Environ. Sci. Technol. 1993, 27, 2885-2895
Exploratory Study on the Combustion and PAH Emissions of Selected Municipal Waste Plastics Lloyd Wheatley,tp* Ylannls A. Levendls,'v* and Paul Vouross
Department of Mechanical Engineering and Department of Chemistry, Northeastern University, Boston, Massachusetts 02 115 Polynuclear aromatic hydrocarbon (PAH) emissions from the combustion of selected synthetic polymers (plastics), commonly found in municipal solid waste streams, were monitored using bench-scale furnaces. Experiments were conducted burning aerosols of poly(styrene), poly(ethy1ene), poly(propylene), poly(methy1 methacrylate), and poly(viny1 chloride) particles, in size cuts in the range of 150-300pm. Most experiments involved dilute dispersions of polymer particles free-falling in a drop-tube laminarflow furnace, at gas temperatures of 750-1150 "C and residence times of 0.5-2 s in air. Upon melting and pyrolysis, the polymer particles burned with luminous envelope flames, whose temperature and duration were measured pyrometrically in the near infrared. Moreover, the gas temperature and residence time were also measured to investigate the effects of both the flame and the postflame conditions on the PAH emission levels. PAHs were captured in the gas and solid phases using XAD adsorbers and glass fiber filters, respectively, and analysis was conducted with gas chromatography. Results showed that the temperatures and durations of sooting luminous flames, in air, were in the range of 1700-2900 OC and 10100 ms, respectively, with PVC flame durations being the shortest and those of poly(ethy1ene) the longest. Furthermore, the particle flame temperatures and durations did not change substantially with the gas temperature, in the range of 950-1150 "C. Thus, changes in PAH concentrations were mostly attributed to postflame conditions. Effective destruction or minimization of toxic hydrocarbon species was found only at the highest postflame gas temperature and longest residence times explored in this study, i.e.. 1150 OC and 2 s. 1. Introduction
Plastics account for 8% of the total municipal refuse by weight but up to 20% by volume (I). Plastics are used in household applications and packaging and, by and large, are disposable and nonbiodegradable. Poly(ethy1ene) is the leading polymer in total production. In 1990,8million t of poly(ethy1ene) found its way to the waste stream in the United States alone (I). Also, during the same year, 2.7 million t of poly(styrene), 1.1 million t of poly(propylene), and 0.5 million t of PVC were discarded. As the applications and usage of plastics are expected to increase in the future, the amount found in the waste stream will also increase. The growing concern over land use and closing of existing landfills has led many communities to consider construction of energy-producing municipal solid waste (MSW) incinerators. There is apprehension, however, over toxic emissions released from such facilities with the contri-
bution from the combustion of plastics being still unclear (1-11). Toxic organic emissions are associated with products of incomplete combustion and depend on the operating conditions (steady or upset) of incinerators, as well as the chemical and physical characteristics of the MSW fuel. With social awareness growing about toxic incinerator emissions, research to relate such emissions to both MSW components and combustion parameters is essential (12). However, to associate the inputs and outputs of the combustion of a fuel as complex as MSW, it was thought best to start by examining the combustion of individual components under controlled conditions, in (and beyond) the range of those found in incinerators. This work concentrated on plastics. In contrast to previous laboratory studies on the combustion of plastics in bulk, this investigation involved the combustion of aerosols (particle dispersions) of plastics, falling by gravity in a drop-tube furnace. This mode of burning better simulates combustion in pulverized refuse-derivedfuel (RDF) incinerators. Moreover, the present technique, where small particles were burned in a high-temperature vertical furnace, ensured ignition and combustion (in a flame) of the particles. This facilitated monitoring of both flame and postflame zone temperatures and durations (residence times). The combustion of five different polymers (plastics) in powder form [poly(styrene), poly(ethylene), poly(propylene), poly(methy1methacrylate),and poly(viny1chloride)] was examined. To compare with the above experimental technique and with results reported in the literature, a limited number of experiments were also conducted in a muffle furnace burning fixed quantities (beds) of polymer particles, in a mode simulating Mass Burn incineration. In all experiments, the combustion effluent was monitored for principal organic hazardous constituents, mainly polynuclear aromatic hydrocarbons (PAHs), using gas chromatography. The goal was to identify trends in emission levels with combustion parameters, but not necessarily to thoroughly identify and quantify all individual components. 2. Literature Review on Combustion and PAH Emissions from Plastics
t Presentaddress: BVI Electricity Corp.,P.O.Box 268, RoadTown, Tortola, British Virgin Islands. Department of Mechanical Engineering. $ Department of Chemistry.
The combustion of polymers is very complex since chemical reactions and transport phenomena take place in the condensed phase and the gas phase, as well as at the interface between the two (11). Melting and evaporation of thermoplastics are combined with thermal degradation or pyrolysis, gas diffusion, and mass transfer as well as homogeneous oxidation. Thermoset plastics partially devolatilize and form char residues, and thus in addition to the above phenomena, they undergo heterogeneous oxidation. To a smaller extent this is also true for a few thermoplastics, such as PVC. Along with the major products of combustion (i.e., COz, HzO, CO, etc.),
0 1993 American Chemical Society
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the burning of polymers also generates hydrocarbon pyrolyzates as products of incomplete combustion. Such compounds are polycyclicaromatic hydrocarbons (PAHs), sulfur- and nitrogen-containing PAHs, polychlorinated dibenzodioxins (PCDDs),polychlorinated dibenzo furans (PCDFs), polychlorinated biphenyls (PCBs), aliphatic hydrocarbons, and hydrogen chloride (HC1) (9). These compounds are found as gaseous combustion effluents as well as condensate on fine particles (fly ash or soot). As early as 1775,Pott (8)observed that the combustion products from home fires might have been the cause of cancer among chimney sweepers. Since then, a multitude of studies have identified many of the above compounds (like several PAHs, dioxins, furans, and polychlorinated biphenyls) as potentially mutagenic and carcinogenic species. Thus, minimization of these speciesin the effluent of combustion systems is of paramount importance. However, there are no regulatory standards on the emissions of PAHs, PCDDs, PCDFs, and aliphatic hydrocarbons released by combustion systems. Therefore, policy must be established based on the results of scientific studies. Previous emission studies burned polymers in bulk at gas temperatures below 1000"C. No data on the existence of flame or its temperature and duration were reported. Emissions from poly(ethylene), poly(styrene), PVC, poly(methyl methacrylate), poly(acrylonitrile), cellulose, etc. were studied in some detail (13-23),but the results were often inconclusive. Most of these related investigations were conducted by placing small samples (0.05-2 g) of polymer in small nickel or ceramic boats inside externally heated horizontal tube (muffle) furnaces (13-20). Furnace wall and/or gas temperatures were monitored by thermocouples and were in the range of 600-950 "C. Sampling involved the use of glass fiber filters or Pyrex wool traps to separate the particulates from the exhaust stream. Gas-phase products were condensed in cold traps or adsorbed on activated charcoal and other chemicals, such as XAD-2 and XAD-4. Removal of hydrocarbon species from the collected matter and the condensate was accomplished by Soxhlet extraction (2-8 h) and was followed by concentration to small volumes (2-10 mL) using rotary evaporators. Identification and quantification of compounds were typically accomplished by gas chromatography (GC) with flame ionization detection (13-20),with mass spectrometry (GC/ MS) being often used to provide more detailed identification of compounds (15-19). The general chromatographic operating conditions were as follows: initial temperature 50 "C, final temperature 250-300 "C, and column heating rate of 6 "C/min. The first comprehensive study on the oxidative pyrolysis of 23 natural and synthetic polymers (plastics and fabrics) was carried out in 1973 by Boettner et al. (13) at a maximum temperature of 800 "C. A series of low carbon number aliphatics (C1-C6) hydrocarbons were identified in the products. Many higher carbon number compounds (CS-C~~) were also observed, but few were identified. Complete combustion was not achieved in that study. It is interesting to note that large amounts of benzene were detected in the oxidative pyrolysis of PVC, in the same temperature range (ca. 500 "C) that HC1 evolves. Morikawa (14)monitored the emissions of benzo[alpyrene and soot from the combustion of poly(ethylene), poly(styrene), poly(propylene), poly(acrylonitrile), PVC, 2888
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PMMA, and cellulose over the temperature range of 600900 "C. Morikawa's results showed that all synthetic polymers produced 1-2 orders of magnitude greater amounts of soot than cellulose. Generally, soot evolution increased with temperature, and poly(styrene), PVC, and poly(acrylonitri1e) produced the largest amounts of soot. At the highest temperatures of that study, poly(styrene) emitted 3-4 times more soot than poly(ethylene), but the soot of the latter contained larger amounts of benzo[a]pyrene, whose yield reached 1.5 mg/g of sample at 900 "C. The concentration of benzo[alpyrene in the soot emitted from PVC and poly(acrylonitri1e)was low despite the large soot evolution. Since no direct relationship between benzo[alpyrene and soot was found, it was concluded therein that PAHs may not necessarily be intermediary products of soot. However, since benzol'alpyrene constitutes only a minor fraction of the total PAHs, it may not be entirely appropriate to use it as an indicator of the total PAH activity (24). Hawley-Fedder et al. (15-18)monitored the products of oxidative pyrolysis of poly(ethylene),poly(styrene),and PVC. The operating temperatures of their combustion unit were 800, 850, 900, and 950 "C, and maximum residence times were 13s. Results forpoly(ethy1ene) (15) indicated that, in the temperature region between 800 and 900 "C, hydrocarbons ranging from C ~to OC36 were found to be the principal constituents in both the gaseous and condensed phase. Such species consisted of a,@-olefins, a-olefins, and n-hydrocarbons, with diene present in the largest quantities. At 950 "C, similar hydrocarbons were also present but so were PAHs, with benzo[blfluoranthene and benzo[alpyrene identified as major constituents in the gas phase and phenanthrene and acenapthalene in the condensed phase. Apparently, the higher temperature (950 "C) promoted pyrolysis and PAH formation. It was concludedthat, under those operating conditions, complete combustion of poly(ethy1ene) did not take place, despite the fact that the sample appeared to have ignited. However, pyrolytic degradation did take place. It is expected that in the flame region species with low molecular weight, such as benzene or phenol, vaporize while higher molecular weight aromatic structures polymerize (25). Combustion of poly(styrene) (16)was found to generate 3-4 times more soot than poly(ethy1ene) and over 90 different hydrocarbon compounds were identified in the combustion effluents. The condensates in the cold traps (gas phase) and the Pyrex wool (condensed phase) were analyzed separately by GUMS. In the gas phase, eight significant peaks were identified in the temperature range of 800 to 900 "C, but only four survived at the highest temperature (950 "C), i.e., indene, naphthalene, biphenyl, and phenanthrene. In the condensed phase the following compounds were identified: (a) at 800 "C, methylnaphthalenes and methylbiphenyls, methylfluorenes, vinylnaphthalenes, diphenylethene, diphenylacetylene, diphenylmethane, and 1,3-diphenylpropane;(b) at 900 "C more PAHs were produced, with naphthalene and phenanthrene being the most abundant. Overall, complete combustion was not achieved. Peak PAH production was detected at 850-900 "C-significantly decreasing at 950 "C-and the majority of PAHs were associated with the particulate matter. Combustion of PVC (17) showed the following trends: The total number of compounds identified in the GUMS
total ion chromatogram (TIC) decreased with increasing temperature. At 800 "C, 24 compounds were found in the cold traps and 53 in the glass wool trap, and at 950 "C, 16 and 28 compounds were found, respectively. At the lower temperatures, the most predominant species were (i) in the gaseous phase: naphthalene, indene, a chlorinated benzene monomer, and phenol; (b) in the condensed phase: naphthalene, phenanthrene, and chrysene. At 950 "C the major compounds identified were a benzofluoranthene isomer and a benzopyrene isomer. Chlorinated species were present in the cold traps, but they were not detected in the glass wool traps, suggesting that they were primarily present in the gaseous phase. Overall, the total amount of PAHs was greatest at 950 "C, which can be partly attributed to large concentrations of naphthalene. Interestingly enough, the most complete combustion was noticed at the intermediate temperature of 900 "C. Elomaa et al. (19)also examined the presence of PAHs in soot produced from the combustion of poly(styrene), poly(propylene), and wood. Experiments were conducted in a furnace at a gas temperature of 700 "C, as well as in ambient temperature air. The amount of PAHs was found to be proportional to the soot production. Poly(styrene) produced more soot and PAHs than either poly(propy1ene) or wood. Phenanthrene was found to be the most abundant of the PAHs in all cases. Benzo[alpyrene was present in small quantities. The authors concluded that reducing the amount of soot is essential for the reduction of PAHs. Klusmeier et al. (20)conducted combustion experiments on poly(styrene) in the temperature range 400-1000 "C. They identified benzene, toluene, ethylbenzene, m,pxylene, styrene, isopropylbenzene, a-methylstyrene, indene, divinylbenzene, naphthalene, 172-methylnaphthalene, biphenyl, diphenylmethane, 1,l-diphenylethane, 1,2diphenylethane, 1,2-diphenylpropane,phenanthrene, and anthracene in the products of combustion. Styrene was present in the highest concentration, and its maximum production occurred at 600 "C. Work has also been conducted to monitor the thermooxidative behavior of polymers at low temperatures. Hoff et al. (21) monitored the products of oxidative degradation of low-density poly(ethy1ene)at -280 "C. The products of oxidation contained aldehydes, organic acids, and ketones. PAHs were not reported under these conditions. Lattimer and Kroenke (22) examined the oxidative and inert pyrolysis products of PVC at temperatures around 550 "C, and they detected pure conjugated aromatic components such as benzene, styrene, naphthalene, biphenyl, and anthracene formed mostly via intramolecular cyclization. The most abundant of these pyrolyzates was benzene, evolving concurrently with HC1. They also detected mixed aromatic pyrolyzates such as toluene, indene, and methylnaphthalene, formed by scission and intermolecular transfer reactions. Pasternak et al.(23)also examined low-temperature (440 "C) pyrolysis products of PVC, and they detected large concentrations of organic condensables, like heptene and pentene as well as smaller concentrations of dodecane and pyrene. It can be concluded from the above lengthy, but nevertheless not all-inclusive,literature review that a wide variety of different PAHs form when plastics pyrolyze and burn at high temperatures. While many different PAHs have been painstakingly identified in the literature, no clear trend with any combustion parameter has been reported. For instance, in the gas temperature region
between 800 and 950 "C, the PAH emissions from the combustion of poly(ethy1ene) were reported to increase with temperature; those from poly(styrene) followed the opposite trend, i.e., decreased with increasing temperature; while PVC showed a minimum production of PAHs at 900 "C (14-16). Little information has been reported on the effect of residence time on emissions. Moreover, in many cases it is unclear whether ignition of the volatilized polymers occurred or the reported results reflect products of oxidative pyrolysis. The present study is aimed at exploring particle/furnace temperature-residence time trends and at identifying conditions that minimize the PAH emissions from the combustion of plastics. Since previous researchers have reported occasional difficulties in achieving complete combustion below 900 "C, the gas temperature range explored in earlier studies was extended to 1150 "C. To provide for these higher temperatures, a vertical furnace employing hanging glass heating elements was used. Experiments involved small polymer particles to ensure their ignition and to facilitate complete combustion in the furnace. Particles of narrow size distribution were fluidized and conducted to a furnace, where they burned in very dilute dispersions, and the organic products of combustion were monitored. Separate pyrometric observations were conducted on single polymer particles, burning under identical conditions, and were used to couple in-flame combustion behavior and emissions. Combustion occurred in an axially isothermal radiation cavity at constant gas temperature. This facilitated the calculation of gas residence times, which were in turn controlled by varying the gas flow rates. 3. Experimental Techniques and Procedure 3.1. Description of Polymers. Five different organic polymers (plastics) were burned in this study in powder form. All polymers were purchased from Aldrich Chemicals. Poly(styrene) (PS) [CH2CH(C&)ln was purchased in pellet form, and it was crushed into a powder immersed in liquid nitrogen in a house hold blender. The resulting powder was fractionated by sieving, and particles in the size cut of 250-297 pm were used in the experiments. This polymer consists of 92.3 % carbon and 7.7 % hydrogen. The glass transition temperature is 100 "C, the melting point is 237.5 "C, and the specific gravity is 1.047. The minimum decomposition temperature is 364 "C (ref 11,p 225). Poly(viny1 chloride) (PVC) [CH2CH(Cl)I, was purchased in powdered form consisting of polydisperse spheroid particles. The powder was fractionated by sieving, and particles in the size cut of 150-180 pm were used in the experiments. The chemical composition of this polymer is 38.44% carbon, 4.84% hydrogen, and 56.73 % chlorine. The minimum decomposition temperature has been reported to be 260 "C (ref 11,p 225). Upon heating beds of PVC particles in porcelain boats to 650 "C for a length of time sufficient for devolatilization (30 min), dark char residues were generated, -10% by weight. Poly(ethy1ene) (PE) [CHZCH~I,was purchased in polydisperse powdered form. Particles in the size cut of 212-250 pm were used in the experiments. The chemical composition of this polymer is 85.7 % carbon and 14.3% hydrogen. This sample has a melting point of 115.0 "C Envlron. Scl. Technol., Vol. 27, No. 13, 1993
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-(A) Poly(styrene)
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(a)poly(styrenel.(b) PVC, ( c ) polylelhylene). (d) poly(propylene). and (e) ply(nmthyl methacrylate).
and a specific gravity of 0.915. The minimum decomposition temperature for HDPE is 404 "C (ref 11, p 225). Poly(propy1ene) (PP) [CHzCH(CHs)I, was purchased in powdered form consisting of polydisperse spheroid particles. The powder was fractionated by sieving, and particles in the size cut of 25@297 Fm were used in the experiments. This polymer consists of 85.7 % carbon and 14.3% hydrogen. This sample has a glass transition temperatureof26 OC,meltingpointof 189 'C,andspecific gravityof0.85. The minimum decomposition temperature is 387 "C (ref 11. D 225). Poly(methy1 methacrylate) (PMMA) [CH2C(CH3)(CO~CH3)I~waspurchased in polydisperse powdered form. The powder was fractionated by sieving, and particles in the size cut of 18@212 jm were used in the experiments. This polymer consists of 60% carbon, 8% hydrogen, and 32% oxygen. It has a glass transition temperature of 114 "C and a melting point of 180 "C. The minimum decomposition temperature is 327 OC (ref 11,p 225). SEM 2888
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EnYlrOn. Sci. Technol.. VOI. 27, NO. 13. 1993
photographs of all the above particles are shown in Figure 1. 3.2. Combustion of Powdered Polymers. Combustion of the polymers was conducted in two externallyheated, laminar-flow furnaces: avertical droptube furnace and a horizontal muffle-type furnace. A. Vertical Furnace. The bulk of the experiments was conducted in a vertical furnace, fitted with an alumina tube, with a resulting radiation zone 7 em in diameter, 25 cm long (26). Wall temperatures in the range of 80C-1200 OC were easilyachieved using Kanthal aluminum disilicide of 1650"C). The gas temperature heating elements (Tmm and velocity profiles were measured with a suction thermometer and a hot wire velocimeter, respectively (26). Gas temperature profiles were generally isothermal throughout the length of the radiation cavity, and a t the centerline they were =50 "C below the wall temperatures. Pulverized plastics were introduced at the top of the furnace injector using a particle fluidizer,as shown in
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Figure 2. A certain quantity of polymer was placed in a glass vial, where it was fluidized in air, and it was subsequently conducted to the furnace through a slender tube. The vial wasadvanced by aconstant velocity syringe pump. The feed tube was kept stationary, its entrance was beveled, and it was always held half-immersed in the surface of the particle bed as the vial moved upward. The vial and the tube were vibrated to facilitate fluidization. Upon introduction in the radiation cavity, the particles were dispersed (forming aerosols) and were ignited and burned while falling by gravity. The airflow rate in this furnace was in the range of 5-12 Lpm, adjusted according to the gas temperature so as to provide gas residence times of 1 or 2 s (26). A total of 2 g of polymer powders was humedineachrun,supplied tothefumaceatratesranging from 1.6 to 2.3 g/h (depending on the polymer density and particle size). The resulting average equivalence ratio, @, definedastheactualfueltoairratiooverthestoichiometric fueltoairratio,wasintherangeof0.18-0.27.PVC burned with the lowest q5 (0.18) because of the high chlorine content.
B. Horizontal Furnace. Experiments were also conducted in a horizontal split-cell furnace, fitted with a quartz tube 4 cm in diameter, 87 em long, a t a wall temperature of 800 "C. The gas temperature profile was measured with a thermocouple, inserted in the tube. The temperature was found to increase axially in the first half of the furnace, but was fairly constant in the second half of the furnace, ca. 750 O C . Thus, the first half length of the furnace acted as an air preheater. Two porcelain boats loaded with 1g of sample each (2 g total) were placed in the middle of the quartz tube, Figure 3. Upon reaching the predetermined wall temperature, the top half of the furnace was opened, and the quartz tube, containing the samples, was promptly placed inside. The airflow rates were 9 or 19 Lpm, and the corresponding gas residence times in the second half of the tube (downstream of the sample) were of 1.1and 0.5 s, respectively. These periods correspond to the postcomhustion residence times in this furnace a t a temperature of ca. 750 "C. Sampling for particulates and hydrocarbons was conducted a t the exit of the furnace. Several minutes after the ignition of the Environ. Scl. Technol..
VoI. 27. NO. 13. 1993
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Ceramic Lining Clam0
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sample took place, the tube was removed from the furnace and the sampling unit was disconnected. 3.3. PAH Extraction and Concentration. Glass fiber filters and adsorbent powders (XAD-2 and XAD-4) were precleaned by Soxhlet extraction for 16 h in methylene chloride. XAD adsorbents were chosen because they exhibit excellent recovery efficiency for the compounds being examined (27,28). The sampling train arrangements are shown in Figures 2 and 3. Upon completion of each combustion run, both filters and adsorbents were placed in the Soxhlet apparatus again and extracted for 16 h in methylene chloride. The extract was subsequently concentrated to a volume of 1.5 mL using a rotary evaporator and transferred to a Teflon-sealed screw-capvial for storage and analysis. 3.4. Chromatographic Analysis. Gas chromatography was used for sample analysis. The instrument was a Hewlett Packard (HP5890) gas chromatograph equipped with flame ionization detection (FID). Identification of compounds was achieved by matching the retention times of sample extracts with the retention times of known compounds found in a coal tar extract standard (SRM 1597), provided by NIST (NationalInstitute of Standards and Technology). A DB-5 Ohio Valley capillary column was used, 0.25 mm i.d. and 30 m in length. The GC operating parameters were as follows: initial temperature 100 "C, initial time 2 min, heating rate 4 "C/min, and final temperature 280 "C. The injector temperature was set at 300 "C, the detector temperature was set at 320 "C, and the column head pressure was set at 1.34 atm (20 psi) to give a gaseous flow rate of ca. 2 mLimin (27). 3.5. Single-Particle Pyrometry. To monitor the temperature and duration of the flame envelope around plastic particles burning in the vertical furnace, a threecolor pyrometer was developed. The technique and the instrument used are briefly described herein; details are given elsewhere (29, 30). Pyrometric observations were conducted from the top of the furnace injector, by replacing the particle feeder with a light collection stage which incorporates a particle introduction port. Thus, by monitoring the flight of single polymer particles falling in the drop-tube furnace along the paths of their luminous trajectories, their combustion history was recorded (30). The radiation emitted by each burning particle was transmitted by a single-fiber optic cable from the furnace to the three-color near-infrared pyrometer (29,30). The 2890
Environ. Scl. Technol,, Val. 27, No. 13, 1993
pyrometer used dichroic spectrum separators to channel the radiation through interference filters to silicon photodetectors. Working wavelengths for the three channels were selected at 0.64, 0.81, and 0.998 pm. The signals were amplified 109times and were recorded, at a frequency of 33 kHz per channel, by an IBM-AT microcomputer equipped with a Data Translation DT2828 A/D high-speed board, using the software package Asyst. 4. Results and Discussion
In the present laboratory-scaleexperiments, as in actual incinerators, the products of thermal decomposition of plastics exhibited both in-flame and postflame (thermal) oxidation. In the vertical oven,in-flameoxidation occurred during the luminous combustion stage. This was monitored by the pyrometer and lasted 15-120 ms for the particle size range of this study (150-300 fim). Postflame (thermal) oxidation occurred upon extinction of the flame as the products of combustion passed through the remaining length of the tube at the furnace temperature. 4.1. In-Flame Combustion. In-flame combustion was monitored in the vertical furnace only, by burning single polymer particles. The conditions were similar to those under which the combustion of dilute dispersions of particles took place. Signals generated by the luminous combustion of particles were recorded by the three-color pyrometer, throughout each particle's burn time. Flame temperatures were obtained by two-color signal ratios using a Planckian approach, as described in refs 29 and 30. In the latter work, the combustion behavior of monodisperse batches of particles of different sizes was studied in detail. Experiments were conducted in air at gas temperatures, Tg,of 750,950, and 1150 O C . Representative three-color signals and corresponding two-color ratio temperaturetime profiles of four individual polymer particles are shown in Figure 4. Profiles obtained from particles of the same kind and size were quite repeatable. PVC and poly(styrene)burned with bright sooting flames that produced strong signals, Figure 4, panels a and b. PVC burned the fastest (15 ms), but it was also the smallest in particle size, containing ca. 4 times less mass than poly(styrene). The flame temperature of PVC rose sharply to 3000 K and then declined to ca. 2000 K, where glowing combustion of the resulting chars was observed briefly (a few ms) (30). The combustion of poly(styrene) particles lasted for 90
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Figure 4. Three-color optical pyrometry radiation Intenslty-time and temperature-time proflles of single particles burning in air at T,, = 1223 K: (a) a PVC particle In the size range of 150-180 pm: (b) a poly(styrene) particle In the slze range of 250-297 pm; (c) a poly(ethylene) particle in the size range of 212-250 pm; and (d) a poly(propylene)particle in the slzerange of 250-297 pm. The nonlumlnous flames of PMMA were not monitored. The wavelengths depicted In the lntenslty profiles were (-) 640, (- -) 810, and 6s.) 998 pm. The wavelength palrs represented In the temperature proflles were (-) 998/810, (- -) 998/640, and (.) 810/640 pm. The baselines of the slgnalare displaceddue todifferent sensitivity to the backgroundfurnace radlatlon.
ms, at an approximately constant temperature (ca. 2500 K). Poly(ethy1ene) and poly(propy1ene) exhibited combustion characteristics similar to each other, i.e., delayed ignition, low luminosity flames, long burn times (120 ms), and temperatures close to 3000 K. These temperatures are in the vicinity of calculated adiabatic flame temperatures with preheated air (30). While the former two materials ignited at the top of the radiation cavity of the furnace, the latter two, i.e., PE and PP, ignited at midlength. Thus, the postflame residence times of their gases were actually half of the nominal 1 and 2 s. Particle signals and temperature-time profiles were obtained for particles burning at gas temperatures of 950 and 1150 "C. It was noticed that differences among the temperature-time profiles of similar particles burning at either of the above temperatures were minimal. This may be attributed to the short combustion durations and the resulting nearly adiabatic and relatively thick flames that
minimized convective losses. Hence, in the above range of gas temperatures and particle sizes, the influence of the gas temeprature was not pronounced. However, further work is needed in this area, and this phenomenon will be explored using particles of different sizes. At 750 OC ignition was erratic, the signal to noise ratio was low, and therefore, results at that temperature are not reported. Flame combustion of PMMA was nonluminous, which is indicative of the low smoke production of this polymer. Faint blue flames were noticed at all gas temperatures but could not be recorded with the present apparatus. Combustion of beds of particles, placed in ceramic boats, in the horizontal (muffle furnace) also generated flames above the samples, but the lifetime, size, and temperature of the flames were not monitored. 4.2. Postflame Observations a n d Monitoring of PAHs. Upon completion of the flame combustion period, the oxidation or pyrolysisproducts traversed the remaining length of the radiation cavity, i.e., the postflame region of the vertical furnace at approximately the furnace gas temperatures. This is because dilute dispersions of particles were generated and the products of combustion quickly cooled to the gas temperature. Since flame combustion lasted for at most 120 ms, Le., less than 10% of the total furnace residence time in all experiments and since flame temepratures were found to be rather independent of the gas temperatures in the region of 950-1150 "C, the observed concentration changes were mainly attributed to the conditions in the postflame region. Results are described in the following for each polymer. A. Poly(styrene). Heating poly(styrene) at temperatures above 350 "C releases products that contain monomer, dimer, and trimer in concentrations around 4045% (25) generated by end-chain scission and depolymerization. Formation of soot follows by direct production from aromatic compounds. Indeed, this was the most sooting polymer, along with PVC, in the present experiments. Postflame oxidation in the horizontal furnace at Tg~ 7 5 "C 0 for 1s resulted in intense chromatographic activity throughout the spectrum (Table I, Figure 5a). Consistent with previously reported work (16),naphthalene and methylnaphthalenes, phenanthrene, biphenyl, and methylfluorenes were the most prominent PAHs. Dibenzofuran was also identified. On the contrary, there was distinctly less activity in the vertical furnace at the same gas temperature, both at 1-and 2-s residence times (Figure 5b). This was partially attributed to incomplete depolymerization and oxidation, since a fair amount of resolidified polymer was observed on the removal of the glass fiber filter from the bottom of the furnace. In addition to other compounds, substantial quantities of naphthalene and fluoranthene were detected in the above experiments. Increasing the vertical furnace gas temperature to -950 "C resulted in significantly more active chromatograms for both residence times considered (Figure 512). 2-Methylanthracene, benzo[blnaphtho[2,1-d]thiophene, and benzo[blfluoranthene, though present at 750 OC, increased in concentration with stronger peaks being observed at the 2-s residence time. Furthermore, many more peaks appeared, corresponding to compounds with carbon number Clo-C14, such as dibenzofuran. An almost identical chromatogram was obtained at Tgof 1150 "C, 1-sresidence time, Figure 5d, but when the residence time was increased to 2 s at the same temperature, a dramatic reduction in both number and concentration of Environ. Sci. Technoi., Vol. 27,No. 13, 1993 1891
Table I. PAH Compounds in Polymer Combustion Products* compound
HF (0.5 8 , 750 "C)
HF (1.18, 750 "c)
VF (1 8, 750 "C)
VF (2 s, 750 "c)
VF (1s,
950 "C)
VF (2 s, 950 "C)
VF (1 s, 1150 "C)
VF (2 s, 1150 "C)
naphthalene 493 1,2,4,3,5 1 1 2 1, 2,4 1,2 1,295 benzothiophene 493 3,5 1 1 1 115 2-methylnaphthalene 493 1,2,4,5 4 2 1-methylnaphthalene 3 2,4,3,5 2 194 1 biphenyl 4 1,2,4,3,5 1 4 4 1 acenaphthylene 4,3 2,395 2 195 acenaphthalene 3 2,5 dibenzofuran 193 1 1 1 1,2,5 1,2 fluorene 493 2,4,3,5 2 2 1 2-methylfluorene 1,2,3 1,4 1 195 1 1 I-methylfluorene methylfluorene 1 1 dibenzothiophene 5 4 195 1 192 phenanthrene 4 1,2,4, 3,5 4 293 5 L4 4 anthracene 4 1,294 4 1,4 1 carbazole 4 192 4 2,5 1,2,4,5 2 3-methylphenanthrene 3 293 1 L2,3 1,2 1 2-methylphenanthrene 3 1,2,3,5 1 3 5 5 1 2-methylanthracene 3 2,3,5 1 1,3 1 1 4H-cyclopenta[defl phenanthrene 4 1,4,3,5 3 194 4- and/or 9-methylphenanthrene 3 192 2 L2,5 1-methylphenanthrene 4 L4 1 1 4 1 1 2,4 5 fluoranthene 3 1,293 acephenanthrylene 4 2,395 1 2 4 2 phenanthro[4,5-bcdl thiophene 5 3 pyrene 493 1,2,4,3,5 2,5 4H-benz[deflcarbazole 3 2,3,5 3,5 benzo[a]fluorene 493 1,2,4,3 4 3 1 1 benzo[ blfluorenelmethylpyrene 1,2,4,3 3 1-methylpyrene benzo[b]naphtho[2,1-dlthiophene 4 2,495 1 1, 2 1 1 1,4 benzokhi] fluorathene 4 4 1 1,2,3,5 4,5 1 benzo[c]phenanthrene 1 4 4 benzo[b]naphtho[l,2-dl thiophene benzo[b]naphth0[2,7-d1thiophene 2 4H-cyclopenta[cdlpyrene 4 3 5 2, 5 4 172 benz[a]anthracene 4 4 4 4 4 4 chrysene/triphenylene 3 3 benzo[blfluoranthene 4,3 1,234 4 1 195 195 2 benzou] fluoranthene 4,3 1,4,3 194 3 192 1,4 L4 benzo[k]fluoranthene 4,3 L4,3 3 1 1 benzo[a] fluoranthene 4 1,2,4,5 2,3 2 benzo[el pyrene 3 1,2,3,5 395 4 4 2 benzo[a]pyrene 3 192,395 4 3 194 1,4 4 perylene 4 4,5 3 indeno[7,1,2,3-cdeflchysene 4 4,3 dibenz [ail anthracene 2 1 1 1 1 1 indeno[ l,2,3-cd]pyrene 493 4 dibenz[a,hlanthracene 19.2 3 pentaphene 3 4 benzo[b] chrysene 493 493 picene 433 493 benzo[ghilperylene 493 4 anthrathrene a Abbreviations: 1, poly(styrene); 2, PVC; 3, poly(ethy1ene);4, poly(propy1ene); 5, PMMA; HF, horizontal furnace; VF, vertical furnace; time refers to postflame residence time; temperature is the gas temperature.
species was observed (Figure 5e). For instance, biphenyl and, to a lesser extent, naphthalene survived the highest temperatures, but phenanthrene did not. On the basis of the comparison of retention times with the NIST standard, 4H-cyclopenta[cdlpyrene was identified as the strongest peak in all chromatograms, eluted at =37 min. The relative concentration of this compound diminished significantly at 1150 "C and 2 s. Moreover, the signals of dibenzofuran, phenanthrene, anthracene, benzofluoranthenes, and benzoanthracenes disappeared under these conditions, while naphthalene was still recorded. B. Poly(viny1 chloride). This polymer is known to evolve hydrogen chloride gas, to undergo chain stripping reactions, and to aromatize by hydrogen evolution at 2892
Envlron. Sci. Technoi., Vol. 27, No. 13, 1993
roughly 450 "C. At higher temperatures, cross-linking between chains results in carbonized residues (25). Negligible monomer yield upon thermal decomposition has been reported (25). Chromatographic analysis of the effluent of the horizontal furnace at Tg= 750 "C revealed an intense activity of many species throughout the spectrum, especially at the 1.1-s residence time. Strong peaks of naphthalene and phenanthrene were identified as observed elsewhere (17,19, 22), and furthermore, fluoranthene and anthracene (17)and substantial peaks of benzo[alpyrene and benzo[elpyrene (19) were identified. Moreover, the presence of carbazole, pyrene, and methylated compounds of naphthalene and phenanthrene were also detected. Both the number of peaks and their
t
e: 1150°, 2 seconds, vertical
-
d: 1150°, 1 second, vertical
c: 950a, 2 seconds, vertical
w
m
b: 750°, 2 seconds, vertical
.I.,
.
I
a: 750°, 1 second, horizontal
TIME, minutes
Figure 5. Gas chromatogram of poly(styrene)burning in (a) the horizontal furnace at T, = 750 O C , 1 s residence time; (b) the vertical furnace at 750 O C , 2 s; (c) the vertical furnace at 950 O C , 2 s; (d) the vertical furnace at 1150 OC, 1 s; and (e) the vertical furnace at 1150 O C , 2 s.
intensity were reduced when combustion took place in the vertical furnace at gas temperatures of 950 and 1150 "C. Naphthalene and methylated naphthalenes were present, along with dibenzofuran, carbazole, fluoranthene, and phenanthrene. Again, the most complete combustion occurred at 1150 "C,especially at the 2-s postflame residence time, where most of the organic constituents of the spectrum disappeared. A peak that was identified as naphthalene was strong a t the lowest temperature, but decreased in intensity as the temperature or residence time increased and was barely detectable at all at 1150 "C and 2 s. Carbazole was fairly strong at the lower temperatures, but again was destroyed at the highest temperature and longest residence time. A list of all the identified compounds is given in Table I. C. Poly(ethy1ene). Poly(ethylene), like other poly(olefines), does not decompose to the monomer. Chain scission and chain-transfer reactions are important during thermal decomposition and generate a wide range of alkanes and alkenes (25). Experiments conducted in the horizontal furnace at Tg= 750 OC resulted in extremely active chromatograms with the characteristic pattern of triplets of peaks, eluting every 3 min, that were identified by previous researchers (ref 15and references cited therein) as being a,w-olefin,a-olefin, and n-hydrocarbon, with chain lengths of 8-23 hydrocarbons. This pattern provides evidence of pyrolytic degradation but incomplete combustion (15). Several of the PAHs that were identified in ref 15 were also detected herein, including naphthalene, fluorene, phenanthrene, fluoranthene, and crysene (see Table I). Incomplete combustion was also observed in the vertical furnace at the gas temperature of 750 "C.This can be explained on the grounds that, at 750 OC, flame combustion of poly(ethy1ene)did not start until the bottom half of this drop tube furnace, and the organic constituents did not have enough time to undergo postflame thermal
destruction. Thus, during these experiments, the gas residence time in the postflame region of the furnace was a fraction of the nominal total (1 or 2 8 ) . During experiments a t 950 and 1150 "C, the particles ignited and burned in the middle section of the furnace providing postflame residence times half of the nominal total (1or 2 s), i.e., 0.5 or 1 s. Nevertheless, examination of the collection stage showed that effective combustion was achieved under these conditions, and the corresponding chromatograms exhibited minimal activity. Most of the components that were present in the effluent of the horizontal furnace experiments diminished or disappeared when poly(ethy1ene) was burned in the vertical furnace, especially at the highest temperature (see Table I). D. Poly(propy1ene). Poly(propy1ene) decomposes in a fashion similar to poly(ethylene), but it is less stable due to the presence of tertiary carbons in the chain (25).Again, intense activity of organic constituents in the effluent of the horizontal furnace was detected. This was represented by a series of strong peaks distributed throughout the entire spectrum of the chromatogram. Among the compounds identified were naphthalene and methylnaphthalene, biphenyl, fluorene, phenanthrene, and methylphenanthrene, anthracene, pyrene, and benzo[a]fluorene (see Table I). However, the number and intensity of compounds reduced significantlyin the experiments conducted in the vertical furnace. Again, the lowest number of species and concentrations in the chromatograms of this polymer were obtained at 1150 "C and 2-s nominal residence time (1-s actual time). Aside from these conditions, a group of O in all unidentified compounds smaller than C ~ appeared other chromatograms. Naphthalene, methylnaphthalenes, biphenyl, acenaphthylene, fluorene, phenanthrene, anthracene, carbazole, benzo[al pyrene, and benzofluoranthenes were among some of the more notable PAHs identified in the vertical furnace experiments (see Table Environ. Sci. Technol., Vol. 27, No. 13, 1993 2803
c: 950°, 2 seconds, vertlcal
:
d -
2
b: 950°, 1 second, vertical
r
-L
I
a: 750°, 1 Second, horizontal
0
5
10
15
20
25
30
35
40
45
50
55
60
65
i
70
TIME, minutes
Flgure 6. Gas chromatogram of poly(methy1methacrylate) burning in (a) the horizontal furnace at T, = 750 "C,1 s resldence time; (b) the vertical furnace at 950 O C , 1 s; (c) the vertical furnace at 950 "C,2 s; and (d) the vertical furnace at 1150 "C,2 s.
I). Benz[alanthracene and to a lesser degreephenanthrene and benzo[cl phenanthrene survived the highest temperatures. E. Poly(methy1 methacrylate). This polymer thermally decomposes almost entirely to the monomer with the decomposition temperature markedly dependent on the polymerization method used to produce it (25). The chromatograms of the combustion products of this polymer exhibited the greatest self similarity, for all conditions explored, Figure 6. Once again, combustion at high temperature was more effective, and compounds smaller than CIS that were present in the horizontal furnace experiments were destroyed. Combustion of PMMA occurred in a nonsooting mode and, thus, exhibited the most benign activity in the chromatograms. At all conditions there was a group of compounds between C I ~ and C18, of which only one compound, 4- and/or 9-methylphenanthrene, was identified herein. Moreover, a peculiar appearance of a group of compounds smaller than C10, including naphthalene was noted at 1150 "C. Dibenzofuran, phenanthrene, carbazole, and benzo[blfluoranthene were present at 950 "C, while small peaks of naphthalene, benzothiophene, acenaphthylene, and fluoranthene survived the highest temperature (1150 "C). All of the above results reflect the combined PAHs in the gaseous and condensed phases (mainly soot). While generally representative of these materials, some small variations may be encountered with different grades of polymers, since the purity and molecular weight of the polymer can markedly affect not only the decomposition rates but also the mechanism of decomposition. 5. Summary and Conclusions
A preliminary laboratory study was conducted aimed at exploringoverall trends in the combustion and emissions of selected municipal waste plastics. Tests were conducted at furnace temperatures and residence time conditions pertinent to municipal incinerators that burn pulverized refuse-derived fuel (RDF). Small polymer particles in 2894
Environ. Sci. Technol., Vol. 27, No. 13, 1993
controlled size cuts, in the range of 150-300 pm, were used to ensure complete burning in the limited size of the benchscale furnaces employed herein. Gas temperatures varied between 750 and 1150 "C, and postflame gas residence times were in the range of 0.5-2 s. The oxygen partial pressure was kept constant at 0.21 atm throughout these experiments. Two different furnaces and combustion modes were used; a horizontal furnace where packed beds of particles were burned and a vertical furnace where combustion of dispersed particles was accomplished. The former furnace was used to compare results with experimental findings under similar conditions, reported in the literature. The comparison was satisfactory since the same characteristic products (and groups of products) were recognized. The latter furnace was subsequently used to simulate conditions of RDF combustion, achieve complete combustion of polymer particles, extend the upper limit of temperatures used in previous studies, and, finally, facilitate observations in both the flame (and char) and postflame combustion zones. This approach made it possible to identify trends in the PAHs production with temperature and residence time. Particles of poly(styrene) and PVC were observed to burn with luminous envelope flames in air, which is a characteristic of high soot concentrations. Combustion of poly(ethy1ene) and poly(propy1ene) was substantially less luminous, and that of PMMA was nonluminous at all (blue flames). Flame temperatures for the first four polymers were in the range of 2500-3000 "C, in air preheated to 950-1150 "C. Flame combustion lasted for several tens of milliseconds for the particle size range of this study. PVC combustion also incorporated a brief residual char oxidation stage following the combustion of volatiles. The overall flame (or flame char for PVC) temperature and duration, however, did not change substantially with gas temperature in the above region (950-1150 "C). Thus, changes in PAH emissions were mainly attributed to the postcombustion gas temperature and residence time. The lowest furnace temperature
explored, 750 "C, was not sufficient to sustain uniform ignition of the particles; hence, observations were not made therein. A clear trend in the emissionsof unburned hydrocarbons was found in these experiments, indicating that destruction of organic compounds was enhanced with increasing postflame residence time and temperature. Minimum emissions of hydrocarbons were achieved at the highest gas temperature (1150 "C) and residence time (2 s) used herein. Further work is needed to explore the influence of ambient oxygen concentration, the effect of even higher gas temperature, and possible synergistic effects (catalytic or inhibitory interactions) in the combustion and emissions of mixed polymer feeds. A future investigation is planned where detailed identification and quantification of selected organic constituents of the combustion effluent, in both the gaseous and condensed phase, will be conducted with GUMS techniques.
Acknowledgments The authors are grateful to Ajay Atal and Mohamed Mahmoud for technical support. Thanks are also due to Bill Fowle for the SEM micrographs. This work was supported by the National Science Foundation Initiation Grant CTS-8908652 and with funds provided by Northeastern University.
Literature Cited Kreith, F. Solid Waste Management: 1989-1990 State Legislation, 47-91-NCSL-1, 1991. Callari, J. Plast. World 1989, 47, 12-15. Karasek, F. W.; Viau, A. C. J . Chromatogr. 1983,265,7988. Karasek, F. W.; Viau, A. C.; Guiochon, G.; Gonnord, M. F. J. Chromatogr. 1983,265, 79-88. Manning, J.;Chehaske, J.;Fraley, P. Sampling and Analysis of Air Toxics from Municipal Waste Combustors. Proceedings from the Second International Conference on Municipal Waste Combustion; 1991; pp 885-910. Shaub, W. M.; Tsang, W. Environ. Sci. Technol. 1983,17, 721-730. Clement, R. E.; Karasek, F. W. J . Chromatogr. 1982,234, 395-405. Smith, A. €3.; Goeden, H. M. Combust. Sci. Technol. 1990, 74, 51-61. Eklund, G.; Stromberg, B. Chemosphere 1983,12,657-660.
Giugliano,M.; Cernuschi, S.;Ghezzi, U. Chemosphere 1989, 19,407-411. Cullis, C. F.; Hirschler, M. M. The Combustion of Organic Polymers, 1st ed., Clarendon Press: Oxford, 1981;Chapter 3. Senkan, S. M. Environ. Sci. Technol. 1988,22 (41,368-370. Boettner, A.; Ball, G. L.; Weiss, B. Combustion Products from the Incineration of Plastics; EPA-67012-73-049;U.S. Government Printing Office: Washington, DC, 1973. Morikawa, T.J . Combust. Toxicol. 1978, 5, 349-360. Hawley-Fedder, R. A.; Parsons, M. L.; Karasek, F. W. J. Chromatogr. 1984, 314, 263-273. Hawley-Fedder, R. A.; Parsons, M. L.; Karasek, F. W. J. Chromatogr. 1984,315, 201-210. Hawley-Fedder, R. A.; Parsons, M. L.; Karasek, F. W. J . Chromatogr. 1984, 315, 211-221. Hawley-Fedder, R. A. PbD. Dissertation, Arizona State University, May 1984. Elomaa, M.; Saharinen, E. J . Appl. Polym. Sci. 1991, 42, 2819-2824. Klusmeier, W.; Ohrbach, K. H.; Kettrup, A. Thermochim. Acta 1986,103, 231-237. Hoff,A.; Jacobbson, S. J . Appl. Polym. Sci. 1981,26,34093423. Lattimer, R. P.; Kroenke, W. J. J. Appl. Polym. Sci. 1980, 25, 101-110. Pasternak, M.; Zinn, B. T.; Gardner, R. 0.;Browner, R. F. Combust. Flame 1984,55, 117-120. Haynes, B. S. Soot and Hydrocarbons in Combustion. In Fossil Fuel Combustion, 1sted.; Bartok, W., Sarofim, A. F., Eds.; John Wiley: New York, 1991. Beyler, G. Thermal Decomposition of Polymers. In Society of Fire Protection Engineers Handbook of Fire Protection Engineering; National Fire Protection Association: Quincy, MA, 1988. Cumper, J. G.; Levendis, Y. A.; Metghalchi, M. ASME Publication HTD-148. ASME: Pittsburgh, 1990; pp 8996. Wheatley, L. H. Masters Thesis, Northeastern University, Boston, 1992. EPA. Test Method TO-13,PAH Determination in Ambient Air. Levendis, Y. A.;Estrada,K. R.;Hottel, H. Rev. Sci.Instrum. 1992,63 (7), 3608-3622. Panagiotou, T.; Levendis, Y. A. A Study on the Combustion Characteristics of PVC, Poly(styrene),Poly(ethylene), and Poly(propy1ene) Particles under High Heating Rates. Submitted to Combus. Flame.
Received for review July 19, 1993. Accepted August 13, 1993.'
* Abstract published in Advance ACS Abstracts, October 1,1993.
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