Utilizing Carbon Dioxide as a Reaction Medium to Mitigate Production

Sep 5, 2012 - Synergetic Sustainability Enhancement via Utilization of Carbon Dioxide as Carbon Neutral Chemical Feedstock in the Thermo-Chemical ...
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Utilizing Carbon Dioxide as a Reaction Medium to Mitigate Production of Polycyclic Aromatic Hydrocarbons from the Thermal Decomposition of Styrene Butadiene Rubber Eilhann E. Kwon,†,‡ Haakrho Yi,‡ and Marco J. Castaldi*,† †

Department of Earth and Environmental Engineering [HKSM], Columbia University in the City of New York, New York, New York 10027, United States ‡ Bio-Energy Research Team, Research Institute of Industrial Science and Technology (RIST), Kwang-Yang-City 545-090, South Korea ABSTRACT: The CO2 cofeed impact on the pyrolysis of styrene butadiene rubber (SBR) was investigated using thermogravimetric analysis (TGA) coupled to online gas chromatography/mass spectroscopy (GC/MS). The direct comparison of the chemical species evolved from the thermal degradation of SBR in N2 and CO2 led to a preliminary mechanistic understanding of the formation and relationship of light hydrocarbons (C1−4), aromatic derivatives, and polycyclic aromatic hydrocarbons (PAHs), clarifying the role of CO2 in the thermal degradation of SBR. The identification and quantification of over 50 major and minor chemical species from hydrogen and benzo[ghi]perylene were carried out experimentally in the temperature regime between 300 and 500 °C in N2 and CO2. The significant amounts of benzene derivatives from the direct bond dissociation of the backbone of SBR, induced by thermal degradation, provided favorable conditions for PAHs by the gas-phase addition reaction at a relatively low temperature compared to that with conventional fuels such as coal and petroleum-derived fuels. However, the formation of PAHs in a CO2 atmosphere was decreased considerably (i.e., ∼50%) by the enhanced thermal cracking behavior, and the ultimate fates of these species were determined by different pathways in CO2 and N2 atmospheres. Consequently, this work has provided a new approach to mitigate PAHs by utilizing CO2 as a reaction medium in thermochemical processes.

1. INTRODUCTION The production of tires in the U.S. results in the generation of a significant quantity of waste tires (∼300 million waste tires per year), which has led to disposal and environmental problems.1,2 Thus, an ideal system for tire disposal would permit material and energy recovery in an environmentally benign way.3,4 Scrap tires are already being using as a fuel cofeed in cement kilns [i.e., tire-derived fuel (TDF) having a heating value of 32−37 MJ kg−1].5,6 Tires are composed of a polymer mixture, which consists mainly of natural rubber (NR), called polyisoprene (IR), and styrene butadiene rubber (SBR).1,2,7−9 The most commonly used synthetic rubber in the tire industry is SBR containing 25 wt % styrene.10 In addition to these polymers, black carbon, fiber, extender, and vulcanizing agents are added during manufacture.7,11 This heterogeneity results in complex thermal decomposition pathways, which lead to a poor understanding of the decomposition mechanisms and the ultimate production and fate of emissions during the thermochemical process (i.e., combustion, pyrolysis, and gasification).2,8,12,13 Thus, considerable work has inevitably been limited to understanding the overall process behavior and thermal kinetic behavior of the thermal degradation of waste tires.8,13 The fundamental level of thermal degradation and the level of pollutants derived from © 2012 American Chemical Society

the thermal degradation of tires have not been investigated extensively.14,15 The formation of benzene derivatives and polycyclic aromatic hydrocarbons (PAHs) is of particular concern because of the adverse health effects associated with such species.16,17 For example, PAHs, a group of ubiquitous organic contaminants, are included in nonmethane hydrocarbons (NMHCs) and are a subject of public concern because of their demonstrated carcinogenic and mutagenic potential.18 Thus, the United States Environmental Protection Agency (USEPA) has listed 14 hazardous air pollutants (HAPs), which include a group of 16 PAHs.18,19 The major source of PAH emissions is from the products of incomplete combustion.17,20−22 Over recent decades, there has been a large amount of interest in the mechanism of PAH and soot formation in combustion, and considerable progress has been made in our quantitative understanding of these processes.17,21,23−28 However, the PAH formation mechanisms for alternative unconventional solid fuels such as refuse-derived fuel (RDF) and TDF are not well Received: Revised: Accepted: Published: 10752

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Table 1. Ultimate and Proximate Analysis of Waste Tire ultimate analysis (weight %)

proximate analysis (weight %)

C

H

N

S

O

volatile

fixed carbon

ash

metal

LHV/MJ kg−1

84.4

6.73

0.39

1.61

1.44

62.2

32.3

4.34

1.14

34.9

vacuum pump, was maintained at 300 °C to minimize the condensation and/or adsorption of PAHs onto its surface. This temperature was determined from the vapor pressure of the expected PAHs.17,20,21 The GC was equipped with a capillary column (0.25 mm ×30 m, HP-MS5), which was directly interfaced to a quadrupole mass spectrometer. Identification of species was accomplished by matching both the gas chromatographic retention times to those of the pure components and the mass spectral fragmentation patterns to those species found in standard MS libraries. Permanent gases and C1−2 chemical species were determined using a Carboxen-1010 (Supelco #25467) connected to the thermal conductive detector (TCD). A micro-GC (Agilent 3000A) was used to obtain C4 species. The concentrations of chemical species were determined by multilevel calibrations using a Restek PAH standard (Lot #A03448), a Sigma Aldrich aromatic standard (PIANO Aromatic Lot#2102), and a Japanese indoor air standards mixture (Lot# 4M7537-U). The high-purity SBR test sample was purchased from SigmaAldrich Chemicals (St. Louis, MO) in granular form. Initial test-sample weights were typically 50 mg, and all samples came from the same chemical batch. The reported concentrations were diluted since there was a considerable flow of purge and protective gases in the TGA, and the exact starting masses of the samples varied slightly. Therefore, all concentrations were normalized to parts per million by volume (PPMV) (50 mg)−1. Three sets of experiments for each individual condition were carried out and averaged to ensure reproducibility and precision. The typical experimental error was within ±1.5%; however, error bars are not shown in an effort to keep the plots clear. In addition, the proximate and ultimate analysis of a waste tire is summarized in Table 1.

understood. There have been recent studies where PAH has been identified from the combustion and pyrolysis of waste tires.1,7,10,14,29,30 The main focus of this study is the investigation of the role of CO2 used as a reaction medium in the pyrolysis of SBR by means of the quantification and identification of the benzene derivatives and PAHs released. The molecular structure of SBR consists of a butadiene backbone supporting an aromatic ligand, which serves to accelerate PAH formation in a N2 atmosphere. Thus, experimental findings may lead to a strategy for mitigating the levels of aromatic (benzene) derivatives and PAHs by means of the direct comparison of the pyrolytic products in N2 and CO2 atmospheres. This study provides detailed information on the identities and concentrations of hazardous air pollutants including 50 major and minor species, which serves to clarify the thermal degradation and the origin and fate of PAHs.

2. MATERIALS AND METHODS 2.1. Thermogravimetric Analysis (TGA). All experiments were carried out using a NETZSCH STA 499 F1 Jupiter thermogravimetric analysis (TGA) unit capable of TGA and differential temperature analysis (DTA) measurements. The TGA unit used for the experimental work enabled a temperature increase from ambient temperature to 1250 °C at heating rates of 10−1500 °C min−1. The three embedded mass-flow controllers controlled the flow rates of the purge gas and protective gases in the TGA unit. The apparatus was controlled using the NETZSCH software Proteus for continuous data acquisition and control. The temperature ramp rates were provided as an input to the software, which then controlled the furnace to achieve the heating rates used for these tests. All data were recorded digitally, and S-type thermocouple readings were compared simultaneously to the target temperature and time. The gases used for the experiments were ultrahigh-purity gases purchased through AirTech. The TGA unit has two inlet ports. One is for the protective gas of pure nitrogen or an inert gas (i.e., helium and argon), which shrouds the balance mechanism from any heat or effluent gases and was set at a flow rate of 20 mL min−1. The second port was used for the introduction of the gas mixture of interest to provide the desired atmosphere during the experiment. The flow rate of the atmosphere inlet was 100 mL min−1; combined with the protective gas flow of 20 mL min−1, this yielded a total flow past the test sample of 120 mL min−1, which was maintained for all the experiments performed. 2.2. Measurements Using a GC/MS Unit. The effluent of the TGA was sent using a valving system to either a micro-GC (Agilent 3000A) or a GC/MS (Agilent 9890/5973) for the identification and quantification of chemical species from the TGA. The lag time of the sample from the TGA furnace to the injection block was calculated to be less than 2 s on the basis of the volume of the transfer line (NETZSCH accessory, 3 mL). The injection block of the GC/MS contained both a 10- and 6port valve assembly (Valco Instrument, Houston, TX). The sampling system, which includes transfer lines coupled to a

3. RESULTS AND DISCUSSION 3.1. Characterization of the Thermal Degradation of SBR in CO2. Eight TGA experiments were carried out, varying the heating rate from 10 to 40 °C min−1 in N2 and CO2 atmospheres over a temperature range from ambient to 1000 °C. Only the experimental data at heating rates of 10 and 40 °C min−1 are shown in an effort to keep the plots clear. The lines correspond to the left axis, indicating the weight loss fraction. The data points correspond to the right axis, which shows the rate of change of mass with time (i.e., the differential thermogravimetric analysis [DTA]). Figure 1 shows that the thermal degradation of SBR is almost identical in terms of the onset/end temperatures of the thermal degradation, even in different atmospheres. The thermal degradation rates shown as the slopes in Figure 1 are also identical. All samples were completely consumed above 500 °C. Therefore, subsequent data are presented up to a temperature of 700 °C. The absence of differences in Figure 1 implies that devolatilization is the main thermal decomposition mechanism of SBR. Thus, the CO2 cofeed impact on the pyrolysis of SBR is limited to the unknown reaction between the chemical species evolved from the pyrolysis of SBR and CO2. The consideration of well-known reactions such as Boudouard reaction must be 10753

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would be released into the gas phase during ligand scission. Other C4 hydrocarbons (i.e., 1,3-butadiene and butene) were also identified and quantified, but the concentration profiles of 1,3-butadiene and butene were not established. As the temperature increases in the TGA, any C4 hydrocarbons are likely to crack to release C1−2 hydrocarbons (i.e., methane and ethylene) and hydrogen. Surprisingly, this cracking behavior was enhanced in the presence of CO2, as shown by the concentration profiles of n-butane and hydrogen in Figure 2. For example, the concentration of hydrogen is significantly lower and begins to form after the n-butane concentration begins to decline. This is consistent with the typical hydrogen mechanism (i.e., unsaturated carbon bonds are reduced by the attachment of a hydrogen atom to each carbon). Apart from the C4 hydrocarbons, the other major chemical species identified and quantified was styrene. The concentration profiles of the styrene that evolved from the pyrolysis of SBR in N2 and CO2 are compared in Figure 3. For example, the

Figure 1. Representative TGA and DTA curves for the SBR sample in N2 and CO2 atmospheres at various heating rates.

excluded, because the Boudouard reaction is thermodynamically favorable in temperature regimes above 720 °C.31 The shift in the TG curves and maxima in the DTA curves in Figure 1 were due to the longer reaction time associated with slower rates of temperature increase. However, all final mass conversions reached 99.9% or more during the tests. Thus, the identification and quantification of the effluent from the TGA unit is imperative for obtaining detailed information on the CO2 cofeed impact on the pyrolysis of SBR. 3.2. CO2 Cofeed Impact on Pyrolysis of SBR via Monitoring of Pyrolytic Products. In the previous work carried out by the authors, the chemical structure of the SBR molecule was assembled using the Spartan interface in the MOPAC program.10 The relative bond dissociation energies indicated that the likely first bond-scission reactions are between the ligand carbon and the butadiene base α-carbon.1,10 This implies that the first reaction during the thermal breakdown of SBR releases an aromatic radical species, which was validated in previous work.1,10 In order to confirm the influence and/or role of CO2 in terms of the chemical species from the pyrolysis of SBR, the concentrations of n-butane and hydrogen were initially quantified for a rate of 20 °C min−1. As shown in Figure 2, the concentration of n-butane reaches a maximum at 390 °C, which is consistent with the maxima of the DTG curves in Figure 1 and then decreases because the SBR sample is exhausted. As discussed above, n-butane species

Figure 3. Concentration profiles of styrene from pyrolysis of SBR in N2 and CO2.

maximum concentrations of styrene in N2 and CO2 reached ∼760 and ∼670 PPMV (50 mg SBR)−1, respectively, at temperatures corresponding to the temperature of the DTA maximum in Figure 1. One interesting feature is that the concentration of styrene in the presence of CO2 was reduced, which is consistent with the previous observation in Figure 2. This enhanced cracking behavior would provide favorable conditions for mitigating the generation of PAHs, because the previous work carried out by the authors showed that benzene derivatives can be precursors of PAHs by means of gas-phase addition/combination reactions.1,10 Thus, in order to obtain more detailed information on the CO2 cofeed impact on the generation of PAHs, simple benzene derivatives, with the exception of styrene, were identified and quantified from the effluent of the TGA unit coupled to the GC/MS heated at a rate of 10 °C min−1 in N2 and CO2, and the resulting concentrations were then plotted. Among the various benzene derivatives, toluene and ethylbenzene were most commonly observed from the cleavage of the styrene backbone followed by hydrogen. Because of the low heating rate (10 °C min−1), the devolatilization reactions are slow, so any detectable concentrations of these generated chemical species are diluted. For example, the four most prevalent gaseous species (C4 hydrocarbons, styrene, toluene, and ethylbenzene) in the thermal decomposition of SBR accounted for over 90% of the total output in PPMV.

Figure 2. Concentration profiles of n-butane and hydrogen from pyrolysis of SBR in N2 and CO2. 10754

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The concentration profiles of toluene and ethylbenzene suggest relative bond dissociation energetics that drives the decomposition of styrene at any given temperature. For example, the concentration of toluene is higher than that of ethylbenzene because bond dissociation from styrene to form toluene occurs spontaneously. Furthermore, the possibility of the evolution of toluene from ethylbenzene by means of thermal decomposition cannot be excluded. The concentrations of toluene and ethylbenzene in Figure 4 decreased in the

Figure 4. Concentration profiles of toluene and ethylbenzene from SBR thermal degradation in N2 and CO2.

presence of CO2, which is consistent with the observations in Figures 2 and 3. Thus, this observation would imply that the thermal cracking of benzene derivatives is readily enhanced in the presence of CO2. In addition, blocking the gas-phase addition reaction due to the observed enhanced thermal cracking behavior induced by CO2 could impede the growing steps for the generation of PAHs. One interesting feature from the GC/MS measurements was the detection of biphenyl in the N2 atmosphere, with a concentration exceeding that of toluene (182 PPMV). However, the concentration of biphenyl in the CO2 atmosphere was significantly lower (74 PPMV), validating the assumption that the gas-phase addition reaction is impeded in a CO2 atmosphere. As a result, monitoring of the formation of benzene derivatives with structural isomers would validate the role of CO2 in the thermal degradation mechanism by showing how small fragments from the SBR backbone react with benzene derivatives from the SBR base. Thus, this study has identified and quantified 37 benzene derivatives with structural isomers (4, 7, 16, and 5 structural isomers of benzene derivatives having molecular weights of 106, 120, 134, 148, and 162, respectively) from the thermal degradation of SBR. Benzene derivatives such as 1,2,4-trimethylbenzene and o-, m-, and p-xylene provide evidence of aromatic ligand scission. These benzene derivatives appear to be formed by gas-phase addition reactions. This is further corroborated by the detection of multiple fused-ring PAH structures, which can only occur as a result of gas-phase addition reactions of aromatic species. A more detailed explanation of the gas-phase addition reaction is presented in Figure 5. All chemical species plotted in Figure 5 are structural isomers with molecular weights of 120 (except pxylene). As discussed above, the structural isomers of benzene derivatives not only provide evidence of the gas-phase addition reaction, but also clarify the hierarchical growing steps of benzene derivatives. More importantly, the concentration profiles of the chemical species in Figure 5 illustrate the CO2

Figure 5. Concentration profiles of phenyl-C2−3 from pyrolysis of SBR in N2 and CO2.

cofeed impact on the pyrolysis of SBR and the formation of PAHs. One interesting feature is that the concentrations of the quantified chemical species in Figure 5 decrease discernibly (by approximately 23−30%) in the presence of CO2. As discussed above, this observation indicates that the cracking behavior and/or blocking of the gas-phase addition reaction can be enhanced in the presence of CO 2. For example, the concentration of p-xylene begins to rise before the 1-methyl4-ethylbenzene and 1,2,4-trimethylbenzene concentrations begin to increase. This is consistent with the gas-phase addition reaction, as shown in Figure 5. For instance, p-xylene may be consumed as a precursor to from 1,2,4-trimethylbenzene and 1methyl-4-ethylbenzene. Chemical species such as p-xylene and 1-methyl-4-ethylbenzene were observed at relatively lower temperatures, which may be attributed to their steric stability. In addition, the concentrations of m-xylene and o-xylene were lower than that of p-xylene by a factor of 1/10. In particular, the relatively low concentration of m-xylene is associated with the generation of 1-methyl-3-ethylbenzene. However, it is difficult to explain why the concentration profiles of m-xylene follow a similar pattern to those of p-xylene at temperatures in the range 300−400 °C. Figures 4 and 5 show the major benzene derivatives, but other minor benzene derivatives were also observed. Because the production of these major and minor benzene derivatives occurred concurrently, a clear dependence of one on the other could not be discerned. However, the concentration profiles of benzene derivatives in the presence of CO2 were decreased significantly, which is consistent with the previous observations in Figures 2−5. Thus, the thermal cracking and impeding of gas-phase addition can be enhanced in the presence of CO2, 10755

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addition/combination reaction. This observation clearly shows that the utilization of CO2 as a reaction medium for the thermochemical process can be an effective methodology for mitigating PAH generation. Simultaneously, the utilization of CO2 can modify the pyrolytic end products through the enhancement of thermal cracking. Finally, the authors pyrolyzed a real tire sample to observe the CO2 cofeed impact on the mitigation of benzene derivatives; the representative chromatogram is shown in Figure 8. The same experimental observation as discussed

limiting the diversity of benzene derivatives as well as mitigating PAH formation. In this regard, the utilization of CO2 would be environmentally beneficial. For the validation and confirmation of the CO2 cofeed impact on the mitigation of PAH formation, the concentration profiles of representative two-, three-, and fourring PAHs such as naphthalene, anthracene, phenanthrene, and pyrene are presented. For example, the concentrations of threeand four-ring PAHs are compared directly to that of naphthalene in Figure 7. In general, the concentration of

Figure 8. Chromatogram of pyrolytic products at 650 °C in N2 and CO2.

above was observed. For example, the utilization of CO2 as a reaction medium in the thermochemical process not only reduced the amounts of condensable hydrocarbons significantly (i.e., ∼30−50% reduction in tar) but also modified the end products of the thermochemical process. In general, condensable hydrocarbons mostly consist of significant amounts of benzene derivatives and PAHs. Figure 8 also suggests that the CO2 cofeed impact is more effective, which is proportional to a heating rate. Unfortunately, the optimal experimental conditions for the use of CO2 have not been established in this work; thus, further study to investigate the optimizing the CO2 cofeed impact should be established.

Figure 6. Concentration profiles of phenyl-C5−6 from pyrolysis of SBR in N2 and CO2.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 1-212-854-6390; fax: 1-212-854-7081; e-mail: [email protected]. Notes

Figure 7. Concentration profiles of two-, three-, and four-ring PAHs from pyrolysis of SBR in N2 and CO2.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the Waste-to-Energy Research Technology Council at Columbia University for their partial support to this project.

naphthalene should be dominant in the lower temperature range during the evolution of PAHs. However, our work clearly shows that the concentration of pyrene reached its maximum concentration in the lower temperature range. In addition, anthracene showed higher concentrations than naphthalene. Thus, it can be postulated that highly concentrated benzene derivatives recombine through another mechanism that does not include naphthalene as an intermediate species. This probably involves cleavage of the butadiene base with the aromatic ligand still attached. The PAH formation sequence does not simply involve the building of larger PAHs from smaller units to construct complex polymer structures. It is possible to generate large PAH molecules by alternative reaction pathways that circumvent the typical addition mechanism. Thus, the discernible reduction in PAHs in the presence of CO2 in Figure 6 can be explained easily by the enhanced thermal cracking and impeding of the gas-phase

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REFERENCES

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