Pyrolysis of various biomass residues and char utilization for the production of activated carbons

J. Anal. Appl. Pyrolysis 85 (2009) 134–141

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Pyrolysis of various biomass residues and char utilization for the production of activated carbons J.F. Gonza´lez a, S. Roma´n a,*, J.M. Encinar b, G. Martı´nez b a b

Department of Applied Physics, Extremadura University, Avenida de Elvas s/n, 06071 Badajoz, Spain Department of Chemical Engineering and Physical Chemistry, Extremadura University, Avenida de Elvas s/n, 06071 Badajoz, Spain

A R T I C L E I N F O

A B S T R A C T

Article history: Received 25 June 2008 Accepted 18 November 2008 Available online 3 December 2008

Biomass samples (almond shell, walnut shell, almond tree pruning and olive stone) were subjected to thermoanalytical investigation to evaluate their thermal behaviour and its correlation with their lignocellulosic composition. Then, the pyrolysis process of these materials was studied in terms of the energy content of the phases generated (gas and liquid). Finally, the feasibility of obtaining effective adsorbents from the char generated was studied. With this aim, the char was used to prepare activated carbons (ACs) by steam gasification at fixed activation temperature and time, identical for the four chars. The differences found in the porosity development of the activated carbons were related to the lignocellulosic composition of the raw material. It is shown that the four biomass residues used are versatile precursors that allow the preparation of adsorbent materials with different textural characteristics. ß 2008 Elsevier B.V. All rights reserved.

Keywords: Pyrolysis Biomass Energy Steam activation Activated carbon Porosity

1. Introduction Among the technologies concerning clean energy generation, the energy exploitation of biomass is an interesting challenge since it is clean (zero net CO2 emission), it is unlimited, and it minimizes the disposal problems associated with the generation of agricultural by-products. Moreover, biomass exploitation allows the possibility of generating added value products such as chemicals or activated carbons (ACs) which means an attractive economic and technological solution. It has been widely reported the feasibility of using agricultural by-products as renewable source of energy by means of pyrolysis and gasification processes [1–5]. The thermal degradation characteristics of lignocellulosic materials are profoundly influenced by their chemical composition (cellulose, hemicellulose and lignin) [6], and TG and DTG curves provide semiquantitative understanding of the thermal degradation processes occurring during thermochemical conversion under various atmospheres. Thus, the comparative thermogravimetric study of different materials can provide useful information about the differences associated to their lignocellulosic composition. On the other hand, three phases are produced when biomass is subjected to pyrolysis processes: char, liquid and gas. The distribution of these phases is a function of operating parameters

* Corresponding author. Tel.: +34 924 289619; fax: +34 924 289619. E-mail address: [email protected] (S. Roma´n). 0165-2370/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jaap.2008.11.035

(mainly temperature and heating rate). The most interesting one from the energetic point of view is the gas (mainly composed by H2, CO, CO2, CH4, etc.) which has a Higher Heating Value (HHV) high enough to be used for the total energy requirements of a biomass waste pyrolysis plant [7] and might also be employed in internal combustion engines, gas turbines and other operating devices [8]. The liquid phase generated (known as tar) is the oil mainly composed of oxygen-containing structures (derivatives of phenol, dihydroxybenzenes, guaiacol, syringol, vanilin, veratrol, furan, acids) [9] and can be used directly as fuel or added to petroleum refinery feedstock [10], although its use involves some nuisances such as high water content (that are detrimental to ignition), presence of corrosive organic compounds, etc., and may also be an important source of chemicals. The char is a solid carbonaceous residue with a high content in fixed carbon (>75%), which can be used directly as fuel, as briquettes [4,5] or as precursor for activated carbons production [2,5]. ACs are porous materials that are able to adsorb certain amounts of compounds in the liquid and gaseous phases; this property makes these solids very interesting to be used in many industrial applications. The porosity of activated carbons is conditioned, among other factors, by the carbonaceous precursor, the activation method used and the operating parameters. There is a huge amount of research on the production of ACs from agricultural by-products by physical [11–14] or chemical activation [15]. Most of these works study the influence of operating parameters on the activation process, but the research focussed on the influence of the lignocellulosic composition of the raw material on a concrete

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activation process is scarce. The investigation concerning this field is very interesting because in the industrial production of ACs around 50% of the precursors used by the manufacturers are lignocellulosic [16]. The aim of this work is to study the exploitation of several biomass residues carrying out the following specific objectives: - To conduct thermogravimetric analysis (under inert atmosphere) on the four biomass residues employed, estimating the relationship between the devolatilization curves and the lignocellulosic composition of the materials. - To study the pyrolysis process in terms of the energy exploitation of the three phases generated, determining the HHV of the gas, tar and char. - To produce activated carbons from the char obtained from each material, by steam activation under common activation conditions, evaluating the influence of the parent material composition on the textural characteristics of the carbons produced.

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placed on a basket inside the reactor and a flow of N2 was supplied (150 mL min1) during the heating (and cooling) stages. Once the activation temperature was attained, water steam (with a flow of 0.19 g min1, diluted in 150 mL min1 of N2) was fed as activating agent (that is, steam partial pressure was equal to 0.61). The steam activation of the char was performed at a temperature of 850 8C and an activation time of 30 min. The choice of these activation conditions is based on previous studies about the development of porosity in some of these materials [22,23] which demonstrated that a high temperature favours the porosity development in steam activation. Having the former argument in mind we chose a short activation time in order to see the effect of the parent char used on the pore size distribution more clearly (during such period of time we might obtain both micro or mesoporous materials, but for large activation times we will possibly achieve mesoporous materials). The activated carbons were named as S– X, where S stands for steam and X represents the parent material.

2. Experimental 2.3. Characterization of activated carbon samples 2.1. Materials The materials employed in this study were walnut shell, almond tree pruning, almond shell and olive stone, which are extensively generated in Europe. These precursors will be named as WS, ATP, AS and OS, respectively. They were all obtained from local manufacturers and shredded to 1–2 mm. The ultimate analysis was made using a LECO CHNS (EA 1108) analyzer and the proximate analysis of the four materials was made according to standard methods [17–19]. The HHV of the materials was determined with a Parr 1351 calorimeter bomb (norm ISO 1928). The lignocellulosic composition of the materials was determined according to Van Soest method [20]. 2.2. Methods 2.2.1. Thermogravimetric analysis Biomass samples were subjected to thermogravimetric analysis using a SETARAM SETSYS EVOLUTION instrument controlled by a PC. The analyses were made using N2 with a flow rate of 20 mL min1, and heating rate of 5 8C min1 up to a final temperature of 700 8C. The sample mass used in all runs was equal to 160 mg. 2.2.2. Pyrolysis The pyrolysis process was performed in a bench scale system in which the different phases generated (char, tar and gas) were quantified. This equipment has been described elsewhere [2,5] and was composed basically of a vertical furnace fed with N2 (150 mL min1), a system where tars and condensable liquids were stored and a gas collecting system. The temperature employed in all the cases was 600 8C and the dwell time 60 min. An initial mass of around 20 g was used in all the cases. The gas composition was determined by a 4000 HRGC KONIK gas chromatograph provided with two thermal conductivity detectors, connected to two columns: Porapack Q and Carboxen-1000 (15 ft length, 1/8 inch diameter). The HHV of the char and tar generated in the pyrolytic process was determined as described in Section 2.1. The chars were named according to the nomenclature: C-WS, C-ATP, C-AS and C-OS, for walnut shell, almond tree pruning, almond shell and olive stone, respectively. 2.2.3. Activation The activation processes were carried out in an installation that has been described previously [21]. The char (5 g) was

2.3.1. Gas adsorption The chars and the activated carbons were characterized by adsorption of N2 at 77 K (AUTOSORB-1, Quantachrome). Moreover, the chars were also characterized by adsorption of CO2 at 273 K, using a volumetric manual system. Both adsorption measurements were made after outgassing at 300 8C during 12 h to a residual vacuum of 105 Torr. About 1.5 g of sample was used in each adsorption experiment. 2.3.2. Mercury porosimetry and density measurements An AUTOPORE 4900 porosimeter from Micromeritics was used to obtain the curves of mercury intrusion, determining the values of meso and macropore volumes and mercury density. Helium densities were measured with a Quantachrome stereopycnometer, following the usual method [24]. From mercury and helium densities, the total pore volume was calculated [25]. 2.4. Calculation of textural characteristics of carbons 2.4.1. N2 adsorption isotherms Different methods were used to analyse the N2 adsorption isotherms at 77 K. One estimate of apparent surface area was obtained by applying the Brunauer, Emmett and Teller (BET) equation [26], yielding the SBET values; the Dubinin–Radushkevich method (DR) was used to determine the micropore volume (Vmi) [27]. Moreover, the aS-method was used to estimate the external surface (SEXT) [28], using a non-porous activated carbon made from olive stones by Rodrı´guez-Reinoso et al. [29] as reference material. Finally, the volume of mesopores (Vme) was calculated from the isotherm as the difference between the volume of N2 adsorbed at P/ P0 = 0.95 and Vmi as determined previously. The N2 values used for these determinations were: 0.162 nm2 as N2 molecular area, 0.808 g cm3 as the density of liquid N2 and 0.33 as the b factor [30]. 2.4.2. CO2 adsorption isotherms In the case of the char, since it may possess a very narrow and incipient microporosity (which cannot be measured with 77 K nitrogen adsorption due to the negligible adsorption kinetics at this temperature), carbon dioxide adsorption at 273 K was used to evaluate its micropore volume, Vmi(CO2), by applying Dubinin– Radushkevich equation [27]. In this case 0.187 nm2 was taken as CO2 molecular area at 273 K, 0.818 g cm3 as the density of CO2 at 273 K and 0.35 as the b factor [30].

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3. Discussion of results The ultimate and proximate analyses of these raw materials are shown in Table 1. It can be seen that these materials have a high content of volatile matter and low content of ash, nitrogen and sulfur, which is interesting with respect to their applications in gasification and pyrolysis processes. 3.1. Thermogravimetric study Fig. 1 shows the effect of temperature on the residual weight fraction (TG) of the four materials. The thermograms show that the thermal decomposition of all the materials starts to be noticeable at about 150 8C and then there is a continuous decrease in the sample weight. The origin of these weight losses can be explained by considering the lignocellulosic composition of the precursors. Subjected to thermal treatment, hemicellulose starts degrading first at around 200–260 8C, followed by cellulose at 240–350 8C [6,31]; in the case of lignin, which is more thermostable [32], its range of decomposition is wider, and overlaps with the weight loss of the former components in such a way that its contribution is very difficult to determine, and has been identified in the ranges ˜ ez et al. [33] 280–500 8C [11], and 175–800 8C [32]. Garcı´a-Iba´n have identified two pyrolysis zones; the first one (called active zone) can be attributed to the volatile compounds generated during the decomposition of hemicellulose and cellulose, and the second one (passive zone) might be due to lignin (and some cellulose) conversion. Fig. 2 shows the rate of weight loss (DTG) of the residues; one can see that the first significant peak stands at about 180–200 8C. This weight loss might correspond to moisture and hemicellulose elimination and is higher in the case of AS and ATP, followed by WS. Then, two overlapping peaks and a flat tailing section are observed. The first peak is associated to hemicellulose decomposition and the second peak is mainly attributed to the combined decomposition of hemicellulose and cellulose. As lignin decomposes slowly over a broad range of temperature, it provides the tailing section of the DTG curve. DTG profiles show that there are significant differences between the rate of decomposition of the four materials. Some correlations between the lignocellulosic composition of the materials and the thermal profiles can be made. With this purpose, four different temperature ranges were defined, according to the changes in rate detected in the TG curves; these zones are shown in TG curves (Fig. 1). Table 2 shows the temperature intervals corresponding to each range and the weight loss values (m/m0) for each precursor. From Fig. 1, it can be inferred that AS starts its degradation at the lowest temperature, and presents the higher weight loss in range 2, which is consistent with its higher hemicellulose content. On the contrary, OS is the material which seems to be harder to decompose at the first range, which might be related to its lowest hemicellulose content, as it is the case. The larger cellulose content of WS is related to its higher weight loss in range 3. On the other hand, it is also the material showing the smallest lignin content, in accordance with the lowest value of range 4. As stated by Mansaray

Fig. 1. TG curves of biomass samples.

Fig. 2. DTG curves of biomass samples.

and Ghaly [34], higher contents of cellulose matter give rise to a greater extent of devolatilization. This is consistent with results since WS was the material with higher extent of devolatilization (it showed the greatest weight loss at 600 8C). The decline of range 4 is more sharply for ATP and OS, as these materials are more resistant, due to their higher lignin fraction. 3.2. Products from pyrolysis The pyrolysis yields and the HHV values of the three phases are given in Table 3. One can see that the four materials present similar char yields, decreasing in the order: ATP > OS > AS > WS. Antal [35] stated that the carbon yield is higher for materials with greater lignin contents, which is reasonably in agreement with results on lignocellulosic analysis. The liquid yields are more variable, and walnut shell shows the highest one, which might be related with its highest hemicellulose + cellulose content (60.8%). With respect to gas yields, they practically exhibit the same

Table 1 Ultimate and proximate analysis of biomass residues. Material

WS ATP AS OS a

Ultimate analysis (%)

Proximate analysis (%) a

C

H

N

S

O

45.10 51.30 50.50 44.80

6.00 6.50 6.60 6.00

0.30 0.80 0.20 0.10

0.00 0.04 0.01 0.01

48.60 41.36 42.69 49.09

Determined by difference to 100%.

Fixed carbon

Moisture

Volatile matter

Ash

15.90 16.00 9.10 13.80

11.00 10.60 10.00 10.40

71.80 72.20 80.30 74.40

1.30 1.20 0.60 1.40

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Table 2 Lignocellulosic composition of the raw materials and values of weight loss at selected intervals. Lignocellulosic composition of the raw materials

Hemicellulose Cellulose Lignin

WS

OS

ATP

AS

20.7 40.1 18.2

17.1 30.8 32.6

20.1 33.7 25.0

25.5 32.5 24.8

Values of weight loss and temperature intervals included in Fig. 1 18 zone 0.114 (315 8C) 28 zone 0.255 (315–380 8C) 38 zone 0.257 (380–440 8C) 48 zone 0.086 (440–600 8C)

0.102 0.251 0.240 0.118

(315 8C) (315–373 8C) (373–425 8C) (425–600 8C)

0.123 0.200 0.244 0.117

(315 8C) (315–380 8C) (380–438 8C) (438–600 8C)

0.126 0.282 0.170 0.109

(315 8C) (315–375 8C) (375–410 8C) (410–600 8C)

Table 3 Pyrolysis yields and HHV of the pyrolysis products. Material

WS ATP AS OS a

HHV of the products from pyrolysis (MJ kg1)

Pyrolysis yields (%) Char

Liquid

Gasa

Raw material

Char

Tar

Gas

23.96 26.75 24.61 25.02

30.64 19.42 20.28 23.42

45.4 53.83 55.11 51.56

18.36 19.41 18.33 18.73

31.89 33.20 29.20 32.04

5.43 5.08 3.87 7.23

6.24 5.63 6.75 6.39

Determined by difference to 100%.

sequence found in their volatile matter content. However, since these yields were calculated by subtraction they can be influenced by the determination of those of the other fractions. The HHV of the chars is low but could be used directly as fuel, manufacture of type-A briquettes, provided that their fixed carbon content is above 76% [36,37], or as precursor for activated carbon production [2,5], as it was the case of this work. The HHV of the tar generated is low but could be used as fuel in boiler or gas turbines. Many detrimental characteristics (such as high water content, high viscosity, poor ignition characteristics, corrosiveness or instability) have to be improved in order to make these tars comparable to fossil fuels. However, combustion tests have shown that biomass pyrolysis oils burn efficiently in standard or slightly modified engines with rates similar to those for commercial fuels [38]. Also, Schro¨der et al. [39] have investigated the use of pyrolysis oils for the production of electricity by diesel engines. However, the most recommendable is to try to favour the cracking of these tars, since they mean a very important obstacle in the refrigeration and cleaning system of the gas used to generate electric power. Apart from that, tar cracking could result in an increase of the energy content of the gas. The HHV related to these gases makes them suitable to be used to confer part of the heat needed for the subsequent activation processes (also to vaporize the water steam used in further activation of the char). Fig. 3 shows the molar concentration of the main molecules composing the gas phase: H2, CH4, CO and CO2. One can see that the maximum concentration is attained for all components in the range 3–10 min, depending on the molecule considered and the raw material. In general, the gas productions found for WS and OS are lower, which is in coherence with the lower values found for the gas yield in these residues. 3.3. Textural characteristics of chars As above mentioned, the pyrolysis processes were carried out at 600 8C during 1 h. We choose this temperature since, according to TG analyses, most volatile matter has been removed under these conditions. Although the optimal carbonization temperature depends on the parent material, we used the same pyrolysis conditions in order to compare the results obtained. RodriguezReinoso et al. [40] studied the influence of temperature on the

pyrolysis of almond shells, finding that the carbonization temperature did not seem to appreciably affect the porous structure of the carbonized material. However, the same research group did not find the similar trend when carbonizing olive stones [41]. Fig. 4 shows the N2 adsorption isotherms at 77 K (a) and the corresponding aS-plots (b) of the chars. The textural parameters determined by application of aS, DR and BET methods are given in Table 4. All these isotherms are essentially of type I, characteristic of microporous solids [42]. However, there are significant variations of the N2 volumes adsorbed and the exact shape of the isotherms, depending on the nature of the parent material. First, C-ATP and C-WS present a much more developed pore structure as it can be seen from the values of N2 adsorbed at 77 K, while the cases of C-AS and C-OS seem to correspond to a char in which the incipient porosity is not easily accessible to N2 at 77 K. Secondly, the pore size distribution was quite different depending on the char; the isotherm of C-ATP exhibited a high increase of N2 adsorbed volume at low P/P0, which corresponds to a high adsorption potential and is indicative of a narrow pore size distribution. On the other hand, samples C-WS, C-AS and C-OS exhibited a much slower approach to the plateau, indicating the presence of a broader pore size distribution. The aS-plots shown in Fig. 4b also show the previous decreasing sequence in micropore volumes and from the slopes of the linear adjustment it can be elucidated that the external areas of the chars are very low. The complementary use of N2 (77 K) and CO2 (273 K) isotherms is important for the characterization of activated carbons because: (i) it resolves the problems of activated diffusion effects associated with N2 adsorption at 77 K [43], (ii) it provides a characterization of micropore volumes of different sizes: narrow (CO2) and wider (N2). The values of micropore volume determined from CO2 adsorption data (Vmi(CO2)) have been included in Table 4. From the differences found in the micropore volumes determined by N2 and CO2 it follows that for the four materials the accessible porosity and the extent of adsorption depend on the adsorbate (and temperature), increasing with CO2. This confirms that the constrictions in these samples are of similar size to the dimensions of N2 or CO2 molecules. When the width of a constriction is very close to the diameter of the adsorbate molecule, the rate of entry of the adsorbate increases with increasing temperature [44]. These chars

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Fig. 4. N2 adsorption isotherms at 77 K (a) and a-plots (b) of the chars obtained.

Fig. 3. Molar concentration of gases during the pyrolysis of: walnut shell (a), almond tree pruning (b), almond shell (c) and olive stone (d).

and (Vmi(CO2)). This fact could be related to the rate of thermal degradation observed in these materials (see Figs. 1 and 2). For example, AS exhibits the highest initial degradation rate which could impede the removal of the volatile matter causing the robust distribution of the carbonaceous matrix, responsible for this restrictive access to N2 at 77 K. On the other hand, ATP presents a lower degradation initial rate favouring the progressive elimination of volatile matter which could result in a less constricted surface, more accessible to nitrogen adsorption. Mercury intrusion and pore size distribution curves are shown in Fig. 5a and b, respectively. The data obtained from Hg and He measurements have been collected in Table 5. It can be seen from Fig. 5a, that the Hg intrusion volumes found in these samples decrease in the sequence C-ATP > C-OS > C-AS > C-WS. Pore size distribution curves (Fig. 5b) exhibit only one strong maximum situated towards lower pore diameter values in the order CATP > C-OS > C-WS > C-AS. It has to be pointed out the great volume of macropores found in C-ATP, followed by C-OS in contrast with the higher mesopore volume in samples C-WS and especially C-AS (see the increase of the Hg intrusion volumes at lower pore diameters for these samples). This fact might indicate certain relation between the development of macroporosity with the lignin content (higher for OS and ATP) and the mesoporosity development with the hemicellulose + cellulose content (higher for WS and AS). Concerning total pore volume measurements, it decreases in the same sequence described for Hg intrusion curves (see Table 4). 3.4. Steam activation of chars

might have therefore constrictions and/or very narrow pores as a result of the carbonization process. However, this restricted accessibility of N2 is less in the case of C-ATP and C-WS, as indicated by the lower value of the difference between (Vmi(N2))

The degrees of burn-off attained after the activation of the chars under the conditions described before (850 8C, 30 min) were the following: 58.5, 52.7, 49.4 and 37.75 for S-ATP, S-AS, S-OS and S-

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Table 4 Textural parameters as determined from adsorption of N2 at 77 K and CO2 at 273 K of the chars.

C-WS C-ATP C-AS C-OS

SBET (m2 g1)

Vmi (cm3 g1)

Vme (cm3 g1)

SEXT (m2 g1)

%SINT

Vmi(CO2) (cm3 g1)

280 204 42 53

0.138 0.097 0.023 0.028

0.022 0.021 0.071 0.008

15 10 2 10

94.5 95.1 95.3 81.4

0.458 0.426 0.305 0.399

Table 5 Textural characteristics of chars as determined by Hg porosimetry and He density.

WS, respectively. The loss in the sample weight during activation is due to the combined effect of devolatilization and loss of fixed carbon resulting from the activation reaction which is water gas reaction: C + H2O ! CO + H2, with some presence of water gas shift (CO + H2O ! CO2 + H2) and Boudouard reaction (C + CO2 ! 2CO). In view of our results, towards steam, C-WS is the least reactive and C-ATP shows the highest value of burn-off, in spite of being the

least consumed during devolatilization. C-AS and C-OS present an intermediate trend. The N2 adsorption isotherms on the activated carbons and the corresponding aS-plots are shown in Fig. 6a and b, respectively. It can be seen that isotherms of carbons S-AS, S-WS and S-OS are of type I (thus, very microporous) [42], and S-ATP is more classifiable as type I with some contribution of type II, which corresponds to a microporous material with some mesoporosity. It has to be highlighted that although S-OS isotherm is essentially of type I; the slight increase of N2 volume observed at high relative pressures also indicates the presence of some mesoporous contribution. Concerning the as-plots, S-ATP presents a high slope, which indicates that this sample contains a great external surface. On the other hand, the shift in the start of the linear region of this curve to higher relative pressure indicates a broader pore size distribution. The textural characteristics determined from N2 isotherms have been collected in Table 6. It can be deduced that the raw material exerts a great influence on the porosity development found after activation under these conditions. The four samples differ

Fig. 5. Mercury intrusion (a) and pore size distribution (b) curves of chars.

Fig. 6. N2 adsorption isotherms at 77 K (a) and aS-plots (b) of activated carbons.

From Hg porosimetry

C-WS C-ATP C-AS C-OS

From He stereopicnometry

VmeP

VmaP

rHg

rHe

Vt2

0.191 0.135 0.256 0.115

0.488 1.370 0.540 0.934

0.665 0.478 0.595 0.523

1.523 1.631 1.795 1.588

0.848 1.480 1.124 1.282

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Table 6 Textural characteristics of the activated carbons as determined from adsorption of N2 at 77 K.

S-WS S-ATP S-AS S-OS

Table 7 Textural characteristics of the activated carbons as determined by Hg and He measurements.

SBET (m2 g1)

Vmi (cm3 g1)

Vme (cm3 g1)

SEXT (m2 g1)

%SINT

From Hg porosimetry

792 1080 601 813

0.442 0.568 0.341 0.455

0.082 0.382 0.034 0.100

35 277 10 53

95.61 74.40 98.41 93.48

VmeP (cm3 g1)

VmaP (cm3 g1)

rHg

rHe

(g cm3)

(g cm3)

Vt (cm3 g1)

0.223 0.288 0.353 0.184

1.009 3.035 1.278 0.918

0.467 0.243 0.396 0.509

2.044 2.289 2.038 2.118

1.654 4.555 2.035 1.491

significantly on their pore size distributions. On one extreme, we have carbon S-AS, which is highly microporous, with the highest %SINT value and an almost negligible external surface. On the other extreme, we can find S-ATP, which contains a large volume of mesopores and SEXT, as it can be seen from Table 6. S-WS and S-OS represent an intermediate situation. Regarding the lignocellulosic composition of the four materials, it can be noticed that, although the micropore and mesopores volumes do not seem to be related to these fractions, some relationship is found between them and the microporous structure of the carbons. Thus, the values of %SINT are higher as the material presents higher hemicellulose + cellulose content. Fig. 7 shows the mercury intrusion (a) and pore size distribution (b) curves of the activated carbons. Table 7 shows the values of mesopore and macropore volumes, mercury density, helium density and total pore volumes. It is noticeable the huge difference found in the macropore volume of S-ATP in comparison with the remaining samples. This is in agreement with the value of external surface area found for this sample (SEXT = 277 m2 g1) and confirms the wide pore size distribution of this carbon. The wide pore size distribution found in this material could be related to its slower

S-WS S-ATP S-AS S-OS

From He stereopicnometry

rate of thermal decomposition in the second temperature range. The mesopore volumes are rather similar for the four carbons, finding the highest one in the case of S-AS, which exhibits an outstanding wide peak in the mesopore range (located at a pore size of 13–20 nm, see Fig. 7b). S-AS is therefore the activated carbon exhibiting the narrowest pore size distribution, which could be associated with a high cellulose + hemicellulose content. 4. Conclusions Several biomass materials were subjected to pyrolysis process and the energetic exploitation of the phases generated was studied. Then the effectiveness of using the chars in the preparation and characterization of effective adsorbents was analysed. It was found that all these materials are a prospective starting material for the preparation of high quality activated carbons. However, the rate of thermal decomposition of the parent material (which is related to its lignocellulosic composition) plays a determinant role on the porosity of the activated carbon produced. In the particular case of the precursors described here, it was found that materials decomposing slowly at the first stages of pyrolysis are likely to yield activated carbons with a broader pore size distribution (greater volumes of meso and macropores), as it was the case of almond tree pruning. Thus, the obtention activated carbons with high values of external areas could be related to significant lignin fractions. On the other hand, the materials showing a high initial thermal decomposition rate (induced by high hemicellulose fractions, as in almond shell) could be more prone to yield highly constricted chars and very microporous carbons. The knowledge of the porous structure of activated carbons when subjected to a common activation process could be very useful in industrial applications. In this work, ACs from almond tree pruning will be more appropriate for applications in adsorption from solution (the large size pores are needed for faster transport of the adsorptive to the microporosity), and ACs from the other residues would be appropriate for gas adsorption applications. Acknowledgments The authors express their gratitude to the ‘‘Junta de Extremadura-Consejerı´a de Economı´a, Comercio e Innovacio´n’’ for the financial support through projects 2PRO4B016/PRI07A088 and 2PR01A034. S. Roma´n and G. Martı´nez thank the ‘‘Junta de Extremadura’’ for their research grants. References

Fig. 7. Mercury intrusion (a) and pore size distribution (b) curves of the activated carbons.

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