Development of a Tar Reforming Catalyst for Integration in a Ceramic

filter element with catalyst particles of a high active surface area. Therefore, in the present work, the third catalyst integration method is applied...
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Ind. Eng. Chem. Res. 2007, 46, 1945-1951

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Development of a Tar Reforming Catalyst for Integration in a Ceramic Filter Element and Use in Hot Gas Cleaning Manfred Nacken,*,† Lina Ma,‡ Karen Engelen,‡ Steffen Heidenreich,† and Gino V. Baron‡ Pall Filtersystems GmbH Production Site Schumacher, Zur Flu¨gelau 70, 74564 Crailsheim, Germany, and Department of Chemical Engineering (CHIS), Vrije UniVersiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium

The development of a suitable tar reforming catalyst for integration in a ceramic hot gas filter element is presented. Calcined dolomite, CaO-Al2O3 mixed oxide, and MgO were used as catalyst supports with a grain size fraction between 0.1 and 0.3 mm for Ni doping. The activity of the catalysts with regard to the benzene and naphthalene conversion with and without additional Ru doping was examined. A MgO supported Ni catalyst with a NiO loading of 6% was found to be the most active catalyst in naphthalene reforming in the presence of 100 ppmv H2S at 800 °C and a filtration velocity of 90 m/h using model biomass gasification gas containing 5 g/N m3 naphthalene. Complete naphthalene conversion was still achieved after 100 h. After integration of the MgO-Ni catalyst in the ceramic filter element, a differential pressure increase of 6.8 mbar at 25 °C was measured, thus indicating the technical relevance of the developed catalyst for use in hot gas cleaning of biomass gasification gas. Introduction With the aim of a reduction of the CO2 emissions in the field of power generation advanced gasification technologies like the integrated gasification combined cycle (IGCC) process,1 using biomass as a CO2 renewable feedstock has attracted more and more attention. The motivation is the higher energy output of these gasification technologies in comparison to combustion technologies. In addition, the use of cleaned biomass derived syngas for the production of synthetic fuels in a subsequent Fischer-Tropsch synthesis finds increasing interest against the background of increasing prices for crude oil and natural gas. The main obstacle for the broad implementation of advanced gasification technologies is the reduction of the tar content in the syngas to prevent condensation of tars downstream of the gasifier in the piping, gas engine, or gas turbine. Conventional techniques for catalytic tar removal are based on a two-step cleaning process by using a separate filter unit followed by a catalyst unit or, vice versa, a catalyst unit upstream of a filter unit. The disadvantage of a catalyst unit after a filter unit is the necessary reheating of the particle free gas to the catalyst operating temperature. If a catalyst unit is placed upstream of a filter unit, there is the problem of fast deactivation of the catalyst by particle deposition. Other options for catalytic tar removal2,3 such as an optimized fluidized bed gasification process by using, for example, dolomite,4 olivine,5 or Ni based catalysts as in-bed additives6 are either not mechanically stable enough for long-term operation or are quickly deactivated by carbon deposition, respectively. Therefore, technically and economically, the most effective way is to combine particle filtration and catalytic tar removal in one unit by using a catalytic hot gas filter.7-9 There are several possibilities to realize the concept of a catalytic filter. The usually selected way is the catalytic activation of a porous ceramic hot gas filter element by developing a catalytic coating on the porous support.7-11 However, this procedure leaves little * To whom correspondence should be addressed. Tel.: +49 7951/ 302-152. Fax: +49 7951/302-280. E-mail: Manfred.Nacken@ europe.pall.com. † Pall Filtersystems GmbH Production Site Schumacher. ‡ Vrije Universiteit Brussel.

Figure 1. Schematic design of a cylindrical catalyst particle layer based catalytic filter element.

freedom in catalyst layer thickness and structure and is more complex to manufacture. Another method to manufacture a catalytic filter element would be to insert the catalytic component in the ceramic grain and binder mixture during the commonly applied manufacturing process of ceramic filter elements. However, in doing this, the accessible surface of the catalytic component would suffer a strong loss of active surface after grain sintering as relatively high sintering temperatures are applied in the manufacturing of hot gas filter elements. A third way to integrate a catalyst in a ceramic hot gas filter element requires a modification of the design of the ceramic hot gas filter candle by using a porous inner tube fixed at the head of the candle to allow the integration of a catalyst particle layer. The design of such a catalytic filter element is sketched in Figure 1. The advantage of this kind of catalyst integration is the higher flexibility with regard to potential applications for catalytic filters, as the development of an appropriate highly active catalyst particle system is generally less time-consuming in comparison to the development of a catalytic coating. In addition, there is the potential to provide a high catalyst capacity by filling the free hollow cylindrical volume of the catalytic filter element with catalyst particles of a high active surface area. Therefore, in the present work, the third catalyst integration method is applied. The development of a suitable tar reforming catalyst for use as a catalyst particle layer in a ceramic hot gas filter element is described to provide a catalytic filter element

10.1021/ie060887t CCC: $37.00 © 2007 American Chemical Society Published on Web 02/22/2007

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with a high tar removal performance in hot gas cleaning of biomass gasification gas. Several research groups have shown that Ni based catalysts show high performance in tar conversion10-17 but are very sensitive to deactivation by H2S18 or carbon formation.12,15 However, they are of technical interest because of their relatively low cost. Therefore, in this work, Ni was also selected as the main catalytically active component. The first target of the presented work was to study the effect of the variation of the catalyst support material, the preparation conditions, and the NiO loading as well as the effect of doping with ruthenium and the effect of H2S addition on the catalytic activity to develop a highly active tar reforming catalyst. The second target of the presented work was to demonstrate the technical relevance of the developed catalyst for use in hot gas cleaning of biomass gasification gas under the conditions of its integration as a catalyst particle layer in a ceramic hot gas filter candle. Experimental Procedures Catalyst Preparation. Several catalyst support samples were prepared or used as delivered by sieving out the grain size fraction between 0.1 and 0.3 mm. The catalyst support samples denoted as CaAl and CaAlT consisting of 30.2 wt % calcium oxide (CaO) and 69.8% alumina (Al2O3) were obtained after complete calcination of a mixture of 99.5% CaCO3 and g99% pure γ-Al2O3 or of 99.9% CaCO3 and 98% γ-Al2O3, respectively. The catalyst support sample MgCa was prepared by complete calcination starting from 99% dolomite (MgCO3CaCO3). For a part of the examinations, 98.5% pure MgO was used as a catalyst support material, denoted as Mg in the following discussions for simplification. The catalyst samples denoted as CaAl2.5Ni, CaAlT2.5Ni, CaAl6Ni, and Mg6Ni were obtained after impregnation of the corresponding catalyst support materials with a solution of 0.42 or 1 mol of nickel nitrate hexahydrate in 1 L of pure 2-propanol by the incipient wetness technique to achieve a NiO loading of 2.5 or 6 wt %, respectively, after complete drying and subsequent thermal decomposition at 400 °C. For the examination of the catalytic activity of the catalyst sample Mg6Ni over a time period of 100 h, the described sample preparation was scaled up by a factor of 25.6. The obtained sample was denoted as Mg6Ni(S) accordingly. In the case of the sample CaAl2.5NiAcAc, a solution of nickel nitrate hexahydrate and acetylacetone (HAcAc) with a molar ratio of HAcAc/Ni ) 2 in 2-propanol was used to achieve the desired NiO loading. Further catalyst samples denoted as CaAl2.5NiRu, CaAl6NiRu, and Mg6NiRu were prepared by coimpregnation of the appropriate catalyst support powders with a solution of 5.1 g of ruthenium trichloride hydrate and the appropriate amount of nickel nitrate hexahydrate in 1 L of pure 2-propanol to achieve a RuO2 loading of 0.25 wt % after complete drying and subsequent thermal decomposition at 400 °C. Preparation of the Catalytic Filter Candle Prototype. A cylindrical hot gas filter element of the type DIASCHUMALITH (N 10-20) of a dimension of 60/40 mm (outer/ inner diameter) with an effective filtration length of 300 mm (commercially available standard candle length is 1520 mm) and a filtration fineness of 0.3 µm was equipped with a porous inner tube fixed at the head of the candle to provide an unfilled noncatalytic filter candle. The catalytic filter candle prototype was prepared by filling this noncatalytic filter candle with the catalyst Mg6Ni(S) with a grain size fraction between 0.1 and 0.3 mm. The filling procedure was performed by vibrating the

candle to achieve maximum packing density. In this way, a hollow cylindrical catalyst particle layer of the dimension 40/ 20 × 300 mm (outer/inner diameter × length) was integrated in the ceramic hot gas filter candle. Measurement of the Catalytic Activity. The catalytic activity of all catalyst samples of the same grain size fraction between 0.1 and 0.3 mm was measured using a quartz glass tube reactor with an inner diameter of 7 mm, in which a catalyst particle layer of 10 mm thickness was inserted between two layers of glass wool of 10 mm thickness on the upstream and downstream side of the reactor. The latter was heated electrically to the corresponding operating temperature in the range of 700900 °C. A model biomass gasification gas consisting of 50 vol % N2, 12% CO, 10% H2, 11% CO2, 5% CH4, and 12% H2O was used, and a constant GHSV (gas hourly space velocity) of 2080 h-1 was adjusted at all temperature points. This corresponds to a filtration velocity (superficial velocity with respect to the catalyst particle layer surface) of 90 m/h. The catalyst samples containing NiO or RuO2, respectively, were reduced in situ during the increase of the temperature to 700 °C using the model biomass gasification gas to simulate the in situ activation of catalytic filter elements under real plant conditions. More details about the experimental setup are described in a previous work.10 Benzene and naphthalene were used as tar model compounds with a concentration of 15 and 5 g/Nm3, respectively. The benzene or naphthalene conversion, respectively, at each temperature point was measured as an average value of 960 measurement points over a time period of 1 h by mass spectrometry. Each conversion measurement was reproduced, and a measurement error of (1% has to be taken into account with respect to the conversion values given in the figures (estimated from calibration). In the catalyst deactivation experiments, the benzene and naphthalene conversion measurements were performed with model biomass gasification gas containing 100 or 200 ppmv H2S, respectively. BET Surface Area. For the determination of the BET (Brunauer-Emmett-Teller) surface area, the catalyst samples were dried for 5 h at 200 °C under vacuum. The measurements were performed on a Quantachrome Autosorb-3 instrument with N2 at 77 K with a measurement error of 0.03 m2/g. Differential Pressure. For the measurement of the differential pressure of the catalytic filter element prototype, the head and bottom part of the candle were vertically fixed and sealed in an appropriate test bench. The differential pressure was measured by passing air from the inside to the outside of the candle at the appropriate volume flows between 5.1 and 14 m3/h at 25 °C for the target filtration velocities between 90 and 248 m/h related to the outer surface of the candle. Results and Discussion Catalyst Support Materials Before and After Ni Doping. Previous studies on the performance of different catalyst systems in tar reforming15,19-21 have shown that a MgO support or a Al2O3 based support material doped by basic oxides reduce the deactivation of Ni based catalysts by carbon deposition. Therefore, a CaO-Al2O3 system and a MgO-CaO system (calcined dolomite) were selected as catalyst support materials. The latter type of support material was also selected because of its intrinsic catalytic activity in tar conversion under biomass gasification conditions.4,22 For this reason, both catalyst support materials were examined on their intrinsic tar reforming activity and their activity after

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Figure 2. Benzene conversion in the absence of H2S as a function of the reaction temperature of the undoped and Ni doped CaO-Al2O3 and MgOCaO catalyst support materials.

Ni doping at 700, 800, and 900 °C using benzene as a tar model compound in a H2S free model biomass gasification gas as also applied in previous work.11 Additionally, thermal conversion of benzene was examined at these temperatures to evaluate the proper contribution of the undoped and doped catalyst support materials in benzene reforming (Figure 2). The achieved benzene conversions of the MgO-CaO and CaO-Al2O3 catalyst support materials increase slightly with increasing reaction temperature from 700 to 900 °C but do not exceed values of about 5-7% after subtracting the contribution of thermal conversion. In spite of these low activities, the results indicate that the reforming activity of the CaO-Al2O3 system is slightly higher at all reaction temperatures in comparison to the MgO-CaO system, even if a conversion measurement error of up to (1% has to be taken into account. The corresponding benzene conversion results after doping with 2.5% NiO show complete benzene conversion in the case of the CaO-Al2O3 support material at all reaction temperatures, whereas in the case of the Ni doped calcined dolomite, the benzene conversion increases linearly with the reaction temperature from 2% at 700 °C to complete conversion at 900 °C. Evidently, there is a strong effect of the kind of the support material on the catalytic activity toward benzene reforming. The BET surface area values of 2 and 9.2 m2/g of both undoped catalyst support materials CaO-Al2O3 or MgO-CaO, respectively, reveal a large difference in the internal porosity. After doping of both support materials with the same amount of NiO, a higher internal porosity in the case of the calcined dolomite would lead to a larger part of NiO integrated in the internal porous system of the bulk of the grains that is hardly accessible by the benzene molecule to be catalytically converted. From this results a lower NiO concentration on the surface of the grains in comparison to the CaO-Al2O3 material. Thus, complete benzene conversion was only measured at a very high reaction temperature of 900 °C, where the risk of carbon deposition by an incomplete reforming reaction is strongly reduced. Because of these superior properties in benzene reforming, CaO-Al2O3 was chosen as a support material for additional examinations by substituting the used highly pure precursors for this support material by more cost-effective precursors in technical purity. No deviation of the complete benzene conversion at 700, 800, and 900 °C was found by using the CaO and Al2O3 precursors of technical purity (not shown in Figure 2). Thus, a scale up of the CaO-Al2O3 catalyst support manufacturing with subsequent doping with 2.5 wt % NiO would be possible. H2S Deactivation. In real biomass gasification gas, a H2S concentration of 50-200 ppmv has to be taken into account depending on the kind of feedstock to be used.23 Accordingly, the benzene reforming activity of the CaO-Al2O3 systems using the two types of catalyst support precursors doped with 2.5%

Figure 3. Benzene conversion as a function of the type of catalyst support precursor and complexation of Ni in the presence of 100 ppmv H2S using a 2.5% Ni doped CaO-Al2O3 catalyst system.

Ni (samples CaAl2.5Ni and CaAlT2.5Ni) were examined in the presence of 100 ppmv H2S (Figure 3). A strong decrease in benzene reforming activity from about 99% benzene conversion at 900 °C to only 15% benzene conversion at 700 °C was observed with decreasing temperature in the case of the sample CaAl2.5Ni. This is due to the increasing blocking of the active sites of the Ni catalyst by H2S with decreasing temperature. Evidently, the amount of H2S adsorbed on the active surface of Ni decreases at higher temperatures, thereby liberating active Ni sites and allowing a nearly complete conversion of benzene at 900 °C. In the higher temperature range between 850-900 °C, nearly the same benzene conversion values were achieved by using the CaO and Al2O3 precursor of technical purity (sample CaAlT2.5Ni), confirming the comparable catalytic activity as found in the absence of H2S. For improving the catalytic activity of the catalyst system CaAlT2.5Ni in the temperature range below 900 °C under H2S atmosphere, a modified catalyst system was prepared by using acetylacetone as a complexing agent to form a stable acetylacetonate chelate complex of Ni according to the following equation (sample CaAlT2.5NiAcAc):

2 HAcAc + Ni(NO3)2 f 2 HNO3 + Ni(AcAc)2 By this way, the dispersion of the catalytically active compound over the CaO-Al2O3 support material should be improved. Surprisingly, a decrease in activity by 9 and 23% down to conversion rates of 90 and 45% at 900 or 850 °C, respectively, in comparison to the system CaAlT2.5Ni was observed (Figure 3). Possibly, the stronger binding of the Ni ion to the acetylacetonate ligands in the stable chelate complex prevents a strong binding of Ni to the support surface. This results in a poorer dispersion of Ni on the support during evaporation of the solvent leading finally to the formation of larger Ni crystallites and a lower benzene reforming activity. As result of these investigations, it can be concluded that the CaAl2.5Ni system shows the highest activity in benzene reforming. Therefore, it was selected for further tests on its reforming activity in the absence and presence of H2S by using naphthalene as a new tar model compound. This second tar model compound was selected to simulate more precisely the catalytic tar cracking under real plant conditions. However, naphthalene is less stable than benzene and is decomposed to CO, CO2, H2, and in part to benzene as a stable intermediate.10 Comparison of Benzene and Naphthalene Reforming Activity. As already shown in Figure 2, complete benzene conversion was found in the whole temperature range from 700 to 900 °C in the absence of H2S using the catalyst system CaAl2.5Ni. In contrast to this, complete naphthalene conversion is only achieved at temperatures g800 °C under the same experimental conditions (Figure 4).

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Figure 4. Benzene and naphthalene conversion in the absence and presence of 100 ppmv H2S using a 2.5% Ni doped CaO-Al2O3 catalyst system (sample CaAl2.5Ni).

At 700 °C and in the absence of H2S, only 67% naphthalene conversion was measured. This lower conversion is probably due to the deactivation of the catalyst by carbon deposition because of the relatively low temperature that is evidently not sufficient for complete reforming of naphthalene as a compound with intrinsic H2 deficiency. In the presence of 100 ppmv H2S, naphthalene was completely converted at 900 °C as in the case of benzene (Figure 4). This result supports the previously mentioned assumption of less strong adsorption of H2S at 900 °C. At lower temperatures of 850 and 800 °C, a naphthalene conversion of 90 and 55%, respectively, is achieved, whereas only 68 and 26% of benzene conversion, respectively, is measured under comparable conditions. These results show that the naphthalene reforming activity of the Ni doped catalyst also decreases with decreasing temperature but to a lower extent than the benzene reforming activity. The higher naphthalene conversion in the presence of H2S can be explained by the stronger adsorption of naphthalene on the Ni surface. The latter is due to the larger surface of the two condensed benzene rings containing aromatic compounds for interaction with the active surface of the Ni catalyst in comparison to benzene. Thus, in the competition with the H2S molecules for the active sites of the Ni catalyst, a higher rate of naphthalene molecules can be converted. An increase of the NiO loading from 2.5 to 6% (sample CaAl6Ni) leads only to a slight increase of naphthalene conversion to 97 or 57% at 850 and 800 °C, respectively, in the presence of H2S (not shown in Figure 4). The problem of fast deactivation of the Ni catalyst by carbon deposition as a result of incomplete reforming reactions of condensed aromatics with a high H2 deficiency is often observed in Al2O3 supported Ni catalysts.19,20 This problem can be overcome by decreasing the acidity of the support material to suppress the adsorption of polyaromatic intermediate compounds, these being precursors for carbon deposits.24 Another solution would be the use of precious metals that seem to increase the hydrogenation performance and hence to decrease carbon deactivation by intermediately formed carbon.25 Use of a MgO Support and Ru Doping. Therefore, a MgO supported Ni catalyst with a NiO loading of 6 wt % (sample Mg6Ni) and a RuO2 doped CaO-Al2O3 supported Ni catalyst with NiO and RuO2 loading of 2.5 and 0.25 wt %, respectively, were prepared (sample CaAl2.5NiRu). First, they were tested on their catalytic performance in naphthalene conversion in the absence of H2S in comparison to the already tested samples CaAl6Ni or CaAl2.5Ni, respectively. Moreover, the effect of an increase of the NiO loading from 2.5 to 6 wt % in the case of the Ru doped catalyst CaAl2.5NiRu was examined (Figure 5). Complete naphthalene conversion was observed independently of the kind of catalyst system at a reaction temperature of g800 °C. At a reaction temperature of 700 °C, an increase in naphthalene conversion by 30% leading to complete conversion was

Figure 5. Naphthalene conversion as a function of the reaction temperature of a MgO supported Ni catalyst and two CaO-Al2O3 supported Ni catalysts of different NiO loading before and after additional Ru doping.

observed by using MgO instead of CaO-Al2O3 as a catalyst support material for the doping with 6% NiO (Figure 5: samples CaAl6Ni and Mg6Ni). This increase in the reforming activity or prevention of carbon deactivation by incomplete reforming, respectively, is partly due to the lower internal porosity of the MgO grains, of which the BET surface (0.15 m2/g) is less than a tenth of the BET surface of the CaO-Al2O3 grains (2 m2/g). This would prevent the inclusion and hence the loss of a large amount of the catalytically active Ni in the bulk of the grains. However, using the same CaO-Al2O3 support material, an increase of the NiO loading from 2.5 to 6% from which an increase in the concentration of the accessible Ni at the grain surface should result does not lead to complete reforming at 700 °C (Figure 5: samples CaAl2.5Ni and CaAl6Ni). Therefore, the stronger basicity of MgO in comparison to CaO-Al2O3 thereby preventing strong adsorption of polyaromatic compounds and catalyst deactivation by carbon deposits might also contribute to the strong increase in naphthalene conversion. More detailed structural information with respect to the pore size distribution of the two different undoped and Ni doped CaO-Al2O3 and MgO support materials and the amount of Ni catalyst inside the internal pore structure of the grains would be necessary to confirm these assumptions, which are an objective of further work. A similar increase of the reforming activity from 70% to complete naphthalene conversion at 700 °C was observed by doping a CaO-Al2O3 supported Ni catalyst of low NiO loading with RuO2 (Figure 5: sample CaAl2.5NiRu). The in situ formed Ru metal evidently allows a very efficient reforming, if one takes into account that the doping concentration is relatively low and that one part might be entrapped in the internal pore structure of CaO-Al2O3. This result is consistent with studies of Rostrup-Nielsen and Bak Hansen,26 who found that Ru exhibits a more than 3-fold higher activity in steam reforming of methane in comparison to Ni. A similar reinforcing effect was found by doping a CaOAl2O3 supported Ni catalyst of high NiO loading with RuO2 (Figure 5: sample CaAl6NiRu). However, an additional naphthalene conversion measurement at 600 °C, resulting in 39% conversion in the case of sample CaAl6NiRu and 63% using sample CaAl2.5NiRu (not shown in Figure 5), indicates that a high Ni concentration lowers the catalytic activity. Probably, Ru is partly covered by Ni as Ru doping was performed in a coimpregnation procedure. The effect of using MgO instead of CaO-Al2O3 and the effect of Ru doping of the two CaO-Al2O3-Ni systems (CaAl2.5NiRu and CaAl6NiRu) were also studied in the presence of 100 ppmv H2S. Additionally, Ru doping of the MgO supported Ni catalyst was examined (Figure 6).

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Figure 7. Naphthalene conversion of the catalyst systems Mg6Ni and CaAl2.5NiRu as a function of the H2S concentration at 800 and 900 °C at a filtration velocity of 90 m/h. Figure 6. Naphthalene conversion as a function of the reaction temperature of a MgO supported Ni catalyst and two CaO-Al2O3 supported Ni catalysts of different NiO loading before and after additional Ru doping in the presence of 100 ppmv H2S.

The conversion values of the catalyst samples Mg6Ni, CaAl6Ni, CaAl2.5Ni, CaAl2.5NiRu, and Mg6NiRu as a function of the reaction temperature show that in the presence of 100 ppmv H2S and a filtration velocity of 90 m/h, the only samples that allow complete naphthalene reforming at 800 °C are the MgO based catalyst system Mg6Ni and the Ru doped catalyst system CaAl2.5NiRu. In direct comparison of the conversion values of the samples CaAl6Ni and Mg6Ni as well as CaAl2.5Ni and CaAl2.5NiRu at 800 °C, an increase of naphthalene conversion by 43 or 45% was achieved by changing the kind of the support material or by Ru doping, respectively. These results indicate a similarly strong reinforcing effect of the MgO support material and Ru doping on catalytic naphthalene reforming also in the presence of H2S. At a reaction temperature of 750 °C, naphthalene conversions of 89 or 77% are achieved for the samples Mg6Ni and CaAl2.5NiRu, respectively, pointing out the slightly higher activity of the MgO-Ni based catalyst system. It is noteworthy that in case of sample CaAl2.5NiRu, Ru doping has led to an increase of the naphthalene conversion by about 60% in comparison to the sample CaAl2.5Ni at this temperature. This result confirms the strong promoting effect of Ru on naphthalene reforming. Moreover, this promoting effect is not affected by H2S with decreasing reaction temperatures from 800 to 750 °C. This indicates the stronger resistance of Ru against H2S poisoning in comparison to Ni. Combining Ru doping with a higher NiO loading of 6% using the two kinds of support material leads in both cases to a decrease in catalytic activity at 800 and 750 °C, more strongly in the case of sample CaAl6NiRu than in the case of sample Mg6NiRu. This may be due to the covering of the Ru component by Ni as already mentioned in the discussion of Figure 5. In the case of the MgO support, the decrease in activity is not clear, as without ruthenium doping, complete naphthalene conversion was found at 800 °C, and also a higher conversion value was measured at 750 °C (Figure 6). Further work focusing on the Ni dispersion, Ni crystallite size, and Ru dispersion in dependence of the kind of the support material is necessary but would go beyond the limit of this article to explain this decreasing effect on the catalytic activity. With respect to the catalytic performance of the investigated systems in Figure 6, the following sequence of catalytic activity in naphthalene reforming in the presence of 100 ppmv H2S and a filtration velocity of 90 m/h can be derived:

Mg6Ni > CaAl2.5NiRu > Mg6NiRu > CaAl6NiRu > CaAl6Ni > CaAl2.5Ni Catalytic Performance of the Developed Catalysts under Severe Application Conditions. In real biomass gasification

Figure 8. Naphthalene conversion of the catalyst systems Mg6Ni and CaAl2.5NiRu as a function of the filtration velocity at 800 and 900 °C at a H2S concentration of 100 ppmv.

applications, there may be deviations in the H2S content depending on the composition of the used biomass feedstock. Moreover, an increase of the filtration velocity (e.g., a doubling of the filtration velocity) would be of large economical interest, as the number of necessary filter candles can be reduced. However, filtration velocities usually applied in the envisaged hot gas filtration applications of ceramic filter candles range from 70 to 120 m/h. Therefore, with respect to the application of catalytic hot gas filter elements for biomass gasification gas cleaning, additional examinations of the catalytic performance of the best catalyst systems Mg6Ni and CaAl2.5NiRu were performed at 800 and 900 °C by doubling in the first case the H2S concentration (Figure 7) and in the second case the filtration velocity (Figure 8), whereas all other experimental parameters were kept constant. A comparison of the naphthalene conversions of Mg6Ni and CaAl2.5NiRu at 800 °C show that a nearly complete naphthalene conversion (99%) was even achieved in case of sample Mg6Ni in spite of the doubling of the H2S concentration from 100 to 200 ppmv, whereas in the case of sample CaAl2.5NiRu, a decrease from nearly complete to 96% naphthalene conversion was observed. At a reaction temperature of 900 °C, complete naphthalene conversion was achieved independently of the catalyst system and H2S concentration. The corresponding conversion values of sample Mg6Ni and CaAl2.5NiRu at 800 °C after doubling of the filtration velocity from 90 to 180 m/h leads to a slightly reduced catalytic activity of 94% naphthalene conversion in the case of Mg6Ni, whereas in the case of sample CaAl2.5NiRu, a stronger decrease down to 90% was observed. An increase of the reaction temperature from 800 to 900 °C leads independently to the increase of the filtration velocity to complete naphthalene conversion in both catalyst systems. Thus, the measured naphthalene conversions in Figures 7 and 8 at 800 °C show clearly that the MgO based catalyst system is the more active system of both catalyst systems under these severe application conditions of doubled H2S concentration or filtration velocity, respectively. Thereby, the validity of the previously given sequence in catalytic activity toward naphthalene reforming is confirmed.

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Figure 9. Naphthalene conversion of the catalyst Mg6Ni(S) over time in the presence of 100 ppmv H2S at 800 °C, a filtration velocity of 90 m/h, and simultaneous monitoring of the differential pressure of the catalyst particle layer.

previous work concerning the development of a catalytic filter element for combined particle separation, NOx removal, and VOC total oxidation, it was shown that by applying a catalytic coating technology on a hot gas filter candle, a differential pressure increase of about 7 mbar was caused by catalytic coating of the porous filter element support.27 Thus, the found increase of the differential pressure by the catalyst particle layer can be assessed to be in an acceptable range. Future work will be focused on pilot testing of this novel tar reforming catalytic filter element under real biomass gasification gas conditions to evaluate the catalytic performance of this prototype in removal of real tars and to define the optimum operating conditions for an efficient and cost-effective hot gas cleaning of biomass gasification gas. Conclusion

Figure 10. Differential pressure of a catalyst particle layer based catalytic filter element and an unfilled reference candle as a function of the filtration velocity at 25 °C by using air.

Examination of the Catalytic Activity Over a Time Period of 100 h. For the assessment of the technical applicability of the most catalytically active system Mg6Ni for use as catalyst particle layer in a ceramic filter element, the preparation of this catalyst system was scaled up to the one kilogram range. After that, the obtained sample Mg6Ni(S), whose BET surface area was measured to 1.95 m2/g, was tested in a long-term naphthalene conversion measurement over a time period of 100 h at 800 °C and a filtration velocity of 90 m/h in the presence of 100 ppmv H2S (Figure 9). At the same time, the differential pressure of the 10 mm thick catalyst particle layer in the quartz glass tube was monitored. Figure 9 shows complete naphthalene conversion over the whole examined time period of 100 h in the presence of 100 ppmv H2S. Therefore, there is no increase in the differential pressure of the catalyst particle layer that would be expected in the case of incomplete naphthalene conversion due to carbon deposition. Moreover, this experiment shows that the catalyst preparation method is scalable. Thus, the developed tar reforming catalyst seems to be suitable to be used as a catalyst particle layer in a ceramic hot gas filter element. Differential Pressure of the Catalytic Filter Candle Prototype. A catalytic filter candle prototype with an effective candle length of 300 mm and the unfilled reference candle were tested on their differential pressure at 25 °C as a function of the filtration velocity by using air (Figure 10). Figure 10 shows that the differential pressure of the catalytic filter candle and the unfilled candle increases linearly with the filtration velocity within the presented filtration velocity range. At a filtration velocity of 90 m/h, the catalyst particle layer based catalytic filter element exhibits a differential pressure of 19.8 mbar at 25 °C. The appropriate differential pressure at 800 °C would be 47.9 mbar by taking the temperature dependence of the gas viscosity into account. Moreover, Figure 10 shows that the differential pressure contribution of the catalyst particle layer is 6.8 mbar. In a

Several tar reforming catalyst systems of different NiO loadings were developed by using different catalyst support materials, a complexing agent, and ruthenium as a promotor. Structural and chemical properties of the support material, like the internal porosity and the basicity, were found to have a strong effect on the catalytic tar reforming properties. Ruthenium is an effective promoter for a CaO-Al2O3 supported Ni catalyst with a low NiO loading of 2.5% toward naphthalene reforming and leads to complete naphthalene conversion at 800 °C in the presence of 100 ppmv H2S and at a practically relevant filtration velocity of 90 m/h. This high catalytic reforming activity was even exceeded by a MgO supported Ni catalyst with a high NiO loading of 6%, allowing additionally a nearly complete naphthalene conversion even in the presence of a H2S concentration of 200 ppmv. Complete naphthalene conversion at 800 °C in the presence of 100 ppmv H2S over a time period of 100 h after scale-up of this catalyst to the one kilogram scale shows its suitability to be used as a catalyst particle layer in the presented novel design of a catalytic filter element. A differential pressure characterization of a first catalytic filter candle prototype of 300 mm length also confirms the technical feasibility with regard to the hot gas filtration requirements. Thus, this novel tar reforming catalytic filter element with the integrated MgO supported Ni catalyst seems to be of technical interest for use in hot gas cleaning of biomass gasification gas. Literature Cited (1) Larson, E. D.; Marrison, C. I. Economic Scales for First-Generation Biomass-Gasifier/Gas Turbine Combined Cycles Fueled from Energy Plantations. J. Eng. Gas Turbines Power 1997, 119, 285. (2) Devi, L.; Ptasinski, K. J., Janssen, F. J. J. G. A Review of the Primary Measures for Tar Elimination in Biomass Gasification Processes. Biomass Bioenergy 2003, 24, 125. (3) Sutton, D.; Kelleher, B.; Ross, J. R. H. Review of Literature on Catalysts for Biomass Gasification. Fuel Process. Technol. 2001, 73, 155. (4) Delgado, J.; Aznar, M. P.; Corella, J. Biomass Gasification with Steam in Fluidized Bed: Effectiveness of CaO, MgO, and CaO-MgO for Hot Raw Gas Cleaning. Ind. Eng. Chem. Res. 1997, 36, 1535. (5) Corella, J.; Toledo, J. M.; Padilla, R. Olivine or Dolomite as InBed Additive in Biomass Gasification with Air in a Fluidized Bed: Which Is Better? Energy Fuels 2004, 18, 713. (6) Garcia, L.; Benedicto, A.; Romeo, E.; Salvador, M. L.; Aruzo, J.; Bilbao, R. Hydrogen Production by Steam Gasification of Biomass using Ni-Al Coprecipitated Catalysts Promoted with Magnesium. Energy Fuels 2002, 16, 1222. (7) Saracco, G.; Montanaro, L. Catalytic Ceramic Filters for Flue Gas Cleaning. 1. Preparation and Characterization. Ind. Eng. Chem. Res. 1995, 34, 1471. (8) Saracco G. Coupling Catalysts and High-Temperature Resistant Filters. In High Temperature Gas Cleaning; Dittler, A., Hemmer, G., Kasper, G., Eds.; Institute for Mechanical Engineering and Mechanics, University of Karlsruhe: Karlsruhe, Germany, 1999; pp 627-640.

Ind. Eng. Chem. Res., Vol. 46, No. 7, 2007 1951 (9) Fino, D.; Saracco, G. Gas (Particulate) Filtration. In Cellular Ceramics: Structure, Manufacturing, Properties, and Applications; Scheffler, M., Colombo, P., Eds.; Wiley VCH: New York, 2005; pp 416-438. (10) Zhao, H.; Draelants, D. J.; Baron, G. V. Performance of a NickelActivated Candle Filter for Naphthalene Cracking in Synthetic Biomass Gasification Gas. Ind. Eng. Chem. Res. 2000, 39, 3195. (11) Ma, L.; Verelst, H.; Baron, G. V. Integrated High Temperature Gas Cleaning: Tar Removal in Biomass Gasification with a Catalytic Filter. Catal. Today 2005, 105, 729. (12) Baker, E. G.; Mudge, L. K.; Brown, M. D. Steam Gasification of Biomass with Nickel Secondary Catalysts. Ind. Eng. Chem. Res. 1987, 26, 1335. (13) Simell, P.; Kurkela, E.; Stahlberg, P.; Hepola, J. Catalytic Hot Gas Cleaning of Gasification Gas. Catal. Today 1996, 27, 55. (14) Narva´ez, I.; Corella, J; Orı´o, A. Fresh Tar (from a Biomass Gasifier) Elimination over a Commercial Steam-Reforming Catalyst. Kinetics and Effect of Different Variables of Operation. Ind. Eng. Chem. Res. 1997, 36, 317. (15) Bangala, D. N.; Abatzoglou, N.; Martin, J.-P.; Chornet, E. Catalytic Gas Conditioning: Application to Biomass and Waste Gasification. Ind. Eng. Chem. Res. 1997, 36, 4184. (16) Aznar, M. P.; Caballero, M. A.; Gil, J.; Martin, J. A., Corella, J. Commercial Steam Reforming Catalysts to Improve Biomass Gasification with Steam-Oxygen Mixtures. 2. Catalytic Tar Removal. Ind. Eng. Chem. Res. 1998, 37, 2668. (17) Wang, W.; Padban, N.; Ye, Z.; Olofsson, G.; Andersson, A.; Bjerle, I. Catalytic Hot Gas Cleaning of Fuel Gas from an Air-Blown Pressurized Fluidized-Bed Gasifier. Ind. Eng. Chem. Res. 2000, 39, 4075. (18) Hepola, J.; Simell, P. Sulphur Poisoning of Nickel Based Hot Gas Cleaning Catalysts in Synthetic Gasification Gas. I. Effect of Different Process Parameters. Appl. Catal., B 1997, 14, 287-303. (19) Bangala, D. N.; Abatzoglou, N.; Chornet, E. Steam Reforming of Naphthalene on Ni-Cr/Al2O3 Catalysts Doped with MgO, TiO2, and La2O3. AIChE J. 1998, 44 (4), 927.

(20) Chen, I.; Chen, F.-L. Effect of Alkali and Alkaline-Earth Metals on the Resistivity to Coke Formation and Sintering of Nickel-Alumina Catalysts. Ind. Eng. Chem. Res. 1990, 29, 534. (21) Horiuchi, T.; Sakuma, K.; Fukui, T.; Kubo, Y.; Osaki, T.; Mori, T. Suppression of Carbon Deposition in the CO2-Reforming of CH4 by Adding Basic Metal Oxides to a Ni/Al2O3 catalyst. Appl. Catal., A 1996, 144, 111. (22) Delgado, J.; Aznar, M. P.; Corella, J. Calcined Dolomite, Magnesite, and Calcite for Cleaning Hot Gas from a Fluidized Bed Biomass Gasifier with Steam: Life and Usefulness. Ind. Eng. Chem. Res. 1996, 35, 3637. (23) Kurkela, E.; Ståhlberg, P. Air Gasification of Peat, Wood, and Brown Coal in a Pressurized Fluidized-Bed Reactor. I. Carbon Conversion, Gas Yields, and Tar Formations. Fuel Process. Technol. 1992, 31, 1. (24) Beltrame, P.; Torrazza, S. Acitivity of Variously Supported Rh Catalysts for Toluene Steam Dealkylation. J. Catal. 1979, 60, 472. (25) Bartholomew, C. H.; Weatherbee, G. D.; Jarvi, G. A. Effects of Carbon Deposits on the Specific Activity of Nickel and Nickel Bimetallic Catalysts. Chem. Eng. Commun. 1980, 5, 125. (26) Rostrup-Nielsen, J. R.; Bak Hansen, J.-H. CO2-Reforming of Methane over Transition Metals. J. Catal. 1993, 144, 38. (27) Nacken, M.; Heidenreich, S.; Hackel, M.; Schaub, G. Catalytic Activation of Ceramic Filter Elements for Combined Particle Separation, NOx Removal, and VOC Total Oxidation. Appl. Catal., B 2007, 70, 370.

ReceiVed for reView July 10, 2006 ReVised manuscript receiVed January 10, 2007 Accepted January 18, 2007 IE060887T