Energy & Fuels 2008, 22, 2443–2449
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Activation of Canadian Coals in a Fixed-Bed Reactor: Effect of the Particle Size on Product Quality Ajay K. Dalai,*,† Narayan C. Pradhan,† Jian Liu,‡ Amitabha Majumdar,‡ and Eric L. Tollefson‡ Catalysis and Chemical Reaction Engineering Laboratories, Department of Chemical Engineering, UniVersity of Saskatchewan, 57 Campus DriVe, Saskatoon, Saskatchewan S7N 5A9, Canada, and Department of Chemical and Petroleum Engineering, UniVersity of Calgary, 2500 Northwest UniVersity DriVe, Calgary, Alberta T2N 1N4, Canada ReceiVed December 17, 2007. ReVised Manuscript ReceiVed April 1, 2008
Three Canadian coals, namely, Bienfait lignite, Montgomery sub-bituminous C, and Coal Valley high volatile bituminous C were activated in a fixed-bed reactor. For each coal, two different sizes of particles in the ranges of 0-1.25 mm (fines) and 1.25-2.5 mm (granules) along with cylindrical pellets of 3.18 mm in diameter and 7 ( 2 mm long were activated. The qualities of the products were determined by measuring iodine and methylene blue numbers, specific Brunauer-Emmett-Teller (BET) surface areas, bulk densities, and ash contents. The specific surface areas and iodine and methylene blue numbers of bituminous coal products were lower than the values obtained with the lignite and sub-bituminous coals, although the product yields were higher. Products obtained from pellets were found to have superior quality compared to that obtained from fines. The ash content of the feed coal influences the quality of the product activated carbon. It was established that a firstorder reaction between steam and coal pellets occurred in the process. The activation energies for the process were also determined.
Introduction Low-, medium-, and high-ranking coals are all used in different parts of the world to produce various grades of activated carbons. Lower rank coals upon activation produce relatively soft, friable products. To produce harder granular carbons, these coal may be pulverized and then pelletized or briquetted using a suitable binding agent. The pellets or briquettes are then granulated to produce the desired particle size range prior to activation.1 Activated carbon has been manufactured in the past using coal fines, which have been granulated in the presence of water and then dried, carbonized, and activated. The adsorptive capacity of the product can be enhanced by mixing the granules with a 12.5-25% solution of sulfide liquor before drying. The activation of these materials has been performed at 700-750 °C for 20-30 min.2 Yamaguchi3 developed a method for the production of activated carbon by adding 1-40 wt % rice bran to pulverized coal, carbonizing the mixture, crushing it to the desired size, and then activating it. Poheshkin et al.4 prepared activated carbon by granulation of powdered coal, treating it with oxygen (air), followed by carbonization and activation. The mechanical strength of the pellets was enhanced when the coal was treated with steam at 100-200 °C at 0.1-10 atm before pelletization. Extrudates prepared by mixing bituminous coal fines with organic additives have been steam* To whom correspondence should be addressed. Telephone: +1-306966-4771. Fax: +1-306-966-4777. E-mail:
[email protected]. † University of Saskatchewan. ‡ University of Calgary. (1) Wilson, J. Fuel 1978, 60, 823. (2) Perederie, M. A.; Suinova, S. I. Manufacture of Granular Activated Carbons. Otkytiya, Izobret, 1989; Vol. 46, p 83. (3) Yamaguchi, T. Manufacture of Activated Carbon. Japan Kokai Tokkyo Koho JP 02221108, 1989. (4) Poheshkin, A. N.; Surinova, S. I.; Kostomarova, M. A. Granular Activated Carbon. USSR, SU 1,263,623, 1981.
activated after carbonization at both low and high temperatures.5 The mechanical strength of the activated carbon obtained from high-temperature carbonization has been reported to be more than that obtained from low-temperature carbonization. Several investigations have been conducted in our laboratories over the past few years to identify the most suitable Canadian coals and favorable conditions for their activation using (a) fixed-bed reactor, (b) a spouted-bed kiln system, and (c) an internally stirred horizontal kiln system.6–9 It was observed that the lower rank coals, such as sub-bituminous and lignite coals, can be activated more easily and produce better quality activated carbons than high-rank coals, such as semi-anthracite, under similar activation conditions.5 It was also observed that coal fines produced during grinding were approximately 50-60% of the material depending upon the grinding and sieving conditions used during preparation of the feed material to yield particle sizes (granules) largely in the range of 1.25-2.5 mm.8 The quantity of the fines fraction suggests that it should be activated so that the economics of this process could be improved. The fines could not be used as such in a pilot plant study because of the dusting problems. Pelletization and activation are important steps if commercialization of the process is to be considered. As expected, during the activation process of the coal fines, higher pressure drops were experienced in comparison to those observed for larger particles (1.25-2.5 mm) (5) Liu, L.; Liu, Z.; Yang, J.; Huang, Z; Liu, Z Carbon 2007, 45, 2836. (6) Tollefson, E. L.; Hall, E. S. Final Report, Agreement U-80-7, Alberta Department of Energy and Natural Resources, Sept 1981. (7) Dalai, A. K.; Zaman, J.; Hall, E. S.; Tollefson, E. L. Fuel 1996, 75, 227. (8) Dalai, A. K.; Chowdhury, A. I.; Hall, E. S.; Zaman, J.; Tollefson, E. L. Fuel 1996, 75, 384. (9) Azargohar, R.; Dalai, A. K. Microporous Mesoporous Mater. 2005, 85, 219.
10.1021/ef700764d CCC: $40.75 2008 American Chemical Society Published on Web 06/03/2008
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Figure 1. Schematic of the fixed-bed reactor setup for coal activation.
in a fixed-bed reactor, thus creating an operating problem if fines are activated on a commercial scale in this manner. Pelletization of the fines followed by activation can avoid this problem and is, therefore, the focus of this work. The objectives of this study are (1) to prepare and characterize activated carbons from three different coals mined in Canada, namely, Bienfait lignite A, Montgomery sub-bituminous C, and Coal Valley bituminous C, (2) to evaluate activated carbon in the pellet form prepared from the fines of these three coals to show whether pelletization followed by activation can be successfully used for the coal fines, and (3) to compare the properties of the pelletized products obtained with those of coal granules and coal fines when activated. Three different particle sizes, namely, 0-1.25, 1.25-2.5 mm, and cylindrical pellets (3.18 mm in diameter and 7 ( 2 mm long) were used in this work. These different particles will be referred to hereafter as “fines”, “granules”, and “pellets”, respectively. From our previous experience with the steam activation of the above-mentioned coal fines and granules, times ranging from 0.5 to 4.0 h and temperatures from 600 to 800 °C were used for the present investigation. The properties of the activated carbons obtained from the pellets were compared to those of the fines and granules by consideration of their relative iodine and methylene blue numbers, specific surface areas, yields, ash contents, and bulk densities. Experimental Section Setup. A fixed-bed, stainless-steel tubular reactor (inner diameter of 22 mm, outer diameter of 25.4 mm, and length of 675 mm) was used to produce activated carbon from the selected coal samples. The reactor was placed in a furnace mounted vertically on a steel frame. The steam used for activation was generated in a boiler, and its flow was controlled by metering water from a burette into a boiler. Air and nitrogen flows into the reactor were controlled using a pair of rotameters. Coal samples undergoing activation were placed on a ceramic fiber pad held on a steel web welded to the wall of the reactor at ∼25 mm below the center of the reactor. Bed temperatures were recorded using K-type thermocouples. A plug of glass wool was inserted in the unheated lower part of the reactor tube to collect tar condensate. Unreacted steam was condensed, and the condensate volume was noted for monitoring steam conversion. A schematic diagram of the setup is shown in Figure 1. Materials and Methods. The main reactant materials for the process were coal, steam, and air. Nitrogen was used as a diluent and
Dalai et al. carrier for the steam into the reactor. In each run, approximately 20 g of dry coal sample was used. During the preheating of the coal, a nitrogen flow of 150 cm3 (STP)/min was passed through the reactor containing the coal sample. At the same time, the reactor and boiler were heated by setting the electrical power transformer regulators at predetermined levels. The coal sample was carbonized in the fixedbed reactor at the heating rate of 1 °C/min. The carbonization was continued until the desired temperature was achieved. At this point, a nitrogen flow rate at 30 cm3 (STP)/min and a flow rate of water required for steam production at 0.04 g min-1 (g of dry coal)-1 were maintained through the reactor. At the end of the run, steam flow was discontinued and nitrogen flow was maintained at 340 cm3 (STP)/ min during cooling of the product samples. The Coal Valley, high-volatile matter containing bituminous C coal was supplied by Luscar-Sterco Ltd., Edmonton, Alberta, Canada. Bienfait lignite coal was obtained from the Bienfait Coal Company Ltd., Bienfait, Saskatchewan, Canada, while subbituminous C coal was provided by Manalta Coal Ltd. from its Montgomery mine near Hanna, Alberta, Canada. Coal feeds of three different size ranges were prepared from each of the three coal varieties. The collected coal samples were first airdried to remove surface moisture and then screened using a sieving unit equipped with 10, 5, 2.5, and 1.25 mm screens. Particles larger than 5.0 mm were crushed in a Massco crusher (built by Mine and Smelter Corp., Denver, CO). The 5.0 × 2.5 mm (-8 + 16 mesh) particles were passed through a Bico attrition mill (supplied by Hoskin Scientific Ltd., Vancouver, Canada) and rescreened to optimize yields of the 1.25-2.5 mm particles. The yield of undersize product between 0 and 1.25 mm represented 50-60 wt % of the total coal fed to the grinder. These fines were retained so that their activation could be investigated. A part of the fines was mixed up to 4.0% wheat bran and then extruded using a pellet mill to produce particles 3.18 mm in diameter and 7 ( 2 mm long. Analytical Methods. The activated carbon products were characterized by determining the following physical properties: iodine number, methylene blue number, specific surface area, bulk density, and ash content. The iodine number is the milligrams of iodine adsorbed by 1 g of carbon at a filtrate concentration of 0.02 N iodine. The methylene blue number is the milligrams of methylene blue adsorbed by 1 g of carbon in equilibrium with a solution of methylene blue, having a concentration of 1.0 mg/L. In general, the iodine number and the amount of methylene blue adsorbed are considered as a measure of the adsorption capability of adsorbents for low and high molar mass solutes, respectively. The iodine number corresponding to the activated carbon produced was determined according to the standard procedure10 as described below. A mass of 0.1 g of activated carbon was placed in a 250 cm3 dry Erlenmeyer flask and fully wetted with 10 cm3 of diluted HCl (5% by weight). A total of 100 cm3 of the iodine solution (0.1 mol/L) was poured into the flask, and the mixture was vigorously shaken for 30 s. After filtration, 50 cm3 were titrated with 0.01 mol/L sodium thiosulfate. For the determination of the amount of methylene blue, the activated carbon (1 g) was added into a 100 cm3 aqueous solution containing 0.5 g/L of methylene blue and stirred for 5 days at 30 °C. After filtration, the aqueous phase concentration of methylene blue was determined with a UV-vis spectrophotometer at a wavelength of 663 nm. The Brunauer-Emmett-Teller (BET) surface areas of the products were measured with the high-speed surface area analyzer (Micromeritics, Norcross, GA) by determining the quantity of nitrogen required to form a single layer of molecules on a certain mass of sample. The bulk densities (g/cm3) of the coal feed and the product were determined by weighing the sample and measuring the volume occupied in a 10 or 20 mL graduated cylinder. The percentage ash was determined as per the American Society for Testing and Materials (ASTM) D3174 method by heating 1-2 (10) Gregova, K.; Petrov, N.; Minkova, V. J. Chem. Technol. Biotechnol. 1993, 56, 78.
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Table 1. Properties of Coals Used for Activation coal company rank
Bienfait Montgomery Coal Valley Bienfait Coal Ltd. Manalta Luscar-Sterco lignite A sub-bituminous C bituminous C
moisture (wt %) ash (wt %) bulk density (g/cm3) original dry
31.0 13.2
23.3 14.3
0.67 0.69
0.70 0.68
7.6 12.4 0.78 0.75
Table 2. Product Yield As a Function of Steam Activation Time at 700 °C for Various Coals Having Different Particle Sizes yield (%, dry basis) from Bienfait Montgomery Coal Valley activation lignite sub-bituminous bituminous time (h) A coal C coal C coal
particle size 0-1.25 mm
1.25-2.5 mm cylindrical pellets (3.18 mm in diameter and 7 ( 2 mm long)
1.5 3.0 4.0 0.5 1.0 1.5 2.0 1.0 1.5 2.0
37.0 23.2 21.8 41.1 31.0 21.6 16.6 47.8 41.7 33.7
52.7 38.4 28.3 69.7 58.0 47.0 36.0 59.2 48.5 33.1
60.0 58.5 52.0 61.1 60.7 55.0 54.0 62.8 61.5 55.7
g of dried sample contained in a porcelain crucible in a furnace for 6 h at 680 °C in the presence of air and weighing the ash. Product yield (wt %), determined for each run, was calculated as 100 × (wt % ash of original dried coal/wt % ash of activated product).
Results and Discussion Coal Analysis. The ash and moisture contents of the coals tested are given in Table 1. The moisture content was determined by ASTM D3173 method. Bienfait coal is listed as “lignite A”, whereas Montgomery and Coal Valley coals are classified as “sub-bituminous C” and “bituminous C”, respectively. The equilibrium moisture content of the lower ranked Bienfait lignite was 31%, while that of Montgomery sub-bituminous C was 23.3%. These values are considerably higher than that of Coal Valley bituminous C having a moisture content of 7.6%. Bienfait lignite A and Montgomery sub-bituminous C coals were similar with respect to their calorific values, although the latter is higher on a moisture- and ash-free basis. The densities of the three coals as received increase from the lignite to the bituminous coal as shown in Table 1. Product Yield. The yields as functions of the activation time and particle size for the three selected coals activated at 700 °C are given in Table 2. In all cases, the product yield decreases with an increasing activation time because of a longer steam-carbon reaction. For any particular particle size range and at a fixed activation time at 700 °C, the yield deceases in the order: bituminous > sub-bituminous > lignite, indicating that the lignite coal is very reactive and bituminous coal is least reactive. At this temperature, higher yields were obtained from pellets prepared from all of the coals compared to those from fines and granules, indicating that the extent of the steam-carbon reaction is lowest in runs with these pellets (Table 2). The extent of pore development in these materials in terms of specific surface area, iodine number, and methylene blue number is described later in this paper. The effect of the particle size on the product yield was most pronounced at the highest activation temperature (750 °C) as shown in Table 3. Bienfait fines do not react with steam well in 0.5 h at 750 °C, whereas the granules and pellets from this
Table 3. Product Yield as a Function of the Steam Activation Time at 750 °C for Various Coals Having Different Particle Sizes yield (%, dry basis) from particle size 0-1.25 mm 1.25-2.5 mm cylindrical pellets (3.18 mm in diameter and 7 ( 2 mm long)
Bienfait Montgomery Coal Valley activation lignite sub-bituminous bituminous time (h) A coal C coal C coal 0.5 1.0 1.5 0.5 1.0 1.5 0.5 1.0 1.5
57.0 29.8 24.6 31.2 28.7 26.2 28.1 26.5 24.6
57.1 43.4 41.2 54.6 34.8 24.7 38.7 26.5 21.2
56.7 52.0 49.6 80.1 75.5 71.0 56.5 45.4 34.7
coal react giving rise to 31% and 28% yields, respectively, in 0.5 h. The fines reacted when the time were increased from 0.5 to 1.0 h, showing clearly that the reactivity of the fines is due to carbonization. The yields from the bituminous pellets and fines at 750 °C are almost the same after 0.5 h of activation time, but the yield with granules is quite high. It is known that higher activation temperatures are required for the activation of this coal.11 Unlike the results at 700 °C, pellets were more easily activated at 750 °C, giving rise to lower yields than the fines for lignite and sub-bituminous coals. This indicates that the relative rates of the steam-carbon reaction and carbonization increase more in pellets than in the fines with an increase in the temperature from 700 to 750 °C. This could be due to the fact that, at the higher activation temperature, the generation of gases in pellets increases diffusion of the reactants through the pores and causes a higher pressure drop with fines in the reactor (which had been observed in the runs) causing the activation process to be less efficient in fines. Activation of Bienfait Lignite A. The iodine number and specific surface area are considered the most desirable product parameters in judging the quality of the activated carbon produced. Figure 2 shows the iodine number and surface area as functions of the yield of the products from fines and pellets of Bienfait coal at 700 °C. The maximum iodine number and surface area for pellets were 522 and 441 m2/g, respectively, which are much higher than the values obtained for fines (454 and 400 m2/g). Therefore, the pellets appear to have been activated effectively at 700 °C, causing an increase in the surface area and iodine number values. Table 4 shows that the pellets prepared using fines from Bienfait coal when activated at 750 °C for 1 h gave a product
Figure 2. Iodine number and specific surface area as functions of the product yield from Bienfait lignite coal at 700 °C.
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Dalai et al.
Table 4. Properties of Products Obtained from Various Coals Activated under Different Conditions activation coal Bienfait Lignite A
particle size 0-1.25 mm 1.25-2.5 mm cylindrical pellets
Montgomery sub-bituminous C
0-1.25 mm 1.25-2.5 mm cylindrical pellets
Coal Valley bituminous C
0-1.25 mm 1.25-2.5 mm cylindrical pellets
temperature (°C)
time (h)
surface area (m2/g)
iodine number
methylene blue number
650 700 750 650 700 760 650 700 750 700 750 700 750 700 750 700 750 700 750 700 750
2.0 1.5 1.0 2.0 1.0 0.5 4.0 2.0 1.0 1.5 1.0 2.0 1.5 2.0 1.0 4.0 1.0 2.0 1.0 2.0 1.5
410 400 372 505 460 460 498 441 430 329 454 640 394 506 427 345 314 300 450 364 426
460 454 402 550 530 505 552 522 519 442 414 620 354 585 524 415 428 422 571 449 510
125.0 116.1 83.0 116.0 105.0 120.0 41.6 31.3 73.0 107.8 109.0 60.0 107.2 81.2 71.8 74.0 76.6 62.5 67.0 25.4 51.5
with an iodine number of 519, whereas the fines fraction when activated at 750 °C for 1 h resulted in a product with a low iodine number of 402, indicating that the latter had not been activated well, resulting in a low iodine number. This comparison also indicates that pelletizing increases the rate of activation and thereby provides a superior product. This is probably due to the low pressure drop in pellets during the steam-carbon reaction, making the process more efficient compared to the case with fines. This supports the conclusions resulted previously in the Product Yield section. The methylene blue number is correlated in Figure 3 with the product yield for fines and pellets from lignite coal activated at 700 °C. The highest methylene blue numbers for fines and pellets are 116 and 60 at yields of 37.0 and 33.7%, respectively. It should be noted that the highest iodine numbers of 454 and 522 for fines and pellets also occurred at these product yields. As evident from Table 4, higher values of the methylene blue number were obtained from fines and granules than that obtained from pellets. The study indicates that, although the pellets were produced from fines, the activated carbon products obtained from these materials have different characteristics. The pellets develop a large fraction of micropores (50
nm) capable of sorbing the large methylene blue molecules. These results again indicate that the activation of pellets compared to that of fines is impeded by diffusional barriers because of their structures. It also may be concluded that fines can be pelletized to produce a product that may be satisfactorily activated to yield high iodine numbers and surface areas and intermediate methylene blue numbers. Activation of Montgomery Sub-bituminous C. Figure 4 illustrates the effect of the activation time as indicated by the product yield on the iodine numbers and surface areas of activated carbons prepared from fines and pellets of Montgomery sub-bituminous coal at 700 °C. It is apparent that both the iodine numbers and surface areas increase with a deceasing product yield, i.e., with an increasing activation time for pellets. These values then pass through maxima or decrease for the products obtained using coal fines. This indicates that, for this sub-bituminous C coal, a low yield resulting from an excessive activation time can result in products of lower quality. At intermediate yields, better product quality can be achieved as seen for the pelletized materials. Figure 4 also indicates that the quality of the product obtained from pellets, in terms of iodine number and surface area, is better than that of the products from fines. The highest surface areas are 470 and 520 m2/g for products from fines and pellets, respectively, when the yield is 38%. The same products are having iodine numbers of 445 and 550.
Figure 3. Methylene blue number as a function of the product yield from Bienfait lignite coal at 700 °C.
Figure 4. Iodine number and specific surface area as functions of the product yield from Montgomery sub-bituminous coal at 700 °C.
Canadian Coals in a Fixed-Bed Reactor
Figure 5. Methylene blue number as a function of the product yield from Montgomery sub-bituminous coal at 700 °C.
Figure 6. Iodine number and specific surface area as functions of the product yield from Coal Valley bituminous coal at 700 °C.
The effects of the activation temperature and time on the development of the optimum iodine number for activated carbons obtained from this sub-bituminous coal are given in Table 4. The corresponding surface areas and methylene blue numbers are also presented in this table. The study shows that granules activated at 700 °C for 2 h gave the highest values for the iodine number (620) and surface area (640 m2/g) when the yield was 36%. Pellets activated under the same conditions also produced good quality activated carbon with the iodine number and surface area of 585 and 506 m2/g, respectively, with a yield of 33.1%. These values for the two carbons are in the range of those of commercially available activated carbon. The methylene blue number of this coal product increases with a decreasing product yield for pellets as shown in Figure 5. Values for fines pass though a maximum of 108 (Figure 5). This high methylene blue number is accompanied by a relatively low iodine number of 442 (Table 4), suggesting that the coal is activated to a state in which the micropore structure (50 nm) are formed in its place. The products from the coal fines have relatively high values for the methylene blue number compared to pellets indicating extensive development of the macropores. The lower methylene blue numbers obtained for the pelletized samples suggest that the activation process is in part controlled by diffusional processes within the larger particles and that, by further activation, higher values may be obtained. Activation of Coal Valley Bituminous C. Iodine numbers and specific surface areas for the products obtained from the activation of bituminous coal are plotted in Figure 6. These values increase with a decreasing product yield to some extent,
Energy & Fuels, Vol. 22, No. 4, 2008 2447
Figure 7. Methylene blue number as a function of the product yield from Coal Valley bituminous coal at 700 °C.
indicating that higher activation times are essential to develop the micropores in the bituminous coal irrespective of the particle sizes. In all cases, especially for fines, the iodine numbers and surface areas were relatively low. It was previously established that bituminous coal granules required a higher activation temperature (in the range of 850 °C) to produce activated carbon in the commercial range.8 The need for such severe conditions can be seen from the data in Table 4, in which the relationship between the activation temperature and optimum activation time for the development of the iodine number has been tabulated. As the activation temperature increases from 700 to 750 °C, the optimum iodine number increases from 422 to 571 for granules with yields of 59 and 45%, respectively, and from 449 to 510 for the pellets with the corresponding yields of 65.3 and 34.5%. The fines produced activated carbons of somewhat lower iodine numbers and surface areas. In view of the relatively high yields and low iodine numbers compared to the results for subbituminous coals, the results imply that further activation may be beneficial. The methylene blue number of the products activated at 700 °C from Coal Valley coal is shown in Figure 7 as a function of the product yield. Again, it is apparent that the methylene blue number tends to increase with a decrease in the product yield. This change corresponds to the development of a greater fraction of macropores with an increasing activation time. Table 4 indicates that the fines activated at 700-750 °C have higher methylene blue numbers than those of the pellets activated under these conditions. In general, more severe conditions are required to activate this coal than the other two lower rank coals to develop more micro- and macropores. Ash Content. Ash, which includes all of the mineral matters in coal, is present in the activated carbon product. The percentage ash content of the product increases with the activation time and with a decreasing yield. This effect is illustrated in Figure 8 for coals activated at 700 °C. The ash content of the lignite coal activated at 700 °C increased from 8.6 (initial value) to 32.5-62.4%. The lignite pellets activated at 650 °C for 4 h gave an activated carbon of medium quality with an iodine number of 552 and a surface area of 490 m2/g. The ash content was relatively low at 41.7%, showing the effects of a lower activation temperature. This indicates the need to find suitable steams of coal with a low initial ash content as one of the criteria for producing high-quality activated carbon products. The product with the highest iodine number had a relatively low initial ash content of 10.5% and was obtained from subbituminous coal. Bituminous coal granules when activated at 750 °C for 1 h, the best conditions identified for this coal, had
2448 Energy & Fuels, Vol. 22, No. 4, 2008
Figure 8. Effect of the steam activation time at 700 °C on ash content of products obtained from various coals.
an iodine number of 571 and contained 30% ash. This is comparable to the 30.5% ash content of the activated subbituminous coal granules activated at 700 °C for 2 h with the highest iodine number of 620 (Table 4). The ash contents of the products from sub-bituminous and bituminous coals increased from 10.5 (initial value) to 20.9-47.6% and from 12.8 (initial value) to 18.3-43.2%, respectively (Figure 8). Also, the ash contents of the products from lignite coal are lower as noted above. Such high ash contents, however, impose limits for the development of surface areas and the sorptive capacities by the residual carbon. In the case of commercially available products, the ash contents are as high as 12% (in Petrodarco activated carbon) and as low as 7.8% (in Calgon SGL activated carbon).12 It is estimated that the initial ash content of the feed material would be 30-50% of these values, suggesting that the materials of low ash content should be carefully chosen for the activation purposes. It is concluded, therefore, that the ash content of the feedstock coal should be as low as possible to give a high-quality activated carbon product. Bulk Density. The bulk density of the activated carbon product generally tends to decrease with the activation time because of the gasification of carbon from the interior of the particles. Figure 9 shows such trends for fines and pellets obtained from all of the three coals. Kinetics of the Steam Activation Process. ReactiVity of Pelletized Coals. The reactivity of the pelletized coal was calculated by the equation R)-
1 dW Wo dt
where Wo is the weight of the dry pelletized coal load (in grams) and W is the weight of the pelletized activated carbon product. The rate of coal conversion was determined by fitting the Newton interpolating polynomial function to each set of conversion-time data and then taking the first derivative. Figure 10 demonstrates changes in the reaction rate with coal conversion for each pelletized coal at 700 °C. It clearly shows that each coal has its own characteristic rate-conversion curve. The chemical reactivity of each coal depends upon its rank. The (11) Yeshaskel, A. Activated carbon: Manufacture and regeneration. Chemical Technology Review number 117, Noyes Data Corporation, NJ, 1978. (12) Liu, J. Preparation and characterization of activated carbon from western Canadian coals. M.Sc. Thesis, University of Calgary, Calgary, Alberta, Canada, May 1995.
Dalai et al.
Figure 9. Effect of the steam activation time at 700 °C on the bulk density of products obtained from various coals.
Figure 10. Reaction rate as a function of the conversion for coal pellets activated at 700 °C.
maximum reaction rate for the sub-bituminous coal is about 2 times that of the least reactive bituminous coal. The variations in the rate-conversion curves are due to the fact that different coals vary greatly from one to another with respect to their pore structures and the changes in the surface properties during the activation process. To account for such phenomena, it is important to note by comparing Figures 2, 4, and 6 that the highest specific surface areas and iodine number values are developed at the peaks of the reaction rate curves as shown in Figure 10. Thus, the rapid decrease of reaction rates can be explained by the decease of carbon concentrations and, importantly, the drastic changes in pore structures and specific surface areas. At the beginning of the reaction, the pore structure and surface area of the coal are not fully developed. Therefore, a relatively small reaction rate is to be expected. During activation, the effect of the steam-coal reaction generates a large fraction of micropores, which contribute to the specific surface area. The existing pores are further enlarged as a result of the effects of the internal diffusion and chemical reaction on the overall rate. The reaction rate is significantly accelerated by the pore structures and coal surface area changes. The reaction rate reaches its maximum, but a further drastic reaction between carbon and steam damages the existing pores. More importantly, the reaction may change so that it is concentrated at the surface of the coal and destroys the surface area being created. The reaction rate is thereby significantly reduced. It should be noted that, for all of the coals tested, the nature of the curves will change with the temperature, coal particle size, coal rank, or possibly some other unexpected side reactions between steam and trace matter within the coal.
Canadian Coals in a Fixed-Bed Reactor
Figure 11. Evaluation of the rate constant for the first-order steam-carbon reaction.
From Figures 2, 4, 6, and 10, it can be concluded that the low-rank coals have a higher concentration of active sites, a higher porosity and surface area, and hence a greater use of the active sites. Thus, the reaction rate is usually higher, and it is easier to reach their maximum surface areas. Thus, the sorptive capacities of low-rank coals can be more readily developed. Reaction Order, Rate Constant, and Apparent ActiVation Energy. In general, the rate of reaction depends upon the coal reactivity and activation conditions. This study has shown that Bienfait lignite is more reactive than Coal Valley bituminous coals. After a series of runs for the preparation of activated carbons, it is possible to give global values in the calculation of reaction rate constants, activation energies, as well as reaction order. After several reaction models were tested, it was found that the results concerning the activation rate could be correlated to obtain the reaction order and the rate constant using a model proposed by several investigators.13–16 This model assumes that the activation rate only depends upon the amount of coal present in the reactor and that the reaction is first-order with respect to the solid phase. To be sure that the reaction is first-order, several standard methods were employed. It uses an integration method that gives a good match with experimental data. The first-order reaction model for coal activation is given by -dC/dt ) kC where C is the fraction of coal remaining in the reactor at time t and k is the order-specific rate constant. Integration of the equation for a given set of conditions gives -ln C ) kt The negative natural logarithm of C for the pelletized coals is plotted as a function of the reaction time to give a straight line, with k as the slope (Figure 11). If the lines are extrapolated, then they all pass almost exactly though the origin. From the study of the first-order reaction between steam and char, it can be concluded that the rate constants obtained at 700 °C, as shown in Figure 11, are 0.417, 0.378, and 0.213 h-1 for Bienfait, Montgomery, and Coal Valley pelletized coals, respectively. However, reaction orders have been reported from 0.6 to 1.0 from different investigators.16,17 The existence of different reaction orders from different researchers may be explained on (13) Dutta, S.; Wen, C. Y. Ind. Eng. Chem. Prod. Res. DeV. 1977, 16, 20. (14) Dutta, S.; Wen, C. Y. Ind. Eng. Chem. Prod. Res. DeV. 1977, 16, 31. (15) Leonhardt, P.; Sulimma, A.; Heek, K. H. V.; Juntgen, M. Fuel 1983, 62, 200. (16) Fung, D. P. C.; Kim, S. D. Fuel 1983, 62, 1337.
Energy & Fuels, Vol. 22, No. 4, 2008 2449
Figure 12. Arrhenius plot for the steam-carbon reaction.
the basis that the different rank coals have their own characteristics. The apparent activation energies were calculated from different reaction temperatures. An Arrhenius plot showing the exponential dependency of the rate constant on the reaction temperature for 650, 700, and 750 °C is given in Figure 12. For all three coals, straight lines were obtained by a least-squares fit of the natural logarithm of k plotted against the inverse of the absolute temperature. The apparent activation energies calculated from the Arrhenius equation gave 190.0, 198.8, and 224.8 kJ/mol for Bienfait, Montgomery, and Coal Valley coals, respectively. These values are in reasonable agreement with those reported by Kwon et al.17 The results also show that low-rank coals have a smaller value of the activation energy and that they can be easily activated when producing activated carbon. Conclusions The following conclusions can be drawn from this study: (1) Fines from the comminution of Bienfait lignite, Montgomery sub-bituminous, and Coal Valley bituminous coals can be successfully palletized and activated to produce activated carbon products that are superior to those products obtained from the activation of fines. (2) Pellets prepared from Bienfait lignite and Montgomery sub-bituminous coals can be activated more easily than those prepared from Coal Valley bituminous coal. All of these coals have potential, but finding coal steams low in ash content is a critical requirement. (3) Dependent upon activation conditions, the properties of activated granular coal and pellets are superior in most respects to those of products from fines. For color body removal from water, the latter has superior properties. (4) Bienfait lignite and Montgomery subbituminous coals upon activation yield products that compare favorably with some commercial, activated carbon products with respect to specific surface area and iodine and methylene blue numbers. Pellets prepared from Coal Valley bituminous coal require more severe activation conditions but yield satisfactory products. If supplies of these coals of lower ash content could be found, improvements in product quality in each case could be achieved. (5) A first-order reaction between steam and pelletized coals was established, and the rate constants were calculated. The activation energies for the activation of pelletized coals were 190.0, 198.8, and 224.8 kJ/mol for lignite, subbituminous, and bituminous coals, respectively. EF700764D (17) Kwon, T. W.; Kim, J. R.; Kim, S. D.; Park, W. H. Fuel 1989, 68, 416.
