MCM41

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Hot Coal Gas Desulfurization Using Regenerable ZnO/MCM41 Prepared via One-Step Hydrothermal Synthesis Mengmeng Wu,* Lei Jia, Huiling Fan, and Jie Mi*

Energy Fuels 2017.31:9814-9823. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/26/18. For personal use only.

Key Laboratory of Coal Science and Technology of Shanxi Province and Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, China ABSTRACT: ZnO/MCM41 sorbents with bimodal pore structure were prepared via one-step hydrothermal synthesis. The structure of fresh, sulfided, and regenerated sorbents was characterized using XRD, TEM, SEM, nitrogen adsorption isotherms, and XPS techniques. The results indicate that the desulfurizer with 35% ZnO shows the highest sulfur capacity (11.0 g of S/100 g of sorbents). The optimum regeneration conditions were 650 °C, 3000 h−1, and 6% O2 concentration. Both ZnO and ZnSiO4 were observed in the regenerated sorbents. The increase of regeneration cycles leads to higher content of the Zn2SiO4. However, it results in no significant effect on the performance of sorbents during five sulfidation/regeneration cycles. It may be due to the transformation of Zn2SiO4 into ZnO under conditions of desulfurization. After five successive sulfidation/regeneration cycles, the sorbents still maintain high sulfur capacity (9.8 g of S/100 g of sorbents) and mechanical strength (80 N/cm).

1. INTRODUCTION The gasification of coal, petroleum coke, biomass, and their mixture is one key technique for clean utilization of these resources.1,2 However, the release of gasous sulfur species such as hydrogen sulfide and carbonyl sulfide is inevitable during the gasification process.2 H2S does damage to the pipeline and gas turbine.1−3 It also causes poisoning of catalyst used in the subsequent chemical synthesis.1−3 In addition, it leads to pollution when released into air. Therefore, the removal of H2S is necessary prior to utilization of gasification products either as fuel gas or as synthesis gas.2−4 The techniques of removing H2S involve wet and dry desulfurization. The former is mainly used for rough desulfurization. It owns large handling capacity, while it cannot realize the high-efficient usage of the sensible heat from the raw gas.3 Meanwhile, deep desulfurization is often impossible.2−5 Single and mixed metal oxide is often used in mid- and high-temperature dry desulfurization. Iron oxide is competitive among the sorbents reported due to high sulfur capacity and low cost. Nevertheless, it cannot realize desulfurization with high efficiency.2,5 In the view of desulfurization depth, ZnO is a most attractive sorbent due to favorable sulfidation thermodynamics.6 It can reduce the concentration of H2S from several thousand ppm to sub ppm levels.6 Zinc oxide is most widely used at midtemperatures (350−400 °C).7,8 A higher desulfurization temperature can result in unfavorable thermodynamics but a larger reaction rate.6−8 However, ZnO can easily be reduced to elemental Zn under the strong reducing atmosphere at high temperatures (>600 °C).9 The ZnO could be stabilized by addition of TiO2 resulting from the formation of Zn2TiO4 with a spinel structure, while the activity reduction of the sorbents is irresistible.7,10 According to Novem’s study,11 the optimum sulfidation temperature during the IGCC (integrated gasification combined cycle) process ranges from 350 to 550 °C due to lower overall process cost. Based on the analysis above, the reduction of ZnO may be avoidable, and higher desulfurization depth is achievable when using ZnO as sorbent for coal gas desulfurization at 500 °C. © 2017 American Chemical Society

In comparison with unsupported sorbents, supported desulfurizers are more preferred because of higher reaction activity, mechanical strength, and thermal stability.7 Mesoporous carriers can supply more mesopores (>2 nm), which is conducive to the mass transfer. Furthermore, ordered mesoporous support12 (MCM41, MCM48, and KIT6) is more attractive due to high surface, large volume, and uniform mesopores.12,13 MCM41 can be used as a good support for sorbents removing H2S for dry (or low H2O content) hot coal gas although the hydrothermal stability of MCM48 and KIT6 is better than MCM-41.14 In order to obtain the sorbents with high loading content, two steps are required. The first step including calcinations treatment to remove structure-directing

Figure 1. XRD spectra of ZnO/MCM41 with different preparation methods. Received: June 6, 2017 Revised: August 23, 2017 Published: August 25, 2017 9814

DOI: 10.1021/acs.energyfuels.7b01615 Energy Fuels 2017, 31, 9814−9823

Article

Energy & Fuels

Figure 2. XRD results of MCM41 and ZnO/MCM41. treated mixture was heated in a Teflon-lined autoclave at 100 °C for 48 h. The products were filtrated and washed to neutral. Subsequently, the solid product was dried at 80 °C and calcined at 550 °C for 6 h. A series of samples with 10 wt % ZnO were obtained. They are named as S-A (ct: 4 h; zs: zinc nitrate solution; st: 50 °C), S-B (ct: 2 h; zs: zinc nitrate solution; st: 25 °C), S-C (ct: 4 h; zs: zinc nitrate solution; st: 25 °C), S-D (ct: 4 h; zs: fresh Zn(OH)2 precipitate; st: 50 °C), S-E (ct: 2 h; zs: fresh Zn(OH)2 precipitate; st: 25 °C), and S−F (ct: 4 h; zs: fresh Zn(OH)2 precipitate; st: 25 °C), respectively. The pure MCM 41 was prepared via optimum synthesis conditions without addition of zinc source. 2.2. Characteristic of Sorbent. X-ray diffraction (XRD) analysis was performed by D/MAX2500 (Pigaku, Japan) using Cu Kα radiation. The crystallite size of ZnO was estimated from (101) XRD peak using the Scherrer formula:15

agent (P123, cetyltrimethylammonium bromide) is the synthesis of ordered mesoporous carrier.12,13 Then the subsequent step involves impregnation, precipitation, or sol−gel method.12,13 It is notable that the calcination operation is also inevitable in the second step. Consequently, the complexity of the synthesis procedure becomes the drawback of the current preparation process. In addition, supported Zn-based sorbents could remove H2S at higher temperatures compared to supported Cu-based7 sorbents. They also have high efficiency in comparison with supported Fe-based or Mn-based5 sorbents. As reported in the literature,12,13 Zn-based sorbents supported on a mesoporous carrier own lower sulfur capacity. Furthermore, the related studies mainly focus on the desulfurization evaluation. The regeneration ability is vital for the application of sorbents. However, regeneration (especially multicycle regeneration) of these sorbents is less reported. The main objective of this paper is to investigate the feasibility of one-step hydrothermal synthesis of MCM41 supported ZnO sorbents. The regeneration properties under different conditions were investigated. Furthermore, the performance and the structure of sorbents during multiple sulfidation/regeneration processes are also analyzed.

d101 =

0.9λ β101 cos θ101

(1)

where d101 and λ are the grain size and the wavelength (0.154056 nm) of the X-ray radiation, respectively, and β101 and θ101 represent the fullwidth at half-maximum intensity and scattering angles of the (101) peak, respectively. The morphology of samples was investigated by transmission electron microscope (TEM, JEM-2100) and scanning electron microscopy (SEM, TESCAN MAIA3). The elemental compositions of samples were analyzed by energy dispersive spectroscopy (EDS). ESCALAB250Xi spectrometer (Thermo Fisher Scientific, U.S.A.) using an Mg Kα source was used for XPS analysis. C 1s was used for the calibration of binding energy. The properties of pore structure were investigated by on JW-BK122W Porosity Analyzer (Beijing JWGB Sci & Tech Co., Ltd.). The Brunauer−Emmett−Teller (BET) equation was used for calculating the surface area of samples. 2.3. Desulfurization Evaluation. A fixed-bed quartz reactor (17 mm in inner diameter, 650 mm in length) was used to investigate the sulfidation properties of sorbents. A total of 10 mL of sorbents located in the central part of the quartz tube was heated to 500 °C under N2 atmosphere at atmospheric pressure. Then the stream was switched to the reaction gas containing 0.25% H2S, 39% H2, 27% CO, 12% CO2, and N2 (balance gas) and was passed through the sorbents with cylindrical shape (3 × 3 mm). The volume hourly space velocity

2. EXPERIMENTAL SECTION 2.1. Preparation of ZnO/MCM41 Sorbent. First, 10.0 g of Na2SiO3·9H2O (analytical pure) was dissolved into 60 mL of deionized water. Then, based on mole ratio (1:5) of cetyltrimethylammonium bromide (CTAB) to SiO2, an appropriate amount of CTAB was dissolved with 100 mL of deionized water at 50 °C under constant stirring. The resulting solution was added dropwise to the aforementioned solution. The pH of the mixture was adjusted to 10 with 1 mol·L−1 H2SO4 solution. The resulting white sol was stirred for a certain time (ct). Then, certain amount zinc source (zs) was added to the white sol with continual stirring at specified temperature (st). H2SO4 solution (1 mol·L−1) was used for adjusting the pH of the obtained mixture. The resultant mixture had a molar composition of 1 CTAB:0.2 SiO2:120 H2O. After stirring for another 2 h, the 9815

DOI: 10.1021/acs.energyfuels.7b01615 Energy Fuels 2017, 31, 9814−9823

Article

Energy & Fuels

2.4. Regeneration. The regeneration properties were also evaluated in the same reactor. The reactant gas with a composition of 2−8 vol. % O2 and balance gas N2 was introduced into the reaction tube after the used sorbents were heated to the regeneration temperature (550−700 °C) in N2. The regeneration space velocity varied over the range of 2000−4000 h−1. The iodometry17 was used to analyze the concentrations of outlet SO2. The regeneration rate (Rr, %) is caculated by the following equation:

Rr =

S b − Sa Sb

(2)

where Sa and Sb represent the mass of sulfur containing in the sorbent after and before regeneration, respectively. The measurement for sulfur content and mechanical strength of sorbents was described in the previous study.17

3. RESULTS AND DISCUSSION 3.1. Effect of Preparation Method. Six preparation methods were used, and the small-angel XRD patterns of the resulting ZnO/MCM41 are presented in Figure 1. Initially, the CTAB surfactant as structure-directing agent first forms hexagonal micelles.18 Then, before the addition of zinc sources, the products from the hydrolysis of silicate and polymerization reactions among hydrolysis products could adsorb on the surface of micelles due to van der Waals forces and other weak interactions.19,20 However, the Zn2+ or Zn(OH)2 could also interact with the micelles, which effects the stability of the skeleton structure of MCM41. As shown in Figure 1, compared to zinc nitrate (corresponding to S-A, S-B, and S-C), using Zn(OH)2 as zinc source (corresponding to S-D, S-E, and S-F) results in much stronger diffraction peaks assigned to MCM4118−20 under the same synthesis conditions. It is attributed to the fact that Zn2+ with stronger interaction with hexagonal micelles leads to more adverse effect on the ordered character of MCM41. Additionally, in comparison with S-D and S-E, the structure of S−F is more ordered deduced from stronger peaks observed in small-angle XRD patterns. It suggests longer ct and lower st are favorable for the formation of an ordered mesoporous structure. It is notable that both the strong peak (belonging to the (100) reflection) and two weak signals (corresponding to the (110) and (200) reflections) indexed to its typical hexagonal regularity of the channel18,21 appear in the XRD patterns of S-F. Furthermore, the peaks assigned to ZnO are also observed in the large-angle XRD patterns (Figure 2, discussed later) of supported sorbents. It indicates that one-step synthesis of ZnO/MCM41 sorbents but maintaining the ordered mesoporous structure at the same time is feasible. 3.2. Effect of Loading Content. The optimal preparation method was used to obtain pure MCM41 and a series of sorbents with different loading contents. The sorbents with 15, 20, 25, 30, 35, 40, and 45% zinc oxide are denoted as S-15, S-20, S-25, S-30, S-35, S-40, and S-45. Three typical diffraction peaks assigned to hexagonal regularity of the channel are observed in both MCM41 and ZnO/MCM41. It suggests that the supported sorbents still maintain the ordered pore structure. Strong diffraction peaks (2θ = 31.8°, 34.4°, 36.3°, 47.5°, 56.6°, 62.9°, and 68.0°) [PDF no. 36-1451] belonging to the ZnO phase22 was observed in the large-angle XRD patterns of ZnO/MCM41 with higher (>20%) loading content. However, for sorbents with lower ( 0.45). It reveals that there are larger pores in the supported desulfurizer. As expected, pores with sizes ranging from 10 to 50 nm are observed in the pore size distribution curves. The hierarchical structures via the self-assembly of ZnO nanoplate and ringent ZnO particles are responsible for the 9818

DOI: 10.1021/acs.energyfuels.7b01615 Energy Fuels 2017, 31, 9814−9823

Article

Energy & Fuels

ZnO in S-20, S-30, and S-35 is observed. The decrease is attributed to a less active site per unit surface or pore volume of sorbents with more ZnO but lower surface area (or pore volume). Further increase of the content of zinc oxide leads to the decline in the breakthrough time, sulfur capacity, and UR. It may be due to worse dispersion of the active component for S-40 and S-45. In addition, the sulfur capacity of all sorbents is higher than that reported in the literature (0.38−2.45%).12 3.3. Effect of Regeneration Conditions on Desulfurization Performance. Used S-35 was for investigating the effect of the regeneration condition. Only ZnS is observed (shown later) for used sorbents. The regeneration curves of ZnO/MCM-41 (S-35) are presented in Figure 6. Figure 7 shows the XRD patterns of sorbents after regeneration. The regeneration reaction is mainly dominated by chemical reaction (eq 3) at the initial stage, while diffusion through the dense ZnO layer is the major resistance of the regeneration process in the later stage.7,24,25 As expected, a large part of the SO2 is released, and the reaction rate is larger at the initial stage of regeneration process (see Figure 6), after which the regeneration reaction becomes slow. It takes significantly longer time before the SO2 concentration of the outlet reaches zero when used sorbents is regenerated at 550 and 600 °C. It is attributed to the fact that the rate of the regeneration reaction at low temperature (9.8 g of S/100 g of sorbents) and mechanical strength (>80 N/cm) during five successive sulfidation/regeneration cycles.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86 351 6018598. E-mail: [email protected]. *Tel.: +86 351 6018598. E-mail: [email protected]. ORCID

Mengmeng Wu: 0000-0001-7805-9385 Jie Mi: 0000-0002-9374-2307 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (21506143 and 51272170) and Natural Science Foundation of Shanxi Province (201701D221041).

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REFERENCES

(1) Wang, X.; Liu, Y.; Sun, Z.; Che, D. Energy Fuels 2011, 25, 4865. 9823

DOI: 10.1021/acs.energyfuels.7b01615 Energy Fuels 2017, 31, 9814−9823