Article pubs.acs.org/EF
Pyrolysis Characteristics of Waste Tire in an Analytical Pyrolyzer Coupled with Gas Chromatography/Mass Spectrometry Kuan Ding, Zhaoping Zhong,* Bo Zhang, Zuwei Song, and Xiaoxiao Qian Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing, Jiangsu 210096, People’s Republic of China ABSTRACT: To investigate the pyrolysis characteristics of waste tire, an analytical pyrolyzer coupled with gas chromatography/ mass spectrometry (Py−GC/MS) setup had been proposed. Waste tire was pyrolyzed under different temperatures. Results showed that the primary pyrolysis products of waste tire at 600 °C were alkenes rather than alkanes or aromatics. Isoprene (18.7%) and D-limonene (22.9%) represented the main compounds of chain alkenes and cyclic alkenes, respectively. The degradation procedure of D-limonene was also investigated. It could be indicated that, when the temperature was 500 °C and below, isomerized alkenes of D-limonene were the main products. With the temperature increasing to 600 °C and above, aromatics began to raise. According to the product distribution, eight pyrolysis reaction pathways of D-limonene were proposed. Pyrolysis of waste tire under different temperatures proved that the reaction pathways of D-limonene were reliable. Moreover, a thermogravimetric analyzer coupled with Fourier transform infrared spectroscopy (TG-FTIR) investigation also consolidated the main processes proposed by Py−GC/MS. These findings provide some references for the pyrolysis mechanism of waste tires.
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characteristics of waste tire. Product distribution under 600 °C was analyzed. To understand further the pyrolytic mechanism of waste tire, one of the main products D-limonene was also pyrolyzed in Py−GC/MS. On the basis of the pyrolyzed products, the degradation pathways of D-limonene were put forward. Variation of the products from pyrolysis of waste tire under different temperatures was also analyzed to verify these pathways. Finally, a thermogravimetric analyzer coupled with Fourier transform infrared spectroscopy (TG−FTIR) study was put forward to verify the above theory proposed by the Py− GC/MS method. The purpose of this study is to provide some basic theories on release disciplines of primary pyrolytic products from waste tire. It is believed beneficial to understand the pyrolysis mechanism of waste tires, to promote efficient utilization of these resources.
INTRODUCTION Waste tires is an ideal material for thermochemical treatment because of their high volatile content, moderate sulfur content, low ash content, and high calorific value.1 As an efficient way to use recycled waste tires, pyrolysis has attracted much attention in recent years. This approach overcomes both energy recovery and emission reduction problems.2 In comparison to other thermochemical processes, such as combustion or gasification, pyrolysis could obtain solid, liquid, and gas products. All of them are of high utilization value. Besides, emissions of pollutant gases, such as polyaromatic hydrocarbons (PAHs), benzene, and phenol-like substances, are controlled during the pyrolysis process.3 Waste tires and derived fuels have been pyrolyzed in various reactors, such as a conical fixed-bed reactor,4 horizontal oven,5 batch reactor,6 fluidized bed,7 spouted bed,8 moving screw bed,1 and vacuum reactor,9 to obtain gas, solid, and liquid products. Liquid product, generally referred to as pyrolysis oil, is most promising to be used as a fuel or chemical material.10−13 It is generally recognized that pyrolysis oil consisted of multiple valuable hydrocarbons, including D-limonene, benzene, toluene, xylene, etc. Typical composition of pyrolysis oil from waste tire comprises 59.3−75.6% aromatics and 19.8−37.0% aliphatics (mainly alkanes) during the temperature range of 400−600 °C.14,15 On the basis of the composition of pyrolysis oil, pyrolysis mechanisms of waste tire have also been proposed. However, these mechanisms somehow consist of many secondary reactions. As such, an experimental method with fewer secondary reactions is expected to be developed. The analytical pyrolyzer coupled with gas chromatography/ mass spectrometry (Py−GC/MS) has been widely applied in the study of biomass pyrolysis.16−19 These applications prove that the Py−GC/MS reactor could suppress secondary reactions by sweeping pyrolyzed volatiles away from the reaction zone within a very short time. In this study, a Py−GC/ MS reactor was adopted to investigate the primary pyrolysis © 2015 American Chemical Society
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MATERIALS AND METHODS
Raw Materials. Waste tire powder with a particle size smaller than 100 mesh was chosen for the experiment. Raw material was dried at 105 °C for 24 h before use. Physical properties of the waste tire were shown in Table 1. Experimental Methods. Fast pyrolysis of waste tire was undertaken in a Py−GC/MS apparatus. Quality of raw material was strictly controlled within 0.50 ± 0.01 mg. The pyrolyzer model was CDS 5200 Pyroprobe, with a platinum filament to heat a micro quartztube reactor. Raw material was positioned in the middle of the quartz tube, with quartz wool fixed on both sides. Pyrolysis was carried out at a series of temperatures (500, 550, 600, 650, and 700 °C) for 20 s. Other operating conditions of the pyrolyzer were set as follows: heating rate of 20 °C/ms, heat preservation area temperature of 270 °C, valve box temperature of 275 °C, and transfer line temperature of 280 °C. Received: February 2, 2015 Revised: March 18, 2015 Published: March 20, 2015 3181
DOI: 10.1021/acs.energyfuels.5b00247 Energy Fuels 2015, 29, 3181−3187
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Energy & Fuels Table 1. Physical Analysis of Raw Material (on an Air-Dried Basis)a elemental analysis (%)
a
proximate analysis (%)
C
H
N
S
Ob
ash
volatile
moisture
FC
HHV (MJ/kg)
69.44
4.84
0.50
1.62
0.43
21.65
56.94
1.52
19.89
38.50
FC, fixed carbon; HHV, higher heating value. bBy difference.
D-Limonene, as one of the main pyrolytic products of waste tire, was also pyrolyzed in the analytical pyrolyzer. The temperature was set at 400, 500, 600, and 700 °C, with a pyrolysis time of 10 s. Because Dlimonene was a kind of liquid material and might volatilize easily before pyrolysis at a high temperature, a specific sample loading method was proposed, which is shown in Figure 1. A moderate
Figure 1. Schematic diagram of the method.
D-limonene
Aliphatic hydrocarbons account for nearly 70% of all of the products. All of them are C5−C10 alkenes. Among them, chain alkenes (33.32%) and cyclic alkenes (35.32%) share the similar ratio. Chain alkenes come from the depolymerization of unsaturated bonds in natural rubber (NR) and polybutadiene rubber (BR). Isoprene (number 3 in Table 2), as the most abundant chain alkene, is mainly formed by either β-scission of NR or the degradation of D-limonene. Cyclic alkenes could be formed in several ways: mainly from the degradation of NR and others from the decomposition of BR or the cyclization of butadiene through the Diels−Alder reaction. D-Limonene (number 26 in Table 2) is the most abundant compound of the cyclic alkenes, which makes up 22.9% of all of the detected products. It is generated from the disintegration of NR and could be easily transformed to other chain alkenes, cyclohexenes, and aromatics. The transformation procedure is quite complicated and will be discussed in detail later. For other cyclic alkenes, their formation mechanism from BR could be found in ref 21, except cyclopentadiene. However, it is clear that cyclopentene could be dehydrogenized to cyclopentadiene. Aromatic hydrocarbons only make up 13.1% of all of the products, including toluene, xylene, styrene, etc. All of them are monocyclic aromatic hydrocarbons (MAHs), with no polycyclic aromatic hydrocarbons (PAHs) detected. Most of the aromatics could be formed by transformation of D-limonene, while the others are developed through the Diels−Alder reaction. The former transformation approach will be considered in the next part, while the latter development method could be found elsewhere.14 Besides, high-molecularweight acids (including n-hexadecanoic and octadecanoic acids) are also found to be rich in the products. From the above depiction, product distribution in Py-GC/ MS is somewhat different from that of pyrolytic oil from waste tire. Williams22 summarized studies of the recent year on pyrolysis of waste tires and pointed out the main aliphatic compounds in oil are alkanes instead of alkenes. In addition, a higher proportion of aromatics in the pyrolysis oil of waste tire had been widely reported. These differences could be explained by the discrepancy of the analytical pyrolyzer and other reactors. In an analytical pyrolyzer, the heating rate could be as high as 20 000 °C/s and pyrolytic volatiles are swept away from the reaction zone within milliseconds. Other reactors are unlikely to have both such a high heating rate and such a transient volatile residence time as the analytical pyrolyzer. Therefore, the Py−GC/MS system is helpful to understand the primary pyrolysis mechanisms of the raw material. From this, it can be concluded that the primary pyrolysis products of waste tire are alkenes rather than alkanes or aromatics, in particular, isoprene and D-limonene. With a decrease of the heating rate and an increase of the gas residence time, the contents of alkanes and aromatics increase. Decomposition Procedure of D-Limonene. As mentioned above, D-limonene is one of the main compounds from primary pyrolysis of waste tire. It has been considered as one of the main decomposition products of NR, and the mechanism could be obtained from other references.9,23,24 It is believed
sample loading
amount of quartz sand (about 15 mg) fixed by quartz wool on both sides was filled in the quartz tube. Then, accurately controlled 0.5 μL of D-limonene was injected into the middle of quartz sand by a microinjector. Besides, the heat preservation area temperature of the pyrolyzer was programmed to heat from 20 to 150 °C at about 10 °C/ s to prevent D-limonene from evaporating. Using this method, the residence time of D-limonene at the high temperature zone was effectively extended. As a result, the volatile fraction of D-limonene was diminished. Pyrolyzed volatiles were led into GC/MS (Agilent 7890A/5975C) straight after pyrolysis. The injector port temperature was kept at 285 °C, with a split ratio set to 60:1. GC was performed using a HP-5 ms capillary column (30 m × 0.25 mm × 0.25 μm). Helium (99.999%) was used as a carrier gas at a constant flow rate of 1 mL/min. The oven temperature was programmed from 50 °C (kept for 1 min) to 290 °C (kept for 2 min) at a heating rate of 8 °C/min. The GC/MS interface temperature was held at 295 °C. The mass spectrometer was operated in electron ionization (EI) mode at 70 eV and obtained from m/z 35 to 550 amu. Chromatographic peaks were identified with reference to the National Institute of Standards and Technology (NIST) MS library. Thermogravimetric analysis (TGA, Setaram TG92) coupled with Fourier transform infrared spectroscopy (FTIR, Bruker Vector 22) was also adopted to confirm the conclusions proposed by the Py−GC/MS investigation. About 15 mg of waste tire was pyrolyzed in TGA up to 800 °C at the heating rate of 20 °C/min and under a nitrogen atmosphere. Pyrolysis products from TGA were scanned in situ by FTIR from 4000 to 600 cm−1 with a scanning interval of 4 s.
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RESULTS AND DISCUSSION Products from Pyrolysis of Waste Tire. Pyrolysis of waste tire was carried out at 600 °C for 20 s, with products listed in Table 2. Gaseous products include carbon dioxide and 2-butene, which sum up to 6.53% among all of the products. 2Butene is considered as gas because of its low boiling point (1 °C). Williams and Brindle20 investigated the pyrolysis of tires in a fixed-bed reactor and found that gases were composed of alkane gases (methane, ethane, butane, and isobutane), alkene gases (ethene, propene, butene, and butadiene), hydrogen, carbon monoxide, and carbon dioxide. However, most of these gases cannot be detected by the mass spectrometer in the GC/ MS analyzer because of their low molecular weight. 3182
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Table 2. Typical Pyrolysis Products from Waste Tire in Py−GC/MS (Reaction Temperature, 600 °C; Pyrolysis Time, 20 s)a number
RT (min)
compound
MF
RPA (%)
gases 1 2 aliphatic hydrocarbons 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 aromatic hydrocarbons 28 29 30 31 32 33 others 34 35 36 37 a
1.376 1.456
carbon dioxide 2-butene, (Z)-
CO2 C4H8
1.581 1.902 2.084 2.123 2.513 2.933 3.068 3.192 4.103 4.26 4.652 5.301 5.744 6.245 6.324 6.468 6.502 7.006 7.097 7.441 7.545 7.737 7.955 8.061 8.393
1,3-butadiene, 2-methyl2-pentene, 2-methyl1,3-cyclopentadiene, 5-methyl2,4-hexadiene, (Z,Z)1,3-pentadiene, 2,3-dimethyl1,3-cycloheptadiene cyclopentene, 1,5-dimethyl1-methylcyclohexa-2,4-diene 1,3-dimethyl-1-cyclohexene cyclohexene, 4-ethenyl1,3-cyclohexadiene, 5,6-dimethyl1,6-dimethylhepta-1,3,5-triene 2,4,6-octatriene, 2,6-dimethyl1,5-heptadiene, 2,3,6-trimethylcamphene 1,3-cyclopentadiene, 5,5-dimethyl-1-propyl1,3,7-octatriene, 2,7-dimethylcyclohexene, 1-methyl-4-(1-methylethenyl)2,4,6-octatriene, 2,6-dimethyl2,6-octadiene, 2,6-dimethyl3-carene 1,5-heptadiene, 2,5-dimethyl-3-methylene1,3,8-p-menthatriene D-limonene 1,3,7-octatriene, 3,7-dimethyl-
C5H8 C6H12 C6H8 C6H10 C7H12 C7H10 C7H12 C7H10 C8H14 C8H12 C8H12 C9H14 C10H16 C10H18 C10H16 C10H16 C10H16 C10H16 C10H16 C10H18 C10H16 C10H16 C10H14 C10H16 C10H16
3.262 4.906 5.347 6.704 7.342 9.243
toluene benzene, styrene benzene, benzene, benzene,
C7H8 C8H10 C8H8 C9H12 C9H12 C10H12
15.564 22.652 24.94
1,3-dimethyl 1-ethyl-3-methyl1,2,3-trimethyl1-methyl-4-(1-methylethenyl)-
quinoline, 1,2-dihydro-2,2,4-trimethyln-hexadecanoic acid octadecanoic acid undetermined
C12H15N C16H32O2 C18H36O2
6.533 0.436 6.097 68.634 18.703 1.507 2.609 1.400 0.463 0.951 1.074 0.395 0.723 0.710 0.551 0.503 0.592 0.720 0.732 0.412 0.678 0.714 2.139 3.934 1.134 2.237 2.402 22.911 0.440 13.107 3.032 4.583 1.395 1.548 1.359 1.190 11.727 1.026 4.203 2.032 4.466
RT, residence time; MF, molecular formula; MW, molecular weight; and RPA, relative peak area.
degradation of D-limonene. Their mutual transformation rules are discussed as follows. At the temperature of 400 and 500 °C, only four products are detected by GC/MS, including cyclohexenes and alkatriene. The yield of each compound increases while the temperature rises from 400 to 500 °C, yet the selectivity (relative peak area) remains steady. None of aromatics has been measured under these relatively low temperatures. The degradation mechanism of D-limonene is proposed in Figure 2. When the temperature is lower than 500 °C, isomerization dominates the main reaction mechanisms. That makes the degradation process of Dlimonene monotonous. A detailed reaction pathway in this case could be described as follows. D-Limonene (a) isomerizes first to cyclohexenes (XIII, i, and g). Then, cyclohexene (i) continues to isomerize to VIII and XI separately. Meanwhile, cyclohexene (g) takes an internal cleavage of the carbon− carbon bond at the allylic position, giving rise to a biradical
that D-limonene would be further decomposed under a relatively high temperature. The pyrolysis mechanisms of Dlimonene had been studied in the past,25 whereas these mechanisms were not complete and were not connected with the temperature tightly. Therefore, experiments were designed to pyrolyze D-limonene in a Py−GC/MS reactor to investigate the pyrolysis discipline of this compound. Product distribution is shown in Table 3. Because the amount of raw material was strictly controlled, the GC/MS peak area could qualitatively represent the yield of products. On the other hand, the relative peak area represents the percentage of one compound in all of the detected products; therefore, product selectivity could be expressed by the relative peak area. The main pyrolysis products of D-limonene include cyclohexenes, cyclohexadienes, octatrienes, and benzenes. When the temperature rises, the total yield (peak area) of all of the products increases gradually. This indicates that a higher temperature would promote the 3183
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1.81 1.53 14.3 0.75 6.11 5.55 2.31 48.18 4.7 2.39 6.55 2.69 1.81 1.31 100
diene (h). Next, by intramolecular hydrogen transfer, alkatriene (XII) is obtained. This cleavage−hydrogen-transfer pathway is undertaken at a lower temperature, which should be distinguished from the following discussed similar pathway at a higher temperature. The content of cyclohexene (VIII) has absolute superiority among all of the products, which means that the reaction pathway of a → i → VIII is highly competitive. When the temperature rises to 600 °C, benzenes and smallmolecular-weight compounds begin to generate and turn the transformation pathways of D-limonene into bloom. Their formation mechanism starts from a high-temperature internal cleavage of the C−C bond at the allylic position of D-limonene (a), producing biradical diene (b). If biradical diene (b) undertakes allylic rearrangement [isomer biradical diene (f) comes into being], followed by β-scission, isoprene (III) is formed. Meanwhile, the biradical diene (b) would also transform into alkatriene (c) through intramolecular hydrogen transformation. Afterward, alkatriene (c) could be converted in two ways. The first way is isomerizing to a conjugated alkatriene (d), followed by an intramolecular diene synthesis, producing cyclohexadiene (e). Then, the loss of ethane leads to the formation of xylene (VI). At last, toluene (V) is generated by the demethylation of xylene (VI). The second way is similarly to the first way yet without aromatization. Alkatriene (c) isomerizes to another alkatriene (IX) and next cyclizes to cyclohexadiene (XIV). However, aromatization of XIV would not occur until the temperature rises to a higher level. Both yield and selectivity of VIII and XII decrease in comparison to the temperature lower than 500 °C, which signifies that isomerization reactions have been restrained. With a higher temperature at 700 °C, more aromatics appear and the transformation mechanism of D-limonene has been further enriched. Cyclohexadiene (e) aromatizes to ethyltoluene (VII) by the loss of methane. Analogously, cyclohexadiene (XIV) also converts to trimethylbenzene (X) by the loss of methane. These reactions exist only at a higher temperature, which implies that their activation energy is relatively high. From Figure 2, the following reaction pathways could be summarized: pathway 1, a → XIII; pathway 2, a → i → VIII; pathway 3, a → i → XI; pathway 4, a → g → h → XII; pathway 5, a → b ↔ f → III; pathway 6, a → b → c → d → e → VI → V; pathway 7, a → b → c → d → e → VII; and pathway 8, a → b → c → IX → XIV → X. The first four pathways exist in the entire temperature range, while the rest of pathways only proceed when the temperature is higher than 600 °C. Influence of the Temperature. Waste tire was pyrolyzed at 500, 550, 600, 650, and 700 °C to examine the proposed pyrolysis mechanism. The product distribution is shown in Figure 3. With increasing the temperature, the total yield of pyrolyzed products shows an overall increasing trend. On one hand, the yield of chain alkenes increases obviously with increasing the temperature from 500 to 550 °C. When the temperature continues to rise from 550 to 700 °C, the yield of chain alkenes keeps ascending mildly. On the other hand, selectivity of chain alkenes increases before the temperature increases to 600 °C and decreases at higher temperatures. In addition, the yield and selectivity of cyclic alkenes change slightly with the temperature. Therefore, the yield and selectivity of alkenes reach an optimization at 600 °C. As one of the main alkene compounds, D-limonene decreases in both yield and selectivity with increasing the temperature. At temperatures lower than 600 °C, D-limonene transforms to
× × × × × × × × × × × × × × × 109 108 108 108 109 × × × × ×
× 109 × 108
× 108 × 108
1.42 0 7.24 0 8.45 3.54 0 39.2 7.61 0 20.22 8.9 2.14 1.29 100 a
× 109
× 109 × 108 × 108
× 109
× 109
× 108 × 108 × 107
× 109
1.381 1.422 1.552 1.84 3.199 4.834 6.622 6.745 7.045 7.292 7.469 8.363 8.775 9.886 I II III IV V VI VII VIII IX X XI XII XIII XIV total
The peak area of volatized D-limonene was neglected. RT, residence time; MF, molecular formula; PA, peak area; and RPA, relative peak area.
1.14 0 5.82 0 6.80 2.85 0 3.15 6.12 0 1.63 7.16 1.72 1.04 8.04
× 108
RPA
× 108
RPA
0 0 0 0 0 0 0 71.26 0 0 14.47 11.87 2.4 0 100 0 0 0 0 0 0 0 5.18 0 0 1.05 8.62 1.74 0 7.26
PA RPA
0 0 0 0 0 0 0 71.43 0 0 13.94 12.93 1.7 0 100 0 0 0 0 0 0 0 3.97 0 0 7.75 7.19 9.47 0 5.56
PA MF RT (min) number
name
C3H6 C4H8 C5H8 C6H12 C7H8 C8H10 C9H12 C10H16 C10H16 C9H12 C10H16 C10H16 C10H16 C10H16
500 °C 400 °C
Table 3. Pyrolysis Products from D-Limonene under Different Temperaturesa
propene 2-butene 1,3-butadiene, 2-methyl2-pentene, 3-methyltoluene benzene, 1,3-dimethylbenzene, 1-ethyl-3-methylcyclohexene, 1-methyl-4-(1-methylethylidene)2,4,6-octatriene, 2,6-dimethylbenzene, 1,2,3-trimethyl1,4-cyclohexadiene, 1-methyl-4-(1-methylethyl)1,3,7-octatriene, 3,7-dimethylcyclohexene, 3-methyl-6-(1-methylethenyl)1,3-cyclohexadiene, 1,5,5,6-tetramethyl-
600 °C PA
2.17 1.84 1.72 8.98 7.35 6.68 2.78 5.79 5.66 2.87 7.88 3.24 2.18 1.58 1.20
PA
108 108 109 107 108 108 108 109 108 108 108 108 108 108 1010
700 °C
RPA
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Figure 2. Schematic diagram of the decomposition process of D-limonene.
Figure 3. Pyrolysis products from waste tire under different temperatures (pyrolysis time of 20 s). MAH, monocyclic aromatic hydrocarbon; BAH, bicyclic aromatic hydrocarbon.
Figure 4. TG−FTIR curves of waste tire pyrolysis: (A) thermogravimetry (TG)−differential thermogravimetry (DTG) curves and (B) FTIR curves under different temperatures. Waste tire was pyrolyzed in TGA under a nitrogen atmosphere and heating to 800 °C at the heating rate of 20 °C/min. SV, sketching vibration; BV, bending vibration.
the alkene yield. When the temperature reaches 600 °C and above, a portion of D-limonene converts to aromatics. Reaction
other alkenes through pathways 1−4. Consequently, the species of alkenes increase, which is responsible for the increment of 3185
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peaks are found at 442 °C. It confirms that CO2 releases at a relatively high temperature. From the analysis above, the pyrolysis process of waste tire in TGA could be summarized as follows. First, alkenes, such as isoprene and D-limonene, release from degradation of rubbers at a relatively low temperature. Then, primary released alkenes undertake a series of reactions, including ring-opening and recyclization reactions at a higher temperature, generating a number of alkane groups. This process happens to be analogical with the decomposition procedure of D-limonene in the Py, which is proposed in the preceding part.
pathways 6−8 are promoted. As a result, both yield and selectivity of monocyclic aromatics, such as toluene, xylene, and trimethylbenzene, increase obviously. These disciplines are consistent with the degradation procedure of D-limonene. Besides, it is worth noticing that bicyclic aromatics appear when the temperature reaches 650 °C and above, including indenes and naphthalenes. Consequently, from the perspective of promoting the production of alkenes and MAH and restraining the yield of PAH, 600 °C would be an appropriate temperature in the Py−GC/MS reactor. TG−FTIR Investigation of Waste Tire Pyrolysis. TG− FTIR curves from pyrolysis of waste tire are shown in Figure 4. Prior to discussion, some crucial differences between TGA and the Py should be emphasized. First, the heating rate of TGA was much lower than that of the Py. In other words, TGA represented slow pyrolysis, while the Py represented flash pyrolysis. Second, the gap between the apparatus set temperature and raw material actual temperature in the two reactors was not close. For example, a temperature lag of about 100 °C in the Py was noted because of the high heating rate and the poor thermal conductivity of raw material.26 However, the temperature lag in TGA was not obvious. Moreover, pyrolysis products in TGA were swept into FTIR continuously along with the raise of the temperature. Whereas in the Py, pyrolysis products were led into GC/MS one time after reaction. In conclusion, pyrolysis products in TGA and the Py at the same temperature could not be compared directly. However, principles proposed by the TG−FTIR method could supplement and confirm that studied by Py−GC/MS. The main pyrolysis stage of waste tire in TGA lies between 197 and 512 °C, with the DTG (differential thermogravimetry) peak temperature of 387 °C. The DTG peak is overlapped by several single peaks. Therefore, the Gauss peak fit method was employed to separate these single peaks. As seen in Figure 4A, three independent peaks have been obtained, lying in the temperature range of 197−506, 316−456, and 371−512 °C. Their corresponding peak temperatures are 352, 387, and 442 °C. Figure 4B shows the FTIR data at the three temperatures. Peaks at 880−995 and 3040−3100 cm−1 represent the bending vibration and stretching vibration of CH and CH2, respectively. These peaks indicate the presence of alkenes. As the temperature increases from 352 to 442 °C, vibration of the alkene functional groups becomes weaker and weaker. It proves that the DTG-fitted curve at 352 °C mainly stands for liberation of alkenes, mainly isoprene and D-limonene. Peaks at 2850−3000 cm−1 represent the stretching vibration of CH3, CH2, and CH groups. Meanwhile, peaks at 1350−1470 cm−1 represent the bending vibration of CH3 and CH2 deformation. These peaks become strongest at 387 °C. Some of the cyclic olefins (primarily D-limonene) undertake ring-opening and recyclization reactions, generating many alkane groups. This corresponds to pyrolysis pathways of D-limonene at 600 °C and above, which consist of a number of ring-opening and recyclization reactions. Peaks around 3013 and 1508 cm−1 represent the stretching vibration of C−H and CC (in the ring), respectively. These peaks represent aromatic rings. The former peak is covered by a strong vibration peak of alkane groups, and the later peak is very weak. Therefore, it can be figured that only a small amount of aromatic compounds are produced. This feature could also be found in Table 2, meaning that aromatics only make up 13% among all of the products from pyrolysis of waste tire at 600 °C in the Py. Peaks around 2349 cm−1 are characteristic peaks of CO2, and the highest
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CONCLUSION The high heating rate and transient volatile residence time make Py-GC/MS system beneficial for investigation of primary pyrolysis mechanisms of the raw material. In comparison to pyrolysis oil, the primary pyrolytic products of waste tire in the Py were alkenes rather than alkanes or aromatics. Isoprene and D-limonene represented the main chain alkenes and cyclic alkenes, respectively. Both of them came mainly from the degradation of natural rubber. According to the degrading products of D-limonene, eight reaction pathways had been proposed. It was indicated that, under a low temperature (≤500 °C), the isomerization reaction dominated the conversion process. When the temperature rose to 600 °C and above, reactions, such as internal cleavage of the C−C bond, intermolecular hydrogen transfer, and cyclization to aromatics, became active. Consequently, more aromatics were generated at a higher temperature. Waste tire were pyrolyzed under different temperatures in the Py. Results showed that a higher temperature would increase the yield of products. The yield and selectivity of alkenes achieved the optimal at 600 °C. Degradation pathways 6−8 of D-limonene were promoted when the temperature rose to 600 °C and above. Moreover, bicyclic aromatics, such as indenes and naphthalenes, were also detected when the temperature was higher than 650 °C. The TG−FTIR experiment exhibited that waste tire went through ring-opening and re-cyclization reactions as the temperature exceeded a relatively high degree. These reactions were the most important reactions in the waste tire pyrolysis process. It certified that the proposed pyrolysis pathways of Dlimonene in the Py were reasonable. This study provides some basic research findings on the pyrolysis mechanism of waste tires. It is believed that these findings could make some contributions to recycling and utilization of waste tires more efficiently.
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AUTHOR INFORMATION
Corresponding Author
*Telephone/Fax: +86-25-83794700. E-mail:
[email protected]. cn. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This paper was sponsored by the National Key Basic Research Program of China (973 Program, 2011CB201505).
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NOMENCLATURE PAH = polycyclic aromatic hydrocarbon DOI: 10.1021/acs.energyfuels.5b00247 Energy Fuels 2015, 29, 3181−3187
Article
Energy & Fuels
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Py−GC/MS = analytical pyrolyzer coupled with gas chromatography/mass spectrometry TG−FTIR = thermogravimetric analyzer coupled with Fourier transform infrared spectroscopy FC = fixed carbon HHV = higher heating value EI = electron ionization NIST = National Institute of Standards and Technology TGA = thermogravimetric analysis NR = natural rubber BR = polybutadiene rubber MAH = monocyclic aromatic hydrocarbon RT = residence time MF = molecular formula MW = molecular weight RPA = relative peak area PA = peak area BAH = bicyclic aromatic hydrocarbon DTG = differential thermogravimetry SV = sketching vibration BV = bending vibration
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DOI: 10.1021/acs.energyfuels.5b00247 Energy Fuels 2015, 29, 3181−3187
