Desulfurization of Hot Coal Gas over Regenerable Low-Cost Fe2O3

Article Cite This: Energy Fuels XXXX, XXX, XXX-XXX

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Desulfurization of Hot Coal Gas over Regenerable Low-Cost Fe2O3/ Mesoporous Al2O3 Prepared by the Sol−Gel Method Mengmeng Wu,*,† Teng Li,† Hongyu Li,‡ Huiling Fan,† and Jie Mi*,† †

Key Laboratory of Coal Science and Technology of Shanxi Province and Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, China ‡ Shanxi Province Energy Products Quality Supervision and Inspection Institute, Taiyuan 030012, China ABSTRACT: Low-cost Fe-based sorbent supported on mesoporous alumina (Fe2O3/Al2O3) was prepared by the sol−gel method. The performance of sorbents was evaluated at 500 °C over a fixed-bed reactor. A mixture of SO2, O2, and N2 in various proportions was used to regenerate the sorbents after desulfurization. The structure of sorbents before and after desulfurization was characterized using N2 adsorption, XRD, SEM, and XPS techniques. The results indicate that high sulfur capacity (12.5%) and utilization rate (88.6%) of Fe2O3 are achieved for the sorbents (35% Fe2O3) calcinated at 500 °C for 3 h. The sorbents should be regenerated at above 500 °C due to the formation of Fe2(SO4)3 at low temperature, while increasing regeneration temperature from 600 to 650 °C leads to a decrease of the regeneration rate resulting from the production of Al2(SO4)3 and Al3Fe5O12. The main sulfur-containing regeneration product is elemental sulfur under the optimum regeneration conditions. γFe2O3 and α-Fe2O3 coexist in the regenerated sorbents. In addition, the Fe2O3/Al2O3 desulfurizers show good performance with durable regeneration ability during five desulfurization/regeneration cycles.

1. INTRODUCTION

both desulfurization−regeneration performance and economy are taken into consideration. In comparison with metal oxide desulfurizer without carrier, supported sorbents owe their better reaction activity to better dispersity and thermal stability.1−3,9 The carrier can be classified into two main categories which are (i) microporous supporter1,2,4 (activated carbon, modified activated coke, carbon fiber, zeolite) and (ii) mesoporous materials3,10−12 (Al2O3, SiO2, SBA-15, MCM-41, MCM-48, KIT-6). Compared to metal oxide supported on a microporous carrier, sorbents with mesoporous supporter have higher desulfurizing depth and utilization rate of active components.1,2,10−12 It is attributed to the fact that the latter faces less possibility of pore plugging and lower mass transfer resistance during the desulfurization process.10−12 However, using an expensive silicon source (tetraethoxysilane) or template (hexadecyltrimethylammonium bromide, Pluronic P123) is usually necessary during the synthesis process of ordered mesoporous supporter.10 Consequently, their large-scale applications are impossible. Metal oxide supported on mesoporous silica is widely used for desulfurization at low temperatures. Mesoporous Al2O3 is usually chosen as the carrier for hot gas sulfidation,13−15 and the supported sorbents (MnO2,9 MnAl2O4,14 Mn−Fe−Zn−O 15 ) show good desulfurization properties at high temperatures. The regeneration performance of used sorbents is also important for their industrial application. O2 regeneration is most common.16,17 However, the reaction is highly exothermic.17 Therefore, diluted oxygen or air is used as an alternative atmosphere in order to avoid local overheating of sorbents.16 Consequently, the product of the regeneration reaction is very

Coal gasification is the key technology of the modern coal chemical industry. However, the sulfur contained in the solid coal will transfer to the raw coal gas during the gasification process,1 and the main gaseous sulfur-containing compounds are H2S and COS. Hydrogen sulfide could result in the corrosion of generator blades used in the IGCC (integrated gasification combined cycle) process or poison of catalyst for syngas conversion reactions.2 It also causes environmental pollution when released to the atmosphere. Therefore, it is necessary to reduce H2S concentration to the required level. Compared to wet desulfurization, dry cleaning owns higher sulfidation efficiency and wider operation temperature range.2−4 Furthermore, mid- and high-temperature desulfurization technology could utilize the sensible heat more efficiently.3,4 Novem5 pointed out that the overall IGCC process efficiency gains may not be sufficient when it is operated at above 550 °C due to higher cost with increasing operating temperature. The optimum sulfidation temperature range appears to be 350−550 °C because of the lower overall process cost ascribed to technical viability and process efficiency.5,6 Various metal oxide (such as ZnO, Fe2O3, MnO2, CaO, and CeO2) sorbents have been developed.7 Zinc oxide sorbent is widely used in cleaning of syngas owing to its favorable desulfurization thermodynamics and high desulfurization efficiency.8 However, at temperatures above 600 °C, reduction of ZnO in the strong reducing atmosphere of hot coal gas followed by vaporization of elemental Zn is inevitable.7 Iron oxide sorbent with low cost has greater capacity for sulfur removal per gram of sorbent.4 More importantly, complete regeneration of Fe-based desulfurizer requires lower temperature compared to Zn-based sorbents and other metal (Mn, Ca, Ce, etc.) oxides.6 Consequently, it is an ideal sorbent when © XXXX American Chemical Society

Received: August 26, 2017 Revised: October 30, 2017 Published: October 31, 2017 A

DOI: 10.1021/acs.energyfuels.7b02512 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 1. Character of prepared Al2O3. (a) XRD patterns. (b) N2 adsorption isotherms and pore size distributions. (c), (d) SEM images.

Table 1. Pore Structure Properties of Al2O3 and Fe2O3/Al2O3 with Different Preparation Conditions

a

loading content (%)

calcination temperature (°C)

calcination time (h)

surface area (S, m2 g−1)

decrease in surface area per unit sulfur capacity (DS, m2 g−1)

0 15 25 35 40 35 35 35 35 35

500 500 500 500 550 600 650 550 550

2 2 2 2 2 2 2 3 4

239 195 (166)a 155 (153)a 146 (142)a 117 (106)a 119 (113)a 101 (99)a 98 (96)a 115 (108)a 111 (97)a

543 25 37 119 49 17 22 48 117

pore volume (V, cm3 g−1) 0.503 0.417 0.366 0.336 0.256 0.347 0.280 0.273 0.343 0.323

decrease in pore volume per unit sulfur capacity (DV, cm3 g−1)

(0.316)a (0.357)a (0.261)a (0.156)a (0.280)a (0.269)a (0.261)a (0.322)a (0.291)a

0.0189 0.0011 0.0069 0.0108 0.0054 0.0010 0.0013 0.0017 0.0027

average pore size (nm) 8.2 7.6 7.6 7.2 6.9 9.1 8.7 8.4 8.5 8.2

(7.0)a (6.8)a (5.9)a (5.0)a (8.4)a (8.3)a (8.2)a (8.2)a (7.8)a

Sorbents after sulfidation.

and the rate of first reaction will be accelerated.16 In addition, the elemental sulfur could be separated by condensation, while the tail gas can be recycled as regeneration gas.16 In this study, mesoporous Al2O3 was prepared by a simple and low-cost method. Then, the Fe-based sorbents were obtained by the sol−gel route. The influence of preparation conditions and of Fe2O3/ Al2O3 was presented. The effect of regeneration parameters on used sorbents under SO2−O2 atmosphere was also discussed. In addition, the desulfurization properties of sorbents during multicycle operation were investigated.

diluted SO2. H2O could react with the metal sulfide oxide by the reverse of the desulfurization reaction. However, the reaction rate is very slow.18 Except for H2S and SO2, large amounts of elemental sulfur were produced when FeS, MnS, and Cu2S were regenerated in H2O−O2 atomosphere.18 However, very large H2O/O2 ratios in the regeneration free gas are necessary.18 Regeneration and Claus reaction could be coupled in one reactor when the used sorbents reacted with SO2.19 Bakker20 regenerated the used Mn-based sorbent with 50 mol % SO2 at 850 °C, while Mi21 investigated the regeneration behavior of modified semicoke-supported Fe/Zn/ Ce sorbent under the atmosphere containing 8−20 mol % SO2. In both cases, directly produced elemental sulfur was achieved. However, metal oxide sorbents with stronger affinity for hydrogen sulfide are more difficult to be regenerated with elemental sulfur as one of the products based on thermodynamic analysis.16,18 Therefore, for Fe-based sorbent, it takes a very long regeneration time before complete regeneration.16,18 If partial oxidation using SO2−O2 atomosphere is applied to regeneration of desulfurizers, the thermal effect resulting from two reactions of metal sulfide with SO2 and O2 could be complementary. Therefore, sorbent deterioration through sintering may be prevented. Also, sulfur will be produced,

2. EXPERIMENTAL SECTION 2.1. Al2O3 Supporter. An amount of 34.2 g of sugar (AC grade) and 37.51 g of Fe(NO3)3·9H2O (AC grade) was dissolved in 360 mL of deionized water. Then the pH was adjusted to 10 by the addition of the required drops of NH3·H2O. The resulting white sol was obtained. The solid material in the sol was collected by the centrifugation method (10 min, 6000 rpm). Then the sample was dried at 120 °C for 10 h. Finally, the resultant sample was calcined at 550 °C in a muffle furnace for 3 h. As shown in Figure 1a, the typical diffraction peaks (at 2θ = 19.7°, 37.0°, 45.7°, and 66.6°) assigned to the γ-Al2O3 phase22 are observed. The N2 adsorption isotherms of Al2O3 (Figure 1b) show a type IV sorption isotherm, indicative of the mesoporous characteristic. B


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of 500−650 °C, while the volumetric space velocity ranged from 6000 to 5000 h−1. The concentrations of SO2 were measured by iodometry.23 The regeneration rate (Rr, %) is expressed by the following formula

The results of pore size distribution (Figure 1b) of the supporter indicate that the pore size is mainly ranging from 2 to 9 nm. As shown in Figures 1c and d, the surface of Al2O3 displays porous structure with large pores (about several hundred nanometers wide). The surface area (239 m2 g−1) and pore volume (0.503 cm3 g−1) of Al2O3 listed in Table 1 are closer to or larger than reported in the literature.9 Such a pore structure is beneficial for the loading of iron oxide. 2.2. Fe2O3/Al2O3 Sorbent. An amount of 12.2 g of FeCl3·6H2O (AC grade) was dissolved in 150 mL of deionized water. Then the NaOH (0.5 mol L−1) solution was added into it drop by drop with constant stirring. After 0.5 h, the resulting precipitation was filtered and washed by deionized water at least 3 times. The treated precipitation was then added into 60 mL of FeCl3 (0.2 mol L−1) solution. The brown mixture was kept stirring for 0.5 h and then maintained static for 1 h. Finally, the sol of Fe(OH)3 was obtained. The Al2O3 powder prepared was added into the Fe(OH)3 sol with different mixing ratios under constant stirring at 60 °C. The mixture was kept stirring until complete water evaporation. The sample was grinded into powder and dried at 60 °C for 10 h. The resulting sample was washed using deionized water untile no Cl−1 was detected. Then, it was heated at 60 °C. Subsequently, the treated powder was calcinated at 500, 550, 600, or 650 °C for 2, 3, or 4 h. Finally, a series of Fe2O3/Al2O3 sorbents with different Fe2O3 content (15, 25, 35%, and 40%) were obtained. 2.3. Characteristic of Sorbents. The X-ray diffraction (XRD) device (D/MAX-2500) using Cu Kα radiation was used to obtain XRD patterns. The grain size of Fe2O3 and Fe1−xS was estimated from the (111) and (220) XRD peak using the following Scherrer formula,23 respectively.

d=

0.9 λ β cos θ

Rr =

SC of sorbent theoretic SC of sorbent

(3)

where Sb and Sa denote the amount of sulfur contained in the desulfurizer before and after regeneration, respectively. The method of measurement for sulfur content in sorbents was reported elsewhere.23

3. RESULTS AND DISCUSSION 3.1. Effect of Preparation Conditions on Desulfurization Properties. 3.1.1. Effect of Loading Content. The pore

(1)

where d and λ are grain size and the wavelength of the X-ray radiation, respectively. The β and θ represent the full-width at half-maximum intensity and scattering angles of the XRD peak, respectively. The XRD peaks were fitted using Gaussian functions. Scanning electron microscopy (SEM, MAIA3 TESCAN) was used to analyze the morphology of samples. N2 adsorption of the samples was measured at −196 °C with an ASAP 2460 Micromeritics apparatus. The Brunauer− Emmett−Teller (BET) method was used to calculate the surface area, while the pore size distribution of samples was obtained by the Barrett, Joyner, and Halenda (BJH) method. 2.4. Desulfurization Tests. The desulfurization performance of the sorbents (1.0 g, 60−80 mesh) was tested in a fixed-bed reactor with a quartz tube (14 mm in inner diameter) at atmospheric pressure using a simulated hot coal gas (0.27% H2S, 39% H2, 27% CO, 0.12% CO2, and N2 as balance gas). The sorbent was first heated to 300 °C under N2 atomosphere at the rate of 15 °C min−1, and then the stream was replaced by the gas mixture (39% H2, 27% CO, and 34% N2) with the same flow rate. When the sorbent was heated to 500 °C at the heating rate of 10 °C min−1, the inlet gas was switched to the reaction gas with the volume space velocity of 3000 h−1. The gas volume was adjusted by a mass flow controller, while the temperature of the sorbent bed was measured with a thermocouple in the center of the sorbent bed. The H2S concentrations of inlet and exit were analyzed by a gas chromatograph equipped with a flame photometric detector. When the H2S outlet concentration reaches 540 ppm, the corresponding time is defined as breakthrough time (BT), while sulfur capacity (SC, %) calculated by the formula described elsewhere24 was the ratio of the mass of sulfur captured before BT to that of sorbent. The utiliztion rate of Fe2O3 (UR, %) was calculated by the following equation:

UR =

S b − Sa Sb

Figure 2. XRD analysis of Fe2O3/Al2O3 sorbents with different loading content.

Figure 3. Breakthrough curves of Fe2O3/Al2O3 sorbents with different loading content (reaction conditions: 0.27% H2S, 39% H2, 27% CO, 0.12% CO2, N2 as balance gas; 500 °C).

structure analysis of Fe2O3/Al2O3 with different loading fractions is listed in Table 1. In comparison with the Al2O3 carrier, the loading of iron oxide (15, 25, 35, and 40%) leads to the decrease of surface area (S) by 18.4, 35.1, 38.9, and 51.0%, respectively. The pore volume (V) also decreases by 17.1, 27.2, 33.2, and 49.1%, respectively. Larger S and V are favorable for the desulfurization of Fe2O3/Al2O3 due to smaller mass transfer resistance. Also, the decrease in the average pore size is observed. The results indicate that a part of pores in Al2O3 are

(2)

2.5. Regeneration Tests. The regeneration experiment of used sorbents (2 mL, 1.95 g) was performed at the same reactor. The reactant stream contains 2 vol % O2, 2−5 vol % SO2, and N2 as the balance gas. The regeneration temperature was varied over the range C


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Figure 4. XRD analysis of used Fe2O3/Al2O3 sorbents with different loading content.

Figure 7. XRD spectra of used Fe2O3/Al2O3 sorbents with different calcination temperatures.

Figure 5. Breakthrough curves of Fe2O3/Al2O3 sorbents with different calcination temperatures (reaction conditions: 0.27% H2S, 39% H2, 27% CO, 0.12% CO2, N2 as balance gas; 500 °C).

Figure 8. Breakthrough curves of Fe2O3/Al2O3 sorbents with different calcination time (reaction conditions: 0.27% H2S, 39% H2, 27% CO, 0.12% CO2, N2 as balance gas; 500 °C).

Figure 6. XRD spectra of Fe2O3/Al2O3 sorbents with different calcination temperatures.

Figure 9. XRD spectra of fresh Fe2O3/Al2O3 sorbents with different calcination time.

occupied by Fe2O3. The XRD patterns of fresh sorbents with different content of iron oxide are shown in Figure 2. The diffraction peaks at 2θ = 24.1°, 33.2°, 35.6°, 40.9°, 49.5°, 54.1°, 62.4°, and 66.0° are ascribed to the α-Fe2O3 [PDF# 33-0664]. Furthermore, with more loading content of the Fe2O3, the

much stronger the intensity of these peaks. The crystallite size of α-Fe2O3 listed in Figure 2 is estimated from the (111) XRD peak using the Scherrer formula.23 The results suggest that the grain size of iron oxide was only 8.8 nm when the mass fraction D


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Figure 10. XRD spectra of used Fe2O3/Al2O3 sorbents with different calcination time.

Figure 12. XRD spectra of regenerated Fe2O3/Al2O3 sorbents with different regeneration conditions. Figure 11. Difference in SO2 concentration at the exit and inlet due to regeneration of Fe2O3/Al2O3 sorbents with different temperatures.

of Fe2O3 was 15%, while increasing loading content results in a significant increment (>14.8 nm) of crystallite size. Furthermore, only a slight change in grain size of iron oxide is observed between two sorbents with 25 and 35% Fe2O3, and the size of the Fe2O3 crystallite increases by 20% with increasing Fe2O3 content from 35 to 40%. Fe2O3 grains with smaller size could provide larger contact area between Fe2O3 and H2S, which facilitates the desulfurization of Fe2O3/Al2O3. Figure 3 presents the breakthrough curves of Fe2O3/Al2O3 sorbents. Breakthrough time (BT), sulfur capacity (SC, %), and the utilization rate of the active component (UR, %) are used to evaluated the desulfurization performance of sorbents.12,14 In comparison with sorbents containing 15% Fe2O3, BT values of sorbents increase by 47 and 90% with the increase of loading concentration (25 and 35%), respectively, and the SC increases by 52 and 101%, respectively. However, further increase in Fe2O3 content results in much lower BT (245 min), SC (9.3%), and lowest UR (57.7%). It is due to the nonuniformity of the active component with such high loading content. In addition, a monotonic decreasing relation between UR (57.7−88.7%) and Fe2O3 content is observed. It is attributed to the better dispersion of iron oxide in sorbents with lower loading content. As shown in Figure 4, peaks of α-Fe2O3 disappear in XRD patterns of four used sorbents, while diffraction peaks (at 2θ =

Figure 13. Difference in SO2 concentration at exit and inlet due to regeneration of Fe2O3/Al2O3 sorbents with different space velocity.

26.0°, 30.2°, 35.6°, 57.5°, and 62.9°) belonging to γ-Fe2O3 [PDF# 39-1346] appear. It is consistent with incomplete conversion (UR: 57.7−88.7%) of Fe2O3. The results also suggest transformation of the crystal type from the α-Fe2O3 to γ-Fe 2 O 3 phase occurs under strong reducing reaction atmosphere. The diffraction peak assigned to Fe1−xS [PDF #29-0726] at 2θ of 43.7° appears in the XRD patterns of four sulfidated sorbents with the initial Fe2O3 content ranging from E


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of O2− (radius: 0.140 nm) by S2− (radius: 0.184 nm).25 However, there is less decrease (by 14.8, 1.3, 2.7, and 9.4%, respectively) in the surface area (S). Two parameters denoted as DS (decrease in S per unit sulfur capacity, m2 g−1) and DV (decrease in V per unit sulfur capacity, cm3 g−1) are also used to evaluate the effect of sulfidation on the pore structure. It is interesting that there is a smaller decrease in the DS and DV (see Table 1) of Fe2O3/Al2O3 with the highest sulfur capacity in comparison with sorbents containing 15 or 40% iron oxide. It is an advantage of the former sorbents. In addition, no obvious change in the average pore size of four sorbents after removing H2S occurs. 3.1.2. Effect of Calcination Temperature. The breakthrough curves and XRD spectra of sorbents (containing 35% Fe2O3) with different calcination temperatures are illustrated in Figures 5 and 6. The sorbents calcinated at 500, 550, 600, and 650 °C are named as S-500, S-550, S-600, and S-650, respectively. An increase of BT, sulfur capacity, and UR with increasing calcination temperature from 500 to 550 °C is observed. However, further elevation of calcination temperature led to the decrease of BT, sulfur capacity, and UR. Diffraction peaks assigned to the α-Fe2O3 appear in the XRD patterns of S-550, S-600, and S-650, while typical peaks (at 2θ = 24.1°, 40.9°, and 62.4°) are very weak in intensity or not observed for S-500. Such a crystal structure of α-Fe2O3 may be the reason for the poor performance of S-500. It is notable that the decrease (by 5.0 and 19.3%) in S and V of S550 with the largest sulfur capacity is larger than that of S600 and S650. Moreover, the DS (49 m2 g−1) or DV (0.054 m3 g−1) values of S550 are higher than the other two sorbents (Table 1). Although the change of pore structure due to sulfidation is unsatisfactory for S550, S550 shows the best desulfurization performance. It may be related to the crystallized structure of the active component. The grain size (27.0 nm) of Fe2O3 in S550 is smaller than that (31.7 and 33.4 nm, respectively) in S600 and S650. There is less possibility of aggregation of crystalline at lower temperatures, and Fe2O3 crystallite with smaller size can provide larger contact area between Fe2O3 and H2S.8 Consequently, S-550 shows better desulfurization properties. Additionally, stronger peaks assigned to Fe1−xS appear in the XRD patterns (Figure 7) of used sorbents, and the grain size of Fe1−xS estimated from the (220) XRD peak varied from 7.2 to 11.5 nm. The order agrees with the order of the calcination temperature of sorbents. 3.1.3. Effect of Calcination Time. Prolonging calcination time is beneficial to the structure stability of the sorbents.3,12,20 However, the sintering of Fe2O3 nanoparticles is inevitable. Thus, proper calcination time is necessary in order to obtain

Figure 14. Difference in SO2 concentration at the exit and inlet due to regeneration of Fe2O3/Al2O3 sorbents with different volume ratios of SO2 to O2.

Table 2. Pore Structure Properties of Used Fe2O3/Al2O3 after Regeneration under Different Conditions temperature (°C)

space velocity (h−1)

500 550 600 650 600 600 600 600 600 600

6000 6000 6000 6000 9000 12000 15000 12000 12000 12000

a

VSO2/VO2

surface area (S, m2 g−1)

pore volume (V, cm3 g−1)

average pore size (nm)

2:2 2:2 2:2 2:2 2:2 2:2 2:2 3:2 4:2 5:2

106a 45 61 87 21 83 89 75 68 72 64

0.322a 0.170 0.189 0.288 0.060 0.284 0.293 0.267 0.191 0.202 0.194

8.4a 11.3 8.9 9.3 9.2 9.3 9.4 9.6 8.1 8.2 8.2

Properties of used Fe2O3/Al2O3 before regeneration.

15 to 40%, suggesting the conversion of metal oxide to metal sulfide. However, the (220) diffraction peak belonging to Fe1−xS at 2θ of 53.1° is undetectable for sorbents (with 15 and 25% Fe2O3) after desulfurization. It may result from low content of Fe1−xS. For sorbents with the highest sulfur capacity, the grain size of Fe1−xS estimated from the weak (220) peak is 7.2 nm, which is much smaller than that of α-Fe2O3 in fresh sorbents. For sorbents with loading content of 15, 35, and 40%, sulfidation leads to a significant decrease in the pore volume (by 24.1, 22.3, and 39.0%, respectively) due to the replacement

Table 3. Regeneration Properties of Fe2O3/Al2O3 after Regeneration under Different Conditions temperature (°C)

space velocity (h−1)

VSO2/VO2

SO2−X (g/g sorbent)

SO2−Y (g/g sorbent)

SO2−Y/SO2−X

500 550 600 650 600 600 600 600 600 600

6000 6000 6000 6000 9000 12000 15000 12000 12000 12000

2:2 2:2 2:2 2:2 2:2 2:2 2:2 3:2 4:2 5:2

0.174 0.151 0.129 0.061 0.043 0.066 0.135 0.018 0.006 0.018

0.077 0.060 0.110 0.090 0.148 0.520 0.067 0.493 0.691 0.376

0.44 0.40 0.85 1.49 3.44 7.88 0.50 27.39 115.17 20.89

F


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Energy & Fuels Table 4. Pore Structure Properties of Sorbents during the Multicycle Sulfidation/Regeneration Process surface area (S, m2 g−1)

pore volume (V, cm3 g−1)

average pore size (nm)

cycles

sulfidation

regeneration

sulfidation

regeneration

sulfidation

regeneration

1 2 3 4 5

106 48 47 46 41

72 47 44 41 40

0.322 0.263 0.201 0.174 0.162

0.269 0.261 0.183 0.163 0.160

8.4 12.4 10.4 11.0 11.2

8.9 13.6 11.1 11.7 11.8

Figure 15. Desulfurization properties of sorbents during successive desulfurization/regeneration cycles.

Figure 17. XRD patterns of sorbents after regeneration.

sorbents (Table 1). It is favorable for its application as a desulfurizer. In addition, there was yellow sulfur on the inside surface of the downstream quartz tube for all sulfidation experiments. It is attributed to the catalytic action of Fe1−xS in the heterogeneous thermal decomposition of hydrogen sulfide.25 A similar phenomenon was also reported by Liu.25 In general, the optimum preparation conditions are 35% Fe2O3, 550 °C (calcination temperature), and 3 h (calcination time), and the utilization rate (88.6%) of iron oxide is higher than that reported (68.8−79.1%) in the literature.4 3.2. Effect of Regeneration Conditions. The used Fe2O3/Al2O3 (35% Fe2O3) calcinated at 550 °C for 3 h was chosen as sulfidated sorbent. The regeneration properties and structure changes of sorbents are presented in Figures 11−14 and Tables 3 and 4. As mentioned in Section 1, the aim of regeneration using SO2−O2 atmosphere in our experiments is to obtain elemental sulfur and decrease the possibility of (or prevent) sintering of active component during the regeneration process. Actually, under SO2−O2 atmosphere, a part of SO2 from the inlet of the fixed bed for regeneration is consumed by regeneration of Fe1−xS by SO2 (Reaction-A, eq 4), while oxygenation of Fe1−xS by O2 (Reaction-B, eq 5) produces SO2. 3 − 3x 1−x 7 − 4x Fe1 − xS + SO2 → Fe2O3 + S (4) 4 2 4

Figure 16. XRD patterns of sorbents after sulfidation.

sorbents with the best desulfurization performance. Figures 8 and 9 present the breakthrough curves and XRD patterns of sorbents with different calcination time. Increasing calcination time from 2 to 3 h results in larger BT, SC, and UR. However, further increase in calcination time leads to the decrease (by 11.6, 4.3, and 4.3%, respectively) in BT, SC, and UR. The grain size of α-Fe2O3 is 27.0, 31.8, and 34.2 nm for fresh sorbents calcinated for 2, 3, and 4 h, respectively. Proper structure stability and pore structure may be responsible for better desulfurization properties of sorbents calcinated for 3 h. After desulfurization, no diffraction peaks assigned to Fe2O3 appear in XRD patterns (Figure 10) of sorbents. It agrees with high SC (11.9 to 12.5%) and UR (84.9 to 88.6%) of three sorbents, and the crystalline size of Fe1−xS is 9.6, 11.3, and 11.3 nm for three sorbents after desulfurization, respectively. It is notable that the DS (48 m2 g−1) or DV (0.017 m3 g−1) values of sorbents with highest sulfur capacity are both lower than that of the other two

Fe1 − xS +

7 − 3x 1−x O2 → Fe2O3 + SO2 4 2

(5)

It is speculated that Reaction-B is the major chemical reaction at the earlier stage of the regeneration process due to significantly favorable thermodynamics compared to ReactionA. However, more SO2 is produced via Reaction-B, as the regeneration progresses and the SO2 concentration (≥57 140 mg/m3) of the inlet are high. It is unfavorable for Reaction-B G


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Figure 18. SEM images of fresh and used sorbents: (a) fresh sorbent (35% Fe2O3, calcinated at 550 °C for 3 h), (b) sorbents after fifth sulfidation, and (c) sorbents after fifth regeneration.

Figure 19. Fe 2p3/2 XPS spectra of sorbents before and after desulfurization/regeneration.

Figure 20. O 1s XPS spectra of sorbents before and after desulfurization/regeneration.

based on the principle of Le Chatelier,26 while it facilities Reaction-A as SO2 is the reactant of this reaction. Furthermore, the reaction heat from exothermic18 Reaction-B could provide a part of the heat for endothermic20 Reaction-A. Consequently, a

great amount of SO2 participates in Reaction-A after the early stage of regeneration. Figures 11, 13, and 14 show the difference in SO2 concentration at the exit and inlet due to regeneration of Fe2O3/Al2O3 sorbents under different conH


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Figure 21. S 2p XPS spectra of sorbents before and after desulfurization.

Table 5. Binding Energies (BEs) and Contents of Different Species Obtained from XPS Spectra of O 1s and S 2p O1s Oa

S2p

Ob

Oc

Sa

samples

BE (eV)

content (%)

BE (eV)

content (%)

BE (eV)

content (%)

fresh sorbent 1st sulfidation 1st regeneration 2nd sulfidation 2nd regeneration 3rd sulfidation 3rd regeneration 4th sulfidation 4th regeneration 5th sulfidation 5th regeneration

530.0 530.1 530.1 530.1 530.1 530.1 530.1 530.1 530.2 530.1 530.1

55.5 45.0 46.9 43.8 52.5 50.2 52.3 40.7 30.7 20.0 48.1

531.6 531.6 531.6 531.7 531.6 531.7 531.6 531.6 531.7 531.6 531.7

39.9 39.4 45.6 46.9 41.4 41.5 39.0 58.1 57.4 46.9 44.7

533.4 533.4 533.4 533.4 533.4 533.4 533.4 533.4 533.4 533.4 533.4

4.6 15.6 7.5 9.3 6.1 8.3 8.7 1.2 11.9 33.1 7.22

a

BE (eV)

Sb content (%)

BE (eV)

Sc content (%)

BE (eV)

content (%)

a

a

a

a

a

a

161.8

49.1

164.0

13.0

a

a

a

a

168.6 168.9 168.4 168.7 168.4 169.0 169.0 169.0 168.9 168.9

37.9 100 30.6 100 21.2 100 29.8 100 25.2 100

161.6

58.1

164.0

11.3

a

a

a

a

161.6

58.9

163.8

19.9

a

a

a

a

161.6

45.8

163.8

24.4

a

a

a

a

161.8

44.4

164.0

30.4

a

a

a

a

Undetectable.

mass of sorbent (g), respectively; t1 and t2 correspond to the time when the CEC reaches 0 for the first and second time during the regeneration progress. It is notable that yellow sulfur was observed on the inside surface of the downstream quartz tube for all regeneration experiments. 3.2.1. Effect of Regeneration Temperature. Two phases of Fe2O3 are observed (Figure 12) in regenerated sorbents. Furthermore, it is mainly γ-Fe2O3 at 500 °C, while α-Fe2O3 is the main phase of iron oxide in sorbents regenerated at higher temperatures (>500 °C). Fe2(SO4)3 is only observed in sorbents regenerated at 500 °C due to the stability of ferric sulfate at 500 °C. It is consistent with its lowest regeneration rate (82.3%). As described above, there is a positive correlation between SO2−Y/SO2−X and the ratio of the elemental sulfur in the regenerated gas products (S and SO2). The value of SO2−Y/SO2−X at 500 °C is slightly greater than that at 550 °C. It may be attributed to the fact that a part of SO2 was consumed for the production of Fe2(SO4)3 at 500 °C. The values of SO2−Y/SO2−X increase with temperature due to the fact that exothermic Reaction-B is thermodynamically unfavorable at higher temperature. The highest regeneration rate (Rr,

ditions. The vertical coordinates are obtained by subtracting the inlet SO2 concentration from the outlet SO2 concentration. The amount of consumed SO2 via Reaction-A and SO2 produced via Reaction-B is denoted as SO2−A (g/g sorbent) and SO2−B (g/ g sorbent), respectively. Then, based on the analysis above, SO2−X and SO2−Y (calculated by eqs 6 and 7) listed in Table 3 represent (SO2−B minus SO2−A at the earlier stage of regeneration) and (SO2−A minus SO2−B after the early stage of regeneration), respectively. It is obvious that higher ratio of SO2−Y to SO2−X reflects larger percentage of elemental sulfur in the regenerated gas products (S and SO2). SO2 −X =

VHSV × Vsorbent × msorbent

∫0

SO2 −Y =

VHSV × Vsorbent × msorbent

∫t

t1

1

C ECdt × 10−3 t2

C ECdt × 10−3

(6)

(7)

In eqs 6 and 7, VHSV and CEC represent volumetric space velocity and the change in SO2 concentration at the exit and inlet, respectively; Vsorbent and msorbent are the volume (mL) and I


Article

Energy & Fuels 93.1%) appears when the sorbents were regenerated at 600 °C. However, further increase of temperature leads to the decrease in the Rr and SO2−Y. Al2(SO4)3 and Al3Fe5O12 appear in XRD patterns of sorbents regenerated at 650 °C. It is responsible for the decrease above. The pore structure properties of used Fe2O3/Al2O3 after regeneration at different temperatures are listed in Table 2. As expected, the average pore size of used sorbents increases after regeneration. However, regeneration leads to the decrease in the surface area and pore volume of four used sorbents. The production of Al2(SO4)3 is the main reason for the larger decrease amount (61 m2 g−1) of used sorbents after regeneration at 500 °C, while the largest decrease (by 80.2%) corresponding to the highest regeneration temperature can be explained by the formation of Al2(SO4)3 and Al3Fe5O12 and possible sintering of sorbents at 650 °C. The lowest decrease in S and V is observed when the sorbents are regenerated at 600 °C. It agrees with the good regeneration (at 600 °C) performance of used sorbents. 3.2.2. Effect of Regeneration Space Velocity. Increasing space velocity from 6000 to 12 000 h−1, a greater amount of SO2−A is observed. It is attributed to lower mass transfer resistance at higher space velocity.15,20 However, further increase of space velocity leads to lowest Rr (88.3%) and the decrease in SO2−Y and SO2−Y/SO2−X. It is due to the fact that the shorter SO2−Fe1−xS contact time at high space velocity (15 000 h−1) is unfavorable for the regeneration reaction. The results of pore structure analysis suggest that the lowest decrease in S and V is observed for used sorbents undergoing regeneration at space velocity of 12 000 h−1. It is consistent with the highest Rr (94.6%) under this regeneration space velocity. As shown in Figure 12, it is mainly α-Fe2O3 at the lowest space velocity, while γ-Fe2O3 is the main phase of iron oxide in used sorbents after regeneration at higher space velocity (9000 and 12 000 h−1). Theoretically, exothermic Reaction-B is more favorable for the production of α-Fe2O3, while endothermic Reaction-A (oxygen oxidation reaction) is more beneficial to the production of γ-Fe2O3. A possible explanation for the different crystalline phases of Fe2O3 is that higher mass ratio of SO2−A and more SO2−A at space velocity of 9000 or 12 000 h−1 is unfavorable for the production of αFe2O3. 3.2.3. Effect of Volume Ratio of SO2 to O2. Based on reaction equilibrium theory, a larger volume ratio of SO2 to O2 is favorable for Reaction A, while it is unbeneficial to Reaction B. As expected, significantly higher ratio (115.17) of SO2−Y/ SO2−X is observed with increasing the volume ratio of SO2 to O2 (3:2 and 4:2). It suggests that elemental sulfur is the main sulfur-containing regeneration product at larger VSO2/VO2. The amount and mass ratio of SO2−A and the Rr (95.5%) of used sorbent reach maximum value when the volume ratio of SO2 to O2 is 4:2 during the regeneration process. However, further increase of volume ratio of SO2 to O2 leads to the significant decrease in SO2−Y and SO2−Y/SO2−X. As Reaction-A is endothermic, higher concentration of SO2 results in more reaction intensity of Reaction A. It could lead to the decrease of temperature of the local sorbent bed. Consequently, the decrease in SO2−Y occurs when the volume ratio of SO2 to O2 is 5:2. The results of XRD patterns (Figure 12) indicate that γFe2O3 is the main phase of iron oxide in used sorbents after regeneration. Also, no Fe1−xS is observed. It agrees with high Rr values of used sorbents. The results of pore structure analysis indicate that the lowest decrease in S and V is observed for used sorbents undergoing regeneration when the volume ratio of

SO2 to O2 is 4:2. It is consistent with the highest Rr (95.5%) under this regeneration condition. Additionally, regeneration leads to no significant difference in the average pore size of sorbents except for the lowest volume ratio of SO2 to O2. 3.3. Desulfurization and Regeneration of Sorbents during Multiple Sulfurization/Regeneration Cycles. Desulfurization properties of sorbents during five sulfurization/regeneration cycles are presented in Figure 15. The results indicate that SC values decrease by 10% after the second sulfidation, while there is only a slight decline in the SC after the third desulfurization. Furthermore, the utilization of Fe2O3 still maintains above 77.7% after the fifth sulfidation, which is higher or closer than that (68.8−79.1%, first sulfidation) of sorbents reported in the literature.4 It is favorable for the application of Fe2O3/Al2O3 sorbents in the field of hot coal gas desulfurization. Strong diffraction peaks belonging to Fe1−xS appear in the XRD patterns (Figure 16) of sorbents after sulfidation. However, very weak peaks assigned to γ-Fe2O3 are also observed for sulfidated sorbents. It is mainly due to incomplete (77.5 to 88.6%) conversion of Fe2O3. The γ-Fe2O3 and α-Fe2O3 coexist in the regenerated sorbents (Figure 17), and very few diffraction peaks corresponding to Fe1−xS appear. It results from incomplete (Rr values ranging from 93.1 to 95.8%) regeneration of used sorbents. Incomplete conversion of Fe2O3 or Fe1−xS during the multiple sulfurization/ regeneration progress is responsible for the decrease of the SC values. The data of pore structure analysis listed in Table 4 indicate that regeneration leads to a significant decrease (by 32.1 and 16.5%, respectively) in the surface area (S) and pore volume (V) of sorbents during the first desulfurization/ regeneration cycle. However, the decline in S due to regeneration varies from 2 to 10% after the first cycle. The smaller decrease (by 8.9, 6.3, and 2.1%, respectively) in V resulting from regeneration during the third, fourth, and fifth cycle is also observed. Furthermore, the S and V of sorbents maintain above 40 m2 g−1 and 0.160 cm3 g−1 after the fourth regeneration. The results may be attributed to the stable structure of sorbents during multiple desulfurization/regeneration processes. It agrees with the properties of sorbents after the third desulfurization/regeneration cycle. Additionally, the average pore size of sorbents becomes larger after each regeneration due to the replacement of S2− (radius: 0.184 nm) by O2− (radius: 0.140 nm). The morphology of fresh and used sorbents is shown in Figure 18. A large number of Fe2O3 nanoparticles with size of about 50 nm appear in the fresh sorbents, while larger (>150 nm) particles are also observed due to the aggregation of nanosized iron oxide. After the fifth sulfidation, most of the particles in used sorbents own larger size (>150 nm), and coalescence between the particles occurs. In addition, there are lots of nanoparticles with size of about 50 nm in the fifth-regenerated sorbents, indicating the recovery of Fe2O3 structure. The morphology agrees with pore structure properties of sorbents. The surface species of sorbents during multiple sulfurization/ regeneration cycles were analyzed by XPS. The spectra of the Fe 2p3/2, O 1s, and S 2p of sorbents before and after desulfurization/regeneration are presented in Figures 19, 20, and 21. Table 5 lists the binding energy (BE) values and contents of different species obtained from XPS spectra. As shown in Figure 19, the XPS peaks of surface Fe atoms in fresh sorbents or regenerated sorbents shift toward higher binding energy after desulfurization due to the conversion of Fe2O3 to metal sulfide. The XPS patterns of the O 1s could be fitted by J


Article

Energy & Fuels three peaks. The peaks located at 530.1 ± 0.1 (Oa) and 531.6 (or 531.7) eV (Ob) are assigned to lattice oxygen and oxygen ions in the oxygen vacancy regions,27−29 respectively, while the set of curves at 161.7 eV (Oc) located at 534.4 are attributed to oxygen in the −OH group or adsorbed water.27−29 The concentrations of lattice oxygen on the surface of sorbents decrease by 4.4−34.0% after sulfidation resulting from the replacement of O2− by S2−. High content (39.4−58.1%) of Ob (oxygen vacancy) on the surface of the sulfidated sorbents is beneficial to the adsorption of SO2 or O2 under SO2−O2 regeneration atmosphere according to the principles of catalysis.28,29 The peaks in the S 2p XPS spectrum of sulfidated sorbents can be deconvoluted into three peaks centrated at 161.6−161.8 eV (Sa, Fe1−xS),30,31 163.8−164.0 eV (Sb, S8),32,33 and 168.4−169.0 eV (Sc, S6+),30−32,34 respectively. As listed in Table 5, Fe1−xS is the major (44.4−58.9%) sulfur-containing species on the surface of sorbents after desulfurization. It agrees with XRD results. There is also 13.0−30.4% elemental sulfur on the surface of used sorbents. It confirmed the presence of sulfur during the sulfidation process as mentioned in Section 3.1. The observation of Sc species could be attributed to the oxidation of elemental sulfur caused by grinding of used sorbents.1 As the regeneration reaction produces sulfur, most of elemental sulfur can release from the reactor with the acid of gas stream at desulfurization temperature (500 °C). However, a small part of sulfur is hard to diffuse from the inner sorbents to the gas phase due to the impact of diffusion resistance.1 Consequently, this part of elemental sulfur could be oxidized to sulfur-containing (S6+) species during the pretreatment of sorbents for XPS tests. As expected, weak peaks assigned to Sc species appear in the XPS patterns of the surface S atom in regenerated sorbents.

ORCID

Mengmeng Wu: 0000-0001-7805-9385 Huiling Fan: 0000-0001-8616-6578 Jie Mi: 0000-0002-9374-2307 Notes

The authors declare no competing financial interest.

■

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

■

4. CONCLUSION Fe-based sorbent supported on mesoporous alumina (Fe2O3/ Al2O3) with low cost was successfully prepared via the sol−gel method. The performance of desulfurization and regeneration of sorbents was evaluated using a simulated hot coal gas and a mixture of SO2, O2, and N2 in various proportions over a fixedbed reactor, respectively. The results suggest that high utilization rate (88.6%) of Fe2O3 and sulfur capacity (12.5%) were achieved when the sorbents (35% Fe2O3) were calcinated at 500 °C for 3 h. The regeneration temperature should be higher than 500 °C, at which Fe2(SO4)3 appears in the regenerated sorbents. The optimum regeneration temperature, volumetric space velocity, and the ratio of SO2 to O2 in the regenerating gas are 600 °C, 12 000 h−1, and 4:2, respectively. Elemental sulfur is the main sulfur-containing regeneration product under these regeneration conditions. The decline in the regeneration rate is observed with increasing regeneration temperature from 600 to 650 °C. The formation of Al2(SO4)3 and Al3Fe5O12 is responsible for this decrease. γ-Fe2O3 and αFe2O3 coexist in the regenerated sorbents. Additionally, the sorbents show good performance of desulfurization and regeneration during successive sulfurization/regeneration cycles due to less change in the structure of the sorbents.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*Telephone: +86-351-6018598. E-mail: wumengmeng111@ 126.com (Wu, M.) *E-mail: [email protected] (Mi, J.). K


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Desulfurization of Hot Coal Gas over Regenerable Low-Cost Fe2O3

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