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Isoconversional kinetic study of the thermal decomposition of sugarcane straw for thermal conversion processes
Accepted Manuscript Isoconversional kinetic study of the thermal decomposition of sugarcane straw for thermal conversion processes Yesid Javier Rueda-Ordó ñez, Katia Tannous PII: DOI: Reference:
S0960-8524(15)01024-X http://dx.doi.org/10.1016/j.biortech.2015.07.062 BITE 15295
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
19 June 2015 17 July 2015 18 July 2015
Please cite this article as: Rueda-Ordó ñez, Y.J., Tannous, K., Isoconversional kinetic study of the thermal decomposition of sugarcane straw for thermal conversion processes, Bioresource Technology (2015), doi: http:// dx.doi.org/10.1016/j.biortech.2015.07.062
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Isoconversional kinetic study of the thermal decomposition of sugarcane straw for thermal conversion processes
Yesid Javier Rueda-Ordóñeza e-mail: [email protected]
Katia Tannousa* e-mail: [email protected]
a
School of Chemical Engineering, University of Campinas –500 Albert Einstein
Avenue, Cidade Universitária “Zeferino Vaz”, 13083-852, Campinas, SP, Brazil
*Corresponding author. Tel.: +55 19 3521-3927 Fax: +55 19 3521-3910 *e-mail: [email protected]
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Abstract The aim of this work was investigate the kinetics of the thermal decomposition reaction of sugarcane straw. The thermal decomposition experiments were conducted at four heating rates (1.25, 2.5, 5 and 10 °C/min) in a thermogravimetric analyzer using nitrogen as inert atmosphere. The kinetic analysis was carried out applying the isoconversional method of Friedman, and the activation energies obtained varied from 154.1 kJ/mol to 177.8 kJ/mol. The reaction model was determined through master plots, corresponding to a two-dimensional diffusion. The pre-exponential factor of 1.82*10 9s-1 was determined by linearization of the conversion rate equation as a function of the inverse of absolute temperature, concerning to activation energy of 149.7 kJ/mol, which are in the order of magnitude for biomass thermal decomposition reported in literature. Finally, the theoretical and experimental conversion data showed a very good agreement, indicating that these results could be used for future process modeling involving sugarcane straw.
Keywords Activation energy; biomass; model free; pyrolysis; thermogravimetry.
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1. Introduction Biomass presents interesting characteristics as raw material for several energy generation processes, as combustion, gasification, liquefaction, and pyrolysis. The biomass or its derivative products (e.g., ethanol, methane, bio-oil, and char) when used as fuels release CO2, which was absorbed from the atmosphere during the photosynthesis process, and consequently, does not have an increase in the global emissions (Abbasi and Abbasi, 2010). The vegetal biomass is composed mainly by hemicellulose, cellulose and lignin, as well as low moisture, extractives, and mineral matter contents (White et al., 2011). According to Sluiter et al. (2008), extractives can be inorganic material, non-structural sugars, chlorophyll, waxes and minor components.
Annually, Brazil produces and processes more than 300 million metric tons of sugarcane, which correspond to a quarter of the 1300 million tons planted worldwide. The sugarcane straw is composed by dry leaves, green leaves and tops of the sugarcane plant. Each ton of stalks corresponds to 140 kg of dry residues. Part of the sugarcane is mechanically harvested and the straw is left on the ground, and other great part is burned before the harvesting of sugarcane in order to reduce the cost of this operation. The São Paulo and Minas Gerais states in Brazil, which are the major producers of sugarcane (around two thirds of the production) are committed for the total removal of burning process until 2018 (Leal et al., 2013). This process has been controlled by the Brazilian government by law No. 11241 (September 19th, 2002) to promote the gradual reduction of the sugarcane burning until 2031. Due to the high generation of this agricultural residue in the ethanol production, it turns necessary to find one or more
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energetic applications for the sugarcane straw. The combustion, gasification or pyrolysis processes could be some possibilities.
In the last few years, there has been a growing interest in the sugarcane straw, and some studies have been conducted on pyrolysis application. Moraes et al. (2012) analyzed the chemical composition of volatile compounds produced during the sugarcane straw pyrolysis, identifying more than 120 compounds, mainly oxygenated ones (acids, aldehydes, alcohols, phenols, ethers and ketones). Mesa-Pérez et al. (2013) investigated the pyrolysis in a bubbling fluidized bed reactor, obtaining maximum of bio-oil and biochar yields of 35.5 wt% and 48.2 wt%, respectively, at a temperature of 470°C. An important contribution to the field was done by these authors, exhibiting the importance and viability of the sugarcane straw as an energetic source.
The biomass pyrolysis process is composed by three stages: drying, devolatilization, and carbonization (Basu, 2010). The drying or dehydration stage occurs in the temperature range between room temperature and 150 °C. The devolatilization stage is characterized by the hemicellulose and cellulose contents volatile release, in which the higher mass loss is registered in the temperature interval between 150 °C and 400 °C. The hemicelluloses are mainly composed by xylose (hardwoods), mannose (softwoods) and other polysaccharides such as glucose, galactose, and arabinose, and the devolatilization temperature is between 200 °C and 300 °C (Di Blasi, 2008). The cellulose, according to Vassilev et al. (2012), is composed by a long network of hydrogen bonds, which links long chains one to another, providing thermal stability and resistance. The cellulose devolatilization temperature is between 250 °C and 350 °C (Di
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Blasi, 2008). Above 400 °C, begins the carbonization stage and the char formation, which is the main component of the final solid residue in the pyrolysis process. According to Pasangulapati et al. (2012), the lignin is the higher precursor of char, about 50% yield, whereas the contribution of cellulose and hemicellulose are very low, 1% for cellulose and 7% for hemicellulose.
Table 1 presents the activation energy obtained by different isoconversional methods for biomasses (Friedman, Flynn-Wall-Ozawa, Coats-Redfern modified, and Vyazovkin methods), classified in fiber (Yao et al., 2008; Ounas et al., 2011; Mishra and Bhaskar, 2014), wood (Poletto et al., 2010; Slopiecka et al., 2012; Anca-Couce et al., 2014), and woody shell residues (Ceylan and Topçu, 2014; Baroni et al., 2015; Rueda-Ordóñez et al., 2015). Yao et al. (2008) determined the activation energy for ten different species of fibers, considering a general trend, remains around 168 kJ/mol, with the exception of hemp, jute, and rice straw, which presented values, 180.9 kJ/mol, 183.1 kJ/mol, and 197.6 kJ/mol, respectively. According to the authors, the higher is activation energy, the higher is the biomass stability comparative to the others fibers.
Ounas et al. (2011) studied the thermal decomposition of sugarcane bagasse and olive residue from Morocco. The authors divided the kinetic results into two conversion sections from 0.00 to 0.50 and 0.50 to 0.80 concerning the hemicellulose and cellulose reactions, respectively. The activation energies for hemicellulose were between 168-180 kJ/mol and 153-162 kJ/mol, and for cellulose were between 168-180 kJ/mol and 153162 kJ/mol, for sugarcane bagasse and olive residue, respectively. The woody residues showed in Table 1, pine (Poletto et al., 2010; Anca-Couce et al., 2014), eucalyptus
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(Poletto et al., 2010), and poplar (Slopiecka et al., 2012), presented activation energy around of 155 kJ/mol, lower than the reported for fibers. The woody shell residues such as hazelnut husk (Ceylan and Topçu, 2014), Tucumã endocarp (Baroni et al., 2015), and Brazil nut woody shell (Rueda-Ordóñez et al., 2015) presented the lowest activation energy compared with the fibrous and woody biomasses, around 140 kJ/mol. Thus, summarizing the data presented in Table 1, the activation energies of biomass thermal decomposition in inert atmosphere using isoconversional methods remained between 150 kJ/mol and 200 kJ/mol.
The aim of this work was the determination of the kinetics of the thermal decomposition reaction and the physicochemical characteristics of sugarcane straw in order to investigate the viability of this biomass as a source for thermal conversion processes.
2. Material and methods 2.1. Material used In this work, sugarcane straw (Saccharum officinarum Linnaeus) was used as raw material composed mainly by dry leaves. The biomass was ground (batches of 10 g in 20 s) in a hammer mill (Tigre S.A., CV2, Brazil) coupled to an induction motor of 3800 rpm, CV5 (General Electric, 25.4062.405, Brazil) and separated by sieving using granutest sieves with mesh of 28 and 35 from Tyler series and then obtained the mean diameter between sieves of 510 µm. After that, a rotary cone sample divider (model laborette 27, Fritsch, Germany) was utilized to obtain a representative sample to apply in the thermogravimetric analysis.
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Sample characterization The sample characterization was divided into four analyses: proximate, ultimate, heating value, and chemical composition. These analyses were performed using the following standards: moisture content (ASTM E871–82), Ash content (ASTM E1755– 01), volatile content (ASTM E872–82) and fixed carbon by difference on a dry basis; elemental analysis (CHN-O) using an elemental analyzer (Perkin Elmer, Series II 2400, USA), and the oxygen content was determined by difference on a dry and ash free basis. The higher heating value (HHV) was determined using an oxygen bomb calorimeter (IKA, C200, Germany), applying the standard ASTM D240-09. The lower heating value (LHV) in MJ/kg was determined using Eq. (1), in which C, H, O, S, and W are the percentage of carbon, hydrogen, oxygen, sulphur and moisture in the biomass, respectively. The sulphur content in Eq. (1) was not taken into account because the percentage was lower than 0.1% and the equipment resolution was not sufficient for an accurate measurement.
LHV=4.187[81C+300H–26(O–S)–6(W+9H)]
(1)
The chemical compositions determination of hemicellulose and cellulose were carried out using the ANKOM A200 filter bag technique based on Van Soest’s method. The lignin content in the sample was determined by the Klason’s method based on acid hydrolysis (Sluiter et al., 2012).
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2.2. Experimental set-up The thermal decomposition experiments were performed with a thermogravimetric analyzer Shimadzu Corporation (TGA-50, Japan). This apparatus measures the sample mass with a resolution of 0.001 mg. The samples were installed in an open sample platinum pan (6 mm internal diameter and 2.5 mm depth) with an initial mass of 3.0±0.2 mg. The temperature was controlled from room temperature (25 ± 3 oC) up to 900 ºC at seven heating rates, 1.25 ºC/min, 2.5 ºC/min, 5 ºC/min, 10 ºC/min, 15 °C/min, 20 °C/min, and 40 °C/min. The International Confederation of Thermal Analysis and Calorimetry (ICTAC) since 2011 (Vyazovkin et al., 2011) recommend the use of at least three temperature ramps lower than 20 °C/min to perform kinetic analysis from thermogravimetry. The highest heating rate chose in this work was to evaluate the effect of transport phenomena in the sample that could affect the thermogravimetric curves, and consequently the kinetic parameters.
The acquisition rate of sample mass was 12 sample/min corresponding to a step of 5 s. Inert gas of high purity (N2=99.996%, 4.6 FID, White Martins, Campinas, Brazil) with flow rate of 50 mL/min was used.
2.3. Mathematical approach Firstly, the TG data was converted to normalized mass, W, using the Eq. (2), where m and m0 are the mass at a time t, and the initial mass, respectively.
=
(2)
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Afterward, the DTG curves were determined with the thermogravimetric analyzer software TA-50WS (Shimadzu Corporation) and normalized (dW/dt) using Eq. (3).
=
(3)
For the kinetic analysis, it was selected the main volatile release range from the normalized mass data. Then, using Eq. (4), were converted in conversion, α, in which W0, W, and Wf are the normalized mass at the beginning, at a time t, and at the end of the main volatile release, respectively. The experimental conversion rate (dα/dt)exp is given by Eq. (5).
=
− −
=− −
(4)
(5)
The theoretical conversion rate (dα/dt)theoretical is given by Eq. (7), in which E is the activation energy, A is the pre-exponential factor, R is the universal gas constant (8.314 J/mol K), and T is the absolute temperature.
− =
(6)
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− = ! "#$ = ! "#$
(7)
2.4. Isoconversional method The kinetic analysis of the thermal decomposition reaction was done applying the isoconversional method of Friedman (1964) for four heating rates between 1.25 ºC/min. to 10 ºC/min.
The method developed by Friedman (1964) solves the Eq. (7) using natural logarithm, as presented in Eq. (8). In this method, the conversion was evaluated from 0.05 to 0.95 with a step of 0.05, resulting in 19 conversion levels. For each conversion level evaluated, it was determined a value of (dα/dt)exp obtained by Eq. (5) and its temperature associated. After that, it was plotted for each heating rate the value of ln(dα/dt) as a function of the 1/T, resulting in a straight line for each conversion, then the slopes are multiplied by the universal gas constant to obtain the activation energies.
%
= % + % "# −
(8)
Reaction model determination The reaction model was determined following the recommendations provided by the International Confederation of Thermal Analysis and Calorimetry (ICTAC). According to Vyazovkin et al. (2011) the use of master plots is the correct way to determine the reaction model showed in Table 2. However, since its conception raised from the same definition of the isoconversional methods, the master plots emerged from the definition
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of the integrated solution of the conversion rate equation (Eq. 7). For that, the Eq. (7) is modified in order to obtain the derivative of conversion with respect to the temperature, and therefore, introducing the heating rate effect, β=dT/dt, is obtained the Eq. (9). From the integration of the reaction model, f(α), it was obtained the function g(α) (Table 2). = "# − = ( = ( − " # ' '
(9)
The last term of the right side of Eq. (9) is an integral without analytical solution. In order to overcome this problem, according to Flynn (1997) the integral is replaced by (E/R)[p(x)] with x=E/(RT), as detailed in Eq. (10).
− " # ) = ( '
2 −+,- " # +,-"− # = * −( / 1 3 = " # 0 '
'
(10)
In Eq. (10), the unknown pre-exponential factor is directly affected by the reaction model, g(α), and therefore, it is necessary to eliminate its influence. Thus, it was selected a conversion reference point (α=0.5), and performed the ratio g(α)/g(0.5) as presented in Eq. (11), which represents the theoretical master plots. Also, the experimental master plots could be obtained from the ratio p(x)/p(0.5).
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)"# " # ' " # = = )". 5# " .5 # ' " .5 #
(11)
In order to obtain the reaction model which better describes the decomposition reaction, the theoretical and experimental master plots are plotted as a function of conversion. Then, the model which better represents the experimental data at the different heating rates (1.25 ºC/min, 2.5 ºC/min, 5 ºC/min, and 10 ºC/min) is considered the suitable for the reaction description.
The solution of p(x) function in Eq. (11) was performed applying the 8th degree rational approximation developed by Pérez-Maqueda and Criado (2000), and presented in Eq. (12). The activation energy used to solve the x=E/RT in Eq. (12) was the average of the set of activation energies provided by the isoconversional method of Friedman (1964).
" # = /
+,-"− #
6 + 6 7 + 887 5 + 09:0 9 + ⋯ < 1/ 8
+ 60 6 + 009 7 + 0857 5 + 0760 9 + ⋯
= 0 < ⋯ + 6=7 + 566589 + 89957 + =560 1 ⋯ + 88=0 = + 6:909 0 + 56098 + 9=0
(12)
Pre-exponential factor determination The pre-exponential factor determination was carried out as a forward step, with the knowledge of the reaction model, f(α). Therewith, rearranging Eq. (7) and plotting the values of ln[(dα/dt)/f(α)] as a function of 1/T , it was obtained a straight line, in which the intercept represents lnA, and with the slope is determined the final activation energy.
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This activation energy should not differ more than 10% from the values provided by the Friedman’s isoconversional method (Janković, 2008).
A B >? @ C = >? − " #
(13)
2.5. Evaluation of data modeling The quality of fit (< 5%) was evaluated through the average deviation percentage (AVP) proposed by Órfão et al. (1999), presented in the Eq. (14), where N is the number of TG experimental data, and SS, the least square method (Eq. 15). In Eq. (15) the experimental conversion (αexp) was determined by Eq. (4), and the theoretical conversion (αtheoretical) was performed with the fourth order Runge-Kutta method, solving Eq. (7), implemented in an Excel spreadsheet (software MS Office Excel 2007 version 12.0.6683.5002). The number of TG experimental data considered were 1687, 688, 392, and 168 for the heating rates of 1.25 ºC/min, 2.5 ºC/min, 5 ºC/min, and 10 ºC/min, respectively.
DE = F H
GG = I
M
GG H
J"# , − "# , L
(14)
0
(15)
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3. Results and discussion 3.1. Biomass characterization Table 3 presents the proximate and ultimate analyses of sugarcane straw obtained in this work and compared with the results obtained by Mesa-Pérez et al. (2013), Leal et al. (2013), and Hoi and Martincigh (2013). Considering the composition of the sugarcane straw as dry leaves, green leaves and tops, Mesa-Pérez et al. (2013) did not present clearly the part of plant used, while Leal et al. (2013) and Hoi and Martincigh (2013) analyzed separately these three parts.
In general, the proximate and ultimate analyses, as well as heating value are in the same magnitude order excluding the moisture content. From the proximate analysis, it was observed that the moisture was around 8% lower than Mesa-Perez et al. (2013) and Leal et al. (2013) results. However, the latest authors present a large range of moisture from dry leaves to green leaves with an increasing of 55 wt.%, and from dry leaves to tops around 70 wt.%.
Regarding to the ash content, our result (3.85 wt.%) was similar to the average of 3.56 ±0.81 wt.% obtained from Leal et al. (2013), whilst Mesa-Perez et al. (2013) showed a significantly higher value (16.4 wt.%). Concerning to the volatile contents, our result (86.6 wt.%) was comparative to the average of 81.47 ±2.71 wt.% obtained from Leal et al. (2013), nevertheless Mesa-Perez et al. (2013) showed a higher difference (12.6 wt.%) probably due the high mineral contents in their samples. About fixed carbon, it was observed that the results of Mesa-Perez et al. (2013) and Leal et al. (2013) are closed (13 wt.% and 14.57 wt.%, respectively), however our results presented lower
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data. From the ultimate analysis, it was observed that the values of C, H, N, S, and O are quite similar with the difference lower than 3 wt.% with exception of Hoi and Martincigh (2013) for green leaves. Comparing the higher heating values the variation (max. 6.8%) could be considered negligible with exception of Leal et al. (2013) for tops of plant (13.5%).
Finally, the properties obtained in this work does not differ significantly with the values obtained by other authors who studied the sugarcane straw, indicating that exists a uniformity of the material under study, providing reliability in the raw material.
In Table 4 is presented the chemical composition of sugarcane straw compared with Saad et al. (2008), Luz et al. (2010), and Costa et al. (2013) works, and also are presented the results of researches (Luz et al., 2010; Rocha et al., 2011; Aboyade et al., 2013) about sugarcane bagasse in order to correlate both biomasses.
Our results of sugarcane straw concerning hemicellulose content was slightly higher (6 wt.%) than the average reported in literature (26.9±0.6 wt.%). Nevertheless, the literature results of sugarcane straw and bagasse no quite difference (26.46 wt.%) was observed. The cellulose content in our sample was 39.81 wt% similar obtained by Saad et al. (2008), but 6 wt.% higher than (~33 wt.%) other sugarcane straw from literature. Comparing this component between sugarcane bagasse and straw, the average value from literature of bagasse was 44.5±0.9 wt.%, which was 5 wt.% and 11% higher than our sample and other sugarcane straw, respectively. Concerning to the lignin content in our sample presented 5 wt.% of variation in comparison with literature and between
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biomasses, i.e., for sugarcane straw was from 21.5 wt.% to 26.1 wt.%, and for bagasse from 21.1 wt.% to 23.5 wt.%.
Finally, both biomasses on a dry basis presented close characteristics. This is very important to the reactor designs, where could be used the same technology for sugarcane straw or a mixture of both biomasses.
3.2. Thermal decomposition analysis Figure 1(a) and Figure 1(b) present the normalized mass (W) and the normalized DTG (dW/dt) as a function of temperature, according to Eq. (2) and (3), respectively. The thermal decomposition of the sugarcane straw was divided into three stages such as dehydration, devolatilization and carbonization.
Dehydration The first stage in a thermal conversion process corresponds to the sample dehydration, and takes place in the temperature range between the room temperature and 150 °C. The moisture content in the sugarcane straw shown in Fig. 1a were 9.32%, 8.13%, 7.17%, 7.31%, 8.35%, 7.37% and 9.35% for the heating rates of 1.25°C/min, 2.5 °C/min, 5 °C/min, 10 °C/min, 15 °C/min, 20 °C/min, and 40 °C/min, respectively. In the DTG curve (Fig. 1b) this stage presented a peak in the temperature range of 25 °C – 100 °C for all the heating rates. Above 150 °C up to 200 °C, was not registered significant mass release (<0.5%).
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Volatile release The second stage or intermediate step in a pyrolysis process is the volatile release, which takes place in the temperature range between 200 °C – 450°C. At this stage, the biomass is completely dried, and its main components (hemicellulose, cellulose and lignin) react with the temperature increasing. Different gaseous compounds are released, and a fraction of them are condensable, which are the precursors of the liquid fraction (Di Blasi, 2008). The volatilized mass of sugarcane straw as shown in Fig. 1a was 69.71%, 69.63%, 68.29%, 69.73%, 69.05%, 67.87%, and 69.70% for the heating rates from 1.25 °C/min to 40 °C/min, respectively. Also, in the same figure is observed the overlapping of the curves at 15 °C/min, 20 °C/min and 40 °C/min, indicating that the use of heating rates higher than 10 °C/min involves mass and heat transfer effects which cannot be measurable. In our case, the last three heating rates are not recommended to determine the kinetic parameters.
The normalized DTG curve, shown in Fig. 1b, was characterized by the formation of two well defined peaks at 298.7 °C and 347.3 °C, 311.6 °C and 360.3 °C, 324.7 °C and 370.8 °C, and 340.0 °C and 385.0 °C, for 1.25 °C/min, 2.5 °C/min, 5 °C/min, and 10 °C/min, respectively. However, for the heating rates higher than 10 °C/min the curve pattern (two peaks) suffered modifications, at 15 °C/min and 20 °C/min the curve pattern turns into a shoulder followed by a peak, and at 40°C/min the curve is almost only one peak. This behavior indicates that the reaction time is too short to obtain a correct representation of the complexity of thermal decomposition of sugarcane straw. Also, because in TG analysis is assumed that the inter-particle and intra-particle transport phenomena effects are avoided.
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According to the literature review presented in the Introduction section, the hemicellulose component reacts between 200 °C and 300 °C corresponding to the first peak in the DTG curve (Fig. 1b). The cellulose component reacts between 250 °C and 350 °C corresponding to the main peak, and the lignin is the responsible for the final remaining solid. However, the latest component reacts in a broad temperature range, from 200 °C to 600 °C, which means that when hemicellulose and cellulose react, the lignin also is decomposing, but not governing the overall reaction.
Carbonization The final stage in a pyrolysis process is the carbonization occurring at temperatures above 450 °C. As observed in Fig. 1a, the mass released in this stage was 12.46%, 10.72%, 8.29%, 5.67%, 15.73%, 16.66% and 16.70% for the heating rates from 1.25 °C/min to 40 °C/min, respectively. The final residues, formed by the mineral content and the char fraction, were 8.61%, 11.52%, 16.30%, 17.30%, 7.65%, 8.09%, and 4.31% for the same heating rates, respectively. In Fig. 1b the DTG curves above 450 ºC presented a small shoulder decreasing its amplitude with the temperature increasing, resembling a tail shape. This behavior is due to the lignin thermal decomposition, which is characterized by low reaction rate and broad temperature range. However, the curves at 1.25 ºC/min, 20 ºC/min, and 40 ºC/min presented different curve patterns with higher conversion rates and higher volatile release. The behavior at 20 ºC/min and 40 ºC/min confirms that were affected by transport phenomena.
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In conclusion, it was observed that the higher the heating rate, the higher the final remaining mass. In other words, decreasing the heating rate, the reaction time is increased, allowing the occurrence of more reactions, breaking the polymeric linkages of biomass compounds and increasing the volatile production, and therefore, greater biomass decomposition.
3.3. Isoconversional method results The isoconversional method of Friedman was used to estimate the activation energy. Due to the heat and mass transfer effects, the kinetic analysis were used only the four lower heating rates (1.25 °C/min, 2.5 °C/min, 5 °C/min, and 10 °C/min) in order to obtain reliable kinetic parameters. The biomass main decomposition occurred from the normalized mass, W, of 90% to 25% for all the heating rates, related to the temperature range between 200 °C and 450 °C (Fig. 1a), and thus, it was the range chosen for the kinetic analysis.
Fig. 2 presents the derivative of conversion with respect to temperature (dα/dT) as a function of conversion (α). Two peaks of the DTG curves were observed to the volatile release corresponding to hemicellulose, cellulose, and lignin decomposition reaction, as described in the volatile release section. Besides that, the conversion range from 0.00 to 0.45 was related to the temperature range from 200 °C to 320 °C for all heating rates, concerning mainly to hemicellulose and lignin decomposition reactions. At the last conversion level (α=0.45), the DTG curve was marked by a minimum, becoming clear the division between hemicellulose and cellulose main reactions.
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It is important to remark in this conversion range, that the small effect of heating rate on the variation of the conversion with temperature (dα/dT) leads similar conversion curve patterns for the four heating rates. Also, verified by the similar shape in the normalized mass curve (Fig, 1a) between 85% and 55%.
For the conversion range between 0.45 and 1.00, the main decomposition reaction corresponds to cellulose and lignin. In Fig. 2, the curves of dα/dT as a function of conversion were similar for the heating rates of 1.25 °C/min and 2.5 °C/min, but for 5 °C/min and 10 °C/min the derivative decreased with the increasing of the heating rate. In this case, the normalized mass curves (Fig, 1a) changed their shapes from 50% to 30%.
The activation energies obtained varied from 154.1 kJ/mol to 177.8 kJ/mol, representing a variation of 15%, and average value of 163.4 kJ/mol. This variation is explained by the fact that the Arrhenius rate law was developed for the study of the kinetics of gases which are well described by a one-step global reaction, whereas in the solid-state kinetics the activation energy presents a strong dependency with the conversion evolution (Janković, 2008). In the conversion range from 0.1 to 0.2, the average of activation energy remained in 154.66±0.44 kJ/mol, corresponding to the temperature interval between 200 °C- 300 °C, related to the biomass components depolimerization, which is the first step in the thermal decomposition with low energy requirements.
Above 0.2 up to 0.5 an increasing from 153.9 kJ/mol to 174.0 kJ/mol was observed. This increasing of the activation energy was due to the polymeric differences between
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hemicellulose and cellulose. According to Di Blasi (2008) the hemicellulose is the least thermally stable of the biomass components, and therefore, its activation energy in an inert thermal decomposition process is lower than the cellulose one. The cellulose is formed by links of β (1-4)-glucose (Liu et al., 2009), very stable links, thus requiring more energy for the breaking and decomposition. The maximum activation energy was achieved when the cellulose decomposition was reached, corresponding to 0.5 of conversion and no higher energy was necessary to continue the biomass decomposition. Therefore, after this conversion (0.5 to 0.8), the activation energy decreased from 177. 5 kJ/mol to 160.0 kJ/mol.
Above the conversion of 0.8, the biomass was almost decomposed and a carbonaceous solid was formed constituting the majority of the residue, and the activation energy became stable in 155.8±1.86 kJ/mol, concerning mainly to the lignin thermal decomposition reaction, as commented in the Introduction section.
3.4. Reaction model and pre-exponential factor determination Reaction model determination The reaction model determination is a fundamental step in kinetic studies, since it is a theoretical function that helps to describe and improve the understanding of the reaction. In biomass studies applying isoconversional methods, it was observed that it is commonly assumed a first or nth order reaction model (Damartzis et al., 2011; Slopiecka et al., 2012; Ceylan and Topçu, 2014; Baroni et al., 2015). Nevertheless, according to Vyazovkin et al. (2011) is not recommended the assumption of a reaction model
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without a previous verification through master plots, since other reaction model could be more suitable to describe the reaction.
In Fig. 3 is presented the theoretical and experimental master plots as a function of conversion. The theoretical curves (lines) are for a few integrated reaction models, g(α), (F1, D1, D2, D3 and D4) which could represent the experimental behavior at the different heating rates evaluated. It is observed in Fig. 3 that the experimental master plots (symbols) at 1.25 °C/min (х), 2.5 °C/min (◊),5 °C/min (∆) and 10 °C/min (о) are consistent with the theoretical master plot of the reaction model D2 (Valensi-Carter equation) corresponding to two-dimensional diffusion. According to Khawam and Flanagan (2006), the two-dimensional diffusion is derived of the assumption of a cylindrical shape solid with a reaction occurring radially from the external boundary to the center, increasing the reaction zone with time. In this present study, the twodimensional diffusion (D2) provides an approximation to the real behavior of the biomass decomposition reaction, and taken into account that the sample of sugarcane straw used in this work is a long fiber with cylindrical shape, the mathematical form is correct. From a kinetic point of view, it is widely discussed the approximation of the thermal decomposition reaction as a one-step reaction, since it is governed by multiple complex reactions affected by the interactions between biomass components.
Pre-exponential factor determination The pre-exponential factor was determined as a forward step, with knowledge of a suitable reaction model, f(α), which provided a good representation of the experimental data. The pre-exponential factor was determined by linearization of ln[(dα/dt)/f(α)] as a
22
function of the 1/T, as mentioned in the subsection Pre-exponential factor determination of the Experimental section, and presented in Fig. 4. The linear equation obtained is given by Eq. (16), in which the slope and intercept were 18.007 and 21.322.
ln[(d α/dt)/f(α)] =18.007(1/T) + 21.322
R2 = 0.9658
(16)
The scattering of the experimental data in Fig. 4 was reflected in the determination coefficient, R2, of 0.9658. This slight scatter effect is due to the formation of two peaks (Fig. 2a), associated to multiple and complex reactions in the biomass thermal decomposition, as stated by Vyazovkin et al. (2011).
The intercept represents lnA, corresponding to a pre-exponential factor A=1.82*109 s-1. The activation energy (E= 149.710 kJ/mol) was obtained through the multiplication of slope by universal gas constant (8.314 J/mol K). This value is slightly below (2%) of the lowest activation energy obtained by Friedman’s method (154.1 kJ/mol), but it is in agreement with the range 150-200 kJ/mol previously analyzed for biomass in the Introduction section and also presented in Table 1.
3.5. Evaluation of the kinetic parameters Fig. 5 presents theoretical (αtheoretical) and experimental (αexp) conversion curves obtained at different heating rates. These curves were obtained assuming the thermal decomposition reaction of sugarcane straw as a one-step global reaction (Eq. 7). It is observed that from the conversion of 0.00 to 0.20, the theoretical data is slightly underestimated, and above 0.80 is overestimated. However, the conversion between
23
0.20 and 0.80, the theoretical curve is a good reproduction of the experimental data. The variation in the extremes suggests a reaction model modification.
In Table 5 are presented the kinetic parameters determined for the sugarcane straw. The kinetic parameters obtained, i.e., activation energy, pre-exponential factor, and reaction model, remained constant for all the heating rates analyzed. This fact confirms the independency of the process with the heating rate, i.e., was successfully avoided the inter-particle and intra-particle effect of the transport phenomena.
The activation energy of 149.710 kJ/mol obtained in this research was in the literature range for biomass (Table 1), normally between 140 kJ/mol and 200 kJ/mol. Also, the reaction model determined for the one-step global reaction provides a better description of the experimental behavior than the reaction model normally used in literature, which is a first or nth reaction order. This fact is in agreement with the results and conclusion of Baroni et al. (2015), who studied the use of first order reaction model to describe the biomass thermal decomposition, concluding that using this reaction model the results are overestimated.
4. Conclusions The sugarcane straw in Brazil would be a potential biomass for thermo-chemical conversion processes due to their thermal characteristics and availability. Also, based on the similarities with bagasse it would be possible to mix both biomasses to use industrially with the current technologies. The reaction model, which better described the reaction, was the two-dimensional diffusion, combined with activation energy of
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149.71 kJ/mol and pre-exponential factor of 1.82*109 s-1. Finally, the reaction kinetics was well described by a one-step global reaction (AVP<3%), indicating that these results could be used for future process modeling involving sugarcane straw.
Acknowledgements The authors gratefully acknowledge the financial support from the Coordination for the Improvement of Higher Level Personnel (CAPES), Brazil.
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16. Liu, Q., Zhong, Z., Wang, S., Luo, Z., 2011. Interaction of biomass components during pyrolysis: A TG-FTIR study. J. Anal. Appl. Pyrolysis. 90, 213-218. 17. Luz, S.M., Gonçalves, A.R., Del’Arco Jr, A.P., Leão, A.L., Ferrão, P.M.C., Rocha, G.J.M., 2010. Thermal properties of polypropylene composites reinforced with different vegetable fibres. Adv. Mater. Res. 123-125, 1199-1202. 18. Mesa-Pérez, J., Rocha, J., Barbosa-Cortez, L., Penedo-Medina, M., Luengo, C., Cascarosa, E., 2013. Fast oxidative pyrolysis of sugar cane straw in a fluidized bed reactor. Appl. Therm. Eng. 56, 167-175. 19. Mishra, G., Bhaskar, T., 2014. Non isothermal model free kinetics for pyrolysis of rice straw. Bioresour. Tech. 169, 614-621. 20. Moraes, M., Georges, F., Almeida, S., Damasceno, F., Maciel, G., Zini, C., Jacques, R., Caramão, E., 2012. Analysis of products from pyrolysis of Brazilian sugar cane straw. Fuel. Process. Technol. 101, 35-43. 21. Órfão, J.J.M., Antunes, F.J.A., Figueiredo, J.L., 1999. Pyrolysis kinetics of lignocellulosic materials – three independent reactions model. Fuel. 78, 349-58. 22. Ounas, A., Aboulkas, A., El harfi, K., Bacaoui, A., Yaacoubi, A., 2011. Pyrolysis of olive residue and sugarcane bagasse: Non-isothermal thermogravimetric analysis. Bioresour. Tech. 102, 11234-11238. 23. Pasangulapati, V., Ramachandriya, K.D., Kumar, A., Wilkins, M.R., Jones, C.L., Huhnke, R.L., 2012. Effects of cellulose, hemicellulose and lignin on thermochemical conversion characteristics of the selected biomass. Bioresour. Tech. 114, 663-669. 24. Pérez-Maqueda, L.A., Criado, J.M., 2000. The accuracy of Senum and Yang’s approximations to the Arrhenius integral. J. Therm. Anal. Calorim. 60, 909-15.
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32. Vassilev, S.V., Baxter, D., Andersen, L.K., Vassileva, C.G., Morgan, T.J., 2012. An overview of the organic and inorganic phase composition of biomass. Fuel. 94, 1-33. 33. Vyazovkin, S., Burnham, A.K., Criado, J.M., Pérez-Maqueda, L.A., Popescu, C., Sbirrazzuoli, N., 2011. ICTAC kinetics committee recommendations for performing kinetic computations on thermal analysis data. Thermochim. Acta. 520, 1-19. 34. White, J.E., Catallo, W.J., Legendre, B.L., 2011. Biomass pyrolysis kinetics: A comparative critical review with relevant agricultural residue case studies. J. Anal. Appl. Pyrolysis. 91, 1-33. 35. Yao, F., Wu, Q., Lei, Y., Guo, W., Xu, Y., 2008. Thermal decomposition kinetics of natural fibers: Activation energy with dynamic thermogravimetric analysis. Polym. Degrad. Stab. 93, 90-98.
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Table 1. Activation energy of several biomasses obtained by isoconversional methods
Table 2. Integral and differential form of several reaction models for solid-state kinetics (adapted from Vyazovkin et al., 2011)
Table 3. Proximate and ultimate analyses and heating values of sugarcane straw
Table 4. Chemical composition of sugarcane straw and bagasse
Table 5. Kinetic parameters of sugarcane straw
Fig. 1 Normalized mass (a) and normalized DTG (b) as a function of temperature. Fig. 2 Derivative of conversion with respect to temperature as a function of conversion.
Fig. 3 Theoretical and experimental master plots for the thermal decomposition of sugarcane straw at the heating rates of (х)1.25 °C/min, (◊)2.5 °C/min, (∆)5 °C/min, (о)10 °C/min as a function of conversion.
Fig. 4 Linearization of the conversion rate equation as a function of the inverse of absolute temperature for our experimental data of the thermal decomposition of sugarcane straw.
Fig. 5 Conversion curves as a function of temperature. Modeled data as solid lines, and experimental data at the heating rates of (■)1.25 °C/min, (□)2.5 °C/min, (●)5 °C/min, (○)10 °C/min.
30
Table 1. Activation energy of several biomasses obtained by isoconversional methods Reference
Yao et al. (2008)
Ounas et al. (2011)
Biomass Bagasse Bamboo Cotton stalk Hemp Jute Kenaf Rice husk Rice straw Wood-maple Wood-pine Sugarcane bagasse Olive residue
Mishra and Bhaskar (2014) Poletto et al. (2010) Slopiecka et al. (2012) Anca-Couce et al. (2014)
Activation energy (kJmol-1) FDa FWOb CRMc VZd 168.5 169.5 168.7 164.1 162.8 161.9 165.3 169.9 169.1 180.9 177.9 177.7 183.1 184.2 184.3 169.8 170.3 169.6 168.2 167.4 166.5 197.6 195.9 196.9 156.0 155.8 154.3 161.5 161.8 160.4 199.5 210.0 178.3 188.5 -
Rice straw
195.0
179.4
178.4
179.6
Pine wood Eucalyptus wood Poplar wood Beech wood Pine wood Hazelnut husk Tucumã endocarp Brazil nut woody shell
160.5
158.0 155.5 158.6 131.1 147.3
157.3 183.7 143.6 127.8 144.6
145.0
144.5
145.7
142.7
-
Ceylan and Topçu (2014) Baroni et al. (2015) Rueda-Ordóñez et al. (2015) a Friedman method b Flynn-Wall-Ozawa method c Coats-Redfern modified method d Vyazovkin method
31
Table 2. Integral and differential form of several reaction models for solid-state kinetics (adapted from Vyazovkin et al., 2011)
Reaction model Power law Power law Power law Power law Phase–boundary Controlled reaction (contracting area) Phase–boundary Controlled reaction (contracting volume) Avrami Erofe’ev (m=2) Avrami Erofe’ev (m=3) Avrami Erofe’ev (m=4) First order nth order One–dimensional Diffusion Two–dimensional Diffusion Valensi-Carter equation Three–dimensional Diffusion Jander equation Three–dimensional Diffusion Ginstling–Brounshtein a Differential form b Integrated form
Code P1 P2 P3 P4
f(α)a 4α3/4 3α2/3 2α1/2 2/3α–1/2
g(α)b α1/4 α1/3 α1/2 α3/2
R2
2(1–α)1/2
[1–(1–α)1/2]
R3
3(1–α)2/3
[1–(1–α)1/3]
A2 A3 A4 F1 Fn
2(1–α)[–ln(1–α)]1/2 3(1–α)[–ln(1–α)]2/3 4(1–α)[–ln(1–α)]3/4 (1–α) (1–α)n
[–ln(1–α)]1/2 [–ln(1–α)]1/3 [–ln(1–α)]1/4 –ln(1–α) [1–(1–α)(1–n)]-(1–n)
D1
1/2α
α2
D2
[–ln(1–α)]–1
(1–α)ln(1–α)+α
D3
(3/2)(1–α)2/3[1–(1–α)1/3]–1
[1–(1–α)1/3]2
D4
(3/2)[(1–α)–1/3–1]–1
(1–2α/3)–(1–α)2/3
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Table 3. Proximate and ultimate analyses and heating values of sugarcane straw This work
Moisture Asha Volatilesa Fixed carbona C H N S O
Mesa-Pérez et al. (2013)
Leal et al. (2013)
DL GL Proximate analysis wt.% 8.42±0.30 10.4 13.5 67.7 3.85±0.21 16.4 2.7 3.7 86.64±0.53 74.0 84.5 80.6 9.51±0.53 13.0 11.6 15.7 Ultimate analysis wt.% a,b 42.94±0.25 43.2 46.2 45.7 6.26±0.16 6.7 6.2 6.2 0.31±0.05 0.3 0.5 1.0 – 0.2 0.1 0.4 46.65±0.18 33.2 43.0 42.8 Heating value MJ/kg 18.61 18.0 17.4 17.4 15.72 17.0 – –
HHV LHV a wt.% on dry basis. b wt% on ash free basis DL–dry leaves GL–green leaves T–tops
T 82.3 4.3 79.3 16.4 43.9 6.1 0.8 0.1 44.0
Hoi and Martincigh (2013) DL GL T – – – –
– – – –
45.2 39.1 5.8 3.0 – – – – 47.4 45.1
16.4 18.3 – –
– –
– – – – 41.5 5.0 – – 46.7 – –
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Table 4. Chemical composition of sugarcane straw and sugarcane bagasse
Reference This work Saad et al. (2008) Luz et al. (2010) Costa et al. (2013)
Luz et al. (2010) Rocha et al. (2011) Aboyade et al. (2013)
Sugarcane straw wt.% Hemicellulose Cellulose 33.28 39.81 26.2 39.4 27.4 33.3 27.1 33.5 Sugarcane bagasse wt.% 28.6 43.8 27.0 45.5 23.8 44.2
Lignin 21.63 21.5 26.1 25.8
Extractives 5.28 – 10.6 –
23.5 21.1 22.4
2.8 4.6 9.7
34
Table 5. Kinetic parameters of sugarcane straw One-step global reaction f(α)
E (kJ/mol)
lnA (ln s-1)
D2=[-ln(1-α)]-1
149.710
21.322
β (°C/min)
1.25
2.5
5
10
AVP (%)
2.3%
2.2%
2.3%
3.1%
35
Fig. 1 Normalized mass (a) and normalized DTG (b) as a function of temperature.
36
Fig. 2 Derivative of conversion with respect to temperature as a function of conversion.
37
Fig. 3 Theoretical and experimental master plots for the thermal decomposition of sugarcane straw at the heating rates of (х)1.25 °C/min, (◊)2.5 °C/min, (∆)5 °C/min, (о)10 °C/min as a function of conversion.
38
Fig. 4 Linearization of the conversion rate equation as a function of the inverse of absolute temperature for our experimental data of the thermal decomposition of sugarcane straw.
39
Fig. 5 Conversion curves as a function of temperature. Modeled data as solid lines, and experimental data at the heating rates of (■)1.25 °C/min, (□)2.5 °C/min, (●)5 °C/min, (○)10 °C/min.
40
Highlights First study about the kinetics of thermal decomposition of sugarcane straw Similar kinetic, thermal and physicochemical characteristics with sugarcane bagasse Two dimensional diffusion reaction model represented the thermal decomposition reaction Mathematical model proposed for conversion rate was in good agreement with experimental data Current industrial technology for sugarcane bagasse is suitable for straw processing
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S0960-8524(15)01024-X http://dx.doi.org/10.1016/j.biortech.2015.07.062 BITE 15295
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
19 June 2015 17 July 2015 18 July 2015
Please cite this article as: Rueda-Ordó ñez, Y.J., Tannous, K., Isoconversional kinetic study of the thermal decomposition of sugarcane straw for thermal conversion processes, Bioresource Technology (2015), doi: http:// dx.doi.org/10.1016/j.biortech.2015.07.062
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Isoconversional kinetic study of the thermal decomposition of sugarcane straw for thermal conversion processes
Yesid Javier Rueda-Ordóñeza e-mail: [email protected]
Katia Tannousa* e-mail: [email protected]
a
School of Chemical Engineering, University of Campinas –500 Albert Einstein
Avenue, Cidade Universitária “Zeferino Vaz”, 13083-852, Campinas, SP, Brazil
*Corresponding author. Tel.: +55 19 3521-3927 Fax: +55 19 3521-3910 *e-mail: [email protected]
1
Abstract The aim of this work was investigate the kinetics of the thermal decomposition reaction of sugarcane straw. The thermal decomposition experiments were conducted at four heating rates (1.25, 2.5, 5 and 10 °C/min) in a thermogravimetric analyzer using nitrogen as inert atmosphere. The kinetic analysis was carried out applying the isoconversional method of Friedman, and the activation energies obtained varied from 154.1 kJ/mol to 177.8 kJ/mol. The reaction model was determined through master plots, corresponding to a two-dimensional diffusion. The pre-exponential factor of 1.82*10 9s-1 was determined by linearization of the conversion rate equation as a function of the inverse of absolute temperature, concerning to activation energy of 149.7 kJ/mol, which are in the order of magnitude for biomass thermal decomposition reported in literature. Finally, the theoretical and experimental conversion data showed a very good agreement, indicating that these results could be used for future process modeling involving sugarcane straw.
Keywords Activation energy; biomass; model free; pyrolysis; thermogravimetry.
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1. Introduction Biomass presents interesting characteristics as raw material for several energy generation processes, as combustion, gasification, liquefaction, and pyrolysis. The biomass or its derivative products (e.g., ethanol, methane, bio-oil, and char) when used as fuels release CO2, which was absorbed from the atmosphere during the photosynthesis process, and consequently, does not have an increase in the global emissions (Abbasi and Abbasi, 2010). The vegetal biomass is composed mainly by hemicellulose, cellulose and lignin, as well as low moisture, extractives, and mineral matter contents (White et al., 2011). According to Sluiter et al. (2008), extractives can be inorganic material, non-structural sugars, chlorophyll, waxes and minor components.
Annually, Brazil produces and processes more than 300 million metric tons of sugarcane, which correspond to a quarter of the 1300 million tons planted worldwide. The sugarcane straw is composed by dry leaves, green leaves and tops of the sugarcane plant. Each ton of stalks corresponds to 140 kg of dry residues. Part of the sugarcane is mechanically harvested and the straw is left on the ground, and other great part is burned before the harvesting of sugarcane in order to reduce the cost of this operation. The São Paulo and Minas Gerais states in Brazil, which are the major producers of sugarcane (around two thirds of the production) are committed for the total removal of burning process until 2018 (Leal et al., 2013). This process has been controlled by the Brazilian government by law No. 11241 (September 19th, 2002) to promote the gradual reduction of the sugarcane burning until 2031. Due to the high generation of this agricultural residue in the ethanol production, it turns necessary to find one or more
3
energetic applications for the sugarcane straw. The combustion, gasification or pyrolysis processes could be some possibilities.
In the last few years, there has been a growing interest in the sugarcane straw, and some studies have been conducted on pyrolysis application. Moraes et al. (2012) analyzed the chemical composition of volatile compounds produced during the sugarcane straw pyrolysis, identifying more than 120 compounds, mainly oxygenated ones (acids, aldehydes, alcohols, phenols, ethers and ketones). Mesa-Pérez et al. (2013) investigated the pyrolysis in a bubbling fluidized bed reactor, obtaining maximum of bio-oil and biochar yields of 35.5 wt% and 48.2 wt%, respectively, at a temperature of 470°C. An important contribution to the field was done by these authors, exhibiting the importance and viability of the sugarcane straw as an energetic source.
The biomass pyrolysis process is composed by three stages: drying, devolatilization, and carbonization (Basu, 2010). The drying or dehydration stage occurs in the temperature range between room temperature and 150 °C. The devolatilization stage is characterized by the hemicellulose and cellulose contents volatile release, in which the higher mass loss is registered in the temperature interval between 150 °C and 400 °C. The hemicelluloses are mainly composed by xylose (hardwoods), mannose (softwoods) and other polysaccharides such as glucose, galactose, and arabinose, and the devolatilization temperature is between 200 °C and 300 °C (Di Blasi, 2008). The cellulose, according to Vassilev et al. (2012), is composed by a long network of hydrogen bonds, which links long chains one to another, providing thermal stability and resistance. The cellulose devolatilization temperature is between 250 °C and 350 °C (Di
4
Blasi, 2008). Above 400 °C, begins the carbonization stage and the char formation, which is the main component of the final solid residue in the pyrolysis process. According to Pasangulapati et al. (2012), the lignin is the higher precursor of char, about 50% yield, whereas the contribution of cellulose and hemicellulose are very low, 1% for cellulose and 7% for hemicellulose.
Table 1 presents the activation energy obtained by different isoconversional methods for biomasses (Friedman, Flynn-Wall-Ozawa, Coats-Redfern modified, and Vyazovkin methods), classified in fiber (Yao et al., 2008; Ounas et al., 2011; Mishra and Bhaskar, 2014), wood (Poletto et al., 2010; Slopiecka et al., 2012; Anca-Couce et al., 2014), and woody shell residues (Ceylan and Topçu, 2014; Baroni et al., 2015; Rueda-Ordóñez et al., 2015). Yao et al. (2008) determined the activation energy for ten different species of fibers, considering a general trend, remains around 168 kJ/mol, with the exception of hemp, jute, and rice straw, which presented values, 180.9 kJ/mol, 183.1 kJ/mol, and 197.6 kJ/mol, respectively. According to the authors, the higher is activation energy, the higher is the biomass stability comparative to the others fibers.
Ounas et al. (2011) studied the thermal decomposition of sugarcane bagasse and olive residue from Morocco. The authors divided the kinetic results into two conversion sections from 0.00 to 0.50 and 0.50 to 0.80 concerning the hemicellulose and cellulose reactions, respectively. The activation energies for hemicellulose were between 168-180 kJ/mol and 153-162 kJ/mol, and for cellulose were between 168-180 kJ/mol and 153162 kJ/mol, for sugarcane bagasse and olive residue, respectively. The woody residues showed in Table 1, pine (Poletto et al., 2010; Anca-Couce et al., 2014), eucalyptus
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(Poletto et al., 2010), and poplar (Slopiecka et al., 2012), presented activation energy around of 155 kJ/mol, lower than the reported for fibers. The woody shell residues such as hazelnut husk (Ceylan and Topçu, 2014), Tucumã endocarp (Baroni et al., 2015), and Brazil nut woody shell (Rueda-Ordóñez et al., 2015) presented the lowest activation energy compared with the fibrous and woody biomasses, around 140 kJ/mol. Thus, summarizing the data presented in Table 1, the activation energies of biomass thermal decomposition in inert atmosphere using isoconversional methods remained between 150 kJ/mol and 200 kJ/mol.
The aim of this work was the determination of the kinetics of the thermal decomposition reaction and the physicochemical characteristics of sugarcane straw in order to investigate the viability of this biomass as a source for thermal conversion processes.
2. Material and methods 2.1. Material used In this work, sugarcane straw (Saccharum officinarum Linnaeus) was used as raw material composed mainly by dry leaves. The biomass was ground (batches of 10 g in 20 s) in a hammer mill (Tigre S.A., CV2, Brazil) coupled to an induction motor of 3800 rpm, CV5 (General Electric, 25.4062.405, Brazil) and separated by sieving using granutest sieves with mesh of 28 and 35 from Tyler series and then obtained the mean diameter between sieves of 510 µm. After that, a rotary cone sample divider (model laborette 27, Fritsch, Germany) was utilized to obtain a representative sample to apply in the thermogravimetric analysis.
6
Sample characterization The sample characterization was divided into four analyses: proximate, ultimate, heating value, and chemical composition. These analyses were performed using the following standards: moisture content (ASTM E871–82), Ash content (ASTM E1755– 01), volatile content (ASTM E872–82) and fixed carbon by difference on a dry basis; elemental analysis (CHN-O) using an elemental analyzer (Perkin Elmer, Series II 2400, USA), and the oxygen content was determined by difference on a dry and ash free basis. The higher heating value (HHV) was determined using an oxygen bomb calorimeter (IKA, C200, Germany), applying the standard ASTM D240-09. The lower heating value (LHV) in MJ/kg was determined using Eq. (1), in which C, H, O, S, and W are the percentage of carbon, hydrogen, oxygen, sulphur and moisture in the biomass, respectively. The sulphur content in Eq. (1) was not taken into account because the percentage was lower than 0.1% and the equipment resolution was not sufficient for an accurate measurement.
LHV=4.187[81C+300H–26(O–S)–6(W+9H)]
(1)
The chemical compositions determination of hemicellulose and cellulose were carried out using the ANKOM A200 filter bag technique based on Van Soest’s method. The lignin content in the sample was determined by the Klason’s method based on acid hydrolysis (Sluiter et al., 2012).
7
2.2. Experimental set-up The thermal decomposition experiments were performed with a thermogravimetric analyzer Shimadzu Corporation (TGA-50, Japan). This apparatus measures the sample mass with a resolution of 0.001 mg. The samples were installed in an open sample platinum pan (6 mm internal diameter and 2.5 mm depth) with an initial mass of 3.0±0.2 mg. The temperature was controlled from room temperature (25 ± 3 oC) up to 900 ºC at seven heating rates, 1.25 ºC/min, 2.5 ºC/min, 5 ºC/min, 10 ºC/min, 15 °C/min, 20 °C/min, and 40 °C/min. The International Confederation of Thermal Analysis and Calorimetry (ICTAC) since 2011 (Vyazovkin et al., 2011) recommend the use of at least three temperature ramps lower than 20 °C/min to perform kinetic analysis from thermogravimetry. The highest heating rate chose in this work was to evaluate the effect of transport phenomena in the sample that could affect the thermogravimetric curves, and consequently the kinetic parameters.
The acquisition rate of sample mass was 12 sample/min corresponding to a step of 5 s. Inert gas of high purity (N2=99.996%, 4.6 FID, White Martins, Campinas, Brazil) with flow rate of 50 mL/min was used.
2.3. Mathematical approach Firstly, the TG data was converted to normalized mass, W, using the Eq. (2), where m and m0 are the mass at a time t, and the initial mass, respectively.
=
(2)
8
Afterward, the DTG curves were determined with the thermogravimetric analyzer software TA-50WS (Shimadzu Corporation) and normalized (dW/dt) using Eq. (3).
=
(3)
For the kinetic analysis, it was selected the main volatile release range from the normalized mass data. Then, using Eq. (4), were converted in conversion, α, in which W0, W, and Wf are the normalized mass at the beginning, at a time t, and at the end of the main volatile release, respectively. The experimental conversion rate (dα/dt)exp is given by Eq. (5).
=
− −
=− −
(4)
(5)
The theoretical conversion rate (dα/dt)theoretical is given by Eq. (7), in which E is the activation energy, A is the pre-exponential factor, R is the universal gas constant (8.314 J/mol K), and T is the absolute temperature.
− =
(6)
9
− = ! "#$ = ! "#$
(7)
2.4. Isoconversional method The kinetic analysis of the thermal decomposition reaction was done applying the isoconversional method of Friedman (1964) for four heating rates between 1.25 ºC/min. to 10 ºC/min.
The method developed by Friedman (1964) solves the Eq. (7) using natural logarithm, as presented in Eq. (8). In this method, the conversion was evaluated from 0.05 to 0.95 with a step of 0.05, resulting in 19 conversion levels. For each conversion level evaluated, it was determined a value of (dα/dt)exp obtained by Eq. (5) and its temperature associated. After that, it was plotted for each heating rate the value of ln(dα/dt) as a function of the 1/T, resulting in a straight line for each conversion, then the slopes are multiplied by the universal gas constant to obtain the activation energies.
%
= % + % "# −
(8)
Reaction model determination The reaction model was determined following the recommendations provided by the International Confederation of Thermal Analysis and Calorimetry (ICTAC). According to Vyazovkin et al. (2011) the use of master plots is the correct way to determine the reaction model showed in Table 2. However, since its conception raised from the same definition of the isoconversional methods, the master plots emerged from the definition
10
of the integrated solution of the conversion rate equation (Eq. 7). For that, the Eq. (7) is modified in order to obtain the derivative of conversion with respect to the temperature, and therefore, introducing the heating rate effect, β=dT/dt, is obtained the Eq. (9). From the integration of the reaction model, f(α), it was obtained the function g(α) (Table 2). = "# − = ( = ( − " # ' '
(9)
The last term of the right side of Eq. (9) is an integral without analytical solution. In order to overcome this problem, according to Flynn (1997) the integral is replaced by (E/R)[p(x)] with x=E/(RT), as detailed in Eq. (10).
− " # ) = ( '
2 −+,- " # +,-"− # = * −( / 1 3 = " # 0 '
'
(10)
In Eq. (10), the unknown pre-exponential factor is directly affected by the reaction model, g(α), and therefore, it is necessary to eliminate its influence. Thus, it was selected a conversion reference point (α=0.5), and performed the ratio g(α)/g(0.5) as presented in Eq. (11), which represents the theoretical master plots. Also, the experimental master plots could be obtained from the ratio p(x)/p(0.5).
11
)"# " # ' " # = = )". 5# " .5 # ' " .5 #
(11)
In order to obtain the reaction model which better describes the decomposition reaction, the theoretical and experimental master plots are plotted as a function of conversion. Then, the model which better represents the experimental data at the different heating rates (1.25 ºC/min, 2.5 ºC/min, 5 ºC/min, and 10 ºC/min) is considered the suitable for the reaction description.
The solution of p(x) function in Eq. (11) was performed applying the 8th degree rational approximation developed by Pérez-Maqueda and Criado (2000), and presented in Eq. (12). The activation energy used to solve the x=E/RT in Eq. (12) was the average of the set of activation energies provided by the isoconversional method of Friedman (1964).
" # = /
+,-"− #
6 + 6 7 + 887 5 + 09:0 9 + ⋯ < 1/ 8
+ 60 6 + 009 7 + 0857 5 + 0760 9 + ⋯
= 0 < ⋯ + 6=7 + 566589 + 89957 + =560 1 ⋯ + 88=0 = + 6:909 0 + 56098 + 9=0
(12)
Pre-exponential factor determination The pre-exponential factor determination was carried out as a forward step, with the knowledge of the reaction model, f(α). Therewith, rearranging Eq. (7) and plotting the values of ln[(dα/dt)/f(α)] as a function of 1/T , it was obtained a straight line, in which the intercept represents lnA, and with the slope is determined the final activation energy.
12
This activation energy should not differ more than 10% from the values provided by the Friedman’s isoconversional method (Janković, 2008).
A B >? @ C = >? − " #
(13)
2.5. Evaluation of data modeling The quality of fit (< 5%) was evaluated through the average deviation percentage (AVP) proposed by Órfão et al. (1999), presented in the Eq. (14), where N is the number of TG experimental data, and SS, the least square method (Eq. 15). In Eq. (15) the experimental conversion (αexp) was determined by Eq. (4), and the theoretical conversion (αtheoretical) was performed with the fourth order Runge-Kutta method, solving Eq. (7), implemented in an Excel spreadsheet (software MS Office Excel 2007 version 12.0.6683.5002). The number of TG experimental data considered were 1687, 688, 392, and 168 for the heating rates of 1.25 ºC/min, 2.5 ºC/min, 5 ºC/min, and 10 ºC/min, respectively.
DE = F H
GG = I
M
GG H
J"# , − "# , L
(14)
0
(15)
13
3. Results and discussion 3.1. Biomass characterization Table 3 presents the proximate and ultimate analyses of sugarcane straw obtained in this work and compared with the results obtained by Mesa-Pérez et al. (2013), Leal et al. (2013), and Hoi and Martincigh (2013). Considering the composition of the sugarcane straw as dry leaves, green leaves and tops, Mesa-Pérez et al. (2013) did not present clearly the part of plant used, while Leal et al. (2013) and Hoi and Martincigh (2013) analyzed separately these three parts.
In general, the proximate and ultimate analyses, as well as heating value are in the same magnitude order excluding the moisture content. From the proximate analysis, it was observed that the moisture was around 8% lower than Mesa-Perez et al. (2013) and Leal et al. (2013) results. However, the latest authors present a large range of moisture from dry leaves to green leaves with an increasing of 55 wt.%, and from dry leaves to tops around 70 wt.%.
Regarding to the ash content, our result (3.85 wt.%) was similar to the average of 3.56 ±0.81 wt.% obtained from Leal et al. (2013), whilst Mesa-Perez et al. (2013) showed a significantly higher value (16.4 wt.%). Concerning to the volatile contents, our result (86.6 wt.%) was comparative to the average of 81.47 ±2.71 wt.% obtained from Leal et al. (2013), nevertheless Mesa-Perez et al. (2013) showed a higher difference (12.6 wt.%) probably due the high mineral contents in their samples. About fixed carbon, it was observed that the results of Mesa-Perez et al. (2013) and Leal et al. (2013) are closed (13 wt.% and 14.57 wt.%, respectively), however our results presented lower
14
data. From the ultimate analysis, it was observed that the values of C, H, N, S, and O are quite similar with the difference lower than 3 wt.% with exception of Hoi and Martincigh (2013) for green leaves. Comparing the higher heating values the variation (max. 6.8%) could be considered negligible with exception of Leal et al. (2013) for tops of plant (13.5%).
Finally, the properties obtained in this work does not differ significantly with the values obtained by other authors who studied the sugarcane straw, indicating that exists a uniformity of the material under study, providing reliability in the raw material.
In Table 4 is presented the chemical composition of sugarcane straw compared with Saad et al. (2008), Luz et al. (2010), and Costa et al. (2013) works, and also are presented the results of researches (Luz et al., 2010; Rocha et al., 2011; Aboyade et al., 2013) about sugarcane bagasse in order to correlate both biomasses.
Our results of sugarcane straw concerning hemicellulose content was slightly higher (6 wt.%) than the average reported in literature (26.9±0.6 wt.%). Nevertheless, the literature results of sugarcane straw and bagasse no quite difference (26.46 wt.%) was observed. The cellulose content in our sample was 39.81 wt% similar obtained by Saad et al. (2008), but 6 wt.% higher than (~33 wt.%) other sugarcane straw from literature. Comparing this component between sugarcane bagasse and straw, the average value from literature of bagasse was 44.5±0.9 wt.%, which was 5 wt.% and 11% higher than our sample and other sugarcane straw, respectively. Concerning to the lignin content in our sample presented 5 wt.% of variation in comparison with literature and between
15
biomasses, i.e., for sugarcane straw was from 21.5 wt.% to 26.1 wt.%, and for bagasse from 21.1 wt.% to 23.5 wt.%.
Finally, both biomasses on a dry basis presented close characteristics. This is very important to the reactor designs, where could be used the same technology for sugarcane straw or a mixture of both biomasses.
3.2. Thermal decomposition analysis Figure 1(a) and Figure 1(b) present the normalized mass (W) and the normalized DTG (dW/dt) as a function of temperature, according to Eq. (2) and (3), respectively. The thermal decomposition of the sugarcane straw was divided into three stages such as dehydration, devolatilization and carbonization.
Dehydration The first stage in a thermal conversion process corresponds to the sample dehydration, and takes place in the temperature range between the room temperature and 150 °C. The moisture content in the sugarcane straw shown in Fig. 1a were 9.32%, 8.13%, 7.17%, 7.31%, 8.35%, 7.37% and 9.35% for the heating rates of 1.25°C/min, 2.5 °C/min, 5 °C/min, 10 °C/min, 15 °C/min, 20 °C/min, and 40 °C/min, respectively. In the DTG curve (Fig. 1b) this stage presented a peak in the temperature range of 25 °C – 100 °C for all the heating rates. Above 150 °C up to 200 °C, was not registered significant mass release (<0.5%).
16
Volatile release The second stage or intermediate step in a pyrolysis process is the volatile release, which takes place in the temperature range between 200 °C – 450°C. At this stage, the biomass is completely dried, and its main components (hemicellulose, cellulose and lignin) react with the temperature increasing. Different gaseous compounds are released, and a fraction of them are condensable, which are the precursors of the liquid fraction (Di Blasi, 2008). The volatilized mass of sugarcane straw as shown in Fig. 1a was 69.71%, 69.63%, 68.29%, 69.73%, 69.05%, 67.87%, and 69.70% for the heating rates from 1.25 °C/min to 40 °C/min, respectively. Also, in the same figure is observed the overlapping of the curves at 15 °C/min, 20 °C/min and 40 °C/min, indicating that the use of heating rates higher than 10 °C/min involves mass and heat transfer effects which cannot be measurable. In our case, the last three heating rates are not recommended to determine the kinetic parameters.
The normalized DTG curve, shown in Fig. 1b, was characterized by the formation of two well defined peaks at 298.7 °C and 347.3 °C, 311.6 °C and 360.3 °C, 324.7 °C and 370.8 °C, and 340.0 °C and 385.0 °C, for 1.25 °C/min, 2.5 °C/min, 5 °C/min, and 10 °C/min, respectively. However, for the heating rates higher than 10 °C/min the curve pattern (two peaks) suffered modifications, at 15 °C/min and 20 °C/min the curve pattern turns into a shoulder followed by a peak, and at 40°C/min the curve is almost only one peak. This behavior indicates that the reaction time is too short to obtain a correct representation of the complexity of thermal decomposition of sugarcane straw. Also, because in TG analysis is assumed that the inter-particle and intra-particle transport phenomena effects are avoided.
17
According to the literature review presented in the Introduction section, the hemicellulose component reacts between 200 °C and 300 °C corresponding to the first peak in the DTG curve (Fig. 1b). The cellulose component reacts between 250 °C and 350 °C corresponding to the main peak, and the lignin is the responsible for the final remaining solid. However, the latest component reacts in a broad temperature range, from 200 °C to 600 °C, which means that when hemicellulose and cellulose react, the lignin also is decomposing, but not governing the overall reaction.
Carbonization The final stage in a pyrolysis process is the carbonization occurring at temperatures above 450 °C. As observed in Fig. 1a, the mass released in this stage was 12.46%, 10.72%, 8.29%, 5.67%, 15.73%, 16.66% and 16.70% for the heating rates from 1.25 °C/min to 40 °C/min, respectively. The final residues, formed by the mineral content and the char fraction, were 8.61%, 11.52%, 16.30%, 17.30%, 7.65%, 8.09%, and 4.31% for the same heating rates, respectively. In Fig. 1b the DTG curves above 450 ºC presented a small shoulder decreasing its amplitude with the temperature increasing, resembling a tail shape. This behavior is due to the lignin thermal decomposition, which is characterized by low reaction rate and broad temperature range. However, the curves at 1.25 ºC/min, 20 ºC/min, and 40 ºC/min presented different curve patterns with higher conversion rates and higher volatile release. The behavior at 20 ºC/min and 40 ºC/min confirms that were affected by transport phenomena.
18
In conclusion, it was observed that the higher the heating rate, the higher the final remaining mass. In other words, decreasing the heating rate, the reaction time is increased, allowing the occurrence of more reactions, breaking the polymeric linkages of biomass compounds and increasing the volatile production, and therefore, greater biomass decomposition.
3.3. Isoconversional method results The isoconversional method of Friedman was used to estimate the activation energy. Due to the heat and mass transfer effects, the kinetic analysis were used only the four lower heating rates (1.25 °C/min, 2.5 °C/min, 5 °C/min, and 10 °C/min) in order to obtain reliable kinetic parameters. The biomass main decomposition occurred from the normalized mass, W, of 90% to 25% for all the heating rates, related to the temperature range between 200 °C and 450 °C (Fig. 1a), and thus, it was the range chosen for the kinetic analysis.
Fig. 2 presents the derivative of conversion with respect to temperature (dα/dT) as a function of conversion (α). Two peaks of the DTG curves were observed to the volatile release corresponding to hemicellulose, cellulose, and lignin decomposition reaction, as described in the volatile release section. Besides that, the conversion range from 0.00 to 0.45 was related to the temperature range from 200 °C to 320 °C for all heating rates, concerning mainly to hemicellulose and lignin decomposition reactions. At the last conversion level (α=0.45), the DTG curve was marked by a minimum, becoming clear the division between hemicellulose and cellulose main reactions.
19
It is important to remark in this conversion range, that the small effect of heating rate on the variation of the conversion with temperature (dα/dT) leads similar conversion curve patterns for the four heating rates. Also, verified by the similar shape in the normalized mass curve (Fig, 1a) between 85% and 55%.
For the conversion range between 0.45 and 1.00, the main decomposition reaction corresponds to cellulose and lignin. In Fig. 2, the curves of dα/dT as a function of conversion were similar for the heating rates of 1.25 °C/min and 2.5 °C/min, but for 5 °C/min and 10 °C/min the derivative decreased with the increasing of the heating rate. In this case, the normalized mass curves (Fig, 1a) changed their shapes from 50% to 30%.
The activation energies obtained varied from 154.1 kJ/mol to 177.8 kJ/mol, representing a variation of 15%, and average value of 163.4 kJ/mol. This variation is explained by the fact that the Arrhenius rate law was developed for the study of the kinetics of gases which are well described by a one-step global reaction, whereas in the solid-state kinetics the activation energy presents a strong dependency with the conversion evolution (Janković, 2008). In the conversion range from 0.1 to 0.2, the average of activation energy remained in 154.66±0.44 kJ/mol, corresponding to the temperature interval between 200 °C- 300 °C, related to the biomass components depolimerization, which is the first step in the thermal decomposition with low energy requirements.
Above 0.2 up to 0.5 an increasing from 153.9 kJ/mol to 174.0 kJ/mol was observed. This increasing of the activation energy was due to the polymeric differences between
20
hemicellulose and cellulose. According to Di Blasi (2008) the hemicellulose is the least thermally stable of the biomass components, and therefore, its activation energy in an inert thermal decomposition process is lower than the cellulose one. The cellulose is formed by links of β (1-4)-glucose (Liu et al., 2009), very stable links, thus requiring more energy for the breaking and decomposition. The maximum activation energy was achieved when the cellulose decomposition was reached, corresponding to 0.5 of conversion and no higher energy was necessary to continue the biomass decomposition. Therefore, after this conversion (0.5 to 0.8), the activation energy decreased from 177. 5 kJ/mol to 160.0 kJ/mol.
Above the conversion of 0.8, the biomass was almost decomposed and a carbonaceous solid was formed constituting the majority of the residue, and the activation energy became stable in 155.8±1.86 kJ/mol, concerning mainly to the lignin thermal decomposition reaction, as commented in the Introduction section.
3.4. Reaction model and pre-exponential factor determination Reaction model determination The reaction model determination is a fundamental step in kinetic studies, since it is a theoretical function that helps to describe and improve the understanding of the reaction. In biomass studies applying isoconversional methods, it was observed that it is commonly assumed a first or nth order reaction model (Damartzis et al., 2011; Slopiecka et al., 2012; Ceylan and Topçu, 2014; Baroni et al., 2015). Nevertheless, according to Vyazovkin et al. (2011) is not recommended the assumption of a reaction model
21
without a previous verification through master plots, since other reaction model could be more suitable to describe the reaction.
In Fig. 3 is presented the theoretical and experimental master plots as a function of conversion. The theoretical curves (lines) are for a few integrated reaction models, g(α), (F1, D1, D2, D3 and D4) which could represent the experimental behavior at the different heating rates evaluated. It is observed in Fig. 3 that the experimental master plots (symbols) at 1.25 °C/min (х), 2.5 °C/min (◊),5 °C/min (∆) and 10 °C/min (о) are consistent with the theoretical master plot of the reaction model D2 (Valensi-Carter equation) corresponding to two-dimensional diffusion. According to Khawam and Flanagan (2006), the two-dimensional diffusion is derived of the assumption of a cylindrical shape solid with a reaction occurring radially from the external boundary to the center, increasing the reaction zone with time. In this present study, the twodimensional diffusion (D2) provides an approximation to the real behavior of the biomass decomposition reaction, and taken into account that the sample of sugarcane straw used in this work is a long fiber with cylindrical shape, the mathematical form is correct. From a kinetic point of view, it is widely discussed the approximation of the thermal decomposition reaction as a one-step reaction, since it is governed by multiple complex reactions affected by the interactions between biomass components.
Pre-exponential factor determination The pre-exponential factor was determined as a forward step, with knowledge of a suitable reaction model, f(α), which provided a good representation of the experimental data. The pre-exponential factor was determined by linearization of ln[(dα/dt)/f(α)] as a
22
function of the 1/T, as mentioned in the subsection Pre-exponential factor determination of the Experimental section, and presented in Fig. 4. The linear equation obtained is given by Eq. (16), in which the slope and intercept were 18.007 and 21.322.
ln[(d α/dt)/f(α)] =18.007(1/T) + 21.322
R2 = 0.9658
(16)
The scattering of the experimental data in Fig. 4 was reflected in the determination coefficient, R2, of 0.9658. This slight scatter effect is due to the formation of two peaks (Fig. 2a), associated to multiple and complex reactions in the biomass thermal decomposition, as stated by Vyazovkin et al. (2011).
The intercept represents lnA, corresponding to a pre-exponential factor A=1.82*109 s-1. The activation energy (E= 149.710 kJ/mol) was obtained through the multiplication of slope by universal gas constant (8.314 J/mol K). This value is slightly below (2%) of the lowest activation energy obtained by Friedman’s method (154.1 kJ/mol), but it is in agreement with the range 150-200 kJ/mol previously analyzed for biomass in the Introduction section and also presented in Table 1.
3.5. Evaluation of the kinetic parameters Fig. 5 presents theoretical (αtheoretical) and experimental (αexp) conversion curves obtained at different heating rates. These curves were obtained assuming the thermal decomposition reaction of sugarcane straw as a one-step global reaction (Eq. 7). It is observed that from the conversion of 0.00 to 0.20, the theoretical data is slightly underestimated, and above 0.80 is overestimated. However, the conversion between
23
0.20 and 0.80, the theoretical curve is a good reproduction of the experimental data. The variation in the extremes suggests a reaction model modification.
In Table 5 are presented the kinetic parameters determined for the sugarcane straw. The kinetic parameters obtained, i.e., activation energy, pre-exponential factor, and reaction model, remained constant for all the heating rates analyzed. This fact confirms the independency of the process with the heating rate, i.e., was successfully avoided the inter-particle and intra-particle effect of the transport phenomena.
The activation energy of 149.710 kJ/mol obtained in this research was in the literature range for biomass (Table 1), normally between 140 kJ/mol and 200 kJ/mol. Also, the reaction model determined for the one-step global reaction provides a better description of the experimental behavior than the reaction model normally used in literature, which is a first or nth reaction order. This fact is in agreement with the results and conclusion of Baroni et al. (2015), who studied the use of first order reaction model to describe the biomass thermal decomposition, concluding that using this reaction model the results are overestimated.
4. Conclusions The sugarcane straw in Brazil would be a potential biomass for thermo-chemical conversion processes due to their thermal characteristics and availability. Also, based on the similarities with bagasse it would be possible to mix both biomasses to use industrially with the current technologies. The reaction model, which better described the reaction, was the two-dimensional diffusion, combined with activation energy of
24
149.71 kJ/mol and pre-exponential factor of 1.82*109 s-1. Finally, the reaction kinetics was well described by a one-step global reaction (AVP<3%), indicating that these results could be used for future process modeling involving sugarcane straw.
Acknowledgements The authors gratefully acknowledge the financial support from the Coordination for the Improvement of Higher Level Personnel (CAPES), Brazil.
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Table 1. Activation energy of several biomasses obtained by isoconversional methods
Table 2. Integral and differential form of several reaction models for solid-state kinetics (adapted from Vyazovkin et al., 2011)
Table 3. Proximate and ultimate analyses and heating values of sugarcane straw
Table 4. Chemical composition of sugarcane straw and bagasse
Table 5. Kinetic parameters of sugarcane straw
Fig. 1 Normalized mass (a) and normalized DTG (b) as a function of temperature. Fig. 2 Derivative of conversion with respect to temperature as a function of conversion.
Fig. 3 Theoretical and experimental master plots for the thermal decomposition of sugarcane straw at the heating rates of (х)1.25 °C/min, (◊)2.5 °C/min, (∆)5 °C/min, (о)10 °C/min as a function of conversion.
Fig. 4 Linearization of the conversion rate equation as a function of the inverse of absolute temperature for our experimental data of the thermal decomposition of sugarcane straw.
Fig. 5 Conversion curves as a function of temperature. Modeled data as solid lines, and experimental data at the heating rates of (■)1.25 °C/min, (□)2.5 °C/min, (●)5 °C/min, (○)10 °C/min.
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Table 1. Activation energy of several biomasses obtained by isoconversional methods Reference
Yao et al. (2008)
Ounas et al. (2011)
Biomass Bagasse Bamboo Cotton stalk Hemp Jute Kenaf Rice husk Rice straw Wood-maple Wood-pine Sugarcane bagasse Olive residue
Mishra and Bhaskar (2014) Poletto et al. (2010) Slopiecka et al. (2012) Anca-Couce et al. (2014)
Activation energy (kJmol-1) FDa FWOb CRMc VZd 168.5 169.5 168.7 164.1 162.8 161.9 165.3 169.9 169.1 180.9 177.9 177.7 183.1 184.2 184.3 169.8 170.3 169.6 168.2 167.4 166.5 197.6 195.9 196.9 156.0 155.8 154.3 161.5 161.8 160.4 199.5 210.0 178.3 188.5 -
Rice straw
195.0
179.4
178.4
179.6
Pine wood Eucalyptus wood Poplar wood Beech wood Pine wood Hazelnut husk Tucumã endocarp Brazil nut woody shell
160.5
158.0 155.5 158.6 131.1 147.3
157.3 183.7 143.6 127.8 144.6
145.0
144.5
145.7
142.7
-
Ceylan and Topçu (2014) Baroni et al. (2015) Rueda-Ordóñez et al. (2015) a Friedman method b Flynn-Wall-Ozawa method c Coats-Redfern modified method d Vyazovkin method
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Table 2. Integral and differential form of several reaction models for solid-state kinetics (adapted from Vyazovkin et al., 2011)
Reaction model Power law Power law Power law Power law Phase–boundary Controlled reaction (contracting area) Phase–boundary Controlled reaction (contracting volume) Avrami Erofe’ev (m=2) Avrami Erofe’ev (m=3) Avrami Erofe’ev (m=4) First order nth order One–dimensional Diffusion Two–dimensional Diffusion Valensi-Carter equation Three–dimensional Diffusion Jander equation Three–dimensional Diffusion Ginstling–Brounshtein a Differential form b Integrated form
Code P1 P2 P3 P4
f(α)a 4α3/4 3α2/3 2α1/2 2/3α–1/2
g(α)b α1/4 α1/3 α1/2 α3/2
R2
2(1–α)1/2
[1–(1–α)1/2]
R3
3(1–α)2/3
[1–(1–α)1/3]
A2 A3 A4 F1 Fn
2(1–α)[–ln(1–α)]1/2 3(1–α)[–ln(1–α)]2/3 4(1–α)[–ln(1–α)]3/4 (1–α) (1–α)n
[–ln(1–α)]1/2 [–ln(1–α)]1/3 [–ln(1–α)]1/4 –ln(1–α) [1–(1–α)(1–n)]-(1–n)
D1
1/2α
α2
D2
[–ln(1–α)]–1
(1–α)ln(1–α)+α
D3
(3/2)(1–α)2/3[1–(1–α)1/3]–1
[1–(1–α)1/3]2
D4
(3/2)[(1–α)–1/3–1]–1
(1–2α/3)–(1–α)2/3
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Table 3. Proximate and ultimate analyses and heating values of sugarcane straw This work
Moisture Asha Volatilesa Fixed carbona C H N S O
Mesa-Pérez et al. (2013)
Leal et al. (2013)
DL GL Proximate analysis wt.% 8.42±0.30 10.4 13.5 67.7 3.85±0.21 16.4 2.7 3.7 86.64±0.53 74.0 84.5 80.6 9.51±0.53 13.0 11.6 15.7 Ultimate analysis wt.% a,b 42.94±0.25 43.2 46.2 45.7 6.26±0.16 6.7 6.2 6.2 0.31±0.05 0.3 0.5 1.0 – 0.2 0.1 0.4 46.65±0.18 33.2 43.0 42.8 Heating value MJ/kg 18.61 18.0 17.4 17.4 15.72 17.0 – –
HHV LHV a wt.% on dry basis. b wt% on ash free basis DL–dry leaves GL–green leaves T–tops
T 82.3 4.3 79.3 16.4 43.9 6.1 0.8 0.1 44.0
Hoi and Martincigh (2013) DL GL T – – – –
– – – –
45.2 39.1 5.8 3.0 – – – – 47.4 45.1
16.4 18.3 – –
– –
– – – – 41.5 5.0 – – 46.7 – –
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Table 4. Chemical composition of sugarcane straw and sugarcane bagasse
Reference This work Saad et al. (2008) Luz et al. (2010) Costa et al. (2013)
Luz et al. (2010) Rocha et al. (2011) Aboyade et al. (2013)
Sugarcane straw wt.% Hemicellulose Cellulose 33.28 39.81 26.2 39.4 27.4 33.3 27.1 33.5 Sugarcane bagasse wt.% 28.6 43.8 27.0 45.5 23.8 44.2
Lignin 21.63 21.5 26.1 25.8
Extractives 5.28 – 10.6 –
23.5 21.1 22.4
2.8 4.6 9.7
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Table 5. Kinetic parameters of sugarcane straw One-step global reaction f(α)
E (kJ/mol)
lnA (ln s-1)
D2=[-ln(1-α)]-1
149.710
21.322
β (°C/min)
1.25
2.5
5
10
AVP (%)
2.3%
2.2%
2.3%
3.1%
35
Fig. 1 Normalized mass (a) and normalized DTG (b) as a function of temperature.
36
Fig. 2 Derivative of conversion with respect to temperature as a function of conversion.
37
Fig. 3 Theoretical and experimental master plots for the thermal decomposition of sugarcane straw at the heating rates of (х)1.25 °C/min, (◊)2.5 °C/min, (∆)5 °C/min, (о)10 °C/min as a function of conversion.
38
Fig. 4 Linearization of the conversion rate equation as a function of the inverse of absolute temperature for our experimental data of the thermal decomposition of sugarcane straw.
39
Fig. 5 Conversion curves as a function of temperature. Modeled data as solid lines, and experimental data at the heating rates of (■)1.25 °C/min, (□)2.5 °C/min, (●)5 °C/min, (○)10 °C/min.
40
Highlights First study about the kinetics of thermal decomposition of sugarcane straw Similar kinetic, thermal and physicochemical characteristics with sugarcane bagasse Two dimensional diffusion reaction model represented the thermal decomposition reaction Mathematical model proposed for conversion rate was in good agreement with experimental data Current industrial technology for sugarcane bagasse is suitable for straw processing
41