, which could be due to the volatilization of metallic zinc under the strong reducing atmosphere.2,11 Partial loss of the active components may also cause reduction in the desulfurization performance, since the first step of

Figure 14. O 1s XPS spectra of fresh and regenerated sorbents.

of O 1s did not change significantly. The strong peak at around 530.1 eV is characteristic of the lattice oxygen. The other two

Figure 15. Analyses of O 1s XPS spectra of fresh and regenerated sorbents. F

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and larger agglomerated particles, although the pore structure was recovered. This could be due to the fact that the regeneration reaction under O2 is exothermic and leads to a high reaction bed temperature and severe sintering of the sorbent, resulting in decreased desulfurization performance (breakthrough time reduced by 18.17%). The elemental mapping graphs of ZF and ZR sorbents are also shown in Figure 17. It can be seen that the Zn element is uniformly spread in all the sorbents. On the other hand, the elemental mapping spectrum of the ZP sorbent showed a sulfur phase, and the intensity of the oxygen phase decreased to some extent compared to that of fresh sorbent. In addition, the elemental mapping spectrum of the sorbent ZR showed that the intensity of the oxygen phase was obviously amplified; however, sulfurcontaining species were detected. On the basis of the aforementioned analysis, the elemental mapping results are consistent with XRD and XPS results. 3.8. Changes in the Sorbent Structure. As shown in Figure 18, the adsorption−desorption curves of ZP, ZF, and ZR

peaks located at 531.3 and 532.4 eV could be attributed to adsorbed oxygen and the oxygen of SiO2.22,36 According to the results of XPS, the content of oxygen showed similar changes as the content of Zn, which is also detrimental for the desulfurization reaction. Figure 16 shows the S 2p spectra of

Figure 16. S 2p XPS spectra of regenerated sorbents.

the sorbent after regeneration. Peaks appearing at 161.9 and 169.1 eV were attributed to the S2− and SO2 residuals or incomplete regeneration of the sorbent, respectively.37 The formation of sulfate may be responsible for the reduced activity of the sorbent. 3.7. SEM−Elemental Mapping Analyses of Sorbents in Sulfurization−Regeneration Cycles. Figure 17 shows the SEM−elemental mapping spectra of ZP, ZF, and ZR sorbents. The morphologies of the ZF and ZR sorbents differed significantly. The sorbent ZF had a clearly visible porous structure. However, the sorbent ZR showed a sintered texture

Figure 18. N 2 adsorption−desorption curves and pore size distributions of ZP, ZF, and ZR sorbents.

sorbents show similar profiles. According to the multilayer adsorption theory and IUPAC, the hysteresis loop of adsorption desorption curve exhibited the typical type III adsorption isotherm.38 Table 2 and Figure 18 present the comparisons of the structural parameters of ZP, ZF, and ZR sorbents used in the present study. The N2 adsorption results indicate that the pore size distribution follows the order of ZR sorbent > ZF sorbent > ZP sorbent, which is consistent with the results of SEM analysis. Thus, it is clear that the regeneration process plays an important role in expanding the pore size. As shown in Table 2, the values of specific surface area, pore volume, and pore diameter of sorbent ZP were 10.98 m2/g, 0.041 cm3/g, and 11.53 nm, respectively, while those of sorbent ZF (subjected to in situ regeneration) were 15.19 m2/g, 0.047 cm3/g, and 16.21 nm, respectively. All the structural parameters of ZF show varying degrees of increase compared to the corresponding values for ZP sorbent. The in situ regeneration of precursor sorbent is an oxidation process, in which the sulfur atoms are substituted by oxygen atoms. Since the atomic radius of the former (0.184 nm) is greater than that of the latter (0.140 nm), the in situ regeneration process will increase the size of the new pores compared to the original structure and form the hierarchical pores. These results are

Figure 17. SEM−elemental mapping images of the ZP, ZF, and ZR sorbents. G

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consistent with the experimental results from SEM analysis. At the same time, the surface area and pore volume of the regenerated sorbent decreased. This might be attributed to the structural sintering caused by high-temperature and prolonged processing during the regeneration. Overall, the results indicate that changes in sorbent pore structure are detrimental to the progress of the desulfurization reaction.

4. CONCLUSION This work presents the preparation of the zinc oxide/red clay sorbent by an in situ regeneration method. The effects of regeneration temperature, space velocity, and concentration of O2 on the regeneration performance of this sorbent were thoroughly investigated in the present work. The zinc oxide/ red clay sorbent displayed good reusability during the desulfurization−regeneration cycles, and after four desulfurization−regeneration cycles, the breakthrough time was reduced by 18.17%. The optimum conditions for preparing the sorbent were found to be an in situ regeneration temperature of 650 °C, regeneration space velocity of 3000 h −1 , and oxygen concentration of 6 vol %. According to the characterization results, the loss in desulfurization reactivity can be attributed to the unfavorable changes in the chemical and physical characteristics of the sorbent: (i) XRD analysis results show the presence of sulfur-containing species such as ZnSO4 and ZnS in the sorbent after regeneration, which not only reduced the regeneration rate but also lowered the desulfurization activity of the sorbent. (ii) Pore structure was more welldeveloped after the in situ regeneration, but agglomeration of the active species occurred after regeneration and lowered the reactivity of desulfurization and regeneration processes. (iii) The N2 adsorption and SEM results reveal that larger surface area and pore volume of sorbent play important roles in the diffusion and adsorption of H2S molecules via the interaction of highly dispersed active sites with H2S. The results of this work may provide a more in-depth understanding of the sorbent structure−activity relationship and lead to the development of more efficient sorbents for the desulfurization process.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86 351 6018280. Fax: +86 351 6018280. ORCID

Jie Mi: 0000-0002-9374-2307 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51272170/21276172). REFERENCES

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DOI: 10.1021/acs.energyfuels.6b02127 Energy Fuels XXXX, XXX, XXX−XXX