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KINETICS, CATALYSIS, AND REACTION ENGINEERING Performance of a Nickel-Activated Candle Filter for Naphthalene Cracking in Synthetic Biomass Gasification Gas Hongbin Zhao, Dirk J. Draelants, and Gino V. Baron* Department of Chemical Engineering (CHIS), Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium
The concept of a ceramic candle filter for high-temperature gas filtration, catalytically activated with nickel using a deposition-precipitation method with urea, was introduced to perform the simultaneous removal of tar and particles from hot biomass gasification gas. Tar cracking over nickel-activated ceramic filter substrates was studied with a synthetic biomass gasification gas (free of H2S and dust) under industrial hot gas filtration conditions. Naphthalene was used as the model compound for high-temperature biomass gasification tar. The main parameters investigated were reaction temperature (750-900 °C), gas velocity (2.5-6 cm/s), and catalyst loading (0.5 and 1 wt % nickel). It was found that when typical filtration gas velocities are used, naphthalene can be completely converted mainly to syngas (H2 and CO) at 800-900 °C over activated filter substrates with a catalyst loading of only 1 wt % nickel. Introduction Environmental impacts of power production have been a topic of large public interest in recent years. Consequently, environmentally friendly and more efficient ways to produce energy are under intense research and development. Biomass integrated gasification combined cycle (IGCC) technology has been demonstrated to be one of such ways.1 The process concepts are mostly based on pressurized air-blown fluidized-bed gasification followed by a combined gas turbine and steam turbine cycle.2 At a smaller scale, an atmospheric air-blown updraft or fluidized-bed gasifier followed by a gas engine is economically more viable, given its lower investment cost. Within the gasifier, the biomass is gasified at 700-1000 °C and converted to gases (H2, CO, CO2, H2O, CH4, light hydrocarbons), tars (highly aromatic compounds), nitrogen compounds (mainly NH3), sulfur compounds (mainly H2S), ash particles, and alkali metals. The gas composition of an air-blown fluidizedbed biomass gasifier after the cyclones is typically 50% N2, 10% H2, 10% CO, 15% CO2, 10% H2O, 5% CH4, 5-20 g of tar/Nm3, 4000 ppmv NH3, 50-200 ppmv H2S, 5-30 g of particles/Nm3, and 1 ppm-wt alkali metals.3,4 Before being fed to the downstream equipment, the gasification gas has to be cleaned from gas contaminants such as particles, alkali metals, tars, and ammonia. This treatment of the hot fuel gas is a critical stage in the overall energy conversion process.5 Established methods use wet scrubbing and can produce high fuel gas purity. Their disadvantages relate to the disposal of contaminated water, complexity, and the associated heat loss. Alternative methods, called hot gas cleaning, currently under development, involve catalytic high-temperature decomposition of tar and ammonia followed by filtration * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: +32-2-6293246. Fax: 32-2-6293248.
to remove particles and chemical sorption of alkali metals on regenerative reactants. Apparently, hot gas cleanup methods may lead to a higher energy efficiency and lower cost. From the work of the Technical Research Center of Finland (VTT), it appeared that commercial nickelbased steam-reforming catalysts were very efficient at 900 °C for decomposition of both tars and ammonia in biomass gasification gas.6 Other research groups also confirmed the tar conversion potential of nickel-based catalysts in gasification gases.7-15 In most cases, nickelbased catalysts are applied in a packed-bed configuration in which the catalysts have a low effectiveness factor mainly due to internal mass-transfer limitation. On the other hand, porous ceramic filters have been demonstrated in the past decade for high-temperature and high-pressure particle control to meet environmental and turbine equipment specifications in advanced power generation systems on both the laboratory and pilot plant scale.16,17 Generally, ceramic filters for hot gas filtration applications can be classified into two types: candle filters and cross-flow monolith filters. Many different materials (oxides, non-oxides, and mixed) may be used for any of these filter types. One major development area of ceramic filters is associated with advanced ceramic materials and manufacturing techniques that will improve the filter element long-term tolerance to thermal/chemical degradation and shortterm durability to thermal/physical shock.18 The other is applications of ceramic filters in combined processes for high-temperature gas cleaning, for instance, catalytically modified fly ash filters for NOx reduction with NH3.19 In the ongoing project, the concept of catalytic candle filters for hot biomass gasification gas cleaning is studied, where particle removal and decomposition of tar and ammonia are integrated in one process unit.
10.1021/ie000213x CCC: $19.00 © 2000 American Chemical Society Published on Web 07/29/2000
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Figure 1. Schematic presentation of a catalytic candle filter.
This simplifies the entire gas-cleaning process with a potential reduction in investment costs, which is an important factor to make biomass gasification a viable alternative. Furthermore, the gas flows now through the catalytically active pores, and internal diffusion is no longer a limiting factor for tar conversion as in a packed or fluidized catalyst bed, which was one of the main motivations to study this system. Figure 1 shows a schematic representation of such a catalytic candle filter. The basis is a conventional candle filter with an asymmetric configuration, i.e., a thin filtration membrane supported on a mechanically stable large-pore substrate. The catalytically active zone is situated in the porous structure of the substrate. When the particulate-laden gas flow enters the catalytic filter, the particles are trapped on the outside surface where a dust cake is formed. Tar components in gaseous form and ammonia flow through the catalytic filter and are catalytically converted. The filtration behavior of candle filters itself is already well-known and will not be discussed here since particles retained on the outer surface of the filter should not affect the catalyst deposited in the interior of the supporting substrate. However, the simultaneous filtration step indirectly imposes some limitations on the catalytic step, which must be taken into account during the development of the catalytic filter: (1) Nickel loading: only a limited amount of catalyst may be deposited inside the filter; otherwise, the pressure drop during filtration will increase unacceptably. (2) Low surface: the available surface area for depositing a catalyst is low since the filter support body consists of nonporous alumina grains of a few hundred microns in diameter. (3) Contact time: full conversion of tars and ammonia must take place over a distance of 1 cm (the wall thickness of the filter) at a typical face velocity of 2-4 cm/s used in industrial filtration, which gives an overall contact time of only 0.25-0.5 s. At present, a preparation route to incorporate pure nickel or calcium and nickel into the preformed filter substrates has been developed. In this paper, the performance of nickel-activated filter substrates for naphthalene cracking was evaluated with a synthetic biomass gasification gas (free of H2S and dust). The catalytic reaction tests were carried out using gas velocities typical for industrial hot gas filtration. Experimental Section Preparation of Catalytic Filter Substrates. R-Al2O3-based filter substrates produced by an industrial
filter partner (Schumacher, Germany) were used in this work. The substrates had a mean pore radius of about 26 µm, a pore volume of 0.1 mL/g, and a BET specific surface area of 0.33 m2/g. They are disk-shaped with a diameter of 3 cm and a thickness of 1 cm and are representative for the candle filter support body on a small scale. These filter substrates were catalytically modified with nickel using a precipitation-deposition method with urea, followed by a calcination and reduction step. This technique allows us to deposit up to 2 wt % of nickel oxide by one preparation cycle. A fairly uniform distribution of nickel precursor throughout the substrate can be obtained. One preparation cycle hardly changes the porosity of the filter substrate, and hence the flow resistance for the gas will be almost unaffected compared with that of the blank filter. Details about the catalyst preparation can be found in a previous publication.20 Reaction Setup. The facility used is a laboratoryscale reaction setup and is shown in Figure 2. The reactor tube (internal diameter 3 cm and length 50 cm), custom-made of dense R-Al2O3 to minimize catalytic wall effects, contained a rigid catalytic filter substrate in the middle and was heated horizontally in a tube furnace. The temperature near the catalytic filter substrate was measured from the inlet and outlet side of the reactor using two thermocouples in an alumina well. The pressure drop across the reactor was monitored by a differential pressure sensor. The reactor outlet was operated under atmospheric pressure. The gas-mixing section allowed us to feed a representative dust-free biomass gasification gas (N2, H2, CO, CO2, H2O, CH4) to the reactor with or without addition of tar, NH3, and H2S. All the gas flows were controlled by mass flow controllers while water was introduced into the preheated gas stream by a peristaltic pump (Technicon Auto Analyzer). Naphthalene as the tar model compound was introduced by passing N2 gas through a heated packed bed of naphthalene crystals. The total gas flow rate can be set to obtain a superficial gas velocity near the filter disks comparable to the face velocity used in candle filtration (1-4 cm/s). The inlet gas was preheated at 150 °C before and after introduction of water and naphthalene to prevent condensation. The gas leaving the reactor was kept at 150 °C and was led either into a tar-sampling train or into a gas analysis line. The tar-sampling train was composed of two washing bottles in a cooling bath at -20 °C. The first bottle was empty to condense most of the water and naphthalene while the second one contained dichloromethane (DCM) as solvent to absorb benzene and other volatile reaction products. After sampling (duration: 15-30 min), the bottles were rinsed with DCM and then the content of the tar components in the solvent was analyzed off-line by gas chromatography (HP 6890 GC equipped with HP-5 column and FID). The gas coming from the gas analysis line was led to an online gas chromatograph (Varian 3400 GC with silica gel column and TCD) to determine the content of the main gas components (N2, H2, CO, CO2, CH4) after removal of tar and water by a cold trap (0 °C) and sulfuric acid washing. In addition, the reactor outlet gas composition was qualitatively followed by an on-line mass spectrometer (Balzer QMA 125) for N2, H2, CO, CO2, H2O, CH4, and benzene to monitor any immediate change in
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Figure 2. Scheme of the laboratory setup for performing catalytic tar cracking tests.
behavior of the catalytic filter substrate during the reaction tests. Tar Model Compound and Gas Feed. Tar is an undesirable byproduct in the production of fuel gas by biomass pyrolysis and gasification. According to the pyrolysis or gasification operation temperature, tar can be classified into low-temperature (450-650 °C) and high-temperature (above 650 °C) tar.6 Both of them contain a wide spectrum of compounds including aliphatics, aromatics, hydroaromatics, heterocyclics (with oxygen, nitrogen, and sulfur as the heteroatoms), and the assorted chemical combinations of these. In this project, tar is derived from a fluidized-bed gasifier fed with biomass, which usually has freeboard temperatures up to 950 °C. According to VTTs work,6 this kind of high-temperature tar consists mainly of highly stable compounds such as benzene (60-70 wt %), naphthalene (10-20 wt %), and other polyaromatic hydrocarbons (10-20 wt %), which can amount to 15-20 g of tar/Nm3 in biomass gasification. In studies of tar cracking using a separate catalyst bed, basically two types of tar sources are applied, one directly drawn from a biomass gasifier and the other from model compounds. Among tar model compounds used are n-heptane,21 toluene,22 benzene,23,24 and naphthalene.15 Use of well-defined model compounds relieves the constraints on simultaneous running of a small gasifier and can simplify the interpretation of the reaction data. In this work, naphthalene was used as the tar model compound with a concentration of 5 g/Nm3 (875 ppmv) in the synthetic biomass gasification gas used, which consisted of 51 vol % N2, 12 vol% CO, 10 vol % H2, 11 vol % CO2, 11 vol % H2O, and 5 vol% CH4. Reaction Test Procedure. The rigid activated filter disk with a nickel loading of 0.5 wt % or 1 wt % was fixed in the middle of the alumina reactor tube using an alumina-based cement. Prior to each run, the catalytic disk was pretreated in 10 vol % H2 in N2 at 900 °C overnight. An experimental run involved the stepwise decrease of the reactor temperature from 900 °C to 750 °C in steps of 50 °C at fixed inlet flow rate. Table 1 gives an overview of the fixed flow rates used (wet and normal
Table 1. Reaction Parameters Investigated for the 0.5 and 1 wt % Nickel-Modified Filter Disk inlet flow (NmL/min) 247 395 590
face velocity space velocity at at 900 °C normal conditions (cm/s) (s-1) 2.5 4 6
0.58 0.93 1.39
reaction temperature (°C) 750, 800, 850, 900 750, 800, 850, 900 750, 800, 850, 900
conditions: 0 °C and 1 atm), together with their respective space velocity (at normal conditions and based on the reaction volume) and superficial gas velocity (in the reactor tube at 900 °C). Every experimental point (except some at 900 °C) was monitored for at least 1 h, with a maximum of 2 h. The reaction obtained an apparent steady state within 15-min operation time at the selected temperature as indicated by the on-line MS analysis. For each temperature, the outlet gas was analyzed for benzene, naphthalene, and the permanent gases. The conversion of naphthalene and methane was calculated from their respective inlet and outlet molar flow rates:
X ) (Min - Mout)/Min × 100%
(1)
The yield of H2, CO, H2O, and CO2 was obtained on the basis of their respective inlet and outlet molar flow rates:
YG ) (Mout - Min)/Min × 100%
(2)
The yield of benzene was calculated using the following equation on the basis of molar flow rates:
YB ) Mout-benzene/Min-naphthalene × 100%
(3)
Results and Discussion Effect of Operation Parameters on Naphthalene Cracking. (1) Effect of Nickel Loading. Figures 3 and 4 show the naphthalene conversion as a function of the reaction temperature and gas velocity with the
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Figure 3. Naphthalene conversion as a function of reaction temperature and gas velocity over a 0.5 wt % nickel-modified and blank filter disk.
Figure 4. Naphthalene conversion as a function of reaction temperature and gas velocity over a 1 wt % nickel-modified and blank filter disk.
0.5 and 1 wt % nickel-modified filter disk, respectively. It can be seen that naphthalene was converted almost completely above 800 °C in the two cases. Below 800 °C the conversion of naphthalene on the 0.5 wt % nickelmodified disk is only slightly lower than that with the 1 wt % nickel-modified disk. Accordingly, there seems to be no important difference in performance between the two tested catalytic filter substrates within the operation window applied. This means that doubling the catalyst loading did not appreciably enhance the catalytic activity. It probably indicates that the available pore wall of the ceramic disk for nickel deposition is nearly completely covered with nickel for a loading of only 0.5 wt %. Thus, adding more nickel will not increase the available nickel surface significantly. It was noted that the pressure drop across the catalytic filter substrate in all the reaction tests remained stable. In addition, after the reactor cooled to room temperature in N2 flow, no carbon deposit on the ceramic tube wall of the reactor nor on the disks was found. Furthermore, on the basis of the measured inlet and outlet gas composition, the carbon material balance was calculated over the reactor system for all experi-
ments. The closure of the carbon balance was within 2%. These observations indicate that no detrimental carbon deposition occurred in the substrates. The net carbon deposition on the catalyst surface is a complex dynamic phenomenon. It is the result of the balance between the reactions that form the carbon precursors and the reactions with for instance H2 and H2O that remove these precursors from the catalyst surface. This balance is influenced by several factors, such as temperature, catalyst composition, nature of support, gas atmosphere, tar content, reactor, catalyst geometry, etc. Apparently, the overall conditions in our experiments were such that the net balance is in favor of carbon removal from the surface since we could not identify carbon deposition on the catalytic filter substrates, even at the lowest temperature tested (750 °C). As a reference, the reaction of naphthalene in the reactor containing a blank filter substrate was examined under the conditions applied for the nickel-activated filter substrates. The results are shown in both Figures 3 and 4. Apparently, naphthalene was significantly cracked when no nickel catalyst was present: up to 18% of naphthalene conversion at the lowest gas velocity (2.5 cm/s) and highest reaction temperature (900 °C). This cracking of naphthalene may be attributed to the minor catalytic effect of the ceramic reactor wall and the pore wall of the ceramic filter substrate together with homogeneous gas-phase reaction. Commercially available nickel-based catalysts consist of 10-20 wt % nickel on a low-surface area ceramic support. It was found that internal diffusion starts to play a part in porous catalyst particles with diameters higher than about 0.2-0.3 mm.25 Consequently, this leads to low efficient use of packed beds of commercial nickel-based catalysts, which have practical particle diameters of the order of millimeters.26 However, in the case of the catalytic filter, a low loading of nickel still exhibits enough catalytic activity for naphthalene conversion. This is because all the nickel is situated on the outer surface of the nonporous R-alumina grains of a few hundred microns in diameter and is instantly available for the gas when it flows through the active filter pores. Consequently, due to better mass transfer in the filter structure, the nickel catalyst is used more efficiently than that in a packed bed, and almost complete conversion is obtained, even with short contact times. (2) Effects of Gas Velocity and Reaction Temperature. As shown in Figures 3 and 4, complete tar cracking was achieved above 800 °C with any gas velocity (defined at 900 °C) lower than 4 cm/s, which is typically used in hot gas filtration. Even with a gas velocity of 6 cm/s, which is higher than normal in hot gas filtration, the conversion remains as good as complete. However, below 800 °C the conversion of naphthalene significantly decreased as the reaction temperature decreased and when the gas velocity increased. As expected, reaction temperature and gas velocity play important inter-related roles in the heterogeneous catalytic reaction. Apparently, the reaction temperature requirement (800-900 °C), imposed by the catalytic reactions, when using a catalytic filter remains the same as that with a conventional packed-bed reactor as used by other research groups. As mentioned before, the simultaneous filtration step imposes some limitations on the catalytic step by using a catalytic ceramic filter, i.e., a low catalytic surface area
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Figure 5. Methane conversion as a function of reaction temperature and gas velocity over a 1 wt % nickel-modified filter disk.
Figure 8. Yield of H2O as a function of reaction temperature and gas velocity over a 1 wt % nickel-modified filter disk.
Figure 6. Yield of H2 as a function of reaction temperature and gas velocity over a 1 wt % nickel-modified filter disk.
Figure 9. Yield of CO2 as a function of reaction temperature and gas velocity over a 1 wt % nickel-modified filter disk. Table 2. Yield of Benzene as a Reaction Product of the Catalytic Naphthalene Cracking over a 1 wt % Nickel-Modified Filter Disk at Different Temperatures and Gas Face Velocities face velocity at 900 °C temperature
2.5 cm/s
4 cm/s
6 cm/s
750 °C 800 °C 850 °C 900 °C
15.4% NDa ND ND
15.7% 2.1% ND ND
12.3% 1.6% ND ND
a
Figure 7. Yield of CO as a function of reaction temperature and gas velocity over a 1 wt % nickel-modified filter disk.
available for catalytic reactions and rather high gas flow applied in industrial filtration. On the basis of this work, it appears that the two gas-cleaning operations in
Not detectable.
biomass gasification, namely, catalytic tar cracking and particle removal, could be integrated using catalytic candle filters within the optimal operational window used for hot gas filtration. Basic Reactions in the Catalytic Filter Substrate. Figures 5-9 show the conversion of CH4 and the yield of respectively H2, CO, CO2, and H2O with the 1 wt % nickel-activated filter substrate. As expected by the presence of nickel, CH4 was partially converted on the catalytic filter substrates (Figure 5). It was found that the content of both H2 (Figure 6) and CO (Figure 7) increased markedly. H2O (Figure 8) was always consumed during the reaction. The content of H2O at
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the reactor outlet was obtained by calculation on the basis of the global material balance. From Figure 9 we cannot draw a conclusion from the change in the CO2 content, as changes are small. At 750 and 800 °C, benzene was identified as one of the reaction products. Table 2 lists the amount of benzene in the tar samples at the reactor outlet for the 1 wt % nickel filter substrate. It is evident that at 750 °C for all gas velocities tested, benzene in a significant amount was derived from the catalytic naphthalene cracking since no benzene formation was detected during the tests with the blank disk. Industrially, the steam reforming of methane or naphtha is carried out with a nickel-based catalyst in tubular reactors to produce syngas (CO and H2).27,28 On the basis of this industrial practice, various commercial nickel-based catalysts were also tested for tar cracking. It was pointed out that CO2/H2O reforming of hydrocarbons is responsible for tar cracking over nickel-based catalysts, resulting in an increase in the content of syngas in the fuel gas.6,29 According to our work, both naphthalene and methane are converted to syngas by consuming H2O. It seems that mainly the steamreforming reaction of hydrocarbons takes place over the catalytic filter substrates, which is in agreement with the results obtained by Rostrup-Nielsen and Bak Haansen.30 Conclusions The feasibility of tar removal from a sulfur-free and dust-free biomass gasification gas with a nickel-activated candle filter has been positively demonstrated. Above 800 °C, almost complete conversion of the tar model compound naphthalene to H2 and CO was found with gas velocities required for simultaneous particle removal. These results are encouraging for further study of more complex tar mixtures consisting of benzene, naphthalene, and higher polyaromatics and also of the influence of 50-200 ppmv H2S in the gasification gas, which is a known poison for nickel catalysts. This implies that more complex sulfur-tolerant catalysts for tar cracking will need to be explored. Acknowledgment The supply of ceramic filter substrates by Schumacher is much appreciated. This work was financed by Research in Brussels (RIB-96/32) and Interuniversity Attraction Poles (IUAP4-11) for Hongbin Zhao and by the Flemish Institute for the Promotion of Scientific and Technological Research in the Industry (IWT) for D. Draelants. Nomenclature Nm3 ) cubic meter at normal conditions (0 °C, 1 atm) NmL ) mL at normal conditions (0 °C, 1 atm) X ) conversion of naphthalene or methane, % YG ) the yield of gases, % YB ) the yield of benzene, % Min ) the inlet molar flow rate, mmol/min Mout ) the outlet molar flow rate, mmol/min Mout-benzene ) the outlet molar flow rate of benzene, mmol/ min Min-naphthalene ) the inlet molar flow rate of naphthalene, mmol/min ND ) not detectable
Subscripts G ) gas B ) benzene in ) inlet of reactor out ) outlet of reactor
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Received for review February 10, 2000 Revised manuscript received May 5, 2000 Accepted May 24, 2000 IE000213X
