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Application of the advanced distillation curve method to characterize two alternative transportation fuels prepared from the pyrolysis of waste plastic Megan E. Harries, Bidhya Kunwar, Brajendra K. Sharma, and Thomas J. Bruno Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02068 • Publication Date (Web): 15 Sep 2016 Downloaded from http://pubs.acs.org on September 20, 2016
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Application of the advanced distillation curve method to characterize two alternative
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transportation fuels prepared from the pyrolysis of waste plastic*
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Megan Harries1, Bidhya Kunwar2, Brajendra K. Sharma2 and Thomas J. Bruno1,**
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*Contribution of the United States Government; not subject to copyright in the
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United States.
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1. Applied Chemicals and Materials Division, Material Measurement Laboratory, National
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Institute of Standards and Technology, Boulder, CO
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2. Illinois Sustainable Technology Center, University of Illinois, Champaign, IL
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**Corresponding author. Tel.: +1 303 497 5158; email:
[email protected].
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Abstract
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The properties of alternative fuels must be evaluated to determine their suitability for use in
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existing engines, either as blends with conventional petroleum products or as stand-alone
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substitutes (drop-in replacements). Alternative fuels made from waste feedstocks are especially
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attractive since they serve the dual purposes of fuel production and environmental pollution
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mitigation. In this study, two alternative liquid fuels generated by the pyrolysis of waste
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polypropylene corrugated advertisement plastics were assessed using a composition-explicit
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distillation approach, which provides a rich data matrix including fuel volatility, composition,
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hydrocarbon classification, and energy content in each distillate volume fraction. The data
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collected using this approach provide valuable information for the development of equations of
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state to describe the thermodynamic behavior of these complex fluids. The analysis showed that,
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despite significant differences in composition, the distillation curves and associated data of the
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alternative fuels show promising similarity to their conventional counterparts.
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Keywords: composition-explicit distillation, ADC, pyrolysis oil, fuel characterization
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1. Introduction
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There is a great deal of interest in the development of alternative liquid fuels for transportation
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and heating applications, both to extend petroleum stocks and to improve performance. One of
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the primary impediments to the adoption of new fuel sources is the cost of modifying or
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replacing existing fuel-burning infrastructure to use a fuel with different combustion and
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thermophysical properties. For this reason, fuels with characteristics similar to existing fuels are
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desirable. Thus, it is important to measure and compare the fuel properties (chemical and
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thermophysical) of alternative fuels to those of conventional fuels. The degree of similarity in the
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major properties can indicate whether the fuel could be used directly (as a drop-in replacement)
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or blended with conventional fuel for use in existing combustion engines.
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Fuel properties are fundamentally determined by composition, but not all properties are equally
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sensitive to varying composition. Volatility is one property that is very sensitive to composition;
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as such, it is frequently used as a fuel design tool.1 The volatility measurement in the laboratory
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is expressed as the distillation (or boiling) curve. The alternative fuels evaluated in this study, a
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gasoline and a diesel fuel refined from polypropylene pyrolysis crude oil, were measured using
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the advanced distillation curve (ADC) method, which combines the volatility measurement with
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a composition-explicit data channel that provides chemical information for each distillate volume
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fraction (DVF).
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The ADC method was developed at NIST as a robust, data-rich volatility measurement approach
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that has been described in depth in previous publications.2-7 The ADC improves on the well-
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known ASTM International D86 distillation standard in several significant ways.8 The ADC
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provides: (1) a composition-explicit data channel for each distillate fraction (for qualitative,
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quantitative and trace analysis), (2) temperature, volume and pressure measurements of low
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uncertainty that are true thermodynamic state points that can be modeled with an equation of
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state, (3) consistency with a century of historical data measured using ASTM D86, and (4) an
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assessment of the energy content of each distillate fraction. Sampling very small volumes of the
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distillate yields a composition-explicit data channel with nearly instantaneous composition
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measurements. Chemical analysis of the distillate fractions provides some understanding of how
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the composition of the fluid varies with volume fraction and distillation temperature, even for
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complex fluids. The significant advantage offered by the ADC method is the ability to model the
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distillation curve with an equation of state.9
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Currently, numerous feedstocks are used in pyrolysis reactors and gasifiers, including municipal
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refuse, various types of plastic, agricultural waste, and other biomass.10 Indeed, our lab
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previously performed ADC characterization on a crude oil produced from pyrolysis of swine
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manure.5, 11 The appeal of alternative fuels sourced from waste is clear: the widespread use of
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feedstocks like plastic trash would simultaneously reduce the need for fossil fuels and the
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demand placed on landfills. Plastics constituted only 0.5 wt.% of municipal solid waste (MSW)
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in United States in 1960 but were 12.5 wt. % (18% by volume) in 2010.12 According to a 2012
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EPA report, 251 million tons of municipal solid waste was generated in the US, of which only
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34.5 % gets recycled and composted. Even after MSW recovery through recycling and
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composting, plastic was the second largest component (18 %, 29 million tons) behind food waste
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(21 %) of the 164 million tons discarded in 2012. Less than 10 % of plastic waste is recovered,
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recycled or utilized for energy production, with the remainder ending up in landfills.12 More than
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50% of waste plastic is polyethylene (#2 HDPE and #4 LDPE) and polypropylene (#5).13
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Besides decreasing the use of landfills, life cycle analyses have demonstrated that pyrolysis of
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MSW is net energy positive.14
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In prior work, fuels derived from polypropylene (PP) corrugated or honeycomb advertisement
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plastic (for example, political campaign signage) were synthesized in a semi-pilot-scale
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continuous auger pyrolysis reactor.15 PP was thermally cracked at 500 °C, producing a crude
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pyrolysis oil. Subsequently, the crude pyrolysis oil was distilled in an automated stainless steel
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spinning band distillation system, resulting in two fractions: one resembling the volatility of
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automotive gasoline (collected at vapor temperatures 35-185 °C) and the other, diesel fuel (185-
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350 °C).15 In this report, these fuels will be referred to as PP gasoline and PP diesel fuel.
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2. Experimental Section
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Both polypropylene-based fuels described in the above paragraph were stored at room
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temperature before measurement. No phase separation was observed in the fluids, but they were
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still mixed by manual agitation before each distillation. For each distillation curve measurement,
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200 mL aliquot of fuel was first measured into a boiling flask (the kettle) at atmospheric pressure
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using a 200 mL volumetric pipet. The flask was heated in an aluminum enclosure controlled by a
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model-predictive temperature controller. The temperature program was designed to lead the
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kettle temperature by about 20 °C. Temperatures were measured with two thermocouples, one
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submerged in the boiling liquid (recording the kettle temperature or Tk) and one suspended in the
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bottom of the take-off position of the distillation head (recording the head temperature or Th).
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The thermocouples had been previously calibrated in fixed point cells.16 The temperature Tk is
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the thermodynamically consistent value that is characteristic of the extant mixture in the kettle as
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it changes throughout distillation. Recording Th serves three purposes: (1) it allows comparison
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with historical measurements made using the D86 distillation method, and the relationship
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between Th and Tk provides (2) diagnostic information about apparatus performance and (3) an
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indication of potential azeotropy.17, 18 The distillations were performed at ambient atmospheric
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pressure in a laboratory located 1655 m above sea level, where typical atmospheric pressure is 83
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kPa. The electronic barometer used to measure the ambient pressure had been previously
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calibrated by use of a fixed cistern mercury barometer, temperature-corrected for the density of
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mercury and the expansion of the brass scale. Its uncertainty was 0.03 kPa. The temperatures
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reported here were adjusted to standard atmospheric pressure (101.325 kPa) using the modified
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Sydney Young equation, a correction that is approximately 7 – 8 °C.19, 20 We note that the
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correction term in this equation is based on a correlation of n-alkanes. The PP gasoline and PP
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diesel fuel studied here do not have compositions strictly comparable to conventional gasoline
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and diesel fuel; however, as will be seen, the component vapor pressures for the fuels are
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comparable to those in conventional fuels. Thus, the correction term for PP gasoline
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corresponded to a carbon chain of 8 and that for diesel fuel corresponded to a carbon chain of 12,
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as has been our practice with gasolines and diesel fuels in the past.21
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For each of the two fuels, we performed three replicate distillations, one of which included
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sampling for composition. While more extensive measurements would decrease uncertainty to
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some extent (discussed below), sample availability was a limiting factor in this work. During
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each sampling distillation, samples of approximately 7 uL were taken at 10 % DVF intervals
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(every 20 mL) using a 10 uL chromatographic syringe.
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Preliminary characterization of the composite (undistilled) fuels informed the choice of a solvent
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that would not interfere with a chromatographic analysis of the components of the fuels. Samples
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from the PP gasoline distillations were diluted in chromatographic grade n-dodecane. Samples
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from the PP diesel fuel distillations were diluted in chromatographic grade n-hexane. Both
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solvents were tested for purity using protocols established at NIST.22 All samples were analyzed
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using gas chromatography-mass spectrometry (GC-MS). Peaks were identified by analyzing
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their mass spectra with assistance from the NIST/EPA/NIH Mass Spectral Database and on the
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basis of retention indices.23 A calibrant solution containing known concentrations of three
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representative hydrocarbons, alkenes with carbon chain lengths 6, 10 and 14, was used to
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calibrate the instrument by external standardization and calculate molar concentration from raw
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area counts.24 The GC-MS methods used are detailed in Table 1. Baseline resolution was attained
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in both chromatographic programs.
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Table 1. Summary of GC-MS methods used to characterize the fuels. Fuel
Method
PP gasoline
30 m, 5 percent phenyl polydimethylsiloxane column with a coating thickness of 0.1 µm; 100:1 split injection via automatic sampler, temperature of injector = 200 °C at 55 kPa (8 psig) head pressure, injection volume = 1 µL, column temperature of 30 °C for 7 min, followed by temperature program at 15 °C/min to 150 °C; hold for 1 min; scan m/z from 33 to 200
PP diesel fuel
30 m, 5 percent phenyl polydimethylsiloxane column with a coating thickness of 0.1 µm; 70:1 split injection via automatic sampler, temperature of injector = 340 °C at 27.5 kPa (4 psig) head pressure, injection volume = 1 µL, column temperature of 30 °C for 10 min, followed by temperature program at 1 °C/min to 200 °C; 50 °C/min to 325 °C; hold for 2 min; scan m/z from 33 to 400
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3. Results and Discussion
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Engineers are able to predict operational parameters (including engine starting ability, fuel
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system icing and vapor lock, fuel injection schedule, etc.) of complex liquid fuels from their
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characteristic distillation curves.25, 26 ASTM D86 has traditionally been used for this purpose;
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however, we have found that the ADC method allows us to model the fluid with an equation of
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state as well as make these operational predictions. Throughout our analysis of the PP fuels, we
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found it useful to compare our measurements with previous ADC work on relevant
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conventionally-sourced fuels, both commercially available and research grade samples. In the
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remaining sections, we refer to six gasolines and four diesel fuels selected for comparison. A
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summary describing these fuels is presented in Table 2 and can be used as a reference throughout
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the remainder of this section.
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Table 2. Summary of relevant previously measured fuels. Gasolines 91 AI SQ gasoline21
Premium summer quarter automotive gasoline formulated with no oxygenate additive and obtained from a commercial source; antiknock index = 91.
91 AI WQ gasoline18 Premium winter quarter automotive gasoline formulated with no oxygenate additive and obtained from a commercial supplier; antiknock index = 91. 91 AI gasoline used
An unleaded, unoxygenated gasoline obtained from a commercial source
in dieseline
and not containing ethanol or other oxygenates such as ethers. This fuel
mixtures
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had an antiknock index (average of the research octane number (RON) and the motor octane number (MON)) of 91, with the RON reported by the supplier as 97.
100LL avgas6
Low-lead (
