Py–FTIR–GC/MS Analysis of Volatile Products of Automobile Shredder Residue Pyrolysis

Automobile shredder residue (ASR) pyrolysis produces solid, liquid, and gaseous products, particularly pyrolysis oil and gas, which could be used as renewable alternative energy resources. Due to the primary pyrolysis reaction not being complete, the yield of gaseous product is low. The pyrolysis tar comprises chemically unstable volatiles before condensing into liquid. Understanding the characteristics of volatile products will aid the design and improvement of subsequent processes. In order to accurately analyze the chemical characteristics and yields of volatile products of ASR primary pyrolysis, TG–FTIR–GC/MS analysis technology was used. According to the analysis results of the Gram–Schmidt profiles, the 3D stack plots, and GC/MS chromatograms of MixASR, ASR, and its main components, the major pyrolytic products of ASR included alkanes, olefins, and alcohols, and both had dense and indistinguishable weak peaks in the wavenumber range of 1900–1400 cm−1. Many of these products have unstable or weaker chemical bonds, such as =CH–, =CH2, –C=C–, and –C=CH2. Hence, more syngas with higher heating values can be obtained with further catalytic pyrolysis gasification, steam gasification, or higher temperature pyrolysis.


Introduction
The dismantling and recycling of end-of-life vehicles (ELVs) is not only an important way to save resources and realize sustainable development but also bears huge social and environmental responsibility. As of the end of 2019, China's car ownership exceeded 260 million. According to the international scrap ratio of 4-6%, the quantity of ELVs in China will be large in the future. Automobile shredder residue (ASR) accounts for about 20-25% of a vehicle's weight [1][2][3][4][5][6] and consists of plastic, textile, leather, rubber, foam, wood, paper, glass, sand, metals, and other materials [3][4][5][6][7][8].
Organic components account for about 60-85% of ASR [9,10], for which thermal conversion technology represents a viable resource recovery process. Pyrolysis and gasification have gradually become the main means to dispose of ASR, since they can reduce the volume and quality of landfill with lower cost, while energy recovery can be carried out simultaneously [11][12][13]. ASR pyrolysis produces solid, liquid, and gaseous products, particularly pyrolysis oil and gas, which could be used as renewable alternative energy resources.
The yields and characteristics of ASR pyrolytic products depend on the feed (e.g., fractions of organic versus inorganic), temperature, residence time, and carrier gases used [12]. There is a great range of variations possible, and the values used in a given process design must be chosen depending on which products are the process targets. Harder and Forton analyzed the variations in the physical characteristics of ASR, provided a review of the current technical developments in pyrolysis processes, and emphasized that the energy content of ASR is crucial to the design of a thermal process [14]. All specimens were combined and mixed well to ensure a representative sample for the determination, and the uniformity of all specimens regarding the particle size (approximately ≤0.1 mm) was achieved. The reaction mechanism of ASR was unknown, so keeping the initial mass of each sample at a fixed constant for all measurements was safe [35]. Thus, the mass of all samples for the TG experiments was 100 ± 0.1 mg, considering the size of the sample pan and the accuracy of balance in the thermogravimetric analyzer.

Experimental Procedure
The TG-FTIR-GC/MS coupling technique is widely used in the analysis of organic pyrolysis behaviors. Therefore, we studied the pyrosis behaviors of ASR and its five main components with the TG-FTIR-GC/MS coupling technique. As shown in Figure 1, GC/MS (Perkin-Elmer SQ 8, Waltham, MA, USA) and FTIR (Perkin-Elmer TL 9000, Waltham, MA, USA) equipment was connected with a TG analyzer (Perkin-Elmer TGA 8000, Waltham, MA, USA), and all transmission lines were heated and maintained at 270 • C to prevent the condensation of any volatile disintegration products. Pyris-Manager software was used to analyze the TG and derivate TG (DTG) data, TimeBase Version 3.1.0 (Perkin-Elmer, Waltham, MA, USA) and Spectrum Version 10.5.3 (Perkin-Elmer, Waltham, MA, USA) were applied for the analysis of FTIR spectra data, and the mass spectra data were analyzed using TurboMass Ver 6.0.0 (Perkin-Elmer, Waltham, MA, USA).
The reaction evolution of plastic, textiles, rubber, leather, foam, ASR, and MixASR was recorded for a full range of temperatures from 30 to 800 • C at a constant heating rate of 10 • C/min under atmospheric control with high-purity (99.99%) nitrogen at a rate of 20.0 mL/min. The pressures of pneumatic gas and balance gas were set to 0.1 and 0.2 MPa, respectively. This nitrogen flow rate ensures an inert atmosphere for the sample during the run, whereas a small sample and slow heating rate ensures that the heat transfer limitations can be ignored. Gaseous products from the TG analyzer were directly collected in the gas cell and determined instantaneously using the Fourier transform infrared spectrometer. The resolution in FTIR was 4 cm −1 , and the scan range was from 4000 to 450 cm −1 . The FTIR spectra of the gaseous products were obtained continuously with the baseline modified. After the FTIR measurements, the gas products were instantaneously swept to the mass spectrometer. The oven temperature was heated and maintained at 270 • C, and the ionization energy under the electron-impact (EI) of gas volatiles was analyzed at 70 eV. The mass-to-ion ratio (m/z) ranged from 45 to 400 and was scanned 200 times per minute.
The TG-FTIR-GC/MS coupling technique is widely used in the analysis of organic pyrolysis behaviors. Therefore, we studied the pyrosis behaviors of ASR and its five main components with the TG-FTIR-GC/MS coupling technique. As shown in Figure 1, GC/MS (Perkin-Elmer SQ 8, Waltham, MA, USA) and FTIR (Perkin-Elmer TL 9000, Waltham, MA, USA) equipment was connected with a TG analyzer (Perkin-Elmer TGA 8000, Waltham, MA, USA), and all transmission lines were heated and maintained at 270 °C to prevent the condensation of any volatile disintegration products. Pyris-Manager software was used to analyze the TG and derivate TG (DTG) data, TimeBase Version 3.1.0 (Perkin-Elmer, Waltham, MA, USA) and Spectrum Version 10.5.3 (Perkin-Elmer, Waltham, MA, USA) were applied for the analysis of FTIR spectra data, and the mass spectra data were analyzed using TurboMass Ver 6.0.0 (Perkin-Elmer, Waltham, MA, USA).

FTIR Analysis of Volatile Products
FTIR is an effective analytical instrument for detecting functional groups and characterizing covalent bonding information. In this study, the Fourier transform infrared spectra of pyrolytic product of plastic, textile, rubber, leather, foam, ASR, and MixASR were recorded to certify the basic chemical group of thermal decomposition product. As shown in Figure 2, Gram-Schmidt profiles of plastic, textile, MixASR, and ASR had only one significant peak, while the Gram-Schmidt profiles of rubber, leather, and foam had two peaks. Below 300 • C, the pyrolysis reactions of ASR, MixASR, or its main components were slow or nonexistent. Over 300 • C, the pyrolysis reactions of rubber, leather, and foam were rapid; the first peak appeared at 346, 322, and 371 • C, respectively, while the second peak appeared at 536, 533, and 452 • C, respectively. Pyrolysis reactions of plastic and textile mainly occurred in the temperature range of 400 to 600 • C, and the only peak appeared at 525 and 515 • C, respectively. Reaching 620 • C, the pyrolysis reaction of plastic, rubber, leather, textile and foam was basically completed, and the curve dropped sharply. The pyrolysis reaction of MixASR mainly occurred in the range of 400-600 • C, and the peak temperature was 526 • C. In addition to the five major organic components, ASR contains 24.6% of other substances in this study, so Gram-Schmidt profiles of ASR and MixASR were very different, but peaks both appeared at 526 • C. The reason is that plastics and fibers accounted for a high proportion of ASR and MixASR, which were the main substances affecting pyrolytic products.
According to the 3D stack plots of MixASR and ASR in Figure 3, the wavenumber ranges of absorption peaks are different. The main absorption peaks in the 3D stack plots of plastic and textiles also appear in the same wavenumber ranges in the 3D stack plots of ASR and MixASR. However, the intensity of the absorption peak at a certain position of MixASR or ASR is not the linear superposition result of the intensities of the absorption peaks at the same position of the five components. In short, the pyrolysis product of ASR is not a simple superposition of the pyrolysis products of its components, but the pyrolysis characteristics of the main components have the greatest influence on the product distribution, especially plastic and textiles. basically completed, and the curve dropped sharply. The pyrolysis reaction of MixASR mainly occurred in the range of 400-600 °C, and the peak temperature was 526 °C. In addition to the five major organic components, ASR contains 24.6% of other substances in this study, so Gram-Schmidt profiles of ASR and MixASR were very different, but peaks both appeared at 526 °C. The reason is that plastics and fibers accounted for a high proportion of ASR and MixASR, which were the main substances affecting pyrolytic products.  According to the 3D stack plots of MixASR and ASR in Figure 3, the wavenumber ranges of absorption peaks are different. The main absorption peaks in the 3D stack plots of plastic and textiles also appear in the same wavenumber ranges in the 3D stack plots of ASR and MixASR. However, the intensity of the absorption peak at a certain position of MixASR or ASR is not the linear superposition result of the intensities of the absorption peaks at the same position of the five components. In short, the pyrolysis product of ASR is not a simple superposition of the pyrolysis products of its components, but the pyrolysis characteristics of the main components have the greatest influence on the product distribution, especially plastic and textiles.   With TimeBase and Spectrum software (Perkin-Elmer, Waltham, MA, USA), the Gram-Schmidt profiles of samples were analyzed, and the FTIR spectra of ASR, MixASR, its main components at 200, 300, 400, 500, 600, 700 • C, and temperatures of peaks of Gram-Schmidt profiles are shown in Figures 4 and 5. These functional groups are useful for distinguishing some specific chemical compounds, like CO 2 , alkanes, olefins, benzene, alcohols, and other organic components. According to Figures 4a-e and 5, it can be observed that the locations of the absorption bands were almost the same for the same sample, but the absorbance intensities were different After pyrolysis, the temperature was higher than 300 • C. This indicated that the major categories of volatile products were not affected by the pyrolysis temperature, but the yield of each main product was still affected by the pyrolysis temperature. Strong absorption peaks appeared at 3100-2700, 1700-1550, and 1500-1250 cm −1 for ASR and its main components, which were assigned to the stretching of =CH 2 , -CH 3 , -CH 2 , -CH-, and C=C bonds. Furthermore, the absorption intensity of the corresponding macromolecules such as alkanes, aldehydes, and alcohols increased with pyrolysis temperatures and increased sharply at the Polymers 2020, 12, 2734 7 of 17 peak temperatures of the Gram-Schmidt profiles, then sharply decreased. The bands of stretching of =CH-, stretching of -C=C-, and plane -CH-bending -C=CH 2 were assigned to the alkenes. The strong absorption peaks of 2930, 2850, and 1460 cm −1 certify the existence of the methylene (-CH 2 -) group. The formation mechanisms of-CH-and C=C bonds were cleavage of alkyl side chains and β bond scission of alkenes. CO 2 is represented by the peak between 2395 and 2250 cm −1 for ASR and its main components, and the production of CO 2 increased with temperature, peaked at the first pyrolysis peak temperature, and then decreased sharply. After exceeding 500 • C, peaks appeared between 720-570 and 1050-1150 cm −1 , which were assigned to the symmetric stretching of C-O. Meanwhile, the CO at 2109-2173 cm −1 can also be detected in the entire pyrolysis process. In addition to these substances, each component of ASR had relatively independent functional groups, but which eventually appeared in the mixture FTIR spectrum.
peaked at the first pyrolysis peak temperature, and then decreased sharply. After exceeding 500 °C, peaks appeared between 720-570 and 1050-1150 cm −1 , which were assigned to the symmetric stretching of C-O. Meanwhile, the CO at 2109-2173 cm −1 can also be detected in the entire pyrolysis process. In addition to these substances, each component of ASR had relatively independent functional groups, but which eventually appeared in the mixture FTIR spectrum.
The major pyrolytic products of ASR and MixASR were much the same, which included alkanes, olefins, and alcohols, and both had dense and indistinguishable weak peaks in the wavenumber range of 1900-1400 cm −1 . Obviously, many of these products have unstable or weaker chemical bonds, which can be further pyrolyzed under further high-temperature (>800 °C) or catalytic pyrolysis.  For plastic, the major pyrolytic products were alkanes and olefins, such as styrene, 3-ethyl-3-methyl heptane, and 2,4-dimethyl-1heptene, since the bonds of stretching =CH2, -CH 3 , -CH 2 , -CH-and bending RHC=CH 2 were assigned. The major pyrolytic products of textiles were CO 2 , alkanes, olefins, benzene, and alcohols, e.g., 2,6-dimethylnonane, styrene, and 2-methyl-1-pentene. Except for CO 2 , alkanes, olefins, and benzene series, the major pyrolytic products of rubber and leather also included alcohols, since the bands of stretching of alcoholic C-OH were assigned to the alcohols, e.g., 1-phenyl-2-propanol. The major pyrolytic products of foam were different; ketones were identified by the bonds of stretching keto C=O, e.g., 1-isopropoxy-propan-2-one. Compared with other components, there are more alkanes and alcohols in foam pyrolysis products.
The major pyrolytic products of ASR and MixASR were much the same, which included alkanes, olefins, and alcohols, and both had dense and indistinguishable weak peaks in the wavenumber range of 1900-1400 cm −1 . Obviously, many of these products have unstable or weaker chemical bonds, which can be further pyrolyzed under further high-temperature (>800 • C) or catalytic pyrolysis.   MixASR Figure 5. The FTIR spectrum of ASR and MixASR at different pyrolysis temperatures.

GC/MS Analysis of Volatile Products of ASR Pyrolysis
In the accurate identification of complicated volatile species, MS analysis is a useful complementary technique for TG-FTIR because TG-FTIR cannot measure homodiatomic species. In order to further accurately analyze the compound types and yields of volatile products of ASR pyrolysis, GC-MS analysis technology was used. The detectable compound was that with a

GC/MS Analysis of Volatile Products of ASR Pyrolysis
In the accurate identification of complicated volatile species, MS analysis is a useful complementary technique for TG-FTIR because TG-FTIR cannot measure homodiatomic species. In order to further accurately analyze the compound types and yields of volatile products of ASR pyrolysis, GC-MS analysis technology was used. The detectable compound was that with a concentration above the detection limit of GC-MS, while the identifiable compound by the NIST library was that with a comparatively large peak area. Since there is no distinguishable peak after 15 min, the chromatogram for the range from 0 to 15 min is shown in Figure 6. Moreover, during the identification of chemical compounds of volatile products, the radicals and functional groups evolved at peaks were based on the results of FTIR analysis. The identification results including compound names and yields of volatile pyrolytic products of ASR and its main components were summarized and are provided in Tables 2  and 3, and Appendix A. According to the GC-MS analysis results, several points were concluded as follows: (1) The identified gaseous pyrolytic products were both composed of alkanes, olefins, alcohols, and benzene series, which were consistent with the analysis results of FTIR. The gaseous pyrolytic products of MixASR and ASR were not the simple superposition of pyrolytic gaseous products of its components, but main compounds of MixASR and ASR also appeared in the pyrolytic products of its main components, especially plastic and textile, e.g., styrene, 1-hexene, toluene, ethylbenzene, 2,4-dimethyl-1heptene, and 11-methyl-1-dodecanol.  Tables 2 and 3. The ASR pyrolysis product contains 3.7385% alkanes, 26.5539% alkenes, and 9.3305% benzene series. The MixASR pyrolysis products contains 0.4045% alkanes, 2.1909% olefins, and 17.739% benzene series. From the comparison of the yield of various pyrolysis products, it can also be seen that the pyrolysis products of ASR or MixASR are not the linear superposition of the pyrolysis products of its main components. This shows once again that the main components of ASR have obvious interactions in the pyrolysis process, and this effect affects the product distribution. (4) Based on the GC-MS analysis results of ASR and its main components, it can be seen that the yields of olefin and benzene series are high in the pyrolytic products, especially styrene. In pyrolytic products of plastic, textiles, foam, ASR, and MixASR, styrene accounted for 13.62, 11.64, 11.93, 17.18, and 20.68%, respectively. However, these substances are chemically unstable and can be further reacted by improving the process to generate more CO and H 2 .

Analysis, Summary, and Discussion
According to the Py-FTIR-GC-MS analysis, the pyrolysis process of ASR could be concluded as follows. Firstly, small molecule alkenes and cycloalkenes were generated from the degradation of ASR due to chain scission at low temperature. Then, the released alkenes and cycloalkenes underwent a series of reactions to form benzene and benzene derivatives as the pyrolysis temperature increased.
The main pyrolysis products were concentrated in alkenes, olefins, alcohols, and aromatic hydrocarbons. The yields of volatile products of ASR pyrolysis varied greatly with changing pyrolysis temperature. Considering that plastic and textiles account for a high proportion of ASR, and other organic matter is affected by them during the pyrolysis process [21], the design of pyrolysis temperature can refer to the pyrolysis characteristics of plastics and fibers. Since most volatile products are chemically unstable, more syngas with a greater calorific value could be obtained by catalytic pyrolysis and gasification [36][37][38].

Conclusions
In this work, efforts were made to investigate the pyrolysis characteristics of ASR and its main components (i.e., plastic, textile, leather, rubber, and foam) through TG-FTIR-GC-MS. The mass variation of samples and the gaseous products were measured simultaneously. Comparing the results of FTIR and GC-MS analysis, good consistency between these two approaches was observed, and the main conclusions can be summarized as follows: (1) The main volatile products of ASR and its main components are alkanes, olefins, alcohols, and benzene series, and their proportions in the pyrolysis products are 3.7385, 26.5539, and 9.3305%, respectively. Many of these volatile products have unstable or weaker chemical bonds, such as =CH-, =CH 2 , -C=C-, and -C=CH 2 . Hence, more syngas can be obtained with further high-temperature (>800 • C) pyrolysis [39,40]. Catalytic pyrolysis and gasification are important research directions for obtaining syngas with a greater calorific value [36][37][38]. (2) According to the Gram-Schmidt profiles and the 3D stack plots of MixASR, ASR, and its main components, the pyrolysis product of ASR is not a simple superposition of the pyrolysis products of its components, but the pyrolysis characteristics of the main components have the greatest influence on the product distribution, especially plastic and textiles. Some hazardous gas exists in pyrolytic products of ASR, such as benzene and toluene, which are harmful to the human body and environment. Therefore, the elimination of toxic and hazardous substances must be considered in the design of pyrolysis process.
In brief, the TG-FTIR-GC-MS coupling technique provided a deeper insight into the understanding of pyrolysis behaviors of ASR. This is helpful for the design of further pyrolysis processes for obtaining more syngas with higher heating value. It could also provide basic information for an industrial pilot or industrialization of the ASR or polymer pyrolysis process.

Conflicts of Interest:
The authors declare no conflict of interest.

Abbreviations
The following abbreviations are used in this manuscript: