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Pt-Rh/MgAl(O) Catalyst for the Upgrading of Biomass-Generated Synthesis Gases S. Albertazzi,*,† F. Basile,† J. Brandin,‡ J. Einvall,§ G. Fornasari,† C. Hulteberg,‡ M. Sanati,| F. Trifiro`,† and A. Vaccari† aDipartimento di Chimica Industriale e dei Materiali, Alma Mater Studiorum, Bologna UniVersity, Viale Risorgimento 4, Bologna 40136, Italy, Chemical Engineering, and DiVision of Ergonomics and Aerosol Technology, Lund UniVersity, Lund 22100, Sweden, and School of Technology and Design/Chemistry-Bioenergy, Va¨xjo¨ UniVersity, Va¨xjo¨ 35195, Sweden ReceiVed September 12, 2008. ReVised Manuscript ReceiVed October 30, 2008
The impact of the main contaminants usually present in the gas generated during gasification of biomass was studied on a Pt-Rh/MgAl(O) reforming catalyst, which was exposed to solutions of K2SO4, KCl, and ZnCl2 and also to a leached solution of biomass fly ash by aerosol technology on a laboratory scale. The catalyst was exposed to a product gas of a bench-scale downdraft gasifier as well. Untreated and exposed catalysts were extensively characterized, and the extent of deactivation was examined in the steam reforming of methane under industrial-like conditions. The above treatments mainly affected the metal dispersion, but the catalyst achieved acceptable performances even after having been exposed to the biomass-generated gas. The noble metals system showed a lower activity than that of a highly loaded Ni commercial-like catalyst in gasifying conditions, but sintering and carbon formation were less pronounced.
1. Introduction Biomass appears to be the most attractive feedstock to supplement fossil fuel at least for four reasons.1 First, biomass is a renewable source. Second, biomass is a practically CO2neutral source, because the carbon dioxide generated in the combustion of biomass is absorbed by photosynthesis when new biomass is growing. Third, it appears to have significant economic potential because of the fact that fossil fuel price demonstrated the ability to exceed $100 per barrel. Finally, biomass has a great advantage over wind and solar energy: biomass is always available on demand, and it can be stocked; therefore, energy and fuel generation can be planned at a constant rate. Therefore, the production of biomass-derived vehicle fuel on a large scale will help to reduce greenhouse gas and pollution, increase the security of European energy supplies, and enhance the use of renewable energy. However, the challenge is to develop efficient conversion technology. Currently, biomass gasification is considered as one of the most promising thermochemical technologies converting biomass into gaseous media for the production of power or secondary fuels.2 The Va¨rnamo Biomass Gasification Centre in Sweden, having a fuel throughput of 18 MW, is a unique plant and an important site for the development of this technology.3 At the present time, the Va¨rnamo plant is the heart of the CHRISGAS European project, whose aim is to convert the produced gas into a mixture * To whom correspondence should be addressed. Telephone: 00390512093677. Fax: 00390512093679. E-mail:
[email protected]. † Bologna University. ‡ Chemical Engineering, Lund University. § Va ¨ xjo¨ University. | Division of Ergonomics and Aerosol Technology, Lund University. (1) Demirbas, A. Prog. Energy Combust. Sci. 2007, 33, 1–18. (2) Bridgwater, A. V.; Toft, A. J.; Brammer, J. G. Renewable Sustainable Energy ReV. 2002, 6, 181–248. (3) The Va¨xjo¨ Va¨rnamo Biomass Gasification Centre. Available at http:// www.vvbgc.com (accessed Sept 11, 2008).
of CO and H2 (also known as syngas), thus having an added value higher than power.4 The predominant commercial technology for syngas generation from natural gas has been and continues to be steam reforming, in which methane, other light hydrocarbons, and steam are catalytically and endothermically converted into hydrogen and carbon monoxide:5 CH4 + H2O a CO + 3H2 (∆H 0298 ) 206 kJ ⁄ mol) From a thermodynamic point of view, for the equilibrium to shift to the right, high temperature and low pressure are required. However, 10-20 bar is normally applied when gas-to-liquid (GTL) plants are present downstream, because they require high pressure and a further compression of the syngas would lower the overall efficiency. Special equipment would be necessary to compress (hot!) the syngas; therefore, it also is quite troublesome from a practical point of view. The commercial plants commonly use supported nickel catalysts.6 The main reasons for catalyst deactivation are carbon deposition, sulfur poisoning, and sintering of Ni particles.7 Actually, the contaminants present in the product gas (tars, ash, NH3, H2S, HCl, and alkali and heavy metals) are also poisonous for the Ni-based reforming catalysts. Tars may be defined as organic contaminants, with a molecular weight greater than that of benzene. Gasification in the range of 700-900 °C usually produces polyaromatic compounds with four and five rings.8 Upon condensation, tars block pipelines and foul equipment. Ash consists of inorganic components derived from the biomass (4) The CHRISGAS Project. Available at http://www.chrisgas.com (accessed Sept 11, 2008). (5) Wilhelm, D. J.; Simbeck, D. R.; Karp, A. D.; Dickenson, R. L. Fuel Process. Technol. 2001, 71, 139–148. (6) Twigg, M. W. Catalyst Handbook, 2nd ed.; Manson Publishing Ltd.: London, U.K., 1996. (7) Bartholomew, C. H. Appl. Catal., A 2001, 212, 17–60. (8) Torres, W.; Pansare, S. S.; Goodwin, J. G., Jr. Catal. ReV. 2007, 49, 407–456.
10.1021/ef800765e CCC: $40.75 2009 American Chemical Society Published on Web 12/04/2008
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Table 1. Brief Description of the Studied Catalysts sample name Pt/Rh Pt/Rh Pt/Rh Pt/Rh Pt/Rh Pt/Rh Pt/Rh Pt/Rh Pt/Rh
untreated untreated spent K2SO4 K2SO4 spent KCl ZnCl2 ash gas gas spent
description sample as supplied after reaction in the SRM laboratory rig exposed for 2 h to aerosol particles containing K2SO4 Pt/Rh K2SO4 after reaction in the SRM laboratory rig exposed for 2 h to aerosol vapor containing KCl exposed for 2 h to aerosol vapor containing ZnCl2 exposed for 2 h to aerosol containing boiler ash salt streamed with a product gas from the bench gasifier Pt/Rh gas after reaction in the SRM laboratory rig
making the interpretation of the results more difficult, and finally because of the carbon formation and deactivation upon feeding methane, which are lower than feeding other hydrocarbons; therefore, the catalytic activity should remain stable during the short tests in the laboratory rig. 2. Experimental Section
feedstock (SiO2, MgO, CaO, K2O, Na2O, P2O5, etc.). A part of ash melts in the gasifier, and it can be removed from the bottom during replacement of the bed material; otherwise, agglomeration may occur. However, the part that leaves with the product gas (fly ash) contains most of the alkali metals, responsible for hot corrosion above 600 °C. Nitrogen, sulfur, and chlorine compounds are also of concern because they are precursors of NOx and SO2 and also because they can poison the catalysts placed in the downstream equipment. Therefore, an optimal gas cleaning stage and the development of more tolerant catalysts are key points for successful application of the gasification process for fuel production at large scale. For this purpose, over the past few years, many researchers have been studying active phases alternative to existing commercial Ni-based catalysts. The steam reforming of oxygenated compounds (acetic acid, phenol, acetone, and ethanol) was studied on Pt, Pd, and Rh supported on alumina and a ceria-zirconia sample.9 The supported Rh and Pt catalysts were the most active for the steam reforming of these compounds, while Pd-based catalysts performed poorly. The activity of the promising Pt and Rh catalysts was also studied for the steam reforming of a bio-oil obtained from beechwood fast pyrolysis. The same good results were obtained by a Rh/Ce catalyst for the steam reforming of ethanol,10 with the best performance among Pt-, Pd-, and Rupromoted Ce catalysts. Natural gas reforming carried out on noble metals11 concluded that the Rh and Pt catalysts were more active than Ir, Pd, and Ru materials. On the basis of these results, a catalyst consisting of Pt/Rh on a MgAl spinel was prepared and exposed to K2SO4, KCl, ZnCl2, and a solution of biomass fly ash. Sub-micrometer-sized selected aerosol particles were generated by aerosol technology, and dependent upon the temperature in the reactor and the salt melting point, the catalysts were exposed to either the aerosol particles of the salt or the salt vapor. Moreover, the catalyst was streamed with a product gas coming directly from a 10 kWth bench-scale downdraft gasifier as well. Untreated and exposed catalysts (Table 1) were characterized by inductively coupled plasma (ICP) mass spectrometry (MS) and atomic emission spectrometry (AES), CHNSO analysis, N2 adsorption/ desorption at -196 °C, X-ray powder diffraction (XRPD), temperature-programmed oxidation combined with mass spectrometry (TPO-MS), thermogravimetry (TG), and H2 chemisorption. Lastly, the extent of deactivation was examined in the steam reforming of methane (SRM), in different operating conditions. The steam reforming of pure methane is appropriate to investigate the effects induced by the above treatments on the catalytic performances, first because of the wide know how present in the open literature, second because of the limited number of side reactions that can complicate the system, thus
2.1. Catalyst Preparation. A batch of 100 g of Pt/Rh on Al/ Mg spinel was prepared according to the following procedure. A slurry made up of MgO and Al2O3, a Bohemite binder, and water was dried at 150 °C for 24 h. The dried mixture was calcined for 2 h at 800 °C and impregnated with a Pt(NO3)2 solution, dried at 150 °C, and calcined at 800 °C. The platinate catalyst was impregnated by a RhCl3 solution and heated as described previously. The resulting catalyst, having a total content of noble metals of 4.5 wt % and a Pt/Rh atomic ratio of 4.0, has been pressed and then crushed into particles having a size range of 0.4-0.8 mm. 2.2. Catalyst Exposure to Model and Real Contaminants from Biomass-Generated Gas. The substances chosen to study the effect of inorganic compounds on the reforming catalysts were KCl, K2SO4, and ZnCl2 based on biomass combustion results.12 Because the inorganic part of biomass is expected to contain almost all elements of the periodic table, another attempt was carried out to investigate the influence of impurities present in a real feed, by collecting a sample of fly ash from an existing circulating fluidized bed biomass combustion boiler having an output of 104 MW (66 MWth and 38 MWe) used in the city district for combined heat and power production. This sample was assumed to contain a realistic mixture of the inorganic contaminants, which the reforming catalyst can be expected to be exposed to in the full-scale gasifier. The main compounds detected by ICP-AES and ICP-MS in the ash salt were potassium (38 wt %), sulfur (16 wt %), sodium (5.6 wt %), and calcium (1.9 wt %). The instrumental setup for aerosol particle generation has been already shown elsewhere.13 The aerosol particles were generated by a pneumatic atomizer (Palas GmbH AGK-2000). The atomizer reservoir contained the salt solution. The carrier gas (N2) and the generated aerosol particles were passed through an inertial impactor to separate large solution droplets before entering a tubular reactor electrically heated. The catalyst bed was placed in the center of the tubular reactor at 800 °C. A continuous flow of 0.5 LN/min containing the generated aerosol particles or the salt vapor was sent through the catalyst bed for 2 h. The concentration of the K2SO4, KCl, and ZnCl2 solution was 50.0 g of salt/L of deionized water. The sample of ash salt solution was prepared by sequentially boiling the fly ash in deionized water, in the following way: 50 g of ash was boiled in 1 L of deionized water for 30 min, and then the solution was filtered; another 50 g of fresh ash was then added to the solution, which was boiled for a further 30 min and then filtered. This procedure was repeated for several times until 665 g of fly ash were processed (the last batch was of 15 g). A higher salt concentration in the ash salt solution was not possible because of the low solubility of some of the components. At the deposition temperature (800 °C), K2SO4 should be the only compound present in the solid phase (melting point of K2SO4, 1067 °C), while KCl and ZnCl2 are present in the vapor phase (melting point of KCl, 773 °C; ZnCl2, 283 °C), but the ash-salt might contain both vapor and solid phases at 800 °C. The efficiency of deposition (amount flushed/amount deposited on the solid) was comprised between 80 and 85% for all salts. Lastly, the catalyst was streamed with a gas coming directly from a bench-scale downdraft gasifier as well, by connecting a benchscale reformer to a 10 kWth downdraft gasifier fed with wood chips and using air as the gasifying agent (Figure 1). The main compounds
(9) Rioche, C.; Kulkarni, S.; Meunier, F. C.; Breen, J. P.; Burch, R. Appl. Catal., B 2005, 61, 130–139. (10) Salge, J. R.; Deluga, G. A.; Schmidt, L. D. J. Catal. 2005, 235, 69–78. (11) Ross, J. H. R. Catal. Today 2005, 100, 151–158.
(12) Pagels, J.; Strand, M.; Rissler, J.; Szpila, A.; Gudmundsson, A.; Bohgard, M.; Lillieblad, L.; Sanati, M.; Swietlicki, E. Aerosol Sci. 2003, 34, 1043–1059. (13) Einvall, J.; Albertazzi, S.; Hulteberg, C.; Malik, A.; Basile, F.; Larsson, A. C.; Brandin, J.; Sanati, M. Energy Fuels 2007, 21, 2481–2488.
Pt-Rh/MgAl(O) Catalyst
Energy & Fuels, Vol. 23, 2009 575 flow was controlled by means of mass flow meters, and deionized water was fed using a high-performance liquid chromatography (HPLC) pump. The temperature was measured by means of a wired thermocouple inserted in the catalytic bed. The test rig was followed by a gas chromatograph for the online determination of the produced gas, equipped with two different channels, with one of those dedicated to the H2 measurement (N2 as a carrier), to improve accuracy (0.1%). Different operational conditions were considered: pressure (P), temperature (T), steam/carbon ratio (S/C), and gas hourly space velocity (GHSV). Three different conditions have been chosen: (A) industrial-like condition (Tout ) 900 °C, P ) 10 bar, S/C ) 2.5, GHSV at 900 °C, and 10 bar ) 3600 h-1), where the highest conversion and yield in syngas are obtained, useful for providing screening data for a possible scale up (the pressure of 10 bar is generally requested for downstream units, such as a Fischer-Tropsch plant for diesel production); (B) low-conversion condition (Tout ) 600 °C, P ) 20 bar, S/C ) 1.7, GHSV at 600 °C, and 20 bar ) 7200 h-1), where small differences between the catalyst should be better highlighted; and (C) high GHSV condition (Tout ) 700 °C, P ) 10 bar; S/C ) 1.7, GHSV at 700 °C, and 10 bar ) 24 000 h-1), useful to evaluate the possibility to work with a smaller reformer but maintaining the same productivity of syngas.
3. Results and Discussion Figure 1. Schematic presentation of the gasifier system of CATATOR.
detected in the wood chips by CHNSO were carbon (46.50 wt %), sulfur (0.02 wt %), hydrogen (6.1 wt %), nitrogen (0.23 wt %), and oxygen (46.20 wt %). A raw measurement of gas-phase elements by MS gave the following results: H2S, 19.7 vol ppm; KCl, 42.9 vol ppm; and ZnCl2, 3.6 vol ppm. K2SO4 was not detected, possibly because the most of it may have condensed in the pipeline upstream of the instrument. 2.3. Characterization of the Catalysts. Elemental analysis (ICP-MS, ICP-AES, and CHNSO) was performed to detect both the amount of the salts deposited on the treated samples and the amount of carbon deposition on the sample exposed to the gasifier product gas. CHNSO data have been obtained by means of a CE Instruments EA 1110. XRPD analyses were carried out using a Philips PW1050/81 diffractometer equipped with a graphite monochromator in the diffracted beam and controlled by a PW1710 unit (λ ) 0.154 18 nm). A 2θ range from 10° to 80° was studied at a scanning speed of 140°/h. Noble metal crystallite sizes were calculated using the Scherrer equation on the best resolved peak (39.8° 2θ Cu KR) and assuming spherical crystallites. TG analyses were carried out on a TGA 2050 TA Instruments, by flowing air at a heating rate of 10 °C min-1 up to 900 °C. TPO analyses were carried out with a 5:95 O2/He (v/v) (total flow rate of 20 mL/min) gas mixture up to 900 °C on a ThermoQuest TPDRO 1100. A Boc Edwards EXC 120 mass spectrometer was used to analyze the gas coming out from the TPDRO 1100. Specific surface area assessment was performed by means of N2 adsorption/desorption at -196 °C with a Micromeritics ASAP 2020 instrument. The samples were pretreated by heating up to 150 °C until a pressure of 30 µmHg was reached, then kept for 30 min at this temperature, and finally heated up to 250 °C and maintained for 30 min. H2 chemisorption analysis was performed by the same Micromeritics ASAP 2020 instrument, equipped with the Chemi Tool. The sample was pretreated with hydrogen at 350 °C for 2 h and then with helium at the same temperature for 1 h (to remove the physi-adsorbed hydrogen) before performing the assay at 35 °C. 2.4. SRM Laboratory Tests. The SRM reaction was carried out in a laboratory rig using a reactor made of Incoloy 800HT to withstand high temperature and pressure as well as an aggressive atmosphere (Figure 2). The dimensions of the reactor tube were 50 cm in height, 30 mm in external diameter, and 9 mm in wall thickness. The reactor was inserted in an oven with two hot zones of 20 cm, capable of reaching a temperature of 1000 °C. The gas
3.1. Characterization Results. The characterization results of the studied samples are shown in Table 2. The Pt/Rh dispersion, defined as the ratio of the amount of Pt and Rh sites on the catalyst surface (i.e., the metal available for interaction with reactants), measured by H2 chemisorption, to the total amount of Pt and Rh sites, decreased in the treated samples, because of both the adsorption of salt particles (by direct deposition and/or condensation of vapors inside the pores) and the exposure environment (by sintering because of streaming water vapor at 800 °C). Discriminating between the above effects (presence of salt and exposure environment) is not of great interest, because the catalyst fed with a biomass-generated syngas is exposed to both effects anyway. However, blank samples were quickly screened, revealing losses in dispersion slightly lower than those occurring in the salt-exposed samples. Instead, it is interesting to highlight that the dispersion decreased the least (among salt-exposed samples) in the Pt/Rh KCl catalyst, probably because of the formation of small and dispersed PtCl particles, also confirmed by the low metallic crystallite size calculated by XRPD-Scherrer. This process has been explained in the open literature14 by either a physical splitting of the metal particles or a spreading of metal monolayers over the surface. The Pt/Rh gas sample presented a dispersion value only slightly lower than that of the Pt/Rh untreated sample, being exposed to neither the aerosol environment nor any salt. After the SRM reaction, the values further decreased, except for the Pt/Rh K2SO4 spent sample (according to the decreased metallic crystallite size). It is important to point out that none of the samples were subjected to any activation treatment before the catalytic tests; therefore, the reaction stream affected the mobility of the noble metal particles on the catalyst surface and, in the situation of the K2SO4-exposed sample, had a positive effect on dispersion. This may be explained by both a cleaning of surface and a redispersion effect, with the latter being relevant in samples having very low dispersion values. The metallic surface area per gram of sample, also measured by H2 chemisorption, is the total active metal surface area available for the interaction with the adsorbate and is related to metal dispersion. Specific surface area analysis by N2 adsorption/ desorption at -196 °C highlighted a decrease in the K2SO4(14) Forzatti, P.; Lietti, L. Catal. Today 1999, 52, 165–181.
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Figure 2. Laboratory rig used for the catalytic tests. Table 2. Characterization of the Studied Catalysts sample Pt/Rh Pt/Rh Pt/Rh Pt/Rh Pt/Rh Pt/Rh Pt/Rh Pt/Rh Pt/Rh
untreated untreated spent K2SO4 K2SO4 Spent KCl ZnCl2 ash gas gas spent
D (%) metallic surface area (m2/gsample) surface area (m2/gsample) metallic crystallite size by XRPD (nm) 3.64 0.72 0.45 1.02 1.51 0.24 0.51 2.94 1.92
0.48 0.10 0.06 0.13 0.20 0.03 0.07 0.39 0.25
and ZnCl2-exposed samples only, probably because of the sizes of these deposited molecules. The Pt/Rh untreated and Pt/Rh K2SO4 samples did not show any relevant decrease in specific surface area after the catalytic tests, while that value halved in the Pt/Rh gas sample. That sample presented the highest surface area value at the beginning, even higher than that of the untreated sample, probably because of the deposition of amorphous carbon on the surface (rather than inside the pores), thus apparently increasing bulk surface area. During reaction, the carbon was probably swept away, and therefore, the value of the Pt/Rh gas spent sample resulted close to that of Pt/Rh K2SO4 spent catalyst. Further discussion will support this consideration. XRPD patterns (Figure 3) are similar for all of the samples, showing the reflections of Pt/Rh, the MgAl spinel phase, and a low content of alumina phase. Except for minimal differences in bulk crystallinity, no relevant changes in the textural properties of the sample occurred during salt exposition and time-on-stream, thus confirming the stability of the support in the treatment and reaction conditions. The metallic crystallite size remained almost unchanged after the salt exposure (Table 2) but decreased in the streamed samples (Pt/Rh gas, Pt/Rh untreated spent, and Pt/Rh K2SO4 spent), probably because of a redistribution effect determined by the reaction stream. It is important to remark that the sample exposed to KCl showed the lowest crystallite size, probably because of the formation of small and dispersed PtCl particles. The presence of carbon deposits, likely formed by tars condensing on the catalyst surface rather than by methane and CO dissociations, was estimated to be 2.18 wt % in the Pt/Rh gas by CHNSO analysis (Table 2), matching the loss of weight of 2.0 wt % (calculated with respect to the carbon-free Pt/Rh untreated spent sample) measured by TG analysis (Figure 4)
16 16 12 10 18 10 18 22 11
61 52 61 37 32 60 64 49 59
elemental analysis (wt %) K, 0.0224 K, 0.3150 K, 0.0824 K, 0.3740 Zn, 0.0205 K, 0.1040 K, 0.0677; C, 2.18 C, Pt/Rh K2SO4 spent > Pt/Rh untreated spent) matches the trend of methane conversion (a higher dispersion value in the spent sample presented in Table 2 is related to a lower methane slip in test C, thus corresponding to a higher methane conversion). Therefore, the Pt/Rh gas catalyst, after being discharged from the gasifier, needed 4-5 h of time-on-stream in a clean feed to recover the activity of the Pt/Rh untreated sample. To complete this work, the catalytic performances of the Pt/ Rh untreated and Pt/Rh gas samples have been compared, in terms of methane conversion, yield in hydrogen, and selectivity in CO, to those of a commercial-like Ni-based catalyst that was deeply investigated elsewhere16 upon the same treatments and operative conditions. The Pt/Rh untreated catalyst performed slightly better than the Ni untreated in the last test only, while in the previous ones, no relevant differences can be detected (Figure 6). Differently, the Ni gas sample showed higher performances than the analogous Pt/Rh gas catalyst in all tested conditions, but the amount of carbon deposits was higher (3.32 wt %) and the sintering was more pronounced as well. The high amount of Ni loaded on the commercial-like sample (15.3 wt %) countervailed the higher loss of active sites, with respect to the Pt/Rh catalyst, thus justifying the higher activity. 4. Conclusions The presented work was an attempt to study on a laboratory scale the impact of some of the contaminants from biomass gasification on a homemade Pt-Rh/MgAl(O) reforming catalyst (4.5 wt %, Pt/Rh ) 4:1 as an atomic ratio). To mimic (16) Albertazzi, S.; Basile, F.; Brandin, J.; Einvall, J.; Fornasari, G.; Hulteberg, C.; Sanati, M.; Trifiro`, F.; Vaccari, A. Biomass Bioenergy 2008, 32, 345–353.
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Figure 5. TPO-MS patterns of the Pt/Rh gas sample.
Figure 6. Comparison of the catalytic performances between the Pt/Rh catalyst and a Ni commercial-like catalyst: CCH4 % ) 100 × (CO + CO2)/(CO + CO2 + CH4); YH2 % ) CCH4 % × H2/(3 CO + 4 CO2); SCO % ) 100 × CO/(CO + CO2); outlet compositions measured by GC. Table 3. Upgraded Gas Composition in the SRM of the Studied Samples with Comparison to Equilibrium Values: (A) Tout ) 900 °C, P ) 10 bar, Vcat ) 5 mL, τ ) 1 s, CH4 ) 218 STP mL/min, H2O(l) ) 0.401 mL/min, GHSV ) 3600 h-1, S/C ) 2.5; (B) Tout ) 600 °C, P ) 20 bar, Vcat ) 5 mL, τ ) 0.5 s, CH4 ) 1517 STP mL/min, H2O(l) ) 1.900 mL/min, GHSV ) 7200 h-1, S/C ) 1.7; and (C) Tout ) 700 °C, P ) 10 bar, Vcat ) 5 mL, τ ) 0.15 s, CH4 ) 2269 STP mL/min, H2O(l) ) 2.841 mL/min, GHSV ) 24 000 h-1, S/C ) 1.7 % (dry gas)
CH4
CH4 equilibrium
H2
H2 equilibrium
CO
CO equilibrium
CO2
CO2 equilibrium
A, Pt/Rh untreated A, Pt/Rh K2SO4 A, Pt/Rh gas B, Pt/Rh untreated B, Pt/Rh K2SO4 B, Pt/Rh gas C, Pt/Rh untreated C, Pt/Rh K2SO4 C, Pt/Rh gas
1.8 1.8 5.3 46.5 41.3 50.6 26.1 22.9 20.8
0.9 0.9 0.9 46.6 46.6 46.6 21.2 21.2 21.2
74.9 69.2 69.0 42.8 44.3 38.3 56.0 57.3 56.8
75.5 75.5 75.5 42.3 42.3 42.3 61.4 61.4 61.4
17.3 21.8 19.7 2.5 5.3 2.5 10.4 12.1 14.5
17.7 17.7 17.7 2.0 2.0 2.0 8.6 8.6 8.6
5.9 7.2 6 8.2 9.1 8.6 7.6 7.7 7.9
5.9 5.9 5.9 9.1 9.1 9.1 8.8 8.8 8.8
the exposure to a product gas of a biomass gasifier, the studied catalyst was exposed to solutions of K2SO4, KCl, ZnCl2, and biomass fly ash by aerosol technology. Moreover, the catalyst was streamed with a product gas coming directly from a 10 kWth bench-scale downdraft gasifier. The main effects of the treatments were (i) the decreased surface area when exposed to salts, such as K2SO4 and ZnCl2, and (ii) the loss of metal dispersion and metallic surface area.
The amounts of deposited salt decreased after reaction, thus highlighting the cleaning of the surface by the reaction stream. Carbon deposits were also detected in the sample streamed with the product gas of the downdraft gasifier. The catalytic performances of the noble metal-containing catalyst seemed not to be sensibly affected by K2SO4 exposure. After being streamed with the product gas of a bench-scale downdraft gasifier, the early tests showed a worse syngas
Pt-Rh/MgAl(O) Catalyst
composition; nevertheless, in the last test, the catalyst recovered full activity. Finally, a commercial-like Ni catalyst showed catalytic performances slightly better than those of the Pt/Rh sample after being exposed to the biomass generated gas, because of the high Ni load that counterbalanced the phenomena of sintering and carbon formation, which were more pronounced. However, these phenomena may lead to short lifetimes, quickly deactivating the catalyst charged in the plant reactor. Because stability is
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usually more relevant than activity in an industrial application, the noble metal system is worthy of being subdued to further and deeper investigation. Acknowledgment. The financial support of the EC 6th Framework Programme (CHRISGAS Project contract number SES6CT2004-502587), the Swedish Energy Agency, and the Swedish Research Council are gratefully acknowledged. EF800765E
