Article pubs.acs.org/EF
Fly Ash from the Combustion of Forest Biomass (Eucalyptus globulus Bark): Composition and Physicochemical Properties R. P. Girón, I. Suárez-Ruiz,* B. Ruiz, E. Fuente, and R. R. Gil Instituto Nacional del Carbón (INCAR-CSIC). Francisco Pintado Fe 26. 33011-Oviedo, Spain ABSTRACT: In this work, the composition (chemistry and petrography) and the physical (textural) and thermal (pyrolysis and combustion) properties of fly ash derived from the combustion of forest biomass (Eucalyptus globulus bark), used as feedstock in a pulp and paper plant located in Northern Spain, have been investigated. Moreover, this original biomass has also been characterized in order to determine its composition and some of its properties. To concentrate the unburned carbons from the fly ash, the raw fly ash was subjected to two different procedures: dry sieving and agglomeration with used vegetable oil processes. It was concluded that the first procedure that gives rise to fractions with a high content in unburned carbons and good textural properties is the more effective procedure. Accordingly, these fractions were investigated as precursor materials in the preparation of activated carbons. As for the inorganic part of the fly ash, this is mainly of a basic character with a predominance of alkaline and alkaline-earth elements.
1. INTRODUCTION Concern about the global environment and other issues mostly related to fossil fuel utilization has led to an increasing demand for renewable energy sources. Among these, the use of biomass and wastes as energy sources is of increasing interest in many countries. Biomass is described1 as a carbohydrate material that is derived from forestry and agricultural operations, including forestry residues (logging, saw-milling, and wood residues), dry, semidry or wet agricultural residues, energy crops, and uncontaminated wood processing residues. Biomass from forest operations includes bark-wood, wood fuel chips, whole-wood chips, and whole-tree chips. A number of different forms of biomass and wastes can be used as fuels, burned, digested, or cocombusted with coal to produce energy. Examples of these energy sources include wood, straw, bagasse, olive oil waste, poultry litter, energy crops, etc. The process of energy production from biomass that is widely available is considered to have net zero CO2 impact,2 or almost zero, because the emissions produced counteract the carbon dioxide fixed by the biomass during its growth. In addition, the combustion of solid biomass is a proven technology for heat and power production, where the technologies of fluidized bed and grate furnace combustion are mainly used.1,3 However, the use of biomass as fuel in combustion or cocombustion is not exempt from problems. For example, Tortosa Masiá2 observed that the presence of inorganic species may induce a process of agglomeration, deposition, and corrosion in the heat transfer surfaces of the boiler. Another issue for concern is that the combustion of biomass generates two solid wastes, fly ash and slag, besides gases, and although the amount of fly ash produced is low when compared with fly ash from coal combustion, there is still the problem of disposal in dumps if the wastes are not to be reused. Moreover, with the increasing restrictions on disposal of this type of materials, because of environmental, economic, and space requirements, there is great interest in finding new outlets for these secondary raw materials, whose properties therefore need to be investigated in depth. © 2012 American Chemical Society
The present study is focused on fly ash derived from the combustion of forest biomass (bark of Eucalyptus globulus). This biomass is used as a feedstock in a biomass boiler system for power generation in a paper plant, which generates an excess of 1500 t/yr of fly ash. The fly ash is composed of an inorganic fraction, which is the predominant component, and a secondary organic fraction made up of unburned carbons. The properties of these fly ashes depend on the type and nature of the feed fuels, the type of combustion, and the operating conditions in which the process develops, among others. Therefore, the objective of this work is to investigate the main characteristics of the fly ashes generated after the combustion of E. globulus bark biomass by determining the composition and the physico-textural and thermal properties, with a view to the potential utilization of both the organic and the inorganic fractions. To this end, the unburned carbons present in the raw fly ash were concentrated by dry sieving and agglomeration with used vegetable oil. In this way, a precise characterization of this organic fraction was carried out to evaluate its potential use as precursor material in the preparation of activated carbons. A potential precursor of activated carbon should have a high carbon content and a low ash content in order to produce activated carbon with good textural properties such as a high BET surface area. In addition, the main characteristics of the original biomass were also investigated.
2. SAMPLING AND ANALYTICAL PROCEDURES For this study, fly ashes were obtained from the combustion of forest biomass in a paper plant located in Asturias (North Spain). The biomass boiler system (a grate boiler), which operates at a temperature of 750 °C, is fed with the bark of E. globulus (a very common allochthonous tree in this region), at times blended with small amounts of other fuels (e.g., pet coke) to enhance the efficiency of the combustion process. This biomass, with a size of several centimeters Received: October 12, 2011 Revised: February 8, 2012 Published: February 8, 2012 1540
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and its filamentous structure still intact, is transported by a conveyor belt from the stock piles and introduced into the combustion boiler system by a worm drive. In the pulp and paper plant, the amount of biomass consumed which is about 106 000 t/yr and generates 1500 t/yr of fly ash. Fly ash was sampled from the electrostatic precipitators of the biomass power plant for a period of a month. Every week 20 kg of fly ashes was collected. The total sample, 80 kg of fly ash, was quartered once dried. A representative sample labeled CVBE was used for this study. Next, the sample was dry sieved at 500, 212, and 150 μm to yield the following five fractions: >500 μm, >212 μm, >150 μm, 500−212 μm, and 150 agglomerate 10% CVBE agglomerate 10% 500−212
7.3 0.6 2.9 72.8 26.3 19.5 16.4 3.5
2.3 0.6 2.9 15.6 17.9 18.5 6.4 3.5
anisotropic fragments (vol %)
fused and dense (vol %)
fused and porous (vol %)
fragments (vol %)
inorganic fraction (glass, oxides, carbonates, quartz, etc.) (vol %)
0.5 1.0 0.4 6.8 10.4 5.1 22.5 5.0
0.5 0.6 2.6 0.0 0.7 1.7 2.3 11.0
3.8 3.4 0.6 0.0 0.1 0.5 3.4 1.0
1.5 1.4 0.0 0.0 0.0 0.0 2.0 0.2
84.1 92.4 90.5 4.8 44.7 54.8 47.0 75.8
Particle size of the sieved fractions are in μm.
which could reach significant values in ashes from biomass, enhances the development of a low ash melting point. For comparison, the indices calculated for the ashes of various forestry biomasses are also shown in this table. The predictive indices obtained for E. globulus bark ash are relatively similar to those of ashes from other biomass, particularly those from wood and Jack pine bark. This is because the main component of the ashes of this kind of forestry biomass is mainly calcium, followed by silica and magnesium in lower percentages. The K and Na contents, particularly K content, in the ash from E. globulus bark are also low in comparison with values found in ashes from wood bark.1 The K content (expressed as oxide) for beech and birch yellow barks are ranging from 2.6 to 8.0 wt %. However, the K content in the eucalyptus bark ash is 2.93 wt % (Table 2). Taking this into account, together with the indices shown in Table 2, the fouling trend and the melting temperature of E. globulus bark ashes may be expected to fall within the same range as those of the wood ashes,15 which exhibited similar composition and predictive indices. 3.2. Petrography of Fly Ash Derived from Forest Biomass Combustion. The petrographic composition of the raw fly ash (sample CVBE) derived from the combustion of E. globulus bark and its fractions is shown in Table 3. The raw fly ash shows a predominant inorganic fraction with an unburned carbon content of about 16 vol % The microscopic components of the fly ashes have been organized into two categories: organic and inorganic components (Table 3), following the system proposed by Suárez-Ruiz et al.20 For the identification and classification of various types of unburned carbons in this fraction, the following combined criteria were taken into account: (i) optical texture (anisotropic/isotropic character of the specific particle); (ii) fused/unfused character; (iii) structure and morphology of the fly ash carbons (such as massive and dense particles, vesiculate particles, and those with porosity, irregular particles, etc.); and (iv) origin. Taking into account that this forest biomass has been blended with a proportion of pet coke, it is also necessary to include this element in the classification of unburned carbons. The unburned carbons derived solely from the combustion of the E. globulus bark are always isotropic and may appear as unfused and porous. They retain the original structure of the walls and cell cavities (Figure 1a,b) of the tree bark. They are only slightly modified during the process of biomass combustion and are the predominant organic particles in the raw fly ash (CVBE raw sample in Table 3). Another type of
unburned carbon present in these ashes but in lower percentages is the isotropic but fused and porous particles (Figure 1c,d) that have degasification vacuoles. Also present are small isotropic fragments made up of particles of less than 15 μm that cannot be assigned to any of the categories of unburned carbons previously described. In the raw fly ashes (CVBE sample) unburned carbons derived from pet coke were also found (Table 3). These unburned carbons are always anisotropic and can be fused and dense (Figure 1e) or fused and porous (Figure 1f). The latter is the predominant type of anisotropic carbons found in raw fly ash. Small anisotropic fragments derived from pet coke were also observed. The highest concentrations of unburned carbons in the sieved fractions were found in the granulometric fraction (>500 μm), where these particles represent a value of 95.2 vol % of the total. The >212 and >150 μm granulometric fractions also tend to concentrate significant amounts of unburned carbons (Table 3). The partitioning of the various categories of unburned carbons among the sieved fractions follows a similar trend in all cases except for the finest fraction. Thus, the isotropic, unfused, and porous unburned carbons are predominant in all cases followed by the isotropic, fused, and porous particles and fragments, all of which are derived from the combustion of biomass. The unburned carbons from pet coke are scarce (Table 3). In the case of the finest fraction (500, 500−212, >212, >150, and 500, 500−212, and 500, >212, and >150 μm fractions, the last fraction has the highest weight due to its high ash content. In comparison with the sieved fractions, the weight recorded for the agglomerate fraction from the CVBE raw sample is low (Table 4). In relation to its chemistry, the carbon content, which is low in the CVBE raw fly ash, increases in the fractions as the particle size increases, and therefore, the sieved >500 μm fraction shows the highest carbon content (Table 4 and Figure 3a), confirming the data obtained from microscopic analysis (Table 3). Figure 3a shows acceptable relationships (correlation coefficient r = 0.93) between the volume of unburned carbons, as determined by microscopic analysis and the carbon content for the raw fly ash and its fractions. Consequently, in the discussion below, the unburned carbon content of the fly ash samples will be used instead of the carbon content. The agglomerate fraction shows an intermediate carbon content, mainly because of the content of isotropic unfused and porous unburned carbons from biomass, as in the case of the sieved fractions. Regarding the total sulfur content, the highest values correspond to the raw CVBE sample of fly ash, the sieved fraction with the lowest particle size, and the
agglomerate fraction (Table 4). The total sulfur content, which is very low in the feed fuel (E. globulus bark), increases considerably in the CVBE raw fly ash. This is because this fuel was combusted together with pet coke to increase the efficiency of the process, and as was pointed out, pet coke was found (Figure 1e,f) as unburned carbon in the fly ash (5.8 vol %). Thus, the increased presence of total sulfur in the fly ash and its fractions is most probably related to the presence of unburned carbons from pet coke, as can be seen in Figure 3b. It also follows that the higher the unburned carbon content from pet coke, the higher the total sulfur content (r = 0.89). The highest N concentration is found in the >500 μm fraction, which shows a close relationship with the unburned fraction. H has the highest value in the agglomerate fraction probably due to the contribution of H from the vegetable oils used in the process of unburned carbon concentration.
Figure 3. Relationships between (a) the carbon content (from ultimate analysis) of the raw CVBE fly ash samples and their sieved and agglomerate fractions and the total unburned carbons determined from petrographic analysis and (b) the total sulfur content of all the fly ash samples and the unburned carbons from pet coke.
Figure 2. XRD spectrum corresponding to the raw CVBE fly ash. A, anhydrite (CaSO4); Ca, calcite (CaCO3); Ge, gehlenite (Ca2Al2SiO7); L, calcium oxide (CaO); Q, quartz (SiO2).
Table 4. Proximate and Ultimate Analysis for the Raw CVBE Fly Ash Sample and Its Fractionsa samples raw CVBE fly ash 500 >212 >150 agglomerate 10% CVBE
wt (%)
moisture (%)
ash (%, dbb)
C (%, db)
H (%, db)
N (%, db)
Stotal (%, db)
93.66 4.17 2.17 6.34 16.89 4.70
0.47 0.78 1.02 8.81 3.74 1.88 2.30
85.85 83.42 90.98 22.43 67.69 83.75 67.92
3.93 3.69 6.51 59.65 18.01 14.03 18.62
0.27 0.35 0.19 0.72 0.29 0.48 0.92
0.09 0.03 0.04 0.46 0.15 0.05 0.13
1.99 3.01 0.30 0.40 0.33 0.09 1.72
The weight obtained for each fraction is also included. Particle size of the sieved fractions is in μm. The agglomerate fraction 10% 500−212 is not included here. bdb: dry basis. a
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Table 5. Chemistry in Terms of Major and Minor Oxides, Basicity Modulus (Mb), and the Si/Al Ratio of the HTA from the CVBE Raw Sample and Its Fractionsa samples raw CVBE 500 >212 >150 agglomerate 10% CVBE a
SiO2 (%)
Al2O3 (%)
Fe2O3 (%)
CaO (%)
MgO (%)
Na2O (%)
K2O (%)
TiO2 (%)
P2O5 (%)
Mb (CaO+MgO/ SiO2+Al2O3)
Si/Al (SiO2/ Al2O3)
35.56 34.53 60.23 43.79 58.20 63.99 33.37
7.63 7.70 9.45 11.74 9.49 8.84 8.78
3.64 2.66 3.19 4.00 3.27 3.16 2.96
42.81 43.94 17.72 24.56 18.38 15.84 42.27
5.58 5.34 3.07 5.66 3.42 3.11 7.13
1.35 1.05 1.18 3.74 1.79 1.54 1.00
3.43 3.14 4.02 4.64 4.24 2.34 2.57
150 agglomerate 10% CVBE agglomerate 10% 500−212
2.40 2.59 2.61 1.95 2.19 2.52 2.29 2.36
2.29 2.68 2.54 1.48 2.08 2.40 2.14 2.19
pore vol.
V TOT (mm3/g)
surface area (m2/g)
mercury density (g/cm3)
total pore volume (cm3/g)
10.20 7.30 11.80 131.10 69.60 25.70 4.70 1.90
12.1 4.3 23.0 291.6 153.2 53.1 2.8 0.9
0.70 0.71 1.41 0.43 0.83 1.20 0.70 1.21
1.02 1.01 0.33 1.83 0.74 0.44 0.99 0.40
porosity (%)
d> 50 nm (%)
50 > d > 5.5 nm (%)
70.96 72.45 46.06 78.13 61.94 52.56 69.29 48.62
99.10 99.07 98.69 99.34 99.36 99.50 99.26 98.57
0.90 0.93 1.31 0.66 0.64 0.50 0.74 1.43
As determined from the adsorbed volume of N2 at 0.95 relative pressure (p/p0). bParticle size of the sieved fractions is in μm.
were obtained, those for Ca, Mg, and Na oxides are exceptionally high, which suggests that the fly ash derived from the combustion of E. globulus bark is of the calsialic type according to chemical classification reported elsewhere.21,22 3.4. Textural Properties. 3.4.1. Helium Density. As can be seen in Table 6 and Figure 4, there is a general decrease in
The chemistry of the high temperature ashes (HTA) in terms of major and minor oxides obtained for the CVBE raw sample and its fractions is shown in Table 5. The basicity modulus (Mb) and the Si/Al ratios are also included. The inorganic content of the raw CVBE fly ash in terms of SiO2 + Al2O3 + Fe2O3, which serves to characterize the fly ash from the combustion of coals into classes F or C, reaches a value, in this case, of about 46.8%, which is lower than the value established by the ASTM C 618-05 norm for class C fly ash (those derived from the combustion of sub-bituminous and lignite coals). This is because the fly ashes from the combustion of E. globulus bark have a higher amount of alkaline-earth elements, as in the case of the ashes from other types of forest biomass.1 As can be seen, CaO and, in lesser proportion, SiO2 are predominant in all the analyzed samples. In the raw CVBE fly ash, Ca is clearly predominant over Si. This Ca is present as calcite and calcium oxide, as identified by XRD. Si is found as glassy material, quartz, and silicates of various composition (Figure 2). Other minerals such as iron oxides and calcium sulfate were also identified. The concentration of these components in the fly ash fractions, particularly in the case of Si and Ca are variable depending on the particle size. The Ca concentration does not show a clear trend (Table 5). Highest values in Ca were found in the finest fraction (500 μm, which is type I according to the BDDT classification,25 presents the largest nitrogen adsorption (Figure 5b) and the highest unburned carbon content (95.2 vol %), followed by fractions >212 μm and >150 μm which exhibit lower unburned carbon and carbon contents (Tables 3 and 4). In comparison with the textural properties of other fly ashes from coal combustion,13,24,26−28 the fly ashes from forest biomass, such as those analyzed in this work, show better textural properties than ash samples from class F fly ash13 (from combustion of bituminous and anthracitic coals) and properties that are relatively similar or even better than those shown by class C fly ash13 (from the combustion of sub-bituminous and lignite coals). The isotherms of the granulometric fractions >212 and >150 μm show a type H4 hysteresis loop, which is associated with capillary condensation in the slit-shaped mesopores. As already mentioned, no adsorption properties (Table 6) were found in the agglomerate fractions despite their unburned carbon content (Table 3). Thus, the behavior of these fly ash fractions reveals certain differences that are reflected in the resulting isotherms and adsorption properties: (i) with respect to the unburned carbon content, which is related to the particle size of the fractions, and (ii) with respect to the physical process used in the concentration of unburned carbons. For similar concentrations of unburned carbons, the adsorption properties are acceptable in the sieved fractions but not in the case of the agglomerate fractions. This could be related to the content in unburned carbons derived from pet coke. 3.4.3. Total Pore Volume VTOT. Figure 6 also illustrates the inherent porosity of the fly ashes. The results for the total pore volume, VTOT (for total fly ash) calculated from the amount of N2 adsorbed by the fly ashes at the relative pressure (p/p0) of 0.95 are variable, as can be seen in Table 6. Values higher than 25 mm3/g fly ash were obtained for the >150, >212, and >500 μm sieved fractions, and they follow an increasing trend (r = 0.96) with the increase in the amount of total unburned carbon in the samples (Figure 6a). For the other sieved fractions and for the raw CVBE sample, the VTOT is always below 25.0 mm3/g and the total pore volume is negligible (Table 6), despite the unburned carbon content. Although the content in unburned carbons derived from the combustion of pet coke in these samples is relatively low (150, >212, and >500 μm, especially the latter sieved fraction (292 m2/g fly ash). The relationship between the surface areas and the amount of unburned carbons present in the samples
the relationship is not so good when they are included in the statistical correlation with the sieved fractions. 3.4.2. N2 Adsorption Isotherms at 77 K. The adsorption isotherms obtained for the raw CVBE sample and the sieved fractions are shown in Figure 5. Here, the adsorption isotherms
Figure 5. N2-77 K adsorption isotherms for (a) the raw CVBE fly ash, the 500−212 and 500, >150, and >212 μm sieved fractions. Agglomerate fractions are not included in the graph because of their extremely low VTOT and BET surface area.
for the agglomerate fractions are not included due to the poor results obtained. As can be seen in this figure, the different fractions of fly ash show different adsorption behaviors. The raw CVBE sample and its fractions 500−212 and 500 μm and >212 μm sieved fractions, which also present the highest moisture content (Table 4). For the other samples, the mass loss below 200 °C was negligible. The temperature range between 200 and 550 °C shows a maximum peak only in the case of the raw CVBE sample and 500 μm and >212 μm. In both cases, the weight loss in this region (Table 7) seems to be due to the release of CO and CO2 from the decomposition of carbonate (magnesium carbonate shows a maximum weight loss during its pyrolysis in this temperature range). The 550−700 °C and >700 °C regions are those in which maximum weight loss was observed in all the samples, though for different reasons (Figure 9 and Table 7). Carbonate decomposition, particularly in the case of raw CVBE sample and 500 μm and >212 μm sieved fractions, the small peak located at around 600 °C (Figure 9) mainly represents the decomposition of some of the unburned carbons that are present in significant amounts in these samples, with some contribution from carbonate decomposition. In the temperature range >700 °C, there is only one shoulder in the >500 μm and >212 μm samples, which could be attributed to the release of CO2 during the decomposition of the organic fraction. This is confirmed by the pyrolysis of the pure unburned carbons (Table 7 and Figure 9) that shows the highest weight loss at temperatures >700 °C. This is a different behavior to that observed in the pyrolysis of the original biomass (E. globulus bark). In this case, the maximum weight loss was observed at lower temperatures (temperature range 200−550 °C, Table 7), the maximum peak being located at 340 °C. As for the pyrolysis of unburned carbons, it should be pointed out that, despite a strong decomposition at temperatures higher than 700 °C, at the end of the pyrolysis process there still remain some unburned carbons, which were observed in the microscopic examination
the BET surface areas on a carbon basis for the sieved fractions range from 81 m2/g·C for the finest fractions to 726 m2/g·C for the fraction >212 μm. As regards the sieved fractions of increasing particle size, there is an increase in BET surface area from >212 μm, to 212−500 and >500 μm fractions. However, the surface areas on a carbon basis are significantly higher for the >212 μm sieved fraction than in the case of the >500 μm fraction. This might be thought to be due to the presence of the larger particles in the sieved fraction >500 μm. Although the >500 μm fraction contains a large porosity (Table 6, Figure 8b), this porosity is deep within the large particles and so, it is relatively inaccessible to gas, which is not the case of the >212 μm sieved fraction. The >212 μm sieved fraction contains, in addition to large particles of unburned carbons, other particles of a smaller size in which the porosity is accessible to gas. The inclusion of this gas in any measurement will give a greater BET surface area, as is shown in Figure 8a. This hypothesis seems to be supported by the value of the surface area on a carbon basis obtained for the 212−500 μm sieved fraction that shows a similar value to that of the >500 μm sieved fraction. A similar conclusion can be extracted for the VTOT data (also on a carbon basis) shown in the Figure 8b. The VTOT for the sieved fraction >212 μm is the almost the sum of those obtained for fractions 212−500 and >500 μm. Thus the higher BET surface areas and VTOT per gram of carbon in the >212 μm fraction seem to be related to the size of the unburned carbons present in the fraction rather than to any variation in the amount of various types of unburned carbons derived from the biomass. The agglomerate fraction from the CVBE fly ash shows the lowest BET surface area and VTOT on a carbon basis of all the analyzed samples. This could be a consequence of the agglomerating process and the oils that are incorporated into the particles and block up the pores, thereby preventing gas from being adsorbed. However, the presence of unburned carbons from pet coke maybe also have a negative influence. In comparison with the textural properties of other fly ashes from coal combustion13 the fly ashes from forest biomass such as those analyzed in this work show better textural properties than class F fly ash (derived from combustion of bituminous and anthracitic coals) and properties that are relatively similar or even better than those shown by class C fly ash (from the combustion of sub-bituminous and lignite coals) 3.4.5. Porosimetry. Table 6 shows the mercury porosimetry results for the raw CVBE fly ash and its fractions. The pore distribution is presented as the percentage of macropores or pores with a diameter >50 nm and pores (mesopores) with a diameter between 50 and 5.5 nm. This type of fly ash and its sieved fractions display an average total pore volume of 0.33 and 1.83 cm3/g ash (for the sieved fractions 500−212 and >500 μm, respectively), and a variable porosity of 46−78%. Thus, porosity is made up of macropores that account for more than 98% of the total pore volume (Table 6); the amount of mesopores is almost negligible (lower than 1.5%). The microporosity was found to be variable according to the N2 absorption results (Figures 5 and 6) described above, maximum microporosity corresponding to the sieved fraction >500 μm due to its high unburned carbons content (Table 3). As for the agglomerate fractions, their porosity, pore volume distribution, and total pore volume values are similar to those described for the sieved fractions (Table 6). Taking into account all the samples analyzed here, no good correlation was found among the total of unburned carbon content (as a proxy of the LOI or the C content) and the porosity (%) and 1550
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Figure 9. Thermograms from pyrolysis under N2 atmosphere obtained for the raw CVBE fly ash sample and its sieved fractions (in micrometers).
Table 7. Total Weight Loss during Pyrolysis in N2 Atmosphere and Rates of Weight Loss at Various Temperature Ranges for the CVBE Raw Sample and Sieved Fractions (in μm) total weight loss
a
weight loss
samples
(%)
700 °C
raw CVBE 500 >212 >150 agglomerate 10% CVBE unburned carbons MgCO3 CaCO3 E. globulus bark
22.68 24.96 4.07 18.92 14.29 5.75 23.55 37.37 57.42 44.88 83.90
0.36 0.31 0.26 6.15 4.72 0.36 naa 10.74 2.86 0.44 8.46
2.09 2.48 0.65 3.37 2.27 0.88 na 5.17 52.86 0.27 69.95
6.48 6.53 1.41 3.18 2.49 1.85 na 4.28 1.29 7.28 3.18
13.75 15.64 1.75 6.22 4.81 2.66 na 17.18 0.41 36.89 2.31
na: not applicable.
samples. These sieved fractions show the lowest total weight loss (Table 7) because of their low Ca and Mg contents and the low content in unburned carbons. In fact, in these samples, silica is the predominant inorganic component (Table 5). Despite this, taking into account both the composition of the 500−212 μm and >150 μm samples (Tables 3−5) and the fact the shapes of their thermograms are relatively similar to those of the >500 μm and >212 μm samples, it is possible that the
of the pyrolysis residues. For the samples with highest inorganic content (raw CVBE sample and 700 °C shows a significant peak (Figure 9) corresponding to the decomposition of calcium carbonate. This is confirmed by the pyrolysis results of pure calcium carbonate (Table 7). As for the 500−212 μm and >150 μm ash fractions, their behavior during pyrolysis is different from that of the other 1551
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Figure 10. Relationships between the calcium and magnesium content (as oxides) and the total weight loss, and the weight loss in the temperature ranges 550−700 °C and >700 °C, in the pyrolysis of the fly ash samples.
weight loss in the temperature ranges 550−700 °C and >700 °C represents the decomposition of unburned carbons. The relationship between the weight loss during the pyrolysis process (total weight loss and weight loss in the temperature ranges 550−700 °C and >700°) of the raw CVBE sample and sieved fractions and the content in Ca and Mg (as oxides) is shown in Figure 10. In the case of the total weight loss versus the CaO and MgO content, the correlation coefficient is
moderate (r = 0.91). This is because the contribution of other inorganic components and of the unburned carbons to the total mass loss has not been taken into account. On the other hand the total weight loss (Table 7) of the agglomerate sample (10%) is similar to that of the raw CVBE and 212 >150 agglomerate 10% CVBE unburned carbons MgCO3 CaCO3 E. globulus bark
15.48 15,24 6.01 79.17 29.54 21.17 32.66 91.88 57.60 43.62 95.85
0.18 0.24 0.25 5.48 2.24 0.71 na 11.08 3.50 0.00 9.00
2.53 0.76 3.67 68.17 23.45 17.67 na 75.55 52.97 0.10 85.34
8.72 8.98 1.52 2.30 2.30 1.97 na 2.25 1.04 17.30 1.51
4.52 5.26 0.57 3.22 1.55 0.82 na 3.00 0.09 26.22 0.00
a
For comparison, the rate of weight loss during combustion in the same conditions of pure substances, unburned carbons, MgCO3, and CaCO3, is also included.
have been differentiated (Table 8). These intervals of temperature are relatively coincident with those described for the pyrolysis process of these samples. The weight loss at temperatures below 200 °C is mainly due to the release of water. This weight loss is more acute in the case of the fly ashes with the highest moisture content (Table 4) due to their high carbon content (Table 4). One exception is the agglomerate
temperature ranges of the carbonate decomposition and a shoulder at the highest temperatures related to the pyrolysis of the organic fraction. 3.5.2. Combustion Behavior. The combustion curves under air atmosphere for the studied samples and the total weight loss are shown in Figure 11 and Table 8,respectively. According to the profile of the combustion curves four temperature ranges 1553
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Figure 12. Relationships between the total unburned carbon content and (a) the total weight and (b) the weight loss in the temperature range of 200−550 °C. Relationships between the calcium and magnesium content (as oxides) and the weight loss in the temperature ranges (c) 550−700 °C and (d) >700 °C in the combustion process of the fly ash samples. Graphic (a) takes into account the CVBE raw sample, the granulometric fractions, and the agglomerate sample. Graphics (b, c, and d) take only into account the CVBE raw sample and the sieved fractions.
However, it should be pointed out that the magnesium carbonate decomposition that occurs at these temperatures may also contribute to weight loss in this temperature range, as can be seen in Table 8, although no correlation was found between the magnesium content (as oxide) and the weight loss of these samples. A temperature range with a significant rate of weight loss particularly in the case of the raw CVBE sample and the 700 °C in Table 8). In these stages, the contribution to mass loss by a few remaining unburned carbons that are combusted at these high temperatures cannot be discarded. This is confirmed by the thermograms from the combustion of pure unburned carbons that also exhibit a weak shoulder and some weight loss. In a microscopic examination of the combustion residues obtained at 1000 °C, no unburned carbons were detected. Only mineral matter was observed, mainly in the form of glassy material and oxides.
10% CVBE sample. Despite its relatively significant unburned carbon content the weight loss at this stage is low because the moisture content is also low. As was discussed above, this sample exhibits a low porosity probably because the oil used to agglomerate the unburned carbons is still retained inside the pores. The temperature range between 200 and 550 °C is the interval in which the unburned carbons are combusted, and so, the weight loss in this stage is acute, particularly in the samples that have highest unburned carbon content (sieved fractions >500 μm, >212 μm, and >150 μm). The combustion of pure unburned carbons (Table 8) shows the maximum weight loss (its maximum peak being located at 450 °C) in this interval of temperatures confirming the results previously discussed. In the case of the combustion of the original biomass (E, globulus bark), the maximum weight loss (maximum peak located at 337 °C) is also produced in the temperature range 200−550 °C (Table 8). A similar combustion behavior was observed for biomass from different types of rice, straw, and pine sawdust.30,31 In the other fly ash samples, the mass loss at this stage (Table 8) is less important because the unburned carbon content is very low (Table 3). In general, the weight loss in the temperature range 200−550 °C clearly influences the total weight loss of the samples during the combustion process. The relationships between the amount of unburned carbons in the studied samples, the total weight loss, and the weight loss in the temperature range 200−550 °C is shown in Figure 12 a,b. 1554
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CONCLUSIONS The conclusions obtained in this work can be summarized as follows: • The biomass of Eucalyptus globulus bark shows a composition and calorific value similar to that described in the literature for other forestry biomass. In this biomass, the inorganic fraction is mainly of a basic character, with a predominance of alkaline-earth elements. This explains why the predictive deposition indices calculated from the ash composition and mainly related to fouling and slagging propensities are similar to those generally described for this kind of biomass. In combustion performed via thermogravimetric analysis, in the conditions used in this work, maximum weight loss was observed to take place at the temperature of about 337 °C. • The fly ash obtained from the combustion of this type of biomass is predominantly made up of an inorganic fraction of a basic character, in which calcium is the most prominent element. The organic fraction is mainly made up of isotropic, unfused, and porous unburned carbons, some of which inclusively still preserve the vegetal structure of the wall and cell cavities. To improve the combustion efficiency of the boiler, a certain amount of petroleum coke was added to the biomass. This is reflected in the fly ash by the presence of specific unburned carbons from pet coke and by a substantial increase in the amount of the total sulfur content compared to the amount present in the original biomass. • The composition data for the sieved and agglomerate fractions derived from the raw fly ash show that the process of dry sieving is more effective than oil agglomeration for concentrating unburned carbons. The sieved fraction with a size >500 μm shows the highest concentration of unburned carbons (and therefore highest C content) from biomass, with a total absence of unburned carbons from pet coke. • According to the results of the textural characterization of the fly ash from biomass combustion, the amount, type, and size of the unburned carbons clearly influence the textural properties. • Globally the amount and type of unburned carbons are the main factors that control the textural properties of a sample. Thus, it is the total sized fraction >500 μm that shows the highest adsorption properties (N2−BET surface area, VTOT, and porosity) because it contains the highest proportion of unburned carbons. The textural properties are even better when they are calculated on a C basis for the >212 μm fraction. This seems to be related with the size and the type of unburned carbons contained in the fraction and the fact that the porosity is accessible to gases. • The thermal behavior of the fly ash samples during pyrolysis in a N2 atmosphere is variable depending on the unburned carbon, Ca, and Mg contents. In any case, at the end of the pyrolysis processes, the unburned carbons have not totally decomposed. Optical microscopy revealed the presence of particles of unburned carbons with a good vegetal structure in the residues after pyrolysis at 1000 °C. • During combustion, the unburned carbons decompose at a temperature of about 450 °C, and no microscopic organic residues are left at the end of the combustion
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processes (1000 °C). The different profiles of the combustion curves for the fly ashes mainly depend on the unburned carbon content and the alkaline-earth carbonate decomposition, and accordingly, different behaviors with respect to the weight loss were observed in the samples studied. • Taking into account the unburned carbon content and the good textural properties shown by the >500 μm sieved fraction, this is the most appropriate sample to be used as precursor material for obtaining activated carbons.
AUTHOR INFORMATION
Corresponding Author
*Phone: +34985119090. Fax: +34985297662. E-mail: isruiz@ incar.csic.es. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The financial support for this work was provided by a research project from the PRI-Asturias, PC 07-015. R.P. Girón acknowledges the CSIC for a predoctoral research grant JAE (No. PR2005-0168). The authors thank CEASA (ENCE-Navia) from North Spain for providing the samples. J. R. Montes and J. Villanueva (from INCAR-CSIC, Spain) are also thanked for preparing the samples.
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