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Experimental Study of Model Biogas Catalytic Steam Reforming: 2. Impact of Sulfur on the Deactivation and Regeneration of Ni-Based Catalysts Mojdeh Ashrafi,* Christoph Pfeifer, Tobias Pro¨ll, and Hermann Hofbauer Institute of Chemical Engineering, Vienna UniVersity of Technology, Getreidemarkt 9/166, 1060 Vienna, Austria ReceiVed February 5, 2008. ReVised Manuscript ReceiVed June 13, 2008
To investigate the effect of sulfur in biogas on steam reforming, the effectiveness and deactivation behavior of Ni-based catalysts in the presence of H2S are experimentally studied. A model biogas, using a constant molar ratio of CH4/CO2 ) 1.5, is contacted to supported nickel catalysts in a laboratory-scale fixed-bed reformer. Various partial pressures of H2S, in the range of 15-145 ppm, are introduced to the feed gas, and the catalyst activity is characterized in terms of methane conversion. Tests are carried out at atmospheric pressure in the temperature range of 700-900 °C. The results show that the catalyst activity strongly depends upon operating temperature. The effectiveness of the H2S concentration on catalyst deactivation behavior also differs for various operating temperatures, which shows a stronger effect at 800 °C compared to 700 or 900 °C. To investigate the impact of the type of catalyst, a comparison of two different catalysts is carried out. To study the regenerability of the catalyst, the effects of sudden sulfur removal from the feed gas, temperature increase, and oxidative treatment are studied. On the basis of the results, the activity of the catalyst will be regained rather quickly when H2S is removed from the feed gas at 900 °C. For practical operation with Ni-based catalysts and biogas at small scale, temperatures of 900 °C seem necessary for the catalytic reforming process.
1. Introduction Biogas produced from anaerobic digestion of biomass is an attractive renewable energy source typically used for small- to medium-scale combined heat and power production and attracts increasing attention as a feedstock for the chemical industry.1–3 Through biogas steam reforming, H2-rich synthesis gas can be produced, from which gas engines benefit in terms of higher efficiency and lower NOx emissions compared to direct combustion of raw biogas.4,5 Furthermore, steam reforming allows for renewable synthesis gas production from biogas or even green hydrogen supply.6–11 * To whom correspondence should be addressed. Telephone: (43) 1-58-801-159-74. Fax: (43) 1-58-801-159-99. E-mail: mashrafi@ mail.zserv.tuwien.ac.at. (1) Kaltschmitt, M.; Hartmann, H. Energie aus biomasse; Springer Publishing: Berlin, Germany, 2001. (2) Kaltwasser, B. RegeneratiVe Energieerzeugung durch anaerobe Fermentation organischer Abfaelle in Biogasanlagen, 1st ed.; Bauverlag Publishing: Wiesbaden and Berlin, Germany, 1980. (3) Biogas Handbook BaVaria; Bavarian State Ministry of the Environment: Rosenkavalierplatz 2, 81925 Mu¨nchen, 2004; Chapters 1-3. (4) Herdin, G. R.; Gruber, F.; Klausner, J.; Robitschko, R.; Plohberger, D. Use of hydrogen and hydrogen mixture in gas engines and potentials of NOx emissions. Presented in ARES-ARICE Symposium on Gas Fired Reciprocating Engines, Canada, 2005. (5) Gruber, F.; Herdin, G. R. The use of H2-content process gas in gas engines. Presented in ASME International Combustion Engine Division, Spring Technical Conference, Chicago, IL, 1997. (6) Zhang, Z. G.; Xu, G.; Chen, X.; Honda, K.; Yoshida, T. Process development of hydrogenous gas production for PEFC from biogas. Fuel Process. Technol. 2004, 85, 1213–1229. (7) Xu, G.; Chen, X.; Honda, K.; Zhang, Z. G. Producing H2-rich gas from simulated biogas and applying the gas to a 50 W PEFC stack. AIChE J. 2004, 50 (10), 2467–2480. (8) Ashrafi, M.; Pfeifer, C.; Proell, T.; Hofbauer, H. Experimental study of model biogas catalytic steam reforming: 1. Thermodynamics optimization. Energy Fuels 2008, 22, 4142–4149.
The thermodynamics and catalytic performance of model biogas steam reforming have been determined in some previous studies.8–11 The application of these results in an industrial reformer, in which a real biogas is used, requires the avoidance of the poisoning effect of some components presented in real biogas to be accounted. One of the most severe and yet commonly encountered poisoning problems is that caused by chemisorption of sulfur impurities on metal catalysts in different processes, such as steam reforming. Sulfur is invariably present as inorganic and/ or organic sulfides in most naturally occurring feedstocks.12 In the case of nickel catalysts (which are mostly used in steam reforming), there have been some attempts to characterize the effect of sulfur compounds on the catalyst activity in steam reforming of natural gas or naphtha. Generally, sulfur compounds lower the activity of nickel catalysts for steam reforming, and the extent of poisoning depends upon the following variables: (1) the kind of sulfur compound, (2) the nature of the catalyst, (3) the operating conditions of the experiment, and (4) the organic compounds undergoing steam reforming.12–15 In the state of the art steam reforming of natural gas, sulfur (9) Kolbitsch, P.; Pfeifer, C.; Hofbauer, H. Catalytic steam reforming of model biogas. Fuel 2008, 87, 701–706. (10) Effendi, A.; Zhang, Z. G.; Hellgardt, K.; Honda, K.; Yoshida, T. Steam reforming of a clean model biogas over Ni/Al2O3 in fluidized- and fixed-bed reactors. Catal. Today 2002, 77, 181–189. (11) Effendi, A.; Hellgardt, K.; Zhang, Z. G.; Yoshida, T. Optimizing H2 production from model biogas via combined steam reforming and CO shift reactions. Fuel 2005, 84, 869–874. (12) Twigg, M. V. Catalyst Handbook, 2nd ed.; Manson Publishing: London, U.K., 1996. (13) Rostrup-Nielsen, J. R.; Sehested, J. Hydrogen and synthesis gas by steam and CO2 reforming. AdV. Catal. 2002, 47, 65–139. (14) Rostrup-Nielsen J. R. Catalytic Steam Reforming; Springer Publishing: New York, 1984.
10.1021/ef8000828 CCC: $40.75 2008 American Chemical Society Published on Web 09/10/2008
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compounds are usually reduced to less than 0.5 ppm in the feed gas to the reformer, and this is performed using a hydrodesulphurization catalyst in combination with a bed of zinc oxide as discussed in detail by Twigg and Rostrup-Nielsen.12,13 Because sulfur is an inherent compound of biogas, its impact on the steam reforming catalyst is of great importance.3 To our knowledge, there are no studies that treat the effects of sulfur on the catalytic steam reforming of biogas. This study is especially interesting, because in small-scale applications of biogas steam reforming, the desulfurization of the feed stock has economically less performance compared to operating at more tolerant conditions toward sulfur poisoning. Hydrogen sulfide (H2S), which may reach concentrations up to 2 vol % in biogas, is a stable compound at the steam reforming process.3,14 On the other hand, most of the sulfurcontaining organic compounds will be transformed into H2S under process conditions.13,16 Therefore, H2S is selected as the sulfur-containing component in this investigation. The main objectives of this work are to determine the amount of hydrogen sulfide tolerable for stable operation on the catalyst or operating conditions that would not lead to an instant loss of catalyst performance for a given H2S loading, respectively. In addition, it is determined whether or not the damage caused by sulfur is reversible and, if so, how can the catalyst performance be recovered. The results will be applied to optimize reformer operation at the anaerobic biomass fermentation plant in Strem, Austria, where a test facility for autothermal biogas steam reforming with subsequent combustion of the reformate in a 500 kWel gas engine is currently investigated.
significantly. For temperatures higher than 750 °C, CH4 conversion is observed to remain practically constant and independent of the steam content in the feed. Therefore, the reactor temperature should be operated within the range of 700-750 °C. In addition, it has been observed that the hydrogen content of the synthesis gas reaches a maximum value at a temperature range of 650-700 °C. High steam/carbon molar ratios always improve the reformer performance, which should be optimized according to economics and heat integration.27 However, steam/ carbon molar ratios above 3 should be applied to achieve acceptable high conversion and effectively avoid coke formation. 2.2. Sulfur Chemisorption on Nickel. The group 8 metal catalysts are highly susceptible to sulfur poisoning, and nickel is more sensitive to sulfide formation than other group 8 metals.13 There are at least three factors that influence not only the degree of sulfur poisoning of precious metal catalysts but also the ease of their regeneration and even a promotional effect. These three factors are the nature of the metal and the sulfur-metal and sulfur-support interactions, which in turn may be influenced by catalyst preparation.17 There is a general agreement that a layer of sulfur covers the metal surface and effects the process. The poisoning of the nickel catalyst may occur even when the concentration of hydrogen sulfide ought not to cause the formation of a bulk compound of sulfur and nickel. Therefore, hydrogen sulfide may be assumed to be retained by a chemisorption process.18–20 According to previous studies, the loss of activity of Ni-based catalysts through sulfur compounds can be due to16,18,21 (1) strong sulfur chemisorption on the nickel surface, which prevents the further adsorption of reactant molecules
2. Theory
H2S + Nisurface T Nisurface-S+H2
2.1. Biogas Steam Reforming. The global steam reforming mechanism of biogas consists of four reversible reactions, which are Methane steam reforming reaction: ◦ ( ) (1) CH4+H2O T CO + 3H2 ∆rH298 K ) +206 kJ/mol ◦ ( ) CH4+2H2O T CO2+4H2 ∆rH298 K ) +165 kJ/mol (2)
Water-gas shift reaction: ◦ ( ) CO + H2O T CO2+H2 ∆rH298 K ) -41 kJ/mol
(3)
Methane carbon dioxide reaction or dry reforming of methane: ◦ ( ) CH4+CO2 T 2CO + 2H2 ∆rH298 K ) +247 kJ/mol (4)
Because of the high CO2 portion in the biogas during CH4 reforming with water vapor, dry reforming of methane (reaction 4) may proceed as well. The water-gas shift reaction is typically faster than methane reforming in the presence of reforming catalysts. The concentrations of H2, CO, H2O, and CO2 in the synthesis gas depend upon the availability of elements after (partial) CH4 conversion and the operating temperature. A previous study for pure mixtures of CH4 and CO2 (model biogas)8 shows that biogas steam reforming must ideally be carried out at high steam/carbon molar ratios in the reformer, to achieve maximum conversion and avoid coke formation. With respect to the operating temperature, it is found that at temperatures below 700 °C the CH4 conversion decreases (15) Marecot, P.; Paraiso, E.; Dumas, J. M.; Barbier, J. Deactivation of nickel catalysts by sulphur compounds: I. Benzene hydrogenation. Appl. Catal., A 1992, 80, 79–88. (16) Sehested, J. Four challenges for nickel steam reforming catalysts. Catal. Today 2006, 111, 103–110.
(5)
and (2) the reconstruction of the Ni surface (i.e., sulfur can modify the chemical nature of the active sites or result in the formation of new compounds), which may modify or decrease the adsorption rates of reactant gases. Sulfur is quantitatively withheld by nickel until saturation.13 Stable saturation of sulfur is observed at a certain range of PH2S/PH2 (from 1-10 × 10-6 to 100-1000 × 10-6, according to the investigations by Rostrup-Nielson)14 above which bulk sulfide is formed. Below this certain PH2S/PH2 range, the saturation layers become unstable and the equilibrium coverage is dependent upon the temperature.14,18 In the case of stable saturation, the surface layer has a well-defined structure like that of two-dimensional sulfides as is described by RostrupNielsen.18 It has also been shown that the adsorption of hydrogen sulfide on the nickel catalyst is very strong at low temperatures, while poisoning of the nickel catalyst occurs with about 5 ppm of sulfur in the feed gas at a temperature of 800 °C and concentrations of the order of 0.01 ppm poison the catalyst already at 500 °C. The explanation for this phenomena is that the poisoning process can be represented by a simple exothermic (17) Shawal Nasri, N.; Jenny, M.; Valerie, A. D.; Williams, A. A comparative study of sulfur poisoning and regeneration of precious-metal catalysts. Energy Fuels 1998, 12, 1130–1134. (18) Rostrup-Nielsen, J. R. Chemisorption of hydrogen sulfide on supported nickel catalysts. J. Catal. 1968, 11, 220–227. (19) Marecot, P.; Paraiso, E.; Dumas, J. M.; Barbier, J. Deactivation of nickel catalysts by sulphur compounds: II. Chemisorption of hydrogen sulfide. Appl. Catal., A 1992, 80, 89–97. (20) Den Besten, I. E.; Selwood, P. W. The chemisorption of hydrogen sulphide, methyl sulphide, and cyclohexene on supported nickel catalysts. J. Catal. 1962, 1, 93–102. (21) Rakass, S.; Qudghiri-Hassani, H.; Abatzoglou, N.; Rowntree, P. A study of surface properties and steam reforming catalytic activity of nickel powders impregnated by n-alkanethiols. J. Power Sources 2006, 162, 579– 588.
4192 Energy & Fuels, Vol. 22, No. 6, 2008 Scheme 1. Flow Chart of the Experimental Setup
adsorption process.12,21–23 Sulfur chemisorption measurements have been performed by Alstrup et al. to predict the equilibrium sulfur surface coverage at various practical reaction conditions.24 In principle, it is possible to regenerate the sulfur poisoned catalyst by removal of the retained sulfur.13,14,18,25 The regeneration process can be performed by treatment with hydrogen (the reverse of reaction 5), but the driving force is extremely small. In addition, sulfur may also be removed by oxidation of the catalyst.24 Adsorption studies of H2S on nickel indicate an increasing reversibility of adsorbed sulfur with an increasing temperature. For steam reforming of methane, the reversibility has been demonstrated at a temperature of 800-900 °C.26 It has also been proven that the nickel catalyst regained the initial activity after the removal of the sulfur compounds in the feed. However, at industrial scale, this normally results in slow regeneration because the rate of diffusion-controlled elution decreases exponentially with time.25 According to RostrupNielsen,18 steam has no influence on the chemisorption equilibrium but the adsorbed sulfur can easily be removed by steaming at temperatures above 600 °C, if this involved complete oxidation of the catalyst and if the catalyst contains no alkali. When alkali compounds are present, steaming results in the formation of sulfates.25 Some studies have shown that it might be possible to regenerate the sulfur-poisoned nickel catalysts by removing S from the catalyst as SO2 by controlled exposure to oxygen (a very low O2 partial pressure) or to species that dissociate to oxygen.26
Ashrafi et al. 47 mm in diameter and, because of its short length in the present experiments (less than 100 mm), the reactor is operated at nearly atmospheric pressure and any pressure profile inside the reactor can be neglected. The heaters are controlled in such a way that all temperature measuring points (TMP1, TMP2, and TMP3) have the same constant temperature. The apparatus has been described in detail previously by Ashrafi et al.8 In all of the tests discussed below except the air treatment experiment, a fresh catalyst bed is used. In each experiment, the catalyst bed is heated up slowly (heating rate equal to approximately 6-7 °C/min) under N2 flow to the desired temperature. Then, a model biogas/steam mixture is introduced and H2 production from biogas steam reforming over the fresh catalyst is observed. When the biogas steam reforming rate has reached a steady state and remained in this condition for at least 1 h (only the last hour is shown in the diagrams), the desired H2S concentration (diluted in N2) is added to the feed stream. In each experiment, the catalyst bed is exposed to H2S to stabilize the catalyst activity for at least 2 h. Some tests are continued to study the catalyst regeneration. After the experiment, the catalyst bed is cooled to room temperature under N2 flow before opening the reactor vessel. The detailed descriptions of the experiment execution is given in Ashrafi et al.8 In these experiments, model biogas, containing 60% CH4 and 40% CO2, is used throughout the investigation. In this work, two commercially available catalysts from the company Sued-Chemie both with the description “G-90” are used. The catalyst are in spherical form and termed in the following: catalyst A (diameter ) 5-7 mm) and catalyst B (diameter ) 2-4 mm). 3.2. Definitions. The parameters, concerning the description of reactions, reaction conditions, and also evaluation of the results, are defined as follows: Steam/carbon molar ratio in the reformer (S/C):
S/C )
YH2O,in YCH4,in
(mol/mol)
(6)
Here, a general definition of organic carbon is considered. In biogas, organic carbon is present only as CH4, while the carbon in CO2 does not count because CO2 does not have any influence on carbon formation. Space velocity (SV):
SV )
V˙in 3 -1 -1 (m h kg ) mcat
(7)
The conversions of substance x:
conversion (x) )
N˙x,BG - N˙x,SG (%) N˙x,BG
(8)
3. Experimental Section 3.1. Experimental Setup. A schematic diagram of the experimental equipment used is given in Scheme 1. It consists of three main sections: feed section, reaction section, and analysis section. The fixed bed reactor and preheater/evaporator section are made of stainless steel (1.4841, X15CrNiSi25-20). The catalytic bed is (22) Beurden, P. V. On the catalytic aspect of steam reforming. ECN, http://www.ecn.nl/docs/library/report/2004/i04003.pdf, 2004. (23) Chahar, B. S. Sulfur poisoning of nickel catalysts: Methanedeuterium exchange reactions. Ph.D. Thesis, Rice University, 1981. (24) Alstrup, I.; Rostrup-Nielsen, J. R.; Rqen, S. High temperature hydrogen sulfide chemisorption on nickel catalysts. Appl. Catal. 1981, 1, 303–314. (25) Rostrup-Nielsen, J. R. Some principles to the regeneration of sulphur-poisoned nickel catalysts. J. Catal. 1971, 21, 171–178. (26) Hepola, J.; Simell, P. Sulphur poisoning of nickel-based hot gas cleaning catalysts in synthetic gasification gas: II. Chemisorption of hydrogen sulphide. Appl. Catal., B 1997, 14, 305–321. (27) Ashrafi, M.; Pro¨ll, T.; Hofbauer, H. Biogas upgrading to hydrogen rich gas by steam reforming: Comparison and optimization of plant configurations. Proceedings of the World Bioenergy Conference, Sweden, 2006.
4. Results and Discussion 4.1. Effect of Hydrogen Sulfide Concentration. Poisoning effects are always correlated with the poison concentration in the feed stream, which, of course, is the important parameter in practical operation. Therefore, the first set of H2S poisoning investigations is carried out with different H2S partial pressures (15, 31, 52, 77, 108, and 145 ppm). This concentration range corresponds to the concentration of this component in the Strem biogas plant in Austria (150-200 ppm), which can be reduced to approximately 30 ppm after desulphurization by introduction of oxygen/air to the process. Other experimental parameters are listed in Table 1. It is previously shown that, under this operating conditions and feed gas ratios, catalyst A exhibits high catalytic activity, no coke deposition, and the system is close to thermodynamic equilibrium.8 Figure 1 presents the behavior of catalyst activity by introducing H2S and showing the effect of H2S partial pressure on methane conversion.
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Table 1. Parameters of the Experiments To Study the Influence of H2S Concentration on Catalyst Deactivation parameter
unit
value
CH4/CO2 ratio S/C ratio space velocity reactor temperature catalyst type catalyst amount catalyst bed high residence time
mol/mol mol/mol m3 kg-1 h-1 °C
1.5 3.07 ( 0.13 18.31 ( 0.50 700 A 110 62 0.20
g mm s
Figure 1. Influence of the H2S concentration of the feed on catalyst activity.
Table 3. Actual Parameters of the Experiments To Study the Influence of Temperature on Catalyst Deactivation by H2S temperature (°C)
H2S concentration (ppm)
S/C (mol/ mol)
SV (m3 kg-1 h-1)
700 700 800 800 900 900
31 108 31 108 31 108
3.12 3.19 2.76 2.81 2.51 3.06
18.48 18.77 17.14 17.33 16.20 18.34
Figure 2. Influence of temperature on H2S poisoning. Concentration of H2S ) 31 ppm.
Table 2. Parameters of the Experiments To Study the Influence of Temperature on Catalyst Poisoning by H2S parameter
unit
value
CH4/CO2 ratio S/C ratio space velocity reactor temperature catalyst type catalyst amount catalyst bed high residence time
mol/mol mol/mol m3 kg-1 h-1 °C
1.5 2.91 ( 0.40 17.71 ( 1.51 700 A 110 62 0.20
g mm s
Sulfur poisoning, using H2S as the poisoning compound, results in an exponential decrease in the catalyst activity. As shown in Figure 1, the conversion of methane decreases dramatically. Even at a hydrogen sulfide concentration of 30 ppm, the reforming catalyst showed an 86% drop in activity after only 12 h. It is also clear from Figure 1 that at 700 °C the final activity of the catalyst after poisoning is not a strong function of the H2S concentration. At high sulfur concentrations, the stabilization of the catalyst activity is faster than at low sulfur levels. The conversion decreases obviously, as soon as a high concentration of H2S is injected; however, for less H2S concentrations, the obvious decrease is not observed immediately and it takes more time until the poisoning effect is stabilized. In all of the tests, the catalyst activity maintains a constant value after poisoning at a low conversion level. At this temperature, the CH4 conversion after deactivation is comparable to that of an empty pipe experiment, as presented previously in Ashrafi et al.8 4.2. Effect of Temperature on Hydrogen Sulfide Poisoning. The adsorption equilibrium also depends upon the temperature of the gas phase as mentioned before. To compare the degree of sulfating on the surface of the catalyst at different temperatures, the same catalyst is used at two different concentrations of H2S, 31 and 108 ppm, and three different temperatures, 700, 800, and 900 °C. Other parameters are listed in Table 2.
Figure 3. Influence of temperature on H2S poisoning. Concentration of H2S ) 108 ppm.
During each experiment, it is observed that the used peristaltic pump of water promotes a constant flow rate, but this varies however with each experiment. Thus, S/C values and therefore also SV values are not exactly the same in all experiments. Table 3 shows the actual S/C and SV values for each experiment. The activity is measured as a function of time at each temperature until a steady-state activity is reached. The results are shown in Figures 2 and 3. It can be observed that the sulfur is adsorbed more strongly at low operating temperatures and the methane conversion will be enhanced at high temperatures to a large extent. The conversion of methane at 900 °C remains in an acceptable range even at 108 ppm H2S. On the other hand, at 700 °C, even a trace content of 31 ppm H2S can degrade the catalyst performance totally. At 108 ppm H2S, the catalyst maintains at 900 °C about 86% of the original methane conversion in comparison to the 8% observed at 700 °C. At 800 °C, the catalyst containing 108 ppm H2S is almost inactive, whereas at 31 ppm, it still maintains about 55% of its original activity. Consequently, the influence of the H2S concentration is most pronounced at 800 °C, while
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Figure 4. Effect of H2S on CH4 conversion during exposure to 108 ppm H2S for two different Ni-based catalysts.
Figure 5. Catalyst performance recovery by H2S removal.
at 700 °C, any concentration leads to quantitative deactivation and, at 900 °C, the CH4 conversion rate is constantly high. 4.3. Deactivation Characteristics of Two Different NiBased Catalysts. Figure 4 shows the effect of H2S on CH4 conversion during exposure to 108 ppm H2S for two different Ni-based catalysts. CH4 conversion decreases dramatically for both Ni-based catalysts used in the presence of H2S. The change in the methane steam reforming rate for both catalysts can be due to sulfur chemisorption on the surface of these materials. However, according to the results in Figure 5, catalyst A provides a little higher resistance toward sulfur adsorption than catalyst B. 4.4. Regeneration of H2S-Poisoned Catalyst: Hydrogen Sulfide Removal. In these experiments, the reactor is operated with biogas, steam, and H2S until the product gas composition becomes stable. After detecting poisoning and its operation for at least 2 h in the steady-state condition, the content of H2S is totally switched off and the biogas steam reforming rate after H2S removal is investigated. Figure 5 presents methane conversion over the catalyst bed in the presence and after removal of H2S. In this figure, period 1 or the first hour refers to methane steam reforming without H2S, period 2 or the next 7 h refers to steam reforming in the presence of H2S, and period 3 refers to steam reforming after H2S removal. The result shows that, in principle, it is possible to remove sulfur from a poisoned catalyst simply by decreasing the sulfur content of the feed. Figure 5 also shows the effect of the reactor temperature on the regeneration during the experiments. Similar to H2S poisoning, the regeneration rate has a close relationship with the temperature. At 700 °C, the rate of regeneration is lower as compared to that of 900 °C. At 700 °C, the degree of sulfating over the catalysts slightly decreases after H2S removal and reexposure in the biogas/steam gas mixture, indicating the slow
Ashrafi et al.
Figure 6. Catalyst performance recovery by temperature enhancement.
sulfur removal from the surface. At 900 °C, the activity of the catalyst is regained rapidly after H2S removal from the feed stream for both H2S concentrations studied. 4.5. Regeneration of H2S-Poisoned Catalyst: Temperature Enhancement. In this part, after poisoning was detected at 700 °C and 145 ppm H2S content, the catalyst is heated stepwise from 700 to 900 °C, in 100 °C steps, and in the presence of H2S. At each temperature, the performance was observed for more than 1 h in steady state. Finally, at 900 °C, H2S is totally terminated. Figure 6 illustrates the enhancement of methane conversion by the increase of the temperature as a function of time. Figure 6 shows that the increase of the temperature can also be used to regenerate the catalyst activity, indicating an increasing reversibility of adsorbed sulfur with an increasing temperature. The catalyst activity begins to be recovered rapidly after a temperature increase and reaches a steady-state value, which shows the strong dependence of sulfur adsorption on the temperature. Finally, at 900 °C, the catalyst activity is totally regained. The CH4 conversion without H2S is naturally higher at 900 °C than at 700 °C. This can be seen from Figures 4 and 5 and is also reproduced by Figure 6. 4.6. Regeneration of H2S-Poisoned Catalyst: Air Treatment. As mentioned before, it has been reported that regeneration of sulfur-poisoned nickel catalysts can be obtained by controlled exposure to oxygen. However, this effect has not been observed in our case at 700 °C. To investigate the reversibility of the H2S poisoning by air treatment, the poisoned catalyst is treated repeatedly with air and tested again in biogas steam reforming. The air treatment is repeated 2 times at 700 °C, with different air partial pressures. After the catalyst is deactivated completely using a model biogas/H2O/H2S mixture at 700 °C and H2S concentration of 109 ppm, the reactor is flushed with an air/N2 mixture with a ratio of 3:100 for 1.5 h. The activity of the catalyst is measured through steam reforming after the first air treatment for about 3/ h. Then, the reactor is followed again by an air/N mixture 4 2 with a ratio of 30:100 for more than 1.5 h, and the steam reforming is repeated again. After each treatment, no enhancement of catalyst activity is observed but a minor decrease is detected. Such a result shows that the H2S poisoning is not reversible with air treatment at 700 °C and the adsorbed sulfur can not be oxidized in the presence of air. Ni may be oxidized to NiO and react with the carrier Al2O3 to form inactive NiAl2O4. Most likely, the catalyst has been transformed into NiAl2O4 in the presence of air. It cannot be excluded, despite the negative result presented, that the addition of even smaller amounts of free O2
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than investigated in the present study could have a positive effect for selective sulfur removal without oxidizing the catalyst. 5. Conclusion Although desulfurization technologies can decrease the amount of hydrogen sulfide present in the biogas, the remaining minor concentrations of sulfur cannot be tolerated by a nickelbased catalyst, where even ppm levels can rapidly deactivate the catalyst. The H2S-poisoning phenomenon proceeds quickly depending upon the H2S level in the gas and operating temperature and seems to be a steady process after the detected poisoning. An equilibrium sulfur level is probably formed on the catalyst particles in the bed, and after this, the poisoning effect remained is steady and does not increase as a function of time (maximum testing time of 15 h). It is important to remark that the poisoned Ni catalyst keeps an appreciable residual activity at 900 °C, but at 700 °C, the catalyst activity decreases rapidly even by low amounts of H2S. It is found that H2S-poisoned catalysts can effectively be recovered by increasing the temperature. The extent of catalyst regeneration by H2S removal increases with an increasing temperature.
The application of high temperatures is identified as the only effective measure to enhance sulfur resistance of nickel-based catalysts. For small-scale biogas applications, where trace removal of H2S is relatively expensive, it is likely that allowing higher H2S contents in the feed gas and, at the same time, operating at higher reformer temperatures is economically advantageous. Acknowledgment. The authors gratefully acknowledge the financial support by the Renewable Energy Network Austria (ReNet Austria), Energy from Biogas (Austrian funds program KNET/KIND), as well as the company Süd-Chemie for providing the catalysts.
Nomenclature BG ) biogas cat ) catalyst in ) inlet stream of the reformer m ) mass, kg N˙x ) molar flow rate of component x, mol/h SG ) synthesis gas S/C ) steam/carbon molar ratio, mol/mol SV ) space velocity, m3 kg-1 h-1 V˙x ) operating volume flow rate of component x, m3/h YA ) concentration of component A in a mixture, mol % ∆rH ) enthalpy change of a reaction, kJ/mol EF8000828
