Fundamental Understanding of the Thermal Degradation Mechanisms

Jul 6, 2009 - The thermal decomposition of waste tires has been characterized via thermo-gravimetric analysis (TGA) tests, and significant mass loss h...
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Environ. Sci. Technol. 2009, 43, 5996–6002

The thermal decomposition of waste tires has been characterized via thermo-gravimetric analysis (TGA) tests, and significant mass loss has been observed between 300 and 500 °C. A series of gas chromatography-mass spectrometer (GC-MS) measurements, in which the instrument was coupled to a TGA unit, have been carried out to investigate the thermal degradation mechanisms as well as the air pollutant generation including volatile organic carbons (VOCs) and polycyclic aromatic hydrocarbons (PAHs) in a nitrogen atmosphere. In order to understand fundamental information on the thermal degradation mechanisms of waste tires, the main constituents of tires, poly-isoprene rubber (IR) and styrene butadiene rubber (SBR), have been studied under the same conditions. All of the experimental work indicated that the bond scission on each monomer of the main constituents of tires was followed by hydrogenation and gas phase reactions. This helped to clarify the independent pathways and species attributable to IR and SBR during the pyrolysis process. To extend that understanding to a more practical level, a flow-through reactor was used to test waste tire, SBR and IR samples in the temperature range of 500-800 °C at a heating rate of ∼200 °C. Lastly, the formation of VOCs (∼1-50 PPMV/10 mg of sample) and PAHs (∼0.2-7 PPMV/10 mg of sample) was observed at relatively low temperatures compared to conventional fuels, and its quantified concentration was significantly high due to the chemical structure of SBR and IR. The measurement of chemicals released during pyrolysis suggests not only a methodology for reducing the air pollutants but also the feasibility of petrochemical recovery during thermal treatment.

energy and useful petrochemical recovery from waste tires have been considered as an environmentally desired end use (7-12). Considerable work has been done in order to investigate the overall process behavior (6, 13-16) and the thermal and kinetic behavior of waste tires (17-20). For example, previous tire pyrolysis studies have been carried out with pyrolysis reactors (21, 22) to investigate the overall process behavior. The study of tire pyrolysis using simultaneous thermal analysis (STA) including thermo-gravimetric analysis (TGA), differential thermal analysis (DTA), and differential scanning calorimetry (DSC) has been carried out to investigate fundamentals of the waste tire pyrolysis process (23-26). Investigations of converting energy and procuring useful petrochemicals from waste tires have been carried out. Moreover, the control of emissions from scrap tire combustion and pyrolysis has been done. For example, a laboratory investigation was performed on emissions from the batch combustion of waste tire chips in fixed bed and techniques and conditions that minimize toxic emissions were identified (22, 27-32). However, thermal degradation and air pollutant generation mechanisms of tires due to their heterogeneous constituents: natural rubber (NR, poly-isoprene (IR)), SBR, accelerator, sulfur, and reinforced material (33, 34) is poorly understood. The reactions that impact the byproduct and pollutant evolution are highly dependent on temperature and heating rates as well as reactant (sample) composition. While industrial size units will likely have very high heating rates due to fuel introduction methodology, the heating rates investigated were to obtain an initial understanding of the dominant processes occurring. This type of data can facilitate in the development of predictive models that can be eventually used to determine the outcomes of different conditions. The objectives of this study are to understand the thermal degradation mechanisms of waste tires during the pyrolysis process. The contributions from SBR and IR during the pyrolysis process are described to verify the mechanism of thermal degradation of waste tires and to envision the feasible recovery of petrochemicals by means of the thermal treatment. A plausible pollutant reduction methodology is proposed with regard to experimental findings and the proposed thermal degradation mechanisms of waste tires. For instance, of particular concern is the generation of VOCs and PAHs from the pyrolysis process of heterogeneous fuels, such as waste tires. Lastly, to understand the thermal degradation at to a more practical level in terms of temperature and heating rate (∼200 °C/min), a flow-through apparatus has been used to test waste tire samples and their main constituents in the temperature range of 500-800 °C.

Introduction

Experimental Section

Fundamental Understanding of the Thermal Degradation Mechanisms of Waste Tires and Their Air Pollutant Generation in a N2 Atmosphere EILHANN KWON AND MARCO J. CASTALDI* Department of Earth and Environmental Engineering [HKSM] Columbia University in the City of New York, New York 10027

Received February 20, 2009. Revised manuscript received June 11, 2009. Accepted June 26, 2009.

Production of tires in the U.S. results in the generation of a significant quantity of waste tires (290 million scrap tires in 2003) that has led to disposal and environmental problems (1-4). Chemical and mechanical properties enabling a high performance by the tire to withstand any given operating environment or mechanical stress have been investigated rigorously and optimized for each type of vehicle. This thermo-mechanical resilience makes it extremely difficult to directly reuse or recycle the scrap rubber materials without mechanical pretreatment or thermal treatment (5, 6). Thus, * Corresponding author e-mail: [email protected]. 5996

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The tire sample was filed into small pieces (0.5 mm) and its ultimate and proximate analysis is shown in Supporting Information Table S1. The high-purity SBR and IR test samples were purchased from Sigma-Aldrich Chemicals (St. Louis) in a granular form ranging in size between 0.075 and 0.085 mm and a lump, respectively. Initial test sample weights were typically about 10 mg as samples came from the same batch. A Netzsch STA 409 PC/4/H TGA unit capable of TG and DTA measurements has been used where the Netzsch software permits continuous data acquisition and controls a silicon carbide furnace to achieve the programmed heating rates. The TGA test was done over a temperature range from 10.1021/es900564b CCC: $40.75

 2009 American Chemical Society

Published on Web 07/06/2009

FIGURE 1. Thermograms of a waste tire, SBR and IR in a nitrogen atmosphere.

TABLE 1. EDX Element Analysis for a Tire and Tire Residual after Thermal Treatment atmosphere tire sample tire residual

pure N2 pure N2

temperature

C Wt%

S Wt%

Ambient (25 °C) 94.14% 1000 °C 98.19%

2.6% 0.0%

ambient to 1000 °C at a heating rate of 10 °C/min, in which the temperature was digitally recorded and simultaneously compared with an S-type thermocouple reading to ensure meeting the target temperature. The gases used for the experiments were ultra high purity and ordered from TechAir (New York). The flow rates were set using an Aalborg thermal mass flow meter (GFCS-01038) certified by Aalborg Inc.. The TGA apparatus has two inlet ports for protective and purge gas streams and the total flow was maintained at 100 mL/min for all experiments. In order to check the thermal degradation at a temperature range from 500 to 800 °C and a heating rate of 200 °C/min, a vertical tubular reactor (SI Figure S1) made of 6.35 mm OD quartz tubing (Chemglass CGQ-0800T-13) fastened with 6.35 mm stainless ultra-torr vacuum fittings (Swagelok SS-4-UT6-400) was used in order to maintain airtight connections. The experimental temperature was achieved using a splithinged furnace (Multiple Unit, Hevi Duty Electric Company) over a temperature range from 500 to 800 °C and the temperature was simultaneously compared with a K-type thermocouple to ensure that the target temperature was met. In order to hold the filed tire sample, a quartz-fritted disk (Chemglass CGQ-0207-03) was used.

The effluent from the TGA unit and the vertical tubular reactor was sent to the GC-MS (Agilent 9890/5973) for identification and quantification of chemical species. The sampling system, which includes the transfer lines coupled to a vacuum pump, was maintained over 300 °C using Omega heating tape (STR-101 series) to mitigate the condensation and/or adsorption of PAHs onto the surface (35). The lag time of the sample from the TGA furnace to the injection block was calculated to be less than 3 s, based on a 5 mL volume transfer line. Morphological and structural information of the residuals resulting from waste tire pyrolysis were acquired using a Hitachi S4700 high resolution SEM (scanning electron microscope) in plane or cross sectional view. Used in conjunction with the SEM, EDX (energy dispersive X-ray spectroscopy) detects the elements present in a selected area of the SEM image providing qualitative and quantitative information.

Results and Discussion The representative data from a TGA test using tire, SBR, and IR samples under the conditions of 10 °C/min heating rate over a temperature range from ambient to 1000 °C in N2 are shown in Figure 1. Further thermal degradation of any of the samples is not observed after 500 °C. Therefore, subsequent data is presented up to temperatures of 500 °C and our study is mainly investigated at the temperature range between 300 and 500 °C, where significant mass loss is observed. The derivative thermo-gram (DTG) shows the thermal degradation rate for tires, SBR, and IR, which displays the rate of that

FIGURE 2. Chromatogram from the thermal degradation of a tire at 360 °C in N2. VOL. 43, NO. 15, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Limonene formation mechanism from IR. thermal degradation of tires (-0.5%/sec) is relatively slower than that of either SBR (-0.25%/sec) or IR (-0.21%/sec). In addition, the DTG for a tire sample is complex due to the various tire constituents. The temperature range of thermal degradation of a tire (∼280-500 °C) is wider than that for SBR and IR and this shows that a tire is more thermally stable and needs a longer retention time during the pyrolysis process. A residual mass of approximately 40% was observed from the tire sample that did not thermally degrade even at 1000 °C. The tire residual was analyzed with SEM/EDX in order to investigate morphological information and determine an elemental analysis of the tire residual. EDX test results are summarized in Table 1 and an SEM image of the tire residual at 1000 °C is shown in SI Figure S2. As shown in Table 1, the major residual element of tires following thermal treatment was found to be carbon. Thus, the tire residual can be a potential candidate for producing of activated carbon as a byproduct from the pyrolytic decomposition of waste tires. The weight fraction of sulfur, used as a vulcanizing agent, was as high as 2.6% before the thermal degradation though there was no observation of sulfur after the thermal treatment. Significant amounts of sulfur were released in the form of Thiophene during the pyrolysis process. In addition, an SEM image (SI Figure S2) visualized a high surface area char as a feasible source of activated carbon (36, 37) This residual can be used as an adsorbent for air pollutants including VOCs and PAHs and then can be combusted as an energy source for the pyrolysis of waste tires. Total activated carbon recovery from waste tires can be ≈1.16 × 106 tons based on waste tire production in the U.S. In order to investigate the thermal degradation mechanisms of waste tires, a series of GC-MS measurements of the chemical releases from the TGA unit were carried out over a temperature range from 300 to 500 °C. A representative chromatogram from a tire at 360 °C is shown in Figure 2, and major chemical species have been labeled with their chemical structures. In addition to all of the chemical species identified in Figure 2, various other hydrocarbons were observed and this is summarized in SI Table S2. 5998

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FIGURE 4. Concentration of styrene and limonene from various SBR and IR ratios. The labeled chemical species shown in Figure 2 suggest that these similar chemical structures of species derived from tire pyrolysis process can be directly ascribed to IR and SBR. For instance, chemical species such as limonene and 1-methyl-4-(1-methylethyl)-benzene were only observed during the pyrolysis process of IR while styrene and 4-ethenylcyclohexene were observed during the pyrolysis process of SBR. Toluene and ethylbenzene are observed during both the thermal degradation of IR and SBR, which suggests that the formation mechanism of Toluene and ethylbenzene may be coupled or may follow distinct but related pathways in both SBR and IR. As shown in SI Table S2, C4-5 chemical species such as butane and pentane were most dominant among the identified hydrocarbons, which were derived from SBR and IR, respectively. Previous work done with SBR in N2 has shown that C4-hydrocarbons were generated from direct bond scission on the monomer followed by hydrogenation (38). For example, as shown in SI Table S2, saturated and unsaturated hydrocarbons with the same carbon base such as n-butane, butene, and 1,3-butadiene, were observed, which led to an understanding of the hierarchical hydro-

FIGURE 5. Concentration of styrene and limonene from the pyrolysis of a tire, SBR, and IR in N2 at 600 °C.

FIGURE 6. Concentration profiles of phenyl-C1-3 from pyrolysis of IR(a) and concentration of phenyl-C2-3 and from pyrolysis of a tire(b). VOL. 43, NO. 15, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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genation or dehydrogenation steps. Many cyclic hydrocarbons in SI Table S2 were observed and cyclohexene was most dominant among them. However, the concentration of cyclic hydrocarbons is much lower when compared to that of the aliphatic C4-5 species. C8-13 aliphatic hydrocarbons were observed, but the concentrations of these chemical species were not significant. As discussed before, significant sulfur identified using SEM/EDX was observed principally in the form of heterocyclic hydrocarbons such as thiophene. This suggests that the vulcanization agent, sulfur, mostly reacts with C4-hydrocarbons derived from SBR. This can be regarded as a positive aspect in pyrolysis of waste tires in that the vulcanization agent does not trigger formation of the various chemical species containing sulfur. This direct bond scission on the monomer followed by hydrogenation or dehydrogenation was also observed from the pyrolysis of IR; thus, Isoprene (2-methyl-1,3-butadiene), 2-methylbut-2-ene, and 2-methyl-1-but-3-yne were observed together as intermediate forms. However, the concentration of Isoprene was not comparable to that of limonene in that limonene was the most dominant chemical species from the pyrolysis of IR. A limonene formation mechanism from IR is presented in Figure 3. First, both R-scission and β-scission by the thermal cleavage were considered, but major bond dissociation was observed by means of β-scission (39-41). The observation of 2-methylbut-2-ene in SI Table S2 and Figure 3 would indicate evidence of its origin as a byproduct of β-scission. Reaction 1 of Figure 3 illustrates β-scission by thermal cleavage in which this β-scission initiates two radicals. As shown in reactions 2 and 3 of Figure 3, one of the radicals generated from reaction 1 rapidly proliferates more free radicals resulting in the generation of limonene. Meanwhile, reaction 4 terminates the other free radical reaction and thus explains the abundance of limonene during pyrolysis of tires and IR. Thus, the concentration of limonene is approximately a factor of 100 times as large as that of isoprene. However, limonene formation does not fully explain the thermal degradation mechanisms of tires in that the interaction between SBR and IR is not considered. Thus, thermal degradation for various mass ratios of SBR and IR was investigated at 390 °C to understand the interaction between SBR and IR, and the findings appear in Figure 4. One interesting observation is that the reaction between SBR and IR is nearly negligible. This finding enables us to better understand the thermal decomposition mechanism of waste tires. Thus, understanding thermal degradation of SBR and IR can be a key in being able to explain the tire thermal degradation mechanism. In addition, it could be possible to estimate the relative weight fractions of IR and SBR in a tire by measurement of its pyrolysate limonene to styrene ratio. The reported concentrations are diluted since there was a considerable flow of purge and protective gas in the TGA and vertical tubular reactor. Therefore, all concentrations have been normalized to PPMV/10 mg. In order to extend this understanding to a more practical set of operating conditions at a heating rate of 200 °C/min, a flow through apparatus has been used to test waste tire samples in the temperature range of 500-800 °C. The amount of styrene and limonene represent the dominant concentrations during the pyrolysis process even at a heating rate of 200 °C/min. The concentrations of styrene and limonene observed at 600 °C from a tire, SBR, and IR are shown in Figure 5. The concentration of styrene and limonene from SBR and IR is observed to reach a level of approximately 210 and 270 PPMV/10 mg of sample, respectively. The concentrations of these chemical species from the pyrolysis of tires are apparently lower due to a 40% residual as shown in Figure 1 and Table 1. In addition, Figure 5 shows that our tire samples whose styrene to limonene rate is 6000

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FIGURE 7. Identified benzene derivatives from the pyrolysis of tires at 600 and 800 °C. generated that the 50% SBR value, must be comprised of more SBR based in Figure 4. There are many benefits that can be gained from the waste tire pyrolysis process. However, the thermal degradation mechanism of waste tires discussed above can trigger the production of various gas phase precursors that lead to favorable conditions for the generation of air pollutants such as VOCs and PAHs. For example, representative VOCs were presented qualitatively in Figure 2. In order to investigate PAH formation accelerated by benzene derivatives as well as to understand the connection between VOCs and their role as precursors to PAH formation (38), the chemicals released from a TGA unit over a temperature range from 300 to 500 °C have been extensively analyzed using a GC-MS (42, 43). Representative phenyl-C1-3 species from the thermal degradation of IR and the concentration profiles of phenylC2-3 from the pyrolysis of tires are illustrated in Figure 6a and b, respectively. The concentration of m-xylene in Figure 6a begins to rise before the appearance of 1-methyl-3ethylbenzene. This is consistent with the gas phase addition reaction since m-xylene may have been consumed as a precursor to form 1-methyl-3-ethylbenzene. The generation of toluene from IR by means of a Diels-Alder reaction differs from that derived from the direct bond scission of SBR backbone: a chemical reaction between a conjugated diene (isoprene) and a substituted alkene (ethane), commonly termed the dienophile. An intermediate form such as 1-methyl-cyclohexene in Table 2 was identified, and its relative concentration compared to toluene was observed to be much lower by at least a factor of 10 due to fast dehydrogenation. This also leads to a lower concentration of isoprene. To investigate more detailed information regarding the growth of benzene derivatives by means of gas phase reactions, the concentration profiles of phenyl-C2-3 structural isomers are presented in Figure 6b. The concentration of m-xylene is relatively high and begins to form before 1-methyl-3-ethylbenzene and 1,2,4-trimethylbenzene. This is consistent with the gas phase reactions. In addition, these benzene derivatives were also detected at a heating rate of 200 °C/min over a temperature range from 500 to 800 °C. These chemical species at 600 and 800 °C are shown in Figure 7. An interesting observation from Figure 7 is the presence

FIGURE 8. Identified PAH concentrations at 600 °C. of phenyl-C5-6 species. Their concentrations were not significant in the low temperature work. For example, the concentration of n-pentyl-benzene (∼4.5 PPMV/10 mg of sample) was more than that of toluene (∼4 PPMV/10 mg of sample). This suggests that the generation of n-pentylbenzene is derived not only from recombination by gas phase reactions but also from direct bond scission from the main backbone of the tire polymer. These particular pathways resulting in the formation of aromatic derivatives show a robust preference for PAH generation because there are multiple routes by which benzene and other substituted aromatics can be created. Thus, compared to conventional fuels, tire pyrolysis has a higher likelihood of accelerating PAH formation. The concentration profiles of representative three ring PAHs formed during pyrolysis in a nitrogen atmosphere are presented in SI Figure S3. In addition to three ring PAHs, pyrene, benzo[g,h,i]perylene, and dibenzo[a,h]perylene were also observed but discernible trends could not be established. Of particular interest is the observation that the three-ring PAH concentrations in SI Figure S3, such as acenaphthene and fluorene are comparable to naphthalene. Of all PAH evolution concentrations, that of naphthalene should be dominant in the lower temperature range. Our work shows that the concentration of three ring PAHs reaches the maximum concentration at a lower temperature range. It can be postulated that highly concentrated benzene derivatives recombine through another mechanism that does not include naphthalene as an intermediate species. The PAH formation sequence does not involve the building of larger PAH’s from smaller units, which can be alternative pathway that circumvent the typical addition mechanism (38). These phenomena are also observed at a heating rate of 200 °C/min. Thus, the concentration of identified PAH species with molecular weights of 128 to 276 from the pyrolysis of a tire, SBR, and IR at 600 °C are shown in Figure 8. Based on all experimental findings, PAH formation from the pyrolysis of a tire in a nonoxidizing environment would be inevitable. Thus, the mitigation of benzene derivative

precursors is one of the more feasible strategies to prevent PAH formation. Thermal processes enabling more complete combustion such as multioxidation by secondary air or auxiliary hydrogen injection will lead to diminished PAH formation. For instance, the thermal degradation of SBR and IR under auxiliary hydrogen injection (3%) (44) yields more cracked hydrocarbons (C1-3 hydrocarbons) and leads to a significant decrease of benzene derivatives. Thus, recycling the pyrolysis products, including H2, into the reactor can be a feasible way to minimize PAH formation (45, 46) In addition, shredded tire chips rather than whole tires should be used as the feedstock in combustion processes since they enable a more thorough thermal treatment and lower concentrations of PAHs due to fewer products of incomplete combustion that can serve as PAH precursors. Clearly these findings are particular to this study’s temperature range and heating rate. It is anticipated that a predictive mechanism could be validated with this data, thus enabling extrapolation of these findings to conditions relevant to other processes.

Acknowledgments We gratefully acknowledge the Waste-to-Energy Research Technology Council (WTERT) at Columbia University for partial support of this project.

Supporting Information Available One table and two additional figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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