Fuel Processing Technology 152 (2016) 116–123 Contents lists available at ScienceDirect Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc Research article Tar abatement for clean syngas production during biomass gasification in a dual fluidized bed L.F de Diego a,⁎, F García-Labiano a, P Gayán a, A Abad a, T Mendiara a, J Adánez a, M Nacken b, S Heidenreich b a b Department of Energy and Environment, Instituto de Carboquímica (ICB-CSIC), Miguel Luesma Castán 4, 50018 Zaragoza, Spain Pall Filtersystems GmbH Production Site Schumacher, Zur Flügelau 70, 74564 Crailsheim, Germany a r t i c l e i n f o Article history: Received 30 March 2016 Received in revised form 26 May 2016 Accepted 31 May 2016 Available online xxxx Keywords: Biomass gasification Syngas cleaning Dual fluidized-bed Catalytic filter a b s t r a c t Syngas obtained from biomass gasification needs to fulfil strong purity requirements before being used as raw material in power energy generation or chemicals manufacturing The use of hot catalytic filter candles inside the freeboard of fluidized bed gasifiers allows obtaining clean syngas without dust and low tar content The tar removal efficiency of four different catalytic filter designs was evaluated with real biomass tar produced in situ in a dual fluidized bed gasifier (DFBG) The tar conversion reached at the outlet of the fluidized bed gasifier was larger for the candles with catalytically active layer design If a monolith is also incorporated, the tar conversion increases up to 95% which is one of the highest values obtained up to date In this case, the tar content at the outlet of the catalytic filter was as low as 0.2 g/Nm3 (N2 free, d.b.) © 2016 Elsevier B.V All rights reserved Introduction Biomass gasification represents a promising technology to produce energy from a renewable source with zero CO2 emissions Gasification allows transforming biomass in a gas with high content of H2 and CO which account for more than 70% of the energy stored in the biomass Among the available technologies for biomass gasification, dual fluidized bed gasifiers (DFBG) allow reaching high gasification efficiencies [1], as it has been shown in some operating gasification plants in Austria [2] and Sweden [3] In a DFBG, steam gasification takes place in a bubbling fluidized bed (BFB) where biomass is converted to syngas Following this, the residual char is transferred to a circulating fluidized bed (CFB) which acts as a combustor, where the char is oxidized and therefore heat is generated to be used in the subsequent gasification process Nevertheless, other gasification products also present in the gasification gas can lead to operational problems in the further use of the syngas generated as raw material in power energy generation or chemicals manufacturing One of these products is the solid particles leaving the fluidized bed In recent years, the use of ceramic and metallic filters for particle filtration at hot conditions has been investigated [4–6] Another product is tar, composed by those organic compounds with a molecular weight larger than benzene [7] In order to prevent tar condensation and therefore fouling, it is desirable that the tar content is decreased down to 30 mg/Nm3 or even lower if the gasification gas is to be used in downstream units such as gas engines or turbines [8] ⁎ Corresponding author E-mail address: ldediego@icb.csic.es (L.F de Diego) http://dx.doi.org/10.1016/j.fuproc.2016.05.042 0378-3820/© 2016 Elsevier B.V All rights reserved If the gas is intended for syngas or methanol production or for use in a fuel cell, then more severe restrictions are applied and the tar content should be further reduced to values between 0.1 and mg/Nm3 [9] In recent years, catalytic hot gas filters for tar abatement have been developed as a cost-effective way to upgrade biomass gasification gas [10–12] A catalytic filter candle is normally placed in the freeboard of a fluidized bed where gasification takes place The incorporation of a catalytic filter inside the gasifier presents several advantages On one hand, it contributes to maintain the thermal efficiency of the biomass conversion process and on the other hand, particle entrainment is avoided Therefore, a hot and clean gas is obtained at the outlet of the gasifier with reduced investment costs Three different types of manufacture processes for catalytic filters have been described in literature [13,14] i Incorporation of a catalytic component in the ceramic grain and binder mixture during the ceramic filter manufacture process ii Modification of the design of the ceramic filter by including a porous inner tube fixed at the head of the filter candle to allow the integration of a catalyst fixed bed iii Catalytic coating on the porous support of a conventional hot gas ceramic filter The first process was early discarded due to the low surface area of the catalytic filters produced The high temperatures used in the manufacturing process led to grain sintering and therefore to losses in the active surface of the catalyst [13] Catalytic filters produced under the other two processes have been optimized and tested under different L.F de Diego et al / Fuel Processing Technology 152 (2016) 116–123 conditions Fig presents a scheme of the different configurations of these two catalytic filters Catalytic filters with a catalyst fixed bed (FB) are shown in Fig 1A They present a high catalytic potential given their flexibility to integrate a custom-made tar reforming catalyst and their capability to integrate high amount of this catalyst in the hollow cylindrical space inside the filter candle considering the limitations imposed by the total weight of the candle and the price of the catalyst incorporated However, face velocities referred to the outer surface of the catalytic filter candle should be limited to allow enough residence time for the catalytic tar reforming reaction [15] Studies about the optimum composition of the fixed bed catalyst can be found in literature Nacken et al [13] tested several tar reforming catalyst systems of different NiO loadings They evaluated the effect of the variation of the catalyst support material, the preparation conditions, the NiO loading and the effect of doping with ruthenium on the catalytic activity of the tar reforming catalyst The catalytic activity tests were conducted using naphthalene as model tar compound The highest catalytic reforming activity was found for a MgO supported Ni catalyst with a NiO loading of wt% With this catalyst, complete naphthalene conversion at 800 °C during 100 h operation even in the presence of H2S was reached Therefore, this catalytic filter was tested in a larger scale A catalytic filter candle of adequate dimensions was manufactured and inserted in the freeboard of a bubbling fluidized bed gasifier where crushed almond shells were used as feedstock [17] Gas and hydrogen yields were notably increased with the use of this catalytic filter and tar content at the outlet of the catalytic filter was between 0.7 and 0.95 g/Nm3 (N2 free, d.b.) Besides, stable performance of the filter was observed after 22 h of gasification The catalytic filters with catalytic coating on the porous support of the conventional hot gas ceramic filter are denoted as catalytic layer filters (CL) (Fig 1B) This design of catalytic filter had already been tested for combined particle separation and NOx removal from laboratory [18] to pilot scale [19] The advantages of the catalytic layer filters is that higher face velocities can be used compared to the fixed bed catalytic filters and therefore the size and weight of the catalytic filter could be reduced This also implies that for the same outer diameter and superficial velocity higher residence time can be achieved when compared to the fixed bed catalytic filters Besides a simplification of the manufacture process compared to the fixed bed filters is also expected [15] The possibility of integration of a tar reforming catalyst as a catalytic layer by catalytic activation of 10 mm thick alumina based filter disks was first 117 demonstrated [10,20,21] Then, several studies were carried out aiming at finding suitable catalytic systems for the pore walls of ceramic filters which combine high surface support materials and active catalysts [11, 15] In these studies, MgO and CaO-Al2O3 were used as supports as well as MgO-CaO and MgO-Al2O3 In some cases, they were doped with La2O3, olivine or ZrO2 In all cases, the coated filters were catalytically activated by impregnation with the appropriate aqueous solution of nickel nitrate hexahydrate to adjust the NiO loading amounts of and 60 wt% related to the amount of catalyst support [15] The catalytic activity was evaluated in all the cases using naphthalene as model tar compound Promising materials were selected to manufacture filter candles to be tested in the freeboard of a bubbling fluidized bed [22] In these experiments, tar conversion extent obtained by means of the catalytic filter was around 58% with final tar contents in the gas around 0.8 g/Nm3 (N2 free, d.b.) Methane was also partially converted (28%) As a result, a significant increase in the gas yield (15%) and in hydrogen concentration was reported Modifications and improvements of the first design of catalytic layer filters (CL) have been recently presented First, the replacement of SiC as filter material with another material which could withstand the high gasifier freeboard temperatures (between 800 and 850 °C) was accomplished SiC was initially used due to the high heat conductivity and good thermal shock resistance for cyclic back pulse cleaning of the catalytic filter [16] However, it was replaced by Al2O3 which allowed long operating times at 850 °C [23] One of the new configurations for catalytic layer filter candles included an additionally integrated catalyst: a catalytically activated Al2O3-based hollow-cylindrical monolith integrated in the hollow cylindrical space of the catalytic filter candle [24] The incorporation of the monolith increased the Ni load of the catalytic filter Promising results were obtained with this new design of catalytic layer filter candles (CL + M) Using the same experimental apparatus at the same operating conditions [23], tar conversion of 93.5% was reached with the use of the catalytically activated monolith in comparison with the 58% tar conversion of the catalytic layer SiC candle Results were also better than those obtained for SiC candle of fixed bed design, where 79% tar conversion was obtained The final tar content of the clean gas was around 0.25 g/Nm3 (N2 free, d.b.) A catalytically activated ceramic foam as additional reforming step for integration into the hollow-cylindrical space of the catalytically activated filter candle was used (CL + Foam) The catalytic activity of this combination at different superficial velocities was first examined using naphthalene as model tar compound [25] Based on these results, a catalytic filter of combined Fig Scheme of configurations of catalytic filters: (A) fixed bed and (B) catalytic layer Adapted from Hackel et al [16] and Nacken et al [15] 118 L.F de Diego et al / Fuel Processing Technology 152 (2016) 116–123 design was developed allowing a reduction of the manufacturing costs compared to those previously reported for catalytic filter candles of fixed bed design, because the catalyst grain filling procedure to realize the fixed bed was avoided [24] The combined catalytic filter was tested with naphthalene as model tar compound and also in experiments in a bubbling fluidized bed gasifier [12] showing good performance The hydrogen content was increased up to 56% (from 39% without catalytic filter) and the tar content was equal to 0.14 g/Nm3 (N2 free, d.b.) In the present work, a comparison of the tar abatement performance of catalytic filters with different designs is presented Four catalytic filter designs were tested: fixed bed (FB), fixed bed with catalytically active inner tube (FB + CL), catalytic layer (CL) and catalytic layer with additional monolith (CL + M) The fact that not a biomass tar model compound but real biomass tar produced in situ in a dual fluidized bed gasifier was used in all the catalytic activity tests adds novelty and applicability to the results presented Experimental 2.1 Dual fluidized bed gasification plant Biomass gasification was carried out in a bench-scale dual fluidized bed gasification plant located at ICB-CSIC (Fig 2) and described in a previous work from the authors [26] The gasification plant consisted of two interconnected fluidized beds The gasifier was a bubbling fluidized bed where biomass was fed in The biomass used was pine wood with an average particle size of 0.5–2.0 mm The proximate and ultimate analyses of biomass are shown in Table Steam was used as gasifying agent for biomass The gasifier bed consisted of Fe/olivine in the size range 0.1–0.25 mm Fe/olivine material was prepared by impregnation Iron nitrate (Fe(NO3)3·9H2O) was dissolved in heated water and olivine was added to the iron aqueous solution The excess water was Table Proximate and ultimate analysis of pine wood (wt.%) Moisture Ash Volatiles Fixed carbon C H N S Cl High heating value (kJ/kg) 6.30 1.10 77.30 15.40 46.60 6.00 0.20 0.004 0.002 18,235 eliminated and the sample dried before being calcined for h at 1000 °C The final content of Fe in the Fe/olivine used was 16% A more detailed description of the preparation method of the Fe/olivine can be found elsewhere [26] After biomass gasification, the solids leaving the gasifier were transferred to the combustor through another bubbling fluidized bed acting as a loop seal to avoid mixing gaseous atmospheres The char which was not gasified was burned in the combustor Hot particles were then returned to the gasifier through a riser The tar produced in situ during biomass gasification was used in the catalytic activity tests of the different filter candles used in the present work The catalytic filters were located downstream the gasifier prior to the tar measurement as it is shown in Fig The filter was placed inside a reactor and heated by a furnace to control the temperature inside the filter Once the tar has been collected for measurement, several gas analysers were used to determine the composition of the gas product streams: CO, CO2 and CH4 concentration was measured in a non-dispersive infrared (NDIR) analyser and H2 using a thermal conductivity detector Moreover, the presence of C2-C3 hydrocarbons was also analysed off-line using a gas chromatograph (HP 5890) with a Poropack Fig Scheme of the dual fluidized bed gasifier L.F de Diego et al / Fuel Processing Technology 152 (2016) 116–123 N column It was possible to by-pass the catalytic filter in order to determine the composition of the gaseous stream at the inlet of the catalytic filter 2.2 Tar sampling and analysis Tar sampling and analysis was based on the European Tar Protocol [27] Moisture and tar were collected in impingers filled with isopropanol Water content of the tar was determined using the Karl-Fischer titration method (CRISON Titromatic KF1S) A gas chromatograph (Agilent 7890A) coupled with a mass spectrometer (Agilent 5975C) was used in the determination of the concentration of the different tar compounds in the samples collected from the impingers The GC was fitted with a capillary column (HP-5) and a flame ionization detector 2.3 Catalytic filters Four different catalytic filter designs were tested in the biomass gasification unit previously described Their scheme is shown in Fig All of them were supplied by Pall Filtersystems GmbH with the following specifications: - DeTarCat FB (Fixed Bed) DeTarCat FB + CL (catalytically active inner tube) DeTarCat CL (Catalytically active Layer) DeTarCat CL + M (catalytically active monolith inside) In all cases, a 21 mm-height filter segment taken from the corresponding full-size filter candle was used for testing Results can be considered as representative on small-scale of the behaviour of the full size filter candle The segment was covered with ceramic caps at the bottom and upper part of the filter In the catalytic filter fixed bed design (DeTarCat FB), MgO powder with a BET surface of about 0.15 m2/g and catalytically impregnated with NiO was filled as fixed catalyst bed into the cylindrical space between two porous silicon carbide tubes The silicon carbide tube has an open pore volume of 38 vol% In a second embodiment of the fixed 119 bed design, the inner porous tube was made of silicon carbide and additionally catalytically activated by a MgO-NiO coating (DeTarCat FB + CL) For the DeTarCat CL design, a porous alumina based filter tube with an open pore volume of 45 vol% was catalytically impregnated with MgO-Al2O3 supported NiO In the fourth design tested, the catalyst amount of the catalytic layer design was further increased by integration of an alumina foam tube into the interior of the alumina filter element tube (DeTarCat CL + M) The alumina foam tube with an open pore volume of 71 vol% was also catalytically impregnated with MgO-Al2O3 supported NiO Table presents a summary of the main characteristics of the four catalytic filter designs tested 2.4 Experimental plan Table summarizes the tests performed with the different catalytic filters tested Once the steady state was reached, tars produced in the gasifier were measured bypassing the catalytic filter This measurement was considered as a reference test and corresponds to the tar composition at the catalytic filter inlet After the reference was set, the gasification gas was forced to pass through the catalytic filter Tar measurements at the outlet of the catalytic filter were then performed and the clean gas was sent to the analysers To determine the tar conversion efficiency of the catalytic filter, the tar reference data were compared to the tar measurements after passing through the catalytic filter It was intended that the amount and composition of tar at the inlet of the catalytic filter were similar for all the experiments performed Two gasification parameters were maintained roughly constant in order to reach this condition First, the temperature in the biomass gasifier was set to 800 °C in all the cases The second parameter was the H2O/biomass ratio In our experiments, it varied between 0.52 and 0.68, which produced a variation in the characteristics of the tar produced in the gasifier and it was considered in the treatment of the results Regarding the catalytic filter operating conditions, the influence of two parameters was evaluated On one hand, two temperatures of the catalytic filter were tested, 800 and 850 °C, according to the recommendations made by the supplier On the other hand, the face velocity was varied in order to determine its influence on the performance of the Fig Gas path through the catalytic filter segments for the four different catalytic filter designs: (A) fixed bed (FB), (B) fixed bed with catalytically active inner tube (FB + CL), (C) catalytic layer (CL) and (D) catalytic layer with additional monolith (CL + M) Ceramic filter ; catalytic fixed bed ; catalytic layer ; catalytic foam ; ceramic caps (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 120 L.F de Diego et al / Fuel Processing Technology 152 (2016) 116–123 Table Characteristics of the catalytic filters used DeTarCat Fixed bed design Catalytic filter configuration Sample notation Filter support Maximum temperature in operation (°C) Filter candle dimensions (A × B × C × D)a (mm) Composition Catalyst support density (g/cm3) FB SiC 800 70 × 50 × 30 × 16 MgO supported Ni 1.7552 (FB) NiO density (g/cm3) 0.1053 (FB) Differential pressure (mbar) (25 °C; face velocity = 90 m/h) 18.5 DeTarCat Catalytic layer design FB + CL SiC 800 70 × 50 × 30 × 16 CL Al2O3 900 60 × 40 1.7086 (FB) 0.0110 (CL) 0.1026 (FB) 0.0124 (CL) 51.6 0.0210 (CL) CL + M Al2O3 900 60 × 40 × 34 × 15 MgO-Al2O3 Ni 0.0147 (CL) 11.1 0.0175 (CL) 0.0483 (M) 0.0144 (CL) 0.0222 (M) 13.4 a FB design: A: catalytic filter outer diameter B: FB outer diameter C: FB inner diameter D: inner tube inner diameter CL design: A: CL outer diameter B: CL inner diameter C: monolith outer diameter D: monolith inner diameter filter for tar abatement The face velocity was defined as the ratio between the gas flow and the filter external area Finally, the cumulative time of each type of filter was also presented in Table The total operation time for each type of catalytic filter is bold marked Results and discussion 3.1 Comparison of tar conversion Fig presents the comparison of the amount of tar and the corresponding tar conversion obtained at the catalytic filter outlet when the four different catalytic filters were used The values are represented versus the corresponding values of face velocities used In Fig 4, closed symbols are used to represent tar reference values and open symbols for the tar content at the catalytic filter outlet The amount of tar in the gas at catalytic filter inlet oscillates between 2.5 and 4.5 g/Nm3 for all the experiments performed In the experiments with the catalytic filter with a fixed bed design (FB), the temperature in the filter was set to 800 °C This was the maximum operating temperature allowed by the manufacturer considering that SiC is the filter support material The effect of the face velocity on the tar content and conversion is clearly seen The highest the face velocity, the larger the tar content at the catalytic filter outlet and therefore, the lower the tar conversion reached This fact can be attributed to a decrease in the residence time of the gasification gas inside the filter when the face velocity increases as it has been observed before by the authors for this type of catalytic filters [28] The tar conversion decreased from 85 to around 50% when the face velocity increased from 40 to 87 m/h At the lowest face velocity tested (40 m/h) the tar content was 0.7 g/Nm3 If an internal catalytically active inner tube is added to the catalytic filter with a fixed bed design (FB + CL), the resulting filter configuration improves tar removal Experiments were performed at the same temperature in the filter as the experiments with the catalytic filter with fixed bed design (FB), i.e 800 °C In this case, the effect of face velocity on the tar content at the catalytic filter outlet is softened The tar content in the experiment at the lowest face velocity (46 m/h) was 0.8 g/Nm3 Tar conversion values decreased with the increase in the face velocity, although the decrease was not as sharp as in the previous experiments with the FB catalytic filter Tar conversion values were around 75% The use of catalytic filters with catalytically active layer (CL) allowed an improvement in tar conversion when compared to FB and FB + CL catalytic filters at 800 °C Moreover, it is possible to operate at higher temperatures than with the other two catalytic filters as Al2O3 is used as filter support material in the CL catalytic filters At 850 °C, the tar content at the outlet of the CL catalytic filter was decreased to 0.3 g/Nm3 This corresponds to a tar removal efficiency of 88%, higher than that found with FB-based catalytic filters It must be also mentioned that the difficulty in the tar removal process increases as tar content decreases The results obtained with the CL catalytic filter were further improved when a monolith was integrated in the hollow cylindrical space of the catalytic filter (CL + M) Actually, the best results in this work were obtained using this catalytic filter configuration Again, temperatures up to 850 °C could be reached with this type of filter, which also contributed to its better performance in tar removal For face velocities around 70 m/h, the tar removal efficiency at 800 °C was 80% and it increased up to 95% at 850 °C The later efficiency value corresponds to a tar content at the outlet of the catalytic filter 0.2 g/Nm3, which can be considered as excellent taking into account the very high tar content at the inlet of the catalytic filter (about 4.5 g/Nm3) These results Table Experimental tests with the four DeTarCat catalytic filter elements Test Type of DeTarCat catalytic filter Tgasifier (°C) H2O/biomass dry (g/g) Tfilter (°C) Face velocity (m/h) Cumulative time (h) 10 11 12 FB FB FB FB FB + CL FB + CL FB + CL CL CL CL + M CL + M CL + M 800 800 800 800 800 800 800 800 800 800 800 800 0.59 0.52 0.52 0.59 0.66 0.66 0.66 0.64 0.59 0.68 0.66 0.66 800 800 800 800 800 800 800 800 850 800 850 850 40 67 75 87 46 70 80 83 72 69 96 67 1.0 2.2 3.2 4.0 1.7 2.9 3.9 1.2 2.5 1.2 2.7 4.0 L.F de Diego et al / Fuel Processing Technology 152 (2016) 116–123 121 Fig (A) Tar amount and (B) tar conversion as a function of the face velocity for the different designs of catalytic filters tested at 800 or 850 °C (Tg = 800 °C) Closed symbols = reference values; open symbols = values at the catalytic filter outlet confirm previous findings by Rapagnà et al [23] which also pointed to the CL + M design for catalytic filters as the most promising for efficient biomass tar removal above the FB or CL catalytic filter designs In the comparison between fixed bed (FB) and catalytically active layer (CL) catalytic filters it should be born in mind that due to their design characteristics higher residence time can be expected for a CL catalytic filter than for the same fixed bed filter with the same candle outer diameter and at the same superficial velocity Considering this, the values of the gas hourly space velocity (GHSV) were calculated for the experiments performed with the different types of catalytic filters tested with the aim of facilitating the comparison Table summarizes the values obtained In the case of FB catalytic filters, larger GHSV values were observed compared to the other three catalytic filters Therefore, shorter residence times for the gasification gas in the catalytic filter can be expected Nevertheless, these values were close to those reported by Nacken et al [11] for similar FB catalytic filters in experiments to evaluate the catalytic activity using naphthalene as model tar compound They performed 50 h long–term tests at 800 °C and observed 100% naphthalene conversion at GHSV of 3120 h−1 and 99.3% conversion at GHSV of 4160 h−1 Another aspect to take into account is the pressure drop in the catalytic filter Measurements were performed during operation with the four types of catalytic filters Results are shown in Fig As expected, higher face velocities lead to a higher pressure drop through the catalytic filter in all types of filters Nevertheless, the pressure drop registered for the FB + CL catalytic filter is notably higher than for the rest of the filters tested It oscillated between 30 and 43 mbar while for the rest the values varied between and 23 mbar in the tests This fact represents an additional disadvantage for the further use of this type of catalytic filter In comparison to that, the pressure drop measured for the CL + M catalytic filter is not especially higher when compared to that of FB and CL catalytic filters, because the catalytically activated monolith creates no additional differential pressure under the applied superficial flow conditions This result together with the high tar conversion obtained in the experiments with this catalytic filter makes this configuration the most promising for further development among all those studied in the present work 3.2 Comparison of syngas and tar composition The composition of syngas and tar at the inlet and outlet of the catalytic filters was measured Selected operating conditions of 800 °C in the catalytic filter and face velocities around 70–80 m/h were chosen in all the cases in order to compare the results for the different filters Table presents the operating conditions and experimental results for the different catalytic filters including the syngas composition in dry and N2 free basis From these values, the H2 production and conversion of the other gases was calculated as the ratio between the variation of moles through the catalytic filter (outlet minus inlet) and the moles at the inlet In all the cases, an increase in H2 at the outlet of the catalytic filter was observed, as it was reported before by other authors [12,22] The largest increment was observed for the FB filter, probably due to a high catalyst amount present in the fixed bed Among the CL catalytic configuration, the largest H2 production was observed for the CL + M filter This result agrees with the observed by other authors when using this type of catalytic filter [23] Tar composition is also plotted in Fig for more clarity In all the cases, the major tar compounds at the inlet of the catalytic filter were naphthalene, indene and biphenylen At the catalytic filter outlet, naphthalene was the major compound and in some cases almost the only tar compound that could be detected in significant level However, it suffered a significant drop during its passage through the catalytic filter Table Gas hourly space velocity (GHSV) Type of DeTarCat catalytic filter Tfilter (°C) GHSV (h−1) FB FB + CL CL CL + M 800 800 800–850 800–850 1800–3900 1470–2550 2090–2545 1550–1915 Fig Differential pressure in the catalytic filter as a function of the face velocity for the different designs of catalytic filters tested at 800 or 850 °C (Tgasif = 800 °C) 122 L.F de Diego et al / Fuel Processing Technology 152 (2016) 116–123 Table Operating conditions and experimental results for the different catalytic filters Catalytic filter Filter temperature (°C) Filter gas velocity (m/h) Filter pressure drop (mbar) Operating conditions Biomass (g/h) H2O/biomass dry (g/g) Gas composition (vol%) Gasifier (N2 free dry basis) CO CO2 H2 CH4 C2H4 C2H6 C3H8 Combustor O2 CO2 Tar (g/Nm3 dry) H2O content (vol%) Tar conversion (%) Tar composition (g/Nm3 dry) Styrene Phenol Benzofuran Indene Naphthalene Biphenylen Fluorene Phenanthrene Anthracene Fluoranthene Pyrene a DeTarCat FB DeTarCat FB + CL Inlet Outlet 800 800 67 16 Xa (%) Inlet Outlet 800 800 70 39 258 0.52 21.7 37.8 27.2 9.2 3.3 0.4 0.3 19.1 32.4 37.6 7.5 2.7 0.3 0.2 15.6 2.7 3.89 42.9 0.21 0.00 0.23 0.96 1.94 0.24 0.04 0.15 0.02 0.05 0.05 DeTarCat CL Xa (%) Inlet Outlet 800 800 83 20 244 0.66 3.3 0.6 62.3 −4.3 −3.9 −11.9 −21.7 15.7 47.0 24.9 8.6 3.1 0.4 0.3 13.7 44.0 32.4 7.0 2.4 0.3 0.2 15.6 2.6 1.12 34.1 71 15.6 1.9 3.04 43.6 0.00 0.00 0.00 0.00 1.11 0.00 0.00 0.00 0.00 0.01 0.00 0.14 0.23 0.07 0.46 1.52 0.33 0.05 0.12 0.03 0.03 0.02 DeTarCat CL + M Xa (%) Inlet Outlet 800 800 77 22 246 0.64 −2.3 4.8 45.7 −8.8 −13.3 −16.0 −25.3 16.9 43.6 27.3 8.5 3.1 0.4 0.2 14.2 42.6 32.9 7.2 2.6 0.3 0.2 15.3 1.8 0.84 37.9 72 16.0 1.9 3.41 44.3 0.02 0.00 0.00 0.03 0.73 0.03 0.00 0.03 0.00 0.00 0.00 0.17 0.28 0.08 0.53 1.64 0.35 0.06 0.15 0.03 0.04 0.03 Xa (%) 244 0.68 −8.8 6.0 31.4 −7.2 −6.2 −7.8 −20.9 18.4 40.5 27.9 9.2 3.3 0.4 0.3 16.6 36.6 35.8 7.7 2.8 0.3 0.2 15.9 1.8 0.93 39.6 73 16.3 1.7 3.42 45.5 15.9 1.8 0.69 34.3 80 0.02 0.00 0.00 0.02 0.81 0.04 0.00 0.03 0.00 0.00 0.01 0.17 0.24 0.08 0.57 1.54 0.32 0.13 0.16 0.04 0.09 0.04 0.00 0.00 0.00 0.01 0.65 0.01 0.00 0.02 0.00 0.00 0.00 1.8 2.0 44.8 −5.5 −4.2 −15.3 −24.7 H2 production or gas conversion Naphthalene conversion was 42.8% for the FB catalytic filter and 52.0% for the FB + CL filter In the case of the catalytic layer filters, naphthalene conversion reached 50.6% for the CL filter and 57.8% for the CL + M filter Fig Tar composition at the inlet and outlet of the catalytic filter for the different catalytic filters tested with Tfilter = 800 °C and face velocity = 70–80 m/h (Tgasif = 800 °C) Considering all the results presented above, some guidelines for future optimization of the use of catalytic filters in tar abatement during biomass gasification could be indicated Obviously, primary measures for tar reduction should be applied in the gasifier in order to decrease the tar concentration at the gasifier outlet (i.e the catalytic filter inlet) as much as possible Temperature and gas flow to be treated are the most important variables affecting the design of a catalytic filter Although high temperatures favor tar conversion, some materials used in catalytic filters manufacture can limit its use at temperatures below 800 °C, i.e SiC In addition, the syngas flow to be cleaned would determine the number of catalytic filters to be used Low face velocities, large filter diameters and wall thickness with increasing catalyst load lead to an increase in the residence time for tar and hydrocarbons in the catalytic filter, favoring tar abatement However, these parameters should be optimized so that a compromise is reached between the tar abatement efficiency and the weight and pressure drop through the filter, considering that the catalytic filter would hang in the freeboard of the gasifier In this sense, the values of the parameters used in this work can be considered as normal in a future application of these catalytic filters in an industrial gasifier Regarding the results obtained in our experiments, the lowest values obtained were 0.2 g/Nm3 This value is still far from the limits set for applications with high quality gas requirements (b1 mg/Nm3) such as methanol production or the combustion in a fuel cell However, it would be easier to reach the specifications for combustion in gas engines or turbines Although different values are given in literature, the maximum allowable concentration would be 100 mg/Nm3 [9] Modeling calculations based on experimental results for the fixed bed catalytic filters presented in this work showed that tar contents below 0.1 g/Nm3 L.F de Diego et al / Fuel Processing Technology 152 (2016) 116–123 can be reached with catalytic filter thickness of 25 mm However, pressure drop and weight should be reduced in this configuration Catalytic layer filter design could reach similar tar abatement efficiency with less restriction regarding pressure drop and weight Conclusions Four different design configurations of catalytic filter for hot gas conditioning have been tested in biomass gasification experiments These are based on different possibilities to include the catalyst: in a fixed bed or inside a catalytically active layer This novel tar abatement technology has been evaluated using real biomass tar produced in situ in a dual fluidized bed gasifier (DFBG) The effect of temperature and face velocity was evaluated in order to optimize tar abatement The most promising catalytic filter design is the catalytic layer integrated with a catalytically activated alumina foam tube (CL + M design) High tar removal efficiencies up to 95% at 850 °C with corresponding tar contents down to 0.2 g/Nm3 have been achieved This design provides a technically feasible solution for combined tar and particulate removal with high performance at acceptable differential pressure under operating conditions Further optimization of the catalytic filter design would be needed in order to use the syngas in gas engines or turbines However, for applications with more restricted requirements, such as methanol production or the use in a fuel cell, additional cleaning downstream the gasifier would be needed Acknowledgments This work was supported by the European Commission (EC Project UNIQUE No 211517-ENERGY FP7-2008/2011) and the Spanish Ministry MINECO (ENE2014-56857-R) T Mendiara thanks for the “Ramón y Cajal” post-doctoral contract awarded by the Spanish Ministry of Economy and Competitiveness Cristina Igado is also acknowledged for her contribution to the experimental work References [1] G Xu, T 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