Abstract
To help better utilize corn stalk (CS), pyrolysis behavior of materials from different parts of the CS including corn stalk without pith, corn root, and corn leaf were analyzed using thermogravimetric analysis (TGA) at heating rates of 5, 10, 20, and 25 °C/min. The apparent activation energies determined by the Friedman method for corn stalk without pith, corn root, and corn leaf were in the range of 26.4 to 103.6 kJ/mol, 37.6 to 69.5 kJ/mol, and 35.0 to 103.9 kJ/mol, respectively, depending on the conversion. The main thermal decomposition occurred within a temperature range of 200 to 350 °C (±10 °C). Most of the volatile materials decomposed at less than a 0.8 conversion rate. At greater than a 0.8 conversion rate, the remaining material was mainly char, and the decomposition of char proceeded at higher conversion rates. Different pyrolysis characteristics in the CS indicated that different treatments should be chosen according to different parts for achieving the optimum conversion rate in practical applications.
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Thermal Decomposition Properties of Materials from Different Parts of Corn Stalk
Siwei Huang,a Qinglin Wu,b,* Dingguo Zhou,a,* and Runzhou Huang a
To help better utilize corn stalk (CS), pyrolysis behavior of materials from different parts of the CS including corn stalk without pith, corn root, and corn leaf were analyzed using thermogravimetric analysis (TGA) at heating rates of 5, 10, 20, and 25 °C/min. The apparent activation energies determined by the Friedman method for corn stalk without pith, corn root, and corn leaf were in the range of 26.4 to 103.6 kJ/mol, 37.6 to 69.5 kJ/mol, and 35.0 to 103.9 kJ/mol, respectively, depending on the conversion. The main thermal decomposition occurred within a temperature range of 200 to 350 °C (±10 °C). Most of the volatile materials decomposed at less than a 0.8 conversion rate. At greater than a 0.8 conversion rate, the remaining material was mainly char, and the decomposition of char proceeded at higher conversion rates. Different pyrolysis characteristics in the CS indicated that different treatments should be chosen according to different parts for achieving the optimum conversion rate in practical applications.
Keywords: Corn stalk; Different parts; Pyrolysis; Apparent activation energy; TGA
Contact information: a: College of Materials Science and Engineering, Nanjing Forestry University, Long Pan Road, 210037, China; b: School of Renewable Natural Resources, Louisiana State University Agricultural Center, Baton Rouge, LA 70803, USA;
* Corresponding authors: qwusfm@gmail.com and dgzhou@njfu.com.cn
Project Supported: A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions
INTRODUCTION
Biomass as a degradable and renewable energy resource has gained the attention of more and more researchers. Pyrolysis is the first process of the conversion of biomass materials into gas, oil, (Caballero et al. 1997a), chemicals, and char products. The pyrolysis process of biomass materials is complex and affected by many factors such as composition, size, and heating rate (Caballero et al. 1997b, Granada et al. 2013). Therefore, a kinetic study of the process becomes a very important aspect for utilizing biomass materials.
In China, the annual production of corn stalk (CS) reaches 400 million tons. The problem of making full use of CS has attracted much attention in the last decade (Kim and Agblevor 2007; Lv and Wu 2012; Granada et al. 2013; Li et al. 2014). Corn stalk is considered a sustainable resource and a replacement for fossil fuel and chemicals. Pyrolysis technology may potentially be used to convert biomass materials to a valuable product, such as pyrolysis oil.
Previous studies indicated that chemical inhomogeneity and structure in different corn stalk fractions were the main reasons for the different processing behaviors (Maiti et al. 2007; Lv et al. 2013). Hemicellulose, cellulose, and lignin are the three main components of biomass and their thermal decompositions occur at 220 to 315 °C, 315 to 400 °C, and 160 to 900 °C, respectively (Miao and Zhang 2012), to vaporize their extractives (Rao and Sharma 1998).
Ash chemistry is also a key factor that influences the use of biomass materials for direct combustion and gasification (Ross et al. 2008). Compositions and structure have been the main factors used in the cited studies to explain pyrolysis behaviors. For example, Wang et al. explored the effects of chemical inhomogeneity on pyrolysis behavior and liquefaction reaction of corn stalk fractions at a single rate of temperature rise (Wang et al. 2014a,b). In studying sample decomposition kinetics, it is necessary to better understand different reaction mechanisms during the decomposition process.
In this study, we investigated pyrolysis characteristics of materials from the different parts of CS at four different heating rates (i.e., 5, 10, 20, and 25 °C/min) in order to compare their thermal stability properties to guide future chemicals and bio-fuel processing through thermal pyrolysis. The Friedman method was used to determine the apparent activation energy of different parts of CS to better understand the reaction mechanisms and pyrolysis characteristics during the thermal decomposition process.
EXPERIMENTAL
Materials
The CS was collected from an agricultural corn field in Si Yang, Jiang Su Province (China). The CS samples were manually separated into corn stalk without pith, corn root, and corn leaf. All materials were washed with tap water to remove impurities and then dried in an oven at 80 °C for 8 h. Each sample was milled and then screened to obtain particles with sizes between 40 and 60 mesh. Finally, the collected materials were dried in an oven at 103 C for 4 h before testing.
Methods
The chemical composition analysis of the CS was carried out according to the standard methods ASTM D1107-96 (1996), ASTM D1110-84 (2013), ASTM D1103-60 (1971), and ASTM D1106-96 (2013). The elemental analysis was performed using a Vario EL III elemental analyzer (CHONS-O, Germany). The elemental and chemical composition of the CS samples is shown in Table 1. As shown, the root had the lowest contents of cellulose, hemicellulose, lignin, and ash among the three materials.
Table 1. Elemental and Chemical Composition Analysis of Samples
The pyrolysis of samples was conducted with a NETZSCH TG 209F3 thermo-gravimetric analyzer (NETZSCH GmbH, Germany) from 35 °C to 700 °C at four different heating rates of 5, 10, 20, and 25 °C/min under a nitrogen (99.999%) flow rate of 30 mL/min. A 3 to 5 mg sample was placed in the Al2O3 ceramic pan of the instrument. Before starting each run, nitrogen was used to purge the furnace for 30 min in order to prevent any unwanted oxidative decomposition.
RESULTS AND DISCUSSION
Thermogravimetric Analysis (TGA)
In this study, the results of thermogravimetric experiments are expressed as a function of conversion 
, which is defined in Eq. 1,
(1)
where Wo, Wt, and Wf are initial, time t (min), and final mass (g) of the sample, respectively.
The degree of conversion results versus temperature for the TGA dynamic experiments at the heating rates of 5, 10, 20, and 25 °C/min of corn stalk with no pith, corn root, and corn leaf are shown in Figs. 1 through 4. The curves of all samples exhibited two weight loss regimes in the region of main decomposition. The early minor weight loss in the samples can be attributed to desorption of moisture as bound water on the surface and the pores of samples (Uzun and Sarioglu 2009). Cellulose, hemicellulose, and lignin are the three main components of biomass materials. Hemicellulose is the easiest to pyrolyze, followed by cellulose and then lignin, the most difficult one (Vafakhah et al. 2014). The first stage weight loss of samples between 230 °C and 310 °C can be attributed to the pyrolysis of hemicellulose (Wang et al. 2008). The decomposition of cellulose between 310 °C and 600 °C caused the appearance of the second stage. The decomposition of lignin was not obviously observed in this study due to its wide decomposition temperature range between 200 °C and 700 °C. This resulted in an overlap of the weight loss curve of lignin with the curves of the other chemical components (Wang et al. 2014a). It was observed that the degradation of CS with no pitch was easier than the other materials at initial reaction stage. This was ascribed to the degradation of higher content of hemicellulose and extractive compounds with poor thermal stability, such as amino acids, fructopyranose, and protein in CS with no pitch (Lv et al. 2013). And the degradation of corn leaf was easier than the other two at the second stage due to the higher cellulose in the leaf.
Fig. 1. The effect of pyrolysis rate of corn stalk without pith, corn root, and corn leaf at the heating rate of 5 °C/min
Fig. 2. The effect of pyrolysis rate of corn stalk without pith, corn root, and corn leaf at the heating rate of 10 °C /min
Fig. 3. The effect of pyrolysis rate of corn stalk without pith, corn root, and corn leaf at the heating rate of 20 °C /min
Fig. 4. The effect of pyrolysis rate of corn stalk without pith, corn root, and corn leaf at the heating rate of 25 °C /min
Differential Thermogravimetric Analysis
The differential rate of conversion, i.e., d /dt, was obtained from differential thermogravimetric analysis (DTG) at the different heating rates of 5, 10, 20, and 25 °C/min (Figs. 5 through 8). A large fraction of samples pyrolyzed between 200 °C and 400 °C, which was attributed to hemicellulose and cellulose decomposition. Table 2 shows the characteristic temperatures of the devolatilization reaction of the samples at different heating rates of 5, 10, 20, and 25 °C/min. The maximum value of corn root, corn stalk without pith, and corn leaf at a heating rate 5°C/min occurred at 338.5, 326.2, and 322.0 °C, respectively. When the heating rate reached 20 °C/min, the maximum value of corn root, corn stalk without pith, and corn leaf occurred at 350, 345, and 330 °C, respectively. The maximum rate of conversion at 5 °C/min was lower than that at 10, 20, and 25 °C/min as shown in Figs. 5 through 8. Corn stalk with no pith had small left shoulders between 120 and 230 °C compared other materials. This was due to the degradation of extractive compounds with poor thermal stability, such as protein, fructopyranose, and amino acids. The appearance of shoulders in the temperature range 230 °C and 300 °C was attributed to the decomposition of hemicellulose. Comparison of the four different heating rates, listed in Table 2, showed that the higher heating rate, the bigger maximum value.
Fig. 5. DTG curves of corn stalk without pith, corn root, and corn leaf at the heating rate of 5 °C/min
Fig. 6. DTG curves of corn stalk without pith, corn root, and corn leaf at the heating rate of 10 °C/min
Fig. 7. DTG curves of corn stalk without pith, corn root, and corn leaf at the heating rate of 20 °C/min
Fig. 8. DTG curves of corn stalk without pith, corn root, and corn leaf at the heating rate of 25 °C/min
Table 2. Characteristic Temperatures of Devolatilization Reaction of the Samples
o=onset; p=DTG peak; s=shift
Heating rate tends to affect the peak temperature and the maximum rate of conversion because the amount of thermal energy influences the heat transfer between the surroundings and the interior of the samples (Yang et al. 2007; Wang et al. 2014b). But the heating rate had no obvious effect on the residue amount.
Kinetic Parameters of Samples
The kinetic analysis was widely used to assess the thermal stability of biomass materials in the last decades. Many different “model-free” methods have been used to determine the pyrolysis kinetic parameters from the thermogravimetric data (Yao et al. 2008; Zhu et al. 2012). The Friedman method (Friedman 1964) is one of the most commonly used methods.
The rate of conversion, 
, in thermal decomposition is expressed as:
(2)
The rate constant 
is generally expressed by the Arrhenius equation (Mishra and Bhaskar 2014),
(3)
where E is the apparent activation energy (kJ/mol), R is the gas constant (8.314 J/K mol), A is the pre-exponential factor (min-1), and T is the absolute temperature (K).
The temperature independent conversion function, 
was expressed as:
(4)
Substituting Eqs. 3 and 4 into Eq. 2 and taking a natural logarithm, the equation 2 yields:
(5)
The apparent energy of activation E , based on Eq. 5, can be determined from the relationship between 
and 1/T. Thus, a family of parallel straight lines of slope – E/R can be obtained, from which the apparent energy of activation E corresponding to the selected conversion can be obtained. The relationship between ln(d /dt ) and 1/T at various conversions from 0.1 to 0.8 is shown in Figs. 9 through 11. For instance, the 0.1 conversion of corn leaf was obtained at the temperatures of 232.56, 243.90, 250.00, and 256.41 °C. At these temperatures, the values of were -6.026, -5.384, -4.742, and -4.541, respectively. From the slope of vs. 1/T at the conversion of 0.1, the activation energy was calculated as 35.041 kJ/mol, and listed in Table 3.
Table 3 shows that the activation energy for the pyrolysis of corn stalk without pith, corn root, and corn leaf was in the range of 26.4 to 103.6 kJ/mol, 37.6 to 69.5 kJ/mol, and 35.0 to 103.9 kJ/mol, respectively, depending on the conversion. This indicated that corn root was converted more cleanly than other materials under the same conversion condition. This was due to the lowest lignin and ash being present in corn root. The activation energy of corn stalk with no pith and corn root increased slowly with increasing the conversion from 0.1 to 0.5. When the conversion increased from 0.6 to 0.8, the activation energy of corn stalk without pith increased sharply (Fig. 12).
Table 3. Calculated Kinetic Parameters for the Pyrolysis of Samples
Fig. 9. Typical iso-conversion plot of the Friedman method, corn stalk without pith
Fig. 10. Typical iso-conversion plot of the Friedman method, corn root
Fig. 11. Typical iso-conversion plot of the Friedman method, corn leaf
Fig. 12. Iso-conversion plots for activation energy versus conversion
CONCLUSIONS
- Thermogravimetric analysis of corn stalk without pith, corn root, and corn leaf was carried out to investigate pyrolysis characteristics and to calculate kinetic parameters. The Friedman method was used to determine the apparent activation energy of CS materials. The apparent activation energies determined by the differential method for corn stalk without pith, corn root, and corn leaf were in the range of 26.4 to 103.6 kJ/mol, 37.6 to 69.5 kJ/mol, and 35.0 to 103.9 kJ/mol, respectively, depending on the conversion rate. Corn root converted more cleanly than other materials under the same conversion condition. This was due to the lowest lignin and ash being present in the root.
- The reaction mechanism of corn with no pith, corn root, and corn leaf of pyrolysis process can be kinetically characterized by two successive reactions.
- The main thermal decomposition occurred within a temperature range of 200 to 350 °C (±10 °C). Most of the volatile materials decomposed at less than a 0.8 conversion rate. However, the material remaining after a greater than 0.8 conversion was mainly char, and the decomposition of char proceeded at higher conversion rates.
- The higher the heating rate, the bigger the maximum weight loss rate. The apparent activation energy values of different parts of the CS revealed the difference in pyrolysis behaviors. The result indicated that different treatments should be chosen according to different parts of CS material in order to achieve the most efficient use of the resource.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the financial support from the Priority Academic Program Development of Jiang Su Higher Education Institutions (No. 164020890) and by Nanjing Forestry University.
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Article submitted: September 24, 2014; Peer review completed: December 11, 2014; Revised version received and accepted: January 29, 2015; Published: February 4, 2015.
DOI: 10.15376/biores.10.2.2020-2031