Environ. Sci. Technol. 2002, 36, 797-808
Polynuclear Aromatic Hydrocarbon and Particulate Emissions from Two-Stage Combustion of Polystyrene: The Effects of the Secondary Furnace (Afterburner) Temperature and Soot Filtration JUN WANG,† HENNING RICHTER,‡ JACK B. HOWARD,‡ YIANNIS A. LEVENDIS,† 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
Laboratory experiments were conducted in a two-stage horizontal muffle furnace in order to monitor emissions from batch combustion of polystyrene (PS) and identify conditions that minimize them. PS is a dominant component of municipal and hospital waste streams. Bench-scale combustion of small samples (0.5 g) of shredded styrofoam cups was conducted in air, using an electrically heated horizontal muffle furnace, kept at Tgas ) 1000 °C. Upon devolatilization, combustion of the polymer took place in a diffusion flame over the sample. The gaseous combustion products were mixed with additional air in a venturi and were channeled to a secondary muffle furnace (afterburner) kept at Tgas ) 900-1100 °C; residence time therein varied between 0.6 and 0.8 s. At the exits of the primary and the secondary furnace the emissions of CO, CO2, O2, NOx, particulates as well as volatile and semivolatile hydrocarbons, such as polycyclic aromatic hydrocarbons (PAH), were monitored. Online analyzers, gravimetric techniques, and gas chromatography coupled to mass spectrometry (GC-MS) were used. Experiments were also conducted with a high-temperature barrier filter, placed just before the exit of the primary furnace to prevent the particulates from entering into the secondary furnace. Results demonstrated the beneficial effect of the afterburner in reducing PAH concentrations, including those of mutagenic species such as benzo[a]pyrene. Concentrations of individual PAH exhibited a pronounced afterburner temperature dependence, typically ranging from a small decrease at 900 °C to a larger degree of consumption at 1100 °C. Consumption of PAH was observed to be the dominant feature at 900 °C, while significant quantities of benzene and some of its derivatives, captured by means of carbosieve/Carbotrap adsorbents, were formed in the afterburner at a temperature of 1000 °C. In the primary furnace, about 30% of the mass of the initial polystyrene was converted into soot, * Corresponding author phone: (617)373-3806; fax: (617)3732921; e-mail:
[email protected]. † Northeastern University. ‡ Massachusetts Institute of Technology. § U.S. Army Natick, RD&E Center. 10.1021/es0109343 CCC: $22.00 Published on Web 01/16/2002
2002 American Chemical Society
while the total mass of PAH represented about 3% of the initial mass of combustible. The afterburner reduced the particulate (soot) emissions by only 20-30%, which indicates that once soot is formed its destruction is rather difficult because its oxidation kinetics are slow under typical furnace conditions. Moreover, increasing the afterburner temperature resulted in an increasing trend of soot emissions therefrom, which might indicate competition between soot oxidation and formation, with some additional formation occurring at the higher temperatures. Contrary to the limited effect of the afterburner, high-temperature filtration of the combustion effluent prior to the exit of the primary furnace allowed for effective soot oxidation inside of the ceramic filter. Filtration drastically reduced soot emissions, by more than 90%. Limited soot formation in the afterburner was again observed with increasing temperatures. The yields of both CO and CO2 were largely unaffected by the temperature of the afterburner but increased at the presence of the filter indicating oxidation therein. A previously developed kinetic model was used to identify major chemical reaction pathways involving PAH in the afterburner. The experimental data at the exit of the primary furnace was used as input to these model computations. A first evaluation of the predictive capability of the model was conducted for the case with ceramic filter and a temperature of 900 °C. The afterburner was approximated as a plug-flow reactor, and model predictions at a residence time of 0.8 s were compared to experimental data collected at its exit. In agreement with the experimental PAH concentration, only a minor impact of the afterburner treatment was observed for most species at 900 °C. OH was deduced to be the major reactant with a mole fraction about 4 orders of magnitudes higher than that of hydrogen radicals. Evidence for the need of further work on the quantitative assessment of oxidation of PAH and their radicals is given.
1. Introduction Numerous environmental catastrophes resulting from improper disposal practices of many different kinds of wastes have caused an increased public awareness of the growing problem of waste generation. Treatment and disposal of municipal and medical solid waste (MSW) is a subject of concern and controversy, particularly since locally enforced stricter air pollution standards have recently led to the closure of many on-site incinerators. Over the years, research efforts have concentrated on the thermal destruction of municipal solid waste (MSW) using pyrolysis and oxidative pyrolysis (incineration) (1-3). Results obtained in recent years suggest that thermal destruction of MSW offers several advantages over alternatives such as landfilling, since it provides maximum volume reduction, permanent disposal, and minimum odor duration as well as detoxification. In the case of incineration, the combustor conditions must be carefully selected to effectively burn the wastes and minimize the emissions of toxic gaseous pollutants. Good mixing of air with the fuel is paramount (2-4), but this is a difficult task, given that in mass burn incinerators the fuel is in solid form of various size and shapes or, occasionally, in liquid form. As a result complex diffusion flames are formed. Thus, aftertreatment of the exhaust gases of the primary combustion chamber, upon mixing with additional air, may be necessary VOL. 36, NO. 4, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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in a secondary combustion chamber (afterburner). This afterburner can deal with the micro- and macroscale mixing inefficiencies of the primary furnace, flow stratification, batch charging, diffusion flame equivalence ratio variation, depletion of oxygen, and reaction quenching on furnace walls (4). Along with the major products of combustion, the burning of wastes generates hydrocarbon pyrolysates as products of incomplete combustion, including the rather problematic polycyclic aromatic hydrocarbons (PAH), chlorinated compounds such as polychlorinated dibenzodioxins (PCDD), polychlorinated dibenzofurans (PCDF), polychlorinated biphenyls (PCB), chlorobenzenes and chlorophenols, halogen gases and hydrogen halides, nitrogenous compounds, and particulate matter, including soot and metal fumes such as mercury, lead, cadmium, arsenic, etc. This study focuses on the combustion of plastics that are contained in waste streams, choosing polystyrene as an example. It examines the emissions of polycyclic aromatic hydrocarbons (PAH) and particulates from incomplete combustion because of health-related issues with such emissions. These include the potential mutagenicity and even carcinogenicity of some PAH, and the cardiovascular and pulmonary ailments that particulates may cause (5-7). Incineration of high-volume waste plastics, which typically consist of polyethylene (65 wt %), polystyrene (22 wt %), polypropylene (9 wt %), and poly(vinyl chloride) PVC (4 wt %) (9, 10), is a major source of PAH and therefore necessitates detailed investigation. Polystyrene (PS) may be considered as a surrogate for polymeric material containing aromatic chemical structures and as such is representative for a significant fraction of municipal solid waste. Polystyrene (PS) has a variety of applications, ranging from packaging of consumer goods and medical supplies to home insulation. It has also been commonly used to simulate wood for decorative purposes. Previous research has shown that combustion of PS leads to the formation of a significantly larger amount of PAH and soot than the other plastics, most likely because of its aromatic structure (11, 12). The combustion of shredded polystyrene cups in the work presented herein is part of a broader investigation in this laboratory, focusing on burning wastes and examining their combustion characteristics and the related emissions. The emissions of particulates (containing both soluble and insoluble compounds), polycyclic aromatic hydrocarbons (PAH), NOx, CO, and CO2, and light hydrocarbons from batch combustion of PS were examined. Condensed particulate material was solvent-extracted, and the resulting extract was chemically characterized. The goal was to identify trends in composition and levels of emissions after treatment of the primary furnace effluent in a secondary furnace (afterburner) at various temperatures. In this work the primary furnace acted as a gasifier/ combustor. Its main function was to convert the solid fuel into gaseous form, partially oxidize it, mix it with additional air, and channel it to the afterburner. In a first approximation, the afterburner was envisioned as a plug-flow reactor, which allows for its description by means of kinetic modeling. Related work has recently examined the effects of the primary furnace temperature on the emissions from burning polystyrene, keeping the afterburner temperature constant (13). This study focuses on the temperature conditions of the afterburner to minimize emissions. In addition to examining the effects of temperature, this work also examines the impact of high-temperature filtration on the final combustion products. The filter is a honeycomb design made of silicon carbide, which can withstand high temperatures. The filter was located inside the primary furnace, prior to its exit, and it was continuously regenerated at the high temperatures therein. Its filtration efficiency was previously measured to be 99% for highly respirable submicron particles (14), which are considered to have significant health hazardous effects. 798
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2. Literature Review on PAH and Particulate Emissions from Burning Polystyrene Polystyrene is an amorphous, high molecular weight linear polymer; it is constituted of covalently bound units of the styrene monomer, which is composed solely of carbon and hydrogen [-CH2CH(C6H5)-]n. An important feature is the presence of a benzene ring in its structure which is responsible for its tendency to form significant amounts of soot in the case of incineration (15). Combustion of polymers is a very complex process since chemical reactions (both pyrolytic and oxidative) and transport phenomena take place in both the condensed phase and the gas phase as well as at their interface. This is especially true for chemical pathways leading to PAH and soot particle formation; their description by complex reaction networks is a topic of ongoing research. When a polymer is burned, there is generally a two-stage pyrolysis/combustion process occurring. Initially, heating of the polymer causes decomposition and evolution of pyrolysates; this is followed by oxygen diffusion into the pyrolysis product cloud, ignition, and establishment of a diffusion flame. In the diffusion flame, a significant amount of soot is formed and it agglomerates in particle chains (12). While still not completely understood, the mechanism of the thermal degradation of polystyrene is considered to be a radical chain reaction. Thermal decomposition of polystyrene has been studied under a variety of different conditions, e.g. in a vacuum, in inert and oxidative atmospheres. Michal (15) studied the thermo-oxidative degradation of polystyrene between 400 and 800 °C. Parikh (16) observed that no degradation of pure polystyrene occurred below 227 °C, while the polymer decomposed entirely at 332 °C during a residence time of 2 h. In an evaluation of the National Bureau of Standards Toxicity Test Method, in three laboratories, the mean temperature at which a sample of polystyrene spontaneously ignites was found to be 520 °C ( 62 °C (17). There is general agreement that the major volatile component produced from the thermal degradation of polystyrene under these conditions is the styrene monomer itself (16). Other commonly detected products are oligomers of styrene, such as the dimer and trimer, toluene, benzene, ethylbenzene, and R-methylstyrene. In addition, species such as benzaldehyde and benzoic acid are formed under oxidative conditions (16). A fair amount of studies in this laboratory and elsewhere have examined the combustion and emissions of commercial polymers, most often including polystyrene (18, 19). Some of these studies have investigated the PAH and soot emissions in addition to other pollutants, such as CO, benzene, and light hydrocarbon species, from the combustion/pyrolysis of polystyrene under batch or steady-flow conditions in various types of furnaces (9, 12, 13, 20-30), a brief review of which is given in ref 13. Burning of polystyrene has been reported to produce large amounts of PAH and soot, more than other common polymers, under most combustion conditions. A trend of decreasing total PAH yields has been observed either as the global equivalent ratio decreases in the fuel-lean domain or as the furnace temperature increases in an elevated temperature range (well above 1000 °C) (12, 23). Soot however, once formed, resists oxidation even at high furnace temperatures at the typically short residence times in furnaces. Many PAH species have been identified with the potentially hazardous phenanthrene being one of the most abundant. Soot particles are agglomerates of small spherical particles of diameters, most commonly, between 0.01 and 0.05 µm. Despite the structural similarity between soot particles and the inorganic particles produced from volatilized ash, the genesis of soot is much less well understood than that of the
FIGURE 1. Schematic of the two stage laminar experimental apparatus and a photographic illustration of a combustion event. The primary and secondary furnaces are separated by a mixing section, where additional air is introduced in a venturi. In one series of experiments a barrier honeycomb ceramic filter was inserted in the primary furnace, as shown. inorganic particles due to the extreme complexity of hydrocarbon chemistry in fuel-rich flames. The elementary composition of soot is changing in the high-temperature region of the flame. Its elementary C/H ratio is typically 8:1, but young soot particles containing considerably more hydrogen are in general present earlier in the flame. During the cooling of the combustion products, soot particles can adsorb hydrocarbon vapors, such as polycyclic aromatic hydrocarbons. According to Milliken’s (31) conceptual idea soot formation in premixed flames is the result of competition between the rate of soot precursor formation and the rate of oxidation of these species. A sharp rise in the critical C/O ratio necessary for soot formation occurs with decreasing temperature. Observation of soot in the exhaust indicates that chemical processes leading to soot precursors are faster than their oxidation, while the sharp rise of the critical C/O ratio at lower temperatures might mean that the soot formation process has essentially stopped under these conditions. Previous studies (32, 33) focused on the critical temperatures which are believed to control soot formation. The approach to avoid soot formation and growth should be to prevent soot particles from forming, because incipient particle formation controls the total mass of soot formed in any process using all kinds of different fuels. Soot formation has been shown to be a high activation energy process controlled by one or more critical steps. It has been shown that soot formation in diffusion flames can be reduced by decreasing the flame temperature due to a decrease of the pyrolysis rate of gas-phase hydrocarbons and thus the soot precursor formation rate (32). This concept was experimented within the combustion of polystyrene (13), and it was found that a primary furnace gas temperature in the vicinity of 600 °C minimized the formation and emissions of PAH and soot. Chung and Tsang (26) and Chung and Lai (27) showed that a reduction of soot formation in polymer flames may be achieved by (a) reducing the effective fuel/air ratio in the flame by decreasing the rate of polymer devolatilization and (b) affecting chemical and physical soot formation steps in the gas phase by introducing soot suppressors, such as combinations of one alkali and one alkaline-earth metal additives. It was shown that significant soot reduction can be achieved by improving mixing between the oxidizer (air) and the products of PS devolatilization since this mixing not only reduces the effective fuel/air ratio in the flame but also creates dilution and cooling effects. Related studies on other fuels have been conducted in order to gain a deeper insight in the physical and, especially,
chemical phenomena which led to the formation of soot particulates in hydrocarbon combustion and pyrolysis (34, 35). Soot formation was studied using increasingly complex experimental methods including optical techniques (36, 37) and molecular beam sampling (38). The development of detailed kinetic models describing soot formation and oxidation has been attempted for well-defined systems such as premixed flames (39) and perfectly stirred reactors (40). For more complex experimental setups, e.g. diffusion flames, flamelet models have been tested (41). Oxidation of soot inside the furnace is necessary for the reduction of particulate emissions. The rate of soot oxidation depends on the temperature, the residence time at that temperature, the partial pressure of oxygen, and the particle characteristics, such as particle size, porosity, pore size distribution, total surface area, density, etc. Effective mixing with air is also a very important parameter. The effect of soot oxidation both in an afterburner and in a high-temperature filter is explored in this work. A brief discussion on soot oxidation kinetics is included in a later section.
3. Experimental Techniques and Procedure 3.1. Fuel Characteristics. Polystyrene (PS) in the form of shredded styrofoam cups was used as the fuel. The fuel was cut in small pieces (≈5 × 5 mm) and was placed in porcelain boats. The elemental composition of the fuel is the same as of the polystyrene investigated in a previous study (13) (92% C, 8% H, 0.04% S) and was determined by Galbraith Laboratories. The fuel had a high content of volatile material, and 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.). The glass transition temperature of PS has been determined to be 100 °C, the melting point is 237.5 °C, and the specific gravity is 1.047. The minimum decomposition temperature has been reported to be 364 °C (42). 3.2. Experimental Apparatus, Horizontal Muffle Furnaces. Batch combustion experiments, involving fixed beds of shredded polystyrene fuel, were conducted in a horizontal, split-cell, electric muffle furnace (1 kW max.) fitted with a quartz tube, 4 cm in diameter, and 87 cm long. This primary furnace was connected to a secondary muffle furnace (the afterburner), as shown in Figure 1. The dimensions of the secondary furnace were 2 cm in diameter and 38 cm long. The effluent of the first furnace passed through a venturi (8 VOL. 36, NO. 4, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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mm in diameter) where it was mixed with four radially positioned perpendicular jets of preheated air, discharging at the periphery of the venturi (Figure 1). The estimation of the penetration of the radially placed air jets at the venturi provided an indication of good mixing of the effluent gas stream from the first furnace with the additional air. Calculations on the momentum flux ratios of jets in crossflow, accounting for measured temperatures at the venturi centerline and at the jet exits, showed that the four jets placed perpendicularly to the venturi penetrated effectively the effluent stream to the centerline of the venturi (13). The mixed charge was then sampled at the exit of the primary furnace and was subsequently channeled to the afterburner. The wall temperature of the secondary furnace was varied in the range of 900-1100 °C, while that of the first furnace was kept constant at 1000 °C throughout these experiments. The gas temperature profile in the primary furnace was measured by a suction pyrometer (23). The gas temperature was found to increase in the first half-length of the primary furnace but was fairly constant in its second half, for the most part 25 °C below the wall temperature. Hence, the first half length of this furnace acted as an air preheater. The filter was inserted in the primary furnace and was heated to the temperature therein, i.e., close to 1000 °C. The gas temperature in the unheated section of the quartz tube between the two furnaces was 250-300 °C. The gas temperature in the secondary furnace was constant for nearly its entire length, approximately 25 °C below its wall temperature. Upon reaching the predetermined wall temperature, a porcelain boat loaded with 0.5 g of sample was inserted from the tube’s entrance, and it was positioned in the mid-length of the quartz tube. To insert the samples quickly in the furnace the porcelain boats were placed at the tip of the inner surface of a half-tube (a quartz cylinder longitudinally split along the centerline). The other end of the half-tube was mounted at the entrance glass fitting of the furnace. At the start of every experiment, the fitting was opened, and the attached half-tube was extracted from the furnace (it was preheated), the sample boat was mounted at its tip and was quickly inserted in the furnace, the entire operation taking a few seconds. All experiments were conducted in atmospheric pressure air (21% oxygen). The air flowrate in the first furnace was 4 L min-1, and the gas residence time of the gases between the sample and the venturi was a fraction of a second. The flowrate of additional air supplied at the venturi, through the four jets, was 2 L min-1. The calculated Reynolds numbers, Re, in the venturi was 425. The residence time of the gases in the secondary furnace was calculated to be in the range of 0.6-0.8 s, according to the temperature therein, and the Reynolds number Re was in the range of 75-85. Sampling was conducted at the exits of both furnaces. While the typical duration of the luminous diffusion flame over the boat was in the order of 1 min, sampling lasted for several minutes to complete the combustion process. This was determined by continuously monitoring the CO2 emissions. 3.3. Monitoring of Combustion Emissions. Polycyclic aromatic hydrocarbon (PAH) emissions as well as NOx, CO, CO2, and particulates (mostly soot and, possibly, traces of minerals) from the combustion of polystyrene were monitored at the exits of the two furnaces. The PAH were sampled by passing half of the effluent through each of the sampling stages consisting of a Graseby sampling head with a filter stage and a glass cartridge containing XAD-4 adsorbent. The sampling stage was placed adjacent to the furnace to minimize losses as it can be seen in the schematic of the experimental setup shown in Figure 1. Prior to each sampling stage the effluent of the furnaces was mixed with a 2 L min-1 flow of nitrogen gas. This dilution nitrogen flow took place in the annulus of two concentric tubes; the inner tube was perforated and, therefore, enabled the mixing of the nitrogen 800
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with the furnace effluent (Figure 1). Hence, the effluent was cooled and inerted. Subsequently, the particulate emissions were trapped on the upper portion of the sampling stage on a 90 mm diameter by 1 mm thick Whatman glass fiber filter with a nominal pore size of 0.45 µm. Gas-phase polycyclic aromatic hydrocarbon emissions were adsorbed on the bed of Supelco XAD-4 resin. In order to ensure complete recovery the length of the XAD-4 bed was more than twice its diameter. After this the effluent passed through a mildly heated Permapure dryer, where the moisture was removed, it was then monitored with the following online analyzers: for NOx using a Beckman 951A chemiluminescent NO/NOx, for SO2 by means of a Rosemount Analytical 590 UV, and for CO and CO2 with Horiba infrared instruments. The output of the analyzers was recorded using a Data Translation DT-322 data acquisition board in a microcomputer. The integrated acquisition system used DT VPI to support data acquisition from within a Hewlett-Packard HP VEE visual programming environment and automatically formed Microsoft Excel data files corresponding to the different experiments. The signals from the analyzers were recorded for the duration of the experiment and subsequently were converted to partial pressures. 3.4. Extraction and Sample Preparation. After 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 25 µL internal standard containing 50 µg each of naphthalene-d8, acenaphthene-d10, anthracene-d10, chrysene-d12, and perylene-d12 was applied to each of the glass bottles containing the samples. The XAD-4 resin and cellulose filters used to trap the PAH emissions were oven-dried at 100 °C prior to use. To ensure the purity of the XAD-4 resin and cellulose filters, blanks of XAD-4 resin and celluose filters were also extracted and analyzed. In addition, a combustion blank was used in which the furnace was operated in the presence of the XAD-4 and cellulose filters, but with no fuel present. Target compounds that appeared in any of the blanks were appropriately qualified based on their concentration. Combustion experiments were at least duplicated, and preferably triplicated, to ensure the reproducibility of the combustion technique. A Dionex ASE 200 Accelerated Solvent Extractor 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. In the next step, they were filled with methylene chloride. After thermal equilibration at 40 °C, the cells were pressurized to 33 atm for a period of 15 min. Following this 15 min soak time the cells were each flushed with 80% of the cell volume using fresh methylene chloride and finally purged for 90 s with nitrogen. The methylene chloride extracts were collected in separate bottles and concentrated. Two extraction cycles were used per cell in order to achieve complete extraction. The total extraction time for the 2 cycle process was 25 min and about 45 mL of methylene chloride was used for the XAD-4 resins, while about 20 mL was implemented for the extraction of the filter papers. The original bottles that stored the combustion resins and filter papers were rinsed twice with 1 mL of methylene chloride and added to the vials containing the methylene chloride extracts. 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 cells have been thoroughly cleaned and inspected to ensure that small resin particles have not become trapped between the stainless steel cell body and the seals in the end caps of the cell. The samples were concentrated under vacuum to a final volume
of 10 mL and were analyzed by gas chromatography coupled to mass spectrometry (GC-MS). 3.5. Analysis by Gas Chromatography Coupled to Mass Spectrometry. Chemical analysis of extracts was conducted by means of gas chromatography coupled to mass spectrometry (GC-MS) using a Hewlett-Packard (HP) Model 5890 gas chromatograph equipped with a HP Model 5971 mass selective detector. The operating conditions of the GC-MS system and the data reduction were described previously (28, 43). The instrument was tuned in accordance with EPA semivolatile criteria prior to the GC-MS analysis of each set of samples. It passed initial and continuing calibration criteria, and no data were qualified as a result of calibrations. Relative response factors were calculated and all criteria were met. This was to be expected due to the well-controlled experimental conditions and the lack of environmental interferents or matrix effects. Each of the target compounds as well as the tentatively identified compounds were quantitated using the appropriate deuterated internal standard. The use of surrogates was not required for this analytical methodology because of the well-controlled experimental conditions, and experience has indicated that no matrix interactions occur between the target compounds and the components used in these experiments. 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 per gram of fuel combusted. Values that were less than 1 µg/g were 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. Standard solutions containing 50 µg each of five characteristic standards, i.e., naphthalene-d8, acephenanthrylened10, phenanthrene-d10, chrysene-d12, and perylene-d12, were diluted to 10 mL and analyzed as an instrument blank in order to provide an indication of extraction efficiency for each of the internal standards. In the case of samples with an extraction efficiency of less than 50% for any of the internal standards, the experiments were repeated. To assess the reproducibility of the PAH analysis, triplicate analyses were performed, and results are described in a following section. 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. 3.6. Extraction Recovery. The average recovery and statistical evaluation of the blanks, samples, and standards for a similar set of experiments has been provided previously (13). The extraction recovery was well above 50% for the internal standards in every sample analyzed which is in agreement with The National Functional Guidelines recommending a recovery of greater than 50% for the internal standards. Extraction recoveries for the internal standards resulting from the extraction of XAD-4 resins and filter papers with the ASE 200 automated solvent extractor were as follows: 64.8 ( 10.6% for naphthalene-d8, 76.2 ( 7.6% for acenaphthene-d10, 79.9 ( 6.2% for phenanthrene-d10, 91.0 ( 10.3% for chrysene-d12, and 103.1 ( 23.9% for perylene-d12. Remarkably the percent difference (ratio of standard deviation to average recovery) is low demonstrating the excellent reproducibility of this extraction technique for the analysis of XAD-4 resins and filter papers. 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 of this technique. The application of this technique would have to be substantially modified for the extraction of samples in which complex matrices are present. The lower recovery efficiencies observed for the more volatile of the internal standards most likely result from the vacuum concentration of the samples from 50 mL to 10 mL.
4. Results and Discussion 4.1. Oxygen Profiles and Evolution of CO and CO2 During Batch Combustion. Upon introduction of the 0.5 g of shredded styrofoam, placed in porcelain boats, to the preheated primary furnace the fuel bed heated, devolatilized, and ignited. A gaseous diffusion flame formed over the fuel bed (Figure 1), and a plume was visible moving to the exit of the furnace. The oxygen partial pressures, PO2, at the exit of the two furnaces experienced transient profiles, indicative of the derivative of the fuel devolatilization and its subsequent oxidation in the gaseous diffusion flame. The minima in the PO2 profiles ranged between 4 and 10% (of 1 atm) in the runs, indicating that globally there was excess oxygen for the oxidation of fuel pyrolysates in both furnaces. Since combustion of the polymer occurred in an unsteady diffusion flame one should be cautious about reporting an equivalence ratio, φ, even a global one. However, having said that, an overall φ of 0.83 may be calculated for the global fuel-air mixture in the first stage based on a sample mass of 0.5 g and an air flow rate of 4 L min-1, assuming perfect mixing and a reaction time of 100 s. The average integrated oxygen mole fractions at the exits of both furnaces were between 11% and 17%, as shown at the presence or absence of the filter in Figure 2a. The fact that more oxygen was consumed at the presence of the filter indicates better overall oxidation of fuel pyrolysates in the system. Integrated CO2 yields also reflect this, as the profiles were higher at the presence of the filter, see Figure 2b, CO2, CO, and H2O being the major oxidation products. The fact that CO2 yields also increased when the effluent went through the secondary furnace may be attributed to additional combustion therein. The effect of increasing the afterburner temperature on the CO2 yields was not significant. Peak CO partial pressures were 2.5-5% (of 1 atm), much lower than those of CO2 (8-20%). The presence of the barrier filter caused a consistent drastic increase in CO at the exit of the primary furnace, in respect to the experiments without the filter usage. This is attributed to the oxidation of the soot that was collected in this ceramic filter, which was maintained at a high temperature (approximately 1000 °C) during the experiments. Combustion of carbonaceous particles at elevated temperatures causes a higher release of CO emissions relative to CO2 (44). Furthermore, the CO emissions from the afterburner are also consistent with the results of ref 44, where the presence of soot in the furnace was found to be a source of CO. When the ceramic filter was not installed in the primary furnace, large amounts of soot traversed the afterburner. Partial oxidation of soot in the afterburner, as shown in Figure 2e, generated additional CO at the exit of the afterburner. Some CO perhaps oxidized to CO2; however, the net result was an increase in CO. Indeed, the CO yields increased by 20-25% with the afterburner treatment of the effluent, see Figure 2c. To the contrary, when the ceramic filter was installed in the primary furnace, insignificant amounts of soot traversed the afterburner. Hence little, if any, CO was generated in the afterburner and much of the entering CO was oxidized. As a result, the CO yields dropped by a factor of 2, see Figure 2c. The effect of the operating afterburner temperature on the CO emissions did not seem to be significant in the examined range. VOL. 36, NO. 4, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Typical profiles of (a) integrated average O2 partial pressures, (b) CO2 yields, (c) CO yields, (d) cumulative PAH yields, (e) particulate yields, and (f) data scatter in PAH yields. Data were collected at the exits of the primary furnace and the secondary furnace, with no filter present (N) or with the ceramic filter installed (F), and are shown as a function of the secondary furnace temperature. The primary furnace temperature was kept constant at 1000 °C. 4.2. Yields of PAH from the Combustion of Polystyrene. The cumulative emission yields of all detected polycyclic aromatic hydrocarbons (PAH), in a molecular weight range 802
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from 116 amu (indene) to 278 amu (benzo[b]chrysene and isomers), are plotted in Figure 2d. More than 120 semivolatile PAH compounds were detected. Plots of selected individual
FIGURE 3. Yields of selected individual major PAH components (µg/g) at the exits of the primary furnace and the secondary furnace, with no filter present (N) or with the ceramic filter installed (F). Yields are shown as a function of the secondary furnace temperature; the primary furnace temperature was kept constant at 1000 °C. component yields of PAH are shown in Figure 3. Generally, only 2-3 ring PAH were present in the gas phase and therefore mostly collected on the XAD-4. The heavier multiring compounds were found in the condensed (solid) phase, as part of the collected particulates on the filters. Isomers, i.e., different species with identical molecular mass and chemical formula, were also identified in the extracts. The number of possible isomers and therefore the complexity of the mixture
increases significantly with the number of carbon atoms, i.e., with the molecule size. This was further complicated by the presence of PAH, substituted by short saturated or unsaturated carbon chains, including methyl. This significantly increases the number of possible isomeric positions, particularly in the case of larger PAH. Treatment of the effluent of the primary furnace in the afterburner, after supply of additional air at the venturi, led VOL. 36, NO. 4, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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to a decrease of total PAH emissions (Figure 2d). Destruction of PAH was drastically enhanced with increasing afterburner temperature, in the investigated range between 900 and 1100 °C. The placement of the ceramic filter prior to the exit of the primary furnace lead to some reduction of PAH but to a smaller extent than the use of the afterburner. The repeatability of the PAH measurements is briefly illustrated in Figure 2f, which depicts cumulative amounts, and is discussed in more detail in ref 13. Profiles of selected predominant individual PAH are shown in Figure 3. Again four plots appear on each graph; please note that the yields at the exit of the first furnace do not depend on the afterburner temperatures. They are presented with straight lines, which are the average of all experiments (three afterburner temperatures, three repeats at every temperature, i.e., nine runs). In most cases the extra treatment in the afterburner reduced the concentration of the PAH species. This is mostly attributed to oxidation, as the oxygen concentrations in the afterburner were fairly high, see Figure 2a, the residence time was sufficiently long, 0.6-0.8 s, and the gases are expected to be well-mixed. This explanation is also supported by the fact that in most cases increasing the afterburner temperature further reduced the concentration of PAH species. This is to be expected as the oxidation kinetics of the PAH species are depending on temperature. The effect of the ceramic filter on PAH emissions was rather ambiguous, while the filter successfully retained most particulates, as shown in Figure 2e. In most cases the filter reduced the amounts of the individual (and cummulative) PAH species collected at the exit of the first furnace. However, at the exit of the secondary furnace the yields of PAH species were higher in the case when the ceramic filter was inserted in the apparatus, despite the lower inlet concentrations. This is attributed to the lower inlet concentration of oxygen (by 2-3% atm) in the second furnace during the experiments with the filter, see Figure 2a. This oxygen defficiency was caused by the additional oxidation of soot and hydrocarbons in the ceramic filter section of the primary furnace, which was evidenced by the rise in CO2 emissions, Figure 2c, and the dramatic decrease in soot emissions, Figure 2e. This trend could have been remedied by a higher supply of additional air at the venturi, during the experiments with the ceramic filter. An alternative explanation for this effect could be that the soot present in the secondary furnace without filtration acts as a sink for PAH, since PAH addition to soot might contribute to its further growth. Conversely, in experiments with filtration, the lack of soot in the second stage effluent would reduce this PAH depletion pathway, allowing for the higher second-stage PAH levels. Due to the high wall temperature (1000 °C) of the primary furnace in these experiments, no significant quantities of styrene dimers and trimers, identified at lower temperatures (9), were detected in the present study. It can be concluded that posttreatment of the effluent of polystyrene incineration opererated at temperatures of practical interest, i.e., between 900 and 1100 °C, after mixing with additional air, leads to a significant decrease of the emissions of individual PAH, including health hazardous compounds such as benzo[a]pyrene. Nevertheless, not only the release of PAH but also the emissions of particulates has to be limited. In this task the effect of the afterburner alone is insufficient, as is discussed in the next section. Thus, a high-temperature barrier filter was employed. The effect of the filter in reducing both PAH and particulate emissions, in conjunction with the afterburner, is promissing but not yet optimized in the current experimental setup. 4.3. Yields of Particulates from the Combustion of Polystyrene. The particulate yields at the exits of the two furnaces are shown in Figure 2e. The particulate matter was black in color and was shown to be soot, i.e., consisting mostly 804
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of carbonaceous material and extractable polycyclic aromatic hydrocarbons (PAH). This soot was a product of the highly luminous diffusion flame, formed during combustion of the polymer. Consistent with the constant primary furnace temperature (1000 °C), the corresponding particulate yields at the outlet of the first stage changed only slightly in this set of experiments and appear as nearly-straight lines in Figure 2e, indicating good reproducibility. Since the temperature of the primary furnace was fairly high (1000 °C), copious amounts of soot were generated in the even hotter diffusion flames, yielding amounts equal to almost one-third of the original mass of the burned polymer. To derive this result, the emission yields at the exit of the primary furnace shown in Figure 2e had to be multiplied by two since its effluent was split in two streams. The effect of the secondary furnace was beneficial in oxidizing particulate emissions from the primary furnace. The soot fraction that was oxidized in the afterburner was in the order of 20% at higher temperatures to 30% at lower temperatures. This finding illustrates the fact that once soot is formed in a flame, its destruction in a postflame is rather difficult, because of its slow oxidation kinetics under typical afterburner furnace gas temperatures and residence times, as discussed in the following section. Furthermore, there is evidence that at increasing afterburner temperature there may be mechanisms present that even form soot therein, since soot emissions increased with increasing afterburner temperature. This is contrary to what was to be expected if oxidation kinetics of soot were acting alone. This might indicate tightly balanced soot oxidation and formation rates with a relatively quicker increase of the latter one with increasing afterburner temperature. A second set of experiments was performed with hightemperature filtration of the exhaust gas of the primary furnace, before entering the afterburner. These experiments were conducted in order to investigate soot formation in the second stage without being affected by particles formed previously. Apart from a gain in the understanding of the formation of soot, the potential environmental benefit of high-temperature filtration in the primary furnace, collecting and burning the soot in the filter, was also assessed. In these experiments the high temperature filter removed 95% of the particulates from the effluent of the primary furnace, see Figure 2e. Since only some brown tars and oils were detected in the paper filter of the first stage, located after the ceramic filter, but not much black soot (solid carbon), the fraction of soot that was captured by the ceramic filter was higher than 95%. As this effluent stream passed through the afterburner the particulate emissions were reduced further at the lowest temperature. But as the afterburner temperature increased the particulate emissions also increased, indicating soot formation in the afterburner. Nevertheless, even at 1100 °C the particulate emissions at the second stage were much lower (by 85%) than those recorded at the absence of the ceramic filter. 4.4. Oxidation of Soot as a Function of the Afterburner Temperature. As mentioned above, soot was generated in the diffusion flame, which formed over the polymer bed in the primary furnace. At the absence of the ceramic filter, soot in the primary furnace effluent entered the secondary furnace (afterburner) which had been preheated to temperatures of 900, 1000, and 1100 °C. Due to the mixing of the effluent of the primary furnace with additional air at the venturi, the soot particles were expected to undergo oxidation in the afterburner. At higher afterburner temperatures the oxidation of soot should have been more efficient. However, as mentioned above, besides oxidation of soot there was also evidence of formation. To calculate the oxidation reaction rates for the soot in the afterburner furnace, the experimentally derived input and output masses were taken into account.
FIGURE 4. Intrinsic reaction rates of carbonaceous particulates (soot) to oxygen at an average integrated partial pressure in the afterburner of 0.146 atm. Comparison of experimental results with kinetic expressions, see the Supporting Information. The apparent reaction rate of soot oxidation was calculated as follows:
minlet - mexit t Assuming that for the small particle size of soot molecular diffusion limitations were negligible, the intrinsic oxidation reaction rate was approximated by
Rin )
∆m minlettS
where ∆m ) minlet - mexit is the loss in the mass of the collected quantity of soot through oxidation in the afterburner (g); minlet is the mass of soot collected at the primary furnace exit (inlet of the secondary furnace) (g); Rin is the intrinsic reaction rate of oxygen per unit mass of carbon per unit surface area per second (g/cm2-s); t is the residence time (0.6-0.8 s in the present work, depending on the gas temperature in the secondary furnace); and S is the surface area per gram of soot (cm2/g). Measurements of the surface area of diesel soots, conducted previously in our laboratory using CO2 adsorption, showed results in the neighborhood of 200 m2/g (2 × 106 cm2/g). This value was used in the equation above in order to calculate the intrinsic reaction rates in the afterburner based on experimental data. The resulting reaction rates are shown in Figure 4, compared to values deduced by means of three empirical expressions as given by Smith (45) for a variety of carbonaceous chars, by Nagle and StricklandConstable (46) for pyrolytic graphite, and by Neeft et al. (47) for diesel soot. Details of these relationships are shown in the Supporting Information. Intrinsic oxidation reaction rates deduced from the experimental data remained nearly unchanged over the investigated temperature range, while all three models predicted a significant increase with temperature. This discrepancy is likely to reflect the contributions of soot formation, which occurs in parallel to oxidation at the investigated temperatures.
4.5. Carbon Balance. The check of the carbon balance between reactants and products is an essential tool for the assessment of the suitability of the chosen experimental procedure. Based on the C/H ratio of polystyrene (1:1), the contribution of carbon to the mass of the fuel represents 92 wt %. The exhaust of the primary furnace has been split in two equal streams, one being collected and analyzed and one channeled to the afterburner prior to its characterization. Therefore, the entire mass of carbon identified at the exits of the primary furnace and of the afterburner should be equal to the mass the carbon contained in the initial fuel. The comparison of the total mass of CO, CO2, soot, and PAH led to a closure of the carbon balance by more than 85%. Light hydrocarbons which have not been taken into account and possible losses during sample collection and preparation are likely sources of error. Nevertheless, a lack of recovery of less than 15% confirms the suitability of the approach used in the present study. 4.6. 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 gas temperature of 1000 °C polystyrene generated some NOx, but it was always found to be at low levels (6-20 ppm). No SO2 emissions were detected, indicating that during the fuel-rich combustion of polystyrene its sulfur content (traces of 0.04 wt %) was converted to other compounds, possibly H2S, or remained in the soot as sulfates. 4.7. Emissions of Light Hydrocarbons. In the present and prior studies, the use of XAD-4 adsorbent has been shown to be suitable for efficient and nearly quantitative recovery of light polycyclic aromatic hydrocarbons (PAH), such as naphthalene and acenaphthylene, present in the gaseous effluent of fuel rich combustion processes. The presence of such two- and three-ring PAH in the exhaust gases after filtration in the sampling stage can be explained by their relative low boiling point, which does not allow the capture of a significant amount of these compounds on celluose filter papers prior to their solvent extraction. In addition to polycyclic aromatic hydrocarbons (PAH) of low boiling point, also light aliphatic hydrocarbons and one-ring aromatic species in particular benzene and its derivatives, are of great interest in order to assess potential pollution stemming from combustion of polystyrene and other municipal solid waste (MSW). Light hydrocarbons, such as acetylene and one-ring aromatics, have been shown to play an essential role in the chemical mechanism leading to larger and larger PAH and finally to soot (35); therefore, their measurement is expected to provide a deeper insight in chemical processes involved in the incineration of polystyrene. To collect light hydrocarbons prior to analysis by gas chromatography coupled to mass spectrometry (GC-MS), sample tubes containing Carbosieve and Carbotrap adsorbents were inserted in the exit lines of the primary furnace and of the afterburner after the cellulose filter and XAD-4. Preliminary results showed the presence of aliphatic species such as 1,3-butadiene, 1-buten-3-yne, and of aromatic compounds such as benzene and styrene. In addition, small quantities of naphthalene and acenaphthylene were detected but were negligible in comparison to the amounts identified on the XAD-4 adsorbent. This finding represents a valuable confirmation of the collection efficiency of the XAD-4 filter. No quantification of light hydrocarbons were conducted in the present work; relative concentrations showed a significant dependence on the temperature of the afterburner but without a homogeneous trend for all investigated species. Of particular interest is the observed temperature dependence of the concentrations of benzene and styrene at the exit of the afterburner. An unambiguous consumption of these species relative to their concentrations at the exit of the primary stage could be observed at afterburner temVOL. 36, NO. 4, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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peratures of 900 and 1000 °C, while at 1100 °C formation of these species occurred. Further work assisted by kinetic modeling will include detailed analysis of the correlation between the concentration of light hydrocarbons and the formation of PAH and soot. 4.8. Comparison with Emissions from Combustion of Other Fuels. Results of the present investigation have also been compared to previous work studying emissions from the combustion of fuels other than polystyrene, such as latex gloves and cotton pads (48). Such materials are particularly abundant in hospital waste streams for which incineration is, in general, the only way of destruction of chemical and biological contaminants. Emissions were compared at the exit of the primary furnace operated at 1000 °C. Particulates, i.e., mostly soot, as well as cumulative PAH emissions from batch combustion of polystyrene were lower than those from burning latex gloves but more than an order of magnitude higher than those from burning cotton pads. Emission yields of CO and CO2 from polystyrene combustion were shown to be comparable to those from latex and higher than those from cotton pads. The comparison of the combustion products of different fuels in the primary furnace and the investigation of the effect of posttreatment in the afterburner at different temperatures, assisted by kinetic modeling, will facilitate the identification of chemical reaction pathways.
5. Kinetic Modeling Update Numerical modeling by means of detailed kinetic modeling is a powerful tool to gain deeper insight in the chemical processes occurring in combustion processes. Complex networks of chemical reactions are used for the description of fuel consumption, the formation of stable and unstable intermediates, and the oxidation to final products such as CO, CO2, and H2O. In the case of insufficient availability of oxygen for a quantitative oxidation of the initial fuel and the primary as well as subsequent products of its oxidative or pyrolytic breakdown, growth to polycyclic aromatic hydrocarbons (PAH) of increasing size and ultimately to soot might occur. PAH and soot have been shown to be potentially health hazardous when released in atmospheric aerosols (5-7) or deposited in landfills together with other solid combustion products (49, 50). Oxygen deficiency is either corresponding to the overall fuel-to-oxygen ratio or related to local mixing imperfections in the combustion system. Under the experimental conditions of the present work, overall excess of oxygen is ensured in the first stage as well as in the afterburner. The presence of PAH and soot as observed experimentally at the exit of the first stage is due to the local lack of oxygen, inherent to the complexity of the combustion of solid fuels. The necessity of polystyrene gasification prior to its oxidation makes transport phenomena, at least locally, to be the ratedetermining step. Therefore, realistic numerical modeling of the primary furnace of the experimental setup used in this work requires the coupling of a detailed kinetic mechanism with the description of a complex flow field and will not be attempted at the present stage. The situation is different for the second stage, i.e., the afterburner. Posttreatment of the exhaust gas of waste incinerators is commonly used in practical devices. However, as seen in the present set of experiments, its effect on PAH and soot formed in the primary stage is often unsatisfactory. Kinetic modeling is a potentially powerful tool for the assessment of the chemical processes occurring in the afterburner and was first tested for the experimental setup of this work (13). As discussed above, the addition of air via the venturi (Figure 1) is expected to result in a well-mixed flow, externally heated. As a first approximation the afterburner is treated as a plug flow reactor in order to allow modeling using complex reaction mechanisms. 806
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In the present work the SENKIN code, a part of the CHEMKIN software package, has been used (51, 52) to predict the evolution of the concentrations of major pollutants, in particular PAH, in the afterburner. The comparison of predictions with experimental data allows for assessment of the predictive capability of the kinetic model. The model used in the present work describes by means of a detailed reaction set the combustion processes, including the growth of PAH up to a molecular mass of 300 amu (C24H12). It consists of 234 species and 961 reactions and is based on a reaction network developed and tested in premixed flames for lowpressure conditions (53, 54). In the atmospheric pressure version, pressure dependence of chemically activated reactions has been taken into account by means of quantum Rice-Ramsperger-Kassel analysis (55). The comparison of model predictions with experimental data available for ethylene combustion at 1 atm in a well-stirred reactor/plugflow reactor setup (56) confirmed the predictive capability of the pressure-adapted model (57). The experimental conditions studied in the present work, i.e., using a ceramic filter between the two stages, are particularly suitable for the assessment of the predictive capability of the kinetic model for the description of the afterburner in staged incineration of solid fuels such as polystyrene. The formation of solid soot particles as well as their reaction with gaseous species are not included in the current version of the model. The operation of the afterburner without the presence of solid species at its inlet allows therefore the wide exclusion of the effect of reactions of soot with gaseous species which are not taken into account by the model. Model calculations for an afterburner temperature of 900 °C were conducted and compared to the corresponding experimental data. Concentrations measured beyond the ceramic filter at the exit of the first stage for major PAH as well as for CO, CO2, and O2 were used as input of the plug flow computation together with mole fraction for N2, H2, H2O, C2H2, and phenylacetylene which were estimated by means of a well-stirred reactor calculation (13). No radicals were expected to survive through the filter; therefore, their concentrations were assumed to be negligible at the input of the afterburner. Also, as shown previously (13) radical concentrations are largely thermodynamically controlled, i.e., depend on temperature. Therefore, radical concentrations included in the input of the plug-flow reactor computation have no significant impact on the resulting reactions. Radical concentrations corresponding to the operating conditions (in particular equivalence ratio and temperature) are computed by the kinetic model for given reaction times. In cases for which previous experimental data were available, good predictive capabilities of the model for radical species such as H, O, and OH have been observed (53, 54). The temperature of the afterburner maintained in this experimental work was assumed to be constant and used as input of the model computations. In agreement with the experimental data, only a small consumption of most PAH was predicted in the afterburner for the case with the ceramic filter at 900 °C. Regarding PAH not detected or in minor concentrations at the exit of the first stage, only some insignificant formation occurred in the afterburner. For example, a mole fraction of about 2.4 × 10-9 of coronene (C24H12), the largest PAH included in the model, was predicted at a residence time of 0.8 s in the afterburner. The lack of reaction in the system is mostly due to computed very small mole fractions of H (e3 × 10-10) and OH (e5 × 10-6) radicals in the system. Such radical species are needed for the formation of PAH radicals prior to PAH decay or their further growth. Oxidation reactions of only a selected number of PAH, e.g., naphthalene, pyrene, and benzo[a]pyrene, are included in the current version of the model. A good understanding of the oxidation of PAH and their radicals is
Literature Cited
FIGURE 5. Rate of production of pyrene; total and contributions of individual reactions. Temperature of the secondary furnace at 900 °C, at the presence of the ceramic filter. likely to be critical for the correct assessment of PAH degradation by posttreatment. As an example, the case of pyrene oxidation was examined in the present work. Significant concentrations of pyrene have been found in different combustion systems (53, 56), and pyrene has been previously selected as surrogate of PAH contamination (58, 59). Based on the suggestions of Wang and Frenklach (60), oxidation of pyrene with O and OH as well as of pyrenyl radicals with molecular oxygen is included in the model. Different from most of the other PAH, and in contradiction to the experimental data, nearly all pyrene was depleted in the model prediction. This indicates the inadequacy of this description of pyrene oxidation for the conditions of the present work. The net rates of production of pyrene at different residence times as well as the contributions of individual reactions to pyrene formation and consumption were determined by means of the postprocessor included in the CHEMKIN software package (52) and are shown in Figure 5 for reaction times up to 10 ms. Reaction times up to 0.9 s were explored, but no additional reaction was observed. No significant contribution of pyrene formation reactions was shown to occur in the system. The dominant individual pyrene consumption reaction was its oxidation with O radicals to 4-phenanthryl leading to the loss of one condensed ring. Pyrene depletion via reactions with OH leading to the corresponding pyrene radicals also represents a major pathway. This significant underprediction of pyrene indicates the need for further investigation of pyrene oxidation with O radicals. However, removal of this reaction from the model led only to a small increase of the predicted pyrene mole fraction; therefore, detailed analysis of subsequent reactions of pyrene radicals, in particular their oxidation, is necessary and will be undertaken in future work. Also, possible additional pyrene formation pathways will be explored. A first conclusion seems to indicate the possibility of operating the afterburner at significantly smaller reaction times due to the lack of reactions in the system at larger residence times, of course, given that mixing is adequate.
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 Calculation of the reaction rate of soot in the afterburner. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review May 2, 2001. Revised manuscript received October 15, 2001. Accepted November 1, 2001. ES0109343
