Porosity Development during CO2 and Steam Activation in a Fluidized

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Energy & Fuels 2000, 14, 142-149

Porosity Development during CO2 and Steam Activation in a Fluidized Bed Reactor A. Linares-Solano,*,† C. Salinas-Martıńez de Lecea,† D. Cazorla-Amoro´s,† and I. Martıń-Gulloń‡ Inorganic Chemistry Department and Chemical Engineering Department, Universidad de Alicante, P.O. box 99, E-03080 Alicante, Spain Received April 29, 1999

Steam- and CO2-activated carbons from a Spanish HV bituminous coal char were obtained at 800-900 °C in a batch laboratory scale fluidized bed reactor. The N2 and CO2 adsorption isotherms and scanning electron microscopy allow for sample characterization. The development of porosity upon activation, using a fluidized bed reactor, is different depending on the activating agent. A much better activation development is attained with steam. Thus, total micropore volume linearly increases with the activation degree, attaining an apparent specific surface area of 1150 m2/g at 50% burnoff for steam-activated carbons. Although CO2 activation also produces a linear increase vs burnoff, much lower pore volume values are obtained, reaching an equivalent specific surface area of only 800 m2/g at 50% burnoff. SEM analysis indicates that CO2 produces external burning of the carbon particles. This different behavior of CO2 and steam activation was also observed for several runs carried out with almond shell char. The explanation of this behavior may be to a partial diffusion control in the CO2 gasification reaction, under fluidized bed conditions.

Introduction The porosity developed in an activated carbon can vary depending on numerous factors, such as the starting raw materials and the type of activation process carried out. Chemical activation may produce different porosity developments depending on the activating reagent (KOH- or H3PO4-based carbons),1 with minor but appreciable differences if the reaction conditions vary even for the same reagent (e.g., different reagent ratio, final temperature treatment, or gas flow rate). Physical activation consisting in the partial gasification of a char with an oxidant gas, mainly steam or carbon dioxide (or a mixture of both), at high temperatures between 800 and 1050 °C, could also produce different pore size distributions.2 There is not a clear agreement about how steam and carbon dioxide develop porosity in carbons. Most authors,3-8 using different raw materials, reported that * Corresponding author. Telephone: +34-96-5903545. Fax: +3496-5903454. E-mail: [email protected]. † Inorganic Chemistry Department. ‡ Chemical Engineering Department. (1) Derbyshire, F.; Jagtoyen, M.; Thwaites, M. In Porosity in Carbons; Patrick, J. W., Ed.; Edward Arnold: London, 1995; pp 227252. (2) Bansal, R. C.; Donnet, J. B.; Stoeckli, H. F. Active Carbon, Marcel Dekker: New York, 1988. (3) Tomkov, K.; Siemeniewska, T.; Czechowski, F.; Jankowska, A. Fuel 1987, 56, 121. (4) Ku¨lh, H.; Kashani-Motiagh, M. M.; Mu´lhen, H. J.; v. Heek, K. H. Fuel 1992, 71, 879-882. (5) Alcan˜iz-Monge, J.; Cazorla-Amoros, D.; Linares-Solano, A.; Yoshida, S.; Oya, A. Carbon 1994, 32, 1277-1283. (6) Rodriguez-Reinoso, F.; Molina-Sabio, M.; Gonzalez, M. T. Carbon 1995, 33, 15-23. (7) Ryu, S. K.; Jin, H.; Gondy, D.; Pusset, N.; Ehrburger, P. E. Carbon 1993, 31, 841.

steam-activated carbons present a wider micropore size distribution than CO2 ones, when activated in a fixed bed reactor. Regarding the adsorption capacity (in terms of total micropore volume, BET surface area or iodine number), most of the authors found similar values when activating with either steam or carbon dioxide, with minor differences. Thus, some authors found either slightly higher values for steam activation,3,4 or slightly higher values for carbon dioxide activation5-7 while others reported similar values.8 Thus, the differences that result after CO2 and steam activation are still a matter of debate and concern.9-10 Rotary kilns and multiple hearth reactors are preferred by the industrial manufacturers for the production of physically activated carbons. In this type of reactors, large differences compared to fixed bed reactor were found in the development of porosity by CO2 and steam.11 Thus, Wigmans11 found that steam produces a much better development of porosity than CO2 in a rotary reactor. Fluidized bed reactors have received little attention by the industry, although they present as advantages a much better heat and mass transfer over other reactor types, as well as more control over temperature. Among several references found concerning the production of activated carbons in a fluidized bed reactor,12-19 Hashimoto et al.18 and Satya Sai et al.19 compared the steam (8) Linares-Solano, A.; Martin-Gullon, I.; Salinas-Martinez de Lecea, C.; Serrano, B. Fuel, in press. (9) Walker, P. L., Jr. Carbon 1996, 34, 1297-1299. (10) Alcan˜iz-Monge, J.; Cazorla-Amoros, D.; Linares-Solano, A. Carbon 1997, 35, 1665-1668. (11) Wigmans, T. Carbon 1989, 27, 13-22. (12) Watanabe, F.; Yanagase, S.; Sugiyama, S. Nippon Kagaku Kaishi 1972, 12, 2313-2317. (13) Kudo, K.; Hoshoda, H.; Mitsui, S.; Sasaki, K. Hokkaido Kogyo Kaihatsu, Shikenso Hokoku 1973, 8, 38-41.

10.1021/ef9900637 CCC: $19.00 © 2000 American Chemical Society Published on Web 12/04/1999

Porosity Development in a Fluidized Bed Reactor

and carbon dioxide activations of bituminous coal and coconut shell char, respectively. Hashimoto et al.18 found that the BET specific surface area attained with CO2 was half of that attained by steam activation, observing that CO2-activated carbon particles presented colored ash at the surface. They described these results as “unexpected”, pointing out that the explanation could be either the presence of oxygen in the CO2 stream or a catalytic phenomenon produced by ash components, resulting in the development of macropores. As commented before, Wigmans11 found results similar to those obtained by Hashimoto et al.18 Recently, Satya Say et al.19 studied the production of coconut shell-based activated carbons in a fluidized bed reactor, analyzing the influence of activating gas (steam and CO2), temperature, particle size, gas velocity, and bed height. From their results, it seems that steam activation produced higher adsorption capacity than carbon dioxide, but unfortunately the comparison of these two activating agents at similar burnoff could not be accomplished because the evolution vs burnoff degree was not presented. Our research group developed a research project funded by the EU to study the production of activated carbons from a previously selected bituminous coal, from Puertollano basin, Central Spain.20 The main tasks of this project were to study the influence of the operation variables in the production of activated carbons at laboratory scale, and the design and operation of a multistage fluidized bed reactor at pilot plant scale.8,17 The present work analyzes the use of a batch laboratory scale fluidized bed reactor for the production of activated carbons from a previously carbonized Spanish bituminous coal, dealing with both steam and carbon dioxide. The results obtained will be compared to those previously reported in a laboratory scale fixed bed reactor.8 In addition, the CO2 activation of an almond shell char, in the fluidized bed reactor, is compared with previously published literature related with the same raw material.21 This work tries to deepen the understanding of the steam and CO2 activation in different reactors and with different reaction conditions to explain the apparently contradictory results found in the literature. Experimental Section Raw Material. A HV bituminous coal (UA14) from Maria Isabel Mine (TECSA, HUSA), in Puertollano basin (Spain), was used to carry out the main part of this work. Table 1 shows the proximate analysis. The initial ash content of this coal was reduced by hydro(14) Kudo, K.; Hoshoda, H.; Mitsui, S.; Sasaki, K. Hokkaido Kogyo Kaihatsu, Shikenso Hokoku, 1973, 8, 42-47. (15) Veselov, V.; Mkhorin, K. E.; Kozhan, A. P.; Orlik, V. N. Khim. Teknol. 1983, 5, 14-17. (16) Klose, E.; Heschel, W. Freiburg Forschungh. 1983, A-672, 101113. (17) Martin-Gullon, I.; Asensio, M.; Font, R.; Marcilla, A. Carbon 1996, 34, 1515-1520. (18) Hashimoto, K.; Miura, K.; Yoshikawa, F.; Imai, I. Ind. Eng. Chem. Process Des. Dev. 1979, 18, 72-80. (19) Staya Sai, P. M.; Amhed, J.; Kirshnaiah, K. Ind. Eng. Chem. Res. 1997, 36, 3625-3630. (20) Munõz-Guillena, M. J.; Illan-Gomez, M. J.; Martin-Martinez, J. M.; Salinas-Martinez de Lecea, C.; Linares-Solano, A. Energy Fuels 1992, 6, 9-15. (21) Cazorla-Amoros, D.; Ribes-Perez, D.; Romań Martinez, M. C.; Linares-Solano, A. Carbon 1996, 34, 869-878.

Energy & Fuels, Vol. 14, No. 1, 2000 143 Table 1. Proximate Analysis and Carbonization Yields (850 °C, 1 h soaking time) of the Raw Materials Used in This Work sample

moisture

volatile

ash

fixed carbon

carbon yield

UA14 AS

5.0 1.0

28.0 75.8

8.0 0.4

59.0 22.8

65.0 23.1

pneumatic wash treatment, based on density variations among particles with different ash content. Further analysis and details can be found elsewhere.8 In addition, Marcona type almond shells (AS), from Alicante area,21 were also used to carry out selected runs. Its proximate analysis is also shown in Table 1. Experimental setup. Prior to the activation runs, UA14 and AS were first carbonized in a batch horizontal fixed bed reactor in a nitrogen atmosphere at 850 °C. Initial sample weight in the crucible was around 25 g, whereas gas flow was fixed at 100 cm3/min (STP). Carbonization yields of the chars are shown in Table 1. From the chars obtained, the fraction 0.71-1.40 mm was selected for further activation experiments. Both CO2 and steam activations were performed in a batch fluidized bed reactor, where the scheme can be observed in Figure 1. The reactor (A) is quartz made, with an internal diameter of 3 cm. The gas distributor is a quartz porous plate, to ensure a satisfactory gas distribution. The reactor is placed inside an electric furnace (F), whose temperature is measured by a thermocouple (TC) placed inside the reactor, just 2 cm above the distributor. A peristaltic pump (P) makes liquid water flow through an electric furnace (D) to generate steam, which crosses vertically upward the reactor, and finally to vent. In the case of CO2 activation, high purity CO2 (minimum of 99.999%) supplied by a cylinder is used, measuring the gas flow by a rotameter. Steam activation with UA14 was carried out at 800 °C with twice the minimum fluidization velocity (3 g H2O/ min) at four different reaction times, while a CO2 activation series UA14 (four reaction times) was carried out at 850 °C and twice the minimum fluidization velocity (4000 cm3/min STP). Additional CO2 runs were carried out at 875 and 900 °C at the same gas flow to analyze the influence of temperature. As a consequence of the results obtained with CO2 activation, three more runs were carried out with the sample UA14 in the same reactor at 850 °C but at fixed bed conditions, with a volumetric gas flow rate of 80, 500, and 1000 cm3/ min. Finally, a single CO2 activation experiment at 850 °C with twice the minimum fluidization conditions was executed with the AS char, to compare with previous results obtained of this same raw material.21 The starting weight of char was approximately 5 g in all activation runs in the fluidized bed reactor. As commented before, the results obtained in the present work are compared to those reported by LinaresSolano et al.,8 where activated carbons from the same bituminous coal char were produced with steam and CO2 in a tubular fixed bed reactor. In that work,8 the initial char weight in the crucible was around 5 g in both steam and CO2 activation, while gas flow rates were 80 cm3/min (STP) in CO2 activation, and 100 cm3/ min (STP) of 1:1 H2O:N2 mixture in steam activation. Sample Characterization. In this work, the burnoff is defined as the mass of carbon reacted to the mass of fixed carbon in the starting char. Samples burnoff is

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Linares-Solano et al.

Figure 1. Scheme of the laboratory-scale fluidized bed reactor.

Figure 2. Burnoff of a bituminous char vs reaction time for steam activation at 800 °C in a fluidized bed reactor, compared with those results obtained in a fixed bed reactor8 at 850 °C.

determined experimentally by gravimetric difference between the final mass of sample and the initial mass of char. To characterize the adsorption capacity, nitrogen at 77 K and carbon dioxide at 273 K adsorption isotherms were determined in an automatic conventional volumetric apparatus (Quantachrome Autosorb 6). Narrow micropore (pore size below 0.7 nm, VCO2) and total micropore (pore size below 2 nm, VN2) volumes are calculated applying the DR equation to the CO2 and N2 isotherm data, respectively. For the almond shell-based activated carbon, BET specific surface area was calculated from the 77 K nitrogen isotherm data in order to compare the results with those reported previously.21 Activated carbon texture was observed with a scanning electron microscope (SEM), to analyze possible external differences between the activation with carbon dioxide and steam. Results CO2 and Steam Activation of the Bituminous Char. Burnoff degree for the steam activation of char UA14 in the fluidized bed reactor at 800 °C, and in the fixed bed reactor at 850 °C8 vs the activation time are plotted in Figure 2. The activation time required to reach a given burnoff is considerably smaller in a fluidized bed than in a fixed reactor, even though the

Figure 3. Burnoff of a bituminous char vs reaction time for CO2 activation in fluidized bed (present work) and fixed bed reactors.8

operating temperature is lower. This increase in reactivity can be attributed to a better gas-solid contact in a fluidized bed, and also to the fact that the ratio of steam flow rate vs initial weight of char is 80 times higher in the fluidized bed runs. Figure 3 shows the corresponding plot for the CO2 activation. In this case, the burnoff data are represented for the CO2 activated carbons prepared in the fluidized bed reactor at 850 °C, for those reported in the fixed bed reactor at 850 °C from the same starting char,8 and for those carbons produced at 875 and 900 °C in the fluidized bed reactor. As in the case of steam activation, carbon dioxide gasification kinetics are higher in the fluidized bed than in the fixed bed reactor; however in this case the increase is not so pronounced. Furthermore, reaction rate in the fluidized bed considerably increases when increasing the temperature from 850 to 900 °C. On the other hand, comparing both Figures 2 and 3, it can be noticed that steam gasification is three times faster than the carbon dioxide one, in agreement with previous works.22 In summary, CO2 and steam activation performed at fluidization conditions allow for a decrease of the process time compared to a fixed bed reactor which, from a practical point of view, is interesting if the final material has an appropiate development of porosity. With respect to the adsorption capacity, the 77 K nitrogen adsorption isotherm, for the whole relative (22) Moulijn, J. A.; Kapteijn, F. In Carbon and Coal Gasification; Figueiredo, J. L., Moulijn, J. A., Eds.; Kluwer Academic Publishers: Dordrecht, 1986; p 291.


Energy & Fuels, Vol. 14, No. 1, 2000 145

Figure 5. Total micropore volume vs burnoff for carbons activated with both steam and carbon dioxide in fluidized bed and fixed bed reactors.8

Figure 4. N2 adsorption isotherm at 77 K for activated carbons prepared in (a) fluidized bed with steam at 800 °C (58.9% burnoff) and carbon dioxide at 850 °C (58.6%), and (b) the corresponding isotherms of samples activated at fixed bed conditions (b) with CO2 (68% burnoff) and steam (66% burnoff)8 at 850 °C.

pressure range, gives semi-qualitative information about the pore size distribution and the total adsorption capacity. Figure 4a shows the nitrogen adsorption isotherms of two samples at similar burnoff degrees (60%), one activated with steam and the other with CO2, produced in the fluidized bed reactor, whereas Figure 4b shows the isotherms of the corresponding activated carbons obtained from the same raw material in a fixed bed reactor, of similar burnoff degrees.8 In Figure 4a, the amount adsorbed is much lower for the CO2-based carbon than the steam one, for both produced in a fluidized bed reactor. This fact agrees with the previous work of Hashimoto et al.,18 also done with a bituminous coal in a fluidized bed reactor, but in complete disagreement with those results obtained in a fixed bed reactor from the same starting char,8 and most published literature.4-7 The isotherm of the steam sample presents a wide knee at low relative pressures, indicating a wide micropore distribution, with a light linear slope for higher relative pressure values, indicating widening of the micropores into mesopores. These findings previously described are typical for the steam activation.5-6 On the other hand, CO2-based carbon, in fluidized bed, presents an isotherm with a closer knee at low relative pressures (small micropores), but from 0.3 and above, a light linear slope too, which may indicate little existence of pores of the border micro-meso, but some higher size mesopore development. From the comparison of both Figures 4a and 4b, it can be observed that

isotherms corresponding to steam activation are similar in shape and absolute values for samples obtained in both fluidized and fixed bed reactors, indicating that steam activation produces a very similar pore development at both reactor conditions, although the reaction rate is much higher in the fluidized bed. On the other hand, the isotherms of the CO2-activated samples are different in shape and absolute values. In the fixed bed the amount adsorbed is as high as the steam-activated sample, with a knee intermediate between the fluidized bed CO2 sample (not so closed) and steam sample (not so wide). In CO2 activation, it is clear that reactor conditions definitively affect both reaction rate and porosity development. Figure 5 shows the evolution of the total micropore volume, calculated from the 77 K nitrogen isotherm data, vs burnoff, for activated carbons produced in the fluidized bed reactor with steam and carbon dioxide. In this case, the total pore volume increases linearly with the burnoff in the range studied for both CO2 and steam activation. Nevertheless, while steam-activated carbons present acceptable pore volume values (ca. 0.4 cm3/g at 60% burnoff), the microporosity attained with CO2 as activating agent is considerably lower (ca. 0.27 cm3/g). Figure 5 also includes the previous results obtained by Linares-Solano et al.8 at fixed bed conditions with steam and carbon dioxide. It can be observed that the total micropore volumes are similar for steam-activated carbons regardless of the reactor type. On the other hand, CO2-based carbons produced in a fixed bed reactor present adsorption capacities similar to those of steambased carbons, but not those obtained in fluidized bed. When comparing CO2 and steam activation, previously reported contradictory results appear. In a fixed bed, micropore volumes obtained for both agents are similar, whereas in a fluidized bed CO2 produces carbons with much lower adsorption capacity. It is important to point out that steam develops the same porosity in fixed and fluidized bed reactors, while in the latter the reaction time is considerably reduced. At the same time, carbon dioxide activation is much faster in fluidized bed, but it affects drastically the pore development. Figure 6 shows the evolution of the narrow micropore volume (calculated from the 273 K CO2 isotherm data, VCO2) for the UA14-based carbons obtained in the fluidized bed reactor vs burnoff, where the data plotted at 0% burnoff corresponds to the value of the starting char. For the two main series of CO2 and steam activation at 850 and 800 °C, respectively, it can be noticed that the narrow pore volumes attained are nearly the same, exhibiting the same tendency vs

146


Figure 6. Narrow micropore volume vs burnoff for activated carbons in a fluidized bed reactor with steam at 800 °C and CO2 and 850, 875, and 900 °C.

Figure 7. Supermicropore volumes (VN2-VCO2) vs burnoff for steam- and CO2-activated carbons in fluidized bed and fixed bed reactors.8

burnoff, a slight increment of the narrow porosity with burnoff. On the other hand, CO2 activations at 875 and 900 °C produce, as expected, a similar but slightly smaller narrow pore volumes with respect to those obtained at 850 °C. From Figure 6 it can be noticed that both CO2 and steam-activated carbons present similar narrow micropore volumes, but from Figure 5 it is clear that the steam series presents much higher values than the CO2 series of total micropore volumes, as mentioned above. This indicates that the higher adsorption capacity of the steam carbons may be attributed to a higher amount of wider micropores rather than more narrow micropores. It is accepted that VCO2 characterizes the narrow micropores or submicropores up to 0.7 nm, and VN2 measures micropores and even some narrow mesopores. Therefore, the difference between both would give the pore volume of the supermicropores, micropores above 0.7 nm up to the border of the mesopores. For samples at a low activation stage, the VCO2 values are sometimes higher than VN2, because the very narrow porosity may produce diffusion problems to N2 accessibility at 77 K. In those cases, the supermicropore volumes are considered zero. Figure 7 shows the supermicropore volumes vs burnoff for fluidized bed CO2- and steam-activated carbons, and the corresponding values obtained in the fixed bed activation also with steam and CO2. In the case of the fluidized bed activation, the supermicropore volume is much higher for steam carbons than for CO2 ones, whereas these results do not occur in the fixed bed activation. Additionally, the CO2 fluidized bed activation does not develop supermicropores from early stages of activations, remaining zero at high burnoff degrees (40%). The above observations confirm that the CO2 activation process, under fluidized bed conditions, is less


Figure 8. BET surface area vs burnoff for almond shell charbased carbon activated with CO2 in a fluidized bed, compared with previous results with the same raw material with steam (fixed and fluidized bed) and CO2 (fixed bed).21

suitable than CO2 activation in fixed bed and steam activation in either fluidized or fixed bed conditions. In summary, steam produces the same activated carbons and the same pore development when the activation process is carried out in either fixed bed or fluidized bed reactors. While CO2-activated carbons present similar micropore volumes compared to steambased carbons (but narrower micropore size distribution) when those are produced in fixed bed reactors, the adsorption capacity considerably decreases when the CO2 activation is produced in a fluidized bed reactor. The two opposite and contradictory results, pointed out by different authors, are found here for the same raw material. Similar micropore volumes with narrower micropore size distribution are found for CO2 activation in fixed bed reactor, which is in agreement with previous results;5,6,10 also, low adsorption capacity for carbon dioxide activation with respect to steam in a fluidized bed, which is in agreement with Hashimoto et al.18 As stated in the Introduction, Hashimoto et al.18 considered their results as unexpected, having the CO2-activated carbon ash colored at the surface, mentioning as a possible explanation the presence of O2 in the CO2 stream, or a possible catalytic effect of the ash material. Nevertheless, if their results and those of the present work agree (except the ash-colored surface), they might indicate that the reason of this agreement can be possibly other than “unexpected”. CO2 Activation with an Almond Shell Char in a Fluidized Bed. To validate the results obtained for the carbon dioxide activation in a fluidized bed reactor, a single run with another raw material, almond shell char, was carried out. Cazorla Amoros et al.21 conducted a study about the calcium-catalyzed steam and CO2 activation of an almond shell char, compared to the noncatalyzed activation. In their noncatalyzed process, they produced steam carbons in both fluidized bed and fixed bed reactors (obtaining similar adsorption capacities) at the same activation conditions as in the present work with char UA14, and CO2-activated carbons in a fixed bed reactor. The experiment accomplished in the present work allows us to complete the above study on the effect of activation in fixed and fluidized bed. Figure 8 shows the BET specific surface area vs burnoff for the carbons reported by Cazorla-Amoros et al.,21 and the value obtained in this work (CO2-flu). Steam carbons present the same surface area and tendency vs burnoff, regardless of the reactor, which is very similar to the activated carbon produced by CO2 activation in a fixed


bed. Again, as in the case of the char UA14, the activated carbon produced with CO2 in fluidized bed conditions presents a much lower adsorption capacity (below 1000 m2/g at 65% burnoff) than that obtained at fixed bed conditions (1600 m2/g at similar burnoff). The unexpected results of Hashimoto et al.18 (i.e., much poorer adsorbent properties after activation with CO2 compared with steam in a fluidized bed) are confirmed in this work with two different raw materials (bituminous coal char and almond shell char). Additionally, a proper development of porosity by CO2 activation can be obtained if the activation is carried out in a fixed bed reactor. These results, obtained with three different raw materials, need a further and detailed analysis to understand the differences between CO2 activation under fixed and fluidized bed conditions. CO2 Activation of Bituminous Char at Different Flow Rates. The main difference between the operation of a fixed bed and fluidized bed reactor is the gas flow rate, which is the unique responsibility of the fluidization of the char particles. This produces a larger mass or molar ratio oxidant/carbon for a fluidized bed activation. In this work and those of Linares-Solano et al.8 and Cazorla-Amoros et al.,21 the initial char mass is the same for both activations (around 5 g), while the CO2 flow rate was 100 cm3/min for the fixed bed reactor and 4000 cm3/min for the fluidized bed reactor, giving ratios of CO2 mass flow/initial char mass of 0.036 and 1.44 g CO2/g char • min, respectively. Thus, the ratio CO2/char in a fluidized bed is 40 times higher. In the case of steam activation, this ratio is even bigger because the steam activation in the fixed bed was carried out with diluted steam in nitrogen at a 1:1 molar ratio, obtaining 0.007 and 0.60 g H2O/g char • min for fixed and fluidized beds, respectively. Carbon gasification reaction, with both steam and carbon dioxide, follow at atmospheric pressure a Langmuir-Hisselwood23 kinetics, where gasification reaction products, hydrogen, and carbon monoxide, respectively, inhibit the reaction

kaPH2O dX ) (1 - X)n; dt steam 1 + KH2PH2 + KH2OPH2O

( )

kaPCO2 dX ) (1 - X)n (1) dt CO2 1 + KCOPCO + KCO2PCO2

( )

where X is the conversion degree (the mass of carbon reacted to the initial mass of fixed carbon in the starting char), t is the time, Pi is the partial pressure of the i compound, ka the reaction rate kinetic constant, Ki are functions of one or more rate constants, and n is the reaction order. The reaction order with respect to the burnoff degree is 1 if the gas-solid reaction follows the volume reaction model,24 where the reaction takes place uniformly at any point of the particle, and consequently the reaction follows a kinetics under chemical reaction control. If the reaction order with respect to the burnoff is below 1 (mostly 0.66), the heterogeneous reaction (23) van Heek, K. H.; Mu¨lhen, H. J. In Fundamental Issues in Control of Carbon Reactivity; Kluwer Academy Publishers: Dordrecht, 1991. (24) Levenspiel, O. Chemical Reactor Engineering, John Willey & Sons: New York, 1999.


Figure 9. Narrow and total micropore volumes of bituminous char-based carbons activated with CO2 at different gas flow rates.

follows the reaction grain or unreacted core model;24 in this latter case the reaction is advancing through spherical surfaces due to the external and/or internal reactive diffusion. For a proper development of porosity by physical activation of carbons, either steam or CO2, gasification must take place under reaction control, to allow the gas reactive molecules to reach the char particle core, and develop porosity and keep the particle size constant. Otherwise, external burnout of the char particle, with no porosity development, will take place if the reaction follows the reaction grain model. The critical reaction rate, when the process changes from chemical to diffusion control, is inherent to the nature of the reaction itself and the nature of the different compounds acting as reactives and products. Martin-Gullon et al.25 studied the steam gasification kinetics of this same char UA14 in a thermobalance, between 800 and 900 °C, using only steam and steam partially diluted with CO/H2 (reaction products). It was observed that the reaction products strongly inhibit the reaction, but the reaction was first order with respect to the burnoff at any reaction condition studied, indicative of kinetics under chemical control. This agrees with the results presented in this work for steam-activated carbons in fixed bed and fluidized bed, where the gas flow rate and, consequently, the gas atmosphere in the carbon surroundings varies considerably. However, as CO2-activated carbons present large differences of porosity when activated under fixed and fluidized bed conditions, the results could be explained if the reaction is under chemical control in a fixed bed, and under partial diffusion control in fluidized bed. Furthermore, CO2 activation at gas flow rates between those used in fixed and fluidized bed conditions, may help to understand the present results. Three different runs were carried out at 850 °C in the fluidized bed reactor at three different CO2 flow rates with different times to obtain carbons at around 5060% burnoff degree. Selected flow rates are 80, 500, and 1000 cm3/min (STP), all of them below the 4000 cm3/g used for the fluidized bed runs. It is important to note that in all of these flow rates, the particles are in fixed bed conditions. Figure 9 shows both narrow and total micropore volumes vs gas flow rate. The samples included correspond to burnoff degrees of 55, 53, 60, and (25) Martin-Gullon, I.; Asensio, M.; Marcilla; A.; Font, R. Ind. Eng. Chem. Res. 1996, 35, 4139-4146.

148



Figure 10. SEM photographs of bituminous char-based carbons, activated with (A,B) steam and CO2 in fixed bed and (C,D) steam and CO2 in fluidized bed.

58 for flow rates of 80, 500, 1000, and 4000 cm3/min (STP), respectively. It can be observed that only the carbon activated with 80 cm3/min present a higher micropore volume (ca. 0.4 cm3/g, as in the steam activation), whereas the other two samples at fixed bed, and the sample at fluidized bed conditions have similar microporosity (ca. 0.3 cm3/g), which are smaller than that obtained with the lowest CO2 flow stream. In conclusion, rather than the use of fluidization conditions during CO2 activation being responsible for this behavior, a different CO2 flow/carbon ratio may produce very distinct porosity development, i.e., the composition of the gas atmosphere in the surroundings of the particle (reactive/products ratio) may strongly influence the porosity development. These experiments confirm that CO2 activation at low flow rate proceed through chemical control, and by increasing the flow rate, diffusional limitations may become important, producing a nonappropiate development of porosity. These conclusions are consistent with similar results obtained by Hashimoto et at.18 (using a fluidized bed reactor) and Wigmans11 (using a rotary reactor at very high temperatures and gas flows). Activation under partial diffusion control should produce a lower microporosity, and consequenly, external burning. SEM Analysis. The SEM pictures of Figure 10 show the external appearance of the different particles of the UA14-based activated carbons (around 60% burnoff) produced by steam and carbon dioxide, at both fixed and

fluidized bed conditions. Figure 10A-C (steam in fixed bed, CO2 in fixed bed, steam in fluidized bed, respectively) show particles with similar properties; flat surfaces with big cracks separating each other. Figure 10D (carbon dioxide in fluidized bed) reflects a very different external texture for the particles. Thus, the surfaces are not flat and present many holes, indicating that the gasification took place mainly over the external surface. This agrees with the previous finding of activation under diffusion control for the carbon dioxide activation in fluidized bed, and fixed bed with moderate and high gas flows-to-carbon ratio. Conclusions Physical activation carried out in a fluidized bed reactor, with either steam or carbon dioxide, considerably reduces the necessary reaction time compared to fixed bed conditions. For both raw materials tested, bituminous coal char and almond shell char, steam activation in a fluidized bed produces activated carbons similar (both in pore size distribution and in total adsorption capacity) to those obtained in fixed bed conditions. Consequently, steam activation under fluidized bed conditions produces activated carbons, in a considerably lower reaction time, with well-developed micropore volumes (0.4 cm3/g at 60% burnoff). In the case of carbon dioxide activation in a fluidized bed (also for both raw materials tested), the increase in


the reactivity negatively influences the pore development compared to CO2 activation carried out at fixed bed conditions and lower gas flows. Thus, CO2 gasification at fluidized bed conditions (high gas flow vs fixed carbon) takes place under diffusion control, obtaining a much lower porosity development with significant external burning. In summary, steam activation produces similar carbons at the same temperatures regardless of the gas flow/carbon ratio, i.e., the type of reactor used. On the


other hand, carbon dioxide activation may not be appropiate for producing carbons on an industrial scale, where higher temperatures and gas flow rates are regularly used. Acknowledgment. The authors thank Steel and Coal European Community (project 7220-EC-758) and CICYT (project AMB-96-0799) for financial support. EF9900637

Porosity Development during CO2 and Steam Activation in a Fluidized

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