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A kinetic study on pyrolysis and combustion characteristics of oil cakes: Effect of cellulose and lignin content
JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY Volume 39, Issue 4, April 2011 Online English edition of the Chinese language journal Cite this article as: J Fuel Chem Technol, 2011, 39(4), 265−270
RESEARCH PAPER
A kinetic study on pyrolysis and combustion characteristics of oil cakes: Effect of cellulose and lignin content Ramakrishna Gottipati*, Susmita Mishra Department of Chemical Engineering, National Institute of Technology, Rourkela, India 769008.
Abstract: Pyrolysis and combustion characteristics of three different oil cakes such as Pongamia (Pongamia Pinnata), Madhuca (Madhuca Indica), and Jatropha (Jatropha Curcas) were investigated in this study. The cellulose and lignin contents of oil cakes play very important role in pyrolysis and combustion processes. A kinetic investigation of three oil cakes was carried out and major part of the samples decomposed between 210°C and 500°C. Pyrolysis and combustion were carried out with the mixtures of cellulose and lignin chemicals in different ratios and compared with the oil cakes. The biomass with higher cellulose content showed faster rate of pyrolysis than the biomass with higher lignin content. However at higher temperatures (>600°C) all the oil cakes exhibited similar conversion at low heating rate in N2 atmosphere. Apparent activation energies increased for Madhuca and Pongamia oil cakes indicating the presence of more cellulose whereas, low activation energy of Jatropha confirms more lignin content. Keywords: pyrolysis; combustion; cellulose; lignin
Biomass is recognized as the third largest primary energy resource in the world. In many developing countries, the fraction of biomass energy consumed ranges from 40% to 50% since the countries have large agriculture and forest area[1]. India, one of the developing countries, has huge potential of the biomass resources, about 321 million tonnes per year in agro-ecological zones[2]. Oil cake, a solid residue that was discarded after extraction of oil seeds, contains lignin and cellulose in varying ratios. Moreover due to increasing demand of biodiesel, load of oil cakes have increased tremendously and about 2 tonnes of oil cake is dumped as a waste for every tonne of biodiesel production. It is necessary to study the pyrolysis and combustion characteristics of oil cakes which can be used as renewable energy resource in developing countries. Pyrolysis studies of waste biomass materials have been widely conducted[3–10]. However, fundamental combustion and pyrolysis characteristics for the oil cakes have not been elucidated precisely yet, based on the main constituents of biomass such as cellulose and lignin. This study focuses on (1) studying the global kinetic parameters of oil cakes during the pyrolysis (2) estimating the effect of cellulose and lignin on pyrolysis and (3) comparing the pyrolysis characteristics of simulated oil cakes with pure cellulose and lignin chemicals.
1
Material and methods
The raw biomass of Pongamia, Madhuca and Jatropha oil cakes are collected from the local oil processing industry, Orissa, India. These oil cakes were crushed and sieved to produce the particles with the size less than 1 mm. Fundamental tests on pyrolysis of oil cakes and simulated biomass were conducted using a thermo-gravimetric (TG) analyzer (SHIMADZU, DTG-60H). The experimental conditions of the TG experiments are shown in Table 1. The samples were heated from room temperature to final temperature at the heating rates of 5°C, 10°C and 20°C/min. Experiments were conducted in different atmospheres such as N2 and Air. Pyrolysis temperature was fixed at 700°C for the preliminary experiments of pure lignin. Thermogravimetric analysis (TGA) results of three oil cakes were expressed as a function of conversion (x), which was defined as:
x = ( w0 − w) ( w0 − w∞ )
(1)
where, w0 is the initial mass of sample; w is the mass of the pyrolyzed sample; w∞ is the final residual mass. 1.1
Proximate analysis of oil cakes
Received: 30-Sep-2010; Revised: 18-Jan-2011 * Corresponding author. Tel.: +91 66 2462255; Fax: +91 661 2462999, E-mail: [email protected] Copyright © 2011, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.
Ramakrishna Gottipati et al. / Journal of Fuel Chemistry and Technology, 2011, 39(4): 265−270 Table 1 Experimental conditions of Thermogravimetric analysis Oil cakes :
Sample
Pongamia, Madhuca
Temperature raising rate
5, 10 and 20°C/min
Gas flow rate
40 mL/min
Pyrolysis Combustion
atmosphere
N2
final holding temperature
700°C
atmosphere
air
final holding temperature
700°C
Table 2 Proximate analysis of Pongamia, Madhuca and
Table 3 Cellulose and lignin contents of Pongamia, Madhuca and
Jatropha oil cakes
a
Jatropha oil cakes
Moisture
Ash
Volatiles
Fixed
/%a
/%
/%
carbon
Pongamia
10.29
10.17
76.45
3.09
Madhuca
10.09
14.63
70.76
4.52
Jatropha
9.22
8.07
74.94
7.84
Sample
ASTM E1756, Standard test method for determination of moisture in biomass
Volatile and ash analyses were carried out using a thermogravimetric analyzer. The sample was heated under an inert atmosphere to 850°C, the weight loss during this step represented volatile component. The gas atmosphere was then switched to air to burn off fixed carbon, while the temperature was reduced to 800°C. Finally, any residue left after the system was cooled to room temperature was considered as ash[11]. The moisture, ash, fixed carbon and volatile contents of the oil cakes studied were illustrated in Table 2. Nitrogen was used as carrier gas at a flow rate of 40 mL/min through out the experiment. 1.2 Determination of cellulose and lignin contents of oil cakes The cellulose and lignin contents of oil cakes were estimated. Cellulose undergoes acetolysis with acetic/nitric reagent to form acetylated cellodextrins which get dissolved and hydrolyzed to form glucose units on treatment with 67% H2SO4. On dehydration with H2SO4, glucose forms 5-hydroxymethyl furfural which on reaction with anthrone gives a green coloured product. The colour intensity measured at 630 nm indicates the cellulose content in the sample studied. The lignin content was determined by finding kappa number of respective oil cakes[12]. Table 3 shows the cellulose and lignin contents of three oil cakes.
2
and Jatropha oil cakes
Chemicals: Cellulose and Lignin
Results and discussion
Cellulose
Lignin
Hemicellulose
/%
/%
+ Othersa
Pongamia
13.24
18.45
68.31
Madhuca
27.99
14.91
57.1
Jatropha
15.31
23.79
60.9
Biomass
a
by difference
2.1
Figure 1 shows the degree of conversion versus temperature for dynamic experiments at heating rate of 5°C/min for Pongamia, Madhuca and Jatropha oil cakes. The minor weight loss of samples at the initial stage was attributed to desorption of moisture as bound water on the surface and the pores of the samples. The sudden increase in the weight loss after 200°C can be attributed to the decomposition of oil cakes that leads to volatile release. The decomposition of Madhuca and Jatropha oil cakes commenced at 233°C, where as Pongamia oil cake decomposed earlier at 213°C. Fig. 2 and Fig. 3 showed the decomposition of oil cakes at 10°C/min and 20°C/min. The major decomposition occurred between 230°C and 575°C. The TG curves of three oil cakes had two weight loss regimes in the region of main decomposition. The first weight loss occurred approximately between 230°C and 350°C for all the samples, and the second occurred between 350°C and 580°C for Pongamia, where as for Madhuca and Jatropha oil cakes it extended up to 650°C suggesting early release of volatile components in Pongamia due to early decomposition. It was further observed that with increasing the heating rate during pyrolysis from 5°C/min to 20°C/min, at higher temperature >500°C difference between pyrolysis characteristics of Madhuca and Jatropha became less till it observed similar trend at 20°C/min heating rate. Conversion for Pongamia oil cake improved from 0.7 to 0.95 at 500°C. 2.2
Proximate analysis of the oil cakes shown in Table 2 and Table 3 represents the cellulose and lignin contents of the oil cakes. Jatropha oil cake with maximum lignin content (23.79%) has high percentage of fixed carbon (7.84%).
Thermogravimetric analysis (TGA)
Pyrolysis behavior of oil cakes, cellulose and lignin
Figure 4 shows profiles of fraction of the mass decrease of combustibles for three types of oil cakes, comparing with the pure cellulose and pure lignin chemicals.
Ramakrishna Gottipati et al. / Journal of Fuel Chemistry and Technology, 2011, 39(4): 265−270
Fig. 1 Pyrolysis of Pongamia, Madhuca and Jatropha oil cakes at
Fig. 2 Pyrolysis of Pongamia, Madhuca and Jatropha oil cakes at
heating rate of 5°C/min
heating rate of 10°C/min
a: Pongamia; b: Madhuca; c: Jatropha
a: Pongamia; b: Madhuca; c: Jatropha
Fig. 3 Pyrolysis of Pongamia, Madhuca and Jatropha oil cakes at
Fig. 4 Residual fraction of combustibles for oil cakes, cellulose and
heating rate of 20°C/min
lignin during pyrolysis
a: Pongamia; b: Madhuca; c: Jatropha
a: Pongamia; b: Madhuca; c: Jatropha; d: lignin; e: cellulose
Cellulose and lignin are generally recognized as main components in biomass[13]. The cellulose and lignin chemicals are tested as references since the biomass mainly consists of these compounds. From the figure, the pyrolysis starts after 0.3 h (200°C) for all the three oil cakes. The combustibles in the biomass react at the two stages during pyrolysis. In the initial stage, instantaneous weight loss can be observed due to cellulose volatilization. In the next stage, the rate of weight loss decreased due to decomposition of manure, mainly lignin. It can be inferred that cellulose chemical decomposes at high decomposition rate within short time, while decomposition rate of the lignin chemical is less and extends to grater time interval than that of cellulose. By comparing these results for actual biomass samples with those for the cellulose and lignin chemicals, it can be concluded that the Jatropha oil cake has more lignin content compared to other two oil cakes. Generally, the lignin is harder to decompose than the cellulose since lignin contains benzene rings[14].
2.3 Combustion behavior of oil cakes, cellulose and lignin The combustion behavior of oil cakes, cellulose and lignin were shown in Fig. 5. Combustion was carried out at the heating rate of 10°C/min. The results showed that the reaction rate for all the samples during combustion became faster than that during pyrolysis. The decomposition time of all the samples shifted towards lesser value due to reactivity of oxygen. Even the reactivity of the lignin was low, during combustion the decomposition occurred at lower temperatures compared to the pyrolysis, because of the oxygen diffused into the pores formed by the volatilization of cellulose at early temperatures. These results suggest that cellulose content in the biomass may enhance the ignition characteristics and decomposition of lignin since the cellulose compounds have the structure of branching chain of polysaccharides and no aromatic compounds, which are easily volatilized at early temperatures[15].
Ramakrishna Gottipati et al. / Journal of Fuel Chemistry and Technology, 2011, 39(4): 265−270
Fig. 5 Residual fraction of combustibles for oil cakes, cellulose and
Fig. 6 Residual fraction of combustibles of cellulose, lignin and
lignin during combustion
their mixtures during pyrolysis
a: Pongamia; b: Madhuca; c: Jatropha;
a: lignin; b: cell20%, lig80%; c: cell40%, lig60%;
d: lignin; e: cellulose
d: cell60%, lig40%; e: cell80%, lig20%; f: cellulose
combustion. 2.5
Fig. 7 Residual fraction of combustibles of cellulose, lignin and their mixtures during combustion a: lignin; b: cell20%, lig80%; c: cell40%, lig60%; d: cell60%, lig40%; e: cell80%, lig20%; f: cellulose
2.4 Pyrolysis and combustion behavior of simulated samples The TG experimental results suggest that the cellulose and lignin content in the biomass affects the reactivity qualitatively. In order to quantitatively elucidate the effect of cellulose and lignin content on the pyrolysis characteristics for the biomass, the simulated biomasses were prepared by mixing of the cellulose and lignin chemicals in various ratios. Figs. 6 and 7 shows mass decrease fraction of combustibles for several simulated biomasses with different cellulose and lignin contents during pyrolysis and combustion at heating rate, 10°C/min. The results of pyrolysis show that the overall reaction rate decreases with an increase of the lignin content. While for combustion, the similar tendency like pyrolysis is obtained especially before 0.6 h, after that the mass suddenly decreased due to the lignin combustion suggesting that the lignin in the biomass controls the reaction rate during
Differential thermogravimetric analysis (DTG)
The differential rate of conversion, dx/dt, was obtained from differential thermogravimetric analysis (DTG). Typical DTG curves of three oil cakes, cellulose and lignin for a heating rate of 10°C/min were shown in Figs. 8(a) and (b). A large fraction of oil cake samples were pyrolyzed between 150°C and 380°C, which was attributed to the decomposition of hemicellulose and cellulose[16,17]. Within this temperature range, the decomposition of hemicellulose and cellulose were conspicuous for Madhuca and Pongamia oil cakes, but it is not in case of Jatropha. There was no well defined peak beyond 400°C and the peaks which were attributed to further devolatilization of residual charcoal occurred at 554°C, 567°C and 572°C for Madhuca, Pongamia and Jatropha oil cakes, respectively. The flat tailing sections of the DTG curves were attributed to lignin, which is known to decompose slowly over a very broad temperature range shown in Fig. 8(b). 2.6 Kinetic parameters for Pongamia, Madhuca and Jatropha oil cakes Kim and Liou et al[18,19] used the differential method to obtain the pyrolysis kinetic parameters from the thermogravimetric data. The rate of conversion, dx/dt, in thermal decomposition was expressed as: dx (2) = kf ( x) dt The reaction rate constant k was expressed by the Arrhenius equation:
⎛− E⎞ k = A exp⎜ ⎟ ⎝ RT ⎠
(3)
Ramakrishna Gottipati et al. / Journal of Fuel Chemistry and Technology, 2011, 39(4): 265−270
Fig. 8 Variation of the instantaneous reaction rate with temperature for pyrolysis of (a) Pongamia, Madhuca, and Jatropha oil cakes, and (b) Cellulose and lignin chemicals at the heating rate of 10°C/min Table 4 Activation energies (kJ/mol) for pyrolysis of Pongamia, Madhuca and Jatropha oil cakes Sample
Conversion 10
30
40
50
60
70
80
90
Pongamia
27.55
21.03
177.59
159.83
122.94
145.30
148.80
88.80
16.64
Madhuca
37.11
103.78
105.70
129.73
172.97
196.82
356.75
285.40
237.83
Jatropha
28.56
34.67
37.87
38.46
40.68
36.73
28.65
13.63
8.32
The temperature independent conversion function, f(x), was expressed as: (4) f ( x ) = (1 − x ) n Substituting Eqs. (3) and (4) in to Eq. (2) and taking a natural logarithm, the above equation yields: E ⎛ dx ⎞ (5) ln ⎜ ⎟ = ln A + n ln(1 − x) − RT ⎝ dt ⎠ From the relationship between ln(dx/dt) and 1/T, apparent energy of activation E, can be determined. The apparent activation energies at each conversion for Pongamia, Madhuca and Jatropha oil cakes are shown in Table 4. From Table V the apparent activation energies increased randomly for Pongamia, Madhuca oil cakes with increasing conversion up to 30% and 70%, respectively. Below 70% of conversion both hemicellulose and cellulose were decomposed and the corresponding activation energies obtained in between 37 kJ/mol and 356 kJ/mol. The reported activation energy values are 145 to 285 kJ/mol for cellulose, 90 kJ/mol to 125 kJ/mol for hemicellulose and 30 kJ/mol to 39 kJ/mol for lignin[20]. In case of Jatropha, the activation energies are very low compared to the other two oil cakes indicating the more lignin content.
3
x /%
20
burned for combustion. Change in cellulose and lignin contents greatly influenced the pyrolysis rate. For the samples with high cellulose content, the pyrolysis rate became faster, whereas biomass with high lignin content has slower pyrolysis rate. Thus, the composition of cellulose and lignin in the biomass was one of the important parameters that affect the pyrolysis and combustion characteristics greatly. Nomenclature A T E wt k w0 n w∞ R x t
pre-exponential factor, s–1 pyrolysis temperature, K activation Energy, kJ/mol weight of sample at time, t, g pyrolysis rate constant, s–1 initial weight of sample, g reaction order final weight of sample, g gas constant = 8.314 J·g·mol–1·K–1 conversion of samples pyrolysis time, s
References
Conclusions [1] Gani A, Naruse I. Effect of cellulose and lignin content on
The role of cellulose and lignin contents in pyrolysis and combustion behaviors of oil cakes (Pongamia, Madhuca and Jatropha) was investigated. The pyrolysis of oil cake samples proceeded mainly with the two stages. The first stage showed rapid mass decrease due to cellulose decomposition. For second stage, lignin decomposed for pyrolysis and its char
pyrolysis and combustion characteristics for several types of biomass. Renewable Energy, 2006, 32(4): 649–661. [2] Sudha P, Ravindranath N H. Land availability and biomass production potential in India. Biomass Bioenergy, 1999, 16(3): 207–221.
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[12] Mission M, Haron R, Kamaroddin M F A, Amin N A S. Pretreatment of empty palm fruit bunch for production of chemicals via catalytic pyrolysis. Bioresour Technol, 2009, 100(11): 2867–2873. [13] Hajaligol M, Waymack B, Kellog D. Low temperature formation of aromatic hydrocarbon from pyrolysis of cellulosic materials. Fuel, 2001, 80(12): 1799–1807. [14] Sharma R K, Wooten J B, Baliga V L, Lin X, Chan W G, Hajaligol M R. Characterization of chars from pyrolysis of lignin. Fuel, 2004, 83(12): 1469–1482. [15] Yang H, Yan R, Chen H, Lee D H, Zheng C. Characteristics of hemicellulose, cellulose and lignin pyrolysis. Fuel, 2007, 86(12/13): 1781–1788. [16] Alvarez V A, Vazquez A. Thermal degradation of cellulose derivatives/starch blends and sisal fibre biocomposites. Polym Degrad Stab, 2004, 84(1): 13–21. [17] Kim S-S, Agblevor F A. Pyrolysis characteristics and kinetics of chicken litter. Waste Manage, 2007, 27(1): 135–140. [18] Kim S-S, Kim S. Pyrolysis characteristics of polystyrene and polypropylene in a stirred batch reactor. Chem Eng J, 2004, 98(1/2): 53–60. [19] Liou T-H, Chang F-W, Lo J-J. Pyrolysis kinetics of acid leached rice husk. Ind Eng Chem Res, 1997, 36(3): 568–573.
[11] Sricharoenchaikul V, Pechyen C, Aht-Ong D, Atong D.
[20] Vamvuka D, Karkas E, Kastanaki E, Grammelis P. Pyrolysis
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pyrolysis of physic nut (Jatropha curcas L.) waste. Energy Fuels,
lignite. Fuel, 2003, 82(15/17): 1949–1960.
2008, 22(1): 31–37.
RESEARCH PAPER
A kinetic study on pyrolysis and combustion characteristics of oil cakes: Effect of cellulose and lignin content Ramakrishna Gottipati*, Susmita Mishra Department of Chemical Engineering, National Institute of Technology, Rourkela, India 769008.
Abstract: Pyrolysis and combustion characteristics of three different oil cakes such as Pongamia (Pongamia Pinnata), Madhuca (Madhuca Indica), and Jatropha (Jatropha Curcas) were investigated in this study. The cellulose and lignin contents of oil cakes play very important role in pyrolysis and combustion processes. A kinetic investigation of three oil cakes was carried out and major part of the samples decomposed between 210°C and 500°C. Pyrolysis and combustion were carried out with the mixtures of cellulose and lignin chemicals in different ratios and compared with the oil cakes. The biomass with higher cellulose content showed faster rate of pyrolysis than the biomass with higher lignin content. However at higher temperatures (>600°C) all the oil cakes exhibited similar conversion at low heating rate in N2 atmosphere. Apparent activation energies increased for Madhuca and Pongamia oil cakes indicating the presence of more cellulose whereas, low activation energy of Jatropha confirms more lignin content. Keywords: pyrolysis; combustion; cellulose; lignin
Biomass is recognized as the third largest primary energy resource in the world. In many developing countries, the fraction of biomass energy consumed ranges from 40% to 50% since the countries have large agriculture and forest area[1]. India, one of the developing countries, has huge potential of the biomass resources, about 321 million tonnes per year in agro-ecological zones[2]. Oil cake, a solid residue that was discarded after extraction of oil seeds, contains lignin and cellulose in varying ratios. Moreover due to increasing demand of biodiesel, load of oil cakes have increased tremendously and about 2 tonnes of oil cake is dumped as a waste for every tonne of biodiesel production. It is necessary to study the pyrolysis and combustion characteristics of oil cakes which can be used as renewable energy resource in developing countries. Pyrolysis studies of waste biomass materials have been widely conducted[3–10]. However, fundamental combustion and pyrolysis characteristics for the oil cakes have not been elucidated precisely yet, based on the main constituents of biomass such as cellulose and lignin. This study focuses on (1) studying the global kinetic parameters of oil cakes during the pyrolysis (2) estimating the effect of cellulose and lignin on pyrolysis and (3) comparing the pyrolysis characteristics of simulated oil cakes with pure cellulose and lignin chemicals.
1
Material and methods
The raw biomass of Pongamia, Madhuca and Jatropha oil cakes are collected from the local oil processing industry, Orissa, India. These oil cakes were crushed and sieved to produce the particles with the size less than 1 mm. Fundamental tests on pyrolysis of oil cakes and simulated biomass were conducted using a thermo-gravimetric (TG) analyzer (SHIMADZU, DTG-60H). The experimental conditions of the TG experiments are shown in Table 1. The samples were heated from room temperature to final temperature at the heating rates of 5°C, 10°C and 20°C/min. Experiments were conducted in different atmospheres such as N2 and Air. Pyrolysis temperature was fixed at 700°C for the preliminary experiments of pure lignin. Thermogravimetric analysis (TGA) results of three oil cakes were expressed as a function of conversion (x), which was defined as:
x = ( w0 − w) ( w0 − w∞ )
(1)
where, w0 is the initial mass of sample; w is the mass of the pyrolyzed sample; w∞ is the final residual mass. 1.1
Proximate analysis of oil cakes
Received: 30-Sep-2010; Revised: 18-Jan-2011 * Corresponding author. Tel.: +91 66 2462255; Fax: +91 661 2462999, E-mail: [email protected] Copyright © 2011, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.
Ramakrishna Gottipati et al. / Journal of Fuel Chemistry and Technology, 2011, 39(4): 265−270 Table 1 Experimental conditions of Thermogravimetric analysis Oil cakes :
Sample
Pongamia, Madhuca
Temperature raising rate
5, 10 and 20°C/min
Gas flow rate
40 mL/min
Pyrolysis Combustion
atmosphere
N2
final holding temperature
700°C
atmosphere
air
final holding temperature
700°C
Table 2 Proximate analysis of Pongamia, Madhuca and
Table 3 Cellulose and lignin contents of Pongamia, Madhuca and
Jatropha oil cakes
a
Jatropha oil cakes
Moisture
Ash
Volatiles
Fixed
/%a
/%
/%
carbon
Pongamia
10.29
10.17
76.45
3.09
Madhuca
10.09
14.63
70.76
4.52
Jatropha
9.22
8.07
74.94
7.84
Sample
ASTM E1756, Standard test method for determination of moisture in biomass
Volatile and ash analyses were carried out using a thermogravimetric analyzer. The sample was heated under an inert atmosphere to 850°C, the weight loss during this step represented volatile component. The gas atmosphere was then switched to air to burn off fixed carbon, while the temperature was reduced to 800°C. Finally, any residue left after the system was cooled to room temperature was considered as ash[11]. The moisture, ash, fixed carbon and volatile contents of the oil cakes studied were illustrated in Table 2. Nitrogen was used as carrier gas at a flow rate of 40 mL/min through out the experiment. 1.2 Determination of cellulose and lignin contents of oil cakes The cellulose and lignin contents of oil cakes were estimated. Cellulose undergoes acetolysis with acetic/nitric reagent to form acetylated cellodextrins which get dissolved and hydrolyzed to form glucose units on treatment with 67% H2SO4. On dehydration with H2SO4, glucose forms 5-hydroxymethyl furfural which on reaction with anthrone gives a green coloured product. The colour intensity measured at 630 nm indicates the cellulose content in the sample studied. The lignin content was determined by finding kappa number of respective oil cakes[12]. Table 3 shows the cellulose and lignin contents of three oil cakes.
2
and Jatropha oil cakes
Chemicals: Cellulose and Lignin
Results and discussion
Cellulose
Lignin
Hemicellulose
/%
/%
+ Othersa
Pongamia
13.24
18.45
68.31
Madhuca
27.99
14.91
57.1
Jatropha
15.31
23.79
60.9
Biomass
a
by difference
2.1
Figure 1 shows the degree of conversion versus temperature for dynamic experiments at heating rate of 5°C/min for Pongamia, Madhuca and Jatropha oil cakes. The minor weight loss of samples at the initial stage was attributed to desorption of moisture as bound water on the surface and the pores of the samples. The sudden increase in the weight loss after 200°C can be attributed to the decomposition of oil cakes that leads to volatile release. The decomposition of Madhuca and Jatropha oil cakes commenced at 233°C, where as Pongamia oil cake decomposed earlier at 213°C. Fig. 2 and Fig. 3 showed the decomposition of oil cakes at 10°C/min and 20°C/min. The major decomposition occurred between 230°C and 575°C. The TG curves of three oil cakes had two weight loss regimes in the region of main decomposition. The first weight loss occurred approximately between 230°C and 350°C for all the samples, and the second occurred between 350°C and 580°C for Pongamia, where as for Madhuca and Jatropha oil cakes it extended up to 650°C suggesting early release of volatile components in Pongamia due to early decomposition. It was further observed that with increasing the heating rate during pyrolysis from 5°C/min to 20°C/min, at higher temperature >500°C difference between pyrolysis characteristics of Madhuca and Jatropha became less till it observed similar trend at 20°C/min heating rate. Conversion for Pongamia oil cake improved from 0.7 to 0.95 at 500°C. 2.2
Proximate analysis of the oil cakes shown in Table 2 and Table 3 represents the cellulose and lignin contents of the oil cakes. Jatropha oil cake with maximum lignin content (23.79%) has high percentage of fixed carbon (7.84%).
Thermogravimetric analysis (TGA)
Pyrolysis behavior of oil cakes, cellulose and lignin
Figure 4 shows profiles of fraction of the mass decrease of combustibles for three types of oil cakes, comparing with the pure cellulose and pure lignin chemicals.
Ramakrishna Gottipati et al. / Journal of Fuel Chemistry and Technology, 2011, 39(4): 265−270
Fig. 1 Pyrolysis of Pongamia, Madhuca and Jatropha oil cakes at
Fig. 2 Pyrolysis of Pongamia, Madhuca and Jatropha oil cakes at
heating rate of 5°C/min
heating rate of 10°C/min
a: Pongamia; b: Madhuca; c: Jatropha
a: Pongamia; b: Madhuca; c: Jatropha
Fig. 3 Pyrolysis of Pongamia, Madhuca and Jatropha oil cakes at
Fig. 4 Residual fraction of combustibles for oil cakes, cellulose and
heating rate of 20°C/min
lignin during pyrolysis
a: Pongamia; b: Madhuca; c: Jatropha
a: Pongamia; b: Madhuca; c: Jatropha; d: lignin; e: cellulose
Cellulose and lignin are generally recognized as main components in biomass[13]. The cellulose and lignin chemicals are tested as references since the biomass mainly consists of these compounds. From the figure, the pyrolysis starts after 0.3 h (200°C) for all the three oil cakes. The combustibles in the biomass react at the two stages during pyrolysis. In the initial stage, instantaneous weight loss can be observed due to cellulose volatilization. In the next stage, the rate of weight loss decreased due to decomposition of manure, mainly lignin. It can be inferred that cellulose chemical decomposes at high decomposition rate within short time, while decomposition rate of the lignin chemical is less and extends to grater time interval than that of cellulose. By comparing these results for actual biomass samples with those for the cellulose and lignin chemicals, it can be concluded that the Jatropha oil cake has more lignin content compared to other two oil cakes. Generally, the lignin is harder to decompose than the cellulose since lignin contains benzene rings[14].
2.3 Combustion behavior of oil cakes, cellulose and lignin The combustion behavior of oil cakes, cellulose and lignin were shown in Fig. 5. Combustion was carried out at the heating rate of 10°C/min. The results showed that the reaction rate for all the samples during combustion became faster than that during pyrolysis. The decomposition time of all the samples shifted towards lesser value due to reactivity of oxygen. Even the reactivity of the lignin was low, during combustion the decomposition occurred at lower temperatures compared to the pyrolysis, because of the oxygen diffused into the pores formed by the volatilization of cellulose at early temperatures. These results suggest that cellulose content in the biomass may enhance the ignition characteristics and decomposition of lignin since the cellulose compounds have the structure of branching chain of polysaccharides and no aromatic compounds, which are easily volatilized at early temperatures[15].
Ramakrishna Gottipati et al. / Journal of Fuel Chemistry and Technology, 2011, 39(4): 265−270
Fig. 5 Residual fraction of combustibles for oil cakes, cellulose and
Fig. 6 Residual fraction of combustibles of cellulose, lignin and
lignin during combustion
their mixtures during pyrolysis
a: Pongamia; b: Madhuca; c: Jatropha;
a: lignin; b: cell20%, lig80%; c: cell40%, lig60%;
d: lignin; e: cellulose
d: cell60%, lig40%; e: cell80%, lig20%; f: cellulose
combustion. 2.5
Fig. 7 Residual fraction of combustibles of cellulose, lignin and their mixtures during combustion a: lignin; b: cell20%, lig80%; c: cell40%, lig60%; d: cell60%, lig40%; e: cell80%, lig20%; f: cellulose
2.4 Pyrolysis and combustion behavior of simulated samples The TG experimental results suggest that the cellulose and lignin content in the biomass affects the reactivity qualitatively. In order to quantitatively elucidate the effect of cellulose and lignin content on the pyrolysis characteristics for the biomass, the simulated biomasses were prepared by mixing of the cellulose and lignin chemicals in various ratios. Figs. 6 and 7 shows mass decrease fraction of combustibles for several simulated biomasses with different cellulose and lignin contents during pyrolysis and combustion at heating rate, 10°C/min. The results of pyrolysis show that the overall reaction rate decreases with an increase of the lignin content. While for combustion, the similar tendency like pyrolysis is obtained especially before 0.6 h, after that the mass suddenly decreased due to the lignin combustion suggesting that the lignin in the biomass controls the reaction rate during
Differential thermogravimetric analysis (DTG)
The differential rate of conversion, dx/dt, was obtained from differential thermogravimetric analysis (DTG). Typical DTG curves of three oil cakes, cellulose and lignin for a heating rate of 10°C/min were shown in Figs. 8(a) and (b). A large fraction of oil cake samples were pyrolyzed between 150°C and 380°C, which was attributed to the decomposition of hemicellulose and cellulose[16,17]. Within this temperature range, the decomposition of hemicellulose and cellulose were conspicuous for Madhuca and Pongamia oil cakes, but it is not in case of Jatropha. There was no well defined peak beyond 400°C and the peaks which were attributed to further devolatilization of residual charcoal occurred at 554°C, 567°C and 572°C for Madhuca, Pongamia and Jatropha oil cakes, respectively. The flat tailing sections of the DTG curves were attributed to lignin, which is known to decompose slowly over a very broad temperature range shown in Fig. 8(b). 2.6 Kinetic parameters for Pongamia, Madhuca and Jatropha oil cakes Kim and Liou et al[18,19] used the differential method to obtain the pyrolysis kinetic parameters from the thermogravimetric data. The rate of conversion, dx/dt, in thermal decomposition was expressed as: dx (2) = kf ( x) dt The reaction rate constant k was expressed by the Arrhenius equation:
⎛− E⎞ k = A exp⎜ ⎟ ⎝ RT ⎠
(3)
Ramakrishna Gottipati et al. / Journal of Fuel Chemistry and Technology, 2011, 39(4): 265−270
Fig. 8 Variation of the instantaneous reaction rate with temperature for pyrolysis of (a) Pongamia, Madhuca, and Jatropha oil cakes, and (b) Cellulose and lignin chemicals at the heating rate of 10°C/min Table 4 Activation energies (kJ/mol) for pyrolysis of Pongamia, Madhuca and Jatropha oil cakes Sample
Conversion 10
30
40
50
60
70
80
90
Pongamia
27.55
21.03
177.59
159.83
122.94
145.30
148.80
88.80
16.64
Madhuca
37.11
103.78
105.70
129.73
172.97
196.82
356.75
285.40
237.83
Jatropha
28.56
34.67
37.87
38.46
40.68
36.73
28.65
13.63
8.32
The temperature independent conversion function, f(x), was expressed as: (4) f ( x ) = (1 − x ) n Substituting Eqs. (3) and (4) in to Eq. (2) and taking a natural logarithm, the above equation yields: E ⎛ dx ⎞ (5) ln ⎜ ⎟ = ln A + n ln(1 − x) − RT ⎝ dt ⎠ From the relationship between ln(dx/dt) and 1/T, apparent energy of activation E, can be determined. The apparent activation energies at each conversion for Pongamia, Madhuca and Jatropha oil cakes are shown in Table 4. From Table V the apparent activation energies increased randomly for Pongamia, Madhuca oil cakes with increasing conversion up to 30% and 70%, respectively. Below 70% of conversion both hemicellulose and cellulose were decomposed and the corresponding activation energies obtained in between 37 kJ/mol and 356 kJ/mol. The reported activation energy values are 145 to 285 kJ/mol for cellulose, 90 kJ/mol to 125 kJ/mol for hemicellulose and 30 kJ/mol to 39 kJ/mol for lignin[20]. In case of Jatropha, the activation energies are very low compared to the other two oil cakes indicating the more lignin content.
3
x /%
20
burned for combustion. Change in cellulose and lignin contents greatly influenced the pyrolysis rate. For the samples with high cellulose content, the pyrolysis rate became faster, whereas biomass with high lignin content has slower pyrolysis rate. Thus, the composition of cellulose and lignin in the biomass was one of the important parameters that affect the pyrolysis and combustion characteristics greatly. Nomenclature A T E wt k w0 n w∞ R x t
pre-exponential factor, s–1 pyrolysis temperature, K activation Energy, kJ/mol weight of sample at time, t, g pyrolysis rate constant, s–1 initial weight of sample, g reaction order final weight of sample, g gas constant = 8.314 J·g·mol–1·K–1 conversion of samples pyrolysis time, s
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