Desulfurization Behavior of CeriumâIron Mixed Metal Oxide Sorbent in

Article pubs.acs.org/IECR

Desulfurization Behavior of Cerium−Iron Mixed Metal Oxide Sorbent in Hot Coal Gas Bo Guo,†,‡ Liping Chang,*,† and Kechang Xie† †

Key Laboratory of Coal Science and Technology, Taiyuan University of Technology, Ministry of Education and Shanxi Province, Taiyuan, Shanxi 030024, P.R. China ‡ College of Environmental Science and Engineering, Taiyuan University of Technology, Taiyuan, Shanxi 030024, P.R. China S Supporting Information *

ABSTRACT: Ce−Fe mixed metal oxide sorbent C2F3B850 was prepared by using cerium oxide and red mud (an iron oxide waste from a steel company). The ability of C2F3B850 (CeO2/red mud = 0.85) was evaluated over a fixed-bed reactor in a simulated coal-derived gas. It was found that the main active components of sorbent C2F3B850 were cerium and iron oxides, the addition of red mud in CeO2-based sorbent could decrease sulfurization temperature, and 500 °C is the best temperature for sulfurization in the range of 500−700 °C. After eight successive sulfurization−regeneration cycles over C2F3B850, the regenerated sorbent still performed well with no apparent deterioration. The average breakthrough sulfur capacity corresponding to H2S concentrations below 50 ppmv is 9.97 (g S/100 g sorbent). Sorbent C2F3B850 during successive sulfurization− regeneration cycles was characterized by means of X-ray photoelectron spectroscopy, X-ray diffraction, and nitrogen adsorption− desorption techniques. The results reveal that the C2F3B850 sorbent with good durability, high efficiency, and high mechanical strength is suitable for desulfurization of hot coal-based gas in the chemical industry.

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INTRODUCTION Hot gas desulfurization of coal-derived fuel gases is an essential process in emerging power generation technologies, such as the integrated gasification combined cycle (IGCC) and the coalbased polygeneration system (an advanced technology of the clean and efficient utilization of coal with the advantages of low cost, high efficiency, and low environmental impact). A number of metal oxide and mixed metal oxides were tested as sorbents for the removal of H2S from gas at high temperatures.1−9 Among the oxides, cerium oxide has attracted attention as a durable high-temperature desulfurization sorbent due to its potential to produce elemental sulfur during the regeneration phase and its fitness for reducing gas compositions. One disadvantage of CeO2 in H2S removal is its lower sulfur capacity compared to that of the iron- and copper-based conventional oxide sorbents. In the work of Kobayashi and FlytzaniStephanopoulos,4 both the reducibility and sulfurization of cerium oxide were improved by the presence of copper and copper-modified cerium oxide used as a desulfurization sorbent over a wide temperature window. Iron oxide-containing waste materials are the most abundant and inexpensive resources. In the study by Slimane and Abasian,10 four metal oxide waste materials were investigated and the results indicated that the sorbent based on an iron oxide waste material, in the as-received as well as processed forms, were the most reactive and exhibited the highest effective capacity for sulfur. In our earlier research,3 it has been found that red mud (an iron oxide waste from a steel company) containing iron oxide as the main active species was considered to be one of the most suitable sorbents based on the properties of good absorption capacity, physical strength, and regenerability. © 2014 American Chemical Society

By considering the attractive regeneration properties of cerium oxide and the high H2S sorption capacity of red mud, a Ce−Fe mixed metal oxide sorbent was prepared and its desulfurization activity in a simulated coal-derived gas and regenerability were tested through successive sulfurization− regeneration cycles. To understand the surface and structural properties of the sorbent, in the second part of this work, the sorbent before and after desulfurization and regeneration was characterized by X-ray photoelectron spectroscopy (XPS), Xray diffraction (XRD), and nitrogen adsorption−desorption (BET).

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EXPERIMENTAL SECTION Preparation of Ce−Fe Mixed Metal Oxide Sorbent. Sorbent C2F3B850 was prepared by using cerium oxide from pyrolysis of cerium nitrate, red mud as active component precursors, and bentonite as a binder. Cylindrical pellets were formed by blending, adding water, and extruding. The extrudates were dried and then calcinated at 850 °C for 4 h to form the sorbent for desulfurization in hot coal gas.11 Compositions of the sorbent and red mud are given in Table 1 and Table S1 of Supporting Information. Test for the Desulfurization Performance of Sorbent. Desulfurization performance of prepared sorbent was evaluated using a fixed-bed reactor with a 20 mm i.d. quartz tube in a simulated coal-derived gas. In a typical test, about 20 mL (about 18.36−18.88 g) of weighed sorbent was first placed in the reactor before the experiment and then heated to the Received: Revised: Accepted: Published: 8874

February 24, 2014 May 5, 2014 May 5, 2014 May 5, 2014 dx.doi.org/10.1021/ie500752n | Ind. Eng. Chem. Res. 2014, 53, 8874−8880

Industrial & Engineering Chemistry Research

Article

Atomic ratios were then calculated from the intensity ratios normalized by atomic sensitivity factors.13−15 The phases of the sorbent before and after desulfurization were characterized by powder X-ray diffraction using an X-ray diffractometer (Rigaku D/max-2500, Japan) with monochromatized Cu Kα radiation, power setting of 40 kV, scan range from 5° to 80° at a scanning speed of 0.25°/min. The specific surface areas and the pore size distribution of the sorbents before and after sulfurization and regeneration were determined by nitrogen adsorption and desorption using a BET instrument (CE Sorptomatic 1990, Italy).

Table 1. Compositions of Sorbent C2F3B850 (Weight Percent)

a

sorbent

CeO2

red-mud

bentonite

soluble starcha

C2F3B850

26

39

30

5

Based on the total weight.

required temperature by an external oven under N2 gas. Feeding gases from cylinders and with purity of 99.99% were adjusted by mass flow controllers to achieve the desired composition (H2S, 0.35%; H2, 39%; CO, 27%; CO2, 12%; and N2 balance gas) and gas flow rate. A schematic diagram of the fixed-bed reactor for sorbent evaluation is shown in Figure 1. The analytical system

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RESULTS AND DISCUSSION Desulfurization Performance of Sorbent. The capacity of the sorbent C2F3B850 for removing H2S at different sulfurization temperatures is shown in Figure 2.

Figure 1. Schematic diagram of a fixed-bed reactor for sorbent evaluation (1, gas cylinder; 2, reducing valve; 3, steadying flow valve; 4, flow meter; 5, mixer; 6, water-thermostat; 7, furnace; 8, thermocouple; 9, temperature controller; 10, sorbent; 11, analysis of inlet; 12, absorption bottle).

Figure 2. Effect of sulfurization temperature on the breakthrough curve of sorbent C2F3B850 (H2S, 0.35%; H2, 39%; CO, 27%; CO2, 12%; N2 balance gas).

comprises a gas chromatograph with a thermal conductivity detector (TCD) and a flame photometric detector (FPD) for low concentrations of sulfur compounds with sensitivity higher than 0.1 ppmv. The breakthrough curve is defined as a plot of outlet concentration of H2S versus time. In addition, the amount of sulfur captured by sorbent at breakthrough onset (lower than 50 ppmv sulfur in outlet, i.e., Cout = 0 ppmv) is denoted as the breakthrough sulfur capacity, which can be calculated by the following equation:9,12 SC (g S/100 g sorbent) M = WHSV × S × [ Vm

∫0

tb

(C in − Cout)dt ] × 10−4

Here, SC is the effective sulfur capacity of sorbent, WHSV the weight hourly space velocity (L h−1 g−1), MS the molar weight of sulfur (32.06 g mol−1), Vm the molar volume of H2S at 1 atm and 25 °C (24.5 L mol−1), and tb the breakthrough time of sorbent (h); Cin and Cout are the inlet and outlet concentration of H2S (ppmv), respectively. Characterization for Sorbent. XPS analysis of sorbent was collected on a ESCALAB 250 spectrometer with Mg Kα radiation (1253.6 eV). The XPS signals were estimated by calculating the integral of each peak, after smoothing, subtracting a nonlinear background, and fitting the experimental curve to a 70% Gaussian−30% Lorentzian function.

Figure 3. Sulfurization−regeneration cycles over C2F3B850 sorbent with WHSV = 3000 mL h−1 g−1; the inset is breakthrough sulfur capacity. Sulfurization conditions: T = 500 °C; H2S, 0.35%; H2, 39%; CO, 27%; CO2, 12%; N2 balance gas. Regeneration conditions: T = 700 °C; O2, 5%; H2O, 15%; N2 balance gas; desulfurizer size, 3 mm.

The H2S breakthrough time decreased with the rise of sulfurization temperature in the range of 500−700 °C. It is different from the change trend of single cerium oxide sorbent with temperature, in which H2S breakthrough time increased from 600 to 800 °C.16 This result also differs from the report of 8875

dx.doi.org/10.1021/ie500752n | Ind. Eng. Chem. Res. 2014, 53, 8874−8880


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To investigate the desulfurization ability and durability of sorbent C2F3B850, eight successive sulfurization−regeneration cycles over C2F3B850 were carried out in a simulated coalderived gas at 500 °C. Sulfurization breakthrough curves and sulfur capacities for eight successive cycles are shown in Figure 3. It can be found that the desulfurization precision of sorbent was going well with increase of sulfurization−regeneration cycle times and the output concentrations of H2S can be reduced to below 20 ppmv in cycles 2−8 before sorbent bed breakthrough, but the breakthrough time was shortened slightly over 5 cycles. The breakthrough sulfur capacity (inset in Figure 3) corresponding to H2S concentrations below 50 ppmv increased from a minimum value of 4.39 (g S/100 g sorbent) in cycle 1 to a maximum value of 12.32 (g S/100 g sorbent) in cycle 5 and then decreased to 9.68 (g S/100 g sorbent) in cycle 6, 10.11 (g S/100 g sorbent) in cycle 7, and 10.77 (g S/100 g sorbent) in cycle 8. Compared to the value of cycle 1, the increase of breakthrough sulfur capacity in cycles 2−8 should be attributed to the changes in the structural properties of the sorbent. Little pulverization but some cracks in sorbent C2F3850 were found after the eighth regeneration. There is no apparent deterioration in performance as the number of cycles increases. XPS Characterization of C2F3B850. XPS analysis was conducted to understand the surface chemical state of Ce, Fe, S, O, and Ca in the sorbent C2F3B850 before and after desulfurization. XPS Ce 3d core level of C2F3B850 before desulfurization (fresh sorbent), after the first sulfurization (first-

Figure 4. Comparison of XPS Ce 3d core level of C2F3B850 sorbent at different conditions.

Xie et al.,7 in which it was found that when the sulfurization temperature increased from 420 to 620 °C, the breakthrough time of sorbent F8C2AS (Fe2O3/CeO2 desulfurizer) was longer and breakthrough times at both 520 and 620 °C were much longer than that at 420 °C. This indicates that the addition of red mud can decrease the optimal sulfurization temperature of the CeO2-based sorbent. Considering the breakthrough time of sorbent C2F3850, it is reckoned that 500 °C is the best temperature for sulfurization in the range of 500−700 °C.

Figure 5. Fitting results of XPS Ce 3d spectra of C2F3B850 sorbent at different conditions. 8876



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Figure 6. (A) Fe 2p, (B) S 2p, (C) O 1s, and (D) Ca 2p XPS spectra of C2F3B850 sorbent at different conditions.

Supporting Information. It is found that the Ce 3d peaks shift slightly to the higher binding energy after the first sulfurization and to much higher binding energy after the eighth sulfurization. This indicated that valence electron density of Ce increased. This is probably a result of interactions of components in the sorbent. XPS Fe 2p core level of C2F3B850 before desulfurization (fresh sorbent) and after the first sulfurization, the eighth sulfurization, and the eighth regeneration are shown in Figure 6A. According to the XPS results shown in Figures 4 and 6A, the energy positions of the Fe 2p3/2, the relative contents of Fe and Ce, and the ratio of Fe to Ce are summarized in Table S3 of Supporting Information. It is found that Fe 2p3/2 peak positions are all lower than the standard value of Fe 2p3/2 peak (710.6 eV).21 This indicated that Fe had some interactions with other atom. According to the electronegativity order Fe (1.83) > Ce (1.12), iron can obtain electrons from cerium. As shown in Tables S2 and S3 of Supporting Information, the binding energy of Fe 2p3/2 decreased when the binding energy of Ce 3p increased at the same condition of the sorbent. The shifts of Ce 3p and Fe 2p3/2 peaks suggest that Ce and Fe chemically interact with each other in the sorbent, which results in the increase of the ratio of Fe/Ce on the surface after sulfurization, especially after the eighth sulfurization. The synergetic effect of Ce−Fe was also observed by Perez-Alonso et al.22 The Ce and Fe metal cations influenced each other to change their

sul), the eighth sulfurization (eighth-sul), and the eighth regeneration (eighth-reg) are shown in Figure 4. As shown in Figure 4, XPS spectra after the eighth regeneration were similar to that of fresh sorbent. To identify XPS Ce 3d peaks, the labels V and U indicate Ce4+ and Ce3+, respectively, in Ce 3d XPS spectra of C2F3B850, and the fitting results of XPS Ce 3d spectra of C2F3B850 at different conditions are shown in Figure 5. As shown in Figure 5, the peaks V0, V1, and V2 and V0′, V1′, and V2′ can be ascribed to Ce4+ 3d5/2 and Ce4+ 3d3/2, respectively. The decrease of intensity of these six peaks indicated the decrease of the Ce4+ content on the surface of the sorbent after the first sulfurization and the eighth sulfurization. The peaks U0 and U1, and U0′ and U1′ can be ascribed to Ce3+ 3d5/2 and Ce3+ 3d3/2, respectively. The increase of intensity of these four peaks indicated the increase of the Ce3+ content on the surface of the sorbent after the first sulfurization and the eighth sulfurization. These peaks represented the presence of both Ce4+ and Ce3+ on the surface of C2F3B850.17−20 The ratio of Ce3+/Ce4+ (inset in Figure 5) increased from about 0.62 to 1.06 after the first sulfurization, to a maximum of 1.31 after the eighth sulfurization, and then recovered to 0.61 after the eighth regeneration, which was similar to that of fresh sorbent. The values of V0, V1, V2, V0′, V1′, and V2′ and U0, U1, U0′, and U1′ shown in Figure 5 are summarized in Table S2 of 8877



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Figure 7. XRD patterns of C2F3B850 sorbent at different conditions (1, α-Fe2O3; 2, SiO2; 3, CeO2; 4, Ce2O2S; 5, Ce2S3; 6, FeS).

As shown in Table S1 of Supporting Information, CaO is one of the components of red mud. The Ca 2p XPS spectra of C2F3B850 are displayed in Figure 6D. The signals at 347.5 and 351.1 eV indicated existence of Ca on the surface of fresh sorbent, whereas the signals disappeared after the first sulfurization, the eighth sulfurization, and even the eighth regeneration. This reveals the shift of Ca from surface to bulk of the sorbent. XRD Characterization of C2F3B850. The XRD patterns of C2F3B850 are displayed in Figure 7. In order to see clearly, the enlarged figures corresponding to two ranges of 2θ from 20° to 38° and from 40° to 66° are shown panels B and C of Figures 7, respectively. For fresh sorbent, the diffraction peaks can be attributed to CeO2 [PDF 44-1001], α-Fe2O3 [PDF 33-0664], and SiO2 [PDF 47-1144]. The absence of diffraction peaks of CaO, MgO and Al2O3, which are components of red mud, suggests that these metal oxides are highly dispersed on the channel walls of C2F3B850. After the first sulfurization and the eighth sulfurization, the sample showed diffraction peaks of FeS [PDF 23-1120], Ce2S3 [PDF 20-0269], Ce2O2S [PDF 26-1085], and CeO2. This indicated that metal oxide was gradually transformed into metal sulfide and the active particles aggregated slightly during desulfurization. Meanwhile the presence of CeO2 means the fluorite oxide-type structure of CeO2 is stable and CeO2 is reduced partially during the desulfurization. After the eighth regeneration, XRD patterns of the sample were

oxidation states both on the surface and in the bulk. The Ce ions could act as carriers of the oxidation and reduction reactions, and the oxygen transfer could be accelerated during the desulfurization process of coal gas.23 The change of the ratio of Ce4+/ Ce3+ (inset in Figure 5) is also due to the interactions of Ce and Fe. S 2p XPS spectra of C2F3B850 are shown in Figure 6B. Compared to that of fresh sorbent, the peaks at about 168.6 and 162.1 eV appeared in the S 2p spectra after the first sulfurization and the eighth sulfurization, which correspond to the S 2p signal of surface metal sulfides and SO2 due to partial oxidation of sulfided sorbents in air.24 After the eighth regeneration, the decrease of peak intensity at 168.6 eV is ascribed to SO2 residuals or incomplete regeneration of the sorbent and the absence of a peak at 162.1 eV is due to complete regeneration. O 1s XPS spectra of C2F3B850 are shown in Figure 6C. O 1s XPS spectra showed a peak at a binding energy of 529.0− 529.5 eV, which is generally accepted as lattice oxygen of CeO2, Al2O3, Fe2O3, and SiO2 existing in the sorbent. A broad peak in the higher BE (binding energy) region was evident at all four conditions, which can be assigned to the oxygen of surface adsorbed hydroxyl and other groups.25−27 The peak in the region of low BE decreased after the first sulfurization and almost disappeared after the eighth sulfurization due to the fact that some surface oxygen was replaced by sulfur atoms. 8878



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strength is suitalble for desulfurization of hot coal gas in the chemical industry.

similar to that of fresh sorbent. This is related to the highperformance of C2F3B850 in successive sulfurization−regeneration cycles. BET Characterization of C2F3B850. To investigate the change of sorbent structure, BET surface area, pore specific volume, and pore diameter distribution of C2F3B850 before desulfurization (fresh sorbent) and after the first sulfurization, the eighth sulfurization, and the eighth regeneration are listed in Table S4 of Supporting Information. Compared to those of the fresh sorbent, BET surface area and pore specific volume reduced by 10.3% and 37.7%, respectively, after the first sulfurization and 36.7% and 43.4%, respectively, after the eighth sulfurization because of the shift of O to S atom in the structure of the sorbent. The volume of metal sulfide is larger than that of its corresponding oxide. During desulfurization, the higher the sulfur capacity is (inset in Figure 3), the more decrease of BET surface area and pore specific volume there is. After the eighth regeneration, BET surface area and pore specific volume recovered to 93.1% and 92.5%, respectively, compared to those of fresh sorbent. Multiple exchanges of O and S atoms in successive sulfurization and regeneration cycles resulted in the change of pore structure of the sorbent. As shown in Table S4 of Supporting Information, the pores at the range of 20−200 Å, which was positively correlated with sulfurization conversion in our previous study,16 increased from 34.78% of fresh sorbent to 50.97% after the eighth regeneration. This indicates that the change of pore structure is beneficial to the proceeding of desulfurization.

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ASSOCIATED CONTENT

S Supporting Information *

Compositions of red mud (Table S1); energy positions in XPS Ce 3d5/2 and Ce 3d3/2 spectra of C2F3B850 sorbent at different conditions (Table S2); energy positions in XPS Fe 2p3/2, the relative contents of Fe and Ce, and the ratio of Fe to Ce of sorbent C2F3850 at different conditions (Table S3); BET surface area, pore specific volume, and pore diameter distribution of sorbent C2F3B850 at different conditions (Table S4). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel./Fax: +86-351-6010482. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of the National Basic Research Program of China (2012CB723105) and National Natural Science Foundation of China (20976117).

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REFERENCES

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CONCLUSION Ce−Fe mixed oxides sorbent C2F3B850 was prepared by using cerium oxide and red mud as the main active components. The addition of red mud in CeO2-based sorbent could decrease optimal sulfurization temperature of sorbent C2F3B850, and the best desulfurization temperature is 500 °C in the range of 500−700 °C. In a simulated coal-derived gas, C2F3B850 performed high reactivity and sulfur capacity over eight sulfurization−regeneration cycles with no apparent deterioration. The breakthrough sulfur capacity corresponding to H2S concentrations below 50 ppmv was from a minimum of 4.39 (g S/100 g sorbent) in cycle 1 to a maximum of 12.32 (g S/100 g sorbent) in cycle 5, and the average value was 9.97 (g S/100 g sorbent). The recyclability of C2F3B850 means that it can be applied in the chemical industry in the desulfurization in hot coal gas. XPS results showed that XPS spectra of Ce 3d and Fe 2p in C2F3B850 after the eighth regeneration were similar to those of fresh sorbent. The increase of ratios of Fe/Ce or Ce3+/Ce4+ on the surface after sulfurization suggested that there existed an interaction between Ce and Fe and that this interaction was beneficial to sulfurization. XRD results showed that CeO2 was partially reduced during the desulfurization and it turned again into the original form after regeneration, which shows that the fluorite oxide-type structure of CeO2 was stable during successive sulfurization−regeneration cycles. BET characterization of sorbent C2F3B850 indicated that the surface area and pore specific volume had a slight decline after the eighth regeneration compared to those of fresh sorbent whereas the change of pore diameter distribution was beneficial to desulfurization. All of the above results reveal that the C2F3B850 sorbent with good durability, high efficiency, and high mechanical 8879



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Desulfurization Behavior of CeriumâIron Mixed Metal Oxide Sorbent in

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