Activated Carbons from Spanish Coals. 2. Chemical Activation

The present paper analyzes the chemical activation of Spanish coals with hydroxides. ..... Figure 3 shows the HK curves for the activated carbons UA1-...
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1108

Energy & Fuels 1996, 10, 1108-1114

Activated Carbons from Spanish Coals. 2. Chemical Activation M. J. Illa´n-Go´mez, A. Garcı´a-Garcı´a, C. Salinas-Martı´nez de Lecea, and A. Linares-Solano* Departamento de Quimica Inorganica, Universidad de Alicante, Alicante, Spain Received October 2, 1995X

Activated carbons from Spanish coals have been prepared by chemical activation with alkali and alkaline-earth hydroxides. The paper analyzes the following aspects: presence of water during pyrolysis, extension of the impregnation and drying processes, activating agent, temperature and time of pyrolysis, activating agent/coal ratio, coal rank, and mineral matter content. KOH and NaOH activation provides very successful results in which N2 apparent surface areas of 2500 m2/g are obtained; contrarily, Ca(OH)2 produces a small porosity development. A pyrolysis temperature of 700 °C yields activated carbons with a wider pore size distribution than those prepared at 500 °C. The pyrolysis time does not affect the microporosity development but an important reduction in the ash content is observed when the time is increased. The use of an activating agent/coal ratio of 2/1 compared to 1/1 has favorable effects: (i) a decrease in the ash content and (ii) a wider pore size distribution. The addition of a drying step before pyrolysis greatly increases the nitrogen surface area and decreases the ash content. Porosity development depends on the coal rank. Low rank coals yield activated carbons with wider pore size distributions. The chemical activation process presents the great advantage, compared to physical activation, of removing the inherent mineral matter of the coal. Activated carbons obtained by chemical activation exhibit much lower ash content than the corresponding original coals. Ash reduction, depending on the coal used, could be very noticeable; the highest reduction observed corresponds to coal UA10 (11.3% ash) that, after NaOH activation, results in an activated carbon with only 0.5% ash content.

Introduction It is well-known that materials of low cost and high carbon content can be used as precursors for the production of activated carbon and that the current trend is toward increasing use of cheap and readily available precursors as coals and lignocellulosic materials.1,2 One of the key requirements expressed in the “Medium Term Guidelines for Technical Coal Research 1994 to 1999” of the European Community of Steel and Coal (ECSC) is the improvement of the competitive position of coal. For this purpose, special research programs in the area of development and application of new and improved products derived from coal have been recommended. The use of coals as activated carbon precursors could be an alternative to develop the market for this special use of coal. The use of Spanish coals as feedstocks for the production of activated carbons is of interest to our country as reflected by several projects being carried out at different universities and research institutes, granted by the ECSC, Ocicarbon, and the Science Ministry. This interest is a consequence of the large coal production (31.7 metric tons in 1993), and reserves3 and the current exclusive use of the Spanish coal as feedstocks for combustion processes. Abstract published in Advance ACS Abstracts, July 15, 1996. (1) Smisek, M.; Cerny, S. Active Carbon, Manufacture, Properties and Applications; Elsevier: Amsterdam 1970. (2) Bansal, R. C.; Donnet, J. P.; Stoeckli, F. Active Carbon; Dekker: New York, 1988. (3) Inventario de Recursos de Carbo´ n en Espan˜ a; Centro de Estudios de la Energı´a, Ministerio de Industria y Energı´a: Madrid, 1979. X

The mineral matter content, abundantly present in Spanish coals,3 seems to be the most serious drawback for their use as activated carbon precursors. This point was analyzed in previous publications,4,5 in which activated carbons from Spanish coals were prepared by a two-stage CO2 activation. The results showed that mineral matter did not affect porosity development upon CO2 activation. However, as expected, it produced an undesirable effect, by increasing the inert part of the samples. A suitable high-volatile bituminous coal for an activated carbon precursor should have a maximum of 10 wt % ash content. Activated carbons can also be prepared by chemical activation1,2,6 in which the carbonaceous precursor is impregnated with a given chemical agent and pyrolyzed. The pyrolysis process is usually carried out at a much lower temperature than that used in the physical activation process. The pyrolyzed mixture is leached in water or acid solutions to recover the activating agent. Several chemicals have been reported to be useful for this purpose. The most widely used are phosphoric acid, zinc chloride, and alkali metal compounds.1,2,6 Though the activation mechanism is not well-known and different for each agent, the common (4) Mun˜oz Guillena, M. J.; Illa´n Go´mez, M. J.; Martı´n Martı´nez, J. M.; Linares Solano, A.; Salinas Martı´nez de Lecea, C. In Extended Abstracts, International Carbon Conference, Parı´s; 1990; p 4. (5) Mun˜oz Guillena, M. J.; Illa´n Go´mez, M. J.; Martı´n Martı´nez, J. M.; Linares Solano, A.; Salinas Martı´nez de Lecea, C. Energy Fuels 1992, 6, 9. (6) Yehaskel, A. In Activated Carbon: Manufacture and Regeneration; Noyes Data Corp.: Park Ridge, NJ, 1978; p 51.

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Activated Carbons from Spanish Coal

feature of these substances is their dehydrogenation properties that decrease pyrolytic decomposition. This inhibits formation of tar, enhancing the yield of the process.7-13 These chemical reagents may promote the formation of cross-links, leading to the establishment of a rigid matrix, less prone to volatile loss and volume contraction upon heating to high temperatures. During the pyrolysis, a transformation of the original carbon structure is observed, which may induce partial gasification of the material.14,15 As a result of the pyrolysis process a much more ordered structure is produced, the porosity of which is more developed.1,2 An important advantage of the chemical activation with respect to the physical activation is the lower temperature (400-700 °C) and shorter time in which the process is accomplished. Also, the global yield of the chemical activation tends to be greater since the chemical agents reduce the production of tars and other volatile products such as acetic acid and methanol.1,7,10,12 In a preliminary study,16 the effect of the activating agents H3PO4, ZnCl2, KOH, and Ca(OH)2 was analyzed using seven coals ranging from anthracite to highvolatile bituminous B. In that study the following conclusions were obtained: (i) H3PO4, in the case of Spanish coals, due to their high mineral matter content, is not a good activating agent since it increases the ash content of the resulting activated carbons; (ii) ZnCl2 and KOH produce a large porosity development mainly in the case of low-rank coals; and (iii) in the case of KOH an important effect of the presence of water during the pyrolysis was observed. In the literature, several studies have been published concerning the chemical activation of coals with hydroxides.13,14,17-26 However, none of them accomplish a detailed study of the different variables that can influence the characteristics of the activated carbons. The present paper analyzes the chemical activation (7) Ibarra, J. V.; Moliner, R.; Palacios, J. M. Fuel 1991, 70, 727. (8) Yamashita, Y.; Ouchi, K. Carbon 1982, 20, 47. (9) Srivastava, S. K.; Saran, T.; Sinha, J.; Ramachandram, L. V.; Rao, S. K. Fuel 1988, 67, 1683. (10) Khan, M. R.; Jenkins, R. G. Fuel 1983, 65, 1203. (11) Khan, M. R.; Jenkins, R. G. Fuel 1989, 68, 1336. (12) Mazumdar, B. K.; Banerjee, D. D.; Ghosh, G. Energy Fuels 1988, 2, 224. (13) Verheyen, V.; Jagtoyen, M.; Derbyshire, F. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1993, 38, 414. (14) Ehrburger, P.; Addoun, A.; Addoun, F.; Donnet, J. B. Fuel 1986, 65, 1447. (15) Wigmans, T.; Haringa, H.; Moulijn, J. A. Fuel 1984, 63, 870. (16) Illa´n Go´mez, M. J.; Mun˜oz Guillena, M. J.; Salinas Martı´nez de Lecea, C.; Linares Solano, A.; Martı´n Martı´nez, J. M. In Extended Abstracts, International Carbon Conference, Paris; 1990; p 68. (17) Wennerberg, A. N.; O’Grady, T. M. U.S. Pat. 4 082 694, 1978. (18) O’Grady, T. M.; Wennerberg, A. N. In Petroleum Derived Carbons; Basha, J. D., Newman, J. W., White, J. L., Eds.; American Chemical Society: Washington, 1986; p 32. (19) Perry, G. J. Proceedings, Japan-Australia Workshop on Structural Characterization and Use of Australian Coal, Osaka, Japan; 1989. (20) Guy, J. P.; Verheyen, T. V.; Felber, M. P.; Heng, S.; Perry, G. J. Proceedings of the 4th Australian Coal Science Conference, Brisbane; 1990; p 380. (21) Verheyen, T. V.; Guy, P. J.; Felber, M. D.; Perry, G. J. Energeia (Center of Applied Energy Research) 1991, 2, 1. (22) Jagtoyen, M.; Groppo, J.; Derbyshire, F. Fuel Process. Technol. 1993, 34, 85. (23) Jagtoyen, M.; Derbyshire, F. Proceedings of the 9th International Pittsburgh Coal Conference, “Coal-Energy and the Environment”; 1992; p 483. (24) Jagtoyen, M.; Toles, C.; Derbyshire, F. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1993, 38, 400. (25) Jagtoyen, M.; Ratbone, B.; Derbyshire, F. Proceedings of the 10th International Pittsburgh Coal Conference, “Coal-Energy and the Environment”; 1993; p 390. (26) Arakawa, H.; Hasabe, T.; Sato, S. U.S. Pat. 3,950,267, 1976.

Energy & Fuels, Vol. 10, No. 5, 1996 1109 Table 1. Characteristics of the Coals Studied ash content (wt %) sample

coal

raw

dem

UA1 UA8 UA10 UA18 UA11

anthracite high-volatile bituminous A high-volatile bituminous B medium-volatile bituminous subbituminous-lignite

5.6 17.0 11.3 13.2 33.6

3.3 1.0 4.3 1.4

of Spanish coals with hydroxides. The following variables were evaluated: presence of water during pyrolysis, extension of the impregnation and drying steps, activating agent, temperature and time of pyrolysis, activating agent/coal ratio, coal rank, and mineral matter content. Experimental Section Five coals from different Spanish coal seams, ranging from anthracite to subbituminous, have been studied. Elemental and proximate analyses as well as calorific values have been used to characterize the raw coals.5 Table 1 includes sample nomenclature, coal rank, and ash content. The as-received samples were ground and sieved to a particle size between 0.71 and 1.68 mm. To study the effect of the mineral matter, the raw coals were demineralized through a treatment with HCl and HF according to the procedure previously described.5 The values of the ash contents in the demineralized coals are also presented in Table 1. The experimental procedure used during the activation process was as follows: One gram of coal is mixed, by stirring, with 5 mL of a solution that contains 1 or 2 g of the activating agent depending on the ratio of activating agent/coal (A/C) used (1/1 or 2/1). This mixture is submitted to two different processes: (i) the “wet process” (WP), consisting of a stage of soaking during 12 h at room temperature or (ii) the “dry process” (DP), which is 12 h of drying of the coal slurry at 110 °C. Using coal UA18, several samples were prepared by combining the two previous processes, keeping constant the global contact time (12 h). The DP time changed from 2 to 12 h. Also, two samples were prepared using a shorter DP time of 1 or 2 h and without WP. Subsequently, the product was submitted to a pyrolysis process in a horizontal cylindrical furnace (65 mm i.d.) in an N2 atmosphere, with a flow of 80 mL/min and heating rate of 5 °C/min to the pyrolysis temperature (500 or 700 °C), that was maintained for 1 or 2 h. The samples pyrolyzed were washed repeatedly with a 5 M solution of HCl and later with distilled water until free of chloride ions. Once the activating agent is removed, the sample is dried at 110 °C for 12 h. The textural characterization of activated carbons has been carried out through physical adsorption of gases (N2 at -196 °C and CO2 at 0 °C) using a conventional gravimetric system (McBain type); previously, the samples were outgassed for 10 h at 250 °C and a final pressure of 1.3 × 10-3 Pa. To calculate the apparent surface area from data of CO2 and N2 adsorption isotherms, the Dubinin-Raduskevich (DR) equation27 was used. Also, the Horvath-Kawazoe (HK) model28 has been applied to the N2 adsorption isotherm data to obtain the micropore size distribution. Mercury porosimetry, accomplished in a Carlo Erba 2000 porosimeter, allows the determination of the pore volume for pore size above 7.5 nm. From the experimental data, the different pore volumes have been estimated with the following procedure: (1) the micropore volume (Vmicro) from CO2 adsorption using the DR27 equation; (2) the supermicropore volume (Vsuper) by difference of Vmicro of N2 (DR equation) and Vmicro of CO2; (3) the mesopore volume (27) Dubinin, M. M.; Raduskevich, L. N. Zhur. Fiz. Khim. 1949, 23, 469. (28) Horvath, G.; Kawazoe, K. J. Chem. Eng. Jpn. 1983, 16, 470.

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Table 2. KOH Activation of Coal UA8: Effect of Water and Activating Agent/Coal Ratio (T ) 700 °C)

a

sample

UA8-1

UA8-2

UA8-3

procedurea ratio A/C ash (wt %) SCO2 (m2/g) SN2 (m2/g) Vmacro (cm3/g) Vmeso (cm3/g) yield (wt %)

WP 1/1 11.6 774 970 0.617 0.074 26

DP 1/1 6.8 1420 1598

WP 2/1 7.1 1273 2029 0.596 0.074 25

78

WP, wet process; DP, dry process.

(Vmeso) deduced from N2 and mercury porosimetry; from N2 adsorption the mesopore volume is obtained by difference in the volume adsorbed at P/P0 ) 0.7 and 0.2, and this is added to that obtained from mercury porosimetry using the pore size range from 7.5 to 50 nm in diameter; (4) the macropore volume (Vmacro) from mercury porosimetry, pore size > 50 nm in diameter.

Results and Discussion The effects of the following variables were studied: the presence of water during activation; the length of time of the impregnation and drying stages; the nature of the hydroxide; the temperature and time of pyrolysis; the activating agent to coal ratio; the coal rank; and the amount of mineral matter. Effect of Water. In a previous study,16 a reduction in the ash content of the activated carbons prepared by KOH activation was reported which seemed to be strongly dependent on the use of a drying step prior to pyrolysis. In the current study coal UA8 was selected to analyze the effect of the presence of water during the activation process (impregnation and pyrolysis). The coal, after being impregnated with KOH solution, was submitted to two different processes (WP and DP) described under Experimental Section. The results are shown in Table 2 (samples UA8-1 and UA8-2). The activation with potassium hydroxide produces activated carbons with a lower ash content with regard to those of the original coal (from 17 to 11.6). This effect is more important when the sample is submitted to a DP before the pyrolysis (from 17 to 6.8); in addition, the yield of the process is much higher in this case and the characteristics of the activated carbon are better (see Table 2). These facts are of great interest since one of the problems that prevents the utilization of Spanish coals, as precursors for activated carbon preparation, is their high ash content. The decrease in the mineral matter content of coals treated with alkali hydroxides has been reported. Thus, among others, Wang et al.29 observed ash removal by caustic digestion of coal (NaOH) followed by acid washing. In that study, the digestion temperature was higher (190 °C vs 110 °C) and an acid washing (1 M HCl) was used additionally. Under these experimental conditions, the removal of quartz and kaolinite was reported to be easy. Our results agree with those of Wang et al.29 because the majority of the mineral components of our coals are quartz and kaolinite. Conversely, other authors24 have observed that the mineral matter content of the coal is not modified by activation with KOH at ratios of KOH/C lower than 1.3, (29) Wang, Z. Y.; Ohtsuka, Y.; Tomita, A. Fuel Process. Technol. 1986, 13, 279.

Figure 1. Micropore size distribution curves (HK model); effect of KOH/coal ratio (coal UA8; T ) 700 °C; DP).

but it is noticeably increased when higher activating agent/coal ratios are used. KOH activation of coal UA8, which has very narrow pores and low surface area,5 allows one to obtain high surface area activated carbons. These results confirm the well-known ability of alkali metal hydroxides to develop the porosity of the raw material.13,14,17-22,24,25,30,31 The extent of the porous development reached depends on the KOH/coal ratio and the procedure used. High apparent surface areas can be obtained by selection of the appropiate conditions. It should be noted that, again, the drying stage favors the process of activation, since it permits more development of porosity. Note in Table 2 that SCO2 of sample UA8-2 is almost double that of UA8-1, both being prepared using the same KOH/coal ratio. Interestingly, the use of a drying process and a coal ratio of 1/1 gives a microporous carbon with a quite homogeneous microporosity. This observation is inferred because in sample UA8-2 the CO2 and N2 apparent surface areas are quite similar.32 Furthermore, Figure 1, which shows the micropore size distribution curves obtained from N2 adsorption results by applying the HK model, confirms that the sample UA8-2 has a quite homogeneous microporous size distribution formed by narrow micropores in which only the primary adsorption process takes place.33 On the contrary, the use of a 2/1 ratio and the absence of the drying process (UA8-3) favor the development of a wider microporosity distribution; as is shown in Figure 1, larger micropores with a diameter of ca. 1.3 nm are created. In the larger micropores the adsorption of N2 can also occur, by a secondary adsorption process (pore filling). The same observation could be deduced directly from Table 2ssample UA8-3 has a SCO2 much lower than SN2 because the supermicroporosity of this sample is well developed.32 (30) Otowa, T.; Shiraishi, M.; Tanibata, R.; Tanaka, N. In Extended Abstracts, International Carbon Conference, Essen, Germany; 1992; p 944. (31) Verheyen, V.; Jagtoyen, M.; Derbyshire, F. In Extended Abstracts, 21st Biennial American Carbon Conference, Buffalo; 1993; p 474. (32) Rodrı´guez-Reinoso, F.; Linares-Solano, A. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Dekker: New York, 1988; Vol. 21, p 1. (33) Gregg, S. J.; Sing, K. S. W. In Adsorption, Surface Area and Porosity; Academic Press: London, 1982; p 242.

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Activated Carbons from Spanish Coal

Energy & Fuels, Vol. 10, No. 5, 1996 1111 Table 3. Effect of Activating Agent (A/C ) 2/1; T ) 700 °C; DP) coal UA1-1 UA1-2 UA1-3 UA10-1 UA10-2 UA10-3

Figure 2. Effect of WP and DP time on surface area for coal UA18 (KOH/C ) 1/1; T ) 700 °C).

These observations reveal that this stage is determinant in the activation process. Presumably during the DP, the higher KOH-coal temperature contact (110 °C) will enhance KOH reaction with the mineral matter and the dehydrogenation process of the coal, producing a new structure which, upon heat treatment, inhibits the subsequent evolution of volatiles and tars, developing better the porosity of the resulting activated carbon. Effect of Time of Drying and Impregnation. As has been discussed, the 12 h of the DP, prior to the pyrolysis step, improves the characteristics of the activated carbons. The SCO2 increases noticeably from 774 to 1420 m2/g (Table 2). In this section, the effect of the extent of the WP and the DP is analyzed in coal UA18, keeping constant the KOH/coal ratio of 1/1, the pyrolysis temperature (700 °C), and a total KOH-coal contact time of 12 h , three samples have been prepared. Figure 2 summarizes the results obtained. The N2 apparent surface area increases from 1237 m2/g, for the sample dried during 12 h, to 1632 and 1729 m2/g for samples dried during 4 and 2 h, respectively. The ash content for the sample dried during 12 h (sample UA181) was 4.5%, while in the sample dried for 4 h (sample UA18-2) the ash content decreased to 3.5%. These data indicate that a decrease in the drying time improves the results by reducing the mineral matter content of the final product and by increasing the apparent surface area. On the basis of these results, it was considered interesting to avoid a long DP and to eliminate the WP. The initial step of the impregnation, before the drying stage, was suppressed and contact times (DP) of only 1 and 2 h were tested. The N2 apparent surface areas of these samples are also plotted in Figure 2. Comparison of these two results confirms that a decrease of the DP increases the apparent surface area of the activated carbon. However, the results in Figure 2 also show a decrease in the SN2 when, using a DP time of 1 or 2 h, the WP is avoided. This indicates that some contact time between KOH and coal, at room temperature during the WP, is suitable before the mixture is heated at 110 °C. Thus, combining a short DP time (i.e. 1 h) with a given appropriate WP time (not studied but probably smaller than 10 h) will give the best results. The positive effect of reducing the contact time, observed in Figure 2, is interesting from a practical point of view,

activating ash SCO2 SN 2 Vmacro Vmeso yield agent (%) (m2/g) (m2/g) (cm3/g) (cm3/g) (%) KOH NaOH Ca(OH)2 KOH NaOH Ca(OH)2

0.9 0.5 3.6 1.0 0.5 6.8

1785 1740 451 1570 1579 440

1790 1980

0.107 0.669

0.042 0.012

2078 1630

0.120 0.010

0.120 0.010

48 38 84 23 8 53

although it is not well understood. Further research has to be carried out to get insights on these observations. Effect of the Activating Agent. In the preliminary study16 a large porosity development was obtained using KOH as activating agent. This work has been devoted to a deeper analysis of the activation with hydroxides. The selected activating agents have been KOH, NaOH, and Ca(OH)2, which are the most widely investigated in the literature.14,17,18,20,21,29 The study has been carried out with coals UA1 and UA10. Conditions include an A/C ratio of 2/1, DP conditions, a pyrolysis temperature of 700 °C, and a time of 2 h. Table 3 presents the results of the N2 (-196 °C) and CO2 (0 °C) adsorption isotherms and of the mercury porosimetry for activated carbons prepared with KOH, NaOH, and Ca(OH)2 of coals UA1 and UA10. From the results it can be asserted that the Ca(OH)2 has little effect as activating agent and produces a poorer development of the porosity and a smaller decrease of the ash content than KOH and NaOH. KOH and NaOH have similar behaviors, producing a decrease of the ash content up to very favorable unexpected limits, and make activated carbons with a large adsorption capacity. Note that the NaOH reduces the ash content of the two activated carbons to the very low value of 0.5% (from 5.6% for UA1 and from 11.3% for UA10). The KOH reduces the ash content to about 1.0%. The greater efficiency of the NaOH or KOH compared to the Ca(OH)2 must be related to the role of these activating agents in the activation mechanism. Thus, potassium and sodium are capable of penetrating into the coal matrix, provoking a separation of graphitic layers that favors the subsequent development of the porosity during the pyrolysis.34,35 Matsukata et al.36 found that Ba and Sr species sink into the bulk of carbon in the course of heat treatment, while this behavior is difficult for Ca species. For KOH and NaOH the resulting activated carbons have quite high surface area and present the unique characteristic of having very similar CO2 and N2 surface areas (except UA10 activated with KOH). The development of the different ranges of porosity and the yield of the activation process do not seem to follow a common standard for the two coals studied. Regarding coal UA1, the activated carbons obtained present the following features: (a) they are substantially microporous, reaching very high values of CO2 microporous surface area (close to 1800 m2/g) and similar SN2 values, (b) a yield of the process of the order of 40-50%, and (c) an ash content less than 1%. In the case of coal UA10, the activation with NaOH (UA10-2) produces results very (34) Go´mez Serrano, V.; Sa´nchez-In˜iguez, F.; Valenzuela-Calahorro, C. Fuel 1991, 70, 1083. (35) Marsh, H.; Wilkinson, A.; Machnikowski, J. Fuel 1982, 61, 834. (36) Matsukata, M.; Kikuchi, E.; Morita, Y. Fuel 1992, 71, 705.

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Figure 4. Nitrogen adsorption isotherms (77 K) of activated carbons from coal UA11; effect of pyrolysis temperature (KOH/ coal ) 2/1, DP). Table 4. Effect of Temperature and Time of Pyrolysis: KOH Activation of Coal UA11 (KOH/Coal ) 2/1; DP) sample T (°C) t (h) ash (%) SCO2 (m2/g) SN2 (m2/g) yield (%) UA11-1 UA11-2 UA11-3

Figure 3. Micropore size distribution curves (HK model); effect of activating agent (activating agent/coal ) 2/1; T ) 700 °C; DP): (a) coal UA1 and (b) coal UA10.

similar to those of coal UA1, though the yield of the process is much smaller. KOH activation produces an activated carbon with SN2 considerably higher than SCO2, indicating that its microporous size distribution is wider than that of the activated carbons prepared from coal UA1. Figure 3 shows the HK curves for the activated carbons UA1-1 and UA1-2 (Figure 3a) and UA10-1 and UA10-2 (Figure 3b). Regarding coal UA1 (Figure 3a), the effective pore diameters for activated carbons prepared using both activating agents are very similar, but a larger micropore volume is obtained with NaOH activation. Contrarily, for coal UA10 (Figure 3b), the KOH activation produces an activated carbon with a larger effective pore diameter and larger micropore volume. In previous studies,14,20 KOH was considered to be much more effective than NaOH; however, this study shows that NaOH could be as effective or slightly more than KOH and that both activating agents can be used to prepare high CO2 apparent surface area activated carbons. Effect of the Temperature and Time of Pyrolysis. One of the advantages presented by the chemical activation of coals compared to the physical activation is the lower temperature and time of the process. In this section the results for two temperatures (500 and 700 °C) and two pyrolysis times (1 and 2 h) are presented. Figure 4 presents the N2 adsorption isotherms of two activated carbons prepared from coal

500 700 700

2 2 1

8.1 5.3 14.2

1329 1517 1503

1390 2449

38 17 27

UA11 using a KOH/coal ratio of 2/1 and activation temperatures of 500 and 700 °C, respectively. The shapes of the isotherms are quite different, indicating different porosities of both activated carbons. The development of porosity (much higher for the sample pyrolyzed at 700°C) as well as the pore size distribution varies considerably (sample pyrolyzed at 700 °C presents a much wider N2 adsorption isotherm knee), providing evidence of the important effect of the pyrolysis temperature. The characteristics of these activated carbons are presented in Table 4. A pyrolysis temperature of 500 °C produces an almost exclusive development of narrow microporosity (micropore less than 0.7 nm) confirmed by the fact that the CO2 surface area is similar to that of N2; contrarily, the pyrolysis at 700 °C produces both micropores and supermicropores (regarding the important differences found between CO2 and N2 surface areas, N2 being much higher than CO2). The increase in pore size with the pyrolysis temperature must be related to the gasification of the coal, catalyzed by potassium. Ehrburguer et al.14 reported the formation of potassium carbonate and the evolution of CO2 and CO during the pyrolysis step. At 500 °C, the K2CO3 formed is not decomposed; therefore, no activation by CO2 evolution will occur at this temperature. At 700 °C the carbonate already decomposes and the evolved CO2 leads to further gasification (as it is confirmed with the lower yield of 17 vs 38%). It is interesting to note that an activated carbon with a final ash content of 5.3% and with a very high surface area (2449 m2/g) can be obtained from coal UA11, which has an initial ash content of 33.6%. As was noticed by Wang et al.,29 not all of the inorganic components of a coal can be eliminated with the same facility. Thus, coal UA11 that, in principle, should be the worst raw material for activated carbon preparation due to its very high ash content, can be converted into

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Activated Carbons from Spanish Coal

an activated carbon with very suitable adsorbent characteristics and only 5.3% in ash. The results in Table 4 show that using a pyrolysis temperature as low as 500 °C, an activated carbon with a surface area of about 1400 m2/g being essentially microporous (deduced upon observing that the CO2 and N2 apparent surface areas are practically equal32) can be obtained. Furthermore, the yield of the process is double with respect to that produced with a pyrolysis temperature of 700 °C, confirming the above statements about the K2CO3 decomposition effect. Finally, the pyrolysis time does not seem to affect the development of the microporosity, but it is important for the reduction in ash content (the ash content is reduced almost 3 times by increasing the pyrolysis time from 1 to 2 h). Effect of the Activating Agent/Coal Ratio (A/C). The A/C ratio widely varies in the literature.13,14,17,18,22-25,30 In the present work the ratios 1/1 and 2/1 have been tested in several coals using different activating agents. Table 2 compiles the results for coal UA8 activated with KOH, pyrolyzed at 700 °C, and using the WP. From a comparison of the results for ratios 1/1 and 2/1 (samples UA8-1 and UA8-3) it can be deduced that an increase in the A/C ratio produces a favorable effect. From the point of view of the ash content, a greater reduction (from 32 to 58%) is produced. The activating agent upon reaction with the mineral matter produces a digestion effect that dissolves, through subsequent water washings, the mineral matter. This effect is more intense as the ratio A/C increases. Regarding porosity development, a very important enhancement is observed that affects all ranges of pore sizes, producing an activated carbon with a wide distribution of porosity, as in the case of the sample prepared with an A/C ratio of 2/1. The increase in N2 surface area with the KOH/coal ratio was also observed previously. An example is the work of Jagtoyen et al.13,23-25,31 Nevertheless, contradictory results have also been found; thus, Otowa et al.,30 using a much higher KOH/coal ratio, observed an increase in the N2 surface up to a KOH/coal ratio of 4/1, while for a greater ratio (6/1) they detected a decrease of the surface area (of approximately 20%). The reason for obtaining a wider distribution of porosity with a KOH/coal ratio of 2/1 compared to 1/1 has not been previously discussed but can be inferred considering the results of Ehrburger et al.,14 previously commented on. The higher the KOH/ coal ratio, the greater will be the formation of K2CO3 and hence of CO2 (from carbonate decomposition) and CO (from carbon gasification). As the extent of carbon gasification increases, the widening in pore size distribution and the lowering in the process yield are observed. These arguments are in good agreement with the results of Table 2, where the two KOH/coal ratios are compared. Effect of the Coal Rank. CO2 and N2 apparent surface areas of coals UA1, UA10, and UA11 activated with KOH in the conditions previously described (KOH/ coal ratio of 2/1, 700 °C, and use of the DP) have already been shown in Tables 3 and 4. CO2 surface areas are very large in all cases, but slightly decrease from the anthracite (sample UA1) to the subbituminous-lignite (sample UA11). A very interesting result is the possibility of obtaining a very high surface area activated carbon with a very narrow microporosity. Note the

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Energy & Fuels, Vol. 10, No. 5, 1996 1113

Figure 5. Effect of coal rank in the porosity development of activated carbons prepared with KOH (KOH/coal ) 2/1; T ) 700 °C; DP).

exceptional case of the anthracite (coal UA1) with an SCO2 ) SN2 of about 1800 m2/g. This narrow pore size distribution is not at all frequent on activated carbons and is not obtained from most of the coals. However, as Figure 5 shows, the pore size distribution changes with decreasing coal rank. Activated carbon from low rank coal (UA10) presents a much wider porosity distribution (a higher volume of supermicropores and mesopores is developed) than activated carbon from high-rank coal (UA1). The different pore size development must be related to (i) the different effect that KOH will have upon the rank of the coal, during the stage of drying (some cross-linkings are formed that may help to maintain the original porous structure of the coal and, hence, its subsequent development of the porosity during the thermal treatment) and (ii) the different gasification rates of the coals upon their rank by the CO2 evolved from K2CO3 formed. It is well-known that the lower the rank, the higher its reactivity and hence the higher will be the degree of gasification and the resultant porosity development. In line with the above arguments it is found that the activation yield decreases with the lowering in the coal rank. Effect of the Mineral Matter. A treatment of demineralization, first with HCl and subsequently with HF, has been used to reduce the ash content of the coals (see Table 1). The ash reduction with the HCl/HF treatment shows to be very dependent on the coal studied. Nevertheless, in the most unfavorable case, corresponding to the anthracite (coal UA1), the reduction goes beyond 50%. The chemical activation of these demineralized coals and the study of the characteristics of the activated carbons obtained will permit us to analyze the effect of the mineral matter in the activation process. The influence of the mineral matter could be through the interaction with the activating agent,11 with the consequent consumption of activating agent, and/or from the different reactivity of the demineralized coal (with respect to the original coal).5 In any case, the textural properties of activated carbons will be influenced. The influence of the mineral matter has been analyzed for KOH activation using the A/C ratio of 1/1 or 2/1, DP treatment, and 700 °C pyrolysis temperature. Some of the results of apparent surface areas, residual ashes (after the activation and washing), and yields of

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1114 Energy & Fuels, Vol. 10, No. 5, 1996

Illa´ n-Go´ mez et al.

Table 5. KOH Activation: Effect of Mineral Matter (A/C ) 1/1; T ) 700 °C; DP) coal

ash (%)

SCO2 (m2/g)

SN2 (m2/g)

yield (%)

UA8-2 UA8D-1 UA10-4 UA10D-1 UA11-2a UA11D-1a

6.8 0.9 3.5 0.8 5.3 1.3

1420 1653 1272 1504 1517 1547

1598 2284 1696 1638 2449 2716

78 42 46 21 17 11

a

The A/C ratio was 2/1.

the process, obtained in coals UA8, UA10, and UA11, are collected in Table 5. The data in Table 5, and the study accomplished in other coals, show that the mineral matter content, independently of the coal, affects the yield of the process and the surface area assessed by N2 adsorption. The micropore surface area, determined by CO2, is only slightly affected. Thus, the yield of the process is quite inferior when demineralized coals are used, probably due to their greater reactivity.5 The activation of demineralized coals provides activated carbons with a slightly larger microporous volume and apparent surfaces areas (N2 surface area) larger than those obtained with the raw coals. Therefore, the activation of the demineralized coals provides wider pore size distribution as is shown by the difference in apparent surface area determinated by N2 and CO2. Its greater degree of activation is responsible of an important increase of the supermicro- and mesoporosity of these samples in relation to the raw coals. The above points are in agreement with the activation mechanism as discussed by Ehrburger et al.14 Conclusions Spanish coals, even those possessing a high mineral matter content, are feasible materials for activated

carbon preparation by chemical activation. A direct relationship between the rank of the coal and the porous texture of the obtained activated carbon has been found. Coals of different ranks develop microporosity, but an increase in the development of the largest pores is observed with decreasing rank. Coals of lower rank present a wider porosity distribution. Demineralization of coals favors the development of the porous structure. However, activated carbons with apparent surface areas of N2 as high as 2500 m2/g can be obtained using raw coals and KOH as activating agent. KOH has been shown to be a very effective activating agent for coal activation. Moreover, the procedure presents the advantage of reducing the ash content without the need of a coal demineralization step. For this purpose a drying step before pyrolysis is beneficial: the samples present higher surface area and lower ash content. However, the time of the drying step should not be too long to get high apparent surface areas. NaOH activation follows the same behavior as that of KOH. The activated carbons produced have apparent surface areas of 2000 m2/g, and the microporosity can vary depending on the different experimental variables. Very narrow micropore size distributions (micropores around 0.7 nm) could be obtained by selecting the appropriate coal precursor, activating agent, and temperature. Acknowledgment. This study was made possible by financial support from DGICYT (Project AMB921032-CO2-O2) and OCICARBON (C-23-435). EF950195+