Energy Fuels 2010, 24, 3256–3261 Published on Web 04/28/2010
: DOI:10.1021/ef100081w
Effects of Ca-Based Catalysts on Biomass Gasification with Steam in a Circulating Spout-Fluid Bed Reactor Yurong Xie, Jun Xiao, Laihong Shen,* Jun Wang, Jing Zhu, and Jiangang Hao Thermoenergy Engineering Research Institute, Southeast University, Nanjing 210096, Jiangsu Province, China Received January 23, 2010. Revised Manuscript Received April 7, 2010
The catalytic performances of natural Ca-based catalysts (dolomite and limestone) and synthetic Ca-based catalysts, used for improving biomass steam gasification, were fully investigated in a circulating spout-fluid bed reactor. In comparison to the catalytic role in biomass gasification, the synthetic Ca-based catalyst, 20% CaO/Al2O3 (20CaAl), displayed a better catalytic effect on the biomass carbon conversion reaction and tar reforming reaction than natural Ca-based catalysts, but the latter played a better role in the reforming reaction of light hydrocarbon because it contained small quantities of iron oxides. For the two types of Ca-based catalysts, the optimal operating temperature was about 860 °C to upgrade the quality of gas product and increase H2 yield. The catalytic activity of synthetic Ca-based catalysts was visibly improved with the increment of CaO loading, and the preferential range of CaO loading was 12.5-20%. In comparison to the lifetime, 20CaAl (the synthetic catalyst) displayed better stability in the catalytic role than natural Ca-based catalysts.
content of the gaseous product.8-10 In this paper, we only focus on the effects of the former on biomass gasification with steam in a circulating spout-fluid bed reactor. For alkaline earth-based catalysts, more reports are primarily used for investigating the catalytic roles of natural rocks (dolomite and limestone) in promoting biomass gasification.11-15 Olivares et al.11 investigated the role of dolomite used in bed material in upgrading the quality of gaseous product and found that the additive caused the tar content to decrease from 12 to 2-3 g/ m3n and the H2 content to increase from 25-28 to 43 vol %. Walawender et al.15 performed a series of experiments using limestone as the bed additive in a fluidized-bed gasifier and reported that the gas composition, heating value, and yield were all influenced by the presence of 25 wt % limestone in the bed. However, the two rocks are easily deactivated because of particle attrition and agglomeration in the fluidized-bed reactor.16 Moreover, the two problems maybe inhibit the use of calcined rocks as catalysts for promoting biomass gasification, to some degree. Until now, to our knowledge, there are still no effective methods for increasing the mechanical strength and reducing agglomeration of a natural Ca-based catalyst [calclined limestone (CL) and calcined dolomite (CD)]. In addition,
1. Introduction Biomass gasification is a promising technology for producing syngas for chemical synthesis (biofuels, methanol, or chemicals) or power generation (hydrogen fuel cell, gas turbine, or engine).1 However, this procedure also produces large amounts of undesirable tarry materials (condensable organic compounds). When the product gas is used for gas turbines and engines, the tar eventually fouls the processing equipment and causes some troubles.2 Hence, reducing the tar content of the gaseous product, derived from biomass gasification, becomes a key factor for commercializing biomass gasification. There are two primary methods for eliminating tar: physical separation and chemical conversion.3-6 In comparison to physical separation, the latter receives more attention and has been widely investigated. Among the chemical conversion methods, tar catalytic conversion is considered as the highest potential technology to inhibit tar formation.7 In terms of cost performance, alkaline earth-based catalysts and transition-metal-based ones have attracted more interest from scholars because of the perfect activities for improving biomass gasification and reducing heavy/light hydrocarbon *To whom correspondence should be addressed. E-mail: lhshen@ seu.edu.cn. (1) Swierczynski, D.; Courson, C.; Bedel, L.; Kiennemann, A.; Guille, J. Chem. Mater. 2006, 18 (7), 4025–4032. (2) Matsuoka, K.; Shimbori, T.; Kuramoto, K.; Hatano, H.; Suzuki, Y. Energy Fuels 2006, 20 (6), 2727–2731. (3) Lopamudra, D.; Ptasinskik, J.; Janssen, F. Biomass Bioenergy 2003, 24 (2), 125–140. (4) Hasler, P.; Nnssbaumer, T. Biomass Bioenergy 1999, 16 (6), 385–395. (5) Bentzen, J. D.; Hummelshoj, R. M.; Elmegaard, B.; Henriksen, U. Proceedings of the Conference Efficiency, Cost, Optimization, and Simulation (ECOS), University of Twente, Enschede, The Netherlands, 2000; pp 97-108. (6) Sutton, D.; Kelleher, B.; Ross, J. Fuel Process. Technol. 2001, 73 (3), 155–173. (7) Pfeifer, C.; Hofbauer, H. Powder Technol. 2008, 180 (1-2), 9–16. (8) Delgado, J.; Aznar, M. P.; Corella, J. Ind. Eng. Chem. Res. 1996, 35 (10), 3637–3643. r 2010 American Chemical Society
(9) Simell, P. A.; Lepp€alahti, J. K.; Bredenberg, J. B. Fuel 1992, 71 (2), 211–218. (10) Corella, J.; Aznar, M. P.; Gil, J.; Caballero, M. A. Energy Fuels 1999, 13 (6), 1122–1127. (11) Olivares, A.; Marı´ a, P. A.; Miguel, A. C.; Javier, G.; Frances, E.; Corella, J. Ind. Eng. Chem. Res. 1997, 36 (12), 5220–5226. (12) Han, J.; Kim, H. Renewable Sustainable Energy Rev. 2008, 12 (2), 397–416. (13) Garcı´ a, X. A.; Alarc on, N. A.; Gordon, A. L. Fuel Process. Technol. 1999, 58 (2-3), 83–102. (14) Gusta, E.; Dalai, A. K.; Uddin, M. A.; Sasaoka, E. Energy Fuels 2009, 23 (4), 2264–2272. (15) Walawender, W. P.; Ganesan, S.; Fan, L. T. Symposium Papers on Energy from Biomass and Wastes, IGT, Chicago, IL, 1981; pp 517-527. (16) Weerachanchai, P.; Horio, M.; Tangsathitkulchai, C. Bioresour. Technol. 2009, 100 (3), 1419–1427.
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gaseous product, namely, that collecting samples of gasification products began and periodically the sample was carried out. 2.3. Sample Analysis. The volume of gaseous product was measured by a flow meter, and the components of gaseous products (H2, CO, CO2, and CH4) were analyzed off-line by the gas chromatograph analyzer (6890N type, Agilent Technologies) equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). The liquid products (tar), derived from tar trapper, were first separated from water by the method of methylene dichloride extraction. Then, the tar amount was determined by a gravimetric method including evaporation of the solvent (60 °C and atmosphere pressure) to a constant sample weight.
Table 1. Proximate and Ultimate Analyses of the Biomass Sample proximate analysis (wt %, ad. basis) moisture fixed carbon volatile ash
ultimate analysis (wt %, ad. basis) 7.07 14.22 61.28 17.43
low heating value (MJ/kg)
C H O N S 14.54
35.12 4.57 34.91 0.79 0.11
there are few reports on the synthetic Ca-based catalyst used for promoting biomass gasification. To obtain stable catalytic properties of a Ca-based catalyst, we will pay more attention to the investigation of the catalytic effect of a synthetic Ca-based catalyst on biomass gasification in this study.
3. Results and Discussion 3.1. Effect of the Bed Material (Catalyst) Type on Biomass Gasification. The four kinds of bed material, including CL, CD, Al2O3, and 20CaAl, were used for fully investigating the effects on biomass gasification with steam. The results were presented in Figure 2 and Table 3. As observed, Al2O3 plays a weaker role in biomass gasification with steam and provides the lowest amount of total gas yield and the highest tar yield, as shown in Table 3. The other bed materials, CL, CD, and 20CaAl, better promote the procedure of biomass gasification, upgrading the quality of gas product and increasing the H2 component. From Figure 2 and Table 3, it can be seen that the three CaO-contained bed material display better catalytic roles in biomass gasification and enhance the conversions of tar and carbon. In comparison to Al2O3, it can be deduced that the catalytic activity of 20CaAl is closely connected with the component CaO and the catalytic action of CL and CD is mainly ascribed to their primary component CaO. This conclusion is consistent with the ones of many previous works,17-19 which have demonstrated that CaO enhance biomass gasification and improve the quality of the gas product. Although the active components are all CaO, there are marked differences in product distributions of the three CaO-contained bed material, as shown in Figure 2 and Table 3. CL and CD play better roles in the yields of H2 and CO2 than 20CaAl, while 20CaAl better promotes the conversion reactions of carbon and tar. This phenomenon may be closed to the component of bed material and the dispersion degree of active ingredient. According to Table 2, it is found that both CL and CD contain some Fe2O3. By the acts of iron oxide, the quality of H2-rich gas may be further upgraded as the following reaction paths:20-22
2. Experimental Section 2.1. Feedstock and Catalysts. The properties of rice straw selected as feedstock are listed in Table 1. This biomass was milled and sieved, and the particle size used in this paper is between 0.6 and 0.8 mm. Natural ores (limestone and dolomite) and a synthetic Ca-based catalyst as bed material are fully investigated in this study. The natural ores were first crushed and sieved to obtain the proper particle size, 0.15-0.60 mm. Then, these crushed particles were calcined for 3 h at 900 °C in a muffle furnace in an air atmosphere. The synthetic Ca-based catalyst is prepared by the impregnation method using an aqueous solution of Ca(NO3)2 and the Al2O3 supporter, which was supplied by Aotai Co., Ltd., Nanjing Province, China. After water was evaporated, the catalyst was further dried for 10 h at 150 °C and then calcined for 3 h at 900 °C under an air atmosphere. The properties of catalysts (bed material) are shown in Table 2. 2.2. Experimental Apparatus and Procedure. Biomass gasification was performed in a circulating spout-fluid bed reactor at atmosphere pressure. The schematic description of the experimental apparatus is shown in Figure 1. The experimental apparatus mainly consisted of a circulating spout-fluid bed reactor, vapor generator, biomass feeder, sampler, temperature controller, etc. The circulating spout-fluid bed reactor is composed of a spout-fluid bed and back feeder, which are made of 1Cr18Ni9Ti stainless-steel material. The spout-fluid bed is a rectangular bed, with a cross-section of 10030 mm2 and a height of 1000 mm. Below the bed, a set of 60° conical distributors, connected with a tube with an 8 mm inner diameter, is installed to enhance the fluidization of the mixture of bed material and biomass. The tube is used to introduce the spouting stream with biomass to the bottom of the spout-fluid bed. The back feeder allows fly ash separated by cyclone to return to the spout-fluid bed through the loop seal with a cross-section of 3020 mm2. A set of 4.5 kW electric heaters to supply biomass conversion with external heat is installed on the upper and lower parts of the reactor. Prior to each test, bed material was first put into the reactor, maintaining the static bed height of about 200 mm, and then the electric heaters were turned to preheat the circulating spoutfluid bed. When the predetermined temperature was reached, 3 g min-1 steam flow (gasification agent), 2 L min-1 N2 flow (fluidizing agent), and 6.5 L min-1 N2 flow (spouting agent) were introduced to the spout-fluid bed from different pipelines, as shown in Figure 1. In addition, 1.5 L min-1 N2 flow was introduced to the loop seal to return the separated fly ash to the spout-fluid bed. After the stabilities of the temperature and pressure drop, 1.5 g min-1 biomass, transported by a screw feeder, was fed into the gasification reactor by way of gas delivery. Fuel gas from the spoutfluid bed was separated from fly ash by the act of the cyclone, and the fly ash was returned to the bed through the loop seal. After the operation became steady at a time of 20 min, part of the fuel gas was introduced to the sampler used for trapping tar and collecting
Fex Oy þ Cn H m f Fex - a Oy - b þ CO2 =CO
ð1Þ
Fex Oy þ tar f Fex - a Oy - b þ CO2 =CO þ :::
ð2Þ
Fex Oy þ CO f Fex - a Oy - b þ CO2
ð3Þ
Fex - a Oy - b þ H2 O f Fex Oy þ H2
ð4Þ
(17) Zhou, H. C.; Jin, B. S.; Zhong, Z. P.; Xiao, R.; Huang, Y. J. J. Power Eng. 2006, 26 (5), 699–702. (18) Xu, G.; Murakami, T.; Suda, T.; Kusama, S.; Fujimori, T. Ind. Eng. Chem. Res. 2005, 44 (15), 5864–5868. (19) Xie, Y. R.; Shen, L. H.; Xiao, J.; Xie, D. X.; Zhu, J. Energy Fuels 2009, 23 (10), 5199–5205. (20) Fukase, S.; Suzuka, T. Can. J. Chem. Eng. 1994, 72 (2), 272–278. (21) Urasaki, K.; Tanimoto, N.; Hayashi, T.; Sekine, Y.; Kikuchi, E.; Matsukata, M. Appl. Catal., A 2005, 288 (1-2), 143–148. (22) Uddin, M. A.; Tsuda, H.; Wu, S. J.; Sasaoka, E. Fuel 2008, 87 (4-5), 451–459.
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Table 2. Properties of Ca-Based Catalystsa properties
CD
CL
0CaAl
particle size (mm) surface area (m2/g) pore volume (cm3/g) average pore diameter (nm)
0.15-0.6 39.29 0.28 28.52
Physical Properties 0.15-0.6 0.15-0.6 32.37 190.18 0.23 0.57 26.17 12.48
CaO SiO2 Al2O3 Fe2O3 MgO others
94.20 0.31 0.32 0.80 0.89 0.07
Chemical Composition (%) 59.00 0.28 0.30 99.95 0.35 39.87 0.02 0.05
5CaAl
12.5CaAl
20CaAl
0.15-0.6 164.69 0.55 13.35
0.15-0.6 114.58 0.45 15.62
0.15-0.6 77.83 0.41 21.20
5.00
12.50
20.00
94.93
87.42
79.90
0.07
0.08
0.10
a
CD, calcined dolomite; CL, calclined limestone; 0CaAl, 0% CaO/Al2O3; 5CaAl, 5% CaO/Al2O3; 12.5CaAl, 12.5% CaO/Al2O3; 20CaAl, 20% CaO/Al2O3.
Figure 1. Schematic description of the experimental apparatus.
From reactions 1-4, iron oxide enhances the water-gas shift reaction and the reforming reactions of tar and light hydrocarbon (CnHm), which all contribute to the increment of H2 yield. For the Ca-based catalyst, active ingredient CaO displays better catalytic selectivity for the reactions of tar and carbon but a poor role in the reforming reaction of light hydrocarbon and the water-gas shift reaction. Therefore, for CL and CD, the better improvement of the yield and concentration of H2 primarily results from the effects of iron oxide on reactions 1-4. For the synthetic Ca-based catalyst, 20CaAl displays an optimal role in the two conversion reactions of tar and carbon, as shown in Table 3. The reason for this phenomenon is that the catalytic activity of CaOcontained bed material is partly dependent upon the dispersion degree of CaO. Lizzio et al.23 have testified the similar conclusion that increasing the dispersion degree of CaO (23) Lizzio, A. A.; Radovic, L. R. Ind. Eng. Chem. Res. 1991, 30 (8), 1735–1744.
Figure 2. Effect of the bed material on gas yields at 860 °C.
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Table 3. Gasification Results with Various Bed Material at Different Temperatures bed material operating temperature (°C) gas component (vol %, dry basis) H2 CO CO2 CH4 H2 yield (mol/kg biomass) CO2 formation ratio (%)a tar yield (g/Nm3) carbon conversion (%) total gas yield (Nm3/kg biomass) a
CL
CD
Al2O3
20CaAl
CL
CD
Al2O3
20CaAl
860
860
860
860
900
900
900
900
52.30 16.07 25.08 6.55 29.42 49.02 4.57 91.41 1.26
51.51 16.80 24.60 7.08 29.20 47.68 3.73 93.95 1.29
48.86 16.65 26.84 7.65 22.80 42.26 8.77 82.02 1.05
49.46 17.57 24.61 8.38 27.46 46.70 3.67 95.93 1.24
51.74 17.26 24.04 6.96 29.28 46.52 3.53 93.37 1.27
50.96 17.51 23.89 7.62 29.09 46.61 2.76 95.65 1.28
48.64 17.69 25.60 8.06 23.67 42.57 6.80 85.40 1.09
49.32 18.29 23.65 8.74 27.54 45.14 2.55 96.73 1.25
CO2 formation ratio = (carbon amount forming CO2/total carbon amount of biomass) 100%.
could evidently improve the catalytic effect of the Ca-based catalyst. For 20CaAl, it is synthesized by the impregnation method, which is propitious for dispersing the active ingredient (CaO) on the alumina supporter. Therefore, the effects of 20CaAl on tar and carbon are enhanced with improving the dispersion of the active ingredient. In addition, there is another important influencing factor for the catalytical activity of 20CaAl: physical structure. Because of the better physical structure (as shown in Table 2), the procedure of gas diffusion is accelerated in the inner of 20CaAl, so that more gasification agent is activated24,25 and reacted with carbon and tar to upgrade the quality of H2-rich gas, respectively. From the above discussion, it can be seen that the synthetic Ca-based catalyst displayed a better catalytic effect on the biomass carbon conversion reaction and tar reforming reaction than natural Ca-based catalysts but the latter played a better role in the reforming reaction of light hydrocarbon and the water-gas shift reaction. 3.2. Effect of the Operating Temperature on Biomass Gasification. Typically, biomass gasification with steam in the fluidized bed is carried out in the range of 800-900 °C.16,26 On the basis of the previous research,27 the efficiency of biomass gasification is very low when the operating temperature is below 860 °C, although efficient catalysts are used as bed material. Therefore, the two temperature points, 860 and 900 °C, are selected for investigating the effect of the operating temperature on biomass gasification in this paper. As shown in Table 3, the operating temperature has a significant effect on the gas component, H2 yield, tar yield, carbon conversion, etc. With an increasing operating temperature, the components of H2 and CO2, CO2 formation ratio, and tar yield are decreased, while the CO component and carbon conversion are increased. The main reason for this phenomenon is that primary reactions of biomass gasification are visibly influenced because of the increasing temperature. These reactions are given as following:28,29 C þ CO2 S2CO þ 172:5 kJ=mol ð5Þ C þ H2 OSCO þ H2
þ 131:5 kJ=mol
C þ 2H2 OSCO2 þ 2H2
þ 90:0 kJ=mol
ð7Þ
CO þ H2 OSCO2 þ H2
- 41:2 kJ=mol
ð8Þ
Cn Hm þ 2nH2 OSnCO2 þ ð4n þ m=2ÞH2 þ Q > 0 kJ=mol
ð9Þ
Cn Hm þ nH2 OSnCO þ ðn þ m=2ÞH2 þ Q > 0 kJ=mol
ð10Þ
tar þ H2 O f CO2 þ H2 þ light hydrocarbon þ ::: þ Q > 0 kJ=mol
ð11Þ
tar þ H2 O f CO þ H2 þ light hydrocarbon þ ::: þ Q > 0 kJ=mol
ð12Þ
tar f CO þ CO2 þ H2 þ light hydrocarbon þ ::: þ Q > 0 kJ=mol
ð13Þ
According to the thermodynamics principles, reaction 8 is exothermic; thus, the reaction is inhibited with the increment of the operating temperature. Therefore, the secondary reaction of CO, derived from reactions 5, 6, 10, 12, and 13, is also inhibited in different degrees, so that the CO net yield is gradually increased instead of H2 and CO2, as described in reaction 8. However, the other reactions are all endothermic; thus, an increasing operating temperature is beneficial for the conversion reactions of tar and carbon. On the basis of the kinetics principles, an increasing operating temperature could further improve the rates of reactions 5-7 and 9-13 and increase the efficiency of biomass gasification. Hence, the increment of the operating temperature results in increasing the carbon conversion and CO component but decreasing the H2 component, CO2 component, CO2 formation ratio, and tar yield. On the basis of consideration of the quality of H2-rich gas and energy consumption, the preferential operating temperature should be at about 860 °C, when the three Ca-based catalysts are selected as bed material. 3.3. Effect of the CaO Loading of the Synthetic Catalyst on Biomass Gasification. The catalytic effects of synthetic catalysts loading various CaO contents on the product distributions of biomass gasification are listed in Figure 3 and Table 4. With the increment of CaO loading, the H2 component gradually increases from 48.86 to 49.46%, the CO2 component
ð6Þ
(24) Kyotani, T.; Hayashi, H.; Tomita, A. Energy Fuels 1991, 5 (5), 683–688. (25) Freund, H. Fuel 1986, 65 (1), 63–66. (26) Saxena, R. C.; Seal, D.; Kumar, S.; Goyal, H. B. Renewable Sustainable Energy Rev. 2008, 12 (7), 1909–1927. (27) Xie, Y. R.; Shen, L. H.; Xiao, J.; Wang, J.; Zhu, J. Proceedings of the Combustion Conference CSET, Chinese University of Science and Technology, China, 2009. (28) Ni, M.; Leung, D. Y. C.; Leung, M. K. H.; Sumathy, K. Fuel Process. Technol. 2006, 87 (5), 461–472. (29) Chaudhari, S. T.; Dalai, A. K.; Bakhshi, N. N. Energy Fuels 2003, 17 (4), 1062–1067.
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Xie et al. Table 5. Distributions of Bed Material CD bed material particle volume (mL) 0.15 mm>Da 0.15 e D e 0.6 mm D>0.6 mm a
CL
20CaAl
fresh
spent
fresh
spent
fresh
spent
450
33 370 82
450
21 380 73
450
5 440 8
Particle size of bed material.
Figure 3. Effect of the CaO loading of the synthetic catalyst on the gas component at 860 °C. Table 4. Effect of the CaO Loading of the Synthetic Catalyst on the Product Distribution of Biomass Gasification with Steam at 860 °C bed material (catalyst) operating temperature (°C) gas yields (mol/kg biomass) H2 CO CO2 CH4 CO2 formation ratio (%) tar yield (g/Nm3) carbon conversion (%) total gas yield (Nm3/kg biomass)
0CaAl 5CaAl 12.5CaAl 20CaAl 860
860
860
860
22.9 7.8 12.58 3.59 42.26 8.77 82.02 1.05
25.11 8.87 13.16 4.19 44.82 6.67 89.29 1.15
26.64 9.41 13.57 4.39 46.53 3.87 93.85 1.21
27.38 9.73 13.62 4.64 46.7 3.67 95.93 1.24
Figure 4. Lifetime test for various catalysts at 860 °C (2, carbon conversion; O, H2 yield).
CaO loading; in addition, the other reason is that active ingredient CaO plays poor roles in reforming reactions 9 and 10. Under the acts of the two factors, CH4, derived from reactions 11-13, is not better converted as reactions 9 and 10; hence, the net CH4 yield is evidently increased with increasing the CaO loading. On the basis of the above discussion and the product distribution, the optimum CaO loading should be in the range of 12.5-20%, which plays very perfect roles in increasing the conversion amounts of tar and carbon (Table 4) and has better cost performance than the others. 3.4. Lifetime Test for Various Catalysts. As discussed above, CL, CD, and 20CaAl have exhibited perfect roles in promoting biomass gasification and upgrading the quality of H2-rich gaseous product. For the three catalysts (bed material), the long lifetime determines whether they could be used for promoting industrial application of biomass gasification. Therefore, this main purpose of the test is to measure the stabilities of the three catalysts. As shown in Figure 4, the carbon conversion and H2 yield of 20CaAl nearly remain the same along the run time, while the two values for CL and CD are gradually decreased. According to these variation trends, it can be deduced that 20CaAl has better stability than CL and CD. In addition, the poor stability of CL and CD is ascribed to the deactivation of bed material (CD and CL) and the deterioration of the fluidization situation with the increment of run time. According to Table 5, it can be seen that CD and CL produce more small and large particles than 20CaAl. For CD and CL, the reason for the formation of small particles is that the two bed materials are quite softer and become erosive;11,17 as shown in Figure 5 and Table 6, the reason for large particles is connected with three kinds of features on the surfaces of large particles: A, B, and C. The feature of A is associated with the reaction of biomass ash and the small particles, derived from bed material; the feature of B
slowly decreases from 26.84 to 24.61%, and the rest have little variation. In comparison to the gas yields of 0CaAl (Al2O3), the largest amplitudes of gas yields (H2, CO, CO2, and CH4) of synthetic catalyst are 19.56, 24.74, 8.27, and 29.25%, respectively. According to the amplitudes and gas yields (Table 4), the decrement of the CO2 component is ascribed to the smallest amplitude and the increments of the other components are closely connected with the larger amplitudes. The reason for these differences in the distributions of gas yields is that the catalytic selectivity of the synthetic catalyst for biomass gasification is visibly improved with the increment of CaO loading. According to Table 4, it can be seen that the increment of CaO loading better promotes the conversions of tar and carbon to produce more gaseous product by accelerating reactions 5-7 and 9-13. From the amplitudes of gas yields, it can be seen that the increment of CaO loading plays a positive role in raising gas yields other than the CO2 yield. According to the main reactions 5-13, the explanation for this phenomenon is perhaps that the increment of CaO loading further promotes CO2 consumption to accelerate carbon conversion, as described in reaction 5. In addition, the fact that active ingredient CaO increases the H2 component probably inhibits reaction 8 to produce CO2, because reaction 8 is primarily influenced by the thermodynamics at 860 °C. Therefore, the amplitude of the CO2 net yield is relatively smaller, while the amplitude of the CO net yield is visibly increased. For the larger amplitude of the CH4 yield, it is closely connected with reactions 11-13, which are further improved to increase tar conversion to produce more light hydrocarbon by the act of increasing 3260
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Figure 6. X-ray diffraction (XRD) characters of used Ca-based catalysts.
active CD and CL are severally transformed to form inter CaSiO3 and CaMgSi2O6, as shown in Figure 6. Hence, large particles directly result in the reduction of the catalytic role of bed material in biomass gasification with an increasing run time. In comparison to CD and CL, 20CaAl displays an excellently stable catalytic role in biomass gasification, because active ingredient Ca-based material is formed, CaO(Al2O3)2, which probably does not only maintain the activity of Ca-based material but also inhibits the interaction of Ca-based material and biomass char/ash. Hence, 20CaAl still keeps perfect stability of the catalytic activity and the fluidizing performance along the run time. After careful comparison, it can be concluded that 20CaAl, the synthetic Ca-based catalyst, is more suitable for promoting biomass gasification than CD and CL (natural Ca-based catalysts).
Figure 5. SEM images of large particles (D>0.6 mm): (a) CD and (b) CL. Table 6. Energy Spectrum Analysis of Large Particles (D>0.6 mm) of CD and CLa CD
C O Na Mg Si K Ca Mn Fe
The influences of Ca-based catalysts on biomass gasification are investigated in the circulating spout-fluid bed reactor. The important catalytic properties of Ca-based catalysts are achieved as follows: (1) The Ca-based catalysts, CL, CD, and 20CaAl, promote the procedure of biomass gasification, upgrading the quality of gas product and increasing H2 yield. In addition, CL and CD display a preferable catalytic role in reforming reactions of light hydrocarbon because they contain iron oxides, while 20CaAl displays better a catalytic effect on the biomass carbon conversion reaction and tar reforming reaction. (2) In terms of the quality of H2-rich gas and energy consumption, the optimal operating temperature should be at about 860 °C, when CL, CD, and 20CaAl are selected as bed material. (3) For the synthetic catalyst, the optimum CaO loading should be in the range of 12.5-20%, which plays perfect roles in promoting biomass gasification and improving the quality of gaseous product. (4) In comparison to natural Ca-based catalysts (CD and CL), the synthetic Ca-based catalyst (20CaAl) is more suitable for promoting biomass gasification and maintains preferable catalytic activity with the increment of run time.
CL
A
B
C
A
B
C
7.1 27.8
5.9 26.63 0.6 1.92 35.22 15.87 12.75 0.61 0.5
20.88 34.49
9.53 30.52 0.44 0.79 13.99 5.37 38.31 0.43 0.62
9.5 21.42 0.84 1.06 35.98 18.68 10.84 0.79 0.89
37.89 24.74 0.32 0.67 18.6 7.16 9.59 0.41 0.62
18.54 14.23 1.11 30.35 0.43 0.44
4. Conclusion
2.52 28.16 11.85 2.1
a The numbers listed in the table represent the weight percentage of various elements.
represents the surface morphology of amorphism and is derived from biomass ash; and the feature of C primary displays the properties of the biomass char, which is not completely gasified. From the above discussion, it can be deduced that the co-actions of biomass ash, biomass char, and small particles of bed material result in the formation of large particles. With the increment of run time, the amount of large particles is sharply increased, so that the fluidizing performance of bed material is seriously destroyed and the disturbance formed between bed material and reactants is visibly weakened. In addition, the bed materials, forming large particles, gradually lost catalytic activities, because the
Acknowledgment. This work was supported by the Special Fund of the National Priority Basic Research of China (2007CB210208 and 2010CB732206). 3261
