Pyrolysis of Lignins: Experimental and Kinetics Studies - Energy & Fuels

Sep 12, 2002 - Since, this group has low dissociation energy (60−70 kcal/mol), this is the main source of CO at lower temperature. At higher .... Da...
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Energy & Fuels 2002, 16, 1405-1412

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Pyrolysis of Lignins: Experimental and Kinetics Studies D. Ferdous, A. K. Dalai,* S. K. Bej, and R. W. Thring† Catalysis and Chemical Reaction Engineering Laboratories, Department of Chemical Engineering, University of Saskatchewan, Saskatoon, SK S7N 5C9, Canada Received February 15, 2002

Lignins are generally used as a low grade fuel in the pulp and paper industry. In this work, pyrolysis of Alcell and Kraft lignins obtained from the Alcell process and Westvaco, respectively, was carried out in a fixed-bed reactor and in a thermogravimetric analyzer (TGA) using helium (13.4 mL/min/g of lignin) and nitrogen (50 mL/min/g of lignin), respectively. The reaction temperature was increased from 300 to 1073 K, while the heating rates were varied from 5 to 15 K/min. The gaseous products mainly consisted of H2, CO, CO2, CH4, and C2+. With increase in heating rate from 5 to 15 K/min both lignin conversion and hydrogen production increased from 56 to 65 wt % and from 25 to 31 mol %, respectively for fixed-bed pyrolysis reaction of Alcell lignin at 1073 K, whereas at the same condition the conversion and hydrogen production increased from 52 to 57 wt % and from 30 to 43 mol % for Kraft lignin. The distributed activation energy model (DAEM) was used to analyze complex reactions involved in the lignin pyrolysis process. In this model, reactions are assumed to consist of a set of irreversible first-order reactions that have different activation energies. This model was used to calculate the activation energy, E, the distribution of activation energy f(E), and the frequency factor k0 for the pyrolysis of Alcell and Kraft lignins in a thermogravimetric analyzer (TGA). For the pyrolysis in TGA, the activation energies for Kraft and Alcell lignins varied from 129 to 361 kJ/mol with maximum distribution at ∼250-270 kJ/mol and from 80 to 158 kJ/mol with maximum distribution at ∼118-125 kJ/ mol, respectively.

Introduction The increased interest in the conversion of wood and its components, for producing alternative fuels and chemicals necessitates a fundamental understanding of processes involving pyrolysis of biomass. Knowledge of the kinetics of thermal reactions is vital for predicting the pyrolysis behavior of biomass materials such as lignin.1 The pyrolysis of lignin is highly complex and depends on several factors such as its composition and processing conditions such as heating rate, reaction temperature, carrier gas flow rate, etc. The pyrolysis of lignins and related materials generally involve two types of reactions: (1) primary and (2) secondary. In primary pyrolysis, products are formed directly from decomposition of original biomass material. On the other hand, in the secondary pyrolysis volatiles evolved in the primary pyrolysis process undergo further reactions, such as coking, cracking, and other complex reactions involving free radicals. During thermal decomposition of lignin, relatively weak bonds break at lower temperature whereas the cleavage of stronger bonds take place at higher temperatures.2 At higher temperature (>500 °C), the aromatic rings are rearranged and condensed releasing hydrogen. CO is produced from two types of * Corresponding author. Tel.: (306) 966-4771. Fax: (306) 966-4777. † Department of Chemical Engineering, University of New Brunswick, P.O. Box. 4400, Fredericton, NB E3B 5A3, Canada. (1) Klein T. M.; Virk S. P. Ind. Eng. Chem. Fundam. 1983, 22, 3545. (2) Caballero, J. A.; Font, R.; Marcilla, A. J. Anal. Appl. Pyrolysis 1997, 39, 161-183.

ether groups. The first is the ether bridge joining subunits. Since, this group has low dissociation energy (60-70 kcal/mol), this is the main source of CO at lower temperature. At higher temperature, the dissociation of diaryl ether causes the additional formation of CO at higher temperature. CH4 is produced readily from a weakly bonded methoxy group -OCH3- (bond energy 60 kcal/mol).3 This means that the measured reaction rate over a range of temperatures must be a cumulative effect of many independent reactions. On the other hand, the thermal degradation of lignocellulosic materials are generally influenced by heat and mass transfer processes in reactors, which affect significantly the activation energy of the process and the preexponential factor. Kinetic models in the literature for the pyrolysis of biomass, lignin, coal, and other organic solids involve simple first-order reaction models.4-7 Other authors8-10 (3) Sada, E.; Kumazawa, H.; Kudsy, M. Ind. Eng. Chem. Res. 1992, 31, 612-616. (4) Dombarg, D.; Rossinakaya, G.; Sergeeva, V. Thermal Analysis, Proceedings Fourth ICTA Budapest 1974, 2, 211-215. (5) Ramiah, M. V. J. Appl. Polym. Sci. 1970, 14, 1323-1337. (6) Suuberg, E. M.; Peters, W. A.; Howard, J. B. Ind. Eng. Chem. Process Des. Dev. 1978, 17, 37-46. (7) Hajaligol, M. R.; Howard, J. B.; Longwell, J. P.; Peters, W. A. Ind. Eng. Chem. Process Des. Dev. 1982, 21, 457-465. (8) Caballero, A. J.; Font, R.; Marcilla, A.; Conesa, A. Ind. Eng. Chem. Res. 1995, 34 (3), 121-129. (9) Koufopanos, C. A.; Maschio, G.; Lucchesi, A. Can. J. Chem. Eng. 1989, 67, 5-84. (10) Chen, Y.; Charpenay, S.; Jenson, A.; Serio, A. M.; Wo´jtowicz, A. M. Chemical and Physical Process in Combustion, Fall Technical Meeting, The Eastern States Section 1997, 147-150.

10.1021/ef0200323 CCC: $22.00 © 2002 American Chemical Society Published on Web 09/12/2002

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have carried out complex analysis incorporating physical transport and chemical reaction mechanisms. In these models, the authors have assumed that the activation energy of the pyrolysis reactions is independent of temperature. Detailed kinetics studies with respect to lignin were carried out by many investigators.4,5,8,9,11-20 Domberg4 estimated apparent activation energy and other kinetic parameters by assuming lignin pyrolysis as a single reaction. Ramiah5 estimated kinetic parameters from the thermogravimetric analysis (TGA) of cellulose, hemicellulose, and lignin between room temperature and 873 K by assuming first-order reaction. The reaction rate constant, k, and activation energy, E, were calculated by static and dynamic thermogravimetric analysis (TGA). The activation energy varied from 54.34 to 79.42 kJ/mol for the temperature range of 517-582 K. Chan and Krieger12 estimated the kinetic parameters for lignin degradation in a microwave. They reported a single activation energy (25.08 kJ/mol) and frequency factor (4.7 × 102 min-1) for the temperature range of 433-953 K. Nunn et al.13 studied the kinetics of milled wood lignin pyrolysis. They proposed a first-order decomposition model and correlated the yield of different products through a single reaction. They reported activation energy and frequency factor for the temperature range of 600-1440 K as 81.2 kJ/mol and 3.39 × 105 min-1. Caballero et al.8 reported a more complex model for the thermal decomposition of Kraft lignin in the temperature range of 423-1173 K. In this model, it was assumed that lignin is formed by “fractions”. A given fraction can decompose only if the temperature of the lignin is greater than or equal to characteristic temperature, TR, of this fraction. The distribution function of this characteristic temperature was defined by “C curve” according to the following equation:

studied kinetics of lignicellulosing biomass materials and their major components (cellulose, hemicellulose, and lignin). In this model, complex reaction networks were considered. Thermogravimetric and differential scanning calorimeter (DSC) curves at different heating rate were evaluated by the method of least squares to obtain pseudo-first-order models including parallel, successive, and competitive reaction schemes. From this study, the activation energy of milled wood lignin was calculated to be 34-65 kJ/mol. A simplified model known as the distributed activation energy model (DAEM), for determining kinetic parameters for complex reactions such as coal pyrolysis reaction has been proposed by Miura.21 Unlike the models described above, in this model, reactions are assumed to consist of a set of irreversible first-order reactions that have different activation energies. The DAEM has been widely used to analyze complex reactions such as pyrolysis of fossil fuels, thermal regeneration reaction of activated carbon, and to represent the change in overall conversion and the yield of a given component during pyrolysis reactions.6,22-26 However, the literature related to the pyrolysis of lignin is scarce. In this work, an attempt has been made to obtain experimental data for pyrolysis of Alcell and Kraft lignins at different heating rates (5 to 15 K/min) and temperatures up to 1073 K using a fixed-bed reactor and to obtain kinetic parameters for these lignins in a TGA using DAEM. The TGA unit was used for kinetic calculations in which heat and mass transfer problems were eliminated. Knowledge gained through a systematic kinetic study of lignin could also be applied to thermal processing of other biomass materials producing fuels.

∫0∞ C dTR ) 1

Alcell lignin was obtained from the Alcell process. This process is an ethanol-based auto catalyzed solvent pulping technology, operated by Repap Enterprises Inc., in a demonstration plant in Miramachi, New Brunswick, Canada. Kraft lignin is the residue obtained from Westvaco Kraft pulping operations in South Carolina. The proximate and elemental analysis of the two fuels are given in Table 1. The carbon (C), hydrogen (H), and nitrogen analyses of lignins were performed on a CHN analyzer (PerkinElmer 2400) and ash content was obtained by carring the samples at 1073 K for 6-8 h using ASTM method no. 286694. The ash content was calculated as a percentage of dry lignin. This procedure was used to remove the total volatiles presents in the samples. Therefore, it was assumed that the percentage of total volatile matter present in the fuels ) 100 - ash content - moisture content. Data for the pyrolysis reaction were generated using two different types of experimental setup such as a fixed-bed reactor and a thermogravimetric analyzer. Fixed-Bed Reactor. Pyrolysis of lignins was carried out at atmospheric pressure in a continuous down-flow fixed-bed microreactor (see Figure 1). The lignins for all reaction were

(1)

In eq 1, C represents the C curve. If the temperature of the system at a given moment is T, the area included under the C curve between zero and TR ) T corresponds to the portion of biomass that can undergo decomposition. According to this model activation energy varied linearly between 72.4 kJ/mol at 473 K and 174 kJ/mol at 973 K. Caballero et al.14 used the same approach to study the primary pyrolysis of Kraft lignin at a heating rate of 20 K/ms in the temperature range of 723-1173 K in a pyroprobe 1000 pyrolyser. Varhegyi et al.15 (11) Iatridis, B.; Gavalas, G. R. Ind. Eng. Chem. Prod. Res. Dev. 1979, 18 (2), 127-130. (12) Chan, W.-C. R.; Kreiger, B. B. J. Appl. Polym. Sci. 1981, 26, 1533-1553. (13) Nunn, R. T.; Howard, J. B.; Longwell, P. J.; Peters, A. W. Ind. Eng. Chem. Proc. Des. Dev. 1985, 24, 844-851. (14) Caballero, A. J.; Font, R.; Marcilla, A. J. Anal. Appl. Pyrolysis 1996, 36, 159-178. (15) Va´rhegyi, G.; Antal, J. M.; Jakab, E.; Szabo, P. J. Anal. Appl. Pyrolysis 1997, 42, 73-87. (16) Caballero, A. J.; Font, R.; Marcilla, A.; Garcia, N. A. Anal. Appl. Pyrolysis 1993, 27, 221-224. (17) Caballero, A. J.; Font, R.; Marcilla, A. J. Anal. Appl. Pyrolysis 1996, 38, 131-152. (18) Caballero, J. A.; Conesa, J. A.; Font, R.; Marcilla, A. J. Anal. Appl. Pyrolysis 1997, 42, 159-175. (19) Tang, K. W. U.S. Forest Service Papers. No. FPL1 1967 71. (20) Sada, E.; Kamazawa, H.; Kudsy, M. Ind. Eng. Chem. Res. 1992, 31, 612-616.

Experimental Section

(21) Miura, K. Energy Fuels 1995, 9, 302-307. (22) Pitt, G. J. Fuel 1962, 41, 267-274. (23) Reynolds, J. G.; Burnham, A. K. Energy Fuels 1993, 7, 610619. (24) Solomon, P. R.; Hamblen, D. G.; Carangelo, R. M.; Serio, M. A.; Deshpande, G. V. Energy Fuels 1988, 2, 405-422. (25) Grant, D. M.; Pugmire, R. J.; Fletcher, T. H.; Kerstein, A. R. Energy Fuels 1989, 3, 175-186. (26) Niksa, S.; Kerstein, A. R. Energy Fuels 1991, 5, 647-665.

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Table 1. Elemental and Proximate Analyses of Lignins Used ultimate analysis C H N O H/C proximate analysis moisture ash volatile matter a

Alcell, wt %

Kraft, wt %

66.41 5.98 0.00 27.41 0.09

59.92 5.90 1.62 32.56a 0.10

2.70 0.15 97.15

4.00 2.25 93.75

sulfur included.

temperature. The product leaving the reactor was condensed in a liquid trap, cooled with the tap water, and separated into liquid (tar) and gaseous fractions. The gaseous product was collected over saturated brine solution. The product gas mixture was analyzed using a chromatograph (Hewlett-Packard 5880A) for hydrocarbons with a FID using a packed SP2100 column (10′ × 1/8′′) and for H2, CO, and CO2 with a TCD using a chromosorb 102 column (6′ × 1/8′′). Temperature programming (40-150 °C) was used in GC for a complete analysis of gaseous hydrocarbons and permanent gas. Thermogravimetric Analyzer. The thermogravimetric instrument consists of a TGA 50 thermobalance and a Mettler TA 4000 thermoprocessor connected with a computer. TGA runs were carried out for both Alcell and Kraft lignins. In this study, about 10 mg of a lignin sample was placed in an alumina pan and heated at 5-15 K/min up to a temperature of 1073 K. The carrier gas (N2) flow rate was maintained at 50 mL/min for each experiment. A separate blank run was conducted using an empty pan under identical conditions and these data were used for baseline correction during the evaluation of the sample TGA profile. To calculate the kinetic parameter for the lignin pyrolysis work, we have taken the time-temperature data for different heating rates while heating-up the sample from 298 K to 1073 K.

Results and Discussion

Figure 1. Experimental setup. used in powder form. The size of the particle was 0.043-0.053 mm. The bed length for the fixed-bed reactor for a sample size of 0.5 g was ∼82.55 mm. The reactor was 550 mm long with 11 mm i.d. and was filled with 0.5 g of lignin, which was mixed with 4 mm quartz chips with a mass ratio of 1:8 in order to avoid the pressure drop in the reactor due to softening, sticking together, and clogging the bed of lignin in the reactor during the pyrolysis process. Separate tests on melting point of each lignin indicated that they all softened at ∼110 °C. Without quartz-chips, the pressure drop in the reactor was high due to the plugging up of the reactor bed by the molten lignin.27 The lignin was held on a plug of quartz wool, which was placed on a supporting mesh at the center of the microreactor. The top of the lignin sample was covered with another quartz wool plug. The carrier gas (helium) was passed from the top while heating the reactor using a temperature controller (Shimaden SR22). The temperature was measured with a thermocouple placed at the center of the lignin bed. Volume of gas produced as a function of temperature and time were recorded and the corresponding gas samples were analyzed in order to quantify and to calculate average molecular weight of the product gas. For studying the effects of heating rate (5 to 15 K/min) on the pyrolysis reaction, experiment was carried out at 1073 K and the carrier gas flow rate of 13.4 mL/min. It took approximately 30-160 min to achieve 1073 K in the fixedbed reactor. The run was continued for another 30 min at this (27) Iqbal, M.; Dalai, A. K.; Thring, R. W.; Bakhshi, N. N. 33rd International Engineering Conference on Energy Conversion, Colorodo Springs, CO, August 2-6, 1998.

It is known that the pyrolysis reaction of lignin is strongly influenced by temperature, heating rate, and the nature of carrier gas. Also, the nature of lignin, its composition, and various functional groups have significant effects on the lignin, conversion, and product yields. As discussed earlier, the processes involving the production of Kraft and Alcell lignins are different. Therefore, the elemental and proximate analyses of these two lignins are quite different as given in Table 1. It is seen that the total carbon in these lignins varies from 60 to 66.4 wt % and sulfur is present only in Kraft lignin. The total volatiles in these materials are quite high (97.15 to 93.75 wt %), whereas the moisture content was in the range of 2.7-4.0 wt %. In this work, the effects of heating rate on the conversion of lignins, the yield of char, tar, and gas, and on the product gas compositions have been studied for the pyrolysis of lignins in a fixed-bed reactor. For the pyrolysis of lignin in the fixed-bed reactor as well as in TGA, the conversion was defined as

conversion ) [amount of lignin (before reaction) amount of solid (called char) (after reaction)]/ [amount of lignin (before reaction)] (2) The conversion is based on the weight of the lignin sample “as received”. The amount of liquid and gaseous volatiles is the same as the quantity in the numerator in eq 2. In the following sections, the data generated in the fixed-bed reactor and TGA are discussed. Analysis of Fixed-bed Reactor Data. The pyrolysis of Alcell and Kraft lignins was carried out earlier28 to produce hydrogen and gas with medium heating value. To understand the lignin pyrolysis reaction, before doing kinetic study, the pyrolysis of Alcell and Kraft lignins were carried out extensively at different carrier gas flow (28) Ferdous, D.; Dalai, A. K.; Bej, K. S.; Thring, W. R.; Bakhshi, N. N. Fuel Process. Technol. 2000, 70, 9-26.

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Table 2. Overall Mass Balance for the Pyrolysis of Kraft Lignin at 1073 K, the Carrier Gas Flow Rate of 33 mL/ min/g of Lignin, and Heating Rate of 15 K/min

rates (13.4-33 mL/min/g of lignin), temperatures (350800 °C), and heating rates (5-15 K/min). Initially each experiment was performed 4 to 5 times in order to get reproducibility of experimental data. Therefore, most of the data in this paper indicate an average value for 4 to 5 experiments under similar experimental conditions. The standard deviation of reproducible results was within 2.5 wt %. We indent to clear that certain results were deemed nonreproducible and therefore excluded in calculating mean and standard deviation. A typical mass balance for a pyrolysis experiment using Kraft lignin at a carrier gas flow rate of 33 mL/min/g of lignin and heating rate of 15 K/min is given in Table 2. It may be noted that the amounts of gas and char were determined experimentally, whereas the tar quantity was obtained from mass balance and was ∼20 wt % of the lignin used in the reactor. This was due to the fact that the tar was distributed in the condenser as well as in the transport line from the reactor to the gas collector via tar condenser. This could be due to production of fine aerosols from tar upon entering the water condenser. It was difficult to account for all the tars produced just by measuring its quantity present in the condenser. The gaseous products mainly consisted of H2, CO, CO2, CH4, and C2+. For the production of H2, both these lignins were used at 1073 K and high heating rate of 15 K/min. The carrier gas flow rate (13.4-33 mL/ min/g of lignin) did not have any significant effect on the lignin conversion which was 64 wt % for Alcell lignin and 55-58 wt % for Kraft lignin at these conditions.29 However, the production of gas increased from 35 to 45 wt % and tar decreased from 27 to 18 wt % with a decrease in carrier gas flow rate from 33 to 13.4 mL/ min/g of lignin. As the carrier gas flow rate is decreased, the tar undergoes secondary reaction, thereby decreasing its yield and increasing gas yield. It was concluded that with a decrease in carrier gas flow rate the volume of product gas decreased from 820 to 736 mL/g for Kraft and from 820 to 762 mL/g for Alcell lignin. At these conditions the production of hydrogen decreased from 43 to 66 mol % for Kraft lignin and form 31 to 46 mol % for Alcell lignin. With a decrease in temperature from 1073 to 623 K both conversion and production of hydrogen increased from 25 to 57 wt % and 0 to 43 mol %, respectively, for Kraft lignin. It was further con(29) Ferdous, D. Masters Thesis, University of Saskatchewan, 2000.

Figure 2. Effect of temperature on V/V* during pyrolysis of Alcell lignin in a fixed-bed reactor for three different heating rates at a carrier gas (helium) flow rate of 13.4 mL/min/g of lignin.

Figure 3. Effect of temperature on V/V* during pyrolysis of Kraft lignin in a fixed-bed reactor for three different heating rates at a carrier gas (helium) flow rate of 13.4 mL/min/g of lignin.

cluded that for the production of H2, both these lignins could be used at 1073 K, at a high carrier gas flow rate, and at a high heating rate. Low carrier gas flow rate is required at 1073 K to produce medium heating value gas. The total synthesis gas production was over 60 mol % from these lignins indicating that it could be an excellent candidate for liquid fuel production through Fischer-Tropsch synthesis.28 In the following investigation the carrier gas flow rate was maintained at 13.4 mL/min/g of lignin. The conversion of lignin at different temperatures (300-1073 K) for three different heating rates (5-15 K/min) is given in Figures 2 and 3. As expected, conversion increased with increasing temperature. From these results it is observed that at lower temperature range the conversion at lower heating rate (5 K/min) is higher than that at higher heating rate (10-15 K/min) but after a certain temperature (850-900 K) the conversion at lower heating rate was leveled off but at other two heating rates the conversion increased with temperature. For example above 1023 K for Alcell and 875 K for Kraft, the conversion is higher at higher heating rate. It may be noted that the char samples, which contained original ash, were not further heated over long

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Table 3. Effect of Heating Rate on the Yield of Char, Tar, Gas, and Product Gas Composition for the Pyrolysis of Alcell and Kraft Lignin at 1073 K and the Carrier Gas (helium) Flow Rate of 13.4 mL/min/g of Lignin Alcell

Kraft

heating rate

heating rate

yield

5 K/min

10 K/min

conversion (wt %) char (wt %) tar (wt %) gas (wt %) gas composition (mol %) H2 CO H2 + CO CO2 CH4 C2+ total gas volume (mL/g of lignin)

56 44.0 14.1 41.9

60 40.0 19.3 40.7

24.9 37.6 62.5 13.9 21.5 2.1 642

29.7 37.3 67.0 12.9 18.6 1.5 694

soak times. These were collected from the reactor at the end of each experiment. After reaction most of the char contained ash and carbonaceous material which were not attempted for further conversion. It was confirmed later from the steam gasification reaction of these lignin that ∼85 wt % Kraft lignin could be converted to gaseous fuel at the steam flow rate of 15 g/h/g of lignin30 indicating that char is capable of forming volatiles further by reacting with steam. As the lignin conversion was high at relatively higher temperature, further experiments were conducted at 1073 K. The effects of heating rate on the conversion of lignins, yield of gas, tar, and char, and the product gas composition at reaction temperature of 1073 K are given in Table 3. The basis for the percentages quoted in Table 3 is the lignin “as received”. The heating rate was varied from 5 to 15 K/min, while carrier gas flow rate was maintained at 13.4 mL/min/g of lignin. From Table 3 it is seen that, higher conversion was obtained at higher heating rate. For example, conversion increased from 56 to 65 wt % and from 52 to 57 wt % with increase in heating rate from 5 to 15 K/min for Alcell and Kraft lignin, respectively. Table 3 shows the final conversion of Alcell and Kraft lignins after holding the sample at 1073 K for half an hour. This table shows that the overall conversion at these conditions increased with increase in heating rate. These could be due to higher conversions beyond 850-900 K at higher heating rtes as shown in Figures 2 and 3. These results are in agreement with those reported by Ekstrom et al.,31 who indicated an increase in overall conversion with increase in heating rate for the pyrolysis of biomass material. Our experimental data on lignin conversion are comparable with the result reported in the literature. Avni et al.32 also reported the maximum conversion of ∼ 62 wt % at 1073 K at the heating rate of 10 K/min during pyrolysis of alkali lignin in the Du Pont Model 951 thermogravimetric analyzer, which is similar to that in the present study (60 wt % for Alcell lignin at the heating rate of 10 K/min at 1073 K). However, the products obtained by Avni et al.32 were somewhat (30) Ferdous, D.; Dalai, A. K.; Bej, K. S.; Thring, W. R. Can. J. Chem. Eng. 2001, 80, 850-1023. (31) Ekstrom, C.; Rensfelt, E. Spec. Workshop Fast Pyrolysis Biomass Proc. 303-326, Sol. Energy Res. Inst., Technol. Rep. SERI/CP622-1096, SERI/CP, 1980, 303-326. (32) Avni, E.; Davoudzadeh, F.; Coughlin, R. W. Fundamental of Thermochemical Biomass Conversion; Elsevier: New York, 1985.

15 K/min

5 K/min

10 K/min

15 K/min

65 35.0 21.1 43.9

52 48.3 2.7 49.0

53 47.5 3.5 49.0

57 43 4.6 52.4

31.3 32.9 64.2 14.3 19.5 2.0 820

30.0 30.0 60.0 14.1 24.0 1.9 670

36.1 26.4 62.5 13.5 22.5 1.5 706

43.0 25.0 68.0 10.0 20.8 1.2 820

different from that obtained in our study. They have reported the production of CH4, CO, CO2, H2, C2H6, C2H4, H2O, CH3OH, and CH3COCH3, whereas we obtained all except H2O, CH3OH, and CH3COCH3. The gas yield also increased with increase in heating rate. This is probably because of higher conversion. Table 3 also shows that the production of tar is increased at the cost of char with increasing heating rate for both cases. However, the effect of heating rate was quite similar in both the cases. The product gas composition and total gas volume of Alcell and Kraft lignins are also given in Table 3. With increase in heating rate from 5 to 15 K/min, production of synthesis gas increased from ∼62 to ∼64 mol % and from 53 to 68 mol % for Alcell and Kraft lignins, respectively. The production of synthesis gas from Kraft lignin was higher than that from Alcell lignin at a higher heating rate because of higher hydrogen production. It is probably because of 10% high H/C ratio in Kraft lignin compared to that of Alcell lignin.29 For Alcell lignin the total gas volume was increased from 642 mL/g of lignin to 820 mL/g of lignin, whereas for Kraft lignin the total gas volume was increased from 670 to 820 mL/g of lignin. So, at a heating rate of 15 K, the gas yield was over 800 mL/g lignin and the total synthesis gas (CO + H2) production was over 60 mol % of total product gas with a molar ratio of 1:1. This gas could be an excellent candidate for Fischer-Tropsch synthesis for the production of liquid fuels due to its high CO content as compared to synthesis gas obtained from steam reforming of methane, naphtha, etc. Thermogravimetric Analysis. Kinetic analysis for the pyrolysis of lignin in a fixed-bed reactor was also performed using 0.5 g of lignin sample.29 The activation energy for Alcell and Kraft lignins was varied from 23 to 79 kJ/mol and 17 to 89 kJ/mol for the temperature range of 440-900 K and 410-940 K. The small activation energy for the pyrolysis reaction in a fixed-bed reactor indicates that there may be significant mass and heat transfers effect during pyrolysis in a fixed-bed reactor. The distributed activation energy model (DAEM) has been used successfully for coal pyrolysis reactions.6,22-26 However, the previous methods used in this model were complex. Miura21 developed a simple method for DAEM to investigate the kinetic parameters for the coal pyrolysis.

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In the present study TGA equipment has been used for determining kinetic parameters using DAEM model. The pyrolysis of lignins in a TGA has also been carried out at three different heating rates in order to calculate the kinetic parameters. In this study, since only 10 mg of sample was used, the mass transfer effects were neglected. This simplified the kinetic analysis significantly. The focus in this study is to estimate the k0 and f(E) for the pyrolysis reaction using the TGA method, where k0 is the frequency factor and E is the activation energy. f(E) is the distribution curve of the activation energy to represent the differences in the activation energies of many first-order irreversible reactions. The DAEM model can be represented as follows when it is applied to represent the change in total volatiles:21

1-

V ) V*

∫0∞ exp(-k0∫0t e -E/RT dt)f(E) dE

(3)

where V ) total volatiles evolved by time t; V* ) effective volatiles content of the pyrolysis material; k0 ) frequency factor; E ) activation energy; f(E) ) distribution function of activation energy; T ) temperature; R ) gas constant. According to this model, the distribution curve f(E) satisfies the following equation:

∫0∞ f(E) dE ) 1

(4)

In eq 4, f(E) represents a distribution curve of the activation energy to represent the differences in the activation energies of several first-order irreversible reactions. The distribution curve f(E) is generally assumed by a Gaussian distribution. According to Gaussian distribution f(E) can be written as

f(E) )

1 -(E-E0)2/2σ2 e σx2π

(5)

where E represents activation energy, E0 is the mean activation energy, and σ is the standard deviation of E. Estimates of f(E) and k0 were made using the following procedure: 1. V/V* vs T relationships at three different heating rates were established, where V is the total volatiles evolved by time t, V* is the total possible volatile materials (gas plus tar) that may be obtained from the lignins. Total volatiles evolved by time was obtained experimentally. V* was obtained by subtracting the ash from total dry mass of lignin samples. 2. The specific reaction rates of volatilization at several but same V/V* values at different temperatures and heating rates were calculated using eq 6:

k ) dV/dt/(V* - V)

(6)

where k ) nominal specific rate, min-1; dV/dt ) change in volatile production with time, g/min. Then Arrhenius plots of ln k vs the inverse of temperature (1/T) at different V/V* values were made. 3. Activation energies from the Arrhenius plots at different levels of V/V* were determined. V/V* was plotted against activation energy, E. 4. V/V* was differentiated with respect to E to obtain f(E).

Figure 4. Effect of temperature on V/V* during pyrolysis of Alcell lignin in a thermogravimetric analyzer for three different heating rates.

5. k0 corresponding to each E value was calculated at different heating rates using eq 7:

k0 )

0.545aEs RT2e -Es/RT

(7)

where, k0 is the frequency factor, a is the heating rate in K/min, Es is the true activation energy, and T is the temperature corresponding to different V/V* and activation energy Es. To apply the DAEM for the analysis of TG data experiments were performed very carefully. TGA experiments were performed at three different heating rates (5-15 K/min) until the temperature reached at its final value of 1073 K for both Alcell and Kraft lignins. The weight loss as a function of temperature and time was obtained for these heating rates, which indicated ∼100 wt % conversion for the temperature range of 800-1073 K for both Alcell and Kraft lignins. So, no time-temperature history was recorded for holding the sample at 1073 K and during cooling. The detailed procedure in obtaining kinetic parameters from TGA data are discussed above. Weight loss data (V) were used directly to calculate the kinetic parameters. V* was calculated as described earlier. V/V* as a function of T, obtained during the pyrolysis in the thermogravimetric analyzer are given in Figures 4 and 5 for both Alcell and Kraft lignins. From these Figures it is seen that the conversion of lignin increased with an increase in temperature and heating rate for Kraft lignin. This trend agrees with the trend reported in the literature.9 But for Alcell lignin the heating rates have negligible effects on conversion at different temperatures. Suuberg et al.33 also reported no change in specific tar or gas yields from coal as heating rate increased from 270 to 10 000 K/s. However, the heating rates have some influence on the volatilization in the temperature range of 600-800 K. For example, at 700 K the fraction of V* that is volatilized is only 40% at a heating rate of 15 K/min compared to that (85%) at a heating rate of 5 K/min. This is due to higher reaction time at low heating rate. (33) Suuberg, M. E.; Peters, A. W.; Howard, B. J. Ind. Eng. Chem. Process Des. Dev. 1978, 17, 37-46.

Pyrolysis of Lignins

Energy & Fuels, Vol. 16, No. 6, 2002 1411

Table 4. Variation of Frequency Factor with Activation Energy for the Pyrolysis of Alcell and Kraft Lignin in TGA Alcell

Kraft

temperature, K

activation energy, kJ/mol

frequency factor, min-1

temperature, K

activation energy, kJ/mol

frequency factor, min-1

545 600 640 710 767 805

129 194 229 252 275 361

6.2 × 1011 2.4 × 1016 1.7 × 1018 9.9 × 1017 1.47 × 1018 9.3 × 1022

507 618 663 704 721 776

80 101 116 124 139 158

3.3 × 107 5.7 × 107 1.98 × 108 2.39 × 108 1.8 × 109 1.84 × 109

Figure 5. Effect of temperature on V/V* during pyrolysis of Kraft lignin in a thermogravimetric analyzer for three different heating rates.

Figure 6. Arrhenius plots of nominal rate constants for the pyrolysis of Alcell and Kraft lignins in a thermogravimetric analyzer at different V/V* values.

This is prominent in case of Kraft lignin, probably due to the presence of a wide variety of non homogeneous materials. Nominal rates (k) were calculated using eq 6. The Arrhenius plot for both Alcell and Kraft lignins are given in Figure 6. From the slope of the linear plots, the activation energy was calculated at different V/V* and are given in Table 4. The figure indicates that the activation energy increased with increase in temperature. This was due to higher conversion i.e., higher value of V/V* at higher temperature (see Figures 4 and 5). The ranges of activation energy were 129-361 kJ/mol and 80-158 kJ/mol for Alcell and Kraft lignin for the temperature range of 530-815 K and 490-810 K, respectively (see Table 4). The frequency factors (k0) were also calculated using eq 7 at different activation energies. The plots of frequency factor as a function of activation energy are given in Figure 7. The figure indicates that the heating

Figure 7. Variation of frequency factor with activation energy at different heating rates during pyrolysis of Alcell and Kraft lignins in a thermogravimetric analyzer.

rates have very little effect on the frequency factor. In this case, k0 is not constant and varies significantly with activation energy. For example, for Alcell lignin, the k0 value increased from the order of 1011 to 1022 min-1 while activation energy increased from 129 to 361 kJ/ mol and for Kraft lignin the k0 value increased from 107 to 109 while activation energy increased from 80 to 158 kJ/mol (see Table 4). Because of higher volatility during pyrolysis of Alcell lignin, at a fixed temperature the activation energy and frequency factor for this lignin are higher than that from Kraft lignin. The activation energy and frequency factor obtained from Kraft lignin are very close to those reported by Koufopanos et al.9 The activation energy and frequency factor obtained for Alcell lignin from TGA data are not comparable with those reported by other authors using TGA.5,15 From Table 4 it is seen that the range of frequency factor for Alcell and Kraft lignins are quite different, which confirms different reaction schemes for Alcell and Kraft lignins during the TGA experiment. The plot of V/V* as a function of activation energy was made.29 f(E) was calculated by differentiating V/V* with E, and was plotted as a function of activation energy for both Alcell and Kraft lignins (see Figure 8). This Figure shows that distribution curve for f(E) spreads over 80-158 kJ/mol for Kraft lignin and 129-361 kJ/mol for Alcell lignin and has a maximum at ∼118-125 kJ/mol and ∼250270 kJ/mol for Kraft and Alcell lignin, respectively. From Figure 8 it is evident that the distribution curves obtained from the experiment for Alcell and Kraft lignin are somewhat different and statistically inferior from the Gaussian distribution. The activation energy values and frequency factor ranges were less for Kraft lignin compared to Alcell lignin, indicating that the former lignin is more reactive at similar temperature conditions, as obtained from fixed-bed experiments.

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Ferdous et al.

be a function of E,21 represented by

k0 ) 2 × 1016e0.05(E-251) s-1

Figure 8. Effect of activation energy on estimated distribution curves f(E) and comparison with statistical Gaussian distribution for the pyrolysis of Alcell and Kraft lignins in a thermogravimetric analyzer.

(8)

V/V* vs T was calculated for three different heating rates by inserting f(E) and k0 in eq 3. The calculated V/V* were quite fitted with the experimental V/V* for the mean activation energy of 254.3 and 123.0 kJ/mol with a standard deviation of 29.0 and 11.0 kJ/mol for Alcell and Kraft lignins, respectively. Figure 9 shows the comparison of calculated and experimental V/V* for the pyrolysis of Alcell lignin in TGA. The relationship between the experimental and calculated values based on the actual f(E) and temperature time history is compared with the 45° line fit. From Figure 9 it is seen that in general calculated V/V* agrees with experimental V/V*. However, there is some deviation between the experimental and the calculated value of V/V* at lower temperature (