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CO2 activation of char from Quercus agrifolia wood waste
PERGAMON
Carbon 39 (2001) 1367–1377
CO 2 activation of char from Quercus agrifolia wood waste a, b *, Alfredo Aguilar Elguezabal ´ ´ Alejandro Robau Sanchez , b Luis de La Torre Saenz a
b
´ , CIPIMM, Havana, Cuba Centro de Investigaciones para la Industria Minero Metalurgica ´ Departamento de Catalisis , Centro de Investigaciones en Materiales Avanzados, CIMAV, Chihuahua, Chih. 31109, Mexico Received 12 October 1999; accepted 15 September 2000
Abstract Three series of activated carbons were prepared from Quercus agrifolia wood using a two-step process, carbonization followed by physical activation with CO 2 . Samples were characterized by N 2 and CO 2 adsorption. Three activation temperatures were used, 800, 840 and 8808C, covering the 18–85 wt.% yield range by variation of residence time. Activated carbons with a well-developed porous structure, predominantly microporous with high BET surface areas, were obtained. No direct relationship was found between exposed BET surface area (the surface where activation reaction takes place), evolution and gasification rate variation. Porosity development appears to be strongly influenced by the kinetic reaction stage, the reactant gas concentration gradient being an ultimate factor that induces porosity evolution. 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Activated carbon; D. Porosity
1. Introduction For many years an established and prolific field within carbon research has been the description of synthesis methods and characterization of carbonaceous porous materials such as activated carbons, chars and composites produced from a wide variety of sources. Agricultural by-products are a quite important and renewable source of raw materials for activated carbon production, and our recent efforts have focused on the preparation of activated carbon from the Quercus agrifolia tree, a plant which is used in the wood industry in United States and Mexico. In Chihuahua, a state in northern Mexico, this tree is very common and its contribution important to agricultural by-products generated during wood processing. Disposal of these residues has become an ecological problem; an attractive economic and technological solution could be its conversion into activated carbon. ´ *Corresponding author, Departamento de Catalisis, Centro de Investigaciones en Materiales Avanzados, CIMAV, Miguel de Cervantes 120 Complejo Industrial Chihuahua, C.P. 31109 Chihuahua, Mexico. Tel.: 152-14-439-1111; fax: 152-14-4391112. ´ E-mail address: [email protected] (A.R. Sanchez).
Previous workers have reported activated carbons obtained from different agricultural wastes such as walnut shells [1], eucalyptus globulus and peach stones [2], olive stones [3], almond shells and peach, plum and cherry stones [4], apricot stones [5], oil palm shells [6], Japanese cypress [7], olive-seed waste residues [8] and Uruguayan eucalyptus wood [9]. It has been extensively proved that any cheap material with high carbon content can be used as raw material for the production of active carbon. There are two principal processes for its preparation: physical and chemical activation [10]. Physical activation, the method used in this study, involves previous carbonization of a carbonaceous material, followed by activation of the resulting char at a temperature of 800–10008C in the presence of suitable oxidizing gases such as steam or carbon dioxide, activation being basically a controlled combustion reaction process. Several elements make study of gasification reactions a challenging and demanding enterprise. The synergic effect of carbon dioxide and steam during controlled combustion processes in internally heated furnaces (i.e. rotary kilns) is a complex phenomenon due to several known reactions that take place. Steam is not only a combustion product but is added to the activation kiln atmosphere to increase its partial pressure.
0008-6223 / 01 / $ – see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 00 )00253-0
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Because of the above-mentioned facts, it is advisable to study activation reactions separately since it would be quite difficult to establish the effect of each gasification reaction and consequently its effects in the final porosity and surface properties of carbon in the presence of several oxidizing gases. CO 2 was chosen as the activating agent for the present study. Steam activation of Quercus agrifolia will be discussed in a further publication. In an attempt to clarify discrepancies associated with an unclear tendency in the effect of use of CO 2 or steam on the development of porosity of activated carbons in previously published ´ works, Rodrıguez-Reinoso et al. [11] concluded that CO 2 produces an opening, followed by a widening, of narrow microporosity under conditions given for reported results. In their work, Gonzalez et al. [3] used both vertical and horizontal furnaces for CO 2 activation and report that the use of a horizontal furnace seemed to be advantageous for microporosity development. Aiming to obtain activated carbons with a well-developed microporosity and correspondingly high surface area from Quercus agrifolia, results reported in the abovementioned works concerning activating agent and equipment [3,11] were followed, studying the relationship between activation parameters and obtained activated carbon properties.
2. Materials and methods
2.1. Experimental procedure 2.1.1. Carbonization Quercus agrifolia wood waste was obtained from local wood manufacturers. Carbonization of samples was performed in a Labline Heatech economy furnace with a built-in microprocessor control containing a purpose-made steel retort provided with inlet connections that made possible the inert gas (nitrogen) feeding used during the heating and cooling steps, so that oxygen from the air does not burn off the sample. There were two additional pipes attached to the retort lid; one of them worked as an exhaust fume chimney and the other as a protective tube for the thermocouple. The lid was tightly fastened to the retort in order to prevent air infiltration. The adequate carbonization temperature and retention time corresponding to a yield of |32–35 wt.%, were previously determined from a TGA analysis. Charges of 1000 g of raw precursors were placed in the retort, which was positioned in the heating zone in such a way that the whole charge was exposed to the same temperature. The system was then heated at a fixed rate of 208C / min to 4508C and held at this temperature for 120 min. The average carbonization yield obtained was 34 wt.%. The carbonized material was then crushed and sieved to obtain a granulated fraction between 0.5 and 1.4
mm, carefully homogenized according to Ref. [12]. Since at 4508C the carbonization process is not completed, it is important to ascertain the weight loss undergone by the precursor from 4508C to each one of the applied activation temperatures, thus facilitating the determination of exact weight loss due to the activation process alone. Consequently, additional runs of carbonization up to corresponding temperatures were carried out. For 800, 840 and 8808C overall carbonization yields of 27, 26 and 25 wt.% were obtained, respectively.
2.1.2. Activation Activation was performed in a Thermolyne F21100 horizontal tube furnace. A fused silica tube was introduced into the furnace chamber, closed at both ends with rubber stoppers with metal tubes inside. These tubes were used for gas flow inlet and outlet as well as for a thermocouple. Ten grams of char were placed inside a stainless steel net that was put in the center of the furnace preheated to the corresponding temperature and previously purged with CO 2 . The carbon dioxide flow was 500 ml / min. Selected activation temperatures were 800, 840 and 8808C and experiments were performed covering the 18– 85 wt.% yield ranges by changing residence time for those three series. Activation temperature and yield are included in sample nomenclature. 2.1.3. Characterization Porosity of the obtained samples was measured according to physical characterization performed using an automatic system (Quantachrome Autosorb 1), through physical adsorption of nitrogen at 21968C and CO 2 at 08C. Changes in the porosity of produced carbons were examined using parameters that do not characterize adsorbent materials completely yet provide helpful and clear information on the evolution of different pore size ranges. The first parameter chosen was micropore volume (Vwm , WM-wide microporosity), calculated from the application of the Dubinin–Raduskhevich (DR) equation to the N 2 adsorption isotherms [13]. By including experimental data up to a relative pressure of P/P0 5 0.3, one can obtain the micropore volume corresponding to wide microporosity [14]. The second parameter was total pore volume (Vt ) measured as the amount of nitrogen adsorbed at P/P0 5 0.95. In type I isotherms this represents pore volume for both micropores and mesopores. Mesopore volume (Vme ) was then calculated as the difference between Vt and Vwm . An additional adsorption parameter, narrow microporosity volume (Vnm , NM-narrow microporosity), was deduced from the DR equation applied to CO 2 adsorption isotherms [15] up to P/P0 5 0.03. The apparent N 2 surface area obtained from the BET equation within the 0.001–0.2 relative pressure range was calculated too because, despite its controversial interpretation for activated carbons [16,17], this parameter provides
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a value that can be useful when comparing characteristics of obtained samples. Instead of considering a single surface area with which to characterize a porous solid, Byrne and Marsh [18] proposed distinguishing between three definitions of this parameter: geometric surface area (GSA), which does not include meso- and microporosity and refers to the geometric shape area of a particle, total surface area (TSA), which does not have a precise definition and is considered to be the maximum surface area which a porous solid can exhibit and the effective surface area (ESA), which would be the area that the material appears to exhibit, as measured from adsorption isotherms using a defined adsorptive. It can be considered that the nitrogen isotherm, according to BET, provides a measure of the ESA and, at the same time, ESA can be considered as that area available to a reacting gas molecule to remove a carbon atom within the porosity. The latter concept was used in the analysis gasification behavior presented in this study.
3. Results and discussion Experimental results are presented in Table 1. Results refer to three series according to activation temperatures used: 800, 840 and 8808C series.
3.1. Gasification behavior The following analysis was made using values deduced from N 2 adsorption isotherms. Microporosity volumes obtained from CO 2 adsorption isotherms were not included
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because obtained values suggest that their contribution to total microporosity is negligible, and the trends discussed hereafter were not affected by Vnm values. This fact will be the subject of further discussion. Weight losses (yield) as a function of residence time for obtained carbons are plotted in Fig. 1; the influence of temperature and residence time on the activation reaction yield is easily noticed. The evolution of apparent activation energy has been included in this figure for further discussion. Although the overall gasification rate shows a constant trend very close to a linear behavior (Fig. 1), more noticeable differences can be found when point-to-point values in each series are considered (Fig. 2a). In this figure the evolution of the gasification rate (reacted carbon in the considered time interval, wt.% / h) can be viewed as a function of normalized residence time. Gasification rates seem to vary depending on applied activation temperature and residence time and undergo changes from 30 to 12, 50 to 33 and 60 to 51 wt.% / h for 800, 840 and 8808C series, respectively, reaching their highest value at 8808C (60 wt.% / h) at early stages of activation. The lowest gasification rate values are found at 8008C and for largest residence time (12 wt.% / h), the gasification rate increasing about five times from the lowest to the highest temperature. It is discernible in Fig. 2a that for lower temperatures (800 and 8408C) and residence times around 60% of longest residence time used, the gasification rate decreases, but from this point forward a less considerable variation is shown. Nevertheless at 8808C, a linear and constant decrease in the gasification rate is perceived. Fig. 2b depicts evolution of the intrinsic gasification rate
Table 1 Results obtained from characterization of obtained series Activation temperature
8008C series
8408C series
8808C series
Sample
Char E-800-85 E-800-76 E-800-64 E-800-54 E-800-51 E-800-42 E-840-76 E-840-66 E-840-47 E-840-37 E-840-18 E-880-70 E-880-58 E-880-47 E-880-36
Gasification rate wt.% / h
Intrinsic gasification rate, IGR wt.% / h m 2
29.8 24.3 18.0 15.4 12.2 11.6 48.4 34.3 35.1 31.7 32.7 59.8 56.5 53.2 51.0
0.065 0.045 0.027 0.017 0.013 0.013 0.065 0.050 0.039 0.028 0.028 0.066 0.070 0.057 0.047
From CO 2 isotherms DR micropore volume Vnm cc / g 0.032 0.034 0.052 0.043 0.033 0.020 0.014 0.023 0.034 0.020 0.011 0.009 0.026 0.044 0.030 0.016
From N 2 isotherms BET surface area m2 / g
DR micropore volume Vwm cc / g
Mesopore volume Vme cc / g
Total pore volume Vt cc / g
408 510 570 774 931 903 900 588 772 1034 1201 1173 723 899 982 1197
0.168 0.206 0.232 0.315 0.387 0.374 0.376 0.249 0.329 0.438 0.512 0.505 0.303 0.379 0.416 0.509
0.009 0.037 0.044 0.071 0.102 0.092 0.040 0.029 0.054 0.113 0.159 0.171 0.049 0.086 0.113 0.141
0.177 0.243 0.276 0.386 0.490 0.466 0.416 0.277 0.383 0.551 0.671 0.676 0.352 0.466 0.529 0.650
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Fig. 1. Evolution of yield as a function of residence time.
(wt.% / h m 2 ) as a function of normalized residence time. The intrinsic gasification rate (IGR) was deduced by relating weight loss to the corresponding residence time (gasification rate) and to the average exposed BET surface area, the latter being calculated as the average conformed by the surface area of the previous experimental point and the area corresponding to the analyzed point, postulating that CO 2 can only react with surface sites which contribute to the measurable BET surface area. This assumption has been made on the basis of porosity results, since the contribution of wide micropores is predominant in the overall micropore volume. Evolution of the exposed BET surface area as a function of normalized residence time can be viewed in Fig. 2c. In an attempt to relate gasification rate evolution to exposed BET surface area (the surface on which the reaction is assumed to take place), data plotted in Fig. 2a–c were scrutinized. Exposed BET surface area (Fig. 2c) shows an immediate enlargement up to 60% of the longest residence time at 8008C. Any further increase in residence time does not enlarge this value. At 840 and 8808C, the exposed BET surface area shows a sustained increment even up to the longest applied residence time. At 8008C there is a decrease in IGR up to 50–60% of total residence time (see Fig. 2b). From this range on, the decrease is less marked and can be considered a stabilization of the gasification rate. Similar behavior is detected at
8408C. There is a decrease in IGR at 8808C also, which continues up to the longest applied residence time, becoming less marked beyond 70% of the longest time. IGR exhibits different lowering trends at early stages of activation, more marked ones than at longer residence times when a less marked decrease is noticed; this behavior is common for all temperatures applied. Nevertheless, there is a constant increase in exposed BET surface area (and thus an increase in the exposed reaction surface according to above-mentioned assumptions) at 840 and 8808C. The above-mentioned analysis suggests that the exposed BET area does not have a marked influence on the gasification rate and that previous assumptions of free and unblocked reaction surface sites are not correct. At 8008C a simultaneous decrease in both exposed BET surface and IGR can be observed, but it was not considered as conclusive evidence. At 840 and 8808C, despite the general increasing trend of exposed BET surface area, IGR declines. One could suppose that as the gasification reaction goes on and internal porosity subsequently develops in carbon particles, there is a resulting enlargement in the path that must be followed by CO 2 molecules up to the surface sites where they might react and from them to the gas bulk, in the case of reaction products. This would imply a retarding effect on the course of the reaction, evidenced through a decrease in the gasification rate as residence time in-
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activation energy was calculated for similar residence times and its behavior was observed as a function of reaction advancement. Magnitudes varied from that associated with diffusion-controlled processes (less than 20 kcal / mol), at early stages of the reaction, to values that indicate a transition to a chemical-controlled stage at higher degrees of activation. Fig. 1 indicates calculated apparent activation energy values, and zones delimited by diffusionand chemical-controlled reaction can be observed. In our case critical residence time, up to which diffusion governs the process, appears to be around 45 min. As a result, the lower gasification rate at advanced stages of the activation process should not be associated with a diffusion-limiting step. Finally this gasification rate behavior is considered as the consequence of the suggested inhibiting mechanism of reaction, possibly caused by formation of carbon–carbon monoxide complexes on the internal surface of carbon [10]. These complexes would block a considerable amount of reaction sites, limiting access of CO 2 and hence retarding overall reaction. At 8808C, although an IGR reduction is observed, it is less striking than that corresponding to lower activation temperatures. The lower extent of reaction attained in this case does not allow to establish if a constant value of IGR would be reached at some further point in its progress.
3.2. Porosity development Fig. 3a gives plots of micropore, mesopore and total pore volume as a function of yield, and the evolution of BET surface area as a function of the yield is shown in Fig. 3b. Results were deduced from N 2 adsorption. Different porosity progressions are found for the series analyzed. Because the behavior observed is close to a linear relationship between yield and residence time for all obtained series (Fig. 1), further discussion will be referred only to yield.
Fig. 2. (a) Gasification rate as a function of normalized residence time. (b) Evolution of intrinsic gasification rate, as a function of normalized residence time. (c) Exposed BET internal surface area as a function of normalized residence time.
creases. In other words, diffusion would be the raising and controlling step of the reaction and it could be another reason for the gasification rate to lessen. If this were the case, the apparent activation energy of the process should be consistent with a diffusion-ruled reaction. In order to clarify this assumption, apparent
3.2.1. 8008 C series To produce this series, the longest residence times were used in order to obtain satisfactory degrees of activation, since at this temperature gasification rates are relatively low. Further experimental points were not made when a marked lessening tendency in obtained porosity was observed. In general terms, both microporosity and mesoporosity increased up to 55 wt.% yield (Fig. 3a). Further activation did not result in a higher porosity, but quite the opposite. The contribution of both microporosity and mesoporosity to this diminishing at yield values lower than 60 wt.% was different. While the micropore volume, after shrinking a little, sustained a constant value, mesoporosity decreased after a rising trend continued up to around 55 wt.% yield. BET surface area shows a similar behavior to micropore
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Fig. 3. (a) Evolution of pore volume as a function of yield. (b) Evolution of BET internal surface area as a function of yield.
volume (Fig. 3b), since the two parameters are directly related.
3.2.2. 8408 C series The lowest yield values were attained in this series. The constant increase in all pore volumes reaches its highest quantity at 35 wt.% yield. Lower yields result in a decrease in micropore volume while mesopore volume continues to rise. Total pore volume increases slightly below 35 wt.%
yield due to higher mesopore volume, despite a shrinking micropore volume. In the same manner, BET surface area decreases according to micropore behavior (Fig. 3b).
3.2.3. 8808 C series In this series, due to the small amounts of starting material (and consequently small quantity of material obtained) and the high reactivity shown, activation beyond 35 wt.% was not carried out. An increase in both micro-
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pore and mesopore volumes as well as in BET surface area values is observed (Fig. 3a and b). However, micropore volume apparently exhibits a more marked raising trend than mesopore volume (see Table 1). Pore volume evolution can then be summarized in the following manner for all series. 1. Microporosity is enlarged at 8008C and its highest attained value maintained almost constant below 50 wt.% yield. Within the same yield range mesoporosity decreases. 2. At 8408C the highest values of micropore volume at 36 wt.% yield are obtained. Further activation results in a decrease in this parameter while mesoporosity still goes up. 3. At 8808C both microporosity (more noticeable) and mesoporosity increase constantly for all samples obtained and reach a maximum comparable to that obtained for 8408C series with the same yield. There were clear differences in ascending and descending trends observed in both micropore and mesopore volumes in the series produced. In Fig. 4a evolution of micropore–mesopore volume ratio (Vwm /Vme ) is plotted as a function of yield for all series. As a consequence of constant micropore volume values attained in 8008C series and the simultaneous decrease in mesopore volume for yields less than 55 wt.%, a rising tendency of Vwm /Vme ratio is found related to the mentioned yield range. The values of Vwm /Vme at early stages of activation suggest that mesoporosity is being modified at a higher rate than microporosity. At 8408C and 8808C series, similar behavior is observed up to a 45 wt.% yield. Below this point both microporosity and mesoporosity appear to maintain almost the same variation in ratio; while at 8408C mesoporosity seems to be modified at a slightly faster rate than microporosity for yields less than 36 wt.%, at 8808C both mesoporosity and microporosity are being altered at the same ratio below 50 wt.% yield. For a complementary analysis of porosity, it is useful to express results per unit of starting char [3,18]. These results are plotted in Fig. 4b. There is an initial increase in micropore volume for all series at yields higher than 55 wt.%, a decrease beginning as the yield gets lower. In 8008C series mesoporosity follows the same tendency as microporosity. At 8408C mesoporosity still increases up to values around 35–38 wt.% yield, with a further decrease at lower yield values. At 8808C mesopore volume rises up to 48 wt.% yield, decreasing slightly below this value. Analysis of Fig. 4a and b gives interesting information. According to these figures, microporosity is destroyed below 50 wt.% yield for all series, but it is important to note that both mesoporosity and microporosity undergo the same process at 8008C (Fig. 4b) while the Vwm /Vme ratio rises abruptly (Fig. 4a). At 8408C, although micropore volume is decreasing below 50 wt.% and mesopore
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volume continues to increase up to 38 wt.%, the Vwm /Vme ratio remains almost unchanged. Similar behavior can be observed below 50 wt.% yield at 8808C, where micropore volume decreases while mesopore volume stays almost at the same value though the Vwm /Vme ratio varies very slightly. These results indicate that porosity is created at lower stages of activation for yields higher than 55 wt.%, while for larger activation degrees widening of previously created micropores and external burn off of particles become significant, in agreement with results reported in Ref. [11]. Observed tendencies when comparing Figs. 3a, 4a and 4b show that at 8008C mesopores are widened (and converted into pores of non-detectable radii using applied adsorption technique) at a faster rate than micropores below 50 wt.% yield.
3.2.4. CO2 adsorption When analyzing microporosity, it is advisable to establish differences between wide and narrow microporosity. This can be achieved using different adsorptives such as CO 2 , which appears to measure a narrower porosity range than N 2 , which detects wider micropores [19]. CO 2 adsorption isotherms were determined for the samples studied. Micropore volumes deduced from the DR equation are presented in Table 1, and Fig. 5 shows evolution of Vnm as a function of yield. There is a common trend for all series: an initial increase followed by a brusque decrease. Those yield values in which the observed trend changes are different for each series. For 8008C series, this takes place when a 75 wt.% yield is reached, while for 840 and 8808C turning points are 65 and 55 wt.% yield, respectively. Differences between the micropore volumes calculated from the DR equation applied to CO 2 and N 2 isotherms have been related to the porous structure of activated ´ carbons in previous studies published by Rodrıguez-Reinoso and Linares-Solano [19] and Garrido et al. [20]. According to the classification proposed, group C includes activated carbons with medium-to-high burn off, with a wider and very heterogeneous microporosity; for these carbons, N 2 micropore volume (Vwm ) is higher than that of CO 2 (Vnm ). Activated carbons presented in this study belong to this group. It can be noticed (Table 1) that for all samples, Vwm . Vnm but the contribution of Vnm to both microporosity and total porosity is also very small. For this reason, in the previously presented analysis concerning gasification behavior, Vnm was not included. The structure of the lignocellulosic material used in this study presumably does not favor the formation of narrow micropores to a considerable extent, at least under the carbonization and activation conditions here employed. For a brief evaluation of the progress of both Vwm and Vnm , Fig. 6 shows the evolution of the Vwm /Vnm ratio as a function of yield for all series. As yield decreases, it can be
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Fig. 4. (a) Evolution of Vwm /Vme ratio, as a function of yield. (b) Evolution of pore volume referred to initial weight as a function of yield.
noticed that after a slight initial decline in the Vwm /Vnm ratio, there is a sharp increase which begins at similar yield values to those at which Vnm decreases for each series (see Fig. 5). This behavior is a consequence of narrow microporosity widening, corroborated through simultaneous growth of Vwm . It is interesting that the Vwm /Vnm ratio maintains its rising trend at 800 and 8408C for yields less than 50 and 35 wt.%, respectively, although Vwm has attained almost constant values (Fig. 3a). Consideration of the data shown in Figs. 4a, 5 and 6
reveals several details. At 8008C, while Vnm decreases for yields below 75 wt.%, both Vwm and Vme still increase up to 55 wt.% yield. Below this yield, Vwm reaches a fairly stable magnitude while Vme decreases. The Vwm /Vnm ratio shrinks while the Vwm /Vme ratio increases rapidly. As a result, it appears that wide microporosity is preserved. For the 8408C series both Vwm and Vme maintain a rising trend up to 35 wt.% as yield lessens. For any further diminishing yield both Vnm and Vwm decrease slightly as Vme increases. However, the Vwm /Vnm ratio is still growing
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Fig. 5. Evolution of micropore volume (Vnm ) deduced from carbon dioxide DR equation as a function of yield.
at the same time as the Vwm /Vme ratio remains almost unaffected, indicating that Vnm is widening at a faster rate than Vwm (see Figs. 4a and 6). At 8808C both Vwm and Vme show a sustained increment up to the smallest yield even as Vnm decreases. As to Vwm /Vme and Vwm /Vnm ratios, the same behavior described for 8408C series is detected. However, some discrepancies in pore volume changes can be noted. A fraction of the narrow microporosity (Vnm ) is widened and becomes part of the wide microporosity
(Vwm ). At the same time, a portion of the wide microporosity (Vwm ) is widened and incorporated into mesopores (Vme ). Apparently there is not an equivalence between both widened micropore volumes (Vnm converted to Vwm and Vwm converted to Vme ). The observed divergence is more evident for 800 and 8408C series. It might be attributed to a possible overlapping of micropore volumes deduced from N 2 and CO 2 isotherms, so that N 2 would also detect part of the microporosity measured by CO 2 adsorption. For this reason, Vnm was not included in total porosity since
Fig. 6. Evolution of Vwm /Vnm ratio, as a function of yield.
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there are no guarantees of a well-defined measurement between Vnm and Vwm . This illustrates the need for complementary techniques that would allow a more precise description of microporosity, mainly for highly activated and heterogeneous carbons.
3.3. Some final considerations According to the previously discussed transition from a diffusion-controlled process at early stages of activation to a chemical-controlled step while the reaction goes on, it can be assumed that reactant concentration is lower in the interior of the particle and the external burning of the particle will be more relevant [3]. This behavior is suggested by the evolution of Vwm /Vme and Vwm /Vnm ratios observed in all series, but to different extents; it is more evident at 8008C, when for yields less than 55 wt.% microporosity is preserved while mesoporosity is destroyed, although its ratio increases. Similar microporosity behavior has been reported previously for different carbonaceous materials [11,21]. Besides differences in the strength of chemical bonding on the location of a reacting carbon atom (more weakly bonded sites tend to be consumed preferentially at the beginning of the reaction), a carbon atom must be accessible to the activating agent. The lower the bonding energy and the better accessibility, the more probable it is that at a given place carbon lattices are intensively attacked by the gasifying agent and the porosity enlarged. Since gas transport depends on pressure, temperature, rate and degree of conversion at a given yield, the growth of porosity will be less marked in micropores if the reaction rate controlled by reaction temperature exceeds a certain value [21]. This supports the proposed explanation. If it is assumed that most reactant gases must pass through mesoporous system in order to get into micropores and thus induce further volume enlargement (or reduction, if enough widening takes place), one could expect a sharp concentration gradient between the external surface and micropores. As a consequence, reactant concentration will be lower in micropores than in mesopores; a portion of the latter would widen and become pores of undetectable diameter with the adsorption techniques applied in this study. Since the absolute contribution to total porosity is less for mesopores than for micropores, mesoporosity decreased would be less noticeable unless Fig. 4a was analyzed, which indicates that mesopores disappear faster than micropores at 8008C. This explanation is also suitable for 8408C and 8808C series. In this case both microporosity and mesoporosity decrease for lower yields, but ratios remain unchanged. This would imply that reactant concentration in both micropores and mesopores has reached a similar magnitude, so that the concentration gradient mentioned was more marked between external and internal parts of the carbon particle. Evidence of this mechanism
could be the stable Vwm /Vme ratio, since the rate at which both mesoporosity and microporosity are being modified is similar while one observes a reduction in microporosity destruction as temperature increases.
4. Conclusions The experimentation and resulting analysis presented in this study have focused on the description of the activation suitability of Quercus agrifolia wood waste with CO 2 as well as on characterization of the activation process and the activated samples obtained. All three series of activated carbons were prepared under similar condition of CO 2 flow; different activation progressions through residence time variation were obtained for each temperature. According to the results and analysis, as well as to information found in other published studies [3,11], the following conclusions may be drawn. 1. Quercus agrifolia wood waste can be considered a suitable raw material for activated carbon preparation. Activated carbons with well-developed heterogeneous porous system and high surface areas were obtained. It can be observed that developed micropores are predominantly supermicropores. Porous heterogeneity of the activated carbons obtained makes porosity analysis complicated, since applied adsorption techniques do not permit a sharp distinction between ‘narrow’ and ‘wide’ microporosity. 2. Assumptions concerning a link between gasification rates and both exposed BET surface area and free active reaction sites did not prove correct. Decrease in the gasification rate while the activation reaction advances could be related to inhibition caused by carbon–carbon monoxide complexes formed on the carbon surface. 3. Microporosity increases at early stages of the activation process up to around 55 wt.% yield. Further activation causes decreases in mesoporosity at lower activation temperatures. As temperature rises, mesoporosity continues to increase for values below 55 wt.% yield. 4. Transition from diffusion to a chemical-controlled process seems to be the ruling mechanism that affects porosity development. Variation in the reactant gas concentration gradient, established between external and internal parts of carbon particles appears to be the ultimate factor that determines both microporosity and mesoporosity evolution. It may be interesting to perform a similar analysis applied to other precursors with different starting properties using the same premises relating to the gasification rate and porosity evolution, though the pore structure appears to be a function mainly of experimental conditions and does not depend on the precursor used [4]. Further research is in
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progress in order to accomplish a comparison between raw materials with sharp differences in its initial characteristics.
References [1] Hu Z, Vansant EF. Synthesis and characterization of a controlled-micropore-size carbonaceous adsorbent produced from walnut shell. Micropor Mater 1995;3(6):603–12. [2] Arriagada R, Garcia R, Molina-Sabio M, Rodriguez-Reinoso F. Effect of steam activation on the porosity and chemical nature of activated carbons from Eucalyptus globulus and peach stones. Micropor Mater 1997;8(3–4):123–30. [3] Gonzalez MT, Rodriguez-Reinoso F, Garcia AN, Marcilla A. CO 2 activation of olive stones carbonized under different experimental conditions. Carbon 1997;35(1):159–62. ´ ´ ´ [4] Gonzalez JC, Gonzalez MT, Molina-Sabio M, RodrıguezReinoso F, Sepulveda-Escribano A. Porosity of activated carbons prepared from different lignocellulosic materials. Carbon 1995;33(8):1175–7. [5] Gergova K, Eser S. Effects of activation method on the pore structure of activated carbons from apricot stones. Carbon 1996;34(7):879–88. [6] Hussein MZ, Tarmizi RSH, Zainal Z, Ibrahim R, Badri M. Preparation and characterization of active carbons from oil palm shells. Carbon 1996;34(11):1447–9. [7] Abe I, Hitomi M, Ikuta N, Kawafune I, Kera Y. Properties of porous carbons prepared from the wood of Japanese cypress – effect of carbonization time at 9008C. Carbon 1996;34(11):1455. [8] Al-Khalid TT, Haimour NM, Sayed SA, Akash BA. Activation of olive-seed waste residue using CO 2 in a fluidized-bed reactor. Fuel Process Technol 1998;57(1):55–64. [9] Tancredi N, Cordero T, Rodriguez-Mirasol J, Rodriguez JJ. Activated carbons from Uruguayan eucalyptus wood. Fuel 1996;75(15):1701–6.
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´ [10] Rodrıguez-Reinoso F. Physical activation: basis and technology. In: Science and technology of activated carbon, short course, Universidad de Alicante, 1994, pp. 4–114. ´ ´ [11] Rodrıguez-Reinoso F, Molina-Sabio M, Gonzalez MT. The use of steam and CO 2 as activating agents in the preparation of activated carbons. Carbon 1995;33(1):15–23. [12] Practice E300, Annual Books of ASTM Standard, vol 15.05, 1995. [13] Gregg SJ, Sing KSW. In: Adsorption, surface area and porosity, London: Academic Press, 1997, pp. 195–247. ´ ´ Martınez ´ [14] Rodrıgues-Reinoso F, Garrido J, Martın JM. The combined use of different approaches in the characterization of microporous carbons. Carbon 1989;27(1):23–32. ˜´ ´ D, Alcanız-Monge [15] Cazorla-Amoros J, de la Casa-Lillo MA, Linares-Solano A. CO 2 as an adsorptive to characterize carbon molecular sieves and activated carbons. Langmuir 1998;14:4589–96. [16] Marsh H. Origin and structure of porosity in carbons. In: Science and technology of activated carbon, short course, Universidad de Alicante, 1994, pp. 4–14. [17] McEnaney B. Characterization of activated carbon I: porosity. In: Science and technology of activated carbon, short course, Universidad de Alicante, 1994, pp. 14–25. [18] Byrne JF, Marsh H. Introductory overview. In: Patrick JW, editor, Porosity in carbons: characterization and applications, UK: Halsted Press, 1995, pp. 2–46. ´ [19] Rodrıguez Reinoso F, Linares-Solano A. Microporous structure of activated carbons as revealed by adsorption methods. In: Thrower PA, editor, Chemistry and physics of carbon, vol. 21, New York: Dekker, 1988, pp. 1–141. ´ ´ [20] Garrido J, Linares-Solano A, Martın-Martınez JM, Molina ´ Sabio M, Rodrıguez-Reinoso F, Torregosa R. Use of N 2 vs. CO 2 in the characterization of activated carbons. Langmuir 1987;3:76–81. [21] Muhlen HJ, van Heek KH. Porosity and thermal reactivity. In: Patrick JW, editor, Porosity in carbons: characterization and applications, UK: Halsted Press, 1995, pp. 131–48.
Carbon 39 (2001) 1367–1377
CO 2 activation of char from Quercus agrifolia wood waste a, b *, Alfredo Aguilar Elguezabal ´ ´ Alejandro Robau Sanchez , b Luis de La Torre Saenz a
b
´ , CIPIMM, Havana, Cuba Centro de Investigaciones para la Industria Minero Metalurgica ´ Departamento de Catalisis , Centro de Investigaciones en Materiales Avanzados, CIMAV, Chihuahua, Chih. 31109, Mexico Received 12 October 1999; accepted 15 September 2000
Abstract Three series of activated carbons were prepared from Quercus agrifolia wood using a two-step process, carbonization followed by physical activation with CO 2 . Samples were characterized by N 2 and CO 2 adsorption. Three activation temperatures were used, 800, 840 and 8808C, covering the 18–85 wt.% yield range by variation of residence time. Activated carbons with a well-developed porous structure, predominantly microporous with high BET surface areas, were obtained. No direct relationship was found between exposed BET surface area (the surface where activation reaction takes place), evolution and gasification rate variation. Porosity development appears to be strongly influenced by the kinetic reaction stage, the reactant gas concentration gradient being an ultimate factor that induces porosity evolution. 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Activated carbon; D. Porosity
1. Introduction For many years an established and prolific field within carbon research has been the description of synthesis methods and characterization of carbonaceous porous materials such as activated carbons, chars and composites produced from a wide variety of sources. Agricultural by-products are a quite important and renewable source of raw materials for activated carbon production, and our recent efforts have focused on the preparation of activated carbon from the Quercus agrifolia tree, a plant which is used in the wood industry in United States and Mexico. In Chihuahua, a state in northern Mexico, this tree is very common and its contribution important to agricultural by-products generated during wood processing. Disposal of these residues has become an ecological problem; an attractive economic and technological solution could be its conversion into activated carbon. ´ *Corresponding author, Departamento de Catalisis, Centro de Investigaciones en Materiales Avanzados, CIMAV, Miguel de Cervantes 120 Complejo Industrial Chihuahua, C.P. 31109 Chihuahua, Mexico. Tel.: 152-14-439-1111; fax: 152-14-4391112. ´ E-mail address: [email protected] (A.R. Sanchez).
Previous workers have reported activated carbons obtained from different agricultural wastes such as walnut shells [1], eucalyptus globulus and peach stones [2], olive stones [3], almond shells and peach, plum and cherry stones [4], apricot stones [5], oil palm shells [6], Japanese cypress [7], olive-seed waste residues [8] and Uruguayan eucalyptus wood [9]. It has been extensively proved that any cheap material with high carbon content can be used as raw material for the production of active carbon. There are two principal processes for its preparation: physical and chemical activation [10]. Physical activation, the method used in this study, involves previous carbonization of a carbonaceous material, followed by activation of the resulting char at a temperature of 800–10008C in the presence of suitable oxidizing gases such as steam or carbon dioxide, activation being basically a controlled combustion reaction process. Several elements make study of gasification reactions a challenging and demanding enterprise. The synergic effect of carbon dioxide and steam during controlled combustion processes in internally heated furnaces (i.e. rotary kilns) is a complex phenomenon due to several known reactions that take place. Steam is not only a combustion product but is added to the activation kiln atmosphere to increase its partial pressure.
0008-6223 / 01 / $ – see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 00 )00253-0
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Because of the above-mentioned facts, it is advisable to study activation reactions separately since it would be quite difficult to establish the effect of each gasification reaction and consequently its effects in the final porosity and surface properties of carbon in the presence of several oxidizing gases. CO 2 was chosen as the activating agent for the present study. Steam activation of Quercus agrifolia will be discussed in a further publication. In an attempt to clarify discrepancies associated with an unclear tendency in the effect of use of CO 2 or steam on the development of porosity of activated carbons in previously published ´ works, Rodrıguez-Reinoso et al. [11] concluded that CO 2 produces an opening, followed by a widening, of narrow microporosity under conditions given for reported results. In their work, Gonzalez et al. [3] used both vertical and horizontal furnaces for CO 2 activation and report that the use of a horizontal furnace seemed to be advantageous for microporosity development. Aiming to obtain activated carbons with a well-developed microporosity and correspondingly high surface area from Quercus agrifolia, results reported in the abovementioned works concerning activating agent and equipment [3,11] were followed, studying the relationship between activation parameters and obtained activated carbon properties.
2. Materials and methods
2.1. Experimental procedure 2.1.1. Carbonization Quercus agrifolia wood waste was obtained from local wood manufacturers. Carbonization of samples was performed in a Labline Heatech economy furnace with a built-in microprocessor control containing a purpose-made steel retort provided with inlet connections that made possible the inert gas (nitrogen) feeding used during the heating and cooling steps, so that oxygen from the air does not burn off the sample. There were two additional pipes attached to the retort lid; one of them worked as an exhaust fume chimney and the other as a protective tube for the thermocouple. The lid was tightly fastened to the retort in order to prevent air infiltration. The adequate carbonization temperature and retention time corresponding to a yield of |32–35 wt.%, were previously determined from a TGA analysis. Charges of 1000 g of raw precursors were placed in the retort, which was positioned in the heating zone in such a way that the whole charge was exposed to the same temperature. The system was then heated at a fixed rate of 208C / min to 4508C and held at this temperature for 120 min. The average carbonization yield obtained was 34 wt.%. The carbonized material was then crushed and sieved to obtain a granulated fraction between 0.5 and 1.4
mm, carefully homogenized according to Ref. [12]. Since at 4508C the carbonization process is not completed, it is important to ascertain the weight loss undergone by the precursor from 4508C to each one of the applied activation temperatures, thus facilitating the determination of exact weight loss due to the activation process alone. Consequently, additional runs of carbonization up to corresponding temperatures were carried out. For 800, 840 and 8808C overall carbonization yields of 27, 26 and 25 wt.% were obtained, respectively.
2.1.2. Activation Activation was performed in a Thermolyne F21100 horizontal tube furnace. A fused silica tube was introduced into the furnace chamber, closed at both ends with rubber stoppers with metal tubes inside. These tubes were used for gas flow inlet and outlet as well as for a thermocouple. Ten grams of char were placed inside a stainless steel net that was put in the center of the furnace preheated to the corresponding temperature and previously purged with CO 2 . The carbon dioxide flow was 500 ml / min. Selected activation temperatures were 800, 840 and 8808C and experiments were performed covering the 18– 85 wt.% yield ranges by changing residence time for those three series. Activation temperature and yield are included in sample nomenclature. 2.1.3. Characterization Porosity of the obtained samples was measured according to physical characterization performed using an automatic system (Quantachrome Autosorb 1), through physical adsorption of nitrogen at 21968C and CO 2 at 08C. Changes in the porosity of produced carbons were examined using parameters that do not characterize adsorbent materials completely yet provide helpful and clear information on the evolution of different pore size ranges. The first parameter chosen was micropore volume (Vwm , WM-wide microporosity), calculated from the application of the Dubinin–Raduskhevich (DR) equation to the N 2 adsorption isotherms [13]. By including experimental data up to a relative pressure of P/P0 5 0.3, one can obtain the micropore volume corresponding to wide microporosity [14]. The second parameter was total pore volume (Vt ) measured as the amount of nitrogen adsorbed at P/P0 5 0.95. In type I isotherms this represents pore volume for both micropores and mesopores. Mesopore volume (Vme ) was then calculated as the difference between Vt and Vwm . An additional adsorption parameter, narrow microporosity volume (Vnm , NM-narrow microporosity), was deduced from the DR equation applied to CO 2 adsorption isotherms [15] up to P/P0 5 0.03. The apparent N 2 surface area obtained from the BET equation within the 0.001–0.2 relative pressure range was calculated too because, despite its controversial interpretation for activated carbons [16,17], this parameter provides
´ et al. / Carbon 39 (2001) 1367 – 1377 A.R. Sanchez
a value that can be useful when comparing characteristics of obtained samples. Instead of considering a single surface area with which to characterize a porous solid, Byrne and Marsh [18] proposed distinguishing between three definitions of this parameter: geometric surface area (GSA), which does not include meso- and microporosity and refers to the geometric shape area of a particle, total surface area (TSA), which does not have a precise definition and is considered to be the maximum surface area which a porous solid can exhibit and the effective surface area (ESA), which would be the area that the material appears to exhibit, as measured from adsorption isotherms using a defined adsorptive. It can be considered that the nitrogen isotherm, according to BET, provides a measure of the ESA and, at the same time, ESA can be considered as that area available to a reacting gas molecule to remove a carbon atom within the porosity. The latter concept was used in the analysis gasification behavior presented in this study.
3. Results and discussion Experimental results are presented in Table 1. Results refer to three series according to activation temperatures used: 800, 840 and 8808C series.
3.1. Gasification behavior The following analysis was made using values deduced from N 2 adsorption isotherms. Microporosity volumes obtained from CO 2 adsorption isotherms were not included
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because obtained values suggest that their contribution to total microporosity is negligible, and the trends discussed hereafter were not affected by Vnm values. This fact will be the subject of further discussion. Weight losses (yield) as a function of residence time for obtained carbons are plotted in Fig. 1; the influence of temperature and residence time on the activation reaction yield is easily noticed. The evolution of apparent activation energy has been included in this figure for further discussion. Although the overall gasification rate shows a constant trend very close to a linear behavior (Fig. 1), more noticeable differences can be found when point-to-point values in each series are considered (Fig. 2a). In this figure the evolution of the gasification rate (reacted carbon in the considered time interval, wt.% / h) can be viewed as a function of normalized residence time. Gasification rates seem to vary depending on applied activation temperature and residence time and undergo changes from 30 to 12, 50 to 33 and 60 to 51 wt.% / h for 800, 840 and 8808C series, respectively, reaching their highest value at 8808C (60 wt.% / h) at early stages of activation. The lowest gasification rate values are found at 8008C and for largest residence time (12 wt.% / h), the gasification rate increasing about five times from the lowest to the highest temperature. It is discernible in Fig. 2a that for lower temperatures (800 and 8408C) and residence times around 60% of longest residence time used, the gasification rate decreases, but from this point forward a less considerable variation is shown. Nevertheless at 8808C, a linear and constant decrease in the gasification rate is perceived. Fig. 2b depicts evolution of the intrinsic gasification rate
Table 1 Results obtained from characterization of obtained series Activation temperature
8008C series
8408C series
8808C series
Sample
Char E-800-85 E-800-76 E-800-64 E-800-54 E-800-51 E-800-42 E-840-76 E-840-66 E-840-47 E-840-37 E-840-18 E-880-70 E-880-58 E-880-47 E-880-36
Gasification rate wt.% / h
Intrinsic gasification rate, IGR wt.% / h m 2
29.8 24.3 18.0 15.4 12.2 11.6 48.4 34.3 35.1 31.7 32.7 59.8 56.5 53.2 51.0
0.065 0.045 0.027 0.017 0.013 0.013 0.065 0.050 0.039 0.028 0.028 0.066 0.070 0.057 0.047
From CO 2 isotherms DR micropore volume Vnm cc / g 0.032 0.034 0.052 0.043 0.033 0.020 0.014 0.023 0.034 0.020 0.011 0.009 0.026 0.044 0.030 0.016
From N 2 isotherms BET surface area m2 / g
DR micropore volume Vwm cc / g
Mesopore volume Vme cc / g
Total pore volume Vt cc / g
408 510 570 774 931 903 900 588 772 1034 1201 1173 723 899 982 1197
0.168 0.206 0.232 0.315 0.387 0.374 0.376 0.249 0.329 0.438 0.512 0.505 0.303 0.379 0.416 0.509
0.009 0.037 0.044 0.071 0.102 0.092 0.040 0.029 0.054 0.113 0.159 0.171 0.049 0.086 0.113 0.141
0.177 0.243 0.276 0.386 0.490 0.466 0.416 0.277 0.383 0.551 0.671 0.676 0.352 0.466 0.529 0.650
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Fig. 1. Evolution of yield as a function of residence time.
(wt.% / h m 2 ) as a function of normalized residence time. The intrinsic gasification rate (IGR) was deduced by relating weight loss to the corresponding residence time (gasification rate) and to the average exposed BET surface area, the latter being calculated as the average conformed by the surface area of the previous experimental point and the area corresponding to the analyzed point, postulating that CO 2 can only react with surface sites which contribute to the measurable BET surface area. This assumption has been made on the basis of porosity results, since the contribution of wide micropores is predominant in the overall micropore volume. Evolution of the exposed BET surface area as a function of normalized residence time can be viewed in Fig. 2c. In an attempt to relate gasification rate evolution to exposed BET surface area (the surface on which the reaction is assumed to take place), data plotted in Fig. 2a–c were scrutinized. Exposed BET surface area (Fig. 2c) shows an immediate enlargement up to 60% of the longest residence time at 8008C. Any further increase in residence time does not enlarge this value. At 840 and 8808C, the exposed BET surface area shows a sustained increment even up to the longest applied residence time. At 8008C there is a decrease in IGR up to 50–60% of total residence time (see Fig. 2b). From this range on, the decrease is less marked and can be considered a stabilization of the gasification rate. Similar behavior is detected at
8408C. There is a decrease in IGR at 8808C also, which continues up to the longest applied residence time, becoming less marked beyond 70% of the longest time. IGR exhibits different lowering trends at early stages of activation, more marked ones than at longer residence times when a less marked decrease is noticed; this behavior is common for all temperatures applied. Nevertheless, there is a constant increase in exposed BET surface area (and thus an increase in the exposed reaction surface according to above-mentioned assumptions) at 840 and 8808C. The above-mentioned analysis suggests that the exposed BET area does not have a marked influence on the gasification rate and that previous assumptions of free and unblocked reaction surface sites are not correct. At 8008C a simultaneous decrease in both exposed BET surface and IGR can be observed, but it was not considered as conclusive evidence. At 840 and 8808C, despite the general increasing trend of exposed BET surface area, IGR declines. One could suppose that as the gasification reaction goes on and internal porosity subsequently develops in carbon particles, there is a resulting enlargement in the path that must be followed by CO 2 molecules up to the surface sites where they might react and from them to the gas bulk, in the case of reaction products. This would imply a retarding effect on the course of the reaction, evidenced through a decrease in the gasification rate as residence time in-
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activation energy was calculated for similar residence times and its behavior was observed as a function of reaction advancement. Magnitudes varied from that associated with diffusion-controlled processes (less than 20 kcal / mol), at early stages of the reaction, to values that indicate a transition to a chemical-controlled stage at higher degrees of activation. Fig. 1 indicates calculated apparent activation energy values, and zones delimited by diffusionand chemical-controlled reaction can be observed. In our case critical residence time, up to which diffusion governs the process, appears to be around 45 min. As a result, the lower gasification rate at advanced stages of the activation process should not be associated with a diffusion-limiting step. Finally this gasification rate behavior is considered as the consequence of the suggested inhibiting mechanism of reaction, possibly caused by formation of carbon–carbon monoxide complexes on the internal surface of carbon [10]. These complexes would block a considerable amount of reaction sites, limiting access of CO 2 and hence retarding overall reaction. At 8808C, although an IGR reduction is observed, it is less striking than that corresponding to lower activation temperatures. The lower extent of reaction attained in this case does not allow to establish if a constant value of IGR would be reached at some further point in its progress.
3.2. Porosity development Fig. 3a gives plots of micropore, mesopore and total pore volume as a function of yield, and the evolution of BET surface area as a function of the yield is shown in Fig. 3b. Results were deduced from N 2 adsorption. Different porosity progressions are found for the series analyzed. Because the behavior observed is close to a linear relationship between yield and residence time for all obtained series (Fig. 1), further discussion will be referred only to yield.
Fig. 2. (a) Gasification rate as a function of normalized residence time. (b) Evolution of intrinsic gasification rate, as a function of normalized residence time. (c) Exposed BET internal surface area as a function of normalized residence time.
creases. In other words, diffusion would be the raising and controlling step of the reaction and it could be another reason for the gasification rate to lessen. If this were the case, the apparent activation energy of the process should be consistent with a diffusion-ruled reaction. In order to clarify this assumption, apparent
3.2.1. 8008 C series To produce this series, the longest residence times were used in order to obtain satisfactory degrees of activation, since at this temperature gasification rates are relatively low. Further experimental points were not made when a marked lessening tendency in obtained porosity was observed. In general terms, both microporosity and mesoporosity increased up to 55 wt.% yield (Fig. 3a). Further activation did not result in a higher porosity, but quite the opposite. The contribution of both microporosity and mesoporosity to this diminishing at yield values lower than 60 wt.% was different. While the micropore volume, after shrinking a little, sustained a constant value, mesoporosity decreased after a rising trend continued up to around 55 wt.% yield. BET surface area shows a similar behavior to micropore
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Fig. 3. (a) Evolution of pore volume as a function of yield. (b) Evolution of BET internal surface area as a function of yield.
volume (Fig. 3b), since the two parameters are directly related.
3.2.2. 8408 C series The lowest yield values were attained in this series. The constant increase in all pore volumes reaches its highest quantity at 35 wt.% yield. Lower yields result in a decrease in micropore volume while mesopore volume continues to rise. Total pore volume increases slightly below 35 wt.%
yield due to higher mesopore volume, despite a shrinking micropore volume. In the same manner, BET surface area decreases according to micropore behavior (Fig. 3b).
3.2.3. 8808 C series In this series, due to the small amounts of starting material (and consequently small quantity of material obtained) and the high reactivity shown, activation beyond 35 wt.% was not carried out. An increase in both micro-
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pore and mesopore volumes as well as in BET surface area values is observed (Fig. 3a and b). However, micropore volume apparently exhibits a more marked raising trend than mesopore volume (see Table 1). Pore volume evolution can then be summarized in the following manner for all series. 1. Microporosity is enlarged at 8008C and its highest attained value maintained almost constant below 50 wt.% yield. Within the same yield range mesoporosity decreases. 2. At 8408C the highest values of micropore volume at 36 wt.% yield are obtained. Further activation results in a decrease in this parameter while mesoporosity still goes up. 3. At 8808C both microporosity (more noticeable) and mesoporosity increase constantly for all samples obtained and reach a maximum comparable to that obtained for 8408C series with the same yield. There were clear differences in ascending and descending trends observed in both micropore and mesopore volumes in the series produced. In Fig. 4a evolution of micropore–mesopore volume ratio (Vwm /Vme ) is plotted as a function of yield for all series. As a consequence of constant micropore volume values attained in 8008C series and the simultaneous decrease in mesopore volume for yields less than 55 wt.%, a rising tendency of Vwm /Vme ratio is found related to the mentioned yield range. The values of Vwm /Vme at early stages of activation suggest that mesoporosity is being modified at a higher rate than microporosity. At 8408C and 8808C series, similar behavior is observed up to a 45 wt.% yield. Below this point both microporosity and mesoporosity appear to maintain almost the same variation in ratio; while at 8408C mesoporosity seems to be modified at a slightly faster rate than microporosity for yields less than 36 wt.%, at 8808C both mesoporosity and microporosity are being altered at the same ratio below 50 wt.% yield. For a complementary analysis of porosity, it is useful to express results per unit of starting char [3,18]. These results are plotted in Fig. 4b. There is an initial increase in micropore volume for all series at yields higher than 55 wt.%, a decrease beginning as the yield gets lower. In 8008C series mesoporosity follows the same tendency as microporosity. At 8408C mesoporosity still increases up to values around 35–38 wt.% yield, with a further decrease at lower yield values. At 8808C mesopore volume rises up to 48 wt.% yield, decreasing slightly below this value. Analysis of Fig. 4a and b gives interesting information. According to these figures, microporosity is destroyed below 50 wt.% yield for all series, but it is important to note that both mesoporosity and microporosity undergo the same process at 8008C (Fig. 4b) while the Vwm /Vme ratio rises abruptly (Fig. 4a). At 8408C, although micropore volume is decreasing below 50 wt.% and mesopore
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volume continues to increase up to 38 wt.%, the Vwm /Vme ratio remains almost unchanged. Similar behavior can be observed below 50 wt.% yield at 8808C, where micropore volume decreases while mesopore volume stays almost at the same value though the Vwm /Vme ratio varies very slightly. These results indicate that porosity is created at lower stages of activation for yields higher than 55 wt.%, while for larger activation degrees widening of previously created micropores and external burn off of particles become significant, in agreement with results reported in Ref. [11]. Observed tendencies when comparing Figs. 3a, 4a and 4b show that at 8008C mesopores are widened (and converted into pores of non-detectable radii using applied adsorption technique) at a faster rate than micropores below 50 wt.% yield.
3.2.4. CO2 adsorption When analyzing microporosity, it is advisable to establish differences between wide and narrow microporosity. This can be achieved using different adsorptives such as CO 2 , which appears to measure a narrower porosity range than N 2 , which detects wider micropores [19]. CO 2 adsorption isotherms were determined for the samples studied. Micropore volumes deduced from the DR equation are presented in Table 1, and Fig. 5 shows evolution of Vnm as a function of yield. There is a common trend for all series: an initial increase followed by a brusque decrease. Those yield values in which the observed trend changes are different for each series. For 8008C series, this takes place when a 75 wt.% yield is reached, while for 840 and 8808C turning points are 65 and 55 wt.% yield, respectively. Differences between the micropore volumes calculated from the DR equation applied to CO 2 and N 2 isotherms have been related to the porous structure of activated ´ carbons in previous studies published by Rodrıguez-Reinoso and Linares-Solano [19] and Garrido et al. [20]. According to the classification proposed, group C includes activated carbons with medium-to-high burn off, with a wider and very heterogeneous microporosity; for these carbons, N 2 micropore volume (Vwm ) is higher than that of CO 2 (Vnm ). Activated carbons presented in this study belong to this group. It can be noticed (Table 1) that for all samples, Vwm . Vnm but the contribution of Vnm to both microporosity and total porosity is also very small. For this reason, in the previously presented analysis concerning gasification behavior, Vnm was not included. The structure of the lignocellulosic material used in this study presumably does not favor the formation of narrow micropores to a considerable extent, at least under the carbonization and activation conditions here employed. For a brief evaluation of the progress of both Vwm and Vnm , Fig. 6 shows the evolution of the Vwm /Vnm ratio as a function of yield for all series. As yield decreases, it can be
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Fig. 4. (a) Evolution of Vwm /Vme ratio, as a function of yield. (b) Evolution of pore volume referred to initial weight as a function of yield.
noticed that after a slight initial decline in the Vwm /Vnm ratio, there is a sharp increase which begins at similar yield values to those at which Vnm decreases for each series (see Fig. 5). This behavior is a consequence of narrow microporosity widening, corroborated through simultaneous growth of Vwm . It is interesting that the Vwm /Vnm ratio maintains its rising trend at 800 and 8408C for yields less than 50 and 35 wt.%, respectively, although Vwm has attained almost constant values (Fig. 3a). Consideration of the data shown in Figs. 4a, 5 and 6
reveals several details. At 8008C, while Vnm decreases for yields below 75 wt.%, both Vwm and Vme still increase up to 55 wt.% yield. Below this yield, Vwm reaches a fairly stable magnitude while Vme decreases. The Vwm /Vnm ratio shrinks while the Vwm /Vme ratio increases rapidly. As a result, it appears that wide microporosity is preserved. For the 8408C series both Vwm and Vme maintain a rising trend up to 35 wt.% as yield lessens. For any further diminishing yield both Vnm and Vwm decrease slightly as Vme increases. However, the Vwm /Vnm ratio is still growing
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Fig. 5. Evolution of micropore volume (Vnm ) deduced from carbon dioxide DR equation as a function of yield.
at the same time as the Vwm /Vme ratio remains almost unaffected, indicating that Vnm is widening at a faster rate than Vwm (see Figs. 4a and 6). At 8808C both Vwm and Vme show a sustained increment up to the smallest yield even as Vnm decreases. As to Vwm /Vme and Vwm /Vnm ratios, the same behavior described for 8408C series is detected. However, some discrepancies in pore volume changes can be noted. A fraction of the narrow microporosity (Vnm ) is widened and becomes part of the wide microporosity
(Vwm ). At the same time, a portion of the wide microporosity (Vwm ) is widened and incorporated into mesopores (Vme ). Apparently there is not an equivalence between both widened micropore volumes (Vnm converted to Vwm and Vwm converted to Vme ). The observed divergence is more evident for 800 and 8408C series. It might be attributed to a possible overlapping of micropore volumes deduced from N 2 and CO 2 isotherms, so that N 2 would also detect part of the microporosity measured by CO 2 adsorption. For this reason, Vnm was not included in total porosity since
Fig. 6. Evolution of Vwm /Vnm ratio, as a function of yield.
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there are no guarantees of a well-defined measurement between Vnm and Vwm . This illustrates the need for complementary techniques that would allow a more precise description of microporosity, mainly for highly activated and heterogeneous carbons.
3.3. Some final considerations According to the previously discussed transition from a diffusion-controlled process at early stages of activation to a chemical-controlled step while the reaction goes on, it can be assumed that reactant concentration is lower in the interior of the particle and the external burning of the particle will be more relevant [3]. This behavior is suggested by the evolution of Vwm /Vme and Vwm /Vnm ratios observed in all series, but to different extents; it is more evident at 8008C, when for yields less than 55 wt.% microporosity is preserved while mesoporosity is destroyed, although its ratio increases. Similar microporosity behavior has been reported previously for different carbonaceous materials [11,21]. Besides differences in the strength of chemical bonding on the location of a reacting carbon atom (more weakly bonded sites tend to be consumed preferentially at the beginning of the reaction), a carbon atom must be accessible to the activating agent. The lower the bonding energy and the better accessibility, the more probable it is that at a given place carbon lattices are intensively attacked by the gasifying agent and the porosity enlarged. Since gas transport depends on pressure, temperature, rate and degree of conversion at a given yield, the growth of porosity will be less marked in micropores if the reaction rate controlled by reaction temperature exceeds a certain value [21]. This supports the proposed explanation. If it is assumed that most reactant gases must pass through mesoporous system in order to get into micropores and thus induce further volume enlargement (or reduction, if enough widening takes place), one could expect a sharp concentration gradient between the external surface and micropores. As a consequence, reactant concentration will be lower in micropores than in mesopores; a portion of the latter would widen and become pores of undetectable diameter with the adsorption techniques applied in this study. Since the absolute contribution to total porosity is less for mesopores than for micropores, mesoporosity decreased would be less noticeable unless Fig. 4a was analyzed, which indicates that mesopores disappear faster than micropores at 8008C. This explanation is also suitable for 8408C and 8808C series. In this case both microporosity and mesoporosity decrease for lower yields, but ratios remain unchanged. This would imply that reactant concentration in both micropores and mesopores has reached a similar magnitude, so that the concentration gradient mentioned was more marked between external and internal parts of the carbon particle. Evidence of this mechanism
could be the stable Vwm /Vme ratio, since the rate at which both mesoporosity and microporosity are being modified is similar while one observes a reduction in microporosity destruction as temperature increases.
4. Conclusions The experimentation and resulting analysis presented in this study have focused on the description of the activation suitability of Quercus agrifolia wood waste with CO 2 as well as on characterization of the activation process and the activated samples obtained. All three series of activated carbons were prepared under similar condition of CO 2 flow; different activation progressions through residence time variation were obtained for each temperature. According to the results and analysis, as well as to information found in other published studies [3,11], the following conclusions may be drawn. 1. Quercus agrifolia wood waste can be considered a suitable raw material for activated carbon preparation. Activated carbons with well-developed heterogeneous porous system and high surface areas were obtained. It can be observed that developed micropores are predominantly supermicropores. Porous heterogeneity of the activated carbons obtained makes porosity analysis complicated, since applied adsorption techniques do not permit a sharp distinction between ‘narrow’ and ‘wide’ microporosity. 2. Assumptions concerning a link between gasification rates and both exposed BET surface area and free active reaction sites did not prove correct. Decrease in the gasification rate while the activation reaction advances could be related to inhibition caused by carbon–carbon monoxide complexes formed on the carbon surface. 3. Microporosity increases at early stages of the activation process up to around 55 wt.% yield. Further activation causes decreases in mesoporosity at lower activation temperatures. As temperature rises, mesoporosity continues to increase for values below 55 wt.% yield. 4. Transition from diffusion to a chemical-controlled process seems to be the ruling mechanism that affects porosity development. Variation in the reactant gas concentration gradient, established between external and internal parts of carbon particles appears to be the ultimate factor that determines both microporosity and mesoporosity evolution. It may be interesting to perform a similar analysis applied to other precursors with different starting properties using the same premises relating to the gasification rate and porosity evolution, though the pore structure appears to be a function mainly of experimental conditions and does not depend on the precursor used [4]. Further research is in
´ et al. / Carbon 39 (2001) 1367 – 1377 A.R. Sanchez
progress in order to accomplish a comparison between raw materials with sharp differences in its initial characteristics.
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