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Pyrolytic production of microporous charcoals from different wood resources
Journal of Analytical and Applied Pyrolysis 98 (2012) 15–21
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Pyrolytic production of microporous charcoals from different wood resources Aleksandra Cyganiuk, Olga Gorska, Andrzej Olejniczak, Jerzy P. Lukaszewicz ∗ Nicholas Copernicus University, Faculty of Chemistry, ul. Gagarina 11, 87-100 Torun, Poland
a r t i c l e
i n f o
Article history: Received 22 August 2011 Accepted 20 June 2012 Available online 1 July 2012 Keywords: Biomass pyrolysis Salix viminalis Wood chars Porous structure
a b s t r a c t A series of porous chars has been obtained by heat treatment of unconventional raw materials, including plants belonging to short rotation woody crops (Salix viminalis, Salix fragilis). The pyrolysis conditions (1–3 h, 600–900 ◦ C) were the same for the production of all chars, e.g., mesoporous and microporous chars. Salix viminalis wood exhibited an advantage over the other materials, because the obtained material had microporous structure such as carbon molecular sieves. Similar properties (surface area, total pore volume, pore size distribution) were observed for charcoals produced from pine wood (Pinus silvestri), but the thermal stability of these properties was inferior. Furthermore, we have also discussed economical and environmental issues associated with the exploitation of wood resources. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Active carbon is a kind of material that has been commonly applied in various processes for thousands of years [1]. There has been a growing interest in active carbon obtained via pyrolysis of natural materials for domestic, medical, and above all, industrial applications, despite the discoveries of various new carbon-based materials (fullerenes, carbon nanotubes, etc.). The pyrolysis of cellulose-rich materials is probably the most traditional and oldest way to obtain active carbon. Generally, different varieties of wood serve as raw materials. However, the latest trends in environmental protection are focused on preserving natural resources like trees and forests. This has made the all wood-based active carbon production technologies more and more controversial. Therefore, the current study proposes to exploit the renewable hard wood resources, i.e., dried wood of Salix viminalis [2]. Salix viminalis belongs to the group of short rotation woody crops (SRWCs) [3,4], which are agriculturally grown and harvested as a “green” fuel. Salix viminalis offers high yield of up to 25 tons/hectare/year which is achievable with a moderate care and chemical protection [5]. Low cost and easy renewal makes Salix viminalis a very attractive raw material for the production of active carbon, provided that the obtained charcoals exhibit acceptable surface properties, like high surface area and well defined and stable pore structure. The pioneering studies [2] proved that standard pyrolysis of this type of wood (Salix viminalis-originated) can yield a strictly nanoporous charcoal. Owing to the extremely narrow pore size distribution, such active charcoals act as nearly perfect carbon
∗ Corresponding author. Fax: +48 56 6544477. E-mail address: [email protected] (J.P. Lukaszewicz). 0165-2370/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jaap.2012.06.008
molecular sieve (CMS). The applications of CMS range from gas separation [6,7] to gas storage (including hydrogen) [8]. Therefore, the search for inexpensive and “green”, i.e., environment friendly production of active charcoal (including CMS) is still of interest among industrial technologists as well as academics. It seems necessary to prove whether the unique pore structure of Salix viminalis-originated CMS is an exclusive property of the raw material applied for its production. Therefore, in the current study, we attempted to compare the pore structure of a series of charcoals obtained using an oxygen-free heat treatment of unconventional raw materials (mostly wood). In general, more conventional types of wood (oak, bamboo, pine, etc.) and/or other raw materials (coconut shells, fruit stones, old tires, etc.) were widely tested for the production of active charcoals. Such charcoals present a wide spectrum of properties with regard to the pore structure of s produced in this way charcoals. For example, some authors claim that a different pore structure can be obtained for the active charcoals obtained from the same raw material like pine wood [9,10]. The differences mainly result from the alterations in the pyrolysis procedure and the kind of chemical treatment of wood prior to pyrolysis and/or after pyrolysis. More spectacular differences in pore structure of active charcoals from the same raw materials can be observed, especially if additional activation of the ready charcoals was carried out. 2. Materials and methods 2.1. Charcoal production Each raw material was selected basing on the ease of its collection, resulting either from its frequent occurrence or from being a kind of waste in food processing. First, all the dried raw
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materials (nine types of cellulose-rich materials including seven types of wood) were preliminary pyrolysed at the temperature of 600 ◦ C for 1 h. The load of 10 g of raw wood was used in the first carbonization. Pieces of the raw material were placed in a tubular furnace. The temperature of the stove was increased by 10 ◦ C/min until the final temperature was achieved. A constant flow of nitrogen (99.99%) was maintained during the whole procedure: heating up to the desired temperature, 1 h long pyrolysis at constant temperature, and cooling down to room temperature. Subsequently, all the samples were subjected to a secondary heat treatment at one of the temperatures of 600, 700, or 800 ◦ C for 1 h. Several samples were heated for an extended time period of 3 h (as presented in Tables 1 and 2). In such a case, one can investigate the influence of more severe conditions of pyrolysis (extended time and elevated temperature) on the pore structure of the investigated active charcoals. Some Salix viminalis-originated charcoals were obtained in a one-step manner. It consisted of heat treatment of dried wood for 1 h at the temperatures of 600, 700, 800, and 900 ◦ C (samples denoted as SV61, SV71, SV81, SV91). In contrast to other samples, the 1 h heat treatment was not followed by a secondary pyrolysis. All the investigated samples are detailed in Table 1.
Table 1 Samples history and symbols. Raw material
Elder (wood) Sambucus nigra Common pine (wood) Pinus silvestri Norway maple (wood) Acer platanoides Brittle willow (wood) Salix fragilis Basket willow (wood) Salix viminalis
Common brich (wood) Betula pendula
2.2. Pore-structure characteristics The pore structure and specific surface area were determined using a widely accepted method, exploiting the phenomenon of low-temperature adsorption of the chemically neutral gases [11,12]. Nitrogen served as an inert adsorptive, and nitrogen adsorption isotherms were recorded at the temperature of liquid nitrogen (−196 ◦ C) using automatic sorption apparatus Micromeritics ASAP 2010. The standard software provided by the manufacturer of ASAP 2010 was employed for the regression of primary adsorption data obtained (nitrogen adsorption vs. relative partial pressure of the adsorptive). Two regression models were applied: BET [13] for the calculation of specific surface area of charcoals and H–K method (Horvath–Kawazoe (H–K) [14]) for the calculation of pore-size distribution function PSD [15], which characterizes the pore structure of the solids. No changes were introduced to the standard calculation procedures offered by the commercial software [16]. Prior to measurements, all the tested
Plum tree (fruit stone) Prunus domestica Pistachio tree (fruit shells) Pistacja vera Black poplar (wood) Populus nigra
Abbreviation Unmodified materials
Modified materials
BC6161, BC6171, BC6181 SZ6161, SZ6171, SZ6181 KZ6161, KZ6171, KZ6181 WK6161, WK6171, WK6181 SV61, SV71, SV81, SV91 SV6161, SV6171, SV6181, SV6163, SV6173, SV6183 BB6161, BB6171, BB6181 PS6161, PS6171, PS6181 LP6161, LP6171, LP6181
–
DSZ
–
–
SV6Zn38 DSV
DLP6Zn38
TC6161, TC6171, TC6181
charcoal samples were degassed in vacuum (10−3 Torr) at an elevated temperature (250 ◦ C) for extended time (3 h) before nitrogen adsorption measurements. 2.3. Other tests The samples’ morphology was investigated using an electron microscope (LEO 1430 VP, Electron Microscopy Ltd.) supplied with
Table 2 Specific surface area of investigated samples. Calculations made basing on BET model. Raw material
Specific surface area SBET (m2 /g) Unmodified materials
Elder (wood) Sambucus nigra Common pine (wood) Pinus silvestri Norway maple (wood) Acer platanoides Brittle willow (wood) Salix fragilis Basket willow (wood) Salix viminalis
Common brich (wood) Betula pendula Plum tree (fruit stone) Prunus domestica Pistachio tree (fruit shells) Pistacja vera Black poplar (wood) Populus nigra
BC6161 5.3 ± 0.1 SZ6161 257.5 ± 9.9 KZ6161 144.8 ± 4.9 WK6161 234.2 ± 7.4 SV61 286.9 ± 10.1 SV6161 275.8 ± 10.7 SV6163 358.2 ± 10.9 BB6161 229.1 ± 8.6 PS6161 137.1 ± 5.5 LP6161 217.7 ± 8.4 TC6161 94.0 ± 3.3
BC6171 5.0 ± 0.3 SZ6171 317.5 ± 12.4 KZ6171 125.7 ± 4.0 WK6171 239.5 ± 8.5 SV71 336.6 ± 10.8 SV6171 296.7 ± 10.5 SV6173 311.7 ± 9.5 BB6171 251.8 ± 8.9 PS6171 51.1 ± 2.5 LP6171 187.4 ± 7.3 TC6171 5.0 ± 0.3
BC6181 1.7 ± 0.1 SZ6181 183.4 ± 6.8 KZ6181 63.7 ± 1.9 WK6181 8.1 ± 0.1 SV81 321.9 ± 10.2 SV6181 94.7 ± 2.8 SV6183 64.4 ± 2.1 BB6181 73.3 ± 2.8 PS6181 2.1 ± 0.1 LP6181 42.5 ± 1.8 TC6181 4.7 ± 0.1
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two detectors for secondary electrons (SEs), backscattered electrons (BSE), and X-ray elemental analyzer (EDX) (Quantax 200 – XFlash 4010, Bruker). Such experimental setup led to the determination of the elemental composition of specific areas and details visible on the scanning electron images. Thermal analysis DSC/DTA/DTG/TG (heating rate 10 ◦ C/min, N2 atmosphere, 20–25 mg sample weight) was performed by means of TGA-DTA Thermal Analysis SDT 2960 instrument to determine the temperature stability of the obtained charcoals.
3. Results and discussion 3.1. Nitrogen adsorption at −196 ◦ C Low-temperature nitrogen adsorption data led to the determination of specific surface area of the investigated charcoal samples. The values differed very much from one another (see Table 2) with respect to the type of raw material applied for the production. Additionally, considerable alterations were observed for charcoals obtained from the same raw material, but at different temperatures of pyrolysis. In general, more severe pyrolysis conditions (800–900 ◦ C) were found to initiate a collapse of carbon network leading to a sharp decrease in the specific surface area of nearly all of the samples treated in this way. This is a result of graphite planes/crystallites growth at high temperature which leads to vanishing of pores. Pore is charcoals obtained by wood pyrolysis are in fact slit like vacancies between randomly oriented graphite planes and crystallites. In general, the charcoal samples obtained at less severe conditions (600–700 ◦ C) from the same type of wood exhibited more developed surface area. For some raw materials (dried wood), a slight increase in the BET surface area was observed, with an increase in the pyrolysis temperature from 600 to 700 ◦ C (Salix viminalis, Salix fragilis, Pinus silvestri, Betula pendula), which was followed by the mentioned above structural collapse. The charcoals obtained from Pinus silvestri were observed to be the most resistible to temperature-caused collapse of pore structure and BET surface area (SBET ). There was no dramatic decrease in SBET between charcoals denoted as SZ6171, SZ6181. Such a decrease was visible in the case of other charcoals (when compared to most pairs of samples denoted as “71” and “81” in Table 2), including Salix viminalis-originated samples. As mentioned earlier, charcoals other than Pinus silvestri-originated charcoals were not very resistible to the rise in the temperature of pyrolysis. A dramatic structural collapse was visible upon increase in pyrolysis temperature from 700 to 800 ◦ C, in particular, among charcoals obtained from fruit stones of Prunus domestica, Salix fragilis, B. pendula, Acer plataroides, and Populus nigra. The charcoals obtained from Sambucus nigra (samples BC61, BC71, BC81) did not exhibit any significant level of specific surface area (from 1.6 to 5.2 m2 /g), which dramatically differed from other results ranging from 94 to 286 m2 /g. In addition, it was possible for Salix viminalis-originated charcoals to avoid the collapse, provided that the primary pyrolysis was not followed by a secondary heat treatment (samples SV61, SV71, and SV81). For such charcoals (one-step production), a temperature of 900 ◦ C was necessary to initiate the collapse (charcoal denoted as SV91) after 1 h long pyrolysis. The collapse was also observed at 800 ◦ C, when the time of pyrolysis was long enough, i.e., 3 h instead of 1 h. In general, the extension of pyrolysis time from 1 h to 3 h caused a decrease in SBET even for one of the most resistible type of wood: observed when comparing Salix viminalis-originated pairs of samples SV61 and SV63, SV71 and SV73, SV81 and SV83 (one-step production). Even within the same genus of plants serving later as raw materials, such as Salix family (Salix viminalis and Salix fragilis), there was only a partial similarity with respect to the SBET values and their
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change upon increasing pyrolysis temperature. Both types of wood yielded charcoals of relatively high surface area, but Salix fragilisoriginated charcoals underwent much more dramatic collapse of charcoal matrix at the temperature of pyrolysis of 800 ◦ C. In general, PSD determination is a complex matter which results are very sensitive to the kind of adsorptive, i.e., gas being physically adsorbed by the solid under investigation. Adsorption data regression model plays a crucial role too. Usually Ar, N2 and CO2 are applied for this purpose. According to some studies [17,18] the usefulness of N2 is limited due to its slow diffusion at −196 ◦ C into micropores which linear diameter is below 2 nm. Physical adsorption of CO2 (at 0 ◦ C or 25 ◦ C) was proposed as new standard procedure for PSD determination regardless of some specific properties of the gas: - CO2 is not chemically inert (evident acidic properties and supposed specific reaction with base centers on the surface), - CO2 it not a dipole but high negative charge is cumulated on both O atoms (specific molecular interactions cannot be excluded), - CO2 has a specific longitudinal shape (“long” 3-atom linear molecule may trigger orientation issues). However, some research suggested inapplicability of N2 preferring CO2 in contrast to other studies proving comparativeness of Ar, N2 and CO2 results regarding PSD [19]. Latest announcements describing theoretical simulation of Ar, N2 and CO2 adsorption [20,21] in a model pore structure proved particular usefulness of Ar as a probe gas. N2 adsorption led to PSDs only slightly differing from Ar-based PSDs. According to the studies, applicability of CO2 seems to be rather limited in the case of charcoal surface with implanted surface functional groups since CO2 -based PSDs suggested the presence of non-existing pores. Thus, despite some objections, N2 adsorption at −196 ◦ C may still serve as a standard experiment leading to a reliable qualitative information on the run of PSD curves for real charcoals. In the light of the statements above, the obtained Salix viminalis-originated charcoals posses molecular sieve-type pore structure dominated by ultra fine pores which size is placed in sub-nanometer range. The described above observations can be supported using the nitrogen adsorption isotherms and PSD functions calculated using H–K method. The most characteristic example curves are depicted in Figs. 1a, 2a, and 3a. Almost all the recorded isotherms were I-type isotherms, according to the IUPAC classification [22]. Furthermore, the inert nature of nitrogen demonstrated the existence of micropores in the carbon matrix. A slight tendency toward adsorption–desorption hysteresis was observed among the charcoal samples obtained from A. plataroides, B. pendula, and fruit stones of Prunus domestica and Sambucus nigra. In the case of Salix viminalis-originated charcoals (Fig. 1, samples SV61, SV71, SV81) as well as Salix fragilis-originated charcoals (Fig. 2, samples WK6161, WK6171, WK6181), the hysteresis was not observed. In general, the hysteresis is a sign indicating that wider pores (mainly mesopores) can be present in the carbon matrix beside very fine pores (micropores). Such a supposition was confirmed by the determination of pore-size distribution function (adsorption data regressed by means of H-R model, software offered by ASAP 2010 manufacturer). Figs. 1b, 2b, and 3b present the mentioned above functions for Salix viminalis-, Salix fragilis-, and B. pendula-originated charcoals. It can be observed that only for Salix family-originated charcoals (SV61, SV71, SV81 samples and SV6161, SV6171, SV6181 samples, and WK6161, WK6171 samples) and for Pinus silvestri-originated charcoals, the PSD functions (Fig. 4b) are very narrow, i.e., linear dimensions of pores are placed within the range of 0.8–1.0 nm. The PSD functions presented in Figs. 4 and 5, and those corresponding to SZ and KZ charcoals are much more complex. The run of the latter PSD functions is not discrete, i.e., the pores of various
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Fig. 1. Nitrogen adsorption isotherms (a) on charcoals obtained from Salix viminalis wood (SV samples) and corresponding pore size distribution functions (b).
Fig. 2. Nitrogen adsorption isotherms (a) on charcoals obtained from Salix fragilis wood (WK samples) and corresponding pore size distribution functions (b).
Fig. 3. Nitrogen adsorption isotherms (a) on charcoals obtained from Betula pendula wood (BB samples) and corresponding pore size distribution functions (b).
Fig. 4. Nitrogen adsorption isotherms (a) on charcoals obtained from Pinus silvestri wood (SZ samples) and corresponding pore size distribution functions (b).
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Fig. 6. SEM image of Salix vimianlis originated charcoal surface (SV61 sample).
Fig. 5. Nitrogen adsorption isotherms (a) on charcoals obtained from Acer platanoides wood (KZ samples) and corresponding pore size distribution functions (b).
linear dimensions (from 0.8 to 2.0 nm) considerably contribute to the total pore volume. Furthermore, the PSD curves for the KZ samples (Fig. 5) can be even considered as a proof of bimodal pore size characteristic. Interesting evolution of surface parameters (PSD function and SBET ) was observed for charcoals obtained from Pinus silvestri (Fig. 4). The SBET values for such charcoals was relatively high, i.e., SZ6161 = 257 m2 /g, SZ6171 = 317 m2 /g, and SZ6181 = 183 m2 /g. The changes corresponded to the general trend described earlier which comprised the increase in the SBET values on rise in the pyrolysis temperature from 600 to 700 ◦ C, and the subsequent decrease in the SBET values when the pyrolysis temperature approached 800 ◦ C. The PSD function (Fig. 4b) for 600 ◦ C-obtained charcoal SZ6161 was very narrow and resembled the narrow PSD function for the corresponding Salix viminalis-originated charcoal (SV61 and SV6161 samples). For the Salix viminalis-originated charcoals, the determined PSD functions remain very narrow, despite the increasing pyrolysis temperature (SV71, SV6171, SV81, SV6181 samples). Furthermore, a partly similar phenomenon was observed for Pinus silvestri-originated charcoals (SZ6161, SZ6171, SZ6181 samples), but with a gradual shift of the PDS maximum from micropore range (subnanometer range) toward the beginning of the mesopore range (ca. 1.8 nm). At the same time, a visible broadening of the main peak in PSD function was also observed. Thus, Pinus silvestri wood as a raw material was found to generate microporous charcoals (similar to Salix viminalis-originated charcoals); however, this unique (MSC-type) pore structure was less thermally stable than that of Salix viminalis-originated charcoals. In general, Salix vimivalis seems to be an outstanding raw material among all the investigated samples, because the produced charcoals obtained from it instantly presented several positive features as follows:
- Oxygen-free pyrolysis of random pieces of Salix viminalis wood generated strictly monoporous charcoal whose structure resembled a perfect molecular sieve (extremely narrow PSD function). - Stable molecular sieve-type pore structure of Salix viminalisoriginated charcoals was thermally stable, i.e., it was able to resist extended pyrolysis time at elevated temperatures. - The dominating pore size corresponding to the maximum at the narrow PSD function (SV samples) was in the subnanometer range and moderately dependent on the extended pyrolysis time and elevation in the production temperature. Apart from the positive physicochemical features mentioned earlier, economical and environmental issues favor the use of Salix viminalis as a raw material for active charcoal application. The growth of Salix viminalis plants is very fast in agriculture plantations and nonagricultural areas like side spaces of highways and railway lines. Salix viminalis is a renewable resource providing unlimited amounts of hard wood which could not be obtained from any other investigated raw material. Its production may easily fulfill the expectations of charcoal manufacturers, apart form the increasing demand from energetic industry. Application of Pinus silvestri for active charcoal production can be considered as a competitive production method of charcoals exhibiting pore structure resembling CMS. However, the growth of Pinus silvestri is very slow, i.e., 40–60 years are necessary to exploit the forests of Pinus silvestri. The long period of growth of these trees is associated with the parallel gradual formation of a local ecosystem in the pine woods involving innumerable types of plants and animals. Cutting pine woods not only causes damage to the particular trees, but also results in the collapse of the whole local ecosystem. On the contrary, Salix viminalis plantations (3 years from implantation to the first harvest) and its short cycle (cutting each year) resemble farming of typical agriculture plants like corn, beats or potatoes. Thus, similar surface properties of SV and SZ charcoals can be easily obtained for Salix viminalis-originated charcoal from economical and ecological point of view. 3.2. Morphology and elemental composition Figs. 6 and 7 present a series of SEM images recorded for selected samples of investigated charcoals. Some characteristic details can be observed for wood-originated charcoals (Fig. 6, sample SV61). Above all, it is a macro structure derived from the original tissue structure in the plant which has been transformed into carbon matrix upon heat treatment. This can be regarded as a kind of plant
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Table 3 Weight content of selected elements in carbon obtained by two step carbonization (primary: 600 ◦ C, 1 h and secondary: 600 ◦ C, 1 h). Method: SEM–EDX (LEO 1430VP + EDX XFlash 4010, magnification 500×). Raw material
Symbol
Betula pendula
BB
Sambucus nigra
BC
Salix viminalis
SV
Salix fragilis
WK
Populus nigra
TC
Pinus silvestri
SZ
Acer platanoides
KZ
Pistacja vera (fruit stones) Prunus domestica (nut shells)
PS LP
Weight content [%]
C IC C IC C IC C IC C IC C IC C IC C IC C IC
C
O
Mg
Na
P
K
Ca
84.8 27.3 79.5 54.3 81.4 45.7 81.9 33.8 80.5 44.7 85.6 81.7 84.8 71.2 86.3 35.8 79.0 44.4
14.6 42.6 18.9 29.8 15.8 35.4 16.7 21.7 16.9 35.2 13.5 17.2 14.0 19.8 13.2 38.7 17.4 34.7
– 0.5 0.5 0.4 – 0.2 0.2 0.1 0.3 0.3 0.1 0.3 0.1 0.1 – – – 0.4
– – – – – – – – – – – – – – – – 0.7 4.1
– 0.7 0.2 – 0.5 0.3 0.2 0.3 0.4 0.3 – – 0.1 0.1 – – – –
0.7 1.1 0.9 0.5 1.6 1.1 1.0 1.7 1.6 1.2 0.3 0.2 0.5 0.5 0.5 0.4 1.9 1.2
– 27.8 – 14.5 0.7 17.4 – 42.6 0.4 18.0 0.5 0.5 0.4 8.2 – 24.4 – 3.7
Residuals: Al, Cr, Fe, Cl, S, Si
IC data represent the elemental composition of inorganic additives; C data correspond to the elemental composition of the basic carbon-based matrix. C, carbon matrix; IC, inorganic crystallite.
“fingerprint.” Such a “fingerprint” looks different for the charcoals obtained from non-wood raw materials like Pistacja vera (LP charcoals) fruit shells and Prunus domestica (PS charcoals) fruit stones (Fig. 7). Thus, the structure and shape of tissues in fruit shells and stones are, in general, very different from those transporting water and minerals along the main stem and branches of a plant. Hence, the morphology of PS and LP samples differ much from that of the remaining charcoals that are wood-originated charcoals (BC, SZ, KZ, WK, SV, BB, TC samples). SEM images were recorded using two types of detectors: SE and BSE. The BSE images demonstrated that some details in the secondary electron images (SE) have a different chemical composition, i.e., contain elements of atomic number higher than 12 (carbon) or 16 (oxygen). Typically, the details (white spots) resemble small cubic microcrystals (Fig. 6). The electron microscope was equipped with an EDX probe that could determine the elemental composition of some spots selected on the surface of the investigated charcoal samples. For each sample, two areas were selected based on the BSE images: “white” cubic microcrystals and “black”
Fig. 7. SEM image of Pistacja vera (fruit shells, LP6161 sample) originated charcoal surface. White crystallites are Ca-based residuals (total Ca weight content below 0.1%) of mineral substances in living plants.
areas. We assumed that “black” spots were parts of carbon matrix which developed from the organic matter forming cell walls in raw materials applied for charcoal production. The “white” decorations were considered to be a residual of inorganic components that were also present in raw materials. The data on elemental composition obtained from EDX measurements are presented in Table 3 which confirmed the mentioned above hypothesis of the origin of “white” (rows denoted as “IC”) and “black” spots (rows denoted as “C”). The elemental composition of “black” spots is typical for charcoals obtained by pyrolysis of organic matter like wood, i.e., consisting mainly of carbon and oxygen [23]. The “white” crystallite-looking details can be considered as spots where calcium derivatives are concentrated. With regard to the history of the investigated samples, it is reasonable to consider that CaCO3 is the main component of “white” spots. Minor amounts of other elements (magnesium, potassium, phosphorus) could also be observed in both the “black” and the “white” spots. The overall concentration of calcium concentration in the charcoal samples was as low as 0.1% (weight), which was obtained from the EDX measurements covering large surface areas, with EDX analysis field covering the black and white spots in proportions typical for a particular charcoal sample. The low content of elements other than carbon and oxygen has no influence on the mechanism of N2 adsorption at −196 ◦ C.
Fig. 8. TG curves recorded the samples SV61, SV71 and SV81.
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3.3. Thermal investigations Fig. 8 presents the weight-loss curves with the increasing temperature of heat treatment (TG curves) for a series of Salix viminalis-originated charcoals (SV61, SV71 and SV81 samples). It can be observed that SV charcoals become unstable above 600 ◦ C, exhibiting accelerated weight loss. The highest weight loss was observed for the less thermally elaborated charcoal SV61, i.e., charcoal obtained after 1-h long pyrolysis at 600 ◦ C. If the pyrolysis temperature was higher (700 ◦ C/800 ◦ C), the mass decrease above 600 ◦ C during TG analysis was lower. However, it is prove that some changes still proceed like graphitization. Such changes may be expected during the secondary heat treatment since SV61, SV71, SV81 sample were not subjected to the secondary treatment and in the performed TG experiment conditions analogous to the secondary treatment were simulated. Similar observations were also observed for most of the investigated charcoals. The observations can be considered as a sign of charcoal instability at elevated temperatures, whose effect corresponds to the above-described collapse of carbon matrix resulting in the lowering of BET surface area. The high mass loss for SV61 charcoal proved that a subsequent heating of such charcoal up to 700 ◦ C or 800 ◦ C could cause some changes, like graphitization [24,25] and evolution of gas products [26]. The effect was more intensive than for SV71 and SV81 samples. For example, SV81 sample underwent only the first heat treatment at elevated temperature (up to 800 ◦ C) before TG measurements, which was associated with intensive release of some volatile products. Thus, the subsequent heating of SV81 sample during the TG-curve determination caused less striking weight loss than that observed in SV61 or SV71 samples. 4. Conclusions In general, the unique molecular sieve-type pore structure could be achieved by careful pyrolysis of several sorts of lignin/celluloserich raw materials (mostly dried wood). The mentioned above CMS-type structure within the carbon matrix exists in a very fragile state, which easily undergoes a thermally induced collapse. Thus, the CMS-type structure can be considered as an intermediate state existing until an intensive formation of graphite sheets which begins in elevated temperatures. In addition, extended time of pyrolysis accelerates the collapse of the pore structure owing to graphitization. Furthermore, the CMS-type structure easily vanishes upon increased pyrolysis time and temperature in most of the investigated charcoals. In the case of Salix viminalis-originated charcoals (SV samples), the mentioned above discrete pore structure was well developed and stable. This property demonstrates the benefits of using Salix viminalis wood as a raw material for active charcoal production. In addition, this conclusion is supported by the economical and environmental aspects, because Salix viminalis is easy to grow at renewable plantations. References [1] A. Dabrowski, Adsorption from theory to practice, Advances in Colloid and Interface Science 93 (2001) 135–224.
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[2] J.P. Lukaszewicz, A. Arcimowicz, B. Klemp-Dyczek, Sposób wytwarzania ˛ sit molekularnych, Polish Patent P 380164 (2006). weglowych [3] L. Christersson, L. Sennerby-Forsse, The Swedish program for intensive short rotation forests, Biomass and Bioenergy 6 (1994) 141–144. [4] P. Borjesson, L. Gustavsson, L. Christersson, S. Linder, Future production and utilisation of biomass in Sweden: potentials and CO2 mitigation, Biomass and Bioenergy 13 (1997) 399–412. [5] M. Labrecque, T.I. Teodorescu, S. Daigle, Biomass productivity and wood energy of Salix species after 2 years growth in SRIC fertilized with wastewater sludge, Biomass and Bioenergy 12 (1997) 409–417. [6] A.I. Shirley, N.O. Lemcoff, Air separation by carbon molecular sieves, Adsorption 8 (2001) 147–155. [7] I. Mochida, S. Yatsunami, Y. Kawabuchi, Y. Nkayama, Influence of heattreatment on the selective adsorption of CO2 in a model natural gas over molecular sieve carbons, Carbon 33 (1995) 1611–1619. [8] F. Rodriquez-Reinoso, Combined and hybrid adsorbents, in: J.M. Loureiro, M.T. Kartel (Eds.), NATO Security Through Science Series, Springer, Dordrecht, 2006, pp. 133–144. [9] K. Laszlo, G. Dobos, G. Onyestyak, E. Geissler, Influence of silicon doping on the nanomorphology and surface chemistry of a wood-based carbon molecular sieve, Microporous and Mesoporous Materials 100 (2007) 103–110. [10] F. Márquez-Montesinos, T. Cordero, J. Rodríguez-Mirasol, J.J. Rodríguez, Powdered activated carbons from Pinus caribaea sawdust, Separation Science and Technology 36 (2001) 3191–3206. [11] Standards DIN 66135-1, DIN 66135-2, DIN 66135-3, DIN 66135-4, Mikroporenanalyse mittels Gasadsorption, Deutsches Institut fuer Normung, Berlin, 2001. [12] Standard DIN 66134, Bestimmung der Porengrößenverteilung und der spezifischen Oberfläche mesoporöser Feststoffe durch Stickstoffsorption; Verfahren nach Barrett, Joyner und Halenda (BJH), Deutsches Institut fuer Normung, Berlin, 1998. [13] Standard ISO 9277, Determination of the Specific Surface Area of Solids by Gas Adsorption Using the BET Method, International Organization for Standardization, Geneva, 1995. [14] G. Horvath, K.J. Kawazoe, Method for the calculation of effective pore size distribution in molecular sieve carbon, Journal of Chemical Engineering of Japan 16 (1983) 470–475. [15] N.A. Seaton, J.R.B. Walton, N. Quirke, A new analysis method for the determination of the pore size distribution of porous carbons from nitrogen adsorption measurements, Carbon 27 (1989) 853–861. [16] Operator’s Manual, Micromeritics ASAP 2010 Accelerated Surface Area and Porosimetry System, Micromeritics Instrument Corporation, Norcross, 1995. [17] J. Garrido, A. Linares-Solano, J. Martin-Martinez, M. Molina-Sabio, R. Rodriguez-Reinoso, R. Torregrosa, Use of nitrogen vs. carbon dioxide in the characterization of activated carbons, Langmuir 3 (1987) 76–81. [18] D. Lozano-Castello, D. Cazorla-Amoros, A. Linares-Solano, Usefulness of CO2 adsorption at 273 K for the characterization of porous carbons, Carbon 42 (2004) 1231–1236. [19] J. Jagiello, M. Thommes, Comparison of DFT characterization methods based on N2 , Ar, CO2 , and H2 adsorption applied to carbons with various pore size distributions, Carbon 42 (2004) 1225–1229. [20] S. Furmaniak, A.P. Terzyk, P.A. Gauden, P.J.F. Harris, P. Kowalczyk, Can carbon surface oxidation shift the pore size distribution curve calculated from Ar, N2 and CO2 adsorption isotherms? Simulation results for a realistic carbon model, Journal of Physics: Condensed Matter 21 (2009) 15005–15014. [21] S. Furmaniak, A.P. Terzyk, P.A. Gauden, P.J.F. Harris, P.P. Kowalczyk, Do all carbonized charcoals have the same chemical structure? 1. Implications of thermogravimetry-mass spectrometry measurements, Journal of Physics: Condensed Matter 22 (2010) 85003–85012. [22] K.S. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, P.A. Pierotti, J. Rouquerol, T. Siemieniewska, Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity (IUPAC Recommendations), Pure and Applied Chemistry 57 (1985) 603–619. [23] J.A. Fuwape, Charcoal and fuel value of agroforestry tree crops, Agroforestry Systems 22 (1993) 175–179. [24] R.E. Franklin, Crystallite growth in graphitizing and non-graphitizing carbons, Proceedings of the Royal Society of London: A A209 (1951) 196–218. [25] J.G. Zhao, L.X. Yang, F.Y. Li, R.C. Yu, C.Q. Jin, Electrical property evolution in the graphitization process of activated carbon by high-pressure sintering, Solid State Sciences 10 (2008) 1947–1950. [26] M. Meszaros, E. Jakab, G. Varhegyi, J. Bourke, M. Manley-Harris, T. Nunoura, M.J. Antal Jr., Do all carbonized charcoals have the same chemical structure? 1. Implications of thermogravimetry-mass spectrometry measurements, Industrial and Engineering Chemistry Research 46 (2007) 5943–5953.
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Journal of Analytical and Applied Pyrolysis journal homepage: www.elsevier.com/locate/jaap
Pyrolytic production of microporous charcoals from different wood resources Aleksandra Cyganiuk, Olga Gorska, Andrzej Olejniczak, Jerzy P. Lukaszewicz ∗ Nicholas Copernicus University, Faculty of Chemistry, ul. Gagarina 11, 87-100 Torun, Poland
a r t i c l e
i n f o
Article history: Received 22 August 2011 Accepted 20 June 2012 Available online 1 July 2012 Keywords: Biomass pyrolysis Salix viminalis Wood chars Porous structure
a b s t r a c t A series of porous chars has been obtained by heat treatment of unconventional raw materials, including plants belonging to short rotation woody crops (Salix viminalis, Salix fragilis). The pyrolysis conditions (1–3 h, 600–900 ◦ C) were the same for the production of all chars, e.g., mesoporous and microporous chars. Salix viminalis wood exhibited an advantage over the other materials, because the obtained material had microporous structure such as carbon molecular sieves. Similar properties (surface area, total pore volume, pore size distribution) were observed for charcoals produced from pine wood (Pinus silvestri), but the thermal stability of these properties was inferior. Furthermore, we have also discussed economical and environmental issues associated with the exploitation of wood resources. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Active carbon is a kind of material that has been commonly applied in various processes for thousands of years [1]. There has been a growing interest in active carbon obtained via pyrolysis of natural materials for domestic, medical, and above all, industrial applications, despite the discoveries of various new carbon-based materials (fullerenes, carbon nanotubes, etc.). The pyrolysis of cellulose-rich materials is probably the most traditional and oldest way to obtain active carbon. Generally, different varieties of wood serve as raw materials. However, the latest trends in environmental protection are focused on preserving natural resources like trees and forests. This has made the all wood-based active carbon production technologies more and more controversial. Therefore, the current study proposes to exploit the renewable hard wood resources, i.e., dried wood of Salix viminalis [2]. Salix viminalis belongs to the group of short rotation woody crops (SRWCs) [3,4], which are agriculturally grown and harvested as a “green” fuel. Salix viminalis offers high yield of up to 25 tons/hectare/year which is achievable with a moderate care and chemical protection [5]. Low cost and easy renewal makes Salix viminalis a very attractive raw material for the production of active carbon, provided that the obtained charcoals exhibit acceptable surface properties, like high surface area and well defined and stable pore structure. The pioneering studies [2] proved that standard pyrolysis of this type of wood (Salix viminalis-originated) can yield a strictly nanoporous charcoal. Owing to the extremely narrow pore size distribution, such active charcoals act as nearly perfect carbon
∗ Corresponding author. Fax: +48 56 6544477. E-mail address: [email protected] (J.P. Lukaszewicz). 0165-2370/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jaap.2012.06.008
molecular sieve (CMS). The applications of CMS range from gas separation [6,7] to gas storage (including hydrogen) [8]. Therefore, the search for inexpensive and “green”, i.e., environment friendly production of active charcoal (including CMS) is still of interest among industrial technologists as well as academics. It seems necessary to prove whether the unique pore structure of Salix viminalis-originated CMS is an exclusive property of the raw material applied for its production. Therefore, in the current study, we attempted to compare the pore structure of a series of charcoals obtained using an oxygen-free heat treatment of unconventional raw materials (mostly wood). In general, more conventional types of wood (oak, bamboo, pine, etc.) and/or other raw materials (coconut shells, fruit stones, old tires, etc.) were widely tested for the production of active charcoals. Such charcoals present a wide spectrum of properties with regard to the pore structure of s produced in this way charcoals. For example, some authors claim that a different pore structure can be obtained for the active charcoals obtained from the same raw material like pine wood [9,10]. The differences mainly result from the alterations in the pyrolysis procedure and the kind of chemical treatment of wood prior to pyrolysis and/or after pyrolysis. More spectacular differences in pore structure of active charcoals from the same raw materials can be observed, especially if additional activation of the ready charcoals was carried out. 2. Materials and methods 2.1. Charcoal production Each raw material was selected basing on the ease of its collection, resulting either from its frequent occurrence or from being a kind of waste in food processing. First, all the dried raw
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materials (nine types of cellulose-rich materials including seven types of wood) were preliminary pyrolysed at the temperature of 600 ◦ C for 1 h. The load of 10 g of raw wood was used in the first carbonization. Pieces of the raw material were placed in a tubular furnace. The temperature of the stove was increased by 10 ◦ C/min until the final temperature was achieved. A constant flow of nitrogen (99.99%) was maintained during the whole procedure: heating up to the desired temperature, 1 h long pyrolysis at constant temperature, and cooling down to room temperature. Subsequently, all the samples were subjected to a secondary heat treatment at one of the temperatures of 600, 700, or 800 ◦ C for 1 h. Several samples were heated for an extended time period of 3 h (as presented in Tables 1 and 2). In such a case, one can investigate the influence of more severe conditions of pyrolysis (extended time and elevated temperature) on the pore structure of the investigated active charcoals. Some Salix viminalis-originated charcoals were obtained in a one-step manner. It consisted of heat treatment of dried wood for 1 h at the temperatures of 600, 700, 800, and 900 ◦ C (samples denoted as SV61, SV71, SV81, SV91). In contrast to other samples, the 1 h heat treatment was not followed by a secondary pyrolysis. All the investigated samples are detailed in Table 1.
Table 1 Samples history and symbols. Raw material
Elder (wood) Sambucus nigra Common pine (wood) Pinus silvestri Norway maple (wood) Acer platanoides Brittle willow (wood) Salix fragilis Basket willow (wood) Salix viminalis
Common brich (wood) Betula pendula
2.2. Pore-structure characteristics The pore structure and specific surface area were determined using a widely accepted method, exploiting the phenomenon of low-temperature adsorption of the chemically neutral gases [11,12]. Nitrogen served as an inert adsorptive, and nitrogen adsorption isotherms were recorded at the temperature of liquid nitrogen (−196 ◦ C) using automatic sorption apparatus Micromeritics ASAP 2010. The standard software provided by the manufacturer of ASAP 2010 was employed for the regression of primary adsorption data obtained (nitrogen adsorption vs. relative partial pressure of the adsorptive). Two regression models were applied: BET [13] for the calculation of specific surface area of charcoals and H–K method (Horvath–Kawazoe (H–K) [14]) for the calculation of pore-size distribution function PSD [15], which characterizes the pore structure of the solids. No changes were introduced to the standard calculation procedures offered by the commercial software [16]. Prior to measurements, all the tested
Plum tree (fruit stone) Prunus domestica Pistachio tree (fruit shells) Pistacja vera Black poplar (wood) Populus nigra
Abbreviation Unmodified materials
Modified materials
BC6161, BC6171, BC6181 SZ6161, SZ6171, SZ6181 KZ6161, KZ6171, KZ6181 WK6161, WK6171, WK6181 SV61, SV71, SV81, SV91 SV6161, SV6171, SV6181, SV6163, SV6173, SV6183 BB6161, BB6171, BB6181 PS6161, PS6171, PS6181 LP6161, LP6171, LP6181
–
DSZ
–
–
SV6Zn38 DSV
DLP6Zn38
TC6161, TC6171, TC6181
charcoal samples were degassed in vacuum (10−3 Torr) at an elevated temperature (250 ◦ C) for extended time (3 h) before nitrogen adsorption measurements. 2.3. Other tests The samples’ morphology was investigated using an electron microscope (LEO 1430 VP, Electron Microscopy Ltd.) supplied with
Table 2 Specific surface area of investigated samples. Calculations made basing on BET model. Raw material
Specific surface area SBET (m2 /g) Unmodified materials
Elder (wood) Sambucus nigra Common pine (wood) Pinus silvestri Norway maple (wood) Acer platanoides Brittle willow (wood) Salix fragilis Basket willow (wood) Salix viminalis
Common brich (wood) Betula pendula Plum tree (fruit stone) Prunus domestica Pistachio tree (fruit shells) Pistacja vera Black poplar (wood) Populus nigra
BC6161 5.3 ± 0.1 SZ6161 257.5 ± 9.9 KZ6161 144.8 ± 4.9 WK6161 234.2 ± 7.4 SV61 286.9 ± 10.1 SV6161 275.8 ± 10.7 SV6163 358.2 ± 10.9 BB6161 229.1 ± 8.6 PS6161 137.1 ± 5.5 LP6161 217.7 ± 8.4 TC6161 94.0 ± 3.3
BC6171 5.0 ± 0.3 SZ6171 317.5 ± 12.4 KZ6171 125.7 ± 4.0 WK6171 239.5 ± 8.5 SV71 336.6 ± 10.8 SV6171 296.7 ± 10.5 SV6173 311.7 ± 9.5 BB6171 251.8 ± 8.9 PS6171 51.1 ± 2.5 LP6171 187.4 ± 7.3 TC6171 5.0 ± 0.3
BC6181 1.7 ± 0.1 SZ6181 183.4 ± 6.8 KZ6181 63.7 ± 1.9 WK6181 8.1 ± 0.1 SV81 321.9 ± 10.2 SV6181 94.7 ± 2.8 SV6183 64.4 ± 2.1 BB6181 73.3 ± 2.8 PS6181 2.1 ± 0.1 LP6181 42.5 ± 1.8 TC6181 4.7 ± 0.1
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two detectors for secondary electrons (SEs), backscattered electrons (BSE), and X-ray elemental analyzer (EDX) (Quantax 200 – XFlash 4010, Bruker). Such experimental setup led to the determination of the elemental composition of specific areas and details visible on the scanning electron images. Thermal analysis DSC/DTA/DTG/TG (heating rate 10 ◦ C/min, N2 atmosphere, 20–25 mg sample weight) was performed by means of TGA-DTA Thermal Analysis SDT 2960 instrument to determine the temperature stability of the obtained charcoals.
3. Results and discussion 3.1. Nitrogen adsorption at −196 ◦ C Low-temperature nitrogen adsorption data led to the determination of specific surface area of the investigated charcoal samples. The values differed very much from one another (see Table 2) with respect to the type of raw material applied for the production. Additionally, considerable alterations were observed for charcoals obtained from the same raw material, but at different temperatures of pyrolysis. In general, more severe pyrolysis conditions (800–900 ◦ C) were found to initiate a collapse of carbon network leading to a sharp decrease in the specific surface area of nearly all of the samples treated in this way. This is a result of graphite planes/crystallites growth at high temperature which leads to vanishing of pores. Pore is charcoals obtained by wood pyrolysis are in fact slit like vacancies between randomly oriented graphite planes and crystallites. In general, the charcoal samples obtained at less severe conditions (600–700 ◦ C) from the same type of wood exhibited more developed surface area. For some raw materials (dried wood), a slight increase in the BET surface area was observed, with an increase in the pyrolysis temperature from 600 to 700 ◦ C (Salix viminalis, Salix fragilis, Pinus silvestri, Betula pendula), which was followed by the mentioned above structural collapse. The charcoals obtained from Pinus silvestri were observed to be the most resistible to temperature-caused collapse of pore structure and BET surface area (SBET ). There was no dramatic decrease in SBET between charcoals denoted as SZ6171, SZ6181. Such a decrease was visible in the case of other charcoals (when compared to most pairs of samples denoted as “71” and “81” in Table 2), including Salix viminalis-originated samples. As mentioned earlier, charcoals other than Pinus silvestri-originated charcoals were not very resistible to the rise in the temperature of pyrolysis. A dramatic structural collapse was visible upon increase in pyrolysis temperature from 700 to 800 ◦ C, in particular, among charcoals obtained from fruit stones of Prunus domestica, Salix fragilis, B. pendula, Acer plataroides, and Populus nigra. The charcoals obtained from Sambucus nigra (samples BC61, BC71, BC81) did not exhibit any significant level of specific surface area (from 1.6 to 5.2 m2 /g), which dramatically differed from other results ranging from 94 to 286 m2 /g. In addition, it was possible for Salix viminalis-originated charcoals to avoid the collapse, provided that the primary pyrolysis was not followed by a secondary heat treatment (samples SV61, SV71, and SV81). For such charcoals (one-step production), a temperature of 900 ◦ C was necessary to initiate the collapse (charcoal denoted as SV91) after 1 h long pyrolysis. The collapse was also observed at 800 ◦ C, when the time of pyrolysis was long enough, i.e., 3 h instead of 1 h. In general, the extension of pyrolysis time from 1 h to 3 h caused a decrease in SBET even for one of the most resistible type of wood: observed when comparing Salix viminalis-originated pairs of samples SV61 and SV63, SV71 and SV73, SV81 and SV83 (one-step production). Even within the same genus of plants serving later as raw materials, such as Salix family (Salix viminalis and Salix fragilis), there was only a partial similarity with respect to the SBET values and their
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change upon increasing pyrolysis temperature. Both types of wood yielded charcoals of relatively high surface area, but Salix fragilisoriginated charcoals underwent much more dramatic collapse of charcoal matrix at the temperature of pyrolysis of 800 ◦ C. In general, PSD determination is a complex matter which results are very sensitive to the kind of adsorptive, i.e., gas being physically adsorbed by the solid under investigation. Adsorption data regression model plays a crucial role too. Usually Ar, N2 and CO2 are applied for this purpose. According to some studies [17,18] the usefulness of N2 is limited due to its slow diffusion at −196 ◦ C into micropores which linear diameter is below 2 nm. Physical adsorption of CO2 (at 0 ◦ C or 25 ◦ C) was proposed as new standard procedure for PSD determination regardless of some specific properties of the gas: - CO2 is not chemically inert (evident acidic properties and supposed specific reaction with base centers on the surface), - CO2 it not a dipole but high negative charge is cumulated on both O atoms (specific molecular interactions cannot be excluded), - CO2 has a specific longitudinal shape (“long” 3-atom linear molecule may trigger orientation issues). However, some research suggested inapplicability of N2 preferring CO2 in contrast to other studies proving comparativeness of Ar, N2 and CO2 results regarding PSD [19]. Latest announcements describing theoretical simulation of Ar, N2 and CO2 adsorption [20,21] in a model pore structure proved particular usefulness of Ar as a probe gas. N2 adsorption led to PSDs only slightly differing from Ar-based PSDs. According to the studies, applicability of CO2 seems to be rather limited in the case of charcoal surface with implanted surface functional groups since CO2 -based PSDs suggested the presence of non-existing pores. Thus, despite some objections, N2 adsorption at −196 ◦ C may still serve as a standard experiment leading to a reliable qualitative information on the run of PSD curves for real charcoals. In the light of the statements above, the obtained Salix viminalis-originated charcoals posses molecular sieve-type pore structure dominated by ultra fine pores which size is placed in sub-nanometer range. The described above observations can be supported using the nitrogen adsorption isotherms and PSD functions calculated using H–K method. The most characteristic example curves are depicted in Figs. 1a, 2a, and 3a. Almost all the recorded isotherms were I-type isotherms, according to the IUPAC classification [22]. Furthermore, the inert nature of nitrogen demonstrated the existence of micropores in the carbon matrix. A slight tendency toward adsorption–desorption hysteresis was observed among the charcoal samples obtained from A. plataroides, B. pendula, and fruit stones of Prunus domestica and Sambucus nigra. In the case of Salix viminalis-originated charcoals (Fig. 1, samples SV61, SV71, SV81) as well as Salix fragilis-originated charcoals (Fig. 2, samples WK6161, WK6171, WK6181), the hysteresis was not observed. In general, the hysteresis is a sign indicating that wider pores (mainly mesopores) can be present in the carbon matrix beside very fine pores (micropores). Such a supposition was confirmed by the determination of pore-size distribution function (adsorption data regressed by means of H-R model, software offered by ASAP 2010 manufacturer). Figs. 1b, 2b, and 3b present the mentioned above functions for Salix viminalis-, Salix fragilis-, and B. pendula-originated charcoals. It can be observed that only for Salix family-originated charcoals (SV61, SV71, SV81 samples and SV6161, SV6171, SV6181 samples, and WK6161, WK6171 samples) and for Pinus silvestri-originated charcoals, the PSD functions (Fig. 4b) are very narrow, i.e., linear dimensions of pores are placed within the range of 0.8–1.0 nm. The PSD functions presented in Figs. 4 and 5, and those corresponding to SZ and KZ charcoals are much more complex. The run of the latter PSD functions is not discrete, i.e., the pores of various
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Fig. 1. Nitrogen adsorption isotherms (a) on charcoals obtained from Salix viminalis wood (SV samples) and corresponding pore size distribution functions (b).
Fig. 2. Nitrogen adsorption isotherms (a) on charcoals obtained from Salix fragilis wood (WK samples) and corresponding pore size distribution functions (b).
Fig. 3. Nitrogen adsorption isotherms (a) on charcoals obtained from Betula pendula wood (BB samples) and corresponding pore size distribution functions (b).
Fig. 4. Nitrogen adsorption isotherms (a) on charcoals obtained from Pinus silvestri wood (SZ samples) and corresponding pore size distribution functions (b).
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Fig. 6. SEM image of Salix vimianlis originated charcoal surface (SV61 sample).
Fig. 5. Nitrogen adsorption isotherms (a) on charcoals obtained from Acer platanoides wood (KZ samples) and corresponding pore size distribution functions (b).
linear dimensions (from 0.8 to 2.0 nm) considerably contribute to the total pore volume. Furthermore, the PSD curves for the KZ samples (Fig. 5) can be even considered as a proof of bimodal pore size characteristic. Interesting evolution of surface parameters (PSD function and SBET ) was observed for charcoals obtained from Pinus silvestri (Fig. 4). The SBET values for such charcoals was relatively high, i.e., SZ6161 = 257 m2 /g, SZ6171 = 317 m2 /g, and SZ6181 = 183 m2 /g. The changes corresponded to the general trend described earlier which comprised the increase in the SBET values on rise in the pyrolysis temperature from 600 to 700 ◦ C, and the subsequent decrease in the SBET values when the pyrolysis temperature approached 800 ◦ C. The PSD function (Fig. 4b) for 600 ◦ C-obtained charcoal SZ6161 was very narrow and resembled the narrow PSD function for the corresponding Salix viminalis-originated charcoal (SV61 and SV6161 samples). For the Salix viminalis-originated charcoals, the determined PSD functions remain very narrow, despite the increasing pyrolysis temperature (SV71, SV6171, SV81, SV6181 samples). Furthermore, a partly similar phenomenon was observed for Pinus silvestri-originated charcoals (SZ6161, SZ6171, SZ6181 samples), but with a gradual shift of the PDS maximum from micropore range (subnanometer range) toward the beginning of the mesopore range (ca. 1.8 nm). At the same time, a visible broadening of the main peak in PSD function was also observed. Thus, Pinus silvestri wood as a raw material was found to generate microporous charcoals (similar to Salix viminalis-originated charcoals); however, this unique (MSC-type) pore structure was less thermally stable than that of Salix viminalis-originated charcoals. In general, Salix vimivalis seems to be an outstanding raw material among all the investigated samples, because the produced charcoals obtained from it instantly presented several positive features as follows:
- Oxygen-free pyrolysis of random pieces of Salix viminalis wood generated strictly monoporous charcoal whose structure resembled a perfect molecular sieve (extremely narrow PSD function). - Stable molecular sieve-type pore structure of Salix viminalisoriginated charcoals was thermally stable, i.e., it was able to resist extended pyrolysis time at elevated temperatures. - The dominating pore size corresponding to the maximum at the narrow PSD function (SV samples) was in the subnanometer range and moderately dependent on the extended pyrolysis time and elevation in the production temperature. Apart from the positive physicochemical features mentioned earlier, economical and environmental issues favor the use of Salix viminalis as a raw material for active charcoal application. The growth of Salix viminalis plants is very fast in agriculture plantations and nonagricultural areas like side spaces of highways and railway lines. Salix viminalis is a renewable resource providing unlimited amounts of hard wood which could not be obtained from any other investigated raw material. Its production may easily fulfill the expectations of charcoal manufacturers, apart form the increasing demand from energetic industry. Application of Pinus silvestri for active charcoal production can be considered as a competitive production method of charcoals exhibiting pore structure resembling CMS. However, the growth of Pinus silvestri is very slow, i.e., 40–60 years are necessary to exploit the forests of Pinus silvestri. The long period of growth of these trees is associated with the parallel gradual formation of a local ecosystem in the pine woods involving innumerable types of plants and animals. Cutting pine woods not only causes damage to the particular trees, but also results in the collapse of the whole local ecosystem. On the contrary, Salix viminalis plantations (3 years from implantation to the first harvest) and its short cycle (cutting each year) resemble farming of typical agriculture plants like corn, beats or potatoes. Thus, similar surface properties of SV and SZ charcoals can be easily obtained for Salix viminalis-originated charcoal from economical and ecological point of view. 3.2. Morphology and elemental composition Figs. 6 and 7 present a series of SEM images recorded for selected samples of investigated charcoals. Some characteristic details can be observed for wood-originated charcoals (Fig. 6, sample SV61). Above all, it is a macro structure derived from the original tissue structure in the plant which has been transformed into carbon matrix upon heat treatment. This can be regarded as a kind of plant
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Table 3 Weight content of selected elements in carbon obtained by two step carbonization (primary: 600 ◦ C, 1 h and secondary: 600 ◦ C, 1 h). Method: SEM–EDX (LEO 1430VP + EDX XFlash 4010, magnification 500×). Raw material
Symbol
Betula pendula
BB
Sambucus nigra
BC
Salix viminalis
SV
Salix fragilis
WK
Populus nigra
TC
Pinus silvestri
SZ
Acer platanoides
KZ
Pistacja vera (fruit stones) Prunus domestica (nut shells)
PS LP
Weight content [%]
C IC C IC C IC C IC C IC C IC C IC C IC C IC
C
O
Mg
Na
P
K
Ca
84.8 27.3 79.5 54.3 81.4 45.7 81.9 33.8 80.5 44.7 85.6 81.7 84.8 71.2 86.3 35.8 79.0 44.4
14.6 42.6 18.9 29.8 15.8 35.4 16.7 21.7 16.9 35.2 13.5 17.2 14.0 19.8 13.2 38.7 17.4 34.7
– 0.5 0.5 0.4 – 0.2 0.2 0.1 0.3 0.3 0.1 0.3 0.1 0.1 – – – 0.4
– – – – – – – – – – – – – – – – 0.7 4.1
– 0.7 0.2 – 0.5 0.3 0.2 0.3 0.4 0.3 – – 0.1 0.1 – – – –
0.7 1.1 0.9 0.5 1.6 1.1 1.0 1.7 1.6 1.2 0.3 0.2 0.5 0.5 0.5 0.4 1.9 1.2
– 27.8 – 14.5 0.7 17.4 – 42.6 0.4 18.0 0.5 0.5 0.4 8.2 – 24.4 – 3.7
Residuals: Al, Cr, Fe, Cl, S, Si
IC data represent the elemental composition of inorganic additives; C data correspond to the elemental composition of the basic carbon-based matrix. C, carbon matrix; IC, inorganic crystallite.
“fingerprint.” Such a “fingerprint” looks different for the charcoals obtained from non-wood raw materials like Pistacja vera (LP charcoals) fruit shells and Prunus domestica (PS charcoals) fruit stones (Fig. 7). Thus, the structure and shape of tissues in fruit shells and stones are, in general, very different from those transporting water and minerals along the main stem and branches of a plant. Hence, the morphology of PS and LP samples differ much from that of the remaining charcoals that are wood-originated charcoals (BC, SZ, KZ, WK, SV, BB, TC samples). SEM images were recorded using two types of detectors: SE and BSE. The BSE images demonstrated that some details in the secondary electron images (SE) have a different chemical composition, i.e., contain elements of atomic number higher than 12 (carbon) or 16 (oxygen). Typically, the details (white spots) resemble small cubic microcrystals (Fig. 6). The electron microscope was equipped with an EDX probe that could determine the elemental composition of some spots selected on the surface of the investigated charcoal samples. For each sample, two areas were selected based on the BSE images: “white” cubic microcrystals and “black”
Fig. 7. SEM image of Pistacja vera (fruit shells, LP6161 sample) originated charcoal surface. White crystallites are Ca-based residuals (total Ca weight content below 0.1%) of mineral substances in living plants.
areas. We assumed that “black” spots were parts of carbon matrix which developed from the organic matter forming cell walls in raw materials applied for charcoal production. The “white” decorations were considered to be a residual of inorganic components that were also present in raw materials. The data on elemental composition obtained from EDX measurements are presented in Table 3 which confirmed the mentioned above hypothesis of the origin of “white” (rows denoted as “IC”) and “black” spots (rows denoted as “C”). The elemental composition of “black” spots is typical for charcoals obtained by pyrolysis of organic matter like wood, i.e., consisting mainly of carbon and oxygen [23]. The “white” crystallite-looking details can be considered as spots where calcium derivatives are concentrated. With regard to the history of the investigated samples, it is reasonable to consider that CaCO3 is the main component of “white” spots. Minor amounts of other elements (magnesium, potassium, phosphorus) could also be observed in both the “black” and the “white” spots. The overall concentration of calcium concentration in the charcoal samples was as low as 0.1% (weight), which was obtained from the EDX measurements covering large surface areas, with EDX analysis field covering the black and white spots in proportions typical for a particular charcoal sample. The low content of elements other than carbon and oxygen has no influence on the mechanism of N2 adsorption at −196 ◦ C.
Fig. 8. TG curves recorded the samples SV61, SV71 and SV81.
A. Cyganiuk et al. / Journal of Analytical and Applied Pyrolysis 98 (2012) 15–21
3.3. Thermal investigations Fig. 8 presents the weight-loss curves with the increasing temperature of heat treatment (TG curves) for a series of Salix viminalis-originated charcoals (SV61, SV71 and SV81 samples). It can be observed that SV charcoals become unstable above 600 ◦ C, exhibiting accelerated weight loss. The highest weight loss was observed for the less thermally elaborated charcoal SV61, i.e., charcoal obtained after 1-h long pyrolysis at 600 ◦ C. If the pyrolysis temperature was higher (700 ◦ C/800 ◦ C), the mass decrease above 600 ◦ C during TG analysis was lower. However, it is prove that some changes still proceed like graphitization. Such changes may be expected during the secondary heat treatment since SV61, SV71, SV81 sample were not subjected to the secondary treatment and in the performed TG experiment conditions analogous to the secondary treatment were simulated. Similar observations were also observed for most of the investigated charcoals. The observations can be considered as a sign of charcoal instability at elevated temperatures, whose effect corresponds to the above-described collapse of carbon matrix resulting in the lowering of BET surface area. The high mass loss for SV61 charcoal proved that a subsequent heating of such charcoal up to 700 ◦ C or 800 ◦ C could cause some changes, like graphitization [24,25] and evolution of gas products [26]. The effect was more intensive than for SV71 and SV81 samples. For example, SV81 sample underwent only the first heat treatment at elevated temperature (up to 800 ◦ C) before TG measurements, which was associated with intensive release of some volatile products. Thus, the subsequent heating of SV81 sample during the TG-curve determination caused less striking weight loss than that observed in SV61 or SV71 samples. 4. Conclusions In general, the unique molecular sieve-type pore structure could be achieved by careful pyrolysis of several sorts of lignin/celluloserich raw materials (mostly dried wood). The mentioned above CMS-type structure within the carbon matrix exists in a very fragile state, which easily undergoes a thermally induced collapse. Thus, the CMS-type structure can be considered as an intermediate state existing until an intensive formation of graphite sheets which begins in elevated temperatures. In addition, extended time of pyrolysis accelerates the collapse of the pore structure owing to graphitization. Furthermore, the CMS-type structure easily vanishes upon increased pyrolysis time and temperature in most of the investigated charcoals. In the case of Salix viminalis-originated charcoals (SV samples), the mentioned above discrete pore structure was well developed and stable. This property demonstrates the benefits of using Salix viminalis wood as a raw material for active charcoal production. In addition, this conclusion is supported by the economical and environmental aspects, because Salix viminalis is easy to grow at renewable plantations. References [1] A. Dabrowski, Adsorption from theory to practice, Advances in Colloid and Interface Science 93 (2001) 135–224.
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