Investigation of the Anisotropic Behavior of Wood Char Particles

Energy & Fuels 2006, 20, 2233-2238

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Investigation of the Anisotropic Behavior of Wood Char Particles during Gasification Ulrik Henriksen,*,† Claus Hindsgaul,† Bjørn Qvale,† Jan Fjellerup,† and Anker Degn Jensen‡ Department of Mechanical Engineering, Technical UniVersity of Denmark, Nils Koppels Alle´ , Building 402, 2800 Kgs. Lyngby, Denmark, and Department of Chemical Engineering, Technical UniVersity of Denmark, Søltofts Plads, Building 229, 2800 Kgs. Lyngby, Denmark ReceiVed April 3, 2006. ReVised Manuscript ReceiVed May 16, 2006

Wood is a strongly anisotropic material, and likewise, the char produced by pyrolysis of wood is characterized by a strong anisotropy. This anisotropic behavior allows relatively easy transport of gas in the longitudinal (L) direction of the wood, but the transport is much less easy in the radial (R) and tangential (T) directions. Despite this, this property has normally not been included in mathematical model descriptions of gasification of thermally thick particles. The present paper describes a study of the influence of the anisotropy on the reactivity of thermally thick char particles during gasification of wood using macro TGA equipment. The char particles, in the form of slabs (approximately 50 × 70 × 10 mm), were produced by pyrolysis of wood slabs that had been cut from the trunk of beech trees. The char slabs were grouped into three categories according to the orientation of the normal to the greater surface of the slabs L, R, or T (see Figure 8). When the smaller surfaces were coated with alumina silicate, the gasification agent could only enter the interior of the slabs through the greater surfaces. Thermally thick char particles from beech and pine reacted more slowly if the gas was transported in the R and T directions than in the L direction. In the reported study, the difference was between 25 and 35%. For lower values of conversion, the difference in reactivity was considerably greater, but for higher values of conversion, the reactivity was almost the same in all directions. For increasing conversion, a considerable cracking was seen and it was concluded that the gasification agent could penetrate through the cracks with an increased reactivity as a result.

Introduction The designation “thermal gasification of biomass” comprises in reality a number of processes. The most important are drying, pyrolysis, and gasification. In general, gasification of char has been observed to be considerably slower than the pyrolysis, thus limiting the process rates and the output of a plant of given physical dimensions. Because of this, the char gasification rate is important for the design and modeling of gasifiers, and the present study has addressed char gasification alone. Biomass char from fixed bed gasifiers will usually be in the form of rather large particles that may be considered thermally thick.1 This means that the char reaction rate is not determined solely by chemical kinetics but also by heat and mass transport. Wood is highly anisotropic (orthotropic), and this anisotropy is carried through the pyrolysis process to the resulting char. The central topics of the present work are the consequences of this anisotropy and its importance. Structure of Wood and Char. Wood consists mainly of cells, in which a void is surrounded by cell walls. These cell walls consist of cellulose, hemicellulose, and lignin. The cells are interconnected through a number of small holes in the walls, the so-called pits, as shown in Figure 1. The diameter of the pits is 1 or 2 orders of magnitude smaller than the diameter of the cell cavity (equal to lumen).2,3 The cells are very long compared to their width, and they spiral slightly in the * To whom correspondence should be addressed. E-mail: [email protected]. † Department of Mechanical Engineering. ‡ Department of Chemical Engineering. (1) Dasappa, S.; Paul, P.; Mukunda, H. S.; Shrinivasa, U. The gasification of wood-char spheres in CO2-N2 mixtures: Analysis and experiments. Chem. Eng. Sci. 1994, 49, 223-232.

Figure 1. Pits between tracheid cells (X) and pits between tracheids and ray cells (XX) are seen. The wood is softwood (magnification of 1200×), with the courtesy of W. A. Coˆte´.2

lengthwise direction but are essentially aligned with the longitudinal direction of the stem of the tree, as shown in Figure 2. The length of the cells is usually 20-200 times greater than (2) Tsoumis, G. Science and Technology of Wood, Structure, Properties, Utilization; Van Nostrand Reinhold: New York, 1991. (3) Lewin, M., Goldstein, I. S., Eds. Wood Structure and Composition; Marcel Dekker: New York, 1991.

10.1021/ef060140f CCC: $33.50 © 2006 American Chemical Society Published on Web 07/04/2006

2234 Energy & Fuels, Vol. 20, No. 5, 2006

Henriksen et al.

Figure 2. Hardwood increment. E, vessel cells; X, fiber cells. Short arrows point out ray cells, with the courtesy of the N. C. Brown Center.3 Figure 5. Simple model of the char structure. It can be seen that the gas transport in the R and T direction requires passage through a higher number of holes than the gas transport in the L direction.

It can be expected that, during the gasification of “thermally thick” particles, where mass transport is the limiting factor, the reactivity would be larger if the transport of gases primarily occurred in the L direction rather than in the R and T directions. However, in the past, the mathematical modeling of gasification of thermally thick particles1,4-6 has assumed that the char was isotropic (see Figure 4). This investigation was performed to see the effect of anisotropy on the rate of conversion during gasification of thermal thick char particles. Experimental Section Figure 3. Char particle from beech. The sample has been pyrolyzed at 600 °C.

Figure 4. Char sample, which is partial, gasified in a two-stage gasifier. The pits are clearly seen (white arrow points out a pit). The ray cell walls may be partly gasified away or broken.

the diameter. Hardwood has large cells as well as small fiber cells in the longitudinal direction. A small number of cells (ray cells) and channels are oriented radially (in the radial direction, R). No cells or channels are oriented tangentially to the stem of the tree (in the tangential direction, T). As mentioned, the basic character of the structure of the wood has been carried over into the char. Figure 3 shows a small beech char particle. Figure 4 shows a section of a partly gasified beech char particle. Pits can be seen clearly, and part of the cell wall has disappeared. This disappearance may have been caused by mechanical break down or chemical reactions. Disregarding the details of the structure inside the cell wall, the appearance of the structure of the char is fairly simple (Figure 5).

General Method. The influence of the anisotropic structure on the char gasification was studied by gasifying “thermally thick” char particles in a macro TGA reactor. The results were represented by the degree of conversion as a function of time and by the rate of conversion as a function of the degree of conversion. The char particles were produced by pyrolysis of slabs that had been cut in different directions, from a beech stem. The char samples were modified in such a way that the direction of diffusion of the intruding gases could be limited to one direction. Macro TGA Reactor. The macro TGA equipment consisted of a reactor containing a char sample. The reactor was suspended in a scale and placed in an oven, which was heated to a constant temperature (see Figure 7). Nitrogen was added during the heating process. The heating rate was 24 °C/min. When the final temperature was reached, it was kept at this temperature for approximately 1/ h to achieve stability. After this stabilizing period, the nitrogen 2 was replaced by the gasifying agent. In the present experiment, this consisted of 50 vol. % nitrogen and 50 vol. % steam. The flow rate of steam was approximately 750 g/min. The gasification agent was preheated to the reactor temperature. In the present experiments, the reactor temperature was approximately 800 °C. During the gasification process, the mass loss was recorded. When the rate of mass loss stopped, all of the carbon contained in the sample had been converted. The addition of the gasification agent was stopped 15 min thereafter. A stream of nitrogen then cooled the sample. The results of these experiments were represented by a relationship between char mass loss and time. Char Sample. The char samples originated from beech and pine. The samples were in the form of slabs of a size of about 50 × 70 (4) Di Blasi, C. Dynamic behavior of stratified downdraft gasifications. Chem. Eng. Sci. 2000, 55, 2931-2944. (5) Gøbel, B.; Henriksen, U.; Qvale, B.; Houbak, N. Dynamic modelling of char gasification in a fixed-bed. In Proceedings of the Conference: Progress in Thermochemical Biomass ConVersion; Tyrol, Austria, September 17-22, 2000; 15 p. (6) Dasappa, S.; Paul, P.; Mukunda, H. S.; Shrinivasa, U. Wood-Char Gasification: Experiments and Analysis on Single Particles and Packed Beds; 27th International Symposium on Combustion, The Combustion Institute, Boulder, CO, 1998; pp 1335-1345.

Anisotropic BehaVior of Wood Char Particles

Energy & Fuels, Vol. 20, No. 5, 2006 2235

Table 1. Overview of the Conducted Experimentsa test series

sample mass (g)

reactor temperature (°C)

L R T

9.7 10.5 8.5

9.9 11.3 10.5

805 805 805

731 732 733

L R T

9.3 9.4 9.5

11.8 12.4 12.8

805 805 805

beech

736 781 721

T T L

9.0 9.2 2.5

11.5 12.7 10.2

805 805 805

pine

740 741 739

L R T

9.2 10.6 10.2

11.0 10.6 10.2

805 805 805

test number

1

beech

715 716 717

2

beech

3 a

sample width (mm)

wood type

category

comment

stopped at X ) 0.5 stopped at X ) 0.12

The category nominates the geometrical orientation in the wood stem, from which it was cut. X is the conversion.

Figure 6. Sketch of a char sample. The four small sides have been coated with alumina silicate.

Figure 8. Three categories of samples cut from the stem. The categories L, R, and T correspond to the direction where the gas intrusion through the big sides can take place.

Figure 7. Macro TGA equipment.

× 10 mm (see Figure 6). This geometry was chosen to approximate one-dimensional conditions as closely as possible and because this sample size was quite close to the sample size used in practice. The samples were sorted into three categories in accordance with their original orientation in the trunk of the tree. Category L was cut such that the greater surface was perpendicular to the longitudinal direction (see Figure 8). Category R was cut such that the greater surface was approximately perpendicular to the radial direction. Category T was cut such that the greater surface was perpendicular to the tangential direction. The samples were cut from the stem with a fine saw. Subsequently, the samples were dried and heated at a rate of 2 °C/min until the final pyrolysis temperature of 600 °C was reached. This low heating rate was chosen to avoid cracking and to produce similar samples. In most real gasifiers, higher heating rates will be seen. During the pyrolysis, the sample shrank, resulting in a char sample that was smaller than the original wood sample. The shrinkage was the smallest in the L direction and largest in the T direction. This meant that the samples in given categories were all of slightly different shape and size. The char samples were not subjected to further mechanical treatment, for fear of blocking pores and channels by dust, which could affect the experimental results. The wood samples were cut to different thickness, and a subsequent sorting of the char samples gave an experimental series with almost the same thickness of the samples in a given category (see Table 1). The sides of the char samples were coated with a thin layer of alumina silicate. This silicate would sinter during the heating, thus forming a gas proof layer, which would ensure that the gas transport

in and out of the char sample would take place through the big surfaces only. After the coating process and before the macro TGA experiment would take place, the samples were dried. Test of the Method To Control Gas Intrusion. To test that the coated alumina silicate would be in place and blocking the influx of gas, two tests were performed. In the first test, a char sample was manufactured according to the previous description. The sample was placed in the macro TGA equipment and heated to 800 °C in a nitrogen atmosphere. After 2 h, the sample was cooled in the same atmosphere. When the sample was examined, it could be seen that the alumina silicate formed an unbroken surface. In the second test, the whole surface was coated by alumina silicate. The sample was placed in the macro TGA equipment, and a gasification experiment at 800 °C was performed. After 2 h in a gas mixture of 50% nitrogen and 50% steam (the gasification agent), almost no char conversion had taken place. Only 2 wt % of the char had reacted after 2 h. It could thus be concluded that the method was successful in preventing gas from entering the char sample at the place where the char was coated. Furthermore, nothing indicated that the used silicate would have a catalytic effect on the char conversion.

Results As previously described, the wood samples were cut, dried, pyrolyzed, coated with alumina silicate on the edge, dried again, and finally tested in the macro TGA equipment. The density of the char samples (before coating with silicate) was 375 kg/m3 ( 5%. The ash content on a dry basis was 2.5 wt %. An overview of the experiments that were carried out has been presented in Table 1. It was decided to present the results in terms of the degree of conversion as a function of time and the rate of conversion as function of the degree of conversion. The


Henriksen et al.

Figure 9. Degree of conversion as a function of time for the two char samples with different thickness (test 721 and 731, with a thickness of 2.5 and 9.5 mm, respectively). Furthermore, a graph based on a Langmuir-Hinshelwood correlation (L-H corr.) is also shown.

Figure 10. Degree of conversion as a function of time for test series 1.

degree of conversion is defined by

X(t) ) 1 -

W(t) Wo

(1)

The rate of conversion is defined by dX/dt. Wo is the mass of the convertible char before gasification, and W(t) is the present amount of char during gasification. For a degree of conversion of 0, no char has been converted, and for a degree of conversion of 1, all char has been converted. It was expected that the fastest conversion would occur for category L samples. To confirm that mass and energy transport were the limiting factors for the char conversion, two experiments of category L were considered (see Figure 9). One char sample consisted of slabs with a thickness of 2.5 mm (test number 721), and the other sample consisted of slabs with a thickness of 9.5 mm (test number 731). The degree of conversion using chemical kinetics was expressed by a LangmuirHinshelwood correlation, where the constants were found using a traditional TGA experiment.7 In traditional TGA experiments, mass and energy transport effects are absent. It was seen that the degree of conversion of the thin sample (test number 721) was very close to the calculated degree of conversion based on chemical kinetics. This means that limitations from mass and energy transport can be neglected for this sample gasified under the present conditions. The char sample with the largest thickness was converted considerably slower than the sample with the smallest thickness (see Figure 9). It was thus concluded that mass and energy transport limited the conversion of char samples with a width of about 10 mm and larger. According to ref 8, the effective thermal conductivity of char from biomass could be calculated as the sum of the thermal conductivity of the solid and gas phases of the void of the material. Using this, it could be assumed that the difference in the char thermal conductivity in the different directions would be relatively small and that this difference would not have a significant influence on the conversion rate. Discussion Three experimental series were considered. Test series 1 consisted of three experiments with beech, one for each of the categories 715(L), 716(R), and 717(T) (see Table 1). Figure 10 (7) Gøbel, B. Dynamisk modellering af forgasning i fixed koksbed (in Danish). Ph.D. Thesis, Department of Mechanical Engineering, Technical University of Denmark, Denmark, Et-PhD.99-04. (8) Larfeldt, J.; Leckner, B.; Melaaen, M. Modelling and measurements of heat transfer in charcoal from pyrolysis of large wood particicles. Biomass Bioenergy 2000, 18, 507-514.

Figure 11. Rate of conversion (dX/dt) as a function of the degree of conversion (X) for test series 1.


shows that the category L was converted faster than the categories R and T. It was noticed that the form of the conversion curve was different for R and T compared to L. For R and T, the rate of the degree of conversion was small in the beginning but increased with time (see Figure 11). When the degree of conversion was about 0.4, the conversion rate was almost as high as for the L category. The samples in category R and T were converted about 35% slower than category L. It is noticed that the samples did not have exactly the same width, and this may have affected the conversion. Test series 2 consisted, as series 1, of three samples of beech char, one for each of the categories 731(L), 732(R), and 733(T). The chosen samples had almost the same thickness. The degree of conversion as a function of time was calculated and shown in Figure 12. The same pattern was seen as for series 1, although the difference between the conversion rates was smaller (see Figure 13). The R and T samples were converted about 25% slower than the L samples. From Figure 13, it was seen that, for smaller

Anisotropic BehaVior of Wood Char Particles




degrees of conversion, the rate of conversion was significantly larger for the L sample than for R and T. As seen for series 1, this tendency is disappearing as the degree of conversion exceeds 0.4. Test series 3 consisted of three experiments with pine wood, one from each of the category 740(L), 741(R), and 739(T). The samples from category L were converted faster than the samples from categories R and T (see Figure 14). The time for total conversion was a bit longer than for the experiments with beech char. In comparison to beech char, the pine char showed bigger differences of the rate of conversion when L samples was compared to R and T samples (see Figure 15). When the degree of conversion was smaller than 0.6, the rate of conversion for L was much larger than for R and T samples (see Figure 15). The R and T samples were converted about 30% slower than the L sample. It was noticed that the samples did not have exactly the same thickness and that this might affect the conversion curve.

Energy & Fuels, Vol. 20, No. 5, 2006 2237

Figure 16. Char sample from category T. The sample is partly gasified. The degree of conversion is approximately 0.12. Parts of the silica have been removed. The cracks can easily be seen but are smaller and more regular than the sample at 50% conversion (Figures 17 and 18).

When the char structure is considered, it is clear that gas with ease can be transported in the longitudinal direction (through the cell lumen in the longitudinal direction and through the pits to the next long cell), only with difficulty in the radial direction (through the cell lumen and through the pits and parallel with these through the radial cells and channels), and with even more difficulty in the tangential direction (through the cell lumen and through the pits). This was confirmed by the measurement of diffusion in wood char in three directions,9 where it is shown that the resistances to diffusion in the R and T directions respectively were approximately 30 and 50 times larger than in the L direction. This was the same order of magnitude as the ratio between the length and diameter of the cell cavity, which indicated that the resistance against gas transport was caused by the pits between the cavities (see Figure 5). An explanation of the fact that the category L was converted faster than the categories R and T could be the char structure. This does, however, not explain why the conversion rate of the R and T samples increased with the degree of conversion to almost the same rate as the category L samples. To examine this further, two experiments were conducted with a category T sample (781 and 736). In these experiments, at the point in time when respectively 12 and 50% of the char was converted, the gasification agent was replaced with nitrogen. This immediately stopped the conversion of char. The sample was cooled and removed from the reactor. Figures 16-18 show photos of the samples after conversion. In the sample converted to X ) 0.12, large cracks in the surface can easily be seen (see Figure 16). For the sample converted to X ) 0.5, more drastic cracks and decomposition of the surface are seen (see Figures 17 and 18). The cracks were either parallel with the L or R direction. The gas hereby had access to the longitudinally oriented cells through these cracks and again through these cells access to even more of the slab volume. When the cracks were formed and spread, the conversion rate increased, which was confirmed by the experiments. It must be noticed that for a char sample that has been heated to 800 °C in the macro TGA reactor in an 100% nitrogen atmosphere for 2 h (for testing the silica coating), no cracks in the surface were observed on this sample after the test. This shows that, for a degree of conversion at X ) 0, no cracks were formed. An explanation for the appearance of the cracks was that, during conversion, the char would shrink with an increasing degree of conversion. As the conversion increased, the local (9) Turkdogan, E. T.; Olsson, R. G.; Vinters J. V. Pore Characteristics of Carbons; Edgar C. Brain Laboratory for Fundamental Research, United States Steel Cooporation Research Center: Monroeville, PA, 1970; Vol. 8, pp 545-564, Pergamont Great Britain.


Henriksen et al.

local conversion rate would result in creep that can lead to mechanical tension in the char and cracking. Conclusion

Figure 17. Char sample from category T. The sample is partly gasified, and the degree of conversion is approximately 0.5. The cracks can easily be seen.

Figure 18. Enlarged section of the char sample shown in Figure 17. The cracks can easily be seen.

conversion rate would vary through the sample. It would be greatest at the surface because of the high temperature and high concentration of the gasification agent. This variation of the

Wood is characterized by a high degree of anisotropy, and this property is propagated to the char produced by the pyrolysis of wood. This anisotropic character would permit easy transport of gas in the L direction but not in the R and T directions. The influence of the anisotropic property on the conversion rate of thermally thick particles of wood char during gasification was studied using a macro TGA test stand. The char that was studied had been produced by pyrolysis of beech and pine and was in the form of slabs (approximately 50 × 70 × 10 mm) that had been cut from tree trunks. The samples were sorted in three groups according to whether the greater side was perpendicular to the L, R, or T direction. When the small sides were coated with alumina silicate, the entrance of the gasification gas through these sides was prevented. Gasification gas thus entered the sample only through the greatest side. It should be concluded that the rates of conversion of the thermally thick char particles from beech and pine were smaller when the transport of gas took place in the R and T directions than in the L direction. These differences were significant when the degrees of conversion were smaller than 0.4-0.6, but at higher degrees of conversion, only small differences were seen. The difference in times of total conversion was between 25 and 35%. At increasing degrees of conversion, extensive cracking of the char was observed visually, and it can be concluded that these cracks allowed the gasification gas to enter the sample with an increased rate of conversion as a result. Overall, it could be concluded that the anisotropic property of beech and pine char had a limited but significant influence on the rate of conversion of thermally thick particles. Acknowledgment. The Ministry of Energy financed this project. EF060140F

Investigation of the Anisotropic Behavior of Wood Char Particles

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