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Simultaneous removal of COS and H2S from hot syngas by rare earth metal-doped SnO2 sorbents
Fuel 181 (2016) 1020–1026
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Full Length Article
Simultaneous removal of COS and H2S from hot syngas by rare earth metal-doped SnO2 sorbents Yi Yang, Yixiang Shi ⇑, Ningsheng Cai Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, China
h i g h l i g h t s Simultaneous removal of COS and H2S by 40 M percent La-doped SnO2 at 350 °C. Small pore size and particle size of the sorbents ascribing to the addition of rare earth metal. High improvement of catalytic hydrolysis of COS by La-doped SnO2 and Y-doped SnO2.
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Article history: Received 13 January 2016 Received in revised form 14 March 2016 Accepted 4 May 2016 Available online 10 May 2016 Keywords: Simultaneous removal COS hydrolysis H2S Rare earth metal SnO2
a b s t r a c t The pure SnO2 and rare earth metal (Y and La)-doped SnO2 were synthesized using a co-precipitation method for the simultaneous removal of COS and H2S from hot syngas at 350 °C. The pure SnO2 sorbent exhibited a poor catalytic hydrolysis effect for COS conversion at moderate temperatures of 300–400 °C due to the formation of SnS during the desulfurization. SnS caused a decrease in the COS conversion because the activity of SnS was considerably lower than that of reduced SnO2 for the hydrolysis of COS. The addition of Y and La resulted in the formation of numerous smaller pores and increased the total pore volume of the sorbents. The good dispersion of rare earth metal resulted in the improvement of COS conversion, which produced a higher breakthrough sulfur capacity. As the La content increased, the COS conversion reached nearly 100% and was no longer the restrictive procedure for the simultaneous removal of COS and H2S. The total breakthrough sulfur capacity was 148.4 mg/g when 40 M percent of La was doped into the SnO2 sorbent. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Hot coal syngas desulfurization with metal oxides in the integrated gasification combined cycle (IGCC) has the potential to improve the economic benefits [1]. Coal syngas contains not only hydrogen sulfide (H2S) but also other sulfurous gases including carbonyl sulfide (COS). In comparison to H2S, COS is more difficult to remove due to its low reactivity with metal oxides. Therefore, the formation of COS in hot syngas will lower the hot syngas desulfurization efficiency. To overcome this limitation, studies of COS removal by sorption, catalytic hydrolysis and hydrogenation conversion have been reported. Among the COS removal technologies, indirect removal by COS conversion reaction and H2S removal is more attractive due to the high removal efficiency [2]. COS conversion reaction
⇑ Corresponding author. E-mail address: [email protected] (Y. Shi). http://dx.doi.org/10.1016/j.fuel.2016.05.007 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.
COS þ H2 O ¼ CO2 þ H2 S COS þ H2 ¼ CO þ H2 S H2S removal
MeOx þ xH2 S ¼ MeSx þ xH2 O During hot syngas desulfurization with a certain amount of CO and CO2, when these two reactions occur in separated reactors, COS might be detected in the outlet if the metal oxide sorbent has a low COS conversion catalytic activity [3]. However, when the H2S sorbent is active for COS hydrolysis, the simultaneous removal of COS and H2S is possible, and will improve the COS conversion, especially when a large amount of CO and CO2 exists in the coal syngas. This simultaneous removal of COS and H2S is also beneficial for the economic evaluation of the hot syngas desulfurization because investment in COS conversion section can be avoided. Many metal oxides have been used for the hot syngas desulfurization [4]. Among these metal oxides, zinc oxide, manganese oxide and iron oxide are candidates for the simultaneous removal of COS and H2S. ZnO is a well-known sorbent for high efficiency
Y. Yang et al. / Fuel 181 (2016) 1020–1026
1021
Nomenclature Qfeed gas Msulfur msorbent Vmol
the total feed gas flowrate (mL min1) the atomic weight of sulfur (32 g mol1) the weight of sorbent (g) the molar volume under standard conditions of 273.15 K and 1 atm (22.4 L mol1)
desulfurization. Studies of the catalytic conversion of COS to H2S with ZnO have been performed by Sasaoka et al. [5,6]. Both ZnO and ZnS formed from ZnO were active for the conversion of COS to H2S. Shangguan et al. [3] prepared the Al2O3 and K2CO3 modified ZnO sorbent, and an improved desulfurization performance was reported. Loading K2CO3 can increase the basicity of ZnO based sorbents, which increases the COS catalytic hydrolysis rate. Wakker et al. [7] reported that the MnO or FeO sorbent effectively removed COS from the feed gas. The sorbent can also be used to remove both H2S and COS from the fuel gas as well. Although the sorbent can directly react with COS, when water is added, COS can be converted into H2S, and then, H2S can be captured by the sorbent. Sakanishi et al. [8] found that Fe impregnation of activated carbon substantially enhanced COS and H2S removal from gases, which primarily enhanced the desulfurization of H2S to form metal sulfide. In addition, COS may be preferably adsorbed as COS itself in the pores. Yu et al. [9] demonstrated that a lignite charsupported Fe and Fe–Mo sorbent could effectively remove H2S and COS from coal syngas with high efficiency, and COS was converted to H2S. In addition to these three metal oxides, SnO2 is a steam regenerative H2S sorbent for moderate temperature desulfurization [10]. Due to steam regeneration, H2S can be concentrated for further utilization in the chemical process. On the other hand, COS may react with SnO2 above 425 °C and has the ability for the COS conversion [10–12]. Therefore, SnO2 is a suitable H2S sorbent candidate for the simultaneous removal of COS and H2S. In the previous studies, the catalysts employed for COS hydrolysis have included Al2O3, ZrO2, TiO2 and activated carbons with promoters consisting of alkali metal oxides, alkali earth metal oxides and transition metal oxides [2]. Recently, a novel type of rare earth oxysulfide catalysts was investigated for the hydrolysis of COS [13,14]. A rare earth metal Ce has been used to improve the catalytic effect for COS removal [15,16]. Compared to traditional catalysts, the rare earth oxysulfide catalysts showed the superiority to Al2O3-based catalysts in anti-poisoning of oxygen [14]. In the simultaneous removal of COS and H2S, COS was converted to H2S and captured by the sorbent. During the regeneration process, the sulfided sorbent reacted with oxygen to regenerated the sorbent for cyclic utilization. Thus, the characteristics of antipoisoning of oxygen is beneficial for the rare earth metals used in the improvement of COS catalytic conversion of the sorbent. However, the application of both SnO2 and rare earth metaldoped SnO2 for the COS removal has not been previously studied. Rare earth metals include yttrium and lanthanide series. La showed the best catalytic activity on COS conversion among the lanthanide series metals [14]. The addition of rare earth metals (Y and La) to the SnO2 has been previously studied, and the results indicate improved SnO2 physiochemical properties [17,18]. So, the objective of this study was to investigate the catalytic effect of SnO2 and the promotion of rare earth metal (Y and La) addition on COS hydrolysis and the simultaneous removal of COS and H2S. In this study, pure SnO2, Y-doped SnO2 and La-doped SnO2 were synthesized using a co-precipitation method. The physical and chemical properties of the SnO2 based sorbents were characterized by Xray diffraction (XRD); Brunauer, Emmett and Teller (BET) theory;
Cin Cout t
the inlet sulfurous gas concentration (%) the outlet sulfurous gas concentration (%) the breakthrough time (min)
and scanning electron microscopy (SEM). The COS removal tests of the SnO2 based sorbents were conducted in a fixed-bed reactor. 2. Experimental 2.1. Preparation of SnO2 based sorbents The rare earth metal-doped SnO2 sorbents were prepared using a co-precipitation method at room temperature. 0.02 mol of SnCl45H2O and a rare earth metal nitrate with an M/Sn atomic ratio of 1:9, 1:4 and 1:2.5 (M = Y or La) were dissolved in 100 mL of deionized water. A 5 mol L1 ammonia solution was added into the solution with vigorous stirring to adjust the pH to 10. The resulting slurry was maintained for 6 h prior to filtering the mixture via vacuum filtration and washing with deionized water until the pH reached 7. Finally, the precipitate was dried in air at 80 °C overnight and calcined at 500 °C for 3 h. The pure SnO2 sorbent was prepared using the previously mentioned precipitation method without rare earth metal nitration. All of the prepared sorbents were crushed and sieved through 97–150 lm. The rare earth metal doped-SnO2 sorbents are referred to as xM–SnO2 (x = molar ration of M/Sn). 2.2. Characterization of SnO2 based sorbents The textural properties of the sorbents were determined using an ASAP 2020 (Micromeritics, USA) by determining the nitrogen adsorption–desorption isotherms at 196 °C. The BET specific surface area was determined from the adsorption data. The pore volume and average pore diameter were determined by applying the Barret–Joyner–Halenda (BJH) model to the isotherm desorption branch. The phase composition analysis of the sorbents using XRD was performed on a Rigaku D-max 2500 X-ray diffractometer with Cu Ka radiation. The experimental data were digitally collected by a ‘‘step by step” scanning method in a 2h angle interval of 10–70°. The sorbent surface morphology was determined by a scanning electron microscopy (ZEISS MERLIN VP compact, Germany). 2.3. COS removal tests of the SnO2 sorbents The COS removal tests of SnO2 and the rare earth metal-doped SnO2 sorbents were conducted in a fixed-bed reactor with an internal diameter of 10 mm. The sorbent containing 0.5 g of active SnO2 with inert SiO2 was charged into the reactor and heated to 400 °C in a N2 atmosphere. Prior to the test, the sorbent was reduced using 50% H2 and balanced with N2 at 400 °C for 2 h, and then, the reactor temperature was adjusted to the required temperature. Next, a feed gas containing 0.2% COS and 2.4% H2O balanced with N2 was controlled by mass flowmeters (D07-19B, Beijing Sevenstar Electronics) and introduced from the top of the reactor with a gas hourly space velocity of 3000 h1. The concentration of H2S and COS in the outlet was analyzed using a gas chromatograph (GC9A, Shimadzu) equipped with a flame photometry detector (FPD)
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whose detection limit was 1 ppmv. Since GC-9A was used for low concentration analysis, an inert N2 was applied to dilute the gas concentration to 1/10 of the original gas concentration. Thus, the breakthrough time was defined as the time when the total outlet sulfurous gas concentration reaches 20 ppmv, which reached 1% of the total sulfurous gas in the feed gas. The breakthrough sulfur capacity of the sorbents was calculated using the following equation:
Z t Q feedgas Msulfur mg sulfur ¼ q ðC in C out Þdt g sorbent msorbent V mol 0
Table 1 Specific surface area, pore volume and average pore diameter of SnO2 and rare earth metal doped SnO2. Samples
Specific surface area (m2/g)
Pore volume (cm3/g)
Average pore diameter (nm)
SnO2 11.1% Y– SnO2 11.1% La– SnO2 25% La– SnO2 40% La– SnO2
41.12 76.19
0.090 0.117
5.7 4.5
77.67
0.101
3.7
72.85
0.121
4.2
98.81
0.192
5.4
3. Results and discussion 3.1. Characterization of the SnO2 based sorbents The XRD results in Fig. 1 indicate that pure SnO2 prepared by precipitation exhibited typical peaks which were assigned to SnO2. When a small amount (11.1%) of Y and La was doped into SnO2, the position of the main peaks do not change, which indicates that the crystal phase of the sorbent was still SnO2. No secondary phases were detected in the rare earth metal-doped samples, which indicates that the rare earth metal oxides and SnO2 form a solid solution in this preparation [17]. The formation of the solid solution also contributes to good dispersion of the rare earth metal cations in the SnO2. As the rare earth metal dopant concentration increased, the diffraction peaks became gradually weaker and broader. The crystallinity of SnO2 was unclear and was amorphous when 40 M percent of La was doped into SnO2. In Table 1, the specific surface area of SnO2 was 41.12 m2/g. When Y or La was doped in the SnO2, the specific surface area of the sorbents was 76.19 and 77.67 m2/g, respectively. Compared to pure SnO2, 11.1% Y or La dopant increased the specific surface area of the sorbent by nearly 85%. The pore volume derived from the BJH desorption also increased, and the sorbents have a smaller pore size. Fig. 2(a) shows the pore distribution of SnO2 and rare earth metal-doped SnO2. The average pore diameter decreased, and numerous small pores with sizes of approximately 3–4 nm were observed. Based on the BET results, the pore feature of the sorbents improved due to the addition of rare earth metals, which is beneficial for the gas–solid reaction. As La concentration increased, an increase in both of the total pore volume and pore size was observed, as
Fig. 2. Pore distribution of SnO2 and rare earth metal doped SnO2.
Fig. 1. XRD patterns of SnO2 (a), 11.1% Y–SnO2 (b), 11.1% La–SnO2 (c), 25% La–SnO2 (d), and 40% La–SnO2 (e).
shown in Fig. 2(b). A higher La concentration blocks some small pores and results in a relatively high ratio of large pores. The SEM photographs of pure SnO2 and rare earth metal-doped SnO2 are shown in Fig. 3. Fig. 3(a) showed that in pure SnO2, the small particles of SnO2 are connected to each other to form larger particles. When La and Y were doped into SnO2, the size of the sorbent decreased compared to that of SnO2, shown in Fig. 3 (b) and (c). Leite et al. [17] reported that rare earth metal-doped SnO2 could prevent small SnO2 particle from increasing in size due to the formation of a segregation layer of rare earth cations on the particle surface. Fig. 3(c) indicates that many small particle with distinct interval exist in the 11.1% La–SnO2. This feature in the
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Fig. 3. SEM photographs of SnO2 (a), 11.1% Y–SnO2 (b), 11.1% La–SnO2 (c), 25% La–SnO2 (d), and 40% La–SnO2.
sorbent structure provides more space for interactions between COS and H2S with the sorbent. According to Fig. 3(d) and (e), with the La content increased, the particle size grew up and small pores were blocked which was coincide with the pore distribution.
3.2. Simultaneous COS and H2S removal by SnO2 based sorbents Prior to the COS removal test, a thermodynamic equilibrium calculation was performed for COS conversion under experimental conditions, shown in Table 2. The results indicate that nearly all of the COS can be converted to H2S by COS hydrolysis under the experimental conditions. Almost no thermodynamic equilibrium limit was observed for COS conversion. The results of the COS removal test with pure SnO2 at 300, 350 and 400 °C are shown in Fig. 4. At 300 °C, pure SnO2 exhibited very poor catalytic ability for COS hydrolysis because initially, COS breakthrough was observed. Even if all of the H2S is captured by Table 2 Thermodynamic equilibrium calculation of COS conversion at the experimental condition. Temperature (°C) Equilibrium COS concentration (ppmv)
300 0.1820
350 0.3258
400 0.5350
the sorbent, COS conversion remained low. Therefore, pure SnO2 at 300 °C is unfavorable for COS removal. When the temperature was increased to 350 and 400 °C, no COS and H2S was detected in the outlet during the first stage. When breakthrough occurs, COS was detected before H2S. The delay time between COS breakthrough and H2S breakthrough was 180 min and 30 min at 350 and 400 °C, respectively. After COS breakthrough, the conversion of COS decreased in all of the three tests. Although an increase in the reaction temperature is beneficial for COS conversion, COS conversion remains the restrictive procedure for the simultaneous removal of COS and H2S. When COS could not be converted to H2S, the utilization of the sorbent was low because only a portion of the sorbent near the outlet can be used. During the sulfidation process of H2S and the sorbent, the reduced SnO2 was gradually converted to SnS. Therefore, the sorbent in the reactor changes with the sulfidation time. Initially, the reduced SnO2 may play a catalytic role in COS hydrolysis. Once SnS formed and occupied the surface of the sorbent, the activity of SnS for COS hydrolysis becomes very important. Fig. 5 shows the outlet COS concentration after the feed gas passes through the SnS bed. SnS was inactive for the COS catalytic hydrolysis when the temperature was 300 °C. The conversion of COS was 0. When the temperature increased to 350 and 400 °C,
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Fig. 5. Performance of COS hydrolysis by SnS at 300, 350 and 400 °C.
Fig. 4. COS and H2S breakthrough curves of SnO2 at 300, 350 and 400 °C.
the COS conversion was nearly 7% and 18%, respectively. According to these experiments, SnS exhibits very poor catalytic ability for COS hydrolysis at moderate temperatures of 300–400 °C. For pure SnO2, when the COS in the feed gas cannot be converted to H2S, the utilization of the sorbent in the reactor was low. However, in sour gas, which contains COS and H2S, the problem can be worse. Because H2S is the main sulfurous gas in the feed, the sorbent will sulfide into SnS more quickly than when only COS exists. The formation of SnS has a negative effect on COS hydrolysis, and the removal of COS can be worse using reduced
Fig. 6. Performance of COS removal by Y–SnO2 and La–SnO2 at 350 °C.
SnO2. Therefore, the catalytic characteristics of pure SnO2 for COS hydrolysis need to be improved. 3.3. Effect of rare earth metal-doped SnO2 on COS hydrolysis In this study, rare earth metals (Y and La) were chosen for doping SnO2 to improve the catalytic hydrolysis of COS, and the results
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are shown in Fig. 6. For both Y- and La-doped SnO2 sorbents, the COS breakthrough time was 300 and 290 min, respectively. In addition, the delay time between COS breakthrough and H2S breakthrough was limited to 60 min and 30 min, respectively. In comparison to 160 min with pure SnO2, the removal of COS and H2S was substantially better. For 11.1% Y–SnO2, after breakthrough, the COS concentration increased slower than the H2S breakthrough. While for 11.1% La–SnO2, the COS concentration increases quicker than H2S breakthrough. Using the same molar ratio for doping, Y–SnO2 exhibited a better performance for COS conversion. When La and Y were doped into SnO2, the foreign cations were well dispersed in the SnO2 based on the XRD results. Another key improvement in the rare earth metal dopant is the surface of the doped SnO2 was rich in rare earth metals [17,18]. This surface enrichment is beneficial for improving of COS hydrolysis because the rare earth metal dopant was added to improve the catalytic characteristics of pure SnO2. This surface rich property results in the best utilization of the catalyst, and the negative impact of SnS formation on COS hydrolysis can be limited to a low level. Based on the sorbent characteristics and experimental test, a schematic representation of the simultaneous removal of COS and H2S by rare earth metal-doped SnO2 is shown in Fig. 7. When rare earth metals were doped into SnO2, the sorbent, whose surface is rich in rare earth metals, creates a surface and distributed catalytic effect on COS hydrolysis. When COS was converted into H2S, the product (i.e., H2S) can be captured by the reduced SnO2. As mentioned above, SnS had a negative effect on COS hydrolysis. Although rare earth metals can provide a continuous effect on COS hydrolysis, the decrease in COS conversion was due to the coverage of the rare earth metals by SnS because the rare earth metal dopant was small. Within the sulfidation process, a decrease in COS conversion was observed. 3.4. Effect of La content on the COS and H2S removal When 11.1% La or Y was doped into SnO2, both of them showed a similar improvement on COS and H2S removal. However, a COS conversion decrease with the sulfidation process still existed. In order to achieve a very high COS and H2S removal performance, La-doped SnO2 was chosen to study the effect of rare earth metal content on COS conversion, and different La doping contents for SnO2 were prepared. Fig. 8 shows that the results of COS conversion and H2S removal by SnO2 doped with different amounts of La. When the La content increased to 25%, the breakthrough of COS and H2S nearly occurred simultaneously. The COS conversion was improved, and the H2S breakthrough was faster than the COS breakthrough, which indicates that the conversion of COS by the 25% La–SnO2 was better than that by the 11.1% La–SnO2. When
Fig. 8. Effect of La content on COS and H2S removal.
the La content was further increased to 40%, no COS was detected in the test, which indicates that the COS conversion reach 100%. With 100% conversion of COS, the COS conversion is no longer the restrictive procedure in the simultaneous removal of COS and H2S. Fu et al. reported that the surface La content increased linearly with the La content when La dopant concentration exceeded 10% [18]. The XRD results indicate that with the increase of La content, the diffraction peaks of SnO2 became weaker and unclear, but there was no secondary phases. Therefore, the La dopant was still well dispersed on the surface of the sorbent even the crystal structure of SnO2 became amorphous. In addition, with the La content increase, the pore size became larger and the total pore volume increased. Within the increase in La content from 11.1% to 25% and 40%, the sorbent can provide more catalytic sties contacting with COS to improve COS conversion. When 40% La was doped to SnO2, the sorbent became amorphous but there was still no independent phase of La2O3 or new species detected by XRD. The sorbent had a highest specific surface area and total pore volume in all the SnO2 based sorbents which contributed to the 100% COS conversion. Table 3 lists the breakthrough time and sulfur capacity of different SnO2 based sorbents. When the COS conversion is improved, the breakthrough sulfur capacity can be increased 1.7 times from 54.86 to 148.4 mg/g. The addition of rare earth metals makes SnO2 a promising sorbent for the simultaneous removal of COS and H2S.
4. Conclusions A novel rare earth metal-doped SnO2 sorbent was synthesized by co-precipitation for the hot syngas desulfurization at 350 °C. The pure SnO2 sorbent exhibited a poor catalytic hydrolysis ability for COS conversion at moderate temperatures of 300–400 °C. The formation of SnS was responsible for the decrease in the COS con-
Table 3 Breakthrough sulfur capacity of La–SnO2 sorbents at 350 °C.
Fig. 7. Schematic of simultaneous removal of COS and H2S by rare earth metal doped SnO2.
a b
Sorbents
BT timea (min)
BT sulfur capacityb (mg/g)
SnO2 11.1% Y–SnO2 11.1% La–SnO2 25% La–SnO2 40% La–SnO2
160 300 290 510 620
54.86 94.96 88.78 137.66 148.40
20 ppmv of total outlet sulfurous gas (COS and H2S) was detected by GC. Based on the sorbent weight.
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version because the activity for COS hydrolysis of SnS was considerably lower than that of SnO2 in the same temperature range. The addition of Y and La to the SnO2 sorbent improved the COS conversion activity and results in the sorbents processing a higher sulfur capacity. According to the XRD and BET results, the addition of Y and La results in the formation of smaller pores as well as good dispersion of foreign cations in the SnO2. Due to improved COS conversion, the sorbent exhibited better simultaneous removal of COS and H2S. The 40% La–SnO2 resulted in a breakthrough sulfur capacity of 148.4 mg/g with nearly 100% COS conversion. This result indicates that rare earth metal-doped SnO2 is a promising sorbent for the simultaneous removal of COS and H2S at moderate temperatures of 300–400 °C. Acknowledgements This work is supported by the National Natural Science Foundation of China (51476092), National High Technology Research and Development Program of China (2011AA050601) and Shanxi Province Science and Technology Major Projects (MH2015-06). References [1] Cheah S, Carpenter DL, Magrini-Bair KA. Review of mid- to high-temperature sulfur sorbents for desulfurization of biomass- and coal-derived syngas. Energ Fuel 2009;23:5291–307. [2] Rhodes C, Riddel SA, West J, Williams BP, Hutching GJ. The low-temperature hydrolysis of carbonyl sulfide and carbon disulphide: a review. Catal Today 2000;59(3–4):443–64. [3] Shangguan J, Zhao YS, Fan HL, Liang LT, Shen F, Miao MQ. Desulfurization behavior of zinc oxide based sorbent modified by the combination of Al2O3 and K2CO3. Fuel 2013;108(11):80–4. [4] Westmoreland PR, Harrison DP. Evaluation of candidate solids for hightemperature desulfurization of low-BTU gases. Environ Sci Technol 1976;10 (7):659–61.
[5] Sasaoka E, Taniguchi K, Hirano S, Uddin MA, Kasaoka S, Sakata Y. Catalytic activity of ZnS formed from desulfurization sorbent ZnO for conversion of COS to H2S. Ind Eng Chem Res 1995;34(4):1102–6. [6] Sasaoka E, Taniguchi K, Uddin MA, Hirano S, Kasaoka S, Sakata Y. Characterization of reaction between ZnO and COS. Ind Eng Chem Res 1996;35(7):2389–94. [7] Wakker JP, Gerritsen AW, Moulijn JA. High-temperature H2S and COS removal with MnO and FeO on c-Al2O3 acceptors. Ind Eng Chem Res 1993;32 (1):139–49. [8] Sakanishi K, Wu ZH, Matsumura A, Saito I, Hanaoka T, Minowa T, et al. Simultaneous removal of H2S and COS using activated carbons and their supported catalysts. Catal Today 2005;104(1):94–100. [9] Yu JL, Yin FK, Wang SY, Chang LP, Gupta S. Sulfur removal property of activated-char-supported Fe–Mo sorbents for integrated cleaning of hot coal gases. Fuel 2013;108:91–8. [10] Nielsen PEH, Sigurðardóttir ID. Development and characterization of steam regenerable sorbents for hot gas desulfurization in coal gasification based combined cycle plant. In: Clift R, Seville JPK, editors. Gas cleaning at high temperatures. Netherlands: Springer; 1993. p. 454–69. [11] Han GB, Park N-K, Lee JD, Ryu SO, Lee TJ. A study on the characteristics of the SO2 reduction using coal gas over SnO2–ZrO2 catalysts. Catal Today 2006;111 (3–4):205–11. [12] Han GB, Park N-K, Yoon SH, Lee TJ. Catalytic reduction of sulphur dioxide with carbon monoxide over tin dioxide for direct sulphur recovery process. Chemosphere 2008;72(11):1744–50. [13] Ma JX, Fang M, Lau NT. The catalytic reduction of SO2 by CO over lanthanum oxysulphide. Appl Catal, A 1997;150(2):253–68. [14] Zhang YQ, Xiao ZB, Ma JX. Hydrolysis of carbonyl sulfide over rare earth oxysulfides. Appl Catal, B 2004;48(1):57–63. [15] Zhao H, Zhang DX, Wang FF, Wu TT, Gao JS. Modification of Fe–Mn mixed oxide COS removal sorbent by rare-earth oxides addition. Process Saf Environ 2009;87(4):274–80. [16] Wang HY, Yi HH, Tang XL, Ning P, Yu LL, He D, et al. Catalytic hydrolysis of COS over calcined CoNiAl hydrotalcite-like compounds modified by cerium. Appl Clay Sci 2012;70:8–13. [17] Leite ER, Maciel AP, Weber IT, Lisboa-Filho PN, Longo E, Paiva-Santos CO, et al. Development of metal oxide nanoparticles with high stability against particle growth using a metalstable solid solution. Adv Mater 2002;14(12):905–8. [18] Fu C, Wang JB, Yang MG, Su XL, Xu J, Jiang BL. Effect of La doping on microstructure of SnO2 nanopowdes prepared by co-precipitation method. J Non-Cryst Solids 2011;357(3):1172–6.
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Fuel journal homepage: www.elsevier.com/locate/fuel
Full Length Article
Simultaneous removal of COS and H2S from hot syngas by rare earth metal-doped SnO2 sorbents Yi Yang, Yixiang Shi ⇑, Ningsheng Cai Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, China
h i g h l i g h t s Simultaneous removal of COS and H2S by 40 M percent La-doped SnO2 at 350 °C. Small pore size and particle size of the sorbents ascribing to the addition of rare earth metal. High improvement of catalytic hydrolysis of COS by La-doped SnO2 and Y-doped SnO2.
a r t i c l e
i n f o
Article history: Received 13 January 2016 Received in revised form 14 March 2016 Accepted 4 May 2016 Available online 10 May 2016 Keywords: Simultaneous removal COS hydrolysis H2S Rare earth metal SnO2
a b s t r a c t The pure SnO2 and rare earth metal (Y and La)-doped SnO2 were synthesized using a co-precipitation method for the simultaneous removal of COS and H2S from hot syngas at 350 °C. The pure SnO2 sorbent exhibited a poor catalytic hydrolysis effect for COS conversion at moderate temperatures of 300–400 °C due to the formation of SnS during the desulfurization. SnS caused a decrease in the COS conversion because the activity of SnS was considerably lower than that of reduced SnO2 for the hydrolysis of COS. The addition of Y and La resulted in the formation of numerous smaller pores and increased the total pore volume of the sorbents. The good dispersion of rare earth metal resulted in the improvement of COS conversion, which produced a higher breakthrough sulfur capacity. As the La content increased, the COS conversion reached nearly 100% and was no longer the restrictive procedure for the simultaneous removal of COS and H2S. The total breakthrough sulfur capacity was 148.4 mg/g when 40 M percent of La was doped into the SnO2 sorbent. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Hot coal syngas desulfurization with metal oxides in the integrated gasification combined cycle (IGCC) has the potential to improve the economic benefits [1]. Coal syngas contains not only hydrogen sulfide (H2S) but also other sulfurous gases including carbonyl sulfide (COS). In comparison to H2S, COS is more difficult to remove due to its low reactivity with metal oxides. Therefore, the formation of COS in hot syngas will lower the hot syngas desulfurization efficiency. To overcome this limitation, studies of COS removal by sorption, catalytic hydrolysis and hydrogenation conversion have been reported. Among the COS removal technologies, indirect removal by COS conversion reaction and H2S removal is more attractive due to the high removal efficiency [2]. COS conversion reaction
⇑ Corresponding author. E-mail address: [email protected] (Y. Shi). http://dx.doi.org/10.1016/j.fuel.2016.05.007 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.
COS þ H2 O ¼ CO2 þ H2 S COS þ H2 ¼ CO þ H2 S H2S removal
MeOx þ xH2 S ¼ MeSx þ xH2 O During hot syngas desulfurization with a certain amount of CO and CO2, when these two reactions occur in separated reactors, COS might be detected in the outlet if the metal oxide sorbent has a low COS conversion catalytic activity [3]. However, when the H2S sorbent is active for COS hydrolysis, the simultaneous removal of COS and H2S is possible, and will improve the COS conversion, especially when a large amount of CO and CO2 exists in the coal syngas. This simultaneous removal of COS and H2S is also beneficial for the economic evaluation of the hot syngas desulfurization because investment in COS conversion section can be avoided. Many metal oxides have been used for the hot syngas desulfurization [4]. Among these metal oxides, zinc oxide, manganese oxide and iron oxide are candidates for the simultaneous removal of COS and H2S. ZnO is a well-known sorbent for high efficiency
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1021
Nomenclature Qfeed gas Msulfur msorbent Vmol
the total feed gas flowrate (mL min1) the atomic weight of sulfur (32 g mol1) the weight of sorbent (g) the molar volume under standard conditions of 273.15 K and 1 atm (22.4 L mol1)
desulfurization. Studies of the catalytic conversion of COS to H2S with ZnO have been performed by Sasaoka et al. [5,6]. Both ZnO and ZnS formed from ZnO were active for the conversion of COS to H2S. Shangguan et al. [3] prepared the Al2O3 and K2CO3 modified ZnO sorbent, and an improved desulfurization performance was reported. Loading K2CO3 can increase the basicity of ZnO based sorbents, which increases the COS catalytic hydrolysis rate. Wakker et al. [7] reported that the MnO or FeO sorbent effectively removed COS from the feed gas. The sorbent can also be used to remove both H2S and COS from the fuel gas as well. Although the sorbent can directly react with COS, when water is added, COS can be converted into H2S, and then, H2S can be captured by the sorbent. Sakanishi et al. [8] found that Fe impregnation of activated carbon substantially enhanced COS and H2S removal from gases, which primarily enhanced the desulfurization of H2S to form metal sulfide. In addition, COS may be preferably adsorbed as COS itself in the pores. Yu et al. [9] demonstrated that a lignite charsupported Fe and Fe–Mo sorbent could effectively remove H2S and COS from coal syngas with high efficiency, and COS was converted to H2S. In addition to these three metal oxides, SnO2 is a steam regenerative H2S sorbent for moderate temperature desulfurization [10]. Due to steam regeneration, H2S can be concentrated for further utilization in the chemical process. On the other hand, COS may react with SnO2 above 425 °C and has the ability for the COS conversion [10–12]. Therefore, SnO2 is a suitable H2S sorbent candidate for the simultaneous removal of COS and H2S. In the previous studies, the catalysts employed for COS hydrolysis have included Al2O3, ZrO2, TiO2 and activated carbons with promoters consisting of alkali metal oxides, alkali earth metal oxides and transition metal oxides [2]. Recently, a novel type of rare earth oxysulfide catalysts was investigated for the hydrolysis of COS [13,14]. A rare earth metal Ce has been used to improve the catalytic effect for COS removal [15,16]. Compared to traditional catalysts, the rare earth oxysulfide catalysts showed the superiority to Al2O3-based catalysts in anti-poisoning of oxygen [14]. In the simultaneous removal of COS and H2S, COS was converted to H2S and captured by the sorbent. During the regeneration process, the sulfided sorbent reacted with oxygen to regenerated the sorbent for cyclic utilization. Thus, the characteristics of antipoisoning of oxygen is beneficial for the rare earth metals used in the improvement of COS catalytic conversion of the sorbent. However, the application of both SnO2 and rare earth metaldoped SnO2 for the COS removal has not been previously studied. Rare earth metals include yttrium and lanthanide series. La showed the best catalytic activity on COS conversion among the lanthanide series metals [14]. The addition of rare earth metals (Y and La) to the SnO2 has been previously studied, and the results indicate improved SnO2 physiochemical properties [17,18]. So, the objective of this study was to investigate the catalytic effect of SnO2 and the promotion of rare earth metal (Y and La) addition on COS hydrolysis and the simultaneous removal of COS and H2S. In this study, pure SnO2, Y-doped SnO2 and La-doped SnO2 were synthesized using a co-precipitation method. The physical and chemical properties of the SnO2 based sorbents were characterized by Xray diffraction (XRD); Brunauer, Emmett and Teller (BET) theory;
Cin Cout t
the inlet sulfurous gas concentration (%) the outlet sulfurous gas concentration (%) the breakthrough time (min)
and scanning electron microscopy (SEM). The COS removal tests of the SnO2 based sorbents were conducted in a fixed-bed reactor. 2. Experimental 2.1. Preparation of SnO2 based sorbents The rare earth metal-doped SnO2 sorbents were prepared using a co-precipitation method at room temperature. 0.02 mol of SnCl45H2O and a rare earth metal nitrate with an M/Sn atomic ratio of 1:9, 1:4 and 1:2.5 (M = Y or La) were dissolved in 100 mL of deionized water. A 5 mol L1 ammonia solution was added into the solution with vigorous stirring to adjust the pH to 10. The resulting slurry was maintained for 6 h prior to filtering the mixture via vacuum filtration and washing with deionized water until the pH reached 7. Finally, the precipitate was dried in air at 80 °C overnight and calcined at 500 °C for 3 h. The pure SnO2 sorbent was prepared using the previously mentioned precipitation method without rare earth metal nitration. All of the prepared sorbents were crushed and sieved through 97–150 lm. The rare earth metal doped-SnO2 sorbents are referred to as xM–SnO2 (x = molar ration of M/Sn). 2.2. Characterization of SnO2 based sorbents The textural properties of the sorbents were determined using an ASAP 2020 (Micromeritics, USA) by determining the nitrogen adsorption–desorption isotherms at 196 °C. The BET specific surface area was determined from the adsorption data. The pore volume and average pore diameter were determined by applying the Barret–Joyner–Halenda (BJH) model to the isotherm desorption branch. The phase composition analysis of the sorbents using XRD was performed on a Rigaku D-max 2500 X-ray diffractometer with Cu Ka radiation. The experimental data were digitally collected by a ‘‘step by step” scanning method in a 2h angle interval of 10–70°. The sorbent surface morphology was determined by a scanning electron microscopy (ZEISS MERLIN VP compact, Germany). 2.3. COS removal tests of the SnO2 sorbents The COS removal tests of SnO2 and the rare earth metal-doped SnO2 sorbents were conducted in a fixed-bed reactor with an internal diameter of 10 mm. The sorbent containing 0.5 g of active SnO2 with inert SiO2 was charged into the reactor and heated to 400 °C in a N2 atmosphere. Prior to the test, the sorbent was reduced using 50% H2 and balanced with N2 at 400 °C for 2 h, and then, the reactor temperature was adjusted to the required temperature. Next, a feed gas containing 0.2% COS and 2.4% H2O balanced with N2 was controlled by mass flowmeters (D07-19B, Beijing Sevenstar Electronics) and introduced from the top of the reactor with a gas hourly space velocity of 3000 h1. The concentration of H2S and COS in the outlet was analyzed using a gas chromatograph (GC9A, Shimadzu) equipped with a flame photometry detector (FPD)
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whose detection limit was 1 ppmv. Since GC-9A was used for low concentration analysis, an inert N2 was applied to dilute the gas concentration to 1/10 of the original gas concentration. Thus, the breakthrough time was defined as the time when the total outlet sulfurous gas concentration reaches 20 ppmv, which reached 1% of the total sulfurous gas in the feed gas. The breakthrough sulfur capacity of the sorbents was calculated using the following equation:
Z t Q feedgas Msulfur mg sulfur ¼ q ðC in C out Þdt g sorbent msorbent V mol 0
Table 1 Specific surface area, pore volume and average pore diameter of SnO2 and rare earth metal doped SnO2. Samples
Specific surface area (m2/g)
Pore volume (cm3/g)
Average pore diameter (nm)
SnO2 11.1% Y– SnO2 11.1% La– SnO2 25% La– SnO2 40% La– SnO2
41.12 76.19
0.090 0.117
5.7 4.5
77.67
0.101
3.7
72.85
0.121
4.2
98.81
0.192
5.4
3. Results and discussion 3.1. Characterization of the SnO2 based sorbents The XRD results in Fig. 1 indicate that pure SnO2 prepared by precipitation exhibited typical peaks which were assigned to SnO2. When a small amount (11.1%) of Y and La was doped into SnO2, the position of the main peaks do not change, which indicates that the crystal phase of the sorbent was still SnO2. No secondary phases were detected in the rare earth metal-doped samples, which indicates that the rare earth metal oxides and SnO2 form a solid solution in this preparation [17]. The formation of the solid solution also contributes to good dispersion of the rare earth metal cations in the SnO2. As the rare earth metal dopant concentration increased, the diffraction peaks became gradually weaker and broader. The crystallinity of SnO2 was unclear and was amorphous when 40 M percent of La was doped into SnO2. In Table 1, the specific surface area of SnO2 was 41.12 m2/g. When Y or La was doped in the SnO2, the specific surface area of the sorbents was 76.19 and 77.67 m2/g, respectively. Compared to pure SnO2, 11.1% Y or La dopant increased the specific surface area of the sorbent by nearly 85%. The pore volume derived from the BJH desorption also increased, and the sorbents have a smaller pore size. Fig. 2(a) shows the pore distribution of SnO2 and rare earth metal-doped SnO2. The average pore diameter decreased, and numerous small pores with sizes of approximately 3–4 nm were observed. Based on the BET results, the pore feature of the sorbents improved due to the addition of rare earth metals, which is beneficial for the gas–solid reaction. As La concentration increased, an increase in both of the total pore volume and pore size was observed, as
Fig. 2. Pore distribution of SnO2 and rare earth metal doped SnO2.
Fig. 1. XRD patterns of SnO2 (a), 11.1% Y–SnO2 (b), 11.1% La–SnO2 (c), 25% La–SnO2 (d), and 40% La–SnO2 (e).
shown in Fig. 2(b). A higher La concentration blocks some small pores and results in a relatively high ratio of large pores. The SEM photographs of pure SnO2 and rare earth metal-doped SnO2 are shown in Fig. 3. Fig. 3(a) showed that in pure SnO2, the small particles of SnO2 are connected to each other to form larger particles. When La and Y were doped into SnO2, the size of the sorbent decreased compared to that of SnO2, shown in Fig. 3 (b) and (c). Leite et al. [17] reported that rare earth metal-doped SnO2 could prevent small SnO2 particle from increasing in size due to the formation of a segregation layer of rare earth cations on the particle surface. Fig. 3(c) indicates that many small particle with distinct interval exist in the 11.1% La–SnO2. This feature in the
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Fig. 3. SEM photographs of SnO2 (a), 11.1% Y–SnO2 (b), 11.1% La–SnO2 (c), 25% La–SnO2 (d), and 40% La–SnO2.
sorbent structure provides more space for interactions between COS and H2S with the sorbent. According to Fig. 3(d) and (e), with the La content increased, the particle size grew up and small pores were blocked which was coincide with the pore distribution.
3.2. Simultaneous COS and H2S removal by SnO2 based sorbents Prior to the COS removal test, a thermodynamic equilibrium calculation was performed for COS conversion under experimental conditions, shown in Table 2. The results indicate that nearly all of the COS can be converted to H2S by COS hydrolysis under the experimental conditions. Almost no thermodynamic equilibrium limit was observed for COS conversion. The results of the COS removal test with pure SnO2 at 300, 350 and 400 °C are shown in Fig. 4. At 300 °C, pure SnO2 exhibited very poor catalytic ability for COS hydrolysis because initially, COS breakthrough was observed. Even if all of the H2S is captured by Table 2 Thermodynamic equilibrium calculation of COS conversion at the experimental condition. Temperature (°C) Equilibrium COS concentration (ppmv)
300 0.1820
350 0.3258
400 0.5350
the sorbent, COS conversion remained low. Therefore, pure SnO2 at 300 °C is unfavorable for COS removal. When the temperature was increased to 350 and 400 °C, no COS and H2S was detected in the outlet during the first stage. When breakthrough occurs, COS was detected before H2S. The delay time between COS breakthrough and H2S breakthrough was 180 min and 30 min at 350 and 400 °C, respectively. After COS breakthrough, the conversion of COS decreased in all of the three tests. Although an increase in the reaction temperature is beneficial for COS conversion, COS conversion remains the restrictive procedure for the simultaneous removal of COS and H2S. When COS could not be converted to H2S, the utilization of the sorbent was low because only a portion of the sorbent near the outlet can be used. During the sulfidation process of H2S and the sorbent, the reduced SnO2 was gradually converted to SnS. Therefore, the sorbent in the reactor changes with the sulfidation time. Initially, the reduced SnO2 may play a catalytic role in COS hydrolysis. Once SnS formed and occupied the surface of the sorbent, the activity of SnS for COS hydrolysis becomes very important. Fig. 5 shows the outlet COS concentration after the feed gas passes through the SnS bed. SnS was inactive for the COS catalytic hydrolysis when the temperature was 300 °C. The conversion of COS was 0. When the temperature increased to 350 and 400 °C,
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Fig. 5. Performance of COS hydrolysis by SnS at 300, 350 and 400 °C.
Fig. 4. COS and H2S breakthrough curves of SnO2 at 300, 350 and 400 °C.
the COS conversion was nearly 7% and 18%, respectively. According to these experiments, SnS exhibits very poor catalytic ability for COS hydrolysis at moderate temperatures of 300–400 °C. For pure SnO2, when the COS in the feed gas cannot be converted to H2S, the utilization of the sorbent in the reactor was low. However, in sour gas, which contains COS and H2S, the problem can be worse. Because H2S is the main sulfurous gas in the feed, the sorbent will sulfide into SnS more quickly than when only COS exists. The formation of SnS has a negative effect on COS hydrolysis, and the removal of COS can be worse using reduced
Fig. 6. Performance of COS removal by Y–SnO2 and La–SnO2 at 350 °C.
SnO2. Therefore, the catalytic characteristics of pure SnO2 for COS hydrolysis need to be improved. 3.3. Effect of rare earth metal-doped SnO2 on COS hydrolysis In this study, rare earth metals (Y and La) were chosen for doping SnO2 to improve the catalytic hydrolysis of COS, and the results
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are shown in Fig. 6. For both Y- and La-doped SnO2 sorbents, the COS breakthrough time was 300 and 290 min, respectively. In addition, the delay time between COS breakthrough and H2S breakthrough was limited to 60 min and 30 min, respectively. In comparison to 160 min with pure SnO2, the removal of COS and H2S was substantially better. For 11.1% Y–SnO2, after breakthrough, the COS concentration increased slower than the H2S breakthrough. While for 11.1% La–SnO2, the COS concentration increases quicker than H2S breakthrough. Using the same molar ratio for doping, Y–SnO2 exhibited a better performance for COS conversion. When La and Y were doped into SnO2, the foreign cations were well dispersed in the SnO2 based on the XRD results. Another key improvement in the rare earth metal dopant is the surface of the doped SnO2 was rich in rare earth metals [17,18]. This surface enrichment is beneficial for improving of COS hydrolysis because the rare earth metal dopant was added to improve the catalytic characteristics of pure SnO2. This surface rich property results in the best utilization of the catalyst, and the negative impact of SnS formation on COS hydrolysis can be limited to a low level. Based on the sorbent characteristics and experimental test, a schematic representation of the simultaneous removal of COS and H2S by rare earth metal-doped SnO2 is shown in Fig. 7. When rare earth metals were doped into SnO2, the sorbent, whose surface is rich in rare earth metals, creates a surface and distributed catalytic effect on COS hydrolysis. When COS was converted into H2S, the product (i.e., H2S) can be captured by the reduced SnO2. As mentioned above, SnS had a negative effect on COS hydrolysis. Although rare earth metals can provide a continuous effect on COS hydrolysis, the decrease in COS conversion was due to the coverage of the rare earth metals by SnS because the rare earth metal dopant was small. Within the sulfidation process, a decrease in COS conversion was observed. 3.4. Effect of La content on the COS and H2S removal When 11.1% La or Y was doped into SnO2, both of them showed a similar improvement on COS and H2S removal. However, a COS conversion decrease with the sulfidation process still existed. In order to achieve a very high COS and H2S removal performance, La-doped SnO2 was chosen to study the effect of rare earth metal content on COS conversion, and different La doping contents for SnO2 were prepared. Fig. 8 shows that the results of COS conversion and H2S removal by SnO2 doped with different amounts of La. When the La content increased to 25%, the breakthrough of COS and H2S nearly occurred simultaneously. The COS conversion was improved, and the H2S breakthrough was faster than the COS breakthrough, which indicates that the conversion of COS by the 25% La–SnO2 was better than that by the 11.1% La–SnO2. When
Fig. 8. Effect of La content on COS and H2S removal.
the La content was further increased to 40%, no COS was detected in the test, which indicates that the COS conversion reach 100%. With 100% conversion of COS, the COS conversion is no longer the restrictive procedure in the simultaneous removal of COS and H2S. Fu et al. reported that the surface La content increased linearly with the La content when La dopant concentration exceeded 10% [18]. The XRD results indicate that with the increase of La content, the diffraction peaks of SnO2 became weaker and unclear, but there was no secondary phases. Therefore, the La dopant was still well dispersed on the surface of the sorbent even the crystal structure of SnO2 became amorphous. In addition, with the La content increase, the pore size became larger and the total pore volume increased. Within the increase in La content from 11.1% to 25% and 40%, the sorbent can provide more catalytic sties contacting with COS to improve COS conversion. When 40% La was doped to SnO2, the sorbent became amorphous but there was still no independent phase of La2O3 or new species detected by XRD. The sorbent had a highest specific surface area and total pore volume in all the SnO2 based sorbents which contributed to the 100% COS conversion. Table 3 lists the breakthrough time and sulfur capacity of different SnO2 based sorbents. When the COS conversion is improved, the breakthrough sulfur capacity can be increased 1.7 times from 54.86 to 148.4 mg/g. The addition of rare earth metals makes SnO2 a promising sorbent for the simultaneous removal of COS and H2S.
4. Conclusions A novel rare earth metal-doped SnO2 sorbent was synthesized by co-precipitation for the hot syngas desulfurization at 350 °C. The pure SnO2 sorbent exhibited a poor catalytic hydrolysis ability for COS conversion at moderate temperatures of 300–400 °C. The formation of SnS was responsible for the decrease in the COS con-
Table 3 Breakthrough sulfur capacity of La–SnO2 sorbents at 350 °C.
Fig. 7. Schematic of simultaneous removal of COS and H2S by rare earth metal doped SnO2.
a b
Sorbents
BT timea (min)
BT sulfur capacityb (mg/g)
SnO2 11.1% Y–SnO2 11.1% La–SnO2 25% La–SnO2 40% La–SnO2
160 300 290 510 620
54.86 94.96 88.78 137.66 148.40
20 ppmv of total outlet sulfurous gas (COS and H2S) was detected by GC. Based on the sorbent weight.
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version because the activity for COS hydrolysis of SnS was considerably lower than that of SnO2 in the same temperature range. The addition of Y and La to the SnO2 sorbent improved the COS conversion activity and results in the sorbents processing a higher sulfur capacity. According to the XRD and BET results, the addition of Y and La results in the formation of smaller pores as well as good dispersion of foreign cations in the SnO2. Due to improved COS conversion, the sorbent exhibited better simultaneous removal of COS and H2S. The 40% La–SnO2 resulted in a breakthrough sulfur capacity of 148.4 mg/g with nearly 100% COS conversion. This result indicates that rare earth metal-doped SnO2 is a promising sorbent for the simultaneous removal of COS and H2S at moderate temperatures of 300–400 °C. Acknowledgements This work is supported by the National Natural Science Foundation of China (51476092), National High Technology Research and Development Program of China (2011AA050601) and Shanxi Province Science and Technology Major Projects (MH2015-06). References [1] Cheah S, Carpenter DL, Magrini-Bair KA. Review of mid- to high-temperature sulfur sorbents for desulfurization of biomass- and coal-derived syngas. Energ Fuel 2009;23:5291–307. [2] Rhodes C, Riddel SA, West J, Williams BP, Hutching GJ. The low-temperature hydrolysis of carbonyl sulfide and carbon disulphide: a review. Catal Today 2000;59(3–4):443–64. [3] Shangguan J, Zhao YS, Fan HL, Liang LT, Shen F, Miao MQ. Desulfurization behavior of zinc oxide based sorbent modified by the combination of Al2O3 and K2CO3. Fuel 2013;108(11):80–4. [4] Westmoreland PR, Harrison DP. Evaluation of candidate solids for hightemperature desulfurization of low-BTU gases. Environ Sci Technol 1976;10 (7):659–61.
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