Thermal and Kinetic Analysis of Coal with Different Waste Plastics

Jan 22, 2018 - Results show that basic structural features of carbon in PVC and LC are quite different; LC had different carbon rings that constituted...
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Thermal and Kinetic Analysis of Coal with Different Waste Plastics (PVC) in Cocombustion Qi Wang, Jianliang Zhang, Guangwei Wang,* Haiyang Wang, and Minmin Sun School of Metallurgical and Ecological Engineering, University of Science and Technology, Beijing 100083, China ABSTRACT: In this study, the structure characteristics and combustibility of anthracite (LC), waste plastics (PVC), and their blends were investigated using laser particle size analyzer, Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, scanning electron microscopy, and thermogravimetric analysis. Results show that basic structural features of carbon in PVC and LC are quite different; LC had different carbon rings that constituted carbon layers and some aliphatic chains in the gap of carbon layers, while PVC is composed mainly by long chains. PVC presents high reactivity mainly due to its more disordered crystalline structure. Through kinetic analysis, it is found that the combustion processes of PVC, coal, and their blends have a very consistent fit with the double parallel reactions random pore model (DPRM). The combustion of the blends is divided into two stages, and for each stage, the activation energy first decreases and then increases with the increase of the PVC. When the proportion of PVC is 80%, each stage of the activation energy achieves the smallest value (first stage 43.9 kJ/mol, second stage 31.4 kJ/mol).

1. INTRODUCTION Poly(vinyl chloride) (PVC) has been widely used in machinery manufacturing, packing, transport, wires, and cables due to its good resistance.1,2 However, it greatly increases the waste rate of PVC. At present, PVC is the second most used plastic behind polyethylene around the world, and the production of PVC is still growing at an annual rate of 30%. Nowadays, the elimination of these waste plastics has become a hot spot and keystone of environmental pollution.3,4 PVC is usually burned or buried with little recycling in China.5 It causes pollution to the environment and a waste of resources. In order to relieve the waste of resources and take advantage of the heat in PVC, it is envisioned that PVC could be used as a solid fuel to provide heat for industrial production. The main constituents of PVC are different from coal. From proximate analysis, volatile contents in PVC are higher and solid carbon is lower. From elemental analysis, hydrogen content in PVC is much higher, from which it can be inferred that more H2O and less CO2 will be produced by PVC than coal during burning, which can reduce the emissions of carbon and mitigate the greenhouse effect. And also the sulfur content of PVC is much lower than that of pulverized coal. The use of PVC instead of pulverized coal can reduce the emissions of SO2. Meanwhile, the calorific value of PVC is higher than that of coal. Above all, PVC is used as a solid fuel that is clean, green, and efficient.6 PVC is used as a solid fuel, which will undergo complex multiphase reactions, including gas−gas reactions, gas−solid reactions, and gas−liquid reactions,7,8 The combustion process of PVC is different from those of pulverized coal and heavy oil due to different physical properties and chemical properties. Therefore, the combustion characteristics of PVC need to be explored for use as a solid fuel. In this regard, national experts and scholars have done much research.9−12 They pay much attention to the pyrolysis temperature,13,14 combustion characteristic index, ignition characteristic index, combustion kinetics,15−18 pyrolysis products and catalytic pyrolysis,19,20 and granulation conditions.21 Liu et al.22 studied the combustion of plastic by thermal © XXXX American Chemical Society

analysis−mass spectrometry (TA-MS). Combustion mechanism was discussed, and the kinetic characteristics were analyzed by thermogravimetry. Liu et al.23 studied the combustion process of mixed samples with different heating rate and different proportion of pulverized coal and waste plastics by thermogravimetric analysis. Because different carbon species have different proportions of amorphous carbon and crystal carbon, Yu et al.24 characterized the degree of graphitization of different carbonaceous materials by Raman spectroscopy. Liu et al.25 analyzed the pyrolysis of polymers in plastics by Fourier transform infrared spectroscopy. Therefore, it is necessary to explore the microphysical structure of carbon in PVC and pulverized coal, to explain the difference in combustibility between PVC and pulverized coal and provide theoretical guidance for using PVC as a solid fuel. Thermal analysis and kinetics study have been widely used to characterize the combustion behavior of fuel.26 Through the thermodynamic analysis, the combustion characteristics and mechanism of the elaboration can be understood. Nonisothermal methods27−30 are commonly used; because only the overall reactivity of PVC can be studied by isothermal methods and the reaction temperature often changes, kinetic parameters can more accurately reflect the reaction process. By this method, many experts studied the combustion process of PVC and pulverized coal or their blends. To date, research on their microstructure has been rarely reported. In this paper, in order to explore the structural differences of PVC and coal and links between structure and combustibility, the particle size distribution of anthracite and PVC was studied by chemical analysis method, and the graphitization degree of both was determined by Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. Then a double parallel reactions random pore model (DPRM)31,32 was employed to Received: October 24, 2017 Revised: January 21, 2018 Published: January 22, 2018 A

DOI: 10.1021/acs.energyfuels.7b03268 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

3 mm × 1.5 mm alumina crucible. Compressed air was used with a 100 mL/min flow rate. In this paper, the method of nonisothermal thermogravimetry was used to analyze the combustion characteristics of LC and PVC. All tests were performed under nonisothermal conditions from room temperature to 1200 °C at three different heating rates: 5 °C/min, 10 °C/min, and 20 °C/min. In order to ensure the accuracy and reproducibility of the results, each test was repeated three times. Fractional conversion (TG) curve and reaction rate (DTG) curve of the reaction process were automatically collected by the computer. The combustive conversion degree (X) is calculated from eq 1: m − mt X= 0 m0 − m∞ (1)

calculate kinetic parameters, which can explain the combustion process of anthracite and PVC.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. For this study, anthracite (LC), the Chinese name of which is Ling yuan, and PVC plastic were used. The samples were put in a drying oven for 4 h at 110 °C before the experiment and then crushed with a jaw crusher and sieved to lower than 200 mesh. The proximate analysis was carried out using the standard procedures GB/T 212-2001. The elemental analysis was performed in accordance with an elemental analyzer. The LC and PVC powder were blended in a mortar for 10 min with the mixing ratios of LC100%/PVC0%, LC80%/ PVC20%, LC60%/PVC40%, LC40%/PVC60%, LC20%/PVC80%, LC0%/PVC100%. 2.2. Sample Characterization. 2.2.1. Microstructure Test. The particle size distribution of the PVC and LC was measured with a laser particle size analyzer (LMS-30). The microscopic morphology of the samples was examined by a Quanta 250 Environmental scanning electron microscope in a secondary electron emission mode. 2.2.2. Raman Spectroscopy. Raman spectroscopy was measured using a laser confocal microscopic Raman spectrometer (JY-HR800, HORIBA Scientific, Edison, NJ). Incident illumination of wavelength 532 nm (10 s, 20 mW) was provided by He-Ne laser as the light source, and any annealing effects were considered negligible.33 The measurement range of this experiment is 1000−1800 cm−1, and the scanning process in Raman spectroscopy mainly depends on the vibration modes of different phonons of the material. The accurate peak intensities of Raman spectra were analyzed through numerical decomposition by assuming a Gaussian fit for all the peaks. 2.2.3. FTIR Spectroscopy. Fourier transform infrared spectroscopy measures the spectral range of 500−4000 cm−1 on a NEX-US 470 FTIR spectrometer using the KBr pellet technique. KBr tablets were prepared by mixing 1 mg of sample and 200 mg of KBr for 5 min, and the mixture was pressed into the sample cell of the infrared spectrometer. A total of 36 scans were performed with a 4 cm−1 spectral resolution. The specific positions of the different functional groups in the IR spectrum are listed in Table 1.34,35

where m0 represents the initial mass of the sample, m∞ denotes the remaining mass at the end of reaction, and mt is the mass of the sample at the time of t. The characteristic parameters of the experimental study are the starting weight loss temperature, Ti (temperature at which the weight loss exceeds 5% of the combustion process), burnout temperature, Tf (temperature at which the weight loss exceeds 95% of the combustion process), maximum burning rate, (dG/dt)max, and average burning rate, (dG/dt)mean. The combustion reactivity of LC and PVC can be evaluated through these characteristic parameters. The comprehensive combustion characteristic index, S,26,27,30 is determined by the eq 2 as follows:

S=

band no. band position (cm 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

3419−3359 3080−3035 2975−2955 2925−2919 2900 2863 2848 1745−1730 1721−1695 1615−1585 1500−1450 1460−1450 1380 1300−1000 900−700 880−860

17

849

18

776−730

19

730−720

)

dG dt

max

dG dt

mean

Ti 2Tf

(2)

2.4. Kinetic Models. In this paper, the combustion process of PVC and blends can be divided into two stages. The two stages are expressed by eq 3 and eq 4. Equation 3 is considered as the dehydrochlorination process, and eq 4 is considered as the combustion of the volatile and carbon residue matter. Both stages can be expressed by the Random Pore Model (RPM).31,30,36 In addition, it is assumed that two parallel reactions are controlled by the chemical reaction step.

Table 1. Band Assignments for Different Functional Groups in FTIR Spectra −1

( ) d( )

2CHCl → CC + 2HCl

(3)

volatile + carbon + O2 → CO2 + H 2O + ash

(4)

The RPM model takes into account the pore structure and its evolution during the reaction. Because the pores grow during the initial stage of combustion and are destroyed due to coalescence of neighboring pores, the combustion reaction equation of PVC and blends may be expressed as the following formula:

functional groups O−H or N−H stretching vibration aromatic nucleus C−H stretching vibration aliphatic CH3 asymmetric stretching vibration aliphatic CH2 asymmetric stretching vibration aliphatic C−H asymmetric stretching vibration aliphatic CH3 symmetric stretching vibration aliphatic CH2 symmetric stretching vibration aliphatic (grease, acid, ketone, aldehyde) (CO) aromatic (carbonyl/carboxyl groups) (CO) aromatic nucleus (CC) (C−C)ar stretching aliphatic chains (CH3, CH2) Symmetric deformation (−CH2−) (bending) phenolic deformation (C−O−C) (stretching) aromatic bonds; (C−H) (out-plane bending) aromatic nucleus (CH), one adjacent H deformation aromatic nucleus (CH), two adjacent H deformation aromatic nucleus (CH), three to four adjacent H deformations alkanes side rings [(CH2)n, n > 4]

⎡ −E ⎤ dX = A exp⎢ × (1 − X) × ⎣ RT ⎦⎥ dt

1 − ψ ln(1 − X )

(5)

where ψ is a parameter of particle structure, associated with the initial porosity, ε0, and pore length, L, and the expression is ψ = 31,36

4πL0(1 − ε0) S0 2

,

wherein S0 is the initial surface area. Integrating up to conversion, X, eq 5 gives

∫0

X

dX (1 − X ) 1 − ψ ln (1 − X)

=

A β

∫T

T

e−E / RT dT

0

(6)

⎛ ⎛ E ⎞ ⎛ T − T0 ⎞ ⎜ ⎟ × ⎜ X = 1 − exp⎜− A exp⎜− ⎟ ⎝ RT ⎠ ⎝ β ⎠ ⎜ ⎝ ⎞ E ⎛ A exp − RT ⎞⎟ ⎛ T − T ⎞⎟ ⎜ 0 × ⎜1 + ⎟⎟ × ⎜⎝ β ⎟⎠⎟⎟ ⎜ 4ψ ⎝ ⎠ ⎠

(7)

Integrating and rearranging eq 6 gives

(

2.3. Combustion Test. The HCT-3 type thermogravimeter was used for the combustion test. About 5.0 mg of sample was filled in a

)

The combustion processes of PVC and the blends are divided into two stages; each stage can be expressed by formula 7. The weight loss of B

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Energy & Fuels Table 2. Proximate and Ultimate Analysis of LC and PVCa proximate analysis (dry basis, wt %)

a

ultimate analysis (dry basis, wt %)

sample

FCd

Ad

Vd

Cd

Hd

Od

Nd

Sd

LC PVC

76.93 4.10

9.02 0.01

14.05 95.89

77.38 55.98

3.61 6.14

1.35

0.9 0.54

0.9 0.17

Note: FC, fixed carbon; V, volatile matter; A, ash.

the phases changes with the LC and PVC ratios. The overall process can be expressed by the following formula:

⎛ ⎛ E ⎞ ⎛ T − T0 ⎞ ⎜ X1 = 1 − exp⎜− A1 exp⎜− 1 ⎟ × ⎜ ⎟ ⎝ RT ⎠ ⎝ β ⎠ ⎜ ⎝ ⎞ E ⎞ ⎛ A1 exp − RT1 ⎟ ⎛ T − T ⎞⎟ ⎜ 0 × ⎜1 + ⎟⎟ × ⎜⎝ β ⎟⎠⎟⎟ ⎜ 4ψ ⎝ ⎠ ⎠

(8)

⎛ ⎛ E ⎞ ⎛ T − T0 ⎞ ⎜ X 2 = 1 − exp⎜− A 2 exp⎜ − 2 ⎟ × ⎜ ⎟ ⎝ RT ⎠ ⎝ β ⎠ ⎜ ⎝ ⎞ E ⎛ A 2 exp − RT2 ⎞⎟ ⎛ T − T ⎞⎟ ⎜ 0 × ⎜1 + × ⎟⎟ ⎜⎝ β ⎟⎠⎟⎟ ⎜ 4ψ ⎝ ⎠ ⎠

(9)

(

ultimate analysis of LC and PVC are shown in Table 2. It can be seen that PVC has high volatiles and very little fixed carbon, but LC has high fixed carbon and little volatiles. And PVC has higher content of hydrogen than LC, which may make the combustibility of PVC better than that of LC. It is speculated that the combustibility of blends may become better as the PVC ratio increases. 3.1.1. Particle Size Analysis. Figure1 presents the particle size distribution of LC and PVC. It can be observed that the predominant fraction of LC is about 10−100 μm, whereas the main range of PVC is above 100 μm. The original data of the particle size distribution and the mass percentage that under different particle size is calculated and obtained (Table 3). From the table, it can seen that the particle size less than 38 μm accounted for 87% of LC and for PVC, particle size greater than 90 μm accounted for nearly 90%. It can be clearly seen that the average particle of LC is 14.74 μm and that of PVC is 171.35 μm. So there is a deviation in the particle size distribution between LC and PVC. 3.1.2. Scanning Electronic Microscopy Analysis. There is some amorphous carbon and graphite carbon in LC and PVC. The structure of amorphous carbon is irregular and isotropic, but graphite carbon is anisotropic.26 The morphology of PVC and LC particles was characterized by SEM, which is presented in Figure2. Consistent with the particle size analysis, it can be found that large amounts of particles in LC tend to be irregular, smaller, sharped edged, and fluffy; while particles in PVC are larger, porous, and coarse. So it can be concluded that the majority carbon in PVC is amorphous carbon, while carbon in LC is graphite carbon. 3.1.3. Raman Analysis. The Raman spectra of the PVC and LC samples in the range 800−1800 cm−1 with λ0 = 532 nm are presented in Figure 3. Because the functional group of LC is very complicated, the volume integral method is carried out for peak fitting the spectra of LC. The LC curve exhibits double broad and overlapping peaks with intensity maxima at −1350 cm−1 and −1590 cm−1, which correspond to the D and G bands of disordered graphite.37 The G band in the Raman spectrum is mainly the extension of the benzene ring, and the D band is mainly composed of six or more (less than graphite) fusion benzene ring structure. So the ratio AD/AG can represent the degree of aromatic ring growth.38 The two peaks VL + VR between the G and D peaks represent the vibrations of the alkyl functional groups as well as the extension of the half-quadrant benzene ring that have 3−5 fused benzene rings (amorphous carbon structure).39 Therefore, the ratio AD/(AVL + AVR) can show the extent of the amorphous carbon structure. According to Table 3, the S band corresponds to the sp2−sp3 or sp3-rich carbonaceous structure, such as the Calkyl−Caryl structure and methyl carbon to aromatic ring.38,39 The area of S peak indicates the cross-linking density and displacement composition of the carbon. The band standing on the left of G band is assigned to GL, and GL is caused mainly by carbonyl CO structure. The remaining two weak bands, SL and SR, represent the ether bond and the benzene-related structure.40 The functional groups

)

(

)

The overall reaction extent can be expressed a linear addition function of X1 with X2,

X = ε1X1 + ε2X 2

(10)

where ε1 and ε2 are initial values of X1 and X2, respectively, and they indicate the initial weight fractions of LC and PVC. The relationship between ε1 and ε2 is expressed as

ε1 + ε2 = 1

(11)

The DPRM model combines the combustion processes of two stages, which is expressed by eq 12:

⎛ ⎛ ⎜ ⎛ E ⎞ ⎛ T − T0 ⎞ ⎜ X = ε1⎜1 − exp⎜− A1 exp⎜− 1 ⎟ × ⎜ ⎟ ⎝ RT ⎠ ⎝ β ⎠ ⎜ ⎜ ⎝ ⎝ ⎞⎞ E ⎞ ⎛ A1 exp − RT1 ⎟ ⎛ T − T ⎞⎟⎟ ⎜ 0 × ⎜1 + ⎟⎟ × ⎜⎝ β ⎟⎠⎟⎟⎟ ⎜ 4ψ ⎟ ⎝ ⎠ ⎠⎠ ⎛ ⎛ ⎜ ⎛ E ⎞ ⎛ T − T0 ⎞ ⎜ + ε2⎜1 − exp⎜− A 2 exp⎜ − 2 ⎟ × ⎜ ⎟ ⎝ RT ⎠ ⎝ β ⎠ ⎜ ⎜ ⎝ ⎝

(

)

E ⎛ A 2 exp − RT2 ⎜ × ⎜1 + ⎜ 4ψ ⎝

(

⎞

) ⎞⎟ × ⎛ T − T0 ⎞⎞⎟⎟ ⎟⎟ ⎠

⎜ ⎝

β

⎟⎟⎟ ⎠⎟⎟ ⎠⎠

(12)

Equation 12 was used to determine the kinetic parameters by employing a nonlinear least-squares fitting method. The whole process was carried out on a program compiled with c++ programming language.

3. RESULTS AND DISCUSSION 3.1. Fundamental Characteristics of Samples. The results of proximate analysis, ultimate analysis, particle size distribution, FTIR spectra, Raman spectra, and SEM micrographs revealed the fact that there are differences in chemical and structural properties of PVC and LC. The proximate and C

DOI: 10.1021/acs.energyfuels.7b03268 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 1. Particle size distribution of (a) LC and (b) PVC.

Table 3. Results from the Analysis of Particle Size Distribution sample

425 μm

average particle size

LC PVC

87.26 0

10.36 2.43

2.14 16.24

0 56.52

0 32.13

0 1.88

0 0

14.74 171.35

Figure 2. SEM images of the samples: (a) LC; (b) PVC. D

DOI: 10.1021/acs.energyfuels.7b03268 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 3. Raman spectra for selected samples at room temperature: (a) LC; (b) PVC.

Table 4. Assignment of Bands from Raman Spectra (1800-1000 cm−1) of LC band name

band position, cm−1

functional group

bond type

GL G VL VR D SL S

1690 1590 1500 1420 1350 1269 1185

carbonyl group CO aromatic ring quadrant breathing; alkene CC aromatics with 3−5 rings; semicircle breathing of aromatic rings; amorphous carbon structure

sp2 sp2 sp2, sp3 sp2, sp3 sp2 sp2, sp3 sp2, sp3

SR

1109

aromatics having six or more fused benzene rings but less than that in graphite aryl-alkyl ether; para-aromatics Caromatic−Calkyl; aromatic (aliphatic) ethers; C−C on hydroaromatic rings; hexagonal diamond carbon sp3; C−H on aromatic rings C−H on aromatic rings; benzene (ortho-disubstituted) ring

sp2

bands of samples are obtained and shown in Table1. LC and PVC have different spectral characteristics and functional groups. The organic compounds in the sample can be divided into three categories: aromatic, aliphatic, and oxygen functional groups.39 The spectral bands at 900−700 cm−1 represent the aromatic bending mode, and the 2800−3000 cm−1 spectral region represents the fat chain and the aromatic structure extension mode. The 1750−1000 cm−1 spectral band represents the oxygencontaining functional groups.38 Three out-of-plane C−H deformation bands are observed in the 900−700 cm−1 region for LC. These bands are assigned to aromatic structures with isolated aromatic hydrogens (871 cm−1), two adjacent hydrogens per ring (802 cm−1), and four adjacent aromatic hydrogens (750 cm−1).30 The absorbance is very small for PVC in the 900−700 cm−1 spectral band, indicating that benzene ring structure of PVC is less. The broad absorption band around 1620 cm−1 is assigned to the stretching vibration of CC bands in aromatic structure, and it can be concluded that strength of the LC peak is higher than that of PVC, which means LC has higher metamorphic grade and its carbon structure is more orderly. There are bands at 2932 and 2864 cm−1 in LC due to aliphatic −CH, −CH2, and −CH3 stretching vibration, assigned to aliphatic −CH bending vibration,30 while a clear absorption peak appeared at 2600− 3100 cm−1 in PVC, and it is caused by the bond of CH−Cl.25 The structures reflected by Raman and FTIR show a similar trend and are consistent with each other. On the basis of structural information provided by Raman and FTIR, different carbon rings constituted the carbon layers of LC, while a relative amount of functional groups hang off the carbon layers. The different carbon rings determined from Raman spectra are composed of six or more fusion benzene ring, half-quadrant benzene ring, methyl

represented by the different positions in the Raman spectra are listed in Table 4.37 The major vibration peaks in Raman spectra of PVC are at 1094 cm−1, 1178 cm−1, 1318 cm−1, and 1426 cm−1, and they are caused by the expansion and contraction of carbon atoms, the rocking vibration of carbon atoms and chlorine atoms, the asymmetric rocking vibration of carbon atoms and hydrogen atoms, and the bending vibration of carbon atoms and hydrogen atoms, respectively. It can be seen that PVC is mainly composed of long fatty acid chains. 3.1.4. FTIR Analysis. The functional groups of LC and PVC are determined by FTIR spectra in the region 500−4000 cm−1, which are depicted in Figure 4. The assignment of absorption

Figure 4. Normalized infrared spectra, 500−4000 cm−1, of LC and PVC. E

DOI: 10.1021/acs.energyfuels.7b03268 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 5. Fractional conversion curve and reaction rate curve of samples: (a) LC; (b) PVC.

blends at the ratio of 20/80 in second stages show two peaks, other blends only have one wide peak in the DTG curve for the second stage. This can be explained because when the ratio of PVC is low and there is low content of volatiles in the blends, the combustion of volatiles is not clear, and the process is contained by the combustion of char like content, the combustion of coal. It can be seen in DTG curves that with the increase of PVC, the peak of the first combustion stage is significantly increased, while the peak of the second stage is reduced, which is also due to the high content of CH−Cl matter in PVC and little content of residual char. The effect of heating rates of the curves for LC and PVC blends is similar. With the increased heat rate, TG curves moved into high temperature zone caused by hysteresis in high heating rate conditions. In order to seek the influences of PVC addition on combustion of blends, TG−DTG method was used to explore the combustion characteristic parameters of LC, PVC, and blends, and the parameters under different heating rates, 5 °C/min, 10 °C/min, and 20 °C/min, are shown in Table 5. It can be see that the values of Ti and Tf of PVC are lower than those of LC, while Ti and Tf of blends lie in the range between of those of PVC and LC. The lower Ti can be explained as PVC has high content lower bond energy of C−Cl that can be cracked in the low temperature range, which is beneficial for low value of Ti. With the increase of PVC content, Tf decreases mainly due to the simple structure of carbon residue. In the study of RoßteuscherCarl et al.,21 the lower bond energy C−Cl bond is cracked and CC bonds form in the main chain, which is the carbon residue of the PVC. Compared with carbon layers of the LC sample, the functional groups of carbon residue in PVC are small and disordered, which contributes to the low value of Tf. From the combustion rate data in the Table 5, it can be found that with increased PVC content, the maximum combustion rate dx/dtmax (s−1) first decreases and then increases with lowest value obtained at 40% of PVC content, and a similar relation can be found to mean combustion ratem dx/dtmean (s−1). As heating rate increased, starting weight loss temperature, Ti, burn out temperature, Tf, peak reaction rate, dx/dtmax (s−1), mean reaction rate, dx/dtmean (s−1), and combustion characteristic index, S, all increased. For the sample LC40%/PVC60%, as heating rate increased from 5 °C/min to 20 °C/min, combustion characteristic index S increased from 7.4 × 10−14 to 5.18 × 10−13. It can be concluded that combustion characteristics are improved by increase of heating rate. With the increase of PVC content, the combustion characteristic index S increases; this can be explained as PVC has low Ti and LC has high Tf, for separate combustion; they are

carbon to an aromatic ring, and benzene-related structure. The functional groups hanging off the carbon layers mainly contain aliphatic −CH, −CH2, or −CH3 detected with FTIR spectra of LC.37 While PVC is composed mainly by CH−Cl long chains So the graphitization degree of LC is higher than that of PVC. 3.2. Thermogravimetric Analysis. The combustion characteristics of PVC and LC were tested at three different heating rates. Fractional conversion (TG) curve and reaction rate (DTG) curve of LC are shown in Figure 5a. For the TG curve at a heating rate of 10 °C/min, as an example, due to low content of volatile substances in LC, the combustion process that occurred at 358−616 °C only shows one combustion stage of fixed carbon. And there is only one high and wide peak in the DTG curve. The TG curve and DTG curve of PVC are shown in Figure 5b. The combustion curve a heating rate of 10 °C/min of PVC can be divided into two stages: (I) peak in DTG curve is high and sharp between 219 and 347 °C with the weight loss of 58.3%, which corresponds to the dehydrochlorination process;25 (II) the DTG curve of PVC shows two peaks: one peak is low and flat between 347 and 512 °C with weight loss of 15.2%, which relates to the precipitation and combustion of volatiles, and one peak is higher occurring at 512−598 °C with weight loss of 26.5%, which indicates the combustion process of residual char in PVC. In addition, the combustion temperature of the samples in the following order: PVC < LC. Therefore, PVC is easier to burn than LC, while LC has higher thermostability with more fixed carbon. Comparing the combustion curves of LC and PVC at different heating rates, it is found that the initial temperature (Ti) and the burnout temperature (Tf) of the reaction gradually increase with the increase of the heating rate, and the TG curve shifts to high temperature area due to thermal hysteresis. Because combustion reactions of these samples are all endothermic, more time is needed to transform heat to low internal temperature for samples under high heating rates, which delay the diffusion rate, resulting in a significant tailing of the TG curve. The combustion characteristics of blends were tested at three different heating rates: Fractional conversion (TG) and reaction rate (DTG) curves are shown in Figure 6. From the TG curves at 5 °C/min heating rate, the burning process can be obviously divided into two stages occurring at 242−381 and 372−637 °C. In the first stage, the combustion process occurred, and the weight loss of LC/PVC blends are 15.05%, 24.12%, 41.07%, and 57.61% at ratios of 80/20, 60/40, 40/60, and 20/80, respectively. In the second stage, both decomposition and combustion of blends occurred, with weight loss of 84.95%, 75.88%, 58.93%, and 42.39%, respectively. While the DTG curve of LC/PVC F

DOI: 10.1021/acs.energyfuels.7b03268 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 6. Fractional conversion curve and reaction rate curve of different ratio of blends at three different rates.

Table 5. Combustion Characteristic Parameters of LC, PVC, and Blends at Different Heating Rates char LC

LC80%/PVC20%

LC60%/PVC40%

LC40%/PVC60%

LC80%/PVC20%

PVC

heating rate (°C/min) 5 10 20 5 10 20 5 10 20 5 10 20 5 10 20 5 10 20

Ti (°C) 398.7 418.9 441.1 282.6 296.7 305.3 268.6 281.1 292.5 267.1 278.3 302.8 242.1 251.9 261.6 254.1 255.7 258.9

Tm (°C)

Tf (°C)

503.9 532.7 559.3 495.5 519.6 543.8 482.8 506.9 523.8 452.3 483.7 499.1 323.2 322.9 357.3 307.5 319.7 331.9

563.9 595.0 637.4 553.6 593.5 634.8 550.9 587.7 630.4 552.7 588.4 619.5 552.0 581.3 613.6 525.4 555.8 588.6

dx/dtmax (s−1) −4

9.7 × 10 1.6 × 10−3 2.7 × 10−3 8.9 × 10−4 1.4 × 10−3 2.0 × 10−3 6.6 × 10−4 1.1 × 10−3 1.8 × 10−3 8.0 × 10−4 1.5 × 10−3 2.7 × 10−3 9.1 × 10−4 1.6 × 10−3 3.0 × 10−3 1.2 × 10−3 2.0 × 10−3 3.8 × 10−3

dx/dtmean (s−1) −4

3.0 × 10 6.2 × 10−4 1.0 × 10−3 2.6 × 10−4 5.2 × 10−4 8.2 × 10−4 3.1 × 10−4 5.1 × 10−4 8.1 × 10−4 3.6 × 10−4 7.2 × 10−4 1.1 × 10−3 4.2 × 10−4 8.2 × 10−4 1.4 × 10−3 5.0 × 10−4 9.7 × 10−4 1.9 × 10−3

S × 1013

tg (min)

0.3 1.0 2.2 0.5 1.4 2.8 0.6 1.8 3.4 0.7 2.3 5.2 1.2 3.6 9.8 1.8 5.2 18.1

33.0 17.6 9.8 54.2 29.7 16.6 55.7 30.7 16.9 57.1 31.0 15.8 62.0 32.9 17.6 54.3 30.0 16.5

indicates that PVC has abundant volatiles and little fixed carbon, while LC has much fixed carbon and a small amount of volatiles. In general, the volatile phase is easier to burn than fixed carbon, which can roughly explain that the flammability of PVC is better than that of LC. Through the particle size analysis and SEM analysis, combustibility can be explained from a microscopic perspective. It can be concluded that majority carbon in PVC is amorphous, while graphite carbon is present in LC. Raman analysis can reveal the functional groups of graphite carbon in LC

concentrated in low temperature region and high temperature region, respectively. However, when they are mixed together, the whole combustion process will be prolonged and intensity will be lowered. Similar phenomena were also found by Wang et al.30 in study of combustion of coal and biomass. 3.3. Correlation between Structural Characteristics and Combustibility. Thermogravimetric analysis shows that the flammability of PVC is better than that of LC, which is closely related to their structure. The results of proximate analysis G

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Energy & Fuels Table 6. Calculated Kinetic Parameters of Different Samples samples LC LC80%/PVC20% LC60%/PVC40% LC40%/PVC60% LC20%/PVC80% PVC

ε1 0 0.11 0.23 0.41 0.52 0.63

A1 (s−1) 0 8.77 × 103 3.66 × 103 3.28 × 103 1.71 × 104 1.39 × 105

E1 (kJ/mol) 0 93.4 84.6 64.7 43.9 50.7

ε2

A2 (s−1)

1 0.89 0.77 0.59 0.48 0.37

7.42 × 10 1.01 × 102 8.90 × 101 7.41 × 101 1.12 × 102 1.08 × 103 3

E2 (kJ/mol)

ψ1

ψ2

R2

91.6 84.8 71.3 55.8 31.4 40.4

0 0.01 0.02 0.04 0.07 0.08

0.02 0.05 0.11 0.17 0.23 0.36

0.9994 0.9994 0.9992 0.9993 0.9994 0.9993

example, the activation energy of the first stage is 64.7 kJ/mol, and it is 55.8 kJ/mol for the second stage. For the combustion of first stage, the evaporation of HCl runs at low temperatures, wherein the movement of the molecule is slow and the energy used to activate the reaction is small, Meanwhile, the interface reaction and diffusion is also differently affected by temperature. The higher the temperature, the more interface reaction and diffusion. Then, the first stage of the dehydrochlorination process produces a porous carbon structure that provides good kinetic conditions for air to diffuse to the surface, leading to lower activation energy in the second stage. The fact that activation energies in the high temperature range are smaller than those in low temperature is related to the study of Okasha et al.42 The combustion of the blends is divided into two stages, and for each stage, the activation energy first decreases and then increases with the increase of the PVC. When the proportion of PVC accounted for 80%, the activation energy of each stage achieves the smallest value (first stage 43.9 kJ/mol, second stage 31.4 kJ/mol). Normally the lower the activation energy is, the higher the flammability will be. However, it can be seen from the comprehensive combustion index S that the flammability has been reduced with increases of PVC ratio, which is not consistent with the law of activation energy. In fact, the flammability is affected by the activation energy and the pre-exponential factor. For the low activation energy conditions, the more activated molecules, the flammability is high in the same temperature range. The pre-exponential factor indicates the number of collisions between activated molecules, the greater the factor, the higher the flammability. For example, it can be seen from Table 6 that activation energy of PVC is higher than that of LC20%/ PVC80%. However, its flammability is also higher than that of LC20%/PVC80% due to its larger former exponential, which is named compensation effect in kinetic studies and is very common in the study of gasification of coal, biomass, and other carbon containing materials.30,42−45

and amorphous carbon in PVC. On the basis of the structural information provided by Raman analysis, it can be seen that the basic structural features of carbon in PVC and LC are quite different, with LC showing the existence of different carbon rings that constitute carbon layers and some aliphatic chains in the gap of carbon layers, and PVC being composed only by long chains. Carbon rings are difficult to burn relative to long chains due to their stability characteristics. So the flammability of PVC is better than that of LC. The relative amount of functional groups hanging off the carbon layers was detected with FTIR spectra, which was analyzed in detail in section 3.1.4. The combustion process of PVC and LC included random scission, depolymerization, and intramolecular transfer reaction.41 For the combustion of LC, aliphatic −CH, −CH2, and −CH3 was first burned and then generates much heat, which can promote the combustion of fixed carbon. The C−H bonds in LC are the weakest section in the main chain, where the decomposition of LC initiated, and then C−C bonds in the benzene ring break due to a large amount of heat afford by the combustion of hydrocarbons. By comparing the TG−DTG curves, we can further explain the excellent combustion performance of PVC. In the combustion process of PVC, HCl was first precipitated below 347 °C due to lower bond energy of C−Cl, whose characteristic peaks are at 3120−2520 cm−1, corresponding to the first stage of PVC combustion.25 At the second stage, −C−H could be detected in the FTIR spectrum; the bond of −C−H is cracked related to the precipitation of volatiles. Meanwhile, dehydrochlorinated PVC cracking formed double bonds in the main chain, and those can be cracked producing aromatic compounds, which is the carbon residue for PVC.41 From above, it can be seen that the combustion of PVC mainly consists of precipitation of HCl, combustion of volatiles, and residual char. Volatiles not only play a role in heating the residual char but also change the pore structure of the coke surface in the PVC, resulting in more voids on the surface. It is can be inferred that mixed combustion of LC and PVC has synergistic effects. Due to precipitation of volatiles of the PVC, the combustion of fixed carbon is promoted in LC and PVC, which can be confirmed by the following discussion of dynamics. 3.4. Kinetic Analysis. In order to investigate the effect of blending on the combustion characteristics of PVC and LC, a double parallel reactions random pore model (DPRM) was used to calculate the kinetic parameters of the burning process by nonlinear fitting, and the results shown in Table 6 and Figure7.show the experimental and calculated curves of weight loss temperature at different heating rates. It can be seen that the correlation coefficient, R2, is above 0.9992 for all samples with different ratio of LC and PVC, indicating that the DPRM model has the ability to accurately express the combustion process. From the activation energy shown in Table 6 for each sample, the activation energies for the first stage and second stage of the burning process is different. And it is higher in the first stage compared to the second stage. Taking LC40%/PVC60% as an

4. CONCLUSIONS The microstructure of LC and PVC showed most particles in LC tend to be irregular, smaller, sharped edged, and fluffy; while particles in PVC are larger, porous, and coarse. On the basis of structural information provided by Raman and FTIR, different carbon rings constituted the carbon layers of LC, while a relative amount of functional groups hang off the carbon layers. The different carbon rings determined from Raman spectra are composed of six or more fusion benzene ring, half-quadrant benzene ring, methyl carbon to an aromatic ring, and benzenerelated structure. The functional groups hanging off the carbon layers mainly contain aliphatic −CH, −CH2, and −CH3 detected with FTIR spectra of LC. PVC is composed mainly by CH−Cl long chains, which lead to the higher combustion reactivity of PVC than LC. Simultaneously, the kinetic parameters were obtained by applying DPRM model. For the two stages in combustion, the activation energy first decreases and then increases H

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Energy & Fuels

Figure 7. Experimental and calculated curves of weight loss temperature at different heating rates.

ORCID

with the increase of the PVC. When the proportion of PVC is 80%, the activation energy of each stage achieves the smallest value (first stage 43.9 kJ/mol, second stage 31.4 kJ/mol).

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Qi Wang: 0000-0002-6684-7250 Notes

The authors declare no competing financial interest.

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AUTHOR INFORMATION

ACKNOWLEDGMENTS This work was supported by the National Science Foundation for Young Scientists of China (51704019), Baosteel Group Co.,

Corresponding Author

*Tel: +8618811397101. E-mail address: [email protected] (G. Wang). I

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Energy & Fuels

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LTD, of Shanghai for the Key Joint Project (U1260202), Fundamental Research Funds for the Central Universities (FRFTP-15-063A1), and the National Basic Research Program of China (973 Program) (2012CB720401).

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