Catalytic Activity in Naphthalene Reforming of Two Types of Catalytic

May 21, 2010 - Pall Filtersystems GmbH Werk Schumacher, Zur Flügelau 70, 74564 Crailsheim, Germany, and Department of. Chemical Engineering (CHIS) ...
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Catalytic Activity in Naphthalene Reforming of Two Types of Catalytic Filters for Hot Gas Cleaning of Biomass-Derived Syngas Manfred Nacken,*,† Lina Ma,‡ Steffen Heidenreich,† and Gino V. Baron‡ Pall Filtersystems GmbH Werk Schumacher, Zur Flu¨gelau 70, 74564 Crailsheim, Germany, and Department of Chemical Engineering (CHIS), Vrije UniVersiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium

The catalytic activity in the reforming of naphthalene of two different types of catalytic filter candles for biomass-derived syngas cleaning are presented and compared at superficial velocities of 72 and 90 m/h, respectively. The tests were performed in a laboratory setup on a fixed catalyst bed and small sections of full size candles. In a 50 h long-term test of the first candle type of fixed bed design, complete naphthalene conversion was found at 800 °C, 100 ppmv H2S, and an average superficial velocity of 135 m/h for the integrated Ni catalyst. Doubling the naphthalene concentration from 5 to 10 g/Nm3 has also led to complete conversion in a short-term test at a superficial velocity of 90 m/h. Because of easier manufacturing of the second candle type, several candles of a catalytic layer design were prepared by using different MgO-containing support materials and adjusting NiO loadings between 35 and 194 wt %. As compared to previous work, a conversion increase from 58 to 74% was achieved at 800 °C, 100 ppmv H2S, and a superficial velocity of 90 m/h that was further improved to 87% by decreasing the superficial velocity to 72 m/h. In the absence of H2S, 96 or 97% conversion was achieved, respectively. Introduction Energy and fuel production by thermochemical conversion of biomass is of increasing interest under the pressure to minimize global CO2 emissions. Among thermochemical conversion processes, gasification is favorable because of its high energy efficiency and the various potential applications of the produced syngas like electricity generation, hydrogen production, and synthesis of chemical compounds for use as fuel.1 Advanced techniques for biomass gasification are in most cases of the dual fluidized bed gasifier type, where a gasification and combustion reactor are connected by a hot circulating bed to provide the endothermic energy for the pyrolysis and reforming reactions during steam gasification of biomass.2,3 It was shown that the gravimetric tar content can be reduced down to contents of about 2 g/Nm3 using olivine as the catalytically active bed material.3,4 Using Ni-doped olivine,5 the tar content can further be reduced down to levels of 0.5 g/Nm3,3,6 but the appropriate transfer to technical scale is prevented by the loss of Nicontaining bed material during the process. Apart from this, these contents are still not low enough to prevent tar condensation in downstream units such as a gas engine or turbine after nonpressurized gasifiers, if compression of the producer gas at ambient temperature is necessary. They require a reduction of the tar content down to 30 mg tars/Nm3 or even lower.7 Therefore, an additional second catalytic tar reforming unit is necessary, which has to be protected from particle deposition by a filter to prevent fast catalyst deactivation. The technical feasibility of such a reactor unit with respect to its stable catalytic performance depends on the tar inlet content and catalyst operating temperature, which should be in the range of 2 g/Nm3 and 800 °C, respectively.8,9 The energy consuming reheating of the producer gas after hot gas filtration to the catalyst operating temperature leads to a substantial decrease of the * To whom correspondence should be addressed. Tel: +49 7951/ 302-152. Fax: +49 7951/26511. E-mail: Manfred.Nacken@europe. pall.com. † Pall Filtersystems GmbH Werk Schumacher. ‡ Vrije Universiteit Brussel.

energy efficiency of the whole process. This can be circumvented using a tar reforming catalytic hot gas filter. In this way, the energy of the hot producer gas is directly used for the catalytic tar reforming reaction occurring during the flow through a thickness of a 10 mm Ni-containing bed or layer after passing the fine filtering outer membrane of the candle (Figure 1). In addition, investment costs are saved by using only one compact catalytic hot gas filter unit instead of a hot gas filter and a separate catalytic reformer unit. Moreover, tar reforming catalytic filter candles, if successfully tested under real gas conditions, could directly be integrated inside the gasifier freeboard to further reduce the investment costs.10-13 In the present work, two different types of catalytic filter candles of 1.5 m length manufactured by two different catalyst integration methods are presented with respect to their activities in the reforming of naphthalene as a tar model compound, dependent on their composition and application conditions. The first type was manufactured by integrating a highly active Ni catalyst, developed in previous work,14 as a 10 mm thick fixed

Figure 1. Structure of a catalytic filter candle with a fixed bed design with a 70 mm outer diameter (a) and with a catalytic layer design with a 60 mm outer diameter (b).

10.1021/ie901428b  2010 American Chemical Society Published on Web 05/21/2010

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Table 1. Composition and Notation of the Prepared Catalytic Filter Elements of the Catalytic Layer Design

a

filter element support

composition of catalyst support (wt %)

NiO related to catalyst support (wt %)

notation

SiC SiC SiC SiC mSiC

MgO-Al2O3 (70-30) MgO-Al2O3 (70-30) MgO-Al2O3 (70-30) + 30 wt % La2O3a MgO-Al2O3-SiO2-Fe2O3 (60-15-20-5) MgO-Al2O3-ZrO2 (68-22-10)

108 189 63 35 194

MgAl-108Ni MgAl-189Ni MgAl30La-63Ni MgAlSiFe-35Ni ZrMgAl-194Ni

Related to the loading amount of MgO-Al2O3.

catalyst bed inside the hollow space of a commercial size cylindrical hot gas filter candle and a porous inner tube (Figure 1a). In contrast to the previous work, a full commercial length hot gas filter candle that has an outer diameter of 70 mm instead of 60 mm and an inner tube with an outer diameter of 30 mm instead of 20 mm was developed. The outer diameters were changed to allow operation of the catalytic filter candle at a lower superficial velocity and an appropriately higher residence time. The advantage of this “fixed bed design” for a tar reforming catalytic filter candle is that the Ni catalyst can be externally manufactured and examined. Moreover, a large amount of catalyst can be integrated into the hollow cylindrical space of the catalytic filter candle to provide a high catalyst capacity for tar reforming. For simplification of the manufacturing procedure, the development of an alternative type of a catalytic filter candle with a “catalytic layer design” (Figure 1b) has been started, where the Ni catalyst is deposited on the pore walls of a silicon carbide hot gas filter element.15 An additional design advantage of this catalytic filter candle type is a higher residence time in direct comparison with the fixed bed design at the same candle outer diameter and superficial velocity. For this reason, the standard outer/inner diameter of a hot gas filter candle of 60/ 40 mm could be selected. The feasibilty of the catalytic layer approach by use of a porous Al2O3-based filter has already been shown by applying the urea method for Ni integration.16,17 As already described in previous work15 also in this follow-up work another preparation technique for preparing Ni-containing catalytic layers was applied based on a two-step impregnation technique. After the first scale-up of this procedure up to 1.5 m candle length in the previous work, catalytic performances of 97% naphthalene conversion under H2S-free conditions and 58% in the presence of 100 ppmv H2S were measured by means of a candle segment in model biomass gasification gas with 12 vol % water and a naphthalene inlet content of 5 g/Nm3. Applying another model biomass gasification gas with 30 vol % water and a naphthalene content of 1.63 g/Nm3, a similar activity of 99.1% at 800 °C was measured on a 250 mm long candle with the same catalytic layer composition.18 With respect to an improvement in the catalytic activity in the presence of H2S, in this work, five new tar reforming catalytic filter candles with a catalytic layer design are examined for their catalytic activity in the presence and absence of H2S by examining the effect of different MgO-containing catalyst support layers and NiO loadings. In addition, the effects of porosity and operating parameters such as temperature and face velocity on the catalytic activity are examined. The model gas examinations of both designs were performed under a similar and technically relevant volume flow/candle ratio to allow a direct comparison of the catalytic activity in model tar reforming of both types of catalytic filter candles and to define the appropriate application and

filtration conditions for combined particle separation and efficient tar reforming. Experimental Procedures Preparation of Full-Size Catalytic Filter Candles with Fixed Bed and Catalytic Layer Designs. For the preparation of a full-size, fixed bed type catalytic filter candle, a Ni catalyst in the form of grains with grain sizes between 0.1 and 0.3 mm was integrated as a fixed bed catalyst in a hot gas filter candle. The Ni catalyst was manufactured by incipient wetness impregnation of a MgO grain support with a NiO precursor solution, according to the same procedure as already described in previous work, to adjust a NiO loading of 6 wt %.14 The appropriate Ni catalyst amount of type Mg6Ni(S)14 was filled into the hollow cylindrical space of a 1520 mm long silicon carbide (SiC)-based catalytic filter candle with a 70 mm outer diameter and 50 mm inner diameter and an integrated porous inner tube with a 30 mm outer diameter. Catalytic layer type catalytic filter candles were prepared according to the two-step impregnation procedure described in a previous work.15 Four SiC-based candles of the type DIASCHUMALITH with dimensions 60/40 mm × 1520 mm (outer/ inner diameter × length) and 38% porosity (SiC) and one modified SiC-based candle with the same dimensions, the same outer membrane, and 50% porosity (mSiC) were impregnated. The same MgO-Al2O3 precursor, as already reported,15 was used. Whereas in the previous work the MgO-Al2O3 support was loaded with 6 and 60 wt % of NiO, here, distinctly higher loadings of 108 and 194 wt % were used (Table 1). Lanthanum nitrate hexahydrate was used as the La2O3 precursor. A 1:1 mixture of the MgO-Al2O3 precursor and raw olivine from Magnolithe were used in one preparation. In deviation of the standard impregnation procedure, the mSiC-based candle was impregnated with a ZrO2-doped MgO-Al2O3 precursor suspension with a solid content of 4.25 wt %. All MgO-containing precursor suspensions were fine wet-milled. After a thermal treatment step to fix the catalyst support layer, distinct NiO loadings were adjusted by impregnation with appropriate Ni nitrate hexahydrate solutions followed by drying in an air stream at 25 °C and a final sintering step at 900 °C.15 Table 1 describes the composition of the catalytic layers of the prepared catalytic filter candles with the corresponding notations. Determination of the BET Surface. The BET (specific) surfaces of the catalytically activated filter candles were measured by use of monolithic pieces with 8 mm × 10 mm dimensions on a Quantachrome autosorb-3 with krypton as the test gas. The corresponding experimental error was 0.005 m2/g. Measurement of Catalytic Activity. The performance of the fixed bed type catalytic filter candle was determined by measuring the catalytic activity of a 10 mm bed of catalyst Mg6Ni(S) under the target operating conditions of the full-size candle. The measurements were performed under the same experimental conditions as described in the previous work using

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a quartz glass tube reactor with a 7 mm inner diameter and a bed height of 10 mm.14 An electrically heated tube reactor, a mass spectrometer, and a model biomass gasification gas composed of 50 vol % N2, 12% CO, 10% H2, 11% CO2, 5% CH4, and 12% H2O with a naphthalene concentration of 5 g/Nm3 for short- and long-term tests and 10 g/Nm3 for short-term tests were used. The tar model compound naphthalene was introduced by passing N2 gas through a heated packed bed of naphthalene crystals. At all examined temperature points between 700 and 900 °C, a constant GHSV (gas hourly space velocity) value of 2080 h-1 was adjusted at a superficial velocity of 90 m/h. The catalytic activity of the five full-size catalytic layer type catalytic filter candles was determined by measuring the naphthalene conversion on a disklike segment with a 28 mm outer diameter and 10 mm thickness that was drilled out from the middle of each 1.5 m long catalytic filter candle and fixed in an Al2O3 reactor tube. This method allowed the direct determination of the catalytic activity of the full-size candle, as a homogeneous catalytic activation over the filter length can be expected by the applied impregnation procedure. The corresponding GHSV at a superficial velocity of 90 m/h was adjusted to 2291 h-1. A measurement error of (2% (absolute percentage) is to be taken into account in the naphthalene conversion measurements. For all activity measurements, a naphthalene inlet content of 5 g/Nm3 was adjusted, if another content was not explicitly given. The naphthalene conversion measurements were performed in the absence of H2S at a constant superficial velocity of 90 or 72 m/h, respectively, at the temperature points 700, 800, and 900 °C by increasing the temperature. H2S deactivation studies were performed by decreasing the temperature from 900 to 700 °C in 50 °C steps. In the case of a fixed bed type catalytic filter candle, 50 h long-term activity tests were performed with the fixed bed catalyst Mg6Ni(S) at a face velocity of 135 m/h at 800 °C in the presence of 100 ppmv H2S, corresponding to a GHSV of 3120 h-1, followed by an increase in the face velocity to 180 m/h (GHSV ) 4160 h-1) for further 50 h to adjust real flow conditions as in the operation of the full-size candle. No pretreatment of the fixed bed catalyst and the catalytically activated filter segments was applied. For activation, the NiOcontaining samples were reduced in situ with a model biomass gasification gas at the corresponding highest temperature point in the case of the short-term tests. Measurement of Differential Pressure. The differential pressure of 1.5 m long, cylindrical, fixed bed design and catalytic layer design catalytic filter candles was examined at 25 °C by using air and adjusting different superficial velocities related to the effective filtration surface of 0.318 or 0.27 m2, respectively. The candles were vertically fixed in an appropriate differential pressure test rig by sealing the front and end part of the filter candle and passing the appropriate volume flows from the inner to the outer surfaces of the candles. The resulting flow resistance was measured as differential pressure with a measurement error of 0.2 mbar. Noncatalytic filter candles with the same dimensions were measured as well, to determine the differential pressure contribution of the integrated catalyst filling or catalytic layer, respectively. Results and Discussion Catalytic Activity of the Filter Candle in the Fixed Bed Design. As a result of screening different developed Ni catalysts for integration in a ceramic filter element for use in hot gas cleaning in previous work, a MgO-supported Ni catalyst denoted

Mg6Ni(S) was found to exhibit the highest performance in naphthalene reforming.14 In the presented 100 h long-term test of a 10 mm thick fixed bed of Mg6Ni(S) under model biomass gasification conditions in the presence of 100 ppmv H2S and applying a superficial velocity of 90 m/h at 800 °C, complete naphthalene conversion was found at an naphthalene inlet concentration of 5 g/Nm3. With this result, the general feasibility to integrate Mg6Ni(S) as a fixed bed catalyst into the hollow space between a ceramic hot gas filter candle and a porous inner tube was shown, as still an acceptable differential pressure was found.14 Therefore, in this work, this fixed bed design of a catalytic filter candle was further developed to a full-size candle. For hot gas cleaning at 800 °C in raw syngas containing H2S, the volume flow per catalytic filter candle should be maximized to reduce the size and the cost of a catalytic hot gas filter. For this reason, in comparison to the long-term test at 90 m/h in the previous work,14 additional 50 h long-term tests of the Ni catalyst Mg6Ni(S) at higher superficial velocities of 135 and 180 m/h were performed (not shown in a figure). During the first 50 h run at a superficial velocity of 135 m/h, complete naphthalene conversion was found in the presence of 100 ppmv H2S. After an increase in the superficial velocity up to 180 m/h for further 50 h, no deactivation of the Ni catalyst was visible. Only a slight decrease in the naphthalene conversion from complete conversion to an average conversion of 99.3% was measured, which is due to the lower residence time of 0.2 s as compared to 0.27 s in the 10 mm thick catalyst bed. The results show the suitability of a hot gas filter candle with 70/50/30 mm (outer diameter/inner diameter/outer diameter of inner tube) dimensions with an appropriate average superficial velocity of 135 m/h through the 10 mm thick integrated fixed bed catalyst to achieve complete naphthalene conversion. The corresponding superficial velocity related to the outer surface of the candle is then 72 m/h at 800 °C, well within the typical range for hot gas filtration between 60 and 120 m/h. When passing a volume flow of 22.9 m3/h through the candle at a superficial velocity of 72 m/h, the differential pressure of the catalytic filter candle of fixed bed design is only 23 mbar at 25 °C, an acceptable value for hot gas cleaning. In comparison to a reported commercial Ni reforming catalyst,19,20 which shows 97% tar conversion in a short-term test at comparable GHSV values between 3063 and 4582 h-1 (residence time, 0.2-0.3 s), 800 °C, and a tar inlet content of MgAlSiFe-35Ni (46%), MgAl108Ni (45%), MgAl-189Ni (43%) It is noteworthy that at 800 °C the lowest active sample MgAl30La-63Ni of these SiC filter candle series in the absence of H2S (Figure 3) is the most catalytically active system in the presence of H2S. This can be explained by analyzing the extent of deactivation in dependence of the composition of the catalytic layer system. In the following, the sequence of decreasing H2S deactivation at 800 °C is shown with the conversion reduction in absolute percentage in parentheses: MgAl-108Ni (48.6%) > MgAlSiFe-35Ni (37.4%) > MgAl-189Ni (33.8%) > MgAl30La-63Ni (13.7%) This sequence shows that in general a higher catalytically active system is more strongly deactivated by H2S due to a larger Ni active surface and its better accessibility. A completely different result is found in sample MgAlZr-194Ni using the new more porous filter element support. Distinctly higher naphthalene conversions were measured at reaction temperatures g750 °C with 74% naphthalene conversion at 800 °C and 96% conversion at 900 °C. The extent of H2S deactivation at the target operating temperature of 800 °C was only 22.7%. This is a surprising result. Evidently, the new ZrO2doped MgO-Al2O3 catalyst support layer deposited on this new filter element support seems to have a strong influence in inhibiting H2S deactivation, as potentially a similarly high deactivation extent as in sample MgAl-108Ni (48.6%) could theoretically be possible. It is to be noted, too, that the corresponding BET value of sample MgAlZr-193Ni of 0.61 m2/g is lower than the one of sample MgAl-189Ni (1.11 m2/ g) with a similar NiO loading. This is a confirmation of the above finding that not in every case the specific surface can be correlated with the catalytic activity. From this, it can be

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Figure 5. Naphthalene conversion of two catalytically activated filter elements as a function of the reaction temperature at different superficial velocities.

Figure 6. Naphthalene conversion of two catalytically activated filter elements in the presence of H2S as a function of the reaction temperature and different superficial velocities.

concluded that it is possible to increase the naphthalene conversion up to 74% in the presence of 100 ppmv H2S at 800 °C, if a basic oxide-supported Ni catalyst system as a reforming catalyst with a suitable NiO loading is deposited on a filter element support with increased porosity. For comparison, in the previous work,14 it was shown that if using a MgO-supported Ni catalyst, this catalyst shows no deactivation at 800 °C in the presence of 100 ppmv H2S in a long-term test of 100 h at the same face velocity of 90 m/h. This result is due to the 2.5-fold higher Ni content in this fixed bed catalyst as compared to the catalytic layer system MgAlZr-194Ni. It is assumed that in this fixed bed Ni catalyst a large active and accessible Ni surface is provided, so that 100 ppmv H2S only deactivates one part of the active Ni surface. A sufficient part of the active surface is kept free from H2S blocking under these conditions to allow complete reforming of naphthalene. However, in the case of a catalytic layer type catalytic filter candle, the Ni catalyst amount that can be integrated is limited. Thus, also, alternative ways to increase the naphthalene conversion at 800 °C in the presence of H2S have to be examined like the increase of the residence time by reducing the superficial velocity from 90 to 72 m/h. Appropriate naphthalene conversion examinations in the absence (Figure 5) and presence of 100 ppmv H2S (Figure 6) were performed using the most catalytically active SiC-based filter candle MgAl30La-63Ni and the more porous mSiC-based catalytic filter candle MgAlZr-194Ni. Without H2S, an increase of the naphthalene conversion by 16% absolute percentage is found in the case of MgAl30La-63Ni at 800 °C, if the superficial velocity is reduced from 90 to 72

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m/h (Figure 5). In case of the candle MgAlZr-194Ni, a naphthalene conversion increase by only 1% up to 97% conversion is achieved at 800 °C when the superficial velocity is decreased from 90 to 72 m/h. Evidently, a highly active catalytic layer system seems to profit in a distinctly lesser extent from the superficial velocity decrease than a lower active catalytic layer system. In the presence of 100 ppmv H2S, an increase of the naphthalene conversion by 16% at 800 °C is found for the catalytic layer system MgAl30La-63Ni (Figure 6). Thus, the 20% face velocity reduction has the same activity-increasing effect in the presence of H2S as in its absence. An activityincreasing effect of the 20% face velocity reduction is also found in case of the catalytic layer system MgAlZr-194Ni, leading to an increase of the naphthalene conversion by 13%, so that a maximum naphthalene conversion of 87% is achieved. There is an additional benefit in a 20% differential pressure reduction in both examined catalytic layer design tar reforming filter elements exhibiting a differential pressure of 18.5 or 15.4 mbar, respectively, at 25 °C and a superficial velocity of 90 m/h. A further increase of the catalytic activity should be possible by either using a more porous ceramic filter element support or increasing the amount of porous filter element material and surface, allowing for higher NiO loading. Moreover, the use of an H2S-resistant noble metal exhibiting a higher reforming rate than Ni is to be examined, too. Additionally, there is the possibility to enhance the catalytic activity of developed catalytic layers by minor O2 addition as was shown in a 250 h longterm test.23 Conclusions Two types of full-size catalytic filter candles were presented and examined for their catalytic activity for hot gas cleaning of biomass-derived syngas by laboratory testing on small samples of the full-size units. The fixed bed design catalytic filter candle has proven higher catalytic performance than the most catalytically active candle of the series of five developed catalytic layer design catalytic filter candles. Complete naphthalene conversion at 800 °C in a 50 h long-term test was found in the presence of 100 ppmv H2S at a typical superficial velocity of 72 m/h, whereas the most catalytically active mSiC-based candle of catalytic layer type with relatively high porosity shows 74% naphthalene conversion in a short-term test at a superficial velocity of 90 m/h. This result is explained with the integrated higher and better accessible Ni catalyst amount for naphthalene reforming in the fixed bed design catalytic filter candle, so that pilot testing of this type of catalytic filter candle at a superficial velocity of 72 m/h is recommended on the basis of the present results. A reduction of the superficial velocity from 90 to 72 m/h in the case of the most active catalytic layer type catalytic filter candle leads to a substantial increase of the naphthalene conversion up to 87%. This result shows the suitability of the catalytic layer design for pilot testing, too, if the catalytic activity could be further improved, for instance, by decreasing the NiO loading on a ZrO2-doped MgO-Al2O3 support, or other measures that were identified. Acknowledgment We are grateful to Jana Bullinger for the preparation work and characterization of the filtration properties.

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ReceiVed for reView September 11, 2009 ReVised manuscript receiVed April 26, 2010 Accepted April 28, 2010 IE901428B