Thermogravimetric analysis was used to investigate oxy combustion of corncob and stover. The biomass samples were heated from ambient temperature to 900°C at different heating rates of 10, 30, and 50 K/min. Both biomass samples showed similar weight loss patterns with three zones, corresponding to dehydration, devolatilization, and char combustion, but displayed different degradation temperatures. Increasing heating rate was found to shift the degradation patterns to higher temperatures. Decomposition rates of cob and stover may have been influenced by their lignocellulosic composition. The kinetic parameters of the thermal degradation process were also determined and compared using the Flynn-Wall-Ozawa and Kissinger-Akahira-Sunose methods. Both methods were found to give similar values and patterns of activation energy against conversion fraction. The average values were found to be in similar magnitude to those reported in the literature, around 170 and 148 kJ/mol for cob and stover, respectively.
Corn is an important agricultural crop in the world and the north of Thailand. It is widely planted for food production and animal feeding. After harvesting, there is a large amount of corn residues. Cob and stover (stalk and leaves) are the main residues. About 0.15 kg of cobs and 0.72 kg of stover are generated for every 1 kg of dry corn grains produced [
The utility industry burning coal is the main sector responsible for major CO2 emissions [
Oxy combustion is an innovative combustion technology for burning fuel using pure oxygen instead of air. In this technique, because there is no nitrogen, NO
The aim of this research is to investigate the combustion behavior of corncob and stover under oxygen environment. Nonisothermal thermogravimetric analysis is used to evaluate thermal characteristics and kinetics of corn residues under highly oxidative condition. The findings should provide useful data for future development of efficient biomass combustion applications.
Samples of corncob and stover were collected from agriculture land in Nan, Thailand. They were naturally dried, crushed to small particle size, and sieved with 120 meshes. The samples of biomass material were subsequently kept in a ziplock plastic bag and stored at room temperature until they were needed for experiment. Typical properties of the samples are shown in Table
Properties of corncob and stover on a dry basis.
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Volatile | Fixed carbon | Ash | C | H | O | N | Cellulose | Hemicellulose | Lignin | ||
Corncob | 80.2 | 16.7 | 3.1 | 49 | 6 | 44.7 | 0.3 | — | — | — | [ |
82.2 | 16.9 | 0.9 | 45.5 | 6.2 | 47 | 1.3 | — | — | — | [ | |
— | — | — | 47.4 | 5.9 | 38.1 | 0.7 | 30 | 38 | 3 | [ | |
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Corn stover |
74.9 | 17 | 8.2 | 47.4 | 5 | 38.1 | 0.8 | — | — | — | [ |
78.7 | 17.6 | 3.7 | — | — | — | — | 51.2 | 30.7 | 14.4 | [ | |
— | — | — | 46.6 | 5 | 40.1 | 0.8 | 48 | 29 | 6 | [ |
The experiments were performed using a Mettler Toledo thermogravimetric analyzer model TGA/SDTA 851e at 0.1
Kinetic analysis was carried out to determine apparent activation energy for the associated thermal degradation. The analytical techniques can be divided into two classes: model-fitting and model-free (isoconversional) kinetics. The model-fitting method is based on a single heating rate, which is a disadvantage because the activation energy varies with the heating rate, due to mass and heat transfer effects. The isoconversional method is preferred by researchers because, with the use of multiple heating rates, it is sufficiently flexible to allow for a change in mechanism during reactions and mass transfer limitations [
The degree of weight loss was shown as conversion,
In this work, features of the thermal degradation characteristics were shown as the change of mass loss with temperature (TG) and mass loss rate (DTG) profiles. Nonisothermal thermogravimetric analysis of corncob was carried out at different heating rates of 10, 30, and 50 K/min in an oxygen atmosphere and the results are plotted in Figure
(a) TG and (b) DTG curves of corncob.
(a) TG and (b) DTG curves of corn stover.
The temperature values at each stage and those characteristics in which the reactions took place are presented in Table
Thermal degradation characteristics of corn residues under oxygen atmosphere.
Stage | Heating rate = 10 K/min | Heating rate = 30 K/min | Heating rate = 50 K/min | |||||||||||||
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Corncob | 1st | 25 | 100 | 74 | 0.29 × 10−3 | 5.05 | 25 | 100 | 92 | 0.8 × 10−3 | 5.22 | 25 | 100 | 98 | 1.32 × 10−3 | 5.06 |
139 | 90.6 | 163 | 90.32 | 163 | 90.85 | |||||||||||
2nd | 190 | 90.18 | 271 | 1.42 × 10−3 | 31.23 | 194 | 90.08 | 289 | 4.1 × 10−3 | 31.57 | 204 | 90.54 | 295 | 7.2 × 10−3 | 31.95 | |
340 | 39.75 | 372 | 36.76 | 378 | 36.59 | |||||||||||
3rd | 340 | 39.75 | — | 372 | 36.76 | — | 378 | 36.59 | ||||||||
550 | 6.38 | 747 | 4.92 | 776 | 7.1 | |||||||||||
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Corn stover | 1st | 25 | 100 | 69 | 0.26 × 10−3 | 3.84 | 25 | 100 | 98 | 0.9 × 10−3 | 6.05 | 25 | 100 | 101 | 1.66 × 10−3 | 6.18 |
132 | 92.21 | 157 | 89.22 | 161 | 89.4 | |||||||||||
2nd | 167 | 91.5 | 275 | 1.17 × 10−3 | 37.24 | 177 | 88.83 | 313 | 3.1 × 10−3 | 47.96 | 177 | 89.14 | 315 | 5.58 × 10−3 | 47.32 | |
368 | 34.91 | 390 | 36.89 | 391 | 37.81 | |||||||||||
3rd | 368 | 34.91 | — | 390 | 36.89 | — | 391 | 37.81 | — | |||||||
514 | 8.41 | 878 | 6.27 | 858 | 8.86 |
It can be seen that the peak devolatilization temperature of stover was higher than cob but the mass loss rate was smaller. The thermal degradation characteristic was affected by the chemical composition of biomass. Difference in their constituent components resulted in different thermal behavior. According to the biochemical composition, both kinds of feedstock samples contained small lignin content, but the fractions of cellulose and hemicelluloses were relatively distinct. This indicated that different behaviors were expected in the devolatilization process. The thermal decomposition of the individual components of lignocellulosic materials was studied by several researchers [
The degradation rate of the char combustion process in the third stage was lower than the devolatilization of the second step. The degradation rates of cob and stover were similar. Degradation of stover appeared to have a higher starting temperature and ends at a higher final temperature than cob. At a relatively high heating rate, the combustion of these char occurred within a much wider temperature range. A constant combustion rate was also observed. This observation generally agreed with the results reported by other researchers [
The decomposition process was shifted to a higher temperature zone as the heating rate increased. The peak degradation temperature during the first weight loss stage of cob and stover was increased from 74 and 69 to 98 and 101°C, respectively. For the second mass loss stage, it was increased from 271 and 275 to 295 and 315°C, respectively. The translation of the TG curves to higher temperature at higher heating rates was due to the reduction of residence time which was not sufficient for heat transfer to permeate into the center of the particle. Thus, the thermal decomposition process was delayed. To have the same weight loss, the degradation temperature was expected to be higher. Gai et al. [
From TG analysis data, the kinetic parameters of overall weight loss were derived from the FWO and KAS methods. Figure
Conversion at any temperature: (a) cob, (b) stover.
Linear plots for determination of the activation energy at different values conversion for corncob: (a) FWO method and (b) KAS method.
Linear plots for determination of the activation energy at different values of conversion for corn stover: (a) FWO method and (b) KAS method.
Variation of apparent activation energy of corn residues with conversion rate.
The characteristic of devolatilization occurred in conversions of around 20–60%. The average
Thermal degradation of corncob and stover under a highly oxidative environment was investigated using TGA at different heating rates of 10, 30, and 50 K/min. Kinetic parameter in terms of the apparent activation energy was determined and compared using the FWO and KAS methods. Oxidative thermal degradation of both samples was found to occur in three mass loss stages: water evaporation, devolatilization, and char combustion. Increasing heating rate was observed to result in increasing mass loss rates, but the start of the thermal decomposition was delayed to higher temperatures. Different lignocellulosic composition of the feedstock may affect degradation behavior differently. Stover appeared to have higher hemicellulose and lower cellulose contents than cob and, hence, lower initial degradation temperatures. From the FWO and KAS methods, the activation energy was found to vary with the conversion fraction. The activation energy calculated by the FWO and KAS methods was similar. The average was about 172 and 149 kJ/mol for cob and stover, respectively.
Preexponential factor (s−1)
Apparent activation energy (kJ/mol)
Reaction function
Integral function of conversion
Reaction rate constant (s−1)
Universal gas constant (8.314 × 10−3 kJ/mol·K)
Temperature (K)
Initial temperature (K)
Initial mass of sample (kg)
Mass at any time (kg)
Final mass at the end of reaction (kg)
Degree of conversion (—)
Heating rate (K/min).
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors wish to acknowledge the supports from Thailand Research Fund (RSA5680011), the Energy Policy and Planning Office, Ministry of Energy, and Chiang Mai University.