Fluidization of Scrap-Wood Materials - American Chemical Society

Ind. Eng. Chem. Res. 1999, 38, 3115-3120

3115

GENERAL RESEARCH Fluidization of Scrap-Wood Materials: The Influence of the Degree of Thermal Decomposition on the Hydrodynamic Properties Joaquin Reina, Enrique Velo, and Luis Puigjaner* Department of Chemical Engineering, ETSEIB, Universitat Polite` cnica de Catalunya, Diagonal 647, 08028 Barcelona, Spain

This work focuses on the hydrodynamics of fluidizing scrap-wood particles from five different sources. The objective of this study is to carry out the analysis of the hydrodynamic properties of these particles (size, density, shape factor, and specific surface) when submitted to the same thermal decomposition that takes places in a fluidized-bed pyrolysis reactor. Correlations to describe the change of some properties with the treatment temperature are proposed. Introduction Gasification or pyrolysis processes constitute an economically attractive route for residue utilization (lignocellulosic materials or urban and industrial wastes), allowing either the synthesis of high-value chemical products or power generation. Among others, scrap wood is one of the wastes generated in a larger quantity and diversity. According to its origin, wood can be classified in wood from conifers or softwood (pine, red fir, and larch), hardwood from leafy trees (oak, beech, ash, etc.), and softwood from leafy trees (birch, poplar, linden, etc.). The fluidized-bed technology, widely used for the gasification or pyrolysis of wood residue, requires the knowledge of both the process kinetics and particle hydrodynamics for its development and operation. In such processes, wood undergoes a progressive loss of weight, due to its thermal degradation, that modifies the basic hydrodynamic properties of its particles inside the reactor. The hydrodynamics of a fluidized bed depends on particle parameters such as size, apparent density, sphericity, minimum fluidization, and terminal velocity. The possible formation of networks between particles is also a key factor when studying the hydrodynamics of fibrous materials. The system under study is formed by residual wood particles from different origins. These particles are different in shape as well as in size, therefore constituting a polydisperse system. For such systems, it is not advisable to use the classic definition for particle intrinsic and hydrodynamic parameters. In this case, it is better to treat them as apparent values. Although various studies on wood fluidization have been published, they do not take into account the change that the matter undergoes with the temperature and by so much the change in its characteristic hydrodynamics. In a fluidized-bed feedstock, the shape of wood particles depends on the method used for their size reduction. Figure 1 shows several types of wood after milling and sieving between 0.75 and 1.2 mm. It can be * To whom correspondence should be addressed. E-mail: [email protected].

seen that particle appearance and structure strongly depend on the source of the scrap wood. If these particles are submitted to thermal decomposition at high temperature, the loss of weight and carbonization produce a change in shape that depends not only on the temperature of treatment but also on the original structure of the particle system. The shape factor is commonly used to quantify the shape of the particles. It is defined as the ratio between the surface of a sphere and the actual surface of the particle, both having the same volume. This value is comprised between 0 and 1. Using this parameter, Lucas et al.1 classified solid particles in three different groups. Group I characterizes particles with a shape factor between 0.8 and 1, particles with a shape factor between 0.5 and 0.8 are in group II, and group III is characterized by a shape factor between 0.1 and 0.5. Particles belonging to group I were identified by the same authors as spherical particles, those in group II as irregular particles, and those in group III as difficultto-fluidize particles. Experimental Section Five different types of scrap-wood particles (forest clearing, demolition of buildings, scrap slot machines, old furniture, and scrap palettes) were used in this study. Wood was milled and sieved. The study on the hydrodynamic performance and properties of the scrap-wood particles was carried out by submitting the material to thermal conversion at different temperature levels (250, 500, 700, and 900 °C). It was thus possible to analyze how the properties of the materials change because of a thermal decomposition as occurs in gasification or pyrolysis processes. Experimental Setup for the Carbonization of the Sample Wood Particles. Thermal treatment of the scrap-wood samples was carried out in a laboratory oven (W. C. Heraeaus Hanau, type KR 170) specially adapted to allow a continuous flow of nitrogen over the sample. A rotameter was used to measure the nitrogen flow rate, which was always set at 100 L/h. Samples were placed in a stainless steel (ANSI 316) cylindrical

10.1021/ie9806388 CCC: $18.00 © 1999 American Chemical Society Published on Web 06/24/1999

3116 Ind. Eng. Chem. Res., Vol. 38, No. 8, 1999

Figure 2. Scheme of the experimental facility for fluidization experiments: (1) fluidized-bed column; (2) cyclone; (3) air flowmeter; (4) pressure drop meter.

Figure 1. Electron micrographs of the different wood particles used: (a) raw material; (b) material treated at 500 °C; (c) material treated at 900 °C.

crucible (70 mm i.d. and 71 mm height). A K-type thermocouple was placed inside the crucible to measure the temperature of the sample. Five samples of each type of wood were conveniently selected. One of them was kept without thermal treatment. The remaining samples were treated at 250, 500, 700, and 900 °C, respectively. Experiments were started

by preheating the oven to the set temperature while nitrogen was added. The sample was thereafter introduced and kept inside the oven for 5 min. The oven was then turned off and allowed to cool overnight while passing nitrogen. The thick layer of ash formed on the top of the sample was always removed by injecting air over the crucible after extracting it from the oven at room temperature. Experimental Setup for Hydrodynamic Experiments. The equipment (see Figure 2) is similar to that used by Geldart.2 The main part is a glass column (51 mm i.d. and 1000 mm height) where particles are supported by a metallic mesh with 2304 orifices/cm2. A Lapple-type cyclone is placed at the top end of the column in order to remove fine particles from the air stream. The pressure drop through the fluidized bed is measured using a water manometer. The air pressure is also measured at the column inlet by a Bourdon gauge. Air is supplied from two calibrated rotameters (0-2 and 0-11 m3/h). Experiments were conducted with air flow rates between 0.4 and 6 m3/h at room temperature and atmospheric pressure. In each experiment, the gas flow rate was increased stepwise from zero to its maximum value (fluidization stage). Then, the gas flow rate was gradually decreased (defluidization stage). At every step, the bed depth and the pressure drop were registered. Particle Size Calculation. Methods to determine the particle size are well described in the literature.3,4 Sieving, elutriation, microscopy, sedimentation, and permeability can be mentioned among others. As seen in the electron micrographs of Figure 1, wood is a fibrous material with low density. Because of particle shape irregularities, sieving alone was discarded as the way to calculate the particle size. Instead, sieving coupled with microscopy was used to obtain more precise measurements. The wood particles can be considered as small cylinders (see Figure 1) whose diameter (d) is smaller than their length (l). The ratio d/l depends on the wood source. A graduated lens was used to measure the mean cylinder length for every sieved fraction of particles. In this way, the basic dimensions of the particles could be defined. For each particle fraction a different shape was

Ind. Eng. Chem. Res., Vol. 38, No. 8, 1999 3117 Table 1. Characteristic Dimensions of Particles for Each Sieving Fraction (All Dimensions in mm) X1 (2-1.20 mm) dm

lm

X2 (1.20-0.71 mm)

deeq

dm

lm

X3 (0.71-0.315 mm)

deeq

dm

lm

deeq

dm

lm

deeq

0.51 0.51 0.51 0.51 0.51

1.75 2.00 2.50 2.50 2.00

0.88 0.92 0.99 0.99 0.92

0.25 0.25

0.75 1.00

0.41 0.45

0.25

1.50

0.51

1.5 1.5 1.5 1.5 1.5

0.84 0.84 0.84 0.84 0.84

0.25 0.25 0.25 0.25

0.5 0.75 0.5 0.7

0.36 0.41 0.36 0.40

forest demolition slot machines furniture palettes

1.6 1.6 1.6 1.6 1.6

4.5 5.0 5.5 5.0 5.0

2.58 2.67 2.76 2.67 2.67

0.95 0.95 0.95 0.95 0.95

Raw Particles 3.25 1.64 3.0 1.60 3.5 1.68 3.5 1.68 3.5 1.68


1.6 1.6 1.6 1.6 1.6

4.5 4.0 4.5 4.5 3.5

2.58 2.48 2.58 2.58 2.37

0.95 0.95 0.95 0.95 0.95

Treated at 250 °C 3.0 1.60 0.51 2.5 1.50 0.51 2.5 1.50 0.51 3.0 1.60 0.51 2.5 1.50 0.51

X1 (1.20-0.89 mm)

X2 (0.89-0.514 mm)

dm

lm

deeq

dm


1.0 1.0 1.0 1.0 1.0

4.0 3.5 3.5 3.0 3.25

1.81 1.74 1.74 1.65 1.69

0.7 0.7 0.7 0.7 0.7


1.0 1.0 1.0 1.0 1.0

3.25 2.5 3.5 3.5 3.5

1.69 1.55 1.74 1.73 1.73


1.0 1.0 1.0 1.0 1.0

3.0 2.25 3.5 3.0 3.0

1.65 1.50 1.73 1.65 1.65

lm

deeq

dm

lm

deeq

Treated at 500 °C 2.5 1.23 2.25 1.18 2.5 1.22 2.25 1.18 2.25 1.20

0.38 0.38 0.38 0.38 0.38

1.5 1.0 1.5 2.0 1.75

0.69 0.69 0.69 0.76 0.73

0.15 0.15 0.15 0.15 0.15

0.75 0.75 0.75 0.75 0.75

0.28 0.32 0.28 0.28 0.28

0.7 0.7 0.7 0.7 0.7

Treated at 700 °C 2.0 1.2 2.0 1.14 2.25 1.18 2.25 1.18 2.25 1.18

0.38 0.38 0.38 0.38 0.38

1.5 1.5 1.5 1.5 1.5

0.69 0.69 0.69 0.69 0.69

0.15 0.15 0.15 0.15 0.15

0.75 1.0 0.75 0.75 0.75

0.28 0.32 0.28 0.28 0.28

0.7 0.7 0.7 0.7 0.7

Treated at 900 °C 2.25 1.18 1.75 1.10 2.00 1.14 2.00 1.14 2.00 1.14

0.38 0.38 0.38 0.38 0.38

1.5 1.5 1.5 1.5 1.5

0.69 0.69 0.69 0.69 0.69

0.15 0.15 0.15 0.15 0.15

0.75 0.75 0.75 0.75 0.75

0.28 0.32 0.28 0.28 0.28

forest

demolition

slot machines

furniture

palettes

20 250 500 700 900

1.14 1.01 0.82 0.77 0.67

1.48 1.41 0.91 0.87 0.84

1.63 1.40 0.92 0.82 0.78

1.57 1.48 0.96 0.86 0.80

1.69 1.42 0.94 0.84 0.80

observed by microscopy. Fine particles tended to be more spherical while big particles were more elongated. For each sieved fraction, the equivalent diameter was calculated as the diameter of a sphere with the same volume according to 3

(1)

These results are illustrated in Table 1. The mean diameter for each type of wood (for different treatment temperatures) is then calculated from the following equation:

∑Xi/di

dms ) 1/

X4 (0.25-0.042 mm)

lm

treatment temp (°C)

deeq ) x3/2dm2lm

X3 (0.514-0.25 mm) dm

Table 2. Mean Diameter of Particles (in mm) at Different Treatment Temperature

(2)

Results are shown in Table 2. Particle Density. The method proposed by Casal et al.5 was used for the measurement of the density and the shape factor of particles. This method, based on the bed permeability, was selected in view of its simplicity and accuracy and also because it is very easy to reproduce the experiments. According to this method, the density is determined from the plot of ∆p/H vs u.

deeq

X4 (0.315-0.177 mm)

Two set of values were found, one corresponding to the bed before its expansion (∆p/H1) and the other to the expanded bed (∆p/H2) (see Figure 3). The apparent density of the bed for each case (before and after its expansion) was calculated from the following expression:

Fb ) M/V

(3)

The particle density was then calculated from

[ { ( )} ] [ { ( )} ] ∆p1/H1 Fb2 2 1/3 Fb1 ∆p2/H2 Fb1 ∆p1/H1 Fb2 2 1/3 1∆p2/H2 Fb1

Fb2 -

Fapp )

(4)

Results are given in Table 3. Shape Factor. Let us define the parameter A as

A ) [Kµu(1 - )2/(3dp2)]

(5)

where K ) 180 for particles of irregular shape. The bed porosity, , takes two different values: 1 for the bed before expansion and 2 for the expanded bed. Both can be calculated from the bed density as

) 1 - Fb/Fapp

(6)

When both values of the bed porosity are used and experimental values of ∆p/H vs A are plotted, a single straight line can be found (see Figure 4) of slope 1/f 2.


Figure 3. Plot for calculating the apparent density: (a) furniture; (b) palettes.

Figure 4. Plot for calculating the sphericity factor: (a) furniture; (b) palettes.

Table 3. Properties and Hydrodynamic Parameters of Particles

be calculated by

Fapp (kg/m3)

φ

S (cm2/cm3) uf (m/s) ut (m/s)


621 759 529 621 505

Raw Particles 0.69 76 0.35 116 0.24 153 0.32 119 0.33 108

0.82 0.82 plug 0.82 plug

3.63 5.26 3.87 4.33 3.80


491 589 360 509 411

Treated at 250 °C 0.72 83 0.37 115 0.64 67 0.33 123 0.60 70

0.68 0.68 plug 0.68 plug

2.26 2.96 2.18 2.52 2.29


Treated at 500 °C 438 0.74 99 475 0.79 83 323 0.64 102 345 0.63 99 335 0.82 78

0.41 0.41 plug 0.41 plug

1.58 2.20 1.47 1.67 1.71


422 469 320 337 326

Treated at 700 °C 0.74 105 0.78 109 0.66 111 0.82 85 0.80 89

0.33 0.38 0.44 0.38 0.52

1.53 1.82 1.35 1.57 1.57


408 436 304 322 315

Treated at 900 °C 0.73 123 0.78 92 0.65 118 0.80 94 0.79 95

0.29 0.33 0.35 0.33 0.39

1.34 1.35 1.24 1.47 1.22

The results obtained for the shape factor φ (see Table 3) are in good agreement with changes observed in Figure 1. Specific Surface. Once the shape factor and the particle diameter are known, the specific surface can

S ) 6/φdp

(7)

Terminal Velocity. The graphic correlation suggested by Heywood was used to measure the terminal velocity of particles. After the Heywood number, defined as NH ) (2/3)Ar is calculated, the graphical correlation gives the value of Ret, from which the terminal velocity of every kind of particle can be calculated. The values found for all of these properties are summarized in Table 3 according to the nature of the particle and the treatment temperatures. Results and Discussion Mean Diameter. For wood samples treated at temperatures higher than 250 °C, it has been observed that particles can be described as cylinders with well-defined diameter and length. For raw wood samples, dimensions are not easily defined because of the irregular form of the particles. The analysis of the mean diameter suggests that the principal thermal decomposition of wood takes place at temperatures that vary between 300 and 400 °C. When the effects of the thermal decomposition on the particle size are compared, it can be seen (Table 1) that this effect is different depending on the particle original size. It has been observed that the thermal decomposition has more influence on the particle diameter than on its length. This is reflected by analysis of the dimensions shown in Table 1 or by visualization of the micrographs shown in the Figure 1. In the scrap-wood samples tested, it has been observed (see Table 2) that the larger percentage of reduction in the mean particle diameter occurs for the samples from slot machines and scrap palettes.

Ind. Eng. Chem. Res., Vol. 38, No. 8, 1999 3119 Table 4. Parameters for the Variation of the Particle Density with the Treatment Temperature (Eq 8) forest demolition slot machines furniture palettes

Table 5. Parameters for the Variation of the Terminal Velocity with the Treatment Temperature (Eq 9)

B0

B1

B2 × 104

r

675 770 528 646 514

-0.566 -0.828 -0.660 -0.770 -0.550

3.70 5.20 4.80 2.60 3.14

0.99 0.99 0.97 0.98 0.99

Particle Density. Results obtained for the density of the five types of scrap wood suggest that two of them can be classified as softwood (wood from slot machines and scrap palettes), while the rest (forest, demolition, and furniture) can be classified as hardwood. Nevertheless, because of the evolution shown by the furniture wood with temperature, it would also be classified as softwood. This classification also agrees with the reduction of size observed in Table 2, where softwoods (slot machines and palettes) show a higher decrease in size compared to hardwoods (forest and demolition), with that of furniture wood being in between. The variation of the apparent density (Fapp) with the treatment temperature can be well correlated, for all of the types of residual wood, by a second-degree equation:

F ) B0 + B1T + B2T 2

(8)

Values for constants B0, B1, and B2 for each type of scrap wood are given in Table 4. Shape Factor. Results obtained in this study for the shape factor show that scrap-wood particles are very irregular. Raw particles, without thermal treatment, have shape factor values in the range of nonspherical particles (i.e., fibers, rashig rings, or spirals). It has also been observed that the thermal decomposition has a strong influence on the shape factor. Table 3 shows that the shape factor increases with temperature. Values near 0.8 are obtained for most of the wood types for temperatures higher than 500 °C. These values are, according to the classification suggested by Lucas et al.,1 in the limit between irregular-shaped and spherical particles. This is confirmed by visualization through a microscope, as in Figure 1. It is clear that irregular particles become well-defined cylinders after thermal treatment. The effect of the temperature on this parameter is influenced by the nature of the particles when the temperature is lower than 250 °C. The observed variation in shape is lower for particles classified as wood from leafy trees or hardwood than that for particles classified as wood from conifers or softwood. This is due to the fact that softwoods retract more with increasing temperature and thermal decomposition thus produces a larger deformation of the particles. The increase in the shape factor with the treatment temperature produces a change in the fluidization characteristics of wood particles. According to the classification of Lucas et al.,1 particles belonging to group III can be reclassified as group II particles after thermal decomposition, and hence difficult-to-fluidize particles become easy-to-fluidize particles. In effect, slot machine and scrap palette wood particles fluidize for treatment temperatures higher than 700 °C. The complete fluidization velocity is shown in Table 3. Velocity of Complete Fluidization. This velocity, defined as the gas velocity at which the bed is visually well-fluidized, could only be defined for scrap woods


C0

C1

C2 × 106

r

3.68 5.27 3.93 4.41 3.80

-0.006 -0.009 -0.007 -0.008 -0.006

4.10 5.48 5.21 5.80 3.77

0.99 0.98 0.99 0.99 0.98

classified as leafy tree wood and decreases upon increasing the thermal treatment temperature. Therefore, the nature of the particles has also in this case a remarkable influence, and stable fluidization for most of the particles tested is doubtful or highly unlikely. Specific Surface. The smaller diameter particles possess larger specific surfaces. However, results obtained in this work show that the particles with larger specific surfaces are those that have a smaller shape factor and a larger diameter, i.e., those that present a higher irregularity (fibers). It has been observed that, upon an increase in the treatment temperature, the specific surfaces of the particles decrease, precisely because of the fact that the thermal decomposition increases the shape factor, making the particles less irregular. Terminal Velocity. The influence of the thermal decomposition temperature on the terminal velocity can be calculated from the Heywood number when taking measured wood particle properties into account. In doing so, the calculated values can be correlated by a second-order equation:

ut ) C0 + C1T + C2T 2

(9)

Values for constants C0, C1, and C2 for each type of scrap wood are given in Table 5. Hydrodynamic Behavior of the Particles under Study. The hydrodynamic behavior observed for all types of tested particles corresponds to Geldart C type powders. Upon an increase of the gas flow, channeling may occur. If these channels do not appear, the bed is completely lifted. This is the case of slot machine and scrap palette particles (softwoods) crude or treated at 250 and 500 °C, whose particle density is smaller. Most of the tested samples fluidize well. Only samples corresponding to slot machines and scrap palette, in its raw form or thermally treated to temperatures up to 500 °C, do not fluidize. This is due to the fact that they are fibrous materials with low density. Nevertheless, when these materials are treated at very high temperature, their characteristics change in a way that allows fluidization. Because they are fibrous materials, it is adequate to speak about bonds or adherence between particles. The fluidizing gas produces interlacing between unattached fibers (surface friction), forming nets that retain part of the fine material. With increase in the gas flow, these particle links become expanded and can even be broken. The solid may begin to fluidize or to be lifted as a piston when the driving force is not sufficient to break such nets. Conclusions The experimental hydrodynamic study carried out on five different types of scrap-wood particles (forest residue, buildings demolition, scrap slot machines, old


furniture, and scrap palettes) treated at different temperatures allows us to conclude the following: (1) The hydrodynamic behavior and properties of the residual wood particles are different according to the origin of the wood and the thermal treatment at which it is submitted. (2) All types of particles studied can be classified as Geldart C type powders. Nevertheless, some differences in fluidization properties have been observed between scrap-wood particles classified as softwood and those classified as hardwood. (3) Changes observed for the apparent density, and the terminal velocity of particles with the temperature of treatment, can be approximated by a second-order equation. (4) The particles shape factor increases upon increasing the thermal treatment temperature. Because of this change, very irregular particles become rounded particles. Some particles that are difficult to fluidize in their raw form can be fluidized after carbonization. (5) It is also shown that the temperature range in which a greater change of the particle properties is observed coincides with the temperature range where the main thermal decomposition of these types of materials takes place (250-450 °C). Acknowledgment This work was financed in part by the Commission of the European Communities (ECSC R&D Program, Contract 7220-ED-081). J.R. acknowledges the financial support from the Instituto de Cooperacioń Iberoamericano. List of Symbols A ) constant used to calculate the shape factor Ar ) Archimedes number, dp3gFf(Fap - Ff)/µ2 Bi ) ith coefficient in eq 8 Ci ) ith coefficient in eq 9 di ) size of particles in the ith fraction, mm dms ) mean diameter of a powder containing a mixture of sizes, mm dm ) mean diameter of particles by sieving, mm

dp ) particle diameter, mm deeq ) equivalent diameter of particles, mm g ) acceleration due to gravity, 9.81 m/s2 H ) bed depth, m K ) constant lm ) mean length of particles, mm M ) mass of wood in the bed, kg NH ) Heywood number ∆p ) pressure loss across the bed, Pa Ret ) Reynolds number at terminal velocity S ) specific surface, cm2/cm3 uf ) velocity of complete fluidization, m/s ut ) terminal velocity, m/s V ) volume of the bed, m3 Xi ) fraction held in the ith sieve Greek Symbols ) voidage in the bed φ ) shape factor µ ) gas viscosity, Pa‚s Fapp ) apparent density of particles, kg/m3 Fb ) bed density, kg/m3 Ff ) gas density of particles, kg/m3

Literature Cited (1) Lucas, A.; Arnaldos, J.; Casal, J.; Puigjaner, L. Improved Equation for Calculation of Minimum Fluidization Velocity. Ind. Eng. Process Des. Dev. 1986, 25, 426. (2) Geldart, D. Estimation of Basic Particle Properties for Use in Fluid-Particle Process Calculations. Powder Technol. 1990, 60, 1. (3) Arnaldos, J.; Coll, T.; Llop, M. F.; Perales, J. F. Caracterizacioń de Partıćulas So´lidas. Ing. Quı´m. 1990 July. (4) Kaye, B. H.; Trottier, R. The Many Measures of Fine Particles. Chem. Eng. 1995, April. (5) Casal, J.; Lucas, A.; Arnaldos, J. A New Method for Determination of Shape Factor and Particle Density. Chem. Eng. J. 1985, 30, 155.

Received for review October 5, 1998 Revised manuscript received April 20, 1999 Accepted April 28, 1999 IE9806388

Fluidization of Scrap-Wood Materials - American Chemical Society

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