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Energy & Fuels 2005, 19, 22-27
Novel Catalyst for Cracking of Biomass Tar Tiejun Wang,* Jie Chang, and Pengmei Lv Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China
Jingxu Zhu Faculty of Engineering Science, The University of Western Ontario, London, Ontario, Canada N6A 5B9 Received May 7, 2003
Cracking of biomass tar was investigated over Ni/dolomite catalyst prepared by the incipient wetness method using modified dolomite as precursor. Modified dolomite was prepared by mixing Fe2O3 powders with natural dolomite powders to increase Fe2O3 content for higher activity of tar cracking. Four other catalysts (natural dolomite, modified dolomite, ICI-46-1, and Z409) were tested and compared with Ni/dolomite catalyst. The effects of temperature, steam-to-carbon, and space velocity on tar conversion were explored. Ni/dolomite is shown to be very active and useful for tar removal. A 97% tar removal is easily obtained at catalyst temperature of 750 °C and space velocities of 12 000 h-1. The minimum S/C ratio for Ni/dolomite was 2.5 at a catalyst temperature of 750 °C to prevent the formation of the coke on the catalyst. No obvious deactivation of catalyst was observed in 60 h on-stream tests. Compared with the Ni-based catalysts (ICI46-1, Z409), Ni/dolomite catalyst is cheap and has also excellent activity and anticoke ability.
Introduction As a CO2 neutral source of renewable energy, biomass has several environmental advantages for industrial exploitation, especially in small- to medium-size plants located close to the sites where feedstocks are available.1 Remarkable progress has been achieved in recent years in the design of biomass gasifiers. However, gas cleaning is still the bottleneck in advanced gas utilization that limits the development of the use of biomass for electricity generation, indirect liquefying, and other specific applications.2,3 Hot dry gas cleaning and upgrading are nowadays the best solution using calcined dolomite or steam reforming (nickel-based) catalysts. Corella and co-workers started testing these catalysts in 1984, and a lot of data have been already generated in this concrete hot gas cleaning process.4-7 However, these nickel-based catalysts are deactivated by coke formation and are more expensive.8-10 Orı´o et al. * Corresponding author. Telephone: +86-20-87057751. Fax: +8620-87057789. E-mail:
[email protected]. (1) Courson, C.; Makaga, E.; Petit, C.; Kiennemann, A. Catal. Today 2000, 63, 427-437. (2) Zhao, H.; Draelants, D. J.; Baron, G. V. Catal. Today 2000, 56, 229-237. (3) Sutton, D.; Kelleher, B.; Ross, J. R. H. Fuel Process. Technol. 2001, 73, 155-173. (4) Narva´ez, I.; Corella, J.; Orı´o, A. Ind. Eng. Chem. Res. 1999, 36, 3317-327. (5) Corella, J.; Aznar, M. P.; Gil, J.; Caballero, M. A. Energy Fuels 1999, 13, 1122-1127. (6) Aznar, M. P.; Caballero, M. A.; Gil, J.; Martı´n, J. A.; Corella, J. Ind. Eng. Chem. Res. 1998, 37, 2668-2680. (7) Narva´ez, I.; Orı´o, A.; Aznar, M. P.; Corella, J. Ind. Eng. Chem. Res. 1996, 35, 2110-2120. (8) Rapagna`, S.; Jand, N.; Kiennemann, A.; Foscolo, P. U. Biomass Bioenergy 2000, 19, 187-197.
investigated several different dolomites (from Norte, Chilches, Malaga, and Sevilla) for oxygen/steam gasification of wood in a downstream catalytic reactor. The higher activity of the dolomites was attributed to their higher Fe2O3 contents.11 The present authors modified the natural dolomite with addition of Fe2O3 powder to increase its Fe2O3 content for higher activity. Using modified dolomite as precursor, the Ni/dolomite catalyst was prepared by the method of incipient wetness. The objective of this paper is to check the characteristics of this cheap Ni/dolomite catalyst in order to reduce the cost of the hot gas cleaning process. Experimental Section Feedstock. The biomass tar was provided by Hongyuan Renewable Energy Co. (1-MW-scale biomass gasification and electric power system) located in Guangdong Province, China. This MW-scale demonstration plant converted the wood powder residue into electricity through biomass air gasification-power generation technology. The operation parameters are gasification temperature (780 °C), equivalence ratio (ER ) 0.26), gasification efficiency (70%), conversion efficiency of carbon (80%) and LHV of the fuel gas (5.8 MJ/m3). The system includes a Venturi purifying tube and three water scrubbers for gas cleaning which eliminates the tar content in the fuel gas from 35 to 5.2 g/m3. However, the tars in the form of aerosols are very difficult to remove by scrubbing systems. (9) Moersch, O.; Spliethoff, H.; Hein, K. R. G. Biomass Bioenergy 2000, 18, 79-86. (10) Marquevich, M.; Czernik, S.; Chornet, E.; Montane´, D. Energy Fuels 1999, 13, 1160-1166. (11) Orı´o, A.; Corella, J.; Narva´ez, I. In Developments in Thermochemical Biomass Conversion; Bridgwater, A. V., Boocock, D. G. B., Eds.; Blackie Academic & Professional: London, U.K., 1987; pp 11441157.
10.1021/ef030116r CCC: $30.25 © 2005 American Chemical Society Published on Web 12/22/2004
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Figure 1. Schematic of the experimental setup. Table 1. Element Distribution of Ni/Dolomite Catalyst Composition element C O Na Mg Al Si Ca Fe Ni total
wt %
at. %
0.63 44.40 1.93 1.43 0.40 1.85 38.40 6.86 4.10
1.29 65.76 1.91 1.34 0.34 1.79 23.85 2.63 1.69
100.00
100.00
Even at temperatures well below the boiling point a certain amount of each component remains in the vaporous phase. A specific oil ejector was designed and fixed before the gas engine. It could remove 30% of tar aerosols in the fuel gas. The feedstock in our tests was collected from the outlet of the oil ejector. It was black viscous liquid at room temperature. Catalysts. Natural dolomite comes from a Chinese mine and contains 31 wt % CaO, 20 wt % MgO, 45 wt % CO2, 0.7 wt % SiO2, 0.1 wt % Fe2O3, and 0.5 wt % Al2O3. The specific surface area of the dolomite is 1.1 m2/g. It was calcined for 3 h at 900 °C under an air atmosphere. The modified dolomite was prepared by mixing Fe2O3 powders with natural dolomite powders (Fe2O3: dolomite ) 5:95). The mixture was calcined at 900 °C for 3 h under an air atmosphere, then pressed, crushed, and sieved to 20-40 mesh. The Ni/dolomite catalyst was prepared by the incipient wetness method using an aqueous solution of Ni(NO3)2 and 20-40 mesh modified dolomite. After evaporating water, the catalyst was dried at 120 °C for 12 h and calcined at 900 °C for 4 h under an air atmosphere. The elemental analysis of catalyst was shown as Table 1. Natural dolomite and two different commercial steam-reforming catalysts (ICI-46-1, Z409) have been tested in this work. The ICI-46-1 catalyst was manufactured by ICIKatalco. It contains 24 wt % NiO, 14 wt % SiO2, 29 wt % Al2O3, 13 wt % MgO, 13 wt % CaO, and 7 wt % K2O. The Z409 catalyst was manufactured by Qilu Petrochemical Co., China. It contains 22 wt % NiO, 23 wt % Al2O3, 11 wt % MgO, 13 wt % CaO, 11 wt % SiO2, 5 wt % Fe2O3, and 7 wt % K2O. Commercial steam-re-forming catalysts were crushed and sieved to particle size of 20-40 mesh to avoid big wall effects since the catalytic reactor had 10 mm i.d. The Ni/dolomite catalyst was characterized by elementary analysis performed on THERMO ISIR1000, by powder X-ray diffraction (XRD) on a Siemens D500TT diffractometer using Cu KR radiation, by scanning electron microscopy (SEM) on a Topcon EM 002B apparatus coupled to energy dispersion X-ray spectroscopy, and by thermal analysis on A&D HM-200.
Apparatus. Experiments were carried out in a bench-scale reactor unit (Figure 1). It consisted of a catalytic packed-bed quartz reactor (8.0 mm i.d.) mounted inside an electric furnace. The biomass tar was initially loaded in a reservoir located inside a second electric furnace, where it was maintained at 0.1 MP and constant temperature above its melting point. A constant flow of nitrogen was bubbled through the melted biomass tar, and the stream of saturated nitrogen flowed to the reactor entrance through a heating line. A cold trap located at the reactor outlet was used to recover the nonconverted biomass tar. Water was pumped with an HPLC pump into the vaporizer to generate the steam. The steam flowed to the reactor entrance through a heating line. A second cold trap located at the reactor inlet was used to measure the content of biomass tar in the stream of saturated nitrogen, which indicated how much tar was fed into the reactor. Temperature in the catalytic bed was measured with 0.5 mm o.d. thermocouples (type K), placed inside the catalyst bed with a 1/8 in. o.d. sheath. Analysis. During each run, samples of exit gas and tars were periodically (every 5-10 min) taken. The tar sampling system consists of five impinger flasks of 300 cm3 each. The first trap is a flask impinger containing 200 cm3 of a CS2 solvent that exhibits excellent solubility for biomass tar in our tests. The last four traps were placed in an ice + salt bath (-5 °C) to condense tar and water. After sampling of a small amount of gas (500 mL), the overall sample (coming from the five flasks) was diluted with enough pure water until a homogeneous phase is obtained. Because the CS2 and H2O produce no response in a flame ionization detector (FID), the tar sample is directly analyzed by a HP4890 gas chromatogram with blank column (length, 1 m; diameter, 0.52 mm) and FID, which is based on the measurement of the total amount of hydrocarbon in the sample.9,12,13 The cool, dry, clean exit gases was sampled and analyzed by an on-line gas chromatogram (model GC-2010, Shimadzu, Japan), which is fitted with a GScarbon plot column (30 m × 0.530 mm × 3.00 µm) and FID and TCD detectors. Helium was used as carrier gas.
Results and Discussion Accuracy of Tar Analysis Method. Because of the amount of tars in the inlet stream (about 5 g/m3 similar to that of real fuel gas after water scrubbers), high accuracy of the tar analysis system is crucial for obtaining a reliable tar value. The negative effect on the accuracy of the system may be the impurity of the (12) Demirbas, A. Energy Convers. Manage. 2002, 43, 897-909. (13) Hasler, P.; Nussbaumer, T. Biomass Bioenergy 2000, 18, 6166.
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Wang et al.
Figure 2. FID signal during analysis cycle.
Figure 3. X-ray diffractograms of Ni/dolomite calcined at 900 °C. References: CaO (37-1497 JCPDS file), Ca(OH)2 (4-0733 JCPDS file), and NiO (22-1189 JCPDS file).
CS2 solvent. Figure 2 gives an example for the output signal during one analysis cycle. The signal of an FID is almost directly proportional to the amount of organically bound carbon atoms for most biomass tar components. Only one peak appeared when a 0.1 µL tar sample was injected into gas chromatogram. The tiny peak appeared when 0.1 µL of CS2 solvent (containing no tar) was injected into the gas chromatogram. The appearance of this tiny peak was attributed to the impurity of the CS2 solvent. However, the area of this tiny peak was much smaller than that of the tar sample. For the absolute value of hydrocarbons, a high accuracy with less than 0.1% of scatter can be achieved. The minimum tar concentration that could be detected is about 10 mg/m3. Characterization of the Ni/Dolomite. The XRD diagram of catalyst calcined at 900 °C is presented in Figure 3. It shows that the dolomite phrase is maintained as the two phrase with a slight shift compared respectively to the JCPDS file of the CaO and Ca(OH)2 references. The most particular area is situated at 2θ between 37.5° and 43.5°, characteristic of the cubic NiO phrase (JCPDS 22-1189). Scanning electron microscopy
(SEM) of the catalyst (Figure 4) shows porosity of this support (holes of diameters ranging from 1 to 5 µm) and a deposit of almost spherical uniform grains (probably NiO) with size between 0.2 and 0.3 µm could be observed and hid completely the surface of the support. After the sample was calcined at 1100 °C, the porosity observed on Ni/dolomite catalyst disappeared, more compact grains were formed, and the local crystal was increased. Activity of Catalyst. To determine the catalytic activity of the Ni/dolomite catalyst, natural dolomite, modified dolomite, and two other commercial catalysts (ICI-46-1, Z409), the tar conversion in the catalytic bed was studied. From the measured tar contents in the gas stream at the catalytic bed inlet and exit, the tar conversion was calculated for all catalysts under wellknown experimental conditions. This overall tar conversion includes all catalytic reactions (for tar removal) as well as thermal reactions. Conversion was determined at different combinations of temperature, steam-tocarbon ratios, and space times individually. Effect of the Catalytic Bed Temperature. The effect of the catalytic bed temperature is shown in Figure 5 for space time of 0.018 (kg of catalyst)‚h/m3
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Figure 4. Scanning electron microscopy (SEM) micrograph of Ni/dolomite calcined at 900 °C.
Figure 5. Tar conversion in the catalytic bed versus temperature (space velocity ) 12 000 h-1; S/C ) 0).
Figure 7. CO content in the exit gas versus temperature (space velocity ) 12 000 h-1; S/C ) 0).
Figure 6. H2 content in the exit gas versus temperature (space velocity ) 12 000 h-1; S/C ) 0).
Figure 8. CO2 content in the exit gas versus temperature (space velocity ) 12 000 h-1; S/C ) 0).
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Figure 9. CH4 content in the exit gas versus temperature (space velocity ) 12 000 h-1; S/C ) 0).
Figure 10. C2+ content in the exit gas versus temperature (space velocity ) 12 000 h-1; S/C ) 0).
Wang et al.
Figure 12. Composition of the exit gas versus S/C ratio (Tb ) 800 °C).
Figure 13. Tar conversion versus space time (Tb ) 800 °C; S/C ) 2.5).
Figure 11. Tar conversion versus S/C ratio (Tb ) 800 °C).
Figure 14. Tar content in the exit gas versus time on stream (S/C ) 2.5; Tb ) 800 °C).
reactant gas (Tb, wet). No steam was injected into the catalytic bed to achieve the steam-to-carbon (S/C) ratios of 0. The catalytic bed ranged in temperature from 650 to 850 °C. Tar conversion ranged from 84 to 98% with Ni/dolomite catalyst, 87 to 99% with ICI-46-1 catalyst, 82 to 98% with Z409 catalyst, 43 to 95% with calcined dolomite catalyst, and 44 to 97% with modified dolomite.
The higher activity of modified dolomite was attributed to its higher Fe2O3 content, which was in agreement with the results of Orı´o et al. 11 The Ni/dolomite catalyst showed excellent activity for tar decomposition similar to other nickel-based catalysts (ICI-46-1, Z409). Figures 6-10 show the increase and decrease extent of different gases. The varying tendency of gas composi-
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Energy & Fuels, Vol. 19, No. 1, 2005 27
Figure 15. Thermal analysis of Ni/dolomite catalyst after coke formation. Ambience: dry air, 9.8 mg sample, 10 mg TG scale, 100 µV sensitivity, 10 mV/min DTG scale, 10 K/min rate of heating, and 28 mL/min air flow.
tion with Ni/dolomite catalyst is similar to that of commercial nickel-based catalysts (ICI-46-1, Z409), but different from that of natural dolomite and modified dolomite catalysts. The content of H2 was improved greatly with all catalysts. The natural dolomite and modified dolomite catalysts could not adjust the gas composition greatly. However, the Ni/dolomite, ICI-46-1 and Z409 catalysts could adjust the gas composition greatly at high temperature (above 750 °C) by dry reforming reaction (eq 1). There is generally an increase in the hydrogen and carbon monoxide content of the exiting gas, with elimination or reduction of the hydrocarbon and methane content.
CnHm + nCO2 f 2CO +
m H 2 2
(1)
Effect of the S/C Ratio. Steam re-forming of biomass tar was investigated with five catalysts (natural dolomite, modified dolomite, Ni/dolomite, ICI-46-1, and Z409) at catalytic bed temperature of 750 °C. We used 2.0 g of catalyst and a tar flow rate of 0.62 g/min for all the experiments and varied the steam flow rate to achieve the desired S/C ratios. Figure 11 plots the tar conversion for each catalyst as a function of the steamto-carbon ratios of 1.3, 2.5, 3.8, 5.2, and 6.5. The S/C ratio had a minor effect on tar conversion in the range studied, although conversion generally tends to increase as the S/C ratio rises. Figure 12 is a plot of the gas composition as a function of the S/C ratio. A high-steam partial pressure makes the water gas shift equilibrium toward hydrogen formation. However, the partial pressure of the organic compound in the gas stream is lower due to the dilution as the S/C ratio rise. All of those affect the apparent kinetic constant, making the reaction rate constant in the S/C ratio. Effect of Space Time. The effect on tar conversion of the gas residence time in the catalytic bed, expressed as space time, is shown in Figure 13 for the different catalysts. The Ni/dolomite catalyst is very active for tar conversion. They need a space time of only 0.020 (kg of catalyst)‚h/m3 (Tb, wet) to get conversions of around
98%. This value for the space time is equivalent to a space velocity of around 12 000 h-1 or to a residence time of less than 0.1 s. This means a very high intrinsic catalytic activity and usefulness for tar elimination. Catalyst Life. Several tests were made for the Ni/ dolomite catalyst in 2 weeks to get data on the catalyst activity vs time on stream. Test of 60 h on-stream (Figure 14) showed an initial loss of activity, but the activity stabilized after 10-15 h with no further deactivation in a 60 h test period. The carbon content on the catalyst was less than 0.8 wt % after this time period. Thermal analysis of catalyst after coke deposition was presented as Figure 15. Two obvious endothermic peaks were observed. One was located at 400-500 °C, and the other was located at 500-600 °C. The former may be the coking on the surface of nickel, and the latter may be the coking on the surface of modified dolomite. Compared with the area of two endothermic peaks, it indicated that the coking formation on modified dolomite was primary. With saturated wet air as regeneration gas, the catalyst was regenerated at 750 °C. The catalytic activity was restored within 0.5 h. Conclusions The Ni/dolomite catalyst is highly effective at the removal of biomass tar and adjustment of the gas composition to syngas quality. Its activity is similar to the commercially available nickel-based re-forming catalysts (ICI-46-1, Z409). The excellent activity of Ni/ dolomite catalyst may be attributed to the utilization of modified dolomite (high Fe2O3 content, good pore size, and distribution) as precursor and strong nickeldolomite interaction. The Ni/dolomite catalyst is effective and relatively cheap, which may be lead to reduction in the cost of the hot gas cleaning process. Acknowledgment. Financial support from the “OneHundred-Scientist Program” of the Chinese Academy of Sciences to J.C. is gratefully acknowledged. EF030116R
