Environ. Sci. Technol. 2008, 42, 6224–6229
Development of Nano-NiO/Al2O3 Catalyst to be Used for Tar Removal in Biomass Gasification J I A N F E N L I , †,‡,§ R O N G Y A N , * ,‡ B O X I A O , § DAVID TEE LIANG,‡ AND LIJUAN DU† Department of Chemical and Environmental Engineering, Wuhan Polytechnic University, Wuhan 430023, China, Institute of Environmental Science and Engineering, Nanyang Technological University, Innovation Center, Block 2, Unit 237, 18 Nanyang Drive, Singapore 637723, and School of Environmental Science & Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
Received January 21, 2008. Revised manuscript received May 13, 2008. Accepted May 14, 2008.
The objective of this study is to develop a novel supported nanoNiO catalyst for tar removal in biomass gasification/ pyrolysis, to significantly enhance the quality of the produced gases. For this purpose, the supported nano-NiO/γ-Al2O3 catalyst was prepared by deposition-precipitation (DP) method. Different analytical approaches such as XRD, BET, TEM and SEM/ EDX were used to characterize the synthesized catalysts. The results showed that the prepared nano-NiO/γ-Al2O3 catalysts had a coated structure with a loading of NiO in catalysts over 12 wt %, and they had also a higher BET surface area over commercial nickel based catalysts. The active components of catalyst were spherical NiO nanoparticles coated on the surface of supports with a size range of 12-18 nm. Furthermore, the activity of the catalysts to remove tar in the process of biomass pyrolysis was also investigated using a bench-scale combined fixed bed reactor. The experiments demonstrated that the tar yield after addition of the catalyst was reduced significantly; the tar removal efficiency reached to 99% for catalytic pyrolysis at 800 °C, and the gas yield after addition of the catalyst increased markedly. The compositions of gas products before and after addition of the catalyst in the process also changed significantly. The percentages of CO2 and CH4 in the product gas after addition of the catalysts were obviously reduced, while those of the valuable H2 and CO strongly increased. Therefore, using the prepared NiO/γ-Al2O3 catalyst in biomass gasification/pyrolysis can significantly improve the quality of the produced gas and meanwhile efficiently eliminate the tar generation.
1. Introduction Biomass gasification is one of the promising technologies converting biomass to bioenergy. One of the major issues in biomass gasification is to deal with the tar formed during the process (1). Oil vapor condenses at reduced temperature to form tar, thus causing problems of blocking and fouling in downstream equipments and becoming an environmental * Corresponding author phone: (65)67943244; fax: (65)67921291; e-mail:
[email protected]. † Wuhan Polytechnic University. ‡ Nanyang Technological University. § Huazhong University of Science and Technology. 6224
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concern (2). Considerable efforts have therefore been directed on tar removal from fuel gas. The main approaches applied so far include physical processes such as filters or wet scrubbers, and thermal processes of high temperatures cracking and catalytic cracking. Among these methods, catalytic cracking, which could operate at relatively lower temperatures and generate high tar removal efficiency, is recognized as the most efficient method to diminish the tar formation in the gas mixture. Catalytic pyrolysis or gasification for tar reduction has been extensively reported in the literatures (3–5). Numerous materials have been tested, only a few have been found to be active catalysts for tar cracking in biomass gasification or pyrolysis. Ni-based catalysts are found to be the most popular types and also the very effective ones for hot gas cleaning (6–8). Currently the developments of novel nickel-based catalyst with improved performance are being carried out. In recent years, nanomaterials have attracted extensive interests for their unique properties in various fields in comparison with their bulk counterparts (9, 10). Therein, nanometer-sized NiO (nano-NiO) particles have attracted much attention for their catalytic properties (11). In particular, for saving cost, nano-NiO particles can be loaded on the surface of distinct carriers (such as alumina, Al2O3) to prepare the supported catalyst. Several methods have been used to synthesize the supported catalysts, involving impregnation, ion exchange, and deposition-precipitation (DP) (12). The impregnation method because of incipient wetness, often leads to poor dispersions while the ion-exchange method, on the other hand, frequently yields high and homogeneous dispersions, but also yields low metal contents (13). The DP method consists of the precipitation of a metal (oxide) phase onto support by addition of a base to a metal salt solution containing support in solution. Deposition-precipitation of metal precursors on supports, when accompanied by adsorption, is recognized to yield high and homogeneous dispersion in relatively short time, even at high metal contents. Thus, the DP method was chosen in the present investigation for the preparation of catalyst. This study targeted to develop a novel and low-cost supported nano-NiO catalysts for tar removal in biomass catalytic gasification or pyrolysis. γ-alumina was used as a support, and urea was used as a precipitant; nickel salt was used as a metal-phase source. The supported nano-NiO/ γ-Al2O3 catalystwerethuspreparedbydeposition-precipitation (DP) method, followed by a series of characterization using XRD, BET, TEM, and SEM/EDX. Following that, catalytic activity of the prepared supported catalyst was evaluated in a bench-scale combined fixed bed reactor by comparison of biomass pyrolysis behaviors with and without catalysts.
2. Experiment and Methods 2.1. Catalyst Preparation. The incorporation of the precursor of nano-NiO into the porous γ-Al2O3 support substrates (>99% purity, ∼3 mm microsphere) was performed by deposition-precipitation (DP) method, and the preparation conditions of nano-NiO were selected according to our previous study (14). The γ-Al2O3 supports were purchased from the Johnson Matthey Company, and they were kept under a vacuum of a few torr for 1 h to exclude the air in the pores of support. First, a stoichiometric amount of reactants, that is, Ni(NO3)2 · 6H2O (nickel nitrate hexahydrate, 0.08 mol) and CO(NH2)2 (urea 0.32 mol) were accurately weighed in a beaker and dissolved subsequently into 100 mL deionized (DI) water. The mixture was stirred with a magnetic stirrer at room temperature until a homogeneous solution was 10.1021/es800138r CCC: $40.75
2008 American Chemical Society
Published on Web 07/02/2008
obtained. Whereafter, the homogeneous solution was transferred into a glass vessel containing 32.64 g of γ-Al2O3 supports, sealed, and placed in oil bath heating at 115 °C for 2.5 h of deposition-precipitation, resulting in precipitation of the nickel precursor (a light green sediment) on γ-Al2O3 supports. After the reaction was complete, the mixture was cooled to ambient temperature and then filtered. The light green solid spheres were washed with DI water to neutral and colorless for removing the possible absorbed ions and chemicals. The light green spheres were then dried in an oven at 90 °C for 6 h. Finally, the dried spheres were calcined in a muffle furnace at 400 or 700 °C (as a reference) for 1 h in air atmosphere, respectively. This process would result in the precipitated nickel precursor on support to decompose to nickel oxide. The resulting catalyst products (i.e., nanoNiO/γ-Al2O3) were then collected for further analysis. All the chemicals such as nickel nitrate hexahydrate, urea, and alumina used in the preparation of catalyst were analytical grade, and they were used without further purification. Deionized water was used throughout the preparation process. 2.2. Catalyst Characterization. The crystalline structures of the supported catalyst product were identified using a Shimadzu XRD-6000 X-ray diffractometer employing Cu KR radiation (λ ) 0.15418 nm). The X-ray tube voltage and current were 40 kV and 30 mA, respectively. The sample was scanned from 10° to 85° (2θ) with a scanning rate of 4°/min. The average crystallite size of the nano-NiO product was estimated according to the Scherrer equation and further confirmed by the transmission electron microscopy (TEM) results. TEM modeled FEI TECNAIG2 working at 100 kV was used to understand the grain size and morphology of nano-NiO particles dispersed on the support surface. For some selected catalyst samples, the elemental distribution of surface and inside the supported catalyst was examined by investigation of a polished radial cross section of the disk using the scanning electron microscope (SEM) coupled with energy dispersive X-ray spectroscopy (EDX). The analysis was performed on a JSM 5310 LV scanning microscope with an energy dispersive detector. The electron probe worked at an acceleration voltage of 15-25 kV and the K alpha line of Ni was used as analytical energy line for the X-ray detector. The metal content in the supported catalysts was measured by X-ray fluorescence (XRF). The equipment used is Philips, PW2400, made by PANalytical in Netherlands. The catalyst samples were first ground in R-alumina mortars to make sure the particles possess enough affinity to form selfsupported disk for XRF analysis. All samples yield satisfactory disks after they were pressed at 10 tons of ram load with 2 min of bleeding time. The sample disks were then put into the sample cell of a XRF machine that automatically do a scanning and gives quantitative results over each found element as weight percentage (oxide form) of each major element found. Accelerated Surface Area Porosimetry (ASAP 2010) instrument, which used liquid nitrogen at 77K, was applied to measure the BET surface area of support and catalyst for estimating its catalytic activity. 2.3. Evaluation of Catalytic Activity. The activity of the supported NiO/γ-Al2O3 catalyst in biomass catalytic pyrolysis was evaluated. The experimental system shown in Figure 1 consisted essentially of a fixed bed pyrolysis unit (underlayer primary pyrolysis bed) combined with another catalytic fixed bed and with a continuous screw feeding system, a gas cleaning section containing a cyclone solid collector and a quartz wool filter, and a cooling system for the separation of water and condensable organic vapors (tar), as well as various gas measurement devices. The free-fall reactor in underlayer fixed bed was 200 mm in diameter (i.d.) × 400 mm in height, and the catalytic fixed reactor was 88 mm in
FIGURE 1. Flow scheme of bench scale reactor. diameter (i.d.) × 1200 mm in height, constructed of stainless steel with temperature control. Both reactors were heated externally, and they could be separately temperature controlled. Biomass pyrolysis with or without catalyst was conducted respectively, under the same process conditions except for certain exceptions mentioned in the text. When pyrolysis without catalyst was conducted, the under-layer bed was controlled at 800 °C, while the catalytic bed was empty, and its temperature was maintained at 200 °C for prevention of vapor condensation. However, for catalytic pyrolysis of biomass, the catalytic bed was packed with the NiO/γ-Al2O3 catalyst (1 kg) calcined at 400 °C. The primary pyrolysis bed temperature was always maintained at 800 °C, and the catalytic bed temperature was maintained at 600, 700, and 800 °C, respectively. Sawdust particles (in particle size 0.15-2 mm) were used as raw materials. They were stocked in a hopper and fed into the reactor via a screw feeder at a rate of 1 kg /h, when the desired temperature was reached. The generated vapors in the hot reactor with fine particles were drawn out by the vacuum pump, which passed through the cyclone and quartz wool filter for the removal of fine particles. The liquid products were condensed when the gas was passed through the watercooling tube and two ice-water condensers in series. After every experiment, the residue collected inside the primary pyrolysis bed reactor and the cyclone were combined and recorded as solid charcoal. The cooling tube and condensers were weighed, and the weight difference before and after the experiment was recorded as the liquid (including water and tar) yield. The total gas yield (wt %) could thus be calculated by difference based on the mass balance of the fed biomass in a specific time period at a constant feeding rate. Those uncondensable gases were precleaned through a glass wool filter and dried by silica gels prior to analysis. The gas products were analyzed using a dual Channels Micro-Gas Chromatograph (Micro-GC, Varian, CP-4900) with a thermal conductive detector (TCD). The detailed measuring procedure can be found in our previous publication (15). Each trial was maintained for a period of 30 min, and several repeat runs were carried out under identical conditions to ensure the repeatability of the process and the reproducibility of the experimental data (product yield) was calculated to be within (2%.
3. Results and Discussion 3.1. Analysis and Characterization of the Catalyst. 3.1.1. Appearance and XRF Analysis. The supported nano-NiO/γ-Al2O3 catalysts prepared by DP method were found to be spherical in shape, essentially, NiO nanoparticles were coated on the support of a γ-Al2O3 sphere (3 mm diameter) as eggshell structure. The apparent color of the supported nano-NiO/ γ-Al2O3 catalysts had an obvious change as a function of the calcination temperature. The catalyst products derived from the precursors calcined at 400 °C for 1 h was dark in color, while that obtained at 700 °C was in light blue color. It was VOL. 42, NO. 16, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. XRD pattern of the catalyst samples calcined at 400 and 700 °C. probably relative to the different valences of nickel or crystal structures associated at different temperatures, which will be confirmed by XRD analysis in the following text. The content of metal oxide in the supported catalysts was measured using XRF. The NiO contents in catalysts prepared at 400 and 700 °C were 12.2% and 12.7%, respectively, which indicated that the calcinations temperature did not exert a significant effect on Ni loading. For the purpose of comparison, the impregnation method for preparing catalyst was also tested where γ-Al2O3 was impregnated in nickel salt, followed by calcinations under the same conditions. The results showed that the NiO loading of catalysts prepared by impregnationwerealllessthan5%.Thedeposition-precipitation (DP) of metal precursors on supports was thus recognized to yield higher metal loading over the impregnation method. The similar findings were reported by Chang (13) on preparation of rice husk ash supported nickel catalyst and by Burattin (16) on Ni/SiO2 materials. 3.1.2. XRD Analysis. X-ray diffractions of catalyst products were carried out to identify the phases present in the supported catalyst samples (NiO/γ-Al2O3), and the results are presented in Figure 2. The XRD profiles of catalyst samples obtained at calcination temperature 400 °C showed that NiO (2θ ) 37.3, 43.3, 62.9, 75.3, and 79.5°) and γ-Al2O3 were the main phases, while NiAl2O4 (nickel aluminate) (2θ ) 45.0, 56.8, 60.4, and 66.0°) was not observed at 400 °C calcination; however at a higher calcination temperature of 700 °C, the NiAl2O4 phase, which characterizes a spinel structure, appeared in the XRD profiles of catalyst samples. The similar finding was also reported by Fajrdo et al. (17) on catalytic activity of Ni-impregnated Al2O3 spheres. According to Scherrer’s equation, the calculated average particle size of nanocrystalline NiO on surface of 400 °C catalyst samples was about 15 nm. This would be confirmed by its TEM images. The presence of lots of NiO phase suggested the availability of catalytic sites after the activation because NiO has generally a high reactivity in oxidation. 3.1.3. TEM and SEM/EDX Analysis. TEM and SEM/EDX analyses were performed on catalysts obtained at different calcination conditions, and only the representative images of the supported catalyst obtained at 400 °C calcination for 1 h were presented here. The TEM micrograph of NiO nanoparticles on catalyst surface is shown in Figure 3. It can be seen that the NiO nanoparticles were sphere shaped. The size of nanoparticles was between 12 and 18 nm, which coincided with the XRD results of catalysts. The SEM appearance image of NiO/γ-Al2O3 catalyst surface is shown in Figure 4. The surface of catalyst was scraggy, the deposit of NiO nanoparticles on the surface of support was multilayer, and NiO nanoparticles displayed a 6226
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FIGURE 3. TEM micrograph of nanoparticles on NiO/γ-Al2O3 catalyst surface.
FIGURE 4. SEM image of NiO/γ-Al2O3 catalyst surface. fairly uniform spatial distribution on the surface. These are all positive factors for the catalytic application (4, 8). At the same time, a certain extent of aggregation of NiO nanoparticles was still observed. To further confirm the structure and element distribution on the surface and inside catalyst, the samples were cut, and the corresponding elemental distributions on the surface of cross-section and inside catalyst were obtained by EDX with a broad analysis window. SEM cutaway photograph of NiO/ γ-Al2O3 catalyst with EDX analysis is shown in Figure 5. The EDX analysis showed that the inside of catalyst consisted exclusively of the elements Al and O at 46.84% and 53.16%, respectively. But at the surface of catalyst three elements (Ni, Al, and O) were mainly observed at 52.04%, 5.44%, and 42.53%, percent. This further confirmed that the prepared catalyst by DP method was a typical coated structure as eggshell, where the NiO nanoparticles mainly coated on the surface of γ-Al2O3 sphere. The core parts were γ-Al2O3, and the shell layers were enriched in NiO nanoparticles. Meantime, the above observations also indicated that no Ni was found inside the catalyst and that the main composition of catalyst surface was nickel oxide with few Al-bearing compounds that can be attributed to the interaction of NiO nanoparticles with alumina support. In commercially available Ni-based catalysts, however, Ni was found in every part of the catalyst with different concentrations (18). With respect to the heterogeneous catalysts, the active sites of catalyst are the available surface and not the inside, thus the coated
FIGURE 5. SEM cutaway photograph of NiO/γ-Al2O3 catalyst with EDX analysis.
TABLE 1. Surface Properties of Catalysts Measured by N2 Physisorption catalyst sample and support
BET surface area (m2/g)
micropore area (m2/g)
external surface area (m2/g)
total pore volume (cm3/g)
average pore diameter(nm)
support, γ-Al2O3 catalyst calcinations at 400 °C, 1 h catalyst calcinations at 700 °C, 1 h
130.1 124.6 111.8
13.3 10.0 8.6
116.8 114.7 103.2
0.41 0.37 0.37
12.5 12.3 13.4
structure of NiO/γ-Al2O3 catalyst prepared by DP method in this study is supposed to be more effectual than the commercial Ni-based catalyst and it can also save cost significantly, resulting from the much reduced of usage of nickel salt in the preparation process. 3.1.4. BET Analysis. The surface properties of the γ-Al2O3 support and the catalysts were evaluated from the nitrogen adsorption-desorption isotherms using ASAP 2010 instrument, and the results are summarized in Table 1. All supported catalysts exhibited nitrogen isotherms with the same shape as that of the support. The catalysts had a relatively high specific surface area, but a slight reduction of the BET surface area and pore volume compared with that of γ-Al2O3 support. This decrease could be attributed to not only the presence of NiO particles partially blocking the porous network of support but also the increase in the density of the materials after the incorporation of a nickel loading as high as above 12 wt %. Table 1 showed that the BET specific and external surface areas of the supported catalysts in current study were much higher than that of commercial nickel based catalyst, which were reported less than 90 m2/g of BET surface area and 85 m2/g of external surface area (18). This was most likely the result of the smaller size of NiO nanoparticles on NiO/γAl2O3 catalysts. The higher BET specific and external surface areas of the NiO/γ-Al2O3 catalysts indicated its high possibility of application as an efficient catalytic material. 3.2. Evaluation of the Catalyst for Tar Removal in Biomass Pyrolysis. 3.2.1. Comparison of Product Yield before and after Addition of Catalyst. Table 2 shows the percentage mass yield of the pyrolysis products in relation to the mass of biomass fed from the primary bed pyrolysis without catalyst and catalytic pyrolysis of sawdust. It can be seen that the char yields of sawdust pyrolysis at different conditions fell in a narrow range (11.6-11.9 wt %), which was similar with that reported in the previous literature (19). This result also reflected the reproducibility of the trials in the free-fall pyrolysis reactor, as char yield did not change with the catalytic bed temperature. The pyrolysis was undertaken in
TABLE 2. Product Yields from Pyrolyzing Sawdust with and without Catalysts (wt % of Biomass, daf, as received) conditions
gas
tar
water
char
primary pyrolysis without catalyst at 800 °C catalytic pyrolysis with NiO/γ-Al2O3 catalyst 600 °C 700 °C 800 °C catalytic pyrolysis with commercial catalyst 600 °C 700 °C 800 °C
62.9
18.2
7.1
11.8
77.8 80.9 85.2
1.9 0.8 0.2
8.4 6.5 3.0
11.9 11.8 11.6
71.4 78.1 82.5
7.6 3.3 1.6
9.2 6.8 4.2
11.8 11.8 11.7
the primary bed under identical conditions; thus the percentage weight of char should have been identical. The results shown in Table 2 also indicated that the gas percentage mass yield at 800 °C pyrolysis in the absence of catalyst was 62.9 wt %; however, when the pyrolysis vapors were passed through the catalytic bed with nano-NiO/γAl2O3 catalyst there was a marked increase in gas yield. Meantime, the yield of gases increased from 77.8 to 85.2 wt % as the catalytic bed temperature increased from 600 to 800 °C. The significant increase in gaseous products yield was attributed to be predominantly through secondary cracking of the pyrolysis vapors on catalyst in the catalytic bed reactor. A similar finding on other catalysts for biomass pyrolysis was also reported by Williams et al. (20) For the pyrolysis of sawdust in the absence of catalysis, the derived liquid was brown and of low viscosity, whereas the catalyzed liquid product was light yellow in color. In particular, for the catalytic pyrolysis of sawdust at 800 °C, scarcely any visible tar was observed in the line after the catalytic reactor or in the condensers for tar collecting. The collected condensed portions were further separated into aqueous and oil fractions using the standard ASTM D244 VOL. 42, NO. 16, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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and IP 291.1 methods, to determine the total tar yield. In the absence of catalyst, the tar yield from the primary pyrolysis was 18.2 wt % (∼ 78.5 g/Nm3), as shown in Table 2. After the addition of the nano-NiO/γ-Al2O3 catalyst, it fell to only 1.9 wt % (∼ 7.8 g/Nm3) at 600 °C, and was reduced further to 0.8 wt % (∼3.2 g/Nm3) at 700 °C and 0.2 wt % (∼ 0.75 g/Nm3) as the temperature of catalytic bed was increased to 800 °C. The influence of temperature on the final products yield of biomass pyrolysis in the absence of catalyst can be found in our previous publication (21). Although in the absence of catalyst, the total yield of liquid decreased slightly with pyrolysis temperature increasing from 600 to 800 °C while that of gases increased markedly, with presence of catalyst the influence of temperature on the final products yield was much more significant, especially the efficiency of tar removal markedly increased with the increase of catalytic bed temperature. The similar tendency was reported in previous literatures (20, 22). The marked reduction of tar yield after addition of catalysts was also accompanied by a significant increase in gas yield. The primary pyrolysis of sawdust mainly occurred in the under-layer pyrolysis bed, and the role of catalyst in catalytic bed was to enhance the secondary cracking of tar thus to reduce the content of tar in vapor. For a comparison, the commercial nickel-based catalyst (NiO content of 15 wt %) was also used in pyrolyzing sawdust under exactly the same conditions. In Table 2, it could be seen that the catalytic efficiency of commercial nickel-based catalyst for tar removal was inferior to that the nano-NiO/ γ-Al2O3 catalyst, in the studied temperature range of 600-800 °C. In particular, the tar removal with presence of nanoNiO/γ-Al2O3 catalyst at 700 °C exceeded that of commercial nickel-based catalyst at 800 °C. This indicated the developed nano-NiO/γ-Al2O3 catalyst could have a high performance in the catalysis of biomass pyrolysis, even at a relatively low temperature. Various nickel-based catalysts were reported in previous literatures for tar removal and improvement of the produced gas quality. For instance, a nickel-based catalytic filter was developed by Baron et al. (23) to achieve 99.0% tar conversions at optimal operating condition of 850 °C, but with only 77% tar reduction observed at 800 °C. A coprecipitated catalyst of Ni/Al for biomass catalytic pyrolysis was prepared with various pretreatments, the resulting tar and gas yield was 2.7-7.3 wt % and 61.2-80.0%, respectively, at 700 °C pyrolysis temperatures (24). Corella et al. (25, 26) tested seven commercial Ni-based catalysts (NiO content of 12-25 wt %) and reported that they all showed to be very active, with about 95% tar removal easily obtained at 800-850 °C. However, their results were all based on crushed particles of the catalyst, for commercial application the effectiveness factors of 1-10% (only) might have to be applied. In this study, the tar removal efficiency exceeded 99% at 800 °C, indicating that the prepared NiO/R-Al2O3 catalyst was ideal for tar removal in biomass pyrolysis with a high efficiency in comparison with the commercial and other nickel-based catalysts mentioned above. 3.2.2. Comparison of Gas Composition before and after Addition of Catalyst. Table 3 shows the percentage in volume of product gases for the primary bed pyrolysis at 800 °C and catalytic pyrolysis at 600, 700, and 800 °C, respectively. The results showed that the main gas products were H2, CO, CO2, and CH4 with less C2 hydrocarbons (C2H4 and C2H6). For the pyrolysis of sawdust in the absence of catalysis, CO2 and CO were the main components of the product gas. In catalytic pyrolysis with nano-NiO/γ-Al2O3 catalyst, the contents of H2 and CO in gas increased significantly and also increased with the temperature of catalytic bed. They became finally the predominant gas components, with 49.2% H2 and 42.2% CO generated at 800 °C. In contrast, the CO2 content in gas without catalyst was 39.3%, which was sharply reduced to 6228
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TABLE 3. Gas Composition from the Pyrolysis of Sawdust with and without Catalyst (vol %) conditions primary pyrolysis without catalyst at 800 °C catalytic pyrolysis with NiO/γ-Al2O3 catalyst 600 °C 700 °C 800 °C catalytic pyrolysis with commercial catalyst 600 °C 700 °C 800 °C
H2
CO
CO2
CH4
C2H4 C2H6
18.8 22.3 39.3 16.5 2.6
0.5
42.3 22.5 27.6 45.1 33.6 16.7 49.2 42.2 5.9
6.1 1.1 3.7 0.7 2.1 0.5
0.4 0.2 0.1
37.0 21.6 30.9 42.6 27.8 22.5 45.5 36.1 14.2
8.7 1.4 5.8 1.0 3.4 0.7
0.4 0.3 0.1
27.6% at 600 °C with nano-NiO/γ-Al2O3 catalyst and further dropped to 5.9% as the catalytic temperature increased to 800 °C. At the same time, the contents of CH4 and some C2 hydrocarbons in the gases with catalyst were decreased and they continued to decrease with the increasing temperature of catalytic bed. For an easy comparison, the gas product percentages with presence of commercial nickel-based catalyst were also listed in Table 3. It is clear that the nanoNiO/γ-Al2O3 catalyst demonstrated a better performance in improving gas product quality than the commercial one, with higher H2 and CO but lesser CO2 contents observed, even at a lower reaction temperature. The aforementioned results indicated that the prepared NiO/γ-Al2O3 catalyst enhanced markedly the cracking of tar in vapor and of hydrocarbons such as CH4 and CnHm in gaseous products to convert them to valuable H2 and CO gas because of the catalysis assisted decomposition of tar and possibly the lengthening of vapor residence time inside the catalytic bed. Particularly the content of H2 and CO in gas components were enhanced significantly, while that of CH4 was decreased. According to the mechanism of tar and biomass pyrolysis in the literature (21, 27), the yields of H2 and CO could be regarded as an indicator for secondary reactions of oil (tar). Thus the higher H2 and CO content after addition of the catalyst indicated that secondary reactions of oil vapor on the NiO/γ-Al2O3 catalyst was drastic, which suggested that the NiO/γ-Al2O3 catalyst had demonstrated an excellent catalytic activity for tar removal. It was evidenced that the prepared nano-NiO/γ-Al2O3 catalyst could improve significantly the quality of the produced gas and remove efficiently tar presented in the vapor phase of biomass pyrolysis. Taking up to evaluate systematically the developed catalyst, the further study on catalyst lifetime, the possibility of quick deactivation and regeneration and the effect of various carriers would be performed in near future.
Acknowledgments The authors gratefully acknowledge the support from the Programme of Introducing Talents of Discipline to Universities in China (Project B06019) and the science research foundation of Hubei Provincial Department of Education, China under grant number D200618006.
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