Activated Carbons from Spanish Coals. 3. Preoxidation Effect on

Energy & Fuels 1997, 11, 785-791

785

Activated Carbons from Spanish Coals. 3. Preoxidation Effect on Anthracite Activation B. Serrano-Talavera, M. J. Mun˜oz-Guillena, A. Linares-Solano,* and C. Salinas-Martı´nez de Lecea Departamento de Quı´mica Inorga´ nica, Universidad de Alicante, 03080 Alicante, Spain Received July 1, 1996X

A Spanish anthracite, with 5.6 wt % ash content, has been submitted to a two-stage activation process to explore its use as an activated carbon precursor. Several oxidation treatments, with two oxidizing agents (air and nitric acid), have been carried out to study the degree of coal oxidation and its influence on both the char porosity and the char activation. Two activating agents, CO2 and steam, have been used to prepare different burn-off samples to analyze the preoxidation effect on the porosity of the resulting activated carbons. The extent of the oxidation degree, followed by TPD experiment in He, increases with the severity of the oxidation treatment in the following order: air 4 h < air 8 h < 4 M HNO3 , 15 M HNO3. All the oxidation treatments carried out in this study introduce important changes in the reactivity of the resulting chars and on the porous development. The more intense the preoxidation treatment is, and hence the amount of oxygen added to the anthracite, the higher is the porosity of the resulting chars. Activated carbons prepared from preoxidized chars, using CO2 or steam, present much larger porous development than the activated carbons coming from the original coal. The results show that the anthracite needs, prior to the pyrolysis process, an oxidation treatment to be used as an activated carbon precursor, as happens with caking coals. High surface area activated carbons (about 1300 m2/g at a 50% burn-off) can be obtained using nitric acid which has proved to be the most effective preoxidation agent. Comparison of both series of activated carbons, prepared in CO2 and steam, shows that steam develops the porosity of the resulting activated carbons more than CO2.

1. Introduction Activated carbon consumption is continuously being increased because they are used in important areas such as waste, drinkable water treatments, atmospheric pollution control, poisonous gas separation, solvent recovery, etc.1-4 In addition, as is well-known, any cheap carbonaceous material can be used as a raw material for the production of activated carbons. In line with this, several Spanish coals have attracted our attention and some of them, ranging from anthracite to subbituminous, have been analyzed as precursors for the preparation of activated carbons by physical5 and chemical activation.6 The former study showed that the rank of the coal precursor influenced the pyrolysis process and the porosity of the char. This in turn affected the rate of activation and ultimately the porosity of resulting activated carbons. The lower rank coals investigated were more suitable activated carbon precursors than higher rank coals.5 Interestingly, a quite singular behavior was observed Abstract published in Advance ACS Abstracts, May 15, 1997. (1) Smisek, M.; Cerny, S. Active Carbon Manufacture, Properties and Aplications; Elsevier Science Publishers: Amsterdam, 1970. (2) Bansal, R. C.; Donnet, J.; Stoeckli, F. Active Carbon; Marcell Dekker: New York, 1988. (3) The Economics of Activated Carbons 1990; Roskill Information Services Ltd.: London, 1990. (4) Jankowska, H.; Swiatkowoski, A.; Choma, J. Active Carbon; Ellis Horwood Limited and Wydawnictwa Nukowo-Technicze: Poland, 1991. (5) Mun˜oz-Guillena, M. J.; Illa´n-Go´mez, M. J.; Martı´n-Martı´nez, J. M.; Salinas-Martı´nez de Lecea, C.; Linares-Solano, A. Energy Fuels 1992, 6, 9-15. (6) Illa´n-Go´mez, M. J.; Garcı´a-Garcı´a, A.; Salinas-Martı´nez de Lecea, C.; Linares-Solano, A. Energy Fuels 1996, 10, 1108-1114.

with an anthracite coal during the pyrolysis step which, besides its high rank, was explained as due to some thermoplastic properties. That affected negatively its subsequent pore development upon activation. In agreement with many published results,7-10 but different from them because they dealt with bituminous coals, an oxidation treatment prevented the above-mentioned thermoplasticity, allowing the pore volume to develop upon activation. The main objective of this paper is the preoxidation study of this unexpected thermoplastic anthracite and its subsequent effect on the activation process. In addition, the interest of this research is based on the use of a relative low ash content coal (5.6%) because Spanish coals typically have much higher ash contents.11,12 Two oxidizing agents, air and nitric acid, and different oxidation treatments have been carried out. Two activating agents, CO2 and steam, have been used to analyze the influence of the preoxidation treatments on the porosity of the resulting activated carbons.

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(7) Mahajan, O. P.; Komatsu, M.; Walker, P. L. Jr. Fuel 1980, 59, 3-10. (8) Maloney, D. J.; Jenkins, R. G. Fuel 1985, 64, 1415-22. (9) Tromp, P. P. J.; Kapteijn, F.; Boon, J. J.; Moulijn, J. A. Int. Conf. Coal Sci. 1987, 537-541. (10) Alvarez, T.; Fuertes, A. B.; Pis, J. J.; Ehrburger, P. Fuel 1995, 74, 729-735. (11) Inventario de Recursos de Carbo´n en Espan˜a; Centro de Estudios de la Energı´a; Ministerio de Industria y Energı´a; Madrid, Spain, 1979. (12) Couch, G. R. Lignite Resources and Characteristics; IEA Coal Research: London, 1989.

© 1997 American Chemical Society

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Table 1. Properties of Coal UA1 (on a Dry Basis) moisture (%) volatile matter (%) ash (%) fixed carbon (%) calorific value (kJ/kg) (dmmf) micropore volume (cm3/g)

1.46 6.91 5.82 86.01 32810 0.069

%C %N %H %O %S

87.06 1.07 3.37 7.84 0.66

a Deduced from CO adsorption at 273 K, applying the DR 2 equation.

2. Experimental Section The characteristics of the coal used and its chemical analysis are compiled in Table 1. The coal, denoted as UA1, comes from Hullas del Coto Corte´s, S.A. (Asturias, Spain). The particle size used is lower than 0.71 mm. The application of the ASTM standard to the proximate analysis data and calorific value of Table 1 reveals that the rank of this coal corresponds to an anthracite. The coal rank has been determined from the reflecting power of the vitrinite. The reflectance value (percentage of incident light reflected) of 2.60 confirms that the coal UA1 is an anthracite in the bordering area of semianthracite.13-15 Different oxidation processes have been carried out with (1) air, at 200 °C during 4 and 8 h, samples denoted as UA1A4 and UA1A8, respectively; (2) a 4 M HNO3 solution, at 100 °C for 1 h, using a weight volume ratio (W/V) of 1:6, sample named as UA1N4; and (3) a 15 M HNO3 solution, at 80 °C until dryness, using a W/V of 1:10, sample UA1N15. After the oxidation process with nitric acid the coal was washed with distilled water until free of nitrate ions, according to the procedure found in the literature.16 Once the nitrate ions were removed, the samples were dried at 100 °C for 12 h. The oxidation degree reached has been analyzed by temperature-programmed desorption (TPD) experiments in helium. The quantification of oxygenated products (CO and CO2), emitted by decomposition of the different oxygen surface groups, of the original and the oxidized coal, allows to determine the degree of the oxidation of the samples. The experiments were carried out in a fixed bed flow reactor coupled to a mass spectrometer (VG-Quadrupole), as described elsewhere.17 The pyrolysis behavior of the anthracite was analyzed by TG-DTA (Stanton Redcroft Series 780), as reported elsewhere.5 Activated carbon preparation was carried out by carbonization, of the original and preoxidized raw material, followed by activation in both CO2 and steam. Both carbonization and activation were carried out in a horizontal furnace (12 cm i.d.). The carbonization was conducted in N2 flow (80 mL/min) at 1123 K for 2 h, using about 30 g of sample. The chars have been named by prefixing the letter C to the name of the coal. The activation was carried out using 4-5 g of sample. The flow gases used were CO2 (80 mL/min) and steam/N2 mixture (100 mL/min) with a partial pressure of steam of 0.05 MPa. The activation temperature was 1123 K and different activation times have been used. The nomenclature includes the activation degree after the name of the char, separated by a hyphen. In the case of the activation in steam the letter s is added. The samples were characterized by physical adsorption, CO2 (273 K) and N2 (77 K), in a conventional gravimetric system and by mercury porosimetry (Carlo Erba Model 200). The pore volumes were estimated from CO2 and N2 adsorption data and (13) Hessley, R. K.; Reasoner, J. W.; Riley, J. T. Coal Science; Wiley Interscience: New York, 1986. (14) Cahiers de L’Utilisation Du Carbon; Editions Technic: Paris, 1989. (15) Van Krevelen, D. W. Coal. Typology-Physics-Chemistry-Constitution; Elsevier Science Publishers: Amsterdam, 1993. (16) Burriel-Martı´, F.; Arribas-Jimeno, S.; Lucena-Conde, F.; He´rnandez-Mendez, J. Quı´mica Analı´tica; Paraninfo: Madrid, Spain, 1992. (17) Linares-Solano, A.; Salinas-Martı´nez de Lecea, C.; CazorlaAmoro´s, D. Energy Fuels 1990, 4, 467-474.

from mercury porosimetry as follows:18 (i) narrow micropores ( 50 nm) was obtained from mercury porosimetry. The apparent surface area was assessed by applying the BET equation19 to the N2 adsorption data; N2 adsorption at 77 K was not carried out on chars due to the well-known activated diffusion problems19,20 caused by constrictions that are present in these types of solid.3,15

3. Results and Discussion 3.1. Coal Oxidation Degree. The degree of sample oxidation, obtained with the different preoxidation treatments carried out in this study, was followed by TPD experiments in He (see Figure 1) from which the amount of CO and CO2 and the total oxygen content of the sample have been assessed. Table 2 compiles the results obtained. The extent of the oxidation degree increases with the oxidation treatment in the following order: air 4 h < air 8 h < 4 M HNO3 , 15 M HNO3. The oxygen introduced by the 8 h air (twice that created by 4 h in air) and the 4 M HNO3 are quite similar and much lower than that reached by the 15 M HNO3 (see Table 2). The TPD curves of some of the oxidation treatments are presented in Figure 1, for the raw anthracite (Figure 1a), for the 8 h air oxidation (Figure 1b), and for the 4 M HNO3 oxidation (Figure 1c). The curves of Figure 1, typical of those reported for other carbonaceous materials,17,21 present, among them, noticeable differences that merit being pointed out. It can be observed that the higher-temperature CO2 peak that appears in the original raw coal at around 980 K (Figure 1a) remains unchanged in the samples treated in air and decreases in samples treated in HNO3. This higher CO2 peak may be due to some inherent CaCO3 present in the mineral matter of the coal, since the evolution temperature is coincidental with that of the CaCO3 decomposition in similar conditions.17 Moreover, this CO2 peak decreases in the 4 M nitric acid oxidized sample (Figure 1c) and does not appear in the 15 M HNO3, in agreement with the well-documented removal of inorganics by acid treatments, an example of which is the work of Samaras et al.22 On the other hand, the different oxidizing treatments carried out give rise to important differences in the TPD profiles. The oxidation by nitric acid creates different types of oxygenated species than the air oxidation. The nitric acid (Figure 1c) produces, as is well-known,17,21 a large amount of carboxylic groups which evolves to CO2 at temperatures of about 650 K. In contrast, the oxidation with air (18) Linares-Solano, A.; Almela-Alarco´n, M.; Salinas-Martı´nez de Lecea, C.; Mun˜oz-Guillena, M. J.; Illa´n-Go´mez, M. J. COPSII; Rodrı´guez-Reinoso, F., Rouquerol, J., Sing, K. S. W., Unger, K. K. Eds.; Elsevier Science Publishers: Amsterdam, 1991; pp 367-377. (19) Gregg, S.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: London, 1982. (20) Rodrı´guez-Reinoso, F.; Linares-Solano, A. In Chemistry and Physics of Carbon; Thrower, P. A., Eds., Marcell Dekker: New York, 1989; Vol. 21, pp 1-146. (21) Otake, Y.; Jenkins, R. G. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1987, 32, 310. (22) Samaras, P.; Diamadopoulos, E.; Sakellaropoulos, G. P. Carbon 1994, 32, 771-776.

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Figure 1. TPD in He of UA1 coal (a), preoxidized in air for 8 h (b) and preoxidized coal in nitric acid 4 M (c). Table 2. Desorbed CO and CO2 from Surface Groups by Raw and Oxidized Coals coal

UA1

UA1A4

UA1A8

UA1AN4

UA1AN15

CO (µmol/g) CO2 (µmol/g) total oxygen

294 240 774

589 107 803

819 579 1977

1044 469 1982

4484 4058 12600

produces anhydride groups, instead of carboxylic groups, that evolves as CO2 at much higher temperature (850 K).17

The results of Table 2 and of Figure 1 clearly show that the different oxidation treatments analyzed in this study increase the oxygen content of the sample. This oxygen increase is important because, as will be shown next, it affects the reactivity and the porosity of the resulting chars and their subsequent activation process. 3.2. Preoxidation Effect on Pyrolysis and Char Characteristics. Previous studies5 carried out with the UA1 char showed a low porosity development upon activation in CO2. The microporous surface of this UA1 char activated up to a 50% burn-off area, determined from CO2 adsorption, was only 400 m2/g. This behavior was attributed to the fact that the microporous surface area of the original coal (181 m2/g) decreased upon carbonization (100 m2/g) as a result of a pore collapse caused during the pyrolysis process by some thermoplastic properties of the coal. The TG-DTA technique helped to follow and to confirm it by means of a temperature-programmed reaction in air.15,23 When pore collapse is not present, as happens with the original coal (not subjected to the pyrolysis step) the TG-TDA curves for the reaction in air exhibited a uniform reactivity, accompanied by a single exothermic DTA peak, indicative of a homogeneous rate of combustion. Contrarily, when the coal is subjected to a pyrolysis step, both the TG-DTA curves of the char for the reaction in air show a nonuniform reactivity evidenced by the appearance of two-step TG curves along with bifurcated DTA peaks. Interestingly, these two observations disappeared when the coal was subjected, prior to the pyrolysis step, to a preoxidation treatment in air at low temperature. The results of Figure 2 present the pyrolysis step and the TG-DTA curves for the UA1 coal oxidized in air (sample UA1A4, Figure 2a) and in HNO3 (sample UA1N4, Figure 2b). The figure shows that both oxidized samples present a unique reactivity in the TG curve and one single peak in the DTA. This indicates, in agreement with results reported before,15,23 that the oxidation treatments carried out avoid the appearance of the two-step TG curves and of the bifurcated DTA of unoxidized sample. Thus, the oxidation treatment as was reported before,15 seems to avoid the anomalous thermoplastic behavior of this anthracite during the pyrolysis step. Consequently, the use of this anthracite as an activated carbon precursor (of interest to us due to its low ash content) suggests the need of an oxidation treatment prior to the carbonization process, similarly to many results reported with bituminous coals.5,9,10,15,24 The oxygen introduced in the structure of these bituminous coals during the oxidation treatment favors the cross-linking of the aromatic and hydroaromatic blocks of the coal, preventing the arrangement produced during the carbonization of the carbonaceous matrix, responsible for the plastic phase formation.7-9,15 Effect on Char Reactivity. The different oxidation treatments carried out in the present paper introduce important changes in the reactivity of the resulting chars in air, CO2, and steam. Figure 3 shows, for example, the TG profiles of a TPR in air for an air-oxidized char (CUA1A8) and for a nitric acidoxidized char (CUA1N15). The char reactivity to air (23) Mun˜oz-Guillena, M. J.; Linares-Solano, A.; Salinas-Martı´nez de Lecea, C. Fuel 1992, 71, 579-583. (24) Ruiz, B.; Parra, J. B.; Alvarez, T.; Fuertes, A. B.; Pajares, J. A.; Pis, J. J. Appl. Catal. 1993, 98, 115-123.

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Figure 4. Evolution of burn-off versus time of CO2 activation for chars coming from UA1.

Figure 2. TG-DTA profiles of the pyrolysis and combustion of the oxidized coals in air for 4 h (a, top) and in nitric acid 4 M (b, bottom) coming from UA1 coal.

Figure 5. Histogram of the different types of pores for chars coming from UA1 (the numbers of the column correspond to their total oxygen content). Table 3. Microporosity of Chars from UA1 Coal

Figure 3. TG profiles of the combustion of the preoxidized chars in air for 8 h and in nitric acid 15 M coming from UA1 coal.

increases with the oxygen content of the samples and this also occurs with the rate of the activation process in CO2 and steam, both at 1123 K. Figure 4, which presents the degree of activation versus the reaction time in CO2, shows that the chars prepared from the previously oxidized anthracite are activated faster than the char coming from the original coal. It can be seen that all the oxidized samples lie on a band (see Figure 4) with higher reactivity than the char prepared from the unoxidized coal. This increase in reactivity may be

sample

Sm (m2/g)

Vm (m2/g)

CUA1 CUA1A4 CUA1A8 CUA1N4 CUA1AN15

100 229 248 295 582

0.040 0.087 0.095 0.113 0.222

produced both by the coal thermoplasticity reduction which produces (as shown next) an increase in the porosity of the chars and/or by oxygen surface group formation that creates active sites. Effect on Char Porosity. Table 3 compiles the surface area and the micropore volumes of the different chars, deduced from the CO2 adsorption data by applying the DRK and the DR equations,19 respectively. Figure 5 presents, in a histogram mode, the different pore volumes of the chars, obtained from CO2 adsorption and mercury porosimetry, to show the effect of the preoxidation treatment carried out. It can be clearly observed from the data of Table 3 and Figure 5 (the numbers of the columns indicate the oxygen content) that the more intense the preoxidation treatment is the higher is the porosity of the resulting chars. Comparing similar degrees of sample oxidation (4 M HNO3 and 8 h air), the nitric acid creates more macroporosity than air oxidation, as could be expected because in the former the sample is oxidized in solution, the 15 M HNO3 being the one that produces the highest porosity of the char.

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Figure 6. Evolution of the microporosity with burn-off (a) CO2 and (b) steam activation.

As shown later, the porosity of the char is the key parameter for the activation process because the degree of porosity development upon activation is directly related to the porosity of the starting char. 3.3. Preoxidation Effect on the Activation Process. The preceding results have a direct consequence on the porous development of the resulting activated carbons and hence on the activation process. Effect on Narrow Microporosity. The evolution of the narrow microporosity (pore size < 0.7 nm), assessed from CO2 adsorption, has been plotted as a function of the degree of activation in Figure 6 for the CO2 and steam activation (Figure 6, a and b, respectively). Activated carbons prepared from preoxidized chars, both by CO2 and steam activation, present larger microporous volume than the activated carbons coming from the original coal because (i) the raw coal loses porosity during the pyrolysis process (note that the anthracite presents higher microporous volume than the char obtained from it) and (ii) the preoxidation of the coal prior to its charring has created porosity. Two interesting observations should be pointed out from this figure: (i) the development of microporosity upon activation depends very much on the degree of preoxidation of the coal (that is on its oxygen content). Thus, the activated carbon series that present the greatest micropore volume is the one obtained by 15 M HNO3 oxidation which produces the char that has the highest micropore volume; (ii) the porosity development varies parallel in all the series (in both activating agents); that is, the porosity development follows the same order as

Figure 7. N2 adsorption isotherms at 77 K for (a) steam and (b) CO2 activated carbons coming from UA1 coal.

the oxidation treatments carried in the coal. The figure also shows that the rate of microporous volume development is not constant during the activation process; the increase in the micropore volume is much larger in the first stages of the activation (up to about 20 % BO; BO ) burn-off) than for higher degree of activations. In other words, the narrow micropores (