Comprehensive Application of Oolitic Hematite for H2S Removal at

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Comprehensive application of Oolitic Hematite for H2S Removal at High Temperature: Performance and Mechanism hanlin wang, Tianhu Chen, Haibo Liu, Wengai Li, Xuehua Zou, Can Wang, and Mengxue Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b04261 • Publication Date (Web): 25 Feb 2019 Downloaded from http://pubs.acs.org on March 5, 2019

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Comprehensive application of Oolitic Hematite for H2S Removal at High Temperature: Performance and Mechanism Hanlin Wang, Tianhu Chen1, Haibo Liu, Wengai Li, Xuehua Zou, Can Wang, Mengxue Li Lab for Nano-mineral and Environmental Materials, School of Resources and Environmental Engineering, Hefei University of Technology, Hefei, 230009, China

Abstract: Iron oxide dry desulfurization is one method for the removal of hydrogen sulfide. This work investigates the physicochemical properties and performance of oolitic hematite in desulfurization at high temperature. The effects of particle size, sulfidation temperature and gas speed on the desulfurization performance were evaluated in a fixed bed reaction, also a desulfurization regeneration cycle experiment was conducted. The characterization via XRF, XRD, SEM and XPS were employed to illustrate the desulfurization mechanism. The results show that the particle size and sulfidation temperature significantly influenced the desulfurization performance. At a sulfidation temperature of 700oC and gas speed of 2.4 cm/s, the 180-200 mesh oolitic hematite showed excellent desulfurization performance with a desulfurization capacity of 358.9 mg/g in the first reaction. Moreover, the use of a regeneration cycle slightly increased the

1

Author to whom correspondence should be addressed.

Email: [email protected]

1

ACS Paragon Plus Environment

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desulfurization performance because the active components were further exposed, and the total desulfurization capacity after 8 cycles can reach 2805.6 mg/g. The desulfurization products are pyrite and pyrrhotite with low crystallinity at 400-500°C, and pyrrhotite is the sole product if the reaction temperature exceeds 600°C. The increase in reaction temperature favored the crystallinity of pyrrhotite. The evolution process of reaction between oolitic hematite and hydrogen

sulfide

was

suggested

as

follows:

hematite→magnetite→pyrite/pyrrhotite. The experimental results indicate that this type of oolitic hematite is a promising material for desulfurization due to excellent performance, large reversal capacity and low cost.

Keywords: Oolitic hematite; Hydrogen sulfide; Desulfurization capacity; Desulfurization mechanism

1 Introduction Gasification of clean coal is considered an important technology for ensuring the future energy supply of China. However, hydrogen sulfide is inevitably produced in the coal gasification process. This substance not only causes serious equipment or pipeline corrosion, which can lead to dangerous accidents 1 but also affects the activity of the catalyst in further application to the coal gas conversion reaction, leading to catalyst poisoning and reduced production efficiency 2. Therefore, it is necessary to reduce the hydrogen sulfide concentration in the gas 2


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stream from a percentage to the ppm level, which is a challenging problem. In addition, the coal gasification process should be conducted at 1100-1300°C to obtain a high thermal efficiency 3-4. With this background, if the waste heat of the coal gasification process can be used to remove hydrogen sulfide from coal gas under high temperature conditions, the efficiency of clean conversion and utilization can be improved. High-temperature dry desulfurization is one of the most promising methods that remove hydrogen sulfide from coal gas based on the reaction of various regenerable metal oxides with hydrogen sulfide to form metal sulfide and achieve the purpose of desulfurization. Current studies by domestic and international scholars have primarily focused on zinc oxide, iron oxide, manganese oxide, cerium oxide, composite metal oxide and their admixtures as a prepared desulfurizer for this purpose

5-9.

According to previous studies, iron oxide as an

ideal material for hydrogen sulfide removal, which has the advantages of fast reaction speed, high sulfurization rate, rich resource availability, low cost, superior regeneration performance and outstanding desulfurization capacity

10-11,

and it has been widely used to purify hydrogen sulfide from coal gas. Fan HL et al. 12 experimentally investigated the waste material of red mud from steel factory bound with kaolinite, diatomite, bentonite and a type of natural brick clay to make a prepared desulfurizer, which exhibited a high desulfurization capacity and stable regeneration performance. TH Ko et al.

13

used red soil from different

regions of Taiwan as a desulfurizer in high temperature desulfurization of coal 3


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gas. Sahu RC et al.

14

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researched the performance of red mud, which is an

industrial waste, for hydrogen sulfide removal from gases. Oolitic hematite is a nanomineral resources that mainly contains nanohematite

15

and is widely distributed in Hunan, Hubei, Guizhou, and Jiangxi

Provinces in China and other places. At present, the detected reserves of oolitic hematite has exceed 10 billion tons in China. However, due to the restriction of the genetic and sedimentary environment, hematite is usually associated with siderite, goethite, chlorite, clay minerals and phosphorus-bearing minerals that are cemented or intertwined to form oolitic hematite ore

16.

Thus, oolitic hematite

displays a low iron grade, high phosphorus content and fine particle composition, which make it difficult to separate and utilize. However, oolitic hematite is a nanomineral that exhibits a large specific surface area, a porous structure, high chemical reactivity and other nanoproperties similar to prepared environmentally and friendly nanomineral materials applied in environmental pollution treatment, and thus it is has great potential application prospects 17. Therefore, in this study, oolitic hematite originating from the Heishiban region of Enshi, Hubei Province, China was selected as a desulfurizer for purification of hydrogen sulfide from synthesis gases at high temperature. The characterizations (XRF, XRD, SEM and XPS) of oolitic hematite and desulfurization products were employed to understand the desulfurization performance, renewability and mechanism of oolitic hematite and hydrogen sulfide.

4


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2 Experimental 2.1 Catalyst material and preparation Oolitic hematite was collected from the Heishiban region of Enshi, Hubei Province, China, and its main components are hematite, a small amount of quartz, illite and apatite

18.

The chemical composition is Fe2O3=82.90%, SiO2=8.16%,

Al2O3=4.05%, P2O5=1.33%, CaO=1.90%, MnO=0.26%, MgO=0.71%, and TiO2=0.17%. After crushing and screening, we obtained oolitic hematite of 60-80, 100-120, 140-160, and 180-200 mesh, which was stored in sealed bags. The average Brunauer-Emmett-Teller (BET) specific surface area of different particle size is 11.25 m2/g.

2.2 Reaction apparatus and gas analysis A quartz tube with an inner diameter of 6 mm and a length of 400 mm was used as a fixed-bed reactor, and 0.5 g oolitic hematite was placed in the constant temperature zone of a tube furnace controlled by a temperature-recording controller. The reaction apparatus was heated in a nitrogen atmosphere and stabilized after reaching a target temperature. The fuel was switched to hydrogen sulfide (3% hydrogen sulfide + 97% nitrogen), and the gas velocity was controlled by a D07-19BM mass flow controller (Beijing Qixing Huachuang Co., Ltd.). In the experiment, the concentration of hydrogen sulfide from the outlet of the reactor was tested every 3 minutes by a GMA-3366 gas-phase molecular absorption spectrometer (Shanghai Beiyu Analytical Instrument Co., Ltd.). The 5


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tail gas is exhausted after multistage absorption by 5% NaOH (mass fraction). The hydrogen sulfide desulfurization performance of the oolitic hematite was evaluated via sulfidation-regeneration cycles in the fixed-bed reactor. In the regeneration cycle experiment, after the desulfurizer was deactivated, synthetic air was introduced into the reactor to oxidize the desulfurization product, which was transformed to iron oxide, and the previous desulfurization reaction was continued to examine the desulfurization performance of oolitic hematite after multiple regeneration cycles. The ratio of the mass of hydrogen sulfide removed by the desulfurizer to the mass of the desulfurizer indicates the desulfurization capacity and is calculated by the following formula: 𝑏

Desulfurization capacity(mg/g) =

𝑇 × 𝐶 × 𝑆 ― ∫𝑎𝑓（𝑡）𝑑𝑡 𝑀

(1)

In the formula, T－reaction breakthrough time (min), C－concentration of hydrogen sulfide in the reactor outlet (mg/mL), S－hydrogen sulfide gas flow rate 𝑏

(mL/min), ∫𝑎𝑓（𝑡）𝑑𝑡 mass of hydrogen sulfide not removed after reaction (mg), and M－mass of oolitic hematite (g).

2.3 Characterization The crystalline structure information of the oolitic hematite and desulfurization products was obtained by X-ray diffraction analysis (Japanese D/max-RB) with a Cu Kα X-ray source. The scan angle (2θ) was 3° to 70° with a step size of 0.02°. The distribution and valence state of elemental components of 6


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oolitic hematite before and after the reaction were detected by a Shimadzu 1800 X-ray fluorescence (Shimadzu Co., Kyoto, Japan) and X-ray photoelectron spectrometer (American Thermo ESCALAB250Xi). A Hitachi SU8020 field emission scanning electron microscope (FE-SEM) located the Center of Analytical Testing in Hefei University of Technology was used to observe the surface morphology of the oolitic hematite and desulfurization products. All of the characterization information for oolitic hematite and the desulfurization products was used to investigate the desulfurization performance, renewability and mechanism.

Fig. 1 Sketch of the desulfurization experimental device.

3 Results and discussion 3.1 Effect of particle size and reaction temperature 7


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To understand the effect of particle size and sulfidation temperature on the desulfurization performance, the process was performed on various particle sizes of oolitic hematite in a temperature range of 400-700°C, and the results are shown in Fig. 2 and Fig. 3 (a). Obviously, in the scope of this experiment, the impacts of particle size and sulfidation temperature on the desulfurization performance are significant. As the particle size of oolitic hematite decreases or the sulfidation temperature

increases,

the

penetration

times

are

prolonged,

and

the

desulfurization capacity gradually increases. The breakthrough times for 60-80 mesh are evaluated as approximately 36, 42, 57 and 72 min for 400, 500, 600 and 700°C, respectively. Interestingly, when the particle size of oolitic hematite decreases to 140 mesh, the desulfurization capacity and penetration time tend to become stable. As shown in Fig. 3 (a), with oolitic hematite as the desulfurizer, the desulfurization capacity gradually increases with the increase in the sulfidation temperature. When the reaction temperature reaches 700°C, the desulfurization capacity of the 180-200 mesh material reaches the maximum value, which is 380.8 mg/g. The value for the 60-80 mesh is the minimum, and the desulfurization capacity is 261 mg/g. In addition, at the end of the reaction at 700°C, it is obvious that yellow crystals of sulfur appear at the outlet of the reactor, and the equilibrium concentration of hydrogen sulfide is lower than the initial concentration. To explain this result, the tail gas is collected and a certain amount of hydrogen is detected by gas chromatography. The results were found to be comparable to those given in the literature, showing that hydrogen and 8


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sulfur originate from the decomposition of hydrogen sulfide catalyzed by desulfurization products at 800°C 19. According to the theory of thermodynamics and molecular dynamics. In the one hand, as the temperature increases, the rates of chemical reaction and diffusion increase, which are beneficial to improvement of efficiency of desulfurization

20.

In the other hand, the desulfurization reaction is exothermic.

When the temperature rises, the equilibrium constant of the reaction decreases, making the equilibrium concentration of hydrogen sulfide increase, which is not conducive to the desulfurization reaction. When the sulfidation temperature reaches 600°C, the impact of particle size on desulfurization is weakened, indicating that the interaction of oolitic hematite and hydrogen sulfide is greatly affected by the sulfidation temperature. The main reason for the influence of particle size on the desulfurization is that the smaller particles of oolitic hematite have a larger surface area and more surface active sites, which expands the probability of contact between hydrogen sulfide and oolitic hematite. And the diffusion resistance of hydrogen sulfide on the surface of oolitic hematite is smaller, which is favorable for adsorption and reaction between hydrogen sulfide and oolitic hematite such that the desulfurization capacity is improved. At the same time, the formation of the desulfurization product on the surface layer of the larger-size particles prevents hydrogen sulfide from further reacting with the inner layer of oolitic hematite, which results in poor desulfurization efficiency 21-22.

In addition, it can be seen from the SEM analysis (Fig. 7) that the surface of 9


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oolitic hematite can fractured at high temperature, which is benefit to the reaction of hydrogen sulfide with inner active component on large particle size. There are two aspects compete with each other so that the effect of temperature and particle size on desulfurization capacity presents a complex law.

(a)

0.8

0.6

0.6

60-80 100-120 140-160 180-200

0.4 0.2 0

20

40 60 80 Reaction time/min

C/C0

0.8

0.0

0.2

100

0.0

1.0

0.8

0.8

0.6

0.6

60-80 100-120 140-160 180-200

0.4 0.2 0

20

40

60

80

100

60-80 100-120 140-160 180-200

0.4

1.0 (c)

0.0

(b)

1.0

C/C0

C/C0

1.0

C/C0

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

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0

20


(d)

0.4

60-80 100-120 140-160 180-200

0.2

120

0.0

100

0

20

40

60

80

100

120

Reaction time/min Reaction time/min (a)℃ 400℃ (b)℃ 500℃ (c)℃ 600℃ (d)℃ 700℃

Fig. 2 Breakthrough curves of different sizes of oolitic hematite under different temperatures (sulfidation condition: 2.3 cm/s, 3% H2S, 97% N2 balance gas; C/C0 is the H2S concentration ratio of effluent to initial).

10


Page 11 of 29

(a)

(b) Blank 2.4 cm/s 3.5 cm/s 4.7 cm/s

1.0

150 100 50 0 70

0 60

0 50

0 40

6080

180 0 -20 140 0 -16 0 100 r -12 0 be

M

h es

nu

Desulfurizing capacity/℃ mg/g℃

300 250 200

0.8 0.6

C/C0

Desulfur izing cap acity/℃ m g/g℃

400 350

/℃ re tu ra pe m Te

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

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0.4 0.2 0.0 0

m

10

20

250 228.5 200

212.3

195.9

150 100 50 0

2.4

3.5

4.7

60

70

Gas speed/(cm/s)


Fig. 3 Desulfurization capacity of different sizes of oolitic hematite under different temperature (a) and breakthrough curves and desulfurization capacity of 60-80 mesh of oolitic hematite with different gas speeds (b) (sulfidation condition: 700°C, 3% H2S, 97% N2 balance gas).

3.2 Effect of gas speed In practice, a certain requirement for the gas speed of the reaction tower exists in the desulfurization process of coal gas at high temperature. The lower velocity prolongs the desulfurization time, resulting in reduced efficiency of the desulfurizer. In this experiment, the effects of three different gas speeds of 2.4 cm/s, 3.5 cm/s and 4.7 cm/s on the desulfurization performance were investigated. Figure 3 (b) shows the gas speed dependencies of the breakthrough curve of 60-80 mesh oolitic hematite for different gas speeds (quartz as a blank) at 700°C. It can be observed from the figure that within the scope of this experiment, the 11


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best gas speed of hydrogen sulfide is 2.4 cm/s. With the increase in gas speed, the desulfurization capacity of the oolitic hematite gradually decreases, and the penetration time is also shortened. The smaller gas flow rate means that the residence time of hydrogen sulfide in the reaction bed is longer, which is beneficial to the desulfurization reaction between hydrogen sulfide and oolitic hematite. In contrast, although the gas-solid mass transfer membrane between hydrogen sulfide and oolitic hematite is thinner as the gas flow rate increases, the efficiency of gas-solid mass transfer is increased, but the residence time of the hydrogen sulfide in the reaction bed is shortened. Hydrogen sulfide does not reach the inner surface of the oolitic hematite and is carried away by the gas flow, which results in shortening of the penetration time, although it is not a disadvantage to the progress of the desulfurization reaction. These results indicate that the reaction system between oolitic hematite and hydrogen sulfide is primarily affected by the chemical reaction and gas-solid phase diffusion.

3.3 Regeneration and recycling In practical engineering applications, the requirements for desulfurizers include high desulfurization performance, ease of regeneration, worse transference ,ease of recycling and lower production costs

23-24.

Therefore, 60-80

mesh oolitic hematite is considered as the desulfurizer, with switching to synthetic air and regeneration for 2 h at 700°C after the desulfurization reaction. The gas speed of synthetic air is 3.5 cm/s during the entire regeneration process to 12


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ensure complete regeneration of the oolitic hematite. Fig. 4 shows that the desulfurizer still maintains high desulfurization activity, and no poisoning inactivation occurs after 8 regeneration and recycling tests. With repeated regeneration and recycling, the more active components are exposed on the oolitic hematite, and the greater the desulfurization capacity will be. The total desulfurization capacity of 8 regeneration and recycling cycles reached 2805.6 mg/g, indicating that oolitic hematite is a desulfurizer with a stable desulfurization capacity and is recyclable. The desulfurization capacity of oolitic hematite is comparable to that reported previous studies by researchers, as shown that in Table 1, which lists the desulfurization capacity of various desulfurizers at different temperatures. The desulfurization performance depends on the reaction conditions and materials, and the desulfurization capacity of different desulfurizers varies at different temperature. In comparison, oolitic hematite should be used in removal of hydrogen sulfide because it has a high desulfurization capacity, wide temperature range and low cost. Therefore, oolitic hematite is promising material for removal of hydrogen sulfide from coal gasification.

13


Energy & Fuels

400

Desulfurizing capacity/℃ mg/g℃

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

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350

337.2

300 250

261.2 250.2 260.1

348.1 315.5

369.9 326.4

282.8

200 150 100 50 0 1

2

3 4 5 6 7 Reaction and regeneration times

8

9

Fig. 4 Desulfurization capacity of oolitic hematite after regeneration cycle (sulfidation condition: 700°C, 2.4 cm/s, 3% H2S, 97% N2 balance gas; regeneration condition: 700°C, 3.5cm/s, 2h, 20% O2, 80% N2 balance gas).

Table 1. Desulfurization capacity comparison Desulfurizer

Range of temperature /°C

Desulfurization capacity /g·S/100 g sorbents

25 400 500 600 515-565 350 450-500 850 400-700

1.98 1.89 1.83 1.39 2.38-2.91 7.24 7.54 18-19 12.3-24.7

Red mud Red soil CuO/SBA MnOx/ACs ZnO-based MnxOy/Al2O3 Oolitic hematite

References 14

13

25 26 27 4

This work

3.4 X-ray diffraction Fig. 5 (a) shows the XRD patterns of 180-200 mesh oolitic hematite and desulfurization products at different reaction temperatures. It can be observed from the figure that natural oolitic hematite respectively at 24.2°, 33.2°, 35.7°, 14


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40.9°, 49.6°, 54.2°, 57.7°, 62.6°, 64.1° is the hematite characteristic diffraction peak, and a small amount quartz, chlorite diffraction peaks. It can be infer that the main components of oolitic hematite from Enshi, Hubei is hematite (α-Fe2O3) with a few impurity minerals such as quartz (α-SiO2), phosphorite, chlorite etc. combine with investigated by XRF. When the reaction temperature is 400°C, the main phase in the desulfurization product is still hematite, but the intensity of peak are weaken. Meanwhile, pyrite and pyrrhotite began to appearance in 56.5° and 43.9°, respectively. It indicating that the reaction of oolitic hematite with hydrogen sulfide is incomplete at 400°C. When the reaction temperature reaches 500°C, the characteristic peaks of pyrrhotite begin to appear at 30.0°, 34.0°, 43.9° and 53.1°. Meanwhile, as the reaction temperature increases, the characteristic peak intensities of hematite become weaker, and the characteristic peak intensity of pyrrhotite gradually increases. When the reaction temperature reaches 700°C, the main phase of the desulfurization product is pyrrhotite. Additionally, the intensity of the characteristic peak of pyrrhotite at 700°C is stronger than at 600°C, and the grain sizes of pyrrhotite (D700°C =22.5 nm﹥D600°C=19.1 nm) were calculated from Scherer’s formula and the XRD results, indicating that the crystallinity of pyrrhotite is higher in the desulfurization product at 700°C. Although the particle size of the desulfurizer is larger, as shown in Fig. 5 (b), the presence of magnetite is detected in the 60-80 mesh desulfurization product at 700°C. A possible reason is responsible for this observation that the first reaction products of oolitic hematite with hydrogen sulfide are pyrite and magnetite, 15


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which are all converted into pyrrhotite eventually. The reaction of hydrogen sulfide and magnetite generated in the inner layer is hindered due to passivation of the desulfurization product formed on the surface layer, and thus magnetite remains in the inner layer. The XRD analysis results of oolitic hematite and the desulfurization products show that oolitic hematite begins to transform into pyrite, magnetite and a small amount of pyrrhotite at 400°C and is completely converted to pyrrhotite at 700°C, and the smaller the particle size of the oolitic hematite, the better the formation of pyrrhotite will be.

H:Hematite P:Pyrrhotite Q:Quartz Py:Pyrite C:Chlorite Py (a) Py Q Py Py

(b)

700℃

H H

H Py Py H H

Py

600℃

H Py

H Py P

H

H

H

500℃

HH400℃

P

H H H C

C 10

20

H 30

40

H

Py Py

Py

Py Py

H

Py

HH

180-200

140-160

Py

M Py Py H

50

Py:Pyrrhotite M:Magnetite Q:Quartz Py Py Py Py Q

Intensity/a.u.

Py

Py Py

Intensity/a.u.

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

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100-120

M

M

Raw 60

70

20

2 degree

30

40

50

60

60-80 70

2 degree

Fig. 5 XRD pattern of oolitic hematite before and after desulfurization under different temperatures (a), different sizes of desulfurization product under 700°C (b). 3.5 X-ray photoelectron spectroscopy The XPS spectra of S (2p) and O (1s) of the oolitic hematite and the desulfurization product formed at different reaction temperatures are shown in Fig. 6. The left picture Fig. 6 (a) shows the S (2p) 3/2 curve. According to previous 16


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studies, the binding energies at 162.6 eV and 162.8 eV can be considered as FeS2 28,

those at 161.5 eV and 161.7 eV can be regarded as FeS 29, and those 163.4 eV,

163.8 eV and 165.0 eV can be considered as S or S8 30-31. It can be observed that oolitic hematite is absent of substantial sulfur. As the reaction temperature increases, the proportion of S- in the desulfurization product gradually decreases, and the proportion of S2- and S0 gradually increases. Combined with the XRD of the oolitic hematite before and after the reaction, the following sulfide reaction route of hydrogen sulfide and the oolitic hematite is speculated: Fe2O3 → Fe3O4,FeS2→Fe1-XS. Based on the O (1s) curve shown in the right figure of Fig. 6 (b), the peak fitting of the characteristic peak of O (1s) shows that the oolitic hematite produces two peaks at binding energies of 530 eV and 532 eV. The lower binding energy indicates the lattice oxygen of the oolitic hematite, and the higher binding energy denotes the surface adsorption oxygen on the oolitic hematite 32. As the reaction temperature increases, the proportion of lattice oxygen at the binding energy of 530 eV is reduced significantly, indicating that the lattice oxygen in the hematite is involved in the sulfurization reaction.

17


Energy & Fuels

S-raw

O-raw

℃ a℃

℃ b℃ 532 530.3

162.6

S-400

O-400 532

163.8 161.7 530.3

S-500

161.5

S-600

162.6

Intensity(a.u.)

Intensity(a.u.)

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

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163.8

162.8

O-500 532

530.3

O-600 532

165.0

161.5

530.3

S-700

O-700 532 161.5

163.4 162.8

530.3

159 160 161 162 163 164 165 166 Binding energy(ev)

526

528

530 532 534 Binding energy(ev)

536

Fig. 6 XPS spectra of S (2p) and O (1s) of raw and desulfurization products under different temperatures.

3.6 Scanning electron microscopy Fig. 7 presents the SEM images and EDS information of the oolitic hematite formed at different temperatures. It can be observed from Fig. 7 (a) that oolitic hematite has a scaly morphology, the surface of the single scale body is flat, and these features are superimposed on each other to form a close-packed body. Fig. 7 (b) is also associated with the result of XRD analysis, and it can be observed that when the reaction temperature is 400°C, the main phase of the desulfurization 18


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product is hematite, but the scale morphology of the oolitic hematite has generally disappeared, and the fractures form many small sheet-like morphologies. From the results of SEM of the reaction product at 500°C (Fig. 7(c)), it can be observed that the sheet-like morphology of the desulfurization product is further reduced, and a certain amount of the granular morphology is formed, and aggregates in the form of flakes and granules are deposited. The EDS analysis shows that the elemental composition is 52.0% iron, 17.2% oxygen and 28.1% sulfur at 500°C. Combining the results of XRD and XPS, it can be inferred that these scaly and granular aggregates are mainly hematite, pyrite and pyrrhotite. When the reaction temperature is increased to 600°C or 700°C, the reaction product is granulated and displays massive particles (Figure 7 (d) and Figure 7 (e)). Associated with XRD analysis results, it can be found that these massive particles are pyrrhotite, and the crystallites have larger crystal grains at 700°C than the reaction product at 600°C, which is cons istent with the XRD analysis.

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Fig. 7 SEM images and EDS of raw and desulfurization products under different temperatures (a-raw, b-400°C, c-500°C, d-600°C, e-700°C).

3.7 Reaction mechanism The XRD results show that the desulfurization products are mainly pyrrhotite. Associated with the state of sulfur before and after the reaction in XPS, the main process of the desulfurization reaction is the sulfuration reaction of oolitic hematite and hydrogen sulfide. The reaction scheme is shown in Fig. 8. When the reaction temperature is 400°C, the reaction between oolitic hematite and hydrogen sulfide produces magnetite, pyrite and a small amount of pyrrhotite. As the reaction temperature increases to 500°C, pyrite begins to decompose to form pyrrhotite and elemental sulfur, followed by a process in which elemental sulfur reacts with hematite to form magnetite. The magnetite that has been produced further reacts with elemental sulfur and hydrogen sulfide, and it is completely converted to pyrrhotite at 700°C. The reaction equations are given as follows 33-36: 3𝐹𝑒2𝑂3 + 𝐻2𝑆→2𝐹𝑒3𝑂4 +𝑆𝑂2 + 𝐻2𝑂

∆𝐻𝜃𝑚 = ―282.2𝑘𝐽/𝑚𝑜𝑙（2）

𝐹𝑒3𝑂4 + 4𝐻2𝑆→𝐹𝑒𝑆2 +2𝐹𝑒𝑆 + 4𝐻2𝑂

∆𝐻𝜃𝑚 = ―144.3 𝑘𝐽/𝑚𝑜𝑙

𝐹𝑒2𝑂3 +3𝐻2𝑆→𝐹𝑒𝑆 + 𝐹𝑒𝑆2 +3𝐻2𝑂

∆𝐻𝜃𝑚 = 1209.9 𝑘𝐽/𝑚𝑜𝑙（4） ∆𝐻𝜃𝑚 = 355.1 𝑘𝐽/𝑚𝑜𝑙

(1 ― 𝑥)𝐹𝑒𝑆2→𝐹𝑒1 ― 𝑥𝑆 + 𝑆 6𝐹𝑒2𝑂3 +𝑆→4𝐹𝑒3𝑂4 +𝑆𝑂2

（3）

（5）

∆𝐻𝜃𝑚 = ―102.4 𝑘𝐽/𝑚𝑜𝑙 （6）

(1 ― 𝑥)𝐹𝑒3𝑂4 +5(1 ― 𝑥)𝑆→3𝐹𝑒1 ― 𝑥𝑆 + 2(1 ― 𝑥)𝑆𝑂2 21


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∆𝐻𝜃𝑚 = ―2161.2 𝑘𝐽/𝑚𝑜𝑙 （7）

Fig. 8 Reaction route of oolitic hematite and hydrogen sulfide. According to the process of the desulfurization reaction, sulfur vapor and hydrogen are present in the outlet tail gas. It is speculated that catalytic cracking of hydrogen sulfide by the sulfide product might occur during the reaction. It is indicated that hydrogen sulfide participates in the reaction in two ways at high temperature. The main reaction is the reaction of the oolitic hematite and hydrogen sulfide at high temperature to form pyrrhotite. The secondary reaction is the desulfurization product that acts as a catalyst to prompt the decomposition of hydrogen sulfide to produce hydrogen and sulfur 37. 𝐻2𝑆

𝐹𝑒1 ― 𝑥𝑆 ∆

𝐻2 +𝑆

（8）

During the regeneration process, the desulfurization product is reconverted to hematite by high-temperature air oxidation. The reaction process is given as follows: 𝑂2

𝐹𝑒1 ― 𝑥𝑆 ∆ 𝐹𝑒2𝑂3 +𝑆𝑂2

（9）

By analyzing the above desulfurization reaction pathway, it can be found 22


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that oolitic hematite at high temperature not only removes hydrogen sulfide but also realizes the regeneration of hematite and recovery of sulfur. The SO2 produced by the regeneration reaction can be recovered by catalytic oxidation to produce sulfuric acid or by absorption to form gypsum.

4 Conclusions Oolitic hematite from the Heishiban region of Hubei, Enshi is a type of desulfurizer with a stable desulfurization capacity and is reproducible. The desulfurization performance is primarily affected by particle size, sulfidation temperature and gas speed. A sulfidation temperature of 700°C, superficial gas speed of 2.4 cm/s, and 180-200 mesh oolitic hematite show excellent desulfurization efficiency with a desulfurization capacity of 358.9 mg/g in the first reaction, and the desulfurization performance slightly increases with the regeneration cycle. The total desulfurization capacity of 60-80 mesh oolitic hematite after 8 cycles of regenerations reaches 2805.6 mg/g. The characterization results of the oolitic hematite and desulfurization products indicate that the phase evolution of oolitic hematite reacts with hydrogen sulfide can be written as follows: hematite→magnetite→pyrite/ pyrrhotite. The reaction products of pyrite and pyrrhotite are of low degree in the range of 400-500°C. Pyrite is unstable above 600°C, pyrrhotite is the sole product, and the degree of crystallinity of pyrrhotite changes as the temperature increases.

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Acknowledgments Financial supports from National Natural Science Foundation of China (No. 41572029, 41772038, 41872040) are acknowledged.

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