Ind. Eng. Chem. Res. 1998, 37, 341-346
341
Sulfur-Deactivated Steam Reforming of Gasified Biomass Jeroen Koningen and Krister Sjo1 stro1 m* Kungl Tekniska Ho¨ gskolan, Chemical Engineering and Technology, Chemical Technology, S-10044 Stockholm, Sweden
The effect of hydrogen sulfide on the steam reforming of methane has been studied. Methane is the most difficult component to convert by steam reforming in the mixture of hydrocarbons, which is produced in biomass gasification. Two catalysts were subjected to hydrogen sulfide levels up to 300 ppm so as to study the effect of sulfur on their deactivation. These catalysts were the C11-9-061, from United Catalyst Inc., and the HTSR1, from Haldor Topsøe. The activation energy of the sulfur-deactivated steam-reforming reaction was calculated to be 280 and 260 kJ/mol, for each catalyst, respectively. The high values most probably originate from the fact that the degree of sulfur coverage of the nickel surface is close to 1 for these experiments. Even under these severe conditions, steam reforming of methane is possible without any carbon formation. The HTSR1 catalyst exhibits a very high sulfur-free activity, resulting in a performance in the presence of hydrogen sulfide higher than that for the C11-9-061 catalyst. By using the HTSR1 catalyst, the reactor temperature can be lowered by 60 °C in order to reach comparable levels of conversion. Introduction Gasification is a well-known technique to convert solid fuels into gaseous products. Biomass as a feedstock for gasification has recently regained interest, since it implies a possible means to decreasing the use of fossil fuels, which are assumed to be responsible for the greenhouse effect. More specifically, for the case of Sweden, biomass is considered to be a potential alternative to the decreased use of nuclear power. Wood, energy crops, and waste from the pulp and paper industry are examples of biomass, which can be gasified at temperatures up to 900 °C. This produces a mixture of gaseous, liquid, and solid compounds. The gaseous products are mainly hydrogen, carbon monoxide, carbon dioxide, methane, and other low-molecular hydrocarbons. The room temperature liquid products are water and an organic fraction, often called tar and consisting of higher hydrocarbons and (poly)aromatic substances. The solid char residue contains mainly carbon and some inorganic components, which are present in the biomass. Nitrogen and sulfur, on the other hand, which also are incorporated in the biomass structure, are released into the product gas during gasification as ammonia and hydrogen sulfide. A typical value for the amount of hydrogen sulfide in gasified biomass is 100 ppm (Lindman et al., 1981; Simell et al., 1996). Gasified biomass typically represents a low-to-medium heating value gas (5-15 MJ/Nm3), making it a usable feedstock for combustion processes (Hallgren, 1996). Another promising application of gasified biomass is in the production of synthesis gas for methanol or ammonia synthesis. Furthermore, the product gas can be used to produce electrical power in fuel cells or by combustion in a gas turbine. To prevent downstream tar condensation, the product gas from a biomass gasifier is commonly led through a catalytic cracking unit, in which the tar is converted in * Author to whom correspondence should be addressed. Telephone: +46 8 790 8248. Telefax: +46 8 108579. E-mail:
[email protected].
the presence of steam. The tar compounds are converted relatively easily, and a cheap dolomite-based catalyst is sufficient for this purpose (Simell et al., 1996). Methane from the gasified biomass is the most difficult to convert (Ekstro¨m et al., 1985; RostrupNielsen, 1984a) and has to be steam-reformed over a nickel-based catalyst at temperatures up to 1000 °C. This increases the hydrogen and carbon monoxide yield, which is of importance for the before-mentioned downstream applications. Since methane is the limiting component in the steam reforming of gasified biomass, the study concentrated on this component. Hydrogen sulfide (H2S) is known to deactivate nickelbased steam-reforming catalysts by chemisorption on the metal surface. Numerous studies of this phenomenon have revealed that the metal-sulfur bond is so strong that catalytic activity is substantially reduced, even at extremely low (ppb levels) gas-phase concentrations of hydrogen sulfide (Bartholomew et al., 1982). To fully exploit the activity of the catalyst, it would be necessary to remove the hydrogen sulfide before the steam-reforming reactor. Using conventional sulfur cleaning, which is run at ambient temperatures, requires the product gas to be initially cooled down and then reheated after sulfur removal. To circumvent this procedure, which has a negative effect on process efficiency, steam reforming has to be run without cleaning the gas prior to the reactor. The aim of this study has therefore been to investigate the steam reforming of methane from gasified biomass, in the presence of variable amounts of hydrogen sulfide. These amounts were in the range of 50-200 ppm, which is typical for gasified biomass. In addition, this study investigates the possibility to describe sulfur deactivation and steam reforming in a combined kinetic equation. This can provide important data for reactor modeling exercises. Theory The term methane steam reforming is used to describe the reaction between methane and steam to
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342 Ind. Eng. Chem. Res., Vol. 37, No. 2, 1998
produce hydrogen and carbon monoxide:
CH4 + H2O H 3H2 + CO
(1)
The simultaneously occurring water-gas shift reaction and reactions producing carbon are important side reactions. The water-gas shift reaction is relatively fast, and is assumed to reach equilibrium at the high temperatures, characteristic for steam reforming. The activation energy for thermal steam reforming is close to 420 kJ/mol. A catalyst is therefore required to reach feasible conversion levels at temperatures below 1100 °C. The group VIII metals from the periodic table are all active for the reaction. Yet nickel is almost exclusively used in industrial installations, since it combines high activity and stability and is at a reasonable price. Excellent reviews by Rostrup-Nielsen (1984a) and van Hook (1980) supply abundant information about the steam-reforming reaction and its characteristics. Numerous studies report kinetics of the steamreforming reaction under sulfur-free conditions. There is common agreement in that the reaction is first-order in methane, whereas some authors have found the reaction rate to be dependent on steam and hydrogen concentrations (Rostrup-Nielsen, 1984a; van Hook, 1980). Assuming a first-order reaction in methane concentration, the reaction rate constant for steam reforming in a tubular reactor can be expressed by
k)
FCH4 Wcat
ln
(1 -1 x)
(2)
Wcat the where FCH4 is the flow of methane in catalyst weight in g, and x the conversion of methane. The reaction rate constant, k, is considered to follow an Arrhenius-type temperature dependency. The activation energy for sulfur-free steam reforming of methane is in the order of 110 kJ/mol. In the derivation of eq 2, the increase in molar flow, caused by the steamreforming reaction, is not taken into account. The mechanism responsible for the deactivation of nickel-based steam-reforming catalysts is the dissociative chemisorption of hydrogen sulfide on the nickel surface. This phenomenon is dependent on temperature and the ratio of partial pressures of hydrogen sulfide to hydrogen, pH2S/pH2. The deactivation reaction can be written as
Ni + H2S H NiS + H2
(3)
Two-dimensional chemisorbed hydrogen sulfide is stable at conditions where the formation of bulk nickel sulfide is thermodynamically unfavored. Only at high pH2S/pH2 ratios and low temperatures will bulk nickel sulfide be formed (Bartholomew et al., 1982). An adequate description of the adsorption behavior of hydrogen sulfide on nickel includes a dependency on the degree of coverage ΘS. From a series of experiments by Alstrup et al. (1981), run in the temperature range of 500-800 °C and pH2S/pH2 ratios of 7-50 × 10-6, the following correlation for an adsorption isotherm of the Ni-S system was found:
pH2
[
( )( )
1 - ΘS ) kS exp Nm3/h,
pH2S
with ∆H°0 ) -289 kJ/mol, ∆S° ) -19 J/K, and R ) 0.69. The degree of sulfur coverage in this study was below 0.9 in the temperature range above 800 °C. Equation 4 is not valid for very high degrees of coverage, but has been demonstrated to describe other experimental results at lower pH2S/pH2 ratios, namely down to ppb levels (McCarty and Wise, 1980). In spite of the strongly deactivating effect of sulfur, the steam-reforming reaction is not completely inhibited (Morita and Inoue, 1965). This study also shows that switching from a feed containing small amounts of organic sulfur compounds, equal to amounts of 100 to 200 ppm of hydrogen sulfide, to a sulfur-free feed restores completely the activity of the catalyst. A plausible explanation lies in the regenerating effect of steam in combination with hydrogen. A study of the regeneration of steam-reforming catalysts, which were deactivated by an amount of 50 ppm of H2S, shows that the activation energy for such a regeneration reaction is close to 240 kJ/mol. This additionally indicates the strength of the nickel-sulfur bond (Ferretti et al., 1990). The chemisorption of hydrogen sulfide on nickel can even have positive effects, mainly in the suppression of carbon formation during steam reforming (RostrupNielsen, 1984b). In this study, the steam-reforming reaction at high temperatures and pH2S/pH2 ratios up to 27 × 10-6 (corresponding to sulfur coverages of up to 0.9) was investigated. The adsorption isotherm, found by Alstrup et al. (1981) (eq 4), was expressed by an Arrhenius-type correlation:
) exp ∆H°0(1 - RΘS)RT -
∆S° R
]
(4)
-ES pH2S RT pH2
-0.3
(5)
with kS ) 0.293 and ES ) 35.8 kJ/mol. The reaction rate over a sulfur-free (r0) and a sulfur-deactivated (rd) catalyst can be correlated according to (Rostrup-Nielsen, 1982)
rd ) (1 - RβΘS)n r0
(6)
The parameter R, expressing the number of sulfur atoms per nickel atom in the case of complete saturation of the nickel surface, was calculated to be close to 0.5. The constant β is the number of nickel atoms that are quenched by each adsorbed sulfur atom and is assumed to be around 2. The constant n is the number of nickel atoms which is required for the steam-reforming reaction. It is concluded in the study that three nickel atoms most likely act as an ensemble on which the steamreforming reaction occurs. Combining eqs 5 and 6 makes it possible to calculate a combined activation energy for both steam reforming as well as the deactivating effect of the hydrogen sulfide chemisorption. The study predicts the value for activation energy as 220 kJ/mol. An interesting aspect of nickel-based catalysts is that they are also active in the decomposition of ammonia. This makes them applicable in the high-temperature cleaning of fuel gas for gas turbines (Mojtahedi and Abbasian, 1995). A recent study on the stability of ammonia decomposition catalysts mentioned the nickelbased catalyst HTSR1 as displaying a surprising activity in the presence of high amounts of hydrogen sulfide (Gupta et al., 1993).
Ind. Eng. Chem. Res., Vol. 37, No. 2, 1998 343
Experimental Section A laboratory catalyst testing unit, consisting of a furnace with a fixed-bed reactor, a gas-mixing section, and a gas-cooling and -analysis section, was used in the experiments. The tubular reactor was made of hightemperature stable Inconell steel and had a length of 54 cm and an inner diameter of 15 mm. A fixed bed of 15 g of catalyst material, crushed to a size fraction of 0.7-1.4 mm, was placed in the middle of the reactor. The temperature in the catalyst bed was measured by means of a thermocouple, which was placed in the bed. The feed gas in the reactor was composed by mixing the various components from high-pressure cylinders using mass-flow controllers. Water was fed to an evaporator, a heated tube filled with porous material producing a steady flow of steam. All lines downstream of the evaporator were heated and insulated to prevent water condensation. A cooling section after the reactor, consisting of a countercurrent cooler followed by a bed of phosphorpentoxide, was used to remove the water without adsorbing the hydrogen sulfide. A blank experiment showed that methane was not converted through gas-phase reactions in an empty reactor. The product gas was analyzed for H2, CO2, CO, CH4, N2, and H2S, using a Balzers QMG421C Quadrupole mass spectrometer. The spectrometric data were converted to concentrations by numerically solving a calibration matrix, which contained calibration factors and mass numbers for each component. The concentration of carbon monoxide was checked by calculating the carbon mass balance for each measurement. Two different types of catalyst were examined. The first, manufactured by United Catalyst Inc. and coded C11-9-061, was tested more extensively. This catalyst contained 11-20 wt % of nickel on an R-alumina carrier. The surface area of the catalyst was 1-3 m2/g. The second catalyst was manufactured by Haldor Topsøe and coded HTSR1. Additional information on this nickel-based catalyst was not released by the manufacturer. Prior to all experiments, the catalyst samples were prereduced for 2 h at a constant temperature of 900 °C, using a flow of 600 NmL/min of 25 vol% H2 and 75 vol% N2. The feed gas simulated gas produced by a biomass gasifier and contained 15 vol% H2, 5 vol% CH4, 15 vol% CO, 15 vol% CO2, 25 vol% N2, and 25 vol% H2O (corresponding to 0.375 mL/min of liquid water). Small amounts of hydrogen sulfide (25, 50, 100, 200, and 300 ppm) were introduced into the feed gas from a highpressure cylinder, containing 2000 ppm of H2S in nitrogen. The total inlet gas flow was 2 NL/min of wet gas for all experiments, corresponding to a space velocity of 5700 h-1 (defined as methane feed in m3/h divided by the catalyst volume in m3). The experiments were carried out at atmospheric pressure and in the temperature range of 750-1050 °C (Figure 1). Results and Discussion Analysis of the Experimental Results. To enable reliably reproducable experiments of deactivation by hydrogen sulfide, certain requirements have to be satisfied. One of the most important is that the reactor should neither adsorb nor desorb sulfur in quantities that are significant compared to the introduced amounts present in the gas phase (Bartholomew et al., 1982). Since the used reactor is made of Inconell, and thus
Figure 1. Laboratory unit for catalyst testing: (1) feed gas mixing section, (2) water pump and evaporator, (3) furnace with tubular reactor and catalyst bed in the middle, (4) condensor, (5) separator, (6) gas meter, and (7) product gas outlet to mass spectrometer.
adsorbs hydrogen sulfide, this requirement is not completely fulfilled. Adsorption of hydrogen sulfide on a metallic reactor wall can indeed have a disturbing effect, especially when measuring very low levels of hydrogen sulfide. Nevertheless, this effect is considered to be of slight influence on the experiments, at the high hydrogen sulfide concentrations which are the central part of this study. Great care was taken to measure the steady-state level of methane concentration, which was defined by a variation of less than 0.01 vol% in methane concentration. The steady-state values in most experiments were reached within 30 min after introduction of hydrogen sulfide in the gas flow. In some cases, a breakthrough of the bed was observed, caused by adsorption of hydrogen sulfide, as it came in contact with fresh catalyst. In these cases, the steam-reforming reaction was continued until the levels of methane and hydrogen sulfide had stabilized completely. For each experiment, the carbon balance was calculated and found to be correct within 1-2%. The catalyst beds were visually checked for coke formation, but it was never observed. A second requirement, formulated by Bartholomew, is that the reactor is gradientless with respect to reactant concentrations and the catalyst bed temperature. However, since the steam-reforming reaction is not equimolar, and also produces hydrogen, the gas composition is not constant, introducing an inaccuracy into the experiments. Calculations showed that the effect of increased gas volume, as a result of the steamreforming reaction, can be neglected. The temperature of the catalyst particles was measured in the middle of the bed and was considered to be constant over the length of the bed. This was confirmed by experiment, using the same amount of catalyst and an equal amount of inert material. This resulted in a doubling of the height of the catalyst bed, but it had no effect on conversion, indicating that the bed is isothermal. This is probably not the case for very high methane conversions, because of the fact that the steam-reforming reaction is endothermic, so that these experimental results were not used for further analysis. Hydrogen Sulfide Chemisorption and Reaction Kinetics. Most of the studies mentioned in the theoretical part of this paper deal with low hydrogen sulfide
344 Ind. Eng. Chem. Res., Vol. 37, No. 2, 1998 Table 1. Degree of Coverage as a Function of Temperature and Amount of Hydrogen Sulfide ΘS temp (°C)
25 ppm
50 ppm
100 ppm
200 ppm
300 ppm
700 750 800 850 900 950 1000 1050
0.948 0.935 0.921 0.906 0.889 0.871 0.852 0.832
0.957 0.947 0.936 0.923 0.910 0.895 0.880 0.863
0.965 0.957 0.948 0.938 0.927 0.915 0.902 0.889
0.972 0.965 0.958 0.949 0.940 0.931 0.921 0.910
0.975 0.969 0.963 0.955 0.947 0.939 0.930 0.920
Figure 3. Arrhenius curves for the C11-9-061 catalyst. Values for the activation energy EA,tot are listed in Table 2. ], 25 ppm; 2, 50 ppm; 4, 100 ppm; 9, 200 ppm; 0, 300 ppm. Table 2. Results of Arrhenius Curves (C11-9-061)
Figure 2. Experimental results on the C11-9-061 catalyst. Methane conversion for various H2S concentrations. [, 0 ppm; ], 25 ppm; 2, 50 ppm; 4, 100 ppm; 9, 200 ppm; 0, 300 ppm.
concentrations, corresponding to pH2S/pH2 ratios of up to 100 × 10-6. The reason for this is twofold. Firstly, such low sulfur levels are of most interest in the context of steam reforming of natural gas. Secondly, small amounts of hydrogen sulfide can have positive effects on the operation of steam-reforming catalysts and are therefore of scientific interest. In the case of gasified biomass, the amount of released hydrogen sulfide and the hydrogen concentration in the product gas result in pH2S/pH2 ratios in the range of (150 × 10-6)-(1500 × 10-6). This is caused by higher sulfur levels (up to 300 ppm) and by the fact that chemisorption studies often use pure hydrogen feed gas with a small amount of hydrogen sulfide, whereas gasified biomass contains only 20 vol % hydrogen. These two effects give values for the pH2S/pH2 ratios which can be 10-fold the values in many chemisorption studies. The most important implication of these high values is that the degree of coverage, ΘS, as calculated from eqs 4 and 5, reaches values close to one depending on the temperature. In Table 1, the degree of coverage is listed as a function of temperature and the amount of hydrogen sulfide. The hydrogen partial pressure used for the calculations was equivalent to 20 vol % of the gas stream. It becomes clear from Figure 2 that methane steam reforming is still possible at these high sulfur coverages. The presence of both steam and hydrogen results in a continuous regeneration of the nickel surface. This makes it, at least partly, available for the steamreforming reaction. It is obvious that this regenerating effect is weakest at low temperatures and high hydrogen sulfide concentrations. From the shape and position of the various conversion curves in Figure 2 it can be concluded that the effect of temperature is much more pronounced than the effect of the amount of hydrogen sulfide.
H2S amount (ppm)
EA,tot (kJ/mol)
25 50 100 200 300
284.2 283.4 287.6 287.2 286.7
The × symbols in Figure 2 represent earlier experiments, run in the same laboratory unit, 10 years ago (Sjo¨stro¨m, 1984). The hydrogen sulfide concentration was in this case 50 ppm, but the lower space velocity can explain the higher methane conversions. Since the degrees of coverage in Table 1 are considerably higher than practically all previous studies on the adsorption of hydrogen sulfide, it is questionable whether the equations derived in the before-mentioned studies can be used to describe the experiments in this study. Nevertheless, an Arrhenius-type temperature dependency was assumed. This assumption is confirmed by plotting the values for the reaction rate constant resulted in Figure 3. The calculated activation energies for the various experimental series are listed in Table 2. The values for the combined activation energy, calculated from this study, are much higher than the value of 220 kJ/mol, predicted by Rostrup-Nielsen (1984b). The error in the activation energy values was estimated from experimental uncertainties and calculated to be (10 kJ/mol. In this respect, the consistency of the activation energy values for all experiments is remarkable. The results confirm that the deactivation by hydrogen sulfide, of this nickel-based steam-reforming catalyst, can be described by a simple first-order kinetic model, and an Arrhenius-type temperature dependency. The activation energy seems to be independent of the degree of coverage. This might be explained by the hypothesis that the dependency of the activation energy, on the degree of coverage, has several threshold values. The lower one would then result in the activation energy reported by Rostrup-Nielsen (1984b); the higher one would account for the higher values, reported in this study. HTSR1 Catalyst. The second catalyst which was tested was reported to have a particular behavior toward hydrogen sulfide in the decomposition of ammonia. According to Gupta et al. (1993), the catalyst’s tolerance to hydrogen sulfide is a function of tempera-
Ind. Eng. Chem. Res., Vol. 37, No. 2, 1998 345
Conclusions
Figure 4. Comparison between the C11-9-061 and HTSR1 catalysts. Methane conversion for various H2S concentrations. ], 0 ppm; 4, 100 ppm; 0, 200 ppm. Open symbols refer to C11-9-061 and closed symbols to HTSR1. Table 3. Results of Arrhenius Curves (HTSR1) H2S amount (ppm)
EA,tot (kJ/mol)
100 200
264.0 266.8
ture. At 800 °C and above, no catalyst deactivation is observed for hydrogen sulfide levels up to 2000 ppm. Yet the HTSR1 catalyst deactivates rapidly at lower temperatures. Although steam reforming and ammonia both take place on a nickel catalyst, the effect of, for example, carrier material and alkali on the activity is not the same for these two reactions (Rostrup-Nielsen, 1973). This indicates that different sites on the nickel surface are active in each of the reactions. Therefore, the deactivation behavior of the HTSR1 catalyst will not necessarily be identical for the steam-reforming reaction. The results of the experimental series on the HTSR1 catalyst are depicted in Figure 4, together with the corresponding curves for the C11-9-061 catalyst. From Figure 4 it becomes clear that the HTSR1 catalyst is very active at sulfur-free conditions, in comparison to the C11-9-061 catalyst. Nevertheless, the effect of small amounts of hydrogen sulfide on the activity is as dramatic in both cases. This could be expected since both catalysts are nickel-based and inherently sensitive to hydrogen sulfide. The higher sulfur-free activity of the HTSR1 catalyst also results in a higher activity in the presence of hydrogen sulfide, in comparison to the C11-9-061 catalyst. The HTSR1 catalyst reaches comparable conversion levels at temperatures around 60 °C lower. The temperature shift in the conversion, when the amount of hydrogen sulfide is doubled, for example, from 100 to 200 ppm is in the same range for both catalysts (ca. 20 °C). No threshold value for the temperature as was reported for ammonia decomposition could be observed. This confirms the assumption that a different type of nickel site is active in the steam-reforming reaction. Values of the activation energy, for the HTSR1 catalyst, are listed in Table 3. The activation energy also seems to be independent of the degree of coverage in this case. The higher activity of this catalyst and the lower value for activation energy might be explained by certain promotors. These promotors facilitate the steam-reforming reaction, even in the presence of substantial amounts of hydrogen sulfide.
Very small amounts of hydrogen sulfide substantially affect the performance of methane steam-reforming catalysts. This effect can be reduced by increasing the temperature, at which the steam reforming is run. This study shows that this is even valid for the high amounts of hydrogen sulfide, which are released into the product gas during biomass gasification. A temperature increase of 100-150 °C restores the methane conversion, for a typical commercial catalyst, to the corresponding sulfur-free levels. The nickel surface is almost completely covered by sulfur atoms under these circumstances, which is shown by the very high activation energy value of the steam-reforming reaction. This value was calculated to be 280 kJ/mol. The degree of coverage, which is close to 1, can be an explanation for the fact that this value is considerably higher than those reported in other studies. A good example of recent developments toward catalysts with a high activity, even in the presence of hydrogen sulfide, is the HTSR1 catalyst. Although this catalyst is also nickel-based, its very high sulfur-free activity leads to a reasonable performance in the presence of hydrogen sulfide. In comparison with the C119-061 catalyst, the reactor temperature can be lowered by 60 °C to reach the same level of methane conversion. Acknowledgment The authors thank Magnus Berg, Lars Waldheim, and Torbjo¨rn Nilsson (TPS Termiska Processer AB) for their useful comments and discussions. Financial support from the Swedish National Board for Industrial and Technical Development (NUTEK) is also gratefully acknowledged. Finally, the support of Haldor Topsøe A/S in supplying the HTSR1 catalyst is much appreciated. Literature Cited Alstrup, I.; Rostrup-Nielsen, J. R.; Røen, S. High Temperature Hydrogen Sulfide Chemisorption on Nickel Catalysts. Appl. Catal. 1981, 1, 303-314. Bartholomew, C. H.; Agrawal, P. K.; Katzer, J. R. Sulfur Poisoning of Metals. In Advances in Catalysis; Eley, D. D., Pines, H., Weisz, P. B., Eds; Academic Press: New York, 1982; Vol. 31, pp 135-242. Ekstro¨m, C.; Lindman, N.; Pettersson, R. Catalytic Conversion of Tars, Carbon Black and Methane from Pyrolysis/Gasification of Biomass. In Fundamentals in Thermochemical Biomass Conversion; Overend, R. P., Milne, T. A., Mudge, L. K., Eds.; Elsevier: London, 1985; pp 601-618. Ferretti, O.; Mare´cot, P.; Demicheli, M.; Gonzalez, G.; Duprez, D.; Barbier, J. Study of the stability and the poisoning by sulfur of Ni on R-alumina steam reforming catalysts (E Ä tude de la stabilite´ et de l’empoisonnement par le soufre de catalyseurs de vapore´formage Ni/alumine a). Bull. Soc. Chim. Fr. 1990, 127, 347352. Gupta, R. P.; Krishnan, G. N.; Hung, S. L. NH3/H2S Advances. U.S. DOE Report METC-93/6131; Research Triangle Institute: Research Triangle Park, NC, 1993. Hallgren, A. Theoretical and Engineering Aspects of the Gasification of Biomass. Ph.D. Thesis, Lund University, Lund, Sweden, 1996. Lindman, N.; Engstro¨m, S.; Rensfelt, E.; Waldheim, L. A New Synthesis Gas Process for Biomass and Peat. Energy from Biomass and Wastes V, Symposium Papers, January 26-30, 1981, Lake Buena Vista; The Institute of Gas Technology: Chicago, 1981; pp 571-611.
346 Ind. Eng. Chem. Res., Vol. 37, No. 2, 1998 McCarty, J. G.; Wise, H. Thermodynamics of Sulfur Chemisorption on Metals. I. Alumina-Supported Nickel. J. Chem. Phys. 1980, 72 (12), 6332-6337. Mojtahedi, W.; Abbasian, J. Catalytic Decomposition of Ammonia in a Fuel Gas at High Temperature and Pressure. Fuel 1995, 74 (11), 1698-1703. Morita, S.; Inoue, T. Allowable Concentrations of Organic Sulfur Compounds for Various Methane-Reforming Catalysts. Int. Chem. Eng. 1965, 5 (1), 180-185. Rostrup-Nielsen, J. R. Activity of Nickel Catalysts for Steam Reforming of Hydrocarbons. J. Catal. 1973, 31, 173-199. Rostrup-Nielsen, J. R. Sulfur Poisoning (Steam Reforming and Methanation). NATO Adv. Study Inst. Ser., Ser. E 1982, 54 (Progress in Catalyst Deactivation), 209-227. Rostrup-Nielsen, J. R. Catalytic Steam Reforming. In Catalysis, Science and Technology; Andersson, J. R., Boudart, M., Eds.; Springer-Verlag: Berlin, 1984a; Vol. 5.
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Received for review June 23, 1997 Revised manuscript received November 3, 1997 Accepted November 25, 1997 IE970452T
