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Zhang, D., Wang, F., Yi, W., Li, Z., Shen, X., and Niu, W. (2017). "Comparison study on pyrolysis characteristics and kinetics of corn stover and its digestate by TG-FTIR," BioRes. 12(4), 8240-8254.


The pyrolysis potential of corn stover digestate (CSD) was compared with corn stover (CS). The effects of anaerobic digestion (AD) on pyrolysis were investigated at different rates by a thermogravimetric analyzer coupled with Fourier transform infrared spectrometry (TG-FTIR). The distributed activation energy model (DAEM) was used to show the differences in the kinetics. The results indicated that the AD process improved the thermal stability with lower mean reactivity (RM), higher solid residue (S850), and decreased release of observed gaseous productions, except for CH4. The release of CH4 from CSD was higher than that of CS, especially in the temperature range of 430 °C to 520 °C. The activation energies (E) of CS and CSD were 184 kJ/mol to 293 kJ/mol (conversions were 0.1 to 0.8) and 99 kJ/mol to 331 kJ/mol (conversions were 0.1 to 0.9), respectively. The activation energies decreased after AD at the same conversion level. The calculated TG data of CS and CSD from the kinetic parameters were in good agreement with the experimental curves.

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Comparison Study on Pyrolysis Characteristics and Kinetics of Corn Stover and Its Digestate by TG-FTIR

Deli Zhang,a,b Fang Wang,a,b Weiming Yi,a,b,* Zhihe Li,a,b Xiuli Shen,a,b and Weisheng Niu c

The pyrolysis potential of corn stover digestate (CSD) was compared with corn stover (CS). The effects of anaerobic digestion (AD) on pyrolysis were investigated at different rates by a thermogravimetric analyzer coupled with Fourier transform infrared spectrometry (TG-FTIR). The distributed activation energy model (DAEM) was used to show the differences in the kinetics. The results indicated that the AD process improved the thermal stability with lower mean reactivity (RM), higher solid residue (S850), and decreased release of observed gaseous productions, except for CH4. The release of CH4 from CSD was higher than that of CS, especially in the temperature range of 430 °C to 520 °C. The activation energies (E) of CS and CSD were 184 kJ/mol to 293 kJ/mol (conversions were 0.1 to 0.8) and 99 kJ/mol to 331 kJ/mol (conversions were 0.1 to 0.9), respectively. The activation energies decreased after AD at the same conversion level. The calculated TG data of CS and CSD from the kinetic parameters were in good agreement with the experimental curves.

Keywords: Corn stover; TG-FTIR; Anaerobic digestion; Pyrolysis kinetics; DAEM

Contact information: a: School of Agricultural Engineering and Food Science, Shandong University of Technology, Zibo, Box 255049, China; b: Shandong Research Center of Engineering and Technology for Clean Energy, Zibo, Box 255049, China; c: College of Engineering, Shenyang Agricultural University, Shenyang, Box 110866, China; *Corresponding author:


Anaerobic digestion (AD) of biomass is a promising technology for energy utilization and environmental protection. AD plants are typically used to produce heat and electricity through the combined heat power (CHP) system or as a natural gas source. Due to the rich contents of nutrients and fibrillar components, the digestate of AD plants is mainly applied for fertilizer. The supplies of the digestate are sustained, but the demands of the soil and vegetation are seasonal. With the excessive accumulation of the residues, the contradiction between supply and demand has become more serious. Moreover, due to the remaining heavy metal, pathogenic bacteria, and antibiotics in the residues, there is potential environmental pollution and harm to the human body caused by digestate. Therefore, more ways to utilize the digestate are needed (Monlau et al. 2015a; Fabbri and Torri 2016).

Pyrolysis technology could convert biomass to biofuels or biomaterials in high temperature without oxygen. Previous studies have indicated the potential advantages of using pyrolysis with the digestate as a feedstock. Liang et al. (2015) showed that the selectivity of pyrolytic products with phenols could be improved by AD, especially for the yield of 4-vinylphenol. Similar conclusions were reported by Wang et al. (2016) in the bio-pretreatment of methanogen. Neumann et al. (2016) discussed a novel combination that had a digestate and thermo-catalytic reforming process that was used to improve the quality of the bio-oil with low viscosity and acid number; its calorific value was 35.2 mJ/kg. Inyang et al. (2010) considered the influence of AD on biochar property, showing that the bio-char derived from digestate was superior for soil amelioration, contaminant remediation, or wastewater treatment compared with raw materials. The phosphate and heavy metal removal potential of digestate derived bio-char is comparable to commercial activated carbons (Yao et al. 2011; Inyang et al. 2012). The energy recovery with pyrolysis of digestate is enhanced compared with AD alone. Additionally, a 6.1 MJ/kg VS energy yield was achieved from the pyrolysis of digestate from chicken manure and corn stover (Li et al. 2014). Monlau et al. (2015b) estimated that the digestate pyrolysis increased the production of electricity by 42% compared with the AD stand-alone plant based on a full scale AD plant. However, most studies on biogas residue pyrolysis have focused on the features and applications of productions and energy efficiency analysis. Few detailed studies have investigated the behaviors of evolved gases in real time during pyrolysis. Li et al. (2017) showed that AD had discriminative effects on pyrolysis kinetics for sludge, food waste, vinasse, and cow manure. Hence, it is indispensable to consider the thermal degradation and kinetics compared with corn stover (CS) and its digestate (CSD), which are some of the main resources for AD plants. These gas release characteristics and kinetics are helpful to approach the pyrolysis mechanism of biogas digestate.

Thermogravimetric analysis coupled with Fourier transform infrared spectrometry (TG-FTIR) has been widely used to examine the pyrolysis of biomass such as lignocellulose, marine algae, and their derived components (Wang et al. 2014; Ceylan and Goldfarb 2015; Tian et al. 2016). This combined method monitors the mass loss of materials with a constant heating rate and distinguishes the functional groups of volatiles during pyrolysis in real time. In addition, the study of kinetics is an important part of biomass pyrolysis. Numerous kinetic analysis models have been reported in the thermal analysis. The distributed activation energy model (DAEM) computing method was improved by Miura (1995) and is widely applied for biomass to show the relationships between activation energy and conversions (Cai et al. 2013; Cao et al. 2014; Soria-Verdugo et al. 2014).

The objective of this paper was to support the application of CSD on pyrolysis. The weight loss characteristics and gas evolution rules for CS and CSD were investigated to explore the effects of AD on the pyrolysis by TG-FTIR. The DAEM method was used to analyze the pyrolysis kinetics of CSD compared with CS. Additionally, the changes in the physicochemical properties between CS and CSD were considered. The research is advantageous in the further applications of pyrolysis or in other thermal conversion technology linked with the AD process.



CSD was prepared from a batch of AD at 35 °C for 35 d. The mass ratio of substrates and the inoculum was a 7:3 based on the total solid (TS) content at 10%. The CS was collected from Zibo in the district of Shandong Province. Before the analysis, the CS and CSD were pulverized to particle sizes below 1 mm and dried at 80 °C for 24 h.

Elemental and Lignocellulosic Analysis

The elemental composition of CHNS was conducted on an elemental analyzer (Vario EL cube, Langenselbold, Germany), and the oxygen content was accounted by difference. The cellulose, hemicellulose, and lignin contents were determined by an extraction unit (FIWE6, VELP Co., Usmate, Italy), following the procedure developed by Van Soest et al. (1991).

TG-FTIR Analysis

A thermogravimetric analyzer (TA Instruments, New Castle, USA) coupled with Fourier transform infrared spectrometry (Thermo Scientific Nicolet iS 50, Waltham, Ma, USA) was used to study the pyrolysis behaviors and kinetics parameters of CS and CSD. The experiments operated from 30 °C to 850 °C under heating rates of 10, 20, and 30 °C/min. The carrier gas was high-purity nitrogen flowing at 60 mL/min to ensure the inert gas atmosphere. The temperature was maintained at 210 °C in the transfer line between the two instruments. The spectrum range of the FTIR was 4000 cm-1 to 400 cm-1, and the resolution was 2 cm-1. A total of 8 ± 0.05 mg of sample was used with each experiment to reduce the variation caused by sample amount.

Analysis of Kinetic Parameters

In the model of DAEM, the reaction consisted of a series of irreversible and independent first-order parallel reactions following the continuous function distribution. The change in total volatiles at different temperature was represented as follows (Miura 1995),



where V is the volatile content at temperature TV* is the effective volatile content, k0 is the frequency factor corresponding to the E value, α is the heating rate, and f(E) is the distribution curve of the activation energy. The distribution curve f(E) is normalized to satisfy Eq. 3.


In this study, the kinetic parameters were estimated by the integral method proposed bMiura and Maki (1998). Equation 1 was simplified to Eq. 4,


where Eis chosen to satisfy Φ(Es, T) = 0.58 at a given temperature. Both activation energy (E) and pre-exponential (k0) factor can be estimated by the Arrhenius equation (Eq. 5); the slope and intercept of the Arrhenius plot are used to calculate the E and k0, respectively.


To predict TG curves with the kinetic parameters calculated from the DAEM, the method used by Ceylan and Kazan (2015) and Li et al. (2017) was adopted. A linear equation is expressed as follows,


where m and n are the slope and the intercept of the plots of ln(1/T2) versus 1/T at a given heating rate.

An expression to calculate temperature with different conversion rates can be determined by Eq. 5 and Eq. 6 simultaneously as Eq. 7.


Plots of V/V* versus T from DAEM data were compared with experimental data.


Properties of CS and CSD

The AD process changed the elemental contents and components of CS. As shown in Table 1, the carbon content was increased slightly. The oxygen was decreased noticeably, which is favorable for feedstock that is transformed into high-quality bio-oil with pyrolysis (Cen et al. 2016). The crystalline regions of cellulose and the presence of lignin were the main obstacles for the biodegradation of lignocellulose. Since hemicellulose contains more branch chains and amorphous region, it biodegraded more easily than cellulose. And the lignin was almost undegraded in anaerobic conditions. Therefore, the content of hemicellulose and cellulose decreased by 36.2% and 25.2%, respectively. The lignin content increased from 7.04% to 16.86%. The biodegradation of CS led to the variation of the element. In addition, the content of ash increased from 3.92% to 11.59%, which was caused by the addition of salt additives and the concentration effect in biogas residue with AD.

Table 1. Ultimate and Lignocellulosic Analysis of CS and CSD

Thermogravimetric Analysis of CS and CSD

Figure 1 shows the thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of CS and CSD under 30 °C/min. The thermal weight loss was divided into three stages within the temperature range of 30 °C to 850 °C according to the peak of the DTG curves; the stages were moisture evaporation and slight degradation, main pyrolysis, andcarbonization.

The weight losses of two samples in the first stage were 6.6 and 7.8%, corresponding to the moisture content of the samples. There were noticeable differences in the second stage during the temperature range of 230 °C to 550 °C between CS and CSD, with a weight loss of 64.5% and 50.6%, respectively. The maximum weight loss rate (Dmax) of CS was much higher than CSD, which was attributed to the changes of compositions in AD process. The pyrolysis characteristics of the three main components were different, the degradation of cellulose and hemicellulose occurred more quickly, and the lignin had a wider pyrolysis temperature range (Quan et al. 2016). Moreover, there was a stronger shoulder peak in the DTG curve of CS than that of CSD, which could be explained by the degradation of hemicellulose in the AD process (Skodras et al. 2006).

Fig. 1. Weight loss and derivative weight loss curves for CS and CSD

In the carbonization stage, there was a slight weight loss in CS and CSD of 5.6% and 6.1%, respectively. The solid residue that was at 850 °C temperature (S850) of CSD was higher than CS. This result agreed with the changes of components in the course of AD process. The pyrolytic parameters of CS and CSD are shown in Table 2. Mean reactivity (RM) was used to describe the pyrolysis reactivity based on the method in Ghetti et al. (1996), which was defined as the ratio of Dmax and Tmax for the pyrolysis peak of the second stage. The lower value of RM and higher value of S850 in CSD implied that the AD process inhibited the thermal volatile release of CS.

Table 2. Pyrolytic Parameters of CS and CSD

Compared with CS, a visible change was observed in the TG-DTG curves of CSD. The AD improved the thermal stability of CS. This result was attributed to the decrease in cellulose and hemicellulose contents and the increase of the relative content of lignin. The increase in ash content was also a potential factor. Alkaline and alkaline earth metals promote the formation of char during biomass pyrolysis (Fahmi et al. 2007).

FTIR Analysis of Pyrolysis

The emission of gases during the pyrolysis of CS and CSD under 30 °C min-1 is shown in Fig. 2, where the distribution of absorbance at different temperatures was in agreement with the DTG curves. The tendency of both samples was similar, but the absorption intensity of CSD was weakened. The characteristic absorptions of the volatiles are listed in Table 3 and Fig. 3 (Tihay and Gillard 2010; Chen et al. 2015; Lazdovica et al. 2017). Only a small portion of low molecules and organic volatiles were released at 240 °C at the initiation of main pyrolysis stage. As the temperature increased, the gas production increased with an intensification of pyrolytic degradation, as shown in Fig. 3(b). At 345 °C, which was approximately the maximum weight loss temperature, abundant and complex organic volatiles were released, including carbonyl groups and oxhydryl groups, which represent the carboxylic, ketone, aldehyde, alcohols, and levoglucosan (LG). COis the primary component of incondensable gases. However, with the temperature further increasing, the organic volatiles almost disappeared as shown in Fig. 3(d). And, slight CO2 and CH4 were observed, caused by cracking of C-H and C-O bonds and aromatization reactions.


Fig. 2. 3-D TG-FTIR diagram of CS and CSD pyrolysis at 30 °C/min

Fig. 3. FTIR spectrum of evolved gases during pyrolysis for CS and CSD at 30 °C/min

The TG-FTIR test conditions of CS and CSD were identical, so the change in absorbance for a given wavenumber was linear with the corresponding gas yield according to Beer-Lambert law (Gao et al. 2013). The evolution of absorbance intensity of gaseous components is shown in Fig. 4; the components were identified by their characteristic bands listed in Table 3.

Table 3. Characteristic IR Spectra Absorptions of the Volatiles

The evolution profile of the CO2 from CSD appeared in two peaks, where the higher peak occurred at 347 °C and the lower peak at 682 °C. The evolution profile was different in CS where there was only one peak. At a low temperature, CO2 mainly originated from the primary decomposition of hemicellulose through decarboxylation and decarbonylation. The further fracture of the C-O and carboxyl groups of lignin were the main sources of CO2 production at high temperature (Yuan et al. 2015). The second peak of CSD was possibly caused by the degradation of lignin. The evolution curves of CO for both CS and CSD had a sharp peak and reached a maximum at approximately 350 °C. After approximately 600 °C, the release of CO increased relatively slowly. According to Fu et al. (2012), the CO released below 450 °C was formed by the cracking of ether and carbonyl groups. At high temperatures (> 600 °C), CO was mainly produced from the secondary pyrolysis of char decomposition (Yang et al. 2007), which caused the CS to have a higher curve. The H2O evolution of both CS and CSD exhibited two peaks. The two peaks were mainly caused by the evaporation of free water and the dehydroxylation of carbohydrates, respectively.

Compared with CS, the release of CO2, CO, and H2O from CSD was much weaker. Below 400 °C, the phenomenon was attributed to the lower contents of cellulose and hemicellulose of CSD caused by AD. When the temperature was higher than 400 °C, the weaker intensity of the secondary pyrolysis of organic volatiles may be the reason for the appearances.

The FTIR profiles of CH4 from both CS and CSD showed a complete peak and a shoulder peak at approximately 400 °C and 500 °C, respectively. This pattern was different from other gaseous components where the first peak of CSD was slightly higher than that of CS. The second peak of CSD was approximately 2.5 times higher from 430 °C to 520 °C, which stayed consistent with the ratio of lignin content in Table 1. Lignin is the primary source for CH4. The first peak was mainly generated from the cleavage in lignin side chains, while the second peak was attributed to the profound rupture of aromatic rings and/or secondary pyrolysis (Wang et al. 2009). Based on the research of De La Cruz F. et al. (2014), methoxy groups in phenol moieties are degraded under AD, which could explain the slight difference in the low temperature peak in CH4 at low temperatures.

The characteristic absorbances in the C=O stretching vibration related to acids, ketones, and aldehydes were found at 1800 cm-1 to 1684 cm-1. The C-O-H stretching vibration related to alcohols and LG were found at 1131 cm-1 to 1058 cm-1. According to Gao et al. (2010), acids, ketones, and aldehydes were mainly produced by hemicellulose and cellulose pyrolysis, especially in the hemicelluloses. After the AD process, cellulose and hemicellulose degradation decreased the carbonyl organic compounds. The same reason could be used to explain the reduction in alcohols and LG that were mainly from cellulose pyrolysis. In addition, the lower organic release was one reason for the lower secondary pyrolysis of CSD.

Fig. 4. Comparison of pyrolysis product evolution curves for CS and CSD

Kinetic Analysis

Three different heating rates ranging from 10 °C/min to 30 °C/min were used to deduce the dependence of activation energy (E) with the conversions during pyrolysis. Because thecoefficient of determination (R2) of the CS fitting curve at the conversion of 0.9 was rather low, the corresponding activation energy was ignored. Meanwhile, the other R2 of the lines drawn at various conversions were greater than 0.95, indicating it was well fitted by the method. Hence, CS conversions varying from 0.1 to 0.8 and 0.1 to 0.9 of CSD were considered in this study. The regression lines for Arrhenius plots of ln(α/T2) and 1/T are shown in Fig. 5. According to Eq. 5, the E and k0 at different levels of conversions V/V* were estimated from the slope and intercept of all lines.

Fig. 5. Arrhenius plot of ln (α/T2) vs. 1/at selected V/V* values for CS and CSD

The activation energy variation ranged from 184 kJ/mol to 293 kJ/mol for CS and 99 kJ/mol to 331 kJ/mol for CSD, as shown in Fig. 6. These values were inconsistent with those of corn stover, groundnut shell, and cotton husk (Ma et al. 2013; Bhavanam and Sastry 2015). The pre-exponential factors for the corresponding activation energies were calculated to be in the order of 1016 to 1024 in CS and 1012 to 1022 in CSD.

Fig. 6. E vsV/V* estimated from the Arrhenius plot for CS and CSD

With the increase in conversion, the activation energy curves of both samples increased; in particular, a sharp raise appeared when the conversion values were higher than 0.7. The E of CSD at 0.1 conversion was remarkably lower than CS, which is attributed to the low corresponding temperature of approximately 130 °C. The E values of CSD pyrolysis were lower than that of CS at the same conversion level, indicating that the AD process reduced the activation energy of CS.

A reaction with lower apparent activation energy requires less energy to break down the chemical bonds between atoms (Ceylan and Kazan 2015). So the lower E of CSD was favorable for pyrolysis. Similar trends were reported by Li et al. (2017) using the AD substrates of lignocellulose biomass, vinasse, and cow manure. The phenomenon is possible caused by the increase in the proportion of lignin and ash. The activation energy of lignin is lower than that of cellulose and hemicellulose (Manya et al. 2003; Barneto et al. 2010); alkaline salts in biomass, whether added or innate, reduce the activation energy (Nassar 1999).

To predict TG curves, the values of m and n were obtained with Eq. 6, as shown in Fig. 7. All of the R2 were higher than 0.98. Using Eq. 7, the experimental curves and the calculated plots of the conversion V/V* vsT of CS and CSD at 30 °C/min were compared in Fig. 8. The DAEM plots were in well agreement with the experimental data implying a favorable validity of kinetic analysis.

Fig. 7. Linearization of ln(1/T2vs. 1/T curves for different heating rates of CS and CSD (10, 20, and 30 °C/min)

Fig. 8. Comparison of experimental curves and calculated plots for CS and CSD


  1. The anaerobic digestion (AD) process changed the components of corn stover (CS) and led to a visible change in the TG-DTG curves. The mean reactivity (RM) decreased from 0.095 to 0.049% min-1 °C-1, and the solid residue (S850) increased from 24.3% to 36.2%. Thus, the AD improved the thermal stability of CS.
  2. Compared with CS, the release of gaseous products from corn stover digestate (CSD) were reduced, including CO2, CO, H2O, and various organic volatiles. However, the release of CH4 was increased, especially within the temperature range from 430 °C to 520 °C, where the product was improved by approximately 2.5 times. A possible explanation is the modifications in the lignin from the AD.
  3. The activation energies (E) of CS and CSD were 184 kJ/mol to 293 kJ/mol and 99 kJ/mol to 331 kJ/mol at the conversions from 0.1 to 0.8 and 0.1 to 0.9, respectively. They had a similar distribution where they rose as the temperature increased. However, the distribution of the CSD was lower than the CS at the same conversion level. The kinetic parameters calculated by the distributed activation energy model (DAEM) were properly fitted with the experimental devolatilization curves.
  4. The lower E distribution and better thermal stability compared to CS indicated that the CSD was favorable for pyrolysis with the target product of char.


The authors are grateful for support for this research from the National Natural Science Foundation of China (51276103 and 51536009), the Distinguished Expert of the Taishan Scholars of the Shandong Province, and the Higher Education Superior Discipline Team Training Program of the Shandong Province.


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Article submitted: June 27, 2017; Peer review completed: September 9, 2017; Revised version received and accepted: September 12, 2017; Published: September 19, 2017

DOI: 10.15376/biores.12.4.8240-8254