MS study

Energy Conversion and Management 199 (2019) 111964

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Thermal behavior, kinetics and fast pyrolysis characteristics of palm oil: Analytical TG-FTIR and Py-GC/MS study

T

Yingyun Qiaoa,c, Bo Wanga,b, Peijie Zonga,c, Yiliang Tiand, Fanfan Xua,c, Dawei Lia,c, Fulai Lie, ⁎ Yuanyu Tiana,c, a

State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao 266580, PR China Sinopec Dalian Research Institute of Petroleum and Petrochemicals, China Petrochemical Corporation, Dalian 116045, PR China c Shandong Engineering and Technology Research Center of High Carbon Low Carbonization, China University of Petroleum (East China), Qingdao 266580, PR China d Department of Chemical Engineering, China University of Petroleum Beijing at Karamay, Karamay 834000, PR China e School of Geosciences, China University of Petroleum (East China), Qingdao 266580, PR China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Palm oil Pyrolysis Kinetic TG-FTIR Py-GC/MS Products distribution

In this work, the palm oil was selected as the model compound of waste cooking oil and the pyrolysis experiments were performed first by TG-FTIR and then via Py-GC/MS. TG-FTIR analysis showed that the temperature range of palm oil pyrolysis was 385.2 °C–488.4 °C and the volatile products were mainly CO, CO2 and a large number of light hydrocarbon. According to kinetic analysis, palm oil pyrolysis followed the 1.5 order reaction model. The apparent activation energy and pre-exponential factor calculated by Coats-Redfern method were 275.257 KJ mol−1 and 4.252 × 1020 s−1. The fast pyrolysis experiments were performed at different final temperatures (500 °C, 550 °C, 600 °C, and 650 °C) and different heating rates (0.2 °C ms−1, 1 °C ms−1, 5 °C ms−1 and 10 °C ms−1). The GC/MS analysis indicated that the pyrolysis products mainly included saturated hydrocarbons, unsaturated hydrocarbons, aromatic hydrocarbons, carbonyl compounds and other oxygenates. As the final temperatures increased, the relative content of carbonyl compounds decreased and the yields of unsaturated hydrocarbons showed a fluctuant trend. The undesirable aromatic compounds were only detected at 600 °C and 650 °C. As the heating rate increased, the yields of carbonyl compounds increased, while the yields of unsaturated hydrocarbons decreased. The possible fast pyrolysis pathway of palm oil showed that palm oil underwent a series of parallel reactions and continuous reactions to form the final products. The most suitable reaction conditions of palm oil pyrolysis should be further determined by the production purpose and detailed technical and economic analysis.

1. Introduction

waste cooking oil to become carcinogenic [3]. The leachate from improperly disposed waste cooking oil can also pollute the environment and soil, and the harmful substances may be transferred to the human body through concentration by the food chain [4]. Thus, pollution from waste cooking oil has become a major concern for modern society, and it is necessary to vigorously develop the harmless utilization of waste cooking oil. Waste cooking oil with triglycerides as the main component is a good substitute for traditional fossil fuels [5,6]. Among all chemical compositions of biomass, lipids have the highest heating value and the lowest oxygen content [7]. The current methods for waste cooking oil treatment and recovery mainly include pyrolysis, transesterification, hydrotreating, gasification, solvent extraction, and membrane technology. Among all of the above techniques, pyrolysis technology is

The production of waste cooking oil has shown a spectacular growth trend in recent years. According to the report of the US Energy Information Administration (EIA), the annual production of waste cooking oil in the United States was approximately 378,000 m3, while the production of waste cooking oil in the European Union was approximately 7 × 105 to 106 tons per year [1]. It is estimated that the annual consumption of edible oil worldwide will reach 660 million tons by 2050, and the output of waste cooking oil will increase accordingly [2]. As a hazardous waste, waste cooking oil has an adverse effect on human health and the environment. The structure of cooking oil can be altered by oxidation reaction after use. This process produces toxic hydroperoxides, such as 4-hydroxy-2-alkenals, which can cause the

⁎

Corresponding author at: State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao 266580, PR China. E-mail address: [email protected] (Y. Tian).

https://doi.org/10.1016/j.enconman.2019.111964 Received 20 May 2019; Received in revised form 17 August 2019; Accepted 19 August 2019 0196-8904/ © 2019 Published by Elsevier Ltd.

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Table 1 Proximate analysis, ultimate analysis and high heating value of the sample. Proximate analysis (wt%)a

Ultimate analysis (wt%)a

C/O and C/H (molar ratio)

VM

FCb

ash

C

H

S

N

Ob

O/C

H/C

97.32

2.68

0

76.92

11.88

0.24

0.22

10.74

0.10

1.85

a b c

HHV (MJ kg−1)c

40.92

On dry basis. Calculated by difference. HHV = 0.328∙C + 1.4036 H-0.0237 N + 0.0929 S−[1−(40.11 H Ash/100 C)].

GC/MS analysis. The conclusions of this work can play an important role in the optimization and upgrading of waste cooking oil and other lipid pyrolysis processes.

considered to be an advantageous disposal method for waste cooking oil due to its environmental friendliness and economic viability [8–10]. Waste cooking oil is converted to gases, oils, and char by pyrolysis. The main advantage of pyrolysis technology is that these three-phase products all have both energy and chemical value. The equipment for pyrolysis is very similar to the equipment for conventional petroleum refining, so this process is particularly promising in areas where the petroleum processing industry is mature [9]. Palm oil is the most consumed vegetable oil in the world [11]. Therefore, pyrolysis experiments using palm oil as a model compound for waste cooking oil are important for the harmless and high-value use of waste cooking oil and palm oil production waste. The pyrolysis behaviors of triglycerides and waste cooking oil have been extensively studied by previous researchers [8,12,13]. However, most previous studies were usually performed at relatively low heating rates [14]. Although there is evidence that the heating rate has a significant effect on the pyrolysis of triglycerides, little research has been done on the fast pyrolysis or flash pyrolysis of triglycerides due to the limitations of experimental apparatuses. Asomaning et al. studied the pyrolysis of oleic acid and indicated that high heating can inhibit the formation of undesired products such as aromatics [15]. With the development of the free-fall reactor and gas-phase catalytic process, the study of fast pyrolysis characteristics including the reaction mechanism and product distribution have become more meaningful [16,17]. The pyrolyzer coupled with gas chromatography/mass spectrometry (PyGC/MS) is a commonly used technology in pyrolysis studies, in which the pyrolyzer can provide different reaction conditions including different heating rates and different final temperatures, and the GC/MS can separate and identify the pyrolysis products accurately. In addition to Py-GC/MS, the thermogravimetric analyzer (TG) coupled with Fourier transform infrared (FTIR) analysis is another important technology for investigating pyrolysis behaviors. The TG analyzer can record the changes in the sample weight in real-time and provide data support for calculating pyrolysis kinetic parameters, and the FTIR detector can discern the gaseous products accurately in the meantime. It is worth noting that TG experiments can only be carried out at relatively low heating rates due to the limitations of heat conduction and to ensure that the pyrolysis reaction is under kinetic control, and only smallmolecule gases can be detected by the FTIR. Combining TG-FTIR and Py-GC/MS are important methods for studying the pyrolysis behaviors of samples, but studies combining these two technologies to investigate the pyrolysis of triglycerides are still rare. In this work, palm oil was selected as the model compound for waste cooking oil. To deeply understand the pyrolysis behavior, kinetic parameters, and fast pyrolysis process of palm oil deeply, pyrolysis experiments were performed using TG-FTIR and Py-GC/MS. The pyrolysis behavior and volatile emissions characteristics were investigated by TG-FTIR firstly. The kinetic parameters including the apparent activation energy and pre-exponential factor were also calculated according to TG data. Then, fast pyrolysis experiments were carried out by Py-GC/MS within the main reaction temperature range which was determined by TG analysis. The detailed product information and the effects of pyrolysis conditions including the heating rate and the final temperature on the distribution of products were also investigated by

2. Material and methods 2.1. Material The original material (palm oil) of this study was purchased from Sinopharm Chemical Reagent Co., Ltd. Before the experiments, the palm oil was dried in a vacuum oven at 75 °C for 6 h to remove the slightly soluble moisture. The contents of carbon, hydrogen, sulfur, nitrogen, and oxygen in the palm oil were analyzed using an elemental analyzer (EA3000, ELEMENTAR Company, Germany). The proximate analysis including determination of volatile matter (VM), fixed carbon (FC) and ash was conducted by an automatic proximate analyzer (JHGF-3, Jinhui Coal Quality Analysis Instrument Co., Ltd., China). The higher heating value (HHV) of palm oil was calculated based on the results of the ultimate analysis and the proximate analysis [18]. The results of the proximate analysis, ultimate analysis and high heating value of the sample are shown in Table 1. It can be seen from Table 1 that the content of VM of palm oil is close to 100%, and that palm oil produces no ash after combustion. A previous study has indicated that substances with high contents of VM and low contents of ash can provide more available energy after pyrolysis [19]. Therefore, waste cooking oil and lipid-rich biomass, such as palm shells and coconut shells, are suitable raw materials for use in the pyrolysis process for energy production.

2.2. TG-FTIR analysis TG combined with an on-line FTIR is a commonly used technology for investigating pyrolysis behaviors, kinetic parameters, and gaseous products distributions during the pyrolysis of samples. In this work, the TG analysis was performed using a thermogravimetric analyzer (STA 449F3 NETZSCH Company, Germany), while the gaseous products released during the pyrolysis were monitored online by an FTIR (TENSORⅡ, BRUKER Company, USA). The furnace of the TG analyzer used in this study is a high-speed furnace (NETZSCH Company, Germany), which can provide the heating rate range from 20 °C min−1 to 1000 °C min−1. To ensure that the pyrolysis of palm oil is under kinetic control, the heating rate of the TG experiment should not be set too high. In this work, approximately 5 mg of sample was heated from 30 °C to 900 °C at the heating rate of 50 °C min−1 in an ultrahigh-purity N2 flow (100 ml min−1). The gaseous products entered the gas cell of the FTIR through a Teflon transfer line with a length of 1 m and an inner diameter of 2 mm. During the test, the temperature of the transfer line and the gas cell was maintained at 200 °C to reduce the possibility of gas condensation. The spectrum was collected at a resolution of 4 cm−1 over the wavenumber range of 4000–500 cm−1, and the spectrum scan frequency was set as 32 times per minute.

2

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2.3. Calculation of kinetic parameters

Table 2 The most commonly used mechanism functions and their integral forms.

The pyrolysis process of palm oil is a heterogeneous chemical reaction, and the kinetic parameters of pyrolysis can be obtained from TG data. In this work, the mass loss of samples can be expressed by Eq. (1) [20]:

α =

m0 - m v = m0 - mf vf

Model Shrinking core model Diffusional

(1)

D2

where α is the conversion rate, m0 is the initial mass of the sample, m is the mass of the sample at any pyrolysis time t, mf is the final mass of the sample, v is the mass of the volatiles at any time t and vf is the total mass of the volatiles produced during the whole reaction process. The TG experiment is a nonisothermal pyrolysis process. However, after the derivation, the Arrhenius equation is applicable to this process and the reaction rate can be described as follows:

dα −E = k (T ) f (α ) = Aexp ⎛ a ⎞ f (α ) dt ⎝ RT ⎠

R1 R2 R3 D1

D3

Nucleation growth model Reaction order model

(2)

Power law

A2 A3 A4 F1 F1.5 F2 F3 P2/3

−1

where A denotes the pre-exponential factor (min ), Ea denotes the apparent activation energy (J/mol), R is the universal gas constant (8.314 J/mol∙K), T is the absolute temperature (K), and f(α) is the mechanism function that represents the reaction model, which depends on the controlling mechanism. The heating dT/dt can be defined as β, so a new equation can be expressed as Eq. (3):

β

−E dα = A exp ⎛ a ⎞ f (α ) dT ⎝ RT ⎠

P2 P3

n=1 n=2 n=3 1-D diffusion 2-D diffusion 3-D diffusionJander n=2 n=3 n=4 n=1 n = 1.5 n=2 n=3 2/3Power law 2-Power law 3-Power law

f(α)

g(α)

1 2(1-α)1/2 3(1-α)2/3 1/2α

α 1-(1-α)1/2 1-(1-α)1/3 α2

[−ln(1-α)]−1 2/3

3/2(1-α)

(1-α)ln(1-α) + α 1/3 −1

[1-(1-α)

]

[1-(1-α)1/3]2

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)3/2 (1-α)2 (1-α)3 (2/3)α−1/2

[−ln(1-α)]1/2 [−ln(1-α)]1/3 [−ln(1-α)]1/4 −ln(1-α) 2[(1-α)−1/2-1] 2(1-α)−1-1 [(1-α)−2-1]/2 α3/2

2α1/2

α1/2

3α1/3

α1/3

theoretical curve gives the best match to the experimental curve, the most suitable mechanism can be obtained.

(3)

2.4. Py-GC/MS analysis

α

It is assumed that α = ∫ dα / f (α ) , the Eq. (3) can be expressed as a

α

g (α ) = ∫ 0

Py-GC/MS is a system frequently-used to investigate pyrolysis products distributions. The pyrolyzer can provide different pyrolysis conditions and the GC/MS can identify products precisely. In this study, a CDS-5000 series pyrolyzer (CDS company, USA) coupled with gas chromatography/mass spectrometry (436-GC/SQ-MS, Tianmei Company, China) was selected to perform the pyrolysis experiment and analyze the products. Before the test, approximately 0.3 mg of sample was placed in the center of a quartz tube (2.5 cm × 2 mm, CDS company, USA), and the ends of the quartz tube were closed with quartz wool. During the test, the quartz tube was surrounded by a platinum wire in the probe and the probe could provide different heating conditions. The aim of this experiment was to investigate the effects of pyrolysis temperature and heating rate on the distribution of the products of palm oil pyrolysis. To determine the effect of the pyrolysis temperature, the final temperatures were set as 500 °C, 550 °C, 600 °C, and 650 °C with a heating rate of 10 °C ms−1. To determine the effect of the heating rate, experiments were performed from room temperature to 600 °C at different heating rates of 0.2 °C ms−1, 1 °C ms−1, 5 °C ms−1, and 10 °C ms−1. The residence time of each experiment was set to 10 s. High-purity helium (1.0 ml min−1) was chosen as the carrier gas. Chromatographic separation of volatile products was performed using a capillary quartz column (60 m × 0.25 mm × 0.25 μm, DB-5MS, Agilent company, USA). Before the separation, the heating procedure of the chromatographic column was set as follows: (i) holding at 40 °C for 3 min, (ii) heating from 40 °C to 180 °C at a heating rate of 3 °C min−1 and holding for 3 min, (iii) heating from 180 °C to 280 °C at a heating rate of 4 °C min−1 and holding for 3 min, (iv) heating from 280 °C to 300 °C at a heating rate of 10 °C min−1.

0

new form:

A T − Ea dα ⎞ dT = ∫ exp ⎛ f (α ) β T0 ⎝ RT ⎠

(4)

where g(α) is the integral form of 1/f(α). There are two main mathematical methods to calculate pyrolysis kinetic parameters, one is the model-free (isoconversional) method and the other is the model-fitting (model-based) method [21]. The modelfitting method can calculate both the apparent activation energy (Ea) and the pre-exponential factor (A) simultaneously using only one heating rate. The Coats-Redfern (C-R) method is one of the commonly used model-fitting methods and it has the minimum relative error compared with other model-fitting methods [22]. The equation of the C-R method can be expressed as Eq. (5)[23]:

g (α ) AR E − a ln ⎡ 2 ⎤ = ln βEa RT ⎣ T ⎦

(5) 2

According to Eq. (5), a straight line can be plotted by ln[g(α)/T ] vs. 1/T, with -Ea/R as the slope and ln[AR/βEa] as the intercept. Ea and A can be calculated from this straight line. An accurate assumption of the reaction mechanism function f(α) is essential for the accurate calculation of pyrolysis kinetic parameters. Table 2 lists the commonly used pyrolysis mechanism functions and their integral forms [24]. Criado has reported a method by which to choose the most suitable mechanism function, and this method can be described as follows [25]: 2

f (α ) g (α ) Z (α ) T (dα / dt )α = =⎛ α ⎞ Z (0.5) f (0.5) g (0.5) ⎝ T0.5 ⎠ (d 0.5/ dt )0.5 ⎜

⎟

(6)

3. Results and discussion

where T0.5, f(0.5), g(0.5) and (dα/dt)0.5 are the temperature, the mechanism function, the integral of the mechanism function and the conversion change rate when α = 0.5. The left side of the Eq. (6) f(α)g (α)/f(0.5)g(0.5) is the theoretical curve, which represented the characteristics of each conversion function. The right side of the Eq. (6) is the experimental curves obtained from the TG data. When one

3.1. TG-FTIR analysis 3.1.1. Thermogravimetric analysis Fig. 1 reveals the TG curve and the corresponding differential analysis of the TG (DTG) curve of palm oil pyrolysis. It can be seen from 3

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functional groups. It could be seen from Fig. 2(a) that in the initial pyrolysis stage (385 °C), two weak absorption peaks of CO2 were observed at the wavenumbers of 600 cm−1 and 2400 cm−1. This phenomenon means that the oxygen-containing functional groups in palm oil cleave first and produce CO2. Previous studies have indicated that the bond energy of oxygen-containing functional groups such as CeO and C]O was lower than that of CeC and CeH, so these groups were easily cleaved at low temperatures [26,27]. As the pyrolysis temperature increased to 412 °C, in addition to the absorption peaks of CO2, the absorption peaks of eCH]CH2 (990 cm−1), CO (2178 cm−1) and eCeH (2960 cm−1) could also be identified, which meant that the palm oil released some CO, CO2 and low-carbon hydrocarbons in this stage. When the pyrolysis temperature reached to Tm, many absorption peaks could be observed. The absorption peaks of eCH]CH2 (910 cm−1 and 990 cm−1), eCH3 (1365–1380 cm−1, 2870 cm−1 and 2960 cm−1), C]C (1620–1680 cm−1) and ]CeH (3000–3100 cm−1) indicate that palm oil pyrolysis releases a large number of hydrocarbons, especially olefins [20]. Asomaning et al. studied the pyrolysis behavior of oleic acid and found that hydrocarbons and acids were the main products [15]. The absorption peaks of CO (2178 cm−1) and CO2 (600 cm−1 and 2400 cm−1) were also observed at this temperature, and this was because the deoxygenation, decarbonylation, and decarboxylation of palm oil all accompany the release of CO and CO2[28]. Due to the influence of association, the wavenumbers of C]O and eOH in the carboxylic acid shift toward the low-frequency region in FTIR analysis. The absorption peaks of C]O (1740–1770 cm−1) and eOH (3000–3200 cm−1) could be identified at the temperature of Tm, and this meant that palm oil released carboxylic acids and other carbonyl compounds such as ketones and aldehydes during pyrolysis. The absorption peak of CeO could be observed within the wavenumber range of 1000–1260 cm−1, but the absorption peak of eOH in alcohol could not be observed within the wavenumber range of 3200–3550 cm−1. This phenomenon may due to the fact that the pyrolysis of palm oil produces some low-carbon ethers, but no low-carbon alcohol compounds. When the temperature increased to Tf (488 °C), only inconspicuous absorption peaks of CO and CO2 could be observed in the FTIR spectrum. According to the Lambert-Beer law, it is widely accepted that the absorbance intensity of IR is linearly related to the concentrations of gaseous products [29]. Thus, changes in the absorbance intensity at different pyrolysis temperatures can reflect the yield of gaseous products. The evolution patterns of some gaseous products and functional groups including CO2, CO, eCH3, eCH]CH2, C]O, and CeO are shown in Fig. 2(b). As presented in Fig. 2(b), the main temperature range of gaseous products release is concentrated between 400 °C and

Fig. 1. TG and DTG curves of palm oil pyrolysis at the heating rate of 50 °C min−1.

Fig. 1 that the DTG peak is narrow and sharp, which means that the temperature range of palm oil pyrolysis is narrow and the mass loss rate of palm oil is fast. There is a small front peak in the DTG curve at the temperature of 420 °C, which indicates that the palm oil reacts more strongly at this temperature than at nearby temperatures. The mass loss rate is the fastest when the temperature reaches the temperature corresponding to the main peak of the DTG curve. The pyrolysis parameters including the initial pyrolysis temperature (Ti), the temperature of maximum mass loss rate (Tm), the final pyrolysis temperature (Tf) and the residual mass ratio after pyrolysis (Mf) are also shown in Fig. 1. The pyrolysis reaction of palm oil occurs within the temperature range of 385.2 °C–488.4 °C, and the mass loss rate is the fastest at the temperature of 440.0 °C. It can be seen from the TG curve that there is a large amount of material decomposition during the palm oil pyrolysis process and that the Mf is only 1.65%. The proximate analysis of palm oil in the previous section also indicates that the VM content is very high, while the FC content is very low, further, there is no ash.

3.1.2. FTIR analysis To investigate the gaseous products release pattern of palm oil during pyrolysis, an FTIR spectrometer was coupled with a TG analyzer in this study. The on-line FTIR spectrometer can provide a 3-dimensional spectrum including information of the IR absorbance, wavenumber and temperature. The FTIR analysis of palm oil is shown in Fig. 2, where Fig. 2(a) is the FTIR spectra at different temperatures and Fig. 2(b) is the evolution patterns for some gaseous products and

Fig. 2. FTIR analysis of palm oil pyrolysis: (a) FTIR spectra at different temperatures, (b) the evolution pattern for some gaseous products and functional groups. 4

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520 °C. This temperature range is slightly delayed compared to that from DTG data due to the time required for gas transport and FTIR scanning. CO2 has two absorption peaks at 350 °C and 470 °C, respectively, while other products have only one absorption peak at the temperature of 470 °C. Thus, the pyrolysis process of palm oil from TGFTIR analysis can be summarized as follows. Firstly, the palm oil is converted to unsaturated fatty acids by cracking reactions. This process occurs at lower temperatures and accompanied by the release of CO2. Then, the released fatty acids undergo a series of pyrolysis reactions such as cracking, deoxygenation and decarbonylation, and produce large amounts of alkanes, olefins, ethers, and carboxylic acids. The mass of palm oil reduces rapidly at this stage. As the pyrolysis temperature continues to increase, high molecular weight tar undergoes secondary cracking reactions and releases a small amount of gas. Nevertheless, FTIR can only identify functional groups and small-molecule gaseous products, thus, the reaction path obtained from TG-FTIR is not convincing enough and a more detailed pyrolysis mechanism could be determined by Py-GC/MS analysis [30].

Fig. 4. The fitting plot of Coats-Redfern method.

3.2. Kinetics analysis

Table 3 Kinetic parameters of palm oil pyrolysis.

It can be seen from the Eq. (5) that the form of the mechanism function seriously affects the calculation result, so determining the most suitable mechanism function is the key to calculating kinetic parameters using the model-fitting method. The most suitable mechanism function of palm oil pyrolysis can be determined by the Criado method According to Eq. (6) and Table 2, the fitting curves of the Criado method are shown in Fig. 3. It can be seen from Fig. 3 that the experimental curve has the highest degree of fitting to the F1.5 theoretical curve, thus the mechanism function of palm oil can be described as a reaction order model and the reaction order is 1.5. The f(α) and g(α) of the F1.5 mechanism function can be obtained from Table 2. According to Eq. (5), the plot of ln[g(α)/T2] vs. 1/T depicts a straight line and Ea and A can be calculated from the slope and intercept of this straight line. Vyazovkin et al. reported that the α range from 0.05 to 0.95 and a step-size of no less than 0.5 were advantageous to determine a more accurate value of Ea in kinetics analysis [31]. Therefore, in this work, the range of α was selected from 0.05 to 0.95 and the step-size was selected as 0.05 to calculate the kinetic parameters. Fig. 4 gives the fitting plot of the C-R method at different α values. According to this plot, the Ea and A of palm oil pyrolysis can be calculated as 275.257 KJ mol−1 and 4.252 × 1020 s−1, respectively. The results of the kinetic analysis are shown in Table 3. A previous study has indicated that the value of A can reflect the control mechanism of the pyrolysis reaction. When the value of A is lower than 10−9 s−1, the reaction is a surfacecontrolled process, while when the value of A is higher than 10−9 s−1, complexes can be transferred on the surface freely [32]. For the palm

Kinetic analysis Ea (KJ mol 275.257

−1

)

Mechanism function A (s

−1

)

R 20

4.252 × 10

2

0.998

f(α)

g(α) 3/2

(1-α)

2[(1-α)−1/2-1]

oil in this study, the value of A is higher than 10−9 s−1, which means that the pyrolysis reaction is not surface controlled.

3.3. Py-GC/MS analysis 3.3.1. Pyrolysis products Py-GC/MS analyses were conducted in this study to investigate the effect of the pyrolysis conditions on the distribution of pyrolysis products. Fig. 5 shows a total ion chromatogram (TIC) of the pyrolysis products at a heating rate of 1 °C ms−1 and a pyrolysis temperature of 600 °C. By matching the peaks of the TIC with the PerkinElmer NIST library, most likely reaction products can be identified. Several major pyrolysis products are also shown in Fig. 5. To investigate the effect of the pyrolysis conditions on the distributions of the products, pyrolysis experiments were conducted at different heating rates (0.2 °C ms−1, 1 °C ms−1, 5 °C ms−1, and 10 °C ms−1) and different final temperatures

Fig. 5. Total ion chromatogram of palm oil pyrolysis at 1 °C ms−1 and 600 °C.

Fig. 3. The fitting curves of Criado method. 5

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50.33% to 30.27%. This means that a high final pyrolysis temperature can deepen the degree of decarbonylation of palm oil. The conversion of triglycerides into hydrocarbon fuels and chemical products by thermal deoxidation is an important way to utilize them, so the deeper the degree of decarbonylation, the higher the added value of the products. As the final pyrolysis temperature increased, the yields of unsaturated hydrocarbons showed a fluctuating trend. The peak-area percentages of unsaturated hydrocarbons were 32.07% at 500 °C, 47.95% at 550 °C, 42.51% at 600 °C, and 48.26% at 650 °C, respectively. Lam et al. studied the pyrolysis behavior of waste palm oil using a microwave heated bed and reached a similar conclusion [13]. This phenomenon is because as temperature increases, the pyrolysis degree of palm oil becomes higher, and more unsaturated hydrocarbons can be produced. However, the reactivity of unsaturated hydrocarbons also increases as the pyrolysis temperature increases. At high temperatures, unsaturated hydrocarbons can be converted into other substances by secondary reactions such as saturation reactions, addition reactions, and aromatization reactions. In this work, aromatic hydrocarbons are only detected at 600 °C and 650 °C. A previous study also indicated that the relative content of aromatics increases as the pyrolysis temperature increases [15]. Aromatic compounds are undesirable substances in oil pyrolysis because they have high toxicity and carcinogenicity, and easily produce char through secondary reactions [36]. According to the above analysis, the most suitable final temperature for the palm oil pyrolysis process should be determined according to the production purpose and detailed technical and economic analyses. Fig. 6(b) displays the effect of the final temperature on the carbon number distribution of the pyrolysis products. It is well known that the

(500 °C, 550 °C, 600 °C, and 650 °C). The possible reactions of palm oil during pyrolysis include cracking reactions, deoxygenation reactions, decarbonylation reactions, aromatization reactions, rearrangement reactions, etc.[33]. To simplify the analysis, the pyrolysis products are classified into five series depending on the type of reaction, which are saturated hydrocarbons, unsaturated hydrocarbons, aromatic hydrocarbons, carbonyl compounds and other oxygenates. Among them, the carbonyl compounds include ketones, aldehydes, acids, and esters, while other oxygenates do not include carbonyl compounds. Maher et al. studied the thermal decomposition of stearic acid and found that the pyrolysis products could be classified into four series, namely alkanes, alkenes, aromatics and carboxylic acids [34]. At the same time, pyrolysis products are also classified according to the carbon number. The calibrated peak-area percentage is used to represent the relative content of each substance [35]. The pyrolysis products of palm oil at different heating rates and different temperatures are listed in Tables S2 and S3. It is worth noting that due to the limitations of the apparatus, low-molecular weight gases including hydrogen, methane and carbon monoxide cannot be detected by GC/MS.

3.3.2. Effect of the final temperature The product distribution depends on the pyrolysis conditions, and it is well known that the final temperature is a key factor during the pyrolysis process. Fig. 6 presents the distributions of the products in different series and different carbon numbers at different final temperatures and the changes in the relative content of the major products. It can be seen from Fig. 6(a) that as the final pyrolysis temperature increased, the relative content carbonyl compounds decreased from

Fig. 6. Effect of final temperature: (a) different products series, (b): different carbon number, (c) (d): main products trend. 6

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degree of pyrolysis deepens as the final temperature increases. It can be seen from Fig. 6(b) that as the final temperature increased, the peakarea percentage of small-carbon number products (C1-C10) increased, while the peak-area percentage of large-carbon number products (C16C20) decreased. Fig. 6(c) and Fig. 6(d) present the fluctuation tendencies of the peak-area percentage for some main products relative to the final temperature. As shown in these two figures, the peak-area percentage of 2-butene increased from 2.30% to 5.94% as the final temperature increased, while the peak-area percentage of 1,7- hexadecadiene decreased from 1.60% to 0.34%. The effects of the final temperature on the yields of 1-heptene, 1-nonene, and 11-hexadecen-1ol were not obvious. The peak-area percentages of 1-hexene, 1-decene, and oleic acid increased to the maximum when the final temperature reached to 550 °C, and then the contents decreased as the final temperature increased. However, the trend of 1-tridecene was quite the opposite. The peak-area percentage of 1-tridecene decreased first and then increased, and the minimum was 1.42% at the final temperature of 550 °C. Hexadecanoic acid is the most abundant fatty acid in palm oil and its relative content can reflect the pyrolysis degree of palm oil. As shown in Fig. 6(d), the content of hexadecanoic acid dropped sharply when the pyrolysis final temperature exceeded to 600 °C, which meant that the degree of pyrolysis of palm oil increased dramatically.

percentage of hexadecanoic acid increased from 15.81% to 28.57% as the heating rate increased from 0.2 °C ms−1 to 10 °C ms−1. This phenomenon can be explained by the fact that some of the hexadecanoic acids do not react during high heating rates due to the limitation of heat transfer. 3.3.4. Fast pyrolysis pathway According to Fig. 5, Tables S2 and S3, unsaturated hydrocarbons are the main products of palm oil fast pyrolysis. Palm oil undergoes cracking reactions and decarboxylation reactions to form a large number of long-chain alkanes and alkenes [34]. It can be seen from Fig. 5 that heptadecane and 1-tridecene are the major products of palm oil pyrolysis. Due to the lower bond dissociation energy of the CeC bond at the middle of the chain compared to that of bonds at both ends of the chain, the hydrocarbon chain preferential produces homolytic cleavage and forms small-molecule olefins. The pyrolysis of organic matter including triglycerides and hydrocarbons follows the mechanism of a free-radical chain reaction [39]. The chain reaction propagates through β-scission, which leads to the breaking of CeC bonds and the substantial formation of small-molecule unsaturated hydrocarbons such as propene, 1-heptene, and 1-octene. The process of β-scission can also activate the substrate and form a large number of new free radicals [40]. The dehydrogenation of alkanes can also produce unsaturated hydrocarbons, however, the dissociation energy of CeH is greater than that of CeC, and the dehydrogenation reaction is more difficult to occur than cracking reaction [41]. Some cyclic olefins such as cyclopentene and cyclopentadiene and aromatic hydrocarbons were found among the pyrolysis products. Idem et al. studied the pyrolysis of canola oil and found that a large number of C5 cyclic compounds were formed [42]. The addition of small-molecule olefins followed by dehydrocyclization and double-bond conjugation can form cyclic olefins. Conjugated dienes and dienophiles can also be arranged into C6 cyclic compounds by the Diels-Alder reaction [30]. A previous study has shown that another possible route for the pyrolysis of triglycerides to form cyclic compounds was the intramolecular cyclization of alkenyl radicals and terminal double bonds [43]. Six-membered cyclic compounds continue to dehydrogenate to form aromatic hydrocarbons, while olefins with carbon numbers greater than six can also directly undergo aromatization to form aromatic hydrocarbons. Only a small amount of saturated hydrocarbons was found among the pyrolysis products in this study. However, Asomaning et al. studied the pyrolysis of oleic acid and found that the n-alkanes were the main products [15]. The differences observed between the two studies can be explained by differences in the reaction conditions used. In Asomaning’s study, the pyrolysis experiment was conducted in a batch microreactor, and the final temperature and residence time were set as 450 °C and 8 h, respectively. However in this study, the experiment was carried out by Py-GC/MS, the sample was rapidly heated to the reaction temperature within a few milliseconds, and the pyrolysis products were quickly carried away by the carrier gas. It is plausible that the secondary hydrogenation saturation reaction of olefins is the predominant mechanism of saturated hydrocarbon formation. Carbonyl compounds containing ketones, aldehyde, acids, and esters are also major products of palm oil fast pyrolysis. The main fatty acids contained in palm oil are hexadecanoic acid (C16:0), oleic acid (C18:1), stearic acid (C18:0) and linoleic acid (C18:2) [13]. It can be seen from Fig. 5 that a large amount of n-hexadecanoic acid was produced after the fast pyrolysis of palm oil. This phenomenon may be due to the low thermal conductivity of palm oil [2]. Under the conditions of a high heating rate and a short residence time, a large amount of carboxylic acid vapor that has not reached the reaction temperature is carried into the GC/MS by the carrier gas. The presence of O in the carbonyl group is an electron-withdrawing atom, which can weaken the CeC bond adjacent to the carbonyl group and therefore the fatty acid tends to first undergo decarboxylation during pyrolysis. However, during fast pyrolysis or flash pyrolysis, a lot of energy accumulates on

3.3.3. Effect of the heating rate The heating rate is another important factor affecting the pyrolysis process [37]. The effect of the heating rate on the pyrolysis process is mainly reflected in two aspects. On one hand, the energy received by the sample per unit time increases as the heating rate increases, so the probability of breaking chemical bonds which have high dissociation energy becomes higher. On the other hand, the heat transfer may become a rate-control step for the pyrolysis reaction when the heating rate is too high. In Py-GC/MS analysis, the volatile products are carried away by the carrier gas in time, thus, a high heating rate can shorten the reaction time and reduce the occurrence of secondary reactions [38]. The effects of the heating rate on the distributions of different products series are shown in Fig. 7(a). As the heating rate increased, the peak-area percentage of carbonyl compounds increased from 29.72% to 43.92%, while the peak-area percentage of unsaturated hydrocarbons decreased from 57.70% to 42.51%. The reason for this phenomenon is that due to the limitations of heat transfer, the degree of pyrolysis decreases as the heating rate increases. The contents of other compounds were relatively low, so the effects of the heating rate on the distributions of these compounds were not obvious. Fig. 7(b) indicates the effects of heating on the carbon number distribution of the pyrolysis products. It can be seen from Fig. 7(b) that the variation of relative content of products with carbon numbers in the range of C1–C5 and C6–C10 displayed a tendency of first decreasing and then increasing with the increase of the heating rate. The peak-area percentage of products with carbon numbers in the range C11–C15 exhibits a decreasing trend as the heating rate increases. In contrast, the relative content of products with the carbon numbers in the range C16–C20 increased with the heating rate increased. The heating rate had no significant effect on the distribution of products with carbon numbers greater than C20. Fig. 7(c) and (d) present the effects of the heating rate on the yields of the main products. It can be seen that the yields of different products have different sensitivities for the heating rate. As the heating rate increased, the relative contents of heptadecane, 2-heptadecanone, and 2octadecanoic acid were almost unchanged. The heating rate had a significant effect on the yield of 1-decene. The peak-area percentage of 1-decene decreased to the minimum when the heating rate reached 5 °C ms−1, and then increased remarkably as the heating rate increased. The relative content changes of 2-butene, 2-pentene, 1-heptene, 1-octene, and 1-decene also showed similar trends. Conversely, the peakarea percentages of 1-tridecene and palmitic acid vinyl ester first increased and then decreased as the heating rate increased. The peak-area 7

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Fig. 7. Effect of heating rate, (a): different products series, (b): different carbon number, (c) (d): main products trend.

behavior. The unsaturated sites in fatty acids can enhance the cleavage of CeC bonds at the β-site, which is more pronounced at high heating rates. Palm oil is a mixture of complex unsaturated and saturated triglycerides. The unsaturated triglycerides are likely to undergo a cleavage reaction prior to decarboxylation and decarbonylation [42]. This is also the reason why a large amount of palmitic acid was detected in the GC/MS analysis, while there was almost no oleic acid. The initial decomposition of palm oil can form large amounts of heavy oxygenated hydrocarbons and unsaturated hydrocarbons. As shown in Fig. 8, the heavy oxygenated hydrocarbons continue to decompose to form hydrocarbons and other oxygenates by decarboxylation, decarbonylation, and β-scission, and the unsaturated hydrocarbons are converted into other small-molecule hydrocarbons by β-scission and elimination. Different types of hydrocarbons can be converted into each other by isomerization and hydrogen transfer reactions, which is why a large number of isomers were detected in the GC/MS analysis. The polymerization of olefins can form dienes and acetylenes, and the dienes can continue to combine with olefins to form C6 + cycloolefins by the DielsAlder addition reaction. At the same time, hydrocarbons can also form cyclic hydrocarbons directly by cyclization. At high temperatures, C6 + cycloolefins can form aromatics by aromatization. It is worth noting that the scheme shown in Fig. 8 is only the most likely reaction pathway inferred from the GC/MS analysis, but it is not the only pathway by which the pyrolysis products can be generated.

the sample within a short time and the chemical bonds are broken in a disorderly manner, instead of being sequentially broken according to the dissociation energy. The direct cleavage of CeC bonds in fatty acids and triglycerides that have not been decarbonylated can produce large amounts of carbonyl compounds. According to the results of FTIR and GC/MS analysis, the other oxygenates of palm oil fast pyrolysis mainly included CO, CO2, and trace amounts of alcohols and ethers. Both decarbonylation and decarboxylation reactions are accompanied by the formation of CO and CO2. The irregularly broken bonds of fatty acids and the addition reactions of olefins with water can form alcohols and ethers, but these reactions are more difficult to produce, and the yields of alcohols and ethers are also lower. The pyrolysis pathway of triglyceride has been widely studied in batch and semi-batch reactors previously [9]. Alencar et al. studied the pyrolysis behavior of tropical vegetable oils and proposed 16 possible types of reactions [44]. Kim et al. studied the lumped kinetics of waste lubricating oil pyrolysis and indicated that the formation of products occurred through a series of continuous and parallel reactions [45]. The pyrolysis pathway is strongly dependent on the reaction conditions. Due to the limitations of instrumentation, the pathway of the fast pyrolysis or flash pyrolysis of triglycerides has not been extensively studied. Fig. 8 gives a possible pathway of palm oil fast pyrolysis. Previous studies have shown that triglycerides were often first cleaved to produce fatty acids, which were then decarbonylated to form hydrocarbons and oxygenates [12,28,33]. The TG-FTIR analysis also indicated that CO2 was the first released gaseous product. The degree of unsaturation of triglyceride has a remarkable effect on its pyrolysis 8

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Fig. 8. The possible pyrolysis mechanism of palm fast pyrolysis.

4. Conclusions

Appendix A. Supplementary data

The purpose of this work was to investigate the pyrolysis behaviors, kinetics, products distribution and fast pyrolysis mechanism of palm oil. For this purpose, the experiments were performed by TG-FTIR and PyGC/MS. The TG analysis revealed that the pyrolysis of palm oil mainly occurred at the temperature range of 385.2 °C–488.4 °C, the mass loss rate reached the fastest at the temperature of 440.0 °C and the mass residue after pyrolysis was only 1.56%. The results of FTIR analysis showed that the volatile products during palm oil pyrolysis were mainly CO, CO2 and a large amount of light hydrocarbon. The reaction of palm oil pyrolysis followed the 1.5 order reaction model and the Ea and A calculated by Coats-Redfern method were 275.257 KJ mol−1 and 4.252 × 1020 s−1. A large number of fast pyrolysis products including saturated hydrocarbons, unsaturated hydrocarbons, aromatic hydrocarbons, carbonyl compounds and other oxygenates were identified by Py-GC/MS analysis. As the final pyrolysis increased, the relative content carbonyl compounds decreased from 50.33% to 30.27% and the yields of unsaturated hydrocarbons showed a fluctuant trend. As the heating rate increased, the peak-area percentage of carbonyl compounds increased from 29.72% to 43.92%, while the peak-area percentage of unsaturated hydrocarbons decreased from 57.70% to 42.51%. Palm oil underwent complex parallel reactions and continuous reactions during fast pyrolysis. The most possible fast reaction pathway of palm oil was proposed in this work.

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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (21576294 and 21706287), Taishan Scholar Foundation of young expert (tsqn201812028), Qingdao Municipal Science and Technology Bureau (18-6-1-101-nsh and 16-6-2-51-nsh), the Fundamental Research Funds for the Central Universities (18CX05022A) and Major Science and Technology Innovation Project of Shandong Province (2018CXGC0301). 9

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