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Fluidization of Biomass Particles in a Gas-Solid Fluidized Bed Wenqi Zhong,* Baosheng Jin, Yong Zhang, Xiaofang Wang, and Rui Xiao School of Energy and EnVironment, Southeast UniVersity, Nanjing 210096, People’s Republic of China ReceiVed June 22, 2008. ReVised Manuscript ReceiVed August 15, 2008
Studies on the fluidization of biomass particles and binary mixtures of biomass particles with fluidization mediums were carried out. The biomass particles used were wood chip, mung beans, millet, corn stalk, and cotton stalk, and the fluidization mediums employed were silica sand, continental flood basalt (CFB) cinder, and aluminum oxide. Experiments were performed in a rectangular biomass fluidized bed (cross-sections of 0.4 × 0.4 m in a dense region and 0.5 × 0.5 m in a freeboard region, with a height of 4.4 m). The minimum fluidization velocity (UMF) of approximate sphere biomass particles (wood chip, mung beans, and millet) and long thin biomass particles (corn stalk and cotton stalk) in different transection diameters and ratios of length/ diameter were tested. Furthermore, the UMF of binary mixtures of biomass particles with fluidization mediums of different particle densities and diameters was obtained. The results showed that the UMF of long thin biomass increases with an increasing transection diameter and aspect ratio of length/diameter, while long thin biomass with the aspect ratio over a certain value could not be fluidized; the UMF of binary mixtures increase with an increasing density and diameter of fluidization medium and an increasing mass fraction of biomass. On the basis of experimental data, new correlations were developed for predicting the values of UMF. Comparisons of the predicted UMF by the correlations with experimental data in both the present work and literature were carried out. It was found that the present proposed correlations reasonably well-predicted the UMF of biomass particles and binary mixtures of biomasses with fluidization mediums.
1. Introduction The transformation of biomass into fuel and chemicals is becoming increasingly popular worldwide as a way to mitigate global warming and diversify energy sources.1 A number of thermo-chemical processes are under development worldwide, e.g., combustion, gasification, and pyrolysis. Many of these processes are based on fluidization; however, biomass particles have peculiar shapes, sizes, and densities, which make them difficult to fluidize and handle.2 Thus, the fluidization characteristics, including minimum fluidization velocity, ways of achieving fluidization, mixing and segregation, and residence time distributions, are of interest for the design and optimization of these processes and equipment. Fluidized beds have been applied widely in dealing with biomass because of their advantages of high heat transfer, uniform and controllable temperatures, favorable gas-solid contacting, and the ability to handle a wide variation in particulate properties.3 Many valuable efforts have been performed in understanding the fluidization characteristics of biomass and the mixture of biomass with fluidization medium, e.g., refs4-11. However, limited work that has been reported on the influence of key particle properties (e.g., particle size * To whom correspondence should be addressed. Telephone: +86-2583794744. Fax: +86-25-83795508. E-mail:
[email protected]. (1) Briens, C.; Piskorz, J.; Berruti, F. Biomass valorization for fuel and chemicals productionsA review. Int. J. Chem. React. Eng. 2008, 6, R2. (2) Cui, H.; Grace, J. R. Fluidization of biomass particles: A review of experimental multiphase flow aspects. Chem. Eng. Sci. 2007, 62, 45–55. (3) Lv, P. M.; Xiong, Z. H.; Chang, C. Z.; Chen, Y.; Zhu, J. X. An experimental study on biomass air-steam gasification in a fluidized bed. Bioresour. Technol. 2004, 95, 95–101. (4) Pilar Aznar, M.; Gracia-Gorria, F. A.; Corella, J. Minimum and maximum velocities for fluidization for mixtures of agricultural and forest residues with second fluidized solid. I. Preliminary data and results with sand-sawdust mixtures. Int. Chem. Eng. 1992, 32, 95–102.
and particle density) and the presence of particles of extreme shapes (e.g., long thin stalks or flat chips) on the fluidization.2 As an important parameter of fluidization hydrodynamic characteristics, the minimum fluidization velocity not only quantitatively indicates the amount of drag force needed to attain solid suspension in the gas phase but also constitutes a reference for the evaluation of the intensity of the fluidization regime at higher velocity levels.12 Until now, there is a lack of data on the minimum fluidization velocity of particular forms of agricultural biomasses, including straw, corn stalk, and cotton stalk, even less than the correlation of minimum fluidization velocity. Thus, theoretical attempts as well as experiments aiming at grasping interesting and helpful information on the minimum fluidization velocities of biomass fuels are expected. In the present work, experiments on the fluidization of biomasses (wood chip, mung beans, millet, corn stalk, and cotton (5) Pilar Aznar, M.; Gracia-Gorria, F. A.; Corella, J. Minimum and maximum velocities for fluidization for mixtures of agricultural and forest residues with a second fluidized solid. II. Experimental results for different mixtures. Int. Chem. Eng. 1992, 32, 103–113. (6) Laytner, F.; Grace, J. R.; Epstein, N.; Pinder, K. L. Mobility of wood wafers in a gas-fluidized bed. In Fluidization VIII; Large, J. F., Lague´rie, C., Eds.; Engineering Foundation: New York, 1995; pp 93-102. (7) Chen, K. F.; Chen, Sh. M. Fluidization properties of high-consistency fiber suspensions. Exp. Therm. Fluid Sci. 1997, 14, 149–159. (8) Rao, T. R.; Bheemarasetti, J. V. R. Minimum fluidization velocities of mixtures of biomass and sands. Energy 2001, 26, 633–644. (9) Kozanoglu, B. U.; Welti Chanes, J.; Garcı´a Cuautle, D; Santos Jean, J. P. Hydrodynamics of large particle fluidization in reduced pressure operations: an experimental study. Powder Technol. 2002, 125, 55–60. (10) Suarez, J. A.; Beaton, P. A. Physical properties of Cuban coffee husk for use as an energy source. Energy Sources 2003, 25, 953–959. (11) Abdullah, M. Z.; Husain, Z.; Yin Pong, S. L. Analysis of cold flow fluidization test results for various biomass fuels. Biomass Bioenergy 2003, 24, 487–494. (12) Frmisani, B.; DeCristofaro, G.; Girimonte, R. A fundamental approach to the phenomenology of fluidization of size segregating binary mixtures of solids. Chem. Eng. Sci. 2001, 56, 109–119.
10.1021/ef800495u CCC: $40.75 2008 American Chemical Society Published on Web 10/21/2008
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Figure 2. UMF of cotton stalk and corn stalk in different ratios of length/diameter. Figure 1. Schematic diagram of the experimental system.
stalk) and binary mixtures of biomasses with fluidization mediums [silica sand, continental flood basalt (CFB) cinder, and aluminum oxide] were carried out in a gas-solid fluidized bed. It focused on examining the effects of particle size, density, and shape on the minimum fluidization velocity and obtaining helpful experimental data. Furthermore, new correlations were developed for predicting the values of minimum fluidization velocity based on the experimental data. 2. Experimental Section Experiments were carried out in a gas-solid fluidized bed. The schematic diagram of the experimental system is illustrated in Figure 1, which consists of a fluidized bed, a gas supply system, a differential pressure acquisition system, and a digital visual acquisition system. The fluidized bed is rectangular-shaped, which has a cross-section of 0.4 × 0.4 m in the low-dense region and a cross-section of 0.5 × 0.5 m in the upper freeboard region. The total height of the bed is approximately 4.4 m. A Roots-type blower supplied the fluidizing gas. Gas flow rates are regulated by an orifice flowmeter. The fluidizing gas entered into the gas chamber first and then entered the bed via the gas distributor. The gas distributor was composed of 48 equally spaced nozzle-type tuyeres with four horizontal orifices. For visual observation, the front wall of the low-dense region was made of glass. The right wall of the bed could be opened for the loading and discharging of biomass particles and fluidization mediums. Five pressure taps were located on the left wall: one was in the gas chamber, and the others were along the bed heights of 0.05, 0.15, 0.68, and 2.96 m, respectively. Pressure drops through the bed were measured by a differential pressure sampling apparatus. The pressures were measured and then converted into voltage signals by a multichannel differential pressure signal transmitter with a scale of 0-10 kPa. The voltage signals were sent to a computer through an A/D converter. The experimental biomass particles were wood chip, mung bean, millet, corn stalk, and cotton stalk. The fluidization mediums were silica sand, CFB cinder, and aluminum oxide. The particle densities were measured by an electronic specific gravity balance (MD-300S). For the measurements of low-density particles, i.e., less than 1000 kg/m3, particles were enclosed in a multiporous metal box first and then immerged into measurement medium. By this method, the voidage can be calculated by subtracting the volume of the metal box. The medium used in the measurements was distilled water. The diameters of approximate sphere particles were measured by a laser particle size analyzer (Mastersizer 2000). The transection diameters and lengths of long thin particles were measures by a vernier caliper. To obtain accurate mean values, three measurements were carried out.
Experiments were conducted at room temperature and atmospheric pressure. The fluidizations of biomass particles with and without fluidization mediums were experimented. For the experiment of biomass fuel with fluidization medium, the initial arrangement of the bed was such that the particles were well-mixed. To obtain such a state, two species were placed into the bed layer by layer in sequence. Starting from the fixed-bed state, the gas velocity was slowly increased until it reached a larger value to obtain a complete fluidized state and then decreased the gas velocity to measure the minimum fluidization velocity according to the pressure drop line. It has been widely accepted that the interaction of the pressure drop line of the fixed bed with that for the fluidization state is denoted the minimum fluidization condition, and its corresponding superficial gas velocity is defined as the minimum fluidization velocity. Similar to most previous investigations (e.g., Rao and Bheemarasetti8 and Kozanoglu et al.9), the value of minimum fluidization velocity measured with a decreasing gas velocity from the complete fluidized state was performed in the present work to ensure that the measured values could make the bed fluidized.
3. Results and Discussion 3.1. Experimental Minimum Fluidization Velocities. 3.1.1. Long Thin Biomass Particles. Long thin biomass is one of the familiar biomass fuels but with extreme shape, e.g., cotton stalks and corn stalks. Here, extreme shape refers to not familiar shape, for example, long thin stalks and flat chips, according to Cui and Grace,2 and long thin biomass refers to that which covers a larger aspect ratio of length/transection diameter (generally 1 < L/dpt < 20). Because of the lower energy consumption of cutting very long biomass stalks to a certain length than grinding to particles, long thin biomass fuels are of increasing interest in industrial applications. For example, they are very appropriate to be dealt with in gas-solid fluidized beds. However, there has been no study on their hydrodynamic characteristics in gas-solid fluidized beds until now.2 In the present experiment, the fluidization of two kinds of long thin biomasses particles was investigated. Figure 2 plots the varieties of minimum fluidization velocity of cotton stalk (Fp ) 365 kg/m3, dpt ) 5 mm) and corn stalk (Fp ) 274 kg/m3, dpt ) 4 mm) with increasing ratios of length/ diameter. Here, the diameter dpt of long thin biomass refers the diameter in its transection. It is found that the minimum fluidization velocities of both biomass fuels increase with increasing aspect ratios of length/diameter. For a given ratio of length/diameter, an increase in particle transection diameter leads to an increasing minimum fluidization velocity, as presented in Figure 3.
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Figure 3. UMF of cotton stalk in different transection diameters.
Figure 4. UMF of mung beans in different mass fractions with fluidization mediums.
It was observed in the experiments that long thin biomass with a ratio of length/diameter over a certain value could not be fluidized. Taking cotton stalk for example, to become fluidized, the aspect ratio of length/diameter should be L/dpt < 18 when the transection diameter of long thin biomass is 3.4 mm, while when the transection diameter increases to 6.5 mm, the aspect ratio L/dpt should be not larger than 12. However, the range of the aspect ratio is not fully determined at this time, because the ratio is related to many other factors besides the transection diameter. The above-mentioned trend might be due to the phenomenon that long thin biomasses are usually bridging and enwinding each other, which gives an adverse effect to fluidization. In this case, large fluidizing gas is needed for fluidization, which is especially visible for the biomass with a large ratio of length/diameter. Besides, unlike particulate fluidization of particles, the fluidizing of long thin biomass is found to be aggregative fluidization. 3.1.2. Approximate Sphere Biomass with Fluidization Medium. Biomass, especially long thin biomass, cannot be easily fluidized because of their peculiar shapes, sizes, and densities. For proper fluidization and processing in the reactor, a second fluidization medium, usually an inert material, such as silica sand, CFB cinder, and aluminum oxide, are used to facilitate fluidization of biomass. They also act as a heat-transfer medium in the reactor. In the present work, the fluidizations of biomass particles including the above-mentioned long thin biomass with fluidization mediums were tested and their minimum fluidization velocities were determined. Figure 4 shows the minimum fluidization velocity of mung bean (Fp ) 1640 kg/m3, dp ) 3.2 mm) with two kinds of fluidization mediums (silica sand, Fp ) 2700 kg/m3, dp ) 1.0 mm; CFB cinder, Fp ) 1870 kg/m3, dp ) 2.8 mm) at different
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Figure 5. Effect of the fluidization medium diameter on the UMF of the binary mixture.
Figure 6. Effect of fluidization medium density on the UMF of the binary mixture.
biomass mass fractions. The data shown in Figure 4 cover a range of effective diameters from 1.0 to 3.2 mm. The minimum fluidization velocity of pure mung bean was determined as 1.07 m/s, and the minimum fluidization velocities of pure silica sand and CFB cinder were determined as 0.56 and 0.92 m/s, respectively. For the binary mixture of mung bean with fluidization mediums, it can be seen that the minimum fluidization velocity increases with an increasing mass fraction of biomass. In this case, the effective particle diameter of the mixture increases, which leads to an increasing minimum fluidization velocity. The effect of the fluidization medium diameter on the minimum fluidization velocity of the binary mixture of wood chip (Fp ) 564 kg/m3, dp ) 0.89 mm) and silica sand is shown in Figure 5. It can be seen that the minimum fluidization velocity increases with an increasing diameter of fluidization medium at a given biomass fuel mass fraction. When the density of the fluidization medium is increased, the minimum fluidization velocity of the binary mixture increases, as presented in Figure 6. The minimum fluidization velocities of binary mixtures of approximate sphere biomass with fluidization medium have similar characteristics of fluidization medium in the fluidization. In these cases, the mass fractions are lower when compared to the fluidization mediums. Both the effective particle diameter and the effective particle density for the binary mixture increase when the diameter and density of the fluidization mediums increase. As a result, the minimum fluidization velocities of binary mixtures increase. 3.1.3. Long Thin Biomass with Fluidization Medium. Figure 7 shows the minimum fluidization velocities of long thin cotton stalk in different ratios of length/diameter with siliceous sand
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Figure 8. UMF of cotton stalk in different diameters with different fluidization mediums.
Figure 7. UMF of cotton stalk in different ratios of length/diameter with fluidization mediums: (a) silica sand and (b) CFB cinder.
and CFB cinder. It can be seen that the minimum fluidization velocities increase with an increasing mass fraction of biomass ranges from 0 to 5%. The present experiments performed under the cases that the mass fraction of long thin biomass is no more than 5%. In the CFB combustion of cotton stalk, the mass fraction of biomass to the fluidization medium is always less than 5% because of the low packed density and high volume ratio of the biomass in the bed corresponding to the higher mass fraction of the solids. Take biomass CFB for example, the fluidization medium with about 500 mm in height is generally used to improve fluidization; the height of long thin biomass (e.g., cotton stalk) reaches about 500 and 720 mm when the mass fraction of biomass is 5% for the fluidization medium of silica sand and aluminum oxide, respectively. It can be imagined that the bed could not be well-fluidized when the height of long thin biomass is higher than that of fluidization medium. Figure 7 indicates that the minimum fluidization velocities of long thin biomass with fluidization medium increase with increasing aspect ratios of length/diameter. In comparison to the minimum fluidization velocity of long thin biomass only and long thin biomass with fluidization medium, it is found that the fluidization can be achieved with a larger aspect ratio of long thin biomass with fluidization medium. Taking cotton stalk of 5 mm transaction diameter for example, the bed could not be fluidized when the aspect ratio L/dpt is larger than 16 in a pure biomass fluidized bed (Figure 3), while a fluidized bed with fluidization medium even when the aspect ratio L/dpt reached 20 can be well-fluidized (Figure 7). This indicated that fluidization medium improves the fluidization of long thin biomass.
The minimum fluidization velocities of long thin biomass in different transection diameters with three kinds of fluidization mediums are presented in Figure 8. As shown in this figure, the minimum fluidization velocities increase with increasing biomass transection diameters for each fluidization medium. Additionally, the minimum fluidization velocities of binary mixtures of long thin biomass with fluidization medium have similar characteristics of fluidization medium in the fluidization, the minimum fluidization velocity increases when the density of the fluidization medium increases. 3.2. Correlation of Minimum Fluidization Velocity. 3.2.1. Determination of the UMF Correlation. Knowledge of the minimum fluidization velocity facilitates the study of reaction kinetics because it allows for a rational use of the gas in the gas phase as an excess over that required for minimum fluidization. It would therefore be useful to be able to predict this velocity instead of having to measure it for each new situation. In the literature, several equations are proposed for predicting the minimum fluidization velocity mainly based on particle and gas properties, i.e., densities of solid and gas, sphericity, particle diameter, and voidage at minimum fluidization velocity (e.g., Ergun14 and Lippens and Mulder15). These expressions require parameters of the sphericity and voidage at minimum fluidization velocity, which are difficult to measure experimentally. Coltters and Rivas16 proposed a new relationship without the necessity of experimental determination of bed voidages and shape factors for the prediction of minimum fluidization velocity, which was in very well agreement with the experimental data. However, there has been little investigation on the minimum fluidization velocity correlation of mixtures of solids with different particle sizes, especially the mixtures of biomass particles with different sizes and densities. Cheung et al.17 proposed an equation for predicting minimum fluidization velocities of binary mixtures of particles in different sizes, which is given by (13) Clarke, K. L.; Pugsley, T.; Hill, G. A. Fluidization of moist sawdust in binary particle systems in a gas-solid fluidized bed. Chem. Eng. Sci. 2005, 60, 6909–6918. (14) Ergun, S. Fluid flow through packed columns. Chem. Eng. Prog. 1952, 48, 89–94. (15) Lippens, B. C.; Mulder, J. Prediction of the minimum fluidization velocity. Powder Technol. 1993, 75, 67–78. (16) Coltters, R.; Rivas, A. L. Minimum fluidation velocity correlations in particulate systems. Powder Technol. 2004, 147, 34–48. (17) Cheung, L.; Nienow, A. W.; Rowe, P. N. Minimum fluidization velocity of a binary mixture of different sized particles. Chem. Eng. Sci. 1974, 24, 1301–1303.
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()
umf ) us
ub us
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xb 2
(1)
This equation depends upon the mass fraction of the particles in the mixture and the individual minimum fluidization velocity for each particle fraction, in which us and ub are minimum fluidization velocities of small and larger particles, respectively, and xb is the mass fraction of the larger particles. Rao and Bheemarasetti8 found that this correlation predicted lower values compared to their experimental values. Thus, they proposed a new correlation, that is dpe2(Fpe - Fg)g 1650µg
umf )
(2)
where dpe and Fpe are effective particle diameter and effective density of the mixtures, respectively. However, eq 2 is not very convenient for use because there is a coefficient included in effective particle diameter that is difficult to determine, detailed information could be seen in Rao and Bheemarasetti.8 In the present work, the minimum fluidization velocity is considered to be characterized by particles with different shapes, sizes, densities, and compositions. The determination of correlation for predicting the minimum fluidization velocities of biomass is based on the general expression proposed by Coltters and Rivas.16 The relationship is presented as follows
[
umf ) kXR ) k
() ]
dp2(Fp - Fg) Fp µg Fg
1.23 R
(3)
Equation 3 is a general expression that may be used to estimate the minimum fluidization velocity in a system. The precise knowledge of bed voidage at minimum fluidization velocity is avoided in this equation, which is very important given the experimental difficulties in determining those parameters, especially when beds of irregular shape and coarse particles are handled.16 dp and Fp in this equation refer to the single material. To use this equation for the biomass mixtures, quantities such as effective particle diameter dpe and effective density Fpe of the irregular shape biomass and binary mixture of biomass with fluidization medium are used. The effective density of the mixture is calculated using the following general equation: Fpe )
w1F1 + w2F2 ) x1F1 + x2F2 w1 + w2
(4)
in which w1 and w2 are the weights of particles in the binary mixture, x1 and x2 are the mass fractions of particles in the binary mixture, with x1 < x2, and F1 and F2 are the densities of particles in the binary mixture. To determine the effective diameter dpe, it is assumed that the effective particle diameter for mixtures depends upon the individual particle sizes, densities, and their composition. The following equation is defined for calculating the effective particle diameter for mixtures:
[( )( )]
dpe ) dp1
F1 dp2 F2 dp1
w2/w1
) dp1
[( )( )] F1 dp2 F2 dp1
x2/x1
(5)
In eq 5, dp1 and dp2 refer to the effective particle diameters of composition in binary mixtures. While dp1 is the diameter of the particle that is in less mass fraction of the mixture, it does not matter if dp1 is the biomass or the fluidization medium. Combining eqs 3-5, the minimum fluidizing velocity yields
[
umf ) kXR ) k
( ) ]
dpe2(Fpe - Fg) Fpe µg Fg
1.23 R
(6)
Figure 9. Determination of the UMF correlation of the low-effective density particulate system.
It is well-known that the minimum fluidization velocity is sensitive to parameters such as solid and fluid densities, the nature of solids and fluids, etc. Additionally, the minimum fluidization velocity is quite sensitive to the density difference because of the buoyancy. The particle-fluid density ratio can be related to the drag exerted from the fluid on the particles and to the void fraction. In the present work, correlations were determined for the low- and high-effective density particulate systems, respectively. For low-effective density particulate system (0 < Fpe < 1000 kg/m3), the correlation of minimum fluidization velocity is determined as
[
umf ) 1.2 × 10-4X0.633 ) 1.2 × 10-4
( ) ]
dpe2(Fpe - Fg) Fpe µg Fg
1.23 0.633
(7)
The determination of eq 7 based on the experimental values is presented in Figure 9. It can be seen that the minimum fluidization velocity is a function of parameter X. Besides, the equation fits the experimental data in an excellent manner, with closer scatter of the data points around this line. The fitting of the experimental data to eq 7 has a correlation coefficient of R ) 0.99. This equation can be used to predict the minimum fluidization velocities of not only the binary particulate system but also biomass, only when the effective density ranges from 0 to 1000 kg/m3, because the experimental data used to determine the correlation includes the data of low-density biomass. For the high-effective density particulate system (Fpe > 1000 kg/m3), the correlation of minimum fluidization velocity is determined as umf ) 1.45 × 10-3X0.363 ) 1.45 × 10-3
[
dpe2(Fpe - Fg) × µg
( ) ] Fpe Fg
1.23 0.363
(8)
The determination of eq 8 based on the experimental values is presented in Figure 10. The minimum fluidization velocity is a function of parameter X. The equation fits the experimental data in a reasonable manner. A closer scatter of the data points around this line is seen. The fitting of the experimental data to eq 8 has a correlation coefficient of R ) 0.96. The correlation is applicable for the high-effective density binary particulate system, with the effective density larger than 1000 kg/m3.
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Figure 12. Comparison of present experimental UMF with predicted data by the correlations of the present work and in the literature. Figure 10. Determination of the UMF correlation of the high-effective density particulate system.
and Bheemarasetti8 correlations contain certain errors, with some errors beyond 40%. Not considering the difference in measurement, when the data under different operating conditions were checked, it was found these two correlations failed to predict the minimum fluidization velocities of binary mixtures of long thin biomasses with fluidization mediums. As a result, the relative errors of predictions to experiments were found to be very large. 4. Conclusions
Figure 11. Comparison of predicted UMF by eqs 7 and 8 with experimental data of the present work and in the literature.
3.2.2. Verification of the UMF Correlation. Figure 11 shows the comparison of predicted minimum fluidization velocities by eqs 7 and 8 with experimental data. Experimental data were obtained from the present work and the experiments by Rao and Bheemarasetti8 and Abdullah et al.11 It is found that the present correlations quite satisfactorily predicted the experimental results. The mean relative error of the predictions to the experiments is 14.7%. This indicates that the present correlation is available. However, the prediction is overestimated the minimum fluidization velocities than experiments by Rao and Bheemarasetti,8 with a mass fraction of biomass less than 10% and a mean relative error over 40%. This error could not generate the difference in the measurement way. Both present experiments and Rao and Bheemarasetti8 employed the method of decreasing superficial gas velocity. Besides, it is known that the data measured with decreasing superficial gas velocity is somewhat larger than that measured with an increasing superficial gas velocity, but the difference is generally no more than 5%. Thus, the error might be due to using mean diameters of fluidization mediums in predictions, while their experiments were carried out with a wide distribution of particle diameters of fluidization medium. To further verify the present correlations, Figure 12 gives the comparison of present experimental minimum fluidization velocities with predicted data by the correlations of the present work and in the literature. The present correlation predicts the experimental results reasonably well, with a mean relative error of 13.2%. While the predictions by Cheung et al.17 and Rao
Experimental data of minimum fluidization velocities for five kinds of biomass particles (wood chip, mung beans, millet, corn stalk, and cotton stalk) and binary mixtures of biomasses with three kinds of fluidization mediums (silica sand, CFB cinder, and aluminum oxide) were obtained in a rectangular fluidized bed (cross-sections of 0.4 × 0.4 m in a dense region and 0.5 × 0.5 m in a freeboard region, with a height of 4.4 m). New correlations were developed for predicting the values of minimum fluidization velocities based on the experimental data. The notable findings are that the minimum fluidization velocity of long thin biomass increases with an increasing diameter and aspect ratio of length/diameter, while long thin biomass with the aspect ratio over a certain value could not be fluidized, and the minimum fluidization velocity of the binary mixture increases with an increasing density and diameter of fluidization medium and an increasing mass fraction of biomass. Besides, the present proposed correlations could predict the minimum fluidization velocities of biomass particles and the binary mixture of biomass with fluidization medium reasonably well. Acknowledgment. Financial support from the National Natural Science Foundation of China (50676021 and 50706007) is sincerely acknowledged.
Nomenclature dp ) sphere mean diameter or volume sphere equivalent diameter, mm dpe ) effective particle diameter of binary mixtures, mm dpt ) transection diameter of the long thin particle, mm dp1, dp2 ) effective particle diameter of composition in binary mixtures, mm k ) parameter in the UMF correlation ub ) minimum fluidization velocity of larger particles, m/s us ) minimum fluidization velocity of small particles, m/s umf ) minimum fluidization velocity, m/s UMF ) minimum fluidization velocity
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w1, w2 ) weight of composition in binary mixtures, kg X ) parameter in the UMF correlation, m/s x ) mass fraction of biomass in the binary mixture xb ) mass fraction of the larger particles in the binary mixture x1, x2 ) mass fraction of composition in binary mixtures
Fg ) gas density, kg/m3 Fp ) particle density, kg/m3 Fpe ) effective particle density, kg/m3 F1, F2 ) particle density of composition in binary mixtures, kg/m3 µg ) gas viscosity, kg m-1 s-1
Greek Letters R ) parameter in the UMF correlation
EF800495U
