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Energy & Fuels 2000, 14, 552-557
The Fast Pyrolysis of Biomass in CFB Reactor Dai Xianwen, Wu Chuangzhi, Li Haibin, and Chen Yong* Guangzhou Institute of Energy Conversion, CAS, No. 81, Xianlie Zhong Road, Guangzhou, 510070, China Received July 23, 1999
With the circulating fluidized bed (CFB) as reactor, an integrated facility was developed for the fast pyrolysis of biomass. The main chemical processes in CFB can be modeled, and the bed is divided into two zones corresponding to the pyrolysis and secondary reactions. The pyrolysis of wood powder was processed varying the bed temperature, particle size of wood powder, and the feeder position. Based on the variation of the pyrolysis gas composition and the bio-oil ingredients, analysis of the experimental data highlights the important effects of temperature, heating rate, and residence time. The main trend is that (1) the higher temperature and longer residence time contribute to the secondary reactions, which lead to less liquids; (2) the lower heating rate favors the carbonization, also reduces the liquid production. The analysis of bio-oil components shows that most compounds in bio-oil are non-hydrocarbons and alkanessaromatics and asphalt are relatively low.
Introduction Since environmental pollution is becoming serious, it has important significance to make use of the clean and renewable resource biomass as a substitution for fossil energy. On the other hand, biomass is just waste before it can be treated and recycled. It will become pollutant to the environment due to landfill and pile-up, or malignant sources resulting in accident fire. The biomass pyrolysis for liquids is a promising technology.1 It offers a number of unique advantages, of which the most significant is that the liquid product can be stored and transported.2 Pyrolysis is thermal degradation either in the complete absence of oxidizing agent or with such a limited supply that gasification does not occur to an appreciable extent or may be described as partial gasification. Moderate temperature about 500 °C, high heating rates of up to a claimed 1000 °C/s or even 10000°C/s, and very short residence time less than 2 s are the perfect reaction conditions.3 To get these strict conditions, many technologies were developed, such as entrained flow reactor, vacuum furnace reactor, vortex reactor, rotating reactor,4 and CFB reactor.5 Because the CFB reactor has many advantages, for example, the simple structures, high production capacity, favorable conditions of heat and mass transfer, and the convenience of operation, etc., the CFB was used as the main reactor in this study. To reduce the operation cost, part of the pyrolysis gas was used as the carrier gas, while the rest and the pyrolysis char were recycled as heat. * Author to whom correspondence should be addressed. Fax: 8620-87608586. E-mail:
[email protected]. (1) Diebold, J. P.; Bridgwater, A. V. Developments in Thermochemical Biomass Conversion; Bridgwater, A. V., Boocock, D. G. B., Eds.; Blackie: Glasgow, 1997; pp 5-26. (2) Huffman, D. R.; Vogiatzis, A. J.; Bridgwater, A. V. Advances in Thermochemical Biomass Conversion; Bridgwater, A. V., Ed.; Blackie: Glasgow, 1994. (3) Integrated Energy Systems in China; Food and Agriculture Organization of the United Nations, 1994; pp 167-179.
Table 1. Proximate and Ultimate Analysis of Wood Powder proximate analysis
wt %
volatile matter fixed carbon ash moisture H.H.V. (kJ/kg)
70.7 21.2 2.3 5.8 18032.5
ultimate analysis
wt %
C H O N S ash
47.9 7.0 41.4 0.1 0.3 2.3
Experimental Section Materials. With quartz sand as circulating particle, the pyrolysis of pine wood powder for liquids was studied in CFB. The particle sizes of two wood powder samples were 0.38 mm and 0.73 mm. The proximate and ultimate analyses of the wood powder are summarized in Table 1. Since the wood powder has large amounts of volatile matter (70.7 wt %), pyrolysis of it for liquids is an efficient method for utilizing wood powder. Facility. Since the CFB has been successfully used in many fields as an efficient, no bubble and high production capacity reactor,6 with CFB as reactor, an experimental facility which included the heating, reaction, measurement and control devices is designed and constructed. Its processing capacity is 5 kg/h. A schematic of the apparatus is shown in Figure 1. The system has five components: (i) a variable speed rotary feeder which can be placed at two different positions along the CFB, (ii) a CFB reactor assembly with two cyclone separators, (iii) cooling system, (iv) combustion chamber, (v) gas circulating pump and preheating system. The CFB reactor was constructed from the heat-resistant stainless steel pipe of 100 mm i.d. The distributor plate was made of a stainless steel plate having 88 holes of 3 mm i.d. The reactor height from the (4) Wagenaar, B. M. Ph.D. Thesis. University of Twente, The Netherlands, 1994. (5) Bridgwater, A. V. Proceedings of conference on bio-oil production and utilization; Estes Park, CO, 1994; pp 24-26. (6) Knowlton, T. M.; Hirsan, Hydrocarbon Process. 1978, 149-156.
10.1021/ef9901645 CCC: $19.00 © 2000 American Chemical Society Published on Web 04/26/2000
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Figure 2. Model of the chemical processes of CFB. the experiment to ensure the pyrolysis vapor and tar’s burning up. Even then, sometimes the pipes from CFB exit to the condenser were clogged by the mixture of tar and sand. So these pipes must be dismantled to clean up after the facility cooled. The gas entrance could be removed, from here the quartz sand was collected. To study the effects of temperature, heating rates, and vapor residence time, the reactor temperature, particle size of wood powder, and feeder position were changed to experiment in this study. Figure 1. Schematic diagram of the experimental apparatus employed. distributor to the gas exit was 2.9 m, which provided gasresidence time about 1.5 s. The standpipe of L valve 1 was made of a 2.3 m-high stainless steel of 100 mm i.d, while the horizontal pipe was 230 mm in length and 60 mm i.d, and the two L valve aeration taps were 110 210 mm above the axial line of their own horizontal pipe. The combustion chamber was a fluidized bed, it was made of a 500 × 310 × 1500 mm stainless steel cube box in which four 1.5 m-long gas-preheating pipes of 60 mm i.d were fixed. The condenser was a 2.4 m-long water jacket of 80 mm i.d, through which the gasescaping pipe of 40 mm i.d was equipped. Pressure taps were mounted flush with the reactor 0.05 m (Pt), 1.4 m (Pb) above the distributor and one of the L valve aeration taps (Pl). The temperature profile along the reactor was measured with K-type (chromel-alumel) thermowells 0.4 m (T1), 2.5 m (T2) above the distributor and gas-exit point (T3) of the reactor. Experimental Procedure. The combustion chamber was the main energy source of this apparatus, in which the shredded charcoal powder was burned to provide heat for the whole system. The carrier gas (air first, then pyrolysis gas) was pumped into preheating pipe by the circulating pump to be heated. The temperature reached about 400 °C, then the gas moved into the electrical heating pipe, in which it was heated to about 500 °C.Through the distributor plate, the gas entered the CFB and carried quartz sand from a rotary feeder to circulate. When bed temperature rose to a proper degree about 500 °C, wood powder was fed by a rotary feeder with a variable speed motor from a sealed hopper. With favorable conditions of heat and mass transfer, the wood powder was pyrolyzed immediately and brought out of CFB quickly. After the quartz sand and pyrolysis char were separated by the two cyclones, respectively, the pyrolysis vapors were quenched when they were through the condenser. Thus, the liquid product-bio-oil could be collected, and the noncondensable gases were extracted by the circulating pump. The separated sand was circulated through L valve 1, while the recycled char and unreacted wood powder were sent to the combustion chamber to burn through L valve 2. During the experiment, the CFB was operated under a little negative pressure. It was necessary to pump the air into this system for a long time after
Chemical Processes in the CFB Reactor The circulating fluidized bed could be divided into two zones corresponding to the main chemical processes,7 it was modeled as Figure 2. (1) Pyrolysis Zone. In this zone, feedstock was loaded into the bed and pyrolyzed very quickly. Since the feedstock particles were small and the heat exchanged rapidly, the heating rate was very high. For example, a small particle at 0.1-0.2 mm diameter, could be heated at the rate of about 103 °C/s in an atmosphere at 1000 °C. In this zone, the main chemical process could be described as
biomass f char + tar + H2O + gas(CO2, CO, CH4, CnHm, H2) Temperature was another essential factor affecting the pyrolysis besides heating rate. Because the relatively high temperature was favorable to form more noncondensable gas and decrease the tar yield, so moderate and carefully controlled temperature was needed. (2) Reduction and Cracking Zone. Before the pyrolysis vapors were quenched by the condenser, further reactions had taken place; for example, the tar cracked and the char was reduced. These processes produced more noncondensable gas such as CO and H2. Some CnHm also cracked at the same time. The main reactions could be expressed as
C + H2O f CO + H2; C + CO2 f 2CO CH4 + H2O f CO + 3H2; CO + H2O f CO2 + H2 Tar f CH4 + H2O + CnHm + H2 Pyrolysis char contributed to secondary cracking by catalyzing secondary cracking in the vapor phase; rapid
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Table 2. Comparison of Gas Compositiona facility
operation condition
gas composition (mol %) LHVb H2 CH4 CO CO2 CnHm (MJ/m3)
CFB 1 D ) 0.41 24.62 gasifier T ) 920 2 D ) 0.38 1.89 T ) 450 R ) 0.8 3 D ) 0.38 3.01 T ) 550 R ) 0.8 this 4 D ) 0.38 7.01 study T ) 550 R ) 1.5 5 D ) 0.73 2.16 T ) 550 R ) 0.8 6 D ) 0.73 5.51 T ) 550 R ) 1.5
13.16 29.03 29.21 2.7
3.98
13.48 49.13 32.8
13.56 23.71
2.68 18.28 39.74 36.29
26.65
4.41 24.13 36.35 28.1
23.23
2.13 15.68 41.8
38.23
27.26
4.32 20.37 37.25 32.55
25.39
a T, temperature (°C); R, gas residence time (s); and D, wood powder diameter(mm). b LHV ) (30.0*CO + 25.7*H2 + 85.4*CH4 + 151.3*CnHm)*4.2 (kJ/m3).
object of gasification is to get high quality gas product. Thus the high temperature of up to 900 °C is wanted to increase the gas product and decrease the tar, while the relatively long residence time contributes to the secondary reactions including char reduction, tar cracking, shift reaction, etc. So the amount of CO2, CO, CH4, and H2 is far more, and the amount of CnHm is less in the gas product of gasification. By contrast, the objective of fast pyrolysis is to obtain more liquid product; it determines the operation conditions of moderate temperature and short residence time to increase the liquid production rate. Such operation conditions lead to the higher amount of CnHm and less amount of CO, CH4, and H2, which indicate that the degree of pyrolysis is not excessive. Table 2 also shows the difference of results at various operation conditions in this CFB reactor. The main trend is that the higher temperature and longer residence time lead to less CO2 and more CO and H2. CH4
Figure 3. Gas composition corresponding to different operation conditions.
and complete char separation was therefore desirable. Since the vapor residence time influenced the yield and quality of the liquid product, the shorter this reaction zone, the better the product. Results and Discussion The Composition of Gas Product. Although gas product was not the main research object, the variation of its composition contributed to understand the mechanism of pyrolysis. The comparison of gas composition is shown in Table 2. The smaller wood powder diameter provides more heating surface area, so it can be considered as the indication of the heating rate. Table 2 shows that the composition of the gas product from CFBG (CFB gasifier)7 is quite different from the gas sample of this study. This difference is the very reflection of the different pyrolysis mechanism between biomass gasification and fast pyrolysis for liquids. The
varies slightly. The bigger particle diameter decreases the heating rate and favors the carbonization, so the amount of gas product is less. The LHV is determined mainly by the amount of CnHm. It must be pointed out that only the changing trend of H2 can definitely reflect the degree of secondary reactions. More H2 suggests that secondary reactions take place more extensively. In contrast to the H2 , other components will increase or decrease when the secondary reactions take place. Such features can be derived from the previous discussion of chemical processes. The gas composition corresponding to different operation conditions is shown in Figure 3. The Components of Bio-oil. The bio-oil sample was dissolved by chloroform first, then the solution was dried, and its weight was measured. Adding ligroin, the insoluble part is asphalt, while the soluble part (which was composed of alkanes, aromatics, and non-hydro(7) Jianzhi, W.; Bingyan, X.; Zhenfan, L.; Xiguang, Z. Biomass Bioenergy 1992, 3 (2), 105-110.
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Figure 4. Gas chromatogram of alkanes fraction. Conditions: 30m × 0.25 mm × 0.25 µm, HP-5 capillary column; carrier gas, He; temperature program, 75 °C (hold 5 min) to 285 °C at 3 °C/min (hold 40 min); injector and detector temperature: 280 °C. For identification of peaks, see Table 4.
Figure 5. Gas chromatogram of aromatics fraction. Column and flow conditions as in Figure 4. Table 3. Weight Percent of Each Fraction alkanes (wt %)
aromatics (wt %)
non-hydrocarbon (wt %)
asphalt (wt %)
31.04
13.47
39.10
16.39
carbons) was injected into the SiO2/Al2O3 column. That which could pass through the column was alkanes. Aromatics and non-hydrocarbons were washed down with the benzene and methanol as solvents, respectively. The separated fractions of alkanes, aromatics, and non-hydrocarbons were analyzed by HP 5972-II gas chromatography-mass spectrometry (GC-MS) system (Figure 4-Figure 6), while the asphalt was detected by PE1725X infrared spectrometry instrument (Figure 7).
The weight percent of each fraction is shown in Table 3, and the main identified components are listed in Table 4. Table 3 shows that most compounds in bio-oil are nonhydrocarbons and alkanes, aromatics, and asphalt are relatively low. The percent of asphalt indicated the pyrolysis degree, more sufficient pyrolysis led to less asphalt. From Table 4 and Figures 4-6, it can be concluded that the bio-oil is extremely complex and is composed of hundreds of compounds. The main components include phenol, phenanthrene, anthracene, naphthalene, and some species of acid. Although a number of components are identified by GC-MS, many species
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Figure 6. Gas chromatogram of non-hydrocarbons fraction. Column and flow conditions as in Figure 4.
Figure 7. Infra-red spectrometry of asphalt fraction (solvent: CHCl3).
of interest are not identified due to8 (a) their low concentration levels below the MS detection levels, and (b) the overlapping of certain species such as aromatics by phenols which are major constituents with excessive peak tailing. Due to the large amount of oxygen in bio(8) Sheu, Y.-H. E.; Philip, C. V.; Anthony, R. G. J. Chromatogr. Sci. 1984, 22, 497-505.
oil, the components of bio-oil are affected severely by oxidation. The Technical Discussion. (1) The carrier gas preheating is important, and it is necessary to take steps to control the preheating temperature strictly. Using the combustion chamber and the electrical heating jacket to preheat together not only improves the utilization
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Table 4. The Main Identified Components name of fraction alkanes
aromatics
non-hydrocarbon
retention time
area % (to the specified fraction)
compound name
22.87 23.19 12.81 21.60 22.67 23.31 24.49 30.04 37.52 37.84 41.60 41.78 47.06 50.03 52.03 58.80 9.15 13.74 17.57 22.79 23.10 25.23 26.83 45.13 50.63 51.58 55.95 60.25 84.43
1.715 0.805 2.125 1.112 1.864 1.673 1.030 1.974 6.537 1.978 2.333 3.165 2.313 1.955 6.437 5.102 4.964 6.638 2.426 1.604 2.235 0.815 0.939 3.209 5.518 1.600 1.290 31.517 1.247
valencene 1H-3a,7-methanoazulene naphthalene 1,1′-biphenyl naphthalene, 1,8-dimethylnaphthalene, 2,3-dimethylacenaphthylene 9H-fluorene anthracene phenanthrene anthracene, 2-methy1anthracene, 9-methy1pyrene phenanthrene, 2,3,5-trimethylphenanthrene, 1-methyl-7chrysene phenol, 2-methoxy2-methoxy-4-methyl phenol phenol, 4-ethy1-2-methoxybenzaldehyde, 4-hydroxy-3-methoxybenzaldehyde, 4-hydroxy-3-methoxyphenol, 2-methoxy-4-(1-propenyl)ethanone, 1-(4-hydroxy-3-methoxyphenyl)hexadecanoic acid 9,12-octadecadienoic acid(z,z)octadecanoic acid 1-phenanthrenecarboxylic acid 1-phenanthrenecarboxylic acid stigmast-4-en-3-one
rate of the combustion chamber’s heat and reduces the electricity usage, but also provides the convenience of controlling and regulating the temperature of carrier gas. (2) To quench the vapors quickly, an efficient cooling method is needed. Cooling the collected bio-oil first with ice water, then pumping the cooled bio-oil into condenser to cool the vapors directly has obvious effect. If it is hard to collect any bio-oil at the beginning; some gauged water or alcohol can be used alternatively, but it should be removed from the final result. (3) The pipe clogging arising out of the mixture of tar and quartz sand is a serious problem and must be solved. An electrical heating jacket may be equipped from the CFB exit to the condenser to avoid the vapors’ condensation inside these pipes. Even then, it was necessary to pump the air into this system for a long time after the experiment to ensure the pyrolysis vapors and tar’s burning up. Conclusion (1) The CFB bed can be divided into pyrolysis and
secondary reaction zones corresponding to the main chemical processes. To obtain high quality bio-oil, the secondary reaction zone should be short enough. (2) The bed temperature, heating rates, and the vapor residence time have important effect on the pyrolysis gas composition and the bio-oil yields. The main trend is that (1) the higher temperature and longer residence time contribute to the secondary reactions, which lead to less liquids; (2) the lower heating rate favors the carbonization, also reducing the liquid production. (3) Most compounds in bio-oil are non-hydrocarbons and alkanes, aromatics, and asphalt are relatively less. (4) The temperature controlling, vapors’ quenching, pipe block, and the gas leakage are the usual problems during the experiment; they should be noticed. Acknowledgment. The authors thank the GuangDong government foundation for the financial support during this investigation. EF9901645
