Activated Carbon Produced from Charcoal Obtained by Vacuum

produced during conventional, atmospheric carbonization process. Steam activation of charcoal obtained by vacuum pyrolysis yields an activated carbon ...
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Energy & Fuels 2001, 15, 1263-1269

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Activated Carbon Produced from Charcoal Obtained by Vacuum Pyrolysis of Softwood Bark Residues Naizhen Cao,†,‡ Hans Darmstadt,†,‡ and Christian Roy*,†,‡ De´ partement de ge´ nie chimique, Universite´ Laval, Que´ bec, Qc, G1K 7P4, Canada, Institut Pyrovac Inc., 333 rue Franquet, Ste-Foy, Qc, G1P 4C7, Canada Received March 26, 2001. Revised Manuscript Received May 23, 2001

Vacuum pyrolysis of softwood bark residues yields besides oil, charcoal as second valuable product. The activation behavior of the vacuum pyrolysis charcoal was compared to charcoal produced during conventional, atmospheric carbonization process. Steam activation of charcoal obtained by vacuum pyrolysis yields an activated carbon with a higher surface area than by using charcoal produced during atmospheric carbonization process. This can be related to the more open pore structure of the vacuum pyrolysis charcoal. Steam activation of the vacuum pyrolysis charcoal was studied at temperatures ranging from 760 to 850 °C. Activated carbons with surface areas above 1200 m2/g and micropore volumes above 0.40 cm2/g were obtained. Depending on the activation conditions, carbons with properties (surface area, pore volume, pore structure and methylene blue value) similar to commercial grades were produced.

Introduction Activated carbon can be produced from various biomass materials. With the increasing ecological and economical significance of environmental protection, the use of waste biomass as feedstock material for the production of activated carbons is attracting increasing interest.1-6 Bark residues are produced by the pulp and paper industry in large quantities during the debarking operation. This waste material is causing significant environmental pollution, in particular in Canada with its important forest industry.7 An advanced pyrolysis process called Pyrocycling, which has been recently demonstrated at the industrial scale,8 can transform bark residues into 36-43% oil, 28-31% charcoal, 1620% gas and 11-17% water. The pyrolysis oil contains valuable fine chemicals, whereas burning of the pyrolysis gas supplies a portion of the heat required by the process.8 The pyrolysis process is conducted at a total †

Universite´ Laval, Que´bec. Institut Pyrovac Inc. * Corresponding author. Tel.: (418) 656-7406. Fax: (418) 656-2091. E-mail: [email protected]. (1) Karaosmanoglu, F.; Tetik, E. Charcoal from the pyrolysis of rapeseed plant straw-stalk. Energy Source 1999, 21, 503-510. (2) Wu, S. L.; Iisa, K. Kinetics of NO reduction by black liquor char. Energy Fuels 1998, 12, 457-463. (3) Bacaoui, A.; Yaacoubi, A.; Bennouna, C.; Dahbi, A.; Ayele, J.; Mazet, M. Characterisation and utilisation of a new activated carbon obtained from Moroccan olive wastes, Aqua (Oxford) 1998, 47, 68-75. (4) Edgehill, R. U.; Lu, G. Q. Adsorption characteristics of carbonized bark for phenol and pentachlorophenol, J. Chem. Technol. Biot. 1998, 71, 27-34. (5) Tancredi, N.; Cordero, T.; Rodriguez-Mirasol, J.; Rodriguez, J. J. Activated carbons from eucalyptus wood. Influence of the carbonization temperature. Separ. Sci. Technol. 1997, 32, 1115-1126. (6) Valenzuela, C. C.; Gomez, S. V.; Hernandez, A. J.; Bernalte, G. A. Use of waste matter after olive grove pruning for the preparation of charcoal. The influence of the type of matter, particle size and pyrolysis temperature. Bioresource Technol. 1992, 40, 17-22. (7) Martel, S.; Tremblay, J.-M. Direction du de´veloppment de l′industrie des produits forestiers. Les e´corces au Que´bec. Ministe`re des ressources naturelles du Que´bec, Canada, 1999. ‡

pressure of 20 kPa and a temperature of approximately 450-500 °C.9 The Pyrocycling process is distinct from other pyrolysis processes which are usually performed under atmospheric pressure.10 Charcoal obtained by vacuum pyrolysis was found more reactive in CO2 gasification than charcoal from atmospheric carbonization.11 It was furthermore confirmed by thermogravimetry (TG) that charcoal from vacuum pyrolysis reacts faster with steam as compared to charcoal produced during atmospheric carbonization.12 This behavior was assigned to differences in the pore structure of charcoal from vacuum pyrolysis and atmospheric carbonization. An important difference between the two processes is the residence time of the organic vapors formed during pyrolysis from the decomposing feedstock in the reactor. The residence time is considerably shorter during vacuum pyrolysis, as compared to atmospheric carbonization. Therefore, side reactions of these hydrocarbons such as the formation of carbonaceous deposits on the surface and in the pores of the charcoal are limited. Charcoal from vacuum pyrolysis has, therefore, a more “open” pore structure and is more easily activated than charcoal from atmospheric pyrolysis.12 These findings suggest that charcoal (8) Roy, C.; Blanchette, D.; de Caumia, B.; Dube, F.; Pinault, J.; Belanger, E.; Laprise, P. Industry scale demonstration of the Pyrocycling process for the conversion of biomass to biofuels and chemicals. Presented at First world conference and exhibition on biomass for energy and industry, Sevilla, Spain, 2000, James & James, London, UK. (9) Roy, C.; Blanchette, D.; Caumia, B. Horizontal moving and stirred bed reactor". Canadian Patent 2,196,841, 1997. International Patent 98, 902, 153.0, 1998. US Patent 8,811, 172, 1997. (10) Bridgewater, A. V.; Peacocke, G. V. C. Fast pyrolysis processes for biomass. Renewable and Sustainable Reviews 2000, 4, 1-73. (11) Plante, P.; Roy, C.; Chornet E. CO2 gasification of wood charcoals derived from vacuum and atmospheric pyrolysis. Can. J. Chem. Eng. 1988, 66, 307-312. (12) Cao, N. Z.; Darmstadt, H.; Soutric, F.; Roy, C. Thermogravimetric study on the steam activation of charcoal obtained by pyrolysis of bark residue. Carbon, 2001, in press.

10.1021/ef0100698 CCC: $20.00 © 2001 American Chemical Society Published on Web 07/04/2001

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Table 1. Properties of Selected Commercial Activated Carbon Samples

sample

feedstock material

Calgon F-300 Asbury #5597 B & S 207A Norit Darco KBB Pica PX714 a

Coal Wood Coal Wood Wood

activation method

physical form; typical use

Steam Steam Steam Chemical Chemical

Pellets. Odor and color removal Powder. Sugar production Pellets. Food industry Powder; Color removal Pellets. Odor removal

surface area [m2/g] external BET DR (t-plot) 901 907 977 1605 1730

993 1060 1102 1502 1853

63 147 63 611 502

MBa value [mg/g] 105 183 125 270 300

pore volume [cm2] micropores mesopores (t-plot) (P/P0 ) 0.95) 0.38 0.36 0.42 0.50 0.58

0.10 0.22 0.09 0.90 0.69

Methylene blue.

from vacuum pyrolysis should be an attractive feedstock material for the production of activated carbon. In the present work, the steam activation of charcoal obtained by vacuum pyrolysis of bark residue was compared to the activation of charcoal produced at a temperature 700 °C and a pressure of 1 atm. These are “typical” conditions for the pyrolysis/carbonization step prior to physical activation in the traditional production of activated carbon. The effect of the activation temperature and time on the different properties of the activated carbons derived from the vacuum pyrolysis charcoals was determined. Furthermore, the experimental activated carbons were compared to commercial grades. Samples and Experimental Techniques Pyrolysis of the Various Bark Residue Samples. The bark residue originating from different softwood trees (70%v/v fir, 28%v/v spruce and 2%v/v pine, hemlock spruce and larch) was provided by Daishowa Inc., Que´bec City, Canada. It consisted of approximately 2/3 (wt./wt.) bark and 1/3 (wt./wt.) wood. Charcoal was produced in a pyrolysis process development unit (PDU), operated in a semi-continuous mode and in a 500 mL horizontal laboratory reactor operated in a batch mode, respectively. In the PDU, charcoal was obtained by vacuum pyrolysis of the dried feedstock material (1012% moisture) with a particle size between 0.5 and 40 US standard mesh, a feeding rate of 39 kg/h at a temperature of 450 °C and a total pressure of 20 kPa. The residence time of the solids was 20 min (run H32A). Detailed information on the PDU used can be found elsewhere.13 The atmospheric carbonization of the same feedstock material with a particle size between 10 and 20 US standard mesh was performed in the 500 mL laboratory reactor (run CW 005). Approximately 40 g of the feedstock was placed in a horizontal stainless steel tube and was heated under a flow of 120 mL/min of nitrogen to 700 °C at a heating rate 8 °C/min. This temperature was held for 2 h. Then the sample was allowed to cool under the same nitrogen flow. For comparison, a charcoal sample produced in the industrial scale Pyrovac demonstration vacuum pyrolysis plant with a capacity of 3000 kg/h was also used as activation feedstock. The pyrolysis temperature and pressure were similar to those of the PDU (run 990418CH2). Details on the demonstration plant and the pyrolysis conditions can be found elsewhere.8 (13) Roy, C.; Blanchette, D.; Korving, K.; Yang, J.; de Caumia, B. Development of a novel vacuum pyrolysis reactor with improved heat transfer potential. Developments in Thermochemical Biomass Conversion; Bridgewater, A. V., Boocock, D. G. B., Eds.; Blackie Academic and Professional; London, UK, 1997; pp 351-367.

Samples of commercial activated carbons were obtained from different producers: Norit Americans Inc., Marshall, TX, USA; Calgon Carbon Co., Huntington, WV, USA; Barnebey & Sutcliffe Co., Columbus, OH, USA; Pica USA, Inc., Columbus, OH, USA and Asbury Carbons Inc., Asbury, NJ. Properties of these samples are listed in Table 1. Steam Activation. The steam activation was performed in a 1000 mL vertical laboratory reactor. Approximately 20 g of charcoal particles between 10 and 20 US standard mesh were placed on a metal screen located in the middle of the reactor. Prior to the experiment, the reactor was purged with 120 mL/min nitrogen for at least 2 h. Then, the charcoal sample was heated in the same nitrogen flow to the desired final activation temperature at a heating rate of 5 °C/min. When the activation temperature was reached, the gas flow was switched to steam (100% H2O, 0.1 g/min) and the sample was held at the specified temperature during a predetermined activation time (1 to 15 h). After activation, the sample was allowed to cool in a nitrogen flow. For some experiments, the sample was allowed to cool immediately after it had reached the selected activation temperature. During such experiments, the samples were not exposed to steam. The steam was generated by heating and vaporizing water in electrically heated metal tubes. The water flow was regulated by a C/L 77120-52 peristaltic pump (Barnant, IL). In additional experiments some activation parameters were changed. In two runs, the charcoal samples were heated at rates of 10 and 20 °C/min, respectively, (as opposed to 5 °C/min) to the specified activation temperature. In the experiments comparing the activation of charcoal from atmospheric carbonization and vacuum pyrolysis, the charcoal mass was approximately 10 g (instead of 20 g). Characterization of the Charcoal and Activated Carbon. An Autosorb-1 MP apparatus from Quantachrome, Boynton Beach, FL, USA was used for the gas adsorption experiments of the carbons. Prior to the adsorption experiment, the sample was outgassed at 200 °C until the pressure increase in the closed sample cell was lower than 1.2 Pa/min (0.009 Torr/min). A typical final dynamic pressure was 0.125 Pa (0.001 Torr). Adsorption of nitrogen at -196 °C and of carbon dioxide at 0 °C was used to characterize the activated carbon and charcoal samples. The BET14 (P/P0 from 0.05 to 0.15) and DR15 (P/P0 from 2 ‚ 10-6 to 2.9 ‚ 10-2) equations were used to calculate the apparent specific surface area. Cross sectional areas of 16.2 and 21.0 Å2/ (14) Brunauer, S.; Emmett, P.; Teller, E. J. Adsorption of gases in multimolecular layers. J. Am. Chem. Soc. 1938, 60, 309-319. (15) Dubinin, M. M.; Radushkevich, L. V. Proc. Acad. Sci. USSR 1947, 55, 331.

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molecule were used for nitrogen and carbon dioxide, respectively. Since the BET method is widely used, in the text only the BET surface areas are discussed. However, the DR areas are listed in the corresponding tables as well. The micropore size distribution was calculated using the software provided by Quantachrome, which is based on the density functional theory (DFT).16 The micropore volume and external surface was calculated using the t-plot method,17 whereas the mesopore volume was obtained by subtracting the micropore volume from the volume adsorbed at P/P0 ) 0.95.18 The proximate analysis was performed using a Mac400 instrument from LECO, St. Joseph, MI, while the concentration of C, H and N was determined with a LECO CHN-600 apparatus. The methylene blue (MB) adsorption capacity was determined according to ASTM D3860 method. Results and Discussions Activation of Charcoal from Vacuum Pyrolysis vs Charcoal from Atmospheric Carbonization. The thermal decomposition temperature and pressure during vacuum pyrolysis (450 °C, 0.2 atm) is considerably lower as compared to typical conditions of atmospheric carbonization prior to the activation step (700 °C, 1 atm). It is, therefore, reasonable to assume that the properties of the charcoal produced in the two processes are different. Consequently, in the present work the activation of charcoal samples from vacuum pyrolysis and atmospheric carbonization was compared. Despite the quite different reaction conditions used, the charcoal yields of the two processes were very similar (32.1 vs 31%, Figure 1). A previous TG study12 had shown that a temperature of 450 °C during vacuum pyrolysis is not high enough to achieve complete decomposition of the bark residue feedstock, whereas the higher temperature of the atmospheric carbonization (700 °C) is sufficient to decompose the feedstock material. An incomplete decomposition during the vacuum pyrolysis is also indicated by the observation that the charcoal from vacuum pyrolysis lost more than one-third of its weight upon further heating to 800 °C (Figure 1). The low charcoal yield of the vacuum pyrolysis process can be explained by the low pyrolysis pressure. As already mentioned, during the pyrolysis, the gas atmosphere in the reactor consists of organic vapors formed from the decomposing feedstock material. During biomass pyrolysis, the evolving macromolecule vapors contain a high concentration of oxygen functional groups,9,19 which can easily undergo condensation reactions. These condensation reactions when they occur ultimately lead to solid products that increase the charcoal yield. Under (16) Olivier, J. P.; Improving the models used for calculating the size distribution of micropore volume of activated carbons from adsorption data. Carbon, 1998, 36, 1469-1472. (17) Lippens, B. C.; De Boer, J. H. “Studies on pore systems in catalysts V. The t method”. J Catalysis 1965, 4, 319-323. (18) Rodriguez-Reinoso, F.; Molina-Sabio, M.; Gonzalez, M. T. The use of steam and CO2 as activating agents in the preparation of activated carbons. Carbon, 1995, 33, 15-23. (19) Roy, C.; Lu, X.; Pakdel, H.; Amen-Chen, C. Wood composite adhesives from softwood bark-derived vacuum pyrolysis oil. Biomass, A Growth Opportunity in Green Energy and Value-Added Productes, Proc. 4th Biomass Conf. Am. Overend, R. P., Chornet, E., Eds.; Elsevier Science Ltd., Oxford, UK, 1999; pp 521-526.

Figure 1. Activation of charcoal from vacuum pyrolysis versus activation of charcoal from “typical” atmospheric carbonization (S.A.: surface area).

vacuum conditions, the vapors are quickly removed by the vacuum pump from the reactor. Thus, during vacuum pyrolysis condensation reactions are limited and the charcoal yield is reduced as compared to atmospheric pyrolysis. The specific surface area of the charcoal from atmospheric carbonization (580 m2/g, DR eqn., CO2 data) was higher as compared to the charcoal from vacuum pyrolysis (307 m2/g, DR eqn., CO2 data, Figure 1). As already mentioned above, during vacuum pyrolysis the temperature was not high enough to achieve complete decomposition of the feedstock. Thus, the low surface area of the charcoal from vacuum pyrolysis is caused by the presence of nondecomposed or not totally decomposed feedstock material in the pores of the charcoal. However, upon heating to 700 °C the decomposition of this material was completed, charcoal pores were liberated and the surface area increased to 557 m2/g (DR eqn., CO2 data). This value was similar to the surface area of the charcoal from atmospheric pyrolysis (Figure 1). However, there were differences in the pore structure of the two materials. For the charcoal produced by atmospheric carbonization, it was not possible to obtain equilibrium nitrogen adsorption data at -196 °C. Such behavior is observed for carbonaceous solids with very narrow micropores, where the diffusion of the nitrogen in the micropores is so slow that adsorption equilibrium is not reached within a reasonable time.20 Samples with narrow micropores can be studied by carbon dioxide adsorption at 0 °C. The DFT pore size distribution calculated from these data showed indeed that the (20) Rodriguez-Reinoso F. Linares-Solano A. microporous structure of activated carbons as revealed by adsorption methods. Chem. Phys. Carbon 1989, 21, 1-146.

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Figure 2. Micropore size distribution of charcoals from vacuum and atmospheric pyrolysis (from CO2 data) and of the activated carbons (from N2 data) produced from these charcoals, normalized to the same height.

micropores in the charcoal from atmospheric carbonization are very narrow. The pore size distribution showed a maximum at a width of 8 Å (Figure 2). The situation was different for the charcoal from vacuum pyrolysis, heated to 800 °C. For this material, it was possible to obtain the nitrogen adsorption isotherm, a first indication that the micropores of this charcoal were wider as compared to the charcoal from atmospheric carbonization. This was also confirmed by the DFT pore size distribution, which exhibited a maximum at a width of 9-10 Å and indicated the presence of larger micropores with width of up to 14 Å (Figure 2). The different pore size distribution of the two charcoal samples influences the steam activation of these materials. Upon steam activation at 800 °C during 2 h, the charcoal from vacuum pyrolysis yielded a carbon with a higher surface area (958 m2/g) as compared to the charcoal from atmospheric carbonization (812 m2/g, Figure 1). Furthermore, the activated carbon from vacuum pyrolysis charcoal had larger micro and mesopore volumes as compared to the activated carbon from “atmospheric” charcoal. All these parameters indicate that the activation of the charcoal from vacuum pyrolysis will be faster as compared to the charcoal from atmospheric carbonization, as reported by Cao et al.12 This can be attributed to the wider pores of the charcoal from vacuum pyrolysis. At the beginning of the activation, the pores are narrow. The rate-limiting step during activation is the diffusion of the oxidizing agent (steam in the present case) in the pores. Thus, the activation of the charcoal from vacuum pyrolysis exhibiting wider pores is faster as compared to charcoal from atmospheric carbonization with narrower pores. Differences in the chemical nature of the charcoal from vacuum pyrolysis and atmospheric pyrolysis may contribute as well to the faster activation of the vacuum pyrolysis charcoal. During atmospheric carbonization, the decomposition of the bark feedstock took place at higher temperatures as compared to vacuum pyrolysis (700 vs 450 °C). Since high temperatures favor the

Cao et al.

healing of defects in the charcoal structure, it is reasonable to assume that the number of defects in the charcoal from atmospheric carbonization is lower as compared to vacuum pyrolysis charcoal. During activation, oxidation occurs first at these defects. Thus, activation of the “high-defect” vacuum pyrolysis charcoal will be faster as compared to charcoal from atmospheric pyrolysis. Steam Activation of Vacuum Pyrolysis Charcoal, Surface Area of the Activated Carbons. In the previous section, the reactivity of vacuum and atmospheric charcoal during activation was compared. However, no attempt was made to optimize the activation conditions. In this section, the effect of activation temperature and time of vacuum pyrolysis charcoal was studied. Steam activation of the vacuum pyrolysis charcoal was performed at 760, 800 and 850 °C during different times. As also observed elsewhere,21 for the temperatures studied, the burnoff initially increased linearly with increasing activation time (Table 2). At an activation temperature of 850 °C, it was observed that the specific surface area increased as well linearly with the activation time. After activation during 6 h, a value of 1570 m2/g was reached (Figure 3). The burnoff at this activation time was already very high (91%). When the activation time was further increased all organic material was oxidized and only ash with a very low specific surface area (7 m2/g) was left. For activation temperatures of 760 and 800 °C, an initial increase of the surface area with increasing activation time was also observed. However, there were differences between the different activation temperatures. First, the increase of the surface area with increasing activation time was slower. Furthermore, the maximum surface area which could be reached at a certain activation temperature decreased with the temperature. An additional difference existed between activation at 760 °C and the two higher temperatures. For all activation temperatures, the surface area initially increased with increasing burnoff, until a maximum surface area was reached. At 800 and 850 °C, this initial increase was identical (for burnoff < 80%), whereas activation at 760 °C yielded, for the same burnoff, activated carbons with a smaller surface area (Figure 4). It was observed in a previous TG investigation that two temperature regimes exist for the steam activation of bark residue charcoal.12 The crossover between the two regimes occurs at approximately 760 °C. The results of the present investigation indicate that in the low temperature regime mesopores are preferably formed (Table 2). After activation at 760 °C for 4 h the burnoff was 61 %. Practically the same burnoff was reached after activation for 2 h at 800 °C. The activated carbon produced at 760 °C had more mesopores (0.31 cm2/g) than micropores (0.22 cm2/g), whereas the sample produced at 800 °C exhibited more micropores (0.33 cm2/ g) than mesopores (0.16 cm2/g). Activation at 850 °C also yielded at comparable burnoff more micro than mesopores (Table 2). At a given pore volume, the surface area of solids with micropores is larger as compared to mesoporous solids. Thus, the more mesoporous activated (21) Cazorla-Amoros, D.; Ribes-Perez, D.; Roman-Martinez, M. C.; Linares-Solano, A. Selective porosity development by calcium-catalyzed carbon gasification. Carbon 1996, 34, 869-878.

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Table 2. Surface Area and Pore Volume of Activated Carbons Obtained by Steam Activation of Vacuum Pyrolysis Charcoal activation conditons T [°C] t [h] 760 760 760 760 800 800 800 800 800 800 800 850 850 850 850 850 850 850 850 850

0 4 8 11 0 1 2 4 6 7 8 0 1 2 4 4c 4d 4e 6 7

surface area [m2/g] burnoff [%]a

BET

DR

external (t-plot)

MBb value [mg/g]

43 61 83 84 47 42 62 66 81 82 86 50 46 65 73 75 75 64 91 92

398 659 848 855 562 558 763 867 1286 1305 1004 521 598 808 1187 976 900 1022 1570 7

717 922 917 644 628 862 949 1448 1347 1087 582 641 873 1176 1057 1006 1092 1598 -

209 294 310 54 39 128 221 433 495 340 50 98 152 333 214 186 309 464 -

46 95 35 44 112 159 46 97 157 225 -

pore volume [cm2] micropores (t-plot)

mesopores (P/P0 ) 0.95)

0.22 0.26 0.27 0.24 0.24 0.30 0.32 0.42 0.41 0.33 0.23 0.25 0.33 0.44 0.38 0.34 0.35 0.58 -

0.31 0.41 0.44 0.07 0.05 0.16 0.23 0.62 0.71 0.48 0.06 0.12 0.20 0.42 0.30 0.26 0.42 0.62 -

a Relative to the charcoal produced at 450 °C. b Methylene blue. c The charcoal sample was heated at a rate of 10 °C/min (as opposed to 5 °C/min) to the activation temperature. d Heating rate 20 °C/min. e Activation of charcoal from the industrial scale by pyrovac demonstration plant8.

Figure 3. Specific surface area of steam activated vacuum pyrolysis charcoal as a function of the activation time.

Figure 4. Specific surface area of steam activated vacuum pyrolysis charcoal as a function of burnoff.

carbon produced at 760 °C had a smaller surface area than the carbons produced at temperatures above 800 °C. For comparison, a charcoal sample from the industrialscale demonstration plant was steam activated at 850 °C for 4h. The burnoff and specific surface area of the produced activated carbon were somewhat lower as compared to the activation in the PDU (Table 2)

reflecting differences between the two reactors. However, the results also indicate that charcoal from the demonstration plant can yield a high-surface area activated carbon. There are indications that the rate at which the charcoal is heated to the activation temperature has an important influence on the properties of the produced activated carbon. In separate experiments, vacuum pyrolysis charcoal was heated to 850 °C at rates of 5, 10 and 20 °C/min, respectively. At this temperature, the three charcoal samples were activated for 4 h. The surface areas and pore volumes of the activated carbons significantly decreased with increasing heating rate. For example, the surface area dropped from 1187 m2/g (rate 5 °C/min) to 976 and 900 m2/g (rates 10 and 20 °C/min, respectively, Table 2). As mentioned before, during heating of the vacuum pyrolysis charcoal to the activation temperature, decomposition reactions take place. There are two possible reaction pathways for the organic vapours. The macromolecules may diffuse to the external surface of the charcoal particle and then into the gas phase. Another possibility is that the vapor-phase molecules undergo consecutive cracking and condensation reactions, which ultimately lead to carbonaceous deposits in the pores of the solid residue. The faster the decomposition, the higher the concentration of vapors. A higher concentration of vapors favors condensation reactions and, therefore, the formation of deposits in the pores. The decomposition rate increases of course with the heating rate. Thus, the pore structure of a charcoal heated at a high rate to the activation temperature will be less open as compared to a charcoal heated at a lower rate. During activation, the deposits in the charcoal heated at a higher rate hinder the diffusion of the oxidizing agent in the charcoal pores, causing a slower activation. Pore Volume and Pore Size Distribution of the Activated Carbons. During activation, new micropores are created and existing micropores are widened. It was

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Figure 6. Correlation between the oxygen and volatiles concentration of bark residue, bark residue derived charcoal and activated carbon.

Figure 5. Micropore size distribution of activated carbons produced at 850 °C and of some commercial grades, normalized to the same height.

observed in the present investigation that for not too high burnoff, the volume of both the micro and mesopores increased (Table 2). This increase was more pronounced for the mesopores than for the micropores. Thus, the carbons became more mesoporous with increasing activation time. As already mentioned, the relative concentration of mesopores was higher for carbons produced at 760 °C as compared to carbons produced at higher temperatures. For the carbon with the highest surface area produced at 760 °C, the volume of mesopores was 1.6 times larger than the volume of micropores; whereas for the corresponding carbons produced at 850 °C the two volumes were similar. The size distribution of the micropores also changed during activation. For charcoal heated to 850 °C, the micropore size distribution exhibited a maximum at 9-10 Å. Upon activation, the maximum of the micropore size distribution was shifted to larger widths (Figure 5). After activation for 6 h the maximum of the micropore size distribution was at 12 Å. These changes strongly influence the adsorption properties of the activated carbon. With increasing micropore diameter, the activated carbon becomes more suitable for large adsorbates. The widening of the micropores also accelerates the transport of the adsorbate in the micropores and the increasing relative concentration of mesopores facilitates the transport of the adsorbate to the micropores. An indicator for the adsorption capacity for large adsorbates is the methylene blue (MB) value. The MB molecule is approximately 18 Å long and 9 Å wide.22 For the experimental activated carbons, the MB value increased linearly with the external surface (Table 2).

Proximate and Ultimate Analysis. The ash content (2.5 wt. %) of the bark residue is relatively high as compared to other biomass materials. During pyrolysis and activation, the ash compounds are enriched in the solid product. The activated carbon contained, therefore, relatively high concentrations of ash, up to 23.5 wt. % (Table 3). The ash compounds influence the properties of the activated carbon. Since the surface area of the ash components is very low (7 m2/g), they “dilute” the high-surface area organic portion of the activated carbon. Thus, a feedstock material with a lower ash concentration should yield an activated carbon with a higher surface area. There is also an influence of ash compounds on the activation reaction. It is well know that charcoal activation is catalyzed by certain ash compounds.21,23 This was also experimentally confirmed for the bark residue charcoal used in the present investigation.12 Thus, activation of low-ash charcoal may be slower as compared to the high-ash bark residue charcoal. The bark residue had a high concentration of oxygen functional groups; this is also reflected by a high volatile matter concentration (Table 3). During pyrolysis at 450 °C and subsequent heating to the activation temperature, oxygen groups are removed and both the oxygen and volatile matter concentration decrease. In fact, a linear relation exists between the oxygen and the volatile matter concentration of the bark residue and the charcoal and activated carbon derived from this feedstock (Figure 6). The only exception is the carbon activated at 850 °C for 6 h. The volatiles concentration of this samples was considerably higher as compared to the other activated carbons. This may be related to the structural order of the activated carbon. In a previous work, the activation of the same samples was studied by surface spectroscopic methods. It was found that during activation, first the structural order of the activated carbon increased, whereas in later stages of the activation the material became less ordered.12 The initial increase of the structural order is due to a preferred oxidation and removal of little-ordered struc(22) Graham, D. Characterization of Physical Adsorption System. III. The separate effects of pore and surface acidity upon the adsorbent capacities of activated carbons, J. Phys. Chem. 1995, 59, 896. (23) Murakami, K.; Shirato, H.; Ozaki, J.-I.; Nishiyama, Y. Effects of metal ions on the thermal decomposition of brown coal. Fuel Process. Technol. 1996, 46, 183-194.

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Table 3. Proximate Analysis and Ultimate Analysis of Activated Carbons Obtained by Steam Activation of Vacuum Pyrolysis Charcoal ultimate analysis [wt %]a

proximate analysis [wt %] sample

fixed carbon

ash

volatile matter

C

H

N

Ob

softwood bark feedstock charcoal activated carbon temperature [°C] time [h] 850 0 1 2 4 6

21.0

2.5

76.5

54.4

6.5

0.5

41.1

55.9

5.0

39.1

78.3

3.8

1.0

21.1

85.4 84.7 86.0 77.7 49.8

10.5 7.8 7.5 12.5 23.5

4.1 7.4 6.6 9.8 26.7

93.1 90.7 93.3 89.5

1.0 1.0 0.7 0.7

1.0 0.8 0.6 0.8

4.9 7.5 5.4 9.0

a

Organic portion. b By difference.

tures. This leaves behind more ordered structures. However, at a certain point when no little-ordered structures are left, the highly ordered structures will also be oxidized, leaving less-ordered structures behind. Thus, the high volatile matter concentration of the carbon activated at 850 °C for 6 h may be attributed to a high concentration of this less ordered carbon material. The structural order of activated carbon certainly influences the adsorption behavior. Comparison between Activated Carbons from Vacuum Pyrolysis Charcoal and Commercial Grades. A large number of commercial activated carbons is available on the market. The properties of every grade are optimized for specific applications. Of the five commercial grades studied, three activated carbons (Calgon F-300, Asbury #5597 and B&S 207A) are predominately microporous with micropore volumes of 0.37 to 0.42 cm2/g, surface areas of approximately 900 to 1050 m2/g and methylene blue values of approximately 100 to 180 mg/g (Table 1). In the present work, experimental activated carbons with similar properties were obtained by steam activation at 800 and 850 °C during 6 and 4 h, respectively (Table 2). The micropore size distribution of these carbons and the abovementioned commercial grades was also similar (Figure 5). These similarities suggest that the activated carbons from bark residue charcoal have the potential to replace some commercial grades. The only significant difference was that the experimental activated carbons had higher mesopore volumes as compared to the commercial grades. This should facilitate the diffusion of the adsorbate to the micropores. However, a high mesopore volume reduces the apparent density of the activated carbon. This might be disadvantageous for volume

constrained applications. The two commercial grades produced by chemical activation (Norit Darco KBB and Pica L-6299) had considerably larger surface areas and pore volumes (Table 1) as compared to the commercial activated carbons produced by steam activation. Of the experimental activated carbons, only the sample produced by steam activation at 850 °C for 6 h had a similar surface area and pore volume. This sample, however, had a high ash content and a high concentration of volatile matter (Table 3). Conclusions Vacuum pyrolysis of softwood bark materials yields oil and approximately 32 wt. % charcoal. The charcoal obtained by vacuum pyrolysis has a more open pore structure as compared to charcoal from atmospheric carbonization and is therefore more reactive during steam activation. Upon steam activation at 800 and 850 °C, carbons with surface areas above 1200 m2/g and micropore volumes above 0.40 cm2/g were produced. These properties and the micropore size distribution are comparable to some commercial activated carbons. Thus, activated carbons produced by vacuum pyrolysis followed by steam activation have the potential to replace some commercial activated carbon grades. Acknowledgment. The authors are thankful to Daishowa Inc., Que´bec City, for supplying the sample of bark residue, to Dr. A. Schwerdtfeger for reviewing the paper and to Norit Americans Inc., Calgon Carbon Co., Barnebey & Sutcliffe Co., Pica USA, Inc., and Asbury Carbons Inc., for having gratefully supplied commercial grade activated carbons samples. EF0100698