Article pubs.acs.org/EF
Advanced Chemical Characterization of Pyrolysis Oils from Landfill Waste, Recycled Plastics, and Forestry Residue Rebecca L. Ware,† Steven M. Rowland,‡,§ Ryan P. Rodgers,‡,§ and Alan G. Marshall*,†,‡ †
Department of Chemistry and Biochemistry, 95 Chieftain Way, Florida State University, Tallahassee, Florida 32306, United States Ion Cyclotron Resonance Program, National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310, United States § Future Fuels Institute, Florida State University, Tallahassee, Florida 32310, United States ‡
S Supporting Information *
ABSTRACT: Waste material pyrolysis has proven useful for the production of pyrolysis oils; however, the physical properties and chemical composition of pyrolysis oils are greatly influenced by the feedstock. It is well established that lignin- and celluloserich material produces pyrolysis oils high in aromatic oxygen-containing compounds, whereas pyrolysis oils produced from other sources such as plastics and household wastes are far less characterized. Here, three fast pyrolysis oils produced from landfill waste, recycled plastics, and pine forestry residue are compared by elemental analysis, Fourier transform infrared spectroscopy (FT-IR), comprehensive 2D gas chromatography (GC×GC), Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), and liquid chromatography. GC×GC, FT-ICR MS, and liquid chromatography provide insight into the chemical composition of pyrolysis oils, whereas FT-IR analysis identifies functional groups. Landfill and plastic pyrolysis oils were found to contain higher hydrocarbon content that resulted from little or no cellulosic material in their feedstock. In contrast, pine pyrolysis oil is more aromatic and contains a higher abundance of polar species due to the number of oxygen functionalities. The hydrocarbons in plastic pyrolysis oil are more saturated than in landfill and pine pyrolysis oils. Due to their lower oxygen content, landfill and plastic pyrolysis oils are more attractive than pine pyrolysis oil as potential fuel candidates.
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INTRODUCTION Ever-increasing waste production by a growing world population and continued industrialization pose potential problems with groundwater contamination, the release of methane gas, potential fire or explosions hazards, and the spread of disease due to rodents and insects, if not properly managed.1 From 1975 to 2013, in the USA alone, the amount of municipal solid waste doubled.2 Waste management techniques, such as landfills and incineration, carry with them serious environmental risks and generation of harmful combustion byproducts. Recycling avoids many of these environmental concerns; however, many waste materials do not qualify for recycling practices. As of 2013, only ∼35% of municipal solid waste generated in the U.S. was recycled, leaving ∼170 million tons of waste to be disposed of by other methods.2 Pyrolysis has proven useful for the conversion of biomass and waste materials into fuel sources, and the chemical composition of biomass-derived pyrolysis oils has been studied extensively by gas chromatography (GC), mass spectrometry (MS), Fourier transform infrared spectroscopy (FT-IR), elemental analysis, nuclear magnetic resonance (NMR), and various extraction methods.3−9 Pyrolysis of municipal solid wastes offers a unique opportunity to reduce costly and environmentally harmful waste storage practices and to generate alternative fuel streams. Common feedstocks for pyrolysis oils are composed primarily of lignin, cellulose, and hemicellulose biomass.10−12 These feedstocks typically have high oxygen content, and thus produce pyrolysis oils that are also high in oxygen content.13−19 One of the most common techniques used for the analysis of © XXXX American Chemical Society
pyrolysis oils is gas chromatography. GC analysis of plantderived pyrolysis oils reveals that the constituents are primarily oxygen-containing compounds, such as phenols, furans, and ketones.13,16,20−22 In contrast, GC analysis of polyethylene- and polypropylene-derived pyrolysis oils reveals that the primary components are gasoline-range hydrocarbons.23,24 Although GC analysis is commonly used for pyrolysis oil characterization, such analysis is limited to the volatile and nonpolar components. Fourier transform ion cyclotron resonance (FTICR) MS provides access to less volatile highly oxygenated compounds, and several studies have shown that plant-derived pyrolysis oils are enriched in compounds containing 1−15 oxygen atoms per molecule, that are not observed by GC analysis.14,15,18,19 The high content of heteroatoms in pyrolysis oils, especially oxygen, is the greatest hindrance to their use as a substitute for crude oil. The chemical composition of pyrolysis oil is greatly influenced by the feedstock.3 Pyrolysis oils derived from plant biomass typically contain between 10 and 30 wt % oxygen due to the cellulosic/lignin-based (high oxygen content) starting material.4,13,14,25−28 Islam et al. showed that the oxygen content of paper waste-derived pyrolysis oils can be as high as 53 wt %.27 In pyrolysis oils produced from different waste plastics, the feedstock composition affects not only the amount of oil produced but also the structures present in the oil.29,30 Pyrolysis of heavily mixed feedstocks, such as municipal solid Received: March 25, 2017 Revised: June 26, 2017
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DOI: 10.1021/acs.energyfuels.7b00865 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 1. Bulk Elemental Analyses, and H/C and O/C Ratios bio-oil
wt % carbon
wt % hydrogen
wt % nitrogen
wt % oxygen
H/C ratio
O/C ratio
landfill plastics pine
73.2 ± 0.96 85.0 ± 0.18 49.2 ± 0.68
9.40 ± 0.15 13.3 ± 0.03 6.89 ± 0.26
0.48 ± 0.03 0.30 ± 0.01 0.05 ± 0.05
4.43 ± 0.07 1.21 ± 0.05 37.6 ± 0.60
1.53 1.87 1.67
0.045 0.011 0.575
was below the limit of detection for these pyrolysis oils and is therefore not reported. FT-IR. Infrared analysis was performed with a PerkinElmer Spectrum 100 ATR FT-IR spectrometer. Sufficient sample was used to cover the zinc selenide crystal and scanned over the frequency range 4000−500 cm−1. FT-ICR MS. All pyrolysis oil samples were dissolved to a concentration of 1 mg/mL in either toluene (landfill and plastic) or toluene:methanol (50:50, v/v) (pine) (HPLC grade, JT Baker, Phillipsburg NJ). All samples were further diluted to a concentration of 100 μg/mL (in toluene) for positive-ion (+) atmospheric pressure photoionization (APPI) FT-ICR MS analysis. Samples were dissolved in toluene to promote dopant-assisted photoionization. FT-ICR MS analysis utilized a custom built 9.4 T FT-ICR mass spectrometer described elsewhere.15,34−37 Ions were generated in a Thermo Scientific APPI source equipped with a Krypton lamp that emits 10.0 and 10.6 eV photons, a nebulizer temperature of 300 °C, sheath gas of 60 p.s.i., auxiliary gas flow of ∼4 L/min, and a sample flow rate of 50 μL/min. Fifty ∼4.6 s time-domain transients were coadded for each spectrum. Mass spectral calibration was performed with Predator software, and formula assignments and imaging were conducted with PetroOrg software.35,38 Silica Gel Fractionation. A silica gel fractionation was performed to obtain three fractions from each pyrolysis oil: the saturated and aromatic hydrocarbons and the polar compounds. Two grams of silica gel was dried overnight at 100 °C prior to column (10 mL disposable pipet) packing. After conditioning with 10 mL of cyclohexane (HPLC grade, JT Baker, Phillipsburg NJ), approximately 20 mg of sample dissolved in 2 mL of cyclohexane was loaded onto the column and allowed to equilibrate for 15 min. Saturated hydrocarbons were eluted in the first fraction with 20 mL of 100% cyclohexane. Next, 20 mL of a mixture of cyclohexane and DCM (90:10, v/v) eluted the aromatic hydrocarbons. The third fraction (polar species) was eluted with 10 mL of DCM:methanol (50:50, v/v) and 10 mL of methanol. All fractions were desolvated under dry N2 gas.
waste, allows for potentially cooperative interaction of components (biomass, plastics, etc.) during pyrolysis, and can result in more desirable products.3,31 To date, a direct comparison of pyrolysis oils produced by comparable methods but from differing feedstocks has not been conducted. Here, we provide a direct comparison of three fastpyrolysis oils produced from landfill waste (landfill), recycled plastics (plastic), and pine forestry residue (pine) by comprehensive two-dimensional gas chromatography (GC×GC), FT-ICR MS, elemental analysis, and FT-IR. Samples were also fractionated on silica gel to determine the quantity of the saturated and aromatic hydrocarbons as well as the polar species in each pyrolysis oil.
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EXPERIMENTAL METHODS
Feedstock Composition. The exact composition of the feedstock for the landfill and plastic pyrolysis oils is unknown because the exact percentage of components in real world municipal wastes is variable, and the feedstocks were not sorted prior to pyrolysis. On the basis of the average composition of waste collected in South Korea, where the oil was produced, landfill waste was composed of mostly miscellaneous combustibles (29%), paper (23%), plastic (13%), and wood (11%).32 Recycled plastics were composed of polyethylene, polypropylene, and polystyrene. Sample Preparation. Pyrolysis oil samples were obtained from PER North America (landfill; Duluth, GA), GenAgain Technologies, L.L.C. (plastic; Lithia Springs, GA), and The University of Georgia Driftmier Engineering Center (pine; Athens, GA). Landfill and plastic feedstocks were pyrolyzed with the PER process; reactor temperature was between 300 and 400 °C without a catalyst, enabling oil recovery between 20% and 35%.33 Pyrolysis conditions for pine are described elsewhere.28 Prior to all analyses, water was removed from pyrolysis oils by passing each oil through a pipet packed with sodium sulfate (anhydrous). The volatile components were removed by flowing dry nitrogen gas (N2) over the samples prior to analyses. GC×GC-MS. Two-dimensional gas chromatography equipped with time-of-flight (TOF) MS detection was performed with a Leco Pegasus 4D system (Leco Corp., St. Joseph, MI) equipped with a cryogenic modulator. A dimethyl polysiloxane column (60 m × 0.25 mm ID, and 0.25 μm film thickness, SGE, Inc.) was utilized for the first dimension and a 50% phenyl polysilphenylene-siloxane column (1.4 m × 0.25 mm ID, and 0.1 μm film thickness, SGE, Inc.) was used for the second dimension. Samples were dissolved in dichloromethane (DCM) at a concentration of 30 mg/mL, and 1 μL was injected with a split ratio of 5:1. The helium carrier gas flow rate was 1 mL/min. The inlet temperature for the gas chromatograph was 300 °C. The temperature of the first-dimension oven was set at 40 °C and held for 0.5 min before ramping to 340 °C at a rate of 3 °C/min. This temperature was held for 10 min. The second-dimension oven was set 5 °C higher than the first, and followed the same temperature ramp. The modulator offset was +10 °C with a modulation period of 6 s and a hot pulse of 0.8 s. The data were acquired and processed with ChromaTOF software (version 4.50) from LECO Corp. Elemental Analysis. Elemental analysis was performed as described elsewhere.14 C, H, N, and O were determined with a sample mass between 1 and 2 mg. Calibration of the instrument was provided by analysis of 2,5-bis(5-tert-butyl-2-benzoxazolyl)thiophene (BBOT) standard for O and a lubricant oil standard for C, H, and N (CE Elentech, Lakewood, NJ). Each sample analysis sequence also included BBOT or lubricant oil to ensure accuracy over time. Sulfur
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RESULTS AND DISCUSSION Bulk Elemental Analysis. Bulk properties were investigated to validate class distributions (heteroatom content trends) provided by FT-ICR MS. Elemental analysis (Table 1) shows that the weight percent of carbon is greater in landfill (73.2%) and plastic (85.0%) pyrolysis oils than in pine pyrolysis oil (42.9%). Conversely, the oxygen content of the pine pyrolysis oil (37.6%) is significantly greater than that of landfill (4.43%) and plastic (1.21%). Abundant volatile components and incomplete combustion, especially in the landfill pyrolysis oil, account for the reported total mass balance of 5); however, the hydrocarbon class is low in abundance relative to landfill and plastics pyrolysis oils. Compounds with DBE > ∼14 or carbon numbers > ∼40 are not commonly observed by GC×GC analysis with cryogenic modulation. Thus, comparison of the GC×GC results (Figure 1) to the hydrocarbon compositional data (Figure 3), demonstrates the potential of FT-ICR MS as a complementary technique for molecular-level analysis. The landfill GC×GC results capture a high abundance of 1−3-ring aromatics and alkanes. The FT-ICR MS results extend that compositional trend to ∼C65 and DBE = 30 (∼9-ring aromatics), well beyond the GC limit. Similarly, the plastic GC×GC results expose an abundance of alkanes with a minor contribution from 1- to 3ring aromatic species that extends beyond C70 and DBE = 25 (∼7-ring aromatics) by FT-ICR MS analysis. Thus, for compound classes detected by both GC×GC and FT-ICR MS, the combination of the compositional results yields increased knowledge for a wide range of aliphatic and aromatic species. Although extension of the analytical window for high boiling hydrocarbons by FT-ICR MS is quite useful, the main advantage of the technique is the detailed compositional analysis of species not observed by conventional analytical methods. Figure 4 shows the DBE vs carbon number plots for the O1−O4 heteroatom classes for landfill, plastic, and pine pyrolysis oil. Both the landfill (O1−O3) and plastics (O1 and
carbon number for the hydrocarbon class (Figure 3) reveal that landfill and plastic pyrolysis oils exhibit bimodal distributions of
Figure 3. (+) APPI FT-ICR MS-derived isoabundance-contoured plots of DBE versus carbon number for the hydrocarbon class from dried landfill, plastic, and pine pyrolysis oils. The hydrocarbon class relative abundance is displayed at the upper left in each panel for each oil type.
aliphatic (low DBE) and aromatic (high DBE) species. For instance, aliphatic or mono-olefinic species have DBE of 0 or 1, whereas mono-, di-, and triaromatic hydrocarbons have DBEs of 4, 7, and 10. The most abundant hydrocarbons in the landfill pyrolysis oil are highly aromatic species that occupy compositional space (DBE ≈ 7−30 and carbon number ≈ 15−40) near the planar polyaromatic hydrocarbon (PAH) limit.39,40 In contrast, the plastic pyrolysis oil is primarily composed of aliphatic species of low DBE, ≈ 2−7, and carbon number, ≈ 15−80. Aromatic hydrocarbons in the plastic pyrolysis oil are likely due to polystyrene in the feedstock. Note that, although the bulk elemental H/C values for these 2 samples are within the ranges commonly observed for crude oil (H/C ≈ 1.5), the molecular-level data afforded by FT-ICR MS exposes a bimodal distribution at the two compositional extremes (very high aromaticity (H/C ≈ 0.6−0.8) and very high aliphatic character (H/C ≈ 1.8−2.0). See Figure S1) that is not available from bulk analysis. The bulk elemental analysis H/C values reflect
Figure 4. (+) APPI FT-ICR MS-derived isoabundance-contoured plots of DBE versus carbon number for select oxygen-containing heteroatom classes (O1−O4) from dried landfill, plastic, and pine pyrolysis oils. D
DOI: 10.1021/acs.energyfuels.7b00865 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
Figure 5. (+) APPI FT-ICR MS-derived isoabundance-contoured plots of DBE versus carbon number for all oxygen-containing heteroatom classes (O1−O16) from the dried pine pyrolysis oil.
oil. The greater concentration of aromatic carbon in pine is illustrated by the absorbance at ∼1600 cm−1, in agreement with FT-ICR MS data showing that pine pyrolysis oil is composed of compounds that occupy compositional space close to the planar PAH limit. The presence of carbonyl groups is noted by absorbances at 1700 cm −1 for landfill and pine oils, whereas the plastic oil has very few or no carbonyl groups. The absorbance in the carbonyl region has previously been attributed to carboxylic acids;41−43 however, the scale-expanded segment in Figure 6 (bottom) shows a slightly different absorbance for landfill, potentially indicating the presence of non-carboxyl carbonyl groups. Further work is needed to determine the oxygen functionalities as well as structures for the hydrocarbon species in landfill and plastic pyrolysis oils. Fractionation. To better understand the mass distribution of compound classes, we performed a separation on silica gel to fractionate the pyrolysis oil samples based on polarity. The three resulting fractions correspond to saturated and aromatic hydrocarbons and polar compounds. Gravimetric results are shown in Table 2. Silica gel separation reveals that the plastic pyrolysis oil contains 72.9% saturated hydrocarbons and 6.9% aromatic hydrocarbons, whereas the landfill oil contains 38.5% saturated hydrocarbons and 16.1% aromatics hydrocarbons. Those differences are likely due to the more saturated nature of the plastic feedstock, composed mainly of high density polyethylene and polyethylene terephthalate. Likewise, the
O2) pyrolysis results display bimodal distributions for the oxygen-containing compounds that match the hydrocarbon distributions, whereas pine shows a single distribution of aromatic compounds that lies close to the planar PAH limit. DBE vs carbon number plots for all oxygen classes from the pine pyrolysis oil (O1−O16) are shown in Figure 5 and exhibit the same trends as O1−O4 classes. Moreover, oxygen compounds are not easily observed by GC×GC analysis, so that the assigned oxygen compounds observed by GC×GC MS, even in pine pyrolysis oils, account for only a small portion of oxygenates present in pyrolysis oil samples. Utilization of FTICR MS provides unique insight into the composition of oxygen compounds in pyrolysis oils that is not available by any other technique. FT-IR Analysis. One limitation of mass spectral-based analyses is the lack of information regarding chemical functionality, especially with ionization techniques such as (+) APPI that ionize a wide variety of compounds. To gain information into functional groups present in these samples, infrared spectroscopy was utilized (Figure 6, top). The most pronounced differences between landfill/plastic and pine are the large absorbances at ∼3400 and ∼1050 cm−1 for pine pyrolysis oil, due mainly to carboxylic acids and alcohols. Also, note the high methyl- and methylene- absorbances at ∼2900 and 2850 cm−1 for landfill and plastic pyrolysis oils due to a larger amount of aliphatic carbon compared to pine pyrolysis E
DOI: 10.1021/acs.energyfuels.7b00865 Energy Fuels XXXX, XXX, XXX−XXX
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Further work is needed to determine the functionalities present in oxygen heteroatom classes and structures for the hydrocarbon species within landfill and plastic pyrolysis oils.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b00865. (+) APPI FT-ICR MS-derived isoabundance-contoured plot of H/C ratio versus carbon number (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Phone: +1 850-644-0529. Fax: +1 850-644-0133. E-mail:
[email protected]. ORCID
Ryan P. Rodgers: 0000-0003-1302-2850 Alan G. Marshall: 0000-0001-9375-2532 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by NSF Division of Materials Research (DMR-1157490) and the State of Florida. The authors thank K. C. Das, PER North America, and GenAgain Technologies for providing the samples; and Nathan K. Kaiser, Greg T. Blakney, Donald F. Smith, and John P. Quinn for continued assistance in instrument maintenance and data analysis. The authors also thank Yuri E. Corilo for providing data processing and imaging software.
Figure 6. Top: FT-IR spectra for dried landfill (blue), plastic (red), and pine (green) pyrolysis oils. Bottom: Scale-expanded carbonyl region of FT-IR spectra for dried landfill, plastic, and pine pyrolysis oils.
Table 2. Gravimetric Results (Mass %) for Silica Gel Fractionation of Landfill, Plastic, and Pine Bio-oils fraction
landfill (%)
plastics (%)
pine (%)
saturated hydrocarbons aromatic hydrocarbons polar species total recovery
38.5 16.1 37.1 91.6
72.9 6.9 11.6 91.3
9.7 15.2 74.5 99.3
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pine pyrolysis oil is produced from lignin and cellulosic-based starting material and is composed mainly of polar compounds (74.5%). The distribution of compounds classes suggests that landfill and plastic pyrolysis oils are far more comparable to crude oil than plant-derived pyrolysis oils.
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CONCLUSIONS Comparisons of three fast pyrolysis oils reveal significant differences between plant- and non-plant-derived oils. Landfill and plastic pyrolysis oils have high hydrocarbon content, a result of little or no cellulosic/lignin material in their feedstock. In contrast, pine is more aromatic and contains a higher mass fraction of polar species due to the large number and relative abundance of oxygen functionalities. Low oxygen content in landfill and plastic pyrolysis oils makes them auspicious candidates as renewable fuels. The combined approach of bulk properties determination through elemental analysis and FT-IR and molecular analysis through GC×GC MS and FTICR MS provides insight that far surpasses any one technique alone. Specifically, FT-ICR MS allows for the detection and elemental composition assignment of high DBE and carbon number compounds, as well as polar compounds, which, due to their high boiling point, are inaccessible by other techniques. F
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