Size and Structural Variations of Biomass Solid Fuels During the Rapid Pyrolysis E. Biagini, P. Narducci, L. Tognotti Dipartimento di Ingegneria Chimica - Università di Pisa - ITALY INTRODUCTION Scanning Electron Microscopy (SEM) analysis has been extensively used to evaluate the structural variations in coal particles after different thermal treatments. SEM images are very definite, can be magnified to obtain accurate details or, vice versa, enlarged to give a global vision of a pulverized sample with minor loss in precision. They have been used to evaluate the macroporosity of coal char particles, as well as the particle swelling, after different devolatilization conditions [1]. The oxidative reactivity of chars from three different high Volatile Matter (VM) coals has been related to the solid structure [2]. The changes in char particle morphology during gasification of high VM coals in CO2 has been also studied [3]. SEM analysis has been applied to biomass chars as well. Wornat et al. [4] noticed structural changes during the combustion of switchgrass chars, conversion of the biomass char particles leading to smaller diameters, smaller aspect ratios and a more lace-like appearance. Della Rocca et al. [5] observed the destruction of cell contents during the pyrolysis of wood sawdust. Biagini et al. [6] assessed the effect of the heating rate after the devolatilization of biomass fuels, as for swelling phenomena and structural variations. Few works developed quantitative methods based on SEM analysis [7]. Generally, the approach was qualitative, analysis of SEM images being extremely time consuming. Solid particles are heterogeneous as for dimensions and morphology, especially for biomass fuels, so that the number of images required to represent a significant sample from a statistical point of view is quite high. This is the reason why SEM analysis was mostly carried out to give representative examples of the average aspect of a sample of particles. The aim of this work is to apply an image analysis program to several SEM images of a fuel and its char obtained at severe conditions during the devolatilization. Variations in size, shape and structure are quantified evaluating the distributions of results obtained on a sample of numerous particles. Biomass fuels are studied because of their heterogeneous nature and the important variations caused by the high VM content. EXPERIMENTAL Two biomass residues are used as starting materials, namely, wood pellets (C 49.4, H 6.1, N 0.99, S 0.16% daf; VM 77.0, FC 20.8, ash 2.2% dry) and olive residue (C 51.2, H 6.7, N 0.83, S 0.05% daf; VM 78.4, FC 20.5, ash 1.1% dry). They are crushed and sieved and the fraction 125-300 µm is used in this work. High Heating Rate (HR) chars are obtained by fast pyrolysis of the raw fuels in a wire mesh reactor (Pyroprobe CDS-2000). This is a commercial reactor on laboratory scale, which performs severe thermal conditions: the nominal HR is 20000°C/s, the maximum temperature is 1400°C. A sample of 2-3 mg is dried at 105°C in a flowrate of pure nitrogen, then the most severe conditions are programmed till the maximum conversion of devolatilization. A detailed theoretical analysis of the system was carried out for evaluating the actual conditions at which a pulverized fuel is undergone during the experiment [8]. In the conditions used in the present
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Size and Structural Variations of Biomass Solid Fuels During the Rapid Pyrolysis
E. Biagini, P. Narducci, L. Tognotti Dipartimento di Ingegneria Chimica - Università di Pisa - ITALY
INTRODUCTION Scanning Electron Microscopy (SEM) analysis has been extensively used to evaluate the structural variations in coal particles after different thermal treatments. SEM images are very definite, can be magnified to obtain accurate details or, vice versa, enlarged to give a global vision of a pulverized sample with minor loss in precision. They have been used to evaluate the macroporosity of coal char particles, as well as the particle swelling, after different devolatilization conditions [1]. The oxidative reactivity of chars from three different high Volatile Matter (VM) coals has been related to the solid structure [2]. The changes in char particle morphology during gasification of high VM coals in CO2 has been also studied [3]. SEM analysis has been applied to biomass chars as well. Wornat et al. [4] noticed structural changes during the combustion of switchgrass chars, conversion of the biomass char particles leading to smaller diameters, smaller aspect ratios and a more lace-like appearance. Della Rocca et al. [5] observed the destruction of cell contents during the pyrolysis of wood sawdust. Biagini et al. [6] assessed the effect of the heating rate after the devolatilization of biomass fuels, as for swelling phenomena and structural variations. Few works developed quantitative methods based on SEM analysis [7]. Generally, the approach was qualitative, analysis of SEM images being extremely time consuming. Solid particles are heterogeneous as for dimensions and morphology, especially for biomass fuels, so that the number of images required to represent a significant sample from a statistical point of view is quite high. This is the reason why SEM analysis was mostly carried out to give representative examples of the average aspect of a sample of particles. The aim of this work is to apply an image analysis program to several SEM images of a fuel and its char obtained at severe conditions during the devolatilization. Variations in size, shape and structure are quantified evaluating the distributions of results obtained on a sample of numerous particles. Biomass fuels are studied because of their heterogeneous nature and the important variations caused by the high VM content.
EXPERIMENTAL Two biomass residues are used as starting materials, namely, wood pellets (C 49.4, H 6.1, N 0.99, S 0.16% daf; VM 77.0, FC 20.8, ash 2.2% dry) and olive residue (C 51.2, H 6.7, N 0.83, S 0.05% daf; VM 78.4, FC 20.5, ash 1.1% dry). They are crushed and sieved and the fraction 125-300 µm is used in this work. High Heating Rate (HR) chars are obtained by fast pyrolysis of the raw fuels in a wire mesh reactor (Pyroprobe CDS-2000). This is a commercial reactor on laboratory scale, which performs severe thermal conditions: the nominal HR is 20000°C/s, the maximum temperature is 1400°C. A sample of 2-3 mg is dried at 105°C in a flowrate of pure nitrogen, then the most severe conditions are programmed till the maximum conversion of devolatilization. A detailed theoretical analysis of the system was carried out for evaluating the actual conditions at which a pulverized fuel is undergone during the experiment [8]. In the conditions used in the present
work, values on the order of 5000-10000°C/s can be expected as for the effective HR of the sample. SEM images are obtained for the high HR chars and the parent materials. Globally, 200-300 particles are collected for each sample and this number is considered significant from a statistical point of view. An image program is used to analyze the SEM images. Each particle is delimited with its outline, which can be easily defined by the program depending on the difference of the optical intensity. Axial and equatorial sizes are measured and the equivalent spherical diameter Dse is defined as
Dse = (Dx Dy2)1/3 (1)
This definition represents only a rough approximation of the size of the particle. A more accurate study of the shape should be carried out in order to characterize the morphology of the solid. The roundness is a measure of the shape of a particle and is calculated as the following ratio
Roundness = p2 / 4πA (2) where p is the perimeter of the particle and A its area. The roundness is 1 for a sphere, 1.27 for a square, and the higher the roundness respect to unity, the more remarked the difference respect to a sphere. This parameter allows to define the shape of a particle, quantifying the difference of almost spherical particles from oblong or slab particles. The fractality is a measure of the indented outline of the particle and is defined as the ratio between the fractal perimeter and the geometric perimeter of the particle. Particles with fractality near unity are very smooth, whereas a particle with a fractality 1.3 is very indented. Finally, the heterogeneity is used here as a measure of the scabrous nature of the particle surface. It is defined as the fraction of pixels within the outlined area, deviating more than 10% from the average optical intensity. It varies in the range from 0 to 1. Values of the heterogeneity near 0 denote uniform particle surfaces. The higher the heterogeneity, the more heterogeneous the surface of the particle.
RESULTS AND DISCUSSION Pulverized samples are well distributed to obtain SEM images of sufficiently separate particles. This preparation allows the major number of particle to be easily captured by the image analysis program which draws the outline of each particle. The example given in Figure 1 shows two typical SEM images of wood and olive residue, which can be easily analyzed. Minor manual operations are required to split superimposed particles or to correct erroneous outlines caused by ambiguous variations in the optical intensity. Figure 1 is magnified enough to include several particles (30-50), with no loss in the precision of the outline of each particle. The difference in size, shape and superficial scabrousness can be qualitatively appreciated even observing each image. A sample of 200-300 particles represents a significant number from a statistical point of view and this gives a distribution of results for a morphological characterization. Clearly, a distribution of results gives more information respect to average values, offering a description of the expectable deviations respect to the average. Figure 2 shows the equivalent spherical diameter, the roundness and the fractality of both materials compared with the respective results for the high HR chars. The average values and the standard deviations are listed in Table 1. Different phenomena should be borne in mind before commenting the results. A large amount of volatile compounds (on the order of 70-80% on a dry basis) is released during the rapid pyrolysis in a very short time. This can produce a high porous solid and generate interactions with the softening of the matrix caused by the rapid heating up. The subsequent trapping of volatiles can cause swelled surfaces which vary the shape of the particle. The thermal
decomposition of chemical bonds and the melting of some compounds can break the fibers of the original material and this can shrink the particle or, even, lead to fragmentation. Furthermore, agglomeration phenomena can be caused by low melting compounds (especially, alkali in biomass ashes) which can stick contacting surfaces of separate particles, giving clusters of particles divisible with difficulty. As for the size, the distribution of wood char is shifted at lower Dse values respect to the parent material. This can be also verified comparing the average values of Dse in Table 1. Vice versa, a slight increase in Dse can be observed for olive residue and its char. As for the shape, the roundness of char particle is nearer to unity respect to the parent material in both cases. This denotes a more spherical shape for char particles and can be due to swelling phenomena. A particle of olive residue is compared with a particle of char in Figure 3 to give a qualitative confirmation. Some surfaces are swelled. Solid matrix seems to trap re-solidified bubbles. These structures are empty and this can be observed for some particles which result broken. Finally, as for the fractality, minor variations are recorded in both cases considering the average values and the distributions.
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Figure 1. SEM images of wood (a) and olive residue (b).
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Figure 2. Comparison of distributions of equivalent spherical diameter Dse, roundness and fractality of wood and olive residue and their respective high HR chars.
Table 1. Average values and standard deviation (in brackets) for morphological distributions Wood Olive residue raw char raw Char Dse [µm] 260 (96) 223 (70) 227 (87) 233 (77) Roundness 2.12 (0.735) 2.04 (0.915) 1.80 (0.43) 1.67 (0.475) Fractality 1.09 (0.025) 1.09 (0.029) 1.09 (0.028) 1.08 (0.027) Heterogeneity 0.384 (0.105) 0.447 (0.095) 0.345 (0.133) 0.425 (0.117)
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Figure 3. SEM images of olive residue (a) and its high HR char (b).
Furthermore, the image analysis can be also carried out in the outlined area. Obviously, the analysis of the surface of a particle (that is the measurement of superficial holes, fissures, porous and variations in superficial height) creates major problems owing to the onerousness (in terms of time) of the procedure and the difficulty in the definition of characteristic parameters. In fact, the delimitation of the external outline can be easily done considering the difference between the particle and the background tones. Vice versa, the optical intensity of the particle surface is very complex and the shape of a porous can be defined only adopting strong assumptions which can compromise the uniqueness of the measure. For instance, the results strongly depend on the brightness of the image or the contrast done during the analysis, so that the dimension of a porous can be measured with poor accuracy. On the other hand, the manual analysis of the images is discouraged by the high number of images required to represent a statistical distribution of results. Besides, the complexity of this kind of analysis seems to be not repaid in terms of morphological characterization. In fact, the estimation of the effective surface is important for processes which require a matter or thermal transfer, e.g. char oxidation (or gasification) where oxygen (or the gasifying agent) has to diffuse to the surface, removal of reaction products regenerating the active sites, radiation from or to an external surface of heat transfer. A scabrous surface is clearly more efficient in the transfer than a smooth surface, with the same geometrical area. However, a detailed characterization of the superficial scabrousness can be considered unnecessary. The quantification of the scabrousness of the surface could be obtained introducing a single parameter relating the real (scabrous) surface to the geometrical (smooth) surface. The heterogeneity defined in the experimental section is adopted in the present work as a measure of the superficial scabrousness. The distribution of pixels with different optical intensity is reported in Figure 4 for three different particles, namely, two raw wood particles with evident different surface aspect and a particle of the high HR char. The relative brightness in the abscissa of this graph is referred to the average optical intensity of each particle: negative values represents darker tones, positive values brighter tones compared to
the average value, which is assumed 0. SEM images of all particles are also shown in Figure 4. The particle with the most uniform surface exhibits the narrowest distribution, with the most pronounced peak. The heterogeneity of this particle is 0.358. The second particle of raw wood shown in Figure 4 has a more scabrous surface, which implies a wider distribution of relative brightness, a flatter peak and a lower value of the heterogeneity (0.509). Finally, the char particle has a surface even more scabrous and indented, and this means a very wide distribution, a very flat peak and a very high value of the heterogeneity (0.600). The distributions of the heterogeneity for both materials and the respective chars are compared in Figure 5. The curves of chars are moved to higher values respect to those of parent materials, the average values passing from 0.384 to 0.447 for wood and from 0.345 to 0.425 for olive residue, respectively. This means a more scabrous surface for char particles respect to parent materials, that is a higher number of surface holes, larger porous and major differences in superficial height. As a matter of fact, though the heterogeneity is a good parameter to quantify the scabrousness of a particle surface, the subsequent relation of this index to obtain the effective surface needs a deeper investigation. In fact the measurement of the effective surface of a particle requires a tridimensional image (that is different angle shot of the same object) or, at least, the measurement of the difference in height at the surface. At present, these studies are in progress and need a major effort in the image analysis procedure.
Figure 4. Distribution of relative brightness (respect to the average optical intensity of the surface of each particle) for different particles of wood and its high HR char.
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Figure 5. Comparison of distributions of heterogeneity of wood and olive residue and their respective high HR chars.
CONCLUSIONS Morphological variations during the pyrolysis of solid fuels are important indications of the devolatilization process and suggest also fundamental characteristics of subsequent steps (char oxidation or char gasification). A detailed image analysis is carried out on a significant number of particles of chars obtained in a wire mesh rector (high temperature and high heating rate) from biomass fuels. A methodological approach is developed in order to evaluate size, shape, fractal perimeter and surface scabrousness of char particles. Results are discussed and compared with those obtained on parent materials. Shrinking, swelling, fragmentation and structural phenomena can be statistically quantified from the distributions of values obtained. The size decreases for wood char (the softening of the solid matrix and the decomposition of chemical structure shrunk the particles during the rapid pyrolysis), whereas it remains practically constant for olive residue. The roundness decreases significantly in both cases and this denotes a more spherical shape caused by swelling phenomena. Variations in fractality are minor. A more appropriate parameter to quantify the scabrousness of the particle surface seems to be the heterogeneity, defined considering the variation in the optical intensity. It increases significantly for the more scabrous char particles respect to the more uniform particles of parent material.
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