Environ. Sci. Technol. 2001, 35, 3541-3552
Polycyclic Aromatic Hydrocarbon and Particulate Emissions from Two-Stage Combustion of Polystyrene: The Effect of the Primary Furnace Temperature J U N W A N G , † Y I A N N I S A . L E V E N D I S , * ,† HENNING RICHTER,‡ JACK B. HOWARD,‡ AND JOEL CARLSON§ Northeastern University, Boston, Massachusetts 02115, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, and U.S. Army Natick, RD&E Center, Natick, Massachusetts 01760
A study is presented on laboratory-scale combustion of polystyrene (PS) to identify staged-combustion conditions that minimize emissions. Batch combustion of shredded PS was conducted in fixed beds placed in a bench-scale electrically heated horizontal muffle furnace. In most cases, combustion of the samples occurred by forming gaseous diffusion flames in atmospheric pressure air. The combustion effluent was mixed with additional air, and it was channeled to a second muffle furnace (afterburner) placed in series. Further reactions took place in the secondary furnace at a residence time of 0.7 s. The gas temperature of the primary furnace was varied in the range of 5001000 °C, while that of the secondary furnace was kept fixed at 1000 °C. Sampling for CO, CO2, O2, soot, and unburned hydrocarbon emissions (volatile and semivolatile, by GCMS) was performed at the exits of the two furnaces. Results showed that the temperature of the primary furnace, where PS gasifies, is of paramount importance to the formation and subsequent emissions of organic species and soot. At the lowest temperatures explored, mostly styrene oligomers were identified at the outlet of the primary furnace, but they did not survive the treatment in the secondary furnace. The formation and emission of polycyclic aromatic hydrocarbons (PAH) and soot were suppressed. As the temperature in the first furnace was raised, increasing amounts of a wide range of both unsubstituted and substituted PAH containing up to at least seven condensed aromatic rings were detected. A similar trend was observed for total particulate yields. The secondary furnace treatment reduced the yields of total PAH, but it had an ambiguous effect on individual species. While most low molecular mass PAH were reduced in the secondary furnace, concentrations of some larger PAH increased under certain conditions. Thus, care in the selection of operating conditions of both the primary furnace (gasifier/ burner) and the secondary furnace (afterburner) must be exercised to minimize the emission of hazardous pollutants. The emissions of soot were also reduced in the afterburner but not drastically. This indicates that soot is indeed resistant to oxidation; thus, it would be best to avoid its formation in the first place. An oxidative pyrolysis temperature of PS in the vicinity of 600 °C appears to accomplish 10.1021/es0105109 CCC: $20.00 Published on Web 07/27/2001
2001 American Chemical Society
exactly that. An additional afterburner treatment at a sufficiently high temperature (1000 °C) may be a suitable setting for minimization of most pollutants. To obtain deeper understanding of chemical processes, the experimental results were qualitatively compared with preliminary predictions of a detailed kinetic model that describes formation and destruction pathways of chemical species including most PAH observed in the present work. The modeling was performed for the secondary furnace assuming plug-flow conditions therein. The experimentally determined chemical composition at the outlet of the primary furnace was part of the input parameters of the model calculation.
Introduction The pyrolysis and combustion behavior of polystyrene (PS) is of technological interest since it is one of the omnipresent commercial plastics in insulation, pipes, foams, containers, appliances, rubber products, automotive instruments, and panels. Thus, the products of its combustion are of interest to both fire technology and waste-to-energy incineration. PS is the third most prominent polymer (plastic); its annual production rate currently exceeds 1 100 000 t in the United States alone. As most applications of PS are in disposable goods (packaging, cups, plastic cutlery, etc.), a large fraction of the polymer produced ends in the municipal waste stream (MSW). With landfill space dwindling, burning this nonbiodegradable material in waste-to-energy plants is an attractive option. Thermal destruction of MSW offers several advantages as it provides maximum volume reduction, permanent disposal, and detoxification. However, to avoid exposure of populations to health-hazardous emissions from such facilities, combustion conditions must be carefully selected. Correlations between emissions and operating parameters, such as furnace temperature, residence time, fuel loading, excess air, and combustion staging, must be investigated. Emissions of special concern are CO, NOx, particulates, and polycyclic aromatic hydrocarbons (PAH), which are often mutagenic. Thermal degradation (pyrolysis) of PS ([-CH2CH(C6H5)-]n) occurs first by chain scission and then by random scission (1). Heating PS above 350 °C releases the styrene monomer but also releases the dimer and trimer. Such depolymerization may produce up to 80 wt % monomer, depending on the heating rate and temperature (2). In fire cases, released styrene is an irritant and has narcotic effects. Other hydrocarbons formed during pyrolysis have been identified as benzene, toluene, ethyl- and dimethylbenzene, naphthalene, and other PAH (3-5). PAH compounds form via sequential hydrogen abstraction/acetylene addition (HACA mechanism) (6) and grow by reactive coagulation of smaller PAH (7). The formation of soot has been linked to PAH formation/destruction mechanisms. Burning of plastics occurs via a complicated interaction of thermal degradation, pyrolysis, combustion of pyrolysis products, and mass and energy transport in both the solid/ liquid and gaseous phases. Upon ignition, a diffusion flame surrounds the pyrolyzing/devolatilizing polymer. Formation and destruction of PAH and soot depend on the flame temperature, the residence time, and the availability of * Corresponding author phone: (617)373-3806; fax: (617)373-2921; e-mail:
[email protected]. † Northeastern University. ‡ Massachusetts Institute of Technology. § U.S. Army Natick. VOL. 35, NO. 17, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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oxygen. Incineration of MSW traditionally required that furnaces operate at high reaction temperatures, excess air levels, and long residence times (such as 2 s at 1000 °C). While such conditions are known for efficient destruction of solid wastes, they may allow some of the pollutants to increase to unacceptable levels (2, 3). Determination of optimum furnace temperature is essential for proper design/operation of incinerators for minimum emissions. This parameter is explored in the present work. However, since combustion of solid polymers takes place in gaseous diffusion flames, a broad range of local equivalence ratios and temperatures exists in the flame, and products of incomplete combustion may form in oxygen-starved regions. In this work, mixing the gaseous effluent of such diffusion flames with additional air and then channeling the ensuing flow to a second furnace for further oxidation was investigated. As confirmed in previous experiences in this laboratory, once soot forms from the combustion of fuels, it is very resistant to oxidation. Therefore, it was deemed beneficial to explore conditions in the combustion of polymers that minimize the formation of soot and its precursors in the first place. For these reasons, combustion of polymers was conducted in two stages. The primary furnace served as the solid fuel (PS) gasifier and partial oxidizer. The secondary furnace served as a premixed gaseous fuel burner. This arrangement also facilitated mechanistic insights into PAH formation/consumption since the secondary furnace could be treated as a plug-flow reactor.
Literature Review on PAH and Soot Emissions from Polystyrene Pott in the Sixteenth Century first related combustion products from home fires with cancer among chimney sweepers, as mentioned in ref 10. Since then, it became increasingly clear that products of incomplete combustion, especially some polycyclic aromatic hydrocarbons (PAH), are carcinogenic, hence their identification and minimization became imperative. Previous laboratory investigations examined unsteady (11-15) or steady-state combustion of polystyrene (16-19, 21-25). Morikawa (11) monitored the emission of benzo[a]pyrene, a suspected carcinogen, and soot from combustion of polyethylene, polystyrene, polypropylene, and other plastics as well as cellulose at 600-900 °C and found that soot evolution increased with temperature. Polystyrene produced the largest amounts of soot, 3-4 times more than polyethylene, but the soot of the latter contained larger amounts of benzo[a]pyrene. Cellulose produced the least amount of soot. Hawley-Fedder et al. (12) also found that the combustion of polystyrene at 800-950 °C, generates 3-4 times more soot than polyethylene and identified 90 different hydrocarbon compounds. Only indene, naphthalene, biphenyl, and phenanthrene survived in the gas phase at 950 °C. In the condensed phase, methylnaphthalenes and methylbiphenyls, diphenylacetylene, diphenylmethane, and 1,3-diphenylpropane were identified at 800 °C, while at 900 °C, additional PAH were observed with naphthalene and phenanthrene being the most abundant. PAH production significantly decreased at 950 °C. The majority of PAH, in particular those of high molecular weight, were associated with the particulate matter. Elomaa and Saharinen (13) monitored condensed PAH in soot produced from the combustion of polystyrene, polypropylene, and wood at 700 °C. The amount of PAH was found to be proportional to the soot production, and again polystyrene produced more soot and PAH than either polypropylene or wood. Phenanthrene was the major PAH, with yields 15 times higher than benzo[a]pyrene. Combustion of PS was conducted by Klusmeier et al. (14) at 400-1000 °C. Styrene was identified to be the main product, 3542
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and its maximum yield (64%) occurred at 600 °C. Other compounds identified were benzene, toluene, ethylbenzene, m-, and p-xylene, isopropylbenzene, 2-methylstyrene, indene, divinylbenzene, naphthalene, 1,2-methylynaphthalene, biphenyl, diphenylmethane, 1,1-diphenylethane, 1,2-diphenylethane, 1,2-diphenylpropane, phenanthrene, and anthracene. Wheatley et al. (15) studied the batch combustion as well as the steady flow combustion of plastics in this laboratory, from 800 to 1200 °C, 1-2 s gas residence times. Qualitative results on PAH emissions were obtained using gas chromatography (GC-FID). Increasing the gas temperature or the residence time, in the aforementioned ranges, the number and yields of PAH decreased dramatically. That investigation was followed by a comprehensive study on the burning characteristics and emissions from steadystate combustion of polystyrene and other polymers at Northeastern University over the past several years. To monitor combustion emissions, powders of polymer particles were injected downward at predetermined constant flow rates in a drop tube furnace. Emissions of PAH, soot, CO, CO2, and NOx were measured at various global equivalence ratios and gas temperatures (16-19). Identification and quantification of PAH were achieved by gas chromatography coupled to mass spectrometry (GC-MS). A parallel investigation on the combustion behavior and flame temperatures of similar size polymer particles was conducted in similar apparatus and conditions using high-speed photography and pyrometry. To facilitate such studies, spherical and monodisperse polymer particles were produced (20) or were commercially purchased, in the size range of 30-350 µm. Observations on combustion of both singles and groups of particles were conducted (19-22) and revealed the intense sooting tendency of polystyrene flames. Falling PS particles formed flames with long wakes, resulting in strings of highly agglomerated soot. The higher the particle number density, the more pronounced the tendency for longer wake formation. Almost one-third of the initial mass of PS was converted to soot under fuelrich conditions (equivalence ratio, φ ≈ 1.5) (19). Condensed PAH accounted for up to 10% of the mass of the soot (17). The fraction of soot particles with an aerodynamic diameter of 2 µm and less (PM2) ranged from 16 to 35 wt %, inversely proportional to the equivalence ratio in the range 0.5 < φ < 1.5. An investigation by Durlak et al. (25) addressed the emissions from combustion of PS particles (100-300 µm) flowing steadily upward in a drop-tube furnace at 800-1000 °C. They found that the total mass and the number of PAH species decrease with increasing gas temperature and decreasing PS feed size, while the mean diameter of the particulates (soot) increases. Most condensed PAH species were found to be concentrated in the smaller aerosol sizes. In another study, Bockhorn et al. (26) examined the thermal degradation of PS under isothermal conditions as well as the evolution of monomer, dimer, and trimer pyrolysates. Rate equations for the different species were formulated, and a kinetic model for the reaction rates of polymer degradation was evaluated. Park et al. (27) examined the oxidative pyrolysis of PS under steady flow conditions in a horizontal furnace at 400 °C followed by a second stage treatment at 400-850 °C, 2 s residence time. That study observed maximum trimer (17.5%) and dimer (4.3%) at the lowest temperatures, maximum monomer (46.6%) and maximum benzaldehyde (11.9%) concentrations at 550 °C, while maximum benzene (5.7%) occurred at 600 °C. As the levels of these compounds decreased with increasing temperature, common PAH such as naphthalene, biphenyl, benzofuran, and anthracene were detected. Their concentration diminished with increasing temperature, and at 850 °C complete conversion to CO2 and
FIGURE 1. Schematic of the experimental two-stage laminar-flow reactor and photograph of the diffusion flame forming over the polystyrene sample inside the quartz tube of the primary furnace.
TABLE 1. Fuel Composition and Energy Contenta fix. carbon volatiles ash C H (%) (%) (%) (%) (%) 1
99
0
92
8
S (%) 0.04
N O Cl (%) (%) (%) 0
0
0
high heating value (MJ/kg) 44.5
The melting point of PS is 237.5 °C, the specific gravity is 1.047, and the minimum decomposition temperature is 364 °C (1). a
H2O was achieved in 2 s. This is in agreement with results of Panagiotou et al. (16, 17) at very low fuel feed rates (low φ). You et al. (28) examined the two stage pyrolysis of PS and the resulting emissions of PAH and particulates, collecting data only at the exit of the combined stages. The temperature in the first stage was raised by 40 °C/min to 500 °C, while the temperature of the second stage was fixed between 900 and 1200 °C. At 900 °C, large amounts of benzene were observed while a temperature of 1000-1100 °C was required for formation of significant amounts of PAH and soot. The above brief review indicates that polystyrene when burned generates more PAH and soot emissions than other plastics, presumably because of the aromatic structure of PS. To minimize such emissions, one has to optimize conditions such as furnace temperature, residence time, fuel loading rate, mixing with air, etc. The present investigation is a systematic study on the effects of incineration temperature and afterburner treatment, upon mixing with additional air, in two-stage combustion of polystyrene.
Experimental Techniques and Procedures Fuel Characteristics. The fuel was polystyrene in the form of Styrofoam cups, cut in small pieces (≈5 × 5 mm), and was placed in a porcelain boat. The elemental composition is given in Table 1, as determined by Galbraith Laboratories. Apart from carbon and hydrogen, a small amount of sulfur (0.04 wt %) was detected. The heating value of PS is comparable to that of premium hydrocarbon fuels (gasoline, diesel oil, etc.). Experimental Apparatus for Staged Combustion of Polystyrene. Batch combustion experiments, involving fixed beds of shredded PS, were conducted in a 1-kW horizontal, split-cell, electric muffle furnace fitted with a quartz tube, which was 4 cm i.d. and 87 cm long. This primary furnace was connected to a secondary muffle furnace (the afterburner), which was 2 cm i.d. and 38 cm long (see Figure 1). The effluent of the first furnace passed through a venturi (8 mm i.d.) where it was mixed with four radially positioned perpendicular jets of preheated air (to 100 °C below the furnace wall temperature) at the periphery of the venturi
(Figure 1). Half of this mixed charge was then sampled at the exit of this furnace while the other half was channeled to the secondary furnace. The wall temperature of the primary furnace was varied in the range of 500-1000 °C. The gas temperature profile was measured by a suction pyrometer using a procedure described by Wheatley et al. (15). The gas temperature was found to increase in the first half-length of the furnace but was fairly constant in the second part of the furnace, for the most part 25 °C below the wall temperature in the aforementioned range, in absence of a combustion event. Hence, a porcelain boat loaded with 0.5 g of sample was inserted from the tube’s entrance, and it was positioned at half-length of the quartz tube. To expediently insert a sample in the furnace, a porcelain boat was placed at the end of the inner surface a quartz cylinder longitudinally split along its centerline. The opposite end of this cylinder was mounted at the entrance glass fitting of the furnace. All experiments were conducted in air. The airflow rate in the first furnace was 4 L min-1, and the gas residence time between the sample and the venturi was in the range of 80130 ms, depending on the furnace temperature. The flow rate of additional air supplied at the venturi was 2 L min-1. The Reynolds number (Re) in the venturi varied between 425 and 590, depending on the temperature measured therein. The gas temperature between the two furnaces dipped to 250-300 °C. The gas temperature in the secondary furnace was constant for nearly its entire length, 25 °C below its wall temperature, which was set at 1000 °C for all runs. The residence time of the gases therein was 0.7 s, and Re ) 80. Sampling was simultaneously conducted at the exits of both furnaces. Luminous diffusion flame durations, over the boat, were in the order of 1 min, i.e., the overall burning rate was 500 mg/min. To estimate the penetration of the additional air jets at the venturi and assess mixing with the effluent gas from the primary furnace, the approach outlined by Lefebvre (29) was used. Accordingly, the maximum penetration length (Ymax) of multiple perpendicular round jets into a tubular duct (in this case the venturi) is given by
Ymax ) 1.25djJ0.5[m ˘ g/(m ˘ g+m ˘ g)] where the momentum flux ratio (J) is
J ) (FjUj2)/(FgUg2) These terms involve the mass flow rates (m ˘ ), the densities (F), and the velocities (U) of the gas in the circular duct (g) and the air in the round jets (j) as well as the diameter of the jets (dj). Calculations using measured temperatures at the venturi centerline and at the jet exits showed that the four VOL. 35, NO. 17, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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radially placed jets penetrate the effluent to its centerline. This is indicative of good mixing of the streams in the venturi. Combustion Emissions Monitoring. Polycyclic aromatic hydrocarbon (PAH) emissions as well as NOx, CO, CO2, and particulates (mostly soot including tars and, possibly, traces of minerals) from the combustion of polystyrene were monitored at the exits of the two furnaces. The two sampling stages were placed at the exits of the two furnaces (see Figure 1), each handling half of the flow of the primary furnace. PAH and particulates were collected using Graseby sampling heads with a filter stage and a glass cartridge containing Supelco XAD-4 adsorbent. Prior to each sampling stage, the effluent of the furnaces was mixed with 2 L min-1 nitrogen gas. Dilution with nitrogen took place in the annulus of two concentric tubes; the perforated inner tube enabled the mixing of the nitrogen with the furnace effluent for quenching. Subsequently, the particulate emissions were trapped on a 90 mm diameter, 1 mm thick Whatman glass fiber filter with a nominal pore size of 0.45 µm. Then, gas-phase PAH were adsorbed on the bed of XAD-4 resin. The length of the XAD-4 bed was more than twice its diameter. Upon exiting the XAD-4, the effluent passed through a sample tube containing 100 mg of Supelco Carbosieve G adsorbent, which retains volatile (C1-C8) organic molecules. Upon removing moisture by a mildly heated Permapure dryer, NOx was monitored using a Beckman 951A chemiluminescent NO/ NOx analyzer; SO2 was monitored using a Rosemount Analytical 590 UV analyzer; CO/CO2 was monitored using Horiba infrared analyzers; and O2 was monitored using a Beckman 350 paramagnetic analyzer. Data were recorded using a Data Translation DT-322 board in a microcomputer. The integrated acquisition system used DT VPI within a Hewlett-Packard HP VEE visual programming environment. The signals from the gas analyzers were recorded for the duration of the experiment and subsequently were converted to partial pressures and, upon numerical integration, to mass yields. Extraction and Concentration of PAH Emissions. Following the combustion experiments, the filters and resins were removed and placed in separate glass bottles with Teflon-lined caps and stored at 4 °C. Prior to extraction with methylene chloride, a 50-µL internal standard containing 100 µg each of naphthalene-d8, acenaphthene-d10, anthracene-d10, chrysene-d12, and perylene-d12 was directly applied to the filters and resins in each bottle. To ensure the purity of the XAD-4 resin and cellulose filters, blanks of XAD-4 resin and filter were also extracted and analyzed. In addition, combustion blanks were analyzed during which the furnace was operated in the presence of the XAD-4 and filter but in the absence of fuel. Target compounds that appeared in any of the blanks were appropriately qualified. To substantially reduce the methylene chloride used in the extraction process, a Dionex ASE 200 accelerated solvent extractor was used in place of the Soxhlet extraction system methods previously reported (16, 17, 30). This revision in the extraction method necessitated the quality assurance evaluation of the automated solvent extraction system to ensure the extraction suitability, efficiency, and reproducibility of the proposed method. The Dionex ASE 200 was used for extracting the organic compounds from both the XAD-4 resins and the cellulose filter papers. The XAD-4 resins were transferred to 33-mL extraction cells while the filter papers were transferred to 11-mL extraction cells. The extraction cells were allowed to initially equilibrate at 40 °C in the ASE 200 system for 1 min. The extraction cells were then filled with methylene chloride and allowed to thermally equilibrate at 40 °C; the cell was pressurized to 500 psi for a period of 15 min. Following the 15-min soak time, the cells were each flushed with 80% of the cell volume with fresh methylene chloride and finally purged for 90 s with nitrogen. The 3544
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methylene chloride extracts were collected in separate bottles for concentration. Two extraction cycles were used per cell. The total extraction time for the two-cycle process was around 25 min. About 45 mL of methylene chloride was used for the XAD-4 resins while 20 mL was used for the extraction of the filter papers. No more than 30 mL of XAD-4 resin could be placed within a 33-mL extraction cell due to the expansion of this resin in methylene chloride. The samples were concentrated under vacuum to a final volume of 10 mL for analysis by gas chromatography coupled to mass spectrometry (GC-MS). Analysis by GC-MS. The GC-MS system consisted of a Hewlett-Packard (HP) model 6890 GC equipped with a HP model 5973 mass selective detector. The GC-MS conditions and data reduction were described previously (16, 17, 30). The instrument was tuned in accordance with EPA semivolatile criteria prior to the GC-MS analysis of each set of samples. The instrument passed initial and continuing calibration criteria. Each of the target compounds and the tentatively identified compounds were quantified using the appropriate deuterated internal standard. In a departure from the EPA method, the GC-MS system was run in the full scan mode and not in a single ion monitoring mode. This was done to ensure the identification, quantification, and reporting of tentatively identified compounds. The use of the full scan mode does not significantly modify the method except to raise the lower reporting limit to about 1 µg of component/g of fuel combusted. Values that are less than 1 µg/g are technically nondetects. Analysis of the combustion system blanks indicated the presence of xylenes, bis(2-ethylhexyl) phthalate, and siloxanes in quantities sufficient to reject all positive results for these compounds in the combustion extracts. The standard solutions containing 100 µg each of five characteristic standards were diluted to 10 mL and analyzed both as an instrument blank and to provide an indication of extraction efficiency for each of the internal standards. Samples with an extraction efficiency of less than 50% for any of the internal standards were repeated. To assess the reproducibility of the PAH analysis, duplicate analyses were performed. The source of the experimental error results from a combination of sampling, extraction, concentration, and analysis techniques. The experimental procedure was kept consistent in all evaluations to ensure the validity of relative trends. The recovery efficiency and reproducibility of the automated solvent extraction technique described herein was judged to be a suitable replacement for conventional Soxhlet techniques based on quality assurance and quality control evaluations. Nevertheless, the application of this technique would have to be substantially modified for the extraction of samples in which complex matrixes are present.
Results and Discussion Oxygen Profiles and Evolution of CO and CO2 during Combustion. Upon introduction of a batch of 0.5 g of PS to the preheated primary furnace, it heated and devolatilized. In most cases a gaseous diffusion flame formed over the fuel bed (Figure 1), and a plume of particulates moved to the exit of the furnace. The oxygen concentration at the exits of the primary and secondary furnaces experienced transient profiles (see Figure 2), indicative of the derivative of the fuel devolatilization and subsequent oxidation in the gaseous diffusion flame. The wall temperature of the primary furnace was varied between 500 and 1000 °C. Minimum oxygen concentrations show that globally neither furnace’s atmosphere was starved for oxygen during the combustion events (see Figure 3a). The oxygen concentrations at the exit of the secondary furnace were lower than those at the exit of the primary furnace, both values already including the additional
FIGURE 2. Typical profiles of CO, CO2, and O2 at the exits of the primary furnace and of the secondary furnace (afterburner). air introduced in the venturi, since further conversion of volatile pyrolysates and primary combustion products takes place in the secondary furnace. A dip in the minimum oxygen concentrations was experienced at 600 °C for both furnaces. Peak CO2 partial pressures at the exit of the primary furnace varied between 7 and 15%, depending on the temperature, which is consistent with the results of detailed balancing calculations for combustion of polystyrene taken quantitatively as styrene in the gas phase at equivalence ratios (φ) between 1 and 2. The peak CO2 partial pressures at the exit of the secondary furnace were higher that those at its input, indicating additional combustion. Similarly, the maximum CO partial pressures at the exit of the secondary furnace were also higher (1-6%) than those at its entrance (typically 1%). The overall yields of CO and CO2 were obtained by integrating the time-dependent profile; results are shown in Figure 3b. Minimum yields of both CO2 and CO were obtained at 500 °C, which indicates the presence of large amounts of unburned pyrolysates. Maximum CO2 yields were obtained at 600 °C, but CO was still low. At higher temperatures, CO2 dropped while CO did not change significantly. The drop of CO2 yields, as the gas temperature rises beyond 600 °C, is related to an increase in the generation of products of incomplete combustion, such as soot and PAH, discussed in the next section. The corresponding drop of the CO2:CO ratio is in agreement with thermodynamic equilibrium. Finally, the CO2 and CO yields from the second furnace followed mildly opposing trends, as CO2 decreased with the primary furnace temperature and CO first increased but then did not change significantly. Yields of Particulates from Combustion of Polystyrene. It is known that the flame temperature, which is related to the furnace temperature, is a dominant physical parameter that affects soot formation in diffusion flames (31). According to the work of Chung and Lai (32), reducing the flame temperature in diffusion PS flames leads to a decrease of the fuel pyrolysis rate and, therefore, of the formation rate of potential soot precursors. Indeed, this was confirmed in the experiments herein as the furnace temperature was varied. The particulate yields at the exits of the two furnaces are shown in Figure 3c. At 500 °C, but also to some extent at 600 °C, the captured particulates resulted from an opaque-white aerosol that evolved during gasification of the polymer. At 500 °C no luminous flame was seen forming over the sample, while at 600 °C a low luminosity flame was observed, not over the sample but close to the venturi. This white aerosol consisted of primary pyrolyzation products of polystyrene, as discussed in the following section. The particulate yield reached a minimum at 600 °C and increased with temperature thereafter. Above 600 °C, particulates were black and appeared to be soot (carbonaceous material including extractable PAH). The second furnace was beneficial in
removing some of the particulate emissions from the first furnace, especially in the case of low first furnace temperatures, where particulates consist mainly of condensed pyrolysates. At primary furnace temperatures above 700 °C, where particulates consist essentially of soot, the effect of the afterburner was not as drastic. This illustrates that once soot is formed, its destruction is difficult because of its slow oxidation kinetics under typical furnace temperatures and residence times as investigated in this laboratory (33). Thus, it may be beneficial from an environmental point of view to implement incineration conditions that minimize the formation of soot and ensure the complete destruction of its precursors. Yields of PAH from Combustion of Polystyrene. The cumulative emission yields of all detected semivolatile PAH covering the mass range from 116 (indene) to 278 amu (benzo[b]chrysene and isomers) are plotted in Figure 3d. More than 120 PAH compounds were detected by GC-MS. Plots of styrene dimer and trimer are shown in Figure 3, panels e and f. Plots of selected individual component yields (combined gas and condensed phase) of PAH are presented in Figure 4. The separation between gas-phase and condensed-phase PAH, respectively, collected by the XAD-4 and the filters is illustrated in Figure I in the Supporting Information. Generally, only 2-3-ring PAH were present in the gas phase, while heavier multi-ring compounds were found in the condensed (solid) phase. This observation is consistent with boiling points and van der Waals interactions between PAH and particulate surface increasing with molecular mass. Most of the naphthalene was found in the gas phase, most of the pyrene was found in the condensed phase, and nearly all of the benzo[a]pyrene was found in the condensed phase. The exact distribution of various species in the two phases, however, depends on the temperature of the sampling stage (36, 37). Often different species with identical molecular mass and chemical formula, i.e., isomers, could be identified in the extracts. The analysis was further complicated by the presence of PAH partially substituted by short saturated or unsaturated carbon chains, including methyl. Overall, cumulative PAH emissions (excluding the styrene oligomers) increased with the gas temperature of the primary furnace (in the range of 500-1000 °C), while the temperature of the second stage was kept constant at 1000 °C (see Figure 3d). However, in comparison to those at the exit of the primary, the PAH emissions from the secondary furnace were much lower, by factors of 2-3. Thus, the effect of the afterburner was beneficial in minimizing the final emissions of cumulative PAH. At the lowest combustion temperature in the first stage (500 °C), large yields of styrene dimer and trimer were detected at the effluent of the primary furnace, totaling 22 VOL. 35, NO. 17, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. (a) Minimum O2 partial pressures at the exits of the primary and secondary furnaces. (b) CO and CO2 yields (mg/g). (c) Cumulative particulate yields (mg/g). (d) Cumulative PAH yields (mg/g). (e) Yields of styrene dimer (µg/g) and (f) trimer (µg/g) as a function of the primary furnace temperature. Tafterburner ) 1000 °C in all cases. mg/g of fuel, as shown in Figure 3, panels e and f. In fact, most of these compounds were detected in the condensed phase on the filter and were the major component of the opaque-white combustion-generated aerosol. The concentrations of these styrene oligomers in the effluent of the primary furnace diminished at higher temperatures. In all cases, these species were completely depleted in the afterburner at 1000 °C. Light organic species that could be retained 3546
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neither on the filter nor on the XAD such as 1,3-butadiene, acetone, or single-ring aromatics were collected using Carbosieve adsorbents and analyzed by means of GC-MS after thermal desorption. As the collection efficiency of Carbosieve traps has not been assessed yet, quantitative results are left for future work. Profiles of 16 major PAH component yields (micrograms per gram of sample) are shown in Figure 4. Emissions were
FIGURE 4. Yields of individual PAH components (µg/g) at the exits of the primary and secondary furnaces as a function of the primary furnace temperature. Tafterburner ) 1000 °C in all cases. very low at the lowest primary furnace temperatures of this study; thereafter, they all increased with rising furnace
temperatures at the exits of both furnaces. At every temperature, it was observed that at the exit of the primary VOL. 35, NO. 17, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 5. Yields of particulates (in mg/g) as well as fluoranthene, benzo[a]pyrene, and pyrene (in µg/g) of triplicate experiments at the exits of the primary and secondary furnaces as a function of the primary furnace temperature. Tafterburner ) 1000 °C in all cases. furnace: (a) the yields of naphthalene were much higher than those of acenaphthylene, which is in agreement with previous observations on other fuels (21, 30, 34, 35); (b) the yields of acenaphthylene were comparable to those of fluorene; (c) the yields of phenanthrene were comparable to those of naphthalene and generally higher than those of all other PAH, in agreement with previous studies on polystyrene (12-14), which also observed similar yields to those herein (i.e., in the range of a few milligrams per gram); (d) fluoranthene was the next abundant compound; (e) the yields of anthracene, pyrene, chrysene, and acephenanthrylene were comparable to each other, and all were a little less than fluoranthene; (f) the yields of cylopenta[cd]pyrene were still lower by factors of 2-4 as compared to those of pyrene but comparable to the yields of benzo[a]pyrene, benzo[g,h,i]fluoranthene, and benzo[g,h,i]perylene; (g) the yields of perylene were even less. The afterburner reduced the concentrations of most individual PAH compounds. However, some PAH remained unaffected, and unfortunately, a few appeared to have been enhanced. Of the 16 compounds shown in Figure 4, the following were substantially reduced: naphthalene, biphenyl, fluorene, phenanthrene, anthracene, chrysene, acephenanthrylene, benzo[b]fluoranthene, and benzo[g,h,i]perylene. The following species appeared to have remained unaffected: fluoranthene, pyrene, benzo[a]pyrene, and benzo[g,h,i]fluoranthene. The concentration of the following species increased at middle and high temperatures: acenaphthylene, cyclopenta[cd]pyrene, and perylene. The increase in these three species was not drastic, typically within a factor of 2. This phenomenon was observed only in high-temperature runs, i.e., when significant amounts of PAH were present at the entrance of the afterburner. In conclusion, at the lowest primary furnace temperatures, i.e., 500 and 600 °C, the concentrations of all PAH species were reduced by the afterburner treatment. At those conditions, the influence of the afterburner was deemed to be highly beneficial in reducing PAH and particulate emissions. An overall comparison can be made with results of previous tests burning other fuels in the same primary laboratory furnace operated at 1000 °C (30, 35) under similar experimental conditions. The comparison was only possible at the exit of the primary furnace, as those previous studies were conducted at the absence of an afterburner. The cumulative PAH yield from the combustion of polystyrene was 38 mg/g of sample at 1000 °C (see Figure 3d) (multiply primary furnace yields by 2 since the effluent was split). This yield is comparable to those from burning similar quantities of latex, 46 mg/g, or waste tire chips, 22-40 mg/g; but it is much higher than those from burning bituminous coal or cotton pads, both at 0.6 mg/g. Regarding the emissions of particulates, PS generated 312 mg/g, which exceeded the 3548
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yields from latex at 264 mg/g and especially those of cotton at 18 mg/g. The particulate emissions of PS were indeed very high, accounting for one-third of the initial mass of the polymer. Approximately 6% of their mass, i.e., 19 mg of 312 mg/g of sample, was extractable with methylene chloride. Overall, the emissions of PAH, particulates, and CO appeared to be low when PS was gasified/burned in oxidative pyrolysis conditions at relatively low temperatures, such as 600 °C, followed by an afterburner treatment at 1000 °C for a residence time in the order of 1 s. Such conditions are recommended. Further work has been conducted on evaluating the effects of the afterburner temperature as well as those of high-temperature filtration (38). Emissions of NOx and SO2. Yields of NOx from batch combustion of PS were only recorded at the exit of the primary furnace. At the lowest gas temperature of 500 °C, polystyrene pyrolyzed without a flame and NOx was not emitted. Combustion at higher temperatures generated some NOx, but it was always found to be at low levels (6-20 ppm). SO2 emissions were not detected, indicating that during the batch combustion of polystyrene its sulfur content (0.04 wt %) was converted to other compounds or remained in the soot. Experimental Uncertainty. In all cases a minimum of two combustion experiments were conducted; if the results were not in good agreement, a third experiment was done. In most cases, the third experiment was added regardless of the agreement of the first two runs to probe the repeatability of the technique. Typically, the agreement for individual PAH species, soot measurements, major gaseous species, etc. was within 10%. The worst cases observed in this study experienced disagreements in the order of (30%, relative to the arithmetic means of the values. Errors may arise from variabilities in the combustion technique itself, i.e., packing of the fuel beds, manual sample insertion, ignition location, the ensuing flame formation, etc. Small losses and variabilities may subsequently affect the sampling, transferring, and storage of filters, adsorbents, etc. Finally, uncertainties may be expected in the analytical techniques. Figure 5 illustrates the repeatability in measurements of three important PAH species (fluoranthene, pyrene, and benzo[a]pyrene) as well as of overall particulate yields.
Kinetic Modeling A deeper insight in the chemical processes responsible for formation and consumption of PAH and soot requires the use of numerical modeling by means of complex reaction networks. Kinetic modeling has become an increasingly powerful tool for the prediction of PAH concentrations in well-defined combustion processes such as premixed flames (39-43) and jet-stirred and plug-flow reactors (44), while the complex connection of physical and chemical processes occurring in most large-scale setups makes their numerical
description particularly challenging and often requires the additional understanding of turbulence or diffusion phenomena. As discussed above, the first stage (the primary furnace) consists of a preheat zone followed by reaction in a diffusion flame type combustion. The second stage, the afterburner, can be describedsat least in a first approximationsas a plugflow reactor. The outlet of the first stage (after the injection of additional air in the venturi) was input to the afterburner containing light gaseous species, PAH, and particulates. In view of the complexity of physical and chemical processes therein, no attempt at complete modeling, which would include chemical kinetics, in the first stage was made. However, in the afterburner the relatively well-defined flowfield allowed for the modeling of PAH formation and consumption using the plug-flow reactor code (45) of the CHEMKIN package (46) developed at Sandia National Laboratories. The atmospheric pressure version of a kinetic model developed at MIT and describing PAH formation up to a molecular mass of about 300 amu (42, 43) was used. Pressure-dependent kinetic data were adapted to atmospheric pressure by means of the Quantum Rice-Ramsperger-Kassel (QRRK) treatment of chemically activated reactions (47), and the atmospheric pressure version of the model has been tested successfully for a sequential wellstirred/plug-flow reactor setup (44). The experimental results obtained at the exit of the primary furnace were input of the plug-flow reactor model computations. The duration of each combustion event (≈100 s) was determined from the recorded time-dependent CO, CO2, and O2 profiles. Integrated mole fractions have been used for these species as well as for individual PAH accumulated over the reaction time at the exit of the primary furnace. Styrene dimer and trimer detected at the outlet of the primary furnace were assumed to decompose quantitatively to monomer, and their mole fractions were converted correspondingly. All model inputs and outputs were expressed in mole fractions. While on-line analyzers for CO, CO2, and O2 provided their readings in this unit, yields of PAH and styrene oligomers were converted dividing the number of moles of individual species collected by the total number of moles flowing through the reactor during the combustion event. This total molar flow was determined by addition of the number of moles of monomeric styrene included in 0.5 g of polystyrene, i.e., 4.81 × 10-3 mol, to the sum of the air flows channeled to the primary furnace and the venturi (0.2724 mol in 100 s). The latter number is significantly larger than the molar amount of styrene. Therefore the increase of the number of moles due to oxidation of styrene affected the resulting mole fractions only in the limits of uncertainty. They split into two flows after the venturi was taken into account. The residence time in the afterburner was identical for all reported runs (≈0.7 s) since its temperature was constant at 1000 °C. Mole fractions deduced from the experimental data could therefore be directly compared with model predictions. In addition to experimentally determined inputs, mole fractions for H2, H2O, acetylene, and phenylacetylene have been estimated using well-stirred reactor kinetic modeling (48) of a stoichiometric styrene/air mixture, based on Burke and Schumann’s flame sheet concept (49, 50). The temperature of the well-stirred reactor modeling was approximated by the adiabatic flame temperature calculated by means of the STANJAN software (51), corrected for heat losses by radiation as well as polystyrene evaporation and decomposition. Adiabatic flame temperatures were computed for φ ) 1.0 and 0.83, at wall temperatures from 500 to 1000 °C. The equivalence ratio φ ) 0.83 corresponds to an overall fuel/air mixture in the first stage based on a sample mass of 0.5 g and an air flow rate 4 L min-1. Perfect mixing was
assumed. The flame temperature calculation was based on equilibration between styrene, benzene, acetylene, ethylene, O2, CO, CO2, H2O, H, O, OH, and H2. Equilibrium temperatures between 2560 and 2750 K and between 2460 and 2680 K were obtained for the φ ) 1.0 and 0.83 cases, respectively, for the aforementioned wall temperature range. Establishment of partial equilibria in the reaction zone of diffusion flames, in particular CO + OH T CO2 + H, has been reported (52). Therefore, the comparison of integrated experimental {XCO2}/{XCO}ratios with those predicted by the well-stirred reactor kinetic modeling should allow for a realistic adjustment of the adiabatic flame temperature, reflecting heat losses. Kinetic modeling at φ ) 1.0 in a wellstirred reactor at 2100 K yielded a {XCO2}/{XCO}ratio of 14, fairly close to the experimental ratios of 18.3, 12.3, and 14.4 for furnace temperatures of 500, 600, and 700 °C, respectively. Adjustment of the equilibrium adiabatic temperature by about 350-500 K reflects heat losses, imperfect mixing, and the assumption of monomeric styrene as fuel, i.e., neglecting polystyrene depolymerization and gasification. Therefore, a temperature of 2100 K has been used for the estimation of input mole fractions for the plug-flow computation of H2, H2O, acetylene, and phenylacetylene by means of well-stirred reactor modeling. Predicted mole fractions have been corrected for the addition of air between the primary furnace and the afterburner. Experimental {XCO2}/{XCO}ratios of 9.1, 9.0, and 3.2 for primary furnace temperatures of 800, 900, and 1000 °C, respectively, were in good agreement with ratios of 10.4 and 3.5 obtained by modeling at 2200 and 2500 K, and φ ) 1.0 in a well-stirred reactor. Input mole fractions for the plugflow computation of H2, H2O, acetylene, and phenylacetylene at 800, 900, and 1000 °C were determined by modeling at 2200 K in the first two cases and 2500 K in the last case. The total amount of dimers and trimers converted to styrene monomer mole fractions were input to the plugflow reactor computations since quantitative depolymerization was assumed to occur at the afterburner temperature of 1000 °C. In addition, mole fractions of the 16 individual PAH shown in Figure 4 were taken into account. The total mass of material used as input to the plug-flow computation (styrene dimers and trimers, 16 PAH) was compared with the total mass of collected material at the exit of primary furnace (styrene dimers and trimers, total PAH) and represented 99, 64, 60, 80, 84, and 92% at 500, 600, 700, 800, 900, and 1000 °C. Plug-flow reactor computations have been performed for the afterburner at 1000 °C under all input conditions investigated herein. To assess the sensitivity of the plug-flow reactor, model predictions to the approximated input parameters; additional computations were performed varying the input data: (a) The acetylene mole fraction was increased by 3 orders of magnitude to 5.0 × 10-3. (b) Xacetylene ) 5.0 × 10-3 and Xbenzene ) 5.0 × 10-4 were added. These mole fractions are results of a well-stirred reactor computation of a styrene/air mixture at φ ) 2.0, at 2100 K. (c) Same as b with addition of XH ) 2.07 × 10-3, XO ) 2.72 × 10-3, and XOH ) 8.22 × 10-3. These mole fractions are computational results of a well-stirred reactor at φ ) 1.0 at 2560 K (the adiabatic flame temperature for φ ) 1.0 and furnace temperature of 500 °C). (d) Maximum experimental CO and CO2 and minimum O2 mole fractions were used as inputs instead of the integrated average values. (e) Increase of the temperature of the plug-flow model computation from 1000 to 1827 °C, i.e. 2100 K. Without considering case (e), comparisons of the corresponding model predictions for different PAH, at a residence VOL. 35, NO. 17, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 6. Comparison between model predictions and experimental data in the afterburner at 1000 °C for different temperatures in the primary furnace. All quantities are given in mole fractions. 600 °C: (a) acenaphthylene, (b) phenanthrene, (c) fluoranthene, and (d) cyclopenta[cd]pyrene. 800 °C: (a) acenaphthylene, (b) phenanthrene, (c) fluoranthene, and (d) chrysene. 1000 °C: (a) acenaphthylene, (b) phenanthrene, (c) fluoranthene, and (d) acephenanthrylene. time of 0.8 s show variations within a factor of 2, despite a spread of input parameters well beyond the uncertainty of their approximation. However, model predictions are strongly affected by temperature. Nearly identical results for light gas species, such as C2H2, H2O, H, O, or OH, in the base case and cases a-d show the importance of thermodynamics, while in case e, at 2100 K, a drastic increase of the PAH removal rate occurred. This finding indicates that afterburner treatments at more than 1000 °C represent a potential approach for efficient removal of PAH. At a primary furnace temperature of 500 °C, it was observed experimentally that most of the material collected at the exit of the primary furnace consisted of styrene dimers and trimers while only relatively small amounts of PAH were formed (see Figures 3 and 4). The model predictions confirmed the trend of a significant removal of PAH in the afterburner as well as formation of small amounts of benzo3550
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[ghi]fluoranthene not detected at the exit of the primary furnace. Monomeric styrene, used as surrogate for the detected dimers and trimers, was predicted to be depleted completely. Experimentally, styrene dimers and trimers were found to be reduced by 3 orders of magnitude but were still detectable after treatment in the afterburner. Preliminary results of model predictions and the comparison with experimental data at the inlet and the exit of the afterburner, i.e., at tres ) 0 and 0.7 s are shown in Figure 6 for acenaphthylene, phenanthrene, fluoranthene, and cyclopenta[cd]pyrene for 600 °C in the primary furnace. The 2-fold depletion of acenaphthylene was predicted correctly and appeared much earlier than the experimental residence time of 0.7 s. Phenanthrene, fluoranthene, and cyclopenta[cd]pyrene were predicted to be nearly unaffected by afterburner treatment (Figure 6). In these preliminary computations, oxidation was included in the kinetic model
only for benzene, naphthalene, pyrene, benzo[a]pyrene, and anthanthracene (C22H12) or radicals of these species. Acenaphthylene depletion is probably related to the removal of 1-naphthyl radicals by reaction with molecular oxygen to 1-naphthoxy radicals followed by formation of 1-naphthol and decay to indenyl radicals, included in the model. 1-Naphthyl is connected to acenaphthylene by reaction with acetylene: 1-C10H7 + C2H2 S acenaphthylene + H. At a primary furnace temperature of 800 °C an increase of the acenaphthylene concentration was observed experimentally while a decrease has been predicted by the model (Figure 6). This discrepancy is likely to be due to the unreasonably strong oxidation of 1-naphththyl, a dominant acenaphthylene precursor, with O2 molecules in the model. 1-Naphthyl is also a precursor of fluoranthene, and its removal may explain the slight underprediction of the latter species at the exit of the afterburner. The consistently pronounced underpredictions of naphthalene, pyrene, and benzo[a]pyrene in the afterburner are evidence of the inadequate description of oxidation by molecular oxygen, as is currently included in the kinetic model. Similarly, at a primary furnace temperature of 1000 °C, acenaphthylene is underpredicted significantly. The slightly faster predicted consumption of phenanthrene as compared to that at lower primary furnace temperatures is consistent with the increase of the acetylene concentration predicted for the afterburner. Growth reactions leading to larger species are therefore enhanced. The depletion of acephenanthrylene in the afterburner can be explained thermodynamically. Isomerization between fluoranthene and acephenanthrylene was included in the model, and equilibrium concentrations were predicted. Details of PAH oxidation chemistry will be addressed in future work, and the model will be improved accordingly. Additional experiments, currently undertaken, varying the temperature of the afterburner and the supply of oxygen after the first furnace are expected to contribute largely to a better understanding of PAH oxidation chemistry.
Acknowledgments The authors acknowledge Mr. Francesco Ayala Sanchez and Dr. Ajay Atal for their assistance. This project was funded by NSF with Grant CTS-9908962; special thanks are due to Dr. Farley Fisher for valuable discussions.
Supporting Information Available One figure showing the yields of naphthalene, phenanthrene, pyrene, and cumulative PAH in gas and condensed phases. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review January 5, 2001. Revised manuscript received May 30, 2001. Accepted June 6, 2001. ES0105109
