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Biomass Air-Steam Gasification in a Fluidized Bed to Produce Hydrogen-Rich Gas Pengmei Lv,* Jie Chang, Zuhong Xiong, Haitao Huang, Chuangzhi Wu, and Yong Chen Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, No. 81 Xianlie Zhong Road, Guangzhou, 510070 People’s Republic of China
Jingxu Zhu Department of Chemical and Biochemical Engineering, The University of Western Ontario, London, Ontario, Canada, N6A 5B9 Received August 12, 2002
The characteristics of biomass air-steam gasification in a fluidized bed for hydrogen-rich gas production are studied through a series of experiments. The gasifying agent, air, was supplied into the reactor from the lower part of the reactor, and steam was added into the reactor above the biomass feeding location. The effects of reactor temperature, steam-to-biomass ratio, equivalence ratio ER, and the biomass particle size on gas composition and hydrogen production are investigated. From the experimental results, it can be seen that the higher reactor temperature, the proper ER, proper steam-to-biomass ratio S/B, and smaller biomass particle size will contribute to more hydrogen production. The highest hydrogen yield, 71 g H2/kg biomass (wet basis), was achieved at a reactor temperature of 900 °C, ER of 0.22, and S/B of 2.70. It is shown that under proper operating parameters biomass air-steam gasification in a fluidized bed is one effective way for hydrogen-rich gas production.
Introduction Hydrogen is regarded by some researchers as the ideal fuel because it is environmentally benign and can be produced domestically from a variety of renewable biomass resources. Biomass energy is an abundant resource all over the world. The transformation of biomass into hydrogen-rich gas provides a potentially competitive means for producing energy and chemicals from renewable sources.1 It has been demonstrated by Cox et al.2 that high hydrogen yields can be achieved through an adequate reactor design and the control of gasification conditions. Although many experimental works have been performed concerning biomass gasification,2-15 only a few * Corresponding author. Phone: 86-20-87787136. Fax: 86-2087608586. E-mail:
[email protected]. (1) McKinley, K. R.; Browne, S. H.; Neill, D. R.; Seki, A.; Takahashi, P. K. Energy Sources 1990, 12, 105-110. (2) Cox, J. L.; Tonkovich, A. Y.; Elliott, D. C.; Baker, E. G.; Hoffman, E. J. Second Biomass Conference of the Americans: Energy, Environment, Agriculture, and Industry Proceedings; National Renewable Energy Laboratory: Golden, Colorado, 1995; pp 657-674. (3) Turn, S.; Kinoshita, C.; Zhang, Z.; Ishimura, D.; Zhou, J. Int. J. Hydrogen Energy 1998, 8, 641-648. (4) Rapagna´, S.; Jand, N.; Foscolo, P. U. Int. J. Hydrogen Energy 1998, 7, 551-557. (5) Narva´ez, I.; Orı´o, A.; Aznar, M. P.; Corella, J. Ind. Eng. Chem. Res. 1996, 35, 2110-2120. (6) Delgado, J.; Aznar, M. P.; Corella, J. Ind. Eng. Chem. Res. 1996, 35, 3637-3643. (7) Delgado, J.; Aznar, M. P. Ind. Eng. Chem. Res. 1997, 36, 15351543. (8) Aznar, M. P.; Caballero, M. A.; Gil, J.; Marte´n, J. A.; Corella, J. Ind. Eng. Chem. Res. 1998, 37, 2668-2680. (9) Corella, J.; Orı´o, A.; Aznar, P. Ind. Eng. Chem. Res. 1998, 37, 4617-4624.
of them put the emphasis on hydrogen production.2-4,11,14 Cox et al.2 combined biomass gasification and hydrogen separation into a single process step through the use of a membrane reactor, which showed good performance through concurrent separation of the hydrogen. Turn et al.3 utilized an oxygen-nitrogen-steam mixture as the gasifying agent to study the effects of reactor temperature, equivalence ratio, and steam-to-biomass ratio: over the ranges of their experimental conditions examined, the hydrogen yield varied from 23 g H2 kg-1 (of dry, ash-free biomass) at an equivalence ratio of 0.37-60 g H2 kg-1 (of dry, ash-free biomass) at an equivalence ratio of 0.0. Rapagna et al.4 performed catalytic biomass steam gasification runs in a bench scale plant consisting essentially of a fluidized-bed gasifier and a secondary catalytic fixed-bed reactor: they mainly studied the influence of the operating conditions in the catalytic converter on the production of H2. Many other researchers investigated the effect (10) Courson, C.; Makaga, E.; Petit C.; Kiennemann A. Catal. Today 2000, 63, 427-437. (11) Garcı´a, L.; Sa´nchez, J. L.; Salvador, M. L.; Bilbao, R.; Arauzo J. In Proceedings of the Bioenergy The Seventh National Bioenergy Conference; Elsevier Applied Science Publishers Ltd: London, England, 1996; pp 859-865. (12) Caballero, M. A.; Corella, J.; Aznar, M. P.; Gil, J. Ind. Eng. Chem. Res. 2000, 39, 1143-1154. (13) Herguido, J.; Corella, J.; Gonzales-Saiz, J. Ind. Eng. Chem. Res. 1992, 31, 1274-1282. (14) Hofbauer, H.; Rauch, R.; Foscolo, P.; Matera, D. In Proceedings of the2000 1st World Conference on Biomass for Energy and Industry; James & James (Science Publisher) Ltd: London, UK, 2000; pp 19992001. (15) Di Blasi, C. Ind. Eng. Chem. Res. 1996, 35, 37-46.
10.1021/ef020181l CCC: $25.00 © 2003 American Chemical Society Published on Web 04/08/2003
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Table 1. Proximate and Ultimate Analysis of Pine Sawdust moisture content (wt % wet basis) higher heating value (kJ/kg) proximate analysis (wt % dry basis) volatile matter fixed carbon ash ultimate analysis (wt % dry basis) C H O N S
8 20540 82.29 17.16 0.55 50.54 7.08 41.11 0.15 0.57
of catalysts on gas composition.5-12 Until now the catalysts studied are mainly calcined dolomite, nickelbased steam reforming catalysts. Jose´ Corella and collaborates6,7 explored the effectiveness, life, and usefulness of calcined dolomite, magnesite, and calcite for cleaning hot gas from a fluidized-bed biomass gasifier with steam. They also used commercial steam reforming catalysts to improve biomass gasification with steamoxygen mixtures.8-9 Courson et al.10 developed Ni catalysts by the use of metallic nickel as active phase grafted on olivine to study steam and dry reforming of biomass produced gas. Herguido et al.13 explored the effect of feedstock type on biomass steam gasification. Until now, extensive literature has been published on biomass gasification with air, steam, steam-oxygen, and O2-enriched air, but relatively little reports were found related to biomass air-steam gasification and even less involving hydrogen production from biomass gasification. On the basis of the above considerations, one smallscale fluidized bed was developed in this study to explore the characteristics of hydrogen production from biomass air-steam gasification, so as to obtain useful data for the design of industrial units. Experimental Section Feed Materials. Pine sawdust obtained from a timber mill was used as the feedstock for experiments. The pine sawdust was sieved into four size ranges for use, and they were 0.60.9, 0.45-0.6, 0.3-0.45, and 0.2-0.3 mm, respectively. The proximate and ultimate analyses of the biomass are reported in Table 1. From the ultimate analysis of biomass, it can be formulated as CH1.7O0.6. Apparatus. The tests were performed in an atmospheric pressure, indirectly heated, fluidized-bed gasification system, which is shown schematically in Figure 1. Its major components are the fluidized-bed gasifier, biomass feeding system, steam and air providing and preheating system, gas metering, cleaning and sampling system, temperature control system, and gas off-line analysis system. The reactor is made of 1Cr18Ni9Ti stainless steel pipe and is externally heated by two electric furnaces. The total height of the reactor is 1400 mm, with a bed diameter of 40 mm and a freeboard diameter of 60 mm. Along the total height of the reactor, there are five temperature and pressure taps for temperature and pressure control. Below the reactor, one air distributor was installed for better air distribution. The distributor is 3 mm in thickness, and 25 holes (i.d. 1 mm) were perforated uniformly into it. The biomass was fed into the reactor through one screw feeder driven by a variable speed metering motor. The air was used as the fluidizing agent and came from the air compressor. Before the air entered into the reactor, it was preheated to 65 °C in the preheater for better
Figure 1. Schematic diagram of biomass air-steam gasification in a fluidized bed. (1) PID temperature controller, (2) steam generator, (3) steam flow meter, (4) air compressor, (5) valve, (6) flow meter, (7) fluidized bed, (8) electric furnaces, (9) air distributor, (10) air preheater, (11) valve, (12) screw feeder, (13) biomass hopper, (14) temperature control, (15) cyclone, (16) flue gas meter, (17) dry ice trap, (18) cotton filter, (19) gas sample pump, and (20) gas sample bag. performance. The steam of 154 °C was produced in a steam generator (model SZ0.015-0.40, Guangzhou Zhongli Boilers Auxiliary Machine Co., Ltd., Guangdong, China). Before steam flowed into the reactor above the biomass feeding point, it was metered by a steam flow meter. The produced gas flow exits the reactor, then passes through a cyclone, which is heated to 200 °C to prevent the tar contained in the gas condensing in it. The pine sawdust feed rate of four different particle sizes was determined over a range of screw speeds prior to testing. To ensure the reliability of tests data, the mass balance calculation is performed for each test. At the beginning of the experiment, the fluidized bed was charged with 30 g silica sand (particle size 0.2-0.3 mm) as bed material, which helped in stable fluidization and better heat transfer. Then the two electric furnaces were turned on to preheat the fluidized-bed reactor; meanwhile, the air preheater was turned on. In the interval of reactor preheating, the steam was prepared for the test. After the bed temperature reached the desired level and was kept steady, the air compressor was turned on to force the air through the preheater, air distributor, and into the reactor. When the bed temperature again turned steady, the screw feeder was turned on at the desired rotate speed and the test began. Typically, it took 15 min for the test conditions to reach a stable state. Three samples were taken at an interval of 3 min after the test ran in a stable state. Sampling and Gas Analysis. After the char carried in the produced gas was separated in the cyclone, the gas stream was passed through a dry ice trap and a cotton filter for drying and cleaning. The cool, dry, clean gas was sampled using gas bags and analyzed on a gas chromatograph (model GC-2010, SHIMADZU, Japan), which is fitted with a GS-carbon plot column (30 m × 0.530 mm × 3.00 µm) and FID and TCD detectors, and standard gas mixtures were used for quantitative calibration.
Results and Discussion Effect of Reactor Temperature. It is known that temperature plays an important role in biomass gasification. In the present work, the reactor temperature was increased from 700 to 900 °C in 50 °C increments to investigate the effect of temperature on gas composition and hydrogen yield. The test results are presented in Table 2 and Figure 2.
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Table 2. Experimental Results of Different Reactor Temperatures reactor temperature (°C) feed rate (kg/h) biomass particle size (mm) air (m3/h) steam rate (kg/h) equivalence ratio steam-to-biomass ratio dry, inert-free, gas composition (vol %) H2 O2 CH4 CO CO2 C2H4 C2H6 C2H2 gas yield (m3/kg biomass) (wet basis) residence time (s)
700
750
800
850
900
0.445 0.3-0.45 0.5 1.2 0.22 2.70
0.445 0.3-0.45 0.5 1.2 0.22 2.70
0.445 0.3-0.45 0.5 1.2 0.22 2.70
0.445 0.3-0.45 0.5 1.2 0.22 2.70
0.445 0.3-0.45 0.5 1.2 0.22 2.70
21.48 0.71 9.12 42.89 20.51 4.44 0.37 0.47 1.43 2.88
28.18 0.85 8.16 39.32 19.45 3.57 0.20 0.26 1.51 2.82
32.10 0.25 7.46 37.73 18.55 3.47 0.17 0.27 2.23 2.34
36.33 0.51 7.25 34.40 18.88 2.29 0.04 0.30 2.45 2.22
39.40 0.49 6.10 33.42 19.36 0.96 0.00 0.27 2.53 2.18
Figure 2. Hydrogen yield as a function of temperature.
The main reactions involved in the gasification process are given below, in which the value of the reaction heat refers to the temperature 298.15 K.
CO + H2O ) CO2 + H2 + 41 kJ/mol
(1)
CH4 + H2O ) CO + 3H2 -206 kJ/mol
(2)
CH4 +2H2O ) CO2 + 4H2 -165 kJ/mol
(3)
C + H2O ) CO + H2 -131 kJ/mol
(4)
C + CO2 ) 2CO -172kJ/mol
(5)
From Table 2, it can be found that the H2 concentration increases with temperature and that the content of CH4 and CO shows a decreasing trend, which indicates that more CH4 and CO reacts with steam to produce added H2 through reactions 1-3. As Table 2 shows, there is still a large quantity of CO in the gas, from which it can be supposed that reactions 4 and 5 happen simultaneously in the steam gasification process. C2H4 and C2H6 concentration decreases with temperature, and Turn et al. 3 found the same trend, which can be attributed to higher temperature supplying more favorable conditions for thermal cracking and steam reforming. From Table 2, it can also be found that the content of C2H4 is far more than that of C2H6 and C2H2. As the temperature increases from 700 to 900 °C, the gas yield increases from 1.43 to 2.53 m3/kg biomass (wet
basis) and residence time (which means the time that gases pass the whole gasifier in this paper) decreases from 2.88 to 2.18 s. Figure 2 shows that hydrogen yield increases from 22 to 70 g H2/kg biomass (wet basis) in the temperature range, a more sharp increase than that of the H2 concentration, from 21.48 to 39.40%. The latter grew slower because there still existed a large amount of CO that did not react with steam owing to the short residence time. Effect of Equivalence Ratio. As shown in eq 6, ER is defined as the actual oxygen-to-fuel ratio divided by the stoichiometric oxygen-to-fuel ratio needed for complete combustion. At the present study, ER was varied from 0.19 to 0.27 by changing the air flow rate and holding the other conditions constant. The tests results of varying ER are reported in Table 3 and Figure 3.
ER )
weight oxygen (air)/weight dry biomass stoichiometric oxygen (air)/biomass ratio
(6)
Table 3 indicates that the hydrogen content varied little in the range of the ER studied, while gas yield first increases and then decreases with ER. As a result, the hydrogen yield showed the same trend with gas yield as that seen in Figure 3. ER affects gasification temperature under the condition of autothermal operation. A higher value of ER corresponds to a higher gasification temperature. In the stage of ER varying from 0.19 to 0.23, because the temperature controller of electric furnaces cannot compensate for the heat loss caused by biomass pyrolysis and steam reforming reactions, the operation temperature was not kept constant in the lower part of the reactor. Therefore, the actual temperature of the steam gasification increased as ER varied from 0.19 to 0.23. Therefore, there is more gas and hydrogen produced as ER changed from 0.19 to 0.23. Correspondingly, the CO concentration experiences an increasing trend as ER increases from 0.19 to 0.23, while the CO2 concentration shows an opposite trend. In the stage of ER varying from 0.23 to 0.27, oxidization reaction 7 becomes more important than steam gasification reaction 8 because of the increased oxygen content. It is obvious that reaction 8 produces 2.25 mol more permanent gas than reaction 7. Then the gas yield
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Table 3. Experimental Results of Different ER Values equivalence ratio (ER) air (m3/h) feed rate (kg/h) biomass particle size (mm) reactor temperature (°C) steam rate (kg/h) steam-to-biomass ratio dry, inert-free, gas composition (vol %) H2 O2 CH4 CO CO2 C2H4 C2H6 C2H2 gas yield (m3/kg biomass) (wet basis) residence time(s)
0.19
0.21
0.23
0.25
0.27
0.50 0.512 0.3-0.45 800 0.8 1.56
0.55 0.512 0.3-0.45 800 0.8 1.56
0.60 0.512 0.3-0.45 800 0.8 1.56
0.65 0.512 0.3-0.45 800 0.8 1.56
0.70 0.512 0.3-0.45 800 0.8 1.56
32.24 0.25 7.90 37.80 17.97 3.40 0.16 0.28 2.13 2.23
31.13 0.21 8.10 39.50 17.26 3.40 0.14 0.26 2.25 2.06
31.44 0.23 7.61 40.06 16.85 3.39 0.19 0.23 2.37 1.93
31.11 0.25 7.37 39.68 18.07 3.15 0.15 0.23 2.18 1.93
31.86 0.31 6.67 38.24 19.87 2.73 0.10 0.22 1.88 1.99
Table 4. Experimental Results of Different Steam-to-Biomass Ratios steam-to-biomass ratio steam rate (kg/h) feed rate (kg/h) biomass particle size (mm) air (m3/h) reactor temperature (°C) equivalence ratio dry, inert-free, gas composition (vol %) H2 O2 CH4 CO CO2 C2H4 C2H6 C2H2 gas yield (m3/kg biomass)(wet basis) residence time (s)
0
0.61
1.35
2.02
2.70
0 0.445 0.3-0.45 0.5 800 0.22
0.27 0.445 0.3-0.45 0.5 800 0.22
0.6 0.445 0.3-0.45 0.5 800 0.22
0.9 0.445 0.3-0.45 0.5 800 0.22
1.2 0.445 0.3-0.45 0.5 800 0.22
33.22 0.57 6.05 42.96 15.91 1.22 0.04 0.03 1.46 2.86
31.84 0.29 9.87 34.19 21.49 2.40 0.05 0.48 1.57 2.77
29.78 0.30 8.34 40.35 16.85 3.79 0.21 0.39 2.39 2.25
30.81 0.27 8.20 39.26 17.41 3.58 0.15 0.33 2.34 2.28
32.10 0.25 7.46 37.73 18.55 3.47 0.17 0.27 2.23 2.34
Figure 3. Hydrogen production as a function of ER.
of per kg of biomass (wet basis) decreases from 2.37 to 1.88 m3 as ER increases from 0.23 to 0.27. This can also explain the results that the content of CO, CH4, C2H6, and C2H2 decreases and CO2 concentration increases as ER varies from 0.23 to 0.27.
CH1.7O0.6 + 1.125O2 ) CO2 + 0.85H2O
(7)
CH1.7O0.6 + 1.4H2O ) CO2 + 2.25H2
(8)
For hydrogen yield, there exists an optimal value of ER, 0.23. At this value, the hydrogen yield per kg of biomass reaches 54 g. Because much of the CO cannot
Figure 4. Hydrogen yield as a function of steam-to-biomass ratio.
convert to H2 through the water gas shift reaction (WGSR), the change in the hydrogen concentration exhibits no distinct trend. Through the above analysis, it can be understood that it is infeasible to apply a too-small or a too-large ER on biomass air-steam gasification. An ER too small will lower reaction temperature, which is not favorable for biomass steam gasification. An ER too large will consume the more produced H2 through the oxidization reaction.
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Table 5. Experimental Results of Different Biomass Particle Sizes biomass particle size (mm) feed rate (kg/h) steam rate (kg/h) steam-to-biomass ratio air (m3/h) reactor temperature (°C) equivalence ratio dry, inert-free, gas composition (vol %) H2 O2 CH4 CO CO2 C2H4 C2H6 C2H2 gas yield (m3/kg biomass)(wet basis) residence time (s)
0.6-0.9
0.45-0.6
0.3-0.45
0.2-0.3
0.512 0.8 1.56 0.6 800 0.23
0.512 0.8 1.56 0.6 800 0.23
0.512 0.8 1.56 0.6 800 0.23
0.512 0.8 1.56 0.6 800 0.23
31.61 0.34 6.63 37.25 21.18 2.65 0.10 0.24 1.53 2.38
31.62 0.17 7.44 37.57 19.49 3.26 0.17 0.26 1.93 2.14
31.44 0.23 7.61 40.06 16.85 3.39 0.19 0.23 2.37 1.93
30.47 0.24 7.79 40.59 16.75 3.64 0.26 0.26 2.57 1.85
Effect of Steam-to-Biomass Ratio (S/B). In this test, the steam rate was varied from 0 to 1.2 kg/h while keeping all other conditions constant to investigate the effect of steam feed rate on gas composition and hydrogen yield. The test results are presented in Table 4 and Figure 4. As shown in Table 4 and Figure 4, the effect of S/B on hydrogen yield is similar to ER. Over a S/B range from 0 to 1.35, hydrogen yield has a nearly linear increase. Gas and hydrogen yield begins to decrease when the S/B exceeds 1.35 and 2.02, respectively. This can be understood that additional low-temperature steam fed into the reactor causes the reaction temperature to drop, causing the gas yield to decrease. Over a S/B range from 1.35 to 2.70, CO, CH4, and CnHm concentrations decrease gradually, while the fraction of CO2 exhibits an opposite trend. This can be explained by the fact that there are more steam reforming reactions of CO, CH4, and CnHm taking place because of the added steam. From Table 4, it can also be found that the hydrogen concentration varies little over the S/B range. This result appears to be in disagreement with Turn et al.’s3 conclusion, in whose study the hydrogen concentration increased with higher S/B. This difference possibly comes from the different operating conditions, and in Turn et al.’s3 study, the steam rate was kept constant while changing the biomass feed rate in order to vary S/B. Figure 4 shows the addition of steam made hydrogen yield increased greatly, from 35.49 g H2/kg biomass (wet basis) (S/B ) 0) to 52.08 g H2/kg biomass (wet basis) (S/B ) 2.02), an increase of 47%. From the analysis of the test data of varying S/B, it can be agreed that the introduction of steam in biomass steam gasification does benefit in increasing the gas and hydrogen yields. However, excessive steam will lower the reaction temperature and cause gas and hydrogen yields to decrease as Figure 4 illustrates. Effect of Biomass Particle Size. In the biomass steam gasification process, the pyrolysis reaction of biomass particle cannot happen until it is heated to a certain temperature, and the size of biomass particles affects the heating rate. Then it can be inferred that the size of biomass particle will have the effect on the produced gas composition and gas yield.
Figure 5. Hydrogen yield as a function of biomass particle size.
In this research, four size fractions of biomass particle were selected to explore the effect of biomass particle size on gas composition and hydrogen yield. It is generally accepted that the gas yield and composition are related to the heating rate of the biomass particles: high heating rates produce more light gases and less char and condensate.15 From Table 5 it can be found that CH4, CO, C2H4, and C2H6 concentrations increase and that CO2 shows an opposite trend as the biomass particle size decreases. As biomass particle size decreases, the gas yield increases from 1.53 to 2.57 m3/ kg. The smallest particle fraction produces 1.04 m3 more gas per kg of biomass (wet basis) than the largest particles. As shown in Figure 5, for per kg of biomass the smallest particles produces 56 g H2, and the largest particles produce 35 g H2. The smallest particle yields 21 g more hydrogen than the largest ones for per kg of biomass (wet basis). This can be explained by the fact that, for small particle sizes, the pyrolysis process is mainly controlled by reaction kinetics. As the particle size increases, the product gas inside the particle is more difficult to diffuse outward and the process is mainly controlled by gas diffusion.
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Conclusions From the analysis of the four critical parameters (temperature, equivalence ration, steam-to-biomass ratio, and particle size), it can be found that the hydrogen yield is most sensitive to ER. The temperature plays a very important role in the process. A higher temperature will be more favorable for gas and hydrogen yield, and this result is different from that of the study aimed at producing fuel gas of higher heating value. Both S/B and ER affect gasification temperature. A too-high S/B and a too-low ER will lower reaction temperature, and then will cause hydrogen yield to decrease. There exist optimal values for S/B and ER. In the present work, the optimal values for S/B and ER are 2.02 and 0.23, respectively. Biomass particle size also has an influence on gas composition and hydrogen yield, and smaller particles will produce more gas.
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The highest hydrogen yield 71 g H2 per kg of biomass is achieved at the condition of temperature 900 °C, ER of 0.22, and S/B of 2.70. Much CO is present in the produced gas, from which potential hydrogen can be produced through WGSR. It is shown that under proper operating parameters biomass air-steam gasification in a fluidized bed is one effective way for hydrogen-rich gas production. Acknowledgment. The financial support received from the National Natural Science Foundation of China (Project 20206031), the Guangdong Province Natural Science Foundation (Project 010876), and the “OneHundred-Scientist Program” of the Chinese Academy of Sciences to Prof. J. Chang is gratefully appreciated. EF020181L
