Energy & Fuels 2008, 22, 1233–1238
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Preparation of Hydrogen via Catalytic Gasification of Residues from Biomass Hydrolysis with a Novel High Strength Catalyst Wen-zhi Li,† Yong-jie Yan,*,† Ting-chen Li,† Zheng-wei Ren,† Miao Huang,† Jun Wang,‡ Ming-qiang Chen,‡ and Zhi-cheng Tan*,§ Department of Chemical Engineering for Energy Recourses, East China UniVersity of Science and Technology, Shanghai 200237, China, Department of Chemical Engineering, Anhui UniVersity of Science and Technology, Huainan, Anhui 232001, China, and Thermochemistry Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China ReceiVed July 3, 2007. ReVised Manuscript ReceiVed NoVember 8, 2007
Producing fuel alcohol via hydrolysis of lignocellulosic biomass leaves a considerable amount of residues waiting for treatment. A study was carried out on the preparation of hydrogen via catalytic gasification of residues from biomass hydrolysis with a novel Ni/modified dolomite binary catalyst, which was prepared by a two-step coprecipitation method and proved available for hydrogen production in terms of both activity and strength. The effects of four operation parameters, that is, the fluidized bed temperature, the catalytic fixed bed temperature, the particle size of the catalyst, and S/B (i.e., the mass ratio of steam to biomass material fed into the fluidized bed per unit time), on hydrogen yield were investigated. The results indicate that hydrogen yield increases with an increase in the temperature of either the fluidized bed or the downstream catalytic fixed bed or the S/B ratio or a reduction in the particle size of the catalyst. The optimum range for each of the four operation parameters from a comprehensive consideration is as follows: 800–850 °C for both the fluidized bed temperature and the catalytic fixed bed temperature, 1.5–2 for the S/B ratio, and 2.0–3.0 mm for the particle size of the catalyst. Furthermore, the gas product from catalytic gasification of residues from biomass hydrolysis contains less CO and CO2 and has a higher H2/CO ratio compared with that of the sawdust. The hydrogen yield of the former is also much higher than that of the latter. These suggest that residues from biomass hydrolysis are an even better gasification material than the original sawdust. This paper provides a novel effective method for modifying the calcined dolomite, which endows the catalyst with satisfactory strength while retaining high activity, and opens a new promising way for utilizing the residues from biomass hydrolysis.
1. Introduction Attention has been focused on the issue of development of new energy sources, especially renewable energy sources friendly to the environment at the time of global awareness of energy and environment problems. Biomass, one of the renewable resources, is considered to be the raw material that can be converted into liquid fuel, and its abundance supports the argument on economic feasibility.1–3 So far as reported, a good number of researchers4–6 have made their efforts in the field of process technology for fuel alcohol from waste lignocelluloses, * Corresponding authors. Phone: +86 021 64253409 (Y.-j.Y.); +86 411 84379199 (Z.-c.T.). Fax: +86 021 64253409 (Y.-j.Y.); +86 411 84685940 (Z.-c.T.). E-mail:
[email protected] (Y.-j.Y.);
[email protected] (Z.-c.T.). † East China University of Science and Technology. ‡ Anhui University of Science and Technology. § Chinese Academy of Sciences. (1) Hamelinck, C. N.; van Hooijdonk, G.; Faaij, A. P. C. Biomass Bioenergy 2005, 28, 384–410. (2) Wyman, C. E. Trends Biotechnol. 2007, 25, 153–157. (3) Radlein, D. St. A. G.; Mason, S. L.; Piskorz, J.; Scott, D. S. Energy Fuels 1991, 5, 760–763. (4) Canettieri, E. V.; Jackson de Moraes Rocha, G.; Andrade de Carvalho, J., Jr.; Batista de Almeida e Silva, J. Bioresour. Technol. 2007, 98, 422–428. (5) Yuan, C.-m.; Yan, Y.-j. Chin. J. Process Eng. 2004, 4, 64–68. (6) del Campo, I.; Alegria, I.; Zazpe, M.; Echeverría, M.; Echeverría, I. Ind. Crops Prod. 2006, 24, 214–221.
and a two-step process of acidic hydrolysis has been accepted as one of the methods for the preparation of fuel alcohol from biomass.7 Residues left over from the hydrolysis process contain, apart from a little amount of unconverted hemicellulose and cellulose, mostly lignin, which accounts for 35–45% of the biomass feedstock, and therefore, the economy of the process cannot be justified without comprehensive utilization of this amount of solid waste. Danner8 investigated the joint process of fuel alcohol manufacture and electricity generation via gasification of the leftover residues. Marcus Oehman and Christoff Boman9 developed the technology to produce briquette from the hydrolysis residues for domestic fuel. Hydrogen is a kind of clean energy. Apart from the established processes for hydrogen production from fossil fuels, much effort has been made to utilize biomass as a raw material.10–12 However, to the best of our knowledge, no reports (7) Galbe, M.; Zacchi, G. Appl. Microbiol. Biotechnol. 2002, 59, 618– 628. (8) Danner, H. Publishable Final Report for Contract JOR3-CT97-7049, 2001. (9) Öhman, M.; Boman, C. Energy Fuels 2006, 20, 1298–1304. (10) Kim, M.-S.; Baek, J.-S.; Yun, Y.-S.; Sim, S. J.; Park, S.; Kimd, S.-C. Int. J. Hydrogen Energy 2006, 31, 812–816. (11) Hanaoka, T.; Yoshida, T.; Fujimoto, S.; Kamei, K.; Harada, M.; Suzuki, Y.; Hatano, H.; Yokoyama, S.-y.; Minowa, T. Biomass Bioenergy 2005, 28, 63–68. (12) Satrio, J. A.; Shanks, B. H.; Wheelock, T. D. Energy Fuels 2007, 21, 322–326.
10.1021/ef700375s CCC: $40.75 2008 American Chemical Society Published on Web 12/21/2007
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regarding utilization of residues from biomass hydrolysis for hydrogen production have appeared so far. Tar, a product formed in gasification of biomass, must be either removed or converted into some other light gas product before entering the subsequent working procedure. A catalytic conversion process coupled with a gasification process is a general method adopted in hydrogen production from biomass feedstock, and dolomite, as a catalyst, is more often preferred for its activity and cheapness. Olivares et al.13 reported that 2 wt % of dolomite blended with biomass as the feedstock in gasification greatly decreased the content of tar while the hydrogen yield rose compared with the case in the absence of dolomite. Guan Hu et al.14 reported on the gasification of almond stones’ shell under a steam atmosphere employing olivine or dolomite as the catalyst and found that calcination could improve their catalytic activity. It was also concluded that calcined dolomite was inappropriate for use in a fluidized bed due to its fragility. Tiejun Wang et al.15 prepared modified dolomite by mixing Fe2O3 powders with calcined dolomite powders to increase Fe2O3 content for higher activity of tar cracking. Pérez et al.16 found that downstream located calcined dolomite significantly cleaned and upgraded the flue gas from a biomass gasifier and also increased the gas yield. José Corella et al.17 demonstrated how the effectiveness of the dolomite in the second reactor was only a little bit higher than that for the inbed location when the gasifying agents were H2O-O2 mixtures. However, no noticeable chemical differences (between the two locations of the dolomite) in gasification of biomass with air were found. José Corella et al.18 again compared the advantages and disadvantages of the dolomite and olivine as the tar elimination catalysts. Dolomite was shown to be 1.40 times more effective for in-bed tar removal than the raw olivine, but it generated 4–6 times more particulates in the gasification gas than olivine. Lopamudra Devi et al.19 demonstrated that a total tar amount of 4.0 g m0-3 could be reduced to 1.5 and 2.2 g m0-3 using dolomite and olivine, respectively. Ruiqin Zhang et al.20 utilized a calcined dolomite guard bed to crack heavy tars for prolonging the lifetime of the downstream catalyst. Lopamudra Devi21 pointed out that, although dolomite has been proven to be a very effective bed additive in terms of tar reduction, it has some critical limitations. Dolomite is softer and thus gets eroded by the silica sand particles (fluidized heat transfer medium). Also, some dolomite particles break during the calcination and give rise to a large production of fines. Thus, there is a great problem of carryover of solids from the bed. An alternative of dolomite can be olivine which has higher attrition resistance. It is not as abundant as dolomite, however. Just as some of the above studies pointed out, the calcined dolomite is fragile, so it is easily crushed under the impact of gas flow and the powdered dolomite might lead to a high (13) Olivares, A.; Aznar, M. P.; Caballero, M. A.; Gil, J.; Francés, E.; Corella, J. Ind. Eng. Chem. Res. 1997, 36, 5220–5226. (14) Hu, G.; Xu, S.; Li, S.; Xiao, C.; Liu, S. Fuel Process. Technol. 2006, 87, 375–382. (15) Wang, T.; Chang, J.; Lv, P. Energy Fuels 2005, 19, 22–27. (16) Pérez, P.; Aznar, P. M.; Caballero, M. A.; Gil, J.; Martin, J. A.; Corella, J. Energy Fuels 1997, 11, 1194–1203. (17) Corella, J.; Aznar, M.-P.; Gil, J.; Caballero, M. A. Energy Fuels 1999, 13, 1122–1127. (18) Corella, J.; Toledo, J. M.; Padilla, R. Energy Fuels 2004, 18, 713– 720. (19) Devi, L.; Ptasinski, K. J.; Janssen, F. J. J. G.; van Paasen, S. V. B.; Bergman, P. C. A.; Kiel, J. H. A. Renewable Energy 2005, 30, 565–587. (20) Zhang, R.; Brown, R. C.; Suby, A.; Cummer, K. Energy ConVers. Manage. 2004, 45, 995–1014. (21) Devi, L.; Ptasinski, K. J.; Janssen, F. J. J. G. Biomass Bioenergy 2003, 24, 125–140.
Li et al. Table 1. Ultimate Analysis of Raw Materials hydrolysis residues (wt %) sawdust (wt %)
Cd
Hd
Od
Ad
46.83 43.17
7.04 6.27
20.77 48.89
25.36 1.67
pressure drop through a blockade in a fixed bed or lots of catalyst loss as fly ash. In this study, a novel Ni/modified dolomite binary catalyst was prepared and characterized. Its application in the hydrogen production through catalytic gasification of the residues from biomass hydrolysis indicates that modification of dolomite is an effective way to increase the strength of the catalyst, therefore overcoming the above-mentioned shortcoming while retaining its catalytic activity. The gasification of sawdust was also carried out employing the Ni/modified dolomite as a downstream catalyst to compare with that of the residues from biomass hydrolysis. Some meaningful results are obtained and are very helpful for utilization of residues from biomass hydrolysis through gasification. 2. Experimental Section 2.1. Sample Preparation. Two types of gasification samples were prepared. One was sawdust, and the other was a binary mixture of residues with a mass ratio of 1:1 from hydrolysis of sawdust and rice husk. All samples were subjected to adequate air drying under sunshine prior to gasification experiments. Table 1 lists the ultimate analysis of the samples on a dry basis, among which oxygen was obtained by difference. 2.2. Preparation and Characterization of Catalysts. 2.2.1. Calcined Dolomite Catalyst. The as-received dolomite sample, which was from Anhui Province of People’s Republic of China, was calcined for 10 h at a temperature of 900 °C in a muffle furnace. The calcined dolomite was then cooled, ground, and sieved to desired sizes. The catalyst thus prepared is called calcined dolomite catalyst. The composition of the as-received dolomite was as follows: MgO, 21.20 wt %; CaO, 31.60 wt %; Fe2O3, 0.19 wt %; Al2O3, 0.30 wt %; SiO2, 0.13 wt %; loss, 46.58 wt %. 2.2.2. Ni/Dolomite Catalyst. A suspension of calcined dolomite (100–120 mesh) in distilled water was prepared and a nickel nitrate solution (1 M) was fed into the stirred suspension dropwise quantificationally, and a kind of coprecipitate was formed. The coprecipitate together with the suspension was dried at 110 °C for about 12 h until it became a somewhat wet and sticky precipitate. The precipitate was then shaped into a spherical particle of the desired diameter and calcined in air at 900 °C for 10 h. The catalyst thus prepared containing 18 wt % NiO from nickel nitrate was called Ni/dolomite catalyst. 2.2.3. Ni/Modified Dolomite Catalyst. A suspension of the calcined dolomite (100–120 mesh) in distilled water was prepared and a magnesium nitrate solution (1 M) was fed into the stirred suspension dropwise quantificationally, and a kind of coprecipitate is formed. The coprecipitate together with the suspension was dried at 110 °C for about 12 h until it became a somewhat wet and sticky precipitate. The precipitate was calcined in air at 900 °C for 10 h and followed by milling and sieving to 100–120 mesh. This primary coprecipitate was called the modified dolomite. The modified dolomite was again mixed with distilled water to make a suspension. The nickel nitrate solution (1 M) was fed into the stirred suspension dropwise quantificationally, and a secondary coprecipitate was formed. The secondary coprecipitate was dried, shaped, and calcined as described in the above Ni/dolomite catalyst preparation process. The catalyst thus prepared containing 1 wt % MgO from magnesium nitrate and 18 wt % NiO from nickel nitrate was called Ni/modified dolomite catalyst. 2.2.4. Strength Test and XRD Analysis of Dolomite. The strength of the calcined dolomite and the modified dolomite with a diameter
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Figure 1. Schematic diagram of the experimental apparatus: 1, biomass hopper; 2, screw feeder; 3, fluidized bed; 4, cyclone; 5, ash hopper; 6, catalytic fixed bed; 7/11, condenser; 8, filter; 9, middle temperature shift reactor; 10, low temperature shift reactor; 12, steam generator; T, temperature control; V, valve; P, pressure gauge.
of 1 mm was tested by using a ZQJ-II intellectualized particle strength tester, made in Dalian, China. The crystal structures of the calcined dolomite and the modified dolomite were characterized by powder X-ray diffraction (XRD) on a Japan Rigaku D/max 2550VB/PC diffractometer using Cu (40 KV, 100 mA) radiation. It was detected in the angle range 10–90 at 12°/min. 2.3. Experimental Apparatus. The experimental apparatus, as indicated in Figure 1, includes two main components, that is, the fluidized bed and the catalytic fixed bed. The capacity of it is 2 kg/h air-dried feedstock. The fluidized bed, with dimensions of 1500 mm in height and 100 mm in diameter, is made of stainless steel which can endure high temperatures up to 1000 °C. A set of 3 kW electric heaters is installed on the upper and lower parts of the fluidized bed, respectively, and a pressure gauge is fixed on its upper middle part. The catalytic fixed bed, with dimensions of 1105 mm in height and 150 mm in diameter, is also made of stainless steel which can endure high temperatures up to 1100 °C and equipped with the same electric heaters as those on the fluidized bed. The subsequent middle and low temperature shift reactors were not used in this study and would be employed for further study on the CO shift reaction. 2.4. Experimental Procedures. The fluidized bed and the fixed bed were preheated before each test. When the predetermined temperatures were reached, superheated steam (800 °C), acting as both a gasifying agent and a fluidizing gas, was introduced into the fluidized bed from the undersurface of the distributor. Preheated nitrogen gas (600 °C) with a certain flow rate was also introduced into the fluidized bed as a compensating fluidizing gas to ensure fluidization for some run carried out at a low S/B ratio. At the same time, nitrogen gas was introduced to produce pressure head on the feedstock in the hopper to balance the pressure from the fluidized bed, and test materials started to be fed by the screw feeder to the fluidized bed. Collecting samples of gas product began at a time of 20 min after the operation became steady every 5 min, and for each run, 10-15 samples were collected. The gas sampling site was located at the outlet of condenser 7. 2.5. Gas Product Analysis. The gas analysis was conducted by applying an Agilent Technologies 6820 chromatographer. The detector (TCD) temperature was set to 250 °C. The temperature program of the column oven was from 50 to 230 °C with a heating rate of 60 °C/min and a 5 min hold time at 230 °C. The carrier gas was high pure argon, and the column inlet pressure was 0.28 MPa. The sample injection volume was 1000 µL. The composition of the standard gas employed for calibration was as follows: CH4, 4.82%; CO2, 16.10%; CO, 14.20%; H2, 25.20%; N2, 39.68%. The sum of H2 and CO yields was defined as the hydrogen yield in this paper considering CO transform technology is a mature one and also for the sake of discussion convenience.
3. Results and Discussion 3.1. Effect of Fluidized Bed Temperature on Hydrogen Yield and Gas Composition. The fluidized bed temperature is an important parameter influencing the experimental results. To investigate its effect on hydrogen yield, the experiments were
Figure 2. Effect of fluidized bed temperature on gas composition.
Figure 3. Effect of fluidized bed temperature on hydrogen yield.
carried out according to the following conditions. Test materials: hydrolysis residues (e40 mesh); feeding rate, 0.4 kg/h; S/B ) 2 (mass ratio of steam to biomass material fed into the fluidized bed per unit time); catalyst, Ni/modified dolomite (3 mm); catalytic fixed bed temperature, 800 °C; fluidized bed temperature, varying from 650 to 900 °C with a step of 50 °C. The experimental results are shown in Figures 2 and 3. Figure 2 indicates that the H2 content increases as the fluidized bed temperature increases, whereas the CH4 content decreases. This is because reactions 1 and 2 are both endothermic reactions, and therefore, temperature increase shifts the equilibrium to the right. CH4 + 2H2O ) CO2 + 4H2 - 165 kJ
(1)
C + H2O ) CO + H2 - 131 kJ
(2)
Meanwhile, the gasification process of the hydrolysis residues is, on the whole, an endothermic process, so temperature increase favors hydrogen formation from the point view of chemical equilibrium. Hydrocarbon content decreases as the fluidized bed temperature increases. At higher temperatures, hydrocarbon chain breakage leads to the formation of alkenes and methane, which are further converted into hydrogen through steam reforming reactions. These reactions are endothermic, so temperature increase results in a decrease of the equilibrium concentration of methane and alkenes, thus in turn promoting the chain breakage of the hydrocarbon. The above discussion also justifies the hydrogen yield increasing trend as the fluidized bed temperature increases shown in Figure 3. It can be seen from Figure 3 that the hydrogen yield increases quite quickly as the temperature increases from 650 to 800 °C, and the yield curve becomes flat within the range from 800 to 900 °C. The optimum fluidized bed temperature lies between 800 and 850 °C considering hydrogen yield and energy consumption comprehensively.
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Figure 4. Effect of catalytic fixed bed temperature on hydrogen yield
Figure 5. Effect of particle size of catalyst on hydrogen yield.
3.2. Effect of Catalytic Fixed Bed Temperature on Hydrogen Yield. The catalytic fixed bed temperature is another important parameter influencing the experimental results. To investigate its effect on hydrogen yield, the experiments were carried out according to the following conditions. Test materials: hydrolysis residues (e40 mesh); feeding rate, 0.4 kg/h; S/B ) 2; fluidized bed temperature, 800 °C; catalyst, Ni/modified dolomite (3 mm); catalytic fixed bed temperature, varying from 650 to 900 °C with a step of 50 °C. The experimental results are shown in Figure 4. It can be seen from Figure 4 that the hydrogen yield rises from 163.97 g/kg of biomass (daf) to 184.24 g/kg of biomass (daf) as the catalytic fixed bed temperature increases from 650 to 900 °C. Thus, the effect of the catalytic fixed bed temperature on hydrogen yield is similar to that of the fluidized bed temperature. High catalytic fixed bed temperature also benefits hydrogen yield thermodynamically and kinetically. It can be predicted that hydrogen yield would still be on the rise if the catalytic fixed bed temperature exceeds 900 °C, the highest catalytic fixed bed temperature that can be reached in this study due to restrictions of the experimental apparatus. On the basis of consideration of hydrogen yield and energy consumption, the preferential catalytic fixed bed temperature lies within the range 800–850 °C. 3.3. Effect of Particle Size of Catalyst on Hydrogen Yield. To investigate the effect of the particle size of the catalyst on hydrogen yield, the experiments were carried out according to the following conditions. Test materials: hydrolysis residues (e40 mesh); feeding rate, 0.4 kg/h; S/B ) 2; fluidized bed temperature, 800 °C; catalytic fixed bed temperature, 800 °C; catalyst, Ni/modified dolomite (3 mm); the catalyst particle size varying from 2 to 5 mm. The experimental results are shown in Figure 5. Hydrogen yield as indicated in Figure 5 increases remarkably as the size reduces from 5 to 3 mm; however, further size
Li et al.
Figure 6. Effect of S/B on hydrogen yield.
reduction is not capable of influencing the yield apparently. Catalytic cracking of tar belongs to complex multiphase reaction. The tar conversion process on the surface of the catalyst can be assumed as the following. Tar in the main gas stream diffuses continuously first to the exterior surface and then to the pore of the catalyst; meanwhile, adsorption and reaction of tar on the surface of catalyst occurs. Finally, desorption of tar cracking product from the surface of the catalyst and its subsequent diffusion to the main gas stream occur. Reduction of particle size in the packed bed can bring about changes in the hydrodynamic behavior of the gas flow, which, in turn, affects the observed kinetics. The smaller the particle size, the greater the exterior area of unit weight of the catalyst, but the greater the pressure drop of the bed will be; on the other hand, resistance from internal diffusion reduces as gas species are transported within the particles and the effectiveness factor increases. In the case that reaction proceeds at higher temperatures, mass transfer can often dominate the whole process because of the physical dimensions of the catalyst particle, and for this reason, a catalyst size of 2–3 mm is preferred. 3.4. Effect of the S/B Ratio on Hydrogen Yield. The S/B ratio means the mass ratio of steam to biomass material fed into the fluidized bed per unit time. To investigate the effect of the S/B ratio on hydrogen yield, experiments were carried out according to the following operation conditions. Fluidized bed temperature: 800 °C, catalytic fixed bed temperature: 800 °C, Test materials:hydrolysis residues (e40 mesh), catalyst: Ni/ modified dolomite(3.0 mm),S/B: from 0.5 to 2.5.The experimental results are shown in Figure 6. Figure 6 indicates that hydrogen yield is dependent on the S/B ratio if the ratio is less than 1.5, whereas it is nearly independent of the S/B ratio when the ratio exceeds 1.5. Increasing the S/B ratio can shift the equilibria of reactions 1 and 2 to the right, thus increasing the hydrogen yield. Though it is an effective way to increase hydrogen yield, higher S/B ratios impose more of an energy requirement on the gasification process. According to the carbon content in the test materials, the S/B ratio is required at 1.1–1.4, provided that this content of carbon reacts completely with water. Thus, the appropriate S/B ratio is in the range 1.5–2.0 for the purpose of higher conversion of carbon. 3.5. Catalytic Effects of Three Catalysts on Hydrogen Yield. To compare the effects of the three types of catalysts prepared in this study on hydrogen yield, experiments were carried out according to the following operation conditions: Fluidized bed temperature, 800 °C. Test materials: hydrolysis residues (e40 mesh); S/B ) 2; catalysts, the three types of catalyst as prepared in section 2.2; catalytic fixed bed temperature, from 650 to 900 °C. The experimental results are shown in Figure 7.
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Figure 7. Effect of catalysts on hydrogen yield. Table 2. Comparison of Gasification of Sawdust with Residues from Biomass Hydrolysisa Ftemperature
700
750
800
Figure 8. X-ray diffractograms of calcined dolomite.
(°C) 850
900
RH (vol %) 2 SH (vol %) 2 RCO (vol %) 2 SCO (vol %) 2 RCO (vol %)
63.35 66.66 75.88 76.87 80.06 28.35 34.64 39.84 42.91 44.75 12.56 11.72 9.23 10.10 8.81 22.56 21.72 20.23 20.10 19.81 21.48 19.48 13.08 11.52 9.75 SCO (vol %) 46.48 41.48 38.08 35.52 34.08 RCH (vol %) 1.49 1.31 1.19 0.93 0.89 4 SCH (vol %) 1.51 1.35 1.21 0.91 0.85 4 Rhydrocarbon (vol %) 1.12 0.83 0.62 0.58 0.49 Shydrocarbon (vol %) 1.10 0.81 0.64 0.56 0.51 RH /CO 2.95 3.42 5.80 6.67 8.21 2 SH /CO 0.61 0.84 1.05 1.21 1.31 2 Rhydrogen yield (g/kg (daf)) 169.56 178.71 182.77 183.74 184.30 Shydrogen yield (g/kg (daf)) 83.08 88.21 92.47 94.18 94.60
Figure 9. X-ray diffractograms of modified dolomite.
a
“F” indicates fluidized bed, and “S” and “R” indicate items from gasification of sawdust and residues from biomass hydrolysis, respectively.
It can be seen from Figure 7 that the hydrogen yield increases as the catalytic fixed bed temperature rises for each catalyst. On the whole, tar or hydrocarbon cracking and reforming reaction is an endothermic process; thus, temperature rise favors tar cracking and therefore leads to higher hydrogen yield. On the other hand, temperature rise results in higher reaction rate, which means higher conversion can be obtained under a given reaction temperature. Under the same catalytic fixed bed temperature, hydrogen yield from gasification by applying either Ni/dolomite or Ni/modified dolomite catalyst is much higher than that from gasification by applying calcined dolomite. This is due to the higher catalytic activity of the component Ni in the catalysts. It can also be seen from Figure 7 that the catalytic activity of Ni/dolomite and Ni/modified dolomite is very similar, which demonstrates that modification of dolomite does not influence its catalytic activity markedly. 3.6. Effect of Test Material Types on Hydrogen Yield. It is interesting to compare hydrogen yield from catalytic gasification of hydrolysis residues and the original sawdust employed for hydrolysis. The catalytic gasification of sawdust was carried out according to the experimental conditions of section 3.1 for hydrolysis residues, and the gasification results of these two materials are listed in Table 2. The changing trends of hydrogen yield and each gas species’ content from gasification of hydrolysis residues are the same as those from gasification of sawdust, whereas the CO and CO2 content from the former is less than half of that from the latter, and the H2/CO ratio and hydrogen yield of the former are 5 times and twice those of the latter, respectively. This might be partly ascribed to the lesser oxygen content of the hydrolysis residues, which is mostly lignin, compared with that of cellulose and hemicellulose in
sawdust. Another possible cause for this difference might arise from the difference in structure and thermochemical reactivity of the two test material types. As we know, lignocellulosic biomass consists mainly of three components (cellulose, hemicellulose, and lignin) bound together tightly. However, the binding forces between lignin and the other two components were no longer present, since they were mostly converted to single sugars via hydrolysis. Therefore, the compact structure of the sawdust or rice husk was destroyed and the hydrolysis residue became more active during the gasification process. The above results and discussion suggest that the biomass hydrolysis residue is better than the original biomass in terms of producing hydrogen via gasification. 3.7. Effect of Modification on Catalyst Strength. The strength of the catalyst is crucial for downstream catalytic processes. Both the calcined dolomite and the modified dolomite were shaped into a ball with a diameter of 1 mm for strength test by an intellectualized particle strength tester. It was found that the strength of modified dolomite was 200 N, while that of calcined dolomite was 0.8 N.This indicates that magnesium nitrate acts as an excellent inorganic binder which can dramatically improve dolomite strength through modification. There might be subtle interactions between dolomite and magnesium nitrate during the modification process, and probably new phases were formed, since this dramatic strength improving was not likely realized by merely mixing such a low percentage of modifier with dolomite physically. However, unfortunately, this cannot be demonstrated by the crystal structures of the two catalysts, as characterized by the XRD analysis shown in Figures 8 and 9, since there are little differences between them, which might be due to the extremely low percentage of the added binder in the dolomite. The strength improving mechanism needs further study and is under investigation in our laboratory.
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Figure 10. Bed pressure vs time.
The bed pressure drop and catalyst loss at different times were tested, respectively, to investigate the effect of catalyst strength on them. The bed pressure drop test is as follows. The calcined dolomite and the modified dolomite are, respectively, put into a fixed bed, forming a catalyst bed with a height of 3.0 cm; the operation temperature is 800 °C, and N2 with a flow rate of 2.0 m3/h at room temperature is introduced into the catalyst bed from the bottom. The test results are shown in Figure 10. It can be seen from Figure 10 that the bed pressure drop increasing with time is very slow when the filled catalyst is modified dolomite (maximum pressure drop is merely 3.0 cm of H2O during the 36 h test period). When the calcined dolomite is applied, it is also slow during the first 12 h; after that, however, it begins increasing quicker and drastic increasing occurs from 20 to 36 h; finally, the extremely high bed pressure interferes with feeding, making operation actually impossible to continue. The particle size and packing state of the modified dolomite remained unchanged on the whole after the test, whereas the calcined dolomite was crushed and the steel net supporting it blocked. This suggests the necessity of dolomite modification on one hand. On the other hand, dolomite modification is also a requirement in terms of catalyst loss during operation. The following catalyst loss test illustrates this. Two hundred grams of the calcined dolomite or modified dolomite with a diameter of 1 mm were used as a fluidized medium in a fluidized bed to test its loss with time. The fluidizing gas is N2 at room temperature with a flow rate of 1.0 m3/h, and the bed operation temperature is 800 °C. The results are shown in Figure 11. It can be seen that the loss of calcined dolomite is 167.3 g during the 6 h testing period, whereas that of modified dolomite is only 29.3 g. The much greater loss of calcined dolomite is due to its fragility. Under the impact of fluidizing gas, most were crushed and powdered and finally flew away. However, modified dolomite exhibited excellent resistance to impact. 4. Conclusions Hydrogen production via catalytic gasification of hydrolysis residues was carried out in this study. Among the three catalysts
Figure 11. Loss of catalyst vs time.
applied, the Ni/modified dolomite binary catalyst proved the best considering its strength and activity comprehensively. The Ni/modified dolomite catalyst was prepared by a two-step coprecipitating method with the calcined dolomite, nickel nitrate, and a minute amount of magnesium nitrate, which acted as an excellent binder for modifying calcined dolomite and made its strength much higher. The effects of the fluidized bed temperature, catalytic fixed bed temperature, S/B ratio, and particle size of the catalyst on the hydrogen yield of hydrolysis residue gasification employing the Ni/modified dolomite catalyst were investigated. Hydrogen yield is favored by increasing fluidized bed temperature, catalytic fixed bed temperature, and the S/B ratio and by decreasing the size of the catalyst. Hydrogen yield and energy consumption been considered comprehensively, the appropriate operation conditions determined were as follows: fluidized bed temperature, 800–850 °C; catalytic fixed bed temperature, 800–850 °C; the S/B ratio, 1.5–2.0; particle size of catalyst, 2–3 mm. Hydrogen production from hydrolysis residues has the following advantages compared with that from sawdust. There is a lower CO2 and CO content and higher H2 content in the gas product, which favors the subsequent gas treatment. In addition, most of the resin type substance in the biomass has been dissolved in the water during the hydrolysis process, which makes the leftover residues brittle. Thus, feeding of hydrolysis residues is easier than that of sawdust and it also can be more evenly mixed with quartz sand, the heat transfer medium. Therefore, better heat transfer performance of the fluidized bed can be obtained. Acknowledgment. This research project was financially supported by the National Natural Science National Hi-tech Research and Development Programme (AA514020-02), Shanghai Development Foundation for Key Technologies (041612002). EF700375S
