Biomass Gasification with Air in a Fluidized Bed - ACS Publications

702

Energy & Fuels 1999, 13, 702-709

Biomass Gasification with Air in a Fluidized Bed: Exhaustive Tar Elimination with Commercial Steam Reforming Catalysts Jose´ Corella,* Alberto Orıó, and Jose-Manuel Toledo Department of Chemical Engineering, University ‘Complutense’ of Madrid, 28040 Madrid, Spain Received October 15, 1998

Seven different commercial nickel-based catalysts for steam reforming of light hydrocarbons and of heavier hydrocarbons were tested for tar removal in a flue gas from an atmospheric fluidized bed biomass gasifier, using air as the gasifying agent. The catalysts were provided by BASF AG, ICI-Katalco, Haldor Topsoe a/s, and United Catalyst Inc. The facility used is a small pilot plant, and the catalytic reactor operates in full flow with a real gasification gas. A guard bed with a calcined dolomite is used to decrease the tar content at the inlet of the catalytic bed to a level below 2 g/m3n. The variables studied include the temperature (730-850 °C) of the catalytic bed, gas residence time, steam content in the flue gas, and composition of the reacting atmosphere. All catalysts for steam reforming of naphthas provide a similar and very high activity. Values of the apparent activation energy and preexponential factor are given and analyzed for the most active catalysts. The catalyst life is also studied. No deactivation is observed with times-onstream of up to 65 h.

Introduction It is well-known in the field how the raw gas produced in biomass gasification in a fluidized bed has to be cleaned of tars and particulates for most of its applications. Due to the very low content of chlorine and heavy metals in biomass, these components are not usually a problem in the thermochemical processing of biomass. Elimination of tars and particulates requires two different technologies. This paper is concentrated on tar removal only. Between wet and hot gas cleaning, the latter is preferred because it really destroys the tars instead of transferring them to a liquid phase, which would need further and expensive treatment. Among the possible hot gas cleaning methods, two are emerging and being adopted by most of the institutions and companies working on biomass gasification: they are based on the use of calcined dolomites or of steam reforming (nickel-based) catalysts located downstream from the biomass gasifier. This paper is concentrated on using these nickel-based catalysts only. The state-of-the-art methods for steam reforming catalysts for hot gas cleanup and upgrading in biomass gasification have already been reviewed.1-3 To compare (1) Narvaéz, I.; Corella, J.; Orıó, 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 (2), 317-327. (2) Caballero, M. A.; Aznar, M. P.; Gil, J.; Martıń, J. A.; France´s, E.; Corella, J. Commercial Steam Reforming Catalysts to Improve Biomass Gasification with Steam-Oxygen Mixtures. 1. Hot Gas Upgrading by the Catalytic Reactor. Ind. Eng. Chem. Res. 1997, 36 (12), 5227-5239. (3) Simell, P. Catalytic Hot Gas Cleaning of Gasification Gas. VTT Publication No. 330; Technical Research Centre of Finland: Espoo, Finland, 1997.

the results from different institutions on the activity of different catalysts, a single first-order (regarding tar content) kinetic equation was proposed for tar elimination4,5 and adopted by the institutions working in Europe in this matter under the EU financed projects AIR2-CT93-1436 and JOR3-CT95-0053. According to such a kinetic model, the catalytic activity for tar removal depends (i) on the composition of the reacting gas atmosphere and (ii) on the tar composition which, in turn, depends on the design and operating conditions of the upstream gasifier which generates such tar.6,7 Catalytic tar elimination over nickel-based catalysts mainly proceeds by steam and dry (CO2) reforming reactions, although there can be simultaneous thermal reactions of cracking and, perhaps, of hydrocracking.8 The steam and CO2 contents in the flue gas are going to have an important role in the overall tar elimination, and the effectiveness of a catalyst for tar removal can (4) Corella, J.; Narvaéz, I.; Orıó, A. Criteria for Selection of Dolomites and Catalysts in Biomass Gasification. In New Catalysts for Clean Environment; VTT Symposium 163; Maijanen, A., Hase, A., Eds.; VTT: Espoo, Finland, 1996; pp 177-183. (5) Corella, J.; Narvaéz, I.; Orıó, A. Fresh Tar (from Biomass Gasification) Destruction with Downstream Catalysts: Comparison of their Intrinsic Activity with a Realistic Kinetic Model. In Power Production from Biomass II; VTT Symposium 164; Sipilla¨, K., Korhonen, M., Eds.; VTT: Espoo, Finland, 1996; pp 269-275. (6) Narvaéz, I.; Corella, J.; Orıó, A. Biomass Gasification with Air in an Atmospheric Bubbling Fluidized Bed. Effect of Six Operational Variables on the Quality of the Produced Raw Gas. Ind. Eng. Chem. Res. 1996, 35 (7), 2110-2120. (7) Corella, J.; Caballero, M. A.; Gil, J.; Aznar, M. P.; Brage, C. Seminar on Power from Biomass III, Espoo, Finland, Sept. 14-15, 1998. (8) Aznar, M. P.; Caballero, M. A.; Gil, J.; Martıń, 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 (7), 2668-2680.

10.1021/ef980221e CCC: $18.00 © 1999 American Chemical Society Published on Web 03/02/1999

Biomass Gasification with Air in a Fluidized Bed

be different for different gas compositions. The composition of the flue gas depends, of course, on several factors, but the most important one is the gasifying agent used. The effectiveness of some nickel-based catalysts in biomass gasification with pure steam and with steamO2 mixtures have been shown in previous papers.2,8,9 In the present work, the activity of the catalysts is studied only in biomass gasification with air (with some moisture in the flue gas, of course). Focusing this study on catalytic hot gas clean up in biomass gasification with air, the effect of many operation variables has been studied by Narvaéz et al.1 using the G1-25S catalyst from BASF AG. In such work, as well as in the simultaneous work carried out by Simell in Finland,3 much knowledge was already gained for this application. Nevertheless, in the market there are many commercial steam reforming catalysts which could be better than the G1-25S one. For this reason, seven other commercial catalysts were obtained from four manufacturers (BASF AG, Haldor Topsoe a/s, ICIKatalco, United Catalysts Inc.). Testing these catalysts to check their usefulness in hot gas clean up is now the main objective of this paper. The catalyst screening is carried out in this work with a real gasification gas and with experimental conditions similar to those foreseen for future commercial application. The change in the composition of the flue gas by the catalytic reactor being already well-known,10 this paper is focused only on the catalytic removal of the tar.

Energy & Fuels, Vol. 13, No. 3, 1999 703

Figure 1. Tar* conversion over different types of Ni catalysts (for naphtha or methane reforming) vs a pseudo space time at different averaged temperatures of the bed of catalyst (T3,c ) 660-850 °C) and different steam-carbon ratios (H2O/C* ) 0.8-6.1 mol/at-g) on the gas stream.

Experimental Section The facility used is a small pilot plant based on a bubbling and atmospheric fluidized-bed gasifier with a 6.0 cm i.d. with a continuous feeding of biomass (near the gas distributor plate) of around 1 kg biomass/h. This facility has been already described in detail.6,10 In the experiments shown in this paper, the gasifier bed was only silica sand (with some char formed in the gasification) without dolomite. The biomass used was small pine wood chips whose characterization can be found in previous work.1 After the gasifier there is a hot (500-550 °C) ceramic candle filter and then a guard bed with calcined dolomite in it whose activity has been shown by Orıó et al.11 This guard bed of dolomite not only decreases the tar content (till about 1 g/m3n) but also changes the tar composition and reactivity. The main experimental conditions in the biomass gasifier and in the bed of dolomite have been shown in a previous paper.10 Safety precautions were taken into account, of course, because of the high H2 and CO contents in the flue gas and the toxicity of the tar obtained. The flue exit gas was burnt in a torch out of the building without any problem, the people doing the tests had to follow some precaution rules, and there was an extractor up of the pilot plant with a high flow rate (speed fan). Catalytic Bed. The catalytic bed has been described in detail by Corella et al.10 It was located in the facility to receive the full flow generated in the gasifier. The catalytic reactor (9) Aznar, M. P.; Corella, J.; Delgado, J.; Lahoz, J. Improved Steam Gasification of Lignocellulosic Residues in a Fluidized Bed with Commercial Steam Reforming Catalysts. Ind. Eng. Chem. Res. 1993, 32 (1), 1-10. (10) Corella, J.; Orıó, A.; Aznar, M. P. Biomass Gasification with Air in Fluidized Bed: Reforming of the Gas Composition with Commercial Steam Reforming Catalysts. Ind. Eng. Chem. Res. 1998, 37 (12), 4617-4624. (11) Orıó, A.; Corella, J.; Narvaéz, I. Performance of Different Dolomites on Hot Raw Gas Cleaning from Biomass Gasification with Air. Ind. Eng. Chem. Res. 1997, 36 (9), 3800-3808.

Figure 2. Tar* conversion over different Ni catalysts (for naphtha or methane reforming) vs averaged temperature of the bed of catalyst at different pseudo space times (0.0140.040 kg/(m3/h)) and different steam-carbon ratios (H2O/C* ) 0.8-6.1 mol/at-g) on the gas stream. had an internal diameter of 60 mm. It had two thermocouples in it, in the center and in the wall (inner side). Such two thermocouples were moved at least once during the experiment along the height of the bed to measure the temperature profiles, which have been shown in a previous paper.10 Due to the relatively big size of the catalytic reactor, the differences in temperature between the axis and the wall and between the inlet and exit were about 5-20 °C. These gradients were always measured, and they are taken into account in the analysis of the reaction kinetics. For this reason, abscissas in Figures 4-8 indicate an interval and not just a point. Besides the catalyst type, the main operating variables and their ranges studied in this work have been the temperature of the catalytic bed (T3,c,av), between 730 and 850 °C; the gas residence time, expressed as space time (τ′3) or as space velocity (SV3), between 0.010 and 0.035 kg/(m3/h) or between 24 000 and 80 000 h-1 (reactor temperature), respectively; the steam content in the flue gas, between 7 and 30 vol %; and the (H2O/C*) ratio in the flue gas, C* being the amount of molecules (given as at-g) susceptible to being reformed (CH4, C2, C3, and tars) in this process, between 0.8 and 5.2 mol/at-g. The values of these parameters in each test, run or experiment are shown in Table 1.

704 Energy & Fuels, Vol. 13, No. 3, 1999

Figure 3. Arrhenius representation for the tar* elimination reaction over some Ni catalysts for naphtha reforming at two different intervals of the steam-carbon ratio for the BASF G150 catalyst.

Figure 4. Arrhenius representation for the tar* elimination reaction over a heavy naphtha reforming catalyst (BASF G150) at H2O/C* < 1.1.

Figure 5. Arrhenius representation for the tar* elimination reaction over a reforming catalyst for naphtha with a medium molecular weight (Haldor Topsoe R-67) at H2O/C* > 2.0. Catalysts Used. The catalysts used have been described and characterized (BET surface area, pore volume, pore size distribution, etc.) in two previous papers.2,10 They were com-

Corella et al.

Figure 6. Arrhenius representation for the tar* elimination reaction over a reforming catalyst for naphtha with a medium molecular weights (BASF G1-25/1) at H2O/C* > 2.0.

Figure 7. Arrhenius representation for the tar* elimination reaction over a heavy naphtha reforming catalyst (BASF G150) at H2O/C* > 2.0.

Figure 8. Arrhenius representation for the tar* elimination reaction over a heavy naphtha reforming catalyst (ICI-46-1) at H2O/C* > 2.0. mercial steam reforming catalysts from four manufacturers: BASF AG, ICI Katalco, Haldor Topsøe a/s, and United Catalysts Inc. These commercial catalysts in their original size and shape are “quite big” rings (of around 15 mm external diameter). For this reason, commercial fixed-bed steam reformers suffer from several problems which seriously affect



Table 1. Some Main Experimental Conditions in and Results from the Catalytic Bed experimental conditions

results

run no.

catalyst

T3,C,av (°C)

τ′3 (kg/(m3/h))

SV3 (h-1)

H2O (% vol)

H2O/C* (mol/at-g)

Xtar,3 (%)

kapp,3 (m3/kg-h)

k′app,3 (Nm3,wet/kg-h)

k′′app,3 (Nm3,dry/kg-h)

25 26 27 31 32 50 33 34 38 39 40 47 52 59 62 63 35 36 41 42 43 51 53 54 44 45 46 55 56 61 64 57 58 65

ICI 57-3 UCI UCI UCI UCI RKS-1 ICI 46-1 ICI 46-1 ICI 46-1 ICI 46-1 ICI 46-1 ICI 46-1 ICI 46-1 ICI 46-1 ICI 46-1 ICI 46-1 R-67 R-67 R-67 R-67 R-67 R-67 R-67 R-67 G1-50 G1-50 G1-50 G1-50 G1-50 G1-50 G1-50 G1-25/1 G1-25/1 G1-25/1

740 800 800 800 800 800 740 800 800 800 800 815 795 810 845 775 800 800 800 800 800 785 815 810 660 800 729 820 790 815 740 815 805 785

0.022 0.020 0.019 0.017 0.017 0.018 0.018 0.027 0.014 0.014 0.016 0.017 0.019 0.015 0.014 0.015 0.014 0.015 0.043 0.047 0.051 0.018 0.020 0.019 0.035 0.026 0.021 0.018 0.015 0.017 0.018 0.015 0.016 0.016

61 600 67 000 70 300 79 000 79 500 73 000 57 800 38 600 78 400 73 600 67 700 71 800 68 500 89 700 92 200 89 900 73 800 70 700 28 900 26 600 24 100 74 400 67 000 69 900 34 900 44 500 58 200 68 800 79 200 73 000 73 500 88 400 81 600 82 300

19.7 10.1 14.6 9.8 7.1 12.2 18.8 18.1 16.7 19.7 13.5 17.5 13.2 29.0 11.9 21.7 29.5 17.8 25.0 27.6 23.3 13.8 12.4 27.3 7.1 8.2 7.9 14.5 16.6 11.9 14.5 23.7 10.9 13.1

5.2 2.4 2.2 1.0 1.4 2.3 3.7 4.8 2.6 3.3 2.1 3.3 3.0 4.2 2.3 3.2 6.1 2.4 4.6 4.9 4.3 3.2 2.1 6.2 0.7 1.1 0.8 2.1 2.1 1.1 2.1 3.5 2.0 2.6

69.4 76.0 87.2 93.5 86.4 92.0 98.5 99.9 97.3 94.5 98.1 99.4 99.3 97.7 98.5 97.5 97.5 98.7 99.8 98.1 98.1 97.5 99.6 99.0 93.6 96.2 88.5 99.4 98.1 90.4 97.7 98.4 98.6 97.8

55 72 109 163 120 139 230 253 267 201 252 300 255 255 294 250 257 290 146 85 77 221 278 243 78 123 102 285 257 140 212 275 262 235

15 18 28 41 30 35 62 64 68 51 64 75 65 64 72 65 65 74 37 22 20 57 70 61 23 31 28 71 66 35 57 69 66 61

12 17 24 37 28 31 50 53 57 41 55 62 57 46 63 51 46 61 28 16 15 49 61 45 21 29 25 61 55 31 49 53 59 53

their operation and performance. These include low catalyst effectiveness due to internal mass transfer resistance, low heat transfer rates, and large temperature gradients. Some gas bypassing or channeling could also occur if the bed height is small (a few centimeters), as in this work. For these reasons, the catalysts were crushed and a size of -1.6 + 1.0 mm was used: The small size of the catalysts is the main difference between this and simultaneous work being performed at pilot scale in which the catalysts are used full size and shape.12 Although the size of the catalysts used is relatively small, internal diffusion controls the overall process, according to previous studies.1 The umf of these crushed catalysts under the operating conditions was known.13 So, the experiments were designed in such a way that (u0,3/umf) is between 1 and 3. The bed was smoothly fluidized to avoid big temperature gradients (even being smothly fluidized the bed was not isothermal, as stated above). Tar Sampling, Analysis, and Measurement. Until mid1998, each institution and/or company working in this area was using its own method for tar sampling and analysis. This means that the amounts given here for the tar content could (12) Corella, J.; Caballero, M. A.; Aznar, M. P.; Gil, J. Work in progress. (13) Delgado, J.; Corella, J.; Aznar, M. P.; Araguës, J. L. The umf at Elevated Temperatures and with Gas from Biomass Gasification in Fluidized Bed for Dolomite, Silica Sand and a Steam Reforming Catalyst. In La Fluidization-Recents Progre´ s en Ge´ nie des Proce´ de´ s; Laguerie, C., Guigon, P., Eds.; Lavoisier: Parı´s, 1991; Vol. 5, pp 6269. (14) Milne, T. A.; Evans, R. J.; Abatzoglou, N. Biomass Gasifier “Tars”: Their Nature, Formation, Destruction, and Tolerance Limits in Energy Conversion Devices. In Making a Business from Biomass in Energy, Environment, Chemicals, Fibers and Materials; Overend, R. P., Chornet, E., Eds.; Elsevier Science Ltd.: Oxford, U.K., 1997; Vol. 1, pp 729-738.

probably not be the same as the amount obtained (for the same circumstances) by other institutions.6,14,15 This situation forced to the IEA (Thermal Gasification Task), USDOE, and the European Commission (Brussels, DGXVII) to organize and celebrate two meetings (held in Brussels on March 18-20, 1998, and in Wu¨rzburg (Germany) on June 6-11, 1998) to define tar (in biomass gasification) and to find, establish, and agree on a protocol to standardize measurements concerning tar. The analysis for tar in this work has not been made under a standard procedure (not existing then). Tar sampled and analyzed here will be called tar*, thus taking into account that the tar amount measured by these authors can be different if it is measured in another institution. Due to the importance of the tar content meaning for this work, a big effort was made to check the values shown in this paper. For instance, some samples of condensates containing tar were first sent to NREL (Golden, CO) to be there analyzed by Molecular Beam Mass Spectrometry.16 Other samples of tars were taken by solid-phase adsorption (SPA) and sent to KTH (Stockholm, Sweden) to be characterized, after their desorption, by gas chromatography by the method described by Brage et al.17 These tar characterizations allowed us to know the tar composition before and after the catalytic (15) Abatzoglou, N.; Evans, R.; Milne, T. A. Biomass Gasifier “Tars”: Their Nature, Formation and Conversion. IEA/NREL Report, 1998; draft version. (16) Evans, R. J.; Milne, T. A. Chemistry of Tar Formation and Maturation in the Thermochemical Conversion of Biomass. In Development Thermochemical Biomass Conversion; Bridgwater, A. V., Boockock, D. G. B., Eds.; Blackie Academic & Professional: London, U.K., 1997; Vol. 2, pp 803-816. (17) Brage, C.; Sjo¨stro¨m, K.; Yu, Q.; Chen, G.; Liliedahl, T.; Roseń, C. Application of solid-phase adsorption (SPA) to monitoring evaluation of biomass tar from different types of gasifiers. In Biomass Gasification and Pyrolysis; Kaltschmit, M., Bridgwater, A. V., Eds.; CPL Press: 1997; pp 218-227.


Corella et al.

reactor.18 Nowadays, a detailed and realistic tar characterization at the inlet and outlet of the catalytic reactor is already done by Corella and co-workers, and this will be presented in a forthcoming paper,18 but here tar will be refereed as just only one lump (tar*). Tar sampling and analysis methods followed in this paper (described previously1) were also compared with the ones used by VTT.3 To do this, some dry (CO2) ice was placed in the cold traps following the same conditions as the ones published by VTT.3 Thus, comparing the three methods used here (ours, the VTT one,3 and the results provided by KTH after the analysis of the same tar samples sent there), it is found that the method used to trap and calculate the tar content followed in this paper (tar*) does not take into account most of the benzene, about 50% of toluene, and about 10-20% of lighter PAHs. They are not collected by the method used in this paper. So, the authors consider that, overall, the tar* contents given here are quite accurate but that they are about a 30% (by average) less than the tar contents measured by SPA or by using the VTT method. Kinetic Analysis. The tar (tar*) content in the flue gas is measured, thus, at the inlet and at the exit of the catalytic bed. An overall tar* conversion, Xtar,3 (which includes thermal reactions), is calculated for the catalytic bed. These tar* measurements are made at different times-on-stream (in the same test). Xtar,3 is calculated several times in the same test. Averaged (respect to time-on-stream) values for Xtar,3 are used here, but its range or interval of error is also measured and used in the kinetic analysis. Several institutions working on this subject agreed years ago on using the same reacting network and kinetic analysis to compare their respective results. The agreed standard network and kinetics were only one lump for tar, one single reaction for the overall tar elimination, and a simple first-order kinetic equation. With this basis and working under plug or piston flow conditions, an apparent and overall kinetic constant (kapp,3) is then calculated by

kapp,3 ) -ln(1 - Xtar,3)/τ′

(1)

This apparent and overall kinetic constant includes the partial pressures of H2O and CO2 which are also, as tar, reactants but can be considered constants in the catalytic bed because their content does not vary much (in the catalytic reactor) since its content in the flue gas is much bigger than the tar one.10 kapp,3 serves as an index to describe the catalyst activity for the overall tar removal and to compare results obtained by different authors and/or institutions (once checked that the units used for τ′ are the same and that they are speaking of a similar tar). Since the error’s interval for Xtar,3 is known, the error’s interval for kapp,3 is also calculated. It will shown in Figures 4-8 concerning kapp,3.

Results Tar* Conversion. The values of tar* conversion obtained with the seven commercial catalysts used are given in Table 1. They are shown in Figure 1 at different gas residence times and in Figure 2 at different temperatures of the catalytic bed. From these two figures the following observations were made. (1) Most tests were made under experimental conditions in which tar* conversion was very high (usually between 97% and 99.5%). (2) The commercial catalysts used provide two different groups of results: The catalysts for steam reforming of light hydrocarbons such as natural gas (ICI 57-3, UCI C11-9-061, and (18) Caballero, M. A.; Gil, J.; Aznar, M. P.; Corella, J. To be published.

Topsoe RKS-1) give tar* conversions clearly lower than the ones obtained with the catalysts for steam reforming of heavier hydrocarbons or naphthas (ICI 46-1, Topsoe R-67, BASF G1-25/1 and G1-50). This second family or group of catalysts is more active for this process than the first family or group. (3) High (98%) conversions are already obtained at very low gas residence times in the catalytic bed (τ′3 ) 0.014 kg of catalyst/(m3/h)). Apparent Kinetic Constant for Catalytic Tar* Removal (kapp,3). kapp,3 was calculated in all tests using eq 1. The values obtained for the four best catalysts are shown in Figure 3. Values shown in this figure are the averaged ones with respect to the time-on-stream. One important fact appears in Figure 3: the (H2O/ C*) ratio is very important in this process. Working at two different levels for the (H2O/C*) ratio, the catalyst activity is very different. The importance of the steam content in the flue gas for the catalytic tar removal was already determined by Aldeń et al.19 and Narvaéz et al.,1 and now it is confirmed by the results shown in Figure 3. The importance of a ratio similar to this one is also well-known in industrial steam methane (or natural gas) reforming (in such case C* is CH4 + C2 + C3) where typical steam-to-carbon molar feed ratios are 2.04.0.20,21 Temperature Dependence. The results obtained with the four best catalysts are represented according to the Arrhenius coordinates in Figure 4 for the BASF G1-50 catalyst with H2O/C* < 1.1, Figure 5 for the Haldor Topsoe R67 catalyst with H2O/C* >2.0, Figure 6 for the BASF G1-25/1 catalyst with H2O/C* > 2.0, Figure 7 for the BASF G1-50 catalyst with H2O/C* > 2.0, Figure 8 for the ICI 46-1 catalyst with H2O/C* > 2.0. Experimental points are shown in these figures as rectangles to take into account the error calculated for kapp,3 and the interval measured for T3,c. Experimental points shown in Figures 5 and 8 could be surprising but are true in these tests. Parameters of the Arrhenius Equation for the Catalytic Overall Tar Removal. After a careful analysis of the data shown in Figures 4-8, the lines which best fit such data were drawn. Such lines are shown in Figure 9, which provides two important conclusions: (1) The four commercial catalysts for steam reforming of naphthas tested here (and indicated in Figure 9) have a similar activity. (2) The H2O/C* ratio in the catalytic bed is more important (under some intervals) than the type of catalyst used. Perhaps the H2O/C* ratio is not the best index to determine the catalyst activity; perhaps the CO2/C* ratio should also be taken into account because CO2 reforms tars as well as H2O does, as indicated by Corella et al. in previous work.10 But the fact is that Figure 9 proves how important the H2O/C* ratio (under some intervals) is to eliminate tars. Remember that Narvaéz et al.1 already proved the importance of the H/C ratio in this (19) Aldeń, H.; Hagstro¨m, P.; Hallgren, A.; Waldheim, L. HighTemperature Catalytic Gas Cleaning for Pressurized Gasification Processes. In Biomass for Energy and the Environment; Chartier, P., Ed.; Pergamon Press: Oxford, U.K., 1996; Vol. 2, pp 1410-1415. (20) Adris, A. M.; Lim. C. J.; Grace, J. R. The Fluidized-Bed Membrane Reactor for Steam Methane Reforming: Model Verification and Parametric Study. Chem. Eng. Sci. 1997, 52, 1609-1622. (21) Basini, L.; Piovesan, L. Reduction on Synthesis Gas Costs by Decrease of Steam/Carbon and Oxygen/Carbon Ratios in the Feedstock. Ind. Eng. Chem. Res. 1998, 37, 258-266.



Figure 9. Arrhenius representation for the tar* elimination reaction over some reforming catalysts for naphtha with medium and heavy molecular weights.

Figure 10. Arrhenius representation comparing the activity of some reforming catalysts in the tar* elimination reaction at H2O/C* > 2.0, considering Eapp ) 40 kJ/mol.

Table 2. Parameters of the Arrhenius Equation for the Different Catalysts Tested and From a Direct or First Fitting of Data

Table 3. Preexponential Factor of the Arrhenius Equation, for the Different Catalysts Tested, Using an Averaged Value of 40 kJ/mol for the Apparent Energy of Activation

catalyst

Eapp (kJ/mol)

k0,app (m3(wet,T3,c)/kg-h)

BASF G1-50 H2O/C* > 2.0 H2O/C* < 1.1 ICI 46-1 BASF G1-25/1 TOPSOE R-67

30 ( 10 30 ( 10 30 ( 12 50 ( 10 50 ( 20

8800 ( 400 3500 ( 500 8000 ( 1000 69000 ( 9000 68000 ( 8000

process in the catalytic bed. H2O/C* and H/C ratios are not the same, but they have a similar meaning. From lines shown in Figure 9 and with the Arrhenius equation, the values for the apparent activation energy (Eapp) and for the preexponential factor (ko,app) are calculated. They are shown in Table 2. It is well-known that on increasing the activity of a catalyst, the apparent activation energy decreases. With this principle in mind, results shown in Table 2 would indicate that the catalysts BASF G1-50 and ICI 46-1 are more active for this application than the BASF G125/1 and the Topsoe R-67 catalysts. The so-called compensation effect between Eapp and ko,app is also known by experts in chemical kinetics. Several pairs of values of these two parameters can fit the experimental data well.22 With this compensation effect in mind and Table 2, all the experimental data were readjusted to an averaged value of Eapp of 40 kJ/ mol. The results from this new fitting are shown in Figure 10 and in Table 3. With this fitting all the differences in activity between the catalysts are charged to ko,app. If Eapp is the same, this parameter, ko,app, is an index or indication of the catalyst activity. Table 3 would thus serve as the ranking of the four best catalysts according to their activity. Criticism on the Kinetic Analysis and Model Used Here. First, the averaged value for Eapp of 40 kJ/ mol merits further comment. It is a very low value for an endothermal reaction, even considering and/or admitting that (i) the catalysts are very active (a fact which generates low values for the activation energy), (ii) the authors are speaking of an apparent parameter (22) Schwab, G. M. On the Apparent Compensation Effect. J. Catal. 1983, 84, 1-7.

catalyst

k0,app (m3(wet,T3,c)/kg-h)

BASF G1-50 (H2O/C* > 2.0) ICI 46-1 BASF G1-25/1 TOPSOE R-67

23 460 23 100 22 800 22 300

(Eapp), (iii) the work has been carried out with some internal diffusion control (above 0.2 mm the internal diffusion plays a role in this process according to Narvaéz et al.,1 and (iv) there has been significant experimental error as indicated in Figures 4-8. After having studied this fact in depth, the authors conclude that (i) this value for Eapp can be accepted because it is an experimental and true finding, (ii) this value does not correspond to a true activation energy but to an apparent one, and (iii) the kinetic model and the reacting network used here are the simplest ones. The true kinetic model and reacting network are much more complex than the ones used here, and eq 1 is an oversimplification. Second, the value of 40 kJ/mol has caused Corella et al. to be developing (for the tar removal reaction) some more accurate reacting networks and kinetic models. A good characterization of the tar, with several reacting sublumps, is needed for that purpose.18 Third, in the whole kinetic treatment, kapp has been calculated for very high conversions. One of the main goals of the authors was to get high conversions, to test the catalysts under conditions near to an industrial application. Then the authors made a kinetic analysis of the results. To obtain more reliable kinetic data, experiments should also have been made with lower values of conversion. The obtained high conversions are interesting from the viewpoint of process technology and indicate quite successful efforts of the authors at optimizing a process concept, but it is difficult to optimize the process and derive kinetic data simultaneously. Deactivation Tests. Each test or experiment usually lasted 3 h under stationary state. In a typical test, three samples were taken at the inlet and exit of the catalytic


Figure 11. Apparent kinetic constant at different times-onstream with several steam reforming catalysts. Operating conditions: T3,c ) 800 °C; ER ) 0.28-0.32.

bed. Nevertheless, for commercial application, the life of the catalyst has to be high. Simell et al.23 feels that catalyst (monoliths in such case) lifetimes of 2-5 years are needed to get an economical and feasible catalytic cleaning. To study such high lifetimes are absolutely out of the scope of this work, but some tests were made with relatively high (for these authors) lifetimes, of up to 65 h. In these long-term tests, the facility was stopped at night and continued the following day. Most of these experiments were carried out at 800 °C in the catalytic bed and with an equivalence ratio in the gasifier of 0.28-0.32. The results are shown in Figure 11. It is observed that in the experimental conditions used here, most catalysts do not deactivate. The activity (k3,app) remains near constant. This is a promising fact which should be confirmed in tests with times-on-stream higher than the ones here used. Conclusions The commercial catalysts for steam reforming of naphthas or heavier (than natural gas) hydrocarbons are more active for tar elimination (in a flue gas from a fluidized bed biomass gasifier) than the ones for steam reforming of light hydrocarbons or natural gas. The four tested commercial catalysts for steam reforming of naphthas have a similar activity. The differences in activity are less than 10%. These nickel-based catalysts are very active for tar removal. High (98%) tar conversions are obtained with very low space times, equivalent to very high space velocities (of around 20 000 h-1, normal conditions). The catalyst activity for tar removal is not a problem, but for scaling-up this process, one must remember that all values of catalyst activity reported here are for crushed particles of the catalyst. For future commercial application of these results, using the commercial “big sizes” of the catalysts, the effectiveness factors reported by (23) Simell, P.; Stahlberg, P.; Solantausta, Y.; Hepola, J.; Kurkela, E. Gasification Gas Cleaning with Nickel Monolith Catalyst. In Development Thermochemical Biomass Conversion; Bridgwater, A. V., Boockock, D. G. B., Eds.; Blackie Academic & Professional: London, U.K., 1997; Vol. 2, pp 1103-1116.

Corella et al.

Narvaéz et al.1 have to be used and then the activities of the catalysts might be 1-10% (only) of the ones here reported. The steam to carbon-to-be-reformed ratio in the flue gas plays an important role in this process, and it should be always higher than 2.0 mol of H2O/at-g of C*. An apparent activation energy (Eapp) of 40 kJ/mol fits the results obtained here with all the commercial catalysts tested. This value is considered very low, even considering and admitting internal diffusion control for the particle sizes used. This low value for Eapp is due to the fact that only one overall reaction (for tar removal) has been considered with single first-order kinetics. Both the reacting network and the kinetic equation are more complex than the ones here used. This low value for Eapp has caused Corella et al. to be developing new networks and kinetic models for the tar removal process. From the results shown in Figure 10 or in Table 3, the best catalyst would be the BASF G1-50 but the differences between the four catalysts tested are not bigger than 10%. So, the authors conclude that all four commercial catalysts for naphthas tested here have a very similar activity for tar removal or destruction and that, under certain limits, the composition of the reacting gas atmosphere (of which the H2O/C* ratio depends) is very important. No catalyst deactivation at high temperatures (higher than 800 °C) has been detected in this work, but it must be remembered that (i) a guard bed was used (it decreased the tar contents at the catalytic bed inlet to values of around of 1 gtar/m3n) and (ii) the maximum time-on-stream used was around 60 h. For future commercial application, long tests (higher times-onstream) should be made to confirm the high life of the catalyst. Acknowledgment. This work has been performed under the EU, DGXII, JOULE3 Program, Project No. JOR3-CT95-0053. The authors thank the financial support received from the EU, DGXII, JOULE3 program. The experimental work by J. M. Ruiz, I. Teijeiro, and J. Urbań is recognized and appreciated. The authors also express their gratitude to BASF AG, Haldor Topsoe a/s, ICI-Katalco, and United Catalysts Inc. for the provision of samples of the steam reforming catalysts. Nomenclature dp ) particle diameter of the catalyst (by sieving), mm. Eapp ) apparent activation energy from the Arrhenius equation and based on a single first-order reaction and only one lump for tar, kJ/g-mol ER ) overall equivalence ratio (air-to-fuel weight ratio used in the experiment divided by the air-to-fuel weight ratio for stoichiometric combustion), dimensionless H3 ) height of the catalytic bed, cm (H2O/C*)3 ) steam-to-carbon ratio at the inlet of the catalytic reactor, mol/at-g hs ) moisture of the biomass fed, wt % k0,app ) preexponential factor of the Arrhenius equation for the catalytic tar elimination, m3(wet,T3,C)/kg-h kapp,3 ) apparent kinetic constant for tar elimination based on a single first-order kinetic equation, m3(wet,T3,C)/kgcat-h


k′app,3 ) the same, m3n (wet)/kgcat-h k′′app,3 ) the same, m3n (dry)/kgcat-h m3n ) cubic meter of gas at normal conditions (1 atm, 0 °C) SV3 ) space velocity in the catalytic reactor, defined as m3 (wet,reactor temperature)/m3cat‚h, h-1 t ) time on stream, h tar* ) tar sampled and measured according to the UCM’s method and described in ref 1 T1,b ) temperature in the gasifier bed, °C T2,c,av ) averaged temperature in the axis of the secondary reactor or guard bed, °C T3,c ) temperature in the center (axis) of the third or catalytic reactor, °C T3,c,av ) the same, averaged between the bottom and the top of the bed, °C


u3,o , u3,e , u3 ) superficial gas velocity in the catalytic bed at the inlet, at the exit, and averaged between inlet and exit, cm/s umf,3 ) minimum fluidization velocity (at the temperature and with the gas composition inside the catalytic reactor) of the catalyst, cm/s W3 ) weigth of catalyst in the catalytic bed, g Xtar ) tar conversion, dimensionless Greek Symbols τ3 ) pseudo space time in the catalytic bed, defined as H3/u3, s τ3′ ) the same, kgcat/m3(wet,T3,c,av)/h EF980221E

Biomass Gasification with Air in a Fluidized Bed - ACS Publications

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