Energy & Fuels 2006, 20, 11-16
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Activation of Pyrolytic Lignite Char with CO2. Kinetic Study R. Murillo, M. V. Navarro, J. M. Lo´pez, T. Garcı´a, M. S. Calle´n, E. Aylo´n, and A. M. Mastral* Instituto de Carboquı´mica, CSIC, M Luesma Castan 4, 50018-Zaragoza, Spain ReceiVed April 22, 2005. ReVised Manuscript ReceiVed NoVember 2, 2005
In this paper, an approach to lignite char partial gasification with carbon dioxide in its physical activation is carried out. Four gas-solid kinetic models (the volume reaction model, modified volume reaction model, changing grain size model, and random pore model) are reviewed in order to obtain the most accurate description for the activation reaction. Both structural and nonstructural models have been used to fit the experimental data obtained in thermobalance experiments. Prior to the determination of the kinetic parameters, a thorough study was performed to achieve the required conditions to carry out the experiments under a kinetic control regime. The final equation proposed to describe the reaction accurately is based on the random pore model. This model, in addition to fitting the conversion versus time plots at different temperatures, is able to offer a physical explanation for the maximum observed in the reaction rate for a conversion of around 30%.
Introduction Activated carbon is a highly porous carbon produced from a carbon-rich material.1 These porous solids are widely used adsorbents and have recently been involved in strategic applications as new cooling systems without CFCs,2 gas storage for the automotive industry,2,3 and hot gas cleaning processes.4 For such applications, the need for an optimum porous texture of the involved adsorbent has already been demonstrated in terms of mean pore diameter, microporous volume, and pore size distribution, PSD. Various methods have already been proposed in order to get a required porosity by direct elaboration or by pore size reduction or widening. Pore size widening can be mainly carried out by two different methods: chemical and physical activation. These processes have usually been accomplished by means of heating a carbon-containing precursor such as coal, peat, coconut shell, or any other inexpensive materials with a high carbon content.5 On one hand, chemical activation processes are performed at the same time as the heating step with the addition of chemical activation agents as dehydrating and oxidants agents.6 On the other hand, the physical activation is carried out in a different step, after the carbonization process, with a suitable oxidizing agent, mainly carbon dioxide or steam. This activation process involves a partial conversion of the material, which modifies the initial pore structure. The aim of this paper is to focus on the second step of the physical activation, where higher degrees of porosity develop* Corresponding author phone: +34 976 733977; fax: +34 976 733318; e-mail:
[email protected]. (1) Rouquerol, F.; Rouquerol, J.; Sing, K. Adsorption by Powders and Porous Solids. Principles, Methodology and Applications; Academic Press: San Diego, CA, 1999. (2) Follin, S.; Goetz, V.; Guillot, A. Ind. Eng. Chem. Res. 1996, 35, 2632-2639. (3) Lozano-Castello´, D.; Alcan˜iz-Monge, J.; de la Casa-Lillo, M. A.; Cazorla-Amoro´s, D.; Linares-Solano, A. Fuel 2002, 81 (14), 1777-1803. (4) Mastral, A. M.; Garcı´a, T.; Murillo, R.; Calle´n, M. S.; Lo´pez, J. M.; Navarro, M. V. Energy Fuels 2004, 18 (1), 202-208. (5) Ismadji, S.; Sudaryanto, Y.; Hartono, S. B.; Setiawan, L. E. K.; Ayucitra, A. Bioresour. Technol. 2005, 96 (12), 1364-1369. (6) Bansal, R. C.; Connet, J. B.; Stoeckli, F. ActiVate Carbon; Marcel Dekker: New York, 1988.
ment are achieved. To develop the optimum porous solid, it is necessary to understand the reactivities and kinetics of the char, since it provides information for the proper design of the activation process. The study has been done in a thermobalance where small changes in weight can be measured. Therefore, conditions such as flow rate, bed height, and particle diameter can be taken into account. This fact allows us establishing the conditions for chemical control, when the gaseous reactant is able to reach the carbon atoms at the available intraparticle surface. Several models have been developed specially for coal gasification that can be used to describe the kinetic behavior of lignite char activation.7 The shrinking core model;8 the volume reaction model, VRM;9 and the modified volume reaction model, MVRM,10 have been used to analyze the results obtained in gasification processes with carbon dioxide conducted with different solids.9-12 A good agreement between experimental and theoretical data can be obtained as well as an expression for the reaction rate in terms of temperature, CO2 partial pressure, and solid conversion. Nevertheless, an important point in the activated carbons’ production is to know the structural evolution of the solid, which has been studied for gas-solid reaction with models such as the changing grain size model, CGSM,13 and random pore model, RPM.14,15 With these models, a prediction of the final solid properties can be reached in terms of the initial solid properties and the reaction conditions. (7) Molina, A.; Mondrago´n, F. Fuel 1998, 77 (15), 1831-1839. (8) Levenspiel, O. Chemical Reaction Engineering; Wiley: New York, 1975. (9) Kasaoka, S.; Sakata, Y.; Tong, C. Int. Chem. Eng. 1985, 25 (1), 160175. (10) Lee, J. S.; Kim, S. D. Energy 1996, 21 (5), 343-352. (11) Murillo, R.; Navarro, M. V.; Lo´pez, J. M.; Aylo´n, E.; Calle´n, M. S.; Garcı´a, T.; Mastral, A. M. Ind. Eng. Chem. Res. 2004, 43, 7768-7773. (12) Bhat, A.; Ram Bheemarasetti, J. V.; Rajeswara Rao, T. Energy ConVers. Manage. 2001, 42, 2061-2069. (13) Georgakis, C. D.; Chang, W.; Szeckely, J. A. Chem. Eng. Sci. 1979, 24, 1072. (14) Bhatia, S. K.; Perlmutter, D. D. AIChE J. 1980, 26 (3), 379. (15) Dunham, G. E.; Miller, S. J.; Chang, R. EnViron. Prog. 1998, 17 (3), 203-208.
10.1021/ef0501187 CCC: $33.50 © 2006 American Chemical Society Published on Web 12/15/2005
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In this paper, a thorough study is performed on lignite charCO2 reaction kinetics, applying different kinetic models to acquire a deeper knowledge of the solid development. This is the first step for a more complex study where the modeling of a reactor and the properties prediction of the activated carbons produced could be performed. Experimental Section The raw material used for the kinetic study was lignite supplied by RWE-Rheinbraun with the following ultimate and proximate analysis: C(daf), 68.9%; H(daf), 4.8%; N(daf), 0.8%; moisture (ar) 21.9% and ash (ar) 3.3%. The lignite was pyrolyzed in a stainless steel, swept fixed batch reactor (15.75 mm i.d. and 75 cm height) placed in an installation described in a previous work.16 A stream of 2 l N/min of nitrogen was introduced to remove the volatiles produced in the reaction. These volatiles were condensed in a cold trap, and no further analysis was performed. The lignite particles with a diameter between 0.2 and 0.5 mm (60 g) were placed in the middle of the reactor where a thermocouple continuously measured the reaction temperature. A constant heating rate of 8 °C/min was set until the final reaction temperature of 900 °C was achieved. The final temperature was held for 3 h. Once the reactor was cooled, the solid product was recovered, crushed, and shifted between 0.1 and 0.2 mm and was stored under an inert atmosphere. In the second step, the char was activated with CO2 in a thermobalance (SETARAM TG/DTA-92) with a sensitivity of 1 µg. The experimental method used has been described in detail elsewhere.17 To ensure the reproducibility of the method, five experiments were performed under the same conditions, obtaining a 5% RSD. Different initial sample weights were tested, and the amount introduced ranged between 2.5 and 10 mg. Different flow rates were also tested (2.6-6.6 cm/s) to study the external transport influence on the reaction conditions. The reaction order was determined from a set of experiments at 12%, 20%, 30%, and 40% CO2 inlet concentrations. Finally, five experiments were conducted at 650, 700, 750, 850, and 900 °C to study the kinetic constant. The experimental conversion (Xexp) was calculated according to eq 1, where w0 is the initial sample weight, wi is the sample weight at any time, and wash is the ash weight (the stable weight after reaction). Xexp ) 100 ×
w 0 - wi w0 - wash
(1)
Char textural characterization was accomplished by N2 and CO2 adsorption isotherms at 77 and 273 K, respectively, using an ASAP 2000 (Micromeritics) apparatus. The surface area, the micropore volume, and the PSD were calculated from the N2 adsorption data applying the Brunauer-Emmett-Teller (BET) equation, the Dubinin-Radushkevich equation (DR), and DFT theory, respectively. The total pore volume was considered to be the volume of adsorbed N2 at a relative pressure of 0.95. In addition, from the CO2 adsorption isotherm, the narrow micropore volume was obtained applying the DR equation. The sample was analyzed by mercury porosimetry, using a POREMASTER GT (33/60) from Quantachrome. The software, version 4.02, was used to calculate PSD for meso- and macropores. This apparatus can develop pressures between 0.01 and 230 MPa with the mercury intrusion in pores from 105 to 6 nm width.
Results and Discussion Lignite Carbonization. In the process of lignite pyrolysis, a 41.08% solid yield is obtained with an ash content of 6.84% (16) Mastral, A. M.; Murillo, R.; Callen, M. S.; Garcia, T. Fuel Process. Technol. 1999, 60 (3), 231-242. (17) Murillo, R.; Navarro, M. V.; Lopez, J. M.; Garcia, T.; Callen, M. S.; Aylon, E.; Mastral, A. M. J. Anal. Appl. Pyrolysis 2004, 71 (2), 945957.
Table 1. Proximate and Ultimate Analysis of the Char Obtained in the Lignite Pyrolysisa
% moisture % ash % volatile matter % fixed carbon %C %H %S %N
lignite char (ar)
lignite char (mf)
lignite char (daf)
0.40 6.84 7.51 85.25 85.89 0.85 1.07 1.76
0 6.87 7.54 85.59 86.23 0.81 1.07 1.77
0 0 8.10 91.90 92.59 0.87 1.90
a
ar: as received basis. mf: moisture-free basis. daf: dry and ash-free basis.
Figure 1. Pore size distribution of the lignite-derived char (mercury porosimetry). Table 2. Textural Properties of the Lignite-Derived Char SBET (m2/g) total pore volume (cm3/g) total micropore volume (cm3/g) narrow micropore volume (cm3/g) exponent nDA in Dubinin-Astakhov equation adsorption characteristic energy in Dubinin-Astakhov equation (J/mol) average micropore width (nm)
278 0.19 0.14 0.243 1.9 21 000 1.08
(see Table 1), which is a medium value for solids used as row material for activated carbon production. The char volatile matter content is 7.5% (see Table 1) despite using extreme conditions for carbonization. However, this trend has also been observed in several works18-22 with different lignite coals and pyrolysis conditions. Several analyses have been done to obtain a more accurate knowledge of the sample. Mercury porosimetry analysis was carried out to characterize the range of porosity from meso- to macropores. The PSD obtained from the mercury intrusion curve is shown in Figure 1. A heterogeneous PSD between 15 and 300 nm with a porosity of 0.19 is observed, close to the reported value in the literature for similar materials.22,23 Different equations were applied to the N2 and CO2 isotherms of the sample in order to study the pores in the range of micro- and mesoporosity. In Table 2, a high value for BET surface area can be observed, probably as a result of the microporosity in (18) Sinag, A.; Sinek, K.; Tekes, A. T.; Misirlioglu, Z.; Canel, M.; Wang, L. Chem. Eng. Process. 2003, 42, 1-5. (19) Kasaoka, S.; Sakata, Y.; Shimida, M. Fuel 1987, 66, 697-701. (20) Skodras, G.; Orfanoudaki, T.; Kakaras, E.; Sakellaropoulos, G. P. Fuel Process. Technol. 2002, 77-78, 75-87. (21) Kwon, T. W.; Kim, J. R.; Kim, S. D.; Park, W. H. Fuel 1989, 68, 416-421. (22) Shchipko, M.; Kuznetsov, B. Fuel 1998, 77 (6), 527-532. (23) Yasyerli, N.; Dogu, T.; Dogu, G.; Ar, I. Chem. Eng. Sci. 1996, 31 (11), 2523-2528.
ActiVation of Pyrolytic Lignite Char
Figure 2. Micropore size distribution of the lignite-derived char (N2 DFT method).
Figure 3. Flow rate influence on lignite char activation (0.1-0.2 mm particle size, 850 °C, 20% CO2, and 5 mg of initial sample weight).
the sample because the total micropore volume is around 75% of the total pore volume (see Table 2). This important contribution of microporosity to the total porosity in the sample can also be deduced from the DFT micropore size distribution (see Figure 2). In this figure, an important proportion of pores between 0.5 and 0.8 nm is observed. This relevant presence of micropores in the char has been explained because of the reaction of the carbon atoms in the pore walls during the pyrolysis step with the oxygen present in the original lignite.22 Char Activation Conditions. Experimental sets have been carried out to determine the influence in the reaction rate of different parameters in order to obtain the kinetic expressions under kinetic control conditions. In all of the cases, the heating and cooling rates were fixed at 20 °C/min, and a delay of 2 min was necessary to stabilize the CO2-N2 mixture passing through the sample. Flow Rate Influence. In Figure 3, the conversion for experiments done with 2.6, 4, 5.3, and 6.6 cm/s flow rates is plotted. There is not a clear trend between flow rate and reaction rate; therefore, no external mass-transfer control is assumed for flow rates faster than 2.6 cm/s. The flow rate was fixed to the conservative value of 4 cm/s for the rest of the experiments. Particle Size Influence. The particle size used to perform the experiments was 0.1-0.2 mm because no internal diffusion control has been found in the literature18,21,23-25 for lignite char particles smaller than 1 mm. Sample Weight Influence. To study the influence of the reactant gas diffusion through the solid bed, experiments were carried out with different initial weights. In Figure 4, the results (24) Liliedhal, T.; Sjortrom, K. Fuel 1997, 76 (1), 29-37. (25) Ye, D. P.; Agnew, J. B.; Zhang, D. K. Fuel 1998, 77 (11), 12091219.
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Figure 4. Initial sample weight influence on lignite char activation (0.1-0.2 mm particle size, 850 °C, 20% CO2, and 4 cm/s).
Figure 5. CO2 concentration influence on lignite char activation (0.1-0.2 mm particle size, 850 °C, 4 cm/s gas speed, and 5 mg of initial sample weight).
from these experiments are plotted. Although there is a change in the reaction rate for all of the weights used, from 10 to 2.5 mg, a weight of 5 mg was chosen because there are two more aspects to take into account: (i) to have a representative amount of sample and (ii) to minimize the influence on weight measures from the variations in the gas flow with temperature. Finally, to study the kinetic parameters, the conditions to perform the reactions were fixed as follows: heating rate, 20 °C/min; flow rate, 4 cm/s; particle size, 0.1-0.2 mm; and sample weight, 5 mg Kinetic Models. Four different kinetic models were tested: VRM, MVRM, CGSM, and RPM.9,13,14,26 The fundamental aspects of these kinetic models have been described in detail in a previous work as well as the methods used to fit the experimental data.11 As it was explained in the Experimental Section, two sets of experiments were carried out in order to find the main parameters of the models. The results of both sets of experiments are shown in Figures 5 and 6. The model parameters and the calculated and optimized values of the fitting parameters are shown in Table 3. Experimental data were fitted to an 80% conversion because, in industrial activation processes, it is unusual to achieve further conversions. In Figures 7 and 8, there are examples of the fitting method used for VRM and MVRM. In these figures, it is shown that a better agreement between the experimental and fitted lines is obtained for MVRM than for VRM because of the successful modification introduced. The method used to obtain the reaction order and the Arrhenius parameter when the CGSM is applied can be observed in Figure 9a and b. In all cases, the linear (26) Dutta, S.; Wen, C. Y. Ind. Eng. Chem. Process Des. DeV. 1977, 16 (1), 20.
14 Energy & Fuels, Vol. 20, No. 1, 2006
Figure 6. Temperature influence on lignite char activation (0.1-0.2 mm particle size, 20% CO2, 4 cm/s gas speed, and 5 mg of initial sample weight).
Murillo et al.
Figure 7. Conversion data based on the volume reaction model for different CO2 concentrations.
Table 3. Kinetic Parameters Obtained for the Four Kinetic Models Used
Figure 8. Conversion data based on the modified volume reaction model for different temperatures.
behavior with temperature shown by the kinetic constants confirms that the experimental conditions used are appropriate to use in a chemical reaction regime, free of transport or diffusional limitations6 over the whole range of temperatures. The four models used show high values of activation energy (from 204.247 to 212.201 kJ/mol), which also confirms that all the experiments have been performed under kinetic control and there is no influence of CO2 diffusion as expressed above. These values are in the range of the values found in the literature; for example, Kovacik et al.27 obtained values between 189 and 271 kJ/mol applying the volume and grain models for the gasification of several coal chars.
Figure 10 shows calculated and experimental results of conversion versus time for the reaction carried out at 850 °C. All four models show a good agreement between experimental and theoretical data in conversion terms. However, this does not mean that all of the models can properly explain the experimental results. Because of the different reaction mechanisms described in the models, probably only one of these mechanisms can be applied to this particular situation. As a consequence, we need to look for other kinetic factors to choose an appropriate model for the accurate description of lignite char activation kinetics. One singularity can appear in the partial gasification of char with CO2, a maximum in the reaction rate versus conversion plot. Figure 11 is an example of this behavior for the sample studied. In this figure, there is also a comparison of the experimental data and the fitted data obtained with the kinetic models. VRM and CGSM cannot describe this maximum, which is due to the fact that both models describe the reaction rate as continuously decreasing.13,26 Consequently, both models can be discarded, although they could fit the conversion versus time results. Only two of the four models used are able to describe this maximum. MVRM was proposed to analyze the rate of gasification quantitatively and takes into account that the apparent reaction rate constant can change as the gasification reaction proceeds. This variation is based on the changes in the activation energy during reaction, and no physical changes in the solid are taken into account. The best fitting result (see Figure 11) is obtained with the RPM, which assumes the reaction rate to be directly proportional to the internal solid surface and its changes during the reaction. RPM describes the pore system growth with the reaction, which (27) Kovacik, G.; Chambers, A.; Ozum, B. Can. J. Chem. Eng. 1991, 69, 811-815.
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Figure 9. Reaction order and Arrhenius plot fitting with data based on the changing grain size model.
RPM has been shown to be able to fit the thermogravimetric data of the study and to produce an overall reaction rate expression based on the char textural parameters obtained from mercury porosimetry. The regression coefficient for the reaction rate is in the range of the coefficients obtained for the other methods reviewed (Table 3). In addition, compared to the other models, RPM shows a more appropriate description of the reaction produced in the solid because of its better fitting of both conversion to time and reaction rate to conversion data. A further study with experimental samples with different porosity development should be carried out in order to validate its use in describing the variation of the solid pore structure with carbon conversion. Figure 10. Comparison of the experimental and calculated conversion vs time (0.1-0.2 mm particle size, 850 °C, 20% CO2, 4 cm/s gas speed, and 5 mg initial sample weight).
Figure 11. Comparison of the experimental and calculated reaction rates (0.1-0.2 mm particle size, 850 °C, 20% CO2, 4 cm/s gas speed, and 5 mg of initial sample weight).
initially produces an increase in the internal surface with a subsequent increase in reaction rate. When the pores overlap, there is a decrease in the internal surface and, therefore, in the reaction time, and as a consequence, a maximum is reached in the reaction rate. Furthermore, this model is based on the textural parameters of the raw material to express the changes in the solid surface during the reaction in the following expression:
S ) So(1 - x)x1 - ψ ln(1 - x)
(2)
By this equation, a fast initial increase of the porosity surface is supposed, reaching a maximum at around a 10-30% conversion.
Conclusions A lignite coal char was partially gasified with CO2 in a thermobalance in order to obtain the main kinetic parameters for this reaction. First of all, the influence of different reaction conditions, such as particle size, flow rate and sample weight, was determined, and the conditions to avoid diffusional effects were fixed. To obtain the kinetic parameters, four different models were tested, all of them showing high regression coefficients when fitting conversion-time data. However, when the reaction rate versus conversion is studied, a maximum conversion between 10 and 30% is observed, which can only be described by MVRM and RPM. The final equation proposed to describe the reaction is based on RPM because this model considers not only the maximum present in the reaction rate with the conversion but also the evolution of the apparent surface of the solid on the basis of its initial textural parameters. Given the statements of this model, a maximum in the solid apparent surface is also expected to occur between 10 and 30% conversion, where the maximum in reaction rate was reached. Acknowledgment. This work has been partially supported by the European Union, Energy and Transport Commission (Contract 7220/Pr 067) and by the General Council of Arago´n (D. G. A., Spain) through the Pre-Doc. Grants of T.G. and J.M.L. R.M. and M.S.C. would like to thank the Spanish Science and Technology Ministry for the Ramo´n y Cajal Program contracts.
Nomenclature C ) Gasifying agent concentration (mol m-3) CGSM ) Changing grain size model Ea ) Activation energy (kJ mol-1) k ) Intrinsic constant rate (m s-1)
16 Energy & Fuels, Vol. 20, No. 1, 2006 ko ) Pre-exponential factor in Ahrrenius equation (m s-1) Lo ) Length of a system that is made by the random overlapping of cylindrical surfaces whose size distribution is Vo(r) (cm cm-3) MVRM ) Modified volume reaction model n ) reaction order nDA ) Dubinin-Astakhov equation exponent R ) Universal gas constant (8.314 J mol-1 K-1) RPM ) Random pore model RSD ) Relative standard deviation So ) Surface area of a system that is made by the random overlapping of cylindrical surfaces whose size distribution is Vo(r) (cm2 cm-3) SBET ) Surface area obtained by applying the BET equation to the N2 adsorption isotherm (m2 g-1) Vo(r) ) pore volume distribution
Murillo et al. VRM ) Volume reaction model wash ) Sample ash weight (mg) wi ) Sample weight at any time (mg) w0 ) Sample initial weight (mg) X ) Solid fractional conversion Xexp ) Solid experimental conversion Greek Symbols Fmolar ) Molar density of the reactant solid (kg m-3) Ψ ) Bathia and Perlmuter structural parameter o ) Total volume of a system that is made by the random overlapping of cylindrical surfaces whose size distribution is Vo(r) EF0501187
